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Northern England British regional geology

Fifth Edition Authors: P Stone, D Millward, B Young, J W Merritt, S M Clarke, M McCormac, D J D Lawrence

Bibliographical reference: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M, and Lawrence, D J D. 2010.  Northern England British Regional Geology Fifth edition. Keyworth,  Nottingham:  British  Geological  Survey.

British Geological Survey

Fifth Edition

Authors: P Stone, D Millward, B Young, J W Merritt, S M Clarke, M McCormac, D J D Lawrence

Contributors: R P Barnes, A S Butcher, D C Entwisle

©  NERC 2010 All rights reserved. British Geological Survey, Nottingham  2010. First published 1935.Second edition 1946. Third edition 1953. Fourth edition 1971. Reprinted with additional bibliography 1978. ISBN 978 085272 652 5. Printed in Scotland by Scotprint, Haddington.

The grid, where it is used on the figures, is the National Grid taken from Ordnance Survey mapping. Maps and diagrams in this book use topography based on Ordnance Survey maps.

Definitions  of  stratigraphical units mentioned in the text may be found via the British Geological Survey’s internet site in the Lexicon of Named Rock Units. ©  Crown Copyright reserved Ordnance Survey licence number 100017897/2010.

Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or  the authority that commissioned the work. You may not copy or adapt thispublication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

Your use of any information provided by the British Geological Survey (BGS) is at your own risk. Neither BGS nor the Natural Environment Research  Council  gives  any  warranty, condition or representation as to the quality,  accuracy  or  completeness of the information or its suitability  for any use or purpose. All implied conditions relating to the quality or suitability of the information, and all liabilities arising from the supply of the information (including any liability arising in negligence) are excluded to the fullest extent permitted by law.

(Front cover) Lindisfarne Castle, in the south of Holy Island, Northumberland, stands on one of the segments of the Holy Island Dyke, a feeder to the Whin Sill-swarm. The dolerite dyke was intruded into lower Carboniferous strata during earliest Permian times. The bench feature in the dolerite in the foreground is interpreted as part of a step-and-stair transgression of bedding during dyke intrusion (P637429). (Photographer: F MacTaggart, British Geological Survey).

Foreword to the fifth edition

Northern England is underlain by a wide variety of rocks with an exposed geological history spanning almost 500 million years. Its mineral wealth, in terms of both coal and metal, has driven the industrial development of parts of the region and has contributed much to the prosperity of the United Kingdom as a whole. The underlying geological diversity also imparts to the region its scenic range and high landscape value. Two of England’s National Parks, the Lake District and Northumberland, form substantial parts of the area described here whilst a third, the Yorkshire Dales, overlaps its southern margin. Recognised Areas of Outstanding Natural Beauty include the Northumberland Coast, the Solway Coast, and the North Pennines, the last of these areas is also designated a UNESCO European and Global Geopark.

The first edition of this guide to the geology of Northern England was written by T Eastwood and published in 1935. The second and third editions were closely based on the original work, though with some additions by F M Trotter and W Anderson. The fourth edition was prepared by a team of authors led by B J Taylor, but was still acknowledged to be based  on the previous editions by Eastwood. The fourth edition was published in 1971, but its preparation was mostly carried out in the late 1960s, before the full effects of the plate tectonic revolution had permeated all aspects of geology. This fact alone would justify a new edition, but the huge volume of additional data generated over the last 50 years makes it long overdue. The fresh insights and interpretations presented here owe much to those who have generated this data: colleagues within the British Geological Survey, academic researchers, energy and mineral exploration geologists, civil engineers and, increasingly, scientists working with a broadly based, environmental brief.

With its historical pedigree, the Taylor et al. text has continued to provide a valuable source  of data for Northern England and its influence can undoubtedly still be found in this, the new fifth edition, which is nevertheless an entirely new work. The authors have approached their task from a perspective of dynamic geological development, in which tectonic processes, sedimentation and magmatism are integrated, rather than being treated separately in the traditional fashion. The book gives a comprehensive account of the geology of the region that it is hoped will prove informative to a wide range of users, from students and those with a general interest in their surroundings, to professional scientists, planners and engineers.

John N Ludden, PhD Executive Director British Geological Survey

Acknowledgements

Chapter 1 Introduction

This account describes the geology of northern England southwards from the Scottish border to the latitude of Morecambe Bay and Teesside ( Figure 1). It covers England’s four northernmost counties: Cumbria, Durham, Tyne & Wear and Northumberland, with some overlap at the southern margin into Cleveland, Yorkshire and Lancashire. The region is underlain by a wide variety of rocks ( Figure 2) with a geological history spanning almost 500 million years ( Table 1). It is this underlying diversity that imparts to the region its scenic range and high landscape value. Two of England’s National Parks, the Lake District and Northumberland, form substantial parts of the area described here whilst a third, the Yorkshire Dales, adjoins its southern margin. Recognised Areas of Outstanding Natural Beauty include the Northumberland Coast, the Solway Coast, and the North Pennines, the last of these areas also being designated a UNESCO European and Global Geopark.

At the centre of the Lake District (an inlier of Lower Palaeozoic rocks) is Scafell Pike which, at 977 m above sea level, is England’s highest mountain. The eleven principal lakes occupy deep glaciated valleys disposed in a broadly radial pattern around the high fells of the Scafell massif ( Figure 1). Surrounding Scafell is craggy, mountainous terrain formed by Ordovician volcanic rocks and associated igneous intrusions, mostly granitic ( Plate 1). To the north are smoother, more rounded hills underlain by Ordovician sandstone and mudstone; the highest of these hills is Skiddaw at 931 m (equivalent strata form the highest point in the Isle of Man, Snaefell, 621 m). Still farther north, the rolling countryside of the Caldbeck Fells encompasses more Ordovician volcanic rocks and a variety of igneous intrusions. The southern Lake District is characterised by undulating hills and rocky crags made up mostly from Silurian strata; the same rocks extend eastward to form the Howgill Fells massif, rising to over 700 m. Whilst these mountain landscapes have been largely sculpted by glacial erosion, the depositional legacy of the ice age can be seen in many lowland areas. Most spectacular are the major spreads of drumlins — elongate mounds of glacial detritus — that swing westward across the southern and northern margins of the Lake District and extend eastward through the Stainmore gap.

The mineral wealth of the Lake District has long been exploited. In the 16th century the area was one of the world’s principal copper producers and many other minerals have been worked over the ensuing years. All of the mines have now closed, but the most recent activity, at Carrock Fell, continued until 1981. Slate and aggregate quarrying are now the only extractive industries, and also have a lengthy history going back to perhaps the earliest exploitation of the Lake District’s geological resources: the Neolithic stone axe industry of Langdale.

Towards the coast of north-west England, the Lake District fells give way to undulating lowland as the ancient rocks disappear beneath a cover of younger, Carboniferous strata, which now crop out across much of west and south Cumbria. In the south, limestones give rise to areas of distinctive karst scenery (Frontispiece) whilst in the west the Carboniferous sequence includes coal and the limestone contains substantial deposits of haematite. Together, these resources fuelled the now largely historical heavy industries of Workington, Whitehaven and Barrow-in-Furness; deep mining for coal ceased in 1986 but some coal is still won from open-cast sites. On the north-west Cumbrian coast, at St Bees Head, spectacular cliffs are formed by red Triassic sandstone ( Plate 2) and eastwards from there the Permo-Triassic strata extend beneath the gentle lowlands that border the Solway Firth. Around Carlisle the sedimentary sequence extends upwards into the Lower Jurassic.

Farther to the east, the fells of the Bewcastle and Liddesdale areas mark the outcrop of Carboniferous strata along the Scottish Border. From beneath these rocks in north Northumberland emerge the Devonian lava and granite of the Cheviot Hills. The geological features do not precisely respect the Anglo–Scottish border, which in this wild and remote area bisects the igneous massif. The summit of The Cheviot (815 m) lies just on the Northumberland side of the border.

Inland from the sandy bays and ruined castles of the Northumberland coast run a series of ridges formed by hard, resistant rocks. Most of the ridges testify to tough, Carboniferous sandstone, but the most prominent of them arises from the Whin Sill, an intrusive sheet of earliest Permian dolerite. The Whin Sill provides the imposing, defensive foundation of Hadrian’s Wall, frontier of the Roman Empire, for much of its seventy-mile course across the narrowest part of England; it is now a World Heritage site ( Plate 3). South of Hadrian’s Wall is the high, flat moorland of the northern Pennines. The moors rise gently westward to the spectacular west-facing Pennine escarpment at Cross Fell (the highest point in the Pennines at 893 m) that overlooks the broad, fertile lowland of the Vale of Eden. Carboniferous rocks, mostly sandstone and limestone form the fault-controlled escarpment, at the foot of which they overlie Lower Palaeozoic rocks of Lake District character. The Vale of Eden, separating the Pennine escarpment from the main Lake District massif, is mostly floored by Permian and Triassic sandstone.

A distinctive feature of the northern Pennine landscape is the marked terrace featuring of the hillsides caused by the alternation of relatively softer and harder beds within the Carboniferous sedimentary succession ( Plate 4); resistant sandstone or limestone usually forms the steep ‘steps’, and mudstone forms the intervening terraces. The harder rocks also host myriad veins of base metal ore that have been worked since Roman times, and many of the area’s small towns and villages owe their development to the once-prosperous mining industry. Whereas ores of lead, zinc and silver were mostly sought in the 19th century industrial heyday, working of baryte, witherite and fluorspar were of greater importance in the 20th century. All mining has now ceased but its effect remains a major influence on the northern Pennine landscape.

Towards the east coast, the limestone and sandstone of the Pennine uplands give way to the Carboniferous coal-bearing measures of the Northumberland and Durham Coalfield. This was one of Britain’s earliest worked coalfields and supported the steel and shipbuilding industries of Tyneside and Wearside. Coal mining has declined in importance in recent years and, with the closure of the last underground mine in 2005, coal working is now restricted to a handful of opencast operations ( Plate 5). Within the coalfield lies the ancient cathedral city of Durham, one of Britain’s World Heritage Sites, built within a magnificent incised meander of the River Wear. To the east of Durham, the Carboniferous rocks of the coalfield are concealed beneath Permian limestone, the outcrop of which is marked by a distinctive belt of arable country terminated by the limestone cliffs of the Durham coast.

The geodiversity seen across northern England is striking. Throughout the region, the underlying geology has had a strong influence on the scenery and pattern of land use. That influence has been locally enhanced or moderated by the erosive and depositional effects of the Quaternary ice age. Some upland areas were scoured by ice, whilst some lower-lying districts were plastered with a thick layer of glacially transported debris — remember the drumlin swarms to the north and south of the Lake District. The human factor has added a further influence, whether through mineral exploitation or the use of local building materials. And still, of course, the landscape is evolving through both natural and anthropogenic processes. So why does northern England look the way it does? How did this collage of rocks come about? The region’s dynamic geological evolution is explored in the following account.

Regional tectonic framework

England’s northern geographical border coincides, more or less, with one of the most fundamental geological boundaries in Britain. This is the Iapetus Suture, the trace of a long- vanished, Early Palaeozoic ocean obliterated by the convergence and ultimate collision of the ancient continents that it once separated. The Iapetus Ocean (as a forerunner of the Atlantic Ocean it was named after the father of the eponymous Atlas) was initiated during late Neoproterozoic times and grew to a likely maximum width in excess of 1000 km by the end of the Cambrian Period (Figure 3a). Thereafter, subduction at its margins wrought its eventual destruction and drove the series of collisional events that built up the Caledonian Orogen, a major tectonic zone that can be traced from Scandinavia, through Britain and Ireland, and on into Greenland and maritime North America.

To the north of the Iapetus Ocean, in sub-tropical latitudes, lay the continent of Laurentia. The Archaean and Proterozoic crystalline basement rocks of Scotland formed a part of this continent, and subduction of Iapetus oceanic crust beneath its margin led to the sequential accretion of oceanic terranes. The history of the ocean’s destruction at this northern margin is recorded in these: for example, in the Tremadoc to Arenig, Ballantrae Complex ophiolite, the obduction of which occurred at about 470 Ma during the collision of a volcanic arc complex with the continental margin. This collision provoked the Grampian Event of the polyphase Caledonian Orogeny. Thereafter, continued late Ordovician to mid Silurian subduction at the northern margin of the Iapetus Ocean is demonstrated by the Southern Uplands accretionary thrust belt.

At the southern margin of the Iapetus Ocean, at a latitude of about 60° south, lay the shores of the Gondwana continent from which a small fragment broke away early in Palaeozoic times. This fragment, known as Avalonia, drifted north, towards Laurentia, as the intervening Iapetus Ocean closed (Figure 3b). In northern England, Lower Palaeozoic inliers ( Figure 2) reveal parts of the northern margin of Avalonia and show how it developed in response to the changing geotectonic regime. The oldest rocks present in the region are likely to be those from the enigmatic, possibly Neoproterozoic, metasedimentary Ingleton Group. This unit is exposed a little to the south of the Lake District in small inliers around the Craven district. Within the main Lake District inlier, the oldest rocks seen are the Tremadoc to Llanvirn, turbiditic mudstone and sandstone of the Skiddaw Group (a broad correlative is the Manx Group in the Isle of Man) that were deposited in extensional basins along the continental margin of Avalonia as it rifted from Gondwana. As much as 5000 m of strata accumulated with much evidence for large-scale slumping of the unconsolidated sediment.

Along other parts of the Avalonian continental margin, and as close to the Lake District as north Wales, there is evidence from volcanic rocks that southward subduction of the Iapetus Ocean commenced during late Cambrian times, but in the Lake District inlier, the earliest subduction-related magmatism was late Ordovician in age. As a precursor to volcanic activity, the deep-marine strata of the Skiddaw Group were uplifted and eroded. Then, during a short-lived but violently active Caradoc volcanic interval (<5 million years culminating at about 450 Ma — see summary of age data in ( Figure 4)), the Borrowdale and Eycott volcanic groups were erupted. These groups now crop out, respectively, to the south and north of the Skiddaw Group, a disposition inherited from their having originally built up within opposing half-grabens, each originally 40 to 50 km wide. Several thousand metres of lava, ignimbrite and volcaniclastic sediment were preserved in a series of subsiding volcanic calderas. Beneath the volcanoes, granitic plutons were emplaced, coalescing to form the major part of the Lake District Batholith: the exposed intrusions of Eskdale, Ennerdale and Threlkeld are representative.

The relative brevity but great intensity of the Borrowdale–Eycott volcanic episode may have been the result of the subduction of the Iapetus Ocean spreading ridge at its Avalonian margin. Such a phenomenon would have two important outcomes. Firstly, it would allow the disruption of the subducting slab which might explain the abrupt cessation of volcanism at the Avalonian margin. Secondly, it would combine Avalonia with the oceanic plate that was being subducted northwards beneath the Laurentian margin of the ocean. These arguments follow from associating the Lake District volcanic sequences with subduction of Iapetus Ocean crust, but an alternative possibility should also be admitted. In this view, the Borrowdale and Eycott volcanicity was generated by subduction of Tornquist Sea crust during convergence of Avalonia with Baltica, and should be linked with the contemporaneous volcanic rocks (known mostly from boreholes) that occur along the eastern margin of Avalonia in what are now the English Midlands and parts of Belgium. In truth, it is probably wrong to regard these two possibilities as being discrete alternatives, since the processes involved must have been tectonically linked and spatially continuous.

Thermal subsidence followed the cessation of volcanic activity and granite intrusion, allowing marine transgression across the eroded remains of the Borrowdale and Eycott volcanoes during late Ordovician times. The Dent Group, the lowest part of the Windermere Supergroup and of Ashgill age, encompasses a range of shallow marine lithofacies with some volcanic rocks produced during the final throes of volcanicity ( Figure 4). It was followed during the Llandovery by accumulation in deeper water of a thin condensed sequence of marine mudstone, the Stockdale Group. Meanwhile, the convergence of Laurentia and Avalonia continued, with the Southern Uplands accretionary thrust terrane testament to the Caradoc to Wenlock subduction of Iapetus Ocean crust beneath the margin of Laurentia. As the ocean narrowed, the fossil faunas preserved at its opposing margins became progressively more cosmopolitan.

The inevitable collision occurred at some time during the mid Silurian, but was something of a tectonic anticlimax. It was not a mountain-building event of orogenic proportions and its effects are hard to find in the tectonic record preserved in the rocks. There was instead something of a tectonic continuum, as the Southern Uplands accretionary thrust terrane overrode Avalonia and continued southwards as a foreland fold and thrust belt. A load-induced, flexural foreland basin advanced ahead of the thrust front and was an influential control on sedimentation during the accumulation of the mid to late Silurian parts of the Windermere Supergroup. Subsidence and sedimentation rates both reached their maximum during late Silurian (Ludlow) times with deposition of the Coniston Group, up to 2000 m or maybe more of turbidite sandstone that accumulated during the course of less than two graptolite biozones, probably no more than one million years. Thereafter, the later Ludlow and Pridoli succession reflects a slowing of the subsidence rate and a commensurate filling of the sedimentary basin. It would seem that convergence between Laurentia and Avalonia ceased, the foreland basin failed to migrate southwards, and isostatic adjustments reversed the earlier effects of loading.

The tectonic effects seen within the exposed rock sequence, though created by collision-related processes, give little indication of the deeper structure of the suture zone. This is more usefully modelled from geophysical data. A number of seismic lines have traversed the Iapetus Suture Zone and have generally been interpreted in terms of a north-west-dipping reflective zone projecting to the surface close to the northern coast of the Isle of Man and thence striking north-east beneath northern England ( Figure 5). Regional interpretations of gravity and magnetic data present a rather more complicated picture in which Avalonian-type crust is caught up in a compound suture zone that extends well to the north beneath the Southern Uplands terrane ( Figure 6).

The continental collision between Laurentia and Avalonia was not an orthogonal event. A wealth of evidence shows that sinistral strike-slip movement was important during the later stages of convergence, and indeed may have been the dominant final effect. One result was the establishment of a late Caledonian fault pattern involving conjugate systems trending north-west and east-north-east across both the Southern Uplands terrane and the Lower Palaeozoic outcrop in northern England. These faults were to have a profound structural influence during subsequent episodes of extensional tectonism when their reactivation controlled late Palaeozoic basin development and geometry. More immediately, in the transtensional tectonic regime pertaining to Early Devonian times, strike-slip basins opened across the region and were filled with the clastic, terrestrial sediments of the Old Red Sandstone lithofacies.

It is something of a geological paradox that the Lower Palaeozoic rocks of the Avalonian margin did not experience their maximum deformation as a result of the Laurentia–Avalonia collision. Instead, that event ground to a halt and was followed by a compressive hiatus; it was not until the Mid Devonian Acadian Orogeny that pervasive deformation and cleavage formation occurred. Stratigraphical constraints, the relationship of cleavage fabrics to dated granite intrusions, and dating of white mica formed within the cleavage planes, all combine to suggest an Emsian age, of about 400 Ma. The metamorphic grade attained then, albeit very low, still implies a substantial overburden, now eroded, of nonmarine Old Red Sandstone strata. A minimum 3500 m pretectonic thickness for this cover has been estimated in the southern Lake District, and tectonically driven subsidence of the underlying Avalonian crust is clearly required to accommodate such a thickness of nonmarine sediment.

One possible mechanism is flexure of the Avalonian footwall by continued shortening along the Iapetus Suture Zone. This has an immediate attraction in that it links the well-established tectonic models for the Southern Uplands — imbricate thrust belt — and southern Lake District — foreland basin — but sadly fails on several scores. In local terms, in the Windermere Supergroup foreland basin in the southern Lake District, a decelerating subsidence rate allowed the basin to fill during latest Silurian and earliest Devonian times; the implication is that the convergence rate declined rapidly well before the onset of Acadian deformation. Further, the age of that deformation is consistent across Britain and Ireland, which is incompatible with a prograding, flexural mechanism. A more likely explanation for Early Devonian subsidence derives from the sinistral strike-slip model, whereby the Old Red Sandstone accumulated in transtensional basins, formed during the hiatus in compression that separated the mid Silurian, final convergence of Laurentia and Avalonia, from the Early Devonian, Acadian Orogeny. The Acadian Orogeny itself was probably initiated by collision of another rifted Gondwanan fragment, Armorica, with the south side of Avalonia (Figure 3c).

Accompanying or closely following the Acadian deformation was the intrusion of lamprophyre and felsic dyke swarms and a range of granitic plutons ( Figure 4) with, in the case of the Cheviot Pluton, associated volcanicity. The subvolcanic batholith that had previously formed beneath the Lake District during the Ordovician magmatism was now enlarged by the intrusion of Acadian components at its margins: the Skiddaw granite in the north and the Shap granite in the south. In addition, the several granitic plutons of the substantial North Pennine Batholith were emplaced beneath the central part of northern England and now underpin the structural high of the Alston Block ( Figure 7). These igneous phenomena are all more readily accommodated within a transtensional tectonic model than within one requiring convergence-driven crustal flexure.

The disposition of Acadian and earlier structures and intrusions then became the principal control on the pattern of renewed extension late in Devonian times and through the Carboniferous. A major structural control on sedimentation in the Nothumberland–Solway Basin is the Maryport–Stublick–Ninety Fathom (M–S–NF) fault system ( Figure 7) that, together with most of the other large synsedimentary faults, appears to be inherited from reactivated, pre-existing basement structures. In particular, the east-north-east-trending M–S–NF faults formed by extensional reactivation in the hanging wall of the Iapetus Suture Zone. They define the northern margin of the Lake District and Alston blocks, structural horsts underpinned by buoyant granitic massifs. The southern margin of the Alston Block (and northern margin of the Stainmore Trough) is formed by the Closehouse–Lunedale–Butterknowe fault system. A broadly similar structural control extends farther south, beyond the confines of the area described in this account, to define the Askrigg Block (underpinned by the Wensleydale granite pluton) and the adjacent Craven/Bowland Basin. To the west, the Isle of Man is formed by a separate structural block above the Manx Pluton.

Throughout the region there is a probability that structural control inherited from the underlying Lower Palaeozoic and older basement continued from Carboniferous into Permian and later times. Separating the Alston and Lake District blocks is the Vale of Eden half-graben, a mostly Permian extensional feature controlled at its north-east margin by the north-west­trending Pennine Fault system. To the west of the Lake District Block, the north-west-trending Lake District Boundary Fault fulfils a similar structural role at the margin of the Irish Sea Basin. Basement control by structures within the Iapetus Suture Zone has also been invoked to explain aspects of Carboniferous volcanism and the intrusion of the early Permian Whin Sill-swarm at about 300 Ma. But consideration of these phenomena is to get a little ahead of the geological story.

Carboniferous sedimentation patterns across northern England were strongly controlled by the subsiding basins and the intervening, more stable blocks. All were developed within a much larger basin system that extended from Ireland, eastwards through north Wales, northern and central England — where it is known as the Pennine Basin — to the North Sea. Within northern England, the stable blocks underpinned by granite batholiths are characterised by thin and incomplete sedimentary sequences; the subsiding basins were infilled with sediment and contain thick and relatively complete sedimentary successions. By this time the region formed part of the southern margin of Laurussia, a ‘supercontinent’ created by the amalgamation of Avalonia, Laurentia, Baltica and other terranes of Asian Russia. The British sector drifted slowly north through equatorial latitudes. An initially arid climate became progressively more hot, humid and wet during that northward drift, and then reverted to arid conditions towards the end of the period. The tectonic regime was broadly extensional, with dextral strike-slip becoming progressively more important.

The Carboniferous sedimentary record is the result of a complex interplay between several factors: subsidence rates, changes in sea level, deposition of limestone in shallow water, and the progradation of major sandstone deltas into the subsiding basins where mudstone accumulated in the deeper water. Early Carboniferous sedimentation was in fluvial and lacustrine to paralic environments, building up the dominantly clastic Inverclyde and Ravenstonedale groups. With continuing subsidence, a greater marine influence is seen in the succeeding strata, the largely deltaic Border Group and the reef to detrital carbonates of the Great Scar Limestone Group. Through the middle part of the Carboniferous succession, a major delta system built out into the subsiding basin and the Yoredale Group accumulated. Cyclic sedimentation was a particular feature, with the sandy, alluvial and deltaic flats subject to intermittent marine incursions that allowed the development of limestone. The colonisation of the delta tops by lush swamp vegetation, and its subsequent burial and conversion to coal, has been a key economic factor in the modern history of the region. This phenomenon is a particular feature of the upper Carboniferous, Pennine Coal Measures Group. Finally, towards the end of the Carboniferous Period, deposition of the Warwickshire Group occurred in a fluvial to deltaic plain setting but under increasingly arid, oxidising climatic conditions; coal is largely absent and the strata are generally reddened.

By late Carboniferous times, the thermal subsidence that had largely controlled sedimentation patterns earlier in the period began to wane. Instead, the region experienced the peripheral effects of a major orogenic collision as, far to the south, Laurussia and the huge Gondwana continent came together to unite the Earth’s land areas into the single vast expanse of Pangaea (Figure 3d). The compressive deformation associated with these events, the Variscan Orogeny, was most intense across mainland Europe, and the southern parts of England, Wales and Ireland, south of the so-called ‘Variscan Front’. Related deformation in the north of England was relatively weak and manifest largely by dextral transtension. It spanned an interval of approximately 15 Ma from the late Carboniferous to the early Permian, and was accompanied by the large-scale intrusion of basaltic magma to form the Whin Sill-swarm and the associated Northern England Tholeiitic Dyke-swarm ( Figure 4).

Renewed extension in Permian times reactivated the broadly north-trending Caledonian structures to form the margins of depositional half-graben basins such as the Vale of Eden; pre-existing east–west faults experienced mainly strike-slip reactivation at this time. Global sea level during Permo-Triassic times was relatively low, and the northern British region was located far from the contemporary coastline, within the interior of Pangaea and a little to the north of the contemporary Equator (Figure 3d): about 10°N at the beginning of the Permian, drifting to about 30°N by the end of the Triassic. Thus, the onshore sequences are largely the result of terrestrial sedimentation in an arid environment. There was also much weathering and erosion of the newly created upland areas, with as much as 500 m of Carboniferous strata worn away in some places. Several late Permian marine transgressions are evident, particularly in the North Sea sequence and its onshore continuation in northern England, but more general marine transgression did not reach the region until late in the Triassic.

The lowest Permian strata of northern England, comprising the Appleby and Rotliegendes groups, are mostly aeolian and fluvial sandstones with conglomerates derived locally from the sides of the fault-defined depositional basins. Later in the Permian, the limestone and evaporite deposits of the Cumbrian Coast and Zechstein groups were produced by the series of marine inundations and the subsequent evaporation of the trapped sea water. A broadly similar sedimentary pattern continued through much of the Triassic Period. Deposition was initially of sandstone and mudstone, the Sherwood Sandstone Group, effected by ephemeral rivers on braided alluvial plains and playa mudflats. Later in Triassic times, the marine influence was stronger, as seen in the sequences of mudstone, evaporitic gypsum and limestone, the Mercia Mudstone and Penarth groups, which are now preserved in the Solway/Carlisle Basin and to the south of the Lake District along the northern shores of Morecambe Bay.

It was the break-up of the Pangaea ‘supercontinent’ during Late Triassic times that brought an end to the prolonged period of mainly terrestrial conditions across northern England. Marine transgression extended across an ever-widening area until, by Early Jurassic times (Figure 3e), global sea levels were relatively high and most parts of the region were submerged. Calcareous marine mudstone from this time, part of the Lias Group, is preserved in the centre of the Solway/Carlisle Basin. A period of uplift and erosion in Mid to Late Jurassic times is recorded by a widespread unconformity, with the maximum effect seen in North Sea basinal sequences. The commensurate fall in sea level continued through the Early Cretaceous interval and an extensive unconformity developed across the surrounding land areas, until rising sea levels brought renewed marine transgression through the later part of the Cretaceous. Continental drift had carried Britain to the latitude of about 35°N by the end of the Triassic and a slow northwards drift continued during the Jurassic and Cretaceous periods. The climate was strongly seasonal with warm, relatively dry summers and cool wet winters.

The later Mesozoic geological history is obscure, with no sedimentary rocks from that interval, or the succeeding Cenozoic, preserved in northern England. There is some evidence that normal and possibly strike-slip faulting continued, and in the Solway/Carlisle Basin sedimentation probably continued into Early Cretaceous times. Conversely, around that same interval, the Lake District and Alston blocks experienced further erosion during development of a widespread unconformity. Thereafter it is likely that renewed subsidence allowed deposition of the Upper Cretaceous Chalk Group across the region, with the maximum post-Variscan burial of northern England probably achieved towards the end of the Cretaceous Period.

At the end of the Cretaceous and into the early Palaeogene Period, regional uplift began as a precursor to the major magmatism associated with the initial opening of the Atlantic Ocean (Figure 3f). This was driven by development of the proto-Icelandic mantle plume, which had its maximum impact in what is now the Hebridean province of western Scotland and Northern Ireland and in Greenland, areas that were then adjacent. There, from about 60 Ma to 55 Ma, immense volumes of basaltic magma were erupted with the coeval intrusion of plutons, sill-swarms and swarms of dykes. Some of the latter, emanating from a volcanic centre on Mull, run south-eastwards across northern England, more than 400 km from their source; examples include the Cleveland and Acklington dykes ( Figure 4). Other dykes run from Northern Ireland across the Isle of Man and into the English Midlands.

And so to the present

Additional impetus was given to the regional uplift of northern England in Miocene times as a distant effect of the Alpine Orogeny. This resulted from collision between the European and African plates, but its main structural effects are not seen much beyond southern England and Wales. Around northern England, its influence was restricted to likely uplift of the Irish Sea and Solway/Carlisle basin sequences. Overall, the Palaeogene to Neogene uplift episodes created erosive conditions across northern England that have continued to the present day. Up to 2500 m of strata may have been removed in this time, including all of the Upper Jurassic and Cretaceous succession. Further erosion of Carboniferous rocks revealed the Caledonian, Lower Palaeozoic basement across the structural highs of the Lake District, the Isle of Man and, less extensively, the Alston Block; to the north, the Southern Uplands block was similarly exhumed.

Erosion was locally much more vigorous from about 2.6 million years ago as glacial conditions were established across northern England during the Quaternary ‘ice age’. In fact this was a period of alternating cold and more temperate interludes, with the most recent of the latter, commencing only about 10 000 years ago continuing to the present day. Ice sheets repeatedly built up on the higher ground and fed glaciers that eroded deep valleys, for example those now occupied by the lakes of the Lake District. Sometimes the ice sheets reached the sea, but at other times Irish Sea or North Sea ice encroached onto the land areas. This, and the multiphase nature of glacial advance and retreat led to complex variations in ice flow direction through time, resulting in a complicated pattern of glacial landforms. As the ice sheets waxed and waned so relative sea level rose and fell: short-term, ‘eustatic’ changes were directly related to the growth of the ice sheets, but longer term, ‘isostatic’ changes arose from the depression of the Earth’s crust by the enormous weight of ice, and its slow recovery once the ice had melted. These effects have led to a range of submerged and perched coastal features, the most prominent of which are raised beaches and cliff-lines now abandoned several metres above present sea-level.

Since the last retreat of the ice from northern England, the human species has become a significant geological agent, modifying landscapes and sedimentary patterns through deforestation, agriculture, mining and urban development, whilst driving many fellow species to extinction. Global warming is once again leading to a renewed rise in sea level, but this time accentuated by an anthropogenic contribution. How these changes will affect the future geological record remains to be seen. In the meantime, across northern England, we continue to rely on geological sources of many raw materials, particularly for construction and road building, whilst attempting to mitigate the effects of geologically induced hazards such as landslides and subsidence.

Chapter 2 Early Ordovician: Skiddaw and Manx groups

Stratigraphical framework

The Skiddaw (English Lake District) and Manx (Isle of Man) groups include the oldest rocks now seen in northern England. Their strata were deposited contemporaneously during the early to mid Ordovician, in relatively close proximity, on the northern margin of Avalonia ( Figure 3). The thickness, duration and geographical extent of the two groups suggest that they were deposited in a large basin complex with a long history of subsidence. Although they are composed of broadly similar, mud-rich turbidite lithologies, the two groups differ considerably in detail and so deposition by localised sedimentary systems seems likely. For the geologist the Skiddaw and Manx groups’ strata are challenging. Lithological uniformity, complex structure and a lack of distinctive stratigraphical ‘marker’ beds combine to keep substantial parts of their outcrops enigmatic, with much detail currently unresolved.

During the early Ordovician, Avalonia formed the southern margin of the Iapetus Ocean and lay at about latitude 60°S within the circum-polar ‘Atlantic’ faunal province. This was characterised by an abundant, but low diversity, graptolite fauna and a sparse trilobite population. Examples of the graptolite fauna are widespread in the Skiddaw Group (from which a few trilobites have also been recovered) but macrofossils are rare in the Manx Group. Both groups contain acritarch floras that are the key to their biostratigraphical correlation ( Figure 8). A selection of Skiddaw Group fossils is shown in ( Plate 6), though the generally small and unsubstantial — yet biostratigraphically crucial — graptolites are under-represented.

The Skiddaw Group comprises a thick (about 5000 m) succession consisting largely of turbiditic sandstone and mudstone. The principal outcrop is in the main Skiddaw inlier of the northern Lake District, with the smaller Ullswater and Bampton inliers nearby to the south-east, and the Black Combe and Furness inliers farther away to the south ( Figure 9). To the east of the Lake District, the Skiddaw Group appears in inliers of Lower Palaeozoic rock at Cross Fell and Teesdale, and has been proved by boreholes to underlie Carboniferous strata across much of the Alston Block. From its graptolite fauna and acritarch flora can be established an Ordovician biostratigraphy spanning the Tremadoc, Arenig and Llanvirn epochs ( Figure 8). The only evidence for a pre-Ordovician component is provided by probable Cambrian acritarchs from a single locality close to Eycott Hill, in the north-east of the main Skiddaw inlier.

In the largest of the inliers (480 km2 including the eponymous mountain of Skiddaw), two distinct stratigraphical belts are present — the Northern Fells Belt and the Central Fells Belt ( Figure 10) — separated by a major wrench and thrust fault system. This structure, the Causey Pike Fault, can be traced across the Lake District ( Plate 7) and eastwards to Cross Fell ( Figure 9) where it separates Skiddaw Group sequences that are stratigraphically equivalent to those forming the two belts of the main, Skiddaw inlier. The Causey Pike Fault coincides with a major geophysical lineament that is interpreted as a basement shear zone into which a concealed, approximately 400 Ma, elongate granitic body has been intruded.

To the north of the Causey Pike Fault, in the Northern Fells Belt, are preserved some 5000 m of mainly mudstone turbidites that were deposited between the Tremadoc and the early Llanvirn, although the acritarch flora from the Eycott Hill area suggests that sedimentation may have commenced in the Cambrian. Wacke-type sandstone beds occur sporadically throughout the succession but become dominant at two levels where they make up the Watch Hill and Loweswater formations ( Figure 11). Their provenance was largely within an old, inactive, continental volcanic arc lying to the south-east. South of the Causey Pike Fault, in the Central Fells Belt, the Buttermere Formation is a major olistostrome that was emplaced from the south during the late Arenig. The olistostrome is overlain by 1000 m, perhaps more, of upper Arenig to Llanvirn siltstone and mudstone with sporadic volcaniclastic interbeds. These strata make up the Tarn Moor Formation, which also crops out farther south-east as inliers around Ullswater and Bampton.

It has been traditional to correlate the Skiddaw Group with the Ingleton Group, a superficially similar turbiditic lithofacies that crops out still farther south in the Craven inliers ( Figure 9), and for which a description is given in the British Regional Geology guide for The Pennines and adjacent areas. However, despite some common features, there are significant compositional and structural differences between the two groups and it is possible that they are not closely related.

The Manx Group is dominated by laminated mudstone and siltstone ( Plate 8) but also includes much thin- to medium-bedded, typically fine-grained sandstone, and intrabasinal debris flows of pebbly mudstone. Small amounts of volcanic rock occur locally and are apparently interbedded with the sedimentary strata. The succession is essentially steeply dipping to vertical and strikes north-north-east to east-north-east, so that the different formations have lenticular outcrops parallel to the long axis of the island ( Figure 12). First described as the ‘Manx Slate’ the rocks were long-regarded as being of Cambrian age then, more recently, thought to be entirely of Ordovician (Arenig) age. In the latest development, newly discovered graptolites have established that part of the succession is Silurian, and most probably Wenlock, in age. The Silurian strata have now been separated as the Niarbyl Formation of the Dalby Group and can be informally associated with the Windermere Supergroup (see Chapter 4).

The stratigraphy and order of succession of the Manx Group have long been a matter of debate. From recent work, the rocks are considered to crop out in north-west younging succession across most of the island ( Figure 12), although they are folded and disrupted by strike-parallel faults which locally repeat parts of the succession. The oldest and youngest parts of the sequence are structurally repeated in the north of the outcrop. It is difficult to estimate the thickness of the individual formations, but most are likely to comprise from several hundreds of metres up to about 1000 m of strata. The Lonan Formation is thicker, probably in excess of 2000 m. The total thickness of the exposed succession is likely to exceed 5000 m, but seems unlikely to have accumulated anywhere as a single entity.

Biostratigraphical control on the Manx Group is principally provided by acritarchs supported by a very sparse graptolite fauna. A Tremadoc to Arenig age range has been established ( Figure 8), although the presence of some long-ranging forms leaves open the possibility that the base of the sequence may extend down into the Upper Cambrian. In general, and within a background of mudstone and siltstone, wacke-type sandstones are common in the older parts of the Manx Group (e.g. Lonan and Glen Dhoo formations), whereas quartz-arenite sandstones become locally important around the middle (e.g. Mull Hill and Creg Agneash formations); mudstone–siltstone sequences with little interbedded sandstone (e.g. Maughold and Creggan Moar formations) form the younger parts of the group. The depositional style is turbiditic and invites a general comparison with the Northern Fells Belt of the Skiddaw Group. Mass-flow deposits of pebbly mudstone in the Upper Arenig Lady Port Formation are possible equivalents of the Buttermere Formation olistostrome in the Skiddaw Group’s Central Fells Belt.

Both the Skiddaw Group and the Manx Group display a complex pattern of very low-grade metamorphism, elevated locally by contact effects around large granitic intrusions. Late diagenetic grades are found in the relatively soft mudstone that makes up, for example, the Tarn Moor and Bitter Beck formations of the Skiddaw Group. These rocks probably preserve syndepositional burial metamorphism augmented (at least in the Skiddaw Group) by the effects of burial beneath the Borrowdale Volcanic Group. Slightly higher grades of regional metamorphism were probably imparted by a combination of thermal effects during late Ordovician magmatism (i.e. associated with eruption of the Borrowdale Volcanic Group) and tectonic effects during Early Devonian (Acadian) deformation. Acadian contact metamorphism is illustrated by the hornfelsed aureoles around the Skiddaw Granite in the Lake District, and the Foxdale Granite in the Isle of Man. A more detailed account of the metamorphic effects is given in Chapter 5.

Skiddaw Group succession

Introduction

In the Northern Fells Belt the Skiddaw Group comprises five formations overlain by the Eycott Volcanic Group ( Figure 11). In ascending order these formations are: Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile. Boundaries between the formations are transitional.

The Bitter Beck Formation comprises thinly laminated, dark grey mudstone, silty mudstone and siltstone with minor amounts of fine grained, wacke-type sandstone. The latter is concentrated in the lower part of the formation where it forms up to 20 per cent of the succession in thin to medium beds (3–30 cm). In places there is abundant slump folding with dislocations sub-parallel to bedding. The formation is at least 500 m thick and lies within the upper Tremadoc murrayi graptolite Biozone ( Figure 8).

The Watch Hill Formation comprises interbedded lithic sandstones (locally coarsening to microconglomerate) and mudstones. The sandstone is typically fine grained with abundant siltstone and mudstone intraclasts; bed thickness ranges up to about 1 m. Sandstone to mudstone proportions change vertically and laterally throughout the succession, but are typically around 60–70 per cent sandstone and 30–40 per cent mudstone. The maximum thickness of the formation is somewhere between 550 m and 800 m but it thins both westwards, to about 100 m, and eastwards, to 40 m. This pattern might have originated through deposition in a system of diverging channels, but interpretation in these terms is precluded by evidence derived from basal flute casts that shows a consistent palaeocurrent flow from an easterly direction. The age of the formation is within the latest Tremadoc murrayi graptolite Biozone ( Figure 8). Equivalent strata further east in the Cross Fell inlier there form the Catterpallot Formation ( Figure 11).

The Hope Beck Formation comprises laminated and thinly bedded mudstone with sporadic, thin sandstone and pebbly mudstone beds. Bioturbation is common, with burrows mainly subhorizontal to bedding and sometimes filled with faecal pellets. The sandstone is quartz-rich lithic wacke, generally medium to coarse grained, but including numerous beds with granules and pebbles. The clast assemblage is dominated by monocrystalline plutonic quartz and polycrystalline metamorphic quartz. Pebbly mudstones are matrix supported (about 50 per cent matrix) and probably represent debris flows. The thickness of the formation is between 600 m and 800 m with a biostratigraphical range from latest Tremadoc to early Arenig ( Figure 8), spanning the phyllograptoides (more-or-less equivalent to the traditional approximatus) graptolite Biozone and extending up into the varicosus graptolite Biozone.

The Loweswater Formation is dominantly formed of quartz-rich wacke-type sandstone in which the individual grains are predominantly of monocrystalline quartz, mostly strained but with some larger, unstrained grains. The sandstone is mostly fine to medium grained, occurring in both parallel-and cross-laminated beds ( Plate 9). The bed thickness increases gradually upwards to a maximum of 1 m near the middle of the formation; concurrently the maximum grain size increases to very coarse grained, though most beds are still fine to medium grained. The upper part of the formation is a mirror image of the lower part, bed thickness and grain size decreasing gradually while the percentage of siltstone and mudstone increases. Overall, the sandstone beds are slightly lenticular. Most have planar bases, but channels, groove casts and flute casts occur locally. Palaeocurrent flow was mainly from the southern quadrant. Small scale slump folds, up to a few metres across are widespread. The Loweswater Formation has a maximum thickness of approximately 900 m, decreasing northwards to an estimated 450 m. Biostratigraphically it occupies a position in the middle part of the Arenig, spanning the upper varicosus and lower simulans graptolite biozones ( Figure 8).

The Kirk Stile Formation is between 1500 m and 2500 m thick but some of this apparent variation may be due to the repetition of strata through the stacking of slumped masses. The principal lithology is thinly laminated mudstone deposited from low-density turbidity currents. Interbedded with the mudstone are local lenticular units dominated by lithic wacke; these sandstone-rich units are typically thinly bedded with parallel- and cross-lamination. Strained quartz is the most abundant grain type, feldspar is rare, but various lithic grains, mostly volcaniclastic siltstone and altered mafic volcanic rock, are relatively common. In the upper part of the formation, sporadic units of sedimentary breccia and slumped strata, from 2 to more than 40 m thick, include intrabasinal clasts up to about 0.5 m in diameter contained in a silty mudstone matrix. Biostratigraphically, the formation ranges from the mid Arenig to the early Llanvirn, spanning the simulans to artus graptolite biozones ( Figure 8); coeval strata, though with subtle lithological differences, occupy the Furness and Black Combe inliers in the south of the Lake District.

Late Arenig and Llanvirn: Central Fells Belt

South of the Causey Pike Fault, the Skiddaw Group comprises two divisions: the massive and chaotically disrupted olistostrome of the Buttermere Formation, unconformably overlain by the mudstone-dominated Tarn Moor Formation. The latter contains interbeds of tuffaceous sandstone and bentonitic claystone, both indicative of contemporaneous volcanicity. The Tarn Moor Formation also crops out in the small inliers of Ullswater and Bampton, and farther east in the Cross Fell inlier has an equivalent in the Kirkland Formation ( Figure 11), albeit that the volcaniclastic component appears to be greater at Cross Fell. The Buttermere Formation olistostrome has a likely counterpart at Cross Fell in the Murton Formation, although the level of sedimentary disruption in the latter is less than in the former.

The Buttermere Formation is at least 1500 m thick, comprising mudstone, siltstone and sandstone turbidite olistoliths set in an argillaceous matrix. The olistoliths range in size from granules to blocks up to a kilometre or more across, though most are in the 5–10 m range; the largest are formed by wacke-type sandstone. Both clasts and matrix are intensely deformed by minor folds and shears, many of which were generated during emplacement of the olistostrome. Fossils from the Buttermere Formation (rare trilobites as well as the more widespread graptolites and acritarchs) indicate a range of ages from early Tremadoc to late Arenig whilst the overlying Tarn Moor Formation contains graptolites of latest Arenig and Llanvirn age. Hence the age of olistostrome emplacement is inferred to be in the late Arenig, possibly at about the boundary between the gibberulus and cucullus graptolite biozones. From the geometry of the major slump folds it can be inferred that the olistostrome was emplaced by down-slope movement towards the north.

Evidence for large-scale soft-sediment movement in the Murton Formation, the stratigraphical equivalent of the Buttermere Formation in the Cross Fell inlier, largely derives from the many exposures that show complex polyphase folding and shearing characteristic of slumped deposits. The interpretation is supported by the range of biostratigraphical ages present, from early Arenig to early Llanvirn, and their apparently random distribution. In lithology, the Murton Formation is mainly thinly-bedded grey siltstone with subordinate thicker interbeds of sandstone. It is probable that the Murton Formation illustrates an only partially disrupted part of the succession that was completely disaggregated in the Buttermere Formation olistostrome.

The Tarn Moor Formation comprises late Arenig to early Llanvirn mudstone and siltstone, with minor volcaniclastic turbidite and bentonite beds. The formation rests unconformably on the highly disrupted Buttermere Formation, although the time-break across the unconformity would appear to be very small. The top of the Tarn Moor Formation is cut out by the unconformable base of the overlying Borrowdale Volcanic Group and the youngest Skiddaw Group strata remaining (from the murchisoni Biozone) are those encountered during construction of the Tarn Moor aqueduct tunnel. The thickness of the formation is probably around 1000–1500 m. In its lower part, laminated mudstone is similar in overall character to the Kirk Stile Formation of the Northern Fells Belt, with which it partly overlaps in age, spanning the cucullus graptolite Biozone of the upper Arenig and the artus Biozone of the lower Llanvirn. The upper part of the formation is characterised by mudstone with up to five per cent of bentonite and volcaniclastic interbeds. The bentonite ranges up from very thin laminae to discrete beds a few centimetres thick. Graptolites indicate the artus Biozone in the Skiddaw and Bampton inliers but range up into the murchisoni Biozone in the Ullswater Inlier.

In both its lithology and fossil content the Tarn Moor Formation is similar to the Kirkland Formation of the Cross Fell inlier. The thickness of the latter cannot be estimated accurately since neither its base nor its top are exposed, but at least 1000 m of strata may be present. A twofold lithological division is possible: a lower sequence of volcaniclastic turbidites interbedded with mudstone and siltstone, and an upper sequence dominantly comprising black graptolitic mudstone. From both of these putative divisions the Kirkland Formation has yielded abundant graptolites representing the artus Biozone.

Farther east, the restricted exposures of the Skiddaw Group in the Teesdale inlier show cleaved, dark grey mudstone that contains graptolites and acritarchs of probable early Llanvirn age. A broad correlation with the Kirkland and Tarn Moor formations seems likely.

Late Arenig to early Llanvirn: southern inliers

In the Black Combe inlier (approximately 50 km2, ( Figure 9)) the dark grey siltstones and mudstones seen in its south-eastern part become paler in colour towards the north-west as a result of secondary metasomatism and metamorphism. Within the unaltered part of the sequence, laminae and thin beds of sandstone occur sporadically and in one instance, low in the exposed sequence, thicken and coalesce into the 300 m of the Knott Hill Sandstone Formation. The altered part of the sequence in the north-west of the inlier is affected by a pervasive cleavage fabric and intruded by numerous granitic sheets. The boundary between the dark, unaltered and the pale, metasomatised mudstones is transitional over a few tens of metres in a zone trending north-east to south-west across the south-east slopes of Black Combe. It is inclined to the north-west at about 40o, sub-parallel to the cleavage, and is interpreted as a mainly tectonic contact.

The thickness of the Skiddaw Group in the Black Combe inlier is uncertain because bedding is generally ill defined and, where discernible, reveals that the silty mudstone is folded on a scale that varies from less than one to several tens of metres, with no consistency in the direction and amount of dip. It is likely that at least some of the deformation arose from slumping of the accumulated sediment prior to its consolidation. The structural complexity combined with the lack of stratigraphical control makes estimates of the thickness speculative but as much as 2000 m of strata might be preserved. Sparse assemblages of graptolites and acritarchs suggest a late Arenig age.

The western margin of the Black Combe inlier is formed by the Lake District Boundary Fault, a regionally important structure trending approximately north–south. To the north of Black Combe and close to Ravenglass, two small inliers of Skiddaw Group strata are contained within the fault zone. The tectonised slivers have also been thermally metamorphosed by the adjacent Eskdale granitic pluton and now comprise hornfelsed black mudstone containing metamorphic biotite and chlorite with sporadic tourmaline and chiastolite; some indurated and recrystallised sandstone is also present. Correlation with the rest of the Skiddaw Group is unclear.

In the south of the Lake District, the Skiddaw Group is also exposed, though poorly so, in a structurally complex inlier (or series of inliers) on the Furness peninsula ( Figure 9) where it is unconformably overlain by the Dent Group, the lowest division of the Windermere Supergroup and of Ashgill age (see Chapter 4). There is no good indication of the sequence or thickness of the Skiddaw Group hereabouts since the inlier is extensively drift covered, with few natural exposures. Most information has been gathered from quarry sections (notably those of the Furness Brick Company), temporary exposures and boreholes.

The rocks present are dominantly dark grey mudstones with sporadic silt laminae up to 2 mm thick. When freshly exposed in quarry sections, the mudstone is almost coal-black, but pyritic inclusions give rise to deep ferruginous stains on weathering. A faint lamination is apparent in places but strong cleavage, intense crushing and minor contortions all combine to obscure sedimentary detail. Graptolites and acritarchs have been recovered locally and indicate an age within the late Arenig to early Llanvirn range, making the Furness strata a little younger than the Skiddaw Group in the Black Combe inlier, a little to the north ( Figure 11).

Manx Group succession

Late Tremadoc to early Arenig turbidite facies

The oldest Manx Group rocks seen at outcrop comprise the Glen Dhoo Formation in the north of the Isle of Man, and the Lonan Formation, which crops out over much of the southern and eastern part of the island ( Figure 12). Both of these formations contain late Tremadoc to early Arenig acritarch floras. They are dominated by mudstone with a variable proportion of siltstone and fine-grained sandstone forming thin beds that are commonly either planar- or cross-laminated. Locally, immature fine-grained sandstone, in beds ranging up to 2 m thick, is dominant within the Glen Dhoo Formation. This formation also includes, near Peel, a minor occurrence of andesitic volcanic breccia that contains an acritarch flora in its mudstone matrix. Sandstone-dominant members are more widely developed in the Lonan Formation, where they make up much of the south-east coast of the island ( Plate 10). Two distinct sandstone types are present, one forming the Keristal Member, and the other forming the Santon and Ny Garvain members.

Although the Keristal Member is only a few metres thick and locally lenticular, it is laterally persistent over at least 15 km as beds of distinctive, pale grey and fine-grained quartz arenite. The beds are ungraded or weakly graded, commonly massive or with parallel-lamination, but in some cases with ripple cross-lamination in thinner beds. They may be organised in either thinning-up or thickening-up sequences. In thinning-up sequences, the basal bed is commonly strongly erosional into underlying thin-bedded turbidites. The Keristal Member represents a short-lived system of sand-rich gravity flows that tapped a source of relatively clean quartz sand.

The Santon Member (at least 600 m thick) and the along-strike but slightly younger and much thinner Ny Garvain Member, are characterised by well-bedded sequences of grey, fine-grained wacke intercalated with the more typical facies of the Lonan Formation. Sandstone beds grade upwards into the intervening mudstone, parallel- and ripple cross-lamination is common, and some bed bases preserve flute marks and horizontal burrows. Locally, beds of pale grey quartz arenite punctuate the less-mature wacke successions. The members are interpreted as the deposits of relatively high-concentration turbidity flows; the palaeocurrents, as determined from sole marks, were directed consistently towards the west.

The base of the Lonan Formation is not seen but at least 2500 m of its strata are exposed. The top is transitional into overlying formations and can be readily defined relative to the lithologically distinct Creg Agneash Formation in the north-east and Mull Hill Formation in the south ( Figure 8) and ( Figure 12). The transition is more subtle in the centre of the island where the Lonan Formation passes directly up into the lithologically similar Maughold Formation.

The relatively high-concentration quartz arenite turbidity flows represented only locally in the Lonan Formation become dominant in the overlying Creg Agneash and Mull Hill formations. Both are characterised by sequences of white or pale grey, quartz arenite interbedded with a variable amount of mudstone. The sandstone beds are massive or weakly graded from medium sand to silt. A few beds have coarse-grained sandstone bases with mudstone rip-up clasts. Internal structure is typically a weakly defined, upward thinning parallel-lamination, in some cases with a thin ripple cross-laminated division at the top. Slump folding on a scale of tens of centimetres to metres is commonly developed, with close to tight disharmonic folds of widely varying orientation. The top of the Mull Hill Formation is not seen due to faulting. The Creg Agneash Formation becomes more mudstone-dominated upwards and passes transitionally into the Maughold Formation as sandstone beds decrease in abundance. The age of the Lonan Formation beneath the Santon Member is constrained by acritarch floras from several localities that are indicative of the late Tremadoc to early Arenig interval.

Acritarchs from the Santon Member itself established correlation with the earliest Arenig phyllograptoides graptolite Biozone ( Figure 8) but graptolites, though present, are poorly preserved and allow only a general Arenig age to be allocated.

Early to mid Arenig mudstone­-dominated facies

The upland spine of the Isle of Man is composed of mudstone-dominated formations ( Figure 12). The Maughold Formation is mainly composed of dark grey, laminated mudstone in bedded or disrupted facies, but locally includes a significant proportion of either siltstone or quartz arenite interbeds. Bioturbation is common as spots or as discordant silt-filled or mud-filled burrows. In the south of the outcrop, pebbly mudstone locally forms a substantial part of the succession, though it is absent in the north-east. This lithology is matrix supported, with clasts generally up to a few centimetres in diameter but ranging up to a maximum of 30 cm. The clasts are mostly intraformational but include rare fragments of a fine-grained igneous rock.

The contact of the Maughold Formation with the Barrule Formation is faulted, but an originally conformable stratigraphical contact between the two formations seems likely. The outcrop of the Barrule Formation, duplicated by thrusting, consists of homogeneous or faintly laminated grey to black mudstone interpreted as hemipelagic sediment deposited in an anoxic sea-bed environment. The formation may be several hundred metres to 1 km thick, depending on internal structure, and passes gradationally into the Injebreck Formation with the appearance of distinctive silty laminae.

The Injebreck Formation is in many respects similar to the Maughold Formation. It is dominated by dark mudstone with pale grey silty laminae and a variable proportion of interbedded siltstone and fine-grained sandstone; the latter may be either wacke or quartz arenite. The sandstone beds typically have sharp tops and bases and are parallel- and ripple cross-laminated. Pebbly mudstone occurs locally in the south-west of the outcrop, but becomes dominant through much of the lower part of the formation in the north-east. Clasts range up to 10 cm across, occasionally to 35 cm, and are matrix supported. They consist mainly of intrabasinal material but include some fine-grained igneous rock types.

The Injebreck Formation appears to pass stratigraphically up into the Glen Rushen Formation, although the evidence is equivocal. The Glen Rushen Formation is composed predominantly of laminated, grey mudstone with a variable proportion of pale siltstone laminae that are generally less than 1 mm thick. Bioturbation is absent and the laminae are laterally persistent. Thin beds of quartzose sandstone or pebbly mudstone occur sporadically. The Glen Rushen Formation passes stratigraphically upwards into the Creggan Moar Formation with a gradual increase in the proportion of siltstone laminae and the appearance of very thin manganiferous ironstone beds.

The mudstone-dominated facies reflects deposition from hemipelagic fall-out and low-concentration turbidity flows into a periodically oxygenated marine basin. Sandstone-rich intervals record periods of medium- to high-concentration turbidity flow, with some of the introduced sediment being highly quartzose. Periodic basin instability resulted in disruption and resedimentation of the bedded facies as pebbly mudstone.

The age of the Maughold, Barrule and Injebreck formations is poorly constrained. Their interpreted stratigraphical position ( Figure 8), above the Tremadoc to lower Arenig Lonan Formation and below the middle Arenig Creggan Moar Formation, implies an early to mid Arenig age. Samples from all of the formations have yielded sparse, low-diversity assemblages of acritarchs consistent with the lower Arenig (varicosus graptolite Biozone) in the Lake District. Most samples from the Glen Rushen Formation have yielded similar sparse assemblages but two samples close to its transition into the Creggan Moar Formation have yielded acritarchs diagnostic of the mid Arenig in the Lake District.

Mid to late Arenig manganiferous iron stone bearing facies

Two units that crop out in the north-west of the Isle of Man are biostratigraphically constrained - as the youngest parts of the Manx Group, ranging from mid to late Arenig in age ( Figure 8). The Creggan Moar Formation crops out in sequence above the Glen Rushen Formation but is structurally truncated along its western margin. The Lady Port Formation forms a tectonically isolated sliver on the north-west coast of the Isle of Man ( Figure 12).

The Creggan Moar Formation was formed mainly from low-concentration turbidity flows into oxygenated bottom waters, with deposition punctuated by intermittent episodes of chemical precipitation. The formation is dominated by thinly bedded, dark grey mudstone, siltstone and very fine-grained pale sandstone. The siltstone and sandstone beds are typically parallel-laminated, rarely cross-laminated, and grade through bioturbated tops into the mudstone. Very thin beds of manganiferous ironstone occur intermittently or in clusters and weather distinctively to shades of reddish brown or dark brown to black. Parallel-laminated siltstone is locally dominant, forming thick uniform successions with rare mudstone or manganiferous beds. Quartz arenite occurs sporadically as pale grey, faintly laminated, thin to medium beds.

The age of the Creggan Moar Formation was only recently constrained as mid to late Arenig ( Figure 8). Acritarchs, sparse near the transition from the Glen Rushen Formation but becoming more abundant and more varied elsewhere, commonly include species indicative of the mid Arenig. However, one locality in the middle of the outcrop has yielded a variety of forms that suggest a late Arenig age. The top of the Creggan Moar Formation is not seen; in the north the Glen Dhoo Formation has been thrust above it and in the south it is faulted into juxtaposition with Silurian rocks of the Dalby Group.

The Lady Port Formation is heterogeneous and extensively disrupted, recording major basin instability. The dominant lithology is matrix-supported pebbly mudstone containing a varying proportion of sandstone and siltstone clasts, commonly up to 20 cm in size but also including rafts of bedded strata up to at least tens of metres in size. Fragments of intact lithostratigraphy include thinly-bedded wacke, and mudstone with siltstone laminae that resemble parts of the Glen Rushen Formation. Alternating with the pebbly mudstone are rare sections of interbedded millimetre-scale manganiferous ironstone bands, siltstone and fine-grained quartzose sandstone that form a lithofacies similar to that of the Creggan Moar Formation. The Lady Port Formation has yielded poorly preserved acritarchs considered to be of late Arenig age, but the likelihood that material has been reworked from underlying formations means that this can only be a maximum.

Depositional regime

In the Isle of Man and towards the bottom of the Manx Group succession, the Peel volcanic assemblage is associated with turbidite strata of late Tremadoc to early Arenig age. Its significance in the context of sedimentary basin development is best illustrated by comparison with the Ribband Group, the contemporaneous sequence preserved to the south-west in Ireland, which contains several andesitic volcanic intercalations. The composition of these suggests that subduction-related, arc volcanism was coeval with sedimentation during the late Tremadoc and continued intermittently through much of the Arenig. However, since the volcanic influence appears to decrease markedly towards the north-east, the nature of the depositional environment might also change. Any such change would require active subduction to be distanced from the Skiddaw Group’s area of deposition, probably by one or more major transform faults. Hence, any comparison of the depositional settings of the Manx and Skiddaw groups must recognise that they may have been compartmentalised by major structures.

The thickness, duration and geographical extent of the Skiddaw and Manx groups suggest that they were deposited in a large basin (or series of basins) with a long history of subsidence. The provenance of the Skiddaw Group was largely within an old, inactive, continental volcanic arc lying to the south-east and there is a marked absence of any juvenile volcaniclastic input prior to the Llanvirn. An extensional continental margin therefore seems a more likely site for deposition than an inter-arc or back-arc zone. In this apparent absence of coeval volcanism until late in its depositional history, the Skiddaw Group contrasts with the Manx Group (which contains minor volcanic rocks) and the more distant Ribband Group in south-east Ireland, as discussed above. There is also a major contrast with contemporaneous developments in the Welsh Basin, where there was considerable subduction-related volcanicity during the Tremadoc and Arenig.

Despite the presence of the rare volcanic interbeds, the likely provenance of the Manx Group sandstones was broadly comparable to, but generally more mature than, that of Skiddaw Group sandstones. Petrographical and geochemical evidence suggests a heterolithic source area containing a continental basement, an extinct volcanic arc and a cover of clastic sedimentary rocks. The quartz-arenite units in the Manx Group are probably peripheral representatives of the ‘Armorican’ quartzite lithofacies that was widely developed on the northern margin of Gondwana during the early Ordovician. An analysis of palaeocurrents and facies architecture, shows that all of the Manx Group turbidite succession was derived broadly from the south-east, i.e. from the direction of Gondwana, with the distributory fan systems swinging towards the west within the depositional basin. Again, a general similarity with the Skiddaw Group emerges, though sedimentation of the two groups was probably fed by different distributory systems.

Some idea of the possible basin configuration during Skiddaw Group deposition can be gained from an examination of the widespread slump folds ( Plate 11). Within the Northern Fells Belt these are predominantly orientated towards the south-east and although the origin of the larger folds has been questioned there remains evidence of a likely south-east-facing palaeoslope, at least during the Tremadoc and early Arenig. Conversely, south of the Causey Pike Fault in the Central Fells Belt, the Buttermere Formation is a large olistostrome emplaced towards the north-west; only relatively thin debris flow beds are seen in the Northern Fells belt at an equivalent stratigraphical level. From the contrasting scale and style and orientation of the slump movements it seems likely that the depositional basin had a steep, faulted southerly margin.

Palaeocurrent evidence from the Skiddaw Group’s two periods of submarine fan development (essentially the Watch Hill and Loweswater formations) suggests that each occurred within a different basin configuration. During deposition of the first (Watch Hill), axial flow was along a trough orientated approximately east–west. Subsequent deposition (Loweswater) was spatially more complex, with greater influence by sea-floor topography. The latter may have been influenced by syndepositional extensional faults trending north-west to south-east within the depositional basin, with the intervening fault blocks tilted to the north­east. The overall picture that emerges is of a north-facing extensional half-graben system with the major boundary fault (or faults) on the south-eastern side. The observed stratigraphical contrast across the Causey Pike Fault would certainly have been made more conspicuous if the basin was originally composite, and so perhaps that structure was initiated as a north-west-downthrowing normal fault partitioning the Skiddaw Group depositional basin. Additional support for this idea is provided by model ages derived from isotope data, which show a marked difference in provenance maturity across the Causey Pike Fault.

The illite crystallinity and clay mineral assemblages of the Skiddaw Group and Manx Group mudstones might also support an extensional basin interpretation. These data suggest that early burial metamorphism, characterised by late diagenetic to low anchizonal grades, occurred under a higher geothermal gradient (at least 35ºC/km) than would normally be expected in an ensialic basin and was probably related to high heat flow during crustal extension and thinning. However, there must remain some uncertainty as to the relationship of burial metamorphism in the depositional basin during its putative extension (Tremadoc to Llanvirn), and that during subsequent suprasubduction zone basin uplift and volcanicity (Llanvirn to Caradoc; discussed below) when a high geothermal gradient would also be expected.

Basin uplift and volcanism

The first indications of volcanic activity during Skiddaw Group deposition are seen in the early Llanvirn, with sporadic interbeds of volcaniclastic turbidite sandstone and bentonite ash in the Tarn Moor and Kirkland formations of the Central Fells Belt ( Figure 11). Despite the local abundance and substantial thickness of volcanic interbeds in the dominantly mudstone succession, the juvenile input was restricted to the discrete volcaniclastic beds whilst the background provenance remained relatively ancient; model ages approaching 2 Ga have been obtained from the Tarn Moor Formation mudstones and are compatible with derivation from a Proterozoic source. The Llanvirn initiation of volcanism therefore seems to have been either at some distance from the depositional basin, or to have involved relatively small-scale and localised eruptions.

The onset of volcanicity in the early Llanvirn followed the late Arenig emplacement of the major Buttermere Formation olistostrome (and the contemporaneous large-scale slumping preserved in the Lady Port Formation, Manx Group) and seismic activity, as a precursor to the volcanic episode, may have triggered the mass-flow movements. However, the olistostrome was clearly emplaced by down-slope movement into a still-extant basin and so it may equally have been instigated by normal, extensional movement on the basin boundary fault(s). Whatever the trigger mechanism, the extensive slumping throughout much of the Skiddaw Group caused considerable stratigraphical disruption. Further disruption was then inevitable during the uplift of the continental margin and the Skiddaw Group basin, with conversion of the deep marine strata into the subaerial basement to the ensuing, mainly Caradoc, Borrowdale and Eycott volcanic groups. The uplift was most likely caused by the generation and rise of andesitic melts above a developing subduction zone, prior to the climactic volcanic paroxysm.

The magnitude of the pre-volcanic stratigraphical disruption, caused by the combination of gravity-driven, mass slump movement and the subsequent basin uplift, may be gauged by the wide range of biostratigraphical zones determined immediately subjacent to the volcanic rocks that unconformably overlie the Skiddaw Group. South of the Causey Pike Fault, below the Borrowdale Volcanic Group, various Arenig to Llanvirn biostratigraphical levels occur close to the unconformity cut across the Buttermere Formation olistostrome and the overlying Tarn Moor Formation. North of the Causey Pike Fault the Skiddaw Group strata immediately subjacent to the Eycott Volcanic Group range in age from possibly Cambrian to Llanvirn ( Plate 12). In this northern area, much of the variation seems likely to have been caused by fault-block rotation prior to volcanicity since there is stratigraphical coherence over wide areas of the outcrop. Rotation could have been either an extensional or compressional effect and was probably superimposed both on disrupted zones caused by the earlier slump movements and on areas that had escaped such disruption.

It must be stressed that there is no evidence for a compressive tectonic event producing penetrative deformation prior to the eruption of the volcanic rocks. This has been a long-running controversy in Lake District geology but it is now established that the earliest regional cleavage cuts both the Skiddaw Group and overlying volcanic and sedimentary formations, which range up to Pridoli in age. In the Isle of Man, the earliest cleavage cuts across the Ordovician Manx Group, the Silurian Dalby Group and some of the intrusive rocks. This cleavage is unequivocally a product of the Early Devonian, Acadian Orogeny, which is discussed in more detail in Chapter 5. The only pre-volcanic but post-depositional fabric present is a widespread bedding-parallel compaction cleavage, particularly marked in parts of the Skiddaw Group. It is strongest in the most argillaceous lithologies and probably results from burial accentuation of the original bedding lamination either prior to, or during, basin uplift. The high heat flow thought to have been prevalent at the time may have assisted the formation of this fabric by accelerating recrystallisation of the clay minerals.

The volcanic rocks that occur on either side of the main Skiddaw Group inlier, the Eycott Volcanic Group to the north and the Borrowdale Volcanic Group to the south ( Figure 10) are Caradoc in age and had a continental margin, suprasubduction zone origin. This requires that an arc–trench gap, probably exceeding 100 km of forearc, originally extended to the north; a situation supportive of an ensialic setting for the Skiddaw Group basin. A network of volcanotectonic, caldera-related faults disrupts the Borrowdale Volcanic Group with proved displacement commonly in excess of 400 m. It is highly probable that similar structures would have affected those parts of the Skiddaw Group currently exposed, with reactivation of the pre-existing fault framework. Major structures such as the Causey Pike Fault would have been a likely focus for such movement.

Chapter 3 Ordovician: Caradoc magmatism

During Caradoc times, the Lake District was the focus for one of the most intense episodes of igneous activity seen during the geological history of the British Isles. A chain of volcanoes lay along the margin of Eastern Avalonia, at least from eastern Ireland, through the Lake District, the English Midlands and into Belgium. In the Lake District, two subaerial volcanic successions were built up within opposing half-grabens, each originally 40 to 50 km wide. Preserved in the northern one is the Eycott Volcanic Group — more than 3200 m of basaltic, andesitic and dacitic lavas and sills with subordinate pyroclastic rocks ( Figure 13). In the southern half-graben, the Borrowdale Volcanic Group comprises at least 6000 m of basaltic to rhyolitic lavas and sills, along with voluminous pyroclastic and volcaniclastic sedimentary rocks. The Borrowdale sequence contains evidence for the former existence of caldera volcanoes and local extensional basins ( Plate 13). It is one of only a handful of volcanic suites worldwide with rocks that contain garnet phenocrysts. In association with the volcanism, components of the Lake District granitic batholith were emplaced beneath the central part of the region at this time ( Figure 13). In the Isle of Man, mafic sheets at Poortown, east of Peel, are intruded into Arenig sedimentary rocks and appear to be geochemically similar to the Lake District’s Caradoc volcanic rocks. Also in the Isle of Man, the biotite-bearing Dhoon Granodiorite Pluton and associated Oatlands Complex may be a product of the Caradoc magmatic phase, but might equally have been emplaced later, at the time of the Acadian Orogeny (see Chapter 5).

The extensional tectonic regime and successive episodes of caldera collapse are seen as the main mechanisms for preservation of the subaerial volcanic rocks in the Lake District. Such occurrences are rare in the geological record and the Borrowdale Volcanic Group is a particularly well exposed example. Most subaerial volcanoes, particularly those constructed with abundant pyroclastic deposits, are more usually disintegrated explosively, weathered, eroded and redeposited in subaqueous sedimentary regimes. The depth of erosion present in the Lake District provides a three-dimensional model of the volcanic system that is rarely available in modern, silicic caldera volcanoes.

The Borrowdale Volcanic Group is also noteworthy in that it provides evidence for the emergence of terrestrial arthropods from a freshwater, rather than marine, environment. In the south-west of the outcrop bedding surfaces in sedimentary rocks exhibit trails and trackways attributed to a myriapod-like organism; this is one of the earliest known records of the existence of terrestrial arthropods. The traces were preserved in shallow-water, lacustrine sediments subjected to episodic drying-out, as demonstrated by the presence of desiccation cracks, and it would seem likely that the organisms survived subaerial conditions, at least temporarily.

Timing and structural setting

Age and duration of the volcanism

In the central Lake District, the age and time span of the volcanic groups are constrained biostratigraphically by the unconformities with the underlying and overlying sedimentary strata. The volcanic rocks lie within an interval of up to 15 Ma, above the lower Llanvirn, uppermost parts of the Skiddaw Group but below the Caradoc to Ashgill (Cautleyan) base of the Dent Group. At Cross Fell and in the northern Lake District, mid and late Caradoc shelly faunas have been found in the marine strata overlying the volcanic rocks whereas, in the southern Lake District, the basal beds of the Dent Group are of mid Ashgill age. No macrofossils have been found in the volcanic rocks but an acritarch assemblage, obtained from grey mudstone of the Holehouse Gill Formation in the Ulpha valley [SD 182 927], was previously thought to indicate a Caradoc age. However, the ranges of the taxa critical to this interpretation have been extended recently and it is now thought that all of the acritarchs obtained from these rocks were derived from erosion of the Skiddaw Group. The palynological assemblage from the Overwater Formation at the base of the Eycott Volcanic Group suggests a biostratigraphical age not older than Llanvirn but possibly Caradoc.

Precise U-Pb ages determined for zircons from the lower and middle parts of the Borrow-dale volcanic succession are Caradoc (451.6 ± 1.4 and 452.8 ± 0.7 Ma). Palaeomagnetic data from both the Eycott and Borrowdale Volcanic groups have been interpreted to indicate emplacement during the single period of normal magnetic polarity that lasted for about 5 Ma during Caradoc times. Thus, volcanism is now thought to have been restricted to the Caradoc.

Structural controls on the magmatism

The present distribution of the Caradoc igneous rocks, and the margins of the Lower Palaeozoic Lake District Block, probably reflects the positions of the original graben boundary faults ( Figure 13). Though now largely identified from Carboniferous displacements, the Maryport Fault system probably approximates to the northern margin of the Eycott Volcanic Group and, unless there is a very substantial strike-slip displacement on it, the Causey Pike Fault represents the boundary between the Eycott and Borrowdale volcanic fields. The position of the southern margin of the latter is inferred from the dramatic stratigraphical thinning that occurs, from about 6 km in the Duddon valley, to discontinuous remnants in the Furness inliers ( Figure 14). Geophysical evidence for relatively shallow magnetic basement beneath the Windermere Supergroup supports an east–west oriented margin to the half-graben.

The presence of large volumes of low density silicic volcanic and granitic rocks at depth much beyond the Cumbria coast seems unlikely in the light of recent geophysical interpretations, though a separate magmatic centre may be represented by granitic plutons beneath the Isle of Man. Eastward from the Lake District, Borrowdale and Eycott volcanic rocks are present in the Cross Fell Inlier, and farther east, on the Alston Block, pyroclastic rocks in the Teesdale Inlier are assigned to the Borrowdale Volcanic Group. Elsewhere on the Alston Block, the few boreholes through the Carboniferous overburden penetrated only the Skiddaw Group and Devonian Weardale Granite. Either the Ordovician volcanic rocks deposited in the Alston Block area were subsequently eroded, or a significant Caradoc volcanic depocentre did not develop in this area.

Faults with known or inferred Ordovician displacement mainly have the following trends: north to north-north-east, east to east-north-east, and north-west ( Figure 15). The north-west­trending set are principally volcanotectonic, that is, they are associated directly with magma movement, and are concentrated in the Scafell and Haweswater areas. In the northern Lake District, major fractures in the Avalonian basement, with east to east-north-east trend, were reactivated as extensional faults. From north to south, the principal structures involved are now represented by the Watch Hill, Causey Pike, Burtness Comb and Grassguards faults, and by two similar structures inferred to delimit the southern extent of the Borrowdale Volcanic Group. Within the Borrowdale Volcanic Group, some faults of this trend developed progressively during formation of the Scafell Caldera; they include a set of synthetic and antithetic faults in Langdale, which together form a keystone graben.

Most of the north–south faults ( Figure 13) and ( Figure 14) are major structures with a complex movement history and are possibly reactivated, basement fracture zones. Three extensive north–south structures divide the Borrowdale volcanic outcrop into blocks. The Lake District Boundary Fault Zone marks the western margin of the Lower Palaeozoic inlier and coincides with the western margin of the batholith; a single fault in its southern extent, it passes northwards into a network of fault strands. One of the components, the arcuate Thistleton Fault, controlled accumulation of the volcanic rocks and so may also have influenced emplacement of Ordovician components of the subvolcanic batholith. Farther east, the Coniston Fault appears to be a simple structure. By contrast, the Troutbeck Fault System is a 4 km-wide network of anastomosing fault strands, making it the most impressive structure within the inlier. To the west of this structure, the volcanic strata are inclined eastwards, whereas those to the east are inclined westwards, bringing Skiddaw Group to the surface in the Bampton Inlier ( Figure 13). Furthermore, the Troutbeck faults follow the deep structural col, established from geophysical data, that separates the western and eastern parts of the batholith. Other structures in this set that were significant in the volcanotectonic development of the Borrowdale Volcanic Group are the Greendale, Dorehead and Whillan Beck faults ( Figure 14).

Pre-volcanic uplift

In late Llanvirn to earliest Caradoc times, the Lake District was elevated prior to the onset of volcanism, producing a major regional unconformity ( Plate 11). As there is no evidence for orogenic deformation at this time, uplift is attributed to mantle hydration, thermal heating and melt generation resulting from the subduction process. Uplift was facilitated by major fault movements and at least 2 km, possibly up to 5 km, of Skiddaw Group strata were eroded off.

Lenticular conglomeratic units of mudstone flakes and sandstone clasts are found at the base of both the Eycott and Borrowdale volcanic groups, and demonstrate that Skiddaw Group sediments were lithified by the time that the first volcanic rocks were erupted. Weathered Skiddaw Group rocks are preserved locally beneath the unconformity, but generally details of this interval are sparse.

The character of the Overwater and Latterbarrow Sandstone formations at the base of the Eycott and Borrowdale volcanic groups respectively, suggests that, prior to the onset of volcanism, the Lake District lay at or about sea level. Fluvial conditions prevailed during deposition of the lower part of the Latterbarrow Sandstone, giving way to intertidal and possibly marine environments in its middle and upper parts. The Overwater Formation is probably marine.

Borrowdale Volcanic Group

Borrowdale volcanic rocks form the craggy mountains of the central Lake District, but also crop out in the Cross Fell, Teesdale and Furness inliers ( Figure 13). In west Cumbria, a substantial thickness of mainly pyroclastic rocks has been proved in deep boreholes underlying Upper Palaeozoic rocks.

The subaerial Borrowdale volcanic sequence is stratigraphically complex with more than one hundred formations and members designated. These variations record the evolving patterns of volcanism, and thus the development of the volcanic field and its associated depocentres ( Figure 16). The principal development phases and the active depocentres are listed in ( Table 2), and the distribution of the successions shown in ( Figure 17). In the following account only the few most important of the units are referred to by name.

The group can be divided informally into ‘lower’ and ‘upper’ parts, representing two contrasting eruptive phases: an early one, mainly of andesite lavas, formed a field of low-profile volcanoes, whereas a later, dominantly silicic phase featured voluminous eruptions of pyroclastic density currents and resulted in caldera formation. In this account, the term ‘pyroclastic density current’ is used to encompass the spectrum of gravity-driven pyroclastic currents that includes pyroclastic surges and pyroclastic flows as its end members.

