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Geology of the South Wales Coalfield Part 1 The country around Newport (Mon.) (Explanation of One-inch Geological Sheet 249, New Series)

By H. C. Squirrell, and R. A. Downing with contributions by B. Kelk

Bibliographic reference: Squirrell, H.C. Downing, R.A. and Kelk, B. Geology of the South Wales Coalfield Part I The country around Newport (Mon.) (Explanation of One-inch Geological Sheet 249, New Series) London: Her Majesty's Stationery Office, 1969.

Natural Environment Research Council Institute of Geological Sciences

Memoirs of the Geological Survey of Great Britain England and Wales

Geology of the South Wales Coalfield Part I The country around Newport (Mon.) (Explanation of One-inch Geological Sheet 249, New Series)

• H. C. Squirrell, B.Sc., Ph.D., and R. A. Downing, B.Sc. with contributions by B. Kelk, B.Sc., Ph.D.
• Palaeontology mainly by M. A. Calver, M.A., M. Mitchell, M.A., W. H. C. Ramsbottom, M.A., Ph.D., H. A. Toombs, A.L.S. and D. E. White, M.Sc.
• Petrology mainly by J. R. Hawkes, B.Sc., Ph.D.
• Coal Economics by H. F. Adams, M.Sc., F.R.I.C.
• Water Supply by J. B. W. Day, B.Sc.

Third Edition London Her Majesty's Stationery Office 1969 © Crown copyright 1969. Published by Her Majesty's Stationery Office. First published 1899. Third Edition 1969. Printed in England for Her Majesty's Stationery Office by Hull Printers Ltd., Willerby, Hull, Yorks.

The Institute of Geological Sciences was formed by the incorporation of the Geological Survey of Great Britain and the Museum of Practical Geology with Overseas Geological Surveys and is a constituent body of the Natural Environment Research Council

Preface

Preface to the First Edition

This memoir is the first of a series which it is proposed to issue in explanation of the new Geological Survey Maps of the South Wales Coalfield. This great industrial district was one of the earliest to be examined after the institution of the Geological Survey, as a branch of the Ordnance Survey, in 1835. Much of it was mapped by Sir Henry De la Beche himself, the first Director-General, but the Staff included also Professor John Phillips, Mr. D. H. Williams, Mr. (afterwards Sir) Andrew C. Ramsay, Mr. W. T. Aveline, Mr. J. Rees, and Mr. T. E. James. An important area, however, extending from Cwmavon to Carmarthen Bay had already been mapped upon the one-inch Ordnance map by Mr. (afterwards Sir) William E. Logan, and it was arranged that not only should his mapping be incorporated with the official work of the Geological Survey, but that Logan himself should become a member of the Staff.

Few records exist as to the distribution of the Staff at that early time, but so far as regards the area described in the following pages it is known to have been geologically mapped by De la Beche and Williams between the years 1837 and 1845.

In June 1891, it having been represented in the House of Commons that the original maps, admirable as they were pronounced to be at the time of their production, had become obsolete both in topography and geology, a re-survey of the whole area was sanctioned. Within five weeks thereafter this work was begun by Mr. Strahan in the neighbourhood of Newport and Pontypool, and the Sheets 249 and 232 of the New Series of Ordnance Maps on the scale of one inch to a mile were published in 1895–96. In 1893 Mr. Strahan was joined by Mr. Walcot Gibson, who surveyed a small portion of the north-west corner of Sheet 249 and part of Sheet 232, and in 1894 by Mr. J. R. Dakyns, who was engaged in the northern part of Sheet 232. Subsequently this force was increased by the addition of Mr. R. H. Tiddeman and Mr. T. C. Cantrill, who have been engaged further to the south and west in the ground comprised in Sheets 231, 248, and 263. The present Memoir has been prepared by Mr. Strahan, and the lists of fossils have been revised by Mr. E. T. Newton and Dr. F. L. Kitchin.

The original Geological Survey of South Wales was unaccompanied by detailed explanatory Memoirs, but much general information was introduced by De la Beche into his treatise "On the Formation of the Rocks in South Wales and South-Western England", which forms part of the first volume of the Memoirs of the Geological Survey. Analyses of the Iron Ores, at that time extensively worked, and lists of the interesting fossils contained in them appeared in the same Memoirs (The Iron Ores of Great Britain, Part III). A First Report on the Coals suited to the Steam Navy, accompanied by analyses in which the Welsh Coals are discussed, was drawn up by Sir Henry De la Beche and Dr. Lyon (afterwards Lord) Playfair (Chemist to the Geological Survey), and was presented to Parliament. It was published as a Blue Book in 1848, and in the same year appeared in the Second Part of the Second Volume of the Memoirs of the Geological Survey. It was followed by further researches on the same subject by the same authors, which were embodied in a Second (1849) and Third Report (1851), also presented to Parliament, and published among the Parliamentary Papers. Among the early contributions of the Geological Survey to the Geology of South Wales, reference may also be made to an account of the Usk inlier of Silurian rocks and of their fossils, which was given by Professor John Phillips in his Essay on the Malvern Hills, in Part I of the Second Volume of the Memoirs.

One of the results of the present re-survey has been the sub-division of great masses of strata hitherto grouped together. The Old Red Sandstone of South Wales has been shown to include representatives of both the Upper and Lower divisions of other parts of the Kingdom, though, so far as at present examined, the formation appears to exhibit an unbroken sequence such as occurs nowhere else, unless among equivalent strata in Devonshire. The true relations of the Upper and Lower Old Red Sandstone to one another, and to the formations above and below them, can only be elucidated by the examination of a wide area.

The Coal-measures, hitherto represented by one colour only on the map, have long been known to include a mass of sandstone by which an upper is separated from a lower series of coal-producing measures. The representation of these three subdivisions by colour on the map has served well to illustrate the structure of the Coalfield. The anticlinal and synclinal folds into which the measures have been thrown, and the faults by which they are cut through, have been traced out with an amount of detail which was impossible to the original surveyors. It was only after the progress of mining had thrown more light on the geological structure of the ground, and maps on the scale of six inches to a mile were available for use in the field that such a task could be undertaken. The importance of an accurate knowledge of the structure of the Coal-field in view of future mining operations can hardly be over-estimated, and an instance is given in this Memoir where many thousands of pounds were wasted for want of it.

While the revision is in progress the boundaries of the Superficial Deposits are traced at the same time as those of the solid rocks underneath, and are shown by colour on a "Drift Edition" of the map. The examination of the Glacial phenomena has led to results of much interest.

At present the map is published on the one-inch scale only, in two editions, one of which ("Solid Rocks Edition") shows the distribution and connections of the solid rocks, apart from the covering of superficial deposits that so often obscures them; the other ("Drift Edition") represents these deposits as they are distributed over the district, and only marks the solid rocks where they lie bare at the surface.

Manuscript copies of the original six-inch field maps are deposited in the Geological Survey Office, where they can be consulted, and copies can be supplied when desired at the cost of drawing and colouring. One Sheet of Vertical Sections (No. 80) has been published in illustration of the particular area described in the present Memoir.

Our thanks are due to a large number of Colliery Managers, Mining Engineers, and Surveyors for assistance given by them during the re-survey of the Coal-measures. Information and access to plans have in all cases been freely accorded to us.

Arch. Geikie. Director-General. Geological Survey Office, 28 Jermyn Street, London. 25th November, 1899

Preface to the Second Edition

In the ten years which have elapsed since this, the first of a series of Memoirs descriptive of the South Wales Coal-field, was issued, the re-surveying of that important area has been brought to the point of completion. On several subjects, on which it was impossible to form a decided opinion while the greater part of the area remained unknown, further light has now been thrown, but the conclusions then formed have been in the main justified. The delimitation for example of the Pennant Series has proved to be the most suitable subdivision of the Coal Measures from the point of view of illustrating the structure of the Coal Field upon the Maps, but a slight change in the correlation of the seam at the top of the series in adjoining basins has been made, as a result of arguments brought forward by Mr. H. K. Jordan.

The Carboniferous Limestone, since the six-inch survey of this part of its outcrop was completed, has been subjected to the researches of Dr. Vaughan, and though it was known that it was here poorly developed, no zonal comparison with other parts of South Wales had been possible. It appears now that though it thickens rapidly to the south-west, even in that direction all the upper zone is missing, so far as regards the area here described. Elsewhere nearly the whole of the series is in a dolomitic condition and generally devoid of such fossils as permit of the determination of the zones. The exact cause of the attenuation, whether it may be an unconformable overstep by the Millstone Grit or non-deposition, remains doubtful.

In the Old Red Sandstone it is now possible to fix with some certainty the limit between the upper and lower divisions, though there is slender, if any, evidence of an unconformity between the two.

A suspicion that the Silurian sequence in the Usk inlier was not complete, is now strengthened by a comparison with the sequence found to obtain near Llandilo, in ground more recently surveyed.

Again we have to acknowledge the assistance cordially given by the proprietors and managers of collieries in the work of bringing the mining information up to date. Since the publication of the First Edition of this Memoir no fewer than seven first-class collieries have been installed or commenced within the area described. In all cases information respecting them has been freely given.

The revision will be made on the maps as opportunity arises. Permission to publish the six-inch maps having been granted some time after the original six-inch survey was made, those maps have been only recently issued.

J. J. H. Teall Geological Survey Office Director 28 Jermyn Street, London 22nd March, 1909

Preface to the Third Edition

During the 57 years which have passed since the Second Edition of this memoir was published there has been a considerable increase in the knowledge of the strata represented in the Newport district, and also, as a result of the great expansion of the coal-mining industry during this period, there has accumulated a large amount of detailed information concerning the Coal Measures. Because of these factors a complete resurvey of the sheet was undertaken, and the district has been redescribed in the light of the new evidence, as well as the most recent advances in the understanding and classification of the rocks in the surrounding districts.

The resurvey took place during the years 1952–3 and 1957–61, and was made by Mr. R. A. Downing (1952–3 and 1957), Dr. B. Kelk (1960–61), Mr. G. D. E. Lewis (1958–60), Dr. H. C. Squirrell (1957–61) and Dr. A. W. Woodland (1953). The work was carried out under the superintendence first of Mr. V. A. Eyles, and then of Dr. F. B. A. Welch. The areas surveyed by each officer are shown in the list of six-inch maps on p. xiii. Sixteen National Grid six-inch maps covering areas occupied by Carboniferous rocks were published between the years 1963–67. The remaining six-inch maps are available for reference in the library. The new one-inch map in Drift and Solid editions has also been published.

The memoir has been written by Dr. Squirrell and Mr. Downing, partly from notes by Mr. Lewis, with contributions by Dr. Kelk. It was compiled by Dr. Squirrell and edited by Dr. J. R. Earp.

Fossils have been named and palaeontological notes supplied by the following members or former members of the Palaeontological Department: Dr. F. W. Anderson, Mr. M. A. Calver assisted by Mr. M. J. Reynolds, Dr. H. C. Ivimey-Cook, Mr. M. Mitchell, Dr. W. H. C. Ramsbottom, assisted by Mr. Reynolds, Mr. J. D. D. Smith and Mr. D. E. White.

Our thanks are given to the following for their assistance in naming certain fossil groups: Ludlow graptolites by Professor O. M. B. Bulman (University of Cambridge); Old Red Sandstone and Coal Measures plants by Dr. W. G. Chaloner (University College London); Mesozoic ammonites by Professor D. T. Donovan (University College London); Lower Old Red Sandstone fishes by Mr. H. A. Toombs (British Museum, Natural History), who also wrote an account of their palaeontology.

Petrographical determinations and descriptions were provided by Dr. R. Dearnley, Mr. R. W. Elliot, Dr. J. R. Hawkes, Mr. E. A. Jobbins and Dr. P. A. Sabine. X-ray examinations were made by Mr. B. R. Young. An account of the hydrology of the district has been written by Mr. J. B. W. Day.

During the resurvey every possible assistance was willingly given by officials of the National Coal Board, in particular by the Divisional Geologist, Mr. R. H. Price, and his assistant, Mr. J. Malthus, by Mr. H. F. Adams, the Officer-in-Charge of the South Wales Geological Outstation, who has also written a short account of the properties and uses of the coals, by the Opencast Executive, and by numerous members of the surveying staff. We are indebted to everybody concerned and acknowledge their invaluable co-operation and unfailing courtesy with grateful thanks.

K. C. Dunham Director. Institute of Geological Sciences. Exhibition Road, South Kensington, London. SW7 2nd January, 1969

List of six-inch maps

The following list shows the six-inch national grid maps included, wholly or partly, in the One-inch Newport (249) Sheet, with the initials of the surveyors and dates of resurvey. In those northern and eastern marginal maps without asterisk, only the areas within the One-inch Sheet boundary were resurveyed. The officers concerned with the resurvey were R. A. Downing, B. Kelk, G. D. E. Lewis, H. C. Squirrell and A. W. Woodland. Those maps marked with an asterisk have been published, and the remainder are available for reference in the Geological Survey and Museum.

Chapter 1 Introduction

Geographical features

The District^‡1  described in this memoir, covered by the Newport (249) Sheet of the New Series, One-inch Geological Survey Map, extends over 216 square miles of east Glamorgan and south-west Monmouthshire. This district includes the south-eastern part of the South Wales Coal Measures basin, the southern half of the Silurian Usk Inlier, and a large tract of Old Red Sandstone. The district is characterized by a variety of landscapes, ranging from high moorlands in the coalfield, to extensive, flat-lying areas of marine alluvium bordering the Bristol Channel. In the north-western half of the district the deeply-dissected Pennant Measures plateau, and Carboniferous and Old Red Sandstone ridges on the rim of the coalfield, form prominent topographical features rising to over 1000 ft on Mynydd Maen (1553 ft), Mynydd Lan (1264 ft), Cefn Crib (1300 ft) and Mynydd Machen (1192 ft). The south-eastern half of the district is typified by a lower, gently undulating landscape, which seldom rises over 500 ft, and by the alluvial flats south and east of Newport, which are nowhere higher than 25 ft above O.D.

The plateau is drained by deeply incised, southward-flowing rivers, which form a superimposed drainage system. The most important are the Taff, Rhymney, Sirhowy and Ebbw, all of which rise to the north of the district. They reach the lower-lying areas south of the coalfield through gaps cut in the border ridges, the most spectacular being the steep-sided gorge of the Taff at Taffs Well. The deep, east–west aligned Glyn Valley, which joins Crumlin and Pontypool, is now virtually dry, but in times past it must have been a major outlet for easterly-flowing water, or ice. In the east of the district the most important river is the Usk, which enters the area near Caerleon, and flows to the Bristol Channel through Newport. Its main tributaries are the southward-flowing Lwyd and Sôr, which join it at Caerleon, and a number of smaller streams which drain the south-eastern edge of the coalfield, e.g. Malpas Brook, which joins it at Newport.

By far the largest urban centre is the industrial town and port of Newport, with a population of over 100,000, while Pontypool is another important centre. The coalfield area is typified by 'ribbon development', that is, most of the buildings and lines of communication follow the valley floors because of the steepness of their sides, and many of the original villages and small towns have coalesced into continuous stretches of buildings running for miles along the valleys. Because of the limit to expansion there are no large towns, and many of the small towns contain only 3000 to 6000 people. The biggest is Caerphilly with a population of about 40,000.

Before the middle of the 18th century the Newport district was occupied by essentially agricultural communities in the lowlands, and pastoral in the mountain areas, although some of the mineral resources had been worked in small quantities for many years. Records of ironstone workings date back to the middle of the 16th century, coal mining in the Mynyddislwyn Vein at Felin Fach to the 13th century, and lead mining near Rudry to Roman times. It was not until the second half of the 18th century, with the oncoming of the Industrial Revolution, that the district began to be affected to any large extent by industrial activity, though not to such a degree as in areas along the north crop of the coalfield, where the iron industry was concentrated at that time. In the early part of the 19th century the first colliery company (Duncan and Co.,) to be set up in South Wales began working the Mynyddislwyn at Llanfabon, and in the late thirties Thomas Powell, founder of the great Powell Duffryn Company, began his colliery experiences near Gelligaer. It was in the middle part of the 19th century, when the demand for steam coal rose rapidly, that the district was markedly transformed into an industrial area, in which agriculture played a relatively minor, though important, role in the economy, as it does today. During the period 1850–1911 the coal industry underwent phenomenal expansion, and reached its peak during the early years of the present century. The size of the industry in South Wales at that time is evident from the 1911 export figure of nearly ten million tons of coal from Cardiff alone. Throughout this period the life of the coal-mining valleys was almost wholly dependent upon the coal industry, and because of this extreme specialization the area suffered seriously during the economic depression following the First World War. To counteract the effect of the decline in the industry a policy of diversification was introduced, to a small extent before the 1939–45 war, and to a much greater extent after. This resulted in projects such as the Treforest Industrial Estate at Nantgarw, the electrical engineering industry at Blackwood and Aberbargoed, the development of one of the largest aluminium rolling mills in Europe at Rogerstone, and recently, the extensive steel works at Llanwern, east of Newport.

At the present day coal mining is still the most important single industry, and is likely to be for many years, but manufacturing industries are becoming increasingly important in the industrial structure. Agriculture also plays a significant role in the general prosperity of the district, particularly in the areas occupied by Silurian and Old Red Sandstone strata, and the alluvial flats near Newport. The soils are suitable for general mixed farming, though the emphasis at present is on milk production. Within the coalfield soils are generally poor, but the land is suitable for grazing and the farms concentrate on the production of milk and meat. Forestry has become important since 1947, mainly within and around the boundaries of the coalfield, and to a small extent in the Usk Inlier.

Geological sequence

The formations represented on the map and sections, and described in this memoir are summarized below:

Geological history

The oldest known rocks in the district, the shelly mudstones of the Wenlock Shale, were laid down in a shelf sea which extended over the Welsh Borders, and possibly a large part of southern England. The overlying Wenlock Limestone probably reflects a shallowing of this sea for a short time, with the resulting development of an abundant, limestone-forming, shelly fauna. This phase was followed by another deeper water period, during which the thick mudstones of the lower part of the Ludlow Series were laid down. Throughout the remainder of the Ludlovian the sea gradually shallowed, and the final stages were characterized by the incoming of sandy sediments deposited in a deltaic environment. The basal beds of the Old Red Sandstone, the Downton Castle Sandstone, represent a continuation of this type of sedimentation, the boundary with the Silurian being transitional. In the south of the Silurian outcrop the poorly developed Ludlow Bone Bed, marking a short break in deposition, provides a convenient boundary between the systems.

The Downton Castle Sandstone was followed by the red, fish-bearing marls and sandstones of the Raglan Marl and St. Maughan's groups, which were derived under warm, humid conditions from the Caledonian Mountains to the north-west, and deposited on a delta plain. During this period the district was covered periodically by shallow lakes or seas, in which a wide range of calcareous deposits was formed. West of Newport the uppermost beds of the St. Maughan's Group contain coarse conglomerates derived from a land area to the south-west of the district.

The stratigraphical gap between the Brownstone Group and the overlying Upper Old Red Sandstone sediments records the effect of the Svalbardic phase of the Caledonian orogeny. By comparison with the intense movements experienced in Central and North Wales, and as near as south Gloucestershire, the effect of this phase of the orogeny in the Newport district appears to have been relatively slight, for there is no apparent discordance of dip between the two divisions. Towards the end of Old Red Sandstone times the fluviatile environment gradually gave way to the encroaching, shallow, shelf sea in which the shelly mudstones and limestones of the Lower Limestones Shale were deposited. Similar conditions, though without mudstone sedimentation, prevailed during the formation of the thick, organic limestones of the Main Limestone, many of which were penecontemporaneously dolomitized.

As a result of the effects of the Sudetic phase of the Hercynian orogeny, an unconformity, increasing in size eastwards, occurs between the Carboniferous Limestone and Millstone Grit series. Following the break in sedimentation the earliest Millstone Grit sediments deposited were of R₂ age at Taffs Well, and it was not until G, times that the Millstone Grit sea reached its maximum known eastward extent (Pontypool) in this district. The dominantly marine sediments of the Millstone Grit were followed by Coal Measures rocks typified by cyclic sedimentation under freshwater or estuarine conditions, with rare incursions of the sea. At intervals (most frequently in the Lower and Middle Coal Measures), during times of relative stability, the area of sedimentation was covered by vegetation, resulting in the formation of deposits of organic material, which on burial were converted into coal. The lower part of the Upper Coal Measures contains a number of beds of red mudstone and seatearth, which originated partly as flood plain deposits derived from red soils in an upland source area, and partly by the oxidation, under tropical weathering conditions, of grey sediments in situ. Above these red rocks the Upper Coal Measures are characterized by thick accumulations of typically false-bedded, pennant-type sandstone laid down by fast-flowing streams in a deltaic area. During much of Coal Measures times minor uplift in the east of the district resulted in easterly attenuation, including non-sequences within the coalfield, and erosion and unconformity along the south-east crop.

The major stratigraphical break between the Coal Measures and Keuper represents the period in which the main earth movements of the Hercynian orogeny gave rise to the principal structural features of the Newport district. Possibly at some phase during this orogeny local, small scale, lead-baryte mineralization, and perhaps even the hematitization (p. 67), of the Carboniferous rocks of the south crop were effected by deep-seated hydrothermal solutions. This tectonic period was followed by the erosion of an enormous thickness of strata which took place, in the later stages at least, under desert conditions, for the Keuper Marl was deposited in land-locked depressions in the deeper parts of which there were highly saline inland seas. It is widely believed that during the unusual environmental conditions of Triassic times the local hematitization of the Carboniferous Limestone occurred. This period was followed by a sudden invasion of the sea, as evidenced by the presence of fish and molluscs in the Rhaetic sediments, and of a rich, shelly fauna in the overlying shales and limestones of the Lower Lias.

Although the great interval of time between the Lower Lias and the Pleistocene is not represented by any sediments, it is possible that higher Jurassic and some Cretaceous strata were deposited and subsequently eroded. It has been suggested that rocks as high as the Chalk extended over much of south and central Wales, the present-day coalfield plateau being the relic sea floor of a late Mesozoic marine transgression. Following uplift and tilting caused by the Alpine orogeny, possibly in Miocene times, the present-day southward flowing river system was then initiated on the Cretaceous sediments.

Prolonged erosion subsequently removed the Mesozoic cover to expose the Palaeozoic rocks more or less as they are today.

During the Pleistocene Period ice flowed into the district from the north, the main movements being along the valleys formed by the Tertiary erosion. Possibly three major ice advances took place, separated by retreats during warmer periods, but only the effects of the last advance are clearly recorded. The southward and eastward extent of the last glaciation is marked by the thick, end-moraine deposits covering the Old Red Sandstone between Tongwynlais and Newport. At the end of the last glacial episode sea level was at least 100 ft lower than it is now, and the rivers flowed in valleys graded to this much lower level. Since then there has been a gradual rise in sea level, and the lower reaches of the valleys have been infilled with alluvial deposits to the present-day level. Evidence of other very recent events in the geological history of the district is furnished by landslipped slopes and peat-covered flats. H.C.S.

Chapter 2 Silurian

General

Silurian strata crop out over an area of about 12 square miles in the northeast corner of the Newport district, and comprise the southern part of the faulted pericline of the Usk Inlier, which occupies a total area of some 28 square miles. The succession consists of about 1700 ft of Wenlock and Ludlow rocks, which were laid down in the stable 'shelf' area lying to the south-east of the Welsh Geosyncline (Wills 1951, p. 12). The beds comprise a conformable sequence of mainly well sorted mudstones and siltstones, with some limestones and a few sandstones. In general, the oldest rocks, the Wenlock Shale, lie in the central part of the inlier, and the youngest rocks, the Whitcliffe Beds, lie around the margin, but faulting and folding have broken the symmetry of the outcrops, resulting in a complex outcrop pattern.

Wenlock Series

The Wenlock Series occupies about 3 square miles around Cilfeigan Park and Coed Mawr. The succession is similar to that of the Wenlock Series in other parts of the Welsh Borders, and has been classified as follows:

Wenlock Shale

The Wenlock Shale occupies approximately 2 square miles in the central part of the Silurian outcrop; almost half of its irregularly shaped outcrop is bounded by the faults which radiate from the centre of the inlier, and the remainder by its normal junction with the Wenlock Limestone. Because of the disposition of its outcrop the thickness of the Wenlock Shale is difficult to calculate, but it is estimated that about 500 to 600 ft of beds are present. Two broad divisions have been recognized within the Wenlock Shale; the lower comprises mainly mudstones, and the upper comprises mudstones with interbedded sandstones. They have not been mapped separately, because of the vague, transitional boundary between them, the lateral impersistence of the upper division, and the paucity of exposures in a structurally complicated area.

The lower division consists of 400 to 500 ft of soft, poorly-bedded, olive or brown mudstones and silty mudstones, which contain sporadic, thin beds of impure, grey, shelly limestone or impure limestone nodules, and thin, flaggy, light brown, silty sandstones and sandstones. Fossils are generally scattered throughout the succession, though they may be concentrated into richer layers, particularly in the thin impure limestones. Because of their soft nature, the beds of the lower division form low-lying ground in the core of the Usk Anticline north of Cilfeigan Park.

The upper division consists of up to 100 ft of beds, the sandstones of which are buff, yellow or brown, fine to coarse grained, sometimes ripple-marked, and commonly contain casts of crinoid columnals. The mudstones and silty mudstones, which are interbedded with the sandstones, are similar to those-in the lower division. Where they are well developed, the beds of the upper division form higher ground than the lower division, e.g. in the areas west of Cefn-ila Hospital, in Cilfeigan Park and east of Greenmeadow. The sandstones have been quarried for walling and building stone at a few localities. The highest beds of the upper division pass gradually into the Wenlock Limestone; sandy limestones appear at first, followed by the purer limestones of the overlying strata.

The fauna of the Wenlock Shale consists largely of poorly-preserved brachiopods associated with crinoids, bryozoa, gastropods, bivalves and trilobites. The most commonly occurring fossils are crinoid columnals, particularly in the upper division, and the brachiopods Amphistrophia funiculata,^‡2  Atrypa reticularis, 'Camarotoechia' nucula, Howellella sp., Leptaena cf. depressa, orthids, Sphaerirhynchia davidsoni and Strophonella euglypha. Less common fossils include bryozoa, the brachiopods Craniops implicata, Cyrtia exporrecta and Meristina obtusa, the trilobites Acaste downingiae and Dalmanites myops, and the ostracod Beyrichia sp.

Wenlock Limestone

The Wenlock Limestone occupies a narrow outcrop, never more than 275 yd wide, around the Wenlock Shale core of the Silurian outcrop, and also in the core of the minor Llangibby Anticline at Cwrn Dowlais, in the east of the inlier. The outcrop around the Wenlock Shale is broken by five faults, which displace it by as much as two-thirds of a mile, and give rise to five separate outcrops varying in length between 450 and 2200 yd. The Wenlock Limestone forms a low scarp feature in the area around Cilfeigan, but elsewhere crops out on the dip-slope of a scarp formed by the sandy beds of the upper part of the Wenlock Shale. The Wenlock Limestone is easily traced throughout the area for the beds have been worked in many quarries which follow the strike. The limestones were used for building purposes, walling, and the production of lime.

In the area under review the Wenlock Limestone varies in thickness between 30 and 45 ft, though one-third of a mile north of the district rapid thinning to only 2 ft was recorded by Walmsley (1959, p. 487) at Rhadyr. In the vicinity of Green-meadow the Wenlock Limestone is about 30 ft thick, and thickens eastwards, south-eastwards and southwards to 40 to 45 ft in the remainder of the inlier. The basal beds are nowhere exposed, but probably they lie conformably on the Wenlock Shale. The Wenlock Limestone consists of a succession of interbedded, massive and nodular limestone beds. The lowest bed visible is exposed in two quarries, and consists of massive limestone up to 9 ft thick lying 10 to 20 ft above the top of the Wenlock Shale. It is followed by 4 to 16 ft of nodular limestone, which are overlain by a further 6 to 12 ft of massive limestone. Above this second bed of massive limestone the uppermost beds of the Wenlock Limestone are invariably nodular limestones, 4 to 10 ft thick, which pass up fairly rapidly into the silty mudstones of the overlying Elton Beds.

The massive limestones are typically very hard, shelly and crinoidal, blue-grey, fine to coarsely crystalline beds, in which there is very little argillaceous material. A ballstone has been exposed within a massive limestone in Cilwrgi Quarry, near Common Coed-y-paen; it is a lenticular mass of unstratified limestone about 4 ft high containing an abundance of corals, bryozoa and crinoid remains. This is the only known occurrence of a ballstone in the Newport district. The nodular limestones consist of thin beds of argillaceous, bluish grey, poorly fossiliferous limestone nodules, generally not more than 6 inches in diameter, lying in a matrix of, or interbedded with, soft, buff or olive, silty mudstones.

The fauna of the Wenlock Limestone is characterized by an abundance of crinoid columnals and brachiopods, and relatively small numbers of corals, bryozoa, gastropods, bivalves, trilobites and ostracods. The most commonly occurring brachiopods are Atrypa reticularis, 'Camarotoechia' nucula, Leptaena spp., and less commonly Amphistrophia funiculata, Dolerorthis rustica, Leptostrophia filosa and Strophonella euglypha.

Ludlow Series

History of research and classification

The Ludlow Series of the Usk Inlier was first described by Murchison (1839), who recognized the similarity of the beds to those of the Ludlow area, and also that rocks equivalent to the Aymestry Limestone were present. Shortly after, Phillips (1848), on the basis of fossils collected at various horizons, proved the presence of beds equivalent to the Lower Ludlow, Aymestry Limestone and Upper Ludlow of Ludlow. Strahan (1899) apparently agreed with Phillips' classification, but, while making the primary survey of the Usk Inlier, he did not draw the boundaries between the divisions. Later, Gardiner (1917) held the view that the Ludlow Series was unconformably overlain by the Old Red Sandstone, and (1927, p. 528) that 'no Aymestry Limestone or Lower Ludlow Beds occur at Usk'. The most recent investigation was by Walmsley (1959) who completely remapped the area, and made a very detailed study of the stratigraphy. He showed that the Ludlow succession is virtually complete, and is closely comparable with the type sequence of the Ludlow area. He also showed that it is nowhere unconformably overlain by the Old Red Sandstone as suggested by Gardiner (1917), and that the eastern margin of the Silurian outcrop south of Llanbadoc is not faulted against the Old Red Sandstone, as shown on Strahan's maps. Amongst his other findings were the Ludlow Bone Bed, and a conglomerate band containing fish remains at the base of his Upper Llanbadoc Beds.

The present resurvey confirmed the stratigraphy elucidated by Walmsley, and it showed that the nine divisions of the new classification (Holland, Lawson and Walmsley 1963) of the Ludlow Series in the type area of Ludlow are represented at Usk. Thus, the nomenclature of the type area is used in this account, though for mapping purposes it proved most practical to group the nine divisions into four readily-mappable units as follows:

The following table shows the correlation of the above mapped divisions with Walmsley's classification, and the previous, and new classifications of the type area of Ludlow.

Lower Bringewood Beds and Elton Beds

The mudstones, siltstones and thin impure limestones of the Lower Bringewood Beds and Elton Beds occupy nearly 4 square miles of low ground between the feature formed by the Wenlock Limestone and uppermost, sandy beds of the Wenlock Shale, and the scarp of the calcareous Upper Bringewood Beds. Their thickness varies between 750 and 800 ft, of which the uppermost 100 to 150 ft comprise the Lower Bringewood Beds. The junction between the Elton Beds and Lower Bringewood Beds is imprecise, for lithologically and faunally the lower division passes almost imperceptibly into the upper through at least 50 ft of beds.

(a) Elton Beds

The Wenlock Limestone is overlain conformably by the Elton Beds, the fairly rapid passage from one to the other being accomplished within about 20 ft of strata. The Elton Beds comprise buff, brown, olive and grey, shalt' to irregularly and coarsely flaggy mudstones, silty mudstones and silt-stones, in which hard nodules and thin bands of bluish, or greenish grey, impure limestone occur sporadically. In general, coarser-grained rocks become more common in the higher beds, and at the same time they become more calcareous.

The Lower, Middle and Upper Elton beds of the type area are probably represented (see faunal account below) at Usk, but their boundaries could not be mapped because the beds are lithologically more uniform, and much less graptolitic than in the type area (Holland, Lawson and Walmsley 1963). H.C.S.

The fauna of the Elton Beds includes many forms surviving from the underlying Wenlock Series. It is characterized by the occurrence of small brachiopods such as 'Chonetes lepisma', Chonetoidea grayi and Skenidioides lewisii, strophomenids including Amphistrophia [Strophonella] funiculata, Leptaena depressa, Leptostrophia filosa, Mesopholidostrophia [Brachyprion] sp. and Strophonella euglypha, as well as the brachiopods Atrypa reticularis, Dicoelosia [Bilobites] biloba, Howellella elegans, Isorthis amplificata, I. clivosa, I. scuteformis uskensis and Sphaerirhynchia davidsoni, the bivalve Cypricardinia subplanulata, and the trilobites Dalmanites myops [D. vulgaris Salter sp.] and Encrinurus sp. Of these, Dicoelosia biloba, Isorthis amplificata and Sphaerirhynchia davidsoni are confined to the lower part of the Elton Beds. Howellella elegans and Dalmanites myops are usually fairly common. Mesopholidostrophia sp. is well represented at some localitieS and 'Chonetes lepisma' may also be locally abundant, specimens with radial ribbing occurring as well as the more typical smooth form. In the top part of the Elton Beds Dayia navicula and Shaleria aff. ornatella make their earliest appearance, but are not common.

The presence of Dicoelosia biloba in the basal 100 to 150 ft of the Elton Beds strongly suggests that these rocks are equivalent to the Lower Elton Beds of the type area at Ludlow. The graptolite Monograptus cf. varians was collected from the middle part of the Elton Beds; M. varians is apparently restricted to the Middle Elton Beds in the type area (Holland, Lawson and Walmsley 1963, p. 107). Walmsley (1959, p. 490) recorded Monograptus cf. tumescens Wood from the upper part of the Elton Beds indicating that it may be equivalent to the Upper Elton Beds of the type area. It is therefore probable that the three subdivisions of the Elton Beds of the type area are represented in the Newport district. D.E.W.

(b) Lower Bringewood Beds

These comprise very fossiliferous, brown, grey, or greenish grey, silty mudstones and siltstones, in which nodules and thin bands of bluish grey, impure limestone are fairly common. Lithologically they are transitional between the underlying, less calcareous, and more argillaceous Elton Beds, and the overlying, impure limestones of the Upper Bringewood Beds. H.C.S.

The Lower Bringewood Beds are generally more fossiliferous than the underlying Elton Beds, although they contain few forms appearing for the first time. Their fauna is characterized by a marked increase in the occurrence of strophomenids, such as Amphistrophia funiculata, Leptaena depressa, Leptostrophia filosa, Mesopholidostrophia sp. and Strophonella euglypha. Other commonly occurring fossils are the brachiopods Atrypa reticularis, 'Camarotoechia' nucula, 'Chonetes lepisma' (smooth form), Craniops implicata, Howellella elegans, Isorthis aff. orbicularis, Sphaerirhynchia wilsoni, the gastropod Poleumita globosa, and the trilobite Dalmanites myops. Fossils occurring rarely include 'Fardenia' pecten, Gypidula cf. lata, Isorthis cf. clivosa, Protochonetes ludloviensis, Liospira striatissima, Cypricardinia subplanulata, and Encrinurus sp. Of these, Protochonetes ludloviensis, Liospira striatissima and Poleumita globosa were not recorded from lower beds. D.E.W.

Upper Bringewood Beds

The Upper Bringewood Beds consist mainly of hard, greenish grey or bluish grey, irregularly flaggy to massive, nodular, impure limestones and calcareous siltstones. Subordinate amounts of silty mudstones and siltstones also occur. The division is thickest in the east of the inlier, where 100 to 150 ft of calcareous rocks form the most prominent topographical features, e.g. the high cliffs overlooking the Usk Valley in the north-east corner of the district, the steep slopes around the north and east sides of Ty'nycaeau Hill, and the scarps running through Caemaen Wood [ST 3626 9973], Coed Garw [ST 3520 9820] and The Forest. West of The Forest the Upper Bringewood Beds thin rapidly and become less calcareous, and along the western flank of the inlier they are only about 75 ft thick. In this area their feature is subdued, and the highest ground is commonly formed by the overlying Leintwardine and Whitcliffe beds.

The base of the Upper Bringewood Beds is easily traced over much of the eastern part of the inlier, where steepening slopes indicate the passage into the more calcareous beds. In the west however, because they are thinner and less calcareous, their base is more difficult to locate. At the top of the division there is a gradual lithological and faunal transition into the overlying Lower Leintwardine Beds. The rocks gradually become more flaggy and less calcareous, and their characteristic fossils die out. Over most of the inlier the junction is best drawn just below the horizon at which Dayia navicula, the most characteristic fossil of the Lower Leintwardine Beds, appears in abundance. In the south of the inlier, at Llandegfedd, the junction is most conveniently taken at the base of a bed of conglomerate (p. 14). H.C.S.

The calcareous environment in which the Upper Bringewood Beds accumulated encouraged the growth of corals, particularly colonies of the tabulate coral Favosites. With this important addition, the faunal assemblage in these rocks is virtually the same as that listed for the Lower Bringewood Beds, though there are variations in the abundance of some forms. For example, Amphistrophia funiculata, Mesopholidostrophia sp. and Strophonella euglypha are less common, and Leptaena depressa, Protochonetes ludloviensis, and Poleumita globosa are more numerous.

In the Newport district, the top of the Bringewood Beds is the upper range-limit of the following stratigraphically important fossils: the brachiopods Amphistrophia funiculata, 'Fardenia' pecten, Gypidula cf. lata, Mesopholidostrophia sp., and Strophonella euglypha, the gastropods Liospira striatissima and Poleumita globosa, the bivalve Cypricardinia subplanulata, and the trilobite Dalmanites myops. Of these, Liospira striatissima and Poleumita globosa are confined to the Bringewood Beds. D.E.W.

Leintwardine Beds

The Leintwardine Beds are readily divisible into lower and upper divisions, as at Ludlow (Holland, Lawson and Walmsley 1963), though, because the latter is never more than 25 ft thick, it was considered impractical to show them separately on the 6-inch and 1-inch maps.

a. Lower Leintwardine Beds

The outcrop of this division generally occupies the higher part of the dip-slope of the scarp formed by the Upper Bringewood Beds, though along parts of the western flank of the inlier it forms the crest of the scarp. The beds vary in thickness between about 145 ft in the east of the inlier and 70 to 110 ft in the west and south. The succession comprises greenish grey or greenish brown, sometimes calcareous, flaggy siltstones, with sporadic, thin, impure, bluish grey, shelly limestones.

In the south of the inlier the base of the Lower Leintwardine Beds is taken at the horizon of a 10 to 14 in bed of conglomerate, which was first described by Walmsley (1959, p. 493) from trial pits near Llandegfedd. The conglomerate lies unconformably on an eroded bed of calcareous siltstone, indicating a distinct break in deposition. Walmsley found a thinner conglomerate at about the same horizon 1.5 miles to the north-east. During the resurvey a thin layer of siltstone with scattered quartz grains up to 1 mm in diameter was noticed at the approximate base of the Lower Leintwardine Beds in a section [ST 3276 9805] in Darran Plantation 1.33 miles to the north-north-west of Llandegfedd. This unusual occurrence of quartz grains of this size, in a normally silty facies, may represent the feather-edge of the conglomerate found by Walmsley, thus further supporting his view that it was derived from the south. H.C.S.

The fauna of the Lower Leintwardine Beds differs markedly from that of the underlying Bringewood Beds. It is characterized by the common occurrence of several species which are usually rare or absent in the beds below: of the brachiopods, Dayia navicula is abundant, Isorthis orbicularis, Orbiculoidea rugata and Salopina lunata are common and Whitfieldella canalis is also present; the bivalves include Actinopterella tenuistriata, Ctenodonta anglica, Fuchsella amygdalina, Goniophora cymbaeformis, Modiolopsis complanata, Nuculites antiquus, Palaeopecten danbyi, Paracyclas perovalis and Pteronitella retroflexa. Other important fossils are the brachiopods Protochonetes ludloviensis, which occurs more abundantly than in the Bringewood Beds, 'Camarotoechia' nucula, Craniops implicata, Howellella elegans and Sphaerirhynchia wilsoni. 'Chonetes lepisma' (smooth form), Isorthis aff. orbicularis and Leptostrophia filosa are rare and, together with Sphaerirhynchia wilsoni they were not found above these beds. Isorthis orbicularis sensu stricto is also virtually restricted to the Lower Leintwardine Beds.

Near the top of the Lower Leintwardine Beds, one specimen of the graptolite Monograptus leintwardinensis incipiens? (extreme form) has been found. In the type area at Ludlow, Holland, Lawson and Walmsley (1963, p. 157) record M. leintwardinensis incipiens from the Upper Bringewood Beds only—i.e. the Monograptus tumescens Zone. However, Professor O. M. B. Bulman, F.R.S., reports (private communication) that it occurs in association with M. leintwardinensis leintwardinensis in the Usk area—i.e. in the M. leintwardinensis Zone. D.E.W.

b. Upper Leintwardine Beds

This division varies in thickness from about 9 ft near Porth-Ilong [ST 3515 9785] to 14 to 18 ft along the eastern flank of the inlier, and to 20 to 25 ft in the west and south-west. The beds comprise flaggy, greenish grey to buff, sometimes calcareous, silty mudstones and siltstones with a few, thin, impure, bluish grey limestones. Because of the lithological similarity of the Upper and Lower Leintwardine beds the boundary between them is drawn on faunal differences. Similarly, the junction of the Upper Leintwardine Beds and Whitcliffe Beds is essentially a faunal boundary, though the two divisions are also slightly different lithologically (see below). At two localities (p. 24) in the southern part of the inlier the top of the Leintwardine Beds is marked by a bone bed, which equates with the Bodenham Bone Bed of the Woolhope Inlier (Squirrell and Tucker in Curtis and others 1967, p. 19). H.C.S.

The Upper Leintwardine Beds have a most distinctive faunal assemblage, characterized in particular by an abundance of the brachiopod Shaleria ornatella. Another important fossil is the trilobite Calymene neointermedia which is virtually confined to this horizon. The cephalopod Michelinoceras bullatum and the trilobite Acastella spinosa appear for the first time. Commonly associated with these fossils are the brachiopods 'Camarotoechia' nucula, Protochonetes ludloviensis, Salopina lunata and Whitfieldella cattails. Dayia navicula occurs in small numbers only, and other less common forms include Atrypa reticularis, Orbiculoidea rugata, Pteronitella retroflexa (common in bands), Cornulites serpularius, and the smooth ostracod Cytherellina siliqua. A single specimen of Monograptus leintwardinensis leintwardinensis was also recorded.

Of the fossils mentioned above, the following have not been recorded from above the Leintwardine Beds in the Usk area: Atrypa reticularis, Orbiculoidea rugata, Shaleria ornatella, Acastella spinosa, Calymene neointermedia, and Monograptus leintwardinensis leintwardinensis.

The bone bed at the top of the Upper Leintwardine Beds has yielded abundant horny brachiopod fragments, the conodont Spathognathodus primus, numerous other conodont fragments and acanthodian and thelodont denticles. D.E.W.

Whitcliffe Beds

The Whitcliffe Beds crop out mainly on the dip-slope around the flanks of the Usk Inlier, and comprise 130 to 150 ft of mainly greenish grey or brownish grey, thickly bedded or flaggy, sometimes calcareous siltstones with sporadic bands of impure limestone. The uppermost 25 ft are interbedded with yellowish brown or buff, fine to medium-grained sandstones, which become more common as the base of the Downton Castle Sandstone is approached. The upper and lower subdivisions of the Whitcliffe Beds of the type area could not be recognized at Usk.

The basal rocks of the Whitcliffe Beds are more thickly bedded and more calcareous than the underlying Leintwardine Beds, and because they are more resistant to erosion they sometimes form slightly higher ground than the beds below. This is particularly well demonstrated just to the east of Pen-y-parc Farm [ST 3570 9660], where the junction between the Whitcliffe and Upper Leintwardine beds is easily traced northwards along the base of a low, but distinct ridge. A similar topographical expression of the contrasting lithologies is also apparent in the north-east corner of the area, and south of Cwm Dowlais. The top of the Whitcliffe Beds is marked by the base of the Ludlow Bone Bed at a number of localities in the south of the inlier, but elsewhere the boundary has been drawn approximately at the horizon above which sandstones. predominate. H.C.S.

Although the Whitcliffe Beds contain highly fossiliferous strata, the variety of the fauna is restricted. Protochonetes ludloviensis, Salopina lunata and 'Camarotoechia' nucula are characteristically abundant, and Howellella elegansis locally common in the uppermost beds. Bivalves are well represented, including Goniophora cymbaeformis, Modiolopsis complanata, Nuculites antiquus, 'Pterinea' cf. rectangularis and Pteronitella retroflexa. Michelinoceras bullatum and Cornulites serpularius are common. The trilobite Homalonotus knightii and the cephalopod Michelinoceras tracheale are rare and are confined to these beds. Dayia navicula and Whitfieldella canalis are found in the basal beds only. In the uppermost 25 ft, very fossiliferous, commonly decalcified, impure limestone bands contain abundant Loxonerna [Holopella] gregarium and L. obsoletum. Acanthodian jaw fragments, an acanthodian spine and thelodont denticles have also been recorded in these uppermost beds, but rarely in sufficient concentrations to form definite bone beds. D.E.W.

Details

Wenlock Series

Wenlock Shale

Some of the lowest beds of the Wenlock Shale in the Newport district are exposed in a new road cutting [SO 3492 0081] in the northern part of the outcrop, where 20 ft of soft, brown mudstone with a few thin beds of impure limestone contain crinoid columnals, Leptaena cf. depressa, orthids, Strophonella euglypha, Calymene sp. and Dalmanites caudatus. Some 100 yd south-eastwards along the cutting approximately 90 ft of olive mudstones are exposed, in which there are two white clay beds, one in thick and the other 3 in. Shelly fossils are scattered throughout the mudstones. Beds at a slightly higher horizon are visible in another road cutting [SO 3445 0113] towards the north-west, there being 20 ft of dark-weathering, buff, olive and khaki mudstones and thin impure limestones rich in shelly remains including Atrypa reticularis, 'Camarotoechia' nucula, Leptaena cf. depressa and orthids. Higher beds again are visible 300 yd farther west, in a continuous section along a lane [SO 3410 0102], which reveals over 100 ft of poorly fossiliferous, brown-weathering, soft, shaly mudstones.

Other exposures of the lower mudstone division of the Wenlock Shale are present in the stream [SO 3450 0040] flowing through Cilfeigan Park, where there are numerous. sections of buff, olive, grey or green, soft, poorly-fossiliferous mudstones and silty mudstones up to 6 ft thick. Several exposures are also visible in the stream running through Coed Mawr [ST 3440 9870], and along the track [ST 3456 9875] passing through the wood to Old Prescoed Farm. The beds consist of mainly brown or grey mudstones and silty mudstones containing scattered shelly fossils. Similar rocks are exposed west of Cilfeigan Park, in a roadside section [SO 3410 0001] (Locality 2),^‡3  and have yielded crinoid columnals, 'Camarotoechia' nucula, 'C'. cf. borealis, Craniops implicata, Howellella sp., Salopina sp., Sphaerirhynchia davidsoni, Bembexia cf. lloydii, Platyceras sp., Grammysia obliqua, Modiolopsis gradata, Plethomytilus mytilimeris, pterineid, Tentaculites ornatus, Dalmanites cf. caudatus, Beyrichia sp.

Beds in the upper sandstone division of the Wenlock Shale have been quarried [SO 3593 0050] near Cefn-ila Hospital, and 3 ft of brown, flaggy sandstone containing: 'Camarotoechia' nucula and crinoid columnals are exposed. In the west of the outcrop, alongside a footpath [SO 3364 0014] (Locality 1), sandstone debris from solid just below the surface contains crinoid columnals, Amphistrophia funiculata, Howellella sp., Sphaerirhynchia davidsoni, Tancrediopsis sp., Tentaculites ornatus, Acaste downingiae, Beyrichia sp. Very fossiliferous, buff and brown, fine to coarse-grained, fiaggy sandstone debris from a ploughed field [ST 3460 9975] (Locality 3) west of Prescoed yielded crinoid columnals, bryozoa, Amphistrophia funiculata, 'Camarotoechia' nucula, Howellella sp., Leptaena cf. depressa, Meristina obtusa, Salopina sp., Sphaerirhynchia davidsoni, Bucanella trilobata, Platyceras sp., Goniophora sp., Grammysia cingulata var. intermedia, pterineid, Tentaculites ornatus, and Acaste downingiae.

The stream section [ST 3487 9838] south-west of Cwm Farm exposes 3 ft of brown or buff, fine to medium-grained, ripple-marked sandstone containing decalcified crinoid columnals and worm markings. Approximately 110 yd upstream an exposure of sandy limestone in the stream bed represents part of the passage sequence into the Wenlock Limestone. The uppermost beds of the Wenlock Shale are also visible along the roadside [ST 3524 9886] near Cwm Farm, where they comprise 4 ft of buff sandstone and silty mudstone containing abundant crinoid columnals. Similar beds are exposed in the stream nearby [ST 3518 9885].

Wenlock Limestone

Greenmeadow–Common Coed-y-paen–Cwm

The Wenlock Limestone has been quarried extensively in the area around Greenmeadow Farm [SO 3369 0118], and is visible in small exposures at several localities, dipping westwards at between 4° and 12°. In a quarry [SO 3365 0133] near the northern boundary of the district 4 ft of hard, massive, bluish grey, crystalline limestone are overlain by 10 ft of nodular limestone, which passes gradually into buff, silty mudstone. For 200 yd south of Greenmeadow Farm several small sections of the uppermost beds of the Wenlock Limestone are visible; the best occurs in a quarry [SO 3371 0095] 250 yd S.5°E. of Greenmeadow, where 6 ft of impure, nodular limestone with subordinate buff, silty mudstone are overlain by 3 ft of buff, silty mudstone, which comprise the basal part of the Elton Beds. Southwards from there the Wenlock Limestone is poorly exposed for about mile, and its boundaries are difficult to mark accurately because they crop out on an east-west dip slope. Some 500 yd north-west of Pentre-Waun Wood [ST 3370 9950] the outcrop of the Wenlock Limestone is pitted with numerous quarries, some of which are now overgrown. The thickest section [ST 3350 9992] shows 6 ft of hard, irregularly bedded, shelly and crinoidal, crystalline limestone overlain by 4 ft of interbedded buff, silty mudstones and nodular limestones. The following fauna was recorded in this quarry: crinoid columnals, Atrypa reticularis, 'Camarotoechia' nucula, Leptaena rhomboidalis, Leptostrophia filosa, and Strophonella euglypha.

In the narrow outcrop of the Wenlock Limestone between the Coed-y-paen and Pen-y-parc faults limestones are well exposed in numerous quarries and one, Cilwrgi Quarry [ST 3398 9839] (Locality 4), is still being worked. The sequence in this quarry is: nodular, bluish grey limestone interbedded with brown and buff silty mudstone 4 ft on very hard, massive, bluish grey, fine to coarsely crystalline, shelly limestone 8 ft, containing a ballstone approximately 4 ft high and 8 ft wide. The following fauna was collected from this quarry: Favosites gothlandicus, F. gothlandicus forbesi, Halysites catenularius, the first record in Britain of Microplasma lovenianum (White 1966, pp. 148–51), Syringopora fascicularia, Thecia hisingeri, Fenestella sp., crinoid columnals, Amphistrophia funiculata, Atrypa reticularis, 'Camarotoechia' nucula, Protochonetes cf. ceratoides, Howellella sp., Dolerorthis rustica, Leptaena cf. depressa, L. cf. rhomboidalis, Leptostrophia filosa, Resserella cf. elegantula, Sphaerirhynchia davidsoni, Platyceras cf. cornutum, 'Pterinea' cf. posidoniaeformis, Tentaculites ornatus, Acaste downingiae, Dalmanites caudatus and D. cf. myops.

In the quarry [ST 3380 9835] 400 yd E.25°S. of Common Coed-y-paen Church the lowest Elton Beds rest on 11 ft of Wenlock Limestone, the section being:

The quarry [ST 3408 9826] 730 yd E.21°S. of Common Coed-y-paen Church reveals the thickest section in the area, as follows:

Farther east, near the Pen-y-parc Fault, the Wenlock Limestone is again exposed in a number of quarries; the thickest section [ST 3460 9823] shows 8 ft of hard, crystalline limestone overlain by 6 ft of nodular limestone.

In the Cwm Farm area the southward-dipping beds of the Wenlock Limestone extend west–east for 700 yd between the Pen-y-parc and Ty'nycaeau faults. The beds are well exposed in two quarries; the most westerly [ST 3494 9883], shows 8 ft of hard, massive limestone overlain by 6 ft of nodular limestone, which pass up into buff, silty mudstone of the Elton Beds; the other [ST 3536 9877], exposes the following succession:

Prescoed–Cilfeigan–Cefn-ila

In the outcrop between Prescoed and Cilfeigan the gently-dipping limestones have been worked extensively. In a quarry [ST 3500 9980] north-east of Prescoed Farm, 6 ft of hard, massive, coarsely-crystalline, shelly limestone are overlain by 10 ft of nodular limestone with some buff silty mudstone, which pass up quickly to silty mudstone containing sporadic limestone nodules. At Cilfeigan the quarries are very overgrown, but the sequence is visible in an exposure [SO 3535 0003] near Cilfeigan Farm, where 6 ft of hard, massive, bluish grey limestone are overlain by 7 ft of nodular limestone and thin beds of silty mudstone, which pass up into the lowest, silty mudstone of the Elton Beds. In the poorly exposed outcrop between the Cilfeigan and Cefn-ila faults the best exposure is in the disused quarry [SO 3603 0032] near Cefn-ila Hospital, where 4 ft of massive, crinoidal limestone are overlain by 10 ft of nodular limestone, which pass up into the silty mudstone at the base of the Elton Beds.

Cwm–Dowlais

In this vicinity the Wenlock Limestone crops out in a small inlier lying in the core of the north-north-east trending Llangibby Anticline. It is bounded in the north-west by the Cilfeigan Fault. The anticlinal disposition of the limestone beds is well demonstrated by the exposures in the area. The thickest section occurs in the quarry [ST 3663 9900] west of Cwm Dowlais Farm, as follows:

Ludlow Series

Lower Bringewood Beds and Elton Beds

(a) Elton Beds

Some of the lowest Elton Beds are visible along a track [ST 3535 9873] (Locality 7), north-east of Cwm Farm, where they comprise soft, flaggy, silty mudstones with thin limestone bands. The rocks are very fossiliferous, and contain commonly occurring Gypidula cf. lata and Sphaerirhynchia davidsoni. Large strophomenids are also fairly common, e.g. Amphistrophia funiculata, Leptaena cf. depressa and Strophonella euglypha. Rarely occurring species include 'Chonetes lepisma' with a faint radial ornament, and Isorthis amplificata. Slightly higher beds are visible in the stream section [ST 3489 9805] (Locality 8), south-south-west of Cwm Farm. The rocks are olive mudstones, and equate with the middle or upper part of the Lower Elton Beds of the type area. They contain a rich fauna, which includes numerous Chonetoidea grayi, and Isorthis scuteformis uskensis, as well as Dicoelosia biloba, Glassia sp., Tancrediopsis ludensis and Dalmanites cf. nexilis.

Strata equivalent to the Middle Elton Beds of Ludlow are visible in the disused quarry [ST 3519 9935] (Locality 9), south-east of Prescoed. The section shows 20 ft of olive, silty mudstones with sporadic limestone nodules. They have yielded a single specimen of Monograptus cf. varians, as well as other fossils typical of this horizon. Rocks lying high in the Middle Elton Beds, or possibly in the basal part of the Upper Elton Beds, are well exposed in a roadside section [SO 3634 0001] (Locality 10), east of Cilfeigan, which extends for about 100 yd. The beds comprise olive, silty mudstones containing sporadic impure limestone nodules, and have yielded a scattered fauna containing forms such as Howellella elegans, Isorthis sp., Skenidioides lewisii, Loxonema planatum, Poleumita sp., Cypricardinia subplanulata, Acidaspis sp., Dalmanites myops, and Monograptus sp.

Strata which probably equate with the Upper Elton Beds of Ludlow are visible in a 50 yd section [ST 3323 9894] (Locality 11) along a disused track, north-north-west of Common Coed-y-paen. They consist of very fossiliferous, flaggy, silty mudstones containing, in particular, large specimens of Amphistrophia funiculata, Leptaena depressa and Mesopholidostrophia sp. Apart from other typical fossils the fauna also includes simple corals, some of the earliest Dayia navicula and Shaleria aff. ornatella, with numerous ostracods including Cythereiina siliqua and Hemsiella maccoyiana. Rocks at a similar horizon are visible along the lane [ST 3649 9827] (Locality 12) near Coed-y-Fferm. The succession there comprises richly-fossiliferous, flaggy siltstones containing an abundance of 'Chonetes lepisma', and fairly common Dayia navicula. The fauna also includes Chonetoidea grayi, Dalejina hybrida, Isorthis scuteformis uskensis, Lingula symondsii, Protochonetes ceratoides, Calymene cf. intermedia, Otarion sp., and indeterminate graptolites.

(b) Lower Bringewood Beds

In the west of the inlier strata of this division are visible in the stream section [ST 3202 9972] (Locality 14) near Trostra, where flaggy silty mudstones contain a scattered fauna typical of these beds. It includes Amphistrophia funiculata, Atrypa reticularis, Leptaena cf. depressa, Mesopholidostrophia sp., Sphaerirhynchia wilsoni and Cypricardinia subplanulata. Farther south, in the stream [ST 3234 9809] (Locality 15) north-east of Llandewi Court, 5 ft of silty mudstones at approximately the same horizon have yielded similar fossils, as well as Protochonetes ceratoides, P. ludloviensis, Strophonella euglypha and Poleumita globosa. About 200 yd downstream [ST 3248 9825] (Locality 16) the siltstones of the Lower Bringewood Beds contain a rich fauna characterized by large brachiopods, in particular Amphistrophia funiculata, Atrypa reticularis and Strophonella euglypha. The fauna also includes Chonetoidea grayi, Eospirifer radiatus, Gypidula sp., Isorthis aff. orbicularis, Mesopholidostrophia sp., Cypricardinia subplanulata, Dalmanites myops and ostracods.

