Wales: British regional geology

M F Howells

Bibliographical reference: Howells, M F. 2007. Wales British Regional Geology Keyworth, Nottingham: British Geological Survey.

British Geological Survey

Wales British Regional Geology

M F Howells

The grid, where it is used on the figures, is the National Grid taken from Ordnance Survey mapping. Maps and diagrams in this book use topography based on Ordnance Survey maps. © Crown Copyright reserved Ordnance Survey licence number 100037272/2007 Printed in the UK by Halstan Printing Group, Amersham, Buckinghamshire C30/07.

Author: M F Howells formerly British Geological Survey Keyworth, Nottingham NG12 5GG

(Front cover) Crib Coch looking west across Llanberis Pass. The high ridges and the summit of Crib Goch are in rhyolite, an intrusion into the Bedded Pyroclastic Formation that overlies the Lower Rhyolitic Tuff Formation. The well-featured contact dips gently to the right (north) and forms the lower slopes that are scattered with numerous erratic blocks. The distant peak on the left is Snowdon summit (Yr Wyddfa) (Photograph P662384).

Acknowledgements

This book was written following my retirement from the British Geological Survey, when I was an Honorary Research Associate in the BGS and a Visiting Professor at the University of Liverpool. I acknowledge reviews by Adrian Rushton (Cambrian), Mark Williams and Dick Waters (Silurian), Bill Barclay (Devonian and Carboniferous), Colin Waters and Jerry Davies (Carboniferous), Dave Tappin (Mesozoic), Adrian Humpage (Cainozoic), Dick Merriman (metamorphism); Nick Robbins and Paul Shand provided notes on the hydrogeology. Most pertinently, I am aware of the influence of many colleagues and coworkers, over so many years of working in the Principality, who enthusiastically shared both the problems across this spectacular fragment of British geology and all that fun in considering their possible solution.

Scientific editing on behalf of BGS was done by Stewart Molyneux; the series editor is Audrey Jackson.

Illustrations were produced by the cartographic team lead by Ron Demaine and under the management of Roger Parnaby. Pagesetting was by Amanda Hill.

Photographs are from BGS archive or author’s own (MFH) unless otherwise stated.

(Frontispiece) This Landsat^TM image of Wales and adjacent area is a winter mosaic (Band combination 4, 5, 7). The low angle of the sun shows the physical and some geological features. High ground with sparse vegetation cover is greenish blue in colour, forest is maroon/red and other vegetation is orange; towns and cities show as blue-grey.

Foreword to the first edition

Wales has had an influence on the science of geology that is in inverse proportion to its small area. Beneath its land area and adjacent sea are representatives of all the major periods of the stratigraphical column, and some were originally defined here in the 19th and early years of the 20th century. Successive generations of geologists have gathered more and more detail and applied new techniques to achieve our current understanding of the geology.

Since its inception in 1835, the Geological Survey has been mapping and acquiring geological data in Wales. This work has been published as maps, memoirs, sheet explanations and other special publications. In addition there has been a continuous stream of publications, some seminal, from university researchers. This book draws heavily on both sources in order to present an overview of the geology and to draw attention to specific sites of interest.

In the first editions of this British Regional Geology series, Wales was covered by two books — North Wales and South Wales — published in 1935 and 1937 respectively, and there were many later new editions and impressions. The pattern of the geology across the Principality and beneath the adjacent sea bed makes it difficult to maintain presentation of the geology in two volumes about an arbitrary east–west line. The political boundary with England to the east, which loosely approximates to the line of Offa’s Dyke, has been taken as the eastern boundary and the ‘Welsh’ localities (i.e. Lower Palaeozoic) of Shropshire are referred to only briefly. Since the first publications of this series, two main factors have profoundly influenced our understanding of the geology; neither was known to the authors of these early books. The first is the theory of plate tectonics, which was formulated through the 1960s largely as a result of the global exploration of the oceans; the second is the data acquired from exploration of the sea floor around Wales in the latter part of the 20th century, by drilling and by geophysical surveys.

The underlying geology is reflected in the contrasting and spectacular features of the Welsh landscape, which attract a multitude of walkers and tourists to the Principality. The easy accessibility from the major conurbations has resulted in the growth of many field centres for natural history studies. We hope that this book will provide a geological overview to students and visitors who are interested in the geology, as well as a summary for the professional geologist, civil engineer and planner.

John N Ludden, PhD, Executive Director, British Geological Survey Kingsley Dunham Centre Keyworth, Nottingham

Chapter 1 Introduction

This book describes the geology of Wales that is shown on the 1:625 000 scale geological map of the Principality of Wales and replaces earlier editions of this series in which north and south Wales were described in separate volumes. Unlike the earlier editions, some of the most pertinent data, retrieved in recent years, from the offshore areas, Liverpool Bay, Caernarfon Bay, Cardigan Bay and the Bristol Channel, have been included. These data serve to complete the geological history and context of Wales and especially so in reference to plate tectonic models.

Within Wales and the adjacent sea, elements of all the major geological systems are to be found (Table 1), and for much of the country the differences in the geology (Figure 1 and map in back pocket) are reflected in the topography (Figure 2) and the scenery. The main exception is the low-lying coastal area of south Wales, between south Glamorgan and Pembrokeshire, which is underlain by folded Palaeozoic rocks but lacks the topographical features of similar strata at higher elevations inland. The contrast reflects a difference in the erosional history, with the coastal areas having been submerged and repeatedly eroded by the sea in more recent geological times.

In Anglesey and Llŷn, there is the largest outcrop of Precambrian rocks in southern Britain, and smaller outcrops occur in Pembrokeshire, Carmarthenshire and Radnorshire. However, Wales is dominated by Lower Palaeozoic rocks — the Cambrian and Ordovician in Llŷn, Snowdonia National Park and north Pembrokeshire and the Silurian over much of central Wales. The sequence is mainly of marine sedimentary rocks, and the recognition of distinctive facies and thickness changes across the outcrops has allowed detailed reconstruction of a marine basin and its shorelines throughout this time. Within the Cambrian, the sedimentary rocks range from silty mudstone, such as those displayed in the great quarries between Nantlle and Bethesda in the north Wales slate belt, and coarse-grained sandstone, as exposed in the castellated ridges of the Rhinog Mountains. The basin in which the sediments accumulated was initiated by major fractures in the Precambrian basement and its evolution was controlled by periodic tectonic activity along these fractures, causing both uplift and subsidence. During Ordovician times, volcanic activity was a major feature and the thick accumulations of widely variable volcanic rocks profoundly modified the sedimentary environments. The spectacular scenery throughout the Snowdonia National Park is due largely to the volcanic rocks and their resistance to weathering (Plate 1). Similarly, in Pembrokeshire, the indented coastline around the northern headland of St Bride’s Bay (Plate 2) reflects the differing resistance to erosion of the volcanic rocks and the interbedded, less competent sedimentary rocks. In late Ordovician and Silurian times, when major volcanic activity had ceased, sedimentation was dominated by fine silt and mud which, when lithified, uplifted and eroded, produced the characteristically smooth profiles of the hills through much of central Wales and the Denbigh moors (Mynydd Hiraethog).

During late Silurian and early Devonian times, the Lower Palaeozoic basin contracted and the rocks were folded and uplifted in the final phase (Acadian) of the Caledonian orogeny to form part of a continental surface that extended from the position of the Bristol Channel into northern Britain; at this time the sea lay to the south. The major folds of this orogeny in Wales, such as the Harlech and Berwyn ‘domes’ and the intervening Central Wales Syncline, can be discerned in the current topography. Similarly, the line of the Bala Fault Zone and its branch along Tal y Llyn and the Dysynni valley is well featured. The emergent land was the site of fluvial, alluvial and lacustrine sedimentation, which produced the Old Red Sandstone continental facies in late Silurian and Devonian times. These Old Red Sandstone rocks are well exposed from Pembrokeshire in the west, through the scarp feature of the Black Mountains and Brecon Beacons, to Herefordshire in the east, and for much of this outcrop they lie with marked discordance on the eroded surface of the folded Lower Palaeozoic sequence. The rich agricultural terrain in the lower ground of the Old Red Sandstone outcrops contrasts sharply with the relatively impoverished ground at similar altitudes in the Lower Palaeozoic outcrops. The absence of Middle Devonian rocks across Wales reflects uplift, possibly the final expression of the Caledonian orogeny. As a result, the Upper Old Red Sandstone sequence is incomplete and everywhere rests unconformably on older strata. The appearance of marine fossils towards the top of this sequence indicates the initiation of a marine transgression, from the south, which reached its full expression in early Carboniferous times.

The Carboniferous rocks accumulated in shallow basins marginal to a remnant of the landmass, formerly known as St George’s Land but now generally referred to as the Wales–Brabant Massif. This landmass occupied most of the area of central and north Wales. In early Carboniferous times, the influx of coarse detritus from this low-relief landmass was minimal, and conditions were ideal for the precipitation of carbonate to form the extensive limestones that are a dominant feature of the scenery in south Pembrokeshire and Gower in south Wales and on Anglesey, at Great Orme (Plate 3) and on Mynydd Eglwysseg, near Llangollen in north Wales. In addition, they form an almost continuous and well-defined outcrop around the major east–west-orientated syncline of the South Wales Coalfield and, in the north, they overstep on to the Lower Palaeozoic strata on the east side of the Clwydian Hills and in the Vale of Clwyd.

In later Carboniferous times, uplift caused temporary emergence of the depositional surface and much coarse clastic debris was deposited in a complex of brackish deltas that were periodically flooded by marine incursions. Tropical rain forest developed along the low lying coastline; the accumulations of vegetation in swamp conditions developed into layers of peat, which eventually lithified into the coal seams of the Upper Carboniferous. The numerous coal seams are the result of the constant repetition of these processes, which were accommodated by subsidence being related to continued tectonic activity. It is the exploitation of the coal, clay and ironstone within this part of the stratigraphical column that has profoundly modified the scenery of the south Wales valleys over the last two centuries.

The earth movements or tectonism that affected the sedimentation patterns in early Carboniferous times reached its climax in the Variscan orogeny, in the late Carboniferous, when the sequence was folded and uplifted to form part of a mountain chain, the Variscides, which extended eastwards from southern Ireland into north-west Europe. In south Wales, where the affects of this period of deformation are most clearly recognised, an east–west-trending syncline from Pembrokeshire to Monmouthshire forms a major topographical feature, but the most intense folding and faulting is displayed in the coastal sections in Pembrokeshire and south Gower.

Following the Variscan orogeny, a new cycle of erosion and sedimentation was initiated and a different fauna and flora, characteristic of the Mesozoic era, was established. The red sandstones and siltstones of the Permian and Trias comprise material derived mainly from the erosion of the Carboniferous rocks and redeposited in a semi-arid environment. The sequence crops out in the Vale of Glamorgan, in the south, and on the western flank of the Cheshire Basin and the Vale of Clwyd, in the north. Progressively, the Triassic landmass was submerged and, in early Jurassic times, the grey marine mudstone and thin limestones that are so graphically displayed in the cliff sections in south Glamorgan were deposited. Extensive and thick sequences of these Mesozoic rocks also occur in the immediate offshore areas. Whether the later Cretaceous (Chalk) sea completely submerged Palaeozoic Wales is a matter of some contention.

Towards the end of Mesozoic times, the repercussions of the Alpine orogeny in southern Europe constituted the main influence on the configuration of land and sea. Most of the area was uplifted, although throughout much of Cainozoic times a marine basin persisted along the Irish Sea. Offshore hydrocarbon exploration has revealed thick sequences of both Cainozoic and Quaternary strata that were derived entirely from the adjacent land. Onshore, evidence of the Pleistocene glaciation is almost entirely of the last glacial episode when local ice covered most of Wales; the most graphic evidence is displayed in erosional features in the upland areas, both valleys and ridges, and particularly in Snowdonia. At the same time a major ice sheet occupied the Irish Sea, and its southward movement was deflected by the north edge of the Welsh landmass forcing it across Anglesey and Llŷn in the west and into Cheshire and Shropshire in the east.

At the beginning of Holocene times the climate showed marked signs of improvement and, in a short time, a deciduous forest was established over most of Wales. Sea level changes continued subsequent to the melting of the ice sheets and it was not until some 5000 years ago that current sea level was attained. However, the submerged forests which are such a feature around the coast of Wales, together with the extensive tracts of river and estuarine alluvium, indicate the continued changes in the relative positions of land and sea.

History of geological research

Wales has had an influence in the history of geology that is inversely proportional to its size. Subsequent to the establishment of geology as a science in the late 18th century, the original subdivisions of the stratigraphical column and subdivisions at series level, particularly of the Lower Palaeozoic, are littered with the names of hamlets and villages within Wales and its borders. Early references include those to Giraldus Cambrensis, who in the 12th century, recognised pyritous shales in the vicinity of Newport, and to Leland who, in the 16th century, recognised the difference between the anthracite of the Gwendraeth valley and the coking coals of Llanelli. Later, in the early 17th century, George Owen traced the outcrop of the Lower Carboniferous limestones around the south Wales coalfield, and this formed the basis of one of the earliest geological maps. At the end of the 17th century, Gibson published illustrations of plants from the Coal Measures near Neath. In the early 18th century, the eminent naturalist Edward Llwyd correlated, on the basis of their faunas, the (Carboniferous) limestones at Barry with those of Caldey Island and, most importantly, he recognised that they were different from the Jurassic limestones in south Glamorgan.

At the end of the 18th century, William Smith, the ‘father of British geology’, crossed the border into the Principality and outlined the general structure of the south Wales coalfield. He traced the outcrop of the broad elements of the Old Red Sandstone and Carboniferous on his renowned map published in 1815. In north and west Wales his observations were slightly less specific — he referred most of the rocks to ‘killas’ and ‘slate’, although he designated the Coedana granite of Anglesey as igneous rock. Greenhough’s geological map of England and Wales was published in 1820, and parts of south Wales were accurate and detailed.

The most concerted, early phase of investigation began in 1831 when Adam Sedgwick and Roderick Murchison began their study of the Old Red Sandstone and the underlying Lower Paleozoic rocks that occupy most of central and north Wales; what began in concert ended in intense rivalry. Sedgwick addressed himself to unravelling the stratigraphy and structure of Snowdonia and Llŷn. In 1835, he designated the great sequence of rocks, with apparently no top or bottom, to the Cambrian, and subdivided them into Lower, Middle and Upper. Murchison, working from the Welsh borderland, divided his sequence into Upper and Lower Silurian and supposedly ‘unfossiliferous’ Cambrian; by 1839 he had published his major treatise, The Silurian System. As the work progressed it was apparent that Sedgwick’s Upper and Middle Cambrian were essentially equivalent to Murchison’s Lower Silurian and the controversy, partly on geological fact but mainly on stratigraphical terminology, was initiated. Much heat was engendered and its resolution did not occur until 1879, when both protagonists had died and Charles Lapworth proposed that the controversial beds be assigned to a newly designated Ordovician System, between the beds of the Cambrian System at Harlech and Arfon in Merionethshire and Caernarvonshire, and those of the Silurian System through Denbighshire and Montgomeryshire. The Geological Survey, presumably in deference to Murchison who was its Director between 1855 and 1871, continued to refer to the Ordovician as Lower Silurian into the early years of the 20th century.

Sedgwick’s map of north Wales, on the scale of one inch to eight miles, was published in 1845. One year later, Daniel Sharpe produced a map of the same area, on a scale of one inch to five and a half miles, which provided the first clear designation of the structures. The Geological Survey was founded in 1835 with Sir Henry De la Beche as Director and, in its early years, it accomplished an extraordinary amount of work in Wales. Following the publication of the Geology of south Wales in its first volume of memoirs, a mapping programme, on the scale of one inch to one mile (1:63 360), was initiated with A C Ramsay, W T Aveline, A R C Selwyn and J B Jukes as surveyors, and J B Salter as palaeontologist. By 1852, the north Wales map was completed and by 1858, a geological map of Wales, on the scale of one inch to four miles, was published; this was revised in 1879. An important element of this work, which is often referred to and much admired, is a series of horizontal sections that were surveyed in the field on a scale of six inches to one mile, and display detail of the geology through some of the most remote areas of Wales.

Ramsay and his colleagues produced the first comprehensive account of the Geology of north Wales in 1866, and the second edition in 1881 presented the stratigraphy, the variable sedimentary rocks, volcanic rocks and associated intrusions, the structures and the geomorphological evolution in the most extraordinary detail. It was in 1860 that Ramsay marshalled the first systematic arguments, following an earlier reference by Charles Darwin, for the effect of glacial erosion on the landscape. Towards the end of the 19th century, the Geological Survey began mapping at the scale of six inches to one mile in areas of economic importance; this work extended into the early years of the 20th century. The whole Carboniferous outcrop was remapped, between Pembrokeshire and Monmouthshire in south Wales, and from Wrexham to Shotton in north Wales; maps, sections and memoirs were published.

Following the early pioneering work of Sedgwick and Murchison, the contribution of individual researchers has continued to be a dominant feature of geological research in Wales. In 1889, A Harker published his elegant essay on the Bala volcanic rocks of Caernarvonshire in which he interpreted the detail provided by the Geological Survey’s maps and Ramsay’s memoir. Similarly, in 1895, Edward Greenly resigned his position with the Geological Survey in Scotland to work on the complex Precambrian rocks of Anglesey. Supported by his wife, Annie, he completed the mapping, on the scale of six inches to one mile, with parts mapped on the 25 inch scale. This remarkable task and partnership was lovingly described in the classic two volumes, A hand through time (1938); the map (1920) and memoir (1919), published by the Geological Survey, remain standard works of reference and the scientific testament.

The work of O T Jones, dating back to the early years of the 20th century was directed towards unravelling the Lower Palaeozoic stratigraphy and was published in a succession of papers and in his Presidential address to the Geological Society of London in 1938. His research formed the basis of both a more refined structural analysis and a palaeogeographical model of an evolving geosyncline. Much of this work was accomplished with W J Pugh, who succeeded him to the Chair of Geology at Aberstwyth and later became Director of the Geological Survey. At the same time P G H Boswell systematically recorded the detail of the Silurian stratigraphy of Denbighshire, culminating with his publication, The Mid Silurian rocks of north Wales, in 1949. His studies were of mammoth proportions but seem to have been clouded by his prolonged palaeontological and sedimentological controversies with O T Jones.

Subsequent to the earliest surveys, the work of G L Elles and E M R Wood on the Ordovician and Silurian graptolite faunas laid the foundation for accurate biostratigraphical zoning, and this has continued with the palaeontological work of the Geological Survey and university researchers. Similarly, E E L Dixon clarified the stratigraphical relations of the Lower Carboniferous rocks and applied the concept of facies to the formation of different limestones. A E Truman’s stratigraphical zonation of the Coal Measures based on slight modifications in bivalve morphology was one of the earliest evolutionary palaeontological studies. As a result of these, and other studies, there are many Welsh type-sections of the Palaeozoic sequence in Britain.

During the Second World War (1939–45), officers of the Geological Survey concentrated on work in the coalfields, mineral resources and water supply throughout Wales. This work continued through the period of the nationalisation of the coalfields, when the survey officers worked in co-operation with geologists from the National Coal Board until the decline of the industry in the 1980s. This work formed the basis of the revision and publication of a number of geological maps, at a scale of one inch to one mile, and many of their component, uncoloured maps at six inches to one mile, together with a number of memoirs

The latest phase of systematic work began in the mid-1960s, when the Geological Survey (renamed the Institute of Geological Sciences and subsequently the British Geological Survey) returned to map the Lower Palaeozoic strata on a scale of six inches to one mile (1:10 560), beginning with the Silurian of Denbighshire. The work extended across the Conwy valley on to the Ordovician rocks of Snowdonia, using aerial photographs at a scale of about 1:8000 as base maps. Subsequently, the Bangor, Snowdon, Harlech, Bala and Cadair Idris 1:50 000 geological sheets were published. This work developed from a basic mapping exercise into a multidisciplinary study of the sedimentation and structure, and particularly of the physical and chemical characteristics of the volcanic rocks that form such a dominant element of the Ordovician sequence. Latterly, contracts with the University of Cardiff resulted in the publication of the Aberdaron and Pwllheli sheets.

Following publication of the Denbigh (1970) and Aberystwyth (1984) 1:50 000 geological sheets, the problems of the Silurian of central Wales have been followed up systematically in a transect from Aberaeron, in the west, through Rhayader into the vicinity of Builth Wells, in the east. Again the approach is multidisciplinary with stratigraphical palaeontology establishing the finest details of the stratigraphy to elucidate the complex structural evolution and the patterns of the depositional environments.

This outline of the history of geological research in Wales is extremely selective, and the omission of many names is inevitable. The reader is referred to recent memoirs (p.209–214) for a fuller bibliography. The work of Howel Williams on Snowdon formed the template for subsequent studies on the Ordovician volcanic rocks. The work of W G Fearnsides, C A Matley and T S Wilson provided a firm basis for further stratigraphical refinement of the Lower Palaeozoic stratigraphy in Merionethshire and south Caernarvonshire. In the second half of the 20th century a notable contributor was R M Shackleton, not least because he was the main catalyst in the work of N Rast, F J Fitch, R Beavon, D Wood and B Roberts. The work of T N George has left an unmistakeable signature in the geological literature of Wales and D Bassett made a lifetime commitment from his PhD research in Talerddig to his role as Director of the National Museum of Wales. Sir Alwyn Williams’ pioneering work on brachiopods and Ordovician palaeoecology was founded in the Bala district. The work of J R L Allen and his co-workers on the great swathe of the Devonian, Old Red Sandstone facies through south Wales and the borders has ensured its international recognition. G Kelling and his co-workers of the (former Department of Geology) University College of Wales at Swansea contributed to a clearer understanding of the clastic environments of the Carboniferous of south Wales. In recent years, the work of N H Woodcock and a succession of research students at the University of Cambridge, in the unravelling of the Silurian stratigraphy in central Wales, has been clearly integrated with the work of the survey.

Geotectonic setting

The original books in this British Regional Geology series, published in 1935 and 1937 respectively, and the revised editions of 1961 and 1975, were not influenced by plate tectonic theory, which was established in the late 1960s. The theory is the most revolutionary concept in geological thinking since Darwin and Wallace presented their separate papers on the theory of evolution by natural selection to the Linnean Society in 1858. The plate tectonic theory proposes that the surface of the Earth comprises half a dozen or so large plates, which are continuously in motion with respect to each other. It developed from the recognition that areas occupied by the oceans have grown and expanded as magma rose along deep fractures in the Earth. At the surface the magma cools and solidifies to form mid-ocean ridges and new oceanic crust spreads slowly from the axis of the ridge. This crust eventually collides with, and pushes under, the adjacent continental plate (subduction). These processes continuously rearranged the crustal plates throughout the Earth’s history.

During late Precambrian and early Palaeozoic time, England, Wales and south-east Ireland lay on a segment of Eastern Avalonia at the edge of the continent of Gondwana, and Scotland and north-west Ireland lay farther north across the Iapetus Ocean at the margin of Laurentia (Figure 3). At the same time, Scandinavia and northern Europe lay on a small plate, Baltica, which was separated from Laurentia by the Iapetus Ocean and from Eastern Avalonia by the Tornquist Sea.

The Precambrian rocks of Eastern Avalonia, the basement to the Lower Palaeozoic sequence in Wales, comprised several crustal blocks or terranes of arc-magmatic rocks and sedimentary basins that had been accreted on to the edge of Gondwana in late Precambrian times, 680 to 545 Ma. During most of Cambrian times, tectonic movements continued to modify the edge of Gondwana and in early Ordovician (Tremadoc) times subduction of oceanic crust beneath Eastern Avalonia, marked by island-arc volcanic activity, was associated with its separation from Gondwana and its drift northwards. Throughout Ordovician times, Wales was the site of a back-arc basin with voluminous calc-alkaline basaltic and rhyolitic volcanism. It was flanked on its south-eastern margin by the Midlands Platform, and the volcanic arc, forming the northern margin of the basin, extended from south-eastern Ireland to the Lake District. In late Ordovician times, the cessation of major volcanism suggests the initiation of the collision between Eastern Avalonia and Laurentia, which is supported by the faunal evidence. The basin continued to accumulate thick sequences of marine sediments throughout Silurian times.

The progressive convergence of the Avalonia and Baltica plates with Laurentia resulted in collision and the Caledonian orogeny. In Wales, this tectonic event was initiated in mid to late Silurian times (Figure 3) and led to localised deformation and uplift. However, the main effects occurred in early to mid Devonian times when the Welsh Basin was inverted, and the accumulated sedimentary and volcanic rocks were folded, metamorphosed and uplifted. This Acadian phase of the Caledonian orogeny completed the amalgamation of Laurentia, Eastern Avalonia and Baltica, at about 400 Ma. It imposed a series of north-east-trending folds and associated cleavage belts as the dominant tectonic grain on the Lower Palaeozoic rocks.

The Caledonian terrane amalgamation formed the supercontinental plate, Laurussia, which began to converge with Gondwana, to the south, causing northward subduction of the intervening Rheic oceanic crust and, in turn, the formation of a series of rift basins across much of Britain. Initially these basins accumulated detritus shed from the uplifted Caledonian mountain chain, the thick continental ‘Old Red Sandstone’ deposits of the Devonian, and then the limestone, deltaic deposits and coals of the Carboniferous. The collision of Laurussia and Gondwana caused the Variscan orogeny, which is reflected in a mountain belt from Russia across Europe into North America, and their amalgamation into the supercontinent of Pangaea. Most of Wales lay to the north of the collision zone but the advancing deformation front controlled tectonic development across the region. A putative line, the Variscan Front (Figure 4), marking the limit of strong deformation, crosses the Bristol Channel and south Wales, following a west-north-west trend across Swansea Bay and Carmarthen Bay. To the south of the front, on Gower and south Pembrokeshire, Upper Palaeozoic rocks have been strongly folded. North of the front, the Wales–Brabant Massif extended across the English Midlands into Belgium. To the north of this landmass, the Carboniferous rocks in north Wales accumulated in a series of extensional basins linked to the Pennines Province.

Following the construction of Pangaea, thermal relaxation of the crust resulted in the formation of a series of rift zones, which eventually fragmented the supercontinent. About the uplands of Wales, the stress created the graben-like East Irish Sea, Worcester and Cheshire basins. Simultaneously, the tropical coal-generating swamps of late Carboniferous times were replaced by the arid desert conditions of the Permian and Trias. The continental fragmentation of Pangaea, and subsequently Gondwana, continued to influence late Mesozoic basin development in Britain in response to the Alpine orogeny, which was caused by the collision of the northern edge of Africa with the European (Eurasia) continent in Late Cretaceous times. On the margins of the Welsh massif, extensional basins, which formed during Jurassic times, continued to accumulate thick sequences of sediments into the Quaternary.p

Chapter 2 Precambrian and ?Cambrian

Precambrian (Proterozoic) rocks in Wales crop out most extensively on Anglesey and Llŷn but also occur in inliers in Arfon, between Bangor and Caernarfon, and smaller inliers in Pembrokeshire, Carmarthenshire and Radnorshire. Throughout the 20th century, the relationship between the rocks at these localities has been a subject of intense discussion and it is only in recent years that the terrane model provided the most satisfactory explanation of the disparate relationships seen at outcrop. The model proposes that all the outcrops are of late Precambrian age, and that they formed as a series of smaller terranes of markedly different geological characteristics, which were accreted together between 700 and 540 Ma.

Two major terranes have been defined: the Monian Composite Terrane, which includes Anglesey and the main part of the Llŷn outcrops, and the Avalon Terrane, which includes small localities on Llŷn, Arfon, Pembrokeshire and the Welsh borders (Figure 4). These terranes are juxtaposed by the Menai Straits Fault System, a complex of steeply inclined shear zones and brittle faults, but there are no exposures displaying the stratigraphical relationships between rocks on either side. The Monian Composite Terrane is equivalent to the Rosslare Terrane in south-east Ireland and both are bound to the north-west by the Leinster–Lake District Terrane.

Monian Composite Terrane

The rocks on Anglesey were first referred to as Monian by Blake in the 19th century and it was this reference that caught the attention of Edward Greenly who, at the time, was working on the Precambrian rocks of the north-west Highlands of Scotland. Greenly resigned from the Survey in 1895 to study the rocks of Anglesey applying the techniques he had acquired in Scotland. His memoir was published by the Geological Survey in 1919 and was shortly followed by his map in 1920. The rocks that occupy the Monian Composite Terrane on Anglesey are referred to the Coedana Complex, the south-east Anglesey Blueschist Terrane and the Monian Supergroup; on Llŷn it is represented by elements of the Monian Supergroup.

Coedana Complex

The Coedana Complex forms a much denuded outcrop, south-west from Dulas Bay to the vicinity of Llanfaelog. It comprises sillimanite-bearing gneiss and amphibolite, which are intruded by the Coedana Granite. The complex includes the highest grade metamorphic rocks on the island, separating the greenschist facies Monian Supergroup, to the north-west, from the blueschist facies rocks, to the south-east. Because of the contrast in grade and the pressure-temperature conditions of formation, W Gibbons and co-workers have argued that these belts must have been generated in separate terranes and subsequently juxtaposed by faults. The north-west boundary of the complex is a post-Arenig brittle fault zone whereas the south-east boundary is a wide ductile zone.

The gneisses are coarse-grained, foliated rocks with light and dark bands, which E Greenly referred to as ‘acidic’ and ‘basic’, respectively. They consist of metasedimentary and metabasic gneisses with local development of migmatites. Zones of a distinctly schistose facies are common and ghosts of other lithological associations, such as limestone, graphitic shale and orthoquartzite, are also discernible. Metamorphic mineral assemblages indicate that the rocks were formed under middle or upper amphibolite facies conditions. Pelitic migmatites contain sillimanite ± almandine ± biotite ± oligoclase ± quartz ± ilmenite. Basic gneisses are mostly amphibolites with hornblende ± oligoclase ± biotite ± garnet ± clinopyroxene ± ilmenite ± quartz ± apatite ± sphene.

These variations led Shackleton to suggest that the gneisses were the result of the high-grade alteration of the Monian Supergroup. However, the Coedana Granite, the largest intrusion in the central Anglesey zone, is not deformed and U-Pb zircon ages indicate that igneous crystallisation took place at 614 ± 4 Ma. Greenly recognised four lithological types in the intrusion — the normal granite, porphyritic granite, white mica granite and fine veins. The normal granite is generally pink and mottled with grey-green chloritic aggregates whereas the porphyritic granite is characterised by large orthoclase phenocrysts. The finer grained, white mica granite is a marginal facies to the main body and the finest grained granite generally occurs in veins. Although Greenly concluded that ‘the granite intruded and merged with the gneiss’, and that they were contemporary, clear relationships are not exposed. Fine-grained micaceous hornfelses, some bedded or banded, and quartzites are found at both contacts with the granite and as xenoliths within the granite. Greenly recorded some typical hornblende hornfels facies minerals consistent with a postulated depth of 3 to 7 km during emplacement. However, the structural and metamorphic evidence suggests that the granite emplacement postdated the gneisses. Locally, the south-eastern margin of the granite has been mylonitised by shearing associated with the high-strain zone.

Blueschist Terrane

The Blueschist Terrane forms a prominent belt on the south-east side of Anglesey, from the vicinity of Red Wharf Bay in the north-east to Newborough in the south-west. The belt was referred to as the Penmynydd Zone by Greenly, which he described as a zone of fine-grained mica schist with lenses of quartzite, foliated limestone, metagabbro and graphitic schist. The zone is characterised by strong deformation although some relict textures have survived in some of the larger bodies. More recent work has shown that the belt includes rocks of markedly different metamorphic grades, including ophiolitic epidote + sodic amphibole metabasites and phengitic metasedimentary rocks. Most importantly, the occur-rence of blue amphibolite and lawsonite has been distinguished close to the eastern margin. Such rocks indicate high pressure and low temperature conditions which are commonly asso-ciated with subduction zones. Analyses of ⁴⁰Ar/³⁹Ar amphibole and phengite data have yielded uplift ages of about 550 Ma for the blueschist event, and 590 Ma for an earlier greenschist metamorphism, possibly sub-sea floor alteration of ocean crust. Consequently, Greenly’s consideration that the association had been derived from the Gwna Group (see below) together with some acid volcanic rocks and granite is unlikely.

To the east, between Menai Bridge and Red Wharf Bay, the blueschist facies rocks are thrust over the Gwna mélange (Monian Supergroup), and to the west they are separated from the mélange by a steeply inclined mylonitic schist belt, the Berw Fault (Figure 4), with indications of sinistral transcurrent movement. The fault is the most westerly fault of the Menai Straits Fault System, a terrane boundary that was reactivated into brittle fault dis-placements throughout Phanerozoic time. It can be traced south-westwards into the Llŷn Shear Zone, on Llŷn, a mylonitic zone that juxtaposes the Gwna mélange against the Sarn Complex at the north-western edge of the Avalonian terrane. Most of the rocks within the zone are steeply inclined, fine-grained, recrystallised schist and phyllite; both margins are gradational.

Monian Supergroup

The age of the Monian Supergroup on Anglesey has been the subject of controversy over many years. Progressively, its designation to the Precambrian has been claused with the possibility of it being Cambrian. Essentially the sequence is more intensely deformed and metamorphosed than the Cambrian sequence on the mainland. Even so, there is sufficient evidence to suggest that the original lithologies were broadly similar, which together with the sparse palaeontological evidence and the increasingly reliable radiometric dates makes the elevation of the Monian Supergroup into the Cambrian unavoidable.

The subdivisions of the Monian Supergroup are essentially those proposed by Shackleton. Earlier, Greenly recognised the Bedded Series, the Gneisses and the Coedana Granite, within the Mona Complex, and considered that they each reflected successive periods of development. Thus, the Gneisses represented the floor on which the Bedded Series accumulated, and both were later intruded by the granites. Shackleton demonstrated a sharp increase in metamorphic grade from the Bedded Series into the Gneisses and argued persuasively that the latter were probably the highly metamorphosed representatives of the former.

The Monian Supergroup (Figure 5) comprises the Holy Island, New Harbour and Gwna groups, and is broadly equivalent to the Bedded Series of Greenly, who thought that the contacts between each unit in the lowest part of the succession were gradational and estimated a total thickness of some 6000 m. However, Shackleton recognised that although the sequence was continuous, the evidence of facing of the sedimentary structures indicated that it was upside down. The broad elements of Shackleton’s succession are generally agreed and these will be followed in this account.

The Holy Island Group has been subdivided into the South Stack, Holyhead and Rhoscolyn formations although the relationships between them remain ambiguous. The South Stack Formation, up to 400 m thick, is spectacularly exposed (Plate 4a), (Plate 4b) about South Stack on Holy Island (Figure 6) and less well exposed about Rhosgoch, south of Cemaes Bay, on the north side of Anglesey. It comprises a sequence of low grade metamorphosed, grey to white turbiditic sandstone with interbedded blue-grey silty mudstone; its base is not exposed. The turbiditic sandstones form flaggy to massive beds with graded bedding, parallel and cross-lamination and, in places, infilled scours at the base. They are interpreted as submarine outer fan deposits that accumulated in a basin considerably larger than the area enclosing the current outcrops. The possible occurrence of Skolithus-type burrows has raised the possibility of a Cambrian age. The most prominent vector of sediment derivation is from the north-west, and it has been proposed that the basin was bounded by a microcontinent in that direction.

The Holyhead Formation, up to 500 m thick, conformably overlies and is lithologically similar to the South Stack Formation. Exposure is restricted to Holy Island (Figure 6), where it consists of thick, interbedded, metamorphosed turbiditic sandstone and silty mudstone. Quartzites, generally 10 to 25 m thick, are subordinate, but include the Holyhead Quartzite, 150 to 200 m thick, which is correlated with a quartzite, 50 m thick, in the core of the Rhoscolyn Anticline. The quartzites are typically massive and generally structureless, but graded bedding and flute clasts are preserved in places. They are interpreted as coarse-grained turbidites from a major channel system in a mid- to inner-fan setting. Detrital zircons from the South Stack and Holyhead formations have yielded (SHRIMP) a depositional age of 501 ± 10 Ma. The Rhoscolyn Formation, up to 300 m thick, is dominated by turbiditic deposits with well-developed, plane parallel, convolute and cross-laminations indicating a more typical midfan facies.

The New Harbour Group forms the dominant outcrop across north-west Anglesey. Its thickness is estimated as up to 2500 m, but this is extremely conjectural because of the complex internal structures. In the south, the main component is highly deformed chlorite-mica schist with little evidence of the original lithologies or depositional environment. However, in the north, near Amlwch, the sequence is less deformed and metamorphosed. A lower argillaceous sequence, the Bodelwyn Formation, grades up into coarse- to fine-grained volcaniclastic sandstones, the Lynas Formation, in coarsening- and thickening-upward cycles. At some localities, as on Holy Island, thin beds of jasper, pillowed basalts and serpentinite bodies are recorded. The geochemical signature of the volcanic rocks indicates a subduction-related island arc setting.

The Skerries Formation forms three distinct outcrops in north Anglesey, about Church Bay, west of Cemaes Head and west of Bull Bay. All of these outcrops juxtapose the formation, probably in faulted contact, with New Harbour Group or Gwna Group strata. The formation has been subdivided into the Church Bay Tuffs, massive, tuffaceous silty mudstones, and the Skerries Grits, dominated by massive poorly bedded volcaniclastic sandstones with subordinate conglomeratic layers and tuffaceous siltstone beds. The outcrop pattern and contrasting lithologies of the Skerries Formation have been attributed to the interdigitation of a proximal and more distal facies of the New Harbour Group. This interpretation is supported by the petrographical similarities between constituent formations, which indicate a similar provenance. The limited palaeocurrent data suggests that deposition occurred in a north-east–south-west-trending basin, on a fan that prograded from the north-west.

The Gwna Group is one of the most striking elements of Monian Supergroup stratigraphy. It forms two main outcrops, north of Malltraeth Bay, on Anglesey, and between Porth Dinllaen, on Llŷn, and Bardsey Island, where it is the main representative of the supergroup. The group is dominated by a mélange, which contains allochthonous clasts, from millimetres to kilometres across, of a wide range of igneous rocks and sedimentary rocks in a sand- or mud-grade matrix. Greenly considered the mélange to be a tectonic breccia, but Shackleton thought that its distribution, the largely unstratified matrix, and its relatively sharp contacts comprised persuasive evidence that it is an olistostrome or slide breccia resulting from lateral displacement. Locally, a crude, ghost-like internal stratigraphy within the dominant clast lithology can be determined. The emplacement of such a deposit was clearly initiated by a catastrophic event, possibly tectonic instability during closure of the basin in which the Monian Supergroup accumulated, causing uplift of the succession and its subsequent collapse. The possible Coedana Complex signature of some granitic clasts suggests the complex was exposed at the time of the disruption and, consequently, the mélange was deposited after 614 ± 4 Ma.

In Cemaes Bay, north-west Anglesey, an outcrop of the Gwna Group comprises blue-grey silty mudstone with flaggy sandstone and ooidal and stromatolitic limestone bands, although it is possible that this apparently ordered sequence is itself a large raft. The stromatolites have been interpreted as late Precambrian or early Cambrian. Pillowed basalts, breccias and hyaloclastites are a common component throughout the entire Gwna Group outcrop and the exposures at Llanddwyn Island on Anglesey, and at Porth Dinllaen and Aberdaron on Llŷn, are particularly striking. The pillowed basalts generally contain jasper-filled cavities and, in the vicinity of fault planes, sheared limestone and dark blue-grey silicified mudstone clasts have been determined.

In Llŷn, the mélange is very well exposed in the coastal section between Porth Dinllaen and Trwyn Bychestyn, and it forms all of Bardsey Island (Plate 5). The mélange is estimated to be up to 3000 m thick and crude internal subdivisions, based on clast types, can be distinguished. Clasts of bedded, fine-grained volcaniclastic siltstone and sandstone (‘Gwyddel Beds’), up to 800 m across, are prominent at the south-western edge of Llŷn where, together with smaller clasts of quartzite, basalt, limestone and red mudstone, their distribution can be mapped out (Figure 7). Basaltic clasts of massive and pillowed lava, breccia and hyaloclastite are common throughout, and whole and fragmented basaltic pillows within a fine-grained micritic limestone matrix, as on Dinas Fawr, are particularly distinctive. Limestone clasts, clean white and bedded or dolomitised and brown, and pyritic lithologies are common; clasts of white quartzite and red mudstone, although not common, are easily discernible. Blocks of sheared lenticular granite, up to 100 m across, are recognised only in the cliff sections along the north-western and western coasts on Bardsey Island, and Wylfa Head. It has been proposed that there were two major collapse events in the emplacement of the mélange and that ‘flow’ was directed towards the east, although this is difficult to substantiate. The wide range in composition of the clasts indicates sediments characteristic of an active plate margin and rocks of possible ‘exotic’ oceanic origin.

Much of the difficulty in interpreting the relationships of the various components of the Monian Supergroup is the result of deformation and metamorphism. Greenly considered the largest folds to be of the order of kilometres in wavelength and that the great folds seen in the cliff sections about South Stack lighthouse (Plate 4a), (Plate 4b) were secondary. However the examination of any section through the South Stack Formation shows that subsets of minor folds are common, and locally the intensity of associated thrusting has totally destroyed the original bedding. In such a context, a schistose texture is commonly well developed. More recent studies on Holy Island suggest that deformation and greenschist facies metamorphism developed in response to south-east-directed shear. The initial stages of deformation resulted in a bedding-parallel S₁ fabric and sigmoidal tension fissures. It was followed by south-east-verging F₂ folds, including the Rhoscolyn and Penrhyn Mawr anticlines (Figure 6), with north-west inclined foliation characterised by an S₂ pressure solution and crenulation schistosity. Finally, the ductile deformation was replaced by an episode of brittle, south-east-directed thrusting.

The ages of the deformation events continue to be controversial. The clearest stratigraphical evidence is that the supergroup is:

unconformably overlain by Ordovician (Arenig) sedimentary rocks that are less deformed and metamorphosed
that boulders of New Harbour Group rocks occur in mid Ordovician (Caradoc) conglomerates
it contains possible Lower Cambrian acritarchs and stromatolites in the Gwna Group and possible trace fossils in the South Stack Formation

The general equivalence of the lithologies of the Monian Supergroup with both those of the Cambrian, south-east of the Menai Straits Fault System, and with the Cahore Group of south-eastern Ireland, has long been accepted. It is corroborated by the high-resolution microprobe data on zircons from the South Stack and Holyhead formations. Such a correlation would suggest that the separation of Avalonia from Gondwana did not take place until late Cambrian-?early Ordovician times, and that the more intense deformation and metamorphism of the supergroup, in comparison with the Cambrian on the mainland, was the result of localised pre-Arenig shearing during amalgamation. However, the evidence remains that Anglesey basement-type rocks were incorporated into lowermost Cambrian sediments in Arfon and indicates their exposure and proximity at that time.

Avalon Terrane

The Avalon Terrane lies to the south-east of the Menai Straits Fault System and includes all the remaining Precambrian outcrops in mainland Wales and the Welsh Borderland. The lithologies in these outcrops display marked differences, from slightly metamorphosed sedimentary rocks to gneisses, with both volcanic rocks and more deepseated magmatic intrusions. The intermediate to acid calcalkaline igneous rocks have been intepreted as the products of subduction-related arc volcanism. The general lithologies are considered to reflect the development of sedimentary basins and the accumulation of much volcaniclastic sediments on a dominantly igneous basement. The small and disparate outcrops inhibit more detailed palaeogeographical interpretation.

The Sarn Complex, of mainly calc-alkaline granitic to dioritic rocks, lies at the north-western edge of the Avalonian terrane. It forms a narrow outcrop on Llŷn, between Porth Dinllaen and Aberdaron, and is separated from the Gwna Group, to the north-west, by the Llŷn Shear Zone (Menai Straits Fault System). To the east, it is overlain by lower Ordovician, Arenig, sedimentary rocks. The most north-easterly exposures, on Mynydd Cefnamlwch, are of a pale adamellitic granite, which is referred to as the Sarn Granite. To the south-west, both homogeneous and foliated gneissic diorites, with unfoliated granite and dolerite veins, are exposed. Many of the exposures within the complex are of mixed, commonly xenolithic, hybridised, biotite-rich rocks, locally with hornblende and clinopyroxene. One of the best exposures, at Meillionydd, shows composition ranging from diorite to granite with local foliation. The petrology and geochemistry has suggested emplacement in a subduction-related arc environment. A gabbro from within the complex has yielded a U-Pb age of 615 ± 2 Ma. The final disposition of the terranes along the Menai Straits Fault System was probably accomplished prior to the formation of the Sarn Complex and the blueschists.

The Parwyd Gneiss forms a restricted outcrop on the east cliff of Parwyd cove. It comprises coarse, foliated orthogneiss, mainly hornblende-garnet amphibolite, with much green hornblende and lesser pale pink garnet crystals. Locally the composition is more acidic and invariably these rocks display intense cataclasis. The Parwyd Gneiss has been included within the Sarn Complex, although they may not be related. Even so, a whole-rock, Rb-Sr isochron age of 542 ± 17 Ma is comparable with the 549 ± 19 Ma age obtained from a granitic rock within the complex. However, both dates must be regarded as minimum ages, markedly different from the age of the metagabbro within the complex.

The Arfon Group crops out in two areas in Caernarvonshire, in close proximity to the Menai Straits Fault System, and in small outliers on Anglesey. On the mainland, the larger outcrop, between Pen y Groes and Deiniolen, lies on the south-east side of the Aber-Dinlle Fault and the smaller outcrop lies close to Port Dinorwic, on the south side of the Dinorwic Fault. Four formations have been recognised, the Padarn Tuff, Minffordd, Bangor and Fachwen formations, but only the Padarn Tuff Formation has yielded a Precambrian age.

The Padarn Tuff Formation (Plate 6) consists mostly of grey-green, bleach-weathered, acidic ash-flow tuffs. Despite good exposure, there is no indication of separate flow units. The tuffs are strongly welded and contain conspicuous crystals of bipyramidal quartz and albite feldspar. Locally the tuffs are intruded by high level rhyolite intrusions and, elsewhere, thin beds of possible air-fall tuffs have a very restricted distribution. The orientation of welding fabrics in the well-exposed area about Llyn Padarn suggests that the tuffs are at least 800 m thick, and geophysical models indicate a possible thickness of up to 2000 m. The great thickness of the tuffs and the uniform lithology suggests that the rapidly erupted tuffs were entrapped in a volcanotectonic depression. U-Pb dating of volcanogenic zircon indicates that eruption occurred at 614 ± 2 Ma, which confirms a Precambrian age broadly coeval with the Coedana Granite and raises the possibility that Anglesey was close to its current position in early Cambrian times. On Anglesey, the Bwlch Gwyn Tuff (Greenly’s ‘felsite’) and the Baron Hill Formation comprise closely comparable acidic ash-flow tuffs that form isolated exposures within the Berw Shear Zone. However, their contacts are not exposed and it is possible that they occur within fault slices at the edge of the Monian Composite Terrane.

In southern Snowdonia, probable Precambrian rocks, the Brynteg Volcanic Formation, were proved in the Brynteg Borehole, sited on the axis of the Harlech Dome. The sequence comprises sedimentary and volcaniclastic rocks with minor primary volcanic rocks of intermediate composition that were possibly generated in an island-arc setting.

South-west Wales and the borders

The Precambrian, Avalonian terrane rocks of Pembrokeshire occur in a series of inliers from St David’s eastwards, in north Pembrokeshire, and some smaller inliers, from Talbenny eastwards, in south Pembrokeshire. Away from the limited coastal sections, both sets of outcrops are poorly exposed. The rocks are intensely altered, and their relationships and associations are extremely difficult to distinguish. It is not surprising that the outcrops have received so little attention in recent years. The rocks are mainly the products of igneous activity. In 1877, Hicks divided them into two units, the Pebidian, comprising lavas and tuffs, and the Dimetian, a suite of intrusions. These names have persisted to the present day. In the outcrops in the vicinity of St David’s, the Precambrian age is drawn from their position beneath the Cambrian basal conglomerate. In the southern outcrops, a recent radiometric age possibly confirms this conclusion.

The Pebidian comprises volcaniclastic sedimentary rocks, pyroclastic rocks and lavas, which are best exposed south-west of St David’s, in the coastal section westwards from Porthllysgi. Nearly 100 years ago, Green subdivided these outcrops into four units, which would probably be regarded as groups in modern nomenclature. In upward succession, they are the Penrhiw, Treginnis, Caerbwdy and Ramsay ‘groups’. The rocks were described as ‘sericitic tuffs’, ‘greenish acid rocks with bands of halleflinta and conglomerate’, ‘hard gritty rocks with abundant trachytic pumice’ and ‘red keratophyre boulders’, and are difficult to categorise in modern terms. A cursory examination shows that the sequence has been subjected to considerable tectonism and secondary alteration, which is locally so intense that most of the original structures have been obscured. In a recent petrochemical study, the rocks have been characterised as basic to intermediate lavas with related volcaniclastic rocks, and acidic tuffs with minor basic intrusions.

East of St David’s, in the Hayscastle inlier, a lower group of andesitic tuffs overlain by a group of rhyolitic tuffs and flowbanded rhyolite has been distinguished. The rhyolites outcrop between Roch Castle and the Cleddau valley and include the prominent crags at Maiden Castle (Plate 7). The rhyolites are green-grey, bleach-weathered, fine-grained and recrystallised; they are possibly overlain by flaggy fine-grained tuffaceous siltstone. The determination of original lithological characters in the rhyolites at outcrop is made difficult by intense jointing and nodular recrystallisation. In south Pembrokeshire, Pebidian rocks are restricted to thrust slices in the Variscan thrust zone, in a small inlier near Benton. They are highly altered, fine-grained tuffs and breccias with flow-banded and spherulitic (possible nodular) rhyolites.

The Dimetian comprises a suite of granite and diorite intrusions within the Pebidian but, because of poor exposure, the exact relationships are difficult to determine. The most significant intrusion is the St David’s granophyre, which is much altered and sheared and is now a coarse-grained, quartzo-feldspar aggregate with much chlorite. An associated quartz porphyry is regarded as petrographically similar, possibly representing a late-stage intrusion. At Hollybush, a medium- to coarse-grained diorite intrusion, with biotite and large hornblende crystals, has been included within the Dimetian suite.

The inliers at Talbenny and Johnston include diorite and quartz diorite intrusions, which locally grade into rocks of more granitic composition. Thin dykes of quartz dolerite with chilled margins are common. In the Bolton Hill quarry near Johnston, the thrust contact between the diorites and the Carboniferous Coal Measures is frequently exposed during excavation. Close to the Variscan thrust zone, for example near Talbenny, these rocks have been overprinted with a pronounced gneissic texture.

The Precambrian Coomb Volcanic Formation of the Llangynog inlier, south-west of Carmarthen, comprises devitrified rhyolite lavas that are flow banded in places. They are overlain by shallow water, volcaniclastic siltstone, which contains medusoids and trace fossils of Ediacaran age. Basaltic lavas, autobreccias and rare hyaloclastites, which are characteristic of subaqueous emplacement, and acidic tuffs, are interbedded within the siltstone. The rocks are in faulted contact with Ordovician strata and are overlain unconformably by both Cambrian and Old Red Sandstone strata.