The fundamental change in eruption style recognised at the base of the upper part of the group saw the probably contemporaneous emplacement of stratified caldera successions at Scafell and Haweswater (Table 2, phases 4a, b; ( Figure 15). The topographical depression created at Scafell was then infilled with the fluvial and lacustrine volcaniclastic sediments that form the top of the succession there. The products of ensuing eruptions accumulated in fault-controlled depocentres developed along a trend parallel to, and south of, these calderas. The Duddon (phase 5a) and Kentmere (phase 5b) basins developed contemporaneously in the south-west and eastern parts of the Lake District respectively. Subsequently, the Ambleside depocentre became active (phase 6).

Then followed the Lincomb Tarns Formation, the most voluminous ignimbrite preserved within the Borrowdale Volcanic Group (phase 7). Volcaniclastic sedimentation once again dominated in the ensuing Helvellyn succession (phase 8). The major, largely concealed, pyroclastic succession in west Cumbria is probably caldera related (phase 9a). However, the relationship of the West Cumbria volcanic rocks with other Borrowdale volcanic successions is unknown, as is that of the succession in the Cross Fell Inlier (phase 9b); either may contain some of the youngest volcanic deposits.

The later successions contain thick sequences of stratified volcaniclastic rocks and these host abundant contemporaneous sills. Contrasting styles of clastic sedimentation are recorded by these sequences. Episodes of diminished eruptive activity are characterised by units containing abundant fluvial channels and their deposits, and by laminated mudstone and siltstone that accumulated in a lacustrine setting. By contrast, huge volumes of unstable sediment were mobilised almost instantaneously during large-magnitude pyroclastic eruptions or by collapse of existing unlithified materials as a result of seismicity, and then transported by mass-flow processes to form lithologically uniform sheet-like deposits.

Initial phreato-magmatic eruptions (( Table 2), phase 2)

Initially, phreatomagmatic eruptions occurred widely and their products are preserved in units varying from a few metres to more than 600 m thick. The Devoke Water Tuff is the most extensive of these units, covering an area of about 30 km² in the western Lake District. Very similar rocks also occur about 5 km north of Calder Bridge [NY 065 105] and around Ullswater. In the south­west of the Lake District, phreatomagmatic deposits comprise much of the Whinny Bank, Po House and Greenscoe formations. In addition to a juvenile component of non-vesicular basaltic andesite, these rocks contain abundant mudstone and sandstone fragments along with accretionary mud pellets derived by pyroclastic fragmentation of the underlying Skiddaw Group and, north of Calder Bridge, the Latterbarrow Sandstone. They have very little ash matrix and were explosively erupted when non-vesiculated magma came into contact with either surface or ground water, resulting in the construction of tuff rings or cones. Clasts that might have an origin deeper in the crust, or from the mantle, have not been recorded, indicating that the explosions occurred at a shallow depth.

The Whinny Bank Tuff Formation comprises planar-bedded mudstone intercalated with an upwards-increasing proportion of andesite lapilli and lapilli-tuff beds. Much of the mud occurs as accretionary lapilli deposited from fall-out ash and/or from pyroclastic surges. In the Furness Inlier, the mudstone-rich lapilli-tuff and breccia of the Greenscoe Formation were deposited within a valley cut into the Skiddaw Group. There is no unambiguous evidence in the basal parts of these units that establishes whether the initial eruptions occurred either in a subaerial, a lacustrine or a marine environment. If subaqueous conditions did prevail, then water depths were less than about 1 km. However, higher parts of the Devoke Water Tuffs, for example, contain discordances and drapes that are typically subaerial in form. chapter three: caradoc magmatism

Low-profile andesite volcanoes (( Table 2), phase 3)

The Birker Fell Formation is interpreted as the remnants of a plateau-andesite lava field erupted from numerous low-profile volcanoes. This voluminous unit crops out over an area of 315 km2 and probably underlies much of the remainder of the volcanic succession ( Figure 17). Andesite sheets with blocky autobrecciated margins dominate the succession, locally comprising 30 to 90 per cent of the observed thickness. Individual sheets are 10–200 m thick and may be mapped laterally for up to 3 km. The sheets are interpreted as mainly block lavas, though sills may comprise up to about 30 per cent of the formation, for example, on High Rigg [NY 307 214]. Locally, erupting lavas fed block-and-ash flows to form substantial accumulations of breccia.

The andesite sheets are remarkably parallel over considerable distances, as illustrated on High Rigg, east of Derwent Water ( Plate 14). Cavities in autobreccia in the upper part of many sheets are filled with laminated sandstone. The lamination is parallel with bedding in overlying clastic rocks, showing that the sequence was near-horizontal when the cavities were filled. There is no evidence for steep slopes, as are associated with classical stratovolcanoes, and instead, an overlapping cluster of shield-like edifices with relatively small diameters and gently sloping flanks is more likely. The facies model is shown in ( Figure 18).

Systematic geochemical cycles characterise the andesite sequence and may be related to fractionation of discrete magma batches within subvolcanic chambers. Cycles in which the rocks became less differentiated with time represent rapid eruption from a compositionally zoned magma chamber, whereas those in which the rocks became more differentiated imply much reduced eruption and recharge rates such that crystal fractionation processes dominated the composition of each successive unit. In addition, some parts of the succession show almost no geochemical evolution with time implying a balance between eruption, magma recharge, mixing and crystal fractionation processes. The occurrence of different sequences of cycle types at various locations supports the view that the lava pile was constructed from a number of volcanic centres.

Basaltic andesite aa-lavas, typically 5 to 30 m thick, occur in the lower part of the Birker Fell Formation in Eskdale and Wasdale, on High Rigg, Hallin Fell [NY 420 190] and around Haweswater [NY 500 140]. Basaltic compound lava fields are developed near the top of the formation in the western part of the outcrop; one such unit comprises basaltic compound aa-flows up to 310 m thick, ponded within a depression, and at least 3 km3 in volume.

The effusive sheets are separated by lenticular units of volcaniclastic rocks, deposited from mass-flows, sheet floods and ephemeral streams, and possibly by wind. Tephra is readily recycled from the flanks of subaerial volcanoes, and thin volcaniclastic units within the Birker Fell Formation may represent considerable time spans during which the landscape may have been buried and stripped more than once. Where intercalations of volcaniclastic rocks are generally thin or absent, for example around Ullswater, the andesite succession was probably emplaced rapidly. By contrast, the Eagle Crag Member, between Haycock and Brown Knotts, is the most extensive and thickest volcaniclastic intercalation within the andesite pile, and it also hosts abundant peperitic sills. The geometry of the unit and the predominantly subaqueous environment of deposition imply the existence of a basin, perhaps controlled by extensional faults.

The lava plateau had largely a constructional topography, but there are also valleys filled with lavas and pyroclastic rocks, or choked with debris-flow and sedimentary deposits. For example, between Yoadcastle [SD 155 949] and Pike of Blisco [NY 270 044], two silicic ignimbrites, the nodular Little Stand Tuff and the lithic-rich Grey Friar Tuff, and the Great Whinscale Dacite lava were confined within an east-north-east-trending valley, at least 16 km long. The amount of topographical relief at any level within the sequence is difficult to estimate, though prior to the succeeding pyroclastic eruptions of the Scafell Caldera succession, the upper surface of the lava field had a relief of no more than about 110 m.

Silicic lavas and pyroclastic rocks were erupted episodically, particularly in the western Lake District. The main units are welded ignimbrites of the Cockley Beck ( Plate 15) and Crag-house tuffs, and the succession of silicic rocks associated with the Great Whinscale Dacite. The aphyric Great Whinscale Dacite is an unusually extensive lava flow of this composition, having a length of at least 13 km and a minimum volume of 4 km3. Porphyritic dacite lavas and domes, many of them up to 250 m thick, also occur at other levels within the formation.

In the western Lake District, the andesitic to dacitic Craghouse Tuff represents the earliest large-volume, densely welded ignimbrite in the Borrowdale Volcanic Group, and possibly provides the first evidence therein of caldera formation. The ignimbrite sequence crops out south-east of the Ennerdale Intrusion, between the Burtness Comb and Thistleton faults. Thickness variation in the ignimbrite is systematic and fault controlled ( Figure 18). Compositional zoning, from dacite at the base to acid andesite towards the top is typical of many ignimbrites erupted from zoned magma chambers.

Scafell Caldera (( Table 2), phase 4a)

A large, silicic caldera volcano was then built in the Scafell and Langdale areas, centred within the axial region of what is now the Scafell Syncline. The intracaldera succession is preserved, comprising stratified, garnet-bearing, andesitic to rhyolitic pyroclastic rocks, mainly very densely welded ignimbrites ( Figure 19). A complex zone of fault-block rotations helps to define the caldera, at least 15 km across and enclosing a minimum area of 220 km2. It is thought that very much more than 400 km3 of magma were erupted causing piecemeal foundering of the caldera floor as incremental subsidence along growth faults allowed successively erupted units to become ponded within ephemeral, local depressions ( Figure 20). The caldera basin was then progressively filled with sediment.

Activity at Scafell began with eruption of andesitic ignimbrite and the succeeding phreatomagmatic tuff of the Whorneyside Formation. Initially, up to about 300 m of welded ignimbrite, the Wet Side Edge Member, buried the pre-existing lava landscape over a wide area from Borrowdale to the Coniston fells (Figure 20a). The ignimbrite passes up into bedded fine tuff containing accretionary lapilli, block sags and beds that drape irregular surfaces. More widely dispersed than the ignimbrite and up to about 30 m thick, this large-magnitude phreatoplinian tuff resulted from the ingress of water into the erupting magma. Around Borrowdale, ash fell into shallow lakes where it was reworked by waves and redeposited by turbidity currents and debris flows; here also the deposits were intruded by andesitic sills. Widespread, piecemeal subsidence began at this time, leading to deformation of the ash deposit (Figure 20b), whilst the fault-block rotation affecting the Birker Fell Formation in Wasdale and Eskdale is thought to have occurred contemporaneously.

Silicic magma was tapped prior to the end of the andesitic phase, resulting in the uppermost part of the Whorneyside Tuff becoming interbedded with silicic tuffs at the base of the Airy’s Bridge Formation. The Long Top Tuff Member comprises welded ignimbrite that is more than 150 m thick on the south side of the caldera. Thickness variations are locally variable and relate to individual volcanotectonic fault blocks (Figure 20c). The very intensely welded Oxendale Tuff is ponded at the base of the Long Top Member in the area between Langdale and Seathwaite Tarn. Also, water flooded episodically into the caldera, resulting in widespread phreatomagmatic surge and fall-out tuffs throughout the region and these form marker units within the sequence ( Figure 19).

The succeeding very densely welded lapilli-tuffs of the Crinkle Tuff Member (Figure 20d) represent the climactic phase of the eruption though the Crinkle Tuffs have a more restricted distribution than the Long Top Tuff. In the northern part of the caldera, around Borrowdale, the Crinkle Tuffs are locally up to 650 m thick. The Bad Step Tuff, one of the component units of the Crinkle Tuff Member, is very thin in this area, becoming much thicker to the south in Langdale. Slumping and large-scale fault-scarp failure accompanied caldera collapse.

There are only a few examples of tuff-filled dykes and pipes that might represent vent sites for the pyroclastic flows. However, the abundance of very densely welded ignimbrite requires rapid eruption and deposition rates and it seems likely that many of the closely spaced growth faults acted as fissure vents.

The climactic eruptions and piecemeal collapse created an irregular topography. Typically, there is an angular discordance of 8 to 15° between these rocks and the overlying Lingmell Formation, with the Crinkle Tuff inclined more steeply towards the centre of the caldera. Volcanic activity that produced the Lingmell Tuffs was much reduced in intensity, and episodic pyroclastic density currents produced an intercalated sequence of thin units of massive lapilli­tuff, breccia and bedded tuff (Figure 20e). Unstable fault scarps continued to shed landslides and avalanche deposits locally.

Post-collapse magmatism at Scafell included eruption of the Rosthwaite Rhyolite and Scafell Dacite. The former was erupted early in this phase and its intrusive counterpart is also exposed on Rosthwaite Fell [NY 260 120]. The later, dacite lava dome straddles a volcano-tectonic fault. Its eruption created significant relief, against which wedge out successive units of the overlying caldera-basin sedimentary fill; lenses of dacite-rich breccia at several levels within the sedimentary pile testify to the episodic shedding of debris into the basin.

The sedimentary fill of the caldera basin (Seathwaite Fell Formation) involved fluvial, lacustrine and pyroclastic processes. Throughout, sediment transport was dominantly towards the basin centre. Near the base along the north side of the basin, steep-fronted pebble-gravel deltas were fed from the north-north-west. Thinly bedded units of fine-grained sediment are probably turbidites, locally reworked, but the presence of wave ripples suggests maximum water depths of a few tens of metres. Debris-flow deposits were also contributed and there are intervals dominated by cross-stratified gravel-filled channels.

Contemporaneously erupted andesitic tephra was brought in from outside the caldera by sediment gravity-flows. Widespread syndepositional deformation and slumping in these deposits resulted from loading and liquefaction of the sediment, possibly triggered by earthquakes. The Pavey Ark Breccia contains abundant andesite blocks that appear to have been hot and plastic on deposition, and which were sourced from the east.

A significant subaerial silicic eruption occurred during the later stages of caldera infill to form the Glaramara Tuff. The tuff, containing abundant accretionary lapilli, becomes thinner and finer grained southwards, and changes in facies from massive in the north, to stratified and cross-stratified, and to massive and bedded in the south. The tuff aggraded from a rapid succession of pyroclastic density currents, produced during phreatomagmatic eruptions, and built a low-relief tuff ring with a vent probably located just north of the present outcrop.

Haweswater ignimbrite centre (( Table 2), phase 4b)

From Ullswater to Haweswater the low-profile volcanoes of the Birker Fell Formation were buried beneath another stratified sequence of silicic pyroclastic rocks. Initially, pyroclastic density current deposits accumulated south-east of Ullswater, from Place Fell [NY 415 185] to Riggindale [NY 458 114], to form a 10–340 m-thick sequence of andesitic to dacitic tuff and lapilli-tuff. The overlying Whelter Knotts Formation comprises a 550 m-thick sequence consisting of three silicic units. The basal, bedded pyroclastic rocks, which include some of the most garnetiferous rocks seen in the Borrowdale Volcanic Group, are overlain by a flow-foliated felsitic unit, recognised only locally around Haweswater [NY 497 167], and by a succession of massive, eutaxitic, parataxitic and lithic-rich lapilli-tuff facies, locally with intercalations of bedded tuff. Welding varies from weak to very dense, with columnar jointing and rheomorphic fabrics locally developed near the southern part of Haweswater. Near the base of the formation on Kidsty Pike [NY 448 125], are thinly stratified ignimbrites, one unit of which contains abundant siliceous nodules.

Though the outcrop of the Haweswater succession is much disrupted by faults, in particular the Troutbeck Fault Zone, there is a broad, basin-like structure, en échelon to the Scafell Caldera ( Figure 15). Across some faults in the Haweswater area, there are abrupt changes in strike of the strata and in thickness and facies in the Whelter Knotts Formation. There is no evidence for a significant break in deposition in this sequence, suggesting that the two formations were probably emplaced during a single, violently explosive eruptive episode accompanying caldera formation in the Haweswater area during the climax of volcanism.

Duddon Basin (( Table 2), phase 5a)

Following the climactic eruptions at Scafell, pyroclastic, lava-dominated and volcaniclastic­sedimentary sequences up to 3000 m thick accumulated in the south-west of the Lake District. These were emplaced contemporaneously with the waning phase of pyroclastic activity and basin infill at Scafell. The new focus of activity began with two voluminous pyroclastic eruptions centred near Ulpha [SD 190 130] and these seem to have been the prelude to development of the Duddon Basin, a major depocentre bound to the north by the Grassguards Fault and to the south by the inferred Bootle Fell and Kirkby faults which bound the graben.

The first eruption produced the densely welded, andesitic to rhyolitic ignimbrite of the Waberthwaite Tuff Formation, which is ponded to a thickness exceeding 1900 m between the Whillan Beck and Baskill faults ( Figure 15). The geometry of the unit, along with features such as abundant rheomorphic fabrics and intercalated units of mesobreccia, suggest accumulation within a caldera.

The second eruption followed without a significant time break, depositing andesitic tephra over an area of at least 16 km diameter, from Duddon Bridge to north-east of Wetherlam to form the Duddon Hall Tuff Formation. The tuff is similar to the earlier Whorneyside Tuff and represents another large-magnitude phreatoplinian fall-out tuff ( Plate 16). Ash fall-out was accompanied by pyroclastic density currents which produced units of bedded, vesicular and accretionary lapilli-rich tuff. In the Coniston Fells these are intercalated with massive units of scoria and lithic-rich lapilli-tuff. During the eruption, andesite sills were emplaced around Ulpha Park and Green How [SD 189 915]–[SD 205 958].

Near Ulpha, the phreatoplinian tuff is overlain by grey silty mudstone of the Holehouse Gill Formation. The mudstone contains abundant reworked acritarchs and the mud was probably derived from eroded Skiddaw Group rocks. The lithology and abrupt thinning of the formation away from its area of maximum thickness suggests accumulation within a small, enclosed basin at a time when volcanic sediment was in short supply. The thickest development of mudstone approximately coincides with the thickest development of the underlying bedded tuff lithofacies, suggesting that the former may have been deposited within a caldera produced by the phreatoplinian Duddon Hall eruption. An andesite lava shield volcano was constructed in the north-east of the basin at this time.

The overlying rocks mark the onset of volcaniclastic sedimentation throughout the Duddon Basin, but this was interrupted episodically by silicic pyroclastic activity. Up to 1200 m of sandstone, breccia and conglomerate are intercalated with sheets of ignimbrite. The sedimentary units thicken markedly towards the axial region of the basin, suggesting that sediment loading and extensional faulting controlled basin development, though facies and thickness changes also relate to the Stonythwaite Fault. The sedimentary units are rapidly accumulated, coarsening-upwards cycles: turbiditic sandstones dominate the well-bedded lower parts, replaced higher up by increasing volumes of mass-flow deposits. Evidence of fluvial reworking is seen at intervals throughout. Contemporaneous erosion occurred in the Coniston Fells, to the north, and much of the volcaniclastic sediment accumulating in the basin may have been supplied from there.

Kentmere Basin (( Table 2), phase 5b)

South of the Haweswater Caldera, in the Kentmere and Shap fells, at least 3000 m of volcaniclastic sedimentary and pyroclastic rocks are preserved in a region bounded by the Swindale Fault Zone. The Kentmere succession hosts the largest concentration of mafic sills within the volcanic group.

The lowest part of the succession accumulated in a fluviolacustrine environment, with subaqueous sediment fall-out repeatedly interrupted by fluvial activity and the influx of mass-flow deposits. A high proportion of the coarser-grained and conglomeratic upper part of the sequence was deposited from mass flows, which reworked contemporaneously erupted pyroclastic deposits. The instability and rapid accumulation of the sediment pile is shown by the presence of abundant soft-sediment deformation.

The extent, facies, thickness and degree of welding in the succeeding andesitic to rhyodacitic ignimbrite sheet resulted from its emplacement during a single, large magnitude, eruption that produced high-density pyroclastic flows; the scale of the event was comparable with those responsible for other large ignimbrites in the Lake District. A welded basal zone, locally with rheomorphic fabrics, passes up into lapilli-tuff characterised by abundant silicic lithic lapilli; the uppermost part is nonwelded coarse tuff.

East of the Troutbeck Fault, the ignimbrite is conformably overlain by bedded, andesitic, pyroclastic rocks, which are interpreted as the remains of a Surtseyan tuff-cone field. In Woundale [NY 410 080], the pyroclastic rocks comprise massive to weakly stratified lapilli­tuff, overlain by bedded and cross-bedded tuff. These were all emplaced from pyroclastic density currents, whereas intercalated layers of blocks and bombs, with accompanying sag features, indicate episodic ballistic ejections. Channels and discordances resulted from subaerial erosion and/or slumping. Northwards, these facies are intercalated with units of massive lapilli-tuff which, whilst emplaced from pyroclastic flows, show local evidence for reworking by waves and currents.

From Kentmere, east to the Shap Fells, basaltic andesite and subordinate andesite sheets, possibly up to 1200 m thick, overlie the bedded pyroclastic unit. Some of the andesite sheets are sills, but the clinkery autobreccias associated with the basaltic andesite sheets are typical of lavas and the sequence most likely originated as part of a low, shield-like volcano.

Ambleside Basin (( Table 2), phase 6)

In the central southern part of the volcanic group outcrop, volcaniclastic sedimentary rocks of the Tilberthwaite Formation within the Ambleside Basin succession overstep the Duddon and Kentmere successions to overlie the Scafell Caldera succession. A marked unconformity at the base is seen locally on the west side of Coniston Old Man [SD 272 975], where sandstone gently oversteps steeply dipping ignimbrite, and at Tilberthwaite [NY 304 014], where sandstone abuts a steep, cliff-like unconformity in ignimbrite. The sandstone succession is about 1100 m thick near Tilberthwaite, thinning northwards to zero in the central fells. A massive landslide deposit, the Side Pike Complex ( Figure 15), lies beneath the Tilberthwaite Formation south of Langdale.

Well-bedded, fine-grained rocks form the lowest 150 m of the succession south-west of Coniston. Here, stratified pyroclastic fall-out deposits were extensively reworked by traction currents. The abundance of siltstone beds and intercalations of grey and brown, fine-grained sandstone indicates either suspension fall-out from distal eruptions or quiescent episodes with low sediment input.

The overlying rocks comprise massive and thickly bedded coarse-grained or pebbly sandstones ( Plate 17). Weak, diffuse bedding and abundant andesite scoria in some of these beds suggest rapid deposition from voluminous high-density sediment flows generated during pyroclastic eruptions. Some of the scoria was derived from peperitic sills that were emplaced contemporaneously at shallow depths within the sediment pile. Abundant soft-sediment deformation structures attest to rapid subaqueous deposition. A widespread silicic accretionary lapilli-tuff unit emplaced during a phreatomagmatic eruption and a small-volume ignimbrite occur in the uppermost part of this sequence.

Lincomb Tarns ignimbrite centre (( Table 2), phase 7)

The stratified succession of silicic ignimbrites within the Lincomb Tarns Formation is the most widespread in the volcanic group. From the size of its outcrop, which encompasses almost 500 km2, it seems likely that the eruptions buried the entire Borrowdale volcanic graben beneath at least 150 m, and locally more than 800 m, of densely welded, dacitic lapilli­tuff. This represents an eruption of exceptional magnitude, during which caldera formation is implicit. The annular form of the Birkhouse Moor and Hogget Gill faults, and the almost flat-lying stratified nature of the Lincomb Tarns ignimbrites within it, may indicate the location of the caldera.

From south to north, the ignimbrite sequence oversteps sedimentary rocks of the Ambleside Basin and Scafell Caldera to overlie Birker Fell andesites around the Legburthwaite graben. The character of the basal contact changes systematically ( Figure 21). In the east and south, the ignimbrite rests on an erosion surface cut across the underlying strata; for example, north-east of Coniston, the ignimbrite fills a 140 m deep valley and in Kentmere it drapes up to 250 m of relief. Westwards the contact becomes conformable, a change interpreted as showing that there the ignimbrite was deposited on a low-relief depositional plain. Ultimately, around Broad Crag [NY 216 077], the ignimbrite is brecciated, reworked and interbedded with sandstone where the pyroclastic flows entered a body of water. These relationships indicate a palaeoslope inclined to the west and its existence may be explained by pre-eruption doming across the region as the high-level magma chamber was filled.

The eruption began with a sequence of pyroclastic surges, and thin deposits were preserved on the depositional plain. Within the Legburthwaite graben, heterogeneous units of garnetiferous eutaxitic lapilli-tuff, lithic-rich lapilli-tuff and tuff-breccia, at least 280 m thick, accumulated, possibly near to the vent. Densely welded, rheomorphic lapilli-tuff of the Thirlmere Member succeeds the basal rocks in the Legburthwaite graben, Wythburn fells and Helvellyn Basin. Contemporaneous movements on the Birkhouse Moor Fault locally produced a thick development of mesobreccia, whilst slumped masses of lapilli-tuff adjacent to the fault were deformed plastically and sheared whilst they were still hot. More than 300 m of Thirlmere ignimbrite are preserved within the Helvellyn Basin and Legburthwaite graben. The features of the Thirlmere Member are typically found within caldera-fill successions at ignimbrite volcanoes.

In the northern part of the Helvellyn Basin, the Thirlmere Member is succeeded by up to 150 m of massive and bedded fine to coarse tuffs. These rocks seem to represent a period of diminished volcanic activity following the paroxysmal Thirlmere eruption. The overlying heterogeneous sequence of ignimbrites is widely dispersed. South-east from the Helvellyn area, eutaxitic lapilli-tuff passes up into crudely stratified lithic-rich lapilli-tuff, tuff-breccia and breccia. In the south, from Shap to south-west of Coniston, the ignimbrite has a more uniform character and is columnar jointed between Oxen Fell [NY 324 015] and Torver Beck [SD 273 963]. Massive lapilli-tuff in the Ullscarf area [NY 291 122] passes westwards, to the Scafell area, into thinner, bedded-flow units.

Helvellyn Basin (( Table 2), phase 8)

The Lincomb Tarns ignimbrites are succeeded by sedimentary rocks intercalated with some silicic pyroclastic units and lavas. The lower part of the succession extends throughout the central Lake District and beneath the unconformity at the base of the Dent Group. However, the main depocentre appears to have been within the Helvellyn Basin, where at least 1600 m of strata are present. Within the basin, strata progressively steepen south-eastwards across north-east-trending syndepositional faults, thus preserving successively younger beds towards the south-east.

In the Scafell area, inundation followed soon after emplacement of the Lincomb Tarns ignimbrite. The uppermost ash deposits were reworked and the succeeding laminated mudstone, siltstone and sandstone were laid down under relatively quiet, subaqueous conditions. Farther east, the sequence is dominated by sandstone containing abundant soft-sediment deformation structures that testify to rapid sediment accumulation. Intercalated lenticular units of pebbly sandstone and breccia, emplaced as mass flows, thicken and coarsen towards major faults; these units represent alluvial fan deposits generated by contemporaneous fault movements.

Locally, on Seat Sandal and Fairfield Brow [NY 3498 1172] to [NY 3536 1180], and north­east of Hartsop village [NY 418 136], andesite block-lavas rest on the Lincomb Tarns Formation. These lavas probably represent remnants of small andesite volcanoes. The Middle Dodd Dacite, which was erupted some time later, encompasses numerous isolated outcrops within an area of about 26 km2, both within and beyond the margins of the Helvellyn Basin.

Clastic sedimentation in the Helvellyn Basin was interrupted by eruption of voluminous pyroclastic density currents that were ponded to the south of the Birkhouse Moor Fault to form the Helvellyn Tuff. Initially, a rapid succession of pyroclastic flows built up a thick deposit of welded lapilli-tuff but, later in the eruption, large volumes of water episodically entered the vent, producing pyroclastic surge and ash fall-out deposits that were interstratified with continued emplacement of ignimbrite. With a thickness of up to 400 m, the Helvellyn Tuff compares to many of the major Borrowdale Volcanic Group ignimbrites.

A return to fluviolacustrine sedimentation occurred with the overlying Deepdale Formation, the base of which ranges across 60 m of topographical relief. Planar-bedded units of massive, normally graded or laminated beds of fine to coarse-grained sandstone make up much of the sequence. These were deposited mainly by turbidity flows, and locally reworked by traction currents. Intercalated lenticular units of coarse-grained sediment within the Deepdale Formation originated as alluvial fan deposits that built up adjacent to the major basin faults. Once again there are abundant soft-state deformation structures.

Gosforth succession (( Table 2), phase 9a)

A succession, mainly comprising ignimbrite and at least 1000 m thick, has been proved within a half-graben to the west of the Thistleton Fault in west Cumbria ( Figure 22). The sequence is poorly exposed and information has been mainly obtained from the 9.8 km of cores of volcanic rock recovered from 22 deep boreholes drilled near Gosforth. The base of this sequence has not been proved and the rocks cannot be correlated with those to the east. The succession may contain the youngest volcanic rocks preserved in the Borrowdale Volcanic Group.

Units such as the Bleawath Formation, Seascale Hall Member and the Fleming Hall Formation comprise densely welded, dacitic and rhyolitic ignimbrites containing rheomorphic fabrics, together with intercalated lithic co-ignimbrite breccias and breccia formed through fusion of welded tuff blocks. This assemblage suggests that another caldera system was developed in this area. Changes in stratigraphy, lithofacies and thickness between some of the closely sited boreholes resulted from either volcanotectonic faulting or contemporaneous erosion and infill. Higher parts of the succession, proved in a northern group of boreholes, comprise a sequence of rapidly deposited sandstones, overlain by another major silicic ignimbrite, and intruded by andesite sills.

Cross Fell succession (( Table 2), phase 9b)

Three volcaniclastic formations occur in the Cross Fell Inlier and though the lithofacies present are typical of the upper part of the Borrowdale Volcanic Group, none of the units can be correlated with those in the central Lake District. The silicic ignimbrites of the Knock Pike Formation are some of the most geochemically evolved rocks seen.

Contemporaneous intrusive rocks

Basaltic, andesitic, dacitic and rhyolitic sills, individually up to 250 m thick, contributed significantly to the development of the volcanic succession. A very large number are hosted in bedded volcaniclastic units within the Duddon, Kentmere, Ambleside and Helvellyn basin successions. Fewer sills are recognised in the Birker Fell Formation, except in the Eagle Crag Member.

Many of the sills are vesicular and have peperitic margins, the textures indicating the importance of fluidisation during emplacement at shallow levels into water-saturated sediment ( Plate 18). In some cases, the carapace of sediment was so thin that parts of the overburden slumped off and became mixed with peperitic clasts in associated mass-flow deposits. By contrast, podiform andesite sills up to about 30 m thick, and within the Helvellyn Basin succession south of Ullswater, have thin chilled margins indicating intrusion into consolidated rock, possibly at a very late stage in the volcanism.

Whilst some of the sills were emplaced during pyroclastic eruptions, the timing of intrusion of most others is uncertain. Their occurrence at many levels in the volcanic pile implies that there were multiple episodes of injection. Moreover, localised concentrations of andesite intrusions, such as that in the eastern Lake District associated with the Kentmere Basin succession, may represent the sites of volcanoes or, perhaps, the early phase of caldera cycles.

Petrography of the volcanic rocks

The Borrowdale volcanic rocks contain up to 30 per cent phenocrysts, comprising mainly plagioclase feldspar. Some clinopyroxene is typically present in the mafic rocks, with pseudomorphs after olivine occurring in a few. By contrast, orthopyroxene becomes the dominant mafic mineral in the felsic rocks, with orthoclase and biotite in a few of the most silicic. Almandine garnet phenocrysts are locally abundant in some andesite and dacite, but they are most abundant in the silicic ignimbrites. The garnetiferous rocks are peraluminous and the mineral and whole-rock compositions are related. The tendency for some calc-alkaline magmas to become peraluminous may be due to the assimilation of pelitic sediment. In the Lake District the source of this could be Skiddaw Group mudstone, but this was available only at relatively shallow crustal levels.

Eycott Volcanic Group

Inliers of the Eycott Volcanic Group occur along the northern margin of the Lake District and also near Melmerby in the north of the Cross Fell Inlier. The aeromagnetic anomaly associated with these rocks links the inliers beneath the cover of Carboniferous rocks to form a continuous, arcuate belt truncated to the south by the Causey Pike Fault ( Figure 13).

Tabular basaltic andesite and andesite sheets, along with subordinate basalt and dacite, comprise the lower part, up to 2400 m thick. The sheets are generally considered to be lava flows, though some are probably sills. The rocks typically contain phenocrysts of plagioclase with subordinate orthopyroxene and clinopyroxene and, in a few rocks, olivine. Some of the basaltic andesite sheets are notable in containing up to 45 per cent plagioclase phenocrysts up to 30 mm across. This striking, ornamental rock has been referred to as ‘Eycott-type’ basaltic andesite. The lavas are interbedded with thin units of tuff, lapilli-tuff, pyroclastic breccia and volcaniclastic sedimentary rocks.

At the base of the group, the Overwater Formation consists of up to about 8 m of bedded grey siltstone and greenish grey tuffaceous sandstone, intruded locally by sills. The uppermost 800 m of the group (Potts Ghyll Formation) is best developed in the Caldbeck Fells, where it consists of heterogeneous, vitric acid andesitic tuff, overlain by pumice- and lithic-rich lapilli-tuff that is ungraded and very poorly sorted. The Potts Ghyll Formation is interpreted as non-welded ignimbrite.

Facies present in the volcaniclastic rocks suggest that a subaerial environment dominated. With the exception of the thick unit of pyroclastic rocks preserved at the top of the group, the Eycott volcanic rocks closely resemble the Birker Fell Formation of the Borrowdale Volcanic Group and probably developed in a similar manner. Thickening of parts of the succession across major faults indicates that extensional faulting played an important role in accumulation of this sequence.

Lake District Batholith

Emplacement (( Table 2), phase 10)

Outcrops of the major granitic intrusions in the Lake District, including the Ennerdale, Eskdale, Broad Oak and Threlkeld masses have long been known in detail, but it was the pioneering interpretations of gravity anomalies that linked these surface exposures to large concealed granitic masses underlying northern England. The subsurface extent of the Lake District Batholith is more than 1500 km2 and detailed interpretation of the potential-field data permits recognition of a substantial number of components ( Figure 13).

Seismic reflection profiles across the western margin of the batholith exhibit zones of intense, subhorizontal reflections intercalated with lenticular, nonreflective ones; these are interpreted respectively as Skiddaw Group hornfels and granitic sheets. The western margin of the stacked, tabular granite sheets has a saw-tooth-like form, and is broadly coincident with the Lake District Boundary Fault.

Caradoc radiometric ages for the exposed Ennerdale, Eskdale and Broad Oak intrusions suggest that the postulated concealed batholith components beneath were also emplaced during this time ( Figure 4). These intrusions also broadly underlie the Scafell Caldera. The batholith is divided by a narrow ‘neck’ that coincides with the Troutbeck Fault Zone. To the east, there are two bodies, the Haweswater and Shap granites. The former may also be Ordovician, because it underlies a caldera-related pyroclastic succession around Haweswater, but the latter is Devonian and so unrelated to the Ordovician volcanism.

The timing of emplacement of the major plutons relative to the volcanic rocks is not known. However, injection of a stack of tabular granite sheets must have uplifted the volcanic sequence substantially, perhaps causing the present relationship across the Thistleton Fault, where possibly the youngest rocks of the Gosforth succession are juxtaposed against the Birker Fell Formation. Within the volcanic sequence, evidence for episodes of substantial uplift is sparse. Uplift centred on the Helvellyn area prior to eruption of the Lincomb Tarns ignimbrites has been inferred, but this is not spatially associated with the granite ‘highs’ in the western and eastern Lake District. Further, the rhyolite volcanoes at Scafell and Haweswater appear not to have undergone late-stage resurgence due to the injection of later magma bodies, a typical phenomenon at some large silicic caldera volcanoes worldwide. Evidence for substantial uplift during emplacement of the Borrowdale Volcanic Group is generally lacking and it must be concluded that injection of the granites occurred late in the magmatic cycle.

Renewed marine sedimentation (( Table 2), phase 11)

This scenario provides a mechanism for erosion of the volcanic pile prior to the onset of Dent Group sedimentation. In the eastern Lake District, topographical relief of several hundreds of metres is present on the upper surface of the Borrowdale Volcanic Group, but very low relief is present south-west of Ambleside, where the unconformity appears to cut down progressively through almost 3000 m of the volcanic succession. During late Ordovician time, global sea levels were falling and a marine transgression at this time in north-west England, marked by the base of the Dent Group, must have had a local cause. To the north and east of the Lake District, in the Carrock and Cross Fell areas respectively, marine sedimentation began with the Drygill and Dufton Shale formations of Longvillian age, but in the Lake District, onset of sedimentation was at least 2 Ma later, in Cautleyan times. Thermal contraction of the batholith and the subvolcanic lithosphere as they cooled following cessation of magmatism provides a local mechanism for controlling the marine transgression.

Ennerdale Microgranite Pluton (452 +- 4 Ma, U-Pb, zircon)

The Ennerdale Pluton, commonly known as the Ennerdale Granophyre, is a 1–2 km-thick, tabular body that crops out over 53 km2 in the western Lake District, north-west of Wast Water. This mass intrudes the Skiddaw and Borrowdale Volcanic groups and may be faulted against the Eskdale Granite. Porphyritic granophyric granite dominates, but dolerite, dioritic, and hybridised dioritic, granodioritic and melanocratic granitic rocks occur locally, adjacent to the margin of the intrusion. The dolerite possibly represents sidewall cumulates from crystallisation of an early dioritic magma, but emplacement of the more voluminous granitic magma whilst the former was still hot and partly crystallised led to the local formation of hybrid rocks.

Eskdale Granite Pluton (450 ± 3 Ma, U-Pb, zircon)

The Eskdale Granite has an outcrop of 53 km2 and consists of medium-grained muscovite granite, aphyric and megacrystic microgranite, and coarse to very coarse granite. Microgranite is most common in the northern part where the almost horizontal marginal contacts and inliers of hornfelsed volcanic rocks suggest that the roof zone of the intrusion is exposed. It is almost concordant with the base of the Birker Fell Formation, but rises in the northern part of its outcrop, at Wasdale Head ( Figure 13), to within a few hundred metres of the base of the Scafell Caldera succession. Metasomatic recrystallisation to quartz-white mica and quartz-topaz greisens occurs locally within all facies exept the coarse granite. A distinctive quartz-andalusite rock is associated with topaz-greisen adjacent to the contact with the Skiddaw Group near Devoke Water.

Minor mineralisation is associated with the greisens around the Eskdale Granite. Small concentrations of arsenopyrite, native bismuth, bisthmuthinite and molybdenite occur disseminated through parts of the topaz-greisen at Water Crag, near Devoke Water. Coarsely crystalline specular haematite occurs locally as vein-like segregations.

Broad Oak Granodiorite Pluton

The Broad Oak Granodiorite (previously referred to, rather confusingly, as the Eskdale Granodiorite) is a discordant mass, 23 km2 in area, which cuts rocks of the Duddon Basin succession. It is medium grained, with hornblende and, locally, almandine garnet and abundant biotite; it lacks muscovite. There is a marginal microgranodiorite. In the south, the granodiorite displays strong argillic alteration. Its emplacement age relative to the Eskdale Granite cannot be determined because their contacts are not exposed. The granodiorite is considered to be Ordovician and its Wenlock Rb-Sr age of 429 ± 22 Ma to be reset; such resetting is a common feature of both the Ordovician intrusions and rocks of the Borrowdale Volcanic Group (see Chapter 5).

Threlkeld Microgranite Intrusion (451 ± 1.1 Ma, U-Pb, zircon)

Interpretation of gravity anomalies east of Keswick has shown that the outcrops of garnet-bearing microgranite are connected to a largely concealed laccolith, 500–1000 m thick and approximately 12 km2 in area, that lies above the main part of the batholith. The Threlkeld Microgranite was intruded into the Skiddaw Group and basal part of the Borrowdale Volcanic Group. The rock contains microphenocrysts of albite and orthoclase, and phenocrysts of corroded quartz and sporadic almandine garnet. The geochemistry and style of the sericite, carbonate and chlorite alteration are very similar to that of silicic pyroclastic rocks in the volcanic group, suggesting that both rock groups were altered at the same time and under similar conditions.

Carrock Fell Centre

The Carrock Fell Centre was emplaced at the junction between the Skiddaw and Eycott Volcanic groups and comprises mafic, mafic–felsic and felsic intrusions with distinctive geochemical affinities. The earliest intrusions are layered cumulate rocks of the Mosedale Gabbros, which form the southern part of the centre. They were intruded as sub-horizontal sills at the base of the cogenetic volcanic group. However, the K-Ar biotite age of 468 ± 9 Ma for the gabbros does not accord with the probable Caradoc age for the volcanic rocks.

Cutting the Mosedale Gabbros with near-vertical, intrusive contacts are the Carrock Intrusions (452.4 ± 3.1 Ma, U-Pb, zircon). These comprise a main mass of micrographic microgranite, known as the Carrock Granophyre, with a narrow zone of apatite-bearing iron-rich microdioritic rocks along the southern margin, intruded by a comagmatic dyke-like body of microgabbro. Typically, the microgranite comprises zoned albite–oligoclase phenocrysts within micrographic intergrowths of quartz and alkali feldspar, and interstitial spherulitic intergrowths. There are small amounts of Ca-rich pyroxene, amphibole and titanomagnetite. The microgranite is interpreted to represent the subsided roof zone of a near-vertical magma chamber in which crystal fractionation of low-Mg tholeiitic basaltic magma had occurred.

Later, lenticular intrusions of micrographic microgranite were emplaced along the Roughton Gill Fault, at the western extent of the complex. The last intrusion at Carrock Fell was the quartzphyric Harestones Rhyolite. This was emplaced into, and bleached, the Longvillian Drygill Shales now seen within a fault-slice along the northern margin of the complex. The Harestones Rhyolite has a late Silurian Rb–Sr isochron age (419 ± 4 Ma), but it is suspected that this has been reset and that the rhyolite belongs to the Ashgill phase of magmatism. Some geochemical similarities with the Yarlside Volcanic Formation support such an association.

Minor intrusions

Many dykes and small intrusive masses of basaltic, andesitic and rhyolitic composition are associated with the magmatism. Aphyric basalt, andesite and microdiorite masses comprising the Embleton Microdiorite Intrusions are clustered across the Watch Hill Fault, east of Cockermouth, and were probably cogenetic with the Eycott Volcanic Group. Also within the Skiddaw Group, the more widely distributed calc-alkaline, augite-phyric meladiorite, dolerite and olivine-augite hornblendite bodies of the Bassenthwaite Intrusions are geochemically related to the Borrowdale Volcanic Group. The Wasdale Swarm comprising tholeiitic basalt dykes is mainly centred in Wasdale and Eskdale, around the margin of the Eskdale Granite.

In the eastern Lake District, over an area of 19 km2, the Haweswater Intrusions comprise small plugs and dykes of hornblende-bearing dolerite and gabbro, with subordinate microdiorite. The rocks are geochemically similar to the Borrowdale volcanic rocks. There is no evidence to indicate that these intrusions are linked to a major basic mass at depth; to the contrary, the gravity low over the region indicates a concealed shallow granitic body.

The mixed-magma, composite basalt–andesite dykes and small masses of the Pike de Bield Intrusions are concentrated in the area between Scafell and Bowfell. They represent a magma conduit system that exploited pre-existing volcanotectonic faults. The intrusions cut formations up to the Esk Pike Formation, the youngest unit in the area. Contact relationships of some masses within the Seathwaite Fell Formation indicate that the host sediments were not fully lithified at the time of emplacement.

Garnet-bearing dacite dykes in Eskdale and Wasdale were probably coeval with the Scafell Dacite, a lava dome erupted during the final phase of activity at the Scafell Caldera. Pink, aphyric and sparsely microporphyritic rhyolite dykes are locally abundant adjacent to the Eskdale and Ennerdale intrusions. Geochemical similarities with the Ennerdale Microgranite suggest that, like the latter, their Rb-Sr isochron ages of 436 to 428 Ma have been reset.

Petrogenesis

Despite mineralogical and geochemical changes resulting from hydrothermal alteration and low-grade metamorphism, an understanding of the likely types of process involved in the generation and evolution of the magmas has emerged during the last thirty years. The geochemical characteristics of the Eycott Volcanic Group rocks are transitional between medium-K, calc-alkaline and tholeiitic affinities, whilst the Borrowdale volcanic rocks are medium to high-K and calc-alkaline. Both groups have geochemical signatures that are closely analogous to modern orogenic andesite suites emplaced through thick continental crust. The range of mafic magmas present is considered to have been derived by partial melting of a common lithospheric mantle source, enriched to different levels in incompatible elements by subduction-derived aqueous fluids. Subsequent geochemical divergence into suites with tholeiitic, calc-alkaline and transitional affinities resulted from varied amounts of partial melting, different fractionation assemblages and from the assimilation of minor amounts of crustal material.

The Threlkeld and Ennerdale granites, the Broad Oak Granodiorite and associated minor intrusions have similarities with the silicic volcanic rocks which they underpin, and exhibit continental-margin volcanic-arc geochemical signatures. By contrast, many of the distinctive features of the Eskdale Granite are more typical of granites with sedimentary protoliths (‘S-type’), including a restricted high-SiO2 content, peraluminous composition, the presence of muscovite, and specific Sr and O isotope compositions. Given that these granitic rocks were emplaced in close proximity and during a very short period in Caradoc times, the compositional differences must relate to processes such as the amount of crustal material assimilated and differing crystal fractionation histories.

Chapter 4 Late Ordovician to Silurian: Windermere Supergroup

The cessation of Caradoc volcanism and granite intrusion was followed by thermal subsidence and a marine transgression across the eroded volcanic fields of the Borrowdale and Eycott groups. This occurred despite a lowering of global sea level coincident with a late Ordovician ice age. The Windermere Supergroup encompasses the ensuing, mostly sedimentary and more-or-less complete succession that ranges up through the remainder of the Ordovician and the whole of the Silurian ( Figure 23). It crops out across the southern Lake District, from Furness in the west to the Langdale and Howgill fells in the east ( Plate 19).

Ordovician clastic and carbonate sedimentation was largely in a near-shore environment but with some of the basal strata showing fluviatile characteristics; two volcanic intervals represent the final throes of local Ordovician eruption. Marine deposition continued across the Ordovician–Silurian boundary despite the brief ice age at the end of Ordovician times, but the postglacial rise in global sea level produced a deepening depositional environment that allowed accumulation of mudstone and shale throughout the Llandovery. The overlying Wenlock sequence is dominated by laminated hemipelagic mudstone but with the proportion of turbidite sandstone increasing upwards. A calcareous siltstone unit spans the Wenlock– Ludlow boundary, marking a major marine regression, but the laminated hemipelagic mudstone reappears above it and continues upwards into the Ludlow, where it is increasingly swamped by the influx of turbidite sand.

The increase in the proportion of turbidite sandstone upwards into the Ludlow is dramatic, culminating in the deposition of a several-kilometre thickness of strata during the duration of three graptolite biozones (the Gorstian, nilssoni, scanicus and incipiens Biozones). This change in sediment accumulation rate has been ascribed to a rapid increase in subsidence brought about by the closure of the Iapetus Ocean and the collision of its marginal continents. As described in Chapter 1, collision between Laurentia (the northern continent) and Avalonia (the southern margin of Iapetus on which the future north of England was situated) resulted in the former over-riding the latter. It was this progressive loading of the Avalonian margin that produced the marked acceleration in subsidence rate and allowed accumulation of the thick turbidite sequence in what was effectively a foreland basin.

The later Ludlow and Pridoli succession reflects the matching of the subsidence rate by sediment supply rate and a commensurate filling of the sedimentary basin. It would seem that convergence between Laurentia and Avalonia slowed, the foreland basin failed to migrate southwards, and isostatic adjustments increasingly balanced the effects of loading. The middle to upper Ludlow strata are mainly low density, silt- and mud-dominated turbidites, but the youngest Ludlow and the Pridoli deposits include shallow-water sandstones and red beds.

Stratigraphical framework

In biostratigraphical terms there is one important difference between the Ordovician and Silurian parts of the Windermere Supergroup. The former contains a locally rich and varied shelly fauna ( Plate 20) but, until recently, very few graptolites had been recorded; in contrast, the latter is largely graptolitic. Hence the basis of biostratigraphical correlation changes at the Ordovician–Silurian boundary, and the application of either scheme across that boundary is uncertain.

Windermere Supergroup strata crop out across a large swathe of the southern Lake District, from Furness in the west to the Howgill Fells in the east ( Figure 24). Distinctive Ordovician successions form inliers in the Cautley and Dent areas, at the eastern margin of the Lake District outcrop, whilst inliers further east at Cross Fell comprise sequences ranging up from the Caradoc as high as the Wenlock. The onset of marine sedimentation was diachronous, with the oldest basal strata, of mid Caradoc age, seen in the north and east of the outcrop: calcareous mudstones of the Drygill Formation succeed the Eycott Volcanic Group in the north-east of the Lake District, whilst the lowest (volcaniclastic, sandy siltstone) beds of the Dufton Shale Formation in the Cross Fell inlier lie unconformably on volcanic rocks assigned to the Borrowdale Volcanic Group ( Figure 25). In contrast, in the south-west of the Lake District, the basal beds of the Windermere Supergroup, resting unconformably on both the Borrowdale Volcanic Group and (in Furness) the Skiddaw Group, are early Ashgill in age; they comprise the variously clastic or carbonate rocks of the Stile End and Kirkley Bank formations.

The upper Ordovician part of the Windermere Supergroup is a variable succession of mudstone, calcareous siltstone and impure limestone, sandstone and conglomerate. It is most complete and thickest (up to about 600 m) in the Cautley and Dent inliers of the eastern Lake District, but much thinner and interrupted by several non-sequences in the south-western part of the outcrop. In the latter area, it records the transition from shore-face through storm-dominated, mixed carbonate and clastic shelf to deeper shelf deposits. The deeper shelf environment was established earlier in the east allowing the more continuous deposition of the mudstone successions now seen in the Cross Fell inlier (Dufton Shale Formation) and the Cautley and Dent inliers (Cautley Mudstone Formation). Throughout the sequence there is a shallow-marine, shelly fauna that includes trilobites and brachiopods ( Plate 20). This characteristic, together with the widespread (though by no means dominant) carbonate lithofacies, has justified the traditional, informal name ‘Coniston Limestone’ for the south­western Lake District outcrop. Modern, formal stratigraphy designates the succession the Dent Group. Two episodes of silicic volcanism are recorded; one creating the ignimbritic Yarlside Volcanic Formation, and a subsequent, more widespread development of bedded tuffs high in the Cautley Mudstone Formation and at equivalent levels elsewhere ( Figure 25).

At the end of the Ordovician, a combination of global sea-level rise and regional subsidence allowed a deep water, graptolitic lithofacies to be established across the whole region. The mudstone and siltstone of the Stockdale Group span the final graptolite biozone of the Ashgill and the whole of the Llandovery, but are no more than 100 m thick. The Wenlock and earliest Ludlow sequence is characterised by distinctively laminated, hemipelagic, carbonaceous siltstones but includes intermittent turbidite sandstone units that presage the subsequent, major influx of sediment later in Ludlow times. These strata form the approximately 1000 m of the Tranearth Group and include, at the top of the Wenlock, two 15 m thicknesses of calcareous siltstone that establish a temporary shallowing of the depositional environment, probably in response to an eustatic sea-level change. The Stockdale Group extends across the whole of the Lake District outcrop and is also present in the Cross Fell inlier. The Tranearth Group is represented in Cross Fell only by its lowermost formation (Brathay), which is the highest unit of the Windermere Supergroup seen there.

A progressive increase in subsidence rate during the early Ludlow was caused by closure of the Iapetus Ocean and the resultant loading of the Avalonian continental margin by the overriding, leading edge of Laurentia (see discussion in Chapter 2). This increase, and the concurrent increase in the supply of coarse sediment, allowed a dramatic increase in sedimentation rate such that a large thickness, up to 2 km or more, of turbidite sandstone was deposited in the course of a single Gorstian graptolite biozone. The turbidite sandstones, and the intervening units of siltstone and hemipelagite, comprise the Coniston Group, the formalised equivalent of the traditional ‘Coniston Grits’. The ‘standard’ lithostratigraphy from the southern Lake District recognises three major sandstone-dominated formations separated by two siltstone formations, but there is some lateral variation in the relative positions of thick sandstone bodies within the siltstone background of the group, and so correlation at formation level is not wholly satisfactory. This is particularly true from the southern Lake District eastward to the Howgill Fells, where, in the latter area, the sandstone of the Screes Gill Formation may represent a diachronously old base to the Coniston Group, laterally equivalent to the top of the Tranearth Group further west. As an added complication, there appear to be more major alternations between sandstone-dominant and siltstone-dominant ‘formations’ in the Howgill Fells than can be discerned further west.

The later part of Ludlow times saw continued turbidite deposition, but from flows with low density and volume. Over 4000 m of banded siltstone and mudstone (Bannisdale Formation) accumulated with sporadic intercalations of sandstone; a sparse graptolite fauna is present, mostly in thin interbeds of hemipelagite. The Bannisdale Formation is the lowest unit of the Kendal Group, the higher parts of which are still unsatisfactory in stratigraphical terms. Above it, a coarser-grained and more calcareous lithology, containing a shelly fauna, has been described as the Underbarrow Formation but the putative boundaries are certainly diachronous and lateral correlation of its supposed outcrop is highly dubious. The succeeding Kirkby Moor Formation is more securely established. It is largely composed of fine-grained sandstone distinguished by features indicative of a relatively shallow-water depositional environment; the sedimentary basin was by this time clearly being filled. In some places, the sandstones have suffered secondary reddening, whilst in others a distinctive siltstone lithofacies may have been deposited in a tidal environment. Both of these features have been cited in definitions of a Scout Hill Formation, but are probably more appropriately regarded as local variations of the Kirkby Moor Formation. Whichever lithostratigraphical name is preferred for the top of the Kendal Group, its age ranges up into the Pridoli.

Dent Group Succession

The formal stratigraphical name acknowledges the thick and almost unbroken Caradoc to Ashgill sequence present in the Cautley and Dent inliers, in the south-east Lake District. In contrast, the successions further west are interrupted by non-sequences and show much local variation in lithofacies, the likely result of topographical interaction with the post-volcanic marine transgression. The Dent Group successions and their regional correlation are summarised in ( Figure 25). Note that additional Dent Group strata crop out further south in the Craven inliers; they are described in the British Regional Geology guide to the Pennines and adjacent areas.

The oldest component strata of the Dent Group in the Lake District are found in the north and east of the region. In an outlier faulted between the igneous rocks of the Carrock Fell Complex and the Eycott Volcanic Group are the shales of the Drygill Formation, calcareous ashy mudstone containing a Longvillian fauna of trilobites and brachiopods. Rocks of very similar age are also found further east, in the Cross Fell inlier, where they form the base of the Dufton Shale Formation, a more extensive succession consisting mostly of dark grey, calcareous mudstones with sporadic limestone nodules, that ranges up into the Ashgill and contains a varied shelly fauna. In places, the lowermost part of the Dufton Shales sequence, of early Longvillian age, is dominated by volcaniclastic siltstone and fine-grained sandstone known as the ‘Corona’ Beds. Above these beds, the Dufton Shale Formation continues up into the Ashgill, Cautleyan Stage, but with a reappearance of the sandy facies towards the top (Billy’s Beck Member), albeit with a more quartzose composition. In the Cross Fell inlier, the Dufton Shale Formation is overlain with slight unconformity by the decalcified but richly fossiliferous carbonate rocks of the Rawtheyan, Swindale Formation, with the succeeding shales of the Ashgill Formation extending into the Hirnantian. At this level in Cross Fell the localised Keisley Limestone is the best example of an Ordovician, calcareous mud-mound in England and contains a diverse shelly fauna rich in trilobites.

By Hirnantian time, Windermere Supergroup sedimentation had commenced across most of the southern Lake District, with basal strata commonly of Cautleyan age. A mixture of clastic and carbonate facies prevails, interrupted by non-sequences and erosional breaks. These combine to produce a laterally variable lithostratigraphy with an abundance of local names.

The most continuous sequence is seen in the Cautley and Dent inliers, where up to about 370 m of calcareous mudstone contains interbeds of nodular limestone from which has been recovered a benthic shelly fauna of trilobites, brachiopods, bryozoans and corals. This is the Cautley Mudstone Formation. The fauna is characteristic of the Cautleyan and Rawtheyan stages of the Ashgill Series, the stratotype of which is located in one of the inliers (Westerdale) near Cautley. The traditional correlation of these Ashgill stages has been with the anceps graptolite Biozone, but recent graptolite discoveries in the Cautley district suggest that correlation with the earlier, Caradocian, linearis Biozone might be more appropriate. The full implications of this have yet to be assessed but are likely to require wide-ranging stratigraphical revisions. At the local level it introduces considerable uncertainty into the ages and correlations of the Dent Group’s component units.

Across the southern Lake District outcrop the Dent Group has a highly variable basal clastic facies consisting of volcaniclastic sandstone and conglomerate. These deposits are discontinuous (and probably diachronous) and represent a variety of littoral and fluvial environments. They are associated together as the Longsleddale Member of the Stile End Formation, which is otherwise composed of calcareous marine siltstone with intermittent developments of nodular limestone. The Stile End Formation ranges up to more than 400 m in thickness and its distribution probably reflects topographical control on the Late Ordovician transgression.

Overlying the Stile End Formation with apparent conformity are up to 185 m of pale grey and pink, flow-folded rhyolitic rocks that were probably emplaced as a rheomorphic ignimbrite. In places, along both the upper and lower margins of the ignimbrite, are thin clastic accumulations composed mainly of ash, pumice and felsitic rock fragments. This Cautleyan volcanic unit has become widely and informally known as the ‘Stockdale Rhyolite’, but recent stratigraphical formalisation has re-established the original terminology and defined it as the Yarlside Volcanic Formation. It is broadly co-extensive at outcrop with the underlying Stile End Formation, suggesting that both were depositionally constrained by topography, with the rhyolitic ignimbrite filling a pre-existing valley that had already been partially filled with sediment deposited during marine transgression. Variation in the thickness of the ignimbrite can be related to residual topography within the confining valley. The top of the ignimbrite shows evidence of subaerial erosion and has been locally reworked into the base of the overlying Kirkley Bank Formation.

In the south-east of the Lake District, the calcareous siltstone, nodular limestone and fine-grained sandstone of the Kirkley Bank Formation overlie the erosion surface at the top of the Yarlside Volcanic Formation. Farther south-west, and in Furness, the Kirkley Bank Formation rests directly and unconformably on either the Borrowdale Volcanic Group or the Skiddaw Group, though it is uncertain whether basal volcaniclastic units (e.g. the Low Scales Member) should be regarded as parts of the Kirkley Bank Formation or more properly assigned to the Longsleddale Member of the Stile End Formation.

Several local members are defined for the Kirkley Bank Formation, based on the local dominance of the constituent lithologies that combine to a thickness of between 5 and 140 m. In general, a high proportion of concretionary limestone, sporadically crystalline, occurs near the base of the formation ( Plate 21) and is succeeded by graded units of fine sandstone to siltstone with basal accumulations of shelly detritus, extensive parallel-or cross-lamination, and locally pervasive bioturbation. The relationship with the overlying, nodular and bioturbated micritic limestone of the Broughton Moor Formation (up to 6 m thick, where present) is unclear. There is no biostratigraphical evidence for the upper part of the Cautleyan Stage, and instead the fossil evidence indicates a mid Rawtheyan age for the Broughton Moor Formation. This would suggest, at least, a stratigraphical hiatus, but in places there is evidence for continuous deposition from Kirkley Bank to Broughton Moor, and it is possible that the latter formation should more correctly be regarded as a condensed sequence. A possibly equivalent limestone unit faulted against the Dufton Shales in the Cross Fell inlier (the Swindale Formation) is richly fossiliferous and spans the Cautleyan–Rawtheyan stage boundary. In the Furness area, south­west of the Lake District, an assemblage of pebbly mudstone and matrix-supported conglomerate with limestone clasts, the Lumholme Member, rests conformably on Broughton Moor Formation strata. It was deposited as a debris flow and contains reworked mid Rawtheyan conodonts, but has also been assigned to the stratigraphically higher Ashgill Formation.

Closely above the Broughton Moor Formation limestone in biostratigraphical terms is a thin (up to 6 m) volcaniclastic unit. It consists of sandy, pyroclastic material resedimented by gravity flows and is suggestive of contemporaneous volcanic activity. It is variously known as the Appletreeworth Formation (south-west Lake District) or the High Haume Volcanic Formation (Furness) and in these guises it ranges up to about 6 m in thickness. It is not present in the south­east Lake District but reappears further east in the Cautley and Dent inliers. There, as the Cautley Volcanic Member, it comprises up to 25 m of rhyolitic tuff interbedded towards the top of the Cautley Mudstone Formation where a mid Rawtheyan age can be firmly established.

At the top of the Dent Group, spanning the Rawtheyan to Hirnantian stage boundary and present across the full breadth of the outcrop from Furness to Cross Fell, is a pervasively bioturbated, laminated mudstone/siltstone unit, the Ashgill Formation, commonly with a more calcareous and fossiliferous basal facies. Across the southern Lake District, the latter appears as a calcareous siltstone with some nodular limestone interbeds, the Troutbeck Member, but in the Cautley and Dent inliers it is an argillaceous limestone, the Cystoid Limestone Member. A unit of sandstone and pebble conglomerate towards the top of the formation has been identified variously as the Rebecca or Wharfe Member, and may be an equivalent of the stratigraphically ambiguous Lumholme Member mentioned above in the context of Broughton Moor Formation. The Ashgill Formation ranges up to 25 m thick in the southern Lake District, but attains 60 m at Cautley and Dent; the basal calcareous member usually represents between 1 and 3 m of the total whilst the Rebecca and Wharfe members rarely exceed 4 m in thickness. The Furness area appears anomalous in this respect, with the Rebecca Member alone apparently attaining a thickness of 50 m.

Stockdale Group succession

Above the Dent Group there is a marked lithological and faunal change with the abrupt appearance of black, graptolitic shale. This is the characteristic feature of the Stockdale Group which, whilst only attaining a maximum thickness of about 120 m, stratigraphically spans the whole of the Llandovery Epoch ( Figure 23) and ( Figure 26). There are two component formations, a lower Skellgill Formation overlain by the Browgill Formation. These can be recognised throughout the entire breadth of the outcrop, from Furness to Cross Fell, despite the thinness of the succession.

The Skelgill Formation is a condensed sequence consisting mostly of graptolitic black mudstone but with subordinate thin beds of calcareous siltstone. It is never more than 40 m thick but still spans 10 or more graptolite biozones, representing the very top of the Ashgill Hirnantian Stage, and the entire Rhuddanian and Aeronian stages of the Llandovery. The basal mudstone beds, of Hirnantian age, are commonly pyritous but a little paler in colour than the rest of the formation, and widely include thin developments of nodular limestone. This basal sequence is separated as the Spengill Member and contains a mixed, graptolitic and shelly fauna. Thin interbeds of calcareous siltstone increase in abundance towards the top of the formation and in places contain a sparse and dwarfed shelly fauna, but black graptolitic mudstone dominates throughout. Sporadic thin layers of bentonitic claystone attest to contemporaneous falls of volcanic ash.

The Browgill Formation consists largely of pale green, oxic siltstones with only a few thin interbeds of black graptolitic mudstone and intermittent horizons of pale grey, bentonitic claystone derived from volcanic ash. It ranges up to about 90 m in thickness and occupies the whole of the late Llandovery, Telychian Stage. Towards the top of the formation a number of local variations occur. Red-brown mudstones are widely seen right across the Lake District and Cross Fell outcrops, and some are calcareous and contain a sparse, shelly marine fauna. They are best developed in the Howgill Fells where up to 20 m are present and contain thin interbeds of black graptolitic mudstone. There, the red mudstones have been designated the Hebblethwaite Member. Above this member in the Howgill Fells, and more generally at the top of the Browgill Formation whether or not there are red beds present, is about 8 m of pale grey mudstone devoid of any black, graptolitic interbeds. Across the Lake District this unit has been recognised as the Far House Member, but it does not appear to be present at Cross Fell where the topmost beds of the Browgill Formation are red mudstones with black graptolitic laminae.

Tranearth Group succession

Above the Stockdale Group lie several hundred metres of strata dominated by a distinctively laminated, hemipelagic silty mudstone. Two formations composed almost entirely of this lithology mark the base and top of the group (Brathay and Wray Castle formations respectively) and are separated by a development of calcareous siltstone (Coldwell Formation) widely (but not invariably) preceded by an underlying sand turbidite unit (Birk Riggs Formation). The varve-like lamination of the dominant hemipelagite is interpreted as the result of periodical (possibly annual) fluctuations in clastic input interrupting a background accumulation of organic material. The hemipelagite is almost exclusively anaerobic, with a pelagic fauna of graptolites and orthocones.

The Brathay Formation, at the base of the Tranearth Group, was deposited mostly during the early Wenlock (Sheinwoodian). It consists almost entirely of laminated hemipelagite, with a few silt and mud turbidite beds becoming more abundant near the top of the formation ( Plate 22). This trend is particularly apparent in the west of the Lake District outcrop where the Brathay Formation is overlain by the sandy turbidites of the Birk Riggs Formation. It is less pronounced in the Howgill Fells where the latter formation is missing and instead the Brathay Formation extends upwards to the base of the Coldwell Formation. The characteristic hemipelagite lamination is defined by alternating layers rich either in quartz silt or in organic carbon. The silt–carbonaceous couplets have a spacing of about 2 mm, but this compacted spacing may double where the lamination passes through one of the fairly widespread, diagenetic carbonate nodules that become more common and larger up-sequence. Rare bentonitic claystone layers, a few mm thick and derived from volcanic ash, occur throughout. At the base of the formation there is a 10–30 m interval in which the distinctive hemipelagite is interbedded with pale grey, bioturbated siltstone. This transition from the underlying Far House Member (Browgill Formation) has been defined as the Dixon Ground Member and appears consistently across the southern Lake District and in the Howgill Fells. The distinctive hemipelagite lithology of the Brathay Formation is also seen in the Cross Fell inlier but there the biostratigraphical evidence proves only the middle part; neither the base nor the top of the formation are seen. This isolated section of the Brathay Formation is the highest part of the Windermere Supergroup preserved at Cross Fell.

The sandy turbidite beds of the Birk Riggs Formation, up to about 380 m thick, were deposited during the lundgreni graptolite Biozone (Homerian, late Wenlock). They are most thickly developed to the west of Ambleside ( Figure 26) but thin westwards and are not seen across the south-west Lake District, before reappearing in Furness. From its maximum development, the formation also becomes thinner eastwards until, in the Howgill Fells, the Brathay Formation extends upwards through the same biostratigraphical interval to the base of the Coldwell Formation. Throughout the Birk Riggs Formation, sand-rich sections alternate with intervals of laminated hemipelagite in which there may be only a trivial proportion of thin sandy beds. The sandstone beds show the range of features characteristic of deposition from turbidity currents: flute and groove casts on their bases, graded bedding, and an upward progression to parallel- and cross-laminated, silty tops. In general, the highest of the turbidite sandstone beds is separated from the base of the overlying Coldwell Formation by about 25 m of laminated hemipelagite.

An abrupt change of lithofacies is represented by the Coldwell Formation, which consists of two units of intensely bioturbated, calcareous siltstone separated by laminated hemipelagite. The two siltstone units are afforded member status and consist largely of graded units with a sporadic basal coquina of reworked shelly material: the Randy Pike Member lies at the base of the formation, the High Cross Member at the top. The total thickness of the formation is in the 50 to 80 m range in the southern Lake District where a significant proportion of the formation, up to about 50 m, is contributed by the central zone of hemipelagite. Thence the formation thins eastwards towards the Shap area, where it is not present, before reappearing with a 10 m thickness in the Howgill Fells. Some additional lithological variation is apparent within the thin, Howgill Fells succession where slumped mudstones, and limestone clasts contained in a hemipelagite matrix, suggest mass-flow deposition. There are also indications from that area of a lateral transition from calcareous to non-calcareous but bioturbated mudstone. Throughout the outcrop, the calcareous siltstone components of the formation contain a shelly fauna, notable particularly for the trilobites, whilst graptolites are widespread in the hemipelagite interval. The biostratigraphical range that these fossils define spans the boundary between the late Wenlock, Homerian Stage and the early Ludlow, Gorstian Stage. The facies and oxicity changes marked by the Coldwell Formation and its fauna probably reflect a eustatic fall in sea level ( Figure 27).

The topmost formation of the Tranearth Group, the Wray Castle Formation, marks a return to laminated hemipelagite with only minor intercalations of mud or silt turbidite. The lithology is very similar to that seen in the Brathay Formation, but the Wray Castle Formation has a higher proportion of silt and slightly larger lamination spacing than its older counterpart. Rare bentonitic claystone layers, derived from volcanic ash and usually only a few mm thick, occur throughout. Deposition of the Wray Castle Formation was entirely within the earliest Ludlow, nilssoni graptolite Biozone but nonetheless, between 280 and 350 m of strata are preserved in the southern Lake District. The formation thins eastwards so that in parts of the Howgill Fells only a few metres of laminated hemipelagite separate the top of the Coldwell Formation from the thick beds of turbiditic sandstone at the base of the Coniston Group.

Coniston Group succession

The base of the Coniston Group is marked by the first substantial appearance of thick, turbidite sandstone sequences in the Windermere Supergroup succession. The group’s thickness is variable within the 1.5 to 2.5 km range, and since deposition was mostly within a single Gorstian graptolite biozone (scanicus, Early Ludlow), it represents a dramatic increase in sedimentation rate over that responsible for the lower parts of the Windermere Supergroup. Within the southern Lake District, there are three major sandstone-dominated sequences separated by two intervals in which the main lithology is hemipelagic siltstone. These five units have been afforded formation status but further east, in the southern Shap Fells and the Howgill Fells, the lower formations thin, pinch out and merge with the underlying siltstones of the Wray Castle Formation ( Figure 26). As an additional complication, the turbidite sandstones marking the base of the Coniston Group in the Howgill Fells, probably extend to a lower stratigraphical level than do their counterparts further west. Hence the base of the Coniston Group is probably diachronous.

The five-part Coniston Group stratigraphy is most fully developed in the south-west of the southern Lake District outcrop. There the basal sandstone unit, the Gawthwaite Formation, ranges up to about 500 m thick but contains a fairly high proportion of silty, laminated hemipelagite. The individual, graded sandstone beds are rarely more than 50 cm thick and tend to occur as repetitive series of beds cumulatively up to about 15 m thick, alternating with rather thinner runs of hemipelagite in which there are only sporadic sandstone interbeds. Apart from the grading, other sedimentary features indicative of turbidite deposition include flute and groove casts on the bed bases ( Plate 23), and an upward progression within the beds to parallel, cross and convolute lamination. The base of the formation (and hence of the Coniston Group) lies within the earliest Gorstian, nilssoni graptolite Biozone, but the top of the formation is probably within the succeeding scanicus Biozone.

The Gawthwaite Formation is succeeded by 100–250 m of silty hemipelagite with only sporadic interbeds of turbidite sandstone. This is the Latrigg Formation, which is in turn succeeded by another major development of turbidite sandstone, the Poolscar Formation, with a 450–700 m thickness range. The Poolscar Formation has the highest proportion of turbidite sandstone beds (relative to hemipelagite) of any part of the Coniston Group. Some individual beds are very thick, up to several metres, and amalgamation of the sandstone beds is widespread. Another unit of mainly hemipelagite lies above the Poolscar Formation; this is the approximately 100 m thick Moorhowe Formation. Finally, the top of the Coniston Group is formed by the third thick development of turbidite sandstones, the Yewbank Formation. This is 700–750 m thick and appears to be intermediate relative to the two lower sandstone formations in terms of bed thickness and hemipelagite proportion. Like them, the Yewbank Formation shows the characteristic range of sedimentary features produced by turbidity current deposition. All of the higher part of the Coniston Group lies within the scanicus graptolite Biozone.

There are significant lateral changes in the Coniston Group lithostratigraphy such that eastwards, in the Shap Fells area, it is hard to apply the formational terminology established further west ( Figure 26). At the base of the group, the Gawthwaite Formation thins eastwards and finally fails, thus eliminating the means of distinguishing between the lithologically similar, hemipelagite formations above and below, respectively the Latrigg and Wray Castle formations. Higher in the succession, a similar problem is caused by the eastwards thinning and disappearance of the hemipelagic Moorhowe Formation, which accordingly allows the sandstone formations above and below to merge. Hence, in the Shap Fells, the Coniston Group is represented by an upper interval largely of turbiditic sandstone, equivalent to the combined Yewbank and Poolscar formations, and a lower interval of largely hemipelagite equivalent to the Latrigg Formation but extending downwards into the, now indistinguishable, Wray Castle Formation. In effect, the boundary between the Tranearth and Coniston groups has become arbitrary. Some of this lateral variation in lithofacies and stratigraphy may have been influenced by synsedimentary faulting.

Further changes are apparent eastwards into the Howgill Fells. There, a thick unit of turbidite sandstone, the Screes Gill Formation, again defines the base of the Coniston Group. It is typically about 250 m thick but thins southwards. It consists mostly of fine-grained sandstone in beds of variable thickness (but most commonly about 30 cm) that grade upwards to a silty mudstone top. Laminated hemipelagic mudstone is interbedded irregularly throughout the formation and contains a fauna indicative of the nilssoni graptolite Biozone. However, since the underlying Wray Castle Formation is here only about 10 m thick, it seems possible that the base of the Coniston Group is older than is the case further west, where a much thicker Wray Castle succession is seen. It is not clear whether or not the Screes Gill Formation can be correlated in any way with the Gawthwaite Formation, or whether they represent discrete, though possibly overlapping, turbidite fans.

The upper part of the Coniston Group in the Howgill Fells comprises an alternation of sandstone and hemipelagite intervals. The lithologies present are very similar to those seen elsewhere in the group and together make up a thickness of about 1000 m, rather less in the south of the area. A succession has been recognised in which four mainly sandstone units are separated by three much thinner units of laminated silty mudstone. Other divisions can be defined locally but prove to be lenticular on a regional scale. Stratigraphically, this succession is equivalent to the highest four formations further west (from Latrigg up to Yewbank) but precise correlations are not possible.

Kendal Group succession

The lithostratigraphical succession above the Coniston Group is only informally associated within a Kendal Group. It is dominated by the very thick succession (up to 4.5 km) of fine-grained sandstone, banded siltstone, silty mudstone and rare hemipelagite that makes up the Bannisdale Formation. The banding is the result of grading from siltstone (or less commonly from fine sandstone) to mudstone upwards through very thin turbidite beds no more than a few centimetres thick ( Plate 24). The siltstones and fine sandstones are commonly affected by convolute lamination, caused by small-scale dewatering, and this feature, together with ripple cross-lamination, appears to become more abundant towards the top of the formation. The mudstones may contain small calcareous or phosphatic nodules. Interbedded turbidite sandstones are locally common towards the base of the formation and in places, particularly in the south-west of the Lake District outcrop, create a transitional boundary with the underlying Coniston Group; the proportion of sandstone decreases upwards. Localised sandstone members have been previously defined in this transitional interval, but their wider application has proved difficult. The hemipelagite component of the formation represents the continuing background sedimentation and contains graptolites, though the biostratigraphical ranges that they indicate suggest some diachroneity. The age of the formation’s base has been variously described as lying between the Gorstian scanicus Biozone and the base of the Ludfordian leintwardinensis Biozone; there is a consensus that the top of the formation ranges up into the leintwardinensis Biozone. There is a noticeable increase in bioturbation in the highest beds.

Lithostratigraphy is poorly defined in the upper part of the Kendal Group, above the Bannisdale Formation. In particular, the previous recognition of an Underbarrow Formation, intervening between the Bannisdale Formation and the overlying Kirkby Moor Formation, is unsatisfactory since it brings together a variety of disparate, localised facies variations. These probably represent stages in the depositional transition from the Bannisdale to the Kirkby Moor formations, and are best regarded as local members of one or other of those two units.

The most securely defined unit is the Kirkby Moor Formation, which comprises several hundred metres (at least) of thickly bedded (up to about 1 m) fine-grained sandstone, the beds having little internal structure apart from convolute lamination produced during dewatering. There are sporadic channelised bedforms and in a few cases a shelly lag deposit is seen at the base of a bed. There is a little more variation in the lowest 100 m, where the formation is more thinly bedded and finer grained than average. In this part of the formation, thin beds of siltstone and fine-grained sandstone are parallel- and cross-laminated, with sporadic developments of larger scale, hummocky cross-stratification. A shelly fauna suggests an age equivalent to the younger part of the Ludfordian leintwardinensis graptolite Biozone, with the possibility that the top of the formation ranges up into the Pridoli.

The highest beds of the Windermere Supergroup have previously been termed the Scout Hill Formation, but the definition has been principally on faunal grounds. However, at this Pridoli level the faunas are strongly facies-controlled and parts of the Scout Hill unit are lithologically indistinguishable from the Kirkby Moor Formation, apart from a widespread secondary reddening. One distinctive member of thinly bedded siltstone (the Helm Member) might usefully be preserved as a component of the Kirkby Moor Formation, but the remainder of the Scout Hill Formation is best subsumed therein. The result is a Kirkby Moor Formation that extends well up into the Pridoli, and is affected by secondary reddening in its higher part. The Helm Member probably occupies a high stratigraphical level within the Kirkby Moor Formation as thus defined.

Depositional environments

The Ordovician, Caradoc to Ashgill, Dent Group records a marine trangression across the remains of the Borrowdale Volcanic Group arc and its Skiddaw Group basement. Transgression may have been initiated by postvolcanic, thermal subsidence, although the intrusion of the major subvolcanic plutons at about 450 Ma, is more or less synchronous with the onset of Caradoc sedimentation in the north and east of the Lake District region. The ignimbritic Yarlside Volcanic Formation confirms that some local volcanicity continued into the Ashgill.

The transgression was clearly intermittent and topographically controlled, with the assemblage of carbonate and locally derived clastic lithofacies consistent with deposition on a shallow marine shelf. In the southern Lake District outcrop, four depositional cycles were separated by periods either of emergence and erosion or of nondeposition. Further east, in the Howgill Fells and at Cross Fell, the first three of these cycles are not developed and the equivalent stratigraphical interval shows continuous deposition in an oxygenated marine environment (respectively the Cautley Mudstone and Dufton Shale formations). The late Ashgill volcaniclastic mass-flow deposits (Appletreeworth Formation, Cautley Volcanic Member etc.) are apparently marine, but may well have been followed by nondeposition and erosion in a relatively shallow-water setting prior to the Ashgill Formation transgression. Thereafter, a fall in sea level during the Hirnantian glaciation ( Figure 27) may have been the trigger for emplacement of the Ashgill Formation’s mass-flow, pebbly mudstone member.

