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Geology of the Keswick district. Sheet description of the British Geological Survey 1:50 000 Series sheet 29 Keswick (England and Wales)

By D G Woodhall

Bibliographical reference: Woodhall, D G. 2000. Geology of the Keswick district. Sheet Description of the British Geological Survey, 1:50 000 Series Sheet 29 Keswick (England and Wales). 48pp.

Author: D G Woodhall. Contributors: D F Ball, B Beddoe-Stephens, D Millward, R A Hughes and P R N Hobbs

Keyworth, Nottingham British Geological Survey 2000 ISBN 0 85272 374 1.

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: GD 272191/2000. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

© NERC 2000. All rights reserved 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 this publication 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.

(Front cover) Thirlmere reservoir, viewed from Steel Fell [NY 322 121], Helvellyn Screes form the steep slopes (lowest are forested) on the right, Blencathra is the highest of the fells in the background (north). The reservoir is situated entirely within rocks of the Borrowdale Volcanic Group, and is aligned along the main trace, and splays, of the Coniston Fault. Blencathra and hills to the left are formed of Skiddaw Group rocks hornfelsed within the aureole of the Skiddaw granite (MN 506391).

(Rear cover)

Acknowledgements

The accompanying 1:50 000 Series geological map was compiled from 1:10 000 maps surveyed by P M Allen, R S Arthurton, R P Barnes, B Beddoe-Stephens, S D G Campbell, A H Cooper, N C Davis, D J Fettes, R A Hughes, B C Kneller, S C Loughlin, B J McConnell, J W Merrit, D Millward, M G Peterson, D E Roberts, G J Roycroft, P Stone, B C Webb, D G Woodhall and B Young. Lower Palaeozoic faunas were identified and revised by A W A Rushton, and likewise microfaunas by S G Molyneux. Much of the petrology and geochemistry was described by B Beddoe- Stephens. The section on applied geology includes contributions on hydrogeology from D F Ball, and engineering geology characteristics and landslips by P R N Hobbs. Information on landfill sites was provided by the Environment Agency office at Penrith, and details of Sites of Special Scientific Interest and Regionally Important Geological Sites were supplied by English Nature (Cumbria Team). The Sheet Description was compiled by D G Woodhall, and reviewed by B Beddoe-Stephens, R A Hughes, J Merritt, D Millward and P Stone.

Notes

National Grid references in the text are given in the form [NY 323 108] and all lie within 100 km square NY. Symbols in brackets after lithostratigraphical names refer to symbols used on the 1:50 000 Series geological map. BGS services and products related to the Keswick district are listed under Information Sources.

Geology of the Keswick district—summary

The Keswick district is dominated by the rugged mountainous topography of the Lake District National Park, and includes some of the highest fells, notably Skiddaw (928 m) and Helvellyn (930 m). The lakes of Loweswater, Crummock Water, Buttermere, Derwent Water, and Thirlmere are also situated within the district, along with parts of Ennerdale Water, Bassenthwaite Lake and Ullswater. After a long history, mining and quarrying has now all but ceased, but past activities remain evident as mine entrances, waste tips and derelict buildings. Tourism now dominates the local economy and many public footpaths and bridleways traverse the district providing easy access to many geological features. Hill farming is the principal land use.

The solid geology is dominated by sedimentary and volcanic rocks of Ordovician age (495 to 450 million years old) but in the extreme north-east of the district there are small areas of younger rocks (about 380 to 330 million years old) of Devonian and early Carboniferous age. The resurvey of the Keswick district has significantly improved our understanding of the lithostratigraphy of the Ordovician rocks, particularly of the sedimentary Skiddaw Group. New formations and members in the overlying Borrowdale Volcanic Group are introduced with those at the top of the group identified as the youngest known anywhere in the Lake District. These newly defined strata greatly extend our knowledge and understanding of the nature of volcanism, depositional processes and the environment during deposition of the Borrowdale Volcanic Group. Mineralisation is evident throughout the district, and includes deposits exploited by the last operating metalliferous mines in the Lake District, namely Force Crag and Greenside.

There is major unconformity between early Carboniferous rocks that formed about 330 million years ago and unconsolidated Quaternary deposits (drift) most of which accumulated within the last 30 000 years when the district was glaciated. The effects of this are displayed in the present landscape of glacially carved valleys, separated by rugged mountains, and by the widespread cover of glacial deposits.

The new geological maps (‘Solid’ and ‘Solid and Drift’) and this Sheet Description provide valuable information on a wide range of Earth science issues. These include traditional aspects such as sedimentation, volcanism, structure, metamorphism and mineralisation, but also cover applied aspects such as mineral, energy and water resources, waste disposal, foundation conditions, and conservation.

(Table 1) Geological succession in the district.

Chapter 1 Introduction

The Keswick district (Figure 1) lies almost entirely within the northern part of the Lake District National Park, and is dominated by rugged mountains (fells) separated by glacially eroded valleys, parts of which are now occupied by lakes (Figure 2). A tourist-based economy of the district is centred on the town of Keswick, but the main legacy of past industrial activity occurs in the form of disused mines and quarries scattered throughout the district. At the present, hill farming is the principal land use. The geology of this part of Cumbria is portrayed on the 1:50 000 Series geological map of the Keswick district, and is explained in the following account. The district lies almost entirely within the Lake District Lower Palaeozoic inlier (Figure 3). A threefold geological succession within the inlier has been recognised for more than 150 years, but only two of the main groups, the Skiddaw and Borrowdale Volcanic groups, are present. The Skiddaw Group (Cooper et al., 1995) consists of marine mudstone, siltstone and sandstone mostly of early to mid- Ordovician age (Tremadoc to Llanvirn), but with some possibly of Cambrian age. A major unconformity (Millward and Molyneux, 1992) separates these marine strata from up to 8000 m of overlying, predominantly subaerial, volcanic rocks of the Eycott and Borrowdale volcanic groups, both of which are of mid- to late Ordovician (Llanvirn to Caradoc) age. A granite batholith, which underlies the central Lake Distric t (Bott, 1974; Lee, 1986a; 1989), is exposed in south- western and northern parts of the district, as the Ennerdale and Skiddaw intrusions respectively. There have been a number of mineralisation episodes, due to hydrothermal activity associated with the batholith. A small area of Upper Palaeozoic strata extends into the extreme north-east of the district, but rocks of Mesozoic and Cainozoic age, have been removed by erosion. Quaternary superficial deposits (drift) now veneer the Palaeozoic rocks. The geological succession in the Keswick district is given in (Table 1).

The Keswick district was originally surveyed on a scale of six inches to one mile, and published as Quarter Sheet 101SE (one inch to one mile) with an accompanying memoir (Ward, 1876). An economic geology memoir concerned with lead-zinc ores also covers the district (Eastwood, 1921). Subsequently, Hartley (1941) published details of the geology of the area around Thirlmere and Helvellyn. The latest survey, on which this Sheet Description is based, commenced in 1971, and involved collaboration (on the Borrowdale Volcanic Group) with Liverpool University during the period 1990–1995. A 1:25 000 Scale geological map of the Lorton and Loweswater area, in the north-west of the district, was published by the British Geological Survey in 1990.

Geological history

The rocks of the Skiddaw Group, the oldest recorded in the district, were deposited in a deep marine environment during the early to mid-Ordovician times (Tremadoc- Llanvirn) (Table 2), but they may also be partly Cambrian in age. They consist of mudstone, siltstone and sandstone. Deposition occurred on the passive oceanward northern margin of a micro-continent known as Eastern Avalonia (Cooper, et al., 1995; Pickering and Smith, 1995). There is evidence of contemporaneous volcanism during the latest stages (early Llanvirn) of deposition (Tarn Moor Formation). This was possibly related to the onset of subduction of Iapetus Ocean, situated between Gondwana (Eastern Avalonia) and Laurentia, as Eastern Avalonia rifted away from Gondwana, and drifted northwards (Trench et al., 1992).

Subduction of Iapetus Ocean gave rise to more intense volcanism, which produced the Eycott and Borrowdale Volcanic Groups (Fitton et al., 1982; Beddoe-Stephens et al., 1995). The generation of large volumes of andesitic magma brought about regional uplift of the continental margin, and consequently a considerable amount of the Skiddaw Group was eroded away (Millward and Molyneux, 1992; Cooper and Hughes, 1993). The predominantly subaerial volcanism that produced the rocks of the Borrowdale and Eycott volcanic groups is estimated to have lasted for about 5 million years during the Llanvirn-early Caradoc (mid- to late Ordovician) (Piper et al., 1997). The magmas were erupted as lavas and pyroclastic rocks, and intruded as high- level sills and dykes. The earliest volcanism was explosive (Hughes, 1994; Beddoe-Stephens, 1997b), before becoming predominantly effusive, with the development of low-relief cones made up of basaltic andesite and andesite lavas (Birker Fell Formation) (Petterson et al., 1992) (Table 4). However, this gave way to predominantly explosive volcanism, the products of which constitute the upper part of the Borrowdale Volcanic Group (Table 5). The Scafell Caldera formed during the eruption of the earliest of a series of large-volume pyroclastic flows, from which widespread sheets of dacitic and rhyolitic ignimbrite were deposited (Whorneyside and Airy’s Bridge formations) (Branney and Soper, 1988; Branney, 1991; Branney and Kokelaar, 1994) (Table 5). Events after caldera development consisted of long periods of volcaniclastic sedimentation (Seathwaite Fell, Esk Pike and Deepdale formations), which were briefly terminated by the emplacement of more ignimbrite sheets (Lincomb Tarns and Helvellyn formations, along with ignimbrite members within the Deepdale Formation) (Table 5).

In the Keswick district there is a major unconformity between the Lower and Upper Palaeozoic strata (Table 2). Events during this period began with uplift, as components of the Lake District batholith were emplaced (for example Ennerdale and Threlkeld intrusions), but were followed by subsidence and sedimentation of the Windermere Supergroup (late Ordovician and Silurian) as the batholith cooled. The ‘soft’ collision of Avalonia and Laurentia during the Silurian restricted sedimentation to a foreland basin, which formed in front (south) of a foreland fold and thrust belt that developed as the leading edge of Laurentia overrode the Avalonian margin (Kneller and Bell, 1993). All Windermere Supergroup strata were removed during the Early Devonian as a result of uplift and erosion brought about by the Acadian orogeny (Kneller and Bell, 1993; Kneller, King and Bell, 1993). This involved crustal shortening, which occurred during the final accretion of Avalonia onto the margin of Laurentia (Soper et al., 1987). The orogeny was accompanied by the development of north-west- trending folds and cleavage, sinistral fault movement, further southerly directed thrust faults and regional metamorphism (Hughes et al., 1993; Meller, 1998).

The intrusion of the remainder of the Lake District batholith (Skiddaw granite and the concealed Crummock Water intrusion), and associated contact metamorphism, also occurred during the Early Devonian, but took place after the main compressive phase of the Acadian orogeny (Soper et al., 1987). The main phases of mineralisation (copper- tungsten, lead-zinc, baryte) are probably related to periods of hydrothermal activity generated by the batholith.

Uplift brought about by the Early Devonian Acadian orogeny was followed by intense erosion during the rest of the Devonian, but there was localised deposition of valley- fill conglomerates (Mell Fell Conglomerate) (Cooper et al., 1993). Further erosion preceded carbonate deposition (Chief Limestone Group) which commenced during the early part of the Carboniferous. The major unconformity between the early Carboniferous and Quaternary incorporates the remainder of the Late Palaeozoic, the whole of the Mesozoic and most of the Cainozoic eras, a period of about 340 million years. Any deposits of these eras have been removed by uplift and erosion, in particular that at the end of the Cretaceous (Chadwick et al., 1994; Lewis et al., 1992).

During the Quaternary, the Lake District block formed an independent centre of ice dispersal, when all but the highest mountains were buried (Lamb and Ballantyne, 1998). The glacigenic Quaternary deposits in the district were mostly left by the last regional ice-sheet glaciation during the Dimlington Stadial of the Late Devensian, but there is limited evidence for pre-Devensian glaciation. The last glaciation was at its maximum extent about 22 000 years BP. A period of warm climatic conditions between 18 000 and 13 000 years BP (Windermere Interstadial), which followed deglaciation, left no clearly recognisable deposits in the district. However, there is extensive evidence for the return of small glaciers to the heads of some valleys, and widespread periglacial conditions, between 11 000 and 10 000 years BP (Loch Lomond Stadial). Renewed climatic amelioration at around 10 000 years BP brought about the retreat of these glaciers, and marks the onset of the Holocene. Since then, alluvial and lacustrine deposits have accumulated in many of the valleys, and the deposition of peat in both lowland and upland areas followed the gradual re-colonisation of the area by vegetation. Landslips have modified the landscape in places.

Chapter 2 Cambrian and Ordovician

The Cambrian and Ordovician rocks of the Keswick district comprise the Skiddaw Group, and the overlying Eycott and Borrowdale Volcanic Groups. The east-north- east-trending Causey Pike Fault (Cooper and Molyneux, 1990) divides the district into two distinct stratigraphical belts; the Northern Fells Belt and the Central Fells Belt. Cooper et al. (1995) applied these belts to the Skiddaw Group, but the Causey Pike Fault also separates the Eycott and Borrowdale Volcanic Groups (Table 1) and (Figure 1).

Skiddaw Group

The Skiddaw Group, the distribution of which is shown in (Figure 1), consists of wacke sandstone, siltstone and mudstone, mostly of Ordovician (Tremadoc to Llanvirn) age, but with some possibly as old as late Cambrian (Table 3); (Plate 1). These strata accumulated as siliciclastic turbidites in deep water on the passive, northern margin of Gondwana, prior to, or during, the separation of the Eastern Avalonian micro-continent (Cooper et al., 1993; 1995). The sediment was derived from ancient continental-arc volcanic rocks of Gondwana farther south. The presence of olistostromes and other evidence of soft-sediment deformation indicate slope instability, particularly during the late Arenig, which was possibly related to rifting and the separation of Avalonia from Gondwana. The biostratigraphy of the Skiddaw Group is based on graptolite and acritarch assemblages (Table 3) (Jackson, 1961; 1978; Molyneux, 1990; Cooper et al., 1995).

Northern Fells Belt

The Skiddaw Group of the Northern Fells Belt consists of the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirkstile formations (Table 3), but is locally undivided.

Skiddaw Group undivided (SkG)

The Skiddaw Group, in the extreme north-east of the district, consists of an unknown thickness of poorly exposed siltstones and mudstones. In the adjacent Cockermouth district, these strata have yielded acritarch assemblages ranging in age from possible mid- or late Cambrian to latest Tremadoc or earliest Arenig (Millward and Molyneux, 1992).

Bitter Beck Formation (BBF)

The Bitter Beck Formation, of upper Tremadoc age (Araneograptus murrayi Biozone) (Cooper et al., 1995) (Table 3), crops out in the extreme north-west of the district (Figure 1). It consists of at least 500 m of dark grey mudstone and silty mudstone, with subordinate siliciclastic fine-grained sandstone, deposited as distal turbidites, which have been thrust southwards, possibly by as much as 5 km, over the Kirkstile Formation along the Watch Hill Thrust (Hughes et al., 1993; Cooper et al., 1995).