Near the centre of the inlier the Lower Bringewood Beds are well exposed in a quarry [ST 3344 9826] (Locality 17) at Coed-y-paen Farm. The beds comprise about 10 ft of flaggy siltstones with impure limestone nodules, the former containing a typical, rich fauna. The following brachiopods are particularly common: 'Camarotoechia' aucula, Howellella elegans, Isorthis aff. orbicularis, Leptaena depressa and Sphaerirhynchia wilsoni. Also present are Amphistrophia funiculata, Mesopholidostrophia sp., Protochonetes ceratoides, Poleumita globosa and ostracods. Farther east the Lower Bringewood Beds are again well exposed for about 150 yd along a track [ST 3498 9811] (Locality 18) near Ton-y-bedw Farm. The siltstones are very fossiliferous, large brachiopods being particularly common. Also present are simple corals, dalmanellids including Isorthis cf. clivosa, Calymene sp., Dalmanites myops, Encrinurus sp. and ostracods. Another good exposure of these beds is in the quarry [SO 3634 0001] at Ysgwbor-y-gwynt, where 20 ft of flaggy siltstones, and calcareous siltstones with scattered, impure, limestone nodules are visible. They contain a scattered, but typical fauna including Favosites sp., Atrypa reticularis, Leptaena rhomboidalis, Leptostrophia filosa, Protochonetes ceratoides and Strophonella euglypha. An unusual abundance of Protochonetes ludloviensis was also noted.

Upper Bringewood Beds

In the north-west of the inlier the thin Upper Bringewood Beds are typically displayed in the quarry [SO 3206 0094] (Locality 20) south-west of Beech Tree Farm, where 16 ft of bluish grey, thickly bedded, impure limestones with beds of irregular nodules are visible. They contain a typical fauna, which includes Favosites gothlandicus, Atrypa reticularis, Gypidula cf. lata, Leptaena cf. depressa, Leptostrophia filosa, Mesopholidostrophia sp., Protochonetes ludloviensis, Poleumita globose, Cypricardinia subplanulata and Dalmanites myops. The fairly low occurrence of Shaleria aff. ornatella was noted in the basal 6 ft of the section. South from this locality for about 2 miles the narrow outcrop of the thin Upper Bringewood Beds is difficult to map, but the beds are visible in a number of exposures, for example, in the stream [SO 3198 0003] near Trostra, where siltstones with bands of impure limestone contain brachiopods such as Amphistrophia funiculata and Strophonella euglypha as well as Favosites sp.and Dalmanites myops. The beds are also exposed in a quarry [ST 3232 9897] (Locality 23), east-south-east of Sluvad, where about 30 ft of massively bedded, impure limestone contain a scattered fauna which includes Favosites gothlandicus forbesi. About 0.75 mile to the south-west the highest beds of this division are visible in a cliff-section [ST 3276 9805] (Locality 24) in Darran Plantation. They comprise about 15 ft of calcareous siltstones and a few impure limestone bands, containing a scattered fauna which includes Phaulactis angusta, Howellella elegans, Leptaena cf. depressa, Sphaerirhynchia wilsoni, Whitfieldella canalis and Pteronitella retroflexa.

Towards the eastern part of the inlier the thickening Upper Bringewood Beds form the scarp feature in The Forest, where a prominent, wooded ridge runs east-north-east, then west between Pen-y-lan and Porth-llong [ST 3512 9785]. Along this stretch the flaggy, calcareous siltstones with impure limestone bands and nodules are well exposed in a quarry [ST 3407 9729] (Locality 28) near Pen-y-lan, and have yielded a typical fauna which includes Favosites sp., simple coral, Atrypa reticularis, Leptaena depressa and Dalmanites sp.Farther east-north-east strata in the top part of the Upper Bringewood Beds are visible in Porth-llong Quarry [ST 3517 9776] (Locality 25). The rocks are poorly calcareous, consisting of siltstones with a few, thin, impersistent bands and nodules of impure limestone. They are richly fossiliferous and contain many of the forms typical of these beds, in particular Amphistrophia funiculata, Atrypa reticularis, Chonetes lepisma, 'Fardenia' pecten, Gypidula sp., Isorthis aff. orbicularis, Strophonella euglypha, Poleumita globosa and Dalmanites myops. North-east of Porth-llong the Upper Bringewood Beds are well exposed [ST 3626 9976] (Locality 27) along the crest of the ridge running through Caemaen Wood, and yield a typical fauna. Much of Llangibby Castle was built on the outcrop of the Upper Bringewood Beds, and 12 ft of massive, impure limestones are exposed [ST 3632 9745] in a quarry near the ruins of the northwestern tower. Along Craig Foel the Upper Bringewood Beds are well displayed in a quarry [SO 3701 0103] (Locality 22), where 23 ft of impure, nodular, rubbly limestones with siltstone bands are visible. As well as the characteristic fossils, these rocks contain numerous zoaria of the bryozoan Leioclema sp.Another quarry [SO 3706 0092] (Locality 21) nearby shows about 35 ft of well-weathered, flaggy, nodular impure limestones, which have yielded an abundant fauna including Favorites sp., simple corals, Leioclema sp., Leptaena cf. depressa, L. rhomboidalis, Protochonetes ludloviensis, Sphaerirhynchia wilsoni, Strophonella euglypha, Poleumita globosa, Tterinea' aff. aymestriensis, Dalmanites sp. and ostracods.

Leintwardine Beds

(a) Lower Leintwardine Beds

Specimens of the conglomerate marking the approximate base of the Lower Leintwardine Beds near Llandegfedd were collected by Dr. W. H. C. Ramsbottom and Mr. J. V. Stephens in 1952, from two temporary pits [ST 3339 9603] and [ST 3332 9600] dug to a depth of 33 ft 9 in and 29 ft 8 in respectively along the line of a proposed dam. (Site later abandoned). The bed is about 12 in thick, and is described by Dr. Hawkes as follows: "The conglomerate (E33897)^‡4  is composed of rounded pebbles (0.7 mm to 10.00 mm long) set in a matrix of partly recrystallized shell fragments, scattered organic (phosphatic) remains and interstitial silt-grade quartz fragments. The pebbles consist of quartz (many are composite grains and were probably derived from quartz veins and granitic bodies), quartz schist, quartzite, metamorphic quartzite, chert, perthite, acid lava and ?andesitic lava. The conglomerate rests unconformably on a calcareous siltstone which is a protoquartzitic type with an average grain size of 0.04 mm. In addition to quartz and calcite the siltstone contains abundant flakes of sericite, muscovite and chlorite, with scattered grains of secondary iron oxide."

The presence of thelodont denticles and conodonts in the conglomerate was noticed by Mr. J. D. D. Smith (Walmsley 1959, p. 493). The following fauna was collected from rocks immediately above and below the conglomerate in these pits: crinoid columnals, 'Camarotoechia' nucula, Protochonetes ludloviensis, Salopina lunata, Pteronitella retroflexa, Loxonema sp., and conodont fragments.

The only continuous section showing the gradual passage of the Upper Bringewood Beds into the Lower Leintwardine Beds is in a 50-ft cliff [ST 3276 9805] (Localities 24 and 32) in the landslipped rocks of Darran Plantation. The upper 35 ft of siltstones and impure limestones comprise the basal part of the Lower Leintwardine Beds. A short distance above their base, which has been taken at the horizon of a layer of coarse, quartz grains (see p. 14), fossils become more common, and within 3 ft Dayia navicula, the typical brachiopod of the Lower Leintwardine Beds, appears in abundance together with other fossils typical of the Lower Leintwardine Beds, e.g. Protochonetes ludloviensis, Sphaerirhynchia wilsoni, Whitfieldella canalis and Pteronitella retroflexa. These fossils are also found in the overlying 30 ft of Lower Leintwardine Beds, as well as a number of other brachiopods and bivalves. About 120 yd to the south-east a small quarry [ST 3284 9798] (Locality 33) in the lower part of the Lower Leintwardine Beds exposes 8 ft of silty mudstones containing particularly abundant decalcified remains of Dayia navicula, commonly occurring Isorthis orbicularis, Salopina lunata, Fuchsella amygdalina, and other characteristic fossils. Another good section of Lower Leintwardine Beds is found in a quarry [ST 3224 9785] (Locality 31) less than 0.5 mile to the west; about 8 ft of very fossiliferous rocks are visible. One of the most fossiliferous localities in the Lower Leintwardine Beds is in the stream [ST 3243 9749] (Locality 35) south of Darran Plantation. The 11 ft of beds exposed contain most of the typical brachiopods, and also a marked abundance of bivalves. That the strata lie near the top of the division is shown by the presence of a few Shaleria ornatella.

In the east of the hiller more than 10 ft of typical flaggy siltstones of the Lower Leintwardine Beds are visible in a section [ST 3647 9745] below the northern wall of Llangibby Castle. Some 800 yd north-north-east 35 ft of fossiliferous Lower Leintwardine Beds are visible alongside the path [ST 3675 9814] leading to Trestevan Farm. In the north-eastern corner of the area quarrying [SO 3679 0091] (Locality 30) in this division has revealed 10 ft of flaggy silty mudstones containing bryozoa, 'Camarotoechia' nucula, Chonetes' sp., Craniops implicata, Dayia navicula, Howellella elegans, Lingula cf. lewisii, Orbiculoidea rugata, Salopina lunata, Sphaerirhynchia wilsoni, Whitfieldella canalis, Actinopterella tenuistriata, Paracyclas perovalis, Modiolopsis sp.and Calymene sp.

The following three sections in the highest strata of the Lower Leintwardine Beds show their lithological and faunal characters, and also their rapid upward passage into the Upper Leintwardine Beds. In Sôr Brook [ST 3248 9726] (Locality 36) south of Darran Plantation 33 ft of strata are exposed, of which the basal 8 ft are the highest Lower Leintwardine Beds. They are flaggy siltstones, and contain abundant Dayia navicula as well as 'Camarotoechia' nucula, Howellella elegans, Protochonetes ludloviensis, Salopina lunata and a number of bivalves and cephalopods. In Court Perrott Quarry [ST 3341 9594] (Locality 37) near Llandegfedd the basal 12 ft of flaggy siltstones with limestone bands are Lower Leintwardine Beds. Like the beds in the previous locality their fauna is characterized by commonly occurring Dayia navicula. Also present are Protochonetes ludloviensis, Salopina lunata, Whitfieldella canalis, Acastella cf. spinosa and Calymene neointermedia. Near Porth-llong a track-side section [ST 3506 9794] (Locality 34) shows 15 ft of the uppermost Lower Leintwardine Beds. The siltstones contain a large fauna, in which most of the typical fossils are present, and Leptostrophia filosa and Sphaerirhynchia wilsoni range higher than usual into the uppermost 9 ft.

(b) Upper Leintwardine Beds

The rapid faunal changes which typify the basal strata of the Upper Leintwardine Beds are demonstrated in the cliff-section [ST 3248 9726] (Locality 43) along Sôr Brook. Above the 8 ft of Lower Leintwardine Beds exposed, the basal 3 ft of the Upper Leintwardine Beds are characterized by the sudden appearance of commonly occurring Shaleria ornatella, as well as the presence o f numerous Whitfieldella canalis, relatively common Calymene neointermedia, and only a few Dayia navicula. A large fauna of less diagnostic forms is also present, including Campylites longissimus, Howellella elegans, Lingula lewisii, Orbiculoidea rugata, Protochonetes ludloviensis, Salopina lunata, and Cyclonema corallii. Nine bivalve species were also found. The basal 3 ft are overlain by a further 22 ft of Upper Leintwardine Beds, which contain the typically abundant Shaleria ornatella as well as many other forms characteristic of these beds. Dayia navicula occurs in small numbers in all but the highest few feet. Leptaena cf. depressa was recorded at an unusually high level, about 4 ft from the base of the division. The basal Upper Leintwardine Beds are also well exposed in Court Perrott Quarry [ST 3341 9594] (Locality 45), where 8 ft of the flaggy siltstones containing a typical fauna overlie 12 ft of Lower Leintwardine Beds.

A good section within the main part of the Upper Leintwardine Beds is visible in Pen-topyn Quarry [ST 3304 9679] (Locality 44) north-west of Llandegfedd, where 15 ft of flaggy siltstones with limestone bands contain a large fauna of the typical brachiopods and bivalves. A specimen of Monograptus leintwardinensis leintwardinensis was also found here. About 15 ft of beds at a similar horizon are also well exposed in Pen-y-parc Quarry [ST 3577 9653] (Locality 49) east-north-east of Llandegfedd.

The uppermost Upper Leintwardine Beds are well exposed in a quarry [ST 3682 9825] (Locality 42) north of Llangibby Castle. They contain their characteristic fauna, which includes abundant Shaleria ornatella, common Protochonetes ludloviensis and Salopina lunata, and a number of Calymene neointermedia. The top of the division is taken where Shaleria ornatella dies out abruptly, that is, about 7 ft from the top of the section. The same strata are visible along the track [ST 3506 9794] (Locality 41) near Porth-llong, and also in a quarry [ST 3372 9661] (Locality 46) north-west of Llandegfedd. At the latter locality the basal 23 ft of visible beds lie within the Upper Leintwardine Beds, and the top 3 ft lie within the Whitcliffe Beds.

For 120 yd along the lane [between [ST 3435 9703] and [ST 3426 9695] (Locality 47) about 0.75 mile north-north-east of Llandegfedd, the Upper Leintwardine Beds yield a very rich and typical fauna of brachiopods, bivalves and trilobites. The top of the division is marked by the lenticular Bodenham Bone Bed up to 3 in thick, which is found in. the floor of the lane [ST 3426 9695] 750 yd N.2°E. of Glebe House. The bed is a shelly, calcareous siltstone and has yielded abundant broken fragments of horny brachiopods, and a few thelodont denticles together with numerous conodont fragments. Another 3-in thick bone bed containing a similar, though less abundant fauna, was found at the top of the Upper Leintwardine Beds in the lowest beds in a quarry [ST 3650 9729] (Locality 50) just south of Llangibby Castle.

Whitcliffe Beds

The basal strata of the Whitcliffe Beds are well exposed along the track [ST 3506 9794] (Locality 53) near Porth-llong, where 14 ft of the flaggy siltstones contain numerous Camarotoechia' nucula, Protochonetes ludloviensis, Salopina lunata and Whitfieldella canalis, and a small number of bivalves. About 7 ft of beds at a similar horizon are visible at the top of a 41-ft section in a quarry [ST 3682 9825] (Locality 55) north of Llangibby Castle. They contain 'Camarotoechia' nucula, Craniops implicata, rare Dayia navicula, Howellella elegans, Protochonetes ludloviensis, Whitfieldella canalis, Pteronitella retroflexa and Neobeyrichia cf. buchiana. The thickest, continuous section of the basal rocks of the Whitcliffe Beds is visible in the quarry [ST 3650 9729] (Locality 61), south of Llangibby Castle, where 57 ft of the flaggy siltstones overlie 5 ft of Upper Leintwardine Beds. They have yielded commonly occurring Protochonetes ludloviensis and Salopina lunata as well as crinoid columnals, Keilorites? squamosus, Howellella elegans and a number of bivalves and cephalopods. A few Whitfieldella canalis were found in the basal 6 ft only. A specimen of the conodont Ozarkodina typica was found about 35 ft from the base. Beds near the base of the division are also well exposed in the quarry [ST 3568 9752] (Locality 54) near Porth-llong. The 15 ft of flaggy siltstones yielded an abundant fauna of typical brachiopods and bivalves, and also a few specimens of Calymene sp.and Homalonotus knightii.

A continuous section of flaggy siltstones near the middle of the Whitcliffe Beds is. visible along the lane [between [ST 3386 9638] and [ST 3381 9623] (Locality 58) north of Llandegfedd. There the fauna is characterized by an abundance of Protochonetes ludloviensis, Salopina lunata and Pteronitella retroflexa as well as Leioclema sp., Michelinoceras bullatum, M. ibex, Cornulites serpularius, Tentaculites tenuis, and Neobeyrichia buchiana. Some of the highest strata of the Whitcliffe Beds are well exposed in the lane [ST 3156 9978] (Locality 51) south of Trostra Farm, where about 10 ft of flaggy siltstones are characterized by bands of completely decalcified limestone, in the form of soft, brown rottenstone, containing a profusion of moulds of Loxonema gregarium. The interbedded siltstones contain a characteristic fauna, and also Howellella elegans and acanthodian spines. Other good exposures of some of the highest rocks of the Whitcliffe Beds are in the quarry [ST 3652 9709] (Locality 60), south-south-east of Llangibby Castle, where Loxonema obsoletum is very abundant in decalcified limestone bands, and in the stream [ST 3345 9576] (Locality 59) near Brook House, where the beds are richly fossiliferous and contain crinoid columnals, 'Camarotoechia' nucula, Howellella elegans, Protochonetes ludloviensis, Salopina lunata, Loxonema gregarium, L. obsoletum, several bivalves and Cornulites serpularius. Specimens of acanthodian jaw fragments were found in the basal 10 ft of the section, and a thin bone bed containing thelodont denticles in the upper 10 ft. H.C.S.

References

CURTIS, M. L. K., LAWSON, J. D., SQUIRRELL, H. C., TUCKER, E. V. and WALMSLEY, V. G. 1967. The Silurian Inliers of the South-Eastern Welsh Borderland. Geol. Assoc. Guide No. 5.

GARDINER, C. I. 1917. The Silurian Inlier of Usk. Proc. Cotteswold Nat. Field Club, 19, 129–72.

GARDINER, C. I. 1927. The Silurian Inlier of Woolhope (Herefordshire). Quart. J. Geol. Soc., 83, 501–30.

HOLLAND, C. H., LAWSON, J. D. and WALMSLEY, V. G. 1963. The Silurian Rocks of the Ludlow District, Shropshire. Bull. Brit. Mus. (Nat. Hist.) Geol., London, 8, 95–171.

MURCHISON, R. I. 1839. The Silurian System. London.

PHILLIPS, J. 1848. The Malvern Hills, compared with the Palaeozoic districts of Abberley, Woolhope, May Hill, Tortworth and Usk. Mem. Geol. Surv., 2 (1).

STRAHAN, A. 1899. The Geology of the South Wales Coalfield. Part I. The Country around Newport, Monmouthshire, Mem. Geol. Surv.

WALMSLEY, V. G. 1959. The Geology of the Usk Inlier, Monmouthshire. Quart. J. Geol. Soc., 114, 483–521.

WHITE, D. E. 1966. The Silurian Rugose Coral Microplasma lovenianum Dybowski from Monmouthshire. Palaeontology, 9, 148–51.

WILLS, L. J. 1951. A Palaeogeographical Atlas of the British Isles and Adjacent parts of Europe. London and Glasgow.

Chapter 3 Old Red Sandstone

General

The Old Red Sandstone crops out over approximately 65 square miles in the eastern and south-eastern parts of the Newport district. The marls and soft sandstones of the Raglan Marl Group and St. Maughan's Group form low-lying, undulating terrain generally devoted to mixed farming. The harder sandstones of the Brownstone Group form higher ground more suitable for grazing and forestry. The base of the Old Red Sandstone is marked by the arenaceous deposits of the Downton Castle Sandstone, which rest conformably on the Ludlow Series, and the top by a transitional passage from the fluviatile sediments of the Upper Old Red Sandstone into the marine limestones at the base of the Carboniferous Limestone Series.

Classification and correlation

The classification of the Old Red Sandstone strata in the Newport district is based on easily mapped lithological groups, and follows the scheme adopted in the Chepstow (250) and Monmouth (233) sheets, with the exception that the sandstone division called Brownstones is here termed Brownstone Group (the explanation for this is given on p. 28). The subdivisions are as follows:

The position of the Silurian–Old Red Sandstone junction in the classification of the rocks in the Welsh Borders has, since 1839, fluctuated between the horizon of the Ludlow Bone Bed and the top of the Dittonian. The various classifications are summarized by White (1950, fig. 2), and he gives an account of the history of the Silurian–Old Red Sandstone boundary problem from the earliest work by R. I. Murchison in 1833, to the position in 1950, when it was generally agreed that the most satisfactory junction was at the base of the Ludlow Bone Bed. This opinion has prevailed until recently, but there is now increasing international pressure from geologists who believe that a new boundary should be established in the continuous marine sequence which exists in Central Europe. The controversy has been reviewed by Holland (1965, pp. 213–21), who discussed the problems involved, and anticipated Bouček and others' (1966) proposal that the boundary should be positioned at the top of the Prídolí formation in Central Bohemia, an horizon which is said to equate with the base of the 'Psammosteus' Limestone Beds of this district.

The junction between the Raglan Marl Group and the St. Maughan's Group in the Newport district has been drawn at the top of a series of beds up to 100 ft thick, that is characterized by the widespread occurrence of calcareous deposits, and which appear to equate with similar calcareous strata in Shropshire. These comprise King's (1925, p. 383) Downtonian 'zone' 1.8, which varies in thickness from 70 to 140 ft, and contains sandstones, marls and limestone beds. The limestones were named 'Psammosteus'Limestones by King, and the term was then used by later workers (Whitehead and Pocock 1947; Greig and others 1968) to describe a group of calcareous beds. Welch and Trotter (1961) and Mitchell and others (1962), however, in their descriptions of the geology around Monmouth and Chepstow, and Droitwich, Abberley and Kidderminster respectively, name only one 'Psammosteus' Limestone, and this refers to the best developed beds of limestone in the respective districts, though they are not necessarily correlatives. Similarly, Ball and Dineley (1961, p. 189), in the Brown Clee Hill area, recognize a persistent limestone, which they call the main 'Psammosteus'Limestone, though they also use 'Psammosteus' Limestones to describe the group of calcareous beds at the base of their Ditton Series. In the Newport district, the group of strata in which calcareous beds are common have been named the 'Psammosteus' Limestone Beds, and, conforming with usage in the adjacent Chepstow district, the highest bed of limestone, which is by far the best developed, is named the 'Psammosteus' Limestone.

In the northern part of the Old Red Sandstone outcrop of the Welsh Borders the strata equivalent to the 'Psammosteus' Limestone Beds of Newport appear to fall naturally into the Dittonian; the base of the group is marked by a sharp facies change, sandstones and conglomerates, interbedded with limestones, becoming abundant (Allen and Tarlo 1963, p. 142). Greig and others (1968), in the Church Stretton district, find this abrupt facies change a convenient boundary for the base of the Ditton Series, and Ball and Dineley (1961) in part of the same area place the boundary slightly higher, at the base of their main 'Psammosteus' Limestone. In the Newport, Chepstow and Monmouth districts, however, a similar facies change to an arenaceous lithology takes place after the deposition of the 'Psammosteus' Limestone Beds, and thus the top of the group appears to be, on lithological grounds, the most natural position for the boundary between the Downton and Ditton series. White (1950) prefers the base of the Dittonian to be marked by the first appearance of Pteraspis, which is usually immediately above the 'Psammosteus' Limestone, or not far below it.

Taking a broad view, the Downtonian–Dittonian junction lies somewhere within the 'Psammosteus' Limestone Beds 'phase' in the Welsh Borders but, because of the apparent diachronism of facies change mentioned above, and the difficulty of marking a faunal boundary, there is no general agreement as to its exact position. It is commonly drawn at the horizon of a sharp litho-logical change in any particular area, this being a convenient position for mapping purposes.

The boundary between the St. Maughan's Group and Brownstone Group in the Newport district is drawn at the position where the dominantly argillaceous deposition of the former division is replaced by the mainly arenaceous deposition of the latter. This is an easily mapped line throughout the area (though it is probably diachronous) and it equates with the St. Maughan's Group–Brownstones junction in the Chepstow district. The term Brownstone Group was originally applied by Heard and Davies (1924, p. 494) in the area around Cardiff to the same sandstone division mapped by Strahan in the Newport district and incorrectly called Brownstones. As an explanation for the adoption in this memoir of the term Brownstone Group instead of Brownstones, the history of the usage of the name Brownstones and its incorrect application in some areas is now given. In the type area of the Black Mountains, Symonds (1872) recognized, beneath the Upper Old Red Sandstone, two groups, namely, the Senni Beds overlain by Brownstones. In this connotation these groups were first mapped by the Geological Survey on the Merthyr (231) Sheet by T. C. Cantrill in 1897, on Brecon six-inch sheets 38 N.E. and 39 N.W., and subsequently on the Ammanford (230) Sheet in 1907 and Carmarthen (229) Sheet in 1909. During the primary survey of the Newport district, however, Strahan was unable to distinguish Brownstones and Senni Beds, and misapplied the term Brownstones to a group of sandstones which are thought to represent the Brownstones of the type area plus the top part of the Senni Beds. This misapplication was later extended by Eastwood (in Summary of Progress for 1937, p. 29) to the Forest of Dean. To avoid further confusion in correlation therefore, and in accordance with Croft (1953, p. 430) who suggests that a local name be applied when the Brownstones and Senni Beds cannot be distinguished, the term Brownstone Group is used in this memoir.

The correlation of the Old Red Sandstone subdivisions in the Newport district with the Downtonian, Dittonian, Breconian and Farlovian stages is shown in (Figure 2). The boundaries of the subdivisions closely follow the boundaries of the stages, with the exception of the St. Maughan's Group–Brownstone Group junction which probably lies within the Breconian. Croft (1953), in introducing the stage name Breconian, did not accurately define the Dittonian–Breconian junction, but, according to H. W. Ball (personal communication), Croft draws it on his maps (as yet unpublished) of the Black Mountains at the base of the lower of the Ffynnon Limestones, which Ball and Dineley (1961) correlate with the lower of the Abdon Limestones of their Clee Series. This limestone horizon is possibly represented in the Newport district by the Ruperra Limestone (p. 36), the base of which then may mark the Dittonian–Breconian junction.

History of research

One of the earliest references to the Old Red Sandstone of the Newport Sheet was by Sollas (1897), who measured the thickness of the Old Red Sandstone in the south-western part of the district, and considered its relationships with the Silurian of the Rhymney Inlier in the Cardiff (263) Sheet to the south. The most important contributions to the knowledge of the Old Red Sandstone are by Strahan (1899) and Heard and Davies (1924). Strahan, in 1891, commenced the first survey of the area on a scale of six inches to one mile, and his account was published in 1899, followed by a second edition in 1909. He recognized three subdivisions; a lower mass of marls and sandstones 2800 ft thick, which equates with the Raglan Marl Group and St. Maughan's Group of the present classification, the Brownstones, 500 ft thick, which correlate with the Brownstone Group, and an upper division of 200 ft of conglomerates and sandstones which constitutes the Upper Old Red Sandstone. His account gives a general description of the succession, in which he recognized that limestones in the lower division, which equate with the 'Psammosteus' Limestone Beds of this account, tended to reappear at the same horizon. He also noticed a persistent limestone not far below the base of the Brownstone Group, which is the Ruperra Limestone of this memoir. Heard and Davies (1924) made a very detailed systematic petrological investigation of all the Old Red Sandstone sediments, in an effort to trace the source of the rocks and to understand their environment of deposition. They concluded that the sediments were derived from a metamorphic and acid igneous complex, together with some arenaceous and calcareous sediments, and suggested that they were derived from a Pre-Cambrian massif to the north-west. They also described the 'Coed-y-Coedcae Fish Band', which lies immediately above the 'Psammosteus' Limestone, and were the first to recognize the Dittonian age of the strata at that horizon.

Conditions of deposition and palaeogeography

Towards the close of Silurian times the Newport district was covered by a gradually shallowing shelf sea in which shelly marine fossils thrived in a dominantly argillaceous environment. The final stages of Silurian sedimentation were characterized by the incoming of more arenaceous sediments as deltaic conditions established themselves over the area. This gradual change was effected by the major Ardennic phase of the Caledonian orogeny, which primarily affected the geosynclinal area of north and central Wales and regions farther north and north-west, where considerable folding took place forming the Caledonian Mountains (Wills 1951, p. 18). At this time the Newport district was part of a relatively stable region, the south-westerly extension of the Midland Block (Wills 1951, p. 14), on which deposition was more or less continuous, and there is a gradual passage from the Silurian to the Old Red Sandstone. The only visible evidence of a break in deposition is at the horizon of the Ludlow Bone Bed, which is poorly developed in the southern part of the Usk Inlier. This is in contrast to other parts of the Welsh Borders, for example in the Woolhope and May Hill inliers, where a sharper lithological break between the two systems is marked by a well developed bone bed. In the Newport district therefore, the highest sandy sediments of the Ludlow Series pass almost imperceptibly into the Downton Castle Sandstone, the main difference between the divisions being their faunal content; marine fossils are abundant in the Ludlow Series, but absent in all but the basal part of the Downton Castle Sandstone, which is characterized by brackish forms such as Lingula and fish. The Ludlow Bone Bed, where present in the south of the Usk Inlier, marks a pause in sedimentation when there was a period of active current action and winnowing in shallow water, but in the Newport district this is a very temporary phase and hardly interrupts the passage from Silurian to Old Red Sandstone.

The rocks of the Downton Castle Sandstone and Raglan Marl Group were probably deposited in a brackish, deltaic or intertidal environment, the Newport district occupying part of an extensive, low-lying, gradually subsiding delta plain, which was occasionally covered by a shallow, brackish sea. The sediments were derived from the Caledonian Mountains to the north-west, and transported into the area by numerous streams, which, at times of heavy rain, flooded large areas. The formation of thick limestones, for example, at the end of Raglan Marl Group times, marks a quiet period when most of the area was flooded by shallow water in which little or no sediment was deposited. The coarse, arenaceous sediments which lie at the base of the St. Maughan's Group indicate a sudden uplift of the Caledonian Mountains, and the commencement of a period when fluviatile conditions became fully established over a complex deltaic area. Higher in the St. Maughan's Group the sediments are finer grained and concretionary limestones appear, suggesting that conditions of deposition were similar to those which prevailed at the end of Raglan Marl Group times. The locally occurring Llanishen Conglomerate may have been derived from a land area to the south-west, which emerged towards the end of

St. Maughan's Group times. The succeeding sandstones of the Brownstone Group denote renewed uplift of the Caledonian Mountains, and this was later followed by the main phase of the Caledonian orogeny, which resulted in the unconformity between the Lower and Upper Old Red Sandstone. The mainly arenaceous beds of the Upper Old Red Sandstone mark another period when fluviatile conditions were re-established in a deltaic area. Towards the end of that period marine conditions gradually established themselves, and the delta was flooded by a shallow sea in which the basal Carboniferous Limestone sediments were deposited.

Lithology

The grey and buff sandstones of the Downton Castle Sandstone are quickly followed by reddish brown or green marls^‡5  which mark the inception of a variety of depositional environments, giving rise to wide lithological variation throughout the Old Red Sandstone. Red marls, striped or spotted green, predominate in the Raglan Marl Group and St. Maughan's Group, and are subordinate in the Brownstone Group and Upper Old Red Sandstone. Sandstones occur sporadically in the Raglan Marl Group, particularly in the lower part, and more commonly in the St. Maughan's Group, where they are locally very thick in the lower half of the succession. The lowest bed of the St. Maughan's Group is commonly a white or pale grey, sometimes conglomeratic sandstone, which locally contains a rich fish fauna. The Brownstone Group is dominantly sandstone, and the Upper Old Red Sandstone contains a high proportion of sandy beds.

One of the most characteristic features of the Old Red Sandstone is the abundance of calcareous deposits in the Raglan Marl Group and St. Maughan's Group, ranging from limestone nodules in marl (race) to beds of limestone up to 25 ft thick. In literature the beds of limestone are commonly referred to as comstones, and were originally defined as such by Buckland (1821, p. 512, footnote). However, according to Murchison (1839, p. 55, footnote 2) the name 'cornstone' should be restricted to conglomeratic beds (the conglomeratic cornstones of this memoir) and never applied to the beds of purer limestone, which are best called concretionary limestones. The terms will be used in this sense throughout the following description.

Three distinctive types of conglomerates are found in the Old Red Sandstone, namely, the conglomeratic cornstones, the Llanishen Conglomerate and the basal conglomerates of the Upper Old Red Sandstone. The first occur at many horizons in the Raglan Marl Group and St. Maughan's Group and consist of numerous, rounded or lenticular pebbles of mudstone, silty mudstone or fine-grained sandstone up to about an inch in length, set in a silty or sandy matrix of crystalline calcite. The pebbles are generally aligned roughly parallel to the bedding, though are sometimes oblique to it, and give the impression of having been deposited by turbulent water. They are intraformational conglomerates, the fragments probably originating from the erosion of a desiccated surface layer of sediment by a swift flow of water, the rolling action causing small pebbles to form from the hardened sediment. These accumulated in lenticular beds, usually not more than about 5 ft thick, and were later cemented with calcite. The Llanishen Conglomerate is found in thick beds throughout the upper 450 ft of the St. Maughan's Group in the southern part of the Newport district, and commonly consists of hard, brown quartzite pebbles up to 3 inches in diameter cemented by a sandy, calcareous matrix. The origin of the pebbles is discussed on p. 36. The basal conglomerates of the Upper Old Red Sandstone are hard, white or pale grey beds composed of quartzite pebbles in a quartz matrix, and they mark the renewal of deposition following the long period of erosion during all, or part, of Middle Old Red Sandstone times.

The most characteristic feature of the bulk of the sediments in the Old Red Sandstone is their red colour, which results from the presence of anhydrous ferric oxide. There has been considerable speculation in the past to explain the origin of the red sediments, ranging from their formation in an arid climate, to the prevailing view that they originated in a warm, humid upland area in which thick, red, lateritic soils developed, and were deposited in an oxidizing environment. The problem thus involves the climate and tectonic setting of the source area, and site of deposition. As far as the area under consideration is concerned, the source of most of the Old Red Sandstone sediments lay in the Caledonian Mountains to the north-west; these sediments were deposited in an area in which the climate was probably warmer and much drier than the source.

Within the Raglan Marl Group and St. Maughan's Group, rhythmic deposition can be recognized, though the cyclothems are irregular in thickness and frequently interrupted. The ideal cyclothem commences with a period of minor erosion, and the formation of an intraformational pellet-conglomerate (conglomeratic cornstone) with a calcareous matrix. This is followed by the deposition of current-bedded and current ripple-marked sandstones, coarse-grained at the base and becoming finer-grained upwards; these pass gradually into poorly bedded silty marls and marls, which form the thickest part of the sequence. Calcareous deposits develop in the marls, scattered nodules at first, which become more abundant upwards, and gradually coalesce into nodular limestones with very little marl. The final stage is reached with the formation of massive limestone virtually free of sediment. The ideal cyclothem is rarely completely developed, one or more of the lithological units being absent according to local variations in conditions of deposition. A detailed study of this cyclic sedimentation has been made in the northern part of the Welsh Borders by Allen and Tarlo (1963), who compare the Old Red Sandstone cyclothems with those in present day deposits. H.C.S.

Lower Old Red Sandstone

Downton Castle Sandstone

The Downton Castle Sandstone occupies a narrow, discontinuous outcrop around the faulted boundary of the Silurian Usk Inlier. Strahan (1899), and Gardiner (1917), thought that the Downton Castle Sandstone was absent along the eastern side of the inlier due to faulting, but subsequently Walmsley (1959) recognized a complete sequence in the Silurian and basal part of the Old Red Sandstone. Over most of the Usk area the gradual lithological passage from the Silurian to the Old Red Sandstone takes place through up to 20 ft of strata. The highest, silty mudstones of the Ludlow Series are usually interbedded with thin siltstones and sandstones, which commonly contain a typical marine fauna; as the sequence passes from the Ludlow Series into the Downton Castle Sandstone it becomes more and more arenaceous, and the Ludlovian fossils become less abundant, until almost unfossiliferous sandstones predominate. Thus the junction between the Ludlow Series and Downton Castle Sandstone, particularly in those areas where the Ludlow Bone Bed is absent, is difficult to mark precisely; it is most suitably placed at the horizon above which sandstones predominate. At the few localities in the southern and southeastern parts of the Usk Inlier where the Ludlow Bone Bed has been found, the boundary is more clearly defined, for the bone bed marks a minor hiatus in sedimentation, which results in a sharper lithological break between the two divisions.

At the base of the Downton Castle Sandstone the Ludlow Bone Bed rests, without angular discordance, on the Upper Whitcliffe Beds. It is a thin, impersistent, brown or buff, sometimes calcareous, sandy or silty bed up to 3 in thick, and may contain phosphatic nodules. It is succeeded by 5 to 75 ft of pale grey to buff siltstones and sandstones, commonly weathered brown, with subordinate red, green or grey mudstones and silty mudstones. The sandstones are massive or thinly bedded, and are characterized by sedimentary structures such as cross-bedding, ripple marks and washouts. The mudstones occur in thin lenticular beds, or in the form of pellets in the base of sandstones which have resulted from penecontemporaneous erosion of semi-consolidated mudstone by strong current action before sandstone deposition.

The Ludlow Bone Bed and the overlying basal few feet of the Downton Castle Sandstone sometimes contain typical Whitcliffe Beds fossils, the most commonly occurring forms being crinoid columnals, Salopina lunata (J. de C. Sowerby), Protochonetes ludloviensis Muir-Wood, Loxonema obsoletum (J. de C. Sowerby), Cornulites serpularius Schlotheim and Beyrichia aff. torosa Jones. The Ludlow Bone Bed also contains a fish fauna, which includes abundant thelodont and acanthodian denticles, and, more rarely, Cyathaspis banksi (Huxley and Salter), Onchus sp.and Sclerodus sp.Fragments of inarticulate brachiopods, e.g. Lingula, are usually common Above the basal beds the Downton Castle Sandstone is poorly fossiliferous, and only Lingula minima J. de C. Sowerby has been found in any abundance. The occurrence of Sclerodus [Eukeraspis] sp., (Strahan 1899; Walmsley 1959, p. 497) is interesting in view of its rare occurrence in the Welsh Borders.

Raglan Marl Group

The Raglan Marl Group is essentially an argillaceous subdivision, with subordinate arenaceous and calcareous beds, the latter being particularly common in the uppermost 'Psammosteus'Limestone Beds. Because of the soft nature of the succession, its outcrop generally occupies low lying ground and forms heavy, clay soils. North of Newport the group attains a thickness of 1800 to 2000 ft and thins south-westwards to only about 1100 ft north of Cardiff, this latter figure being obtained partly from observations made in the northern part of the adjacent Cardiff (263) Sheet. The boundaries of the Raglan Marl Group are well defined; the junction with the Downton Castle Sandstone is drawn where red marl of the Raglan Marl Group predominates over buff sandstone, the transition being fairly rapid and its position easily mapped around the periphery of the Silurian Usk Inlier. The junction with the St. Maughan's Group is taken at the top of the 'Psammosteus' Limestone, a bed which is easily traced over much of the area.

The Raglan Marl Group consists predominantly of marl or silty marl, usually reddish brown to purple, and green in spots, irregular patches, stripes and ramifying tubes, or along margins of bedding and joint planes. It is poorly bedded and has an irregular, blocky fracture. Numerous, minute flakes of mica are commonly present. A few lenticular beds of sandstone are interbedded with the marls, particularly in the lower part of the sequence. They are most abundant in the north-eastern part of the outcrop, where they form low ridges, but south-westwards, as the Raglan Marl Group thins, they rarely occur. Their colour is usually red, but may be buff or green, and they vary from soft and fine-grained to hard coarse-grained beds. Mica is a common constituent and the sandstones may be soft and shaly on account of its abundance. The best developed sandstone in the Raglan Marl Group lies about 150 ft above the Downton Castle Sandstone; it attains a thickness of 170 ft and has been traced for about 5 miles around the southern periphery of the Usk Inlier. This is exceptionally thick for a sandstone in the Raglan Marl Group, about 20 to 50 ft being more usual.

A variety of calcareous deposits is present in the Raglan Marl Group, most common being small, impure, limestone nodules averaging about half an inch in diameter, which occur in varying abundance in beds of marl. These grade into impure, nodular limestones, in which the nodules are larger and more abundant and the red marl a minor constituent, and finally into massive compact limestones almost free of argillaceous sediment. Other calcareous concretions have the form of greenish grey, ramifying pipes or rods, up to half an inch in diameter and 2 ft in length. They are aligned perpendicular to the bedding, and sometimes packed together in great abundance.

Calcareous deposits are particularly common in the 'Psammosteus' Limestone Beds. Through about 100 ft of strata marls with limestone nodules, and nodular and massive concretionary limestones, become more abundant upwards, the sequence terminating in the well developed 'Psammosteus' Limestone, which is the most persistent limestone in the area and invaluable as a marker horizon. Lithologically it is a nodular, greyish white limestone, commonly red and green stained, in which a characteristic soft, purplish red marl fills the interstices between the nodules. In some areas where it is thickly developed it takes the form of a massive, amorphous-looking, greyish white limestone, sometimes well jointed, almost porcellaneous in appearance and very hard. Bedding is rarely seen. Its thickness varies typically between 5 and 15 ft, though ranging up to 30 ft. No fossils have previously been recorded in this limestone, but Dr. J. R. Hawkes and Dr. B. Kelk report that specimens from Pentre-bach Quarry (see p. 45) exhibit an organic appearance, suggesting the possibility of algal growth. Dr. F. W. Anderson also examined the thin sections, and he believes that they contain traces of what appear to be very poorly preserved algal tubules like those of Girvanella sp.and Ortonella sp.The presence of algae in these limestones indicates that they may be, in part at least, of organic origin, and not, as was previously thought, entirely chemically precipitated. Similar algal features have been observed by Dr. Anderson (in Eyles and others 1949, p. 48) in limestones in the Upper Old Red Sandstone of Ayrshire.

A summary of the few fish remains found in the Raglan Marl Group is included in a report (p. 37) by Mr. H. A. Toombs on the fishes from the Lower Old Red Sandstone.

St. Maughan's Group

The St. Maughan's Group consists of interbedded marls, silty marls, sandstones, conglomeratic cornstones, concretionary limestones and conglomerates. Marls predominate, and they are lithologically similar to those in the Raglan Marl Group. Arenaceous rocks occur more frequently than in the Raglan Marl Group, particularly sandstones in the lower part of the sequence, and conglomerates locally in the upper part. Impersistent beds of conglomeratic cornstone or concretionary limestone occur irregularly throughout the succession. The thickness of the St. Maughan's Group has been estimated to be about 1500 ft in the vicinity of Pontypool, though this may not be an accurate figure, for the exact position of the base of the division cannot be mapped with certainty in this area, and the average dip of the steeply inclined beds is difficult to calculate. Southwards, one mile south of Cwmbran, the thickness increases to about 2000 ft due to the development of sandstones in the basal part of the sequence, and farther south-west, one mile south of Machen, decreases to approximately 1750 ft, of which the upper 450 ft contain the thick beds of Llanishen Conglomerate. On the eastern margin of the Newport Sheet the St. Maughan's Group is about 1500 ft thick, this estimate being based partly on evidence from adjacent ground in the Chepstow (250) Sheet.

Over most of the Newport district an abrupt facies change marks the junction between the Raglan Marl Group and St. Maughan's Group; the 'Psammosteus'Limestone is followed by a sandy sequence containing a characteristic medium to coarse-grained, buff, white, or greenish white sandstone, locally conglomeratic at the base, which contains scattered fish remains. Sandstones higher in the sequence are similar to those in the Raglan Marl Group, usually being reddish brown, sometimes green or buff, fine to coarse-grained and commonly micaceous. This marked change to an arenaceous lithology is well demonstrated along the western limb of the Usk Anticline, particularly between Cwmbran and the Ebbw Valley, where up to 400 ft of sandstone in 600 ft of beds are present. Along the eastern limb of the anticline the change to a sandy lithology is less marked, though the incoming of a high proportion of arenaceous sediment above the 'Psammosteus' Limestone is clearly demonstrated. Southwest of the Ebbw Valley the same change in sedimentation can be recognized, but sandstones immediately above the 'Psammosteus' Limestone are usually not more than about 5 ft thick.

Generally speaking, in the area north-east of the Ebbw Valley, the lower half of the St. Maughan's Group contains up to one third of arenaceous beds (the remainder being marl and limestones), and the upper half considerably less. South-west of the Ebbw Valley, arenaceous beds are infrequent, except in the upper part, which contains thick beds of Llanishen Conglomerate, interbedded with pebbly sandstones, conglomeratic cornstones, concretionary limestones and marls.

The Llanishen Conglomerate characterizes the upper 450 ft of the St. Maughan's Group in the area south and south-west of Machen. The type locality of the conglomerate is in the railway cutting at Llanishen Station [ST1812 8229], 3.5 miles north of Cardiff on the southern margin of the Newport Sheet; it was described by Strahan (1899, p. 17) as a bed of gravelly, red conglomerate occurring 2000 ft above the Silurian and 800 ft below the base of the Brownstone Group. Conglomerate beds of a similar nature and horizon were later described by Strahan and Cantrill (1902, p. 17) in the adjacent Cardiff (263) Sheet, and in more detail by Heard and Davies (1924) around Cardiff. The term Llanishen Conglomerate was first used by Dixon (in discussion, Heard and Davies 1924, p. 516) when he correlated it with the Ridgeway Conglomerate of Pembrokeshire. Pringle and George (1948) included the Llanishen Conglomerate with the Ridgeway Conglomerate in a division called the Red Conglomerate Group. Because the conglomerates, interbedded with marls, sandstones and limestones, occur through approximately 450 ft of strata at the top of the St. Maughan's Group, it is necessary to extend the original usage of the term Llanishen Conglomerate to include the total thickness of these distinctive conglomerate beds.

The origin of the pebbles in the Llanishen Conglomerate and Ridgeway Conglomerate has been a subject of much speculation, for the lithology of the constituent pebbles is difficult to match with any older strata, and fossil remains in the pebbles are rare. Heard and Davies (1924, p. 135) suggest a Pre-Cambrian source to the north-west, but this is unlikely for the sporadic presence of fossils in the pebbles indicates younger strata. Dixon (in discussion, Heard and Davies 1924, p. 516) is of the opinion that the presence of horny brachiopods in pebbles in the Ridgeway Conglomerate (e.g. Kutorgina) indicates a Cambrian origin and suggests that the source is south of Pembrokeshire. Cox (in discussion, Heard and Davies 1924, p. 517) states that the lithology of some of the pebbles of coarser grain in the Cardiff area resembles that of the Lower Cambrian Bronllwyd Grit of North Wales. During the present resurvey of the Newport district a boulder of Llanishen Conglomerate, found in drift deposits near Newport, yielded a fine-grained sandstone pebble containing several fragments of 'Camarotoechia' cf. nucula (J. de C. Sowerby) which are almost certainly of Silurian age.

From the available evidence, it is deduced that a southerly or south-westerly origin for the Llanishen Conglomerate is most likely, for the deposits are confined to the area around the southern part of the South Wales Coalfield in the Newport district. Its most northerly exposure occurs 4 miles north-west of Newport. The large size of some of the pebbles indicates a nearby source, and thus a Lower Palaeozoic land ridge probably existed during late St. Maughan's Group times along the site of the Bristol Channel.

Laterally impersistent conglomeratic cornstones and concretionary limestones occur at many horizons throughout the St. Maughan's Group. One of the best developments of the latter is at an estimated 500 to 600 ft above the Tsanunosteus' Limestone, where two limestones lying about 60 ft apart have been mapped east of Caerleon; the upper, which is the thicker and purer, is known as the Coldra Limestone. Higher, at about 1450 ft above the 'Psammosteus' Limestone, the Ruperra Limestone is a greyish white nodular, or massive bed up to 20 ft thick, and it has been traced for about 2 miles in the area south-east of Machen. Like the 'Psammosteus'Limestone it contains diffuse structures that may be algal in origin (p. 34). Near Pontypool a well-developed limestone (here named the Pontypool Limestone), at least 12 ft thick, lies at the top of the St. Maughan's Group. The position of this limestone in relation to the Ruperra Limestone is not known for certain, but, if the St. Maughan's Group–Brownstone Group junction is more or less synchronous along the 10 mile stretch between Pontypool and Ruperra, the Pontypool Limestone lies some 300 ft higher than the Ruperra Limestone. The two limestones may then be the equivalents of the lower and upper Abdon limestones of the Clee Hills, which Ball and Dineley (1961, p. 198) correlate with Croft's lower and upper Ffynnon limestones of the Black Mountains.

Fish remains in sandstones and cornstones have been found at several horizons in the St. Maughan's Group, and a summary of the occurrences is included in the following account by Mr. Toombs of the Lower Old Red Sandstone fishes of the Newport district. H.C.S.

The Ludlow Bone Bed has yielded the two forms so characteristic of this horizon in its type area, namely Cyathaspis banksi and Sclerodus sp.The accompanying wealth of acanthodian and thelodont denticles cannot yet help stratigraphically. Only Sclerodus sp.has been recorded from the Downton Castle Sandstone and this is within its known stratigraphical range.

So few localities in the Raglan Marl Group have yielded fishes that it is unsafe to generalize, but in this district some forms seem to occur lower in relation to the 'Psammosteus'Limestone than they usually do; there is also overlapping of the Traquairaspis and Pteraspis zones, as in some other areas. In this connection caution is urged in interpreting the significance of isolated occurrences of Traquairaspis pococki (White), T. symondsi (Lankester) and Pteraspis leathensis White, the three forms that have been used to delineate zones above or below the Downtonian–Dittonian junction. Within recent years intensive collecting has greatly increased their known stratigraphical and geographical ranges, but in none of them has any variation of strati-graphical significance been observed and their complete ranges are clearly not yet known. Experience has shown that the reliable determination of these zones depends upon adequate faunal assemblages.

The lowest discovery of fishes in the Raglan Marl Group is at Ty-coch Quarry (p. 44), at about 800 ft below the 'Psammosteus' Limestone. There Kallostrakon sp.was found in abundance, a good deal lower than usual, but within its known range; elsewhere it is a useful horizon marker, commonly occurring just below the Traquairaspis zones, as in the type locality, Bush Pitch, Ledbury. At Alteryn Quarry (p. 45), about 460 ft below the 'Psammosteus' Limestone, Traquairaspis pococki occurs at the lower limit of its known range. Similarly, the only occurrences of Pteraspis leathensis are low ones, at Glen Usk Quarry (p. 44), 10 to 20 ft below the 'Psammosteus' Limestone and Penrhos Farm Quarry (p. 49) just above the Limestone and so in the St. Maughan's Group. Pteraspis sp.also occurs with Traquairaspis sp.at Ivybridge Quarry (p. 44), 10 to 20 ft below the Limestone.

In contrast, the only records of Traquairaspis symondsi are from its most usual horizon, the coarse, white sandstone that so often lies above the 'Psammosteus' Limestone and forms the base of the St. Maughan's Group in much of this area. At Coed-y-Coedcae (p. 52) it is associated with Anglaspis sp.; this association in this kind of sandstone, either a little above, or a little below the 'Psammosteus'Limestone, is a remarkably constant feature in the succession, from the Clee Hills to Pembrokeshire.

From about 500 to 600 ft above the ‘'Psammosteus' Limestone a typical Dittonian succession was proved by Dr. J. M. C. W. Baker (University College, Cardiff) in 1954 in a temporary section for a water-supply tunnel south-west of Christchurch (p. 50). Here the lower beds yielded Pteraspis rostrata (Agassiz) and the upper beds Pteraspis crouchi (Lankester). At Cat's Ash Quarry (p. 50) higher beds have yielded an arctolepid spine and fragments of Pteraspis sp., the fine ornamentation of which is comparable with that of Pteraspis leachi White, but positive identification is not possible.

In addition to the fish remains collected during the resurvey of the Newport district (see detailed account pp. 42 to 57) Mr. Toombs has provided the following list of published records and old collections in the British Museum (Natural History) which are too poorly localized to be of much stratigraphical value, though they are probably from the St. Maughan's Group: Gold Tops, Newport, yielded Kujdanowiaspis sp.[P. 5049] and Pteraspis rostrata monmouthensis White, [P. 5034], the holotype of the subspecies described by White, 1935, p. 391: the specimen P. 8895 referred by Traquair (1898, p. 68) to Psammosteus, and quoted by Strahan (1899, p. 21), is Pteraspis sp.From Maindee, to the east of Newport, there is another specimen (P. 8894) referred by Traquair (1898, p. 68) to Psammosteus: this is probably a fragment of a cephalaspid. A ventral disk of Pteraspis crouchi (P. 25021) also comes from this locality. From Stow Hill Road, Newport, comes the rostrum of Pteraspis crouchi (P. 8898) figured by Lee (1882, pl. 3, f. 9) and referred to by Strahan (1899, p. 21). Specimens simply labelled 'Newport' include Protodus sp.(P. 8900), Pteraspis crouchi (P. 25040), Securiaspis sp.(P. 25032) and Weigeltaspis sp.(P. 25030–31). H.A.T.

Brownstone Group

The outcrop of the Brownstone Group, a dominantly arenaceous subdivision, forms a prominent escarpment which runs from just east of Pontypool southwestwards through Upper Cwmbran, Risca and Machen to the southern boundary of the Newport district. A small outlier of the Brownstone Group, in the form of a south-westerly pitching syncline, crops out over an area of about two fifths of a square mile, one mile east of Risca. The Brownstone Group attains its greatest thickness of 575 to 600 ft east of Pontypool, and gradually thins southwards to 500 ft west of Cwmbran and to 450 ft north of Risca. South-westwards, near Machen, it thins to less than 400 ft, before thickening again to 500 ft at Thornhill this thickness being maintained to the southern boundary of the district.

The upper and lower limits of the Brownstone Group are sharply defined and easily mapped. The lower boundary is marked by the rapid transition from marls to sandstones at the junction with the St. Maughan's Group, and the upper boundary at the base of the coarse, white conglomerates which constitute the lower part of the Upper Old Red Sandstone. The former boundary usually runs along the base of the scarp slope formed by the Brownstone Group, and the latter along the crest of the scarp, or a short distance down the dip slope, though local variations occur.

The sandstones of the Brownstone Group vary considerably in grade from soft, fine-grained, argillaceous sandstones, to hard, coarse-grained, sometimes conglomeratic sandstones. Their bedding varies from flaggy to massive, and false-bedding is a characteristic feature. Mica commonly occurs, sometimes in abundance, giving the rock an almost shaly fissility. Thin, reddish brown and green marls and silty marls occur sporadically. They may contain small, impure limestone nodules, but rarely in sufficient abundance to form distinct beds of concretionary limestone. The upper surfaces of marl beds may show signs of penecontemporaneous erosion, and lenticular pebbles derived from these marls may be seen in the basal sections of overlying sandstones.

The rocks of the Brownstone Group are devoid of fossils which give an indication of their age. Their basal beds are thought to equate with the upper part of the Senni Beds of the Black Mountains and surrounding areas ((Figure 2)), which contain a flora from a relatively high horizon in the Lower Old Red Sandstone (Croft and Lang 1942, p. 159). This is confirmed by the presence of Rhinopteraspis dunensis (Roemer) high up in the Senni Beds (Croft 1953, p. 431), for this species is known to occur in the upper part of the Lower Old Red Sandstone. Thus, if the above correlation of the Brownstone Group is correct, the lower beds are definitely of Lower Old Red Sandstone age. Throughout the remainder of the Brownstone Group sedimentation appears to have been continuous, and it is most likely that all the rocks were deposited during Lower Old Red Sandstone times, though it is possible that deposition continued for a while in Middle Old Red Sandstone times, before uplift and erosion commenced during the Svalbardic phase of the Caledonian orogeny, and gave rise to unconformity between the Brownstone Group and Upper Old Red Sandstone. H.C.S.

Upper Old Red Sandstone

The Upper Old Red Sandstone consists of a laterally impersistent, basal conglomeratic formation overlain by alternations of variably coloured sandstones, silty mudstones and mudstones of fluviatile origin on a subaerial delta, which pass gradually into the wholly marine strata of the succeeding Carboniferous Limestone Series, there being no apparent stratigraphical break between the two series.

The outcrop of the Upper Old Red Sandstone is never more than 500 yd wide, and follows the periphery of the basin of the South Wales Coalfield from Pontypool south-westwards through Risca to Tongwynlais. It generally occupies low-lying ground between the escarpments of the Brownstone Group and Carboniferous Limestone Series. When thickly developed the basal conglomerates form a sharp, scree-covered ridge rising above the bolder, more rounded, escarpment of the Brownstone Group. The land has a thin soil cover, and is generally used for rough grazing and forestry. The basal conglomeratic beds are usually well exposed, and, where thickly developed, give rise to lines of crags along high ground. The junction with the Brownstone Group is easily traced, the hard white conglomerates and quartzites contrasting strongly with the underlying, softer, argillaceous, reddish brown sandstones. The softer beds overlying the conglomerates are poorly exposed by comparison, and are only visible in a few stream sections and artificial exposures. The thickness of the Upper Old Red Sandstone ranges from 250 to 400 ft. At Pontypool the basal conglomerate is about 75 ft thick and the total thickness of the Upper Old Red Sandstone approximately 275 ft. Southwards, the maximum thickness of about 400 ft is attained along parts of the outcrop between Upper Cwmbran and Risca. South-west from Risca, to the southern edge of the Newport Sheet the thickness varies between 250 and 350 ft.

The sudden appearance of white or pale grey coarse conglomerates and sandstones above the reddish brown impure sandstones of the Brownstone Group, marks the renewal of deposition following a period of erosion during Middle Old Red Sandstone times. This unconformity is impossible to demonstrate in the Newport district, for the boundary between the Brownstone Group and the Upper Old Red Sandstone is nowhere exposed, and thus their relationships are not known. From the available field evidence the Upper Old Red Sandstone appears to lie conformably on the Brownstone Group, for there is no readily discernible discordance of dip between them, or evidence to prove overlap of the conglomerate on to different horizons of the Brownstone Group. However, in the adjoining Chepstow and Monmouth districts an unconformity has been proved between the two divisions (Welch and Trotter 1960, p. 4), the Upper Old Red Sandstone overlapping on to the St. Maughan's Group south of Monmouth. Similarly, east of the River Severn, the conglomerate rests upon the Thornbury Beds (equivalent to the St. Maughan's Group or Raglan Marl Group), and in the Tortworth district on beds of Silurian age (Kellaway and Welch 1955, p. 6). It is reasonable to assume therefore, that the Newport district was similarly affected by earth movements and erosion, following the deposition of the Lower Old Red Sandstone, and that the conglomerates probably rest unconformably on an eroded Brownstone Group surface with some discordance of dip.