The postglacial rise in global sea level, coupled with regional subsidence of the Avalonian continental margin (perhaps the initial, flexural response to collision with Laurentia subsequent upon the closure of the Iapetus Ocean — this large-scale tectonic concept was introduced and described in Chapter 1), created the marine environment for Llandovery deposition of the Stockdale Group. Fine-grained sediment was deposited either as hemipelagic fall-out or from small, low-density turbidity flows. The dominance of black, carbonaceous and pyritous mudstone devoid of bioturbation in the Skelgill Formation indicates anaerobic depositional conditions. The prevalence of these anoxic conditions in the early Llandovery can be attributed to physical and biogeochemical changes in the oceans during the deglaciation that followed the Hirnantian ice age.

The lithological transition into the Browgill Formation records an increase in availability of fine-grained clastic detritus, introduced by low-density turbidity flows. There was also an increase in the volcanic ash contribution to the sediment budget, though this does not necessarily mean that the source volcanoes were local. Background conditions continued to be anaerobic until the later stages of Browgill Formation deposition. Then, in the late Llandovery, oxygenated environments became established, as required by the haematite-rich red beds of the Hebblethwaite Member and the absence of black mudstone amidst the grey shales of the Far House Member. There may have been an additional topographical control on the distribution of these oxygenated enclaves.

Anaerobic conditions were re-established early in the Wenlock allowing deposition of the distinctive, laminated mudstone and siltstone that comprises the Brathay Formation. The lamination probably arose from a periodic (and possibly annual) fluctuation in the supply of silt and organic material, with the former being introduced either by hemipelagic fall-out or by low-density turbidity flows. A more substantial input of turbidite sand during the later part of the Wenlock Period created the Birk Riggs Formation. A range of sedimentary features therein, including channels, flute and groove casts on bed bases, and cross-lamination in the upper parts of beds, provides evidence of palaeocurrent orientation and hence of basin geometry. Though some of the evidence is contradictory, the Birk Riggs Formation can be most simply interpreted as a series of small, overlapping turbidite fans dispersing sediment towards the south-west along the axis of a depositional trough.

A late Wenlock, eustatic fall in sea level temporarily restored aerobic conditions; deposition of the calcareous Coldwell Formation resulted ( Figure 27). A varied, benthic fauna became established and included trilobites, brachiopods and bioturbating organisms. The graded, silty beds were probably deposited from waning and possibly storm-generated sediment flows into relatively deep water. Thereafter, towards the end of the Wenlock Period, sea-level rise and accelerating basin subsidence combined to re-establish deeper water, anaerobic conditions with a return to Brathay-style sedimentation; the Wray Castle Formation was the result.

The rate of basin subsidence increased abruptly into the early Ludlow as loading of the Avalonian continental margin by the over-riding, leading edge of Laurentia gathered momentum. Turbidity flows poured into the deepening basin and thick sequences of graded sand and silt built up as the Coniston Group. Its sandstone-dominated formations record intervals of turbidite fan growth; the intervening siltstone-dominated formations record intervals of less vigorous sedimentation that might have been induced by temporary rises in sea-level. The turbidite beds contain a characteristic assemblage of sedimentary features, and analyses of those that provide an indication of palaeocurrent direction show that the turbidite fans were mostly supplied from the north-east and built-out towards the south-west. This pattern is fairly well maintained across the southern Lake District outcrop but appears to be more complex in the Howgill Fells. The composition of the sandstones (and of a derived acritarch microflora) suggests that their constituent sediment was eroded from the Scandian Orogen, then developing at the margin of Baltica. The dominance of turbidity current flow directed towards the south­west, along the axis of the depositional trough, would be compatible with such a source.

The relatively mature ‘recycled orogen’ compositional character of the Coniston Group continues upwards into the dominantly silty Bannisdale Formation, and a similar, south-west directed palaeoflow can also be deduced. The turbidity currents depositing the Bannisdale Formation siltstone were of lower density than those depositing the Coniston Group’s sandy beds, but a very thick sequence resulted nevertheless. Remarkably, approximately 4 km of strata built up in the duration of a single graptolite biozone, which in the late Silurian was probably equivalent to a maximum of 1 million years; a sedimentation rate of at least 4 mm per year. An anaerobic environment had persisted from Coniston Group times, but towards the top of the Bannisdale Formation an increase in bioturbation marks a change to more oxygenated sea-floor conditions. At the same time, subtle compositional changes imply a shift in sediment source.

Both of these trends are more fully developed in the Pridoli Kirkby Moor Formation. The calcareous and bioturbated members (hitherto the Underbarrow Formation) and the locally rich benthic fauna attest to deposition in an oxygenated environment. At the same time, the appearance of sedimentological evidence for storm-influenced deposition suggests a dramatic shallowing of the basin ( Figure 27). In the uppermost part of the formation this tendency culminates in red beds and siltstones that may have been deposited under tidal conditions. It is clear that by Pridoli times the sedimentation rate exceeded the basin subsidence rate and the Windermere Supergroup sedimentary basin was filled. In parallel with this, sediment composition and palaeocurrent distribution patterns suggest that the Scottish Southern Uplands terrane had become the source of the deposited sediment. The long-standing tectono-sedimentary influence of the Iapetus Ocean had ended and the scene was set for the terrestrial Old Red Sandstone.

Southern Uplands and Isle of Man

Some lithostratigraphical components of the Windermere Supergroup crop out well to the south of the Lake District in the Craven inliers and are described in the British Regional Geology Guide to the Pennines and adjacent areas. An extensive sequence is present there, ranging upwards from equivalents of the Dent Group into the lower part of the Coniston Group. Although not formally part of the Windermere Supergroup, regionally associated strata also crop out in northern England within the southernmost inliers of the Scottish Southern Uplands terrane and in a recently discovered thrust slice on the Isle of Man.

The Scottish Southern Uplands terrane developed at the northern, Laurentian margin of the Iapetus Ocean as an accretionary thrust belt. Its construction spanned the Caradoc to Wenlock interval, driven by the subduction of Iapetus Ocean crust beneath Laurentia. Sandstone turbidites with intervening mudstones are the dominant lithologies, and the youngest such strata still preserved, containing a graptolite fauna indicative of the lundgreni Biozone, crop out along the southern margin of the terrane. That southern margin extends south of the Anglo–Scottish border in an inlier of Wenlock turbidites on the south-west flank of The Cheviot. The rocks present are associated with the Raeberry Castle Formation of the Riccarton Group and contain an acritarch microflora in addition to the lundgreni Biozone graptolites. A noteworthy feature of the Riccarton Group is the presence within it of laminated hemipelagic interbeds identical to those seen at the same stratigraphical level in the Brathay Formation. Clearly, by late in the Wenlock, the Iapetus Ocean had narrowed sufficiently to allow development of a common background lithofacies right across its remaining width.

The recent discovery of another lundgreni Biozone fauna, on the north-west coast of the Isle of Man close to Peel ( Figure 12), provoked a radical re-interpretation of the local geology since the fauna was contained in a unit whose supposed correlation with Arenig strata elsewhere on the island had underpinned a previously proposed, large-scale structural model. The Wenlock rocks, designated the Niarbyl Formation (Dalby Group), comprise over 1000 m of turbiditic sandstone and siltstone, laminated hemipelagite and rare bentonite. The similarities to the Brathay–Birk Riggs division of the Tranaerth Group are striking. The base of the Niarbyl Formation is cut out by a major thrust and shear zone that has emplaced it structurally above the Manx Group.

Chapter 5 Devonian: Acadian deformation and magmatism

Convergence of Avalonia and Laurentia

In northern England the last traces of definitively local, subduction-related volcanism are seen in the Ashgill Dent Group. The bentonite layers that occur sporadically through the Silurian groups of the Windermere Supergroup, though demonstrating intermittent ash fall, could have been derived from a distant volcanic source and do not confirm continued southwards subduction of Iapetus Ocean crust beneath Avalonia. This is believed to have ceased by the end of the Ordovician. Instead, continued subduction at the northern, Laurentian margin of the Iapetus Ocean effected continental convergence by the late Llandovery or Wenlock. The initial stages of collision do not seem to have been a dramatic, mountain-building affair, and the first indication of continental proximity was the establishment of a relatively tranquil depositional setting right across the suture zone; the laminated hemipelagites of the Brathay Formation and its coeval equivalents in Scotland and the Isle of Man, in the Riccarton and Dalby groups respectively, were the result. The collision process has been widely described as ‘soft’.

Notwithstanding the ‘softness’ of the initial collision, the accretionary complex that had developed at the leading edge of Laurentia (the Southern Uplands terrane) initially overrode and depressed the north-western margin of Avalonia, resulting in the dramatic increase in subsidence and sedimentation rates recorded in the Ludlow part of the Windermere Supergroup by the Coniston Group and Bannisdale Formation ( Figure 28). Flexure of the Avalonian footwall in response to loading may have been instrumental in the widespread resetting of the Rb-Sr isotopic systems in all Ordovician igneous rocks to a broadly Ludlow age range — about 420–430 Ma, ( Figure 29); the U-Pb and Sm-Nd systems were unaffected. Under these circumstances the resetting would have accompanied a hydration of the rocks that arose from increased permeability through fracturing.

The foreland basin phase of the Windermere Supergroup did not progress; instead the basin filled during the late Ludlow and Pridoli, from about 420 Ma, culminating in deposition of the shallow water lithofacies seen in the Kirkby Moor Formation. The convergence of Laurentia and Avalonia had probably ceased by then and, from the late Silurian onwards, their relative movement became dominated by sinistral strike-slip, which had been intermittently affecting development of the Southern Uplands accretionary complex since late Llandovery time. By about 410 Ma, the Early Devonian, the regional tectonic regime had become sinistrally transtensional, allowing emplacement of a swarm of lamprophyre dykes (detailed below) right across the sutured tracts; dyke intrusion was probably focused above a deep-seated crustal shear zone. Transtensional sedimentary basins also formed during this phase and within them accumulated thick successions of fluvial, Old Red Sandstone lithofacies. Traces of these cover rocks are preserved in the Southern Uplands, and the underlying Lower Palaeozoic strata were probably the source of the Old Red Sandstone sedimentary sheets that spread southwards across the Iapetus Suture into northern England. Few physical traces now remain of this sedimentary cover. The Peel Sandstone of the Isle of Man (described below) may be a remnant, but its former widespread presence provided the necessary cover beneath which Acadian deformation and metamorphism occurred. For example, across the southern Lake District, an original cover at least 3.5 km thick is required above the highest preserved, Pridoli beds of the Windermere Supergroup to achieve the grade of Acadian metamorphism seen therein.

The Acadian Orogeny caused the first regionally significant, penetrative deformation of the Lower Palaeozoic rocks on the Avalonian margin, affecting the Pridoli strata of the Windermere Supergroup as intensely as the Tremadoc strata of the Skiddaw Group. This deformation phase was therefore not related to closure of the Iapetus Ocean, but was instead driven by a subsequent convergence regime that followed the period of Early Devonian transtension. As discussed in Chapter 1, this was most probably linked to the closure of the mid European Rheic Ocean further to the south.

Acadian Orogeny

Stratigraphically, the Acadian Orogeny is constrained as post-Pridoli, pre-Late Devonian. Cleavage development overlapped the generation of metamorphic aureoles around the Devonian granites at Skiddaw and Shap, and these have been dated radiometrically at about 399 and 404 Ma respectively. Further south, in the Ribblesdale (Craven) inlier, metamorphic white mica developed along the cleavage in a Ludlow bentonite has given radiometric ages in the 397–418 Ma range. All the evidence points to the Acadian Orogeny being an Early to Mid Devonian event, most probably of Emsian to Eifelian age.

The Acadian tectonic deformation was superimposed on pre-existing, Late Ordovician and Silurian disruption of the strata in both the Skiddaw and Manx groups. Not surprisingly, the structures of both groups are highly complex, and their apparently polyphase character led initially to proposals for a protracted, polyphase deformation history. The recognition of the earliest deformations as soft-sediment, slump-related phenomena, and definitive confirmation that the first, regional tectonic cleavage was indeed Acadian, have largely resolved the regional interpretational controversies. The possibility of a diachronous cleavage being developed in a southward prograding thrust belt, in continuity with the Southern Uplands accretionary complex and linked to the Windermere Supergroup foreland basin, cannot be sustained. However, at a more local scale, the detailed structures of the Manx and Skiddaw groups still remain open to different interpretations.

The Borrowdale and Eycott Volcanic groups were affected by synvolcanic faulting associated with piecemeal caldera collapse. The most important Acadian structures superimposed on the previously block-faulted volcanic tracts are regional monoclines facing north in the Eycott Volcanic Group and south in the Borrowdale Volcanic Group. These structures define the regional Lake District ‘anticline’ of which the Skiddaw Group forms the core, and are asymmetrically disposed above the margins of the subvolcanic batholith ( Figure 30). The northern ‘monocline’, affecting the Eycott Volcanic Group may well have been initiated much earlier, with geophysical evidence and age relationships suggesting that at least some of the volcanic rocks were rotated to a near-vertical attitude soon after their eruption. In this case, Acadian effects may simply have been to modify the existing structure. A better-defined and more extensive Acadian structure is the southern, Westmorland Monocline; it affects the southern margin of the Borrowdale Volcanic Group outcrop and the overlying Windermere Supergroup strata, and lies above the southern margin of the batholith. Cleavage is strongly developed in the volcanic rocks close to the monocline hinge zone, forming the Tilberthwaite slate belt in which the ‘green slate’ industry is concentrated (see Chapter 12). Another zone of enhanced cleavage, the Honister slate belt, lies further north and may coincide with a step, downwards to the north, in the top surface of the batholith. Overall, the style of deformation is strongly influenced by the disposition of the more rigid rocks in the volcanic succession and by the protective effects of the underpinning batholith.

Across the southern Lake District the steep limb of the Westmorland Monocline forms the northern limb of the Bannisdale Syncline, the large-scale, regional structure that is the primary influence on disposition of Windermere Supergroup strata ( Figure 30). A single slaty cleavage is ubiquitous and congruous to the folding, though the relationship is commonly transecting (albeit by only a small amount) rather than axial planar.

Skiddaw Group structure

The dominant structural feature is a series of polyphase fault-and-thrust zones that trend approximately east-north-east across the main Skiddaw Group outcrop, roughly parallel to the regional strike of bedding and cleavage. Of these, the Causey Pike Fault has the longest demonstrable history and a geographical extent that spans the main Lake District inlier and extends as far as the Cross Fell inlier 30 km to the east. It has a profound stratigraphical effect (equivalent to at least 2 km vertical downthrow to the south), separating discrete parts of the Skiddaw Group succession with marked differences in sedimentary provenance and slump-fold orientation; hence it may well have had a synsedimentary role, partitioning the original depositional basin. Some sinistral strike-slip movement seems likely during the Early Devonian transpressive phase, whilst south-directed thrust movement cutting across the approximately 400 Ma Crummock Water thermal aureole is likely to be an Acadian (sensu stricto) effect.

Farther north, the Gasgale, Loweswater and Watch Hill thrust faults were all most probably initiated as the slide planes of large-scale, synsedimentary slumps. Thereafter, subsequent reactivation during Acadian deformation has created a situation such that, in the hanging walls, minor, upright folds have an axial-plane cleavage that progressively swings into an alignment parallel with the thrust plane, but is then crenulated by a uniformly shallow-dipping cleavage. The Watch Hill thrust is a particularly complex structure and is most probably a compound plexus of faults having different attitudes, movement senses, and ages.

Acadian deformation of the Skiddaw Group occurred against this regional background of pre-existing stratigraphical disruption. The main Acadian cleavage forms a regional arc in the Skiddaw Group, with a west to east variation in trend between north-east–south-west and east–west. It is a variably developed, pressure-solution fabric that is almost slaty in some places but is absent in others. Where a bedding-parallel, sedimentary compaction fabric is well developed, this regional Acadian cleavage may have the appearance of a crenulation fabric. The main cleavage is axial planar to gently plunging, steeply inclined, open to isoclinal folds with amplitudes of hundreds of metres. To the north of the Causey Pike Fault the cleavage mostly dips to the north with associated folds overturned towards the south; the folds are commonly hanging-wall anticlines to south-directed reverse faults. To the south of the Causey Pike Fault (though as far north as Carrock Fell in the east of the outcrop and Mellbreak [NY 144 195] in the west) folds associated with the main cleavage are less common and, where present, are upright or overturned towards the north. In this southern zone, the dip of the main cleavage is more variable, to both north and south. Further south, for example in the Bampton inlier, the Acadian cleavage is only weakly developed and mostly dips steeply to the north; in this inlier the dominant ‘cleavage’ is a strong bedding-parallel fissility.

In the Skiddaw Group of the northern Lake District, the regional Acadian cleavage is commonly crenulated by fabrics that are axial planar to open, gently plunging minor folds with gently inclined axial planes. These may have been generated by vertical shortening in the tectonically thickened sedimentary pile. However, in some cases the attitude of these later, minor folds is variable ( Plate 25); more than one generation may be present or perhaps conjugate sets of crenulations have developed rather than a single cleavage. Nonetheless, the widespread, gently dipping crenulation plane is itself crenulated by later, more variable fabrics, some of which are associated with minor folds linked to a set of south-directed thrusts. These may possibly be related to the late thrust movement established in the Causey Pike fault zone. A range of minor, small-scale structures such as kink bands is also widespread; it postdates all of the cleavage development, and may have been generated late in the Acadian or even long thereafter.

In the south of the Lake District, in the Black Combe inlier, the Skiddaw Group has been severely deformed within a major, south-directed thrust zone. On the north side of this zone the rocks are intensely cleaved, sheared and metasomatised with much quartz-tourmaline veining; south of the thrust zone the combination of slaty and crenulation cleavages is very similar to that seen in the main, northern inlier. However, only a short distance south from Black Combe, in the Furness inlier, only the regional slaty cleavage is present. The Black Combe inlier lies within the hinge zone of the Westmorland Monocline, in a position to experience the maximum focus of Acadian strain above the margin of the subvolcanic batholith. The Furness area, lying south of the monocline, escaped much of the late Acadian deformation.

The overall impression is that a regional cleavage and subsequent, highly domainal crenulation fabrics were all developed during the Acadian Orogeny. It is likely that the sequence of fabrics records a continuum of deformation rather than separate tectonic pulses, with deformation being effected within a regime controlled by south-directed reverse faults.

The interpretation of thrust geometry in the Skiddaw Group is important in establishing the overall convergence regime. The sequence of thrusts seen at outcrop was previously modelled as an imbricate network extending southwards from the leading edge of the Southern Uplands thrust belt ( Figure 31) but, in order to accommodate the apparently steep dips that some of these structures show at outcrop, subsequent reorientation was required. It could only be achieved by compression buttressed against the subvolcanic Lake District Batholith during the propagation of a deep thrust dislocation at the base of the batholith. This in turn was thought to have driven the Westmorland Monocline into mountain front proportions, with commensurate acceleration of subsidence in the Windermere Supergroup foreland basin phase. Such reorientation would have been essentially pre-Acadian and is effectively ruled out by the relationship of the major fault structures to the Acadian cleavage.

A more likely, alternative view of the Skiddaw Group thrusts sees them as early, synsedimentary slump planes caught up in wholly Acadian reactivation entirely unrelated to the Southern Uplands thrust belt. In this interpretation the dominant Acadian components are part of a compressional flower structure generated along the northern margin of the batholith ( Figure 32). If this view is accepted, it follows that a considerable thickness of Skiddaw Group strata probably exists below the oldest exposed level; the tentative recognition of a single Cambrian-aged locality (see discussion in Chapter 2) may be supportive. It also follows that the southward propagation of thrusts at the leading edge of the Southern Uplands thrust belt must have largely taken place at a structural level higher than that currently exposed.

Manx Group structure

The Manx Group succession on the Isle of Man has a relatively simple outcrop pattern due to predominantly steep dip and consistent north-east strike, although the latter swings to the east in the north-east of the outcrop. However, at the exposure scale, dip may vary widely due to two main phases of deformation: the first is a pervasive slaty cleavage associated with gently to moderately plunging folds on a range of scales; the second is a widely developed, gently dipping crenulation cleavage associated with small folds that verge in the direction of dip of bedding. These affect both the Ordovician Manx Group ( Plate 26) and the upper Silurian Dalby Group. The earlier deformation also affects many of the minor intrusions, the Dhoon Grandiorite Pluton, and possibly the metamorphic phases in the aureole of the Early Devonian Foxdale Granite, although not the granite itself. Overall, the broad style and Acadian age of the deformation seen in the Manx Group are both very similar to the equivalent features of the Skiddaw Group, including the cleavage–aureole relationships around the Shap and Skiddaw granite plutons. A late phase of localised deformation affecting the Manx Group as a broadly north-trending, steeply dipping crenulation cleavage associated with upright folds may be Carboniferous in age, although somewhat similar fabrics and structures in the Skiddaw Group are regarded as having a late Acadian origin.

A number of kilometre-scale folds have been identified in the outcrop of the Lonan Formation, but determination of the overall structure of the Isle of Man has, until recently, been limited by the absence of a well-established stratigraphical framework. Early interpretations suggested fold models based on lithological correlations that have since been eliminated on biostratigraphical grounds. More recently, the possibility that major strike-parallel faults may separate tracts of reasonably coherent stratigraphy has been developed into an interpretation wherein a broad zone of south-east-directed thrusting has imbricated the upper part of the sequence and carried it over a folded ‘foreland’ succession ( Figure 33).

Several ductile shear zones run subparallel to the north-east-orientated tract boundary faults within the Manx Group and are generally marked by narrow zones of intense fabric development with indications of predominantly sininstral shear. They would seem to indicate a transition from orthogonal compression to transpression during the later stages of Acadian deformation. In this respect the Isle of Man has more in common with the Scottish Southern Uplands terrane than with the Lake District inlier.

Geophysical data suggest a zone of complex structure in the mid crust, underlying and trending parallel to the zone of faulting in the north-western part of the Isle of Man (see Chapter 2). This, together with the apparent north-westward increase in the structural complexity of the Manx Group, may be related to the proximity of the Isle of Man to the Iapetus Suture. Deep seismic profiles across the suture show a number of north-dipping reflectors in its footwall and these might link with the tract boundary faults in the Isle of Man — and also, by inference, with the major fault structures in the Skiddaw Group.

Borrowdale Volcanic Group structure

Acadian tectonic structures were superimposed on the pre-existing volcanotectonic framework of the Borrowdale Volcanic Group; the main basins were tightened, their contained strata were folded, and a cleavage was locally superimposed. Fault reactivation seems likely, with clear examples in the Coniston and Troutbeck zones, and in general the geometry of the Acadian structures is superficially congruous with that of their volcanotectonic precursors. Three large-scale features of regional importance are the Haweswater, Scafell and Ulpha synclines. The first of these synclines, at Haweswater, is much broken-up by faulting, but the other two are more clearly defined. The Scafell Syncline is several kilometres across and dominates the structure of the Lake District’s Central Fells. Though broken-up by many volcanotectonic faults, it has an overall north-east, Caledonoid trend and axial plunge that decreases north-eastwards from about 30° to subhorizontal. The Ulpha Syncline, further to the south-west, formed as Acadian sinistral transpression was superimposed on the pre-existing, volcanotectonic and extensional Duddon Basin. It is a large but poorly defined, eastward-plunging fold within a high-strain zone on the south side of the Lake District Batholith. It is truncated by the unconformity at the base of the Windermere Supergroup, but the uniform cleavage spanning the unconformity attests to the Acadian influence. A further complication is the position of the syncline close to the hinge zone of the Westmorland Monocline, the major Acadian structure of the region. The attempts to fold the relatively rigid volcanic strata resulted in sheared-out anticline–syncline pairs, for example in association with the Greenburn Thrust and the east-north-east faults to the north of the Shap Granite; these are demonstrably Acadian structures.

It is significant that the large-scale structure of the Borrowdale Volcanic Group is dominated by synclines, and worth noting that previously described structures such as the Nan Bield and Wrynose anticlines are now thought to arise from the cumulative effects of faulting. This general absence of anticlines reinforces the regional interpretation of the major synclines as reactivated and tightened, volcanotectonic basins. It further emphasises the generally rigid response to deformation of the volcanic succession overlying the Caradoc batholith.

Cleavage fabric varies from spaced to slaty and penetrative, with the intensity varying between different lithologies; it is best developed in volcaniclastic sandstones and phyllosilicate-rich rocks, and may show strong refraction across lithologically contrasting strata. Since most of the central Lake District is underlain by the granitoid batholith, the Acadian strain there was relatively low, and so cleavage is only weakly developed. In contrast, zones of relatively high strain occur in places, particularly above the margins of the batholith and in the belt of steeply dipping strata in the Coniston Fells. There, all lithologies possess a steeply inclined cleavage with a uniform strike trend of about 065°, though there is a regionally arcuate trend from about 030° in the south-west of the outcrop (e.g. Millom Park) to approaching 080° in the east (e.g. Haweswater). The cleavage dip shows systematic variations in zones influenced by the distribution of the more significant faults.

Windermere Supergroup structure

The disposition of Windermere Supergroup strata in the southern Lake District is largely influenced by a single large-scale, regional structure — the Bannisdale Syncline. This forms the lower hinge line of the Westmorland Monocline ( Figure 24) and ( Figure 30), for which the overall maximum amplitude is about 10 km. Farther east, in the Howgill Fells, similar-scale structures occur, but their continuity with those seen in the southern Lake District cannot be confirmed. The axis of the Bannisdale Syncline changes in trend and plunge along its length; eastwards, it swings from 055° with a plunge of 30° to the east-north-east (about half of which can be attributed to post-Carboniferous fault rotation), to become subhorizontal with an axial trend of about 065°. The regional structural arc is even more pronounced when minor fold hinges and cleavage strikes are considered. Then, the structural trend varies from about 060° in the south-west, east–west in the Howgill Fells, and about 110° in the Craven inliers ( Figure 34).

A single Acadian cleavage is developed throughout the Windermere Supergroup, as a penetrative, mica-defined fabric in the mudstones, but as a more widely spaced, pressure-solution fabric in the sandstones ( Plate 27). The cleavage is generally inclined steeply (though strongly refracted through different lithologies), but has a less arcuate strike trend than the axial planes to the associated minor folds. Hence, the cleavage shows a small clockwise transection of the fold hinges in the west of the outcrop, passing eastwards into a small anticlockwise transection ( Figure 34). This is part of a wider pattern of transecting cleavage seen across the Avalonian Lower Palaeozoic strata of Britain and Ireland. It has been interpreted in terms of transpressive strains created by the indenting of a rigid basement block that was driven northward during Acadian continental collision at the southern margin of Avalonia.

In the Isle of Man, the Dalby Group strata have a deformation history similar to that seen in the Manx Group, emphasising the Acadian age of the folding and cleavage affecting both units. A strong axial planar cleavage is ubiquitous across the first generation of folds in the Dalby Group, and in the hinge zones is commonly developed as a convergent cleavage fan ( Plate 28). Later folds are more variably associated with crenulation cleavages that are usually gently inclined.

A network of broadly north-trending faults cuts across the main outcrop of the Windermere Supergroup, and many of the individual faults separate tracts with different fold and cleavage patterns. They have accommodated variable flexure and so have changes in displacement along their length. These faults were probably pre-existing, basement fractures that propagated upwards during Acadian deformation, partitioning strain in the Windermere Supergroup strata. The larger examples (e.g. Coniston, Brathay and Troutbeck faults; ( Figure 35) give rise to significant offsets of the supergroup’s basal unconformity, but their displacement decreases upwards into its younger stratigraphical components. Overall, the faults appear to have had early, subvolcanic and/or Early Devonian histories, and then to show Acadian reactivation, before many also fulfilled a post-Acadian role, particularly during Permo-Carboniferous extension.

Regional metamorphism

Only low grades of regional metamorphism pertain in the Lower Palaeozoic strata. They are best defined by illite crystallinity in terms of the anchizone and epizone and were achieved by a combination of sedimentary burial and Acadian tectonism. An important variable during the former was the thermal regime affecting the basins in which sedimentary burial occurred. So, for example, likely extensional basins such as those in which the Skiddaw and Manx groups were deposited, would have had a higher heat flow (up to 50°C/km) than the Windermere Supergroup’s foreland basin that formed in response to convergence and loading. One effect of this contrast might be the widespread presence of a bedding-parallel, burial-induced compaction fabric in the (warm) Skiddaw and Manx groups, but its virtual absence in the (cool) Windermere Supergroup.

In the Skiddaw Group, there is a complicated distribution of grade, some of which can be attributed to post-metamorphic faulting. However, the underlying broad pattern of diagenetic to low epizonal variation is hard to reconcile with a single metamorphic episode. Instead, it would seem likely that the effects of burial have been overprinted by regional metamorphism, developed contemporaneously with folding and cleavage formation and locally influenced by the composition of the affected rock. Similarly, throughout the Manx Group, low-grade regional metamorphism can be linked with deformation, although again the pattern seems strongly influenced by the original composition of the various units.

The western part of the Borrowdale Volcanic Group outcrop lies within a thermal aureole surrounding the Caradoc, Eskdale and Ennerdale intrusions; an inner zone is characterised by secondary hornblende with biotite, and an outer zone by hornblende with actinolite, epidote and chlorite. Beyond these obvious contact metamorphic effects, the volcanic rocks in the eastern part of the outcrop carry a metamorphic mineral assemblage that includes epidote, chlorite and white mica, locally and rarely with prehnite and/or pumpellyite. It is not easy to disentangle the overlapping effects of thermal, contact metamorphism as opposed to those induced by high heat flow during synvolcanic burial, entirely postvolcanic burial, or Acadian metamorphism during deformation but still beneath a thick cover of now-eroded sedimentary rocks. Overall, post-Caradoc, sub-greenschist facies metamorphism of the Borrowdale Volcanic Group seems likely, broadly comparable in grade to the anchizonal metamorphism described below from the Windermere Supergroup. An alternative interpretation of the 420–430 Ma, reset Rb-Sr ages from the volcanic and intrusive rocks relates the resetting to isotopic re-equilibration during burial diagenesis, rather than the hydration during structural flexure discussed previously.

The Windermere Supergroup shows a more uniform pattern of low-grade, burial metamorphism; low anchizone grade in the east, rising westwards to upper anchizone or low epizone. The change can be at least partially explained by the overall eastwards plunge of the Bannisdale Syncline, the regionally dominant structure, supplemented by post-metamorphic faulting. The nature of the sedimentary basin in which burial occurred would suggest a low geothermal gradient of about 25°C/km and, if this is adopted, a burial depth of several kilometres (at least 4 km, maybe more depending on other assumptions) is required for the top of the Bannisdale Formation in order to achieve the metamorphic conditions seen therein. This cannot be achieved from the known upper parts of the Windermere Supergroup stratigraphy, and so requires a considerable thickness of Old Red Sandstone strata to have been originally present across the Lake District, but which has since been lost through erosion.

Early Devonian magmatism

Granitic plutons were emplaced at a high structural level across the Iapetus Suture Zone, within both the Southern Uplands and northern England terranes, during the Early Devonian Acadian Orogeny. In the Lake District, these include the high heat-production Shap and Skiddaw granites, steep-sided equant plutons that were emplaced at the margin of the pre-existing, Caradoc subvolcanic granite masses; other concealed bodies may also have contributed to the expansion of the Lake District Batholith at this time. A separate high heat-flow granitic batholith, including the Weardale Pluton, was also emplaced to the east-north­east, below the northern Pennines. Farther north, forming the high moorlands of the Cheviot Hills, is the Cheviot Volcanic Formation, the remains of a large andesitic volcanic field. The co-magmatic and coeval, subvolcanic Cheviot Granite Pluton is the easternmost member of the Galloway Suite of granitic intrusions that also includes the Criffel and Fleet plutons. Associated with the granites are swarms of microgranite and lamprophyre dykes that cut Lower Palaeozoic rocks throughout northern England and southern Scotland. In the Isle of Man, several granitic plutons and associated minor intrusions have been intruded into the Manx Group.

Skiddaw Granite Pluton (399 ± 0.4 Ma, U-Pb, zircon)

The roof zone of the steep-sided, broadly cylindrical Skiddaw Pluton (about 4.5 km in diameter) is exposed in valleys within the Skiddaw Massif. It is a medium-grained, biotite granite, locally porphyritic and composed of orthoclase, oligoclase, quartz and biotite. The degree of greisen formation increases northwards to Grainsgill where the granite is pervasively converted into quartz–muscovite greisen. Emplaced into the Skiddaw Group, the mass is surrounded by a concentric thermal aureole that grades outwards through garnet, biotite-cordierite, chiastolite and chloritoid zones. Intrusion overlapped the formation of the main cleavage, with chiastolite both overgrowing and being weakly wrapped by the fabric ( Plate 29). Mineralogical studies on the aureole rocks suggest that the granite was emplaced at a depth of 8 to 9 km, whereas the composition of fluid inclusions formed during greisenisation show that that process occurred at a depth of between 2.4 and 4.9 km; uplift and erosion must clearly have followed rapidly after intrusion. The granite is associated with the formerly economic tungsten vein mineralisation that cuts both the granite and the adjacent Carrock Fell Centre (see Chapter 10).

Shap Granite Pluton (404 ± 0.5 Ma, U-Pb, zircon)

In the eastern Lake District, the steeply conical Shap Pluton, approximately 8 km2 in outcrop area, cuts the Borrowdale Volcanic Group and adjacent Windermere Supergroup. The surrounding thermal aureole and interpretation of potential-field data indicate a more extensive subcrop. This decorative pink and grey biotite monzogranite contains up to 60 per cent Carlsbad-twinned orthoclase–perthite megacrysts, up to 5 cm in length, set in a coarse-grained groundmass composed of orthoclase, plagioclase, quartz and biotite ( Plate 30). The megacrysts have been interpreted either as phenocrysts, or as porphyroblasts resulting from late-stage potassium metasomatism; the present consensus favours the former. Microdioritic enclaves and xenoliths of country rock are abundant locally. The main contact metamorphic effect in both the Borrowdale Volcanic Group and Windermere Supergroup was production of a biotite hornfels aureole, though within the latter, sillimanite, andalusite and cordierite occur in the more aluminous rocks, whilst the more calcareous lithologies contain an assemblage that includes vesuvianite, grossularite, diopside, anorthite, tremolite and actinolite. Hydrothermal mineralisation and fissure metasomatism are conspicuous features of the granite and its aureole respectively, the latter being dominated by garnet–epidote–hornblende veins. The aureole overprints cleavage in both the Borrowdale Volcanic Group and Windermere Supergroup, but some of the dykes believed to be cogenetic with the Shap Granite are cleaved at their margins.

Concealed plutons in the Lake District

Concealed plutons in the Lake District To the north of the Ennerdale Microgranite, bleached and recrystallised Skiddaw Group rocks, accompanied by locally abundant tourmaline veins, occur in an elongate, east-north­east-trending zone, 24 km long and up to 3 km wide, adjacent to the Causey Pike Fault. The metasomatic event has been dated at around 400 Ma (Rb-Sr, whole rock) and its effects overprint thermal metamorphism that was probably caused by a concealed elongate granitic mass, referred to as the Crummock Water Intrusion. The geometry of this pluton, modelled from potential-field geophysical data, suggests emplacement along an active fault zone. Another metasomatic bleached zone affects Skiddaw Group rocks in the northern part of the Black Combe Inlier. A radiometric age has not been obtained, but the metasomatism may be related to a concealed Acadian pluton because the metasomatised rocks have very similar mineralogical characteristics and structural relationship to the host mudstones, as those in the Crummock Water zone adjacent to the Causey Pike Fault. An intrusion beneath Black Combe would, like the Shap and Skiddaw plutons, be marginal to the batholith; it could be the source of the Early Devonian microgranite dyke swarm that is concentrated in the metasomatised zone, and also of the tungsten, tin and bismuth mineralisation seen in adjacent areas (see Chapter 10).

Weardale Granite Pluton (399 ± 0.7 Ma, U-Pb, zircon)

Weardale Granite Pluton (399 ± 0.7 Ma, U-Pb, zircon) The northern Pennines are underpinned by an elongate, east-north-east-trending batholith, 60 by 25 km in extent, the existence of which was detected by gravity surveys and by the Rookhope Borehole ( Figure 36). Interpretation of the gravity data suggests that the batholith comprises a cluster of broadly cylindrical plutons, the central one of which is referred to as the Weardale Granite Pluton. This granite, as sampled by the Rookhope and Eastgate boreholes, is aphyric, contains biotite and muscovite, has a shallow-dipping, gneissose­like foliation and is cut by pegmatitic and aplitic facies and by a few tourmaline-bearing veins. The Weardale Pluton is peraluminous in composition and geochemically similar to the Skiddaw Granite. In common with the Shap and Skiddaw granites, it has a high heat-flow value and this is believed to have had an important role in driving the convection cells responsible for the later formation of the zoned North Pennine Orefield. The granite may have been the source of elements such as Sn, F and Bi within the mineralisation (see Chapter 10).

Cheviot Volcanic Formation (395.9 ± 3.8 Ma, Rb-Sr, biotite)

The Cheviot Volcanic Formation is poorly exposed over an area of about 600 km2 and its present thickness is about 500 m. Since it is likely that the coeval granite pluton had a substantial cover, the original thickness of volcanic rocks probably exceeded 2000 m, making the Cheviot volcanic eruptions comparable in scale to the early phase of Caradoc volcanism in the Lake District. The volcanic rocks unconformably overlie steeply dipping, tightly folded sandstone and cleaved mudstone of the Riccarton Group (Wenlock) and are overlain by either Upper Old Red Sandstone Group conglomerates containing abundant andesite and granite clasts, or by Lower Carboniferous beds, only some of which contain andesite fragments. Small exposures of red sandstone within the volcanic outcrop may be intercalated with the volcanic sequence, but fossils have not been found to indicate the biostratigraphical age of the volcanism. The radiometric ages determined show that volcanism occurred at the end of Early Devonian time, during the Acadian Orogeny.

The succession comprises stacked trachyandesite and subordinate trachyte sheets that contain phenocrysts of plagioclase, hypersthene, augite, ilmenite and apatite. There are also a few sheets of biotite-feldspar-phyric trachyte (previously described as ‘mica-felsites’) and, near the base of the succession, one or more sheets of rhyolite that represent eruptions of more fractionated magma. The restricted compositional range and absence of basaltic trachyandesite are in contrast with other Old Red Sandstone volcanic successions farther north in the Midland Valley of Scotland.

The sheets are massive to amygdaloidal and scoriaceous; near horizontal platy jointing is characteristic of many. The more readily weathered tops of the units have been eroded in some areas to form a prominent bench and scarp landscape. The benches reflect the gentle eastward dip of the sequence. The sheets have been interpreted as lavas, erupted in a subaerial setting, though the presence of a significant proportion of sills cannot be ruled out. The nature of this sequence has parallels in the Caradoc volcanic complexes of the Lake District, with the Birker Fell Formation of the Borrowdale Volcanic Group and the main part of the Eycott Volcanic Group; all three may have resulted from similar clusters of lava-producing low-profile volcanoes.

The basal unit of the formation in the south-west of the outcrop comprises up to 60 m of breccia composed of rubbly, angular to subangular clasts of silicic volcanic rock, along with some mudstone fragments. This unit is most likely the product of initial phreatomagmatic eruptions, though a sedimentary origin cannot be entirely discounted. Other intercalations of pyroclastic and volcaniclastic sedimentary rocks are present higher in the succession, though they are sparse. In the uppermost parts of some sheets there are enclaves and fissure-fills of green fine-grained sandstone and siltstone. Fragments of these rocks are commonly seen in streams, suggesting that this lithology is more common in the unexposed parts of the succession.

Cheviot Granite Pluton (395.9 ± 2.9 Ma, Rb-Sr, biotite, whole rock)

The Cheviot Pluton has an outcrop of about 60 km2 and, at a depth of 4 km, a diameter of nearly 20 km. It intruded the Cheviot Volcanic Formation late in the volcanic episode, thermally altering the volcanic rocks, in places for up to 2 km away from the contact. The Cheviot Pluton comprises an outer zone of grey, quartz monzonite to quartz monzodiorite which was emplaced first, followed by an inner zone of medium to coarse monzogranite; the final phase of intrusion is represented by a pink medium-grained granophyric granite. The mineral assemblage includes some clinopyroxene as well as biotite, an unusual feature in rocks of this composition. Exposures of the pluton margin in Common Burn [NT 930 265] and Hawsen Burn [NT 953 225] show a complex of granite dykes and veins penetrating the volcanic rocks.

Minor intrusion suites of northern England

Sporadic calc-alkaline lamprophyre dykes cut all Lower Palaeozoic units in the Lake District, Cross Fell, Cautley, Dent and Teesdale inliers; further south they occur in the Ingleton inlier and to the north the lamprophyre swarm spans the Iapetus Suture with widespread dykes seen in the Southern Uplands terrane. Two groups have been recognised: clinopyroxene– phlogopite–Ca-amphibole-phyric kersantite, and strongly biotite-phyric kersantite and minette. Some dykes show no cleavage whilst others are cleaved, suggesting that intrusion was both pre- and post-tectonic. Their radiometric ages, spanning the range 420 to 402 Ma (late Silurian to Early Devonian), suggest that they were emplaced before the granites.

Minor intrusions and dyke swarms comprising feldspar and quartz-feldspar-phyric microgranite, rhyolite and microdiorite occur in the Lake District and Cross Fell inliers and within the Cheviot massif. In the first of these areas, the minor intrusions are concentrated particularly around Scafell, in Black Combe, in the Duddon valley, and in the vicinity of the Shap Pluton. Some of the Lake District dykes have radiometric ages within error of the Shap and Skiddaw plutons, making them broadly contemporaneous with the Acadian magmatic event. To the south of the Cheviot Pluton a similar swarm of felsic dykes emanates in a broadly radial pattern.

Isle of Man

Intrusive rocks are abundant within the Manx and Dalby groups in the Isle of Man, though the igneous bodies are mostly dykes or small minor intrusions. There are three larger masses: the Dhoon Granodiorite and Foxdale Granite plutons, which have outcrop areas of about 2 km2, and the smaller Oatlands complex ( Figure 12). Granite may underlie much of the island at depth.

The Dhoon Granodiorite Pluton is biotite-bearing and locally porphyritic but it is strikingly altered, with plagioclase replaced by zoisite and muscovite, probably as a result of greenschist-facies metamorphism. There is no radiometric age determination on the granodiorite, but relationships with the deformation sequence affecting the Manx Group rocks indicate that the pluton was emplaced early in the sequence of events. Together with the volcanic-arc affinity suggested by its trace element geochemistry, this could mean that the Dhoon Pluton was emplaced contemporaneously with the Ordovician arc magmatism in the Lake District, though a later age is equally possible. The granitic through to gabbroic Oatlands complex is now very poorly exposed but has been generally considered to have more features in common with the Dhoon Granodiorite than with the Foxdale Granite.

The Foxdale Granite Pluton is muscovite bearing and the accessory mineral assemblage includes garnet; porphyritic microgranite and pegmatitic varieties occur locally. The relationship to the deformation sequence, the relatively high content of radioactive elements, and the likely radiometric age of around 400 Ma, all indicate an association with the Early Devonian granites of northern England.

Many of the dykes and minor intrusions, which range in composition from mafic to felsic, are deformed and metamorphosed and may therefore have been emplaced early in the tectonic history. Altered basalt and basaltic andesite sills at Poortown ( Figure 12) have a volcanic arc signature and, though the age of these sills is uncertain, they may be equivalent to the Caradoc volcanic rocks in the Lake District. A few mafic dykes cut the Wenlock strata of the Dalby Group and so establish that at least some dykes were emplaced during, or later than, the late Silurian. The suite of microgranite sheets occurring along the central spine of the island has been linked to the Dhoon Pluton.

Petrogenesis

The lamprophyric magmas were emplaced during Early Devonian times whilst a transtensional tectonic regime was in operation. The magmas were large-fraction melts of depleted oceanic lithosphere that had been metasomatised by aqueous fluids derived from the dehydration of Iapetus oceanic crust during subduction and by a CO2-rich phase from a deeper mantle source. Variable but small degrees of fractional crystallisation were involved before final emplacement.

By contrast, the broadly calc-alkaline granites and the Cheviot volcanic rocks were emplaced towards the end of the Acadian Orogeny. The Cheviot volcanic rocks show some similarities with other Old Red Sandstone examples in the Midland Valley of Scotland, but there are also differences. For example, the Cheviot rocks are more potassic and, despite having strong enrichment in light rare-earth elements, the Sr content is low.

Despite the orogenic setting of northern England towards the end of the Acadian events, a sedimentary protolith is unlikely as the main source of the Early Devonian granites. Rather, the parental magmas were probably derived from a mafic source that contained residual garnet. These calc-alkaline granites are compositionally evolved rocks, yet show little evidence of an extended history of crystal fractionation. However, as with the Ordovician magmas before them, assimilation of Skiddaw Group sedimentary material is implicated, particularly by the Sr, Pb and O isotope compositions.

Old Red Sandstone

Although a thick cover of Old Red Sandstone strata can be deduced from the metamorphic grade attained by the Lower Palaeozoic rocks of northern England, very little trace of it remains. The Early Devonian was a time of regional sinistral transtension and this seems the most likely mechanism to have driven the tectonic rifting and subsidence that would have been necessary if several kilometres of mainly fluvial sediment were to accumulate. It is probable that much of the Lower Old Red Sandstone cover was then eroded and recycled southwards during subsequent, Acadian basin inversion. Local deposits of Upper Old Red Sandstone strata are post-Acadian, post-tectonic, and range in age from Late Devonian to Early Carboniferous. By the end of the depositional interlude, the region was largely a continental peneplain, and relict exposures of the end-Devonian land surface preserve soil profiles developed in an arid, tectonically quiescent environment.

Isle of Man

A likely remnant of the once-extensive Lower Old Red Sandstone fluvial deposits is the Peel Sandstone Formation, which has a small, faulted outcrop on the north-west coast of the Isle of Man. It comprises red, coarse-grained sandstone and conglomerate, deposited in braided river and alluvial plain environments by a river system flowing from the north-west; there are sporadic, interbedded calcrete (palaeosol) layers and a minor but significant aeolian component. Estimates of the sequence’s thickness range from 500 m up to 2000 m. Slump folds and small-scale, synsedimentary thrust imbrication confirm a depositional palaeoslope inclined towards the south-east. The conglomerates contain a range of clast types and many of the clasts are relatively angular: quartz, quartzite, various volcanic and volcaniclastic lithologies (mostly felsic), sandstone, limestone and shale are well represented. Manx Group clasts are notably absent. The limestone clasts are of particular importance in that some contain Wenlock shelly fossils, whilst others have a possibly Ashgill fauna. The Wenlock fossils provide a maximum age for the Peel Sandstone Group, and from palaeomagnetic results it is regarded as most probably of Early Devonian age (about 400–410 Ma). Despite this likely pre-Acadian age, there is no indication that the Acadian cleavage in the adjacent Manx Group continues into the Peel Sandstones, nor is there any sign of significant tectonic deformation beyond the regional rotation to a dip of up to 50° north-west.

Northern England

In parts of north-west England, it has been customary to recognise a single unit of conglomeratic ‘basement beds’ separating Lower Palaeozoic strata from the Carboniferous limestone succession. However, in parts of Cumbria these ‘basement beds’ are better interpreted as two superimposed units: a lower, laterally discontinuous, red-bed succession, unconformably overlain by an upper unit of Carboniferous, mixed marine and terrigenous, clastic beds, locally including volcanic rocks. Only the rocks of the lower succession are now regarded as of Old Red Sandstone lithofacies and were laid down in a continental, pluvial-desert setting of high to moderate relief. Their deposition was controlled by rift systems whose orientation, sediment provenance and dispersal trends were very different from those associated with the ensuing Early Carboniferous rift regime. There is a complete absence of fossils, hence the age assignments can only be tentative.

Along the eastern flank of the Lake District massif, local conglomerate accumulations are apparently post-tectonic but, in the case of the Mell Fell Conglomerate Formation, are cut by Early Carboniferous dykes. The Mell Fell Conglomerate crops out from Great and Little Mell Fells and the northern end of Ullswater, northwards to the Carboniferous limestone escarpment west of Penrith; there is a small outlier in the Heltondale Beck by Askham village [NY 506 207]. The polygenetic, red-bed conglomerate contains clasts derived mostly from the Borrowdale Volcanic Group and the Windermere Supergroup. Some clasts preserve a cleavage, confirming post-Acadian deposition, and so a Late Devonian (Upper Old Red Sandstone) depositional age seems likely. The depositional environment was one of coalesced alluvial fans and braided river channels in a pluvial, desert setting. The Mell Fell Conglomerate, at about 1500 m, is the thickest of the preserved conglomerate sequences and probably accumulated in a valley controlled by, or eroded along, the Causey Pike Fault.

This raises the possibility of post-Acadian movement on that important regional structure. In some parts of the Cross Fell inlier, the Lower Palaeozoic strata are unconformably overlain by up to about 35 m of coarse clastic rocks that have traditionally been associated as the ‘Polygenetic Conglomerate’ ( Plate 31). Clasts are mainly of Lower Palaeozoic volcanic or sedimentary lithologies and a broad correlation with the Mell Fell Conglomerate has been generally assumed.

The Shap Wells Conglomerate Formation contains interbeds of mudstone and fluvial sandstone and is conformably overlain by the Blind Beck Sandstone Formation. The outcrop is small and restricted to the valley of the Birk Beck, a northern tributary of the River Lune. A broadly similar assemblage occurs farther south, where the Sedbergh Conglomerate Formation is exposed in its type area along the course of the River Rawthey, in disparate outcrops along the Pennine Fault Zone and in the valley of the River Lune, north of Kirkby Lonsdale; the overlying, 15 m thick Nor Gill Sandstone Formation is restricted to outcrops in a tributary of the River Rawthey. The conglomerates are coarse- to very coarse-grained, nonlocally sourced terrigenous deposits laid down in debris flows, coalescent alluvial fans and braided river systems. The Shap Wells conglomerate beds exposed along the Birk Beck interleave with varicoloured mudstone beds that were probably deposited in lakes; either localised, ephemeral and formed after flash floods, or semipermanent and in a playa-type setting. The red sandstone beds of the Blind Beck and Nor Gill formations are mainly lithic and poorly sorted river deposits, but include some beds of aeolian origin. In addition, the Nor Gill Sandstone Formation contains nodular and sheeted calcrete beds that originated as palaeosols.

Chapter 6 Carboniferous: blocks, basins and sedimentary cyclicity

Rocks of Carboniferous age underlie about three-quarters of the Northern England region, either at outcrop or concealed by later beds. They are lithologically varied and accumulated in a range of depositional environments: marine, paralic, deltaic and terrestrial. Early in Carboniferous times, rapidly subsiding, fault-controlled, extensional basins developed between structurally elevated, emergent blocks. The variation in the thickness of lower Carboniferous strata throughout northern England reflects the close association between sedimentation and the widespread contemporaneous faulting. Syndepositional folding, related to dextral movement on some of the major faults, developed from late in Visean times. By the end of the Visean, thermal relaxation was also driving crustal subsidence, resulting in the gradual submergence of the distinctive block-and-basin regional topography, and the depositional onlap of the earlier structural highs.

Although subsidence was influenced by the broad pattern of structures established early in the Carboniferous, it interacted with eustatic changes in sea level, so that cyclical sedimentation patterns became a general feature of the Carboniferous successions. Throughout the period, the principal rock types of limestone, mudstone, sandstone, seatearth and coal succeed each other in a regular pattern of cyclothems, repeated at different scales and with varying degrees of complexity. Individual rock types within each cyclothem reflect changing environmental and depositional conditions. A particularly well-developed and distinctive cyclic succession built up during the late Visean to early Namurian interval and is commonly referred to as the ‘Yoredale facies’.

Stratigraphical principles

In common with Carboniferous rocks elsewhere in Britain, those of northern England have long attracted attention on account of their economic potential. Generations of underground and opencast workings for coal, vein ores, sandstone, mudstone and limestone have provided a wealth of information as to the distribution and nature of the strata. More recently, oil exploration has encouraged deep boreholes and geophysical investigations that have provided further data for concealed strata at depth. As long ago as the late 18th century, a threefold division of the Carboniferous rocks was recognised: the Carboniferous (or ‘Mountain’) Limestone, the Millstone Grit, and the Coal Measures. These lithostratigraphical (rock) divisions became broadly equated with the chronostratigraphical (time) divisions: Dinantian, Namurian and Westphalian. In the 1970s and 1980s a more detailed framework of chronostratigraphical divisions was established for use in the comparison of rocks from area to area within Europe. Three of these divisions — Holkerian, Asbian and Brigantian — are defined from sections in Cumbria. ( Table 3) shows the relationship of the European scheme to the global subdivisions of the Carboniferous.

Chronostratigraphical subdivision is largely based on biostratigraphy, which relies on the identification of key fossils and fossil assemblages, and the recognition of how these changed with time through successions of strata. Such a classification of Carboniferous stratigraphy across Britain and Europe is possible because the key species were rapidly evolving and widely distributed marine animals, and their fossilised remains enable a firm chronostratigraphical framework to be erected. The framework was initially based largely upon macrofauna assemblages, mainly of corals, brachiopods and bivalves; subsequently, microfauna such as foraminifera and conodonts were employed to allow correlation at international level. Particularly useful are the foraminifera, corals and conodonts in the early Carboniferous, and the ammonoids (goniatites) in the late Visean, Namurian and Westphalian. In northern England, goniatites are generally rare and nonmarine bivalves and, more recently, palynomorphs (spores) have proved important zonal indicators. A selection of Carboniferous macrofossils is shown in ( Plate 32).

The application of sequence stratigraphy allows an alternative approach to the description, interpretation and correlation of the Carboniferous succession. Sequences are defined as stratigraphical units bounded by regional unconformities or discontinuities related to cyclical changes of sea level. Sequence stratigraphical models then distinguish between sedimentary units, or systems tracts, produced during different parts of the cycle of variation in relative sea level. So, for example, transgressive systems tracts form when sea level is rising, high-stand systems tracts form when sea level is high, and low-stand systems tracts form when sea level is low. Sequences are on different scales, and are arranged in a hierarchy from 1st order (of longest duration) to 5th order. Sequence stratigraphy emphasises the allocyclic controls on sedimentary successions (tectonism, climate and eustasy) and can provide an improved understanding of depositional history and sedimentary system development. In places, especially in concealed sequences with limited borehole control, it can enable a higher resolution correlation than can be obtained by existing palaeontological methods alone. In northern England, sequence stratigraphy has been applied in the continuing debate regarding the origin of Yoredale cycles (discussed below), but has proved of most value in developing a high-resolution sequence stratigraphic framework for the Westphalian strata. This has enabled an improved correlation of the succession onshore with that offshore, beneath the North Sea.

Despite the advantages of sequence stratigraphy, the recognition of surfaces bounding systems tracts can be very difficult. In view of this, and given the fossiliferous nature of most Carboniferous rocks, biozonation continues to be an important tool for stratigraphical subdivision and provides a chronological framework for the succession. Ideally, the different methods should be used in combination. Hence, the long-established and well-tested use of marine bands as an important key to stratigraphical correlation, equates with identification of maximum flooding surfaces in sequence terminology.

Beds of economic importance, such as sandstone, limestone, coal or claystone, are commonly identified with local names, the principle of lithostratigraphy. Each district has its own set of names, but the same name is commonly used for beds in different locations and at different levels. The British Geological Survey has recently rationalised the nomenclature of British Carboniferous lithostratigraphy, grouping together eight broad types of sedimentary lithofacies associations ( Table 4). These regional group names will be implemented in future BGS publications and have been adopted here; their lithofacies associations and formational components are summarised in ( Figure 37). The correlation between the traditional, district stratigraphies and the newly adopted regional lithostratigraphy is summarised in ( Figure 38). Across northern England the group names reflect an early Carboniferous situation wherein the effect of a pronounced block and basin topography ( Figure 7) produced locally variable successions. Subsequent deposition patterns became progressively (but diachronously) more uniform right across the region as synsedimentary fault control waned. However, even then, cycles of marine flooding and retreat continued intermittently at the trough margins in response to both global sea-level changes and asymmetric, or ramped, subsidence along the block boundary faults.

Depositional controls

The structural framework

Over a hundred years ago, it was appreciated that the Carboniferous of northern England was deposited in a series of troughs or ‘geosynclines’, separated by higher areas. Towards the end of the 20th century, the recognition that these basins and blocks were formed as extensional or transtensional features during a period of lithospheric stretching was a major development in our understanding of the region. This period of crustal extension, in a stress-field with a dominantly horizontal component of north–south tension, began in the Late Devonian and continued until late Visean times; it gradually produced a rifted topography of fault-bounded blocks with intervening graben and half-graben basins. Rifts opened along preexisting lines of structural weakness embedded in the underlying crust, with the Iapetus Suture Zone providing a primary control. For example, the fault system forming the southern margin of the Northumberland–Solway trough is rooted into the Iapetus Suture so that the trough effectively formed by extension in the suture’s hanging wall. The extensional regime allowed the local occurrence of basaltic magmatism.

Rifting was pulsed, with particularly active episodes in Courceyan, Chadian to early Arundian and in mid to late Asbian times, although the magnitude and perhaps the timing of each pulse, appears to have varied significantly from basin to basin. The buoyant and rigid behaviour of the blocks is usually ascribed to the presence beneath them of low-density granitoid plutons, emplaced during Late Ordovician or Early Devonian magmatic events. Within this framework, the Carboniferous record of uplift, tilting and submergence of individual blocks points to a complex history of fault-block rotation and lateral as well as vertical movements along the boundary faults. Further, block boundaries are not always fault-controlled, but may be transitional across the hinge zone into a half-graben. In these cases, large areas between the main blocks and flanking the troughs are now covered by a shelf carbonate succession deposited in a regime that was neither wholly ‘block’ nor wholly ‘basin’. The principal structural units ( Figure 7) are described below.

The Southern Uplands Block and its offshore continuation into the Mid North Sea High, broadly separated the Peel–Solway–Northumberland Basin from the Midland Valley of Scotland. Evidence suggests that, at times, the barrier was breached by a series of narrow north-north-west-trending basins.

Traversing the north of England is the composite Peel–Solway–Northumberland Basin. Interpretation of geophysical profiles across the basin indicates that it developed over the inferred line of the Iapetus Suture, with extension and growth faulting further facilitated by pre-existing intracrustal detachment surfaces such as the Causey Pike Fault. The southern margin of the basin is defined by the Maryport–Stublick–Ninety Fathom fault system. Throughout much of its length the northern margin is also formed by a system of en échelon synsedimentary dislocations including the North Solway, Gilnockie and Alwinton faults. In the early Carboniferous, the Cheviot Block separated the Tweed Basin from the main Northumberland Trough, although the extent of its continuation offshore is still uncertain. The eastern side of the Cheviot Block, onshore, was submerged in the Asbian and its boundary with the Northumberland Trough is poorly defined. The latter basin is a half-graben, with the rock succession thickest close to the major bounding fault in the south ( Figure 39). Seismic data reveal a number of fault-controlled, linear intrabasinal highs that may have been exposed and subjected to contemporary erosion during the early, pre-Chadian period of basin evolution. To the west, the Peel and Solway basins formed as complex and roughly symmetrical grabens.

On its south side, the Northumberland Trough is bordered by the Alston and Lake District blocks. Underlain by the North Pennine Batholith and bounded by faults on three sides, the Alston Block formed a prominent high until the Asbian. The Lake District Block is underpinned by the Lake District granitic batholith and probably remained emergent during early Carboniferous times. Thereafter, southward tilting of its upper surface gradually allowed northward onlap of Arundian and younger strata and it is likely that latest Visean, Namurian and Westphalian strata were deposited over most if not all of the block. The Southern Lake District High is essentially the south-dipping flank of the Lake District Block, which from seismic evidence was dissected during Visean times by a series of small north-trending half-grabens. The Lake District block extends westwards as the Ramsey–Whitehaven Ridge, an elevated tilt-block, bounded to the north-west by the Maryport Fault and to the south-east by the Lagman and Eubonia faults. At its western end, the ridge merges with the Manx Block, a structural high underpinned by the granitic Manx Pluton.

To the west of the Alston Block, the Vale of Eden Basin developed from the Visean onwards as a half-graben structure adjacent to the Pennine Fault system. To the south of the block, the Stainmore Trough is an embayment open to the east. The northern margin of the trough is bounded by the Closehouse–Lunedale–Butterknowle fault system, which links northwards via the Pennine Fault to the Stublick Fault at the northern margin of the Alston Block. In contrast to the floor of the Northumberland Trough, seismic evidence indicates that the floor of the Stainmore Trough is relatively flat, with little evidence of significant intrabasinal faulting. The southern margin of the Stainmore Trough is formed by the Askrigg Block, a northward dipping massif underpinned by the Wensleydale Granite. Thence the basin network extends through the Craven and Lancaster Fells basins and continues westward into the East Irish Sea Basin. The northern margins of the Askrigg, Craven and Lancaster Fells structural units define the southern limit of the Northern England region described here; these units are described in the companion volume for the Pennines and adjacent areas.

The timing of early Carboniferous extension remains a matter for debate. Geophysical interpretation indicates that the earliest, synrift Carboniferous strata in northern England were deposited in the axes of the main troughs. Very thick, early Carboniferous deposits appear to form the lower part of the synrift succession in the Northumberland–Solway Basin and the Stainmore Trough, but their character is unknown since they are located at depths beyond the reach of existing exploratory boreholes. From likely correlations with surface outcrops in northern Cumbria, these early deposits (at least in the Northumberland Trough) may well include fault-related breccio-conglomerate and basaltic lavas erupted at the onset of rifting. Siliciclastic rocks interbedded with limestones of Arundian and Holkerian age were proved in the Seal Sands Borehole [NZ 5379 2380] and may be typical of much of the basin fill in the Stainmore Trough. The synextensional rocks of the region are largely confined to the basinal areas, and are thickest close to the major bounding faults. Up to around 5000 m of synrift strata lie adjacent to both the Maryport–Stublick–Ninety Fathom fault system along the southern margin of the Solway–Northumberland Trough, and to the Closehouse–Lunedale– Butterknowle fault system at the northern margin of the Stainmore Trough. Equivalent strata are largely absent from the structural highs though relatively complete but thin, synextensional sequences do occur on the eastern part of the Cheviot Block and on some of the main intrabasin highs. Only relatively small thicknesses, up to 200 m of beds from the later part of the extensional phase, occur around the margins of the Alston and Lake District blocks.

Both the Peel–Solway–Northumberland Trough and the Craven Basin were inundated from a seaway to the west, while the Stainmore Trough was probably flooded from the east. Borehole evidence shows that deep marine conditions were maintained throughout Dinantian times in the Craven Basin and concealed eastern part of the Stainmore Trough. However, the exposed Dinantian successions of both the Solway–Northumberland and west Stainmore troughs were deposited in relatively shallow marine conditions and there were frequent fluviodeltaic incursions. During later phases of extension, deposition gradually spread more widely until, Asbian to Brigantian times, rapid subsidence gave way to slow downwarping of the major troughs and their adjacent bounding block areas. Minor extensional faulting continued into the ‘postextensional’ phase.

Tectonic influences on local sea level

The position of relative sea level was a primary control on the nature of the Carboniferous sedimentary successions. Active growth of the major structures beneath the region caused the sea to be persistently deeper in some areas, thus influencing both the type and thickness of sediment deposited. In general, the block areas subsided more slowly than the intervening basins, resulting in the accumulation of thinner sequences of Carboniferous rocks over the blocks than in the basins. Despite these marked variations in subsidence rates, it seems that during much of the Carboniferous, sedimentation across northern England everywhere kept pace with subsidence so that the depositional surface across both blocks and basins was at any time almost horizontal.

During the Dinantian and most of the Namurian, limestone and marine mudstone were deposited in maximum water depths of a few tens of metres; coals, seatearths and many of the sandstones represent emergence of a few metres. Lateral changes in lithofacies and stratal thickness point to recurrent syndepositional activity along basin margins and inherited fault lines such as the Closehouse–Lunedale fault system. From the late Namurian, significant marine influence was progressively lost over the entire region and subsidence and river-borne sedimentation were balanced, maintaining a stable delta-top environment through to the late Westphalian. Deposition was increasingly dominated by sand, silt and mud, carried into the region by large prograding river deltas draining a land area far to the north. With time, marine intervals became less frequent and of shorter duration. The likely changes in palaeogeography inherent in this situation are summarised in ( Figure 40).

The initial, rapid fault-controlled subsidence along the early Carboniferous rift axes, was locally accompanied by the eruption of basaltic lavas, now preserved in northern Cumbria and Northumberland, and along the southern margin of the Southern Uplands. Earliest Carboniferous (Tournaisian) sedimentary successions include variable thicknesses of unfossiliferous conglomerate and sandstone derived from erosion of the Late Devonian landscape. The lithofacies closely resemble that of the upper Old Red Sandstone in southern Scotland (Stratheden Group) where, in the Tweed Basin, there is locally a conformable passage from uppermost Devonian into lowermost Carboniferous strata.

Limestone deposition predominated during Visean times in much of south and west Cumbria, in Ravenstonedale and on the Alston and Askrigg blocks. Sedimentary environments ranged from shallow shelf seas to ramp and slope areas, though the central parts of the Alston and Lake District blocks were only partially and briefly submerged. Differential subsidence between the Alston, Cheviot and Southern Uplands blocks and the Tweed and Northumberland basins had been most active during the early part of the Tournaisian and reduced progressively from the Visean onwards.

In late Visean times, uplift of source areas to the north led to southwards progradation of a giant clastic delta complex that rapidly filled the basinal areas of northern England. The Cheviot Block was the first to lose structural independence in the Asbian, with deposition between the Northumberland and Tweed basins becoming uniform and continuous. At the same time, the Alston Block became more closely linked with the Northumberland Trough as the degree of differential subsidence across the Stublick–Ninety Fathom line gradually decreased. Thereafter, from the Brigantian onwards, a similar pattern of cyclic sedimentation developed throughout the region, although relatively thin Brigantian and Namurian successions over the Alston Block indicate that it was still subsiding more slowly than the adjacent Northumberland Trough. This situation persisted until the beginning of the Westphalian, but then uniform subsidence affected both block and basin until at least the Bolsovian (Westphalian C). The Westphalian D red beds of the Canonbie area are the youngest Carboniferous strata now preserved in northern England and their lithofacies shows establishment of a fluvial–terrestrial environment.

Climate

Palaeomagnetic and lithofacies evidence (the latter including worldwide facies distributions) independently suggest that Britain was situated in near-equatorial latitudes for much of the Carboniferous Period (Figure 3 c–d). In northern England, Tournaisian terrestrial strata laid down in small isolated basins include pedogenic horizons (cornstones) indicative of a semi-arid climate; the coeval offshore deposits preserved in the Northumberland Trough are assemblages of interbedded mudstone (some with halite and gypsum pseudomorphs), sandstone and argillaceous dolostone (‘cementstone’) deposited in a marginal marine environment subject to periodic desiccation. The discovery of thick, early Visean anhydrite beds in the Easton Borehole [NY 4412 7170] confirmed largely arid climatic conditions and shallow marine deposition. By the late Visean, Britain lay at the southern margin of the equatorial belt, but experienced fluctuations of climate with the possibility of monsoonal-type rains. Thereafter, during the later part of the Carboniferous, Britain moved into humid, equatorial latitudes, as confirmed by the extent of coal within the sedimentary sequence that accumulated. The end of the Westphalian saw a return to more arid conditions.

From late Visean times onward, the southern hemisphere experienced repeated phases of glaciation as its continental mass (Gondwana, by then the southern part of Pangaea — ( Figure 3)) drifted across the South Pole. The coldest intervals may have brought about short-term, seasonally drier climate farther north, whereas melting ice may have resulted in a wetter equatorial climate, and would certainly have caused a eustatic rise in sea level. It has been proposed, but not established, that glacial fluctuations in southern Gondwana, controlled sedimentary cyclicity elsewhere. Across northern Britain, at least, this glacial/eustatic influence would have interacted with other, more local tectonic effects. A factor in the southern Gondwana glaciation may have been the rapid Carboniferous rise in the atmospheric O2/CO2 ratio that was coincident with the proliferation of land plants. At around this time there was a marked evolutionary expansion of several groups of spore-bearing plants such as sphenopsids (horsetails), lycopsids (clubmosses) and filicopsids (ferns). Two groups of seed producing plant — cordaites and pteridosperms — also expanded, whilst the first conifers, cycads and bennettitales appeared in late Carboniferous times. Other side-effects of the increase in atmospheric oxygen were seen in the coal swamps: the evolutionary rise of large insects, and the high frequency of wild fires.

Sedimentary cyclicity

Sedimentary cyclicity is a feature of the Carboniferous successions. The phenomenon is particularly developed in parts of the upper Visean and lower Namurian successions of the Northumberland and Stainmore troughs and the Alston and Askrigg blocks, which are characterised by the ‘Yoredale facies’; an assemblage wherein each cyclothem is thicker, more extensive, and can be more widely correlated, than is the case for cyclothems elsewhere in the Carboniferous succession. Each Yoredale cyclothem has a limestone at the base, which is overlain sequentially by mudstone, sandstone, seatearth and coal ( Figure 41). The cycles have an average thickness of around 20 m, range up to a maximum of several hundred metres, and are generally thicker in the basins and thinner on the blocks. Their immediate cause was a marine transgression followed by a progressive shallowing and change to fluvial conditions with subsequent emergence and the growth of delta-top swamp vegetation; a series of events repeated many times. In terms of sequence stratigraphy, the limestone was deposited during the high-stand phase with the base representing the sequence boundary, coincident with the transgressive surface. Lowstand facies are represented by palaeosols and coal at the top of some of the cycles.

The origin of typical Yoredale cycles has been much discussed, with tectonic, eustatic and sedimentary mechanisms all proposed. Since broadly similar patterns of relative cycle thickness are found for the Yoredale successions in both block and basin localities, a control on deposition that affected the whole region seems likely and from this perspective glacioeustatic sea-level oscillations are attractive. However, the advance and migration of delta lobes, or local tectonism such as that associated with syndepositional fault movement, both of which mechanisms are independent of sea-level change, can also result in cyclical deposits of mudstone and sandstone. It is most likely that a complex interaction of all of these mechanisms resulted in the deposition of the distinctive Yoredale lithofacies and the many other examples of Carboniferous cyclothems.

Northern, ‘Dinantian’ successions

The early Carboniferous sedimentary succession of the Northumberland Trough is distinct from those seen across the rest of the region. For much of early Carboniferous times, the Northumberland Trough was a narrow gulf-like extension of the open sea, widening to the south-west and with marine influence decreasing towards the north and east. A corresponding reduction in stratigraphically useful fauna means that a detailed biostratigraphical correlation of the succession across the region is difficult, particularly towards the bottom of the sequence. The sedimentary lithostratigraphy reflects the interplay of fluviodelatic and shallow marine depositional systems. The emergent margins of the basin were sources of clastic sediment during the early period of deposition, but for much of Dinantian times axial drainage systems were dominant, building from the north and east towards a shallow sea in the west. Thereafter, marginal clastic deposition adjacent to the active North Solway fault system persisted in the Solway Basin. These variations are illustrated in the representative lithostratigraphical sections and correlations for the Tournaisian to Visean strata that are presented in ( Figure 42).

Inverclyde Group

Along the southern margin of the Southern Uplands Block, the lowest Carboniferous strata are of terrestrial and peritidal facies; they are very similar to the Inverclyde Group of the Midland Valley of Scotland. In recognition of this similarity, recent work has extended the group name and some component formation names into the northern part of the Northumberland Trough ( Figure 37) and ( Figure 38). The position of the Devonian–Carboniferous boundary is not clear, and along the northern margin of the trough, some rocks of Old Red Sandstone lithofacies are included within the Inverclyde Group. Amongst these are coarse conglomerates, developed locally at the base of the group, which now crop out along the flanks of the Cheviot massif and represent the oldest basin-fill deposits currently exposed: examples include the conglomerates at Roddam Dene, Ramshope Burn and Windy Gyle. They are all believed to be coeval with the Kinnesswood Formation of the Midland Valley of Scotland, and so are probably of post-Devonian, early Tournaisian age. The best exposed, at Roddam Dene, is a clast- to matrix-supported, massive, imbricate conglomerate containing subangular to rounded, pebble- to boulder-size clasts of Cheviot andesite with minor amounts of Lower Palaeozoic sedimentary rock and rare Cheviot granite. It is interpreted as the product of ephemeral streams that drained the deeply eroded margins of a Cheviot landmass during the semi-arid weathering conditions of early Carboniferous times.

Another small outcrop of strata reminiscent of the Kinneswood Formation lies on the south-west side of the Cheviot massif around Cottonshope Head [NT 801 061]. There, the Cottonshope basalt lavas (see below) are underlain and interbedded with a sequence of red or grey sandstones, dark mudstone with ochreous concretions, and thin bands of concretionary, carbonate-rich ‘cornstone’. This sedimentary assemblage has been traditionally described as the ‘Lower Freestone Beds’, but with the recent recognition of the cornstone lithology its association with the Kinneswood Formation, and therefore a Tournaisian age, seems probable.

The initial, fault-controlled subsidence of the Northumberland–Solway Trough was accompanied along its northern margin by extrusion of basalt lavas. On the northern flank of the Solway Basin (and cropping out mostly in Scotland) such lavas comprise the Birrenswark Volcanic Formation, and on the south side of the Solway Basin the Cockermouth Volcanic Formation. Farther east, the southernmost exposures of the Kelso Volcanic Formation (again with a mostly Scottish outcrop) impinge on the northern margin of the Northumberland Basin in a small area near Carham [NT 7985 3838], whilst the Cottonshope Volcanic Formation crops out on the south-west flank of the Cheviot massif. The volcanic lavas in the Birrenswark and Kelso formations are mostly composed of mildly alkaline basalt with rare hawaiite or mugearite; they are interbedded with red sandstone and with a few thin units of tuffaceous and volcaniclastic rocks. Up to about 90 m of lava comprise the Birrenswark Formation with up to about 120 m of lava forming the Kelso Formation, though in both cases the thickest part of the succession lies north of the border in southern Scotland. The Cottonshope Formation comprises up to three lava flows of tholeiitic, olivine-phyric basalt with a cumulative thickness of 24 m; a Tournaisian age is established for the lavas from their association with the subjacent Kinneswood Formation strata.

The Cockermouth Volcanic Formation is formally a component of the Ravenstonedale Group but can conveniently be considered here with the other Tournaisian volcanic formations. It consists of four to six lava flows with a cumulative thickness of about 100 m, and consists of tholeiitic, olivine-phyric basalt with subordinate andesite. No clastic rocks are preserved within the lava sequence, though outcrops of lapilli-tuff a little to the south, on Little Mell Fell, have been interpreted as the remains of an infilled vent conduit.

The lavas of the Birrenswark and Kelso volcanic formations interfinger with, and are conformably overlain by the Ballagan Formation ( Plate 33), a sequence of interbedded sandstone, mudstone and argillaceous dolostone (or ‘cementstone’); some of the mudstones contain halite and gypsum pseudomorphs whilst traces of anhydrite have been reported. The overall lithofacies is indicative of a lacustrine to lagoonal depositional environmnent with intermittent marine incursions. In the Tweed Basin, the Ballagan Formation attains a thickness of some 430 m and is conformable with the underlying Old Red Sandstone strata. It thence thins over the Cheviot Block, where it overlaps onto pre-existing topography around the eastern flank of the massif, but then abruptly thickens again southward and westward into the Northumberland Basin. A clastic sequence forms the highest part of the Ballagan Formation in the Langholm area of southern Scotland. This, the Whita Sandstone ( Figure 38), was introduced into the northern part of the Solway Basin from a provenance in the Southern Uplands of Scotland. Though its outcrop does not extend south of the border, it is probably equivalent to thin sandstone beds seen within the upper part of the Ballagan Formation farther south in northern England. Across its northern England outcrop, the Ballagan Formation passes conformably up into the Lyne Formation of the succeeding Border Group, though the transition is diachronous and at the regional scale the two formations are partly lateral equivalents.

Border Group

The Border Group is made up of the Lyne Formation and the Fell Sandstone Formation. The boundaries between the two formations are strongly diachronous, and though in the Bewcastle and Bellingham areas the Fell Sandstone Formation conformably overlies the Lyne Formation, at Brampton the two are lateral equivalents. In the north-east part of the Northumberland Trough, the Lyne Formation is missing and the Fell Sandstone rests unconformably on the Ballagan Formation ( Figure 38) and ( Figure 42).

In its fullest development, at outcrop, the Lyne Formation comprises cyclical sequences of fine-grained subarkosic sandstone, siltstone, mudstone and limestone. Traces of anhydrite are also present but, in the subsurface, the Easton Borehole proved that thick anhydrite deposits are interbedded through about 1300 m of clastic strata (the Easton Anhydrite Member) at about the level of the Bewcastle and Lynebank members. Deposition took place in a fluctuating network of peritidal, deltaic and fluvial environments subject to occasional marine incursions, with the anhydrite probably accumulating under sabkha conditions. The oldest Lyne Formation strata were most likely deposited during the late Tournaisian; fossil evidence establishes a Chadian age but the formational base is nowhere exposed and is most probably strongly diachronous. The Lyne Formation has a likely maximum thickness in excess of 1500 m, and may be very much thicker in the axial part of the Northumberland–Solway Trough. Most of its sandstones were deposited from lobate deltas that intermittently migrated from north-east to south-west along the axis of the developing basin. Limestone beds are generally quite thin, many originated as peritidal accumulations of ooidal pellets whilst some are algal or stromatolitic. The stratigraphically highest assemblage of limestone beds, forming the Cambeck Member, are shelly, algal limestones with an early Visean (possibly Arundian) fauna of bivalves, brachiopods and rare corals. The earliest marine limestones, with a restricted but abundant brachiopod fauna, first appear in the south-west part of the basin and only later did marine incursions extend to the north-east.