In Powys, Precambrian strata crop out in two fault-bound inliers within the Church Stretton Fault Zone. The Old Radnor inlier exposes intensely brecciated sandstone and silty mudstone that are closely comparable with the type Longmyndian sequence seen farther north. The dominantly sandy Strinds Formation and the finer grained Yat Wood Formation have been interpreted as products of a braided alluvial delta and a possible subaqueous delta, respectively. The composite Hunter and Stanner inlier comprises dolerite and gabbro sheets intruded by fine-grained felsite and granophyric dykes. Some of the dykes closely resemble granophyres in the nearby Uriconian of Shropshire, and similar lithologies have been determined as clasts in Longmyndian conglomerates. The acid intrusion on Stanner Hill has been dated at 702 ± 8 Ma. The patterns of alteration in these rocks suggest they may possibly be part of a subvolcanic complex.p

Chapter 3 Cambrian

In earliest Lower Palaeozoic times, Wales, together with Belgium, England, southern Ireland and the Avalon area of eastern Newfoundland, formed part of a north-west-facing continent on the south-east side of the Iapetus Ocean at a latitude between 40º and 60º south (Figure 3). The intense volcanic, tectonic and metamorphic activity that occurred at the edge of this continent in late Precambrian times had dissipated by the time it was encroached upon by a major marine transgression with deposition of a carbonate-poor sequence in early Palaeozoic times. The elements of this continental plate (Avalonia) and its proximity to the Gondwana continent are still uncertain; it was separated from Baltica by the Tornquist Sea. On the north-west side of the Iapetus Ocean the Scottish Lower Palaeozoic carbonate-rich sequence accumulated at latitudes between 25º and 40º south on the edge of the North American continent of Laurentia. In Wales, the structural grain of the Precambrian basement influenced the development of a basin in which shallow and deep water sediments accumulated throughout early Palaeozoic times. The basin was defined by the Menai Straits Fault Zone at the edge of the Irish Sea Landmass in the north-west, and the Welsh Borderland Fracture Zone, including the Pontesford, Church Stretton and Twyi lineaments, at the edge of the Midland Platform in the south-east. Both fault zones continued to influence basin development and sedimentation throughout early Palaeozoic times (Figure 8).

The Cambrian System was defined in Wales, its historical type area, in the early years of the 19th century. However, in parts of the succession, particularly the lower parts, there is a dearth of faunas and the stratigraphy is difficult to establish. Consequently, the base of the Cambrian is now formally defined in south-east Newfoundland. The oldest rocks assigned to the Cambrian system in north Wales overlie an unconformity, but it is possible that some of the rocks below the unconformity may also be of Cambrian age; there is no direct contact between undisputed Cambrian rocks and the Monian Supergroup on Anglesey. There are no agreed international series subdivisions of the Cambrian System. The sequence has been divided into Lower, Middle and Upper parts (Table 2) and these have been casually regarded as series, although on the global scale they have been used inconsistently. Consequently, the British regional series names Comley, St David’s and Merioneth, respectively the Lower, Middle and Upper Cambrian as applied in Britain, have been used informally. Sea level rose progressively from early Cambrian times, with minor fluctuations, into late Cambrian times when three pulses are recognised, possibly caused by a fluctuating ice sheet in the southern hemisphere. Subsequently, there was a pronounced fall in sea level. The radioactive calibration of the time scale has been difficult because of the lack of suitable lithologies and the resetting of isotope systems in metamorphic events, but Welsh rocks have contributed important data nonetheless. On the geological timescale currently in use (2004) the Cambrian system spans the interval 542 to 488 Ma.

Cambrian strata crop out in the Harlech Dome, Arfon and Llŷn, in north Wales, and in the vicinity of St David’s in Pembrokeshire and Llangynog, south-west of Carmarthen, in south Wales. Local lithological correlation is good, although wider correlation is hindered by poor biostratigraphical control in the lowermost beds. The rocks contain some of the earliest remains of marine fossils. The communities are representative of the European Avalonian Province and are totally distinct from those of the North American Laurentian (Pacific) Province observed in equivalent strata in Scotland. The shallowest marine environments generally contain lingulate brachiopods whereas trilobites inhabited the more offshore settings. Trilobites only became firmly established in late Comley times. The richest faunas are found in the dark, blue-grey mudstone of the St David’s Series, which host a variety of sponges, hyoliths, echinoderms and bradoriid ostracods as well as several trilobite species. Most of these animals were benthic and relatively local in distribution, but some species of the large paradoxidid and tiny blind agnostid trilobites are widely distributed, being known from Scandinavia, Siberia, Spain and Newfoundland; they may have been bathypelagic, swimming widely in deep waters. These are the most useful macrofossils for dating the rocks. In the Merioneth Series, black mudstone contains the specialised olenid trilobites that evolved to occupy the most poorly oxygenated environments and characterise the ‘Olenid Biofacies’.

Comley Series

In Arfon, Comley Series (Lower Cambrian) strata overlie the Padarn Tuff Formation in the outcrop between Deiniolen and Penygroes, where they comprise the Fachwen Formation (Figure 9), and in the vicinity of Bangor, where they form the Minffordd and Bangor formations. The contact near Fachwen on the north side of Llyn Padarn was considered to be the Precambrian-Cambrian boundary in the 19th century. The lower formations thicken dramatically from some 50 m on the east side of Llyn Padarn, to about 500 m on the west side and to 2000 m near Bangor; this increase is thought to reflect rapid subsidence associated with intense tectonic activity. The lowermost beds, conglomerates and sandstones, overlie Precambrian welded ash-flow tuffs (Padarn Tuff Formation), and locally thin beds of fine- grained air-fall tuff and ash-flow tuff indicate that volcanism recurred sporadically in the vicinity. The conglomerates, consisting mainly of subangular clasts of welded tuff, and the sandstones with distinctive quartz crystals derived from the underlying tuffs, probably represent rapidly accumulating alluvial fan and fluvial deposits, composed of debris derived from local fault scarps and fluvial systems. Well-rounded clasts of quartzite, mica-schist, granite and jasper indicate more distant derivation from the Precambrian on Anglesey. Towards the top of the sequence, the increasing frequency of silty mudstone interbeds suggest more general subsidence with the establishment of a more extensive marine basin in which mudstone of the overlying Llanberis Slates Formation was deposited.

The Llanberis Slates Formation is the source of the slates that have been worked in the vicinity of Bethesda, Llanberis and Nantlle for over 200 years and accounted for the growth of these communities. The distribution of the formation at outcrop is clearly defined by the numerous trials and small quarries that were excavated beyond the main quarries at Penrhyn and Dinorwic (Plate 8a), (Plate 8b). The formation consists mainly of blue-grey and purple silty mudstone, which accumulated in a broad marine basin that deepened to the south-east. Intermittent instability in the vicinity of the distant shoreline, probably to the north-west, caused incursions of coarse sands in turbidity flows, which locally fill channels in the underlying mudstones and include distinctive rip-up clasts of silty mudstone. These coarse-grained sandstones formed major obstructions to slate extraction and were given local names by the quarrymen; for example, the Dorothea Grit at Nantlle was considered to be the equivalent of Red Grit (Gwenithfaen-Goch) at Penrhyn, but such correlations are difficult to substantiate.

Bedding in the purple silty mudstone is locally well developed, commonly accentuated by sharply contrasting green bands as a result of the reduction of iron; similar reduction produced the characteristic green spotting, which has been used to estimate strain across the slate belt. Large fresh surfaces of slate produced by modern cutting equipment commonly reveal a spectrum of bedding fabrics, from slump folds in continuous beds of silty mudstone and muddy siltstone, to totally disaggregated sediment. The latter probably produce the best slates because they contain no well-defined bedding planes or other lithological contrasts to interrupt the cleavage. Trilobites found in green slates near the top of the formation in Penrhyn Quarry include Pseudatops viola, and they provide the lowest biostratigraphical evidence in the sequence. The fauna has been assigned to the top of the Olenellid Zone, in the middle of the Comley Series. Estimation of the thickness of the Llanberis Slates Formation is difficult because it lies between two more competent formations, and consequently channelled a great deal of the strain when the sequence was folded. However, a minimum of 380 m has been indicated on the published maps.

These lithological subdivisions of the Comley sequence can be traced southwards from Bethesda to Nantlle with some confidence. However, farther to the south, into the Harlech Dome, correlation is more difficult. Here, the lowest part of the sequence comprises pebbly, cross-bedded sandstone with interbedded siltstone and mudstone (Dolwen Formation); the base is not exposed but it was proved in a borehole, overlying the Brynteg Volcanic Formation of probable Precambrian age. The base is defined by conglomeratic volcaniclastic sandstones, although the contained clasts are compositionally different from the underlying strata. The overlying sequence of interbedded sandstone and siltstone indicates several phases of delta building, and an overall gradual progression into deeper water settings. The only body fossil is a specimen of Platysolenites antiquissimus, of Comley age, recovered from the Brynteg Borehole. At the top of the sequence, laminae with euhedral feldspar crystals and fragments of quartz euhedra have been intepreted as air-fall tuffs although the site of the vent has not been determined.

The lower sandstone sequence grades up into blue and purple silty mudstone (Llanbedr Formation), which has been worked for slates at a number of localities near Harlech. The overlying Rhinog Grits Formation, up to 780 m thick, forms the castellate ridges of Rhinog Fawr, Rhinog Fach and Y Llethr that dominate the local topography (Plate 9). The formation is characterised by massive, locally pebbly, graded turbiditic sandstone and mudstone, thin flaggy, fine-grained sandstone interbeds and beds of well-sorted, coarse-grained sandstone. Many of the pebbles are of lithologies similar to Anglesey Precambrian rocks. The formation is spectacularly exposed about the Roman Steps, in Cwm Bychan (Plate 10). The dominance of the turbiditic process in the accumulation of this sequence was first demonstrated in the classic studies by Kuenen in 1953 and Kopstein in 1954. Elements of the Bouma cycle are present throughout various combinations and in the sole marks and current bedding indicate deposition in a submarine fan complex, with sediment transport mainly from the north-east.

Similar coarse sandstones forming the Hell’s Mouth Formation, crop out on the west side of St Tudwal’s peninsula on Llŷn, although their base is not seen (Figure 10). The beds are crudely graded, with cross-laminations, convolute bedding and groove and flute casts which are commonly loaded. In most of the formation only trace fossils are present, but trilobites (Hamatolenus (Myopsolenus) douglasi, Kerberodiscus succinctus and Serrodiscus ctenoa?) associated with hexactinellid sponge spicules and a single inarticulate brachiopod near the top indicate the upper part of the Protolenid-Strenuellid Zone of the Comley Series (Table 2), which is younger than the Pseudatops viola horizon at the top of the Llanberis Slates Formation. However, the broad correlation suggests that the latter developed distally to the fan complex of the Rhinog Grits Formation at the end of Comley Series times.

In Pembrokeshire, the Comley Series (Figure 11) is up to 300 m thick and is represented by the Caerfai Group, which outcrops in both the St David’s and Hayscastle anticlines. A basal conglomerate, which contains pebbles of quartz, quartzite and acid tuff, displays channel structures and large-scale planar cross-stratification, with the foresets directed southwards. These structures, together with the occurrence of vertical cylindrical burrows of Skolithus, have been interpreted as indicating intertidal deposits. The conglomerates are overlain by green feldspathic sandstone (St Non’s Sandstone) with detrital blue amphibole, red mudstone (Caerfai Bay Shales), and plane and cross-laminated, micaceous, feldspathic, purple sandstone (Caerbwdy Sandstone) (Plate 11). The red mudstone forms a convenient marker and is conspicuous in the cliffs at Caerfai, at Castell on Ramsey Sound and at Cwm Mawr near Newgale. Pale tuffs interbedded with the red mudstone have given a radiometric age of 519 ± 1 Ma. The mudstone is the lowest unit from which fossils, although rare, have been recorded. These include Lingulella primaeva and the bradoriid Indiana lentiformis of probable late Comley age. The sequence west of St David’s reflects a nearshore shelf environment, but the lack of symmetrical wave ripples suggests that deposition occurred below wave base (100 to 200 m).

At Llangynog, south-west of Carmarthen, a thin conglomerate and green feldspathic sandstones, the former containing well-rounded pebbles of igneous rocks, overlies the Precambrian rhyolites and volcaniclastic sedimentary rocks and are considered to be of Comley age. St David’s Series

St David’s Series

In earliest St David’s Series (Middle Cambrian) times, a regressive episode about the Harlech Dome is reflected in the thin manganese-bearing shales at the base of the Hafotty Formation. The contact is well exposed at the Hafotty Mines, on the hillside above Llanaber, where coarse-grained, thickly bedded turbidites of the Rhinog Grits Formation pass abruptly into grey, thinly bedded, fine-grained, cross-bedded sandstones and siltstones with distinctive concentrations of manganese, which has been worked extensively in the vicinity. Above, the formation consists mainly of banded siltstone, mudstone and fine-grained sandstone, possibly distal turbidites. One finely laminated ore bed contains 12.3 per cent manganese. The manganese was precipitated in a carbonate form with silica, and is quite unlike the spheroidal manganese nodules on modern sea floors. It has been postulated that the manganese was derived from the intense weathering of a gneissic landmass. Conversely, a diagenetic origin of the manganese has been proposed, and such a model would rule out the need for a manganese-rich provenance. However, it is difficult to envisage a shallow marine basin in which reducing conditions could develop within a sequence that mainly reflects more dynamic processes. Manganese-rich shales and sandstones (Trwyn y Fulfran Formation) occur in the equivalent position overlying the Hell's Mouth Grits Formation on St Tudwal’s peninsula, Llŷn.

Subsequently, incursions of turbiditic sands occurred in both the Harlech area and in Llŷn, although their direction of transport was contradictory, to the north-west and south-west, respectively. The coarse pebbly turbiditic sandstones (Barmouth Formation and Cilan Formation) are similar to the earlier submarine fan deposits; they grade up through thin turbiditic sandstone with interbedded purple and green shale (Gamlan Formation, Ceiriad Formation), with persistent indication of manganese precipitation, into black, pyritous mudstone with lenses and laminae of sulphides and few fine-grained sandstones (Clogau Formation, Nant-y-big Formation). The black mudstone of the Clogau Formation forms a ‘slack’ feature at outcrop and is easily traced around the flanks of the dome. The pyritous character suggests quiescent accumulation in a reducing environment, and it contains the lowest rich fossil fauna in Wales, which is dominated by the trilobite Paradoxides with the small eyeless agnostid trilobites. In addition, the mudstone hosts the main concentration of gold, copper, lead and zinc vein mineralisation through the Dolgellau Gold Belt on the north side of the Mawddach estuary. North of Llŷn and Harlech there is no evidence of St David’s Series strata.

In Pembrokeshire, the series is represented by the Solva and Menevian groups. The Solva Group comprises green and purplish sandstone, siltstone and mudstone, which are well exposed in Solva Harbour and on the west side of Caerbwdy Bay. Apart from a possible unconformity in Caerfai Bay, the sequence conformably overlies the Caerbwdy Sandstone Formation. The sedimentary structures and the trace fossils indicate a shallow marine environment and the trilobites, inarticulate brachiopods, and acritarchs indicate the Paradoxides oelandicus Biozone and possibly the Ptychagnostus gibbus Biozone (Table 2). The abrupt appearance of the trilobite fauna at the base of the Solva Group is related to a change in facies. The Menevian Group is transitional with the underlying Solva Group and consists of thin sandstones and mudstones overlain by dark grey shale with thin beds of lenticular sandstone and fine-grained, pyroclastic debris and, at the top, coarse-grained, massive sandstones with interbedded shales and thin turbiditic sandstones. The dark grey shales are equivalent to those of the Clogau Formation in the Harlech district, and suggest accumulation in a quiescent, marine, oxygen-poor environment, probably associated with deepening of the basin during continued transgression. The shales contain an extensive fauna that indicates the zones of Tomagnastus fissus, Hypagnostus parvifrons and Ptychagnostus punctuosus. The uppermost coarse sandstone contains the brachiopod Billingsella hicksii. The overlying Lingula Flags Formation (see below) consists of alternating beds of siliceous siltstone and micaceous mudstone. It is difficult to correlate, with unconfirmed records of ‘Paradoxides’ and ‘Concoryphe’ that suggest a Mid Cambrian age for at least the lower part of the formation.

Merioneth Series

In the area of Harlech and Llŷn, the period of mud deposition at the end of St David’s Series times was terminated by the third major incursion of mainly fine- to medium-grained turbiditic sand. The incursions overstep progressively to the north so that between Nantlle and Bethesda the sandstones considered to be at the base of the Merioneth Series (Upper Cambrian) lie directly on the Llanberis Slates Formation (Comley Series) (Figure 9). On St Tudwal’s peninsula, the lowermost coarse-grained, turbiditic sandstones, with rounded pebbles of St David’s Series limestones, grade up into thinly bedded, finer grained turbiditic sandstones with interbedded shales containing the trace fossil Nereites?, which indicates a relatively deep-water environment. A similar upward gradation is repeated in the Harlech district and is thought to reflect a late St David's Series regression on the adjacent shelf.

In the Harlech district, the Maentwrog Formation is assigned to the lower part of the Merioneth Series. It comprises grey silty mudstone interbedded with coarse siltstone to fine sandstone turbidites, with thin reworked sandstone laminae and predominantly mudstone above, and passes upwards into the Ffestiniog Flags Formation. The trilobite Olenus, associated with agnostids, occurs at several levels, and indicates at least three subzones of the early Merioneth Olenus Biozone. To the north, between Nantlle and Bethesda, these beds appear to pass laterally into the thickest expression of turbiditic sandstones, which have been given various local names by the quarrymen but are now grouped into the Bronllwyd Grits Formation. Palaeocurrent indicators in the sandstones determine dominant derivation from the north-west. The formation includes manganiferous shales in its lower part, and many of the sandstone beds have conglomeratic bases, with rounded quartz and quartzitic pebbles, up to 20 mm, and silty mudstone flakes, up to 0.25 m across. It is well exposed, through a series of steeply inclined folds in the steep north-facing slope on the south side of Llyn Peris. From Llŷn to Arfon, these variable turbiditic sandstones are overlain by massive or poorly bedded silty mudstone, thinly bedded quartzose siltstone and fine-grained sandstone with ripple marked bedding planes, low-angle cross-lamination and plane parallel lamination (Marchllyn and Ffestiniog Flags formations). The sandstones were deposited mainly as turbidites derived from the north but were modified by wave and current action. This formation is host to the Coed y Brenin porphyry copper deposit just north of Dolgellau. In the area about Nant Ffrancon, a thicker, quartzose sandstone, the Carnedd y Filiast Grit Member (Figure 9), occurs near the top of the Marchllyn Formation, and forms a distinctive feature in the back wall of Cwm Graianog, where the ripple-marked top is separated from the slightly discordant base of an Ordovician sandstone by a thin band of siltstone (Plate 12). The occurrence of the trace fossils Cruziana, Rusophycus and Skolithus indicate deposition in a shallow-water setting with current flow towards the south-west; infilled mudcracks indicate temporary emergence.

Around the Harlech Dome, the Ffestiniog Flags Formation grades up through sandy and silty beds with abundant brachiopods (Lingulella davisii) into dark grey to black, carbonaceous, pyritic and locally uraniferous mudstone (Dolgellau Formation) at the top of the Cambrian sequence. The mudstone, up to 150 m thick, contains laminae, less than 1 mm thick, of disseminated pyrite and siltstone, which define bedding. These beds reflect slow accumulation, mainly in a pelagic setting, in poorly oxygenated bottom waters with restricted circulation. They contain relatively rich trilobite and brachiopod faunas, which indicate the presence of about 12 subzonal assemblages from the P. spinulosa to Acerocare zones (Table 2). Two beds of reworked volcanic rocks, one near the top of the Dolgellau Formation and the other almost coincident with the Dolgellau-Doly-cyn-afon transition, have given radiometric ages of 491 Ma and 489 Ma, respectively.

In south Wales, the upper part of the Cambrian sequence, the Lingula Flags Formation, crops out both in Pembrokeshire and in the Llangynog inlier, south-west of Carmarthen. In both localities, siliceous siltstone and micaceous mudstone are a distinctive component, and cross-laminated, siliceous sandstones indicate deposition in shallow water. The high mica content may reflect local shoreline erosion of Precambrian metamorphic rocks at the southern margin of the Welsh basin. In Pembrokeshire, the sequence is some 700 m thick near St David’s, and the lowest beds contain the earliest Merioneth Series trilobite, Agnostus pisiformis, representing a zone that has not yet been proved in north Wales. Conversely, the higher zones recognised in north Wales cannot be clearly identified, but the abundance of the brachiopod Lingulella davisii supports a broad correlation. To the east of St David’s, the entire Upper Cambrian sequence is overstepped by lower Ordovician (Arenig) strata near Hayscastle, but at Llangynog, black shales, which are lithologically and faunally similar to those of the Dolgellau Formation in north Wales, contain olenid trilobite faunas representing four or five Olenid subzones.

Chapter 4 Ordovician

Ordovician strata form extensive outcrops throughout Wales in which the sequence shows remarkable lateral variation in thickness and lithological content. The form of the Welsh basin throughout Ordovician times remained broadly similar to that which had been established in late Precambrian times. However, the influence on the pattern of sedimentation of both the Irish Sea Landmass in the north-west and the Midland Platform in the south-east was periodically affected by sea level changes. Mudstone deposition, probably the result of sea-level rise, was widespread through the Tremadoc but the bioturbation and lower carbonate content suggest more oxygenated water than previously. Sea level fluctuated in the basin throughout Tremadoc to Llanvirn times, followed by a major transgression on to the adjacent platform in early Caradoc times (the gracilis Biozone transgression). Subsequent lowering of sea level, with low stands in late Caradoc and Ashgill times, was probably caused by glaciation centred on the Gondwana continent. The sequence is dominated by blue-grey mudstone and siltstone with local development of conglomerates and sandstones in nearshore settings and dark grey to black anoxic silty mudstone in distal deep water. The major difference in the sedimentation patterns from the Cambrian was the marked diminution in the influence of turbidites, at least until late Ordovician times when turbiditic and associated sediments that are more characteristic of the Silurian were first deposited. Most importantly, the repeated sequences of volcanic rocks reflect the intense volcanic activity that profoundly modified the local sedimentation patterns through most of Ordovician time. The volcanic activity was facilitated by major deep-seated faults and thinning of the crust at the northern edge of Avalonia. The earliest volcanism, during Tremadoc times, was that of an island arc, related to subduction of Iapetus ocean crust to the north, at the margin of Avalonia. Subsequent volcanism, in Llanvirn and Caradoc times, developed within an extensional back-arc setting.

All but one of the series of the Ordovician used in southern Britain — Tremadoc, Arenig, Llanvirn, Caradoc and Ashgill — were defined in Wales and its borders in the 19th and early 20th centuries, although the Tremadoc was originally placed within the upper Cambrian. These series subdivisions can be applied to the Ordovician of continental Europe and Asia, which lay on the Gondwana plate on the south-east side of the Iapetus Ocean, but are less easily applied to those sequences in northern Britain, North America and Greenland, which lay on the Laurentian plate, on the north-west side of Iapetus (Figure 3).

As currently defined and ratified by the IUGS, the base of the Ordovician System is defined at the base of the Tremadoc Series (Tremadocian Stage of current nomenclature), at a level just below the first appearance of the planktonic dendroid graptolites of the genus Rhabdinopora [Dictyonema], and the top coincides with the base of the acuminatus Biozone at the base of the Silurian. Subdivision of the Ordovician System into global series and stages is nearing completion with only one of the global stages derived from the Anglo-Welsh divisions. Historically the Ordovician System in Wales has been divided into five series, with their constituent stages and substages, on the basis of graptolites and shelly fauna (Table 3); (Figure 12). The graptolites occupied the water column and are generally found in offshore, pelagic, dark grey or black, pyritous and carbonaceous mudstones, which reflect anoxic bottom water conditions and deeper water. Conversely, the well aerated, relatively nearshore sandstone, mudstone and calcareous sediments were an attractive habitat for the shelly faunas. The great variety of shelly fossil assemblages facilitates local detailed correlation, but the interplay between evolution and migration due to changing environmental conditions demands careful interpretation. Because of the separation of the two environments, it is not surprising that correlation between graptolite and shelly faunas can be imprecise. However, there are places in outer shelf settings, as at Builth Wells, where graptolites are mingled with deep benthic faunas containing trilobites. Locally bryozoa, ostracods and echinoderms were common. They provide particular evidence of the rich faunal communities that were established in warm clear seas during Ashgill times.

Tremadoc

From late Cambrian into early Ordovician times there was widespread deposition of mud Tremadoc across the Welsh Basin and its adjacent shelf. In Wales, Tremadoc strata, up to 500 m thick, are exposed around the Harlech Dome, from Arthog on the south side of the Mawddach estuary to the Vale of Ffestiniog and Tremadog in the north. The strata are absent in Arfon and Anglesey, and in Pembrokeshire, but whether they were ever deposited there or were removed during the Arenig transgression is a matter of continued debate. Small outcrops of Tremadoc strata have been proved in Carmarthenshire in south Wales. To the east of Wales, an estimated 1500 m of Tremadoc greenish grey mudstone (Shineton Shale Formation) accumulated on the platform in the Welsh Borderland probably within a half-graben that developed during an extensional phase of rifting on the Midlands Platform.

Where the base of the Tremadoc is seen, as at Ogof Ddû near Criccieth, and near Ffriog in Arthog, there is a marked change, over 2 to 3 m, from black anoxic mudstone of the Dolgellau Formation to paler grey-green, rusty weathered mudstone with phosphatic nodules at the base of the Tremadoc. The Tremadoc succession has been given many local formational names, which the Geological Survey has rationalised into the Dol-cyn-afon Formation, comprising marine mudstone, locally bioturbated, with thin, cross-laminated, silty sandstones and some thicker, lenticular sandstone beds. At Ogof Ddû, the graptolite Rhabdinopora is found 8.5 m above the base of the formation. Around the northern flank of the Harlech Dome, the graptolite flabelliformis and tenellus biozones, and the trilobite pusilla (or salopiensis) and sedgwickii biozones are all present, and the sequence is the most complete in Wales. The formation has been subdivided into members on the basis of coarse- and medium-grained volcaniclastic sandstones, although this division is difficult to sustain laterally. Essentially, the sequence reflects a higher energy marine environment than that which prevailed during deposition of the Dolgellau Formation, although in places, particularly in the mudstone near the top, there is evidence of a more restricted shallow shelf and lagoonal environment reflecting the initiation of emergence.

At Dol-cyn-afon, on the eastern flank of the Harlech Dome, only the lowermost flabelliformis Biozone is proved. It is possible that the higher part of the sequence was removed by erosion before deposition of the subaerial Rhobell Volcanic Group in late Tremadoc times; the group oversteps the Dol-cyn-afon Formation to rest on the Dolgellau Formation. The Rhobell Volcanic Group is the denuded remnant of a basaltic volcano, which was uplifted and then deeply eroded prior to the marine transgression during early Arenig times. Successive basaltic lava flows, locally autobrecciated, overstep eastwards across a highly irregular surface. Locally some primitive basalts contain cognate cumulate blocks composed dominantly of pargasite with calcic clinopyroxene and Ti-magnetite derived from a compositionally stratified magma chamber (Plate 13). The basalts were erupted subaerially from a series of fault-controlled fissures, aligned north-south, which are now marked by a suite of subvolcanic dolerite and dioritic intrusions. Rafts of country rock, which were rotated during magma emplacement, add to the structural complexity. Autobrecciation and hydrothermal alteration have affected all the rocks of the intrusive complex. The complex of subalkaline intrusions within the Cambrian sequence on the Harlech Dome, to the west of Rhobell Fawr, and the porphyry-copper mineralisation at Coed y Brenin are directly related to this magmatic episode.

Along the south side of the Mawddach estuary, the Dol-cyn-afon Formation comprises dark grey mudstone with some laterally impersistent sandstones near the top of the sequence. The mudstone is intensely bioturbated, possibly the result of shallowing prior to emergence. Both the flabelliformis and pusilla biozones have been determined. Around Mynydd y Gader, south of the Ceunant Fault, the formation has been removed by erosion.

In western Llŷn, rocks of the early Tremadoc, flabelliformis and tenellus biozones occur at Pen Benar. They are lithologically similar to those around the Harlech Dome, and sandstones near the top of the sequence probably reflect shallowing of the marine environment.

In south Wales, at Llangynog south-west of Carmarthen, grey-green micaceous mudstone has yielded a graptolite-trilobite fauna of Tremadoc age. The trilobites Pseudohysterolenus and Parabolina argentinensis have not been recorded elsewhere in Britain, but are comparable with forms found in South America. A shallow borehole in Carmarthen Bay, south-west of Worms Head, proved about 2.5 m of dark grey micaceous mudstone of possible tenellus Biozone age. At Treffgarne, north Pembrokeshire, basal Arenig strata overlie basaltic andesite and andesite lavas and associated volcaniclastic rocks that are broadly similar in character to the Rhobell Volcanic Group, although their possible correlation remains equivocal.

Arenig

The Arenig Series in Wales is a transgressive sedimentary sequence that followed a eustatic regression and possible local uplift at the end of Tremadoc times. It is divided into three stages, Moridunian, Whitlandian and Fennian (Table 3). It was first defined at Arenig Fawr to the north-east of the Harlech Dome, but subsequent biostratigraphical work has shown that there the series is incomplete, with most of the Whitlandian and Fennian stages missing. The most complete section is in south Wales where basinal sediments, similar to those of the Tremadoc Series, continued to accumulate. Elsewhere, Arenig rocks crop out around the margin of the Harlech Dome, in Snowdonia, in Llŷn and on Anglesey.

The magnitude of the basal Arenig unconformity is most apparent to the north of the Harlech Dome where Arenig beds overstep on to the Precambrian Padarn Tuff Formation near Bangor, and on to the Monian Supergroup near Red Wharf Bay on Anglesey. Similarly, on Llŷn, Arenig sedimentary rocks overstep almost the whole of the Cambrian sequence in the width of St Tudwal’s peninsula (Figure 10), and similar strata overlie the Precambrian Sarn Complex in the vicinity of Aberdaron. This basal Arenig unconformity (Plate 14) gives an indication of the minimum age for the final juxtaposition of the terranes adjacent to the Menai Straits Fault System. The intense erosion reflected in the unconformity caused dramatic changes in sedimentation across the boundary. Relatively deep-water, pelagic sediments of the Tremadoc were replaced by shallow-water, coarse-grained and locally conglomeratic sand in the lowermost Arenig.

The basal Arenig sandstones are well developed, but variable in lithology and thickness throughout most of the north Wales outcrops (Figure 13). They represent fan delta and shoreface deposits, and even where there is no other evidence of an angular unconformity, the Neseuretus trilobite biofacies indicates shallow water. In the Vale of Ffestiniog, the distinctive Garth Grit is a massive bedded, white quartzitic sandstone that is locally bioturbated and conglomeratic. It is probably a beach or sublittoral deposit. About Rhyd it is up to 130 m thick, but thins eastwards through a more flaggy, volcaniclastic sandstone facies to Blaenau Ffestiniog, and farther to the slate quarries at Croes yr Ddwy Afon where it is intensely bioturbated and chloritic, and forms the hanging wall of the main slate extraction. The clasts are mainly of vein quartz, rhyolite, andesite and siltstone; many are coated with a black, phosphatic encrustation that was once thought to be a bryozoan, ‘Bolopora undosa’, but now is known to be an oncolitic accretionary precipitate of chemogenic or bacteriogenic origin. Similar encrustations are recognised on clasts in the basal Arenig sandstone (Graianog Sandstone) in Cwm Graianog, high above the Nant Ffrancon valley. These basal sandstones contain the trace fossils Phycodes and Teichichnus, and are overlain by a sequence of mudstone, ripple-marked sandstone and, in places, thin, bioturbated, ooidal ironstones. In the sandstone beds, trails of a foraging trilobite, Cruziana, are associated with the burrows Skolithus, and the mudstone contains the graptolites Azygograptus and Didymograptus.

Across Llŷn, conglomerate with clasts of Precambrian rocks is common at the base of the Arenig sequence. At Wîg, near Aberdaron, the conglomerate that overlies the Sarn Complex passes up into siltstone, bioturbated sandstone and dark grey mudstone with phosphatic nodules (Wig Bâch Formation; (Figure 13)a. The mudstone has yielded the trilobite Merlinia selwynii (Salter) and the graptolite Aygograptus eivionicus Elles, indicating an early Arenig age for the transgression at this locality. Eastwards across the peninsula, the base of the Arenig rests progressively on younger strata. T P Crimes, in his detailed studies of the trace fossils, distinguished eight facies in this sequence, which he attributed to transgressive-regressive pulses within the intertidal zone. Basal phosphatic and ferruginous conglomerates, with ‘Bolopora undosa’, and coarse-grained sandstones grade up into medium-grained sandstones and siltstones, with a Cruziana ichnofacies, which in turn grade into finer grained sandstone and mudstone with a fodinichnia ichnofacies. The gradation reflects a progressive shift from a nearshore to an offshore setting. At Nant y Gadwen, the formation consists of siltstone and mudstone with trilobites and graptolites that indicate a fairly complete Arenig sequence and relatively deep-water conditions. At Porth Meudwy, coarse-grained turbiditic sandstones and conglomerates (Porth Meudwy Formation), probably mass flow deposits, are of probable late Arenig age. The sedimentary structures indicate dominant north-east and south-west flow, possibly parallel to a shoreline that was close to western Llŷn and may have been fault controlled. Thin welded tuffs on the north side of Mynydd Rhiw indicate the earliest stages of volcanic activity (Rhiw Volcanic Group), which lasted from late Arenig into Llanvirn times. North of Trwyn Talfarach, a suite of dolerite sills form the dominant feature. At the base of the tuffs is a manganese bed with rhodocrosite, rhodonite and pyrolusite, and a pisoidal ironstone.

The thickest Arenig sequence, up to 1500 m, with a basal, locally conglomeratic sandstone overlain by mudstone (Carmel Formation), is recorded on Anglesey, about Mynydd y Garn, on the north-west side of the Coedana Complex outcrop; the thickness may be accentuated by folding. The main outcrop extends from Rhosneigr, on the south-west coast, to Dulas Bay and Carmel Head, in the north, with smaller outcrops inland from Red Wharf Bay. The coarsening of the basal sandstones towards the north indicates a possible source in that direction. North of the Carmel Head Thrust, the conglomerates are typical beach deposits, possibly remobilised, and they are overlain by coarse-grained, cross-bedded sandstones with brachiopods and trilobites that indicate the nearshore Neseuretus biofacies. The clasts in the conglomerates are entirely of local, Precambrian origin. The basal deposits probably prograded southwards across the Precambrian substrate, from Whitlandian to late Fennian times. Locally derived conglomerates with many angular clasts persist into the uppermost Arenig (Treiorweth Formation). The associated Nantannog Formation comprises mudstone scattered with a similar range of Precambrian clasts, and marked facies change suggests remobilisation. Generally, it overlies the Treiorweth Formation, but in places it replaces it to lie directly on the Carmel Formation. The brachiopod faunas similarly reflect a high-energy environment, and indicate that inundation occurred during Fennian times.

About the Harlech Dome, the Arenig strata are broadly referred to the Allt Lwyd Formation at the base of the Aran Volcanic Group (Figure 13b). The formation can be traced with marked thickness variations from the Moelwyns in the north to Cadair Idris in the south. Coarse-grained sandstones, similar to the Garth Grit farther north, are laterally impersistent at the base. In the historical type area about Arenig Fawr, the sandstones are overlain by alternating beds of dark grey siltstone and light grey, fine-grained, bioturbated sandstone. The lithologies suggest a shallow water environment, and interdigitated volcaniclastic sandstones contain a shelly fauna characteristic of the inshore Neseuretus biofacies. Most of the sequence is lower Arenig (Moridunian), but some localities suggest a mid Arenig, Didymograptus simulans Biozone age. Southward, the main part of the formation comprises well-featured, medium- to thick-bedded, coarse-grained feldspathic sandstone, separated by thinly bedded, flaggy sandstone and silty mudstone. Loaded irregularities in the sandstone bases and Chondrites- and Skolithus-type bioturbation are common. The feldspathic sandstone, with abundant feldspar-phyric basalt clasts similar to the Rhobell Fawr Group, increases in abundance south-eastwards towards Aran Fawddwy, where the Aran Boulder Bed (up to 300 m thick) at the top of the Allt Lwyd Formation contains similar clasts of altered andesite or basaltic andesite. To the east, the Aran Boulder Bed oversteps onto the Mawddach Group, and to the south-west passes laterally into a crudely graded conglomerate. The boulder bed and associated conglomerate are interpreted as an alluvial fan complex, with both fluvial and debris flow deposits.

In south Wales, early Arenig tectonism associated with the development of the basin resulted in a series of fault blocks, which were eroded and transgressed prior to the deposition of conglomerates and coarse-grained, cross-bedded sandstones (Ogof Hên Formation; (Figure 14)). In general, Tremadoc strata are absent throughout Pembrokeshire, and the unconformity is most graphically displayed at Whitesand Bay where the basal sandstone infills incised Cambrian strata. On Ramsey Island, the basal Arenig pebbly sandstone with ‘Bolopora undosa’ rests with slight angular unconformity on the Lingula Flags Formation of the Merioneth Series (Figure 15). The basal sandstone grades up into laminated and crosslaminated sandstone and siltstone, with burrows and trilobite tracks. The overlying mudstone contains a rich shelly fauna of early Moridunian age. Whitlandian strata on Ramsey Island were probably removed by disruption and soft-sediment sliding prior to deposition of black graptolitic mud, offshore in dysaerobic bottom waters. Such conditions persisted into late Fennian times when mud deposition was interrupted by distant volcanic activity, with the emplacement of fine- to medium-grained rhyolitic volcaniclastic turbidites as thin to thick beds within the black mudstone (Aber Mawr Formation). The mudstone contains abundant extensiform graptolites, and passes up into the Lower Llanvirn.

In the Abereiddi area, the basal coarse sandstones, which transgress Upper Cambrian (Merioneth) strata, grade up into micaceous mudstone and feldspathic sandstone turbidites. The sequence has yielded a rich shelly and graptolite fauna of Whitlandian age and the overlying darker grey shale contains blind or nearly blind trilobites of the atheloptic association plus mesopelagic trilobites of the cyclopygid biofacies, which imply water depths of probably more than 300 m. Farther east, at Treffgarne, sandstone turbidites and debris-flow deposits, which unconformably overlie the Trefgarn Volcanic Group, grade up into blue-grey, tuffaceous, cross-laminated sandstone and micaceous shale with dendroid and extensiform graptolites. In the Whitland area, equivalent strata have yielded a Whitlandian fauna and have been interpreted as turbidites and channelled mass-flow deposits passing into shallower water wave-influenced sandstones. The distribution pattern of this facies suggests a shoreline to the south, possibly controlled by fault-block movement of the basement in the Haverfordwest area. The overlying sequence of mudstone and siltstone reflects progressive deepening, and graded turbidites at the base of the Fennian indicate possible tectonic reactivation of the southerly source area. However such activity was brief, as the Fennian strata of hemipelagic dark grey fissile and blocky mudstone (Pontyfenni Formation) yield rich trilobite and graptolite faunas, indicating that open marine conditions prevailed over south-west Wales at that time. In latest Arenig times, the deposition of pale grey mudstone is attributed to falling sea level as a result of eustatic regression.

Llanvirn

Following the end Arenig regression, a blanket of mud with a deep water biofacies, which covered most of Wales and its borders, reflected a period of marine transgression in Llanvirn times. The series is most completely preserved in south Wales and it is there, in Pembrokeshire, that the type section was established. For most of north Wales, Llanvirn strata are extremely restricted, either the result of nondeposition or, more probably, of removal by erosion.

There is a rich trilobite fauna with large-eyed cyclopygids being particularly distinctive, and the series is subdivided into a number of graptolite biozones: D. artus (formerly D. bifidus), D. murchisoni and Hustedograptus teretiusculus (Table 3). The Llandeilo Series was formerly a constituent part of the Ordovician of Wales; it included the Hustedograptus teretiusculus Biozone and part of the Nemagraptus gracilis Biozone. Redefinition of the chronostratigraphy now includes the H. teretiusculus Biozone in the Llanvirn, and the gracilis Biozone is the lowest biozone of the Caradoc Series.

The volcanic activity initiated in late Tremadoc and Arenig times evolved into a major geological event during the Llanvirn. Llanvirn volcanic rocks crop out in western Llŷn, about the Harlech Dome in southern Snowdonia and between western Pembrokeshire and Builth Wells along the southern crop. The volcanism developed in a back-arc basin from subduction of Iapetus Ocean beneath the northern, leading edge of Avalonia.

Between Porth Neigwl (Hell’s Mouth) on southern Llŷn and Llanfairfechan on the coast to the east of Bangor, there is a thick sequence (up to 1500 m) of blue-grey, mudstone and siltstone with laterally impersistent fine-grained sandstones (Nant Ffrancon Subgroup; (Figure 13)a; local pisoidal ironstones indicate brief periods of shoaling. The sequence is relatively well exposed in the lower parts of both the Nant Ffrancon Pass and Llanberis Pass, and there is no evidence for non-sequences or unconformites in either section. However, in southern Snowdonia, between the Cwm Pennant and the Trawsfynydd fault zones, Nant Ffrancon Subgroup rocks of Caradoc age rest directly on Arenig or older strata. Elsewhere, in Llŷn and around the eastern and southern flanks of the Harlech Dome only strata of Abereiddian artus and murchisoni Biozone age are present. Only in Cwm Pennant, south-west of Snowdon, is there any suggestion that Llandeilian rocks may be present, and their general absence is correlated with a widespread mid-Ordovician unconformity.

On Anglesey, Llanvirn rocks outcrop in the central region and contrast sharply with those through most of Arfon. They comprise a sequence of graptolitic mudstone with thick wedges of matrix-supported conglomerates and medium- to coarse-grained sandstones. These coarse deposits, with mainly Precambrian and Monian Supergroup clasts, reflect the continuation of the tectonic activity that influenced the Arenig sedimentation on Anglesey, and are interpreted as having been deposited from debris flows transported over short distances on an irregular sea floor; a wedge of these deposits, up to 650 m thick, is banked against an easterly dipping fault scarp. Rich assemblages of brachiopods and less common trilobites have been determined from the sandstones, and the mudstones have yielded graptolites of the Didymograptus artus and D. murchisoni biozones.

On Cadair Idris and north of the Mawddach estuary, acid tuffs with thin tuffites of the Offrwm Volcanic Formation are the first major expression of the volcanism (Aran Volcanic Group) that dominated southern Snowdonia from Llanvirn through to early Caradoc times (Figure 13b); (Plate 15). The acid tuffs, both welded and nonwelded, are massive-bedded and contain quartz and feldspar crystals and pumice fragments; foliation is locally accentuated by siliceous recrystallisation (Plate 16) and interbeds of fine-grained, devitrified dust tuffs are common. The formation thins markedly eastwards. South of Dolgellau, it oversteps the Arenig sequence to rest on upper Cambrian (Merioneth) strata. Around Tonfannau, several acidic ash-flow tuffs are intebedded with mudstone. The dearth of coarse terrigenous deposits and shallow-water structures suggest that the tuffs were deposited in a relatively deep, probably subsiding basin. A few thinly bedded tuffaceous sandstones indicate reworking of pyroclastic debris into debris flows and turbidites from shallower parts of the basin. The mudstones have yielded a graptolite fauna of early Llanvirn (artus Biozone) age.

The overlying Cregennen Formation comprises dark grey mudstone with impersistent basic tuffs and tuffites, few acid tuffs and some pillowed basalt. The Brin Brith and Cefn Hir members form prominent features between Arthog and Llynnau Cregennen, particularly across Pared y Cefn Hir, but they merge to the east, and thin and wedge out westwards towards Arthog (Figure 13b). The members are typically composite and reflect considerable complex reworking of both acid and basic volcanic debris into flows (Plate 17). Coarse-grained tuffites become finer grained and more thinly bedded upwards and may have been deposited in broad migrating channels; thinner, finer grained tuffaceous deposits are interpreted as low-concentration turbidites. The numerous tabular and contorted clasts of bedded tuff within the coarser deposits suggest collapse and erosion of channel margins. Graptolitic mudstones have yielded an artus Biozone fauna at many localities. The uppermost, Llyn y Gafr Volcanic Formation consists mainly of basalt lavas and bedded basic tuffs, with a few acid tuffs and intercalated graptolitic mudstones. Correlation is imprecise because of marked lateral facies variation and, locally, the intensity of basaltic intrusions. The thick accumulation of pyroclastic breccias near Llyn Gafr comprises blocks of highly vesicular basalt with a sparse matrix of comminuted basaltic debris, and probably represents an accumulation close to a vent. The massive and pillowed lavas represent the first major effusive volcanic episode in southern Snowdonia; the intercalated mudstones, together with the absence of evidence of shallow-water reworking, suggest accumulation well below wave base. However, the abundance of scoriae and cuspate fragments in the basic tuffs also suggest periodic explosive activity. North of the Bala Fault, the basaltic tuffs and thin pillowed basalts of the Melau Formation are possibly lateral equivalents, and contemporaneous acidic activity is represented by an acidic ash-flow tuff and breccia (Brithion Formation) derived from the collapse of a rhyolite dome at Creigiau Brithion on the edge of Aran Fawddwy. The eroded top of the dome suggests temporary emergence.

North of the Harlech Dome and within the Nant Ffrancon Subgroup, acid ash-flow and fine-grained dust tuffs with high level synvolcanic intrusions (Plate 18) and extrusions of locally autobrecciated diorite and rhyolite (Rhiw Bach Volcanic Formation) crop out from Llŷn Morwynion (east of Ffestiniog) to the north-east of Blaenau Ffestiniog. Locally, the tops of the diorite sheets show evidence of shallow marine reworking into laterally impersistent beds of volcaniclastic sandstones. In contrast, the relationships of the rhyolites, for example at Carreg y Fran in Cwm Teigl and Graig Goch north-east of Llynnau Gamallt, are distinctively more intrusive. The volcanic rocks are hosted within dark blue-grey, sparsely graptolitic mudstone of artus Biozone age, which has been extensively quarried and mined for slates in the complex of quarries from Llechwedd to Manod Mawr.

On Ramsey Island, volcanism and sedimentation were profoundly influenced by the north-south, Ramsey Fault (Figure 15). To the east of the fault, debris flow deposits with clasts of rhyolite and Cambrian rocks are interbedded with mudstone and minor siltstone (Porth Llauog Formation). The debris flows were controlled by repeated activation of the Ramsey Fault, which destabilised large masses of volcaniclastic debris to produce both high- and low-density turbidity currents. To the west of the fault, temporary uplift and emergence resulted in littoral conditions, and conglomerates rest unconformably on Cambrian rocks. The conglomerates form the basal member of the Carn Llundain Formation, a thick succession of graded and laminated tuff turbidites with three thick acidic ash-flow tuffs, locally with well-developed columnar jointing, in which ragged pumice clasts are moulded about lithic clasts and crystals. The tuffs are interpreted as deposits entrapped close to a vent by rapid subsidence, and the eruption and emplacement are considered to have taken place under water. High-level intrusion of autobrecciated rhyolite was facilitated by fluidisation of the unlithified sediments. Throughout this volcanic activity the background sedimentation was dominated by black mudstone, and shallowing and emergence was extremely local and temporary.

On the Pembrokeshire mainland, evidence of broadly contemporaneous volcanic activity is preserved near Abereiddi and Fishguard, and its distribution was, to some extent, constrained by the Fishguard-Cardigan Fault Belt. In the Abereiddi-Abercastle area, the historical type area for the Llanvirn series, the Llanrian Volcanic Formation comprises thick rhyolitic tuffs and coarse- and fine-grained volcaniclastic deposits. These are separated by black graptolitic mudstone from overlying pumiceous lapilli tuffs (Abereiddi Tuff Member, previously termed Murchisoni Ash) formed by the youngest volcanic episode in the district (Figure 14). The tuffs were mantled by black pelagic mud with a rich, mid to late Llanvirn fauna of murchisoni and teretiusculus Biozone age. A rich shelly fauna near the top of the overlying Castell Limestone Formation indicates the transition into a shallower setting, spanning the Llanvirn-Caradoc boundary.

Another volcanic centre lay to the east in the vicinity of Fishguard and Strumble Head, and the sequence (Fishguard Volcanic Group) is spectacularly exposed in the sea cliff sections across Pen Caer between Pwll deri and Pen Anglas. The earliest activity produced rhyolite and rhyodacite lavas (locally pillowed) and breccias (Porth Maen Melyn Formation) that are overlain by pillowed basalts and hyaloclastites with intercalated volcaniclastic turbiditic sandstones (Strumble Volcanic Formation) on the north side of Porth Maen Melyn. The fluid basic magmas were emplaced mainly as interdigitating pillowed and sheet flows with the minor tuffs and hyaloclastites indicating periodic explosive activity. The final phase of rhyolitic activity (Goodwick Volcanic Formation) resulted in the extensive emplacement of welded and nonwelded ash-flow tuffs and thick rhyolite lavas, with flow-banded and perlitic cores and autobreccia carapaces. The formation is well-exposed in the cliff section between Goodwick and Penfathach. The primary volcanic elements are associated with much reworked volcaniclastic material, both in debris flow deposits and in the background silty mudstone. Interbedded graptolitic mudstones indicate a mid Llanvirn age close to the artus-murchisoni biozonal boundary. The sequence can be traced eastwards into Mynydd Preseli and, farther, into possible feather-edges near Narberth.

A striking feature of the Llanvirn sequence through western Pembrokeshire is the complex of dolerite, gabbro and dioritic intrusions, with subordinate microgranites and microtonalites, which were clearly coeval with the volcanic activity. They intruded the volcanic pile and associated unlithified mud, and extend west of the outcrop of the volcanic rocks. Perhaps the most prominent are the layered gabbroic complexes of Carn Llidi and St David’s Head (Plate 19), which are probably the same intrusion on the limbs of a north-east trending syncline. Both outcrops display internal layering of gabbroic facies that can be crudely correlated; the marginal quartz-gabbro is considered to be the undifferentiated magma and the other varieties were largely generated by in situ magmatic differentiation. The intrusive suite is also present on the islands of Bishop and Clerks, some 5 km west-north-west of Ramsey Island, and on Mynydd Preseli, south-east of Fishguard, from where the Stonehenge bluestone were excavated.

In eastern Pembrokeshire and Carmarthenshire, blue-black graptolitic mudstone sedimentation persisted through early and mid Llanvirn times (artus and murchisoni biozones). Younger silicified tuffs (Asaphus Ash;(Figure 14) with interbedded siltstone, commonly with the asaphid trilobite Basilicus tyrannus, are correlated with the late Llanvirn teretiusculus Biozone. The overlying flaggy sandstone, siltstone and dark grey mudstone with few volcaniclastic siltstones (Hendre Shales) lie within the teretiusculus and gracilis biozones, and therefore span the Llanvirn-Caradoc boundary.

Towards the east, deep water graptolitic mudstone passes laterally, near Llandeilo, into shallow marine sediments deposited close to the basin margin. The lowermost, early Llanvirn, blue-black and grey shales, with bands of rhyolitic tuff, grade up into sublittoral conglomerate, sandstone and rhyolitic tuff with subordinate mudstone and argillaceous limestone (Ffairfach Group). The group contains a rich shelly fauna. The overlying fossiliferous succession of shallow marine, fine- to medium-grained sandstones with impure limestones (Lower Llandeilo Flags) reflects a major upward transgressive cycle. A Williams interpreted the succession as shoreface coarse sands passing through intertidal to subtidal sands and silts, to shallow shelf mud deposited below wave base.

About the Tywi valley, volcaniclastic debris in the sequence was derived by shallow water reworking of lavas and pyroclastic deposits from the approximately contemporaneous Builth volcanic complex to the north-east. The lowermost dark grey graptolitic mudstone (Camnant Mudstone Formation) was deposited in fairly deep water and is overlain by acid tuffs, coarse agglomeratic tuffs, and spilitic and pillowed porphyritic andesite lavas with local high-level keratophyric intrusions (Builth Volcanic Group;(Figure 14). The sequence was the subject of the classic interpretation by O T Jones and W J Pugh. In particular, they recognised that andesitic boulder beds and coarse feldspathic sandstones (Newmead Formation) were laid down as beach deposits around exhumed sea stacks and cliffs during the emergence of small volcanic islands. The volcanic rocks are overlain by grey mudstone with a few tuffaceous beds and thin argillaceous limestones (Llanfawr Mudstone Formation); the richly fossiliferous mudstone indicates that the sequence extends up into the gracilis Biozone at the base of the Caradoc.