The basal conglomerates equate with the Quartz Conglomerate division of the Chepstow and Monmouth districts, and they consist of very hard, white or pale grey, coarse, quartz conglomerates interbedded with similarly coloured quartzites and subordinate, thin, red, green or grey, silty marls and marls. The conglomerates occur in lenticular beds, up to 80 ft thick along parts of the outcrop, and are thin or absent elsewhere. They are best developed near Pontypool, south-west of Upper Cwmbran, north-east of Risca, at Machen, south and east of Rudry and along Craig Llanishen near Thornhill.

Above the basal conglomerates the Upper Old Red Sandstone comprises mainly soft, poorly cemented, grey, green, yellow or buff, fine to coarse-grained, sometimes false-bedded sandstones, with subordinate red, green or grey marls and thin lenticular beds of conglomerate. They equate with the Tintern Sandstone Group of the adjacent Chepstow district. Many of the sandstones in the upper part of the succession have a calcareous matrix. The boundary between the Upper Old Red Sandstone and Carboniferous Limestone Series is drawn at the base of a thick limestone, in, and above which, the rocks are fully marine. This limestone lies at the base of the Lower Limestone Shale (the basal division of the Carboniferous Limestone Series) and forms a low scarp, which is easily followed throughout much of the district. Below the limestone there is an interdigitation of fluviatile and marine sediments, which constitute a transitional sequence from the Upper Old Red Sandstone into the Carboniferous Limestone. This passage is fairly abrupt in some areas, but in others it is more gradual, and extends for up to 60 ft through the succession. In the latter case, dark mudstones, thin shelly limestones and buff and yellow sandstones containing marine fossils, are interbedded with non-marine Upper Old Red Sandstone sediments. When these circumstances prevail the exact position of the Upper Old Red Sandstone–Carboniferous Limestone boundary is difficult to mark precisely. There is a strong case for drawing it at the lowest horizon above which marine fossils occur, but this would not be a practical line in the Newport district, for such precise mapping requires continuous exposures through the sequence at regular intervals along its crop. The Upper Old Red Sandstone is generally poorly exposed, and the boundary is most satisfactorily drawn at the base of the thick limestone mentioned above, which is the first easily recognized, persistent bed of Lower Limestone Shale facies in the Newport district. Kellaway and Welch (1955, p. 7) draw attention to the fact that this position for the boundary was first selected by Buckland and Conybeare (1824), and it has been widely used by later workers. They also mention that the difficulties of defining the base of the Carboniferous Limestone Series in the Bristol-South Wales region would have been avoided if the Lower Limestone Shale had been included in the Upper Old Red Sandstone. The base of the Main Limestone (which overlies the Lower Limestone Shale), easily traced on lithological and faunal grounds throughout the district, would then mark the boundary between the two systems. However, the present boundary has strong priority claims and its traditional usage is continued in the Newport district.

No fossils are known in the Upper Old Red Sandstone of the Newport district, except in the passage beds into the Carboniferous Limestone Series. In these beds, marine fossils typical of the Lower Limestone Shale were found. They include crinoid ossicles, Fenestella sp., 'Camarotoechia' mitcheldeanensis Vaughan, Leptagonia analoga (Phillips), Syringothyris sp., orthotetids and Myalina sp.H.C.S.

Details

Downton Castle Sandstone

Along the east side of the Usk Inlier the thin development of Downton Castle Sandstone forms a very narrow outcrop and exposures are few. In the lane [ST 3682 9825] north-north-west of Trestevan Farm the transitional nature of the passage from Silurian to Old Red Sandstone is well exposed; the highest, fossiliferous, sandy Whitcliffe Beds pass gradually into the Downton Castle Sandstone, which is only about 5 ft thick. About 600 yd south-south-west of Llangibby Castle the Downton Castle Sandstone thickens up to 30 ft, and forms a prominent north-east to south-west ridge extending for about 0.25 mile. At the base of the sandstone, a temporary section [ST 3636 9694], exposed by ploughing, revealed a brown, sandy Ludlow Bone Bed about 2 in thick, containing thelodont denticles, fragments of inarticulate brachiopods and small phosphatic nodules. South-westwards, in the southern bank of the lane [ST 3546 9570] near Brook House, the Ludlow Bone Bed is again present; it is a coarse-grained, buff or brown, decalcified sandstone up to 3 in thick, in which Walmsley (1959, p. 497) collected the following fauna: 'Dastacoderma' Harley, Cyathaspis banksi (Huxley and Salter), Onchus sp., Sclerodus sp., acanthodian and thelodont scales, together with fragments of Lingula sp., O. rugata and S. longissimus."

At Llandegfedd [ST 3383 9573] the Downton Castle Sandstone thickens to 40 to 50 ft, and forms a wide, dip-slope outcrop. In this vicinity Strahan (1899, p. 17) recorded Sclerodus [Eukeraspis] sp., from a pit (probably the disused quarry [ST 3340 9530] northeast of Ty–Capten Farm) 600 yd south-west of Llandegfedd which showed: "hard grit interbedded with shaly sand and clay". North-westwards, the thickness of 40 to 50 ft is retained, and the Downton Castle Sandstone forms a fairly wide outcrop through Granary and Pentwyn farms [ST 3226 9670] and [ST 3180 9776] respectively as far as Sluvad Farm.

Rapid thinning takes place north of Sluvad Farm, and the Downton Castle Sandstone is probably no more than 5 ft thick for a distance of about 400 yd. Northwards again, the outcrop widens as the sandstones thicken to 30 ft or more north of Trostra Farm [SO 3158 0020]. On this western flank of the Usk Inlier there are few good exposures of Downton Castle Sandstone. The Ludlow Bone Bed is visible in the stream [ST 3181 9753] south of Pentwyn Farm, where it is a thin, pale grey, fine-grained sandstone containing crinoid columnals, decalcified bryozoa, Lingula sp., Protochonetes ludloviensis, Salopina lunata, Loxonema sp., pterineid, Cornulites serpularius, Beyrichia sp.and thelodont denticles.

In the stream running north from Pentwyn Farm, small scattered exposures exhibit typical sandstones of this division; in one section [ST 3168 9800] the sandstone has yielded numerous Lingula minima, which show distinct current orientation, the fossils being aligned roughly parallel to each other. Approximately 200 yd north buff sandstones are exposed in the bank [ST 3169 9816] of a stream, and they contain: crinoid columnals, Howellella elegans (Muir-Wood), Lingula sp., Protochonetes ludloviensis, Salopina lunata, Loxonema obsoletum, Cornulites serpularius, Tentaculites ornatus J. de C. Sowerby, Acastella sp., and Beyrichia sp.These sandstones probably lie very near the base of the Downton Castle Sandstone.

Two small, irregularly shaped outcrops of Downton Castle Sandstone occur 1 mile and 1.5 miles south-west of Llandegfedd. The northerly outcrop is very poorly exposed, and its boundaries were traced mainly on the evidence of superficial, buff or brown sandstone debris. In the southerly outcrop, at Llanfrechfa [ST 3200 9370], the transitional junction between the Ludlow Series and the Downton Castle Sandstone is visible in a stream section [ST 3196 9383]; the uppermost Whitcliffe Beds are very arenaceous, and the-Ludlow Bone Bed appears to be absent. Along the track [ST 3190 9397] leading north from Llanfrechfa House, the same junction is exposed, and there the Ludlow Bone Bed is present as a brown or buff sandstone, about 1 in thick, containing abundant shelly fossils and numerous fish denticles. The following fauna was collected: crinoid columnals, Campylites sp., Lingula minima, Protochonetes ludloviensis, Salopina lunata, Cornulites serpularius, Beyrichia aff.torosa, Cytherellina siliqua (Jones), and thelodont denticles.

Raglan Marl Group

North-east of the Afon-lvyd Valley

The narrow outcrop of the Raglan Marl Group on the western side of the Usk Inlier near Pontypool results from steepening dips, and the effect of the north-south trending Pen-y-Ilan Fault, which cuts out approximately 700 ft of Downtonian beds. Southwards, the outcrop widens as the westerly dip lessens, and the Pen-y-Ilan Fault dies out, until, east of Pontnewydd, the full thickness of approximately 2300 ft is present. The effect of the Pen-y-Ilan Fault is demonstrated by following the prominent sandstone ridge which runs from Lower New Inn [ST 3050 9889] north-eastwards to Pen-y-Ilan [SO 3123 0065], where it is abruptly cut out by the fault, the throw being about 700 ft. The structure is complicated there by the Pen-y-Ilan Fault splitting into two branches, but the overall effect is to throw the sandstone against Whitcliffe Beds of the Ludlow Series.

The Raglan Marl Group is exposed only in a few places west of the Usk Inlier; in the stream running west from Pentwyn Farm [ST 3180 9886] and then turning southwards, typical exposures of red and green marl with sporadic, thin beds of red, flaggy sandstone occur. Between this stream and the Afon-lwyd Valley, several sandstones develop, giving rise to low, sinuous ridges, most of which die out within a short distance. One lying approximately 200 ft above the Downton Castle Sandstone is the most persistent, and has been worked locally for building stone; its outcrop runs southwards from Tymawr-Llanthewy [ST 3146 9817], and swings eastwards around the faulted southern periphery of the Usk Inlier, then continues north-eastwards along a prominent ridge through Llwyn-celyn [ST 3550 9492] and dies out just east of Llangibby Castle. The sandstone is visible in a disused quarry [ST 3656 9641], 1100 yd south of Llangibby Castle, where 10 ft of red, flaggy beds are exposed. Below this sandstone the Raglan Marl Group is well exposed in the stream running north-eastwards from Cwm-ffrwd Farm [ST 3532 9500], mainly red marl with a few silty and sandy beds being visible. H.C.S.

In the Llanhennock area, east of Sôr Brook, the Raglan Marl Group is affected by a shallow syncline trending north-east to south-west through Llanhennock, on the southern flank of the Usk Anticline. To the north and west of this syncline, exposures are few, but, to the south, on the north side of the Usk Valley, several small, scattered exposures are present. One of the uppermost sandstones of this group is exposed in two old quarries near Glen Usk. In one [ST 3620 9255], Glen Usk Quarry, 5 ft 6 in of red, false-bedded sandstone, with a thin, impersistent rubbly limestone near the base of the quarry face, have yielded acanthodian denticles, Onchus sp., Pteraspis sp., P. leathensis, and Pachytheca. The other, Ivybridge Quarry [ST 3592 9275], reveals 5 ft of red sandstone which have yielded acanthodian denticles, cephalaspid and heterostracan fragments, Pteraspis sp., and Traquairaspis sp.

Approximately 700 yd east of Glen Usk the 'Psammosteus' Limestone forms the crest of a low ridge, and it was worked extensively for lime in this vicinity. It is now poorly exposed, the following localities themselves being partly overgrown: a disused quarry [ST 3688 9313] north-east of Glen Usk, showing 5 ft of rubbly limestone with some red marl, and another [ST 3702 9260] east of Glen Usk, exposing 3 ft of rubbly limestone. Westwards, the outcrops thins, and passes to the south of Llanhennock, before swinging northwards to join the north-eastern flank of the previously mentioned syncline. The limestone was worked in shallow diggings [ST 3556 9356] east-north-east of Lan-Sôr-Fawr. Half-a-mile to the north-east of these, the outcrop is cut out by a north-south fault [ST 3634 9408] to the west-north-west of Cefn-henllan Farm. Some 1.5 miles of 'Psammosteus' Limestone outcrop are concealed by the alluvial belt of the River Usk south of Llanhennock. B.K.

Between Sôr Brook and the Afon-lwyd Valley the 'Psammosteus' Limestone crops out in Penrhos Farm Quarry [ST 3398 9198]. where the following sequence is visible:

North of this quarry, shallow workings follow the outcrop of the 'Psammosteus' Limestone, which was burnt for lime. In this vicinity it varies in thickness from 2 ft to more than 10 ft; in the small quarry [ST 3443 9280] south-south-east of Cassandra Farm, 10 ft of massive, rubbly 'Psammosteus' Limestone with some red and green marl is exposed, this not being its total thickness.

Afon-lwyd Valley to Ebbw Valley

In this area the Raglan Marl Group occupies a mainly low-lying outcrop in the core of the Usk Anticline, 3 miles wide in the north-east, narrowing to 1.5 miles at Rogerstone where it is largely drift covered. It is poorly exposed, except in the quarries where the marls have been excavated for brick-making. In Ty-côch Quarry [ST 2935 9343], 0.5 mile south of Cwmbran, approximately 250 ft of marl, containing fine-grained sandstones and impure concretionary limestones, are exposed. In this quarry a large, loose block of impure, concretionary limestone yielded the following fish remains: acanthodian scales and bone fragments, many specimens of Kallostrakon sp., and Onchus sp.The disused Malpas Road Brickworks Quarry [ST 3050 8980], north-west of Newport, reveals nearly 200 ft of the Raglan Marl Group.

In the quarry a normal fault down-throwing westwards disturbs the sequence; west of the fault there are exposed 100 ft of red marls, with green mottling and striping common at the base. Small calcareous nodules are present throughout, and are sometimes. abundant in thin beds. East of the fault the succession is:

Alteryn Quarry [ST 2912 8863], north-west of Newport exposes the following 120 ft of beds at about 460 ft below the 'Psammosteus' Limestone:

Above the quarry a locally developed, fine to coarse-grained, white to grey or brown sandstone with some conglomerate forms a prominent ridge extending north-east to south-west for nearly a mile. H.C.S.

A specimen (E30289) of the sandstone collected from debris on the ridge [ST 2880 8822] is a coarse protoquartzite with sub-angular to sub-rounded quartz grains ranging from 0.4 mm to 3.5 mm across. The average grain size of the quartz is between 0.7 mm and 1.0 mm. Other fragments present include quartzite, quartz-schist, quartz-muscovite-schist, intermediate igneous lava, silty and silty calcareous sediment, perthite and sodium-felspar, and rare muscovite. 'Vermicular' chlorite was noted in one quartz fragment. Many of the quartz fragments are cemented by secondary silica at their margins, and a certain amount of interstitial iron oxide is present.

Abundant weathered felspars are evident in the hand specimen. J.R.H.

In the stream running south-eastwards from Croes-y-mwyalch Reservoir [ST 3032 9228] the Raglan Marl Group is well exposed, and comprises mainly red marls containing calcareous nodules and a few beds of sandstone. Smaller sections are exposed in the streams 250 yd south of Park Farm [ST 3186 9170], and 200 yd south of Church Farm [ST 2900 9010].

The 'Psammosteus' Limestone crops out along the crest of a prominent scarp on the western and eastern flanks of the Usk Anticline. In the north, on the western side of the anticline, the scarp rises from the low-lying ground of the Afon-Iwyd Valley at Pontnewydd, and runs southwards for 3.5 miles through Henllys House [ST 2880 9400] and just west of Pentre-bach Farm [ST 2855 9208] to Bettws Brook [ST 2750 9045], and then becomes obscured by morainic drift. South of Henllys House, for about one mile, the 'Psammosteus' Limestone has been worked extensively, there being numerous shallow pits and trenches along its outcrop. In Pentre-bach Quarry [ST 2852 9243] the following sequence can be seen:

Three specimens (E31451), (E31452), (E31453) collected from the basal bed in the above section are fine-grained, silty limestones with irregular patches composed of larger calcite plates between 0.3 mm and 3.5 mm across. A few fragments of quartz and subgreywacke material (up to 7.5 cm across) occur, being most abundant in specimen (E31451). Some are extensively replaced by calcite. The areas of finer-grained carbonate predominate in the samples. They generally have an organic appearance, and show either rounded, or sinuous outlines. Filamentous structures are evident in some, suggesting the possibility of algal activity and growth. In specimen (E31451) the areas of fine-grained calcite are largely replaced by small rhombs of dolomite. Iron-oxide (hematite) is common in specimens (E31451) and (E31453), occurring chiefly in the areas of fine- grained calcite. Little hematite appears in the dolomitized rock. J.R.H.

In the stream [ST 2834 9299] west-south-west of Pentre-bach Farm, approximately 20 ft of massively bedded and nodular limestones lie on 20 ft of soft, red, fine-grained, micaceous sandstones. Southwards from there, to Pant-yr-eos Brook, the 'Psammosteus' Limestone has not been seen and may be absent locally.

Along the eastern flank of the Usk Anticline, the ridge formed by the ''Psammosteus' Limestone Beds and basal sandstones of the St. Maughan's Group runs from Caerleon Forge [ST 3365 9155] south-westwards along Lodge Wood [ST 3260 9165], through Crindau [ST 3090 8980], where the sandstones are very thin or absent, and along Barrack Hill [ST 3030 8910] and Ridgeway [ST 2930 8820] to the Ebbw Valley. Along this stretch the 'Psammosteus' Limestone Beds are well exposed in the small park [ST 3133 9028] along the bank of the River Usk north of Newport as follows:

Recent excavations at Crindau, on the site of the western portal of the Newport By-Pass Tunnel [ST 3085 8985], have revealed the following section in the uppermost part of the Raglan Marl Group, and basal part of the St. Maughan's Group:

South-west of the Ebbw Valley. Between the Ebbw Valley and the southern margin of the Newport district, the narrow outcrop of the Raglan Marl Group is extensively covered by morainic drift, and thus exposures of the beds are few. The sandstones above the 'Psammosteus' Limestone, which form a prominent ridge north-east of the Ebbw Valley, are virtually absent in this area, and thus the topography is subdued, though locally a scarp develops where the 'Psammosteus' Limestone is unusually thick.

Along the northern flank of the Usk Anticline the Psammosteus' Limestone is exposed at irregular intervals along its sinuous outcrop. In the north-east it appears from beneath the thick, morainic drift cover, cropping out for a distance of about 300 yards near Pen-tre-tai Farm [ST 2615 8820], where it is exposed in Pen-tre-tai Quarry [ST 2599 8799] as follows: St. Maughan's Group (see p. 52 for details) 8 ft on nodular 'Psammosteus' Limestone with a purplish red, marl matrix 12 ft. Half a mile to the south-west, immediately south of Ty'n-y-ffynnon Farm [ST 2525 8747], the 'Psammosteus' Limestone forms a conspicuous east-west ridge for a distance of about 800 yd. The limestone is particularly well developed there, its maximum thickness being about 30 ft. The quarry [ST 2532 8740] south-east of the farm exposes 10 ft of massive, rubbly limestone with prominent, vertical joints containing secondary calcite; purple marl occurs in the joints and within the limestone. In this vicinity two other beds of limestone have been quarried; they lie about 60 and 100 ft respectively below the 'Psammosteus' Limestone, and attain an individual thickness of up to 10 ft. The upper bed is exposed in a small quarry [ST 2534 8733] south-south-east of Ty'n-y-ffynnon Farm, where 6 ft of very massive, light grey limestone with calcite veining are visible. The lower bed is similar in lithology, and 3 ft are exposed in a quarry [ST 2541 8729] south-east of Ty'n-y-ffynnon Farm.

Westwards the outcrop of the 'Psammosteus' Limestone is easily traced as far as the Rhymney Valley, where it disappears beneath the alluvium. South-westwards from the Rhymney Valley its position beneath the morainic drift cover has to be conjectured for about 1 miles, before it is next seen in the stream [ST 2122 8489] running through Coed-y-Coedcae as shown below:

South-west from Coed-y-Coedcae, in the area of Cefn Mably Park, the westerly pitching Cefn Mably Anticline and Cefn Mably Syncline cause the crop of the 'Psammosteus' Limestone to swing through an S-shape before running south-west again to the southern boundary of the Newport district. There are no exposures of the 'Psammosteus' Limestone in the area around Cefn Mably Park, the only evidence for its existence being in two small quarries [ST 2045 8395] and [ST 2078 8430] near Cefn-porth-isa Farm, which contain rubbly limestone debris. A lower limestone was also quarried near Cefn-porth-isa Farm, the crop of which can be traced for about two-thirds of a mile. H.C.S.

Along the southern flank of the Usk Anticline, in the area south-west of the Ebbw Valley, the outcrop of the 'Psammosteus' Limestone is almost entirely drift covered, but it emerges for two-thirds of a mile near the Ebbw Valley, where it was quarried [ST 2778 8644] south-east of Duffryn Court. South-westwards it crops out for 500 yd near Pwll-côch, and is exposed as 4 ft of white, concretionary limestone with irregular patches of purple marl in an old quarry [ST 2349 8267] north-west of the farm. Two beds of limestone were worked there; some fragments of purple sandstone are also present in the quarry. R.K.

St. Maughan's Group

Pontypool to the Ebbw Valley

In the neighbourhood of Pontypool, high dips, folding and faulting reduce the width of the St. Maughan's Group outcrop to less than mile immediately north of the Afon-lwyd Valley. In the cutting [SO 2988 0018] south-west of Pontypool Road Station, red and green marls and sandstones in the lower part of the St. Maughan's Group dip west at 80 to 90°, and they are overturned along part of the section, where they dip at 80° to the east. West and south-west of this cutting, similar high dips and overturning prevail in sandstone exposures along the banks [SO 2955 0005] of the Afon-lwyd.

North of the Afon-lwyd the top of the St. Maughan's Group is locally marked by the Pontypool Limestone, which has been traced for about 0.25 mile; it is visible in a quarry [SO 2940 0063] where 12 ft of massive, greenish grey, red-stained, rubbly, impure limestone are exposed. A specimen (E31455) collected from this locality is a dolomitired limestone containing some detrital quartz grains. Irregular patches of fine-grained dolomite (which appear to replace calcite) form 60–70 per cent of the rock, and they are separated by areas consisting of calcite crystals averaging 0.07 to 0.1 mm across. Patchy hematitization seen in the hand specimen affects both dolomitic and calcitic parts of the rock. This is the highest limestone known in the Lower Old Red Sandstone of the Newport district, and has been seen only at this locality. Southwards from the above-mentioned quarry the bed appears to die out rapidly, and may pass into 9 in of conglomeratic comstone visible in a quarry [SO 2924 0046] west of Pontypool Road Station.

South of the Afon-lwyd, the low-lying outcrop of the poorly exposed, dominantly argillaceous St. Maughan's Group widens to 0.5 mile at Pontnewydd. South from there, a succession of sandstones up to 500 ft thick develops in the lower part of the St. Maughan's Group and forms a prominent scarp running 3.5 miles south from Pontnewydd to Bettws Brook, where it becomes obscured by drift. The distinctive white or buff sandstone which characterizes the base of the St. Maughan's Group over a large part of the Newport district is present in this area. It is visible in a quarry [ST 2875 9360] north-north-west of Pant-glas Farm, 6 ft being exposed. Southwards, about 350 yd, abundant sandstone debris is visible [ST 2872 9328] north-west of Pant-glas Farm; it is white, coarse-grained, red-stained in parts and contains green marl pellets, or irregularly-shaped cavities from which marl has been removed. Rare fish remains are present in this sandstone, which yielded a fragment of Traquairaspis symondsi. About 1200 ft above the base of the St. Maughan's Group in this area, another thick sandstone develops, and forms a conspicuous ridge which runs southwards from Gelli Farm [ST 2730 9290] for nearly 2 miles, before passing under the drift cover. Typical members of this division are exposed in Nant Henllys [ST 2697 9155]. Above this sandstone, forming the top part of the St. Maughan's Group, there are about 200–300 ft of red and green marls with some sandstone, limestone and conglomerate. Typical exposures of the marls are visible along Nant Henllys [ST 2709 9270], and in the section [ST 2578 9330] 50 yd west of Ty-trappau. A rubbly limestone, about 15 to 20 ft thick, has been quarried [ST 2638 9372] north-north-east of Pen-sarn Farm; it is a very local deposit, cropping out for no more than 700 yd before dying out. In the lane side [ST 2564 9220] near Cwrt Henllys Farm there are exposed 4 ft of conglomerate, containing brown quartzite pebbles up to 11 inches in diameter, set in a calcareous matrix. The bed weathers to a loose, gravelly deposit. It is the most northerly exposure of the Llanishen Conglomerate facies in the Newport district. H.C.S.

Christchurch–Llanhennock area to Ebbw Valley

The St. Maughan's Group crops out in the Llanhennock area as an outlier within the syncline previously mentioned (p. 43); in the north-east it is faulted against the Raglan Marl Group. A small stream [ST 3641 9284] north of Glen Usk exposes 4 ft of fine-grained, red sandstone, with interbedded green and red-mottled marl, immediately overlying the Raglan Marl Group. Numerous fragments of coarse, white sandstone typical of the arenaceous strata in the basal beds of the St. Maughan's Group occur on a slight rise on the gently-sloping ground approximately 20 yd north of the stream. B.K.

North of Caerleon, in Penrhos Farm Quarry [ST 3398 8198], the following section is well exposed:

In this quarry Dr. J. M. C. W. Baker found Pteraspis cf. leathensis. The specimen is deposited in the British Museum (Natural History).

About a mile west of Caerleon conglomeratic cornstones are well developed in Mount Pleasant Quarry [ST 3196 9086], where the following succession was recorded:

A specimen of Onchus sp.was found in the above quarry. H.C.S.

South of the Usk Valley, in the Christchurch area, the beds of the St. Maughan's Croup are exposed almost solely by streams cutting into the hillsides, and in a few quarries. An old quarry [ST 3522 8957] west of Coldra, reveals the Coldra Limestone as 4 ft of purple, impure, rubbly limestone overlying 3 ft of white, concretionary limestone. On the north-west side of Chepstow Hill 3.5 ft of Coldra Limestone, overlain by purple sandstone, is exposed in a stream [ST 3522 9052]. The outcrop of the Coldra Limestone passes southwards along the side of Chepstow Hill through an overgrown quarry [ST 3487 9030] by the Roman road north-west of Coldra, where purple sandstones overlying 3 ft of white limestone are exposed. In the stream [ST 3470 8961] north of Christchurch Church, 3 ft of rubbly, purple limestone underlie purple, micaceous sandstone which has yielded specimens of Pteraspis sp.

About 60 ft below the Coldra Limestone lies another persistent limestone, though apparently thinner, and less pure. It was seen in a stream [ST 3567 8985] north of Coldra, as 18 in of very rubbly, purple and green limestone overlain by purple marl. The stream [ST 3509 9061] north-north-west of Coldra exposes this thin limestone, and there it is very sandy. South-south-westwards it appears in another stream [ST 3446 8970], where the section is:

A stream [ST 3426 8944] east of Christchurch Church, exposes a 2-ft thick red and green conglomeratic cornstone. This is seen for 110 yd in the side of the stream, where a northward lateral transition from the conglomeratic cornstone to a white, almost pure, concretionary limestone is visible. B.K.

In 1954 a water supply tunnel (National Grid References of portals: [ST 3438 8925] and [ST 3456 8899]) was driven 346 yd through the St. Maughan's Group south-west of Christchurch. The succession traversed was examined by Dr. Baker who found a rich fish fauna in cornstones and sandstones dipping south-eastwards at about 10°. His collections were deposited in the British Museum (Natural History), and determined by Mr. Toombs as follows:

• Collection A–In beds between the south-east portal and a position about 115 yd along the tunnel; Pteraspis crouchi.
• Collections B and C–In beds between 115 yd and 175 yd from the south-east portal; Cephalaspis sp., Pteraspis rostrata, Pachytheca.
• Collection D–At about 260 yd from the south-east portal; cephalaspid scales. Pteraspis rostrata, Pteraspis sp., Pachytheca and plant fragments.
• Collection E–In beds between the north-west portal and the previous position; cephalaspid fragments. H.C.S.

In Cat's Ash Quarry [ST 3681 9085], by the remains of a Roman road, up to 6 ft of purple sandstone with ripple-drift bedding rest with angular discordance on 5.75 ft of dark, lime-green, current-bedded sandstone. These have yielded: arctolepid spine, cephalaspid fragments and Pteraspis sp.B.K.

In the excavations [ST 3085 8985] for the construction of the western portal of the Newport By-Pass Tunnel, the following section in the St. Maughan's Group was recorded:

The plant-bearing parts of the 4.5-ft bed of silty mudstone occurring 2 ft 2 in above the top of the Raglan Marl Group have yielded (Chaloner and Street 1966, pp. 87–102) Pachytheca sp.and the following assemblage of miospores: Chelinospora vermiculata Chaloner and Streel, Emphanisporites rotatus McGregor, Granulatisporites newportensis Chaloner and Streel, G. sp., Punctatisporites cf. punctatus (Ibrahim), Retusotriletes sp.; from single specimens the following were also recognized: Densosporites sp., Lycospora sp.and Murospora sp.According to Chaloner and Street this assemblage is from an horizon about the same age as the earliest generally accepted record of vascular land plants.

The following section was obtained from a borehole [ST 3189 8768] south-east of Newport Castle, sunk in the bed of the River Usk to explore the foundations of a new bridge:

The quarry [ST 3045 8890] south-east of the Barracks, reveals 20 ft of red marl and silty marl with laterally impersistent, fine-grained sandstone beds up to 2 ft thick. South-westwards, to the Ebbw Valley, only scattered small exposures of red sandstone, commonly current-bedded, can be seen. About 0.25 mile south of the outcrop of the St. Maughan's Group in this area, excavations for the storage tanks of a petrol station [ST 2873 8613] revealed 10 ft of coarse, sandy and clayey gravel resting on 3 ft of red marl and silty marl of the St. Maughan's Group.

South-west of the Ebbw Valley

Along the southern flank of the Usk Anticline, in the neighbourhood of Bassaleg and Castleton, the St. Maughan's Group is extensively covered with drift deposits and the scattered outcrops are poorly exposed. Along the northern flank of the anticline drift deposits are widespread over the lower part of the St. Maughan's Group, while the upper part is relatively well exposed. One of the few exposures in the basal beds is in Pen-tre-tai Quarry [ST 2599 8799] near the Ebbw Valley, where the distinctive, coarse-grained, white sandstone commonly present at the base of the St. Maughan's Group is absent, the following section being present:

The basal beds are next exposed 3.5 miles south-westwards, in the Coed-y-Coedcae stream section [ST 2122 8489], as follows:

From the same conglomeratic sandstone, the Coed-y-Coedcae Fish Band, Heard and Davies (1924, p. 490) recorded "Pteraspidian and cephalaspidian fishes" and quoted A. S. Woodward's identification of Pteraspis sp.; however the "bits of cancellated tissue" which Woodward saw are Anglaspis and Traquairaspis.

North of Coed-y-Coedcae, in the neighbourhood of Nant Cwm Crynant [ST 2050 8550] and Ruperra Castle [ST 2198 8643], the outcrop of the St. Maughan's Group is almost free from drift deposits, and is well exposed. The lower 1300 ft of the 1750 ± ft of beds comprising the St. Maughan's Group in this area, consist of red marls and silty marls, with subordinate, red, fine-grained, flaggy sandstones. They are exposed along Nant Cwm Crynant and in the Coed-y-Coedcae and Cwm Du [ST 2215 8565] stream sections. Above this thick argillaceous sequence the upper part of the St. Maughan's Group comprises 400 to 500 ft of marls, sandstones, limestones and thick beds of Llanishen Conglomerate. The most widespread limestone is the Ruperra Limestone, which attains a thickness of 15 to 20 ft, and has been traced for a distance of 2 miles in this area. It is exposed in a quarry [ST 2242 8653] east-north-east of Ruperra Castle, 6 ft of rubbly limestone being visible, and in Ruperra Quarry [ST 2174 8629], where the section is: fine to coarse, conglomeratic cornstone 12 ft on rubbly concretionary limestone (Ruperra Limestone) 15 ft, calcareous, conglomeratic sandstone 4 ft. H.C.S.

The upper bed (specimen (E31449)) contains pebbles and fragments (averaging 0.5 mm and ranging up to about 6.0 mm across) of protoquartzite, subgreywacke, quartzite, chert, siltstone and intermediate igneous lava. Many of the fragments are extensively replaced by calcite and, in some cases, the position of their original margins is marked by grains of hematite. Apart from calcite the matrix contains scattered, sub-angular and sub-rounded quartz grains, while rare fragments of sodic plagioclase and perthite occur. The calcite crystals mostly range from 0.01 mm to 0.15 mm across, and there are scattered, rounded or sinuous patches of very fine-grained material which could possibly be of organic ( ?algal) origin. Patchy developments of hematite are evident in hand specimen. The Ruperra Limestone (specimen (E31448)) is fine-grained, and contains scattered quartz fragments and a little detrital schistose quartzite, as well as some patchy hematitization. It shows signs of organic structure ( ?algal filaments). Below the Ruperra Limestone in the above section the 4 ft of calcareous, conglomeratic sandstone (specimen (E31450)) is a calcareous protoquartzite with a few, scattered, large lithic fragments (up to 1.75 cm). Rounded to sub-angular fragments of quartz, with subordinate quartzite, chert, felspar, basic lava, ?devitrified rhyolite, silt, rare zircon etc. occur in a matrix composed chiefly of very fine-grained calcite crystals. Locally, recrystallization has produced plates up to about 0.4 mm across. Most of the clastic fragments are coated by hematite. J.R.H.

The Llanishen Conglomerate is well exposed in Nant Cwm Crynant [ST 2040 8565], from which the following section demonstrating the variable nature of the succession in the upper part of the St. Maughan's Group was obtained:

The Llanishen Conglomerate characterizes the highest beds of the St. Maughan's Group in this area, and the beds are well exposed [ST 2118 8635] west of Ruperra Castle, immediately below the base of the Brownstone Group. There, 12 ft of conglomerate containing sandstone pebbles up to 2 inches in diameter set in a sandy, calcareous matrix are visible. At a similar horizon the conglomerates may be seen along a track [ST 2015 8560] in Coed Llwyn-celyn where, in parts, they weather to a coarse gravel. Although the majority of the resulting pebbles are well rounded, a few are angular and have the appearance of dreikanters. H.C.S.

Specimens (E31460), (E31461), (E31462) of reddish brown pebbles proved to be protoquartzites of siltstone grade (average grain size 0.04 to 0.06 mm), containing mainly sub-rounded to sub-angular quartz grains and also felspar, muscovite, chlorite, chert and metamorphic quartz-chlorite rock (in (E31461)-(E31462), and felspar, muscovite, chert, tourmaline and zircon (in (E31460). Disseminated, interstitial hematite occurs in all three specimens. J.R.H. Approximately 3 miles south-west of Ruperra Castle, the Llanishen Conglomerate is visible in a stream [ST 1802 8442] north of Cefn-on Halt, and along the pathway [ST 1794 8416] at Cefn-on Halt, where the following section was recorded:

Brownstone Group

Pontypool–Risca–Machen–Thornhill

Natural exposures of the Brownstone Group are small and sporadic, and the rocks are well exposed only in artificial sections such as quarries, and along lanes and tracks. Near Pontypool, and for about 1 mile to the south, several impersistent beds of marl, up to 25 ft thick, are interbedded with the sandstones of the Brownstone Group, and these form narrow bench-features on the high ground occupied by the outcrop. One of the marl beds is exposed above Griffithstown, where excavations for a small reservoir [ST 2866 9705] have revealed 20 ft of red marl with some sandstone. More typical sections of the Brownstone Group are exposed in the disused quarry [SO 2975 0020] west of Pontypool Road Station, 30 ft of red sandstone with some silty mudstone being visible, and 1 mile to the south, where sandstone dipping at between 30 and 80º is present for 100 yd along the track [ST 2840 9840] leading to Penyrheol Farm. Southwards from there the outcrop of the Brownstone Group narrows as the inclination of the beds increases, and retains its prominent scarp-feature through Upper Cwmbran to near Risca. It is generally poorly exposed along this part of its outcrop, though Pen-y-rhiw mynydd Quarry [ST 2490 9188] exposes approximately 70 ft of massive, false-bedded, reddish brown sandstones, with a few beds of red or green marl up to 3 ft thick, which usually contain small, irregular, red or grey, impure limestone nodules. A specimen (E30288) of sandstone from this quarry is a protoquartzite (average grain size about 0.1 mm) consisting of sub-angular to sub-rounded quartz grains (some with secondary silica overgrowths) and subordinate fragments of siltstone, quartzite, quartz-schist, quartzose rock with 'vermicular' chlorite, chert, muscovite and rare microcline. Interstitial iron oxides are present. The Brownstone Group is also exposed in disused quarries [ST 2530 9010] east-south-east of Risca Station, where up to 30 ft of sandstone with thin beds of marl are visible.

South of the Ebbw Valley the outcrop of the Brownstone Group swings southeastwards and runs through Coed Mawr, where it forms a very conspicuous ridge, then it turns south-west again along Craig Wyllt [ST 2400 8800] towards the Rhymney Valley. This swing of the crop is caused by the Machen Anticline and Machen Syncline, the axes of which trend north-east to south-west. Typical exposures of Brownstone Group may be examined along the track [ST 2450 8855] running through Coed Mawr. South-westwards from the Rhymney Valley the Brownstone Group forms a well-defined ridge along Craig Ruperra, then follows more subdued ground for about a mile where it is exposed in a disused quarry [ST 2155 8654], which reveals 20 ft of flaggy sandstone with thin beds of marl. For about 300 yd along a lane [ST 1840 8485] near Cefn-on Halt the Brownstone Group is exposed in an almost continuous section of green, brown and purple sandstones with thin beds of red marl. Westwards, the scarp formed by the Brownstone Group runs along Craig Llanishen, swings south for about one mile through Coed-y-Wenallt, then turns south-westwards to the edge of the district. Near Thornhill sandstone beds with some marl are well exposed alongside the Cardiff–Caerphilly road [ST 1600 8420] where it rises over the steep scarp slope.

Upper Old Red Sandstone

Pontypool–Upper Cwmbran–Risca

The basal beds of the Upper Old Red Sandstone are particularly well developed near Pontypool, where coarse quartz conglomerates, containing pebbles up to 2 inches in diameter, crop out extensively on the westerly-facing dip-slope in Pontypool Park. They are well displayed in an exposure [SO 2904 0105] south-east of Pontypool Road Station, where they dip at 25° in a westerly direction. Abundant, coarse conglomerate scree covers the dip slope 300 yd farther north. The conglomerate thins rapidly southwards, and south of the Afon-lwyd Valley is replaced, at least in part, by a hard, pale grey sandstone, which is visible in the disused quarries [ST 2872 9992] near Folly Reservoir. A mile farther south, the variable nature of the Upper Old Red Sandstone succession is displayed along the track [ST 2825 9866] leading to Penyrheol Farm, where small exposures show interbedded quartz conglomerates, soft, fine-grained sandstones and red, green and grey marls. The basal conglomerate is thin or absent in this vicinity and for about 1 mile southwards to Upper Cwmbran, but is well developed again for the next 1.5 miles southwestwards to a short distance south of Llanderfel. Along this part of the outcrop it attains a thickness of about 20 ft, for example, along the track [ST 2715 9633] 500 yd south of The Square where it lies on reddish brown and green sandstones of the Brownstone Group. Near the ruins of Llanderfel Monastery some of the uppermost beds of the Upper Old Red Sandstone are exposed in Dowlais Brook [ST 2665 9537]. About 10 ft stratigraphically below the basal limestone of the Lower Limestone Shale the succession comprises flaggy, buff and yellow sandstone containing: plant impressions, a spiriferid, Myalina sp., and a doubtful bivalve. Approximately 20 yd downstream [ST 2666 9536], some 60 ft stratigraphically below the Lower Limestone Shale, the following section was recorded:

South-westwards about two-thirds of a mile, in the entrance to a drainage adit from Henllys Colliery [ST 2608 9445], 8 ft of soft, green, micaceous sandstones are exposed. They lie in the upper half of the Upper Old Red Sandstone, and demonstrate the soft nature of the beds in that part of the sequence. One mile north-east of Risca, below the pennant sandstone prominence of Twmbarlwm, the conglomerates and hard white sandstones of the basal part of the Upper Old Red Sandstone attain a thickness of about 60 ft, and form a well-defined escarpment trending north-east to south-west for nearly a mile, the dip-slope of which is strewn with scree from these beds. The sandstones are protoquartzites, a specimen (E30293) of which consists of sub-angular to sub-rounded quartz grains (average grain size 0.1 mm) with scattered fragments of perthite, silty material, quartzite, chert, quartz-schist, muscovite and tourmaline. A little iron oxide is associated with some of the constituents and many quartz grains are cemented by secondary silica. A sample (E30294) of the conglomerates proved to have similar constituents, and is only different in containing abundant, rounded pebbles of white ?vein-quartz up to 2 or 3 cm across.

Risca–Machen–Thornhill

Near Risca the Upper Old Red Sandstone is well exposed along a stream section [ST 2345 9032] where hard and soft, fine to coarse-grained, grey, green, brown or buff, sometimes conglomeratic, commonly micaceous sandstones are visible. A few beds of red marl and silty marl also occur. Near the junction with the Carboniferous Limestone Series the matrix of the sandstones becomes calcareous, and, at about 50 ft below this junction, a 6-in bed of grey, Shelly limestone, of typical Carboniferous Limestone lithology, is interbedded with these sandstones. H.C.S.

A specimen (E30291) of the calcareous sandstone is a calcareous protoquartzite consisting of sub-angular to sub-rounded quartz grains (average grain size 0.18 mm), with relatively abundant fragments of microcline, perthite and lesser sodic plagioclase. Other fragments include siltstones, quartz-schist, quartzite and muscovite flakes. The calcite cement has an irregular distribution and certain calcitic patches appear to have been partly replaced by very fine-grained carbonate which may be dolomitic. Some quartz grains in the rock are cemented by secondary silica. A little hematite occurs in cracks and as coatings to some felspar and quartz constituents. J.R.H.

Southwards, the Upper Old Red Sandstone is next exposed near Machen, its basal conglomerate forming a slight, north-east to south-west ridge, about 300 yd southeast of Machen. The conglomerate is visible in the road cutting [ST 2293 8790] south-south-east of Machen Church, where 10 ft of hard, massive, quartz conglomerate are present, and also in the small copse 150 yd to the north-west. In the stream [ST 2222 8740] which runs through Draethen Village, and for about 1 mile upstream, the typical, buff, sometimes calcareous sandstones of the upper part of the Upper Old Red Sandstone are visible in numerous, small exposures up to 5 ft thick, dipping at between 30 and 45° to the north-west. Abundant crinoidal remains have been found in some of these sandstones. The basal conglomerate is strongly developed in this vicinity, abundant conglomeratic scree lying on the north-facing dip-slope in Coed-y-Squire [ST 2120 8655] and Coed-y-Bwdrwin [ST 2165 8670]. In Nant Cwm, about one-third of a mile south of Rudry, the Upper Old Red Sandstone is exposed for about 0.5 mile along the stream; at one locality [ST 1920 8600] the following section, demonstrating the variable nature of the sequence, was recorded:

Below the Carboniferous Limestone scarp face of Cefn-onn, typical exposures of Upper Old Red Sandstone are visible along a lane [ST 1845 8525], where the succession consists of alternations of green, yellow and brown micaceous sandstones and red marl, dipping at between 12 and 32° to the north-west. Westwards, the basal conglomerate, inclined at between 40 and 63°, forms the conspicuous ridge of Craig Llanishen [ST 1700 8450] near Thornhill; small sections of the conglomerate are exposed at several localities along this outcrop. A short distance north of Craig Llanishen an excavation [ST 1651 8458] for the foundations of an electricity pylon, exposed beds at about 15 ft below the Lower Limestone Shale. They consisted of white, friable, sandstones underlying yellow, micaceous sandstones containing: Fenestella sp., 'Camarotoechia' mitcheldeanensis Vaughan, Leptagonia analoga (Phillips) and orthotetoids.

From Thornhill the outcrop of the Upper Old Red Sandstone swings south through Coed-y-Wenallt, then west towards Tongwynlais, where the beds are folded by the westerly-pitching Castell Coch Anticline and Tongwynlais Syncline. In a disused quarry [ST 1350 8240] at Tongwynlais alternations of red and yellow, fine to coarse-grained sandstones and red marls are exposed, dipping at between 24 and 46° in a southerly direction. These beds lie in the upper part of the Upper Old Red Sandstone. Nearby, in the same quarry, the transition between the Upper Old Red Sandstone and the Carboniferous Limestone Series is demonstrated in another section [ST 1344 8223]. The following sequence, in northly-dipping beds, was recorded:

The boundary between the Upper Old Red Sandstone and Carboniferous Limestone Series is most conveniently placed at the base of the 10-inch bed of sandy limestone. H.C.S.

References

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Chapter 4 Carboniferous Limestone Series

General classification

The base of the Carboniferous Limestone Series is taken at the base of the Lower Limestone Shale, as defined by Buckland and Conybeare (1824), and the top at the unconformable junction with the overlying sandstones and mudstones of the Millstone Grit Series. The Upper Old Red Sandstone–Carboniferous Limestone boundary problem is discussed in the previous chapter (p. 41). The rocks of the Carboniferous Limestone Series have been subdivided, on a lithological basis, as follows:

History of research

The earliest systematic work on the Carboniferous Limestone Series in the Newport district was by Strahan during the primary survey of the area. In the Memoir (1909a) he gives a general description of the stratigraphy of the beds, which deals mainly with the lithological characters of the sequence. A more thorough investigation was made by Dixey and Sibly (1918) in the area between Taffs Well and Risca; they presented a very detailed and comprehensive account of the stratigraphy of the Carboniferous Limestone Series, which included a classification and map based on the faunal zones established by Vaughan (1905) in the Avon Gorge. Much of this palaeontological work is incorporated in the present account, for the resurvey mapping was based largely on litho-logical, rather than faunal, criteria. A recent study of the Main Limestone (George 1956a), covers the area between Risca and Pontypool.

Dixey and Sibly (1918), in their detailed analysis of the fauna of the Carboniferous Limestone Series in East Glamorgan and West Monmouthshire, recognized assemblages typical of the Cleistopora (K), Zaphrentis (Z) Caninia (C) and Seminula (S) zones established by Vaughan (1905) in the Avon Gorge. They traced these zones first in the area between Bridgend and Misken, on the Bridgend (262) Sheet, where the sequence attains a thicker development than in the Newport district. The result of their work showed that four, broad faunal divisions could be recognized, and these form the basis of their map of the area. The divisions are as follows:

When Dixey and Sibly attempted to trace these zones into the Newport district, they found that in the Taffs Well area faunas typical of the K and S₂ zones were easily recognizable, but the dolomitic nature of the beds between these two zones made it difficult to define a boundary between the ZC₁ zones and the C₂S₁ Zone. They show a boundary line between the two divisions, but it is based only on lithological differences along part of its course, and merely sketched along the remainder. Thus the ZC₁ zones and C₂S₁ Zone are most conveniently grouped together, and the Carboniferous Limestone of the Newport district falls most satisfactorily into 3 divisions as follows:

In this chapter these three divisions will be referred to as K, Z-S₁ and S₂.

History of deposition

At the beginning of Carboniferous times a shallow shelf sea spread over the deltaic area in which the Upper Old Red Sandstone sediments were deposited. The passage from one system to the other is gradual and conformable, and characterized by the interdigitation of continental and marine sediments. The establishment of a marine environment was marked by the influx of a shallow-water fauna consisting of mainly bryozoa, crinoids and brachiopods, which thrived in these conditions and formed the dominant constituent of the thick limestones in the basal part of the Lower Limestone Shale. Limestone formation was followed by a period in which mudstone sedimentation was dominant; this was probably due to subsidence of the sea floor and derivation of fine-grained sediments from an uplifted St. George's Land, which lay to the north-west and covered the area occupied by central and north Wales. These muddy conditions were followed by a clear, shallow-water environment in which the thick, mainly organic limestones of the Main Limestone were deposited. At some later stage these were affected by metasomatic alteration which changed them into dolomitic rocks. Between S₂ Zone times and the incursion of the Millstone Grit sea the history of sedimentation in the Newport district can only be conjectured, for the basal strata of the Millstone Grit rest unconformably upon beds ranging from the S₂ Zone to low in Z-S₁ ((Figure 4)). The thinning and disappearance of higher zones, and the magnitude of the break between the Carboniferous Limestone and Millstone Grit, could be attributed to the uplift of a relatively complete Carboniferous Limestone sequence, followed by extensive erosion to the present sub-Namurian level. However, a more likely concept, and one involving far less tectonic movement and erosion, is that the beds are thin, or absent, due to little or no deposition, followed by a period of limited erosion. Sedimentation was controlled by an area of upwarp lying to the east and south-east of the present South Wales Coalfield, which was ancestral to the Namurian Usk Anticline (George 1956b). The main Sudetic earth movements were probably preceded by minor uplifts of this area during the Carboniferous Limestone period, which would have had an inhibiting effect upon sedimentation, with consequent reduction in thickness from west to east and south-east, and even absence of some beds in the more eastern parts. Later, in upper Visean and/or early Namurian times, a land area emerged for a while over part, or all, of the Newport district, and was subjected to some erosion before being covered by Millstone Grit sediments.

Lower Limestone Shale

The narrow outcrop of the Lower Limestone Shale extends from Tongwynlais north-eastwards through Thornhill and Machen to the Ebbw Valley, then turns north-north-eastwards and runs through Llanderfel and Upper Cwmbran to Pontypool. For mapping purposes the two-fold lithological classification of a lower limestone division and an upper mudstone division, established in the Newport district by Strahan (1909a, p. 19), has been adopted. Dixey and Sibly (1918, p. 125), recognized three divisions; the lower two are equivalent to the lower limestone division of this account, and the upper to the upper mudstone division. The Lower Limestone Shale is conterminous with the Cleistopora (K) Zone (Vaughan 1905).

The Lower Limestone Shale lies conformably on the Upper Old Red Sandstone; there is a passage from one system to the other and the position selected for the boundary is at the base of the first thick limestone lying above the calcareous sandstones of the upper part of the Upper Old Red Sandstone. The junction between the two divisions of the Lower Limestone Shale is fairly abrupt, and easily mapped throughout the area; the hard beds of the lower limestone division commonly produce a low, but prominent ridge, and the soft beds of the upper mudstone division form flat, or gently sloping ground. The boundary between the upper mudstone division and the Main Limestone is taken where the argillaceous rocks of the former pass into the thickly-bedded dolomites of the latter. This junction follows the base of the massive scarp formed by the Main Limestone along much of its outcrop.

In general the Lower Limestone Shale thins from the south-west to the north-east, with local variations. At Tongwynlais the lower limestone division is 200 ft thick, and then thins rapidly north-eastwards, to only 70 ft at Thornhill 2 miles along the crop. Between Thornhill and the Rhymney Valley it varies between 50 and 70 ft, and for a short distance near Rudry it is absent. Between the Rhymney and Ebbw valleys the division thickens gradually to 120–140 ft north of Risca, and between there and Pontypool thins to 50–60 ft, except at Llanderfel where it is 20 ft or less. The upper mudstone division is 150 ft thick at Tongwynlais, and thins to 80–110 ft at Thornhill; this thickness is maintained for 7 miles, to near Risca, then decreases to about 70 ft between Risca and Pontypool. At Llanderfel there is a local thickening to 140 ft.

The lower limestone division consists mainly of thickly bedded or massive, greyish brown-weathering, crinoidal, bryozoan, shelly and oolitic limestones, with subordinate, dark grey mudstones. The crinoidal limestones are the most prominent members, consisting dominantly of crinoid ossicle debris with some bryozoa and shelly fossils, cemented by a crystalline calcite matrix. Locally, this rock is strongly hematitized. The oolitic limestones are most common in the middle and upper parts of the sequence, particularly in the western area of the outcrop towards Tongwynlais. They are generally more massively bedded than the crinoidal and shelly limestones. The subordinate mudstones are more common in the lower part of the succession, and sometimes they occur in almost the same proportion as the limestones. In the middle and upper parts they are infrequent, and occur in thin lenticular beds only. They are usually poorly fossiliferous.

The upper mudstone division consists dominantly of grey, sometimes micaceous, shaly mudstones, and some buff, fine-grained, current-bedded, silty sandstones. These contain subordinate, thin beds and nodules of grey, impure, brown-weathering limestones, which become more abundant in the upper part of the sequence, but never in a proportion greater than the mudstones. The mudstones are generally poorly fossiliferous, but the thin limestones commonly contain a rich fauna of crinoids, bryozoa and brachiopods.

There are no essential differences between the faunas of the lower limestone and upper mudstone divisions, thus the following general palaeontological description applies to the whole Lower Limestone Shale sequence. The fauna is characterized by abundant crinoidal remains, commonly occurring bryozoa, brachiopods and ostracods, and less commonly or rarely corals, bivalves and fish. The crinoids occur in thick accumulations of detached ossicles, and they are commonly the dominant constituent of many of the limestones. Bryozoa, e.g. Fenestella sp., are frequently found in association with the crinoids, and locally occur in abundance. The commonly occuring brachiopods are 'Camarotoechia' mitcheldeanensis Vaughan, orthotetoids including Schuchertella sp., Plicochonetes stoddarti (Vaughan), productoids including Avonia sp., Rugosochonetes vaughani Muir-Wood and Unispirifer tornacensis (de Koninck). The diagnostic coral Vaughania [Cleistopora] vetus Smyth occurs only rarely.

Along the outcrop of the lower limestone division between Tongwynlais and Machen, hematitization is a prominent feature in some of the limestones in the basal part of the sequence. The beds affected are usually crinoidal and bryozoan limestones, in which hematitization varies from the fine dissemination of iron oxide, to concentrations which have been explored with a view to working the rock as an iron ore. A small amount may have been worked in a level near Rhiwbina, but on the whole the deposits have no economic significance. One of the earliest references to the hematitized limestones at Rhiwbina is by Rogers (1861, p. 180), in which he puts forward the theory that the hematite was secreted by the crinoids which constitute the rock. He also gives a full chemical analysis (p. 217) of a specimen of the ore, in which the iron content is given as 47.7 per cent. H.C.S.

A sample collected during the present resurvey is a hematitized bioclastic limestone. Abundant fragments of crinoids, shells and bryozoa occur in a fine-grained (average grain size 0.03 mm), silty, calcite-dolomite matrix, which also contains scattered muscovite flakes. The fossil fragments appear in layers rather than in a random fashion. A little fine-grained hematite is associated with the matrix constituents, but is seen chiefly in the bioclastic material. Many of these fragments are largely replaced by the iron-oxide, others show partial replacement, while a few are still composed entirely of calcite.

Hematite commonly occurs in those parts of the calcite plates once occupied by living tissue, suggesting that the hematitization was not due to later permeating iron-rich solutions. Further evidence against such an origin is provided by a specimen containing two crinoid plates set in a hematite matrix. Also, scattered crinoid plates (usually in twos or threes) are sometimes cemented together by hematite and these structures show signs of erosion (rolling). It seems likely, therefore, that the iron (probably in the form of pyrite) was already present, or being precipitated in and around the fragments, at the time the rock was laid down. Subsequent oxidation of the pyrite to hematite could have occurred during the Triassic period. J.R.H.

Sibly (1919, p. 60) described similar hematitized limestones in Gower, and concluded similarly that the beds received their iron content at the time of deposition, though he suggested also that some secondary enrichment had taken place.

Dolomitization affects the lower limestone division in several areas as follows: (1) for about 0.5 mile along the crop 2.5 miles north-north-east of Risca; (2) north of Machen in the vicinity of Casten Meredydd; (3) north of Craig Llanishen for about 0.5 mile along the crop; (4) Rhiwbina, and (5) Tongwynlais.

Vein mineralization in the Lower Limestone Shale is confined to commonly occurring calcite in the lower limestone division throughout the area, and a small amount of baryte, sometimes hematite-stained, at Llanderfel, near Henllys Colliery and north-east of Machen. A little galena occurs in the baryte near Henllys Colliery.

Main Limestone

The Main Limestone has been, and still is, one of the most extensively quarried formations in the Newport district. The rocks have been used for building purposes, lime, railway ballast etc., and at present are much in demand for refractory and fluxing purposes. Consequently, the Main Limestone is generally well exposed, and the characteristics of the beds and their lateral variations may be examined at numerous localities. The Main Limestone crops out along a narrow strip of country, never more than 0.75 mile wide, extending from the Taff Valley at Taffs Well in the south-west, to Pontypool in the north-east. The division lies conformably on the Lower Limestone Shale, and its upper limit is marked by an unconformity. The base is characterized by the appearance of thickly bedded, dolomitic limestones lying above the mainly argillaceous beds of the upper mudstone division of the Lower Limestone Shale, and this junction can easily be traced along the base of the scarp formed by the hard dolomites and limestones along much of their outcrop. The upper boundary is rarely seen, but is fairly easily mapped, and it is clear from the faunal zoning that the Millstone Grit oversteps on to different horizons of the Main Limestone when traced from the south-west to the north-east (Figure 4). At Taffs Well the Main Limestone consists of strata lying within the Z-S₁ and S₂ zones; eastwards, gradual attenuation takes place, and the S. Zone beds die out 0.5 mile east of the Rhymney Valley. North-eastwards from there the upper beds of Z-S₁ gradually disappear, and between the Ebbw Valley and Pontypool only the lower beds are represented.

Consequent upon the overstep of the Millstone Grit on to the Main Limestone, and attenuation within the sequence itself, the division suffers a notable diminution in thickness when traced from the south-west to the north-east. At Taffs Well it attains its maximum thickness of about 1770 ft, then thins rapidly to 1000 ft at Thornhill 2 miles to the north-east. From there it maintains this thickness as far as the Rhymney Valley, where it thins to about 700 ft. Between the Rhymney and Ebbw valleys further thinning to about 400 ft takes place, and 3 miles north-east of the Ebbw Valley, at Llanderfel, it is represented by not more, and probably less than 40 ft of beds. From Llanderfel gradual thickening takes place northwards, and at Pontypool the Main Limestone is about 300 ft thick.