The Fell Sandstone Formation ( Plate 34) has an arcuate outcrop around the flanks of the Cheviot Massif from Burnmouth in south-east Scotland through Northumberland into the Brampton–Bewcastle areas of Cumbria, thence to Thirlstane on the north coast of the Solway Firth. It was laid down during a time of source area uplift when a fluvial depositional system advanced from the north-east into the Northumberland Trough. The source area involved was a topographic high to the north and east comprising Grampian and Fenno-Scandinavian structural blocks and their Devonian molasse basins. A continuous steady uplift of the source area provided a constant supply of submature clastic sediment, with complementary subsidence of the basin maintaining the fluviodeltaic depositional environment. Deposition was probably effected by several braided river systems, each occupying a belt several kilometres wide, and constrained by intrabasinal faulting to the axial region of the Northumberland Trough. Additional evidence for extensional faulting during deposition of the Fell Sandstone Formation is provided by the presence within it, on the northern side of the Solway Basin, of extrusive lavas — the Kershopefoot Basalt.

The earliest fluvial deposits accumulated in the north-east of the Northumberland Trough where the dominant subarkosic sandstones are interbedded with sparse red mudstones and seatearths, the latter only very rarely associated with thin coals. The fluvial sandstone passes westward and diachronously into a succession of fluviodeltaic and shallow marine deposits and, in the Bewcastle area, splits into at least two major sandstone units separated by finer-grained marine strata that are laterally continuous with the Cambeck Member of the Lyne Formation ( Figure 37). In contrast, towards the central part of the Northumberland Trough, the base of the Fell Sandstone Formation (defined by the Whitberry Marine Band) is conformable above the Lyne Formation ( Figure 38).

The Fell Sandstone Formation attains its maximum thickness of about 350 metres near Harbottle in Northumberland, where it consists almost entirely of sandstone. Elsewhere, the proportion of sandstone to mudstone varies considerably and boreholes show that in places the Fell Sandstone may contain up to 40 per cent of finer-grained lithologies. The diachronous base of the formation becomes generally younger towards the south-west: Chadian in north-east England where the Fell Sandstone directly overlies the Ballagan Formation, and variously Chadian to Holkerian farther south-west where the Fell Sandstone overlies the Lyne Formation ( Figure 38). The youngest Fell Sandstone Formation strata range up into the early Asbian. The succession is largely unfossiliferous, although ostracods and the large bivalve Archanodon jukesi (Bailey) have been recorded together with some plant fossils.

Southern, ‘Dinantian’ successions

A different lithostratigraphy from that described above from the Solway–Northumberland Trough is applied to the rock sequences deposited during Dinantian rifting to the south of the Maryport–Stublick–Ninety Fathom fault system, ( Figure 37) and ( Figure 42). The southern successions described in this section are peripheral to the Lake District and Isle of Man blocks, spread across the Alston Block, or form the lower part of the Stainmore Trough stratigraphy.

Ravenstonedale Group

The oldest strata of the Ravenstonedale Group, the Pinskey Gill Formation, crop out south of the River Lune between Tebay and Kirkby Stephen. There, 50 m of mudstone, fine-grained sandstone, and limestone-dolostone rest on a smooth marine planation surface cut across Silurian rocks and contain an impoverished Courceyan marine fauna. Deposition of the formation was apparently in a shallow embayment with restricted marine circulation at the western limit of the Stainmore Trough, where the beds were probably laid down in somewhat shallower-water conditions than pertained across the main part of the trough. The Pinskey Gill Formation is unique to this district, but shows similarity in stratigraphical position and lithology to the subsurface Raydale Dolostone Formation, which is known only from boreholes located farther to the east in the Stainmore Trough.

In scattered outcrops across Ravenstonedale, the Courceyan marine beds are overstepped by quartz-pebble conglomerate, lithic sandstone and mudstone, gypsiferous in places, which comprise what were formerly called the Basal or Basement Beds ( Plate 35). All of these occurrences are now brought together as the Marsett Formation in the north of England, and the equivalent Langness Conglomerate Formation in the Isle of Man. These formations encompass the first regionally extensive Carboniferous deposits, which originated as an accumulation of contemporaneous regolith, alluvial fan, river and marginal marine sediment. The two formations vary in thickness across the region. In some places only 10 to 30 m of strata are seen, which may be banked around local irregularities in the underlying syndepositional topography. Much thicker deposits, up to 500 m, occur in proximity to known, or suspected, early Carboniferous growth faults such as the North Solway Fault, the Kirkby/Foxfield Fault system north of the Duddon Estuary in Furness, the Butterknowle Fault on the West Pennine Escarpment and the Shag Rock Fault on the Isle of Man. Extensional faulting may also have been a controlling influence on eruption of the basaltic lavas that form the Cockermouth Volcanic Formation and lie near the base of the Marsett Formation on the south side of the Solway Basin. These lavas have been described earlier in association with the volcanic rocks that occupy a similar position in the Inverclyde Group.

The Marsett and Langness Conglomerate formations are imprecisely dated, but their palaeofloras indicates a Courceyan age in many areas. However, the sequences in west Cumbria and the Isle of Man contain internal disconformities and in their upper parts are interbedded with limestones that range in age from Chadian to Arundian, depending on location. Hence the sequences may represent accumulation, or cycles of erosion and re-accumulation, over a long period of time.

In Ravenstonedale, the Vale of Eden and south Cumbria, deposition of the Marsett Formation was ended by a late Chadian marine transgression, extending out from the Craven and Stainmore basins onto the southern margin of the Alston and Lake District blocks. Initially, conditions were supra-to peritidal and the lower parts of the Martin Limestone Formation in south and west Cumbria and the Stone Gill Limestone Formation in Ravenstonedale are largely composed of dolostone, algal limestone (both oncolites and stromatolites are present) and calcareous mudstone with bedding surfaces marked by desiccation features. In the Shap and south Cumbria areas, the marginal, shallow-water marine environment was maintained through to the end of the Chadian and the Shap Village Limestone Formation comprises dolostone, shelly ooidal limestone, cross-bedded calcareous sandstone and pebbly calcarenite. Mudstone seatearths occur in the lower part of the formation, whilst the top is defined by palaeosol developed from a breccia deposit.

The upper part of the Martin Limestone Formation was deposited in a broadly shallow-water marine environment and comprises partially dolomitised limestone, some of which is ooidal. Mudstone, sandstone and calcarenite are not present, and the lithological range is similar to that of the open marine, platform and ramp carbonates that typify the overlying Great Scar Limestone Group. Accordingly, in some accounts, the Martin Limestone Formation is included within the Great Scar Limestone Group. The top of the Martin Limestone Formation is prominently marked by a reddened palaeokarst zone.

Great Scar Limestone Group

The type area of the Great Scar Limestone Group ( Plate 36) is the Askrigg Block, where it accumulated as a platform sequence about 400 m thick and Arundian to Asbian in age. Regionally, the group name is used to unify all equivalent, locally named, thick carbonate platform successions of Dinantian and earliest Namurian age in the north of England ( Figure 38). The greatest exposed thickness is seen in Ravenstonedale where about 800 m of strata range from Chadian up to Asbian in age. The sequence in west Cumbria is a little thinner, at about 740 m, but ranges a little higher stratigraphically, from Chadian up to Pendleian (but with no evidence for Arundian rocks). In contrast, on the Alston Block only about 107 m of strata are present and are largely Asbian in age. In the south of the Isle of Man, the group is up to about 157 m thick and Arundian to Asbian in age. In the north of the island, approximately 7.3 m of strata at the bottom of the Ballavaarkish (Shellag North) Borehole [NX 460 007] are assigned to the Great Scar Limestone Group but may possibly be more closely related to the Yoredale Group; a broadly Dinantian or early Namurian age is the best biostratigraphical estimate that can be made from the poor evidence available.

Chadian to Arundian

The lowest formation in the Great Scar Limestone Group, forming the base of the sequence in the Stainmore Trough and extending north-west to just south of Shap, is the Coldbeck Limestone Formation. A dolostone with a characteristic abundance of algal structures (oncoliths) interbedded with mudstone, it was deposited on a west- to east-sloping marine shelf on which the characteristic Chadian coral Dorlodotia flourished. The conformably overlying Scandal Beck Formation comprises a succession of cyclically bedded bituminous limestone with thin siltstone interbeds.

At the end of the Chadian there was a general reversion to shallow water conditions and block areas suffered erosion. In the Stainmore Trough, where marine environments persisted until late in the Chadian, the Brownber Formation was deposited as a sequence dominantly made up of ooidal limestone, but interspersed with beds of calcareous sandstone and cross-bedded calcarenite. The sandstone beds are laterally discontinuous, due to channelised deposition, and contain distinctive layers of well-rounded pebbles of white quartz and calcite.

In the early Arundian, a fault-controlled acceleration in basin subsidence rates resulted in deeper marine conditions becoming re-established in the Stainmore Trough and across south Cumbria. At first, high-energy, shallow-marine conditions resulted in deposition of the cross-bedded ooidal limestone typical of the Red Hill Formation. Subsequently, water depths increased and the Dalton Formation of south Cumbria, and time equivalent Breakyneck Scar Limestone Formation of Ravenstonedale and the Stainmore Trough, are thick (200–300 m), marine shelf successions of dark grey, well-bedded crinoidal grainstone with siltstone interbeds.

The early Arundian deposition of peripheral shelf limestone was interrupted where rivers flowing from the north-east introduced terrigenous sediment, now represented around Edenside and Ravenstonedale by the Ashfell Sandstone Formation. It comprises the fluviodeltaic deposits of a river that flowed around and across the Alston Block, feeding a delta system within the Stainmore Trough. Within the upper part of the Dalton Formation in south Cumbria are sandstone beds that may have been introduced by the same fluvial system that deposited the Ashfell Sandstone farther to the east. A broad, regional link also seems likely with the approximately coeval fluvial system responsible for deposition of the Fell Sandstone Formation in the Northumberland–Solway Basin.

The early Arundian deposition of peripheral shelf limestones extended to the Castletown area in the southern part of the Isle of Man where a sequence about 90 m thick forms the Derbyhaven Formation. Three members are recognised. The lowest of these, the Turkeyland Member, consists of ooidal and bioclastic grainstone and has a sharply defined basal contact with the Langness Conglomerate. The middle, Sandwick Member, comprises dark, bioclastic packstone with an abundance of mudstone interbeds. At the top of the formation is the dark, pyritic packstone of the Skillicore Member, with an increasing proportion of interbedded fine-grained sandstone indicating a progressive shallowing of the depositional environment. The Turkeyland and Sandwick members correlate, respectively, with the Red Hill and Dalton formations of the south Cumbria succession. The Skillicore Member records a minor input of sandy sediment, though compositional differences make it unlikely to have been derived from the same source as the sands that formed similar interbeds in the Dalton Formation.

Holkerian to Pendleian

The Holkerian saw a significant extension of the shallow marine depositional environment onto the flanks of the Lake District and Alston blocks. Across north-west England the Ashfell Limestone and Frizington Limestone formations (and the equivalent Knockrushen Formation of the Isle of Man) are distinctive, dark grey, argillaceous limestone successions, cross- or tabular-bedded, with sandstone interbeds, especially towards the base. South of Penrith and into Ravenstonedale, the topmost beds record a marine regression and comprise a shallow marine to estuarine facies of cherty, porcellanous limestone, carbonaceous mudstone and ferruginous shell beds. On the Askrigg Block and in the Cockermouth area respectively, the Ashfell Limestone and Frizington Limestone formations are topped by thin fluviodeltaic intervals with a coal developed locally. In south Cumbria, the Holkerian Park Limestone Formation was deposited on a carbonate ramp with little terrestrial input, giving a lithologically uniform succession of pale grey, well-jointed, crinoidal grainstone, commonly crowded with robust gastropod and brachiopod shells.

The late Holkerian marine regression led to early Asbian emergence and erosion of the block and platform areas. However, later in the Asbian, the most extensive of the Dinantian marine transgressions re-established basinal or marine platform conditions across the entire region; even the Lake District Block was inundated at this time.

The character of the Asbian platform carbonate deposits is remarkably uniform across northern England and beyond. The limestones were formed on a sediment-starved marine carbonate platform, subject to a fluctuating relative sea level. This oscillation in water depth is expressed in a cycle of sediment type, with low-energy wackestone grading upwards into high-energy grainstone. Corals, mainly lithostrotionoids, occur commonly both fragmentally and as in situ colonies. During sea-level lowstands the exposed limestone surfaces were subject to karst development and a patchy form of diagenetic calcretisation, traditionally termed ‘pseudobreccia’. During more prolonged emergent intervals, the karst topography was wholly or partly infilled with claystone, formed from the accumulation of residual soils, decayed vegetation and subaerial deposits of volcanic ash. The Asbian limestones today form the most prominent of the scarps and upland plateaux of the north Pennines, and the spectacular glaciated limestone pavements of the Edenside and Kendal areas.

Platform carbonate rocks of early Asbian age comprise the Potts Beck Limestone Formation in Ravenstonedale and the lower part of the Urswick Limestone Formation in south Cumbria. The successions have a relatively shallow-water depositional aspect and siltstone–mudstone interbeds are common. Prominent amongst the latter is the ‘Woodbine Shale’, a 4–5 m thick bed of black pyritous mudstone located about 30 m above the base of the Urswick Limestone Formation; it forms a marked topographical break in many parts of south Cumbria and north Lancashire. A sandstone bed marks the top of the Potts Beck Limestone Formation above Orton Scar [NY 634 097], south of Appleby.

There was no early Asbian sedimentation on the Alston Block nor in west Cumbria west of the Bothel Fault, and in these areas, respectively, the Melmerby Scar Limestone Formation and the lower part of the Eskett Limestone Formation are condensed late Asbian successions. Thicker developments of lithologically identical rocks comprise the upper part of the Urswick Limestone Formation in south Cumbria and the Knipe Scar Limestone Formation of Ravenstonedale and Edenside. The Robinson Limestone Member, the highest component of the Melmerby Scar Formation (and also represented at the top of the Knipe Scar Formation) is separated from the limestones below by a prominent sandstone–mudstone interval at its base. In the Isle of Man, the upper Asbian Balladoole Formation comprises a platform carbonate succession equivalent to the Urswick Limestone Formation of south Cumbria. Locally however, the Balladoole Formation includes bioherms, indicating that the southern margin of the carbonate platform lay nearby.

In general, the youngest beds of the Great Scar Limestone Group are of Asbian age, and are succeeded by the uppermost Asbian to Brigantian strata of the Yoredale Group. An exception to this pattern is seen in west Cumbria where the Eskett Limestone Formation (Great Scar Limestone Group) continues up through the Brigantian and into the Pendleian as a dominantly limestone succession but with abundant thin interbeds of mudstone and sandstone. This upper part of the Eskett Limestone Formation is a time-equivalent deposit to much of the Alston Formation and is conformably overlain by sandstone — the ‘Hensingham Grit’ — at the base of the Stainmore Formation ( Figure 37).

Craven Group

The Craven Group is most fully developed in the Craven Basin area of Lancashire, to the south-west of the Craven Fault system at the margin of the Askrigg Block ( Figure 38), but its strata extend northwards and have limited outcrop on the Isle of Man. There, in the south, reactivation of the basin margin fault system caused the Knockrushen Formation to be succeeded by a 14 m-thick band of hemipelagic, cherty wackestone and mudstone. This band has a distinctive late Holkerian fauna and is the equivalent of the Hodderense Limestone Formation seen farther south in the main part of the Craven Basin. The succeeding Asbian to Brigantian succession in the Isle of Man and north Lancashire is largely of prodelta or basinal facies, with interbedding of black, hemipelagic claystone and limestone turbidites; it is accommodated in the Bowland Shale Formation. The Asbian, Balladoole Limestone Formation (Great Scar Limestone Group) was partly coeval with the Bowland Shale Formation and was subsequently overstepped by it. In the south of the Isle of Man, the lowermost Asbian part of the Bowland Shale Formation includes the Scarlett Point Member ( Plate 37), wherein hemipelagic claystone is interbedded with coarser-grained sediment, including large blocks (olistoliths) of reef carbonate, introduced by turbidity currents and submarine slides from the platform margin to the north. Higher in the Isle of Man succession, in the Brigantian part of the Bowland Shale Formation, submarine debris flows and slides of volcaniclastic rocks and basaltic pillow lavas form the Scarlett Volcanic Member ( Plate 38), and indicate that the basin margin was volcanically active at this time.

Yoredale Group, Visean to Namurian

Towards the end of the Visean, tectonic extension was replaced by a prolonged period of thermal relaxation and crustal sag, resulting in widespread marine transgression and the gradual submergence of the distinctive block-and-basin structure beneath an increasingly terrigenous sediment cover. Thereafter, during the Namurian and Westphalian, a broad region of subsidence developed across the north of England, between the Southern Uplands and the Wales–Brabant massifs, and is known as the Pennine Basin. Within this basin the cycles of Namurian sedimentation were still strongly influenced by the broad pattern of structural features established earlier in the Carboniferous, but this effect was diminished by the beginning of Westphalian times.

The Visean to Namurian Yoredale Group is characterised by repeated upward-coarsening sedimentary cycles (‘Yoredale’ cycles, ( Figure 41)) on a wide range of scales. It is divided, in upward sequence, into the Tyne Limestone, Alston and Stainmore formations; divisions are based largely on the relative abundance of the different rock types within cycles. The base of the group is diachronous and ranges from early Asbian in the Northumberland Trough and Solway Basin, to Brigantian on the Alston and Askrigg blocks, in the Stainmore Trough and in south Cumbria, and to Pendleian in West Cumbria.

The Asbian Tyne Limestone Formation is restricted to the Northumberland–Solway Trough where its presence accounts for the greater part of the diachroneity at the base of the Yoredale Group ( Figure 37) and ( Figure 38). At the end of the Asbian, the sediment-starved carbonate platforms of the Great Scar Limestone Group were overrun from the north by a regionally extensive river-delta system that experienced repeated marine incursions. Simultaneously, the margin of the Craven Basin advanced northwards, bringing deepwater conditions to the former carbonate shelf across the Furness and Cartmel peninsulas of south Cumbria and the River Keer valley in Lancashire. Across the Alston Block and in the Stainmore Trough, the Yoredale Group is divided into two formations, the mainly Brigantian Alston Formation and the Namurian Stainmore Formation; the lithological characteristics of the constituent limestones differ between the two formations and the division also reflects the decrease in limestone abundance in the latter relative to the former. At the boundary between the two formations the Great Limestone (formally, the topmost unit of the Alston Formation) represents the depositional response to a significant transgressive event that took place at the beginning of Namurian times, when marine conditions were established across a wide area of northern England and beyond.

In the Isle of Man and north Lancashire, the equivalent rocks to the Brigantian to Namurian part of the Yoredale Group are of prodelta and basinal facies and, as previously described, are accommodated in the Bowland Shale Formation, where mudstone dominates.

Tyne Limestone Formation

The Asbian, Tyne Limestone Formation displays variable Yoredale-type cyclicity, with the limestone and sandstone components absent from parts of the succession. Where present, the sandstones tend to thicken into troughs and half-grabens. A range of lithofacies is present, which includes strata originating in marine, shelf carbonate and deltaic environments, with some local lacustrine deposits; the shelf-carbonate assemblage is more fully developed farther south as part of the Great Scar Limestone Group ( Figure 37) and ( Figure 38). Much of the formation comprises upward-coarsening cycles, each overlying a thin but extensive bed of marine limestone. The limestone is succeeded in each sedimentary cycle by marine mudstone, which is commonly bioturbated, and then by sandstone topped in places by seatearth and a thin coal seam. These marine to deltaic, Yoredale-type cycles are best developed and up to about 400 m thick in the west of the region, in the Solway Basin and western part of the Northumberland Basin. Thence, in the lower part of the formation, there is a lateral eastward transition into a sequence of lacustrine-deltaic cycles of limestone, mudstone, and sandstone with thick coal seams, traditionally known in Northumberland and Berwickshire as the ‘Scremerston Coal Group’.

The ‘Scremerston Coal Group’ is generally about 300 m thick but this figure increases locally and reaches almost 2000 m around Falstone [NY 725 875] thanks to the effects of synsedimentary fault movement. The sequence comprises sedimentary cycles similar to those of the Tyne Limestone Formation in Cumbria, but with more terrigenous material and with thinner, more argillaceous limestones. Coals are thicker and more numerous than is the case in the Cumbrian succession; the thickest, at 2 m, is the Plashetts Coal which has been widely worked. The marine influence is re-asserted above the ‘Scremerston Coal Group’, with the sedimentary cycles comprising a limestone overlain by mudstone, which commonly shows evidence for storm-driven reworking of the sediment, then by a shallow-marine sandstone. Some of the cycles are topped by a terrestrial development of calcrete, seatearth and coal.

The base of the Tyne Limestone Formation is formally defined in the Solway Basin by a limestone known as The Clattering Band that contains a distinctive fauna of Lithostrotion corals and the brachiopod Semiplanus. Also occurring at the base of the formation, but restricted to the northern margin of the Solway Basin, is the Glencartholm Volcanic Member; up to 180 m of interbedded basaltic and trachytic, pyroclastic and volcaniclastic strata. The upper boundary of the Tyne Limestone Formation is conformable with the base of the Alston Formation, and is taken at the base of another distinctive limestone with algal features and a variety of local names: the Low Tipalt or Callant Limestone in the central part of the Northumberland Trough, the Watchlaw Limestone farther east, and the Peghorn, Askham or Hawes limestones on the Alston and Askrigg blocks.

Alston Formation

The Alston Formation crops out over wide areas of the Alston Block, Edenside and Northumberland. As currently defined, it contains Yoredale Group strata of mostly Brigantian age, which had previously been assigned to several locally defined groups ( Figure 38). Representative lithostratigraphical sections and correlations are summarised in ( Figure 43) with the lithofacies illustrated in ( Plate 39)a, b. The formation is up to about 250 m thick on the Alston block, but thickens considerably into the Northumberland and Stainmore troughs, reaching over 400 m in the former and exceeding 1000 m in the eastern part of the latter.

On the Alston and Askrigg blocks, the base of the Alston Formation is taken at the change from the platform limestone facies of the Great Scar Limestone Group, to cyclical, marine-deltaic Yoredale facies. In most areas this change occurs close below the Asbian– Brigantian boundary, coincident with a band containing Girvanella algal structures, within the equivalent, but variously named, Askham, Peghorn or Hawes limestones. From Coldbeck, through Edenside to Stainmore, and also in upper Teesdale, the base is marked by a laterally persistent, erosionally based sandstone unit, the Wintertarn Sandstone Member (equivalent to the Thorney Force Sandstone on the Askrigg Block). Across the axis of the Stainmore Trough, the Wintertarn Sandstone Member is in places underlain by a thin, but complete Yoredale cycle with the Birkdale Limestone at its base. In these areas the base of the Alston Formation is formally taken at the top of the Robinson Limestone Member of the Melmerby Scar Formation, described earlier in this chapter.

At the top of the Alston Formation lies the Great Limestone Member (= Main Limestone on the Askrigg Block, First Limestone of Cumbria, Dryburn Limestone of north Northumberland). The Great Limestone is of early Namurian age, since a basal Pendleian index fauna has been found in closely adjacent mudstone at several localities in south Northumberland, for example in Greenleighton Quarry [NZ 034 917]. The Great Limestone attains a maximum thickness of over 20 m in Weardale, on the north-east flank of the Alston Block. When traced northwards from there into the Northumberland Trough, the lower part of the member, comprising richly fossiliferous massive limestone beds, maintains a remarkable consistency in thickness of 4 to 4.5 m with up to three distinctive biostromes: the Chaetetes, Brunton and Frosterley bands, each with a rich fauna of corals and sponges (( Plate 32)e — Frosterley Marble). The upper part of the Great Limestone Member contains mudstone interbeds and passes gradually into a dominantly mudstone sequence as it extends northwards across the Northumberland Trough ( Plate 40). The Great Limestone is of economic importance as a host for mineral deposits (see Chapter 10) and the source of a variety of mineral products including limestone flux, ornamental stone and crushed rock for aggregate (see Chapter 12).

The Alston Block succession of the eponymous formation is some 220–250 m thick and consists of about ten main Yoredale cycles. Each of the cycles begins with a limestone bed, rarely exceeding 10 m in thickness, succeeded by a coarsening-upwards unit composed mainly of terrigenous mudstone, siltstone and sandstone. The major limestone units display a remarkable lateral uniformity in thickness, lithological character and fossil content. The intervening clastic beds tend to remain constant in overall thickness, but their lithological make- up is far more variable and some contain channel sandstone bodies. The channels may show significant erosion at their bases and the stratigraphically important limestone beds are cut out locally. Mudstone beds with a shelly marine fauna occur at some levels and may grade laterally into thin secondary limestone developments. Seatearths are recorded at the top and within some cycles and at least one seam of workable coal is recorded in some areas: for example, the high-quality Shilbottle Coal of Northumberland and the Reagill Coal of Edenside.

Limestone units within the Alston Formation ( Figure 43) are generally dark grey, but some are notably bituminous and almost black in colour. They show an overall upward change from massive, fine-grained wackestone with a notable algal component, to current-bedded crinoidal packstone and grainstone. Some limestones comprise a single bed, but others are made up of two or three beds separated by thin mudstone or sandstone intervals. When these composite members are traced across the region from south-west to north-east, the separation of the individual limestone beds increases as the mudstone-sandstone intervals thicken and develop as separate cyclothems. This has given rise to a confusing array of locally named subdivisions, many with ‘upper’ and ‘lower’ derivatives.

In north Lancashire, east of Morecambe Bay, the general Yoredale cyclicity is still evident, but the limestone beds were deposited in deeper water; they are fewer in number, thinner, often composed of crinoid debris, and grade locally into shelly mudstone. Chert is common, and some beds are ferruginous, such as the Undersett Limestone that develops a characteristic ochreous patina where weathered; its correlative on the Alston Block is the Four Fathom Limestone ( Figure 43).

North-west of Morecambe Bay, in Furness and Cartmel, borehole records show that the Pendleian strata have a north to south transition from shelf to basin lithofacies. The succession encountered at Holker and around the Duddon estuary retains the Yoredale cyclical pattern of the Alston Formation, but at Roosecote, Gleaston and south of the Cark Fault, it comprises uniform dark grey, cherty, argillaceous limestone and mudstone. Rocks of the latter character were previously included in the, now obsolete, Gleaston Formation, but in view of their deep water affinities, are now assigned to the Bowland Shale Formation of the Craven Group.

Stainmore Formation

The Stainmore Formation comprises a largely deltaic, cyclical succession of terrigenous mudstone, siltstone, coal (mostly of indifferent quality), and sandstone. Some of the sandstone occurs in channel-fill deposits, fining upwards and with erosional bases cross-cutting the underlying strata. Otherwise the succession is dominated by mudstone–siltstone–sandstone coarsening-up cycles, with beds of limestone mostly confined to the lower part. Very rarely, a mudstone bed contains Namurian Stage, index nautiloids, and though uncommon, these establish the occurrence of intermittent marine incursions. The sandstone lithology is generally fine- to medium-grained, micaceous and often carbonaceous, with cross- and ripple-bed forms; some coarser-grained, channel-fill sandstone bodies are locally present. Lithostratigraphical variations and correlations across northern England are summarised in ( Figure 44) with the lithofacies illustrated in ( Plate 39)c and d.

Traditionally, the general term ‘Millstone Grit’ has been used for the Namurian succession in the north of England, although it had long been apparent that north of the Askrigg Block the lithological assemblage is distinct from the thick development of coarse ‘gritstone’ and marine mudstone of the South Pennines and Derbyshire, type area for the Millstone Grit. Rather, across northern England, the northern succession displays ‘Yoredale-type’ cyclicity, albeit limestone beds are less prominent in many of the cycles in comparison with those of the Alston Formation beneath. The name ‘Stainmore Group’ has also been used as a lithostratigraphical designation for the Namurian sequence in northern England, and that term is now formalised as the Stainmore Formation; it also subsumes the coeval and lithologically similar Hensingham Group of west Cumbria.

The Stainmore Formation does not correspond exactly with the time span of the Namurian. The early Namurian Great Limestone Member, traditionally taken as the basal marker bed of the ‘Stainmore Group’, is now formally included in the underlying Alston Formation ( Figure 38). However, the top of the Stainmore Formation, taken at the base of the Subcrenatum Marine Band (the lowermost unit of the Pennine Coal Measures Group), coincides with the Namurian–Westphalian boundary. Throughout northern England (and also farther south), the middle part of the Namurian, from the late Arnsbergian to the Kinderscoutian, is only represented by either a thin sequence of strata, often only a few metres thick, or, in places, by a non-sequence. The reason for this regional hiatus in deposition is not clear.

The transition southwards from the Stainmore Formation into the characteristic Millstone Grit lithofacies (now formally defined as the Millstone Grit Group) is diachronous and irregular. The overall relationship shows large channel-fill sandstone bodies of ‘Millstone Grit’ lithofacies, encroaching northwards from the Askrigg Block into the more typical, mixed shelf-deltaic lithofacies assemblage of the Stainmore Formation in the Stainmore Trough. In south Cumbria, the Stainmore Formation is overstepped northwards by typical Millstone Grit Group sandstone beds, though in this area their precise stratigraphical position is indeterminate.

Although the cyclical sedimentation pattern of the Stainmore Formation rocks enables lithological correlation to be made across northern England, limestone and calcareous mudstone generally take the place of the dark, goniatite-bearing mudstone that allows the biostratigraphical zonation of the Millstone Grit Group farther south. In much of northern England, goniatites are rare and the majority of the remaining macrofossils, though abundant at many horizons, are not particularly diagnostic of stratigraphical position. Exceptions to this situation are seen in west Cumbria and Stainmore, where successions of mudstone beds containing a characteristic ammonoid fauna have been recorded; in the Stainmore outlier there are key sections in Mousegill Beck [NY 835 123] and other nearby streams. A prospect of much improved correlation is offered by recent (and continuing) palynological studies.

Northumberland, Durham and the Stainmore outlier

Across the Alston Block, about 290 m of Stainmore Formation, Yoredale-type strata conformably succeed the Great Limestone ( Plate 40). The lower part of this succession is dominated by mudstone, whereas the sandstone components are thicker in the upper part. Into the adjacent basins the succession thickens to about 500 m: northwards across the Stublick and Ninety Fathom faults into the Northumberland Trough, and southwards across the Lunedale and Butterknowle faults into the Stainmore Trough. Several Stainmore Formation coals have been worked in Northumberland, the Little Limestone Coal extensively so, and the Crag Coal near Hexham.

The most complete Namurian succession that has so far been biostratigraphically proved is contained in the Stainmore outlier, a narrow fault-block located at the intersection of the Dent and Pennine Faults, south-east of Brough. The outlier preserves 500 m of Namurian sandstone, mudstone and coal with up to 20 marine limestone bands some of which, such as the Upper and Lower Felltop Limestone, can be correlated westwards into other parts of Cumbria. Although there is not a full complement of index nautiloids, comparison of brachiopod and other faunas suggests that all but one of the Namurian faunal stages are represented at Stainmore. Coral remains are a notable feature of the Botany Limestone.

Coarse-grained sandstone bodies with ‘Millstone Grit’ characteristics — a strongly erosive base and channel-like morphology — are present at several levels within all of the Stainmore Formation succession, but do not necessarily correlate from area to area. They are probably relicts of the fluvial systems that fed southwards into the Millstone Grit deltas of the Askrigg Block and the central Pennine Basin — the Millstone Grit Group sensu stricto. In Northumberland such sand bodies include the Rothley and Shaftoe Grits of the Morpeth and Rothbury areas, and the Longhoughton Grits of the Alnwick area. In all of these cases there is evidence of contemporaneous tectonic control on sedimentation. A similar lithofacies dominates the upper part of the Namurian succession on the Alston Block, east from Alston. On the Askrigg Block, the base of the Millstone Grit Group is well defined, taken at the unconformable base of the Pendleian, Grassington Grit. This has a likely correlative, the Lower Howgate Edge Grit, which in the Stainmore Trough is interbedded with Namurian strata of the Stainmore Formation.

North and west Cumbria and the Isle of Man

In west Cumbria, the lower part of the Stainmore Formation succession contains fossiliferous mudstone, siltstone and limestone of Pendleian to Arnsbergian age. Diagnostic faunas were recovered from the BGS boreholes at Distington [NX 9967 2331] and Rowhall Farm [NY 0851 3664] near Maryport, and from commercial boreholes around Ullock [NY 0770 2400] but examples have also been found at outcrop near Hensingham. In the upper section of the Distington Borehole a Yeadonian fauna is present in sandstone beneath Westphalian Coal Measures, whilst at Rowhall Farm the Coal Measures rest directly on the Arnsbergian strata. Accordingly, and since no angular unconformity is apparent, a significant non-sequence is inferred between the strata of Pendleian to Arnsbergian age and those of Yeadonian age. Other historical accounts from both north and west Cumbria that record fossils of Marsdenian to Yeadonian age would seem to confirm the absence of middle Namurian strata. Farther afield, boreholes on the northern point of the Isle of Man similarly record a mudstone succession of Arnsbergian age followed by Yeadonian deltaic sandstone beds. This stratigraphical break seems larger than the Arnsbergian to Kinderscoutian hiatus recorded elsewhere in the north of England.

Six Yoredale-type cycles of Pendleian to Arnsbergian age can be identified in the Distington and Rowhall Farm boreholes. In the first cycle the Hensingham Grit, a prominent sandstone, 20 m thick and with large-scale cross-bedding, is interpreted as the deposit of an extensive river system that prograded from the north and terminated shelf carbonate deposition, here at the level of the First (= Great) Limestone. A similar, but un-named sandstone unit is seen in the River Lowther south of Penrith. These beds are laterally discontinuous and display cross-bedding that indicates a north to south depositional current. After this initial influx of sand, the supply of coarse-grained sediment diminished and a cyclic sequence of mainly fine-grained lithologies — mudstone, siltstone and coal — marks a change from marine to deltaic conditions; intermittent marine incursions are demonstrated by impure limestones and marine mudstones.

Across both north and west Cumbria, Pendleian and Arnsbergian limestone beds are only rarely more than 2 m thick and are commonly composed largely of crinoid debris; mudstones may contain either a marine or a brackish fauna. The Pendleian limestone beds are laterally persistent and bear the same names in most parts of their range; prominent are the Little Limestone and the Crow or Crag Limestone. However, it should be stressed that the names may apply to beds at different levels in different areas and that regional correlation remains tentative. Identifiable macrofossil remains recorded in the limestone and marine mudstone beds include brachiopods, bivalves, trilobites and gastropods, but corals are relatively rare. Age-specific nautiloids are rare, apart from the Arnsbergian index Tylonautilus nodiferus, reported from a number of localities such as the Snebro Gill Beds at Hensingham (west Cumbria) and a single instance of Anthracoceras glabrum in the Caldbeck area (north Cumbria).

South Cumbria and north Lancashire

The Namurian succession bordering Morecambe Bay is known only from boreholes in and north Furness and Cartmel and a poorly exposed faulted outlier in the Keer valley north of Carnforth. The Furness and Cartmel Namurian (previously called the Roosecote Mudstones) has a deep-water depositional character and comprises thick mudstone-siltstone units with thin sandstone interbeds. A Pendleian age is confirmed by the index goniatites Cravenoceras leion, C. malhamense and Eumorphoceras pseudobilingue, which are recorded from the boreholes. In the Keer valley, the succession is of thinly interbedded mudstone, siltstone and fine-grained sandstone. In both areas the sandstone beds are interpreted as deposits of turbidity currents flowing off a ‘Millstone Grit’ delta front advancing broadly from the east. All of these Namurian strata, including the Roosecote Mudstones, are coeval with the Stainmore Formation but are transitional into its lateral equivalents farther south. These Namurian sequences around Morecambe Bay are now generally assigned either to the Bowland Shale Formation of the Craven Group, where mudstone dominates, or to the Pendle Grit Formation of the Millstone Grit Group if substantial sandstone bodies are present.

Pennine Coal Measures Group Westphalian

Across most of northern England sedimentary deposition continued unbroken from the Namurian into the Westphalian. From the later part of the Namurian onwards, significant marine influence was progressively lost over the entire region and deposition was increasingly dominated by sand, silt and mud, carried into the region via large river deltas. These initially drained from a land area to the north or north-east but other provenances were active at different times and by the end of the Westphalian sediment input from the south and south-east was important. Subsidence balanced sedimentation so that a stable delta-top environment was maintained and the resulting Westphalian sequence is essentially continuous over large parts of the region. The Westphalian regional stage (formerly a series) is divided into four substages, originally identified as A to D, the three lowest having since been formalised as Langsettian, Duckmantian and Bolsovian; Westphalian D survives (Table 3 and ( Table 5).

The lithostratigraphical units of Westphalian age present in northern England comprise the Pennine Coal Measures Group and the Warwickshire Group. The Pennine Coal Measures are predominantly grey in colour, nonmarine and characterised by vertically stacked, coarsening-upward cycles commonly up to 15 m thick. Each cycle is composed, in upward sequence, of mudstone, siltstone and sandstone and is capped by a seatearth and coal, though coal forms only a minor part of the sequence. Clay ironstone occurs within some of the mudstones. The Warwickshire Group consists of interbedded mudstone, siltstone and sandstone similar to those of the Coal Measures but distinguished by the presence of primary red beds; the overall colour range is from red-brown to grey, and coal is rare. Secondary reddening of the Coal Measures and older Carboniferous strata is common where these rocks lie close below, or have been exhumed from below the basal Permian unconformity. The zone of reddening can extend 100 m or more below the unconformity and is accompanied by a general oxidation of the rock mineralogy. Farther south in the Pennine Basin, the Warwickshire Group ranges up into the Stephanian.

Westphalian strata were laid down over most (probably all) of the region, but the Variscan tectonics of the late Carboniferous and early Permian deformed them into gentle folds and produced extensive faulting. Uplifted areas were soon eroded, and Westphalian beds were preserved only in the downwarped areas that make up the present coalfields ( Figure 45). The separation of the coalfields is not an original feature, therefore, but a product of Variscan deformation. Subsequent, post-Variscan sedimentation in the region commenced in Permian times and produced an extensive, unconformable cover of Permo-Triassic strata, much of which has since been stripped away by further erosion. ‘Exposed’ coalfields are those where the Westphalian strata crop out at surface; ‘concealed’ coalfields are hidden beneath Permo-Triassic strata.

Westphalian Coal Measures crop out, principally, in the west of the region in the Cumbrian Coalfield, and in the east in the Northumberland and Durham Coalfield which, in Northumberland has some of the best coastal exposures of Westphalian Coal Measures anywhere in Britain. Smaller outliers of Westphalian strata occur around Canonbie on the north side of the Solway Basin and spanning the Anglo-Scottish border, in the Tyne valley along the line of the Stublick Fault, and in the Stainmore Trough ( Figure 45). Between the Cumbrian and Canonbie coalfields the Coal Measures are concealed beneath the Solway syncline, whence the subcrop continues from north Cumbria into the Vale of Eden.

At more than 1600 m, the thickness of Westphalian strata in the Solway Syncline is the maximum seen in northern England as described in this account. There, the Carboniferous rocks are commonly reddened to a depth of more than 100 m beneath the unconformable cover of Permo-Triassic desert sandstone. In the Northumberland and Durham Coalfield, about 850 m of Coal Measures strata are present in the axial zone of the Boldon Syncline, north-west of Sunderland, whilst offshore from Tynemouth the thickness is locally up to 830 m. The Coal Measures in the east and south-east of the Northumberland and Durham Coalfield are also concealed by Permian and Triassic rocks, but in general the zone of sub-Permian reddening is less well-developed east of the Pennines.

Deep mining in the Westphalian strata of the region has ceased since publication in 1971 of the previous edition of this British Regional Geology guide to Northern England; much of the final working was from collieries extending beneath the sea. Since then, new information has been acquired during development of sites for opencast coal extraction ( Plate 41), and much of the region has been geologically resurveyed to modern standards. This has enabled major advances in our understanding of the geological development of the coalfields. In addition, the application of sequence stratigraphy has brought a new approach to interpretations of the Westphalian succession onshore and improved its correlation with the offshore sequences, particularly that beneath the North Sea.

The depositional framework

The Westphalian coal-bearing strata of central and northern England were deposited in the Pennine Basin, by then a single province continuous with the north-west European paralic belt, from which it became separated by later, Variscan folding. The Pennine Basin was bordered by the Southern Uplands and associated smaller-scale landmasses such as the Cheviot Block to the north, and by the Wales–Brabant Massif to the south ( Figure 40). Deposition occurred in a widespread fluviolacustrine environment that developed on a low-lying and largely waterlogged plain subjected both to intervals of emergence and to intermittent marine transgressions that became less frequent with time. The succession throughout most of the Pennine Basin is complete.

The lowest Westphalian strata were deposited in a gently subsiding, lower delta-plain environment under the dominant influence of mainly fluvial delta systems. Southward progradation of this depositional system through time resulted in middle Westphalian strata being deposited in a more proximal, upper delta-plain environment dominated by river distributary channels and lacustrine deltas. Later in the Westphalian, a waning fluvial influence caused re-establishment of lower delta-plain conditions. The basin occupied an equatorial position and experienced a humid, tropical climate characterised by high precipitation rates. This combination of factors provided ideal conditions for the development of a high water table, poorly drained palaeosols, peat swamps and the eventual formation of coal seams. Overall, subsidence and sedimentation were able to keep pace with one another throughout deposition. Periodic emergence allowed development of the coal swamps but these were in turn buried by more river-borne sand and silt carried into the northern England sector of the Pennine Basin from the north and east.

During discrete marine incursions at periods of high global sea level, distinctive thin layers of sediment were deposited over wide areas. These marine bands contain characteristic fossil assemblages and have been the traditional means of stratigraphical correlation. Their modern integration with palynological data remains the primary means of correlation within and between coalfields in the Pennine Basin, and thence, from basin to basin, as far as eastern Europe. The marine bands have also allowed preliminary correlation of Westphalian strata onshore with those offshore beneath the North Sea. They represent geographically widespread, short-lived, marine flooding events and are fundamental to a proper understanding of the succession since they reflect the most significant changes in base level (relative sea level) across the entire alluvial plain. The main marine bands — Subcrenatum, Vanderbeckei and Aegiranum — are used to define the substages of the Westphalian ( Table 5).

An alternative means of long-distance correlation is provided by the beds of volcanic ash — now altered and known as tonsteins — that fell over the coal swamps. Tonstein beds are widely used in Europe for intra- and inter-basinal correlations, and their radiometric dating forms the basis of the modern geochronology for the late Carboniferous.

( Figure 46) illustrates the varied depositional environments within which the Pennine Coal Measures Group of northern England accumulated. There, and in most of the other coalfields throughout the Pennine Basin, the Coal Measures show a broad, threefold subdivision of lithofacies relative to stratigraphy:

The better-developed coals in the middle part of the succession, up to the Vanderbeckei Marine Band, probably reflect changes in relative sea level that occurred at an optimum pace for the initiation and long-term maintenance of coal swamps. The change from poorly developed to well-developed coals is relatively sharp in the Langsettian sequences preserved across the Pennine Basin coalfields of northern England. The subsequent Duckmantian change back to a coal-poor succession is less well defined.

Clastic lithologies

The cyclothemic nature of the Coal Measures has long been recognised and studied. At least 40 such cycles are known from the Pennine Coal Measures Group of County Durham with the main lithologies following one another in each cyclothem, in an ascending order of mudstone at the base (overlying the coal at the top of the underlying cycle), siltstone, sandstone, seatearth, coal ( Figure 41). The nature and origin of the cyclothems have been much discussed, and it appears that no single explanation will suffice. Such cyclicity is a natural reflection of the interplay of sedimentary processes, and the only external mechanism needed to produce them is continuous subsidence. However, it is generally accepted that the periodic changes in sea level leading to marine flooding events are related to global glacial events, in this case the late Carboniferous glaciation of southern Gondwana which at that time lay over the South Pole. Some activity along the Stublick–Ninety Fathom fault system continued during the Westphalian but, in contrast to circumstances earlier in the Carboniferous, contemporaneous fault activity is not considered to have been a major influence on sedimentary patterns. Only local fault-controlled effects have been proposed and these include deposition of the Langsettian of Cumbria, and the stacking of channel sandstones in the Durham Coal Measures.

The intercoal sequences were deposited during the gradual infilling of shallow interdistributary bays and lakes by shallow-water, crevasse-splay delta complexes ( Figure 46). They are interbedded with a number of prominent sandstone bodies that were deposited by the low-sinuosity, distributary channels feeding the crevasse-splay systems. These sandstone bodies can be stacked, one above another, to produce significant thicknesses; for example, in west Cumbria, a substantial sandstone thickness is developed where the Bannock Band Rock directly overlies the Main Band Rock.

Mudstones are generally well bedded and grey, but darker and more fissile when carbonaceous. Siderite commonly occurs in thin layers or as flattened nodules. Many of the mudstones contain a shelly fauna and this is usually indicative of a brackish-water depositional environment; marine mudstones are rare. Siltstones are also grey, commonly laminated and with a range of bioturbation structures. They grade imperceptibly into both mudstone and sandstone, the latter being mostly fine grained and quartzofeldspathic.

Seatearths, as preserved palaeosols, may be developed from any of the clastic lithologies and are gradational into the underlying facies. In contrast, the contact with any overlying coal seam is sharp. The seatearths contain abundant carbonaceous plant material (most commonly Stigmaria rootlets), and bright coaly stringers with disseminated pyrite. The rootlets increase in abundance upwards through the seatearth and disrupt any original lamination that might have been present. The seatearths typically break along irregular fracture surfaces that may be either covered with slickensides, or curved and polished. Some sandstone seatearths have been leached of feldspar and silicified to produce the distinctive hard beds called ganisters.

Coal

Across the low-lying, Langsettian to Duckmantian alluvial plain, coal swamps formed as mires developed on abandoned lacustrine crevasse-splay delta systems. Individual abandoned delta systems are thought to have been up to 10 km wide and 20 km long. The associated mires developed over prolonged periods of time and were not necessarily contemporaneous from one delta system to another. Lateral continuity and synchroneity would have been particularly unlikely along the more active marginal parts of the basin. Thus, variations in coal seam thickness across the alluvial plains are to be expected, although it should be stressed that most of the main coal seams, both in Cumbria and in Northumberland and Durham, can be widely correlated.

Mires develop under waterlogged conditions of rising base level and are able to maintain themselves for thousands of years in water depths of about 1 m, probably the optimum water depth for swamp growth. A lowered water table leads to oxidation and destruction of organic matter as the swamp environment gives way to better-drained conditions, no coals form, only overthickened palaeosols. A rise in water level allows the mire to be buried by clastic sediment. Peat-forming environments in the Westphalian of northern England ranged from low-lying and brackish mires to seasonally flooded forest swamps. The resulting coal seams range up to about 2 m in thickness and so a peat:coal compaction ratio in the order of 10:1 would indicate that peats were originally up to 20 m thick; autocompaction during peat growth would reduce that decompacted thickness.

The character and quality of a coal is determined by the conditions in the swamp in which it was formed and by subsequent burial history. Alluvial plain coals typically have low ash and sulphur contents, the latter indicative of acid conditions. The effects of temperature and pressure over long periods of geological time tend to expel water and volatile constituents from coal. Thus, a coal that has been subjected to elevated temperatures typically exhibits a low volatile content, a high carbon content and high calorific value; it is said to have a high ‘rank’. Conversely, coals that remain high in volatiles and have a relatively low carbon content and calorific value are said to be of low ‘rank’. A notable feature of the Northumberland and Durham Coalfield is the geographical variation in the rank of the coals ( Figure 45), with the highest rank found in west Durham but thence decreasing into other parts of the coalfield. No variation is apparent in the Cumbrian Coalfield’s broadly middle-ranking coals. The difference between the two coalfields arises from their contrasting histories of burial and geothermal heat-flow, the latter influenced by distance from the underlying, high heat-production Acadian granites of the North Pennine Batholith.

Fauna and flora

The Pennine Coal Measures Group of northern England contains an abundant and varied fossil fauna that includes both nonmarine and marine species. Nonmarine invertebrates include worms, gastropods, bivalves, eurypterids, crustaceans, insects and fish, whilst the marine faunas include foraminifera, worms, brachiopods, goniatites and conodonts ( Plate 32). The nonmarine bivalves are of particular importance for biostratigraphical purposes and are the basis of a widely used biozonal scheme ( Table 5). Nonmarine fossils are concentrated in the few metres of argillaceous strata that form the basal zone of the various cyclothems, but many cyclothems have no preserved fauna or contain only a small range of invertebrate fossils. Apart from fish remains, a small range of vertebrate fossils has also been recorded, with amphibian bones recovered from mudstone above the Northumberland Low Main Seam at Newsham in Northumberland. Marine fossils are naturally restricted to the marine bands. However, some marine incursions in the southern part of the Pennine Basin failed to reach the more proximal Northumberland and Durham Coalfield, and most of those that did penetrate to the far north of the basin affected that area for only a relatively brief interval. Accordingly, marine bands in northern England ( Table 5) tend to feature a less varied and abundant fauna than those farther south.

Remains of the coal swamp vegetation are common fossils in the Coal Measures. Masses of compressed plant material make up the coal seams, roots are found in situ in seatearths, drifted leaf fronds and plant stems occur in mudstone and siltstone beds, and chaotic ‘log-jams’ of broken tree trunks and branches are a feature of some of the thicker sandstones. Study of Westphalian floras in northern Britain indicates that the flora of the floodplains was dominated by pteridosperms with some ferns, sphenopsids and lycopods, and that of the peat-forming swamps by lycopods. Today, the lycopod group is only represented by low-growing plants, but during the Westphalian some lycopods were tree-sized, the most familiar being Lepidodendron, with its distinctive bark pattern of rhomboidal scales, and its Stigmaria root system. Calamites, a giant relative of the present-day horsetail Equisetum was also common and grew around lakes and on point bars, whilst a range of pteridosperms grew on the levées alongside meandering rivers.

Following significant studies in the first half of the 20th century, a set of plant biozones was established for the Westphalian. More recently palynology, the study of plant spores of various kinds, has become a standard biostratigraphical technique. It has the great advantage of requiring only a small rock sample as a source of many fossils, and been applied extensively in the correlation of borehole sequences of Westphalian strata from the North Sea and between the offshore and onshore basins. This has been an important development because, whilst correlations of the major Westphalian surfaces and marine bands are not in doubt, more precise local and regional correlations are hindered by the impersistent nature of Coal Measure facies which results in the lateral equivalence of major sandstone bodies and coals.

Westphalian stratigraphy

Stratigraphical summaries for the main coalfields, and the correlations between them, are shown in ( Figure 47).

Northumberland and Durham

The Westphalian Coal Measures succession of the Northumberland and Durham Coalfield accumulated along the northern margin of the Pennine Basin. It has more proximal characteristics, with a smaller marine influence, than are seen to the south in the classic coalfield successions of Lancashire and Yorkshire. Accordingly, fewer marine bands occur in Northumberland and Durham than in coalfields farther south, whilst those that are present are less well developed. In general terms, the Langsettian sequence of Northumberland and Durham has most in common with an upper delta-plain model, and the Duckmantian sequence with an alluvial-plain model.

The base of the Pennine Coal Measures Group in north-east England south of the River Tyne is placed at the Quarterburn Marine Band the inferred correlative of the Subcrenatum Marine Band the base of which defines the base of the Westphalian regionally ( Table 5). However, no definite occurrence of Gastrioceras subcrenatum has been found, nor has the Quarterburn Marine Band been proved north of the river Tyne. There, because the early Westphalian fauna is generally sparse, the base of the Pennine Coal Measures Group has to be taken immediately above the highest occurrence of marine, costate brachiopods. Similar problems exist with regard to the precise position of other boundaries — in Northumberland the Vanderbeckei Marine Band, the base of which marks the base of the Duckmantian (and also the base of the Pennine Middle Coal Measures Group), is indicated only by the presence of a marginal marine Lingula fauna, whereas a wholly marine fauna (though still lacking the diagnostic Vanderbeckei fauna) occurs at the same level farther south in Durham.

Pennine Lower Coal Measures Formation

In the Northumberland and Durham Coalfield, the base of the Pennine Coal Measures was traditionally defined by the lowest workable coal, either the Ganister Clay Coal or locally in Northumberland the Brockwell Coal. These coals are now known to lie some distance above the Quarterburn Marine Band, with the intervening strata mainly consisting of sandstone with only thin and impersistent mudstone or coal interbeds. This is essentially a continuation of the lithofacies seen in the underlying Stainmore Formation of the Yoredale Group and it has, in the past, been referred to as ‘Millstone Grit’. Above the Ganister Clay Coal, the Pennine Lower Coal Measures contain a higher proportion of mudstone and coal, with the thicker, more productive seams in the higher part of the succession, commencing with the Brockwell Coal. Below this level the sandstones are coarse grained with some sufficiently siliceous to be termed ganisters. Above the Brockwell Coal the sandstone beds are thinner and finer grained but the increasingly argillaceous Pennine Lower Coal Measures sequence still comprises at least 50 per cent sandstone; five or six significant coal seams are also present. The Pennine Lower Coal Measures here comprise about 220 m of strata.

Pennine Middle Coal Measures Formation

The Duckmantian to Bolsovian, Pennine Middle Coal Measures comprise up to about 500 m of strata in the Northumberland and Durham Coalfield. The base is defined by the base of the Harvey Marine Band, the local correlative of the Vanderbeckei Marine Band. The Middle Coal Measures contain most of the workable coals, particularly in the lower section of about 180mup tothe High Main Coal. This is the thickest and most widely worked seam over much of the coalfield; it contains excellent quality coal, is locally over 2.5 m in thickness, and formed the original basis for large scale exploitation of the Northumberland and Durham Coalfield. Several marine bands also occur, but like the Pennine Lower Coal Measures, more than 50 per cent of the succession is made up of sandstone. Above the High Main Coal some of these sandstones are coarse grained and massive: examples include the High Main Post (overlying the eponymous coal) and the Seventy Fathom Post, which was worked for grindstones.

Pennine Upper Coal Measures Formation

The base of the Pennine Upper Coal Measures is taken at the base of the Down Hill Marine Band, the local correlative of the more widely recognised Cambriense Marine Band. It is preserved in the Sunderland area, where it is overlain by about 150 m of poorly exposed strata in the much-faulted Boldon Syncline. The main features of this sequence are the predominance of grey argillaceous strata, the presence of only a few thin coals and the general sparseness of the nonmarine bivalve faunas. Nearby, the presence of Pennine Upper Coal Measures strata concealed beneath Permian to the east and north of Sunderland has been inferred on structural grounds, but no stratigraphical information is available. North of the River Tyne, near Killingworth, on the northern (downthrow) side of the Ninety Fathom Fault, the presence of some 155 m of Upper Coal Measures is again inferred entirely on structural grounds by analogy with the Sunderland district.

The Tyne valley outliers

A series of small faulted outliers of Westphalian strata occur along the Tyne valley, forming the Midgeholme, Plenmeller and Stublick coalfields. The outliers are mostly elongated east–west and consist of southward-dipping Coal Measures; they are terminated abruptly to the south against the Stublick–Ninety Fathom fault system. Opencast coal investigations since the 1980s have improved the stratigraphical correlations with nearby areas of the Northumberland and Durham Coalfield, based largely on lithology, augmented by sparse palaeontological data ( Figure 47).

A cumulative thickness of about 200 m is represented through the three inliers, mostly from the Pennine Lower Coal Measures, but extending upwards into the lowermost Middle Coal Measures. Most of the stratigraphically useful horizons lie within the Lower Coal Measures ( Figure 47). The Low Main Sandstone and its equivalents can be traced throughout the outliers, and provides a useful lithostratigraphical marker. The presence of the Victoria Shell Bed above the Slag Coal helps to fix the position of the coal and the underlying sandstone-dominated interval. A marine fauna from the Plenmeller West opencast site is believed to be from the equivalent of the Gubeon (Listeri) Marine Band, which places the horizon some 100 m below the Harvey (Vanderbeckei) Marine Band, the boundary between the Pennine Lower and Middle Coal Measures.

Stainmore outlier

Over 300 m of Westphalian Coal Measures strata are preserved in two small areas in the Stainmore outlier, a narrow fault-block located at the intersection of the Dent and Pennine faults, south-east of Brough. They conformably succeed Namurian strata, and dip steeply eastward to be terminated against faults. The succession spans the Pennine Lower and Middle Coal Measures and is closely comparable, both in thickness and lithology, with the equivalent strata seen in the Durham Coalfield, 25 km to the north-east ( Figure 47). A correlative of the Subcrenatum Marine Band marks the base of the Pennine Lower Coal Measures sequence, whereas the Vanderbeckei Marine Band is replaced by a nonmarine shell bed. Towards the top of the succession there is a well-constrained correlation with the Bensham coal seam of the Pennine Middle Coal Measures in Durham.

Cumbria

The Cumbrian Coalfield ( Figure 45), with its numerous productive coal seams, crops out in a broad arc around the west and north of the Lake District from Egremont in the south­west to Caldbeck in the east. Concealed deposits occur below a cover of Permian and Triassic rocks in the Vale of Eden and north of the Maryport Fault, but have not proved to be economically exploitable. Farther north, the Canonbie Coalfield is situated at the northern margin of the north-north-east trending Solway Syncline and seismic reflection data confirm that the Pennine Coal Measures of Cumbria and Canonbie meet beneath this major structure. Westphalian strata also extend offshore under the Irish Sea and have been encountered in boreholes north of the Isle of Man. Some of the most productive seams were worked from sub-sea collieries with pit-head installations located on the coast at Whitehaven and Harrington.

Working of the Pennine Coal Measures in the Cumbrian Coalfield has always been hampered by a closely spaced set of north-west-trending post-Carboniferous faults. These faults are usually steeply inclined and rapidly switch throw along strike. Also present are a set of north-east-trending, large-throw, low-angled faults which are more widely spaced (5– 10 km); they may represent a postdepositional reactivation of an original set of basin-margin growth faults. The Pennine Lower Coal Measures have a relatively uniform thickness across the coalfield, but in west Cumbria, the Pennine Middle Coal Measures show an overall thickening from south-east to north-west, suggesting that by then the Lake District Block was a positive, though not necessarily emergent, area.

Unlike the situation in Northumberland, the position of the Subcrenatum Marine Band, and hence the base of the Westphalian, is well established from boreholes throughout the Cumbrian Coalfield. In contrast, the Vanderbeckei Marine Band is poorly developed in the Cumbrian Coalfield and its position can only be inferred from the presence of shells or fish remains. The Aegiranum Marine Band and the slightly lower Haughton Marine Band are recorded from the Whitehaven coastal collieries and to the north of Distington. However, over most of the coalfield the Aegiranum and higher marine bands appear to be cut out below the unconformable base of the Whitehaven Sandstone, which itself is no older than the Cambriense Marine Band.

Pennine Lower Coal Measures Formation

In the Cumbrian Coalfield, the Pennine Lower Coal Measures comprise three to four coarsening-upward cycles, each culminating in a thick bed of fluvial sandstone. Coals, though few in number, are laterally persistent. They may occur singly or in groups of two or three clustered immediately below the base of a major sandstone unit.

The lowest 30 m of the Pennine Middle Coal Measures succession are fine grained, with a number of locally prominent Lingula bands. The lowest coal seam, the Harrington Four Foot, is recognised across the whole area. It is located a few metres above the Subcrenatum Marine Band, and immediately below the Harrington Four Foot Rock. This is a major sandstone unit, often the only stratigraphical marker for the base of the Coal Measures in old borehole records. To the north, the Harrington Four Foot Rock grades to siltstone and a set of thin coals known collectively as the Albrighton coals appear. More generally, the succession above the Harrington Rock includes coal seams of variable thickness and quality: the Upper and Lower Threequarters seams, the Wythemoor ‘Parrot’ Seam, and the Micklam Fireclay Seam.

Higher in the succession, the next cycle contains the Sixquarters Seam and, immediately overlying it, the Sixquarters Rock sandstone. This seam is one of the prime economic coals of west Cumbria, both onshore and offshore, whilst the sandstone is a regionally prominent channel sand body that has been worked as a building stone. Above the sandstone a mainly argillaceous succession contains, in upward sequence, the Lickbank, the Eighteen Inch and the Little Main seams. The Little Main has been a popular target for mining; the lower two coals are of lesser interest but increase in thickness and quality offshore.

Pennine Middle Coal Measures Formation

The Vanderbeckei Marine Band, usually represented by a Lingula shell band in west Cumbria, appears 15 metres above the Little Main seam, but does not mark a change in general lithofacies. In the lower part of the formation, mussel bands are developed locally and a number of coals are present. The coals are mainly of indifferent quality, except for the Yard Seam (locally known as the Metal Band), which was worked throughout the coalfield. Above the Yard Seam, the Pennine Middle Coal Measures display a more pronounced cyclicity, with quite thin units of mudstone with coal separating thicker (25–35 m) sandstone-prone intervals. This section contains the two thickest and most widely exploited coal seams of the area: the Main (Cumbria) Band and the Bannock Band. The two associated sandstone units, the Main Band Rock and the Bannock Band Rock, are relatively thick and have erosive bases so that, in places, the underlying coal seam is cut out.

Continuing upwards through the Pennine Middle Coal Measures, the next 50–70 m are mainly argillaceous but contain three major coal seams: the Tenquarters, Slaty and White Metal. In the north-west of the coalfield the lowest seam has an overlying sandstone unit, the Tenquarters Rock, that forms a prominent cliff along the coast, north of Parton. Above the White Metal seam a major sandstone unit is present called the Countess Sandstone from the disused pit of that name. This sandstone forms a 20-m cliff on the coast from Parton south to Whitehaven and is encountered in boreholes throughout the region.

The Countess Sandstone marks the upper limit of major workable seams in the Whitehaven area. Between the top of this sandstone and the base of the Whitehaven Sandstone Formation (discussed below) is an interval subject to marked lateral variation. A number of thin coal seams are present, and the Black Metal and Brassy seams, are identified. Several, however, have no local name, or obvious regional correlative. In the Whitehaven collieries and from Workington northwards, this interval contains a series of mudstone/sandstone cycles, with thin coals, and the presence of the Aegiranum Marine Band indicates the upward passage from Duckmantian into Bolsovian strata.

Pennine Upper Coal Measures Formation

The St Helens Marine Band, recorded from the northern part of the coalfield, is correlated with the Cambriense Marine Band and so some 70 m of overlying strata are classed as Pennine Upper Coal Measures. The rocks are largely sandstone and mudstone, mostly reddened to some degree, with a few thin coals and claystone seatearths.

Canonbie

The Canonbie Coalfield is situated at the northern margin of the north-north-east-trending Solway Syncline, beneath which it links with the Cumbrian Coalfield. It is likely that the individual coal seams of Cumbria and Canonbie persist across this structure and although correlation has not been proved by deep boreholes, it is supported by seismic reflection results. The limited area of exposed Coal Measures strata around Canonbie lies mainly to the north of the border in Scotland ( Figure 45). Interpretation based on boreholes and seismic surveys carried out in the 1980s indicated that the concealed coalfield to the south was larger and had more potential than was previously envisaged. In common with the other coalfields of the region, the Subcrenatum Marine Band has not been found in the Canonbie Coalfield. However, the tentative correlation of a marine band near the base of the Becklees Borehole [NY 3517 7158] with the Templeman’s (Langley) Marine Band of the Cumbrian Coalfield has enabled a generalised stratigraphical succession to be established. The Pennine Lower Coal Measures succession is about 120 m in thickness, the Middle Coal Measures is about 230 m and Upper Coal Measures is about 170 m thick. Coals are present in the Lower Coal Measures but are commonly unnamed and are difficult to correlate. Borehole information indicates that they are generally less than 0.8 m in thickness. The main seams are from the Pennine Middle Coal Measures, principally seven of Duckmantian age ( Figure 47). There are only a few thin coals present in the Upper Coal Measures, most of them being located close to the base. The exception is the aptly named High Coal, which occurs about 170 m above the base of the Pennine Upper Coal Measures and has been proposed as a convenient marker for the base of the overlying Warwickshire Group.

Warwickshire Group

The Pennine Coal Measures Group in the Cumbrian and Canonbie coalfields is overlain by red-bed successions that, together with other, similar late Carboniferous sequences elsewhere in Britain, make up the Warwickshire Group ( Figure 47). Primary reddening, occurring soon after sediment deposition, is a key characteristic but is not uniformly developed throughout, so that in general red beds alternate with unreddened intervals. This phenomenon introduces local uncertainty as to the exact age of the Warwickshire Group strata and their stratigraphical relationship to apparently coeval, but unreddened rocks in the Pennine Coal Measures Group.

Cumbria

In Cumbria, a red-bed succession at least 300 m thick forms the Whitehaven Sandstone Formation. The lithology and sedimentological character distinguish it from the underlying Pennine Coal Measures Group, and its base changes stratigraphical level with respect to coal seam and marine band marker horizons therein.

The most prominent part of the succession is the 100 m or more of pink to red sandstone that forms coastal cliffs at Whitehaven and can be seen in small quarries and natural exposures over much of the central Cumbrian Coalfield. Also included in the formation are 200 m of more lithologically varied, finer-grained red-beds that have been described from two boreholes, at Frizington Hall [NY 019 171] and Millyeat [NY 023 178], and also form small exposures in the valley of the Dub Beck [NY 022 175]. This succession, the Millyeat Member, contains thin interbeds of Spirorbis limestone, coals, mudstone and sandstone. With the exception of these thin limestone beds, the Whitehaven Sandstone Formation in west Cumbria is all but barren of fossil material. Nonspecific plant remains have been reported, along with the occurrence of the zonal bivalves, Anthraconauta phillipsi and A. tenuis. This assemblage suggests, but does not prove, a Bolsovian or Westphalian D age for the upper part of the formation. In the northern sector of the Cumbrian Coalfield, a Westphalian D, Tenuis Zone bivalve fauna, is found in similar, but more fossiliferous, strata in the Cockermouth area.

The lower part of the Whitehaven Sandstone Formation was laid down in a major braided river system that flowed across the area from the north-east. It carried a voluminous and coarse-grained sediment load of different character to that found in Pennine Coal Measures sandstones. Between the fluvial channels were limited areas of coal swamp, showing that the environment was not, at that stage an arid one. Later, the major river system either switched to another location or the sediment supply became restricted; minor river channels continued to deposit laterally impersistent sandstone beds but deposition of fine-grained sediment in interdistributary or lacustrine environments became dominant. Coal-forming conditions occasionally developed, but a change to a drier climate is indicated by the presence of the Spirorbis limestone beds, which formed in shallow and well-oxygenated lakes, possibly brackish due to high evaporation rates.

The Whitehaven Sandstone Formation is faulted against unreddened strata of the Pennine Coal Measures Group, suggesting that reddening was a relatively early process. It seems most likely to have occurred during permeability-controlled, late Carboniferous diagenesis. Whether the process began during a syndepositional change to an arid, oxygenating environment or was accomplished during postdepositional Variscan uplift, remains an open question.

Canonbie

At outcrop, about 290 m of the Warwickshire Group red beds are almost continuously exposed along the banks of the River Esk, but the maximum proved thickness is up to about 530 m, which occurs in the Becklees Borehole close to the central axis of the Solway Syncline. Seismic reflection data indicate that elsewhere in the centre of the syncline the group could be up to about 700 m thick.

Three formations have been recognised within the group, each with distinctive geophysical log signatures that allow them to be readily correlated in the subsurface. The lowermost, Eskbank Wood Formation ranges in thickness between 145 and 175 m. The base of the formation is diachronous, marked by the repeated alternations of grey and primary red-bed strata. Where no core or core descriptions exist, it is difficult to determine the position of this change and it is suggested that the base is taken at the top of the High Coal, which forms a prominent marker horizon. The formation comprises red mudstone, with some fine-to medium-grained sandstone, calcrete palaeosols, thin beds of Spirorbis limestone and Estheria-bearing mudstone. The overlying Canonbie Bridge Sandstone Formation ranges in thickness from 131 to 154 m. The base of the formation is sharp, marked by the incoming of thick units of medium-and coarse-grained cross-bedded channel sandstone. A noticeable feature of these sandstones is their greenish grey colouration, which can be related to the presence of abundant lithic grains. The Becklees Sandstone Formation is the highest unit recognised from the Warwickshire Group of Canonbie and is overlain unconformably by Permian strata. Its full thickness is not known, but up to 200 m are proved in the Becklees Borehole. This fine-grained sandstone has a distinct orange-brown colouration.

Warwickshire Group sedimentation in the Canonbie area largely took place on a well-drained alluvial plain, and was characterised by an early, primary oxidation of the strata. Large braided river systems were common features of the alluvial plain, with palaeocurrent data from the Canonbie Bridge Sandstone showing that the rivers flowed towards the north. Overbank and floodplain mud was deposited between the river channels, where soils were able to form during intervals with low rates of aggradation. The youngest Warwickshire Group strata seen in the Canonbie Coalfield are probably typical of what covered most of northern England prior to late Carboniferous folding and uplift.

Chapter 7 Late Carboniferous to early Permian deformation and magmatism

By Stephanian times, the thermal subsidence that had controlled sedimentation patterns during the Namurian and Westphalian Regional Stages had all but ceased. Far to the east, the Ural Ocean had closed whilst to the south, Gondwana had collided with Laurussia; the supercontinent of Pangaea was the result. The Laurussia–Gondwana collision generated the Variscan Orogeny with a mountain fold belt produced as its culmination. The approximate northern limit of the fold belt extends from the mainland of Europe across southern England and Wales and into Ireland, and is referred to as the Variscan Front. In Stephanian times, northern England lay in the foreland region to the north of the deformation front in an equational latitude ( Figure 3).

In northern England, deformation and magmatism took place during an interval of approximately 15 Ma that is now represented by the major unconformity between upper Carboniferous and lower Permian strata. Although sedimentation in the region had probably continued into the deformation phase, with the developing structures influencing depositional thicknesses, little evidence of the resulting rocks has been preserved. However, studies of conodont colouration (as an indicator of burial depth and consequent heating) suggest that up to about 2000 m of upper Carboniferous strata were removed at this time from north Northumberland. Although tectonic activity in the foreland region was much weaker than in the fold belt to the south some folds were formed, whilst reactivation of preexisting, long-lived faults caused the inversion of the Carboniferous basins. Field evidence suggests that the style of the deformation was profoundly influenced by the distribution of established fault-bound blocks with their granite cores, and the region’s position above the Iapetus Suture Zone.

Traditionally, the late Carboniferous tectonic event across northern England was attributed to east–west shortening, a model based on fold orientations and attributed generally to lateral ‘extrusion’ of fault-bound basement blocks under north–south compression. Associated strike-slip movements arose from reactivation of pre-existing faults that were orientated obliquely to the imposed stress regime. North–south extension towards the end of the tectonic episode then allowed large volumes of tholeiitic basaltic magma to ascend east–west faults and intrude the sedimentary succession to form the Whin Sill-swarm and its associated dykes. Though the intrusions cut late Carboniferous structures in places, elsewhere magma appears to have been emplaced during deformation. This contrast has led to alternative explanations involving wholly separate compression and extension events. Most recently, the contrasting styles of late Carboniferous deformation seen in the Northumberland Basin and adjacent Cheviot Block, have been explained by a single event involving oblique dextral extension. In this model, existing structures within the Acadian basement partitioned strain, allowing compressive structures to develop in some places whilst the region as a whole experienced extension. The extensional regime permitted huge volumes of tholeiitic magma, generated through mantle melting, to invade the sedimentary sequences not only in northern England and the Midland Valley of Scotland, but also across much of north-west Europe.

Late Carboniferous structures

Syn-Variscan tectonism in northern England — the peripheral effects of the orogenic deformation farther south — was dominated by reactivation of faults that had already played major roles in the earlier development of the region. Thus, apportioning fault movements to particular events can be difficult. The clearest evidence for syn-Variscan deformation comes from structures that affect Carboniferous rocks but are truncated by the unconformity at the base of the overlying Permian strata. The principal structural features that date from this episode are summarised in ( Figure 48).

Solway Basin

Seismic reflection profiles across the Solway Basin have been interpreted to show Carboniferous rocks folded into the north-north-easterly trending Solway Syncline and the complementary Carlisle and Bewcastle anticlines; the folds are truncated by the unconformity at the base of the Permian sequence ( Figure 49). The Bewcastle Anticline is asymmetrical with a steeply dipping north-west-facing limb. The fold is cut obliquely by the Goat Island–Lyne Fault, which is inclined south-east but throws down to the north-west; like many similar structures it was formed by reverse reactivation of an earlier synsedimentary normal fault. The eastern margin of the Carlisle Anticline is marked by the Brackenhill Fault, a westerly dipping reverse fault which also shows a significant strike-slip component in its late Carboniferous movements. It was subsequently reactivated as a normal fault during Permian extension.

At the southern margin of the basin, reactivation of the Maryport Fault was accompanied by regional uplift in the hanging wall, which led to substantial erosion and the formation of the unconformity at the base of the Permian succession. As with many of the long-lived faults in northern England, further reactivation of this fault continued to play an important role in controlling the ensuing Permian sedimentation.

Northumberland Basin and Cheviot Block

The Northumberland Basin lacks structures on the scale and intensity of those seen in the Solway Basin. Several small-scale anticlines are associated with the hanging-wall blocks of the Stublick and Ninety Fathom faults at the southern margin of the basin. The distribution of these suggests dextral transpression. In the centre of the basin, the east-north-east-trending Antonstown and Sweethope faults are early Carboniferous normal faults that have opposing inclinations: strata within the hanging walls formed a gentle anticline, probably during late Carboniferous times. A wealth of smaller-scale structures including faults and folds is particularly well displayed on the Northumberland coast ( Plate 42). The relatively simple fault pattern comprises west- to west-south-west-trending conjugate sets of normal dip-slip faults parallel to the major basin-bounding structures. There are also subordinate, separate sets of wrench faults parallel to the normal faults.

Over the Cheviot Block, the southern boundary of which approximates to the Swindon and related faults, structure within the Carboniferous rocks is dominated by abundant east­north-east-trending dextral, and subordinate east-south-east-trending sinistral, strike-slip faults. Oblique to these are two large asymmetrical anticlines with steep, west-facing limbs that are cut by high-angle reverse faults. The Holborn Anticline and its associated Hetton Fault trend north-north-west, and the Lemmington Anticline and the Bolton Fault trend north-north-east; each anticline is approximately 18 km long and 2.5 to 5 km wide, and forms a broad periclinal dome. Farther north, and in contrast to the structures described above, the north-north-westerly trending Berwick Monocline faces east. This structure can be traced for at least 14 km before it gradually dies out southwards, but north of Berwick-upon-Tweed the steep limb is a high angle reverse fault, juxtaposing Carboniferous rocks against Silurian and Old Red Sandstone strata. Again, these features are inferred to arise from reverse reactivation of syndepositional faults.

Vale of Eden and Alston Block

At the eastern margin of the Vale of Eden, the north-west-trending Pennine Fault System has a long, complex history of reactivation and at outcrop it forms a dextral strike-slip duplex. Syn-Variscan movements include formation of an easterly facing monocline that was breached by several westerly dipping reverse faults. Carboniferous rocks in the Vale of Eden form a broad shallow syncline with axial trace orientated a little anticlockwise from the Pennine Fault System; the fold is truncated by the fault system. Subsequent Permian extension on the Pennine Fault System influenced contemporaneous sedimentation within the Vale of Eden half-graben. On the Alston Block, the almost north–south Burtreeford Disturbance is, along its southern part, another east-facing, faulted monocline. This lies on the western flank of the Weardale Pluton, and appears either to predate, or to be contemporaneous with the earliest Permian emplacement of the Great Whin Sill.

Over the centre of the Alston Block, Dinantian and Namurian strata form a gentle, open periclinal fold with an approximately west–east axis, referred to as the Teesdale Dome. This structure may have been initiated during late Carboniferous times, but doming could equally have occurred much later, during Cenozoic uplift. To the east in Durham, Coal Measures strata form a broad, irregular south-east-trending and plunging structure referred to as the Boldon Syncline. Upper Coal Measures strata are preserved in the axial region near Sunderland. Overlying Permian rocks are unaffected by this structure confirming that deformation occurred in late Carboniferous and early Permian times. Offshore, the Vane Tempest Structure is an east-facing north-north-westerly trending asymmetrical anticline.

In south Durham the west–east orientated Trimdon Anticline and its complementary syncline to the south affect Carboniferous rocks, but much of the Middle Coal Measures was eroded from the axial region of the structure prior to deposition of Permian strata in the area. The anticline is asymmetrical with a steep northern limb and developed in the hanging wall of the reactivated Butterknowle Fault, which marks the southern margin of the Alston Block.

Dent Fault System

At the western margin of the Askrigg Block, a complex set of folds and faults known as the Dent Fault System has the form of a positive flower structure in cross-section and a contractional strike-slip duplex in map view. Within the Lower Palaeozoic basement, a precursor of the Dent Fault was reactivated, forcing development of an easterly facing, north-north-east-trending monocline in the Carboniferous cover rocks. The periclinal Taythes Anticline, west of the steep limb of the monocline, formed as a result of interference with earlier, Acadian folds. Rupture of the steep limb of the monocline then produced the westerly dipping Dent Fault, the hanging wall of which was fractured by near-vertical faults ( Figure 50). These structures are linked kinematically through north-north-west to east-south-east shortening during syn-Variscan sinistral transpression.

Early Permian magmatism

The tholeiitic basaltic intrusions emplaced in northern England during earliest Permian times comprise the Whin Sill-swarm and east-north-east-trending dykes of the Northern England Tholeiitic Dyke-swarm. These intrusions have played an important role in the early development of geological science in Great Britain. In northern England, the word ‘sill’ was used to describe a flat-lying layer of rock, and ‘whin’ meant hard. Hence, the term ‘Whin Sill’ may have been in use long before the origin of the rock was known and is most likely the first geological use of the word ‘sill’. The Great Whin Sill was recognised to be of igneous origin early in the nineteenth century, but there was debate as to whether it was an intrusive sheet or a lava flow. Its intrusive origin was finally established through investigations in Northumberland, and the Great Whin Sill became regarded as the type example of a sill.

