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Geology of the South Wales Coalfield, Part II, the country around Abergavenny

By W. J. Barclay

Bibliographic reference: Barclay, W. J. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny, 3rd edition. Mem. Br. Geol. Surv, Sheet 232 (England and Wales).

Geology of the South Wales Coalfield, Part the country around Abergavenny Memoir for 1:50 000 geological sheet 232 (England and Wales)

• Author
• W. J. Barclay
• Contributors
• Stratigraphy and structure D. I. Jackson
• Palaeontology M. Mitchell B. Owens N. J. Riley D. E. White
• Petrography G. E. Strong
• Hydrogeology R. A. Monkhouse

British Geological Survey, Natural Environment Research Council

Third edition. London: Her Majesty's Stationery Office 1989. © Crown copyright 1989. First published 1900. Second edition. 1927 Third edition 1989. ISBN 0 11 884408 3. Printed in the United Kingdom for HMSO Dcl 240411 C20 3/89

• Author
• W. J. Barclay, BSc British Geological Survey, Keyworth
• Contributors
• D. I. Jackson, MA British Geological Survey, Edinburgh
• B. Owens, BSc, PhD, N. J. Riley, BSc, PhD, G. E. Strong, BSc, and D. E. White, MSc, PhD British Geological Survey, Keyworth
• R. A. Monkhouse, BSc British Geological Survey, Wallingford
• M. Mitchell, MA 11 Rider Gardens, Leeds Formerly British Geological Survey, Keyworth

Other publications of the Survey dealing with this district and adjoining districts

Books

• British Regional Geology
• South Wales (3rd Edition), 1970
• The Welsh Borderland (3rd Edition), 1971
• Memoirs
• Geology of the country around Monmouth and Chepstow, 1961
• Geology of the country around Newport (Mon.), 1969 Geology of the country around Pontypridd and Maestcg, 1964
• Sheet Explanation
• Geology of the country around Merthyr Tydfil, 1988
• Mineral Assessment Report
• The hard rock resources of the country around Caerphilly, 1984

Maps

• 1:625 000
• Great Britain (South Sheet) Solid Geology, 1979
• Quaternary Geology, 1977 Bouguer Anomaly, 1986
• Aeromagnetic Anomaly, 1965
• 1:250 000
• Bristol Channel Sheet (51°N 04°W)
• Solid geology, 1988
• Sea bed sediments and Quaternary geology, 1986
• Bouguer Gravity Anomaly, 1986
• Aeromagnetic Anomaly, 1980
• 1:125 000
• Hydrogeological map of South Wales, 1986
• 1:63 360
• Sheet 248 (Pontypridd), Solid, 1963 Sheet 249 (Newport), Drift, 1969
• 1:50 000
• Sheet 231 (Merthyr Tydfil), Solid and Drift editions, 1979
• Sheet 232 (Abergavenny), Solid and Drift, in press
• Sheet 233 (Monmouth), Solid and Drift, 1974
• Sheet 248 (Pontypridd), Drift, 1975
• Sheet 249 (Newport), Solid, 1975
• Sheet 250 (Chepstow), Solid and Drift, 1981
• 1:25 000
• Usk-Cwmbran, Solid and Drift, 1971

Preface to the Third Edition

The resurvey of the Abergavenny district was the final part of a major study, initiated in 1945, covering the main South Wales Coalfield. Mapping at the scales of six inches to one mile and 1:10 000 was carried out between 1968 and 1982 by Mr W J Barclay, Mr D I Jackson, Mr K Taylor, Dr L P Thomas and Dr H C Squirrell. The District Geologists were Dr J R Earp (until 1975), Dr D B Smith (until 1980) and Dr R A B Bazley.

This memoir has been written and compiled by Mr Barclay. Mr Jackson provided contributions to the Silurian, Quaternary and Structure chapters. Dr D E White identified the Silurian faunas and jointly wrote the Silurian chapter. Dr D E Butler assisted with the Devonian biostratigraphy. Mr M Mitchell identified the Dinantian macrofaunas and contributed to the Dinantian biostratigraphy. Dr N Riley was responsible for naming the Dinantian foraminifera and the Namurian and Westphalian macrofaunas. The Dinantian miospores were examined by Dr B Owens.

We are grateful to Dr P L Forey, Dr R S Miles and Ms S V T Young of the British Museum (Natural History) for identifying the Devonian vertebrate remains and to Prof. W G Chaloner of Royal Holloway and Bedford New College, University of London, for his work on the Devonian and Carboniferous floras. Prof. G D Sevastopulo of Trinity College, Dublin, reported on the Dinantian conodonts collected by Messrs M J Reynolds and J Pattison. Petrographical descriptions were given by Mr G E Strong. Mr R A Monkhouse wrote an account of the hydrogeology, and Mr B Vowles of British Coal provided a summary of the properties and uses of the coals of the district. The memoir was edited by Mr I C Burgess.

During work in the coalfield area, many officials of British Coal's Deep Mines Division and Opencast Executive gave their assistance, and we gratefully acknowledge their contribution. We are also indebted to the local authorities and consultants who allowed us to examine borehole cores, and to landowners and farmers for their willing co-operation.

F G Larminie OBE Director British Geological Survey Keyworth Nottingham 1 December 1988

Preface to the Second Edition

The exhaustion of the first edition of this memoir affords the opportunity of making some additions and corrections, rendered necessary by the progress of general geological research and of local coal-mining since the issue of the first edition in 1900.

Sir A Strahan and Dr W Gibson having now left the staff, such re-examination of the ground as was necessary for the purposes of this edition was made in 1925 by Dr T Robertson, under the general supervision of Mr T C Cantrill, who has also acted as editor of the present volume.

In the Silurian area a horizon corresponding to the Aymestry Limestone has been identified. The Upper Ludlow sequence established at Ludlow by Miss G L Elles and Miss I L Slater, and that near Llandeilo described by the Geological Survey, have given us a clearer view of the highest beds of that formation. Progress has been made by Mr W W King in the subdivision of the Old Red Sandstone of the Midlands, and his divisional lines can be broadly recognised in the country around Abergavenny. The Carboniferous Limestone of South Wales has been investigated by A Vaughan, Mr E E L Dixon, Dr T F Sibly, and others, and the application of their work to the present district has given conclusive evidence, on the north-east crop, of an overstep by the Millstone Grit on to the lowest zone of the Avonian sequence. The Millstone Grit has yielded a fauna that justifies its correlation with a part of the Millstone Grit of Yorkshire. Recent work by Messrs D Davies, A Howell, and Prof A H Cox on the flora and fauna of the Coal Measures of Glamorgan, promises to show itself applicable in Monmouthshire also. The development of the collieries has thrown much new light on the structure of this part of the coalfield, especially with reference to movements that took place during the deposition of the Coal Measures, and to the great fault-troughs that cross the district in a north-northwest direction.

Attention from the geological side has been given to the mineral products of the district, and the chapter relating to this subject has been greatly enlarged.

Mr J Pringle has advised on all the palaeontological questions involved, and has named the fossils collected during the revision; and Mr E E L Dixon has read the chapter on the Carboniferous Limestone. The photographs, of which a list is given in Appendix II, were taken by Mr J Rhodes.

Reference to the mining firms will be found in Chapter VI. They have one and all shown the utmost readiness to assist in the collection of additional information, and without the help of their agents and surveyors the account of the Coal Measures would have been inadequate.

The published six-inch maps included in the list on p.x have received some revisions, which in due course will be incorporated in the one-inch map.

John S Flett Director Geological Survey Office 28 Jermyn Street London, SW1 28 October, 1926

Preface to the First Edition

The first part of the Descriptive Memoir on the Geology of the South Wales Coalfield, embracing the country around Newport, was published last autumn. In the Preface to it some account was given of the history of the mapping of South Wales by the Geological Survey. The present publication forms the second part of the same Memoir. It describes the extreme north-eastern corner of the coal-producing area, and includes also a considerable portion of the great tract of Old Red Sandstone which extends over the counties of Hereford, Brecknock, and Monmouth. It has been written mainly by Mr A Strahan, who surveyed the south-eastern part of the map (Sheet 232) and supervised the mapping of the rest of the ground. His colleagues who were associated with him in the field-work have supplied him with descriptions of parts of the districts mapped by them. Thus, Mr W Gibson, among other contributions, has furnished an account of the important mining district which extends from Blaenavon to Rhymney, while Mr Dakyns' notes describe the evidence on which his mapping of the Carboniferous Limestone and Old Red Sandstone of the northern part of the map was founded.

The subdivision of the strata which was adopted for Part I has been continued through the area described in the following chapters. The Old Red Sandstone includes two main subdivisions, the lower consisting chiefly of marls with their 'cornstones', and the upper of red sandstones and conglomerates in which 'cornstones' are less common. The conglomerates, however, so abound in the upper part of the sandstone-subdivision that they have been separately distinguished on the map by colours, though it is doubtful whether this distinction will be practicable in the region lying further west.

This classification agrees in the main with that adopted by the Rev. W S Symonds in his admirable work the 'Records of the Rocks.' To the same observer also we are indebted for much of our information concerning the fossils by which it is proved that this part of the kingdom contains representations of both the Upper and Lower Old Red Sandstone, though their existence had already been demonstrated by Murchison. The great unconformability, however, which separates these groups in Scotland and Ireland has not been detected in Monmouthshire and South Wales, insomuch that it is even a matter of doubt where to draw a line between them.

The Carboniferous Limestone, as developed in this part of South Wales, displays the phenomenon of dolomitisation with unusual clearness. By collecting specimens at definite intervals, every stage in the alteration of an oolitic limestone into a crystalline dolomite has been illustrated. The examination of thin sections of these rocks under the microscope was one of the last of the many important investigations carried out by Prof. W W Watts while engaged on the staff of the Geological Survey.

The Millstone Grit has long been known from the researches of Mr W Adams and Dr G P Bevan to yield an interesting marine fauna. The partial survival of marine forms in the lower part of the Coal Measures, and their gradual replacement by estuarine forms, throws an interesting light on the change of conditions which led to the deposition of the coal-bearing strata in this part of the country.

The Coal Measures, in the lower part of wfiich the argillaceous ironstones so much in request in the early part of the century abound, lie at a gentle angle, and are accessible over broad outcrops. The ores were readily worked, therefore, in open-air excavations or 'patch-works', the great size of which bears witness to the former importance of the industry. Though these local ores are no longer worked, a short account of them is given in the following pages, in view of the possibility of their being again used at some future date. The coals, on the other hand, are more extensively worked now than formerly, and are being followed southwards under the deeper parts of the basin. The northern margin of the Pennant Grit forms one of the most conspicuous physical features of the district, and in its deep indentations contrasts strongly with the escarpment formed by the more highly inclined strata of the south crop.

Two editions of the map are issued. One of these represents the older formations; the other shows also the Superficial Deposits (Drift Edition). The distribution of these drifts indicates that the principal water-partings of the present day formed ice-partings in the Glacial Period, but the fact that the watersheds have been overridden by the ice in places points to the extensive scale of the glaciation. Small but interesting moraines in the deeper cwms remain as relics of the latest glaciers.

Copies of the original field-maps, on the scale of six inches to a mile, are deposited in the Geological Survey Office here, where they can be consulted, or whence MS copies can be supplied if desired.

The principal shaft-sections in the area described in the present instalment of the Memoir have been drawn to one scale and arranged to form Sheet 81 of the Vertical Sections, published separately by the Geological Survey.

We have again to thank Colliery Managers, Mining Engineers, and Surveyors for free access to plans and for their unvarying courtesy in placing at our disposal their great store of mining information.

Arch. Geikie Director General Geological Survey Office 28 Jermyn Street, London 16 June, 1900

Six-inch or 1:10000 maps

The National Grid six-inch to one-mile and 1:10 000 maps included wholly or in part in the Abergavenny sheet are listed below, together with the initials of the surveyors and the dates of survey. The officers involved were W. J. Barclay, R A. Downing, D. I. Jackson, H. C. Squirrel], K. Taylor, L. P. Thomas and A. W. Woodland. Those sheets marked* are only partly surveyed; only the areas within the Abergavenny sheet of these marked‡ are currently available on National Grid maps, the remaining areas (within the Monmouth sheet) being on Six-Inch County maps. Those sheets marked/ are lithoprinted published maps, the others are available as dye-line maps, or photographic copies of manuscript maps.

Notes

• In this memoir the word 'district' refers to the area included in 1:50 000 geological sheet 232 (Abergavenny).
• Figures in square brackets are National Grid references within 100 km square SO.
• Numbers preceded by the letter A refer to the Survey's collection of photographs; those preceded by the letters MR refer to specimens in the Survey's rock collection, and those preceded by E to the sliced rock collection; the letters MPA and SAD refer to micropaleontological samples in the Survey's Biostratigraphy Research Group collection.
• The authorship of fossil species is given in the fossil indexes.

Country around Abergavenny—summary of geology

The district described in this memoir includes the north-east part of the South Wales Coalfield. The market town and tourist centre of Abergavenny lies to the north-east, and occupies a strategic position in the Welsh Borderland, backed by the mountains of upland Wales and facing the lowlands of Gwent.

The exposed rocks range in age from Wenlock to late Westphalian, and have, since Sir Roderick Murchison's visit in the early 1830's, attracted much geological interest. The Wenlock and Ludlow rocks, seen in the northern part of the Usk Inlier, were shallow marine sediments. The later Silurian and Devonian rocks are of continental Old Red Sandstone facies. The Lower Devonian St Maughans Formation is well known for its early vertebrate remains, and the succeeding Senni beds for their primitive vascular plant fossils.

Throughout Carboniferous time the district was situated on the periphery of a depositional basin, with the Wales–Brabant massif to the north and the Usk Axis in the east. Facies changes, condensed and attenuated sequences, depositional hiatuses and unconformities are the results of varying tectonic control on subsidence during the basin's evolution. The Dinantian shelf sequence is excellently exposed in the Clydach and north crop areas, and has proved fertile ground for sedimentological studies in recent years. The Namurian rocks are poorly exposed, but boreholes have provided new information on their attenuation. The coals of the Westphalian Coal Measures have been intensively exploited since the publication of the previous edition of this memoir, by both deep and shallow mining. Opencast workings and many site investigation boreholes have also given much new data.

Valley glaciers in the late Devensian were responsible for most of the glacial and fluvioglacial deposits of the district. The terminus of the Usk glacier was partly in the south-east, and the Usk Valley contains a variety of end-glacial and recessional morainic features and deposits. The landslips of the coalfield valleys, mostly initiated during deglaciation, continue to present engineering difficulties today.

The Gavenny is a small misfit stream occupying a broad valley that was a pre-glacial course of the Afon Honddu. The hills to the east (Skirrid Fawr) and west (Deri) are in Lower Devonian Senni Beds; a prominent landslip is visible on the western flank of Skirrid Fawr. Fluvioglacial gravels occupy the Usk Valley. (A13438)

(P945511) Geological sequence.

Chapter 1 Introduction

General

This memoir describes the geology of the district covered by the Abergavenny (232) Sheet of the 1:50 000 Geological Map of England and Wales (Figure 1). The district lies mostly in the county of Gwent, with the north-west part in Powys and the south-west in Mid Glamorgan. Much of the northern part lies within the Brecon Beacons National Park.

(Figure 2) shows the topography and principal settlements of the district. The market town and tourist centre of Abergavenny originated as the Roman station Gobannium, and was later a Norman settlement, during which period the existing castle was built. The other main population centres are the towns of the coalfield valleys–Rhymney, New Tredegar, Tredegar, Ebbw Vale, Brynmawr, Blaina, Abertillery, Blaenavon, and Abersychan.

The district can be divided into three areas of greatly contrasting scenery, illustrating well the close relationship between geology and physiography–the coalfield, the Black Mountains and the Vale of Gwent. The north-east part of the South Wales Coalfield forms about 60 per cent of the district. It consists of a central plateau region of resistant Pennant sandstones of the Upper Coal Measures, around which are narrow peripheral belts of high moorland underlain by Lower Coal Measures, Millstone Grit, and Carboniferous Limestone. The Pennant sandstone plateau reaches a maximum height of 581 m above OD near Blaenavon, and is deeply dissected by the south-east-trending valleys of the Bargoed Taff, Bargoed Rhymney, Sirhowy, Ebbw Fach, Ebbw Fawr, and Afon Lwyd. The Black Mountains area in the north has an attractive upland scenery made by the sandstone-dominated formations of the Lower Devonian, the Sugar Loaf being the highest hill within the district at 596 m. The Vale of Gwent in the east is underlain by the softer, more argillaceous formations of the Lower Devonian on which rest the glacial and fluvioglacial deposits of the Usk Valley. Within the Vale of Gwent, the more resistant Wenlock and Ludlow series rocks of the Usk Inlier form a higher tract in the extreme south-east of the district.

Although ironworks had been established in the Clydach gorge around 1600, it was the advent of the Industrial Revolution in the mid-eighteenth century which produced the most dramatic change in the district. A series of ironworks was set up around the outcrop of the Coal Measures from Merthyr Tydfil to Pontypool, using local ironstone, coal, and limestone. This area was then the industrial heart of the region. Iron making started to decline in the mid-nineteenth century, and this period saw the major expansion of coal mining to meet world demand for steam coal. The output from deep mining in South Wales reached a peak prior to the first world war since when there has been a steady decline. This decline has accelerated in recent years, and only five collieries remain in 1987 (see p.119). The period between 1970 and 1985 has, however, been one of increased opencast activity and a string of profitable sites has been worked round the outcrop of the Coal Measures, with some reserves remaining to be exploited. In the last few years there has been an increase in the number of small private licensed mines, and about twenty are working in 1987.

Three limestone quarries were in operation during the resurvey, mainly producing crushed rock aggregate, but all have since been abandoned (p.119). With the cessation of steel making at Ebbw Vale, the district has no heavy industry, but has attracted light manufacturing industry, and most of the traditional centres of employment have new industrial estates. The Vale of Gwent is a fertile agricultural area supporting mixed, arable, and fruit farming.

Geological sequence

The sequence represented at outcrop is as shown on the inside front cover (P945511). (Figure 3) shows the general geology of the district.

Outline of geological history

The Wenlock rocks of the Usk Inlier are the oldest exposed. These and the overlying Ludlow sequence were the last deposits of the Welsh Lower Palaeozoic basin, and were laid down on a shallow muddy marine shelf near its south-east margins. Limestones in the sequence record- periods when elastic input was reduced. The basin was filled by late Silurian times when the onset of continental collision produced the Caledonian mountain belt. The Old Red Sandstone rocks were deposited on the southern margin of this belt in an external basin. The Lower Old Red Sandstone rocks are almost entirely continental, fluvial deposits, and coarsen upwards from the mudstone-dominated Raglan Mudstone to the sandstone-dominated Brownstones. Pedogenic carbonates (calcretes) indicate an arid to semiarid tropical climate. The Devonian period saw the main invasion and colonisation of the Caledonian continent by early vascular plants and by the first fish, the armoured jawless ostracoderms. The St Maughans Formation contains abundant fragmented remains of the ostracoderms, and the Senni Beds are well known for their early vascular plants.

The final phases of the Caledonian orogeny occurred in the Middle Devonian when the area was uplifted and eroded. Sedimentation resumed in the Upper Devonian with the Plateau Beds. These represent a variety of depositional settings from marginal marine to continental. Following minor uplift and erosion, the Quartz Conglomerate Group was deposited by streams flowing south from the Caledonian massif.

Major transgression from the south in the early Carboniferous over the Old Red Sandstone coastal plain produced a shallow carbonate shelf on which the Dinantian Carboniferous Limestone was deposited. During much of this period the district, situated on the southern flanks of St George's Land, was marginal to a major subsiding trough to the south. The rocks are almost all of shallow water near-shore facies, and contain several breaks that are commonly associated with emergent features such as palaeokarstic surfaces and palaeosols. The succession represents a period when both tectonic and eustatic controls were operating.

Uplift of the Usk Axis and erosion took place from the late Dinantian to the late Namurian, and is indicated by the unconformity that separates the Dinantian rocks from the Namurian ones. The Dowlais Limestone, of Holkerian age, is the youngest Dinantian formation preserved in the district, but progressive Namurian overstep towards the Usk Axis resulted in all the Dinantian rocks younger than Courceyan being removed in the extreme east.

The Namurian Millstone Grit was deposited on the northeast margin of the South Wales basin in a fluviodeltaic complex that prograded southwards from St George's Land and the Usk Axis.

Similar paralic conditions prevailed throughout the deposition of the Lower and Middle Coal Measures in Westphalian A to Westphalian C times, when the South Wales basin was one of a series that covered the equatorial region of the world. These rocks contain the principal coals, representing peat accumulation in swamps that periodically covered the entire basin. Inter-seam mudstones, siltstones and sandstones were deposited mainly in lakes and channels on a delta plain. Eustatic marine transgression took place across the delta plain on ten known occasions and produced shallow pro-delta marine conditions in which the Coal Measures marine bands were deposited.

The Upper Coal Measures (Westphalian C to Westphalian D) have, near their base, a distinctive suite of red beds, the 'Deri Beds', formed by in-situ oxidation under tropical weathering conditions. The Pennant sandstones that lie above heralded a major palaeogeographical change in the region during which the northward-advancing Variscan mountain belt shed alluvial detritus into a rapidly subsiding trough. Pennant sedimentation was fully established in the district after deposition of the Brithdir Coal, and the sequence consists mainly of immature fluvial greywacke sandstones, with some thin, but laterally persistent inter-distributary mudstone beds. The final phase of the Variscan orogeny took place in late Carboniferous to Permian times when the rocks were uplifted, deformed and eroded.

The known geological history from the Wenlock to the late Carboniferous spans about 140 million years; of the 280 million years since then to the Pleistocene no deposits remain, apart perhaps from the infillings of some swallow holes on the outcrop of the Namurian sandstones which may be the product of Tertiary tropical weathering. Marine planation and pulsed uplift during the Alpine orogeny were probably responsible for the topography on which the ancestral drainage system was imposed, with subsequent modification by tilting and river capture (Clarke, 1936).

By the start of the Quaternary era the broad physiography of the district was established. During the glaciations of the Pleistocene the valleys were deepened and some of the rivers locally blocked and diverted. A range of glacial and fluvioglacial deposits were left behind when the glaciers retreated, most of which belong to the final (late Devensian) glaciation. Remanie patches of drift east of the Afon Lwyd Valley and erratics on the coalfield plateau probably belong to an earlier one. Solifluction and landslip deposits accumulated under periglacial conditions on the valley sides, and these processes continue today under severe weather conditions. The 10 000 years since the Devensian have seen only minor geological change. Flandrian sea level rise during deglaciation led to the deposition of alluvial spreads in the valley bottoms.

Chapter 2 Silurian

Rocks of the Wenlock and Ludlow series crop out in the south-eastern part of the district, forming the northern part of the Usk Inlier, a north-trending pericline, fault-bounded on its north-western margin. The Pfidolf Series, considered to be represented by most of the Raglan Mudstone Formation of the Old Red Sandstone, crops out north and northwest of the inlier. As the Silurian–Devonian boundary cannot be recognised at present within the Raglan Mudstone, for convenience, the whole of this formation, including the Psammosteus Limestone, is described in this chapter. (Figure 4) gives a generalised section of the sequence.

The Wenlock and Ludlow rocks are predominantly mudstones and siltstones, calcareous to varying degrees; limestones and fine- to coarse-grained sandstones also occur. Thin bentonite layers throughout the sequence provide evidence for intermittent volcanic activity. The sediments were deposits of the Welsh Lower Palaeozoic Basin, laid down on a shallow muddy shelf to the south-east of the main basin on the flanks of the Midlands Microcraton (Allen, 1985). The gradual infilling and consequent shallowing of this basin through Ludlow times culminated in the establishment of a marginal marine to distal alluvial plain environment, in which red mudstones of the Raglan Mudstone (Prídolí Series) accumulated.

Previous research

Early work on the Silurian rocks by Murchison (1839), De La Beche (1846), Phillips (1848) and Gardiner (1917, 1927) was reviewed by Walmsley (1959) and later by Squirrell and Downing (1969). Previous editions of this memoir (Strahan and Gibson, 1900; Robertson, 1927) gave brief descriptions of the rocks.

Gardiner (1917, 1927) believed that the Lower Ludlow Beds and Aymestry Limestone were absent, and that the Upper Ludlow Beds were overlain unconformably by the Old Red Sandstone. However, Robertson (1927) showed that the Ludlow succession is complete and in conformity with the overlying Old Red Sandstone. These observations were confirmed by Walmsley (1959), who established the lithological and faunal successions, and related them to the Ludlow type area. He also elucidated the detailed structure and named the major faults.

The southern half of the Usk Inlier, within the 1:50 000 Newport (249) Sheet, was described by Squirrell and Downing (1969). For the Ludlow Series, they used the combined lithostratigraphical and biostratigraphical units which had been established by Holland, Lawson and Walmsley (1963) for the type area of the Ludlow Series, around Ludlow, Shropshire, rather than Walmsley's local lithostratigraphical classification. Walmsley's terminology is used in this account.

More recently, detailed studies of the diverse brachiopod assemblages of the Wenlock and Ludlow series of the Usk Inlier have formed part of regional investigations by Calef and Hancock (1974), Hurst (1975), Bassett (1976) and Watkins (1979).

From a study of brachiopod ecology, Hurst (1975) concluded that the Wenlock–Ludlow series boundary represents a single eustatic transgression throughout the Welsh Borderland. He considered that, in the Usk Inlier, this event initiated deposition of the Wenlock Limestone, which was thus the basal formation of the Ludlow Series, and which was renamed the Usk Limestone. Bassett (1976) showed that there was no evidence for major deepening at the base of the limestone, and placed the base of the Ludlow Series at its top, although firm biostratigraphical evidence is lacking to pinpoint the junction precisely.

The lithostratigraphical classification used in this account, its relationship to the previous one, and its correlation with the standard successions of Much Wenlock and Ludlow are shown in (Table 1).

Wenlock Series

The Wenlock Shale of previous accounts is here subdivided into two formations. The name Glascoed Mudstone Formation is proposed for the calcareous mudstones that crop out in the axial region of the inlier. The sandy and silty beds conformably overlying the Glascoed Mudstone, are here named the Ton Siltstone Formation. A thin limestone lying near the top of the Ton Siltstone is named the Trostrey Limestone Member (p.18 in Institute of Geological Sciences, 1979). Recognition of this bed, which has previously been miscorrelated with the Wenlock Limestone (Walmsley, 1959), explains the anomalous thickness variations quoted for the Wenlock Limestone. We retain the name Usk Limestone (Hurst, 1975) for the Wenlock Limestone, but follow Bassett (1976) in regarding it as the uppermost unit of the Wenlock Series.

Monograptus flemingii flemingii, which has a long stratigraphical range, is the only graptolite recorded from the Wenlock rocks (Bassett, 1974). Consequently, it is uncertain whether the Sheinwoodian Stage is represented at outcrop, or whether the exposed beds belong exclusively to the Homerian Stage.

The Wenlock sediments and faunas record a period of overall shallowing, from deeper water offshore conditions during deposition of the Glascoed Mudstone to shallow-water, near-shore and possibly lagoonal, clastic and carbonate deposition of the Ton Siltstone and Usk Limestone, respectively.

Glascoed Mudstone Formation

The Glascoed Mudstone Formation (new name) is probably equivalent to the upper part of the Coalbrookdale Formation of the Wenlock type area around Much Wenlock, Shropshire, and includes the oldest beds exposed in the inlier. It was formerly included in the Wenlock Shale (Strahan and Gibson, 1900; Robertson, 1927; Walmsley, 1959). The formation occupies the low-lying, largely drift-covered ground in the central axial portion of the inlier, where about 220 m are estimated to crop out, but the base is not exposed.

The rocks consist mainly of silty calcareous mudstones and siltstones, grey where fresh and weathering grey-green to khaki. Limestone nodules are present throughout, and thin bentonitic clays are also a characteristic feature. Bedding plane landslips associated with these clay layers are found where the River Usk has cut into the formation, for example at Graig y Pandy [SO 343 041], Craig y Garcyd [SO 357 027] and near Coed Cox [SO 3415 0213].

The beds are generally very fossiliferous, with a diverse shelly fauna predominantly of brachiopods, including Antirhynchonella linguifera, Baturria edgelliana, Coolinia pecten, Cyrtia exporrecta, Megastrophia (Protomegastrophia) semiglobosa, Meristina obtusa, Pentlandina cf. lewisii plakodis, Resserella canalis, Skenidioides lewisii, Striispirifer plicatellus and Y gerodiscus aff. cornutus. The large solitary coral Ketophyllum cf. turbinatum verrucosum is locally common, and trilobites recorded include Cyphoproetus depresses, Dalmanites spp. and Encrinurus cf. tuberculatus. Molluscs are not common and, as mentioned above, only one long-ranging species of graptolite, M. flemingii flemingii, has been found (Bassett, 1974). The abundant diverse benthic fauna suggests a mid-shelf, shallow marine environment.

Type section: the designated type section is at the Royal Ordnance Factory, Glascoed, 3 km north-west of Usk, where the beds are exposed in a railway cutting [SO 3393 0210] to [SO 3405 0202] near Bryn Farm (see p.10). Walmsley (1982, p.5) gave details of the section. The beds contain a fauna in which Cyrtia exporrecta and Megastrophia (Protomegastrophia) semiglobosa are very common, and Atrypa reticularis, Leptaena depressa, Meristina obtusa and Eoplectodonta duvalii are common.

Ton Siltstone Formation

The Ton Siltstone Formation (new name) consists dominantly of sandy beds and lies at the top of the Wenlock Shale of previous authors. It crops out in a narrow strip around the large diamond-shaped outcrop of the Glascoed Mudstone and below the scarp formed by the Usk Limestone. Much of the outcrop is drift-covered, the best exposures being around Ton, Trostrey and Rhadyr.

The formation is about 100 m thick, and mainly comprises silty sandstones with interbedded siltstones and silty mudstones. The sandstones are coarse grained, green (commonly decalcified to buff and brown) and micaceous. They are mainly poorly bedded, but both parallel-lamination and cross-lamination are found. The siltstones and mudstones are micaceous and predominantly olive-green in colour. The sandstones cap poorly defined, coarsening-upwards cycles and have rippled surfaces. Their thickness increases in progressively younger cycles.

Lying about 15 to 20 m below the top of the formation is the nodular Trostrey Limestone Member (new name), which ranges from 2 to 8 m in thickness.

Generally the most common fossils are the brachiopods Craniops implicates, Salopina conservatrix and a rhynchonelloid closely comparable with Microsphaeridiorhynchus nucula. These are associated with crinoid columnals, commonly attaining a diameter of 7 mm, tentaculitids, bivalves, including species of Grammysia, Nuculites and Ptychopteria, the trilobite Homalonotus sp.and ostracods including Aechmina sp.Bioturbation is widespread, and concentrations of small, black (?carbonaceous, ?algal) amorphous patches are common.

In the more calcareous beds, especially the Trostrey Limestone Member and the basal transitional beds of the formation, a less restricted brachiopod fauna is locally present. This includes Amphistrophia funiculata, Atrypa reticularis, Eospirifer radiates, Meristina obtusa, Resserella canalis and Strophonella euglypha. A 5 cm crinoidal limestone, 4.25 m below the Trostrey Limestone Member, has yielded an assemblage of conodonts indicative of the Sagitta (conodont) Zone, which is known to span the two uppermost graptolite zones of the Wenlock Series and the lowermost of the Ludlow Series (Austin and Bassett, 1967) (see p.11).

The Ton Siltstone Formation represents a late Wenlock regressive event, evidence for which is also found in the neighbouring Cardiff district (Waters and Lawrence, 1987) and elsewhere in South Wales (Bassett, 1974; Hurst and others, 1978). The sediments, with a source area lying close to the south (Bassett, 1974), probably accumulated in shallow water, close to shore. The beds that contain a very sparse bivalve-ostracod fauna may indicate temporary brackish-water or lagoonal conditions.

Type sections: the designated type section of the Ton Siltstone Formation is near the hamlet of Ton, 3.5 km northwest of Usk, where two sections [SO 3294 0247] to [SO 3304 0251] and [SO 3320 0252] to [SO 3324 0255] in the upper part of the formation are exposed in a lane. Details of these sections are given on p.12, and the fauna collected from the lower (Locality 17) is listed in Appendix 3.

The designated type section of the Trostrey Limestone is at Trostrey, 3.5 km north-north-west of Usk, in the cliff of the River Usk [SO 3596 0413] (Locality 13). There, it consists of 9.7 m of silty calcareous mudstones with abundant limestone nodules. Details of the section are given on p.11, and the fauna collected is listed in Appendix 3.

Usk Limestone Formation

This formation, previously called the Wenlock Limestone, was named the Usk Limestone by Hurst (1975). It is about 12 to 14 m thick and consists of nodular to lenticular bedded grey and green-grey shelly and crinoidal grainstones with interbedded green calcareous mudstones. It was formerly extensively worked for lime, both in quarries and underground.

Brachiopods are the major component of the fauna. These include Amphistrophia funiculata, Atrypa reticularis, Dalejina hybrida, Gypidula galeata, Isorthis cli vosa, Leptostrophia filosa, Pholidostrophia (Mesopholidostrophia) deflecta, Sphaerirhynchia wilsoni and Strophonella euglypha. Corals (Acanthohalysites sp., Favosites hisingeri), bivalves (Mytilarca mytilimerus, Ptychopteria sp.), trilobites (Dalmanites sp., Encrinurus variolaris) and crinoid columnals have also been recorded.

The Usk Limestone is close to the southern limit of limestone development at the Wenlock–Ludlow series boundary, and there is no limestone at this level in the Rumney Inlier of the Cardiff district to the south. Its thickness is half that of the Much Wenlock Limestone of the Wenlock Edge, Shropshire type area. Scoffin (1971) deduced a maximum water depth of 30 m for the sea in the region of carbonate deposition.

Ludlow Series

The classification used in this account and its correlation with the Ludlow succession are shown in (Table 1).

Graptolites are rare in the Ludlow rocks, but the junction between the Gorstian and Ludfordian stages can be recognised at the base of the Upper Llanbadoc Beds on the basis of the shelly assemblages.

A fairly rapid change to deeper-water conditions occurred at the Wenlock–Ludlow boundary. This was followed by shallowing, with the succession of lithologies and faunas of the Ludlow strata indicating a gradual change from distal to proximal shelf environments.

Lower Forest Beds

These crop out in four discrete areas and occupy poorly exposed low-lying ground. About 155 m of beds consist mainly of thinly bedded silty mudstones, but with some impure limestones and calcareous mudstones and layers of limestone nodules. The mudstones are characteristically pale green to pale grey-green, locally almost turquoise. Carbonate beds become increasingly common towards the base of the formation. The junction with the underlying Usk Limestone is placed where the nodular limestone facies of the latter is succeeded by the mudstone dominated facies of the Lower Forest Beds in which the limestones are about 10 cm thick, slightly sandy and with sharp tops and bottoms.

In the transitional calcareous beds at the base of the formation many of the large brachiopods already recorded in the Usk Limestone are represented, such as Amphistrophia funiculata, Atrypa reticularis, Gypidula galeata, Leptaena cf. depressa, Leptostrophia filosa and Strophonella euglypha. Higher beds are usually sparsely fossiliferous (Walmsley, 1959, p.490), but concentrations of shells occur at intervals throughout the sequence, characterised by small brachiopods, especially Aegiria grayi, Craniops implicatus, Dalejina hybrida, Dayia navicula, Dicoelosia biloba, Ludfordina pixis, Protochonetes minimus, Shagamella minor and Skenidioides lewisii. The bivalves Colpomya consors, Cypricardinia subplanulata, Mytilarca sp., Nuculites pseudodeltoideus and 'Paracyclas' perovalis and trilobites, including species of Calymene and Dalmanites, are also well represented.

Early Gorstian examples of D. navicula, which is more characteristic of the early Ludfordian, have also been recorded in the south of the inlier (Squirrell and Downing, 1969, p.20), at Woolhope (Squirrell and Tucker, 1960, p.144, fig.1), and to the north of Ludlow (Shirley, 1939, p.360). Their presence may be a persistent feature along the depositional strike of these beds and their lateral strati-graphical equivalents.

The Lower Forest Beds are considered to have been deposited in a distal shelf environment in relatively deep water, although the presence of shelly layers suggests that the sediments were disturbed at intervals by storm activity. Deposition was mainly from suspension, and the sediments were intensely bioturbated, probably by trilobites and soft-bodied burrowers (Watkins, 1979).

Upper Forest Beds

These beds generally crop out in the scarp feature overlooking the lower ground occupied by the Lower Forest Beds, forming either the whole of the feature or the lower part. They are best seen east of the Llanbadoc Fault, from Coed Adam to Llancayo (p.14). Their thickness ranges from 15 m in the west to 40 m in the east. The beds comprise dark green silty mudstones, commonly calcareous, with limestone layers and nodules up to 15 cm thick. There is a gradation at the base spanning about 15 m from the more thinly bedded, less silty, paler mudstones of the Lower Forest Beds. In the west, where exposure is poor, the formation is slightly finer grained and more thinly bedded, and is inseparable from the overlying Lower Llanbadoc Beds.

The fauna is characterised by the return of large brachiopods, including Amphistrophia, funiculata, Atrypa reticularis, Eospirifer radiatus, Leptostrophia filosa, Pholidostrophia (Mesopholidostrophia) lepisma, Sphaerirhynchia wilsoni and Strophonella euglypha.

The distal shelf depositional environment of the Lower Forest Beds probably persisted during accumulation of the Upper Forest Beds, with the presence of shell layers at the bases of siltstones again indicating the influence of traction currents which were possibly storm-generated.

Lower Llanbadoc Beds

The outcrop of this formation closely follows that of the Upper Forest Beds and, in the east, generally forms the highest parts of the fault-broken wooded scarp from Clytha Hill to Llancayo. In the west, where the formation is thinner, the feature is subdued, and although they locally form short wooded escarpments the beds mainly crop out in the intermediate ground between the low-lying outcrop of the Upper Forest Beds and the feature-forming siltstones of the Llangibby Beds. Exposure is generally poor despite the fact that the beds were once dug for lime.

The beds show greater east–west facies variation than any of the other Ludlovian formations. In the east they consist of 35 m of medium and dark green calcareous silty mudstones, rubbly calcareous siltstone and impure nodular limestones interbedded in approximately equal proportions. The argillaceous beds are commonly decalcified at outcrop, and have a brown weathering colour. In the east the base of the formation is placed where the proportion of limestone nodules and layers increases sharply to about 40 per cent of the succession. In the west of the inlier the beds are reduced to about 15 m in thickness and consist mainly of interbedded calcareous silty mudstones and rubbly nodular siltstones with some impure limestones, which cannot be mapped separately from the underlying Upper Forest Beds.

The beds contain a typical late Gorstian fauna, closely comparable with that of the underlying Upper Forest Beds.

A record of Kirkidium (see p.14) is believed to be the first from the Usk Inlier. Shell banks of Kirkidium knightii are characteristic of the Aymestry Limestone in its type area in the Welsh Borderland.

The mixed carbonate-elastic sequence is interpreted as having been deposited in a sheltered, low-energy environment on the inner shelf, protected by carbonate barriers and shell banks situated on the shelf edge.

Upper Llanbadoc Beds

Although the outcrop of this formation is largely drift-free, the beds are poorly exposed. They occupy a variety of topographic settings, including the top of the scarp feature between the Clytha Fault and Trostrey Hill.

Thickness ranges from 30 m in the west to 20 m in the east and lithology appears to be constant across the inlier. Well bedded yellow-green siltstones predominate and are interbedded with silty mudstones and thin coquinal limestone layers and nodules, the latter being particularly concentrated towards the base of the formation. Small landslips around the outcrop of the formation on Trostrey Hill may be caused by thin bentonite layers.

The base of the formation is placed at the sharp change where the dark green nodular-bedded mudstones of the Lower Llanbadoc Beds are overlain by well bedded yellow-green siltstones and silty mudstones. Although largely unexposed the junction is marked by a change in soil from dull olive-green clayey silt below to yellow to khaki sticky clay above. This is well illustrated in a disused cart track north of Coed y Bwnydd [SO 3662 0711]. Near Llandegfedd, on the Newport (249) Sheet, Walmsley (1959, p.493) described a conglomerate at the base of the formation. No exposures of the conglomerate were seen during the resurvey in the Abergavenny district, but a single loose block of a distinctive conglomerate containing black phosphatic pebbles and Lingula was found [SO 3706 0484]. The bed is probably lenticular and confined to scours in an erosion surface marking a local disconformity. Elsewhere in the Welsh Borderland, bored limestone conglomerates are widespread at this level, and have been interpreted as hardgrounds formed during periods of non-deposition (Cherns, 1980).

The sharp lithological change at the junction between the Lower and Upper Llanbadoc beds resulted from increased terriginous supply, and coincides with an equally sharp faunal change, marking the boundary between the Gorstian and Ludfordian stages. There is no evidence for the establishment of deeper-water conditions (Watkins, 1979, p.193). The late Gorstian fauna of the Upper Forest Beds and Lower Llanbadoc Beds, characterised by the common occurrence of large strophomenids, is replaced at the base of the Upper Llanbadoc Beds by an early Ludfordian fauna, in which the brachiopods Dayia navicula and Isorthis orbicularis, together with Sphaerirhynchia wilsoni, are particularly common.

Lower Llangibby Beds

This formation comprises a thin marker unit, narrow in outcrop, ranging in thickness from 12 m in the west to 6 m in the east. It consists of buff weathering thinly bedded silty sandstones and siltstones alternating in beds of 5–10 cm. The fauna is highly distinctive and dominated by Shaleria ornatella, with Microsphaeridiorhynchus nucula, Protochonetes ludloviensis and Salopina lunata locally common. The trilobite Calymene puellaris is also characteristic of this formation. Progressive shallowing into the Lower Llangibby Beds is indicated by coarsening of the siltstones, and reduction in the proportion of argillaceous beds. Concentration of fossil debris into lag deposits at the bases of the siltstone and sandstone layers indicates that traction current and storm-related processes were dominant.

Middle Llangibby Beds

These beds form the broad rounded tops of Clytha Hill and Trostrey Hill in the east and Craig yr Harris in the west, as well as the crest of the strike ridge south of Little Mill adjacent to the Little Mill Fault. They form the crest of an upper feature above the wooded lower scarp of the Lower Llanbadoc Beds, giving a high outer rim to the inlier above the low-lying ground occupied by the Raglan Mudstone. Their thickness ranges from 37 m in the east to 52 m in the west.

The beds consist of siltstones and silty sandstones with some silty mudstones and, locally, unfossiliferous cross-laminated sandstones up to 0.3 m thick. The siltstones are yellow and buff and decalcify at outcrop to give 'gingerbread' brown weathering.

At one locality (see p.15) the base of the formation is marked by a lag deposit containing abundant horny brachiopod fragments. To the south, in the Newport district, Squirrell and Downing (1969, p.15) reported a bone bed at this level, which they equated with the Bodenham Bone Bed of the Woolhope Inlier.

Although fossils are abundant in the Middle Llangibby Beds, there is a distinct lack of diversity, and only the brachiopods M. nucula, P. ludloviensis and S. lunata are common, generally concentrated at the bases of the sandstones.

A depositional hiatus is represented by the lag deposit at the base of the formation. Higher beds are typical of a high-energy proximal shelf position, exhibiting evidence of traction and storm processes. Ripple marks, minor scours, erosion surfaces and low-amplitude subtidal sand waves are developed. Bioturbation, common in the deeper water sediments, is absent, and the lack of diversity in the fauna is further indication of shallowing.

Upper Llangibby Beds

These are poorly exposed with few natural sections, and are seen at only four localities in small quarries and cuttings. The beds underlie a narrow strip of ground outside the feature of the Middle Llangibby Beds. Their thickness ranges from 7 m in the west to 4 m in the east.

They consist of brown-weathering yellow and buff siltstones and silty sandstones, and silty mudstones which are green where fresh. The presence of impure sandy lenses, largely decalcified to rottenstone at outcrop, distinguishes this formation from the Middle Llangibby Beds. The sequence is also slightly more thinly bedded, and lacks the unfossiliferous cross-laminated sandstones of the underlying unit.

The rottenstones are generally crowded with moulds of the turreted gastropod Loxonema, associated with the same brachiopod assemblage present in the underlying Middle Llangibby Beds.

The Upper Llangibby Beds were deposited in a high-energy environment similar to that of the Middle Llangibby Beds. The presence of abundant gastropods may indicate further shallowing.

Přídolí Series

Downton Castle Sandstone

This formation, termed the Speckled Grit by Walmsley (1959), is unexposed in the district, but its outcrop, where drift-free, is marked by sandstone fragments in the soil. It consists of 7 to 12 m of mainly pale yellow, orange-specked feldspathic sandstones. Its base is at the Ludlow Bone Bed which is also unexposed, and seen only as field brash at one locality (p.16). Although little can be deduced about the sedimentation of the Downton Castle Sandstone from the limited evidence within the district, studies throughout the Welsh Borderland (e.g. Allen, 1985) have shown it to be a complex of littoral, shallow marine, sand shoals. The Ludlow Bone Bed is a condensed lag deposit, suggesting a regressive–transgressive event that interrupted the overall transition from the shallow marine Ludlow and early Přídolí rocks and the continental later Přídolí sequence (Allen, 1985).

Raglan Mudstone Formation

The Raglan Mudstone Formation was formerly named the Raglan Marl Group in the adjoining districts (Squirrell and Downing, 1969; Welch and Trotter, 1961), and is equivalent to the lower part of the Red Marl Group (Robertson, 1927). The Pontypool Road Fault traverses the outcrop of the Raglan Mudstone, and since its throw is unknown, the thickness of the formation is also unknown, but a minimum of 600 m is estimated to be present. The sequence consists of red, blocky or structureless calcareous mudstones ('marls'), parallel-laminated mudstones, silty mudstones, siltstones and sandstones. It forms the low-lying agricultural ground in and around the Usk Valley in the south-east of the district. Much of the outcrop is drift-covered, and there are few good natural sections.

A characteristic feature of the formation, and of the overlying St Maughans Formation (p.20) is the presence of pedogenic carbonate, or calcrete. This occurs in a range of developments from small scattered limestone nodules (glaebules) to rubbly, nodular and massive limestone. By analogy with present-day carbonate soils, the calcretes formed in the C soil horizon in a warm to hot semi-arid climate with a low seasonal rainfall of 100 to 500 mm annually (e.g. Allen, 1974). They tended to develop towards the top of the red structureless mudstones, and thickened and matured with time, the massive limestones indicating periods of formation in excess of 10 000 years (Allen, 1974; Leeder, 1975). The Psammosteus Limestone is a mature, rubbly, nodular to massive calcrete that occurs in the northern part of the outcrop. The top of the Raglan Mudstone Formation is placed at the top of this calcrete. Although the mudstones of the formation are predominantly red, calcrete development has produced pale green, purple, and red mottling, and pale green reduction spots and zones are also common. Pale green and mauve mudstones occur close to the base of the formation near Little Mill, and are reminiscent of the Temeside Shales of Shropshire.

The sandstones are predominantly fine grained, calcareous, and micaceous. Pale green, mauve, drab brown and red-brown are the commonest colours. Packets of sandstone up to 3 m thick occur, but they are generally thinner. Channel-fill sandstones with sharp bases resting on the underlying scoured surface contain intraformational mud flake and calcrete debris and exhibit both flat, parallel bedding and cross-bedding. They form the bases of fining-upwards alluvial cycles locally (Allen, 1964), passing upwards into siltstones and mudstones. Thin sheet sandstones commonly form parts of interbedded ripple drift-laminated sandstone-siltstone packets. Bioturbation (cf. Planolites) is common in the siltstones, silty mudstones and fine-grained sandstones. Fish fragments are locally present in the sandstones and siltstones.

The Townsend Tuff, an air-fall tuff complex that occurs widely throughout South Wales and the Welsh Borderland in the upper part of the Raglan Mudstone (Allen and Williams, 1981), has not been recorded in the present district.

Allen (1974, 1985) and Allen and Crawley (1983) have shown that the regionally metamorphosed rocks of northwest Britain were the probable source of the Raglan Mudstone sediments, which are rich in mica, garnet and metamorphic rock grains. The sediments accumulated on extensive coastal mud- and sand-flats. Although lingulids have not been found in the district, they occur elsewhere towards the base of the formation, and point to marine influence when the depositional area was part of the intertidal zone (Allen, 1985). The presence of cf. Planolites may also suggest an intertidal setting. However, most of the Raglan Mudstone shows evidence of alluvial deposition, with the flats being combed by meandering streams subject to crevassing and overbank flooding. The blocky mudstones with calcretes are typical alluvial floodplain deposits, with the Psammosteus Limestone recording a prolonged period of geomorphological stability and low sedimentation rates (Allen, 1985).

Details

Selected localities from which faunal collections were made are listed in Appendix 3. The following notes summarise the details.

Wenlock Series

Glascoed Mudstone Formation

The outcrop is mainly drift-covered and natural sections and exposures are largely restricted to the backscars of landslips on the banks of the Usk and small exposures in Berthin Brook.

A railway cutting [SO 340 020] near Bryn Farm shows the following:

This is the designated type locality of the Glascoed Mudstone (see p.7, and also Walmsley, 1982).

Small exposures occur in Berthin Brook [SO 3431 0202]; [SO 3450 0208]; [SO 3510 0182], and in a tributary to the south-east, e.g. [SO 3565 0151]. A small exposure in the sewage works nearby [SO 3566 0150] (Locality 1) yielded a fauna including Atrypa reticularis, Eospirifer radiatus, Lingula sp., Strophonella euglypha and Encrinurus sp.

Olive-green friable mudstones are exposed at Graig y Pandy [SO 3412 0427], and a faunal collection was made from a small exposure [SO 3417 0423] (Locality 2). A nearby stream section (Locality 3) yielded a varied brachiopod assemblage, including Antirhynchonella linguifera, Glassia sp., Ludfordina pixis, and Ygerodiscus aff. cornutus. About 4 m of interbedded micaceous khaki siltstones and silty mudstones are poorly exposed in a track [SO 3437 0393]. The siltstones contain small brachiopods, including Leangella segmentum, and also numerous examples of Dalmanites sp.Bassett (1974) recorded Monograptus flemingii flemingii from this locality.

There are numerous small exposures in landslip scars and intact slipped blocks at Craig y Garcyd [SO 360 025] containing a diverse fauna, which was collected at several places [SO 3618 0246]; [SO 3596 0257]; [SO 3593 0259] to [SO 3600 0255]; [SO 3578 0270]; [SO 3582 0271], and [SO 3575 0267] (Localities 4, 5, 6, 7, 8 and 9 respectively). The following composite section was recorded:

The fauna from these beds includes Ketophyllum cf. turbinatum Derrucosum, Propora aff. tubulata; Antirhynchonella linguifera, Baturria edgelliana, Eospirifer radiatus, Megastrophia (Protomegastrophia) semiglobosa, Meristina obtusa, Pentlandina cf. lewisii plakodis, Resserella canalis; Dawsonoceras annulatum; Cyphoproetus depressus and Encrinurus tuberculatus. Bassett (1974) recorded Monograptus flemingii fiemingii in a calcareous nodule in fossiliferous grey-green silty shales exposed in a trench section near Monkswood [SO 3435 0245].

Ton Siltstone Formation

The formation is poorly exposed near Bettws Newydd. Small sandstone outcrops on the south-east bank of the River Usk north of Chain Bridge [SO 3487 0575] contain much brachiopod debris, including ?Microsphaeridiorhynchus nucula, and also crinoid columnals. Debris from trenches near Berllan Fedw [SO 3526 0555] yielded Amphistrophia funiculata, Leptaena cf. depressa, cf. M. nucula, Salopina conservatrix; Leptodesma?; acastids; and crinoid columnals, up to 7 mm in diameter. Other small exposures were recorded at Thornbury Cottage [SO 3524 0597] and Coed y Ringoed [SO 3596 0508], and the Trostrey Limestone is partly exposed in a disused pit [SO 3518 0518] (Locality 10), where Dicoelosia biloba is common and Ferganella borealis, Resserella sabrinae, Cypricardinia subplanulata and Acanthopyge hirsuta also occur.

There are some exposures in the bank of the River Usk at Graig y Pandy, e.g. [SO 3415 0474], where 2 m of silty sandstone with possible comminuted plant fragments are seen.

The following section is seen in a disused quarry north-west of Trostrey [SO 3565 0471]:

Exposures of silty sandstone and siltstone [SO 3589 0445], [SO 3593 0441] (Localities 11, 12) near St David's Church, Trostrey yielded the brachiopods Craniops implicatus, cf. Microsphaeridiorhynchus nucula, and Salopina conservatrix; bivalves, and crinoids.

The river cliff and road cutting at Weir Wood, Trostrey [SO 359 041] (see also Walmsley, 1959, p.488 and Austin and Bassett, 1967, pp.274–283) display the following:

Austin and Bassett (1967) recorded the following conodonts from the 0.08 m (2") crinoidal limestone layer: Ligonodina sp., Ozarkodina edithae, O. cf. media, O. sp., Spathognathodus sagitta bohernicus, S. sagitta rhenanus, S. sp., and Trichonodella sp.This assemblage is indicative of the Sagitta (conodont) Zone.

Outcrops of decalcified sandstones with shale clasts and interbedded mudstones [SO 3610 0399] (Locality 15); [SO 3606 0410] (Locality 16) in the sides of the lane leading from the A471 at the southern end of the road cutting described above, towards St David's Church, yielded a restricted shallow-water fauna. This includes cf. M. nucula and S. conservatrix; molluscs, especially Grammysia spp.and trilobites. Black amorphous fragments are common and may be comminuted plant debris. At Locality 15, numerous examples of Homalonotus sp.were collected.

There are exposures in a lane north-west of Ton; the following section, exposing beds near the top of the formation, was recorded [SO 329 024]:

Further exposures in the lane 150 m to the east [SO 3320 0252] (Locality 17) show:

In the stream [SO 3320 0226] in Hill Wood below Ton, 4.25 m of beds are arranged in coarsening-upwards cycles of spheroidal-weathering, green-grey silty mudstone, fissile siltstone and bioturbated sandstone. The mudstones and siltstones contain bivalves and possible plant debris, and the sandstones contain crinoid columnals.

The following composite section is based on outcrops in shallow diggings [SO 3346 0178] (Locality 18) and laneside exposures [SO 3347 0177] (Locality 19) south-east of Ton:

Bassett (1974) gave details of a trench section [SO 3338 0173] to [SO 3370 0215] which exposed 105 m of beds immediately underlying the Usk Limestone.

Intermittent stream exposures [SO 3359 0198] to [SO 3367 0202] show silty mudstones, blue-grey where fresh, weathering to dark green, and containing possible disseminated plant debris and pyrite.

The outcrop of the formation is drift-covered from Trostrey to Rhadyr. On the south bank of the Usk, near Rhadyr, 4 m of olive and brownish green micaceous bioturbated siltstones and silty sandstones with possible plant debris are exposed [SO 3650 0239] close to the Rhadyr Fault. About 23 m of green, brownish yellow weathered poorly bedded silty sandstones and interbedded micaceous silty mudstones with possible plant debris are present in an old railway cutting [SO 3642 0184] to [SO 3650 0176] near Rhadyr, but the section is now overgrown and exposure is poor. The sandstones contain Lingula sp.and the bivalves Leptodesma sp.and Nuculites sp.The Trostrey Limestone is exposed in the cutting [SO 3641 0180] where 0.7 m of platy-bedded nodular limestone containing a fauna including Amphistrophia,funiculata and S. euglypha are seen. An excavation in the north-east bank of the cutting [SO 3648 0178] showed:

Bryozoans including Fenestella sp.and crinoid columnals occur throughout, and the restricted fauna is typical of the late Wenlock arenaceous sequence.

To the south-west of the cutting, the outcrop of the beds above the Trostrey Limestone is marked by brash of flaggy decalcified orange and buff sandstone rich in crinoid columnals and bryozoans.

Usk Limestone

Fossil collections from old quarries [SO 353 060] to the west of Bettws Newydd (Localities 20,21,22) include the tabulate corals Acanthohalysites sp.and Faeosites hisingeri, as well as a diverse brachiopod assemblage, including Amphistrophia funiculata, Horoellella elegans, Leptostrophia filosa, and S. euglypha. Pholidostrophia (Mesopholidostrophia) deflecta was recorded from one of these quarries [SO 3533 0597] (Locality 20).

The following section was measured in the entrance to a collapsed adit [SO 3544 0584], west of Bettws Newydd:

Shallow pits in impure nodular limestone with a varied brachiopod assemblage are present on a gentle dip slope [SO 3603 0515] (Locality 23) in Coed y Ringoed, south of Bettws Newydd.

A disused quarry north of Trostrey [SO 3579 0493] shows the following:

There are no good sections in the Usk Limestone in its outcrop between the Little Mill and Llanbadoc faults, but debris of nodular limestone and bentonitic clay is seen on the banks of the Usk [SO 3414 0488], north of Graig y Pandy and in landslipped material [SO 3407 0457] to the south.

At Be-Penvane [SO 3278 0271]; [SO 3279 0277] the following composite section was recorded:

In the old quarry [SO 3279 0274] (Locality 24) beside the road leading to Tynewydd Farm, 3 m of calcareous mudstones with limestone nodules yielded corals and brachiopods, including Favosites sp., Isorthis clivosa, Leptaena cf. depressa, Orbiculoidea sp.and Protochonetes minimus.

Fifty metres to the north-east [SO 3283 0273], a section reads:

Extensive old quarries, crop diggings, underground pillar and stall workings and lane sections are present around Ton. Exposures in the side of a lane [SO 3295 0193]; [SO 3295 0188] (Localities 25, 26) have yielded a diverse brachiopod fauna, together with Poleumita globosa and Encrinurus variolaris.

A disused quarry [SO 3303 0195] shows:

Walmsley (1959, 1982) described exposures nearby at Ton Farm [SO 332 019] where he recorded A. reticularis and L. depressa together with other brachiopods, corals, crinoids and Dalmanites.

The following section was compiled from a disused quarry and adjacent underground workings [SO 3330 0165]:

About 4.5 m of nodular limestone, resting on 1.9 m of crinoidal limestone, are seen in an old quarry [SO 3365 0135].

Blue-grey coarse-grained bioclastic limestone with large dolomitised crinoid columnals is present in debris from old shallow diggings in the basal limestones between the Cefn Ila and Rhadyr faults, south-west of Rhadyr. Yellow and green bentonite clay debris was recorded nearby. In the only outcrop to the south [SO 3588 0144], lying near the base of the upper nodular unit, 1.5 m of pale green and buff mudstones, silty mudstones and blue-green fine-grained limestone nodules are seen.

Ludlow Series

Lower Forest Beds

These crop out in four separate areas.

Near Graig y Pandy

North-west of the Llantilio–Little Mill fault system, the only exposure is in a small quarry [SO 3405 0491] (Locality 27), and consists of soft green mudstones with a diverse fauna of small brachiopods including Aegiria grayi, Craniops implicatus, Dicoelosia biloba, Leangella segmentum and Ludfordina pixis. Between the faults, a small trackside exposure [SO 3398 0459] (Locality 28) yielded a closely comparable fauna.

North and east of the River Usk

The beds occupy a narrow belt of low-lying drift-covered ground east of the Bettws Newydd–Llanbadoc Fault and below the wooded scarp of the Upper Forest Beds. Soft pale green mudstones near the base of the formation in a field [SO 3602 0648] (Locality 29) contain numerous examples of L. pixis. They yielded an impoverished assemblage of poorly preserved, mainly long-ranging acritarchs, tentatively assigned a Ludlow age by Dr R E Turner (formerly BGS). Mudstones near the top of the formation [SO 3635 0671] below Coed y Bwnydd contain a small variety of Dayia navicula, and other brachiopods.

A disused quarry [SO 3691 0381] (Locality 30) north of Llancayo exposes 12.5 m of thinly bedded, slightly calcareous, green mudstones with some limestone ribs and nodules. They contain a diverse fauna, mainly of brachiopods, including Protochonetes minimus and Skenidioides lewisii; bivalves and trilobites, including Calymene cf. blumenbachii are well represented, and the graptolite Saetograptus sp.was also recorded.

A disused quarry [SO 3720 0362] (Locality 31) lying immediately east of the district boundary exposes flaggy calcareous mudstones with a diverse brachiopod fauna, including D. navicula, Isorthis clivosa, and Pholidostrophia (Mesopholidostrophia) lepisma; Nuculites pseudodeltoideus and other molluscs; trilobites also occur.

South and west of Ton

There are numerous small exposures, pits, and sections in sunken lanes in gently rising drift-free ground between the Usk Limestone scarp and the Upper Forest Beds scarp.

Small stream exposures e.g. [SO 3276 0206] (Locality 32) of blue-grey calcareous mudstones with sandy laminae and sandy limestone ribs and nodules are rich in brachiopods including Gypidula galeata, Leptaena depressa and Sphaerirhynchia wilsoni. A stream exposure [SO 3292 0187] (Locality 33) yielded a similar diverse brachiopod assemblage, tabulate corals and trilobites. The beds at Localities 32 and 33 lie close above the Usk Limestone, with lithologies transitional between the two formations; the fauna, characterised by large brachiopods, is more comparable with that of the Usk Limestone.

A lane section [SO 3270 0237] (Locality 34) east of the faulted junction with the Middle Llangibby Beds exposes 35 m of pale green, brown-mottled mudstones with some silty laminae; some thicker-bedded darker green mudstones are present towards the top. Fossils include A. grayi, Dalejina hybrida, Shagamella minor, and Dalmanites sp.

Rhadyr

Former sections and disused quarries are infilled, and there are no good exposures.

Upper Forest Beds

North-west of the Llantilio–Little Mill fault system, the beds are grouped with the Lower Llanbadoc Beds, and are described below.

These beds form the bulk of the wooded scarps of Coed Adam and Coed y Bwnydd. There are several small exposures near the north of Coed Adam, at one of which [SO 3616 0770] (Locality 35) khaki siltstones and silty mudstones contain a characteristic late Gorstian fauna of large strophomenaceans and trilobites, including Encrinurus rosensteinae. Beds near the top of the formation are exposed in overgrown limestone diggings [SO 3642 0684] on Coed y Bwnydd, where 0.2 m of blue-green, fine-grained limestones and mudstones contain brachiopods including Leptostrophia filosa.

Nodular calcareous siltstones and interbedded blue-green sandy limestones are exposed in a pit at Lower Berllwyd Farm [SO 3675 0613] (Locality 36). They contain a diverse brachiopod fauna in which large strophomenaceans are prominent, together with trilobites such as E. rosensteinae.

A disused quarry [SO 3690 0611] (Locality 37) exposes thinly bedded calcareous sandstones, coquinal limestones, and calcareous mudstones with a typical late Gorstian fauna of brachiopods and molluscs, including Liospira striatissima.

Good exposures are found below Trostrey Wood [SO 3675 0475] to [SO 3688 0460], where 9 m of medium-green poorly bedded mudstones with scattered blue-green sandy or coquinal limestone nodules are present. Diverse brachiopod assemblages occur [SO 3678 0473] (Locality 38), and [SO 3684 0465] (Locality 39), including Amphistrophia funiculata, Coolinia pecten, Eospirifer radiatus and S. euglypha; trilobites are also well represented.

Excellent exposures in beds near the base of the formation are found in a deep track [SO 370 045] (Locality 40) at the south-east end of Trostrey Wood where the following was recorded:

A general collection from this locality includes A. funiculata, C. implicatus, S. wilsoni, and the ostracod Neobeyrichia torosa.

Succeeding beds are exposed in a nearby disused limestone pit [SO 3701 0442] (Locality 41) and comprise 10.5 m of micaceous mudstones and decalcified siltstones, dull green in colour and weathering brown. There are some slightly fissile mudstone beds and, in the topmost 4 m, some blue-green sandy limestone layers up to 0.04 m thick. The mudstones are darker green towards the top of the exposure. A large collection from this locality includes corals, bryozoa, brachiopods, molluscs, trilobites, ostracods, and a graptolite–Saetograptus

A disused quarry [SO 3707 0394] in Coed Shenkin Morris exposes 6 m of calcareous mudstones near the base of the formation. They contain small brachiopods and are interbedded with thin (0.01–0.05 m) shelly limestone coquinas with S. euglypha and with unfossiliferous fine-grained sandy limestone nodules.

Lower Llanbadoc Beds/Upper Forest Beds

A lane section [SO 3467 0555] (Locality 42) west of the Llantilio–Little Mill fault system near Chain Bridge shows the following:

In a gully [SO 3230 0209] (Locality 43) near Little Mill, 2 m of crushed olive-green calcareous siltstones and decalcified argillaceous limestone are exposed faulted against the Raglan Mudstone by the Little Mill Fault. A possible example of Saetograptus sp.was collected from this locality.

Brown-weathering dark green calcareous silty mudstone debris with Leptaena cf. depressa and abundant S. wilsoni is seen at the top of Coed Di-haul [SO 3214 0152] (Locality 44) near the southern boundary of the district.

Lower Llanbadoc Beds

East of the Bettws Newydd Fault, the beds form the higher slopes of Coed Adam and Coed y Bwnydd, where there are many disused overgrown limestone pits, in one of which [SO 3636 0738] 2 m of interbedded shelly limestones up to 0.05 m thick and calcareous mudstones are exposed. The limestones contain A. funiculata and Kirkidium sp.A disused limestone quarry [SO 3682 0647] shows 2.5 m of interbedded dark green, brown-weathering nodular-bedded silty calcareous mudstones and blue-green limestone nodules with the rugose coral Phaulactis sp.and also Leptaena cf. depressa, Leptostrophia filosa, S. wilsoni and Poleumita globosa. A similar fauna was obtained from 2 m of poorly bedded calcareous mudstones with limestone nodules exposed in Coed yr Allt [SO 3647 0525] on the west side of Trostrey Hill. On the northern side of the hill, the highest beds of the formation, as well as basal Upper Llanbadoc Beds, are exposed in a small pit [SO 3694 0552]. The former consist of unfossiliferous nodular-bedded impure limestones and silty calcareous mudstones. 1.5 m of beds lying near the base of the formation and exposed in a disused quarry in Trostrey Wood [SO 3662 0492] (Locality 45) are of dark green and pale brown-green calcareous mudstones interbedded with blue-green sandy coquina' limestone nodules with abundant A. reticularis. A landslip backscar in Round Wood [SO 3708 0459] (Locality 46) exposes 5 m of pale green calcareous parallel-laminated siltstones with some coquina' shelly limestone lenses with rugose corals, a diverse brachiopod assemblage, and the bivalve Mytilarca mytilimeris. Impure fine-grained sandy limestone beds up to 0.06 m thick are present, and become increasingly common upwards.

Riverside exposures [SO 3703 0227] in the Usk near Llancayo reveal 3.5 m of bright green calcareous blocky mudstones with limestone lenses, some of which are coquina' and contain A. reticularis and S. wilsoni.

Upper Llanbadoc Beds

A small exposure [SO 3416 0532] (Locality 47) west of the Fiantilio–Little Mill fault system below Craig yr Harris shows 1.5 m of green siltstones with a characteristic early Ludfordian fauna including abundant Dayia navicula and Sphaerirhynchia, Saetograptus leintwardinensis incipiens is also present. Pale green siltstones with D. navicula were observed in the structurally complex ground east of the Little Mill Fault near Great Beech [SO 3216 0195].

North-east of Coed y Bwnydd, a disused quarry [SO 3669 0711] (Locality 48) exposes 3.8 m of thinly bedded dull green calcareous silty mudstones with limestone nodules which yielded an abundance of D. navicula and Isorthis orbicularis. Bivalves are also well represented, including Cypricardinia suhplanulata, Goniophora cymbaeforrnis and Palaeopecten danbyi.

Basal beds of the formation are seen in the old disused pit [SO 3694 0552] on the northern flank of Trostrey Hill and consist of 0.75 m of yellow-green fissile siltstones and silty mudstones with D. navicula abundant. Small exposures [SO 3676 0499] and [SO 3687 0491] of dark green silty mudstones with D. navicula and I. orbicularis are seen to the south.

LOWER LLANGIRRY BEDS

This formation produces a distinctive weathering soil, and the presence of Shaleria ornatella in field brash facilitates its mapping.

A small digging [SO 3266 0232] (Locality 49) was in buff and pale green siltstones and silty sandstones with rottenstone nodules which are probably decalcified shell coquinas. These yielded a diverse brachiopod fauna including Hyattidina canalis, Protochonetes ludloviensis and S. ornatella.

Almost all the formation is seen in a lane [SO 347 056] (Locality 50) near Chain Bridge, where 11 m of siltstones and silty sandstones in beds 0.05 to 0.10 m thick yielded abundant examples of P. ludloviensis and S. ornatella. Below Craig yr Harris 2.5 m of interbedded pale green, brown-weathering laminated siltstones, silty sandstones and bioturbated silty mudstones [SO 3420 0543] (Locality 51) yielded numerous examples of H. canalis and S. ornatella, together with a diverse bivalve fauna including Cunearnya truncata and Fuchsella amygdalina.

A well-exposed quarry [SO 3662 0857] at Fynnonau Farm, Clytha shows the following:

Exposures in the floor of the track nearby [SO 3666 0852] yielded Leptodesma sp.and Calymene puellaris.

A small exposure [SO 3712 0483] in Cowbottom Wood shows brown-green silty mudstones with D. navictda„44. nucula and S. ornatella interbedded with poorly bedded bivalve-bearing silty mudstones and green siltstones.

Fissile richly fossiliferous siltstones and silty mudstones in a disused quarry [SO 3724 0490] (Locality 52) east of the district boundary contain a typical Ludfordian fauna, with numerous examples of M. nucula, P. ludloviensis, S. lunata and Shaleria ornatella; bivalves are also common.

An exposure at a stream source nearby [SO 3725 0547] (Locality 53) yielded a closely comparable fauna, including C. puellaris.

MIDDLE LLANGIRBY BEDS

This formation is well exposed because of its arenaceous nature.

A roadside section [SO 3393 0549] north-west of Craig y Harris exposes the topmost 5.5 m of the formation comprising green silty mudstones interbedded with slightly decalcified cross-laminated siltstones. Some of the siltstone beds contain lenticular shell coquinas with P. ludloviensis, Salopina lunata and crinoid debris.

The basal 7 m of the formation are exposed in a lane [SO 347 056] near Chain Bridge where the following was recorded:

A lane section [SO 327 023] (Locality 54) exposes 4 m of beds in faulted contact with the Lower Forest Beds (see p.14). The beds consist of buff-yellow and yellow-green siltstones and interbedded parallel-laminated silty sandstones. M. nucula, (common), S. lunata, and bivalves including G. cymbaeformi s and Pteronitella are present in the basal laminae of the sandstones. A concentration of P. ludloviensis was noted in the lowest bed exposed.

The A40 road cutting at Clytha [SO 367 087] (Locality 55) provides a fine section through the topmost 20 m of the formation, summarised as follows:

Decalcified brown siltstone brash with M. nucula, P. ludloviensis and crinoid columnals is widespread on Trostrey Hill and Clytha Hill.

The topmost 7.5 m of the formation are exposed in the railway cutting [SO 3225 0350] north of Little Mill, and consist of thinly bedded olive-green unfossiliferous mudstones, pale green calcareous sandstones and decalcified brown siltstones with P. ludloviensis, S. lunata, bivalves and crinoid debris.

Upper Llangibby Beds

This formation is poorly exposed.

The lowermost beds, comprising siltstones and interbedded shelly rottenstones with Loxonema sp., are exposed in a lane on Star Pitch [SO 3393 0549]. A small quarry [SO 3475 0566] (Locality 56) near Chain Bridge exposes green siltstones and silty mudstones with P. ludloviensis, S. lunata and Loxonema sp.Olive-green silty sandstone and brown rottenstone debris with Loxonema sp.can be collected close to the Little Mill Fault [SO 3171 0141], near the southern boundary of the district.

The following was recorded in the railway cutting [SO 3225 0350] north of Little Mill:

In the eastern outcrop, exposure is limited to the Clytha A40 road cutting [SO 367 087] (Locality 55), where the basal 3.5 m of the formation are seen resting on Middle Llangibby Beds (see p.15). They comprise yellow-green and olive-green micaceous silty sandstones with shelly coquinal lenses which contain P. ludloviensis, S. lunata, Loxonema gregarium, Goniophora cymbaeformis and Nuculites antiquus. Interbedded blocky siltstones are commoner towards the top.

Přídolí Series

Downton Castle Sandstone

There are no sections, and few exposures. The outcrop at the northern part of the inlier around Clytha is almost entirely drift-covered, but buff micaceous siltstone fragments were noted in the subsoil [SO 3690 0849] near Keeper's Cottage.

The outcrop within the Little Mill–Llanbadoc fault system is mainly drift-covered, and where drift-free, for example on the banks of the River Usk [SO 3479 0570] at Chain Bridge, there are no exposures.

A small outcrop in Coed Tynewydd [SO 3253 0232], where the formation is faulted against the Raglan Mudstone by the Little Mill Fault, is marked by purplish-weathered gritty sandstone brash.

Buff sandstone and siltstone debris [SO 3197 0147] was recorded near Great Beech, in a small, partly fault-bounded outcrop. Fine-grained, well-sorted sandstone contains Lingula sp.in life position, and silty micaceous sandstone contains Pachytheca sp., and numerous black fragments of plant or algal origin. The siltstone yielded numerous shells including Lingula sp., P. ludloviensis and S. lunata, and thelodont scales. It is probably the local equivalent of the Ludlow Bone Bed.

Raglan Mudstone Formation

Llanvapley–Llanover–Llanvihangel nigh Usk

There are few good exposures of the mudstone facies in this area; the best was a temporary section at Lower House Farm [SO 3561 1192] where 5 m of red and purple, green-mottled marl with purple pedotubules were seen. There are several exposures of mature calcrete, all of which are correlated with the Psammosteus Limestone, at Upper Court [SO 3311 1092], at Galchen [SO 3505 1390], and Llanthewy Wood [SO 3562 1318]; [SO 3573 1304]; [SO 3585 1330]. The Galchen exposure is a recent one and shows:

Purple, medium-grained sandstone forms a horizontal feature between Aberffrwd and Llansantffraed Court, and is exposed in Aberffrwd Brook [SO 3515 0995]. In the east bank of the Usk, a 2.5 to section [SO 3593 0920] comprises several silty sandstone/siltstone fining-upwards cycles, the finer parts containing cf. Planolites.

Llanover–Goetre

Exposures are mostly confined to small streams in the drift-free ground to the west and south-west of Pant-y-Goitre, in the axial region of the Usk Anticline. Beds consist of thin interbedded red mudstones, siltstones and fining-upwards silty sandstones. Bioturbation (cf. Planolites) is common in the siltstones and sandstones. The best exposures west of Pant-y-Goitre are near Hendre-isaf Farm [SO 3269 0814], Neuadd-newydd Farm [SO 3345 0839] and [SO 3370 0820], a small disused quarry [SO 3436 0877], and in a stream [SO 3415 0846]; at the last locality, the section reads:

South of Pant-y-Goitre, a disused quarry [SO 3482 0769] near Highmead provides an illustrative section:

Small sections [SO 3408 0657]; [SO 3393 0621]; [SO 3394 0610] are present in a lane near Ty Fry. The trace fossil cf. Planolites is common in these exposures; a 3 cm lens of friable grit at the second locality contains fish fragments.

Mamhilad–Little Mill

The Pontypool Road Fault divides the Raglan Mudstone outcrop in this area, and the formation is faulted against the Ludlow Series by the Little Mill Fault to the east.

West of the Pontypool Road Fault, strata towards the top of the formation are poorly exposed, with much of the outcrop occupied by a factory estate. There are several small exposures in a small stream near 'l'y-isha, in one of which [SO 3141 0295] 1.5 m of red and green mudstones overlie 0.75 m of purple medium-grained micaceous sandstone. These exposures lie close to the axis of the Pontypool Monocline (Squirrel] and Downing, 1969, p.242).

Purple marl overlying coarsely micaceous siltstone with cf. Planolites is exposed in a small road cutting [SO 3088 0199] on the A4142.

The Psammosteus Limestone was not seen in this area, and is not recorded from here southwards to Cwmbran (Squirrel' and Downing, 1969). The narrowness of the Raglan Mudstone outcrop is due mainly to the steep dips and the presence of the Pontypool Road Fault and the Little Mill–Pen-y-Llan Fault, but it is possible that penecontemporancous erosion may have removed the topmost Raglan Mudstone strata.

To the east of the Pontypool Road Fault, outcrops are largely concentrated in the structurally complex ground south of Little Mill, where several ridges consisting of groups of thinly bedded sandstones were mapped.

Small pits at Little Mill Brick Works provide good exposures; the following section [SO 3164 0215] is representative, and illustrates the thin-bedded nature of the sequence:

Chapter 3 Devonian

The Devonian Period is represented by the major part of the Old Red Sandstone (Figure 5), a thick pile of continental red beds. As discussed in the previous chapter, these rocks straddle the Silurian–Devonian boundary which is believed to lie within the Raglan Mudstone Formation. They crop out over an area of about 260 km² around the north-east crop of the coalfield. The Raglan Mudstone and St Maughans formations occupy the low agricultural land in the east of the district. In contrast, the Senni Beds and Brownstones, both sandstone-dominated formations, form the bold ring of hills that encircles Abergavenny north of the Usk and also occupythe steep slopes at the base of the coalfield escarpment. The Brownstones are the topmost formation of the Lower Devonian, and are unconformably overlain by the Upper Devonian Quartz Conglomerate Group which crops out in a narrow belt high on the coalfield escarpment, and in a small outlier capping the Sugar Loaf.

Previous research

The earliest reference was by Murchison (1839) who described the rocks of Skirrid Fawr, and recorded Holoptychius from the Darren and some vertebrate remains from near Abergavenny. These early vertebrate faunas attracted the Victorian palaeontologists, who collected many fragments now known to come from the St Maughans Formation (see p.21). One locality subsequently yielded complete cephalaspids (White and Toombs, 1983) (p.21), and a unique pteraspid was obtained from the Senni Beds (White, 1938) (p.27). The district is also well known for the early vascular plants of the Senni Beds (Croft and Lang, 1942; Edwards, 1969).

Stratigraphical studies were carried out by Symonds (1872), Strahan and Gibson (1900), and Robertson (1927). Sedimentological studies were done by Allen (1963, 1964b, 1965), Allen and Tarlo (1963), and Lovell (1978 a,b). Micropalaeontological examinations have been carried out by Mortimer (1967), Richardson and Lister (1969), and Turner (1973).

Classification

Lithostratigraphy

(Table 2) shows the classification used in the resurvey, its correlation with the previous one and with those of the adjoining districts. Symonds' (1872) classification was amended by Strahan and Gibson (1900). Robertson (1927) showed that the Red Sandstone of Strahan and Gibson comprises both the Senni Beds and underlying Brownstones. Squirrell and Downing (1969, pp. 28, 29) proposed the term Brownstone Group in the adjoining Newport district, where the Brownstones and Senni Beds of the Black Mountains are indistinguishable, and where, in their view, Strahan (1909) had misapplied the name Brownstones. This situation holds in the south-east of the present district north of Pontypool. The term Brownstones is however preferred as a valid formational name, regardless of the fact that the formation is in part stratigraphically equivalent to the Senni Beds (Waters and Lawrence, 1987).

Chronostratigraphy

Traditionally, the Old Red Sandstone of the Anglo-Welsh basin has been divided into four local and somewhat ill-defined stages–the Downtonian, Dittonian, Breconian, and Farlovian (Allen, 1974b, 1977; Allen and Tarlo, 1963; Croft, 1953; King, 1925, 1934). The Downtonian is now placed in the Silurian (Bassett, Lawson and White, 1982) and the Dittonian and Breconian stages await formal definition (see e.g. Allen, 1977). The upper part of the Farlovian is of Tournaisian age. Detailed application of the standard European marine Devonian stages is not at present possible, although broad correlations can be made using the vertebrates and palynology (see (Table 3)).

Sedimentation

This has been summarised by Williams (1980) and Allen (1985). The rocks are almost entirely clastic sediments deposited in arid to semi-arid conditions in a warm to hot climate at a latitude of between 20° and 30°S (see, e.g. Allen, 1979). They accumulated on the southern margin of the evolving Caledonian (or 'North Atlantic') continent on a piedmont plain between an equatorial land mass to the north and a deep basin, the 'Rheic Ocean', to the south.

The Lower Old Red Sandstone has been interpreted by Allen and Williams (1979) as an upward-coarsening offlap succession in which the mudstone to sandstone ratio declines upwards and the sandstones generally become coarser in the younger formations. Thus, the mudstone-dominated Raglan Mudstone Formation, spanning the Silurian–Devonian boundary, passes upwards into the St Maughans Formation which consists of cyclical alternations of mudstones and sandstone. This, in turn, is overlain by the sandstone-dominated Senni Beds and Brownstones. The Raglan Mudstone represents deposition in a distal fluvial to marginal marine environment, the St Maughans Formation in a medial fluvial environment, and the Senni Beds and Brownstones in proximal fluvial environments. The distant metamorphic rock source of the Raglan Mudstone (p.10) was cut off prior to deposition of the Brownstones which contain pebbles of Ordovician tuff and lava, Silurian greywacke and shelf sandstones derived from the Lower Palaeozoic terrain of north and central Wales, (Allen, 1974c).

Continuing uplift and resultant southerly migration of the facies belts led to non-deposition and erosion in the AngloWelsh area in Middle Devonian times. Thus an unconformity separates the Brownstones from the Upper Old Red Sandstone rocks.

The Plateau Beds, present only in the west of the district, were deposited partly in a marginal marine setting, and partly in aeolian, and fluvial environments, (Lovell, 1978 a, b). The sediments of the overlying Quartz Conglomerate Group are entirely fluvial, deposited on a southward-dipping palaeoslope (Allen, 1965).

Lower Devonian (Lower Old Red Sandstone)

St Maughans Formation

Originally named the St Maughans Group (Welch and Trotter, 1961), this unit is now given formation status. It is equivalent to the topmost part of the Red Marl Group (Strahan and Gibson, 1900; Robertson, 1927). The formation has a wide outcrop north-east and east of Abergavenny and also occupies a narrow strip at the foot of the coalfield escarpment from Abergavenny to Pontypool. A sequence about 500 m thick consists of mudstones and siltstones (about 75 per cent), and sandstones. Intraformational conglomerates and calcretes form a small but distinctive part of the formation.

In general, the base of the formation is placed at the top of the Psammosteus Limestone, where an abrupt facies change takes place, sandstone appearing in abundance above. Where the limestone is absent, the base of the formation is placed at the facies change. In the south-east of the district around Pontypool, the Pontypool Limestone, a mature, massive to rubbly calcrete occurs at the top of the formation (Squirrell and Downing, 1969). It can be traced for about 4 km north of Pontypool. About 1 km north of Pontypool thick purple sandstones, lithologically similar to the overlying Brownstones, appear below the Pontypool Limestone. Mature calcrete is also present at the top of the formation west of Abergavenny at Graig (p.23). Thick development of intraformational conglomerate rich in calcrete clasts is seen at this level at Llanfoist (p.23). Thus with calcrete found both in situ and reworked in intraformational conglomerates, there is evidence for extensive and prolonged pedogenesis at this level. Squirrell and Downing (1969) suggested that the Pontypool Limestone may correlate with the Upper Abdon Limestone of the Clee Hills, which, in turn, is tentatively correlated with the upper Ffynnon Limestone of the Black Mountains (Ball and Dineley, 1961).

Lithology

A characteristic feature of the formation is the cyclic arrangement of the facies (see e.g. Allen, 1963; Allen and Tarlo, 1963). Intraformational conglomerate, most commonly Type A of Allen and Williams (1979), rests on an eroded, scoured surface of the underlying mudstone or siltstone. This conglomerate, the 'conglomeratic cornstone' of the older literature, is present as lenticular sheets, normally about 0.2–0.5 m thick, in which low-angle planar cross-bedding is sometimes present. The rock is polymictic, consisting of an unsorted mixture of calcrete, siltstone and mudstone clasts in a fine sand or silt matrix. It is in this facies that most of the fish remains, for which the district is well known, are found as disarticulated fragments. Eurypterid remains and plant debris also occur.

The contact with the overlying sandstone is commonly an erosion surface but, in places, intercalation of the two facies is seen. Where the conglomerate is absent, the cycle commences with an erosively based sandstone containing intraformational debris and, locally, fish and plant remains. The sandstones are micaceous, fine- and medium-grained, and show both parallel lamination and large-scale cross-bedding in their basal parts, with rippled cross-lamination above. Average thickness is 1 to 1.5 m and in stream sections these sandstones form waterfalls. The rock is purple and pale green where fresh, but commonly brown where weathered and decalcified. Groove casts and other bottom structures generally indicate a north–south current direction.

The sandstones generally fine upwards and pass into red-brown siltstones and mudstones. This facies is commonly 2 to 3 m thick and is truncated by an erosion surface in which desiccation cracks may be present. Bioturbation is seen in places, and calcrete nodules are common, particularly towards the upper part of the cycle. More mature massive calcretes, the 'concretionary limestones' of the older literature, are seen in places, for example in a stream northwest of Skirrid Fawr [SO 3208 1909] and on the Arwallt [SO 3351 1858]. These are mottled grey and green-grey brecciated micritic limestones. The mudstones are commonly blocky and unbedded with all primary structures destroyed by desiccation and pedogenesis. Green mudstones are present locally and may contain plant debris.

Sedimentation

The sedimentation of the St Maughans Formation and its stratigraphical equivalents throughout Wales and the Welsh Borderland has been detailed in many papers by Allen (e.g. 1962, 1963, 19646, 1970, 1974a) although, in this district, only the Cwm Mill section has been described (Allen, 1963, 1964b; Allen and Tarlo, 1963).

The fining-upwards sequences, from channel-bottom lag conglomerates through channel-fill and point bar sands into floodplain overbank silts and muds, have been interpreted as the deposits of high-sinuosity streams (e.g. Allen, 1974a). More recently, increasing emphasis has been placed on the ephemeral nature of alluvial fan and plain deposition and Collinson (1978) has suggested that some of the intraformational conglomerates could be flash flood deposits. The layer of articulated cephalaspids at Cwm Mill (p.21) is the product of a catastrophic flood which washed the fish out of a river channel and entombed them in overbank silt (Allen, 1963, 1970; Allen and others, 1968; White and Toombs, 1983). In Allen's alluvial model, the fine silt and mud at the top of the cycles are overbank deposits, but it is possible that some may be aeolian in origin. Calcrete formation in the mudstones and siltstones indicates low accretion rates on the flood plain allowing pedogenic processes to operate for periods of thousands of years (e.g. Allen, 1974a, Leeder, 1975; Woodrow and others, 1973).

Palaeontology

The St Maughans Formation contains an abundance of fish-bearing intraformational conglomerates. (Table 4) lists the fossil localities and the faunas collected during the resurvey. Strahan and Gibson (1900) gave a list, repeated by Robertson (1927), of the early vertebrate discoveries. These authors questioned the stratigraphical value of some of these as their locations were only vaguely known. It is, however, possible to he reasonably sure about two of them. Cephalaspis salweyi ((Plate 1), fig. 1; Salter, 1868; H. Woodward, 1881) and Pteraspis rostrata ((Plate 1), fig. 7), listed as coming from a quarry on the east side of Skirrid Fawr, probably came from the small quarry 800 m north-west of Llandewi Court (Locality 17, (Table 4); see p.25). Gethlellydd [sic] from which Pteraspis rostrata was recorded (Woodward, 1891) and Poraspis sericeus was collected ((Plate 1), fig. 8) is likely to be the small quarry 320 m west of Gelli-lwyd Farm (Locality 27, (Table 4)). Discoveries whose locations remain unknown are Cephalaspis lyelli (Woodward, 1891), Climatius ornatus (Murchison, 1839), Palaeaspis sericea (holotype), Scaphaspis lloydii, Pteraspis rostrata ((Plate 1), fig. 6), Rhinopteraspis [Pteraspis] crouchi (Lankester, 1878; 1873; Woodward, 1891) and Ptychacanthus dubius ? (Murchison, 1839; Woodward, 1891).

Stensiö (1932) described Cephalaspis whitei, C.agassizi and Securiaspis kitchini from the Skirrid Fawr quarry (Locality 17; (Plate 1), fig. 3) and the holotype of Stensiopelta woodwardi from 'Asylum Grounds, Abergavenny' ((Plate 1), fig. 2). The latter is probably the Asylum Quarry of Woodward (1891) from which he recorded Rhinopteraspis [Pteraspis] crouchi. He also recorded this species from a boulder in the railway cutting near Maindiff, Abergavenny. Stensis (1932, p. 201) added a note to his identification of S. woodwardi suggesting that, on the basis of observations by Wickham King, these specimens together with those of P.crouchi apparently came from a boulder in drift. The exact source of the holotype of S. woodwardi is thus uncertain. Subsequent collection of S. woodwardi and Pteraspis from the River Gavenny close to the asylum [SO 3042 1452] (Locality 28) by W N Croft may indicate that the material was in situ.

White and Toombs (1983) have recently described the Cwm Mill cephalaspid bed discovered by W N Croft (White, 1950). The following are present: Cephalaspis cradleyensis Stensid (abundant) C. cwmmillensis (one specimen), C.abergavenniensis (one specimen) and C.(Cwmaspis) billcrofti (one specimen). In addition, Croft collected the following from the same locality: Rhinopteraspis crouchi ((Plate 1), fig. 5; White, 1973), plant debris, eurypterid fragments (Hughmilleria? and Pterygotus?), Cephalaspis sp.and pteraspid fragments (GSM collection Zg 854–866, GSM 60296–60302). Turner (1973) recorded what she considered to be an atypical thelodont assemblage from Cwm Mill comprising Turinia pagei, the typical Dittonian species, as well as the Downtonian forms Katopwas grossi and Goniporus alatus. However, Karatajüte-Talimaa (1978, in Russian) has recorded similar assemblages from the Dittonian of Russia.

Plant debris is present locally in the green intraformational conglomerates, and in green mudstones and siltstones.

Prof. W G Chaloner has been able to identify only Pachytheca and axial fragments in the material collected during the resurvey. No spore analysis has yet been done on the St Maughans Formation within the district.

Biostratigraphy

White (1950) made a tentative attempt to zone the Lower Old Red Sandstone of the Anglo-Welsh area by means of its ostracoderm faunas ((Table 3)). Considering the fauna of the formation as a whole, Dr P L Forey (personal communication, 1977) concluded that, in view of the presence of Pteraspis rostrata and Rhinopteraspis [Pteraspis] crouchi, along with Poraspis and Corvaspis, the beds can be assigned to the lower part of the P. crouchi Zone. He regards the fauna as of early or mid-Dittonian age, equivalent to the Gedinnian of the European Rhenish marine succession and the Lochkovian standard stage.

Details

Numbered localities are fish-bearing localities referred to in (Table 4).

West of Abergavenny

Here the beds are in the drift-filled Usk Valley and there are few exposures. Small trackside exposures near the top of the formation north of the Usk at Graig [SO 2530 1635]; [SO 2538 1636] are of interbedded sandstones and rubbly mature calcretes; this horizon may equate with Pontypool Limestone. A cliff section at the same strati-graphical level 1 km to the east [SO 2622 1600] shows:

A thick development of intraformational conglomerate occurs at the same stratigraphical level south of the Usk at Llanfoist [SO 2808 1349] where a section was exposed by a canal burst in 1975 ((Figure 6), Section 3).

North of Abergavenny

A veneer of glacial till covers the outcrop in the Gavenny Valley northwards from the town. Small exposures are present in the river between Abergavenny and Llantilio Pertholcy, the best one being the much documented section in a small tributary above Cwm Mill [SO 3110 1557] ((Figure 6), Section 1). Croft's discovery of a bed of whole cephalaspids (White, 1950) ((Plate 1), fig. 4) subsequently attracted many workers (Allen,1964b; Allen and others, 1968; Allen and Tarlo, 1963; 1968; Turner, 1973; White, 1973; White and Toombs, 1983). Allen (1963) gave a detailed description of the section and its sedimentology. Fauna obtained from this locality is listed on p.21.

West side of Gavenny Valley

Two small quarry exposures [SO 3058 1874] and [SO 3091 1955] are present towards the top of the group on the flanks of Bryn Arw. The more easterly (Locality 1) exposes 1.35 m of intraformational conglomerate with Poraspis? sp.and Kujdanowiaspis sp.A sequence mainly of mudstones near the top of the formation is exposed in the stream in Cwm Bryn-arw.

East side of Gavenny Valley

Fish-bearing intraformational conglomerate exposures on the south-west flanks of Skirrid Fawr (Localities 2 and 3) are listed in (Table 4). The best stream section on the western side of Skirrid Fawr [SO 3100 1887] to [SO 3206 1911] exposes about 8 m of red mudstones to the north-east of a fault. The following is seen south-west of the fault:

Fossiliferous exposures upstream (Localities 5 and 7) are listed in (Table 4).

Northern flanks of Skirrid Fawr

Locality 6 ((Table 4)) is the basal section of several in a roadside ditch near Llwynfranc Farm [SO 3263 1910] to [SO 3287 1914] and comprising sandstones, intraformational conglomerates and mudstones, some of which are green and contain plant fragments.

Good sections are present in a stream above (Localities 8, 9) and below Pant-y-tyle Farm. The section below the farm reads:

Sections further downstream (Localities 10 and 12) are listed in (Table 4).

There are good sections in a small tributary stream which rises below Pen-y-parc, e.g. [SO 3360 1941] and in a disused quarry near Whitehouse farm (Locality 24).

An old quarry west-south-west of Gelli-Iwyd Farm [SO 3504 1822] (Locality 27) shows:

North-east of Skirrid Fawr

Numerous deeply incised streams contain good exposures. In Full Brook, for example [SO 3572 2009], the beds are disturbed by faulting and associated folding. In a tributary of the Trothy north-west of Great House [SO 3712 2056] (Locality 25), the following typical section

To the east of Pothouse (Locality 4), there are numerous exposures in small deeply incised eastward flowing streams, for example Nant-y-Carw and a tributary to the south. The sandstones are generally 1 to 1.5 m thick, and are red-brown, pale green or grey in colour. They are fine grained, micaceous, calcareous, cross-bedded, and rest on the underlying scoured surface of intraformational conglomerate or mudstone; intraformational debris and fish fragments are commonly present in the basal parts. Each sandstone typically forms the basal part of a fining-upwards cycle, and passes up through red-brown siltstone and mudstone about two to three metres thick. Calcrete nodules are common, particularly in the upper part of the mudstone. The sandstones do not in general form features on the interfluves in this area and cannot be mapped and matched between the streams. Also, there is evidence of structural disturbance of the beds in the lower reaches of these streams towards their confluence with Full Brook.

A calcrete comprising cobble-size nodules of grey-green limestone below 0.3 m of massive rubbly limestone was noted on the Arwallt, on the north-east flanks of Skirrid Fawr [SO 3351 1858]. An old quarry to the east [SO 3427 1951] (Locality 16) exposes intraformational conglomerates rich in fish debris.

Llanddewi Sgyrrid–Llannetherine

There are excellent sections in two small streams above Pentre Farm [SO 3400 1782]. Fish fragments from a conglomerate in the easterly stream (Localities 14 and 15) are listed in (Table 4). The following temporary section was recorded nearby in a cutting behind West Cottage [SO 3404 1805].

This is probably the locality from which P. rostrata was collected (Woodward, 1891) (see p.21).

The following was recorded in an old quarry [SO 3354 1767] 650 m north-east of Skirrid Farm (Locality 17):

This is probably the locality from which Cephalaspis whitei, C. agassizi, P. rostrata and Securiaspis kitchini were recorded (Lankester, 1868, 1881; Stensio, 1932; Woodward, 1891) (p.21; (Plate 1)).

Fish remains from a conglomerate in a lane section at Llanvetherine [SO 3652 1730] (Locality 26) are listed in (Table 4).

Abergavenny–Skirrid Each

The Gavenny stream at Pen-y-Fal Hospital (formerly the Asylum), [SO 3043 1451] (Locality 28) shows:

This is the locality from which Croft obtained Stensiopelta woodwardi and Pteraspis sp.(see p.21).

A small unnamed stream in Tredilion Park has sections exposing fish-bearing intraformational conglomerates overlain by sandstones [SO 3140 1481], [SO 3172 1449], [SO 3174 1442]. The last (Locality 23) yielded P. cf. rostrata.

A small stream on the western slopes of Skirrid Fach [SO 3120 1364] to [SO 3127 1365] exposes about 19 m of beds ((Figure 6), Section 2). This section was mentioned by Strahan and Gibson (1900, p.10) and Robertson (1927, pp.12,13). Robertson suggested that the presence of 'concretionary limestone' indicated the 'Psammosteus Limestone Zone', and equated the bed with the one at Llandewi Rhydderch c. [SO 357 131] (see p.16). This correlation is erroneous, for the Skirrid Fach exposures lie at a much higher stratigraphica] level, towards the top of the St Maughans Formation, whereas the limestone at Llandewi Rhydderch is correlated with the Psammosteus Limestone.

Sandstone and conglomerates are exposed in a gully on the southwest flanks of Skirrid Fach [SO 3061 1349] to [SO 3114 0327] and blocks from a ditch nearby [SO 3096 1297] yielded fish fragments (Locality 21).

Skirrid Fach–Llangattock nigh Usk

The best sections are in the tributary of Mynachdy Brook below Tredilion Farm e.g. [SO 3328 1451], in a disused quarry in Quarry Brake [SO 3493 1421], in a disused quarry near Upper Farm [SO 3358 1230], and in the upper reaches of Ffrwd Brook [SO 3289 1174]. These lie towards the base of the St Maughans Formation, and the sandstones are predominantly green, with plant debris being common.

Llanover–Pontypool

The outcrop is largely drift-covered south from Llanover to Goetre where the formation is estimated to be about 490 m thick. The best section south of Goetre is in a stream [SO 305 045] ((Figure 6), Section 4). An intraformational conglomerate in a gully in Lower Wern Wood [SO 3023 0192] consists of granules, pebbles and pellets with interstitial angular to subangular detrital silt- to fine sand-grade quartz cemented by sparry calcite. The pebbles are predominantly of calcareous quartz siltstones and the pellets are micritic calcrete, some with silt-grade quartz (Strong, 1977). A sandstone in a disused pit north-east of Ty-poeth Farm [SO 3029 0154] is a pale brown grey fine- to very fine-grained subgreywacke composed of angular quartz grains with patchy quartzitic texture. Interstitial chlorite and iron oxide are abundant as well as authigenic quartz (as chert), albite and some large detrital mica flakes. Baryte is present on a joint surface (Strong, 1977).

The Pontypool Limestone can be traced from Coed Howell [SO 3015 0515] to the district boundary. It crops out in Mamhilad House Wood, and 2.4 m of mature calcrete are exposed in a small pit [SO 2959 0354]. Specimen (E49437) from the base of the exposure is a granoblastic microsparite with veins and patches of sparite. Specimen E 49438 is a dark grey/cream mottled micrite recrystallised in patches to microsparite. Specimen (E49439) comes from the top of the limestone where red and blue marl infill small pipes. It is a red/grey mottled microsparite with micrite and some wisps of micaceous siltstone. There are also some secondary veins of iron oxide and associated rhombohedral carbonate (?ferroan calcite) (Strong, 1977).

Senni Beds

Originally included within the Red Sandstone of Strahan and Gibson (1900), this formation was recognised within the district by Robertson (1927). It crops out in an extensive area north and west of Abergavenny, along the base of the coalfield escarpment, and as an outlier on Skirrid Fawr. Thickness ranges from about 200 m in the north of the district to 100 m in the south-east. The attenuation in the south-east suggested to Robertson (1927, p.11) the possibility of overlap of the Senni Beds by the Brownstones, but this has not been confirmed by the resurvey. In the extreme southeast, north of Pontypool, a colour transition takes place in which the green sandstones of the Senni Beds pass southwards into purple and red-brown sandstones that are included in the Brownstones.

The base of the Senni Beds is placed where sandstones become predominant in the succession, producing a marked change of slope. Locally, mature calcrete lies at the top of the underlying St Maughans Formation, and the base of the Senni Beds is placed at the top of the calcrete.

The Senni Beds consist mainly of fluvial sandstones, with minor mudstones and siltstones. The sandstones range in colour from olive-green to purple and purplish grey. Generally, green sandstones occupy the upper part of the formation and purple ones occur in the lower part. Green sandstones are present to the west and south of Abergavenny, and also immediately north of the town on the hills of Rholben and Deri, but further north, they pass into purple sandstone facies, and only purple sandstones are present on the northern flanks of the Sugar Loaf, on Bryn Arw and on Skirrid Fawr, north-east of Abergavenny.

The sandstones are mainly cross-bedded and lenticular, though parallel lamination and tabular beds are also found, particularly in the purple sandstones. They are predominantly fine to medium grained but the formation generally coarsens upwards, and coarser gritty sandstones occur towards the top of the formation along with pebbly sandstones. Individual beds show a cyclic, fining-upwards pattern. Clastic mica is particularly common in the olive-green plant-bearing sandstones. The sandstones are generally calcareous, decalcification leading to brown weathering. Intraformational calcareous conglomerates are common, though much less so than in the underlying St Maughans Formation. Intraformational debris also occurs in the bases of sandstone units. Mudstones and siltstones, mainly red-brown but locally green, are present as thin interbeds between the sandstones, draping their scoured tops; thicker and more laterally persistent beds are also common, giving 'slacks'. Calcrete nodules are present in places in the mudstones, but calcrete is more commonly seen as reworked clasts in the conglomerates and in the sandstone bases. Mature calcrete, in the form of tough massive mottled micritic limestone, occurs locally.

Sedimentation

No detailed work has yet been done on the sedimentation of the Senni Beds of the district, though a generalised environmental interpretation of these and the succeeding Brownstones has been given by Allen (1974b). Loeffler and Thomas (1980) provided a more detailed picture of the depositional environment or the Senni Beds of the Brecon Beacons area. Considered by Robertson (1933) to be deltaic, they are now interpreted as fluvial in origin. The intraformational conglomerates and most of the sandstones are in-channel deposits, stacked in multi-storey high energy channel-fill sequences; some of the finer-grained sandstones may be levee or crevasse-splay deposits. The mudstones represent overbank floodplain deposition in temporary lakes and abandoned channels. The green sandstone facies with its vascular plant flora is considered to represent rapid sediment accumulation under shallow water table conditions allowing the plants to flourish and then be preserved under post-burial reducing conditions (see Loeffler and Thomas, 1980, p. 293).

Biostratigraphy

The Primrose Hill Quarry pteraspidid (see below; p.18) discovered by W N Croft Rhinopteraspis cornubica (Allen and others, 1968; White, 1938; White, in discussion of Allen and others, 1968), is the only fossil vertebrate known from the Senni Beds of the district, and this is the only occurrence of this species, taken as the Breconian index fossil, in South Wales and the Welsh Borderland.

Llanover Quarry is the best known and most prolific of the fossil plant localities of the district. Croft and Lang (1942) recorded a flora including Drepanophycus, cf. Psilophyton, Dawsonites and Gosslingia (see p.30 for complete list). Mortimer (1967) and Richardson and Lister (1969) recorded microfloral assemblages containing mid-Dittonian spores together with several later forms.

All the fossil evidence indicates a middle to late Pragian age for the Senni Beds.

Details

West of Abergabenny

In debris from a trench for the Talybont Reservoir dam, Croft and Lang (1942) recorded the following floral assemblage from beds at the top of the formation: Drepanophycus spinaeformis, Gosslingia breconensis, Zosterophyllum cf. australianum, Prototaxites sp., Nematothallus sp.and Pachytheca sp.There are sections in purple channel-fill sandstones in the stream north of there. The formation is very well exposed in the River Usk west of Llangynidr. At Glaw-Coed Wood for example, crags up to 20 m high are of large-scale, cross-stratified, fine- to medium-grained, purple, grey and greenish grey sandstones with some thin intraformational conglomerate layers. Exposures in the river from Llangynidr [SO 1515 2030] to Glan Usk bridge [SO 1922 1990] are predominantly of in-channel cross-stratified and flaggy sandstones, mainly purplish grey and greenish grey in colour. The sandstones are fine to medium grained, fine upwards, and have intraformational debris in their bases; some thin intraformational conglomerate layers are present. Near Pant-teg farm [SO 1646 2003], a mature calcrete up to 0.1 m thick rests sharply on underlying red-brown silty mudstone and has an eroded top overlain by sandstone with intraformational debris at its base.

An old quarry towards the top of the formation on the Myarth [SO 1739 2055] exposes 15 to 22 m of thickly bedded to massive medium- to coarse-grained sandstones. These are grey where fresh, weathered to green, and the bases of the units are conglomeratic and contain subrounded pebbles up to 13 mm, mainly of quartz, but with some black chert. Thin red-brown and green siltstone interbeds have deeply scoured upper surfaces. Two thicker and more laterally persistent mudstone/siltstone beds crop out to the east. The lower bed is exposed in an old quarry near Penmyarth Cottage [SO 1830 2002] where 2 m of red-brown mudstone contain green siltstone layers with plant debris. Thickly bedded purple-brown fine- to medium-grained micaceous sandstone lies below.

Primrose Hill Quarry [SO 2070 2000] ((Figure 7), Section 1) lies at the top of the formation, the junction with the overlying Brownstones lying close above. The sequence consists predominantly of thickly bedded, internally cross-bedded, medium- to coarse-grained green sandstones, pebbly in places. Intraformational mudstone clasts are present in the scoured bases of units and some red-brown mudstone beds, up to 2 m thick occur. It is not known from which bed the ptcraspidid Rhinopteraspis cornubica (see above) was obtained. Fallen blocks of green sandstone in the quarry floor contain red mudstone burrowfills.

Steeply dipping red mudstone and siltstone striking east-northeast–west-south-west and shattered grey sandstone and conglomerate at Pregge Mill, Crickhowell [SO 2164 1931] probably mark the position of the Neath Disturbance (Taylor, 1974).

The formation is well exposed in the Grwyne Fechan at Llanbedr where, for example, a river cliff [SO 2434 2018] consists of large-scale cross-bedded purple and purplish grey channel sandstones. Some units are massive, and some flaggy and there are also intraformational conglomerate layers with large calcrete clasts. To the north [SO 2413 2041] to [SO 2392 2067], purple gritty trough cross-bedded sandstones are exposed in a shallow syncline. Although stratigraphically far below the Senni Beds–Brownstones junction these beds are of typical Brownstones facies.

In an old quarry north-west of Llangenny [SO 2336 1824], 4 m of large-scale planar cross-bedded medium- to coarse-grained purple and green sandstone with layers of intraformational mudstone and siderite clasts are seen. About 70 m below, the following section is exposed in an old quarry [SO 2352 1784]:

The Senni Beds crop out on the lower slopes of the Sugar Loaf, where there are thick and laterally persistent argillaceous beds. The best sections are in the lower outlying hills of Rholben and Deri to the south-east. Basal beds of the formation are well exposed in crags near Tyncwydd [SO 263 160] ((Figure 7), Section 2) and show a transition from a mixed sequence of sandstones and intraformational conglomerates at the base to one in which sandstones are predominant. About 40 m of beds are exposed and the base of the formation is placed above the higher of two well-developed calcretes (see p.23). All the sandstones are fine grained, and purple or red-brown, with pale green leached zones at their bases. Green plant-bearing sandstones which lie at a higher level are intermittently exposed south-west of Pen-y-graig.

North of Abergavenny

Small quarries on the lower slopes of Rholben expose purple sandstones. A large old quarry higher up the sequence [SO 2825 1675] shows the following:

In a small quarry 320 m to the north-west [SO 2805 1704], 2 m of green, channel-fill sandstone overlie a slump bed of deformed sandstone lenses in green mudstone.

There are good exposures in old quarries on Twyn-yr-allt, Deri. The best section reads:

Old quarries to the north [SO 2967 1625] expose up to 5 m of channel-fill sandstones, mainly khaki-green and brown, but some are grey and mauve. Grain size ranges from fine to medium and the bedding geometry is very irregular with large-scale cross-stratification caused by lenticular or wedge-shaped units bounded by erosion surfaces. Some thin silty mudstone partings drape these surfaces. There are some layers of intraformational mudstone and calcrete class conglomerate, including, towards the base of the exposures, a bed of largely randomly orientated decimetre-size grey-green mudstone clasts. Fallen blocks of fine- to medium-grained parallel-laminated sandstone contain vertical burrows 2 mm and 5 mm in diameter infilled with red-brown mudstone.

A similar high-energy multistorey channel-fill purple sandstone sequence at a lower stratigraphical level is exposed in a quarry [SO 2988 1625] comprising about 5 m of wedge-shaped sandstone units and a few red-brown mudstone interbed drapes. Some of the sandstones are highly calcareous giving rubbly weathering. Both fine-grained and medium-grained varieties are present and some sandstones are micaceous. There are a few intraformational conglomerate layers, and intraformational debris in the bases of units consists mainly of mudstone clasts with subordinate siderite and calcrete. Similar beds are exposed in a quarry to the north-east [SO 300 164].

Croft and Lang (1942) recorded Drepanophycus spinaeformis, Gosslingia breconensis, and Pachytheca sp.from unspecified localities on the Deri.

Northwards from the Deri, the green sandstones pass into purple sandstones. No green beds occur on Bryn Arw, where at the southern end, medium- to coarse-grained trough cross-bedded purple sandstones and intraformational conglomerates are very well exposed in crags. Persistent mudstone/siltstone beds lie between the sandstones.

Skirrid Fawr is an outlier of Senni Beds. Trough cross-bedded quartzose intraformational conglomerates and interbedded sandstones lie at the base of the formation and are exposed in crags at the northern end of the hill [SO 3323 1843] and [SO 3330 1829]. Close above lie two prominent thick mudstone beds, exposed at the base of the landslip backscar [SO 3304 1828], where (Figure 7), Section 3 was recorded. (Figure 7), Section 4 was compiled from exposures at the southern end of Skirrid Fawr [SO 3280 1719].

About 20 m of purple sandstones with intraformational conglomerate layers underlying this section are seen in a landslip backscar 300 m to the north.

Abergavenny to Pontypool

Beds near the top of the formation are exposed in the Clydach gorge [SO 2460 1485] to [SO 2444 1451]. They consist mainly of cross-bedded tough grey faintly purple coarse-grained sandstones with some intraformational conglomerate beds. Quartz pebbles are present in some layers. Parallel-laminated soft green micaceous sandstones and red mudstone beds are also found.

In Llanwenarth stream, Govilon [SO 264 137] there are good exposures of flaggy to massive grey and huff sandstones.

Numerous large old quarries are present on the lower slopes of the Blorenge and Llanfoist. For example, about 5.5 m of beds [SO 2717 1352] consist mainly of purplish or greenish grey sandstones, with some intraformational conglomerates and red brown mudstone interbeds. All the sandstones have sharp tops and bases, the latter containing intraformational clasts; one of the mudstone layers has calcretc glaebules. Below the road, in Govilon quarry [SO 2732 1360], 7.6 m of mainly grey and purple sandstones are exposed. Several old quarries on Coed y Person expose purplish or grey sandstones. One of the best sections [SO 2759 1368] reads:

Another quarry [SO 2790 1350] at about the same stratigraphical level exposes a similar sequence.

There are numerous exposures by the old tramway through Glebe Wood from Llanfoist to Cwm Graf. Towards the top of the formation [SO 2819 1288], a green micaceous siltstone channel-fill deposit, 0.1 to 0.8 m thick, underlies 3 m of olive-green and buff sandstones and contains much carbonised plant debris identified by Dr D Edwards (personal communication 1982) as Hostinella. There are also a few isolated sporangia, some of which resemble Zosterophyllum llanoveranum. Across the disused tramway [SO 2814 1282], and at a slightly higher stratigraphical level, 1.9 m of olive-green flaggy micaceous sandstone with green mudstone clasts at the base rest on a scoured surface of red-brown mudstone with calcrete nodules. These beds lie very close to the top of the Senni Beds.

Numerous small old quarries in pale grey, green and purple sandstone are present on the steep slopes below and to the east of Coedyprior Common. The best [SO 2943 1063] exposes about 8 m of a multistorey sandstone sequence. The sandstones are fine grained, purplish grey where fresh, brown where decalcified and leached to pale green at their bases. Beds are up to 2 m thick, one is lenticular, and all have sharp bases and eroded tops. Red to brown mudstone is confined to thin interbeds and scour hollows.

The topmost 20 m of the formation, together with overlying Brownstones are exposed in a lane section [SO 2920 0826] to [SO 2934 0839] (Barclay and Jackson, 1984). The top of the Senni Beds is placed at the top of 2 m of buff and greenish parallel-bedded sandstone. The basal bed of the Brownstones is unexposed, but is 2 m thick, and probably a mudstone.

The disused quarry at Ffawydden [SO 2966 0793] known as Llanover Quarry^‡1  is a classic locality renowned for its Pragian (Siegennian) flora. About 9 m of interbedded olive-green sandstones and green mudstones are exposed (Figure 8). Croft and Lang (1942) recorded a rich floral assemblage, mainly from a 10 inch mudstone layer and comprising the following: Drepanophycus spinaeformis, cf. Psilophyton princeps, Dawsonites arcuatus, Gosslingia breconensis, Zosterophyllum cf. australianum, Z. llanoveranum, Z. sp., Cooksonia sp., Sporogonites exuberans, Sciadophyton steinmanni, Taeniocrada sp., Prototaxites sp., Nematothallus sp.and Pachytheca sp.Edwards (1969) described Z. llanoveranum in detail. Mortimer (1967) and Richardson and Lister (1969) listed spore assemblages from the quarry. W N Croft collected eurypteroid fragments (GSM (BGS) Zg 867–868A).

Small quarries on Pentre Hill [SO 3006 0668], at Craig Ddu [SO 3028 0625], and west of Pen-y-stair Farm [SO 304 058] give typical sections of the Senni Beds.

There are many scattered outcrops in Cwm Wood [SO 302 056] of yellow-green and green-grey calcareous sandstones, weathered to buff-brown. They are arranged in fining-upwards cycles of erosivcly based sandstone, flaggy and cross-bedded at base, and parallel-laminated at the top. Green mudstone clasts and plant debris are present throughout. Similar stacked fining-upwards sandstone units are seen in exposures in Coed Howell [SO 2988 0524].

Sandstones from Mamhilad House Wood [SO 295 035] are subgreywackes with, in addition to quartz grains, a variety of rock fragments including chert, metaquartzite, volcanic rock, mudstone, detrital felspars and white mica. Intraformational conglomerates from the same locality contain a similar variety of grains in addition to pebbles of micritic calcrete (Specimens (E49431), (E49432),(E49433), (E49441), (E49443)–G Strong, 1977).

Brownstones

The Brownstones are predominantly arenaceous. Their main outcrops are around the coalfield escarpment and on the Sugar Loaf, with smaller areas near Llanbedr and on Buckland Hill. Thickness ranges from about 130 m in the south-east to about 400 m in the north-west. The reduced thickness in the south-east may be due to the unconformable overstep of the Upper Devonian rocks towards the Usk Axis (Allen, 1974b). The formation consists mainly of red-brown, purple-brown and pinkish brown cross-bedded calcareous micaceous sandstones. Grain size ranges from fine to coarse, with gravelly, pebbly beds locally present. Red mudstones are largely confined to thin interbed drapes, but thicker and laterally persistent beds also occur. Cross-bedding is typically irregular with lenticular or wedge-shaped channel-fill units defined by cross-cutting erosion surfaces stacked on top of each other to give multistorey sandstone bodies. Cyclicity is evident in most exposures, the beds comprising stacks of upwards-fining units. Intraformational debris in the form of mudstone, siltstone and calcrete clasts is a common constituent of the basal parts of units, overlying scour or erosion surfaces cut in the bed below. Exotic debris includes pebbles of vein quartz, quartzite, jasper, chert and acid lava. Allen (1974b) reported evidence of frequent subaerial exposure in the form of sun cracks and rain prints.

Sedimentation

Like the Senni Beds, the Brownstones are entirely fluvial, with much of the sediment deposited in stream channels. Allen (1974c) and Allen and Crowley (1983) demonstrated that the sediment source was of early Palaeozoic rocks in the Irish Sea–North Wales area. The preponderance of high energy channel-fill sandstones in the district invites comparison with the beds at Ross-on-Wye described by Allen (1983) and interpreted by him as the deposits of low sinuosity, braided sand-bed streams combing a mud-draped flood plain in a proximal setting. The sheet sandstones comprising stacked channel-fill units and interbedded mudstones or siltstones represent a more distal setting and compare with facies sequence B of Tunbridge (1981b), interpreted by him as low-sinuosity, flash-flood channel deposits merging laterally and downslope into muddy floodbasin deposits. The mudstones and siltstones are presumed to have been deposited from slow-moving or still-water bodies on the floodplain, although a windblown origin is also possible (Tunbridge, 1981b).

Biostratigraphy

The beds have, to date, yielded no fauna or flora and thus they can only be assigned an age from indirect evidence. Succeeding the middle to late Pragian Senni Beds, and truncated by a major unconformity of Middle Devonian age, they are late Pragian to Emsian in age. They were included by Croft (1953) in the Breconian local stage.

Details

West of Abergavenny

Disused quarries by the old railway track above Talybont Reservoir provide good exposures, the best [SO 0945 1735] showing 20 m of cross-bedded purple sandstones with mudstone interbeds.

There are good exposures in Nant ddu c. [SO 0925 1555], including the topmost beds of the formation (Taylor and Thomas, 1974).

The best exposure at Llangattock is in a disused quarry [SO 1968 1618] where 6 m of thickly bedded to massive fine- to medium-grained purple sandstones are seen.

North of Primrose Hill Quarry c. [SO 205 206] small exposures show a transitional sequence of purple and greenish sandstones between green sandstones of the Senni Beds and purple-brown sandstones of the Brownstones.

A disused quarry on the hillside west of Llanbedr [SO 2300 2030] exposes strongly cross-bedded purple-grey coarse-grained gravelly sandstone with scattered quartz pebbles, one of which has a long axis of 8 cm.

The formation is present as an outlier on the Sugar Loaf, capped by the Upper Old Red Sandstone. Several thick and laterally persistent mudstones are present towards the base. The upper part of the formation on the steep higher slopes of the mountain contains thinner mudstone layers. There are few good sections. Pale grey-green gritty sandstone debris occurs near the base of the formation on Mynydd Llanwenarth; small disused quarries on the western flanks [SO 2535 1895], [SO 2580 1872], [SO 2562 1827] worked speckled pale purple, grey and brown sandstones. Exposures higher up the mountain [SO 2668 1875], [SO 2670 1868] are mainly of purple and pink coarse-grained gritty sandstones with scattered quartz pebbles and pebbly conglomeratic layers. Bedding is discontinuous and lenticular.

South of Abergavenny

The Brownstones are well exposed in the Clydach gorge from Gilwern [SO 2535 1445] to Clydach [SO 2235 1267], in the stream and along the Heads of the Valleys road. They consist of irregularly bedded sandstones in lenticular units bounded by cross-cutting erosion surfaces. Both gritty rather soft sandstones with much lithic debris and harder quartzose sandstones are present. Intraformational debris mainly comprises red mudstone fragments and some calcrete and siderite clasts; exotic angular to subangular quartz pebbles up to about 10 mm are scattered throughout. Mudstone beds are confined to thin interbed drapes.

The topmost beds and their junction with the overlying Quartz Conglomerate Group are exposed in Nant Sychnant [SO 2253 1270]. The junction is an erosion surface cut in mudstone. The following was recorded:

About 12 m of sandstones are seen in Nant Dyar below the faulted junction with the Quartz Conglomerate Group [SO 2317 1290] and are arranged in lenticular units defined by erosion or scour surfaces. Mudstones are preserved in places in scour hollows. The units exhibit large-scale cross-bedding internally. Both fine-grained purple-brown soft sandstone and harder medium- to coarse-grained purple sandstone are present. Further downstream, in the vicinity of Pont yr Efail [SO 2310 1302], red-brown cross-bedded sandstones, pebbly grits and conglomerates are exposed.

A. disused quarry at Gilwern [SO 2470 1414] exposes about 15 m of beds comprising red-brown and purple, cross-bedded, locally pebbly sandstones with some mudstone partings. Conglomerate layers consist mainly of quartz pebbles but also contain other pebbles and intraformational red and green mudstone clasts.

The formation is well exposed in the Punchbowl [SO 2800 1170] on the eastern flanks of the Blorenge where cross-bedded purple sandstones predominate. A minor cyclicity is seen in places here; this consists of a thin basal unbedded layer resting on an erosion surface cut in red-brown mudstone and overlain by trough cross-bedded sandstones. The sandstones are commonly pebbly, and conglomerate layers contain quartz (mainly) and jasper pebbles and intraformational mudstone clasts. Towards the top of the formation the beds are leached to soft, pale green sands which have been dug in small disused adits nearby [SO 2772 1136].

The basal part of the formation is exposed in a lane section [SO 2920 0826] to [SO 2928 0831];

Beds near the top of the formation are exposed in crags south-west of Mynydd Garn-clochdy [SO 2937 0516] where the following section was compiled:

Upper Devonian (Upper Old Red Sandstone)

The Upper Devonian is represented by the Plateau Beds and the Quartz Conglomerate Group, the latter extending up into the early Carboniferous. The beds crop out in a narrow belt round the coalfield escarpment and in outliers on Pen Cerrig-calch and the Sugar Loaf. (Figure 9) gives a schematic cross-section illustrating the distribution of the constituent formations and the relationships between them.

Plateau Beds

This formation is about 26 m thick in the extreme west of the district and is progressively reduced by overstep of the Quartz Conglomerate Group to the east. The feather edge is in the Blaen Onneu area (Thomas and Taylor, 1974). A thin development of the formation is present immediately north of the sheet boundary at the Darren (Barclay, 1978).

Most of the work done on the Plateau Beds during the resurvey has been previously published (Hall and others, 1973; Taylor and Thomas, 1974, 1975). The sedimentology of the formation has been described by Lovell (1978a,b).

Two lithological units are recognised, Divisions B and C of Lovell (1978a). Division B, the basal unit, consists of about 18 m of thinly to medium-bedded, red-brown, fine- to medium-grained cross-bedded sandstones. The cross-bedding is directed towards the north-west. The base of the formation is placed at the base of the lowest granule-rich mudstone (Croft, in Thomas, 1951; Taylor and Thomas, 1974; Lovell, 1978a). Division C of Lovell comprises a mixed sequence of fine-grained sandstones and mudstones in medium-bedded units. Bioturbation is common and lingulids, brachiopods and fish and plant debris are found. The unit is about 18 m thick, and can be further subdivided into two. The lower 9 m is devoid of fossils, although bioturbated, and consists of a series of fining-upwards cycles. The sandstones are very fine grained, parallel laminated, and have sharp bases. Some channel scour-and-fill structures are seen. Some beds have rippled surfaces and mudcracks are present at some levels. The upper 9 m begin with a fish bearing intraformational conglomerate containing siderite clasts. Above this are cross-bedded channel-fill sandstones; fossil fragments, including brachiopods, occur in intraformational debris in the bases of channels and in thin layers between sandstones in the channel-fill. A variable sequence, mudstone-dominated in some places and sandstone-dominated in others, forms the top of Division C.

Sedimentation

The basal bed of the formation, comprising quartz granule layers in mudstone, was considered by Lovell (1978a) to be a mudflow deposit. The sandstones of Division B, considered by Allen (1965) and Taylor and Thomas (1975) to be fluvial, were thought by Lovell (1978a,b) to be mainly aeolian, with some intercalated water-laid (?wadi) sediments. The mixed sequence comprising Division C was interpreted by Taylor and Thomas (1974, 1975) as being marginally marine with tidal scouring being responsible for the channel forms; intertidal features such as bioturbation, desiccation cracks, ripple cross-lamination and bimodal cross-bedding were also noted. Lovell (1978a, b) found evidence of supratidal, intertidal and possibly subtidal environments.

The Plateau Beds thus record a generally transgressive sequence of events commencing with deposition of distal mudflows, followed by aeolian and fluvial deposition, and culminating in deposition in a marginal marine setting.

Biostratigraphy

The biostratigraphy of the Plateau Beds was described by Butler (in Taylor and Thomas, 1975). Fish fragments are common and Bothriolepis sp., Holoptychius sp., and cf. Pseudosauripterus anglicus have been identified from the Nant ddu–Blaen Dyffryn Crawnon area. Lingulids and calcareous brachiopods have also been recorded (Hall and others, 1973, Taylor and Thomas, 1974, 1975). The vertebrate remains serve only to confirm a late Devonian to early Carboniferous age for the beds. The presence of the spiriferid Cyrtospirifer verneuili, the rhynchonelloid cf. Ptychomaletoechia omaliusi and the bivalve Leptodesma cf. lichas in outcrops on the adjoining Merthyr Tydfil district suggests a Frasnian or early Famennian age for the formation (Allen, 1964a, 1977; Butler in Taylor and Thomas, 1975). Prof. W G Chaloner has compared a fragment of plant from Nant ddu [SO 0897 1549] with Prototaxites which has a known range of late Silurian to Frasnian. This combined with the invertebrate evidence and the presence of Bothriolepis would point to a Frasnian age.

Details

A detailed record of the formation is provided by the Cwar yr Ystrad Borehole [SO 0842 1446] (Taylor and Thomas, 1975) ((Figure 10), Section 1).

The formation is well exposed in Nant ddu [SO 0895 1532] and in cliff exposures from there to Blaen Dyffryn Crawnon [SO 0954 1494]. Details of these localities were given by Hall and others (1973) and Taylor and Thomas (1974, 1975). Lovell (1978a,b) provided further data ((Figure 10), Section 2).

A section in a tributary of Nant Pyrgad [SO 1044 1538] reads:

There are good sections in Nant Pyrgad [SO 1091 1558] where an intraformational conglomerate with lingulids, calcareous brachiopods, and fish fragments lies near the top of the Plateau Beds.

Quartz Conglomerate Group

(Figure 10) gives the principal sections of the Quartz Conglomerate Group. The three constituent formations, named by Lovell (1978a), are, in ascending order, the Wern Watkin Formation, the Craig-y-cwm Formation and the Garn-gofen Formation.

Wern Watkin Formation

The Wern Watkin Formation (new name) corresponds to the pale quartzitic sandstones of Barclay (1975) and equates with the Grey Grits of the Merthyr Tydfil district. It consists largely of pale greenish grey and buff fine-grained flat-bedded quartzitic sandstones. They are generally in beds between 0.1 and 0.5 m thick; interbed contacts are sharp and, in places, erosive. Mudstone, where present, is mainly confined to thin interbeds and drapes on the scoured upper surfaces of the sandstones and to clasts in the bases of the sandstone. The mudstones are mainly pale green, although red-brown mudstones also occur. Internally the sandstones show both parallel lamination and cross-bedding. There is no exotic debris in the formation where it is exposed around the coalfield escarpment, but quartz pebbles are present in the northerly outliers of the Sugar Loaf and Table Mountain, and in the extreme west of the district. A nodular or rubbly calcrete lies at the top of the formation in many places.

The type locality of the formation is a quarry near Wern Watkin Farm [SO 2107 1529] (see p.35).

A regional unconformity separates the formation from the underlying Brownstones over most of the district, except in the west, where it rests unconformably on the Plateau Beds. No angular discordance is seen in the few localities where the junction is exposed, but slight angular discordance is perhaps discernible between the formation and the underlying Brownstones where it caps the Sugar Loaf.

The thickness of the Wern Watkin Formation ranges from about 8 m in the west of the district to about 16 m in the east.

A fish-bearing conglomerate which has also yielded Sanguinolites sp.marks the base of the formation locally in the west of the district (Hall and others, 1973; Taylor and Thomas, 1974, 1975).

Craig-y-cwm Formation

The Craig-y-cwm Formation (new name) corresponds to the quartz pebble conglomerate unit of Barclay (1975). The feather edge of the formation is in the Blaen Onneu–Daren Cilau area. At Daren Cilau only about 5 m are present, but the thickness increases south and east of there to about 12 m in the Clydach area.

The formation consists mainly of quartz-pebble conglomerates, although these occur in lenses and the proportion of conglomerates to finer sediments varies between sections. Quartz pebbles predominate in the conglomerates but quartzitic sandstone, lava and jasper pebbles are also found. Fine-grained quartzitic grey-green sandstones, pebbly sandstones and red-brown mudstones, commonly with calcrete nodules, also occur. Mature rubbly calcretes and intraformational conglomerates are also present locally.

Fish fragments are occasionally found in the conglomerates (Barclay, 1975), and Lovell (1978a) has recorded Bothriolepis.

The type locality of the formation is Craig-y-cwm [SO 2825 0885]. The base of the formation is placed at the top of the persistent calcrete which marks the top of the Wern Watkin Formation, quartz-pebble conglomerates appearing for the first time at this level.

Garn-gofen Formation

The Garn-gofen Formation (new name) is equivalent to the 'sandstones and marls' of Barclay (1975). The formation wedges out in the Blaen Onneu–Daren Cilau area and reaches a maximum of about 20 m at Craig-yr-hafod. It is probably the equivalent of the Tintern Sandstone Group of the Monmouth and Chepstow districts (Welch and Trotter, 1961), and consists of a sequence of interbedded sandstones and mudstones. The sandstones in the basal part of the formation are pale grey-green quartzites; those in the upper part are grey-green, soft, friable, richly micaceous and feldspathic. The latter show parallel lamination and cross-bedding and are, in places, bioturbated. Thin intraformational calcareous conglomerates, consisting largely of calcrete clasts, are found at the top of the formation in the Clydach gorge, and a mudstone clast intraformational conglomerate lens containing a large amount of fish fragments is present in Nant Sychnant (Barclay, 1975).

The formation name is derived from Garn-gofen Farm near Craig-y-cwm [SO 2825 0885], the type locality being Craig-y-cwm. The basal contact with the Craig-y-cwm Formation is sharp and locally slightly erosional, and, at the type locality, is overlain by pale yellow to white micaceous sandstone (Lovell, 1978a).

Sedimentation

Allen (1965) made a detailed study of Upper Old Red Sandstone sedimentation, including sandstone petrography and provenance. The Grey Grits were considered by Taylor and Thomas (1975) to be marginally marine, but Allen (1965) and Lovell (1978a, b) favoured a fluvial depositional environment for these and the Wern Watkin Formation. They are interpreted as the deposits of shallow, possibly ephemeral, sandy braided streams flowing in a generally southerly direction. Abandonment of this alluvial complex is indicated by the presence of an extensive and mature calcrete. The Craig-y-cwm Formation is clearly fluvial and Lovell (1978a) suggested gravelly braided stream bed-load deposition for the conglomerates. The fine-grained sediments with calcretes are presumed to be the floodplain deposits of the alluvial complex, which was derived from the north-west. The Garn-gofen Formation is interpreted as the deposits of meandering streams flowing from the north or north-east. The abundance of elastic mica, garnet and feldspar in this formation suggests erosion of a metamorphic rock source.

Biostratigraphy

Fish fragments, mostly indeterminate, have been recorded from a few localities (Barclay, 1975; Lovell, 1978a). Among the remains collected from Nant Sychnant [SO 2252 1262] is a snout of cf. Osteolepis macrolepidotus; Lovell (1978a) recorded Bothriolepis sp.from the Craig-y-cwm Formation in Cwm Llanwenarth [SO 2545 1143]. Neither these or the bivalve Sanguinolites sp.from Dyffryn Crawnon (Taylor and Thomas, 1975) have precise stratigraphical significance, but a late Devonian to early Carboniferous age can be suggested.

Details

North of the Usk

Basal beds of the Wern Watkin Formation cap Table Mountain [SO 226 206] and the Sugar Loaf [SO 2725 1877]. Their presence on Table Mountain was explained by Robertson (1927, pp.14, 15) on structural grounds, involving downfaulting of the beds in relation to their position about 80 m higher on Pen Cerrig-calch, north of the district boundary. However, beds in the underlying Brownstones can be traced without a break between the two localities and land-slipping is thought to be a more likely explanation, the 10 m of quartzites of Table Mountain having slipped as a discrete mass. About 10 m of buff and white quartzites at the base of the Wern Watkin Formation cap the Sugar Loaf and rest on Brownstones. Viewed from the north, there is a hint of angular discordance between the two formations.

South of the Usk

Darren Fawr to Clydach

Details of the Cwar yr Ystrad Borehole [SO 0842 1446] and of the Nant-ddu, Darren Fawr and Dyffryn Crawnon areas have been published previously (Hall and others, 1973; Taylor and Thomas, 1974, 1975; Lovell, 1978a, b) ((Figure 10), Sections 1 and 2).

There are good exposures at Daren Cilau, Llangattock [SO 1925 1582]; [SO 2006 1592]; [SO 2034 1585] from which Section 3, (Figure 10) was compiled. Crag exposures [SO 1925 1582] show about 15 m of flat-bedded pale grey, green and buff quartzitic sandstones above which are 4 to 5 m of pale green cross-bedded sandstones with quartz pebbles aligned down the foresets. Near the top of these is a 0.3 m thick intraformational rubble bed with calcareous sandstone and calcrete debris. Exposures below the track [SO 2006 1592] show:

The rubble beds contain intraformational debris including calcareous sandstone which, in part, is bound with thin undulose subparallel discontinuous and anastomising calcareous laminae (Specimen E 54525). The laminae have a clear sparry core with a thin turbid micritic envelope, suggesting that the micrite formed a hollow envelope later infilled with spar. In the lower part of the specimen, more diffuse and thicker lamellae contain a few sand grains and grade into the sandstone matrix without a micrite envelope. The sparry material here shows a columnar structure, but with some suggestion of lamination. Although this lower part suggests calcretisation of the host sandstone, R W Sanderson (BGS Internal Report, 1981) considered that the lamination in the upper part is more like algal (stromatolitic) lamination than laminar calcrete.

Quartz pebble conglomerates and interbedded grey green sandstones in disused quarries [SO 2034 1585] yielded one large but indeterminate fish fragment.

A disused quarry [SO 2107 1529] is the type locality of the Wern Watkin Formation. About 8 m of pale yellow fine-grained flat-bedded quartzitic sandstone are present in medium to thick beds, with slightly eroded tops. Green silty micaceous mudstone is present mainly as clasts in the bases of the sandstones and also as thin interbeds.

Disused quarries [SO 2206 1450], [SO 2206 1438] and [SO 2257 1328] also expose beds of the Wern Watkin Formation.

Clydach to Pontypool

The Wern Watkin and Craig-y-cwm formations are well exposed in the Clydach gorge [SO 2239 1266] to [SO 2204 1268]; the topmost beds of the Garn-gofen Formation, comprising soft, friable, micaceous cross-laminated sandstones, green and red-brown mudstones, and intraformational calcareous conglomerates, are also exposed [SO 2202 1267] ((Figure 10), Section 4).

An exposure in the Clydach gorge [SO 2240 1265] shows:

The Wern Watkin Formation is well seen in Nant Sychnant, a tributary of the Clydach [SO 2253 1270] ((Figure 10), Section 5) where a waterfall exposes about 10 m of flat-bedded pale grey-green quartzitic sandstones resting with no apparent discordance on the Brownstones. The Garn-gofen Formation in Nant Sychnant [SO 2252 1262] mainly comprises soft richly micaceous pale grey-green parallel-laminated sandstones stacked in cross-bedded sets. Red mudstones, calcreted and containing buff dolomitic calcrete nodules, are interbedded with the sandstones towards the base. The sandstones display a variety of bioturbation traces; the topmost 0.20 m layer is distinguished by a lack of bedding and has a churned texture with coarser sand nodules presumed to be burrowfills. Cross-bedding and internal lamination are preserved in the underlying 0.35 m bed but are disrupted by a variety of coarser-grained buff sandstone nodules and lenses again presumed to be burrowfills. Two large oblique burrowfills are present in the underlying 0.77 m bed. One is 3 cm wide, closes upwards, and is filled with green siltstone which in turn is burrowed with small sandy burrowfills. The second burrow is 36 cm long, 5 cm wide at the top, tapering towards its base and filled with green siltstone.

A 0.2 m thick lens of gritty mudstone-clast intraformational conglomerate lying about 3 m above the base of the Garn-gofen Formation contains an abundance of fish fragments from which the following were identified by Dr R Miles: an arthrodire plate, an indeterminate placoderm bone, a snout of cf. Osteolepis macrolepidotus (see p.35), a rhipidistian tooth, indeterminate crossopterygian remains and an acanthodian spine.

The Garn-gofen and Craig-y-cwm formations are well exposed in Nant Dyar [SO 2322 1280] to [SO 2317 1286], but only the topmost part of the Wern Watkin Formation is present, faulted against Brownstones ((Figure 10), Section 6). A calcrete lying about 12 m from the top of the section contains veins with malachite and pyrite, the mineralisation presumably due to proximity to the fault.

There are good sections in the stream below Pen-y-galchen, Pwll du [SO 2491 1192] to [SO 2498 1195] ((Figure 10), Section 7). An indeterminate fish fragment was obtained from the base of a conglomerate in the Craig-y-cwm Formation.

Cwm Llanwenarth [SO 2540 1159] to [SO 2534 1180] ((Figure 10), Section 8) provides a good section of the beds. Fish fragments obtained during the resurvey are indeterminate, but Lovell (1978a) recorded Bothriolepis sp.

Garn-ddyrys Tip Borehole No.1 [SO 2572 1191] proved the following:

The Craig-y-cwm Formation is well exposed on the north-east face of the Blorenge [SO 2787 1223] where there are 7 m-high crags of quartz pebble conglomerates. The Wern Watkin Formation is well exposed at Craig-yr-hafod [SO 2780 0994], but exposure of higher beds is patchy ((Figure 10), Section 9).

Craig-y-cwm [SO 2825 0890] offers a more complete section ((Figure 10), Section 10; (Plate 2)) and provides the type sections for both the Craig-y-cwm Formation and the Garn-gofen Formation (Lovell, 1978a).

About 5 m of quartz pebbles conglomerates and trough cross-bedded gravelly sandstones are exposed at Craig-yr-allt [SO 2926 0679] (Plate 3). Quartz-pebble conglomerates are also exposed in disused pits on Mynydd Garn-wen [SO 2889 0434] and [SO 2868 0386] where they are friable and were dug for gravel.

Chapter 4 Dinantian

Dinantian rocks, known traditionally as the Carboniferous Limestone, crop out around the coalfield in a narrow belt which widens to 1 to 2 km in the west of the district. The exposed sequence is thickest there (140 m), but is reduced, mainly by Namurian overstep, to about 25 m on the east crop.

The rocks were deposited following a major northward-directed marine transgression over the Old Red Sandstone alluvial plain. They are mainly shallow water carbonates, and include off-shore shelf, shoal, and peritidal deposits, with lesser amounts of siliclastic marine and terrestrial rocks. The district lay on the northern margin of the South-west Province for much of the Dinantian (p.44); this is reflected in the thinness of the succession and the many breaks within it that are associated with features of emergence such as palaeokarstic surfaces and calcrete palaeosols.

The beds range in age from Courceyan to Holkerian. The absence of the Tongwynlais Formation (Waters and Lawrence, 1987) at the base of the sequence indicates a depositional hiatus (p.44). The topmost beds are unconformably overlain by Namurian rocks.

Previous research

Following early examination by Murchison (1839) and De La Beche (1846), Strahan and Gibson (1900) divided the Carboniferous Limestone into the Lower Limestone Shales and overlying Main Limestone. The west-to-east attenuation of the Main Limestone was thought to be due to internal thinning of its constituent members. Dixon (in Strahan, 1909) was the first to point out the unconformable nature of the Millstone Grit–Carboniferous Limestone junction. Robertson (1927, 1933) and Robertson and George (1929) applied Vaughan's coral-brachiopod zones to the succession, and recognised a westerly developing unconformity at the base of the Upper Seminula(S₂) Zone. George (1954) amended the zonal classification, and identified a lower unconformity at the base of the Upper Caninia (S₁) Zone, previously recognised in Gower and Pembrokeshire (e.g., Dixon and Vaughan, 1912). He also revised Robertson's (1927) lithostratigraphy and provided a sound basis for subsequent work. Reference to the district can be found in many papers by George (e.g. 1952, 1956a,b, 1958, 1969, 1970, 1972, 1974).

Reservations concerning the validity of Vaughan's zonal scheme (in discussion of George, 1954) were confirmed when the succession in the Avon gorge was found to be incomplete (Mitchell, 1972, 1980). Ramsbottom (1973) proposed a classification based on major cycles of transgression and regression, and George and others (1976) erected a new chronostratigraphical classification (Table 5) and (Table 6).

Sedimentological studies have been carried out on the Lower Limestone Shale (Burchette, 1981, 1987; Whitcombe, 1969), the Abercriban Oolite (Raven, 1986; Wright, 1986), and the Llanelly Formation (Riding and Wright, 1980; Wright 1980, 1981a, b, c, 1982a, b, 1986). Other published work includes conodont studies (Austin, 1973; Rhodes, Austin and Druce, 1969), and petrographical studies (Bhatt, 1973, 1975).

Lithostratigraphy

(Figure 11) gives a generalised section of the Dinantian sequence, and lithological divisions used in the resurvey are shown in (Table 5).

The Lower Limestone Shale Group is divided into two formations. The Castell Coch Limestone (Waters and Lawrence, 1987) is the lower, and consists mainly of calcarenites and oolites. The Cwmyniscoy Mudstone (new name) is proposed for the higher formation which consists largely of dark grey silty mudstones. It equates with the upper mudstone division of the Lower Limestone Shale of Squirrell and Downing (1969, p.59). It is named from

Cwmyniscoy quarries, near Pontypool [ST 2830 9940], on the Newport (249) Sheet where 25 m of dark grey, silty, micaceous mudstones are exposed. Thin laminae of calcareous siltstone and very fine-grained sandstone are scattered throughout, and thicker beds of shelly or crinoidal calcarenite occur in the uppermost parts. The base of the formation is placed at the sharp junction with the underlying massive calcarenites of the Castell Coch Limestone. The top is marked by the fairly sharp base of the dolomites of the overlying Main Limestone (Squirrell and Downing, 1969, p.59).

Above the Lower Limestone Shale Group is a variable group of strata, named the Oolite Group by George (1954). It is predominantly oolitic in the adjoining Merthyr Tydfil district where it was renamed the Abercriban Oolite (Taylor and others, 1979), and this term is now applied to the western outcrops of the Abergavenny district. Towards the east, dolomites interfinger with, and replace the oolites, and the beds are entirely dolomitic on the east crop. Here the name Clydach Valley Group is used, the type locality being the Clydach Valley where the two facies interfinger and where most of the constituent formations are defined. The classification used in this account is an emended version of the schemes proposed by George (1954) and George and others (1976). The formations, described in upward succession, are as follows:

Sychnant Dolomite

Sychnant Dolomite (new name): name derived from Nant Sychnant 12256 1251] where 4 m of fine-grained dolomites are present. This is Bed 1 of George (1954, p.288) and was included in the overlying Pwll-y-Cwm Oolite by George and others (1976). Its junction with the underlying Cwmyniscoy Mudstone is gradational, with thin fine-grained dolomite layers intercalated with shale. The base of the formation is placed at the base of the dolomite overlying the highest shale bed.

Pwll-y-Cwm Oolite

Pwll-y-Cwm Oolite: defined by George (1954), it was extended downwards by George and others (1976) to include Bed 1 of George (1954, p.288). It is here redefined to exclude this bed.

Pantydarren Beds

Pantydarren Beds (new name): name derived from disused quarries at Coed Pantydarren [SO 2200 1380] where 5 m of fine-grained, thinly bedded dolomites are present. It equates with Bed 3 of George (1954, p.288). The base of the formation is placed at the sharp junction between the dolomites and the underlying Pwll-y-Cwm Oolite.

Blaen Onnen Oolite

Blaen Onnen Oolite: George and others (1976) proposed this name for Beds 3, 4 and 5 of George (1954, p.288). Ordnance Survey maps show the locality as Blaen Onneu, and the quarry containing the type section is also known as such, but George and others (1976) followed Robertson (1927, footnote p.36) in assuming this to be an error. To avoid further confusion, the name Blaen Onnen is retained; the Blaen Onnen Oolite is defined to exclude the dolomites above and below (Beds 3 and 5 of George, 1954, p.288); it equates with Bed 4 of George (1954, p.288) and with the Middle Oolite of Owen and others (1965). The base of the formation is placed at the base of the massive oolitic grainstones which overlie a prominent rubbly bed 3 m above the base of the quarry face (see p.48).

Coed Ffyddlwn Formation

Coed Ffyddlwn Formation (new name): name derived from Coed Ffyddlwn [SO 2215 1260] in the Clydach gorge below Llanelly quarry. Two facies are represented in the formation; fine-grained dolomites predominate, with a thin peritidal unit at the top named the Darren Ddu Limestone (member). The formation equates with Beds 5 and 6 of George (1954, p.288); the Darren Ddu Limestone is George's Bed 6, the basal part of his Marker Beds and the basal part of the Clydach Beds of George and others (1976). The base of the formation is placed at the sharp junction between fine-grained dolomites and the underlying Blaen Onnen Oolitc. The Darren Ddu Limestone is named from disused quarries at Darren Ddu in the Clydach gorge [SO 2194 1290] where the member consists of 10 m of thinly bedded micritic limestones, fine grainstones, dolomitic mudstones and shales (see p.50).

Gilwern Oolite

Gilwern Oolite (Formation): named by George (1954) as the topmost formation of his Oolite Group (Bed 8, p.288), and equivalent to the Caninia Oolite of Robertson (1927). Bed 7 of George (1954, p.288) was named the Coral Bed and included in his Marker Beds. These were renamed the Clydach Beds by George and others (1976), with the Coral Bed the essential element of the formation as a widespread faunal marker bed, (W H C Ramsbottom, personal communication, 1978). The Coral Bed is here named the Craigy-Gaer Coral Bed (member) from Craig-y-Gaer [SO 2240 1324] where it consists of 2 to 3 m of coarse-grained Shelly crinoidal and skeletal grainstones, and where a sharp, erosive break separates it from the underlying peritidal Darren Ddu Limestone. Although there is in places a prominent parting, possibly a palaeokarstic surface, within the bed, it is, in general, the basal part of a fining-upwards unit of which the bulk is the Gilwern Oolite. It is best, therefore, to be included within the Gilwern Oolite, and the name Clydach Beds of George and others (1976) is discarded.

The Abercriban Oolite and Clydach Valley groups are truncated by a major palaeokarstic surface. Above this is the Llanelly Formation, named by George and others (1976), and equivalent to the Calcite Mudstone Group of George (1954). Four subdivisions of the formation were named by Wright (1981b) as members; in ascending order these are: the Clydach Halt Member, the Cheltenham Limestone, the Penllwyn Oolite (the Linoproductus Oolite of George, 1954 and the Seminula Oolite of Robertson, 1927), and the Gilwern Clay. Wright included a group of quartzitic sandstones within the Gilwern Clay which George (1954) placed within the overlying S₂ Zone limestones. Barclay and Jackson (1982) gave these sandstones separate formational status and named them the Garn Caws Sandstone.

The S₂ Zone limestones (Robertson, 1927) were named the Dowlais Limestone by George and others (1976).

Ramsbottom's (1973) correlation of the succession with his proposed major transgressive–regressive cycles is shown in (Table 5).

Biostratigraphy and chronostratigraphy

Lower Limestone Shale Group

The Castell Coch Limestone and the shelly limestone beds in the Cwmyniscoy Mudstone contain a rich brachiopod assemblage (Table 7) typical of Courceyan shallow water facies. The only record of a Tournaisian cephalopod in the shelf facies of the South-west Province is from near Abergavenny (George, 1952). Spores from the base of the Castell Coch Limestone indicate an early Tournaisian (Tn 2a) age, with the base perhaps becoming younger northwards (Lovell, 1978a). The Lower Limestone Shale Group was not collected systematically for conodonts during the resurvey. The fauna recovered from the Castell Coch Limestone (Table 8) is typical of that which occurs in the shallow-water, high energy facies of the Siphonodella Zone (Groessens, 1976) elsewhere in the South-west Province and Ireland. Polygnathus inornatus occurs in the basal transgressive beds of the formation, and in the beds at the top. The higher occurrence is related to a second transgressive event which established the deeper water conditions of the Cwmyniscoy Mudstone. Siphonodellids and P. inornatus were recorded in the basal beds of the Cwmyniscoy Mudstone by Rhodes and others (1969), and indicate a Siphonodella Zone age.

Abercriban Oolite Group and Clydach Valley Group

Pwll-Y-Cwm Oolite

The Pwll-y-Cwm Oolite contains a restricted brachiopod fauna of little stratigraphical value (Table 7). The conodonts (Table 8) are shallow water forms lacking in diagnostic elements, but probably belong to the Siphonodella–Pseudopolygnathus multistriatus interzone.

Pantydarren Beds

The Pantydarren Beds contain no useful macrofauna and only shallow water conodonts (Table 8). Shale layers near the base of the formation yielded assemblages of corroded and torn miospores (p.50). Verrucosisporites nitidus is recorded throughout the LN-CM zones (Table 6), and Umbonatisporites distinctus is known throughout the HD-CM zones. Dictyotriletes cf. sagenoformis has previously been recorded only from the Viséan, but there are only a few specimens in the assemblages, and they are here only referred to the species and not formally assigned to it (Owens, unpublished BGS report, 1976).

Blaen Onnen Oolite

The Blaen Onnen Oolite contains a sparse shelly fauna on the north crop that includes M. mitcheldeanensis, Syringothyris sp.and Unispirifer sp.Shelly dolomites at this stratigraphical level on the east crop have yielded Michelinia megastoma, Cleiothyridina sp., Syringothyris sp.and Unispirifer tornacensis (Table 7). Conodonts from near the base of the formation in the Clydach Valley include Bispathodus spinulicostatus,, the evolutionary intermediate between B. aculeatus and Pseudopolygnathus multistriatus, and Prioniodina oweni (Table 8). These suggest a position close below the base of the Ps. multistriatus Zone. A conodont assemblage obtained from Gallowsgreen Quarry on the east crop from shelly dolomite at a stratigraphical level corresponding to the top of the Blaen Onnen Oolite contains specimens of Ps. multistriatus with a morphology that suggests the lower part of the Ps. multistriatus Zone. The base of this zone is therefore placed within the Blaen Onnen Oolite (Table 8).

Coed Ffyddlwn Formation

The Coed Ffyddlwn Formation contains no macrofauna in the type area; the equivalent beds on the east crop contain

M. 'megastoma', Zaphrentis konincki?, Syringopora cf. ramulosa, M. mitcheldeanensis and Syringothyris? The conodonts recovered from the formation in the Clydach Valley (Table 8) are restricted shallow water Courceyan forms.

Gilwern Oolite

The age of the Gilwern Oolite is problematical despite the fact that the Craig-y-Gaer Coral Bed has yielded good macrofaunas and microfaunas. Ramsbottom (1973) suggested a correlation with Cycle 3 ( = Arundian), but George and others (1976) included the beds in the Chadian. These authors reported 'occasional foraminifera including Tetraxis cf. paraminimus and ammodiscids' from the Clydach Beds, presumably from the Craig-y-Gaer Coral Bed, which suggested to them correspondence with the V1-age rocks of the Belgian sequence. The presence of Stenoscisma cf. isorhyncha and the evidence of conodonts from the Gilwern Oolite were noted as being in accordance with this age. The Craig-y-Gaer Coral Bed is a critical horizon. George (1954, p.313, figures 10, 11, 12) illustrated a coral from this bed as Hapsiphyllum cf. konincki and suggested a Zaphrentis Zone age by comparison with material from the Bristol district. The original specimens have been re-examined by Mr M Mitchell who considers them to be eroded specimens of Palaeosmilia murchisoni, a species not known in rocks of Tournaisian age.

On the basis of the macrofauna collected from the bed at Craig-y-Gaer during the resurvey, Mitchell (1976, p. 116) suggested correlation with the upper part of the Upper Canini a (C₂S₁) Zone, that is, the Arundian. The fauna includes Michelinia megastoma, Palaeosmilia murchisoni, Composita sp Linoprotonia sp Megachonetes cf. papilionaceous, Stenoscisma isorhyncha and Syringothyris sp.(Table 7). A microfaunal assemblage confirming the Arundian age was reported by Ramsbottom (1976, p. 116), but attempts to repeat and check this record have been unsuccessful, and an examination of the lithology of the original slide suggests sample mixing during laboratory preparation; the record must therefore be discounted. Foraminifera from the bed include Biorbis duplex and Palaeospiroplectammina mellina sub sp.of Fewtrell and others, 1981. These forms, together with the absence of archaediscids, would suggest a Chadian age.

The conodont faunas are characterised by an abundance of Polygnathus mehli, which appears for the first time in the transgressive Graig-y-Gaer Coral Bed. This occurrence above strata of the Ps. multi striatus Zone suggests that the formation lies within the P. mehli Zone. The absence of evidence for the succeeding Mestognathus beckmani Zone, either due to facies control or non-sequence, suggests a late Courceyan or Chadian age.

Llanelly Formation

The Llanelly Formation contains a very restricted fauna. Composita sp.and Linoprotonia cf corrugatohemispherica occur in abundance in the Penllwyn Oolite, but do not give a precise indication of age. A small, facies-controlled conodont assemblage including Spathognathodus scitulus is of little biostratigraphical use. George (1954, p. 307) recorded an abundance of algae, among which Koninckopora inflata was subsequently identified (George, 1954, p.435). Foraminifera including archaediscids were recorded by George (1954) and confirm an Arundian age. Barclay and Jones (1978) recorded the Viséan spore Lycospora pusilla.

Dowlais Limestone

The Dowlais Limestone contains a Holkerian age macro-fauna that includes Lithostrotion cf. araneum (a cerioid coral not know to occur below this stage), L. martini, Syringopora cf. ramulosa, Composita sp., and Linoprotonia hemisphaerica. A shale from the base of the formation at Llanelly quarry (p.63) yielded a diverse miospore assemblage with several of the species that Neves and others (1974) and Clayton (1985) considered to he of stratigraphical significance. Lycospora pusilla and Vallatisporites ciliaris appear at the base of the Pu Zone and range into the Tc Zone, but a Tc Zone age cannot be assigned since Schopfites claviger is known only in the CM and Pu zones (Owens, unpublished BGS report, 1975).

The Tournaisian–Viséan boundary

The positioning of the Tournaisian–Viséan boundary remains problematical. On the conodont evidence of a Courceyan age for the Gilwern Oolitc, the boundary is marked by the unconformity which truncates the formation and which is succeeded by the Arundian Llanelly Formation. The balance of evidence from the foraminifera would favour a Chadian age for the Gilwern Oolitc and thus place the Tournaisian–Viséan boundary at the base of the Craig-yGacr Coral Bed. This would also be its position if the Gilwern Oolite were (on the basis of the macrofauna) Arundian.

Dolomitisation

Dolomites constitute a minor part of the Dinantian sequence in the west of the district, being restricted to thin ankeritic siltstones and dolocretes within the oolites of the Abercriban Oolite. They make up all the sequence in the east, forming the northern part of the east crop dolomite belt (Hird and others, 1987). The Clydach Valley lies in the transitional zone where dolomitised and undolomitised rocks inter-finger. The dolomites fall into two main categories: syngenetic dolomites, and epigenetic or late-stage dolomites.

Syngenetic dolomites

Included in this group is a minor amount of dolomite in the limestones of the Lower Limestone Shale in which Whitcombe (1970) recognised two phases of diagenetic dolomitisation.

The Sychnant Dolomite, Pantydarren Beds, and much of the Coed Ffyddlwn Formation are syngenetic stratabound dolomites, intercalated between undolomitised limestones. George (1954) suggested that the very fine-grained dolomites in these formations might be primary. Sedimentary lamination is preserved and the dolomite is an early penecontemporaneous replacement. A peritidal setting, involving dolomitisation by seepage of magnesium-rich hypersaline fluids, may be invoked (Adams and Rhodes, 1960; Friedman, 1980). In Ramsbottom's (1973) overview of Dinantian sedimentation, these dolomites occur within the regressive phases of the cycles and are an early product of the evaporative process. However, other authors (e.g. Folk and Land, 1975) have argued that reduced salinity of connate water in a mixing zone of phreatic (fresh) and marine (saline) waters also leads to dolomitisation.

Dolomite also occurs in some palaeosols (dolocretes) within the Abercriban Oolitel^‡2 (Wright, 1986), and as lenses within the basal beds of the Llanelly Formation (see p.61).

The lenses may be the deposits of hypersalinc ponds and abandoned channels on the palaeokarstic surface of the Gilwcrn Oolite (cf. Muir and others, 1980).

Epigenetic dolomites

As demonstrated by Strahan and Gibson (1900), George (1954), and Jones and Owen (1967), the pervasive dolomitisation of the east crop, which affects the entire Dinantian sequence, postdates the Variscan deformation. It also post-dates stylolite formation (Strong, 1978). Enlargement of crystal size by aggrading neomorphism of strata-bound dolomicrites may also have taken place at that time. This late-stage replacement dolomitisation of the east crop is clearly related to its marginal position and proximity to the Usk Axis. Several authors (e.g. Wilson, 1975) have pointed out that positive areas such as this are prime sites for dolomitisation, where phreatic water can move down a hydrological gradient.

Palaeogeography and depositional models

The South-west Dinantian Province, of which the district is part, was part of an extensive belt of carbonate deposition that stretched from Ireland to Germany. The palaeogeography of the province is well known (e.g. George, 1972; Wright, 1986). The sediments form a northward-thinning wedge deposited between the deep-water Cornubian basin of south-west England and St George's Land (part of the Wales–Brabant island) to the north. The hinge zone of the wedge was never far north of the present district, and deposition there was mostly in water rarely more than a few metres deep. Minor changes in the rate of tilt of the stable St George's Land block produced either emergence and erosion of the marginal north crop area or its inundation by shallow water (George, 1958, 1972, 1978a). Evidence for tectonic control of subsidence and sedimentation is provided by the unconformable overstep of the Abercriban Oolite and the Clydach Valley Group by the Llanelly Formation, and also the overstep of both the Llanelly Formation and the Abercriban Oolite by the Dowlais Limestone in the adjoining Merthyr Tydfil district. Residual thicknesses of the formations below these breaks show that the overstep was directed to the north-west, normal to the caledonoid Neath Disturbance, which may thus have exerted an important influence (George, 1954, pp.305, 311, fig. 9).

Eustatic sea level control was proposed by Ramsbottom (1973, 1978) as the main factor controlling Dinantian sedimentation. The succession thus comprises a series of major cycles, or mesothems, each initiated by eustatically induced transgression. In general, Ramsbottom's model involves initial deposition to be in the deepest water, producing bedded bioclastic limestones with progressive shallowing leading to oolite formation and eventually to peritidal deposition. In relation to the Abergavenny district, Ramsbottom (1973) regarded the oolites as transgressive, and the intervening dolomites and peritidal deposits as regressive. Superposition of the peritidal Llanelly Formation on the Gilwern Oolite was regarded as normal regressive shallowing.

George (1978b) restated the importance of tectonic control in this area, and there is at present general acceptance of the view that both tectonic and eustatic controls were operating during the Dinantian. Wright (1986) proposed that much of the sequence was deposited on a ramp (Ahr, 1973), as opposed to a shelf, and that the Abergavenny district lay in the inner ramp zone. In this zone, the sequence consists mainly of oolite shoal deposits and back shoal peritidal deposits, with numerous erosion surfaces and discontinuities. Where peritidal deposits overlie these breaks, they are therefore transgressive, and not regressive, as suggested by Rams-bottom (1973).

There is no evidence for a major, long-lived reef in the Miskin–Taffs Well area, as postulated by Bhatt (1976) (George, 1978a).

Lower Limestone Shale Group

The limestone-dominated Castell Coch Limestone forms a scarp feature above which lies a 'slack' produced by the mudstone-dominated Cwmyniscoy Mudstone. Maximum thickness of 60 m occurs in the Pontypool area, thinning northwards to about 45 m in the Clydach area and westwards to about 20 m in the Trefil area. Maximum Namurian overstep of the Dinantian takes place on Mynydd Garn-clochdy where the Millstone Grit rests on the Castell Coch Limestone. Regional studies (Burchette, 1977, 1981, 1987), and comparison with the Cardiff area (Waters and Lawrence, 1987) show that a lower unit, the Tongwynlais Formation, is absent in the district, being onlapped by the Castell Coch Limestone south of the Pontypool. The Castell Coch Limestone is well exposed, both in natural sections and disused pits and quarries, but the Cwmyniscoy Mudstone is, in general, poorly seen.

Castell Coch Limestone

The Castell Coch Limestone can be divided into three units. These are, in upward sequence:

i.        Oolitic and skeletal calcerenites, generally sandy, with sandstones towards base; base marked by thin but persistent lag conglomerate.

ii.      Lagoon phase' deposits comprising micrites, shales and fine-grained limestones.

iii.    Oolite and oolite calcarenite.

i.        A lag conglomerate resting sharply on the Quartz Conglomerate Group forms the base of the Carboniferous Limestone throughout the district, thinning from about 0.7 m at Daren Clog-fawr to a few centimetres in the Clydach. The conglomerate is succeeded by calcareous sandstones, sandy limestones, and cross-bedded shelly, oolitic, and crinoidal calcarenites. The sandstones and sandy limestones weather cariously at outcrop giving a honeycomb appearance.

ii.      There is a transition upwards from the calcarenites into a thin-bedded succession of shales, micrites and oolitic, peloidal fine-grained limestones. The micrites contain shell debris in parts, are generally peloidal, and contain silt- to fine sand-grade quartz. Stromatolitic lamination, fenestrate fabric and vermiform gastropod biostromes are common in these beds (Burchette and Riding, 1977; Lovell, 1978).

3. A wedge of bimodally cross-stratified oolitic calcarenite, thinning from 5 m at Craig-yr-hafod to about 2.5 m on the Blorenge, forms the topmost member of the Castel Coch Limestone on the east crop, but is absent west of the Blorenge. Where seen, it commonly has a hcmatised top.

Cwmyniscoy Mudstone

This consists predominantly of grey calcareous mudstones, with thin interbeds of silty and bioclastic limestone rich in shell debris. Burchette (1981) reported that some of these interbeds are graded and laminated limestone–quartzite couplets with coarse, graded lower portions and laminated upper silty portions.

Sedimentation

The fossiliferous, phosphatic lag conglomerate at the base of the Lower Limestone Shale represents deposition in the littoral zone during initial transgression of the area. The persistence of this bed over the district, spore evidence of a Tn2a age, and absence of the underlying Tongwynlais Formation indicate that the Old Red Sandstone/Carboniferous Limestone boundary in this area is not, as it has been traditionally regarded, a transitional one, but that a non-sequence is present (Lovell, 1978b). Burchette (1981, 1987) related the succession to three depositional cycles. Basal calcarenites represent shoal deposition in water which gradually shallowed and led to regressive progradation of 'lagoon phase' deposits of peritidal facies over the oolite barrier. Subsequent transgression led initially to shoal deposition of a calcarenite barrier complex and subsequently to a deeper shelf environment in which the Cwmynscoy Mudstone was formed. The limestone beds within the shales are believed to be tempestites deposited during storm surges. Interdigitation of fine-grained micritic and dolomitic

Details

(Figure 12) shows the principal sections in the Lower Limestone Shale Group.

West of Abergavenny

The Cwar yr Ystrad Borehole [SO 0842 1446] ((Figure 12), Section 1) proved the basal part of the Cwmyniscoy Mudstone and all the Castell Coch Limestone. Details of this were given in Taylor and Thomas (1975).

A stream section below Cwar yr Ystrad quarry [SO 096 147] was described by Burchette (1981) ((Figure 12), Section 2).

An outcrop of the basal part of the Castel] Coch Limestone at Daren Clog-fawr [SO 1120 1635] exposes the following:

The basal quartz and phosphate pebble conglomerate is exposed to the north-west and is 0.7 m thick there.

A disused quarry [SO 2009 1588] exposes argillaceous limestones with shale and sandstone layers overlying sandy oolitic crinoidal grainstones with sandstone streaks. These in turn rest on calcareous sandstones 7.8 m thick and containing, at their base, the basal quartz pebble conglomerate of the Castell Coch Limestone. Lovell (1978a) recorded a Tournaisian (Tn2a) miospore assemblage from these beds.

The basal oolitic and skeletal calcarenites of the Castell Coch Limestone are well exposed in a roadside section [SO 2238 1277] on the Heads of the Valleys road at Blackrock ((Figure 12), Section 3); overlying silty limestones, micrites and shales are also exposed and include a bed showing synsedimentary deformation. The topmost part of the Cwmyniscoy Mudstone and the transitional beds into the overlying Sychnant Dolomite are seen higher up the same road [SO 2174 1268]. There are good exposures of mudstones of the Cwmyniscoy Mudstone in the Clydach gorge [SO 2160 1252] to [SO 2178 1261] and the Castell Coch Limestone is also well exposed in crags [SO 2178 1261] to [SO 2198 1267] where the following was recorded:

The basal bed of the Castell Coch Limestone is a fossiliferous calcareous conglomerate with well rounded to elongate pebbles of metamorphic and igneous quartz, metaquartzite, spicular chert and dark grey phosphatic siltstone. The fossils include Serpuloides?, Paraconularia sp., Lingula sp., Macropotamorhynchus sp., Orbiculoidea sp., bivalves, turreted juvenile gastropods, ostracods, fish spines and scales.

Nant Sychnant [SO 2253 1261] to [SO 2255 1252] provides an almost complete section of the Lower Limestone Shale ((Figure 12), Section 4). The Castell Coch Limestone crops out below the road to Llanelly Hill and the Cwmyniscoy Mudstone above, the road obscuring about 7 m of beds. This section was described by Burchette (1981). About 9 m of patchily dolomitised oolitic and skeletal calcarenites at the base of the Castell Coch Limestone are succeeded by about 5 m of thinly bedded silty limestones and shales. These, in turn, pass upwards into about 2 m of micrites and shales which contain cryptalgal laminites, evaporite pseudomorphs, and the substrate-attached gastropod 'Serpula advena' (Burchette, 1981). These are hypersaline 'lagoon phase' deposits and are succeeded by cross-stratified silty limestones which contain, near their top, a bed possibly deformed by penecontemporaneous seismic activity. (Lovell, 1978a). The Cwmyniscoy Mudstone comprises mudstones with thin layers of shelly, bioclastic and silty limestones. Burchette (1977, 1981) interpreted these layers as storm beds or tempestites, deposited in an offshore shelf environment.

The basal beds of the Castell Coch Limestone, much dolomitised, are exposed in Nant Dyar [SO 2322 1280] and in Cwm Llanwenarth [SO 2540 1159]; at the latter, thinly bedded dolomites and limestones rest on the basal conglomerate which is 5cm thick. Lovell (1978a) recorded a Tournaisian (Tn2a) miospore assemblage from these beds.

Garn-ddyrys Tip Borehole No.1 [SO 2572 1191] ((Figure 12), Section 5) proved most of the Castell Coch Limestone.

South of Abergavenny

The Castell Coch Limestone is well exposed on the north-east face of Blorenge [SO 2752 1235] ((Figure 12), Section 6). About 2.5 m of bimodally cross-bedded blue-grey oolite and oolitic calcarenite lie near the top of the formation and are overlain by 0.6 m of thinly bedded sandy, crinoidal, shelly calcarenites and calcareous sandstones. A deformed bed within these probably equates with the deformed layer in the Clydach gorge. Numerous old pits to the south, on the south-eastern flanks of Blorenge, expose small sections in the Castell Coch Limestone. The formation is exposed at Craig-yr-hafod [SO 2738 0994], ((Figure 12), Section 7) and at Craig-ycwm [SO 2820 0890] ((Figure 12), Section 8); the topmost oolite has a red hematised top at these localities (Burchette, 1977; Lovell, 1978a; Strahan and Gibson, 1900).

Afon Lwyd

The group crops out in the Afon Lwyd Valley north of Garndiffaith and there are numerous small exposures of the Castell Coch Limestone in the river in Cwmavon. Better sections are confined to disused quarries such as Cwmavon Quarry [SO 2690 0732] where about 23 m of beds are exposed.

Cwmffrwd Halt Borehole [SO 2708 0416] proved the Castell Coch Limestone to be 14.89 m thick and the Cwmyniscoy Mudstone to be 36.22 m.

There is a small inlier of Lower Limestone Shale immediately north of the Trevethin Fault in Pontypool and there are some outcrops, mainly of Castell Coch Limestone, in the river.

East Crop

The Castell Coch Limestone outcrop was formerly much dug on the east crop southwards from Craig-y-cwm and there are numerous pits and quarries. One to the north-east of Mynydd Garn-clochdy [SO 2897 0655] shows:

Beds near the base of the Castell Coch Limestone are exposed below in a disused pit [SO 2912 0667]:

Maximum Namurian overstep of the Dinantian is attained southeast of Mynydd Garn-clochdy where Namurian shale rests on the topmost beds of the Castell Coch Limestone.

Nant-y-mailor quarry c. [SO 2826 0443] exposes the following section in the Castell Coch Limestone:

There are numerous small exposures in the Castcll Coch Limestone on the wooded scarp south of the Trevethin Fault between Twyngwyn and the district boundary.

Abercriban Oolite Group and Clydach Valley Group

Much data have been published on these beds (George, 1954, 1956b; Robertson, 1927; Strahan and Gibson, 1900; Wright, 1986). A generalised section showing constituent formations is given in (Figure 11), and their correlation with previous classifications is shown in (Table 5).

The Clydach Valley Group is very well exposed in numerous disused quarries and in road and stream sections in the type area of the Clydach Valley. It comprises a cyclic sequence of thinly bedded dolomites and massive oolitic grainstones. On the south side of the Clydach Valley, dolomites become increasingly important and the group is entirely dolomitic along the east crop. To the north and west, the dolomites decrease in thickness and oolites become dominant. The formations can be correlated along a series of disused quarries from Clydach to Trefil, although several of the faces are inaccessible. Thicknesses of individual formations generally increase southwards, but the overall thickness is affected by unconformities. The Gilwern Oolite, for example, is 20 m thick on the west side of the Afon Lwyd Valley at Pontypool, but is overstepped northwards by the Llanelly Formation and is thin to absent on the north crop. Namurian overstep reduces the thickness of the beds on the east side of the Afon Lwyd and along the east crop, where they are absent in places. About 20 m of beds are present in the west of the district at Trefil, compared to about 45 m in the Clydach Valley. The Sychnant Dolomite is up to 5 m thick, and consists of thinly bedded finely crystalline dolomitised skeletal packstones which, in places, show small scale cross-stratification and tractional lamination. It is poorly fossiliferous, and contains only a restricted fauna of crinoid debris, bryozoan fragments and gastropods. It has a sharp upper boundary, which is a palaeokarstic surface in the Heads of the Valleys road section (see p.50).

The Pwll-y-Cwm Oolite is a pale grey oolitic grainstone up to 7 m thick and rich in shell and other biotic debris. It is massive to thick-bedded, with stylolite-bounded beds, and with internal cross-bedding discernible in places. It is dolomitised along the east crop, and probably equates with the Brofiscin Oolite of the Cardiff district (Waters and Lawrence, 1987). A palaeokarstic surface truncates the formation at Cwar yr Hendre, and elsewhere the upper boundary is sharp and planar. The Pantydarren Beds are up to 13 m thick and consist of thinly bedded fine-grained dolomites in the Clydach Valley.

Thin spore-rich mudstone interbeds are present towards the base of the formation in the Clydach (p.50), and algal mudstone with ostracods has been recorded from this level (George, 1954, p.295). To the north and west of the type locality at Coed Pantydarren, oolitic grainstone wedges into the dolomite, and at Blaen Onneu the formation is predominantly oolitic, with some thinly bedded dolomites at the base and a rubble bed at the top.

The Blaen Onnen Oolite is a massive, medium grey oolitic grainstone composed of ooids, peloids, and biotic debris.

Maximum thickness of 14 m is at the type locality, thinning taking place southwards towards the Clydach Valley where it is 5 m. South and east of there, it is a coarse crinoidal dolomite and fingers out into finer grained dolomites along the east crop.

The Coed Ffyddlwn Formation, which has a sharp boundary with the Blaen Onnen Oolite, is about 14 m thick in the type area in the Clydach Valley, and comprises two lithologies. The bulk of the formation consists of fine-grained thinly bedded homogeneous dolomites, with some laminated dolomicrites. Concentrations of shell and crinoid debris occur within these beds. Above them, making the topmost 1 to 2 m of the formation is a distinctive unit, the Darren Ddu Limestone, which consists of thinly bedded pale grey micrites and green mudstones, with some peloidal grainstone layers and flat pebble layers. Stromatolitic lamination is present in some of the micritic beds. Small amounts of fine-grained sandstone and siltstone are present in places, detrital quartz is common, and quartz pebble conglomerate has also been recorded (Raven, 1981). Calcretes, layers of fibrous calcite, micritic crusts and desiccation cracks are also found in these beds. The formation is absent at Blaen Onneu, and may be represented by a 0.3 m bed of spar nodules at C waryr-Hendre. Fauna of the Coed Ffyddlwn Formation is restricted to conodonts (p.42), shell and echinoderm debris, some ostrocods, gastropods, and algal remains.

An erosion surface separates the Coed Ffyddlwn Formation from the overlying Craig-y-Gaer Coral Bed, which forms the basal 2 to 3 ni of the Gilwern Oolite. The Craig-yGaer Coral Bed is a coarse-grained shelly, crinoidal grainstone, and locally contains at its base large angular clasts of the underlying Darren Ddu Limestone. Discontinuity surfaces are present in places within the bed, for example 0.4 m above its base at Twyn y Dinas, and 0.7 m above its base at Gilwern Hill. There is an upward gradation from the Craig-y-Gaer Coral Bed into the main part of the Gilwern Oolite. The Gilwern Oolite is a massive, pale grey oolite and oolitic bioclastic grainstone, which weathers to a very pale grey to buff colour at outcrop. The oolitic grainstone contains a variety of biotic fragments including ooids, peloids, shell and echinoderm debris. Micritised intraclasts of oolite are also present, and are particularly concentrated towards the middle of the formation in the Clydach Valley (Raven, 1981). An intricate discontinuity has been observed in the middle of the formation at Darren Ddu (Raven, 1981), and this may be a submarine hardground (Wright, 1986). Thickness ranges up to 20 m at Pontypool where the formation is totally dolomitised. Northward overstep by the Llanelly Formation reduces the thickness to 10 m in the Clydach Valley, and to 3 m at Daren Cilau. It is absent at Blaen Onneu, and may be represented by 4 m of oolitic grainstone at Cwar-yr-Hendre (Figure 13). Below the unconformity, the oolite is brecciated and rubbly to depths of up to several metres as a result of subaerial exposure and dissolution. Pale green and yellow clay fills many of the solution pipes and channels (Plate 7). In places, the primary grainstone fabric of the oolite is replaced by microspar calcrete (Barclay, 1980; Wright, 1981b, 1982b). Cementation of the oolite predated the karst formation in the Clydach Valley area, where an upward sequence of cements laid down in marine phreatic through meteoric phreatic to meteoric vadose diagenetic environments has been recognised and related to marine regression and subaerial exposure (Raven, 1983)^‡3 . No sedimentary structures have been observed in undolomitised Gilwern Oolite, but bimodal cross-bedding is discernible where it is dolomitised on the east crop. The Craig-y-Gaer Coral Bed is richly fossiliferous, and has yielded some corals and many brachiopods (p.43; (Table 7)).

Sedimentation

The Trefil–Clydach area was one of limited subsidence and mainly oolitic shoal deposition, with repeated emergence and subaerial exposure. The spar nodule beds are probably calcrete palaeosols, although Raven (1981) has reported that some are rich in conodont and vertebrate remains, suggesting that they may be a dissolution/reprccipitation phenomenon, perhaps related to fossil water table levels.

The restricted fauna and sedimentary structures of the Sychnant Dolomite suggests deposition in shallow water affected by bottom currents. The early dolomitisation of the sediments is presumably related to a regressive phase which produced the erosion surface that truncates the formation. The ostracods, algal remains, and spore-rich mudstones of the Pantydarren Beds suggests a lagoonal or peritidal origin. The Coed Ffyddlwn Formation also contains evidence of peritidal deposition, particularly in the Darren

Ddu Limestone member, with features such as siliciclastic influxes, calcretes, micritic crusts, desiccation cracks and cryptalgal lamination. Concentrations of shell and crinoid debris in some of the dolomites of the formation indicates the presence of tractional currents in the intertidal zone. The irregular surface which truncates the formation has been interpreted as one of marine erosion, or ravinement by Wright (1986). However, angular discordance seen at Twyn y Dinas, and the large size of the angular clasts of Darren Ddu Limestone in the base of the Craig-y-Gaer Coral Bed at Darren Ddu suggest uplift and subaerial erosion. Subsequent transgression led to deposition of the Craig-y-Gaer Coral Bed, followed by shoal deposition of the Gilwern Oolite, although erosion surfaces within the formation indicate minor cycles of transgression in the overall upward shoaling.

Details

Cwar-yr-Hendre to Clydach

Oolites and oolitic grainstones are predominant, with only thin dolomitic interbeds. Thickness ranges from 19 m at Cwar yr Ystrad to 45 m in the Clydach Valley. The attenuation is achieved by thinning of all the formations, and by northward overstep of the beds by the Llanelly Formation. Cwar yr Hendre and Cwar yr Ystrad quarries c. [SO 090 146] provide excellent and extensive sections ((Figure 13), Section 1). The floor of the quarries is of fine-grained dolomite (?Sychnant Dolomite), which is overlain by 3 to 4 m of oolitic grainstone (?Pwll-y-Cwm Oolite). The top of the oolitic grainstone is an irregular ?subaerial surface which is infilled and overlain by up to 1 m of fine-grained dolomite with green shale (?Pantydarren Beds). The rubbly palaeosol at the top of the sequence is up to 5 m thick and contains Palaeosmilia murchisoni.

There are good exposures of oolitic grainstones on Clo Cadno, and on the flanks of Garn Caws. In a swallow hole [SO 1280 1644], 0.8 m of very coarse-grained crinoidal grainstone passes upwards into cream oolite; this is almost certainly the Craig-y-Gaer Coral Bed and overlying Gilwern Oolite (cf. George, 1954, p.292). About 4 m of the °elite are seen in a nearby swallow hole [SO 1276 1618].

Blaen Onneu Quarry is the type locality of the Blaen Onnen Oolite (George and others, 1976) ((Figure 13), Section 2). About 3 m above the quarry floor is a bed, 0.3 to 0.5 m thick, of radiating fibrous calcite nodules with some green shale and brown dolomitic siltstone. It rests on a palaeokarstic surface, reddened in parts, of oolitic grainstone, and is succeeded by about 14 m of thick-bedded to massive pale grey oolitic grainstone. A spectacular palaeokarstic surface truncates the formation with pinnacles of oolite up to 5 m high blanketed by a rubbly palaeosol (Wright, 1982b). George (1954, p.292 and fig. 4) recorded 1.5 m of beds of the Coed Ffyddlwn Formation overlain by 2.5 m of the Craig-y-Gaer Coral Bed, but there is no trace of these in present outcrops (Taylor, 1974). Fauna collected from the Blaen Onnen Oolite includes Macropotamorhynchus mitcheldeanensis.

Crags and disused quarries provide excellent exposures from Craig y Casten [SO 1740 1675] to Daren Cilau [SO 1980 1585], Chwar Pant-y-rhiw [SO 2020 1560] and Darren [SO 210 150], although they are not everywhere accessible in steep cliffs. (Figure 13), Section 3 gives a composite section for Daren Cilau and Chwar Pant-y-rhiw where the beds lie at the base of cliffs which expose Llanelly Formation and Dowlais Limestone above. The Gilwern Oolite is reduced by overstep to about 5 m of beds, most of which are rubbly and brecciated under the palaeokarstic surface. The Coed Ffyddlwn Formation comprises a thinly bedded peritidal assemblage of micritic and fine-grained peloidal limestones with a rubbly nodule bed at the base. Raven (1981) reported a thin quartz conglomerate bed, and the presence of fibrous and micritic red- and green-stained crusts on some beds. (Figure 13), Sections 4 and 5 show sections at the north [SO 2085 1516] and south [SO 2115 1470] ends of the Darren. (Figure 13), Section 6 shows a composite section of cliff and quarry exposures at Carreg Fawr/Darren Disgwylfa [SO 2191 1432] and Pant-y-Gilwern [SO 2150 1465]. The cliff at Pant-y-Gilwern is largely in Pwll-y-Cwm Oolite. The sequence from the top of the Pwll-y-Cwm Oolite to the top of the Blaen Onnen Oolite is exposed in quarries at Carreg Fawr/Darren Disgwylfa. The 'Lonely Shepherd', a prominent stack left during the quarrying operations [SO 2172 1450], consists of shelly oolitic grainstone at the base of the Blaen Onnen Oolite. Conodonts from this locality are listed in (Table 8).

Disused quarries at Coed Pantydarren [SO 2197 1373] provide the composite section shown in (Figure 13), Section 7. In the northern quarry, massive pale grey oolitic grainstones comprising the Pwll-yCwm Oolite are overlain by fine-grained thinly bedded dolomites,narned from this locality, the Pantydarren Beds. In the southern quarry the Blaen Onnen Oolite is a massive sharp-topped unit. The Coed Ffyddlwn Formation is inaccessible, but the lower part appears to consist of pale grey limestones draped over an ir regular surface of buff dolomite. Higher beds of the formation, comprising thinly bedded dolomites, micritic limestones and green shales are poorly exposed, but the overlying Gilwern Oolite is well seen.

Clydach Valley-north side

Good exposures in crags and old quarries from Craig-y-Gaer to the Clydach river give a composite section ((Figure 13), Section 8). The Heads of the Valleys road provides a section from topmost beds of the Cwmyniscoy Mudstone [SO 2180 1272] to the Blaen Onnen Oolite [SO 2165 1265].

The Sychnant Dolomite has a transitional contact with the Cwmyniscoy Mudstone and consists of 3 m of thinly bedded finely crystalline dolomitised skeletal packstones showing some small-scale cross-stratification and tractional lamination. Crinoid debris is the main faunal element, with some bryozoan fragments, gastropods and burrowing traces. Specimen E 46492 from this outcrop is a medium to dark grey recrystallised dolomite composed of rhombohedral or granular, buff-coloured dolomite and a little calcite in which coarser (0.06 mm) layers containing fine sand–silt grade quartz alternate with fine (0.03 mm) laminae. The dolomite is of replacement origin but the dolomitisation was penecontemporaneous rather than post-lithification, with the primary sedimentary lamination unaffected by the recrystallisation (Sanderson, Internal BGS Report, 1975). The top of the formation is a clint-and-griketype palaeokarstic surface with three clay-lined potholes infilled by the overlying Pwll-y-Cwm Oolite (Barclay, 1974; (Plate 4)). The oolite is almost continuously exposed from the Heads of the Valleys road to its type locality at Pwll-y-Cwm in the Clydach river [SO 2176 1270]. It is a well sorted, medium-grained, pale grey oolite with biotic fragments forming the cores of the ooliths. Scattered shell debris includes spiriferoids and Syringothyris sp.juv.

In an organic residue recovered from the Pantydarren Beds, 0.5 m above their base, the following spores were identified: Verrucosisporites nitidus, Granulatisporites sp., ?Raistrickia sp., Dictyotriletes sp., Convolutispora sp.cf. C. mellita and Hymenozonotriletes sp.A sample from 3 m higher yielded Verrucosisporites sp., V. nitidus, Apiculatisporis sp., Umbonatisporites distinctus, Raistrickia sp., Convolutispora sp., C. mellita, Dictyotriletes cf. sagenoformis, Hymenozonotriletes sp., Reticulatisporites polygonalis, Knoxisporites triradiatus, ?Spelaeotriletes sp.and badly corroded forms of ?Retusotriletes and ?Crassispora. Although both residues contained many miospores, most were extensively corroded and torn (see p.41).

The Pwll-y-Cwm Oolite, Pantydarren Beds and the Blaen Onnen Oolite are well exposed in roadside outcrops on the old Brynmawr road at Blackrock [SO 2178 1274]. The Pwll-y-Cwm Oolite contains Chaetetes sp., Macropotamorhynchus mitcheldeanensis, Syringothyris cf. exoleta and Edmondia sp.; M. mitcheldeanensis, an orthotetoid, cf. Pugulis vaughani and Unispirifer tornacensis were collected from the Blaen Onnen Oolite, which is patchily dolomitised. Con odonts are listed in (Table 8). The sequence can be further examined in quarries off the old tramway leading to Darren Ddu Quarry [SO 2210 1295]. Here the Pwll-y-Cwm Oolite is affected by small-scale low angle faulting.

The Coed Ffyddlwn Formation is well exposed in a small subsidiary quarry on Darren Ddu [SO 2194 1290] and this is the type locality of the Darren Ddu Limestone member. The section is summarised thus:

Conodonts from this locality are listed in (Table 8). The surface at the top of the Darren Ddu Limestone has relief of up to 0.5 m and the basal bed of of the overlying Craig-y-Gaer Coral Bed contains large angular clasts of micrite from the underlying beds.

The Craig-y-Gaer Coral Bed at Darren Ddu Quarry [SO 2217 1298] yielded Dielasma sp., Leptagonia sp., Linoprotonia sp., Megachonetes cf. papilionaceus, an orthotetoid, a rhynchonelloid, Schizophoria sp., a smooth spiriferoid, Bellerophon sp., and a bivalve. The Gilwern Oolite is completely exposed and consists of 10 m of oolite with a rubbly palaeokarstic top. Raven (1981) recorded an erosion surface at half height within the oolite at Darren Ddu, with pure, well-sorted oolite lying above and poorly sorted oolite below. There is also an irregular erosion surface near the base of the Craig-y-Gaer Coral Bed.

Craig-y-Gaer is the type locality of the Craig-y-Gaer Coral Bed. This bed, and the topmost part of Coed Ffyddlwn Formation are exposed around the base of a stack [SO 2238 1330] left during quarrying. The Craig-y-Gaer Coral Bed consists of Shelly and crinoid-rich grainstones (Plate 5). Details of the fauna from this locality were given on p.43, and conodonts are listed in (Table 8). The Gilwern Oolite is exposed in the quarry, the massive white oolite becoming rubbly in the topmost 3 to 4 m under the palaeokarstic surface, and with green and yellow clay infilling solution cavities.

Clydach Valley–south side

The Clydach Valley Group crops out in the Clydach River. bed from Pwll-y-Cwm [SO 2158 1252] to Devil's Bridge [SO 2155 1249] where a spectacular gorge cuts through Pwll-y-Cwm Oolite and Pantydarren Beds. Upstream, the Blaen Onnen Oolite is completely dolomitised to coarse-grained massive granular dolomite. The overlying Coed Ffyddlwn Formation and Gilwern Oolite are well exposed in the banks of the main stream, the latter being dolomitised in this vicinity. The Llammarch Dingle provides a section from the Sychnant Dolomite and Pwll-y-Cwm Oolite exposed at its confluence with the Clydach [SO 2170 1252] to the Gilwern Oolite [SO 2169 1235] which is rubbly and dolomitised throughout. The Blaen Onnen Oolite rests on an irregular surface of Pantydarren Beds (Barclay, 1974) and has yielded M. mitcheldeanensis, an orthotetoid, a smooth spiriferoid and Unispirifer sp.

Nant Sychnant gives a section from the base of the Sychnant Dolomite to the top of the Blaen Onnen Oolite ((Figure 14), Section 2). The Coed Ffyddlwn Formation is best examined in the railway cutting by Llanelly quarry [SO 2282 1248] and in roadside outcrops below the quarry [SO 2228 1251] to [SO 2234 1252]. (Figure 14), Section 2 gives the railway cutting section. The roadside section, the top of which lies about 1 m below the top of the formation, reads:

The Gilwern Oolite is well exposed in Llanelly quarry (Plate 6) where up to 10 m of massive clean white oolite lie at the base of the face and the palaeokarstic top of the formation and associated rubble bed with a variety of solution pipes and fissures are particularly worth examination (Barclay, 1980, Wright 1980, 1982a) (Plate 7). To the east of the quarry, an exposure of the base of the Craig-y-Gaer Coral Bed by the old tramway [SO 2257 1250] yielded a rich fauna: Buxtonia sp., Fasciculophyllum sp. Linoprotonia sp. Macropotamorhynchus mitcheldeanensis, M. sp. (juv)., Megachonetes cf. papilionaceus, an orthotetoid, a rhynchonelloid, Schizophoria sp.spiriferoids, Syringothyris sp. Psephodus sp.,and other fish fragments. Conodonts are listed in (Table 8); 40 m to the east, the Gilwern Oolite is totally dolomitised in the vicinity of the Blaenavon Fault. Farther up the old tramway, the topmost 2 m of Gilwern Oolite are of rubbly microspar calcrete. The relationship between proximity of faulting and dolomitisation of the oolite is again well illustrated near the entrance to Twyn y Dinas old quarries (Robertson, 1927). The Craig-y-Gaer Coral Bed is exposed in the quarries [SO 2290 1263] and rests with slight angular discordance on underlying pale grey micritic limestones, stromatolitic in parts, of the Coed Ffyddlwn Formation (Plate 8). The Craig-y-Gaer Coral Bed, which has an irregular discontinuity 0.4 m above its base, consists of pale grey shelly grainstone fining upwards into cross-bedded oolitic grainstone. The following fauna was collected: Linoprotonia?, M. mitcheldeanensis, an orthotetoid, a productoid (juv.), a rhynchonelloid, Schizophoria sp., a smooth spiriferoid, a spiriferoid, and Bellerophon sp.

Clydach to Blorenge

The Nant Dyar section [SO 2328 1263] to [SO 2342 1246] is shown in (Figure 14), Section 10. In a totally dolomitic succession below the Gilwern Oolite, the Pwll-y-Cwm and Blaen Onnen oolites are unrecognisable in primary lithology and are represented by fine- to medium-grained crinoidal dolomites. A similar situation applies in the Cwm quarries c.[SO 2340 1270] ((Figure 14), Section 11) where a shelly crinoidal dolomite may represent the Blaen Onnen Oolite. Chen layers are present in the Coed Ffyddlwn Formation. The

Gilwern Oolite is completely exposed both here and in old quarries around Gilwern Hill to Pwll du [SO 2465 1260]. An old quarry face and crags [SO 2480 1221] below the track to Gilwern Hill quarries at Pwll du shows:

Clydach Valley Group dolomites are exposed in trackside outcrops to the north [SO 2496 1374]. To the south, the Gilwern Oolite is exposed in a crag [SO 2475 1203] near Pen-y-galchen where 1.8 m of topmost Darren Ddu Limestone are also seen and consist of thinly bedded micritic limestones and shaly interbeds. These beds are also exposed in a stream to the south-west [SO 2465 1180], where they underlie the richly fossiliferous Craig-y-Gaer Coral Bed, here crowded with the brachiopods Buxtonia sp., Megachonetes sp., and Syringothyris. Clydach Valley Group dolomites are well exposed further down this stream.

The Gilwern Oolite is completely exposed in Pwll du quarry [SO 2513 1150], and is about 15 m thick. It is dolomitised immediately to the east and largely remains so for the remainder of its outcrop to the east. Robertson (1927) explained the abrupt change from limestone to dolomite, unrelated to any faulting, by absence of the impermeable Gilwern Clay, overstepped here by the Namurian, thereby allowing downward percolation of magnesium-rich ground water.

A compiled section of exposures in Cwm Ifor is shown in (Figure 14), Section 12 (see also George, 1954). The Gilwern Oolite consists of about 17 m of coarse-grained cross-bedded dolomite and is underlain by dolomicrites and micrites of the Coed Ffyddlwn Formation.

The Craig-y-Gaer Coral Bed is present in primary lithology in a disused roadside quarry [SO 2580 1162]. The Clydach Valley Group is progressively overstepped by the Namurian to the north-east of there, and is probably absent on the north-east flanks of the Blorenge. Details of the sequence proved in Pen-ffordd-goch Borehole [SO 2553 1058] ((Figure 14), Section 13) were given by Barclay and Jones (1978).

Blorenge to Pontypool

George (1956b) and Robertson (1927) gave details of the group in this area. The sequence is almost entirely dolomitised,but the Pwll-y-Cwm Oolite and the Gilwern Oolite can be recognised as coarse-grained dolomites. The intervening beds comprise mainly fine- and fine- to medium-grained dolomites. The sequence thickens southwards.

Cliff exposures at Craig-yr-hafod [SO 2725 1005] ((Figure 14), Section 14) show Namurian rocks resting on thin-bedded dolomicrites of the Coed Ffyddlwn Formation. The underlying coarse-grained bed remains undolomitised in parts where it is an oolitic grainstone; it is probably the Blaen Onnen Oolite. This locality and Graig quarries (see below) are the only ones on the east crop where it can be recognised. To the south, it probably fingers out into crinoidal dolomites.

George (1956b) gave details of Graig Quarry [SO 2723 0738], Blaenavon ((Figure 14), Section 15). Namurian quartzite and conglomerate rest unconformably on thinly bedded dolomicrites with a thin oolitic layer. George recorded rich algal remains from these beds, including codiaceans (commonly Ortonella) and spongiostromes; also reported were calcispheres, ostracods and foranfinifera (including plectogyrids and ammodiscids). A solitary specimen of Hapsiphyllum sp., was recorded from the oolitic layer. George correlated these beds with the Marker Beds, the Coed Ffyddlwn Formation of this account. His correlation of the oolitic layer with the Coral Bed (now the Craig-y-Gaer Coral Bed) is erroneous, as the latter lies at a higher stratigraphical level. George recorded undolomitised oolite in the underlying thick-bedded shaly, crinoidal dolomites, which would suggest correlation with the Blaen Onnen Oolite. The following were collected from these dolomites: Michelinia 'megastoma' (M. sp. nov.), Cleiothyridina sp., Macropotamor hynchus mitcheldeanensis, orthotetoids including cf. Schellwienella, a spiriferoid, Syringothyris sp., Unispirifer tornacensis, and a fish fragment. The overlying fine-grained beds of the Coed Ffyddlwn Formation yielded M. 'megastoma', 'Zaphrentis' konincki? a zaphrentoid (silicified), Cleiothyridina sp., and a spiriferoid. Chert layers above and below the thick-bedded dolomites provided a useful tool for local correlation of sections in the Afon Lwyd Valley.

The section at Gallowsgreen Quarry [SO 2662 0677], Varteg Hill is shown in (Figure 14), Section 16. (see also George 1956a, p.312). The Gilwern Oolite is about 14.5 m thick, and dolomitised to coarse-grained massive dolomite showing cross-bedding. The underlying Coed Ffyddlwn Formation is recognisable as thinly bedded fine-grained dolomites. A coral bed at the southern end of the quarry and lying 3.35 m below the top of the formation contains Syringopora cf. ramulosa; M. 'megastoma' was also obtained from the formation. The massive crinoidal dolomites below have zaphrentoid bands at 7.3, 8.3 and 9.3 m above their base. Both the base and top of these beds are shelly and have yielded Caninia? sp., M. 'megastoma', M. mitcheldeanensis, orthotetoids including Schellwienella sp., spiriferoids and Syringothyris? The fossils are generally poorly preserved and bekitised. A chert layer immediately below the base contains Megachonetes?, an orthotetoid and a smooth spiriferoid.

Four silica types occur in the chert: (i) microgranular quartz replacing the allochems, possibly neomorphic replacement of a first generation aphanocrystalline chert, (ii) scalenohedral crystals of fibrous quartz as outgrowths on crinoid fragments, (iii) chalcedonic quartz, typically fibrous and banded, filling most of the inter-granular space, and (iv) late-stage coarse-grained granular quartz associated with (iii) and infilling some intergranular voids. Dolomitisation followed the silicification (Strong, 1978).

(Figure 14), Section 17 gives a composite section from many small quarries at Wytchwood [SO 2688 0598] to [SO 2685 0550].

The section at Craig Cynfyn Quarry [SO 2716 0302] is given in (Figure 14), Section 18. Coral-rich beds near the top of the quarry contain Michelinia 'megastoma', Syringopora cf. ramulosa and ?Osagia masses. Dolomitised Gilwern Oolite lies above the quarry, but is overstepped eastwards by the Namurian, and the latter rests on Clydach Valley Group dolomites on the east crop east of Company's Wood.

Llanelly Formation

Formerly named the Calcite Mudstone Group (George, 1954), this formation is a distinctive unit of mainly fine-grained and thinly bedded peritidal limestones (Plate 9). Following the earlier records of Gibson and Strahan (1900) and Robertson (1927), George (1954) gave a detailed description of the formation. Recent sedimentological studies have been carried out by Wright (see Wright, 1986 for references).

The formation is 20 m thick at Llanelly quarry, its type locality (p.60). It thins northwards and westwards from the Clydach Valley, mainly by internal attenuation, but also partly by overstep by the overlying Dowlais Limestone; about 7 m are present at Cwar-yr-Hendre. It is absent over much of the east crop, being overstepped by the Namurian on the east side of the Afon Lwyd Valley. The formation is subdivided into four members (Wright, 1981b). (Figure 15) gives a generalised section.

The Clydach Halt Member comprises a variable sequence of detrital conglomeratic limestone, clays with micritic nodules and clasts, calcretes and sandstones. It is a lenticular unit, and fills hollows in the underlying palaeokarstic surface of the Gilwern Oolite. It is generally only up to 1 to 2 m thick, but reaches 4 m at Cwm quarries, its type locality.

The Cheltenham Limestone Member generally makes up the bulk of the Llanelly Formation and consists mainly of thin- to medium-bedded fine-grained limestones with pale green clay interbeds. The limestones consist of a variety of lithologies including coarse intraclastic and bioclastic grainstone, peloidal grainstones, packstones and wackestones, algal laminites, and lime mudstones (micrites). Wright (e.g. 1986) has described these in detail. The beds show a cyclic arrangement of these facies in places, although in others they are poorly ordered. Where they are cyclic, the beds show grading, the coarser lithologies passing upwards into the finer ones (Strong, 1978, 1979; Wright 1986). Individual beds are thinner on the north crop, compared to the Clydach Valley, and quantities of quartz silt, sand and pebbles increase generally northwards, although the Penffordd-goch Borehole [SO 2553 1058], situated to the east of Clydach, proved 10 m of a sequence of thinly bedded siliciclastic rocks and impure carbonates (Barclay and Jones, 1978). Peloidal rocks are less common on the north crop and the peloids tend to be smaller (Strong, 1979). Desiccation cracks and fenestral ('birds-eye') structures are typical of the micrite facies (Plate 10), and gypsum pseudomorphs have been recorded (Bhatt, 1975; Wright, 1981a, b, 1986). Up to ten palaeosols are present, the most persistent being the Cwm Dyar Pedoderm (Wright, 1981b, 1982b). Fauna is restricted to ostracods, brachiopod, crinoid and cephalopod fragments, a few foraminifera, gastropods and algae. The algae occur mainly as oncoids ((Plate 11); Wright, 1981c), but dasyclad fragments, and stromatolites are also found, and small algal-vermiform gastropod bioherms have been recorded (Wright and Wright, 1981). Burrowing is common in the finer lithologies. The clay interbeds are composed mainly of illite, with minor amounts of mixed layer illite–smectite.

The Penllwyn Oolite (The Linoproductus Oolite of George, 1954) is about 3 m thick and consists mainly of oolitic grainstone. It has a sharp, erosive base, and the basal bed is an oncoid-rich bioclastic grainstone (the Uraloporella Bed of Wright, 1981a; the Composita Bed of George, 1954). The oolite is sandier on the north crop, for example at Blaen

Onneu where quartz grains core the ooids, compared to the Clydach Valley, where biotic fragments form the cores. Cross-bedding, deformed in places, is seen in the oolite at Blaen Onneu, where its topmost rippled surface is veneered by stromatolites (Wright and Wright, 1985). A sheet of in-situ oncoids is present at this level on Gilwern Hill [SO 2395 1317].

The Gilwern Clay is a red-brown, purple and green mottled clay containing a rubbly to platey calcrete at its base, calcrete nodules throughout, and, locally, a rootlet bed and coal at its top (Barclay, 1981; Barclay and Jones, 1978; Wright, 1982b). Wright named it the Llanelly Pedocomplex. It is 7 m thick at Llanelly quarry, and 5 m at Cwar-yr-Hendre. At Blaen Onneu, it is deeply scoured, ranging in thickness from 0.3 to 6 m. The clay comprises illite, chlorite, and mixed layer illite–smectite, with quartz and goethite.

Sedimentation

The Clydach Halt Member represents the deposits of a karstic plain, the beds filling and veneering the underlying surface, which was emergent for a prolonged period (Wright, 1980). The fluvial deposits occur both in confined shallow channels and ponds, and as tabular beds of probable sheet flood origin. The calcretes and clays were formed in the interdistributary areas of the karstic plain. Marine transgression produced a shallow marine peritidal coastal complex in which the beds of the Cheltenham Limestone were deposited. A range of environments, from shallow bays and lagoons to supratidal, sabkha-like flats can be recognised. The graded, shallowing-upwards cycles are typical peritidal cycles (Wright, 1986). The green clay beds capping these cycles are probably soils, with the mixed layer clays perhaps suggesting volcanic dust input. The calcretes record more prolonged periods of emergence and soil formation. The Uraloporella Bed, forming the transgressive base of the Penllwyn Oolite, contains oncoids thought to have formed in a schizohaline lagoon (Wright, 1981c). The base of the bed may mark a surface of marine erosion, or ravinement, with shoreface migration of the Penllwyn Oolite shoal. Shallowing and exposure of the oolite shoal is marked by minor erosion surfaces and supratidal layers (Wright, 1986), The Gilwern Clay represents a major regression during which floodplain clays accumulated in an arid to semi-arid climate; the rootlet bed and coal at its top suggest backswamp deposition in a more humid environment (Wright, 1980).

Garn Caws Sandstone

Formerly included in the S₂ Zone (Holkerian) Dowlais Limestone (George, 1954; George and others, 1976; Robertson, 1927) and included as a subfacies of the Gilwern Clay member of the Llanelly Formation (Wright, 1981b), these beds were named and given separate formational status by Barclay and Jackson (1982).

Maximum thickness is found in the Blaen Onneu area where the formation consists of about 10 m of fluvial quartzitic sandstones and conglomerates with some thin intercalated plant-bearing green-grey clayey mudstone beds. It thins to the east, and at Llanelly quarry only a few centimetres of pebbly gravelly sandstone are present. It is extensively exposed in the type area of Garn Caws [SO 1295 1680] where it consists of cross-bedded quartzites and quartz pebble conglomerates with thin seatearth interbeds (see p.60).

Sedimentation

The formation is a fluvial one, the sandstones being the deposits of distributary channels of a deltaic complex that prograded southwards onto the carbonate shelf during a major regression and lowering of base level. The conglomerates are channel lag deposits and the cross-bedded sandstones probably represent point bar deposition. The channels are cut in the underlying Gilwern Clay, and, at Blaen Onneu, this has been scoured to a depth of up to 5 m. The plant-bearing clay layers are probably inter-distributary lake deposits. Parallel-laminated quartzites with sharp bases and grading upwards into mudstones, as seen in Carno adit (Barclay and Jackson, 1982), may be crevasse splay deposits.

Details

Cwar yr Ystrad to Clydach

The beds are extensively exposed in Cwar yr Ystrad and Cwar-yrHendre quarries ((Figure 16), Section 1), but easy access to exposures is limited. Wright (1981a) gave detailed sedimentological logs of the sections.

The best sections of the Garn Caws Sandstone are found in swallow-holes in one of which [SO 1233 1618] 3 m of fine-grained, mainly grey-green, thin-bedded sandstones with clay interbeds are exposed; 3 m of buff, gritty, parallel-laminated and cross-bedded sandstones are exposed nearby [SO 1266 1624]. The junction between the Llanelly and Garn Caws Sandstone formations is seen in a swallow-hole [SO 1263 1605] where the following was recorded:

There are numerous small exposures of Llanelly Formation oolitic grainstones and peloidal micrites in this area.

Details of the Llanelly Formation and Garn Caws Sandstone encountered in Carno adit [SO 1500 1501] to [SO 1485 1516] were given by Barclay and Jackson (1982).

The section exposed at Blaen Onneu Quarry [SO 1542 1686] ((Figure 16), Section 2) reads:

Wright (1981b,d, 1981a) provided further sedimentological data; of particular interest are large oncoids in the base of the oolitic grainstone (the Uraloporella Bed) with fascicular optic calcite, which he interpreted as the deposits of a schizohaline lagoon; rosettes of replaced gypsum occur at the top of the underlying Cwm Dyar Pedoderm.

The Llanelly Formation is well exposed in cliffs at Llangattock quarries, Daren Cilau ((Figure 16), Section 3), but access to the beds is mainly dangerous and difficult. Wright (1981b) gave sedimentological details. The basal part of the formation is accessible [SO 1997 1579]. The palaeosol, lying on the eroded top of the underlying Gilwern Oolite (p.48) is 4 to 5 m thick and consists of limestone clasts in green and purple clay. It is overlain by 3 to 4 m of thinly bedded limestones, sandstones and clays. The beds are tabular and have pebbly conglomeratic bases. The basal bed is 0.5 m thick, and passes upwards from gravelly conglomeratic limestone with quartz, micrite and limestone clasts to a medium to dark grey peloidal micrite. The overlying green clay layer is overlain by a quartzitic sandstone, conglomeratic at the base, and fine-grained above. The beds were interpreted by Wright (1981b) as a fluvial/floodplain sequence, but the presence of limestones, and the tabular geometry of the beds perhaps suggests a shallow marine, mixed carbonate/ siliciclastic shoreline environment, where reworking of locally derived fluvial input took place.

The lithologies of higher beds in the Llanelly Formation can be examined in the large fallen blocks below the cliffs. Quartz-rich oncolitic limestones, fenestral micrites, rippled surfaces, desiccation cracks and stromatolites are common. Wright (1981b) recorded the alga Koninckopora and the trace fossil Chondrites in these blocks.

Sandy oolitic grainstones and calcareous sandstones of the Llanelly Formation crop out between Daren Cilau and Pant-yGilwern [SO 2113 1471]. At the Darren [SO 2098 1493], a 0.4 m bed of pebbly grit lies near the base of the formation; 3 m higher is a sequence of c.3 m of thinly-bedded limestones and thin quartzitic sandstones.

The section in a quarry [SO 2175 1441] at Darren Disgwylfa is summarised thus:

The Llanelly Formation is well exposed in Craig-y-Gaer quarry [SO 2231 1322]. The basal bed is a peloidal grainstone with gastropods, oncolites and large angular clasts of the underlying Gilwern Oolite. Micrites and microsparites predominate above, with a bed of calcareous quartz pebble grit lying near the top of the exposed sequence.

Clydach Valley

Llanelly quarry [SO 2235 1240] provides complete exposure of the Llanelly Formation ((Figure 16), Section 4; (Plate 6); although access to the entire sequence is possible, the face is dangerous, particularly during and following rain, and great care is required (Barclay and Jones, 1978; Wright, 1980, 1981a, b, c, 1982a, b). The thin-bedded nature of the formation contrasts with the underlying massive Gilwern Oolite.

Small lenses of clayey fluvial sediment fill hollows in the underlying karstic surface and constitute the Clydach Halt Member. They are overlain by the Cheltenham Limestone, which consists of 8.8 m of fine-grained, thinly bedded limestones with clay or shale inter-beds. In parts, the limestones show a cyclic arrangement of fining-upwards lithologies from bioclastic grainstones through peloidal packstones and wackestones to fenestral micrites. A colour change takes place 5 m up from the base where pale grey-green limestones succeed dark grey ones. Overlying these beds, the Penllwyn Oolite comprises 3 m of oolite and oolitic grainstone, with the Uraloporella Bed at their base (Wright, 1981c). Robertson (1927) miscorrelated this bed as the Seminula Oolite.

The overlying Gilwern Clay is a prominent 5 m thick bed of mottled green and purple clay. Calcretc nodules are present in abundance at the base and are scattered throughout. Colour laminations reveal broad folding, probably caused by pedogenic processes. The top 0.5 m of the clay bed is leached to pale grey and passes up into 0.5 m of calcareous mudstone. This, in turn, is overlain by a hard blue-grey micrite with abundant rootlets. A wide shallow channel cuts these topmost beds in the east of the quarry, in the bottom of which is a coals carbonaceous shale. Dowlais Limestone beds complete the channel fill.

The same sequence is exposed in Cwm quarries c. [SO 2340 1270] (Figure 16), Section 5) for which Wright (1981b, 1982b) gave details. The Clydach Halt Member is a variable sequence of c.4 m of clays, rubbly to nodular calcretes and intraclastic conglomerates infilling and draping the karstic top of the underlying Gilwern Oolite. At one point, these beds are cross-stratified and fill a channel cut in the oolite. The multiple calcrete profile, termed the Tyler bout Pedocomplex (Wright, 1982b), and the detrital conglomerates, themselves calcreted, indicate several phases of pedogenesis. The beds also contain lenticular bodies of buff or orange-weathering homogeneous ankeritic siltstone, probably representing infills of ponds or abandoned channels. The basal bed of the Cheltenham Limestone oversteps these beds northwards. Wright (1981b, 1982b) has described two palaeosols lying near the top of the Cheltenham Limestone, the Darrenfelen and Cwm Dyar pedoderms (later renamed geosols, Wright, 1984, 1987); the former is a platy calcrete exhibiting pseudo-anticlinal or gilgai-type soil structure.

Clydach to Blorenge

Large disused quarries on the northern c.[SO 2384 1330] and eastern c.[SO 2465 1260] flanks of Gilwern Hill provide excellent exposure of the Llanelly Formation ((Figure 16), Section 6). Barclay and Jones (1978) recorded Linoprotonia cf. corrugatohemispherica from the Penllwyn Oolite; algal nodules, worm tubes, and Syringopora cf. reticulata were also collected. The Cheltenham Limestone mainly comprises a stacked sequence of fining-upwards cycles, with transgressive oolitic peloidal and bioclastic grainstones at the base and micrites at the top. There are a number of rubbly palaeosols, the overlying transgressive beds infilling the palaeokarstic surfaces. Shallow cross-bedding, suggesting a channel-fill, is present at one point. The Uraloporella Bed, at the base of the Penllwyn Oolitc, forms the roof of a prominent cave at half-height in the upper face of the eastern quarry and consists of a sheet of oncolites. The Gilwern Clay is a mottled purple, green and orange clay, and shows, at the northern end of the quarry, some planar cross-stratification, perhaps suggesting a fluvial origin. The clay is unconformably overlain by Namurian grit at this locality.

Pwll du Tunnel Borehole No.1 [SO 2469 1167] proved 18.1 m of the Llanelly Formation underlying Namurian sandstones.

The Llanelly Formation is completely exposed in PwIl du quarry [SO 2510 1155], but largely inaccessible. The basal beds are dark grey to brown micrites and calcareous mudstones (Barclay and Jones, 1978).

Narnurian overstep progressively reduces the formation east of Pwll du, and it is absent east of the Tumble area. The Pen-fforddgoch Borehole [SO 2553 1058] proved 9.8 m of beds of the Llanelly Formation unconformably overlain by Namurian sandstones and corn-prising a thinly bedded sequence of sandy and silty limestones, micrites, and some quartzitic sandstone. Pebble beds of quartz and micrite pebbles in an argillaceous micritic matrix are present at the base of the formation (Barclay and Jones, 1978).

Afon Lwyd

Barclay and Jones (1978) gave details of the Llanelly Formation in the Forgeside Borehole [SO 2504 0828]. A carbonaceous shale within the Gilwcrn Clay yielded a miospore assemblage including Lycospora pusilla. The Penllwyn Oolite was 3 m thick (from 89.20 to 92.28 m) and contained Linoprotonia cf. corrugatohemispherica.

The Llanelly Formation is mainly dolomitised at outcrop in the Blacnavon area, and was proved in site investigation boreholes in Baker Street. Borehole RI. [SO 2532 0872] showed the following:

The formation is undolomitised to the south-east of the town on the east side of the valley where it is present for about 700 m before being overstepped by Namurian grits. It is absent on the east side of the valley southwards from there.

On the west side of the valley, the Gilwern Clay is overstepped by the Namurian about 500 m south-east of Forgeside station and the underlying beds, all dolomitised, form a narrow outcrop southwards to Gallowsgreen Quarry [SO 2656 0702]. About 2.8 m of the Llanelly Formation is exposed there, unconformably overlain by Namurian quartzite, and consisting of 2.2 m of coarse-grained crinoidal dolomite above dolomicrite.

The formation is largely absent to the south, with Namurian rocks resting on dolomitised Gilwern Oolite, but it is present in the tributary valleys at Talywain and Abcrsychan. At the former [SO 2656 0405], 0.25 m of porcellaneous cream dolomite rest on coarse-grained dolomite of the Gilwern Oolite in stream exposures; 10 m of calcareous mudstones and micrites were proved in the IGS Abersychan borehole [SO 2617 0347] (Barclay and Jones, 1978).

Dowlais Limestone

This is the youngest Dinantian formation present in the district, being unconformably overlain by the Namurian rocks. A maximum thickness of about 80 m is found in the Trefil area and 70 ni are present at Blackrock quarry in the Clydach Valley [SO 2125 1529], but to the east of there it is rapidly overstepped; about 20 m remain at Llanelly quarry, 10 m at Cwm quarry, and it is completely cut out on Gilwern Hill [SO 2450 1290]. It is absent to the east of there, and along the east crop. An unconformity develops westwards at the base, and, in the west of the Merthyr Tydfil district, the formation rests on the Lower Limestone Shale.

Tabular, thick-bedded, medium to dark grey, foetid, bituminous peloidal grainstones, packstones and wackestones with dark grey shale interbeds make up much of the basal part of the formation. Fine-grained dark grey micritic limestones also occur, and sandy limestones and calcareous sandstones are present near the base. More oolitic, massive, paler grey limestones occur in the higher parts of the formation. Patchy late-stage dolomitisation is extensive in the Clydach Valley. The formation is richly fossiliferous. The coral Lithostrotion commonly occurs as in-situ thickets. The brachiopods Composita and Linoprotonia are particularly characteristic, with Composita forming coquinas, and Linoprotonia concentrated in the shaly interbeds. Algal stromatolites and oncoids are present in the lower part of the formation, and trace fossils including Chondrites also occur.

Sedimentation

Following the major regression represented by siliciclastic deposition of the Gilwern Clay and Garn Caws Sandstone, and by the westward-developing unconformity, marine transgression led to the re-establishment of carbonate deposition.

Although no detailed facies analysis has yet been published, a depositional setting in a fluctuating broad shelf lagoon behind an extensive oolite shoal complex has been suggested (Wright, 1982c). A range of facies can be recognised. The peloidal grainstones, and packstones and wackestones are shelf lagoon deposits, with the more oolitic rocks representing either shoal bodies or tidal delta deposits U Scott, personal communication, 1986). The finer-grained micritic rocks contain evidence of subaerial exposure, including rootlet beds and calcrete crusts, and are typical peritidal deposits. Bedding planes littered with oncoids and stromatolites were interpreted by Wright (1982c) as omission surfaces, with large, elongate oncoids perhaps being Chondrites burrowfills, exumed and concentrated by storm events.

Details

Cwar yr Ystrad to Clydach

The formation is about 80 m thick along the north crop from Cwar yr Ystrad to the Clydach Valley.

The basal beds are exposed at Cwar yr Ystrad quarry [SO 0824 1399] and consist mainly of dark grey bituminous limestones with shale partings, sandy limestones and calcareous sandstones.

Dark grey limestones with shale interbeds at the base of the formation are seen in a downfaulted block at the gates to Cwar yr Hendre quarry [SO 0978 1482]. Axophyllum vaughani, Lithostrotion martini, Syringopora cf. ramulosa, Composita ficoidea and Linoprotonia corrugatohemispherica were collected from exposures above the quarry face [SO 1000 1510]. There are numerous exposures of dark grey limestone in swallow-holes south and south-east of the quarries, but no substantial sections.

The formation was formerly extensively quarried at Trefil c. [SO 120 135] where there are cliffs of dark grey, well-bedded, tabular bituminous limestones, weathering to pale grey, and with dark grey shale interbeds.

Details of the beds exposed in Cairn° adit [SO 1581 1385] to [SO 1500 1500] were given by Barclay and Jackson (1982).

The cliffs of Craig y Cilau provide excellent exposures [SO 1832 1650] to [SO 1860 1590]; at the latter, about 24 m of dark grey limestones are exposed, with sandstone streaks and pyrite blebs towards the top where the formation is overlain by Namurian rocks. About 27 m of beds are seen under the Namurian nearby [SO 1893 1558] and consist of sandy and shelly oolite, granular, crinoidal, algal and bituminous limestones. Brachiopods and corals are present in layers, and there is a bed of Lithostrotion martini near the base of the exposures.

There are small disused quarries at Darren [SO 2090 1490] and Darren Disgwylfa [SO 2170 1413], but the best section is provided by Pant Mawr quarries [SO 2200 1310], and summarised thus:

Loose blocks on the quarry floor contain L. martini and Composita ficoidea.

Clydach Valley

Blackrock quarry [SO 2130 1250] (Plate 12) was working during the resurvey but has since been abandoned. Basal Namurian quartzites lie about 10 m above the top of the quarry. About 60 m of tabular limestones are exposed, but the section is largely inaccessible. A small thrust fault, with associated drag folding figured in Strahan and Gibson (1900, p.25) is visible in the face. The rocks are extensively dolomitised. An approximate section reads:

Fallen blocks contain oncolites, Lithostrotion cf. araneum, Syringopora cf. ramulosa, Composita sp., and Linoprotonia hemisphaerica. Lithologies in these blocks include sandy, bioclastic and peloidal grainstones. The formation is rapidly reduced to the east by Namurian overstep.

At Llanelly quarry [SO 2235 1240] the formation is about 20 m thick and consists of well bedded dark grey bituminous limestones, most of which are inaccessible. Fallen blocks include dark micrite, mudstone seatearth, dark grey bituminous oolitic grainstones, wackestones and packstones. Dark grey carbonaceous shale layers are common and are crowded with large productoids (Linoprotonia hemisphaerica). Some bedding planes are covered by Composita ficoidea, others by felts of the trace fossil Chondrites. Stromatolites and oncolites are common on some bedding planes, including large cylindrical oncoids which Wright (1982c) considered to be reworked Chondrites burrowfills. In addition, hryozoa and Punctospirifer scabricosta were collected from the fallen blocks. In the west of the quarry, the basal beds are sandy, with small quartz pebbles at their base. In the eastern part of the quarry, the basal beds infill a broad shallow channel; coaly shale lines the base of the channel (see p.61). An impersistent productoid-bearing carbonaceous shale at the base of the formation yielded a rich and diverse miospore assemblage: Punctatisporites spp., P. irrassus, P.sp. cf. P. nitidus, Calamospora sp., C. parva, Retusotriletes incohatus, Apiculiretusispora sp., Baculatisporites cf. fusticulus, Cyclogranisporites spp., Anaplanisporites baccatus, Verrucosisporites sp., cf. V. baccatus, Converrucosisporites sp., cf. Schopfites claviger, Convolutispora spp., Dictyotriletes spp. D. cf. sagenoformis, Lycospora pusilla, Stenozonotriletes sp., Crassispora trychera, C. cf. trychera, ?Vallatisporites sp., V. ciliaris, Colatisporites decorus, ?C. decorus, Auroraspora solisortus, Rugospora polyptycha, Spelaeotriletes cf. arenaceus.

The formation is 13 m thick on the western flanks of Gilwern Hill, where the basal 9 m are exposed in Cwm quarries [SO 2345 1288]. The basal 1 m is a tough, buff weathering sandy limestone, above which is a sequence of well-bedded medium to dark grey bioclastic limestones with Lithostrotion martini, Syringopora sp., and productoids.

Namurian overstep further reduces the formation eastwards across its outcrop on Gilwern Hill. It is absent above the quarries on the eastern flanks of the hill, and from there southwards along the east crop. It reappears under the coalfield to the west, and about 1 m of the formation was drilled below the sub-Namurian unconformity in Blaenserchan Colliery underground borehole [SO 2435 0213].

Chapter 5 Namurian

Namurian strata crop out over about 26 km² around the margin of the coalfield. They comprise a succession of fluvial, deltaic, and shallow marine sediments deposited on the north-east margins of the South Wales basin. The 'Wales–Brabant' landmass, of which St George's Land was part, lay close to the north, and, in the east, the Usk Axis was an emergent land area for much of the Namurian (Kelling, 1974; Ramsbottom, 1978). The marginal position of the district is illustrated by the incomplete sequence of late Namurian age compared to the more complete, 550 m thick, basinal sequence in the Gower, the predominantly fluvial nature of the sediments, and the shallow-water faunas of the marine bands. The sequence thins from a maximum of about 45 m in the west of the district to under 20 m in the south-east, the attenuation being accomplished by progressive intra-Namurian overlap of the rocks of the Marsdenian by those of Yeadonian age (Figure 17). Northward attenuation also takes place, and about 28 m are present on Mynydd Llangynidr in the most northerly outcrops.

Sandstones are well exposed on the moors but substantial natural sections are confined to Rhymney Bridge and the Pitwellt (Evans, 1971), to Nant Milgatw (Barclay, 1973; Robertson, 1927), and to the Clydach Valley (Jones and Owen, 1966; Owen and others, 1965). Surface boreholes at Rhymney Bridge, Cwm Carno and Pwll du, and underground boreholes at Six Bells and Blaenserchan collieries supplement previous data (Barclay and Jackson, 1982; Barclay and Jones, 1978; Hall and Squirrell, 1972; Robertson, 1927).

Namurian rocks rest unconformably on Holkerian strata over much of their outcrop, but from the Clydach area eastwards the break increases in magnitude, and they rest on progressively lower levels of the Dinantian (Figure 18); along the east crop from Blorenge to Pontypool, they rest on Courceyan strata (Barclay and Jones, 1978; George, 1954, 1956a,b; Jones and Owen, 1966; Robertson, 1927). Maximum overstep takes place on the east side of Mynydd Garnclochdy where Namurian rocks rest on the topmost beds of the Castell Coch Limestone (see p.71, Jackson, 1975).

Classification

(Figure 17) shows the lithostratigraphical and chronostratigraphical classifications. The rocks lying between the Carboniferous Limestone and the Coal Measures have been traditionally known as the 'Millstone Grit' or 'Millstone Grit Series' on account of their stratigraphical equivalence to the Millstone Grit of the Pennines. Given the modification to its upper limit, 'Millstone Grit' is now used as an informal lithostratigraphical name for the rocks of Namurian age (Jones, 1974).

Strahan and Gibson (1900) recognised three divisions: in ascending order: (1) pebbly grits and conglomerates (2) shales with lenticular sandstones and (3) massive sandstone with lenticular shale (the Farewell Rock). Robertson (1927) named (1) the Basal Grit and (2) the Shale Group. Within the Shale Group, he identified the Gastrioceras cancellatum Zone of Bisat's (1924) goniatite-based classification. The identification of the Subcrenatum Marine Band within the Shale Group (Leitch and others, 1958) places only the lower part of this unit, together with the Basal Grit, in the Namurian; the upper part of the Shale Group and the Farewell Rock lie in the Westphalian.

The Pendleian (E₁), Arnsbergian (E₂), Chokierian (H₁), Alportian (H₂) and Kinderscoutian (R₁) stages of the Namurian are not represented. Only the top two stages, Marsdenian (R₂) and Yeadonian (G₁), corresponding to mesothems N10 and N11 (Ramsbottom, 1978), are known to be present (Barclay and Jones, 1978; Jones, 1974).

Stratigraphical summaries were provided by Ramsbottom (1969, 1978), George, (1970), Jones (1970, 1974) and Ramsbottom and others (1978).

The Dinantian–Namurian Junction

The age of some of the basal grits and sandstones assigned to the Namurian is problematical. Fossil evidence is lacking, but they may be middle to late Viséan (Jones, 1970, 1974; Jones and Owen, 1966) (see below).

A Marsdenian age was proved for the basal beds near Trefil, where Bilinguites was collected (Barclay, 1973). A similar age was assigned to the basal beds in the Clydach-Pwll du area on floral grounds (Jones, 1970; Jones and Owen, 1966), but, over most of the district, a Marsdenian age is generally inferred for the basal Namurian rocks rather than diagnostically proved.

Owen and Jones (1961) examined the Dinantian/ Namurian junction at two localities in the district: (i) at Trefil old quarries [SO 1233 1333], where Namurian sandstones unconformably overlie dolomitised and silicified Dowlais Limestone; (ii) in the Clydach gorge, where collapsed masses of shale, sandstones and conglomerate rest on dolomitised Dowlais Limestone. They believed that the foundering of the basal Namurian was due to intra-Namurian dolomitisation and solution, but on subsequent re-examination (Jones and Owen, 1966), considered that the dolomitisation is Triassic and the foundering of Tertiary to Pleistocene age. They further suggested that the intimate nature of the siliciclastic/carbonate junction is evidence of the diachronous cessation of carbonate deposition and the spread of elastic sediments consequent upon uplift of the St George's Land and the Usk Axis. Thus, in their view, the Dinantian carbonate depositional area was contracting from middle to late Viséan times. Against this, however, is the fact that clastic sediments form parts of a cyclic sequence of sandstones and limestones of Holkerian and Asbian age to the east of the Usk Axis, in the Forest of Dean.

Details

On Trefil ddu [SO 1047 1473]; [SO 1074 1391]; [SO 1075 1387]; [SO 1102 1373], at Trefil [SO 1241 1314] and in Ogof Fawr [SO 1274 1518] quartz pebble conglomerate, pebbly grit and sandstone rest on a palaeokarstic surface of Dowlais Limestone. The Rhymney Bridge and Carno New Dam W1 boreholes penetrated the surface, and it is also well exposed in Carno adit (Barclay and Jackson, 1982). The unconformity is exposed in cliffs at Craig y Cilau [SO 1860 1590] and [SO 1893 1558] where Namurian strata, comprising carbonaceous shale overlying basal pebbly sandstone and conglomerate, rest on Dowlais Limestone. Re-examination of the Clydach Heads of the Valleys road section shows that younger Namurian strata lie horizontally over contorted older Namurian strata. Moreover, the contorted strata appear to have foundered while semi-lithified. The foundering would thus appear to be due to Namurian solution, and unrelated to the dolomitisation of the Dowlais Limestone which is clearly related to the joint system, and therefore post-dates the Variscan orogeny. Other localities in the Clydach area where the junction was examined during the resurvey are a roadside cutting near Pont Gwyn [SO 2084 1260], Nant Gwyn [SO 2082 1266], a stream near Penlanfach [SO 2113 1263], roadside exposures above Blackrock [SO 2159 1285] and an old cutting at Darrenfelen [SO 2207 1241]. At the disused Pwll du quarry [SO 2513 1150] Namurian quartzites rest unconformably on the Gilwern Clay member of the Llanelly Formation (Jones and Owen, 1966). In Pwll du Tunnel No.1 Borehole unsorted pebbly quartzitic sandstone fills irregularities in the palaeokarstic surface of the Dowlais Limestone to a depth of 0.6 m, only the basal 2 m of this formation being present below the unconformity.

There are four exposures of the unconformity on the east crop. At Craig-yr-hafod [SO 2730 0991], Namurian grits, including balled up masses, infill a highly irregular palaeokarstic surface with relief of up to 10 m cut in dolomites of the Clydach Valley Group. In a small gully near Gallowsgreen Quarry [SO 2656 0702], well-bedded pale green and grey sandstone, reddish in parts, overlies dolomitised beds of the Llanelly Formation, with a 0.4 m-deep solution pipe filled with bedded pebbly quartzite. At Graig Quarry [SO 2723 0738], quartzitic sandstone-filled pipes extend 7.5 m down into Clydach Valley Group dolomites. At Lasgarn Wood c. [SO 2740 0430], Namurian rocks infill a potholed palaeokarstic surface displaying a clint and grike-type topography, now in part exhumed. In the northern part of the wood [SO 2732 0419], a swallow hole apparently of Namurian age is filled with later Namurian sediments.

At Craig-yr-hafod and Lasgam Wood, north–south faults throw down the Dinantian rocks to the west, but the Namurian rocks to the east. Attenuation of the Dinantian takes place from west to east across these faults, indicating that in this most easterly part of the Namurian outcrop, closest to the then tectonically active Usk Axis, Namurian overstep was achieved, at least in part, by penecontemporaneous step-faulting and erosion of the underlying Dinantian basement.

Stratigraphy and sedimentation

Kelling (1974) summarised the sedimentary history of the Namurian rocks. Sedimentation in the west of the district began during the late Marsdenian in a south-westerly pro-grading deltaic complex. The district was situated on the back margin of the upper delta plain, with mainly fluvial, high energy, in-channel deposition, and much sediment reworking. Shallow lakes in the interdistributary areas were filled by muds and silts and colonised by vegetation. Where not removed by subsequent fluvial scouring, these deposits are preserved as thin rootlet beds. Marine incursions periodically flooded the delta plain. The increase in extent of each succeeding marine band suggests that the eastward expansion of the depositional area was pulsed, and that marine transgressions initiated these pulses (Ramsbottom, 1977, 1978).

Sandstones predominate in the Marsdenian, whereas the Yeadonian exhibits a range of sandstone to mudstone ratios across its outcrop. In the Rhymney sections the Yeadonian consists mainly of mudstones, but the proportion of sandstone increases to the east. The pattern is variable, however; for example, sandstones occupy 84 per cent of the sequence at Blaenserchan, while at Abersychan 2.2 km to the northeast the proportion is 50 per cent (Figure 19).

The sandstones are almost entirely orthoquartzites, their clean and mature nature indicating winnowing and reworking. Conglomerates are common, the pebbles being mainly of vein quartz, but other lithologies are represented and include metaquartzite, jasper, and felsite (Jones, 1974; Kelling, 1974). Clasts of Dowlais Limestone are present in the basal Namurian conglomerate in Carno adit (Barclay and Jackson, 1982), and the basal grits of Lasgarn Wood contain boulders of Gilwern Oolite. The sandstones are mainly medium- to coarse-grained, and occur in lenticular, internally cross-bedded units consisting of stacked fining-upwards cycles. Comminuted plant debris is common in the bases of units, and large calamitid branch casts are a feature of the Marsdenian sandstones (Owen and others, 1965). Interbed contacts are mainly sharp and erosive, but in places the sandstones pass upwards into thin argillaceous rootlet beds.

In the Marsdenian, mudstones, along with siltstones, seatearths and coal smuts are present mainly as thin inter-beds between the sandstones. Marine mudstones are confined to thin, impersistent, and mainly impoverished probable correlatives of the Metabilingue, Superbilingue and Anthracoceras marine bands. These bands become more impoverished and fail eastwards, with each succeeding band covering a wider area.

The Cancellatum Marine Band marks the base of the Yeadonian. With the Cancellatum transgression, the depositional area reached all the area within present outcrops, except perhaps extreme parts of the north and east crops. The consistent thickness (15 to 20 tn) of the Yeadonian beds across the district indicates that the Usk Axis had little effect at that time.

The Cancellatum Marine Band is a richly fossiliferous mudstone west of the Clydach Valley, and contains a near-shore benthic fauna as well as goniatites; to the east, it deteriorates to a Lingula bed. Silty limestone layers and concretions are found in some localities where the band is richly fossiliferous. Although the regressive phase of the Cancellatum Zone consists mainly of rootlet beds and fluvial sandstones over most of the district, the marine band is succeeded by Carbonicola-bearing mudstones and siltstones in the Rhymney Bridge and Pitwellt sections.

The succeeding Cumbriense transgression appears to have been the most widespread of the Namurian marine incursions. The marine band is generally present, and attains maximum thickness in the extreme south-east, in contrast to the overall thinning of the Namurian in that direction. It is poorly fossiliferous and contains a Lingula biofacies assemblage mainly, though some juvenile C. cumbriense have been recorded from the Clydach Valley (Jones and Owen, 1966), and richer brachiopod-bivalve faunas were obtained from Six Bells and Blaenserchan underground boreholes. In these boreholes, the band consists of two marine mudstone phases separated by barren sandstone. This sandstone, and the one overlying the marine band in the Rhymney and Pitwellt sections (the 'Cumbriense Quartzite') may have been deposited in a littoral environment, as distributary mouth bar sands (Kelling, 1974), in contrast to most of the orthoquartzites of the Yeadonian which have a fluvial aspect.

Details

Rhymney–Trefil

Nant Pitwellt provides good sections of the beds between the Superbilingue Marine Band and the top of the Namurian (Evans, 1971). The Superbilingue Marine Band [SO 0946 1088] is overlain by a thin sandstone, on top of which is a coal smut. The coal is succeeded by a channel-fill quartzitic sandstone, above which is the Anthracoceras Marine Band. This marine band is 1.35 m thick and consists of shales with productoids, lingulids and Planolites. The Cancellatum Marine Band [SO 0903 1116] is richly fossiliferous, with the succeeding Carbonicola beds best seen 100 m upstream [SO 0900 1125] and 100 m downstream [SO 0900 1107]. The Cumbriense Marine Band [SO 0898 1112] consists of silty mudstones with lingulids and fish remains. Overall, the sequence is similar to that provided by the nearby Rhymney Bridge section, and the Rhymney Bridge No 1 Borehole ((Figure 19), Section 1).

Quartzites and conglomerates lying near the base of the Namurian are well exposed on Trefil ddu and Mynydd Llangynidr, though good sections are confined to some of the multitude of swallow-holes that pit the area. Marine mudstones (?Metabilingue Marine Band) lying about 7 m above the base of the Namurian are exposed in a swallow hole [SO 1018 1310]. Productoid fragments, Rugosochonetes sp., a calcareous brachiopod and Dunbarella sp.were collected. Lingula- and Orbiculoidea-bearing mudstones, possibly the Superbilingue Marine Band, are exposed in swallow holes [SO 1098 1300]; [SO 1055 1262], the latter also yielding Aviculopecten sp.Quartzites with a few thin siltstone and mudstone seatearth interbeds which overlie this horizon are exposed in the Rhymney River north of the reservoir [SO 1011 1057]; [SO 1015 1062] (Evans, 1971). The Anthracoceras Marine Band was exposed in the reservoir [SO 1026 1041] in 1972 after a prolonged period of drought, and consisted of 3.4 m of mudstones with Lingula mytilloides, Orbiculoidea cf. cincta, productoid fragment, Anthraconeilo sp., Nuculopsis sp., Coleolus sp., Homoceratoides fragment, and Serpulites stubblefieldi. Rhymney Bridge No 1 Borehole [SO 1008 1015] ((Figure 19), Section lb) gave a complete section from the sandstone overlying the Anthracoceras Marine Band to the base of the Namurian. L. mytilloides was obtained from the lower (Superbilingue) marine band, and L. mytilloides and fish remains including Rhabdoderma and Strepsodus were obtained from the higher (Anthracoceras) marine band.

The Cancellatum Marine Band is well exposed in the River Rhymney below the reservoir [SO 1041 1010] to [SO 1038 1016] (Robertson, 1927, Evans, 1971) where about 2.4 m of richly fossiliferous mudstones are present below lenticular orthoquartzite and fine-grained sandstone/siltstone channel-fill units ((Figure 19) Section la). A rich goniatite/brachiopod fauna (see (Table 9)) is present towards the top of the hand, and fragments of the trilobite Brachymetopus edwardsi have also been collected (R Owens, personal communication, 1981). A silty limestone layer 0.8 m from the top of the band yielded 16 fragmentary conodonts identified by M J Reynolds as Idiognathodus delicatus, Idiognathoides attenuatus and Declinognathodus lateralis.

Fine-grained sandstone/siltstone channel-fill deposits, a lenticular ironstone bed up to 0.27 m thick, and succeeding mudstones which overlie the Cancellatum Marine Band are all characterised by the presence of Carbonicola (Evans, 1971). The mudstones also contain Anthraconaia aff. bellula, aff. A. tenisulcata, C aff. pseudacuta, Curvirimula belgicaNaiadites sp., fish fragments and Oeisina arcuata. Further downstream [SO 1042 1000] the Cumbriense Marine Band may be represented by 0.3 m of pyritous fish-bearing mudstone above 2.15 m of mudstones with Naiadites aff. hibernicus and Rhadinichthys sp.and by silty mudstones [SO 1048 0996] which underlie quartzitic sandstones and siltstones at the top of the Namurian. The silty mudstones contain Naiadites sp.and Spirorbis sp.

A 1.7 m mudstone containing pyrite burrowfills and Planolites is exposed in Nant Llechau near Trefil [SO 1194 1197]; it is either the Anthracoceras or Superbilingue Marine Band. North of Trefil, in a swallow hole [SO 1214 1458], 0.25 m of mudstones were excavated under about 7 m of quartzites and conglomerate. The mudstones (?Metabilingue. Marine Band) contained goniatites identified by W H C Ramsbottom as Bilinguites of R₂ age (Barclay, 1973), chonetids and worm traces; this is the only direct goniatite evidence of Marsdenian age in the district. Weathered mudstones, with L. mytilloides, possibly the Superbilingue Marine Band, are exposed in a swallow hole [SO 1207 1472]. Debris probably from the same bed with L. mytilloides was excavated from a small reservoir at Trefil [SO 1248 1298]. Lingula-bearing mudstones are also exposed in several swallow hole sections [SO 1250 1381]; [SO 1275 1290]; [SO 1260 1275]. The last section shows:

A section from below the Superbilingue Marine Band to above the Cancellatum Marine Band in Nant Milgatw [SO 1342 1233] (Barclay, 1973) reads:

The Cancellatum Marine Band is best seen at Milgatw Farm [SO 1331 1166] (Robertson 1927, p.48) where c.1.5 m of richly fossiliferous mudstone (Table 9) are intercalated between quartzites.

The Cumbriense Marine Band is seen in a waterfall in Nant Milgatw [SO 1353 1265] where the following section is exposed:

Carno-Mynydd Llangynidr

Details of the Namurian sequence in Carno adit are given elsewhere (Barclay and Jackson, 1982). The fauna of the Cancellatum Marine Band is listed in (Table 9).

The Carno New Dam boreholes proved the Namurian succession; Borehole VV1 ((Figure 19), Section 2) is typical. Three marine horizons are present in the boreholes-the Anthracoceras and Cumbriense marine bands with a brackish water assemblage of lingulids, fish fragments and P. ophthalmoides, and the Cancellatum Marine Band with a rich goniatite-brachiopod fauna. The Cumbriense Marine Band lies up to 17 m below the top of the Namurian, the bed being overlain by a succession mainly of fluvial quartzitic sandstones. Rapid thinning takes place northwards from here, for the Namurian is estimated to he only about 28 m thick at outcrop on Mynydd Llangynidr. Basal shales and sandstones are well exposed at Craig y Cilau (p.62).

The Clydach area

In the Heads of the Valleys road section [SO 2072 1242] to [SO 2053 1236] (Plate 13) both the Superbilingue and Anthracoceras marine bands are represented by thin faunal mudstone layers in a predominantly arenaceous Marsdenian succession ((Figure 19), Section 3). The former contains Lingula, fish fragments and gastropods, the latter Lingula and Orbiculoidea. The rich Cancellatum Marine Band fauna is listed in (Table 9) (Jones and Owen, 1967). The Cumbriense Marine Band is now unexposed, but the index goniatite has been recorded (Jones and Owen, 1967). A Marsdenian floral assemblage of Lepidophyllum sp., Neuropteris cf. nachstebreckiana, Mariopteris cf. acuta, Paripteris gigantea and Sphenopteris sp.has been recorded from beds near the base of the Namurian (Jones and Owen, 1966).

North of the Clydach gorge, the marine band exposed in the Nant yr Hafod [SO 2084 1310]; [SO 2082 1315] is probably the Cancellatum Marine Band (Robertson 1927). Fossiliferous mudstone from two small spoil heaps at old adit entries in the stream is also from this level (Table 9). An impure carbonate layer from the marine band [SO 2082 1315] yielded conodonts identified by M J Reynolds as Idiognathoides attenuatus, Idiognathodus sulcatus, Idiognathodus delicatus, and Declinognathodus lateralis. To the south-east, the marine band is exposed in Nant Gwyn [SO 2109 1286], but further east it is probably absent, being removed prior to deposition of coarse quartz pebble conglomerate and pebbly grit.

On the south side of the Clydach gorge, in Llammarch Dingle, ((Figure 19) Section 4), Paripteris gigantea (large variety), Alethopteris cf. lonchitica, Neuropteris cf. nachstebreckiana and Mariopteris sp.have been recorded from shale and siltstone layers within predominantly arenaceous beds towards the base of the Namurian (Jones and Owen (1967, p.190) to be the Subcrenatum Marine Band is more probably the Cancellatum Marine Band.

To the east and south, the Cancellatum Marine Band is represented mainly by a Lingula band, though in the Pwll du Tunnel No 1 Borehole [SO 2469 1167] ((Figure 19), Section 6) both it and the Cumbriense Marine Band are burrowed, but otherwise barren, mudstones. A siltstone layer within the basal Namurian sandstones of the borehole, and lying 2.8 m above the Dinantian, yielded plant debris including Neuropteris cf. schlehani.

About 6 to 8 m of basal Namurian quartzite and conglomerate are exposed in the nearby Pwll-du quarry [SO 2513 1150], and a green mudstone layer lying immediately above the unconformable junction with the Gilwern Clay of the Llanelly Formation yielded Neuropteris sp.A section recorded nearby [SO 246 117] (Jones and Owen, 1967, p.190) reads:

Re-examination of the section showed that the 1.52–1.83 m bed of 'grey and blue marl ...' is the Gilwern Clay (Dinantian).

The Blorenge

The IGS Pen-ffordd-goch Borehole ((Figure 19), Section 7 [SO 2553 1058] proved 24 m of Namurian (Barclay and Jones, 1978), and a similar thickness, mainly of quartzites and conglomerates is present on the north-east scarp face of the Blorenge c. [SO 273 123].

Blaenavon-Abersychan area

Boreholes drilled in this poorly exposed tract have been described previously (Barclay and Jones, 1978; Hall and Squirrell, 1972) ((Figure 19), Sections 8, 9, and 11, (Table 9), (Table 10)). About 10 m of quartzites resting on the Dinantian at Craig-yr-bafod [SO 2730 0980] include a mudstone layer 7.8 m above the base containing L. mytilloides, and probably the Cancellatum Marine Band. About 7.5 m of quartzite and conglomerate with clay beds are exposed above the Dinantian at Graig Quarry [SO 2723 0738]. The following section is exposed in a gully north of Gallowsgreen Quarry [SO 2656 0702].

The following section was measured in a stream and swallow hole near Craig-y-felin on Mynydd Garn-clochdy [SO 2782 0608] to [SO 2772 0602]:

East of Mynydd Garn-clochdy, where the sub-Namurian unconformity truncates the underlying Dinantian at the top of the Castell

Coch Limestone (see p.47), the basal bed of the Namurian seen in disused limestone quarries [SO 2935 0538] to [SO 2920 0540] is buff, green-weathered micaceous shale.

In Lasgarn Wood c. [SO 2740 0430] there are numerous exposures of basal Namurian quartzite and grit infilling and blanketing a highly irregular palaeokarstic surface cut in the underlying Dinantian (see p.66).

Underground boreholes

Three boreholes drilled underground at Marine, Six Bells and Blaenserchan colleries give the only data for the Namurian below the coalfield. The Marine Borehole [SO 1895 0398] proved only the topmost 2.6 m of the Namurian. The Six Bells Borehole [SO 2191 0279] proved 17.5 m of beds, and the Blaenserchan Borehole [SO 2435 0213] proved the whole of the Namurian succession ((Figure 19), Section 10). In both the Six Bells and Blaenserchan boreholes, the Cumbriense cycle consists of two marine bands separated by non-marine beds. At Six Bells, a lower Lingula-Planolites bed is separated from a higher, brachiopod-bivalve bed (Table 10) by 2.8 m of non-marine beds. At Blaenserchan, a lower Shelly mudstone (Table 10) is separated by 1.8 m of quartzitic sandstone from a higher mudstone containing Euphemites sp.and Planolites.

Chapter 6 Westphalian

The Westphalian Coal Measures, forming the north-east part of the South Wales Coalfield, occupy about 200 km² in the south-west to central part of the district. Up to 1000 m of measures are preserved in the west, thinning to about 600 m in an attenuated succession on the east crop. The rocks succeed those of the Namurian conformably, and range in age from Westphalian A to Westphalian D (Figure 20).

Almost 40 per cent of the outcrop is a plateau formed by resistant Upper Coal Measures Pennant sandstones. The coalfield valleys dissect the plateau, and the argillaceous rocks of the Middle and Lower Coal Measures crop out on the lower slopes of these valleys and in a peripheral belt around the Pennant escarpment.

(Figure 20) gives a generalised vertical section of the Westphalian rocks. Their lithostratigraphical subdivision is that used throughout England and Wales by the British Geological Survey (Stubblefield and Trotter, 1957; Woodland and others, 1957). The presence of the Subcrenatum, Vanderbeckei and Aegiranum marine bands allows precise chronostratigraphical subdivision of the Lower and Middle Coal Measures into Westphalian stages A, B and C. The Westphalian C–D boundary is less accurately positioned. It has been traditionally placed at the Cefn Glas seam and its correlatives in South Wales (e.g. Crookall, 1955; Ramsbottom and others, 1978), but Cleal (1978) has argued that, on floral evidence, it lies within the Rhondda Beds.

The Lower and Middle Coal Measures contain the principal coals, and also the bedded sideritic ironstones that were once extensively worked. In order to correlate coals across the coalfield, seam nomenclature has been standardised by the BGS and the National Coal Board (now British Coal) in conformity with the nomenclature established by Woodland and Evans (1964) using the seam names of the Taff and Rhondda valleys.

Previous research

The first geological survey, at the one-inch scale, by Dc la Beche and Williams between 1837 and 1845 led to publication of the Old Series One-Inch Geological Sheet 42 and Vertical Sections (Sheets 8 to 10); some details were given in the first Geological Survey memoir (De la Beche, 1846). Prior to this, Sopwith (1843) had produced a geological map and cross-sections of the Ebbw Vale–Sirhovvy Valley area (Turner and Dearman, 1982). Palaeontological investigations were reported by Bevan (1858a,b), Salter (in Rogers, 1861) and Wheelton Hind (1905).

Strahan and Gibson carried out the primary six-inch survey in 1890–95, and the one-inch map was published in 1896. The first edition of this memoir and a revised map were published in 1900. Robertson's revision in the late 1920s led to publication of the second edition of this memoir in 1927 and of a revised one-inch map in 1932.

Contributions to local stratigraphy have been made by Blundell (1952), Sullivan and Moore (1956), Leitch and others (1958), Hall and Squirrel! (1971), and Barclay and Jones (1978). Thomas (1974) gave a summary of the stratigraphy of the South Wales Coalfield. The red beds of the Upper Coal Measures have been studied by Howell and Cox (1924), Blundell and Moore (1960) and Downing and Squirrel! (1965). Sedimentological studies of the Rhondda Beds were carried out by Kelling (1963), and Bluck and Kelling (1963), and the sedimentation of the Coal Measures was summarised by Kelling (1974). Coal seam details and correlations, based on underground workings, were given by Adams (1956, 1967) and by the National Coal Board (1960).

Database

The original colliery shaft sinkings, summarised by Robertson (1927, pp.57–57), give the most complete records, although in some cases their accuracy is doubtful and the positioning of some marine bands conjectural. Two more recent colliery developments have provided modern sections: Abertillery New Mine, driven in 1957–59 and logged by R A Downing and G D E Lewis, and Washery Drift, Blaenavon, driven in 1971–72 and logged by W J Barclay and D I Jackson. During the resurvey, Ogilvie, Markham, Marine, Abertillcry, Six Bells, Big Pit and Blacnserchan collieries were working and visits were made to collect roof faunas and log cross-measures drivages. Ty Trist Colliery, Tredegar, was visited by H C Squirrel! prior to its closure in 1959. Ogilvie Colliery closed in 1974 and Big Pit (now a museum) closed in 1980. Several small working surface levels and drifts were examined during the resurvey, and opencast operations at Trecatty, Dowlais Top, Rhymney, Mountain Pit, Rhyd-y-Blew, the Laurels and Blaenant provided much new information. In addition, many boreholes, drilled mainly for site and tip stability investigations, have been examined.

The following account gives a representative selection of the large amount of data available. The six-inch or 1:10 000 maps of the area are listed on p.x; these contain summaries of the principal shafts, boreholes, sections and outcrops and are available through BGS offices. In addition, open file reports detailing selected shafts, boreholes and sections are available for six-inch sheets SO 20 NE (Blaenavon) and SO 20 SE (Abersychan–Pontypool). Full records of shafts, boreholes and principal sections are held in the BGS National Geosciences Data Centre, Keyworth, and in the BGS Regional Office, Aberystwyth, and are available for inspection. Abandoned coal-mine plans and details of current workings and coal seams are held by British Coal, Cardiff.

Information on opencast workings and exploration is held by British Coal Opencast Executive, Aberdare.

Lithologies and sedimentation

The lithologies of the Lower and Middle Coal Measures of South Wales, and their arrangement in coarsening-upwards coal-capped cycles are well documented (Squirrell and Downing, 1969; Woodland and Evans, 1964). The cycles are typical of those deposited on a fluvial-dominated delta plain (Fielding, 1985), and coarsen upwards from fine mudstones at the base through silty mudstones and siltstones into fine-grained sandstones. The colour of the rocks is mainly restricted to shades of drab grey, with decrease in grain size and increase in carbon content generally leading to darker shades. Sideritic ironstone is common in the finer lithologies, both as bedded tabular sheets and nodules. Seatearths, ranging from mudstones to sandstones, lie in the upper part of the cycles and underlie the coals.

The fine mudstones and silty mudstones, in which most of the body and trace fossils occur, are either blocky and homogeneous or show fine parallel lamination; bioturbation may disrupt the lamination. These beds are lacustrine deposits, laid down in the deeper parts of fresh-water lakes, with mudstone–siltstone oscillations reflecting fluctuating discharge into the lakes. Cross lamination, rippled bedding, (laser bedding and wavy lamination higher in the cycle indicate increasing energy, with tractional currents replacing fall out from suspension as the deltas of the distributary channels prograded into the lakes. The sandstones which form the topmost part of each coarsening-upwards cycle are the mouth bar deposits of these minor distributaries. Sandstones may also, however, interrupt the cycle at any point, and these represent a variety of fluvial depositional settings including distributary channels, crevasse splays, and levee deposits. The channel-fill sandstones are the coarsest, commonly conglomeratic at base, and fining upwards. They rest on the scoured surface of the underlying beds, and exhibit both trough and planar cross-bedding. They are almost entirely quartz arenites, but Pennant-like sandstones appear locally near the top of the Middle Coal Measures (p.91).

Within the lacustrine cycles, the mouth bar sandstones become rooted upwards, indicating that the sediment–water interface became shallow enough to allow plant colonisation. The establishment of vegetation led to reduced depositional energies, and the rooted beds tend to fine upward from sandstones and siltstones into mudstones. Chemical and organic alteration of the sediments by pedogenic and biogenic processes converted them into siliciclastic palaeosols (seat-earths). Peat accumulation followed, with subsequent burial and compaction leading to coal formation.

The base of the mudstones which overlie the coals has been traditionally taken as the base of the lacustrine cycles. Ramsbottom (1984) argued, however, that the widespread extent of the principal coals is evidence that the peat formed in response to a rising water table caused by eustatic marine transgression, and that these coals are in effect 'failed marine bands'. The widespread extent of the marine bands through Europe is generally held to he evidence that the transgressions which initiated them were eustatic. The marine hands overlie the coals, occupying a similar position in the cycle to the non-marine mudstones. They are predominantly composed of mudstones and silty mudstones, deposited in pro-delta bays when the delta plain was flooded to shallow depths. Renewed delta progradation following the marine incursion produced the typical Coal Measures cycle described above, with lacustrine deposits succeeding the marine ones.

Of particular interest within the district is the variety of responses in different parts of the sequence to eastward approach to the contemporary basin margin in the vicinity of the Usk Axis. These generally reflect a transition from the lower to the upper part of the delta plain.

a)      In the sequence from the top of the Subcrenatum Marine Band to the Garw seam, the eastwards failure of the coals, the im persistence of the marine bands, and the preponderance of seatearths and mature channel-fill sandstones in the east point to the proximity of the basin margin, with continuing emergence and reworking of the sediments (p.79).

b)      The sequence between the Garw seam and the Vanderbeckei Marine Band thins from 70 m in the west to 20 m in the east, but the coals generally maintain their thickness and tend to coalesce on the east crop.

c)      In the beds between the Vanderbeckei Marine Band and the Nine-Feet seam, major channelling takes place in the south-east. The Nine-Feet seam is also much affected by washouts in the east, although, where present it may show anomalously high thicknesses.

d)      In the beds between the Nine-Feet seam and the Aegiranum Marine Band, the coals are much affected by washouts, with many channel-fill sandstones in which slump structures are common. The coals tend to have ganisteroid seatearths (Jackson, 1973). There is also evidence that, locally, differential compaction of the sediments led to the sandstones forming topographical highs on which coal was never deposited (p.90).

e)      In the beds between the Aegiranum Marine Band and the top of the Cambriense Marine Band, the well-developed lacustrine coarsening-upwards cycles of the west pass eastwards into a thinner fluvial, sandstone-dominated sequence in which most of the coals are absent. The marine bands also generally fail eastwards.

The Upper Coal Measures were deposited in a different sedimentary regime to that of the underlying beds. Pennant sandstone sedimentation spread diachronously north-east to reach all the district after deposition of the Brithdir coal. Prior to that, Pennant sandstones were deposited locally only in the south-west near the top of the Middle Coal Measures (p.91) and in the Llynfi and Rhondda Beds.

Although both the Llynfi Beds and the Rhondda Beds contain stacked sequences of coarsening-upwards coal-capped lacustrine cycles in the west of the district, alluvial plain facies become predominant eastwards. The coals generally fail eastwards, and mature channel-fill quartzitic sandstones, commonly pebbly and conglomeratic, are abundant. A red-bed facies, comprising red, purple and green mottled mudstones and siltstones, most of which are rooted and show evidence of pedogenic alteration, spread diachronously westwards, commencing in the Llynfi Beds of the south-east crop and covering the whole district in the Rhondda Beds (Downing and Squirrell, 1965). Blundell and Moore (1960) suggested that the reddening was secondary, and of Permo-Triassic age. This was disproved by Downing and Squirrell (1965) who proposed a dual origin of a red, lateritic soil source area and contemporaneous oxidation of the sediments in situ. Kelling (1968, 1974) discounted a red soil source as a major factor, and proposed subaerial weathering and oxidation on an alluvial plain with a low water table.

Kelling (1968) described fining-upwards fluvial cycles in the Rhondda Beds, consisting of channel-fill orthoquartzites and conglomerates passing up into trough cross-stratified sandstones, overlain by overbank siltstones and mudstones. From a study of the cross-bedding, and channel orientations (Bluck and Kelling, 1963; Kelling, 1964, 1969), it was concluded that the basin was closed to the east and receiving sediment from the Usk Axis at that time.

An alternative depositional model for the Rhondda Beds was proposed by Kelling (1974, 1986). He argued that the textural and mineralogical maturity of the orthoquartzites and the roundness of the quartz pebbles, together with a high proportion of discoidal pebbles, were incompatible with a proximal alluvial plain setting. A littoral environment was thus suggested, with the orthoquartzites being beach, barrier and tidal channel deposits, and the argillaceous beds being lagoonal penemarine deposits, subject to periodic emergence and oxidation.

Of the two contrasting depositional models, a proximal alluvial plain is preferred. Although discounted by Kelling (1974, p.206), the maturity of the orthoquartzites may be better explained by continued reworking of the deposits in high-energy channels draining the Usk Axis area where either Namurian or Upper Devonian conglomerates were unroofed. Such a model is more consistent with the sedimentation patterns observed throughout much of the Namurian and Westphalian indicating the continuing activity on the Usk Axis. Indeed, the diachronous spread of the red-bed facies from the axis, along with the evidence of extreme attenuation, non-sequences and local unconformities, point to the westwards migration of the basin margin at that time.

Pennant sedimentation heralded a major palaeogeographical change. Almost all the earlier Westphalian sediments were derived from the north or east, from distributaries draining St George's Land and the Usk Axis. The Pennant sandstones are immature, lithic-rich arenites or subgreywackes deposited by high-energy, low sinuosity braided rivers flowing northwards from the northward-advancing Variscan front. Channel-fill sandstones predominate in the Brithdir, Hughes and Grovesend Beds. The Variscan advance must, however, have been pulsed, as shown by the thin but laterally persistent floodplain lacustrine mudstones within the Pennant sandstones. The thicker mudstones and coals of the Grovesend Beds record prolonged periods of delta plain deposition. The eastward attenuation of the beds above the Brithdir, whether due to unconformity or condensation (p.96) points to the continuing effect of the Usk Axis.

Faunas

Marine faunas

Ten marine bands have been identified. Of these, the Subcrenatum is the most fossiliferous, and contains a shallow-water, near-shore benthonic fauna with an abundance of horny and calcareous brachiopods, gastropods and bivalves. Goniatites, characteristic of the more-offshore goniatitepectinoid biofacies belt (Calver, 1969; Ramsbottom, 1970), are mainly concentrated in a bed near the base of the band (Barclay and Jones, 1978; Hall and Squirrel], 1971).

The other marine bands contain faunas belonging mainly to the Lingula biofacies belt, representing deposition in very shallow pro-delta waters where salinities were reduced by fresh-water flushing. The horny brachiopod Lingula mytilloides is generally the only macrofossil present, although the Vanderbeckei (p.86), Aegiranum (p.91) and Cambriense (p.91) marine bands contain crinoid fragments, sponge spicules and other horny and calcareous brachiopods locally. Arenaceous foraminifera are fairly common in these bands, and are characteristic of the Edmondia Marine Band (p.91). The trace fossil Planolites ophthalmoides is ubiquitous, and typically occurs above the Lingula band.

The Subcrenat um, Vanderbeckei, and Aegiranum marine bands show no effects of proximity to the Usk Axis, maintaining their character eastwards across the district, and indicating that the area of the axis was totally submerged following major marine transgressions. The other marine bands fail eastwards, onlapping against the Usk Axis, and showing an eastward faunal transition from Lingula hands in the west through Planolites bands into barren mudstones in the extreme east.

Non-marine faunas

Non-marine bivalves ('mussels') occur mainly at the base of the lacustrine cycles in the roofs of the coal seams. They are not common, and their scarceness is another indication of the marginal position of the district during much of the Westphalian. With a few exceptions, the bivalves are thus of little use in correlation, in contrast to their value in the more 'distal' parts of the the basin (for example, see Woodland and Evans, 1964, pp.24–26). The principal bivalve horizons are as follows:

1. The 'Carbonicola Bed', containing the pseudorobusta' fauna of large, thick-shelled Carbonicola, lies 9 to 15 m above the Garw seam, and extends from the west of the district eastwards to the Clydach Valley.
2. The Five-Feet-Gellideg seam carries a distinctive canneloid mudstone roof with Curvirimula in parts of the district.
3. The Bute seam has a canneloid mudstone roof in which flattened, broken 'ghost' mussels' occur; only Naiadites has been identified.
4. The roof of the Upper Six-Feet seam ('Threequarter') at Victoria No.6 Pit, Ebbw Vale yielded a bivalve assemblage including the syntype and lectotype of Anthraconaia pumila and the lectotype of A. subcentralis (Bassett, 1972; Weir, 1966) (p.89).
5. The Eighteen-Inch seam roof contains large 'mussels' including Anthraconaia adamsii in the west of the district. The lectotype of this species comes from Ebbw Vale (Bassett, 1972). The Pentre seam also carries A. adamsii in the west (p.92).
6. An Anthraconauta phillipsültenuis fauna occurs with the Brithdir coal in the Ebbw Fawr Valley (p.100).
7. A shell bed in the roof of the Mynyddislwyn contains Anthraconaia tudliamsoni and Anthraconauta tennis, in association with Leaia sp.(see below).
8. Fish remains are commonly associated with the bivalves, but other persistent fish beds are:

1. the roof of the Garw
2. the roof of a thin coal smut 5 m below the Five-FeetGellideg
3. the roof of the Gorllwyn Rider.

Other non-marine fossils include ostracods, Luestheria, crustaceans and trace fossils, most of which are of little use in correlation. The branchiopod crustacean Leaia is abundant in the roof of the Mynyddislwyn coal.

Floras

Plant remains are the most abundant fossils of the Coal Measures, and although most are comminuted to indeterminate debris, well-preserved impressions are common, and these mainly occur in the roof beds of the coals. Although no systematic collections were made during the resurvey, remains were collected from boreholes, underground locations, and from some outcrops and spoil heaps. These have been identified by Professor W G Chaloner, and details are held in the BCS Biostratigraphy Research Group files. Robertson (1927, pp.134–135) listed early plant collections.

Although the plant occurrences add little to the more precise stratigraphical correlations obtained by using the marine bands and the widespread coals, Professor Chaloner makes the following comments on a number of assemblages, in which plants characteristic of Dix's (1934) floral zones occur:

1. The presence of Neuropteris schlehani and Sphenopteris cf. hoeninghausi at the level of the M2 Marine Band (p.79) is typical of Zone C (Westphalian A).
2. The association of Alethopteris decurrens with Neuropteris heterophylla (Nloshii of some authors) and Annularia radiata from the roof of the Yard (pp.85, 86) is typical of Zone D (upper part of Westphalian A).
3. The presence of Asolanus camptotaenia in the Llynfi Beds (p.98) is characteristic of Zones F and G (Westphalian C).
4. Neuropteris scheuchzeri, Sphenophyllum emarginatum and Pecopteris miltoni from the roof of a coal about 5 m below the Cefn Glas (p.101) are typical of Zone H, and are designated species of the Westphalian D.

Stratigraphy

Lower Coal Measures (Westphalian A)

(Figure 21) gives a generalised vertical section. These beds are conveniently divided at the Garw coal, below which the succession comprises quartzitic sandstones (the 'Farewell Rock') and interbedded mudstones, lacking the cyclic, coal-bearing facies characteristic of later strata. The sandstones form gentle slopes north of the Heads of the Valleys road, and on the moorland area of Mynydd-y-garn-fawr. The beds above the Garw are mainly argillaceous, and crop out in a narrow belt round the coalfield. The thickness of the Lower Coal Measures ranges from a maximum of over 150 m in the south-west to about 80 m in the south-east.

Subcrenatum Marine Band to Garw Seam

Details of these beds were given by Robertson (1927), Blundell (1952), Leitch and others (1958), Jones and Owen (1967) and Barclay and Jones (1978). An unpublished sedimentological study was made by Bluck (Kelling, 1974).

Recent surface boreholes at Tafarnau Bach, Rassau and Cwm Carno, and underground boreholes at Blaenserchan, Six Bells and Marine collieries^‡4  have provided new data.

The Subcrenatum Marine Band is exposed at only a few localities, but boreholes have proved it to be present throughout the district, and it is thick and richly fossiliferous. The maximum recorded thickness (23.8 m) is in the Marine Colliery underground borehole. Two marine phases, separated by a seatearth, were recorded in the Blaenserchan Colliery underground borehole.

The succeeding beds bear more resemblance to the Namurian than to the main bulk of the Westphalian. Orthoquartzitic channel-fill sandstones predominate in the Clydach area, but over much of the district they alternate with cycles of mudstones, siltstones and seatearths. The cycles are locally capped by thin coals, and two of the more persistent coals have been mined to a small extent at outcrop. Both these coals have been referred to as the Engine Coal (Blundell, 1952; Leitch and others, 1958; Robertson, 1927). On the north crop these coals have marine mudstone roofs, correlated with the M1. (?Springwood) and M2 (?Honley) marine bands of Leitch and others (1958). Only impersistent representatives of three higher marine horizons (M3, M4 and M5 of Leitch and others) occur within the district, and their correlation is uncertain.

A quartzite unit lies towards the top of the sequence, directly underlying the Garw east and south of the Clydach Valley, but separated from it to the west by mudstones, about 9 m thick at Rhymney Bridge, expanding westwards to 15 m at Dowlais, where the beds include the 'Rosser Veins' ironstones (Robertson, 1933). The quartzite, traditionally referred to in the Abergavenny district as the 'Farewell Rock', was, until the work of Leitch and others (1958), confused with a stratigraphically lower unit of the same name in the Merthyr Tydfil district.

Comparison of the Abersychan Borehole and the Blaenserchan Colliery Underground Borehole ((Figure 24), sections 10 and 11) illustrates the marked expansion in the thickness of the beds above the Subcrenatum Marine Band westwards from the east crop area. The marginal situation of the east crop is indicated by the thinness of the sequence, the presence of cycles comprising channel-fill sandstones and rootlet beds, and the impersistence of faunally impoverished marine beds (Barclay and Jones, 1978; Hall and Squirrell, 1972). Hall and Squirrel] suggested the presence of a major disconformity at the base of the 'Farewell Rock', but Barclay and Jones (1978) proposed that attenuation caused by multiple non-sequences, non-deposition and reworking were perhaps more likely. The abundance of channel-fill sandstones in this upper delta plain setting may also explain the lateral variation of the sequence, with differential compaction leaving older channel-fill sandstones as topographic highs.

Slump beds in the Blaenserchan Colliery Underground Borehole may equate with similar beds at the same stratigraphical level in Pembrokeshire (Kuenen, 1949) which Jenkins (1962) considered to have been caused by contemporaneous seismic activity.

Details

The principal sections are given in (Figure 24).

Subcrenatum Marine Band

The band was formerly well exposed at Rhymney Bridge (Leitch and others, 1958, p.474), but there is now only a small exposure behind the filter station [SO 1046 0989] from which Anthraconeilo sp., Gastrioceras cf. subcrenatum, and Rhabdoderma sp.were obtained.

The marine band is well exposed in Cwm Odyn-ty [SO 1500 1445] where 5 m of fossiliferous mudstones are seen in the banks of the stream (Table 11). The band thickens markedly southwards, as proved by the Carno New Dam boreholes, in which it was up to 16 m ((Figure 24), section 3c). There are some small exposures in the east bank of the Carno river; for example, silty mudstones high in the marine band [SO 1592 1373] yielded Lingula mytilloides and cf. Coteolus, and a small outcrop at the base of the band is visible at low water level at the northern end of Carno Reservoir [SO 1611 1342] (Table 11).

The outcrop of the band in Llammarch Dingle (Jones and Owen, 1967) is now largely obscured, but some fossiliferous mudstones are exposed near the base [SO 2192 1196] ((Figure 24), section 5), and a small exposure at the same level is present in a stream [SO 2298 1209] near Penallt.

Pen-ffordd-goch Borehole [SO 2553 1058] ((Figure 24), section 7) proved the band to be 12 m thick (Barclay and Jones, 1978), but it thins to the north-east. The marine band is unexposed along the east crop, but boreholes at Forgeside [SO 2504 0828] ((Figure 24), section 11) and Abersychan eg. [SO 2617 0347] ((Figure 24), section 12) provided details (Barclay and Jones, 1978; Hall and Squirrell, 1972).

Three underground boreholes proved the marine band below the coalfield; at Marine Colliery [SO 1895 0398] ((Figure 24), section 13), at Six Bells Colliery [SO 2191 0279] ((Figure 24), section 10), and at Blaenserchan Colliery [SO 2435 0213] ((Figure 24), section 11). Sandstones, presumed to he distributary mouth bar sands, are included in the marine band in the Marine Colliery Borehole. Two marine phases, separated by a seatcarth, were recorded at Blaenserchan.

Subcrenatum Marine Band to Garw Seam

Quartzites lying about 9 m below the Garw are exposed in the River Rhymney [SO 1063 0942] south of Rhymney Bridge ((Figure 24), section lb); boreholes on Tafarnau Bach industrial estate gave sections of the beds between the Garw and the quartzities, for example, Borehole No.15 [SO 1125 0999] ((Figure 24), section la). Bioturbated mudstones with fish fragments including Rhizodopsis sauroides contain numerous ironstone layers and pass upwards into the Garw rootlet beds.

Mudstones with Lingula mytilloides and cf. Myalina sp.at the base of a cutting [SO 1080 1080] at Blaen Rhymney ((Figure 24), section lc) are tentatively correlated with the M1 Marine Band (Leitch and others, 1958, p.475). A thin coal above was formerly dug to a small extent (Robertson, 1927, p.49), and was 0.6 m in trial pits to the south of the cutting. Plant-bearing mudstones occur above and below the coal, and Asterophyllites sp., Lepidophloios sp., Lyginopteris hoeninghausi and Neuropteris schlehani were previously recorded (Leitch and others, 1958, p.475). In addition, Sphenophyllum elemarginatum and Sphenopteris aff. hoeninghausi were collected during the resurvey (p.75). Leitch and others (1958) suggested that the M2 Marine Band was absent due to scouring during deposition of the higher sandstone in the section. It is perhaps more likely that the roof of the coal marks the stratigraphical level of the marine band which was not deposited at this locality.

In the Sirhowy–Rassau area the beds between the top of the Subcrenatum cycle and the base of the 'Farewell Rock' consist of two coarsening-upwards cycles capped by thin coals ((Figure 24), section 2). Marine mudstones at the bases of the cycles are correlated as the MI. and M2 marine bands. Spoil from trials in the higher coal at Blaen-y-cwm [SO 1337 1140] contains L. mytilloides. At this locality, Leitch and others (1958) recorded 0.15 m of coal overlain by mudstones with Neuropteris schlehani passing up into black shales with Lingula and fish scales; they correlated the coal with the Engine Coal of Blaenavon (Blundell, 1952). An exposure to the south [SO 1337 1107] shows about 7 m of silty mudstones of the M1 cycle passing upwards into 0.15 to of ganistcr under a 4 cm coal smut. L. mytilloides and Serputites sp.were collected from the roof of the smut (M2 Marine Band). The lower coal and 10 m of overlying mudstones (M1 Marine Band) are exposed in a gully [SO 1337 1153] to the north. In boreholes at Rassau industrial estate ((Figure 24), section 2), the M1 and M2 cycles consist mainly of bioturbated mudstones with Planolites ophthalmoides. Both cycles are underlain by coal smuts and begin with marine mudstone with L. mytilloides and fish fragments. L. mytilloides also occurs towards the top of the M1 cycle, and these two marine phases may equate with the Mla and M lb marine bands of the Gwendraeth Valley (Archer, 1968, p.35).

The M1 cycle thins markedly eastwards between Rassau and Cwm Carno. A cliff section [SO 1638 1230] in Cwm Carno provides a section up to the base of the 'Farewell Rock', and there are good exposures of this quartzite further south [SO 1638 1175] at Carrnehown. ((Figure 24)), section 3 gives a composite section. Lingula-bearing mudstones at the base of the Cwm Carno section were previously correlated with the M1 Marine Band (Leitch and others, 1958), but are now thought to be the topmost part of the Subcrenatum Marine Band (Barclay and Jackson, 1982). Spoil from old levels on the M1 coal in Cwm Carno [SO 1647 1271] contains L. mytilloides, Productus carbonarius and Rhadinichthys sp. The coal, reduced to a smut, and the overlying M1 Marine Band are exposed at low water level in Llangynidr Reservoir [SO 1529 1390] and in the reservoir bank [SO 1518 1394]. The outcrop and spoil from an old digging yielded cf. Accuthopecten sp., Lingula sp., Nuculopsis cf. gibbosa, Rhadinichtbys sp.and Rhizodopsis sauroides.

The M1 Marine Band is exposed in a roadside ditch [SO 1648 1469] on Cein Pwll-coch where it overlies a 0.4 m coal and contains L. mytilloides and Orbiculoidea sp.Disused quarries nearby [SO 1735 1365] give good exposures of the 'Farewell Rock'. The basal part of the 'Farewell Rock' and 6 m of the underlying beds are patchily exposed in a swallow hole [SO 1764 1530] on Mynydd Llangattock. Lingula-bearing mudstones at the base of the exposures are correlated with the M1 Marine Band, and a 0.3 m coal lying 4 m higher carries the M2 Marine Band in its roof.

The best sections in the Clydach gorge area are in Cwm Nantmelyn [SO 2021 1231] and on the Heads of the Valleys road c. [SO 2007 1217]. (Figure 24), section 4 gives a composite section. Leitch and others (1958, p.476) correlated the coal near the base of the section as the Engine Coal, and suggested that the position of the M2 Marine Band lies at the base of the pebbly quartzite above. These authors recorded mudstones with Lingula and Orhiculoidea at a lower stratigraphical level, in the bank of the River Clydach 45 m east of Cwm Nant-melyn; this bed was also examined during construction of the Heads of the Valleys road, where it contained Lingula and productoids (T R Owen, personal communication 1965). It was tentatively correlated by Leitch and others (1958, p.476) as the M1 Marine Band, but it may be the topmost part of the Subcrenatum Marine Band, with the coal at the base of the section being the one that elsewhere carries the M1 Marine Band. Two impersistent fish-bearing layers were recorded higher in the Cwm Nant-melyn section and correlated with the M3 and M4 marine bands (Leitch and others, 1958). One of these layers (?M4) is present on the north side of the Heads of the Valleys road [SO 2008 1222] where it contains Lingula, fish scales and worm traces, but it is absent on the south side of the road where the base of the overlying Farewell Rock has scoured down into a sequence of interbedded sandstones and seat earths.

A section compiled from exposures in Llammarch Dingle [SO 2193 1177] to [2194 1 190] is given in (Figure 24), section 5. Jones and Owen (1967) correlated the coal 18 m below the Garw as the Engine Coal and recorded Lingula sp., Productus sp.and fish scales in its roof. Mudstone (?Ml Marine Band) in spoil from crop levels in the coal [SO 2193 1190] contains fish fragments.

In the Nam Dvar section [SO 2367 1219] (Robertson. 1927, p.47; (Figure 24), section 6), die lowest coal exposed was named the Engine Coal by Jones and Owen (1967). These authors recorded Lingulabearing shale close above and correlated it with the M4 Marine Band, but it is perhaps more likely to be a lower (?M1) horizon.

Details of Pen-ffOrdd-goch Borehole [SO 2553 1058] ((Figure 24), section 7) have been published previously (Barclay and _Jones, 1978).

A section compiled from exposures in Nam Llechau [SO 2557 0971] to [SO 2555 0946] is given in (Figure 24), section 8. Blundell (1952, p.31.4) correlated the coal seam with the Engine Coal and equated it with the Sun Vein of the south-east crop. Calamites sp., Lyginopteris hoeninghausi, Mariopteris acuta, Neuroperis rectinervis and N. aff schlehani were recorded from shales below the coal (Blundell, 1952), and Lingula mytilloides and fish fragments occur in the roof mudstones (?M2 Marine Band). Blundell recorded 15.2 m (50 feet) of 'Farewell Rock', but this includes the quartzite roof of the Garw coal and 5 m of quartzite below the coal.

Two marine hands in Forgeside Borehole [SO 2504 0828] ((Figure 24), section 9) were correlated as the M2 and M3 marine bands (Barclay and Jones, 1978). In the light of the new data from the west, they may be better correlated as the M1 and M2 marine bands.

The 'Farewell Rock' is exposed at a few localities between Blaenavon and Abersychan, for example a disused quarry [SO 2626 0713], the railway cutting at Garndiffaith [SO 2653 0430] and a disused quarry in Abersychan [SO 2649 0361] (Barclay and Jackson, 1984; Barclay, Jackson and Squirrell, 1984).

The succession in the Abersychan area ((Figure 24), section 10) has been described previously (Barclay and Jones, 1978; Hall and Squirrell. 1972).

Only one marine band (?Ml) was present in Six Bells Colliery underground borehole [SO 2191 0279] ((Figure 24), section 12). The overlying sequence contains three rootlet beds which are truncated by scour surfaces. These surfaces are overlain by quartzites and may mark the stratigraphical positions of the M2, M3 and M4 marine bands. A similar sequence was proved in Marine Colliery underground borehole [SO 1895 0398] ((Figure 24), Section 13), although the M1 Marine Band is absent, and the M3 and M4 marine hands may be represented by thin fish-bearing layers.

Slump beds above the Subcrenatum Marine Band in the Blaenserchan Colliery underground borehole [SO 1895 0398] ((Figure 24), section 11) consist of sandstones and siltstones with depositional clips of 55', (p.79). Non-marine faunas occur at two levels, Carbonicola sp.. Curvirimula belgica, C. trapeziforma, Carbonita humilis, and C. secans being collected. A thin bioturbated mudstone layer 2 m below the Garw may equate with one of the higher (?M4) marine bands.

A thin coal, known as the Little, is present in the Mon Lwyd Valley (Squirrel] and Downing, 1969), and has been dug locally on the east side, south of the Trevethin Fault; its precise stratigraphical level is not known. The best exposure of the coal and the quartzites of the 'Farewell Rock' that lie above and below is in a disused quarry [SO 2816 0144].

Garw to Five-Feet-Gellideg

The thickness of these beds ranges from about 50 m in the south-west to 15 m in the south-east. They are predominantly argillaceous in the west, but sandstones occupy about half the sequence in the east. The argillaceous parts are bioturbated, with trace fossils including Planolites, Gyrochorte carbonaria, and Cochlichnus kochi. Numerous layers and nodules of sideritic ironstone were formerly extensively mined.

The Garw is 0.4 to 0.7 m thick. Apart from limited crop workings, it has been deep-mined in the Tredegar and Abertillery–Blaenavon areas. Typically it has a distinctive roof of dark grey mudstones with fish fragments, although quartzite forms its roof in the Blaenavon area. Thick-shelled bivalves, Carbonicola spp.,are abundant in the higher roof beds of the Garw west of the Rhymney Valley, but to the east they are concentrated in a persistent layer that extends to the Clydach Valley. This layer, the 'Carbonicola Bed' rests on a seatearth and lies at the base of a coarsening-upwards cycle. Fish-bearing, bioturbated mudstones occur at this level on the east crop, where they form the lower part of the Bottom

Vein Mine, an ironstone horizon formerly much worked. The cycle is capped by a thin coal which persists over most of the area as a smut, but is thicker at Varteg and known as the Little Jack and Balls (Robertson, 1927, p.63). The overlying cycle starts with fish-bearing mudstones and grades up into the seatearth of the Five-Feet-Gellideg; this seatearth is a sandstone over much of the district.

Details

A selection of recent sections is given in (Figure 25).

West of the River Rhymney

Little is known of the sequence at depth, but an underground borehole [SO 1180 0531] near Pontlottyn is said to have proved the

Garw to be 0.6 m thick and lying 51 m below the Five-FeetGellideg. Anthraconaia sp., Carbonicola ex. gr. pseudorobusta, Curvirimula sp., Naiadites aff. flexuosus, Geisina arcuata and Carbonita sp.were obtained from the mudstones above the Garw in a borehole at Pontlottyn industrial estate ((Figure 25), section 2). The Garw is 0.7 m in the River Rhymney [SO 1072 0928] at Bute Town, and the overlying beds are intermittently exposed downstream. These include the 'Carbonicola Bed' [SO 1083 0910], which yielded Carbonicola aff. pseudorobusta and Elonichthys sp.(see also Evans, 1971), and a fine exposure of the rippled surface of the Five-Feet-Gellideg seatearth with long stigmarian roots [SO 1080 0875].

Rhymney–Sirhowy area

Boreholcs at Tafarnau Bach industrial estate ((Figure 25), section 4) proved the Garw to be 0.4 to 0.9 m thick. Fish fragments in the roof of the coal include Elonichthys sp., Rhadinichthys sp.and Rhizodopsis sauroides. Shelly mudstones (the 'Carbonic°la Bed') 14 m above the coal yielded Carbonicola cf. extenuata, C. pseudorobusta, C. aff. pyrimidata, C. aff. robusta, aff. Curvirirnula belgica, C. trapeziforma, Geisina arcuata, Spirorbis sp.and fish fragments.

The Garw is 0.6 m thick in the Tredegar area, where it was deep-mined at Ty Trist Colliery. Dr Squirrel] noted ostracods, fish fragments, Planolites, and plant debris in its roof.

Ebbw valleys

An old ironstone patch excavation north-east of Beaufort [SO 1698 1240], filled since the resurvey, exposed the 'Carbonicola Bed' resting on mudstone scatcarth and forming the base of a coarsening-upwards cycle. Shale rich in fish remains overlies the 'mussel' hand; the following were collected: Carbonicola sp., Curvirimula trapeziforma, Geisina arcuata, Spirorbis, Elonichthys sp., Rhadinichthys sp., Rhabdoderma sp.and Rhizodopsis sp. A rich 'mussel' fauna was recorded at this locality by Blundell (1952, p.316). Across the Heads of the Valleys road [SO 1695 1228], a recent excavation gives a good section of the 10 m of beds underlying the Five-Feet-Gellideg ((Figure 25), section 6). Boreholes at Church View [SO 1696 1132] ((Figure 25), section 7) proved most of the beds.

Underground boreholes at Marine Colliery ((Figure 25), section 5), Six Bells Colliery ((Figure 25), section 10), and Abertillery New Mine ((Figure 25), section 9) provided recent sections at depth in the Ebbw valleys. The Garw is 0.7 m thick and is currently being worked from Abertillery.

Clydach–Pontypool

There are numerous exposures in old coal and ironstone diggings north of the Heads of the Valleys road near Clydach Terrace. (Figure 25), section 8 is compiled from exposures in the Clydach stream [SO 1792 1329] to [SO 1821 1291]. The Garw and Bottom Vein Ironstone were extensively worked in this area. In boreholes at Rhydw Farm [SO 199 124] ((Figure 25), sections 11), Mr R H Price (personal communication, 1985) recorded the Garw section as: coal (worked) 0.3 to 0.5 m, on mudstone 0.25 m, on coal, inferior, 0.13 m. The head of the Clydach gorge provides excellent exposures and a near complete section of the beds ((Figure 25), section 12).

Quartzite forms the roof of the Garw from Gilwern Hill eastwards to the Blorenge. It is exposed around the Foxhunter car park on the Blorenge, and in old Garw workings [SO 2638 1075]. Plant-rich mudstones intervene at Pen-ffordd-goch [SO 2553 1058] ((Figure 25), section 13) (Barclay and Jones, 1978).

Washcry Drift ((Figure 25), section 14) and Forgeside Borehole ((Figure 25), section 15) (Barclay and Jones, 1978) gave sections of the beds in the Blaenavon area. An average section of the Garw at Big Pit [SO 2400 0909] reads: quartzite (the 'Garw Rock') c.3.2 m, coal 0.6 m, seatearth 0.07 m, coal 0.03 m. The quartzite is locally scoured by channelling, and the siltstone channel-fill beds contain a layer of Carbonicola preserved in ironstone. The Garw is 0.9 m thick north of Varteg, but thins and deteriorates eastwards. Blaenserchan Colliery underground borehole [SO 2435 0213] ((Figure 25), section 16) provides a recent and complete section of the beds between the Garw and the Five-Feet-Gellideg.

Five-Feet-Gellideg to Amman Rider

These beds are predominantly argillaceous, and range in thickness from over 40 m in the west to 11 m in the southeast. (Figure 26) gives selected sections.

The Five-Feet-Gellideg is one of the best coals, and has been extensively mined (Robertson, 1927). The seam is a complex of three coals, the Gellideg, Lower Five-Feet, and Upper Five-Feet. To the west of the Rhymney Valley, these coals are separate, and occupy up to 7 m of beds, but they converge eastwards. Over much of the district, a two-leaf section totalling about 1.5 m (the 'Old Coal'), is typical. A thin rider coal is present in the south of the Ebbw Fawr Valley, and in the Afon Lwyd Valley north of Pontypool. A major north–south washout over 3 km long affects the seam complex from Deri in the Bargoed Rhymney Valley to its outcrop at Rhymney opencast site. In the Tredegar area, the Five-Feet-Gellicleg converges with the overlying Lower Seven-Feet to give a 5 m coal complex.

An upwards-coarsening cycle of fine to silty bioturbated mudstones forms the roof of the Five-Feet-Gellideg in the west of the district; Planolites and C. kochiwere recorded at Ogilvie Colliery, and Curvirimula and fish fragments are present locally. Silty mudstones and siltstones containing plant debris form the roof in the central and eastern parts of the district, with sandstone present at Blaenavon; mudstone scatcarth overlies the coal in the Pwll du–Blorenge area where the succeeding beds comprise seatearths and thin coals.

The Seven-Feet is a single two-leaf seam in the southwest, but two coals and a thin rider are found at outcrop to the north in the Rhymney Valley. At outcrop to the east of the Rhymney Valley, two widely separated coals are correlated as the Lower Seven-Feet and the Upper Seven-Feet. The Lower Seven-Feet, known locally as the Yard, is a two-leaf coal at outcrop over much of the district. The Upper Seven-Feet, known as the Meadow in the Sirhowy and Ebbw valleys has been worked extensively.

The roof beds of the Lower Seven-Feet and of the Upper Seven-Feet are generally plant-bearing mudstones, but Naiadites sp., ostracods and a crustacean carapace were collected from the roof of a rider smut to this coal at Rhyd-yBlew opencast site.

The Yard is generally of poor quality, and unworked at depth. Known as the Three Coals west of the Rhymney Valley, and as the Engine Coal to the east, it has been dug in opencast sites and mined to a small extent from surface adits. It generally consists of two or three leaves giving about 1.3 m of coal.

The beds above the Yard are typically plant-bearing mudstones, but bivalves were collected at Ogilvie and Markham. Where the Amman Rider lies less than 5 m above the Yard the beds are of mudstone seatearth.

The Amman Rider is thin everywhere, and has a high sulphur content. It has been dug in recent opencast operations, but is unworked at depth. Between Brynmawr and Pontypool it is combined with the Yard, and in parts of the east crop the Amman Rider, Yard and Upper Seven-Feet coalesce. The combined seam is known locally as the Meadow Vein.

Details

(Figure 26) gives selected sections of the beds.

West of the River Rhymney

Trecatty opencast site [SO 080 080] ((Figure 26), section 1) and boreholes at Pontlottyn industrial estate ((Figure 26), section 3) provided recent sections of the sequence at outcrop, and underground boreholes at Ogilvie Colliery proved the sequence at depth.

(Figure 26) Selected sections of the beds between the Five-Feet-Gellideg and the Amman Rider. See (Figure 23) for key. Local seam names are on the right side of the sections, standard names on the left. Inset map shows the outcrop of the beds (stippled), locations of sections, and approximate isopachs (in metres)

The Five-Feet-Gellideg is a complex of three coals extending over 7 m in the Trecatty–Pontlottyn area. It is known as the Lower Four-Feet, with the top leaf being named the Lower Four-Feet Yard in the Rhymney Valley. The top leaf (Upper Five-Feet) was known as the Gilwych at Ogilvie Colliery.

The Seven-Feet is represented by a single two-leaf seam in the Bargoed Taff and Bargoed Rhymney valleys; a typical section in the underground workings is coal 0.3 to 0.7 m, 'stone' 0.06 m, coal 0.3 to 0.6 m. The coals are thickest in the extreme south-west, thinning north-eastwards. A thin rider lies 0.56 m above the coal south-west of the Ogilvie pits, and converges with it 1 km to the north-west. Two to three coals represent the Seven-Feet at outcrop; the lowest, named the Little Vein at Trecatty, and the Seven-Feet or Clay at Pontlottyn (where its underclay was formerly mined) is correlated as the Lower Seven-Feet. Two leaves, each about 0.4 m thick and separated by 0.8 m, are present at Trecatty, but a single leaf about 0.6 m thick was worked at Pontlottyn. The Upper Seven-Feet is known as the Little Vein Rider at Trecatty where it is 0.4 m thick. Plant fragments typically occur in the roof beds of these coals; Carbonita cf. humilis was collected from the roof of the Lower Seven-Feet at Pontlottyn.

The Yard is 0.45 m thick in boreholes and drivages at Ogilvie Colliery, but is thicker to the north at Fochriw and at Trecatty where a three leaf sections reads: coal 0.5 m, parting 0.15 In, coal 0.2 m, parting 0.3 m, coal 0.1 m. The roof beds are plant-bearing mudstones at outcrop; Anthracosphaerium sp.and Anthracosia ovum were collected underground at Ogilvie Colliery.

The Amman Rider is a thin coal, generally about 0.3 m thick, and was dug at Trecatty and Royal Arms opencast sites.

Rhymney -Sirhowy area

The Five-Fect-Gellideg ('Old Coal') is washed out in a major north-south channel that extends from Deri in the Bargoed Rhymney Valley to its outcrop at Rhymney opencast site. Three leaves are present at Markham Colliery ((Figure 26), section 6), and the average working section at Ty Trist Colliery was: top coal 0.75 m, seatearth 0.3 m, coal with thin parting 1.4 m. IIard silty mudstone with plant fragments forms the roof in the Sirhowy Valley.

The Lower Seven-Feet ('Yard') was 0.6 m thick at Rhymney opencast site, and plant remains including,4nnularia cf. radiate were collected from its roof. The Upper Seven-Feet ('Meadow') is about 0.65 m thick at Rhymney, with two thin impersistent riders. Naiadites sp.(juv) was obtained from the roof of the lower rider. At depth to the south, the Seven-Feet is represented by one coal known as the New Seam at East Elliot Pit, where it consists of: coal 0.6 m, parting 0.07 m, coal 0.43 m. The seam worked at Markham Colliery as the Meadow Vein (coal 0.71 m, lireclay 0.4 m, coal 0.96 m) is the Seven-Feet and Yard combined. Anthracosia regularly was collected from its roof. The Upper Seven-Feet was known as the Yard, or Little Yard at Tredegar. It was up to 0.8 m thick in boreholes at Charles Street, Tredegar, with a thin rider 1 m above. Indeterminate bivalve fragments were present in the roof of the rider.

The Yard is known as the Engine at outcrop in the Rhymney and Sirhowy valleys. At Rhymney opencast site', it was about 1.2 m thick, in two or three main leaves, and had a mudstone seatearth roof. A standing Lepidodendron trunk was present in these roof beds. It is a single thin, unworked coal at New Tredegar and East Elliot collieries and, as stated previously, is the top coal of the 'Meadow Vein' at Markham Colliery. It has three leaves at outcrop in the Sirhowy Valley, where boreholes at Charles Street, Tredegar showed it to have been partially worked. The top leaf, up to 0.97 m, was the thickest.

The beds between the Yard and Amman Rider are entirely arg,illaceous, and range from less than 5 m at Tredegar to 16 m at Markham Colliery.

The Amman Rider is 0.3 to 0.4 or thick and unworked, apart from at Rhyrnney opencast site.

Ebbw valleys

(Figure 26), sections 7, 8 and 9 are selected from this area.

The Five-Feet-Gellideg ('Old Coal') is thick, of good quality and extensively mined. A typical section at Marine Colliery is: coal 0.63 m, clod c.0.33 m, coal 0.76 m. Towards the southern margins of the district, a 0.75 m rider coal lies 0.2 to 1.6 m above. A two leaf section similar to that at Marine was worked at Rhyd-y-Blew opencast site and at depth in the Ebbw Each Valley at Cwnitillery Colliery.

Canneloid mudstone and clark grey tine mudstones with bivalves lie above the Five-Feet-Gellideg at outcrop, but silty to very silty mudstones with plant fragments form the roof at depth. These beds lie at the base of a cycle which is 5 m thick in the north and up to about 15 m thick underground at Abertillery.

The Seven-Feet is represented by two coals, the Lower Seven-Feet ('Yard'), and the Upper Severs-Feet ('Meadow Vein' or 'Gwarycae'). The Lower Seven-Feet is a two-leaf coal at outcrop, an average section at Rhyd-y-Blew opencast site being: coal 0.83 m, mudstone up to 1.6 nr, coal 0.28 m. Its development is apparently variable at depth; it is only 0.10 m at Marine Colliery, and although absent in some sections in the south of the Ebbw Fach Valley, it is present in Abertillery New Mine where it is 0.75 m thick. The separation between the Lower Seven-Feet and the Upper Seven-Feet is 10 m at outcrop, and ranges between 7 and 11 m at depth. The Upper Seven-Feet has a typical section at outcrop of coal 0.6 m, mudstone 0.15 to 0.45 no coal 0.15 m to 0.30 m. It is generally about 0.7 to 0.8 m thick in the Ebbw Fawr and Ebbw Fach valleys. The seam named the 'Meadow Vein' in workings in the south of the Ebbw Fach Valley is a composite seam comprising the Upper Seven-Feet and Yard (see below).

The roof mudstones of the Upper Seven-Feet typically contain plant fragments, but a thin mudstone bed resting on seatearth 1.6 m above the coal at Rhyd-y-Blew contains in addition bivalve fragments including Naiadites, and a crustacean fragment. The beds between the Upper Seven-Feet and the Yard, consisting almost entirely of mudstones, range in thickness from 7 m at Rhyd-y-Blew to a few centimetres in the south of the Ebbw- valleys.

The Yard ('Engine') consists of two main leaves at outcrop, totalling about 1.2 m, and this thickness is maintained at depth to the south, although it is unworked in the Ebbw Fawr Valley because of its inferior quality. It is known as the Upper Meadow at Marine Colliery. In the Ebbw Fach Valley it is the top leaf of the 'Meadow Vein', an average section being coal (Yard) 0.8 m, seatearth 0.4 m, coal (Upper Seven-Feet) 0.8 m.

Plant-bearing mudstones generally form the roof of the Yard; where the beds between the coal and the overlying Amman Rider are less than a few metres they consist entirely of mudstone seat-earth. At outcrop, the beds thin from 6 m in the west of the Rhyd-yBlew workings to 1.5 m in the east, where Neuropteris heterophylla and Alethopteris decurrens were collected. The two seams converge on the east side of the Ebbw Fawr Valley to give a 0.6 m coal known locally as the Meadow Vein. The separation increases southwards up to 13 m at Six Bells Colliery, where sandstones occupy 54 per cent of the sequence and where N. heterophylla and Annularia radiate were collected.

The Amman Rider is 0.3 to 0.4 m throughout the area and of poor quality. It has only been exploited at Rhyd-y-Blew opencast site.

Clydach -Pontypool

Sections 10 to 14, (Figure 26), are selected from this area.

The Five-Feet-Gellideg ('Old Coal') is variable at its outcrop near Brynmawr; a group of coals is present north of the Heads of the Valleys road, whereas only a single seam was recorded at the Brynmawr roundabout on this road. A two-leaf section is generally present south-east of Brynmawr and on the east crop, for example at Blaenant opencast site where both leaves are about 0.85 m thick and separated by 0.2 m. Exceptions to this are at Washery Drift ((Figure 26), section 11), where three leaves are present, and at British Top Pit, where the upper leaf is washed out. A rider coal, known as the 'New Found Out' (Upper Five-Feet) splits from the coal near Gwenallt, the split widening to 2 m northwards to Golynos.

The Five-Feet-Gellideg roof mudstones have yielded Carbonicola cf. extima, Curvirimula belgica, C. candela, C. aff. trapeziforma, C. subovata, Naiadites sp.,and Carbonita humilis at outcrop north of Brynmawr. The mudstones are at the base of a coarsening-upwards cycle, and at Blaenant contain only burrow traces; a thin cannel coal 6 m above carries Curvirimula aff. trapeziforma and Carbonita Seatearth or sandstone generally overlies the Five-FeetGellideg on the east crop.

The beds between the Five-Feet-Gellideg and the Seven-Feet group are variable both in lithology and thickness. Maximum thickness occurs at Brynmawr where boreholes proved 19 m of beds, comprising one fining-upwards cycle. To the east and on the east crop, the beds consist mainly of mudstone seatearths, thin coals, and thin quartzitic sandstones. The sequence is greatly attenuated near Pontypool where, at Gwenallt for example, only 2.4 m of seatearth separate the coals.

The Seven-Feet is represented by two coals in the Brynmawr- Blaenavon area, the Lower Seven-Feet, known as the Yard, and the Upper Seven-Feet. The Lower Seven-Feet consists of two leaves, the section at Blaenant, for example, reading: coal 1.0 m, mudstone and seatearth 1 to 1.3 m, coal 0.6 m. The Upper Seven-Feet is thin at Brynmawr (see (Figure 26), section 9), and forms part of a group of coals at Blaenant ((Figure 26), section 10). The topmost coal in this group is the thickest, and was worked as the 'Meadow'. It carries the Vanderbeckei (Amman) Marine Band in its roof, and is therefore the standard Yard and Amman Rider combined. In old colliery workings in the Clydach area, the coal below was worked as the 'Meadow' and correlates with the Upper Seven-Feet, and the higher coal (Yard and Amman Rider combined) was known as the Tach. The Upper Seven-Feet is 0.8 m thick at Blaenavon, and known as the Meadow. Southwards, the Upper Seven Feet, Lower Seven-Feet, and Yard converge, giving a composite seam in the Abersychan area, also known as the Meadow Vein. This is a thick, good quality coal and is being worked at Big Arch, Talywain [SO 2591 0382] where an average section is: coal 0.3 m, mudstone 0.06 coal with pyrite 0.15 m, coal with some carbonaceous mudstone 0.08 m, clean coal 2.0 m, carbonaceous mudstone 0.05 to 0.3 m, coal 0.2 m there are, however, several large washouts, and although there is locally up to 3.5 m of coal, its section is variable at the margins of these channels. In the extreme south-east, around Pontypool, the Amman Rider also joins the Seven-Feet and Yard ((Figure 26), sections 13, 14).

The Yard is a separate seam only in Washery Drift, Blaenavon ((Figure 26), section 11) where it is thin and known as the Meadow Vein Rider. Elsewhere, it is either combined with the Upper Seven-Feet or the Amman Rider, or both, as described above. Silty mudstones with plant debris including Alethopteris decurrens and Annularia radiata form the roof of the Yard in Washery Drift, and are overlain by a channel-fill quartzite. Plant remains including cf. Diplotmena schlatzarense, Neuropteris cf. heterophylla, N cf. pseudogigantea and Sphenophyllum miriophyllum were collected from the roof of the combined Seven-Feet and Yard (Meadow Vein) at Blaenserchan Colliery.

The Amman Rider is separate from the underlying Yard in Washery Drift, where it is 0.4 m thick and contains much pyrite. Although correlation is uncertain, it may also he separate in the Varteg area where a 0.6 m coal lying from 2.7 to 7.5 m above the Seven-Feet-Yard may be the Amman Rider. It is absent at Blaenserchan and Llanerch where a major washout removed it and the succeeding coals.

Middle Coal Measures (Westphalian B and part of Westphalian C)

The Middle Coal Measures crop out in a narrow poorly exposed belt around the coalfield. They extend from the base of the Vanderbeckei (Amman) Marine Band (the base of Westphalian B) to the top of the Cambriense (Upper Cwmgorse) Marine Band. Their thickness ranges from 220 m at depth in the south-west of the district to 120 m at outcrop in the south-east (Figure 28). The measures include the Aegiranum (Cefn Coed) Marine Band, which marks the base of Westphalian C.

Vanderbeckei (Amman) Marine Band to Aegiranum (Cefn Coed) Marine Band

(Figure 27) gives generalised vertical sections. The sequence comprises a series of coal-capped cycles and is predominantly argillaceous, although sandstones become increasingly common towards the east, particularly above the Four-Feet seam. The thickness ranges from 130 m in the south-west of the district to 65 m in the south-east, with about 100 m present over much of the central part of the coalfield area. The Nine-Feet, Six-Feet and Four-Feet seams, all generally represented by two leaves, are among the most economically important coals of the district.

There is a large amount of data available for these beds. Apart from colliery shaft sections (Robertson, 1927), Trecatty opencast site and Abertillery New Mine provided recent sections. Other opencast sites, Washery Drift at Blacnavon, and many boreholes have proved parts of the sequence. There are few natural sections. (Figure 28) gives selected sections.

The Vanderbeckei (Amman) Marine Band is present throughout the district, resting on the Amman Rider over much of the area, and on the combined Amman Rider, Yard and Seven-Feet in the east. It is the basal part of a thick upwards-coarsening cycle and consists of mudstones and silty mudstones containing a shallow marine Lingula biofacies belt fauna (Calver, 1969). Lingula mytilloides and fish fragments are generally the only macrofauna present, although productoid, bivalve and crinoid fragments have also been recorded; arenaceous foraminifera are quite common. Planolites ophthalmoides is ubiquitous, and typically occurs in a phase above the Lingula bed. Pyrite burrowfills and other trace fossils are also common. The thickest recorded development of the marine band (6 m) was in Aneurin Place Borehole No.1, Brynmawr (p.89). Normally, Lingula is confined to the basal 0.3 m of the cycle. Above, the cycle comprises parallel-laminated silty mudstones, siltstones, fine-grained sandstones and ironstones. The ironstones are particularly common and were formerly extensively mined (the 'Darren Pins Ironstone'). Bivalves are locally present in the beds immediately overlying the marine band. Higher in the cycle, flaser-bedded siltstones and fine-grained sandstones intensely Not urbated by rootlets and escape burrows underlie the seatearth of the Bute. This seatearth is a ganister in the east of the district.

The Bute is a thin coal over most of the district, and is absent in parts of the Sirhowy and Ebbw valleys. It is thicker in the Abersychan –Pontypool area where it is known as the Yard, and consists of two leaves. It has a distinctive roof of dark grey mudstones with poorly preserved bivalve fragments. The beds between the Bute and the Nine-Feet are between 7 and 12 m thick.

The Nine-Feet generally consists of two leaves, separated by up to 20 m in the west, but combined in parts of the east crop. The Lower Nine-Feet is one of the thickest and most extensively mined seams of the district. The Upper Nine-Feet is thinner and is absent in the central part of the district. Many channels cut through the Lower Nine-Feet east of the Ebbw Each Valley, but where present, the coal has an anomalously thick development locally. In Washery Drift, Blaenavon, for example, 4.2 m of coal are present in 7.8 m of beds. The roof mudstones of both leaves of the Nine-Feet are generally plant-bearing; the bivalves Naiadites carinatus and N. modiolaris were recorded from the roof of the upper leaf at Rhymney (Robertson, 1927, p.135) and isolated fragments, mainly Naiadites, were collected during the resurvey. The old record of Athyris planosulcata from the roof of the Lower Nine-Feet at Beaufort (Bassett, 1972; Robertson, 1927, p.135) is the only one of a marine fossil at this level in the South Wales Coalfield, and is probably dubious. The fossil may have come from the Vanderbeckei Marine Band. Several thin coals, correlated with the Red Vein group, lie in the beds between the Upper Nine-Feet and the Six-Feet in the extreme west of the district, but do not in general persist eastwards.

The Six-Feet lies at the base of an important group of coals termed the Big Vein Group by Robertson (1927), and comprising the Six-Feet, the Lower Four-Feet ('Big Vein') and the Upper Four-Feet ('Elled'). The Six-Feet is generally represented by two leaves, the Lower Six-Feet ('Lower Threequarter') and the Upper Six-Feet ('Threequarter'). These two coals and the Upper Four-Feet are between 0.75 and 1 m thick, with the Lower Four-Feet averaging about 1.5 m in thickness. All the coals have been extensively mined. The Upper Four-Feet is much affected by washouts in parts of its outcrop on the east crop. Barren mudstone with plant remains typically forms the roof of these coals. A bivalve fauna including Anthraconaia pumila and A. subcentralis is recorded from Ebbw Vale (pp.75,89). The beds between the Upper Four-Feet and the base of the Aegiranum (Cefn Coed) Marine Band are predominantly argillaceous in the south-west where they are 38 m thick. Towards the east, quartzitic sandstones increase, and occupy about half of an attenuated (10 m) sequence in the south-east. In the west up to four coal-capped cycles are present, the lowest coal being the Two-Feet-Nine. This coal is thick enough to have been mined in the west, but it thins eastwards, and is either absent in the east or cannot be distinguished from the other coals in this group. In the west, two of these thin coals have barren mudstone roofs which are probably correlatives of the Haughton (Hafod Heulog) and Sutton (Britannic) marine bands. These bands are absent or represented mainly by burrowed layers to the east, and their correlation is uncertain. The highest cycle in this group of beds is capped by a thin smut which is overlain by the Aegiranum Marine Band.

Details

West of the Riser Rhymney

Sections 1, 2 and 3, (Figure 28) are selected from this area.

The Vanderbeckei Marine Band is about 1 m thick at Ogilvie Colliery where arenaceous foraminifera, Lingula mytilloides Planolites ophthalmoides, Gyrochorte carbonaria and pyrite burrowfills were recorded. At outcrop at Trecatty opencast site, the marine band is sparsely fossilifcrous with scattered lingulids and burrowfills.

The Bute is thin and unworked underground. It is 0.75 m thick at "Frecatty where it was named the Brass on account of its high pyrite content. It was 0.3 to 0.5 m in boreholes at Pontlottyn where its roof mudstones contained Naiadites sp.and Spirobis.

The Lower Nine-Feet ('Rhaslas') has an average section: coal 0.2 m, parting 0.01 m, coal 0.99 m, 'rashes' 0.5 m, coal 0.56 m, and has been extensively mined. The separation between it and the Upper Nine-Feet ranges from 15 m at Ogilvie Colliery to 4 m at outcrop. The Upper Nine-Feet is known as the Red Vein in this area, and should not be confused with the Red Vein of the Rhondda Valley which lies at a higher level, and may be represented by a thin coal 6 m above the Upper Nine-Feet at Bedlinog. The Upper Nine-Feet is about 1 m thick. Two coals are present in the beds between the Upper Nine-Feet and the Six-Feet at Trecatty ((Figure 28), section 1), the lower of which is named the Harnlo and the upper the Red Vein.

The Six-Feet ('Threequarter') is a composite seam at depth, totalling about 1 m of coal; at Trecatty opencast site it was about 3 m thick, and was worked as the 'Big'.

The Lower Four-Feet was named the 'Yard' at Ogilvie Colliery where the main worked leaf is 0.75 m thick. At Trecatty it is named the Black where it averages 0.8 m, but is extensively washed out. The Upper Four-Feet (or 'Big Vein' of Ogilvie) lies 5 m above the Lower Four-Feet, and is about 1 m thick.

The Two-Feet-Nine, known as the 'Elled', is generally thin or of poor quality with many partings, and has been mined to only a limited extent. An average section is: coal 0.17 m, partings 0.48 m, coal 0.28 m, parting 0.43 m, coal 0.28 m, parting 0.38 m, coal 0.28 m.

Two thin rider coals between the Two-Feet-Nine and the coal smut underlying the Aegiranum Marine Band at Trecatty may mark the positions of the Haughton and Sutton marine bands ((Figure 28), section 1).

Rhymney -Sirhowy area

Sections 4, 5 and 6, (Figure 28), are selected from this area.

The Vanderbeckei Marine Band was 1 m thick at Rhymney opencast site and contained L. mytilloides, Megalichthys sp.and pyrite burrowfills. Up to 1.6 m of mudstones and siltstones with arenaceous foraminifera, L. mytilloides, Megalichthys sp., Tomaculum and pyrite burrowfills were recorded in boreholes at Charles Street, Tredegar. Some of the lingulids were articulated and in life position.

The Bute appears to be absent at depth in the Sirhowy Valley, but is up to 0.5 m thick at outcrop between Rhymney and Tredegar. Large channels cut through the seam at Rhymney opencast site. It is about 0.35 m thick at Charles Street, Tredegar where it has been worked to a small extent.

The Lower Nine-Feet is known as the Rhaslas in the Rhymney Valley, the Bydylog or Black in the Sirhowy Valley. It is 1 to 1.4 m thick throughout this area and has been much worked.

The separation between the Lower Nine-Feet and Upper Nine-Feet is variable, ranging from 2 m at McLaren Colliery to 11 m at Tredegar. The Upper Nine-Feet, known as the Red Vein or Polka, is 0.8 to 1.2 m thick. It is absent in parts of the Sirhowy Valley, either due to washout or non-deposition; boreholes in Tredegar proved mudstones overlying seatearth at the position of the coal.

The Six-Feet ('Threequarter') is generally about 1 m thick. Around Tredegar it is represented by two closely separated coals, the lower termed the 'Threequarter' and the upper the 'Yard'.

These converge in the centre of Tredegar giving about 3 m of coal. The Lower Four-Feet is up to 15 m above the Six-Feet in the south of the area; the seams converge northwards, and are less than 1 m apart at Tredegar.

The Lower Four-Feet ('Big Vein') is 1 to 2 m thick and separated from the Upper Four-Feet by up to 2 m of mudstones, seatearths and coals. The section in a private mine at Bryn-oer, Rhymney [SO 1178 0897] reads: mudstone with Planolites sp.,coal 0.1 ID (Upper Four-Feet), seatearth 0.3 m, coal 0.6 m, seatearth 1.0 m, coal 0.15 m, seatearth 0.15 m, coal (Lower Four-Feet). Plant remains including Lepidodendron ophiurus, Neuropteris cf. heterophylla, N. cf. lenuifolia and Sphenophyllum saxifragaefolium were collected from the roof of the Lower Four-Feet in Sirhowy No. 9 Pit Borehole No. 1 [SO 1470 0917]. The Upper Four-Feet is about 1 m thick throughout the area. Naiadites obliquus was collected from its roof in a borehole in Tredegar [SO 1399 0924].

The Two-Feet-Nine is about 1.2 to at depth in the area and was worked as the 'Elled', but deteriorates northwards towards its outcrop and its correlation is uncertain. The seam referred to as the Tiled' in the outcrop area around Tredegar is the Upper Four-Feet. The Two-Feet-Nine is probably one of a group of thin coals overlying the Upper Four-Feet in Carreg Bica Drift Mine [SO 1492 0930]. Quartzitic channel-fill sandstones and mudstone seatearths occupy most of the 15 m of beds between the ?Two-Feet-Nine and the Aegiranum Marine Band.

Ebbw valleys

Sections 7, 8, 9 and 10, (Figure 28) are selected from this area.

The Vanderbeckei Marine Band is present throughout and was examined at Rhyd-y-Blew opencast site, Marine Colliery, Six Bells Colliery, Abertillery New Mine and in numerous boreholes. L. mytilloides is ubiquitous, and arenaceous foraminifcra, Dunbarella, and fish fragments were also collected. Spoil from old workings at Brynmawr [SO 1836 1260]; [SO 1894 1271] contains marine mudstone from the band. In Aneurin Place Borehole No. 1 [SO 1925 1238], 6 m of beds include three Lingula bands separated by Planolites ophthalmoides hands. In nearby boreholes for the proposed Brynmawr Bypass, the marine band was 4 m thick, with two Lingula beds. The Bute is thin or absent in the south; 8 cm of dirty coal are present in a miss-measures drift at Six Bells Colliery, for example, and it is absent in the No. 5 Pit ((Figure 28), section 11) and at Abertillery New Mine. At outcrop, it was 0.45 mat Rhyd-y-Blew opencast site and 0.3 to 0.8 m in boreholes at Brynmawr. It is exposed in disused quarries [SO 183 124] west of Brynmawr where it rests on quartzitic seatearth.

Dark grey canneloid mudstones form the roof of the Bute; bivalve fragments and Carbonita evelinae were obtained from them at Rhydy-Blew..

The Lower Nine-Feet ('Black' or 'Byclylog') is about 1.2 m thick throughout the area, but is extensively washed out in the Ebbw Fitch Valley. At the Laurels opencast site [SO 1810 1125] Brynmawr, its thickness ranged from 1 to 2.7 m adjacent to a washout.

The roof beds of the Lower Nine-Feet at outcrop are silty mudstones and siltstones with plant remains and ironstone nodules; two beds of tough quartzite 1 m thick lie 3 m and 7 m above the coal at the Laurels opencast site. Barren silty mudstones fbrm the roof at Marine Colliery, and bivalves were recorded at Six Bells Colliery.

The Upper Nine-Feet ('Top or Upper Black') is present in the south, but is thin or absent in the north and east. It lies between 2.5 m and 9 m above the Lower Nine-Feet. 'Fhe average thickness in the Marine Colliery workings ((Figure 28), section 9) is 1 to 1.3 m, with some thin leaves above and below. It is about 1 m thick at Abertillery New Mine ((Figure 28), section 10); at Beynon Colliery, the section is: coal 0.2 to, seatearth 0.59 ID, coal 0.3 m, seatearth 1.67 m, coal 0.72 m.

The Lower Six-Feet (Tower Threequarter' or 'Little Three-quarter') lies between 14 m and 24 m above the Upper Nine-Feet, and is best at Marine Colliery where it is about 1 m thick. At Six Bells Colliery ((Figure 28), section 11), the section is: coal 0.15 m, seatearth 0.15 m, coal 0.62 m; both coals have many dirt laminae. Further deterioration takes place northwards, with the section at Abertillery reading: coal and seatearth in thin layers 0.13 m, seatearth 0.13 m, coal 0.45 m.

The Upper Six-Feet ('Threequarter') lies 6.5 m above the Lower Six-Feet at Marine Colliery, but the separation decreases northwards and eastwards and, in the Ebbw Fawr Valley it ranges from 0.4 to at Six Bells to 3 m at Beynon Colliery. The coal generally consists of one main leaf about 1 m thick, with some thin or dirty coal below.

Anthraconaia aff. pulchella, A. pumila (lectotype and syntype), A. aff. pumila, A. cf. pumila, A. subcentralis, Anthracosia (?)sp. and Anthracosphaerium (?)sp. were recorded from the roof mudstones of the Upper Six-Feet in No. 6 Pit, Victoria Colliery, Ebbw Vale (Bassett, 1972; Weir, 1966). During the resurvey a few bivalves were collected at Six Bells Colliery; elsewhere only plant remains were found. The separation between the Upper Six-Feet and the Lower Four-Feet ranges from 2.1 m at Beynon Colliery to 15.2 m at Abertillery.