The resurvey mapping of the Main Limestone of the Newport district was based mainly on lithological criteria, and, as a result, the beds have been classified into two broad divisions (p. 59). These are best exposed in the area between Taffs Well and Tongwynlais for which the following description applies. The lower division, which includes the Z-S₁ and basal part of the S₂ zones (Figure 4), consists of about 1420 ft of medium to dark grey, brown-weathering, hard, massively bedded, medium to coarse-grained dolomite, in which crinoid ossicles are the only commonly occurring fossils which have survived dolomitization. They generally occur scattered throughout the rocks, but are sometimes concentrated into rich layers. Poorly preserved corals and brachiopods occur sporadically. Some of the dolomite was originally oolitic, for under the microscope thin sections show ghost outlines which have been preserved in the new structure of the rock. Thin lenticular beds of dark, brittle chert occur at several horizons in the dolomite, but they are generally rare. One of the most characteristic features of the dolomite is the abundant secondary mineralization along bedding planes, joints and faults, and within innumerable cavities which ramify throughout the beds. The most commonly occurring minerals are calcite, baryte, hematite and galena.

The upper division of the Main Limestone (all but the lower beds of the S₂ Zone) consists of about 350 ft of mainly massively bedded, poorly stratified, pale grey, white-weathering, coarse to fine-grained, sometimes current-bedded oolitic limestone, containing crinoids, corals and brachiopods, sometimes in abundance. The boundary between the two lithological divisions appears to be diachronous, for north-eastwards from the Taff Valley it transgresses the faunal boundaries, and the upper division dies out approximately 1 mile north-east of Taffs Well. Between there and Thornhill the Main Limestone is dolomitized throughout. The lower part consists of typical, medium to coarse-grained dolomite, but the upper part is characterized by the presence of fine- grained dolomites and some limestones interbedded with the coarser-grained dolomites. These are the 'dolomite mudstones' of Dixey and Sibly (1918), and lithologically they are very compact, amorphous-looking pale grey or almost white rocks, which commonly have a conchoidal fracture and a distinctive porcellanous appearance; they are sometimes referred to as 'china-stone limestones'. Where thickly developed they are less resistant to erosion than the coarser-grained dolomites and form strike-valleys along the outcrop of the Main Limestone (Plate 4B). Dixon and Vaughan (1912, p. 516) described this distinctive rock in Gower, where they referred it to a deposit characteristic of a 'Modiola' or lagoon-phase, laid down in deep, quiet water. George (1954) studied similar rocks in Breconshire, in the Upper Caninia Zone, and found that there they are algal limestones, with detrital fragments of calcispheres, foraminifera, bryozoa, crinoids, shells and ostracods. He concluded that they were deposited in shallow water, for they show signs of desiccation in the form of sun-cracks, and contain dendritic seaweed fronds. A comparison with the Gower 'Modiola' phase deposits showed that the latter are much less rich in algae and are virtually without bryozoa, crinoids and brachiopods. George suggested that the Gower deposits were laid down in a more restricted environment than those of Breconshire.

In the Newport district the maximum development of the 'Modiola' phase deposits is in the Thornhill–Cefn-onn–Rudry area (Figure 4), where they occur throughout about 400 ft of beds below the S₂ Zone strata. Westwards from Thornhill they die out within a mile. Eastwards and north-eastwards they continue as far as the Ebbw Valley at Risca, where they die out, probably cut out by the Millstone Grit overstep. Above the 'Modiola' phase deposits between Thornhill and the Ebbw Valley, there is a thin series of coarser-grained dolomites similar to those below the 'Modiola' phase. These are the S₂ Zone beds, and according to Dixey and Sibly (1918, p. 155) they are about 100 ft thick at Thornhill, 60 ft in the Cefn-onn area and 50 ft in the Rhymney Valley, just east of which they are cut out completely by the Millstone Grit. Between the Ebbw Valley and Pontypool to the north-east, the Main Limestone, consisting of the lower part of Z-S₁ only, is completely dolomitized, the beds being typical, massive, medium to coarse-grained dolomites with few fossils. Just south of Pontypool, in the Cwm Quarries, the lower 40 ft of the Main Limestone consist of shelly and oolitic limestone, partially dolomitized only, and some dark mudstone; the remaining 200 ft or so are typical dolomites.

In the Newport district it is apparent that the alteration of limestone to dolomite has had a profound effect on the composition, structure and appearance of the rocks. Dolomitization obliterated many of the original structures within the limestone, e.g. dolomitized oolites have lost their concentric structure, though 'ghost' outlines of the ooliths may still be visible owing to the presence of impurities within the original rock. Fossils are also frequently destroyed, though dolomitization affects some fossils more than others. It appears that as a general rule aragonite is more easily altered than calcite, and fine-grained material is less resistant than coarse-grained material. Consequently, strong calcite fossils resist dolomitization, and aragonite shells are very susceptible to alteration. Thus dolomites are often found in which crinoids and brachiopods with thick shells, e.g. productoids, have escaped complete alteration. On leaching during weathering the calcite of the fossils is selectively dissolved leaving dolomite with empty moulds of fossils in which secondary mineralization commonly takes place. The cavernous nature of dolomite may also be explained by the theoretical ten per cent decrease in volume which attends dolomitization. The dolomites of the Newport district are invariably veined by primary vein minerals such as calcite, baryte, hematite and galena, which fill the cavities in the rock. Calcite deposits may be subsequently dolomitized, e.g. near Risca, where there are large cavities in filled with crystalline calcite which has been altered to dolomite, except for a small central core, which has remained unchanged.

Much of the dolomitization of the limestones probably took place during Carboniferous Limestone times, and this was first suggested by Dixon (in Strahan 1909, p. 19) who wrote "From the fact that the dolomite is comparable with that of Gower, and persists as such for upwards of 15 miles, it may be inferred that much of the dolomitization was 'contemporaneous' rather than the result of subsequent alteration along veins or faults. That the rocks were at first calcitic is shown by their containing crinoidal remains, and the expression 'contemporaneous' is intended as implying dolomitization while they were still under the influence of the Carboniferous Limestone sea". George (1956a, p. 315), in his account of the Main Limestone along the east crop of the South Wales Coalfield, came to a similar conclusion, stating that the beds were altered to their present state some time during the Avonian.

Along parts of its outcrop the Main Limestone contains hematite deposits along joints and bedding planes, and in other cavities in the rock. In the vicinity of Garth Wood, Fforest Fawr and Rudry, sufficient concentrations of hematite were found for them to be of economic value; in these areas the ore was formed largely by chemical replacement of dolomite, by iron compounds in the form of hydrated brown or red hematites. Associated minerals are commonly quartz, calcite and baryte. According to Sibly (1927, p. 86) the replacement of the host rock, as opposed to the infilling of pre-existing cavities, is proved by the occurrence of isolated masses of dolomite within the ore and penetration of the parent rock by patches, fingers, etc. of hematite. The ore was worked most extensively at Garth Mine, where it occurred in large masses to a depth of more than 400 ft, and in small amounts from the Fforest Fawr and Rudry areas. Only at Llanharry, south-west of the Newport district, is iron ore being worked from the Main Limestone at the present day.

The origin of the hematite in the Carboniferous Limestone Series has been the subject of considerable speculation for many years. A widely accepted view is that iron-bearing solutions descended into the limestones, the iron being derived either from the denudation of Coal Measures and Old Red Sandstone strata (Etheridge 1868 and 1870), or Triassic rocks (Wethered 1882). In the Forest of Dean, Trotter (1942, p. 75) believes that the hematite was formed when the iron-rich Triassic land surface, which existed during the period of denudation following the Hercynian earth-movements, was lowered on to the upturned edges of the Carboniferous Limestone. However, research by Dr. M. Williams (University of London, Ph.D. thesis, 1958) at the Llanharry Mine, where the Main Limestone contains the richest hematite deposits in Glamorgan, has shown that no definite conclusions can be made about the source or age of the hematite mineralization. It was deduced by Williams. that the primary source was hydrothermal and mineralization was Post-Triassic, possibly Tertiary. He advances the theory that the ore bodies were formed by iron-rich solutions rising upwards from a reactivated source during, or soon after, the earth movements (considered to be of Miocene age) which caused folding and faulting of the Mesozoic rocks of the area.

Extensive dolomitization of the limestones in the Z-S₁ division and basal part of the S₂ Zone resulted in the destruction of most of the fossil remains. In Z-S₁ the only surviving fossils in many of the beds are crinoid ossicles, associated with poorly preserved and indeterminate shell fragments. At some horizons however, the fauna is better preserved, and is characterized by commonly occurring crinoid debris, and less commonly, or only rarely-occurring bryozoa, corals, brachiopods and ostracods. The Z Zone age of the lower beds of the Main Limestone is proved by the occurrence in the south-west of the corals Fasciculophyllum omaliusi (Milne Edwards and Haime) and Zaphrentites delanouei (Milne Edwards and Haime), and the brachiopods Rugosochonetes vaughani and Unispirifer tornacensis. Higher horizons within Z-S₁ are difficult to prove on the basis of the sparse fauna of the beds, but it is assumed they are represented between Z Zone beds in the basal part of the Main Limestone and the S₂ Zone beds in the upper part.

Because of the undolomitized nature of the upper part of the Main Limestone in the Taff Valley area, fossils are far more abundant in the S₂ Zone than in Z-S₁. The most commonly occurring are bryozoa, corals, brachiopods and ostracods. The S₂ Zone is recognized by the association of Composite ficoidea (Vaughan), Davidsonina carbonaria (McCoy) and Linoproductus corrugatohemisphericus (Vaughan) with Carcinophyllum vaughani Salee and Lithostrotion martini Milne Edwards and Haime. H.C.S.

Details

Lower Limestone Shale

Tongwynlais-Thornhill-Rudry-Draethen-Risca

In the neighbourhood of Tongwynlais the outcrop of the 200-ft thick lower limestone division of the Lower Limestone Shale is widened considerably by the effect of the Caste11 Coch Anticline and Tongwynlais Syncline. The limestone has been extensively worked in this vicinity, and the general structure is shown by the inclination of the beds in the quarries. In a disused quarry [ST 1323 8263] east of Castell Coch, the beds lie on the northern limb of the Castell Coch Anticline and dip northwards at about 52°. The following section is visible in the western end of this quarry:

In a quarry [ST 1310 8240] south of Castell Coch, the limestones lie on the southern limb of the Castell Coch Anticline, and dip at angles varying between 30 and 36° towards the south-south-west. The beds there have yielded the following fauna Avonia sp.,' Camarotoechia' mitcheldeanensis, Chonetes aff. failandensis Smith, Eumetria cf. carbonaria (Davidson), Rugosochonetes sp., Schuchertella sp., smooth spiriferoid and ostracods including Paraparchites sp.

In a nearby railway cutting [ST 1308 8237] the limestone beds contain: 'Camarotoechid mitcheldeanensis, 'C.' aff. mitcheldeanensis, Chonetes aff. failandensis, Cleiothyridina sp., Eumetria sp., orthotetoids, smooth spiriferoids and Syringothyris sp.The sharp fold of the Tongwynlais Syncline is demonstrated in another quarry [ST 1350 8230]. There the beds in the northern limb dip steeply southwards and are vertical along: part of their crop; the beds in the southern limb lie at a lower angle and are not seen to dip at more than 34° to the north. In this vicinity the transitional junction between the Upper Old Red Sandstone and Lower Limestone Shale is visible in a section [ST 1344 8223] 600 yd S. 34° E. of Castel] Coch; the details of the succession are given on p. 57, in the detailed description of the Upper Old Red Sandstone.

The highest beds of the upper mudstone division of the Lower Limestone Shale-are exposed in Coed Cefn-garw Quarry [ST 1380 8288]. They comprise light grey, dolomitic shales with thin beds of laminated, fine-grained, dolomitic sandstones and argillaceous, dolomitic limestone containing a rich fauna, especially productoids and crinoid ossicles. preserved in chalcedonic silica. The junction with the Main Limestone is well exposed at this locality. South of Coed Cefn-garw Quarry the lower limestone division is. exposed in a disused quarry [ST 1370 8269] near Ty-isaf Farm, where it consists of alternations of limestone and bluish grey, slightly silty, micaceous mudstone. The limestone is greatly impregnated with hematite and contains: Conularia quadrisulcata J. Sowerby, 'Camarotoechia' aff. mitcheldeanensis, Chonetipustula?, Eumetria cf. carbonaria, orthotetoid, smooth spiriferoids [juvs.], Syringothyris cf. principalis North, and ostracods..

The lower limestone division was quarried [ST 1465 8300] in the vicinity of Rhiwbina, but the workings have since been filled in and no limestone is visible; during the original survey Strahan recorded massive limestone dipping at between 19° and 21° towards the north-west. The limestone, which was slightly dolomitized (Dixey and Sibly 1918, p. 130), was burnt for lime at this locality. About 100 yd north of these quarries an entrance to a disused level marks the site of an exploration for hematite; a spoil heap near the entrance shows the nature of the rock which was mined (described previously on p. 63). Northwards from Rhiwbina, and then swinging eastwards towards Thornhill, the outcrop of the Lower Limestone Shale occupies low-lying ground in Cwm-Nofydd and is generally poorly exposed along this stretch. In a stream section [ST 1573 8447] near Thornhill the lower part of the upper mudstone division is exposed, and consists of grey, shaly mudstone with thin beds of fine-grained, laminated, calcareous sandstone. The following fossils were found in these beds: Cleiothyridina royssii â (Vaughan), smooth spiriferoids and Syringothyris elongata? North. The junction with the lower limestone division is also visible in this stream section.

North of the Brownstone Group ridge of Craig Llanishen, which runs east from Thornhill, the junction between the lower limestone and upper mudstone divisions. is visible in a stream section [ST 1713 8488]. Upstream, higher horizons of the mudstone division are visible, the beds being soft, bluish grey, micaceous, shaly mudstones. containing thin beds of impure, crinoidal and shelly limestone; small ironstone and rare pyrite nodules occur in these beds. Dixey and Sibly (1918, p. 131) found the following fauna in the limestones: "Pustula subpustulosa, Productus cf. bassus, Chonetes cf. hardrensis, Scheliwienella cf. crenistria, Athyris royssii, Syringothyris cf. cuspidata and abundant ostracods". A section nearby [ST 1697 8479], in the lower limestone division, exposes a crinoidal limestone which has been dolomitized, and beds at about the same horizon are visible in a small quarry [ST 1681 8469] near Bwlch-ygelli, where undolomitized, crinoidal limestone lies above dolomitized, crinoidal limestones.

In the vicinity of Cefn-onn, explorations have been made for hematite deposits in the lower limestone division. Three trials in the base of the limestone have been found on the north side of Coed Tranch-yr-hebog [ST 1770 8500]; in the entrance to the most westerly [ST 1762 8499] the following section, passing down into the Upper OldRed Sandstone, is exposed:

North-eastwards, for nearly 2 miles, the lower limestone division forms a low, but prominent ridge below the Main Limestone escarpment of Cefn-onn and Craig-y-Ilan. Along this stretch the lower limestone division is poorly exposed, but it may be examined in a section [ST 1920 8605] south-west of Coedcae-garw, where massive, impure, crinoidal limestone is visible. East of Cwm Farm [ST 2026 8640] the lower limestone division thins out completely for 500 yd; it reappears near Duffryn-tawel Farm [ST 2086 8656] and thickens north-eastwards to about 50 ft for nearly a mile, before thinning again in Draethen, where no more than 20 ft of limestone beds are present. The outcrop of the upper mudstone division is easily followed between Duffryntawel Farm and Draethen, for it occupies a well-developed, level, or gently sloping platform below the massive scarp of the Main Limestone. It is nowhere exposed along this part of its outcrop.

North-east of Machen the configuration of the Lower Limestone Shale outcrop is complicated by the effect of the Machen Anticline and Machen Syncline, and a north-east to south-west aligned fault which runs through Llan-danglws and Tycanol farms [ST 2365 9018] and [ST 2336 8956] respectively. East of the fault the Lower Limestone Shale is folded into the Machen Syncline, and a small outlier of the upper mudstone division is preserved in the core of the fold. In the quarries [ST 2365 8978] east of Castell Meredydd, beds of the lower limestone division are strongly hematitized, and the shells and crinoidal remains are silicified. In the northernmost quarry a vein of baryte with some hematite occurs within the 8 ft of limestone exposed. West of the fault the crop of the lower limestone division branches into two; the northern arm runs east-west along the northern flank of the Machen Anticline, through Castell Meredydd [ST 2256 8872] to the Rhymney Valley, where it swings south and east under the alluvium and joins the southern arm, which runs through Machen and follows the axis of the Machen Syncline. The limestone thus encloses an oval-shaped inlier of Upper Old Red Sandstone about one-third square mile in area. In the crags and disused quarries in the vicinity of Castell Meredydd well-bedded, crinoidal limestone has been extensively dolomitized, and in the upper part of the succession thin, lenticular beds of chert are present; up to 25 ft of limestone are visible at this locality. Some 250 yd west-south-west along the crop dolomitized crinoidal limestone is strongly hematitized in an exposure [ST 2236 8960] north-east of Pandy Farm. Hematitization also affects the crinoidal limestone in the vicinity of Machen, and is displayed in a section [ST 2295 8914] 180 yd E. 20° N. of Machen Church.

On the southern side of the Ebbw Valley the lower limestone division is exposed at several localities which show typical oolitic, crinoidal, and shelly limestones. A well-developed oolite is visible in a disused quarry [ST 2349 9066], and in a stream section [ST 2336 9030] shelly limestones have yielded the following fauna: Fenestella sp., 'Camarotoechia' mitcheldeanensis, 'C.' aff. mitcheldeanensis, Eumetria sp., Schuchertella sp., smooth spiriferoids, Spirtfer?, Syringothyris?, and ostracods including Paraparchites sp.The sequence in this stream is now poorly exposed, but Dixey and Sibly (1918, pp. 126–7) were able to record a fuller succession in the lower limestone division as follows:

400 yards south of the stream section crags of crinoidal limestone up to 10 ft high are exposed.

Risca-Llanderfel-Upper Cwmbran-Pontypool

On the northern side of the Ebbw Valley the 140 ft thick lower limestone division forms a low scarp-feature, which runs north-eastwards for nearly a mile, before being obscured by a cover of head which has accumulated on the slopes below Twmbarlwm. The upper mudstone division, about 130 ft thick, crops out along a narrow, but well-defined gently-sloping platform between the scarps of the lower limestone division below and the Main Limestone above. The lower limestone division is exposed in a quarry [ST 2398 8959] north of Risca Station, where 6 ft of massive, oolitic, crinoidal and shelly limestone are visible. The shelly limestone has yielded the following fauna: Fenestella sp., Avonia?, orthotetoid, U. tornacensis, and ostracods. A short distance to the north-east it is again exposed in some disused quarries [ST 2428 9189] which show up to 25 ft of massive and flaggy, crinoidal, shelly and oolitic limestone. The beds were burnt for lime in this vicinity. Below Twmbarlwm, the outcrop of the Lower Limestone Shale is obscured by a cover of head for about two-thirds of a mile, before emerging again near Pant-yr-yrfa Farm [ST 2535 9240]. North-north-east of this farm, the lower limestone division is dolomitized for at least two-thirds of a mile; it is exposed in two quarries, [ST 2573 9393] and [ST 2560 9370], where small sections of dolomite are visible, and again in a larger quarry [ST 2607 9448] in which 40 ft of thickly bedded, dolomitized limestone pass up into grey and brown mudstones. The dolomitization in this vicinity has apparently obliterated most of the fossil remains, for only crinoid ossicles and a few indeterminate shelly fragments have been found. A small amount of baryte with some galena, which probably occurred in small veins in the dolomite, was seen in the debris in the bottom of the quarry.

Near Llanderfel the lower limestone division thins abruptly to about 20 ft or less, and the upper mudstone division thickens to a maximum of approximately 140 ft. This thinning and thickening is confined to the immediate vicinity of Llanderfel Farm [ST 2659 9538]. A small amount of rock from the lower limestone division was quarried [ST 2664 9539] near the farm, but it is not now exposed; in the stream bed near the small quarry, several blocks of baryte were noticed, which probably originated from veins in the limestone. The shaly beds of the upper mudstone division. dipping at 70° west-north-west, are exposed for a short distance along the stream bed west of the limestone quarry. About 10 ft above the outcrop of the limestone they have yielded the following fossils: Fenestella sp., 'C.' mitcheldeanensis, Leptagonia analoga (Phillips), and, 70 ft higher, a specimen of the fish tooth Psephodus sp.

Between Llanderfel and Penyrheol 2.5 miles to the north-north-east, the Lower Limestone Shale is poorly exposed and follows almost featureless ground. The lower limestone division has been quarried at a few localities, and is well exposed in the quarries [ST 2772 9985] south-west of the Mountain Air Inn, where up to 8 ft of typical, massively bedded, crinoidal and shelly limestone are visible. In the small disused quarry [ST 2827 9889] near Penyrheol Farm, the following section was recorded: massive limestone 4 ft on interbedded grey limestone and mudstone 4 ft 6 in, massive grey limestone 8 ft. Northwards from Penyrheol the lower limestone division can be traced along a line of disused quarries, which include the Cwm Quarries. In the most southerly of these [ST 2834 9934] 15 ft of the lower limestone division is exposed, as well as the complete sequence of the upper mudstone division, which totals 68 ft 9 in. The following succession was recorded:

The mudstones and thin impure limestones of the upper mudstone division have yielded: Vaughania vetus Smyth, bryozoa including Fenestella sp., Avonia?, Buxtonia sp., 'Camarotoechia' mitcheldeanensis, aff. mitcheldeanensis, Cleiothyridina royssii (Davidson non Leveille), C. royssii â (Vaughan), Dielasma sp., Eumetria sp., Lingula sp., orthotetoid, Pustula subpustulosa I. Thomas, Rugosochonetes aff. vaughani, Schuchertella sp., smooth spiriferoid, Spirifer cf. elongata, S. cf. principalis, U. tornacensis, Parallelodon cf. bistriatus (Portlock), ostracods including Paraparchites sp., fish tooth and spine. The basal 15 ft of limestone has yielded: 'C.' mitcheldeanensis, orthotetoid, Plicochonetes stoddarti, Schuchertella sp., and ostracods including Paraparchites sp.In the most northerly of the Cwm Quarries [ST 2836 9978] 40 ft of dark shaly mudstone, with thin, impersistent, impure, shelly limestone beds, lie with an abrupt junction on about 50 ft of hard, massive, grey limestone, which is shelly and crinoidal, particularly in the lower part.

At Pontypool the two-fold division of the Lower Limestone Shale is demonstrated in Pontypool Park [SO 2809 0110]. The lower limestone division is about 50 ft thick, and crops out along a low ridge which runs north-north-east to south-south-west through the park. The beds are exposed in a disused quarry [SO 2871 0097], where 8 ft of crinoidal and shelly limestone pass up into grey mudstones. The upper mudstone division, 70 ft thick here, occupies a narrow outcrop on the low-lying ground between the scarps of the lower limestone division and the Main Limestone dolomites. H.C.S.

Main Limestone

Garth Wood area

In the vicinity of Garth Wood the Main Limestone forms a prominent west-south-west to east-north-east escarpment, through which the River Taff has cut a deep gorge. The division attains its greatest thickness in this area, the beds totalling up to 1770 ft; of this the lower ± 1420 ft are largely dolomitized and the upper ± 350 ft only locally dolomitized. The rocks are well exposed in numerous quarries, though the western part of the outcrop is partially obscured by boulder clay. In one small, disused quarry [ST 1092 8233] partially dolomitized, oolitic limestone underlies 3.5 ft of gravelly boulder clay. The following fossils were found in the oolitic limestone: Carcinophyllum cf. vaughani, Lithostrotion martini, Syringopora sp., Composita sp., Linoproductus aff.corrugatohemisphericus, Linoproductus sp., hemisphaericus (J. Sowerby) group, smooth spiriferoids, Straparollus sp.and ostracods. This fauna indicates that the limestone lies within the S2 Zone. Approximately 700 yd eastwards, a large quarry [ST 1160 8242] reveals 50 ft of thinly bedded, fine to medium-grained, crystalline dolomite containing poorly preserved bands of corals and brachiopods. The quarry face is broken by veins of massive hematite up to 10 in wide. The following fossils were collected from this quarry: C. vaughani, C. cf. vaughani, Lithostrotion aranea (McCoy), Syringopora sp.and Davidsonina carbonaria. These fossils are indicative of the S₂ Zone. Northwards, another quarry [ST 1168 8272] exposes 25 ft of shelly, oolitic limestone, which is locally strongly dolomitized. The limestone yielded fossils of the S₂ Zone, namely: C. vaughani, Cleiothyridina sp., Composita ficoidea, D. carbonaria, L. corrugatohemisphericus and a smooth spiriferoid.

Near the centre of Garth Wood the ground is pitted by a series of shafts and steeply sloping inclines, which lead into the disused hematite workings of Garth Mine. The exposures in the entrances to these workings are generally inaccessible; the beds vary from well-bedded to massive dolomite. The spoil heaps in the area contain hematite, limonite in the form of yellow ochre, quartz, calcite and a considerable amount of oolitic limestone. In the area to the north there are many disused trials for hematite, and the main entrance [ST 1166 8276] to the Garth Mine occurs 500 yd N. 39° E. of Ton-mawr Farm. Sibly (1927, p. 82) gives a full description of the workings in this mine. From the main entrance an adit was driven in a direction S. 29° E. for 420 yd, and then S. 7° E. for 85 yd, where it reached the floor of a large pit which had been sunk to a vertical depth of about 250 ft from the surface. The rocks in the area are mainly fine-grained, grey dolomites, in which there are two 80 to 100-ft thick beds of dolomitized oolitic limestone lying about 100 ft apart. The iron ore occurred mainly in the upper bed of oolite, but also to a certain extent in the beds below. It occurred in large quantities, in bodies of very irregular shape, and improved in quality with increasing depth. It was mined extensively from the beginning of the 19th century until 1884, first from the crop, and then from the main adit which was driven in 1840. Deep working extended for nearly 150 yd east and west of the adit. The only statistics available show that 116000 tons of ore were raised during the last eleven years of working, and 176 tons of ochre during the last three years. Considerable amounts were probably worked before production figures were accurately recorded. The ore was smelted in the disused Pentyrch Iron Works at Taffs Well, about 0.5 mile north-east of the mine.

Large amounts of limestone have been quarried from the steep slope overlooking the Taff Valley, in particular in the Steetley Co. Quarry [ST 1240 8270], where the rock is worked for refractory purposes. There, 200 ft or more of massively bedded, medium to coarse-grained dolomite with calcite veining are visible. Immediately north of this quarry the disused Cwarre Glâs Quarry [ST 1220 8290], exposes a thick section of oolitic limestones in which vein dolomitization is extensive. South of the Steetley Co. Quarry a section along the Barry Railway cutting [ST 1249 8252] shows thickly bedded dolomite which has been folded by the Castell Coch Anticline; the axis of the fold runs east-west across the cutting, at a position approximately 120 yd north of the entrance to the tunnel through Garth Wood Mountain. The axis of the complementary east-west Tongwynlais Syncline lies approximately 220 yd to the south. These structures rapidly die out westwards. The basal part of the Main Limestone is visible in Morgantown Quarry [ST 1220 8210], which is worked by the Steetley Co.; approximately 80 ft of thickly bedded dolomite, with baryte and hematite veining in the north-west corner of the quarry, are exposed. Two thrust planes dipping northwards at 35° are visible in the eastern face. The following fossils were collected from beds near the base of the section: Fasciculophyllum omaliusi, Zaphrentites delanouei, Fenestella sp., and an orthotetoid fragment. They indicate that the rocks are of Zaphrentis (Z) Zone age. Dolomites at a similar horizon are exposed in Ty-nant Quarry [ST 1270 8212], where about 100 ft of beds are visible.

Fforest Fawr–Thornhill

Almost the whole thickness of the Main Limestone is exposed in quarries and railway cuttings along the eastern side of the Taff Gorge. The basal, completely dolomitized limestones, are visible in the 200-ft high face of Castell Coch Quarry [ST 1280 8273]. In the southern part of the quarry the sharp junction between the Lower Limestone Shale and Main Limestone is visible; unfossiliferous, micaceous mudstones of the former division are succeeded by the grey, argillaceous dolomites of the latter. The following fossils were observed on a mudstone parting by Dixey and Sibly (1918, p. 150): "Productus burlingtonensis, associated with Schellwienella cf. crenistria and Syringothyris cf. cuspidata". Within the main mass of dolomite in this quarry fossils are scattered and poorly preserved, and consist mainly of crinoid debris and indeterminate shell fragments. The face of the quarry shows intersecting, almost vertical, joint or fault planes, and in parts the massive dolomite is extremely shattered. In the south of the quarry masses of cemented, angular, dolomite scree rest on dolomite in situ.

North-west of Castell Coch Quarry the Main Limestone is well exposed along two railway cuttings, namely the now-disused, low-level cutting of the Cardiff Railway, and the high-level cutting of the Barry Railway. Along the low-level cutting between Castell Coch Quarry and a position 250 yd north of the Walnut Tree Bridge [ST 1266 8295], the beds exposed are mainly poorly fossiliferous, fine to medium-grained dolomites, with a few beds of dolomitic mudstone in which Dixey and Sibly (1918, p. 150) recorded "Syringopora cf. Reticulata (?) and Productus corrugatohemisphericus". They assign these beds to the C₂S₁ Zone. Continuing northwards the exposures are mainly of S₂ Zone oolitic limestones, patchily dolomitized in the basal part, and containing shelly fossils and corals typical of the zone. At a position 450 yd north of the Walnut Tree Bridge, the beds are disturbed and a southward thrust can be recognized. The S₂ limestones continue to the end of the cutting.

Along the high-level Barry Railway cutting north-north-eastwards from the Walnut Tree Bridge, dolomites of the C₂S₁ Zone are well exposed and in them Dixey and Sibly (1918, p. 150) recorded, at about 110 yd from the bridge: "a band of yellow dolomite containing colonies of Lithostrotion martini with the tissue completely dolomitized." They place this band 35 ft below the base of the S2 Zone. The beds of the S. Zone commence about 140 yd from the bridge; they are well exposed along this cutting, and in Portobello Quarry [ST 1275 8328], and are fully described by Dixey and Sibly (1918, p. 150) as follows: "They are grey slightly crinoidal rocks, fine-grained to medium-grained, and thinly bedded with occasional clay partings. At their base, Cyrtina carbonaria and Seminula ficoides are represented by numerous casts in a band of dark grey dolomites, red-stained in patches. This band, defining the base of S., lies 140 yd from the wall of the Walnut Tree Viaduct, on the eastern side of the cutting. Except in this basal band, fossils other than crinoid ossicles are rare in these dolomites. Productus sp., Seminula ficoides and Carcinophyllum sp., have been identified. The passage into the overlying oolites is well exposed in the same cutting. No definite division line can be drawn, because dolomitization has affected the basal portion of the oolite very extensively, spreading not only in veins along joints, but also in masses which involve the body of the rock. The Barry Railway also affords a good section of the oolites; but these beds are even better displayed in Portobello Quarry, which lies between the two railways, and in the cuttings on the Cardiff Railway. They exhibit the typical Ethology and fauna of the Seminula Oolite, although everywhere veined by dolomite and calcite. In the northern portion of each railway section the beds are much shatter jointed and traversed by occasional faults, probably thrusts. The following corals and brachiopods have been recorded: Syringopora sp., Lithostrotion martini, L. (Nematophyllum) minus (lower beds only), rare, Carcinophyllum sp., Caninia sp.rare, Productus corrugatohemisphericus common, papilionaceous Chonetes common, and Seminula ficoides abundant". During the present survey the following fauna was collected from the S₂ Zone in the Barry Railway cutting: [ST 1277 8320]: Carcinophyllum vaughani, Chaetetes depressus (Fleming), Syringopora cf. ramulosa Goldfuss, Composita ficoidea, Davidsonina carbonaria, Linoproductus sp., smooth spiriferoid, Eoptychia ?, Naticopsis sp. [juv.], Straparollus sp.[juv.], and ostracods including Paraparchites sp.Eastwards from the Barry Railway cutting the mainly undolomitized nature of all but the lower part of the S₂ beds of the Main Limestone continues for two-thirds of a mile along the crop, before dolomitization also affects these beds to a considerable extent.

In Fforest Fawr (the high, wooded area east of the Taff Valley) the Main Limestone has been worked for hematite, which occurs in cavities in the dolomite. Near Castell Coch Quarry ore was worked from a vertical fissure [ST 1280 8285]. The open working in massive dolomite extends east-south-east for about 150 yd; it is up to 8 ft wide at surface level and extends downwards at least 40 ft. To the north-east the disused 'Slide Pit' [ST 1341 8328], occurs near the middle of Fforest Fawr; it is a slant driven through crystalline dolomite in a north-westerly direction, towards a hematite vein aligned at N. 16° E., which was then followed downwards and opened out to a width of 18 ft. Another disused pit [ST 1361 8365] leads into three caves which slant down with the bedding. There appears to be very little large scale hematite veining east of this locality.

On the east side of Nant-y-Fforest, which delimits the eastern boundary of Fforest. Fawr, the lower part of the Main Limestone has been quarried at several localities. In Cefn-garw Quarry [ST 1375 8295] massive, crinoidal dolomites are well exposed in a 70-ft high face. In the southern part of the quarry the rocks are folded into a shallow syncline, and in the northern part by a sharper anticline, the axis of which trends east-west. At the top of the face in the northern part of the quarry the anticline develops into a high-angle thrust to the south. The following fauna was collected from this quarry during the resurvey: Rugosochonetes vaughani, Schellwienella sp., Syringothyris cf. elongata and Unispirifer tornacensis. In the same quarry Dixey and Sibly (1918, p. 152) recorded, in the middle part of the succession, an irregular development of black, argillaceous streaks, extensive silicification resulting in patches. of crinoidal chert and occasional nodules of chalcedony, and irregular, patchy, iron staining. They found the following fossils in these beds, which lie within the Z Zone: "Zaphrentis delanouei(?); Chonetes cf. hardrensis common, Leptaena analoga common, Schellwienella cf. crenistria common, Camarotoechia mitcheideanensis common, Spirifer clathratus, Syringothyris cf. cuspidata and Spinferina octoplicata, mutation p." From the upper beds in the quarry, which are similar crinoidal dolomites, though highly oolitic in parts, they collected "Schellwienella cf. crenistria and Spirifer clathratus". Eastwards about 0.5 mile, Gelli Quarry [ST 1449 8324] exposes 60 ft of fine-grained crystalline dolomite, with calcite and baryte, at the base of the Main Limestone. Half a mile to the north, approximately 100 ft of fine-grained, crinoidal dolomite at a slightly higher horizon are visible in the Bwlch-y-cwm Quarry [ST 1450 8404]. In this neighbourhood the uppermost beds of the Main Limestone are exposed in a disused quarry [ST 1433 8452] where there is a section in grey, crystalline dolomite with quartz, hematite and baryte veining. Another small quarry [ST 1478 8460] reveals medium-grained, crystalline, hematitized dolomite with calcite veining. At this locality Dixey and Sibly (1918, p. 156) collected the following fauna indicative of the S₂ Zone: "L. martini, Carcinophyllum sp., Cyrtina carbonaria and Seminula ficoides."

In the vicinity of Cefn-carnau a small southward-flowing stream [ST 1532 8450] cuts through the outcrop of the Main Limestone, which is exposed at irregular intervals along the stream bed. The rocks are fine to coarse-grained dolomites, except in the upper part of the sequence, where porcellanous, argillaceous dolomites and calcareous mudstones occur. The Main Limestone is about 1000 ft thick in this area.

Around Thornhill the Main Limestone is well exposed in numerous workings for dolomite. Blaen-Nofydd Quarry [ST 1605 8465] exposes the lower part of the Main Limestone, which comprises steeply dipping, massively bedded dolomite veined with hematite, baryte and galena. In the north-west of the quarry the strata are overturned and strike-faulted. Beds at a similar horizon, and also mineral veined are visible in smaller quarries [ST 1582 8468] and [ST 1575 8462] on the western side of the Cardiff-Caerphilly road. Many exposures are also present along the prominent ridge of Cefn-carnau, which runs east from Blaen-Nofydd Quarry. Some of the highest beds in the Main Limestone are visible in a disused quarry [ST 1591 8488] north of Casten Mor Graig, where yellow-weathering, pinkish grey dolomite containing Lithostrotion sp. ?martini group is exposed. Nearby, in another disused quarry [ST 1569 8483], Dixey and Sibly (1918, p. 156) collected the following fauna from the grey dolomite: "Syringopora sp., L. martini, Carcinophyllum sp., Productus corrugatohemisphericus, Cyrtina carbonaria and Seminula ficoides". These fossils indicate that the beds lie in the S₂ Zone.

Cefn-onn–Rudry–Draethen

Immediately west of the line of the Cardiff-Caerphilly railway tunnel the Cefn-on Basic Slag Co. Quarry [ST 1740 8515] exposes approximately 100 ft of massive, bluish grey, crinoidal, dolomitized limestone in the basal part of the Main Limestone. Higher beds are visible in another quarry [ST 1730 8530], where a 15-ft section shows massive dolomite interbedded with thinly bedded, laminated, fine-grained dolomite. In the northern end of this quarry there is a sharp junction between 4 ft of dark, medium to coarse-grained dolomite with veins and patches of calcite and baryte, which rest on light grey, fine-grained dolomite. The following fossils were found by Dixey and Sibly (1918, p. 154) in beds of fine-grained dolomite cropping out along the track skirting the quarry: "Bellerophon, Schellwienella cf. crenistria, Seminula sp., and crinoid-ossicles." They place the beds in the C₂S₁ Zone. In the quarry [ST 1731 8534] immediately to the north-east, strongly dolomitized limestone, with numerous, enlarged joint planes containing hematite, calcite (in the form of dog-tooth spar) and baryte, is visible. In a lane-section [ST 1705 8536], from dolomites in the S₂ Zone high in the Main Limestone, Dixey and Sibly (1918, p. 156) found "Cyrtina carbonaria and Seminula ficoides."

East of the Cardiff-Caerphilly tunnel towards Cefn-on Farm, the Main Limestone forms the wooded scarp-slope of Cefn-onn, along which the dolomite of the lower part of the Main Limestone has been quarried in several places, including the Cefn-on Farm Quarry [ST 1837 8556]. North of Cefn-on Farm beds typical of the upper part of the Main Limestone are exposed in a cutting along a track [ST 1816 8574]; they consist of argillaceous limestones, calcareous shales and some dolomite. Farther along the track [ST 1824 8597], and higher in the sequence, fine-grained, banded, porcellanous, dolomitic limestones interbedded with coarser-grained dolomites are visible. In the disused quarry [ST 1800 8599] 460 yd N. 30° W. of Cefn-on Farm the highest beds of the Main Limestone are exposed; Dixey and Sibly (1918, p. 156) collected the following S₂ Zone fauna from a bed of coarse-grained, white oolite at this locality: "L. martini, Carcinophyllum sp., C. carbonaria and S. ficoides".

South and south-west of Rudry the massive dolomites of the lower part of the Main Limestone form the prominent wooded escarpment of Craig-y-llan [ST 1960 8645] and Coed Cefn-onn [ST 1900 8600]. Along Craig-y-llan the dolomite is veined with baryte and galena, and there is evidence of exploration for these minerals. The dolomite is well exposed in a disused quarry [ST 1975 8640], in crags [ST 1960 8630] and along a footpath [ST 1941 8632] through Craig-y-llan.

In the neighbourhood of Rudry and eastwards towards Draethen, fine-grained 'Modiola' phase deposits are well developed. Dixey and Sibly (1918, p. 153) recognized two of these phases, the lower being up to 250 ft thick and the upper 50 ft, but they were difficult to identify during the present resurvey. The strike-valleys formed by these softer beds are prominent features of the topography in the neighbourhood of Rudry (Plate 4B), but the phases are difficult to map separately for they commonly die out laterally or merge into each other. A strike-valley formed by one of these phases has been traced from just north of Cefn-on Farm, north-westwards through the south side of Rudry, then north of Rudry House [ST 2016 8663], where it is particularly well developed, and on to Coed-Cefn-Pwll-du north-west of Draethen, where it plunges down into the Rhymney Valley 500 yd south of Rhyd-y-Gwern Farm [ST 2168 8840]. The fine-grained dolomites are generally poorly exposed, but they can be examined in a lane-side section [ST 2029 8680] and in two disused quarries [ST 2024 8725] and [ST 2000 8713].

South-east of Rudry the lowermost beds of the Main Limestone have been locally strongly hematitized; ore was worked from a small mine [ST 2040 8644], where a slope was driven down the base of the limestone, and from the workings about 2000 tons of ore yielding 50 % of iron were obtained before 1875, when drainage problems proved insuperable. The hematitized limestone with calcite veining is visible in the entrance to the disused mine.

North-west of Draethen the heavily wooded outcrop of the Main Limestone is widened by the effect of the Machen Anticline and Machen Syncline, the axes of which are aligned parallel to the strike of the beds (Plate 11). In the vicinity of Draethen, and south-west for about 1 mile, the lower part of the Main Limestone has been quarried extensively, and massively bedded dolomite is well exposed in Cwm Leyshon Quarry [ST 2116 8686] and others [ST 2210 8748], [ST 2178 8726], [ST 2081 8664]. Northwest of Draethen the hillside overlooking the Rhymney Valley is, in parts, thickly-covered with cemented dolomite scree (calcrete), which was worked in a small quarry [ST 2185 8776]. The rock is not now exposed, but abundant loose blocks lie on the floor of the quarry. The uppermost beds of the Main Limestone are well exposed in a stream [ST 2138 8844], where an almost continuous section of medium to coarse-grained, sometimes crinoidal dolomite is visible.

In the wooded country to the north-west and west of Draethen numerous disused workings for lead are marked by lines of open workings, shafts and adits. One line of old workings, two-thirds of a mile north-east of Draethen, probably follows an east-west fault, for the spoil around the workings contains brecciated dolomite cemented by veins of calcite, hematite, baryte and galena. A disused quarry [ST 2079 8702] lies along the line of another fault or shatter zone, for it exposes 25 ft of disturbed dolomite containing veins of baryte with galena and hematite. The history of lead mining in the Rudry area dates back to Roman times, for several Roman coins were discovered in a disused mine in Cefn-pwll-du (Strange 1782, p. 18), two-thirds of a mile north-west of Draethen. A large amount of lead was used in the garrison conduits at Caerleon and it is probable that some of it was obtained from this area. In 1840 a subsidiary of a Cornish Mining Company commenced working galena in Coed Liwyn Hir [ST 2110 8690], and the ore is believed to have been smelted near Ffwrnes Blwm Farm [ST 1418 8715]. The last commercial venture was in 1871, in Cwm Leyshon, where, from a shaft [about [ST 2110 8724] 75 ft deep, a lode was traced to the surface. Because of drainage problems the mine was closed in 1872.

Machen-Risca

In Machen Quarry [ST 2220 8870] the basal beds of the Main Limestone are well exposed: approximately 80 ft of well-bedded, crystalline dolomite are visible. In a nearby railway cutting [ST 2213 8857] Dixey and Sibly (1918, p. 153) recorded from the base of the Main Limestone; "8 to 10 ft of thinly bedded dolomitic limestones with some highly argillaceous bands, and an abundant development of crinoidal chert underlying grey dolomite without chert. The lowest bed exposed is a band of dark grey, crinoidal limestone, slightly cherty but non-dolomitic, and containing Chonetes cf. hardrensis and Chonetes crassistria Vaughan (non McCoy) in profusion, together with Spirtfer clathratus and Schellwienella cf. crenistria." North of Machen Quarry, beds in the upper part of the Main Limestone are exposed in a stream section [ST 2221 8907]. They are fine-textured limestones with a porcellanous appearance, interbedded with some dolomite. Dixey and Sibly (1918, p. 155) describe the former as: "calcite mudstones and other fine-grained limestones." They place the lower beds in the section in the C₂S₁ Zone and the upper beds in the S₂ Zone. This is the most easterly record of S₂ beds in the Newport district.

East of Ffwrwm and northwards towards Risca the outcrop of the Main Limestone is pitted extensively by shallow workings for galena, and possibly for the gangue mineral baryte. The veins occur along fault planes or joints, aligned in a general east to west or north-east to south-west direction. It is doubtful whether any large amounts of ore were obtained from this area. South of Risca the basal and middle parts of the Main Limestone are being worked in some new quarries [ST 2335 8980], where up to 150 ft of fine-grained, crinoidal dolomite are exposed. In this vicinity the Main Limestone, 550 ft to 600 ft thick, forms a high, wooded scarp which runs northwards through Coed-y-Mochyn and Dan-y-Graig to the Ebbw Valley. In Dan-y-Graig Quarry [ST 2346 9080] approximately 150 ft of massively bedded dolomite, containing crinoid debris and a few shelly fossils, are visible. Some 400 yd north-west of this quarry, in a railway cutting [ST 2324 9113], Strahan (1909a, p. 21) recorded Millstone Grit shales resting on a smooth textured dolomitic rock–the highest bed of the Main Limestone. At the same locality Dixey and Sibly (1918, p. 155) described the beds as "chiefly dolomitic mudstones with much very fine-grained crystalline dolomite and some coarsely crystalline dolomite". This is the most northerly known occurrence of Wadi°la' phase limestone in the Newport district. They also recorded small exposures of smooth textured oolitic limestone with ostracods near Buck Farm [ST 2326 9094], 200 yd south of the railway cutting.

Risca-Llanderfel-Upper Cwmbran-Pontypool

On the northern side of the Ebbw Valley the Main Limestone is about 400 ft thick, and the dolomites are being worked in Risca Quarry [ST 2365 9145], where over 300 ft of the Main Limestone are exposed. The basal 20 ft of beds in this quarry are interbedded, dark grey, poorly fossiliferous, shaly mudstones and impure, shelly and crinoidal dolomitic limestones. George (1956a, p. 315) recorded the following fossils from these basal limestones: "Schuchertella wexfordensis, Cyrtospirifer cf. tornacensis, Syringothyris sp., Cleiothyridinia sp., and Camarotoechia cf. mitcheldeanensis." The remainder of the succession in the quarry consists of thickly bedded, heavily dolomitized, grey limestones containing crinoidal remains and a few poorly preserved shells and corals. Hematitization along joints and bedding planes is prevalent, chiefly in the lower part, and calcite veining (much of it dolomitized) is common throughout. North-eastwards the Main Limestone thins rapidly, and in a distance of 0.5 mile is reduced to about 190 ft near Ysgwbor-newydd [ST 2428 9198]. There, it forms conspicuous crags for about 300 yd along the strike, in which thickly bedded dolomite dips at between 22° and 30° to the north-west.

North-north-east from Ysgwbor-newydd the Main Limestone forms no feature for 2.5 miles, and the boundaries of its outcrop along this stretch are largely conjectural. It is exposed only in two small quarries [ST 2552 9375] and [ST 2528 9334] west of Blaen-yCwm Farm. At both these localities about 2 ft of dolomite are visible. In the most northerly quarry the rock was burnt for lime. In the vicinity of Llanderfel the Main Limestone thins rapidly, and its outcrop is no more than 20 yd wide at Llanderfel Farm [ST 2659 9538]. An outcrop 60 yd wide was mapped by Strahan, but an exposure [ST 2657 9542] of 10 ft of hard, pale grey quartzite in the low north-north-east to south-south-west ridge running behind the farmhouse, which was originally mapped as limestone, proved the ridge to be formed of the basal sandstone of the Millstone Grit. There is no evidence for the presence of the Main Limestone at this locality, but it is possible that a thin development, amounting to no more than 40 ft, is present.

North-north-east, towards Upper Cwmbran, the boundaries of the Main Limestone are again difficult to mark precisely across the featureless ground, and the main guide is the scattering of solution hollows along its outcrop. At Upper Cwmbran, where the Main Limestone is 175–200 ft thick, vertical or nearly vertical, massively bedded dolomite has been worked in two quarries [ST 2720 9663] and [ST 2728 9670]. In the northerly quarry Strahan (1909a, p. 21) recorded, in the upper part of the sequence, a 2-ft bed containing quartz pebbles up to 0.5 inch in length.

Towards Penyrheol from Upper Cwmbran, the gradually thickening Main Limestone occupies almost featureless ground, but its position is well shown by the abundance of solution hollows along its outcrop, which form circular debris-filled depressions up to 12 yd in diameter. In the vicinity of the Cwmyniscoy Quarries, immediately north of Penyrheol, the Main Limestone is 250 to 300 ft thick and it is being worked in the main quarry [ST 2820 9970] where the following sequence is visible:

The 20 ft of oolitic and shelly limestone yielded the following fauna: bryozoa, 'C.' mitcheldeanensis, Rugosochonetes sp., Schellwienella aspis Smyth, smooth spiriferoids, Syringothyris sp., Leiopteria laminosa (Phillips), ostracod and Psammodus sp.This fauna suggests a ?Zaphrentis (Z) age for the beds.

Nearer Pontypool the Main Limestone is exposed in a disused quarry [SO 2829 0038] at Trosnan Junction, where up to 40 ft of grey, massive dolomite dipping 14° W.N.W. are visible. Westwards, about 100 yd, a small exposure [SO 2815 0038] reveals the junction between the Main Limestone and overlying basal sandstone of the Millstone Grit; medium to coarse-grained, pale grey quartzite lies on an irregular surface of hard, bluish grey dolomite. The junction is again seen in a section [ST 2805 9979] in Cwm Ynis Coy, where 8 ft of hard quartzite overlie 3 ft of dolomite. In Pontypool the dolomites of the Main Limestones form high, poorly exposed ground running through the town, and again on the northern side of the Afon-lwyd Valley, where they form a prominent escarpment on the north side of Pontypool Park. H.C.S.

References

BUCKLAND, W. and CONYBEARE, W. D. 1824. Observations on the South-western Coal District of England. Trans. Geol. Soc., ser. 2, 1, 210–316.

DIXEY, F. and SIBLY, T. F. 1918. The Carboniferous Limestone Series on the southeastern margin of the South Wales coalfield. Quart. J. Geol. Soc., 73, 111–64.

DIXON, E. E. L. 1907. The Geology of the South Wales coalfield. Part VIII. The Country around Swansea. Mem. Geol. Surv.

DIXON, E. E. L. and VAUGHAN, A. 1912. The Carboniferous Succession in Gower (Glamorgan) with Notes on its Fauna and Conditions of Deposition. Quart. J. Geol. Soc., 67, 477–571.

ETHERIDGE, R. 1868. On the Physical Structure of the Northern part of the Bristol Coal Basin, chiefly having reference to the iron ores of the Tortworth area. Proc. Cotteswold Nat. Field Club, 4, 28–49.

ETHERIDGE, R. 1870. On the geological position and geographical distribution of the reptilian or dolomitic conglomerate of the Bristol area. Quart. J. Geol. Soc 26, 174–92.

GEORGE, T. N. 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quart. J. Geol. Soc., 110, 283–322.

GEORGE, T. N. 1956a. Carboniferous Main Limestone of the East Crop in South Wales. Quart. J. Geol. Soc., 111, 309–22.

GEORGE, T. N. 1956b. The Namurian Usk Anticline. Proc. Geol. Assoc., 66, 297–316.

ROGERS, E. 1861. The Iron Ores of Great Britain. Part III. Mem. Geol. Surv.

SIBLY, T. F. 1927. Special Report on the Mineral Resources of Great Britain. Vol. X. Iron Ores. The Haematites of the Forest of Dean and South Wales. 2nd edit. Mem. Geol. Surv.

STRAHAN, A. 1909a. The Geology of the South Wales Coalfield. Part I. The Country around Newport, Monmouthshire. 2nd edit. Mem. Geol. Surv.

STRAHAN, A. 1909b. The Geology of the South Wales Coalfield. Part X. The Country around Carmarthen. Mem. Geol. Surv.

STRANGE, J. 1782. Reverend Mr. William Harris's Observations on the Roman Antiquities in Monmouthshire and the neighbouring counties of Wales. Archaeologia, 6, 6–38.

TROTTER, F. M. 1942. Geology of the Forest of Dean Coal and Iron Ore Field. Mem. Geol. Surv.

VAUGHAN, A. 1905. The Palaeontological Sequence in the Carboniferous Limestone of the Bristol area. Quart. J. Geol. Soc., 61, 181–307.

WETHERED, E. 1882. On the origin of the haematite deposits in the Carboniferous Limestone. Geol. Mag., 9, 523–5.

Chapter 5 Millstone Grit Series

General

Introduction

The narrow outcrop of the Millstone Grit extends from the south-west corner of the Newport district through Taffs Well, Machen, Risca and Upper Cwmbran to Pontypool. The series rests unconformably on the Main Limestone of the Carboniferous Limestone Series and is overlain conformably by the Lower Coal Measures, its upper limit being marked by the base of the Gastrioceras subcrenaturn Marine Band. The Millstone Grit consists of two Ethological divisions, an impersistent, basal sandstone and conglomerate group up to 75 ft thick, overlain by 70 to 175 ft of dominantly argillaceous beds. Only the basal group, where thickly developed locally, forms features of any significance; the argillaceous rocks usually occupy low-lying, marshy ground. The sandstones and conglomerates are generally well exposed, but the overlying mudstones are seldom visible and there are no continuous exposures through any great thickness of strata. Only the Rudry Borehole has provided a complete section through the Millstone Grit (p. 86 and Appendix I).

Because of the scarcity of exposures in the argillaceous rocks of the Millstone Grit few geologists have worked in the area. During the primary six-inch survey Strahan (1909) found no fossils, apart from plant remains in the sandstones, and much of his description applies to strata which are now classified in the Lower Coal Measures. A detailed examination was made by Moore along the south crop (1945, pp. 148–51) and south-east and east crops (1948, pp. 267–72). Along the south crop he found marine fossils at one locality, which Dr. C. J. (now Sir James) Stubblefield believed were from the G zone. Along the south-east and east crops Moore found marine fossils at several localities, but the faunas, due to the absence of diagnostic goniatites, gave no conclusive evidence of their horizon, though he thought that they indicated the presence of the higher zones of the Millstone Grit.

Classification and correlation

At the time of the primary survey of the Newport district the traditional upper limit of the Millstone Grit in the eastern part of the South Wales Coalfield was at the top of a sandstone known as the 'Farewell Rock' in the Newport district and along the north crop, and the 'Cefn Cribbwr Rock' in the Pontypridd district (Woodland and Evans 1964, p. 6). In the Newport district the 'Farewell Rock' is the first thick sandstone below the lowest, economically important seam in the Lower Coal Measures, that is, the Five-Feet-Gellideg, locally known as the Old Coal or Hard Vein. Thus, the Millstone Grit then 'comprised a three-fold lithological classification into upper and lower arenaceous divisions separated by a group of mudstones. This classification was also used by Moore (1948) during his survey along the south-east crop. In recent years however, following detailed systematic work on the faunas of the Upper Carboniferous, it has been internationally agreed that the junction between Namurian and Westphalian (i.e. Millstone Grit and Coal Measures) should be drawn at the base of the Gastrioceras subcrenatum Marine Band. Thus, in the Newport district, the 'Farewell Rock' and approximately the upper one-third of the original middle mudstone subdivision now lie in the basal part of the Lower Coal Measures.

The present classification of the Millstone Grit is based on its sequence of goniatite faunas, and the divisions originate mainly from the work by Bisat (1924, 1928) in Northern England. They are as follows:

• YEADONIAN STAGE (Lower Gastrioceras, G₁) comprising the zones of G. cumbriense and G. cancellatum
• MARSDENIAN STAGE (Upper Reticuloceras, R₂) comprising the zones of R. superbilingue, R. bilingue and R. gracile
• KINDERSCOUTIAN STAGE (Lower Reticuloceras, R₁) comprising the zones of R. reticulatum s.s., R. nodosum and R. circumplicatile
• ALPORTIAN STAGE (Upper Homoceras, H₂) comprising the zones of HomoceraWider prereticulatus, Homoceras undulatum and Hudsonoceras proteus.
• CHOKIERIAN STAGE (Lower Homoceras, H₁) comprising the zones of Homoceras beyrichianum and H. subglobosum
• ARNSBERGIAN STAGE (Upper Eumorphoceras, E₂)
• PENDLEIAN STAGE (Lower Eumorphoceras, E₁)

In Millstone Grit stratigraphy the term 'Marine Band' has a particular significance, for most of the mudstones in the succession are marine, or quasi-marine. It should not be confused with a 'Zone', e.g. Gastrioceras cancellatum Zone, in which there is usually a range of faunal phases (Ramsbottom, Rhys and Smith 1962, p. 114). In the ideal cycle these commence with the 'least-marine' phase (quasi-marine conditions) and, as the environment becomes more saline (and/or the water deepens), they pass gradually into the 'more-marine' phases which commonly contain goniatites. Six phases were recognized by Ramsbottom and others in Derbyshire as follows: (1) Fish phase; (2) Planolites phase; (3) Lingula phase; (4) Mollusca spat phase; (5) Anthracoceras or Dimorphoceras phase; and (6) the typical thicker-shelled goniatite phase. Archer (1968) recognized seven phases in Carmarthenshire. As the goniatites are the only reliable indicators of horizon in the Millstone Grit, it is the goniatite phases in each faunal zone which are commonly designated by the term 'Marine Band', and the term is used in this sense in this chapter.

Although the Millstone Grit is poorly exposed in the Newport district, sufficient faunal evidence has been obtained to show that only beds of upper R₂ age and G₁ age are represented. In the south-west (including a small part of the Cardiff district to the south, see p. 85) the lowest beds of the Millstone Grit lie at the horizon of the Reticuloceras superbilingue Marine Band of upper R₂ age. In the same area higher strata have yielded a fauna from the Gastrioceras cancellatum Marine Band, which is of basal G₁ age. These are overlain by unexposed beds, which almost certainly represent the remainder of the Yeadonian Stage, and then by the Lower Coal Measures, at the base of which the G. subcrenatum Marine Band has been proved.