Whin Sill-swarm

The Whin Sill-swarm underlies about 4500 km2 of northern England, extending from the southernmost outcrops in Lunedale, west as far as the Pennine escarpment, and north to Holy Island ( Figure 51). Four separate sills have been identified from the outcrop distribution, magma-flow directions, and differences in petrology and palaeomagnetic signatures. These are the Farne Islands and Alnwick sills in the north, the Great Whin Sill, which crops out in Teesdale, the Pennine escarpment and along the Roman Wall, and the Little Whin Sill in Weardale. The volume of magma intruded was at least 215 km3 and possibly much more, as the sills appear to thicken and extend eastwards for some distance under the North Sea.

The sill-swarm is composed of quartz-dolerite and this tough rock typically forms picturesque crags and scarps that dominate the landscape of parts of north-east England. In Northumberland, the Farne Islands are almost entirely made of dolerite. North of the Tyne valley, the Romans built an important segment of Hadrian’s Wall along the impressive dolerite scarp that runs for a distance of 25 km. In upper Teesdale, the Great Whin Sill forms waterfalls on the River Tees at High Force [NY 880 284] and at Cauldron Snout [NY 814 286], where the river cascades spectacularly over columnar-jointed crags. Columnar cooling joints are well developed in many other places, for example at Sewingshields Crags [NY 800 700], Longhoughton Quarry [NU 231 153], and Cullernose Point [NU 259 221] ( Plate 43); Low Force [NY 903 277] is a series of rapids where the river flows over columnar-jointed dolerite. Both sills and dykes provide solid foundations for castles at Dunstanburgh, Bamburgh and Holy Island.

The Farne Islands and Alnwick sills were emplaced within Carboniferous rocks overlying the Cheviot Block. From the Farne Islands and Budle Bay to the Kyloe Hills, the Farne Islands Sill is up to about 30 m thick and was intruded into five horizons that range from the upper part of the Fell Sandstone Formation, in the north of the outcrop, to the Eelwell Limestone within the Alston Formation on the Farne Islands. To the south of this, in Beadnell Bay, the horizon of the Alnwick Sill is some 120 m higher in the stratigraphy within the lower part of the Stainmore Formation. The sill’s horizon thence falls stratigraphically southwards to lie at a level within the Tyne Limestone Formation at its most southerly outcrop near Newton Burn [NU 145 068]. The Alnwick Sill varies from about 6 to 21 m thick.

The Great Whin Sill is on average about 30 m thick. The thickest single leaf has been recorded in borings in West Allendale (81 m) and Weardale (90 m, but this was near a small horizon change and may be anomalous). The sill tends to thin towards its northern, western and southern margins ( Figure 52). Along the Roman Wall, the thickness of the sill varies from 20 to 50 m. Multiple sheets occur at outcrop only east of Thockrington [NY 970 800], though in boreholes more than one sheet has been recorded at depth: for example, three dolerite sheets recorded in the Harton Borehole [NZ 3966 6563] total 90 m. In upper Teesdale, the Great Whin Sill intrudes its lowest stratigraphical level within the Melmerby Scar Limestone (for lithostratigraphy see ( Figure 43)). From here, it thins and rises in stratigraphical level in every direction, forming a ‘saucer-shaped’ intrusion. This form is also demonstrated along the sill’s northern outcrop where its level changes systematically from the Oxford Limestone north-eastwards into lowest Namurian strata and westward to its highest stratigraphical level within the Westphalian B (Duckmantian) Coal Measures in the Midgeholme coalfield. The presence of the sill in Coal Measures strata here suggests that before intrusion these rocks had been juxtaposed against Dinantian strata by displacement on the Stublick Fault. The changes in stratigraphical level occur in transgressive steps, separated by significant distances where the intrusion maintains a constant level. In places, the sill transgressions are fault controlled, for example between Steel Rigg and Sewingshields Crags [NY 751 676] to [NY 813 704], but elsewhere the sill rises either gradually or by short steps.

At outcrop near Stanhope in Weardale, the Little Whin Sill is a flat-lying, columnar-jointed dolerite sheet, up to 13 m thick, intruded into the Three Yard Limestone of the Alston Formation. The sill thins westwards and dies out near Ludwell. To the north, in the Rookhope Borehole [NY 937 427], some 6 km north-west of Weardale, the Little Whin Sill is about 2 m thick and it has been encountered north of the River Wear in mines at Stotfield Burn [NY 943 424] and Stanhope Burn [NY 987 413]. In the Rookhope Borehole, the Little Whin Sill lies stratigraphically about 120 m above the Great Whin Sill. In the Woodland Borehole [NZ 091 277], some 15 km to the south-east of Stanhope, the Little Whin Sill is encountered 20 m above the Three Yard Limestone, but it is not present at outcrop in upper Teesdale or in boreholes drilled to this level around Crook to the east.

Sill contacts are sharp though irregular in places, implying that the host rocks were lithified prior to intrusion. Locally, for example at Barrasford Quarry [NY 910 742] in Northumberland, narrow dyke-like masses protrude from the upper surface of the Great Whin Sill into the overlying strata. More widely in the Northumberland section of the Great Whin Sill, blocks of the underlying sedimentary rock have been levered up into the sill during intrusion and now form xenoliths close to its base. Similarly detached rafts of host strata are seen within the body of the Alnwick Sill, for example near Cullernose and at Longhoughton [NU 231 153], but, by contrast, xenoliths are rare in the north Pennines part of the Great Whin Sill intrusion. Between Budle Point and Harkess Rocks, the relationship between the Farne Islands Sill and the sedimentary country rock is extraordinarily complex, with numerous fragments and blocks of sandstone occurring within the sill. Here, the sedimentary rocks were probably disrupted immediately prior to intrusion by precursory hydroclastic activity.

Vesicles are present in the marginal zones of the Great Whin Sill along its northern outcrop, though none have been recorded from Teesdale. Of particular interest, however, are flattened amygdales up to 1.5 m long and 60 cm wide, close to the upper contact of the Farne Islands Sill, best exposed at Harkess Rocks [NU 177 356]. The inner surface of the gas bubbles is revealed where the amygdale has been removed by later erosion. The lower surface of these vesicles has a tachylitic margin up to 2 mm thick and this displays parabaloid ropy flow-structures, similar in miniature to the surface of pahoehoe lava ( Plate 44). The linings of the large vesicles must have remained plastic for long enough to allow flattening, elongation and the development of flow structures by the still molten magma moving through the intrusion. These are thought to be rare features of sills. Below this zone are further zones of abundant smaller vesicles, along with narrow zones of peperitic breccia.

The maximum thermal metamorphic effect on the country rock is seen in upper Teesdale, where limestones are recrystallised for over 30 m from the contact and mudstones are spotted for almost 40 m. The metamorphic effect of the three sheets of dolerite in the Harton Borehole can be detected in rocks at distances of 425 m above and 180 m below the sills. Generally, the rank of coal increases dramatically towards an intrusion: vitrinite reflectance increases, the texture changes and ultimately the coal becomes a natural coke. Relatively pure limestones, such as the Melmerby Scar Limestone, are recrystallised with a saccharoidal texture, and known from the characteristic weathering as ‘sugar limestone’. This rock produces distinctive soils that support a relict arctic alpine flora, including the spring gentian, Gentiana verna, on Cronkley and Widdybank fells, for which the area is renowned.

Impure limestone and calcareous mudstone have been metamorphosed into calc-silicate rocks containing such minerals as garnet, vesuvianite, diopside, feldspar, chlorite, epidote and rare wollastonite. Close to the contact, typically dark mudstones become light-coloured, hard porcellanous rocks known as ‘whetstones’; farther away they develop spots, mostly of chlorite, quartz and illite, but sporadically also with andalusite and cordierite. In several places, layers of pyrite nodules within country rocks close to the margins of sills and dykes have been altered to pyrrhotite, for example at Barrasford Quarry and Wynch Bridge. The presence in the contact zone and in rafts of sedimentary rock of wollastonite and vesuvianite in particular, indicate temperatures estimated to be as high as 720°C. Multiple episodes of metamorphism around upper Weardale have been cited as evidence for emplacement of the Little and Great Whin Sill magmas at separate times.

In the Alston Block, mineral veins of the Northern Pennine Orefield cut both the Whin sills and their associated dykes. The dolerite acts as a competent wallrock, like limestone and massive sandstone, and hence is a favourable host for mineralisation (see Chapter 10). An unusual occurrence of magnetite and niccolite-bearing skarn mineralisation at Lady’s Rake Mine in upper Teesdale has been cited as evidence of the interaction between north Pennine mineralising fluids and Whin Sill contact rocks whilst the latter were still hot.

Northern England Tholeiitic Dyke-swarm

Basalt and dolerite dykes associated with the Whin Sill-swarm are typically 3–10 m wide and have north-east to east-north-east trends similar to the structural grain in the underlying Lower Palaeozoic basement. The dykes produce pronounced magnetic anomalies. occur in four, widely separated subswarms, three of which could be regarded essentially as discontinuous single dykes with en echelon offsets ( Figure 51). Some authors have used the term ‘echelon’ rather than ‘subswarm’.

The dextrally offset northern dykes, belonging to the Holy Island Subswarm, lie at the northern margin of the Cheviot Block. The subswarm has similar palaeomagnetic characteristics to the Farne Islands Sill. South of the Cheviot Hills, the High Green Subswarm can be traced for over 80 km, converging slightly on the Holy Island Subswarm to cross the coastline at Boulmer. Its segments are offset sinistrally and some are up to 65 m wide. This subswarm was emplaced within the Swindon and Cragend–Chartners faults, a zone of east-north-east-trending structures that mark the southern margin of the Cheviot Block. Offsets on the St Oswald’s Chapel Subswarm are broadly sinistral and the subswarm includes the Haltwhistle, Erring Burn, Bavington and Causey Park dykes, the last of which can be traced for some distance offshore from Druridge Bay. Near Hexham, this subswarm swings to a more north-easterly trend, converging on the High Green Subswarm. The High Green and St Oswald’s Chapel dyke subswarms have similar palaeomagnetic signatures to the Alnwick Sill. Finally, near the southern limit of the Great Whin Sill exposures, the Hett Subswarm comprises several dykes to the south of Durham, including the Ludworth Dyke; palaeomagnetic evidence links this subswarm with the Great Whin Sill. On Holy Island [NU 123 416] to [NU 149 419], the Holy Island Dyke is a complex mass which has several short sill-like sectors. Exposures of these exhibit gently dipping planar chilled upper surfaces. Parallel to these are zones of large vesicles in which the skin exhibits ropy flow-structures very similar to those described towards the top of the Farne Islands Sill at Harkess Rocks. A detailed magnetic survey suggests that the intrusion steps upwards through the stratigraphy both to the north and to the east ( Figure 53). Examples such as this strengthen the thesis that the dykes are feeders to the Whin Sill-swarm.

Petrology

The quartz-dolerite of the sills and dykes is composed of labradorite, subophitic augite and iron-titanium oxides with an intersertal, and commonly micropegmatitic, intergrowth of quartz and alkali feldspar. Minor constituents include hypersthene or pigeonite, hornblende, biotite, apatite and pyrite. Fresh olivine has been found only in the Little Whin Sill in the Rookhope Borehole and at Turn Wheel Linn, but pseudomorphs after this mineral have been found in at least some of the other sills and dykes. The chilled margins and many dykes contain some intersertal, pale brown, microlitic glass locally with skeletal ilmenite, but Ca-poor pyroxene is absent.

Grain size in the sills typically increases from the tachylitic or very fine-grained margins to medium grained in the centre. Where the sill is more than about 50 m thick, a pegmatitic zone may be developed about one third of the way down from the top, for example in upper Teesdale and in the Rookhope Borehole. Thus, this facies is rare in Northumberland where the sills are generally thinner than 50 m, though it is exhibited in Keepershield Quarry [NY 896 727], where the sill is 36 m thick. Pegmatitic patches and veins are characterised by clusters of long, feathery augite crystals and intergrown quartz and alkali feldspar. Ca-poor pyroxenes are absent from the pegmatitic areas and iron-titanium oxides are rare, but biotite and hornblende are important minor constituents. Patches and veins of pink aplitic, fine-grained, quartzo-feldspathic aggregates with almost square phenocrysts of sodic plagioclase are also common in Northumberland, for example in Barrasford Quarry, at Cullernose Point and Dunstanburgh. Segregations such as these are typical final products of differentiation seen in the upper parts of many thick sills. Veins and irregular masses of fine-grained basalt, presumably from later pulses of magma, have been recorded locally in both sills and dykes, for example at Swinburne Quarry [NY 947 765]. The percentage of microphenocrysts increases towards the centre of the Great Whin Sill and differences in trace element geochemistry between the chilled margin and the sill interior imply that there was more than one pulse of magma.

The petrographical affinities are dominantly tholeiitic, but geochemically, the sills and dykes are transitional between alkaline and tholeiitic. The Great Whin Sill shows a very slight trend towards iron enrichment, in contrast to the Little Whin Sill which is geochemically homogeneous. It has been suggested from geochemical and mineralogical evidence that the Little Whin Sill may be an early differentiate from the Whin magma. However, the iron-rich nature of the Little Whin Sill and the presence of some resorbed calcic plagioclase crystals suggest that it had already undergone some differentiation prior to its emplacement.

Close to contacts, fault-planes, mineral veins or coal seams, dolerite and basalt may be altered to a pale cream or yellowish brown rock, referred to as ‘white whin’. This is composed of quartz, illite, kaolinite, muscovite, rutile, anatase and carbonates and was formed by the interaction between dolerite and hydrothermal solutions, probably of juvenile origin. Where white whin occurs close to the sill contact, adjacent mudstones show a marked increase in Na2O and the development of abundant albite, suggesting that sodametasomatism has occurred.

In addition to the zones of white whin, a suite of late-stage hydrothermal minerals has commonly developed in joints and vesicles during the final stages of cooling. Quartz-calcite­chlorite veins are abundant locally throughout the Whin Sill-swarm, with smaller amounts of chlorite, bowlingite, sericite, stevensite, albite, anatase and titanite also present. Joint-surface veneers and late-stage veins contain abundant pectolite along with analcime, apophyllite, chabazite, prehnite, stilbite and rare datolite.

Age

In the Midgeholme Coalfield, the Great Whin Sill is intruded into Coal Measures and in the Durham Coalfield, dykes of the Hett Subswarm emplaced within the Coal Measures do not pass up into Lower Permian strata. Near Appleby in the Vale of Eden, pebbles of dolerite are known from Lower Permian breccia (‘brockram’). Thus, the intrusions were emplaced and exposed to erosion during the time interval represented by the unconformity between the upper Carboniferous (Duckmantian–Bolsovian) and lower Permian strata of the region. The age is reinforced by radiometric dates including a K-Ar whole-rock age of 301 ± 6 Ma, a U-Pb baddelyite (ZrO2) date of 297.4 ± 0.4 Ma, both determined from the Great Whin Sill, and an Ar-Ar plagioclase date of 294 ± 2 Ma from the Holy Island Dyke. The Ar-Ar age is younger than the U-Pb date and may support the view that the Farne Islands Sill and Holy Island Dyke Subswarm may have been emplaced later than the Great Whin Sill. Recent palaeomagnetic studies of all the sills and dykes revealed virtual geomagnetic poles that are consistent with the magmatism having occurred in earliest Permian times when the ancient geomagnetic field was reversed.

Emplacement mechanism

The dyke subswarms tend to occur at the margins of the main intrusions within the sill-swarm and it has long been suggested that the sills were fed by the dykes. Similarities in geochemistry and palaeomagnetic signature support this relationship, but a direct connection between them has not been proved in the field. Several examples of basaltic dykes cutting the Great Whin Sill have been cited as evidence that the dyke swarms may represent a slightly later event but, even so, the relationship of the dykes to the sills has been explained in a single emplacement model.

To explain the dykes and sills as the products of a single intrusive phase ( Figure 54), it is envisaged that basaltic magma rose along the dykes at the outer margins of the sedimentary basins until it reached hydrostatic equilibrium. The magma then gravitated downwards into the lower, central parts of the basin succession where it accumulated to form an overall saucer-shape intrusion. On the opposite side of the basin, the magma then advanced up dip under the head of pressure, so that here the outer parts of the intrusion tend to be thin and steeper than bedding, pinching out as they approach the surface. This process should be reflected by magma-flow directions in the dykes and sills, determined from features such as fingers and tongues extending from contacts and from the large vesicles in the Farne Islands Sill and Holy Island Dyke. A study of the alignment of magnetic grains in the rock indicates a more complex pattern of magma flow than is suggested by the model, though the flow direction is predominantly north away from the Hett and Holy Island dykes, and southwards at the southern limit of the Farne Islands Sill at Harkess Rocks.

The large vesicles seen in the Farne Islands Sill and Holy Island Subswarm may have formed by repeated localised pressure falls caused by fluid-induced fracturing of the country rock as the tip of the intrusion advanced. Alternatively, they may have formed by rapid decompression during injection of the magma into the sedimentary pile close to the land surface of the time; substantial uplift and erosion in this region before emplacement is implied by this second mechanism. It has been estimated that the Great Whin Sill took about 60 years to crystallise completely, whilst the Little Whin Sill may have taken only one and a half to two years.

Chapter 8 Permian, Triassic and Jurassic: deserts, rivers and shallow seas

The major continental collision that drove the Variscan Orogeny during late Carboniferous and early Permian times created the Pangaean supercontinent. Within this landmass, Britain lay in a tropical latitude, approximately 10° north of the equator, and drifted slowly northwards to subtropical latitudes of approximately 30° north by early Triassic times (Figure 3d). The depositional environments included widespread deserts, tropical and evaporitic seas, fluvial outwash plains, ephemeral lakes and mudflats.

Erosion of the folded and uplifted Carboniferous strata had generated mature, gently rolling plains across which spread an early Permian desert. By late Permian times, continental extension had opened seaways, flooding low ground across large inland drainage basins. On the western edge of northern England, the Bakevellia Sea developed, covering approximately the area of the present-day Irish Sea and its marginal areas. To the east, the Zechstein Sea covered approximately the area of the present-day North Sea and extended as far to the east as Lithuania and Poland. Early Triassic times saw continental deposition restored over northern England. Large river systems transported sands from the south and aeolian dune fields developed. Further marine transgression in mid to late Triassic times, the result of expansion of the Tethys Ocean from southern Europe, then brought coastal conditions to northern England; open marine conditions followed in Jurassic times.

Today, Permian strata are preserved in northern England to the north-west of the Pennines, around Carlisle, in the Vale of Eden and in west Cumbria, and to the east of the Pennines, over much of County Durham. Permian strata also occur at the north-east end of the Isle of Man but are there entirely obscured by thick superficial deposits. Triassic strata have a similar though more restricted onshore outcrop. In northern England, Jurassic strata are only known from the vicinity of Carlisle; those that succeed the Triassic succession near Middlesbrough fall into another regional district — that of East Yorkshire and Lincolnshire. It is probable that Permian and Triassic rocks once partially covered the Carboniferous strata of the north Pennines, northern Cumbria and Northumberland, and that Jurassic rocks also had a much wider original distribution.

Early Permian

In northern England, and in response to the Variscan Orogeny further south, compressive uplift of the Carboniferous basins continued from late Carboniferous times into early Permian times. As a consequence, the early part of the Permian Period saw considerable erosion and denudation of Carboniferous strata, culminating in a locally irregular, but regionally peneplanar, surface known today as the Permian Unconformity. Extensive oxidisation of pyrite and siderite within the exposed Carboniferous strata resulted in its ubiquitous reddening for several metres below the unconformity. Descriptions of ‘brecciated’ mudstone and porcellanous textures in old borehole journals from the coalfield areas may well indicate the presence of calcrete soils developed on the exhumed early Permian land surface. An upland area, approximately coincident with the present-day Pennines, separated lowlands to the east from those in the west.

West of the Pennines: the Appleby Group

To the west of the ‘proto-Pennines’, east–west orientated extension reactivated large fault structures in the underlying Carboniferous strata, generating a series of isolated rift basins that developed as major centres of Permian deposition. Thick aeolian dune fields (draa) developed within these basins, which were bounded by a low-lying, gently undulating topography cut across Carboniferous rocks. Thin deposits of aeolian sand accumulated in small hollows and depressions in this topography, some of which may have been karstic features in Dinantian limestones. The depressions were separated by broad, bare, rocky, desert surfaces (hamarda), probably with scattered stones coated in desert varnish. Overlooking these low-lying areas were the uplands of southern Scotland, the Lake District and the Isle of Man. There, fluvial sandstones and breccias were deposited in canyons, whilst large alluvial outwash fans and flash-flood deposits spread out around the hills. The palaeogeography is summarised in ( Figure 55)a.

The Penrith Sandstone Formation ( Table 6) contains the oldest Permian strata preserved in north-western England, and is of Guadalupian to Lopingian age. The base of the formation is characterised by alluvial fan and flash-flood breccias that collectively form the Brockram facies (Figure 56a). These deposits comprise coarse, poorly bedded and poorly to moderately sorted breccias with angular clasts of granite, local Carboniferous and Lower Palaeozoic lithologies, and volcanic rocks from the Lake District. Where the Brockram facies is particularly thin and contains clasts coated in desert varnish, it may represent hamarda rather than flash-flood and alluvial fan deposits. The Brockram is only locally present at outcrop, but deposits up to 150 m thick are exposed in quarries in the Brough district, whilst substantial thicknesses have also been recorded in west and south Cumbria. Boreholes in the Irish Sea and at the north-east end of the Isle of Man have proved basal breccias to the Penrith Sandstone’s equivalent there, the Collyhurst Sandstone Formation, that are thought to be lateral equivalents of the mainland Brockram ( Table 6).

Overlying and in places interfingering with the Brockram are the aeolian sandstones that comprise most of the Penrith Sandstone Formation. The deposition of these strata was strongly influenced by variations in pre-Permian topography with the major depositional centres being the intermontane basins of the East Irish Sea and the Vale of Eden. In the Vale of Eden, large sand dunes developed and are preserved at outcrop as red, fine- to coarse-grained, well-sorted, aeolian sandstone with strong cross-bedding. The orientation of the forsets implies a palaeowind direction from the east or south-east. Within these dominantly aeolian deposits are sporadic interbeds of fluvial sandstone that are thought to represent deposition in ephemeral wadis that cut the dune fields, and irregular units of bedded sandstone with sub-angular debris that are thought to represent deposition in interdune ponds.

In the East Irish Sea Basin and offshore around the Isle of Man, drilling has shown that, with the exception of the basal Brockram, the remainder of the Collyhurst Sandstone consists predominantly of aeolian dune deposits. In some areas, well-developed but sporadic mudstone and siltstone partings are present and are thought to represent deposition in a damp interdune or sabkha setting.

Elsewhere, in the Solway Firth and Carlisle basins, the Penrith Sandstone Formation is thinner. It has been proved by drilling in the Carlisle Basin to consist predominantly of red, fine- to very fine-grained sandstone of aeolian origin, with moderately rounded and sorted grains. Seismic data indicate that the Penrith Sandstone in the Carlisle Basin is not physically connected to the outcrop in the Vale of Eden. Nor is there any physical connection between either of these occurrences and lateral equivalents of the Penrith Sandstone to the north in the Dumfries Basin, or with localised occurrences around Canonbie. These spatially separated occurrences of aeolian sandstone suggest that they originated as distinct dune fields that accumulated in small basins separated by higher ground.

East of the Pennines: the Rotliegendes Group

To the east of the ‘proto-Pennines’, the Permian Unconformity dipped gently eastwards beneath the developing North Sea Basin and was bounded by higher ground to the north, south and the west. Strong winds swept across the barren, rocky desert landscape carrying with them abrasive sand particles and scattering small pebbles of resistant lithologies. Ephemeral sheet floods redistributed some of the pebble deposits over the barren rock and into wadi channels between large migrating belts of sand dunes. The rocks deposited in this environment form the Yellow Sands Formation, the oldest Permian strata preserved in north-eastern England ( Table 7). They cover some two-thirds of the Permian Unconformity in that area and are believed to be of comparable age to the Penrith Sandstone Formation of north-west England.

The Yellow Sands Formation consists of weakly consolidated sand, sandstone and breccia. In the north-west of the outcrop most of the formation consists of unconsolidated sand but, to the south-east, breccia and sandstone dominate, with only the upper part of the formation consisting of sand. At outcrop, the Yellow Sands Formation consists entirely of weakly consolidated yellow sand (from which it derives its name) whereas the breccia and sandstone components do not form significant outcrops and are known mainly from boreholes and shafts. Quarrying activities at Eldon Hill, Middridge and East Thickley, all near Bishop Auckland in County Durham, formerly exposed the Permian Unconformity and the basal breccias and sandstone ( Plate 45), but these quarries are now backfilled and the exposures are obscured.

The dominant lithology in the Yellow Sands Formation is an unconsolidated, coarse-grained, siliceous sand containing much pyrite. The yellow colour results from the oxidisation of the pyrite to limonite. In the subsurface, below the oxidised zone, the pyrite is fresh and accordingly the sands are predominantly grey in colour. The individual sand grains are frosted and well rounded and have the classic ‘millet seed’ appearance characteristic of aeolian transport. The thickness of the unconsolidated sand is highly variable. In general terms it appears to form up to eight east-north-east-trending discontinuous belts of aeolian dunes, each 1.5–3.5 km wide and separated by corridors averaging 1 km wide in which the sand is thin or absent. This pattern is modified locally such that in places the cross-bedded dune ridges have thick sand deposits between them ( Plate 46). The primary shape of many of the ridges has been considerably modified by subsequent late Permian events.

The breccias contain rock fragments up to 8 cm long embedded either in a matrix of smaller rock fragments and sand, or entirely in sandstone. The fragments are angular to subangular and consist predominantly of locally derived Carboniferous limestone with some fragments of mudstone and sandstone derived from the Carboniferous Coal Measures. Bluish-grey, hard, well-cemented sandstone is often interbedded with the breccias and contains small pebbles of quartz or, more commonly, fragments of sandstone, siltstone and mudstone. The thickness of the breccia–sandstone association in boreholes is commonly about 60 cm but it is highly variable and can range from as little as a few centimetres up to 15 m. Where the breccia–sandstone association is thin, many of the rock fragments are coated in desert varnish. It is probable that the association was deposited in an arid environment on a desert rock pavement and was partially redistributed by ephemeral sheet floods. Some watercourses may have cut into the Permian Unconformity, as suggested by an exposure on the Durham motorway (A1M) at Cleasby, where 1m of breccia is exposed filling a channel-like hollow in the unconformity surface. Sporadic bands of dolomitic mudstone noted within the breccias in some boreholes probably indicate deposition in temporary ponds associated with flooding events.

Late Permian

Following deposition of the dominantly aeolian strata of early Permian times, a profound environmental change occurred over much of northern England (Figure 55b). The principal factors driving this change were repeated temporary marine incursions into large areas of the region, and the effects of the resultant higher water table on sedimentation in the surrounding areas. The high ground of the Pennines, Lake District, Isle of Man and the Southern Uplands of Scotland suffered continued erosion and, by the end of the Permian Period, were reduced to areas of very low relief or in some areas had become sites of deposition.

West of the Pennines: Cumbrian Coast Group

To the west of the Pennines, the Bakevellia Sea flooded into low-lying areas. Marine conditions were established in the main depositional centres of the Solway Firth and East Irish Sea basins, whilst carbonate platforms developed in many marginal marine areas such as south and west Cumbria. Extensive evaporitic sabkhas surrounded the marine areas and, at times of marine regression, extended into the basin centres. Adjacent to high ground and sources of sediment supply, siliciclastic deposition locally overwhelmed the evaporitic sabkhas. As a consequence of this range of palaeoenvironoments, the strata of late Permian age in north-west England are highly variable.

In areas close to sources of sediment supply, evaporite beds are thin and separated by significant thicknesses of reddish-brown siltstones interbedded with fine-grained sandstone and with some conglomerate and breccia. These strata form the Eden Shales Formation and crop out in northern Cumbria, around the margins of the Carlisle and Vale of Eden basins. The outcrop between the two basins is contiguous, indicating that, unlike the situation in the early Permian, late Permian sedimentation was more widespread and not restricted to isolated basin areas. The siltstones and sandstones of the formation are both regularly and irregularly bedded and are locally structureless. The irregularly laminated rocks were deposited by accretion of wind-blown sand and silt; the more evenly laminated strata were deposited by muddy sheet floods. Towards the base of the formation thin beds of gypsum and anhydrite occur, suggesting the periodic establishment of sabkhas and ephemeral shallow lakes that dried out to leave desiccation cracks in siltstone and to produce dried mud flakes that were incorporated into overlying beds of sandstone and conglomerate. East of Appleby, the basal strata comprise laminated sandstone interbedded with calcareous siltstone containing abundant carbonaceous plant debris. These strata are collectively known as the Hilton Plant Beds and probably accumulated as sheet-flood deposits peripheral to a desert lake. The plant beds are well exposed in Hilton Beck [NY 7196 2058]. Generally in the Vale of Eden sequence, four distinct evaporite beds exist and are known locally as ‘A’ (the lowest) to ‘D’(the highest) Beds. The ‘A’ bed anomalously contains halite in addition to gypsum and anhydrite. It is likely that the halite was deposited subaerially within the silty sediment of a sabkha environment, possibly causing volume increase of the sediment and so elevating it to a subaerial position where desiccation produced mud cracks.

In basinal areas, distant from the sources of sediment supply, dolostone, anhydrite and halite, with subordinate mudstone, built up to form the St Bees Evaporite Formation (Figure 56b), which is the lateral equivalent of the lower part of the Eden Shales ( Table 6). The formation is best known from cored boreholes and mine workings in Cumbria where anhydrite is dominant but with gypsum as a secondary replacement. At a shallow depth the evaporite strata may be removed from the sequence by percolating ground waters leading to collapse of the overlying beds. Offshore the formation is less well known but does contain beds of halite, locally in excess of 100 m thick, in addition to anhydrite. Onshore, in south and west Cumbria, carbonate rocks form a major part of the formation and these, together with the evaporite rocks, appear to have formed in response to cyclic sea-level changes. The carbonate rocks contain locally abundant bivalves, stromatolites and ooids that suggest marine deposition in dominantly shallow water (Figure 56b). Coeval with the marine deposition of the carbonate rocks, gypsum–anhydrite beds formed in sabkhas and shallow saline lakes peripheral to the marine basins. At times of relatively low sea level the evaporitic facies encroached into the central parts of the basins.

Onshore, in west Cumbria, the St Bees Evaporite Formation is overlain by the St Bees Shale Formation (Figure 56c); offshore it is overlain by the Barrowmouth Mudstone Formation ( Table 6). Both of these overlying formations are the lateral equivalents of the upper part of the Eden Shales Formation and share many of its lithological characteristics.

East of the Pennines: the Zechstein Group

To the east of the Pennines, transgression of the Zechstein Sea ended the early Permian continental phase and initiated a prolonged period of marine conditions. The County Durham area lay on the western margin of the Zechstein Sea with the inferred shoreline only slightly to the west of the current Permian outcrop (Figure 55b). Reefs developed just offshore and subparallel to the coast, generating a large, shallow lagoonal environment in which significant evaporation could take place. Periodic marine flooding and evaporation led to cyclic sedimentation of carbonates and evaporites. Traditionally, carbonate–evaporite English Zechstein Cycles have been identified, each consisting of sequentially deposited clastic rocks, carbonates and sulphates and culminating in the precipitation of halite and highly soluble potassium and magnesium salts. Five such cycles characterise the marine Permian deposits of north-eastern England, although the cycles are rarely complete and are usually dominated by carbonate deposition. A slightly different picture now emerges from the application of a modern, sequence-stratigraphical approach where sequences are defined as relatively conformable successions of genetically-related strata bounded by unconformities. The unconformities are the result of relative sea-level fall and hence the initial strata of a sequence are those deposited at times of lowstand, or by the ensuing transgression. In north-east England, this alternative approach identifies seven Zechstein Sequences (ZS) consisting of evaporite– carbonate (rather than the traditional carbonate–evaporite) successions. Correlation between Zechstein Sequences and the lithostratigraphical nomenclature of north-eastern England is given in ( Table 7).

The first sequence (ZS1) began with an initial rapid transgression of the Zechstein Sea that flooded wide areas of early Permian desert. The oldest marine Permian strata preserved in north-east England represent this initial transgression and are the laminated, silty, dolomitic mudstones of the Marl Slate Formation ( Plate 45). The unusual lithology of the formation has prompted alternative interpretations of its origin, either as a shallow-water lagoonal deposit or as a deeper-water basin deposit. The currently prevailing view is that deposition took place in a barren basin with stagnant waters roughly 200 to 300 m deep, though water depths would have varied significantly, particularly in eastern County Durham, where the flooded draa of the Yellow Sands Formation gave rise to significant variations in sea-floor topography.

The Marl Slate Formation can be traced throughout the entire marine Permian outcrop of north-east England. At outcrop it is yellowish brown in colour but when unweathered it can be seen to consist of alternating bands of dark grey and black sediment that are finely laminated and often bituminous. Locally, the clastic units are interbedded with thin beds of dolostone and dolomitic limestone. Thin layers of aeolian sand are present in many places, particularly near the base of the formation, and are interpreted as Yellow Sands material reworked into the Marl Slate succession by rapid marine transgression. Pyrite, galena and sphalerite coat bedding planes and joints throughout the formation and are thought to have a syngenetic origin, although some degree of diagenetic redistribution is likely. The Marl Slate is well known for its fossil fauna and flora. Plant remains are abundant and well-preserved fossil fish have been collected from a number of localities ( Plate 47); their preservation may well have been assisted by syngenetic mineralisation.

The top of the Marl Slate Formation is typically marked by a sharp contact with the overlying deposits although in some places, particularly the south-west of County Durham, the contact appears more transitional with thin beds of Marl Slate lithology interbedded with the lowest beds of the overlying formation, known traditionally as the Magnesian Limestone and divided into lower, middle and upper divisions ( Table 7). These equate formally to five formations: in ascending order, the Raisby Formation (traditionally the Lower Magnesian Limestone), the Ford Formation (traditionally the Middle Magnesian Limestone) and three formations, the Roker Dolostone, Seaham Residue and Fordon Evaporite (Edlington), and Seaham formations spanning the traditional Upper Magnesian Limestone.

Deposition of the Marl Slate partly filled the early Permian sea-floor topography and gave rise to an undulating, eastwards inclined, marine slope on which the overlying Raisby Formation was deposited. Likely water depths in the depositional basin ranged from approximately 100 m (in present-day coastal areas) to 200 to 300 m in the central parts (now offshore). The Raisby Formation has only a narrow outcrop in north-east England although it does extend beneath younger strata to the east. It is composed of yellow or cream-coloured dolostone and pure, grey limestone (although the latter is rare), which are exposed along an escarpment 30 to 60 m high between Hetton-le-Hole and Ferryhill, County Durham. The formation can be divided into three lithological units distinguished by colour, bedding thickness, texture and compositional variations. The lowermost unit comprises rocks of dolomitic composition through to almost pure limestone in regular beds 15 to 30 cm thick. Laminated argillaceous layers with galena, pyrite and sphalerite are common towards the base of the unit, particularly where there is a transition from the Marl Slate. The middle unit of the Raisby Formation is the most commonly seen in outcrop and consists predominantly of dolostone although, in places, calcitic dolostone is common. These rocks are grey to buff coloured, finely crystalline and thinly bedded in layers 5 to 10 cm thick. The bedding is planar on the large scale, but may be very uneven and nodular in fine detail; abundant stylolitic bedding laminae bear thin films of argillaceous residue. Widespread stratified but brecciated levels exist towards the top of the middle unit, often interbedded with gypsum. The upper unit of the Raisby Formation comprises buff to brown dolostone in irregular lenticular beds up to 45 cm thick. Stylolites and brecciated patches are less common than in the underlying middle unit.

Deposition of the Raisby Formation all but eliminated the sea-floor topography leaving a generally smooth, eastward-dipping surface that was steep enough to cause intermittent instability of the partially lithified sediment accumulation. Contorted and chaotic rock structures produced by minor submarine avalanches and slumping can be seen locally, particularly in the coastal cliffs to the south of South Shields. Such structures are most prevalent towards the top of the formation where the effects of depositional slope angle were probably accentuated by a relative fall in sea level of several metres.

Overlying the Raisby Formation is a thin sequence of fine-grained, clastic dolostone that forms the lowermost strata of the Ford Formation and represents initial lowstand deposition of Zechstein Sequence 2 (ZS2). These beds are known informally as the transitional beds as the junction between them and the underlying dolostone of the upper Raisby Formation is usually indistinct. The earliest transitional beds are barren but the abundance of a fossil shelly fauna increases upwards. The transitional beds are interpreted as marine deposits lain down during a gradual and irregular dilution of the highly saline waters in which the upper parts of the Raisby Formation accumulated. The uppermost transitional beds were probably deposited in near-normal marine conditions, with most of the Ford Formation deposited in the subsequent, shallow-water shelf environment. Shells accumulated in sufficient numbers to form an elongate bank on which a reef-forming fauna could gain a foothold. The formation of the shell bank and subsequent shelf-edge reef had a profound influence on the environment and led to contemporaneous but locally highly varied depositional settings.

The reef itself is perhaps the best-known feature of the upper Permian succession of north-east England. During the early stages of reef formation, growth was predominantly upwards, but there was some lateral expansion subsequently. The lower parts are composed of massive dolomitic limestone and dolostone containing a prolific fauna of brachiopods, bivalves and polyzoa. As the reef built upwards to a maximum height of about 60 m, shallower water encouraged more rapid growth of calcareous algae, whilst increasing salinity led to the gradual extinction of much of the earlier fauna. Consequently, the uppermost parts of the reef are largely of algal origin and contain a wide variety of stromatolitic growth forms, many similar to those found today in the intertidal zone of hypersaline lagoons such as those seen at Shark Bay, Western Australia.

On the western (landward) side of the reef, within a shallow and protected lagoonal environment, a varied sequence of granular ooidal and pisolitic carbonates accumulated with the rate of deposition keeping pace with the upward growth of the reef. These rocks form most of the Ford Formation outcrop and are almost universally dolomitised. The recrystallised platy dolostone crystals, up to 5 mm across, give the rock a characteristic texture known locally as ‘felted’, which is mostly confined to the lagoonal beds of the Ford Formation. The uppermost lagoonal strata interdigitate with the reef deposits.

On the eastern (basinal) side of the reef, deposition was comparatively slow except in the immediate vicinity of the reef edge where wedge-shaped, fore-reef talus aprons developed from detrital material eroded from the front of the reef. The earliest talus aprons were buried by continuing reef growth but have been proved underground in sections at Easington Colliery and in boreholes. Later aprons are exposed within County Durham at Blackhall Rocks ( Plate 48) and Crimdon Beck.

The various reef facies of the Ford Formation crop out in a sinuous belt extending south­south-east from Down Hill (near Sunderland) towards Hartlepool. The reef lithologies are often more resistant to erosion than the surrounding strata and in places form distinct topographical features such as Beacon Hill near Easington, County Durham. The extent of the reef southwards from Hartlepool is uncertain, although indirect evidence from gravity surveys and boreholes suggest that it may well continue as far south as Seaton Carew. South and west of Seaton Carew, semi-open shelf conditions prevailed during deposition of the Ford Formation, rather than the lagoonal environment present to the north, and its strata are difficult to distinguish in boreholes from those of the underlying Raisby Formation.

Towards the end of Ford Formation times, construction of the reef was terminated by a fall in sea level of several metres. The uppermost member of the Formation is the Hartlepool Anhydrite, which was deposited in shallow water as the lowstand wedge of Zechstein Sequence 3. It consists of dense aggregates of very finely crystalline laths that form nodular masses of almost pure anhydrite. At outcrop in the Tynemouth area, the anhydrite has been largely removed by solution and all that remains is a solution residue of grey-brown, argillaceous and sandy dolostone.

With continuing deposition on the lagoonal and basinal sides of the reef, the Zechstein Sea became increasingly shallow and saline. By the end of Ford Formation times, the reef was largely buried and exercised only minimal control on sedimentation. The Zechstein Sea was by then a very shallow-water, hypersaline and inhospitable shelf-edge environment.

Deposition of the Roker Formation on the shelf and adjacent basin slope marks the onset of the next major marine transgression. The slope facies is one of the most highly varied and spectacular carbonate units of the Permian succession in northern England. At the base of the formation, the Concretionary Limestone Member comprises thinly bedded granular dolostone that is locally recrystallised and contains calcite concretions ( Plate 49). In the Sunderland area, these concretions are spectacularly well developed and the unit is known informally as ‘cannonball rock’. In the coastal exposures of County Durham, the concretionary limestone can be divided into a lower unit up to 15 m thick containing abundant concretions and an upper unit, up to 20 m thick, in which they are absent. However, overall distribution of concretions is laterally variable. Where concretions are not present, the rock is largely a soft granular dolostone with traces of small-scale cross-bedding and ripple marks. Slightly below the middle of the member is a thinly laminated impure dolostone that is creamy-grey in outcrop but grey and bituminous at depth. It can be split into paper thin, flexible sheets and is know informally as the Flexible Limestone. This unit has yielded fish remains in the Sunderland area whilst plant remains are common throughout its outcrop in northern England. In County Durham and the Tynemouth area, the Concretionary Limestone has been brecciated by collapse following solution of the underlying Hartlepool Anhydrite, and now consists of angular and rounded fragments of grey-brown crystalline limestone in a matrix of brown dolomitic limestone ( Plate 50).

The shelf deposits of Roker Formation consist mostly of the Hartlepool and Roker Dolostones, which are composed almost entirely of soft, granular and ooidal, cross- and ripple-bedded dolostone with little significant variation in lithology. The dolostone units are mainly exposed in coastal cliffs around Roker and Seaham and to the north of Hartlepool, where many have foundered as a result of solution of the underlying Hartlepool Anhydrite. The lithology of the dolostones is consistent with deposition in a shallow-water shelf environment, but the presence of rip-up clasts and minor erosion surfaces may indicate the subaerial emergence of sediment deposited partly within the intertidal zone.

After deposition of the Roker Formation, marked sea-level oscillations have been inferred, the evidence drawn from a curious evaporite that contains sedimentary features characteristic of both shallow and deepwater conditions. This is the Seaham Residue and Fordon Evaporite Formation. Within it, Fordon Evaporites are present today only in the subsurface offshore where they reach a thickness of up to 90 m and consist primarily of gypsum, halite and anhydrite with some dolostone. They formerly extended westwards at least as far as the present coastline, but there they have been largely removed by solution, leaving only the Seaham Residue and resulting in significant foundering of the overlying rocks. The Seaham Residue is up to 9 m thick at its type locality of the coastal cliffs at Seaham Harbour and consists primarily of limestone and a dolomitic clay residue. The formation is believed to represent the lowstand facies marking the initiation of Zechstein Sequence 4, with parasequences within it indicating subordinate sea-level oscillations. In the Stockton area, south of the Seaton Carew Fault, the lateral equivalent of the Seaham Residue and Fordon Evaporites ( Table 7) is the Edlington Formation (traditionally the Middle Permian Marls).

Deposition of the Fordon Evaporite largely completed the filling of the Zechstein Basin. Sedimentation thereafter took place in a relatively shallow water environment, perhaps less than 20 m deep, below the tidal zone and with little or no basin floor relief. Under these conditions the Seaham Formation accumulated. It is relatively uniform in lithology and comprises the transgressive strata of Zechstein Sequence 4. It consists mostly of mudstone (some calcareous), limestone and dolostone and is exposed in the coastal cliffs around Seaham. The whole formation has been severely disrupted and locally brecciated by foundering arising from solution of the underlying Fordon Evaporite, now represented only by the Seaham Residue. Foundering is generally less severe and less extensive than that affecting the stratigraphically lower Concretionary Limestone. The Seaham Formation carries a distinctive assemblage of bivalves and algae with abundant, small tubular remains of the probable alga Calcinema permiana. Algal laminates and sedimentary features such as cut-and-fill structures, low angle cross-lamination, and low amplitude–long wavelength ripples, all increase in abundance upwards through the formation and may indicate shallowing to a high subtidal environment by the end of its deposition.

The limited diversity, but great abundance, of fossil remains within the Seaham Formation indicates unique environmental controls on the plant and animal population, most probably indicating hypersalinity of the water. Coupled with decreasing water depth, this led to the development by the end of Seaham times of sabkha conditions under which the youngest Permian strata of northern England were deposited. The Billingham Anhydrite Formation is now present only in the offshore area where it is 3 to 6 m thick and composed of gypsum and anhydrite with some dolostone. Offshore to the east of Billingham, the anhydrite is overlain by the much thicker Boulby Halite Formation. The Billingham and Boulby formations contain deposits assigned to Zechstein Sequence 5.

The disappearance of permanent open water and the re-establishment of arid conditions over north-east England are indicated by the nature of the succeeding Rotten Marl Formation. It consists of dull, dark red-brown, silty mudstone commonly with scattered halite crystals and cut by a network of veins containing fibrous halite and gypsum. This formation demonstrates the first real clastic input into the basin but is preserved only in the south-east of the region, south of the West Hartlepool Fault, and offshore. Although the Rotten Marl can be distinguished from the overlying Roxby Formation in offshore boreholes, the same distinction is not possible at outcrop in the Durham area where the boundary between the two units is transitional, with the Rotten Marl commonly represented only by a thin residual layer. This is incorporated stratigraphically at the base of the Roxby Formation which, in such a situation, directly overlies the Boulby Halite Formation. In its lower part, the Roxby Formation is a dominantly mudstone sequence, but the frequency and thickness of siltstone and sandstone interbeds increase upwards until the Roxby Formation passes gradually upwards into the Early Triassic Sherwood Sandstone Group.

The sequence stratigraphy of these upper Zechstein strata is somewhat subjective due mainly to the limited data available. The Rotten Marl Formation was deposited at a time of low sea level, on coastal flood plains, in salt pans and in lagoons. It is more proximal than the Boulby Halite Formation, which implies a sequence boundary between the two with the Rotten Marl representing the intiation of Zechstein Sequence 6. The Roxby Formation was deposited at a time of higher sea level and may represent a further sequence, Zechstein Sequence 7, with related shallow water deposits out towards the basin centre, or it may represent the uppermost strata of Sequence 6.

Triassic and Jurassic

In northern England, sedimentation continued into Triassic times, primarily on the sites of the former, but still subsiding, Permian basins. The Pennines remained an upland area, although somewhat more subdued than previously, separating deposition in the west from that in the east (Figure 55c).

North-west England

West of the Pennines, the late Permian to early Triassic interval saw a transition from a mainly marine to a continental environment; the nature of the sediment deposited changed accordingly. A large and extensive fluvial channel system — the Budleighensis River — flowed northward, carrying significant amounts of sediment derived from a distal, southerly source in the mountains of Armorica. This dominant source was supplemented, to varying degrees, by sediment eroded more locally from high ground in the Southern Uplands of Scotland and the English Lake District. The uniform, fine-grained, micaceous, reddish-brown sandstones of the St Bees Sandstone Formation, the lowermost unit of the Sherwood Sandstone Group, were deposited by this river system (Figure 56d). The formation extends over much of onshore west Cumbria and the Carlisle and Vale of Eden basins, and is exposed spectacularly in the cliffs around St Bees Head, Cumbria ( Plate 2) and ( Plate 51).

The base of the St Bees Sandstone Formation is usually taken at the first major sandstone bed above the thinly bedded sandstones and siltstones of the upper Eden Shales and equivalents, although the transition is almost certainly diachronous. The Permian–Triassic boundary occurs within the lower part of the formation ( Table 6). Siltstone and claystone beds are prevalent towards the base and a wide range of sedimentary structures, including parallel lamination, planar-and trough-cross-bedding, and convoluted bedding, is present. Throughout early Triassic times, the Budleighensis river system evolved and matured. Lithology and sedimentary structures towards the base of the formation are indicative of unconfined sheet-flood events that pass upwards into single storey channels with over-bank flood deposits. These in turn give way to large multistorey, stacked channel complexes. The very top of the formation contains increasing amounts of fine-grained sediment, mudstone interbeds and aeolian grains.

Following deposition of the St Bees Sandstone, the extensive, high-energy river system waned and was largely abandoned. The Budleighensis River had been diverted, probably by tectonic movements, to flow eastwards of its former course, across the English Midlands and into the North Sea, depositing its sediment as the ‘Bunter Sandstone’ of eastern England. Consequently, desert conditions prevailed and aeolian dune fields extended over much of north-west England, driven and supplied with sediment by winds from the east and north­east (Figure 56e). Large-scale draas developed, separated from each other by flat, damp interdune areas of limited lateral extent. These aeolian deposits, with their well-rounded grains, formed the dark reddish-brown, fine- to coarse-grained sandstone in the lower part of the Calder Sandstone Formation. Onshore, this formation crops out widely across west Cumbria. It overlies the St Bees Sandstone with a sharp disconformity suggesting that the change in depositional environment was abrupt and was possibly followed by a short period of nondeposition. By late Calder Sandstone times, fluvial conditions had been re-established across north-west England (Figure 56f) and several fluvial sandstone units occur in the upper part of the formation. The medium- to coarse-grained sandstone lithologies of these units suggest a local clastic supply, with the abundance of well-rounded grains suggesting the reworking of aeolian dunes; the orientation of sedimentary structures indicates a source region to the east-north-east.

In the offshore areas of the East Irish Sea and Solway Firth basins, and around the Isle of Man, the equivalent succession to the combined onshore St Bees and Calder Sandstone formations is all assigned to the St Bees Sandstone Formation. The offshore formation has two members: the lower fluvial sandstone of the Rottington Sandstone Member is the lateral equivalent of the onshore St Bees Sandstone, whilst the upper, aeolian sandstone of the Calder Sandstone Member is the lateral equivalent of the onshore Calder Sandstone Formation ( Table 6).

The fluvial influx in late Calder Sandstone times was short-lived and soon desert conditions prevailed once more; aeolian dunes were common with damp areas or interdune ponds between them. Siltstone, medium-grained and planar-laminated sandstone, and coarse- grained and cross-laminated sandstone were all deposited in this environment and together form the Ormskirk Sandstone Formation. Offshore, the formation is known from the East Irish Sea and Solway Firth Basins and the Isle of Man area, with an onshore outcrop in west Cumbria. The base of the formation is marked by a discontinuity coincident with a major break in sedimentation throughout most of north-west Europe.

In the Carlisle and Vale of Eden basins, the St Bees Sandstone is overlain by strata referred to the Kirklinton Sandstone Formation. This unit is broadly equivalent to the Calder and Ormskirk formations which have been recognised locally at outcrop and can be identified in the Silloth No. 1 Borehole and on seismic reflection profiles. The lower part of the Kirklinton Formation is of mixed fluvial and aeolian origin and is equivalent to the Calder Sandstone, though is generally much finer-grained than that lithology in west Cumbria. The upper part of the Kirklinton Formation is for the most part lithologically similar to the aeolian lithofacies of the Ormskirk Sandstone.

The St Bees Sandstone, Calder Sandstone and Ormskirk Sandstone formations, together with their lateral equivalents, form the Sherwood Sandstone Group ( Table 6).

By mid Triassic times, coastal and shallow marine conditions had returned to north-west England with the restoration of the Bakevellia Sea during a major transgression. Marine and evaporitic conditions existed over the East Irish Sea Basin and Isle of Man area, with playa lakes and coastal, low-relief mudflats extending to the east into the Carlisle Basin. The strata deposited in these environments form the Mercia Mudstone Group and comprise mudstone, halite and minor amounts of dolostone, dolomitic mudstone and anhydrite. Strata of the Mercia Mudstone Group are not recognised in onshore west Cumbria, but are preserved beneath Quaternary cover in the south of the Furness District as an extension of the East Irish sea succession.

The Mercia Mudstone Group strata are best preserved in the north of the East Irish Sea Basin and the Isle of Man area where six formations are recognised ( Table 6). The Leyland Formation consists of a basal evaporite deposited in coastal sabkhas at the beginning of the transgression, followed by mudstone interbedded with widely varying proportions of halite. The variations in the succession are thought to result from its deposition in a wide range of spatially restricted environments including small lakes, evaporitic brine pools and salt flats, and restricted shallow marine areas. The range of environments arose partly as a result of topographical variations caused by significant syndepositional faulting contemporaneous with the marine transgression. The overlying Preesall Halite, Dowbridge Mudstone, Warton Halite and Elswick Mudstone formations all have similar lithological characteristics and were deposited in coastal marine conditions, although thickness variations of halite in the later formations suggest a general decrease in marine influence by these times. The youngest strata of the Mercia Mudstone Group in the East Irish Sea Basin and around the Isle of Man are grey-green dolomitic mudstones of the Blue Anchor Formation.

In the Carlisle Basin, the Mercia Mudstone Group is largely restricted to the central areas. At outcrop, the succession is very poorly exposed and all strata are assigned to the Stanwix Shales Formation ( Table 6). However, cored boreholes have proved the presence of the Blue Anchor Formation, whilst the other formations of the East Irish Sea succession can be identified from the geophysical log patterns of the Silloth No. 1 Borehole. The Stanwix Shales Formation’s strata are predominantly reddish brown, locally grey-green mudstones that accumulated on broad playas and in coastal sabkhas or shallow water bodies. Some thin beds of very fine-grained sandstone, and beds of halite and nodular gypsum–anhydrite are present, and indicate deposition in saline lakes and evaporitic mudflats when enhanced subsidence elevated sea level and introduced a greater marine influence.

Up until late Triassic times, north-west England had lain on a northward- and later westward-inclined palaeoslope. In late Triassic times, tectonic movement re-orientated this slope to a southward inclination and allowed transgression of the sea from the south via a new ‘south-west passage’ that lay along the course of the present-day St George’s Channel. Upper Triassic strata in north-west England were formed from the sediments deposited during this transgression and in the ensuing shallow-water, marine environment. The Penarth Group rests disconformably on the Mercia Mudstone and is subdivided into two formations. The marine Westbury Formation is present in the Irish Sea and around the Isle of Man where it consists of dark grey to black, pyritic, non-calcareous mudstone indicative of shallow marine deposition. In the Carlisle Basin, boreholes in the Great Orton area have proved the Westbury Formation, resting unconformably on the Blue Anchor Formation (Mercia Mudstone Group), with a similar lithology. The succeeding Lilstock Formation consists of thinly bedded, grey, argillaceous and calcareous siltstone and sandy limestone. It is present in the Irish Sea and around the Isle of Man, and has been proved onshore in boreholes in the Carlisle Basin. Mudcracks, gutter casts and dewatering structures present in borehole core indicate that it was deposited during a progressive shallowing of the marine environment to a subtidal and lagoonal setting.

The lagoonal and subtidal environments of Lilstock Formation times ended when marine conditions returned to north-west England towards the end of the Triassic Period; thereafter the water progressively deepened into Jurassic times. The only preserved strata from this interval exist in a small outlier within the centre of the Carlisle Basin. There, Lias Group strata are almost entirely covered by superficial deposits with only very limited exposure in stream sections. The full extent of the outlier is therefore conjectural but has been recently revised based on additional evidence from new boreholes in the Great Orton area. At present, rocks from the lower part of the Lias Group are believed to be present over an area of approximately 70 km2 to the west and north-west of Great Orton, and to be faulted against Mercia Mudstone Group strata to the north, south and west. This lowest part of the Lias Group straddles the Triassic–Jurassic boundary. The Upper Triassic strata are grey, moderately calcareous mudstones that are commonly silty and interbedded with thin calcareous siltstones. Towards the base of the Lias Group, a few thin beds of limestone and cross-bedded sandstone are present along with shell accumulations that may represent storm surge deposits within the shallow, Late Triassic sea. The base of the Jurassic system is taken at the lowest occurrence of the ammonite Psiloceras planorbis. The lowermost Jurassic Lias Group strata are very similar to the Upper Triassic mudstones except for the presence of this species. No limestone is present and the succession indicates a progressive deepening of marine conditions throughout early Jurassic times. Younger Jurassic strata are not preserved in north-west England.

North-east England

To the east of the Pennines, the filling of the Zechstein Basin in late Permian times substantially reduced areas of permanent open water and allowed coastal to continental, arid conditions to be re-established by early Triassic times. The Budleighensis River brought vast amounts of sediment from the high ground of Armorica to the south-west. The soft, fine-grained, thickly bedded, dull red sandstones and subordinate red mudstones and siltstones of the Bunter Sandstone are the sediments deposited in this environment; their boundary with the underlying Permian to earliest Triassic Roxby Formation is transitional and diachronous. The Bunter sandstones represents the lowermost Sherwood Sandstone Group strata preserved within north-east England and are present beneath superficial cover south of the West Hartlepool Fault. Surface exposures can be found only on the foreshore at Seaton Carew, and in the intertidal zone at nearby Long Scar where abundant cross-beds, ripple marks and desiccation cracks attest to a shallow-water, coastal origin. From time to time, swamps and peat bogs appear to have become established on the coastal plain. Evidence for this comes from carbonaceous lenses within the Bunter Sandstone at Hartlepool, coupled with widespread, irregular lenses of pale green colour and some more extensive zones of grey to yellow colouration, the likely result of localised chemical reduction of the red sands. In the Stockton area, to the south of the Seaton Carew Fault, the Sherwood Sandstone Group is known from borehole records to be overlain locally by strata of the Mercia Mudstone Group (known traditionally in north-east England as the Keuper Marl), but the mudstone is nowhere exposed. Strata of younger Triassic age or of Jurassic age are not preserved in north-east England.

Chapter 9 Late Mesozoic and Cenozoic tectonics and magmatism

Almost 190 million years elapsed between deposition of the youngest, Lias Group, rocks preserved in the Carlisle Basin and the onset of the Quaternary glacial episodes, yet the geological record in northern England now provides little evidence of the original distribution, lithology, thickness and stratigraphy of the deposits laid down during this time. It is widely agreed that Mesozoic rocks once covered much of the region, well beyond their present distributions in the Solway, Irish Sea and North Sea basins, whereas the nearest Cenozoic sedimentary sequences preserved are in Northern Ireland and in the southern parts of the Irish Sea and North Sea. Some knowledge of the events that occurred during this long interval is crucial to understanding how the region looks today. A broad picture can be pieced together by extrapolating stratigraphical and structural information from contiguous regions, and by deducing the burial history of the surviving rocks and hence the original thickness of the now-eroded cover. Techniques that address the latter problem include: studies of the variation in the physical properties of rocks with depth in boreholes, estimation of palaeotemperatures from the increasing reflectance of vitrinite particles and the colour of spore and conodont fossils (which darken with rising temperature), and from apatite fission-track analysis. The last method is based on the radiation damage suffered by individual crystals and the degree to which they have been annealed by rising temperature. The results have proved controversial, as these methods of investigating the burial history depend on assumptions that are not independently verifiable; a consensus on what is considered to be geologically reasonable has emerged only recently.

It is likely that the Jurassic sedimentation pattern that had been established in the Carlisle, Irish Sea and North Sea basins continued into Early Cretaceous time. However, around that time, areas such as the Lake District and Pennine blocks were eroded once again, as the widespread, late Cimmerian unconformity developed. Regional, post-rift shelf subsidence then dominated across the southern UK during Late Cretaceous times and probably extended to northern Britain, resulting in deposition of a relatively uniform Cretaceous sequence, dominantly of the Chalk Group. Maximum post-Variscan burial of the region is thought to have been attained towards the end of the Cretaceous Period, though in the Cleveland Basin, to the south-east of the region, maximum burial may have been attained a little later, in Oligo-Miocene times.

Northern England was affected by distant events of epic proportions during the Cenozoic Period. Regional uplift in Late Cretaceous to early Paleocene times established the region as land once again and triggered an episode of erosion that has continued, probably with little interruption, until the present day. The cause is believed to have been thermal uplift along the north-west margin of Europe as a precursor to the formation of new oceanic crust and opening of the North Atlantic Ocean. This was initiated by the impact of the proto-Icelandic mantle plume on the base of the lithosphere in a pre-Atlantic region that included the west coast of Scotland, Northern Ireland and eastern Greenland (Figure 3f). This area became the focus of intense magmatism from about 60 to 55 Ma during which time immense volumes of basaltic magma were erupted from fissures and central volcanoes, with the accompanying intrusion of central-complexes, dyke swarms and sills. Swarms of tholeiitic basic dykes emanated from these main centres; some from Mull reached northern England, up to 420 km from their source, whilst dykes from Northern Ireland centres cross the Isle of Man and extend across Wales into the English Midlands.

Later, probably in Miocene times, a further major episode of uplift affected the Irish Sea and Carlisle sedimentary basins, probably as a distant effect of the Alpine Orogeny. This arose from the collision, away to the south, of the African and European plates. As a result of the two major tectonic uplift events in northern England during the last 65 million years, it is estimated that between 700 and 2500 m of strata have been removed by erosion, including the entire cover of Upper Jurassic and Cretaceous rocks.

Jurassic to early Cretaceous extension

Comparison with regions to the south and east, where the stratigraphical record of this interval is more complete, suggests that the extensional basins in and adjacent to, northern England continued to develop through normal faulting well into Early Cretaceous times. In west Cumbria, fault displacement demonstrably postdates the Sherwood Sandstone Group within the hanging-wall block of the Lake District Boundary Fault, whilst several faults in the Carlisle Basin displace Lias Group rocks. Moreover, up to one third of the total displacement on the Keys Fault in the western part of the East Irish Sea Basin has been estimated as due to post-Triassic movement, and thus, by analogy, significant throws may also have occurred at this time on other similarly orientated structures in the region, such as the Pennine and Lake District Boundary Fault zones. Supporting evidence for post-Triassic movements on the latter is provided by Early Jurassic to Early Cretaceous radiometric ages determined on minerals within fault gouge.

Palaeogene magmatism

Palaeogene dykes in northern England have a consistent east-south-east trend and have long been considered to originate from the Mull Centre ( Figure 57). Detailed geochemical studies and a K-Ar radiometric age of 55.8 ± 0.9 Ma on the Cleveland Dyke confirm this connection. The orientation and petrography of the Palaeogene dykes readily distinguish them from the more west-to-east dykes associated with the earlier Whin Sill-swarm. Two conspicuous Palaeogene dyke arrays are present in northern England ( Figure 57) and cause prominent aeromagnetic anomalies. The first is the Acklington Dyke, which intrudes Silurian to Carboniferous rocks from the Scottish Borders, through Northumberland into the North Sea. The second cuts Carboniferous to Jurassic rocks through the Vale of Eden, North Pennines and the North Yorkshire moors; it comprises a set of mostly vertical, en échelon segments that are collectively known as the Cleveland (or Armathwaite) Dyke ( Plate 52). In addition, numerous other, similarly trending dykes of the Blythe and Sunderland subswarms are locally exposed in several parts of Northumberland and on the east coast between Blythe and Sunderland, with more encountered during coal mining in the Northumberland and Durham Coalfield. Locally, inclined lenticular and broadly concordant laccolith-like bodies are linked to the dykes.

Whereas most of the dykes are rarely more than 5 m wide, the Acklington and Cleveland dykes locally attain widths of 30 m, though the Cleveland Dyke is typically 22 to 28 m wide. Collectively these dykes derived from a huge volume of magma. For the Cleveland Dyke alone, this has been estimated to be at least 85 km3. Further, numerical modelling suggests that this dyke represents a single pulse of magma that spread laterally from a magma reservoir beneath Mull at a velocity of up to 18 km per hour, reaching its farthest extent in 1 to 5 days. Where magma rose vertically, en échelon dyke segments were produced.

The contact metamorphic aureoles adjacent to the dykes are typically very narrow and mining records show that coking of coal seams appears to have been much less than that associated with the earlier dykes of ‘Whin Sill’ affinity. During mining, some of the dykes were found to contain cavities up to 4 m by 7 m. These are thought to represent xenoliths of coal that were gasified after incorporation in the magma.

The dykes are dark grey or bluish grey, locally amygdaloidal, fine-grained to glassy basalt and basaltic andesite. Petrologically they are composed of essential plagioclase enclosed within pyroxene, with patches of microcrystalline or glassy mesostasis. Though the Acklington Dyke is not porphyritic, some of the others are conspicuously so. For example, the basaltic andesite Cleveland Dyke contains up to 4 per cent phenocrysts of plagioclase, with subordinate orthopyroxene and clinopyroxene but with no olivine, either as phenocrysts or in the groundmass. Minor olivine is present elsewhere, for example in some of the dykes exposed in the Bellingham area of Northumberland. The Tynemouth Dyke is characterised by conspicuous anorthite phenocrysts.

Throughout its length, the Cleveland Dyke is remarkably uniform geochemically, but chemical and petrographical data indicate a magmatic history that involved fractional crystallisation at several depths in the crust with minor amounts of crustal contamination of the magma. Locally, inclusions of gabbro and cumulate-textured plagioclase represent relicts that crystallised in the shallow magma chambers where the final composition prior to intrusion was produced. Ovoid inclusions of devitrified glassy material represent liquid remaining from the final crystallisation of the dyke.

Olivine dolerite dykes with an east-south-east trend are scattered throughout the Isle of Man and linear aeromagnetic anomalies show that the dyke swarm extends offshore. The dykes seem to be most abundant cutting the southern outcrop of Carboniferous rocks, but a 12 m wide dolerite dyke exposed in the northern part of the island forms a segment of the Fleetwood Dyke, which the aeromagnetic data suggests is one of the most prominent in the Manx region ( Figure 57). It is not a single intrusion but a set of dykes, arranged en échelon and associated with inclined sheets, that was emplaced along a fault zone traversing east­south-east across the Irish Sea towards the Lancashire coast. The Palaeogene dykes of the Isle of Man appear to radiate from a centre in Northern Ireland.

Cenozoic uplift and basin inversion

Estimates of Cenozoic exhumation from apatite fission-track studies suggest that some 700 m of strata, mainly Late Cretaceous and possibly Permian and Triassic in age, were removed from the Scafell area of the central Lake District, whereas erosion of a more complex sequence of Carboniferous to Cretaceous rocks from the area around the West Newton Borehole, just to the north of the Maryport Fault in north Cumbria, amounted to about 1550 m. The difference in these Chapter approximates to the current difference in the elevation of the two localities above sea level.

Compressive uplift of the Carlisle Basin occurred probably during Miocene times. This process is referred to as inversion: pre-existing extensional faults are reactivated with a reverse sense and strata are folded. Up to about 2500 m of a mainly Jurassic and Cretaceous succession are thought to have been removed during basin inversion, with the Lias Group accounting for as much as 1500 m of this thickness within the depocentres, thinning to about 600 on the intervening saddle areas. Any overlying Middle Jurassic to Lower Cretaceous strata were either an originally thin sequence, or had been removed prior to deposition of 600–800 m of Upper Cretaceous Chalk Group rocks that are thought likely to have capped the succession. The Lower Jurassic strata in the outlier near Carlisle are situated on one of the saddle areas and have been preserved because they lie within an area that was less affected by inversion than the depocentres.

Palaeotemperatures inferred from conodont colours in the Lower Carboniferous rocks of north Northumberland and the Permian rocks of Durham suggest that Mesozoic cover in the area is unlikely to have been greater than about 2000 m. Farther south, the removal from the centre of the Cleveland Basin of about 2500 m of post-Middle Jurassic cover is required to explain the depth of burial inferred for this region; Palaeogene rocks may have accounted for up to 1000 m of this thickness. The Lake District and North Pennines may have been emergent throughout Palaeogene times, though it is possible that some strata were laid down on these block areas during Miocene basin inversion.

Inversion of the East Irish Sea basins was associated with the formation of hanging-wall anticlines during reversal of earlier normal faults. These effects are particularly well displayed by the Maryport and Lagman faults which form the margins of the south-west-trending Ramsey–Whitehaven Ridge, and by faults in the Coastal Plain Fault System to the west of the Lake District Boundary Fault Zone. Reverse displacement on faults within the Lake District Boundary Fault Zone seems probable because apatite fission-track studies suggest that Cenozoic uplift of the East Irish Sea was some 500 m greater than that of the Lake District. Displacements on faults in other parts of the region at this time are also likely, notably in the Cheviot Hills, though convincing evidence is lacking.

Though other explanations are feasible, the tantalising prospect of Quaternary fault displacement is suggested by high-resolution seismic reflection data from Holmrook, south­east of Gosforth in west Cumbria. Small steps a few metres in height imaged at the interface between bedrock and the overlying superficial deposits are close to the inferred position of strands within the Lake District Boundary Fault Zone and would be consistent with Quaternary fault reactivation. Furthermore, the impressive Pennine Fault scarp is locally more than 500 m high and appears to be a geomorphologically young feature, as do fault scarps within, and bounding, the Cheviot Hills. Intermittent, small-magnitude earthquakes, felt particularly in the western half of the region, confirm that some faults remain active in northern England. The recently reactivated movement of certain faults in the Magnesian Limestone of eastern County Durham, due to rising groundwater in abandoned coal workings is discussed in Chapter 12.

No Cenozoic sediments are known to have survived in the region, though the products of pervasive weathering during this time may have been preserved locally. Possible examples are to be found around Buckbarrow in the south-west Lake District, in the Cheviot Hills and in the North Pennines. At Buckbarrow, rocks of the Borrowdale Volcanic Group are completely altered to a clay deposit up to at least 10 m thick. Both the Cheviot volcanic rocks and the Cheviot Granite are intensely altered and disintegrated in parts to depths of between 2 and 50 m; the residual sand and clay is referred to as saprolite. At Holwick Scars, in the North Pennines, the Great Whin Sill is rotted to a depth of up to 15 m. In all of these localities, the remarkable preservation of igneous and pyroclastic textures in the ‘deposit’ shows that transformation occurred in situ. Reminiscent of deeply weathered rock profiles seen in some present day humid tropical regions, formation of these deposits in early Cenozoic time would accord with the fact that this was globally one of the warmest episodes of Phanerozoic Earth history. Alternatively, the saprolite may have formed during warm, humid conditions later in Cenozoic times. A different origin seems more likely for the intense, argillic alteration of the Broad Oak Granodiorite in the southern part of its outcrop, contiguous with the Borrowdale Volcanic Group example above. There, the transformation may have been caused by circulating hydrothermal fluids and not by weathering processes.

The enigmatic palaeokarst clays of the Asby area, south-east of Shap, may also originate from this time, though deposition probably occurred during Quaternary ice movements. There, solution hollows in the karst surface of the Dinantian limestones are filled with a massive, tenacious red clay deposit containing sporadic striated pebbles and a flora that includes Carboniferous spores and long-ranging forms that are most likely Quaternary. Abundant leaf cuticle material was probably derived locally from a rich herbaceous flora, but abundant kaolinite and chlorite in the clay suggests that this component may have been derived from deeply weathered granite and volcanic rocks. Thus, these ‘pocket deposits’, which are also known from the karst of Derbyshire, may have been sourced from deeply weathered rocks in the Shap Fells and smeared into the karst surface by ice during Quaternary times.

Chapter 10 Mineralisation

The rocks of northern England host a remarkable variety of epigenetic mineralisation styles, with examples concentrated mainly in the northern Pennines, the Lake District, west and south Cumbria, the Isle of Man and adjacent to the margins of the East Irish Sea Basin. Mineralisation ranges in age from Ordovician to Holocene. Metalliferous veins and related deposits comprise the bulk of this mineralisation, but the area also contains significant deposits of nonmetallic minerals, some of which have been of economic importance. Their essential features and the likely origins of the principal vein suites are discussed below. The deposits are epigenetic, that is they were formed at relatively high levels, near to the surface of the Earth; pressures and temperatures were accordingly low.

Lake District

Epigenetic mineralisation within the Lower Palaeozoic rocks of the Lake District inlier is largely confined to the Skiddaw, Eycott Volcanic and Borrowdale Volcanic groups and the major igneous intrusions ( Figure 58); mineralisation is rare within the rocks of the Windermere Supergroup. Many of the metalliferous veins of the Lake District appear to exhibit a close structural, and perhaps genetic, relationship to the form of the largely concealed Lake District granitic batholith. Veins are typically concentrated above, or close to, ridges in the roof region of the batholith or above its north and south walls. The virtual absence of veins from the Windermere Supergroup is consistent with their close association with the batholith, which does not extend beneath that division.

The mineralogical composition, chemical characteristics and structural relationships of the deposits give evidence for several mineralising episodes, in some cases within the same vein. Deposits range in age and composition to include: Late Ordovician copper-rich assemblages; Early Devonian tungsten-bearing veins; Carboniferous or early Permian lead–zinc mineralisation, locally accompanied by abundant baryte; widespread post-Permian haematite mineralisation; super-gene assemblages, possibly formed in Jurassic or later times. The earliest phases of mineralisation may be the result of hydrothermal activity during the closing stages of Ordovician magmatism, with subsequent episodes during the tectonic evolution of the region. The Lower Palaeozoic rocks of the Lake District, including the granitic batholith, and the Carboniferous and Permo-Triassic rocks of the adjoining basins appear to have been source rocks for the introduced minerals.

Two belts of extensive metasomatic alteration, which affect Skiddaw Group rocks, may give important clues to some of the mineralising processes. In the northern part of Black Combe and in the Crummock Water aureole are zones of intense metasomatic alteration and bleaching of the rocks which exhibit substantial net additions of such elements as As, B, K and Rb and locally Ca, F and Si, with dehydration and depletions of Cl, Ni, S, Zn, C and in places Cu, Fe, Li and Mn. The inferred presence of an elongate granitic body along the northern margin of the Lake District Batholith and the Early Devonian date for the Crummock Water metasomatism provide strong evidence that Skiddaw Group rocks may have been a source of ore metals for some of the Lake District veins.

No clear uniformity of vein orientation is apparent within the Lake District, though it has been observed that copper-bearing veins are commonly orientated east–west with a southerly dip, whereas mainly lead–zinc veins typically trend within 45° of north and dip towards the east. However, there are many exceptions to this pattern. Although evidence exists for vertical zonation of constituent minerals in a few instances, most of the Lake District deposits show no obvious lateral or vertical zonation of constituent minerals. In this they differ from the commonly zoned veins of the nearby northern Pennines.

The following accounts review the deposits in order of their approximate age, with the oldest first.

Graphite deposits

The Seathwaite graphite deposit in Borrowdale [NY 232 125] is unique in the British Isles and has very few parallels elsewhere in the world. Mining here dates back to at least the 16th century and continued intermittently until 1891. The graphite is closely associated with a dioritic intrusion into the lowest part of the Borrowdale Volcanic Group. It occurs in veins and in a series of at least eight individual, steeply inclined, pipe-like bodies developed at the intersection of faults. The ‘pipes’ are each up to 1 m by 3 m across and from 2 m to 100 m in vertical extent. Within these, the graphite forms discrete nodules from 1 mm up to over a metre across, though nodules up to a few centimetres across were probably commonest ( Plate 53). The nodules occur within a buff-coloured matrix of intensely altered dioritic rock. Graphite locally appears to replace the host. Within the nodules the graphite is compact, fine grained and remarkable for its purity. It is accompanied by small amounts of quartz, chlorite, pyrite and chalcopyrite.

The petrographical relationships and the inferred high formation temperature (c. 500°C) suggest a close genetic relationship between mineralisation and the intrusion. As all mafic intrusions in the Lake District are of Caradoc age (with the exception of the Early Devonian lamprophyres), the graphite mineralisation at Seathwaite is also likely to be of Caradoc age. The carbon was derived from organic-rich sediments within the underlying Skiddaw Group and was probably deposited from CO2- and CH4-bearing aqueous fluids.

Copper veins

Veins in which copper-bearing minerals are the main metallic constituents comprise an important suite of deposits that were formerly of considerable economic importance. The greatest concentrations of this type of mineralisation occur at Coniston [SD 280 970], Ulpha [SD 186 923] and Haweswater [NY 482 133], with examples more widely distributed at Black Combe, the Vale of Newlands, and in parts of the Caldbeck Fells ( Figure 58). The peak years for Lake District copper production were firstly during Elizabethan times and then again during the 19th century. Very small amounts of copper ores continued to be raised at Coniston into the early years of the 20th century.

In these deposits, chalcopyrite is the most widespread primary copper sulphide mineral, though tennantite, chalcocite and bornite are locally abundant, the last in situations which suggest some secondary enrichment. Associated ore minerals commonly include abundant arsenopyrite, pyrite and pyrrhotite, with very much smaller amounts of native bismuth, bismuthinite, bismuth sulphoselenides and sulphotellurides, cobalt and nickel minerals, galena and sphalerite. Traces of gold have been identified in a few localities. Gangue minerals mainly comprise quartz, chlorite, dolomite, and locally, stilpnomelane. Magnetite, most of which appears to replace original haematite, is plentiful in a few veins, notably the Bonsor Vein at Coniston ( Plate 54), where mine records suggest the proportion of this mineral increases with depth. Copper-bearing veins in the Consiton area appear to have been widest, up to several metres across, and were most productive where they cut silicic ignimbrites.

Within the Bonsor Vein, temperatures of 350–400°C have been suggested for the deposition of early arsenopyrite and replacement of early haematite by magnetite. Quartz, chlorite, stilpnomelane, calcite, dolomite, pyrrhotite, chalcopyrite, sphalerite and later arsenopyrite were probably deposited at temperatures of around 240°C, with later minerals including pyrite, native bismuth, bismuthinite and galena likely to have been deposited at temperatures as low as 200°C. Limited fluid inclusion studies on quartz from veins regarded as part of this suite in the Vale of Newlands suggest that the mineralising fluids were moderately saline brines (about 5–10 equiv.wt per cent NaCl). The Borrowdale Volcanic Group rocks have been proposed as the source of the metallic elements whereas the Skiddaw Group is considered the most likely source of the sulphur. These veins predate the regional Acadian cleavage and are therefore likely to have formed during or shortly after the final phases of caldera volcanism.