Caradoc

In early Caradoc times, the south-eastern margin of the Welsh Basin continued to be clearly defined by the Welsh Borderland fault system at the edge of the Midland Platform. Subsequently the platform was progressively transgressed (Figure 16), and by late Caradoc times it was totally submerged. At the north-west margin of the basin, the Irish Sea Landmass was probably submerged or subdued in relief for most of Caradoc times. The probable absence of Llandeilian strata over most of north-west Wales suggests that the deposition of the Caradoc sequence was preceeded by uplift and erosion but, over most of the outcrop, there is little lithostratigraphical evidence in support of this break. The only evidence for a major discordance occurs on Anglesey, where a coarse basal conglomerate with Monian Supergroup pebbles overlies Precambrian strata and, less graphically, in southern Snowdonia.

Caradoc strata occur within most of the Ordovican outcrops through Wales. In the northern part of the basin, shallow water sands were commonly deposited, whereas in the southern parts, deep water basinal muds persisted, although locally, as in south Cardiganshire, there were incursions of coarse-grained turbidites. In Snowdonia, the strata are the main component of the Ordovician outcrop and the sequence is dominated by volcanic rocks, which reflect an intense, but restricted, period of volcanic activity. The subdivision of the Caradoc into nine substages (Table 3) was established, on the basis of the rich brachiopod and trilobite faunas, in the Welsh Borderland. There have been some tentative correlations between these stages and the graptolite biozones determined locally in the Welsh Basin and together with the shelly fauna of Snowdonia it has been possible to constrain the volcanic stratigraphy, with some confidence, into two stages, Soudleyan and Longvillian.

In southern Snowdonia, on the northern slopes of Cadair Idris (Plate 15) and Aran Fawddwy, the Llanvirn basaltic sequence (Llŷn y Gafr Volanic Formation) is overlain by a thick succession of silty mudstone with tuffaceous beds and a few thin sandstones (Ty’r Gawen Mudstone Formation) (Figure 13b). Parts of the formation are laterally equivalent to two thick volcanic sequences (Pen y Gadair Volcanic Formation; Benglog Formation), which wedge out westwards towards the Llanegryn Fault. East of the fault, the base of the Ty’r Gawen Formation is marked by pyritic mudstone with phosphate nodules and scattered ooids (Fron Newydd Member); west of the fault, it is markedly unconformable. An impersistent ooidal and pisoidal ironstone, generally 1 to 2 m thick but up to 10 m near Cross Foxes, lies close to the base and has been worked for low-grade iron ore in small pits and levels that clearly mark its outcrop. The ironstone is mainly a massive bed of grain-supported chamosite ooids with rare pisoids. A poor graptolite fauna suggests the Nemagraptus gracilis Biozone.

On Cadair Idris, the Pen y gadair Volcanic Formation consists predominantly of pillowed, massive and autobrecciated basalt lavas with crystal tuffs and a few acidic ash-flow tuffs. The basalts are spectacularly exposed about the summit of Cadair Idris. The formation indicates a renewal of effusive and explosive basaltic volcanism in southern Snowdonia, and its distribution is similar to that of the Llyn y Gafr Formation. The acidic ash-flow tuffs, some welded, indicate contemporaneous explosive acid volcanism, but its source is no longer discernible. Towards the north, a sequence of pillowed basalts, primary and reworked basaltic tuffs and hyaloclastites (Benglog Volcanic Formation), with intercalations of graptolitic mudstone of gracilis or early multidens Biozone age, successively oversteps older formations to rest on Arenig rocks near Bala. On the south-east side of the Harlech Dome, there was local contemporaneous emplacement of acid ash-flow tuffs and rhyolites (Craig y Ffynnon Formation) and, slightly later, a restricted accumulation of andesitic lavas, crystal tuffs and breccias with intercalated mudstones (Pistyllion Formation).

The final episode of volcanism of the Aran Volcanic Group was probably the most explosive and widespread. On Cadair Idris, the Craig Cau Formation comprises tuffs, tuffites and volcaniclastic debris flow deposits with interbedded unfossiliferous mudstones, overlain by acidic ash-flow tuffs. The tuffs and tuffites contain a variable proportion of mud components and mudstone clasts. The contorted shape of some of the clasts indicate that they were incorporated while still unlithified, and suggests that the acid pyroclastic debris was erupted through unlithified mud. Emplacement was probably from a number of centres, and intrusion of the late stage Cadair Idris microgranite into one centre remains a possibility. The upper (welded) tuff can be traced eastwards through Aran Fawddwy and northwards to Arenig as the Aran Fawddwy Formation. It is overlain by silty mudstone of possible Costonian-Harnagian age.

The volcanic sequence throughout southern Snowdonia shows no evidence of significant reworking, suggesting that its eruption and emplacement was largely contained within the marine environment. It is overlain by a thick sequence of mainly thinly bedded turbiditic silty mudstone and siltstone with interbedded laminated dark grey to black mudstone (hemipelagite) (Ceiswyn Formation). The proportion of mudstone increases up-sequence, which probably reflects gradual drowning of the shelf. The graptolite and trilobite faunas indicate that the formation is of early Soudleyan and multidens Biozone age. At its top, a thin bed of black, intensely cleaved, pyritic and graptolitic mudstone, the Nod Glas Formation, marks the top of the Caradoc (Onnian Substage). The mudstone is poorly exposed, but its outcrop can be mapped by the feature that it forms, a depression between Towyn and Dinas Mawddwy.

In the vicinity of Bala, the dominantly mudstone strata of the Caradoc pass laterally into a more silty and fine sandy sequence with three prominent, but thin tuff beds — the Cefn Gwyn Tuff, at the base of the Soudleyan Substage, and the Frondderw and Pont y Ceunant tuffs, within the Soudleyan. Also in this direction the lower part of the Nod Glas Formation has been reported to pass laterally into a calcareous and phosphatic facies (Cymerig Limestone Member, Woolstonian Substage), which overlies thin calcareous tuffs (Gelli Grin Calcareous Ashes, Longvillian Substage) (Figure 13b).

The thick sequence of Caradoc rocks in the Berwyn Hills shows broadly similar sedimentary characters to the sequence farther west. It is dominated by silty mudstone and fine-grained sandstone, with local evidence of a volcaniclastic component, and three prominent tuff horizons — the Cwm Clwyd, Swch Gorge and Pandy Tuff formations. The sequence accumulated in shallow marine to intertidal environments, of generally low energy but with periodic storm events and intermittent local emergence. Faunas are relatively well preserved and provide valuable indices to depth, turbulence and rates of sedimentation. In the Vale of Ffestiniog, between Rhyd and Tremadog, pretectonic gravity sliding has disrupted the sequence close to the base of the Caradoc and the lower part of the Ordovician sequence is missing. A sedimentary mélange, the Rhyd Mélange, oversteps down to Tremadoc strata (Figure 13a). The mélange is most easily recognised where there is a marked lithological contrast between the component clasts and matrix. The scale of the disruption ranges from rafts of Tremadoc and Ffestiniog Flags Formation strata, up to 300 m across, down to grain-to-grain sediment disruption. The top of the deposit grades up into disturbed and folded mudstone, and it is conformably overlain by two volcaniclastic debris-flow deposits, derived from the collapse and subaqueous reworking of associated rhyolite intrusions-extrusions (Moelwyn Volcanic Formation), of probable Costonian age. To the east of Ffestiniog, patches of similar mélange deposits have been determined in the poorly exposed ground on the northern edge of the Migneint.

In northern Snowdonia, slumped beds at the top of the Nant Ffrancon Subgroup reflect seismic activity prior to the first of two major cycles of Caradoc volcanism. The products of this volcanism comprise the Llewelyn Volcanic Group and the Snowdon Volcanic Group (Figure 13a), which dominate both the Caradoc succession and the topography through central and north Snowdonia. The 1st Eruptive Cycle, expressed in the Llewelyn Volcanic Group, is distributed between Conwy in the north, through the Carneddau to the vicinity of Nant Peris in the south. The earliest activity developed from a number of centres, which were broadly contemporaneous. The Conwy Rhyolite Formation comprises intrusive and extrusive rhyolites and acidic ash-flow tuffs about a centre in the vicinity of Conwy Mountain. In the Anafon valley, these acid extrusive volcanic rocks abut and interdigitate with trachyandesite lavas and intrusions of the Foel Fras Volcanic Complex. Farther south, the early phase of the 1st Eruptive cycle comprises extrusive basaltic lavas, the Foel Grach Basalt Formation, and rhyolite lavas and acidic ash-flow tuffs, the Braich tu du Volcanic Formation, exposed high on the flank of Pen yr Ole Wen on the north side of Nant Ffrancon. The sedimentary association throughout this contrasting volcanic sequence is consistently of marine mudstone, and the dearth of volcaniclastic debris suggests the suppression of any significant volcanic edifices. Along the outcrop, the interdigitations, juxtaposition and thickness of the various components are complex and were commonly influenced by active faults.

A number of subvolcanic intrusions are spatially associated with the extrusive volcanic rocks. These are mainly of intermediate, andesite-dacite composition with fewer rhyolites and microgranites. They occur from Penmaenmawr in the north, to Talgau in the Pass of Llanberis, in the south.

This early phase of intense volcanic activity was followed by a prolonged period of sedimentation which, in the area of the Carneddau, indicated progressive shallowing of the marine environment. Late in the interval, conglomeratic alluvial fan and braid-plain facies developed from a source in the north. Locally, these beds were reddened by oxidation during temporary emergence. It was upon this gentle, southerly dipping surface that the first major expression of acidic ash-flow tuff volcanism in Snowdonia (Capel Curig Volcanic Formation) was emplaced. This volcanism began with the eruption of a major ash flow from a centre that was possibly sited in the vicinity of the north Wales coast. From the relationship of the acidic tuff to its substrate, the ash flow is interpreted as having been transported sub-aerially southwards across the site of the Carneddau to a shoreline in the vicinity of the Ogwen valley; thence, it travelled subaqueously (Plate 20a). To the north of the Ogwen valley, in the subaerial setting, the base of the ash-flow tuff is generally planar. However, to the south, on Gallt yr Ogof, where it was deposited in water, the base of the tuff is intruded by ‘flames’ of sediment up to 25 m high, and in the Capel Curig Anticline, about Llynnau Mymbyr, large bodies of tuff within the underlying strata are detached from the main sheet. These relationships reflect the thixotropic yielding of the water-saturated substrate by the rapid emplacement of the ash-flow tuff sheet; the process was facilitated by fluidisation at the contact between the hot tuff and wet sediment. Farther south, in the Lledr valley, similar pods of tuff that were detached from the flow front are the sole representatives of the tuff sheet. The tuffs are welded in both the subaerial and subaqueous environments. The extremely limited reworking of the top of the tuff indicates rapid subsidence and re-establishment of the marine environment. Subsequent ash-flow volcanism developed from the northern centre, and a centre was initiated simultaneously in the vicinity of the Glyders, which is now marked by rhyolite intrusions that were emplaced during the late stages of the volcanic activity. This centre developed a temporarily subaerial edifice, with repeated emplacement of blocky ashflow and slumped tuffs that incorporate a significant amount of epiclastic debris. Dust-rich eruptions during the later stages resulted in accumulations of accretionary lapilli tuffs, and their distribution in the associated marine sedimentary sequence corroborates the site of the eruption centre.

Following the 1st Eruptive Cycle, there was a prolonged period of sedimentation (Cwm Eigiau Formation) prior to the initiation of the second cycle and the emplacement of the Snowdon Volcanic Group. The distribution of the sedimentary facies indicate that marine conditions prevailed over part of the area, but in the south-west, around Moel Hebog, fluvial and deltaic deposition occurred on a palaeoslope that dipped generally towards the north-east. Cross-bedded sandstones and debris flow deposits, interpreted as alluvial plain and fan deltas, pass north-eastwards into shoreface deposits interleaved with the marine sediments (Figure 17). Oxidation of the sediments indicate local emergence and the sediments record the changing position of a shoreline (Figure 18). It was on to this surface that the earliest deposits of the second cycle, the Pitts Head Tuff Formation, were emplaced.

The development of the 2nd Eruptive Cycle, represented by the Snowdon Volcanic Group, was influenced by north-east-orientated, basement-controlled fractures, which defined the Snowdon Graben, and a central Beddgelert Fault Zone. The fractures influenced both the volcanic centres and the distribution of the deposits. Broadly contemporaneous eruptions from a centre in the Crafnant country, in north-east Snowdonia, probably developed along an extension of the same lineaments. The earliest eruptions formed two subaerially erupted ashflow tuffs of the Pitts Head Tuff Formation, which overlies the oxidised alluvial deposits on Moel Hebog. The eruptive centre lay about Llwyd Mawr, where a sequence of compositionally similar tuffs was ponded in a volcanotectonic depression and juxtaposed against Llanvirn mudstone. The lowest outflow tuff can be traced along the west side of the Snowdon massif; it crossed the shoreline into the sea to the north of Cwm Caregog, and continued under water through to the Ogwen Cottage area. Throughout this distance the tuff is essentially complete, with a well defined basal zone that shows a concentration of feldspar crystals, and a massive welded central zone with local rheomorphism, which grades up into a finer grained, nonwelded, locally reworked top. Siliceous nodules are common, generally in patches, although the spectacular examples on Moel Hebog at the base of the central unit, are lithologically controlled. The tuff horizon lies just below the Soudleyan-Longvillian stage boundary, which is the most tightly constrained stage boundary within the Caradoc of central and northern Snowdonia. Thin, fine-grained tuffs at about this level along Y Braich, on the north side of Ogwen valley, and on Curig Hill, east of Capel Curig, may be the distal air fall from this eruption.

Following this early eruption, the north-east-directed palaeoslope was maintained, and the strata deposited in the interval before the next significant eruption thicken in that direction. North of the Pass of Llanberis, these strata comprise a thick succession of coarse- to medium-grained, cross-bedded sandstones with intercalations of siltstone and fine-grained siliceous tuffs that probably represent distal air-fall and water-settled vitric dust. In Cwm Idwal, the narrow quarry excavation south of Ogwen Cottage indicates their past importance as ‘honestones’. Late in the interval, basaltic volcanism was extensive, and is indicated by basalt lavas (and intrusions), hyaloclastites and tuffs, which are well developed from Cwm Cneifion to the Pass of Llanberis and farther into Cwm Llan, on the south side of Snowdon, where they are the sole element. The basalts form a coherent geochemical group, distinct from those erupted later in the cycle.

The emplacement of the Lower Rhyolitic Tuff Formation was concentrated about the Snowdon Centre. Magma movement prior to the first eruption domed and disrupted the partly unlithified sediments in an area about the Beddgelert Fracture Zone, and the earliest, acidic ash-flow tuff activity developed at a centre close to Yr Arddu (Yr Arddu Tuffs) (Plate 20b) and the Nantmor Fracture. The ash-flow tuffs, with a patchy concentration of blocks, accumulated close to their eruptive centre, which was later intruded by a rhyolite. Subsequently, the main caldera-forming activity within the Lower Rhyolitic Tuff Formation (Figure 19) developed progressively along the Beddgelert Fracture Zone. Early fissure eruptions, probably along the line of the Gwynant valley, produced welded ash-flow tuffs with large rafts of the earlier Pitts Head Tuff, south-west of Cwm Tregalan and between Beddgelert and Moel Ddu. To the east of the fracture zone, thick wedges of sedimentary and volcanic breccias, such as those on the south-east side of Llyn Dinas, developed as a result of instability caused by enhanced tectonism in the vicinity of the fracture zone. The activity progressed into the main eruptive phase and the emplacement of more than 500 m of homogeneous, nonwelded acidic ash-flow tuff about the Snowdon massif. The distribution of these tuffs, the associated facies and thickness variations, allowed the caldera structure to be clearly defined. The thickest intracaldera sequence, at Lliwedd, lay close to the eruptive centre and to the north margin of the caldera, along the line of the Llanberis Pass. There the base of the tuffs, marked by a coarse breccia, is exposed in Dinas Mot and Dinas Gromlech. The primary intracaldera tuffs thin south-westwards to Moel Hebog, where they include large rafts of the Pitts Head Tuff, close to the south-western margin of the caldera.

From the north of Llanberis Pass to Cwm Idwal, a nonwelded ash flow tuff outside the caldera margin, the sole primary representative of the caldera-forming activity, is overlain by tuffs and volcaniclastic sediments reworked from the caldera margin. The base of the outflow tuff includes numerous blocks in irregular patches and entrained in layers, and the overlying reworked tuffs display a progressive increase in the sedimentary component. A thick rhyolite near the top of the sequence in the core of the syncline at Cwm Idwal forms a prominent feature along the east limb to Esgair Felen, high above Llanberis Pass; the fine-grained, columnar jointed, devitrified rhyolite is autobrecciated at its top contact, and is overlain by cross-bedded coarse-grained sandstones crowded with rhyolitic debris.

To the east, the outflow tuff, a single primary, nonwelded, ash-flow tuff, about 40 m thick, was transported into deeper water, and forms a persistent outcrop from Betws y Coed and Capel Curig in the south to Conwy in the north. Subsequently the Crafnant Centre (Crafnant Volcanic Group) developed in north-east Snowdonia in a basin dominated by mud deposition. The influence of the Snowdon Centre was minimal. The main activity was of acidic ashflow tuff volcanism, with much slumping and soft-sediment disruption, but to the north, at Tal y Fan, a contemporaneous basic centre is marked by a restricted sequence of basaltic lavas and tuffs with a coeval dolerite sill (Tal y Fan Volcanic Formation). At the Snowdon Centre, the form of the caldera established from the distribution of the tuff facies is corroborated by the distribution of five geochemically distinct groups of rhyolites, which were successively emplaced along specific annular and linear patterns within the structure. The earliest rhyolites were related to a resurgent phase within the caldera, which initiated a period of shallow marine reworking of the intracaldera tuffs, prior to an episode of basaltic volcanic activity (Bedded Pyroclastic Formation) (Figure 20). In Cwm Glas, rhyolitic pebble and cobble conglomerates that lie on the reworked top of the intracaldera tuffs are overlain by a rhyolite lava, which was itself eroded into small stacks. The contemporaneous emplacement of a large rhyolite intrusion, exposed in the face of Cyrn Las, deformed the intracaldera tuffs into a well-defined syncline.

The Bedded Pyroclastic Formation forms extensive outcrops about the Snowdon massif and in the cores of the Cwm Idwal, Moel Hebog and Dolwyddelan synclines. Its base is variably conformable to unconformable on the Lower Rhyolitic Tuff Formation. It is dominated by basaltic volcaniclastic sedimentary rocks, with high level intrusive and extrusive basalts, hyaloclastites and basic tuffites. The formation is well exposed in the outcrops between Cwm Glas and the north-east face of Snowdon (Plate 21); (Plate 22). The succession records the complex interplay between uplift, subsidence, volcanism and sedimentation in a shallow-marine environment, and developed from the shoreline reworking of small Strombolian-type island volcanoes. Shoreline cliffs formed and debris was reworked into offshore fan deltas and distal turbidite aprons.

At Cwm Idwal, in the core of the syncline, a sequence of cross-laminated basaltic tuffites with ripple-marked bedding planes and a late Longvillian fauna overlie the acidic debris flow tuffites at the top of the Lower Rhyolitic Tuff Formation. The overlying pillowed basalt and basaltic breccias, which form the cliffs about Twll Du, are most easily examined in the blocks within the landslip in the slope below. The syncline at Moel yr Ogof, south-west of Snowdon, also has pillowed basalts, pillow breccias and hyaloclastites in the core, which are overlain by bedded basaltic tuffites with two relatively massive basalt flows. At Dolwyddelan, the sequence, predominantly of basaltic tuffites with volcaniclastic turbiditic sandstones and siltstones, thins to the south across the axis of the syncline, reflecting its accumulation in deeper water at some distance from the eruptive centre. The final expression of volcanism at the Snowdon Centre was of acidic activity (Upper Rhyolitic Tuff Formation), which is represented in restricted outcrops high on Crib y Ddysgl ridge, in small synclinal outliers on the ridge east of Lliwedd and in the Dolwyddelan Syncline. The most spectacular outcrop is at Clogwyn y Person (Plate 23) where a thick, acidic ash-flow tuff in the main cliff oversteps bedded acidic tuffs and tuffaceous siltstones to rest directly on the basaltic volcaniclastic sequence. Locally there is evidence for emergence and littoral erosion with basal unconformities, intrusion of rhyolites into wet sediment and dykes feeding rhyolite domes. At the base of the sequence, rounded rhyolite pebbles lie on a slightly undulose unconformity and indicate local emergence and littoral erosion. Along Crib Goch, an intrusive rhyolite, of probably similar age, forms one of the most spectacular outcrops in Wales.

In eastern Snowdonia, at Dolwyddelan, most of the sequence consists of poorly bedded, heterogeneous and ill sorted mixtures of pyroclastic and epiclastic debris. Similarly, at the Crafnant Centre, the last major acidic event was the emplacement of a massive tuff, with little evidence of internal sorting or bedding, as exposed in the high scarps of the Gwydir Forest, west of Llanrwst. It is a heterogeneous mixture of acidic shards, pumice clasts and feldspar crystals in a dominantly mudstone matrix. The mixture is considered to have resulted from an explosive eruption through unlithified mud. However, near Dolgarrog, a final expression of basic volcanism resulted in a restricted accumulation of basaltic lava, pillowed breccia and hyaloclastite asssociated with black graptolitic mudstone (Dolgarrog Volcanic Formation) (Figure 13a). In central and south-west Snowdonia the strata overlying the volcanic sequence have been removed by erosion, but in the core of the Dolwyddelan syncline black pyritic and graptolitic mudstone overlies the volcanic sequence. Similar mudstone occurs throughout north-east Snowdonia (Llanrhychwyn Slates, Cadnant Shales), and the absence of any significant reworking of the underlying volcanic sequence indicates both its containment in a marine environment and rapid subsidence. In the vicinity of the small basic centre at Dolgarrog, a stratified pyrite deposit (Cae Coch) occurs at the basal contact of the black mudstone; the ore deposit formed by hydrothermal exhalation on to the sea floor.

Graptolites in the black mudstone indicate a D. clingani Biozone age and possibly part of the underlying D. multidens Biozone. The accumulation of black mud, which reflects an euxinic environment, was widespread throughout north Wales at this time (Nod Glas Formation). The mudstone is thickest, up to 450 m, in north-east Snowdonia where there is no evidence of significant reworking in spite of the close association with the thick volcanic sequences. The relationships suggest extensive and rapid postvolcanic subsidence within the Snowdon graben.

During the evolution of volcanism at the Snowdon centre both acidic and basic magma bodies were emplaced within the accumulated sequence, but with little surface expression. The intrusions can be broadly subdivided into acidic and basic with only a single intermediate, andesitic intrusion (Llyn Teyrn) exposed. The major acidic plutons include the Tan y Grisiau Granite and the Mynydd Mawr and Ogwen microgranites. The basic intrusions are of dolerite and basalt. The dolerites are a persistent feature of the Caradoc outcrop, but only in association with the extrusive volcanic sequence within the Snowdon Graben.

Emplacement of many of the intrusions stimulated hydrothermal solutions, which pervaded the centre and profoundly altered the sequence in places. The hydrothermal alteration included all mineralogical, chemical and textural changes in rocks resulting from interaction with hot water of varying chemistry. The processes were complex, in many instances changing the bulk chemistry of the volcanic rocks, and elsewhere Cu/Pb/Zn mineralisation was a distinctive feature. Devitrification of the volcanic glass, the dominant element of both the acid and basic volcanic rocks, was intense. Volcanogenic control of Cu, Pb and Zn sulphide deposition is most evident about the Snowdon centre (Figure 21). The extensive small workings for metalliferous sulphides, evident from both excavations and waste tips that occur across the Snowdon massif, date largely from the 19th century. The sulphides occur both in thin veins and in disseminations about lithological contacts. Only in a few cases, as at Britannia Mine in Cwm Glaslyn, have individual veins been worked for more than 200 m along strike. The main metallic minerals, pyrite, chalcopyrite, sphalerite and galena, are associated with calcite and quartz. Most are fracture-fill veins, probably the result of hydraulic fracturing, with a variety of ore textures and, in places, clasts of wall rock. The distribution pattern of old workings indicates a marked concentration along the Beddgelert Fault Zone and particularly within the basic lithologies of the Bedded Pyroclastic Formation, close to its basal contact with the Lower Rhyolitic Tuff Formation. Because of the close relationship between mineralisation and the caldera structure, it is suggested that the metals were derived by leaching of the volcanic rocks by a convecting hydrothermal cell that incorporated meteoric (sea) water and was driven by a residual, subcaldera, magmatic heat source.

On Anglesey, basal Caradoc breccias overstep on to the Monian Supergroup at Carmel Head, and in the south coarse sandstones with interbedded mudstones transgress older Ordovician rocks; the strata have yielded Costonian brachiopod faunas and gracilis Biozone graptolites. At Parys Mountain, a complex association of acidic ash-flow tuff, rhyolite and breccia with some carbonated basalt, probably of Caradoc age, is juxtaposed against mudstone. The volcanic rocks are host to a massive sulphide deposit, which has been worked possibly since the Bronze Age (Plate 24) and is still a subject of intense exploration. Elsewhere, shales with a clingani Biozone fauna, which overlie basal Caradoc sandstones and siltstones, reflect progressive deepening during continued transgression.

On Llŷn, Caradoc strata overlie Arenig to lower Llanvirn strata. Near Pwllheli, acidic ashflow tuffs have been correlated with the Pitts Head Tuff Formation, but the main volcanic sequence of trachyandesite and acidic ash-flow tuffs (Upper Lodge Volcanic Formation) was derived from a local centre (Figure 13a). The Llanbedrog Volcanic Group comprises similar lithologies, but is associated with reworked, locally derived volcaniclastic conglomerate, sandstone and siltstone. Coeval intrusions form a prominent feature (Plate 25), as in the stock-like granodiorite intrusions at Bwlch Mawr and Caer Gribbin on the north coast and the gabbroic dolerite sill at Mynydd Penarfynydd. The sequence is overlain by black mudstone of the Nod Glas Formation.

In south-west Wales, extensive dark grey, silty, graptolitic, pyritous mudstone is Caradoc in age, and indicates that relatively deep water and low energy conditions had persisted since late Arenig times. However, in north Pembrokeshire and south Cardiganshire, the sedimentation was influenced by movement on the Newport Sands Fault. South of the fault, sedimentation was mainly of mud, which now comprises the Pen yr Aber and Cwm yr Eglwys mudstone formations. North of the fault, the upper part of the Cwm yr Eglwys Mudstone Formation interdigitates with and is overlain by turbiditic sandstone, mudstone, slumped beds and conglomerate of the Dinas Island Formation (Plate 26), which is well exposed in the cliff sections between Dinas Head and Poppit Sands. In Carmarthenshire, the deposition of mud graded into more lime-rich beds (Mydrim Limestone) at about the gracilis-multidens Biozone boundary (Figure 14). At Llandeilo, the intertidal to subtidal siltstone and mudstone persisted into early Caradoc times, and grade up into deep-water black shales (Dicranograptus Shales). Along the Tywi Anticline, faulted inliers of blue-grey and black mudstone (St Cynllo’s Church Formation) with a few thin sandstone and bentonite bands are of multidens and clingani Biozone age.

In the northern part of the Builth Wells inlier, coarse basaltic breccias of the Trelowgoed Volcanic Formation are of probable Caradoc age, and are possible correlatives of agglomeratic tuffs, rhyolitic breccias and spilitic basalts interbedded with mudstone and siltstone of gracilis Biozone age, near Llanwrtyd. Localised slump deposits may indicate contemporaneous fault activity at the basin margin. In the inlier and along the Tywi Anticline, impersistent sheets of dolerite intrude Caradoc mudstone, and the irregular peperitic contacts indicate that the sediment was unlithified at the time of emplacement.

Ashgill

Following the Nod Glas anoxic event in late Caradoc times, there was a gradual change of deposition into grey mud and silt, commonly calcareous, in Ashgill times. The change was related to a general shallowing of the basin in response to the maximum extent of the glaciation, which had developed on the Gondwana continent. In places, coarser shoreface and submarine fan deposits accumulated. However, graptolitic mudstone of latest Ashgill persculptus Biozone age marked the initiation of the transgression that continued into the Llandovery.

In north-east Snowdonia, in the vicinity of Conwy, blue-grey mudstone and siltstone with thin, flaggy, cross-bedded sandstone bands have yielded a shelly fauna of Rawtheyan age, indicating a significant hiatus between these beds and the underlying Cadnant Shales. The break is thought to reflect submarine erosion and non-deposition since the basal Ashgill facies, representing a relatively deep offshore environment, overlies the anoxic mudstone with no interverning shallow marine deposits. The mudstone is overlain by thickly bedded, channelled calcarenites (Conwy Castle Grit) laid down on a submarine fan; this episode of sedimentation was probably stimulated by the glacioeustatic fall in sea level. The calcarenites contain rugose corals and allochthonous elements of the Hirnantian fauna.

To the south, the Ashgill outcrop defines the Derwen Anticline and trends south-westwards to Tywyn. It persists into the Bala district and fringes the Berwyn Dome. The calcareous grey silty mudstone and subordinate shelly sandstone overlie Caradoc strata. At Bala, impersistent layers and concretions of fine-grained, pale-grey, muddy bioclastic limestone contain a rich shelly fauna of Rawtheyan age, and Pusgillian strata are also reported. The uppermost dark blue silty mudstone (Foel y Ddinas Mudstone Formation) includes the Hirnant Limestone Member with its distinctive brachiopod fauna, defining the Hirnantian, the highest stage of the Ashgill and the top of the Ordovician. This fauna contains a mixture of genera seen in the older strata as well as newly evolved forms, and represents a fundamental reorganisation of benthic community structure during one of the major episodes of mass extinction in Earth history.

In the vicinity of Corris and the Dysynni valley, the basal succession of mainly massive, pale grey, bioturbated silty mudstone (Broad Vein Formation) has been extensively worked for slate, and interbedded in the upper part is rusty-weathering graptolitic mudstone (Red Vein) of the anceps Biozone. A sparse shelly fauna includes the deep-water Novaspis-cyclopygid Association of Rawtheyan age. The formation grades up into sparsely fossiliferous, dark grey, hemipelagic mudstone (Narrow Vein Formation). In the inliers of west mid Wales, equivalent strata (Nant y moch Formation) comprise a succession of thinly bedded, turbiditic sandstone/siltstone and grey mudstone couplets with a few graptolites that indicate the anceps Biozone. The highest beds are massive, poorly cleaved, silty mudstone with isolated grains and pebbles, and bedded turbiditic quartzose sandstone (Garneddwen Formation), which display lateral variations in thickness, bedding disaggregation, loading and slump folding. The development of the formation was synchronous with the glacio-eustatic regression when sediment input from the shelf increased; equivalent lithologies occur in the Plynlimon and Machynlleth inliers (Drosgol and Bryn Glas formations). To the east, in the Berwyn Dome, the basal unconformity is diminished, but lithologies are generally similar (Dolhir Formation). South-east of the Berwyn Dome, Ashgill strata are poorly represented, but pale brown, micaceous and calcareous mudstone (Trawscoed Mudstone Formation) forms a small exposure beneath the Powis Castle Conglomerate of Llandovery age.

To the south, Ashgill rocks crop out about the Tywi and Rhiwnant anticlines. A thick sequence of pale grey, blocky and bioturbated mudstone with thin sandstone laminae (Tridwr, Nantmel Mudstones and Cribarth formations), of Cautleyan to Rawtheyan age, conformably overlie Caradoc strata, and are overlain by laminated silty mudstone, fine- to coarse-grained sandstone and conglomerate (Yr Allt Formation) of Hirnantian age, deposited in a subtidal or intertidal setting. The change was caused by the onset of the glacio-eustatic regression. To the west, Ashgill rocks crop out around the southern closure of the Central Wales Syncline, towards the Cardiganshire coast, and the eustatic change occurs between the bioturbated blue-grey mudstone with rare cross-laminated sandstone of the Tresaith Formation and the coarser clastic facies of the Llangranog Formation with a rich assemblage of trace fossils. Between Llandeilo and Haverfordwest, thin argillaceous limestones occur within the Cautleyan Stage, and near Whitland, the Mydrim Shales (Caradoc) grade up into the Sholeshook Limestone (Figure 14). The contact with the overlying Slade and Redhill Mudstone Formation is marked by a thin impersistent conglomerate that is overlain by shallow-water, blue-grey mudstone and thin grey sandstone with trilobites of Rawtheyan age. The overlying sequence is dominantly argillaceous, with a rich Hirnantian fauna at the top.

Ordovician volcanism

The extensive Ordovician volcanic activity in the Welsh Basin is consistent with the general model of Ordovician plate tectonics that places the basin in a back-arc setting. The initiation of volcanic activity, in late Tremadoc times, was an island-arc volcanic episode. Subsequently, this subduction-related episode was succeeded by mainly tholeiitic volcanism related to back-arc extension. The volcanism was essentially bimodal, with considerable volumes of tholeiitic basalts of ocean-floor affinity and rhyolites. The relatively minor volume of intermediate, andesite to rhyodacite magma, resulted from low pressure fractional crystallisation of the tholeiitic basalts. The small volume of intermediate magma and the lack of geophysical evidence for the existence of large bodies of mafic residua have been used to suggest that the large volume of acidic magma was the result of partial melting of the crust. However, extensive and concerted studies in central and northern Snowdonia concluded that the whole suite was derived by crystal fractionation from three distinct basalt parents. The parental basaltic magmas resided at subcrustal levels and periodically ascended, together with their rhyolitic derivatives, through a system of deep crustal fractures to upper crustal levels. It is argued that most of the denser intermediate derivatives did not reach upper crustal levels. Batches of rhyolitic magma, derived from basaltic magmas with minor amounts of crustal contamination, temporarily occupied small high-level magma chambers beneath the Snowdon Centre. The model does not envisage the existence of a single large, basic to acid zoned magma chamber or, contrary to earlier interpretations, that wholesale crustal melting was the dominant factor in the production of the rhyolite magmas.

Chapter 5 Silurian

The end-Ordovician, glacio-eustatic low-stand of sea level was reversed during early Silurian times when an ice cap, centred on the south pole, then located on what is now North Africa, began to melt, causing a worldwide marine transgression. Subsequent fluctations in sea level were mainly the result of changes in the volume of ice at the poles, but probably there was also some local tectonic contribution related to the closure of Iapetus Ocean. By late Llandovery (Telychian) times, the Midland Platform was totally flooded although slight changes in platform and basin distribution continued to affect the patterns of sedimentation across Wales. Broadly, it was a period of comparative tectonic stability and warm climate.

Silurian strata occupy the core of the Central Wales Syncline and can be traced in a narrow outcrop between the Harlech and Berwyn domes into the Denbigh moors and Clwydian Hills in north Wales. They also crop out on the eastern limb of the Twyi Anticline, and eastwards towards the Welsh Borderland. Elsewhere, they occur in small inliers, as at Cardiff and Usk, and in folded and thrust slices within the Variscan zone of west Pembrokeshire. It was from these Welsh outcrops and their continuation into the Welsh Borderland that Murchison in 1835 proposed the name of the system, from the ancient tribe, the Silures, and from where the names of three of the four series, Llandovery, Wenlock and Ludlow, were taken. The biostratigraphical subdivisions of the sequence, based on both graptolites and shelly faunas, are well founded (Table 4). It is estimated to span approximately 28 Ma, from 444 to 416 Ma, and throughout this time, faunal provincialism on either side of Iapetus diminished because of the closure of the ocean. Closure of Iapetus probably occurred in the mid to late Silurian, but related deformation events continued into Devonian times.

Silurian rocks through most of central Wales are mainly of mudstone and silty mudstone with interbedded sandstones, which on cursory examination appear to be remarkably uniform or repetitive. The sedimentary structures and faunal content determine their basinal character, and a traverse to the east clearly demonstrates their contrast with penecontemporneous rocks in the vicinity of the shelf in the Welsh Borderland. This marked contrast in facies inhibited a clear understanding for many of the early surveyors, and it was not until O T Jones presented his perceptive Presidential Address, ‘On the Evolution of a Geosyncline’, to the Geological Society in 1938, that the basis was laid for modern interpretation.

More recently, following a long period of research into the shelf areas, there has been a concerted effort by the British Geological Survey and others to unravel the structure of central Wales and to trace the sedimentological changes from shelf to basin. The elucidation of the basinal sequence has been profoundly influenced by the determination of the thickness and sandstone content of the turbidite units, and the amount and type of hemipelagic mudstone. The hemipelagites represent background sedimentation; they comprise black and dark grey laminated mudstone deposited beneath anaerobic bottom waters and pale grey bioturbated mudstone deposited under oxygenated conditions. Many of the formational subdivisions are based on these variations and are thus markedly diachronous.

Llandovery

The type area of the Llandovery Series lies just to the east of Llandovery, in the Tywi valley, and it was here that O T Jones, in 1925, described three ‘lithological stages’, with 13 lithological subdivisions and specific faunas. Following additional work by A Williams and others, it has been these faunal assemblages that have been the basis of Llandovery correlation (Table 4). For example, the Rhuddanian Stage was defined at a section in the vicinity of Cefn Rhuddan Farm, the Aeronian Stage from a forestry road section north of Cwm coed Aeron Farm and the Telychian Stage from a section close to Pen lan Telych Farm.

Marine faunas, particularly shelly faunas that were sensitive to water depth, document the extent of the early Llandovery transgression. Nearshore areas were characterised by the Lingula brachiopod community and progressively deeper environments by the Eocoelia, Pentamerus, Stricklandia and Clorinda brachiopod assemblages. The deep-water basinal environments were characterised by benthic faunas known only from their trace fossils, such as the Nereites assemblage, or by planktonic organisms, particularly graptolites (Figure 22), which litter the laminae of black and dark grey mudstones.

In early Llandovery times, the shelf formed a narrow zone between western Pembrokeshire and Llandovery, but broadened northwards to include the Berwyn Hills (Figure 23). Between Abbeycwmhir at the northern end of the Twyi Anticline and the west Berwyn Hills, Telychian strata overlie Ashgill strata; the unconformity is overlain by graptolitic mudstone and not the coarse clastic deposits that would be expected across a transgressive shoreline. The break represents either nondeposition or removal by slumping. The Llandovery succession is more complete both in the east Berwyn Hills and in the basin to the west. These spatial relationships suggest that the non-sequence indicates the position of the outer shelf and the adjacent upper part of the west-facing submarine slope of early to mid-Llandovery age.

To the south, near Welshpool, Llandovery strata overstep the Ashgill to rest upon strata of Caradoc age. The Meifodia–Clorinda–Stricklandia fauna of the basal conglomerate (Powis Castle Conglomerate) may be indicative of relatively deep water, which suggests that the shoreline in this vicinity was probably fault controlled (Figure 24). In the vicinity of Llandrindod Wells, the shelf facies, which outcrops east of the Garth Fault, comprises distal shelf-ramp mudstone similar to the slope-apron mudstone to the west (Figure 25). Its age ranges from Telychian (Cerig Formation) to early Wenlock (Builth Mudstones Formation), and in the Builth Wells district it rests unconformably on Ordovician strata. In spite of the thickness and facies changes related to penecontemporaneous fault movements, the main influences on sedimentation were changes in sea level. These mudstones (Telychian Stage) crop out around the closure of the Tywi Anticline, between Rhayader and Abbeycwmhir, but, on the eastern side of the anticline, the Telychian sequence rests with marked disconformity both on earlier Llandovery and Ordovician strata. However, slumped and disturbed beds and turbiditic pebbly sandstones (Henfryn Formation) overlie the disconformity, and have been interpreted as the result of coeval movement along the Tywi Lineament. This would confirm an erosional model for the dis-conformity rather than a non-depositional slope.

At Llandovery, the sequence is up to 1200 m thick with possibly one internal unconformity. It has been divided into eight formations comprising mudstone and fine-grained sandstone with parallel and cross-laminations, accompanied by a few thin, fine-grained, micaceous shelly sandstones and calcareous sandstones that are deeply weathered to rotten-stones. However, the biofacies is distinctive, and L R M Cocks and co-workers have correlated the evolutionary lineages in brachiopod faunas to the graptolite and acritarch biozones. The mud-dominated deposition of the shelf sequences reflects relative stability, and the sandstone beds indicate periodic storm events.

A similar mudstone-dominated sequence can be traced south-westwards into the northern limb of a syncline in the vicinity of Haverfordwest, but in the inliers, on the south limb, the facies is more distinctly littoral. For example, at Rosemarket, conglomerates and sandstones (Rosemarket Beds) of Telychian age, with fossils representative of the Pentamerus and Clorinda Communities, unconformably overlie Precambrian rocks, and at Marloes similar strata overlie the Skomer Volcanic Group.

The Skomer Volcanic Group, up to 1000 m thick, is well exposed about Skomer Island, Midland Island and the Marloes peninsula. Because of the proximity of the group to the outcrops of lower Ordovician volcanic rocks, it had been regarded as Arenig in age. However, the volcanic rocks are interbedded with, and overlain by, sedimentary rocks that contain upper Llandovery faunas (Coralliferous Group) and particularly those of the nearshore Lingula and Eocoelia Communities. The conformable boundary with the Wenlock Series is contained within the Coralliferous Group. The most abundant rock type in the Skomer Volcanic Group is basaltic lava and some of the flows are pillowed, indicating subaqueous emplacement. However, other flows with distinctively reddened top surfaces suggest subaerial weathering during temporary emergence. Flow-banded rhyolites, such as that at Garland Stone, are thick, generally restricted in extent and, unlike the basalts, shed much clastic debris into the associated sedimentary environment. Finely laminated, acidic dust tuffs occur low in the sequence and much volcaniclastic material is incorporated into debris flow deposits. A thin acidic ash-flow tuff near the top of the sequence provides a clear datum plane eastwards between Mew Stone and The Neck, on the south and west of Skomer Island, to Midland Island, and just south of Jeffry’s Haven on the mainland. The volcanic group is unconformably overlain by conglomerate and sandstone, which in turn are overlain by green siltstone with thin bentonite beds. Evidence of early Silurian volcanic activity of broadly similar chemical character was determined in a borehole near Maesteg, and farther east in the Tortworth inlier in Somerset. The chemical signature of some of the widespread bentonite bands in the Welsh borders and across the Midland Platform has facilitated correlation, but the source of the volcanic dust has not been identified; it is possible that the activity was sited on the southern edge of Laurentia.

West of Rhayader, a slope-apron facies, deposited within the basin, comprises thinly bedded mudstone turbidites (derived from the east) with interbedded hemipelagites. The mudstone turbidites are variably siltstone/mudstone couplets, structureless or graded mudstone, or silt-laminated mudstone capped by graded and/or structureless mudstone. The hemipelagites are laminated or burrowed; the laminated types display delicate, continuous alternations of dark grey carbonaceous laminae and pale grey silty laminae. The sparse bioturbation and the preservation of organic debris support accumulation in anoxic bottom water conditions. Where bioturbation does occur, the hemipelagites are pale grey and without discernible carbonaceous debris; they indicate relatively oxic bottom conditions and the pale colour reflects destruction of organic carbon by downward migrating oxidation fronts.

The character of the hemipelagites allowed discrimination of anoxic, oxic and mixed slope-apron facies to the west of Rhayader (Figure 25). During Rhuddanian (Cwmere Formation) and late Aeronian (M. sedgwickii Shales) times, the influx of terrigenous silt was reduced and anoxic facies sedimentation was established. With the mixing of the basin waters and a higher silt input, an oxic facies was established in Aeronian times (Derwenlas Formation) and early Telychian times (Rhayader Mudstones Formation). A mixed facies formed across parts of the proximal apron (Tycwtta Mudstones Formation), and at the same time there was anoxic facies deposition farther west. Within these dominantly mudstone sequences a complex of channels and lobes of turbiditic, coarse-grained and pebbly sandstones and conglomerates has been distinguished both on the eastern limb of the Central Wales Syncline (Caban Conglomerate Formation; (Plate 27)) and on the western limb (Ystrad Meurig Grits Formation). The determination of the distribution of the internal facies of both these formations, and their intricate relationship with the slope-apron facies envelope, has illuminated the details of the architecture of this ancient system (Figure 25).

As the sea transgressed eastwards across the Midland Platform in late Llandovery times, the amount of sediment delivered into the Welsh Basin from the east waned considerably. Simultaneously, there was a progressive increase in the influence of northerly directed channel systems from a landmass (Pretannia) to the south.

Reconstruction of the basinal sequences in central Wales, in the Llanilar–Rhayader area, has shown that after the deposition of the Caban Conglomerate Formation there were three main events when sand was distributed widely in the mud-dominated sediments of the basin. The earliest, in early Telychian times (turriculatus Biozone s.l.), forms the striking sequences in the sea cliffs around Aberystwyth (Aberystwyth Grits Group) (Plate 28). The formation comprises turbiditic sandstone and mudstone in variable proportions; the sandstones rarely exceed 0.3 m in thickness. The sequence was one of the earliest to be ascribed to density current deposition, and was described, in a seminal paper, by A Wood and A J Smith. In addition, there are many folds and dislocations within these outcrops that have caused much debate as to whether they are the result of early soft-sediment deformation or later tectonic deformation.

The base of the Aberystwyth Grits Group is exposed at Harp Rock (Plate 29), near Borth, where it conformably overlies the Borth Mudstone Formation, and it crops out to the south, to beyond Newquay. The turbiditic sandstones are generally fine grained, and sedimentary structures such as flute casts, groove casts, bounce casts, grading and convolute lamination are abundant. In the northern part of the outcrop, where the turbidity currents were distal, dilute and non-erosive, thin, black, pyritous, graptolitic and carbonaceous mudstone beds are common. The mudstones indicate brief intervals of anoxic bottom conditions, which later became dominant.

The problems of the distribution of the Aberystwyth Grits Group facies (Figure 26a, b) and its lateral relationships are typical of all these diachronous Llandovery formations. The group shows marked changes from south to north with increasing distance from source, and from west to east towards the Bronant Fault. The Borth Mudstones Formation, which clearly underlies the group near Borth, is, at least in part, the lateral equivalent, and the graptolite faunas are closely comparable; the group lies entirely within the turriculatus Biozone s.l. In sharp contrast to the eastward derivation of the underlying formations, the turbiditic element of the Aberystwyth Grits Group was derived mainly from the south; the current outcrop represents part of a turbidite fan. This change in provenance is considered to reflect contemporaneous tectonic and volcanic activity in Pembrokeshire.

In Telychian times, the eastward margin of the southerly derived turbidite systems migrated progressively eastwards (Figure 25). Within the Central Wales Syncline, the Cwmystwyth Grits Group comprises thinly bedded turbiditic sandstone/mudstone couplets, with lesser hemipelagites. However, it includes abundant thick-bedded turbiditic sandstones, which have been given local names (Rhuddnant Grits and Pysgotwr Grits formations). The thickness of the group and its constituent formations vary considerably, and some of these changes are controlled by major faults. The high-matrix sandstones of the Rhuddnant Grits form beds up to 1.5 m thick, and occur in packages up to 50 m thick. A concentration of high-matrix sandstone at the base (Llyn Teifi Member) is well exposed in the crags about Llyn Teifi. There is no systematic distribution and the sandstone lithologies pass laterally into turbidite sequences that are dominated by thinly bedded sandstone/mudstone couplets (Glanyrafon Formation). However, the dominant transport directions are from the south. On the western limb of the syncline, the lower part of the Rhuddnant Grits Formation spans the late turriculatus Biozone while the top encompasses much of the crispus Biozone. In contrast, most of the formation on the eastern limb lies within the crispus Biozone (Table 4).

Similar high-matrix sandstones are the dominant lithology of the Pysgotwr Grits Formation, which crops out about Llyn Cerrigllwydion and in the upper reaches of Afon Ystwyth. They were derived from the south, and the sequence spans the crispus and griestoniensis biozones. By comparison with modern turbidite systems, both the Rhuddnant and Pysgotwr Grits formations are mainly of the median sandstone-lobe facies, and the amalgamated beds of the Llyn Teifi Member are of proximal settings. The systems did not evolve by the free progradation of sandy fans but were tectonically controlled — the sandy (proximal) facies are anchored by bounding faults and prevented from prograding over less sandy (median to distal) facies.

Into north Wales, Llandovery rocks form a narrow outcrop along the northern limb of the Derwen Anticline to the vicinity of Cerrig y Drudion, and a restricted outcrop, at Capel Garmon, on the west side of the Conwy Valley Fault. Farther north, a narrow outcrop of blue black shales (Gyffin Shales) between Conwy and Deganwy is where G L Elles began her pioneering studies on the stratigraphical importance of the graptolite faunas. The Bryn Dowsi Mudstone Formation, which spans uppermost Ashgill to middle Llandovery, comprises dark grey hemipelagic mudstone that is graptolitic in places, turbiditic mudstone and pale burrow-mottled mudstone. Towards the top of the sequence (Telychian Stage), black graptolitic mudstone is overlain by pale, mottled grey-green, turbiditic and hemipelagic mudstone (Pale Slates). The strata are closely comparable with coeval sequences in mid Wales and suggest tectonic quiescence. Similar, but less complete deep water mudstones occur at Parys Mountain on Anglesey and close to Llanystumdwy on Llŷn. At both localities, there is no indication of the influence or the proximity of the north-western edge of the basin. In south-east Wales, at the south-western edge of the Midlands Platform, borehole and geophysical data have proved thick basinal accumulations of early to mid Llandovery strata, unconformable on Tremadoc strata, in the Woolhope and probably Usk basins.

Wenlock

Wenlock strata form extensive outcrops throughout Wales. From the vicinity of Conwy in the north, they can be traced southwards and eastwards around the Denbigh moors into the core of the Central Wales Syncline, west Berwyn Hills. From there the Wenlock outcrop extends south to Llandrindod Wells, and from there to the south-west forming a narrow impersistent outcrop along the eastern limb of the Twyi Anticline into Pembrokeshire. In addition, Wenlock strata crop out in inliers at Cardiff and Usk.

The late Llandovery marine transgression had, by early Wenlock times, established an extensive shelf sea across the Midland Platform (Figure 27). West of the platform, a broad Welsh Basin extended from Pembrokeshire to Denbighshire, and this may have become shallower farther to the west with possible emergence. However, the dominant source of sediment transported into the basin was from the landmass of Pretannia to the south.

In early Wenlock times, there was restricted deposition of carbonates in the Welsh Borderland but this became more widespread with the accumulation of the Much Wenlock Limestone Formation, of late Wenlock age, across the Midland Platform. It is here, near Much Wenlock, that the boundary stratotypes for the base of the Wenlock Series and its stages were established. At localities on either side of the border, there is evidence of overstep at the base of the Wenlock. For example, at Presteigne, a basal algal- and bryozoan-rich limestone rests on lower Llandovery strata, and at Old Radnor it rests on Precambrian. However, in early Wenlock times, muddy and variably calcareous sediments, the Buildwas and Coalbrookdale formations, were deposited across most of the shelf. The carbonate element diminishes towards the basin edge; the shelf mudstones contain a rich shelly fauna of brachiopods, trilobites and corals, and a sparser graptolite fauna.