Watch Hill Formation (WHg)

The Watch Hill Formation, of upper Tremadoc to lower Arenig age (Table 3), has no crop within the district, but is shown at depth in section 1 on the 1:50 000 Series geological map. The formation consists of between 550 and 800 m of lithic wackes, siltstones and mudstones, emplaced as turbidites and interpreted as submarine fan deposits (Cooper et al., 1995).

Hope Beck Formation (HBF)

The Hope Beck Formation, of Arenig age (Tetragraptus phyllograptoides to Didymograptus varicosus biozones) (Cooper et al., 1995) (Table 3), is present as partly fault- bounded inliers in the north-western part of the district, at Swinside, between Crummock Water and Bassenthwaite Lake (Figure 1). Only the upper 400 m of the formation is present. The rocks consist of normally graded distal turbidites, comprising dark grey siltstone and mudstone, commonly bioturbated, with subordinate siliciclastic medium to coarse-grained sandstone. There are a few beds of matrix-supported, pebbly mudstone interpreted as debris-flow deposits (Cooper et al., 1995).

Loweswater Formation (LWF)

The Loweswater Formation (Cooper et al., 1995), also of Arenig age (Didymograptus varicosus and Didymograptus simulans biozones) (Table 3), crops out mostly in the north-western part of the district (Figure 1). It is about 900 m thick, and is dominated by sandstone turbidites that were deposited during a major period of submarine fan development (Cooper et al., in prep) (Table 3); (Plate 1). Jackson (1961) noted southerly and south-westerly directed palaeocurrents, but Cooper et al. (1995) noted palaeocurrents mainly from the south-east or south-south-east. These were interpreted as having been controlled by basin-floor topography (Moore, 1992). Small-scale slump folds up to a few metres across occur locally, and there is evidence of bioturbation (Cooper et al., 1995).

Kirk Stile Formation (KSt)

The Kirk Stile Formation (Cooper et al., 1995), of Arenig to basal Llanvirn age (Didymograptus simulans to D. artus biozones) (Table 3), dominates the Skiddaw Group in the Northern Fells Belt, and the Causey Pike Fault marks the southern edge of the outcrop (Figure 1). It either rests on, or is faulted against, the Loweswater Formation. Large parts of the Kirk Stile Formation are hornfels within the thermal metamorphic aureoles of the Crummock Water and Skiddaw intrusions. The formation is between 1500 and 2500 m thick, and consists mostly of distal turbidites comprising laminated to thinly bedded dark grey siltstone and mudstone. However, locally, in the middle of the formation there are lenticular units, 80 to 120 m thick, of sandstone-rich turbidites; in the upper part of the formation there are a few debris-flow deposited breccias, up to 40 m thick, which are associated with slumped beds. Slump folding commonly occurs in units 2 to 40 m thick (Plate 2), bounded by less disturbed and undisturbed beds, indicating that the deformation was either syndeposition or early postdepositional.

Central Fells Belt

The Skiddaw Group of the Central Fells Belt comprises the Buttermere and Tarn Moor formations.

Buttermere Formation (BUF)

The Buttermere Formation, of early Tremadoc to late Arenig age (Webb and Cooper, 1988; Webb, 1990; Cooper et al. 1995) (Table 3), dominates the Skiddaw Group of the Central Fells Belt, and crops out south of the Causey Pike Fault between Buttermere, Derwent Water and Threlkeld Common (Figure 1). The formation is an olistostrome deposit, at least 1500 m thick, made up of disrupted, sheared and folded mudstone, siltstone and sandstone turbidite olistoliths (Webb and Cooper, 1988; Hughes, 1994; Hughes and Fettes, 1994; Cooper et al., 1995).

The olistoliths consist of angular to subrounded, clasts, blocks and rafts of siltstone, mudstone and subordinate sandstone in a silty mudstone matrix. They vary in size from less than a centimetre to a kilometre or more across, but many are between 5 and 10 m. Some angular olistoliths were at least partly lithified before being incorporated in the deposit, whereas others are ragged with injection structures from the matrix suggesting they were in a plastic state when redeposited. The matrix around the olistoliths consists of a disaggregated mixture of particles, and fine- grained quartz is abundant. The olistoliths and matrix are intensely deformed by minor folds and shears, many of which formed during emplacement of the olistostrome. A characteristic feature of the formation is the juxtaposition of beds of widely different ages, for example, lower Tremadoc acritarch assemblages within a few metres of those of late Arenig age, as in Swinside Gill [NY 190 177] (Cooper et al., 1995). There is no evidence that the assemblages are mixed.

The Buttermere Formation is mostly undivided, but between Buttermere and Derwent Water the middle part includes the Robinson Member (RBN), which comprises sandstone turbidites (Webb and Cooper, 1988; Webb, 1990; 1992; Cooper et al., 1995). This consists of several large sandstone-rich olistoliths, up to 1 km across and 250 m thick.

The age of olistostrome emplacement is constrained by the Tremadoc to late Arenig age of the olistoliths and matrix in the Buttermere Formation and the latest Arenig to Llanvirn age of the overlying Tarn Moor Formation. Emplacement is inferred to be in the late Arenig, possibly at about the gibberulus-cucullus Biozone boundary, and probably took place in one massive slumping episode involving downslope movement towards the north-west (Webb and Cooper, 1988; Cooper et al., 1995) and/or west (Moore, 1992).

Tarn Moor Formation (TMF)

The Tarn Moor Formation, of upper Arenig to Llanvirn age (Table 3) (Cooper et al., 1995), constitutes the highest part of the Skiddaw Group in the Central Fells Belt. It crops out in the eastern part of the Keswick district (Figure 1). The formation is between 1000 and 1500 m thick and consists mostly of mudstone and siltstone. The base has not been proved, but its age and outcrop pattern indicates that it overlies the Buttermere Formation. Consequently, its lower boundary must be unconformable on the highly disrupted Buttermere Formation (Table 3). Laminated mudstone, with subordinate siltstone, in the lower part of the Tarn Moor Formation contains uppermost Arenig to lowest Llanvirn acritarchs, graptolites (A. cucullus {formerly D. hirundo} and D. artus biozones) (Table 3) and trilobites. Overlying mudstone, with lower Llanvirn (D. artus Biozone) graptolites, incorporates sporadic bentonite beds up to a few centimetres thick, and volcaniclastic sandstone turbidite successions up to 12 m thick. In the neighbouring Appleby district, mudstones intersected by the Tarn Moor Tunnel have yielded evidence of the D. murchisoni Biozone (Wadge et al., 1969; 1972).

Depositional environment of the Skiddaw Group

The Bitter Beck, Hope Beck and Kirk Stile formations are interpreted as mudstone-dominated turbidite fans (Moore, 1992). Coarser clastic material, which is rare in the Kirk Stile Formation, occurs as siltstone and fine- to medium- grained sandstone turbidites of the channel levée and depositional lobe facies associations (Moore, 1992). However, the restricted amount of this material is believed to indicate a high sea-level stand, or sediment starvation due to intrabasinal topography. Widespread slump folds and sediment slides in the Kirk Stile Formation are products of late Arenig or early Llanvirn soft-sediment deformation, which was approximately contemporaneous with the emplacement of the Buttermere Formation olistostrome. Tectonic activity and/or a lowering of sea level were possible causes of this deformation.

The Watch Hill and Loweswater formations represent major influxes of coarse clastic detritus, probably during relatively low sea level stands. The Watch Hill Formation was deposited in either an east-west trough, or on a fan with a northward palaeoflow. The Loweswater Formation marks the height of turbidite fan development within the Skiddaw Group. It incorporates the distributary channel (medium- to coarse-grained sandstone turbidites and channel-fill deposits), depositional lobe (fine- to medium- grained sandstone turbidites) and inter-lobe (siltstone turbidites) facies associations (Moore, 1992). Moore proposed an extensional basin setting for the Loweswater Formation submarine fan, with north-north-west-trending faults that separated fault blocks tilted towards the north-east, and believed that the widespread convolute laminations in the formation were indicative of contemporaneous tectonic activity.

Eycott Volcanic Group (EVG)

The Ordovician Eycott Volcanic Group is restricted to the southern part of Eycott Hill in the extreme north-east of the Keswick district (Figure 1). There are more extensive outcrops in the neighbouring Cockermouth district (Eastwood et al., 1968), where it attains a maximum thickness of about 3200 m, and is dominated by tabular andesite sheets. The latter are continental margin tholeiitic-type rocks (Fitton et al., 1982; Millward, 1999). At Eycott Hill, the group unconformably overlies the Skiddaw Group of the Northern Fells Belt (Millward and Molyneux, 1992; Millward et al., in press), and consists of about 800 m of basaltic andesite and andesite sheets with a few interbedded volcaniclastic rocks. The basaltic andesite and andesite sheets are inferred, but not proved, to be subaerially erupted lavas. These range in thickness from 20 to 90 m, and are typically massive with amygdaloidal zones at the base and top. Some display flow foliation, and/or fine-scale platy jointing parallel to the base, and have autobreccia at the top. Most are feldspar-phyric, but some contain exceptionally large (about 5 cm across) plagioclase phenocrysts, and were referred to as ‘Eycott-type’ basaltic andesites by Eastwood et al. (1968). Intercalated volcaniclastic rocks occur generally in units up to 5 m thick; most are of parallel-bedded, fine- to coarse-grained volcaniclastic sandstone. Locally at the base is a lapilli-tuff interpreted as a pyroclastic fall deposit by Millward et al., (1999). A pinkish weathered welded dacitic ignimbrite, about 20 m thick, is present approximately 370 m above the base of the succession.

Borrowdale Volcanic Group (BVG)

The Ordovician (Llanvirn to Caradoc) Borrowdale Volcanic Group, altogether about 7500 m thick, consists of subaerially erupted basaltic, andesitic, dacitic and rhyolitic lavas and pyroclastic rocks, volcaniclastic sedimentary rocks deposited in shallow subaqeous environments, and numerous high- level sills. These rocks are calc-alkaline in composition with continental margin affinities (Fitton et al., 1982). The contact with the Skiddaw Group, where not faulted, or obscured by igneous intrusion, is mostly unconformable (see Moseley, 1992 for review; McConnell and Kneller, 1993; Hughes, et al., 1993; Hughes, 1994; Campbell, 1995; Beddoe-Stephens, 1997b) (Plate 3). The apparent conformable contact reported by Gough (1965) from the lowest workings of the Greenside mine, near Glenridding, is probably faulted (Hughes, 1995).

Birker Fell Formation (BFA)

The Birker Fell Formation, 1600 to 2400 m thick, crops out between the Ennerdale intrusion and the eastern edge of the district (Figure 1). The formation consists mainly of lavas and sills, mostly andesitic in composition, but with some of basalt, basaltic andesite and dacite, together with subordinate interbedded pyroclastic and volcaniclastic sedimentary rocks (McConnell and Kneller, 1993; Beddoe- Stephens, 1995; 1996; 1997b; Campbell, 1995; Millward, 1998) (Table 4). Petterson et al. (1992) interpreted the formation as a plateau-andesite field made up of a cluster of low-profile volcanic edifices.

Basal, lenticular volcaniclastic sedimentary and pyroclastic rocks crop out in Ennerdale, Borrowdale, Low Rigg, Threlkeld and Matterdale (McConnell and Kneller, 1993; Campbell, 1995; Beddoe-Stephens, 1997b). Volcaniclastic sedimentary rocks near Low Rigg and Threlkeld, consists of a few metres of conglomerate and pebbly sandstone, overlain by up to about 18 m of variably tuffaceous mudstone and siltstone (Ward, 1876; Mitchell et al., 1972; Wadge et al., 1974; Campbell, 1995). The pyroclastic rocks were produced by explosive, phreatomagmatic eruptions and represent the earliest eruptive activity of the Borrowdale Volcanic Group in the district (Beddoe-Stephens, 1997b). Subsequent eruptive activity was mainly effusive, and dominated by the emplacement of aphyric and plagioclase-phyric andesite and dacite sheets (Plate 4), but with subordinate pyroxene-phyric basaltic andesite and garnetiferous andesite, as lavas and/or high-level sills (Beddoe-Stephens, 1999). In the south-western part of the district, in Ennerdale, the lowest of these sheets consists of up to 350 m of massive, aphyric, flow-foliated dacite (McConnell and Kneller, 1993). The middle part of the formation, north-east of Rosthwaite, and on Armboth and Castlerigg fells [NY 295 160]; [NY 285 195], includes extensive sheets of garnetiferous andesite, whereas farther east, on Watermillock Common [NY 380 203], there are local occurrences of andesitic hyaloclastite (Beddoe-Stephens, 1995; 1996; Campbell, 1995). The upper part of the formation, around Thirlmere, on Armboth Fell [NY 305 153] and Whiteside Bank [NY 340 174], includes sheets of basaltic andesite, but adjacent to Glenridding [NY 383 172] dacite lava occurs at a similar stratigraphical level.

A number of members occur in the lower and middle parts of the Birker Fell Formation in the Ennerdale and Borrowdale areas. In Borrowdale, the basal volcaniclastic rocks are overlain by andesite lava, 200 m thick, of the Grange Crags Member (GCG), and lapilli-tuff (ignimbrite) of the Ashness Member (AHT) (Beddoe-Stephens, 1997b) (Table 4). In Ennerdale and Borrowdale, volcaniclastic rocks, 190 to 270 m thick and about 350 m above the base of the formation, constitute the Eagle Crag Member (Eag) (McConnell and Kneller, 1993) (Table 4). This member, which incorporates the ‘slates’ quarried at Honister, has been intruded by numerous andesite sills, and is locally overlain by lava of the Seatallan Dacite (STD). This, along with the andesitic and dacitic pyroclastic rocks of the Craghouse Member (CHT), also crop out in the adjacent Ambleside district where they have been described in detail by Millward et al. (2000). The Craghouse Member is not apparent north of the Burtness Comb Fault, interpreted as syndepositional by Branney et al. (1993a), and McConnell and Kneller (1993). South of this fault, the middle part of the formation includes andesite lava and breccia, 500 m thick, of the Haystacks Member (HYS) which is in part overlain by volcaniclastic rocks, 250 m thick, of the Round How Member (RND) (McConnell and Kneller, 1993) (Table 4).

Whorneyside Formation (Wny)

The Whorneyside Formation is the lowest of the formations of the predominantly volcaniclastic upper part of the Borrowdale Volcanic Group (Table 5). It crops out west of the Coniston Fault (Figure 1), and rests unconformably on the Birker Fell Formation (Branney, 1988; Branney, 1991; Branney and Kokelaar, 1993). The formation attains a maximum thickness of about 140 m, but in the vicinity of Bell Crags, close to the Coniston Fault, [NY 300 147], it wedges out. In places the Whorneyside Formation is complicated by the presence of abundant andesite sills (Branney and Kokelaar, 1993). The lower part consists of a massive, eutaxitic (welded), andesitic lapilli-tuff, up to 100 m thick. It is interpreted as low-volume ignimbrite, with basal fall and surge deposits and constitutes the Wet Side Edge Member (WSE) (Millward et al., 2000). The emplacement of these deposits was accompanied by the initial phase of development of the Scafell Caldera (Branney, 1991; Branney and Kokelaar, 1994). The upper part of the formation comprises bedded tuff, interpreted as a phreato-magmatic ash-fall deposit (Branney, 1991; Branney and Kokelaar, 1993; Millward et al., 2000).