Tungsten veins

A small group of veins, formerly worked at the Carrock Fell Mine [NY 323 329] in Mosedale ( Figure 58), comprise the only known occurrence of tungsten mineralisation in Britain outside of south-west England that has ever attracted commercial interest. Mining for tungsten appears to have begun here in the mid 19th century but proved unsuccessful. Subsequent attempts to resume working all failed, the latest ending in 1981.

The tungsten ores wolframite and scheelite are accompanied at Carrock Fell Mine ( Plate 55) by arsenopyrite, pyrrhotite and pyrite in quartz–muscovite–apatite veins which strike approximately north–south through the Grainsgill cupola of the Skiddaw Pluton, the Carrock Fell Centre, and other rocks associated with them. Minor consitituents of the veins include native bismuth, bismuthinite, bismuth sulphotellurides, molybdenite (with a Re-Os age for mineralisation of about 392 Ma), iron-rich sphalerite and traces of gold. A few specimens of cassiterite have been reported. Significant tungsten mineralisation appears to have been confined to strike lengths of only 1 km centred around the mine, though panned concentrates from surrounding streams suggest that tungsten minerals may be more widely distributed.

The Carrock Fell veins are genetically associated with the Early Devonian Skiddaw Granite, much of which is here metasomatised to greisen. The mineralising fluids are interpreted as moderately saline solutions that were periodically charged with CO2 and enriched in tungsten. Fluid inclusion studies suggest temperatures of 240–295°C for the formation of the greisen, and 265–295°C for wolframite mineralisation. The tungsten-bearing veins are cut by later quartz and galena-bearing veins and there is abundant evidence of a complex series of mineralising events. Supergene alteration has produced a great variety of unusual mineral species.

Elsewhere in the Lake District, small crystals of scheelite have been found, together with some molybdenite, in drusy cavities in the Early Devonian Shap Granite; the molybdenite has given a Re-Os age of about 405 Ma for mineralisation. Scheelite also occurs, accompanied by rare wolframite, in a narrow quartz vein in the Ordovician Broad Oak Granodiorite at Buckbarrow Beck on Corney Fell [SD 135 910]. This locality is of particular interest for the abundance of rare supergene species including cuprotungstite, russellite and bismutoferrite. Although relatively high concentrations of tungsten are known in greisen associated with the Eskdale Granite near Devoke Water, no tungsten minerals have been recorded there.

Apatite–chlorite veins with cobalt

A north-north-east-trending vein which cuts bleached Skiddaw Group mudstones within the Crummock Water aureole at Scar Crag on Causey Pike [NY 206 207] is unique amongst exposed Lake District veins in carrying, as its main constituents, quartz, apatite, chlorite and arsenopyrite with traces of the cobalt minerals alloclasite, cobaltite, glaucodot and skutterudite. Unsuccessful attempts were made in the 19th century to work the vein for cobalt ores. Similar quartz–apatite–chlorite veins, though without sulphides, occur at Brown How [NY 115 158] and Crag Fell [NY 095 148] in Ennerdale. The mineralisation is likely to be an Acadian phenomenon genetically associated with formation of the Crummock Water aureole above a concealed granite.

Tourmaline veins

Small amounts of tourmaline are associated with most of the larger granitic intrusions. However, numerous tourmaline-rich quartz veins up to 2 m wide are especially common within parts of the Crummock Water metasomatic aureole.

Antimony veins

In addition to their presence as minor components in other veins, or as minute inclusions within galena in the lead–zinc veins, antimony minerals comprise the main ore minerals in a small, but distinctive suite of veins within the Lake District. The best known occurrence of antimony mineralisation, and the only one known to have been of economic interest, is that at Robin Hood [NY 228 328], near Bassenthwaite, where a few tonnes of stibnite were mined during the 19th century from an antimony-bearing quartz vein in Skiddaw Group rocks. Other veins dominated by antimony ore minerals include a stibnite–berthierite–zinkenite-rich vein in Skiddaw Group rocks at Wet Swine Gill [NY 314 322], Caldbeck Fells, berthierite and jamesonite-bearing veins in Borrowdale Volcanic Group rocks at Hogget Gill [NY 389 112], near Brothers Water, and stibnite veins reported from St Sunday Crag [NY 360 130], also in the Borrowdale Volcanic Group. The Wet Swine Gill Vein may be associated with the nearby Carrock Fell Mine suite of tungsten-bearing veins. A boulder of pure stibnite weighing up to 50 kg, found in boulder clay near Troutbeck Station [NY 390 270], suggests that further antimony-rich veins could be concealed beneath glacial deposits in the northern Lake District. Few investigations have been undertaken on Lake District antimony mineralisation, the origins and age of which remain uncertain.

Lead–zinc veins

Veins in which lead and zinc minerals are the dominant ores form, like the copper-bearing veins, a very important suite of deposits in the Lake District, where they were formerly of considerable economic importance. The most significant concentrations occur in the Vale of Newlands, at Thornthwaite [NY 223 258], Brandlehow [NY 250 196], Helvellyn [NY 325 148], Hartsop [NY 410 125], Eagle Crag [NY 358 142], Greenside [NY 365 174] and Force Crag [NY 200 215], and around Threlkeld and in the Caldbeck Fells. The Lake District has had a long history as a lead producer: the area’s last major lead mine, Greenside, closed in 1962 though attempts at small-scale production of zinc ore with associated baryte continued at Force Crag until 1990.

Galena and sphalerite are the main primary ore minerals in these veins, accompanied locally by minor amounts of chalcopyrite and tetrahedrite. Minute inclusions of native antimony and antimony sulphosalts are common within the galena. Silver is almost invariably present within the galena, probably within minute inclusions of lead, copper and antimony sulphosalts. Assay values of up to 30 ozs of silver per ton of lead (838 ppm) have been recorded from the Caldbeck Fells and high concentrations of silver are known to be present in tetrahedrite at Eagle Crag and elsewhere. Small specimens of native silver have been reported from supergene assemblages within the near-surface parts of veins at Force Crag and Red Gill on the Caldbeck Fells. The Force Crag Vein contains significant concentrations of manganese oxide minerals in its upper, supergene zone: a few tonnes of manganese ore are understood to have been mined here. Gangue minerals in these veins include abundant quartz, baryte, calcite, dolomite and locally siderite. Baryte is especially common in the upper parts of the Force Crag Vein, which was worked partly for this mineral. Fluorite is a minor constituent of several veins but is present in substantial amounts in veins at Brandlehow and Whitecombe Beck, Black Combe. Quartz pseudomorphs and epimorphs after baryte are common, suggesting that baryte was formerly a major constituent of these deposits.

Depositional temperatures in the range 110–130°C have been suggested. Metals may have been derived from rocks of the Skiddaw Group, the granitic batholith or from Carboniferous sediments in the adjoining Solway–Northumberland Trough by a process of convective leaching involving Carboniferous sea water. Sulphur isotope studies support derivation of sulphur from Carboniferous evaporites present at depth in north Cumbria. The Lake District lead–zinc veins are known to cut and thus postdate the copper-bearing veins, and a limited number of K-Ar determinations of altered vein wallrock has given ages of between 360 and 330 Ma. However, many aspects of the veins invite close comparison with those of the Northern Pennine Orefield and it is possible that the lead–zinc veins of the Lake District may be members of the same suite of veins as their Northern Pennine counterparts, but exposed at a deeper structural level within Lower Palaeozoic wall rocks. If so, a Late Carboniferous or Early Permian age of emplacement seems probable since this has been well established for the northern Pennines area. Evidence from boreholes in the Sellafield area suggests that lead–zinc mineralisation may be similar in age to the main phase of haematite mineralisation in west Cumbria, which is likely to be Permo-Triassic or later as discussed below.

Baryte veins

Baryte is a common gangue mineral in several of the lead–zinc veins but is present in unusually great abundance in several veins in the Caldbeck Fells, notably at Potts Gill, where it was an important commercial mineral. There, the baryte is accompanied only by quartz and manganese oxides with traces of sulphide minerals. These veins, and perhaps the baryte mineralisation at Force Crag, may represent a separate episode of baryte mineralisation, perhaps postdating the lead–zinc mineralisation.

Haematite veins

Although haematite is locally a constituent of a variety of veins within the Lake District, it is the dominant mineral within a distinctive suite of veins within the Lower Palaeozoic rocks. Notable formerly economic concentrations of haematite veins include those within Skiddaw Group rocks at the Knockmurton and Kelton Fell mines near Loweswater and within the Eskdale Granite in mines near Boot. Other less economically significant veins include those within Borrowdale Volcanic Group rocks at Ore Gap [NY 241 072], Grasmere [NY 341 098] and Deepdale [NY 390 150], and in the Ennerdale intrusion. Vein widths of up to 7 m were encountered at Kelton Fell. All of these veins are typically filled with haematite with very small amounts of quartz, dolomite or calcite comprising virtually the only gangue minerals. Haematite is usually present as the mammillated fibrous crystalline variety known as ‘kidney ore’, though compact, massive and crystalline forms (specular ore) are found locally. The characteristic form of the haematite and the almost monomineralic nature of the serves to link them genetically with the very large replacement bodies of haematite within the Carboniferous limestones of west and south Cumbria. Their origin and Permo-Triassic (or younger) age are considered below.

Gold

Although claimed from a number of locations within the Lake District, many of these reports must now be regarded as unreliable. Nevertheless, gold has been reliably recorded from a variety of deposits of different ages including the Carrock Fell tungsten deposit, from a gossan in the Black Combe area, in copper veins in the Coniston area, at Dale Head in the Vale of Newlands and in panned concentrates from various streams in the Cockermouth area. The name ‘Goldscope Mine’ [NY 226 185] in the Vale of Newlands is almost certainly a corruption of an old German name and appears to have no connection with the presence of gold. There is no evidence for any gold ever being recovered commercially from the Lake District.

Supergene mineralisation

Parts of the Lake District, particularly the Caldbeck Fells, are famous for the abundance, variety and beauty of supergene minerals within the near-surface parts of the veins. Whereas many of these are the products of weathering related to modern water-table levels, the presence of such minerals at depths well below the present oxidation zone may be the result of earlier supergene processes, perhaps as early as the Jurassic.

Haematite deposits of Cumbria

The Carboniferous limestones of west and south Cumbria host a large number of haematite orebodies. The west Cumbrian iron orefield, which comprises a comparatively narrow belt of country extending between Lamplugh and Calder Bridge, may be considered in two unequal parts. Haematite was no doubt first discovered in the larger, exposed orefield north of Egremont ( Figure 58) where Carboniferous limestone crops out at the surface. The southern, concealed portion of the orefield extends south of Egremont, where Carboniferous rocks pass beneath the unconformable cover of Permo-Triassic strata. Numerous mines have been worked in west Cumbria, the main centres of operation being Frizington, Cleator Moor and Egremont. The south Cumbrian orefield may also be viewed as two unequal parts. A small number of deposits were worked in the Millom area including, at Hodbarrow Mine [SD 178 785], some of the largest individual orebodies. A rather more extensive group of deposits lies in the area between Barrow, Dalton and Askham in the Furness peninsula. A single haematite deposit at Waterblean [SD 177 824], near Millom, occurs within Upper Ordovician, Dent Group limestone. It seems highly probable that haematite was worked for iron ore in Cumbria in pre-Roman times, though the first documentary records date from the 12th century. The 19th century witnessed the peak of production before large-scale mining ended in south Cumbria in 1968, and in west Cumbria in the late 1970s. Small-scale production for the manufacture of pigment continued until 2006 from shallow underground workings at Florence Mine, Egremont [NY 017 103]. The Cumbrian haematite deposits typically occur as large irregular or flat-lying replacements of limestone, usually associated with or adjacent to faults (Figure 59 a and b). Original features of the host limestone, including bedding planes, stylolites, and fossils are commonly preserved in haematite, whereas mudstone interbeds may continue unreplaced through the ore. Haematite also locally replaces limestone-rich parts of the Brockram at the base of the west Cumbrian Permo-Triassic succession. True vein fillings of haematite occur locally both in the Carboniferous limestones, especially in the Furness area, and within the Lower Palaeozoic rocks of the Lake District, as described above. In addition to these orebody types, the Furness portion of the south Cumbrian orefield contains numerous haematite bodies known as ‘sops’. In these deposits, which are unique to Furness, haematite fills large, roughly conical dissolution hollows in limestone ( Figure 59)c. The centre of each sop is typically filled with sand, probably derived from a previous cover of Permo-Triassic strata.

The Cumbrian deposits are composed almost exclusively of haematite but in a variety of forms. Compact, massive haematite is most abundant, but the area is famous for the fibrous mammillated variety known as ‘kidney ore’ ( Plate 56), the distinctive conical broken fragments of which are termed ‘pencil ore’. Crystallised haematite, or ‘specularite’, is locally present in small quantities. Some manganese oxide minerals locally accompany the haematite and were worked on a small scale in a few mines in the Bigrigg and Askham areas. Other metallic minerals are generally very rare, though galena and copper sulphides have been recorded. Small amounts of chalcocite were raised from one mine at Anty Cross near Dalton-in-Furness, but it seems probable that the copper-bearing vein here predated the haematite orebody. Gangue minerals, which comprise only a very small proportion of the orebodies, include dolomite, calcite, aragonite, baryte and locally some siderite and fluorite.

The origin of the haematite deposits of the area has long attracted attention and controversy. Currently, the most widely accepted genetic models involve the transport of iron derived either from Permo-Triassic sedimentary rocks in the Irish Sea Basin, or from the granites of the western Lake District. Convective leaching of iron was effected by fluids circulating in response to a probable heat source beneath the Irish Sea. The iron-rich fluids were driven up-dip and gained access to the limestones via faults, and in the Furness area via dissolution hollows; large-scale metasomatic replacement of limestone then took place.

In west Cumbria the role of permeable Permo-Triassic rocks has been of crucial importance in determining the distribution of orebodies. Where these rocks rest directly upon Dinantian limestones, ore bodies are common, but where thick mudstones of Namurian, Westphalian or late Permian age intervene, orebodies are generally absent. The local abundance of specular haematite within the interstices of coarse-grained sandstones within the Coal Measures suggests that these too may have acted as mineralising aquifers in suitable structural settings. If so, hitherto undiscovered haematite orebodies could exist to the west of the known west Cumbrian orefield.

It is likely that a former covering of permeable Permo-Triassic or Carboniferous rocks acted as a pathway enabling mineralising fluids to gain access to fractures within the Lower Palaeozoic rocks of the Lake District, where haematite was deposited as fissure fillings. Such a hypothesis is entirely consistent with the comparative abundance of haematite veins in the western parts of the Lake District and their absence from the east of the region.

Fluid inclusion studies of gangue minerals suggest that the mineralising fluids were hypersaline brines at temperatures of up to 120°C. The age of this mineralisation is still a matter of dispute. Arguments have been advanced for a Permian or Early Triassic age, though other evidence favours a post-Triassic age.

Isle of Man

Epigenetic mineralisation within the Isle of Man comprises a range of vein deposits ( Figure 60). The most significant of these are associated with steeply inclined faults in the Tremadoc to Arenig, turbiditic strata of the Manx Group, although some veins in the Foxdale area pass into granite at depth. Relatively little mineralisation is known from the Upper Palaeozoic rocks of the island. Despite a long history of base metal and iron production extending back to the 13th century or perhaps even earlier, the mineral veins of the Isle of Man are still poorly understood. They appear to have a close similarity to some of the Lake District mineral vein assemblages.

Lead–zinc–copper veins

Most of the island’s veins carry dominant lead–zinc mineralisation, with galena and sphalerite as the principal ore minerals. Copper is locally common, mainly as chalcopyrite, though with some tetrahedrite in places. Unlike the nearby Lake District, there is insufficient evidence to distinguish a separate suite of copper-dominated veins and it seems likely that the island’s copper production came from copper-rich portions of veins dominated by lead–zinc mineralisation. Other ore minerals include pyrite, pyrrhotite and some jamesonite. Gangue minerals include quartz, calcite, dolomite, siderite and locally some baryte and fluorite.

The recorded silver contents of Manx galena varies enormously from as little as 3 ozs per ton of lead (84 ppm) to as much as 400 ozs (11 160 ppm) in rare instances at the Foxdale mines where, in addition, tetrahedrite rich in silver has been reported. A comparatively high uranium content is present in solid hydrocarbons within some lead veins, notably those at Laxey and Snaefell.

Although extremely limited, evidence for the age of the island’s lead–zinc mineralisation is consistent with late Carboniferous or Permian vein emplacement. This is similar to the likely age of the suites of veins dominated by lead–zinc mineralisation that are seen in the Lake District and northern Pennines.

Haematite veins

A small group of veins dominated by haematite, with only very minor amounts of quartz, dolomite and calcite, are almost certainly counterparts of the Cumbrian haematite mineralisation. They have been worked from Manx Group wall rocks near Maughold Head [SC 486 924]. Siderite, a scarce mineral in the Cumbrian haematite deposits, appears to have been common locally.

Northern Pennine Orefield

The comparatively thin succession of Carboniferous rocks on the Alston Block are cut by an extensive suite of veins and related deposits which collectively make up the Northern Pennine Orefield. The orefield coincides closely with the uplands of the northern Pennines, but extends eastwards to include parts of the Durham Coalfield. A group of richly mineralised faults close to the southern margin of the Northumberland Trough in the Haydon Bridge area, are generally regarded as comprising an outlying portion of the orefield. The deposits exhibit many characteristics of the worldwide ‘Mississippi Valley’ mineralisation type, though they should be considered as a special fluoritic subtype with similarity to the deposits of the Illinois–Kentucky fields of the USA.

Since at least the 12th century, the orefield has been a significant producer of lead and iron ores; minor amounts of copper have also been produced and a little silver has been won as a by-product of lead smelting. The peak years of metal production were during the 18th and 19th centuries. More recent years saw the rise in importance of zinc mining, together with major production of fluorspar, baryte and witherite. Commercial mining ended in 1999 with the closure of the combined Groverake–Frazer’s Hush Mine [NY 890 440], the area’s last major fluorspar mine (see Chapter 12). In addition to its distinguished history as a mineral producer, the orefield has contributed much to the understanding of hydrothermal deposits, from the theories of mine agents during the early years of geological science, the revision of ideas necessitated by the drilling of the Rookhope Borehole [NY 933 428], through to present-day, fluid inclusion and isotope studies.

Mineral veins in the orefield occupy conjugate sets of normal faults, which typically show maximum throws of only a few metres. Veins have three principal orientations ( Figure 61). Most numerous, and most productive of lead ore, are those trending approximately east­north-east. A smaller number of roughly north-north-west-trending fractures are generally known as ‘cross veins’. Throughout much of the orefield these are barren faults, though locally, especially in parts of Alston Moor, they are associated with significant mineralisation. A handful of roughly east–west fractures, known as ‘Quarter Point’ veins, complete the suite. Unlike the other vein sets, the ‘Quarter Point’ veins typically occupy faults with a marked sinistral transcurrent displacement. They are mostly several metres wide and are composed of spar minerals with comparatively low sulphide values. The orefield’s largest fluorspar orebodies occur in ‘Quarter Point’ veins.

A distinctive feature of the orefield is the close relationship between the geology of the wallrock and the potential of the fault as a mineralised vein. Hard, competent wallrocks such as limestone, sandstone or dolerite of the Whin Sill-swarm provided clean open fissures favourable for the deposition of wide mineralised veins. In weak, incompetent rocks such as mudstone, fault fissures are normally tightly closed and generally poorly mineralised. In Yoredale-type sequences, this relationship results in laterally extensive oreshoots against or between hard beds, alternating with barren intervals between weak wallrocks, giving the ‘ribbon oreshoots’ so characteristic of this orefield and well developed at Killhope ( Figure 62). Veins are typically filled with coarsely crystalline gangue and ore minerals, commonly as crude bands parallel to the vein walls. Vein widths vary from a few millimetres to over 10 m, though most of the worked veins appear to have been less than 5 m wide.

In addition to hundreds of fissure veins, the Northern Pennine Orefield is noted for extensive replacements of limestone wallrock by introduced minerals. Adjacent to many veins, the limestone has suffered extensive metasomatism, in places extending for many metres from the parent vein, for example in the Allenheads area ( Figure 63). The original limestone has been wholly or partly replaced by variable assemblages of quartz, ankerite, siderite, fluorite, baryte, witherite, galena and sphalerite, with smaller amounts of other minerals. These deposits are known locally as ‘flats’ because of their essentially horizontal form. Ore minerals such as galena and sphalerite are commonly more abundant in the ‘flats’ than in the parent veins, and some of the orefield’s most productive metal mines were worked in such ‘flats’. The most extensive and formerly economically important ‘flat’ deposits have been found in the Great Limestone, though significant ‘flat’ mineralisation is also present locally in the Melmerby Scar, Jew, Single Post and Tynebottom limestones. Such metasomatic replacements appear to be best developed where impervious mudstone directly overlies limestone.

Galena and sphalerite are the main ore minerals throughout the orefield, with sphalerite dominant locally, notably in the Nenthead, area. Most of the galena is silver-bearing with typical silver values of between 4 and 8 oz per ton of lead (112 to 223 ppm), though exceptional silver values of up to 90 oz per ton of lead (2511 ppm) have been identified in a few places. The sphalerite normally carries some cadmium and, in a few places, a little mercury. Chalcopyrite is common in small amounts throughout the orefield, but is abundant in a few deposits in the Garrigill area. Sulpharsenides of cobalt and nickel, and small amounts of arsenopyrite and tetrahedrite group minerals, have been identified as minute grains in many of the deposits. Pyrite and marcasite are common, locally accompanied by some pyrrhotite. Small, but significant concentrations of nickel minerals are known from Settlingstones [NY 844 683], Scordale [NY 764 228] and Lady’s Rake [NY 806 342] in Upper Teesdale.

Siderite and ankerite are extremely widespread in both vein and ‘flat’ deposits. Supergene alteration of these minerals to goethite has produced large deposits of workable limonitic or ‘brown haematite’ ores. In addition to siderite and ankerite, the main gangue minerals are fluorite, quartz, baryte and witherite. The presence of the last mineral together with the local abundance of barytocalcite and the more restricted occurrence of alstonite, two unusual double carbonates of calcium and barium, is one of the remarkable, though as yet unexplained, features of this orefield. All three minerals were first described from the Northern Pennines, beyond which they are very rare.

One of the most striking features of the orefield is the marked concentric zoning of the mineralisation ( Figure 61). A large central zone in which deposits typically carry abundant fluorite, for example at Groverake ( Plate 57), is surrounded by outer zones in which the barium minerals baryte and witherite comprise the characteristic gangue minerals, for example at Scraithole ( Plate 58). The separation of these zones is very sharp, with fluorite and barium minerals generally exhibiting a mutually exclusive relationship. Small concentrations of chalcopyrite, commonly accompanied by traces of bismuth minerals, and very rarely by minute amounts of synchesite, monazite and cassiterite, have been described from the innermost parts of the fluorite zone. Some major fluorite-bearing veins appear to pass downwards into veins dominated by quartz and iron sulphides amongst which pyrrhotite is common. Galena concentrations are commonly greatest in the outermost part of the fluorite zone.

The main phase of mineralisation appears to have been emplaced very soon after the intrusion of the Whin Sill-swarm in earliest Permian times. An unusual deposit in Upper Teesdale, with abundant magnetite and galena and local concentrations of niccolite, may result from reaction between metal-rich mineralising fluids and contact rocks of the Great Whin Sill while the latter were still hot. If the small concentrations of baryte, galena, sphalerite and, locally, a little fluorite within the Permian rocks of eastern County Durham are linked with the Northern Pennine deposits, the mineralisation must postdate deposition of the Permian limestones. The baryte mineralisation appears to be largely a late-stage effect, introduced during the waning stages or by the distal portions of the main hydrothermal system.

By analogy with the zonation of minerals around the granites of south-west England, and with support from early geophysical studies, a concealed Late Carboniferous (‘Variscan’) granite beneath the northern Pennines was postulated to account for the mineralisation. However, the proving of an Early Devonian age for the Weardale Granite in the Rookhope Borehole, drilled in 1960–61, clearly demonstrated that the mineralisation could not be the direct result of granite intrusion. Instead, as a ‘high-heat-production’ granite, the Weardale Granite is now considered to have driven a convective circulation of mineralising fluids long after its intrusion. From fluid inclusion and isotopic evidence, mineralising brines were derived by dewatering of adjoining sedimentary basins before scavenging metals from the rock through which they passed. Sources of the required chemical elements probably included Lower Palaeozoic basement rocks, the Weardale Granite itself, Carboniferous sedimentary rocks and the Whin Sills. Minerals were deposited in fissure veins by crystallisation from solutions which cooled as they flowed outwards from emanative centres.

Basin-margin mineralisation

Maryport–Gilcrux –Stublick–Ninety Fathom Fault Zone

An en échelon belt of faulting extends from the Cumbrian coast near Maryport to the Northumberland coast at Cullercoats. It includes the Maryport, Gilcrux, Stublick and Ninety Fathom faults, which together define the boundary between the Lake District and North Pennine blocks, and the Solway–Northumberland Trough. Baryte vein mineralisation, locally accompanied by small concentrations of base metals, is common in a belt up to about 4 km wide, associated with the Maryport–Gilcrux Fault Zone, within the Carboniferous rocks on the northern margins of the Lake District. Concentrations of baryte, accompanied by lead, zinc, copper and locally mercury mineralisation, occur in association with the Stublick Fault in the lower Carboniferous rocks of the Brampton area. Occurrences of barium and lead mineralisation, which lie close to this fault line in the Carboniferous rocks of Northumberland, include the Settlingstones and Fallowfield lead/witherite veins, which have been regarded as peripheral deposits to the Northern Pennine Orefield. Baryte mineralisation is present within the Ninety Fathom Fault in Coal Measures rocks on the Northumberland coast at Cullercoats.

Structural and stratigraphical conditions on the southern margin of the Northumberland– Solway Trough are reminiscent of those that host the base metal deposits of central and Ireland. Hence they invite speculation that similar mineralisation may occur within Lower Carboniferous rocks at depth in northern England. The mineralisation seen at the surface today could then be the product of a partial remobilisation from deeper, concealed deposits. Although some exploration based on this model has been undertaken, no deposit has yet been identified.

Lunedale– Butterknowle Fault Zone

Baryte and lead mineralisation close to or within the Lunedale–Butterknowle Fault Zone, notably at Closehouse Mine, Lunedale ( Figure 61), appears to be part of the main northern Pennine suite of deposits. Where the fault crosses the Magnesian Limestone outcrop in eastern County Durham, it is associated with a narrow belt of baryte and fluorite mineralisation, and small concentrations of copper minerals occur within the fault near Coxhoe [NZ 320 360]. Widespread, but uneconomic, disseminated sulphides have been proved in Coal Measures rocks immediately south of the fault in this area. As with the mineralisation on the northern margin of the block, these occurrences along the southern margin of the Alston Block invite comparison with some of the Irish baryte and base-metal deposits.

East Irish Sea Margin

Deep boreholes in the Sellafield area, drilled as part of an investigation for disposal of low-level radioactive waste, revealed abundant evidence of mineralisation related to the eastern margins of the East Irish Sea Basin. Whereas much of the mineralisation encountered can be matched with that exposed at surface, evidence of hitherto unrecognised mineralisation was revealed, including iron hydroxides, complex Ba-Ca-Mn oxyhydroxides, calcite, pyrite, marcasite, baryte, anhydrite and gypsum — all related to the present-day groundwater system.

Chapter 11 Quaternary: ice sheets and a changing climate

Landscape evolution

The long-term evolution of the landscape of northern England prior to the last glaciation is not well known. Elaborate hypotheses involving several phases of marine erosion and subaerial peneplanation during the Palaeogene and Neogene have been proposed to explain apparent concordant summit levels and topographic benches, but they have not been substantiated and are at variance with the offshore record. Some elements of the landscape are inherited from surfaces that are very old indeed, for example, the sub-Devonian unconformity on the Lower Palaeozoic strata of the Lake District and Howgill Fells, and the sub-Permian surface on the Carboniferous strata of the Alston Block and outer Lake District. The dominant control on topography, however, is bedrock lithology. For example, the Whin Sill, a particularly hard and resistant rock, has created many dramatic features such as the escarpment followed by the Roman Wall ( Plate 3), the cliffs of High Cup Nick on the north Pennine escarpment and High Force in Teesdale. The Fell Sandstone (Border Group) forms several notable escarpments in Northumberland. In contrast, depressions between the escarpments have resulted from the preferential erosion of mudstone and saprolite formed by chemical weathering in the humid tropical climate of the Palaeogene.

The present pattern of drainage ( Plate 59) originated in the early Palaeogene during the creation of fault-bounded basins and massifs peripheral to the opening North Atlantic Ocean. Long-term isostatic uplift associated with granitic plutons positioned beneath the Lake District, Alston Block, Cheviots and the Isle of Man consequently influenced the developing pattern of drainage. The Lake District is a fine example of radial superimposed drainage that was initiated on a domed Chalk surface in the Palaeogene, but then accentuated by glacial erosion beneath ice caps centred over the area during several glaciations. The Howgill Fells exhibit deeply dissected, dendritic drainage incised into Silurian mudstones, and the famous ‘hair-pin gorge’ at Durham is a fine example of an entrenched meander ( Plate 60) that is probably no older than late glacial. A similar age is likely for many examples of river capture, such as that affecting the headwaters of the River North Tyne in Northumberland.

There is little doubt that the glacial and periglacial episodes of the Pleistocene have left the dominant imprint on the landscape. Ice sheets stripped most unconsolidated deposits from the Pennine uplands, leaving large areas of smoothed and polished rock. Considerable glacial erosion occurred locally to form the cirques, arêtes, lake-filled glacial troughs and glacial breaches of the central Lake District. Erosion by meltwater created innumerable clefts and winding, steep-sided glacial drainage channels. These features commonly dissect interfluves and traverse obliquely across hillsides. In Northumberland, the sandstone cuestas have been accentuated where ice flowed parallel to the strike, as in the vicinity of the Roman Wall, but partially obscured where flow was transverse to the strike. Soft mudstones have been preferentially eroded to form strings of poorly drained, peat-filled basins.

Modification of lowland areas occurred mainly through glacigenic deposition, which formed widespread, gently undulating, poorly drained, relatively featureless plains of till interspersed with mounds, ridges and terraces of outwash sand and gravel. Locally, subglacial processes produced swathes of the ice-moulded, drumlin topography that is so characteristic of many Pennine dales and the Vale of Eden. The present drainage system was initiated during deglaciation, with many rivers following routes originally carved out by glacial meltwater. Some rivers were deflected from their preglacial courses, which are now concealed valleys filled with glacigenic material. Subglacial deposition of till has generally smoothed the topography relative to the underlying rockhead surface and without it some coastal areas would be submerged.

Record of climate change

Evidence of global Quaternary environmental change has been found in deep-sea sediments, in cores of ice taken from the summits of the Greenland and Antarctic ice sheets and from extended sequences of interbedded loess and organic deposits from continental Europe and Asia. The onset of glaciation in the Northern Hemisphere probably began in the Late Miocene, suggesting that the climate of the British Isles had begun to deteriorate long before the beginning of the Quaternary Period at 2.6 Ma, when ice-rafted debris first appears in the North Atlantic deep ocean record.

The frequency, rapidity and intensity of climatic change are key features of the Quaternary, with climate alternating between ‘glacial’ and ‘interglacial’ modes; at least 50 significant ‘cold–warm’ oscillations have been recognised. The driving force of climatic change is the long-term cyclical variation in the incidence of solar energy caused by the Earth’s orbital periodicities, which run over roughly 23 ka, 41 ka and 100 ka intervals. These fluctuations have been amplified substantially by additional factors involving physical, biological and chemical interactions together with ‘feedback loops’ between the atmosphere, oceans and ice sheets. Of particular importance to the British Isles are changes in the position of the Gulf Stream. This northward-flowing current of warm surface water is compensated by the return southwards of cold, dense water at depth. Sudden changes in this circulation pattern had a major impact on climate.

An invaluable proxy record of global climate is provided by the relative proportions of the two common isotopes of oxygen contained in the skeletons of calcareous microfossils recovered from deep ocean sediment cores. During glacial periods the oceans’ water becomes relatively enriched in the heavy isotope of oxygen (18O), and the marine (oxygen) isotope stages (MIS) thus obtained now provide a universal means of dividing the Quaternary ( Figure 64). The oxygen isotope record indicates that during the Early Quaternary, when glaciers possibly first developed in the Cumbrian Mountains, each of the principal cold–warm cycles lasted about 40 ka. Following a major change at about 780 ka BP, there have been seven longer, more rigorous glacial–interglacial cycles, although substantial ice sheets appear to have grown in only three or four of them in the Northern Hemisphere. Each of these glacial episodes lasted between 80 and 120 ka and was followed abruptly by an interglacial lasting 10–15 ka; the rapid deglaciations are described as ‘terminations’ ( Figure 64). The glacial periods included long, cold intervals, termed ‘stadials’, and less cold, or even warm, ‘interstadials’ lasting for a few thousand years. Most terrestrial evidence preserved in northern England relates to the last major glacial–interglacial cycle (Devensian and Holocene stages), but older deposits are preserved locally.

High-resolution evidence from Greenland ice cores, coupled with the MIS record, confirms that dramatic climatic changes have occurred on the millennial (and possibly even decadal) scale. Some 24 interstadial intervals are now identified in the Devensian stage alone, compared with the five or six that were recognised previously from the pollen record and formalised in the traditional British chronostratigraphy ( Figure 64). These short interstadials started with abrupt warming and typically cooled over a period of 1 to 3 ka, several being grouped together and superimposed on longer cycles during which temperatures declined gradually ( Figure 65). The culmination of each overall cooling phase was a glacial readvance of regional extent coinciding with a massive discharge of icebergs into the North Atlantic (known as a ‘Heinrich event’), as indicated by the appearance of abundant ice-rafted debris in deep ocean sediment cores.

Lithostratigraphy

A comprehensive lithostratigraphical framework covers all onshore Quaternary deposits in Britain. The Albion Glacigenic Group embraces the older glacigenic deposits previously described as ‘Older Drift’, although few examples occur in northern England. The Caledonia Glacigenic Group embraces all glacigenic tills, gravels, sands, silts and clays that form surface deposits within the limits of the Devensian ice sheet. Being the products of the latest glaciations, these deposits commonly have distinct morphological expression and are equivalent to the ‘Newer Drift’ of previous classifications. Subgroups within the Caledonia Glacigenic Group ( Figure 66) are based on the provenance of the constituent till, which strongly reflects the composition of both the underlying bedrock, and that of the particular suite of rocks that the ice crossed before the subgroup till was deposited. The composition of morainic, glaciofluvial and glaciolacustrine deposits is similarly influenced, though to a lesser extent.

Deposits of the Central Cumbria Glacigenic Subgroup are typically yellowish or greyish brown and contain clasts derived mostly from the Lake District, Shap Fell and the Howgill Fells: lithologies present include basalt and welded tuff (Borrowdale Volcanic Group), Lower Palaeozoic slate and wacke-sandstone, granite and Upper Palaeozoic limestone and sandstone. The clasts were derived by an independent ice cap centred over the Cumbrian Mountains. The upland parts of the Isle of Man are generally mantled by stony diamictons containing clasts of Lower Palaeozoic wacke-sandstone and slate that are also of local origin; these comprise the Manx Glacigenic Subgroup. The Isle of Man became a palaeonunatak relatively early in the deglaciation, allowing periglacial processes to rework the till and the frost-shattered bedrock, resulting in thick deposits of head. The several subsequent glacial readvances affected only the margins and north of the island.

The Irish Sea Coast Glacigenic Subgroup includes deposits derived from the floor of the Irish Sea in addition to southern Scotland, the Vale of Eden, the west Cumbrian Coalfield and the Lake District. They are typically vivid reddish brown or grey, containing clasts of red and yellow Permian sandstone, Lower Palaeozoic wacke-sandstone and siltstone, granite and granodiorite. Welded tuff, mudstone and coal, together with Pleistocene shell fragments and reworked marine microfossils, are common to the south of the Solway Firth. The deposits were laid down from ice that flowed around the north-west flanks of the Lake District to merge with ice sourced in the Southern Uplands of Scotland; the combined ice sheet then flowed southwards through the Irish Sea basin towards the Celtic Sea, intermittently overwhelming the north-eastern end of the Isle of Man.

The North Sea Coast Glacigenic Subgroup includes a suite of brownish grey to reddish brown deposits that contain clasts derived predominantly from the Carboniferous rocks of north-east England (yellow, grey and white sandstones, mudstone, limestone, gannister, coal, dolerite), Lower Palaeozoic and Devonian rocks of southern Scotland (wacke-sandstone and mudstone, granite, andesite, red sandstone) and the Permian and younger strata of the North Sea basin (red marl, gypsum, chalk, glaciomarine diamictons). The subgroup was deposited by ice sourced mainly in the Scottish Borders and central Scotland that flowed south-eastwards along the eastern coast of England as far as north Norfolk.

Early and mid Quaternary

The glacigenic deposits occurring across the central part of northern England are assigned to the North Pennines Glacigenic Subgroup. These are dominated by clasts of Carboniferous lithologies (sandstone, limestone, mudstone, gannister, coal), but include some far-travelled clasts from Scotland and the Lake District (wacke-sandstone and siltstone, granite and granodiorite). Tills are generally dark grey.

Glacigenic deposits of this age are assigned to the Albion Glacigenic Group. The oldest Quaternary sediments known from northern England fill subvertical karstic fissures within the limestones and dolostones of the Zechstein Group that form cliffs along the coast of County Durham. The fissures are generally linear, vary from 1 to 7 m wide and extend 25 m vertically down the cliffs, locally along faults. They contain breccias formed of limestone and dolostone, together with a few clasts of red mudstone derived from higher parts of the Permo-Triassic succession now eroded away. Most of the fissure fillings are unfossiliferous and likely to be either Mesozoic or early Palaeogene in age as they are similar to evaporite dissolution breccias that occur elsewhere within the Permian limestones. However, several fissures contain clay that has yielded assemblages of partially pyritised organic material including peat, seeds, tree trunks, ostracods, mammalian bones and freshwater molluscs. The remains of over a hundred species of plant have been identified in this so-called ‘Castle Eden flora’, the majority of which no longer grow in Britain, or are extinct. The likely Early to Mid Quaternary age is supported by the identification of Mammuthus meridionalis, an elephant that was common in Europe up to the Cromerian. Sparse glacial erratics of supposed Scandinavian origin have been found in fissure fill deposits locally, but how and when these erratics got into the fissures is not clear.

The Mid Quaternary spans five or more major glacial–interglacial cycles, but scant evidence of these events has survived in northern England. The oldest known glacigenic deposit is the Thornsgill Till Formation, a deeply weathered diamicton that is preserved in the valleys of Thornsgill and Mosedale, between Keswick and Penrith, in the Lake District. The till is greater than 14 m thick locally and contains clasts mainly of Borrowdale Volcanic Group and Skiddaw Group lithologies, and Threlkeld microgranite, which suggests that it formed beneath an ice cap centred on the Lake District. Analysis of the weathering profile (see ‘Troutbeck Palaeosol’ below) indicates that the Thornsgill Till predates the Ipswichian and may be as old as the Anglian (MIS 12).

The oldest known till in west Cumbria is the Drigg Till Formation, a reddish brown, stony diamicton containing an ‘Irish Sea Coast’ suite of clasts. It was encountered in boreholes in lower Wasdale and around Drigg, stratigraphically below varved glaciolacustrine deposits of probable Early Devensian age ( Figure 67). Some clasts in the till are weathered, but far less so than in the Thornsgill Till. The presence of Criffel granodiorite and other Scottish clasts suggests that the till was laid down in a major glaciation by an ice sheet rather than by a local valley glacier, possibly during MIS 6. This has implications for landscape evolution, because it strongly suggests that lower Wasdale, one of the classic U-shaped, glacially over-deepened valleys in Britain, was largely created before any Devensian glaciations. A similar MIS 6 age is suggested tentatively for till underlying supposed Ipswichian deposits at Dalton­in-Furness, and the Ayre Lighthouse Formation, which includes some 70 m of sand, gravel and diamicton of Irish Sea and Scottish provenance, resting on bedrock at -142 m OD in the north of the Isle of Man.

Probably the best known pre-Devensian till in northern England is the Warren House Gill Till Formation, or ‘Scandinavian Drift’ of earlier accounts. This dark grey, very compact, pebbly sandy clay diamicton crops out on the coast north of Horden, where it occupies the floor of a buried valley at the mouth of Warren House Gill. Recent removal of colliery waste has uncovered the deposit after many decades of burial. The till, which is up to 5 m thick, is distinctive because it contains Scandinavian erratics to the exclusion of rocks from the British mainland excepting local limestones and dolostones. More than 80 per cent of the erratics have been matched with igneous and metamorphic rocks occurring in the Oslo area of southern Norway, including larvikite and nordmarkite. Chalk, flint, red sandstone, red and green (Triassic) calcareous mudstone and belemnite fragments constitute about 6 per cent of the erratics, and numerous marine shell fragments occur within the lower part of the till. This lithological assemblage crops out in the North Sea Basin to the east and north-east, strongly suggesting that the till was laid down by ice that crossed the North Sea Basin from Scandinavia. The age of the glaciation is uncertain. It rests on fissure fillings similar to those yielding Cromerian fossils nearby and contains shells of extinct ‘Arctic’ species, which have probably been reworked. The till underlies the Warren House Gill Loess Bed, a periglacial deposit containing weathered clasts and concretions that probably formed prior to the Ipswichian Interglacial following retreat of the ice that laid down the till. Consequently, the Warren House Gill Till is now generally assigned to the glaciation of MIS 6.

In County Durham, the Easington Raised Beach Formation, which crops out at Shippersea Bay, north of the former Easington Colliery, probably formed during the Mid Pleistocene. The deposit comprises about 4 m of partly cemented gravel and sand containing marine shells and, possibly, rare Scandinavian erratics. It rests on a bevelled platform cut into limestones and dolostones of the Zechstein Group at about 30 m above sea level. Twelve temperate genera of marine mollusc have been identified from the deposit, and borings by marine molluscs and annelid worms in the rock platform and cobbles confirm that the deposit is in situ.

Much debate has centred on the age of the Easington beach deposit, which is very important in establishing the regional lithostratigraphical succession. It has formerly been assigned to the Ipswichian (MIS 5e), because it yielded an ‘infinite’ radiocarbon age greater than 38 ka BP and contained Scandinavian clasts possibly derived from the Warren House Gill Till, thought to be of MIS 6 age. However, subsequent amino acid analyses on shells of Patella vulgata revealed two age populations attributable to the earlier MIS 9 and 7 interglacials respectively, the older population possibly having been reworked into the younger. Furthermore, a pre-Ipswichian, MIS 7 or older age would be more easily reconciled with the height of the deposit above sea level, as it is much higher than other, more reliably dated Ipswichian raised beaches in northern England, particularly at Sewerby, near Bridlington, which lie approximately at present sea level. The preservation of such an old beach fragment at Easington is explicable because the site lies to the south-east of a particularly hard Permian reef knoll that could have protected it from the passage of ice during one or more glaciations.

Ipswichian Interglacial (MIS 5E)

During this interval of climatic amelioration, the district became cloaked in mixed deciduous forest, which eventually included a peculiarly large proportion of hornbeam and alder in addition to birch, pine, oak and holly. Representative deposits are not common, most being only tentatively dated and lacking in stratigraphical continuity. One well-documented terrestrial deposit that crops out in a river cliff of the Scandal Beck, at the southern edge of the Vale of Eden drumlin field, is the Scandal Beck Peat Bed (Figure 68a). It comprises at least 4 m of organic mud, sand, gravel and compressed peat containing pollen, coleoptera and plant macrofossils indicative of the closing stages of an interglacial. The organic deposits occur in the core of a drumlin, overlain by two units of till, but stratigraphical uncertainties remain because weathered diamicton similar to the lower till unit also occurs beneath the organic deposits, suggesting that the latter may have been ice-rafted, although probably not very far. Another possibly Ipswichian organic deposit was found between tills in a borehole near Dalton-in-Furness, south Cumbria.

Elsewhere in northern England, construction of a cutting on the A19 trunk road at Hutton Henry, near Peterlee, revealed a unit of very strongly compressed peat. It contained well-preserved fragments of moss and some samples contained high percentages of hornbeam pollen. The peat bed occurred 6.7 to 8.5 m below ground level near the base of glacial till, which boreholes revealed rested on a sequence of plastic clays, sands and gravels at least 24.4 m thick. The peat was sheared, folded and streaked out towards the south-west in continuity with the fabric of the enclosing till. Field relationships confirmed that it formed a glacial raft.

Weathering profiles of supposed Ipswichian age have been tentatively identified sporadically across northern England, including the Troutbeck Palaeosol. This truncated palaeosol is developed on the Thornsgill Till (see above) and is overlain by younger till. The palaeosol results from the severe in-situ chemical breakdown of mudstone, volcanic and granitic clasts, many of the last being pitted and bleached; it has also been periglacially disturbed, with frost shattering of stones and cryoturbation. The weathering profile is 14 m deep locally, decreasing in severity downwards, and probably required 100 000 to 150 000 years of temperate conditions to form; these could have occurred cumulatively during the Ipswichian and several earlier interstadials of the Mid Pleistocene.

Boreholes drilled at the northern end of the Isle of Man proved 8 m of shelly silts and sands at depths of -65 to -73 m OD. The shell fauna of the unit, named the Ayre Formation, probably represents an in-situ interglacial marine assemblage that can be correlated with the Ipswichian. On the mainland, lenses of buff-coloured clay within red clay were found beneath till and gravel in a borehole drilled near Wigton, Cumbria. The clays apparently rested on bedrock within a buried channel at about -21 m OD and contained the marine snail Turritella communis, foraminifera and ostracods. These fossiliferous deposits may be Ipswichian in age, but were possibly transported as glacial rafts from the Solway Firth during an early stage in the last glaciation, if not earlier, when Scottish ice flowed up the Vale of Eden.

Early and mid Devensian

Glacigenic deposits of this age are assigned to the Caledonia Glacigenic Group. The continental European record indicates that the Ipswichian Interglacial was halted by rapid climatic deterioration at about 115 ka BP. This was followed by a warmer period at about 100 ka BP (MIS 5c), which probably correlates with the Chelford Interstadial, when mixed birch, pine and spruce forest developed in northern England. Cooling recommenced at about 90 ka BP, followed by another warmer period at about 80 ka BP (MIS 5a), the so-called Brimpton Interstadial. Significant cooling then occurred until about 65 ka BP (MIS 4), when a significant Early Devensian glaciation may have affected northern England. However, uranium-series dating of speleothems from caves in the Craven district indicates that low growth was maintained between 90 and 45 ka BP, implying that a tundra-like environment was more likely than full glaciation. Intervals of faster growth at 76, 57 and 50 ka BP suggest three short, warmer interludes.

The only known representative deposit in northern England for one of these early interstadials is the Mosedale Beck Peat Bed, a poorly preserved deposit of compressed woody peat stratigraphically above the Troutbeck Palaeosol. The lower, silty part of the sequence is dominated by non-arboreal pollen indicative of open grassland, whereas the rest of the peat is dominated by willow that probably formed scrubland. Uranium series dating on twigs has yielded ages of 77 and 91 ka BP, which roughly correlate with MIS 5a.

Boreholes sunk in the 1990s around Sellafield and Drigg in west Cumbria have greatly increased our knowledge of Early Devensian events and contemporaneous relative sea levels. The Maudsyke Till Formation, which overlies the aforementioned Drigg Till in lower Wasdale, records a local, Early Devensian glacial advance ( Figure 67). The till is overlain by varved, glaciolacustrine silt and clay, the Carleton Silt Formation, deposited in a proglacial lake that existed for several thousand years. The glaciolacustrine deposits pass upwards into boreal to Arctic marine rhythmites and shelly sands of the Glannoventia Formation. The gradational transition between the formations at -20 m OD records a marine incursion into the lake basin. Amino acid ratios in shell fragments from the Glannoventia Formation are tentatively commensurate with an age of about 60 ka BP.

The Kiondroughad Formation, known from boreholes in the north of the Isle of Man, comprises 60 m of glacigenic sand, gravel and diamicton containing a suite of clasts typical of the Irish Sea Coast Glacigenic Subgroup. It rests partly on an extensive rock platform at between -41 and -53 m OD, partly on the Ipswichian Ayre Formation, and is tentatively assigned to the Early Devensian. Several boreholes between Luce Bay (off south-west Scotland) and Ramsey Bay (off the north-east coast of the Isle of Man) penetrated laminated glaciolacustrine clays that grade upwards into shelly silts and muddy sands between -50 and -70 m OD. These sequences (Luce Bay Formation) are capped by till. The boreal affinities of the micro-and macrofauna of this formation, and its inferred Early Devensian age, invite correlation with the Carleton Silt and overlying Glannoventia formations of lower Wasdale. However, the contemporary sea level has been calculated to be about -30 m OD during deposition of the Luce Bay Formation, some 10 m below the top of the Glannoventia Formation.

There is apparently no record of the Mid Devensian in northern England apart from some fossil bones: those of woolly mammoth were recovered from Hartlepool Docks and those of woolly rhinoceros from gravel underlying till near West Hartlepool. However, pockets of frost-shattered rock and rubbly periglacial deposits that are quite commonly encountered beneath Late Devensian tills may have formed in this predominantly cold substage. One warmer interlude between 50 and 38 ka BP is known as the Upton Warren Interstadial. Uranium-series dating of speleothems from caves in the Craven district suggest that the period from 44 to 34 ka BP was relatively mild, whereas permafrost or total ice cover then prevailed from 34 to 14.7 ka BP.

Main Late Devensian (MLD) glaciation

Most of the glacigenic deposits now preserved in northern England were laid down during the MLD glaciation, between at least 28 and 14.7 ka BP, when the district was overwhelmed entirely by ice apart from, perhaps, some of the highest peaks in the south-west Lake District. The deposits are assigned to the Caledonia Glacigenic Group.

There is growing evidence from the global sea-level record that the Last Glacial Maximum (LGM) occurred relatively early in the Late Devensian, from about 27 ka BP and lasting for about five thousand years. A period of rapidly rising global sea level starting at about 22 ka BP possibly triggered a large-scale glacial reorganisation of the last British ice sheet, which achieved its maximum position at Dimlington, in Holderness, shortly after 21.6 ka BP. Local readvances occurred during overall glacial retreat, mainly involving mobile coastal ice streams. A subsequent, more controversial event, the Scottish Readvance, has been correlated tentatively with Heinrich Event 1. It is thought to have affected the northern tip of the Isle of Man and north-western Cumbria, and may have been contemporaneous with a readvance in north-east Ireland at about 16.7 ka BP. The whole of the northern England district was probably ice-free by 14 ka BP.

Directions of ice flow have been obtained mainly from the orientation of drumlins and other ice-moulded landforms, together with the distribution of striae and glacial erratics ( Figure 68). However, the ice-flow indicators clearly relate to more than one glacial event and generalised directions of ice flow often conflict with those inferred from detailed mapping, lithostratigraphy, till fabric analysis, satellite imagery or digital terrain models. It is clear that local centres of ice accumulation formed important elements of the MLD ice sheet, separated by relatively fast-flowing, topographically constrained, ice streams ( Plate 59). Accumulation centres were positioned over the Langholm Hills in the Southern Uplands of Scotland, Carter Fell and the Cheviot Hills on the Scottish border, and the high ground of the Alston and Askrigg blocks. There the ice remained relatively sluggish and cold-based, depositing little till and causing relatively little glacial erosion. A major, linear ice divide linked the Lake District and the western Pennines across Shap Fell; it was independent of topography and its position shifted northwards during the glaciation. Palaeonunataks identified in the Lake District demonstrate that the surface of the ice sheet stood at between 800 and 870 m OD. These mountains are capped by frost-shattered rock, blockfields and tors that are apparently separated from glacially moulded bedrock at lower elevations by sharp periglacial trimlines. The surface of the ice sheet has been estimated to be about 700 m OD over the Irish Sea basin during the LGM, when even Snaefell (621 m) on the Isle of Man was buried.

The sequence of events that occurred during the MLD glaciation is not fully understood since there is insufficient geochronological control, some phenomena result from more than one phase of glaciation, and the stratigraphical record is beset with difficulties of regional correlation. The MLD ice sheet was dynamic with migrating ice divides, corridors of fast-flowing ice (ice streams) and fluctuating margins that locally surged into proglacial lakes and across the adjacent sea bed. This resulted in multiple local readvances leading to the juxtaposition of tills of markedly different provenance. There are numerous references in the older literature to ‘tripartite sequences’ comprising ‘lower’ tills, ‘middle’ sands, silts and clays, and enigmatic ‘upper’ tills; modern work has shown that the stratigraphy cannot be so simply rationalised.

The Vale of Eden was an extremely ‘congested’ sector of the former ice sheet, for which some widely published glacial reconstructions are glaciologically implausible. For example, Scottish ice is envisaged to have flowed eastwards across the Solway lowlands and through the Tyne Gap contemporaneously with ice flowing westwards from the Vale of Eden, either adjacently, or at different levels in the ice sheet. It is more likely that ice ceased to flow through the Tyne Gap as a consequence of the westward flow becoming established (Figure 68b). This major glacial readjustment probably resulted from changing mass balances of accumulation areas and shifting ice divides that allowed ice to flow into the Irish Sea basin. The drawdown of ice into the basin may have been triggered by rapid global sea level rise at about 22 ka BP causing accelerated iceberg calving.

The pattern of ice flow across the Pennine uplands can be deciphered from swathes of drumlins and other glacially streamlined landforms, which mainly relate to an earlier phase in the glaciation, prior to the major readjustment (Figure 68a). The picture is more complicated on lower ground towards the North Sea coast, where there are thick and complex glacigenic sequences, but relatively weak drumlinisation. Here a powerful, coastal ice stream abutted, diverted and interacted with more locally sourced ice (Figure 68b).

A controversial factor is the role of Scandinavian ice, which is widely believed not to have crossed the North Sea during the Late Devensian, because lodgement tills of this age are largely absent offshore. However, it is now known that low gradient ice sheets flowing over wet, deformable beds, especially in marine areas, are unlikely to form thick units of stony lodgement till. Instead, thin units of fine-grained diamicton with well-dispersed pebbles and shell fragments are produced that can be mistaken for in situ glaciomarine deposits. It seems increasingly likely that the Scottish and Scandinavian ice sheets did indeed coalesce during at least an early phase of the MLD glaciation, and possibly on two previous occasions during the Devensian. This helps to explain why ice from Scotland and the Tweed Basin evidently encroached back onto the coast between Northumberland and north Norfolk rather than simply flowing offshore.

The MLD glaciation of the Isle of Man

The Isle of Man exhibits a very complex sequence of glacigenic sediments because of its location in the centre of the Irish Sea basin, where it has been affected by successive phases of ice flow from Scotland. The entire island was likely to have been glaciated at the LGM, now constrained to the Late Devensian by a radiocarbon date of about 36 ka BP obtained recently from organic sediments beneath the glaciogenic sequence at Strandhall, in the south of the island; crucially, no glacial deposits underlie the organic layer within a topographic basin otherwise conducive for till deposition. Judging from this new evidence, and contrary to previous interpretation, any Early Devensian glaciations affecting the Irish Sea basin must have been of limited extent.

The lowlands in the north of the island are underlain by up to 250 m of Pleistocene sediment assigned to the Irish Sea Coast Glacigenic Subgroup. The upper part of the sequence is exposed in a 25 km stretch of coastal cliff sections that reveal an exceptional range of glacigenic deposits and large-scale glacitectonic structures ( Plate 61). The products of three glacial episodes are contained within three offlapping and southward-tapering glacigenic formations that are separated by significant unconformities and abut the Bride Moraine, a major push moraine complex ( Figure 69). The Shellag Formation, which formed during the LGM, is composed mainly of deformed, shelly, clay-rich, clast-poor deformation till with subordinate beds of sand and gravel. The stratigraphically higher Orrisdale and Jurby formations are composed of complex interbedded units of deformation till, glaciofluvial outwash, sediment gravity flows, and glaciolacustrine and deltaic deposits that possibly record up to seven readvances of the ice sheet margin. Based on radiocarbon dates on organic sediment within kettlehole basins in the Orrisdale and Jurby formations, the readvances are thought to have occurred between 22.4 and 17.3 ka BP.

The MLD glaciation of north-west England

Erratics of granodiorite and wacke from the Southern Uplands occur in discrete units of till within bedrock depressions and in the cores of some drumlins in the Vale of Eden ( Figure 70). A Devensian age is likely, as the tills are not particularly weathered. The stratigraphical position and fabric of these diamictons, which are assigned to the Gillcambon Till Formation, indicate that Scottish ice flowed up the Vale of Eden, across the Stainmore Gap and towards Teesside and the Vale of York. It was joined in the upper Vale of Eden by ice flowing from the northern Lake District. Scottish ice also flowed eastwards through the Tyne Gap. The timing of this ‘Early Scottish Advance’ is unclear, but it probably occurred during the LGM, if not earlier in the Devensian (Figure 68a). Scottish ice was subsequently displaced from the Vale of Eden, though ice sourced in the Galloway Hills continued to flow through the Tyne Gap for a while, where it was joined by ice flowing from the northern Lake District. Ice funnelled across Stainmore from both the Lake District and Cross Fell, to create the drumlins of the Stainmore Suite. The ice would have reworked previously deposited tills containing Scottish clasts. Ice apparently did not cross the Howgill Fells. The absence of Scottish erratics within the mountainous parts of the Lake District, and the pattern of its glaciated valleys, suggests that throughout the MLD glaciation it was occupied by a local ice cap with radial outlet glaciers.

The shape and distribution of elongate drumlins in the Vale of Eden and around the north of the Lake District ( Plate 59), together with the composition of glacial erratics, indicate unambiguously that ice subsequently flowed down the Vale of Eden, swinging to the west around the northern Lake District and thence into the Irish Sea basin (i.e. in the opposite direction to the earlier situation). From the northern Lake District towards the Solway lowlands at Carlisle there is a northward progression of drumlin style — from spindle-shaped, through less elongate forms, to transverse rogen moraines — that apparently indicates a diminishing rate of flow towards the lower ground where ice was thickest. The glacial reorganisation, which possibly followed partial deglaciation, is also recorded between Appleby and Brough, where the Stainmore Suite of drumlins is overprinted by the younger Howgill Suite, created by ice flowing northwards from an important ice divide crossing the Howgill Fells. The reorganisation is probably recorded stratigraphically at the Scandal Beck interglacial site where, despite possible glacitectonic disturbance, the sandstone-rich till overlying the peat bed, and the uppermost limestone-rich till, have been correlated with the Stainmore and Howgill suites of drumlins respectively.

The upper till deposited in the Vale of Eden following the reorganisation is named the Greystoke Till Formation. Although most clasts are derived from the Lake District and Edenside, it contains some Scottish erratics, especially towards the north, that have probably been reworked from the deposits of the Early Scottish Advance. The Lowca Till and Ravenglass Till members of the Seascale Glacigenic Formation were probably formed at this time in west Cumbria, by ice swinging around the north of the Lake District, picking up clasts from the Cumbrian Coalfield. The tills are generally red, extremely compact and stony; they are up to 10 m thick, but locally infill buried valleys 70 m deep. In lower Wasdale, the Ravenglass Till rests on diamictons dominated by Borrowdale Volcanic Group lithologies (Holmrook Till Member) ( Figure 67), indicating that locally sourced ice from the Lake District was displaced southwards. An increase in the proportions of Scottish erratics in tills occurring toward the top of several multi-till sequences in west Cumbria indicates that Scottish ice eventually became even more dominant.

Extensive deposits of glaciofluvial sand and gravel were laid down in the Tyne Gap and Vale of Eden following the ice sheet reorganisation (Baronwood Sand and Gravel Formation). They occur as ice-marginal fans, flat-topped plateaux, mounds, ridge systems and terraces. Fans and plateaux commonly include sequences that coarsen upwards from laminated silt and clay, through fine- and coarse-grained sand to cobble gravel, indicating that they were deposited as deltas within ice-marginal lakes or ice-walled enclaves within stagnant ice. Ridge systems commonly include cores of cobble gravel partially overlain and surrounded by sand and gravel that fine upwards, an assemblage that is typical of eskers formed subglacially. Terraces and benches are generally formed by dense gravel, laid down as glaciofluvial outwash. All deposits include lenses of, and are locally capped by, red gravelly diamicton. The ‘Brampton Moraine Belt’ east of Brampton exemplifies this suite of landforms. It is an ice-marginal, glaciofluvial complex bounded by ice-contact slopes that face west or north-west and includes the Hallbankgate Esker, which extends towards a major subglacial drainage channel within the Tyne Gap ( Plate 59).

There are numerous glacial drainage channels around the Vale of Eden, including a splendid series that descends northwards obliquely down the western slopes of the North Pennine escarpment. Most channels were formed in the vicinity of the ice margin at transitory positions of actively retreating ice. They record progressive lowering of the ice surface in conjunction with frontal retreat, which led to eventual separation of ice emanating from the Lake District and Scotland with the formation of intervening lakes (higher levels of ‘Glacial Lake Carlisle’). Although some channels within the Vale formed subglacially beneath stagnating ice where steady-state, dendritic, meltwater channel systems developed, in general the ice probably remained active during retreat.

There is a recurrent conclusion in the older literature that one or more major glacial readvances of Scottish ice occurred across the Solway lowlands and the coast of west Cumbria during the later stages of the last glaciation (Figure 68c). Several limits have been postulated for the main event, the Scottish Readvance, that extend across the Irish Sea, link with the Bride Moraine in the north of the Isle of Man and continue into Ireland, but none has proved sustainable. Nevertheless, the readvance concept has recently regained support following the discovery of fresh evidence in the Solway lowlands, west Cumbria, the Isle of Man and Ireland.

Evidence for a readvance of Scottish ice was first reported from the Carlisle district and subsequently two readvances, the ‘Gosforth Oscillation’ and the subsequent Scottish Readvance sensu stricto were established in west Cumbria. It has since been confirmed from extensive investigations in west Cumbria that the Gosforth Oscillation event, in particular, witnessed widespread deposition of fine-grained sediment in the lower reaches of the Eden Valley and in lower Wasdale (Seascale and Aikbank Farm Glacigenic formations) ( Figure 67). The traditional view is that these bodies of sediment were deposited in proglacial lakes dammed at the eastern margin of Irish Sea ice, but alternatively they may have been lain down subglacially within ‘tunnel valleys’. Whatever their origin, the deposits were then overridden and glacitectonically disturbed during several subsequent local glacial readvances of Irish Sea ice that caused minimal subglacial erosion, yet laid down thin, widespread units of sandy or clayey diamicton containing well-dispersed pebbles and shell fragments. This thinly bedded and heterogeneous package of sediments constitutes the Gosforth Glacigenic Formation. There are minority views that tills of the Gosforth Glacigenic Formation are either solely the products of proglacial, cohesive debris flows or, more contentiously, are glaciomarine mud drapes.

The type area for the Scottish Readvance is the St Bees Moraine, where there are splendid cliff sections through proglacially tectonised deposits ( Figure 71) & ( Plate 62). However, the regional significance of the Scottish Readvance at St Bees remains problematic. It has generally been correlated with the creation of the Bride Moraine on the Isle of Man, which has been recently linked with Heinrich Event 1. The Gosforth Oscillation, and likely subsequent readvances, all occurred in response to expansions and contractions of the Irish Sea ice stream, which repeatedly encroached on the Cumbrian coast to override ground previously glaciated by ice emanating from inland sources.

Fine-grained glaciolacustrine deposits of the Great Easby Clay Formation were laid down in the Carlisle area when deglaciation was interrupted by a readvance of Scottish ice, such that ice occupied the Solway Firth and blocked drainage within the Solway lowlands ( Figure 72). The levels of ‘Glacial Lake Carlisle’ thus formed may have first been dictated by the heights of overflow channels within the Tyne Gap, exploiting previously formed subglacial channels, and later by ice-marginal channels to the south-west of Carlisle. A misfit valley linking the modern rivers Caldew and Wampool by way of the Dalston Gap, south-west of Carlisle, functioned as a major glacial spillway ( Plate 59). The Great Easby Clay Formation includes dark reddish brown clays, silts and very fine-grained sands that are generally thinly laminated and locally varved. The laminated deposits contain sparse dropstones and convolute bedding; slump and water-escape structures are common. Sands and gravels deposited in and around the lake to the north have been assigned to the Plumpe Sand and Gravel Formation.

The Great Easby Clay and Plumpe Sand and Gravel formations (formerly known as the ‘Middle Sands’) overlie an extensive landscape unconformity developed on till containing Scottish clasts and probably laterally equivalent to the Gillcambon Till of the Vale of Eden. The age of the unconformity is unknown, but it possibly formed during a partial deglaciation of the MLD ice sheet following the LGM, but before the Scottish Readvance at St Bees.

The glaciolacustrine and glaciofluvial sediments that were deposited in and around Glacial Lake Carlisle, and other more ephemeral lakes, have been disturbed glacitectonically and are capped by red diamictons of the Gretna Till Formation, which is dominated by Scottish clasts. A readvance of Scottish ice clearly occurred there, but synchroneity with the ‘Scottish Readvance’ at St Bees seems unlikely, especially since parts of the Great Easby and Plumpe formations have been incorporated within drumlins sculptured by ice that flowed from the Vale of Eden as well as Scotland. The glacial limit of the readvance of Scottish ice across Lake Carlisle is not clear, but it probably lies just to the south of the city where new NEXTmap imagery ( Plate 59) reveals an abrupt termination of a suite of low, south–north orientated, elongated drumlins.

Deposits of glaciofluvial sand and gravel were laid down during the final retreat of Scottish ice. Several parallel eskers were created by meltwaters that flowed subglacially towards an ice margin in the vicinity of the Dalston Gap overflow channel. They are draped on drumlins formed earlier by ice that flowed in the opposite direction. A prominent terraced plateau at Holme St Cuthbert, north of Maryport, subsequently formed as a delta at the margin of ice lying to the north-west ( Plate 59).

The MLD glaciation of north-east England

At the LGM, north-eastern England was mainly a receiving area for ice, inundated by powerful streams that flowed eastwards across the Pennines via the Tyne and Stainmore gaps. The Tyne stream flowed towards the coast south of Blyth, whereas the Stainmore stream occupied the Teesside lowlands and possibly branched southwards into the Vale of York (Figure 68a). An ice cap on Cross Fell fed eastwards down the upper dales of the Wear and Tees, where till was deposited mainly within the valleys. Another, larger and independent ice cap was positioned over Carter Fell and the Cheviot Hills, where it was almost encircled by a combination of ice flowing from the south-west and a substantial ice stream flowing from the north-west through the Tweed Basin. Blocky periglacial deposits and decomposed bedrock are particularly common within the Cheviot Hills, where there has been relatively little glacial erosion.

As discussed above, a major, undated glacial readjustment occurred within the Vale of Eden, probably following the LGM, eventually leading to a complete reversal of flow. An immediate consequence of this reversal would have been a weakening of the eastward flow of ice through the Tyne Gap and Stainmore. The Tweed ice stream, together with ice from central Scotland, was deflected south-eastwards, parallel to the coast of northern England. Augmented by ice sourced in the Cheviot Hills, it became a powerful ‘North Sea ice stream’ that constrained ice flowing from the Pennines, deflecting it south-eastwards, possibly partially into the Vale of York (Figure 68b). The most likely reason for the deflection is that Scandinavian ice occupied the North Sea Basin. The ‘North Sea ice stream’ laid down the distinctive suite of deposits assigned to the North Sea Coast Glacigenic Subgroup and found along the coast as far south as Norfolk ( Figure 66).

The traditional view is that there followed a substantial retreat of ‘Pennine’ ice and possibly a contraction of the North Sea ice stream too, but the latter remained immediately offshore, obstructing drainage. As the two bodies of ice decoupled and separated northwards, large ice-marginal lakes occupied the Durham lowlands, particularly in the valleys of the rivers Tyne and Wear, west of the escarpment formed by Permian (Zechstein Group) carbonate rocks. The largest was ‘Glacial Lake Wear’, which stood at several levels (132, 90 and 43 m OD) governed by the elevations of spillways that became available sequentially, including the Ferryhill Gap [NZ 300 330]. Another, Glacial Lake Edderacres, lay in the vicinity of Peterlee. A very substantial volume of laminated sand, silt and clay (formerly known as the ‘Middle Sands’) was deposited within these bodies of water ( Figure 73).

There is very good evidence, including glacially deformed glaciolacustrine deposits and the widespread occurrence of an ‘upper’ till towards the coast, that the North Sea ice stream encroached inland during a late-stage readvance. The western limit of this readvance is represented by a belt of morainic deposits strewn with kettleholes lying to the east of Castle Eden (Elwick Moraine Member). There is some evidence for a contemporaneous advance of Pennine ice into the lakes, which suggests that the event was climatically driven rather than simply involving a surge of the potentially unstable North Sea ice stream, but although the sequence of events is generally agreed, the chronology is not yet known.

Tills laid down by Pennine ice (North Pennines Glacigenic Subgroup) are generally dark brown to grey, very compact and stony. They form a gently undulating, 5 to 10 m thick sheet across much of lowland Northumberland and Durham, but within the Tyne Gap, Weardale and Teesdale they are strongly drumlinised and more variable in thickness. The till sheet thins and becomes more patchy across interfluves, thickening locally in the lee of bedrock highs and into concealed valleys, where thicknesses locally exceed 80 m. Erratics of a variety of rock types are common in the till sheet with some, better described as glacial rafts, large enough to have been quarried.

The tills have been named only where it is possible to distinguish more than one unit separated by glaciolacustrine or glaciofluvial deposits of the former ‘Middle Sands’. This so-called ‘tripartite sequence’ is mostly restricted to the Durham lowlands, where the Wear Till Formation (formerly the Durham Lower Boulder Clay) generally lies directly on rockhead, and is the thickest, most laterally extensive, stony and consolidated of the till units ( Figure 74). Its erratics include: andesite, tuff and granite from the Lake District; wacke and granodiorite from southern Scotland; red sandstones probably of both Triassic and Devonian age and from a variety of sources. The upper of the two tills, the Butterby Till Member (formerly the Upper Stony Clays in part), contains a similar suite of clasts to the Wear Till, but is less compact and stony. It is generally thin and patchy, but up to 15 m thick locally. It is widely believed to have formed either as cohesive debris flows from ice margins or as solifluxion flows from surrounding deglaciated slopes. However, its wide extent, relatively consistent thickness and the dispersion of its clasts suggest that it formed subglacially as a deformation till, possibly during a glacial readvance.

The Butterby Till is difficult to distinguish from, and locally passes laterally into, the enigmatic Pelaw Clay. This widespread, surficial unit of reddish brown silty clay contains well-dispersed stones and commonly small calcareous concretions towards the base of the weathering zone. It occurs up to an elevation of about 132 m OD, is generally 0.5 to 2 m thick, though locally up to 9 m, and includes contorted beds of sand. It generally rests on fine-grained glaciolacustrine deposits and is widely believed to have formed by periglacial processes and mass flowage following the draining of the ice marginal lakes.

The predominantly glaciolacustrine deposits sandwiched between the aforementioned tills in the Durham lowlands form the Tyne and Wear Glaciolacustrine Formation (formerly known as the Durham and/or Tyne-Wear complex) which is locally up to 60 m thick. This complex sequence was deposited in Glacial Lake Wear and associated lakes; it tends to fine upwards and southwards, but with fine-grained units successively overlapping coarser ones northwards. Thinly laminated, greyish brown to brownish grey silty clay and micaceous silt predominate with subordinate units of fine-grained sand, gravel and diamicton formed of pebbly sandy clay. Complex interdigitation of units is common, especially towards the coast. Dropstones are common and crustacean traces have been noted in laminated silt in the former Herrington opencast coal site ( Figure 73).

Tills and related deposits laid down by the North Sea ice stream (North Sea Coast Glacigenic Subgroup) crop out up to about 15 km inland of the coast. As seen farther inland, two widespread till units may be distinguished where glaciofluvial or glaciolacustrine deposits intervene, commonly within concealed valleys. In addition, cliff sections along the Durham coast and inland in incised stream valleys (denes) reveal isolated hollows in the carbonate rocks of the Zechstein Group that are filled with cemented gravel and sand beneath the lower of the two tills. These ‘Lower Gravels’ (Limekiln Gill Gravel Formation) typically contain a high (50–60 per cent) proportion of locally derived limestone pebbles and a varied suite of far-travelled clasts including Carboniferous lithologies, andesite, granite, gneiss, schist, flint, quartz, quartzite and dolerite, together with unabraded shell fragments. The age of the gravels is unknown and their genesis unclear. They are almost certainly younger than the Castle Eden Fissure Fills (though they occur in the same general area) and contain a similar suite of pebbles to the overlying till, suggesting that they formed as glaciofluvial outwash in front of the advancing ice sheet. If so, the supposed presence of pristine shell fragments is puzzling.

The lower till of the coastal sequence generally lies directly on rockhead, and is the thickest, most laterally extensive, stony and consolidated of the till units in the North Sea Coast Glacigenic Subgroup. It is generally dark grey to greyish brown, locally reddish brown and up to 15 m thick. It contains clasts mostly of Carboniferous lithologies in Northumberland, with far-travelled rocks from the Southern Uplands and, south of Blyth, additionally from the Lake District; local Permian lithologies predominate south of the Tyne. Along the coast of Durham it is named the Blackhall Till Formation and is overlain extensively by sandy, glaciofluvial and glaciolacustrine deposits of the Peterlee Sand and Gravel Formation (formerly part of the ‘Middle Sands’). The Peterlee Formation commonly coarsens upwards to gravel from red fine-grained sand, silt and clay, but within buried valleys may locally comprise up to 30 m of laminated clay. The cross-bedded Ryhope Sand Member of the formation formed at an early stage when Glacial Lake Wear drained eastwards, subglacially, towards the coast north of Seaham. The Blackhall and Peterlee formations are overlain by the Horden Till Formation (formerly known as the Durham Upper Boulder Clay), which is typically a weathered, brown or reddish brown stony clay containing a relatively higher proportion of clasts from upper parts of the Zechstein Group than is seen in the lower till; it also contains clasts of volcanic rock from the Cheviot Hills.

The youngest widespread till-like deposit in the coastal lowlands is the Prismatic Clay, which derives its name from closely spaced, subvertical, prismatic jointing that probably formed in dry periglacial conditions. It is generally less than 1 m thick, dull brown, and differs from the more widespread Pelaw Clay in being more sandy and containing smaller pebbles. The Prismatic Clay apparently overlies ice wedge casts in the Ryhope Sands and so is probably of periglacial origin. A similar red or reddish brown deposit caps the Teesside Clay Formation, which is widespread beneath the Teesside lowlands up to about 92 m OD. Varves in the laminated Teesside Clay have been tentatively matched with the late glacial Greenland ice-core record, and dated very approximately by thermoluminescence to between 18.0 and 18.5 ka BP.

The fine-grained deposits of the Tyne and Wear Glaciolacustrine Formation commonly occur within buried, concealed valleys that cannot be delineated easily at the surface. These features occur beneath the coastal lowlands, where ice flowed south-eastwards across pre­existing valleys such as the lower reaches of the Tyne and Wear, and they are particularly common to the south of Durham. Most are palaeovalleys cut by drainage flowing directly towards the North Sea before at least the last glaciation. Others, such as beneath the Team Valley, were carved out by ice across the grain of the country. Some concealed channels have been graded to a base level below -30 m OD during periods of low sea level, but others have humped longitudinal profiles and clearly formed subglacially.

Although the locations of many concealed valleys are known from borehole records, it is rarely possible to examine closely the sequences filling them, which are generally complex and difficult to understand. A rare example was revealed in 1999 during excavation of the Maiden’s Hall opencast coal site, 13 km north-north-east of Morpeth. There, an approximately 30 m deep, east–west orientated linear depression cut into bedrock is concealed beneath till of the North Sea Coast Glacigenic Subgroup ( Figure 75) and ( Plate 63). A basal unit of weathered, shelly till of possible pre-Devensian age is confined to pockets at the base of the incision, which are overlain by iron-stained gravel deposited in a braided river environment. The overlying succession includes seven or more distinct cyclic sequences made up of units of laminated silt and clay, sand, gravel and diamicton, many of the latter containing well-dispersed clasts typical of deformation till. A small, truncated, gravel-filled channel preserved beneath one of the diamictons toward the top of the sequence was almost certainly formed by subglacial drainage. The uppermost till unit oversteps the entire succession within the depression to rest on bedrock at its margin. The rhythmic sequence was probably formed entirely subglacially as a result of seven or more surge events of the North Sea ice stream across a ‘tunnel valley’, although readvances of ice across an ice-marginal lake confined within the channel cannot be ruled out.

Most elements of the Maiden’s Hall sequence may be seen in cliff sections at Sandy Bay, Whitley Bay and Whitburn Bay, and it is likely that much of the Tyne and Wear Glaciolacustrine Formation was laid down within tunnel valleys before the overlying ice thinned and broke up to form ice marginal lakes such as Glacial Lake Wear. This would help explain the absence of totally convincing evidence for the extent of the lakes shown in ( Figure 73), such as widespread shorelines and deltas, and the presence of extensive concealed glaciolacustrine deposits that occur towards the Ferryhill Gap and southwards towards Middlesbrough and Darlington.

Following the creation of Glacial Lake Wear and associated lakes, the Pennine ice shrank into separate glaciers that retreated up the major valleys leaving proglacial trains of outwash sand and gravel and moraine, much of which was subsequently reworked into terraces by meltwater. Substantial deposits occur within the South Tyne valley up to Haltwhistle, and westwards into the Tyne Gap col at Gilsland. In Northumberland, meltwaters were concentrated a few kilometres inland from the coast along the former suture zone between Cheviot–Carter Fell ice and the relatively active North Sea ice stream. This resulted in a string of glaciofluvial ice-contact deposits that probably formed sequentially northwards as the two ice masses decoupled and separated. A good example is the complex linear assemblage of features that stretches 13 km south-south-east from Spindlestone [NU 152 233]; components include eskers, flat-topped open crevasse fills, and the well-known Bradford Kames. Another complex of interconnected eskers, kames and plateaux studded with kettleholes subsequently formed to the east, when ice stagnated south-east of Wooler. Further retreat led to the deglaciation of a large topographical depression between the Cheviot Hills and the Fell Sandstone cuesta in northern Northumberland. A glacial lake was impounded there, to the south of the dwindling Tweed ice stream, in which at least 22 m of laminated silts and clays were deposited, and then overlain by alluvium, to form the Millfield Plain.