Within the basin, in the Rhayader area, alternating burrowed, mottled and laminated mudstone spans the Llandovery–Wenlock boundary (Dolfawr Mudstones Formation), and hemipelagites of the Nant-ysgollon Mudstone Formation mark a return to oxygen-poor bottom conditions and deeper water (Figure 27). These slope apron sediments were gradually overwhelmed by deposition of muddy turbidites, seen in the upper part of the Nant-ysgollon Mudstone Formation and in the contemporaneous Penstrowed Grits Formation farther east, part of an extensive sandstone lobe facies that was supplied from the south. The medium- to thick-bedded, high-matrix turbiditic sandstones, similar to those of the Llandovery, are interbedded with thin siltstone turbidites, laminated hemipelagites and few bentonites. The abundant bioclastic debris reflects the proximity to the eastern shelf edge. The grits are the youngest and most easterly sandstone-lobe facies in the southern part of the Welsh Basin. Major slump sheets in the Builth Mudstones Formation developed from steep, west-facing slopes and syndepositional movement, for example on the Garth Fault. The mudstones are dark, blue-grey, laminated and graptolitic; they reflect a further deepening of the basin and anaerobic conditions, which were largely sustained throughout the Wenlock.

Around Montgomery, the Bromleysmill Shale and Aston Mudstone formations similarly define the basin to shelf transition. Non-bioturbated mudstone in the vicinity of Gregynog passes laterally to the south-east into intensely bioturbated and homogenised mudstone with thin limestones in the proximity of Bishop’s Castle at the edge of the Midland Platform.

Through most of central and north Wales, the basinal sequence was dominated by the northwards transport of turbidites (Figure 27). In Denbighshire, Wenlock strata define the broad syncline between the Conwy valley, in the west, and the edge of the Vale of Clwyd, in the east; small outcrops occur farther east in the Clwydian Hills. The Denbigh Grits Formation (Figure 28) is thickest in the Llanrwst district and thins eastwards into the Clwydian Hills. It comprises an alternation of sandstone, grey and dark grey siltstone and mudstone (striped beds) and disturbed beds. The sandstone ranges from fine grained through to coarse grained and pebbly. The proportions of the various lithologies vary markedly, both vertically and laterally. Thick sandstones occur only in the lower half of the succession. The poorly sorted sandstones with an abundant clay matrix, graded bedding and current structures are typical turbidity current deposits. Both proximal and distal deposits have been distinguished. In contrast to the outcrops farther south, transport was from the west to south-west, and was possibly influenced by the scarp of an active Conwy Valley Fault. Disturbed beds are the predominant lithology of the Berllan and Llanddoget formations. They include a complete spectrum of lithologies, and vary from contorted to completely destratified strata resulting from subaqueous slumping, initiated either by seismic shock or by the rapid loading of sediment onto a water-saturated layer.

As the sandstone lobe facies moved westwards in late Wenlock to early Ludlow times, anoxic mud (Nantglyn Flags Formation) was deposited on the slope apron. The formation is dominated by regular alternation of turbiditic, silty mudstone and laminated muddy siltstone bands with locally developed thin calcareous siltstone (ribbon-banded mudstone); calcareous concretions form in discrete layers. Current data indicate that most of the sequence was transported from the west, although the calcareous siltstones indicate derivation from a southerly source. Towards the end of Wenlock times these patterns were interrupted by two episodes during which shelly and bioturbated mottled mudstone was deposited over much of the basin, probably in response to a global marine regression that caused a temporary return to oxygenated depositional conditions and colonisation of the sea floor by benthic organisms.

South-west from Builth Wells towards Llandeilo, along the southern limb of the Tywi Anticline, calcareous mudstone, sandstone and impure limestone become progressively more apparent. The strata contain the distinctive shelly faunas of the shelf facies of the Welsh Borderland. Also in this direction, upper Wenlock strata (Homerian) overstep lower Wenlock strata (Sheinwoodian) on to Llanvirn shales just east of Llandeilo and beyond, reflecting continued tectonic activity along the Tywi Lineament.

In south-east Wales, Wenlock strata crop out in an inlier in the suburbs of Rumney and Pen y Lan in Cardiff and in the core of the anticline in the Usk inlier. At Cardiff, the lowest strata are typically grey-green mudstones, calcareous and silty in places, with thin sandstone bands (Pen y Lan Mudstones). The sandstones are fine grained with planar bases and diffuse and wave-rippled tops. The sequence contains a rich shelly fauna of late Wenlock (Homerian) age, and occupied a shallow marine, mid-shelf environment. The sandstone and bioclastic limestone beds were probably storm induced. The mudstones are sharply overlain by a pebbly, cross-bedded, fine- to coarse-grained sandstone (Rhymney Grit), which lies at the base of a sequence of sandstone, siltstone and mudstone with few thin limestones, conglomerates and an ironstone (Cae Castell Formation). The basal sandstone, containing laminae of carbonaceous debris, was deposited during a temporary shallowing of the marine environment and is interpreted as a subtidal sand bar.

The Wenlock calcareous mudstone and siltstone in the core of the Usk inlier (Glascoed Mudstone Formation) are similar to those at Cardiff. The generally offshore character of the mudstone is progressively replaced by facies with nearshore and possibly lagoonal features, and clastic and carbonate deposition in the overlying silty sandstone (Ton Siltstone Formation) and limestone (Usk Limestone). The limestones are the most southerly development of carbonate precipitation in the late Wenlock.

In south Pembrokeshire, Wenlock strata are exposed in small inliers between Freshwater East and Marloes. At Marloes and Wooltack, the lowest Wenlock strata comprise a thin sequence of blue-grey and green siltstone, mudstone and calcareous sandstone near the top of the Coralliferous Group (late Llandovery to early Wenlock age) which unconformably overlies the Skomer Volcanic Group. The strata contain a rich shelly fauna that are typical of a shelf environment, including the brachiopods Costistricklandia lirata lirata and Eocoelia sulcata, and the coral Palaeocyclus porpita. These beds pass up into flaggy bedded sandstone and siltstone with softer brown and grey sandy mudstones that included thin weathered rottenstone bands (Gray Sandstone Group), which are well exposed (apart from the base) south of the prominent Three Chimneys in Marloes Bay (Plate 30). Fossils are generally rare, but can be common in some rottenstones, particularly in the lower part of the succession, where they include brachiopods and corals that suggest an early to mid Wenlock age. Higher beds have yielded a more restricted fauna with inarticulate brachiopods (Lingula sp.) and indeterminate bivalves that are not diagnostic of age, but these beds are probably still Wenlock. The sequence coarsens upwards, and ripples, scours and cross-bedding are common throughout. It is interpreted as a transition from a marine to shallow marine environment with deltaic incursions and some intertidal influence. Interbeds of red sandstone occur with increasing frequency, reflecting the establishment of a fluviatile floodplain environment in the Old Red Sandstone facies that forms such a prominent feature at the western edge of the bay.

Ludlow

The marine transgression in early Ludlow times had little effect on the broad palaeogeographical framework of the Welsh Basin. However, subsequent prolonged marine regression with a few transgressive pulses finally obliterated the broad sedimentation patterns that had dominated the Welsh Basin since early Cambrian times. The basin silted up and, before the close of the epoch, terrestrial red-bed sedimentation had become firmly established in south-west Wales, although in different places at different times (Figure 29).

Throughout Wales, Ludlow strata conformably overlie Wenlock strata; the type area for the series lies near Ludlow in the Welsh Borderland, and its base is regarded as coinciding with the base of the nilssoni Biozone. In north Wales, Ludlow strata crop out in the hinge areas of the Llangollen and Denbighshire synclines, and in the Clwydian Hills. They also crop out at Long Mountain east of Welshpool, and form a wide outcrop southwards from Newtown across Black Mountain and Clun Forest to Builth Wells. To the south-west, the outcrop is overstepped by the Old Red Sandstone, south-west of Llandeilo. Ludlow strata also form small outcrops in south Pembrokeshire and in the inliers at Cardiff and Usk.

The Gorstian marine transgression resulted in the widespread deposition of mud, increasingly calcareous towards the east, across the shelf in the Welsh Borderland, and this environment was inhabited by a diverse shelly fauna. At the same time, graptolitic mud accumulated in the Welsh Basin, and at the height of the transgression it encroached onto the shelf margin. The source of most sediment was probably the landmass of Pretannia to the south. Thin bentonites within the shelf sequence indicate contemporaneous and distant volcanism.

In the Welsh borders, there is a marked increase in thickness of Ludlow strata westwards across the shelf (Figure 30). Graptolitic mudstone (Oakeley Mynd Formation) at the base of the succession east of Clun Forest is overlain by a thick sequence of interbedded calcareous siltstone and hemipelagite with carbonaceous laminae (Bailey Hill Formation) deposited by turbidity currents and mass sliding on the outer shelf. Upwards, the sequence becomes progressively more typical of a shallow marine environment, with an abundant shelly fauna. Large slump folds lie along the axis of a trough, which persisted between Clun Forest in the north and New Radnor in the south into late Ludlow times, with silt being transported into an area of hemipelagic mud. Graptolites, orthocones and a poor benthic fauna suggest poorly oxygenated bottom waters, below wave base.

Ribbon-banded mudstone of the slope apron can be traced into Denbighshire in the upper part of the Nantglyn Flags Formation, and the Neodiversograptus nilssoni Zone has been identified. The disturbed beds, which are a dominant element in the lowest part of the sequence, decrease upwards (Plate 31), and here consist entirely of mudstone and are generally restricted to the area between Llangerniew and Llansannan. The top of the formation is defined by a facies change to turbiditic sandstone, striped siltstone, mudstone and disturbed beds with rare ribbon-banded mudstone (Elwy Group) (Figure 28). The easterly directed sediment transport along the axis of the Denbigh trough persisted from Wenlock times.

In the well-defined outcrop between Builth Wells and Llandeilo, there is marked thinning of the sequence and an increase in the proportion of impure shelly limestone and decalcified rottenstone. Most distinctively, near Llandeilo, there is a restricted sequence of conglomerate and coarse-grained, cross-bedded, ripple-marked deltaic sandstone (Trichrug Formation) with red beds reflecting temporary emergence. These sandstones indicate an increasing clastic input and the proximity of a landmass to the south. To the south of Builth Wells, in the Wye valley, the river traverses the Ludlow sequence between Llanelwedd and Twmpath. The section is one of those examined by R Murchison, where he recognised the transition from fossiliferous offshore mudstone and siltstone, with slump bedding, into the Old Red Sandstone.

In the Cardiff inlier, sheet sandstones and limestones in a dominantly mudstone sequence (Cardiff Group) are interpreted as sublittoral deposits, laid down above storm wave base in a mid-shelf environment, with a rich and varied fauna. Towards the top of the sequence, amalgamated sandstones and limestones suggests a more proximal shoreline association, deposited around normal wave base. In the Usk inlier, nodular silty limestone is common in the lower part of the sequence in the eastern outcrops, but to the west the carbonate content decreases.

Towards the top of the sequence, shelly conglomeratic limestone shows evidence of hardground development. The sequence reflects the progressive shallowing on a distal shelf environment passing into a nearshore, high-energy, proximal shelf setting with evidence of traction and storm processes. In Pembrokeshire, fluviatile conditions were established during early Ludlow times. At both sides of Marloes Bay, reddened marine, grey and green mudstone and quartzite (Gray Sandstone Group) are conformably overlain by red siltstone with mud-cracked surfaces and calcretes with subordinate sandstones and air-fall tuffs (Red Cliff Formation). By late Ludlow times, the Irish Sea landmass had probably merged with Pretannia. With continuing emergence, the area of marine deposition across Wales was becoming more restricted and, progressively, the basin was silting up.

Přídolí

In Přídolí times, coastal and eventually fluvial/alluvial environments dominated the patterns of sedimentation. In mid Přídolí times, brief marine incursions encroached across the Midlands into the Welsh Borders, but farther west the Old Red Sandstone molasse facies spread across the emergent landmass. The classic localities of the Přídolí sequence lie in the Welsh Borders and particularly around Ludlow, where it comprises ripple-laminated sandstone and olive-green lenticular siltstone (Downton Castle Sandstone Formation). At its base, the Ludlow Bone Bed Member less than 0.3 m thick, contains five or more thin beds, each a few millimetres thick, with fish and brachiopod fragments, which formed as lag sand concentrates in the intertidal zone. Westwards, Přídolí strata crop out in the outliers around Clun Forest and Knighton, and are extensively exposed across the Black Mountains and Mynydd Eppynt. The olive-green mudstone and grey-green micaceous sandstone (Temeside Mudstone Formation and equivalents) are overlain by red-brown to purple-brown micaceous mudstone (Raglan Mudstone Formation;(Figure 33). The latter include subordinate sandstones, which, in association with common calcretes, are stained green and purple. The sandstones form the bases of fining-upwards, alluvial cycles that are rich in minerals and rock fragments typical of a metamorphic terrain, possibly derived from north-west Scotland. The sedimentological features suggest deposition from streams meandering across a coastal plain. These beds, of mid Přídolí age, have yielded a diverse flora of early plant megafossils.

Calcretes are distinctive and common in the Raglan Mudstone Formation. They range from scattered limestone nodules to rubbly and massive limestones, and record periods of emergence and soil development. Some are estimated to have developed over 10 000 years, in relatively hot climatic conditions with low seasonal rainfall. At the top of the succession, the massive ‘Psammosteus Limestone’ is persistent throughout south Wales and the borders and, on miospore evidence, the Silurian–Devonian boundary is considered to lie slightly below it. The ‘limestone’ has been shown to be diachronous relative to the fish zones from which the name was derived. About the Wye valley, the unit has been renamed the Bishop’s Frome Limestone Member. To the south-west, the Přídolí sequence gradually oversteps the underlying Silurian strata. In Pembrokeshire, the Old Red Sandstone facies was in probably established in Ludlovian times (Red Cliff Formation), and north of Milford Haven the boundary is conformable. The Přídolí lies within the lower part of a sequence of calcareous mudstone and siltstone with common calcretes (Milford Haven Group) (Figure 33), which are interpreted as deposits of marginal marine and distal fluvial environments. Thin beds of variably coloured distal air-fall tuffs (termed ‘magenta beds’ by the early geological surveyors) are common, for example in the road section on the east side of Sandy Haven. The tuffs vary from fine-grained, ‘greasy’ weathered dust tuffs to coarse-grained crystal tuffs with flinty fine-grained tops. Most of these tuffs cannot be correlated with confidence but two, the Townsend Tuff and Pickard Bay Tuff beds, have been used extensively as marker horizons. These tuff beds are well exposed on either side of the Ritec Fault, at Little Castle Head, on the north shore of Milford Haven and along Pickard Bay, west of Freshwater West. Both horizons are composite, consisting of three beds of graded, crystal lithic tuff to dust tuff with parallel laminations and rippled tops. The Townsend Tuff Bed can be traced through the Black Mountains, where it lies some 50 m below the Bishop's Frome Limestone, and into the Welsh Borderlands. In the Cardiff inlier, the Raglan Mudstone Formation rests on a thin bone bed that is correlated with the Ludlow Bone Bed.

Caledonian orogeny

The Lower Palaeozoic rocks that accumulated in the Welsh Basin have been assigned to the ‘paratectonic’ (nonmetamorphic) Caledonides. Nevertheless, the contemporaneous movements and unconformities throughout Lower Palaeozoic times were the relatively gentle precursors of the intense tectonic activity that occurred at the culmination of the Caledonian orogeny — the Acadian phase of deformation that occurred towards the end of the Early Devonian. The orogeny resulted from the oblique collision of the continents of Eastern Avalonia and Laurentia following closure of the intervening Iapetus Ocean. In Wales, progressive collision, from late Silurian to mid-Devonian times, resulted in deformation (Figure 31) and low-grade metamorphism (Figure 32) of the Lower Palaeozoic sequence, and the inversion of the basin.

The original development of the basin in late Precambrian times was greatly influenced by fractures that had previously contributed to the construction of the basement mosaic. The north-west margin of the basin was broadly coincident with the Menai Straits Fault Zone, and the south-east margin with the Welsh Borderland Fault Zone; tectonic activity in both these zones, and the similarly aligned Bala Fault Zone, continued to affect sedimentation patterns throughout Lower Palaeozoic times. However, between these major features there are many similar lineaments that contributed progressively to the geological evolution. The development of the broadly north–south-orientated complex anticline in the Cambrian strata of the Harlech Dome was probably constrained by flanking lineaments which had previously distinguished a major centre of deposition; similar orientations are displayed in the fault-controlled Conwy valley and Vale of Clwyd. During Ordovician times in Snowdonia, the development of volcanic activity was controlled by the north-east–south-west-orientated Beddgelert Fracture Zone and the distribution was constrained by the marginal faults of the larger, similarly aligned, Snowdon Graben. On the south side of Snowdonia, the magnetic anomalies across the strikingly featured Bala Fault Zone are most easily interpreted as contrasting basement rocks. Activity continued on the fault affecting the cover rocks for some considerable time.

Within the Silurian outcrops, four prominent lineaments have been distinguished, from west to east, the Glandyfi, Central Wales, Tywi and Pontesford lineaments. The north-trending Glandyfi Lineament, in the Aberystwyth district, marks a regional divide in fold vergence and is coincident with a major fracture, the Bronnant Fault. Contemporaneous movement along this fault caused profound westwards thickening of the Aberystwyth Grits Group. Similarly, the Central Wales Lineament, situated along the axial zone of the Central Wales Syncline, influenced early Silurian sedimentation patterns.

The Tywi Lineament, broadly coincident with the axial zone of the intensely faulted Tywi Anticline, was most active in early Silurian (Telychian) times. The Garth Fault, at the eastern end of the anticline, separated the shelf and basinal sequences during Ashgill and Silurian times, and defined the eastern limit of the pervasive Caledonian (Acadian) deformation. Locally, the Tywi Lineament lies close to the Pontesford Lineament but the latter is associated mainly with a series of north-east-trending faults in the Ordovician inliers at Shelve and Builth. Throughout the basin, there is a plexus of syndepositional and postdepositional faults between these major lineaments.

All the lineaments are considered to reflect the reactivation of basement fractures and their upward propagation into the cover sequences. In their early manifestation, during basement construction, strike-slip movement was probably important, but subsequent movement, in early Palaeozoic times, was predominantly vertical, although there is some evidence, as in the Builth Wells inlier, of strike-slip movement in the late Ordovician. Several of the lineaments influenced the location and form of the later Caledonoid structures, which, in addition, were affected by the lithological contrasts. These affects are most clearly expressed in the contrasts between the Silurian maps of central Wales, in dominantly mudstone sequences, and the Cambrian and Ordovician maps of north Wales, where thick beds of competent sandstones, extrusive volcanic rocks and irregular intrusive bodies are intercalated with the mudstone sequence.

The Central Wales Syncline is flanked, to the west and east respectively, by the Teifi and Tywi anticlines (Figure 31), and these complex open folds can be traced to the southwest into Pembrokeshire where the orientation is generally closer to east–west. When traced into north Wales, the patterns of the folds are less clearly defined. The main syncline persists into the vicinity of the Bala Fault, which separates the north–south-orientated Harlech Dome, in the west, from the broadly east–west-orientated Berwyn Dome and Llangollen Syncline, in the east. North of the Vale of Ffestiniog, the structures are clearly influenced both by the vicinity of the Menai Straits Fault System and the thick competent Precambrian strata of the Bangor and Padarn ridges. The dominant structure is the main, north-east–south-west Snowdon Syncline(s), flanked to the west by the Llanystumdwy and Llŷn synclines and, to the east, by the east–west-orientated, overturned, Dolwyddelan Syncline; this complex synclinorium (nest of synclines) lies entirely within Ordovician strata. In marked contrast is the broad, open, east–west synclinal flexure in the Silurian strata to the east of the Conwy Valley Fault. On Anglesey, the Lower Palaeozoic rocks are strongly folded and overthrust, with the south-east vergence that characterises most of the Caledonoid folds across Wales.

Most of the Lower Palaeozoic rocks of Wales are cleaved and, at any one locality, the cleavage is more closely spaced and more pervasive in mudstones than in adjacent sandstones. Throughout the Lower Palaeozoic sequence, the single penetrative cleavage (S₁) is ubiquitous but its orientation and attitude is highly variable. Locally it is overprinted by a crenulation cleavage (S₂). Cleavage planes are generally defined by foliae of aligned phyllosilicates (micas), opaque and secondary minerals that separate thin laminae of non-foliated quartz, chlorite and muscovite. It is the cleavage in the mudstones that resulted in the development of the slate industry through Wales in the early 19th century, and which subsequently has had such a profound affect on the social, economic and cultural life. The extraction occured throughout the Lower Palaeozoic sequence: in the Lower Cambrian rocks of Bethesda–Nantlle, the Upper Cambrian on Llŷn, the Lower Ordovician at Blaenau Ffestiniog and Pembrokeshire, the Upper Ordovician at Corris and Aberllefenni, the Lower Silurian near Machynlleth, and Middle Silurian at Corwen and Llangollen. Such a concentration of slate extraction in such a small area must be unique. The cleavage varies from a spaced, discontinuous fabric in areas of low grade metamorphism to a closely spaced and more pronounced fabric in the higher grade areas. Most commonly, the cleavage is axial planar to the main folds and generally fans across them. However, locally in north Wales and more generally in the mudstone sequences of central Wales, cleavage transects the axial trace of the folds. For example across the Teifi Anticline and to the west of the Bronnant Fault, there is a systematic change in clockwise transection from 10° to 16° in the north to 4° to 7° in the south. It has been suggested that such clockwise transected folds reflect sinistral transpressive deformation, although the evidence in the Welsh Basin is still equivocal.

The determination of the age of the cleavage has been problematic, but recent work suggests that, in places, it was initiated at an early stage in the deformational history. Also, there is evidence that cleavage was being developed in parts of the basin when other areas were still receiving sediment. It seems likely that compression and the geothermal gradient caused the cleavage to develop progressively through the thick basinal sequence. There is a consensus that the main influence on cleavage development was during early to mid Devonian times (Emsian or Eifelian stages). Cleaved red beds of Přídolí age have been recorded along the Pontesford Lineament and on Anglesey.

The Lower Palaeozoic rocks suffered low-grade, mainly subgreenschist, regional metamorphism during the end-Caledonian orogeny. Nowhere have the original fabrics been obliterated. The most sensitive indicator of metamorphic grade is white mica (crystallinity), which is the dominant mineral of the mudstones, the main sedimentary component within the basin (Figure 32). There is a decline in white mica crystallinity from older into younger rocks: the Cambrian strata are epizonal (low greenschist facies) while Ordovician and Silurian stata are anchizonal (pumpellyite basite facies). However, this simple depth of burial pattern can be modified by inhomogeneous strain, as would be expected in strained mudrocks adjacent to the steep limbs of folded competent layers, crystallinities would be enhanced.

Platy and fibrous stilpnomelane occur in pressure shadows around opaque iron ore grains and along iron oxide residues on pressure solution seams. Porphyroblasts of chloritoid, diagnostic of greenshist facies, are irregularly distributed in high anchizonal rocks. The metamorphosed basic rocks comprise ‘unbuffered’ assemblages of chlorite, actinolite, epidote, calcite and leucoxene, and are of little use in determining pressure/temperature conditions. However less altered assemblages do occur in the centre of some intrusions and the absence of prehnite implies that pumpellyite-actinolite facies conditions were obtained — approximately 325º and 2.25 kilobars pressure.

Chapter 6 Devonian

The Caledonian Orogeny resulted in major changes to palaeogeography and sedimentation patterns across Wales and adjacent areas. The marine basin and adjacent platform that had influenced Lower Palaeozoic sedimentation were uplifted during the collision between Laurentia and East Avalonia. Together with Baltica, these two continents amalgamated to form Laurussia, a continent some distance to the north of Gondwana (Figure 3). The folded and uplifted sequence was a fragment of the extensive Caledonide mountain chain that developed along the length of the collision zone between Ireland and the Scottish Hebrides, and beyond.

The late Silurian and Devonian Old Red Sandstone rocks of Wales accumulated in the Anglo-Welsh Basin, south of the Caledonides, as the synorogenic to post-orogenic (‘molasse’) deposits of the Caledonian Orogeny. At the beginning of Devonian times, the shoreline lay close to south Devon and Cornwall, and later sedimentation in south-west England, the historical type area of the Devonian System, was dominantly marine in the evolving Rheic Ocean. Northwards from Devon, land was continuous into northern Scotland, and Wales and the adjacent areas became the site of extensive fluvial, alluvial and less common lacustrine sedimentation that characterises the Old Red Sandstone. South Wales lay in an external (extramontane) basin some distance south of the mountain chain, while Anglesey probably lay in an isolated basin at its edge. Drainage dispersal was mainly to the south and east on the northern margins of the Rheic Ocean. Palaeomagnetic evidence suggests that the region lay at subtropical latitudes of about 17°S; recent palaeogeographical reconstructions suggest a position about 30°S.

Detailed sedimentological studies have provided new insights into the environments and shown that contemporaneous fault activity was an important influence. The recognition that the limestones (‘concretionery cornstones’) are fossil carbonate soils (calcretes) has been particularly helpful in the interpretation of the depositional environment. The climate was seasonally wet, semi-arid and tropical. Most of the succession is characteristically red because of its iron oxide content.

Work on palynological (miospore) assemblages has refined stratigraphical correlation of successions in the Anglo-Welsh Basin, as has the recognition of widespread lithological markers such as air fall tuffs and emergent surfaces. However, problems of correlation remain between the terrestrial Old Red Sandstone succession and the standard marine Devonian stages, but progress is being made using miospores. The fossils that are present in the Old Red Sandstone illustrate the profound changes that were taking place in the evolutionary record during Devonian times, with the continued colonisation of terrestrial habitats by vascular plants and the expansion of vertebrates, including their eventual emergence on to land. The culmination of the Caledonian Orogeny in the Early Devonian (Emsian) Acadian Phase resulted in erosion and non-deposition throughout Mid Devonian times in most of Wales. In north Wales the Old Red Sandstone is exposed solely on Anglesey. In central Wales it is exposed in isolated outliers on Long Mountain and Clun Forest. Farther south, it crops out widely in the Black Mountains, Brecon Beacons, the Carmarthenshire Fans and on the limbs of several Variscan folds in southwest Pembrokeshire where it is spectacularly exposed in the sea cliffs (Figure 33).

Lower Old Red Sandstone

The base of the Devonian remains imprecisely located in Wales, but is tentatively placed within the uppermost Raglan Mudstone Formation, at a level just above the Bishop’s Frome Limestone Member and equivalent strata. The early Devonian Lower Old Red Sandstone comprises an upward-coarsening sequence of alluvial fan deposits.

The succession in south Wales and the Welsh Borders has been subdivided traditionally into three, loosely defined chronostratigraphical stages, which were locally applied because of the difficulty of correlating with the standard European marine stages. These stages are the Downtonian (corresponding largely to the Silurian Přídolí Series), Dittonian (converted to informal lithostratigraphical usage by some authors as the Ditton Group) and Breconian. In terms of the standard Lower Devonian stages, the highest Downtonian, Dittonian and lower Breconian correspond to the Lochkovian Stage, and the rest of the Breconian correlates with the Pragian and Emsian stages (Figure 33).

Lochkovian

In general, the Lochkovian successions are lithologically comparable with those of the underlying Přídolí Series. However, the Bishop’s Frome Limestone Member (see Chapter 5) at the top of the Raglan Mudstone Formation (Přídolí–lowest Lochkovian) marks a major change in basin architecture and facies, indicating a prolonged period of basin shutdown after which more proximal alluvial fan environments replaced the coastal mud flats and marine-influenced alluvial floodplains of the Přídolí. Sandstone increases progressively upwards towards the top of the Lochkovian. The sandstones are typically cross-bedded, with intraclast beds and intraformational erosion surfaces. In addition, they are less micaceous than the sandstones in the Přídolí successions, and their heavy mineral assemblages are not dominated by minerals from a metamorphic terrane, but by clasts and minerals from a dominantly igneous and sedimentary source. It seems clear that the distant northern metamorphic source of the Přídolí was replaced by the sedimentary and igneous rocks of the inverted Lower Palaeozoic basin.

Traditionally referred to the upper part of the Red Marls Group, the Lochkovian successions have been given a plethora of local formational names. The formations are characterised by the arrangement of their component facies in upwards-fining alluvial cycles. Overall, the formations tend to coarsen upwards, the sandstones becoming thicker and more dominant.

Individual cycles commence with the coarsest facies, usually cross-bedded sandstones resting on an erosion surface. The sandstones are predominantly red-brown and purple, but green sandstones occur in places. Lenticular sheets of calcareous intraformational conglomerate, rich in calcrete clasts, locally contain exotic pebbles. The latter are interpreted as the deposits of major distributaries, whereas those conglomerates dominated by calcrete clasts may be the deposits of shallow, ephemeral, flashy streams. Intraformational clasts may also occur at the bases of some sandstone beds. The sandstones, including laterally accreted sheets, are interpreted mainly as the channel deposits of high-sinuosity streams, and the intraformational conglomerates as channel-lag deposits. Flow in many of the channels was probably seasonal. Fish fragments occur in the conglomerates and the sandstones, and provide a biostratigraphical classification (Protopteraspis, Rhinopteraspis crouchi and Althaspis leachi zones), as do microvertebrate fragments. The large cylindrical trace fossil Beaconites is common in the tops of sandstone bodies. Arthropod crawling traces have been recorded in Pembrokeshire and at Pantymaes Quarry near Sennybridge (Diplichnites gouldi), and Tredomen Quarry near Brecon has yielded the traces of fish tails and fins. The fish occur in finer lithologies, and were probably entombed in overbank deposits during flooding.

The sandstones commonly fine upwards into red siltstone overlain by mudstone. Green mudstone also occurs locally, and has yielded plant fragments and miospores. The finer grained lithologies, like those of the underlying Prˆídolí mudrocks, are pedogenically altered, with common calcrete nodules. More mature, massive to rubbly calcrete limestones that occur sporadically in the highest parts of some cycles formed in seasonally wet, arid and semi-arid climates. Pseudoanticlinal, gilgai-type structures can be seen where the calcretes are well exposed. Sandstone-filled desiccation cracks are common at the tops of mudstone units. The fine-grained lithologies were mainly deposited in floodplain environments that were subject to frequent desiccation and calcic soil formation; at least some of the mudstones and siltstones may have originated as windblown dust, and some may have formed as lacustrine deposits in ephemeral floodplain lakes. Some mudstone bodies are interbedded with thin, point bar-type gravelly lenses of calcrete clasts in low-angle cross-bedded sets, and have pelleted fabrics, indicating deposition of pedogenic, pelleted mud aggregates from bedload in small sinuous channels.

Along the north coast of Milford Haven in Pembrokeshire, between the Ritec and Benton faults, the Gelliswick Bay Formation at the top of the Milford Haven Group comprises the thickest development (up to 1500 m) of the upwards-fining alluvial cycles in Wales and the Welsh Borders. Contemporaneous movement along the faults resulted in the deposition of alluvial fans at the basin margins, and introduced marked thickness variations. South of the Ritec Fault (Plate 32); (Plate 33), the Freshwater West Formation, up to 580 m thick, consists of the Conigar Pit Sandstone Member and the overlying Rat Island Mudstone Member. The Conigar Pit Sandstone mainly comprises a stacked succession of upward-fining cycles of intraformational conglomerate, sandstone and mudstone with calcrete. The most notable feature of the member at its type locality is the occurrence of eleven sandstone-mudstone lateral accretion complexes. The Rat Island Mudstone Member consists of red calcareous siltstone with abundant nodular calcrete.

To the east, Lochkovian strata (the St Maughans Formation) form the lower part of the prominent scarp features through the Carmarthenshire Fans to the Black Mountains. In Monmouthshire, where the St Maughans Formation was exposed during construction of the M4 motorway tunnel at Newport, the strata are closely comparable with those farther west. Equivalent beds that are well exposed in the Sawdde Gorge have been referred to the Llanddeusant Formation.

The St Maughans and Llanddeusant formations comprise cycles of very fine- to medium-grained, grey, pink and red channel sandstones, fining-upwards and overlain by siltstone and mudstone overbanks deposits, as described above. Complete fish specimens have been discovered in the St Maughans Formation at Cwm Mill near Abergavenny. More mature, massive to rubbly calcrete limestones, such as the Pontypool limestones and the Ffynnon limestones, occur at the top of the St Maughans sequence near Pontypool and in the Black Mountains, respectively. A 15 m-thick, green, channel sandstone complex in the St Maughans Formation at Pantymaes Quarry, south of Sennybridge, may have resulted from transcurrent movement and slight northern uplift of the Carreg Cennen Disturbance. The lithological uniformity of the sequence indicates that, in early Devonian times, south Wales lay within the distal part of a vast, broadly south-facing alluvial plain.

On Anglesey, probable Lochkovian strata crop out southwards from the coast between Dulas Bay and Lligwy Bay inland to near Llangefni. The succession is about 500 m thick, and records the burial of a dissected platform by sediments that were derived from the mountain belt to the north. The correlation and age of the oldest beds, which were deposited in a palaeovalley that was geographically isolated from the Anglo-Welsh succession, are uncertain, but much of the overlying succession is similar to that in south Wales, suggesting stratigraphical and sedimentological continuity. Asymmetric folding and an associated cleavage in the sediments are attributed to the Acadian Orogeny, and support a pre-latest Early Devonian age.

At the base of the succession on Anglesey, the Bodafon Formation comprises up to 45 m of conglomerate and pebbly sandstone with clasts of Precambrian and Ordovician rocks, and constitutes a diachronous, laterally variable facies that drapes the underlying palaeotopography. The formation is interpreted as a series of coalescing alluvial fans that were banked against a north-west-facing slope. The overlying Traeth Bach Formation comprises 130 m of red-brown calcareous siltstone with abundant calcrete nodules, three extraformational conglomerate beds, two intraformational conglomerates and sporadic thin sandstone beds. The siltstones are interpreted as playa lake deposits that were extensively calcretised during frequent episodes of emergence, desiccation and soil formation. The sequence is similar to the Raglan Mudstone Formation in south Wales and its top is similarly marked by the thick mature calcrete. The Porth y Mor Formation, 350 m thick, comprises fining-upwards cycles of conglomerate, sandstone and siltstone, the last containing calcrete nodules. The succession contains examples of laterally accreted sandbodies (recognised by epsilon cross-stratification, first identified here by J R L Allen), and the formation is interpreted as having been deposited in the channels of south-eastward-flowing, meandering streams and as overbank deposits on their floodplains. Heavy mineral assemblages rich in zircon, tourmaline and rutile, and lithic grains of feldspathic gneiss and quartz-mica schist were sourced either from the local Mona Complex outcrop, or from high-grade Precambrian metamorphic rocks of north-west Scotland. The uppermost Traeth Lligwy Formation is 25 m thick, and consists of thinly bedded, fine-grained, bioturbated sandstone and siltstone, generally lacking in calcrete and conglomerate. Deposition in more permanent lakes is suggested, an environment that is unique to Anglesey in the Lower Devonian of the Anglo-Welsh Basin.

Pragian–Emsian

The Brecon Beacons are the type area of the Breconian local stage, equivalent to the highest Lochkovian and the Pragian and Emsian stages of the standard chronostratigraphical succession. The strata are preserved across the whole of the south Wales outcrop. They are conformable on Lochkovian strata throughout, apart from south of the Ritec Fault in Pembrokeshire, where a thick conglomerate (the Ridgeway Conglomerate Formation) may be at least in part late Lochkovian to Emsian age and may rest unconformably on the Lochkovian Freshwater West Formation.

In Pembrokeshire, north of the Ritec Fault, the Gelliswick Bay Formation passes conformably up into the Cosheston Group. The group is well exposed in Milford Haven, around the easterly closure of the Burton Anticline, north east of Pembroke Dock (Figure 34). It is characterised by green sandstone, and is correlated with the Senni Formation farther east, but is much thicker (1500 to 1800 m), having been deposited in a zone of active rifting in the hanging wall of the Benton Fault. The group coarsens upwards overall, and is divided into five formations. The lowermost three formations (Llanstadwell, Burton Cliff and Mill Bay formations) comprise predominantly green sandstone with subordinate red sandstone, green intraformational conglomerates and red and green siltstone, all arranged in upward-fining sequences. The uppermost formations (Lawrenny Cliff and New Shipping formations) are similar, but contain coarser grained, red-brown sandstone and conglomerate with a wide variety of sedimentary, intrusive and extrusive igneous and metamorphic clasts. The bed forms and internal sedimentary stuctures have enabled a detailed palaeogeographical reconstruction of a southerly flowing braided to meandering stream system across an extensive alluvial flat. The lowermost three formations and particularly the Mill Bay Formation contain soft-sediment deformation structures, including small-scale ball and pillow structures, convolute lamination, loaded ripples and, most spectacularly, large scale foundering in thick beds with pillow-like clasts, reflecting seismic activity associated with the rifting of the basin.

South of the Ritec Fault, the Freshwater West Formation is overlain, possibly unconformably, by the Ridgeway Conglomerate Formation, which comprises coarse, polymict conglomerate, fine- to coarse-grained sandstone, and siltstone with calcretes. Thickness estimates range up to 465 m. The matrix-supported conglomerates are generally massive, but poor planar or cross-bedding and some clast imbrication is discernible in places. The clasts in the conglomerates are subangular to well rounded and mainly of quartzite, lithic greywacke, siltstone and vein quartz, with a marked influx of phyllite clasts in the higher beds. The formation is interpreted as having been deposited in a braided stream or alluvial fan complex, the cross-bedding, clast imbrication and clast size indicating a nearby source of Precambrian and Lower Palaeozoic rocks to the south. Some clasts have yielded fossils of possible Cambrian–Ordovician age. A northward-prograding alluvial fan occupied the area of Freshwater West, with more distal braided stream environments at West Angle Bay. Beaconites antarcticus occurs in the siltstones and the alga Prototaxites is abundant in intraformational conglomerates; both are thought to have inhabited areas in or close to active river channels. Three-dimensional bedforms and large-scale cross-bedding in some of the siltstones have been interpreted as bedload-transported pedogenic mud aggregates. The formation has yielded only non-diagnostic fossils (some crossopterygian fish fragments and plant fragments have been found), and the age is uncertain. It has been placed variously in the late Lockovian or (if the base is interpreted as an unconformity) as Pragian to Mid Devonian.

To the east, Pragian–Emsian strata are exposed in the scarp of the Carmarthenshire Fans, the Brecon Beacons and the Black Mountains in Monmouthshire. Throughout this outcrop, the alternation of flaggy mudstone and siltstone with fine- to coarse-grained sandstone is persistent. In the lower part of the sequence, the Senni Formation, about 150 to 200 m thick, comprises green or grey-green sandstone with red-brown siltstone and mudstone interbeds. The sandstone beds range from very fine- to medium-grained, with coarser, pebbly varieties appearing at higher levels. They consist mainly of tabular sheets of lenticular, cross-bedded, channel sandstones with internal erosion surfaces. Calcrete clasts occur in the bases of sandstone bodies, along with other intraformational mudstone and siltstone debris. The sand-bodies generally fine upwards, and their tops are commonly truncated by scour or erosion surfaces. The argillaceous interbeds contain calcrete nodules, and there are also more mature, platy, massive and rubbly calcretes. Desiccation cracks are seen locally. The formation is interpreted as the product of low-sinuosity, seasonally flowing, sandy, braided streams in a mid-fan setting, the finer lithologies being floodplain lake, crevasse splay and channel abandonment facies.

The Senni Formation is characterised by the green colour of its sandstone, which is due to the presence of chloritised micas. It is particularly known for its early vascular plant remains, which include Gosslingia breconensis, Hostinella heardii, Krithodeophyton croftii, Sennicaulis hippocrepiformis, Tarella trowenii, Uskiella spargens and Zosterophyllum llanoveranum. High water table conditions, perhaps due to a wetter climate, are invoked for the preservation of the plant remains and the predominantly green colour of the formation. Only three fossil fish localities have been recorded. The Breconian index fossil Rhinopteraspis dunensis (= R. cornubica) is known from Primrose Hill Quarry, Crickhowell (north-west of Abergavenny), Althaspis senniensis from Heol Senni Quarry, south-west of Brecon, Powys, and Pteraspis dixoni and Cephalaspis sp. from Pengau (Pen-y-gau) Farm near Ferryside on the Tywi estuary. R. dunensis occurs in the Mid Siegenian of mainland Europe.

The Brownstones Formation is the highest Lower Old Red Sandstone formation, and comprises a sequence of fining-upward, conglomerate-sandstone-mudstone cycles in which calcretes are poorly developed. It is characterised by red-brown, fine- to coarse-grained fluvial sandstones, with red-brown, locally green, mudstone and siltstone interbeds. The proportion of mudstone decreases upwards, the succession tending to become more sandstone-dominated in its upper parts. The formation is thickest in the Forest of Dean, where 1200 m are present. In its type area, the Brecon Beacons and Black Mountains, the formation is up to about 400 m thick. From there, it thins westwards along the north crop of the South Wales coalfield, by unconformable overstep of the Upper Old Red Sandstone and then the Carboniferous. Eastwards, it thins below the Upper Old Red Sandstone on to the Usk palaeohigh, with 130 m present on the east crop of the coalfield. It also thins southwards, with about 120 m present in Gower. Angular discordance at the pre-Upper Old Red Sandstone unconformity is seen locally, for example on Bannau Brecheiniog in the Carmarthenshire Fans, from Fan Hir to Fan Foel on the Black Mountain, and on the Sugar Loaf near Abergavenny.

In the type area, the sandstones generally form extensive sheets, as seen in the north-facing scarps of the Brecon Beacons (Plate 34). The tabular sandbodies are parallel laminated and trough and planar cross-bedded, and consist of multistorey units of stacked, cross-cutting, channelised bodies. Sandstones also occur as single-bed, nonchannelised sheets and as thin interbeds within the mudstones. The sandstones are red-brown, purple-brown and pinkish, calcareous and micaceous, and range from fine to coarse grained. Mudstone, siltstone and calcrete clasts are common at the bases of the sandstone units, which generally fine upwards, and intraformational calcrete clast conglomerates occupy minor channels locally. The succession is interpreted as the product of a prograding fan system that formed in a semi-arid, seasonally wet climate. The channellised sandstones are the products of mid-fan, wet season, flashy deposition, with more distal floodplain environments represented by sheet flood sandstones and mudstones and siltstones. The fine lithologies have been interpreted traditionally as floodplain mud and silt deposited from suspension in lakes or slow-moving water bodies, but some may be aeolian. Dessication cracks and rain prints on many bedding planes indicate frequent subaerial exposure.

Gravelly, pebbly and conglomeratic sandstones are locally common, particularly in the upper part of the formation in the Forest of Dean and parts of the north and east crops of the South Wales Coalfield. Pebbly beds at Llyn y Fan Fawr, 4 km west of the Swansea valley, were sourced from the east and are perhaps attributable to uplift on the Swansea valley Fault. Similarly, pebbly beds in the Cennen valley between Llandybie and Kidwelly may have been deposited as a result of uplift on the Carreg Cennen–Llandyfaelog Fault. The lithologies of the pebbles at Llyn y Fan Fawr include acid volcanics, lithic arenites and vein quartz, which may have come from locally exposed Precambrian and Cambrian outcrops. Locally derived Ludlow pebbles have been recorded at Caeras in the Cennen valley. Pebble suites at Ross-on-Wye comprise a range of igneous, metamorphic and sedimentary rocks of Lower Palaeozoic age, thought to have been derived from north Wales.

The Brownstones Formation in Wales has yielded only plant and rootlet fragments and trace fossils. However, the Dittonian index fossil fish Althaspis leachi has been recorded from the Wilderness Quarry in the Forest of Dean.

Near Cardiff and Newport in south-east Wales, the St Maughans Formation is overlain by the Llanishen Conglomerate Formation, about 150 m thick and consisting of interbedded purplish red conglomerate, sandstone, siltstone, mudstone and calcrete. The Llanishen Conglomerate is exposed in the Cowbridge and Rogerstone anticlines, and its excavation during the construction of the M4 Motorway yielded much new information. The lithologies are organised in upwards-fining cycles and erosion surfaces are common, especially in the coarser beds; calcrete profiles occur in the finer beds. The sequence is a proximal alluvial facies, the conglomerates and sandstones being the deposits of stream channels that interdigitated, both laterally and downslope, with siltstones and mudstones that accumulated across mudflats. In places, a calcareous cement is leached out at the surface to produce an unconsolidated gravel. The clasts in the conglomerate consist of possible Silurian volcanic or pyroclastic rocks (about 50 per cent), lithic sandstones and pink (Llandovery?) quartzite. The pebbles and cobbles are lilac to purple, subrounded to subangular, and many have dark purple, surficial iron-staining. The lithologies are closely comparable with those of the Ridgeway Conglomerate Formation, in south Pembrokeshire. Internal bedforms and the suite of pebbles in the conglomerate, which are unlike the northerly derived pebbles found in most of the Old Red Sandstone succession, suggest that the Llanishen Conglomerate, like the Ridgeway Conglomerate, was derived from the south.

Throughout south Wales, the generally coarsening-upwards sequence in the Lower Devonian reflects an increasingly proximal alluvial facies that was probably initiated by uplift, to the north. The southwards progradation of the facies belts and migration of the fluvial fall-line at the deformation front resulted in erosion and nondeposition throughout the mid Devonian in most of Wales when the alluvial systems deposited sediment in the sea to the south. The top of the Lower Old Red Sandstone is marked by a profound unconformity.

Upper Old Red Sandstone

Late Devonian Upper Old Red Sandstone strata, referred to the local Farlovian stage, overlie the unconformity that truncates the underlying Breconian and older successions. They are predominantly fluvial, deposited on a southerly facing palaeoslope, but lacustrine, aeolian and marginal marine facies occur sporadically. The succession is thin in comparison with the Lower Old Red Sandstone, with up to a maximum of about 350 m in south-west Pemrokeshire. Stratigraphical correlation, mainly by vertebrate remains and palynology, is imprecise, but an age range from late Frasnian to early Carboniferous has been demonstrated, with much of the succession being Famennian.

In the Pembroke peninsula, the Skrinkle Sandstones Group is well exposed to the south of the Ritec Fault. The group accumulated as a synrift succession of alluvial fan, alluvial plain and lacustrine deposits in the hanging wall of the Ritec Fault. Its distribution is confined to the Tenby–Angle fault block, thickening southwards from 100 m near the Ritec Fault to 330 m at Freshwater West. The beds lie unconformably on the Ridgeway Conglomerate, overstepping it eastwards to rest on the Milford Haven Group. There is a transition into grey Carboniferous beds at the top of the group. The group is subdivided into the Gupton Formation and overlying West Angle Formation. Locally, as at Freshwater West, the Gupton Formation is interpreted as comprising two, axial basin-fill, fluvial, coarsening-upwards sequences of mature clean sandstone. The lower sequence (Lower Sandstone Member) is exposed only at Freshwater West. It is 55 m thick and comprises, in upward succession: small, multistorey sandstone units in a background of mudstone and siltstone; thicker, single-storey sandstone beds; and stacked, pebbly sandstone-based, fining-upwards units. The member is interpreted as the product of a south-eastward prograding terminal fan. The higher sequence (Stackpole Sandstone Member) comprises a lower, mudstone-rich, heterolithic facies and an upper, trough cross-bedded and parallel-laminated sandstone facies association. This member is 6 m thick near the Ritec Fault, thickening to 68 m at Freshwater West. It is interpreted as the product of lacustrine and high-energy braidplain environments.

The overlying West Angle Formation comprises channel-fill conglomerates and sandstone sheets, typical of meandering channel systems, fining up into fluvial sandstones with mudstone and calcrete interbeds. It is characterised by red sandstones, conglomerates rich in igneous, sandstone and phyllite clasts and calcretes. The lower part of the formation (Conglomerate Member), in which conglomerate channel-fills and sheets are typical, is interpreted as the product of meandering channel systems that prograded southwards. The pebbles were probably derived from the Precambrian Pebidian volcanic complex and associated Ordovician volcanic rocks of the north Pembrokeshire coast, either directly or by recycling of the clasts of the Cosheston Group. The upper part of the formation (the Red-Grey Member) comprises fining-upwards fluvial sequences with mudstone and calcrete tops, similar to the meandering channel deposits below, but showing an upward increase in grey beds and grey-green sandstones with plant remains. The grey beds contain nonmarine fossils initially, but there is an upwards increasing marine influence, a precursor to the main transgression at the start of the Carboniferous. The finer sandstones and mudstones contain phosphatised plants, fish teeth and bivalves. Beds with a thin coal, root traces, Planolites, calcretes and a shelly limestone have been interpreted as a lagoonal accumulation, and spores indicate a Late Devonian age. The highest bed of the formation, a yellow-weathered, calcareous sandstone with internal parallel- and cross-lamination, reflects a transgessive barrier beach sequence, and the overlying slumped grey mudstones and calcareous sandstones have yielded spores (VI Biozone flora) of Early Carboniferous age.

East of the Milford estuary, in the Carmarthenshire Fans, Black Mountain and Brecon Beacons, the Upper Old Red Sandstone consists of the Plateau Beds Formation, which is named from its typical topography along this outcrop. The formation rests unconformably on the Brownstones Formation and is best known for its sporadic vertebrate and marine brachiopod faunas. The Afon y Waen Fish Bed is one of several fish-bearing, lenticular conglomerates. The formation consists mainly of red-brown and purple, quartzitic sandstone with subordinate mudstone. Thickness ranges up to a maximum of 58 m in Breconshire. Three divisions have been recognised on the basis of facies, and together they comprise a broadly transgressive sequence from fluvial and possible aeolian deposition to marginal marine environments. A widespread, granule-rich mudstone, up to 8 m thick at the base of the formation, may have been a mudflow. It is succeeded in the Swansea valley area by up to 7.5 m of conglomerate, pebbly sandstone and red mudstone (Division A), perhaps formed in a southerly directed, braided stream environment. At a similar level east of Afon Llia, planar cross-bedded red sandstone displaying north-westerly directed palaeocurrents and intercalated with water-laid sandstones overlie the basal granule bed. These beds (Division B) are about 18 m thick and have been interpreted as aeolian and possible wadi sediments, although a shallow, tidal origin may be more likely. The overlying division (C) is up to 18 m thick. Its lower half comprises fining-upwards, red-brown sheet sandstones, lenticular, channelised, red-brown and purple quartzose sandstones, pebbly sandstones, some fish-bearing conglomerates (including the Afon y Waen Fish Bed) and interbedded red mudstones. This is overlain by a heterolithic sequence of interbedded fine-grained, red-brown sandstones, fish-bearing conglomerate lenses and mudstones. The succession has yielded sporadic brachiopods (including Cyrtospirifer verneuili, Lingula spp., Ptychomalotoechia omaliusi), bivalves (Leptodesma cf. lichas, Pterinopecten sp., Sanguinolites sp.), fish fragments (Bothriolepis, Coccosteus, Holoptychius, Pseudosauripterus anglicus, cf. Rhinodipteryus and Sauripterus), the resting trace Rusophycus, an abundance of burrows (including cf. Planolites) and trails, and plant fragments. This upper division is interpreted as being marginal marine, showing a general upward transition from supratidal to intertidal and subtidal environments. It indicates a transgressive event, the dating of which remains imprecise, but P. omaliusi is confined to the early Famennian in the marine deposits of mainland Europe.

The Plateau Beds Formation is overstepped by the Grey Grit Formation, about 13 m thick, which consists mainly of greenish grey or greenish white, very fine- to fine-grained, cross-bedded, quartzitic sandstones. Thin green mudstone interbeds and quartz pebble layers occur sporadically. Fauna is restricted to fish fragments and the bivalve Sanguinolites, along with some burrows, including forms resembling Skolithos and Arenicolites. Cross-bedding indicates a predominantly southerly or south-easterly flow, but north-east current indicators have also been recorded, as well as some herring-bone cross-bedding. A braided stream environment is suggested, but a shallow marine setting for at least part of the formation cannot be discounted. Between the Cennen valley and the Tawe Valley Disturbance, a thin (4 to 7 m) unit of pale grey to yellow and greenish, tabular-bedded quartz conglomerate and pebbly, quartzitic sandstone (Pont Clydach Formation) oversteps the Plateau Beds and grades up into the Avon Group (Lower Limestone Shales, Carboniferous). Ctenocanthus spines, Planolites and Palaeophycus-type burrows have been recorded, and lingulids occur in mudstone in the uppermost transitional beds. The formation is probably entirely of Carboniferous age.

In the vicinity of Cardiff, the basal Upper Old Red Sandstone (Cwrt yr ala Formation) oversteps the Lower Old Red Sandstone sequence between Creigiau and the Taff valley. comprises thinly bedded, quartzitic sandstone, siltstone and mudstone in fining upwards alluvial cycles, with some thick, commonly pebbly sandstone beds. Nodular calcrete profiles are well developed in the mudstones. The sequence is interpreted as the product of a high-sinuosity fluvial channel system, the mudstones being more distal flood plain deposits. Bothriolepis and Sauripterus have been recorded, as well as Beaconites burrows.

Around the eastern crop of the South Wales Coalfield, the Upper Old Red Sandstone is represented by the Quartz Conglomerate Group, which forms a narrow outcrop on the coalfield escarpment and, most distinctively, a small outlier capping the Sugar Loaf at Abergavenny (Plate 35). The group thickens from about 25 m at Llangattock in the north, to over 70 m at Risca in the south. In the Cardiff area, it rests unconformably on the Cwrt yr ala Formation. At its base, 14 to 20 m of grey-green quartzitic sandstones (Wern Watkin Formation) correlate with the Grey Grit Formation of the Merthyr district. A mature calcrete at the top of the Wern Watkin Formation is overlain by quartz pebble conglomerates (Craig-y-cwm Formation), which contain sporadic fish fragments, including Bothriolepis. At the top of the group, micaceous, feldspathic and garnet-rich sandstones with interbedded red mudstones constitute the Garn gofen Formation. Fish remains from these beds include a fragment of Osteolepis macrolepidotus. In addition, the bivalve Archanodon jukesi was collected from the Quartz Conglomerate Group in Gwent, and Holoptychius nobilissimus has been recorded at Tongwynlais. The group is interpreted as being fluvial in origin, formed in generally southerly prograding alluvial fan systems. The Wern Watkin Formation is of sandy, braided stream origin; the Craig-y-cwm Formation is the product of gravelly, braided stream deposition; and the Garn gofen Formation is very immature and was deposited in high-sinuosity streams, the influx of mica, garnet and feldspar indicating the erosion of a newly exposed metamorphic rock source. Locally the uppermost beds of the Quartz Conglomerate Group in south-east Wales are sharply overlain by the early Tournaisian Tongwynlais Formation (Avon Group) of the Carboniferous Limestone. Spores from 18 m below the top of the group at Tongwynlais north of Cardiff are regarded as earliest Famennian. To the north, there is a depositional break between the uppermost beds of the group and the Carboniferous Limestone.

Chapter 7 Carboniferous

Recent palaeomagnetic and structural studies in the Upper Palaeozoic rocks across Europe have indicated that the Carboniferous sequences in the British Isles accumulated to the north, on current coordinates, of a micro-plate collision zone that lay across central Europe, between Armorica, the Massif Central and the Vosges. On the north side of this zone, the Rheno-Hercynian Basin, floored by ocean crust, was systematically closed by northward-directed thrusting that persisted into late Carboniferous times. Farther to the ‘north’, the British Isles lay on a foreland where, in response to tectonism, a series of faulted basins developed on either side of a landmass, which has been referred to as St George’s Land but is now included in the Wales–Brabant massif (Figure 35). The basins and adjacent platforms became the controlling influence in Carboniferous sedimentation.

The Carboniferous sequence includes a range of shallow marine carbonates, fluviodeltaic and shallow marine siliclastic deposits, lacustrine sediments and coal. The distribution of facies indicates a continuous interaction between terrestrial and marine influences, and the temporal variations reflect both climatic and sea level changes. One of the most discussed features of the sequence is the cyclical patterns that have been variously ascribed to eustatic, tectonic and autocyclic sedimentary mechanisms; all these mechanisms were probably operative.

Biostratigraphical schemes based on microfloras, conodonts and foraminifera complement the long established stratigraphy based on the macrofaunas. The three main lithostratigraphical divisions of Carboniferous Limestone, Millstone Grit and Coal Measures have been recognised since the early 19th century, and have been related to the chronostratigraphical divisions, Dinantian (Tournaisian and Visean), Namurian and Westphalian. The sequence includes the coal, ironstone, clay and limestone, which have been so intensively exploited, making this the most economically important part of the stratigraphical sequence in Wales.

Dinantian

The marine intercalations in the Upper Old Red Sandstone sequence in south Wales are the first indications of the more general subsidence of the area and the northward marine transgression that became established in early Carboniferous times. The repeated fining-upwards alluvial/fluvial red beds of the continental Old Red Sandstone gave way to grey, richly fossiliferous, calcareous mudstone and limestone in early Dinantian times. Coincidentally, there was a distinct change from an arid climate through semi-arid to monsoonal in late Dinantian times. Over much of south Wales, where the Dinantian rocks overlie the Upper Old Red Sandstone, the boundary is conformable and gradual, but locally there is overlap on to older strata.

A Vaughan’s original zonal scheme of Dinantian strata was based on coral and brachiopod assemblages, and because the south Wales succession is similar to that in the Avon Gorge, in Somerset, it was initially referred to as Avonian. Recently it has been shown that the type section in the Avon gorge is incomplete and that coral assemblages were influenced by the lithofacies. Consequently, correlation between different basins is difficult. In 1973, W H C Ramsbottom proposed that the sequence could be divided into six major cycles, or mesothems, distinguished on sedimentary features and faunal and floral assemblages, each cycle indicating a marine regression and transgression of global eustatic significance. The topmost two cycles were divided into a number of minor cycles, or cyclothems. Later biostratigraphical work modified the mesothem boundaries, which T N George and co-workers used as the basis of the Dinantian stage boundaries, although the stages are defined on faunal evidence and not primarily on the cycle boundaries. The concept of the major cycles stimulated much discussion and, the current consensus is that such broad regional facies changes would be obfuscated by facies patterns caused by local, vertical fault movements.

In recent years, it has been shown that the detailed stratigraphy is far more complex than previously envisaged. The stratigraphy has been refined on evidence from the study of conodonts and foraminifera. In addition, detailed study of petrography and textural variation within the limestones and their accurate classification has facilitated a clearer understanding of the environmental changes.

Dinantian strata in south Wales form a broad outcrop in the limbs of the folds in south Pembrokeshire, Gower and the Vale of Glamorgan and a narrow outcrop along the north and east crop between Pembrokeshire and Monmouthshire. In north Wales, the sequence is well exposed about the north-east coast of Anglesey, in the cliffs of Great Orme’s Head and Little Orme’s Head enclosing Llandudno Bay, in the scarp from Colwyn Bay into the Vale of Clwyd, and from Prestatyn to south of Llangollen, on the east side of Halkyn Mountain.

The Dinantian sequence in south Wales is part of the Southern Province, and accumulated at the southern edge of the Wales–Brabant Massif. It has been subdivided into the Avon Group and Pembroke Limestone Group (Figure 36), The distribution of facies was controlled by a combination of sea level rise and differential subsidence, which led to the formation of a carbonate ramp (Figure 37). On the landward side of the ramp, bioclastic and ooidal grainstones and peritidal carbonates reflect transgressive and regressive events, with much evidence of subaerial exposure. In the mid-ramp zone, the sequence of bioclastic packstones and wackestones is much thicker, and accumulated in deeper water below fair-weather wave base. Graded beds indicate storm-influenced deposition and, during transgressive phases, back barrier peritidal facies developed. Across the outer side of the ramp, the thickest sequences accumulated below storm wave base, and bioclastic mudstone and wackestone are associated with reef mounds. In north Wales, at the northern edge of the landmass, a broadly similar sequence accumulated at the edge of the Pennine Province, and it was profoundly influenced by movement along pre-existing fracture zones, including the Welsh Borderland, Bala and Menai Straits fault zones. In detail, the sequence there is more clearly related to that of central and northern England.

Tournaisian

Tournaisian strata are restricted to south Wales, where predominantly continental Upper Old Red Sandstone strata are overlain by largely marine strata of the Avon Group (previously the Lower Limestone Shale Group, and, in Gower, the Cefn Bryn Shales) at the base of the Carboniferous sequence. These shallow marine shelf and shoreline carbonates record a northward-directed marine transgression on to the landmass, and they form an important transition into the succeeding carbonate sequence. The strata reach a maximum thickness of about 145 m in the south crop, thinning to a few metres in the north; the base of the group becomes younger in the same direction.

The Avon Group contains a variety of carbonate and terrigenous lithologies that have been ascribed to three depositional environments: barrier shoal, lagoon and shelf embayment. The barrier shoal association comprises ooidal and skeletal grainstones deposited in a turbulent tidal to near-shore setting. Skeletal fossil debris is abundant, but trace fossils are rare. The lagoonal lithologies include thinly bedded ooidal-skeletal and peloidal packstones with abundant algae and a rich, but low-diversity fauna dominated by Modiolus; evaporite pseudomorphs and desiccation cracks are common. The facies reflects deposition in a shallow-water low-energy environment, probably high intertidal to shallow subtidal. In contrast, the shelf embayment association of grey mudstone and argillaceous limestone is characteristic of a low-energy, but deeper offshore environment. The calcarenites of the barrier shoal association sheltered the lagoon on the landward side from the deep water shelf embayment association on the seaward side.

Between the Vale of Glamorgan and Newport, the lowest part of the group comprises ooidal, peloidal, skeletal and sandy limestones, and interbedded mudstones with some calcretes and ironstones (Tongwynlais Formation). The accumulation was dominated by northward-directed palaeocurrents, in contrast to the southward-directed systems of the underlying fluvial sandstones of the Upper Old Red Sandstone. The areal and sequential patterns of the lithologies reflect four shoaling-upward sedimentary cycles, in environments ranging from coastal plain, peritidal, lagoon, embayment and barrier to open marine (Figure 38). Broadly, the cycles represent progressive deepening and the northward retreat of a barrier shoreline.

To the west, into south Pembrokeshire, the Avon Group has not yet been subdivided, but it is represented in the upper part of the clastic facies of the Skrinkle Sandstone Formation. However, towards the east crop, the Tongwynlais Formation is progressively and completely onlapped by the Castell Coch Limestone Formation south of Pontypool. The thick- and commonly cross-bedded, skeletal and ooidal grainstone is well exposed in the Taff Gorge. The skeletal debris is mainly brachiopod fragments, crinoid ossicles and bryozoan fragments. Red staining is locally intense and is generally associated with late-stage epigenetic dolomitisation. North of Pontypool, where the limestone rests directly on the Upper Old Red Sandstone, a basal pebble conglomerate (a littoral lag deposit) is overlain by transgressive littoral ooidal and skeletal calcarenites, which are correlatives of the middle and upper Tongwynlais Formation to the south, and part of the same transgression. Overall, the Castell Coch Limestone represents an extensive belt of subtidal to intertidal shoals, up to 10 km wide and over 90 km long. A strong tidal influence prevailed across its width and, particularly in the south crop, there was a distinct eastwards to north-eastwards longshore component. At the same time, to the east of the coalfield, along the Wye valley, an ooidal barrier reformed and advanced south-eastwards.

At the top of the Avon Group, dark grey to black, silty and micaceous mudstone, with subordinate thin, bioclastic graded limestones and calcareous siltstones (Cwmyniscoy Mudstone Formation) form a distinct depression east of the Vale of Glamorgan. The sequence thins from about 40 m to some 20 m, and marks a return to deeper water on the shelf, below fair weather wave base; graded bioclastic limestone beds reflect periodic storm events. The formation closely resembles the upper part of the Tongwynlais Formation, and where the Castell Coch Limestone is poorly developed they are difficult to separate.

The uppermost Tournaisian to lower Visean limestones (Black Rock Limestone Subgroup) at the base of the Pembroke Limestone Group, form a prominent feature in Gower and the Vale of Glamorgan, where they comprise two thick bioclastic units separated by an oolite. The group thins from about 500 m to some 120 m across the Vale of Glamorgan. At the base, thin bedded, coarse- to fine-grained, graded skeletal packstones (Barry Harbour Limestone Formation) indicate the continuation of the storm-influenced shelf conditions. Higher in the sequence, the mudstone content diminishes, suggesting a higher energy environment. To the north of the Vale of Glamorgan axis, the lithofacies is dolomitised and overlain by fine-grained, grey, laminated and cross-laminated secondary dolomite, suggesting a more restricted environment. These lithofacies patterns, indicating shallowing into more proximal settings, can be traced into eastern Gower (Shipway Limestone).

Throughout the south-east outcrop, the early sedimentation culminated in the first extensive southward-prograding oolite shoal complex (Brofiscin Oolite Formation) comprising grey, well-sorted ooid grainstones, locally with skeletal grainstones and well developed cross-stratification. The oolite thins from the north side of the Vale of Glamorgan into the Gower where it is represented by a 3 m-thick, cross-bedded bioclastic grainstone with thin ooidal laminae. The overlying limestones (Friars Point Limestone and Tears Point Limestone) are thickly bedded, dark grey, foetid, argillaceous, richly fossiliferous skeletal packstones with shaly partings. Intense bioturbation imparts an irregular, nodular appearance to many of the beds. They range from over 400 m on the coast about Barry to 85 m on the north limb of the Cowbridge anticline, and these limestones are thought to have accumulated below storm wave base. The top of the sequence is extensively dolomitised and was previously referred to as the ‘Laminosa Dolomite’. Several phases of dolomite formation have been determined and the earliest was probably related to penecontemporaneous emergence. In the east crop and in the Monmouth and Chepstow area, the entire thickness is dolomitised, probably the result of uplift in the hinterland causing continental and marine waters to mix in the subsurface.

Across the north crop of the coalfield there is a major break within, or an overall thinning of, the uppermost Tournaisian sequence. About Merthyr Tydfil a variable group of predominantly massive ooidal grainstones and thinly bedded dolomite (Abercriban Oolite Formation) overlies the Avon Group. To the east, as in the Clydach valley, dolomitised and non-dolomitised rocks interfinger, and farther along the east crop, the sequence is totally dolomitised. Here, the pervasive dolomitisation postdates both Variscan deformation and stylolite formation, and it has been ascribed to movement of phreatic water down a hydrological gradient in proximity to the Usk Axis. Along the north-east crop, the sequence has been referred to the Clydach Valley Subgroup with many depositional breaks, some of which are marked by a palaeokarstic surface with an overlying palaeosol and massive and nodular calcrete horizon (Figure 39). In the Heads of the Valleys road section in the Clydach valley, the lowermost Sychnant Dolomite Formation is capped by a palaeokarstic surface and is overlain by an ooidal sequence with four bioclastic units. The lowermost Pwll y Cwm Oolite Formation is a pale grey, cross-stratified ooidal grainstone, rich in shelly debris, and the Blaen Onnen Oolite Formation is a massive grey grainstone. The oolites are separated by thin- to medium-bedded, cyclic limestone packages (Pant y darren Formation), which display five major lithofacies reflecting typical shallowing-upwards peritidal cycles. The Coed Ffyddlwn Formation, overlying the oolites, comprises fine-grained, thinly bedded dolomite with concentrations of shell and crinoid debris; it is separated from the overlying Gilwern Oolite Formation by an erosion surface. The abundance of palaeokarstic surfaces across south Wales suggests fairly broad regional uplift, which was possibly controlled by movement within the Severn Valley Fault Zone. The Tournaisian–Visean boundary probably lies close to the top of the Blaen Onnen Oolite Formation.

The broad lithological divisions of the Black Rock Limestone Subgroup can be traced into the Gower (formerly the Penmaen Burrows Limestone Group that consisted, in upward succession, of the Shipway Limestone, Tears Point Limestone and Langland Dolomite). Farther west, in Pembrokeshire (Plate 36), the sequence is considerably thicker and, as yet, the subdivisions have not been determined. The dominant lithofacies, as exposed in the vicinity of Bosherton and Linney Head on the south limb of the Pembroke Syncline, comprises up to 450 m of bioturbated, well-bedded bioclastic packstone and wackestone. The richly fossiliferous limestones, with crinoid, bryozoan and brachiopod debris, are typical of outer shelf environments. They are overlain by dolomitised build-ups of carbonate mud (formerly referred to as Waulsortian reef mound limestones) of the Berry Slade Formation, up to 150 m thick, which is not included in the Black Rock Limestone Subgroup. The carbonate mud formed a belt, roughly parallel to the depositional strike, which extended into west Somerset and farther into Belgium (Figure 35). The carbonate mud build-up consists of bioclastic calcareous mudstone, packstone and vuggy limestone; it is comparable with the downslope biohermal and biostromal constructions in distal, deep-water ramp settings. The succession accumulated in water deep enough to be unaffected by any of the relative changes in sea level that influenced sedimentation farther east. On the north limb of the syncline in south Pembrokeshire, the limestones thin markedly into a mixed neritic, nearshore sequence and display the first indications of the successive overstepping of strata, which so dominated the subsequent Lower Carboniferous sedimentation.

Visean

Visean strata occur throughout south Wales, and they are the sole representatives of the Lower Carboniferous in north Wales. In south Wales, the precise position of the Tournaisian–Visean boundary in relation to the lithological subdivisions (Figure 36) remains problematical, but there is enough conodont evidence to place it close to the top of the Friars Point Limestone and Tears Point Limestone in the Vale of Glamorgan and the Gower, respectively. The sequence is dominated by highly fossiliferous limestone that is typically medium- to thick-bedded, dark bituminous, argillaceous wackestone to grainstone. On cursory examination, the limestone appears uniform, although detailed examination and comparison with modern carbonate systems indicate a shallowing from deeper water settings during a major marine transgression into a more proximal setting above storm-wave base.

On the south side of the Cowbridge Anticline, dolomitisation at the top of the Friars Point Limestone was probably initiated by an influx of meteoric water and then enhanced both during later emergence and again during post-Dinantian burial. The dolomitisation is particularly prominent (Langland Dolomite) at the top of the Tears Point Limestone Formation in eastern Gower, and here, as elsewhere through the eastern outcrops in early to mid-Chadian times, regression initiated the lowering of the wave base and the extensive development of ooidal shoals (Gully Oolite Formation and Caswell Bay Oolite), which prograded southwards. The oolites comprise two shallowing up sequences, each capped by an irregular surface with persistent stromatolite growth and evidence of subaerial exposure. Farther east, four similar cycles have been determined in the formation. To the west, into western Gower, the formation thickens and becomes more bioclastic and bioturbated. In south Pembrokeshire, about Bosherton and Linney Head, the sequence (Linney Head Limestone Member and Hobbyhouse Bay Limestone Member) is mainly of thinly bedded crinoidal limestones interbedded with richly fossiliferous, dark grey, calcareous mudstone. Carbonate mud mound build-up continued with massive to poorly bedded, dolomitised calcite mudstone interdigitating with bedded limestone at the margins; polyzoan fronds and patches of crinoid debris are the only determinable fossils.

Along the north-east crop, the Gilwern Oolite Formation comprises a thin basal, coarse-grained, shelly ooidal grainstone, which grades up into a massive, pale grey, ooidal bioclastic grainstone at the top of the Clydach Valley Subgroup. Calcrete development at the top of the formation marks a widespread non-sequence (the mid-Avonian unconformity of early surveyors) at the base of the Arundian. The surface is infilled with mottled grey-green clays, representing a fossil soil, with detached blocks of oolite.

Along the south crop, bedded calcareous mudstone (Caswell Bay Mudstone Formation) represents the basal Arundian marine transgression across the Gully Oolite Formation. The mudstone forms a restricted but distinctive sequence at Three Cliffs Bay (Plate 37) and Caswell Bay in Gower, at Tenby and along the north crop, west of Blorenge. In Gower, coarse conglomerates at the base are wholly calcareous, but those along the north crop include much siliclastic debris derived from the Old Red Sandstone. The thickness and lithological variations are attributed mainly to deposition on the irregular, underlying palaeokarst surface. The eroded upper surface was buried beneath lower shoreface sands, and the subsequent sea-level rise established peritidal conditions with the deposition of the High Tor Limestone Formation to the north of a barrier in the vicinity of the Vale of Glamorgan axis. The High Tor Limestone Formation shows significant variations in thickness and facies. It comprises a basal, coarse bioclastic grainstone, with reworked limestone clasts, overlain by well-bedded, fine-grained to coarsely crinoidal, skeletal packstone and wackestone with intercalated thin mudstone and a rich and diverse Arundian fauna. Across the Vale of Glamorgan, it is overlain by a thick bedded to massive oolite (Cefn yr hendy Oolite), which, at its top, is marked by an irregular palaeokarstic surface with a thin palaeosol. This sequence represents a single transgressive (High Tor Limestone) to regressive (Cefn yr Hendy Oolite) cycle, comparable to those in the northern sequences except that it is considerably thicker. In western Gower, the High Tor Limestone Formation, up to 150 m thick, comprises medium- to thick-bedded bioclastic packstone and grainstone with no indication of shallowing events.

Along the north crop, to the east of Merthyr Tydfil, a karstic surface at the top of the Gilwern Oolite is infilled with terrestrial thin fluvial siliclastic beds (Clydach Halt Member) at the base of the Llanelly Formation (Figure 39). The sequence of massive and thickly bedded bioclastic and ooidal grainstones and dolomites contains many depositional breaks, some marked by palaeokarst surfaces, which are well developed on the top of the oolite beds.

The uppermost Penllwyn Oolite displays the same gradation, bioclastic grainstone into ooidal grainstone, as in the lower sequence, and its base marks marine erosion. The shallowing-up cycles represent a shallow marine peritidal coastal complex and are most clearly expressed, and thickest, in the Cheltenham Limestone Member. At the top of the sequence, the Gilwern Clay Member marked a regression, with flood-plain clays accumulating in an arid to semiarid climate, with a rootlet bed and coal at the top, suggesting probable back-swamp deposition and more humid conditions.

Northwards and westwards from the Clydach valley, the Llanelly Formation thins and is overstepped by the Dowlais Limestone Formation. Along the north crop, uplift restricted limestone deposition, and fluvial quartzitic sandstone and conglomerate with thin intercalations of plant-bearing mudstone (Garn Caws Sandstone Formation) occupy a similar position. These were deposited in distributary channels within a deltaic complex that prograded southwards on to the carbonate shelf. The channels are incised, up to 5 m, into the underlying Gilwern Clay. During late Arundian times, the barrier complex that had restricted the earlier peritidal environment and prevented the establishment of open marine conditions migrated progressively northwards. The hummocky cross-stratified packstones and grainstones at the top of the sequence indicate the return of a high-energy shoreface environment that culminated in an oolite shoaling event (Hunts Bay Oolite Subgroup). The subgroup is widespread and shows considerable variations in thickness and facies; it is mainly Holkerian in age. In the Vale of Glamorgan, the progradational Cefn yr hendy Oolite culminated in local emergence. However, the succeeding limestone (Argoed Limestone) marked a marine transgression (Holkerian) and a return to offshore conditions and colonisation by lithostrotionid corals. Simultaneously, an ooidal shoal complex (Cornelly Oolite Formation) prograded southwards and peritidal deposits (Stormy Limestone Formation) accumulated behind it (Figure 36). A karstic surface developed at the top of the subgroup during a marine regression at the end of the Holkerian. To the west, in south Pembrokeshire, the broadly equivalent Stackpole Limestone is richly fossiliferous and bioclastic.

In the north crop, the major regression marked by the siliclastic deposition of the Gilwern Clay and Garn Caws Sandstone was followed by a marine transgression in the Holkerian and the re-establishment of carbonate deposition (Dowlais Limestone Formation). The limestones, up to 80 m thick near Trefil (Plate 48) are progressively overstepped by Namurian strata, and, east of Gilwern Hill, they are totally absent. In the vicinity of Monmouth, the undivided Drybrook Limestone abuts sandstone (Cromhall Sandstone) deposited from a south-flowing fluviodeltaic system that was initiated in early Holkerian times. West of Merthyr Tydfil, the limestone rests directly on the Avon Group, and shows a particularly distinctive alternation of nodular algal limestone in which brachiopods acted as nuclei for algal growth, and fine-grained calcite mudstone (Concretionary Beds). The limestone hosts the widespread cave systems at Dan yr Ogof and Ogof Ffynnon Ddu in the Tawe valley, which developed from phreatic incursions of water utilising joint, fault and bedding plane surfaces.

In the Vale of Glamorgan, a major regression in late Holkerian to early Asbian times initiated reworking of terrigenous quartz sand (Pant Mawr Sandstone Member) and its transport on to the carbonate shelf; on the north crop, the Honeycombed Sandstone occurs in a similar position. These sandstones are characterised by low-angle cross-bedding and bioturbation. The overlying Oxwich Head Limestone Formation of Asbian age comprises thickly bedded, fine- to coarse-grained, recrystallised, grey mottled, skeletal packstones. In Gower, the limestones are the main site of extensive cave systems that probably owe their development to the influence on the subsurface drainage of a thin coal, in the middle of the sequence, and to the overlying mudstone. Ooidal grainstones have been determined near the base, and in the north crop (Penderyn Oolite) they thin markedly. Mottled red and green clay seams (possibly palaeosols) above hummocky and pitted surfaces are common in the lower part of the Asbian sequence in the northern crop and some, as at Locks Common, Porthcawl, are well developed in the south crop.

To the west of Bridgend, the final phase of Dinantian sedimentation (Late Brigantian) is represented by interbedded thin limestone and mudstone with silicified limestones and banded cherts (Oystermouth Formation). An increase in the supply of terrigenous mud marked the transition into the siliclastic deltaic environment of the Late Carboniferous. In the west, this transition was essentially conformable, but, to the east, increasing non-sequence culminated in unconformity. The thinning of the Asbian and Brigantian strata towards the north crop and the vertical and lateral facies variations have been related to three basin-wide fluctuations in the regional subsidence rate. Along the north crop, ooidal grainstone deposition was interspersed with a number of quartz sandstone beds derived from the landmass to the north. In the south crop, the absence of sandstones and the dearth of ooidal limestones indicate more open marine conditions, although, at times, prograding barrier islands probably intervened.

In north Wales, the Dinantian sequence was initiated in Chadian times and records periodic transgression on to the northern edge of the Wales–Brabant Massif. The distribution of facies and the thickness of the mainly shallow-water limestone sequence was influenced by movement along the Welsh Borderland, Bala and Menai Straits fracture zones, and intervening fractures, which formed downfaulted embayments such as the Vale of Clwyd (Figure 40). The sequence (Figure 41) can be correlated more directly with the Craven Basin to the north-east than with south Wales. It has mainly been ascribed to the Clwyd Limestone Group, but a restricted development of the overlying Craven Group has been recognised in the vicinity of Prestatyn.

At the base of the sequence, unfossiliferous red sandstone and mudstone with a few calcretes and laterally impersistent conglomerates, informally named the Basement Beds, could possibly range down into the Tournaisian. The beds represent fluvial and alluvial deposits with few marine incursions, and have been placed in a number of formations, based on their geographical location (Figure 41). Thin, conglomerate-based, fining-upwards sequences record deposition in meandering streams, and striped silty mudstones are interpreted as overbank deposits laid down on a gently inclined coastal plain. On either side of the Alyn Valley Fault that runs subparallel to the east of the Clwydian Range, the Basement Beds are overlain by the Foel Formation (late Chadian), which consists of variably dolomitised and argillaceous limestone with beds rich in oncolites, calcareous sandstone and plant-bearing siltstone. The lithologies are interpreted as a peritidal, aggradational sequence. The sequence thins markedly on to the horst between the Alyn Valley and Vale of Clwyd faults, indicating their contemporary movement. Subsequent marine transgression, in early Arundian times, culminated in the widespread accumulation of open marine, platform carbonates across much of north Wales; these include the Llanarmon Limestone Formation in Flintshire, which now replaces the Llysfaen Limestone Formation of Colwyn Bay and Ochr-y-foel Limestone Formation of Prestatyn. The cross-bedded, peloidal and skeletal grainstones developed through the south-westward migration of high-energy, inner ramp facies. Subsequent north-eastward progradation created a broad, gently inclined carbonate platform.

In late Arundian to early Holkerian times, as the Dinantian transgression resumed, localised uplift and erosion caused major changes in the distribution of facies across the platform. Sea level rose throughout Holkerian times and drowned the platform; the shoal facies was restricted to the margin where coarsely crinoidal and cross-stratified sequences were formed in the upper part of the Llanarmon Limestone Formation, previously the Gop Hill Limestone, at Prestatyn. To the west, there was extensive epigenetic dolomitisation and mineralisation (previously Llandudno Pier Dolomite) and also the bryozoan ‘reef’ of Nant y Gamar. The coeval Leete Limestone Formation of Flintshire and south Denbighshire, now replacing the Dulas Limestone Formation of Colwyn Bay and the Tandinas Limestone Formation, at Penmon, on Anglesey, are peritidal deposits, which accumulated within protected landward embayments. The limestones are commonly oncolitic, subtidal packstones and grainstones. They are interbedded with intertidal fenestral wackestone with root traces and gypsum pseudomorphs, and calcite mudstone (‘birds eye micrites’) in a series of progradational rhythms that range from less than 0.5 m to several metres thick.

Oscillations in sea level in late Asbian times, caused by the onset of the Gondwanan glaciation, resulted in deposition of thick-bedded, massive, mottled or pseudobrecciated skeletal packstones (Loggerheads Limestone Formation including the Llandulas, Great Orme and Penmon limestones; (Plate 3);(Plate 38). The lithologies show that during intervals of high sea level, open-marine subtidal conditions prevailed over much of the platform, and the abundant karst surfaces, mudstone palaeosols and calcretes indicate intermittent regressions with emergence. Late Asbian limestones are the oldest preserved in the Corwen outlier and, at the same time, knoll reefs, at Little Orme, Dyserth and Axton, defined the northern edge of the platform. The reefs comprise fossiliferous wackestone and floatstone, with characteristic stromatactis structures and numerous brachiopods and ammonoids (goniatites). On the basin side of the reef at Dyserth, thin graded packestones and wackestones (Prestatyn Limestone Formation) and the succeeding fine-grained, well-sorted packestones (Gwaenysgor Limestone Formation), with blocks of the reef limestones, represent carbonate detritus derived from the adjacent reef and platform.

During late Asbian times, in south Flintshire, a profound rifting episode is reflected in the thick, late Asbian and Brigantian sequence on the north side of the Bala Fault Zone. In contrast, the sequence is condensed to the south at Minera. Similar thickness variations between Anglesey and the Great Orme, may be due to coeval movement along the Menai Straits Fault Zone. In early Brigantian times, regional subsidence modified the eustatic oscillations, causing widespread changes in the cyclicity. The Cefn Mawr Limestone Formation, including the Bishops Quarry and Traeth Bychan limestone formations, comprise shoaling upward cycles characterised by thin-bedded wackestones and mudstones, rich in corals, brachiopods and, locally, in the foraminifera Saccaminiopsis. At the top of the cycles, skeletal packstones and peloidal and ooidal grainstones are commonly capped by karstic surfaces and calcretes. The cycles are thicker and more varied than those within the Asbian, and record more extensive and rapid rises in sea level. The carbonate platform facies, between Berwyn, Arfon and Anglesey, records the highest inundation. Along the platform margin, near Prestatyn, the older reefs were overwhelmed by turbiditic and hemipelagic mud (Teila Formation) with distinctive ammonoid and bivalve faunas. Farther east, on Halkyn Mountain, seismic activity along the Nercwys–Nant figillt Fault Zone caused the collapse of the adjacent platform.

Throughout Asbian and early Brigantian times, small volumes of siliclastic debris were supplied from the south on to the encroaching carbonate platform. Intermittently, during subaerial exposure, the streams cut deep channels into the carbonate sediments causing shoe-string sandbodies, which are particularly well exposed on Anglesey where they have been named at various localities, such as Helaeth, Benllech, Fedw Fawr and Parc. In the late Brigantian (Minera Formation of Flintshire and Denbighshire, and the Red Wharf Cherty Limestone of Anglesey), beds of shallow marine sandstones within the limestones record an increase in the supply of siliclastic detritus. This has been ascribed to both tectonic rejuvenation of the source areas, as a result of plate collision in southern Europe, and climatic changes associated with the Gondwanan glaciation; the same events contributed to the northward advance of the fluviodeltaic sandstones in early Namurian times.

Silesian

Silesian strata in Wales are dominantly clastic deposits of deep marine to alluvial fan environments. Correlation is based on the richly fossiliferous shale marine bands. Certain marine bands, distinguished by a diagnostic ammonoid (goniatite) fauna, can be traced within and between basins on a regional scale and these provide the basis of a high resolution biostratigraphical framework. Strata bound by successive marine bands represent units of related facies, which have been referred to as cyclothems (Figure 48). Many of the cyclothems are dominated by fluviodeltaic deposits and their variable facies patterns have been related to differing positions in the depositional systems. Recently the concept of sequence stratigraphy has augmented these methods of correlation and interpretation — facies relationships are related to key surfaces possibly caused by fluctuations in sea level. In the fluviodeltaic deposits, these surfaces include a range of transgressive and emergent surfaces, which are referred to as sequence boundaries. The methods of sequence stratigraphy are particularly applicable to successions deposited at times of glacial eustasy, as during the Carboniferous, when high fluctuations in sea level left a distinctive signature in the stratigraphy. The main phase of Carboniferous glaciation was initiated in the early Namurian and peaked in late Westphalian times when an ice sheet covered large parts of the Gondwanan supercontinent; the geographically widespread marine bands developed during periods of glacio-eustatic transgression.

The cyclothems occur on both a large scale, in hundreds of metres of strata, and on a small scale of a metre to tens of metres. The large-scale cyclothems are defined between marine bands, generally coarsen upwards and have been attributed to prograding deltas. The smaller cyclothems also coarsen upwards but culminate in a seatearth and coal seam, reflecting a phase of emergence. Within both cyclothems the vertical patterns of the lithologies are widely variable. In the small scale sequences, these can be related to local patterns of sedimentation, but the controls on the large scale patterns, whether eustatic or tectonic, remain a matter of some considerable debate.

Namurian

Across south Wales, Namurian strata form a persistent outcrop about the main basin and westwards into Pembrokeshire. The sequence (Figure 42) represents a fluviodeltaic to basinal succession which overstepped on to older rocks, for example the Ordovician strata near Haverfordwest. Towards the eastern edge of the basin, along the north crop (Plate 39), the sequence progressively oversteps the north–south-aligned Dinantian zones, reflecting contemporaneous activity along the Usk Axis, which was probably emergent for most of Namurian times. The southern limit of the basin is now concealed below the Bristol Channel. In north Wales, the succession accumulated at the south-western margin of the Pennine Basin; it forms a narrow outcrop between the Dee estuary and Oswestry. Chronostratigraphical classification and correlation is based on the marine bands that define the stage boundaries in the Pennine Basin (Table 5).

In south Wales, the Namurian Millstone Grit sequence, now named the Marros Group, is most complete in Gower where it is represented by some 800 m of mainly prodelta mudstone. To the north and east, the sequence thins due to non-deposition, unconformities and the proximity of the basin margin. It is thinnest in the east crop, near Pontypool (Figure 43), where about 20 m of fluvial quartz sandstones with palaeosols and one or two marine mudstones are present. On the north crop, the sequence is dominated by deltaic braidplain deposits that prograded southwards into the basin. Contemporary movement on faults contributed to facies and thickness variations that were determined mainly by post-rift thermal subsidence of the basin. Uplift along the Carreg Cennen Disturbance (Figure 31) caused emergence of the Penlan Axis, which separated the main basin from the Pembrokeshire sub-basin.

Changes in the sea floor environment from Dinantian to Namurian times resulted in progressively more muddy conditions, and bivalves and ammonoids replaced the coral-brachiopod fauna. However, the fluvial to shallow marine sandstones are largely unfossiliferous apart from drifted plant remains. In the shallow water, close to the basin margins, calcareous shales contain bivalves, gastropods, brachiopods (orthids, chonetids, spirifers, productids, common mud burrowing Lingula and Orbiculoidea cf nitida), few bryozoans and rare zaphrentoid corals. In the upper part of the sequence, shales associated with thin coal seams contain freshwater and brackish water mussels (Carbonicola lenicurvata, Anthraconaia cf. bellula), and indicate the initiation of the swampy conditions that became such a characteristic feature of later, Westphalian times. The designated marine bands refer to those beds with a noticeably enriched fauna, and the distribution of these bands demonstrates most clearly the expansion of the basin throughout the Namurian, with only the upper (Marsdenian and Yeadonian) stages present in the north and east (Figure 42). However, even in Gower, where the succession is most complete, the basal parts of some stages are missing. In the context of sequence stratigraphy, the marine bands represent high-stand flooding surfaces induced by sea level rise. The maximum flooding surfaces are marked by a concentration of thick-shelled ammonoids, calcareous brachiopods and limestones. On either side of these surfaces the character of the faunal assemblages is determined by salinity and water depth which are broadly influenced by distance from the shore. The determined phases with increasing depth are:

Planolites opthalmoides
Lingula
Mollusc spat
Anthracoceras or Dimorphoceras
Ammonoid
Calcareous brachiopod

On this basis, three broad biofacies belts, Lingula biofacies, shelly benthic biofacies and the ammonoid-pectinoid biofacies, have been determined (Figure 44). Thus many of the marine bands of the north crop, closest to the landmass, are of Lingula biofacies, those in Gower and south Pembrokeshire are of the ammonoid-pectinoid biofacies, and a zone of shelly benthic biofacies occurs in between.

The Marros Group comprises a basal quartzitic sandstone (Twrch Sandstone Formation) overlain by a mudstone-dominated sequence (Bishopston Mudstone Formation) and an upper lithic sandstone (Telpyn Point Sandstone Formation;(Figure 42). The lowest Namurian strata (Aberkenfig Formation) were encountered in the Kenfig Borehole near Bridgend and comprise dark grey mudstone and black chert with some sandstones (35 m). The base of the group is markedly diachronous. At Ragwen Point in Carmarthen Bay conodonts (including Gnathodus bilineatus, G. girty simplex, G. girty subspindit and G. commutatus) found in septarian nodules that occur about 20 m above the base suggest a Pendleian (E₁) age. In the Bridgend area there is a non-sequence, which is marked by pebbly lags of chert intraclasts, phosphatic nodules, abraded bone fragments and coprolites. The Pendleian (E₁) stage is absent, and the lowest recorded faunas include Posidonia corrugata, G. bilineatus and Cravenoceras nitidus, indicating a mid- to late-Arnsgergian age (E_2b2–E_2b3). The magnitude of the unconformity increases northwards and eastwards, the basal Namurian strata becoming younger as the underlying Dinantian strata become older. The oldest strata on the north-east crop are of late-Marsdenian (R₂) age. The top of the group lies at the base of the Gastrioceras subcrenatum Marine Band, which is recognised throughout south Wales (Plate 40); (Plate 41).

At Bishopton, in Gower, basal radiolarian chert layers, with Cenosphaera, Carposphaera, Cenellipsis and Sphaerozocum, are local equivalents of the Aberkenfig Formation, as are strata formerly referred to as the Plastic Clay Beds between Drefach and Llygad Llwchwr on the north crop. The age of these beds may range from Brigantian to Arnsbergian, although the lowest proven Namurian zone is E_2b in shales within the overlying Twrch Sandstone Formation near Capel hir bach in Carmarthenshire and some of the Plastic Clay Beds may be decalcified Brigantian.

The Twrch Sandstone Formation, formerly the Basal Grit, is thickest, about 190 m, in the Twrch valley and it ranges in age from Pendleian (E₁) to Marsdenian (R₂). It consists mainly of quartzitic sandstone with conglomerate and thin beds of mudstone and siltstone. Pebbly sandstone with clasts of Dinantian limestone are common along the north crop and are locally clean and mature; some beds of orthoquartzite contain 98 per cent detrital quartz and have been worked for refractory material. The coarsest quartz sandstones fine upwards, and are deposits of braided, fluviodeltaic distributary channels. The channels became progressively more clearly developed through Namurian times and a localised southerly source is indicated in the sediments in the Aberkenfig–Margam sector of the south crop (Figure 45). Mudstone beds record freshwater lacustrine or floodplain deposition, or in some cases marine incursions. Some fine-grained, well-sorted quartz sandstones coarsen upwards or, more commonly, have sharp bases with hummocky and swaley cross-stratification and parallel to low-angle cross-stratification; these beds have been interpreted as storm-dominated delta front and shoreface deposits. The quartzitic rocks form prominent escarpments about the coalfield, and especially where the Upper Old Red Sandstone thins westwards in the Carmarthenshire Fans.

The most detailed facies and environmental interpretation of the sequence is based on the well exposed coastal cliff sections, such as those at Marros Sands and Tenby, in Pembrokeshire. Close to the south end of Ragwen Point, the Twrch Sandstone Formation overlies limestones interbedded with calcareous shales (Brigantian). The contact is erosional, although conodonts from septarian nodules in the overlying sandstones indicate an early Namurian (Pendleian E₁) age and a non-sequence rather than a disconformity at the base of the sequence. The occurrence of mature quartz sandstones and marine mudstones suggests a littoral setting, but the abundance of palaeosol horizons, with roots and trunks of giant club mosses, as exposed at Ragwen Point, are indicative of more continental conditions.

The Bishopston Mudstone Formation, formerly the Middle Shales, consists predominantly of mudstone with minor quartz sandstones. It is thickest in Gower, where at least 20 marine bands have been recorded at Barland Common, Bishopston. However, it is most clearly exposed in the cliff sections at its type locality in Marros Sands, south Pembrokeshire, which have been the subject of detailed sedimentological analysis. Three major coarsening-upwards deltaic sequences have been determined; the lowest sequence is capped by distributary channel sandstones. The cycles consist mainly of interbedded mudstone and siltstone, with Lingula and Sanguinolites, deposited in brackish to marine distributary bays. Fine-grained sandstones represent a range of mouth bar and coastal barrier environments. The marine transgression at the base of the third cycle is marked by the Gastrioceras cancellatum Marine Band, which is widespread in south Wales and indicates a major expansion of the basin. Two thin, but widespread quartz sandstones, the Twelve Feet Sandstone and the Cumbriense Quartzite, occur above and below the marine band in the north-east of the main basin. The latter has been interpreted as a storm-influenced, regressive barrier sand complex.

The Telpyn Point Sandstone Formation in Pembrokeshire is one of many sandstones in south Wales that have been named the Farewell Rock. It has also been termed the Upper Sandstone Group and it may be the equivalent of the Llanelen Sandstone in north Gower. It is well exposed at a number of coastal localities, for example west of Telpyn Point, at Settling Point, south of Broadhaven, in the core of a syncline at First Point and in the core of an anticline, at Waterwynch Bay, north of Tenby. The formation comprises a fining-upward, dominantly fluvial distributary channel sequence. The basal, massive, medium-grained sandstones display internal erosional surfaces marked by lag concentrates and casts of Calamites, and local soft-sediment deformation structures. The overlying sandstones show more typical trough cross-stratification, and are overlain, in turn, by fine-grained sandstones with well developed ripple lamination showing a wide variation in palaeocurrent orientations. Correlation of this fining-upwards pattern along strike has identified an incised north-north-west-trending valley fill with a sharply erosional base that cuts out both the Cumbriense and Anthracoceras marine bands to the west of Cwm Pedol (Figure 46). Up-sequence, the channels are better preserved, and evidence of decreasing stream energy has been attributed to decreasing channel slope as relative sea level rose. This trend culminated in the deposition of the upper siltstone as a channel plug. The palaeosol cap to the fluvial sequence indicates that the valley was filled prior to the flood at the start of the transgression.

To the east of Afon Twrch, the erosional unconformity at the base of the channel extends on to the interfluve, a terrace-like emergent surface beyond the incised valley. On the interfluve, the palaeosol overlies the Cumbriense Marine Band and consequently its base defines a marked basinward facies shift. The widespread fluvial incision surface and the laterally correlative interfluve surface is regarded as a sequence boundary, which can be related to a similar surface in the Namurian sequence in northern England and southern Ireland.

In Pembrokeshire, large-scale cross-stratification indicates that in the north crop the derivation was from the south whereas in the south crop the flow was dominantly towards the east.

This disparity in the direction of the fluviodeltaic systems may be the result of their separation by an active east–west tectonic zone along the line of the Ritec Fault. In response to late Namurian (Yeadonian) uplift, the deltas advanced to the south and east, and their channels deeply incised the shelf and delta slope deposits of the Bishopston Mudstone Formation. As in the main basin, the top of the sequence is marked by the widespread Gastrioceras subcrenatum Marine Band at the base of the Westphalian. However, in the main basin, sandstones of similar lithology occur above the marine band (Figure 46), which suggests that the locus of delta deposition migrated to the north-east.

In Namurian times, north Wales lay at the south-western edge of the Pennine Basin on the north side of the Wales–Brabant Massif. Currently, the only remnant of the Namurian sequence lies on the east side of the Dinantian outcrop, from near Prestatyn in the north to Oswestry in the south (Figure 47). The facies changes across this outcrop, from dominantly sandstone in the south (Cefn y Fedw Sandstone Formation) to predominantly mudstone (Bowland Shale Formation) overlain by lithic sandstone (Gwespyr Sandstone Formation) in the north. All the succession, from E₁ to G₁, is present, but the base is markedly diachronous, with E1 strata probably present but unproved by ammonoids in basinal areas around Holywell and Flint, E2 beds overlying Carboniferous Limestone around Hope Mountain, and G₁ beds forming the base at Rhydymwyn.

At the base of the sequence, the Pentre Chert Formation comprises banded, glassy to impure granular chert with siliceous mudstone and siltstone. A few thin beds of variably silicified, white to light brown, fine- to medium-grained, locally pebbly quartzose sandstone and crinoidal chert also occur. In the north, the formation rests with minor unconformity on the Brigantian Teila Formation, and thickens southwards to rest unconformably on the Brigantian Cefn Mawr Limestone. Within cycles, beds thicken upwards, and the amount of terrigenous material, both as mudstone beds and as a contaminant within the cherts, decreases. A sparse macrofauna includes crinoid, productid and rhynchonellid debris and thin-shelled ammonoids. The chert and mudstone accumulated from suspension in moderately deep water below storm wave-base; the thin sandstone and crinoidal beds are probably storm turbidites. The age of the formation is constrained by the presence of Sudeticeras sp. (splendens/stolbergi group), suggesting of the late-Brigantian P_2b Subzone, in the top of the underlying Cefn Mawr Limestone, and the Cravenoceras malhamense Marine Band (E_1c1) in the overlying Bowland Shale Formation. This suggests an age no younger than the E_1b Subzone.

The Cefn y Fedw Sandstone Formation comprises quartz sandstone, pebbly in places with thin beds of quartz conglomerate, interbedded with subordinate mudstone, siltstone, siliceous mudstone and chert, arranged in coarsening upwards cycles. The grey sandstone is well sorted with low-angle hummocky and cross-stratification; Zoophycus and Rhizocorallium are locally abundant on bedding planes. The formation is thickest north of the Bala Fault, which probably influenced the distribution of the fluvial systems, and here it is represented by three sandstones within the Bowland Shale Formation, The lower sandstone is mainly Pendleian, the middle mainly Chokierian to Alportian, and the upper, early Kinderscoutian to late Mardsenian. To the south, the sandstones merge into a single unit. The formation reflects a fluviodeltaic system, sourced in the Wales–Brabant Massif, which prograded northwards into the Pennine Basin. The Bowland Shales Formation (formerly the Holywell Shales) consists of mudstone with thin beds of fine- to coarse-grained sandstone, ganister (siliceous seatearth) and some thin coals; most of the ammonoid zones have been distinguished. The dark grey mudstone is finely micaceous, fissile to massive and weathers distinctively to red brown. Black, highly fossiliferous marine beds, up to several metres thick, are separated by beds that are largely unfossiliferous except for fish and plant remains. Sideritic ironstone occurs locally as layers and nodules as well as small phosphatic nodules. Palaeosols with sideritic rootlets underlie rare thin coals. The thin, fine- to medium-grained, quartzose and micaceous sandstones display flaser bedding and bioturbation in places. Carbonaceous debris is both disseminated and concentrated in laminae. The lowest marine band determined is the Cravenoceras malhamense (E_1c) Marine Band and all the succeeding stages are present. In the Flint area, the top of the formation is marked by the Gastrioceras cumbriense Marine Band. The formation represents deltaic to prodelta deposition with a marine signature becoming more evident to the north.

The southerly derived influx of sands diminished progressively through Namurian times, and in the late Namurian there was the first influx of sand (Gwespyr Sandstone Formation and the Aqueduct Grit of Ruabon Mountain) derived from the north-east. The brown, fine-grained, variably feldspathic and micaceous sandstones are characterised by large-scale tabular cross-bedding, but low-angle and hummocky cross-stratification also occur. Thinly interbedded sandstone and mudstone with thin coals and seatearths were deposited in delta top lagoons and interdistributary bays. Deltaic sandstones interdigitate with prodelta mudstones of the Bowland Shales Formation.

Westphalian

In south Wales, Westphalian strata occupy the core of the east–west-oriented syncline between St Bride’s Bay in the west and its closure in Monmouthshire in the east (Figure 31). In north Wales, the strata crop out in the Flintshire and Denbighshire coalfields, in the Vale of Clwyd, in restricted outcrops within the Menai Straits Fault System, and about Malltraeth in south-west Anglesey. Each of these outcrops is the downfolded (post-Westphalian) remnant of a more extensive area of sedimentation, with numerous minor folds and a well-developed fault pattern. The tectonic influence is most intense close to the deformation front in Pembrokeshire. The strata in south Wales were deposited during the final infilling of the Variscan foreland basin on the southern edge of the Wales–Brabant Massif, and those in north Wales are part of the infill of the extensional Pennine Basin to the north of the massif.

In south Wales, the lower part of the succession, up to early Bolsovian (Westphalian C), is argillaceous (South Wales Coal Measures Group), and about 1000 m thick (Figure 49). The upper part of the sequence is dominated by lithic sandstone (Warwickshire Group) of Bolsovian and Westphalian D age, and is about 1500 m thick. The lower sequence was deposited in a lower to upper delta plain environment, where intermittent rises in sea level induced marine incursions. Sediment was derived mainly from the north and the east, but initiation of the Variscan orogeny in early Bolsovian times caused uplift to the south. Emergence of the Variscan mountain chain led to an influx of coarse immature debris, which was deposited in alluvial braidplains that prograded northwards into the basin. In late Westphalian times, the delta plain environment was re-established.

Most of the Lower and Middle Coal Measures formations are dominated by mudstone and siltstone in coarsening upward units, cyclothems (Figure 48), in which facies are repeated cyclically. They were deposited in freshwater lakes on delta plains. Within the sequence, marine bands record periodic marine transgression and deposition in prodeltas and wetlands. Fluvial sandstones form extensive tabular sheets from both confined flow and flood events. In the Pennant Sandstone Formation, major distributary, channelised, fining-upwards sandstones are common around the margins of the basin and break the pattern of cyclicity. The cycles are capped by rootlet-bearing palaeosols (seatearths), which become more prominent towards the basin margin, and coal seams which, in the same direction, show evidence of erosion (washout) and splitting. The coals developed from peat accumulation when a rise in water table level with decreasing clastic input caused plant colonisation and the establishment of mires. The thick seams, with low ash and sulphur contents, probably accumulated in raised mires. Mire growth ceased when a rise in the water table formed brackish and fresh water lakes with communities of bivalves. Clastic input was low. The regional extent of the main cycles suggests that fluctuating water table depth was the main influence on sedimentation. However, recent facies interpretations have related the cycles to parasequences and to water table fluctuations caused by eustatic changes. Their thickness ranges from a few metres on the basin margin to up to 30 m in the centre. The correlation of the principal marine bands throughout the Westphalian basins of Europe and America show Maximum flooding that they were formed during eustatic sea level rise linked to the melting of the ice sheets on Gondwana. The grey mudstones in the nonmarine, lacustrine cycles are generally more organic-rich and darker at the base, and paler and siltier upwards. The freshwater bivalve (mussel) communities that thrived in these lakes form the basis of the broad traditional biostratigraphical classification of the Westphalian succession (Figure 49) that was first established in south Wales. However, as with the marine bands, the mussel fauna becomes impoverished towards the basin margin. Plant debris is abundant in the mudstone sequences and petrifactions are common in the sandstones. Spores, together with the remains of mosses and relatives of the ferns and conifers, have also contributed to the biostratigraphical classifications. Thickness variation, coal seam splitting and small-scale synsedimentary growth faulting reflect local tectonic activity. Later, end-Variscan tectonism severely affected the incompetent Middle Coal Measures, causing shortening along bedding planes and reactivation of earlier extensional faults.

The base of the Coal Measures Group is marked by the Subcrenatum Marine Band, which has not been proved in the Swansea district, but is a persistent element of the stratigraphy elsewhere. The ammonoid-brachiopod fauna is well developed and is particularly rich in the condensed sequences at the edge of the basin. In the strata up to the Garw Coal, there are seven marine bands, mostly Lingula bands with Planolites opthalmoides, and few coals. Some bands are only locally preserved and only a few bands persist to the east crop. The M1 Marine Band, represented by two bands in the Gwendraeth valley and on the north crop, is particularly rich in fauna at Margam, where it contains burrowing and crawling molluscs, brachiopods, ammonoids and sponge spicules. The M2 Marine Band lies in the roof of the Crows Foot Coal at Margam, and at an equivalent level in relation to the Sun Vein on the south-east crop. The M3 Marine Band (Cefn Cribwr/Wernffrwd) contains a varied fauna, including Gastrioceras listeri, on the south crop. The M4 Marine Band (Margam) contains abundant foraminifera and the ammonoids Anthracoceras sp. and cf. Domatoceras on the south crop. The M5 Marine Band occurs only in the western part of the north crop and, in addition to the ubiquitous Lingula mytilloides, foraminifera are abundant in Cwm Clydach, in the Gwendraeth valley. The Cwm Berem Marine Band has been recognised only in the Gwendraeth valley. Towards the eastern crop, and the Usk Axis, channel fill sandstones within these strata indicate local emergence and reworking. Similar sandstones are common in the north and south crops, the most extensive being the Farewell Rock along the north crop, and the Cefn Cribwr Rock of the south crop. The sandstones developed as southward-prograding delta lobes. The coals that cap the cycles are mostly thin, the thickest being the Sun Vein about Pontypool and the Crows Foot of Cefn Cribwr. The commercially important coals of the Coal Measures Group were referred to many local names, but were subsequently rationalised, mainly by applying names from the Aberdare area (Figure 49). However, local names were retained in the west of the coalfield.

The Garw Coal is the lowest coal that was worked commercially, although it rarely exceeds 0.75 m in thickness and is generally less. It carries a distinctive fauna of fish fragments in its roof and marks a change from cycles capped by thin coals below, to cycles capped by thick coals above. Above the Garw Coal, cyclothems contain few marine beds whereas seatearths and coals are common; sideritic ironstones are well developed, both as beds and as nodules within the mudstones. The ironstones were the principal source of industrial iron in south Wales and, as in the Merthyr district, were extensively mined. Towards the west, the ironstones are more numerous but their quality decreases. Cycles between the Garw and Gellideg coals commence with mussel-bearing mudstone rich in thick calcite-shelled Carbonicola, the ‘pseudorobusta’ faunas. The succeeding cycles are predominantly argillaceous and capped by the Gellideg, Five Feet, Seven Feet and Yard seams, all worked coals, but subject to splitting and, in the east, merging into composite seams. The thin Amman Rider Coal contains the Vanderbeckei (Amman) Marine Band in its roof, and it marks the Lower to Middle Coal Measures boundary. The band comprises mudstone containing Lingula mytilloides, Planolites opthalmoides and fish fragments, but in the south-east it is richly fossiliferous. The band persists throughout the coalfield and its thickest development, some 6 m, is at Brynmawr. It forms the base of a thick parasequence, which contains ironstones that have been worked, and is capped by the Bute Coal, which is the lowest in a group of commercially important coals – the others being the Nine Feet, Red Vein, Six Feet, Four Feet, and Two Foot Nine coals. The coals between the latter and the Cefn Coed Marine Band are thin and generally of poor quality, but the interval is characterised by fining upwards, channel fill, quartzitic sandstones (Upper and Lower Cockshot Rock and Elled Rock), which indicate derivation from the south and west. The sandstones are the earliest expression of the Pennant Sandstone Formation lithology. In the west, three thin marine bands, Graigog, Mole and Trimsaran, occur in this interval, but only the latter two are present in the central part of the coalfield where they have been named the Hafod Heulog and Britannic, respectively. The Cefn Coed Marine Band, at the base of the Bolsovian, is present throughout the coalfield, and at Aberbaiden, on the south crop, it contains the richest fauna of any marine band in Britain. From a 14 cm layer in the 0.46 m marine band, at least 80 species have been recorded; these include Anthracoceras aegiranum, corals, crinoids, horny and calcareous brachiopods with chonetids being particularly abundant, bivalves, nautiloids, trilobites, ostracods and conodonts. In the southern area, between Pontardawe and Margam, the biofacies reflects clear water and open marine conditions. Farther north, between Cynheidre and Glynneath, the impoverished fauna is more indicative of muddy waters and, in the Cynon and Taff valleys, the band is dominated by brackish water assemblages.

The succeeding beds contain numerous ironstones that have been extensively worked in the Black Pins Mine Ground.

In the sequence between the Cefn Coed and Upper Cwmgorse marine bands, the coals that cap the cycles are generally thinner than those below, but they have been worked. They include the Gorllwyn, Gorllwyn Rider, Eighteen Inch, Lower Pentre, Pentre, Pentre Rider, Abergorky and Hafod seams. In the Abergavenny and Newport districts, tonsteins, indicating distant volcanic activity, have been determined in the roofs of the Gorllwyn Rider and Pentre Rider seams, and within the Lower Pentre seam. Marine bands occur in the roof of the Pentre Rider (Edmondia Marine Band), Abergorky (Shafton) and the Hafod (Cambriense) seams, and the only occurrence of the Carway Marine Band, with foraminifera as its only marine fossil, is in the Gwendraeth valley. The Five Roads Marine Band is best developed in the west of the Gwendraeth valley, where its fauna is dominated by bivalves, including cf. Edmondia goldfussi, E. aff. transversa and Myalina compressa, but it has been recognised in parts of the Pontypridd and Newport districts. The Edmondia (Foraminifera) Marine Band is widespread and generally contains abundant foraminifera, including Agathamminoides, Glomospira, Glomospirella, Hyperammina and Tolypammina. Apart from in the vicinity of the east crop, the Shafton (Lower Cwmgorse) Marine Band occurs throughout the coalfield, but it is best developed in the Gwendraeth valley where it is dominated by Dunbarella macgregori. Similarly missing in the vicinity of the east crop is the otherwise widespread Cumbriense (Upper Cwmgorse) Marine Band, at the Coal Measures –Warwickshire Group boundary; this is richly fossiliferous with the nautiloid Huanghoceras postcostatum and ammonoids Anthracoceras cambriense and Politoceras kitchini. It is the highest marine band in the Westphalian succession in Britain and, in the south-west of the coalfield, it is overlain by green-grey, feldspathic, lithic sandstone, which marks the first major influx of southerly derived Pennant Sandstone Formation sands into the basin. The lithic sands appeared earliest in the south-west, between Cynheidre and Margam, spread diachronously north-eastwards, and reached the north-east edge of the basin after deposition of the Brithdir Coal.

The Pennant Sandstone Formation facies are organised in large-scale cycles, fining upwards from erosively based sandstone into siltstone and mudstone. The sandstone is cross-bedded, conglomeratic at the base, with coal rafts, log clasts, siltstone and ironstone clasts. They represent alluvial deposition in highly sinuous channels, and the overlying mudstone and siltstone represents floodplain deposits. Clasts of Devonian phyllite and spilitic basalt confirm derivation from the south. The formation is subdivided by coal seams into the Llyfni, Rhondda, Brithdir, Hughes, Swansea members, which together with the overlying Grovesend Formation are dominated by sandstone and form the interior plateau of the coalfield. Thin mudstones form ‘slack’ features in the steep valley sides. The lowest Llyfni and Rhondda members have been referred to the Upper Coal Measures Formation in the main part of the basin. The principal coal seams are the Rhondda No. 1 and No. 2, Brithdir, Cefn Glas and Mynyddislwyn. The latter and two higher coals (Small Rider and Big Rider) have an abundance of Leaia in their roofs. The correlation of the Mynyddislwyn seam, in the east, with the No. 3 Llantwit and Swansea Four Feet seams suggests an eastward-developing unconformity, caused by continuing movement along the Usk Axis.

In early Westphalian D times, uplift along the Usk Axis resulted in a lowering of the water table and oxidation of the sediments in the vicinity. Thin coals in the Llynfi and Rhondda members fail towards the east crop, and east of the Taff valley, the red beds of the Deri Formation progressively replace the grey measures of the Pennant Sandstone. Red, purple and green mudstones and siltstones, commonly with rootlets and pedogenic fabrics, are interbedded with mature quartz sandstone that is pebbly in places and probably derived from the Usk Axis. These changes are most apparent in the uppermost, Grovesend Formation, which is lithologically different from those below. The strata are preserved in the Gowerton Syncline where they are predominantly grey mudstones, with coals, seatearths and ironstones. To the east, in the Caerphilly Syncline, the strata are markedly thinner, devoid of coal seams, and include red and purple mudstones and a flora of possible Stephanian age. The lithologies indicate suppression of the alluvial valley environment of the underlying Pennant sandstones and re-establishment of the delta plain. The reddened strata reflect a change of environment from the Coal Measures to ‘New Red Sandstone’ although it is possible that a considerable thickness of late Carboniferous strata over south Wales has been removed.

The Westphalian strata in Pembrokeshire occupy the core of the syncline between Carmarthen Bay and St Brides’s Bay and a small fault-bound outcrop between Newton and Newgale. The faulted southern boundary of the main outcrop juxtaposes Coal Measures Group strata against Precambrian, Silurian and Lower Carboniferous rocks. The northern boundary, with Namurian strata, is less severely disrupted.

In the spectacular cliff sections between Amroth and Wiseman’s Bridge, folded and faulted strata within the communis, modiolaris and similis-pulchra zones are exposed. The basal bed is a pale weathered, laminated ankeritic siltstone with abundant specimens of the nonmarine bivalve Anthracosia regularis. The overlying strata include at least eleven seatearth–coal seam associations and nine mussel bands, indicating a succession of floodplain deposits that accumulated in swamps and freshwater lakes in a coastal plain environment. These are overlain a complex of contiguous minor channels with fining-upwards sandstone to siltstone infills and numerous soft-sediment disruption structures; the complex reflects the advance of a west and north-westerly flowing river system. The uppermost coal is overlain by the Amman Marine Band, which here is a thin mudstone with ammonoids and brachiopods. The overlying sequence is well exposed to the vicinity of Wiseman’s Bridge, and comprises three coarsening-upward cycles capped by thin coals; they are typically coastal plain, swamp, fresh water and brackish lake deposits laid down during marine regression.

On the north side of Waterwynch Bay, the Gastrioceras subcrenatum Marine Band is overlain by an alternating siltstone-mudstone sequence, displaying part of a coarsening-upward regressive cycle. Synsedimentary deformation structures are common, including small-scale faults, slump folds and convolute lamination, and indicate the water saturated state of the sediments. In St Bride's Bay, Lower Coal Measures are exposed in the cliffs between Settling Point and Broad Haven, although locally, as about The Sleekstone, the sequence is intensely faulted. Sandstones in the cliffs and foreshore on the north-west side of Nolton Haven, comprise an overall fining-upward, fluvial channel-fill cycle similar to those of the typical Pennant facies farther east. Similarly, the orientation of cross-bedding, ripple marks and minor channels all indicate transport to the west and north-west, as determined in the main basin.

In north Wales, the main Westphalian outcrop lies between Point of Ayr, at the northern edge of the Dee estuary, and Ruabon, in the south. These strata dip eastwards beneath the younger rocks of the Cheshire plain and re-emerge, farther to the east, in the Lancashire and Staffordshire coalfields. To the west, Westphalian strata crop out through the Trias cover in the Vale of Clwyd and in the Malltraeth syncline, in south-west Anglesey.

The succession has been subdivided into the Pennine Coal Measures Group overlain by the Warwickshire Group (Figure 50). The Pennine Coal Measures Group is lithologically similar to the Coal Measures Group of south Wales; it is subdivided into Lower and Middle Coal Measures formations by the Vanderbeckei (Llay) Marine Band. The Warwickshire Group is a sequence of red and purple mudstones with a few thin limestones, coals and grey mudstones that was deposited in an oxidising environment of an upper delta or alluvial plain.

The basal Gastrioceras subcrenatum Marine Band of the Pennine Coal Measures Group, 1 m of dark mudstone with an ammonoid-pectinoid facies, lies between the leaves of the Gwespyr Sandstone in north Flintshire and immediately above the Aqueduct Grit in Denbighshire. Seven marine bands occur higher in the sequence, and of these the Llay Marine Band (A. vanderbeckei) and the Warras Marine Band (A. hindi), defining Langsettian–Duckmantian (Westphalian A–B) and Duckmantian–Bolsovian (Westphalian B–C) boundaries, respectively, are the most significant stratigraphically. In Flintshire, the Coal Measures, up to 650 m thick, contains some 18 workable steam coal seams of which the Main Coal, the Wall and Bench Coal and the Yard Coal can be traced across most of the district and are the most important. Locally, thick sandstones occur within the dominantly shaly sequence near Buckley, the Hollin Rock succeeds the Hollin Coal and replaces most of the beds up to the base of the Buckley Fireclay Group but wedges out to the west and east. In Flintshire, the Buckley Fireclay Group, at the top of the Pennine Coal Measures Group, comprises compact, fine-grained quartzose and softer feldspathic sandstones. The white and grey sandstones are superficially stained purple, red and yellow. They grade into fine siliceous, purple, black and grey calcareous mudstone (marl), which is the main source of the raw material used in the manufacture of fire- and acid-resisting bricks and tiles. Elsewhere, beds of fine-grained clayey sandstone (ganister) are worked for brick making.

The red measures of the Warwickshire Group are traceable southwards from near Flint in the Dee estuary, to Ruabon and farther into the Shropshire Coalfield near Oswestry. In the southern part of this outcrop, the upward succession comprises the Etruria Formation, Halesowen Formation and the Salop formations. The base of the group is markedly diachronous, ranging from mid-Duckmantian in the Flintshire coalfield to Bolsovian in the Denbighshire coalfield. The Etruria Formation is restricted to the south of Wrexham; it is up to 300 m thick, and consists mainly of red or purple mudstone, mottled yellow, with a few thin beds of grey to black mudstone, carbonaceous smears and thin sandstones. In the upper part there are Spirobis limestones, but the only other fossils recorded are plants, ostracods, fish and rare mussels. The Halesowen Formation consists mainly of grey mudstone, sandy mudstone and sandstone with red and purple bands and mottling in places; a cyclical pattern can be discerned in places, but the coals are very thin. At Pont y Cyfflogyn, a limestone at the base is overlain by a marine band, which probably lies close to the Bolsovian–Westphalian D boundary. Apart from a possible occurrence in the Vale of Clwyd, the formation has not been determined north of Caergwrle. The Salop Formation forms a wide outcrop from Llay, north of Wrexham, to Oswestry. It comprises red, purple and green mottled mudstone and grey feldspathic sandstone, which seem to overstep the underlying formations (Figure 50). Particularly distinctive components are marl breccias and mudstones with abundant hematite grains. In the Vale of Clwyd, similar red beds, resting unconformably on Dinantian limestones have been correlated with the Salop Formation. Less common are grey measures with thin coals and thin limestones. The only fossils determined are plants, Spirobis and tetrapod footprints.

The relationship of the red measures to the underlying coal measures, and the possible unconformity at the base of the Etruria and Salop formations, have been subjects of considerable discussion. Comparison with the sequences farther east suggest that the relationships are most easily explained by interpreting the red measures as a facies that is diachronous; the Etruria Formation in Denbighshire is the lateral equivalent of the Buckley Fireclay Group of south Flintshire. On Anglesey, Westphalian strata crop out on the north-west side of the Berw Fault, in the area of Malltraeth, south-west of Llangefni; the outcrop is largely obscured by glacial deposits and alluvium. The detail of the sequence is derived mainly from shafts and boreholes from 19th century exploration. For much of the outcrop, the strata overlie Dinantian limestone, but in the south-east they rest on Precambrian. The sequence consists of 450 m of grey measures (Pennine Coal Measures Group) with a thick cross-bedded coarse-grained sandstone at the base, referred to the ‘Millstone Grit’, overlain by some 200 m of red beds (Warwickshire Group). Marine shales within the sandstone contain a fauna that compares closely with the Gastrioceras listeri Marine Band in Flintshire. The overall sequence is typically cyclic with as many as 13 coals, four over 1 m thick, which reputedly were mined intermittently from the 15th century to the late 19th century. Mussels from a bed in the middle of the sequence suggests the lower part of the Carbonicola communis Zone. The Warwickshire Group, consisting of red mottled and grey mudstone and red, locally conglomeratic sandstone may rest unconformably on the grey measures, but the evidence is inconclusive. In a narrow outcrop along the Menai Straits, red, pale-green mottled mudstone (Ferry Beds), which unconformably overlies Visean limestone, has been assigned a Westphalian age on lithology. The mudstone is similar to that south-east of Port Dinorwic, which is associated with a distinctively massive boulder bed, crowded with subrounded clasts of locally derived sedimentary and volcanic rocks set in an argillaceous matrix, and with only the most imperceptible indication of horizontal bedding. The mudstone is bound to the south-east by the Dinorwic Fault, and is intensely folded within the Menai Straits Fault System at the northeast edge of the outcrop.

Coal

Coal forms an insignificant proportion, probably less than 2 per cent, of the Westphalian sequence in south Wales, yet its economic, cultural and social significance has been immense. It is traditionally accepted, probably because it is often repeated, that coal was used for cremation on Garth Mountain during the Bronze Age (some 4000 years ago) and that the Romans were the first to work it systematically. Serious extraction began in the Middle Ages, with the initiation of smelting in the Swansea district, of tin and lead from Cornwall, and silver and lead from north Wales. However, it was the 19th century that witnessed the greatest expansion of mining, for iron smelting and the insatiable appetite of steam locomotives and domestic heating. By the end of the 19th century, Cardiff became the most important coal exporting port in the world. The subsequent decline through the 20th century as a result of competition from oil and coal imports has been inexorable. Now in the 21st century, there is one workers’ cooperative deep mine, some small private shallow mines and isolated large-scale machine-driven opencast operations (Plate 42).

One of the distinctive features of the South Wales Coalfield is the variation in ‘rank’ of the coals (Figure 51). The bituminous coals are low rank, soft and friable with a high proportion, 20 to 40 per cent, of volatiles and a carbon content from 84 to 91 per cent; they are good house, gas and coking coals. In contrast, anthracite is a hard lustrous coal of high rank, with a low proportion (3 to 8 per cent) of volatiles, a low hydrogen content, a high (over 93 per cent) carbon content and low ash; it burns at high temperatures with little flame or smoke and is unsuitable for coke manufacture. Steam coals are intermediate between the bituminous and anthracite coals, although they grade into each other.

The sedimentary rocks across the coalfield indicate very low grade metamorphism but the patterns of illite crystallinity indicate an increase into the anchizone grade in the extreme northwest sector of the main basin. This pattern is also displayed by coal rank values, based on volatile content and vitrinite reflectance, which indicate a distinct increase in rank from the south-east to the north-west (Figure 51). The bituminous coal occurs mainly along the south and east crops, steam coal between the Taff and the Neath valleys, particularly about the Rhondda valleys, and the anthracite occurs along the north crop westwards from the Neath valley, especially in the Gwendraeth valley, into Pembrokeshire. These variations in rank have stimulated much debate and controversy over many years. The broad correspondence of the coal isovols (lines of equal volatile matter) to the stratigraphical boundaries indicate that coal rank developed prior to the formation of the main coalfield structure. However, the correspondence is not precise as the isovols drop gently to lower stratigraphical horizons to the south-east.

In the lowermost coal seams the rank expressed in vitrinite reflectance values varies from 1 per cent in the south-east to 4 per cent in the north-west, which reflects a variation in maximum maturity temperature from about 150°C to 325°C. Such a difference is difficult to reconcile solely by different depths of burial as the depocentre for the preserved sequence lies to the south of the area of highest rank. Detailed studies in the Ffos Las opencast site, which lies across the Llanon and Trimsaran disturbances, indicate that rank developed both before and during thrusting. Similarly, a geothermal gradient of 310°C per km in the eastern part of the coalfield (higher in the west) suggests that burial alone cannot be responsible for this gradient. Localised zones of higher temperature have been correlated with in-seam thrust detachments, suggesting that the thrusts channelled both hot fluids and a wide spectrum of mineral phases, including gold traces, into the coals. In north Wales, there are records of coal extraction in both the Flint and Denbighshire coalfields since the 15th century. The last working collieries, at Bersham, near Wrexham, and Point of Ayr, on the Dee estuary, limped through to their final demise in the last years of the 20th century, even though reports of extensive reserves had been a persistent feature of their final years.

Variscan Orogeny

The movements that periodically affected sedimentation through Devonian and early Carboniferous times in Wales were the precursors of the Variscan Orogeny, which climaxed in the late Carboniferous; the orogeny was caused by the collision between the Laurussian and Gondwanan plates. In early Devonian times, rifting developed passive marginal basins in south-west England and elsewhere, and these accumulated marine Devonian and Carboniferous sedimentary and volcanic rocks. Progressive convergence, beginning in mid-Devonian times, inverted these basins and thrust them northwards. The movement caused flexural subsidence ahead of the deformation front and the development of a foreland basin in south Wales, which accumulated detritus eroded from the fold and thrust belt to the south. It is only in the Devonian and Carboniferous sequences in south Pembrokeshire, the Gower and south Glamorgan, that the typically east–west-trending Variscan structures can be clearly distinguished (Figure 31). Two zones of deformation lie on either side of the Variscan Front, a putative line that defines the limit of strong deformation. To the south of the front, the deformation is typical of a fold and thrust belt, particularly in south Pembrokeshire where the Devonian and Carboniferous strata are weakly cleaved and altered to the lowest anchizonal grade of regional metamorphism. To the north of the front, thrusting is more variable, folds are more open (Plate 43) and the rocks are non-metamorphosed.

In Pembrokeshire, south of Milford Haven, the Ordovician and Namurian sequences are intensely folded. The steep-limbed folds have axes that trend close to an east–west orientation; strata in the anticlines range down to Llanvirn age, and in the synclines, as at Bullslaughter Bay, the cores are of Dinantian and Namurian strata. Axial and limb buckling is extremely common, several minor thrusts replace minor folds and fold axes are displaced by several, post-folding cross-faults as in Stackpole Quay. Within the Upper Carboniferous sequence, the Precambrian Johnston Complex and Benton Volcanic Group have been thrust northwards on the Johnston Thrust for some 4 km over the Westphalian (Figure 52).

The Johnston–Benton fault belt and the Ritec Thrust dominate the structure of south Pembrokeshire, as high-level, thin-skinned Variscan features, and the trace of the Ritec Thrust subsequently influenced the erosion of Milford Haven. However both structures had probably influenced sedimentation, as major growth faults in a pre-Variscan extensional regime, since early Silurian (late Llandovery) times; the effect of the Variscan compression was to overprint contractional features at their upper levels. Farther east, the Johnston Thrust can be distinguished in the cliffs at Amroth and from here it can be projected into the broadly north-easttrending, Llandyfaelog–Carreg Cennen Disturbance close to the north edge of the main coalfield.

Within the main coalfield, the complexity of the main Variscan deformation has been most manifestly displayed in the open cast sites; local shortening estimates up to 50 per cent have been proposed, but overall are closer to 30 per cent. Throughout the central and northern coalfield a forethrust system developed, which was balanced by a major backthrust system along the south crop. Many thrusts have been distinguished, particularly in the anthracite 'belt', between Ammanford and the Gwendraeth valley, and in the south crop. The dip along many of these faults steepens markedly, as from 20º to 70º along the Ty’n y Nant and Moel Giliau fault sytems. These structures were controlled by the variable competence of the sedimentary rocks; the competent sandstones of the Pennant Sandstone Formation acted as a passive roof to the thrust system.

To the east of Pembrokeshire, the front is more difficult to delineate although it can be traced close to the north side of the Gower and farther into the Vale of Glamorgan. Within Gower, the folds are not as closely packed as in Pembrokeshire, but vertical and overturned strata, as in the south cliffs of Rhossili Bay, are not uncommon. The major anticlines, as at Rhossili Down and Cefn Bryn, preserve Devonian strata in their cores, while the synclines, as at Oystermouth and Port Eynon, preserve Namurian beds. Parallel thrusts and cross-faults, although present, are not as apparent as farther west. Pre-existing structures beneath the south crop were the main influence in determining the backthrust system and the Variscan folds in the Vale of Glamorgan at the edge of the mountain front.

The syncline of the main basin of the coalfield is most clearly defined east of Llanelli. The wide outcrop patterns, for most of the north crop, and the narrow outcrops, of the east and south crops, reflect its general asymmetry, but there are many local complexities. Within the main syncline, en échelon minor folds, such as the Pontypridd and Maesteg anticlines, and the Caerffili, Gelligaer and Llanelli synclines, progressively deflect into a north-easterly trend as they are traced eastwards into Monmouthshire.

Faults are an important element in the structure of the coalfield and many caused major problems in the extraction of the coal. The dip or cross-faults are the most obvious, predominantly north-north-west-trending in the western and central parts of the coalfield and gradually deflecting to more north-westerly trending in the eastern parts. Many of these faults developed during folding whereas others postdate folding. Two of the most distinctive structural features of the main coalfield, the Neath and Swansea valley disturbances, are northeast–south-west zones of folded and faulted strata that trend into the coalfield from the Devonian outcrops to the north and east (Figure 31). The Neath Disturbance is marked by intense folding of lower Carboniferous strata, as at Penderyn and Craig y dinas. Similar folding, as at Cribarth, marks the Swansea Valley Disturbance. The Neath and Swansea valley disturbances bear a close similarity to the Carreg Cennen Disturbance, which emerges from Devonian strata near Pendine and passes laterally into the Johnston Thrust, in the Coal Measures, near Amroth (see above). The zones clearly represent a long history of pre-Variscan and probably post-Variscan tectonic activity, which suggests they were propagated by basement fractures.

The clear determination of Variscan structures in the areas of central and north Wales, where there are no outcrops of Upper Palaeozoic strata, is impossible. However, it is safe to assume that most of the earlier Caledonoid structures were accentuated in some way. This is supported by the general disposition of the Carboniferous strata marginal to the Lower Palaeozoic landmass, from Oswestry through to Colwyn Bay and Llandudno to Anglesey, which suggests considerable upwarping of major structures such as the Harlech Dome and the Derwen Anticline. Reactivation of the Bala Fault Zone is most graphically demonstrated by its extension, in the Bryneglwys Fault, through to Caergwrle, and its sinistral displacement of the Carboniferous sequence. Similar reactivation is apparent in the displacement of the Lower Carboniferous sequences along the Vale of Clwyd faults, the Menai Straits Fault System, and marginal to the Berw Fault, at Malltraeth, on Anglesey.

Mineralisation

The lead-zinc (Pb-Zn) mineralisation in Central Wales Mining District, between Aberystwyth and Llanidloes, is hosted mainly in Silurian and, less commonly, Ordovician rocks. However, lead isotope data indicates that the mineralising fluids were initially active in early Devonian times, possibly in response to the Acadian phase of the Caledonian Orogeny, and later in Carboniferous times, possibly prior to coalification events in south Wales. Similar associations occur in Silurian strata, in the Llanfair Talhaiarn district in Denbighshire, and in Ordovician strata in the Llanrwst district, on the west side of the Conwy valley.

In north-east Wales, there is extensive mineralisation along Variscan faults within Lower Carboniferous strata; on Halkyn Mountain and in the vicinity of Minera, the mineralisation has been intensively exploited. The deposits are mainly concentrated in the Lower Carboniferous limestones and the overlying Cefn y Fedw sandstone with lesser concentrations in the Basement Beds. The ores are mainly of sphalerite and galena, with little silver, associated with chalcopyrite, baryte and fluorspar. The ores are concentrated along joints, faults and, in the limestones, in cavities along bedding planes where the calcite has been dissolved. The ores tend to be richest where capped with a bed of shale, which restricted the upward movement of the solutions. The solutions are assumed to be magmatic, although their source remains problematic as there is no indication of contemporaneous igneous activity.

In south Wales, iron mineralisation in the Lower Carboniferous limestones has been extensively worked in the Vale of Glamorgan, near Llanharry. The ore consists mainly of colloidal and crystalline hematite, commonly altered to goethite, with quartz, calcite and dolomite. The ore bodies occur along the north-north-west-trending cross-faults. They fill large cavities and enlarged fractures in the limestones, generally beneath the Triassic unconformity, close to their contact with impermeable Namurian mudstones. The ores have been related to hydrothermal and to meteoric groundwater solutions. Probably both solution processes were involved — an earlier hydrothermal (sulphide) phase and a later replacement phase due to meteoric circulation. With the latter, the iron and silica are thought to have been leached from Triassic and Silesian strata, and deposited both as replacements and in cavities.

Chapter 8 Mesozoic

Following the Variscan orogeny and the concomitant continental reorganisation, Northern Europe lay deep within the newly formed supercontinent of Pangaea, and was straddled by one of the largest deserts in the Earth’s history. The desert lay close to the Equator within a belt of easterly trade winds.

In contrast to the largely compressional stress during the Variscan orogeny, the area was subjected to tensional stress and the main physical features that formed in early Permian (Late Palaeozoic) times persisted for most of the Mesozoic. The most significant of these features were fault-controlled depositional basins that were separated by massive blocks or horsts and were markedly different from the basins that had previously influenced Upper Palaeozoic sedimentation. Such basins are situated off the coasts of Wales (Figure 53) and contain thick sequences of Mesozoic strata, which became targets as possible reservoir rocks during oil and gas exploration. To the west of Wales, the Central Irish Sea, Cardigan Bay, St George’s Channel and North Celtic Sea basins are predominantly north-east- to south-west-trending structures (Figure 54). Farther south, the South Celtic Sea and Bristol Channel basins are orientated more closely east–west. The basins were initiated along normal faults, which profoundly influenced early sedimentation.

The St George’s Channel Basin is bound on its south-east margin by the St George’s Channel Fault and, farther to the south-east, by the seaward extension of the Bala Fault. The Cardigan Bay Basin is bound to the east by the Tonfannau Fault. The asymmetry of the infill faulted on its northern side. The gross unconformity at the base of the Mesozoic sequence, with the local absence of both Permian and lower Triassic, reflects prolonged erosion of the uplifted Palaeozoic rocks. At the end of Permian times, a mass extinction, when 30 to 50 per cent of all orders and families were lost, is considered to be the most important event in the biotic record. Its cause, whether sea level change, salinity, temperature, mega-volcanism, asteroid impact or other, is still a matter of debate. The evidence of the event in northern Europe is sparse, but when more permanent marine deposition returned in late Jurassic times the Palaeozoic flora and fauna were replaced by Mesozoic species.

Permian-Triassic

During Permian–Triassic times, most of the area of the British Isles was a desert, with areas of rugged hills, as across Wales, and marginal areas of lower ground. Through Permian times, a marginal shallow sea, Zechstein, lay to the north-east, close to the current position of the North Sea, and it progressively encroached the landmass so that by mid-Triassic times, the Welsh landmass was almost surrounded by water. The Mesozoic era began with the removal of extensive areas of the exposed Carboniferous rocks and, at the same time, there was widespread oxidation of the desert surface. The earliest sediments were locally derived, reddened, waterlaid conglomerates and breccias that were probably generated in alluvial fans. The ‘New Red Sandstone’ represents a molasse facies to the Variscides, comparable with the Devonian, Old Red Sandstone, molasse facies to the Caledonides. Evidence of the fauna and flora in early Permian times is extremely poor, probably due to its low preservation potential and original low biodiversity in the desert environment. In mainland Wales, reconstruction of the early Permian geography is difficult because of the limited outcrop. However, offshore exploration has determined a fault-controlled basin in Cardigan Bay, and another in the east Irish Sea. Periodically, hypersaline lakes were established in these basins.

The occurrence of Permian rocks in north-east Wales has not been positively proved, but two outcrops of red sandstone in the Vale of Clwyd (Figure 40); (Plate 44) have been assigned a Permian age. Along the vale, geophysical surveys have determined three sedimentary basins that are bound on the east by the Vale of Clwyd Fault, which downthrows some 1500 m to the west. On the west side of the outcrop, between Abergele and Rhuddlan, red sandstone (Kinnerton Sandstone Formation) oversteps on to Carboniferous limestone and marks the edge of a small, fault-bound basin that extends northwards into Liverpool Bay (Figure 55a). The friable, red, predominantly aeolian sandstones contain cemented layers with siltstone beds and mud-flake breccias of fluvial origin. An offshore borehole near Llandudno proved 65 m of unconsolidated red sand, with Upper Permian (Kazanian to Tatarian) spores, which has been considered to be equivalent to the Kinnerton Sandstone Formation.

Farther south, in the Denbigh–Ruthin area, exposures of red sandstones are scarce. Some of the more competent, cemented sandstones have been quarried, for example those from Foel Ganol were used in the construction of Ruthin Castle. The false-bedded sandstones are aeolian crescentic dune deposits, and palaeocurrents indicate westerly directed winds. The sand grains are subangular, faceted, coated with limonite and locally show secondary over-growths of quartz. Feldspar is a ubiquitous accessory mineral and ilmenite is the dominant heavy mineral; rutile, tourmaline and zircon are common and garnet is less so. The rich mineral assemblage contrasts sharply with the paucity of the assemblages in the Carboniferous sandstones. Similar sandstones, also assigned to the Kinnerton Sandstone Formation, crop out on the western edge of the Cheshire Basin; the sequence oversteps the Carboniferous in eastern Denbighshire to rest on Ordovician strata near Oswestry and Llanymynech. To the south-west of Anglesey, Permian strata have been inferred in the Central Irish Sea Basin (Figure 55a). In a borehole at the southern margin of St George’s Channel Basin, calcareous mudstone and muddy sandstone, overlying Westphalian strata, have been assigned to the Permian, based on their geophysical signature.

Biostratigraphical evidence of Triassic age is variable and generally sparse. Macrofossils are scarce, but they do indicate that diverse assemblages occur. Plant spores and pollen are more widespread and allow some regional correlation. Three major lithostratigraphical units have been determined. They are, in ascending order, the Sherwood Sandstone, Mercia Mudstone and Penarth groups.

From the elevated edge of eastern Denbighshire and Flintshire, it is easy to envisage the development of the New Red Sandstone molasse facies that occupies the Cheshire Basin to the east. The lowermost, early Triassic, dominantly sandstone sequence (Sherwood Sandstone Group) with a local development of pebbly and conglomeratic sandstone (Cheshire Pebble Beds Formation, formerly the Bunter Pebble Beds) at its base, crops out close to the border, near Oswestry (Figure 55b). The sandstones are typically well cemented, medium to coarse grained with scattered quartzitic pebbles. The internal bedforms suggest deposition in confined channel systems of low-sinuosity rivers; interbedded friable sandstones with faceted grains have been interpreted as wind-blown sand on gravel bars. All the palaeocurrent evidence indicates that the river systems drained persistently to the north-northwest, through the Cheshire Basin.

The top of the Sherwood Sandstone Group and the lower part of the overlying Mercia Mudstone Group are dominated by blocky red mudstone with interlaminated siltstone and mudstone. Halite that is extensively exploited in the Cheshire salt industry also occurs offshore within the Mercia Mudstone Group. The halite indicates that there were ephemeral lakes and the thickest deposits occur in the vicinity of syndepositional faults. The sandstones were deposited in a continental fluvial environment with rivers flowing northwards into the area of Liverpool Bay (Figure 55c). Fine-grained, aeolian sandstones within the sequence record north-easterly prevailing winds, probably during drier winter seasons. Vertebrate tracks have been recorded.

The Mercia Mudstone Group forms the main outcrop in the extension of the Cheshire Basin in Liverpool Bay, north-east of Anglesey. To the south, elements of the three lithostratigraphical groups occur in both the St George’s Channel and Cardigan Bay basins. The Sherwood Sandstone Group is generally finer grained than the onshore equivalent, possibly reflecting a more distal depositional environment. In the centre of the basins, the Mercia Mudstone Group strata lie conformably on the Sherwood Sandstone Group and four lithological units, with variable proportions of mudstone, siltstone, fine-grained sandstone and halite, dolomite and gypsum have been distinguished. Using geophysical logs, these units have been correlated with equivalent sequences in onshore boreholes. The evaporites were precipitated in coastal marine sabkhas. The thickest sequence, some 1700 m, penetrated by a borehole at the southern edge of St George’s Channel Basin (Well 103/2–1), compares with a maximum of only 160 m in the Vale of Glamorgan. The lithologies suggest more basinal conditions than their onshore continental equivalents. The St George's Channel and Cardigan Bay basins are bound on the south-east side by the seaward extension of the Bala Fault Zone, and the asymmetry of the basin infill indicates syndepositional activity.

The first clear indication of the extent of these offshore Mesozoic sequences was provided by the Mochras Borehole (Figure 56), which was drilled between 1969 and 1971, on Shell Island, near Llanbedr, to the south of Harlech. During Triassic times, as today, the site lay close to the western edge of the Welsh landmass. At the bottom of the borehole, at a depth of 1906 m, some 32 m of Triassic sedimentary rocks have yielded a miospore assemblage of Norian to late Rhaetian age. The Triassic sequence has been divided into the Terrigenous Formation, which consists of pale red-brown calcareous sandstone and dark brown shales, overlain by the Carbonate Formation, consisting of grey-green, dolomitised carbonate with some hematitic sandy beds, and suggesting deposition in a playa-lake environment; it is not closely comparable with the Rhaetic facies of south Wales.

Triassic rocks in south Wales are largely restricted to the Vale of Glamorgan, but small patches of red calcareous mudstone in some of the Gower bays and a larger outcrop of red breccia and conglomerate at Port Eynon are probably Triassic karstic remnants. Similar, red-stained breccias within Dinantian limestones in south Pembrokeshire and the Vale of Glamorgan are also possible sites of Triassic collapse cavities. In the Vale of Glamorgan, the lowermost Triassic strata are predominantly brownish red calcareous mudstone and siltstone) (Figure 57) that locally pass laterally into a ‘marginal facies’ of conglomerate, breccia and sandstone. The transition was controlled by the topography, which was dominated by the effects of erosion on the contrasting lithologies within the Cardiff–Cowbridge Anticline. The Carboniferous limestone formed ridges and small hills, and the softer Lower Old Red Sandstone along the axis was the site of a valley in the main basin to the south-east. The Mercia Mudstone Group in south Wales is probably of Norian age. The red mudstone is typically massive and dolomitic with common gypsum nodules and veins. Thin bands of greenish grey mudstone, with mottling, mudcracks and raindrop imprints, occur in places and, where they pass into the marginal facies, they contain thin calcareous sandstone and siltstone beds. The marginal conglomeratic facies is dominated by clasts of Dinantian limestones. Generally, exposures are poor but good coastal cliff sections occur between Barry and Sully islands, and between Ogmore by Sea and Kenfig Pool, where the basal unconformity and Triassic sediments in fissures and caverns in the limestones are commonly well displayed (Plate 45).

The red mudstone is overlain by green and grey-green mudstone, in places thinly laminated with thin dolomitic limestone beds (Blue Anchor Formation), which locally pass laterally into a conglomeratic marginal facies (Figure 57). Red mudstone bands are rare and gypsum nodules are common at a few horizons. Fish, plant and reptilian remains have been recovered from strata near the top of the formation, and sparse palynomorph assemblages have Rhaetian associations. The 14 m section at Lavernock Point displays a range of lacustrine lithologies, and desiccation cracks and evaporites indicate periodic drying out. The plant debris and spores in the upper part of the sequence, and the change in colour from red to green, indicate a change from arid to humid conditions probably associated with increasing marine influence, which culminated in the late Triassic marine transgression.

The Rhaetian marginal facies deposits have been divided into continental and lacustrine shore-zone subfacies. The continental subfacies of red conglomerate, breccia and sandstone, with few siltstone and mudstone beds and nodular calcretes has been subdivided into four lithofacies. The lacustrine subfacies consists predominantly of well sorted breccia, calcareous sandstone, siltstone and mudstone with subordinate nodular dolomites and, in proximity to the mudstones, both fenestral and cryptalgal carbonates. The breccias and conglomerates, with mainly Carboniferous limestone clasts (Plate 46), are well sorted and pass laterally, in as little as 10 m, into red mudstone. Platforms backed by low cliffs are cut into both subfacies; on the west side of Little Island, adjacent to Barry Island, five such platforms occur in a vertical section of 6 m. The marginal conglomeratic subfacies is interpreted as an alluvial fan deposit, and the ill-sorted angular breccias resemble modern scree. Matrix-supported conglomerates and thinly bedded graded sandstones are interpreted as the deposits of concentrated to dilute grain flows. These facies are most easily related to deposition in a major lake that dried out from time to time, with prolonged periods of subaerial exposure. In the lacustrine subfacies, the lateral gradation from scree deposits through shore-zone clastics to red mudstone reflects the passage from a littoral to a sublittoral environment. The platforms indicate variations in the lake level and erosion by wave action. Between Barry and Sully, screes and local sheet flood and stream flood deposits are associated with shore-zone facies in the uppermost part of the sequence.

The Blue Anchor Formation is overlain by about 12 m of dark grey and grey mudstone with subordinate sandstones, siltstones and limestones (Penarth Group), which passes into a more restricted marginal facies. The strata were deposited during the mid to late Rhaetian marine transgression that persisted into early Jurassic times. In the cliff sections about Penarth Head, at St Mary’s Well Bay and north of Lavernock Point, the group overlies an erosion surface corresponding to a significant shoreline recession following deposition of the Blue Anchor Formation. The lowest strata, the Westbury Formation, consist of dark grey, fissile, pyritous mudstone with a few thin beds of limestone and calcareous sandstone. Six depositional cycles have been recognised, each with a basal fine-grained sandstone that fines upwards into mudstone with bivalve-rich layers; bivalves include Rhaetavicula contorta, Chlamys valoniensis and Protocardia rhaetica. South of Penarth, between Lavernock Point and St Mary’s Well Bay, thin limestones occur with the mudstones, and contain a restricted bivalve fauna, dominated by Liostrea bristovi. These are overlain by a thin fossiliferous, locally conglomeratic coarse-grained sandstone commonly referred to as a bone bed (Storrie's fish bed). The bed represents a strand-line deposit of the main marine transgression, which probably engulfed most of the Vale of Glamorgan. The foraminifera, marine ostracods, the brachiopod Orbiculoidea, echinoid fragments, ophiuroids, cirripedes and various marine invertebrates indicate a diverse community in a shallow marine, low-energy environment. The presence of relatively coarse lithologies and uncemented shell accumulations suggest more turbulent phases, possibly storm induced.

At the top of the Rhaetian sequence, the Lilstock Formation consists of grey and grey-green mudstone with few siltstone, sandstone and limestone beds. It has been divided into the Cotham Member overlain by the Langport Member (Figure 57). The junction between the Westbury Formation and the Cotham Member is sharp and locally channelled, up to 40 cm deep, indicating uplift and consequent shallowing. The basal calcareous, pale grey silty mudstone was probably deposited in a restricted marine lagoonal environment, and contains abundant dark mudstone clasts torn from the substrate. The mudstone matrix contains a diverse fauna with Cardinia and abundant ‘Gervillia’ praecursor, together with most of the Rhaetian bivalve taxa. However, up-sequence, the faunas decline and the presence of the foraminifera Dentalina, Euguttulina liassica, Lingulina tenera and Nodosaria and the ostracod Darwinia liassica indicate fresh-water influxes. At the top of the member, a coarse-grained, locally ooidal sandstone infills large polygonal desiccation cracks, up to 8 cm wide.

The Langport Member comprises porcellanous limestone overlain by calcareous mudstone with thin beds of fine-grained sandstone, siltstone and limestone. The pale grey, white-weathered, porcellanous limestone beds are separated by thin mudstone partings. Identifiable bivalves in the limestones include Modiolus, Dimyopsis and Liostrea hissingeri and the mudstones contain foraminifera, ostracods, echinoid, fish and plant debris. The facies records the final marine inundation of the lagoonal environment of the Cotham Member. The overlying calcareous mudstone was deposited in a shallow marine, offshore environment, and the few beds of thin sandstone and argillaceous limestones, some of which are graded, were probably storm generated. The top of the Penarth Group is sharply overlain by ‘paper shales’ of the St Mary’s Well Bay Member, which straddles the Triassic–Jurassic boundary, at the base of the Blue Lias Formation.

To the west of the Vale of Glamorgan, the basal Triassic unconformity can be traced across Swansea Bay and close to the south coast of the Gower. However, the thickest Triassic sequence is contained within the Bristol Channel Basin (Figure 53), bound on its north side by the Central Bristol Channel normal fault zone, an extensional reactivation of a Variscan thrust. The Mesozoic infill of the basin, which was profoundly influenced by the active tectonic extension, thickens west of Lundy Island. The Sherwood Sandstone, Mercia Mudstone and Penarth groups have all been proved, and the lithologies have been determined mainly from the geophysical logs and their comparison with onshore wells. The 960 m of the Mercia Mudstone Group within the basin, compared with about 160 m in the Vale of Glamorgan, is the clearest testimony to syndepositional fault movement. The sequence, predominantly of mudstone and siltstone with a thick central zone of halite, was deposited in basinal conditions some distance from the continental facies of south Glamorgan.

Jurassic

The Rhaetian marine transgression across southern Britain advanced over an extensive area of low relief. Consequently, widespread uniform depositional conditions persisted into early Jurassic times. Throughout the Jurassic much of Europe lay beneath a generally shallow sea, which supported a rich fauna, particularly of ammonites. These allow the Jurassic sequence to be divided into 12 stages and about 75 zones, and facilitate precise correlation. The region probably lay some 10° south of its present latitude; the climate was warm and generally damp and the land areas were densely vegetated. However, seasonal drier conditions are reflected in the restricted development of red beds and in the presence of some anhydrite horizons.

In Wales, Jurassic outcrops are restricted to the Vale of Glamorgan where, south of Bridgend, the Lower Lias sequence forms a dissected, gently inclined surface. However, around Wales, thick Jurassic sequences abut Palaeozoic rocks on all sides — in the Cheshire, Celtic Sea, Bristol Channel and Cardigan Bay basins (Figure 58). The juxtaposition of these sequences is mainly the result of post-Jurassic tectonism although there is evidence for contemporaneous extensional basin development that allowed a thick sequence of Jurassic strata to accumulate. There is no evidence to suggest that the Welsh Palaeozoic terrain formed an upland area in early Jurassic times, but there is evidence of small islands over parts of south Wales.

Lower Jurassic

The base of the Jurassic is placed at the base of the Planorbis Zone, just above the base of the Lias Group, at the first appearance of the ammonite Psiloceras planorbis. Thus the lowermost beds of the Lias Group are of Triassic age but the entire group is described here.

The Lias Group rests conformably on the Penarth Group and, in the Vale of Glamorgan, lies mainly within the Hettangian and lower Sinemurian stages (Figure 59a). It comprises up to 150 m of thinly interbedded limestone and calcareous mudstone, a facies that has been widely recognised in southern Britain and is now termed the Blue Lias Formation. As with the Triassic sequence, sedimentation was influenced by the presence of small islands of Dinantian limestone, and a marginal facies developed in their vicinity (Figure 59b). The sequence has been subdivided into three members on the basis of the ratio of limestone to mudstone.

The lowermost St Mary’s Well Bay Member, exposed between St Mary’s Well Bay and Lavernock Point, comprises about 20 m of mudstone with interbedded limestone in approximately equal proportions; the conformable base of the Planorbis Zone, and therefore the Jurassic, lies some 5 to 7 m above the base. At the base of the member, the Paper Shales consist of delicately interlaminated pale grey calcareous siltstone and silty mudstone with abundant bivalves, Liostrea hissingeri, Modiolus sp., and echinoderm debris. Shallow water, attached suspension feeders dominate the bivalve macrofauna in the overlying strata (Bull Cliff Member), which consists of thin tabular limestone beds with subordinate mudstones. The limestone is mainly blue-grey, burrowed and argillaceous with bivalves, ammonites, brachiopods and echinoderm debris. The mudstones are variably calcareous and contain similar faunas. The mudstone–limestone contacts appear to be gradational because of diagenetic modification. Less common are argillaceous limestones with carbonaceous laminae and laminated bituminous mudstones that occur at three distinct levels in the Planorbis Zone. The lithologies are remarkably persistent over some 10 km in the coastal outcrops and the only lateral variation occurs in individual beds, particularly the nodular limestones, which are more common in the upper part of the member. The marginal facies comprise thin conglomeratic lags with mudstone flakes and concretionary dolomitic pebbles from the underlying Penarth Group, and locally, as at Witland, these deposits infill a markedly incised substrate. The top of the formation is marked by a prominent limestone bed that lies in the lower part of the Liasicus Zone of the Hettangian Stage.

The type section of the overlying Lavernock Shales Member lies between St Mary’s Well Bay and Lavernock Point. The member is up to 12 m thick, lies entirely within the Liasicus Zone and comprises blue-grey, calcareous mudstone with subordinate thin beds of nodular limestone. The mudstone is burrowed; crinoid and echinoid fragments, ostracods, bivalves and a few ammonites are variably distributed. The marginal facies of thinly bedded conglomerates with angular clasts of Dinantian limestone, and interbedded mudstones is well exposed between Whitmore Stairs and Temple.

At the top of the Blue Lias Formation, the Porthkerry Member, up to 120 m thick, is an alternating sequence of limestone and mudstone similar to the St Mary’s Well Member, but without the laminated lithologies. The Porthkerry Member is equivalent to Truman’s ‘Upper Limestone Series’ and is the main component of the spectacular cliffs of the Glamorgan coast (Plate 47). The sequence is characterised by a varied fauna of bivalves, including Pinna and Plagiostoma, with echinoids, ammonites and gastropods; ‘nests’ of Gryphaea are common in the shales and the large branching burrowing systems of Thalassinoides are widespread. Four informal lithostratigraphical units, based on the bedding characters and the limestonemudstone ratios, have been determined. The limestone beds increase in abundance and thickness into the upper part of the Bucklandi Zone, where the beds are replaced locally by the marginal facies that prograded into the offshore area. The member lies mainly in The Angulata Zone and the overlying Bucklandi and Semicostatum zones of the lower Sinemurian.

The distinctive alternations of limestone and mudstone of the Blue Lias Formation reflect deposition in a shallow sea, but the nature of the alternations, whether primary or secondary, has been controversial; a primary origin is supported by the widespread lateral extent of individual beds. Generally aerobic bottom conditions prevailed and burrows infilled with pale limestone are present within the mudstone substrate, but the presence of bituminous shales indicates a lack of oxygen from time to time. Alternatively, it has been argued that the limestones represent diagenetically altered beds of winnowed shell concentrates, deposited within storm wave-base. Studies of sequences showing less secondary modification indicate that mudstone deposition represented deepening and that subsequent slow regression caused progressive accumulation of condensed limestone. The patterns suggest a delicate interplay of eustatic sea-level fluctuations and local tectonism. In the Vale of Glamorgan, primary cycles are difficult to substantiate because of the diagenetic overprint, but it is likely that deposition occurred predominantly within storm wave-base.

The offshore distribution of Lower Jurassic strata (Figure 58), like that of the Permo-Triassic deposits, is controlled by the Bristol Channel and South Celtic Sea basins in the south, and by the St George's Channel and Cardigan Bay basins farther north. Cores and geophysical records indicate some 350 m of Blue Lias Formation on the flanks of the Bristol Channel Syncline where it is dominantly a mudstone sequence with few limestones. The Blue Lias Formation is overlain by mudstone of the Lower Lias Clay (now referred to the Charmouth Mudstone Formation), which is not exposed onshore in south Wales. To the west, in the South Celtic Sea Basin, the Blue Lias sequence thickens to 604 m and the proportion of limestone increases markedly. Similarly, thin Pleinsbachian–Toarcian sequences (Middle Lias and Upper Lias) of the Bristol Channel Basin also thicken westwards.

At the edge of Cardigan Bay, the Mochras Borehole proved the thickest Lower Jurassic sequence (1305 m) in the British Isles; all four stages are present (Figure 60). The sequence is dominated by massive calcareous mudstone and siltstone, which are extensively bioturbated. There is little evidence of coarse clastic debris, slump structures or other indications of a contemporaneous shoreline or fault scarp despite the proximity of the Cambrian sequence on the flank of the Harlech Dome. However, such a thickness clearly demonstrates that the extensional basin development initiated during the Permo–Triassic, or earlier, continued into the Jurassic. The ‘Lower Lias’ is 896 m thick and it is possible that the lowest 150 m of dark grey mudstone in which there are some thin, interbedded limestones and a suggestion of possible rhythms may be the equivalent of the Blue Lias. Offshore seismic reflection profiles suggest that the thickness of ‘Lower Lias’ at Mochras is maintained along the axes of the St George’s Channel and Cardigan Bay basins.

The ‘Middle Lias’ in the Mochras Borehole comprises some 147 m of interbedded siltstone and mudstone, with proportionally more siltstone than the ‘Lower Lias’, few thin limestones, a sandstone and a conglomerate layer; bituminous mudstones are present locally. The beds contain ammonites, belemnites, crinoid debris and plant fragments and are bioturbated in places. A similar thickness, with some thin sandstones at the base, occurs in St George’s Channel Basin. In the South Celtic Sea Basin, the lowermost grey calcareous mudstone passes into red-brown mudstone in places, with some thin sandstone interbeds and a variable silt and glauconite content; two thin coals have been recorded. The change reflects the passage from shallow marine to subaerial conditions.

In the Mochras Borehole, the ‘Upper Lias’ (Toarcian) is 262 m thick, and comprises grey mudstone, with sparse siltstone and limestone beds and calcareous ironstone nodules, overlain mainly by micaceous mudstone with silty interbeds and locally abundant plant debris. To the south-west, in St George’s Channel Basin, a well penetrated 291 m of grey calcareous, slightly micaceous mudstone of an equivalent Toarcian age.

Middle Jurassic

The Middle Jurassic strata, which crop out extensively in England from Yorkshire to the Dorset coast, are shallow water deposits that display complex facies changes and erosion surfaces. It is reasonable to assume that similar strata encroached onto the Welsh landmass (Figure 61) that had emerged from the Jurassic sea by up-warping and a fall in sea level in late Toarcian times, but all evidence has been removed by subsequent erosion. However, Middle Jurassic strata have been preserved in the deeper parts of the Bristol Channel, St George's Channel and Cardigan Bay basins, and these sequences (up to 1000 m) are markedly thicker than those onshore. In the central parts of these basins, sedimentation was probably continuous, but on the basin margins there is evidence of uplift and erosion. Over the St Tudwal’s Arch (Figure 61) an estimated 650 m of Lias Group strata were probably removed, and north of the Pembroke Ridge the Upper Lias sequence is missing. In the Cardigan Bay and St George’s Channel basins, Middle Jurassic strata form the cores of the synclines that plunge south-westwards beneath the Cainozoic cover.

Elements of the onshore stratigraphy have been recognised in the Welsh offshore basins. In St George’s Channel Basin, two units equivalent to the Lower and Middle Inferior Oolite Upper Jurassic and the Upper Inferior Oolite have been distinguished. The lower unit consists of pale grey, fine-grained sandstone with interbedded soft grey mudstone; the upper unit consists of grey calcareous siltstone with fewer sandstone beds. The lithofacies reflect a fluviodeltaic setting. Farther south, in the South Celtic Sea Basin the sequence is thinner and consists predominantly of glauconitic mudstone indicating marine conditions. In the Bristol Channel Basin, the groups are difficult to distinguish in the uniform calcareous marine mudstone sequence.

The Great Oolite Group, largely Bathonian in age, is up to 800 m thick and has a similar outcrop in the offshore basins. All the stratigraphical subdivisions (Fuller’s Earth, Great Oolite, Forest Marble and Cornbrash) of the onshore sequences have been distinguished, but all are thicker. In St George’s Channel Basin, deposition appears to have kept pace with rapid subsidence; the lithologies are broadly comparable with the onshore sequences. The distinctive geophysical signature of the pale nodular limestones and calcareous mudstones of the Cornbrash, which span the Bathonian–Callovian boundary, has been recognised in both the St George’s Channel and Bristol Channel basins. In the latter, the Cornbrash rests unconformably on lower to middle Bathonian beds of the Great Oolite Group. At the top of the Middle Jurassic, the onshore sequence of the Kellaways Formation and Lower and Middle Oxford Clay (now referred to the Peterborough and Stewartby members, respectively) has been determined in the offshore basins. In St George’s Channel Basin, the Kellaways Formation comprises grey-green silty mudstone that passes up into fine- to coarse-grained calcareous sandstone. The geophysical signature seen in the Kellaways Formation is repeated in the overlying Lower and Middle Oxford Clay, reflecting an increase in sandstone towards the top of each depositional cycle. The signature is similar to that which occurs in places in the onshore outcrop and is thought to reflect shallower marginal marine conditions.

In the western Bristol Channel Basin, the Lower and Middle Oxford Clay are mainly olive-grey mudstone without the distinctive geophysical signature seen farther north. However, to the east, the sequence is lithologically similar to the onshore sequence in southern England.

Upper Jurassic

The systematic cycles of basin subsidence and infill continued into the beginning of Late Jurassic times. The thick sequence in St George’s Channel indicates that subsidence kept pace with sedimentation. South of the Pembrokeshire Ridge, only the lowest part of the succession is preserved and, in the Bristol Channel, a marginal marine to deltaic sequence suggests uplift and erosion in the south.

The base of the Upper Jurassic, which coincides with the base of the Upper Oxford Clay (now the Weymouth Member onshore), can be recognised in the geophysical logs of onshore boreholes, and has been correlated offshore. The broad stratigraphical elements of the onshore sequence can be distinguished in the offshore basins although there are significant lithological variations. In St George’s Channel Basin, the Upper Oxford Clay Member, and the West Walton and Ampthill Clay formations have been determined in a sequence of blue-grey silty mudstone and soft grey-brown calcareous mudstone interbedded with loose- to well-cemented sandstones, with some upward-fining cycles. The sandstones are interpreted as fluviodeltaic, coastal plain sand bodies, and a few anhydrite beds indicate periodic desiccation.

In the Bristol Channel Basin, equivalent grey-green, fine-grained sandstones, with common glauconite and lignite fragments and a dwarfed molluscan fauna suggest transitional estuarine or deltaic conditions. The Kimmeridge Clay Formation has been determined in boreholes in both the St George’s Channel and the Bristol Channel basins. In the St George’s Channel Basin, the formation, which rests conformably on the Ampthill Clay Formation, comprises mudstone with traces of glauconite and lignite, interbedded with pale grey limestone and sandstone with a few anhydrite layers. The sequence, up to 958 m thick, reflects considerable syndepositional subsidence. The lower and upper subdivisions of the onshore succession are not clearly defined on the geophysical profiles, and there are significant differences. There is no Cretaceous

Lower Cretaceous indication of the presence of oil shales, and although the sediments suggest marine deposition the microfauna includes some freshwater forms of a lagoonal or fluvial overbank setting. The Kimmeridge Clay Formation forms the core of the Bristol Channel Syncline, and the dark grey mudstone and siltstone with lignite fragments are comparable with the onshore organic-rich shales.

Portlandian strata of the uppermost Jurassic have been determined beneath the Cainozoic cover in the centre of St George’s Channel Basin and at the western edge of the Bristol Channel Basin. In the former, the strata consist of yellow and red-brown, variably calcareous mudstone with few interbedded, coarse-grained loosely cemented sandstones. One borehole in the Bristol Channel Basin proved that the sequence lies directly on the Oxford Clay Formation, although the contact is probably faulted. The beds have been divided into the Portland Group, yellowish grey calcareous mudstone with pale to dark-grey limestone at the top, overlain by the Purbeck Group. The facies associations are characteristic of freshwater conditions. The Jurassic–Cretaceous boundary is currently taken very near the base of the Purbeck Group in onshore sequences. Jurassic strata have been recognised in the small Berw Basin on the north-east side of Anglesey.

Cretaceous

During the Cretaceous Period, over some 77 million years, there were marked changes in the sedimentary patterns that were due to a combination of eustatic sea-level changes and regional tectonic activity. There is evidence of Early Cretaceous uplift in the St George’s Channel Basin, but the most profound change occurred across the Lower–Upper Cretaceous boundary at about 99 Ma. Prior to this time, tensional activity caused rifting and block faulting, which was most graphically expressed in the Rockall Trough, off north-west Scotland, and in the North Sea. Subsequently, this activity ceased and deposition of a widespread blanket of coccolith ooze (chalk) was initiated.

At the present time there are no Cretaceous rocks on mainland Wales, but they are preserved south of the Variscan Front in the North Celtic Sea, South Celtic Sea and Bristol Channel basins (Figure 62); for the most part these basins were those that controlled Jurassic sedimentation. North of the Variscan Front, it is possible that Lower Cretaceous rocks are preserved in the axial part of St George’s Channel Basin, but as yet they have not been proved. A marked fall in sea level in early Cretaceous times exposed much of the British Isles to terrestrial deposition. At the same time, rifting continued to the south and south-west of Wales, and the supply of coarse clastic sediments into the basins was maintained. However from mid Cretaceous times, extensional stress waned, the area subsided and the sequence became progressively marine, overstepping the basin margins.

The thickest sequence, about 3300 m, preserved in the North Celtic Sea basin, spans the complete zonal range of both the Lower and Upper Cretaceous. Less complete sequences occur in the South Celtic Sea and Bristol Channel basins. Four main facies associations have been determined.

Lower Cretaceous

In the North Celtic Sea Basin, the freshwater lagoonal Purbeck facies comprises grey calcareous mudstone with sandstone and thin limestone beds. In the Bristol Channel Basin, silty mudstone and limestone with some gypsum beds have been recorded. The sequence is similar to that in southern England. The Cinder Bed has been identified on logs; this is a marker horizon rich in oyster debris that was formerly taken as the base of the Cretaceous.

The Wealden facies in both the North Celtic Sea and South Celtic Sea basins comprises mudstone, siltstone, silty and muddy sandstones with thicker quartzose sandstone beds, coals and lignites in places. In the South Celtic Sea Basin, the sequence unconformably Middle Jurassic strata. It is less calcareous and less fossiliferous than the Purbeck facies and, unlike the latter, sphaerosiderite is common. The main source of clastic debris was Wales and south-west England, and the sequence accumulated in brackish and fresh water fluvial and alluvial systems; the thicker sandstones are probably distributary sand bodies.

In the Bristol Channel Basin, Wealden strata have been determined in many boreholes along the axis of an east–west-trending syncline that formed during early Cretaceous times. The mottled red-brown and grey mudstones and siltstones are characteristically noncalcareous and, in the eastern part of the basin, interbedded sandstones and mudstones have been interpreted as alluvial deposits. Detailed sedimentological analysis indicates deposition in a floodplain environment with high and fluctuating water tables, and high-energy distributary channels. Wealden Clay mineral assemblages are dominated by kaolinite, chlorite and mica, but at the Purbeck–Wealden transition kaolinite is absent, attributed to changes in the weathering processes in the hinterland. Smectite is present, derived, in part at least, from the alteration of volcanic ash.

During late Lower Cretaceous times, local subsidence was greatest at the south-west end of the North Celtic Sea Basin with the deposition of up to 140 m of Gault and Greensand facies, of Albian age. No equivalent sequence has been distinguished closer to the Welsh coastline. In the eastern part of the South Celtic Sea Basin, shallow-water clastic sedimentary rocks and limestones are exposed at the sea bed, and many of the boreholes proved calcareous glauconitic siltstone and mudstone with several prominent limestones. In the Bristol Channel Basin, thin Gault Clay was determined in one borehole.

Upper Cretaceous

Early in the Late Cretaceous the coincidence of regional subsidence and high sea level led to the widespread deposition of chalk, a limestone composed mainly of the microscopic remains of planktonic algae (coccoliths). By latest Cretaceous times this sea extended far beyond previous limits and covered most of England and Wales with the possible exception of a small island about west and north Wales. Subsidence was more regional than during Early Cretaceous times and thick sequences continued to accumulate within the pre-existing basins rather than on the intervening highs, but this was probably more to do with differential compaction than tectonic activity.

In the South Celtic Sea and Bristol Channel basins, the lowest part of the Upper Cretaceous (Cenomanian) comprises calcareous, glauconitic sandstone, which grades up from the underlying Lower Cretaceous (Albian), and is similar to the gradation observed in many areas of southern England. The sandstone is overlain by glauconitic limestone or calcareous mudstone (Glauconitic Marl), which in turn is overlain by similar mudstone with intercalated siltstone (Chalk Marl†). Chalk sedimentation was diachronous across the region from mid-Cenomanian times. On basin margins deposition of glauconitic sediment continued into the Coniacian. The Chalk is white and hard with minor glauconite and quartz sand, and the influx of planktonic foraminifera corroborates deeper water conditions.

The lowermost unit of the Middle Chalk consists mainly of hard, nodular chalk with interbedded calcareous mudstone. It is closely comparable with the equivalent Melbourne Rock onshore, and has been distinguished in all the offshore basins south of the Variscan Front. Higher in the sequence, the lithologies revert to softer, more argillaceous chalk, which again bears similarities to the onshore equivalent. The geophysical reflector that elsewhere marks the base of the Upper Chalk has not been recognised and this unit is less clearly defined than the lower sequences. The base has been palaeontologically determined in the South Celtic Sea and Bristol Channel basins. Above the base, the chalk becomes progressively softer, although interbedded hardground and flint layers are picked out on the geophysical logs. The well-defined foraminiferal faunal changes across the stage boundaries are coincident with slight changes in chalk lithology.

† The revised stratigraphical nomenclature that has been applied to the Cretaceous sequence in the south of England has not yet been applied to the offshore succession.

Oil and gas

Since the early 1970s, the Mesozoic sequence in the Welsh offshore basins has been the target for hydrocarbon exploration but, as yet, no significant discoveries have been made. The generation and entrapment of hydrocarbons depends on the sedimentation of an organic-rich source rock, the presence of a reservoir in which the hydrocarbons can accumulate, a suitable seal, and a series of tectonic events that allow the hydrocarbons to be generated after burial and then to migrate into the reservoir.

In the North Celtic Sea Basin, in Irish waters, a gasfield located in Albian–Aptian sandstones at Kinsale Head has been in production since 1979. In the South Celtic Sea and Bristol Channel basins, the equivalent sandstones are only thinly developed and the wells that have been drilled have all proved to be dry. The paucity of organic-rich source rocks in both basins is a major problem. Jurassic, organic rich claystones, which are major source rocks in the North Sea and Irish Sea basins, are not widely developed. Silesian strata with vitrinite organic debris have elsewhere proved a rich source for gas and some oil but, without a suitable cap rock, it is likely that the gas would have escaped during Variscan deformation. The low-grade pervasive metamorphism of the Devonian–Carboniferous strata was proved in a few offshore deep wells indicating that there is little possibility of a hydrocarbon source. Additionally, the tectonic history of both the South Celtic Sea and Bristol Channel basins makes successful hydrocarbon exploration unlikely.

North of the Variscan front, the 3000 m of Upper Palaeozoic strata and 6000 m of Mesozoic and Cainozoic sedimentary rocks in St George’s Channel Basin would seem to be a more likely target for hydrocarbon exploration. Drilling objectives have included sandstones in the Upper Lias (Bridport Sands equivalent) and Middle Jurassic, with Cretaceous green-sands, equivalent to those in the Kinsale Head Gasfield, as secondary objectives. Until now the only gas and oil shows have been poor. However, it is possible that further exploration in St George’s Channel Basin may prove the Sherwood Sandstone to be a prospective reservoir and the overlying Mercia Mudstone Formation halites to be an effective cap rock. The Dragon prospect, off Pembrokeshire, is a Jurassic reservoir, which has proved some oil shows. The potential of Upper Palaeozoic source rocks is likely to be better than in the basins to the south of the Variscan front as the rocks are not metamorphosed. In the East Irish Sea Basin, to the north of Wales, a widespread phase of oil and gas accumulated during late Mesozoic times. Oil-prone source rocks include Dinantian limestone, Namurian shale and Westphalian oil shale, cannel coal and marine bands. Gas-prone source rocks comprise Namurian shale and Westphalian coal and mudstone.

Chapter 9 Cainozoic

Towards the end of Cretaceous times, Europe began to be affected by compressional forces as the ocean (Tethys) that had separated Europe from Africa for most of Mesozoic times began to close and the Alpine orogeny was initiated. The culmination occurred in early Eocene to possibly Miocene times when the Alps and Carpathian mountain chains formed at the southern edge of the European craton. Broadly contemporaneous with these events was the opening of the north-east Atlantic with intense igneous activity in north-west Britain and, in the North Sea, extensive subsidence and sedimentation. However, within the craton, between these major tectonic expressions, the Alpine effects in Wales were mainly in the rejuvenation of basement structures and broad crustal warping.

At the end of the Cretaceous, an episode of mass extinction saw the demise of many marine faunas and, more famously, the dinosaurs. The cause has been attributed to a major meteorite impact followed by an intense and prolonged period of climatic change. However, there is little evidence of this catastrophic event in the geological column about Wales where, as in most basinal areas, sedimentation across the boundary was uninterrupted.

Palaeogene–NeogenePalaeogene–Neogene replaces Tertiary, which is now an obsolete term.

Sea level rose progressively throughout most of Cretaceous time, although in the final stages, Campanian to Maastrichtian, there was a reversal of this trend and most of the basins close to the Welsh coast were inverted and, by early Palaeogene times, were subaerially exposed. The uplift of the basins led to erosion and, for the most part, succeeding deposition was mainly terrestrial. During this period, the climate was tropical or subtropical and it was only later, in Neogene times, that there was a gradual reduction in temperature. Palaeogene sediments have been determined in the St George’s Channel, South Celtic Sea and North Celtic Sea basins. Smaller outliers are preserved in half-grabens in the Cardigan Bay and Bristol Channel basins. The only Neogene sediments recorded are the Lower Miocene sediments in the Mochras Borehole.

The morphology of the main Cainozoic basins was controlled by pre-existing structures; the north-easterly trend of the St George’s Channel Basin is Caledonian, and the more easterly trend of the South Celtic Sea and the Bristol Channel basins is Variscan. In the St George’s Channel Basin, the thickest Cainozoic accumulations coincide with those of the Mesozoic, whereas in the east–west-trending basins they are offset laterally. North of the Variscan Front, the absence of the upper part of the Cretaceous sequence in the St George’s Channel and Cardigan Bay basins may be due to non-deposition or, more probably, uplift and erosion, which would also account for the unconformity between the Jurassic and Cainozoic sequences. However, in St George’s Channel Basin, the unconformity has been ascribed to basin inversion with movement along St George’s and associated faults. To the south of the Variscan Front, the South Celtic Sea Basin was also inverted, although uplift was generally more regional.

The sedimentary character of the Cainozoic sequence is best known from the Mochras Borehole, sited close to the faulted margin of the Cardigan Bay Basin, where 524 m of Middle Oligocene to Lower Miocene strata were proved. The eastward-dipping sequence is contained in a half-graben (Figure 63), and rests unconformably on Lower Jurassic strata. The sequence has been divided into three lithostratigraphical units. The Basal Red Unit comprises coarse-grained, cobble conglomerates, up to 14 m thick, with interbedded fining-upward sand to clay sequences, 1 to 2 m thick. The conglomerates are intepreted as alluvial debris is marked by a gradual change in colour from red to grey, and is characterised by upward-fining cycles of silt and clay with a carbonaceous and lignite component. The facies associations are characterisitic of a floodplain environment. The contact with the overlying, uppermost Lignite and Clay Unit is marked by a gradual increase in the proportion of lignite. The olive-green sediments form sharp-based, upward-fining sand to clay sequences laid down in brackish water or on well drained, vegetated surfaces. In many places, internal structures are obliterated by bioturbation, organic debris is scattered throughout the upper parts of the cycles and spherulitic siderite is common. The sequence suggests that swamp conditions, with possible seasonal variations, were established at the height of the water table.

Lithologically similar sequences were intersected by a borehole at Tonfannau, sited on marine alluvium between the Mawddach and Dyfi estuaries, and in two offshore boreholes close to the northern edge of Cardigan Bay. Farther offshore, in St George’s Channel Basin (Figure 63), the thickest Cainozoic sequence is intersected by the St George’s Fault along part of which a salt wall has been emplaced. North-west of the fault, in a faulted syncline, over 1500 m of Palaeogene (Eocene to Oligocene) strata lie unconformably on Cretaceous strata, and four boreholes have shown that the overall characteristics of the sediments are closely comparable with the Mochras sequence. A similar environment is envisaged — a floodplain with fluvial channel sands and overbank deposits. Between St George’s Channel Basin and the Welsh mainland, a small outlier of Eocene to mid-Oligocene sediments has been preserved in the Teifi basin, where 10 m of dark green and pale brown laminated clay, with plant fragments and glauconite, indicate possibly the most northerly marine incursion about Wales. The general disposition of Palaeogene facies across Cardigan Bay reflects a widely meandering river encroaching a swamp-dominated flood plain (Figure 64).

In the South Celtic Sea Basin, over 500 m of Palaeogene sediments have been dated as mid-Eocene to Oligocene on the basis of a sparse microfloral assemblage. One borehole penetrated finely laminated, green, micaceous, glauconitic and lignitic sandstone with interbeds of silty clay and dark brown to black lignite. Lignite is more abundant near the base of the sequence and is thought to reflect deposition in a low-energy marginal marine environment. The upward increase in sandy glauconitic sediments suggests an increasing marine influence, although the persistent lignite content suggests close proximity to a vegetated landmass.

During Cainozoic times, the Bristol Channel Basin was no longer extant, but farther south, to the west of Lundy Island, a new basin, the Bristol Channel Marginal Basin, developed. This basin has a downfaulted northern edge, and 300 m of Cainozoic strata are preserved within it. Seismic records indicate that a lower sequence, possibly sandstone, is overlain by clay and lignite, which have been recovered from the sea floor. East of Lundy Island, 335 m of braided channel and flood plain deposits are preserved in the Stanley Bank Basin on the east side of an extension of the Sticklepath–Lustleigh Fault Zone. Lundy Island is the site of an early Eocene igneous complex — a coarse- to fine-grained granite is itself intruded by numerous south-east-trending dykes of olivine and quartz dolerite, trachyte and trachyandesite. Dolerite dykes of Eocene age have been distinguished in a swarm, offshore, northeast of Anglesey, and many have been determined across Anglesey, Llŷn and Arfon with some extending into Snowdonia and beyond. These Paleocene–early Eocene igneous rocks lie at the edge of a large igneous province (Thulean), which straddled the eastern edge of Greenland, Iceland and extended to the Hebrides and north-east Ireland. The dykes in north Wales are the fringe to a volcanic centre at Mourne in Northern Ireland, although the currently exposed dykes imply a significant cover at the time of their intrusion and, consequently, considerable erosion since Eocene times.

The extensive offshore record of Palaeogene–Neogene sedimentation contrasts with the patchy evidence onshore. Evidence of Palaeogene sediments on mainland Wales is restricted to deeply weathered regoliths, such as the ‘pocket deposits’ on the Carboniferous limestone outcrops between Llandudno and Mold in north Wales. The deposits include both alluvial sediments, introduced by subsidence into solution cavities, stratified sand and clay (products of subtropical, subaerial weathering, with gibbsite and kaolinite) and interbedded lignite, introduced by underground streams. Similar mottled clays, some 15 m thick, occur within limestone cavities at Flimston, near Castlemartin, in south Pembrokeshire, and others are exposed in the quarry at Trefil north of Merthyr Tydfil (Plate 48). Elsewhere in south Wales, in situ weathered profiles in Namurian sandstone along the north crop and in Old Red Sandstone orthoquartzite in the Black Mountains have also been regarded as Cainozoic in age.

The patchy onshore Palaeogene–Neogene record makes it difficult to determine the geological evolution of this period in Wales. The task is further complicated by the variety of processes — continental and regional tectonism with concomitant movement of magma, climate changes and sea level fluctuations — that were particularly active throughout the period. The most clearly defined morphological feature in Wales is the ancient Lower Palaeozoic Caledonian massif, which developed from uplift that culminated in Mid Devonian times. Later, uplift was reactivated through Carboniferous times, and then the massif was enlarged by accretion in the late stages of the Variscan orogeny. However, interpretation of subsequent Mesozoic events and the final emergence in Cainozoic times is inhibited by the extremely restricted outcrops. In early Mesozoic times, the Palaeozoic landmass was probably reduced to a relatively low peneplain on which Triassic and Lias Group sediments were probably deposited and subsequently eroded. Later, with the exception of a restricted area in north-west Wales, the area was submerged beneath late Cretaceous seas. However, no Cretaceous rocks have been found on land, in solution cavities in Carboniferous limestone or in the Cainozoic of the Mochras Borehole. It is possible that the massif was uplifted prior to the transgression and, consequently, the preglacial evolution of Wales was entirely controlled by Cainozoic tectonic events and sea-level changes.

The relationship of the Welsh massif to the Mesozoic — Cainozoic offshore basins has long been a controversial subject, but the debate was intensified during the sinking of the Mochras Borehole and its completion in 1972. The juxtaposition of the thick Mesozoic and Palaeogene sequences in the borehole with Cambrian rocks that are exposed on the flank of the Harlech Dome just 2.5 km to the east, emphasises the influence of the intervening contemporaneous, coast-aligned Llanbedr Fault on sedimentation. This fault has an estimated downthrow of not less than 3750 m on the postulated base of the Triassic, and not less than 1350 m on the base of the Palaeogene.

The contrast in the preserved Mesozoic–Cainozoic sequences in the offshore basins to the north and south of the Variscan Front reflects a marked contrast in deformation. To the north, in St George’s Channel and Cardigan Bay basins, no Cretaceous strata are preserved and the Palaeogene sequence, of mainly Eocene to Oligocene terrestrial rocks, rests on Upper Jurassic rocks. To the south of the Variscan Front, in the Celtic Sea Basin, variable Lower Cretaceous and thick Chalk sequences overlie Jurassic strata, and in the main Bristol Channel Basin the youngest preserved sediments are usually of late Jurassic–early Cretaceous age, although terrestrial Oligocene sediments are preserved in faulted sub-basins. For much of the Mesozoic and Cainozoic, the Welsh massif was not a significant feature and there is evidence of Neogene uplift offshore.

The geomorphological detail of events in Neogene times across the Welsh massif is difficult to determine, but the landforms suggest they were largely erosional. Most probably the evidence of any significant deposits would have been removed by the subsequent glaciation; the numerous quartz pebbles on the Carboniferous limestone surface across Castlemartin peninsula (Plate 49), in south Pembrokeshire, continue to be quoted as being of possible Pliocene age. It was only in late Miocene–early Pliocene times that sea encroached the landmass, possibly along synclinal sags in the Celtic Sea and Bristol Channel basins. Simultaneous uplift of the land surface was probably dome-like and sporadic and, consequently, a stepped profile developed.

Subsequent glacial erosion has obscured many of the geomorphological features that were sculpted through Cainozoic times, and there has been much conflicting interpretation of the features. For example, a possible concordance between north and south Wales in summit heights below 600 m, has been attributed to eustatic uplift, in stepped pulses, and a falling sea level. However, O T Jones saw an inclination from north Wales southwards, which he attributed to subaerial (Triassic) erosion and Cainozoic tilting; such extrapolation is more difficult with the possible residual surfaces over 335 m (1100 feet). Later E H Brown recognised four major 'peneplains', each ranging through 100 m or more, and ascribed them to subaerial erosion and eustatic uplift. When afforded with extended views from the numerous vantage points about Wales the existence of such ‘levels’ or ‘surfaces’, in general terms, is most persuasive but their detail is much more difficult to record. In the Brown analysis, the ‘Low Peneplain’, 215 to 335 m above OD, and probably late Pliocene, is most clearly distinguished on the sides of the Tywi and Teifi valleys and in the inner reaches of the Monmouthshire valleys; in the mountains of north-west Wales it cannot be distinguished. The ‘Middle Peneplain’, 460 to 490 m above OD, and probably an intermediate Neogene stage, is most clearly developed in the scarp and dip slopes of Mynydd Eppynt and the hills to the south-west. However, it is possible that these features, and similar notched features at an equivalent level in the Old Red Sandstone of the Brecon Beacons and in the Pennant Sandstone in the coalfield, may be due to the underlying geology. The ‘High Plateau’, 520 to 580 m above OD, possibly of mid-Cainozoic uplift, was identified mainly by its height rather than its topography as it is totally fragmented and widely distributed between the escarpments in the Brecon Beacons and Black Mountains, the complex features between Plynlimon and Drygarn, and about Llyn Vyrnwy. The uppermost, ‘summit Plain’, 615 to 1100 m above OD, and possibly the sub-Mesozoic surface exhumed, was clearly confined to the summits but their isolation makes their interconnection difficult; just to link the Snowdonia summits in such a way would necessitate a domed warping with an amplitude of some 300 m in a distance of 8 km. To reconstruct a wider surface, between the Snowdonia summits and Pen y Fan (886 m OD) in south Wales, with Arenig Fawr (855 m OD), Cadair Idris (890 m OD) and Radnor Forest (660 m OD), in between, is much more difficult.

The ‘coastal plateau’ is probably the best developed erosion surface (Plate 49), although it is a composite feature in which erosion on the lower platforms excavate and locally obliterate the higher, and older, platforms. The recognition of outliers of higher platforms above lower platforms reflect pulses of emergence, and support the broad subdivision into the ‘200 feet’, ‘400 feet’ and ‘600 feet’ platforms in Gower, Pembrokeshire, Anglesey and Llyn. However, there may be a vertical range up to 60 m due to variations in the rate of uplift. The ‘600 feet’ platform has been recognised around much of the Welsh coast, apart from in the vicinity of the Llanbedr Fault in Merionethshire where, as on Llŷn, Anglesey and Arfon, it has been removed by erosion of the lower platforms. Its apparent uniformity is more easily reconciled by eustatic uplift and a beach rather than a subaerial peneplain. The ‘400 feet’ and the ‘200 feet’ platforms are more clearly defined, but again they show a wide variation in altitude. For example, the ‘200 feet’ platform has been ascribed to features at 50 m above OD in Gower and Pembrokeshire and at 30 m above OD in the Vale of Glamorgan. The only evidence of a possible Neogene or later age is the truncation of the pipe clays (Oligocene) by the ‘200 feet’ platform at Flimston, south Pembrokeshire.

The controlling influence on the drainage patterns across Wales has long been a major topic of geomorphological discussion, and even now that there is little evidence to suggest that the patterns were superimposed from a Chalk cover, there is still much disagreement. Clearly, through the Cainozoic, the intermittent development of the plateau surfaces was a significant influence. The knick points in the long profiles of most river valleys indicate repeated phases of uplift. Of particular interest are the south Wales cave systems in the Dinantian limestones along the north crop. Entries into the Agen Allwedd and Craig y Ffynnon systems, between Mynydd Llangynidr and Llangattwg, lie some 200 m above the current level of the River Usk and this possibly indicates the minimum uplift since Neogene times. In addition, there is much evidence of river capture as the early radial drainage patterns, of the high plateau stage, diminishes and the stream network becomes increasingly constrained by the geology. The drainage patterns across the south Wales coalfield, from the edge of the Old Red Sandstone escarpment, show no evidence of having been influenced by the Variscan fold and fault patterns. Similarly, the drainage of the Lower Palaeozoic terrain in the Tywi, Wye and Severn river systems provides equally striking disregard of the Caledonian structures. In north Wales, the distribution of the drainage patterns about the major structures are broadly radial. The possibility of the removal of most or all of the Mesozoic cover prior to the Neogene would necessitate the development of a mosaic of separate drainage centres about the summits of Snowdonia, the Harlech Dome, Cadair Idris and the Arans, Plynlimon and Drygarn and the Old Red Sandstone escarpment. The classic asymmetry of the drainage, with long gentle profiles of rivers such as the Severn and Wye to the east and the short steep profiles of the westward-flowing rivers (e.g. the Rheidol and Ystwyth), was enhanced by contemporaneous movement along the Llanbedr Fault.

Quaternary

The progressive climatic deterioration across Wales throughout Palaeogene and Neogene times reached its nadir when, as a result of dramatic cooling, ice that accumulated in upland areas and at high latitudes spread southwards from Scandinavia across much of northern Europe. Sediments derived from these ice sheets are attributed to the Pleistocene Epoch and the overlying, ‘Recent’, deposits to the Holocene. Historically these two epochs have formed the Quaternary with its beginning taken at 1.8 million years before present. However at the time of writing, it has been proposed that the beginning of the Quaternary should be taken at 2.6 Ma which was the onset of the fluctuating episodes of cold and warm climate. In northern Europe, the ice movement was complex with alternating periods of temperate, cool, cold and periglacial conditions, and it was not until the Anglian stage (some 0.48 Ma) that there was direct evidence of glaciation in Wales. Although there is indirect evidence of earlier ice ages in both late Ordovician and Carboniferous times, the Quaternary is the first time when ice sheets enveloped Wales and the adjacent sea. This is the period when Homo sapiens evolved and in which we now live.

Pleistocene

Throughout upland Wales, the evidence of glaciation is overwhelming in the cwms, U-shaped valleys, moraines, roches moutonnées and other erosional features, which are entirely the signature of the last, Devensian glaciation that obliterated the evidence of earlier ice cover (Table 6). Concurrent with the development of local Welsh ice, the larger Irish Sea ice sheet encroached Wales from the north and extended southwards across Anglesey and Llŷn into Cardigan Bay, and eastwards passing through the Cheshire plain. Farther south, the ice sheet transgressed from Cardiganshire into Pembrokeshire to impinge on Gower and south Glamorgan (Figure 65). The contact between the Welsh ice and Irish Sea ice was complex, and the unravelling of the different and commonly ambiguous deposits has been the subject of considerable controversy. The southern limit of the Devensian Welsh ice lay close to the south coast, and beyond the ice margin, in south Glamorgan, Gower and south-west Pembrokeshire, there are remnant deposits of an earlier, pre-Ipswichian glaciation.

Until the recent extensive exploration of the offshore areas, the above outline formed the basis of all investigations into the complexity of the surfaces and stratigraphy of the Pleistocene in Wales. Now, the offshore exploration has provided a database that has completely revised the approach to an understanding of these features. The most significant contribution has been the detail of the pre-Devensian events with a stratigraphy from the pre-Anglian, near the base of the Middle Pleistocene, through the Devensian (Figure 66).

The earliest evidence of the Pleistocene on mainland Wales has long been considered to be the deposits of the ‘Older Drift’. These deposits are predominantly tills of the Irish Sea ice and they occur mainly in those areas outside the limit of the Late Devensian (‘Newer Drift’) glaciation. In south Wales, Irish Sea till with shells and clasts of Pembrokeshire igneous rocks have been determined about West Angle Bay in south Pembrokeshire, in the Gower and as far east as Pencoed in the Vale of Glamorgan. In the Gower, these early deposits were followed by the construction of the Paviland moraine, which includes numerous boulders of Namurian sandstones derived from the north crop of the coalfield. The moraine is the oldest (possibly Anglian) remnant of north–south Welsh ice movement. However, the most comprehensive evidence of the earlier Pleistocene events lies within the cave deposits, such as those at Minchin Hole and Bacon Hole in the Gower. Sections through these deposits have shown an early inner beach deposit unconformably overlain by the Patella and Neritoides beaches, of interglacial, Ipswichian age, which are probably equivalent of the nearby Horton and Langland and Broughton raised beaches. The Gower cave deposits have yielded the most complete faunal record for the Ipswichian and early Devensian in Wales; a wide range of mammalian fossils include bison, giant ox and hyena, and fossil remains of birds include curlew, dunlin and shearwater. In addition, worked ivory fragments and polished bone fragments indicate occupation by Palaeolithic man. In north Wales, similar mammalian faunas including hippopotamus bones, dated to the mid-Ipswichian, some 125 000 years ago, have been found in the Cefn and Pontnewydd caves above the Elwy valley. In Pontnewydd Cave, a human molar has been determined, and numerous artefacts, including handaxes, suggest that the cave was used as a butchering site by Palaeolithic man, about 30 000 years ago. The possible Ipswichian age for the raised beach deposits on wave-cut platforms as at Poppit Sands in Pembrokeshire, Red Wharf Bay on Anglesey, and Porth Oer on Llŷn, has been used to constrain the overlying Irish Sea till to the Devensian Stage.

As with the earlier stages, evidence of the Early and Mid Devensian stages is mainly restricted to the coastal sections and the cave deposits outside the limit of the Late Devensian ice sheet. In the Gower the stages are represented by cemented breccias in Bacon Hole Cave, and in north Wales, on Llŷn, possibly by the lowest of two tills in a cliff section at Glanllynnau (Figure 67). In contrast, the signature of the later Devensian ice sheet dominates the Welsh landscape. Ice movement was controlled by topography, and particularly by the major valleys that existed at the end of the Neogene Period. In north Wales, the pattern was essentially radial, eastwards into the low ground of Cheshire–Shropshire, northwards into the Irish Sea and westwards into Cardigan Bay. The pattern suggests that the thickest ice lay between the Arenig Mountains and Harlech Dome but, subsequently, subsidiary loci, with high cwm glaciations, were established in the main massifs in Snowdonia, the Harlech Dome, the Arans and Cadair Idris. South of the Dyfi valley the pattern is slightly more obscure, possibly as a consequence of the smoothness and lower elevation of the late Neogene surface. However, a dominant influence was the ice cap across the Plynlimon–Drygarn range, which supplied ice westward, through the Dyfi, Rheidol, Ystwyth, Aeron and Teifi valleys into the Irish Sea Ice Sheet in Cardigan Bay; eastwards the main outlet was into the Severn valley and, via Rhayader and Builth, into the Wye valley. The complex pattern of erosional and depositional features associated with the Tywi, Usk and Wye glaciers is the result of convergent patterns, with common capture and, locally, flow reversal from the temporary confrontation between large and small bodies of ice. The long, southerly directed, dip-slope feature of the Fans and Brecon Beacons fed ice across the uplands and valleys of the coalfield, although a small ice cap across Craig y Llyn fed ice into the Neath and Cynon valleys on its north side. All the deposits of the Welsh ice were locally derived.

The erosional effects of the glaciation can be seen clearly in the highest ground, and most spectacularly in Snowdonia. Here, the radial growth of cwms, Glaslyn, Glas, Du’r Arddu, and Clogwyn, about the Snowdon summit leaves little of the earlier smooth profile, which is preserved in places across both the Glyders and Carneddau. Similarly, the perfectly proportioned Cwm Cau on the south side of Cadair Idris was excavated into a generally smooth surface at the top of the massif. In mid-Wales, cwms such as Cwm Rheidol form the heads of many of the valleys and, in south Wales, the scarps of both the Old Red Sandstone and the Pennant Sandstone are littered with cwms, many of which, as elsewhere, are occupied by moraine damned lakes; Llyn y Fan Fawr lies beneath the scarp of Mynydd Ddu at the head of the Tawe valley, and Llyn Fawr lies beneath the Pennant Sandstone scarp of Craig y Llyn. Most of the high cwms contain well featured moraines, and those in Cwm Idwal were influential in convincing Charles Darwin, in the early 19th century, that glaciation was the last major geological process to have moulded the Welsh landscape. In south-east Wales, the large arcuate moraines at Llanfihangel Crucorney, north of Abergavenny, and at Glais, in the Tawe valley, mark the maximum extent of the late Devensian ice.

The glaciers sculpted and overdeepened the valleys into characteristic U-shaped profiles, and most of the north Wales examples, such as the Llanberis (Plate 50), Nant Ffrancon (Plate 51) and Conwy valleys, display successive rock basins in their longitudinal profile. In the Conwy valley, bedrock is at 35 m at Llanrwst and some 130 m at Dolgarrog, just 7 km downstream. It is likely that similar depths occur in most of the valleys in north-west Wales. Similarly, in south Wales, boreholes and geophysical surveys in all the major valleys prove pronounced overdeepening. In Snowdonia, the sculpting power of the debris-laden base of the ice is most clearly seen in the striated rock surfaces and roches moutonnées that are ubiquitous features throughout the high cwms, but the most spectacular striated surfaces are those that were temporarily exposed (Plate 52) for the first time in about 10 000 years when Llyn Peris was drained during the construction of the Dinorwic Pump Storage Scheme.

Where the influence of the Irish Sea ice was significantly more powerful than the Welsh ice, it encroached well into the Welsh mainland. In north Wales, the pressure of Welsh Ice was sufficient to constrain the Irish Sea ice to a line that coincides approximately to the present-day coastline for most of the contact, but Irish Sea ice did extend into the Vale of Clwyd and for some considerable distance into the Cheshire–Shropshire plain. In north-west Wales, the influence of the Irish Sea ice is recognised mainly across Anglesey and western Llŷn. It is apparent that during the late Devensian glaciation, the Welsh ice extended onto Llŷn, well outside the main distributaries in the Nant Ffrancon, Llanberis and Gwyrfai passes, and many coastal sections such as those at Glanllynnau and Gwydir Bay show evidence of the fluctuating ice front. At Glanllynnau, the cliff section exposes two tills (Figure 67), of a single glacial episode, separated by fluvioglacial sand and gravel. Kettleholes in the vicinity contain grey silty clay that has yielded palynological data and information on beetle faunas that indicate a level close to, or at, the Pleistocene–Holocene boundary.

The Irish Sea ice in Cardigan Bay was sufficiently powerful to extend high into the Teifi valley and possibly across the col between Llandyssul and Carmarthen to join the Tywi glacier. Farther south, distribution of erratics indicates that the ice rose over the flanks of Mynydd Preseli to spread across south Pembrokeshire into Carmarthen Bay, Gower and the Vale of Glamorgan.

The remnant deposits of the Irish Sea ice on mainland Wales are largely restricted to scattered outcrops of till, sand and gravel in Anglesey and Llŷn. Particularly distinctive are the sections, south of Lleiniog, on the east coast of Anglesey, where some 5 m of till overlies 6 m of coarse sand and gravel; both deposits include a wide range of clasts derived from Scotland, the Lake District and from the floor of the Irish Sea. From Scotland, the clasts of the riebeckite-microgranite of Ailsa Craig in the Firth of Clyde and the Goat Fell granite from Arran are particularly distinctive. From the Lake District, clasts of the Eskdale granite and Ennerdale granophyre have been determined. From the Irish Sea, there are Jurassic and Cretaceous rocks and a suite of marine shells. In south Wales, the drift includes numerous clasts of volcanic rocks whose provenance lay in the Ordovician outcrops of either north Wales or Pembrokeshire.

The highest penetration of the Irish Sea ice on to the mainland occurs at an elevation of 400 m OD on Moel Tryfan on the western edge of Snowdonia. Here within the complex of the Alexandria Slate Quarry are marine shells and sands that were probably derived from the base of the ice sheet and redeposited from the meltwater streams adjacent to, or beneath, the wasting ice sheet. Unfortunately, the exposures within the complex of the Alexandria Slate Quarry have deteriorated, and the most complete descriptions of extensive shelly fauna are those of the first study published in the early 19th century. At that time, the shells were quoted, in the heated discussions of the Glacial Theory, as evidence of the Biblical flood.

Meltwater channels formed both in drift deposits and in rockhead, and are a common feature of the Welsh topography, particularly in the less elevated areas. Commonly the channels are dry and steep sided, and their intricate patterns provide a great deal of information about the glacial drainage system. One of the most spectacular systems lies about the Gwaun valley (Figure 68); the lack of correlation between the channel orientations and contours indicates that in many instances the subglacial water was under great hydrostatic pressure.

The difficulties in understanding the detail of both the Irish Sea and Welsh ice sheets arise because both probably comprised several glacial phases interspersed with more clement interglacial phases. The evidence for this complexity is most clearly seen in the coastal sections about Cardigan Bay from Llŷn to north Pembrokeshire, and at various localities between Langland Bay and Broughton Bay in the Gower. In Cardigan Bay, the original concept of a lower and upper glacial till separated by fluvial sands and gravels, deposited when the ice cover had retreated, is too simplistic. The problem is exacerbated by the nature of the deposits: unconsolidated tills with widely variable proportions of clay, sand and clasts, and patches or impersistent layers of sand, gravel and cobbles. Such lithologies do not lend themselves to clear environmental interpretation, and even less so to stratigraphical correlation. Consequently, there seems to have been as many interpretations as workers. The supposed ‘correlation’ between the deposits of north and south Wales has been based on the assumption that the ‘raised beach’ deposits of Porth Oer on Llŷn, and Red Wharf Bay on Anglesey are equivalent to the Patella beaches of Gower, and are of Ipswichian (last interglacial) age, with the overlying glacial deposits being of Late Devensian age. However, the equivalence of the north Wales deposits has not been substantiated.

During the late stages of glaciation, meltwater flowing from the Welsh ice sheet as it retreated from the Cardigan Bay coastline was impeded by the Irish Sea ice and temporary lakes were formed. The largest of these lay in the Teifi valley whose profile indicates successive lake levels down from the vicinity of Tregaron to the coastline at Cardigan. Recently, the understanding of these lake deposits has been improved as a result of the engineering work subsequent to the landslip through Llandudoch (St Dogmaels) to the south-west of Cardigan. Boreholes proved a sequence of laminated silt and clay, sand, gravel and till. Laminated silt and clay, up to 103 m OD, are interpreted as annual and seasonal varve deposits in a lake that changed its shape with the fluctuations at the front of the Irish Sea ice sheet. At its maximum development the lake drained southwards, into Pembrokeshire, into a lake at a lower level in the Nevern valley, west of Newport. Elsewhere, as at the confluence of the Ely, Taff and Rhymney rivers in suburban Cardiff, widespread outwash fans were deposited by meltwater.

Head and other mass movement deposits mantle both upland and lowland slopes. Much of this originated as periglacial deposits, although talus (scree), solifluction and landslip continue to form. Repeated freezing and thawing in cold climatic conditions has produced the conspicuous block-fields seen in some mountainous areas. Patterned ground is another remnant of very cold conditions and may take the form of stone stripes, circles or polygons (Plate 54).

Offshore, the absence of Miocene to early Pleistocene deposits has been attributed to uplift and erosion during this interval. A significant thickness of Middle and Upper Pleistocene deposits occurs in St George’s Channel, with less in Cardigan and Liverpool bays. In the Bristol Channel, there are large areas with no Middle and Upper Pleistocene deposits. The sequence is most clearly understood from seismic profiles, which display the unconformable relationship with the pre-Quaternary strata. The subdivision of the sequence into six formations (Figure 66) is based largely on the seismic profiles with some supporting borehole data. The formations show considerable lateral variations and interdigitation, and further subdivision into facies and informal members has been possible. The deposits are of widely differing lithologies, ranging from till through coarse cobbles and boulders to sand, silt and clay. Within the sequence, three erosional surfaces, with major incisions in excess of 100 m, has allowed four depositional cycles to be recognised and the overall stratigraphy is broadly similar to that of the North Sea.

The Mochras Borehole intersected some 80 m of till and pebbly gravel, and subsequently these have been correlated mainly with the Western Irish Sea Formation. The thickest element, of clast-supported tills, was assigned to the Sarnau facies, which forms the low, smooth-topped ridges or sarnau that extend seaward from the coastline. Mochras lies at the landward side of Sarn Badrig. The ridges have been intepreted as median moraines between glaciers exiting from major valleys, and as the remnants of late-glacial sandurs. In St George’s Channel, pebbly mud of the St George’s Channel Formation is overlain by till of the Cardigan Bay Formation, which locally includes a thick central unit of silt and sand. Thinner sequences of the Cardigan Bay Formation have been determined onshore and offshore at Llandudno and in Caernarfon Bay. The uppermost Surface Sands Formation is restricted mainly to the nearshore and intertidal areas, as in the shelly sands in Tremadog Bay, where it is up to 30 m thick. The lower part of the formation reflects shallow-water or subaerial conditions, but the upper parts are the product of present-day (Holocene) marine processes. For much of the nearshore areas between south Glamorgan and south Pembrokeshire, the Surface Sands Formation is the dominant element of the thin Pleistocene sequence. Inshore the formation passes into intertidal sands, mudflats and salt-marshes.

Holocene

At the beginning of Holocene times, some 10 000 years before present (BP), the climatic amelioration that had been briefly interrupted during the Loch Lomond Stadial (Younger Dryas) continued. A temperate deciduous mixed forest, with regional variations, developed across Wales, particularly below 500 m OD. The forest was progressively modified by increasing human occupation, but the main clearance did not occur until early Roman times, some 2000 years ago.

At the maximum growth of the Late-Devensian ice sheet, about 21 000 BP, sea level was at its lowest, approximately 100 m below present OD. From Late Devensian into Holocene times, sea level changes were induced by melting of the ice sheet and isostatic rebound, and between 18 000 and 15 000 years BP there was a temporary land bridge between Wales and Ireland. Current sea level was attained about 5000 years ago, although movements have continued. The principal postglacial deposits, apart from the extensive tracts of river alluvium, occur in the vicinity of the coast where, locally, swathes of blown sand impede drainage and separate large tracts of alluvium from the shoreline. Such relationships are common, but the most notable are probably those between Porthcawl and Swansea, east of Pendine, at Ynyslas (Figure 69) near Borth, at Morfa Harlech, at Newborough Warren on Anglesey and at the mouth of the Vale of Clwyd between Abergele and Prestatyn. In Cardigan Bay, the beach material is mainly transported northwards and in many instances, for example at Ynyslas, ridges of beach gravels form a foundation for the dunes. At Ynyslas, the dunes separate the submerged forest and associated beds on the foreshore from Borth Bog to the east; the site has been extensively studied and provides an important record of coastal and environmental changes over the past 7000 years. The submerged forest beds are most commonly exposed at or below high-water mark in a setting where they could not be formed at the present time; tree bases in growth positions are common. In the excavation for the major dock and shoreline installations in Glamorgan, postglacial peat beds were particularly well exposed, down to 30 m below current sea level at Barry. The peat beds are interbedded with marine and estuarine clays, which contain a characteristic fauna that indicates some pauses in the gradual drowning. At Crymlyn Bog (Swansea), the youngest peat, up to 12 m thick, at and above current sea level, was probably formed some 3000 years ago. Associated with the plants in the peat beds are insect remains and mammal bones, particularly of deer species that no longer frequent the Welsh countryside. In the bays of south Wales, the peat beds have yielded flint artefacts, indicating that the forests were temporarily dry enough to encourage Neolithic and early Bronze Age human habitation.

Inland, solifluction processes have been active on hillslopes, and landslips are a prominent feature, and potential hazard, particularly within the deeply dissected valleys across the coalfield in south Wales. The postglacial sea-level changes and variations in the drainage patterns caused extensive terracing of the river alluvium. Most of the gentle upland slopes in Wales are covered with thin peat, which thickens locally in depressions, for example on Plynlimon. One of the most distinctive, romantic and probably thickest swathes of peat is that across Migneint, west of Arenig Fawr, which includes Llyn Conwy close to the source of the Conwy river. The River Teifi, near Tregaron, flows through one of the largest valley peat bogs (Cors Caron), which formed in the floor of the moraine-dammed lake; the oldest peat has been dated at about 10 000 years BP, dating the melting of the ice a little earlier.

Offshore, sea-bed sediments can be broadly divided into mobile sediments and gravelly lag deposits, which are actively involved in the current marine process. Where mobile sediments are absent, the sea bed is mantled by a shelly or pebbly gravel and coarse sand deposit, which undergoes constant winnowing and reworking, and it is this process that forms the mobile layer. In both St George’s Channel and Cardigan Bay, the gravelly deposits pass laterally into giant sand waves, up to 40 m high, and tidal sand ridges. In the central part of Tremadog Bay, the sand passes laterally into mud in which considerable volumes of gas have been recorded. Similar sand waves are well developed to the south and west of Gower in the Bristol Channel.

These offshore, unconsolidated sands and gravels are an important economic resource, which have been dredged in the outer Severn estuary (Figure 70) and in the linear tidal sand ridge, Nash Sand, close to Porthcawl. It is in this area that the proposed Severn Barrage is likely to be sited; the idea for a barrage was originally put forward by Thomas Telford in the early 19th century. There have been two detailed investigations, an ‘outer’ location between Breaksea Point and Warren Point, and an ‘inner’ location between Lavernock Point and Brean Down. Sediment along the line of the inner location is less than 1 m thick, except at sandbanks where it is up to 10 m in places. The superficial sediments are underlain variously by Carboniferous limestone, in the vicinity of the Holm islands, Triassic mudstone, and Lower Jurassic limestone and mudstone. The deep-water channel between the Holm islands determined this to be the prospective site of the turbines.

Chapter 10 Geology and man

This guide has described the geological evolution of Wales over the past 700 million years, drawing particular attention to the major physical events and the initiation of fauna and flora. For many, the most important event would be at the end of this time span in the Quaternary era, when Homo sapiens evolved. The earliest evidence of human occupation is from Pontnewydd Cave in the Elwy valley, where teeth of Lower Palaeolithic (Old Stone Age) hunters and artefacts have been recovered; the association with many bones of bears and wolves suggest that the cave was used as an abattoir during a slightly warmer phase, some 230 000 years ago. In Paviland Cave in Gower, bones, referred to the Red Lady, dated at some 29 000 years, yielded protein that indicates a diet containing fish and vegetables, and suggest a marked development from the carnivorous habits of the Neanderthal community. In nearby Minchin Hole Cave, some researchers have suggested a matching cultural development, with fragments of ivory from mammoth tusks that are claimed to have been shaped, and ochreous bands on the walls that are claimed to be decorative graffiti, but others disagree. It is apparent that, by this time, human evolution was advanced, far closer to now than to its beginnings, and it is reasonable to assume that by Late-Devensian — early Holocene times, about 11 000 calendar years BP, the human population was beginning to be more established, moving inland from the coastal fringes as the influence of the ice waned. Although details of their communal living are restricted, there is a wealth of information through to Neolithic (5000 to 4000 years BP), Bronze and Iron age times, in the mounds, cromlechs and stone and hut circles that enrich the countryside (Plate 55). Pollen records after 3500 years BP show how cultivation encroached onto higher ground. Some 2000 years ago, the Roman legions entered Wales and their legacy, in the road pavements, for example through Bwlch y Ddeufaen and north from Caersws, and the forts, as at Caerleon and Caernarfon, are still discernible, and are now preserved by CADW (Welsh Historic Monuments Executive Agency).

The aboriginal Welsh population was profoundly influenced by the terrain. For example, deviations along the route of Offa’s Dyke, such as between Kington and the Clwydian Hills, reflects both the topography and, therefore, the geology. However, their main concern would have been for secure places to live. The main barriers around the edges of their forts and the houses within them would have been built of timber from the surrounding forests. However, there is evidence in the higher ground, as in Cwm Dulyn, north-east Snowdonia, of hut circles that were probably constructed entirely from the many glacial erratics in the vicinity. The Romans, being the first serious civil engineers to occupy the country, were the first to quarry and fashion local rocks to build and decorate their larger constructions.

For early man, rock such as flint, which could be fashioned into hand axes and arrow heads, represented wealth to be sought after and traded. In south-east England, the Chalk was a rich repository for such flints and the absence of similar rocks in Wales necessitated alternative solutions. There is evidence that fine-grained siliceous rocks were similarly utilised, for example in the ‘axe factory’ above Penmaenmawr fragments worked from the chilled margin of the intrusion were found, and there are many examples of the Ordovician air fall tuffs and metamorphosed siltstone in implements recovered from the Elwy valley caves. Such tuffs continued to be quarried for honestone across Snowdonia into the 18th century, for example the quarry that now channels the footpath south from Ogwen Cottage. The wealth of weather-resistant rock, in outcrop and glacial erratics, with variable shapes and sizes, facilitated their incorporation into the standing stones, stone circles and cromlechs, which developed out of the social and cultural aspects of the societies. The monuments are particularly prominent in western Pembrokeshire and, as at Pentre Ifan, their remnants are impeccably preserved. The bluestone lintels at Stonehenge were derived from the flanks of the Preseli ridge in Pembrokeshire, and how they were transported, by ice, hard labour or a combination of both, is still a matter of debate.

There can be little doubt that Bronze and Iron age man recognised clays that could be fired into domestic pottery and used to channel water, that they used coal and peat for fuel, and that metalliferous minerals were incorporated into their culture. Consequently, it would be surprising if the largest mining districts, such as those at Cwmystwyth, Minera and Parys Mountain, did not have a history that extended back for some 4000 years. Most of the nonferrous metalliferous mines were worked mainly along veins for lead and zinc, which, in places, included both silver and gold in association. Elsewhere, as in the mines in the core of the Snowdon massif, and at Parys Mountain and in Coed y Brenin, copper was the main incentive, and the deposits were related to the late stages of Ordovician volcanic activity. In the Dolgellau and Dolaucothi districts, gold is associated with black mudstones, as is uranium at Tremadoc. Manganese, in both mudstone and as a replacement of ooidal ironstone, was worked on Llŷn and at Harlech. It was probably during the Roman occupation that the extent of the mineral wealth began to be appreciated, and there is hardly an account of a metalliferous mine in Wales that does not make reference to it being first developed during Roman times, or earlier, although the evidence, in many instances, is, at best, questionable. Arguments for the great antiquity of copper, lead and zinc mining at Parys Mountain on Anglesey, across the Snowdon massif and in the Central Wales Mining District are based mainly on mining techniques, which are extremely difficult to date, or on implements, such as hammer stones, which are similarly difficult to restrict to a certain period. Even so, on such evidence, several sites across Wales, including those recently excavated on Great Orme, have been assigned to the late Bronze Age or early Iron Age. However at its peak from the 18th through the 19th century, the mining of the veins in these districts, and those in the Lower Palaeozoic rocks in the Llanrwst and Llanfair Talhaiarn districts was intense and labour intensive. The mines hosted in Carboniferous Limestone, in south Wales, in the outcrop close to the north Wales coast, in the Vale of Clwyd, and at Minera and Llandegla, developed in similar fashion.

Even though there is evidence, as in the Blaenavon area, that coal was being extracted and sideritic iron ore was being smelted in the 15th and 16th centuries, the main development of the industries in both south and north Wales, and at the same time the development of the north Wales slate quarries was directly related to the industrialisation and colonial expansion of Britain in the late 18th century and through the 19th century. It is now almost impossible to envisage the speed of change in those sparsely populated valleys – the sinking of the mines for coal, the development of the railway system for transport to the docks and the urban areas of England, the movement of the labour force from England and the rural areas of central Wales, the expansion and development of the metalliferous industries, the opening up of quarries for sandstone to build the houses, ironstone for smelting, silica sand for furnace bricks, and limestone for flux for the furnaces. The key factor was not just that all of these essential geological elements were there in abundance to feed the force of the industrial revolution, but that they were there in close proximity. The line of the current Heads of the Valleys road cuts a swathe through the heartland, and just one sector, between Blaenavon and Merthyr Tydfil, displayed the fortuitous juxtaposition of the Ebbw, Sirhowy, Rhymney and Taff valleys, with pit heads by the score, and a concentration of the smelting and iron foundries at Blaenavon, Ebbw Vale, Tredegar, Merthyr Tydfyl and Dowlais. The broad extensive limestone plateau between Trefil and Mynydd Llangattock lay a stone’s throw to the north, and the surface water supply was sufficient for the steel and coal ‘masters’ industrial processes. Access to international markets from the ports at Cardiff and Barry was just a day trip away to the south. The importance of this industrial landscape in the early 19th century has been recognised internationally by its recently awarded World Heritage status, and consequently the remnants of the successive layers of its relentless development are guaranteed preservation. The site includes the Big Pit, the colliery museum of the National Museum of Wales, still with some ex-miners as guides. This industrial scenario, based entirely on the geology, extends seamlessly westwards through the Neath, Gwendraeth and Amman valleys, and further into Pembrokeshire. The patterns of development of these labour-intensive industries persisted, with very little change until the middle of the 20th century. The south Wales coal mining industry reached its peak in 1913, when 56 million tons of coal were produced and the industry employed 232 800. There is not a village in the coalfield without graphic evidence of the harshness of the mining industry which shaped their communities, none more so than Senghenydd where 81 men were killed in 1901 and 439 men in 1913, in underground gas explosions at the Universal Colliery. Following nationalisation in 1947, the National Coal Board began to develop new collieries, but by 1955 a programme of pit closures began, and after 1985 mass closures had reduced the mining industry in south Wales to a shadow of its former importance. During this last phase the legacy of the industry continued to impact on the communities and sometimes, as in Aberfan, in a most devastating fashion — on 21st October 1966 a spoil tip that had been sited on a spring line was mobilised into a slurry that engulfed the village school killing 116 children and 28 adults. Today, deep mine activity is at Tower Colliery, Hirwaun, following a successful employees take over, and the Nant Hir No. 2 Colliery, Seven Sisters, also under private licence. Work is in progress in three colleries, Aberperwn (Neath), Black Barn (Torfaen) and Blaencwn in consideration of their possible renaissance. Such activity is difficult to reconcile with those assurances, such a short time ago, that their immediate demise was inevitable. Open cast activity is currently restricted to four sites in the Vale of Neath but applications for development continue to be made. In north Wales, the pattern was repeated in the Flint and Denbigh coalfield, and by the 1980s all coal and steel production had ceased. Even the levels of brick and pottery clay extraction, based on the Coal Measures in the vicinity of Buckley, was reduced to the scale of a cottage industry, somewhat similar to the state of the slate industry in Caernarvonshire and Merionethshire.

Wales was the world leader in the production of roofing slates throughout the 19th and early 20th centuries. Small slate quarries had been opened across the whole of the Lower Palaeozoic outcrop, but the main enterprise lay in Snowdonia, and the development of the rail network, including the narrow gauge, and the ports at Penrhyn, Caernarfon and Dinorwic, ensured the growth of villages such as Bethesda, Llanberis, and Blaenau Ffestiniog. At the turn of the 19th century, some 14 000 men were employed in the industry, but a gradual decline into the middle of the 20th century accelerated into almost total demise with the availability of cheap slates from overseas and the development of the clay and composition roofing tile industry; currently only 300 to 400 are employed. Penrhyn Quarry at Bethesda and Dinorwic Quarry at Llanberis are probably the most impressive excavations, but whereas the former currently operates a small-scale production, Dinorwic is the site of the power generators for the pumped storage hydroelectric scheme that was developed in the 1970s. Today, the scale of this great industry is most clearly reflected in the waste tips which, in places, dominate the landscape; 95% of the extracted rock was rejected. However, many of the narrow gauge railways have been preserved to ferry the tourists up and down the valleys.

Apart from this extremely close link between geology, industry and the provision of employment, the most obvious link between geology and population is seen in the construction of the villages, towns and cities. The determining factor in the use of building stones is their availability. This is nowhere more obvious than in the standing stones, castles and the cathedrals, when, during their construction, transport for any significant distance would have been very difficult. Typical of locally sourced building stones are the Carboniferous Limestone of the Din Lligwy circular huts on Anglesey, the Silurian flags in Conwy Castle and Valle Crucis Abbey, the Cambrian grit in Harlech Castle, Devonian sandstone in Tintern Abbey, and Triassic sandstone in Caldicott Castle; the pattern is repeated consistently. Even in comparison with present-day construction the scale of many of these projects is difficult to understand: that it was 1271 when the ‘mass of masonry’ (Pennant Sandstone) used for building Caerphilly Castle was sourced, excavated and transported to the site. The local availability of different rock types is reflected in the buildings, as in the Precambrian and Cambrian rocks in St David’s Cathedral and the adjacent Bishop’s Palace in north Pembrokeshire, and in the Victorian and Edwardian suburbs of Cardiff and the villages in the Vale of Glamorgan where there are many examples of Carboniferous sandstone and limestone, juxtaposed with Triassic sandstone and Jurassic limestone. The availability of Lower Palaeozoic, Carboniferous and Triassic rocks along the north Wales coast is reflected in the towns between Bangor and Flint. However, whole areas may be virtually dominated by a distinct lithology, and the villages throughout the south Wales coalfield, built from the flags of the Pennant Sandstone, are the best example.

Cursory observation of the small towns and villages of central Wales would suggest a similar uniformity, with the dominant lithologies being blue-grey muddy siltstone and sandstone of the Silurian. However, local differences can be distinguished on closer examination, for example between Llandrindod Wells and Rhayader or Aberystwyth and Machynlleth. At Aberystwyth, the dominant flags incorporated in the buildings of the late 19th and early 20th century are from the turbidite sandstones of the Aberystwyth Grits that are exposed in the cliffs and on the foreshore, and were quarried in the vicinity. At Machynlleth, beyond the outcrop of the Aberystwyth Grits, the sandstone flags are less common, and grey muddy siltstone flags are dominant. To the east, to Llanidloes, Newtown and Welshpool, there is a progressive increase in the influence of Carboniferous and Triassic rocks, and of bricks and tiles sourced in Flintshire, Denbighshire and Shropshire. Similarly, eastwards from Llandovery, the point at which the influence of Old Red Sandstone begins in the buildings of Brecon, Hay-on-Wye, Abergavenny and the intervening villages lies almost directly on the geological boundary. In recent years these local differences, which so clearly reflected the local geology, are being encircled and overwhelmed by a tide of breeze blocks, pebble dash and composition tiles or, even worse, cheap imported slates.

The relentless expansion of the construction industry is matched in the cement and aggregate industries. Large cement works are sourced from quarries in Carboniferous and Jurassic limestones in the Vale of Glamorgan and Carboniferous limestones in Flintshire. High purity and dolomitic limestones are also quarried for metallurgical and refractory use. The Dinantian limestone is the major resource of hard rock aggregate, as in the quarries at Llandulas and Trefil, but locally, as at Old Radnor, the Silurian (Wenlock) limestone, unconformably overlying the Precambrian, makes a significant contribution. Even the shortest journey throughout the Principality passes quarries in a wide range of rock types that have been excavated in the past, if only for local use. In central Wales, the coarse grits in the Lower Silurian have been a rich source, but the recent closure of the Cerrig Gwynion Quarry near Rhayader has meant that Hendre Quarry in the Rheidol valley is the only current operation. The largest excavations of igneous rocks for aggregate are in the granophyre near Trefor on Llŷn and the microdiorite at Penmaenmawr, both with the facility to ship out the aggregate directly. Elsewhere, dolerites are worked from the quarries at Llanelwedd, near Builth Wells and diorites (Precambrian) at Johnston in Pembrokeshire.

Deposits of glacial or alluvial sand and gravel are common, and many have been worked from time to time for local use. However, commercially viable deposits, such as the fluvioglacial sediments at Pantgwyn Mawr (Plate 53) at the northern edge of Cardigan, at Pentir, south of Caernarfon, Bodfari and Wrexham are few. In recent years, dredging in the Bristol Channel has been particularly successful, especially for sand, and it is likely that this will be extended.

The influence that water had in the industrial development of Wales is matched by its current importance as a commercial commodity. The mountainous terrain at the eastern edge of the Atlantic Ocean ensures an average annual rainfall significantly greater than the rest of southern Britain. However, the nature of the geological foundation for most of the country directs most of the rainfall into surface drainage rather than into major underground aquifers, which would lie mainly in post-Carboniferous sequences.

The Triassic sequence in the Vale of Clwyd basin is an aquifer that has been drawn into the public water supply. The sandstones are variously porous and fracture flow is locally important. Superficial deposits conceal most of the sequence and away from the well defined valley margins the supply is mainly artesian. The main extraction is from a series of boreholes in Llannerch Park and some boreholes farther upstream are used seasonally to augment river flow. Similarly, there are many private wells into the Triassic sequence in the Cardiff district. The water-bearing strata of the Carboniferous Coal Measures was a problem in most mines throughout the coalfields and then, ironically, the high concentration of iron and sulphate as a result of the mining has markedly reduced its quality. The aquifers within the thick sequence of Dinantian limestone are important both in north Wales and around the periphery of the coalfield in south Wales. For example, the Schwyll Spring, near Bridgend is sourced from rainfall, stream sinks and sections of the drainage, and supports a significant abstraction licence, whereas wells in the vicinity of Castlemartin are less productive. In north Wales, Ffynnon Asaph (spring) is the key feature of the water supply to Prestatyn.

The Lower Palaeozoic strata through central Wales are weakly permeable and water storage is mainly confined to fractures. Successful wells are as speculative as those drawn from the shallow zone of weathering, particularly at the boundary between the superficial deposits and bedrock, which are such a common feature of the remote hill farms. In some of the larger valleys, sourced by large catchments such as the Rheidol, Teifi, Dyfi and Tywi, sand and gravel within the superficial deposits have provided a reliable source and the Lovesgrove boreholes in the vicinity of Aberystwyth are probably the most significant.

Overall, the vulnerability of groundwater in Wales to pollution from ill-considered farming practice is less than in many other parts of southern Britain. However, in areas of Carboniferous strata, acid and ferruginous discharges are common and the closure of the coal mines did little to control the problem, Similar discharges occur in the vicinity of the metal mines and is nowhere more graphically displayed than in the acrid pool, which reputedly devours abandoned cars rapidly, deep in the hole at Parys Mountain. Of course, it was the salinity and the brackish springs that put the wells into Builth, Llandrindod, Llangamarch and Llanwrtyd, in the early 19th century. The spa waters are derived from slow and deep circulation over very long periods of time — the discharge at Llandrindod Wells is derived from rainwater recharge in Pleistocene times. The most obvious manifestation of the surfeit of water in Wales are the numerous reservoir complexes that litter the valleys. Their construction has ensured the supply of water not only throughout the Principality but also to assuage the thirst of the industrial Midlands and north-west England.

In the 1970s, following the sinking of the Mochras Borehole at Llanbedr, and the discovery of oil and gas in the North Sea, hydrocarbon exploration off the coast of Wales was intense. The surveys yielded a great deal of information about the geology and the structures, but proved no significant concentrations of hydrocarbons. Nevertheless, the presence of the Kinsale Head field, off the south coast of Ireland, to the west, and the Douglas Oilfield and Morecambe Gas field, to the north, is likely to stimulate further exploration.

British Geological Survey publications of Wales

The following memoirs of the British Geological Survey have been used as reference sources throughout this book. Memoirs, maps and other BGS publications are available from BGS offices (see page 207).

Onshore memoirs

The memoirs and Sheet Explanations^‡ are listed by sheet number and the latest edition of the map is shown in italics. † out of print. These may be purchased from BGS libraries as black and white photocopies F facsimile of earlier map available *. 1:63 360 scale map

Sheet No. and name	Date	Authors
	memoir (Rom) map (italic)
92, 93, parts of 94, 105, 106 Anglesey	1919† 1980	Greenly, E
107 and part of 94 & 95 Rhyl and Denbigh	1984 1970;1985	Warren, P T, Price, D Nutt, M J C, and Smith, E G
106 Bangor	1985† 1985	Howells, M F, Reedman, A J, and Leveridge, B E
108 Flint	2004 1999	Davies, J R, Wilson, D, and Williamson, I T
118 Nevin	Sheet 75NW† 1850*
119 Snowdon	1997 1997	Howells, M F, and Smith, M
120 Corwen	1993
121 Wrexham	1927† 1993	Wedd, C B, Smith, B, and Willis, L J
122 Nantwich and Whitchurch	1966 1967	Poole, E G, and Whiteman, A J
133 Aberdaron and Bardsey Island	1993 1994	Gibbons, W, and McCarroll D
134 Pwllheli	2002 1999	Young, T P, Gibbons, W, and McCarroll, D
135 Harlech	1985 1982	Allen, P A, and Jackson, A A
136 Bala	1986
137 Oswestry	1929† 1999	Wedd, C B, Smith, B, King, W B R, and Wray, D A
149 Cadair Idris	1995 1995	Pratt, W T, Woodhall, D G, and Howells, M F
150 Dinas Mawddwy	Sheet 60NW† 1855*
151 Welshpool	In press	Cave, R. South-east part is included in Montgomery memoir
163 Aberystwyth	1986 1989	Cave, R and Hains B A
164 Llanidloes	Sheet 60SW 1850
165 Montgomery	2001 1994	Cave, R, and Hains, B A
177 Aberaeron	1994
178 Llanilar	1997 1994	Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G, and Zalasiewicz, J A
179 Rhayader	1997 1993
180 Knighton	Sheet 56NE† 1850*
193/210 Cardigan and Dinas	2003 2003	Davies, J R, Waters, R A, Wilby, P R, Williams, M, and Wilson, D
194 Llangranog	2006 2006	Davies, J R, Sheppard, T H, Waters, R A, and Wilson, D
195 Lampeter	2006 2006	Davies, J R, Schofield, D I, Sheppard, T H, Waters, R A, Williams, M, and Wilson, D
196 Builth Wells	2004‡ 2005	Schofield, D I, Davies, J R, Waters, R A, Wilby, P R, Williams, M, and Wilson, D
197 Hay-on-Wye	2004 2005	Wilby, P R
209 St David’s	1992
210/193 Cardigan and Dinas	2003 2003	Davies, J R, Waters, R A, Wilby, P R, Williams, M, and D. Wilson
211	1857*† Sheet 41NW One inch Sheet 41 NW, 1857
212	1857*† Sheet 41NE One inch Sheet 41 NE, 1857
213 Brecon	2005 2005	Barclay, W J, Davies, J R, Humpage, A J, Waters, R A, Wilby, P R, Williams, M, and Wilson, D
214 Talgarth	2003 2004	Barclay, W J, and Wilby, P R
226/227 Milford	1978
228 Haverfordwest	1914† 1976	Strahan, A, Cantrill, T C, Dixon, E E L, Thomas, H H, and Jones, O T
229 Carmarthen	1968 1997	Archer, A A Special memoir for sheets 229, 230 and 246
230 Ammanford	1907† 1977	Strahan, A, Cantrill, T C, Dixon, E E L, and Thomas, H H
231 Merthyr Tydfil	1988 3rd edition 1979	Barclay, W J, Taylor, K, and Thomas, L P
232 Abergavenny	1989 3rd edition 1990	Barclay, W J
233 Monmouth	1961 1974	Welch, F B A, and Trotter, F M
244/245 Pembroke and Linney Head	1921† 1983	Dixon, E E L
246 West Gower and Pembrey	1907†	Strahan, A
247 Swansea	In press 1907† 1977	Barclay, W J Strahan, A
248 Pontypridd	1963 Third edition 1992	Woodland, A N, and Evans, W B
249 Newport	1969 Third edition 1997	Squirrell, H C, and Downing, R A
250 Chepstow	1961 1972	Monmouth and Chepstow memoir Welch, F B A, and Trotter, F M
261/262 Bridgend	1990 Second edition 1990	Wilson, D, Davies, J R, Fletcher, C J N, and Smith, M
263 Cardiff	1987 Third edition 1988	Waters, R A, and Lawrence, D J D

Iron ores of Great Britain. 1861. Part III Iron ores of south Wales. Memoir of the Geological Survey of Great Britain. (London: Longman, Green, Longman and Roberts forHMSO.)
Ramsay, A C. 1881. Geology of north Wales. Second edition. Memoir of the Geological Survey of Great Britain. (London: Longman & Co for HMSO.)

South Wales Coalfield memoirs Sheets 229 to 263

Part I The country around Newport. Squirrell, H, And Downing, R A. Third edition. 1969
Part II The country around Abergavenny. Barclay, W J. Third edition. 1989
Part III The country around Cardiff. Waters, R A, And Lawrence, D J D. Third edition. 1988
Part IV The country around Pontypridd and Maesteg. Woodland, A W, And Evans, W B. Third edition. 1964; 1996 reprint
Part V The country around Merthyr Tydfil. Barclay, W J, Taylor, K, And Thomas, L P. Third edition. 1988
Part VI The country around Bridgend, Wilson, D, Davies, J R, Fletcher, C J N, and Smith, M. Second edition. 1990
South Wales Coalfield. The upper Carboniferous and later formations of the Gwendraeth valley and adjoining areas. Archer, A A. 1968

Special monograph

Ordovician (Caradoc) marginal basin volcanism in Snowdonia (north-east Wales). Howells, M F, Reedman, A J, And Campbell, S D G. 1991

Field guide

Geological excursions in the Harlech Dome. Allen, P M, And Jackson, A A. 1985
Regional geochemical atlases
British Geological Survey. 1997. Regional geochemistry of parts of north-west England and north Wales. (Keyworth, Nottingham: British Geological Survey)
British Geological Survey. 1999. Regional geochemistry of Wales and part of west-central England: stream water. (Keyworth, Nottingham: British Geological Survey.)
British Geological Survey. 2001. Regional geochemistry of Wales and part of west-central England: stream sediment and soil. (Keyworth, Nottingham: British Geological Survey.)
Hydrogeology reports
Physical properties of major aquifers in England and Wales, WD/97/34
Physical properties of minor aquifers in England and Wales, WD/00/04

Offshore memoir

United Kingdom offshore regional report: the geology of the Irish Sea. Jackson, D I,Jackson, A A, Evans, D, Wingfield, R T R, Barnes, R P, And Arthur, M J. 1996.
United Kingdom offshore regional report: the geology of the Cardigan Bay and the Bristol Channel. Tappin, D R, Chadwick, R A, JacksonA A, Wingfield, R T R, And Smith, N J P. 1994.

British Geological Survey Maps

British Geological Survey maps and associated literature are available from the Sales Desk, British Geological Survey, Keyworth NG12 5GG (Telephone 0115 936 3241; fax 0115 936 3488; e-mail sales@bgs.ac.uk; Internet address http://www.geologyshop.com), and at the BGS Edinburgh office (Telephone 0131 667 1000), or through the BGS London Information Office, Natural History Museum (Earth Galleries), South Kensington, London SW7 2DE (Telephone 0171 589 4090), or through Stationery Office stockists and all good booksellers. A catalogue of maps and literature is available on request and can be found on the BGS web site.

Small-scale maps include:

1:1 500 000

Gravity anomaly map of Britain, Ireland and adjacent areas. (Colour shaded relief), 1997 Magnetic anomaly map of Britain, Ireland and adjacent areas. (Colour shaded relief), 1998 Tectonic map of Britain, Ireland and adjacent areas, 1996.

1:625 000 (about 10 miles to one inch):

Bedrock geology, UK South, 2007*
Quaternary map of the United Kingdom, South, 1977

Hydrogeology map: England and Wales, 1997

1:250 000 (about 4 miles to one inch)

53N 04W Liverpool Bay Solid 1978, Sea bed sediments and Quaternary 1984
53N 06W Anglesey S 1982, Sea bed sediments 1990, Quaternary 1990
52N 06W Cardigan Bay S 1982, Sea bed sediments 1988, Quaternary 1990
52N 04W Mid-Wales and the Marches S 1990
51N 06W Lundy Solid geology, Sea bed sediments 1983

* the geological map of the region in the back pocket is derived from this map

Figures, plates and tables

(Figure 1) Geology of Wales.

(Figure 2) Topography of Wales and adjacent area.

(Figure 3) Distribution of continents in Late Proterozoic and early Palaeozoic time. Note: England and Wales are greatly exaggerated in size (adapted from Mitchell, 2004).

(Figure 4) Terranes of the Caledonian and Variscan orogens (adapted from Cope et al., 1992).

(Figure 5) Generalised vertical section of the Monian Supergroup.

(Figure 6) Holy Island: simplified map (adapted from Phillips, 1991).

(Figure 7) Outcrop pattern of the ‘Gwyddel Beds’ clasts in the Gwna mélange, south-west Llŷn (adapted from Gibbons and McCarroll, 1993).

(Figure 8) Palaeogeography of the Welsh Basin in mid-St David’s (Mid Cambrian) (adapted from Cope et al., 1992).

(Figure 9) Cambrian succession in north-west Wales (adapted from Rushton and Howells, 1999).

(Figure 10) St Tudwal’s peninsula, Llŷn, sketch map of the geology (adapted from Young et al., 2002)

(Figure 11) Geological sketch map showing the Cambrian rocks of the St David’s — Haycastle area (adapted from Rushton, 1974).

(Figure 12) Ordovician (Llanvirn and Caradoc) graptolites from Wales. i Nemagraptus gracilis (Hall) (X3); ii Dicellograptus intortus Lapworth (X4); iii Normalograptus pollex Zalasiewicz and Rushton (X5); iv Corynoides aff curtus Lapworth (X5); v Orthograptus ex gr. calcaratus (Lapworth) (X5); vi Lasiograptus harknessi (Nicholson) (X5); vii Didymorgraptus murchisoni (Beck) (X4); viii Pseudoclimacograptus angulatus sebyensis Jaanusson (X5); ix Diplograptus foliaceus (Murchison) (X5); x Ensigraptus cf. caudatus (Lapworth) (X5); xi Diplacanthograptus spiniferus Ruedemann (X5); xii Dicranograptus clingani Carruthers (X5); xiii Hustedograptus teretiusculus sensu Elles and Wood (X4); xiv Amplexograptus arctus Elles and Wood (X5).

(Figure 13a) Cartoon (not to scale) illustrating a generalised vertical sequence of the Ogwen Group and equivalents (Arenig—Caradoc) across northern Snowdonia, Llyn and Anglesey (adapted from Rushton and Howells, 1999).

(Figure 13b) Cartoon (not to scale) illustrating a generalised vertical sequence of the Ogwen Group and equivalents (Arenig—Caradoc) across southern Snowdonia and the Berwyn Hills (inset) (adapted from Rushton and Howells, 1999).

(Figure 14) Generalised vertical sections of the Ordovician strata of north Pembrokeshire, west Carmarthenshire and Builth Wells (adapted from several sources).

(Figure 15a) Simplified geological sketch map of Ramsey Island (adapted from Kokelaar et al., 1984).

(Figure 15b) Summary of lithostratigraphy, Ramsey Island (adapted from Kokelaar et al., 1984)

(Figure 16) Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992).

(Figure 17) Graphic logs and environmental interpretation of the Pitts Head Tuff, Moel Hebog (adapted from Orton, 1988 reproduced in Howells et al., 1991).

(Figure 18) Depositional environment prior to the deposition of the Lower Rhyolitic Tuff (adapted from Howells et al., 1991).

(Figure 19) Outcrop and measured sections of the Lower Rhyolitic Tuff Formation (adapted from Howells et al., 1991).

(Figure 20) Bedded Pyroclastic Formation, a generalised section of the south face of Crib y Ddysgl (adapted from Kokelaar 1992).

(Figure 21) Relationship of mineralisation to the Lower Rhyolitic Tuff Formation, Snowdon Caldera (adapted from Howells et al., 1991).

(Figure 22a) Ordovician (Ashgill) and Lower Silurian (Llandovery to Wenlock) graptolites from Wales. i Normalograptus arvulus (Lapworth) X10 ii Monoclimacis cf. crenulata sensu Elles and Wood X10 iii Monograptus triangulatus fimbriatus (Nicholson) X10 iv Spirograptus turriculatus (Barrande) X5 v Atavograptus gracilis Hutt X5 vi Cyrtograptus rigidus cautleyensis Rickards X5 vii Monograptus gemmatus (Barrande) X10 viii Normalograptus persculptus (Elles and Wood) X10 ix Streptograptus exiguus (Nicholson) X10 x Paradiversograptus runcinatus (Lapworth) X10 xi Monograptus flemingii (Salter) X5 xii Monoclimacis cf. griestoniensis sensu Elles and Wood X10 xiii Normalograptus? magnus (Lapworth) X10 xiv Plectograptus? bouceki Rickards X10 xv Pristiograptus pseudodubius (Boucek) X10

(Figure 22b) Silurian (Wenlock and Ludlow) graptolites from Wales. i Saetograptus varians (Wood) X6 ii Gothograptus nassa (Holm) X6 iii Monoclimacis micropoma (Jaekel) X6 iv Pristiograptus cf. deubeli (Jaeger) X6 v Saetograptus colonus compactus (Wood) X6 vi Lobograptus scanicus X6 vii Saetograptus clunensis (Earp) X6 viii Saetograptus leintwardinensis (Lapworth) X6 ix Bohemograptus cf. butovicensis (Boucek) X2 x Bohemograptus bohemicus tenuis (Boucek) X6 xi Spinograptus spinosus (Wood) X6 xii Monograptus uncinatus orbatus Wood X6 xiii Saetograptus leintwardinensis (Lapworth) X6 xiv Monograptus aff. unguliferus Perner X6 xv Pristiograptus jaegeri Holland X6 xvi Lobograptus scanicus (Tullberg) X6

(Figure 23) Palaeography of the early Llandovery basin and shelf (adapted from Cave and Hains, 2001).

(Figure 24) Cartoon illustrating stratigraphy across the southern Berwyn Hills (adapted from Cave in Cope et al., 1992).

(Figure 25) Lithostratigraphical cross-section of central Wales illustrating the Llandovery—Wenlock (Homerian) sequence (adapted from Davies et al., 1997).

(Figure 26a) Provenance of the Aberystwyth Grits (adapted from Cave and Loydell, 1997).

(Figure 26b) Depositional models for the Aberystwyth Grits (adapted from Davies et al., 1997).

(Figure 27) Sedimentation patterns, early Wenlock (adapted from Cave and Hains, 2001).

(Figure 28) Generalised section through the Wenlock—Ludlow rocks of the Denbigh area (adapted from Warren et al., 1984).

(Figure 29) Palaeogeography of the late Ludlow (adapted from Cave and Hains, 2001).

(Figure 30) Cross-section illustrating the facies relationships of the Ludlow Series (adapted from Holland and Lawson, 1963; additional information from R Cave, 2007).

(Figure 31) Tectonic map of Wales — key opposite (adapted from BGS, 1996). BD Berwyn Dome; BF Bala Fault; CSF Church Stretton Fault; CVF Conwy Valley Fault; CWS Central Wales Syncline; DS Dolwyddelan Syncline; HD Harlech Dome; LS Llyn Syncline; LsS Llanystumdwy Syncline; LSZ Llyn Shear Zone; ML Malvern Lineament; MSF Menai Straits Fault; ND Neath Disturbance; PL Pontesford Lineament; SS Snowdon Syncline; TA Tywi Anticline; TeA Teifi Anticline; UA Usk Anticline. Inferred age of structures: magenta Precambrian to Early Palaeozoic; blue Acadian; brown Variscan; green Mesozoic; orange Cainozoic (mainly Alpine). Key to (Figure 31).

(Figure 32) Metamorphic map of Wales (data collated by R J Merriman, see Merriman, 2006).

(Figure 33) Vertical sections illustrating the Old Red Sandstone of Wales (adapted from Dineley, 1992). Thickness in metres.

(Figure 34) Burton anticline north-east of Pembroke dock (adapted from Allen et al., 1982).

(Figure 35) Dinantian palaeogeography (adapted from Freshney and Taylor 1980).

(Figure 36) Vertical sections illustrating the Dinantian of south Wales (adapted from Waters et al., 2007).

(Figure 37) Facies variation in the lower Dinantian of south Wales (adapted from Wright, 1996).

(Figure 38) Dinantian of south Wales, schematic cross-section (adapted from Wilson et al. 1990).

(Figure 39) Vertical sections illustrating the Dinantian of the east crop, south Wales (adapted from Barclay, 1989 and Wright 1996).

(Figure 40) Vale of Clwyd, sketch map of the geology (adapted from Warren et al., 1984).

(Figure 41) Vertical sections illustrating the Visean of north Wales (after Waters et al., 2007).

(Figure 42) Vertical sections illustrating the Namurian of south Wales (adapted from Jones, 1974). CCD Carreg Cennan Disturbance; ND Neath Disturbance; SVD Swansea Valley Disturbance

(Figure 43) Namurian of south Wales: isopach map (adapted from Jones, 1974).

(Figure 44) Namurian of south Wales: distribution of biofacies (adapted from Jones, 1974).

(Figure 45) Environmental interpretation of late Namurian in south Wales (adapted from Kelling et al., 1974).

(Figure 46) Farewell Rock lithofacies (after Hampson et al., 1997).

(Figure 47) Silesian of north-east Wales: schematic section showing selected marine bands (Davies et al., 2004).

(Figure 48) Idealised contrasting cyclothems (after Woodland and Evans, 1964). a–c South Wales Coal Measures Group d Warwickshire Group

(Figure 49) Generalised vertical sections of the Westphalian strata of south Wales showing selected coals and marine bands (adapted from Thomas, 1974).

(Figure 50) Generalised vertical sections of the Westphalian strata of north-east Wales showing selected coals and marine bands (Davies et al, 2004).

(Figure 51) Maps showing rank of coal (%Vm) in the South Wales Coalfield (adapted from White, 1991).

(Figure 52) Cross-section illustrating Variscan structures in Pembrokeshire (adapted from Powell, 1989).

(Figure 53) Mesozoic basins in relation to the Welsh Palaeozoic massif (after Tappin et al., 1994).

(Figure 54) Cross-section illustrating the Mesozoic geology around Wales (after Tappin et al., 1994).

(Figure 55) Distribution of Permian and Triassic strata around Wales (adapted from Tappin et al., 1994).

(Figure 56) Seismic reflection profile across part of Cardigan Bay Basin with interpretation extended to the Mochras Fault (Tappin et al., 1994).

(Figure 57) Schematic cross-section through the Triassic sequence of the Vale of Glamorgan, not to scale (after Wilson et al., 1990).

(Figure 58) Distribution of Jurassic strata in Cardigan Bay and the Bristol Channel (after Tappin et al., 1994).

(Figure 59)a Lower Jurassic sequence of the Cardiff area. (Figure 59)b Schematic cross-section through the Lower Jurassic, Vale of Glamorgan, not to scale (after Wilson et al., 1990).

(Figure 60) Jurassic strata: correlation of Mochras and offshore boreholes (adapted from Tappin et al., 1994).

(Figure 61) Mid Jurassic palaeogeography (after Penn and Evans, 1976; Tappin et al., 1994).

(Figure 62) Correlation of Cretaceous lithologies (after Tappin et al., 1994).

(Figure 63) Palaeogene strata in offshore basins (after Tappin et al., 1994).

(Figure 64) Depositional model for Cardigan Bay, early Palaeogene (after Tappin et al., 1994).

(Figure 65) Glaciation of Wales: flow direction of the Welsh and Irish Sea ice (compiled from various sources).

(Figure 66) Offshore Quaternary stratigraphy (after Tappin et al., 1994).

(Figure 67) Quaternary deposits exposed in cliff sections at Glanllynnau. Note: exaggerated vertical section (adapted from Young, Gibbons and McCarroll, 2002).

(Figure 68) Melt water channels in the vicinity of Fishguard (adapted from John, 1970).

(Figure 69) Superficial deposits south of the Dyfi estuary (adapted from Godwin, 1943). a Map of Borth Bog b Cross-section, Borth Bog and Dyfi estuary

(Figure 70) Licensed aggregate dredging locations in the Bristol Channel (after James et al., 2005; for current holdings see http://www.thecrownestate.co.uk).

Frontispiece This LandsatTM image of Wales and adjacent area is a winter mosaic (Band combination 4, 5, 7). The low angle of the sun shows the physical and some geological features. High ground with sparse vegetation cover is greenish blue in colour, forest is maroon/red and other vegetation is orange; towns and cities show as blue-grey.

(Plate 1) Snowdon massif viewed from the east, at Curig Hill across Llynnau Mymbyr to the Snowdon Horseshoe, Llewedd, Yr Wyddfa, Crib y Ddysgl and Crib Goch. (MFH) (P662385).

(Plate 2) North Pembrokeshire coast viewed westwards from Carn Llidi, across Whitesands Bay to Ramsey Island (C D R Evans) (P662386).

(Plate 3) Great Ormes Head, Llandudno. The Loggerheads Limestone Formation is exposed in the main cliff face with a capping of the Cefn Mawr Limestone and Red Wharf Limestone formations (MFH) (P662387).

(Plate 4)a Folded low-grade metamorphosed sandstone with interbedded silty mudstone, South Stack Formation, Holy Island, Anglesey (P662388).

(Plate 4)b Minor folds, South Stack Formation, Holy Island, Anglesey (MFH) (P662389).

(Plate 5) Gwna Group, mélange with large clasts of white quartzite in unbedded silty mudstone matrix, Trwyn Maen Melyn, Llŷn (P662390).

(Plate 6) Llyn Padarn, Llanberis. Pale crags to the left are acidic ash-flow tuffs of the Padarn Tuff Formation, which are overlain by sandstones, locally conglomeratic, and thin acid tuffs of the Fachwen Formation (P007267).

(Plate 7) Maiden’s Castle, north of Treffgarne. Denuded crags of flow banded, autobrecciated, locally nodular rhyolite (Precambrian) (P662391).

(Plate 8a) Penrhyn Quarry, Bethesda exploited the Llanberis Slates Formation. Nant Ffrancon and the high plateau of the Carneddau can be seen in the distance (P662392).

(Plate 8b) Llanberis Slates Formation: bedding is defined by reduction of iron in the green bands (P662393).

(Plate 9) Rhinog Mountains viewed west-south-west from Foel Boeth. Rhinog Fawr, on right, and Diffwys, in the centre, forming the western limb of the Harlech Dome. Y Garn, on left, apart from main ridge (P662394).

(Plate 10) Roman Steps, south of Cwm Bychan: the flags are turbiditic sandstones and siltstones from the Rhinog Grits Formation (MFH) (P662395).

(Plate 11) Porth Clais, viewed eastwards across steeply dipping coarse-grained, thickly bedded sandstones of the Caerfai Group (MFH) (P662396).

(Plate 12) Cwm Graianog, on the west side of the Nant Ffrancon. Ripple marked bedding plane of the Carnedd y Filiast Grit (Cambrian) which is overlain locally by the Graianog Sandstone (Ordovician, Arenig) across a slight angular disconformity (T P Crimes) (P662397).

(Plate 13a) Photomicrographs (x 10; crossed polarised) of cumulate textures in the Rhobell Volcanic Group (B P Kokelaar). a Pargasite–salite accumulate formed in a magma chamber at more than 30 km below the Earth’s surface. Crystallisation of these minerals from the parental basic magma caused the residual magma to evolve to andesite (P662398).

(Plate 13b) Photomicrographs (x 10; crossed polarised) of cumulate textures in the Rhobell Volcanic Group (B P Kokelaar). b Pargasite crystals, rimmed with actinolite, were brought up from depth and the interstitial melt cooled rapidly on eruption (P662399).

(Plate 13c) Photomicrographs (x 10; crossed polarised) of cumulate textures in the Rhobell Volcanic Group (B P Kokelaar). c Pargasite phenocryst with complex embayments, in fine-grained basalt. The crystal was partly dissolved in the magma before eruption (P662400).

(Plate 13d) Photomicrographs (x 10; crossed polarised) of cumulate textures in the Rhobell Volcanic Group (B P Kokelaar). D. Basalt containing a cluster of intergrown plagioclase (glomerocryst) in a groundmass of flow-aligned feldspar microcrysts (P662401).

(Plate 14) Unconformity at the base of the Arenig, Trwyn Llech y doll, St Tudwal’s peninsula, Llŷn. Arenig sandstones in the cliffs dip gently eastwards and rest on more steeply dipping siltstones of Middle Cambrian age (P662402 from Young et al., 2002).

(Plate 15) Cadair Idris scarp viewed across the Mawddach estuary. Middle to Upper Cambrian in the lowermost slopes pass up into the Ordovician Aran Volcanic Group in the ridges across Bryn Brith, to the left of centre. Acidic and basic volcanic rocks with intercalated sedimentary rocks and associated intrusions crop out in the main scarp (P662403 C D R Evans).

(Plate 16) Acid tuff with foliation accentuated by siliceous recrystallisation, Offrwm Volcanic Formation, Mynydd-y-gader (P662404).

(Plate 17) Bedded basic tuff overlain by coarse basaltic debris flow, Cefn Hir Member, Cregennen Formation, Pared y Cefn Hir (P662405).

(Plate 18) Manod Bach, north of Ffestiniog. A stock-like intrusion of quartz latite composition with extensive autobrecciation (MFH) (P662406).

(Plate 19) Layered gabbro exposed on Carn Llidi (C D R Evans;) (P662407).

(Plate 20a) Wave ripple marks in coarse-grained sandstones, Capel Curig Volcanic Formation, near Cwm Clorad Isaf, west of Capel Curig (P662408).

(Plate 20b) Siliceous nodules, acidic ash flow tuffs, Yr Arddu (P662409).

(Plate 21) Yr Wyddfa, summit of Snowdon from the Miner’s Track below Glaslyn. Acidic ash-flow tuffs of the Lower Rhyolitic Tuff Formation crop out in the foreground with basaltic volcaniclastic rocks and intrusions of the Bedded Pyroclastic Formation in the cliff face (P213255).

(Plate 22) Bedded Pyroclastic Formation, basaltic volcaniclastic sedimentary rocks resting on the reworked top of the Lower Rhyolitic Tuff Formation, Cwm Glas (MFH) (P662410).

(Plate 23) Clogwyn y Person, Upper Rhyolitic Tuff Formation and rhyolite intrusion, overlying Bedded Pyroclastic Formation (P662411).

(Plate 24) Parys Mountain, Amlwch, Anglesey, the main opencast excavation. Copper, lead and zinc have been worked as well as other rarer metals. There is evidence of Bronze Age mining at this site (MFH) (P662412).

(Plate 25) Yr Eifl, Llŷn, viewed from near Llithfaen to the south-west. On the left is the Caer Gribin granophyre and on the right the Garnfor microgranodiorite intrusions (MFH) (P662413).

(Plate 26) Cliffs south-west of Cemaes Head, Pembrokeshire. Folded and faulted, turbiditic sandstones and siltstones of the Dinas Island Formation (Caradoc) (P662414).

(Plate 27) On the hillside above Caban Coch Reservoir are two thick sequences of turbiditic conglomerate of the Caban Conglomerate Formation (P662415).

(Plate 28) Couplets of turbiditic sandstone and mudstone, Aberystwyth Grits Group (Llandovery), Tan y Bwlch, south of Aberystwyth (MFH) (P622416).

(Plate 29) Harp Rock (Craig y Delyn), Borth. The lowest of the three prominent turbiditic sandstones marks the base of the Aberystwyth Grits Group (Llandovery) (P662417).

(Plate 30) Three Chimneys, Marloes Bay. Prominent sandstone beds are separated by softer, weathered silty mudstone, near top of Skomer Volcanic Group (Llandovery) (MFH) (P662418).

(Plate 31) Disturbed bed in Elwy Group, Silurian. The base cuts down into ribbon-banded flags of the Nantglyn Flags Group. The nature of this bed and its base was a point of issue between O T Jones and P G H Boswell. Roadside quarry, Ty’n y ffordd, near Llangerniew, Denbighshire (MFH) (P662419).

(Plate 32) Sea cliffs in steeply inclined Moors Cliff Formation, Manorbier (MFH) (P662420).

(Plate 33) A south-verging fold couplet in the dominantly sandy Milford Haven Group (Old Red Sandstone), St Ann’s Head, Dale peninsula. (P662421).

(Plate 34) Plateau Beds Formation (Old Red Sandstone), Pen y Fan escarpment, Brecon Beacons (P662422).

(Plate 35) The Sugar Loaf viewed across the Usk valley from Govilon. The Sugar Loaf hill is capped by an outlier of Quartz Conglomerate (Upper Old Red Sandstone) with the Senni Beds below (P662423).

(Plate 36) Green Bridge of Wales: well bedded limestone with chert (Arundian) overlain by thick-bedded pale grey limestone (Holkerian) (MFH) (P662424).

(Plate 37) High Tor Limestone Formation dipping south, Three Cliffs Bay, Gower (MFH) (P662425).

(Plate 38) Carboniferous limestone (Leete, Loggerheads and Cefn Mawr limestones) overstep on to the lower Silurian strata in the foreground, Mynydd Eglwyseg (MFH) (P662426).

(Plate 39) Gently dipping Dinantian limestones overlain by Namurian sandstones, typical of the north crop, upper Tawe valley, west of Craig y Nos (P662427).

(Plate 40) Cliff section, east of Amroth, across Namurian – Westphalian boundary, G. subcrenatum Marine Band (MFH) (P662428).

(Plate 41) Henryd waterfall. G. subcrenatum Marine Band at the base of the Lower Coal Measures (P622429).

(Plate 42) Dunraven open cast site, excavation in Middle Coal Measures, in 1969 (P622430).

(Plate 43) Monoclinal fold disrupted by small thrust, Lower Coal Measures north of Broadhaven (P662431).

(Plate 44) Vale of Clwyd viewed from the south-east. The fault-defined eastern margin of the Vale of Clwyd separates Triassic and Carboniferous strata from the Silurian outcrop of the Clwydian Hills (MFH) (P662432).

(Plate 45) Fissure in High Tor Limestone (Dinantian) infilled with Triassic sediment, Ogmore by Sea (MFH) (P662433).

(Plate 46) Triassic blocky scree deposits resting unconformably on the High Tor Limestone (Dinantian), Ogmore by Sea (P662434).

(Plate 47) Cliff section and reefs of alternating limestone and mudstone of the Porthkerry Member (Jurassic) viewed south from Dunraven to Whitmore Stairs (MFH) (P622435).

(Plate 48) Palaeokarstic surface at the top of the Dinantian Dowlais Limestone overlain unconformably by pebbly grit at the base of the Namurian; the grit also infills large solution cavities in the limestones, Trefil Quarry (MFH) (P662436).

(Plate 49) Platform across truncated Dinantian limestones near St Govan’s Head, south Pembrokeshire (MFH) (P662437).

(Plate 50) Pass of Llanberis looking north-west. Classic U-form of a glaciated valley with ribbon lakes, Llyn Peris and Llyn Padarn in the distance and scattered perched blocks across the grassy surface in the foreground (P662438).

(Plate 51) Nant Francon Pass looking north from Y Gribin ridge. Llyn Idwal bordered by moraines lies to the left and Llyn Ogwen at the base of Pen yr Ole Wen to the right (P007200).

(Plate 52) Glacially scoured surface of Cambrian Bronllwyd Grits Formation exposed when the lake was drained during the construction of the Dinorwic Pump Storage scheme, Llyn Peris (P662439).

(Plate 53) Glaciofluvial sand and gravel, Pantgwyn Mawr Quarry, near Cardigan (P662440).

(Plate 54) Patterned ground, stone polygons periglacial features preserved at Foel Grach, Carneddau (P662441).

(Plate 55) Siâmbr Pentre Ifan, Preseli: Neolithic dolmen constructed with slabs of cleaved tuffaceous rock (Fishguard Volcanic Group) derived locally (MFH) (P662442).

(Table 1) Timescale of major geological events in Wales.

(Table 2) Cambrian chronostratigraphy and biostratigraphy used in Wales (and England).

(Table 3) Ordovician series.

(Table 4) Silurian stages, series and biozones.

(Table 5) Namurian stages and ammonoid biozones

(Table 6) Pleistocene chronostratigraphy (adapted from Campbell and Bowen, 1989).