In Borrowdale, the West Side Edge Member ignimbrite contains lithic blocks up to 20 cm across and is coarser grained than anywhere within the Ambleside district, suggesting local derivation. Branney (1991) suggested that the ignimbrite was sourced from a vent located north of its outcrop, but there is no direct evidence for such a vent. In Sour Milk Gill [NY 230 122], the upper part of the Whorneyside Formation includes strata deposited in shallow ephemeral lakes (Branney, 1991; Branney and Kokelaar, 1993), and it is from these strata that trace fossils have been recorded (Johnson et al., 1994). These consist of trails of small nonmarine myriapod-like arthropods of the ichnogenera Diplichnites and Diplopodichinus.

Airy’s Bridge Formation (AIB)

The Airy’s Bridge Formation consists of dacitic and rhyolitic pyroclastic rocks, mostly ignimbrite, in the form of tuff and lapilli-tuff with local intercalations of breccia (Branney et al., 1993b; Branney and Kokelaar, 1994; Millward et al., 2000). These strata were produced by further violently explosive volcanism that accompanied development of the Scafell Caldera (Branney and Kokelaar, 1994) (Table 5). The formation crops out across the Borrowdale fells, where it is up to 700 m thick, and eastwards as far as Thirlmere where it is no more than 40 m thick. It is not apparent east of the Coniston Fault (Figure 1) and (Figure 4). There is a mostly conformable contact with the Whorneyside Formation, but overstep onto the Birker Fell Formation takes place in the vicinity of Bell Crags.

The lower part of the formation consists of dacitic, eutaxitic, garnetiferous tuff and lapilli-tuff, up to 200 m thick and deposited mainly by pyroclastic flows (Branney et al., 1993b), which is the main constituent of the Long Top Member (LTT) (Table 5). The member thins eastwards across the district to only 15 m at Bell Crags [NY 298 147] and 8 m at Hause Point [NY 317 152] on the western side of Thirlmere. Branney et al. (1993b) distinguished a number of marker horizons of bedded tuff and accretionary lapilli-tuff, interpreted as phreatomagmatic fall and surge deposits, which crop out in the Keswick district at Sourmilk Gill [NY 227 120] (Stonesty and Cam Spout tuffs), Base Brown [NY 229 119] and Bell Crags [NY 298 147] (both Hanging Stone Tuff).

The overlying Crinkle Member (Crk) constitutes the upper part of the Airy’s Bridge Formation. In the Borrowdale Fells, the member attains a maximum thickness of about 650 m, and has an impersistent basal unit, up to 40 m thick, of lava-like rheomorphic ignimbrite, which constitutes the Bad Step Tuff (Bdp) (Branney et al., 1992; 1993b; Millward et al., 2000). In places, the stratigraphical position of this tuff is occupied by up to 15 m of eutaxitic lapilli-tuff (Z^R on the 1:50 000 Series geological map), which is considered to be a welded but less rheomorphic, distal equivalent (Branney et al., 1993b). Above these basal strata, the Crinkle Member consists of rhyolitic, intensely welded, garnetiferous, parataxitic lapilli-tuff, emplaced as rheomorphic ignimbrite, together with a few intercalations, up to 5 m thick, of mesobreccia. Parataxitic foliation is defined by abundant high-aspect-ratio (up to 200:1) fiamme, and is commonly steeply dipping or vertical, particularly near faults. The mesobreccias and rheomorphic deformation probably formed in response to syndepositional fault movement. East of the Borrowdale Fells, the basal strata are absent and the member thins eastwards to only about 30 m on the western side of Thirlmere.

Lingmell Formation (Lme)

The Lingmell Formation consists mainly of pyroclastic flow, fall and surge deposits, in part reworked, consisting of garnetiferous eutaxitic tuff and lapilli-tuff, along with some breccia and accretionary lapilli-tuff displaying ripple and low-angle cross-lamination (Kneller, Kokelaar and Davis, 1993, Millward et al., 2000) (Table 5). Lavas occur locally. The volcaniclastic strata thin eastwards from about 75 m across the Borrowdale fells to no more than 10 m at Thirlmere (Figure 1) and (Figure 4). The contact with the underlying Airy’s Bridge Formation is mostly conformable (Kneller, Kokelaar and Davis, 1993). The stratigraphy is complex as a result of abrupt lateral thickness and lithofacies variation across volcanotectonic faults. Many low-angle soft-state faults or slide surfaces are present, and some fine-grained tuffs in the upper part of the formation display numerous small-scale slump structures. The Rosthwaite Rhyolite (RsR) consists of up to 130 m of flow-foliated and folded rhyolite lava, which crops out on Rosthwaite Fell [NY 256 118] (Kneller, Kokelaar and Davis, 1993). Autobreccia occurs at the top and base of the lava, and as lenticular layers within it. The Scafell Dacite (ScD) consists of garnetiferous dacite lava, which crops out in the extreme south of the district between Kirk Fell [NY 195 105] and Green Gable [NY 215 107]. Most of its outcrop lies within the adjacent Ambleside district, and Millward et al. (2000) provide a detailed description.

Seathwaite Fell Formation (Set)

The Seathwaite Fell Formation consists of up to 540 m of subaqueously deposited volcaniclastic sandstone, for example as shown in (Plate 5), with intercalations of pebbly sandstone, breccia and tuff (Kneller and McConnell, 1993; Woodhall, 1998; Millward et al., 2000) (Table 5). There are a number of high-level basaltic andesite and andesite sills. Across the Borrowdale fells eastwards as far as Thirlmere, the formation mostly rests conformably on the Lingmell Formation (Kneller and McConnell, 1993), but east of the Coniston Fault, it rests directly on the Birker Fell Formation (Woodhall, 1998).

The lowest part of the formation consists of up to 60 m of planar bedded volcaniclastic sandstone and siltstone, with many graded beds; it is interpreted as wave- reworked turbidite and/or ash-fall deposits (Kneller and McConnell, 1993). Ripple cross-lamination indicates a south-south-easterly directed palaeocurrent (from-340Þ). On the Borrowdale fells, these strata are overlain by the Cam Crags Member (CCr), which consists of stratified breccia and pebbly sandstone interpreted as fan-delta deposits. This member is up to 275 m thick, but thins eastwards before wedging out in the vicinity of Ullscarf. Farther north, the Bell Crags Member (BCr) lies at a similar stratigraphical level and consists of lenticular, matrix-supported, mass-flow deposited breccia, up to 60 m thick.

The above members are overlain by a varied succession, up to 185 m thick, of siltstone, sandstone, pebbly sandstone and breccia, characterised by a lack of bedding and by widespread disturbance slumping and disruption, which were interpreted as locally channelised mass-flow deposits (Kneller and McConnell, 1993). In Borrowdale, and as far north-east as Bell Crags, these strata are overlain by the Pavey Ark Member (Pav). The lower part, which is that shown on the 1:50 000 Series geological map, consists up to about 80 m of mass-flow-deposited, massive to stratified pebbly sandstone and structureless, matrix-supported breccia. Some of the breccia contains a number of highly irregular vesicular andesitic pyroclasts, which were considered to have been fluid when incorporated in the deposit, and this formed the basis of the overall interpretation of the Pavey Ark Member as the deposits of eruption-related subaqueous ‘gravity-flows’ (Kneller and McConnell, 1993). The upper part of the member consists of up to about 25 m of sandstone, deposited by turbidity currents and suspension sedimentation, with lenticular (channel-fill) units of stratified breccia and trough cross-stratified pebbly sandstone (Kneller and McConnell, 1993). South of Wythburn, the Pavey Ark Member consists of up to 100 m of pebbly sandstone and breccia, containing clasts of vesicular andesite, massive or flow-foliated dacite and with some clasts resembling fiamme. Above the Pavey Ark Member, the formation, consists of up to about 220 m of planar bedded, locally wave ripple cross-laminated, siltstone and fine- to coarse-grained sandstone, with subordinate channel-fill, pebbly sandstone and breccia. The finer grained bedded sandstones have been interpreted as low density turbidites, whereas the coarser grained sandstones, along with the subordinate pebbly sandstone and breccia, are interpreted as the deposits of high-density gravity flows, including debris- flows (Kneller and McConnell, 1993). Palaeocurrent indicators show dispersal to the north-east.

The uppermost part of the formation includes a lenticular unit, up to 6 m thick, of bedded phreatomagmatic ash-fall deposits, and a variable succession of dacitic pyroclastic fall, surge and flow deposits, which constitute the Glaramara Tuff (GMT) (Kneller and McConnell, 1993). The most continuous outcrop, and maximum thickness, is around Glaramara [NY 246 105], but there are isolated occurrences of accretionary lapilli-tuff, up to 2 m thick, adjacent to Harrop Tarn [NY 313 132] and Steel Fell [NY 324 107] (Woodhall, 1998).

Lincomb Tarns Formation (LTa)

The Lincomb Tarns Formation, 200 and 400 m thick, consists mostly of dacitic to rhyodacitic ignimbrite in the form of massive to eutaxitic, but locally parataxitic and rheomorphic, lapilli-tuff, deposited from a series of large-volume pyroclastic flows (McConnell, 1993; Woodhall, 1998; Millward et al., 2000) (Table 5). There is subordinate ash-fall, surge and low- volume pyroclastic flow deposited bedded tuff and lapilli-tuff, and local intercalations of subaqueously deposited volcaniclastic sandstone.

Around Ullscarf and Wythburn, up to 400 m of undivided dacitic lapilli-tuff, the lower part of which is welded with a eutaxitic foliation, rests on the Seathwaite Fell Formation (McConnell, 1993; Woodhall, 1998). The contact varies from conformable to unconformable, and the latter is due to emplacement on an irregular low-relief erosion surface. Similar lapilli-tuff dominates the formation in the various outcrops between Thirlmere, Rydal Head [NY 365 105], High Hartsop Dodd [NY 393 107], and Glenridding [NY 377 168] where the base is faulted.

In the Thirlmere area, there are a number of distinct lithofacies, in the lower part of the formation, which constitute members. On the Wythburn Fells for example [NY 303 135], and around Steel Fell for example [NY 313 115], [NY 323 108], the basal Tarn Crags Member (TCr) rests on the Seathwaite Fell Formation, and typically consists of no more than 10 m of dacitic, bedded tuff and lapilli-tuff, with some eutaxitic layers, interpreted as fall and surge deposits (Woodhall, 1998). These equate with unnamed basal surge deposits in the Ullscarf area (McConnell, 1993). However, at the northern end of Thirlmere around Castle Rock [NY 322 197] and Great How [NY 313 187], up to 280 m of garnetiferous, eutaxitic lapilli-tuff, lithic lapilli-tuff and vitrophyre, petrographically similar to the Tarn Crags Crag Member, rests on the Birker Fell Formation (Millward, 1998).

Overlying strata consist either of undivided lapilli-tuff, as on the Wythburn fells, or a lithofacies dominated by at least 300 m of densely welded, parataxitic lapilli-tuff and vitrophyre which constitutes the Thirlmere Member (Thl) (Woodhall, 1998). The latter has been previously referred to as the Thirlmere Rhyolite (Hartley, 1941), and locally as the Armboth Fell ignimbrite (Beddoe-Stephens, 1997a). In the southern part of Thirlmere, and on Steel Fell, the Thirlmere Member rests either on the Tarn Crags Member or directly on the Seathwaite Fell Formation (Woodhall, 1998). However, elsewhere (for example Armboth Fell [NY 295 168], around Great Dodd [NY 342 205] and adjacent to Birkhouse Moor [NY 365 165]) it either rests directly on, or is faulted against, the Birker Fell Formation (Beddoe- Stephens, 1997a; Millward, 1998). The lowest exposed part of the member consists of at least 70 m of coarse, lithic-rich lapilli-tuff, which is seen locally along the eastern side of Dunmail Raise [NY 334 104] to [NY 330 158]. This lapilli-tuff may equate with a coarse basal breccia, up to 100 m thick, which crops out on the western side of Watson’s Dodd [NY 333 195]. Alongside Thirlmere and Dunmail Raise, the lapilli-tuff is overlain by parataxitic lapilli-tuff, which displays evidence of rheomorphism in the form widespread large-scale deformation, as indicated by variations in the attitude of the parataxitic foliation, and localised small-scale folding. There are local occurrences of mesobreccia, and vitrophyre. The former dominates the member where it crops out on Helvellyn Screes [NY 330 155], and the latter occurs at the top of the member farther south along the eastern side of Thirlmere and Dunmail Raise. The parataxitic lapilli-tuff represents densely welded ignimbrite, which is interpreted as having been emplaced rapidly, and ponded at high temperatures; the rheomorphism and mesobreccia probably formed as a result of contemporaneous faulting.

Along the eastern side of Thirlmere, the Thirlmere Member is overlain by the Raise Beck Member (RBk) which consists of up to 140 m of bedded tuff, in part reworked and locally with volcaniclastic sandstone at the top (Woodhall, 1998). The sandstone was deposited during a hiatus in the volcanism. This member has been intruded by a number of andesite sills, and is overlain by undivided, dacitic lapilli-tuff, which constitutes the upper part of the Lincomb Tarns Formation in the Thirlmere area immediately east of the Coniston Fault. The lapilli-tuff is about 275 m thick on Seat Sandal [NY 344 115], but thins northwards to only 65 m on Whelpside [NY 330 142]. In places the lapilli-tuff contains blocks up to 50 cm across of andesite, dacite and parataxitic lapilli-tuff, and the upper part is finer grained and bedded, with some local [for example Seat Sandal] concentrations of accretionary lapilli.

Esk Pike Formation (ESP)

The Esk Pike Formation consists mainly of volcaniclastic sedimentary and pyroclastic rocks, up to 200 m thick, in the form of sandstone, for example (Plate 5), pebbly sandstone, conglomerate, breccia, tuff, and lapilli-tuff (McConnell, 1993; Woodhall, 1998) (Table 5). The formation is preserved in outliers west of the Coniston Fault, and on High Hartsop Dodd [NY 394 108], but east of the fault it is more extensive although largely concealed beneath younger strata. It rests on the Lincomb Tarns Formation, and the contact is most distinct where it is an erosion surface on the underlying ignimbrite. Deposition probably took place in a fluvio-lacustrine environment similar to that envisaged for the Seathwaite Fell Formation. Transportation was by processes of mass flow, traction and suspension, and probably involved pyroclastic material produced during contemporaneous explosive volcanism, as well as particles derived from the erosion of pre-existing volcanic rocks. Units of mass-flow deposited pebbly sandstone and breccia attain maximum thickness and grain size in proximity to major faults, notably the Browncove Fault, suggesting fault controlled sedimentation (Woodhall, 1998). Some of the intercalated tuff and lapilli-tuff was emplaced as ignimbrite. On Fairfield Brow [NY 352 118] and around Grisedale Tarn [NY 350 121] the Esk Pike Formation is intercalated with basaltic andesite and andesite sheets, possibly lavas, up to 150 m thick (Woodhall, 1998). On Seat Sandal [NY 340 115], basaltic andesite lava, with associated pyroclastic breccia, is thought to be equivalent.