Numerous glacial drainage channels occur in the area. Most were cut sequentially within ice marginal zones during deglaciation, like those previously described along the Northern Pennine escarpment. Others were cut earlier across cols when ice was less constrained by the underlying topography. Fine examples of subglacial channels with humped longitudinal profiles include the Beldon Cleugh [NY 915 505] and East Dipton [NY 962 600] channels to the south of Hexham, and the Butt Hill channel [NY 627 504] that carried water across the main Pennine watershed north-west of Alston. The Humbleton Hill channels and the Throws are two classic, anastomosing channel systems that lie 4 km west of Wooler in the eastern Cheviot Hills.

Late glacial period

The period of time between the diachronous retreat of the MLD ice sheet from any given area, and the beginning of the Holocene at 11.55 ka BP, is referred to as ‘late glacial’. It includes the Windermere Interstadial and the Loch Lomond Stadial, both periods of climatic instability ( Figure 65). Deglaciation commenced in a cold, arid environment and high parts of the district probably witnessed several thousand years of ice-free, periglacial conditions before the onset of rapid warming at about 14.7 ka BP, the beginning of the Windermere Interstadial. The abrupt amelioration in climate occurred when temperate waters of the Gulf Stream returned to the western coasts of the British Isles. River terraces and alluvial fans formed across the district by paraglacial processes, which swept away loose glacial debris before soils became stabilised by vegetation. Masses of ice buried within glacigenic sediments during all phases of the MLD glaciation melted out relatively slowly to form kettleholes.

Organic sequences preserved within kettleholes may contain pollen, spores and the remains of beetles and midges, and hence provide a valuable record of environmental change: good examples have been described near St Bees and from the Isle of Man. Sediment cores from lake basins and bogs also provide extensive records into the Holocene, with notable results recorded from Windermere in Cumbria and Hawes Water in north Lancashire, from Blelham Bog south of Ambleside, and from Bolton Fell and Walton Moss north of Brampton. The lake basins in particular demonstrate the classic ‘tripartite’ late-glacial sequence: cold stage clastic deposits overlain by organic or carbonate-rich interstadial material, overlain by clastic sediment representing the Loch Lomond Stadial. At the start of the interstadial in northern England, summer temperatures rose by 7ºC within a decade or so, peaking at about 18ºC. Freshly deglaciated ground was first colonised by a pioneer vegetation of open-habitat, Alpine species, followed by the immigration of crowberry heath, juniper and dwarf varieties of birch and willow. Eventually, open birch woodland developed with juniper and isolated stands of Scots pine locally. An oscillatory climatic deterioration occurred throughout the Windermere Interstadial ( Figure 65) and it is possible that glaciers had already started to build up in the Cumbrian mountains before more sustained cooling began at about 13 ka BP. Giant deer flourished during the interstadial, but then became extinct.

The Gulf Stream current retreated rapidly to the latitude of northern Portugal at 12.65 ka BP, heralding the start of the Loch Lomond Stadial. Summer temperature across northern England declined to an average of about 11.5ºC and a tundra environment became ubiquitous. Only plant communities tolerant of the Arctic conditions were able to colonise the unstable soils. The stadial witnessed the demise of large herbivores, including the woolly mammoth and woolly rhinoceros, but the role of hunting by Upper Palaeolithic man in the extinctions is still uncertain.

Periglacial processes destroyed the immature soils that had developed during the Windermere Interstadial, creating a range of rubbly deposits collectively known as ‘head’ that are especially well developed in the Howgill Fells and Cheviot Hills. There was extensive development of permafrost and ground ice, the latter melting to create fossil ice wedge casts and pingos (circular depressions) in lowland areas, particularly within spreads of glaciofluvial sand and gravel. Polygonal networks of ice wedge casts developed within the glaciolacustrine deposits of Millfield Plain. Both fluvial and debris-flow activity was enhanced, especially during springtime snowmelts, and slopes were particularly prone to failure. The sand and gravel that underlies the loamy floodplain alluvium in many of the larger valleys was probably deposited from braided rivers during the stadial.

Fossil periglacial features are widespread, particularly on slopes where intermittent viscous flow of seasonally thawed soils (gelifluction) has occurred. On steep, upland slopes, tear- shaped gelifluction lobes have been washed out to form arcuate lobes of boulders. The lower slopes of valleys and headwater basins commonly take the form of smooth, gently sloping gelifluction terraces that terminate in bluffs along the streams. The periglacial climate was particularly severe in upland areas, causing many rock falls and producing abundant frost-shattered rock. Many areas of talus formed during the stadial though most have since become inactive and vegetated: examples include the impressive Wasdale Screes in the Lake District and the block screes below crags of the Whin Sill and Carboniferous sandstone in Northumberland. Granite tors, and some of thermally metamorphosed andesite, formed on Cheviot hilltops. Blockfields and blanket head formed above about 600 m OD in the north Pennines, commonly displaying large-scale patterned ground composed of polygons and stripes.

At least 64 glaciers are thought to have existed in the Lake District during the Loch Lomond Stadial, mostly small ones on the north-eastern sides of summits and ridges, but probably including some plateau icefields with outlet glaciers. The central Lake District boasts the most extensive suite of glacial landforms, typified by the magnificent cirques and arêtes of Helvellyn. Small glaciers are also thought to have occurred beneath the scarp of Cronkley Scar [NY 840 294] in Teesdale, and within the Cheviot Hills at The Bizzle [NT 920 220]. The Isle of Man apparently remained unglaciated. Fresh hummocky deposits in some of the cirque-shaped valleys along the western fault scarp of the northern Pennines have been interpreted as moraines formed during the stadial, but are more likely to result from rotational landslides. Landslides were widely initiated during the late-glacial period, especially during ice sheet deglaciation, when glacially oversteepened slopes became unstable; they are considered further in Chapter 12.

Holocene

The Holocene began abruptly at 11.55 ka BP when the warm Gulf Stream current became re-established, providing an ameliorating influence on the climate of the British Isles. Average summer temperatures rose by about 8ºC within 100 years. The widespread occurrence of bare, unstable soils at the beginning of the Holocene led to intense fluvial erosion and deposition, with enhanced debris flow activity on mountainsides and extensive formation of landslips as the ground thawed. The rivers were generally braided with gravelly beds. Soils gradually became more stable following the establishment of vegetation, firstly by pioneering herbs, shrubs and scrub communities similar to the succession of the Windermere Interstadial, and later by woodland.

Trees colonised the district from the south, at first mainly via the coastal lowlands, which were much more extensive than today owing to lower sea level in the early Holocene (see below). Later arrivals came via the Lancashire lowlands and the Vale of York. Most species spread relatively late into the Cumbrian Mountains, Pennines, Cheviots and particularly the Isle of Man. A distinct early phase of juniper dominance was quickly replaced by birch and willow woodland. Hazel became firmly established on the fringes of the Irish Sea and around Teesside by about 10 ka BP, possibly aided by human immigration. An expansion of elm at about 9.6 ka BP generally preceded oak, which arrived between 9.2 and 8.35 ka BP, especially in low-lying fertile areas. Pine spread into southern Cumbria between 9.5 and 8.7 ka BP, and then became established on the Zechstein Group limestone outcrop in County Durham. The arrival of alder at about 7.8 ka BP coincided with the transition from the drier, ‘Boreal’ climate period to the wetter, ‘Atlantic’ one in the mid Holocene ( Table 8). Lime arrived shortly after and all the elements of the mixed mid Holocene deciduous forest were present by 6.7 ka BP when dense tree cover dominated the landscape of all but the highest parts of northern England. Ash, maple, yew and beech all increased later in the Holocene as a result of human intervention.

The Mesolithic hunter-gatherers began to burn forests in the earliest Holocene, perhaps to increase the availability of food resources, but substantial clearance did not begin until about 6.7 ka BP with the arrival of cereal farming in the Neolithic. This preceded the ‘Elm Decline’ — an important biostratigraphical marker in pollen records that defines the end of maximum forest extension at about 5.6 ka BP. Forest clearance increased between 4.3 and 2.9 ka BP in the Bronze Age, with the most extensive deforestation occurring between 2.4 and 1.8 ka BP in the Iron Age. Despite the spread and retreat of the great Holocene forests, many species of the late glacial tundra flora of the district persist to the present day on the limestone soils of Upper Teesdale, including Dryas octopetala (the mountain avens, a small, white-flowered plant of the Rosaceae), after which was named the ‘Younger Dryas’ stage — the alternative name for the Loch Lomond Stadial used in continental Europe. Many wetland sites have also survived throughout the Holocene and preserve a valuable record of climatic change. For example, analyses of protozoan spores, plant macrofossils and humification in peat cores from raised bogs such as Walton Moss, Cumbria, have identified cyclical wetter and drier episodes ( Figure 76).

Although the imprint of glaciation and periglaciation remains dominant, postglacial processes have superimposed subtle, but distinctive modifications on the landscape of northern England, particularly around the coasts. Steep hillsides have been modified by gullying, slope failure, soil-creep and debris flow; valley floors have been sculpted by fluvial erosion and deposition; tidal inlets have become choked with muddy estuarine alluvium and salt marsh deposits. Periglacial processes continue to operate on ground above about 450 m OD, particularly on Cross Fell but also elsewhere, creating a range of similar, but smaller features to those formed during earlier periglacial episodes. Peat growth accelerated at the beginning of the Atlantic climate period with extensive ombrogenous blanket mires forming over the wet uplands and within poorly drained topographical depressions. Diatomaceous deposits accumulated locally in lake basins where there was relatively little sediment influx, such as at Kentmere, near Kendal. The single-thread, submeandering stream patterns of the present day became established early in the Holocene once soils had been stabilised by vegetation, but catchments have been profoundly affected by subsequent deforestation, land drainage, cultivation, overgrazing, mining, gravel extraction and industrial development.

Sea-level change

At present, sea level is relatively high compared with its position during most of the Quaternary, so much evidence of past fluctuations has been either destroyed by marine erosion or submerged. Relative sea levels around northern England have been determined by both depression of the land under the ice load during glaciation and its subsequent recovery — glacio-isostatic effects, and by changes in global sea levels — eustatic effects. The latter are mainly determined by the amount of water contained within continental ice sheets and global sea level has risen from a lowstand of about -120 m OD twenty thousand years or so ago, as those ice sheets have melted. The interaction of isostatic and eustatic effects means that former sea levels, as portrayed in relative sea level curves ( Figure 77), vary considerably around the coast of Britain. The rate of isostatic uplift was greatest during and immediately after deglaciation and has since fallen exponentially. Some differential crustal rebound may have occurred during the Holocene in north-east England leading to the formation of flights of river terraces, the deeply dissected Durham Denes and the entrenched meander at Durham.

Two distinct sets of raised beach and estuarine deposits occur along the coasts of north-west England, where the amount of glacio-isostatic recovery was sufficiently great to bring about an early period of falling sea level. The older, Late Devensian (‘late glacial’) set, which occur sporadically around the Cumbrian coast, was formed during and shortly after retreat of MLD ice, whereas the younger, far more extensive set formed during the mid to late Holocene. The southern limit of Late Devensian beaches defines an important ‘hinge line’ in Britain stretching between Morecambe Bay and Berwick upon Tweed; net postglacial subsidence having occurred to the south; net postglacial uplift to the north (with the greatest isostatic response in the western Highlands of Scotland where the ice load was at a maximum). Raised beaches found along the North Sea coast to the south of Berwick­-upon-Tweed, where eustatic sea level has been the dominant influence, are most probably all of Holocene age. Though some fragmentary raised beaches, rock platforms and marine planation surfaces in this area have been claimed as Late Devensian, the evidence is equivocal. Equally, claims that deglaciation of the Irish Sea basin was accompanied by widespread deposition of glaciomarine sediments up to 150 m OD cannot be substantiated.

The sea-level curves for northern England reveal lowstands in the early Holocene as glacioisostatic rebound outstripped global sea-level rise. Levels as low as 60 m below OD may have occurred within the northern Irish Sea basin, exposing a land bridge that linked the Isle of Man to Cumbria. Several concealed valleys in north-east England, including the Wear and Tyne, were graded to sea level at about 40 m below OD.

The subsequent postglacial transgression resulted in the formation of the set of Holocene raised beaches and associated estuarine silts, fine-grained sands and clays. Detailed investigations along the Northumberland coast reveal that sea level rose from about -5 m to +2.5 m between 9.0 and 2.5 ka BP ( Figure 77), then fell back slightly, but is currently rising again at between 0.7 and 0.1 mm per year. In contrast, around the coast of Cumbria, sea level rose towards a distinct highstand of about +2 m OD by about 7.0 ka BP in the mid Holocene. The result was the main postglacial shoreline marked by fragmentary raised beach deposits of well-rounded shingle backed by a degraded cliff. The Cumbrian coast railway follows stretches of the raised shoreline to the north of Seascale.

Peat beds and tree stumps (‘submerged forests’) are intermittently exposed on the foreshores of both coasts of the district, where they provide clear evidence of lower Holocene sea level. At St Bees, one such deposit comprises compressed peat with wood fragments, nuts and seeds, interbedded with mud and tufa; a calibrated radiocarbon age of about 8.6 ka BP was obtained on a piece of wood. Another well-known submerged forest occasionally emerges on the foreshore in Hartlepool Bay. A Neolithic human skeleton has been recovered from the peat there together with flint artefacts; the bones yielded a calibrated radiocarbon age of 5.4 ka BP.

Chapter 12 Geology and man

This chapter reviews the relationship between geology and human activity in northern England, an association that can be traced back to the Neolithic stone axe industry of Langdale, in the Lake District, which utilised a volcaniclastic siltstone from the Seathwaite Fell Formation, Borrowdale Volcanic Group. Of particular importance in modern times are water and mineral resources: major coalfields lie on either sides of the Pennines, whilst a range of metal ores and a variety of industrial and bulk minerals have all been extracted. The legacy of mineral extraction exerts a profound influence on land use and development, aspects that are also constrained by the natural characteristics of the region’s geology. Landscape conservation is a significant issue in a region that includes two national parks (and part of a third) and three areas of outstanding natural beauty.

Fuel and energy

Coal

As one of the region’s major mineral products, coal provided the main basis for the development of heavy industries. The Westphalian Coal Measures have been most productive, principally from two main coalfields, the Northumberland and Durham Coalfield in the east and the smaller Cumbrian Coalfield in the west. Small, isolated Westphalian coalfields occur as faulted outliers on the downthrow side of the Stublick Fault Zone. A few seams within older Carboniferous rocks have locally yielded significant quantities of coal. Most notable of these are the Dinantian Plashetts, Shilbottle and Scremerston coals of north Northumberland and the Namurian Little Limestone Coal of the South Tyne valley and Alston area.

The Northumberland and Durham Coalfield is the oldest area of commercial coal mining in Britain with records of working on the banks of the Tyne dating from the 12th and 13th centuries. A slow rise in production over the following few centuries was followed by a massive surge in demand during the Industrial Revolution, and by 1800 annual output had reached 2.5 million tonnes. Production peaked at about 30 million tonnes per year around the turn of the 20th century. For the Cumbrian Coalfield, documentary records of working exist from the mid 16th century, though earlier working is likely. Here too, the heyday of mining came during the latter half of the 19th and first half of the 20th centuries with average annual outputs reaching around 2 million tonnes.

A notable feature of the Northumberland and Durham Coalfield is the geographical variation in the rank of the coal, as discussed in Chapters 6 and 7. A coal’s rank largely determines its end use, with low rank coals best suited to domestic or power station use, medium rank coals to gas and domestic coke making, and high rank coals to metallurgical coke making. Coals of high rank occur in west Durham with rank decreasing thence into other parts of the coalfield ( Figure 45). No variation is apparent in the Cumbrian Coalfield, which produced broadly middle-ranking coals. The region’s highest rank coal, a ‘semi-anthracite’, occurs in the Namurian, Little Limestone seam of the Alston area which, at the time of writing, is still mined from drift workings at the Ayle Colliery.

The earliest workings were close to seam outcrops. Then, as mining techniques improved, the extraction of extensive reserves from deeper shafts and larger collieries became possible; ‘pillar and stall’ extraction, where as much as half the coal was left intact for roof support ( Plate 64), was replaced by longwall methods, in which most of the available coal was recovered. The sinking of Hetton Colliery in 1820 opened a new era for the Northumberland and Durham Coalfield by establishing that workable seams extended seawards beneath the unconformable cover of Permian rocks. These reserves were to sustain the industry through the 20th century with mining eventually extending to almost 7.5 km offshore. Similarly in Cumbria, coal mining finally extended up to 6.5 km offshore beneath the cover of Permo-Triassic rocks.

The second half of the 20th century saw a catastrophic decline in the industry’s fortunes, and with closure in 2005 of Ellington Colliery in Northumberland, deep, underground coal mining in the region finally ended. However, the further development of large-scale opencast mining, which had started during the Second World War, enabled continued recovery of substantial tonnages of coal in both coalfields. Opencast extraction continues today at a few sites ( Plate 5) and ( Plate 41), though accessible reserves are limited.

The coal industry exercised a profound effect on the region’s landscape. As the industry prospered, communities expanded around the collieries and were linked by railway networks, whilst huge quantities of waste rock accumulated in spoil heaps. Although much has been cleared, partly through land reclamation schemes associated with opencast mining, evidence of this once-great industry survives in the pattern of settlements, the abandoned railway lines and the former coal exporting facilities at the coasts.

Peat

Blanket peat bogs cover extensive areas of the region’s uplands, whilst lowland peat occurs more locally on valley floors, the sites of former lakes or coastal plains. Historically, small quantities of upland peat were employed as domestic fuel and substantial amounts were used in the lead smelting industry of the northern Pennines, particularly in the 18th and 19th centuries. During the 20th century, most peat working was to supply the horticultural market, with supplies obtained from lowland peat deposits in north Cumbria at Solway Moss, Bolton Fell and Wedholme Flow. The last two of these sites are now scheduled as Sites of Special Scientific Interest, with Wedholme Flow granted additional Special Area of Conservation status as an active raised bog. There is very little current extraction.

Oil and gas

The onshore region of northern England has only very limited hydrocarbon potential. Despite the generally pessimistic assessment, the Carboniferous rocks at the southern margin of the Northumberland–Solway Trough have attracted some exploration interest focussed on closed anticlines at or near the basin margin. A number of seismic reflection surveys and some drilling projects have been undertaken, though so far without the discovery of viable reserves. The discovery and successful development of commercial oil and gas fields in the central and southern parts of the East Irish Sea Basin have stimulated further exploration in sedimentary basins beneath the waters around the Isle of Man.

Coalbed and mine-gas methane

Coal-bearing rocks naturally generate methane as a by-product of coalification — the low-grade metamorphism of peat through lignite to coal and anthracite. Some of this methane migrates from the source rock as it forms, but some remains, either adsorbed or held as a free gas in voids. Although the residual methane presented a serious hazard in underground mining, only rarely was it ever encountered in exploitable amounts. In one such situation, methane from Haig Colliery, Whitehaven, was used from 1950 onwards to raise steam at the pithead; in 1953 methane from this source was supplied to Whitehaven gasworks. In general, the West Cumbrian coalfield has relatively high seam-methane content, but the high density of faulting limits any exploitable resource. The Northumberland and Durham Coalfield generally has low seam-methane levels.

Geothermal energy

Among the high heat-flow granites in the region, the most promising geothermal prospect is the Weardale Granite. Measurements made in the Rookhope Borehole ( Figure 36), drilled in 1960–61, indicated a heat flow of nearly twice the national average and raised hopes that the Weardale Granite might provide energy by the ‘hot dry rock’ process. In this, cold water from the surface is pumped via a borehole into hot, fractured rocks at depth, where it is heated, and then returned to the surface via an adjacent borehole. This premise was tested during the 1980s. Because Rookhope is remote from major centres of population, a test hole was drilled at Rowlands Gill, about 10 km south-west of Newcastle and above a pluton on the north-east extremity of the North Pennine Batholith ( Figure 36). This pluton, like the Weardale Granite, lay immediately beneath the sub-Carboniferous unconformity. Although the granite was not reached, the heat flow recorded in the Carboniferous rocks suggests that temperatures of around 230°C would be attained at a depth of 7 km.

More recent investigations, encouraged by saline thermal springs in the deepest workings of the abandoned Cambokeels fluorspar mine in Weardale, have examined the local use of geothermal energy in the Eastgate area of Weardale. An exploratory borehole here in 2004 proved groundwater at a temperature of 46°C at a depth of 995 metres. Any exploitation could be via a ‘low enthalpy system’ wherein the hot groundwater is extracted directly from deep aquifers.

Industrial minerals

Limestone and dolomite-rock

Many of the limestones of northern England have been worked in the past, though often only for small-scale, local use. Those of Carboniferous age account for the bulk of production; most are within the Great Scar Limestone Group (Visean), but one major unit, the Great Limestone, forms the uppermost part of the Alston Formation (Yoredale Group, Namurian).

In the earliest use of limestone, quicklime and slaked lime were produced as a soil dressing or to make lime mortar. Across most of the region’s limestone outcrops are numerous small limekilns that operated to meet these demands. With improved transport links, lime burning became centralised at a smaller number of quarries where large reserves with consistent quality could be worked more efficiently. However, the greatest expansion in limestone working resulted from the demand for limestone flux created by the rapid growth in iron smelting during the 19th century. Large quarries in several of the thick Visean limestones of the Furness area supplied the iron works of south Cumbria, Visean and Namurian limestones from west Cumbria were employed in the nearby furnaces, and the Namurian Great Limestone of Weardale was extensively quarried for the iron works of north-east England. The decline of iron making, now restricted in northern England to Teesside, ended the large-scale demand for limestone flux, though burnt lime for use in steel making is still produced at Hardendale Quarry, near Shap.

In south Cumbria, Visean limestones such as the Red Hill, Park and Urswick formations are of high purity (more than 97 per cent CaCO3), and the Park Limestone is worked at Stainton Quarry for use in the chemical and water treatment industries. Most of the south Cumbrian limestones also provide good quality aggregates. Farther east, in the Shap–Ravenstonedale area, similar though generally rather thinner high-purity sections are found in the Knipe Scar and Potts Beck limestones. The equivalent formations in west Cumbria are commonly more argillaceous than those of south Cumbria and Shap, but limestone suitable for general-purpose aggregate is worked from several quarries.

Thick, Visean limestones (Robinson and Melmerby Scar) are worked for aggregate on the Pennine escarpment; they are generally of high purity. The Namurian, Great Limestone is worked as a source of aggregate in the northern Pennines and central Northumberland, and also in north Cumbria (where it is known as the First Limestone). In the Isle of Man, Carboniferous limestones are worked for agricultural lime and crushed rock aggregate at Turkeyland and Billown.

Carbonate rocks comprise most of the Permian Zechstein Group in eastern County Durham. Much of the sequence is dolomitic and has been traditionally referred to as the Magnesian Limestone; it is the main source of dolomite-rock, CaMg(CO3)2, in Britain. Dolomite-rock has been of great importance to the steel industry. It was used in the manufacture of sea-water magnesia (at Hartlepool), a material used in refractory bricks for convertor linings and in other applications. Sea-water magnesia production ceased in 2005 but dolomitic lime is still in demand as a flux in steel making. Significant quantities of dolomite-rock are also used for agricultural lime. The Magnesian Limestone is generally inferior to Carboniferous limestones as a source of aggregate, because of its variable character, lower strength and higher porosity. However, it is extensively quarried for a range of construction uses, mostly for fill and sub-base roadstone.

Cement raw materials

Portland cement production normally takes place where the two main raw materials, limestone and clay (or in some cases mudstone), are available. Some gypsum or anhydrite is also required. The Weardale Cement Works, at Eastgate in Weardale, worked the Great Limestone with shale being obtained from the overlying clastic sequence. Cement making was a major industry in Weardale, until the closure of the works in 2002. Portland Cement was also formerly a by-product of sulphuric acid production from anhydrite at Billingham and Whitehaven.

Gypsum and anhydrite

Permian gypsum and anhydrite beds occur within the Eden Shales and St Bees Shale formations of the Vale of Eden and the Cumbrian lowlands. Permian anhydrite, locally altered to gypsum near rockhead, also occurs in the Billingham Anhydrite Formation of Teesside and south-east County Durham.

Anhydrite was formerly mined at Hartlepool for the manufacture of cement, and at Billingham on Teesside, Sandwith Mine near Whitehaven, and at Long Meg Mine near Lazonby, as a raw material for the manufacture of sulphuric acid. This industry ceased in the early 1970s. Much smaller amounts of anhydrite, mainly for use in the glass and cement industries, were mined until fairly recently at Newbiggin Mine in the Vale of Eden.

Gypsum has been worked for plaster and plasterboard manufacture near Carlisle, at Whitehaven, near Darlington and in the Vale of Eden; an underground mine in the last area, at Kirkby Thore, now supplies the region’s only plaster works. Cheap but high quality synthetic gypsum, or ‘desulphogypsum’ produced as a by-product during desulphurisation of flue gases at the Drax coal-fired power station in Yorkshire, has partly replaced natural gypsum as a raw material for plaster making here.

Halite

Brine pumping from beds of halite (salt) within the Triassic Mercia Mudstone Group supported small-scale salt industries at Point of Ayre on the Isle of Man, and on Walney Island, south Cumbria, during the early 20th century. Salt deposits in both of these areas are continuous with more extensive deposits offshore beneath the east Irish Sea; those at Walney Island continue southwards and were formerly also worked at Preesall in west Lancashire. The deposits beneath Point of Ayre and Walney Island are unlikely to be of future economic interest, either as a source of salt or for the development of storage cavities, because of the thinness of the individual halite beds and the presence of wet rockhead.

Beneath Teesside, halite up to 43 m thick occurs within the Permian Boulby Halite Formation. Following the accidental discovery of this salt bed in 1859, it has been extensively worked in the Greatham area by solution mining and formed the basis of the local chemical industry. Brine extraction for chemical use ceased in 2002 but salt solution cavities are still used for storage purposes.

Sand and gravel

The main use of sand is as a fine aggregate in concrete, mortar and asphalt; gravel is mainly employed as a coarse aggregate in concrete. Appreciable quantities of both are also used as constructional fill material. Within northern England, the sand and gravel resources may be grouped into two broad categories: unconsolidated Quaternary deposits and weakly cemented sands of Permian age.

Extensive deposits of Quaternary, glaciofluvial outwash and postglacial river terrace and alluvial sand and gravel have been worked throughout the region, in places on a considerable scale. Holocene coastal deposits of sand and gravel, including blown sand deposits, are also locally important in southern County Durham and the northern part of the Isle of Man. Some marine sand and gravel is dredged from deposits on the floor of the North Sea.

The commercial potential of any body of sand and gravel depends upon a variety of interrelated factors and the depiction of sand and gravel deposits on geological maps cannot be taken as an indication of a workable deposit. Only in the Brampton, Tyne Valley and Millfield Plain areas are systematic evaluations available. In general, glaciofluvial deposits are more variable in composition and particle size than river gravels. Where Coal Measures rocks comprise a major source of the included particles, for example in parts of County Durham, substantial amounts of coal and mudstone fragments within the deposit will reduce its quality.

Because of the regular grain size and weak cement, the Permian Yellow Sands Formation of County Durham comprises an important resource of good quality building sand, with some also being used for asphalting. Large quarries currently work the Yellow Sands, usually in association with the overlying Magnesian Limestone, at Eppleton, in the Sherburn and Coxhoe areas and at Thrislington, near Ferryhill.

Igneous rock

Many of the region’s igneous rocks have been worked for roadstone and aggregate, but whilst these remain important products, they are now derived from a more restricted number of rock types than hitherto.

The region’s largest outcrop of igneous rock, the Ordovician Borrowdale Volcanic Group in the central Lake District, includes a wide range of rock types but very few of these have been worked other than on a small scale for local use. Andesite and volcaniclastic rocks in the Furness Inlier in south Cumbria were formerly quarried on a large scale at Greenscoe, and silicic lapilli-tuff within the Waberthwaite Formation is still worked at Ghyll Scaur Quarry, near Millom, for road surfacing. This rock has a very high polished stone value and abrasion resistance, making it suitable for anti-skid surfaces. Similar silicic lapilli-tuff was formerly worked for roadstone in a large quarry at Knock Pike in the Cross Fell Inlier. Metamorphosed andesite within the aureole of the Shap Granite has a particularly high abrasion resistance and high relative density, making it well suited for use as roadstone and railway track ballast. This rock is today worked at the Blue Quarry, Shap, where some of the crushed rock is additionally employed in the making of concrete products.

Until several years ago, the Shap Granite was itself worked for crushed rock and roadstone at the Pink Quarry, Shap. Other Lake District intrusions formerly worked for crushed rock and roadstone include the Threlkeld Microgranite, the Eskdale Granite, the Broad Oak Granodiorite and the Embleton Diorite. On the Isle of Man, crushed rock aggregate and roadstone are produced from the Foxdale Granite at Stoney Mountain, and from the dolerite intrusion at Poortown, near Peel.

In the region’s other large volcanic complex, Cheviot, only a small intrusion of microgranite at Alwinton has been substantially worked, at Harden Quarry, for crushed rock and roadstone. Its bright red colour makes it sought after for specialised uses.

Known locally as ‘whinstone’, the dolerite of the Whin Sill-swarm has a high polished stone value, making it ideal for use as a roadstone. It has long been quarried at numerous sites in Northumberland and County Durham and large tonnages are today extracted from quarries at Belford, Longhoughton, Barrasford, Swinburne and Divethill in Northumberland, and from one large quarry near High Force in County Durham. Roadstone and crushed rock aggregate are the main products, with larger blocks occasionally employed as rip-rap or as armour stone for coastal defences.

Palaeogene dolerite dykes have been worked locally for roadstone: the Acklington Dyke at Acklington; the Cleveland Dyke at Barrock Fell near Armathwaite, at Cockfield Fell and at Bolam in County Durham.

Clay and shale

Small brickworks utilising a great variety of clays were formerly common in many parts of the region. Raw materials included Quaternary glacial till, laminated lacustrine clays, alluvial and marine clays and silts; Carboniferous mudrocks have also been used. Until recently, large brickworks at Birtley, near Gateshead, worked substantial deposits of laminated clay within the Team valley. Glacial clays are still worked today for brick-making on a small scale at Thrunton, near Alnwick. Many Coal Measures mudstones are suitable for making ‘common’ bricks and it was usual in the 19th and early 20th centuries for brick works to be attached to collieries.

The brick industry is now based on a small number of plants producing high-quality facing bricks, engineering bricks and related products such as clay pavers. Modern brick-making technology requires raw materials with predictable properties and consistent firing characteristics, with blending of different clays, including fireclays (see below) employed to achieve improved durability and a range of fired colours and textures. Lower and Middle Coal Measures mudstones, quarried in the Bishop Auckland and Gateshead areas, together with mudstone recovered as a by-product of opencast coal extraction, supply large modern brickworks at Bishop Auckland and Throckley. Skiddaw Group mudstones in the Furness area supply a brickworks near Askham.

Fireclay

Fireclay, known in northern England as ‘seggar’, is a palaeosol, a non-marine mudstone upon which grew coal-forming vegetation. It generally occurs as comparatively thin (less than 1 m), homogeneous beds with abundant rootlet traces. Fireclays exhibit a wide range of mineralogical compositions and properties that determine their vitrification characteristics and colour after firing. Consisting essentially of kaolinite, hydrous mica and fine-grained quartz, fireclays have high alumina and low alkali contents, hence the refractory properties.

Historically, fireclays were mainly used in the manufacture of refractory products for use in the iron and steel industries. The fireclay was commonly extracted as a by-product of coal mining but there were also many dedicated fireclay workings. The distinctive, pale buff-coloured firebricks, produced at numerous collieries, remain a conspicuous feature of vernacular architecture in parts of the Northumberland and Durham coalfield. Fireclays were also much used in the manufacture of salt-glazed pipes and sanitary ware.

Demand for refractory products declined sharply after the late 1950s, as a result of changing technology in the iron and steel industry. Today, only very small amounts of fireclay are used for refractory purposes. Significant amounts are still recovered as a by-product of opencast coal extraction and are in demand as raw materials for brick-making, either to produce pale buff-coloured bricks or to blend with other clays.

Sandstone

Although the principal use of sandstone within the region is as a building material, a variety of sandstones have been employed for other purposes.

Hard wacke sandstone within the Silurian Kirkby Moor Formation has very high polished stone values and is quarried for roadstone at Holmscales and Roan Edge, near Kendal. The quartz arenite-dominated Creg Agneash Formation in the Ordovician Manx Group is today worked for crushed rock aggregate at Dreemskerry on the Isle of Man.

Several of the region’s ‘gritty’ Carboniferous sandstones proved ideal as grindstones. Particularly well known were the so-called Newcastle Grindstones obtained from prominent sandstone beds (the ‘Grindstone posts’) in the Middle Coal Measures of the Wrekenton and Springwell areas, near Gateshead.

Siliceous sandstone, including the seatearths found beneath some coal seams, is commonly termed ganister. It was well-suited to the manufacture of silica refractories. Ganisters were formerly worked from the Namurian sequence around Castleside and Healeyfield, near Consett, from Harthope Quarries and elsewhere in Weardale, and at Distington and Branthwaite in west Cumbria.

Deeply weathered, coarse-grained feldspathic sandstones are common in the Carboniferous sequences of north-west County Durham and adjoining parts of Northumberland. They formerly provided excellent sources of natural moulding sand at several localities including Castleside and near Blanchland.

Diatomite

Diatoms are a form of siliceous algae, and substantial accumulations of their fossilised remains occur in some postglacial lake deposits in the Lake District. The only economically workable deposit of diatomite in England was exploited there, between 1924 and 1975 at Kentmere, near Kendal, and was used in the manufacture of insulation. The diatomite bed was between 4.5 m and 11.5 m thick over an area of approximately 30 hectares beneath the alluvium of the River Kent.

Graphite

One of the region’s most famous mineral deposits is the graphite deposit at Seathwaite in Borrowdale. Mining here dates back to at least the 16th century and continued intermittently until 1891. Originally worked for making crucibles and moulds for casting weaponry, the remarkably pure graphite eventually became the basis for the pencil industry which survives today in Keswick, though now using imported raw materials.

Metalliferous and associated minerals

Numerous scattered sites show evidence for the early use of the region’s varied range of ores, but the first documentary evidence for metal mining comes from the Norman period when, in the 12th century, iron, lead and silver were recorded as being produced from the northern Pennines.

With the establishment under Elizabeth I of the Company of Mines Royal, and the introduction of new mining and smelting technology, mainly from Germany, parts of the Lake District assumed world significance for copper production in the 16th century. By the 18th and on into the 19th centuries, mining and smelting in the northern Pennines, the Lake District and the Isle of Man, were at the forefront of the world lead industry. A collapse in metal prices towards the close of the 19th century brought about the closure of most of the region’s mines, but an increasing economic interest in zinc ores (and the accompanying gangue minerals fluorspar, baryte and witherite) helped to offset the worst economic effects. The 19th century also witnessed the heyday of iron mining in south-west Cumbria, an industry that continued on a large scale until 1980.

Iron

Sedimentary ironstones with typical iron content of around 25 per cent are locally common in parts of the Coal Measures. They were of greatest economic significance in north-east England, where prominent beds of clay ironstone were the basis for the establishment of ironworks at Consett, Bedlington, Tow Law, Witton Park and elsewhere. During the 19th century, their working was ended by the discovery of the much larger deposits of Jurassic ores in Cleveland, and the increased availability of high-grade haematite ore from Cumbria.

Clay ironstones are also present locally within the Visean succession. The most economically important were the Redesdale Ironstone Shales, which were worked in the Ridsdale and Bellingham areas of Northumberland in the mid 19th century. Iron from this source had the distinction of being used in the construction of Robert Stephenson’s High Level Bridge over the Tyne at Newcastle.

Iron carbonate minerals, particularly siderite and ankerite are locally abundant within the Northern Pennine Orefield. Substantial deposits of goethite-rich ores, derived by supergene oxidation of primary iron carbonate minerals, occur both in vein outcrops and as large bodies replacing the limestone wall-rock of the veins. Although such ores were almost certainly worked in ancient times ( Plate 65), their greatest exploitation came in the 19th century when large deposits such as those at the Rigg and West Rigg opencasts in Weardale supplied the iron works in Weardale and at Consett, Tow Law and Spennymoor. Iron content of the ores ranged up to 40 per cent Fe, and although this workable level was usually only attained as a result of supergene alteration, a number of deposits rich in primary siderite were also workable, for example at Carricks, Rowantree and Rispey mines in Weardale.

The region’s largest, and richest iron ore deposits were the almost monomineralic, haematite orebodies of west and south Cumbria. Cumbrian haematite generally has an iron content of between 50 to 60 per cent, and a phosphorous content of less than 0.02 per cent making it ideally suited to the production of high-grade steel. Iron working in the area dates from the 12th century and mining is known to have been active in the 17th and 18th centuries. Large-scale mining dates from the 19th and first half of the 20th centuries but ended in the late 1970s with the closure of the combined Beckermet–Florence Mine at Egremont. Small-scale production continued until the late 2000s from shallow workings at Florence Mine, with the output employed in specialised steel making and in pigment manufacture. It has been estimated that about 250 million tonnes of haematite ore have been raised in Cumbria since the mid 19th century.

Copper

The mining and smelting of copper ores from numerous veins, hosted in Borrowdale Volcanic and locally Skiddaw Group rocks in the Coniston, Ulpha and Newlands areas, and in Eycott Volcanic Group rocks in the Caldbeck Fells, placed the Lake District at the forefront of the world’s copper industry during the 16th century. Much larger-scale copper mining, mainly in the 19th century and principally at Coniston, was an important part of the Cumbrian economy. A fall in world copper prices towards the close of the 19th century, combined with difficulties in processing the chalcopyrite-rich ores, led to the decline of the industry. Mining ended in the early 20th century after some years of intermittent and very small-scale activity.

Although chalcopyrite is widely present in the polymetallic veins of the Northern Pennine Orefield, small workable concentrations were only rarely found, mainly in the Garrigill and Tynehead areas. Likewise in the Isle of Man, small quantities of copper ores were worked from only a few of the metalliferous veins, though some waste tips were reprocessed for copper ore in the 1950s.

Lead and zinc

Mineral veins carrying mainly lead and zinc mineralisation (galena and sphalerite are the principal ore minerals) are concentrated within the northern Pennines, the Lake District and the Isle of Man. They comprise the region’s most abundant and widespread, non-ferrous metalliferous resource.

The Carboniferous rocks of the Alston Block and immediately adjoining parts of the Northumberland Trough are particularly important hosts for the mineral veins. In this area, the northern part of the Northern Pennine Orefield ( Figure 61), unambiguous records of lead mining begin in the 12th century. Then, in addition, there was significant production of silver as a by-product of lead smelting. Galena was the sole lead ore mineral at all but a very few mines in the Alston area, where some cerussite was also worked from deposits affected by extensive supergene alteration. Most of the veins had been discovered by the mid 17th century and from then until the close of the 19th century, two major companies, the London Lead Company and the WB Company, dominated the industry. A collapse in world lead prices in the late 19th century meant that only a handful of mines survived into the first half of the 20th century. Thereafter, small amounts of lead ore continued to be produced as a by-product of the mining of zinc ore, and the spar minerals barytes, witherite and fluorspar, until mining for these finally ended in the closing years of the 20th century. That part of the Northern Pennine Orefield lying within the Northern England region produced at least 4 million tonnes of lead concentrates.

Although not as numerous as those in the Pennines, workable lead and zinc-bearing veins are also common in parts of the northern Lake District. Mining here can be traced back over many centuries, though the peak of production dates from the 19th century. The Lake District’s last lead mine, at Greenside, near Ullswater, closed in 1962 with a total recorded output of approximately 250 000 tonnes of lead concentrates. Galena and small amounts of cerussite were the ore minerals, though Dry Gill Mine, in the Caldbeck Fells, also raised a few hundred tonnes of mimetite (a lead chloro-arsenate), for use in the glass industry.

A long history of mining in the Isle of Man ( Figure 60) ended in 1919 with the closure of Laxey Mine ( Plate 66). Galena and sphalerite in vein widths of up to 8 m were recorded there, and a parallel vein 3 km to the west was worked in the Snaefell area until 1898. Unsuccessful attempts to resume mining at Laxey were made in the first half of the 20th century and some lead ore was recovered by reprocessing spoil heaps during the 1950s. Elsewhere in the Isle of Man, the Foxdale and Glen Rushen group of mines worked a major vein structure, up to 5 km long, until 1911, whilst lead and some copper ores were raised from mines in the vicinity of Bradda Head prior to 1883.

Concentrations of supergene smithsonite, locally known as ‘calamine’, are common in parts of the northern Pennines and were worked from several mines in the Alston area from an early date, mainly for the making of brass. The primary zinc ore, sphalerite, was discarded as a waste product until the 19th century, when substantial tonnages began to be raised from orebodies previously worked for lead. The principal zinc mining centres were in the Nenthead area of the northern Pennines, at Force Crag in the Lake District and at Laxey in the Isle of Man. In the northern Pennines substantial tonnages of sphalerite were recovered from lead mine spoil heaps in the Nenthead and Haydon Bridge areas during World War II and in the 1950s. As recently as the 1980s some zinc ore was recovered as a by-product of fluorspar mining at Cambokeels Mine in Weardale. The total zinc production from the northern Pennines is around a third of a million tonnes.

Silver

Most of the region’s lead ores are silver bearing. Medieval references to the ‘Carlisle silver mines’ may refer to deposits in either the northern Pennines or the Caldbeck Fells. Northern Pennine galena ore typically had silver contents ranging from 112 to 224 parts per million (ppm), though values as high as 2510 ppm Ag are recorded locally. At least 170 tons of silver were recovered from northern Pennine lead ores during the 18th and 19th centuries. Reported outputs of up to 4 tons of silver per year during the 12th century, if correct, suggest that much richer silver-bearing lead ores were then available than have been recorded in subsequent years. The lead ores of the Lake District had silver contents of around 800 ppm, which may have been the main reason for their 16th century exploitation. In the Isle of Man, silver values of around 420 ppm were commonly encountered with average values rising to around 1120 ppm Ag for the Laxey ore. Very high silver values exceeding 10 000 ppm were recorded locally at Foxdale, probably because the ore also contained silver-rich tetrahedrite.

Minor metal production

The only British deposit of potentially workable tungsten ores outside of south-west England occurs within quartz veins associated with the Skiddaw Granite at Mosedale, in the northern Lake District. There have been several short-lived attempts at mining the wolfram and scheelite of this deposit at Carrock Fell Mine, the first in 1854, the most recent ending in 1981; some arsenic was recovered as a by-product. Elsewhere in the Lake District, during the 19th century, small quantities of the antimony sulphide, stibnite, were worked at Robin Hood, near Bassenthwaite, and a few tonnes of cobalt and nickel ores were produced from the Coniston copper mines. Manganese ores have been mined from the haematite deposits of west and south Cumbria, and from the upper parts of the Force Crag Mine, near Braithwaite.

Fluospar

Fluorspar is the commercial name for the mineral fluorite (CaF2), an extremely abundant gangue mineral in central parts of the Northern Pennine Orefield. Until late in the 19th century, huge quantities were separated during the processing of lead ores and either discarded on spoil heaps or backfilled into abandoned underground workings. Many fluorite-rich veins, regarded as too poor for lead mining, were left unworked. Towards the end of the 19th century, an increasing range of industrial uses, notably the introduction of open-hearth steel making, created a substantial demand for the mineral. Commercial production then began from a number of mines, either as a by-product of lead mining, or by re-working veins previously mined for lead. Substantial tonnages were also recovered by processing spoil heaps and extracting fluorite-rich fill from abandoned stopes. The rapidly rising demand for fluorspar helped to offset the decline of lead mining during the closing years of the 19th century.

Fluorspar mining was further developed, especially in Weardale and parts of Alston Moor, during the 20th century, with the product supplying a substantial domestic and export market. An unusual use of the mineral was the production of high quality transparent crystals from Boltsburn Mine, Rookhope, which were exported to Austria for the manufacture of specialised lenses. Increasing demand for acid-grade fluorspar for the chemical industry encouraged exploration for further reserves and a reopening of several mines during the 1970s. However, the availability of cheaper fluorspar from new producers eventually made the North Pennine mines uneconomic. The combined Groverake–Frazers Hush Mine, the region’s last fluorspar producer, finally closed in 1999 ( Plate 67). The total recorded production of fluorspar from the northern Pennines exceeds 2 million tonnes.

Barytes

Baryte (BaSO4) is a common gangue mineral in the outer parts of the Northern Pennine Orefield ( Figure 61) and in some of the lead-bearing veins of the northern Lake District. Like fluorite, this mineral was generally regarded as a waste product until industrial uses were found for it in the 19th century and led to its working as one of the region’s major mineral products. For one of its most important early uses, as a white pigment in paint making, only top quality barytes was required and impure or discoloured material was ignored. As other uses emerged, such as heavy aggregate in concrete, for drilling fluids, and a variety of uses in the chemical industry, colour became less important, allowing previously uneconomic deposits to be mined.

Substantial tonnages of barytes have been obtained from several mines in the northern Lake District, notably from Force Crag, Potts Gill and Sandbeds, though the region’s greatest production has come from the northern Pennines with major producers including Cow Green, Silverband, Dun Fell, Lunehead and Closehouse mines. In several of these mines, baryte occurred both in veins and in associated replacement flats in limestone. There was also significant barytes production from vein deposits in the Durham Coal Measures, which were worked alongside coal in mines such as New Brancepth, Ushaw Moor and South Moor. Barytes mining finally ended in about 2002 with the abandonment of the large opencast working at Closehouse, in Lunedale, after several years of small-scale and intermittent production. The region’s total recorded production of barytes is around 1.5 million tonnes.

Witherite

A unique feature of the Alston Block portion of the Northern Pennine Orefield is the local abundance of the rare, barium carbonate mineral witherite (BaCO3) within the outer zone of the field. It is an important gangue in many veins and flats and is the major constituent of several large deposits. Witherite working, for the manufacture of a range of barium chemicals, began in the Alston area as early as 1850, and between 1860 and 1870 two lead mines in the Tyne valley, at Settlingstones and Fallowfield, changed over to witherite production. Over the next century, the region was to be the world’s major commercial source of this mineral. Witherite was worked at several mines in the Alston and Tyne valley areas and also in the adjoining Durham Coalfield, notably at South Moor Colliery where a shaft was sunk in the 1930s specifically for witherite production. The leading producer was Settlingstones Mine, near Hexham, which at its closure in 1969 had accounted for almost two-thirds of the region’s total output of around 1 million tonnes of witherite.

Future prospects

Future commercial interest in the remaining haematite resources in west Cumbria, or in any of the region’s other iron ore deposits, is very unlikely. Possibilities exist for locating further fluorite and lead–zinc mineralisation at depth within the northern Pennines, though investigation is likely to be triggered only by a significant rise in world commodity prices. There is some potential for stratabound base-metal mineralisation associated with the margins of the Northumberland– Solway and Stainmore troughs, but these would be difficult exploration targets.

Building stone

Most of the region’s rocks have been employed to some extent in building. Until the widespread adoption of brick, stone from small quarries was used locally in vernacular architecture, and contributes much to the variety of the landscape. It is only appropriate here to comment on particularly significant uses of building stone, or on its larger-scale extraction.

The earliest organised quarrying and shaping of building stone was probably associated with the building of Iron Age hill forts. Extraction on an ‘industrial’ scale commenced during the Roman occupation with the construction of Hadrian’s Wall and its associated infrastructure. Despite being partially sited along the outcrop of the Whin Sill ( Plate 3), the Wall’s designers selected sandstone as the main construction material, avoiding use of the intractable dolerite. Together with limestone for making lime mortar, the sandstone was obtained from quarries along the course of the Wall, though only a few can today be identified as of Roman origin, for example those at Brampton and near Chollerford. Following the departure of the Romans, Hadrian’s Wall provided a ready source of building stone for subsequent structures. The distinctive squared blocks produced by the Roman masons are today recognisable in castles, churches, farm buildings and even drystone walls.

In the Isle of Man, Ordovician sandstones from the Manx Group were used most spectacularly in the massive walls of Peel Castle. On the mainland, only a few of the varied Lower Palaeozoic rocks of the Lake District have been exploited on a large scale. Prominent amongst these, and comprising some of the region’s best-known building and roofing stones, are the Lake District ‘green slates’. These cleaved, volcaniclastic lithologies occur at several stratigraphical levels within the Ordovician, Borrowdale Volcanic Group, but most of the commercial working has taken place within the Eagle Crag Member of the Birker Fell Formation, for example at Honister, and in the Seathwaite Fell and Tilberthwaite formations in the Coniston, Langdale and Kirkstone areas, where extraction continues today. The slate has been extracted in large quarries, which in some locations, for example at Honister and Coniston, have been enlarged into underground workings. Traditionally, most of the slate was used for roofing and whilst substantial amounts are still produced for this purpose, recent years have seen most green slate employed for architectural or paving stone, where its attractive, mottled appearance can be utilised to good effect ( Plate 68).

Intensely cleaved, bluish-grey mudstone and siltstone from the Silurian part of the Windermere Supergroup have also been important sources of slate. Substantial quarries have been worked in the Brathay Formation near Windermere, but slate production today is restricted to the Wray Castle Formation at Kirkby Quarry, near Kirkby in Furness. In the Isle of Man, some inferior roofing slate was formerly obtained from cleaved Ordovician mudstone of the Barrule and Glen Rushen formations of the Manx Group.

Of the Lake District’s granitic rocks, only the Shap and Eskdale intrusions have been significantly worked for building stone. The durable and distinctive, pink and porphyritic Shap Granite has an attractive texture and takes a good polish; hence it has been widely used for construction and as a monumental or ornamental stone ( Plate 30). The grey Broad Oak Granodiorite has been quarried near Waberthwaite for use as a building and ornamental stone. Neither the Shap nor the Broad Oak stone is currently in regular production, though some is occasionally produced to meet particular requirements. The Isle of Man granites have been used as local building stone but there is no current production for that purpose.

Red, Devonian sandstone from the Isle of Man’s Peel Sandstone Formation has been worked on the Creg Malin headland north of Peel, and used for many buildings in the town, most notably the cathedral. Some Carboniferous limestone was also extracted for local building use from quarries on the island’s south coast, around Castletown.

Many of northern England’s Carboniferous sandstones have long been major sources of good freestone ( Plate 69) and production continues at numerous sites for building and repair work over a wide area. In Northumberland, the Fell Sandstone has a distinctive purplish-pink hue at Doddington Quarries, near Wooler, whereas pale brown sandstone is produced from another part of the Border Group at Cop Crag, near Byrness. Pale buff sandstone of the Yoredale Group is worked at a number of different localities in Northumberland: near West Woodburn–Darney, Blaxter, Cragg and High Nick; at Black Pasture near Chollerford; near Allenheads; and at Millknock Quarry [NY 880 793] near Birtley, which has recently re-opened. Elsewhere, Yoredale Group sandstone is quarried near Nenthead, Cumbria, and at several quarries in the Staindrop, Barnard Castle and Stanhope areas of County Durham. The Low Main Post sandstone from the Coal Measures, obtained from long-abandoned quarries in Durham City, was a major source of stone for the construction of Durham Cathedral ( Plate 60), whilst Springwell Quarries, near Gateshead, have been another source of Coal Measures sandstone. A very flaggy sandstone near the base of the Coal Measures has for centuries been a source of roofing slabs and is still worked for this purpose, and for paving and walling, at Ladycross Quarry, near Slaley, though reserves are believed to be nearing exhaustion.

A distinctive feature of many Cumbrian villages is the use of reddish Permo-Triassic sandstone for building purposes. The Penrith Sandstone provides good, salmon-pink freestone for walling and paving. Numerous quarries mark its outcrop north of Penrith where it is still worked in the Plumpton and Lazonby areas. The red, St Bees Sandstone is particularly well suited to the carving of detailed shapes and ornamentation. It has been widely used across Cumbria and formerly enjoyed a considerable export market as far afield as North America. The stone has been much quarried in the Vale of Eden, the St Bees area and near Barrow, where Furness Abbey stands as a monument to its early use for building purposes.

Except in a few areas, for example around Kendal and Kirkby Stephen, the region’s Carboniferous limestones have not been widely used as building stone. However, a few have been worked for ornamental use. Best known of these is a dark grey to black lithology rich in a solitary coral, Dibunophyllum bipartitum, which is present within the Great Limestone of the northern Pennines. The rock takes a high polish and is known locally as the ‘Frosterley Marble’ since much was formerly worked at Harehope Quarry, near Frosterley in Weardale ( Plate 32). Fine examples of the stone may be seen in churches across the region, including Durham Cathedral, and it has been widely used abroad. Blocks of Frosterley Marble are occasionally recovered today during limestone quarrying at Broadwood Quarry, Frosterley, and shipped to Italy for dressing. In the Isle of Man, flaggy, dark grey bituminous limestone from the Bowland Shale Formation at Poyllvaaish was formerly worked as an ornamental stone and marketed as a ‘black marble’.

Ground engineering

The main engineering considerations briefly identified here include foundation conditions, excavatability and suitability as fill. In the characteristically hilly terrain, other local factors such as gradient of the ground, position of a site on a slope, and how well that site drains will also be of importance. Geohazards are discussed separately in the subsequent section of this chapter.

Although the rocks of northern England are generally suitable for most types of foundations, they may be weakened locally by weathering, dissolution, faulting or mineralisation. The effects of such alteration on the engineering characteristics will depend partly on the rock type and partly on local factors, as will the ease of excavation and appropriateness of the excavated material as fill. Methods used to excavate strong or very strong rocks (mostly igneous or Carboniferous and older if sedimentary) will depend on bedding, faulting and jointing; the weaker rocks (mostly Permian and Triassic) and the unconsolidated Quaternary deposits are much more easily removed. The strong rocks are generally too expensive to extract and crush for fill material unless they have been processed from mine or quarry waste. The weaker rocks and many Quaternary deposits provide suitable fill material, although some mudstones, clays and organic deposits are unsuitable.

Note that in an engineering context, the terms weak, strong, very strong etc., and stiff, soft etc., have a precisely defined meaning.

Bedrock

In the Lake District and the Isle of Man, most of the pre-Carboniferous rocks are strong and often very strong when fresh, but foundation conditions will vary locally with the degree of alteration and the intensity, orientation and fill of discontinuities, including jointing, bedding, cleavage and vein networks.

Carboniferous limestone is strong to very strong but is slightly soluble in acid rain water and groundwater, allowing the formation of karst features such as sinkholes and caves, particularly along zones of weakness such as major joints or faults. The upper few metres of limestone may be more generally weathered into loose, gravel-to boulder-grade material. Sinkholes may be open and easily identified or hidden beneath a superficial cover, or they may be infilled by soft to firm or loose material that has been washed in and which will have dramatically different engineering properties from the surrounding rock. Dissolution cavities at depth may collapse and migrate upwards to the surface. When considering construction on karstic Carboniferous limestone areas, it is important to identify where dissolution cavities occur. Small sinkholes may be spanned by a variety of foundation types including reinforced strip or strengthened raft. Alternatively, piled foundations may be used to transfer the load to deeper, good quality rock.

Carboniferous sandstone is generally moderately strong or very strong when fresh and in general provides good foundation conditions. However, it may weather to sand near surface, producing an uneven engineering rockhead surface. Namurian sandstones are locally weathered to depths greater than 6 m, which may lead to differential settlement.

Carboniferous mudstone and siltstone usually provide good foundation conditions, although, when fully weathered, the mudstone becomes a firm to stiff clay. Since this weathered material has a lower bearing capacity than unweathered rock, it may be necessary to place foundations below the weathered zone. Oxidation of pyrite in the mudstone will produce acid and sulphate-rich ground conditions; subsequent reaction with calcium carbonate will then form gypsum. Where this has occurred, buried ironwork will need to be protected from the acid conditions and the use of sulphate-resisting cement may be required.

Permian rocks generally provide good foundation conditions though weathering may affect their behaviour. In north-east England, the weak sandstone of the Yellow Sands Formation weathers to a very weak condition or even to loose sand. The succeeding Zechstein Group is highly variable. The weak mudstones weather to a firm to stiff clay; dolomitic limestone, moderately strong when unweathered, may also weather to friable sand. In the Newcastle and Sunderland areas, and around Darlington, beds of gypsum have dissolved, leaving residual clay horizons and causing severe disruption to the overlying limestone, leading to collapse and surface subsidence. Similar phenomena in north-west England, notably in the Vale of Eden, arise from gypsum dissolution in the Permian Eden Shales Formation (Cumbrian Coast Group), and may also lead to subsidence. In the Triassic Mercia Mudstone Group rocks of north-west Cumbria, weak to moderately strong mudstone weathers to firm clay, but weak interbeds of halite and gypsum may have been removed by groundwater movement; halite at depths greater than 50 m, and gypsum near to the surface. The degree of disruption to the overlying deposits largely depends on the thickness of beds that have been removed. Where construction is above gypsiferous deposits, surface drainage should not be via soakaways sited near buildings.

The Permian and Triassic sandstones of north-west England, typified by those of the Sherwood Sandstone Group, are generally moderately weak to strong when fresh and provide good foundation conditions. However, weathering of the sandstone to dense sand, sometimes to considerable depths, results in a highly variable engineering rockhead. Calder Formation sandstones are generally weaker, more porous and prone to deeper weathering than the other formations of the Sherwood Sandstone Group.

Superficial deposits

Glacial till, when fresh, is a firm to stiff or very stiff gravelly sandy clay containing pebbles, cobbles and sometimes boulders. It generally provides good foundation conditions for normal foundations but is commonly weathered to a depth of 3 to 4 m, up to 8 m thick in places; this weathered upper layer is weaker and fissured. Bearing capacity near surface may vary depending on the weather conditions, becoming softer when wet and stiffer when dry. The softened near-surface material may need to be removed prior to construction. An abundance of cobbles and boulders within the till, or the presence of interbeds of gravel, sand or laminated clay and silt, may change the foundation conditions locally and engender differential settlement.

Glaciofluvial sand and gravels generally make an adequate foundation for domestic and light industrial buildings, but lateral and vertical variation in density, and the presence of clay and silt lenses, may lead to differential settlement; deeper foundations, perhaps piles, are often required for large buildings. Excavation in saturated sand below the water table may produce ‘running’ conditions.

Laminated clay and silt, often of glaciolacustrine origin, are generally soft to stiff, finely laminated and sometimes include sand laminae. The top metre or so may be stiffer due to desiccation. Such deposits usually have low to moderate compressibility and, generally, a moderate to fairly rapid rate of consolidation due to horizontal drainage along the coarser-grained laminae. Where the water content is high, the low shear strength reduces bearing capacities and foundations must be designed to take this into consideration. Low strength also means that excavations and cuttings will require support. Laminated clay layers have been associated with failure beneath spoil mounds and embankments. Typically, the widespread Pelaw Clay, is generally soft, has lower strength than the laminated silts and clays that lie beneath, and is very unstable in excavations. Laminated clay and silt is usually unsuitable as a fill material.

Fine-grained river and estuarine alluvium may not be suitable for standard foundations as this material often has low bearing capacity, a problem exacerbated by peat or organic clay layers within the alluvium. Such localised variations in character may lead to differential long-term settlement. Excavations will probably require support and dewatering as the water table is likely to be near surface. In general, construction on peat should be avoided unless specialised methods of construction are used.

Artificial deposits, waste disposal and landfilling

Man-made deposits are widespread in northern England, occurring not only in urban and modern mining areas but also in rural areas where mining, quarrying and related industrial activities have taken place. They are potentially very variable and may have contrasting foundation conditions within a short distance; those associated with metal mining are likely to be toxic. Important factors include composition, extent, thickness, method of emplacement and the length of time over which settlement has taken place. In general, buildings should not straddle the man-made material and the enclosing bedrock since there is a strong probability of differential settlement, which may affect not only the building but also the associated services.

Many of the region’s numerous abandoned quarries have been employed for the disposal of domestic and industrial waste. In at least one instance, at the Florence Mine near Egremont, west Cumbria, a large subsidence hollow over old mine workings has been used as a landfill site.

Some landfill sites are well documented, such as landscaped opencast coal workings and recently active, domestic waste tips but many older sites are poorly documented. These older sites may contain a heterogeneous mixture of materials, including methane-generating and toxic components, whilst voids may present an additional problem. Alternatively, the fill material may be a relatively innocuous combination of local waste rock and overburden derived from opencast workings.

The reclamation and landscaping of colliery spoil heaps has involved the redistribution of spoil over large areas. The colliery waste is largely composed of rock fragments but may also contain substantial quantities of coal and pyrite. This can result in combustion, with associated settlement and generation of toxic gases. Weathering of colliery waste may produce high sulphate and acid groundwater. With the decline of the region’s steel industry, large quantities of metal-rich slag have been redistributed during landscaping of the old industrial sites.

Particularly sensitive issues surround the disposal of low-level radioactive waste at Drigg, just to the south of the Sellafield nuclear plant in west Cumbria. The Drigg site is the UK’s national low-level waste disposal site and has been in operation since 1959. The material received typically comprises paper, packing materials, plastic sheeting, protective clothing and redundant machinery, derived from other UK nuclear facilities, hospitals, research laboratories and industrial processes. The waste is compacted and containerised before disposal in concrete vaults built into Quaternary marine and lacustrine silts and clays overlying glacial till. A very large geological database was generated for the whole Sellafield area in the early 1990s, during investigation of the site’s suitability for the deep disposal of low- and intermediate-level radioactive waste. Proposals for further research were rejected at a public enquiry that ran from 5 September 1995 to 1 February 1996.

Geological hazards

The region’s geology has a profound effect on a wide spectrum of land use and environmental issues, and in some circumstances may influence natural hazards that constrain or limit development. Some hazards are discussed briefly below.

Seismicity

The north of England does not face a high risk of major, damaging earthquakes, but neither is the region seismically quiescent. Indeed, the 26 December 1979 Carlisle earthquake, with instrumental magnitude of 4.7 ML, was one of the larger British earthquakes of the 20th century. It was felt over most of southern Scotland, Cumbria and north-east England. Prior to that, an earthquake near Kirkby Stephen on 9 August 1970, with an instrumental magnitude of 4.1 ML, was felt throughout northern England.

The regional seismicity of the north of England is characterised by a band of relatively intense activity up the spine of the Pennines, with some activity to the west of this but with practically nothing to the east ( Figure 78). This pattern terminates at the line of the Solway Firth, north of which there is little activity (although an earthquake near Dumfries on 26 December 2006 had an instrumental magnitude of 3.7 ML). It seems likely that the pattern of seismicity is related to the geometrical reaction of structural components, most likely the major geological blocks, to the overall pattern of crustal stress. There is currently a maximum compressive stress from the north-west or north-north-west arising from the widening of the Atlantic Ocean. Such a regime could induce generally rotational movements of the Pennine and Lake District blocks, with shearing movements on their flanks.

The 1979, 4.7 ML Carlisle earthquake was generated at a depth of about 5 km. There was a marked directionality to energy release, resulting in the earthquake being much more perceptible to the north than to the south. The strongest effects were felt around Carlisle, Longtown (close to the epicentre) and Canonbie (across the border in Scotland), with damage caused to roofs and chimney stacks, debris falling and cracks appearing in walls. A series of significant aftershocks continued until 1981. The fault plane solution of the aftershock sequence, and the lineation of aftershock epicentres, shows lateral movement on a near-vertical plane with a strike of 123°.

There are two contenders for the strongest earthquake in the region, though magnitudes of both have been estimated from historical records: a 5.0 ML earthquake on 11 August 1786 had an epicentre just offshore from Whitehaven and a depth of about 16 km; a 5.1 ML earthquake on 17 March 1843 had an epicentre well offshore from Barrow and a depth of about 15 km.

Landslides

Landslides, developed on a great variety of substrates and in many different geological settings, are a common feature across the region and their deposits are generally underrepresented on existing geological maps. They were widely initiated during the late glacial period, especially during ice sheet deglaciation, when glacially over-steepened slopes became unstable. Thawing of permafrost led to water over-saturation and high pore-water pressures that reduced rock shear strength and facilitated failures. The landslides range from deep-seated rotational slides to relatively shallow translational slides and include progressive multiple slope failures. These landslides may be stable under present-day conditions though human intervention such as excavation or loading, or any alteration of the local groundwater regime, could renew instability. The following brief account touches on only a small selection of illustrative examples.

Very large slope failures, involving many thousands of cubic metres of rock and superficial materials, and covering many hundreds of square metres of hillside, have occurred in several upland parts of the region. Examples of substantial landslides include: Skiddaw Group rocks at Latrigg, near Keswick, at Buttermere Fell on the south face of Robinson, and near Whicham on the south-east flank of Black Combe; Ennerdale microgranite and Skiddaw Group rocks at Crag Fell, Ennerdale [NY 095 147]; mostly Yoredale Group limestone, sandstone and shale at Mason’s Holes and elsewhere on the Pennine escarpment; and Coal Measures on hillsides near Moorside Colliery, west Cumbria [NY 055 216]. Smaller scale slope failures are particularly common in the northern Pennines, where competent, permeable sandstone overlies incompetent and impervious mudstone.

In the Isle of Man, large coastal landslides affect Manx Group strata, for example at Marine Drive to the south of Douglas, with incipient movement opening up deep clefts at The Chasms on the south side of Meall Hill. Another area of extensive coastal landslides affects the mainland to the north of St Bees Head in Cumbria. There, marine erosion of the St Bees Shale Formation has caused extensive rotational slipping of strata in the overlying St Bees Sandstone Formation. Substantial coastal landslides are also seen near Whitehaven where marine erosion has induced slip in seaward-dipping Coal Measures strata.

Slope failures involving mostly superficial deposits are comparatively common. Low-angle failures in till, perhaps resulting from groundwater flow through interbedded sand- or gravel-rich horizons or to movement above water-saturated sands and laminated clays, have been widely recognised in the northern Pennines, notably in parts of East and West Allendale, and along parts of the Coquet Valley in Northumberland. Similar slope failures are active in the steep-sided valley system forming the Durham denes, adjacent to the North Sea coast. Comparable features have been widely noted in the Lake District and Isle of Man.

Limestone and gypsum dissolution

Dissolution of soluble rocks such as limestone and gypsum may create underground voids, collapse of which eventually propagates upwards and causes subsidence. Where such voids are numerous or large, or are actively developing, they may constitute a significant geological hazard. The many Carboniferous limestones of northern England are comparatively thin and extensive cave systems are unusual. However, sinkholes, or dolines, are extremely common in areas of limestone outcrop and in places may present stability problems. The dolomitic rocks which comprise much of the Zechstein Group are much less soluble than ordinary limestone, so dissolution is not widespread, though small solution caves are recorded locally, for example in the Sunderland area. Permo-Triassic gypsum beds in the Carlisle, Vale of Eden and Darlington areas locally exhibit dissolution features, including a phreatic cave system at Kirkby Thore, Cumbria, whilst surface collapses have been reported locally. Amongst the best-known examples are the subsidence hollows known as Hell Kettles, south of Darlington, which are reputed to have appeared suddenly in the late 12th century.

Mining subsidence and fault reactivation

Centuries of coal mining in parts of the region have left a legacy of surface collapse and instability. Even subsidence due to modern coal mining, which is generally predictable and manageable, may become less so if the fill material has a low bearing capacity and so allows further incremental collapse. More significantly, the extent and nature of earlier mining, particularly shallow pillar and stall working, may be less well known. Voids created during this early work may remain open long after abandonment, with collapse taking place unpredictably over many years ( Plate 70). Should such old workings be present above modern longwall workings, the subsidence difficulties are compounded. Although reliable plans exist for a high proportion of the region’s coal workings, small but significant areas of very old workings near seam outcrops are unrecorded.

Groundwater levels typically rise following the reduction of pumping after mine abandonment. In addition to the risks of surface discharge of contaminated water (see below), such groundwater rebound may affect ground stability. Renewed subsidence may be caused in long-abandoned workings, and in some circumstances faults may be reactivated, resulting in the sudden appearance of surface collapse features in linear belts. Numerous examples have recently been identified on the Zechstein Group outcrop above the concealed portion of the Durham Coalfield, though the phenomenon may be more widespread across the region.

Underground extraction of very large bodies of haematite in west and south Cumbria, and of non-ferrous ores and spar minerals in the northern Pennines and Lake District, have created a legacy of localised, but commonly substantial, surface subsidence ( Plate 71).

Minewater discharges

The recovery of groundwater levels following the abandonment, or reduction, of pumping in areas of former mining, may impact upon the local and regional groundwater regime. Mine waters may be highly acidic and contain a variety of chemical elements, some potentially toxic; commonly present are iron, manganese, aluminium, lead, zinc, arsenic and cadmium. In certain circumstances, contaminated groundwater may discharge to the surface via mine openings or through natural pathways such as faults or permeable formations. It may also threaten to contaminate aquifers. Despite a variety of groundwater management measures being in place across northern England, uncontrolled discharges still occur.

Natural and mine gas emissions

Radon is a radioactive gas derived from the decay of the naturally occurring uranium that is found in small quantities in all rocks and soils. The rate of release of radon is largely controlled by the uranium concentration in the source material and the type of minerals in which it resides. Once radon is released, it is quickly diluted in the atmosphere and does not normally present a hazard. However, radon that enters poorly ventilated or enclosed spaces can reach high concentrations. The health risk arises from decay of the gas to form radioactive particles that may be inhaled.

Regional variations in radon levels are related principally to geology. Some granites release radon and the Shap, Skiddaw and Cheviot plutons present a limited hazard. Of greater importance is the well-established link between limestone and high levels of radon emission, with Carboniferous and Permian limestone presenting the greatest hazard in northern England. Radon monitoring is only considered advisable in the limestone areas.

Most coals and coal-bearing rocks generate methane, known to generations of miners as ‘fire damp’ and the cause of numerous catastrophic underground explosions. Methane continues to be released into old workings whence it may be discharged to the surface through abandoned mine openings or through natural pathways such as faults or permeable formations, driven by rising groundwater or falling barometric pressure. Several instances are known of methane discharges igniting.

Air trapped in poorly ventilated, old workings may become depleted in oxygen and enriched in carbon dioxide as a result of the decomposition of timber, coal or sulphide minerals. Such oxygen-deficient air, known to miners as ‘black damp’ or ‘stythe’, can, like methane, be discharged to the surface and being heavier than air it may become a significant hazard in tunnels, cellars, trenches, or any poorly ventilated space. The region has witnessed several potentially hazardous discharges of ‘stythe’, and at least one fatality has occurred.

Hydrogeology and water supply

Water supply across north-east England is largely drawn from surface reservoirs. Of these, the Kielder Water reservoir in the uppermost reaches of the River North Tyne has the largest capacity (200 000 Ml) of any UK reservoir and is the largest artificial reservoir in western Europe. It was opened in 1982 to supply Tyneside, Wearside and Teesside. Water is released from Kielder Reservoir into the River Tyne and can be transferred thence into the Rivers Wear and Tees. Abstracted river water is then used to supplement the traditional supply from groundwater and smaller reservoirs, notably those at Derwent and Cow Green.

In north-west England, the radial lakes of the Lake District are important sources of supply ( Figure 79). Haweswater and Thirlmere have been developed to supply water southwards to Barrow and the Manchester conurbation; supplementary supplies are drawn from Ullswater and Windermere. The west Cumbrian towns are largely supplied from Crummock Water and Ennerdale Water, whilst Wast Water is used as a source of supply for the Sellafield nuclear site. In the Isle of Man there are no productive aquifers and the public supply is currently derived exclusively from surface sources such as the Sulby Reservoir to the west of Snaefell.

Lower Palaeozoic rocks

The impermeable Lower Palaeozoic rocks forming the central Lake District (and most of the Isle of Man) create an important gathering ground for surface water. Low bulk permeability is provided mostly by fissures, which also allow some groundwater infiltration. Variations in fissure width, extent and interconnection produce a range of groundwater conditions and hydraulic yields are unpredictable. Mineral springs rise from the Skiddaw Group near Derwent Water and Keswick, and from Dent Group limestone at Shap; their chemistry suggests the slow circulation of peaty water along calcite-bearing mineral veins.

Carboniferous rocks

The Border Group’s Fell Sandstone Formation contains important aquifers that for over a century, and despite some saline intrusion, have supplied Berwick-upon-Tweed. The Fell Sandstone is hydrogeologically complex and multilayered, with up to seven discrete sandstone aquifers separated by thick, laterally persistent layers of impermeable mudstone. The majority of water flow is via fractures or thin, coarse-grained horizons. Large springs are associated with cross-cutting dykes intruded into fault zones: near Rothbury, Cartington Spring [NU 042 044] and Tosson Spring [NU 030 002] jointly yield in excess of 9000 m3/d.

Limestone, sandstone and conglomerate of the Ravenstonedale and Great Scar Limestone groups act as aquifers throughout their northern England outcrop, confined by intervening argillaceous rock layers. Bulk permeability is due almost entirely to fissures except in some of the calcareous sandstones where a secondary intergranular permeability has developed after leaching of the cement. Intergranular and bulk porosities are always low so that storage capacity is limited, whilst rapid drainage from fissures makes borehole yields unpredictable.

Groundwater in Carboniferous limestone is of a calcium bicarbonate type. Its storage and transmission depends entirely on fissure size, extent and interconnection. Earlier periods of karstic weathering have left passages and enlarged fissures that may now be open, flooded or plugged by sediment. If flow is dispersed through the rock mass there is a water table, relatively slow drainage and limited storage; if restricted to well-defined passages there is no water table, rapid drainage and negligible storage. Abstraction of water may alter hydraulic conditions by unplugging fissures or plugging them with sediment. Limestone aquifers are highly vulnerable to pollution since fissures give ready access to the surface.

Sandstone aquifers in the Yoredale and Pennine Coal Measures groups give good yields. Intergranular permeability occurs in some coarse sandstone but fissures are the main influence on water-bearing properties. Major fissures occur along fault planes, and minor fissures are present along joints and bedding planes. Most close with depth so that permeability decreases downwards within a single aquifer and lower aquifers are generally less permeable than higher ones. Below depths of 250 m, the majority of fissures are closed; at still greater depths, any remaining fissures can give initially high discharges when intercepted, but yields decrease rapidly to zero showing that any recharge at depth takes place very slowly. Only faults are likely to yield persistent discharges at depths below 300 m.

Mine pumping in the Cumbrian Coalfield has abstracted up to about 20% of the reliable yield, dominating the flow pattern, and drawing in sea water to pollute the aquifers. Chemical analyses of mine drainage water reflect the infiltrated sea water, but suggest that normal groundwater is probably a sodium sulphate type with subordinate amounts of chloride. Mine workings in the Northumberland and Durham Coalfield were extensively interconnected and pumping maintained a water table at about 150 m below the ground surface. With the end of mining activity, groundwater levels are rising.

In north-east England, the Yoredale Group’s Stainmore Formation constitutes a multilayered aquifer in which thick sandstone and limestone beds act as individual aquifers confined by impermeable mudstone. In many places springs issue from the base of water-bearing horizons and have provided small-scale village supplies. Groundwater storage and movement is predominantly through joints and fractures with only minor contributions via the rock matrix. Borehole yields are dependent on the number and size of fractures encountered in a productive horizon and many boreholes penetrate more than one productive horizon.

Across the Northern Pennine Orefield, mine workings have modified the original hydrogeological character by interconnecting previously separate aquifers, and by creating preferential flow paths now discharging at mine entrances. Some substantial flows have been recorded from drainage adits (soughs). Langthwaite Level (locally known as Goose Nest) is a good example that, during the late 1960s and early 1970s, supplied the villages of Langthwaite [NZ 005 025] and Arkle Town [NZ 008 019].

Permian and Triassic rocks

Within the Permian to Triassic succession of north-west England, there are several productive aquifers in the Penrith Sandstone Formation (Appleby Group) and the Sherwood Sandstone Group. These aquifers have high groundwater potential and could be exploited either by direct abstraction to supply, or as sources of water for regulating river flow.

The rocks of the Penrith Sandstone Formation have intergranular and fissure permeability with the highest porosity and permeability in the weakly cemented sandstones south of Cliburn [NY 359 527]. Elsewhere, silica cementation produces sharp reductions in intergranular porosity and permeability, whilst similar reductions are associated with the development of calcite and gypsum cements near the contacts with the Brockram and the overlying Eden Shales Formation. The reductions may be offset locally by fissure flow or re-solution of cement. The groundwater is typically of calcium bicarbonate type with moderate to low total dissolved solids and low hardness. Water confined close to the Eden Shales has higher total dissolved solids and sulphate concentrations, whilst harder water close to the Brockram arises from the solution of carbonate cement.

The Sherwood Sandstone Group forms a very large, continuous aquifer. Intergranular porosity and permeability are generally lower than in the Penrith Sandstone Formation due mainly to compaction and cementation, but fissures are well developed and allow water transmission. The groundwater is typically hard, sodium bicarbonate water with a moderate concentration of total dissolved solids. Saline intrusion occurs in areas of heavy pumping near Barrow and probably occurs to some extent all along the Cumbrian coast. Water confined below the Mercia Mudstone Group is harder than elsewhere, has more total dissolved solids and a higher sulphate concentration.

In north-east England, both the Rotliegendes and Zechstein groups contain productive aquifers. In the Sunderland district, the Yellow Sands Formation of the Rotliegendes Group forms an important aquifer with high granular porosity, and has been tapped by many wells and boreholes. The groundwater quality is generally good. The large volume of water held in the Yellow Sands Formation posed a particular problem for drainage of mine workings in the immediately subjacent Coal Measures.