North-east of the Taff Valley the only records of mudstones containing recognizable goniatites are from one surface exposure at Wern-ddu (Moore 1945, p. 149), and from the Rudry Borehole. The goniatites in the surface exposure (p. 86) were considered by Dr. C. J. (now Sir James) Stubblefield to be of G₁ age. In the Rudry Borehole the basal 38 ft 11 in of the 164 ft 11 in (corrected for dip these thicknesses would be about 32 ft and 143 ft respectively) of Millstone Grit are of upper R₂ age, for they are immediately overlain by the G. cancellatuin Marine Band. In the overlying G₁ mudstones the G. cumbriense horizon has been proved, and also the G. subcrenatum Marine Band at the base of the Lower Coal Measures.

The thinner sequence (70 to 125 ft) of mudstones along the south-east and east crops between Risca and Pontypool may represent even less of the Millstone Grit than is present at Rudry, possibly beds of G₁ age only in places, but there is no conclusive faunal evidence. A comparison of the Rudry and Pontypool successions suggests that the fauna of brachiopods, gastropods and fish from the lowest mudstones of the 125 ft of Millstone Grit in Cwm Ynis Coy, Pontypool, is possibly from an horizon not far below the Anthracoceras band (p. 88). This lies nearly 12 ft below the base of the G. cancellatum horizon in Rudry Borehole.

A comparison (Figure 5) of the Millstone Grit successions in the Newport district and at Aberkenfig in the Pontypridd district to the west, shows that rapid overlap takes place in the 12 miles separating the two areas. At Aberkenfig Woodland and Evans (1964, p. 9) showed that only strata of E₁ age appear to be absent from the Millstone Grit which is nearly 900 ft thick. Eastwards from there successively higher beds overlap on to the Carboniferous Limestone Series until, in the Newport district, only beds of upper R₂ and G₁ age are represented.

Thickness and lithology

The sandstones and conglomerates at the base of the Millstone Grit vary in thickness from 0 to 75 ft. They are thin or absent between the south-west corner of the district and the Rhymney Valley, except for a stretch of about a mile to the north and north-east of Rudry, where they are up to 20 ft thick. Between the Rhymney and Ebbw valleys the basal arenaceous strata are particularly well developed for a distance of about a mile, and they are up to 75 ft thick. In the area between the Ebbw Valley and Pontypool they are generally thin or even absent, except at Llanderfel and near Pontypool where they thicken rapidly to as much as 100 ft along short distances. The overlying mudstone division generally varies from 100 to 175 ft between the south-west corner of the district and the Ebbw Valley. North-east of the Ebbw Valley the thickness ranges from 70 to 100 ft as far as the Pontypool area, where it increases slightly to about 125 ft.

The basal arenaceous rocks range from fine-grained sandstones to coarse conglomerates with a few intercalated mudstones. The sandstones are usually white, pale grey, buff etc., with irregular, flaggy to massive bedding. They are mainly siliceous, well cemented and very hard. Massive conglomeratic sandstones and conglomerates (Plate 5A) with pebbles up to 2 inches in diameter are commonly interbedded with the sandstones. The pebbles, which are set in a quartz sand matrix, are usually of white quartz with a small fraction of other varieties of crystalline silica such as smoky and rose quartz, and cryptocrystalline silica such as black chert and red jasper. The overlying, mainly argillaceous division, comprises dark to light grey mudstones and silty mudstones, which are mainly of marine or quasi-marine origin, with a few thin intercalations of fine to medium-grained quartzitic sandstone. Thin beds and nodules of ironstone are commonly present. In the section of the Rudry Borehole cyclic sedimentation can be recognized, but it is poorly developed. Each cyclothem usually comprises only mudstones and silty mudstones with a thin, poor seatearth; coals are absent and sandstones rare. H.C.S.

Details

Taffs Well-Thornhill-Rudry-Machen-Risca

Along the south crop, between the western boundary of the Newport district and Taffs Well, the narrow, west-east aligned outcrop of the Millstone Grit is largely obscured by boulder clay, and the small drift-free areas are very poorly exposed. At the entrance to a small level [ST 0839 8244] 1260 yd W. 39° S. of Soar Farm, several feet of bluish grey, silty, micaceous shale with thin beds of sandstone dip northwards at about 75°. No fossils were found in the shale, but spoil from the level yielded the following poorly-preserved remains: Caneyella multirugata (Jackson), Dunbarella cf. elegans (Jackson), Anthracoceras sp., and Gastrioceras crencellatum Bisat. This fauna is from the Gastrioceras cancellatum Marine Band.

In the south-western part of the Newport district the basal beds of the Millstone Grit are nowhere visible, but during the resurvey they were well exposed in a temporary section a few hundred yards to the south, in the Cardiff (263) Sheet. As the rocks in this section yielded a fauna of important stratigraphical significance, the details are recorded here. In the Creigiau Quarries [ST 0870 8195], in which the dolomitized Main Limestone of the Carboniferous Limestone Series is worked, there is a narrow, east-north-east aligned outcrop of Millstone Grit mudstones lying between two outcrops of Main Limestone, the northern-most of which has been thrust southwards over the Millstone Grit. Differential erosion of the dolomite-mudstonedolomite sequence has resulted in the mudstone outcrop occupying a narrow, flat, shelf-like area, 200 to 400 yd wide, between the two dolomite outcrops which form bold scarp-features. Quarrying in the lower mass of dolomite exposed [ST 0870 8203] the overlying Millstone Grit, the section being:

The fauna from the 2 ft of dark grey mudstone, which lies only 4 ft 1 in above the junction with the Main Limestone, is from the Reticuloceras superbilingue Marine Band.

East of Taffs Well the Millstone Grit outcrop is largely covered with terrace gravel, glacial sand and gravel or boulder clay, for a distance of about 2 miles. Farther east again, towards Thornhill, the outcrop is easily traced for two-thirds of a mile along a narrow belt of low-lying, marshy ground between the more prominent outcrops of the Main Limestone below, and basal sandstones of the Lower Coal Measures above. The rocks are rarely visible along this stretch. A temporary exposure (Moore 1945, p. 149) existed in 1941 on the side of the road south of the Black Cock Inn [ST 1440 8479] about a mile west of Thornhill. It consisted of grey and olive green, calcareous shales lying approximately 30 ft above the Carboniferous Limestone Series. They were very fossiliferous and yielded the following fauna of mussels: " Anthracomya bellula Bolton, A. cf. lenisulcata Trueman, A. sp., C. sp., and Naiadites sp.". Trueman, who indentified these fossils, concluded that the fauna occurred within the lenisulcata Zone, but could not give the exact horizon represented. He thought that the fauna was similar to one from the base of the Lower Coal Measures of Yorkshire (Wray and Trueman 1934, p. 37) and to one in the Lancashire Coalfield (Wright 1934, p. 12) which ranged from near the top of the Millstone Grit into the Lower Coal Measures, and was associated with the goniatite zone of Gastrioceras.

East of Thornhill the unexposed Millstone Grit runs for nearly 1.5 miles through the heavily-wooded area of Wern-ddu. Along the road [ST 1822 8615] bounding the eastern extremity of the wood alternations of light greyish green, fine-grained, laminated sandstone and bluish grey, silty mudstone with plant remains dip northwards at 26°. When Moore (1945, p. 149) surveyed the area, soft, blue shales lying about 50 ft above the Carboniferous Limestone, were exposed below these sandy beds and yielded some poorly preserved goniatites which Dr. C. J. (now Sir James) Stubblefield determined as follows: "the locality seems to yield a definite Gastrioceras (Specimens Al and A2). A7 is somewhat like Gastrioceras crenulatum and A6 like Gastrioceras cumbriense, so I think these are from the G zone though not necessarily the cumbrienseSubzone." Moore also found the following mussels in sandy shales overlying the mudstones: "Carbonicola aff. recta Trueman, C. cf. pseudacuta Trueman, C. sp., and Anthracomya knisulcata Trueman". H.C.S.

In Rudry Borehole (see Appendix I for a complete section) a marine band lies 17 ft 8 in above the Carboniferous Limestone. It occurs between 1561 ft 5 in and 1562 ft 1 in, and the presence of Anthracoceras sp.suggests it is equivalent to the Anthracoceras band in this position along the north crop. The Gastrioceras cancellatum Marine Band lies 11 ft 9 in higher and is 12 ft 8 in thick. The fauna of this band may be divided into: (1) a benthonic fauna with poorly-preserved Gastrioceras1537 ft 9 in to 1338 ft 10 in. (2) A goniatite phase containing the association of G. crencellatum and Agastrioceras carinatum (Frech) which is usual in the upper part of the cancellatum band-1538 ft 11 in to 1541 ft 10 in. (3) A fauna including Gastrioceras cancellatum Bisat and R. superbilingue [the middle goniatite fauna of this banp] together with benthonic elements-1542 ft 6 in to 1547 ft 7 in. (4) A basal benthonic fauna-1547 ft 8 in to 1549 ft 9 in.

Between the G. cancellatum Marine Band and the G. cumbriense horizon the non-marine fauna of Anthraconaia cf. angulosa Pastiels is usual at this horizon, but its occurrence through nearly 26 ft of beds is more than would normally be expected. At the base of these non-marine strata there is some interdigitation with beds con-taming Planolites. The marine sequence between 1448 ft 8 in and 1503 ft 6 in represents the G. cumbriense horizon, but no goniatites are present and the fauna is largely of a benthonic nature. There are five levels at which the fauna is reduced to Planolites, and an equal number of Lingula phases, and there appears therefore to have been considerable variation in salinity and/or depth during the deposition of the band. The benthonic fauna comprises chiefly (apart from Lingula) crinoids, Productus carbonarius de Koninck, Lissochonetes (at one level only) a single bryozoan, and common bellerophontoids and nuculoids. Such a faunal association is of widespread occurrence in near-coastal G. cumbriense band faunas. W.H.C.R.

North-east of Rudry the lowermost silty mudstones of the Millstone Grit are exposed in a stream [ST 1975 8711], and they have yielded: Carbonicola pseudacuta Trueman and Naiadites sp. cf. obliquus Dix and Trueman, forms which are known to occur at about the horizon of the Gastrioceras cancellatum Marine Band. Approximately 200 yd east-north-east of this stream the basal part of the Millstone Grit contains beds of quartzitic sandstone, and some conglomerate, for a distance of nearly a mile along the strike. These arenaceous rocks are visible at a few localities, including the section [ST 2011 8727] at Cwrt-y-llaca where beds of white quartzite dip north-westwards at 26°; loose blocks of quartz conglomerate, probably from the basal beds of the Millstone Grit, are also visible. In Coed-Cefn-pwll-du, in the stream [ST 2095 8771] near Pen-how Farm, blocky, grey mudstones dip north-west at about 26°. Northeastwards along the stream the Millstone Grit comprises mudstone weathered to grey or buff clay at several localities.

Between Machen and Risca the base of the Millstone Grit is marked by a thick development of sandstones and conglomerates, that have been traced for about a mile along the strike. They are visible at several localities, the best sections being along the high ground [ST 2305 8960] above Pont-y-mister in the Ebbw Valley, where there are numerous natural exposures of massively bedded, white, quartzitic sandstone containing abundant, well-rounded pebbles of mainly white quartz and some red jasper up to 2 inches in diameter. The dip generally varies between 20 and 30 degrees to the north-west. About 100 yd north-west of these exposures a local development [ST 2295 8970] of up to 20 ft of white, quartzitic sandstones, traced for only 200 yd along the strike, occurs a few feet below the conjectural position of the top of the Millstone Grit. They form a low ridge, along which a few exposures are visible.

Risca-Llanderfel-Upper Cwmbran-Pontypool

Immediately north-east of the Ebbw Valley the narrow outcrop of the Millstone Grit is largely covered by head for about 1.5 miles, and can then be easily followed along a belt of flat, wet, rush-covered ground as far as Henllys Colliery [ST 2588 9441]. The Henllys Colliery Slope commences in Millstone Grit mudstones, at approximately 25 ft above the top of the Main Limestone, and drifts through the succession into the overlying Lower and Middle Coal Measures. At the time of the resurvey the slope was open for a distance of about 85 yd, and was then blocked by a fall. The walls are rock-lined, but at a few places the lining has fallen away and Millstone Grit mudstones are visible. At approximately 80 yd down the slope, at about 75 ft above the base of the Millstone Grit, the succession comprises blocky, grey mudstones dipping at 25° to the north-west; they are very fossiliferous in parts and have yielded: Lingula mytilloides J. Sowerby, Orbiculoidea nitida (Phillips), Productus carbonarius, Euphemites sp., Retispira sp., Straparollus? [high spired], Palaeoneilo laevirostris (Portlock), Polidevcia aff. acuta (J. de C. Sowerby), Schizodus sp., Gastrioceras sp.[coronate form, no ornament]. The precise horizon of the beds cannot be determined from this fauna, but it suggests that they lie high in the Millstone Grit, and are probably of G₁ age.

A rich fauna was also obtained from the Henllys Colliery Slope by Moore (1948, p. 271) as follows: "Lingula mytiloides J. Sowerby, Productus (Linoproductus) sp., Palaeolima cf. retifera (Shumard), Crurithyris sp., Protoschizodus cf. axiniformis (Portlock), Sanguinolites sp. nov., Nuculana cf. stilla (McCoy), 'Nucula' cf. aequalis J. de C. Sowerby, ?Modiola sp., Bucaniopsis densistriatus Weir, ostracods, probably including Paraparchites sp."

The workings in Henllys Colliery were drained by a heading which opened [ST 2494 9458] into Cwmcarn nearly two-thirds of a mile west of the main entrance to the mine. At this exit mudstone spoil has yielded crinoid columnals, Crurithyris sp., Productus carbonarius, Rugosochonetes sp., Edmondia sp., Limipecten ? juv., Palaeoneilo laevirostris, Sanguinolites?, nuculoids indet., pleurotomarian gastropod fragment, Gastrioceras sp.[internal mould of tubercles]. This fauna is too incomplete for its horizon to be determined. The origin of the spoil is uncertain, for the drainage heading itself runs from the Brithdir Beds in Cwmcarn, to the Lower Coal Measures where it joins the main Henllys Slope. The most likely possibility is that the spoil was obtained while repairing or widening that part of the main slope that passes through the Millstone Grit.

North-north-east from Henllys Colliery the outcrop of the Millstone Grit is readily traced along wet, rush-covered ground for about 500 yd, then is more difficult to follow across the almost featureless country towards Llanderfel and Upper Cwmbran. At Llanderfel there is a development of sandstone up to 50 ft thick at the base of the Millstone Grit, which forms a low north-north-east aligned ridge running behind Llanderfel Farm [ST 2659 9538]. The beds are visible in a small exposure [ST 2657 9542] 40 yd north of the farm, where 10 ft of hard, white and buff, quartzitic sandstone dipping at 70° W.N.W. are present. Nearby, the overlying, soft, grey mudstones of the Millstone Grit are poorly exposed at a few places along the stream [ST 2646 9550] which cuts through their narrow outcrop. Moore (1948, pp. 268–9) found the following fossils in these mudstones: "Lingula mytiloides J. Sowerby, Chonetes cf. laguessianus de Koninck, Chonetes sp., Productus sp.; Aviculopecten cf. scalaris (J. de C. Sowerby) and Sanguinolites cf. angustatus (Phillips)." North-north-east from Llanderfel 6 ft of the basal sandstone are exposed [ST 2654 9583] in the entrance to a small disused level. In spoil nearby Moore (1948, p. 268) found "Lingula mytiloides and Carbonicola sp.", which are probably from some of the lowest Millstone Grit mudstones.

Across the featureless slope north of Upper Cwmbran the unexposed Millstone Grit is difficult to map precisely for about a mile, but then follows a well-defined belt of flat, marshy ground which runs into Pontypool. In Cwm Ynis Coy [ST 2805 9979], just south of Pontypool, 3 ft of Main Limestone are overlain by 8 ft of hard, pale grey sandstones, the basal rocks of the Millstone Grit. Moore (1948, p. 268) recorded a thicker section here comprising some 10 ft of sandstone overlain by blue shales containing "Lingula mytiloides and Productus sp.". The blue shales referred to are probably those which crop out in the bank [ST 2804 9980] on the north side of the stream, where the following section was recorded during the resurvey:

In the mudstones at this locality Lingula sp.has been found by Mr. T. R. Owen (personal communcation).

Northwards from Cwm Ynis Coy the basal sandstones thicken to about 50 ft west of Trosnan Junction [SO 2833 0042], but they appear to die out rapidly northwards. in Pontypool. An exposure [SO 2815 0038] near Trosnan Junction shows 8 ft of medium to coarse-grained, quartzitic sandstone lying on an irregular surface of Main Limestone.

References

ARCHER, A. A. 1968. The Geology of the South Wales Coalfield. Special Memoir. The Upper Carboniferous and later formations of the Gwendraeth Valley and adjoining areas: in parts of the Carmarthen (229), Ammanford (230) and Worms Head (246) sheets. Mem. Geol. Surv.

BISAT, W. S. 1924. The Carboniferous goniatites of the north of England and their zones. Proc. Yorks. Geol. Soc., 20, 40–124.

BISAT, W. S. 1928. The Carboniferous goniatite zones of England and their continental equivalents. Cong. de Strat. Carb., Heerlen, 117–33.

MOORE, L. R. 1945. The Geological sequence of the South Wales Coalfield: the 'South Crop' and Caerphilly basin, and its correlation with the Taff Valley sequence. Proc. S. Wales Inst. Eng., 60, 141–227.

MOORE, L. R. 1948. The sequence and structure of the southern portion of the east crop, of the South Wales Coalfield. Quart. J. Geol. Soc., 103, 261–300.

RAMSBOTTOM, W. H. C., RHYS, G. H. and SMITH, E. G. 1962. Boreholes in the Carboniferous rocks of the Ashover District, Derbyshire. Bull. Geol. Sun. Gt. Brit., No. 19,75–168.

STRAHAN, A. 1909. The Geology of the South Wales Coalfield. Part I. The Country around Newport, Monmouthshire. 2nd edit. Mem. Geol. Surv.

WOODLAND, A. W. and EVANS, W. B. 1964. The Geology of the South Wales Coalfield. Part IV. The Country around Pontypridd and Maesteg. 3rd edit. Mem. Geol. Surv.

WRAY, D. A. and TRUEMAN, A. E. 1934. The Fauna of the Lower Coal Measures in West Yorkshire. In Summ. Prog. Geol. Surv. for 1933, Part II, 37.

WRIGHT, W. B. 1934. The Fresh-water Fauna of the Lower Coal Measures of Lancashire. In Summ. Prog. Geol. Surv. for 1933, Part II, 8.

Chapter 6 Coal Measures: general

Introduction

The south-eastern part of the South Wales Coalfield lies in the north-west of the Newport district, covering an area of over 100 square miles. Up to 3700 ft of Coal Measures resting conformably on the Millstone Grit extend up to the Grovesend Beds which are preserved in the synclinal parts of the Carboniferous basin. As well as many coals and their seatearths, the succession contains rocks ranging from mudstones and silty mudstones through striped beds (thin alternations of quartzitic siltstone and mudstone or silty mudstone) and siltstones to coarse sandstones (both pennant and quartzitic types) and conglomerates. The usual colour of the argillaceous rocks is grey, but red and green mudstones form part of the succession, most commonly in the Llynfi and Rhondda beds. The occurrence of tonstein (in the sense used by Scheere 1955, p. 8, i.e. a mudstone in which the clay mineral is almost exclusively kaolinite) is recorded for the first time in the Middle Coal Measures of South Wales.

The succession has been divided according to the revised classification of the Coal Measures of England and Wales (Stubblefield and Trotter 1957, pp. 1–5, and Woodland and others 1957, pp. 6–13). The new names for the divisions have been used both on the published geological maps of the present area, and in this memoir, and they are shown on the generalized vertical section of the measures in (Figure 6).

As Woodland and Evans (1964) discovered during the resurvey of the adjacent Pontypridd (248) Sheet, much confusion in correlation has arisen in the past because of the indiscriminate naming of coals. Individual seams are commonly known by different names in different parts of the coalfield, and the same names are frequently applied to different seams. Although this was due in part to errors in correlation, much of it arose because, in the early days of mining, each valley possessed its own series of seam names. Gradually, as workings extended, some names became common to adjacent valleys, or even larger areas, but it was not until Woodland and Evans published (in 1959–60) their six-inches to one mile geological maps of the Pontypridd district that standardization of naming over a wide area was attempted. In order to achieve conformity many of the names used by Woodland and Evans have been applied to the equivalent seams in this district, in particular within the succession between the Gellideg and Two-Feet-Nine seams (the sequence which contains the majority of the worked seams). Above the Two-Feet-Nine the names applied in the eastern part of the Pontypridd district are used in this account whenever the correlation is certain, although in a few cases local names have been given precedence, e.g. in the Grovesend Beds. The standard names used in this memoir, and the majority of the previously used local names are given in (Plate 6), (Plate 7) and (Plate 8), and (Figure 6).

The Coal Measures of the Newport district, in particular between the Gastrioceras subcrenatum Marine Band and the Brithdir seam, show a marked reduction in thickness when traced towards the east, and this has been caused by the influence on sedimentation of the Usk anticlinal upwarp, which existed to the east and south-east of the present edge of the coalfield. From the western margin of the district gradual eastward attenuation occurs in all the strati-graphical divisions, and at the same time many coal seams and some of the marine bands die out, until, along the south-east crop, an extremely attenuated sequence remains. In this south-eastern part of the coalfield thinning of individual beds, or groups of beds, is particularly accentuated by non-sequences, and at least one unconformity. In the following general account the beds are firstly described where the sequence is more complete in the west, and then their easterly development and extent is traced. The graphic, generalized vertical sections (Figure 9), (Figure 16) and (Figure 21) are based largely on the sequence in the west, and the easterly variations are portrayed in the vertical sections at specific localities (Plate 6), (Plate 7) and (Plate 8).

Along the south-east crop between Coed-y-Darren [ST 2407 9213] and near Upper Cwmbran [ST 2680 9640] the Coal Measures are particularly attenuated between the Garw and Brithdir seams, but because of the lack of exposures and few mining records in this area, very little is known about the horizons represented at the surface. On the published six-inches to one mile and one-inch to one mile maps of this area, therefore, the boundary lines of the stratigraphical divisions are largely hypothetical. The geological facts available about this small area have been summarized previously (Squirrell and Downing 1964, pp. 127–32). It was suggested that the extreme attenuation can be accounted for by a combination of restricted deposition, non-sequences and unconformity near a shoreline, the present south-east crop having existed near the edge of the Coal Measures basin of deposition for long periods of Lower, Middle and basal Upper Coal Measures times. H.C.S.

Sedimentation

The Coal Measures in South Wales are principally of deltaic origin and the sequence exhibits the typical cyclic pattern associated with such deposition. In detail the nature of the cycIothems^‡6  is not identical throughout the succession (Woodland and Evans 1964), the differences reflecting changes in the environments in which they formed. Ideally a standard cyclothem in the Lower and Middle Coal Measures comprises the following sequence:

• 6. Coal
• 5. Seatearth
• 4. Sandstone passing up into mudstone
• 3. Generally unfossiliferous mudstone with plants towards the top, passing up into sandstone
• 2. Mudstone with non-marine fossils 1. Mudstone with marine fossils

This cyclic unit is repeated many times throughout the sequence, but rarely in the complete form just given. The more usual cycle is represented by phases 1 or 2, 3, 5 and 6. The pattern of rhythmic sedimentation was influenced by relief, depositional environment, rate of subsidence and climate (Wanless 1950), the latter determining the amount of erosion. A further factor is the nature of the sediment and its susceptibility to compaction. Some authors believe that sedimentation and differential compaction, together with uniform subsidence, account for cyclic sequences (van der Heide 1950). However, in eastern South Wales the significance of differential subsidence is clear from the rapid local changes in thickness and lithology in many areas, as typified, for example, in the Rhymney Valley between the Nine-Feet and Two-FeetNine seams, and it is believed that regionally it must also have been important.

In general the changes in sedimentation are related to a balance between rate of subsidence and rate of sedimentation. The marine incursion, which in some cases followed a coal-forming period, occurred when the rate of subsidence exceeded the rate of peat formation, and conversely, the final infilling of the basin towards the close of each cycle, commenced when the rate of sedimentation exceeded subsidence.

Ideally each cycle was initiated by a period of relatively sudden subsidence allowing a marine transgression. Two reasons may be given for suggesting this initial subsidence was sudden. Firstly, the fact that the marine phase immediately succeeds the coal without the intervention of an estuarine phase, despite the fact that such an estuarine phase initiates the cycle in many cases. Secondly, where a marine phase occurs the lower beds in places contain a fauna indicative of deeper water than the upper beds. Alternatively, the absence of an inshore phase below the marine bed may be explained by suggesting that the inshore deposits were reworked by currents as the marine transgression progressed. Of the two views this seems less likely, as no evidence of reworking has been obtained.

The marine phases are principally represented by Lingula-mudstones, though in places a more diversified fauna is found. Normally the marine phase was brought to an end by an increase in the amount of mud entering the basin. It is not usual in the Newport district to find the marine bands succeeded by mudstones with mussels, although instances of this are known. Commonly the cycles do not begin with a definite marine phase; the first beds are more usually mudstones containing mussels and their significance in the sedimentation cycle is similar to that of marine bands. The mussel-bearing mudstone is normally restricted to the immediate roof strata of coal seams. As with the marine phase, the restriction is due to a change in the nature of the sedimentation indicated by a passage from dark, carbonaceous mudstones, associated with the mussels, to the overlying grey, blocky, generally unfossiliferous mudstone. The colour change reveals the decline in organic content per unit thickness as the rate of sedimentation increased (Eagar 1953, p. 179). The remaining deposits forming a cyclothem represent the gradual infilling of a basin of sedimentation, culminating with a land surface and seatearth and coal phases.

It has been suggested that seatearths developed their character before the formation of peat commenced (Stout 1923, Weller 1930, Trueman 1947). Weller (1956) pointed out the occurrence of zones of leaching and calcium carbonate enrichment in seatearths. It seems doubtful whether these could develop under the conditions which prevailed in coal swamps, where the dominant movement of ground water must have been upwards, and not downwards as would be required for excessive leaching. The absence of a close genetic relationship between seatearths and coals seems to be emphasized by the presence of thick seams, for example the Nine-Feet, which may have developed from 40 to 50 ft of peat. The vegetation giving rise to the upper layers of these thick seams could not have been related to the underlying seatearth, and the thickness of seatearths bears little relationship to the thickness of the overlying coals. Seatearths probably represent an aerial, relatively well-drained phase in the cycle, and are fossil soils. The remains of the vegetation growing in the soils decayed and were not preserved. Genetically they are related to the underlying rocks, for example in the upper part of the Middle Coal Measures they tend to be more arenaceous where underlain by sandstones.

The succeeding coal represents a period of slight lowering of the base level, with a consequent rise in the water table producing a swamp-forest environment. Under these conditions considerable thicknesses of organic matter accumulated under anaerobic conditions in stagnant water.

A complete cyclothem represents a period of subsidence allowing transgression of the strand line, followed by a regression as the delta front gradually advanced. The sequence of environments caused by these fluctuations was shallow sea, subaqueous delta with brackish water, subaerial delta and coal swamp. A final stage was that of an inland flood plain which was probably attained at the close of some cycles in the upper part of the Middle Coal Measures, and was largely a feature during the formation of the Upper Coal Measures. The rate of subsidence is difficult to determine, but, if it is correct to assume that the initiation of each cycle was due to a sudden subsidence, and the seatearth-coal phases represent stable conditions, then the rate of subsidence was probably rapid initially, declining gradually with time as the cycle developed. R.A.D.

Lower Coal Measures

The Lower Coal Measures crop out only along a narrow strip of country around the edge of the coalfield. The sequence extends from the base of the Gastrioceras subcrenatum Marine Band to the base of the Amman Marine Band. For descriptive purposes the succession has been divided at the horizon of the Garw. The lower group of beds varies in thickness from over 300 ft in the south-west to under 200 ft in the north-east and east, and the upper group varies from about 250 ft in the west to under 90 ft in the east (Figure 7). Along the south-east crop up to 200 ft of Lower Coal Measures may be present in Coed-y-Darren, but to the north-east their outcrop narrows rapidly, and they reach their minimum total conjectured thickness of only 100 ft to the east of Twmbarlwm. Between Henllys Colliery [ST 2880 9400] and Upper Cwmbran their estimated thickness is 130 to 150 ft, though local thickening occurs at Llanderfel [ST 2659 9538].

The measures of the lower group are very variable, ranging from mudstones to coarse quartzitic sandstones, and containing one persistent coal, the Sun Vein. The beds are known only from exposures and mining records along their outcrop. The upper group consists mainly of argillaceous rocks and contains several coals, a few of which are economically important. The sequence above

Within the Lower Coal Measures marine faunas are known from four horizons, all in the lower group of beds (Figure 8). The Gastrioceras subcrenatum Marine Band at the base is the thickest, but it has not been proved as widely as the marine band above the Sun Vein. The other two are relatively poorly developed, each being known at only one locality. Mussel faunas occur at a few horizons, the most notable being the 'pseudorobusta' fauna, which occurs between the Garw and Gellideg seams, and the 'Yard' fauna. The fish-band above the Garw, persistently developed in this district, has proved to be a useful marker horizon.

(a) Measures below the Garw Seam

The complete sequence of these beds is known only from Rudry Borehole, and the following account is based largely on the succession proved there, though references are made to other localities where relevant details are available. The measures are about 300 ft thick (corrected for dip) in Rudry Borehole, and also around Nant-y-Brynau just east of the River Taff. They thin eastwards to just under 200 ft at Risca and 150 ft near Upper Cwmbran, before thickening again to 175 ft at Pontypool (Figure 8).

The Gastrioceras subcrenatum Marine Band is nearly 50 ft thick in Rudry Borehole, and contains a rich fauna of horny brachiopods, mollusca and fish (Woodland and others 1957, p. 54) (Appendix I). The band has been proved at two localities west of the borehole, and persists to the east. Above the marine band at Rudry the succession is continued by nearly 25 ft of mudstone containing Planolites ophthalmoides Jessen, then by 36 ft of silty mudstones and striped beds with a few P. ophthalmoides overlain by a thin marine band containing bivalves. This marine phase has been equated (Woodland and others 1957, p. 54) with a thicker marine band at Margam, which is thought to be equivalent to the M1 horizon (Leitch and others 1958, pp. 466, 469) of the north crop. It is followed by nearly 50 ft of striped beds and sandstone, then by a thick seatearth underlying the Sun Vein, the probable equivalent of the Crows Foot Coal of the Pontypridd district (Woodland and Evans 1964, p. 35). The Sun Vein is widely known along the south and east crops, and is thick enough to have been worked at a number of localities.

The roof mudstones of the Sun Vein commonly yield a marine fauna (Moore 1945, p. 151 and 1948, p. 271), the most abundant fossils being Lingula mytilloides and Orbiculoidea sp., and others include Productus carbonarius and mollusca. This marine band has been correlated (Squirrell and Downing 1964, p. 127) with the M2 Marine Band of Leitch and others (1958) along the north crop; it is also believed to be present at Margam (Woodland and others 1957, p. 53). Locally it is absent, e.g. in Waun Fawr Quarry near Risca, where only plants have been found above the coal.

The succession is continued at Rudry by nearly 30 ft of unfossiliferous silty mudstones on which rest a thick series of quartzitic sandstones. In Waun Fawr Quarry the equivalent argillaceous beds are nearly 50 ft thick, and some 32 ft above the Sun Vein there are 18 ft of mudstones and silty mudstones containing a poor marine fauna of L. mytilloides and fish. Although no diagnostic fossils have been found in these marine rocks, they may be equivalent to the Cefn Cribbwr Marine Band (Woodland and Evans 1964, p. 35) of the Margam No. 1 Borehole, and the M3 horizon of Leitch and others (1958) along the north crop. They are the only marine beds known at this horizon in the Newport district; their absence in Rudry Borehole has been attributed (Woodland and others 1957, p. 53) to faulting, though it is possible that they have been replaced there by the basal sandstones of the Cefn Cribbwr Rock (Figure 8). This arenaceous division is about 65 ft thick at Rudry, and is followed by 35 ft of argillaceous beds with a few sandstones. In the top part of these a thin Planolites phase (Figure 8) may equate with the Planolites/plant phase beneath the Margam Marine Band at Margam, which Woodland and Evans (1964, p. 37) equate with the M4 horizon of Leitch and others (1958) on the north crop. The 35 ft of beds are represented by thin mudstones with coal around Nant-y-Brynau (p. 128), and by 15 ft of mudstones with an 18-in coal near the base at Pontypool. Although no marine mudstones have been found associated with this seam at these localities, it can be tentatively correlated with the thin coal underlying the Margam Marine Band in the Pontypridd district.

The 30 to 40 ft of measures up to the Garw along the south crop and at Pontypool are mainly sandstones; in Rudry Borehole two irregular erosion surfaces within the beds are considered (Woodland and others 1957, p. 52) to indicate non-sequences, and to account in part for the extreme thinness of this part of the succession when compared with the 320 ft of equivalent beds at Margam.

(b) Garw to Amman Rider

A generalized vertical section of these measures is given in (Figure 9), and the detailed correlation of the principal coals is shown in (Plate 6).

The Garw, proved at a few localities only, varies in thickness from 2 to 6 inches in the western part of the district to 10 in at Rudry and Tirpentwys, and 2 ft near Pontypool. Its roof is typically dark grey, carbonaceous mudstone containing abundant fish remains and rare P. ophthalmoides and mussel fragments.

The measures between the Garw and Gellideg reach their known maximum thickness of 107 ft at Abercynon, and thin eastwards to 34 ft near Pontypool. They consist mainly of mudstones, with a few sandstones which are thickly developed locally. Ironstones, in the form of nodules and thin beds, are common throughout, and have been worked as the 'Spotted Mine' from just above the Garw near Pontypool. Thin, impersistent coals are known at three horizons. One seam lies 10 to 17 ft above the Garw at Abercynon and near Pontypool, and two higher coals with fossiliferous mudstone roofs are known in the southwestern part of the district. P. ophthalmoides and P. sp.occur at several horizons between the Garw and Gellideg. Seventy-seven and 56 ft below the Gellideg at Abercynon and Nantgarw respectively, mudstones contain a 'pseudorobusta' band, which at Nantgarw yielded a typical fauna of Carbonicola acuta (J. Sowerby), C. pseudorobusta Trueman, the ostracod Geisina arcuata (Bean) and fish.

The Gellideg is commonly the bottom coal of a composite seam which also includes the Lower and Upper Five-Feet. It is thickest in the north-west, varying from 24 to 32 in of clean coal, but gradually thins southwards and eastwards to 9 in or less. In the lower parts of the Sirhowy and Rhymney valleys, and in a wide belt along the south and east crops (Figure 10), the Gellideg, if present, cannot be recognized, for it is inseparable from the lower part of the Five-Feet. Over the central and south-western parts of the area the seam is subject to extensive washouts. The roof of the Gellideg, because of the close proximity of the Five-Feet, is most commonly seatearth, or silty mudstone with roots.

The Gellideg and Lower Five-Feet are separated by 2 to 33 ft of argillaceous beds in the north-west, and generally by less than 2 ft in the remainder of the area, though locally this figure may be exceeded, e.g. in parts of Tirpentwys.

The Five-Feet is a single coal in the lower parts of the Rhymney and Sirhowy valleys, and in a wide belt along the south crop between the Taff and Ebbw valleys. Elsewhere it usually consists of two closely lying leaves, the

Lower and Upper Five-Feet, totalling 3 to 4.5 ft. Locally, it is poorly developed, or even absent, e.g. around Windsor. East of the Rhymney Valley the two. leaves are more or less of equal thickness, but to the west there is considerable variation and one of the leaves, most commonly the upper, is usually much thicker than the other. A further split into three leaves takes place at scattered localities. The roof mudstones of the Five-Feet usually contain plant remains only, though at Celynen North they have yielded Planolites montanus Richter and two species of Curvirimula, and, at Wern-ddu Claypit, Moore (1945, p. 157) found "Anthraconauta minima".

The Five-Feet and Seven-Feet are separated in the north-west by 40 to 85 ft of argillaceous beds with thin sandstones, which decrease in thickness southwards and eastwards to between 15 and 40 ft, and, locally, to as little as 7 ft. Thin coals at two horizons within the succession are present in the northwest and thick seatearths underlying the Seven-Feet are common locally e.g. at Oakdale where they reach 20 ft.

The Seven-Feet and the Yard comprise an extensively worked, usually closely lying group of seams, totalling 6 to 9 ft in the west and north-east, and 4 to 6 ft in the remainder of the area. In the upper Rhymney and Sirhowy valleys, and in the south and south-west, the Seven-Feet and the Yard usually consist of one coal each of very variable thicknesses, and locally they combine to form a single, clean coal, which is also joined by the overlying Amman Rider in some southerly areas, e.g. at Bedwas. In the north-west, north-east and east the Seven-Feet and the Yard comprise three, or less commonly four coals, of which the lower two or three constitute the Seven-Feet, and the upper one or two the Yard. The top coal of the Seven-Feet is generally the thickest, usually ranging from 2 to 3.5 ft, and the lower one or two range from 1 to 2.5 ft each. The coals of the Seven-Feet lie within 2 ft of each other over much of the area, except at Celynen South, and in the Abercynon-Albion area, where up to 6 ft and 14 ft of strata respectively intervene.

In the north-west, strata separating the Seven-Feet and the Yard increase rapidly in thickness to over 20 ft, and reach a maximum of 37 ft. The roof of the Seven-Feet is generally seatearth, but, where the interval between the Seven-Feet and Yard is appreciable, it is mudstone, which in Abercynon, Albion and Celynen North has yielded a small fauna including Cochlichnus kochi (Ludwig), Naiadites sp., Lioestheria striata Pruvost (non Miinster) and L. vinti (Kirkby).

The Yard varies from 3 ft or more in the west, to 6 inches in the south-east. In the north-west it consists of two leaves, the lower varying from 1 to 2.5 ft, and the upper from 1 to 10 in. The two leaves are never more than 2.5 ft apart, and usually much less. The roof of the Yard is generally mudstone or silty mudstone, except in the southern half of the area where it is seatearth when the Amman Rider lies close above. In the north-west the mudstones contain a mussel fauna which most commonly contains Anthracosia regularis (Trueman) and Carbonicola venusta Davies and Trueman. Towards the east mussels are rare, and plants more common, e.g. at Tirpentwys.

The Yard and the Amman Rider are separated by up to 45 ft of argillaceous strata in the north, which thin rapidly southwards, so that in the southern half of the area the coals commonly lie within inches of each other or even coalesce. The Amman Rider is commonly a single coal 1 to 2.5 ft thick, but in the northwest it splits into two leaves up to 10 in apart. The lower is 2 to 11 in thick and the upper 12 to 24 in. Locally, the lower leaf splits again giving a three-coal section. The Amman Rider has a high sulphur content and thus is rarely worked.

Along the south-east crop between Coed-y-Darren and Upper Cwmbran very little is known about the beds between the Garw and Amman Rider. In Coed-yDarren, Garw roof mudstones have been found in spoil heaps, but this is the only proof for the presence of any of the beds at outcrop in this area. However, it is possible that the Five-Feet–Gellideg, Seven-Feet, Yard and Amman Rider are all represented along at least part of the outcrop, for they are fully developed in Luther May's Drift in Henllys Colliery, at about 375 yd from the surface. About a mile north-north-east of Henllys Colliery the Seven-Feet and Yard have been worked, as the Meadow Vein, to within 50 yd of the surface, thus it is possible that these two seams and possibly others crop out there. H.C.S.

Middle Coal Measures

The Middle Coal Measures crop out only around the edge of the coalfield. Their lower limit is marked by the base of the Amman Marine Band, and their upper limit by the top of the Upper Cwmgorse Marine Band. In this account the beds have been divided into two groups at the top of the Cefn Coed Marine Band. The lower group varies in thickness from up to 340 ft in the north-west, south-west and north-east, to under 200 ft in the south-east, and the upper group varies from 380 ft in the north-west to less than 150 ft in the east (Figure 15). Along the south-east crop little is known about their true thickness, or the horizons represented, but it is estimated that at the eastern end of Coed-y-Darren the entire Middle Coal Measures sequence is not more than 70 ft thick. The nearest proved thickness of Middle Coal Measures is 398 ft at Cwmcarn Colliery only 1500 yd to the north, illustrating the rapidity of the attenuation in this area. A little farther north-east from Coed-y-Darren the thickness of the Middle Coal Measures calculated from the resurvey boundaries is only 35 ft, but, as the lines are highly speculative, little reliance can be placed on this figure. It is possible that the Middle Coal Measures are unrepresented along the mile or so of ground to the south and east of Twmbarlwm, and this might be attributable to the north-easterly increasing unconformity at the base of the Rhondda Beds in Coed-y-Darren (Squirrell and Downing 1964, pp. 130–1). Between Henllys Colliery [ST 2588 9441] and Upper Cwmbran the Middle Coal Measures appear to thicken to as much as 170 ft.

The lower group of beds is dominantly argillaceous, with a few laterally impersistent sandstones occurring most commonly in the upper part. They contain many coals, amongst which are the most economically important seams in the district. The upper group also consists predominantly of argillaceous beds, but sandstones are more common, occurring in some areas in much the same proportion as the mudstones. The beds contain many coals, but none is of more than minor economic significance. Of particular interest are tonstein bands occurring at two horizons, the lower in the roof strata of the Gorllwyn Rider at Hafodyrynys Colliery, and the upper within the Lower Pentre seam at places in the south-west of the district.

Marine faunas are known from seven horizons within the Middle Coal Measures. The lowest, the Amman Marine Band, was proved in every colliery visited, and it appears to persist throughout the area, though it is likely to be absent along parts of the south-east crop. The Cefn Coed Marine Band also occurs widely, but the other four are more restricted laterally. Mussel faunas are found above several seams, the best developed being those associated with the Nine-Feet, Six-Feet and Four-Feet seams.

(a) Amman Marine Band to Cefn Coed Marine Band

A generalized section of this sequence showing the lithology and main faunas is given in (Figure 16).

The Amman Rider and Bute seams are separated by mainly argillaceous beds with a few thin sandstones. These measures are 15 to 65 ft thick, except in the extreme east, where they thin to only 4 to 6 ft. At their base the Amman Marine Band is 3 to 12 ft thick, and consists of medium to dark grey mudstone containing pyritic 'fucoids' and granules, and a variable fauna. In some collieries it is little more than a Lingula band containing mainly L. mytilloides associated with small numbers of forms such as sponge spicules, Planolites ophthalmoides and fish. In other collieries, however, the more varied fauna found only in the south-eastern part of the South Wales Coalfield is present, and it includes foraminifera, sponge spicules, conulariids, crinoids, horny and calcareous brachiopods, gastropods, bivalves, ostracods and fish.

The Bute, a very variable seam, commonly consists over the western two thirds of the area of two more or less equal leaves 1.5 to 2.5 ft thick, separated by a parting which varies irregularly from as little as 8 in to 28 ft. Towards the north-west the lower leaf is impersistent. In the south-west the Bute thickens rapidly locally, e.g. in Ffynnon Wen Borehole 8 ft 3 in of coal in 4 leaves are present. In the eastern one third of the area the Bute consists of a single coal, which is locally split by thin partings. The roof mudstones of the Bute yielded a limited, non-marine fauna (Figure 16) at localities in the south and central parts of the area, including a few mussels of which Naiadites quadrates (J. de C. Sowerby) is the most common.

In the north-west the Bute is separated from the Nine-Feet by as much as 50 ft of mainly argillaceous beds with thin sandstones, but by as little as 10 inches in the east. Variation in thickness of these beds can be rapid over short distances, e.g. from 50 to 5 ft between Deep Navigation and Albion collieries.

The Nine-Feet has been one of the most extensively worked seams in the district, particularly in the north and east where it is virtually worked out in many collieries. In the upper Taff Valley, and to the east of the Sirhowy Valley, the Nine-Feet is 6 to 12 ft thick and consists either of two coals usually lying within 2 ft of each other, or, in much of the latter area, a single, clean coal, In the upper Taff Valley the lower coal is usually one or two feet thicker than the upper, but, where two coals are present east of the Sirhowy Valley, the upper is usually the thicker.

In the remainder of the coalfield area (delineated by the line of the 2-ft parting in (Figure 17)) the two coals, known as the Upper and Lower Nine-Feet, diverge rapidly and may be as much as 50 ft apart, the measures between comprising mainly argillaceous rocks. The Lower Nine-Feet is usually 2 to 5 ft thick and commonly contains a number of dirt partings, more particularly in the south. The seam may be absent locally because of non-deposition or washout, as in Wyllie and parts of Nantgarw. The mudstones above the seam are rarely fossiliferous, but they yielded a small fauna (Figure 16) at Bargoed. The Upper Nine-Feet also varies from 2 to 5 ft, but is generally a cleaner coal, and is worked much more extensively.

The dark mudstones immediately overlying the Nine-Feet at Deep Navigation and Taff Merthyr carry the following large, crushed shells of the Anthracosia phrygiana fauna; Anthracosia beaniana King. A. ovum Trueman and Weir, A. phrygiana (Wright) and A. sp.intermediate between ovum and disjuncta Trueman and Weir. As in the adjacent, north-east part of the Pontypridd district (Woodland and Evans 1964, p. 51) these are overlain at Deep Navigation by paler mudstones containing the 'Red Vein fauna' of mainly small, 'solid' mussels. Elsewhere large mussels are absent, and only the 'Red Vein fauna' has been found above the seam. It includes variants of Anthraconaia fisheri (Wright), A. obscura (Davies and Trueman), A. pulchella Broadhurst, Anthracosia angulata (Chernyshev), A. faba (Wright), A. nitida (Davies and Trueman), A. ovum and A. phrygiana.

The measures between the Nine-Feet and Six-Feet seams, usually ranging in thickness between 40 and 90 ft, thicken locally, and reach their maximum of 135 ft in the Britannia shafts. As little as 20 to 25 ft separates the seams at some localities in the east and south. The beds are mainly argillaceous with sporadic developments of sandstones, which are thickest where the separation is greatest. In the Taff Valley and central part of the Rhymney Valley, coals of the Red Vein group lie 20 to 60 ft above the Nine-Feet and up to 40 ft below the Six-Feet. They consist of a lower coal, normally less than 10 in thick, separated by up to 20 ft of mudstones, from a thicker, upper coal comprising one to three leaves, which usually lie close together, but may be interleaved within as much as 20 ft of beds. Eastwards, the constituent leaves of the Red Vein group either thin gradually and die out, or coalesce with the overlying Six-Feet, except in Celynen North and South collieries where they can still be recognized individually, lying up to 25 ft below the Six-Feet. The 'Red Vein fauna' of small anthraconaioid Anthracosia found in the Pontypridd district (Woodland and Evans 1964, p. 54) was not found in association with the Red Vein group of coals in the present area.

The Six-Feet is a single, compound seam west of a line of split (Figure 18) which runs through Grosfaen, Bargoed, Oakdale and Nine Mile Point. East of the line two distinct seams occur, the Lower and Upper Six-Feet. In the westerly area the complete seam consists most commonly of 5 to 7 ft of coal in two, more or less equal leaves separated by a thin parting. In the north, and at Bedwas and Nantgarw, further thin partings tend to develop in both leaves. North of the Caerphilly Syncline the Six-Feet is an important seam, and has been worked extensively. East of the line of split the Lower Six-Feet, which is seldom worked, consists of a main coal 2 to 3.5 ft thick, commonly closely underlain by a thin leaf usually not more than a few inches thick, though 22 in are present at Cwmbran. A thin leaf also splits off the top of the main coal at a few localities. The roof rocks of this seam are usually unfossiliferous.

The Lower and Upper Six-Feet are normally separated by 25 to 40 ft of mudstones with thin sandstones, though locally these measures thicken to 55 ft. The Upper Six-Feet is 2 to 4 ft thick, and is normally a single, clean coal, though one or more thin leaves may split from its base.

Along the south crop between Wern-ddu and Risca Colliery the Lower and Upper Six-Feet, lying up to 6 ft apart, comprise the basal coals of the 'Big Vein Group', a closely lying association of all the seams between the Lower Six-Feet and the coal underlying the Hafod Heulog Marine Band.

In much of the west and south-west of the district the mudstone roof of the Upper Six-Feet is characterized by a mussel fauna dominated by species of Anthracosia, with a few Anthraconaia and Naiadites (Figure 16). The most common are variants of Anthracosia angulata, A. atra (Trueman), A. concinna (Wright), A. elliptica (Chernyshev), A. lateralis (Brown) and A. ovum. In more easterly areas the fauna is generally poorly represented by small, scattered, crushed and fragmentary shells only.

The thickness of measures between the Six-Feet and Four-Feet varies considerably, and tends to fluctuate rapidly over short distances. The two seams are most commonly 20 to 70 ft apart, but the separation increases to 105 ft at Bargoed and falls to 12 ft or less at Bedwas, Hafodyrynys, Britannia and the northern part of Abercynon. In the south-east, as at Risca, less than 2 ft separate the seams, the approximate line of split running as shown in (Figure 18). The beds between the seams are usually mudstones, though sandstones develop in the north-east.

The Four-Feet, extensively worked in the north-west, comprises a group of coals subject to complex splitting and thickness variations. In the upper Taff Valley the seam commonly consists of up to four leaves lying close together, the middle two, 1 to 3 ft thick, being the main worked coals. Eastwards, in the upper Rhymney Valley, four or five leaves, totalling 4.5 to 5.5 ft of coal, are usually present, and in the southern part of Penallta they are spread through 30 ft of measures. Traced eastwards again, to the northern part of the Ebbw Valley and the Glyn Valley, the Four-Feet consists typically of a main coal about 3 ft thick, closely overlain by up to three leaves, 6 to 10 in thick, separated by thin dirt partings. In the southern part of the Ebbw Valley and at Cwmcarn the main coal is 1 to 3 ft thick, and is overlain, or underlain, by variable thin leaves.

In much of the southern half of the area to the west of the Ebbw Valley, the Four-Feet splits into two distinct seams, the Lower and Upper Four-Feet, which are probably equivalent to the two, main worked coals of the upper Taff Valley. They are separated by 5 to 24 ft of mudstones. The Lower Four-Feet is very variable, ranging from only 5 in at Llanbradach to 4 ft 7 in in four leaves at Windsor. The overlying mudstones have yielded Cochlichnus kochi and a few Naiadites. The Upper Four-Feet is usually a single, clean coal, 2 to 3 ft thick, though locally thinning to only a few inches. In the south-east, at Wern-ddu, Rudry and Risca, the Lower and Upper Four-Feet lie close together and form the middle part of the 'Big Vein Group'.

The immediate roof of the Four-Feet is generally mudstone, though, in the north-east, sandstones may occur immediately above the seam. At Abercynon a 9-in conglomeratic sandstone is present locally, and at Cwmcarn a dolomitic conglomerate, up to 5 ft thick, lies above the seam. The roof mudstones carry a rich mussel fauna in the upper Taff Valley, particularly at Abercynon and Albion. It includes variants of Anthraconaia cymbula (Wright), A. ellipsoides Weir, Anthracosia acutella (Wright), A. atra, Anthracosphaerium exiguum (Davies and Trueman), Naiadites alatus Trueman and Weir, N. productus (Brown) and N. obliquus? Dix and Trueman. In most other parts of the area the fauna is less well developed and usually consists of only scattered, small, crushed or fragmentary shells.

The Four-Feet and Two-Feet-Nine are 20 to 50 ft apart in the northern half of the area, except locally, where the thickness increases to 70 ft and falls to less than 2 ft, as at Grosfaen. In the southern half of the area, less than 20 ft of beds separate the seams, and along parts of the south crop under 6 ft. The rocks are predominantly argillaceous, only a few thin sandstones occurring in the sequence, except in the north-east where up to 50 ft of quartzitic sandstones comprising the 'Elled Rock' lie immediately, or close above, the Four-Feet.

The Two-Feet-Nine is rarely worked because of its high dirt content. The seam is best developed in, and to the west of the Sirhowy Valley, where 2 to 4.5 ft of coal in 2 to 4 unequal leaves are separated by partings varying from 1 in to 10 ft. Between Wern-ddu and Risca Colliery the seam forms the upper part of the 'Big Vein Group', and probably also includes the thin coal which underlies the Hafod Heulog Marine Band in areas to the west. East of the Sirhowy Valley the Two-Feet-Nine deteriorates rapidly, and in the Ebbw and Glyn valleys consists only of a single coal, 32 in thick at Celynen South, and 12 in or less elsewhere. East of a line joining Pontypool, Abercarn and Risca the seam is either absent, or no more than a few inches thick. The mudstones overlying the seam have yielded only shell fragments.

The Two-Feet-Nine and Cefn Coed Marine Band are usually separated by 40 to 70 ft of variable measures in the west, and 25 to 60 ft in the east. The rocks are poorly exposed along their narrow outcrop, and can be seen in only a few collieries. They contain as many as six thin coals in the west, all of which die out eastwards. To the west of the Ebbw Valley the measures between the Two-Feet-Nine and the coal underlying the Hafod Heulog Marine Band are normally 15 to 30 ft thick, though they may be quite thin locally, e.g. only 2 ft 9 in at Britannia, and, as mentioned above, the two coals coalesce in the south-east. The rocks are commonly argillaceous, but quartzitic sandstones up to 25 ft thick are present at Taff Merthyr, Albion, Universal, Llanbradach and Bedwas. These sandstones probably equate with the Lower Cockshot Rock of the Pontypridd district. The coal underlying the Hafod Heulog Marine Band is normally the first coal above the Two-Feet-Nine, but one other thin coal occurs at Abercynon, and two occur at Deep Navigation. The seam below the marine band varies from a streak to 25 in, and may be split into two or three leaves separated by thin partings. Locally it is absent, as in the Gypsy Lane Borehole, where the Hafod Heulog Marine Band rests directly on seatearth. In the Ebbw Valley and at Tirpentwys the only coal between the Two-Feet-Nine and the seam directly underlying the Cefn Coed Marine Band is believed to be the easterly equivalent of the coal underlying the Hafod Heulog Marine Band, but no marine strata have been proved above it.

The Hafod Heulog Marine Band, only proved west of the Ebbw Valley, is up to 5 ft thick and usually contains little other than Planolites ophthalmoides and horny brachiopods. At Risca the fauna also included the marine ostracod Hollinella, and the brackish-water form Geisina.

The Hafod Heulog Marine Band and the Cefn Coed Marine Band are commonly separated by 10 to 25 ft of measures, though locally by as much as 40 ft. The rocks are usually argillaceous, but at Universal and along the south crop sandstones and mudstones occur in approximately equal proportions. The equivalent beds in, and to the east of the Ebbw Valley are usually dominantly arenaceous. In the north-west of the district one or two thin coals lie within these beds; the lower may equate with the thin co2l underlying the Britannic Marine Band (Woodland and Evans 1964, p. 39) of the Pontypridd district, though no marine strata were proved above it during the resurvey of the present area. At New Rockwood, in the No. 2 Borehole, the lowest known red mudstones in the Newport district were proved between the Hafod Heulog and Cefn Coed marine bands.

The Cefn Coed Marine Band has been identified at several localities throughout the district. It normally comprises up to 5 ft of marine mudstones, but may be thicker locally, e.g. in the Hafodyrynys No. 4 Drift, where 25 ft of marine strata occur. The fauna of the marine band consists of abundant Planolites ophthalmoides and horny brachiopods; more rarely foraminifera, sponge spicules, Paraconularia sp., crinoids, gastropods, bivalves, goniatite fragments, ostracods and fish are pi-sent.

(b) Cefn Coed Marine Band to Upper Cwmgorse Marine Band

A generalized section of these measures, showing their lithology and principal faunas is given in (Figure 21).

The beds between the Cefn Coed Marine Band and the Gorllwyn Rider are 30 to 70 ft thick in the east and south, and 60 to 120 ft in the north-west. At Abercynon they thicken locally to 150 ft. The beds are predominantly argillaceous, but sandstones develop at various horizons in different parts of the area, commonly in the upper part of the succession in the north and north-east, and locally in the lower part in the south. The Gorllwyn is poorly represented in this district, being definitely known only in the shafts at Albion, Abercynon and Grosfaen, where it is a thin coal 15 to 26 ft below the Gorllwyn Rider. Two thin coals 8 to 16 ft below the Gorllwyn Rider at Tirpentwys and Hafodyrynys may also equate with the Gorllwyn horizon.

The Gorllwyn Rider, present almost everywhere, is most commonly a single, clean coal 3 to 36 in thick, though splitting into two leaves at Bargoed, Britannia, Nine Mile Point and Crumlin, and into three leaves at Llanhilleth and Tirpentwys. In a previous publication (Squirrell and Downing 1964, p. 126 and (Plate 7)) the Gorllwyn Rider at Tirpentwys and Crumlin was inadvertently called the Gorllwyn. The Gorllwyn Rider has never been worked commercially on more than a very small scale. The roof mudstones contain Anthraconaia adamsii? (Salter) at Crumlin and Naiadites cf. subtruncatus (Brown) in the Hafodyrynys No. 4 Drift. In the Hafodyrynys Nos. 1, 2 and 4 drifts the beds overlying the seam contain a band of tonstein up to 3.5 in thick, separated from the coal by between 2 in and 5 ft of measures.

The cyclothem above the Gorllwyn Rider is 13 to 25 ft thick in the west, but thins eastwards to 7 ft at Bedwas and Nine Mile Point and to only 8 in at Risca. The beds are mainly mudstones, though sandstones have been proved in some collieries, e.g. Taff Merthyr, where the sequence is mainly arenaceous. The next coal, the Eighteen-Inch, is present only south and west of a line drawn through Cwmcarn, Wyllie and Oakdale, except possibly at Crumlin, where the seam may be represented by the thin coal 21 in above the Gorllwyn Rider. The Eighteen-Inch averages 6 to 17 in, and is usually a single, clean coal. At Taff Merthyr, where the thickness is 26 in, two equal leaves are separated by 11 in of seatearth. In Ffynnon Wen Borehole the mudstones overlying the seam yielded Anthraconaia cf. adamsii.