Middle Dodd Dacite (MDD)

The Middle Dodd Dacite consists of 40 to 260 m of feldspar- phyric dacite lava, in part flow-foliated and locally brecciated at the top, which rests conformably on the Esk Pike Formation (Woodhall, 1998; Millward et al., 2000) (Table 5). In the extreme south-east of the district, it forms outliers on and adjacent to High Hartsop Dodd, but farther north it crops out in inliers near Patterdale [NY 389 150] to [NY 393 160], along Deepdale [NY 391 133] to [NY 398 141], and in Cold Cove [NY 382 139] (Figure 3).

Helvellyn Formation (Hlv)

The Helvellyn Formation consists of dacitic ignimbrite, 80 to 400 m thick, which crops out extensively on the Helvellyn range, in Grisedale, and Striding Edge, eastwards as far as Glenridding (Table 5) and (Figure 1). It rests on the Esk Pike Formation (Woodhall, 1998). The contact is sharp, and, in places, it is either a low relief erosion surface, or is highly irregular due to loading. The latter is probably a result of emplacement onto unconsolidated sediment, possibly beneath shallow water. The ignimbrite consists of massive, feldspar crystal-rich lapilli-tuff, and in places, flow units, 10 to 130 m thick are separated by a few metres of bedded tuff and lapilli-tuff fall deposits. A eutaxitic foliation is conspicuous in the lowest part of the formation, indicating that this part is welded (Plate 6). The maximum thickness of 400 m is attained between Helvellyn and Grisedale, but the formation thins southwards to 200 m along the eastern side of Grisedale, 130 m at Fairfield, and 80 m in Rydal Head. This variation in thickness is due in part to postemplacement erosion, but may also reflect accumulation on block-faulted terrain of moderate to high relief. It is uncertain whether subsidence took place during emplacement.

Deepdale Formation (Dpd)

The Deepdale Formation closely resembles the Seathwaite Fell and Esk Pike formations in that it consists of subaqueously deposited volcaniclastic sandstone, with intercalations of pebbly sandstone, breccia, and pyroclastic rocks (Woodhall, 1998) (Table 5). The formation is most clearly distinguished where it rests on the Helvellyn Formation, but is less readily recognised where there is overstep onto the Middle Dodd Dacite and Esk Pike formations. The depositional environment was probably mainly fluviolacustrine. There are a number of andesite sheets mostly emplaced as high-level sills. Some of the strata equates with the uppermost part of the ‘Felsic and Basic Tuffs’ of Hartley (1941) and it includes an ignimbrite, referred to as the St Sunday Crag Member by Woodhall (1998), which was described by Millward (1980).

The main outcrop lies between Rydal Head and Ullswater, and there are outliers on Fairfield, and along the Helvellyn range, including Striding Edge (Figure 1). Overstep onto the Middle Dodd Dacite and sandstone of the Esk Pike Formation takes place in central and eastern parts of the main outcrop. The basal contact is most distinct where it rests on a fissured, high-relief erosion surface on the Helvellyn Formation (Woodhall, 1998). The Deepdale Formation is at least 650 m thick in the main outcrop, where strata are preserved in a downfaulted basin orientated north-north-east-south-south-west (Figure 4) and (Figure 5). There is evidence for syndepositional movement along these and other faults within the main outcrop.

The Deepdale Formation consists mostly of planar bedded volcaniclastic sandstone made up of massive to normally graded or laminated beds. Depositional mechanisms included combinations of mass-flow, traction and suspension, and many beds are probably turbidites (Plate 5). In places, the succession includes a number of south- to south-east-dipping erosion surfaces, where near horizontal strata are truncated by those which are relatively steeply dipping. These surfaces probably formed as a result of syndepositional uplift and tilting. This, together with rapid sedimentation, produced a variety of small to large-scale deformation structures, such as convolute bedding to slump folding. Pale weathered silicic tuffs, mostly reworked and re-sedimented by mass-flow and tractional processes, occur in the lower part of the formation, and are evidence of contemporaneous explosive volcanism. There are a few units of planar bedded ash-fall tuff and accretionary lapilli-tuff that has not been reworked, mostly in the highest part of the formation.

The sandstone incorporates a number of prominent lenticular masses of pebbly sandstone and/or breccia, or pyroclastic rocks which form members (Woodhall, 1998). The basal Cawk Cove Member (Cwk) consists of up to 115 m of fluvially deposited, stratified pebbly sandstone, made up of detritus derived from the Helvellyn Formation. The Blind Cove Member (Bld) occurs in the middle part of the formation, and consists mainly of pebbly sandstone mass-flow deposits, 2 to about 30 m thick, mostly separated by bedded sandstone. This member varies in thickness from 15 to about 200 m, and is particularly well exposed along St Sunday Crag [NY 368 138] to [NY 371 141]. The Blake Brow Member (Blk) consists of mass-flow deposited breccia and pebbly sandstone, with some intercalations of bedded sandstone. It is up to 200 m thick but is confined to the southern part of the main outcrop, where it occurs above the Blind Cove Member, and locally rests on an eroded surface of andesite lava. In Dovedale, it is represented by a number of much thinner (5 to 20 m) lenticular units within bedded sandstone. The breccia is made up of ignimbrite and andesite clasts up to 0.75 m across, along with a few large rafts, 10 m or more across, of intraclast-bearing sandstone, which were possibly derived from the erosion of newly formed fault scarps.

The St Sunday Crag Member (SSC) consists of dacitic lapilli-tuff, interpreted as ignimbrite, with subordinate tuff, mostly at the base and top, which represents pyroclastic fall and surge deposits (Millward, 1980; Woodhall, 1998). It is 45 to 55 m thick, but is confined to the area around St Sunday Crag, where it directly overlies the Blind Cove Member. The ignimbrite includes densely welded, parataxitic lapilli-tuff, which locally displays evidence of rheomorphism. The Cockley How Member (Cky) is another dacitic ignimbrite, up to 60 m thick, which is present in the middle part of the formation in southern and eastern parts of the main outcrop (Woodhall, 1998). This ignimbrite consists mainly of nonwelded, massive lapilli-tuff, but there is an impersistent, lithologically distinct, basal layer of densely welded, parataxitic lapilli-tuff up to 5 m thick. Although lithologically this closely resembles the St Sunday Crag Member, there are differences in geochemistry (Woodhall, 1998). The Dove Crag Member (DvD) consists of andesitic ignimbrite, at least 50 m thick, which is present only on Hart and Dove crags [NY 370 111] and [NY 374 106]. It forms the highest preserved part of the Deepdale Formation, and is lithologically and geochemically distinct from the ignimbrites of the St Sunday Crag and Cockley How members (Woodhall, 1998). The Dove Crag Member ignimbrite rests on an erosion surface on bedded sandstone, and the lower part is eutaxitic probably as a result of welding.

Contemporaneous intrusions in the Borrowdale Volcanic Group (A)

High-level sills of basaltic andesite, andesite and dacite occur throughout the Borrowdale Volcanic Group. Many were emplaced into unconsolidated volcaniclastic rocks, with the formation of peperitic margins (Branney and Suthren, 1988), but the distinction between lavas and sills remains uncertain in many parts of the Borrowdale Volcanic Group succession (for example Campbell, 1995). This is particularly the case with the Birker Fell Formation. The numerous andesite sheets within the volcaniclastic Eagle Crag Member of that formation, together with similar sheets in the overlying Whorneyside Formation are interpreted as sills. Similar sheets, some with peperitic margins occur sporadically throughout the upper part of the Borrowdale Volcanic Group, particularly within the Seathwaite Fells, Esk Pike and Deepdale formations (Woodhall, 1998).

Geochemistry of the Borrowdale Volcanic Group

Geochemical variations in the Birker Fell Formation are typically calc-alkaline trends of decreasing TiO₂, Fe₂O₃, MgO, CaO, Cr, Ni, V, Sc and Sr against increasing SiO₂ (Beddoe-Stephens, 1995). Immobile and incompatible elements, Zr, Y, Nb, Th and light rare earth elements show linear trends of increasing abundance with SiO₂. In the Borrowdale area, Beddoe-Stephens (1997a) distinguished a group of low-MgO–Cr–Ni basaltic andesites at the base, including the aphyric Grange Crags Member, followed by higher MgO–Cr–Ni basaltic andesites. The upper part of the Borrowdale succession is dominated by highly porphyritic, Al₂O₃-enriched andesite, but also includes some garnet-bearing andesites with higher contents of TiO₂, MgO, Fe₂O₃, Ni, Cr and V, and correspondingly lower Al₂O₃. The composition of the garnet-bearing andesites may be due to mixing with more basic magma, such as that represented by numerous doleritic and microdioritic xenoliths.

In the upper part of the Borrowdale Volcanic Group, the ignimbrites vary from andesite to rhyolite in composition, and most plot in discrete fields on variation diagrams (Beddoe-Stephens, 1997a). Details of the Whorneyside and Airy’s Bridge formations can be found in Millward et al. (in press). The Thirlmere Member of the Lincomb Tarns Formation is rhyodacitic (69–72.5% SiO₂) and plots together with analyses of the Crinkle Member of the Airy’s Bridge Formation (Beddoe-Stephens, 1997a). However, the remainder of the Lincomb Tarns Formation is compositionally more variable and ranges from acid andesite to dacite (58–65% SiO₂). The Helvellyn Formation is more uniformly dacitic in composition (63–65% SiO₂). The ignimbrites within the Deepdale Formation fall into two groups, one rhyodacitic or dacitic, and the other andesitic to dacitic. Ignimbrites of the St Sunday Crag and Cockley How members are rhyodacitic, but are geochemically distinct. The former is more silicic. It is strongly zoned, from a rhyolitic base (78% SiO₂) to rhyodacite (70–74% SiO₂). The Dove Crag Member ignimbrite is andesitic to dacitic in composition (61–67% SiO₂).

Chapter 3 Intrusive rocks

Older (late Ordovician, Caradoc–Ashgill) and younger (Early Devonian) intrusions are distinguishable on the basis of isotopic age determinations and some field characteristics. The Ennerdale (late Ordovician) and Skiddaw (Early Devonian) intrusions are exposed parts of the Lake District batholith, that otherwise lies at shallow depths throughout the district (Figure 5). The older (Ordovician) part of the batholith has been interpreted as a stacked set of subvolcanic, granitic laccoliths that interfinger with wedges of country rock (Evans et al., 1993; 1994), and which were probably emplaced in a supra-subduction zone setting by a number of magmatic pulses (Fettes, in press). The younger part of the batholith consists of cylindrical plutons. Other intrusions consist of the laccolithic Threlkeld microgranite, and compositionally diverse suites of minor intrusions (dykes, sills and plugs).

Older intrusions (Ordovician)

Ennerdale intrusion

The Ennerdale intrusion crops out in the south-west of the district, around Ennerdale Water, where it has intruded the Skiddaw Group and the lowest part of the Borrowdale Volcanic Group (Figure 1) and (Figure 5). The contact with the Skiddaw Group appears to be steep sided (Hughes and Fettes, 1994; Fettes, 1999), but, seismic reflection data reveal that the overall form of the intrusion is a sheet-like mass about 1100 m thick (Lee, 1989; Evans et al., 1993; 1994). The intrusion consists of granophyric-textured microgranite or granite, composed of quartz, plagioclase (some as phenocrysts), K-feldspar, biotite and chlorite, along with accessory epidote, iron oxide, sphene, apatite and zircon (Hughes and Fettes, 1994; Fettes, in press). Granophyric textures are absent at the margins, but increase in abundance towards the centre of the intrusion (Rastall, 1906). Geochemical analyses (Clark 1963; O’Brien et al., 1985; Millward et al., 2000) indicate a calc-alkaline composition (O’Brien et al., 1985) with the characteristics of an I-type granite generated within a volcanic arc environment (Fettes, in press). Hughes and Fettes (1994) concluded, from field evidence, that the intrusion predates the regional deformation, a view confirmed by Hughes et al. (1996) who presented a U–Pb zircon age of 452 ± 4 Ma (Caradoc). Earlier whole-rock K–Ar and Rb–Sr ages of 370 ± 20 Ma and 420 ± 4 Ma, reported by Brown et al. (1964) and Rundle (1979), must represent reset ages.

Threlkeld microgranite

The outcrops of microgranite on Low Rigg [NY 305 230], Threlkeld Knotts [NY 328 238], Bramcrag [NY 320 220] and around White Pike [NY 339 229] comprise the laccolithic Threlkeld intrusion (Hadfield and Whiteside, 1936; Rastall, 1940; Wadge, 1972; Firman, 1978a; Caunt 1984; Lee, 1989; Campbell 1995). Exposures in Bramcrag Quarry [NY 339 229], clearly show intrusion into the lowest part of the Borrowdale Volcanic Group as well as the Skiddaw Group (Wadge et al., 1974; Loughlin, 1999) (Plate 7). The microgranite is composed of plagioclase, subordinate alkali feldspar, garnet and quartz phenocrysts, in a groundmass of quartz, plagioclase, chlorite, iron oxide, and accessory minerals including zircon and apatite (Campbell, 1995). Xenoliths derived from the Skiddaw Group and Borrowdale Volcanic Group occur mostly near the margins (Rastall, 1940; Loughlin, 1999). Geochemically, the intrusion is calc-alkaline in composition (O’Brien et al., 1985). Wadge et al. (1974) reported a Rb–Sr isochron age of 438 ± 6 Ma (Llandovery), but Rundle (1981) recalculated this as 445 ± 15 Ma (Caradoc/Ashgill).

Minor intrusions

Minor intrusions of inferred Ordovician age occur throughout the Skiddaw Group, and consist of dykes, sheets and small plugs of strongly altered lamprophyre, basalt, dolerite, gabbro, andesite, diorite, rhyolite, microgranite and hornblendite. Many lamprophyres were described by Fortey et al. (1994) as pyroxene-phyric, microdiorites. These include dykes and dyke-like intrusions at Scawgill Bridge [NY 178 243] to [NY 177 260] and in the Whinlatter area [NY 165 268], and a plug at Bowness [NY 222 290]. Lamprophyre dykes on Ullock Pike [NY 245 287] where referred to as augite-hornblende meladiorites (Fortey et al., 1994). North-east of Buttermere, close to the Causey Pike Fault, there are dykes and small plugs of intensely altered vesicular, pyroxene-phyric basalt (Fortey, et al., 1994). Farther south-east at Hindscarth [NY 216 165] the minor intrusions include the east-west-orientated Squatt Knotts dolerite (Strens, 1962 in Fortey, et al., 1994). A sill or laccolith of altered dolerite occurs at Castle Head [NY 270 227], near Keswick, and intrusions on the north-west side of Derwent Water are of augite-phyric diorite (Fortey et al., 1994). No isotopic ages have been determined from the older minor intrusions within the district. However, in the adjacent Cockermouth district, Rundle (1979; 1992) quoted a Rb–Sr isochron age of 444 ± 24 Ma (Ashgill) for the ‘Embleton’ diorite, and a mean K–Ar age of 458 ± 9 Ma (Caradoc) for the Dash ‘picrite’.

Basaltic andesite and andesite dykes are widely scattered throughout the Borrowdale Volcanic Group (Beddoe- Stephens, 1995; Campbell, 1995; Woodhall, 1998), and many probably acted as feeders to the various lavas and high-level sills in the group. Andesite dykes intrude the highest part of the Borrowdale Volcanic Group in the extreme south-east of the district around Deepdale.

Younger intrusions (Devonian)

Skiddaw granite

The Skiddaw granite has intruded the Skiddaw Group, and gravity data (Bott, 1974; Lee, 1986a) indicate a flat-topped, steep-sided intrusion, about 9 x 15 km, and elongate in a north-east-south-west direction (Figure 5). However, it is exposed only in three small outcrops (Eastwood et al., 1968), of which only the southernmost, at Sinen Gill [NY 299 281], lies within the Keswick district (Figure 5). Here, the intrusion is a medium-grained biotite granite composed of orthoclase, oligoclase, quartz and biotite, along with accessory minerals including zircon, apatite, ilmenite, pyrrhotite, pyrite, epidote, sphene, brookite and rutile (Rastall, 1910; Rastall and Wilcockson, 1915; Hitchen, 1934; O'Brien et al., 1985). Large phenocrysts of coarse perthite up to 5 cm long contain inclusions of plagioclase, quartz, muscovite, biotite, zircon and apatite (Firman, 1978a). Potassium-argon dating of fresh biotite, from Sinen Gill, gave an age of 399 ± 8 Ma (Early Devonian), recalculated from Shepherd et al. (1976), which is the best minimum estimate for the age of cooling after emplacement (Rundle, 1992).

Minor intrusions

A few altered, biotite-bearing lamprophyre dykes occur in the Skiddaw Group on Sale Fell [NY 193 297] and on Skiddaw Dodd [NY 245 275]. That at Sale Fell has been identified as minette (Ward, 1876; Fortey et al., 1994) and kersantite (Eastwood et al., 1968). The rock is strongly chlorite-calcite-sericite altered so that very little biotite remains (Fortey et al., 1994). Biotite from the Sale Fell intrusion has yielded an Early Devonian potassium-argon age of 402 ± 9 Ma (Rundle, 1979).

Rhyolite and microgranite dykes are locally common within the Borrowdale Volcanic Group, in the south-western part of the district, near the Ennerdale intrusion, with which they are probably genetically linked. Farther east, around Thirlmere, the Armboth dyke (Beddoe-Stephens, 1996; Burton, 1998), and an unnamed 40 m-wide microgranite dyke (Millward, 1998), are not cleaved, unlike the Borrowdale Volcanic Group, therefore suggesting an Early Devonian or younger age.

Chapter 4 Metamorphism

In a recent survey of white-mica (illite) crystallinity of the Skiddaw Group, Fortey (1989) identified burial/regional metamorphic grades ranging from the diagenetic zone, through the anchizone into the lower epizone in the Skiddaw Group. Anchizonal and low epizonal rocks are widespread west of Derwent Water and Bassenthwaite Lake, but elsewhere in the district, ‘low crystallinity’ (diagenetic to anchizonal) rocks predominate (Fortey, 1989, Figure 8). Bedding-parallel growth of illite is assumed to have formed by burial metamorphism, but additional cleavage-parallel growth of this mineral occurred during regional metamorphism and deformation related to the Acadian orogeny.

The inner part of the aureole of the Ennerdale intrusion is characterised by highly indurated and recrystallised fine- grained metasedimentary rocks of the Skiddaw Group, with chiastolite and cordierite porphyroblasts (Hughes and Fettes, 1994). A spaced fracture cleavage cutting the hornfels indicates that the metamorphism, and therefore the intrusion, predated the Acadian orogeny. The outer part of the aureole consists of more weakly indurated and recrystallized rocks, with a penetrative, pressure-solution cleavage. The rocks contain biotite, together with secondary carbonate, and incipient spots of cordierite and/or andalusite.

The Crummock Water aureole is an east-north-east- trending elongate zone of bleached and recrystallised (metasomatised) mudstone and siltstone of the Skiddaw Group, 24 km in length and up to 3 km wide (Cooper et al., 1988). The bleaching is due to carbon loss, accompanied by substantial depletion of Cl, Ni, S, Zn, H2O, and a marked gain in As, B, K and Rb. The rock mineralogy is little affected by these geochemical changes, but small aggregates and porphyroblasts of white mica and chlorite have formed, and there has been tourmaline veining (Fortey and Cooper, 1986). The aureole postdates the regional (Acadian) cleavage, and a Rb–Sr age of metasomatism of 401 ± 3 Ma (Cooper et al., 1988) indicates that it is associated with Early Devonian magmatism.

The aureole of the Skiddaw granite spans the Keswick and adjoining Cockermouth districts, and was described originally by Rastall (1910) and Hitchen (1934). Eastwood et al. (1968) identified three zones within the aureole: an outer zone with chiastolite and spotting: an intermediate zone of massive, hard mudstone with andalusite, cordierite and biotite in a matrix of white mica and chlorite: and an inner zone of cordierite-biotite-hornfels.

The effects of hydrothermal, contact, burial and regional metamorphism have been recognised in the Borrowdale Volcanic Group (Fortey, in Millward et al., 2000; Meller, 1998). Hartley (1941) noted the abundance of carbonate in many specimens from the Thirlmere and Helvellyn areas. Beddoe-Stephens (1996) recognised extensive chlorite- white mica-carbonate alteration, with minor epidote/clinozoisite and rare prehnite in rocks from the Borrowdale area. Millward (1998) observed carbonate and white mica alteration in rocks from the northern part of the Thirlmere area, and suggested that they were probably equivalent to the carbonate-phyllosilicate-epidote zone distinguished by Fortey (in Millward et al., 2000) in the Ambleside district.

Although metamorphosed, the volcanic rocks throughout the district mostly retain original igneous or clastic textures. The feldspars have usually been replaced by varying amounts of white mica, carbonate, albite, quartz and chlorite. Unaltered clinopyroxene persists locally, but is more typically replaced by chlorite, along with sphene and granular opaque oxide. The last two are, in turn, replaced by haematite and ‘leucoxene’ (semi-opaque Ti-oxide). No fresh orthopyroxene remains. Epidote occurs in andesites of the Birker Fell Formation in Borrowdale and adjacent to Thirlmere.

On the basis of petrographical studies of the secondary mineralogy, Meller (1998) suggested that the Borrowdale Volcanic Group was affected by at least four metamorphic events during the Palaeozoic. An initial hydrothermal event probably occurred during the volcanism and resulted in the deposition of chalcedony in vesicles and veins. There was subsequent overprinting by contact and burial metamorphism. The former is defined by the presence of biotite and/or hornblende, with or without hornfelsing, up to a few kilometres from the contact with the batholith (for example Ennerdale intrusion). Burial metamorphism is defined by calc- silicate and calcite-hosted secondary mineral assemblages. A maximum grade of prehnite-actinolite to actinolite-pumpellyite facies was reached during the Devonian immediately prior to the Acadian orogeny. Regional metamorphism associated with this orogeny produced tectonically aligned white mica. Pervasive carbonate veins were thought to be either coeval with the regional metamorphism, or due to a later postmetamorphic event.

Chapter 5 Structure

Large north-east-trending folds, a regional north-east- trending cleavage, and many faults, fault zones and trending thrusts (Figure 6) north-east, north-west, north-south and east-west dominate the structure of the Keswick district. Most structures formed in response to the Acadian orogeny, during the Early Devonian, but pre-Acadian structures are apparent in the Skiddaw and Borrowdale Volcanic groups, and some post-Acadian deformation is suspected.

Pre-Acadian deformation

Pre-Acadian deformation is marked by syndepositional, soft- sediment folding, and the development of an olistostrome, in parts of the Skiddaw Group (Webb and Cooper, 1988; Hughes et al., 1993), and volcanotectonic faulting in the Borrowdale Volcanic Group (Branney and Soper, 1988; Branney et al., 1993a; Branney and Kokelaar, 1994).

Slump folds in the Skiddaw Group were described by Hughes et al. (1993) as intrafolial, disharmonic, recumbent and lobate structures. According to Webb and Cooper (1988), slump folds in the Northern and Central Fells belts have south-east and westwards vergence, respectively, and this opposing vergence suggested that the Skiddaw Group was deposited in a relatively narrow, probably fault controlled basin. However, Hughes et al. (1993) refuted the sedimentary origin of some of the folding in the Northern Fells belt, and consequently the fault-controlled basin interpretation remains controversial.

Many of the faults in the Borrowdale Volcanic Group have been interpreted as volcanotectonic structures on the basis of closely associated thickness and facies changes, local angular unconformities, gravity collapse structures, and ductile deformation structures in ignimbrites (Branney and Soper, 1988; Branney et al., 1993a; Branney and Kokelaar, 1994). Between Kirk Fell [NY 195 105] and Thirlmere, north-west-south-east, north-south and north- east-south-west faults are associated with the northern part of the Scafell Caldera (Branney and Kokelaar, 1994). These faults formed during piecemeal collapse of the caldera, the result of which is a west-south-west orientated depression, the Scafell Syncline. The syncline, which passes into the adjacent Ambleside district, has preserved within it the main products of the caldera collapse, the Whorneyside and Airy’s Bridge formations.

The north-south Coniston Fault was an important structure during deposition of the Borrowdale Volcanic Group (Moseley, 1993; Woodhall, 1998). It appears to mark the eastern limit of the products, and area affected by collapse, of the Scafell Caldera (Woodhall, 1998) (Figure 4). An increase in thickness of the Seathwaite Fell Formation westwards across the fault is consistent with syndepositional dip-slip movement with downthrow of the western side. However, along Dunmail Raise, the juxtaposition of the Seathwaite Fell and Lincomb Tarns formations, with the latter on the east side, is indicative of downthrow to the east.

Soper and Moseley (1978) and Moseley (1993) show the axis of the Scafell Syncline east of the Coniston Fault, between Thirlmere and the southern end of Ullswater. However, the Borrowdale Volcanic Group east of the fault consists of post-Scafell Caldera formations preserved in a broad fault-bounded basin (Woodhall, 1998). This basin is probably, at least in part, of volcanotectonic origin. The Birkhouse Moor and Hogget Gill faults along the basin margin (Figure 6) have experienced hundreds of metres of downthrow to the south and north-west respectively. Within the basin, a number of major north-south- to north- east- orientated faults are associated with thickness and facies variations within the Esk Pike, Helvellyn and Deepdale formations, which indicate that they are also syndepositional structures (Woodhall, 1998).

Acadian deformation

In the Skiddaw Group, north-east-trending folds with a regional, axial planar cleavage are the main regional structures of the Acadian deformation event. The cleavage fabrics vary from penetrative, slaty and pressure solution types, to spaced fractures. The folds have amplitudes up to hundreds of metres, are gently plunging, steeply inclined, and range from open to isoclinal (Hughes et al., 1993). Crenulation cleavages and related folds were produced during later stages of the Acadian event, and are related to a set of south-directed thrust faults, the Watch Hill, Loweswater, Gasgale and Causey Pike thrusts (Figure 6). Soper and Roberts (1971) showed that intrusion of the Skiddaw granite, now known to be of Early Devonian (about 400 Ma) age (Rundle, 1992), postdated the regional Acadian cleavage, but predated the later crenulation cleavages, so constraining the timing of cleavage development. The largest of the south-directed thrust faults is the Causey Pike Fault, which is a major structure that is prominent on satellite imagery and potential-field data (Lee, 1989) (Figure 5) and (Plate 8). It is coincident with the Crummock lineament of Lee (1989), and is possibly an important crustal-scale feature (Chadwick and Holliday, 1991; Kneller and Bell, 1993; Kneller, King and Bell, 1993). Bedding and cleavage are sigmoidally deformed across the Causey Pike Fault suggesting sinistral movement of the fault during the later stages of the Acadian deformation, possibly synchronous with aureole development (Hughes et al., 1993). Reversed movement on the Causey Pike Fault (Plate 8) postdates the Crummock Water aureole.

Acadian deformation of the Borrowdale Volcanic Group developed a north-east-trending regional cleavage, and probably reactivated at least some pre-existing faults. Crenulation cleavages are rare within the Borrowdale Volcanic Group, but have been reported from argillaceous rocks in the lowest few metres in Borrowdale (Hughes, 1994). The regional cleavage is best developed in volcaniclastic rocks, particularly in parts of the Helvellyn and Deepdale formations. The Coniston Fault, one of the most prominent in the Lake District, was reactivated during this period of deformation and experienced strike-slip movement (Moseley, 1993). Acadian deformation tightened the caldera depressions and sedimentary basins into open synclines, for example the Scafell Syncline (Figure 6), but palaeomagnetic evidence suggests that these structures are mainly pre-Acadian (Channell and McCabe, 1992).

Post-Acadian deformation

Post-Acadian deformation is difficult to determine within the Keswick district because of the limited extent, or lack, of upper Palaeozoic and younger strata. According to Soper and Moseley (1978) post-Acadian deformation (referred to as Hercynian) within the Lake District as a whole was mostly end-Carboniferous (Saalian), and involved reactivation of Acadian structures, notably north- west ‘wrench’ faults. Moseley (1993) suggested that post- Carboniferous and post-Triassic dip-slip movement possibly took place along the Coniston Fault. Structures associated with uplift at the end of the Cretaceous (Chadwick et al., 1994) have not been identified.

Chapter 6 Concealed geology

The concealed geology of the central part of the Lake District, including the Keswick district, is known from the interpretation of gravity and aeromagnetic data. Using this data Lee (1986a, 1989) identified a number of important east- north-east-trending geophysical lineaments, of which the Crummock (gravity) and Ullswater (gravity and magnetic) lineaments extend across the Keswick district (Figure 5). These, and a number of less extensive north-east-south-west lineaments, probably mark fundamental basement fractures, which were initiated prior to, and influenced the structural development of, the Borrowdale Volcanic Group and the intrusive form of the Lake District batholith.

The Lake District batholith is a key component of the concealed geology of the central part of the Lake District. Its three-dimensional form was first revealed by Bott (1974; 1978) on the basis of the initial regional gravity survey of the United Kingdom. The form of the batholith has been the subject of further detailed interpretations of regional gravity and aeromagnetic data (Lee, 1986a, 1989; Firman and Lee, 1986) (Figure 6). These form the basis for the configuration of the batholith as shown in the two cross-sections which accompany the 1:50 000 Series geological map, and in (Figure 5). The top of the batholith is less than 2 km below the surface in the southern part of the Keswick district, but is 3 to more than 6 km in the north, except around the Skiddaw granite. High-level components are the Ennerdale and Threlkeld intrusions, and the concealed Crummock granite (Lee, 1989) (Figure 5).

Chapter 7 Mineralisation

The Lower Palaeozoic rocks of the Keswick district host a wide variety of epigenetic mineral veins. In the following section the numbers in brackets refer to the mineralised localities shown in (Figure 7), and are listed in (Table 6). The origin of the mineralisation has attracted research from an early date, and a close structural and genetic relationship between the Lake District batholith and the distribution of vein mineralisation has been advocated (Firman, 1978b). A summary of this and the more important references are given in the Geochemical Atlas for the area (British Geological Survey, 1992). Young (1987) has given a comprehensive review of mineral occurrences and their distribution throughout the Lake District. There is no uniformity of vein orientation, but Eastwood (1921) observed that the many copper veins are orientated east-west, whereas the lead-zinc veins are commonly nearly north-south.