The Zechstein Group contains good aquifers with hydraulic continuity despite the presence of impermeable interbeds. Permeability depends largely on fracturing, with some intergranular storage in reef limestone and where dolostone predominates. Yields are extremely variable, but highest where the fracture density is greatest, commonly in the vicinity of faults. Yields may also be enhanced by collapse and brecciation of the aquifers caused by dissolution of underlying gypsum beds. The groundwater quality is generally good, though the waters are very hard and there is a high vulnerability to surface pollution with high nitrate concentrations reported in places. Sulphate and chloride concentrations increase markedly down-dip due to the presence of gypsum and halite in the confining strata.

Quaternary and Holocene deposits

Superficial deposits are highly variable. Aquifers occur within sand and gravel units and may be confined by interbedded silts, clays or glacial till. All supplies are likely to fluctuate rapidly in response to variations in precipitation. The water is generally hard, due to bicarbonate or sulphate concentration, and may be ferruginous. Brackish water occurs in the marine deposits along the coast. The aquifers are liable to pollution from agricultural and industrial discharges. The main significance of Quaternary and Holocene deposits in the context of regional hydrogeology is that they form the confining bed over the main aquifers and control recharge and influence water chemistry.

Selected bibliography

Chapter 1 Introduction

Brenchley, P J, and Rawson, P F (editors). 2006. The Geology of England and Wales. Second edition. (London: The Geological Society.)

Cope, J C W, Ingham, J K, and Rawson, P F (editors). 1992. Atlas of palaeogeography and lithofacies. Geological Society of London Memoir, No. 13.

Johnson, G A L (editor). 1995. Robson’s Geology of North East England. Transactions of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne, Vol. 41, No.1.

Lawrence, D J D, Vye, C L, and Young, B. 2004. Durham Geodiversity Audit. (Durham County Council.)

Lawrence, D J D, Arkley, S L B, Everest, J D, Clarke, S M, Millward, D, Hyslop, E K, Thompson, G L, and Young, B. 2007. Northumberland National Park — Geodiversity audit and action plan. British Geological Survey Commissioned Report, CR/07/037N.

Oldroyd, D R. 2002. Earth, water, ice and fire: two hundred years of geological research in the English Lake District. Geological Society of London Memoir, No.25.

Woodcock, N H, and Strachan, R A. 2000. Geological History of Britain and Ireland. (Oxford and Edinburgh: Blackwell Science Publishing.)

Woodcock, N H, Quirk, D G, Fitches, W R, and Barnes, R P (editors). 1999. In sight of the suture: the Palaeozoic geology of the Isle of Man in its Iapetus Ocean context. Geological Society of London Special Publication, No.160.

Young, B, Lawrence, D J D, and Vye, C L. 2004. North Pennines Area of Outstanding Natural Beauty: A geodiversity action plan 2004–09. (North Pennines AONB Partnership.)

Chapter 2 Early Ordovician: Skiddaw and Manx groups

Burnett, D J, and Quirk, D G. 2001. Turbidite provenance in the Lower Palaeozoic Manx Group, Isle of Man: implications for the tectonic setting of Eastern Avalonia. Journal of the Geological Society of London, Vol. 158, 913–924.

Cooper, A H, and Molyneux, S G. 1990. The age and correlation of the Skiddaw Group (Early Ordovician) sediments of the Cross Fell inlier (northern England). Geological Magazine, Vol. 127, 147–157.

Cooper, A H, Rushton, A W A, Molyneux, S G, Hughes, R A, Moore, R, and Webb, B C. 1995. The stratigraphy and correlation of the Skiddaw Group (Ordovician) of the English Lake District. Geological Magazine, Vol. 132, 185–211. 264

Millward, D, and Molyneux, S G. 1992. Field and biostratigraphic evidence for an unconformity at the base of the Eycott Volcanic Group in the English Lake District. Geological Magazine, Vol. 129, 77–92.

Rushton, A W A. 1988. Tremadoc trilobites from the Skiddaw Group in the English Lake District. Palaeontology, Vol. 31, 677–698.

Rushton, A W A. 1993. Graptolites from the Manx Group. Proceedings of the Yorkshire Geological Society, Vol. 49, 259–262.

Rushton, A W A, and Molyneux, S G. 1989. The biostratigraphic age of the Ordovician Skiddaw Group in the Black Combe inlier, English Lake District. Proceedings of the Yorkshire Geological Society, Vol. 47, 267–276.

Simpson, A. 1963. The stratigraphy and tectonics of the Manx Slate Series. Quarterly Journal of the Geological Society of London, Vol. 119, 367–400.

Simpson, A. 1967. The stratigraphy and tectonics of the Skiddaw Slates and the relationship of the overlying Borrowdale Volcanic Series in part of the Lake District. Geological Journal, Vol. 5, 391–341.

Soper, N J. 1970. Three critical localities on the junction of the Borrowdale Volcanic rocks with the Skiddaw Slates in the Lake District. Proceedings of the Yorkshire Geological Society, Vol. 37, 461–493.

Soper , N J, and Dunning, F W. 2005. Structure and sequence of the Ingleton Group, basement to the central Pennines of northern England. Proceedings of the Yorkshire Geological Society, Vol. 55, 241–261.

Stone, P, and Evans, J A. 2002. Neodymium isotope characteristics of Ordovician provenance on the Avalonian margin of the Iapetus Ocean. Scottish Journal of Geology, Vol. 38, 143–153.

Webb, B C, and Cooper, A H. 1988. Slump folds and gravity slide structures in a Lower Palaeozoic marginal basin sequence (the Skiddaw Group) NW England. Journal of Structural Geology, Vol. 10, 463–472.

Chapter 3 Ordovician: Caradoc magmatism

Beddoe-Stephens, B, Petterson, M G, Millward, D, and Marriner, G F. 1995. Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District. Journal of Volcanology and Geothermal Research, Vol. 65, 81–110.

Branney, M J, and Kokelaar, B P. 1994. Volcanotectonic faulting, soft-state deformation and rheomorphism of tuffs during development of a piecemeal caldera, English Lake District. Geological Society of America Bulletin, Vol. 106, 507–530.

Branney, M J, and Soper, N J. 1988. Ordovician volcano-tectonics in the English Lake District. Journal of the Geological Society of London, Vol. 145, 367–376.

Branney, M J, and Suthren, R J. 1988. High-level peperitic sills in the English Lake District: distinction from block lavas and implications for Borrowdale Volcanic Group stratigraphy. Geological Journal, Vol. 23, 171–187.

Fitton, J G. 1972. The genetic significance of almandine-pyrope phenocrysts in the calcalkaline Borrowdale Volcanic Group, northern England. Contributions to Mineralogy and Petrology, Vol. 36, 231–248.

Hughes, R A, Evans, J A, Noble, S R, and Rundle, C C. 1996. U-Pb chronology of the Ennerdale and Eskdale intrusions supports subvolcanic relationships with the Borrowdale Volcanic Group (Ordovician, English Lake District). Journal of the Geological Society of London, Vol. 153, 33–38.

Johnson, E W, Briggs, D E G, Suthren, R J, Wright, J L, and Tunnicliff, S P. 1994. Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District. Geological Magazine, Vol. 131, 395–406.

Millward, D. 2002. Early Palaeozoic magmatism in the English Lake District. Proceedings of the Yorkshire Geological Society, Vol. 54, 65–93.

Millward, D. 2004. The Caradoc volcanoes of the English Lake District. Proceedings of the Yorkshire Geological Society, Vol. 55, 73–105.

Millward, D, and Evans, J A. 2003. U-Pb chronology and duration of upper Ordovician magmatism in the English Lake District. Journal of the Geological Society of London, Vol. 160, 773–781.

Millward, D, Beddoe-Stephens, B, Williamson, I T, Young, S R, and Petterson, M G. 1994. Lithostratigraphy of a concealed caldera-related ignimbrite sequence within the Borrowdale Volcanic Group of west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 25–36.

Millward, D, Marriner, G F, and Beddoe-Stephens, B. 2000. The Eycott Volcanic Group, an Ordovician continental-margin andesitic suite in the English Lake District. Proceedings of the Yorkshire Geological Society, Vol. 53, 81–96.

Petterson, M G, Beddoe-Stephens, B, Millward, D, and Johnson, E W. 1992. A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District. Journal of the Geological Society of London, Vol. 149, 889–906.

Piper, J D A, Stephen, J C, and Branney, M J. 1997. Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetisation during a single late Ordovician polarity chron. Geological Magazine, Vol. 134, 481–506.

Chapter 4 Late Ordovician to Silurian: Windermere Supergroup

Barron, H F. 1989. Mid-Wenlock acritarchs from a Silurian inlier in the Cheviot Hills, NE England. Scottish Journal of Geology, Vol. 25, 81–98.

Furness, R R, Llewellyn, P G, Norman, T N, and Rickards, R B. 1967. A review of Wenlock and Ludlow stratigraphy and sedimentation in NW England. Geological Magazine, Vol. 104, 132–147. 266

Ingham, J K. 1966. The Ordovician rocks in the Cautley and Dent districts of Westmorland and Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 35, 455–505.

King, L M. 1994. Turbidite to storm transition in a migrating foreland basin: the Kendal Group (Upper Silurian), northwest England. Geological Magazine, Vol. 131, 255–267.

Kneller, B C. 1991. A foreland basin on the southern margin of Iapetus. Journal of the Geological Society of London, Vol. 148, 207–210.

Kneller, B C, Scott, R W, Soper, N J, Johnson, E W, and Allen, P M. 1994. Lithostratigraphy of the Windermere Supergroup, Northern England. Geological Journal, Vol. 29, 219–240.

McCaffrey, W D. 1994. Sm-Nd isotopic characteristics of sedimentary provenance: the Windermere Supergroup of NW England. Journal of the Geological Society of London, Vol. 151, 1017–1021.

McCaffrey, W D, and Kneller, B C. 1996. Silurian turbidite provenance on the northern Avalonian margin. Journal of the Geological Society of London, Vol. 153, 437–450.

McNamara, K J. 1979. The age, stratigraphy and genesis of the Coniston Limestone Group in the southern Lake District. Geological Journal, Vol. 14, 41–69.

Rickards, R B. 2002. The graptolitic age of the type Ashgill series (Ordovician). Cumbria, UK. Proceedings of the Yorkshire Geological Society, Vol. 54, 1–6.

Rickards, R B, and Woodcock, N H. 2005. Stratigraphical revision of the Windermere Supergroup (Late Ordovician–Silurian) in the southern Howgill Fells, NW England. Proceedings of the Yorkshire Geological Society, Vol. 55, 263–285.

Shaw, R W L. 1971. The faunal stratigraphy of the Kirkby Moor Flags of the type area near Kendal, Westmorland. Geological Journal, Vol. 7, 359–380. Chapter 5 Devonian: Acadian deformation and magmatism

Chapter 5 Devonian: Acadian deformation and magmatism

Cox, R A, Dempster, T J, Bell, B R, and Rodgers, G. 1996. Crystallisation of the Shap Granite: evidence from zoned K-feldspar megacrysts. Journal of the Geological Society of London, Vol. 153, 625–635.

Dewey, J F, and Strachan, R A. 2003. Changing Silurian–Devonian relative plate motion in the Caledonides: sinistral transpression to sinistral transtension. Journal of the Geological Society of London, Vol. 160, 219–229.

Fortey, N J, Roberts, B, and Hirons, S R. 1993. Relationship between metamorphism and structure in the Skiddaw Group, English Lake District. Geological Magazine, Vol. 130, 631–638.

Hughes, R A, Cooper, A H, and Stone, P. 1993. Structural evolution of the Skiddaw Group (English Lake District) on the northern margin of eastern Avalonia. Geological Magazine, Vol. 130, 621–629.

Kneller, B C, King, L M, and Bell, A M. 1993. Foreland basin development and tectonics on the northwest margin of eastern Avalonia. Geological Magazine, Vol. 130, 691–697.

Merriman, R J, Rex, D C, Soper, N J, and Peacor, D R. 1995. The age of Acadian cleavage in northern England, UK: K-Ar and TEM analysis of a Silurian metabentonite. Proceedings of the Yorkshire Geological Society, Vol. 50, 255–265.

Soper, N J, and Woodcock, N H. 2003. The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension? Geological Magazine, Vol. 140, 627–647.

Soper, N J, Webb, B C, and Woodcock, N H. 1987. Late Caledonian (Acadian) transpression in North West England: timing, geometry and geotectonic significance. Proceedings of the Yorkshire Geological Society, Vol. 46, 175–192.

Soper, N J, Strachan, R A, Holdsworth, R E, Gayer, R A, and Greiling, R O. 1992. Sinistral transpression and the Silurian closure of Iapetus. Journal of the Geological Society of London, Vol. 149, 871–880.

Vaughan, A P M. 1996. A tectonomagmatic model for the genesis and emplacement of Caledonian calc-alkaline lamprophyres. Journal of the Geological Society of London, Vol. 153, 613–623.

Woodcock, N H, Soper, N J, and Strachan, R A. 2007. A Rheic cause for Acadian deformation in Europe. Journal of the Geological Society of London, Vol. 164, 1023–1036.

Chapter 6 Carboniferous: blocks, basins and sedimentary cyclicity

Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). 1989. The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. Occasional Publication of the Yorkshire Geological Society, No. 6.

Barclay, W J, Riley, N J, and Strong, G E. 1994. The Dinantian rocks of the Sellafield area, West Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 37–49.

Bott, M H P, Swinburne, P M, and Long, R E. 1984. Deep structure and origin of the Northumberland and Stainmore troughs. Proceedings of the Yorkshire Geological Society, Vol. 44, 479–495.

Burgess, I C. 1986. Lower Carboniferous sections in the Sedbergh district, Cumbria. Transactions of the Leeds Geological Association, Vol. 11, 1–23.

Calver, M A. 1968. Distribution of Westphalian marine faunas in Northern England and adjoining areas. Proceedings of the Yorkshire Geological Society, Vol. 37, 1–72.

Cleal, C J, and Thomas, B A. 1996. British Upper Carboniferous Stratigraphy. Geological Conservation Review Series, No. 11. (Peterborough: Joint Nature Conservation Committee.)

Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A and Wright, V P. 2004. British Lower Carboniferous Stratigraphy. Geological Conservation Review Series, No. 29. (Peterborough: Joint Nature Conservation Committee.)

Dickson, J A D, Ford, T D, and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. Proceedings of the Yorkshire Geological Society, Vol. 46, 203–229. 268

Fairbairn, R A. 2001. The stratigraphy of the Namurian Great/Main Limestone on the Alston Block, Stainmore Trough and Askrigg Block of northern England. Proceedings of the Yorkshire Geological Society, Vol. 53, 265–274.

Fielding, C R. 1984. A coal depositional model for the Durham Coal Measures of North East England. Journal of the Geological Society of London, Vol. 141, 917–931.

Fraser, A J, and Gawthorpe, R L. 2003. An Atlas of Carboniferous Basin Evolution in Northern England. Geological Society of London, Memoir, No. 28.

Garwood, E J. 1913. The Lower Carboniferous succession in the north-west of England. Journal of the Geological Society of London, Vol. 68 (for 1912), 449–586.

Guion, P D, Fulton, I M, and Jones, N S. 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant High. 45–78 in European Coal Geology. Whateley, M K G, and Spears, D A (editors). Geological Society of London Special Publication, No. 82.

Heckel P H, and Clayton, G. 2006. Use of the new official names for the Subsystems, Series and Stages of the Carboniferous System in international Journals. Correspondence. Proceedings of the Geologists’ Association, Vol. 117, 393–396.

Johnson, G A L. 1984. Subsidence and sedimentation in the Northumberland Trough. Proceedings of the Yorkshire Geological Society, Vol. 45, 71–83.

Johnson, G A L, and Dunham, K C. 1963. The geology of Moor House. Nature Conservancy Monograph, No. 2. (London: HMSO.)

Johnson, G A L, and Nudds, J R. 1996. Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 86, 181–226.

Jones, N S, and Holliday, D W. 2006. The stratigraphy and sedimentology of Upper Carboniferous Warwickshire Group red-bed facies in the Canonbie area of SW Scotland. British Geological Survey Internal Report, IR/06/043.

O’Mara, P T, and Turner, B R. 1999. Sequence stratigraphy of coastal alluvial plain Westphalian B Coal Measures in Northumberland and the southern North Sea. International Journal of Coal Geology, Vol. 42, 33–62.

Owens, B, and Burgess, I C. 1965. The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland. Bulletin of the Geological Survey of Great Britain, Vol. 23, 17–44.

Reynolds, A D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumbrian Basin. Proceedings of the Yorkshire Geological Society, Vol. 49, 135–148.

Rippon, J H. 1996. Sand body orientation, palaeoslope analysis, and basin fill implications in the Westphalian A–C of Great Britain. Journal of the Geological Society of London, Vol. 153, 881–900.

Rippon, J H. 1998. The identification of syndepositionally active structures in the coalbearing Upper Carboniferous of Great Britain. Proceedings of the Yorkshire Geological Society, Vol. 52, 73–93.

Rowley, C R. 1969. The stratigraphy of the Carboniferous Middle Limestone Group of West Edenside, Westmorland. Proceedings of the Yorkshire Geological Society, Vol. 37, 329–350.

Smith, T E. 1968. The Upper Old Red Sandstone–Carboniferous junction at Burnmouth, Berwickshire. Scottish Journal of Geology, Vol. 4, 349–354.

Tucker, M E, Gallagher, J, Lemon, K, and Leng, M. 2003. The Yoredale Cycles of Northumbria: High-Frequency Clastic-Carbonate Sequences of the Mid-Carboniferous Icehouse World. Open University Geological Society Journal, Vol. 24, 5–10.

Turner, B R, Younger, P L, and Fordham, C E. 1993. Fell Sandstone Group lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone. Proceedings of the Yorkshire Geological Society, Vol. 49, 269–281.

Ward, J. 1997. Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. 277–296 in Petroleum Geology of the Irish Sea and Adjacent Areas. Meadows, N S, and others (editors). Geological Society of London Special Publication, No. 124.

Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/07/01.

Chapter 7 Late Carboniferous to early Permian deformation and magmatism

De Paola, N, Holdsworth, R E, McCaffrey, K J W, and Barchi, M R. 2005. Partitioned transtension: an alternative to basin inversion models. Journal of Structural Geology, Vol. 27, 607–625.

Francis, E H. 1982. Magma and sediment – I: Emplacement mechanism of late Carboniferous tholeiite sills in northern Britain. Journal of the Geological Society of London, Vol. 139, 1–20.

Goulty, N R, Peirce, C, Flatman, T D, Home, M, and Richardson, J H. 2000. Magnetic survey of the Holy Island Dyke on Holy Island, Northumberland. Proceedings of the Yorkshire Geological Society, Vol. 53, 111–118.

Liss, D, Hutton, D H W, and Owens, W H. 2002. Ropy flow structures: a neglected indicator of magma-flow direction in sills and dykes. Geology, Vol. 30, 715–718.

Liss, D, Owens, W H, and Hutton, D H W. 2004. New palaeomagnetic results from the Whin Sill complex: evidence for a multiple intrusion event and revised virtual geomagnetic poles for the late Carboniferous of the British Isles. Journal of the Geological Society of London, Vol. 161, 927–938.

Shiells, K A G. 1964. The geological structures of north-east Northumberland. Transactions of the Royal Society of Edinburgh, Vol. 65, 449–481. 270

Stephenson, D, Loughlin, S C, Millward, D, Waters, C N, and Williamson, I. 2003. Carboniferous and Permian Igneous Rocks of Great Britain, North of the Variscan Front. Geological Conservation Review, No. 27. (Peterborough: Joint Nature Conservation Committee.)

Woodcock, N H, and Rickards, B. 2003. Transpressive duplex and flower structure: Dent Fault System, NW England. Journal of Structural Geology, Vol. 25, 1981–1992. Chapter 8 Permian, Triassic and Jurassic: deserts, rivers and shallow seas

Chapter 8 Permian, Triassic and Jurassic deserts, rivers and shallow seas

Barnes, R P, Ambrose, K, Holliday, D W, and Jones, N S. 1994. Lithostratigraphical subdivision of the Triassic Sherwood Sandstone Group in west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 51–60.

Benton, M J, Cook, E, and Turner, P. 2002. Permian and Triassic red beds and the Penarth Group of Great Britain. Geological Conservation Review, No. 24. (Peterborough: Joint Nature Conservation Committee.)

Brookfield, M E. 2004. The enigma of fine-grained alluvial basin fills: the Permo-Triassic (Cumbrian Coastal and Sherwood Sandstone Groups) of the Solway Basin, NW England and SW Scotland. International Journal of Earth Sciences, Vol. 93, 282–296.

Holliday, D W, Holloway, S, McMillan, A A, Jones, N S, Warrington, G, and Akhurst, M C. 2004. The evolution of the Carlisle Basin, NW England and SW Scotland. Proceedings of the Yorkshire Geological Society, Vol. 55, 1–19.

Holliday, D W, Jones, N J, and McMillan, A A. 2008. Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the north-eastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK. Scottish Journal of Geology, Vol. 44, 97–110.

Ivimey-Cook, H C, Warrington, G, Worley, N E, Holloway, S, and Young, B. 1995. Rocks of Late Triassic and Early Jurassic age in the Carlisle basin Cumbria (north-west England). Proceedings of the Yorkshire Geological Society, Vol. 50, 305–316.

Jackson, D I, and Johnson, H. 1996. Lithostratigraphic nomenclature of the Triassic, Permian and Carboniferous of the UK offshore East Irish Sea Basin. (Nottingham: British Geological Survey for the United Kingdom Offshore Operators Association.)

Jones, N S, and Ambrose, K. 1994. Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 50, 61–76.

Smith, D B. 1989. The late Permian palaeogeography of north-east England. Proceedings of the Yorkshire Geological Society, Vol. 47, 285–312.

Smith, D B. 1995. Marine Permian of England. Geological Conservation Review, No. 8. (Peterborough: Joint Nature Conservation Committee.)

Tucker, M E. 1991. Sequence stratigraphy of carbonate–evaporite basins; models and applications to the Upper Permian (Zechstein) of northeast England and adjoining North Sea. Journal of the Geological Society of London, Vol. 148, 1019–1036.

Worley, N E. 2005. The occurrence of halite in the Permian A Bed Evaporite, Kirkby Thore, Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 55, 199–203.

Chapter 9 Late Mesozoic and Cenozoic deformation and magmatism

Akhurst, M C, Barnes, R P, Chadwick, R A, Millward, D, Kimbell, G S, and Milodowski, A E. 1998. Structural evolution of the Lake District Boundary Fault Zone in west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 52, 139–158.

Chadwick, R A, Kirby, G A, and Baily, H E. 1994. The post-Triassic structural evolution of north-west England and adjacent parts of the Irish Sea. Proceedings of the Yorkshire Geological Society, Vol. 50, 91–102.

Fitch, F J, Hooker, P J, Miller, J A, and Brereton, N R. 1978. Glauconite dating of Palaeocene–Eocene rocks from East Kent and the time-scale of Palaeogene volcanism in the North Atlantic region. Journal of the Geological Society of London, Vol. 135, 499–512.

Green, P F. 2002. Early Tertiary palaeothermal effects in Northern England: reconciling results from apatite fission track analysis with geological evidence. Tectonophysics, Vol. 349, 131–144.

Holliday, D W. 1999. Palaeotemperatures, thermal modelling and depth of burial studies in northern and eastern England. Proceedings of the Yorkshire Geological Society, Vol. 52, 337–352.

Macdonald, R, Wilson, L, Thorpe, R S, and Martin, A. 1988. Emplacement of the Cleveland Dyke: evidence from geochemistry, mineralogy and physical modelling. Journal of Petrology, Vol. 29, 559–583.

Chapter 10 Mineralisation

Bouch, J E, Naden, J, Shepherd, T J, McKervey, J A, Young, B, Benham, A J, and Sloane, H J. 2006. Direct evidence of fluid mixing in the formation of stratabound Pb-Zn-Ba-F mineralisation in the Alston Block, North Pennine Orefield (England). Mineralium Deposita, Vol. 41, 821–835.

Cooper, M P, and Stanley, C J. 1990. Minerals of the English Lake District – Caldbeck Fells. (London: British Museum, Natural History.)

Crowley, S F, Bottrell, S H, McCarthy, M D B, Ward, J, and Young, B. 1997 34S of Lower Carboniferous anhydrite, Cumbria and its implications for barite mineralization in the northern Pennines. Journal of the Geological Society of London, Vol. 154, 597–600.

Fortey, N J, Ingham, J D, Skilton, B R H, Young, B, and Shepherd, T J. 1984. Antimony mineralisation at Wet Swine Gill, Caldbeck Fells, Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 45, 59–65.

Ixer, R A, Stanley, C J, and Vaughan, D J. 1979. Cobalt-, nickel-, and iron-bearing sulpharsenides from the North of England. Mineralogical Magazine, Vol. 43, 389–395.

Ixer, R A, Young, B, and Stanley, C J. 1996. Bismuth-bearing assemblages from the Northern Pennine Orefield. Mineralogical Magazine, Vol. 60, 317–324.

Lowry, D, Boyce, A J, Pattrick, R A D, Fallick, A E, and Stanley, C J. 1991. A sulphur isotopic investigation of the potential sulphur sources for Palaeozoic-hosted vein mineralization in the English Lake District. Journal of the Geological Society of London, Vol. 148, 993–1004. 272

Millward, D, Beddoe-Stephens, B, and Young, B. 1999. Pre-Acadian copper mineralisation in the English Lake District. Geological Magazine, Vol. 136, 159–176.

Milodowski, A E, Gillespie, M R, Naden, J, Fortey, N J, Shepherd, T J, Pearce, J M, and Metcalfe, R. 1998. The petrology and paragenesis of fracture mineralization in the Sellafield area, west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 52, 215–241.

Shepherd, T J, and Goldring, D C. 1993. Cumbrian hematite deposits, northwest England. 419–443 in Mineralization in the British Isles. Patrick, R A D, and Polya, D (editors). (London: Chapman and Hall.)

Stanley, C J, and Vaughan, D J. 1982. Copper, lead, zinc and cobalt mineralization in the English Lake District: classification, conditions of formation and genesis. Journal of the Geological Society of London, Vol. 139, 569–579.

Vaughan, D J, and Ixer, R A. 1980. Studies of the sulphide mineralogy of north Pennine ores, and their contributions to genetic models. Transactions of the Institution of Mining and Metallurgy, Vol. 89, B99–100.

Young, B. 1985. The distribution of barytocalcite and alstonite in the Northern Pennine Orefield. Proceedings of the Yorkshire Geological Society, Vol. 45, 199–206.

Young, B. 1987. Glossary of the minerals of the Lake District and adjoining areas. (Newcastle upon Tyne: British Geological Survey.)

Young, B, Styles, M P, and Berridge, N G. 1985. Niccolite-magnetite mineralization from Upper Teesdale, North Pennines. Mineralogical Magazine, Vol. 49, 555–559. Chapter 11 Quaternary: ice sheets and a changing climate

Chapter 11 Quaternary:ice sheets and a changing climate

Boardman, J (editor). 1981. Field Guide to Eastern Cumbria. (Brighton: Quaternary Research Association.)

Boardman, J, and Walden, J (editors). 1994. The Quaternary of Cumbria: Field Guide. (Oxford: Quaternary Research Association.)

Bowen, D Q (editor). 1999. A revised correlation of the Quaternary deposits in the British Isles. Geological Society of London Special Report, No. 23.

Bridgland, D R, Horton, B P, and Innes, J B. 1999. The Quaternary of north-east England: Field Guide. (London: Quaternary Research Association.)

Chiverrell, R C, Plater, A J, and Thomas, G S P. 2004. The Quaternary of the Isle of Man and North West England: Field Guide. (London: Quaternary Research Association.)

Ehlers, J, Gibbard, P L, and Rose, J (editors). 1991. Glacial deposits in Great Britain and Ireland. (Rotterdam: Balkema.)

Huddart, D, and Glasser, N F. 2002. Quaternary of Northern England. Geological Conservation Review Series, No. 25. (Peterborough: Joint Nature Conservation Committee.)

Hughes, D P, Mauquoy, D, Barber, K E, and Langdon, P. 2000. Mire-development pathways and palaeoclimatic records from a full Holocene peat archive at Walton Moss, Cumbria, England. The Holocene, Vol. 10, 465–479.

Lambeck, K, and Purcell, A P. 2001. Sea-level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling. Journal of Quaternary Science, Vol. 16, 497–506.

McMillan, A A, Hamblin, R J O, and Merritt, J W. 2004. An overview of the lithostratigraphical framework for Quaternary and Neogene deposits of Great Britain (Onshore). British Geological Survey Research Report, RR/04/04.

Merritt, J W, and Auton, C A. 2000. An outline of the lithostratigraphy and depositional history of Quaternary deposits in the Sellafield district, west Cumbria. Proceedings of the Yorkshire Geological Society, Vol. 53, 129–154.

Shennan, I, and Andrews, J. (editors). 2000. Holocene land–ocean interaction and environmental change around the North Sea. Geological Society of London Special Publication, No. 166.

Zong, Y, and Tooley, M J. 1996. Holocene sea-level changes and crustal movements in Morecambe Bay, northwest England. Journal of Quaternary Science, Vol. 11, 43–58.

Chapter 12 Geology and man

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wagstaff, S J, and Williams, A T. 1997. The Physical Properties of Major Aquifers in England and Wales. BGS Technical Report, WD/97/34; Environment Agency R & D Publication, No. 8.

British Standards Institution. 1999. BS 5930 Code of practice for site investigations. (London: British Standards Institution.)

Brown, E T. 1980. Rock characterization, testing and monitoring, ISRM Suggested Methods. (Oxford: Pergamon Press.)

Cooper, A H. 1998. Subsidence hazards caused by the dissolution of Permian gypsum in England: geology, investigation and remediation. 265–275 in Geohazards in engineering geology. Maund, J G, and Eddleston, M (editiors). Geological Society of London Special Engineering Publication, No. 15.

Dearman, W R, Money, M S, Coffey, J R, Scott, P, and Wheeler, M. 1977. Engineering geological mapping of the Tyne and Wear conurbation, North East England. Quarterly Journal of Engineering Geology, Vol. 10, 145–168.

Gutmanis, J C, Lanyon, G W, Wynn, T J, and Watson, C R. 1998. Fluid flow in faults: a study of fault hydrogeology in Triassic sandstone and Ordovician volcaniclastic rocks at Sellafield, north-west England. Proceedings of the Yorkshire Geological Society, Vol. 52, 159–175.

Hughes, D B, and Clarke, B G. 1997. The glacial tills of Northern England in relation to the stability of screening and spoil mounds at opencast coal sites. Proceedings of the International Association of Engineering Geology Symposium on Engineering Geology and the Environment, Athens. 2419–2424. (Rotterdam: Balkema.)

Hughes, D B, Clarke, B G, and Money, M S. 1998. The glacial succession in lowland Northern England. Quarterly Journal of Engineering Geology, Vol. 31, 211–234.

Jackson, I, and Lawrence, D J D. 1990. Geology and land-use planning: Morpeth- Bedlington-Ashington: Part 1 Land use planning. British Geological Survey Technical Report, WA/90/14.

Knott, S D. 1994. Fault zone thickness versus displacement in the Permo-Triassic sandstones of NW England. Journal of the Geological Society of London, Vol. 151, 17–25.

Musson, R M W. 2007. British Earthquakes. Proceedings of the Geologists’ Association, Vol. 118, 305–337.

Musson, R M W, and Henni, P H O. 2002. The felt effects of the Carlisle earthquake of 26 December 1979. Scottish Journal of Geology, Vol. 38, 113–126.

Patrick, C K. 1978. Hydrogeology and Environmental Geology. Chapters 17 and 18, 250–275 in The Geology of the Lake District. Moseley, F (editor). Yorkshire Geological Society Occasional Publication, No. 3.

Pettifer, G S, and Fookes, P G. 1994. A revision of the graphical method for assessing the excavatability of rock. Quarterly Journal of Engineering Geology, Vol. 27, 145–164.

Ryder, R, and Cooper, A. 1993. A cave system in Permian gypsum at Houtsay Quarry, Newbiggin, Cumbria, England. Cave Science, Vol. 20, 1, July 1993.

Figures, plates and tables

Figures

( Figure 1) Topography of northern England. The geographical relationship of the Isle of Man to the mainland is restored in ( Figure 5) and on the back pocket map.

( Figure 2) Outline geology of northern England. The geographical relationship of the Isle of Man to the mainland is restored in ( Figure 5) and on the back pocket map.

( Figure 3) Series of palaeogeographical reconstructions showing continental movements from the Ordovician to the Palaeogene (after Woodcock and Strachan, 2000).

( Figure 4) Summary chart of radiometric dates for the igneous rocks of northern England.

( Figure 5) Cross-section of the Iapetus Suture Zone modelled largely from seismic data. Section line shown on the accompanying map.

( Figure 6) Deep crustal sections of the Iapetus Suture Zone modelled from gravity and magnetic data. For locations of section lines see ( Figure 5).

( Figure 7) 3D model showing depth to the Lower Palaeozoic ‘basement’ across northern England, viewed from the west. The principal structural features that influence Upper Palaeozoic and later geology are identified and related to a ‘blocks and basins’ sketch map rotated into a top-to-north orientation.

( Figure 8) Biostratigraphical correlation of the Skiddaw and Manx groups.

( Figure 9) Distribution of the Manx and Skiddaw groups in the Isle of Man and north-west England respectively; also shown are the locations of boreholes proving Skiddaw Group strata at depth. The South Borrowdales Lineament is a deep-seated feature, defined by geophysical data, which coincides with the steep limb of the Westmorland Monocline.

( Figure 10) Outline geology of the main Skiddaw Group inliers in the northern Lake District.

( Figure 11) Stratigraphical correlation of the Skiddaw Group between the Lake District and Cross Fell inliers.

( Figure 12) Outline geology of the Manx Group in the central and southern parts of the Isle of Man.

( Figure 13) Geological map showing the principal Ordovician and Devonian igneous bodies and their relationship to major structural features. BrO Broad Oak Granodiorite Pluton, Butt Buttermere Intrusion, CF Carrock Fell Centre, CW Crummock Water Granite, Dun Dunmail Intrusion, En Ennerdale Granite Pluton, Esk Eskdale Granite Pluton, Ha Haweswater Gabbro-Microdiorite Swarm, HG Haweswater intrusion, LG Loweswater Intrusion, Ryd Rydal Intrusion, Sh Shap Granite Pluton, Sk Skiddaw Granite Pluton, Th Threlkeld Microgranite Intrusion, Ulp Ulpha Intrusion, Wa Wasdale outcrop of Esk, Wear North Pennine Batholith, Wen Wensleydale Granite Pluton.

( Figure 14) Schematic north to south cross-section through the Lake District approximately at the end of Borrowdale–Eycott volcanism. The widths of the contiguous Eycott Volcanic Group (EVG) and Borrowdale Volcanic Group (BVG) half-grabens have been obtained from the reduction of Acadian strain in the Cumberland and Westmorland monoclines. Vertical scale is approximately twice that of the horizontal. SkG = Skiddaw Group.

( Figure 15) Map of the Borrowdale Volcanic Group showing the main depositional centres and principal faults with displacements inferred to have occurred during accumulation of the volcanic succession.

( Figure 16) Generalised south–north and west–east cross-sections through the Borrowdale Volcanic Group showing the relationships of the various stratigraphical units. Sills are omitted. All units are formations, except for the Scafell Dacite and Rosthwaite Rhyolite which are members. Approximate lines of section are shown on ( Figure 17).

( Figure 17) Distribution of the lithostratigraphical successions within the Borrowdale Volcanic Group. See ( Table 2) for details of the successions. The lines of cross-sections refer to ( Figure 16).

( Figure 18) Diagram showing the relationship between lithofacies within the Birker Fell Formation of the Borrowdale Volcanic Group. The diagram approximates to a north-west to south-east cross-section from Wasdale to Devoke Water and the upper Duddon valley. The surface labelled base of upper BVG approximates to the topographical relief prior to eruption of the overlying Whorneyside Formation. Numbered features refer to volcanic events discussed in the text: 1 Devoke Water Tuff; 2 Birkby Fell Basalts; 3a Great Whinscale Dacite and Little Stand Tuff; 3b Craghouse Tuff and Seatallan Dacite; 4 Throstle Garth and Wrighthow basalts; 5 Eagle Crag Member.

( Figure 19) Generalised stratigraphy of the succession at the Scafell Caldera volcano, with details of the explosive, caldera-collapse phase and overlying caldera lake infill sequence.

( Figure 20) (opposite) Schematic cross-sections showing development of the Scafell Caldera during piecemeal collapse. The sequence of events is greatly simplified (after Branney and Kokelaar, 1994). A Emplacement of the Whorneyside Formation ignimbrite and succeeding phreatoplinian ash. B Onset of piecemeal subsidence causing deformation of the Whorneyside deposits; burial beneath hot silicic ash of the Long Top Tuffs erupted from new vents. C Further collapse causing ductile deformation of the hot ignimbrite. D Paroxysmal eruptions producing very densely welded ignimbrites of the Crinkle Tuffs and causing renewed subsidence and fault-scarp collapse.E Waning eruptions and silicic dome emplacement; sedimentation in a caldera lake.

( Figure 21) Basal relationships and outcrop of the Lincomb Tarns Formation. The inferred extent of the caldera is indicated.

( Figure 22) Lithostratigraphy of the concealed Gosforth succession, west Cumbria. Thin dykes and sills are omitted.

( Figure 23) General stratigraphical succession for the Windermere Supergroup in its type area of the southern Lake District (after Kneller et al., 1994). The buff-coloured sections in the Formation/Member column show breaks in the preserved succession.

( Figure 24) Generalised distribution of the component groups of the Windermere Supergroup in its southern Lake District outcrop.]

( Figure 25) Stratigraphical correlation across the Dent Group (after Kneller et al.,1994).The buff-coloured sections show breaks in the preserved succession.26).

( Figure 26) Lateral stratigraphical variation through the Silurian sector of the Windermere Supergroup (after Rickards and Woodcock, 2005).:).

( Figure 27) Variations in sedimentation rate and sea level during deposition of the Windermere Supergroup (after Rickards and Woodcock, 2005). Prd = Pridoli.26).

( Figure 28) Migration of the Southern Uplands accretionary thrust complex onto the Avalonian continental margin: the tectonic setting for deposition of the Coniston Group in the early Ludlow.

( Figure 29) Summary of radiometric dates from the Lake District inlier showing the resetting effect at about 420–430 Ma.

( Figure 30) Simplified structural cross-section of the Lake District inlier, showing the likely overburden at the time of Acadian deformation. This was mostly eroded away prior to the deposition of the Carboniferous strata (after Woodcock and Soper. ( Figure 6).5 in Brenchley and Rawson (editors). The Geology of England and Wales, 2006)

( Figure 31) Principal structures affecting the Skiddaw Group modelled in terms of southward thrust propagation ahead of the advancing Southern Uplands accretionary complex (after Hughes et al., 1993). For location of faults see ( Figure 10).

( Figure 32) Principal structures affecting the Skiddaw Group modelled in terms of a deep-seated flower structure rooted along the northern margin of the Lake District Batholith. To clarify the major structures, the Lower Palaeozoic strata are projected above the present surface level (cf. ( Figure 30)). The unconformity at the base of the Carboniferous sequence is shown as a reference level (after Woodcock and Soper. ( Figure 6).5 in Brenchley and Rawson (editors). The Geology of England and Wales, 2006).

( Figure 33) Sketch cross-section illustrating the main structural elements of the Manx Group.

( Figure 34) Variation in the relationship between Acadian cleavage and fold orientation across the outcrop of the Windermere Supergroup in the southern Lake District and Craven inliers (after Soper, Webb and Woodcock, 1987).

( Figure 35) Pattern of faults cutting the Windermere Supergroup in the southern Lake District (after Soper and Woodcock, 2003).

( Figure 36) Contoured top surface of the Weardale Granite Pluton and other components of the North Pennine Batholith. The inset map shows the location of the batholith in northern England in relation to the other major intrusive bodies.

( Figure 37) Summary chart of lithofacies (above) and stratigraphy (opposite) for the Carboniferous successions in northern England (after Waters et al., 2007. BGS Research Report RR/07/01).

( Figure 38) Correlation chart for the traditional district-based Carboniferous lithostratigraphies (named on the figure) and the regional group lithostratigraphy adopted in this account (identified by colour).

( Figure 39) Sketch cross-section from the Cheviot Block to the Stainmore Trough, showing the half-graben structure of the Northumberland Trough (after Chadwick et al., 1995. The Northumberland–Solway Basin and adjacent areas. BGS Subsurface Memoir).

( Figure 40) Distribution of Carboniferous lithofacies across northern England and the surrounding regions (after Cope et al., 1992. Atlas of Palaeogeography and Lithofacies, Geological Society of London Memoir, No. 13).

( Figure 41) Illustrative logs and interpretations for some types of high-frequency clastic sequences within the Yoredale and Pennine Coal Measures groups of northern England (after Tucker et al., 2003).

( Figure 42) Representative sections and correlations for the Tournaisian to middle Visean (Asbian) sequences.

( Figure 43) Representative sections and correlations for the Alston Formation, Yoredale Group,showing correlation with the Eskett Formation of west Cumbria.

( Figure 44) Representative sections and correlations for the Stainmore Formation, Yoredale Group.

( Figure 45) Outcrop of Westphalian strata and the location of the principal coalfields in northern England, showing variation in rank across the Northumberland and Durham Coalfield.

( Figure 46) Schematic illustration of the facies variation within the upper delta plain depositional environment of the Westphalian, Pennine Coal Measures Group (after Guion et al., 1995).

( Figure 47) Representative sections and correlations for the Pennine Coal Measures and Warwickshire groups.

( Figure 48) Map showing the main structures associated with late Carboniferous to early Permian deformation in northern England. Section A–B is shown in ( Figure 49)..35).

( Figure 49) Cross-section through the Solway and Vale of Eden basins. Line of section is shown on ( Figure 48) (after Chadwick et al., 1995. The Northumberland–Solway Basin and adjacent areas. BGS Subsurface Memoir).. 13).

( Figure 50) Cross-section through the Dent Fault System (after Woodcock and Rickards, 2003).

( Figure 51) Outcrop of the early Permian, tholeiitic Whin Sill-swarm and associated dyke subswarms. Key boreholes proving dolerite: Cr Crook; Et Ettersgill; Ha Harton; Lo Longhorsely; Lc Longcleugh; Ro Rookhope; Th Throckley; WB Whitley Bay; Wo Woodland.

( Figure 52) Isopachytes, in metres, on the Whin Sill-swarm. Where more than one leaf of dolerite is present, the total thickness is given.

( Figure 53) Sketch of a south-to-north section through the centre of Castle Hill, Holy Island, showing the alternating dyke-like and sill-like segments of the Holy Island Dyke (after Goulty et al., 2000). Castle Hill and the bench feature are illustrated on the front cover of this book. The bench feature is interpreted as part of a step-and-stair transgression of bedding during dyke intrusion.)

( Figure 54) Diagrams illustrating the mechanism of intrusion of the Whin Sill-swarm. a dykes are intruded to within 1 kilometre of the surface; b lateral intrusion of magma leads to gravitational flow down-dip and ponding of the magma in the centre of the basin; c to achieve hydrostatic equilibrium, magma advances up-dip on the other side of the basin with interfingering at the leading edge. Broken lines denote variation introduced where there are multiple dyke sources (after Francis, 1982).

( Figure 55) Distribution of depositional environments across northern England during Permian and Triassic times (after Cope et al., 1992. Atlas of Palaeography and Lithofacies, Geological Society of London Memoir, No. 13).

( Figure 56) Cartoons illustrating the changes in depositional regime through Permian and Triassic times (after Akhurst et al., 1997. The geology of the west Cumbria district. BGS Memoir).

( Figure 57) Distribution of Palaeogene dyke swarms across northern England and the surrounding regions.

( Figure 58) Principal metalliferous mining sites of Cumbria and the Lake District. Named localities are those mentioned in the text.

( Figure 59) Cross-sections of haematite ore bodies in west and south Cumbria (after Rose and Dunham, 1977. Geology and haematite deposits of South Cumbria. BGS Memoir). a Florence and Ullcoats mines, Egremont, west Cumbria. b Bigrigg Mine, Egremont, west Cumbria. c Park Sop Mine, Furness, south Cumbria.

( Figure 61) Principal geological features of the Northern Pennine Orefield. Named localities are those mentioned in the text (after Dunham, 1990. Geology of the Northern Pennine Orefield (1). BGS Memoir).

( Figure 62) Cross-section through the Killhopehead Vein to demonstrate the lithological control on the ore shoots. For location see ( Figure 61) (after Dunham, 1990. Geology of the Northern Pennine Orefield (1). BGS Memoir).

( Figure 63) Plan view of the ‘flat’ mineralisation at Allenheads Mine. For location see ( Figure 61) (after Dunham, 1990. Geology of the Northern Pennine Orefield (1). BGS Memoir).

( Figure 64) British chronostratigraphy, geomagnetic polarity and a representative oxygen isotope record (ODP 677) from the North Atlantic tuned to orbital timescales. Calendar ages based on historical records, annual layering in ice cores, tree rings, varve counting etc., are somewhat older than conventional radiocarbon ages, which have been calibrated to take this disparity into account using the radiocarbon calibration program of Stuiver, Reimer, and Reimer (2005) CALIB 5.0. [WWW program and documentation]. Conventional radiocarbon ages on shell material have also been adjusted by subtracting about 405 years to take into account the ‘marine reservoir effect’ for British waters.

( Figure 65) Proxy climate record of the last glacial termination based on an ice core (GISP2) obtained from the Greenland ice sheet summit. Blue bands denote periods of time within which Heinrich Events (H0 to H2) occurred. Ice core data provided by the National Snow and Ice Data Center, University of Colorado at Boulder, and the WDC for Paleoclimatology, National Glaciophysical Data Center, Boulder, Colorado.

( Figure 66) Distribution of glacigenic subgroups. The geographical boundaries are approximate, but will be refined as knowledge of the distribution of defining formations of till is improved.

( Figure 67) Schematic transect across lower Wasdale, Lake District, showing relations of the lithostratigraphical units.

( Figure 68) Speculative reconstructions of the last ice sheet. a At about the Last Glacial Maximum (LGM): 28–22 ka BP, but when Scottish ice had ceased flowing across Stainmore, and Scandinavian ice had advanced into the central North Sea Basin, forcing ice from the Pennines and Tweed Basin to flow into the Vale of York. b Following a major glacial reorganisation involving ‘drawdown’ and ‘headward scavenging’ of the Irish Sea ice stream into the Solway lowlands and Vale of Eden. Exact timing and correlation of events is unknown, but North Sea ice pushed farther into the Teesside lowlands once ice from the Lake District ceased flowing across Stainmore. Subglacial glaciofluvial deposition probably occurred within tunnel valleys in the Durham lowlands prior to the creation of Glacial Lake Wear. c Scottish ice advances into the Solway lowlands following retreat of ice sourced in the Lake District. Multiple readvances affect the Isle of Man and the west Cumbrian coast.

( Figure 69) Schematic transects across the north of the Isle of Man showing lithostratigraphical relationships.

( Figure 70) Schematic transect between the northern Lake District and the Vale of Eden showing lithostratigraphical relationships.

( Figure 71) Sketch of part of the push moraine exposed south-east of St Bees beach.

( Figure 72) Schematic transect across the Solway lowlands and Carlisle showing lithostratigraphical relationships.

( Figure 73) Speculative reconstruction of Glacial Lake Wear and associated ice-marginal lakes. Note: North Sea ice probably extended farther inland during an earlier phase when extensive subglacial glaciofluvial and glaciolacustrine sedimentation occurred within tunnel valleys.

( Figure 74) Schematic transect to the south of Sunderland showing lithostratigraphical relationships.

( Figure 75) Transect across a concealed channel at Maiden’s Hall opencast site.

( Figure 76) Changes in mire surface wetness and implied rainfall during the Holocene at Walton Moss, Cumbria.

( Figure 77) Representative relative sea-level curves. Solid lines are based on detailed biostratigraphical evidence; broken lines are based on predictive glacio-hydro-isostatic modelling.Pennine

( Figure 78) Distribution of earthquakes above 3.0 ML in northern England, 1650 to 2006.

( Figure 79) Distribution of surface water from the Lake District reservoirs.

Plates

(Frontispiece) Limestone pavement developed on the outcrop of the Carboniferous Great Scar Limestone Group at Holmepark Fell, Cumbria (P689724).

( Plate 1) The heart of the Lake District: crags of Borrowdale Volcanic Group rocks in Langdale, Cumbria (P005227).

( Plate 2) St Bees Sandstone Formation strata (Permo-Triassic, Sherwood Sandstone Group) at St Bees Head, Cumbria (P220633).

( Plate 3) Hadrian’s Wall and the Great Whin Dolerite Sill at Housesteads Crags west of Hexham, Northumberland (P222329).

( Plate 4) Terraced hillside in the northern Pennines created by strata of the Yoredale Group at Blagill, near Alston, Cumbria (P689653).

( Plate 5) Plenmeller opencast coal workings, Northumberland, photographed in the mid 1980s (P548139). The site is now restored.

( Plate 6) Selection of fossils from the Skiddaw Group: a Trilobite, Cyclopyge sp. from the Buttermere Formation at Beck Grains [NY 0776 1128] (RX1438/1438A), x8; b Pliomerid trilobite from the Hope Beck Formation, River Derwent near Kirkhouse [NY 1690 3290] (Ht 1267), x8; c Graptolite, Didymograptus deflexus from the Loweswater Formation at Barf [NY 217 265] (SM A.17712), x4.5; d Graptolite, Araneograptus murrayi from the Watch Hill Formation at Trusmadoor [NY 2777 3363] (RX 3099), x1; e Acritarch, Stellechinatum sicaforme sicaforme from the Bitter Beck Formation at Bitter Beck (MPK 5366), x800; f Acritarch, Cymatiogalea messaoudensis from the Watch Hill Formation at Watch Hill (MPK 5357), x600; g Acritarch, Stelliferidium trifidum from the Bitter Beck Formation at Bitter Beck (MPK 5371), x700. (P711116).

( Plate 7) View north-west from Robinson Crag [NY 2020 1720]. The foreground rocks, the Newlands Pass and Knott Rigg beyond lie within the outcrop of the Buttermere Formation, in the Central Fells Belt of the Skiddaw Group. Behind Knott Rigg, the Causey Pike Fault runs along the foot of the prominent crags of Wandope and Crag Hill, which lie within the Northern Fells Belt. Grasmoor forms the highest, most distant peak (P005045).

( Plate 8) Thinly bedded, mudstone-rich turbidites from the Lonan Formation, Manx Group, at Douglas Bay, Isle of Man [SC 390 770]. Similar strata make up much of the Manx and Skiddaw groups (P018620).

(Plate 9a) Sandstone turbidites from the Loweswater Formation, Skiddaw Group: a At outcrop on the slopes of Whiteside [NY 1605 2237] (P005003).

(Plate 9b) Sandstone turbidites from the Loweswater Formation, Skiddaw Group: b Detail of the Loweswater Formation at Liza Beck, Grasmoor [NY 1638 2097], to show the lenticular, cross-laminated internal structure and the interbedded mudstone; soft-sediment deformation and boudinage have affected these strata. Field of view is approximately 1 m (P006929).

(Plate 10a) Sandstone turbidite beds from the Lonan Formation, Manx Group: a At outcrop on the Isle of Man coast to the north-east of Onchan Head [SC 410 774] (P018635).

(Plate 10b) Sandstone turbidite beds from the Lonan Formation, Manx Group: b Detail of the Lonan Formation at Marine Drive, south of Douglas [SC 3778 7397] (P018605).

( Plate 11) Slump folds in the Buttermere Formation, Skiddaw Group, at Goat Crag, Buttermere, Cumbria [NY 1888 1629] [P005042].

( Plate 12) Unconformity at the base of the Eycott Volcanic Group at Chapel House Reservoir, Cumbria [NY 2582 3551]. The steeply dipping fabric beneath the unconformity is bedding in the Skiddaw Group accentuated by a bed-parallel compaction fabric (P005198).

( Plate 14) Trap topography in the andesite lava and sill succession of the Birker Fell Formation on High Rigg, St John’s in the Vale, near Keswick. The resistant andesite sheets form the crags, and the more easily eroded interbedded volcaniclastic rocks the prominent terraces (D Millward, P704123). 

( Plate 15) Welded ignimbrite, Cockley Beck Tuff, Duddon valley, south-west Lake District. The streaky (eutaxitic) texture on the rock surface is formed by the collapse and flattening of pumice lapilli that were still hot and plastic on deposition from a pyroclastic flow. The laminated tuff (bottom left) was deposited from a pyroclastic surge. The compass is 18 cm long (P006920).

( Plate 16) Bedded tuffs within the Duddon Hall Formation, Duddon valley, showing planar bedding, grading and alternation between coarse and fine layers. These rocks are interpreted as subaerial ash-fall deposits from a phreatoplinian eruption. The hammer is 37 cm long (P005147).

( Plate 17) Volcaniclastic sedimentary rocks within the Ambleside Basin succession on Torver High Common, south-west Lake District. A range of coarse-grained sandstone and pebble-conglomerate lithofacies were emplaced as mass-flow deposits and in a fluvial environment. The hammer is 37 cm long (P005139). 

( Plate 18) Complex relationships along the upper contact of an andesite sill intruded into laminated volcaniclastic sandstone, Middle Fell, Wasdale, Cumbria. The white weathered zone of siltstone between the laminated sandstone and andesite suggests that water-saturated sediment was fluidised during emplacement of the andesite. The hammer head is 17 cm long (P005180).

( Plate 19) Langdale Fell, on the northern flank of the Howgill Fells, is underlain by Silurian strata from the upper part of the Windermere Supergroup (P668926). 

( Plate 20) Selection of Ashgill shelly fossils from the Dent Group: a Brachiopod, Rafinesquina sp., internal mould of pedicle valve from the Kirkley Bank Formation (High Pike Haw Member, Cautleyan Stage) near Appletree Worth [SD 2465 9285] (RU 4075), x2; b & c Brachiopods, Plaesiomys inflata from the Kirkley Bank Formation (High Pike Haw Member, Cautleyan Stage) in the Coniston area - b = internal mould of transverse pedicle valve (Gsd461) x1.5, c = latex cast from external mould of brachial valve (Gsd271) x1.5; d Brachiopod, Fardenia (?) cf. transversaria, internal mould of pedicle valve from the Kirkley Bank Formation (High Pike Haw Member, Cautleyan Stage) near Appletree Worth [SD 2508 9341] (Zw8345) x1.5; e Trilobite, Mucronaspis olini, small pygidium from the Ashgill Shale Formation (Troutbeck Member, Rawtheyan Stage) north-east of Appletree Worth [SD 2488 9309] (Zs430) x6; f Trilobites, Staurocephalus cf. clavifrons, internal mould of exoskeleton associated with a second, small individual and (top right) a pygidium of Panderia? from the Ashgill Shale Formation (Troutbeck Member, Rawtheyan Stage) at Boo Tarn [SD 2811 9673] (Zw9286) x5. (P711117).

( Plate 21) Thinly interbedded mudstone and nodular limestone from the Applethwaite Member of the Kirkley Bank Formation. The beds dip moderately to the right and are cut by a strong, near-vertical cleavage. Moor Head, Troutbeck [NY 4243 0365]. The hammer head is 17 cm long (P223319).

( Plate 22) Brathay Formation laminated hemipelagite with thin siltstone and mudstone interbeds was worked here at Applethwaite Quarry, Troutbeck [NY 4234 0338] for building slates. The thin white bed (arrowed) near the base of the rock face in the centre of the photograph is a bentonite (altered volcanic ash) band (P223322). 

( Plate 23) ‘Donkey Rock’: an extensive development of flute casts on the base of a vertical bed of Gawthwaite Formation (Coniston Group) sandstone at Eccles Riggs Quarry [SD 2105 8682]. The flow direction of the eroding turbidity current was from the top to the bottom of the quarry face. The hammer is 37 cm long (P005161).

( Plate 24) Thinly interbedded, turbiditic mudstone, siltstone and fine-grained sandstone of the Bannisdale Formation seen here south-east of Raw Ghyll near Staveley [4550 9914]. Bedding is picked out especially by the lines of small cavities produced by the preferential weathering of carbonate-rich concretions in the mudstone layers. The hammer is 37 cm long (P223342).

( Plate 25) Cleavage development in Skiddaw Group (Buttermere Formation) strata near Hollows Farm, Borrowdale [NY 249 174]. The regional Acadian cleavage (S1) is inclined from top left to bottom right and is deformed by minor open folds with an axial planar crenulation cleavage inclined from top right to bottom left (A H Cooper, MNS8736). 

( Plate 26) Tight to isoclinal folds refolded by recumbent open folds in laminated to thinly bedded mudstone and siltstone of the Lonan Formation, Manx Group, at Port Erin, Isle of Man [SC 1959 6929] (P104237).

( Plate 27) Regional Acadian cleavage strongly developed in a siltstone and mudstone sequence from the Birk Riggs Formation (Tranearth Group) at Applethwaite Common, Troutbeck [NY 4233 0297]. The hammer is 37 cm long (P223323).

( Plate 28) Regional Acadian cleavage forming a convergent fan in a syncline of sandstone beds from the Niarbyl Formation (Dalby Group) near Glen Maye, Isle of Man [SC 2237 7987] (P104236).].

( Plate 29) Porphyroblasts of andalusite/chiastolite in Skiddaw Group mudstone within the thermal aureole of the Skiddaw Granite; these examples are mostly postkinematic relative to the well-defined Acadian cleavage, x25 (P054639).

( Plate 30) Specimen of pink Shap Granite containing large K-feldspar phenocrysts, Shap, Cumbria. The largest phenocrysts are about 4 cm long (P519145).

( Plate 31) ‘Polygenetic Conglomerate’ seen in Limekiln Beck near Gamblesby [NY 6260 3980]. The subrounded clasts of Lower Palaeozoic volcanic and sedimentary rocks are contained in a coarse sandy matrix (P006876).  

( Plate 32) Selection of Carboniferous macrofossils: a Crinoid, Woodocrinus? from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland (DL4741, P589464); b Goniatite, Cravenoceras cf. lineolatum from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland (P587665), c Brachiopod, Brachythyris sp. from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland (DL4353, P587666); d Brachiopods, large Gigantoproductids in the Sugar Sands Limestone, Stainmore Formation, at Sugar Sands Bay, Alnwick, Northumberland (P643515); e Corals, a polished surface of ‘Frosterley Marble’ from the Great Limestone Member of the Alston Formation, showing detail of the abundant coral Dibunophyllum bipartitum, Weardale, County Durham (P524822) x0.5; f Trace fossils, Teichichnus-type animal burrows preserved in sandstone from the upper part of the Appletree Limestone cycle of the Tyne Limestone Formation, Hindleysteel Quarry, Henshaw Common, Northumberland [NY 7496 7291] (P222338).

( Plate 33) Mudstone and argillaceous dolostone (‘cementstone’) of the Ballagan Formation (Inverclyde Group) exposed in Akenshaw Burn, Northumberland [NY 609 896] (S Arkley, P709473).

( Plate 34) Crags of coarse-grained sandstone from the Fell Sandstone Formation (Border Group) at Dancing Green Hill, between Doddington and Belford, Nothumberland [NU 063 333] (D Lawrence, P709474).

( Plate 35) ‘Basement Beds’ conglomerate from the Marsett Formation, exposed in Ardale Beck [NY 6610 3490], Cumbria, on the Pennine escarpment to the west of Cross Fell (P006879).

( Plate 36) Great Scar Limestone Group exposed in the western escarpment of Holmepark Fell, Cumbria [SD 540 795], viewed from Farleton Knott (P689698).

( Plate 37) Interbedded hemipelagic limestone and mudstone of the Scarlett Point Member, Bowland Shale Formation, exposed in the sea cliff at Scarlett Point [SC 257 661], Isle of Man (GS1039).

( Plate 38) Pillow lavas of the Scarlett Volcanic Member, Bowland Shale Formation, seen at Close ny Chollagh [SC 245 670], Isle of Man (GS1042).

(Plate 39a) Sedimentary features of the cyclic succession within the Yoredale Group exposed on the Northumberland coast. a Calcrete palaeosol and thin coal (in narrow cleft), marking the top of a cycle, overlain by limestone; between the Oxford and Eelwell Limestones, Alston Formation, south of Berwick-upon-Tweed (P663002).

(Plate 39b) Sedimentary features of the cyclic succession within the Yoredale Group exposed on the Northumberland coast. b Beds of hummocky cross-stratified sandstone within prodelta grey mudstone sequence; Oxford Limestone cycle, Alston Formation, south of Berwick-upon-Tweed (P662990).

(Plate 39c) Sedimentary features of the cyclic succession within the Yoredale Group exposed on the Northumberland coast. c A thin, prodelta clastic sedimentary cycle showing black mudstone coarsening upwards into sandstone, overlying a channel sandstone unit with a sharp base; Howick Limestone cycle, Stainmore Formation, south of Howick Burn, near Alnwick (P643529).

(Plate 39d) Sedimentary features of the cyclic succession within the Yoredale Group exposed on the Northumberland coast. d Cross-bedding in channel sandstone unit; Howick Limestone cycle, Stainmore Formation, south of Howick Burn, near Alnwick (P643531).

( Plate 40) Great Limestone Member of the Alston Formation overlain by ‘Yoredale cycles’ of the Stainmore Formation at Heights Quarry [NY 924 390], Eastgate in Weardale, County Durham (B Young, P548113).

( Plate 41) Strata of the Pennine Coal Measures exposed in the West Chevington opencast site [NZ 2440 9660], Northumberland (P220607).

( Plate 42) Syncline within strata of the Alston Formation (Yoredale Group) on the foreshore at Berwick-upon-Tweed, Northumberland (P689517). line.98).

( Plate 43) Columnar jointed dolerite in the Alnwick Sill at Cullernose Point on the Northumberland coast. In the foreshore is a ‘whaleback’ anticline in the Four Fathom (Sandbanks) Limestone (P643594).

( Plate 44) Ropy flow-structure on the lower, inner surface of a large, flattened vesicle at the top of the Farne Islands Dolerite Sill at Harkess Rocks, Bamburgh, Northumberland coast [NU 177 358]. The hammer head is 17 cm long (P637452).   

( Plate 45) Basal Permian unconformity near the bottom of the face in the disused (and now backfilled) East Thickly Quarry [NZ 2407 2565], County Durham. Thinly bedded dolostone of the Raisby Formation overlies the Marl Slate and Yellow Sands formations, the latter resting on thicker, cross-bedded sandstones of the Pennine Lower Coal Measures Formation (P221601).

( Plate 46) Old Quarrington Quarry, County Durham [NZ 338 377]. The Yellow Sands Formation, overlain by the Marl Slate Formation (here only about a metre thick) and thinly bedded dolostone of the Raisby Formation (P548172).

( Plate 47) Fossil fish remains (Palaeoniscus) from the Marl Slate Formation in the now-backfilled Down Hill Quarry, West Boldon, Sunderland [NZ 349 601] (P552051)

( Plate 48) Blackhall Rocks, north of Hartlepool [NZ 473 380]. Sea cliffs cut in a rubbly dolomitic breccio-conglomerate formed as a talus apron in front of the Ford Formation reef. Wave-rolled blocks of stromatolitic limestone are included (P221294)

( Plate 49) Detail of the Concretionary Limestone Member of the Roker Formation from a coastal exposure near Sunderland [NZ 412 555] (P69303).

( Plate 50) Collapse-brecciated limestone of the Roker Formation produced following dissolution of the underlying Hartlepool Anhydrite. Trow Point, Sunderland [NZ 385 666] (P693034).

( Plate 51) St Bees Sandstone Formation strata at Hole Sike near Ousby, Cumbria [NY 6199 3452]. Cross-bedded, red-brown sandstone overlies finely bedded brown and pale grey micaceous sandstone and siltstone (P221917).

( Plate 52) Dolerite of the Cleveland Dyke exposed in the River Eden at Armathwaite [NY 5035 4538]; the dyke is here intruded into Permian strata of the Penrith Sandstone Formation, Appleby Group (P221883).

( Plate 53) Nodules of graphite set in altered dioritic rock, from the Seathwaite graphite deposit at Borrowdale, Cumbria (P649471).

( Plate 54) Cut section through the Bonsor Vein, from a block recovered from the dumps at Bonsor Mine, Coniston, Cumbria. The vein consists of crude bands between dark greenish grey rhyolitic wall rock. Black magnetite (M) is abundant and is accompanied by small amounts of chlorite. The main ore mineral is metallic yellow chalcopyrite (C). Gangue minerals are quartz (Q) and calcite (L). Scale bar is 10 cm long.

( Plate 55) Tungsten mineralisation in the Harding Vein, at Carrock Fell Mine, Mosedale, Cumbria. Large, black, blade-like crystals of wolframite are set in a quartz gangue. In places, for example just above the hammer, wolframite is partly replaced by pale brown scheelite (arrowed). The hammer head is 17 cm long (P220211).

( Plate 56) Large mass of the mammillated variety of haematite known as ‘kidney ore’. The photograph was taken in the Lonely Hearts Orebody at Florence Mine, Egremont, west Cumbria. The hammer head is 17 cm long (P220237).

( Plate 57) Purple and green fluorite with bands of white quartz and bluish chalcedony, Groverake Vein, Rookhope, Weardale, Co. Durham. Photograph taken in the sub-level above the 60-fathom level, Groverake Mine. The banding is characteristic of the Groverake Vein. The brown patches are superficial iron-staining. The compressed air pipe is about 7 cm in diameter (P223157).

( Plate 58) Scraithole Vein in Scraithole Mine, West Allendale, Northumberland. The vein cuts the Namurian Great Limestone, and consists predominately of witherite with small quantitites of sphalerite and numerous clasts of limestone wall rock. (Photo T F Bridges, 1983; P601083).

( Plate 59) NEXTMap Digital Surface Model (1.2 m vertical resolution) (NEXTMap Britain elevation data from Intermap Technologies). ALB Alston Block; ASB Askrigg Block; B Brampton Kame Belt; CF Carter Fell; CH Cheviot Hills; CR Criffel; DG Dalston Gap; FG Ferryhill Gap; HF Howgill Fells; LH Langholm Hills; MH Moffat Hills; MO Maidens Hall opencast site; MP Millfield Plain; R Rogen moraine; S Shap Fell; SG Stainmoor Gap; SR Scottish Readvance limit at Carlisle; SB Scandal Beck interstadial site; StB St Bees Moraine; T Troutbeck interglacial sites; TG Tyne Gap; TV Team Valley.

( Plate 60) Durham Cathedral and Castle occupy a precipitous promontory within an incised meander of the River Wear. Photograph courtesy of D Robinson.

( Plate 61) Sea cliffs at Shellag Point, Isle of Man, cut in glacigenic sediments of the Bride Moraine that have been subjected to glacitectonic deformation (P649468a).

( Plate 62) Exposures in the push moraine at St Bees, west Cumbria (see ( Figure 71)): a Boulders on the beach washed out of the St Bees Sand and Gravel Member, looking north-west towards St Bees Head (P545250); b Glacitectonised Gutterfoot Sand Member (P666845); c Large anticlinal structure (P666258); d Grey, calcareous, pebbly muddy diamicton (St Bees Till) overlying St Bees Sand and Gravel (P666380).

( Plate 63) Sediments infilling a concealed channel at Maiden’s Hall opencast site (see ( Figure 75)): a Deformation till with bevelled and striated boulders overlying cross-bedded pebbly sand (P543552); b Truncated gravel-filled channel (P543560); c Laminated silt, clay and very fine-grained sand with soft sediment deformation structures (P543550); d Laminated silty clay passing up into a melange of ripped-up silt and clay, capped by deformation till (P543555); e Cross-bedded pebbly sand with northerly palaeocurrent resting on deformation till (P543554); f Fine-grained sand with climbing ripple-drift cross-lamination, increasingly folded and sheared upwards (P543549).

( Plate 64) Pillars of coal, the roof supports left behind in old, shallow mine workings, re-appearing in modern excavations at the Langley opencast site, Northumberland [NZ 200 469] (P711118).

( Plate 65) Queensberry ironstone workings at Cowshill, Weardale [NY 857 410](B Young, P548088). Supergene alteration has converted metasomatic replacement deposits of carbonate ores in the Great Limestone into workable deposits of limonitic ironstones.

( Plate 66) Lady Isabella or Great Laxey Wheel erected in 1854 to pump water from the Laxey Mines, Isle of Man, and now restored as a tourist attraction [SC 432 852] (P018718).

( Plate 67) Groverake fluorspar mine [NY 895 441], Rookhope, County Durham shortly before it closed in 1999 (B Young, P548081). The tall head frame stands on the No.2 or Drawing shaft, and the smaller over the No.1 or Old Whimsey shaft. The outcrop of Groverake vein passes between the two shafts.

( Plate 68) Pets Quarry, Kirkstone [NY 392 071]. The sawn blocks of ‘green slate’ are derived from the volcaniclastic rocks of the Seathwaite Fell Formation (Borrowdale Volcanic Group) (MNS 5415).

( Plate 69) Hown’s Quarry, Consett, Co. Durham [NZ 098 489]: a source of building stone excavated in sandstone from the Pennine Lower Coal Measures Formation (P222553).

( Plate 70) This surface collapse above old mine workings at Dinnington [NZ 209 731], close to Newcastle airport, occurred in November, 2000 (P266690).

( Plate 71) Flooded collapse hollow over the Park Sop haematite workings, south Cumbria [SD 211 753], with the Duddon estuary in the background (P222846).

Tables

Table 1 Geological succession of the rocks and deposits of the northern England region.

Table 2 Summary of the principal phases in the development of the Borrowdale Volcanic Group.

Table 3 Stratigraphical classification of the Carboniferous rocks of northern England. Note 1 Although the Carboniferous Subcommission of the International Commission on Stratigraphy has recommended that the terms ‘Dinantian’ and ‘Silesian’ should no longer be used, they are such fundamental units in the description of British Carboniferous rocks that they are likely to be encountered throughout the currency of this guide.

Table 4 Summary of the lithofacies characteristics of the Carboniferous successions in northern England (after Waters et al., 2007. BGS Research Report RR/07/01).

Table 5 Stratigraphical classification of the Westphalian strata in northern England.

Table 6 Lithostratigraphical subdivision of the Permian, Triassic and Jurassic rocks of north-west England.

Table 7 Lithostratigraphical subdivision of the Permian and Triassic rocks of north-east England.

Table 8 Representative regional deposits and events of the last two glacial–interglacial cycles to affect northern England.

Tables

Table 2 Summary of the principal phases in the development of the Borrowdale Volcanic Group.

Development phase Location, lithofacies and thickness Main events
11 Marine transgression Post-BVG

Marginal to Lake District in late Caradoc times; S Lake District in Ashgill

 Thermal contraction of granites on cooling allowed progressive marine transgression on to Lake District block
10 Batholith emplacement Beneath Scafell and Haweswater calderas. Granite, microgranite and granodiorite Injection of thick tabular granite sheets resulting in uplift and erosion of volcanic field
9b Cross Fell Cross Fell Inlier. Volcaniclastic sedimentary and pyroclastic rocks, one major sheet of silicic lapilli-tuff; >1100 m Volcaniclastic sedimentation interspersed with sporadic pyroclastic eruptions
9a Gosforth succession West Cumbria: stratiform sequence of andesitic and dacitic lapilli-tuff with subordinate volcaniclastic sedimentary rocks and andesite sills; >2500 m Dominantly explosive intermediate and silicic pyroclastic activity producing many densely welded ignimbrites; ?caldera related
8 Helvellyn Basin succession Central Fells, Helvellyn Basin. Stratiform units of volcaniclastic sandstone and breccia; subordinate pyroclastic units and a silicic lava; >1600 m Fluviolacustrine sedimentation within extensional basin with episodic catastrophic influx of eruption-generated sediment-gravity flows; interrupted by small andesite lava shields and ignimbrite emplacement
7 Lincomb Tarns ignimbrite Throughout BVG. Eutaxitic and parataxitic dacitic lapilli-tuff; columnar jointed; >800 m Most widespread and voluminous ignimbrite within BVG. Large magnitude silicic pyroclastic eruptions forming ignimbrite shield
6 Ambleside Basin succession Central south Lake District. Oversteps 5a, b, 4a; bedded volcaniclastic sandstone and breccia; >1100 m Fluviolacustrine sedimentation dominated by catastrophic influx of eruption generated sediment-gravity flows. ?Associated with emplacement of many sills
5b Kentmere Basin succession SE Lake District. Bedded units of volcaniclastic sedimentary rocks, intercalated with sheets of pyroclastic rocks, some lavas; <2800 m Intermediate to silicic ignimbrites; andesitic tuff-cone and lava-shield; fluviolacustrine sedimentation at base; major centre for sill emplacement
5a Duddon Basin succession SW Lake District. Stratified sequence of volcaniclastic sedimentary rocks, andesitic and rhyolitic pyroclastic rocks and andesite lavas Explosive intermediate ?caldera-related pyroclastic eruptions; succeeded by catastrophic eruption-generated sedimentation in extensional basin, episodically interrupted by emplacement of silicic ignimbrites. Many sills emplaced?
4b Haweswater Caldera succession Ullswater, Haweswater. Stratified sheets of dacitic and rhyolitic pyroclastic rocks; garnetiferous; <650 m Large magnitude silicic pyroclastic activity; large-volume ignimbrites associated with caldera collapse
4a Scafell Caldera succession Central Fells. Stratified sheets of andesitic, dacitic and rhyolitic pyroclastic rocks; dacite and rhyolite lavas; garnetiferous. Overlain by bedded volcaniclastic sandstone and breccia; >700 m Andesitic phreatoplinian eruption followed by sequence of large volume silicic ignimbrites with associated development of piecemeal hydrovolcanic caldera; postcaldera silicic lavas and caldera basin sedimentary infill
3 Monogenetic andesite volcanoes Throughout BVG. Andesite lavas and sills, some of basalt, basaltic andesite and dacite; subordinate tuff, lapilli-tuff and sandstone; some units garnetiferous; < 2700 m Construction of subaerial, multiple-vent, low-profile andesite volcanoes; some high volume basalt lava-fields; episodic silicic volcanism including possible caldera formation
2 Initial phreato- magmatism Devoke Water, Calder Bridge, Millom Park, Ullswater. Mafic lapilli-tuff and tuff-breccia; contains much mudstone and sandstone from Skiddaw Group; <600 m Hydrovolcanism, construction of tuff-cone field(s)
1 Pre-volcanic Latterbarrow Sandstone and Overwater Siltstone locally preserved; 0–400 m Regional uplift and erosion of Skiddaw Group. Local evidence of fluvial and ?marine deposition

Table 4 Summary of the lithofacies characteristics of the Carboniferous successions in northern England (after Waters et al., 2007. BGS Research Report RR/07/01).35).

Facies Subfacies Lithologies Depositional environments
Continental and peritidal Continental fluvial clastic (‘Cornstones’) Purple-red conglomerate, sandstone and red mudstone, with nodules and thin beds of concretionary carbonate (calcrete) Alluvial fan, fluvial channel and floodplain overbank; semi-arid climate
Continental and peritidal Peritidal marine  and evaporite  (‘Cementstone’) Grey mudstone, siltstone and sandstone, characterised by nodules and beds of ferroan dolostone (‘cementstone’) and evaporites (mainly gypsum and anhydrite) Alluvial plain and marginal marine flats subject to periodic desiccation and fluctuating salinity; semi-arid climate
Heterolithic clastic and nonmarine carbonate Interbedded grey sandstone, siltstone, mudstone and locally oil shale. Thin subordinate beds of lacustrine limestone and dolostone, seatearth, coal and sideritic ironstone Fluviatile, deltaic, lacustrine and rarely marine, commonly alternating in thin cycles
Open marine platform and ramp carbonate Shallow shelf platform carbonate Blanket bioclastic carbonate with crinoid banks, shelly or coral biostromes and algal (Girvanella) bands; often pot-holed bedding surfaces, overlain by thin bentonite layers Tropical shallow marine; episodic emergence
Open marine platform and ramp carbonate Ramp carbonate Calcareous mudstone with common dark bituminous and bioclastic limestone; local carbonate breccias. Reef knolls or Waulsortian reefs (mud mounds) Early stage of platform carbonate evolution; ramps develop upon gentle inclined shallow marine slopes marginal to platforms
Hemipelagic Dark grey to black mudstone, in part calcareous with calcareous nodules (‘bullions’). Thin sandstone and limestone beds locally common; breccias present may represent proximal turbidites or slump features Quiet and relatively deep basinal; with minor influx of sand-rich turbidites within prodelta region, and carbonate-rich turbidites on carbonate slopes
Mixed shelf carbonate and deltaic (‘Yoredale’) Typically upwards coarsening cycles of limestone, mudstone, sandstone, seatearth or ganister and coal Marine with episodic progradation of lobate deltas
Fluviodeltaic (‘Millstone Grit’) Shallow-water, sheet-like delta Upwards-coarsening cycles of black mudstone, grey siltstone, fine- to very coarse- grained feldspathic sandstone; coal and seatearth common, but thin Cyclic delta progradation with common black shale marine bands resulting from delta abandonment and marine transgression. Common disconformities
Fluviodeltaic (‘Millstone Grit’) Delta-top Condensed, predominantly upward-fining cycles of structureless clayrock to sandstone. Thick high-alumina seatclay, fireclay and bauxitic clay are common. Beds of limestone, ironstone, cannel and coal rare Millstone Grit cyclic sequences are thicker and fewer than Coal Measure ones
Fluviodeltaic (‘Coal Measures’) Upward-fining and upward-coarsening cycles of grey to black mudstone, grey siltstone, fine- to medium-grained sandstone, with frequent seatearth or ganister and relatively thick coals Wetland forest and soils, floodplain, river and delta distributary channel, prograding deltas and shallow lakes
Alluvial (‘Barren Measures’) ‘Red-bed’ Red, brown or purple-grey mudstone, siltstone and sandstone. Minor grey mudstone, thin coal, lacustrine (‘Spirorbis’) limestone and pedogenic limestone (calcrete) Fluvial channel. Red beds oxidised at, or close to, time of deposition; semi-arid climate