The Eighteen-Inch and Lower Pentre are separated in the north-west by 20 to 44 ft of argillaceous beds, with locally developed sandstones. The measures thin southwards and eastwards, and along much of the south crop the two seams either lie within inches of each other, or unite. The Lower Pentre is present over most of the area west of the Ebbw Valley, but elsewhere is known only at Crumlin, Celynen North and Tirpentwys. In the west and south the average thickness is 1 to 2 ft, usually in two unequal leaves separated by a thin parting. To the east the seam thins to a single coal, normally less than 12 in thick, though at Crumlin a split into two leaves again occurs. The parting proved to be tonstein at Bedwas, Nantgarw, Wern-ddu Claypit and in Gypsy Lane Borehole, and tonstein is possibly present in a number of other collieries in the west and south, where a parting in the seam was recorded during the shafts sinkings. Only plant remains have been found in the mudstones overlying this seam.

In the west and north-west the Lower Pentre is separated from the Pentre by 20 to 60 ft of argillaceous beds, with sandstones locally developed. To the east the beds thin gradually, and are 10 ft thick or less at Crumlin and Tirpentwys, and only 2 ft 9 inches in Wern-ddu Claypit. The Pentre averages 8 to 18 inches in the west and north-west, and occurs either as a single coal, or as two, usually unequal leaves separated by a parting, which, although normally thin, may attain 4.5 ft. To the east, the seam thins gradually, and dies out east of the Ebbw Valley, except at Tirpentwys where it comprises two 6-in leaves separated by 12 in of seatearth. Mudstones usually overlie the Pentre, but they have yielded only plants and traces of annelids.

Ten to 30 ft of mainly argillaceous measures intervene between the Pentre and Pentre Rider in the west and north. These thin to the east and south to no more than 2.5 ft at Wern-ddu. Mainly sandstones comprise the 10 to 15 ft of beds between the seams at Bedwas, Risca and Abercarn. The Pentre Rider is present almost everywhere to the west of a line connecting Cwmcarn, Hafodyrynys and Pontypool; to the east it is thought to be absent, but the seam may be present in the No. 2 Drift at Hafodyrynys (p. 176). The Pentre Rider averages 11 to 22 in of clean coal in the west, but thins eastwards to as little as 3 inches in the Ebbw Valley.

The roof strata of the Pentre Rider are very variable. In the Taff, Rhymney and Sirhowy valleys sandstone overlies the seam in a number of collieries, and in one of the cross-measures drifts at Nantgarw up to 15 in of conglomerate are present. Elsewhere it is overlain by the mudstones of the Foraminifera Marine Band, which usually contain a fauna rich in foraminifera including Agathamminoides sp., Glomospira sp., Glomospirella sp., Hyperammina sp.and Tolypammina sp., common P. ophthalmoides and a few L. mytilloides and fish. The bivalves Nuculopsis cf. gibbosa (Fleming) and Schizodus axiniformis (Phillips) occurred near the middle of the band in Rudry Borehole. The upper part of the marine phase in Ffynnon Wen Borehole (Appendix I) contains a similar fauna to that given above, but with the addition of Myalina sp., Geisina sp.and Hollinella cf. bassleri (Knight), an association which suggests that the Five Roads Marine Band may be represented, though in the type area (the Gwendraeth district, Archer, in Eyles 1956, p. 34) foraminifera are not found in the band. A similar development occurs in Rudry Borehole (Appendix I), some 16 ft above the Foraminifera Marine Band, the two marine phases being separated by slightly siltier mudstones containing plant debris.

The beds between the Foraminifera Marine Band and the next persistent seam, the coal underlying the Lower Cwmgorse Marine Band, consist of 35 to 90 ft of variable measures. In a few collieries, e.g. Abercynon, the beds are mainly sandstones, but in most others they are mainly mudstones with varying proportions of sandstone, which occur most commonly in the middle and lower parts. In a small number of collieries west of the Sirhowy Valley, a thin coal called the Abergorky lies close below the coal underlying the Lower Cwmgorse Marine Band. The seam is usually only a few inches thick, though it reaches 12 inches in the Nantgarw shafts. The most westerly known occurrence is in Rudry Borehole, where 2 in of coal occur at this horizon. The beds between the Abergorky and the succeeding coal are up to 20 ft thick, and usually consist of mudstones and seatearths, though sandstones occur locally.

The coal underlying the Lower Cwmgorse Marine Band is present almost everywhere, and is known to be absent only in the Hafodyrynys Nos. 1 and 2 drifts, and in Coed-y-Darren north of Risca. At these localities the presumed horizon of the base of the seam is marked by the top of a poor seatearth. The coal averages 4 to 10 in, though at a few collieries the thickness varies from 12 to 16 in. The roof of the coal is usually the mudstones of the Lower Cwmgorse Marine Band, though sandstones occur locally, e.g. in Ffynnon Wen Borehole. The marine band has been proved at Tirpentwys and in five collieries in the south, and it is likely to be present wherever mudstones overlie the coal. Normally, the marine strata are not more than a few feet thick, but in Rudry Borehole marine fossils extend through some 26 ft of beds, though there is a barren run of about 16 ft in the middle. The fauna consists of common Lingula mytilloides, and more rarely occurring L. cf. squamiformis Phillips, Orbiculoidea sp., Euphemites anthracinus (Weir), Dunbarella sp., ostracods and fish.

The measures between the Lower Cwmgorse Marine Band and the next overlying coal, the Hafod, are 50 to 80 ft thick in the west and 30 to 50 ft in the east. They are mainly argillaceous with subordinate sandstones in the upper part, though at a few collieries, particularly in the west and south-west, sandstones predominate. The Hafod, normally a single, clean coal, is 15 to 26 inches in the north-west, but thins to between 3 and 12 inches in the east and south-east. The seam is absent locally, e.g. in parts of Nantgarw, where thick sandstones replace both the coal and the mudstones which normally overlie it.

The Upper Cwmgorse Marine Band, overlying the Hafod, varies in thickness from only about 2 ft to as much as 28 ft (in Gypsy Lane Borehole) though a thickness of less than 14 ft is more usual. It has yielded a rich fauna in which L. mytilloides and Orbiculoidea cf. nitida (Phillips) are fairly common, in association with more scattered foraminifera, sponge spicules, archaeocidarid spines, horny and calcareous brachiopods, gastropods, bivalves, goniatites and fish (Figure 21). H.C.S.

Upper Coal Measures or Pennant Measures

The Upper Coal Measures are up to 2,800 ft thick, and comprise a large part of the Coal Measures outcrop. Pennant sandstones^‡7  form an appreciable part of the sequence and are responsible for the characteristic, topographical features of the coalfield. These sandstones are interbedded, at irregular intervals, with relatively thin argillaceous horizons containing coals, and this alternation of lithology gives rise to a topography of steep slopes broken by flat, or only gently sloping, bench-like features ('slacks') along which the mudstones, coals and seatearths crop out. These features proved to be of considerable value for mapping and correlation purposes, enabling the mudstones to be traced for considerable distances. Of the many coals occurring in the Upper Coal Measures only three have been exploited to any large extent, namely the No. 2 Rhondda, the Brithdir and the Mynyddislwyn, though a number of others, such as the Coed Caedyrys and Big Rider, have been worked on a smaller scale. The basal sandstones of the series are white, or pale grey orthoquartzites, similar to those found in the Lower and Middle Coal Measures, and these are followed, at a lower stratigraphical horizon in the west than in the east, by typical pennant sandstones. In the Taff Valley pennant-type sedimentation was firmly established by Rhondda Beds times, but it was not until after the formation of the Brithdir seam that this diachronous facies reached the eastern part of the district, e.g. at Hafodyrynys Colliery.

Marine horizons are unknown in the Upper Coal Measures, but mussels and plants are common. Mussel horizons are not distinctive, however, and thus they are of little value in detailed correlation. Two broad zones, the Anthraconauta phillipsii and A. tenuis zones, are separated, somewhat arbitrarily, at the Cefn Glas horizon. Well preserved plant remains were collected from several horizons, and these are listed in the details.

There are differences of opinion concerning the detailed correlation within the Llynfi and Rhondda beds of this district. Moore (1945 and 1948) believes that the No. 2 Rhondda seam is cut out by an unconformity in the Taff Valley, along the south crop and in the Caerphilly Basin; also that a second unconformity reduces the sequence by 160 ft between Abercarn and Cwmcarn collieries. In the north-east of the coalfield Blundell (1952) claims that an unconformity cuts out the top part of the Middle Coal Measures, the Llynfi Beds and the lower part of the Rhondda Beds. The resurvey suggests that the No. 2 Rhondda is present over much of the coalfield area, and that the absence of the seam in some areas can be attributed to non-deposition or washout conditions. The only area where the absence of the coal is probably due to unconformity is within a limited area along the south-east crop. The thinning of the measures between Abercarn and Cwmcarn collieries is explicable as little more than normal attenuation. In the north-east the beds considered by Blundell to be absent are thought to be represented, and the thinner sequence there can again be attributed to attenuation. The evidence supporting the writer's views has been given in some detail previously (Squirrell and Downing 1964, pp. 121–125) and will not be repeated here.

A feature of the Upper Coal Measures of the Newport district is the presence of red and green mudstones, seatearths and, less commonly, siltstones, interbedded with normal grey mudstones and white quartzites. These are the Deri Beds of Howell and Cox (1924), and are found mainly in the Llynfi and Rhondda beds, though there are a few records of red coloration in the upper part of the Middle Coal Measures. The origin of the coloration has been the subject of debate; Howell and Cox implied that the rocks were coloured at the time of deposition, but Blundell and Moore (1960) suggest that the coloration was secondary and of Permo–Triassic age. The writer believes that reddening was contemporaneous or penecontemporaneous with deposition, and that the beds partly originated as flood-plain deposits derived from red soils in an upland source area, and partly by the oxidation under tropical weathering conditions of grey Coal Measures mudstone uplifted to form a subaerial delta. The subject has been fully discussed in a recent publication (Downing and Squirrell 1965, pp. 45–56) and will not be amplified here.

The believed absence in Monmouthshire and east Glamorgan of any representative of the Swansea Beds, which are 1500 ft thick west of the River Neath, led Woodland and others (1957, p. 9) to infer the presence of a large unconformity below the Mynyddislwyn seam. The resurvey has confirmed this to be the most likely explanation for the absence of the Swansea Beds, and thus, conforming with the vertical sections on the published maps of the adjacent Pontypridd (248) Sheet, the unconformity has been drawn between the Mynyddislwyn and the underlying seatearth in sections on the maps of this district.

(a) Llynfi Beds

The Llynfi Beds crop out along a faulted belt of ground, never more than 450 yd wide, around the rim of the coalfield. They extend from the top of the Upper Cwmgorse Marine Band to the base of the No. 2 Rhondda seam. Their maximum thickness of about 340 ft is attained in the south-west (Figure 22) and they thin eastwards by gradual attenuation and small scale non-sequences to around 100 ft at Nine Mile Point and Cwmcarn. The minimum known thickness of Llynfi Beds is 27 ft 10 in in Coed-y-Darren north-east of Risca. The thin succession there may be due partly to internal attenuation, and partly to the unconformity at the base of the overlying Rhondda Beds (Squirrell and Downing 1964, pp. 130–1). North-east of Coed-y-Darren the Llynfi Beds probably thin further along the mile or so of ground to the south and east of Twmbarlwm, and, although a thin outcrop has been shown on the published six-inches to one mile geological map, they could even be cut out completely by the above-mentioned unconformity.

The measures from the Upper Cwmgorse Marine Band to the next overlying coal, the Blackband, comprise 45 to 70 ft of argillaceous beds, with a few sandstones developed locally. The Blackband is persistently developed only in the extreme west of the district, and is usually not more than 8 in thick; its most easterly known occurrence is at Bedwas. Only fish and plant remains have been found in the roof mudstones of this seam.

The Blackband is usually separated from the overlying Tormynydd by only a few feet of mudstone, though up to 21 ft have been recorded at Abercynon. The Tormynydd occurs in the western half of the area only, and is generally 3 to 8 in thick. Its roof is usually mudstone containing plant remains. Between the Tormynydd and the No. 3 Rhondda there are up to 20 ft of mudstones with locally occurring sandstones, e.g. at Abercynon. The No. 3 Rhondda occurs almost everywhere, and varies from a few inches to 3.5 ft. Best developed in the south-west, the seam has been worked on a small scale along the crop between the Taff Valley and Wern-ddu. Its roof is variable, most commonly being mudstone varying to sandstone.

Between the No. 3 Rhondda and the Taldwyn lie 20 to 120 ft of variable measures. In the south-west a coal up to 12 in thick lies 8 to 14 ft above the No. 3 Rhondda, and this may be the equivalent of the Rock Fach of the south crop in the Pontypridd district (Woodland and Evans 1964, p. 66). The Taldwyn is a distinct seam up to 30 in thick in the west, but eastwards it splits and thins, and correlation becomes uncertain, though it is probably represented as far east as the Ebbw Valley. The roof is usually mudstone, though sandstones may occur, as at Abercynon. Above the Taldwyn the succession is continued by up to 46 ft of mainly argillaceous beds to the next overlying coal, the Gilfach. This seam is best developed in the south-west, reaching 37 inches in two unequal leaves up to 5 ft apart. To the north-east and east the coal gradually dies out, and, like the Taldwyn, its correlation becomes uncertain. The roof of the Gilfach is generally mudstone, and has yielded plants and the strap-like markings named as Gyrochorte carbonaria Schleicher.

The measures between the Gilfach and No. 2 Rhondda generally consist of 30 to 80 ft of mainly argillaceous beds in which one thin coal has been recorded in several collieries. In the south-west, rapid local thickening takes place and the strata are 155 ft thick in Ffynnon Wen Borehole, where the succession includes four thin coals with mudstone roofs containing plant remains, Anthraconauta phillipsii (Williamson) and 'Estheria'. About 21 ft below the No. 2 Rhondda at Hafodyrynys Colliery canneloid mudstones with A. phillipsii and Carbonita hundlis (Jones and Kirkby) occur.

(b) Rhondda Beds

The Rhondda Beds crop out mainly along a belt of ground up to 700 yd wide running east from the south-west corner of the district, then north to Pontypool (Figure 23); a small sub-drift outcrop also occurs in the north-west, in the Bargoed Tâf Valley. The Rhondda Beds commence with the No. 2 Rhondda seam, and their upper limit is marked by the Brithdir seam. The beds are thickest in the Taff Valley (670 ft) and thin to about 150 ft along the southeast crop from just north of Risca to Upper Cwmbran (Figure 23). The Rhondda Beds comprise a very variable succession, in which arenaceous and argillaceous rocks occur in approximately equal proportions, except in some easterly areas where the arenaceous strata predominate. Amongst the argillaceous rocks red and green coloured mudstones feature prominently throughout the succession.

The No. 2 Rhondda is the most widely worked seam in the Rhondda Beds, having been exploited extensively in the area between Bedwas and Risca, and on a smaller scale along the south and east crops. Where worked it is usually a single coal 2 to 4 ft thick, with thin, locally-developed partings. The seam is absent in the west-central part of the coalfield area, and is very thin in the south-west around Nantgarw. Its absence is attributed mainly to widespread washout conditions, but non-deposition is also believed to be a cause. In the north the No. 2 Rhondda is present almost everywhere, but although commonly 2 to 3 ft thick it has rarely been worked because of its inferior quality. The roof strata of the No. 2 Rhondda range from conglomerates to mudstones, the latter commonly containing a rich flora which may belong to either Floral Zone G or H of Dix (1934).

In much of the west and central parts of the area the succession is continued by 140 to 195 ft of mainly conglomerates and pennant sandstones, which constitute the Saron Sandstone. Elsewhere mudstones replace the arenaceous rocks to varying degrees, and predominate in some areas, e.g. in the upper part of the Rhymney Valley. To the north-east appreciable thinning takes place and the corresponding strata are represented by probably not more than 50 ft of beds, mainly sandstone, around Tirpentwys and Hafodyrynys. In the northern part of the Rhymney Valley, and also at Wyllie and Nine Mile Point, a bed of rashings, or thin coal, occurs up to 30 ft above the No. 2 Rhondda; this coal horizon may be the equivalent of the Field Vein of the Pontypridd district.

Above the Saron Sandstone lies a group of predominantly argillaceous beds, which are up to 240 ft thick in the west, and contain as many as five coals including the No. 1 Rhondda near the middle, and the No. 1 Rhondda Rider near the top, though the latter may be absent locally. Eastwards, gradual attenuation occurs within this sequence, and in some areas this is associated with the development of more arenaceous strata. In the same direction the coals gradually die out, until, in the south-east, only the No. 1 Rhondda Rider remains within a thin group of mudstones, but, locally, even this coal is absent, as at Crumlin. The coals are not important from an economic standpoint, because of their thin, inferior nature, and only the No. 1 Rhondda, at Llanbradach, has been worked to any appreciable extent.

The roof mudstones of the No. 1 Rhondda are characterized by the presence of the typical zone fossil Anthraconauta phillipsii, but canneloid mudstones from above the No. 1 Rhondda Rider at Llanbradach contain A. aff. tenuis (Davies and Trueman), which is the lowest record of this form in the district, as it is in the Pontypridd district (Woodland and Evans 1964, p. 70).

Between the No. 1 Rhondda Rider and the Brithdir there are 200 to 290 ft of measures in the west, and these thin to 90 ft at Hafodyrynys. They comprise a very variable group of strata, arenaceous and argillaceous rocks occurring most commonly in approximately equal proportions, though in some areas the former predominate, and in others the latter. In the north-west and central areas up to four thin coals lie at irregular intervals within these beds. They rarely exceed 12 in and because of their thin, irregular occurrence correlation of individual horizons is difficult. In the south and east coals are absent. The presence in Rudry Borehole (Appendix I) of Anthraconaia sp.(a genus more characteristic of the Anthraconauta tenuis Zone than the A. phillipsii Zone) together with A. phillipsii, A. tenuis and ' Estheria' sp.at about 50 ft above the No. 1 Rhondda Rider is interesting in view of a similar association in the Pontypridd district (Woodland and Evans 1964, p. 196) about 130 ft above the same seam.

(c) Brithdir Beds

The Brithdir Beds crop out extensively in the lower and upper parts of the Taff Valley, in the lower Sirhowy, Ebbw and Cam valleys, and in a belt of ground, usually not more than 100 yd wide, extending from the south-west corner of the district to Pontypool. The group, extending from the base of the Brithdir to the base of the Cefn Glas, is over 800 ft thick in the south-west, but thins north-eastwards to just under 200 ft to the south-west of Pontypool (Figure 24). The sequence is largely of pennant sandstones, with subordinate mudstones and seatearths, and a few coals.

The Brithdir, at the base, has been worked extensively in a stretch of ground 2 to 3 miles wide, extending south from the northern boundary of the district between the Bargoed Tâf Valley and the easternmost outcrop of the seam. The normal thickness in this worked area is 2 to 4 ft, but to the south the seam. deteriorates rapidly, being represented only by thin, inferior coal or rashes; locally it is absent. In the north-west the roof strata are silty mudstones which have yielded abundant plant remains, e.g. in Bedlinog Colliery. In the remaining part of the area pennant sandstones usually overlie the seam, and form an excellent roof, though thin mudstones with plants may intervene locally.

Above the roof strata of the Brithdir the succession is continued everywhere by pennant sandstones. In the Taff Valley, and parts of the Rhymney and Sirhowy valleys these are interrupted, between 100 and 220 ft above the Brithdir, by a group of mudstones containing the Brithdir Rider. These mudstones are thickest at Nantgarw and Albion, where they are 110 and 70 ft respectively; to the east rapid thinning occurs, and they are not known east of a line drawn through Oakdale and Bedwas. Near the middle of these mudstones a coal, up to 12 in thick, occurs at Nantgarw and Albion, and at the top the Brithdir Rider, up to 11 in thick, occurs at most collieries in the west.

The Brithdir Rider is usually immediately overlain by pennant sandstones, which, 140 to 240 ft higher, pass into the measures associated with the Dirty and Dirty Rider seams; these are present in the area bounded by a line joining Rudry, Risca, Windsor and Nantgarw, and also in the upper Taff Valley. They attain their thickest development of up to 275 ft in a small area east of Rhyd-y-felin in the Taff Valley, where a basal 60 ft of mudstone containing the Dirty Seam, is separated from an upper 15 ft of mudstone containing the Dirty Rider, by 120 to 200 ft of sandstones. A similar succession is also present on the south-westerly slopes of the Taff Valley around Tonteg, but elsewhere the sandstones between the seams are either much thinner, or absent, e.g. in Ffynnon Wen Borehole. A number of coals is known within these measures, but the exact correlation of every seam is not clear. Only the thickest two, the Dirty and Dirty Rider, can be correlated with any certainty. Both are best developed east of Rhyd-y-felin, where the thickness is sufficient for the seams to have been worked from a few levels, though not extensively. The roof mudstones contain Anthraconauta phillipsii, A. cf. tenuis, ostracods and fish.

Above the Dirty Rider the sequence is continued by a maximum of 320 ft of pennant sandstones, with thin mudstones and an occasional coal, to the argillaceous beds underlying the Cefn Glas. The last mentioned are best developed in the west, where 100 to 130 ft of mudstones contain up to four coals, including one up to 24 in thick. Eastwards these measures thin gradually, and the coals die out, until, in the south-east of the area, they are either very thin or absent.

(d) Hughes Beds

The Hughes Beds commence with the Cefn Glas seam, and their upper limit is at the base of the Mynyddislwyn. Their outcrop covers a greater area than any other division of the Pennant Measures. Where completely preserved in the Llantwit–Caerphilly, Abercynon, Blackwood and Llanhilleth synclines, the thickness ranges from 500 to 650 ft, except between Llanhilleth and Tirpentwys where it is about 400 ft. The Cefn Glas consists of 12 to 30 in of coal in the west, south and north-east, but, because dirt partings are common, the seam has only been worked on a small scale at scattered localities. In the upper Rhymney and Ebbw valleys, and in the south-east, the Cefn Glas is usually not more than 12 in thick. The seam and its associated mudstones proved difficult to trace at the surface in the lower Ebbw, Gwyddon and Cam valleys, and consequently the boundary between the Lower and Upper Pennant measures in these areas is largely conjectural.

The roof rocks of the Cefn Glas are commonly mudstones in the west, and pennant sandstones in the east. The mudstones yield a characteristic Anthraconauta tenuis Zone assemblage typified by the first appearance of large A. tenuis, associated with A. phillipsii, Spirorbis, Carbonita spp.and fish.

The mudstones above the Cefn Glas are followed by massive pennant sandstones which, in the south-west and central parts of the area, continue for 100 to 280 ft up to the group of mudstones containing the Daren-ddu and Coed Caedyrys seams. In the north and north-east these argillaceous beds are either thin or absent, and there the pennant sandstones continue, virtually uninterrupted by mudstones, to the thin, argillaceous rocks at the top of the Hughes Beds.

The mudstones associated with the Coed Caedyrys and Daren-ddu seams reach 140 ft in the southern Taff Valley, but they thin and die out towards the north and north-east. Near Senghenydd these measures have yielded Anthraconala aff. pruvosti (Chernyshev) and Anthraconauta phillipsii. The presence of the former genus is interesting in view of its absence from the A. phillipsii Zone of the district. The Coed Caedyrys usually lies near the middle of this argillaceous division, and, where best developed on the eastern side of the Taff Valley near Nantgarw, it consists of up to 43 in of coal with a variable dirt parting. The seam has been worked fairly extensively in this area, and also on a small scale at a few other localities to the north and west. The Daren-ddu lies close to, or at the top of the mudstones, and is usually a thin coal only.

Above the Daren-ddu in the south-west the succession is continued by 380 to 480 ft of pennant sandstones, which contain up to 60 ft of argillaceous rocks near the middle, as well as sporadic, thin, impersistent mudstones throughout. The 60 ft of mudstones include up to three thin coals, and, to the south of Church Village, one of these has been called the Daren-ddu Rider. The highest rocks of the Hughes Beds usually consist of not more than 20 ft of mudstones and seatearths underlying the Mynyddislwyn.

(e) Grovesend Beds

The Grovesend Beds extend from the base of the Mynyddislwyn seam to the highest Coal Measures known in the district. The beds crop out in the Llantwit–Caerphilly Syncline in the south, and in the synclinal belt of the Abercynon, Blackwood and Llanhilleth synclines in the north. The thickest development, some 500 ft, occurs in the south, in the small outcrop to the west of the Taff Valley. East of the Taff a larger outcrop, about 4 square miles, exists around Caerphilly, and there the beds attain a thickness of 460 ft. In the north up to 350 ft of Grovesend Beds extend over about 7 square miles between Nelson and the Rhymney Valley (the Gelligaer area), about 460 ft over 12 square miles between the Rhymney and Ebbw valleys (the Blackwood area), and 160 ft to the east of Llanhilleth (the Cefn Crib area).

The Mynyddislwyn (called the No. 3 Llantwit to the west of the Taff Valley), has been exploited extensively. In the southern outcrops the seam is a single coal, 30 to 55 in thick, but in the northern outcrops it consists of two distinct coals, the Lower and Upper Mynyddislwyn. The former varies from 19 to 36 in, and in some areas contains several dirt partings. Because it is thinner and of poorer quality than the upper coal, workings are only on a small scale. The mudstone and seatearth between the two coals varies from less than 12 inches in the west to over 30 ft in the south-east (Figure 25). The Upper Mynyddislwyn is usually 3 to 4.5- ft of clean coal and is virtually worked out. The immediate roof is usually mudstone, though sandstone occurs in some areas. The former has yielded plant remains, mussels and ostracods, and the abundance and variety of Leaia is a feature of this horizon.

In the northern outcrops, to the west of a line joining Hengoed and Rhos-wen, the Mynyddislwyn is overlain by a maximum of 140 ft of mainly argillaceous beds, but east of the line up to 90 ft of pennant sandstones develop above the roof mudstones of the coal. In the southern outcrops the equivalent beds consist of a variable succession of mudstones and sandstones. Between 50 and 120 ft above the Mynyddislwyn the sequence contains a group of mudstones including a number of coals, one of which is named the Small Rider. In the northern outcrops this seam is usually thin, but the thickness is greater in the south, and small-scale workings west of the Taff have exploited a thickness of some 3 ft, the seam being named the No. 2 Llantwit. The roof mudstones contain typical non-marine mussels and ostracods.

In the Gelligaer area (p. 206) a thick blackband ironstone, not far below the Small Rider, is associated with bituminous shales which have yielded a rich fauna including Anthraconauta phillipsii, A. tenuis, ostracods and Leaia.

Above the mudstones associated with the Small Rider in the northern outcrops the succession is continued by 60 to 110 ft of pennant sandstone, followed by 60 to 160 ft of mainly argillaceous beds in which a number of coals are known. In several areas one of the thickest of these is usually called the Big Rider, but, because of the difficulties of correlation in a thick series of poorly recorded and exposed mudstones, it is not always certain that the seam thus named is the same seam throughout the northern outcrops. In the south the beds above the Small Rider range from 240 to 360 ft of variable measures, and contain several coals, the highest, and thickest being called the Big Rider or No. 1 Llantwit. There is no evidence that the Big Rider of this succession equates with any of the coals by the same name in the north, though it belongs to the same coal-forming period. The roof of the Big Rider carries Anthraconauta phillipsii, A. tenuis, ostracods, Leaia and fish. Above the seam the highest Grovesend Beds are a series of little known sandstones and mudstones up to 180 ft thick near Blackwood, and about 220 ft west of Treforest. A coal of unknown thickness is present near the top of these beds in each area. H.C.S.

References

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BLUNDELL, C. R. K. 1952. The succession and structure of the north-eastern area of the South Wales Coalfield. Quart. L Geol. Soc., 107, 307–33.

BLUNDELL, C. R. K. and MOORE, L. R. 1960. Mid-coal measures "red beds" in the South Wales Coalfield. C. R. 4me Congr. Strat. Carb. Heerlen 1958, 1, 41–8.

DIX, Emily. 1934. The sequence of floras in the Upper Carboniferous, with special reference to South Wales. Trans. Roy. Soc. Edin., 57, 789–838.

DOWNING, R. A. and SQUIRRELL, H. C. 1965. On the red and green beds in the Upper Coal Measures of the eastern part of the South Wales Coalfield. Bull. Geol. Surv Gt. Brit., No. 23, 45–56.

EAGAR, R. M. C. 1953. Variation with respect to petrological differences in a thin band of Upper Carboniferous non-marine lamellibranchs. Liverpool and Manchester Geol. Journ., 1, 161–90.

HOWELL, A. and Cox, A. H. 1924. On a group of red measures or coloured strata in the east Glamorgan and Monmouthshire Coalfield. Proc. S. Wales Inst. Eng., 43, 139–74.

LEITCH, D., OWEN, T. R. and JONES, D. G. 1958. The basal Coal Measures of the South Wales Coalfield from Llandybie to Brynmawr. Quart. J. Geol. Soc., 113, 461–86.

MOORE, L. R. 1945. The geological sequence of the South Wales Coalfield: the 'South Crop' and Caerphilly basin, and its correlation with the Taff Valley sequence. Proc. S. Wales Inst. Eng., 60, 141–227.

MOORE, L. R. 1948. The sequence and structure of the southern portion of the east crop of the South Wales Coalfield. Quart. J. Geol. Soc., 103, 261–94.

PETTIJOHN, F. J. 1957. Sedimentary Rocks. 2nd edit. Harper.

SCHEERE, J. 1955. Contribution à l'étude des tonstein du Terrain houiller beige. Publ. Ass. Étud. Paléont. No. 19. Brussels.

SQUIRRELL, H. C. and DOWNING, R. A. 1964. The attenuation of the Coal Measures in the south-east part of the South Wales Coalfield. Bull. Geol. Surv. Gt. Brit., No. 21, 119–32.

STOUT, W. 1923. Theory of the origin of coal formation clays. Geol. Survey Ohio Bull., 26, Ser. 4, 533–68.

STUBBLEFIELD, C. J. and TROTTER, F. M. 1957. Divisions of the Coal Measures on Geological Survey Maps of England and Wales. Bull. Geol. Surv. Gt. Brit., No. 13, 1–5.

TRUEMAN, A. E. 1947. Stratigraphical problems in the Coal Measures of Europe and North America. Quart. J. Geol. Soc., 102, xlix-xciii.

VAN DER HEIDE, S. 1950. Compaction as a possible factor in Upper Carboniferous rhythmic sedimentation. Rept. 18th Int. Geol. Cong., Pt. 4, 38–45.

WANLESS, H. R. 1950 Late Paleozoic cycles of sedimentation in the United States. Rept. 18th Int. Geol. Cong., Pt. 4, 17–28.

WANLESS, H. R. and WELLER, J. M. 1932. Correlation and extent of Pennsylvanian cyclothems. Bull. Geol. Soc. America, 43, 1003–16.

WELLER, J. M. 1930. Cyclical sedimentation of the Pennsylvanian Period and its significance. Jour. Geol., 38, 97–135.

WELLER, J. M. 1956. Argument for diastrophic control of late Paleozoic cyclothems. Bull. Amer. Assoc. Petrol. Geol., 40, 17–50.

WOODLAND, A. W., ARCHER, A. A. and EVANS, W. B. 1957. Recent boreholes into the Lower Coal Measures below the Gellideg–Lower Pumpquart horizon in South Wales. Bull. Geol. Surv. Gt. Brit., No. 13, 39–60.

WOODLAND, A. W. and EVANS, W. B. 1964. The Geology of the South Wales Coalfield. Part IV. The Country around Pontypridd and Maesteg. 3rd edit. Mem. Geol. Surv.

WOODLAND, A. W. and EVANS, W. B. and STEPHENS, J. V. 1957. Classification of the Coal Measures of South Wales with special reference to the Upper Coal Measures. Bull. Geol. Surv. Gt. Brit., No. 13, 6–13.

Chapter 7 Lower Coal Measures: details

In the following detailed accounts of the stratigraphy of the Coal Measures, except for the beds below the Garw and above the Mynyddislwyn, the district has been divided, for ease of description, into six areas. These are represented on (Figure 26), which also shows the locations of many of the collieries mentioned in the text. A number of local mining terms applying to particular lithologies are quoted in the detailed accounts, as well as in Appendix I, and although most of these have already been explained in the Pontypridd Memoir (Woodland and Evans 1964), for convenience a complete list is given here:

• Balls: nodules of ironstone.
• Band: thin parting in a coal, probably mudstone or seatearth.
• Bast: cannel or cannelly mudstone.
• Bastard: applied usually to non-typical or intermediate lithologies, e.g. a bastard clift is probably a sandy mudstone.
• Clam: probably mudstone.
• Clay: parting in a coal, probably seatearth.
• Clift (or cliff): blocky mudstone or silty mudstone.
• Clod: mudstone or seatearth forming a band in a coal, or that part of the roof which comes away with the coal during working.
• Dirt: any band other than coal within a seam.
• Holing: parting in a coal, probably mudstone or seatearth.
• Kind clift: soft mudstone.
• Mine: ironstone; mine ground in clift with much ironstone in bands or nodules.
• Pins: thin bands of hard rock, usually ironstone.
• Rashings or rashes: either soft, carbonaceous shale with streaks of coal, or highly disturbed, slickensided, comminuted shale or mudstone formed by movement parallel to the bedding and usually associated with the roof or dirt bands in coal seams. Normally rashes should be retained for the former definition, and rashings for the latter.
• Rock: hard, massive sandstone.
• Warricks: ironstone nodules.

A. Gastrioceras Subcrenatum Marine Band to Garw Seam

Surface details: In the extreme west these beds are mainly argillaceous, but on approaching the Taff Valley, they pass laterally into about 150 ft of alternating sandstones and mudstones. Typical, massive, quartzitic sandstones and grey mudstones are visible in a quarry [ST 1170 8320] at Georgetown, and nearby, alongside a small stream, mudstones from a trial pit [ST 1156 8297] yielded the following fauna from the Gastrioceras subcrenatum Marine Band: Caneyella multirugata (Jackson) [including young forms], Dunbarella sp.[fragments], ?Posidonia sp.nov., Anthracoceras sp., Gastrioceras subcrenatum (Frech), mollusca spat including gastropods and Cypridina sp.Mr. Calver comments that in this fauna there are several examples of the large ostracod Cypridina, mainly in the layers where mollusca spat is abundant. This association of Cypridina and spat has been noted in the Namurian (Ramsbottom and others 1962, p. 116), and it is possible that Cypridina was a pelagic form.

East of the Taff, around Nant-y-Brynau [ST 1310 8410], mapping shows that the group comprises approximately 300 ft of mainly arenaceous rocks with some mudstones, in which two coals have been proved and a third inferred. At their base mudstone spoil from a small trial [ST 1299 8402] yielded: Palaeoneilo sp., nuculoid indet. [juv.], Sanguinolites cf. ovalis Hind and Gastrioceras cf. subcrenatum. The goniatite remains are fragmentary, but this fauna is undoubtedly from the G. subcrenatum Marine Band. Coal fragments in the spoil are probably from a seam of unknown thickness lying within the 15 to 25 ft of mudstones above the marine band. Coal in the same mudstones was also found in the spoil from a trial [ST 1385 8446] 1100 yd to the north-east.

The mudstones overlying the G. subcrenatum Marine Band are followed by approximately 100 ft of sandstones with thin mudstones, succeeded by about 30 ft of mudstones in the upper part of which lies the Sun Vein. This seam, of unknown thickness, was worked from a level [ST 1330 8436] in the south bank of Nant-y-Brynau, presumably driven south-eastwards through higher strata to intersect the coal. The mudstone spoil from this level yielded the following fauna from the M2 Marine Band: Planolites ophthalmoides, Lingula mytilloides, and Orbiculoidea cf. nitida. Moore (1945, p. 151) also found Productus sp.in this spoil. Productus carbonarius was found in tip material from a disused shaft nearby, which is believed to have been sunk to the Sun Vein.

The M2 Marine Band is closely followed by about 90 ft of sandstones (the Cefn Cribbwr Rock) with interbedded thin mudstones. The uppermost beds of this sequence are visible in a small quarry [ST 1314 8420] near Nant-y-Brynau as follows:

The sandstones overlying the conjectured coal horizon are at least 30 ft thick, and they are exposed at the entrance to the level [ST 1330 8436] in which the Sun Vein was worked, where the section is:

The 1 ft of black, silty mudstone with fish scales is thought to equate with the roof strata of the Garw seam.

South of Caerphilly Common hard, conglomeratic, quartzitic sandstones in the upper part of this group dip northwards at 40 to 50°, and are exposed at several places along a prominent ridge. In Wern-ddu Claypit the uppermost rocks of the group were described by Moore (1945, p. 156) (Appendix I); the 3-in coal overlain by micaceous shales with fish remains near the base of his section is probably the Garw. Below this coal he recorded approximately 6 ft of fireclays with ironstone nodules, underlain by micaceous, flaggy sandstones. A similar sequence was recorded during the resurvey, in a trench [ST 1674 8546] along the south side of the disused claypit, though the Garw coal was not seen.

Approximately one-third of a mile north-east of Wern-ddu Claypit the uppermost beds of the group are again exposed, in a small stream section [ST 1743 8592]. The Garw is absent, but its dark, micaceous, roof mudstones with fish remains (p. 137) overlie 14 ft of seatearth and silty mudstone with roots on 1 ft of hard, fine-grained, white, quartzitic sandstone with roots. Between this locality and the Rhymney Valley, there are only a few sections in massive, hard, quartzitic sandstones. Half a mile north-east of Rudry the Sun Vein, lying approximately 100 ft above the Gastrioceras subcrenatum Marine Band, was exploited from several small levels for about 0.5 mile along the crop. The section in this area is: strong shale roof, fireclay 3 in, coal 2 ft, fireclay 3 ft. In the mouth of one of the levels Moore (1945, p. 150) collected a rich flora characteristic of Floral Zone C from the higher roof mudstones of this seam. The same seam, 2 to 2.5 ft thick, was worked from levels on both sides of the Rhymney Valley, most extensively in Bovil Colliery [ST 2180 8907] near Chatham, and in Machen Pit where it was subject to washouts (Forster Brown 1865, p. 97). Near Risca the Sun Vein was worked fairly extensively from a level [about ST 2306 9119] near Waun Fawr Brick Works. The following section was recorded in the southern end [ST 2304 9100] of Waun Fawr Quarry:

When beds 5, 6, 7 and 8 above are followed towards the northern end of the quarry, they thin and die out gradually, until bed 4 rests on bed 9.

On the northern side of the Ebbw Valley at Risca the Sun Vein is believed to crop out near the site of the disused Darren Brick Works [ST 2344 9166], but no evidence for its presence was forthcoming during the present resurvey, for the area is now largely obscured by colliery spoil. North of Risca, in the Coed-y-Darren [ST 2400 9210] area, the arenaceous rocks overlying the Sun Vein horizon locally reach about 100 ft in thickness, and form a low ridge which runs along the south side of the wood. The basal 4 ft of hard, quartzitic sandstones of this division are visible in the collapsed mouth of a trial level [ST 2437 9220] for the Sun Vein. The uppermost beds of the division are visible in the mouth of a trial level [ST 2405 9208] in Coed-y-Darren, where several feet of hard, flaggy sandstone dipping north-north-west at 55° are well exposed. A specimen (E30302) of sandstone from this section has been described by Dr. Hawkes as an orthoquartzite consisting of sub-rounded quartz grains (average grain size about 0.1 mm) with rare chert and plagioclase fragments and some accessory tourmaline and opaque oxide.

The spoil from three small levels [ST 2552 9389], [ST 2542 9375] and [ST 2538 9366] in the Sun Vein north-west of Blaen-y-cwm Farm yielded to Moore (1948, p. 271) L. mytilloides and Gastrioceras sp.Spoil from the northernmost of two levels [ST 2579 9431] and [ST 2577 9424] farther north-east yielded L. mytilloides (Moore 1948, p. 271). In this area the sandstones above the Sun Vein form an impersistent, low, but well-defined ridge for about a mile. A small section of the hard, quartzitic sandstones is exposed in the collapsed mouth [ST 2582 9444] of an entrance to the disused Henllys Colliery.

At Llanderfel the uppermost of the sandstones above the Sun Vein horizon are visible in a small exposure [ST 2648 9548] 150 yd north-west of Llanderfel Farm, where about 6 ft of hard, massive, white quartzitic sandstones dip west-north-west at 70° or more. Near Garn-wen Farm [ST 2672 9619] the sandstones above the Sun Vein total about 60 ft, and they have been worked in several shallow excavations [ST 2672 9613] near the farm. North-east of Garn-wen Farm spoil from a small level [ST 2685 9632] in the Sun Vein yielded the following fauna from the M2 Marine Band: Campylites sp., Paraconularia sp., Lingula mytilloides, cf. Leptoptygma sp.[juv.], and Rhabdoderma sp.

Towards Pontypool the sandstones above the Sun Vein thicken gradually, and they are well exposed in Cwm Ynis Coy [ST 2780 9956], where up to 15 ft are visible in the stream bed, and 10 ft in the disused quarry [ST 2784 9946] nearby. At Pontypool the Sun Vein was worked in Twmpath Level [SO 2771 0044] where Strahan recorded: "black shale 5 in on fireclay 18 in, coal 19 in". In this area sandstones above the seam reach 130 ft in thickness, and form a conspicuous ridge for about 1 mile south of the Pontypool–Crumlin railway. The following sequence is exposed in a quarry [SO 2780 0027] just south of the railway:

Above this quarry about 15 ft of argillaceous beds, lying about 70 to 80 ft above the Sun Vein, have been traced for about 300 yd along the strike. They have been explored by two levels [ST 2772 0023] and [SO 2777 0021], which appear to have worked small amounts of coal and ironstone.

The succession below the Garw in the Pontypool area was described by De La Beche (1846, p. 176) as follows:

The origin of this section is not known, but it is substantially accurate. The sequence "underclay, Little Coal, and mudstone with ironstone beds" in the section correlates with the 15 ft of argillaceous beds (described above) lying 70 to 80 ft above the Sun Vein, which were mapped during the resurvey.

Underground details: In the Nantgarw Colliery West Lateral drivages the exact sequence in these measures was confused by structural complexity. A seam called the No. 29 was encountered in the West Lateral 280 Horizon [ST 1145 8412], at a calculated stratigraphical distance of 180 to 200 ft below the Five-Feet-Gellideg (No. 28 Seam). It is overlain by mudstone yielding Lingula mytilloides about 10 ft above the seam, and is believed to be the equivalent of the Sun Vein of the south crop.

In New Rockwood No. 2 Borehole a 'fireclay' lying 524 ft below the surface may mark the horizon of the base of the Garw; it is underlain by 73 ft of mudstones, sandy mudstones and sandstones. Gypsy Lane Borehole (Appendix I) near Nantgarw was drilled to a depth of 249 ft 5 in below the Garw, but as dips varied between 20° and near vertical the exact thickness of beds traversed is not known.

Farther east, the record of the Van Colliery shaft gives no thickness for the Sun Vein, which is separated from the Five-Feet-Gellideg by 210 ft of 'measures'. In the Machen Colliery shaft the Sun Vein is recorded as 6 ft of coal separated from the Five-Feet-Gellideg by 161 ft of 'rock'. Rudry Borehole (Woodland and others 1957, p. 50; and Appendix I) provides the most complete record of this part of the succession. H.C.S.

In the North Taff Area the North Pit Borehole [ST 0818 9448] at Abercynon, proved the following sequence below the Garw:

In Lancaster Pit at Universal Colliery the presumed Garw, 3 in thick, lies at –1180 ft O.D., and is underlain by mudstone with bands of ironstone containing. A 2.5-ft bed of 'green conglomerate' at –1238 ft O.D. At Penallta the 31.5 ft of 'clift' and thin sandstones lying below the Garw represent the maximum thickness of strata , proved below the seam in the North Rhymney and Sirhowy Area.

Llanbradach underground No. 3 Borehole [ST 1493 9088], just east of the shafts, penetrated 151 ft of barren measures below the Seven-Feet. The basal 50 ft of mudstones with a few thin sandstones may be below the horizon of the Garw. At Wyllie a 'fire-clay' at –1370 ft O.D. in the South Shaft probably marks the horizon of the Garw It is underlain by 19 ft of 'rock' on 30 ft of mudstones with a little sandstone.

B. Garw to Amman Rider

North Taff area

The essentially argillaceous sequence below the Gellideg has been proved to a depth, of up to 188 ft below the seam in boreholes at Albion and Abercynon collieries The log of the North Pit Borehole [ST 0818 9448] at the latter colliery is as follows:-

The Garw is usually some 6 in thick and lies about 100 ft below the Gellideg. A further thin coal, 30 ft below the upper seam, has been recorded in an underground borehole at Abercynon.

At Albion and Abercynon the Gellideg and the Five-Feet form a composite seam known as the 'Seven-Feet'. Typical seam sections are:

Farther north, at Deep Navigation and Taff Merthyr, the separation between the seams is greater and consists of seatearth and silty mudstone with roots. The line of split is shown in (Figure 10). North-east of Deep Navigation shafts the lower coals of the Five-Feet form a single coal; a typical section of both the Five-Feet and Gellideg is: coal 39 in on dirt 2 in, coal 13 in, dirt 7 in, coal (Gellideg) 32 in.

In boreholes near the northern boundary of the Deep Navigation take, and just to the north of the Taff Merthyr shafts, the section is similar to that found in parts of Bargoed (see p. 134), and is as follows: coal 18 in on dirt 6 in, coal 9 in, dirt 33 in, coal (Gellideg) 30 in. The change in the thickness of the two leaves of the Five-Feet appears to be due to the dying out of the parting found in the south of the Deep Navigation area, and the incoming of a new parting in the top thick leaf of the seam.

The roof of the Five-Feet varies from a smooth mudstone to hard siltstone. National Coal Board geologists have recorded Cochlichnus kochi in mudstones up to 14 ft above the Five-Feet in boreholes at Taff Merthyr Colliery. Strata in the immediate roof of the Five-Feet commonly yield poorly preserved non-marine mussels.

The measures between the Five-Feet and the Seven-Feet are 55 ft thick in Albion South Shaft and 58 to 69 ft at Abercynon. West of the Llanfabon Fault in Deep Navigation the interval between the seams is believed to be about 46 ft, while to the east of the Gelligaer Fault 40 ft is more normal. Boreholes in the northern part of the Deep Navigation area proved 31 to 34 ft, while near Taff Merthyr shafts the interval is about 28 ft. The sequence is mainly argillaceous with a few thin sandstone beds. Nodules and bands of ironstone are developed, particularly in Albion.

The Seven-Feet is between 36 and 48 in thick at Albion and Abercynon collieries, where it is referred to as the Bottom Yard. Some 14 ft below the seam at Albion a 15-in coal has been recorded, and the same seam occurs 8 to 11 ft below the Seven-Feet at Abercynon where thicknesses of 22 and 30 in have been proved. To the north, in Deep Navigation, the Seven-Feet section is: coal 39 in on dirt 3 in, coal 11 in, dirt 1 in, coal 9 in, dirt and inferior coal 6 in. The upper thick leaf, which contains in places a thin dirt parting, is presumably the equivalent of the Albion and Abercynon Seven-Feet, and the seam below the Seven-Feet at these collieries is the equivalent of the lower two leaves of the Deep Navigation Seven-Feet section. At Taff Merthyr, where the seam has been proved in boreholes, the section is similar to that at Deep Navigation. The Seven-Feet has been worked from Albion, Abercynon and Deep Navigation, although not extensively.

The measures between the Seven-Feet and the Yard are extremely variable. The two seams may be separated by as little as 36 in of seatearth and silty mudstone at Albion, while in the Abercynon area the intervening thickness varies from 18 to 36 ft, consisting of seatearth, where the interval is small, but seatearth and silty mudstone as it increases. The separation is greatest near the shafts, where a sandstone forms the immediate roof of the seam. In other parts of the colliery a blue, soft mudstone containing Cochlichnus kochi occurs between the coal and the sandstone. At Albion the roof mudstones have yielded: C. kochi, Naiadites sp.[juy.] or Curvirimula sp., Lioestheria vinti and L. striata. In the same colliery Moore (1944, p. 200) collected large specimens of Naiadites triangularis from a canneloid shale lying above the coal. At Deep Navigation 36 ft of argillaceous strata occur between the seams west of the Llanfabon Fault, and about 24 ft east of the Gelligaer Fault. To the north boreholes near the Taff Merthyr shafts have proved 35 to 43 ft of argillaceous measures between the seams. At Deep Navigation, to the east of the Gelligaer Fault, a thin coal lies some 6 ft below the Yard Seam and this is presumably equivalent to a 6-in coal 13 ft below the Yard in a borehole near Taff Merthyr shafts.

The Yard seam is known as the Top Yard at both Albion and Abercynon, and a small area has been worked at the former colliery. The section in both collieries is characterized by a distinct upper leaf, which is separated from the main coal by a variable parting, for example: Albion–coal 10 in on dirt 1 in, coal 32 in, Abercynon coal 5 in on dirt 5 in, coal 30 in. To the north, at Deep Navigation and Taff Merthyr, the seam thickness varies between 24 and 30 in. A cannel or canneloid mudstone roof has been noted both at Taff Merthyr and Abercynon. The overlying mudstone, which forms the roof of the seam where cannel is absent, yields Anthracosia regularis. At Albion the roof mudstones have yielded: Anthraconaia fisheri, Anthracosia regularis, cf. Carbonicola oslancis Wright, cf. C. venusta. In Abercynon this fossiliferous mudstone is of varying thickness up to about 10 ft, and is succeeded by a few feet of fine-grained sandstone or siltstone, which in places rest directly on the coal.

Fourteen feet of argillaceous strata separate the Yard and Amman Rider at Taff Merthyr, and generally about 40 ft of almost entirely argillaceous rocks at Albion and Abercynon, although Moore (1943, p. 204) recorded 51 and 67 ft respectively at the last two collieries.

The Amman Rider is about 14 in thick at Albion, and an inferior coal, 6 in thick, lies some 8 ft below it. A similar sequence is found in Abercynon, but the top coal varies considerably from about 4 ft, including thin dirt partings, south of the shafts, to 10 in north of the shafts. The interval between the main coal and the underlying seam varies from 1 to 10 ft. A double coal section occurs in Deep Navigation west of the Llanfabon Fault, the interval between the seams being about 3 ft, but to the east of the Gelligaer Fault three leaves are found, for example coal 20 in on dirt 23 in, coal 5 in, strata 5 ft 2 in, coal 5 in.

North Rhymney and Sirhowy area

The sequence between the Garw and the Gellideg is known only from shaft and borehole records and few details are available. Part of the sequence has been proved in all the collieries in the area, but the Garw only at Penallta, Britannia and Grosfaen. At Penallta the Garw horizon is represented probably by 7 in of soft, dark shale with specks of coal, underlain by 17 in of seatearth, about 100 ft below the Gellideg. The seam is 4 in thick at Britannia and lies 100 ft below the Gellideg. At Grosfaen it is represented in a borehole drilled from the bottom of Grosfaen East Shaft, by black rashings 18 in thick, 101 ft below the Gellideg.

The measures between the Garw and Gellideg consist mainly of mudstone, but with sandstone beds which, although predominantly thin, may attain 15 ft. A thin and impersistent coal horizon is developed about 25 ft below the Gellideg. It is 15 in thick at Oakdale, and is represented by 4 and 16 in of coal and rashings at Bargoed Oakdale collieries. In the central part of the area, and in the south-west, more than 2 ft of strata intervene between the Five-Feet and the Gellideg (Figure 10).

The Five-Feet–Gellideg is known as the Lower Four-Feet in the Rhymney Valley, and as the Old Coal in the Sirhowy Valley. It is a composite seam with a variable section, the individual leaves of which are difficult to correlate in detail throughout the area. The seam is affected by a washout to the west of Grosfaen shafts, which has an apparent north-north-west to south-south-east trend. In the south the seam has not been extensively explored, but washout conditions prevail at least in part of the Penallta Colliery area, and they also affect the seam at Oakdale. The Gellideg, and in places the lower leaf of the Five-Feet, are worked at Grosfaen, Bargoed and Oakdale collieries. In the central part of the area, and in the south-west, more than 2 ft of strata intervene between the Five-Feet and the Gellideg (Figure 10).

The seam section of the Five-Feet–Gellideg at Bargoed is similar to that occurring in parts of Taff Merthyr and Deep Navigation (p. 132), and is typically: coal 12 in on dirt 4 in, coal 29 in, dirt 18 in, coal (Gellideg) 29 in. A thin parting may develop in the lower leaf of the Five-Feet. At Grosfaen the lower part of the Five-Feet has deteriorated, the typical section being: coal 16 in on dirt 7 in, coal 8 in, seatearth 48 in, coal (Gellideg) 29 in. The top leaf of the Five-Feet is split into two approximately equal coals in some districts of the colliery. In the west and east shafts the Five-Feet is separated from the Gellideg by 8 and 16 ft respectively of argillaceous beds with thin sandstones. The Five-Feet–Gellideg at Britannia, where it has been explored near the shafts, is a thick, but dirty seam, consisting of layers of coal, inferior coal and carbonaceous shale. In a cross-measures drivage [ST 156 980] 8 ft 2 in of coal are spread over 15 ft 9 in of strata, the thickest leaf of good quality coal in the section being no more than 22 in. To the south of the shafts the following section was recorded in a borehole [ST 158 977]: coal 42 in on dirt 13 in, coal 50 in, dirt 5 in, coal (Gellideg) 6 in.

Little is known of the seam at Penallta. Four coals are recorded near the bottom of the No. 1 Shaft as follows: coal 36 in on bastard seatearth 20 in, coal 5 in, dirt 72 in, coal 5 in, seatearth 48 in, coal 15 in, but the identification of the two component seams is difficult although the Gellideg has obviously thinned. A borehole near the shafts proved the following section: coal 6 in on dirt 27 in, coal 39 in, measures 10 ft 6 in, coal (Gellideg) 26 in.

The thickness of the Gellideg in the Sirhowy Valley at Oakdale, varies between 9 and 25 in, but is normally about 23 in. Usually the seam is separated from the overlying Five-Feet by 15 to 19 in of seatearth or carbonaceous shale. The Five-Feet is in two leaves, an upper coal about 25 in thick, and a lower coal about 30 in thick, separated by a parting approximately 1 in thick. The roof of the Five-Feet is usually a smooth, virtually unfossiliferous mudstone. At Grosfaen a very thin layer of dark grey, silty mudstone with plants immediately overlies the seam.

The thickness of the measures between the Five-Feet and Seven-Feet is very variable. The maximum is attained north of Britannia shafts and in Bargoed, where 70 ft to over 85 ft is normal. This thins to the north-west, south-west and east; at Grosfaen and Oakdale between 38 and 50 ft are usual, and about 65 ft at Penallta. The sequence is mainly argillaceous. Ironstones are commonly developed, particularly at Oakdale, Grosfaen, and Bargoed, and thin sandstones are characteristic of the sequence in some places, for example, in Penallta (where arenaceous deposits occur in the lower part of the sequence) and in Oakdale. Thick seatearths (e.g. about 20 ft thick) below the Seven-Feet are a feature at Oakdale. Up to two riders are present between the Five-Feet and Seven-Feet at Grosfaen. In the West Pit, an 8-in coal lies 14.5 ft above the lower seam, and boreholes to the north-east [SO 1395 0105] and south-east [SO 1405 0005] of the shafts proved thin coals about 20 ft and 30 ft above the seam. Both at Bargoed and Britannia a 4-in rider coal occurs 35 and 27 ft respectively above the Five-Feet, but it has been proved in only one borehole at each colliery; the seam appears to be absent in other parts of the collieries. The relationship of the riders to the principal seams is uncertain, but they may represent splits from the lower part of the Seven-Feet at Taff Merthyr, and would be, therefore, the equivalents of the thin coals below the Seven-Feet at Abercynon and Albion.

The Seven-Feet at Grosfaen has a three-coal section as follows: coal 12 in on dirt 5 in, coal about 22 in, dirt 5 in on a lower leaf varying from 3 to 13 in. The top two coals are equivalent to the thick upper leaf in the northern part of the North Taff Area. The three-coal section persists into the Bargoed and Britannia areas, but towards the east of these collieries, the two upper leaves reunite, forming a single coal with an average section of 36 in. The lowest leaf of the seam at Grosfaen may either die out towards the east and south-east, or join the upper part of the Seven-Feet, or it may be represented by the apparently impersistent rider recorded in places in the measures below the Seven-Feet at Bargoed and Britannia. The seam referred to as the Seven-Feet at Penallta is composite in character and comprises both the Yard and Seven-Feet of the standard sequence, the lower leaf being the Seven-Feet. Near, and to the east of Penallta shafts about 27 in is a normal thickness for this lower leaf, but where the coal has been worked 600 yards south-south-east of the shafts, 22 in is usual. This information indicates a general thinning towards the south. In the Sirhowy Valley the Seven-Feet comprises the lower leaf of the compound Meadow Vein. At Oakdale the thickness varies between 27 and 43 in but 36 or 37 in is usual. The Seven-Feet has been worked from Grosfaen, Bargoed, Britannia and Oakdale (as part of the Meadow), and also to a limited extent from Penallta (as part of the locally named Seven-Feet which includes the Yard).

The measures intervening between the Seven-Feet and the Yard thin in an easterly and south-easterly direction. The maximum thickness is probably attained in the area between Britannia and Grosfaen shafts. Some 35 ft are present in a heading [ST 156 980] just to the west of Britannia shafts, and 31 and 17 ft occur in Grosfaen West and East pits respectively. In Bargoed North Pit the interval is 14 ft, and rapid easterly thinning reduces the interval to no more than 12 in of seatearth in the east of the Bargoed and Britannia takes. The line indicating a separation of more than 12 in trends north-south through the eastern parts of these collieries, but swings to a north-east to south-west direction in Penallta, passing some 200 to 300 yd southeast of the shafts. Two trial pits [ST 1555 9447] and [ST 1540 9431] to the south of the colliery showed the Seven-Feet and Yard as a single seam, with thicknesses of 36 and 60 in respectively, with, in each case, a dirt parting 4 in from the top. Generally, the interval between the seams at Oakdale is less than 24 in, although locally 48 in have been recorded.