The Lower Four-Feet ('Big Vein') is 1.4 m thick at outcrop and 1.8 In at depth in the south. An anomalous thickness of 3.5 m in Abertillery New Mine is due to reverse faulting.

The beds between the Lower-Four-Feet and the Upper Four-Feet range from 2.5 m at outcrop at the Laurels opencast site to 10.2 m at Beynon Colliery. About 5 to 6 m of beds are present in the south, arid they generally consist of mudstone seatearths and thin coals. At Abertillery New Mine, 9 m of beds include 6 m of sandstone.

The Upper Four-Feet ('Elled') is between 0.8 m and 1 m thick at outcrop at the Laurels opencast site, increasing to 1.1 m to 1.3 m at depth in the south.

The beds between the Upper Four-Feet and the Aegiranum Marine Band are between 15 m and 24 m thick. Beynon Colliery Emergency Drift and Abertillery New Mine ((Figure 28), section 10) provided recent records of them. The basal part was exposed in a cutting at Waun-y-pound [SO 1516 1047] to [SO 1545 1029], Ebbw Vale. The roof beds of the Upper Four-Feet were mudstone seatearth cut by a broad channel filled with plant-rich mudstones. A coal 5.8 m above the Upper Four-Feet (?Two-Feet-Nine) has a section which reads: coal 0.4 m, mudstone seatearth 0.7 m, ganister 0.42 m, coal 0.2 m. A coal 8.7 m above is 0.32 m thick and has mudstone with pyrite burrowfills in its roof. This mudstone may be the Haughton (Hafod Heulog) Marine Band. At the Laurels opencast site [SO 177 109] there are 14 m of plant-rich mudstones with ironstone nodules above the Upper Four-Feet. A thin coal above is overlain by dark grey sulphurous mudstone with pyrite burrowfills and a few shell fragments; the mudstone is also probably the Haughton Marine Band. Quartzitic sandstones form the roof of the Upper Four-Feet at Beynon Colliery. The ?Haughton Marine Band lies in the roof of a coal 11 m above the Upper Four-Feet. The coal consists of: coal and mudstones mixed 0.15 m, seatearth 0.26 m, coal 0.30 m, and was proved in a borehole on Beynon tip [SO 1975 0865] where the ?Haughton Marine Band was represented by mudstones with Planolites and burrowfills.

Clydach -Pontypool

Sections 12, 13 and 14, (Figure 28), are selected from this area.

Blaenant opencast site proved sections of the beds between the Vanderbeckei Marine Band and Bute, and between the Upper Four-Feet and the Aegiranum Marine Band. A roadside section [SO 2190 1156] to [SO 2185 1140] at the head of Cwm Llammarch exposes beds from below the Bute to the Upper Nine-Feet. Washery Drift, Blaenavon ((Figure 28), section 12) proved the beds between the Vanderbeckei Marine Band and the ?Lower Six-Feet.

The Vanderbeckei Marine Band is the basal part of a 14 to thick coarsening-upwards cycle of parallel-laminated mudstones, siltstones, ironstones and fine-grained sandstones at Blaenant opencast site. About 0.3 m thick, the marine band contains crinoid columnals, Lingula mytilloides, Orbiculoidea nitida, Productus sp.and Rhizodopsis sauroides. It was also sampled at Washery Drift, where it is 1.2 m thick, and at Blaenserchan Colliery.

The thickness of the beds between the base of the Vanderbeckei Marine Band and the Bute ranges from 4 m at Pontypool to 14 m at Blaenant.

The Bute is extensively washed out at Blaenant; it consists of two leaves 0.2 m thick, and 0.2 m apart. The basal leaf is 0.47 m thick at Washery Drift. The thickest development of the Bute in the Abergavenny district is in the Varteg-Talywain area, where the coal is known as the Yard and has an average section of: coal 0.55 m, parting 0.2 to 0.4 m, coal 0.7 m. The parting between the two leaves appears to increase southwards, with two coals separated by 3.5 to 4 m of beds at British Top Pits ((Figure 28), section 13) and Navigation Colliery. The coal is absent due to washout in the Blaenscrchan–Llanerch–Cwenallt area.

The roof of the Bute in Cwm Llammarch [SO 2190 1157] contains fish fragments and ostracods; bivalves were recorded at Washery Drift. The beds between the Bute and the Lower Nine-Feet are 4.5 to 9 m, their maximum thickness being at Blaenant opencast site where they consist of channel-fill quartzites.

The Lower Nine-Feet ('Black', 'Bydylog' or 'Bottom Rock') is about 1.4 m thick at Blaenant, and an excavated section in Cwm Llammarch reads: coal 0.68 m, seatearth 0.2 m, coal 0.65 m. There are several coals in the Blaenavon area, as proved in Washery Drift. Two of them were formerly worked, the lower one as the 'Bydylog', and the upper the 'Bottom Rock'. The Lower Nine-Feet and the Upper Nine-Feet combine south of Varteg Hill, the section at Lower Varteg being: coal 1.07 m, parting 0.25 m, coal 0.81 m. The Nine-Feet ('Rock') is 2.4 m at British Top Pits, but is washed out at outcrop for 1 km north of the Trevethin Fault, and westwards from there towards Llanerch Colliery.

The roof mudstones of the Lower Nine-Feet in Washery Drift contained Leaia sp.(with egg cases), an arthropod fragment and bivalve fragments. The separation of the Lower Nine-Feet and the Upper Nine-Feet increases northwards from Varteg Hill, where the coals converge, to about 14 m in the Blaenant–Cwm Llammarch area.

The Upper Nine-Feet ('Horn' or 'Top Rock') is thin to absent in the Clydach–Blaenant area. It is 0.4 m thick at outcrop in Cwm Llammarch, about 1 m in workings at Big Pit, Blaenavon, and 1.2 m at Varteg Hill.

The beds between the Upper Nine-Feet and the Lower Six-Feet are 23 m thick at Milfraen and 15 m from Blaenavon to Varteg; they thin markedly south of Varteg and 4.5 to 5 m of beds separate the Nine-Feet and Six-Feet at Navigation Colliery and British Top Pits.

The Six-Feet consists of two leaves at Blaenant, where an average section reads: coal 0.76 m, parting 0.3, coal 0.53 m (Upper Six-Feet or 'Threequarter'), measures 1.6 to 2 m, coal 0.45 m (Lower Six-Feet or 'Little Threequarter'). A thin coal at Washery Drift is tentatively correlated with the Lower Six-Feet ((Figure 28), section 12); only one coal (?Upper Six-Feet) is recorded in the nearby Coity Pit where it is 1.4 m thick. The section of the Six-Feet at Blaenserchan Colliery is: coal 1.07 to 1.77 m, parting c.1.5 m, coal 0.36 to 0.76 m. The parting is reduced to 0.15 m at Llanerch Colliery, where the seam was known as the Big.

The roof beds of the Six-Feet are generally mudstones and silty mudstones, generally containing plant remains and ironstone nodules, although a few bivalve fragments were noted at Blaenant.

On the east crop sandstone (the 'Threequarter Rock') or 'fireclay' forms the roof locally. The separation of the Six-Feet and Lower Four-Feet ranges from 3 to 12.5 m. At Blaenant, it is 3 to 5 m in the west of the opencast site and increases to 8 to 9 m in the east. It varies markedly in the Varteg area, being 3 m at Lower Varteg, but increasing both to the north and south of there up to 12.5 m.

The Lower Four-Feet, generally known as the Big Vein is 1.1 to 1.5 m throughout the area, although it is absent locally at Blaenavon and Blaenserchan. This absence is probably mainly due to washout, although it is possible that the coal never accumulated near Coity Pit, Blaenavon. There, a borehole [SO 2349 0896] proved 18 m of sandstones overlying the Six-Feet, and these may have been sands forming a topographical high above the Lower Four-Feet peat swamp, being less compactible than the surrounding muds.

The beds between the Lower Four-Feet and Upper Four-Feet are generally between 2 and 4 m thick, with a maximum of 6.3 m at Coity Pit, Blaenavon, and a minimum of 1.4 m at Varteg Hill. They consist of mudstone seatearth, thin coals and carbonaceous mudstone with plant debris.

The Upper Four-Feet ('Elled') is about 1 m thick, but is extensively washed out. It was 0.8 to 1 m thick at the western end of Blaenant opencast site, but mostly washed out in the east. It is 1.2 m thick at Varteg where it was worked as the Red Vein. It is 2.4 m thick at British Top Pits, but is generally washed out in the Blaenserchan–Llanerch area.

The beds between the Upper Four-Feet and the Aegiranum Marine Band range in thickness from 30 m at the west of Blaenant opencast site to 12 m at Pontypool. They consist mainly of quartzitic sandstones and seatearths. The immediate roof beds of the Upper Four-Feet in the west of the Blaenant workings comprise a coarsening-upwards cycle of mudstones, fine at their base and containing plant fragments and some trace fossils. In the east of the site, pebbly quartzite (the 'Elled Rock') forms the roof and cuts down through the coal in many places. A coal 0.2 m thick and lying about 20 m above the Upper Four-Feet has a mudstone roof with Orbiculoidea sp.and pyrite burrowfills. The mudstone is either the Haughton or Sutton Marine Band.

The beds are extensively exposed in the backwall of Blaen Pig opencast site [SO 2313 1136] to [SO 2407 1127]. Boreholes near Coity Pit, Blaenavon [SO 2404 0874]; [SO 2349 0896] and near Blaenserchan [SO 2587 0235] drilled these beds, but the absence of the Upper Four-Feet and Lower Four-Feet makes precise correlation difficult. Quartzitic sandstones, siltstones and seatearths make up most of the sequence.

Aegiranum (Cefn Coed) Marine Band to Cambriense (Uppr Cwmgorse) Marine Band

(Figure 29) gives a generalised vertical section. These beds have a well-developed cyclicity in the west where they are predominantly argillaceous. Up to nine coarsening-upwards coal-bearing delta plain cycles are present, and four of them commence with shallow marine mudstones; the others commence with bivalve or fish-bearing, bioturbated mudstones. From a maximum of 120 m in the south-west, the beds thin to 50 m in the south-east, and become predominantly arenaceous. Northward attenuation also takes place, 70 to 80 m being present on the north crop. The easterly attenuation is not regular, but stepped (inset, (Figure 30)), possibly suggesting basement control of subsidence by north- to north-west-trending growth faults.

The coals in these beds are mostly thin and fail eastwards, with only two persisting to the east crop. Of the marine bands, only the Aegiranum is present throughout the district, the others being absent on the east crop. The transition from the thicker, argillaceous, cyclic sequence of the west to the thinner, sandstone-dominated sequence of the east, and the eastwards failure of the coals and marine hands reflect a transition from the lower to the upper delta plain (p.74).

Tonsteins, reported by Squirrell and Downing (1969) in the adjoining Newport district, occur at three horizons; in the roof of the Gorllwyn Rider, in a parting within the Lower Pentre, and in the Pentre Rider.

Greenish Pennant sandstones are present above the Pentre Rider in the Rhymney and Sirhowy valleys, and between the Shafton (Lower Cwmgorse) and the Cambriense (Upper Cwmgorse) marine bands in the Bargoed Taff Valley. Their presence records an early incursion of Pennant sedimentation into the area, although this generally commenced later here than anywhere else in the coalfield (see p.74).

The Aegiranum Marine Band is present throughout the district, and lies at the base of a thick coarsening-upwards cycle. The marine band is a dark grey mudstone containing L. mytilloides mainly, and passes up into bivalve and fish-bearing mudstones with numerous ironstone layers, the 'Black Pins Mine Ground'. A persistent quartzitic sandstone overlies these mudstones and is in turn overlain by the Gorllwyn group.

The Gorllwyn group of coals is represented mainly by the Gorllwyn Rider, although a thin underlying coal, correlated as the Gorllwyn, is present locally. The Gorllwyn Rider ('Blackband' or 'Black Pins Coal') is sufficiently thick to have been worked from outcrop, and is currently being mined in the Rhymney Valley. A thin tonstein layer in its roof is overlain by black fish-bearing mudstones.

The Eighteen-Inch is a thin split seam in the west where it is known as the Soap Vein. The seam known as such on the east crop is probably the Pentre Rider (see below), with the true Eighteen-Inch absent east of the Ebbw Fach Valley. Smooth, 'soapy' mudstones and ferruginous siltstone, intensely bioturbated by Planolites montanus, form the roof of the coal. Bivalves including Anthraconaia adamsii, A. expansa, A. hindi, Naiadites alatus/melvillei and N. quadratus are recorded (Bassett, 1972; Blundell, 1952); fish fragments and ostracods are also present.

The Lower Pentre is present west of Ebbw Vale and is typically a thin split seam with a hard sandstone and tonstein in the parting. Plant-bearing inudstones with standing Lepidodendron trunks were recorded in its roof at Dowlais Top opencast site [SO 100 175], and bivalves have been recorded in its roof at Tredegar.

The Pentre is a thin coal present only west of Ebbw Vale and has bivalves in its roof including A. adamsii and Naiadites sp.Ostracods and cf. Euestheria sp.are also recorded.

The Pentre Rider is present as a thin coal throughout most of the district. A parting in the coal on the east crop contains a thin tonstein. The coal is known locally there as the 'Soap Vein'. The Edmondia (Foraminifera) Marine Band lies in the roof of the coal, except in parts of the Rhymney and Sirhowy valleys where Pennant sandstone is present at this position.

The marine band is up to 3 m thick and contains L. mytilloides, foraminifera and Planolites ophthalmoides. It forms the base of a coarsening-upwards cycle up to 10 m thick and containing many ironstones. A non-marine phase is recorded near the base of the cycle at two localities, and the marine mudstones above may correlate with the Five Roads Marine Band (Squirrell and Downing, 1969).

The Abergorky is a thin coal present only in the west of the district. At outcrop, pebbly quartzite occupies most of the 4 m thick cycle above. This cycle is capped by a thin coal, generally no more than a smut at outcrop, but sufficiently thick to have been exploited to a minor extent in Cwm Bargoed where it was named the 'Tic Cae.

The Shafton (Lower Cwmgorse) Marine Band lies in the roof of this coal over most of the district. It is absent in a belt near the Dowlais Fault in Cwm Bargoed where quartzitic sandstones fill channels cut through the marine band. It is also absent in part of the east crop near Pontypool. The fauna consists mainly of L. mytilloides and fish fragments, but Dunbarella sp.and Nuculopsis sp.have also been collected.

The beds between the Shafton Marine Band and the Hafod coal are up to 15 m thick and comprise a fining upwards cycle. The cycle is interrupted by Pennant sandstone in Cwm Bargoed, and by quartzite in its outcrop from Rhymney eastwards.

The Hafod, a thin coal or smut in the west of the district, is locally absent in the Ebbw Fach Valley and on the east crop.

The Cambriense (Upper Cwmgorse) Marine Band lies in the roof of the Hafod and is present throughout the district apart from the area of the east crop near Pontypool. It is the more fossiliferous of the two Cwmgorse marine bands. Apart from L. mytilloides, which is abundant, the fauna collected includes Orbiculoidea cincta, 0. cf. nitida, Dunbarella macgregori, D. papyracea, Phestia sharmani, Productus sp.Roundyella sp.,and Euphemites anthracinus.

Details

West of the River Rhymng

Sections 1, 2, 3, (Figure 30) and section 15, (Figure 31) are selected from this area.

The Aegiranum Marine Band is up to 3.5 m thick at Trecatty opencast site, but is absent locally in a large channel filled by up to 20 m of sandstones. L. mytilloides, Orbiculoidea nitida, Serpulites stubblefieldi, Paraconularia sp.,gastropods and conodont fragments were collected.

The Gorllwyn Rider crops out below Dowlais Top opencast site. It is known as the Blackband, ranges from 0.35 to 0.75 m in thickness, and was extensively mined from adits. The overlying coarsening-upwards cycle of silty mudstones and siltstones is well exposed in a line of crags above the coal, and the sequence from the Gorllwyn Rider to above the Pentre Rider is seen in a disused railway cutting at Dowlais Top [SO 097 074]. (Figure 31), section 15 shows the beds exposed during opencast working. The roofs of the Eighteen-Inch and Pentre seams contain Anthraconaia adamsii and ostracods; plant remains and standing Lepidodendron trunks were present in the roof of the Lower Pentre. Pennant sandstone, exposed in the railway cutting, forms the roof of the Pentre Rider (see p.91). The following section was recorded in a small digging at Trecatty opencast site:

A small exposure of the Shafton Marine Band at Pontlottyn [SO 1108 0675] yielded L. mytilloides (juv.), Dunbarella sp.and Spirorbis sp.(attached to the Dunbarella).

Rhymney-Sirhowy area

Sections 4, 5, and 6, (Figure 30) and sections 16, 17 and 18, (Figure 31) are selected from this area.

The outcrop of the Aegiranum Marine Band is obscured by spoil from Rhymney to Tredegar, both it and the succeeding beds having once been dug for ironstone in open excavations or 'patchings'. L. mytilloides can be collected from marine mudstone in the spoil. The topmost part of the Aegiranum cycle was drilled in Sirhowy No. 9 Pit Borehole No. 3 [SO 1464 0891] where Euestheria sp.and fish fragments including Rhabdoderma sp.are scattered through mudstones with ironstone layers ((Figure 31), section 18b). L. mytilloides was collected from the marine band in Carreg Bica drift mine 40 m from the portal [SO 1492 0929].

The Gorllwyn Rider ('Blackband') is currently being worked at Old Unemployed Level, Rhymney [SO 1195 0787], where an average section is: coal 0.68 m, fireclay 0.46 m, coal 0.44 m. The coal and the overlying beds were examined in a small opencast cut at Brynoer [SO 1215 0895], where the section reads:

The coals thin and the parting thickens eastwards towards Tredegar; the section in Sirhowy No. 9 Pit Borehole No. 3 [SO 1464 0891], for example, reads: coal 0.35 m mudstone seatearth 1.51 m, coal 0.15 m, mudstone seatearth 0.37 m, coal 0.34 m.

The coarsening-upwards cycle above the Gorllwyn Rider is interrupted by quartzitic sandstones between Rhymney and Tredegar (see (Figure 31), section 16). The strata from the Eighteen-Inch to above the Edmondia Marine Band were exposed in abandoned opencast workings on Rhyrnney Hill, and were also examined during active working at Mountain Pit opencast site in 1973. (Figure 31), section 16 gives a compiled section. The top leaf of the Gorllwyn Rider and the sequence to above the Pentre Rider are patchily exposed in a crag at Carreg Bica [SO 1489 0930] ((Figure 31), section 18).

The Eighteen-Inch ('Soap Vein') is generally a single leaf coal, about 0.3 m thick at Rhymney, and thinning to 0.2 m at Tredegar. At Mountain Pit opencast site, its roof mudstones contained Anthraconaia adamsii, A. aff. stobbsi, Naiadites sp., Rhizodopsis sauroides and Spirorbis sp.Bioturbated soapy mudstones and ironstones overlie the bivalve bed (the 'Soap Vein Ironstone'). Black intensely bioturbated canneloid mudstone forms the immediate roof of the Eighteen-Inch in Sirhowy No. 9 Pit Borehole No. 3, and passes upwards into mudstones with bivalves including Anthraconaia sp.and cf. Naiadites sp.

At Mountain Pit, the Lower Pentre is split into a lower 0.2 m and an upper 0.1 m leaf by a hard fine-grained sandstone parting with tonstein wisps. The succeeding cycle begins with mudstones with bivalves and plant debris, but locally a sandstone up to 3.6 m thick rests on the coal.

The Pentre is about 0.3 m thick in this area, and at Mountain Pit its roof mudstones contain Naiadites sp.and cf. Euestheria sp.The mudstones form the base of a coarsening-upwards cycle 4 to 5 m thick at Mountain Pit, but sandstones occupy most of the cycle at Carreg Bica.

The Pentre Rider is 0.3 to 0.4 m throughout the outcrop area and, at Mountain Pit, carries the Edmondia Marine Band in its roof. At Carreg Bica, green Pennant sandstones overlie the coal.

The Edmondia Marine Band is about 3 m thick at Mountain Pit and consists of soapy mudstones with arenaceous foraminifera, L. mytilloides, fish fragments and trace fossils.

The Abergorky is about 0.2 to 0.3 m thick, and, on Rhymney Hill, is overlain by up to 4 m of very coarse-grained pebbly quartzite.

The Shafton Marine Band is exposed in a gully at Rhymney [SO 1209 0754] where it rests on a thin coal smut and contains L. mytilloides, Dunbarella sp., Nuculopsis sp., Hindeodella sp., and fish debris including Rhabdoderma sp.,Rhadinichthys V. and Megalichthys? This band and the Cambriense Marine Band are exposed in a stream below Scotch Peter's reservoir, Llyswedog, Tredegar ((Figure 31), section 17). The Shafton Marine Band, first recorded by Blundell (1952, p.321) [SO 1520 0844], comprises 1 m of hard dark grey mudstones with an abundance of L. mytilloides and fish fragments. The coarsening upwards cycle above is about 13.5 m thick and capped by the Hafod, a coal smut 0.02 m thick. The Cambriense Marine Band [SO 1533 0854] is intermittently exposed; fish fragments and Planolites ophthalmoides were recorded.

Ebbw valleys

Sections 7, 8, 9 and 10, (Figure 30) are selected from this area. Abertillery New Mine [SO 3205 2058] ((Figure 30), section 9) gave a section of the entire interval, but the recorded thicknesses appear exaggerated.

The Aegiranum Marine Band was noted at Twynyderyn, Nantyglo [SO 1968 1089] where it rests on a coal smut and consists of black Lingula-bearing mudstone with pyrite burrowfills. It was examined underground in Beynon Colliery emergency exit [SO 1980 0859] where it is 0.3 m thick and contains L. mytilloides in abundance, Orbiculoidea sp productoid fragments and a hindeodellid bar. A bioturbated layer at the top of the marine band is crowded with small lingulids and pyrite burrowfills. The marine band is about 4 m thick in Abertillery New Mine, with L. mytilloides and foraminifera in the basal 2.7 m, fish scales in the topmost 1.3 m, and Planolites throughout. The non-marine phase of the Aegiranum cycle is unexposed, but spoil from pits and adits from Newtown to Nantyglo e.g. [SO 1768 1073], [SO 1848 1021], [SO 1850 1020] yielded the ostracods Carbonita sp., and cf. Geisina subarcuata, and fish fragments including Elonichthys sp.and Rhadinichthys sp.In Beynon Colliery emergency exit, dark grey mudstones with ironstone layers and nodules (the 'Black Pins Mine Ground') contain Euestheria and fish fragments including cf. Rhabdoderma. A nearby borehole [SO 1975 0865] proved most of the Aegiranum cycle; the mudstones lying close above the marine hand were rich in fish debris including a maxilla of Elonichthys sernistriatus (identified by Dr B G Gardiner of the British Museum, Natural History), Platysomus sp., Rhabdoderma sp.and Rhadinichthys sp.A mudstone seatearth above divides the major cycle into two minor ones, the higher cycle containing Naiadites sp.,cf. Euestheria sp.,and worm traces.

The Gorllwyn Rider ('Black Pins Coal') has the following section at Beynon Colliery: coal 0.2 m, sandstone seatearth 0.18 m, coal 0 to 0.03 m, sandstone seatearth 0.32 m, coal 0.07 m. At Abertillery New Mine only 0.15 m of coal are present, with sandstones above and below. The roof at Beynon consists of mudstones with fish fragments; quartzitic sandstones lie close above.

Abertillery New Mine provides the best section of the Eighteen-Inch (' Soap Vein') in the area: coal 0.25 m, parting 0.01 to 0.02 m, coal 0.13 m, canneloid mudstone 0.05 m, coal 0.08 m, seatearth 0.38 m, coal 0.20 m. Its roof consists of pale grey mudstones with ironstone layers, and with large flattened A. adamsii. Spoil from a disused level in the coal near Newtown [SO 1782 1073] contained bivalve fragments, Carbonita humilis, C. secans and fish fragments. Anthraconaia adamsii (lectotype), A. aff. adamsii and A. sp.were collected from Penycae c. [SO 175 100], Ebbw Vale (Bassett, 1972; Weir, 1966). Blundell (1952, pp.319, 320) recorded Anthraconaia adamsii, A. expansa, Naiadites quadratus, fish remains and ostracods from the roof of the 'Soap Vein' in the Brynmawr area. Rogers (1861) recorded annelid worm traces in these beds (the 'Soap Vein Mine Ground').

The Lower Pentre is generally absent in the area, although it is present in South Griffin No. 3 Pit [SO 2001 0659] where it is 0.3 m thick and lies 3 m below the Pentre.

The Pentre is known as the Court and is present throughout the area as a thin coal. It is exposed in the wall of an old excavation on the golf course at Nantyglo [SO 1848 1018] where the section is: coal 0.04 m, shale 0.04 m, coal 0.02 m. A similar section was recorded at Abertillery New Mine. Indeterminate bivalve fragments and Ceisina subarcuata were obtained in spoil from shallow pits to the coal at Nantyglo [SO 1838 1024].

The Pentre Rider is about 0.3 m thick. Sandstones overlie the coal at Marine Colliery, but elsewhere, the Edmondia Marine Band is present. The coal and succeeding beds are exposed in the backwall of disused excavations between Newtown and Nantyglo [SO 1755 0943] to [SO 1793 1247]. The Edmondia Marine Band, which lies at the base of a coarsening upwards cycle, is about 3 m thick, and contains L. mytilloides, arenaceous foraminifera, P. ophthalmoides and fish fragments. In Abertillery New Mine 2.1 m of barren silty mudstones overlie the marine band and are in turn overlain by 0.66 m of mudstones with Planolites and fish scales. The higher fossiliferous band is tentatively correlated with the Five Roads Marine Band. The Abergorky can be recognised as a thin coal in most of the colliery shaft sections, but only a seatearth occurs at this level in Abertillery New Mine.

The Shafton Marine Band has been proved only at Abertillery, but is probably present throughout the area. About 4.6 m of dark grey mudstones with numerous ironstone layers and nodules, and iron pyrite, contain an abundance of Planolites and other trace fossils, but only a few L. mytilloides and Orbiculoidea occur.

The Cambriense Marine Band is more fossiliferous and has been recorded at several localities. Spoil from an ironstone level north-west of Bwlch y Garn [SO 1771 0954] yielded Lingula sp.and cf. Roundyella. The following was obtained from spoil at a nearby disused ironstone adit [SO 1780 0976]: L. mytilloides, Orbiculoidea cf. nitida, orthotetoid?, Dunbarella macgregori, Euphemites anthracinus, gastropods and indeterminate rod-like structures. Mudstones above the adit yielded Anthraconaia cf. stobbsi and Rhadinichthys sp.In boreholes near Rose Heyworth Colliery e.g. [SO 1985 0675] the band was about 2.5 m thick and contained mainly L. mytilloides, fish fragments and worm traces; Orbiculoidea cincta, Phestia cf sharmani, a cephalopod jaw, indeterminate bivalves and Spirorbis sp.were also collected. Anthraconaia sp., Naiadites aff. obliquus, Elonichthys sp., Rhadinichthys sp.,and Platysomus sp.occur in the overlying non-marine phase of the cycle. About 1.8 m thick at Abertillery New Mine, the band yielded foraminifera, L. mytilloides, Orbiculoidea cincta, Euphemites sp., Serpulites stubblefieldi, ostracods, Tomaculum, fish fragments and Planolites. It was sparsely fossiliferous in East Pentwyn Borehole No. 2 [SO 2041 0753], from which Lingula sp.,cf. Carbonita sp., Sphenothallus stubblefieldi and Platysomus. sp.were collected. An overlying 2 m-thick non-marine phase contained Planolites, Elonichthys sp., Rhabdoderma sp., Rhadinichthys sp.,and Platysomus sp.There is a small exposure of the marine band near Cwm Celyn [SO 2029 0888].

Clydach -Pontypool

Sections 11 to 14, (Figure 30) and section 19, (Figure 31) are selected from this area.

The Aegiranum Marine Band is present throughout. It is exposed for 1 km in the backwall of Blaen Pig opencast site [SO 2313 1136] to [SO 2407 1127] where it is a Lingula bed at the base of a coarsening upwards cycle 8 m thick. Two boreholes at Big Pit Tip, Blaenavon [SO 2404 0874], [SO 2349 0896] proved the marine band; sponge spicules, arenaceous foraminifera, L. mytilloides, L. spat, Orbiculoidea sp., a productoid, cf. Dunbarella, turreted gastropod spat and a platform conodont were collected. The backwall of disused opencast workings [SO 2507 0742] south of Blaenavon ((Figure 31), section 19) exposes the band. Greenland Tip Borehole No. 1 [SO 2587 0235] proved the marine hand to be only 0.23 m thick at the base of 4.7 m of broken mudstones. A sparse fauna of L. mytilloides (juv.), Orbiculoidea sp.and a ?productoid fragment was recorded.

The beds from the base of the Aegiranum Marine Band to the Gorllwyn Rider are 10 to 15 m thick, being thinnest in the south.

Quartzitic channel sandstones (the 'Black Pins Rock') overlie the Aegiranum cycle. They are 4.5 m thick in a disused quarry [SO 2356 1147] at Blaen Pig and 2.8 m thick in Greenland Tip Borehole No. 1 [SO 2587 0235].

The Gorllwyn Rider ranges from 0.3 m to about 0.7 ni and has been mined locally from outcrop. It is up to 0.7 m in the backwall of disused excavations in the Black Pins Mine Ground between Garn Terrace and Tir Abraham Harry [SO 242 099]. Three sections illustrate the variation in the seam:

1. Cody Pond [SO 2321 0919]: tonstein 0.04 m, coal and carbonaceous mudstone 0.28 m, coal 0.38 m.
2. Waun Hoscyn opencast site [SO 2507 0742]: coal 0.21 m, shale and coal 0.06 m, coal 0.29 m, mudstone seatearth 0.9 In, coal 0.18 m. The basal leaf of this section may be the Gorllwyn.
3. Rhiw Frank, Abersychan [SO 2623 0254]: tonstein 0.04 m, coal and mudstone 0.09 m, coal 0.07 m, mudstone seatearth 0.09 m, dirty coal 0.06 m, coal 0.28 m, mudstone seatearth 0.39 m, coal 0.36 m.

The coal was 0.53 m thick in the Greenland Tip Borehole No. 1 [SO 2587 0235] near Blaenserchan where it is known locally as the Harmonium Coal.

Black tonstein, succeeded by dark grey carbonaceous mudstone, forms the roof of the Gorllwyn Rider, but quartzitic sandstones generally lie close above and form the roof of the coal north of Blaenavon. The Eighteen-Inch, lies 2 to 3 m above the Gorllwyn Rider. It is thin or absent south of Blaenavon, but to the north-west it has been worked from surface levels, for example at Waun-Pwll- Dwr Colliery [SO 2099 1125] where it is 0.45 m thick. Its thickness appears to be variable, however, and 0.9 m of coal were recorded nearby [SO 2127 1116].

The Lower Pentre is a thin coal north-west of Blaenavon, and is absent to the south. It may be the coal dug to a small extent near Blacnant [SO 2061 1122] where the section reads: coal 0.02 m, seatearth 0.15, coal 0.08 m, seatearth 0.04 m, coal 0.07 m.

The Pentre is unproved north-west of Blaenavon, but was exposed in a track at the disused Waun Hoscyn opencast site [SO 2520 0716] where two 0.2 m thick leaves are separated by 0.06 m of mudstone seatearth. A coal formerly worked at the 'Soap Vein' in a level near Golynos [SO 2535 0424] may be the Pentre, or the Pentre and Pentre Rider combined.

The Pentre Rider is also unproved north-west of Blaenavon, but lies 2.6 m above the Pentre in the Waun Hoscyn track section and comprises: coal smut, tonstein 0.07 m, coal 0.39 tn.

The Edmondia Marine Band was present in this track section also, where it consists of 3.8 m of mudstones with a few ironstone layers and nodules. L. mytilloides and abundant foraminifera were recorded (Jackson, 1973). A higher mudstone may represent the Five Roads Marine Band.

The Shafton Marine Band is probably absent throughout this area. The Cambriense Marine Band was recorded by Blundell (1952, p.230) in a tunnel north-west of Coity Pond where it rests on 0.23 m of coal (the Hafod) above a coarse pebbly sandstone (the 'Coity Grit' of Blundell). L. mytilloides, Orbiculoidea sp., Dunbarella papyraceus, Productus sp.and fish fragments were collected, and L. mytilloides is present in the spoil at the tunnel mouth. Marine mudstones with lingulids, bivalves and fish fragments on the opencast road above Waun Hoscyn [SO 2470 0767] were previously correlated as the Shafton Marine Band (Jackson, 1973), but are more likely to be the Cambriense Marine Band.

Upper Coal Measures (Pennant Measures)

Up to 650 m of Upper Coal Measures are present, and make up about half of the outcrop of the Coal Measures. Much of this outcrop is the Pennant sandstone plateau into which the coalfield valleys are incised. The beds range in age from late Westphalian C to late Westphalian D (see p.72).

(Figure 32) gives a generalised vertical section. The sequence is divided into formations at persistent coals (Woodland and others, 1957). The No. 2 Rhondda coal, marking the base of the Rhondda Beds, is thin or absent on the east crop, making their differentiation from the underlying Llynfi Beds difficult. These two formations are unique in the Coal Measures sequences in containing red beds (see p.74). The Brithdir, marking the base of the Brithdir Beds, is a good coal, and has been much worked. The diachronous inception of Pennant sandstone sedimentation reached most of the district at this level, having reached the extreme southwest somewhat earlier (see p.74). The Brithdir, Hughes, and Grovesend beds are dominated by Pennant sandstones.

Two views have been expressed concerning the extreme attenuation that takes place in the lower part of the Upper Coal Measures between the Ebbw Fach Valley and the east crop. Blundell (1952) proposed that a major unconformity cuts out all the Llynfi Beds and the basal Rhondda Beds in addition to the topmost Middle Coal Measures. Squirrell and Downing (1964, 1969) suggested that, in the east crop area of the Newport district, the sequence is condensed but complete, apart from a local unconformity in the south-east. Within the present district, the absence of the No. 2 Rhondda, which defines the base of the Rhondda Beds (p.98), and the lack of identifiable horizons in the Llynfi and Rhondda beds make precise correlation of these beds difficult, and the nature of their attenuation hard to assess. It is likely, however, that on the east crop the sequence is both highly condensed and affected by eastward-developing unconformities.

The apparent absence of the Swansea Beds, which lie between the Hughes and Grovesend beds to the west of Neath, was considered to be best explained by the presence of an unconformity below the Mynyddislwyn (Squirrell and Downing, 1969; Woodland and Evans, 1964; Woodland and others, 1957). Kelling (1974) suggested that the Swansea Beds may be represented in the east of the coalfield in an attenuated Pennant Measures succession, with the coals of the west absent. This implies that the Usk Axis was more persistent than intermittent in effect, and dispenses with the need to invoke pre-Mynyddislwyn uplift and erosion. Assuming that the correlation of the Mynyddislwyn with the Wernffraith or Swansea Four-Feet is correct (see Woodland and others, 1957, p.9), it is possible that the beds mapped in the east as the Hughes Beds are the attenuated equivalent of both the Hughes Beds and the Swansea Beds of the west. There is no conclusive evidence at present either for this, or for the presence of a pre-Mynyddislwyn unconformity.

Llynfi Beds

The thickness of the Llynfi Beds ranges from about 25 m in the south-east to 80 m in the south-west. Precise thickness in the south-east is difficult to gauge as the Cambriense Marine Marine Band has not been proved and the No. 2 Rhondda coal is absent.

The formation is poorly seen, with exposures generally confined to a few stream sections and feature-forming sandstones. Mudstones and sandstones occur in approximately equal proportions. Pennant sandstones are confined to the Bargoed Taff Valley, with quartzitic sandstones, commonly pebbly and conglomeratic, present in most of the district. Red beds appear towards the top of the formation in the Ebbw Fach Valley and extend from there to the east crop (Squirrell and Downing, 1965). Five thin coals occur in the west, but most of them are absent east of the Sirhowy Valley, and correlation of those present is uncertain.

The Blackband is up to 0.45 m thick, and overlain by pebbly quartzite at outcrop. The thick sandstones above the coal in Bedlinog shaft ((Figure 33), section 2) may be Pennant sandstones. A thin impersistent coal lying 3 to 7 m above the Blackband in the Rhymney Valley may equate with the Tormynycld of the Taff Valley.

The No. 3 Rhondda appears to be the most persistent coal in the Llynfi Beds. Although not proved at outcrop in the west, it can be recognised in shaft sections. It is generally thin, but its probable correlative in Ebbw Vale, the 'Old Man's Coal' is thick enough to have been mined from surface levels. The 'Merthyr Vein' of Llanerch near Pontypool is tentatively correlated with the No. 3 Rhondda, and has also been worked locally.

The Taldwyn and Gilfach seams are present west of the Rhymney Valley and have been mined to a small extent, although they are poor coals with numerous partings. The coals persist into the Sirhowy Valley, but their correlation is uncertain. They fail to the east, or are present only locally. Up to three thin coals lie in the beds between the Gilfach and the No. 2 Rhondda in the Rhymney Valley, but also fail eastwards.

Details

West of the River Rhymney

Sections 1, 2 and 3, (Figure 33) are selected from this area.

The Blackband section in a trial pit [SO 1008 0654] north of Fochriw is: coal 0.07 m, black shale 0.15 m, 'rashes' 0.05 to 0.13 m, coal 0.18 m, grey shale 0.48 m, coal 0.31 m. Pebbly quartzite forms the roof of the coal. The ?Taldwyn crops out in Cwm Golan, and was mined from a level [SO 0895 0533]. The Gilfach was formerly worked to a small extent from crop levels in Cwm Bargoed, and a small area [SO 0992 0602] was dug in an opencast pit at Fochriw in 1973. A trial pit section [SO 1024 0618] illustrates the seam's development in this area: coal 0.18 m, shale 1.47 m, coal 0.30 m, fireclay 0.10 m, coal 0.35 m in 2 leaves. Both the Blackband and the Gilfach have high ash and sulphur contents. The ?Gilfach and a thin coal 7 m above were exposed on a house-building site [SO 114 060] at Pontlottyn in 1969. A higher coal, the 'Penwatingoch House Coal', was formerly worked there with an average section of: rashes 0.38 m, coal 0.25 m, coal 0.25 m, clod 0.18 m, coal 0.43 m. Quartzite forms the roof of this coal.

Rhymney–Sirhowy area

Sections 4 to 7, (Figure 33) and sections A, B and C, (Figure 34) are selected from this area. Nant Tyswg [SO 1350 0571] to [SO 1338 0713] ((Figure 34), section A) provides the best section of the Llynfi Beds.

The Blackband crops out in the River Rhymney at Pontlottyn [SO 1200 0586] where it is overlain by 2 m of a coarsening-upwards cycle of mudstones, rich in plant debris at their base. The cycle is truncated by a scour surface below 3 m of quartz conglomerate. The Blackband was exposed in the road bank in 1973 near Newport House, Rhymney [SO 1216 0626]. Only 0.3 m of plant-rich shale was present below the conglomerate, and the Blackband section was: coal with mudstone partings 0.18 m, mudstone seatearth 0.06 m, coal 0.12 m, mudstone seatearth 0.13 m, coal 0.3 m. The coal, its plant-rich shale roof, and the overlying quartzite are exposed in a waterfall at Tan-y-Bryn, Rhymney [SO 1223 0651]; the coal was formerly dug in shallow pits to the north. Its quartzite roof crops out over a wide area on Rhymney Hill, and at the head of Nant Tyswg; the beds above are exposed to the south in this stream ((Figure 34), section A).

The ?No. 3 Rhondda is exposed in the bank of Scotch Peter's reservoir [SO 1558 0893] ((Figure 34), section C). The underlying pebbly quartzite crops out in the reservoir. This quartzite and a higher one were formerly quarried to the north-west.

Ebbw valleys

Sections 8, 9 and 10, (Figure 33) are selected from this area.

Only 4 m of quartzite separates the top of the ?No. 3 Rhondda (Old Man's Coal) from the top of the Carnbriense cycle at Bwlch y Garn on the east side of Ebbw Vale. The coal was extensively mined on the sides of the Ebbw Fawr Valley, for example at Briery Hill [SO 1675 0835], at Woodland Terrace [SO 1767 0807], and at PenCoedcae Mine [SO 1866 0968]. An average section of the coal is: coal 0.38 m, clod and 'rashes' 0.61 m, coal 0.23 m.

In the Ebbw Fach Valley, boreholes at Rose Heyworth tip ((Figure 34), section ll) and Abertillery New Mine ((Figure 34), section E) provided sections of up to c.40 m of beds, but apart from the pebbly quartzite which overlies the Cambriense cycle, there are no markers to allow precise correlation between the two successions, although they are less than 1 km apart.

East Pentwyn Farm Borehole No. 1 [SO 2050 0755] proved 9 m of Llynfi Beds, including 2.3 m of black shale with ironstones. These shales yielded aff. Anthraconaia sp., Carbonita humilis, C. cf. salteriana, cf. Euestheria sp., cf. Rhadinichthys sp.and Rhabdoderma sp.and may correlate with the roof beds of the Blackband, as recorded in the Newport district (Squirrell and Downing, 1969, pp.181, 301). They rest on plant-rich mudstones in this borehole, from which Annularia aff. radiata, Asolanus camptotaenia (see p.75), Asterophyllites mis, Cyperites sp.and Sphenopteris sp.were collected.

Two thin coals crop out in Cwm Celyn; one, mined locally [SO 2038 0883], is probably the 0.18 m thick coal exposed in Nant Ystruth [SO 2080 0870]. Two thin higher coals were exposed in a trench [SO 2078 0878] where the section reads: coal 0.2 m, seatearth 0.9 m, mudstone 0.45 m, coal 0.1 m. There are exposures of higher beds in Nant Ystruth, for example ostracod-hearing mudstones [SO 2077 0901], above which green and red mottled mudstones lying towards the presumed top of the formation are seen.

East crop

Sections 11 and 12, (Figure 33) are from this area.

The Llynfi Beds crop out in a very narrow and poorly exposed belt down the east crop. Blaenserchan No. 2 Pit and Llanerch Shaft ((Figure 33), sections 11 and 12) provided old sections, but the placing of the base and top of the formation is uncertain. Blaenserchan Tip Borehole No. 1 [SO 2519 0239] ((Figure 34), section F) gave a recent section of most of the beds.

Rhondda Beds

The Rhondda Beds range in thickness from about 35 m in the south-east to 200 m in the south-west. The No. 2 Rhondda coal, which defines the base of the formation, is absent in the east of the district, making the precise position of the base conjectural.

Sandstones and mudstones are present in approximately equal proportions in the west, but sandstones predominate in the attenuated eastern succession. The coals that occur in the west generally fail eastwards, and correlation of those that are present in the east is uncertain.

The sandstones belong to two contrasting facies, Pennant subgreywacke and mature orthoquartzite. The Bargoed Taff Valley lies in the transitional area where the two facies interdigitate. Pennant sandstones, which are typical of the Rhondda Beds throughout most of the South Wales Coalfield to the west, wedge out in this area; orthoquartzitic sandstones, commonly pebbly and conglomeratic, appear in the Bargoecl Taff Valley, and are present to the east.

In the west the argillaceous rocks include the typical grey Coal Measures mudstones, siltstones and seatearths and red beds; they are almost entirely red beds in the east. The red beds, informally termed the 'Deli Beds' by Howell and Cox (1924) from their occurrence in Ogilvie Colliery shafts near Deri ((Figure 35), section 3), comprise a sequence of colour- mottled mudstones interbedded with the orthoquartzites. The colours include pale green, orange, purple, and brick red, and many of the beds are rooted and have other pedogenic features (see p.'74)

The No. 2 Rhondda is present in and west of the Ebbw Fawr Valley, but generally absent to the east. It is mainly thin and inferior, but is sufficiently thick to have been worked from crop levels in the Bargoed Taff Valley, and was being worked during the resurvey at Fochriw.

Thick Pennant sandstones (the 'Saron Sandstone') overlie the No. 2 Rhondda in the Bargoed Taff Valley. Pennant sandstones may also occupy much of the sequence between the No. 2 Rhondda and the No. 1 Rhondda at depth in the Bargoed Rhymney Valley. The sequence ranges from about 20 m in the Ebbw Fawr Valley to about 50 m at depth in the Bargoed Taff Valley. To the east of the Bargoed Rhymney Valley, it consists of interbedded quartzites and red beds.

The No. 1 Rhondda group consists of three mainly thin and inferior coals which are present in the west, but fail eastwards and are absent east of the Ebbw Fawr Valley. The coals lie within 30 m of beds in the west and about 10 m in the Sirhowy Valley. The two higher coals, the Fochriw and the Tyladu have been worked locally in the west, both from surface levels and in opencast pits, and are 0.6 to 0.7 m thick. The Tyladu was also worked at depth in a small area around New Tredegar pits. The Fochriw was being mined at Cae Glas Drift Mine, Fochriw during the resurvey. About 18 m of beds separate the Fochriw and the Tyladu. The roof beds of the Tyladu yielded Anthraconauta aff. phillipsii, Carbonita sp., and fish debris.

The No. 1 Rhondda Rider is present only in the Bargoed Taff Valley and at depth in the southern part of the Bargoed Rhymney Valley where it is thin and of no economic importance. The Tylacourt lies 30 to 40 m above the No. 1 Rhondda Rider. This coal is inferior, with numerous partings, but has been worked to a small extent from surface levels in the Bargoed Taff and Bargoed Rhymney valleys. It extends to the Sirhowy Valley, and is absent to the east.

About 40 m of beds separate the Tylacourt from the Brithdir; these include Pennant sandstones in the Bargoed Taff Valley, but to the east they consist of intercalated red beds and quartzites.

Details

West of the River Rhymney

Sections 1, 2 and 3, (Figure 35) are selected from this area.

The No. 2 Rhondda was formerly mined from levels in Cwm Bargoed e.g. [SO 0825 0499]; [SO 0905 0470] where the section is: conglomeratic sandstone 0.6 m, inferior coal with partings 0.33 m, seatearth 0.2 m, coal 0.48 m. A small area was dug in an opencast pit at Fochriw in 1973, and the coal was being mined at Cae Glas Drift Mine, Fochriw during the resurvey. An average section is: coal 0.23 m, clod 0.28 m, coal 0.23 m, 'rashes' 0.07 m, coal 0.15 m.

Mudstones form the immediate roof of the No. 2 Rhondda in the Bargoed Taff and Bargoed Rhymney valleys, but Pennant sandstones (the 'Saron Sandstone') occupy much of the overlying beds in the Bargoed Taff Valley and in the south of the Bargoed Rhymney Valley ((Figure 35), sections 1 and 2). Quartzitic sandstone and conglomerates form the roof on the west of the Rhymney Valley.

The No. 1 Rhondda group comprises three coals. The middle coal (Fochriw) and the topmost one (Tyladu) have been worked from surface levels and in opencast pits in this area. The Fochriw is 0.6 to 0.7 m thick and was being worked at Cae Glas Drift Mine, Fochriw during the resurvey. An average section of the sequence at Pontlottyn opencast site [SO 110 057] was coal (Tyladu) 0.61 m, measures 18 m, coal (Fochriw) 0.69 m.

Dark grey mudstones form the roof of the Tyladu seam, and spoil from levels at Pontlottyn e.g. [SO 1200 0532] yielded Anthraconauta aff. phillipsii, Carbonita sp., and fish fragments including Elonichthys sp.and Rhabdoderma sp.

The No. 1 Rhondda Rider is a thin coal in the Bargoed Taff Valley; it is absent in the Bargoed Rhymney Valley, although it appears in the south in the Grosfaen pits, immediately south of the district boundary.

The Tylacourt was formerly worked to a small extent at levels in Cwm Bargoed e.g. [SO 0910 0315] and at Plantation Level in the Bargoed Rhymney Valley; at the latter, it lies 42 m below the Brithdir and comprises 1.6 m of thin coals and intercalated shales.

Pennant sandstones overlie the coal at outcrop in Cwm Bargoed, whereas feature-forming quartzite and conglomerate are present in the Bargoed Rhymney Valley.

Rhymney–Sirhowy area

Sections 4, 5, 6 and 7, (Figure 35) are selected from this area.

The No. 2 Rhondda is about 0.5 to 0.6 m thick. Up to 1.5 m of mudstones lie in its roof at outcrop and are succeeded by pebbly quartzitic sandstones. It crops out in Nant Tyswg [SO 1350 0570] where it was formerly mined in Kilmarnock Level. The coal has not been recognised at outcrop in the Sirhowy Valley and is probably absent. Its inferred position within the Tredegar Trough (p.108) is at the base of a pebbly quartzite which was once quarried near St James' Hospital, Tredegar [SO 1508 0805]. The coal is present at depth further south in the Sirhowy Valley ((Figure 35), sections 6, 7).

The Fochriw and Tyladu seams crop out around Twyn Abertyswg in the Rhymney Valley, where both have been dug in shallow pits. The Fochriw has a quartzite roof which was formerly quarried [SO 1282 0609]. Dark grey mudstone in the roof of the Tyladu contains abundant Anthraconauta sp. The Tyladu ('Rock Vein') was worked in a small area from New Tredegar No. 3 and No. 4 pits where its section is: mine ground 0.1 m, 'clod' 0.1 m, coal 0.61 m, 'clod' 0.05 m, fireclay 0.61 m. Both the Fochriw and Tyladu were worked in crop levels in the Sirhowy Valley. The ?Fochriw was proved in Peacehaven Borehole No. 5 [SO 1544 0732] where the following was recorded: sandstone 2.4 tn, mudstone seatearth 0.25 m, coal, mainly inferior 0.1 m, mudstone seatearth 1.33 m, coal, inferior at top 0.3 m, quartzite 5.4 m.

The Tylacourt was 0.76 m thick, with dirt partings in Bedwellty Tip Borehole No. 2 [SO 1529 0582]. There are 37 m of measures between this coal and the Brithdir, consisting of sandstones, green siltstones and red mudstones. The topmost part of this sequence can be seen in a landslipped mass at New Tredegar [SO 1385 0442].

Ebbw valleys

Sections 8, 9 and 10, (Figure 35) are selected from this area.

The No. 2 Rhondda is represented by a thin coal on both flanks of the Ebbw Fawr Valley. It has variable thickness in the Marine Colliery shafts ((Figure 35), section 5), being 0.2 m in No. 2 shaft and 0.66 m in No. 1 shaft. It is absent further south at Six Bells and is also generally absent in the Ebbw Fach Valley, although trials [SO 1947 0799] at Blaina indicate its presence there.

The No. 1 Rhondda group is represented by two coals on the west flanks of the Ebbw Fawr Valley. A landslip backscar [SO 1657 0825] exposes the lower coal, 0.5 m thick, and the 3 m of beds between it and the higher coal. The higher coal is unexposed, but was worked by crop levels, and was termed the Ebbw Vale No. 2 seam by Blundell (1952). He recorded Anthraconauta phillipsii in spoil from the levels; A. aff. tenuis, Carbonita pungens. and fish fragments were collected during the resurvey. This fauna suggests correlation with the roof beds of the Tyladu seam of the Rhymney Valley. Blundell correlated the coal with Cwmdu No. 2 seam of the Taff Valley, which Woodland and Evans (1964) correlated as the No. 1 Rhondda. Blundell also recorded a coal 24 m above which he named the Ebbw Vale No. 1 seam, and correlated with the Cwmdu No. 1 seam of the Taff Valley. Woodland and Evans (1964) correlated the Cwmdu No. 1 seam with the No. 1 Rhondda Rider; however the Ebbw Vale No. 1 seam is probably the Tylacourt.

The No. 1 Rhondda group is represented on the eastern flanks of the Ebbw Fawr Valley by a coal (?Tyladu) formerly dug in levels north of Waunlwyd. This is probably the 0.5 m coal proved in Waunlwyd Tip Borehole No. 6 [SO 1811 0611]. It has a quartz grit roof which was formerly quarried at Waunlwyd.

Much of the Rhondda Beds succession in this area consists of pebbly course-grained quartzites and conglomerates; these form features, and are well exposed in the interfluves between the Sirhowy, the Ebbw Fawr and the Ebbw Fach valleys. Boreholes at Waunlwyd tip, Ebbw Vale provided detailed sections of the lower part of the formation, which consists mainly of khaki-green mudstone seatearths, interbedded quartzitic sandstones, and a few thin inferior coals and 'rashes'. The beds at the top of the formation are exposed in a disused quarry [SO 1683 0705] at Victoria, Ebbw Vale, and consist of 3.7 m of quartzite above 3.7 m of coloured argillaceous beds.

Brynmawr–Pontypool

The Rhondda Beds comprise an attenuated sequence of 30 to 40 m of red beds, quartzites and conglomerates. There are no coals in the area north of Pontypool, and the absence of the No. 2 Rhondda makes the precise positon of the base of the formation conjectural. The No. 2 Rhondda is present south of the Trevethin Fault. Squirrell and Downing (1969) correlated it with a coal known locally as the 'Soap Vein', but it is more likely that it is a thin coal lying 13 m above, and that the 'Soap Vein' is the No. 3 Rhondda.

Brithdir Beds

The Brithdir Beds extend from the base of the Brithdir coal to the base of the Cefn Glas coal. They range in thickness from 60 m in the south-east to 140 m in the west. The formation consists largely of Pennant sandstones which appear in major quantity for the first time at this level throughout most of the district (see p.74). Subordinate mudstones and seatearths underlie 'slacks' between the feature-forming sandstones.

The Brithdir (Tillery, Red Ash) is generally a good coal and has been extensively mined although it deteriorates on the east crop. Two small private mines were operating in the Bargoed Rhymney Valley during the resurvey. The coal is a single, 1 m thick seam west of the Ebbw Fawr Valley, but is in two leaves to the east separated by up to 7 m. Plant-rich mudstones typically form the roof of the coal, with Pennant sandstone close above. Floral assemblages indicative of Dix's Zone H were listed by Sullivan and Moore (1956, p.411). These authors also first recorded bivalves from the roof beds of the lower Brithdir in the Ebbw Fawr Valley (p.102), with species of Anthraconauta indicating a horizon at or near the boundary of the Phillipsii and Tenuis zones.

Up to 20 m of Pennant sandstones overlie the Brithdir, and are overlain by a mudstone sequence up to 12 m thick at the top of which lies the Brithdir Rider coal. These mudstones and the coal are absent east of the Ebbw Fach Valley. The Brithdir Rider is a thin coal of poor quality and no economic importance. Neuropteris flexuosa, characteristic of Dix's Zone H, occurs in its roof mudstones.

About 25 m of Pennant sandstones overlie the Brithdir Rider. These are succeeded by up to 25 m of mudstones which crop out in a 'slack' that can be traced throughout the district, apart from in a 500 m-wide 'washout' zone trending south-east from Mynydd James to Twyn Grwyd, and in a smaller area on Cefn Coch (p.102). These mudstones contain a number of thin, inferior, and impersistent coals, named the Cefn Glas group (Woodland, 1969); plant debris is abundant in the mudstones, and bivalves are locally present (p.102).

Details

West of the River Rhymney

(Figure 36), section 1 is selected from this area.

Details of the Brithdir in the Bargoed Taff Valley, including a floral assemblage from its roof in Bedlinog Drift [SO 100 019] where the coal is about 1.5 m thick with some thin partings, were given by Squirrell and Downing (1969, p.194). The coal is about 1.3 m thick in the Bargoed Rhymney Valley, where it was being worked in three levels during the resurvey [SO 1023 0493]; [SO 1124 0497]; [SO 1090 0406].

About 20 m of Pennant sandstones overlie the Brithdir, above which is a thin, impersistent mudstone bed with the Brithdir Rider at its top; this coal is thin and inferior. The succeeding Pennant sandstones, about 60 m thick, are overlain by up to 25 m of argillaceous beds underlying the Cefn Glas. A coal within these mudstones and 7 m below the Cefn Glas was explored by trial pits on Gelligaer Common where is was up to 0.75 m thick and said to be of good quality. The mudstones were exposed in a gas pipeline trench to the east of the Dowlais Fault on Cam y Bugail [SO 1022 0380]. They are 9.35 m thick and contain seven thin inferior coals including the Cefn Glas at their top. They thicken on the east side of the Bargoed Rhymney Valley; Ogilvie Tip Extension Borehole No. 2 [SO 1191 0402], starting about 5 m below their top proved 19.1 m of mudstones, seatearths and thin coals ((Figure 36), section I a).

Rhymney–Sirhowy area

(Figure 36), sections 2, 3 and 4 are from this area.

The Brithdir averages about 0.9 m in the Rhymney Valley, 0.6 to 0.7 m in the Sirhowy Valley, and has been much worked. Good cliff sections of the Brithdir Beds are found in landslip backscars in the Rhymney Valley at Troed-rhiw'r-fuwch [SO 128 045] and New 'l'redegar [SO 139 046] ((Figure 36), section 2). At the latter, the Brithdir Rider consists of 0.6 to 0.8 m of inferior coal. There are also good quarry sections of the Brithdir Beds in the Sirhowy Valley at Bedwellty [SO 1520 0610] and at Troedrhiwgwair [SO 1571 0750] ((Figure 36), section 3). The Brithdir Rider and associated mudstones lie near the top of the Bedwellty quarry, the coal section being: inferior coal and rashes 0.48 m seatearth 0.3 m, inferior coal 0.07 m.

The mudstones below the Cefn Glas give a prominent 'slack' throughout this area. A coal lying near their base was worked under Mynydd Bedwellty [SO 1445 0536] where an average section is: coal 0.1 m, mudstone 0.1 m, coal 0.5 m. Spoil from the workings yielded a Westphalian D flora of Neuropteris scheuchzeri, Sphenophyllum emarginaturn, and Pecopteris miltoni (p.75). Borcholes at Markham Colliery tip proved the mudstones to be up to 6.4 m thick, with the Cefn Glas at their top and a thin coal 3.5 m below it.

Ebbw valleys

Sections 5 to 9, (Figure 36) are from this area.

The Brithdir is a single coal on the west side of the Ebbw Fawr Valley, about 0.6 m thick at outcrop above Victoria, Ebbw Vale. Two leaves are present on the east side of the valley, separated by 4.8 m at Red Ash levels [SO 1830 0848]. The top leaf, about 0.55 m thick, was worked. Anthraconauta phillipsii, A. cf. phillipsii, and A. aff. tenuis were recorded in spoil from an adit to the south c. [SO 1867 0780] (Sullivan and Moore, 1956, p.413). Spoil from levels on the lower leaf further south in Cwrn Merddog [SO 1885 0690] contains plant debris including Mariopteris cf. nervosa, Neuropteris cf. ravinervis, Pecopteris miltoni, and P. cf. oreopteridea. Only one leaf is present at depth in the Ebbw Fawr Valley, 0.7 m thick at Craig Fawr Red Ash Colliery [SO 1910 0327]. Two leaves are present at outcrop on the west side of the Ebbw Fach Valley, the section at West Blaina Red Ash levels [SO 1932 0800] being: coal 0.58 m, 'rock' 1.2 to 3 m, 'clife 0.61 to 1.52 m, coal, 0.71 m. Black canneloid mudstone debris in spoil from these levels, probably from the roof of the lower leaf, is rich in bivalves, with ostracods and fish fragments. Sullivan and Moore (1956) listed the following: Anthraconauta phillipsii, A. cf. phillipsii and A. aff. tenuis. In addition, A. phillipsii inter tenuis, Carbonita evelinae and C. humilis were collected during the resurvey. A similar fauna was recorded by Sullivan and Moore from a tip further north c. [SO 1897 0868], along with a large floral assemblage dominated by neuropterids and representative of Floral Zone H. Between 1.75 m and 2.4 m of measures separate the leaves of the Brithdir southwards to Rhiw Colbren levels [SO 208 047] near Abertillery, but only one leaf is present at outcrop further south and in workings from Aberbeeg North Colliery, where it is 0.61 to 1.22 m thick (Squirrell and Downing, 1969, p.199). On the east side of the Ebbw Fach Valley two leaves are separated by up to 5 m of beds. The lower is the thicker and has been extensively worked; its section at Llanerch Padarn Red Ash Level [SO 2165 0654] is coal 0.2 to 0.38 m, stone 0.02 to 0.25 m, coal 0.25 to 0.38 m, fireclay 0.1 to 0.25 m, coal 0.53 to 0.68 m.

The Brithdir Beds have a narrow terraced outcrop on the flanks of the Ebbw Fawr and Ebbw Fach valleys. The sandstones form bold steep slopes, and the mudstones underlying the Brithdir Rider and Cefn Glas crop out in the intervening 'slacks' or benches. Many boreholes were drilled through the formation on Marine Colliery tip; Borehole MN 3 [SO 1826 0434] is shown in (Figure 36), section 5.

Pennant sandstones, about 16 m thick, overlie the Brithdir on the west side of the Ebbw Fawr Valley and also on the east side of the valley near Cwm, but mudstones overlie the coal in Cwrn Merddog and on the west of the Ebbw Fach Valley.

The argillaceous beds underlying the Brithdir Rider are 11 to 15 m thick on the west side of the Ebbw Fawr Valley, but thin eastwards and are absent in the Ebbw Fach. They are exposed at Cwm [SO 1883 0480] where they consist of silty mudstones and mudstone seatearth with several thin inferior coals. Neuropteris flexuosa was obtained, and indicates a Zone H age. In the Ebbw Fawr Valley, the Brithdir Rider is a thin inferior coal at the top of the mudstones; it is absent in the Ebbw Fach Valley.

About 25 m of Pennant sandstones, conglomeratic at their base, overlie the Brithdir Rider. Where this coal and the underlying mudstones are absent, the beds between the Brithdir and the mudstones below the Cefn Glas consist mainly of Pennant sandstone up to 30 m thick in the west, but thinning to about 20 m on the east side of the Ebbw Fawr Valley.

The mudstones underlying the Cefn Glas range from 8 to 15 m in thickness in the Ebbw Fawr Valley. They are 8 m thick in the Ebbw Fach, but are locally absent in a 400 m wide channel belt extending from Blaina south-east to Twyn Grwyd. The mudstones are completely exposed in a disused quarry [SO 1694 0630] at Victoria, Ebbw Vale and the topmost part is exposed in a gully [SO 1878 0358] above Marine Colliery where it consists of a thin dirty coal below 4 m of mudstone seatearth. An exposure [SO 1899 0827] near the base of the mudstones at Llanerch-y-pant, Blaina yielded Anthraconauta aff. phillipsii showing some tendency towards A. tenuis.

Brynmawr -Pontypool area

Sections 10 and 11, (Figure 36) are from this area. The Brithdir Beds are about 60 m thick, and consist almost entirely of Pennant sandstones, apart from 6 to 7 m of mudstones underlying the Cefn Glas.

The best exposures of the beds are in the backscar of Pant-gas landslip [SO 2514 0315] and in British quarry [SO 2510 0358].