In a genetic classification of Lake District mineralisation, Stanley and Vaughan (1982) recognised, in some cases in the same vein, Early Devonian copper-tungsten-bearing veins, early Carboniferous lead-zinc- dominated mineralisation, some baryte deposition in the late Carboniferous and post- Triassic supergene assemblages. There is field evidence suggesting that some copper mineralisation predates the regional cleavage-forming event in the Early Devonian (Acadian orogeny) (Millward et al., 1999). The main phases of mineralisation are probably related to periods of hydrothermal activity within the batholith.

The copper veins typically contain chalcopyrite, arsenopyrite and pyrite, in varying proportions, in a gangue dominated by quartz, chlorite and dolomite. Important concentrations of this mineralisation occur at Honister (Table 6), locality 42, and south-west of Derwent Water adjacent to Manesty (localities 21, 23) and Dale Head (4), and other locations in the Vale of Newlands (Millward et al., 1999). Stanley and Vaughan (1982) ascribed an Early Devonian age to these veins. However, Millward et al. (1999) cited field evidence indicating that, among others, the veins at Honister, Manesty and Dale Head, were strongly cleaved during the Acadian orogeny, and therefore must predate this event. They suggested a genetic link to the final phases of the Caradoc magmatism, on the basis of mineralisation style and its relationship with the volcanic rocks.

A small vein, previously exploited for cobalt mineralisation, occurs as the Scar Crag Vein on Causey Pike (locality 10). This north-north-east-trending vein cuts the bleached Skiddaw Group mudstones in the Crummock Water aureole, and appears to be unique in the Lake District in having apatite, chlorite and arsenopyrite as its main constituents, with traces of cobalt minerals. Stanley and Vaughan (1982) assigned an Early Devonian age to the Scar Crag Vein.

Antimony veins are known from several widely scattered Lake District localities, including a few in the Keswick district. Berthierite occurs in quartz veins at Hogget Gill, near Patterdale [NY 389 112], and stibnite has been reported in quartz veins on St Sunday Crag, near Helvellyn [NY 360 130] (Davidson and Thompson, 1948; Fortey et al., 1984). Ward (1876) and Postlethwaite (1913) noted the discovery of erratic boulders of stibnite weighing up to 50 kg, in till at Troutbeck Station [NY 390 270]. This suggests that at least one as yet undiscovered substantial antimony vein is present, presumably concealed beneath Quaternary deposits in the northern part of the district. Stanley and Vaughan (1982) were uncertain of the age of the antimony mineralisation, but Fortey et al. (1984) inferred an Early Devonian age for that on Carrock Fell in the Cockermouth district. The widespread occurrence of antimony minerals as inclusions in galena in the Early Carboniferous lead-zinc veins, reported by Stanley and Vaughan (1981), is consistent with remobilisation from a suite of earlier antimony deposits (British Geological Survey, 1992).

The lead-zinc veins typically contain galena and sphalerite, accompanied by minor chalcopyrite, and with silver as an important impurity. Stanley and Vaughan (1981) have described the widespread occurrence of native antimony and antimony sulphosalts as inclusions in the galena. Tetrahedrite occurs locally, for example at Eagle Crag (Table 6), locality 50. Gangue minerals include quartz, baryte, calcite and dolomite. Fluorite is a minor constituent in a few deposits, such as in the Brandlehow vein (locality 5). Quartz pseudomorphs and epimorphs after baryte at some localities suggest that this mineral was formerly an important constituent. Major concentrations of lead-zinc mineralisation occur in the Newlands Valley (for example localities 1, 7, 8, and 13), Threlkeld (29), Brandlehow (5), Helvellyn (55), Eagle Crag (50), Greenside (49) and Force Crag (9). At Force Crag (locality 9) there is evidence for vertical zonation of the mineralisation; abundant baryte with some manganese oxides and relatively scare sulphides gives way downwards to more sulphide-rich mineralisation with very little baryte (Young and Cooper, 1988). At Greenside (locality 49) there is similar zonation, along with an increase in chalcopyrite in the sulphide assemblage at depth (British Geological Survey, 1992).

Stanley and Vaughan (1982) assigned an Early Carboniferous age to the lead-zinc veins. The metals in these veins may have been derived from Lower Palaeozoic sedimentary rocks, possibly in part due to convective leaching by Carboniferous sea water, as well as from basement granites.

A deposit of remarkably pure graphite occurs at Seathwaite near Borrowdale (Table 6), locality 18. It is associated with a basalt intrusion in the Borrowdale Volcanic Group, and appears to lack any counterpart in Britain or elsewhere. The uniqueness of the deposit has attracted much geological and mineralogical attention (British Geological Survey, 1992 and references therein). The graphite occurs as nodules in a series of pipe-like deposits within altered dolerite. Minor amounts of arsenopyrite, chalcopyrite, galena, pyrite and sphalerite occur in the deposit. Strens (1965) explained its origin in terms of the catalytic reduction of large volumes of carbon monoxide, and Parnell (1982), by derivation from deeply buried carbonaceous rocks, but no wholly satisfactory mechanism for these processes has been identified. Ineson and Mitchell (1974) reported K–Ar ages of 382 and 376 Ma for the dolerite and graphite respectively, which suggest a genetic association.

Haematite, similar to that of the limestone replacement deposits of west and south Cumbria, occurs in veins within the Lower Palaeozoic rocks of the Lake District, and a few occur in the Keswick district. Adams (1995) mentions north-south, east-west veins around Flouten Tarn (Table 6), localities 36–38, and north-south and north- west-south-east veins in the Providence Mine, adjacent to Little Tongue Gill (56). The spoil from the Deepdale mine is reputed to have yielded kidney ore, along with some iron-stained baryte (Adams, 1995). Rose and Dunham (1977) and Dunham (1984) suggest derivation of the iron from red, iron-rich Permo-Triassic sediments in the Irish Sea Basin, but origin of these deposits is controversial. A post-Triassic age for the haematite mineralisation seems the most favourable (Rose and Dunham, 1977; Shepherd and Goldring, 1993).

Chapter 8 Devonian and Carboniferous

Devonian

Devonian strata are represented by the Mell Fell Conglomerate (Wadge, 1978, and references therein), which crops out in the extreme north-east of the district, mostly on Great Mell Fell [NY 400 255], and rests unconformably on the Skiddaw and Borrowdale Volcanic groups (Figure 2). No fossils have been recorded, but the conglomerate has been variously referred to either the Old Red Sandstone or basal Carboniferous. A Devonian age was preferred by Wadge (1978) because the conglomerate was found to be lithologically distinct from the basal Carboniferous strata, and because the conglomerate rests on strata deformed during the Early Devonian Acadian orogeny, a Middle to Late Devonian age is preferred. Thickness estimates range from 275 m (Dakyns et al., 1897) to 1500 m (Capewell, 1955), with the former estimate preferred by Wadge (1978). The conglomerate, referred to as a molasse deposit by Cooper et al. (1993), is polygenetic and contains subangular to rounded clasts, about 0.1 to 0.3 m across but some are up to 1 m, in a purplish red sandstone matrix cemented by calcite or iron-oxide (Wadge, 1978). The cobble and boulder-sized clasts are mostly of greywacke sandstone derived from the Windermere Supergroup, but there are pebble-sized and smaller clasts of limestone, calcite, volcanic rocks and feldspar crystals (Capewell, 1955).

Carboniferous

Carboniferous strata consist of the Chief Limestone Group (Seventh Limestone), which unconformably overlies the Eycott Volcanic Group, and locally the Mell Fell Conglomerate, in the extreme north-eastern part of the district (Figure 1). The strata are a continuation of basal marine Lower Carboniferous (Dinantian) rocks present in the adjacent Cockermouth district (Eastwood et al., 1968). In the Keswick district, there is about 75 m of pale grey, bedded limestone with interbedded calcareous mudstone, siltstone and cross-bedded sandstone.

Chapter 9 Quaternary geology

The extensive Quaternary glacigenic deposits within the district mostly relate to the most recent ice-sheet glaciation, which took place during the Late Devensian, Dimlington Stadial (26 000 to 13 000 years BP) (Pennington, 1978; Boardman, 1991; 1992; 1996). This has stripped the landscape of most earlier glacial and interglacial deposits, but those of a pre-Devensian glaciation are locally preserved (Boardman, 1991). Deposits of the Windermere Interstadial (13 000 to 11 000 years BP) are restricted to lake-bed sediments, but there are both deposits and landforms that relate to a restricted glaciation during the Loch Lomond Stadial (11 000 to 10 000 years BP). Evidence for this and earlier glaciations are summarised in (Figure 8). All glaciers disappeared rapidly about 10 000 years ago as temperatures rose at the onset of the Flandrian Stage (10 000 years BP to present). During this stage, sedimentation has been confined mostly to valley floors and lake basins.

Pre- Devensian

An intensely weathered, pre-Devensian till, the Thornsgill Formation of Boardman (1991; 1994), crops out at several localities in the valleys of Mosedale and Thornsgill becks in the north-eastern part of the Keswick district, but is not shown on the 1:50 000 Series geological map (Figure 8). This till is up to 14 m thick, rests on the Skiddaw Group, and contains erratics derived from the west. Microtextures suggest ice movement in an east-north-east direction (Boardman, 1991; fig. 114) (Figure 8). The till underlies a peat bed, compressed beneath a younger till, and Boardman (1994) tentatively assigned this peat to the end of the last interglacial (Ipswichian) or to an early Devensian interstadial.

Dimlington Stadial

Widespread deposits of till, although not dated, are probably of the Dimlington Stadial (Figure 8). The till, which probably equates with the Threlkeld Till of Boardman (1991), is typically a relatively impermeable unsorted deposit in which rock debris ranging in size from sand to boulders occurs in a matrix of clay, silty-clay or silt. It includes lenticular deposits of glaciofluvial sand and gravel. Till commonly underlies featureless ground, but in places it forms low drumlins elongated parallel to the ice-flow direction (Boardman, 1994; 1996). Numerous drumlins around the town of Keswick are orientated north- west-south-east, and others farther east, around Threlkeld, are orientated north-east-south-west (Figure 8). The overall direction of ice-movement was from south to north, but in the Vale of Threlkeld it moved north-eastwards, constrained by Blencathra, before turning northwards (Figure 8). Some of the highest mountains in the district may have remained just above the maximum altitude of the Dimlington Stadial ice sheet at its maximum thickness (Lamb and Ballantyne, 1998) (Figure 8). The district was ice-free by about 14 000 years BP (Pennington, 1978).

Loch Lomond Stadial

Conspicuous landforms and glacigenic deposits of the Loch Lomond Stadial are confined to north- and east- facing heads of valleys. The landforms consist of corries, partly bounded by steep slopes and cliffs. In the Lake District, Sissons (1980) has identified evidence of 64 valley glaciers of the Loch Lomond Stadial, and 37 of these lie wholly or partly within the Keswick district (Figure 8). The extent of these glaciers is inferred from the distribution of moraines, and other linear till ridges (Sissons, 1980; Evans, 1994) (Plate 9). These features are typically made up of unsorted cobble and boulder gravel. Periglacial slope deposits, mostly in the form of stratified scree, are exposed along or close to the A592 from Keswick northwards along Basenthwaite Lake (Boardman, 1977: 1996). In some roadside exposures, till of the Dimlington Stadial is overlain by scree (Boardman, 1996).

Flandrian

Silt, sand and gravel deposited during the Flandrian (Holocene) forms alluvium beneath present-day floodplains, river terraces, numerous alluvial fans and debris cones, and lacustrine deposits (Plate 10). Peat covers many smooth upland areas, and occurs locally in some lowland areas. Alluvial fans occur downstream from abrupt changes in stream gradient, and are common where streams meet the alluvial plains of larger watercourses. Pollen assemblages from organic deposits, which underlie an alluvial fan in the Seathwaite valley, indicate that human- induced vegetation changes, possibly following settlement of the area between about 900 and 1000 AD, immediately preceded fan development (Parker et al., 1994).

Chapter 10 Applied geology

The Keswick district lies almost entirely within the boundaries of the Lake District National Park (designated as such in 1951), which altogether consists of 2280 square kilometres of upland area of outstanding natural beauty. Hill farming is the principal land use, but tourism now dominates the local economy. There has been a long history of metal mining and slate quarrying, and the aftermath of these activities remains, in the form of mine entrances, mine waste tips and derelict buildings in many parts of the district (Plate 11). Most mining ceased before the National Park was established, but some persisted until 1992 when Force Crag Mine was finally abandoned. Geological issues covered in this chapter include:

• metalliferous minerals
• nonmetalliferous minerals
• construction materials
• geothermal resources
• hydrogeology and water supply
• waste disposal
• ground stability
• seismicity
• conservation

Metalliferous minerals

The central and northern parts of the Lake District metalliferous ore-field are located within the Keswick district. It includes the Greenside Mine, which was the most successful of all the Lake District metal mines. Some of the mineralisation localities (Figure 7), (Table 6), such as at Goldscope, Brandelhow, Barrow, Force Crag, the Thornthwaite mines, Threlkeld, Greenside, Eagle Crag and Hartsop Hall, were extensively mined over long periods. In contrast there are many trials that were followed by little or no commercial exploitation. Waste products from abandoned mine workings are likely to cause pollution where water tables are high or flooding is likely. Collapse of the workings is an ever-present hazard to engineering operations in the district. Many old workings may be unrecorded, and details of many others are probably incomplete.

The earliest records of mining date back to the 16th century, but at Goldscope, Adams (1995) mentions open mine workings that may be several hundred years earlier, and a possible reference to the mine in a 13th century land inventory. Copper mining declined in the 17th century as no new sources were discovered, and lead mining declined towards the close of the 19th. The decline of lead mining was partly offset by the rising demand for industrial minerals such as baryte. Mining activity peaked during the late 18th and 19th centuries when ore was extracted from workings as deep as 910 m (3000 ft) at Greenside, 165 m (540 ft) at Goldscope, 125 m (420 ft) at Brandlehow and 55 m at Yewthwaite. Lead, zinc and baryte was extracted from the Force Crag Mine, situated at the head of the Coledale valley. Economic quantities of lead and zinc were extracted from the Low Force workings (Young and Cooper, 1988; Adams, 1995). Between 1984 and 1992, a combination of poor market conditions, problems with the processing plant and eventual collapse of the lowest level forced the operators to abandon the mine in March 1992. Greenside Mine operated almost continuously over a period of 200 years, and its total yield was over 200 000 tons of lead concentrate and 2 000 000 ozs of silver, all from a single vein (Gough, 1965; Firman, 1978b; Adams, 1995). The last ore was extracted in April 1961. When the mine was abandoned in 1962, the workings were 430 m (1420 ft) below Lucy Tongue Level, and 910 m (3000 ft) below the top of the hill. In the deepest workings the vein became barren of ore on reaching the Skiddaw Group. Elsewhere, the mines at Goldscope, Long Work, Thornthwaite, Rachel Wood and Threlkeld were all abandoned between 1917 and 1928 (Table 3). Hartsop Hall mine closed down in 1942.

According to Adams (1995), small amounts of haematite were exploited during the 18th (about 1700) and 19th (1873–1876) centuries from the Providence (Tongue Gill) Mine, near Grasmere. Ore was supplied to a furnace in Langdale, and 300 tons of ore was raised.

Non-metalliferous minerals

The High Force workings at the Force Crag Mine have yielded mainly baryte, which did not become saleable until the late 1860s (Adams, 1995). In 1939 the mine yielded 35 000 tons of baryte, however, soon after 1947 a drop in the price of baryte caused the mine to close. Renewed working in 1960 lead to the discovery of a new deposit. Between 1966 and the mid-1980s there were various attempts to work the lead-zinc ore as well as the baryte. The latter is also abundant in the upper workings at Greenside, but was not of commercial significance. The graphite mine at Seathwaite gave rise to the Keswick pencil industry about 1790. However, this was late in the mine’s history, because workings are known to date back to 1555 (Adams, 1995).

Construction materials

Slate has been extracted, since at least as early as 1870, from an extensive system of surface and underground quarries on either side of the Honister Pass, between Buttermere and Borrowdale. The slate consists of green, cleaved volcaniclastic rocks of the Eagle Crag Member of the Birker Fell Formation. The worked area includes three distinct quarry locations, Dubs [NY 210 135], Yew Crag [NY 224 143] and Honister Crag [NY 215 140]. Only Honister Crag has been worked recently. At the turn of the 20th century most of the slate was used for roofing purposes, but by the Second World War the use of tiles predominated. Since about 1950 the best slate has been used either for monumental (decorative) or architectural purposes, but the latter has declined since about 1970. Although slate waste can be used for a variety of purposes, including slate powder as an inert filler, the manufacture of concrete blocks and roofing tiles, and in the manufacture of lightweight aggregates (Stephens Associates, 1988), there is no record of this use in the Lake District.

There is potential use of mine spoil as a crude aggregate, particular for sub-base and farm track construction. Areas of useable spoil exist at a number of mines, for example Barrow and Yewthwaite. Microgranite was extracted from Threlkeld and Bram Crag quarries for use as an aggregate. Threlkeld Quarry ceased operation in 1980, and matters concerning its restoration, landscaping and potential uses have been under consideration (Lake District National Park, 1994). The quarry is now the site of a mining museum.

Locally derived stone has been used for construction purposes. Glacial erratics and rocks extracted from small local quarries have been used in buildings and in particular in the construction of many miles of dry stone walls. Sand and gravel occur within the glacial and postglacial deposits of the district, but no large extensive deposits are known, and none have been worked for commercial use. Although there is a variable thickness of blanket peat across the district, none appears to have been exploited for fuel or horticultural use.

Geothermal energy

The Lake District batholith has been investigated as a potential hot dry rock geothermal resource, because of its size, and possible above-average content of the main heat-producing radio-elements, uranium, thorium and potassium (Lee, 1986b; Lee et al., 1984; 1987; Webb and Brown, 1984; Webb et al., 1987; Wheildon et al., 1984). Measurements in boreholes on the Skiddaw and Shap granites, (Cockermouth and Kendal districts, respectively) showed elevated heat-flow values of 100.9 and 77.8 mWm², respectively, caused by above-average radiogenic heat production of 4.2 and 5.2 µWm³. No heat flow measurements were made in the Keswick district. The Skiddaw and Shap granites are limited in surface extent, and therefore the measurements are probably not representative of the rest of the batholith (Lee, 1986b). Measurements of surface heat production from exposed parts of the batholith are 5.2 and 4.2 µW/m³ for the Shap and Skiddaw intrusions, but only 1.9 to 2.8 µW/m³ for the Threlkeld, Ennerdale and Eskdale intrusions (Lee, 1986b, table 3.4). The low measurements for the last three may be a function of uranium depletion, either by weathering or oxidation by hydrothermal fluids. Heat flow over the central Lake District is possibly significantly above the national average, but more heat flow boreholes are needed to confirm this.

Hydrogeology and water supply

The mountainous terrain of the district equates with a high annual rainfall (about 2500 mm) and consequently there is abundant surface water in the form of lakes and rivers (Figure 2). However, the bedrock that underlies the district generally has low permeability and intergranular porosity, with groundwater flow and storage restricted to joints and fractures in weathered rock (Patrick, 1978) and along fault planes. As a consequence, there are no regionally significant aquifers in the district, most water supplies being taken from the surface sources. Recharge to bedrock aquifers is restricted by a combination of poorly permeable rock, clayey drift cover and steep mountain slopes. The annual infiltration rate is estimated at less than 40 mm (Patrick, 1978). Groundwater is also present in superficial deposits within the narrow, glaciated valleys in river gravels and other granular material. These sources usually have a shallow water table and can transmit larger volumes of groundwater compared to bedrock. Hillside till is another source of groundwater from springs.

The overall volume of groundwater present in the district is low, but it is still an important source of supply for many houses. British Geological Survey well records include 78 spring supplies in use, 35 shallow wells and 2 boreholes. Most sources that are in use are located on the lower slopes of hillsides in valleys close to dwellings, but there are many other springs present in the area that are too remote for exploitation. Spring supplies are in use particularly around Bassenthwaite Lake and to the east of Keswick.

Thirlmere reservoir was created from a pre-existing lake, the level of which was raised following the construction of a 16.5 m-high masonry dam at the northern end (Hoyle and Sankey, 1994). The 4.8 km-long reservoir holds 41 000 million litres, when full, and is now owned by the North West Water Authority. The water is carried by gravity in a ‘cut and cover’ aqueduct from Thirlmere to Manchester, a distance of 155 km, and is treated south of Dunmail Raise. It is also used in conjunction with other water sources to supplement supplies to Cumbria and central Lancashire.

Waste disposal

There are at present no operational landfill sites, for waste disposal, in the Keswick district. A site at Town Cass, Keswick [NY 261 229] was operational until January 1987, and a wide range of domestic, commercial and industrial wastes was deposited. The site was reinstated in May of that year. There were former landfill sites at Thornthwaite Mine [NY 224 259], Bog House [NY 238 245], Threlkeld Quarry [NY 329 246], Rannerdale Knott [NY 165 185], Holmcragg Wood [NY 251 172], Mill Moss [NY 396 157] and Rosthwaite Bridge [NY 256 149], which were operational prior to 1976. However, the type and quantity of waste deposited on these sites is not known.

Ground stability

There is only a small amount of geotechnical data available for the district. The engineering behaviour of soils and bedrock in the district is summarised in (Table 7) and (Table 8), and the slope stability of these materials, in (Table 9). The Skiddaw Group mudstones weather readily to form a weak material consisting of slabs and prisms. The weathering of these rocks may result in slope stability (see below) and maintenance problems (Hencher and McNicholl, 1995), particularly in cuttings. The strength of the Borrowdale Volcanic Group rocks is reduced by the presence of small-scale veins filled with weaker haematite, calcite and dolomite. The porosity of the intact volcanic rocks is typically very low with values less than 1%, but which may vary across formations.

Site investigations for the Keswick northern by-pass produced geotechnical data on the Quaternary deposits in the area north and north-west of the town, in particular the till, which gave problems in construction. These included resilience against compaction and susceptibility to water content change when used as a fill, and unpredictable ground water conditions in cuttings (Cocksedge and Hight, 1978). Throughout the district, till varies widely in its geotechnical properties, which reflect variations in the proportion of clay/silt to cobble and boulder-sized clasts. The age and mechanisms of formation of most of the landslides in the district are unknown, but in hard rock areas most were considered to be rock-falls (Anon, 1987). The largest landslide in the Keswick district is on Buttermere Fell, at Gatesgarth [NY 204 156]. This covers an area of about 200 hectares and is a deep-seated landslide in the Buttermere Formation, possibly of interglacial origin, caused by the presence of down-slope dipping, east-west faults, and glacially over-steepened slopes (Webb, 1990). The back-scarp is below Littledale Edge at Hackney Holes. Smaller landslides occur east of Keswick, on the south-east side of Latrigg [NY 285 247] and [NY 282 242], along Glenderaterra Beck [NY 262 298], and at 15 other locations. About half of these involve till and other Quaternary deposits. Landslides in till are prevalent below 300 m above OD, and frequently have their origin in solifluction (Boardman, 1982).

Seismicity

Earthquakes are not uncommon in north-west England, but few such events appear to have occurred within the district (Musson et al., 1984; Musson, 1994). Musson et al. (1984) list events in Keswick on 6 July 1787 and 9 to 14 July 1901. The former was felt over an area of about 500 square kilometres including Threlkeld and Penrith, and an epicentre just north of Helvellyn is suspected. The 1901 events were possibly aftershocks of the Carlisle earthquake on 9 July of that year. A number of events have occurred in the adjacent Ambleside district, notably at Grasmere in 1867, 1885 and 1911, with magnitudes of 2.7, 3.1 and 3.1 respectively, and at Ambleside in 1988 (Musson, 1994; Millward et al., in press). These events were felt widely throughout the central Lake District, as was the Whitehaven earthquake in 1786, and the Carlisle earthquakes of 1901 (magnitude range 3.2 to 4.1), 1915 (magnitude 3.4), 1979 and 1980 (magnitude range 3.2 to 4.7). In common with the rest of the Central Lake District, the Keswick district is not especially prone to seismic activity, but as with Britain as a whole, earthquakes of unusual intensity could occur.

Conservation and geological heritage

The town of Keswick has long been a haven for geologists. A local man, Jonathon Otley (1766–1856) studied the terrain around the town, and is remembered today by a road name close to the town centre, a plaque on his cottage, and a special section commemorating his life and work in the town museum. More recently, localities around the town have been recognised, by English Nature, as being of national importance, providing valuable Earth science teaching sites. These have received new designations as SSSI (Site of Special Scientific Interest) and RIGS (Regionally Important Geological/ Geomorphological sites), and consequently have official recognition and some degree of protection from development or damage (Smith, 1996). There are nine SSSI and forty RIGS sites in the Keswick district. Many of the SSSI sites are small, for example Comb Beck [NY 182 150] and Throstle Shaw [NY 237 272], and have specific aspects of geological interest such as minersalisation or Quaternary deposits. However, some are much larger, for example Buttermere Fells and Helvellyn/Fairfield, and have a wide range of aspects of geological interest, such as lower Palaeozoic stratigraphy, Caledonian structures, mineralisation, geomorphology and Quaternary deposits. There are 11 sites within the district described in the Geological Conservation Review series volumes published by the Joint Nature Conservation Committee. Five are concerned with geological structures (Treagus, 1992), and six with volcanic and intrusive igneous rocks (Stephenson et al., 1999).

Information sources

Further geological information held by the British Geological Survey relevant to the Keswick district is listed below. It includes published maps, memoirs and reports, along with open- file maps and reports. Other sources include borehole records, mine plans, fossils, rock samples, thin sections, hydrogeological data and photographs.

Searches of indexes to some of the collections can be made on the Geoscience Index System in British Geological Survey libraries. This is a developing computer-based system, which carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographical information system linked to a relational database management system. Results of the searches are displayed on maps on the screen. At the present time (1999) the data sets are limited and not all complete. The indexes, which are available, are listed below:

• Index of boreholes
• Topographical backdrop based on 1:250 000 scale maps
• Outlines of British Geological Survey maps at 1:50 000, 1:10 000 and 1:10 560 scale and County Series maps
• Chronostratigraphical boundaries and areas from British Geological Survey 1:250 000 maps
• Geochemical sample locations on land
• Aeromagnetic and gravity data recording stations
• Land survey records

Details of geological information available from the British Geological Survey can be accessed from the BGS Home Page at http://www.bgs.ac.uk.

Maps

Geological maps

• 1:625 000
• United Kingdom (North Sheet) Solid geology, 1979; Quaternary geology, 1977.
• 1:253 440
• 3 Carlisle, Keswick and part of the Isle of Man, Solid geology, 1959
• 1:250 000
• Sheet 54N 04W Lake District, Solid geology, 1980
• 1:50 000
• Sheet 22 Maryport, Solid geology, Solid and drift geology, 1980, 1995E
• Sheet 23 Cockermouth, Solid geology, 1977, Solid and drift geology, 1975
• Sheet 24 Penrith, Solid and drift geology, 1974
• Sheet 38 Ambleside, Solid geology, 1996, Solid and drift geology, 1998
• Sheet 37 Gosforth, Solid geology, in press Sheet 28 Whitehaven, Solid geology, in press
• 1:25 000
• Sheet NY12 Lorton and Loweswater, Solid and drift, 1990.
• 1:10 000 and 1:10 560
• Details of the original geological surveys are given on the 1:50 000 Series geological sheets. Copies of the fair-drawn maps of these early surveys are available for public reference at the British Geological Survey Library, Edinburgh.
• The maps at 1:10 560 (six-inch) or 1:10 000 scale covering all or part of 1:50 000 Series Sheet 29 Keswick are listed below, along with the surveyors’ initials and dates of survey. The surveyors were: P M Allen, R S Arthurton, R P Barnes, B Beddoe-Stephens, S D G Campbell, A H Cooper, N C Davis, D J Fettes, R A Hughes, B C Kneller, S C Loughlin, B J McConnell, J W Merrit, D Millward, M G Petterson, D E Roberts, G J Roycroft, P Stone, B C Webb, D G Woodhall and B Young. The maps are not published, but are available for public consultation in the British Geological Survey libraries in Edinburgh and Keyworth, and can also be consulted at the London Information Office in the Natural History Museum, South Kensington. Dyeline copies (black and white) may be purchased from the British Geological Survey sales desk.

Geophysical maps

• 1: 1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:250 000
• Sheet 54N 04W Lake District; Aeromagnetic anomaly map, 1977, Bouguer gravity anomaly map, 1986.
• 1:50 000
• Geophysical information maps (GIM) are available on a plot-on- demand basis for all 1:50 000 Series geological sheets in the United Kingdom. These summarise graphically the publicly available geophysical information held for each sheet in the BGS databases. Features include:
• Regional gravity data: Bouguer anomaly contours and location of observations
• Regional aeromagnetic data: total field anomaly contours and location of digitised data points along flight lines.
• Gravity and magnetic fields plotted on the same base map at 1:50 000 scale to show correlation between anomalies.
• Separate colour contour plots of gravity and magnetic fields at 1:125 000 for easy visualisation of important anomalies.
• Location of local geophysical surveys
• Location of public domain seismic reflection and refraction surveys
• Location of deep boreholes and those with geophysical logs.

Geochemical atlases

• The geochemical Baseline Survey of the Environment (G-Base) is based on the collection of stream sediment and stream water samples at an average density of one sample per 1.5 square kilometres. The fine (minus 150 mm) fractions of stream sediment samples are analysed for a wide range of elements, using automated instrumental methods.
• The samples from the Lake District were collected in 1978–1980. The results (including Ag, As, Ba, Be, Bi, B, CaO, Cd, Co, Cr, Cu, Fe₂O₃, Ga, K₂O, La, Li, MgO, Mn, Mo, Ni, Pb, Rb, Sb, Sn, Sr, TiO₂, U, V, Y, Zn and Zr in stream sediments, and pH, conductivity, fluoride, bicarbonate and U for stream waters) are published in atlas form (British Geological Survey, 1992). The geochemical data, with location and site information, are available as hard copy for sale or in digital form under licensing agreement. The coloured geochemical atlas is also available in digital form (on CD-ROM or floppy disc) under licensing agreement. British Geological Survey offers a client-based service for interactive GIS interrogation of G-BASE data.

Hydrogeological map

• 1:625 000
• England and Wales, 1977

Publications

The various memoirs, books, reports and papers relevant to the district are listed in the reference section. British Geological Survey Technical and other reports are not widely available, some may be purchased from British Geological Survey or consulted at the British Geological Survey and other libraries.

More details of the general geology can be found in British Geological Survey Technical Reports by Beddoe Stephens (1995, 1996, 1997a, b), Branney and Kokelaar (1993), Branney et al. (1993a, b), Campbell (1995), Fortey (1989a), Hughes (1994), Hughes and Fettes (1994), Kneller and McConnell (1993), Kneller, Kokelaar and Davis (1993), McConnel and Kneller (1993), Millward (1998), McConnell (1993), Lee (1989), Rundle (1992) and Woodhall (1998). Memoirs such as that by Ward (1876) are out of print but can be consulted in British Geological Survey libraries. The district lies within the Northern England part of the British Regional Geology series of publications (Taylor et al., 1971), which are readily available in British Geological Survey and some other bookshops.

Information on mineral resources can be found in the memoir by Eastwood (1921), now out of print but available for library consultation, and in a Technical Report by Millward and Young (1984). Minerals present in the district are given in the glossary of Lake District minerals by Young (1987), which is available from the British Geological Survey and other bookshops.

Seismic information appears in Technical Reports, such as those by Musson (1994) and Musson et al (1984). There is a collection of internal British Geological Survey biostratigraphical reports, not listed in the references, details of which are available from the Biostratigraphy Group in the Keyworth office. Geothermal energy assessment forms the basis of detailed British Geological Survey reports (Lee et al., 1984; Webb and Brown, 1984; Wheildon et al., 1984), and an HMSO publication (Lee, 1986b). An analysis of satellite imagery (Landsat) also appears in a technical report (Berrange, 1991). Popular publications consists of a 1:200 000 scale full colour satellite image poster, for the Lake District and surrounds, and a multimedia CD-ROM, entitled Discovering geology: the Lake District, available for Mac/PowerMac and IBM compatible computers.

Other information

Borehole and Site Investigation record collection

Collections of records of borehole and site investigations, relevant to the district, are available for consultation at the British Geological Survey, Edinburgh, where copies of most records can be purchased. The collection consists of the sites and logs of about 110 boreholes. Index information, including site references, for these boreholes have been digitised. The logs are either hand- written or typed and many of the older records are driller’s logs.

Mine and quarry information

The British Geological Survey maintains a collection of information on mining and quarrying in the district in the form of notes and plans; there are notes and or plans for 18 mines (including Force Crag and Greenside) and 1 quarry (Threlkeld) in the district.

Material collections

British Geological Survey photographs

More than 100 photographs illustrating aspects of the geology of the district are deposited for reference in the British Geological Survey libraries at Edinburgh and Keyworth. Sheet albums of the more recent photographs are also held in the British Geological Survey information office in London. The photographs depict details of the various rocks exposed, both naturally or in excavations, and also some general views. Copies of the photographs can be purchased as black and white or colour prints, and 35 mm transparencies, at a fixed tariff, from the Photographic Department, British Geological Survey, Edinburgh.

Petrological Collections

The petrological collections for the district consist of more than 500 rock specimens and thin sections. Most specimens and thin sections are from the Borrowdale Volcanic Group and related intrusions.

Palaeontological Collections

The collections of biostratigraphical specimens are taken from surface and temporary exposures, mostly within the Skiddaw Group. Registered samples are held by the British Geological Survey at Keyworth.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references may be purchased from the Library subject to the current copyright legislation.

ADAMS, J. 1995 Mines of the Lake District Fells. (Skipton: Dalesman Publishing Company).

AKHURST, M C, and 24 others. 1997. The geology of the west Cumbria district. Memoir of the British Geological Survey, Sheets 28, 37 and 47 (England and Wales).

ANONYMOUS 1987. Review of research into landsliding in Great Britain. Geomorphological Services Ltd. Reports to the Department of Environment. Series A, Vol. 1, South-east England and East Anglia.

BEDDOE-STEPHENS, B. 1995. Mapping and related studies of the Borrowdale Volcanic Group on parts of 1:10k sheets NY12NE and NY12NW. Part of the 1:50k Sheet 29 (Keswick). British Geological Survey, Mineralogy and Petrology Short Report MPSR/95/22.

BEDDOE-STEPHENS, B. 1996. Mapping, petrology and geochemistry of the Borrowdale Volcanic Group on 1:10k sheets NY21NE and NY21NW. Part of the 1:50k Sheet 29 (Keswick). British Geological Survey, Mineralogy and Petrology Short Report, MPSR/96/16.

BEDDOE-STEPHENS, B. 1997a. Geochemistry of the Borrowdale Volcanic Group in the Keswick District. British Geological Survey, Mineralogy and Petrology Short Report, MPSR/97/10.

BEDDOE-STEPHENS, B. 1997b. The initiation of Borrowdale volcanism in the Ordovician Lake District as indicated by basal volcaniclastic deposits. British Geological Survey Technical Report, WG/97/14.

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.

BEDDOE-STEPHENS, B. 1999. Falcon Crag. 149–153 In Caledonian Igneous rocks of Great Britain, STEPHENSON, D and 6 others. Geological Conservation Review Series, Vol. 17, Joint Nature Conservation Committee, Peterborough.

BERRANGE, J P. 1991. Linear analysis of Landsat Thematic Mapper imagery of the English Lake District and environs. British Geological Survey Technical Report, WA/91/30.

BOARDMAN, J. 1977. Stratified screes in the northern Lake District. Proceedings of the Cumberland Geological Society, Vol. 3, 233–237.

BOARDMAN, J. 1979. Pre-Devensian weathered tills near Threlkeld Common, Keswick, Cumbria. Proceedings of the Cumberland Geological Society, Vol. 4, 33–44.

BOARDMAN, J. 1982. Glacial geomorphology of the Keswick area, Northern Cumbria. CumberlandGeological Society. Vol. 4, 115–134.

BOARDMAN, J. 1991. Glacial Deposits in the English Lake District. 175–83 in Glacial Deposits in Great Britain and Ireland. EHLERS, J, GIBBARD, P L, and ROSE, J (editors). (Rotterdam: Balkema).

BOARDMAN, J. 1992. Quaternary landscape evolution in the Lake District -- a discussion. Proceedings of the Cumberland Geological Society, Vol. 5, 285–315.

BOARDMAN, J. 1994. Mosedale. 165–172 in The Quaternary of Cumbria: Field Guide. BOARDMAN, J and WALDEN, J (editors). (Oxford: Quaternary Research Association).

BOARDMAN, J. 1996. Classic landforms of the Lake District, Classic Landform Guides. (Sheffield: Geographical Association).

BOTT, M H P. 1974. The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden. Journal of the Geological Society of London, Vol. 130, 309–331.

BOTT, M H P. 1978. Deep structure. 25–40 in The geology of the Lake District. MOSELEY, F (editor). Occasional Publication of the Yorkshire Geological Society, No. 3.

BRANNEY, M. J. 1988. The subaerial setting of the Ordovician Borrowdale Volcanic Group, English Lake District. Journal of the Geological Society of London, Vol. 145, 887–890.

BRANNEY, M J. 1991. Eruption and depositional facies of the Whorneyside Tuff Formation, English Lake District: An exceptionally large-magnitude phreatoplinian eruption. Geological Society of America Bulletin, Vol. 103, 886–897.

BRANNEY, M J, KOKELAAR, B P, and MCCONNELL, B J. 1992. The Bad Step Tuff: a lava-like rheomorphic ignimbrite in a calc- alkaline piecemeal caldera, English Lake District. Bulletin of Volcanology, Vol. 54, 187–199.

BRANNEY, M J, and KOKELAAR, B P. 1993. The Whorneyside Tuff Formation in the Central Fells. British Geological Survey Technical Report, WA/93/40.

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 volcanotectonics 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 Group stratigraphy. Geological Journal, Vol. 23, 171–187.

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Figures, plates and tables

Figures

(Figure 1) Simplified geological map (solid) of the Keswick district.

(Figure 2) Topography of the district.

(Figure 3) The Lake District Lower Palaeozoic inlier showing the extent of the Skiddaw, Eycott and Borrowdale Volcanic groups, the Windermere Supergroup, and major igneous intrusions, relative to that of the Keswick district.

(Figure 4) Borrowdale Volcanic Group stratigraphy depicted in an east-west transect (not to scale).

(Figure 5) Three-dimensional form of the Lake District batholith based on an interpretation of gravity and aeromagnetic data by Lee (1989. fig 8.1), showing its form within the Keswick district, along with the main geophysical lineaments.

(Figure 6) Structural map of the district; rock units as in (Figure 1), but also including the Eagle Crag Member (Eag) of the Birker Fell Formation.

(Figure 7) Mineralisation localities in the district.

(Figure 8) Quaternary glaciation in the district.

Plates

(Front cover) Thirlmere reservoir, viewed from Steel Fell [NY 322 121], Helvellyn Screes form the steep slopes (lowest are forested) on the right, Blencathra is the highest of the fells in the background (north). The reservoir is situated entirely within rocks of the Borrowdale Volcanic Group, and is aligned along the main trace, and splays, of the Coniston Fault. Blencathra and hills to the left are formed of Skiddaw Group rocks hornfelsed within the aureole of the Skiddaw granite (MN 506391).

(Plate 1) Thinly bedded turbiditic sandstone and siltstone at the top of the Loweswater Formation on the south side of Whiteside End [NY 1660 2169]. This is a transitional lithology, locally developed, at its contact with the overlying Kirk Stile Formation. It has been quarried on a small scale for roofing flags (D 3809).

(Plate 2) Slump folds in hornfelsed Skiddaw Group siltstones of the Kirk Stile Formation, within the Crummock Water aureole at Lad Hows [NY 1729 1925]. The folds are commonly disharmonic with sheared limbs. Obvious plastic deformation of the bedding weathers more conspicuously than in unaltered siltstones (D 3829).

(Plate 3) The contact between the Skiddaw Group and Borrowdale Volcanic Group as seen in a stream section at Causeway Foot, adjacent to the Keswick–Ambleside road [NY 2907 2197]. Basal volcaniclastic conglomerate of the Borrowdale Volcanic Group rests with a slight angular unconformity on dark grey mudstones of the Skiddaw Group. The hammer rests on the contact (L 2031).

(Plate 4) High Rigg [NY 307 214], viewed from St Johns in the Vale, showing prominent trap topography in the andesite dominated Birker Fell Formation in the lower part of the Borrowdale Volcanic Group (Photo: D Millward).

(Plate 5) Bedded volcaniclastic sandstone of the Deepdale Formation exposed on the south side of Link Cove, near Fairfield [NY 3704 1188]; the lithology shown is typical of the Seathwaite Fell, Esk Pike and Deepdale formations in the upper part of the Borrowdale Volcanic Group (MN 506671).

(Plate 6) Eutaxitic lapilli-tuff forming the welded lowest part of the ignimbrite of the Helvellyn Formation, Rydal Head near Fairfield [NY 3568 1079]. Similar rocks occur in the Lincomb Tarns Formation, and other ignimbrites in the upper part of the Borrowdale Volcanic Group (MN 506725).

(Plate 7) The Threlkeld microgranite in Bramcrag Quarry, Threlkeld [NY 3202 2197]. Looking upwards at the quarry face, the pale coloured microgranite is overlain by volcaniclastic rocks at the base of the Borrowdale Volcanic Group (L 2040).

(Plate 8) The Causey Pike Fault, in this case a thrust fault, at Causey Pike [NY 2196 2086]. The upper two-thirds of the photo comprises hornfelsed Skiddaw Group siltstones, within the Crummock Water aureole. The sloping contact just below the hammer is the Causey Pike Thrust, and the underlying rock consists of sheared siltstones of the Buttermere Formation (D 3802).

(Plate 9) View north-eastwards along the lower part of the Grisedale valley, from Ruthwaite Lodge [NY 3553 1355]. Hummocky morainic deposits of a Loch Lomond Stadial glacier that occupied the upper part of the Grisedale valley are apparent in the foreground. Place Fell forms the skyline in the centre (MN 506730).

(Plate 10) View westwards from the summit of Place Fell [NY 406 169] along the lower part of Glenridding valley with part of Ullswater in the foreground. The village of Glenridding is situated on an alluvial fan developed where Glenridding Beck flows into Ullswater. Farther up the valley, in the background on its right (north) side, are extensive spoil heaps from the Greenside Mine workings. The fells on the skyline consist of Whiteside Bank (far left), Raise (centre) and the southernmost part of Stybarrow Dodd (far right) (MN 506838).

(Plate 11) Force Crag Mine, Braithwaite [NY 2010 2166]. In the foreground is the settling pond for tailings from the treatment plant, housed in the buildings in the middle distance. Behind the buildings are large areas of spoil from the mine levels. The prominent crag, crossed by waterfalls, is Force Crag, which is made up of Skiddaw Group mudstones (Kirk Stile Formation). The Force Crag vein forms a conspicuous straight gully immediately to the right of Force Crag (D 3856).

(Plate 12) Small landslip on the north side of Wythburn valley, below Castle Crag [NY 3057 1175], about 1.5 km south-west of the southern end of Thirlmere. The landslip probably occurred during very wet weather in February 1995, and involved the localised remobilisation of gravely head deposits that partly veneer the valley side. Castle Crag, on the skyline, is made up of volcaniclastic rocks of the Esk Pike Formation (MN 506384).

(Back cover)

Tables

(Table 1) Geological succession in the district.

(Table 2) Summary of the geological history of the district.

(Table 3) Biostratigraphy and lithostratigraphy of the Skiddaw Group.

(Table 4) Stratigraphy, depositional processes and environment, and volcanological interpretation of the lower part of the Borrowdale Volcanic Group.

(Table 5) Stratigraphy, depositional processes and environment, and volcanological interpretation of the upper part of the Borrowdale Volcanic Group.

(Table 6) Details of mineralisation localities in the Keswick district, see (Figure 7) for localities.

(Table 7) Engineering considerations of bedrock in the Keswick district.

(Table 8) Engineering considerations of Quaternary, man-made and landslide deposits in the Keswick district.

(Table 9) Slope stability of Quaternary, man-made and landslide deposits, and bedrock, in the Keswick district

Tables

(Table 4) Stratigraphy, depositional processes and environment, and volcanological interpretation of the lower part of the Borrowdale Volcanic Group

(Table 5) Stratigraphy, depositional processes and environment, and volcanological interpretation of the upper part of the Borrowdale Volcanic Group

(Table 6) Details of mineralisation localities in the Keswick district, see (Figure 7) for localities

(Table 7) Engineering considerations of bedrock in the Keswick district