Where the Yard is but a short distance above the Seven-Feet the intervening measures are seatearth or carbonaceous shale. As the thickness increases mudstone develops with sandstone and ironstone bands, the actual roof of the Seven-Feet commonly being a slightly silty mudstone. Canneloid shale or dark grey mudstone with plants forms the immediate roof in places. At Britannia, in an underground heading [ST 156 980] 200 yards west of the shafts, two thin beds of rashings occur 19 and 24 ft above the Seven-Feet.

The Yard forms the upper parts of the Seven-Feet of Penallta and the Meadow Vein of Oakdale, and is known as the Seven-Feet Rider at Grosfaen, Bargoed and Britannia. Workings in the seam are restricted to areas where it forms a composite seam with the Seven-Feet. The thickness of the Yard varies from 24 to 40 in south of Britannia shafts. In the north-west, limited information suggests that the coal is thin, for example, 13 inches in Grosfaen East Pit and 18 inches in Bargoed North Pit. However, farther east, where it approaches the Seven-Feet, 30 in is the usual thickness. A variation of between 29 and 45 in is known in Oakdale; east of a north-south line passing through the shafts 36 in is invariably exceeded, and west of the shafts less than this is usual. Where the Yard is worked in Penallta a thin rider is present above the main coal. Similar sections occur in Oakdale, west of the shafts, in Bargoed North Pit and in Grosfaen:

The greater thickness of the Yard in Oakdale east of the shafts is due to the union of the main and rider coals.

The thickness of the measures between the Yard and the Amman Rider exceeds 25 ft north-west and north of a line passing through Penallta, Bargoed and Oakdale shafts. A maximum known thickness of 38 ft is attained in the north of Oakdale, but over most of the northern area the thickness probably varies between 25 and 30 ft. South-east of this line there is a gradual reduction, and little more than 6 ft are present in the extreme south of Penallta. Where the Yard and Amman Rider lie close together the intervening measures consist entirely of seatearth. Towards the north, once the parting exceeds 10 to 15 ft, mudstones are developed and the roof of the Yard carries a mussel fauna of crushed and solid shells. At Oakdale the crushed shells occur in the immediate roof with the solid shells in the higher roof. The fauna includes: Anthraconaia cf. curtata (Brown) [short form], A. sp. cf. williamsoni (Brown), Anthracosia regularis, Carbonicola venusta, Naiadites sp. cf. productus and N. sp.(intermediate between productus and quadratus). The following plants were found at Oakdale, from about 15 ft above the Yard seam; Alethopteris decurrens (Artis), Cordaites sp., and Neuropteris heterophylla (Brongniart).

The measures between the Yard and Amman Rider are entirely argillaceous, except near, and to the north-west of Penallta shafts, where thin sandstones tend to occur.

The Amman Rider is recorded both as a single and double coal in the north, but in Penallta it is invariably a double coal. The single coal section varies between 16 and 24 in, while the double section comprises an 18- to 24-in upper leaf, separated from a lower leaf of between 2 and 11 in, by a dirt parting up to 19 in. R.A.D.

Nantgarw-Risca area

The Garw is poorly developed in this area, and is absent locally, e.g. in a stream [ST 1743 8592] in Wern-ddu (p. 128). It is usually less than 6 in thick, though it reaches a known maximum of 10 inches in Rudry Borehole. Its roof is commonly dark, carbonaceous, micaceous, silty mudstone, which characteristically yields a fauna of fish remains, e.g. in a trench [ST 1674 8548] close to Wern-ddu Claypit, where an acanthodian spine and Rhabdoderma sp.were found. The roof mudstones also yielded fossils in Gypsy Lane and Rudry boreholes (Appendix I).

The Garw is separated from the Gellideg by 45 to 90 ft of argillaceous beds with a few sandstones. Two, thin, imperistent coals occur within these beds at Nantgarw, and one was recorded in Rudry Borehole. The argillaceous rocks within this group of measures commonly contain Planolites and fish remains. In the Nantgarw No. 1 West Cross-measures 280 Horizon, at about 56 ft below the Gellideg, mudstones have yielded: cf. Planolites sp.[small var.], Spirorbis sp., Carbonicola acuta, C. pseudorobusta, Geisina arcuata and fish remains including Rhabdoderma sp.

The Five-Feet–Gellideg (Lower Black, Old or Hard) has been worked fairly extensively at Risca, Nine Mile Point and Bedwas, and on a small scale along the south crop at South Cambria, Lan, Nantgarw, Ffwrnes Blwm, Rudry, Coed-y-Cefn and Black Vein collieries (Figure 26). In some areas it is absent due to either non-deposition, as in Gypsy Lane Borehole, or washouts, as in parts of Wyllie, Llanbradach and Windsor. Where present the seam usually varies in thickness between 2 and 6 ft, sometimes with thin partings, but to the west it commonly splits into two or three distinct leaves. Only at a few localities can the Five-Feet and Gellideg seams be recognized individually, as given below:

The roof of the Five-Feet–Gellideg is mudstone or silty mudstone over a large part of the area, and sandstone locally, for example, at Nine Mile Point. The mudstone commonly contains plant remains, sporadic mussels and fish scales. At Bedwas, Moore (1945, p. 172) found the following fauna: "Anthraconauta minima (Ludwig), Naiadites aff. flexuosa Dix and Trueman, Naiadites sp., and fish scales". He also recorded (1945, p. 157) a rich flora in the mudstone above the coal in Wern-ddu Claypit, which he assigns to Floral Zone D.

The measures between the Five-Feet–Gellideg and Seven-Feet seams are thickest in the Nantgarw area, where they vary between 30 and 60 ft. North and eastwards from there the separation is generally between 15 and 45 ft, though locally it may be much thinner, e.g. in Wern-ddu Claypit, where it is only 9 ft. The strata consist predominantly of mudstones, silty mudstones and seatearths, with sandstones in thin beds, e.g. in Ffynnon Wen Borehole, where about 10 ft of sandstone lie a short distance above the Five-Feet–Gellideg. The seatearth below the Seven-Feet varies considerably in thickness; it is usually only a few feet thick, but reaches 20 ft in Ffynnon Wen Borehole, and Moore (1945, p. 173) recorded 40 ft at Nine Mile Point.

The Seven-Feet, Yard and Amman Rider are most conveniently dealt with together, for over most of the area they usually form a composite seam in which the constituent leaves are separated by thin partings only, or they even merge into a single, clean coal. In the north and east of the area, at Universal, Wyllie, Nine Mile Point, Risca and the northern part of Llanbradach, the Amman Rider splits away from the other coals, which generally lie close together, the separation being as much as 35 ft. The composite seam (most commonly known as the Meadow, Brass or Nine-Feet) has been worked fairly extensively in the north and eastern part of the area at Risca, Nine Mile Point, Wyllie, Llanbradach and Windsor, and on a small scale along the south crop at South Cambria, Lan, New Rockwood, Van, Coed-y-Cefn and Black

Vein collieries. It varies in thickness between 3 and 8 ft, and in the central, southern and western parts of the area commonly shows a division into three leaves, or rarely four, which vary considerably in thickness, though the middle leaf is generally the thickest. The following sections illustrate some of the variations:

At Bedwas, in parts of Nantgarw and in Ffynnon Wen and Rudry boreholes the Seven-Feet, Yard and Amman Rider merge into a seam in which the individual coals are virtually impossible to recognize.

In the north and eastern part of the area the Seven-Feet and Yard usually consists of two coals at Universal and Wyllie, and three at Nine Mile Point and Risca, where a thin leaf splits off the upper coal. In this vicinity the Amman Rider is generally separated from the Seven-Feet and Yard by 2 to 35 ft of measures. The following sections are fairly typical of the collieries named:

North Ebbw area

Below the Gellideg only 30 ft of sandstones, mudstones and seatearths have been proved in collieries in this area, and in the North Shaft at Celynen North they include a 6-in coal lying 16.5 ft below the Gellideg. The combined Five-Feet and Gellideg usually comprises 5 to 6.5 ft of coal in 3 or 4 closely lying leaves. Average sections are as follows:

Local variations in the seam sections occur in all the collieries, e.g. at Celynen North up to 5 ft of mudstone with ironstone nodules separate the top and middle leaves in western parts of the colliery, and at Celynen South up to 12 ft of sandstone locally separate the same coals. The roof of the Five-Feet-Gellideg is usually mudstone or silty mudstone, or rarely sandstone. The mudstone is generally poorly fossiliferous, and has yielded only fragments of mussels, and at Celynen North and Crumlin Planolites montanus, Curvirimula cf. candela (Dewar) and C. subovata (Dewar). The siltier rocks commonly contain plant remains.

The strata between the Five-Feet-Gellideg and Seven-Feet seams vary in thickness from 20 to 50 ft. They usually consist of seatearths, mudstones with ironstone nodules and silty mudstones; a few thin beds of sandstone occur in the thicker sections, e.g. at Celynen North.

The Seven-Feet (Meadow Vein) has been mined fairly extensively in the Ebbw Valley, and where the overlying Yard seam lies close above it may also be included in the worked section. Typical seam sections are:

In the southern part of Celynen South and at Abercarn the strata between the two leaves of the Seven-Feet thicken to 10 ft, and commonly include hard, quartzitic sandstones. The beds between the Seven-Feet and Yard are usually seatearths, though mudstones may occur immediately above the former where the parting thickens to a few feet. Such mudstones at Celynen North have yielded abundant plant remains, Spirorbis sp.and Naiadites sp.The roof of the Yard is most commonly the seat-earth of the overlying Amman Rider, which varies in thickness as follows: Crumlin and Celynen North, 8 in to 3 ft; Celynen South, 1 to 4 ft; Abercarn, up to 7 ft. At Llanhilleth the roof is mudstone or silty mudstone with abundant plant remains, which, with the overlying seatearth, totals 10 to 15 ft.

The Amman Rider is everywhere a single coal, and varies in thickness between 1 and 2 ft.

Tirpentwys-Cwmcarn area

At Tirpentwys the Garw (Rough Coal) was 10 in thick in an underground borehole [SO 2501 0017]; in the Glyn Colliery shafts and at Blaendare it is 22 in. In a railway cutting [SO 2750 0036] at Pontypool, Strahan recorded 2 ft of coal, probably the Garw, lying a short distance above the 'Farewell Rock'. There are no records of workings in the Garw.

At Upper Race, south of Pontypool, in disused patch-workings [ST 2788 9880] for the Spotted Mine, spoil heaps contain the distinctive Garw roof mudstones, which have yielded the following abundant fish scales: Elonichthys sp., Rhabdoderma sp., Rhadinichthys sp.and Rhizodopsis sp.Moore (1948, p. 247) found "Coelacanthus sp." in mudstone at the same locality.

The measures between the Garw and Five-Feet-Gellideg total 54 ft at Tirpentwys and 34 ft at Glyn and Blaendare; they consist mainly of interbedded sandstones, mudstones and ironstones. In the Glyn shafts 6.5-ft of measures containing beds of ironstone lie 4 ft above the Garw, and these are at the same horizon as the ironstones excavated in the patch workings at Upper Race, and in the disused ironstone level [ST 2778 9927] in the same area. They appear to have been the most extensively worked ironstones in the area, particularly in Glyn and Cwm Lickey collieries, and they were worked in small quantities in Cwmbran Colliery and Parfitts Level [ST 2721 9698] at Upper Cwmbran. At Glyn and Blaendare a thin coal, 5 to 7 in thick, lies above these ironstones, at about 12 ft above the Garw. At Tirpentwys the same coal is 10 in thick, and lies 17 ft above the Garw.

At Glyn the Five-Feet-Gellideg is underlain by 21 ft of sandstone, and the same bed, 22 ft thick, has been recorded at Blaendare. At the surface this sandstone has been traced for 0.5 mile south of the reservoir [ST 2770 9920] at Upper Race, immediately west of the patch workings for ironstone. It is visible in small exposures at a few localities along this stretch, where it is a hard, pale grey or white quartzite. Sandstone at the same horizon is visible in the Pontypool-Crumlin railway cutting [SO 2750 0036], where 8 ft of fine-grained sandstone with silty mudstone are exposed about 6 ft above the position of the 2-ft thick Garw recorded by Strahan.

The Five-Feet-Gellideg is typically in three leaves at Tirpentwys. The bottom coal (Gellideg) varies from 1 to 3 ft, and is usually separated from the middle coal by only 1 to 2 ft of measures, but these thicken locally up to 15 ft and contain hard, quartzitic sandstones which may carry water. The middle coal is 1 to 2 ft thick, and the upper about 2 ft; they are normally separated by 1 to 3 ft of seatearth, which thicken locally to 8 ft and include sandstones. The roof of the seam is commonly seatearth or mudstone with roots and plant remains, or, locally, hard, fine-grained sandstone.

A three-coal section is also present in the disused Mynydd Maen Colliery near Pontypool, where the seam was worked extensively north of the Glyn Valley. There the partings between the coals are not as variable as at Tirpentwys. South of the Glyn Valley the Five-Feet-Gellideg generally consists of two leaves; a bottom third leaf, up to 9 in thick, is present locally, e.g. at Cwm Lickey and parts of Hafodyrynys. At Hafodyrynys, the section, coal 21 in on seatearth 8 in, coal 25 in, recorded in the No. 2 Drift, is fairly typical of the colliery; the roof is mudstone with roots. A similar section occurs at Cwmcarn, where the seam is being worked a mile north of the shafts. The roof there is coarse, silty mudstone with plant remains, which passes up quickly into striped beds then sandstone. The seatearth is commonly sandy. In Cwm Lickey Colliery the Five-Feet-Gellideg totals 2.5 to 3 ft in three leaves lying close together; the bottom leaf is up to 9 in thick, the middle leaf 21 in and the upper leaf no more than 7 in. Along the east crop the Five-Feet-Gellideg, comprising two leaves, was exploited in several levels and drifts driven from the surface. West of Pontypool it was worked in Wern Hill [SO 2759 0080], Blaendare [SO 2757 0029] and Victoria [SO 2755 0022] collieries, and farther south from Blaendare Mine [ST 2724 9920] at Upper Race.

The measures between the Five-Feet-Gellideg and Seven-Feet vary between 7 and 25 ft; they consist of seatearths and mudstones, which locally, as at Glyn, Tirpentwys and Cwmcarn, are interbedded with sandstones. The seatearth below the Seven-Feet has been worked on a small scale from Cwmbran and Henllys.

The combined Seven-Feet and Yard (Meadow Vein) is 4.5 to 9.5 ft thick, and consists of three or four leaves generally separated by seatearth. The lower two or three leaves comprise the Seven-Feet, and the upper leaf the Yard. They have been worked extensively in the northern part of the area, and on a smaller scale in the south. At Tirpentwys, Hafodyrynys, Cwmcarn and Cwmbran the seam comprises three coals; the lower varies from 15 to 33 in, and is separated from the middle coal by 2 to 12 in of seatearth. The middle main coal is the thickest, usually between 21 and 54 in, and is generally separated from the upper by 6 to 18 in of seatearth. Locally however, for example, near the Cwmcarn shafts, the parting may be as much as 7.5 ft. The upper coal varies between 4 and 15 in. At Glyn and Blaendare collieries the combined Seven-Feet and Yard comprises four coals, there being two leaves below the thick, main coal, both between 10 and 24 in thick. At Tirpentwys and Cwmcarn the roof of the top coal, the Yard, is mudstone and silty mudstone with plant remains, passing up into striped beds and some sandstone. The following flora was collected at Tirpentwys: Annularia radiata (Brongniart), A. sphenophylloides (Zenker), Asterophyllites charaeformis (Sternberg), A. equisetiformis (Schlotheim), Calamites sp., Lepidodendron simile Kidston, Lycopodites carbonaceus (Feistmantel), Neuropteris heterophylla, N. obliqua (Brongniart), N. cf. tenuifolia (Schlotheim), Sphenophyllum cuneifoliunt (Sternberg), S. saxifragaefolium (Sternberg) and Sphenopteris striata Gothan. At Hafodyrynys the roof is seatearth.

Along the east crop the combined Seven-Feet and Yard has been worked on a small scale from levels driven on the seam. It was seen by Strahan in the entrance to a level [SO 2742 0044] 0.5 mile west of Pontypool, at Lower Race, which presumably worked the seam, and it was also exploited in Mountain Level c.[ST 2756 9928] at Upper Race.

The measures between the Yard and Amman Rider are 5 in to 45 ft thick. At Tirpentwys they comprise about 30 ft of mainly silty mudstone with a few thin sandstones. At Hafodyrynys, Glyn, Cwm Lickey and Cwmbran the thickness generally varies between 35 and 45 ft, though locally it decreases to 15 ft. Southwards, at Henllys, it is 16 ft, one mile north of the Cwmcarn shafts 3 ft, and near the Cwmcarn shafts as little as 5 in. The section in a drift [ST 2363 9363] 150 yd N. 30° W. of the Cwmcarn shafts is:

At Glyn, Cwm Lickey and Cwmbran up to 10 ft of mudstone containing ribs of ironstone, the Meadow Vein Mine, lie about 30 ft above the Yard, and have been worked in small amounts.

The Amman Rider (John Williams Coal) varies from 10 to 27 in of clean coal; it is. nowhere worked because of its high sulphur content.

South-east crop area

The only surface evidence of these measures is in spoil heaps from crop workings [ST 2382 9200] in Coed-y-Darren, where the characteristic roof rocks of the Garw, dark, silty mudstones containing fish remains, were found.

The section of the Five-Feet–Gellideg in Luther May's Drift [ST 2555 9449], a short heading driven from workings in the Nine-Feet in Henllys Colliery, is: coal 24 in on parting 30 in, coal 18 in. In Darren Mine [ST 2342 9181] north of Risca the seam is unrecorded, and may be represented by sandstone. The combined Seven-Feet and Yard section in Luther May's Drift is: coal 12 in (Yard) on seatearth 9.5 in, coal 45.5 in, seatearth 14 in, coal 18.5 in. It likewise appears to be absent in Darren Mine, and may be represented there by sandstone. Recently, (Squirrell and Downing 1964, p. 129) it was suggested that the Seven-Feet and Yard are represented by a thin coal in Darren Mine, but this is now considered to be the Amman Rider (see below).

The Amman Rider (John Williams Coal or Brass Vein) in Luther May's Drift is 27 in thick, and lies l3.5 ft above the Yard. In Darren Mine it is probably represented by 26 in of 'coal and clay'. H.C.S.

References

DE LA BECHE, H. T. 1846. On the Formation of the Rocks of South Wales and South Western England. Mem. Geol. Surv., 1, 1–296.

FORSTER-BROWN, T. 1865. The Caerphilly Mineral District. Proc. S. Wales Inst. Eng., 4, 86–131.

MOORE, L. R. 1945. The Geological Sequence of the South Wales Coalfield: The "South Crop" and Caerphilly Basin and its Correlation with the Taff Valley Sequence. Proc. S. Wales Inst. Eng., 60, 141–252.

MOORE, L. R. 1948. The Sequence and Structure of the Southern Portion of the East Crop of the South Wales Coalfield. Quart. J. Geol. Soc., 103, 261–300.

MOORE, L. R. and COX, A. H. 1944. The Coal Measure sequence in the Taff Valley and its correlation with the Rhondda Valley sequence. Proc. S. Wales Inst. Eng., 59, 189–304.

RAMSBOTTOM, W. H. C., RHYS, G. H. and SMITH, E. G. 1962. Boreholes in the Carboniferous rocks of the Ashover district, Derbyshire. Bull. Geol. Surv. Gt. Brit., No. 19, 75–142.

WOODLAND, A. W., ARCHER, A. A. and EVANS, W. B. 1957. Recent boreholes into the Lower Coal Measures below the Gellideg-Lower Pumpquart horizon in South Wales. Bull. Geol. Surv. Gt. Brit., No. 13, 39–60.

WOODLAND, A. W. and EVANS, W. B. 1964. The Geology of the South Wales Coalfield. Part IV. The Country around Pontypridd and Maesteg. 3rd edit. Mem. Geol. Surv.

Chapter 8 Middle Coal Measures: details

A. Amman Marine Band to Cefn Coed Marine Band

North Taff area

The measures between the Amman Rider and the Bute are normally between 20 and 25 ft thick, but east of the Gelligaer Fault in Deep Navigation Colliery their thickness is 18 ft, and in boreholes near Taff Merthyr shafts only 15 ft have been proved. The sequence comprises mudstones and silty mudstones with, in places, thin sandstone and ironstone bands. The Amman Marine Band, some 3 ft thick, is present at the base and has been identified in all the collieries apart from Deep Navigation, where the strata were inaccessible at the time of the resurvey. The marine band has yielded the following fauna at Albion: sponge spicules (pyritized), Lingula mytilloides, Levipustula piscariae (Waterlot), Orbiculoidea sp., Megalichthys sp.[scale].

In the south of the area the Bute consists of two coals, an upper leaf, normally about 24 in thick, separated by a parting of some 20 in from a thinner lower coal with an average thickness of 20 in, but with a known range of 10 to 30 in. To the north, in Deep Navigation, the coal sections are similar but the interval between the seams has increased to between 17 and 28 ft. Still farther north at Taff Merthyr, boreholes near the shafts proved the lower leaf to be 20 to 24 in thick and the upper leaf about 33 in thick. The interval between the seams is about 30 ft. No workings are known in either the lower or upper leaves. Planolites sp.and Gyrochorte carbonaria have been found some 3 to 7 ft above the lower leaf in Penallta.

Between the Bute and the Nine-Feet the measures vary from about 5 to 50 ft; the known minimum and maximum thicknesses have been recorded at Albion and Deep Navigation collieries respectively. The mainly argillaceous sequence includes thin, lenticular sandstones.

Compression structures have caused considerable variation in the section of the Nine-Feet, but in general, disregarding tectonic pinching and duplication, a single thick coal with few dirt partings splits into upper and lower seams to the north-east, this being part of the general regional split of the seam towards the east (see (Figure 17)). The undisturbed section in Albion and Abercynon consists of some 9 to 12 ft of coal including thin dirt partings. At Abercynon the seam may be divided into upper and lower leaves by 4 to 6 ft of measures. To the north in Deep Navigation, Taff Merthyr and Merthyr Vale collieries the Nine-Feet consists, in general, of a lower coal 4.5 to 5.5 ft thick separated by a dirt parting from an upper coal 2.5 to 3.5 ft thick. Both seams may be composite in character and the lower coal at Deep Navigation commonly contains a band of ironstone. North-east of the Deep Navigation shafts some 14 ft of strata intervene between the two coals, and in Taff Merthyr the split develops rapidly from under 2 ft near the shafts to over 34 ft in the eastern part of the colliery. Sections illustrating variations in the nature of the Nine-Feet are given below:

The seam has been worked from Albion, Abercynon, Deep Navigation and Merthyr Vale, though not extensively. The roof of the Nine-Feet commonly comprises a dark carbonaceous mudstone about 12 in thick, which contains in some localities, e.g. Deep Navigation, crushed mussels including Anthraconaia pulchella, A. aff. robertsoni (Brown), Anthracosia beaniana, A. nitida and A. phrygiana. About 3 ft above the seam grey mudstones contain the following fauna of small, solid mussels in Albion, Abercynon and Deep Navigation: cf. Anthraconaia pulchella, Anthracosia cf. aquilina (J. de C. Sowerby), A. sp. cf. concinna, A. cf. nitida, A. cf. ovum and A. cf. phrygiana.

The thickness of the measures between the Nine-Feet and the Six-Feet varies from about 66 to 100 ft; in the south 90 to 100 ft appear to be usual, whereas in the north the range is normally between 70 and 90 ft. The sequence is predominantly mudstone, but includes ironstones, both as nodules and thin bands, and interbedded thin sandstones. Thin or poor quality coal seams occur at two horizons and represent the Red Vein group of seams. This nomenclature should not be confused with the seam name applied to the upper coal of the Nine-Feet in parts of the Newport district.

The lower coal of the Red Vein group is about 6 in thick and lies between 20 and 62 ft above the Nine-Feet (the maximum being attained in Abercynon Colliery) and 10 to 30 ft below the upper coal of the group. The upper coal of the Red Vein group is a composite seam, commonly in three leaves. It lies 6 to 40 ft below the Six-Feet, the intervening strata being mudstone and seatearth. In Deep Navigation up to four seams, occurring 6 to 15 ft below the Six-Feet, represent the same horizon, and a similar series of thin seams extending over a vertical range of almost 20 ft have been proved in borings at Taff Merthyr. There the interval between the Six-Feet and the upper leaf of the Red Vein group ranges up to almost 40 ft. Both seams of the Red Vein group in Taff Merthyr become part of the Lower Six-Feet of the North Rhymney Area.

The Six-Feet normally consists of about 6 ft of coal, with possibly two very thin dirt partings in the south of Albion. Towards the east of the colliery area there is a tendency for a three-coal section to develop, and this persists into the adjoining North Rhymney Area. Elsewhere the Six-Feet normally comprises two coals. A typical section in the west of Deep Navigation is: coal 42 in on dirt and inferior -coal 22 in, coal 39 in. Generally both the upper and lower coals thin in an easterly direction towards Penallta. North-east of Deep Navigation shafts the upper coal tends to split into a number of thin leaves, but in Taff Merthyr the upper coal is about 36 in thick, and the thickness of the lower coal is somewhat less than that recorded to the south. Borehole evidence in the north-east of Taff Merthyr suggests that a 3-coal section may be usual in this area. The Six-Feet has been worked extensively from Albion, Abercynon and Deep Navigation collieries, but virgin areas remain in the north and between Abercynon and Deep Navigation.

Eighty feet of measures separate the Six-Feet and Four-Feet seams in Albion Colliery North Shaft, but the interval decreases to the north and in Abercynon ranges from 50 ft in the south of the colliery to 12 ft in the north. North of the Abercynon Syncline the interval again thickens and in Deep Navigation is normally 50 ft. This is maintained into the Taff Merthyr area, but in the extreme north of this colliery take the two seams have approached to within 14 ft. The measures in the north comprise mainly mudstones with a few thin sandstones, but in the south the sequence is more arenaceous and includes striped beds, composed predominantly of siltstone and sandstone. In Albion a sandstone 50 ft thick occurs 4 ft above the top of the Six-Feet, and in places rests on the coal. The following fossils were collected from the mudstone roof of the seam: Anthracosia cf. elliptica, A. lateralis, A. aff. lateralis, A. sp. nov.cf. simulans Trueman and Weir, cf. Anthracosphaerium propinquum (Melville), Naiadites productus, N. sp. nov.

The Four-Feet is a composite coal extensively worked throughout the area. The following sections are typical in the south of the area:

A section examined in Albion included 7 in of cannel in the top of the seam. West of Deep Navigation shafts the worked section is about 4 ft 6 in and this represents the middle two coals in the above sections. The worked section is underlain by 15 in of inferior coal, and probably is overlain by a thin coal, as in Taff Merthyr, where a typical section is: coal 14 in on dirt 1 in, coal 52 in (including a dirt parting of variable thickness), inferior coal 4 in. In the north-eastern districts of Taff Merthyr the section is: coal 15 in on dirt 6 in, coal 27 in, dirt 10 in, coal 30 in; the lower two coals are worked. This section splits in Grosfaen Colliery, the lower coal forming the thin riders at the top of the Upper Six-Feet, and the centre coal develops into the Four-Feet of Grosfaen. Whether the upper thin seam also becomes part of the Four-Feet, or forms the lower leaf of the Grosfaen Two-Foot-Nine is uncertain.

A thin coal, up to 8 in thick, occurs 5 to 10 ft below the Four-Feet in the Taff Bargoed Valley. The Four-Feet is affected by a wash-out about 0.5 mile wide coursing south-west from Bedlinog. It has been encountered in workings from Merthyr Vale, Deep Navigation and Taff Merthyr; within the area affected only isolated pockets of coal are likely to remain.

The thickness of the measures between the Four-Feet and the Two-Feet-Nine seams is usually between 15 and 20 ft, although in parts of the take of Abercynon the base of the Two-Feet-Nine is 40 ft above the Four-Feet. At Taff Merthyr the interval varies from 12 to 70 ft, and in Bedlinog Shafts it is 75 ft. The sequence consists primarily of seatearth and mudstone, and the roof mudstones of the Four-Feet commonly carry a characteristic fauna of mussels. The following were found at Albion: Anthraconaia ellipsoides, Anthracosia acutella, A. atra, A. sp.cf. atra [short form], Anthracosphaerium exiguum, Naiadites aff. alatus, N. cf. productus, N. obliquus?. Plant remains may also be found with the mussels in the roof mudstones, as shown in the following collection from Abercynon: Neuropteris sp.cf. scheuchzeri, N. tenuifolia, Sphenophyllum myriophyllum Cr™épin, Anthraconaia cf. cymbula, Anthracosia sp., Naiadites alatus?, N. productus. Locally, in Abercynon, the roof of the seam is a 9-in band of conglomeratic sandstone. This is succeeded by a silty mudstone with plant remains.

The Two-Feet-Nine is a composite seam with a very variable section. Although only extremely small areas have been worked, the seam is exposed in roads driven for air bridges. The section in Albion is: coal 34 in on dirt 3 in, coal 15 in; the lower part of the top coal is of inferior quality. A thin seam, some 3 in thick, occurs about 7 ft above the Two-Feet-Nine in the colliery shafts. At Abercynon the bottom coal appears to have split into two leaves to give a three-coal section as follows: coal 24 in on dirt 1 in, coal 9 in, dirt 1 in, coal 6 in. The general section of the Two-Feet-Nine in Deep Navigation comprises a lower leaf about 42 in thick overlain by a number of thin coals. Farther north in Taff Merthyr and Bedlinog the seam has split into a number of thin coals.

The sequence between the Two-Feet-Nine and Cefn Coed Marine Band is visible only in the East Return at Deep Navigation, where the beds dip in an easterly direction into the Llanfabon Fault plane [ST 1027 9750]. The succession in this heading is as follows:

The Hafod Heulog Marine Band yielded a very sparse fauna of Lingula mytilloides.

Elsewhere in the area the sequence is known only from details recorded during the sinking of the colliery shafts. In the Bargoed Tâf Valley three coals occur between the Two-Feet-Nine and the Cefn Coed Marine Band. The highest seam carries the marine band, and the first seam above the Two-Feet-Nine carries the Hafod Heulog Marine Band. The sequence in the south of the area is not so clear, but possibly in a full sequence 4 seams (or at Albion 5 seams, if the thin coal overlying the Two-Feet-Nine is included) intervene between the Two-Feet-Nine and the Cefn Coed Marine Band. The lowest of these coals is about 2 ft thick in the Abercynon shafts, but the other two are always less than 12 in. The seam below the Cefn Coed Marine Band is 2 or 3 in thick, and has frequently not been recorded in shaft sections, suggesting an impersistent development. The thickness of the measures in this part of the sequence varies between 50 and 70 ft, and the beds consist principally of mudstone with ironstones although sandstones are not uncommon. In particular, at Taff Merthyr Colliery, the Two-Feet-Nine is overlain by 27 ft of sandstone including two beds of 'hard white rock' presumably quartzite, and the strata between the presumed horizon of the Hafod Heulog Marine Band and the succeeding coal are more than half sandstone. The thin coal above the Two-Feet-Nine in the Albion Colliery shafts is succeeded by 17 ft of sandstone. The Cefn Coed Marine Band is exposed in the East Return at Deep Navigation, where the following fauna was found in the roof of a 2-in coal: Lingula mytilloides, costate productoid fragment, Straparollus sp., Hollinella sp.

North Rhymney and Sirhowy area

The Amman Rider and Bute are separated by about 50 ft of strata in the northwest. At Oakdale the sequence is 40 ft thick, but in Penallta it is variable and ranges from about 25 to 55 ft. Near and to the east of the shafts some 40 to 55 ft separate the two seams, but 30 to 34 ft are generally present in the south of the take. The Amman Marine Band is present in all the collieries in the area and has yielded Ammodiscus sp., sponge spicules, Paraconularia sp., Lingula mytilloides, costate productoids, small turreted gastropods and Rhizodopsis sp.At Britannia 8 ft of marine strata overlie the Amman Rider, at Grosfaen the band is just under 4 ft thick, while 34 ft occur at Penallta. Typically, the lower part consists of pyritic, grey mudstone containing very small specimens of L. mytilloides, overlain by grey mudstone without pyrite, but with larger L. mytilloides. A section of the band at Grosfaen is:

The sequence above the marine strata is essentially argillaceous, though sandstone beds, up to 5 ft thick, but usually less than this, are developed, for example, near the middle of the sequence at Britannia and near the top at Oakdale. The horizon of a thin coal (believed to be the Lower Bute) which occurs in this sequence is discussed more conveniently below.

The seam referred to as the Bute is probably the equivalent of the upper leaf of the double coal characteristic of the Bute in areas to the west. In this area the upper leaf is locally named the Brass, from its tendency to contain pyrite, a name which also has been applied to the Amman Rider. Throughout the area the seam is normally between 18 and 24 in. The sections in Oakdale suggest a deterioration in the quality in the east. Towards the south of the Penallta take a thin coal horizon may be developed above the main coal giving the following typical section: coal or coal and rashings 3 in on dirt 14 in, coal 17 in.

The lower leaf of the Bute appears to be impersistently developed and to nip out in the east. In Penallta a seam of coal or coal and rashings some 18 in thick is present between 8 and 28 ft below the Bute (i.e. Upper Bute) at several localities and a 7-in inferior coal can be seen in a 'roof-up' near Grosfaen shafts, between the Amman Rider and the Bute (upper) although it is not recorded in either shaft section. A seatearth about 35 ft below the Bute (upper) in Bargoed North Pit may represent the same horizon. There is no indication of this coal in Oakdale.

The measures between the Upper Bute and the Nine-Feet are 26 ft thick in Grosfaen West Pit, this being the maximum known in the area. In the vicinity of Penallta shafts 10 to 15 ft occur, but in the east and south-east of the take this has thinned to between 7 and 10 ft. Near Britannia shafts 7 ft separate the two seams and in Bargoed shafts 9 and 12 ft occur. Farther east at Oakdale, variable thicknesses between 7 and about 20 ft have been recorded. Seatearth, commonly with ironstone nodules, comprise the sequence where the interval between the seams is thin, but mudstones are developed in the lower part as the measures thicken. These are commonly silty and may contain rootlets and plants, as at Britannia. The seatearth below the Nine-Feet in Penallta shafts contains a thin seam of coal or rashings about 8 or 9 ft below the seam.

The Nine-Feet forms a single seam only in the south-west and in the extreme east; elsewhere two distinct seams occur, the upper being known as the Red Vein and the lower as the Rhas Las in the Rhymney Valley, but at Oakdale they are referred to as the Upper and Lower Rhas Las. As a result of compressive forces and the incompetent nature of such thick seams, the coals are either abnormally thick or thin in many localities, and the seam sections may vary considerably within very limited areas.

Generally the Lower Nine-Feet consists of a lower coal between 3 and 4.5 ft thick overlain by one or two thin riders, more than one rider tending to occur as the separation between the Upper and Lower Nine-Feet increases. The lower leaf of the seam is also split in some localities into two coals by a thin parting, which, as in Grosfaen, may consist of pyrite. The following sections illustrate the nature of the seam in the area, but they should not be taken to represent the section throughout the take of the collieries named, for each variation can usually be found in each colliery.

At Bargoed the roof mudstones of the Lower Nine-Feet have yielded Anthraconaia pumila (Salter), cf. Anthracosia nitida and Naiadites cf. productus. The interval between the Upper and Lower Nine-Feet reaches a maximum of over 50 ft just to the west of Grosfaen shafts. From there it decreases to the south and east. About 38 ft are present in Grosfaen shafts, about 30 ft near Bargoed shafts and in the Penallta and Britannia takes the separation ranges up to 25 ft. In the south-west 12 in of carbonaceous mudstone separate the two seams, and in the Oakdale take the interval 3 decreases in an easterly direction from about 3.5 ft, until the Nine-Feet is represented by 8 ft of clean coal. These figures indicate the general trend, but locally the parting varies markedly within relatively limited areas.

The Upper Nine-Feet in the north-west of the area is normally about 36 in thick, but varies between 21 and 43 inches. Farther south, at Britannia and Penallta, a somewhat thicker section of between 48 and 54 in occurs, while towards the east at Oakdale 24 to 30 in is usual. The seam is subject to splitting and this commonly takes the form of thin splits from the base of the seam, as for example in this section measured at Britannia: coal 36 in on dirt 22 in, coal 5 in, dirt 5 in, coal 5 in.

The roof mudstones of the Nine-Feet at Oakdale have yielded: Gyrochorte carbonaria, cf. Planolites montanus, Anthracosia beaniana [small], A. cf. nitida, Naiadites cf. productus [juv. or stunted]. At Penallta a mussel band occurring in grey, blocky mudstone about 39 in above the seam includes: Anthraconaia cf. pulchella, Anthracosia cf. angulata, A. cf. nitida. Elsewhere in the area the mudstone roof of the seam carries a poorly represented fauna of crushed shells and shell fragments.

Near Britannia shafts the Nine-Feet and Six-Feet are separated by over 100 ft of measures; 135 ft are present in Britannia North Pit. Over the remainder of the area the thickness is less than this. Generally the interval is greater in the south and east than in the north-west. In Grosfaen and Bargoed 25 to 40 ft is the usual range, the average probably being about 30 ft. About 70 ft occur in the north of Oakdale, 50 ft in Oakdale North Pit and between 50 and 70 ft in the Penallta take. The sequence is essentially of an argillaceous nature. Ironstone bands and nodules are relatively common and thin sandstones are developed in places. Where the maximum thickness is attained near Britannia shafts, the central part of the sequence comprises sandstones interbedded with mudstones, and the sandstones constitute almost 50% of the total sequence between the Nine-Feet and the Six-Feet. The Red Vein group of coals, prominent in areas to the west, becomes part of the Lower Six-Feet in the north of the area. In Penallta the position of the lower coal in the group possibly may be indicated by rashings about 20 ft above the Nine-Feet proved in a borehole 4200 ft east of the shafts. Thin coals 5 to 10 ft below the Six-Feet probably represent the upper part of the group. To the north these coals become part of the Six-Feet.

In the south of the area the Six-Feet is a compound seam, generally consisting of three leaves, although with a greater number in places as a result of splitting, commonly of the centre leaf. At Penallta the upper and lower leaves vary between about 14 and 42 in, while the middle leaf is normally about 18 in. The following section indicates the general nature of the seam in the colliery; coal 20 in on dirt 1 in, coal 14 in, dirt 4 in, coal 38 in. Variations in the seam at Britannia are illustrated by the following sections, all measured in workings to the south-west of the shafts:

The seam has been worked at Britannia and extensively at Penallta.

North of Britannia the Six-Feet splits into numerous coals and the section is very variable. The upper part of the seam, the Upper Six-Feet, is referred to locally as the Yard, the main coal of which is worked at Bargoed and Grosfaen. The lower part, that is the Lower Six-Feet, is known as the Three Quarter. It is a dirty seam only -worked to a small extent at Bargoed. The nature of the Six-Feet at Bargoed may be illustrated by the following section: Upper Six-Feet–coal 12 in on dirt 2 in, coal 6 in, dirt 7 in, coal 3 in, dirt 11 in, coal 31 in (worked section), dirt variable, Lower Six-Feet–coal 9 in, dirt 5 in, inferior coal 13 in, dirt 15 in, coal 20 in, dirt 4 in, coal 10 in.

A few feet below the Lower Six-Feet thin seams representing the Red Vein group occur. A major split develops between the Upper and Lower Six-Feet to the north and west of Bargoed shafts and north of Oakdale shafts. In the north of the Bargoed and Oakdale takes the intervening measures are about 50 and 70 ft thick respectively, while some 25 to 30 ft are present in the vicinity of Grosfaen shafts. The separation of the Upper and Lower Six-Feet in Bargoed Colliery appears to be accompanied by splitting of the Lower Six-Feet, for a trial pit [SO 1566 0071] in the northern part of the colliery take proved the various coals of the Lower Six-Feet interleaved through some 25 ft of measures. Argillaceous strata normally comprise the sequence between the Upper and Lower Six-Feet, but where the maximum thickness is attained in the north, sandstones are also developed. Plants are conspicuous in the roof of the Lower Six-Feet at Oakdale.

The general section of the two parts of the Six-Feet in Grosfaen is:

The lower part of the Lower Six-Feet probably represents the Red Vein group.

At Oakdale the Lower Six-Feet is not worked. It varies between 24 and 36 in and a thin coal up to 8 in thick commonly occurs within 10 ft of the base of the main leaf. The upper Six-Feet is composite in character and the main leaf, which is worked, is normally between 27 and 38 in. The following section indicates the general nature of the seam, but each stratum is subject to variation in thickness: coal and rashing 12 in on seatearth 48 in, coal 2 in, dirt 1 in, coal about 36 in, dirt 2 in, coal 2 in, dirt 2 in, coal 2 in. The thin coals occurring above the main coal of the Upper Six-Feet in Grosfaen, Bargoed and Oakdale are believed to equate with the lower part of the Four-Feet as developed at Britannia and Penallta (see below). The mudstone roof of the Upper Six-Feet carries a poor fauna of mussels at Penallta and Bargoed. The following were collected in the former colliery: Anthracosia cf. concinna, A. elliptica, A. lateralis? and Naiadites sp., together with cf. Planolites sp.Locally in Bargoed Colliery the roof consists of arenaceous rocks. At Grosfaen, where silty mudstone forms the roof strata, plants were the only fossil remains recorded during the survey.

The thickness of the measures between the Six-Feet and the Four-Feet is extremely variable. In the south-west, at Penallta, 50 ft are usually present. These decrease to about 20 ft in the south of the take, and also towards the north where 25 ft is normal in Grosfaen. At Oakdale 30 to 50 ft occur near the shafts, but this thickness decreases towards the north. The main variations occur in Britannia and Bargoed collieries. The two seams are separated by no more than 18 in of carbonaceous shale in the Britannia shafts, yet just over 1 mile to the north 105 ft of measures occur between the seams in the Bargoed shafts, and about 0.5 mile farther to the north-north-east the interval between the main leaves of the Upper Six-Feet and the Four-Feet is less than 4 ft. It is worth noting that despite these rapid variations within a very limited area the interval between the Nine-Feet and the Four-Feet remains fairly constant at 130 to 150 ft. As the thickness variations of the sequence between the Nine-Feet and the Upper Six-Feet are in general the inverse of those noted between the Upper Six-Feet and the Four-Feet, the thinning of the beds between the Six-Feet and the Four-Feet in the north of Bargoed Colliery is part of the general northerly thinning already noted at Oakdale, and between Penallta and Grosfaen. The measures separating the seams are mainly argillaceous where the thickness is less than 50 ft, but sandstones are developed where more than this occurs. Thin sandstone bands are present at Penallta in the lower part of the sequence, and sandstones up to 20 ft thick are recorded near and to the south-east of Bargoed shafts, the measures exceeding 75 ft there.

The Four-Feet, usually referred to as the Upper Four-Feet in the Rhymney Valley, is a composite seam throughout the area. The principal coal displays little variation over the northern part of the area, but the seam as a whole is subject to considerable splitting closely related to the splitting of both the Six-Feet and the Two-Feet-Nine. At Penallta the following four-coal section is typical: coal 14 in on dirt 2 in, coal 20 in, coal and mudstone 8 in, coal 24 in, dirt 2 in, coal 6 in. In the south of the take these seams are interleaved through some 30 ft of measures. At Britannia the main coal is about 38 in thick, which in the south of the take is divided by a thin dirt parting as follows: coal 15 in on dirt in, coal 22 in. The precise relationship of this coal to the Penallta Four-Feet is not clear, but the latter undoubtedly splits towards the north and the upper part probably equates with the thin riders above the main coal at Britannia, which lie within 30 to possibly some 50 ft of measures above the main coal. The section of the Four-Feet (excluding the thin riders above the main coal) in the Britannia shafts, where it is separated from the Six-Feet by a parting no more than 18 in thick is: coal 34 in on dirt 22 in, coal 3 in, dirt 14 in, coal 18 in, dirt 9 in, coal 8 in, dirt 21 in, coal 3 in. The main coal in Bargoed and Grosfaen collieries has a fairly consistent thickness of 36 in. The lower leaves of the seam at Britannia appear to split from the main coal to form the riders above the Upper Six-Feet at Bargoed and Grosfaen. A series of thin seams is also developed above the main coal in these collieries, as indicated in the following section: coal 14 in on dirt 12 in, coal 8 in, dirt 4 in, coal 6 in, dirt 6 in, coal 10 in, seatearth 72 in, coal 33 in. These thin seams equate with the riders above the main coal of the Four-Feet at Britannia. In Grosfaen the thin riders form the lower part of the seam referred to as the Two-Feet-Nine at that colliery (see p. 151). At Oakdale, where the Four-Feet is known as the Big Vein, the main coal again averages 36 in, though varying between 30 and 40 in. The two thin coals normally lying within a few feet of the base of the main coal, together with the thin rider seams of the Upper Six-Feet, are probably equivalent to the lower part of the Britannia Four-Feet. Locally, most of these thin seams split from the main coals and lie within 20 ft of measures near the middle of the sequence between the principal leaves of the Upper Six-Feet and Four-Feet. No rider coals have been seen in sections of the measures above the Four-Feet at Oakdale. The main leaf of the Four Feet has been worked extensively in the north, but less so in the south. A poor fauna of crushed mussels occurs in the smooth grey mudstone forming the roof of the main leaf of the Four-Feet at Grosfaen and Britannia, and a similar fauna together with plant fragments is present in Oakdale. In Britannia the roof of the thin rider a few feet above the main leaf also carries a fauna of crushed mussels.

The thickness of the measures between the Four-Feet and the Two-Feet-Nine in Penallta varies from about 4 to 15 ft. At Oakdale the interval is about 45 ft. Elsewhere direct comparisons are difficult because of the splitting of the upper part of the Four-Feet, as developed at Penallta, into several thin leaves, but it may be useful to indicate the general interval in the north-west between the main leaf of the Four-Feet, that is the coal normally worked, and the Two-Feet-Nine. In the Britannia shafts this amounts to 61 ft, though 1600 yards north-east and 1100 yards south-west of the shafts it is known to be only some 35 ft. Approximately 20 to 35 ft are present in Bargoed, while in Grosfaen 10 to 15 ft is normal. A predominantly argillaceous sequence occurs between the main leaf of the Four-Feet and the Two-Feet-Nine. Where the measures are relatively thin, as at Grosfaen and in some districts in Penallta, carbonaceous mudstones and/or seatearth are the principal rock types; elsewhere mudstones containing ironstones, both as nodules and bands, are equally important.

The Two-Feet-Nine has been little worked. The seam section is rather variable. At Penallta it comprises normally two leaves about 15 in thick, separated by a dirt parting up to 24 in. A similar two-coal section is present [ST 171 986] north-east of Britannia shafts, but near and south-west of the shafts the available information indicates a single coal about 32 in thick. South-east of the Bargoed shafts the Two-Feet-Nine is a dirty seam comprising 14 in of coal underlain by some 36 in of inferior coal and carbonaceous mudstone. A multiple coal section is present in the South Shaft and in the workings west of the shafts as detailed below:

The lower part of the seam referred to as the Two-Feet-Nine at Grosfaen actually equates with the upper section of the Four-Feet, and the following record of the sequence in the West Pit above the main leaf of the Four-Feet indicates the general nature of the measures: coal 12 in on dirt 2 in, coal 10 in, dirt 19 in, coal 18 in, dirt 2 in, coal 14 in, dirt 22 in, coal 12 in, dirt 1 in, coal 12 in, dirt 66 in, coal 39 in (main leaf of the Four-Feet). The upper three coals possibly equate with the Two-Feet-Nine of the remainder of the area. Though little is known of the Two-Feet-Nine in Oakdale, the available information suggests that the section is variable. In the North Shaft the seam is recorded as 35 in of coal, but 500 yards to the north-north-east the following composite section occurs in a cross-measures drivage; coal 13 in on dirt 2 in, inferior coal 2 in, dirt 2 in, coal 8 in, seatearth 36 in, dirt 1 in, coal 2 in.

At Penallta one or two thin coals occur in the 18 to 36 in of thinly bedded carbonaceous mudstone and seatearths overlying the Two-Feet-Nine, Above this the sequence is continued by mudstone and sandstones of variable thickness up to a 10-in coal, about 20 ft above the Two-Feet-Nine. The 3 to 5 ft of measures forming the immediate roof of the Two-Feet-Nine in parts of Britannia are similar to the sequence described at Penallta, but in other districts of the colliery the roof may be thin quartzitic sandstone. The overlying measures up to the 7-in coal some 12 to 16 ft above the Two-Feet-Nine comprise seatearths and silty mudstones. The thin leaves of coal occurring in the roof measures of the Two-Feet-Nine at Penallta and Britannia may well form the upper part of the Two-Feet-Nine in the remainder of the area. In the Bargoed shafts a sandstone about 9 ft thick forms the roof of the Two-Feet-Nine, and this is succeeded by measures, mainly seatearths, some 7 to 9 ft thick, which are overlain by a 12-in coal. A coarse-grained quartzitic sandstone rests on the Two-Feet-Nine in an air-bridge west of the Grosfaen shafts, but in the shafts themselves the roof is 'clift' which, with seatearths, comprises the 23 ft of measures between the Two-Feet-Nine and the next succeeding coal, a seam 12 in thick. The roof of the Two-Feet-Nine in Oakdale North Shaft is a sandstone about 7 ft thick. 'Cliff, interbedded with a further 5 ft of sandstone, completes the sequence up to the first coal above the Two-Feet-Nine.

The Hafod Heulog Marine Band occurs in the mudstone roof of the thin coal usually lying 12 to 27 ft above the Two-Feet-Nine, which is the lowest of the three thin coals developed between the Two-Feet-Nine and the Cefn Coed Marine Band. The lower marine band has been identified at Penallta, Britannia and Grosfaen collieries. The mudstones have yielded only Lingula mytilloides, L. cf. elongata Demanet and Orbiculoidea cf. nitida. The position of the band at Bargoed is uncertain, as the relevant part of the sequence is inaccessible, and only two coals occur in the shafts between the Two-Feet-Nine and the presumed position of the Cefn Coed Marine Band. Whether the lower seam equates with the first or second coal above the Two-Feet-Nine in the remainder of the area is not known.

Generally an argillaceous sequence of seatearths and mudstones intervenes between the Hafod Heulog and Cefn Coed marine bands. A coal, 10 to 19 in thick, lies 4 to 16 ft above the base of the lower marine band, and a further thin coal about 6 in thick underlies the Cefn Coed Marine Band. The latter seam is normally about 20 ft above the base of the Hafod Heulog Marine Band, but at Oakdale, where the sequence is not now accessible, the interval is 36 ft if the presumption is correct that the Cefn Coed Marine Band overlies the two thin beds of rashings 64 ft above the Two-Feet-Nine.

An examination of the records of the sequence in the Grosfaen Colliery shafts suggest that three seams intervene between the Two-Feet-Nine and the Cefn Coed Marine Band, though no more than two are recorded in each shaft. The thin coal below the Cefn Coed Marine Band, and the marine band itself, appears to be 'washed out' in the West Pit. There the middle seam of the three coals is overlain by 3.5 ft of 'clift' and 18 ft of sandstone. In the East Pit the lowest of the three seams appears to be absent.

The Cefn Coed Marine Band is exposed in the West levels [ST 130 948] at Penallta, where, in 30 to 36 in of mudstone, the following fossils were found: Paraconularia sp., Lingula mytilloides and Orbiculoidea cf. nitida. The marine band was also proved in the South Level, 400 yd south of No. 2 Pit, where it yielded L. mytilloides, O. cf. nitida, Edmondia ?, Anthracoceras sp.[fragments] and ostracods (cf. Geisina juv.).R.A.D.

Nantgarw–Risca area

The succession between the Amman Rider and Bute usually comprises predominantly argillaceous beds with some sandstone, which vary inconsistently in thickness from 10 to 40 ft. The Amman Marine Band occurs at the base, and generally consists of 4 to 10 ft of blocky, medium to dark grey mudstone with pyrite granules and ironstone nodules. It was proved by Moore (1945) in Risca, Nine Mile Point, Bedwas, Llanbradach, Windsor and New Rockwood collieries, and in Wern-ddu Claypit. During the resurvey it was also found at Wyllie and Nantgarw, and in the former colliery it proved to be richly fossiliferous, e.g. in the Golden Mile cross-measures [ST 1786 9347] as follows: cf. Planolites ophthalmoides, Paraconularia sp., Lingula mytilloides, Orbiculoidea sp., Levipustula piscariae. In Windsor it is up to 9 ft thick and yielded sponge spicules, Paraconularia sp., crinoid columnals [pentagonal], Levipustula sp., L. mytilloides, 0. cf. nitida and a small, turreted gastropod.

Above the Amman Marine Band the mudstones pass gradually into siltier beds, which normally continue to the base of the Bute seatearth. Locally, e.g. in Ffynnon Wen Borehole, up to 10 ft of sandstone occur below the Bute seatearth. The Bute has been worked only on a very small scale along the south crop at South Cambria, Lan, New Rockwood, Blackbrook, Van and Rudry collieries. The seam varies in thickness from 8 in to 10.5 ft. Over much of the area it is formed of two leaves totalling 2 to 5 ft, which are separated by 1 in to 20 ft of argillaceous beds. Typical sections are:

At a few localities the Bute consists of a single coal only, e.g. Nantgarw South Shaft (3.5 ft), Nantgarw No. 1 East Cross-measures 380 Horizon (3 ft), Bedwas shafts (15 in) and Black Vein Colliery shafts (1.5 ft). Locally, in the south-west, the Bute is exceptionally well developed, as below:

The mudstone roof of the lower leaf of the Bute at Wyllie contains: Anthracosia cf. ovum [juv.], Anthracosphaerium exiguum and Naiadites quadratus? The upper leaf is usually overlain by fossiliferous mudstones, which in Windsor yielded: cf. Planolites montanus, Anthraconaia modiolaris (J. de C. Sowerby), A. aff. williamsoni, Naiadites sp.(intermediate between quadratus and productus), Leaia cf. pruvosti Raymond and Lioestheria vinti.

The measures between the Bute and Nine-Feet seams usually vary irregularly in thickness between 10 and 40 ft. The beds consist dominantly of mudstones, silty mudstones and seatearths, and a few thin sandstones. The sections in Wern-ddu Claypit (Moore 1945, p. 155), and in Ffynnon Wen and Gypsy Lane boreholes (Appendix I) are typical.

The Nine-Feet is split into two main coals (Lower and Upper Nine-Feet) in the west and central part of the area (Figure 17), and in the east, at Nine Mile Point, Risca, Rudry and Wern-ddu, the coals lie close together, or merge completely. The Lower Nine-Feet is absent in the Wyllie shafts, Gypsy Lane Borehole and Windsor Underground Borehole [ST 1206 8794], probably due to non-deposition. Where present it usually varies in thickness between 2 and 5 ft. It is in places a single, clean coal, but it commonly contains dirt partings, as in the Universal Colliery Lancaster Shaft, where six partings totalling 1 ft 9 in occur in 3 ft 8 in of coal.

In the west and central part of the area the argillaceous measures separating the Lower and Upper Nine-Feet vary in thickness as follows: Ffynnon Wen Borehole 45 ft, Llanbradach 40 ft, Nantgarw 12 to 23 ft, Universal 12 ft, New Rockwood No. 2 Borehole 14 ft. These strata generally contain a high proportion of seatearth. The Upper Nine-Feet has been worked fairly extensively at Wyllie, Bedwas, Llanbradach, Universal and Windsor, and to a limited extent in most of the collieries along the south crop. It is usually a single, clean coal, and generally varies in thickness between 2 and 5 ft, though it may be thicker locally, as in Gypsy Lane Borehole, where it is 7.5 ft.

In the east the combined Lower and Upper Nine-Feet is known as the Black Vein. It has been worked very extensively at Nine Mile Point, Risca and the Black Vein Colliery, and to a small extent along the south crop. The seam usually varies in thickness between 3 and 9 ft, and commonly consists of two coals separated by a thin parting, though in some parts it is a single, clean coal. The following sections are fairly typical:

In Wern-ddu Claypit (Moore 1945, p. 155) the seam is exceptionally thick: coal 66 in on rashings and fireclay 18 in, coal 3 to 12 in, fireclay with coaly rashings 30 in, coal 54 in, rashings 2 in, coal 9 in, rashings 1 in, coal 6 in.

The roof of the Nine-Feet is usually mudstone, which in several collieries contains the distinctive dwarf mussel fauna of this horizon, e.g. at Nine Mile Point, in the shaft pillar [ST 1926 9136] and [ST 1921 9134], as follows: Anthraconaia aff. fisheri [small, cf. Weir 1966, pl. 39, figs. 5, 7], A. cf. obscuras, A. pulchella, Anthracosia cf. angulata, cf. A. nitida and A. ovum? Mr. Calver reports that the small size and indifferent preservation of this fauna make the generic identification of individuals a matter of uncertainty, but it probably represents an Anthraconaia aff. pulchella assemblage, with trends towards Anthracosia nitida and A. ovum.

The measures separating the Nine-Feet and Six-Feet vary irregularly from 20 to 100 ft. Abrupt changes in thickness occur, e.g. at Nantgarw the measures thin from 100 ft in the shafts to 21 ft in the Main Cross-measures 380 Horizon 800 yd to the south, and then thickens again to 80 ft in the No. 1 East Cross-measures 380 Horizon 900 yd farther east. The measures consist predominantly of mudstones, silty mudstones and seatearths, and a few thin sandstones. Only in New Rockwood No. 2 Borehole are there many sandstones, and there they occur fairly commonly throughout. Within the measures in the western and north-western parts of the area there are one or more thin seams which comprise the Red Vein group of coals. These generally occur in the upper part of the sequence, and in some parts lie very close to the Six-Feet. The Red Vein group totals 8 to 40 in of coal in one to three leaves in up to 15 ft of strata. Relevant sections are:

In the southern, south-eastern and eastern parts of the area the Red Vein coals either join the overlying Six-Feet, or die out.