The Brithdir had the following average section at Blaentillery Drift Mine [SO 2270 0820], Blaenavon: coal 0.2 m, stone 0.07 m, coal 0.23 m, clod 0.07 m, coal 0.61 m. A similar section was present in workings to the south under Mynydd Varteg, Twyn Du and Waen Wen. The coal is 0.7 m thick at Blaenserchan Colliery; a large north-west trending washout affects the seam under Mynydd Llanhilleth.

Pennant sandstones form the roof of the Brithdir over most of this area, but argillaceous beds overlie the coal in the Cwm Sychan–Llanerch area. These are 2.7 m thick in Blaenserchan No. 2 Pit [SO 2437 0210], and consist of mudstone with thin coals and coaly shales. About 55 m of Pennant sandstones overlie the Brithdir. The succeeding mudstones which underlie the Cefn Glas are exposed in the Pant-gas landslip backscar [SO 2514 0315], and the topmost 2.7 m, consisting of thin shaly coals and seatearths, are exposed in the south-west corner of British quarry [SO 2510 0358].

Hughes Beds

The Hughes Beds (Figure 32) extend from the base of the Cefn Glas coal, known locally as the Tillery Rider, to the base of the Mynyddislwyn seam. Their outcrop covers much of the Pennant sandstone plateau, and is the largest of the Coal Measures formations. About 150 m of beds are present, their thickness being almost constant across the district, although some northward attenuation is indicated by comparison with a thickness of about 200 m in the Newport district to the south. Pennant sandstones dominate the formation, with only thin and impersistent mudstones. The Cefn Glas coal is thin, dirty and impersistent.

Details

West of the River Rhymney

The section of the Cefn Glas in a gas pipeline on Cam y Bugail (p.100) was: Pennant sandstone on silty mudstone 0.4 m, coal 0.03 m, plant-rich mudstone 0.5 m, coal 0.15 m, 'rashes' 0.05 m, clay 0.05 m, coal 0.12 m. It was sufficiently thick on the east side of the Bargoed Rhymney Valley at Deri to have been mined from crop levels [SO 1282 0230].

A thin mudstone that lies 60 m above the Cefn Glas within the Dowlais Trough (p.107) on Gelligaer Common [SO 099 035] may equate with the Darren ddu seam of the Taff Valley; it is absent to the east.

The complete Hughes Beds sequence is present only within the Dowlais Trough, where at the top of the formation, c.4 m of mudstones underlie the Mynyddislwyn.

Rhymney -Sirhowy area

The Cefn Glas coal is proved only in the south of the Sirhowy Valley, where boreholes at Markham Colliery tip showed it to be 0.4 to 0.5 m thick, with shale laminae near the top.

The Cefn Glas is overlain by intraformational conglomerate with coal, mudstone, and ironstone clasts. The conglomerate lies at the base of a sequence of about 130 m of Pennant sandstones. A disused quarry at Markham Colliery [SO 1675 0215] provides the best section of these sandstones. The whole Hughes Beds sequence is present only within the Tredegar Trough (p.108) on Cefn Manmoel and on Mynydd Pen-y-fan.

Ebbw valleys

The Cefn Glas crops out at the top of the prominent 'slack' formed by the mudstones at the top of the Brithdir Beds, and is mainly a thin dirty coal. On the west side of the Ebbw Fawr Valley, it is 0.12 m in a disused quarry [SO 1694 0630] at Victoria, Ebbw Vale; further south, it was absent in some of the Marine Colliery tip boreholes, its thickest development being in Borehole 15 [SO 1785 0474]: dirty coal 0.5, 'rashes' 0.1 m, coal 0.22 m. It is exposed in a gully south of the tip [SO 1878 0358] (p.102), where its section is: dirty coal 0.15 m, parting 0.05 m, coal 0.03 m. On the east side of the Ebbw Fawr Valley, the coal is thick enough to have been worked from crop levels in Cwm Merddog. A small exposure in the roof of the worked coal [SO 1912 0599] shows: Pennant sandstone on dirty coal 0.15 m, clay 0.18 m, coal 0.07 m, clay 0.1 m. seatearth 0.35. Boreholes at Six Bells Warm Turn tip e.g. [SO 2129 0249] showed the Cefn Glas to be impersistent, ranging from 0 to 0.2 m. It is exposed in the backscar of Darren Ddu landslip on the west side of the Ebbw Each Valley (p.117) where it is 0.35 m thick. On the east side of the valley, its section in the Blaina landslip backscar is: 'rashes' 0.16 m, mudstone seatearth 0.73 m, 'rashes' 0.36 m.

A few thin impersistent mudstones occur within the Pennant sandstone-dominated sequence above the Cefn Glas. One such mudstone, lying 30 m above the Cefn Glas, is exposed in a subsidence fissure [SO 2260 0592] on Twyn Pentre where 0.26 m of green silty mudstones overlie 1.06 m of black shales with Anthraconauta aff. phillipsii, Spirorbis sp.and an abundance of ostracods including Carbonita cf. agnes, C. cf. pungens, C. salteriana, and C. cf. scalpellus.

Brynmawr-Pontypool area

The Hughes Beds form outliers on Cefn Coch, Coity Mountain, and also form the plateau southwards from Twyn Du. Pant-glas landslip backscar [SO 2514 0315] provides the best section of the beds in this area.

The Cefn Glas is a thin, inferior coal. Its section in Blaenserchan No. 1 Pit [SO 2437 0210] is: coal 0.3 m, 'rashes' 0.15 tn. At Pantglas landslip, 0.17 m of coal overlie 0.14 m of 'rashes', and the coal is succeeded by about 52 m of Pennant sandstones.

Grovesend Beds

The Grovesend Beds (Figure 32) are confined to a narrow down-faulted block in the Dowlais Trough, and larger outcrops on Mynydd Pen-y-fan and Mynydd Llanhilleth. Most of the outcrop on Mynydd Llanhilleth that lies within the district boundary has been stripped for opencast mining of the Mynyddislwyn seam.

The Mynyddislwyn is a good coal throughout, with two leaves separated by a parting that ranges from 0.11 m on Gelligaer Common to 0.68 m on Mynydd Llanhilleth. The combined thicknesses of the leaves range from 1.6 to 2.1 m.

The immediate roof mudstones of the Mynyddislwyn are characterised by large numbers of Leaia, as well as Anthraconauta phillipsii-tenuis, A. tenuis, and ostracods. These roof beds lie at the base of about 25 m of mudstones on Gelligaer Common, but only 5 m are present on Mynydd Pen-y-fan. Up to 3.4 m of mudstones overlie the Mynyddislwyn on Mynydd Llanhilleth, but over much of the outcrop area, Pennant sandstone forms the roof of the coal.

The Small Rider lies about 30 m above the Mynyddislwyn, most of the intervening beds in the east being Pennant sandstone. It is 0.3 to 0.4 m thick, and lies within 5 m of mudstones. A thin, inferior coal lies at the top of these mudstones and is overlain by about 30 m of Pennant sandstones. A sequence of 20 m of mudstones above, capped by the Big Rider, is preserved only on Mynydd Pen-y-fan.

The Big Rider has the following section on Mynydd Peny-fan: coal, 0.61 m, clod 0.15 m, coal 0.76 m (Squirrell and Downing, 1969, p.212). The Pennant sandstones forming its roof are the youngest strata of the district.

Details

The Mynyddislwyn was formerly worked in a level [SO 1109 0184] on Gelligaer Common, where Squirrell and Downing (1969, p.224) noted that the coal was overthrust to the south by ice action. The coal section is: shale with Anthraconaia williamsoni, Leaia leidyi and plant fragments, coal 1.43 m, 'rashes' 0.11 m, coal 0.46 m (Robertson, 1927, p.101). On Cefn Manmoel [SO 176 044], the coal was formerly worked from outcrop, but is much affected by small-scale faulting. Its outcrop on Mynydd Pen-y-fan is marked by a line of crown holes above extensive crop workings. An average section of the seam in the workings, and in exploratory boreholes is: coal 0.6 to 0.9 m, parting 0.45 m, coal 0.6 to 0.9 m. The seam has been largely removed by opencast mining on Mynydd Llanhilleth where a typical section is: coal 1.2 m, seatearth 0.68 m, coal 0.79 m.

Anthraconauta aff. phillipsii, A. phillipsii-tenuis, A. tenuis, A. aff. tenuis and Carbonita sp.were collected from tipped roof mudstones of the Mynyddislwyn on Mynydd Pen-y-fan [SO 1842 0243]; [SO 1907 0313]. The only sections of the beds above the Mynyddislwyn are to be found in the backwall of the opencast workings on Mynydd Llanhilleth e.g. [SO 2370 0133]:

Chapter 7 Structure

The main structural elements of the district are shown in (Figure 37). The structure of the coalfield area, as illustrated by the structure contours in the Nine-Feet Seam, is shown in (Figure 38). Three major tectonic cycles, Caledonian, Variscan and Alpine, are represented in the sedimentation and deformation of the exposed rocks of the district.

The Caledonian cycle is represented initially by shallow marine shelf sedimentation on the south-east margin of the Lower Palaeozoic Welsh Basin. The early stages of the orogenic phase had little tectonic effect within the district, but caused the facies transition from the Ludlow and early Prídolí marine sediments to the later Prídolí and Devonian continental red-bed sequence for which the newly created Caledonian mountain belt to the north provided a source. The culmination of the Caledonian orogeny in the middle Devonian resulted in uplift and erosion, as shown by the unconformity between the Upper and Lower Old Red Sandstone. This unconformity may increase south-eastwards towards the Usk Axis; westward overstep towards the axis in the Monmouth district (Allen, 1974) provides further evidence of early movement on this structure.

The Variscan tectonic cycle began in Upper Devonian times, extension and rifting producing the Carboniferous basin in the foreland of the Variscan belt. Two major structural highs exerted control on the subsidence and sedimentation of the basin until late in the Westphalian, St George's Land in the north, and the Usk Axis in the east. Unconformities, non-sequences, variations in thickness and facies within the Carboniferous rocks reflect the interplay of these basements structures. In the Dinantian carbonate sequence unconformities and facies variation are mainly due to the proximity of St George's Land, the Neath Disturbance perhaps acting as a structural hinge zone that controlled subsidence and uplift (George, 1954). Earth movements occurring from late Dinantian to Namurian times led to reactivation of the Usk Axis, folding, uplift and erosion, as shown by the unconformable overstep of the Dinantian rocks by the late Namurian sequence. Minor east–west thrusting in the Dinantian rocks (Roberts, 1972) may also have taken place then. The pulsed expansion of the Namurian basin in the Marsdenian (p.66) produced a typical basin margin onlap situation with 'Steer's Head' geometry (Dewey, 1982; George, 1970, fig. 24).

Within the Lower and Middle Coal Measures the eastward attenuation and facies changes clearly demonstrate the continuing presence of the Usk Axis, which formed the eastern margin of the basin for much of Westphalian A to C times. Subsidence of the basin to the west of the axis may have been augmented by minor growth faulting along deep-seated north-west-trending fractures (p.91; Robertson, 1927) which were reactivated during late Variscan basin inversion as extensional faults (see p.106).

At or about the Westphalian C–D boundary active uplift and subaerial exposure of the Usk Axis produced the red bed facies and local unconformities in the basal Upper Coal Measures (p.95). Coeval with this uplift, the northward-advancing Variscan foreland was producing a fundamental change in palaeogeography and sedimentation, with the Variscan mountain belt shedding large quantities of alluvial sediment (the Pennant Measures) northwards into a rapidly subsiding trough. A land mass in the Bristol Channel, which supplied small amounts of sediment intermittently throughout the Westphalian, has been suggested as the source (Kelling, 1974). However, the presence of late Variscan dextral transcurrent faulting in the Bristol Channel has been postulated (e.g. Higgs, 1986; Holder and Leveridge, 1986), so that the source of the Pennant sediments may now lie a considerable distance to the west.

Eastward attenuation of the Upper Coal Measures, whether by internal thinning or unconformity (see p.96), shows that the Usk Axis was still active until late in Westphalian D times. Late Variscan inversion and compression of the Carboniferous sediments against the stable block of St George's Land accentuated the broad east–west depositional sagging and produced the main coalfield syncline. The main effects of this compression are seen in the south and west of the coalfield, the Abergavenny district remaining relatively undisturbed. Squirrell and Downing (1969, p.249) suggested that this was due to the district's distance from Pembrokeshire, the main area of impact of the north-driven sediment wedge on the north-east-trending 'Caledonoid' foreland. Other factors may have been (1) that the district was a shadow region protected from the collision by the Usk Axis (Squirrell and Downing, 1969, p.251), and (2) that the north-east part of the coalfield may itself have acted as a relatively rigid block of competent strata, as shales are confined to a 'sandwich-filling' between sandstone-dominated successions, in contrast to the thicker mudstonedominated sequences to the south. A further possibility (Williams and Chapman, 1986) is that a decollement in the Lower Palaeozoic basement resolved much of the Variscan stress in the district, although there is, as yet, no evidence of such a structure. The amount of accommodation by bedding plane slip is difficult to gauge; although the movement may have been small for individual planes, most seatearths are slickensided, and many coals have slickensided layers ('rashings'), so the cumulative effect of these small movements may have been considerable. Minor east–west thrusts in the Dinantian rocks (Roberts, 1972) may also have contributed.

The cross-faults of the coalfield (p.107) were formed during an extensional phase that followed the main compressive phase of the Variscan orogeny (Squirrell and Downing, 1969, p.251). Some may have formed by reactivation of minor Westphalian growth faults. They are largely tensional fractures that define narrow grabens best developed in the Upper Coal Measures and there is now no evidence of any earlier transcurrent movement along them (Anderson, 1951; Robertson, 1927; Squirrell and Downing, 1969).

The Neath Disturbance (p.108), like the Cribarth (Swansea Valley or Tawe) and Carreg Cennen disturbances, is a major 'Caledonoid' Variscan compression zone that overlies a crustal fracture with a prolonged hisuvry of movement and reactivation (Owen and Weaver, 1983). Owen (1954) proposed that late-stage Variscan sinistral wrenching occurred along it, but there is no convincing evidence for such movement, either in the present district or in the adjoining Merthyr Tydfil area (Barclay and others, 1988).

The effects of the Alpine orogeny within the district are difficult to assess, although the present topography is generally held to have been initiated on a surface that was uplifted and tilted during this period (Clarke, 1936; George, 1980). Weaver (1975) suggested that the Neath Disturbance may have been reactivated, but George (1980) discounted any Neogene vertical movement. Squirrell and Downing (1969, p.251) considered the possibility that the coalfield cross-faults were the product of Alpine tensional stress, but concluded that most of the movement took place in the Variscan.

Structures affecting the Silurian rocks

Folding

The Usk Anticline is the principal fold, bringing the Wenlock Series to the surface in its core. Initial folding was in the Middle Devonian (Allen, 1974). Further folding took place from the late Dinantian to early Namurian (p.64; George, 1956b), and the structure is likely to have been intensified during the final phase of the Variscan orogeny. In the Usk area, Walmsley (1959) named the structure, the Coed-y-paen Pericline. The axis of the pericline trends due north, and plunges increasingly steeply northwards. The fold has a broad axial region, and dips on the anticline limbs generally average about 15°, ranging from 4° to 22° on the eastern limb and 4° to 43° on the western one. On the latter, dips are steepest in the structurally complicated ground adjacent to the Little Mill Fault. Northwards, across the Pontypool Road and Little Mill faults, the position of the axis of the anticline is unknown in the poorly exposed ground, but it may be displaced sinistrally by up to 1 km by these faults. Further north, the axis trends to the north-north-east towards Pant Brook [SO 357 128] where its position is shown by the Psammosteus Limestone outcrop. A postulated north-east-trending fault, the Llanvapley Fault, appears to offset the axis dextrally.

A small subsidiary north–south syncline is postulated on the western limb of the Usk Anticline north of Little Mill. It lies between the Little Mill Fault and the Pontypool Road Fault, has Raglan Mudstone in its core, and a small fault-bounded inlier of easterly-dipping Ludlow Series beds on its western limb 500 m north of Little Mill.

Faulting

The Pontypool Road Fault is the northwards continuation of the Pontypool Monocline (Squirrell and Downing, 1969, pp.242, 251). It forms the north-western boundary of the Usk Inlier. Its north-east trace cuts the axis of the Usk Anticline and its continuation in the Monmouth district is known as the Llantilio Fault (Welch and Trotter, 1961, p.13). The amount of vertical westerly downthrow on the fault is unknown, as there are no marker horizons within the Raglan Mudstone; Walmsley (1959, p.503) estimated a figure of up to 365 m. Welch and Trotter (1961, p.13) implied sinistral wrench of about 3 km along this structure in the Llantilio Crossenny area, and there may be up to 200 m of sinistral wrench movement within the present district.

The Little Mill Fault is the northwards continuation of the Pen-y-llan Fault of the Newport district (Squirrell and Downing, 1969, p.239). It throws down west and may have up to 600 m of sinistral strike-slip movement. Its vertical displacement is 380 m near Chain Bridge [SO 3480 0567] and at least 280 m near Little Mill. A north–south splinter fault [SO 3266 0235] has a calculated throw of 210 m. The main fault splays into two north-eastwards near Little Mill. The two splays converge 1 km north-west of Bettws Newydd, and the fault joins the Pontypool Road Fault 1 km to the north.

The Llanbadoc Fault has been extended northwards from its position as mapped by Walmsley (1959) to reconcile the revised mapping of the Wenlock Series south-west of Bettws Newydd, and to link with an ill-defined fault, the Bettws Newydd Fault, in the northern part of the inlier. The throw of the Llanbadoc Fault in the Trostrey area is down east a minimum of 150 m.

Other important faults are the Clytha Fault (Walmsley, 1959) in the north of the inlier, and the Cefn Ila and the Rhadyr faults, which extend into the district from the Newport district (Squirrell and Downing, 1969). Several cross-faults have been mapped, their positions being marked by dry valleys.

Structures affecting the Devonian and Carboniferous rocks

The structure of the Devonian and Carboniferous rocks is relatively simple over most of the district, apart from in the Devonian rocks in the north-east along the axis of the Usk Anticline, and in a narrow belt along the Neath Disturbance in the north-west.

Folding

The broad disposition of the rocks, their strike swinging from north-east–south-west in the west of the district to north-north-west–south-south-east in the east (Figure 37), reflects their position on the northern limb of the South Wales Syncline, with the westerly-dipping beds in the east lying on the flank of the Usk Anticline.

Superimposed on this overall structure at depth in the coalfield are a number of narrow north-west-trending folds formed concurrently with the cross-fault troughs. The eastnorth-east-trending Llanhilleth Syncline occurs to the south of the Trevethin Fault.

Faulting

The major faults fall into two groups: north-west-trending cross-faults and north-east-trending 'Caledonoid' faults.

Cross-faults

The principal faults of the coalfield are a suite of north-west-trending normal faults. These were named 'cross-faults' or 'dip faults' by Woodland and Evans (1964, p.239) in the Pontypridd district where they are aligned approximately parallel to the regional dip and perpendicular to the major fold axes. The term 'cross-faults' was extended to the Newport district (Squirrell and Downing, 1969, p.243), and is used in this account, although the faults are more parallel to the regional strike in the east. They occur in pairs that define narrow grabens at the surface. The eastern faults of each pair are generally synthetic and truncate the western (antithetic) faults at depth. Many parallel smaller antithetic, synthetic and en-échelon fractures occur, and the main master joint sets have the same trend (Roberts, 1966). Mining has led to reactivation of the main faults by release of stress, producing scarps and fissures in the competent Pennant sandstones.

The Rhymney Fault and an associated drag fold affect the Namurian and basal Westphalian rocks in Nant Pitwellt [SO 0900 1137] (Evans, 1971). This fault may extend southeastwards and link with an unnamed fault [SO 105 080] in the Lower and Middle Coal Measures at Bute Town.

The Rhos and Dowlais faults define a graben, named the Dowlais Trough, which traverses the south-west of the district. Mining subsidence scarp fissures mark the surface positions of these faults, particularly the Dowlais Fault on Cam y Bugail (Plate 14). An en-échelon fracture takes up the movement on Gelligaer Common to the south-east. The Rhos Fault has a throw of 119 m in the Brithdir, and 91 m in the Seven-Feet. The Dowlais Fault throws the Brithdir 100 m, and the Seven-Feet 82 m. Where seen underground immediately south of the district [SO 1148 0218], the Dowlais Fault was marked by a 37 m wide shear zone of steeply dipping disturbed strata (Barclay, 1971). A subsidiary fault within the Dowlais Trough, antithetic to the Rhos Fault, faults down a narrow block of Grovesend Beds on Gelligaer Common.

The Tredegar Trough is bounded on the west by the Tredegar Fault and on the east by the Sirhowy Fault. The trough extends from Mynydd Pen-y-fan on the southern margins of the district north-west to Tredegar and on to the Namurian and Dinantian terrain near Trefil. On Mynydd Pen-y-fan, the Tredegar Fault, known also as the Transport Fault, is replaced by an unnamed en-échelon fault 200 m to the west. The Sirhowy Fault, known as the Pen-y-fan Fault on and to the south-east of Mynydd Pen-y-fan, is the bigger fault, and truncates the Tredegar Fault close under the level of the Brithdir. North-west of Mynydd Pen-y-fan, from Manmoel to Troedrhiwgwair, the trough is marked by subsidence scarp fissures. The Sirhowy and Tredegar faults throw down the Brithdir coal about 64 m and 18 m respectively.

Two north-trending faults affect the rocks in the Aberbeeg–Llanhilleth area. The Aberbeeg Fault in the west has a throw of 26 to 34 m down east in the Brithdir at Aberbeeg [SO 206 018], but splinters and dies out 2.2 km to the north. The Llanhilleth Fault throws down west, and has a maximum throw east of Aberbeeg where the Brithdir and Nine-Feet seams are displaced by 33 to 36 m. This fault also dies out northwards, and appears to be truncated by the north-north-west-trending Abertillery Fault. This fault throws down the Nine-Feet Seam 33 m to the east at Abertillery, but dies out 1.5 km to the north.

A narrow graben affecting the lowest Westphalian rocks has been mapped north-west of Beaufort. The western fault is here named the Rassau Fault, and the eastern one the Llangynidr Fault. Steep westerly dips in Namurian grit exposed at low water level in the eastern corner of Llangynidr Reservoir [SO 1537 1410] mark the surface position of the latter.

The Brynmawr Trough is defined by the Brynmawr Fault to the west and the Clydach Bridge Fault to the east. The trough extends from Cefn Onneu in the north to Blaina, where the Brynmawr Fault splinters, swings to the south-south-east, and may connect with the Abertillery Fault. The Clydach Bridge Fault continues south-east from Blaina and extends to the east crop where it dies out about 1 km north of Pontypool. The Brynmawr Fault has a throw of over 40 m in the Middle Coal Measures near Brynmawr, but it diminishes to the south, and at depth; for example, the FiveFeet-Gellideg is displaced by 21 m to the east of Blaina. The Clydach Bridge Fault was exposed at Blaenant opencast site [SO 2010 1155] where it was a single fracture dipping 50° to 55° to the west and throwing down the Upper Four-Feet Seam by 14 m. The fault plane steepens southwards and its throw increases to 62 m in the Five-Feet-Gellideg beneath Mynydd James [SO 218 083]. The Greenland Fault is parallel and complementary to the Clydach Bridge Fault. It comprises two step faults at the surface, and a tight anticlinal drag fold occurs at depth near its northern termination on Twyn Gwryd. It extends from there south-east to Pontypool where it is truncated by the Trevethin Fault. It is exposed in the backscar of Pant-glas Slip [SO 0046 0291] and in a cliff near Llanerch Colliery [SO 2579 0248]. At the former, the fault plane, showing vertical slickensides, dips 62° to the southwest and displaces sandstones of the Hughes Beds; at the latter, it dips more steeply, and juxtaposes Brithdir Beds against Rhondda Beds.

The Blaenavon Trough extends southwards from the Clydach Valley where it is 200 m wide, its width increasing to 1.2 km at Blaenavon. The Blaenavon Fault, which is the western hounding fault and the more dominant, swings southwards over Coity Mountain and is truncated by the Clydach Bridge Fault. It has a throw of 75 m in the Bottom Vein Mine north of Garn-yr-erw, but this decreases to about 45 m at Blaenavon and to 20 m near its southern termination. The eastern fault, the Carreg Maen Taro Fault, can be traced as far as the outcrop of the topmost Dinantian rocks south-east of Blaenavon. It can be located in the backwall of the abandoned Blaen Pig opencast site [SO 2370 1146] where it has a throw of 12 m. The north-north-west-trending Nant Dyar Fault splays from this fault 400 m to the north-west.

Caledonoid faults

Neath Disturbance

The Neath Disturbance, the principal 'Caledonoid' (eastnorth-east-trending) structure, crosses the north-western part of the district. The disturbance is a complex zone of east-north-east-trending faults and upright folds between Neath and Bryniau Gleision [SO 085 157] (Barclay and others, 1988; Owen, 1954; Owen and Weaver, 1983). The main element of the disturbance in the latter area is the Dinas Fault, which is displaced dextrally by the Tredegar Fault close to the west of the district boundary, and has a northerly downthrow ranging from 15 to 36 m (Owen, 1954, p.348). Owen showed it to continue north-eastwards toward Crickhowell, but its vertical displacement at Darren Fawr [SO 090 160] is small and there appears to be no northerly downthrow of the rocks immediately to the east, otherwise Upper Devonian rocks would cap Tor y Foel [SO 116 194]. The fault is therefore shown on the 1:50 000 map as terminating immediately east of Darren Fawr.

From Blaen Dyffryn Crawnon [SO 105 107] to Blaen Onneu [SO 168 170], the main element of the disturbance is the Blaen Onneu Fault, which has a southerly downthrow. This fault has a complex relationship with a series of cross-faults, being displaced by some and apparently truncating others; this would suggest that the cross-faults and the Blaen Onneu Fault are part of a set of conjugate shears and are at least partly contemporaneous. The Blaen Onneu Fault appears to die east of Blaen Onneu, and is replaced to the north-east by an en-échelon northerly downthrowing fault, here named the Coedycerrig Fault.

The Coedycerrig Fault is located at Pregge Mill, Crickhowell [SO 2164 1931] where disturbed steeply dipping sandstones of the Senni Beds crop out (Taylor, 1974). The northerly downthrow on the fault in this area is estimated, by comparison of levels of the Carboniferous and Upper Devonian rocks on Pen Cerrig-calch (north of the district boundary) with those on the north crop and on the Sugar Loaf, to be about 200 m (Owen, 1954, p.348).

To the north-east of Crickhowell, the fault follows the valley of the Grwyne Fawr, and has a throw of about 150 m.

Beyond the district boundary near Forest Coal Pit, it continues north-east in Cwm Coedycerrig.

Trevethin Fault

Within the district, the Trevethin Fault is the only Caledonoid fault in the coalfield. It is the north-east continuation of a belt of similar trending structures in the adjoining district to the south, and is the en-échelon replacement of the Glyn Fault (Squirrell and Downing, 1969, pp.246, 247). The Trevethin Fault throws down south, the maximum vertical displacement being over 100 m at Trevethin [SO 284 021]. It dies at the surface about 1 km south-west of Pontypool, being absent at Llanhilleth opencast site, but it can be traced at depth in colliery workings for a further 2 km. The Trevethin Fault thus differs from the cross-faults of the coalfield which are generally more persistent in the Pennant Measures. A complementary anticline on the north side of the fault brings the Castell Coch Limestone to the surface at Pontnewynydd [SO 27 02], and was recorded in coal workings at Gwenallt (Robertson, 1927, p.116). To the south of the fault lies the Llanhilleth Syncline, in the axial region of which is a previously unrecorded outlier of Lower Coal Measures strata that includes the Garw and Five-Feet-Gellideg coals (Jackson, 1975). The Glyn Fault has a northerly downthrow, and its en-échelon relationship with the Trevethin Fault would suggest that they are the pairs of a dextral transcurrent system, with an intervening transtensional pull-apart zone. There is, however, little displacement of the Lower Old Red Sandstone outcrops to the east of the coalfield, and the faults apparently die before reaching the Pontypool Road Fault. The Llanhilleth Syncline also dies out eastwards, suggesting that it formed concurrently with the Trevethin Fault. Minor east-north-east-trending faults, most of which throw down to the north, have been mapped to the north of the Trevethin Fault, around Coedcae [SO 294 025], on Mynydd Garn-wen [SO 287 041], and in Lasgarn Wood [SO 278 038].

Chapter 8 Quaternary

Pleistocene

The period from the end of the Carboniferous to the Quaternary is unrepresented in the district, apart possibly from the infillings of some swallow holes, which have been interpreted as the product of Tertiary tropical weathering (BattiauQueney, 1973,1984).

During the Pleistocene, South Wales was glaciated on several occasions. The early glaciations, termed the Irish Sea and Paviland glaciations by Bowen and others (1986), predate the interglacial raised beach deposits of the South Wales coast. These deposits have been attributed to three separate high sea-level events, on the basis of amino-acid studies and uranium series ages, and they pre-date the late Devensian glaciation to which most of the glacial deposits of the district are assigned (Bowen and others, 1985; 1986).

Following their initial study by Symonds (1872), the drift deposits were systematically described by Strahan and Gibson (1900), whose account was reproduced in Robertson (1927). Charlesworth (1929) delimited the late Devensian 'Newer Drift' within the district, and noted the evidence for a local glacier in the Afon Lwyd Valley. Pocock (1926) examined the drift deposits of the Usk Valley, and suggested that there was evidence for several phases of deposition and erosion. Clarke (1936) studied the evolution of the drainage of the Black Mountains and concluded that two glaciations had affected the area, whereas Woodland and Evans (1964) found evidence of only one glaciation, of late Devensian age, in the nearby Pontypridd district. Squirrell and Downing (1969) considered that the coalfield valley deposits of the Newport district were Devensian, but suggested that the high-level drift deposits and erratics on the interfluves might belong to an earlier glaciation. They were also doubtful about the age of gravelly deposits on the eastern slopes of the Afon Lwyd Valley, but suggested that their distance from the limit of the late Devensian deposits might indicate an earlier glaciation.

Welch and Trotter (1961) described the glacial deposits of the adjacent Monmouth district as 'gravelly Boulder Clay', designated them 'Morainic Drift' and attributed them to the late Devensian 'Newer Drift'.

Lewis (1966), in a controversial interpretation, considered the Usk Valley deposits to be the terminal deposits (the 'Breconshire End-Moraine') of a late Devensian mid-Wales ice cap, with the Brecon Beacons unglaciated and undergoing solifluction processes at that time. Implicit in this hypothesis is that the South Wales Coalfield was also unglaciated, thereby dating its glacial deposits as early Devensian, or older. This interpretation won few supporters, and Lewis (1970a) returned to the conventional view of the existence of a late Devensian Breconshire ice-cap.

Williams (1968a) suggested a multiglacial stratigraphy for the Usk Valley deposits comprising an older glaciation (the 'Upper Usk Maximum'), and two more recent glaciations (the 'Vale of Gwent Maximum' and 'the last glacial advance') separated by an interstadial. Lewis (1970a,b) proposed a pulsed retreat of the late Devensian Usk glacier, with a subsequent readvance as far as Llangynidr. Trotman (1963) provided pollen analysis of the late Devensian–Flandrian sequence at Waen Ddu (p.113).

Bowen (1973) modified Charlesworth's (1929) 'Newer Drift' limit, and provided stratigraphical summaries (Bowen, 1973, 1974, 1977, 1978, 1981; Bowen and others, 1986). The present resurvey has further refined the limits of the 'Newer Drift' in the Ebbw Fach, Afon Lwyd and Usk valleys, and confirmed the presence of high-level gravelly deposits east of the Afon Lwyd that may belong to an earlier glaciation. Morainic deposits in the cwms of the district are probably attributable to the Devensian late-Glacial (Loch Lomond) readvance (Ellis-Gruffydd, 1977; Walker, 1980).

Glacial deposits

The glacial deposits include till ('boulder clay'), sandy till, glacial sands and gravels, morainic drift, lacustrine silts and clays, fluvioglacial outwash deposits and fan gravels. They are conveniently divided into two groups: (1) the coalfield deposits, and (2) those of the Usk Valley and its tributaries.

Coalfield deposits

On the moorlands north of the coalfield valleys, the late Devensian ice-sheet was continuous. On the lower ground to the south, the ice was funnelled into a series of valley glaciers, of which those in the east were least powerful. The Pennant sandstone interfluves between the valleys formed narrow nunataks that increase in height eastwards, Twyn Ffynhonnau Goerion on Coity Mountain, Blaenavon being the second highest summit (581 m above OD) in the South Wales Coalfield.

Till drapes the sides of all the coalfield valleys, apart from Ebbw Fach and Afon Lwyd where it is more patchy. It also forms a dissected till plain to the north of the valleys. Where fresh, the till is a stiff grey clay, but it weathers to brown and yellow clay in the upper few metres. It is charged with clasts ranging in size up to boulders; these are mainly of Coal Measures and Millstone Grit sandstones, although Old Red Sandstone and Carboniferous Limestone lithologies are also represented. Striated erratics are occasionally found in fresh sections. The till contains varying amounts of sand, and includes sand lenses which may cause problems in engineering works. In the valleys, the tills become sandier downstream and towards the valley bottoms, and grade into impure glacial gravels. Glacial rafting of large grit bodies occurs locally (p.111).

Original constructional topography is rarely preserved, but morainic mounds are present at Abertysswg in the Rhymney Valley, and at Trefil and Pochin in the Sirhowy Valley. In sections in the Sirhowy Valley at Tredegar (p.112) and in some boreholes in Tredegar and near Brynmawr (p.112),the till overlies a variable thickness of Coal Measures mudstone and coal debris. This appears to be locally derived head, and was presumably deposited in hollows during periglacial conditions prior to the onset of the last glaciation. Alternatively, it may be deformation till (Francis, 1975). Thin beds of overconsolidated lodgement till are recorded in some boreholes.

Glacial sand and gravel is largely confined to narrow zones in the valley bottoms, and consists mainly of dirty, poorly sorted cobble and boulder gravel deposited englacially, subglacially or as outwash.

Lacustrine laminated silts and clays have been recorded in a few boreholes, for example at Brynmawr (p.112), where 2 m of such deposits underlie till. They were laid down subglacially, or in ice-dammed glacial lakes.

Deposits of the Usk Valley and associated areas

Most of the Usk Valley within the district was excavated in pre-Devensian times. An early base level may be preserved as the remnant of a high-level bench at 280 m on the Blorenge. A more widespread bench, about 30 m lower occurs on both sides of the valley, and is best seen above Llanwenarth Breast and on the Blorenge (p.114). It probably marks the pre-late Devensian base level and is not a remnant of the 'Low Plateau' of Wales (Brown, 1960; Evans, 1973). The late Devensian Usk glacier was largely confined to the pre-existing valley, but excavated it to a lower base level. The valley bottom and those of its tributaries contain a variety of glacial, fluvioglacial, and glaciolacustrine deposits, mostly laid down as the main glacier, its tributaries and offshoots melted and retreated. Spreads of kame and kettle moraine mark stillstands or slight readvances in the glacier's retreat (Barclay and Jackson, 1977; Lewis, 1979a,b; Williams, 1968a). The present River Usk largely follows its pre-glacial course, apart from the Talybont to Llangynidr section (p.112) and the Brynderwen gorge (p.114), both of which appear to be late Devensian diversions.

Sandy till drapes the higher flanks of the valleys, its sandy nature reflecting the dominance of sandstones in the Old Red Sandstone source area. Many of the clasts in the till are well rounded, and were derived from older fluvial or fluvioglacial deposits of the Usk Valley of which there is now no trace. As in the coalfield valleys, the tills grade downslope into glacial sands and gravels, the division between the two being mainly arbitrary. The glacial sands and gravels commonly form hummocky kame and kettle topography; Dakyns and others (1896) classified all the Usk Valley bottom deposits as glacial sand and gravel, contemporaneous with the till.

Fluvioglacial sands and gravels in the bottom of the Usk Valley are up to 30 m thick. They are poorly sorted in general, contain well-rounded pebbles, cobbles and boulders, and are locally cemented by carbonate. They are typical of valley-train proglacial outwash deposits, laid down as bars in high-energy braided streams. Sand lenses within the gravels are the infillings of small channels and pools on and between the gravel bars during waning current stages. The upper surface of the gravels is kettled in some places, but elsewhere is terraced into kame terraces and flights of erosional benches. Fan-shaped bodies of fluvioglacial gravels at the confluence of the Usk and some of its tributaries are mapped as fluvioglacial fan gravels.

A stony clay overlies the gravels locally; sections exposed during construction of the new A40 trunk road between Abergavenny and Raglan revealed a variety of relatively stoneless silt and fine sand lenses filling hollows in the surface of the underlying gravels and tills. These lenses represent alluvial, colluvial and aeolian (loessic) deposition.

Morainic drift has been mapped in two settings: terminal moraines and moraine complexes. The terminal moraines are typically sharp, fresh ridges lying across some of the tributary valleys and cirques of the Usk. Three such moraines are present in Cwm Onneu (p.113),representing three halts in the retreat of a minor glacier. Moraine complexes are mainly kettle and kame moraine, and have been mapped as morainic drift where they comprise a chaotic mixture of ice-contact tills, clays and gravels. Where one type of deposit is dominant, the complex has been distinguished accordingly.

Lacustrine deposits consist of silts, sand and clays deposited in temporary glacial lakes. The largest spreads occur in the Rhian-goll Valley (p.113), in the Gavenny Valley near Llanvihangel Crucorney (p.113), and near Llanvapley. The last-mentioned occurrence represents the western extension of a lake site at Llwynderi in the adjoining Monmouth district (Welch and Trotter, 1961). Late-Glacial silts and clay were proved below the peat cover at Waen Ddu (p.113) by Trotman (1963). Similar deposits are likely to be present in the other cirques of the district.

Details

The coalfield area

Heads of the Valleys area

A till sheet covers much of the moorland north of the Heads of the Valleys Road between Merthyr Common and Mynydd Llangattock, veneering an irregular topography. The pre-glacial channel of Nant Pitwellt to the north of Rhymney Bridge [SO 0924 1098] is filled with till (Evans,1971). Boreholes at Tafarnau Bach proved up to 13 m of till, and temporary excavations [SO 1205 1090] revealed up to 7 m of sandy till with coal rafts and striated erratics, as well as a till-based quartzite mass. The largest glacial raft known in this area is the quartzite mass in Shon Sheffrey's reservoir [SO 1296 1152] (Strahan and Gibson, 1900, p.94 and fig. 10). Coarse boulder gravels are present in the stream banks to the north of the reservoir.

Eastwards from the Sirhowy Valley, the till sheet generally thickens to a maximum of about 25 m on the west bank of the ice-marginal Carno stream near Rassau (Barclay and Jackson, 1981). To the east of the stream, there is a patchy till cover up to 5 m thick.

A two-layer drift sequence was recorded in trial pits at Rhydw Farm, Brynmawr c.[SO 1997 1242] by Mr R J Price (personal communication, 1982). The upper layer, 0.6 to 1.8 m thick, is till consisting of very silty clay with clasts up to boulder size, and rests on weathered rock-head where the drift is thin. The lower discontinuous layer is up to 3.2 m thick and consists of locally derived angular Coal Measures debris in a dark grey clayey silt. It is probably pre-late Devensian head.

Bargoed Taff and Bargoed Rhymney valleys

Up to 13 m of till occurs in Cwm Golau [SO 0892 0562] and 22 m of 'clay with boulders' were recorded in Bedlinog No. 1 shaft [SO 0975 0163]. Evidence that ice mounted the col at the head of Nant Llwynog and moved southwards was given by Squirrell and Downing (1969, p 224). Till lines the Bargoed Rhymney Valley and its tributaries, with coarse ill sorted fluvioglacial gravels in the main valley bottom. Ogilvie Colliery shafts proved the gravels to be up to 27 m thick.

Rhymney Valley

Gravels filling the valley bottom are up to 25 m thick, and pass upslope into till. Constructional mounds of till are present on the valley side at Abertysswg [SO 130 053] (Evans, 1971). A till sheet about 3 m thick covers the moorland drained by Nant Tyswg, a tributary of the Rhymney. Lenses of sand and gravel have been mapped within the till near Heathcock [SO 1297 0765], [SO 129 073].

Sirhowy Valley

Sections in the bank of the River Sirhowy [SO 1347 1050], [SO 1390 1030] at Dukestown consist of an upper pale brown sandy till with abundant grit cobbles and boulders overlying locally derived dark grey debris of Coal Measures mudstone, coal and seatearth (p.111). The junction between the two units is highly irregular. Boreholes nearby at Charles Street proved 5 to 9 m of till, and in one borehole [SO 1399 0993] the till rests on a lm bed of mudstone debris in a clayey matrix. Boreholes in central Tredegar proved a variable thickness of head above till, with locally derived debris present in one borehole below the till.

Mounds of locally derived drift are present on the surface of the valley-fill deposits c. [SO 157 054] north of Pochin; c. 31 m of drift were proved in Pochin shafts [SO 1620 0461]. The Sirhowy Valley is constricted to the south, and the abundance of erratics to the east on Coed y Llanerch c. [SO 172 044] shows that a tongue of ice mounted the interfluve between the Sirhowy and Ebbw valleys at that point. Coarse fluvioglacial gravels fill the Sirhowy Valley bottom around Markham, where boreholes proved up to 21 m of drift on the west side of the river.

Ebbw Fawr Valley

Up to 9 m of stiff, stony till were noted at Rhyd-y-Blew opencast site [SO 1578 1113], and 17.7 m of till were recorded in Gwaun Helig Borehole No.5 [SO 1599 1008]. To the south, glacial deposits are largely confined to the bottom of the valley, with head draping the valley sides. Cwm Merddog housed a small tributary glacier, and about 3 m of till lies in the bottom of the valley. Glacial gravels fill the main valley bottom south of Marine Colliery [SO 1890 0392]; 11.8 m were proved in the No 2 Shaft, and 4 m of coarse, unsorted, dirty gravels are exposed nearby [SO 1897 0365]. Sections along a forestry track [SO 2007 0238] on the west side of the valley revealed up to 6 m of gravel, with boulders up to 1 m in diameter.

Ebbw Fach Valley

Boreholes for the Brynmawr bypass c. [SO 192 113] proved up to 7 m of till, locally overlying up to 2 m of lacustrine sand, silt and clay. To the south of Nantyglo, the valley is lined with solifluction deposits consisting of yellow or orange clay with quartzite debris and, locally, Pennant sandstone and coal fragments.

North of Abertillery, the steep lower slopes above the alluvium are strewn with tumbled grit and quartzite blocks and underlain by gravelly till; this material is affected by shallow landslipping. A small exposure at Six Bells [SO 2203 0302] consists of subrounded conglomerate and quartzite erratics derived from the outcrop of the Rhondda Beds. Downstream of Six Bells, the Ebbw Fach flows south-west in a narrow V-shaped valley to where it joins the Ebbw Fawr at Aberbeeg. There is no till in this section of the Ebbw Fach, but the small tributary valley of Cwm Llwydrew [SO 225 027] is drift-choked, unlike all the east bank tributaries to the north. A spread of till occurs to the east-south-east on Mynydd Llanhilleth, although opencast operations stripped the southern part. Either the Ebbw Fach glacier was blocked at Six Bells and was diverted into Cwm Llwydrew, depositing its terminal moraine on Mynydd Llanhilleth, or Cwm Llwydrew–Cwm Du was the pre-glacial course of the Ebbw Fach, and its present course from Six Bells to Aberbeeg is a more recent glacial spillway.

Afon Lwyd Valley

Gravelly, locally derived till occurs at Forgeside, Blaenavon, and is exposed in several gullies e.g. [SO 2471 0876]. Forgeside Borehole [SO 2504 0828] proved 10.2 m of this material (Barclay and Jones, 1978). A thin till, deposited by a weak glacier, is confined to the valley bottom, and occurs only as far south as Pontypool, marking the southern limit of the late Devensian glaciation. Thin remanie patches of gravelly till with small Pennant sandstone fragments occur on the Carboniferous Limestone outcrop high on the east side of the valley. They are probably pre-Devensian, and equivalent to the high-level deposits seen to the south (Squirrel] and Downing, 1969).

Usk Valley and tributaries

Talybont–Llangynidr

Up to 17 m of sandy deposits were recorded in boreholes in Glyn Collwn on the site of Talybont Reservoir [SO 1007 1927]. Fluvioglacial gravels occur downstream of the dam, and gravelly till veneers the lower slopes of the valley upstream. Lewis (1970a) suggested that the deposits in the boreholes were pro-glacial lake deposits laid down to the south of the terminus of a glacier which flowed south down the valley as far as the dam site during a late readvance. The presence of till upstream from the dam suggests that the valley contained a north-flowing tributary glacier to the main Usk glacier (Lewis, 1970b).

A thin spread of till and abundant erratics occur on the col between Glyn Collwn and Llandetty, the till extending up to about 360 m above OD. A tongue of diffluent ice from the Glyn Collwn glacier must have passed through the col, dropping steeply into the Usk Valley at Llandetty. Sands and gravels with a kame and kettle form fill the bottom of the valley from Llandetty to Llangynidr. Lewis (1966; 1970a) assigned these deposits to a late readvance of the Usk glacier. The kame and kettle topography is particularly impressive around Glaw-Coed Wood c.[SO 138 201]. Difficulties reported during water pipe-laying were due to the variation of the deposits and the presence of fine sand lenses (Williams,1968a). The main ridges of gravel lie north of the river between Glaw-Coed Wood and Llandetty Hall. They are parallel to the present river course and were interpreted by Lewis (1966) as possible crevasse-fills.

The Dyffryn Crawnon Valley has a typical U-shaped glaciated profile, but sandy and gravelly deposits in the bottom are of local origin and mapped as head. Cwm Claisfer housed a tributary glacier, and up to 3 m of till are present. A large fan-shaped spread of fluvioglacial gravels and gravelly till lies at the confluence of Nant Claisfer and the Usk. The best sections are in the south bank of the Usk [SO 1700 1962] where up to 15 m of gravelly till consists of boulders of Old Red Sandstone in a red-brown clayey sand matrix, locally cemented by carbonate.

Llangynidr to Crickhowell

Gravels form a high-level bench on the north side of the River Usk from Cil-with-fach [SO 1395 2030] to Gliffaes [SO 1705 1992], a feature interpreted by Williams (1968a) as a kame terrace. High level fluvioglacial gravel terraces are also present in the Usk Valley at Glan Usk Park, and are benched on the south side at two levels, 6 m and 12 m above the river flood plain. On the north side of the river, the terrace extends northwards on both flanks of the Rhian-goll; a broad alluvial area c.[SO 190 206] probably marks a former lake site, boreholcs proving silts, clays and peat above gravel. Myarth Hill to the west c.[SO 175 205] is a good example of a boat-shaped nunatak trimmed by a glacier which divided at the western end of the hill and coalesced to the east. The pre-Devensian course of the Usk was probably to the north of the hill, between Bwlch and Tretower, north of the district boundary.

An isolated patch of boulder clay occurs at Blaen Onneu at about 445 m above OD [SO 159 171], and an extensive spread of till covers the area from Cwm Onneu south to Craig y Cilau. Cwm Onneufach contains evidence of three stillstands in the retreat of a minor tributary glacier. Three moraines [SO 1961 1614], [SO 1913 1585], [SO 1875 1620] have alluvial flats where lakes were dammed; the highest and youngest lake site is Waco Ddu [SO 1865 1650]. Trotman (1963) recorded the following sequence:

Trotman (1963) placed the base of Zone IV(i.e. the base of the Flandrian) within Bed 3, the late-Glacial climatic amelioration (Zone I–II boundary) occurring during deposition of Bed 2.

The oldest moraine [SO 1961 1614] is a small sharp ridge of till, the topmost 7 m of which consist of angular Carboniferous Limestone pebbles (95 per cent), and Old Red Sandstone pebbles (2 per cent) in a brown clay matrix. The basal part consists of up to 2 m of red-brown till comprising c.60 per cent of angular Old Red Sandstone clasts up to boulder-size in a sandy clay matrix.

Crickhowell -Llanvihangel Crucorney–Abergavenny

The Usk glacier forked at Crickhowell, a smaller glacier flowing north-east to the Grwyne Fawr Valley. The latter deposited a thin but widespread sandy clay till, with a tributary glacier in Cwrn Cumbeth depositing up to 9 m of sandy till e.g. [SO 2192 1985]. In the Usk Valley at Llangattock, hummocky gravelly moraine is partly buried by a wide fluvioglacial gravel fan at the confluence of Nant Onneu-fach and the Usk. A temporary section in 1980 [SO 2154 1864] in the moraine at Crickhowell showed the following ice-contact sequence:

Mounds of moraine occur south of the Llangattock gravel fan, and 5 m of gravels were exposed in a road cutting [SO 2210 1723] on the A4077. There are good exposures of gravelly moraine in the banks of the River Usk west of Gilwern; the following section [SO 2400 1539] was recorded:

A bank 30 m high [SO 2440 1534] comprises clayey sandy till above gravels; the gravels are unsorted of boulder- to cobble-grade at their base, and cemented by carbonate in part. A similar sequence of gravelly till above gravels occurs in the morainic deposits north of the confluence of the Usk with the Grwyne Fawr at Glangrwyney. Further north, the Grwyne Fawr cuts through a two-layer sequence of sandy till overlying glacial sands and gravels. Fluvioglacial gravels form a bench on the east bank of the Grwyne Fechan at Llanbedr c. [SO 245 202], and a lens of sand occurs within sandy till nearby [SO 238 203]. The till surface is ketticd to the south.

East of Llanbedr, sandy till fills the bottom of the Grwyne Fawr Valley, and about 23 m are exposed east of Coed Mill [SO 2620 2005]. Gravels underlie the till, and crop out at the surface locally, for example near Tre-wysgoed [SO 274 206] where up to 8 m are seen. A gravelly moraine lies to the east c. [SO 281 207] where the Grwyne Fawr swings northwards on the district boundary. Morainic gravel mounds, with intervening lacustrine flats occur in the northern part of the dry valley between Forest Coal Pit [SO 290 207] and Bettws [SO 298 193]. Lewis (1970a) proposed a complex pulsed retreat of the Gwryne Fawr glacier and its offshoots in this area, but, since only the part of the deposits that lies within the present district has been mapped, a synthesis is not attempted here.

An extensive spread of hummocky sands and gravels in the north of the Gavenny Valley constitutes the Llanvihangel Crucorney Moraine (Clarke, 1936; Lewis, 1970a). Up to 6.2 m of cobbles in a silt to sand matrix were seen in trial pits [SO 3238 2024], [SO 3231 2008]. Flat alluvial areas to the east and south of the moraine probably mark former glacial lake sites, and a trial pit [SO 3230 2000] showed:

Sandy till up to 10 m thick lines the Gavenny Valley to the south, and forms constructional mounds around the confluence of the Bettws and Gavenny valleys.

Gravelly moraine is present on the south side of the Usk at Gilwern, and is trimmed in part by a high-level fluvioglacial gravel fan. A borehole [SO 2542 1496] proved 1.2 m of gravel on 4.9 m of stony till above 6.1 m of gravels; 5 m of boulder gravel are exposed nearby [SO 2606 1490].

The Cibi Valley is filled with sandy till that has a kettled surface. A borehole [SO 2873 1727] is said to have proved 43.9 m of drift; if correct, this is the deepest recorded rock basin in the district. The remnant of a gravelly moraine occurs further south [SO 2915 1648].

Abergavenny is underlain by coarse fluvioglacial gravels whose surface is veneered by fan deposits under much of the town. Elsewhere, the gravels are either kettled or have kame terrace form (Lewis, 1970a).

East of Abergavenny

Skirrid Fawr and the area to the north-east are drift-free, but an extensive spread of sandy till lies to the south. A maximum thickness of 10 m was noted near Llanthewy-Skirrid [SO 3445 1646], but the till is generally about 2 m thick. It has a fine sand to silty loam matrix, becoming more clayey with depth, and contains clasts that are predominantly well rounded. It commonly fills the bottoms of the small valleys, with the interfluve areas covered by a thinner veneer of drift, or by a scatter of erratics and patches of remanie drift. The sandy till extends east to the adjoining Monmouth district where it was described by Welch and Trotter (1961) as gravelly boulder clay and depicted on the 1:50 000 map as Morainic Drift. Within the present district, the material lacks constructional form and the freshness of features described in the area to the east (Welch and Trotter, 1961), and has been mapped as till. Several lenses of fluvioglacial gravels occur in this area, for example at Waen ddu [SO 325 153]. Outwash sands and gravels with a moundy valley-train form are present at Llandewi Court [SO 342 169].

Abergavenny–Llanover

Gravelly deposits, displaying fine kame-and-kettle topography, occur in the Llanfoist area. They also underlie the Usk alluvium, filling a buried channel at least 15 m deep at the A465 road bridge [SO 2999 1304] (Williams, 1968b). The bench at c.250 m above OD on the lower slopes of the Blorenge (p.111) has a thin veneer of till.

The Punchbowl [SO 2810 1165] is a fine example of a glacial cirque, and a small alluvial area marks a former lake site that was ponded behind a morainic bar. Sandy clay till veneers the lower slopes of the east crop escarpment, grading downslope into gravelly till and glacial gravels. The gravels have a kettled topography east of Llanellen.

To the east of the River Usk, cuttings and temporary pits made in 1980 along the new A40 provided sections through the predominantly gravelly deposits. Fluvioglacial outwash gravels occupy the lowest ground alongside the River Usk and its floodplain. Up to 20 m of these gravels, cemented locally, are exposed in the river bank [SO 3027 1213]; [SO 3122 1108] to [SO 3172 1044].

A feature of the temporary sections along the A40 was the presence of lenses and lenticular sheets of clayey silt, silty clay or silty sand up to 3 m thick. These fill hollows in the irregular surface of the underlying deposits. They are mainly colluvial deposits formed in periglacial conditions, with some possibly being aeolian. Much of the route of the A40 from the A465 interchange [SO 3070 1255] to Llangattock Lodge [SO 3191 1058] lies on very gravelly till, depicted on the map as glacial sand and gravel. Clay content of the till is generally low, and the deposit becomes increasingly gravelly downwards. Rotting and disintegration of Devonian sandstone clasts add to the sand content.

Excavations for bridge foundations [SO 3127 1146] showed a complex sequence:

The chaotic nature of the sequence, proved in boreholes to exceed 25 m, indicates deposition on or in close association with ice.

From Llangattock Lodge [SO 319 106] to Llangattock nigh Usk the A40 lies on fluvioglacial gravels overlain by up to 3 m of clayey silt. Up to 5 m of the gravels were seen in bridge foundations at Penpergwm [SO 3234 1013] where they were well rounded and of pebble- to boulder-grade; at least 25 m of drift were proved by boreholes. Bridge foundations [SO 3211 1001] at Llangattock nigh Usk were in gravels which fine upwards and are overlain by clayey silt lenses. Good sections were seen in these gravels at Bryn [SO 334 098].

On the western flanks of the Usk Valley, the east crop escarpment is breached by Ochram Brook, the two tributaries of which originate in late-glacial lake sites occupying cirque bottoms. The northern cirque, Craig-yr-hafod [SO 2745 1015], shows two stillstands in the retreat of a small tributary glacier, with two lake levels ponded behind moraines. Rotational landslipping may have contributed to the formation of Craig-y-cwm [SO 286 092], the second cirque, which has a peaty lake flat enclosed by nivation ridges. Downslope of both cirques are extensive spreads of hummocky sandy till, which appear to overlie the Usk Valley deposits, and probably date from the late-Glacial (Loch Lomond) readvance.

A temporary section [SO 2897 0999] in the steep valley side at Brynbedw showed up to 1 m of clean sand in fining-upwards cycles between a lower stony till and an upper sandy till. The sand was probably deposited in an ice-marginal stream or lake between the glacier and the valley wall.

Llanover to Usk

Between the River Usk and the east crop escarpment is a low-lying area of undulating relief in which the terminal deposits of the Usk glacier were laid down. Emerging from the confines of the Usk Valley below Abergavenny, the glacier spread out into a piedmont ice-sheet. Till covers much of the area between Llanover and Monkswood, with an upper limit that falls uniformly from 150 m above OD east of Coed y Prior to 45 m above OD near Llancayo. The till is a sandy variable deposit that typically has a profile comprising brown stony loam on red-brown stony, sandy and silty clay on stiff red stony clay. A change in the composition of the till takes place at about 85 m above OD, with more sandy till above and clayey till below. The clasts are mainly of Namurian and Upper Devonian quartzites and conglomerates of pebble and cobble size, and show a general increase in roundness towards the present River Usk and towards the south-east margin of the till deposits. Much of the till surface is kettled, with enclosed alluvial hollows. The broad spur north of Croes-y-pant [SO 313 045] and the area west of Goytre Wharf [SO 312 065] have a fluted topography of north-west-trending till ridges. Temporary sections [SO 3195 0660] on the A 4042 nearby exposed 3 m of weathered sandy till that becomes more clayey towards the base.

To the east of the A4042, on the broad spur between Berthin Brook and Nant y Robwl, a thin soliflucted veneer of sandy till (?flow till) overlies an extensive, well-sorted, medium-grained sand. About 1.5 m of the sand are exposed in the sides of minor dry interconnecting valleys that have cut through the till in the Coed yr AlltCefn Mawr area [SO 33 04]; [SO 33 03], near the southern limit of the Usk glacier. The valleys are flat-floored, steep-sided, up to 60 m wide and 20 m deep. They were probably late-glacial melt-water channels. Some have right-angle bends and blind terminations, suggesting a subglacial, crevasse-controlled origin. The sand apparently thickens south-eastwards, and is reputed to be over 21 m thick near Penpedairheol, the covering sandy till thinning in that direction. Less sandy (?melt-out) till, with fewer but larger, boulder-size erratics, forms wet ill-drained tracts and uniformly sloping sheets at a lower level. The Arctic glacier model of Boulton (1972), involving lower and upper tills with intervening sands, can be readily applied to this area.

Deposits of impure gravel lie between Pant-y-Goitre [SO 350 086] and Brynderwen [SO 354 065], the River Usk cutting through them in the Brynderwen gorge. They form gently rolling ground in most of their outcrop west of the river, apart from in the south, where, as in the area to the east of the river, they have a complex moundy topography. In the Brynderwen gorge, the gravels form a bluff 22 m high, and up to 32 m are present on the east bank. Sandy till overlies the gravels near Hillgrove [SO 3611 0840] and near Claltan Coppice [SO 3620 0630]. The clasts in the gravels are mainly of Lower Old Red Sandstone rocks, but Upper Old Red Sandstone and Carboniferous Limestone rocks are also represented.

A dissected spread of fluvioglacial gravels, locally benched and partially buried by younger alluvial deposits, occupies a belt alongside the River Usk from Llanover eastwards to Ty-mawr [SO 335 090]. Extensive areas of these gravels occur on the east bank of the Usk [SO 360 040] south and west of Trostrey, north of Chain Bridge [SO 345 065] and north-east of Great Estavarney [SO 355 035]. The gravels, which show a crude upward-fining, are composed predominantly of pebbles and cobbles, with a maximum clast size of about 0.4 m (Plate 15).

The gravels have a kettled topography south-east of Llanellen [SO 315 100] and at Kemeys Commander [SO 345 045], but over much of their outcrop they are benched into flights of irregular and unpaired terraces. The terraces are best developed near Llancayo [SO 365 030] where individual risers range from 1.3 m to 5 m over a distance of 600 m. The terraces decline downstream and are buried by the floodplain alluvium, of the River Usk. A red stony clay caps the gravels around Trostrey, and is overlain locally by sand and silt arranged in fining-upwards alluvial/colluvial cycles.

Good exposures are confined to the banks of the River Usk, for example south-east of Trostrey [SO 3613 0386] where the section reads:

Fluvioglacial gravels occur along the north side of the ice-marginal Berthin Brook from Little Mill [SO 322 030] to north of Bryn Farm [SO 340 023]. The gravels extend eastwards to Rhadyr [SO 365 020], marking the former, pre-late Devensian course of Berthin Brook.

Head

Head is used for a range of scree, colluvial, solifluction and gelifluction deposits, mostly formed in periglacial conditions during deglaciation, but these processes remain operative at the present day under severe weather conditions. The composition of the head depends on the source rocks, and ranges from clay with boulders to silts and fine sands. Locally, the deposit shows a downhill stratification parallel to the slope; there is a gradation between head and translational earth and debris flows that commonly form the toe regions of landslip complexes (p.116). Head is widespread throughout the district, draping many valley slopes. Some of the most extensive spreads occur in the coalfield valleys, originating from spring lines where thick Pennant sandstone aquifers overlie weak, impermeable mudstones. The thickest recorded deposits are at Waun Avon [SO 220 103] where about 6 m of clay with boulders occur. Many of the slopes of the east crop are covered by grit boulders. Block screes are present locally, for example on Waun-Pwll mawr [SO 265 087], and grade downslope into solifluction deposits.

Flandrian

Deposits of Flandrian age have formed since the melting of the Devensian glaciers about 10 000 years ago. The base of Pollen Zone IV is taken as the start of the Flandrian Period (Mitchell and others, 1973).

Calcareous tufa

Small areas of calcareous tufa have been mapped at Blaen Onneu [SO 1573 1690], deposited by lime-rich waters emanating from the base of the Abercriban Oolite.

Cave deposits

The Dinantian rocks of the district contain an extensive system of caves and passages including, below Mynydd Llangattock, Agen Allwedd which is 24 km long and one of the largest in South Wales. Agen Allwedd contains a sequence of sediments in which several cycles have been identified (Bull, 1978, 1980). Laminated silt and mud, termed the cap mud, is of widespread occurrence. Palaeomagnetic studies (Noe1,1983; Noel and others, 1979) showed it to have a stable depositional remanent magnetism with a polarity indicating sedimentation within the last 12 000 years. It was deposited when the cave was flooded after the late Devensian glaciation, in Pollen Zone III times, 10 800 to 10 300 years BP (Bul1,1980; Noel and others, 1979). Coarser sediments pre-dating the cap mud and separated from it by a depositional hiatus may represent deposition during the onset and waning of the Devensian glaciation, the maximum of which was about 17 000 years BP (Bull, 1980). A further hiatus followed the deposition of the cap mud between 10 300 and 7000 years BP. A thin organic layer was deposited between 7000 and 5000 years BP.

Terrace deposits

Only minor amounts of terrace deposits are present within the district, all related to the River Usk, and are mainly classified as First Terrace deposits, with some Second Terrace deposits. First Terrace deposits consist of red clayey silt and clay, and their top lies only 0.3 to 0.6 m above the floodplain level. Where the floodplain is absent or very narrow, as, for example, in gorge sections of the River Usk, narrow strips of First Terrace deposits lie up to 6 m above low river discharge level, and consist of reddish brown sand. Second Terrace deposits may be clay or gravel, and are separated from those of the First Terrace by a step of 1.5 m.

Alluvium

Flat-topped alluvial deposits fill the bottoms of most of the river valleys of the district, and consist mainly of overbank silt and fine sand above channel-lag gravels. The deposits underlie the floodplains of the major rivers, and are exposed in their banks. The coalfield valleys have rivers with narrow floodplains, and many are now obscured in part by colliery and ironworks spoil. The youngest alluvial deposits in these valleys commonly contain coal and shale debris derived from this waste.

The Usk Valley contains the greatest volume of alluvial deposits, with a floodplain up to 1 km wide in places and up to 4 m above low river level. The deposits consist of red-brown silt and fine sand, with gravel lenses. The higher layers contain coal granules and shale chips.

Alluvial deposits not associated with the present drainage system have been mapped in the numerous enclosed hollows, and abandoned channels in morainic deposits, and these are especially common in the terminal area of the Usk glacier between Llanellen and Monkswood. Alluvial fans occur where tributaries join the main river, and large examples in the district are mainly found in the Usk Valley, for example at Glangrwyncy [SO 235 165], and south of Llanellen [SO 310 095].

Peat

Peat deposits are mainly concentrated in the moorland areas north of the coalfield valleys where they accumulated in ponded hollows on the collecting ground of the coalfield glaciers. The largest spread is on Mynydd Llangattock [SO 180 155] where up to 1.8 m of peat were recorded. Small patches occur on the Pennant sandstone plateau, the greatest concentration being on Coity Mountain in the east of the coalfield. The thickest known, and the only dated peat deposit in the district is at Waen Ddu [SO 1865 1650] where 4.95 m of peat range in age from Zone VI to VIIb (Trotman,1963) and are underlain by lacustrine muds and silts (p.113).

Foundered strata

The outcrop of the Namurian and Dinantian rocks is marked by many swallow holes, giving an interstratal karst topography (Thomas, 1973, 1974). Many of these solution hollows contain collapsed Namurian debris, but only the larger occurrences have been depicted as foundered strata. Grit debris was said to be over 5 m thick in a gas pipeline trench in one such occurrence at Pant-y-Gilwern [SO 213 146]. There is a concentration of areas of foundered Namurian grit in the Dowlais Limestone at Blaen Onneu, and a possible area of foundered Dinantian strata was noted on Garn Caws [SO 126 165].

Landslips

Landslips are abundant in the coalfield valleys (Knox, 1937) where the presence of thick Pennant sandstone above weak Rhondda Beds mudstones on the steep valley sides provides favourable conditions for mass movement. Most of the major landslips, for example Troed-rhiw'r-fuwch and New Tredegar in the Rhymney Valley, Bournville and East Pentwyn in the Ebbw Fach Valley, and Pant-glas in the Afon Lwyd Valley, have three elements; translational earth and debris flow in the toe region, rotational or slumping movement involving the main mass of the slip, and toppling and rock fall involving the detachment of Pennant sandstone blocks from the backscar area. The last process is aided locally by reactivation of fault planes and the formation of subsidence fissures by undermining, for example at the Darren Ddu landslip in the Ebbw Fach Valley [SO 1984 0606]. Debris flow movement has locally been initiated or enhanced by loading of colliery spoil, for example at Cwm in the Ebbw Fawr Valley [SO 1880 0534]. Although most of the movement took place in periglacial conditions during deglaciation when the support of the valley glaciers was removed, mass movement continues today and the instability of many valley slopes presents engineering problems. Some major slips are recent; for example the Bournville landslide [SO 207 068] (Plate 16) was apparently initiated in 1893, and the East Pentwyn landslide [SO 206 075] in 1954 (Gostelow, 1977). A comprehensive study of the landslips in the coalfield area has been published previously, including a catalogue in which each slip is classified and its current condition assessed (Conway and others, 1984).

Outside the coalfield, there are only minor amounts of landslip, the most extensive being at Graig [SO 260 160] and Skirrid Fawr [SO 325 175]. Both involve basal Senni Beds sandstones and the underlying less competent argillaceous beds of the St Maughans Formation. An apron of slips on the western flanks of Skirrid Fawr includes a classic example of a rotational slip [SO 329 183], involving an intact mass of Senni Beds sandstones. A good example of a debris flow occurs in the Dinantian in the Clydach Valley [SO 2250 1314] where the topmost beds of the Cwmyniscoy Mudstone, weakened by a spring at their contact with the overlying Clydach Valley Group have flowed 170 m. Bedding plane slides promoted by thin bentonites occur in the Silurian Glascoed Mudstone, for example at Graig y Pandy [SO 342 046] and Craig y Garcyd [SO 357 027].

Made Ground

Made Ground is largely confined to the coalfield area where there are large volumes of mining waste from coal and ironstone mines and opencast workings. The thickest deposits come from the deep coal mines, although some of the older tips have been reworked for small coal, and many derelict tip areas have been recently landscaped. Following the Aberfan disaster in 1966, the stability of the major colliery tips was investigated by drilling and remedial works carried out where necessary. Several tips show evidence of flow slide movement similar to the Aberfan failure, for example Bedwellty tip [SO 1545 0555], Marine tip [SO 184 045], and West Blaina Red Ash tip [SO 1955 0810]. The evolution of slopes on the tips of the Blaenavon area has been studied by Haigh (1978).

Chapter 9 Economic geology

Coal

Coal has been the principal economic product for 150 years and remains so today, despite a rapid decline of the industry in the district since the war. Robertson (1927) provided an account of coal mining activities in the 1920's and Adams (1967) summarised the data collected by the National Coal Board's Coal Survey (National Coal Board, 1960, 1962). The district was included in a review by Thomas (1961). Robertson and Adams gave much data on seam workings and coal properties, and only a brief summary is included here, together with an account of present mining activities. Variation in coal rank is shown in (Figure 39).

The values shown are British Coal codes, as follows:

There is an east-west increase in rank across the district with low rank, high volatile coals on the east crop and the highest rank steam coals in the south-west. Mr D B Vowles, British Coal, has provided the following contribution (1984):

"The Coal Measures succession is thin and therefore all the workable seams are readily accessible. Most of the output has been from the main productive group of coals of the Lower and Middle Coal Measures, and this is continuing at the present time. This group includes the following seams, all of which have been worked to a greater or lesser extent: Two-Feet-Nine, Four-Feet, Six-Feet, Upper Nine-Feet, Lower Nine-Feet, Yard, Upper Seven-Feet, Lower Seven-Feet, Five-Feet-Gellideg, and Garw. The general mining conditions have been good since there is an absence of major compressional structures and the dips of the strata are generally low and fairly steady. This has resulted in the intensive exploitation of the seams in the past and even thin seams like the Garw have been mechanised. These coals are suitable for a variety of markets both on their own account and for blending with other coals. The main use of coals with rank codes 202, 203 and 204 is for steam raising and they have also been used for domestic purposes. The coals with rank code 301, and especially 301a, are among the best metallurgical coking coals in the country whereas coals with rank codes 204 and 501 are valuable constituents of coking blends. At the time of writing, colliery closures and mergers are the result of the depletion of the reserves due to intensive exploitation in the past."

Some of the thinner seams of the Middle Coal Measures, particularly the Gorllwyn Rider and Pentre Rider, have been worked in the past from crop levels, and the former is being mined at present, at Rhymney. The Rhondda coals have been worked locally and are still being mined at Fochriw. The Brithdir (Red Ash) and the Mynyddislwyn were once extensively mined as house coals and the former is currently being worked from several levels. At the start of the resurvey of the coalfield area (1968) seven deep mines were in operation–Ogilvie (with connections to Elliot and McLaren shafts), Markham, Marine, Abertillery New Mine, Big Pit, Six Bells and Blaenserchan colleries. At the time of writing (1987) five remain, Ogilvie and Blaenavon having closed, although the latter remains open as a museum.

Opencast mining has accounted for much of the output of the district since 1970. Royal Arms, Trecatty, Rhymney, Rhyd-y-blew, the Laurels and Blaenant sites worked the seams from the Five-Feet-Gellideg to the Two-Feet-Nine. Trecatty also worked some higher thin seams. Waunavon, Blaen Pig, Llanhilleth (on the Mynyddislwyn) and Waun Hoscyn were older sites of major extraction. Smaller sites include Fochriw (No. 1 Rhondda group), and Pwll-du (Five-Feet Gellideg to Upper Four-Feet). The latter is one of a few sites that remain to be re-exploited.

Iron ore

Siderite ("clay ironstone") was formerly extensively mined and the iron-making provided the base on which the economic development of the district and indeed British 19th century prosperity was founded. Rogers (1861) recorded evidence for ancient wind furnaces on the Blorenge and there were forges in Pontypool in 1425. Iron was being smelted in the Blaenavon area in 1577 and the Clydach gorge was also the site of an early ironworks about 1600. By about 1740, the use of charcoal for smelting had so deforested the region that the industry had declined almost to extinction. The advent of the Industrial Revolution about 20 years later produced a major expansion, with local coal used for smelting. The main ironworks were at Pontypool, Aberyschan, Blaenavon, Nantyglo, Beaufort, Ebbw Vale, Tredegar and Rhymney, all sited on the outcrop of the Lower and Middle Coal Measures, in which the ironstones occur both as thin tabular beds ('pins') and as nodular concretions ('balls'). Initially dug at outcrop in opencast excavations ('patches'), the ironstones were subsequently deep-mined from pits and adits. By the middle of the 19th century extraction was becoming too expensive and the industry went into decline; by the end of the century it had died.

Rogers (1861) and Strahan (1900) gave detailed accounts of the ironstones which were worked, including analyses. Robertson (1927) gave a generalised section giving the positions of the main beds.

Limestone and dolomite

Lime-burning for agriculture was formerly widespread using the Silurian limestones; the Usk Limestone was mined at Ton. The Devonian calcretes, including the Psammosteus Limestone and the Pontypool Limestone, were dug locally for this purpose also, as well as some of the other calcretes and intraformational conglomerates of the Old Red Sandstone.

Apart from lime-burning, the Dinantian limestones were much worked for flux in the iron industry, the oolites and dolomites of the Clydach Valley Group, the Abercriban Oolite and the Dowlais Limestone being the main source.

The physical properties of the limestones were described by Harrison and others (1983). Most of the limestones have adequate strength as crushed rock aggregate for most constructional uses, and are highly resistant to abrasion, but they have low polish resistance and are unsuitable for road surfacing. The Gilwern Oolite is the purest limestone in the sequence, with impurities ranging between 0.3 and 0.9 per cent. The Pwll-y-Cwm Oolite and the Blaen Onnen Oolite are less consistently pure, with 1 to 3 per cent of insoluble residue.

Many of the dolomites of the Clydach Valley Group are of metallurgical-grade suitable for refractory and fluxing purposes, with consistently high MgO values and relatively low amounts of silica, alumina, and iron.

During the resurvey, three quarries were working Dinantian limestones, their output being mainly crushed rock aggregate. All three quarries are abandoned in 1987. Cwar-yr-Hendre–Cwar yr Ystrad [SO 090 146] worked the Llanelly Formation and Abercriban Oolite, and supplied some material for flux to the Ebbw Vale steelworks. Blaen Onneu [SO 155 168] worked the Llanelly Formation, Blaen Onnen Oolite and part of the Pwll-y-Cwm Oolite. Blackrock [SO 215 126] worked the Dowlais Limestone.

Sandstone

Virtually all of the sandstones exposed in the district have been quarried to a minor extent. Sandstones in the St Maughans Formation and the Senni Beds were once worked in many small quarries for building flags and tiles. The Brownstones were also quarried locally and there are old adits in friable decalcified sandstones at the top of this formation on the Blorenge. Robertson (1927) recorded the presence of workings for road metal in unconsolidated pebbly grits of the Quartz Conglomerate Group near Pontypool. The Wern Watkin Formation has a number of old quarries, its flat-bedded nature presumably making it suitable for flagstones. The Dinantian Garn Caws Sandstone has a very high resistance to polishing and abrasion and was dug to a minor extent at Blaen Onneu Quarry. The high silica content (98 per cent) of the 'Farewell Rock' resulted in it being quarried at one or two places for use as a refractory in steel making. Likewise, the ganister underlying the Old Coal was once mined. The Black Pins Rock and Soap Vein Rock were formerly mined on a small scale for building stone, and other Coal Measures sandstones have all been quarried locally, particularly the quartzites of the Llynfi and Rhondda beds and the Pennant sandstones. There are no sandstone quarries in operation in the district at present.

Brick clays and fireclays

Robertson (1927) gave an account of the brickmaking industry in 1925. There are no workings at present, although some seatearth was extracted during opencast coal excavations, and some colliery spoil used for brickmaking. The Raglan Mudstone was, until recently, quarried at Little Mill brickpit, Pontypool and there are other older pits in the red marls of the Lower Old Red Sandstone. The bulk of brick production was from the shales of the Lower and Middle Coal Measures. Most of the worked fireclays were also in these beds. Robertson recorded that red mudstones in the Rhondda Beds were being dug for brickmaking in 1925.

Sand and gravel

The situation in 1925 (Robertson, 1927) remains today. No commercial exploitation of the resources of the district has taken place.

Hydrogeology and water supply

The Abergavenny district is mostly within Hydrometric Area 56, with the south-west part in Area 57 and the northeast in Area 55. The water resources are administered by the Welsh Water Authority. The district is in the area covered by an hydrogeological map of South Wales published on a scale of 1:125 000 by the British Geological Survey in 1986. The principal rivers are the Usk, flowing from the northwest to the south-east, and the Rhymney, Sirhowy, Ebbw Fawr, Ebbw Fach and Afon Lwyd, flowing south through the coalfield valleys.

The average annual rainfall ranges from about 1500 mm in the south-west to about 800 mm in the north-east (data supplied by the Welsh Water Authority, in Monkhouse and Richards, 1983). The mean annual evaporation over the district generally is of the order of 450 mm.

Robertson (1927) indicated that most of the water supply in the area came from surface sources, mainly from springs and reservoirs, and this is still the case. The largest reservoirs are Talybont [SO 100 190] in the north-west and Llandegfedd [SO 330 000] to the south-east of the district.

The coal mines of South Wales are extremely wet, and large quantities of drainage water are pumped. In this area, the major discharge is from Ogilvie [SO 120 030], amounting in 1975 to 1.7 million cubic metres (m³). The mine drainage flows into the southern streams and tends to affect their quality. The hydrogeological properties of the sequence are as follows:

Silurian The Wenlock, Ludlow and Pfidoli series strata are largely argillaceous and yield little groundwater. Shallow wells in the limestones may yield small supplies, but are vulnerable to surface pollution.

Devonian The sandstones of the St Maughans Formation, the Senni Beds, the Brownstones and the Upper Old Red Sandstone are locally used aquifers. There is generally sufficient cementation of the sandstones to inhibit intergranular flow, and groundwater is mostly restricted to fissures in the weathered zone. The overall resources are theoretically quite large, possibly exceeding 40 million m³/a, but these are reduced by difficulties of exploitation, and high-yielding boreholes are rare. Springs issuing from the bases of the sandstones are common, but usually yield less than 10 m³/d. Locally, discharges of more than 50 m³/d are recorded, and a group of springs in the Senni Beds to the north of Abergavenny yields over 1000 m³/d. There are several boreholes, mainly of 300 mm diameter or less, but few penetrate to depths over 30 m. Exceptionally, yields of over 500 m³/d have been recorded, but the mean is less than 50 m³/d.

Carboniferous The strata in the higher part of the Dinantian sequence tend to be more permeable than in the lower part, although much variation has been recorded (Welsh Water Authority, 1980). Groundwater is contained wholly within fissures and, although these are relatively large, they tend to be sparsely distributed. An unusual feature is the presence of concentrated flow paths through the limestones leading to some of the major springs, such as Shon Sheffrey's [SO 126 118] and Fynnon Gisfaen [SO 207 123]. The groundwater resources are replenished both by infiltration into the outcrop and by recharge through collapse dolines in the overlying Namurian strata. The total resources theoretically exceed 50 million m³/a, but the technical difficulties of exploitation are likely to restrict this to 5 million m³/a (Welsh Water Authority, 1980). In any case, the extraction of groundwater from boreholes would be at the expense of flow from springs many of which are used at present. The groundwater quality is usually good. Often, the residence time of the groundwater between infiltration and discharge is short, and the composition of the water remains little changed. For the same reason, pollution from surface sources may occur. In cases where the residence times are longer, the water tends to have a greater magnesium sulphate content, although the degree of mineralisation is usually insufficient to render the water impotable.

Springs are very common, ranging in size from a few m³/d to more than 25 000 m³/d. The discharges vary seasonally, generally reaching peak values by early summer and falling to their lowest levels by early winter.

Boreholes depend for their yield on intersecting water-bearing fissures and this makes their construction rather speculative; dry boreholes are common. A study of public supply boreholes in the limestones suggests that, where a yield is obtained, it averages about 300 m3/d, with a 25 per cent probability of more than 1000 m³/d. While a larger-diameter borehole would in theory have more chance of intersecting fissures, the diameter appears to have little effect on pumping rates. Few boreholes have been drilled for groundwater in this district, and, until recent years, none exceeded 80 m in depth.

As part of an investigation from 1979 to 1982, boreholes were drilled to 130 m depth at Rhymney Bridge [SO 104 091], to 141 m at Trefil [SO 123 126], to 187 m at Dros-y-Lyn [SO 121 121] and to 220 m at Rhydw [SO 199 124]. Nearly 900 m³/d were obtained from Rhydw and Rhymney Bridge, but the other two boreholes yielded less than 130 m³/d (Welsh Water Authority, 1982).

Under normal circumstances, the alternation of sandstones and argillaceous beds within the Namurian Millstone Grit is characteristic of a multi-layered aquifer. However, in this district, the beds tend to contribute more to the recharge of the underlying limestones than to afford a resource in their own right. The groundwater quality is usually good with low hardness and a low chloride ion concentration, but, occasionally, iron concentrations exceed 0.3 mg/1.

Springs are common, usually of low yield, although discharges of more than 50 m³/d have been recorded. The seasonal fluctuations are usually smaller than those of the Dinantian limestones since the groundwater flow through the aquifer is much slower. Several of the springs rising from the Millstone Grit are apparently fed by the limestones beneath.

Yields of boreholes are usually less than 100 m³/d. An exception is at Carno [SO 159 137] where a borehole intersected an adit (Barclay and Jackson, 1982) and obtained nearly 500 m³/d, but much of the water probably came from the Dinantian limestones. Most of the groundwater is contained within fissures in the sandstones, and it is usually not worth drilling to depths of more than 50 m. Artesian flow from completed boreholes is common.

The Westphalian Coal Measures may be divided hydrogeologically into the Upper Coal (or Pennant) Measures, consisting predominantly of sandstone, and the Middle and Lower Coal Measures in which the dominant lithology is mudstone. Large quantities of groundwater are contained in the sandstones, and the interbedded mudstones separate the aquifer into discrete layers. Mining subsidence has tended to cause hydraulic continuity between these layers, and, in some locations, into the coal workings; combined with the mine dewatering, the result has been an extensive lowering of groundwater levels, even in the higher strata.

The outcrop of the higher part of the Upper Coal Measures is the high coalfield plateau, deeply dissected by the steep-sided coalfield valleys. A significant amount of the rainfall (up to 250 mm/annum) infiltrates to become groundwater. For the Lower and Middle Coal Measures the equivalent figure is nearer 150 mm/annum. Only about 5 per cent of this water is pumped out, the remainder contributing to the baseflow of the coalfield rivers, and usually emerging as springs at the bases of sandstones. Yields from the Coal Measures are variable, both spatially and seasonally. At Rhymney [SO 110 082], a borehole of 200 mm diameter and 115 m depth yielded nearly 650 m³/d on test, whereas another at Aberbeeg [SO 210 020] was abandoned for lack of yield. In those few areas remote from coal workings, it may be possible to obtain reasonable yields of potable water, but the Coal Measures cannot be regarded as a reliable aquifer in this district.

Springs issuing from the Coal Measures are common, with yields from the Upper Coal Measures sandstones being greater and locally exceeding 200 m³/d. In the Middle and Lower Coal Measures, there are few springs discharging more than 30 m /d, although this may be due in part to the interception of spring flow by mine drainage.

The Upper Coal Measures contain calcium bicarbonate and calcium sulphate waters with a low mineral content; more highly mineralised sodium bicarbonate and sodium sulphate waters are normally confined to the Middle and Lower Coal Measures (Ineson, 1967). In consequence, drainage from mines in which the water is derived predominantly from the Upper Coal Measures generally consists of water suitable for public supply.

Mine drainage fell by one-third between 1950 and 1972 (Rae, 1978), a trend that is still continuing with the closure of pits. As the coal workings become filled with water, and as groundwater levels rise in the surrounding strata, there is a substantial increase in groundwater storage. However, this is offset by the stored water being in general of poor quality and unsuitable for public supply without extensive treatment.

Quaternary The extensive deposits of glacial and fluvioglacial sands and gravels of the Usk Valley provide some supplies, amounting to about 300 000 m³/a, but their full potential as a groundwater source is imperfectly known. Boreholes in these deposits require sand screens. Depths do not generally exceed 10 m, and larger yields are usually sought by increasing the borehole diameter. At Abergavenny [SO 299 142], an excavated shaft of 3.6 m diameter was said to have yielded 9 m3/d. The groundwater in these deposits is close to the ground surface and impervious cover is usually absent. Accordingly, precautions are necessary to guard against pollution from overground sources.

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Appendix 1 Shafts, boreholes and cross-measures drifts

(Table 12) gives a summary of colliery shaft records, including the depths to the base of selected horizons. This appendix lists, by 1:10 000 or six-inch maps, the more important boreholes and cross-measures drifts. Complete records of these and the other records from the district are available for inspection at the BGS regional office, Aberystwyth and at the National Geosciences Data Centre, Keyworth. Copies can be purchased at a fixed tariff. Each entry in the following list gives the BGS permanent record number, location of the borehole or cross-measures drift, its stratigraphical range and reference to any published work.

• SO01SE/1 Cwar Yr Ystrad Borehole [SO 0842 1446]. SL + 544 m. Lower Limestone Shale Group to + 525.2 m, Wern Watkin Formation (Grey Grits) to + 505.8 m, Plateau Beds to + 479.6 m, Brownstones drilled to + 465.7 m. Taylor and Thomas, 1975
• SO10NW/44 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/45 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/46 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/47 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/48 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/49 Sirhowy No. 9 Pit boreholes c. [SO 146 090]. Middle Coal Measures.
• SO10NW/71 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/72 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/73 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/74 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/75 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/76 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/77 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/78 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/79 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/80 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/81 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/82 Pontlottyn Industrial Estate boreholes c. [SO 106 081]. Middle and Lower Coal Measures
• SO10NW/97 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/98 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/99 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/100 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/101 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/102 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/104 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/105 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/106 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/107 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/108 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/109 Tredegar Central Area Development boreholes c. [SO 141 093]. Middle Coal Measures
• SO10NW/119 Charles Sheet boreholes c. [SO 139 099]. Middle and Lower Coal Measures
• SO10NW/120 Charles Sheet boreholes c. [SO 139 099]. Middle and Lower Coal Measures
• SO10NW/121 Charles Sheet boreholes c. [SO 139 099]. Middle and Lower Coal Measures
• SO10NW/122 Charles Sheet boreholes c. [SO 139 099]. Middle and Lower Coal Measures
• SO10SW/10 Ogilvie Tip Extension boreholes c. [SO 119 039]. Upper Coal Measures; Brithdir Beds
• SO10SW/11 Ogilvie Tip Extension boreholes c. [SO 119 039]. Upper Coal Measures; Brithdir Beds
• SO10SW/17 Ogilvie Tip boreholes c. [SO 116 035]. Upper Coal Measures; Brithdir Beds and Rhondda Beds
• SO10SW/18 Ogilvie Tip boreholes c. [SO 116 035]. Upper Coal Measures; Brithdir Beds and Rhondda Beds
• SO10SW/19 Ogilvie Tip boreholes c. [SO 116 035]. Upper Coal Measures; Brithdir Beds and Rhondda Beds
• SO10SW/20 Ogilvie Tip boreholes c. [SO 116 035]. Upper Coal Measures; Brithdir Beds and Rhondda Beds
• SO10SW/21 Ogilvie Tip boreholes c. [SO 116 035]. Upper Coal Measures; Brithdir Beds and Rhondda Beds
• SO10NE/11 Rose Heyworth Tip borehole c. [SO 199 066]. Upper and Middle Coal Measures
• SO10NE/12 Rose Heyworth Tip borehole c. [SO 199 066]. Upper and Middle Coal Measures
• SO10NE/18 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/19 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/20 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/21 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/22 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/23 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/24 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/25 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/26 Waun Lwyd Tip boreholes c. [SO 180 063]. Upper Coal Measures; Rhondda Beds and Llynfi Beds
• SO10NE/32 Beynon Tip borcholes c. [SO 197 087]. Middle Coal Measures
• SO10NE/33 Beynon Tip borcholes c. [SO 197 087]. Middle Coal Measures
• SO10NE/38 Peacehavcn boreholes c. [SO 155 070]. Upper Coal Measures; Rhondda Beds
• SO10NE/39 Peacehavcn boreholes c. [SO 155 070]. Upper Coal Measures; Rhondda Beds
• SO10NE/40 Peacehavcn boreholes c. [SO 155 070]. Upper Coal Measures; Rhondda Beds
• SO10NE/41 Peacehavcn boreholes c. [SO 155 070]. Upper Coal Measures; Rhondda Beds
• SO10NE/42 Peacehavcn boreholes c. [SO 155 070]. Upper Coal Measures; Rhondda Beds
• SO10NE/43 Bedwellty Tip boreholes c. [SO 153 057]. Upper Coal Measures; Rhondda Beds
• SO10NE/44 Bedwellty Tip boreholes c. [SO 153 057]. Upper Coal Measures; Rhondda Beds
• SO10SE/16 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/17 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/18 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/19 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/20 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/21 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/22 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/23 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/39 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/40 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/41 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/42 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/43 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/44 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/45 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/46 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/47 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/48 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/49 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/50 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/51 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/52 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/53 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/54 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/55 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/56 Marine Tip boreholes c. [SO 185 044]. Upper Coal Measures; Hughes Beds, Brithdir Beds and Rhondda Beds
• SO10SE/24 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/25 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/26 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/27 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/28 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/29 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/30 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/31 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/32 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/33 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/34 Markham Tip boreholes c. [SO 168 017]. Upper Coal Measures; Hughes Beds, Brithdir Beds
• SO10SE/35 Marine Colliery underground borehole [SO 1895 0398]. Starts at Five-Feet-Gellideg ( - 173.7 m); Lower Coal Measures to - 256.6 m; Millstone Grit drilled to - 259.2 m
• SO11SW/36 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/37 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/38 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/39 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/40 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/41 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/42 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/43 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/44 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/45 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/46 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/47 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/48 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/49 Tafarnau Bach Industrial Estate boreholes c. [SO 120 105]. Lower Coal Measures
• SO11SW/76 Dros-y-lynn Borehole [SO 1212 1212]. SL c. + 393 m, Millstone Grit to + 349 m, Dowlais Limestone drilled to + 205 m
• SO11SW/77 Charles Street boreholes [SO 138 100]. Middle and Lower Coal Measures
• SO11SW/78 Charles Street boreholes [SO 138 100]. Middle and Lower Coal Measures
• SO11SE/66 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/67 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/68 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/69 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/70 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/71 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/72 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/73 Rassau Industrial Estate boreholes c. [SO 147 118]. Lower Coal Measures
• SO11SE/70 Glanffrwd Terrace boreholes c. [SO 165 106]. Middle and Lower Coal Measures
• SO11SE/71 Glanffrwd Terrace boreholes c. [SO 165 106]. Middle and Lower Coal Measures
• SO11SE/78 Church View boreholes c. [SO 170 112]. Lower Coal Measures
• SO11SE/79 Church View boreholes c. [SO 170 112]. Lower Coal Measures
• SO11SE/80 Church View boreholes c. [SO 170 112]. Lower Coal Measures
• SO11SE/81 Church View boreholes c. [SO 170 112]. Lower Coal Measures
• SO11SE/82 Church View boreholes c. [SO 170 112]. Lower Coal Measures
• SO11SE/113 Carno Adit [SO 1640 1270]. Millstone Grit, Dowlais Limestone and Llanelly Formation. Barclay and Jackson, 1982
• SO11SE/114 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/115 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/116 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/117 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/118 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/119 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/120 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/121 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/122 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/123 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/124 Carno New Dam boreholes c. [SO 160 135]. Lower Coal Measures and Millstone Grit
• SO11SE/125 Aneurin Place boreholes c. [SO 192 123]. Middle and Lower Coal Measures
• SO11SE/126 Aneurin Place boreholes c. [SO 192 123]. Middle and Lower Coal Measures
• SO11SE/127 Aneurin Place boreholes c. [SO 192 123]. Middle and Lower Coal Measures
• SO11SE/128 Aneurin Place boreholes c. [SO 192 123]. Middle and Lower Coal Measures
• SO11SE/129 Rhydw Borehole [SO 1995 1244]. SL c. + 353 m. Lower Coal Measures and Millstone Grit to + 105 m; Dowlais Limestone drilled to + 133 m
• SO11SE/130 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/131 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/132 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/133 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/14 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/135 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/136 Brynmawr Bypass boreholes c. [SO 195 118]. Middle and Lower Coal Measures
• SO11SE/237 Rhydw Farm boreholes c. [SO 199 124]. Lower Coal Measures
• SO20NW/1 Abertillery New Mine [SO 2050 0580]. Upper Coal Measures (Llynfi Beds), Middle and Lower Coal Measures
• SO20NW/20 Rose Heyworth Tip boreholes c. [SO 201 063]. Upper and Middle Coal Measures
• SO20NW/21 Rose Heyworth Tip boreholes c. [SO 201 063]. Upper and Middle Coal Measures
• SO20NW/22 Rose Heyworth Tip boreholes c. [SO 201 063]. Upper and Middle Coal Measures
• SO20NW/23 Rose Heyworth Tip boreholes c. [SO 201 063]. Upper and Middle Coal Measures
• SO20NW/19 Washery Drift, Big Pit [SO 2358 0936]. Middle and Lower Coal Measures
• SO20NW/24 Big Pit Tip boreholes c. [SO 235 090]. Middle Coal Measures
• SO20NW/25 Big Pit Tip boreholes c. [SO 235 090]. Middle Coal Measures
• SO20NW/26 Big Pit Tip boreholes c. [SO 235 090]. Middle Coal Measures
• SO20NW/27 Big Pit Tip boreholes c. [SO 235 090]. Middle Coal Measures
• SO20NW/63 East Pentwyn Farm boreholes c. [SO 205 075]. Upper Coal Measures (Llynfi Beds) and topmost Middle Coal Measures
• SO20NW/64 East Pentwyn Farm boreholes c. [SO 205 075]. Upper Coal Measures (Llynfi Beds) and topmost Middle Coal Measures
• SO20NW/65 East Pentwyn Farm boreholes c. [SO 205 075]. Upper Coal Measures (Llynfi Beds) and topmost Middle Coal Measures
• SO20SW/11 Blaenserchan Colliery underground borehole No. 1 [SO 2435 0213]. Starts at Seven-Feet ( - 7.42 m); Lower Coal Measures to - 85.6 m, Millstone Grit to - 108.5 m, Dowlais Limestone drilled to - 109.5 m
• SO20SW/14 Six Bells Colliery underground borehole [SO 2191 0279]. Starts at Five-Feet-Gellideg ( - 134.11 m); Lower Coal Measures to - 206.3 m, Millstone Grit drilled to - 227.8 m
• SO20NE/6 Waun-Pwll mawr Borehole [SO 2729 0774]. SL c. + 403 m. Drift to + 399.9 m, Millstone Grit to + 383.6 m, Clydach Valley Group drilled to + 377.3 m. Barclay and Jones, 1978
• SO20NE/7 Forgeside Borehole [SO 2504 0828]. SL c. + 345 m. Drift to + 334.8 m, Lower Coal Measures to + 290.6 m, Millstone Grit to + 263.8 m, Llanelly Formation drilled to + 252.1 m. Barclay and Jones, 1978
• SO20NE/21 Dragon's Site boreholes c. [SO 205 094]. Lower Coal Measures
• SO20NE/22 Dragon's Site boreholes c. [SO 205 094]. Lower Coal Measures
• SO20NE/35 Baker Street boreholes c. [SO 253 087]. Llanelly Formation
• SO20NE/36 Baker Street boreholes c. [SO 253 087]. Llanelly Formation
• SO20SE/1Abersychan boreholes c. [SO 264 035]. Lower Coal Measures and Millstone Grit. Hall and Squirrell, 1972
• SO20SE/2 Abersychan boreholes c. [SO 264 035]. Lower Coal Measures and Millstone Grit. Hall and Squirrell, 1972
• SO20SE/3 Abersychan boreholes c. [SO 264 035]. Lower Coal Measures and Millstone Grit. Hall and Squirrell, 1972
• SO20SE/4 Abersychan boreholes c. [SO 264 035]. Lower Coal Measures and Millstone Grit. Hall and Squirrell, 1972
• SO20SE/5 IGS Abersychan Borehole [SO 2617 0347] SL + c.201 m. Drift to + 194.7 m, Lower Coal Measures to + 168.9 m, Millstone Grit to + 147.6 m, Llanelly Formation drilled to + 137.6 m. Barclay and Jones, 1978
• SO20SE/41 Cwmffrwd Halt Borehole [SO 2708 0416] SL + 608 m, Drift to + 604.6 m, Clydach Valley Group to + 593.9 m, Cwmyniscoy Mudstone to + 579 m, Castel' Coch Limestone to + 542.8 m, Quartz Conglomerate Group drilled to + 531.8 m
• SO20SE/55 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/56 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/57 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/58 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/59 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/60 Greenland Tip boreholes c. [SO 258 023]. Middle Coal Measures
• SO20SE/61 Blaenserchan Tip Borehole No. 1 [SO 2519 0239] SL + c.304 m; Upper Coal Measures; Brithdir Beds, Rhondda Beds and Llynfi Beds drilled to + 224.8 m
• SO21SW/17 Pwll du Tunnel boreholes c. [SO 246 117]. Millstone Grit, Dowlais Limestone and Llanelly Formation
• SO21SW/18 Pwll du Tunnel boreholes c. [SO 246 117]. Millstone Grit, Dowlais Limestone and Llanelly Formation
• SO21SE/13 Pen-ffordd-goch Borehole [SO 2553 1058] SL + c.479 m. Lower Coal Measures to + 412.7 m, Millstone Grit to + 388.4 m, Llanelly Formation to + 378.6 m, Clydach Valley Group drilled to + 338.8 m. Barclay and Jones, 1978
• SO21SE/29 Garnddyrys Tip Borehole No. 1 [SO 2572 1191] SL + 375.3 m. Lower Limestone Shale Group to + 353.2 m, Garngofen Formation drilled to + 346.3 m

Appendix 2 List of Geological Survey photographs Sheet 232

Robertson (1927, Appendix II, p.137) listed photographs then available. These can be examined in the library of the British Geological Survey, Keyworth. The following were taken during the resurvey and are deposited in the library of the British Geological Survey, Keyworth. They are available for purchase as black and white prints, colour prints, or 2 x 2 in colour transparencies. Dates of the photographs are given, and National Grid references are in square brackets.

_Silurian

Devonian

Carboniferous Dinantian

Namurian

Westphalian

Quaternary

Views

• Catalogued under Sheet 231

Appendix 3 Lists of selected Wenlock and Ludlow series fossil localities and fossils

A Selected localities

A complete list is held by the Biostratigraphy Research Group, BGS, Keyworth. National Grid references are in square brackets, followed by BGS registered specimen numbers.

Glascoed Mudstone Formation

Ton Siltstone Formation

Usk Limestone

Lower Forest Beds

Upper Forest Beds

Lower Llanbadoc Beds

Upper Llanbadoc Beds

Lower Llangibby Beds

Middle Llangibby Beds

Upper Llangibry Beds

B List of fossils collected

The name of each fossil is followed by the locality number (see preceding list). The qualification of the specific identification follows the number, where appropriate; qualification of the entire identification precedes the number.

Hydrozoa

• Stromatopora sp.22

Anthozoa

• Acanthohalysites sp.21
• auloporoid 41D
• Favosites hisingeri Milne-Edwards and Haime 20
• Favosites sp.24, 38, ?41D, ?41F, ?42B
• heliolitid 18
• Ketophyollum cf. turbinatum verrucosum (Hisinger) 5
• Ketophyllum? 6
• Phaulactis sp.?41C, ?41D, ?41F, 46
• Propora aff. tubulata (Lonsdale) 5
• Pycnactis sp.36
• Rhabdocyclus? 35
• rugose corals-unclassified 18, 24, 42B, 45, 55
• tabulate coral-unclassified 33

Bryozoa

• bryozoans-unclassified 5, 9, 11, 12, 15–7, 30, 35–7, 40, 41H, 45, 51B, 51C
• Fenestella sp.16
• Leioctenza? 41A, 41B, 41F, 41H

Brachiopoda

• Aegiria grayi (Davidson) 2, 27–30, 34, 35, 41D, 41E
• Aegiria? 19
• Amphistrophia funiculata (McCoy) 17, 18, 20, 21, 23–6, 32, 35–40, 41A, 41D-G, 45, 46
• Antirhynchonella linguifera (J de C Sowerby) 3, 5, 6?, 9
• Atrypa reticularis (Linnaeus) 1, 3, 14, 16, 18–26, 33–9, 41E-H, 44B, 45, 46, 48
• Atrypa sp.5, 9, 27
• Baturria [Cordatomyonia] edgelliana (Davidson) 4
• Coolinia pecten (Linnaeus) 9, 36, 38, 40, 41D-G
• Coolinia sp.29
• Craniops implicatus (J de C Sowerby) 10, 11, 23, 25, 27, 30, 32, 36, 40, 41A, 41E, 41G, 42B, 44B, 46, 47, 50, 51B, 52, 55
• Craniops sp.14
• Cyrtia exporrecta (Wahlenberg) 9
• Cyrtia sp.10
• Daleji na hybrida (J de C Sowerby) 2, 22, 23, 25, 26, 30–2, 38
• Dalejina? 21, 26, 41A
• Dayia navicula (J de C Sowerby) 31, 41A, 41D, 41E, 47, 48, 51B, 52
• Dayia? 31
• Dicoelosia biloba (Linnaeus) 9, 10, 27
• Eoplectodonta sp.2
• Eospirifer radiatus (J de C Sowerby) 1, 6, cf. 13E, 17, 23, 26, 39, 41C, 41H, 46
• Eospirifer sp.2, 29, ?34, ?41E
• Glassia obovata (J de C Sowerby) 29
• Glassia sp.2, 3, ?27, 28, 31
• Gypidula galeata (Dalman) 20, 26, 32, 33, 36, 37
• G. cf. lata Alexander 41H
• Gypidula spp. 5, 41F
• Howellella elegans (Muir-Wood) 21, 28–30, 32, 35, 37, 38, 41E, 41G, 42A, 42B, 44A, 47, 48, 50, 51B-D, 52, 53
• Howellella sp.4, 12, 13F, 17, 20, ?22, 27, 33, ?36, ?51B
• Hyattidina canalis (J de C Sowerby) 49, 50, 51B-D, 52, 53
• Isorthis amplificata Walmsley 25
• clivosa Walmsley 2, 9cf., 24, 26, 29–32, 33cf., 34cf., 47, 51D
• orbicularis (J de C. Sowerby) 47–9, 52
• Isorthis sp.?20, ?24, 31, ?33, 36–8, 40, ?41A, 41B, 41E, 41G, 42A, 44A, 44B, 45, 47
• Katastrophomena? 5
• Leangella segmentum (Lindstrom) 3, 9, 10, 27
• Leptaena depressa (J de C Sowerby) 5cf., 24cf., 25cf., 32, 33cf.,  35cf., 36cf., 40cf., 41Dcf., 41Fcf., 44Acf., 46cf.
• Leptaena sp.?10, 21, 24, 28, 33, 41A, 44B
• Leptostrophia filosa (J de C Sowerby) 21, 24–6, 30, 32, 36–40,  41A, 41E-G, 42A, 43, 52
• Lingula cf. lata J de C Sowerby 30
• Lingula spp. 1, 31, 51A
• Ludfordina pixis Kelly 3, 27–9
• Megastrophia (Protomegastrophia) semiglobosa (Davidson) 5, 8, 9
• Meristina obtusa (J Sowerby) 7, 18
• Meristina? 22
• Microsphaeridiorhynchus nucula (J de C Sowerby) 37, 41B, 47–50, 51B-D, 52–5
• cf. M. nucula 11, 12, 15, 16, aff.16, 17, 21
• Microsphaeridiorhynchus sp.14
• Morinorhynchus? 19
• Nucleospira cf. pisum (J de C Sowerby) 29
• Orbiculoidea rugata (J de C Sowerby) 47, 48, 52
• Orbiculoidea sp.13E, 24, 30, 31 orchid 13E
• Pentlandina cf. lewisii plakodis Bassett 9
• Pholidostrophia (Mesopholidostrophia) deflecta Bassett 20
• P. (M.) lepisma 31, 32, 34, 36–8, 40, ?41A, 41B, 41D, 41F-H, 46
• Pholidostrophia (Mesopholidostrophia) sp.2, 9, ?21, 29, 30, 35, 45
• Plectatrypa? 16
• Protochonetes ceratoides (Reed) 28, 31cf., 36
• P. ludloviensis Muir-Wood 42A, 47–50, 51B, 51C, 52–6
• P. minimus (J de C Sowerby) 9, 24, 25, 29cf., 30, 36
• Protochonetes sp.21, 41A, 41D
• Resserella canalis (J de C Sowerby) 9, 19
• Resserella sabrinae Bassett 10
• Resserella? 5, 19, 41A, 41B, 41D
• Salopina conservatrix (McLearn) 11, 12, 14cf., 15cf., 16
• Salopina lunata (J de C Sowerby) 47, 49, 50, 51B, 51C, 52–6
• Salopina? 13F, 40, 41A, 51B
• Shagamella minor (Salter) 31, 34, 35, 41D, 41E, 41G, 42B, 48
• Shaleria ornatella (Davidson) 31 aff., 48–50, 51A, 51C, 52, 53
• Shaleria sp.?25, 35, ?51B, 51D
• Skenidioides lewisii (Davidson) 2, 3, 10, 29–31
• Sphaerirhynchia wilsoni (J Sowerby) 22, 23, 25, 26, 32, 33,
• 36–40, 41A, 41B, 41G, 41H, 42B, 43, 44A, 46–8
• Sphaerirhynchia sp.?1, 14, 18, ?21, ?41D
• ?Stegerhynchus borealis (von Buch) 10, 19
• Striispirifer pticatellus (Linnaeus) 9, 41D, 41E
• Strophochonetes? 10
• Strophonella euglypha (Dalman) 1, 20, 21, 24, 26, 34–40, 41F, 41H
• Strophonella sp.33, 38, ?41G, 44B
• Ygerodiscus aff. cornutus (Davidson) 2, 3

Gastropoda

• bellerophontid 1–3
• Euomphalopterus alatus alatus (Wahlenberg) 41B
• E. alatus subundulatus (Salter) 42A, 43
• Gyronema octavia (d'Orbigny) 30
• Liospira striatissima (Salter) 37
• Liospira? 45
• Loxonema gregarium (J de C Sowerby) 55
• L. planatum Longstaff 3
• Loxonema spp. 31, 55, 56
• Plectonotus boucoti Peel 13Eaff., 15
• Poleumita globosa (Schlotheim) 26, 43
• Poleumita? 5, 42A

Bivalvia

• Actinodonta? 16
• Cardiola sp.51C
• Cleidophorus? 41A
• Colpomya consors Reed 30
• `Ctenodonta' anglica (d'Orbigny) 52
• Ctenodonta? 41A
• Cuneamya truncata (McCoy) 51C
• Cypricardinia subplanulata Reed 10, 41D, 48
• Cypricardinia? 37
• Fuchsella amygdatina U de C Sowerby) 47, 51C, 52
• Fuchsella? 35, 47
• Goniophora cymbaeformis (J de C Sowerby) 48, 54, 55
• Goniophora? 30
• Grammysia aff. pembrokensis Williams 15
• G. aff. triangulatus (Salter) 15, 16
• Grammysia sp.51B, ?51C
• Leptodesma (Leiopteria) spp. 42B, 48, 51C, 52, 53, 55
• `Modiolopsis' gradatus (Salter) 17
• Modiolopsis spp. ?29, 30, 55
• Mytilarca mytilimeris (Conrad) 20, 42B, 46
• Mytilarca sp.6, 30, 31
• 'Wuculana' sp.29
• Nuculites antiquus (J de C Sowerby) 55
• N. pseudodeltoideus Reed 31
• Nuculites spp. ?2, 14, 17, 51C, ?52
• Orthonota rigida (J. de C Sowerby) 41A, 51B
• Palaeoneilo aff. sera Reed 41C
• Palaeopecten danbyi (McCoy) 48
• Paracyclas' perovalis (Salter) 30cf., 31, ?41A, 48
• Pteronitella retroflexa (Wahlenberg) ?48, ?51C, ?51D, ?52,?54, 55
• Ptychopteria (Actinopteria) tenuistriata (McCoy) 51B, 51C
• Ptychopteria (Actinopteria) spp. 11, 15, 17, 19, 25, 37, 38, ?41B, 47

Cephalopoda

• Dawsonoceras annulatum (J Sowerby) 7
• Kionoceras angulatum (Wahlenberg) 30, 31, 41A
• Michelinoceras? aff. recticinctum (Blake) 41E
• 'Orthoceras' ibex J. de C. Sowerby 30cf., 52; 55
• D. tracheate? J. de C. Sowerby 55

Tentaculitida

• tentaculitids-unclassified 13A, 14, 35, 42B, 51B

Hyolithida

• Hyolithes forbesi (Sharpe) 30,41D

Trilobita

• Acanthopyge hirsuta (Fletcher) subsp. 10
• Acaste downingiae (Murchison) 14
• Acaste sp.15
• Acastella? 51C
• Acastocephala? 9
• Calymene cf. blumenbachii Brongniart 30
• C. puellaris Reed 51Bcf., 51C, 53
• Calymene spp. 16, 19, 35, 38, 41A, 41D, 41E, 51A-C, 52
• cheirurid 5
• Cyphoproetus depressus (Barrande) 9
• Dalmanites spp. 3, 4, 6, 9, 13E, 19, 21, 24, 26–31, 33, 34, 36–9, 41E, 46
• Encrinurus rosensteinae Tripp, Temple and Gass 35, 36, 38, 41C, 41Ecf.
• E. tuberculatus (Buckland) 2cf., 8
• E. variolaris (Brongniart) 25
• Encrinurus spp. 1, 3, 7, 9, 38, 39, 44B
• Hemiarges aff. bucklandi (Milne-Edwards) 10
• Homatonotus sp.15
• homalonotid ?16
• phacopid 13E, 14
• Proetus (Lacunoporaspis) obconicus Lindstrom 51B
• proetid 35, ?41A, 52

Ostracoda

• Aechmina sp.14, 19
• beyrichiids-not classified 13E, 14, 16, 19, 27–30, 35, 37, 41A, 41C-E, 41C, 42B
• Cytherellina siliqua (Jones) 40
• ?Hemsiella maccoyiana (Jones) 40
• Neobeyrichia torosa (Jones) 40, 46cf.
• Primitia cf. punctata Jones 10

Crinoidea

• crinoid fragments 10–12, 13A, 13E, 14–21, 29, 30, 35–7, 41G, 41H
• Placocystites sp.11, 12

Machaeridia

• machaeridial plates 10

Graptolitoidea

• Saetograptus leintwardinensis incipiens (Wood) 47
• S. leintwardinensis leintwardinensis (Lapworth) 51A
• Saetograptus spp. 30, 41A
• saetograptid - indet. 43

Miscellaneous

• Cornulites serpularius Schlotheim 56
• fish fragments 13E

Index of Post-Ludlow series fossils

No distinction is made here between a positively determined genus or species and examples doubtfully referred to them (i.e. with the qualifications aff., cf., .ex. gr or ?), nor between spat, juvenile or adult forms. Fossils identifiable at generic level only (e.g. Anthraconaia sp.) are listed after the named species. Page numbers in italics refer to figures, plates and tables.

• acanthodian spine
• Acuthopecten sp.
• Agastrioceras carthatum Frech
• Alethopteris decurrens (Artis)
• A. lonchitica (Schlotheim)
• algae
• algal remains/nodules
• Alitaria sp.
• ammodiscids
• Amphissites sp.
• Anaplanisporites baccatus (Hoffmeister, Staplin & Malloy) Smith & Butterworth
• Angyomphalus sp.
• annelids
• Annularia radiata Brongniart
• Anthracoceratites sp.
• Anthraconaia adamsii (Salter)
• A. bellula (Bolton)
• A. expansa (Hind)
• A. hindi (Wright)
• A. lenisulcata (Trueman)
• A. pulchella Broadhurst
• A. pumila (Salter)
• A. stobbsi (Dix and Trueman)
• A. subcentralis (Salter)
• A. williamsoni (Brown)
• A. sp.
• Anthraconauta phillipsii (Williamson)
• A. phillipsii inter A. tenuis
• A. tenuis (Davies and Trueman)
• A. sp.
• Anthraconeilo laevirostrum (Portlock)
• A. taffiana
• A. undulata (Phillips)
• A. sp.
• Anthracosia ovum Trueman and Weir
• A. sp.
• Anthracosphaerium sp.
• Apatognathus sp.
• Apiculatisporis sp.
• Apiculiretusispora sp.
• archaediscids
• arthrodire plate
• arthropod fragment
• Asolanus camptotaenia Wood
• Asterophyllites equisetiformis (Schlotheim)
• A. sp.
• Athyris planosulcata (Phillips)
• Auroraspora solisortus Hoffmeister, Staplin & Malloy
• Aviculopecten dorlodoti Delepine
• A. gentilis (J. de C. Sowerby)
• A. sp.
• Avonia sp.
• Axophyllum vaughani (Salee)
• Baculatisporites fusticulus Sullivan
• Bellerophon sp.
• bellerophontoids
• Bilinguites sp.
• Biorbis duplex Strank
• Bispathodus aculeatus (Branson and Mehl
• B. aculeatus aculeatus (Branson and Mehl)
• B. aculeatus plumulus (Rhodes, Austin and Druce)
• B. spinulicostatus (E.R. Branson)
• bivalves (marine)
• (non-marine)
• Bothriolepis sp.
• brachiopods
• Brachymetopus edwardsi Owens
• bryozoan fragments
• rhabdomesid cryptostome
• burrow traces/burrowfills
• Buxtonia sp.
• calamitid branch casts
• Calamites sp.
• Calamospora parva Guennel
• C. sp.
• Cancelloceras cancellatum (Bisat)
• C. crencellatum (Bisat)
• C. cumbriense (Bisat)
• Caneyella sp.
• Caninia sp.
• Carbonicola extenuata Eagar
• C. extima Eagar
• C. pseudacuta Trueman
• C. pseudorobusta Trueman
• C. pyrimidata (Brown)
• C. robusta (J. de C. Sowerby)
• C. sp.
• Carbonita agnes (Jones)
• C. evelinae (Jones)
• C. humilis (Jones and Kirkby)
• C. pungens (Jones and Kirkby)
• C. salteriana (Jones)
• C. scalpellus (Jones and Kirkby)
• C. secans (Jones and Kirkby)
• C. sp.
• Carcinophyllum sp.
• Cavusgnathus sp.
• Cephalaspids
• Cephalaspis abergavenniensis White and Toombs
• C. agassizi Lankester
• C. (Cwmaspis) billcrofti White and Toombs
• C. cradleyensis Stensio
• C. cwmmillensis White and Toombs
• C. lyelli Agassiz
• C. salweyi Egerton
• C. whitei Stensio
• C.sp.
• Chaetetes sp.
• Chondrites
• chonetids
• Cleiothyridina roysil (Davidson)
• C.sp.
• Climatius ornatus (Agassiz)
• Cloghergnathus sp.
• Clydagnathus cavusformis Rhodes, Austin & Druce
• C. darensis Rhodes, Austin & Druce
• C. unicornis Rhodes, Austin & Druce
• C. sp.
• Cochlichnus kochi (Ludwig)
• codiaceans
• Colatisporites decorus (Bharadwaj & Venkatachala) Williams
• Coleolus sp.
• Composita ficoidea (Vaughan)
• C. sp.
• Cooksonia sp.
• conodonts
• Converrucosisporites sp.
• Convolutispora mellita Hoffmeister, Staplin & Malloy
• C. sp.
• corals
• Corvaspis sp.
• Crassispora trychera Neves & Ionnides
• C. sp.
• crinoid columnals/debris/fragments/ossicles
• crossopterygian remains
• crustaceans
• Curvirimula belgica (Hind)
• C. candela (Dewar)
• C. subovata (Dewar)
• C. trapezifornza (Dewar)
• C. sp.
• cyathaspid
• Cyathaspis sp.
• Cyclogranisporites spp.
• Cyperites sp.
• Cypricardella rectangularis (McCoy)
• Cyrtospirifer verneuili (Murchison)
• dasyclad fragments
• Dawsonites arcuatus Halle
• D. sp.
• Declinognathodus lateralis (Higgins and Bouckaert)
• Dictyotriletes sagenoformis Sullivan
• D.sp.
• Dielasma sp.
• Diplotmena schlatzlarensis Stur
• Donaldina ashtonensis (Bolton)
• Drepanophycus spinaeformis Goepp
• Dunbarella carbonaria (Hind)
• D. macgregori (Currie)
• D. papyracea (J. Sowerby)
• D. sp.
• Echinoconchus sp.
• echinoderm debris
• Edmondia sp.
• Elonichthys semistriatus Traquair
• E. sp.
• Euchondria sp.
• Euestheria sp.
• Eumetria carbonaria (Davidson)
• Euphemites anthracinus (Weir)
• E. jacksoni (Weir)
• E. sp.
• eurypterid remains/fragments
• Fasciculophyllum sp.
• fish debris/fragments/remains/scales/spines/tooth
• foraminifera
• arenaceous
• Gastrioceras subcrenatum Schmidt
• G. spp.
• gastropods
• turreted
• Gezicina arcuata (Bean)
• G. subarcuata (Jones)
• Gnathodus cuneiformis Mehl & Thomas
• G. simplicatus Rhodes, Austin & Druce
• G. sp.
• goniatites
• Goniporus alatus (Gross)
• Gosslingia breconensis Heard
• Gosslingia
• Granulatisporites sp.
• Gyrochorte carbonaria Schleicher
• Hapsiphyllum konincki (Milne-Edwards & Haime)
• H. sp.
• hexactinellid spicules
• Hindeodella sp.
• hindeodellid bar
• Hindeodus cristulus (Youngquist & Miller)
• H. sp.
• Holoptychius
• Homoceratoides
• Hostinella
• Huanghoceras sp.
• Hughmilleria
• Hyalostelia sp.
• Hymenozonotriletes
• Idiognathodus delicatus Gunnell
• I. sulcatus Higgins and Bouckaert
• Idiognathoides attenuatus (Harris and Hollingsworth)
• Katoporus grossi Karatajiite-Talimaa
• Knoxisporites triradiatus Hoffmeister, Staplin & Malloy
• Koninckopora inflata (de Koninck) Lee
• K. sp.
• Kujdanowiaspis sp.
• Leaia leidyi (Lea)
• L. sp.
• Leiopteria sp.
• Lepidodendron ophiurus (Brongniart)
• Lepidodendron
• Lepidophlois sp.
• Lepidophyllum sp.
• Leptagonia analoga (Phillips)
• L. sp.
• Leptodesma lichas Hall
• Lingula mytilloides J. Sowerby
• L. sp.
• lingulids
• Linoprotonia corrugatohemispherica (Vaughan)
• L. hemisphaerica (J. Sowerby)
• L. sp.
• Lissochonetes sp.
• Lithostrotion araneum (McCoy)
• L. martini Milne-Edwards & Haime
• Luciellina sp.
• Lycospora pusilla (Ibrahim) Somers
• Lyginopteris hoeninghausi (Brongniart)
• Macropotamorhynchus mitcheldeanensis (Vaughan)
• M. sp.
• Marginifera sp.
• Mariopteris acuta (Brongniart)
• M. nervosa (Brongniart)
• M. sp.
• Martinia sp.
• Megachonetes papilionaceous (Phillips)
• M. sp.
• Megalichthys sp.
• Merospirifer sp.
• Metacoceras sp.
• Michelinia megastoma Phillips
• M. 'megastoma' (M. sp.nov.)
• 'mussels'
• Myalina sp.
• Naiadites alatus/melvillei Trueman and Weir
• N. carinatus (J. de C. Sowerby)
• N. flexuosus Dix and Trueman
• N. hibernicus Eagar
• N. modiolaris (J. de C. Sowerby)
• N. obliquus Dix and Trueman
• N. quadratus (J. de C. Sowerby)
• N. sp.
• Nematothallus sp.
• neuropterids
• Neuropteris flexuosa Sternberg
• N. heterophylla Brongniart
• N. loshii Brongniart
• N. nachstebreckiana Leggewie & Schonefeld
• N. pseudogigantea H. Potonie
• N. ravinervis Bunbury
• N. rectinervis Kidston
• N. scheuchzeri Hoffman
• N. schlehani Stur
• N. tenuifolia (Schlotheim)
• N. sp.
• Nuculopsis aequalis (J. de C. Sowerby)
• N. gibbosa (Fleming)
• N. wewokana (Dirty)
• N. sp.
• Onchus sp.
• oncoids
• oncolites
• Orbiculoidea cincta (Portlock)
• O. nitida (Phillips)
• O. sp.
• orthotetoids
• Ortonella
• Osagia
• Osteolepis macrolepidotus Agassiz
• ostracoderm faunas
• ostracods
• Pachytheca sp.
• Palaeaspis sericea (Lankester)
• Palaeolima simplex (Phillips)
• Palaeosmilia murchisoni Milne-Edwards & Haime
• Palaeospiroplectammina mellina (Malakhova)
• Paraconularia sp.
• Parallelodon semicostatus McCoy
• P. sp.
• Paripteris gigantea (Sternberg)
• Patellilabia sp.
• Patrognathus variabilis Rhodes, Austin & Druce
• Pecopteris miltoni (Artis)
• P. oreopteridea (Schlotheim)
• Phestia acuta (J. Sowerby)
• P. attenuata (Fleming)
• P. sharmani (Etheridge)
• placoderm bone
• Planolites montanus Richter
• P. ophthalmoides Jessen
• P. sp.
• plant debris/fragments/remains
• Platyconcha hindi Longstaff
• Platysomus sp.
• plectogyrids
• Pleurophorella hindi (Bolton)
• Plicochonetes sp.
• Polidevcia acuta (J. de C. Sowerby)
• Polygnathus communis communis Branson & Mehl
• P. inornatus Branson
• P. mehli Thompson
• Poraspis sericeus Lankester
• P. sp.
• Posidonia sp.
• Prioniodina oweni Rhodes, Auston & Druce
• Productus carbonarius de Koninck
• P. sp.
• productoids
• Prolotaxites sp.
• Psephodus sp.
• Pseudopolygnathus dentilineatus Branson
• P. minutus Metcalfe
• P. Primus Branson and Nehl
• P. sp.
• Pseudosauripterus anglicus (A. S. Woodward)
• Psilophyton princeps Daws
• Psdophyton
• pteraspid
• pteraspidid
• Pteraspis rostrata (Agassiz)
• P. sp.
• Pterygotus
• Ptychacanlhus dubius Agassiz
• Ptychomaletoechia omaliusi (Gosselet)
• Ptychomphalus sp.
• Pupils vaughani (Muir-Wood)
• Punctati spathes irrassus Hacquebard
• P. nitidus
• Hoffmeister, Staplin & Malloy
• P. spp.
• Punctospirifer scabricosta North
• Raistrickia sp.
• Reticulatia americanus (Dunbar and Condra)
• Reticulatispordes polygonalis (Ibrahim) Smith & Butterworth
• Retispira striatus (Fleming)
• R. undata (R. Etheridge jun.)
• R. sp.
• Retusotrilites incohatus Sullivan
• R. sp.
• Rhabdoderma sp.
• Rhadinichthys sp.
• Rhinopteraspis cornubica McCoy
• R. [Pteraspis] crouchi Lankester
• rhipidistian tooth
• Rhipidomella michelini (Leveille)
• Rhizodopsis sauroides (Williamson)
• R. sp.
• rhynchonelloids
• Roundyella sp.
• Rugosochonetes hindi Muir-Wood
• R. vaughani Muir-Wood
• R. sp.
• Rugospora polyptycha Neves & Ioannides
• Sanguinolites ovalis Hind
• S. sp.
• Scaphaspis lloydii Lankester
• Schellwienella sp.
• Schizodus antiquus Hind
• S. axiniformis Phillips
• Schizophoria hudsoni George
• S. sp.
• Schopfites claviger Sullivan
• Schuchertella sp.
• Sciadophyton steinmanni Krausel & Weyland
• Securiaspis kitchini Stensio
• Selenimyalina sp.
• Serpula advena
• Serpulites stubblefieldi (Schmidt and Teichmuller)
• S. sp.
• Serpuloides
• siphonodellids
• Soleniscus sp.
• Solenomya sp.
• Spathognathodus scitulus (Hinde)
• S. stabilis (Branson & Nehl)
• S. sp.
• Spelaeotriletes arenaceus Neves & Owens
• S. sp.
• Sphenophyllum emarginatum Brongniart
• S. miriophyllum Crepin
• S. saxifragaefolium (Sternberg)
• Sphenopteris hoeninghausi -Brongniart
• S. sp.
• Sphenothallus stubblefieldi Schmidt and
• Teichmuller
• Spirorbis sp.
• sponge spicules
• spongiostromes
• Sporogonites exuberans Halle
• spiriferid
• spiriferoids
• smooth
• Stenoscisma isorhyncha (McCoy)
• Stenozonotriletes sp.
• Stensiopelta woodwardi (Stensio)
• Strepsodus sp.
• stroboceratid
• stromatolites
• Syringopera geniculata Phillips
• S. ramulosa Goldfuss
• S. reticulata Goldfuss
• S. sp.
• Syringothyris elongata North
• S. exoleta. North
• S. principalis North
• S. sp.
• Taeniocrada sp.
• Taphrognathus sp.
• Tetraxis paraminimus Vissarionova
• thelodonts
• Tomaculum
• Tornquistia polita (McCoy)
• trilobite
• Turinia pagei (Powrie)
• Umbonatisporites distinctus Clayton
• Unispirifer tornacensis (de Koninck)
• U. sp.
• Vallatisporites cdiaris (Lubu) Sullivan
• V. sp.
• vascular plants
• Verrucosisporites baccatus Staplin
• nitidus (Naumova) Playford
• V. sp.
• worm tubes/traces
• Zaphrentis (Sychnoelasma) konincki (Milne-Edwards & Haime)
• zaphrentoids
• Zosterophyllum australianum Lang and Cookson
• Z. llanoveranum Croft and Lang
• Z. sp.