The Six-Feet can be recognized as an individual seam over all the area, except around Wern-ddu, Rudry and Risca where it forms the basal member of the 'Big Vein Group' (p. 108). The Six-Feet has been worked on a large scale at Windsor, Universal and Llanbradach in the north-west, and on a small scale at Nine Mile Point, Wyllie and Bedwas, and along the south crop at South Cambria, Lan, New Rockwood and Van. In the north-west part of the area the Six-Feet usually consists of two leaves of fairly equal thickness, generally between 2 and 3 ft; the upper leaf may contain a thin parting near the middle. The following sections are typical:

Around Nantgarw and at Bedwas (Moore 1945, p. 185) the Six-Feet is very variable and contains up to five partings. The mudstone roof of the seam yielded a mussel fauna, consisting commonly of species of Anthracosia, at several collieries including Nantgarw, Windsor, Wyllie and Llanbradach. The following fauna was found at Wyllie: Anthraconaia pulchella, A. cf. williamsoni, Anthracosia cf. angulata, A. cf. beaniana, A. cf. ovum, Naiadites productus and Lioestheria sp.[with circular markings, ?egg cases].

The beds between the Six-Feet and Four-Feet comprise 12 to 45 ft of mainly argillaceous strata with a few thin beds of sandstone, except in the Wem-ddu–Rudry–Risca area, where the seams lie close together and form part of the 'Big Vein Group'. The variation in thickness may be rapid, as at Nantgarw, where it fluctuates between 20 and 45 ft within the colliery take. The minimum thickness of 12 ft is at Bedwas.

The Four-Feet consists of two leaves split by a thin parting at Universal; east and south of there the two leaves separate into two distinct seams, the Lower and Upper Four-Feet. The former has been worked at Windsor, and both on a limited scale along the south crop between South Cambria and Blackbrook collieries. In the Universal shafts the Four-Feet comprises two leaves separated by a thin parting: upper coal 16 in, clod 3 in, lower coal 20 in. East and south of Universal the Lower and Upper Four-Feet are separated by 5 to 24 ft of argillaceous measures with occasional thin sandstones near the middle. The Lower Four-Feet varies considerably from a 5-inch coal at Llanbradach, to 4 ft 7 in of coal in four leaves at Windsor. Some of the other variations are:

The Upper Four-Feet is usually a single clean coal, and is best developed in the west where it averages 2 to 3 ft. It thins north-eastwards to only 4 in at Llanbradach, and thickens again to 1 ft 9 in at Wyllie and to 2 ft 2 in at Nine Mile Point. The roof of the seam is usually mudstone, though in parts of the Nantgarw take it is formed by the seatearth of the overlying Two-Feet-Nine. The mudstones contain a rather poor mussel fauna. At Windsor they yielded Anthracosphaerium aff. propinquum and Naiadites cf. productus.

The Four-Feet is separated from the Two-Feet-Nine by up to 17 ft of seatearths and mudstones. The maximum separation occurs in the north-west, at Universal, and gradually diminishes southwards and eastwards, until the two seams join in the vicinity of Wern-ddu, Rudry and Risca. The Two-Feet-Nine, rarely worked because of its high dirt content, consists of 1 to 5 ft of coal, commonly in three unequal leaves separated by partings, thus:

Locally it consists of two leaves, e.g. at Bedwas, or only one, e.g. in New Rockwood No. 2 Borehole. The roof varies from seatearth to sandstone. It is usually seatearth around Nantgarw, Windsor and Wyllie, mudstone at Universal and Llanbradach, and sandstone at Bedwas and New Rockwood. Only plant remains have been found in these strata.

The measures between the Two-Feet-Nine and the Hafod Heulog Marine Band vary in thickness from 4 to 45 ft. The separation may vary rapidly, e.g. the thickest sequence occurs in the Nantgarw shafts, and the thinnest in Gypsy Lane Borehole only one mile to the north-east. The beds are usually argillaceous, and commonly include a high proportion of seatearths which may contain thin coal streaks. Sandstones occur at a few localities; at Universal 17 ft of sandstone lie in the upper part of the cyclothem, at Llanbradach 16 ft with a basal conglomerate lie in the lower part of the cyclothem, and at Bedwas the 17 ft of beds are recorded as nearly all sandstone.

The coal underlying the Hafod Heulog Marine Band varies from a thin streak, e.g. in the Nantgarw Main Cross-measures 280 Horizon, to 25 inches in three leaves in the Universal shafts, as follows: coal 16 in on clod 24 in, coal 3 in, rashing 18 in, coal 6 in. Locally, e.g. in Gypsy Lane Borehole and Nantgarw Main Cross-measures 380 Horizon, coal is absent, and the marine band lies directly on seatearth.

In the area of the 'Big Vein Group' (p. 108) all, or more usually part, of the group has been worked extensively at Risca and the Black Vein Colliery, and on a smaller scale at Machen, Rudry and Van collieries along the south crop. The worked coal thickness, whether all or part of the 'Big Vein Group', is currently known locally as the Big Vein, but in the past the constituent seams were called, in descending order, the Red, Grey, Big and Bed veins, as shown in the section at Risca given below. The naming varied slightly from area to area, e.g. in Wem-ddu Claypit the Bed Vein is included in the Big Vein of the Risca section. The only known complete sections of the 'Big Vein Group' are:

A less detailed section at Machen Colliery was given by Forster Brown (1865, p. 93) as follows: coal (Red) 16 in on soft rubbish 10 in, Grey Vein stone 7 in, coal (Grey) 16 in, soft rubbish 2 ft, coal (Big) 8 ft.

The Hafod Heulog Marine Band, consisting of up to 4 ft of dark, blocky or shaly, pyritiferous mudstone, was seen, but not named, by Moore (1945) in Wern-ddu Clay Pit (p. 154), and in the workings at Llanbradach (p. 183), Windsor (p. 266) and Bedwas (p. 188). In Wern-ddu it comprises 4 in of black, sulphury, paper shales immediately overlying the top coal of the 'Big Vein Group', or it is separated from it by a thin, laterally impersistent conglomerate. During the resurvey it was also proved in the Nantgarw Main Cross-measures 280 Horizon, in Ffynnon Wen and Gypsy Lane boreholes and at Risca. It usually contains only horny brachiopods in association with Planolites ophthalmoides, though at Risca, in a drift [ST 2151 9176] 240 yd N. 42° E. of the No. 1 Shaft, where it rests on the top coal of the 'Big Vein Group' (see section above), the mudstones yielded P. ophthalmoides, Hollinella sp.[pyritized], and several Geisina sp.[pyritized]. Mr. Calver comments that the occurrence of Geisina is rare in the Lower simills-pulchra Zone, although when found it is usually in the strata adjacent to marine incursions, particularly where the marine element is poorly developed. Thus, this record is in accord with the interpretation of Geisina as a brackish-water form.

The beds between the Hafod Heulog and Cefn Coed marine bands vary in thickness from 10 to 40 ft; over much of the area they are dominantly argillaceous, with commonly occurring seatearths and bastard seatearths. At Universal in the northwest, and along parts of the south crop, sandstones and argillaceous rocks occur in approximately equal amounts. In New Rockwood No. 2 Borehole 8 ft of red and grey-mottled mudstone, with some sandstone, lie 2 ft below the Cefn Coed Marine Band. This is the lowest known occurrence of red measures in the Newport district.

The Cefn Coed Marine Band, consisting of 6 in to 4 ft of blocky, commonly pyritiferous, grey mudstone with ribs and nodules of ironstone, has been proved at several localities, and elsewhere its position can be conjectured with a high degree of certainty, for it lies at the base of a thick mudstone division which rests on a thin coal, seatearth or sandstone. Moore (1945) has proved the marine band in Sexton's Heading [ST 1773 9004] at Bedwas (p. 188), in Llanbradach (p. 226) Windsor (p. 226) and Wern-ddu Claypit (p. 161). During the resurvey it was also found at Nantgarw and in Ffynnon Wen and Gypsy Lane boreholes. Like the Hafod Heulog Marine Band it generally contains only Lingula and Orbiculoidea, associated with P. ophthalmoides, though at Llanbradach Moore also found: "Dunbarella [Pterinopecten] papyraceus (J. Sow.) and Productus sp.," and crinoid ossicles and fish remains. At Windsor it also contains foraminifera, including Glomospirella sp., and sponge spicules.

North Ebbw area

The measures between the Amman Marine Band and the Bute vary in thickness from 32 to 65 ft, and are dominantly mudstones and seatearths with a few thin sandstones. The Amman Marine Band has been proved in all the principal collieries, except Abercarn, and consists of 3 to 6 ft of blocky, dark grey mudstone commonly containing pyrite granules and nodules, and pyritized burrows. Thin beds and nodules of ironstone are usually present in the upper part of the marine band. At Celynen North the marine band yielded the following rich fauna, which suggests the proximity of a shoreline environment: sponge spicules [pyritized], crinoid columnals [pentagonal], Levipustula piscariae, Lingula mytilloides, Orbiculoidea sp., gastropods [small; pyritized], cf. Tomaculum sp.[clusters]. A slightly different assemblage was found at Crumlin as follows: Campylites sp., Planolites ophthalmoides, sponge spicules [pyritized], crinoid columnals [pentagonal], L. mytilloides, Orbiculoidea sp., Aviculopecten sp.[fragment], and fish remains. At Llanhilleth and Celynen South the marine band yielded only L. mytilloides [large and abundant at the former colliery].

The Bute, locally known as the Yard, varies from 20 to 27 in. Over most of the area it is a single coal, but in the Abercarn shafts it is split as follows: coal 6 in on shale 4 in, coal 14 in. The seam has not been worked in this area. The roof of the Bute is usually mudstone, or silty mudstone with plant remains. At Celynen North it is seatearth. The Bute and Nine-Feet are separated by 4 to 34 ft of mainly argillaceous measures. The maximum separation occurs at Llanhilleth, where the intervening beds are predominantly seatearths, and the minimum at Celynen North, where only 4 ft of seatearth lie between the seams.

The Nine-Feet, known locally as the Black Vein, has been worked very extensively, and all but the most inaccessible reserves are exhausted. It is generally a single, clean coal, and usually varies in thickness from 5 to 10 ft; locally it may reach as much as 14 ft. At Crumlin the section shows a parting: coal 6 ft on dark mudstone 1 in, coal 3 ft. The upper foot or so is commonly inferior coal or rashes, as at Llanhilleth, Crumlin and Celynen North. The roof of the Nine-Feet is usually hard, well-bedded mudstone containing scattered plant fragments, and sporadic, small, flattened, mussels. The following fauna was collected at Crumlin: cf. Anthraconaia pulchella, Naiadites cf. productus [juv.], and Lioestheria striata (Pruvost non Mfinster).

The Nine-Feet and Lower Six-Feet are separated by 35 to 70 ft of argillaceous beds with a few thin sandstones. At Celynen North, where the separation is 65 to 70 ft, a thin, double coal lies about 40 ft above the Nine-Feet; in the shaft the section is: coal 14 in on 'strong clift' 4 ft, coal 5 in. These coals are probably equivalent to part of the Red Vein group of seams in more westerly areas. They are also represented in Celynen South, where two thin coals lie a short distance below the Lower Six-Feet.

The Lower Six-Feet (worked only at Celynen North) is 2 to 3 ft thick, and usually consists of a main coal with a thin leaf lying either above, and/or below the coal, e.g.:

The 6-in coal in the Crumlin East Main Drift contains a pale blue mineral, which, on X-ray examination, proved to be melanterite, a decomposition product of pyrite. The roof of the Lower Six-Feet generally consists of up to 5 ft of seatearth or rashes, which are overlain by mudstone or silty mudstone with plant remains.

The thickness of the beds between the Lower and Upper Six-Feet usually varies between 25 and 45 ft, though at Crumlin up to 55 ft are known, and in the Abercarn shafts only 10 ft. The measures comprise mudstones and silty mudstones with beds of ironstone, and a few thin sandstones.

The Upper Six-Feet is generally a single coal varying in thickness from 2 to 3.5 ft. It has been worked fairly extensively in the southern part of Crumlin, and at Celynen North and Celynen South. Except at Llanhilleth the roof is mudstone or silty mudstone with plant remains or, more rarely, fragments of non-marine mussels. At Llanhilleth the roof varies from mudstone to striped beds or sandstone.

The Upper Six-Feet and Four-Feet are separated by 20 to 45 ft of mudstone, silty mudstone and seatearth, with a few thin beds of sandstone. A thin coal, 8 to 15 in thick, lies 6 to 15 ft above the Upper Six-Feet in all the collieries except Llanhilleth; it is probably a leaf split from the Upper Six-Feet.

The Four-Feet varies in thickness from 1.5 to 5 ft, and has been worked fairly extensively. At Llanhilleth a typical section is: coal 6 in on thin parting, coal 8 in, thin parting, coal 8 in, seatearth 15 in, main coal 36 in. In the remainder of the area the Four-Feet is usually a single coal varying from 1.5 to 3 ft thick. Locally, thin leaves develop above or below the seam, e.g. in the north-east of the Celynen North take: coal 1.5 in on mudstone 8 in, coal 1 in, seatearth 10 in, main coal 34 in. The roof of the Four-Feet is commonly up to 3 ft of seatearth, overlain by mudstone and silty mudstone with plant remains and sporadic fragments of non-marine mussels. In parts of Llanhilleth it is silty mudstone, or striped beds, overlain by seatearth. In Crumlin roof mudstones have yielded Naiadites cf. obliquus and arthropod fragments.

The measures between the Four-Feet and the Cefn Coed Marine Band were examined in Crumlin Colliery, the section in the North Big Vein Drift [ST 2107 9807], 270 vd N. 10° W. of the North Shaft, being:

Up to 30 ft of arenaceous beds (the 'Filed Rock') lie above the roof strata of the Four-Feet in the Llanhilleth, Crumlin and Celynen North shafts, but they are virtually absent in Celynen South and Abercarn. The Two-Feet-Nine, not worked in this area, is best developed in Celynen North, where it is nearly 3 ft thick; the seam thins northwards to only 4 inches in the Llanhilleth shafts, and southwards to about 12 in at Celynen South. It is absent in the Abercarn shafts.

The 3-in coal lying 23 ft above the Two-Feet-Nine in the above drift section is probably the seam which underlies the Hafod Heulog Marine Band in more westerly parts of the Newport district. It is visible again in the North Black Vein Return [ST 2104 9812], and the section at approximately 130 yd N. 13° W. of the North Shaft is: hard sandstone 4 ft on silty mudstone 1 ft, seatearth and mudstone with plant remains 5 in, inferior coal 2 in, silty mudstone with roots. The coal is also present in the shafts at Llanhilleth and Abercarn, where it is overlain by sandstone, and in Celynen South, where it is overlain by seatearth.

The Cefn Coed Marine Band, as well as being visible at the top of the drift in the above section at Crumlin, is also exposed in a nearby heading [ST 2106 9821], 420 yd N. 7° W. of the North Shaft, as follows:

The above fauna is interesting in view of the abundance and large size of the crinoid remains, some of the columnals measuring 16 mm in diameter. The horizon of the marine band is readily identified in the remaining collieries in the Ebbw Valley Area.

Tirpentwys-Cwmcarn area

The measures between the Amman Rider and Bute vary in thickness at Tirpentwys from 35 to 85 ft, at Hafodyrynys, Cwmcarn, Henllys and Glyn from 10 to 23 ft, and at Cwmbran and Cwm Lickey from 4 to 6 ft. During the resurvey the Amman Marine Band, consisting of grey, blocky mudstone and silty mudstone with ironstones and pyrite granules, was proved at Tirpentwys, Hafodyrynys and Cwmcarn. In the Hafodyrynys No. 4 Drift it contained crinoid columnals [pentagonal], Lingula mytilloides, Hollinella cf. bassleri and cf. Tomaculum sp., and in the No. 2 Drift, crinoid columnals, L. mytilloides, gastropod [juv.; pyritized], Edmondia ? [juv.; pyritized] and Cypridina sp.[pyritized]. In a drift [ST 2362 9364] at Cwmcarn the band yielded cf. Planolites ophthalmoides, well preserved examples of Levipustula piscariae, L. mytilloides and cf. Tomaculum sp.Some clusters of the last mentioned show the individual pellets to have a longitudinal groove. Similar pellets are discussed and figured by Calver in Smith and others (1967, p. 184, pl. v, Figure 6), and a faecal origin ( ?gastropod) is suggested.

A collection from the marine band at Cwmcarn, submitted in 1960 by Mr. D. E. White, on behalf of the National Coal Board, contained crinoid columnals [pentagonal], L. mytilloides, Levipustula cf. piscariae, Productus carbonarius, turreted gastropod [pyritized internal mould], cochliodont tooth.

The Amman Marine Band was formerly proved in Blaendare Colliery, where Salter (1861, p. 228) recorded "Productus scabriculus Sowerby", in 'mine ground' above an Engine Coal, clearly the Amman Rider. Along the east crop Moore (1948, p. 276) found Lingula mytilloides and Orbiculoidea sp.in spoil from crop workings, 350 yd south of the Penyrheol reservoir at Blaendare, which are from the Amman Marine Band.

Between the Amman Marine Band and the Bute the measures are mainly argillaceous, with sandstones developing in some parts, e.g. at Tirpentwys, where 25 ft of hard sandstone with some silty mudstone lie a few feet above the Amman Marine Band in the Hafodyrynys No. 4 Drift. In the No. 2 Drift, 1 ft of cannel occurs at the base of the 9 ft of seatearth which lie below the Bute. The Bute usually varies between 12 and 33 in, and is absent locally, e.g. in the Hafodyrynys No. 4 Drift. A small area of the seam was worked at Mynydd Maen. At Hafodyrynys, Mynydd Maen and Cwm Lickey it is 22 to 33 in thick, and at Tirpentwys 12 to 24 in. In Cwmbran it is doubtfully represented by three thin coals which lie very close to the overlying Nine-Feet thus: coal 6 ft (Nine-Feet) on clod 10 in, coal 8 in, shale 2 in, coal 6 in, clod 3 in, coal 6 in. At Cwmcarn the seam section was recorded in a drift [ST 2367 9363] 150 yd N. 30° W. of the shafts: coal 20 in on seatearth 7 ft, cannel 1 in, coal 14 in. Where the Bute lies close to the Nine-Feet, e.g. at Cwmcarn, its roof is commonly seatearth, and where it lies some distance below the Nine-Feet, as at Tirpentwys, it is silty mudstone with plants.

The measures between the Bute and Nine-Feet vary between 10 in and 30 ft of mainly mudstone and seatearth, with a few thin sandstones. They are 26 to 30 ft thick at Tirpentwys, 26 ft at Cwm Lickey, 15 ft at Glyn and 8 ft at Cwmcarn. In the Hafodyrynys No. 2 Drift 15 in of seatearth separate the seams, and at Cwmbran only 10 in.

The Nine-Feet varies from 6 to 12 ft, and is generally a single clean coal, though thin partings may develop locally. In most pits it is virtually worked out. Only in the disused Mynydd Maen Colliery, where it is reputed to be largely washed out, are there no records of workings in the seam. At Tirpentwys, seam thicknesses of up to 12 ft are not uncommon, particularly in the area around the shafts. At Hafodyrynys, Glyn, Cwm Lickey, Cwmbran and Henllys the Nine-Feet is generally 7 to 9 ft thick, and at Cwmcarn slightly thinner. The roof of the Nine-Feet normally consists of blocky mudstone or silty mudstone, which commonly contains plant remains. Rarely, e.g. at Tirpentwys, the mudstone contains small, poorly preserved non-marine mussels. In Cwmcarn the roof is locally a 9-in bed of sandy, ironstone conglomerate.

The measures between the Nine-Feet and Lower Six-Feet vary between 24 and 95 ft; they consist mainly of silty mudstones with some mudstone and sandstone. The Lower Six-Feet was worked fairly extensively at Glyn, Blaendare and Albion, and in small amounts at Hafodyrynys, Cwmbran and Cwmcarn. Typical variations in the seam sections are:

At Hafodyrynys a section in a staple pit [ST 2490 9850] proved all the measures between the Nine-Feet and Four-Feet as follows:

In the Hafodyrynys No. 2 Drift the Lower Six-Feet is poor: coal 3 in on seatearth 24 in, coal 0.5 in, seatearth 15 in, coal 0.5 in. In Cwmbran the lower leaf is unusually thick: coal 28 in on seatearth 10 in, coal 22 in. The roof of the seam usually consists of mudstone or silty mudstone with plant remains, or seatearth where the seam lies close to the overlying Upper Six-Feet.

The beds between the Lower and Upper Six-Feet vary in thickness from 35 to 55 ft at Cwmbran, Cwm Lickey, Glyn and Hafodyrynys, but at Tirpentwys and Cwmcarn they diminish to 6 ft. They are mainly mudstones and seatearths, with a few thin coals and sandstones.

The Upper Six-Feet is normally 28 to 48 in of clean coal: it is thin or absent locally, and several partings separating very thin leaves may develop in the lower part of the seam, e.g. in the Hafodyrynys No. 4 Drift (Appendix I). Fairly large amounts of the Upper Six-Feet have been worked at Glyn, Blaendare, Mynydd Maen, Cwm Lickey, Tirpentwys and Cwmcarn. In the Hafodyrynys No. 2 Drift the Upper Six-Feet is the bottom leaf of the composite seam termed the Big Vein. The section compares with that in the staple pit 900 yd E. 15° S. of the shafts thus:

At Cwmcarn a section in the Bovril Drift [ST 2356 9363], 250 yd north-north-west of the shafts, spans the succession between the Lower Six-Feet and the Four-Feet:

The roof of the Upper Six-Feet is very variable; it is commonly seatearth, or mudstone and silty mudstone with plant remains, and in some areas sandstone, as in parts. of Glyn, Cwmbran and Cwmcarn. In the north-east of the Cwmcarn take sandstone fills partial or complete washouts in the Upper Six-Feet. A few, small, poorly-preserved non-marine mussels have been found in the mudstone roof in Tirpentwys.

The measures between the Upper Six-Feet and Four-Feet are thickest at Tirpentwys, generally 45 to 55 ft, and thinnest in parts of Hafodyrynys, e.g. only 2.5 ft in the No. 2. Drift. They are mainly argillaceous with some arenaceous beds, e.g. at Glyn, Cwmbran and Cwmcarn, where 8 to 21 ft of sandstone lie immediately, or a short distance above, the Upper Six-Feet. Sandstones at this horizon crop out [ST 2750 9979] at Upper Race south of Pontypool; there, up to 30 ft of sandstones, interbedded with silty mudstones and a few, thin, impersistent coals, are exposed. At the base of the bluff formed by the sandstones, the presence of coal fragments in small spoil heaps suggests that the Upper Six-Feet, was worked to a small extent.

The Four-Feet is a composite seam and usually varies from 2.5 to 6 ft. In the Cwm Lickey shafts only 12 in of coal are present at this horizon, but it is possible that there washout conditions prevail. The seam has been worked on a large scale at Tirpentwys, Glyn, Cwm Lickey, Albion and Blaendare, and on a smaller scale at Hafodyrynys. and Cwmcarn. Typical sections are:

At Hafodyrynys the quality of the Four-Feet deteriorates southwards due to the appearance of thin partings in the main coal, and increasing amounts of inferior coal and rashes in the overlying leaves. Near the Cwmcarn shafts the seam consists of the main coal only, and north-west of the shafts of the main coal overlain by two thin leaves in about 3 ft of seatearth, rashes, etc. Also at Cwmcarn 6 in to 2 ft of cannel lie immediately above the seam.

West of Pontypool Strahan (1909) recorded coal in the cellar of the large house called Waun-Wem [SO 2719 0050] as the Nine-Feet, but the resurvey suggests that it is the Four-Feet. At Upper Race the Four-Feet has been mined in several small levels [around ST 2746 9984] near Darren Farm, and in the New Vein Level [ST 2736 9885], 200 yd E. 25° S. of the Cwm Lickey shafts. In spoil heaps near the entrance to the New Vein Level, Moore (1948, p. 278) obtained abundant Lingula, and during the resurvey Orbiculoidea sp.and crinoid columnals were found. Moore believes that the spoil is from the New Vein Level, and has suggested that the marine fossils may have been obtained from the marine band below the Cefn Coed Marine Band (the Hafod Heulog Marine Band). The present author believes, however, that the spoil in question was derived from the nearby Cwm Lickey Colliery, and is from the Amman Marine Band, encountered either in cross-measures drivages, or workings for ironstone.

The roof of the Four-Feet is generally mudstone or silty mudstone with plant remains. In the Hafodyrynys No. 1 Drift the mudstones have yielded Anthraconaia cf. cymbula [juv.] and Naiadites cf. daviesi Dix and Trueman [juv.], and in the No. 2 Drift Naiadites cf. obliquus [juv.]. Beds of hard, fine to medium-grained sandstone are not uncommon, and in some areas form the immediate roof to the seam. In Cwmcarn up to 5 ft of dolomitic conglomerate locally overlie the seam. It may form the immediate roof, but is usually separated from the coal by cannel, seatearth, mudstone or silty mudstone. The following section [ST 2359 9346] was measured at approximately 100 yd west-north-west of the shafts:

The conglomerate in this section is a dolomitic conglomerate with dolomite, quartzite and chert pebbles. The rock is composed of rounded pebbles up to several centimetres long of very fine-grained, chocolate-coloured dolomite, white granulitized quartz or quartzite, and chert in a ground mass of mainly angular quartz grains up to 0.5 mm across cemented by fairly coarse, ankeritic dolomite. Quartz grains of the groundmass are embedded in the margins of the dolomite pebbles, and dolomite of the groundmass penetrates the margins of the quartzite pebbles. P.A.S.

The measures between the Four-Feet and Cefn Coed Marine Band normally vary in thickness from 55 to 80 ft. They comprise mainly sandstones, with interbedded seatearths, mudstones and silty mudstones; a few thin coals occur locally. They were seen only in the Hafodyrynys Nos. 1, 2 and 4 drifts and in Blaendare Fireclay Mine [ST 2721 9913] south of Pontypool. The Four-Feet is overlain by up to 50 ft of mainly arenaceous beds, which comprise the Elled Rock'. Along the east crop the sandstones have been traced to the west of Pontypool, where they form a low, north-south ridge, and for a short distance at Upper Race, 100 yd north-east of Darren Farm [ST 2735 9970]. South-east of Cwm Lickey Colliery the Elled Rock' is well developed, and forms a prominent scarp which can be followed as far south as Upper Cwmbran. The sandstones are exposed in the bluff [ST 2754 9844] above White House as follows:

A specimen (E30298) of the Elled Rock' from a section [ST 2688 9682] near Upper Cwmbran is a protoquartzite consisting of sub-angular to sub-rounded quartz grains (average grain size about 0.15 mm) and scattered fragments of chert, quartzite and silty material, with a little interstitial sericite and accessory iron oxide and tourmaline. J.R.H.

The Two-Feet-Nine is an impersistent seam in this area. It is absent in Tirpentwys, Glyn and Cwmbran. In the Cwm Lickey and Cwmcarn shafts it is a 4-in coal lying 47 ft and 16 ft respectively above the Four-Feet. In the West Shaft at Hafodyrynys, and in the nearby underground No. 1 Borehole [ST 2412 9843], it lies 33 ft above the Four-Feet and is 20 in and 12 in thick respectively. The seam dies out towards the south and south-east of the Hafodyrynys take. In Blaendare Fireclay Mine it is an impersistent coal up to 6 in thick, lying about 25 ft above the Four-Feet.

In the Hafodyrynys No. 4 Drift, the Cefn Coed Marine Band is underlain by three thin seams totalling 20 in of coal in 8 ft of strata. Elsewhere in the area coal is absent at this horizon, and the marine band rests on either seatearth, as in the Glyn-Cwm Lickey area, or arenaceous rocks as in the Hafodyrynys Nos. 1 and 2 drifts and Cwmcarn. In Blaendare Fireclay Mine the section is:

The Cefn Coed Marine Band in the Hafodyrynys No. 4 Drift consists of 28 ft 10 in of smooth, grey, blocky mudstone containing abundant, small, pyrite nodules, and has yielded the following fauna: cf. Planolites ophthalmoides [vague], Lingula mytilloides, Orbiculoidea cf. nitida, small pyritized gastropods (cf. Ianthinopsis), pectinoid fragment. In the Hafodyrynys No. 1 Drift however, it is only 6 in thick, and contains L. mytilloides, Euphemites anthracinus, and fish remains including Elonichthys sp., Pleuroplax attheyi (Barkas) and Rhabdoderma sp.

South-east crop area

The Amman Marine Band crops out in Coed-y-Darren near Risca, where medium grey mudstone in spoil heaps [ST 2382 9200] from crop workings has yielded L. mytilloides and Levipustula sp.The marine band may occur in Darren Mine [ST 2342 9181], in 'mine cliff' lying above 26 in of 'coal and clay' thought to mark the Amman Rider horizon. Farther north crinoid columnals and fish fragments were found in mudstone in the large spoil heap [ST 2600 9435] near the entrance to Henllys Colliery, and these may be from the marine band in the Henllys workings.

The Nine-Feet was not seen at the surface during the resurvey, but in Cwmbran Colliery it was worked to within 325 ft of its presumed outcrop near Gam-wen Farm [ST 2672 9619]. In the upcast shaft [ST 2608 9508] of Henllys Colliery, at a depth of 321 ft, it dips to the west at between 55° and 58°. It was also proved in Luther May's Drift in Henllys Colliery, where it is 8 ft 10 in thick. The resurvey suggests that the Nine-Feet crops out in the vicinity of Henllys Colliery, but Moore (1948, p. 295) concluded that the seam is cut out by an unconformity before it reaches the surface. The problem has been fully discussed by Squirrell and Downing (1964, p. 129). In Darren Mine the believed Nine-Feet is 39 in thick.

The Four-Feet was proved in a small trial [ST 2689 9673] at Upper Cwmbran, 200 yd south of the entrance to the disused Cwmbran Colliery Slope. Its outcrop is easily traced in this vicinity, at the base of the overlying sandstone (Elled Rock) which forms a prominent north-south ridge. This sandstone is about 80 ft thick above the entrance to Cwmbran Colliery, and is well exposed 100 yd farther south [ST 2688 9673], where up to 15 ft of massively bedded sandstone with thin beds of mudstone are visible. Southwards again, the sandstone dies out within 250 yd, for it is absent in the stream [ST 2684 9658] 400 yd W. 30° S. of The Square, where only argillaceous beds are present.

In Darren Mine the beds above the presumed Nine-Feet include 95 ft of 'rock', near the middle of which lies 4 ft of 'coal and shale' which may be the equivalent of the Lower Six-Feet and associated measures. Higher in the sequence the No. 2 Fireclay, which lies 23.5 ft below the believed horizon of the Cefn Coed Marine Band in the Darren Mine, was worked extensively in No. 7 Level [ST 2321 9169].

The uppermost beds of this group of measures crop out along the north side of the track [ST 2385 9204] passing through Coed-y-Darren, and nearby, in the cliff [ST 2385 9204] immediately above a small level on the north side of the same track, where the section is:

B. Cefn Coed Marine Band to Upper Cwmgorse Marine Band

North Taff area

This sequence, with the exception of the lowest few feet, is nowhere exposed in the area and details are derived entirely from the records of shaft sinkings. It is impossible to base too much reliance on lithological changes between the various shafts, for these may be more apparent than real due to the number of individuals involved in compiling the logs of the sections.

The first seam above the Cefn Coed Marine Band is normally the Gorllwyn, but in the North Taff Area this coal has been recorded only in Albion and Deep Navigation shafts. At Albion the seam is 3 in thick, and lies about 70 ft above the presumed position of the Cefn Coed Marine Band, while at Deep Navigation it is represented by a bed of rashing 4 ft thick lying 80 ft above the marine band. In Abercynon and Taff Merthyr shafts the Gorllwyn Rider is the first coal above the Cefn Coed Marine Band. The interval between the two horizons varies from about 50 ft at Taff Merthyr to 150 ft at Abercynon. Where the Gorllwyn is present it lies about 20 ft below the Gorllwyn Rider. Argillaceous measures comprise most of the sequence between the Cefn Coed Marine Band and the Gorllwyn Rider, except at Abercynon where arenaceous beds are developed at two horizons and represent almost 50 % of the sequence, thereby accounting for the greater thickness there. At Taff Merthyr the cyclothem is completed by 8 ft of sandstone. The Gorllwyn Rider is 8 to 12 in thick throughout the area.

Above the Gorllwyn Rider the sequence in the south is continued up to the Eighteen-Inch by 15 to 25 ft of argillaceous strata, but sandstone is developed at Deep Navigation and the measures are mainly sandstone at Taff Merthyr. The Eighteen-Inch is about 1 ft thick, except at Taff Merthyr where it is recorded as a double coal as follows: coal 13 in on dirt 11 in, coal 13 in. Twenty to 30 ft of argillaceous measures separate this seam from the Lower Pentre, which is normally a double coal as follows: coal 12 in on dirt 2 in, coal 6 in. At Deep Navigation the top leaf is split by a thin dirt parting, and at Taff Merthyr only one coal has been noted.

The thickness of the measures between the Lower Pentre and the Pentre vary from 20 to 30 ft and consist of mudstone and seatearth in the north, but sandstones are important in the south. In Abercynon North Shaft the measures are mainly 'hard white rock', presumably quartzite. The Pentre varies from 8 to 20 in of coal. At Taff Merthyr an upper leaf, 15 in thick, is separated from a 5-in lower leaf by 28 in of seatearth.

The measures between the Pentre and Pentre Rider are mainly mudstone and seatearth and increase in thickness from about 10 ft in the south to 26 ft in the north. Normally the Pentre Rider, a seam about 18 in thick, is overlain by the Foraminifera Marine Band, but because of the inaccessibility of the beds it was not proved in this area during the resurvey. The marine band is unlikely to be present throughout the area for at Albion, Abercynon and Deep Navigation the roof of the seam is recorded as sandstone. At Deep Navigation the Pentre Rider is succeeded by 25 ft of arenaceous deposits and 40 ft of mudstone and seatearth. This takes the sequence up to a 4-in coal assumed to be the Abergorky. A mainly sandstone sequence continues up to a 2-in coal, which is believed to be overlain by the Lower Cwmgorse Marine Band.

The measures between the Pentre Rider and the next succeeding coal at Abercynon and Deep Navigation are 70 ft thick, and in the former colliery consist almost entirely of sandstone. At Abercynon the upper seam is in two leaves, a top coal of 7 in separated from a lower 3-in coal by about 2.5 ft of 'clod'. Probably the lower leaf is the Abergorky and the upper leaf carries the Lower Cwmgorse Marine Band in its roof.

At Albion and Taff Merthyr the correlation is obscure; only one seam intervenes between the Pentre Rider and the Hafod. This is 6 in thick at Albion, and is separated from the Pentre Rider by 74 ft of measures including several sandstones. At Taff Merthyr the intervening coal is only 2 in thick and occurs 88 ft above the Pentre Rider, from which it is separated by argillaceous ground with an arenaceous development in the central section. It is not possible to say whether this seam represents the Abergorky or the thin coal normally found above the Abergorky, which is overlain by the Lower Cwmgorse Marine Band. From information regarding the position of the marine band in other parts of the Newport district the coal is probably that carrying the marine band.

The sequence between the presumed position of the Lower Cwmgorse Marine Band and the Hafod consists mainly of sandstone at Albion, Abercynon and Deep Navigation, but at Taff Merthyr argillaceous beds predominate, sandstone only occurring below the Hafod seatearth. The thickness of this part of the sequence varies from 53 to 79 ft, and appears to attain a maximum in the centre of the area.

The Hafod varies in thickness from 15 to 26 in. The Upper Cwmgorse Marine Band probably occurs in the roof of the seam, but nowhere is this accessible in the area.

North Rhymney and Sirhowy area

The measures in this part of the sequence are known mainly from the records of shaft sections. The sequence is exposed to a limited extent in the West and North-West levels at Penallta Colliery, both of which pass through the trough-faulted area between the Gelligaer and Llanfabon faults. None of the seams in the sequence is worked, and hitherto they have not been named. In this account all but two of the seams are named, the basis for the nomenclature being a somewhat tentative correlation with the Rhondda Valley sequence.

Over most of the area the first seam above the Cefn Coed Marine Band is believed to be the Gorllwyn Rider. A 4-in seam possibly equating with the Gorllwyn occurs about 67 ft above the presumed base of the marine band in Grosfaen East Pit, but it has not been proved elsewhere. The interval between the marine band and the Gorllwyn Rider varies from about 120 ft in the Grosfaen West Pit to about 60 ft at Bargoed. The lower part of the sequence is argillaceous. This passes up into 'clift' with sandstone bands, which is commonly overlain by a sandstone about 10 ft thick occurring just below the Gorllwyn Rider. The 48 ft of sandstone near the middle of the sequence in the Grosfaen West Pit, may well have originated as a channel sand in view of its absence in the adjacent East Pit, and the presence there of 'clift' and 'mine' ground including the possible equivalent of the Gorllwyn.

The Gorllwyn Rider is a double coal in Bargoed and Britannia, the following section being typical: coal 18 in on dirt 3 in, coal 20 in. At Penallta and Grosfaen the seam is a single coal about 24 and 18 in thick respectively. Although the coal is not present in the Oakdale North Pit the horizon is indicated probably by 28 in of 'clod' resting on 10.5 ft of sandstone, 76 ft above the presumed position of the Cefn Coed Marine Band. Between 7 and 16 ft of measures separate the Gorllwyn Rider from the Eighteen-Inch coal in the Grosfaen and Penallta shafts. At Bargoed and Britannia the interval is about 20 ft. The Eighteen-Inch is not present in Oakdale North Pit, but a seatearth 34 ft above the presumed position of the Gorllwyn Rider may indicate its base. The measures between the two coals are entirely argillaceous, except at Britannia, where 6 ft of sandstone occur in the middle of the section, and at Oakdale where sandstones are developed towards the top of what is believed to be the equivalent section in this colliery.

The thickness of the Eighteen-Inch is rather variable. Between 9 and 18 in seems to be typical, but only 3 in are present in Grosfaen West Pit and the seam is absent in Oakdale. The coal is succeeded up to the Lower Pentre by 15 ft of measures at Oakdale, about 20 ft at Bargoed and Britannia, and 29 and 44 ft in the Penallta shafts. Sandstones appear to be developed locally, otherwise the sequence is argillaceous.

The Lower Pentre is represented by coal only at Oakdale, where a 9-in seam lies between thin beds of rashings. The horizon is indicated by thin rashings at Penallta, and in Britannia and Bargoed South shafts, but rashings 78 in thick are present in Britannia North Pit. The seam is absent in Grosfaen shafts and in Bargoed North Pit.

The interval of about 60 ft between the Lower Pentre and the Pentre is considerably higher at Bargoed than in the collieries to the south-east. Only in Britannia South Pit is this figure approached. There, 40 ft occur, though only 26 ft are present in the North Pit. The difference between the two is largely due to the presence of 20 ft of sandstone in the middle of the sequence in the South Pit, whereas only 6 ft occur in the North Pit. The similar thickness of the measures between the Eighteen-Inch and the Pentre at Bargoed and Grosfaen may be taken to suggest that the interval between the Lower Pentre and the Pentre is increasing towards the north. However, the absence of the Lower Pentre at Grosfaen precludes proof of this supposition. Most of the 16 ft of measures comprising the Oakdale section are sandstone. 'Strong clift' (presumably siltstone) with sandstone bands occurs in the upper part of the sequence in Bargoed. At Penallta the sequence is entirely argillaceous, as is the case below the Pentre in Grosfaen shafts.

Two thin leaves of coal separated by an interval varying from 3 in to 4.5 ft constitute the Pentre at Bargoed and Britannia. Where the parting is thickest it is composed of rashings and seatearth or 'clift'. In Penallta, Grosfaen and Oakdale shafts a single coal occurs, which is 7, 8 and 13 in thick respectively; the seam is absent in Grosfaen West Pit. In the West and North-West levels at Penallta the seam presumed to be the Pentre consists of two leaves a few inches thick, separated by 1 or 2 in of dirt.

Throughout the area the interval between the Pentre and the Pentre Rider is between 12 and 30 ft and consists almost wholly of argillaceous measures. The Pentre Rider is a variable seam. Where recorded in Penallta the thickness ranges from 5 to 16 in, but the seam is absent in the No. 2 Pit. The general thickness in the Rhymney Valley is probably about 12 in. At Oakdale, in the Sirhowy Valley, no more than 4 in occur in the North Pit, but this seam is underlain by 63 in of seatearth and 10 in of rashings. The absence of the seam in Penallta No. 2 Pit suggests an impersistent development common to so many of the thin coals in the Middle Coal Measures above the Cefn Coed Marine Band.

The Pentre Rider is separated from the Abergorky by 21 ft at Oakdale and 45 to 60 ft over the remainder of the area. Arenaceous beds occur either immediately above, or a few feet above, the Pentre Rider throughout most of the area, and the higher sequence also contains sandstone bands. Only at Penallta is the sequence between the seam and the Abergorky mainly 'clift', though even there 6 ft of sandstone occurs 5 ft above the lower seam in the No. 2 Pit, and the middle part of the sequence in the No. 1 Pit includes sandstone bands. Six to 8 ft of sandstone occur above the Pentre Rider in the Britannia shafts; this has increased to some 34 ft in Bargoed North Pit and as much as 39 ft in Grosfaen. The sandstone is commonly succeeded by 'clift', with sandstone bands, which extends up to the seatearth below the Abergorky. At Oakdale almost the entire 20 ft of measures between the seams are sandstone. The Abergorky is represented by a thin coal, sometimes associated with rashings, or by a bed of rashings alone. It is not recorded in Britannia South Pit or Grosfaen shafts, but in the West Pit of the latter colliery, the horizon is indicated by a seatearth.

The interval between the Abergorky and the succeeding thin coal increases from some 3 ft at Penallta and 7.5 ft at Oakdale to about 20 ft at Bargoed and Grosfaen; 16 ft are present in the centre of the area at Britannia. Seatearth forms the sequence where the lower thicknesses prevail, but towards the north-west sandstones are developed. The thickness of the seam above the Abergorky is everywhere between 4 and 16 in except at Grosfaen; there the seam is absent in the East Pit though 3 in of rashings occur in the other shaft. The Lower Cwmgorse Marine Band usually occurs in the roof of this coal, but in the West Level at Penallta the badly exposed roof has weathered to a mottled clay, and in the North-West Level the immediate roof consists of 12 in of mudstone overlain by sandstone, with the sandstone in places resting on the coal. No marine fossils have yet been found in the roof where argillaceous measures are accessible.

Over most of the area the thin coal above the Abergorky is separated from the Hafod by 56 to 61 ft of strata, though about 73 ft occur in the Penallta shafts. The lower part of the section is argillaceous, but sandstone, up to about 12 ft thick, normally occurs below the seatearth associated with the Hafod. This is the highest of the three sandstone developments found between the Cefn Coed Marine Band and the Hafod, the others being below the Gorllwyn Rider and above the Pentre Rider. In the Bargoed South Pit the Hafod seatearth is underlain by 9 ft of 'green clift', and this is the lowest record of coloured beds in the area.

At Penallta the Hafod is about 12 in thick (in the No. 2 Pit the section is split by 2.5 in of dirt into a 9.5-in top coal and a 4-in bottom coal) but elsewhere the coal is 20 to 24 in. The seam usually carries the Upper Cwmgorse Marine Band, and this is exposed in the North-West and West levels at Penallta. The thickness of the marine strata is controlled by the undulating base of the overlying erosive, conglomeratic sandstone which cuts out the coal locally in the West Level. The maximum thickness of the band is about 7 ft. Marine fossils are relatively abundant, though they consist almost entirely of Lingula mytilloides. Other fossils recorded include Dunbarella sp.[fragment] and fish remains including Rhadinichthys sp.and platysomid scales. R.A.D.

Nantgarw-Risca area

The measures of this group are visible only in small exposures along their narrow outcrop. The sequence was fully exposed in Wern-ddu Claypit (Moore 1945, p. 15354) (Appendix I), but it is now largely obscured by debris and vegetation. Much of the knowledge of the beds has been derived from shaft sections, boreholes and underground drivages, most important of which are the cross-measures headings and deep boreholes at Nantgarw, and Sexton's Heading at Bedwas Colliery. Little is known of the sequence in the area west of the Taff Valley, and along the south crop between Rudry and the Sirhowy Valley.

Between the Cefn Coed Marine Band and the Gorllwyn Rider (the Gorllwyn is absent in this area) lie 40 to 70 ft of mainly argillaceous beds. They contain a few sandstones, best developed at Llanbradach and Risca, where up to 20 ft of arenaceous strata are present in the middle and basal parts of the sequence respectively. At Bedwas Moore (1945, p. 189) recorded 17 ft of coarse, pink and white quartzites containing beds of ganister and coarse conglomerates lying immediately below the seatearth of the Gorllwyn Rider.

The Gorllwyn Rider is present everywhere, except at Wyllie and along the south-crop at Rudry and Wern-ddu. It is from 3 to 26 in thick and is usually a single coal. At Nine Mile Point it is 26 in thick, and is split into two equal leaves separated by a 1-in parting. The roof of the Gorllwyn Rider is invariably mudstone, which may be black and carbonaceous, as in Ffynnon Wen and Gypsy Lane boreholes. It commonly contains plant remains, and has also yielded Cochlichnus kochi, Gyrochorte carbonaria and a fragment of Naiadites ?

The thickness of the strata between the Gorllwyn Rider and Eighteen-Inch in the western part of the area varies from 13 to 23 ft. The beds are mudstones and seat-earths, except at Nantgarw, New Rockwood and Windsor, where up to 10 ft of arenaceous beds develop in the middle of the cyclothem. Eastwards the thickness diminishes rapidly to 7 ft at Bedwas and Nine Mile Point, and to only 8 in at Risca. The Eighteen-Inch is present over much of the area; it is absent at Wyllie, and possibly in Rudry Borehole, though there it may be represented by the 24 in of rashes lying 6 in below the Lower Pentre. In the Nantgarw No. 2 East Cross-measures 280 Horizon and New Rockwood No. 2 Borehole it probably forms the basal part of a composite seam, 42 and 46 in thick respectively, which includes the Lower Pentre in its upper part. Where it is separated from the Lower Pentre the Eighteen-Inch varies from 6 to 17 in, and is nearly always a single coal. In Wern-ddu Claypit the Eighteen-Inch lies at the base of 10 ft of beds which Moore (1945, p. 154) recorded as: "Yard Vein (probable position) within belt of sandy shales and fireclays". During the resurvey this part of the sequence was examined in greater detail, and the 'Yard Vein' was found to comprise all the seams between the Eighteen-Inch and Pentre Rider inclusive (Appendix I). The roof strata of the Eighteen-Inch are mudstones, except where the Lower Pentre lies close above, and then they are seatearths or mudstones with roots. In Ffynnon Wen Borehole mudstones have yielded Anthraconaia cf. adamsii.

The beds between the Eighteen-Inch and Lower Pentre are thickest at Universal, Llanbradach and Windsor, where they vary from 13 to 19 ft of argillaceous strata. To the south and east considerable attenuation takes place, e.g. Gypsy Lane Borehole 9 ft, Nantgarw shafts 7 ft, Nantgarw No. 1 East Cross-measures 380 Horizon only 7 in. Over all the eastern part of the area, and along the south crop as far west as New Rockwood, it is invariably less than 4 ft. In Rudry Borehole it may be as small as 6 in if the 2 ft of rashings below the Lower Pentre is the equivalent of the Eighteen-Inch. In the Nantgarw No. 2 East Cross-measures 280 Horizon and New Rockwood No. 2 Borehole the Eighteen-Inch and Lower Pentre appear to form a single, clean coal.

The Lower Pentre is very variable, and in parts of the area contains a thin parting of tonstein, which was first recognized by Mr. Lewis at Bedwas, in Sexton's Heading, at a position [ST 1775 8995] 710 yd from the beginning of the drift. It was subsequently found in the same coal at other localities as shown below:

According to Mr. Young 'specimens (Bedwas–(E31559), (E31560), (E31561), (E31562); Wern-ddu–(E31556)-(E31557); Nantgarw–(E31558)) show from X-ray examinations (X-ray films (X4288), (X4289), (X4290), (X4291), (X4285), (X4286) and (X4287) that this tonstein consists of well-ordered kaolinite with a trace of quartz'. The same tonstein was also seen in the Cardiff-Caerphilly railway cutting [ST 1713 8610] near Wern-ddu Claypit as follows: coal 6.5 in on tonstein 2 in, coal 9 in, seatearth 2 in, coal 11 in, seatearth 13.5 in, coal 8 in (probably the Eighteen-Inch). A specimen (E29271) has been described by Dr. Dearnley as 'a dark grey kaolinite mudstone; the rock is extremely fine-grained with abundant, typical crystals of kaolinite up to. 0–3-0.5 mm across and small, scattered quartz grains in a groundmass which is optically continuous over large patches, but which apparently has a slightly higher birefringence than the kaolinite crystals. This may be due to a small amount of Mite.' The X-ray report from Mr. Young indicates the presence of abundant kaolinite, a little illite and a very small quantity of quartz (X-ray film X2111). The rock may be classified as a tonstein in the sense of a mudstone containing abundant kaolinite crystals.

Other Lower Pentre sections now inaccessible, suggest the presence of tonstein, e.g.:

The Lower Pentre thins to 15 in at Nine Mile Point, 10 in at Wyllie, 9 in at Risca and to only 6 in at Rudry. There is no evidence for the presence of tonstein in the coal at any of these localities. The Lower Pentre has been exploited to a small extent along the south crop between the western edge of the sheet and Caerphilly Common. The roof of the seam consists of mudstone with plant remains, or, where the Pentre lies close above, of seatearth or mudstone with roots. A rich flora was recorded by Moore (1945, p. 189) at Bedwas, which he assigned to an horizon low within Floral Zone F.

The mudstones, seatearths, and sporadic sandstones between the Lower Pentre and Pentre vary in thickness from 2 ft 9 in to 37 ft. They are thickest in the west, usually more than 20 ft, but only 14 ft in the Nantgarw shafts. In the east they are usually less than 20 ft, though up to 30 ft at Bedwas. The strata are thinnest in Wernddu Claypit, comprising only 2 ft 9 in of seatearth. In Gypsy Lane Borehole 12 ft of sandstone lie 7 ft above the Lower Pentre, and at Nine Mile Point 8 ft of sandstone lie 4 ft above the same seam. At Risca sandstone comprises 11.5 ft of the 13 ft 3 in of measures between the two seams.

The Pentre varies in thickness from to 2.5 in. It is best developed at Nantgarw, as 14 in or more of clean coal, or as two unequal leaves with a parting up to 9 in thick. It is also in two leaves at Llanbradach and in Sexton's Heading at Bedwas. It is thinnest in Gypsy Lane Borehole. In the northern and eastern parts of the area it is usually 8 to 16 in thick, but only 2 in at Risca. The roof of the Pentre is commonly mudstone, or seatearth where the overlying Pentre Rider lies close above. In New Rockwood No. 2 Borehole, and at Nine Mile Point and Risca, it is sandstone.

The strata between the Pentre and Pentre Rider vary irregularly from 2.5 to 21 ft. The minimum development is 2.5 ft of seatearth at Wern-ddu; the maximum is at Bedwas, where the beds are mainly mudstones and seatearths with a little sandstone. At Nantgarw, not more than 10 ft of mainly seatearths are present. In the western and central parts of the area the measures are argillaceous, but at Nine Mile Point and Risca they are dominantly sandstones. The Pentre Rider is a single coal and varies from 4 to 22 in, though mostly it ranges between 11 and 22 in. It is thickest at Nantgarw and New Rockwood and thinnest in Rudry Borehole. No workings are known.

The roof of the Pentre Rider is mudstone at Nine Mile Point, Bedwas, Universal and in the eastern part of Nantgarw, and in Rudry, Ffynnon Wen, Gypsy Lane and New Rockwood No. 2 boreholes. Elsewhere it is mainly sandstones, interbedded with a few mudstones. In the Nantgarw No. 1 East Cross-measures 380 Horizon up to 15 in of conglomerate make the immediate roof. The Foraminifers Marine Band has been proved in the roof mudstones in Ffynnon Wen, Gypsy Lane and Rudry boreholes (Appendix I), in the Nantgarw No. 1 West Cross-measures 280 Horizon on the No. 12 Seam, in Sexton's Heading [ST 1779 8986], 607 yd from the beginning of the drift, and in Wern-ddu Claypit. It is characterized by abundant foraminifera associated with P. ophthalmoides and rare Lingula (Appendix I). In Rudry and Ffynnon Wen boreholes the Five Roads Marine Band may be represented in slightly higher strata (p. 114).

Between the roof strata of the Pentre Rider and the thin coal underlying the horizon of the Lower Cwmgorse Marine Band lie 35 to 65 ft of mainly sandstones at Universal, Windsor, Llanbradach and parts of Nantgarw, but mainly mudstones at Wyllie, Nine Mile Point and Risca, and in Ffynnon Wen and Gypsy Lane boreholes. Elsewhere, argillaceous and arenaceous rocks occur in approximately equal proportions. In Wern-ddu Claypit Moore (1945, p. 153) recorded 1 ft of red shales lying near the top of these beds. The Abergorky seam lies 35 to 51 ft above the Pentre Rider; it occurs in the Nantgarw shafts and main cross-measures drivages, in Ffynnon Wen Borehole, in Sexton's Heading, in Rudry Borehole and in Wern-ddu Claypit. It is a single coal, 3 to 15 in thick at Nantgarw, and usually less than 2 in elsewhere. Its roof is usually mudstone or seatearth. The persistent seam which underlies the horizon of the Lower Cwmgorse Marine Band is usually a single coal 3 to 15 in thick, and is commonly overlain by mudstone, though in parts of Nantgarw, e.g. the Main Cross-measures 280 Horizon, and in Ffynnon Wen Borehole, it is roofed by sandstone.

The Lower Cwmgorse Marine Band has been proved in Gypsy Lane Borehole, in Nantgarw No. 1 West Cross-measures 280 Horizon (on the No. 10 Seam), in Sexton's. Heading at Bedwas, in Wern-ddu Claypit and in Rudry Borehole. In Sexton's Heading [ST 1784 8977] it is 7 ft thick and yielded abundant Lingula mytilloides, as well as Orbiculoidea sp., Euphemites anthracinus and Hollinella sp.

The Lower Cwmgorse Marine Band is separated from the Hafod by 35 to 78 ft of variable measures. Around Nantgarw the sequence is composed almost exclusively of sandstones over a large part of the take. To the north and east from there beds are dominantly argillaceous, and sandstones are restricted to the middle and upper strata. In Wern-ddu Claypit the 8 in of coal and coaly shale lying 8.5 ft below the Hafod is the only known occurrence of coal at that horizon in the area. In Rudry Borehole the upper part of the cyclothem is characterized by the presence of grey, or greenish grey, bastard seatearths containing sphaerosiderite, interbedded with mudstones and quartzitic sandstones. Approximately 11 ft below the Hafod 3 ft of bastard seatearth is mottled red.

The Hafod is a single coal and varies from 3 to 14 in. It appears to be present everywhere, except in the Nantgarw No. 2 East Cross-measures 280 and 380 horizons where it is replaced by sandstone. The Upper Cwmgorse Marine Band was proved in Gypsy Lane, Ffynnon Wen and Rudry boreholes (Appendix I), and in some of the headings at Nantgarw. In Sexton's Heading [ST 1789 8967] it yielded the following fauna: sponge spicules [pyritized], Planolites ophthalmoides, Lingula mytilloides, L. sp.[broad form], Orbiculoidea cf. nitida, turreted gastropod [fragment], Polidevcia cf. acuta (J. de C. Sowerby), Megalichthys sp.[scale], fish tooth indet. and faecal remains.

North Ebbw area

The sequence between the Cefn Coed and Upper Cwmgorse marine bands is known only from the records of colliery shafts, and in drivages to the north of the Crumlin shafts. Between the Cefn Coed Marine Band and the Gorllwyn Rider the sequence comprises 35 to 60 ft of argillaceous measures with varying amounts of sandstone. At Abercarn and Celynen South, a few thin sandstones are interbedded with the mudstones in the 20 ft of beds overlying the Cefn Coed Marine Band, but they die out northwards. At Llanhilleth, Crumlin and Celynen North up to 37 ft of quartzitic sandstones lie a few feet below the Gorllwyn Rider, but they thin southwards to only 10 ft at Celynen South, and are absent at Abercarn. At Crumlin the 50 ft of variable beds between the Cefn Coed Marine Band and Gorllwyn Rider are visible in a heading [ST 2111 9818] 390 yd north of the North Shaft.

The Gorllwyn Rider (Soap Vein) varies from 8 to 36 in; it is best developed at Llanhilleth and Crumlin, and deteriorates southwards. At Crumlin it comprises two leaves, the lower varying from 8 to 12 in, and the upper from 20 to 27 in. In a heading [ST 2120 9820] 400 yd N. 5° E. of the North Shaft the section is: coal 24 in on seatearth 12 in coal 10 in Other sections are!

The roof of the Gorllwyn Rider is invariably mudstone; in Crumlin it contains abundant plant remains and Anthraconaia adamsii?.

Between the Gorllwyn Rider and the Pentre Rider lie 23 to 72 ft of measures with up to three thin coals. In the north the sequence is dominantly argillaceous with a little sandstone, but southwards it becomes more arenaceous, until, at Abercarn, it is predominantly sandstone. At Crumlin the 4-in coal which lies 21 in above the Gorllwyn Rider may be the Eighteen-Inch seam; it is absent elsewhere. At Crumlin the Lower Pentre and Pentre lie 20 ft and 32 ft respectively above the supposed Eighteen-Inch. Although the correlation is uncertain the Lower Pentre and Pentre appear to be present at Celynen North, but at Celynen South and Abercarn the Lower Pentre is absent and at Llanhilleth both are missing. Comparative sections are: