The geology of the South Wales Coalfield Part 6 The country around Pontypridd and Maesteg (explanation of one-inch geological sheet 248) Memoirs of The Geological Survey of Great Britain

By Austin W. Woodland, and W. B. Evans

Bibliographic reference: Woodland, A.W. and Evans, W.B. 1964. The geology of the South Wales Coalfield Part IV The country around Pontypridd and Maesteg (explanation of one-inch geological sheet 248)

Department of Scientific and Industrial Research. Memoirs of the Geological Survey of Great Britain England and Wales

The geology of the South Wales Coalfield Part IV The country around Pontypridd and Maesteg (explanation of one-inch geological sheet 248)

By Austin W. Woodland, B.Sc., Ph.D., and W. B. Evans, B.Sc.

Third Edition. LondonHer Majesty's Stationery Office 1964

Preface

Preface to the first edition

This, the fourth part of the Memoir descriptive of the South Wales Coal Field, is an explanation of the New Series Map Sheet 248, which includes the important part of the Coal Field extending from Pontypridd to Cwmavan, and the greater part of the range of the best steam-coal.

The ground was originally surveyed on the Old Series One-inch Map, Sheet 36, by Sir H. T. De la Beche, Sir W. E. Logan and Mr. D. H. Williams. Most of it was probably the work of De la Beche himself, but it is likely that the western part was included in the survey made by Logan before he became connected with the Geological Survey. The maps which he had prepared were exhibited in 1837 at a meeting of the British Association at Liverpool, and attracted the attention of De la Beche, with the result that they were handed over to the Geological Survey and incorporated in the official maps, while Logan himself became a member of the staff. Sheet 36 was published in or before the year 1845.

The re-survey was made on the six-inch scale under the superintendence of Mr. Strahan, and was published in 1899. The south-eastern part of Sheet 248 was surveyed by Mr. Strahan, the south-western part by Mr. Tiddeman, and the northern part by Mr. Gibson. Each geologist contributes an account of the area surveyed by himself to the present volume, and the whole has been edited by Mr. Strahan.

Two sub-divisions of the Coal Measures, namely, the Pennant Series and the Lower Coal-series, occupy nearly the whole area. In the latter occurs a group of seams which yield the well-known smokeless steam-coal. The measures are illustrated in the three sheets of Vertical Sections, Nos. 83, 84, 85, to which frequent reference is made in these pages, and in the Plate bound up in this volume. It is noticeable that though the seams can be recognised for long distances in an east and west direction, yet both they and the measures associated with them change so rapidly southwards that only a general correlation by groups is possible. In this part of the Coal Field some massive sandstones, much resembling Pennant, develop in the upper part of the Lower Coal-series.

The Pennant Series shows a no less remarkable expansion both in a southward and westward direction. At the same time its upper part becomes split up by shales and coal-seams, which have yielded the bulk of the coal near Neath, Swansea and Llanelly. Thus both the top and the bottom of the Pennant is less distinct than in Monmouthshire, where the sub-division consisted almost exclusively of sandstone.

The Upper or Llantwit Measures occur in two small outliers only in the area comprised in Sheet 248.

In the South Crop, where the high dip usual in the southern margin of the Coal Field prevails, the Millstone Grit and what may be the top of the Carboniferous Limestone come into view, but they are partly overspread by Triassic strata, which have been laid down unconformably upon their truncated edges. The Coal Measures themselves are partly thus covered, and it is worthy of note that the Triassic conglomerates consist not of Coal Measure rocks, but of fragments of Carboniferous Limestone.

The abnormal condition of the Rhaetic Group in this part of South Wales is well exhibited near Pyle, but a full account of these interesting rocks is reserved for the description of the region round Bridgend, where they are more widely distributed.

The structural geology has been treated in great detail in view of its importance in connection with mining. The course of the Pontypridd anticline and the development of the Moel Gilau Fault along the same line of disturbance are among the most important features. Mr. H. K. Jordan's conclusions with respect to this great fracture receive confirmation from the recent work.

By an examination of the glacial deposits it has been possible to trace the route taken by an ice-flow which had its source in Brecknock. It followed the main valleys across the Coal Field, such as those of the Taff, Neath and Tawe, but, though it filled them to overflowing, it failed to surmount the great Pennant scarp of Carn Mosyn. Here, on the other hand, was generated a subordinate ice-flow which followed the Rhondda, Ogmore, Garw and Llynfi valleys, but which, both to the east and west, amalgamated with the Brecknock ice and joined in the general press southwards.

Chapter X summarises the observations on the principal economic products. The character of the coals, being a matter which affects the Coal Field as a whole, will be treated in a general memoir more suitably than in the explanation of a single sheet.

The Map is published in two Editions, on one of which (the Solid Edition) glacial deposits are omitted, while on the other (the Superficial Edition) such deposits are indicated by colour, as well as those portions of the Solid Geology which are not concealed by Drift. MS. six-inch Maps, geologically coloured, are deposited in this office, where they can be consulted. Copies can be obtained at cost price.

We have again to express our thanks for valuable assistance rendered by Colliery Managers, Mining Engineers and Surveyors.

J. J. H. Teall. Director. Geological Survey Office,28, Jermyn Street, London, 28th February, 1903

Preface to the second edition

THE preparation of a Second Edition of this Memoir gives the opportunity of adding recent information, more especially with relation to the mining developments which have taken place since the publication of the First Edition in 1903. Thus, while the workings on the steam-coals along the south crop west of the Ogwr have come to a standstill, with the exception of the establishment of the Cribbwr Fawr Colliery near Pyle, the Rock-Fawr Seam is now extensively mined by slopes along the southern margin of the Pennant. East of the Ogwr several coals at about the same horizon are being worked, but it is still uncertain which of them corresponds to the No. 2 Rhondda Seam. North and north-west of Llantrisant, deep shafts have recently been sunk through the Pennant to reach the steam-coals at the Cwm Colliery near Llantwit Fardre, and at the Coed-Ely Colliery in the Ely Valley. In the Ogwr Valley, and especially in the Avan Valley and its tributaries, developments have followed the establishment of docks at Port Talbot; but in the Rhondda Valleys there has been but little change in mining exploration.

The revisions of the Memoir, apart from the descriptions of the works referred to, have been mainly editorial. Mr. R. H. Tiddeman having left the staff in February, 1902, the task of collecting fresh information was entrusted to Dr. Gibson and Mr. Cantrill. Their duties have been greatly facilitated by the readiness with which mining engineers and colliery officials have supplied details. Valuable assistance has also been given by Mr. H. J. Randall and Mr. W. A. Williams, of Bridgend. Special reference also must be made to the completion by Mr. H. K. Jordan of his researches on the South Wales Coalfield.

Some new collieries referred to in this volume will not be found on the edition of the one-inch map, issued in 1902; but sufficient details are given in the text to enable the reader to locate them.

The six-inch geological maps covering the area included in the one-inch Sheet 248, and the sections of the principal shafts in the area, are quoted in lists on pp. vii and viii.

A. Strahan, Director. Geological Survey Office, 28, Jermyn Street, London, 19th June, 1916

Preface to the third edition

The years since the publication of the Second Edition of this memoir have seen a considerable expansion in mining in the Pontypridd district, as well as a greatly increased understanding of Coal Measures stratigraphy, palaeontology and sedimentation generally. It was therefore decided that a resurvey of the district should be undertaken instead of a revision similar to that of the Abergavenny (Sheet 232) and Merthyr Tydfil (Sheet 231) memoirs, published in 1927 and 1933 respectively. During the course of this resurvey, particular attention has been given to the correlation of the Pennant strata, and this has resulted in a revised classification of these rocks. Detailed examination of the Lower and Middle Coal Measures, with their productive coals, has involved much time in the examination of underground exposures at all the working collieries. In addition, many hundreds of new records of shafts, boreholes and mine-roadways have been acquired.

The present account together with the published six-inch and one-inch maps represents a great advance in the understanding of the geology of the district. The structure of the Coal Measures has received much attention. Major thrusts, and gravity slides of a type not previously recognized in Britain, are described for the first time. These, together with the cross-faults and innumerable incompetence-structures which affect the productive measures nearly everywhere, are interpreted as reflecting the continuous evolution of Armorican movements in South Wales.

The resurvey was started in 1945 by Dr. A. W. Woodland, who in 1947 was joined by Mr. W. B. Evans. The work was carried out under the superintendence first of the late R. W. Pocock, and then of Dr. V. A. Eyles, and was completed in 1954. The areas surveyed by the two officers are shown in the list of six-inch maps on p. xiii. Thirty-six county maps have been published between the years 1957–60. The new one-inch map was published in a Drift Edition in 1960, and a Solid Edition in 1963.

The memoir has been written by Dr. Woodland and Mr. Evans, and edited by Dr. F. B. A. Welch. Animal fossils have been identified, and much help given throughout the work by Mr. M. A. Calver, Dr. W. H. C. Ramsbottom, and Dr. C. J. Stubblefield: Dr. J. Weir has also identified some of the non-marine shells. The plants were named for the most part by Dr. R. Crookall, but Dr. W. G. Chaloner has determined those more recently collected. The fossils were mainly collected by the two authors.

Throughout the resurvey great assistance has been forthcoming from numerous officials of the coal industry. Before nationalization, the surveyors, in particular those of the Powell Duffryn and Ocean Coal Companies, gave unstinted help, and in later years co-operation by the National Coal Board has been on a no less generous scale; and we gratefully acknowledge our thanks for this assistance. We also acknowledge the assistance given by the officers of the Coal Survey Laboratory at Cardiff, and in particular to Mr. H. F. Adams, the present Officer-in-Charge, who has supplied a short section (pp. 293–6) on the quality and utilization of the coals of the district.

C. J. Stubblefield, Director. Geological Survey Office, Exhibition Road, South Kensington, London, S.W.7 7th November, 1963

List of six-inch maps

The following list shows the six-inch 'county' maps included, wholly or partly, in the area described by this memoir, with the initials of the surveyors and the dates of re-survey. In those marginal maps without asterisk, only the areas lying within the One-inch Sheet boundary were re-surveyed. The officers concerned with the re-survey were W. B. Evans and A. W. Woodland. All maps marked with an asterisk were published uncoloured between 1957 and 1960.

Chapter I Introduction

Geographical setting and industrial background

The Pontypridd (248) Sheet of the One-Inch Geological Map of England and Wales depicts most of the central part of the South Wales Coalfield lying between the Taff and Neath valleys, together with the northern margins of the Vale of Glamorgan (Figure 1). The greater part of the district is occupied by Coal Measures, but in the extreme south small areas of Millstone Grit crop out in the rim of the coal basin. This southern area includes small patches of Mesozoic rocks which, farther south between Bridgend and Cardiff have extensive outcrops.

Except for a narrow southern strip the entire district is formed by a deeply-dissected plateau of Pennant Measures, the so-called Blaenau Morgannwg, the surface of which falls gradually southwards. In the north, heights of 1500 to 1800 ft are attained on Cefntyle-brych (1756 ft), Mynydd Tynewydd (1694 ft), and Moel yr Hyrddod (1817 ft), but in the south the hills that overlook the South Crop are only 800 to 1000 ft high. This southerly falling plateau is interrupted by a gentle ridge which runs southwards from Cefn Ffordd (1969 ft), in the Merthyr Tydfil district immediately to the north, towards the heads of the Ogwr Fawr and Garw valleys. Here are situated Werfa (1866 ft), the highest point within the district, the heights above Craig Fawr (1832 ft), Mynydd Caerau (1823 ft), and Mynydd William Meyrick (1769 ft). The plateau clearly post-dates the development of the major structures of the district, for its level is unaffected by both the folds and the faults. Lithological variations in the rocks that form its surface are reflected in minor steps which may have been accentuated by discontinuous uplift. It has been suggested that this high ground represents a series of unwarped platforms, but since the higher levels are those most perfectly preserved it appears simpler to regard it as a single warped surface, showing in this district a fall of rather less than 1° to the south.

The rivers draining the plateau are deeply incised to depths of a thousand feet or more. They are narrow and steep sided and, as yet, have destroyed little of the plateau surface. In the east, the Taff and its tributaries, the Cynon, the Clydach, the Rhondda Fach, and the Rhondda Fawr, drain south-eastwards. The Ogwr Fach and Fawr (or Ogmore), the Garw and the Llynfi all run more nearly southwards; while the Avan, Pelena, and Neath trend south-westwards. With the exception of the Taff, Cynon, and Neath, all rise within the Pennant plateau, and apart from the Neath, none shows any adjustment to the underlying structures. The river system is clearly superimposed, and it has been claimed by some to have originated upon a now-removed cover of Chalk; the apparent lack of erosion on the plateau surface, however, suggests that the streams are consequents which developed as the surface was uplifted, and reflect in their varying directions the changing slopes of that surface. In this connexion the Cefn Ffordd-Werfa ridge marks the divide between streams which flow southeastwards, and those that drain southwards or south-westwards.

South of the Pennant plateau a continuous hollow extends from Margam to Rhiwsaeson along the strike of the mudstones of the Lower and Middle Coal Measures, and carries no significant strike streams. This is flanked still farther south by a series of low scarps and ridges formed by the sandstones of the lowest Coal Measures and Millstone Grit, which stretches from Kenfig Hill in the west, through Cefn Cribwr and Cefn Hirgoed to Penprysg, where it passes beneath glacial drift into the Bridgend district to the south.

Until the nineteenth century the district was settled only by scattered farming communities little visited by, or known to, the outside world. The small-scale digging of coal had for long supplemented wood as a source of fuel, certainly since Elizabethan times and possibly since as early as the thirteenth century, when the monks of Margam Abbey are reputed to have worked coal. These dayholes were as much a part of the rural scene as were the village forges or the small woollen mills.

The early years of the Industrial Revolution saw little change in this pattern of life. Despite the smelteries and collieries around Swansea and Neath, the ironworks along the North Crop eastwards from Hirwaun, and the growth of a major area of house-coal production in the Monmouthshire valleys, in only two parts of the present district did industries of any size exist. Near Maesteg and Cwmavon, and along the western part of the South Crop, where local iron-ores were easily accessible, small ironworks, presumably with their ancillary collieries, were established prior to 1830, and around Dinas there were sizeablemines exploiting the house-coals of the Pennant and sending them by canal, and later by railway, to Cardiff. Even in 1848 a traveller to the Rhondda could write 'The people of this solitudinous and happy valley are a pastoral race, almost entirely dependent on their flocks and herds for support The air is aromatic with wild flowers and mountain plants—a sabbath stillness reigns.'

The real impetus to coal production came when it was realized that the Lower and Middle Coal Measures of much of the district contained seams of a quality ideal for steam-raising. The rapid increase in the number of steam-engines of all kinds, particularly coal-fired steam ships, led to an immense demand for these coals. In 1828 the first colliery producing steam-coal for sale outside the district was opened near Merthyr, and within two years workings began in the Cynon Valley. By 1843 Lletty Shenkin Colliery, apparently the first steam-coal pit in the district, had been sunk, followed in 1845 by Aberaman Colliery. In 1851 a trial pit at Cwmsaerbren (later to become Bute Colliery) proved the extension of these seams beneath the Rhondda, and within the next thirty years most of the collieries in this valley were established, though in the more isolated valleys of the Avan, the Garw and the Ogmore, new pits were still being sunk or old ones deepened in the early years of the twentieth century.

The industry reached its peak immediately before the First World War, at least 70 per cent of its output being for the overseas market. The dependence of the world's shipping on the coals of the district was reflected by the frequency with which the words 'Naval', 'Navigation' or 'Ocean' appeared in the names of collieries or colliery companies. Although production figures for the district are not available, the Rhondda valleys alone produced over 9 million tons in 1914.

After this date the gradual world-wide replacement of coal by oil as a marine-fuel led to an inevitable contraction in demand and production, which has continued ever since. In 1957 the total output from the Rhondda had fallen to 2,380,000 tons, and for the district as a whole it was only 8,247,000 tons in 1961, barely 4 per cent of the country's output. Even so, it still contributes over one third of the output of the South Wales Coalfield. Of late years the emphasis has shifted somewhat from the steam-coals to the coking-coals farther south.

Geological sequence

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

Geological history

The oldest exposed rocks are of Millstone Grit age, though it is practically certain that at depth both the Carboniferous Limestone and the Old Red Sandstone are continuous beneath the district. The deposition of the Carboniferous Limestone was followed by minor uplift accompanied by progressive erosion of the uppermost beds towards the east. Renewed subsidence allowed the Millstone Grit sea to re-enter the area and to deposit its sediments above the plane of unconformity.

During the ensuing Coal Measures times, numerous episodes of limited subsidence and associated sedimentation were interspersed with periods of stability, during which luxurious vegetation spread across the silted area of deposition, producing accumulations of humic matter which on burial became coal seams. Unlike most other British coalfields the later Coal Measures were characterized by the entry of vast quantities of sand, now preserved as the Pennant Sandstones. During their accumulation, slight uplift resulted in erosion and unconformity in the eastern areas of the coalfield.

A major gap in the stratigraphical record separates the Coal Measures from the marls and conglomerates of the Keuper. Within this period the Armorican earth-movements produced in the earlier formations a large proportion of the structures observed at the present time. Uplift, accompanied by erosion, resulted in the removal of much of the strata already deposited, so that in the south of the district Keuper rocks now rest on the Millstone Grit, and in the area still farther south on beds as low as the Old Red Sandstone. It is also probable that considerable thicknesses of Upper Carboniferous rocks no longer preserved in the district were removed at this time.

The Triassic strata are believed to have formed under desert conditions. It is clear from their relationship with the Pennant near Llantrisant that they were laid down at the foot of a Pennant escarpment which occupied to a large extent its present position, though it does not follow that the present surface of the Pennant plateau is of Triassic origin.

The marine Rhaetic and basal Liassic rocks which follow were formed in a sea which lapped round this area of high ground, and in consequence they exhibit marginal facies.

Since the time interval between the Lias and the Pleistocene is not now represented by any deposits, any attempt to reconstruct the geological history must be largely speculative. It appears likely, however, that the district underwent at least one complete cycle of subaerial erosion, which removed any cover of later Mesozoic rocks and which cut the present surface of the coalfield plateau. At a relatively recent date this surface has been uplifted and tilted, and its slight degree of dissection makes it difficult to believe that this uplift could be earlier than the mid-Tertiary, to which period the post-Liassic faults may also belong. To certain platforms developed in the district to the south, a Pliocene age has been tentatively ascribed, which, if correct, sets an upward limit to the age of uplift.

Finally, during the Pleistocene the district was invaded by glaciers which produced minor modifications of the landscape; since the melting of the ice, few changes of significance have taken place. A.W.W., W.B.E.

Chapter 2 Millstone Grit Series and earlier Carboniferous beds

Introduction

The Millstone Grit Series crops out along a strip of country never more than three-quarters of a mile wide in the central southern margin of the district, extending from the neighbourhood of Pen-y-fai Common (half a mile south of Aberkenfig) in the west, to Penprysg in the east. In the former area the beds are faulted against Triassic rocks, and westwards to Swansea Bay they are known only in boreholes near Kenfig Pool, which lies in the north-western corner of the Bridgend district. With the exception of numerous lines of crags marking the outcrops of the harder sandstones, exposures are poor and no good continuous sections exist. Most of the knowledge of the shales and their accompanying faunas is derived from isolated exposures in the Ogmore Valley immediately south of Aberkenfig.

At the time of the original six-inch survey of the district little was known concerning the systematic palaeontology of the Upper Carboniferous. Few fossils were collected and no use was made of the faunal evidence for classifying and separating the various main divisions of strata. The upper limit of the ' Millstone Grit' was fixed arbitrarily at the top of a thick sandstone, the Cefn Cribbwr Rock (see Woodland and others 1957, p. 57), which forms the bold feature of Cefn CribwrThe ridge named 'Cefn Cribwr' on the present one-inch geological map (1960, 1963) was in previous editions called 'Cefn Cribbwr'. In this memoir the shorter spelling is used only in topographical references. and Cefn Hirgoed as far eastwards as Penprysg. This sandstone was believed to correspond with the Farewell Rock of the North Crop of the Coalfield which similarly marked the upper limit of the Millstone Grit in that area. In more recent years great advances have been made in the study of the fossils of the Upper Carboniferous and in accordance with international practice the Geological Survey now takes the dividing line between the Millstone Grit Series (Namurian) and the overlying Coal Measures (Westphalian) at the base of marine strata characterized by the presence of Gastrioceras subcrenatum C. Schmidt. During the course of the present survey the G.subcrenatum Marine Band has been identified at a level approximately 450 ft below the top of the Cefn Cribbwr Rock. This thickness of strata, formerly included as the uppermost beds of the Millstone Grit, is now accordingly assigned to the lowest Coal Measures.

Along this portion of the South Crop of the Coalfield the G. subcrenatum Marine Band is known only from the National Coal Board's Margam Park No. 1 Borehole, the site of which is some four miles west-north-west of the nearest outcrops at Pen-y-fai. This borehole serves to fix the position of the marine band with reference to the overlying beds, but unfortunately its relations to the beds below are obscure owing to the intervention of major structural complications. Using the evidence of this borehole it is nevertheless possible to infer the position of the top of the formation at outcrop, especially as the Gastrioceras cumbriense Marine Band, which should lie not far below, is known at several surface localities.

The basal beds of the Series, together with the uppermost beds of the Carboniferous Limestone, crop out in the area just south of the Pontypridd Sheet margin. This area, which lies between Litchard (1.25 miles north of Bridgend) and the main Bridgend–Maesteg road, on the margins of six-inch sheets Glamorgan 34 S.W. and 40 N.W., is included in the present account.

Classification

The systematic stratigraphy and palaeontology of the Millstone Grit Series is founded on the researches of W. S. Bisat and later workers in the Mid-Pennine Province of Northern England, and is based on the sequence of goniatite faunas. Comprehensive accounts of the classification at present in use in that area are described by Bisat (1924, 1928) and R. G. S. Hudson (1945) with later modifications. The main divisions are defined as follows (reading from above downwards):

• G₁ Stage (Lower Gastrioceras) comprising the zones of G. cumbriense and G. cancellatum
• R₂ Stage (Upper Reticuloceras) comprising the zones of R. superbilingue, R. bilingue and R. gracile
• R₁ Stage (Lower Reticuloceras) comprising the zones of R. reticulatum s.s., R. eoreticulatum and R. circumplicatile
• H Stage (Homoceras) comprising the zones of H. eostriolatum, H. beyrichianum and H. subglobosum
• E₂ Stage (Upper Eumorphoceras)
• E₁ Stage (Lower Eumorphoceras)

Following Bisat's work several workers (D. G. Evans and R. O. Jones 1929; Dix 1931; Robertson 1933 and Ware 1939) have described the Millstone Grit of the North Crop of the Coalfield and of North Gower to the west of the present area. These authors have shown that the goniatite sequences of these areas are closely comparable with those of the Pennines, and that the 'zonal' scheme outlined above is of general application to South Wales. In spite of the paucity and scattered nature of the exposures in the area at present under review, a fairly complete picture of the Millstone Grit Series can be pieced together. The generalized succession for the area south and south-east of Aberkenfig, together with the principal proved faunas, is given in (Figure 2). The compilation of the sequence from the mapping and location of fossil localities is not in doubt, but in two instances the exact super-position of the faunas is inferred by comparison with outside areas. Thus, the relationship of the Gastrioceras cancellatum horizon to that of G. cumbriense is not clear in the field and the Homoceras eostriolatum fauna is known only from spoil material.

Relations of the Millstone Grit Series to the Lower Carboniferous

It has long been established that the Millstone Grit in South Wales rests with a varying degree of unconformity on the Carboniferous Limestone. This was first demonstrated by the Geological Survey (see, for example, Strahan 1909, p. 20; Strahan and others 1914, p. 151) in the extreme eastern and north-western outcrops of the main coalfield where the effects of unconformity are now known to be at their greatest. Subsequently, numerous authors describing the zonal characters of both the Millstone Grit and the Carboniferous Limestone, have done much to elucidate the details of the unconformity; among these may be mentioned: Dixey and Sibly 1918, p. 123; Dixon 1921, p. 157; 0. T. Jones 1925; Dixon and Pringle 1927, p. 124; George 1927, p. 51; Evans and R. 0. Jones 1929, p. 164; Robertson 1927, p. 43, 1933, pp. 32–33; Robertson and George 1929, p. 4; Dix 1931; Ware 1939, p. 199.

The sequence appears most complete in the Tenby area and in North Gower, where there is little or no evidence of disconformity, and sedimentation has every appearance of having been continuous. Nevertheless shales (associated with cherts) of Upper Eumorphoceras (E₂) Age appear to rest directly on Upper Limestone shales ('Black Lias'), now known to be of Upper Posidonia (P₂) Age (see below), and thus non-sequence even here is inferred by the apparent absence of any strata representative of the Lower Eumorphoceras (E₁) Age. Northwards and eastwards from these areas the extent of the unconformity gradually increases by progressive erosion of Lower Carboniferous strata and by non-deposition of successive horizons of the Millstone Grit.

Difficulties in fixing a definite line of division between the two formations in the present area have been discussed by T. N. George (1933, pp. 254–7) and little can be added to the conclusions and opinions there expressed. The sequence of the area south of Aberkenfig shows strong similarities to that in North Gower. Limestones, associated with dark patchy chert, are interbedded with calcareous mudstones, and crop out between Litchard and the railway line north of Coity Junction. Some of the limestones are dark and argillaceous, some are fine-grained and compact, while a few are crinoidal; rotten-stone weathering is common. The lithology and fauna of these beds are typical of the 'Black Lias' of the Oystermouth area and it seems probable that they represent the same beds. In 1948 these strata were penetrated in a borehole near Kenfig Pool some 6.5 miles to the west, where they yielded goniatities of P₂ age (see Ramsbottom 1954, p. 54). This essentially calcareous facies is therefore confirmed as being of Lower Carboniferous age. Close above these beds in the area south-east of Aberkenfig occurs a thin layer of striped radiolarian cherts, followed by hard thinly bedded blue-grey siliceous mudstones about 30 ft thick. These beds are characterized by numerous conodonts and Posidonia,but so far have failed to yield any goniatites. There is an imperceptible passage from these beds into a massive quartzitic sandstone some 30 to 40 ft thick, immediately above which occurs a calcareous mudstone, weathering to rotten-stone and yielding abundant conodonts and Cravenoceratoides?,indicating a possible high E₂ age. A very short distance above this horizon goniatites of H age occur. Thus (excluding the sandstone) little more than 30 ft of strata at best can be referred to the E division.

In North Gower there are 400 to 500 ft of beds, mainly argillaceous, referable to E₂, the bottom part of which is characterized by black cherts, and beneath which occur striped cherts with radiolaria. At Kenfig Pool at least 110 ft of beds with numerous developments of black chert occur above similarly striped cherts. In the Litchard area no genuine black cherts or cherty mudstones have been noted, and it is inferred that nearly the whole of the E₂ strata are missing, the beds failing from the bottom upwards. Despite the failure of the black cherts the striped radiolarian cherts persist with the same relations to the 'Black Lias' as at Kenfig Pool and in North Gower; the striped cherts therefore appear to have a closer affinity to the Lower Carboniferous than to the E₂ division of the Millstone Grit. From this it would appear that the best place to draw a dividing line between the two formations is at the top of the striped cherts with radiolaria.

Lithology and fauna

The Millstone Grit Series of the present area is composed mainly of mudstones and shales with intercalated beds of sandstone and gritstone up to rarely more than 50 ft in thickness. In the Aberkenfig area (see (Figure 2)) the total thickness of strata is about 900 ft. East and north-east there must be considerable thinning caused by progressive cutting-out of the lower beds by unconformity. The sandstones in the top 600 ft (R₂ and G) are impersistent while those of the lower beds (E₂–R₁) seem to be more strongly developed. However, the division of the Series into two clearly defined lithological parts—the Shale Group above and the Basal Grit below—is not nearly so evident on the South Crop as it is to the north of the Coalfield (see Robertson, 1933, p. 49).

The sandstones are usually very massive and quartzitic, and in places coarse and even conglomeratic. White quartz grains and pebbles make up the bulk of the last-named types, but pink quartz and jasper have been observed. The quartzose beds are commonly coarsest nearest the top of the individual posts. The outcrops of these sandstones make parallel, bracken-covered bands with sporadic prominent crags, which stand out clearly from the wettish, bracken-free and rush-covered outcrops of the accompanying mudstones.

The mudstones are of typical Upper Carboniferous type. In the upper part of the Series (R₂ and G) marine mudstones and shales appear to be confined to definite horizons, probably separated by non-marine strata, the whole showing a marked degree of rhythmic sedimentation. The lower beds, however, appear to be almost wholly marine and the rhythmic character is less developed.

The lowest mudstones represent a conodont–Posidonia faunal phase in dark mudstones, but goniatites determined as Cravenoceratoides? were found at one locality. Abundant conodonts occur both as assemblages of the different form genera and as individuals, and it is possible that some of the assemblages are excretory (Rhodes, 1952, p. 888). The occurrence of Posidonia cf. corrugata (R. Etheridge jun.) in these beds tends to confirm the Eumorphoceras age indicated by the presence of Cravenoceratoides?.

The Homoceras and Lower Reticuloceras faunas from this area consist of typical goniatite-lamellibranch associations closely comparable with those of similar age in the north of England. They are confined to a single group of mudstones and shales and are probably not more than 80 ft thick. The only Upper Reticulocerasfauna collected—from the Pygmaeoceras sigma Bed—is of similar phase.

The principal marine bands of Lower Gastrioceras Age carry 'shelly' faunas, such as have been previously listed from the North Crop of the South Wales Coalfield by Robertson (1927, pp. 48–50; 1933, pp. 63–5). They include, in addition to goniatites and lamellibranchs, many brachiopods, gastropods and nautiloids; faunas of such richness are uncommon in the Millstone Grit except in North and South Wales. Some of the species in these bands appear to belong to undescribed forms. The fauna collected from the Gastrioceras cumbriense Marine Band is considerably more varied than that from the G. cancellatum Band; it includes in particular the characteristic lamellibranch Aviculopecten cf. losseni (von Koenen) commonly found with G. cumbriense Bisat wherever the latter occurs.

Notable in this area is the occurrence of a horizon near the top of the Series which contains the worm burrows of Planolites,together with Lingula mytilloides.Such an association indicates that conditions may not have been favourable for the establishment of a fully marine fauna. A.W.W.

Details

As explained in the foregoing, the main outcrops of the Millstone Grit Series in the Pontypridd district occur in the area immediately south and south-east of Aberkenfig. (Figure 3) shows the solid geology of this area and the locations of the principal localities described below.

The basal beds of the Series crop out at two localities. The first is in a cutting [SS 8984 8220] on the west side of the Bridgend-Maesteg road near the northern end of the Pen-y-fai Mental Hospital (locality 1 of (Figure 3)). Here the following section, now much overgrown, can be made out:

The following fauna has been obtained from the black mudstones: Spirifer?; Posidonia cf. corrugata (R. Etheridge jun.); fish fragments; and conodont assemblages including Hindeodella sp., Ozarkodina sp.and cf. Polygnathus communis Branson and Mehl. The base of the formation is not exposed, and a short distance to the south the Quarella Sandstone (Rhaetic) is brought in by faulting. The same beds are poorly exposed on the east side of the Bridgend-Abergwynfi railway [SS 9040 8220], about 600 yd N. of Coity Junction (locality 2). Here the same characteristic conodont assemblages are found in the lower beds close upon striped radiolarian cherts. The silty mudstones above are seen to pass gradually upwards into massive quartzitic sandstones about 20 ft of which are exposed in the cutting immediately south of the bridge over the road [SS 9029 8244], 200 yd S.W. of Pen-y-cae Farm.

This sandstone is succeeded by about 100 ft of mudstones and shales, the top part of which is exposed in the road side [SS 9038 8250] at the hairpin bend 150 yd W. by S. of Pen-y-cae (locality 3). Here 10 ft of blue and blue-black shales were noted, containing: Sphenopteris sp.; Posidoniella sp.; Anthracoceras or Dimorphoceras,Reticuloceras cf. moorei Bisat and Hudson, R.stubblefieldi Bisat and Hudson, and Reticycloceras sp.;this assemblage indicates the presence of the R. nodosum Subzone of the R. eoreticulatum Zone of R₁ Age.

Beds near the top of the same group of mudstones were cut in a sewer-trench [SS 8945 8221], some 80 yd long and aligned almost due east-west, situated 420 yd N. by E. of All Saints' Church, Pen-y-fai (locality 4). The excavation throughout its length was in iron-stained, blue-black shales with a few nodules of ironstone, and three distinct horizons were recognizable. At the western end of the trench the shales yielded Ephippioceras sp., Reticuloceras cf. adpressum Bisat and Hudson and R.aff. paucicrenulatum Bisat and Hudson; these forms are indicative of the R. circumplicatile Zone. In the middle portion of the trench Dunbarella aff. rhythmica (Jackson), Hudsonoceras ornatum (Foord and Crick), Reticuloceras sp.and Reticycloceras sp.were obtained; suggesting the development of the R. eoreticulatum Zone. In shallow foundations nearby, shales with goniatites of the R. nodosum Bisat and Hudson group, associated with crinoid columnals and Posidoniella cf. minor (Brown), confirmed the existence of this Zone, though the exact relationship of the two faunas was not clear. At the eastern end of the trench a fauna indicating the upper part of R₁ occurred: Dunbarella cf. rhythmica,Posidoniella cf. minor; Homoceras?,and Reticuloceras cf. reticulatum s.s.

At both this Pen-y-fai locality and near Pen-y-cae, exposures occurred in only the top portion of the shale-'slack', some 20 ft at most. However, about 200 yd N.N.W. of the main entrance to the Parc-gwyllt Mental Hospital a reservoir [SS 9165 8275] has been excavated over a considerable portion of a shale-'slack' lying between two well-marked developments of quartzitic sandstone. The spoil from these excavations (locality 5), now grown over, yielded the following fauna: Posidoniella sp.; Homoceras magistrorum Hodson, H. aff. eostriolatum Bisat, H. henkei H. Schmidt, Homoceratoides prereticulatus Bisat, Reticuloceras cf. adpressum, R. paucicrenulatum and R. sp.(with delicate close non-crenulate transverse striae, lingua slight or absent). These collections show that several faunal horizons are present extending from the uppermost Homoceras eostriolatum Zone of H Age to the basal R. circumplicatile Zone of R₁.

The basal beds of this shale-'slack' are seen in a quarry [SS 9130 8270], 400 yd to the W. by S. at a point 100 yd N. of Parc-gwyllt Fach Farm (locality 6). Here the following section can be seen:

The lowest beds of weathered mudstone contain abundant conodonts including Hindeodella sp.,Ozarkodina sp., Polygnathodella? and Streptognathodus cf. elegantulus Stauffer and Plummer, associated with Posidoniella sp.Close above, a bed with poorly preserved goniatites, identified as Cravenoceratoides?,is suggestive of E Age. This further supports the view that the whole of H should be present in the shales immediately above.

It appears from the contained fossils that the shales lying beneath the reservoir belong to the same horizon as those exposed in a parallel line of strike farther south in the Pen-y-fai sewer-trench and at the hairpin bend near Pen-y-cae. It therefore seems likely that strike-faulting of some considerable magnitude separates the two areas, though no positive evidence of this dislocation is to be seen in the field. The hypothetical position of such a fault necessary to explain the repetition of the faunas is shown on (Figure 3). This corresponds approximately with the position of a strike fault postulated, for different reasons, by R. H. Tiddeman during the original six-inch survey. In the area immediately north and north-west of the Parc-gwyllt Hospital Tiddeman mapped a small inlier of Carboniferous Limestone, based on the presence of chert fragments, which were correlated with the chert-beds of the 'Black Lias' in the area west of Litchard. During the present survey a number of shallow pits dug to bed-rock in the area of the supposed Carboniferous Limestone revealed only buff-weathered mudstone and silty mudstone of typical Millstone Grit type. Fragments of cherty limestone used as hard-core during the construction of the hospital were, however, observed, but were presumably imported. On the basis of this evidence, therefore, the inlier of Carboniferous Limestone is omitted from the present maps, though the strike fault of Tiddeman is retained.

The R₁ shales are succeeded by what is probably the thickest quartzitic sandstone of the Millstone Grit in this area. An impersistent band of mudstone or shale is developed near the middle of the sandstone, which crops out in the neighbourhood of Pen-y-cae (close to which a small quarry [SS 9054 8248] is dug in about 12 ft of massive quartzite) and again along a roughly east-west line at the northern end of the reservoir referred to above. Small quarries [SS 9110 8286] about 300 yd N.W. of Parc-gwyllt Fach show 6 to 8 ft of quartzitic sandstone, while sporadic crags continue the line of outcrop to the east and west.

Beds of definite R₂ Age are nowhere exposed in the Aberkenfig area. General stratigraphical considerations, however, suggest that they occupy most, if not all, of the broad featureless area of Pen-y-fai Common to the west of the Ogmore River, and much of the central portion of Cefn Hirgoed to the east. Features, associated with isolated crags, show the existence of several impersistent quartzitic sandstones in this portion of the sequence, which may have a total thickness of some 300 to 400 ft. Some three miles to the east of the Ogmore Valley, in a small stream [SS 9550 8222] 100 yd E.N.E. of Gwastadwaun Farm (830 yd N.W. by W. of Pencoed Station), grey slightly silty shales yielded: Aviculopecten sp., Posidoniella sp.; Anthracoceras sp.,cf. Homoceras striolatum (Phillips), Gastrioceras aff. lineatum Wright and Pygmaeoceras sigma (Wright). Grey and blue-grey shales and mudstones are exposed in the stream for about 200 yd to the north-west, but no other faunal horizons were found. The beds show evidence of minor folding and faulting, and their exact position in the sequence is not known, but they appear to be at a stratigraphically low level in the group here assigned to R₂

About 100 ft of beds, in which no fossils have been found, are to be seen in the west bank of the Ogmore River at the north-eastern corner of Pen-y-fai Common. These would appear to belong to the top part of R₂ or possibly the lowest part of G. The section may be summarized as follows:

Both the Gastrioceras cancellatum and G. cumbriense marine horizons are well exposed in the stream which marks the northern boundary of Pen-y-fai Common, at the southern end of Aberkenfig village. In the stream-bank immediately east of Pen-y-fai road [SS 8943 8298], about 70 yd E. of Brookland House, the following section was measured (locality 7):

The basal grey mudstones yielded: Nuculana cf. stilla (McCoy), Nuculopsis aff. gibbosa (Fleming), Posidonia cf. gibsoni Salter; cf. Coelogasteroceras dubium (Bisat), Agastrioceras carinatum (Fresh), Anthracoceras sp., Gastrioceras crencellatum Bisat. The goniatites indicate the upper part of the G. cancellatum Marine Band or Zone.

On the west side of the Pen-y-fai Road [SS 8940 8297], and only a short distance below the section just described, about 12 ft of grey mudstones and shales overlie some 10 ft of flaggy sandstones. Near the top the shales yielded: Lingula mytilloides J. Sowerby; Caneyella multirugata (Jackson); cf. Coelogasteroceras dubium,poorly preserved Gastrioceras sp.Some 4 ft lower down a shelly horizon contained crinoid columnals, together with the following brachiopods: Martinia?, Productus (P.) carbonarius de Koninck, P. (Dictyoclostus) cf. hindi Muir-Wood, and Schizophoria aff. hudsoni T. N. George.

About 90 yd downstream, beneath the bridge carrying the main Bridgend-Maesteg road [SS 8952 8302], the Gastrioceras cumbriense Marine Band crops out and the following section may be seen (locality 8):

Although the G.cancellatum and G. cumbriense marine bands presumably exist along the full extent of Cefn Hirgoed as far east as Penprysg (where the Millstone Grit Series plunges eastwards beneath Glacial Drift), no further exposures are known. Similarly there is no evidence at outcrop (apart from sandstone) of the 200 ft or so of beds presumed to lie between the cumbriense horizon and the base of the Coal Measures. The uppermost beds of the Series were proved in the Margam Park No. 1 Borehole [SS 8194 8581], 2050 yd E. 14° S. of Margam Church, as follows:

Down to the top of the mudstone 56 ft 4 in thick the dip was more or less normal—about 10° to 12°. Below this point, however, the cores exhibited rapid variation of dip up to vertical and the recorded thicknesses are obviously much too great. The borehole was continued a further 750 ft but the beds were highly disturbed and no lower horizons appear to have been proved. The thick quartzitic sandstone of the above section seems to have been repeated three times and the underlying mudstone with Planolites and Lingula at least twice; dips approached the vertical over much of this portion of the borehole which it is assumed was drilled into the lower limb of an underthrust. A.W.W.

References

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.

DIX, Emily. 1931. The Millstone Grit of Gower. Geol. Mag.,68, 529–43.

DIXEY, F. and SIBLY, T. F. 1918. The Carboniferous Limestone Series on the south-eastern margin of the South Wales Coalfield. Quart. J. Geol. Soc., 73 [for 1917], 111–64.

DIXON, E. E. L. 1921. The unconformity between the Millstone Grit and the Carboniferous Limestone at Ifton, Monmouthshire. Geol.Mag.,68, 157–64.

DIXON, E. E. L. and PRINGLE, J.1927. The 'Penlan Quartzite'. Sum. Prog. Geol. Surv.for 1926,123–6.

EVANS, D. G. and JONES, R. 0. 1929. Notes on the Millstone Grit of the North Crop of the South Wales Coalfield. Geol. Mag.,66, 164–77.

GEORGE, T. N. 1927. The Carboniferous Limestone (Avonian) Succession of a Portion of the North Crop of the South Wales Coalfield. Quart. J. Geol. Soc.,83, 38–95.

GEORGE, T. N. 1933. The Carboniferous Limestone Series in the West of the Vale of Glamorgan. Quart.J. Geol. Soc.,89, 221–72.

HUDSON, R. G. S. 1945. The Goniatite Zones of the Namurian. Geol. Mag.,82, 1–9.

JONES, O. T. 1925. The base of the 'Millstone Grit' near Haverfordwest. Geol. Mag.,62, 558–9.

RAMSBOTTOM, W. H. C. 1954. In Sum.Prog. Geol. Surv.for 1953,54.

RHODES, F. H. T. 1952. A Classification of Pennsylvanian Conodont Assemblages. J. Paleont.,26, 886–901.

ROBERTSON, T. 1927. The Geology of the South Wales Coalfield. Part II. Abergavenny. 2nd edit. Mem.Geol. Surv.

ROBERTSON, T. 1933. The Geology of the South Wales Coalfield. Part V. The Country around Merthyr Tydfil. 2nd edit. Mem. Geol. Surv.

ROBERTSON, T. and GEORGE, T. N. 1929. The Carboniferous Limestone of the North Crop of the South Wales Coalfield. Proc. Geol. Assoc.,40, 18–40.

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

STRAHAN, A. , CANTRILL, T. C., DIXON, E. E. L., THOMAS, H. H. and JONES, O. T. 1914. The Geology of the South Wales Coalfield. Part IX. The Country around Haverfordwest. Mem. Geol.Surv.

WARE, W. D. 1939. The Millstone Grit of Carmarthenshire. Proc. Geol. Assoc.,50, 168–204.

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

Chapter 3 Coal Measures : general

Except for a narrow strip along the southern margin the entire district is underlain by Coal Measures rocks. These succeed the Millstone Grit Series conformably and upwards of 6500 ft of beds are preserved between Maesteg and Margam. The sequence is however incomplete, for the highest 1000 ft of measures, such as are seen west of Neath, have been removed by post-Carboniferous erosion, and a further 700 ft, somewhat lower in the succession, are apparently cut out by unconformity. A generalized section of the Coal Measures of the Pontypridd district is given in (Figure 4); it shows the distribution of the thicker sandstones, the principal coal seams, and the known horizons of marine strata, together with the classification used in this memoir.

The Coal Measures of South Wales, in common with those of most of Britain, consist of alternations of mudstones, siltstones and sandstones, together with numerous coals and associated seatearths. Sandstones, although occurring throughout the succession show a remarkable development in the top half, where in developments up to 500 ft or so in thickness, they make up by far the greater bulk of the strata. Persistent sandstones, formerly ascribed to the Millstone Grit (see p. 6), are also widespread in the lowest strata of the formation. Between these two groups the measures are largely argillaceous and it is in these that the major coals are concentrated.

Cyclic sedimentation

The various sediments are arranged in a consistent order forming a succession of repetitive units termed rhythms, cycles, cyclic units or cyclothems. It is thought that sedimentation took place in shallow water on a shelf of considerable extent, and that the cyclothems resulted from intermittent subsidence of this shelf, possibly accompanied on occasions by eustatic changes. The clastic sediments represent the filling up of the depositional area to water-level, and the coals represent the decayed products of the vegetation which, during relatively lengthy stable periods, colonized the former area of deposition. The bulk of the sediments evidently accumulated under brackish or even fresh-water conditions, but the sporadic occurrence of comparatively thin but persistent layers of mudstone with typical marine fossils shows that occasional influxes of the sea took place, most of which spread throughout the area of sedimentation.

A typical Coal Measures cyclothem may be considered as starting with the first sediments accumulating after subsidence (the roof strata of a coal) and ending with the final products of silting up and emergence (the seatearth and its overlying coal). In general the clastic sediments coarsen upwards and the contained fossils suggest that the environment becomes increasingly non-marine. Truly marine forms, when present, lie at or near the base of the cyclothem; they tend to be succeeded upwards by such forms as Lingula,foraminifera and Planolites ophthalmoides,which may foreshadow oncoming brackish conditions; and these in turn by 'Estheria' and the so-called non-marine lamellibranchs or 'mussels', suggestive of even less saline, estuarine facies. The overlying beds are characterized by plants and plant debris, and by the time vegetation became established, as witnessed by the presence of roots in seatearth, the environment was virtually one of fresh water.

Many attempts have been made to define a single 'standard' or 'ideal' cyclothem; but rarely do actual cases conform to this standard. Local variations in conditions of subsidence and deposition inevitably cause modifications to the pattern, and produce a cycle of deposition differing widely in many cases from the hypothetical standard. Four main variations are recognizable in the Coal Measures of the present district (see (Figure 5)).

Below the Gellideg Seam successive subsidences followed one another rapidly, and in consequence coals are insignificant. The south-western part of the district appears to have occupied a relatively 'off-shore' position, for the lowest portions of several cyclothems are occupied by thick marine strata, and these are overlain in turn by thick mudstones containing abundant traces of the burrowing worm, Planolites ophthalmoides,believed to indicate near-marine conditions. These two phases usually comprise the bulk of the individual cyclothems, which in general vary in thickness from 30 to 100 ft, though 200 ft are recorded at Margam Park.

From the Gellideg to the Two-Feet-Nine seams the periods of stability between successive subsidences were greatly prolonged, and consequently the typical cyclothem ends with a thick coal. Marine sediments are known from only one horizon; usually an 'estuarine' phase with 'mussels', up to a few feet thick, forms the base of the cyclothem; and these estuarine muds pass upwards into mudstones, silty mudstones and striped beds, generally rich in plant debris. Only rarely does this sequence end in a widespread sheet of sandstone. In thickness these cyclothems range from 20 to 100 ft and their lateral development is variable. Splitting of coals is common and mere partings in a seam at one locality may be represented by full cyclothems 100 ft thick a few miles away.

In the upper part of the Middle Coal Measures the cyclothems show a close approach to what is generally considered the 'ideal' Coal Measures form. Many of the units begin with marine strata carrying varied faunas; these are followed successively by Lingula,Planolites,'mussel' (rare) and plant phases. The mudstones containing these fossils pass upwards into silty mudstones, striped beds, and, in several instances, into well-developed fine-grained sheet-quartzites, such as the Cockshot Rocks of the Maesteg area. Ganister-like seatearths at the top of the cyclothem are associated with coals of only moderate thickness, 1 to 3 ft being average. Commonly each unit has a total thickness of 40 to 120 ft; a generally more uniform subsidence is deduced from the decreasing importance of seam-splits.

In the Lower and Middle Coal Measures, and particularly among the main seams, sandstones are encountered which do not form part of the cyclothemic sequence as above described. They are characterized by sharp bases, and conglomeratic developments with rolled pellets of ironstone and mudstone are common in the lower layers. The strata on which they rest commonly show signs of local erosion, which was probably contemporaneous with the influx of the sand. They may interrupt any part of a cyclothem and in places the thicker sandstones may occupy the space of several units. They appear to be largely independent of the factors controlling true cyclic deposition and may reflect abnormal conditions in the adjoining land areas. These are the sandstones which are usually associated with 'washouts' in coal seams.

The typical Pennant cyclothems are markedly different, and it is debatable whether their mode of formation is the same as that of the sedimentary cycles previously described. They are seldom less than 100 ft thick, and they may reach over 500 ft, and at least 80 per cent is made up of a virtually continuous body of sandstone. This lies immediately or closely upon a coal, and the base is commonly conglomeratic. Mudstones, up to 20 ft or more thick, may intervene between the coal and sandstone, and may contain Anthraconauta and ostracods, associated at a few horizons with Euestheria,at their base; varied and abundant plants may also be present. The junction between the mudstones and the overlying sandstones is almost invariably sharp and there is commonly evidence of erosion of much or all of the mudstone. In contrast, the top of the sandstone always grades or interdigitates into silty mudstones and mudstones, which are unfossiliferous apart from plant debris. Several horizons of seatearth and associated coal streaks may occur before the cyclothem ends with a coal, commonly dirty, of 1 to 3 ft in thickness. These distinctive Pennant cyclothems are interpreted as reflecting a major change in the geography of the sedimentary basin and its surrounding landmass; reflecting possibly greater elevations of the land precursory to the onset of the Armorican orogeny.

Lithology

Argillaceous rocks

These range from soapy clay to silty or sandy mudstones; they may be very fissile, especially in the roofs of coals, or hard and blocky. They vary from pale grey to black, the latter types usually occurring close to coals. The dark colour is usually due to the presence of finely divided carbonaceous material, but disseminated iron sulphide is responsible in some instances. Mica may be abundant, giving an apparent siltiness to the rock. Silty material is increasingly important higher in the cyclothem, and there is a gradual transition into siltstones. Marine mudstones and shales are usually dark bluish-grey and contain abundant pyrite in the form of cubes, granules, and Tucoids', or replacing organic remains.

Nodules (tails') and bands ('pins') of clay-ironstone are associated with the mudstones, especially in lower beds of the cyclothem; irregularly shaped nodules may also abound in the lower parts of seatearths. They are hard and compact and vary from light grey or brown to black; they consist mainly of ferrous carbonate with varying amounts of calcium and magnesium in solid solution, and may contain varying amounts of admixed clay material. Septaria are common, the cracks being filled with a variety of secondary minerals (see North and Howarth 1928). Hard compact carbonaceous sideritic mudstones, known as blackbands, occur widely at several horizons, usually in the roofs of coal seams. They may be blocky and massive, but are more generally thinly laminated; they are commonly rich in 'mussels' and ostracods.

Striped Beds

Theseconsist of alternations of pale grey quartzitic siltstone and medium grey mudstone or silty mudstone, the individual layers being commonly not more than about one-sixteenth of an inch in thickness, though they may occur in bands up to several inches. The relative abundance of the two components is inconstant; some rocks are almost entirely muddy, with only a few silty stripes; others may consist largely of quartzitic silt with only subordinate wisps of mud. Lens-bedding is common and the rocks may also show evidence of turbulence and slumping.

Arenaceous rocks

These fall into two main classes: quartzitic sandstone, often known as 'cockshoe, and developed characteristically in the Lower and Middle Coal Measures; and 'pennant', largely restricted to the Upper Coal Measures. The former consist of angular quartz grains set in a crypto-crystalline groundmass of secondary silica; feldspars are rare or absent, and carbonaceous material is variable in amount and arranged usually on the bedding. Many sandstones are fine enough to merit the name siltstone, but coarse pebbly layers occur in some towards the base of the Lower Coal Measures. Individual beds are commonly about 5 ft thick; and false-bedding is not normally conspicuous.

Pennant sandstone when fresh is a green or bluish-grey rock, resistant to weathering, but which on prolonged exposure develops a yellowish or brownish surface. Large scale false-bedding is general, though many massive posts up to 20 ft or so thick show little trace of bedding. Locally thinly and regularly bedded tilestones and flagstones are developed, the fissility being commonly due to concentration of detrital mica on the bedding. The microscopic characters of the rock have been described by Flett (see, for example Strahan 1907, p. 58; Strahan and others 1907, p. 93; Strahan and others 1917, pp. 129–30) and Heard (1922). Typically it consists of subangular to sub-rounded grains of fairly even size set in a crypto-crystalline matrix of silica, white mica and clay minerals. The fragments consist mainly of quartz, some 'granitic' or sheared, but feldspars, largely kaolinized, are always present, and fragments of pre-existing rocks are not uncommon Mica is abundant in some varieties. Carbonaceous material may be disseminated throughout the rock as specks of indeterminate character or it may be concentrated as layers of detrital coal. The casts of large prostrate tree-trunks and flattened pieces of timber are common, especially in some of the coarser bands.

Pebbly layers occur sporadically, most of the pebbles consisting of quartz, but shales, slates, cherts, quartzites, jaspers and sandstones are also present. Closely associated with such pebbly horizons, but also occurring at other levels, are beds containing rolled clay-ironstone pellets and 'rafts' and 'pebbles' of coal, pointing to the pene-contemporaneous erosion of only partly consolidated materials. Many of the coal 'pebbles' are rude cuboids, suggesting the break-up of a bed which already possessed a rectangular joint-pattern. The extensive tabular form of many of the 'rafts' makes it likely that the coal material was of a leathery consistency on burial, while the relationships of the raft and the enclosing sandstone show clearly that there has been no further reduction in volume upon subsequent coalification.

Seatearths

The root-beds immediately beneath coals have been altered by the action of humic acids derived from the overlying vegetation. Lithologically they range from clays to coarse sandstones; good quality fireclays are rare and true ganisters virtually non-existent, and so the term seatearth is here used to cover all varieties of coal-floor strata.

Nature and distribution of fossil bands

Marine Bands

Withinthe district marine faunas are known from thirteen horizons (see (Figure 4)). Apart from the Amman Marine Band, they occur in two main groups: five lie below the Garw and the remainder between the Two-Feet-Nine and the base of the Upper Coal Measures. Certain of the bands exhibit distinct faunal phases in sequence which are thought to correspond with variations in contemporary salinity. The band may start with a cannelly fish-phase, which, with increasing salinity, is followed first by horny brachiopods and foraminifera, and then by varied forms such as brachiopods, gastropods, lamellibranchs, and goniatites. Thereafter a decreasing salinity is indicated by a return of the Lingula- and foraminifera-phases followed by that of Planolites ophthalmoides.

The sub-Garw marine bands (see pp. 33–7) are known only along the western South Crop, where they commonly exceed 20 ft in individual thickness. A goniatite-phak is usual, and the bands are generally similar in their development to those in the upper part of the Millstone Grit Series. The Planolites-phase is particularly thick, 30 ft and 40.5 ft being recorded in two of the bands.

The Amman Marine Band is for the most part little more than a Lingula-band,but towards the south-east the fauna becomes increasingly more varied and 'calcareous'.

Of the higher bands, the Hafod Heulog (p. 59), Britannic (p. 59) and Foraminifera (p. 62) horizons carry little other than horny brachiopods, foraminifera, and Planolites.The Five Roads Band (p. 63) is known from one locality, where it contains only Myalina.The others, the Cefn Coed (p. 60), the Lower Cwmgorse (p. 63) and the Upper Cwmgorse (p. 64), although locally Lingula-bands,more usually include varied faunas, especially in the west and south-west of the district.

Non-marine lamellibranchs

Theseoccur sporadically throughout the sequence, being especially common in the roof strata of coals, particularly those between the Gellideg and Two-Feet-Nine, where they are of particular value in correlation. Individual 'mussel'-bands are less continuous than marine bands and few persist throughout the district. Some, moreover, are associated with particular lithologies, and this association, while making them readily recognizable as markers over limited areas, also renders them liable to lateral failure. A zonal system based on variations in the 'mussel' fauna was erected by Davies and Trueman (1927) and extended by later workers (see (Figure 4)). It has formed the basis of studies within the present area by Evans and Simpson (1934) and Moore and Cox (1943). While the zonal system is not sufficiently precise for detailed correlation, a number of restricted faunal assemblages, some indeed confined to single cyclothems, have been recognized within the present district and are described briefly below.

In the sub-Garw measures, 'mussel' faunas are known only from the Margam Park Boreholes (pp. 77–9). The lowest occurs about 250 ft above the base of the Coal Measures, where representatives of the Carbonicola fallax.Wright group, characteristic of the lower part of the lenisulcata Zone, are found. Between the Margam Marine Band and the Garw, shells are known from several horizons. The commonest forms belong to Curvirimula,including variants of C. trapeziforma (Dewar), but Anthraconaia sp. nov.(cf. modiolaris), Carbonicola aff. crispa Eagar, C. aff. pontifex Eagar and C. aff. proxima Eagar have also been identified. These are generally considered to be at about the junction of the zones of Anthraconaia lenisulcata and Carbonicola communis (see also Eagar 1962, pp. 323, 330).

The 'pseudorobusta fauna' is found between the Garw and Gellideg seams. Several more or less distinct layers of dark shaly mudstone carry a characteristic fauna of large Carbonicola with thick, often crushed, shells preserved in recrystallized calcite. These reach about 70 mm. in length and include variants of C. communis Davies and Trueman, C. pseudorobusta Trueman and C. rhomboidalis Hind; they are associated with Curvirimula,as well as with Geisina arcuata (Bean) and Spirorbis sp.Similar shell-bands are known from the same general position in most British coalfields.

The 'Yard fauna' is characterized by 'solids' dispersed through several feet of blocky grey mudstone. It consists of the last abundant Carbonicola,those grouped around C. venusta Davies and Trueman, and the earliest abundant Anthracosia,variants of A. regularis (Trueman). These have a general similarity of shape and it is not always easy to differentiate between them.

The 'Amman fauna' is made up mainly of Anthracosia of the aquilina (J. de C. Sowerby)/ovum Trueman and Weir group, usually poorly preserved as 'ghosts' in smooth grey mudstone immediately overlying the Amman Marine Band. Many individuals show distinct Anthraconaioid characters, and this is presumed to result from the somewhat unusual environment consequent upon the marine incursion (Jenkins 1960, p. 116).

The 'Bute fauna' is widely developed over the western half of the district where 'solid' shells are usually abundant. It consists essentially of an association of Anthraconaia of the modiolaris (J. de C. Sowerby)/williamsoni (Brown) group and Anthracosphaerium of the affine (Davies and Trueman)/exiguum (Davies and Trueman)Iturgidum (Brown) complex. The Bute roof is the lowest horizon at which these two genera are abundant.

The 'Nine-Feet fauna', best developed in the north-eastern areas, is essentially a facies fauna of large crushed Anthracosia (30 to 40 mm.) in dark carbonaceous and ferruginous mudstones. Variants and intermediates of A. aquilina, beaniana King, disjuncta Trueman and Weir, ovum and phrygiana (Wright) are common; shells of the A. phrygiana group showing a marked tilt of growth lines are especially characteristic.

The 'Red-Vein fauna' is less restricted; it occurs above the Nine-Feet and at several other horizons between this seam and the Caerau. Small 'solid' shells occur through blocky grey mudstone and characteristically possess a retiform wrinkling of the periostracum. The assemblage consists mainly of Anthracosia and Anthraconaia,but because of their small size it is difficult to separate the two genera with certainty: they may be referred to Anthracosia aff. nitida (Davies and Trueman) with variants approaching A. angulata (Chernyshev) and Anthraconaia pulchella Broadhurst.

The 'Six-Feet fauna' is best developed in a carbonaceous ironstone of blackband type, though similar but less abundant forms persist in grey mudstone.

The shells, which reach 30 mm. in length, consist almost entirely of Anthracosia and are dominated by variants of A. atra (Trueman). Associated forms include A.aquilinoides (Chernyshev), A. concinna (Wright), A.elliptica (Chernyshev), A. lateralis (Brown) and A.planitumida (Trueman).

The 'Four-Feet fauna' is notable for the fine preservation of the 'solid' shells and the ease with which they can be removed from the enclosing blocky grey mudstone. Anthracosia,Anthraconaia and Anthracosphaerium are all represented in considerable variety. Particularly characteristic is a form identified as Anthraconaia sp. nov.(cf. curtata),and members of the A.librata (Wright)! cymbula (Wright) and Anthracosphaerium exiguum/propinquum (Melville) groups are almost equally typical. The Anthracosia element consists largely of variants of the species A. atra,concinna and planitumida.

The 'Two-Feet-Nine fauna' is composed of rather small (15 to 20 mm.) 'solids' dispersed through grey mudstone and usually associated with nodular ironstone bands carrying Planolites montanus Richter. The main elements are species of the genus Anthracosia including acutella (Wright), atra, concinna,lateralis (Brown), planitumida and simulans Trueman and Weir; Anthracosphaerium propinquum is well represented, as is also an elongate Anthraconaia of the lanceolata (Hind)/ wardi (Hind non Salter) group. Similar faunas occur at several horizons close above the Two-Feet-Nine.

'Mussels' are not numerous in the measures between the Hafod Heulog and Upper Cwmgorse marine bands. Two horizons of note are recorded, both characterized by Anthraconaia.The Caedavid Seam of the Garw valley (p. 158) yields A. adamsi (Salter) preserved as squashed forms in blackband, and in the Rhondda Fawr and Llynfi valleys A. hindi (Wright) and A. warei (Dix and Trueman) have been obtained as 'solids' from grey mudstone above the Eighteen-Inch Seam.

Anthraconaia aff. stobbsi (Dix and Trueman) and Naiadites are found above the White Seam, but above this the 'mussel' faunas show a marked change.

Apart from a few Anthraconaia pruvosti (Chernyshev) found near the middle of the Rhondda Beds, only Anthraconauta is recorded in the Upper Coal Measures.

A. phillipsii (Williamson) is persistent, but A. tenuis (Davies and Trueman), which appears in the Rhondda Beds, becomes increasingly abundant upwards. Both species are confined to a relatively few horizons, at which, however, they are usually very abundant. Despite their individual lack of faunal distinctiveness, the bands tend to be confined to the roofs of particular seams and this gives them an added value as markers.

Other faunal bands

At several horizons beneath the Gellideg there are thin bands of carmelly mudstone rich in fish debris. One of these, lying about 50 ft above the Margam Marine Band evidently passes into a marine band in the west of the coalfield (Woodland and others 1957b, p. 52). Another significant fish-band lies in the roof of the Garw; this persists throughout South Wales, and it may well extend much further afield for the roofs of the Kilburn and the Arley in the Pennine coalfields are almost identical with it.

'Estheria',although locally occurring at numerous levels, shows a remarkable concentration in the roof of the Abergorky. Here Lioestheria vinti (Kirkby) is virtually unaccompanied by any other fossils, and the band is traceable over most of the coalfield. A similar concentration is known from what may be the same horizon in other coalfields, notably the Main 'Estheria' Band of the Nottingham-Derby area (Edwards and Stubblefield 1948, pp. 231–3). L.striata Pruvost (non Munster) is present widely on the roof of the Lower Six-Feet.

Floras

Apart from the coal seams, which consist almost entirely of the products of decayed vegetation, plant remains are ubiquitous. Mostly they consist of indeterminate comminuted debris, but mudstones rich in well-preserved leaves and stems occur. Good roof floras have been obtained locally from the Yard, Bute, Lower Nine-Feet, Upper Nine-Feet, Caerau, No. 2 Rhondda, No. 1 Rhondda, Brithdir and Daren-ddu or Mountain seams. Protracted collecting can produce long lists of species even where the fossils are not abundant, and in this way D. Davies (1921) produced remarkable collections from all the worked seams in the collieries of Clydach Vale and Gilfach Goch.

Formerly plants were used as the main basis for correlation and classification (Kidston 1893–94, 1894). Working mainly in the western areas of the South Wales Coalfield, Dix (1934) proposed a nine-fold zonal subdivision of the Upper Carboniferous, and this scheme was followed closely by Moore and Cox (1943) in their work in the Taff and Rhondda valleys.

Most of the plant species are long-ranged, and the zones each cover considerable thicknesses of strata; the boundaries between the zones are hard to define, and individual seam floras are not readily distinguishable. Consequently no great attention has been paid to the plants during the present survey; they have been collected where readily available, and are listed in their appropriate places in the details.

Classification

Early classifications of the Carboniferous rocks in South Wales were wholly lithological. The traditionally arenaceous Millstone Grit was considered to extend upwards from the Limestone to the top of the 'Farewell Rock', the highest significant sandstone beneath workable coals and ironstones. In the succeeding Coal Measures the existence of a major sandstone formation, the Pennant, between two predominantly mudstone sequences led to a natural three-fold subdivision into Lower Coal Series, Pennant Series and Upper Coal Series, and, with minor variations, this classification was in general use until the present survey. Its drawbacks became apparent when detailed mapping in the present and adjoining Swansea districts disproved the old idea that the Pennant lithology was bounded by two time planes, conveniently marked by the No. 2 Rhondda Seam below and the Mynyddislwyn or No. 3 Llantwit above. This old conception had led the earlier surveyors to serious mis-correlation. Thus in the earlier editions of this memoir the base of the Pennant Series was taken at the Brithdir Seam in the east, at the No. 2 Rhondda in the central areas, and at the Rock Fawr or No. 3 Rhondda along the western South Crop. These three distinct seams each mark the approximate base of massive pennant lithology locally, but they are now known to be spread over some 1800 ft of strata in the area of thickest development.

Over the last thirty or forty years detailed work in all the coalfield areas has demonstrated the continuity over wide areas of many of the bands containing marine fossils. These provide a secure basis for detailed correlation between the various coalfields and as a result the Geological Survey has recently adopted a single classification for the Coal Measures throughout England and Wales (Stubblefield and Trotter 1957). This recognizes three divisions, Lower, Middle and Upper: the division between the Lower and Middle is taken at the base of the marine band which occurs about the middle of the Anthraconaia modiolaris Zone, and which is characterized by Anthracoceras vanderbeckei (Ludwig); and that between Middle and Upper at the top of the highest marine band, that characterized by Anthracoceras cambriense Bisat. The application of this scheme to the South Wales sequence has already been discussed (see Woodland and others 1957a). Although palaeontological in its conception, it does, in South Wales, have the additional merit of restricting the Pennant sandstones, even where most fully developed, to the Upper Coal Measures. In order to retain such a deeply entrenched term this uppermost division has been given the local synonym of Pennant Measures, eventhough it includes the whole of the former Upper Coal Series and the uppermost part of the Lower Coal Series.

South Wales is unique among the major British coalfields in that the Upper Coal Measures occupy most of the surface and are generally a good deal thicker than the combined lower divisions. Further sub-division is therefore desirable, and this has been made possible by the tracing across the coalfield of several coal horizons. A separation into two major groups at the Hughes Seam and its equivalents is supported by both floral and faunal evidence. Crookall (1955, p. 3) suggests that this horizon marks the junction between Westphalian C and D (that is between Staffordian and Radstockian), and it may also be taken as the arbitrary boundary between the 'mussel' Zone of Anthraconauta phillipsii and that of A.tenuis.Both divisions have been further divided into three groups though these have been distinguished only on the six-inch maps. This classification and the reasons for its adoption have already been set out in detail

(Woodland and others 1957a) and the following table summarizes its application within the present district.

Thickness variation

It has long been known that all divisions of the Carboniferous thicken markedly when traced across the coalfield from east to west. Much of this increase in the Coal Measures takes place within the Pontypridd district where, considering the measures up to the Daren-ddu and its equivalents, the highest level at which true comparison is possible, some 2900 ft of beds in the north-east expand to about 6250 ft near Margam. The thickening is not uniform throughout the sequence, being more marked in the lower divisions than the higher.

Detailed knowledge of the beds below the Garw is limited, but at Dowlais, some 5 miles north of Merthyr Vale, they are little more than 150 ft thick (Robertson 1933, pp. 69, 96); in the Margam Park No. 1 Borehole (p. 77) they were proved to be 1146 ft, a seven- or eight-fold increase in 22 miles. The higher beds of the Lower Coal Measures thicken in the same direction from about 200 ft in the Taff and Cynon valleys to about 600 ft at Kenfig Hill, a three-fold increase in 18 miles (Figure 6). In the Middle Coal Measures both the beds between the Amman and Cefn Coed marine bands and those between the Cefn Coed and Upper Cwmgorse thicken westwards from about 300 ft around Pontypridd to 700 or 800 ft between Maesteg and Margam (Figure 7). A similar thickening is seen in the Llynfi Beds, from 250 ft at Merthyr Vale to 700 ft at Margam, but above this the rate falls off progressively; the Rhondda Beds are rather less than 600 ft in the northern Rhondda and Taff valleys and about 1100 ft to the west of Aberkenfig; and the Brithdir Beds about 500 ft in the Rhondda and on the eastern South Crop and 800 ft south-west of Maesteg (Figure 8). As far as can be estimated from their incomplete preservation the Upper Pennant Measures suffer little variation in thickness across the district.

The variation in coal thickness is broadly analogous. Except below the Garw, coals are common throughout the sequence and upwards of 58, at least locally, reach 2 ft in thickness. Although they are fairly evenly distributed there is a concentration of the thicker ones between the Gellideg and Two-Feet-Nine, and almost all the present-day production in this central part of the coalfield comes from this group. In the Cynon Valley 12 coals occur in this portion of the sequence, spread through about 375 ft of measures; in all they total 464 ft or about one-eighth of the total thickness of strata; the individual seams average 3 ft 10.5 in thick and are about 34 ft apart. In the Margam–Kenfig area, on the other hand, splitting of the same coals has produced 18 significant seams totalling about 97.5 ft of coal through 1200 ft of measures; coal makes up about one-twelfth of the total thickness of strata and the individual seams, which average 5 ft 5 in, lie about 75 ft apart.

Seam nomenclature

The naming of coal seams in South Wales has always been chaotic. Within much of the present district attempts were made to apply the names in use around Aberdare, at least to the main productive part of the sequence. Even so, local nomenclatures grew up; sometimes for a single seam, sometimes for an individual colliery, sometimes even for a region such as the South Crop. More confusing still were the many miscorrelations with the Aberdare sequence. Thus not only was the same seam given many names, but the same name was applied to many different seams. During the resurvey the seams between the Garw and Two-Feet-Nine were everywhere correlated with the Aberdare sequence and re-named accordingly, splitting being accommodated by use of the forenames Lower, Middle and Upper. These names appear on the six-inch and one-inch maps, are now in use by the National Coal Board, and form the basis of the following account of the stratigraphy. (Plate 3) shows the detailed correlation of the coals from the Gellideg to Two-Feet-Nine, together with the new names and the local names in former use.

Above the Two-Feet-Nine the coals are less widely worked and the desirability for a single set of seam names is not so great. Simplification into two nomenclatures has, in general, proved sufficient within the Middle Coal Measures, and both sets of names are shown on the one-inch map and used in the following account, see (Plate 4). Within the Pennant, only the No. 1 and No. 2 Rhondda were hitherto in use as regional names, and many coals were unnamed. Seams have been given the names in use where they are best developed, generally in the Taff–Rhondda district, and these names have, on the six-inch maps, been linked with any existing local names. The relationship of the various named sequences is shown in (Plate 5).

Lower Coal Measures

The Lower Coal Measures outcrop within the district is limited to two areas: a narrow strip along the southern margin, and a small tract on the north side of the Moel Gilau Fault west of Bryn. Even in these areas the measures are poorly exposed and are covered by extensive drift deposits. The division falls naturally into two groups at the Garw, a seam which can be easily correlated throughout South Wales and which would seem to be readily correlated in other coalfields (e.g. the King of North Staffordshire, the Arley Mine of Lancashire, the Kilburn of the East Midlands and the Victoria of Durham).

Below the Garw, knowledge of the succession is virtually confined to the extreme south-west of the district, where the Margam Park No. 1 Borehole penetrated the full sequence, and where sporadic exposures are to be seen in the Cefn Cribwr area. Here these measures are about 1100 ft thick; eastwards and north-eastwards considerable thinning must take place, for in the Dowlais area, about 4 miles north of Merthyr Vale, the corresponding strata are leas than 200 ft thick (Robertson 1933, pp. 69, 96), while at Rudry in the Newport district, 8 miles east of Llantwit Fardre, the corresponding thickness is about 300 ft (Woodland and others 1957b, p. 50). The frequency of marine strata and the insignificant coal development illustrate the close links that these lowest Coal Measures have with the underlying Millstone Grit.

The measures above the Garw range in thickness from 200 ft in the north-east to 600 ft in the south-west. The sedimentation is of characteristic Coal Measures type: there are no marine strata and thick coals are developed at regular intervals throughout the sequence.

(a) Measures below the Garw Seam

(Figure 9) shows the lithological and faunal characters of these beds as proved in the Margam Park boreholes, and the following account is based on these sections except where clearly stated.

The Gastrioceras subcrenatum Marine Band is well developed through a thickness of 50 ft, while a few goniatite fragments are also recorded 16 ft above. The lowest 20 ft carries only Lingula mytilloides J. Sowerby, Planolites ophthalmoides Jessen, Orthotetid fragments and fish debris; the succeeding 30 ft contains a rich and varied fauna of goniatites (including G. subcrenatum C. Schmidt), mollusc spat and marine lamellibranchs. To the north of the district at Nant Melyn, one of the headstreams of the River Cynon, similar thick marine beds, nearly 50 ft, are recorded (Leitch and others 1958, p. 474), and the same is true to the west at Cynheidre, 80 ft, and to the east at Rudry, 49 ft (Woodland and others 1957b, pp. 43, 50). Near Glynneath the equivalent beds are only 4 ft thick (Leitch and others 1958, p. 466) and an important sandstone, the 'Farewell Rock', is developed immediately above. It would seem that, except possibly in the northwest, this marine band is present in considerable thickness throughout the Pontypridd district. Despite the great thickness of the band, which contrasts sharply with that in the Pennine province, it is noteworthy that it forms part of a normal cyclic sequence.

Nearly 80 ft of mudstones with sporadic ironstones separate the marine band from a flaggy sandstone 110 ft thick. This sandstone lies at approximately the same horizon as the 'Farewell Rock' of Carmarthenshire (Ware 1939, p. 173) and the head-waters of the River Neath (Jones and Owen 1956, p. 240), where it is about 130 ft thick; eastwards from Margam the sandstone must die out, for it is not represented in the Rudry Borehole (Woodland and others 1957b, p. 50).

The succeeding 120 ft or so of mudstones yield from numerous levels an abundant and rich flora, including such forms as Alethopteris lonchitica (Schlotheim), Mariopteris acuta (Brongniart), Neuropteris schlehani Stur and Sphenopterids, all of which are typical of the Floral Zone C of Dix (1934). Similar floras are recorded from the Nant Llech Plant Beds (Dix 1933, pp. 164–6). These plant beds are overlain by a further 42 ft of mudstone, the lowest 15 ft of which carry 'mussels', including Curvirimula sp., Carbonicola aff. artifex Eagar, C.declinata Eagar and C. fallax,associated with sporadic P.ophthalmoides and fish debris. A similar fauna occurs in the same general position in the Pennine province, as for example, above the Bassy Mine in Lancashire, and the Two-Foot Coal in North Staffordshire.

The overlying cyclothem is 85 ft thick and culminates in a thin coal believed to be that worked in the Cefn Cribwr area under the name Crows Foot or Cors-y-fran. A 25-ft marine band occurs at the base and carries a rich fauna of burrowing and crawling molluscs accompanied by brachiopods, including horny forms and Productids; towards the top of the band a few non-diagnostic goniatites are associated with pyritized sponge spicules, Lingula mytilloides, Hollinella sp.and a few gastropods. The band is split towards the top by a 1 ft 7 in band in which the marine fauna is restricted and associated with Geisina cf. arcuata (an ostracod usually considered to be non-marine). It is noteworthy that at Cynheidre two distinct marine horizons separated by a near-seatearth occur in this position (Woodland and others 1957b, p. 42) and on the North Crop the Ml of Leitch and others (1958, pp. 466, 469) is similarly split by a thin sandstone and shale with plants.

At Margam the rest of the cyclothem above the marine band is represented largely by 33 ft of mudstones carrying Planolites ophthalmoides throughout.

Along the crop east of Kenfig Hill sandstones up to 45 ft thick occur towards the top; these are commonly quartzitic with scattered pebbles. They have been quarried at Cefn Cribwr and Cefn Hirgoed.

The Crows Foot Coal is overlain by a 1 ft 7 in band containing P. ophthalmoides, L.mytilloides and fish debris. This marine band has been recorded at Cynheidre, where it is similarly impoverished, and along the North Crop, where it is believed to be the M2 horizon of Leitch and others (1958). The rest of the cyclothem consists of about 76 ft of mudstones with plants, interbedded with layers of quartzitic sandstone and siltstone.

The succeeding cyclothem is 185 ft thick. Near the base occurs 20 ft 9 in of dark marine mudstones carrying a varied fauna, and here called the Cefn Cribbwr Marine BandOn the recently published one-inch maps this is termed the Gastrioceras listed Marine Band.. The band commences with a brief Lingula-phase followed by various lamellibranchs; towards the middle of the band abundant Gastrioceras listed (J. Sowerby) and G. sp.(with faint spiral ornament) appear associated with Anthracoceras sp.and lamellibranchs, while the uppermost 8 ft yield only Lingula and Planolites.A similar fauna was obtained from a quarry and level spoil near Tycribwr Farm, west of Aberkenfig. This marine band is apparently that referred to as the Wernffrwd Marine Beds in West Gower (Jones, S. H. 1935, pp. 318–21), where G.listeri is also recorded. Away from the South Crop this goniatite is unknown in South Wales. At Cynheidre the horizon appears to be represented by 12 ft of mudstone with Lingula (Woodland and others 1957b, p. 42). On the North Crop its equivalent, the M3 horizon, is widely present though usually thin: at Cwm Gwrelech, for example, Lingula,Productids and Pectinids occur over a thickness of 2 ft 2 in (Leitch and others 1958, p. 465). Because of the occurrence of G. listen this band has been correlated with the Bullion Mine and its equivalents in the Pennine province; certain significant differences, however, are seen when the full sequences of the two areas are compared and it is not impossible that the true correlative of the Bullion Mine lies in one or other of the marine horizons close below the Cefn Cribbwr Band.

The marine band is overlain by 121 ft of quartzitic sandstone, the Cefn Cribbwr Rock (Woodland and others, 1957b, p. 57), which range upwards through 33 ft of striped beds, sandy mudstone and seatearth, to a 5-in coal. The sandstone maintains its thickness from Kenfig Hill to Penprysg and has been quarried extensively.

The sequence is continued upwards by 83 ft of argillaceous measures to another thin seatearth. A Planolitesjplant-debris phase nearly 8 ft thick occurs at the base and is succeeded by some 21 ft of strata containing marine fossils. This band, here termed the Margam Marine Band, begins with about 5 ft of mudstone containing foraminifera, horny brachiopods, Myalina,Bellerophontid gastropods and fish debris, associated throughout with P.ophthalmoides.This is overlain by about 12 ft of mudstones carrying much mollusc spat at top and bottom and abundant foraminifera with Lingula and P. ophthalmoides just above the middle. A 5-inch layer rich in lamellibranchs, such as Dunbarella and Caneyella,together with goniatites, chiefly Anthracoceras sp.,an involute cephalopod (cf. Domatoceras),and conodonts, is followed at the top of the band by 3 ft of dark pyritic mudstone containing only Lingula.The mudstones making up the rest of the cyclothem carry abundant P.ophthalmoides right up to the base of the overlying seatearth. The marine band crops out in part in a stream bed near Bryn-coch, where it is again characterized by abundant Anthracoceras and cf. Domatoceras.The horizon appears to be represented only by a Lingula band on the North Crop (the M4 horizon of Leitch and others) and in the west of the Coalfield, and its absence at Rudry suggests that it dies out eastwards.

The 300 ft of measures upwards to the Garw Seam are distinctive in containing no true seatearths. They are predominantly argillaceous in character, though thin impersistent sandstones characterize the upper portion and ironstones are numerous in the lower part. An abnormally thick cyclothem in this position is typical of the sequence in most British coalfields. P. ophthalmoides is common at intervals throughout these measures and several significant horizons of dark cannelly shale with fish debris also occur. One such fish-bed at the base also carries, in the west of the Coalfield, Lingula and foraminifera. 'Mussels' occur at numerous levels throughout the cyclothem. The lower bands yield Curvirimula.About midway in the group, shells referred to Carbonicola aff. pontifex occur, and similar forms are present at about the same horizon in the Dulais Valley (Woodland and others 1957b, p. 44). Not far below the Garw Carbonicola crispa and C. proxima occur and similar forms are known widely in South Wales, where they are generally taken as representing the highest fauna of the Anthraconaia lenisulcata Zone (Eagar 1962, p. 330).

(b) Garw to Amman Rider

A generalized vertical section of these measures showing seam-splitting and principal faunas is given in (Figure 10), while the lateral variation and detailed correlation of the coals is illustrated more fully in (Figure 27) and ((Plate 3)).

The Garw

The Garw has been proved only at scattered points. About 1.5 ft thick at Aberaman, it thickens south-westwards to almost 3 ft near Kenfig Hill, where it has been worked on a small scale as the Cribbwr Fact'. The roof appears to consist everywhere of dark fissile mudstones yielding much fish debris; sporadic Planolites ophthalmoides occur and suggest an approach to marine conditions.

Between the Garw and the Gellideg the measures are mainly argillaceous, though locally, as for example in the Rhondda valleys, thin sandstones are present in the lower part. In thickness they are less variable than most of the Lower Coal Measures sequence, being 80 to 100 ft in the Cynon Valley and reaching a known maximum of 190 ft west of Bryn. A thin coal is usually present not far below the Gellideg and its roof has also yielded P. ophthalmoides.As elsewhere (Robertson 1929, 1933) ironstone bands are common throughout and especially close above the Garw, and these were formerly worked at Bryndu on the South Crop and east of Cwmavon. Several bands of dark shale or mudstone with abundant thick calcite-shelled 'mussels' occur, the highest lying about 20 to 40 ft below the Gellideg and the lowest about 60 ft above the Garw.

These are the 'pseudorobusta' bands which yield variants of Carbonicola communis, C.martini Trueman and Weir and C. pseudorobusta,commonly associated with the ostracod Geisina arcuata.Between these conspicuous 'mussel'-bands the mudstones carry sporadic Curvirimula throughout.

The Gellideg

The Gellideg (Figure 11) is the lowest coal normally worked in the major collieries. Barely 2 ft thick in the east and north of the district, it thickens steadily south-westwards to reach about 9 ft at Newlands Colliery near Pyle.

Typically it consists of a single clean coal, but in the north, as at Cwmaman and Abergorky, a thin parting lies near the base, while in the southern part of the Rhondda valleys up to 2 ft of bast are present at about the same position. In the Eastern and Western colliery-areas a further parting may be present near the top of the seam. Over large areas the roof is sandy and under these conditions extensive washouts may occur. In general these trend from north-north-east to south-south-west, i.e. roughly at right-angles to the inferred land margin to the north-east. A major washout running from north-west to south-east lies astride the area between Eastern and Western collieries and extends southwards through Britannic Colliery; only its eastern margin has been proved with certainty but the area of barren ground is known to be more than a mile wide. Two distinct faunas are known from areas where mudstone forms the immediate roof: at Fernhill Colliery and Margam Curvirimula subovata (Dewar) is common, generally associated with plants carrying attached Spirorbis;at Maesteg an abundant fauna consisting of the internal moulds of variants of Carbonicola communis and related forms is preserved.

In the extreme north-east of the district, at Mountain Ash and Merthyr Vale, and in the south-east, at Llanha ran and Llantrisant, the Gellideg and Five-Feet seams are only a foot or so apart and both are worked in the same roadways. Westwards they split rapidly, and parallel to the line of split a sandstone, often striped and up to 50 ft thick, is developed within the cyclothem. Still farther to the west this sandstone thins and is replaced by striped beds and mudstones with only thin beds of sandstone, the two seams here being usually 40 to 70 ft apart, a thickness rather less than where the sandstone is best developed.

The Five-Feet

The Five-Feet has a complex section and is subject to widespread splitting. The main elements of its seam-structure are shown in (Figure 12). At its simplest in the extreme east of the district it consists essentially of a single coal about 4 ft thick. To the west thin partings develop, and in the central Rhondda area it consists of two main leaves, each 2 to 2.5 ft thick, with a thin coal lying within the intervening dirt, and further thin coals present at the top and bottom of the complete section. In the north of the Rhondda Fawr the two coals split several feet apart. The Lower and Upper Five-Feet coals are clearly separate in the Ogmore Valley. At Maesteg and in the areas to the south and south-west further partings develop in both the main coals, while the thin middle coal of the Rhondda area thickens and forms a distinct seam. The roof of the Five-Feet rarely carries a fauna, but sporadic Curvirimula have been recorded from the collieries in the north-east of the district and on the South Crop at Llanharan.

Over much of the district the Upper Five-Feet and the lowest coal of the Seven-Feet group are separated by 20 to 30 ft of strata, usually argillaceous, but containing thin sandstones towards the base, especially in the east. At Abergorky and again west of Bryn the Upper Five-Feet and the Lower Seven-Feet are less than 2 ft apart and the two coals have been worked over small areas as a single seam.

The Seven-Feet group

The Seven-Feet group of seams behaves in a complicated fashion; representative sections of the group are given in (Figure 13) and these summarize the splitting and behaviour of the individual coals. Only in the Cynon Valley north of Mountain Ash are all the coals united so as to form a single seam. Three main leaves, totalling in all 5 to 5.5 ft, are clearly represented and this three-fold character remains after the seam splits. The top leaf, about 1 ft thick, and the bottom leaf, varying from 1 to 2 ft, split away southwards and westwards, and in the area south-west of Maesteg the three coals are spread over some 100 ft of measures. In much of the lower Rhondda valleys the Middle and Upper Seven-Feet coals are worked together as one seam; within a limited area around Cwmaman the same is true for the Lower and Middle coals. Over the rest of the district the three leaves form distinct seams of which only the Middle is in general workable; even this is of little value in the Maesteg area since it develops up to three thick seatearth partings. The south-westwards splitting is accompanied by a thickening of all the coals : around Maesteg and the western South Crop the Lower Seven-Feet is up to 3 ft thick and has been worked on a small scale; the Middle Seven-Feet contains up to 5 ft of coal; and the Upper Seven-Feet is 2 ft or so thick. Over considerable areas the Upper Seven-Feet lies in the floor of the overlying Yard Seam, and in the south-east of the district, around Cwm and Llanharan, the Lower Seven-Feet appears to die out, while the Middle and Upper seams combine to form a single coal only about 2 ft thick. Few fossils have ever been recorded from any of the Seven-Feet coals. In the Cynon Valley Anthraconaia cf. curtata (Brown) and A. williamsoni occur sporadically.

The Seven-Feet and the Yard are never far apart; indeed, over much of the southern Rhondda area the Upper Seven-Feet is worked as the bottom coal to the Yard, and at Femhill Colliery the two seams are similarly associated. Elsewhere the separation is rarely more than 20 ft, except at Penrikyber Colliery where about 60 ft has been recorded. The strata are almost wholly argillaceous.

The Yard

The Yard (Figure 14) varies in thickness from about 2 to 6 ft, its thickest development being in the south-east Rhondda area and along the South Crop. Over much of the area the seam consists of two leaves, an upper one 2 to 3.5 ft, separated from a lower, 3 in to 1 ft 9 in thick, by up to 12 in of clod or rashings. Where the seam is thickest there is no separate bottom coal and it is presumed that the two coals have coalesced. A rich diagnostic fauna of Anthracosia regularis and Carbonicola venusta variants is present in much of the district.

Locally the Amman Rider lies close upon the Yard. At Deep Duffryn Colliery the separation is only a few inches and the two coals have been worked together as a single seam over a small area, where the multiple section was confused with that of the Seven-Feet; at Cwm Colliery the Amman Rider appears in the rippings of the Yard workings. Elsewhere varying amounts of mainly argillaceous strata separate the two seams, and in the area between Maesteg and the South Crop 80 to 100 ft of measures may be developed. The Amman Rider itself is a rather dirty sulphurous seam, and it has hardly ever been worked economically. In the south-east at Llanharan a single coal 10 to 24 in is present, but in the north-east the seam consists typically of two leaves, the upper one 12 to 22 in and the lower 8 to 14 in. Traced south-westwards these split and are, for example, 20 ft apart at Dare and 40 ft in the Garw Valley. At Maesteg and on the western South Crop the upper coal develops partings and presents typically a three-leaf section.

Middle Coal Measures

For convenience of description the Middle Coal Measures have been divided into two groups at the Cefn Coed Marine Band.

The lower group, which comprises the upper part of the Anthraconaia modiolaris Zone and the Lower Anthracosia similis–Anthraconaia pulchra Zone, includes the bulk of the main group of worked seams. The generalized section of strata and the principal faunas are shown in (Figure 15), which also summarizes the splitting of the coals. The group thickens westwards from less than 300 ft in the Abercynon area and around Llantrisant to between 600 and 700 ft in the Avan Valley and 800 ft in the extreme south-west (see (Figure 7)). Apart from the marked development of coal the measures are predominantly argillaceous; sandstones occur throughout the sequence but they are mostly impersistent. Non-marine lamellibranchs are found on most of the major seams and have been used extensively in correlation (see pp. 25–6): marine strata are confined to a single band, at the base of the group, and to two, sometimes three, horizons in the uppermost 50 to 150 ft.

The upper group (Figure 24), less than 300 ft thick in the Llantrisant area and more than 800 ft in the south-west, corresponds to the Upper similis-pulchra Zone. Although sandstones may be locally important, particularly in the higher part of the group, few are continuous throughout the district. Coals occur at intervals but they are not comparable in thickness with those of the lower group ; most of them have been worked locally, but none maintains an economic section regionally. 'Mussel' bands are developed at only one or two levels, and more important as aids to correlation are the marine and Euestheria horizons which are found at fairly regular intervals.

(a) Amman Marine Band to Cefn Coed Marine Band

These measures underlie by far the greater part of the district; their outcrop is limited to comparatively narrow strips along the South Crop and on the north side of the Moel Gilau Fault around and to the west of Maesteg.

The Amman Marine Band

The Amman Marine Band (S. H. Jones 1934, p. 429–30) is virtually ubiquitous and is of particular significance since it comprises the only known marine strata associated with the main group of coals. Over most of the district it varies from 1 to 3 ft in thickness, but it may reach 6 ft or more in the eastern collieries and 8 ft in the west. Over large areas it contains little except Lingula mytilloides in fissile silty mudstones. In the Cynon Valley and in much of the southern Rhondda area these Lingula shales are underlain by blocky mudstones containing abundant pyritic burrows, 'fucoids' and granules, associated with sponge spicules, rare gastropods and a few lamellibranchs; still farther south-eastwards, at Cilely, Cwm and Llanharan collieries, calcareous brachiopods and crinoid columnals appear and presage the development of the rich calcareous faunas of the southeastern parts of the Coalfield (Moore 1945, p. 175).

The marine strata are overlain by mudstones which are generally about 30 ft thick, though they are as little as 12 ft around Penrhiwceiber and Ynys-y-bwl and as much as 70 ft in the south-west. In the lowest few feet these mudstones contain abundant though stunted 'mussels', characterized nearly everywhere by a 'ghost'-like preservation; typical forms are cf. Anthraconaia williamsoni,and variants of Anthracosia ovum, A. phrygiana and A.regularis.Here and there solid shells resembling A.concinna occur. Locally to the south of Maesteg a thin coal occurs about 10 ft below the Bute and may be a split from that seam.

The Bute

The Bute (Figure 16) is everywhere of considerable economic importance. Over most of the Rhondda valleys and the areas to the east and south it consists of two distinct leaves separated by a parting more or less in the middle of the seam. The parting thickens steadily to the north-east; in the Rhondda Fach it has increased to about 17 in and in the Cynon Valley is thick enough to make the seam unworkable; genuine splitting takes place in the Taff Valley and at Merthyr Vale the two leaves are separated by as much as 15 ft of mudstone, which carry at their base a fauna of small Anthracosphaerium and Naiadites.To the west and south, into the Avan and Llynfi valleys and on the South Crop, the parting dies out and the seam consists of clean coal. The overall coal thickness varies from 3 ft in the north-west in the Glyncorrwg and Glyncastle colliery-areas, to rather more than 6 ft to the south of the Rhondda Valley; over most of the area the thickness varies from 4 to 5.5 ft. Locally in the northern parts of the Rhondda Fach a thin coal is developed in the base of the seam, and a similar leaf of inferior coal appears at the base of the section in the Pontycymmer and Maesteg areas.

In much of the area south and south-east of Gelli the Lower Nine-Feet lies close above the Bute and the two coals were worked as one seam in the Cilely area. Elsewhere the two seams are distinct, with upwards of 50 ft of measures developed between them. The roof of the Bute is almost barren in the eastern areas, but in the west it carries an exceptionally rich and characteristic 'mussel' fauna. In the upper Rhondda Fawr and in the Corrwg the shells are all preserved as 'solids' and lie in about 12 in of dark mudstone in the immediate roof of the seam. South-westwards from Blaen-gwynfi to Kenfig Hill up to 5 ft of cannelly mudstones intervene between the 'mussel'-band and the coal; these, too, carry abundant shells, which although crushed, appear to consist essentially of the same forms as the solid fauna. Variants of Anthraconaia williamsoni, A. modiolaris,cf.Anthracosia regularis, Anthracosphaerium affine, A bellum (Davies and Trueman), A. exiguum, A. fuscum (Davies and Trueman), A. turgidum, Naiadites quadrates (J. de C. Sowerby) predominate together with N. sp. nov.cf. modiolaris Hind. The overlying beds consist mainly of mudstones, though thin impersistent sandstones may be present and locally cut out the 'musserband. In the south-western part of the district and in the upper Rhondda Fawr a thin coal occurs some 10 to 20 ft below the Lower Nine-Feet.

Nine-Feet Seam

A major split develops in the Nine-Feet Seam (Figure 17),  (Figure 18),  (Figure 19), the line of the 2-ft parting running north-westwards from Pontypridd through the eastern workings of the Ferndale collieries to Maerdy, thence westwards to the area south of Fernhill and Glyncorrwg and into the north-western parts of the district. To the north of this line the single seam has an undisturbed thickness of 7 to 10 ft; but over wide areas it is so affected by intense structural deformation resulting from incompetence that no normal section can be recognized, and the seam may vary in thickness from virtually nothing up to 30 ft. Under these conditions the disturbed seam is associated with considerable developments of black rashings which have resulted from the complete breakdown of a highly carbonaceous roof-shale. These structural difficulties have hitherto precluded large scale development of the coal. A prominent cannel parting near the middle of the seam extends as far west as Fernhill, and continues southwards in the Upper Nine-Feet (see below). The split develops rapidly on another, and lower, dirt parting which thickens until, throughout much of the district, the Lower and Upper Nine-Feet coals are 40 to 60 ft apart, and locally the separation is more than 100 ft.

The Lower Nine-Feet

The Lower Nine-Feet (Figure 18) has been worked extensively. In the collieries of the southern Rhondda Fawr and southwards towards Llanharan and Llantrisant it unites with the Bute Seam (see above). The coal is about 3 ft thick throughout most of the Rhondda area; it thickens south-westwards into the Garw and Llynfi valleys and it thins again to about 4 ft at Newlands. The thickening of the seam west of the Rhondda is accompanied by the development, 6 to 24 in above the base, of a parting, which increases southwards to produce genuine splitting of the seam. Thus at Bryn the lower leaf lies several feet below the upper; at Newlands what appears to be this bottom coal is 16 ft below the main Lower Nine-Feet. West of the Ogmore Valley further thin partings are present in the upper coal. Up to 18 in of cannel occur associated with the main parting in the Maesteg area, while at Western and Cambrian the roof of the coal carries up to 15 in of cannel.

The measures between the Lower and Upper Nine-Feet coals consist mainly of sandy and silty mudstones, and sandstone bands are frequent. A thin coal lies close below the Upper Nine-Feet in the Upper Rhondda and at Newlands. These measures are almost devoid of animal fossils.

The Upper Nine-Feet

The Upper Nine-Feet (Figure 19) is liable to internal disturbance and has not been so widely worked. It is thickest in the Porth area, where it exceeds 7 ft; over most of the district a thickness of around 5 ft is more normal, though in the Maesteg area it thins to less than 4 ft. At Newlands Colliery it has been worked extensively south of the Newlands Thrust, with an abnormally thick but consistent section of 12 to 13 ft of coal with several partings; about half a mile north of the thrust a more normal thickness of about 5 ft is recorded, and no explanation can be offered at present for the very sudden increase in thickness of the seam south of the thrust. Over much of the northern part of the district a parting, usually associated with cannel, appears in the middle of the seam. The cannel is 1 to 2 ft thick in the Rhondda valleys, and is clearly the same as that recorded in the combined Nine-Feet coal to the north-east. South-westwards into the Ogmore and Garw areas a parting, also locally associated with cannel, develops near the top of the seam. Near Maesteg the uppermost few inches of coal split away and a thin cannel is left in the roof of the main seam. In the area between the Rhondda Fawr and Ogmore valleys a thin bottom coal is recorded, and at Werntarw this may be represented by the 24 in of coal which lies 8 ft below the main Upper Nine-Feet.

In the north-east of the district the Nine-Feet roof, where it consists of dark carbonaceous shale, carries an abundant fauna of crushed and semi-crushed large 'mussels', among which have been recognized variants of Anthraconaia salteri (Leitch), Anthracosia aquilina, A. beaniana, A. disjuncta, A.ovum, A. phrygiana and A. retrotracta (Wright). These are overlain by paler blocky mudstone carrying the 'Red Vein fauna' of small solid 'mussels', such as Anthraconaia aff. pulchella,Anthracosia aff. angulata and A.aff. nitida.

The measures between the Nine-Feet and the Six-Feet are amongst the most variable in the sequence (Figure 20), due in the main to the irregular behaviour of the Red Vein group of coals. Usually some 60 to 90 ft of strata are involved, but they may reach as much as 130 to 140 ft in the Clydach Vale–Cwm-parc–Llwyn-y-pia area, and as little as 20 ft at Coedely. However, this statement of variation is not as straightforward as it would seem, for at Cwmaman, where the separation between the Nine-Feet and Six-Feet is about 80 ft, the coals of the Red Vein have coalesced to form a single compound seam about 50 ft above the Nine-Feet. At Western Colliery, on the other hand, the separation is about the same, but the top leaves of the Red Vein lie within 3 ft of the Six-Feet while a thin coal which may represent the bottom leaf of the Cwmaman Red Vein is found only a few feet above the Nine-Feet.

Red Vein of the Maerdy–Cwmaman

The composite single Red Vein of the Maerdy–Cwmaman area totals about 3 to 5 ft of coal in three, rarely four, leaves. Southwards and south-eastwards the component coals split apart and thin markedly, and in the Mountain Ash area they are reduced to insignificance. After splitting in the central Rhondda the upper two leaves again unite on the western side of the Rhondda Fawr and make an important seam, here called the Caerau. This thickens steadily through the Ogmore, Garw and Llynfi valleys, where the double seam reaches 5 to 6 ft: the lower leaf of 33 to 41 in is separated from the upper leaf of 15 to 30 in by an inch or so of rashings. This well-developed seam always lies close below the Six-Feet, and the separation is less than 3 ft over much of the area between the Garw Valley and Llantwit Fardre. One or two thin coals developed between the Nine-Feet and the Caerau throughout this central area represent the lower coals of the Red Vein group.

On the South Crop the Red Vein group comprises two distinct seams, which in the west show marked thickening. The Caegarw (Caerau) of Newlands totals 7.5 ft of coal with several dirt partings in the lower part: at Llanharan a double coal 4 to 5 ft thick includes 8 to 21 in of rashings in the middle. The lower seam, here, for the sake of convenience, retaining the name Red Vein, varies from 5.5 ft at Newlands to about 3 ft at Llanharan, and has two leaves at both localities.

The 'Red Vein fauna' of small Anthraconaioid Anthracosia occurs at various horizons within the group. At Western Colliery and certain of the northern Rhondda pits it is associated with an underlying development of the 'Nine-Feet' A.phrygiana/aquilina fauna on the second thin coal above the Nine-Feet. The first rider seam above the Nine-Feet in the area between the Ogmore and Rhondda Fach valleys carries a characteristic fauna of solid Naiadites of the productus (Brown)/subtruncatus (Brown) group.

The Six-Feet

The Six-Feet (Figure 21) is probably the most important coal horizon in the district; almost certainly it has been worked on a greater scale than any other seam, and in many of the collieries has been completely exhausted. Major splitting is again an important feature and two distinct seams, the Lower and Upper Six-Feet, are present over large areas. A single compound seam exists to the south-east of a line drawn through Merthyr Vale, Mountain Ash, Wattstown, Dinas and Penrhiw-fer to Bryncethin. In most of this area a three-coal section is typical: top coal 22 to 28 in, parting up to 24 in, main coal 36 to 52 in, parting up to 7 in, bottom coal 12 to 24 in. In the Cynon Valley a further parting occurs about 9 to 15 in from the top of the main coal, while in this area also the bottom coal appears to have failed. The parting between the main and bottom coals is reduced to a mere plane in the Cilely–Porth–Pontypridd area; and in the extreme east and south-east, at Cwm and Llanharan collieries, that between the top and main coals dies out altogether, the seam consisting of 5.5 to 7 ft of clean coal overlying a bottom coal of 12 to 15 in.

The major split in the seam develops between the main and top coals, and the separating measures, which are mainly argillaceous, increase rapidly in thickness to 60 ft at Mardy and Dare collieries, and 50 to 70 ft in the Ogmore and Garw valleys. The sections of both split seams remain remarkably similar to the corresponding portions of the full seam just described. The Lower Six-Feet consists for the most part of a main coal, 48 to 66 in, and a bottom coal 16 to 30 in. In the northern parts of the Cynon and Rhondda valleys a parting is present 2 to 9 in from the top of the main coal, and is evidently the same as that already noted in the lower reaches of the Cynon Valley; this parting increases to about 2 to 5 ft in the Tydraw area. The Upper Six-Feet everywhere consists of clean coal usually about 20 to 26 in. in thickness, but locally in the Llynfi Valley it attains 40 in. The two seams reunite on the western side of the Llynfi and in the Corrwg and lower Avan valleys : the combined section is generally similar to that of the eastern parts of the district, though the main coal is rather thicker.

Because of structural complications the exact nature of the Six-Feet on the western reaches of the South Crop is obscure. It appears to consist mainly of a single seam about 6 ft thick, but local splitting has been observed during opencast operations in the Cefn area, west of Aberkenfig.

Over the northern part of the area where the Lower Six-Feet has a separate existence, its roof yields fragmentary 'mussels' and abundant Lioestheria striata.In most of the southern half of the district the Upper Six-Feet (both where it is separate and combined with the lower seam) carries in its immediate roof a striped black ferruginous mudstone, akin to blackband, which contains a characteristic fauna of crushed and semi-crushed Anthracosia: variants of A. acutella, A. aquilina, A. atra,A. barkeri Leitch, A. concinna, A. elliptica,A. lateralis, A. planitumida are associated with the ostracod Carbonita humilis (Jones and Kirkby). Farther north in the central Rhondda area the mudstone is only slightly ferruginous and well-preserved 'solids' of the same species abound. In the extreme north the ferruginous facies is absent; the rich fauna fails and is replaced by sporadic small shells locally associated with a small distinctive variety of Planolites ophthalmoides.At Glyncastle Colliery an inch or two of coal lie close above the composite seam and unite with it at Ynisarwed in the Neath Valley; this thin coal also carries abundant L. striata.

The measures up to the Four-Feet may be as little as 12 ft thick in the northeast and as much as 90 ft in the Garw Valley and at Llanharan. They are largely argillaceous, though impersistent sandstones are locally present in the northern and western areas.

The Four-Feet

The Four-Feet comprises yet another compound group of coals subject to complex splitting (Figure 22). In the northern collieries of the Cynon Valley a single clean coal 5.5 to 6 ft thick has been worked long ago to virtual exhaustion; traced southwards and westwards this coal develops three distinct partings, and throughout the rest of the Cynon area, in the Rhondda Fach, and in the eastern parts of the Rhondda Fawr the combined four-leaved seam has been worked extensively. In the Rhondda Fach the four leaves vary as follows: (top) 12 to 21 in, 16 to 36 in, 27 to 34 in, and (bottom) 12 to 16 in; the partings vary up to 24 in. in thickness. In the north of the Rhondda Fawr, the Avan Valley and the northern collieries of the Garw and Llynfi valleys a single complex seam is again found, though throughout this area the section is more variable, and in places, as at Avon Colliery, comprises up to 12 ft of coal with numerous dirt partings. Over most of the southern Rhondda Fawr, the southern Llynfi and throughout the South Crop two distinct seams separated by as much as 40 ft of mudstone represent the Lower and Upper Four-Feet seams; the sections are variable but the four-fold make-up of the combined seam in the Rhondda Fach and Cynon valleys appears to be recognizable, the split having taken place between the two middle coals of the combined seam.

In the Duffryn Rhondda area thin streaks of coal appear in the seatearth of the combined seam and these thicken and split away when traced southwards through the Llynfi area to the South Crop : at Celtic Colliery the lowest is as much as 70 ft below the main Lower Four-Feet. In the Clydach Vale, Gilfach Goch and Ogmore Vale areas the Four-Feet coals split completely: four individual seams, thinner than their counterparts to the east, are spread over 50 to 80 ft of strata.

Along the South Crop and adjacent areas the Lower Four-Feet roof yields a rather poor fauna of Anthraconaia cymbula, A. obscura (Davies and Trueman), A.subcentralis (Salter), Anthracosphaerium propinquum and Naiadites sp.The top coal of the group, however, carries a highly diagnostic, abundant and varied fauna over almost the entire district; the well-preserved solid 'mussels' include variants and intermediates of the following species: Anthraconaia sp. nov.(cf. curtata), A. cymbula, A.lanceolata, A. librata, A. rubida (Davies and Trueman), Anthracosphaerium exiguum, A. propinquum, A.turgidum, Anthracosia acutella, A. aquilina, A.atra, A. concinna, A. fulva (Davies and Trueman), A. lateralis, A. planitumida and A.simulans.The blocky mudstone with these 'solids' is often underlain by a few inches of smooth dark shale containing crushed shells, mainly Anthracosia and Naiadites preserved as mere films.

The Two-Feet-Nine

The Two-Feet-Nine (Figure 23) is separated from the top coal of the Four-Feet by some 20 to 50 ft. The seam is well developed in the Llynfi, Garw and Ogmore valleys and on the western side of Cambrian Colliery, and throughout this area it has been extensively worked; it consists of 4 to 5.5 ft of clean coal with a further 1 to 1.5 ft lying 6 in to 5 ft below, and up to 12 in of cannel may be developed in the roof. In the Llynfi Valley there is a plane-parting 9 to 18 in below the top of the seam and to the north-west of Coegnant and Caerau another parting develops near the middle of the main coal and thickens rapidly northwards. This line of split can be traced north-eastwards from Bryn to the Maerdy area: to the northwest the upper leaf is generally thin but it reaches a maximum of about 3 ft around Treherbert ; the lower leaf remains associated with its bottom coal and in the north of the Rhondda valleys its roof has yielded many 'mussels' preserved in irony mudstone and reminiscent of the fauna of the Penny-pieces in the areas to the north-west (Ware 1930, p. 473). Characteristic forms are: Anthraconaia cf. rubida,Anthracosia cf. atra, A.cf. aquilina, A.cf. lateralis, A.aff simulans,and A.cf. planitumida.

South of the line of split the seam has been widely mined in the Rhondda valleys north of Llwyn-y-pia and Wattstown; in this area a 3 to 4.5 ft coal has one or two thin leaves in the floor. Southwards from Ferndale and Llwyn-y-pia and eastwards from Cambrian the seam suffers very rapid thinning and through most of the collieries of the southern Rhondda it is less than 2 ft thick. Southwards and westwards from Cwm Colliery it improves just as quickly and at Coedely 4.5 ft of coal was worked, while at Llanharan and Llantrisant the section varies between 5.5 and 6.5 ft of clean coal. In this latter area cannel is again prominant at the top of the seam, and this passes upwards into several feet of black cannelly mudstone overlain by grey shelly mudstones.

Along the rest of the South Crop the seam is also thick. In the Heol-y-cyw area it has been proved by Opencast exploration to be 4 to 4.5 ft, while, although intensely disturbed, it has also been extensively worked by opencast methods between Aberkenfig and Margam.

An abundant and varied fauna of small to medium-sized 'mussels' characterizes the grey mudstones for several feet above the seam along the South Crop and in the mid-Glamorgan valleys as far north as Avon and Park. Characteristic forms include: Anthraconaia cymbula, A. sp. nov.cf. lanceolata,Anthracosia acutella, A. aquilina, A. atra,A. concinna, A. lateralis, A. planitumida,A. simulans Anthracosphaerium propinquum and Naiadites spp.At Cambrian Colliery a rider coal (the exact correlation of which is not known), 7 to 8 in thick, is recorded 3 to 4 ft above the main coal. This carries a 'mussel' fauna in blackband similar to that from the Six-Feet: Anthracosia atra, A. barkeri and A. fulva are especially abundant.

Measures above the Two-Feet-Nine

The measures above the Two-Feet-Nine are poorly exposed along their outcrop and they can be seen in only a few of the collieries. They are only 60 ft thick at Cwm Colliery and rarely exceed 150 ft in the eastern parts of the district generally: westwards they expand to about 300 ft beyond Bryn and at Margam Park. Coals are rarely of great significance and they are seldom named. In the Cynon Valley and the north of the Rhondda Fach an inferior composite seam of three to five leaves—the Three Coals—lies 2 to 50 ft above the Two-Feet-Nine. Westwards and south-westwards it splits and its behaviour is variable: upwards of four or five thin coals may be spread over 50 ft of strata; in places some of the coals fail altogether and at Llanharan only one coal is found between the Two-Feet-Nine and the Hafod Heulog Marine Band coal. In a few localities individual coals are named; as for example, the Fireclay and Coal Tar seams of Maesteg and the Lantern of the Cefn and Kenfig Opencast sites.

At the western end of the South Crop 'mussel' faunas closely comparable with that of the Two-Feet-Nine occur at several horizons including the roof of the Lantern; while at Cwm Cynon Colliery a varied fauna, including Anthraconaia sp. nov.cf. cymbula, Anthracosia aft atra and abundant variants of Anthracosphaerium affine, A. exiguum and A. propinquum,occurs in the roof of the Three Coals.

Above the Three Coals and its equivalents a compact quartzitic sandstone, locally with a sharp base and showing signs of local erosion of the underlying beds, develops within and to the west of the Rhondda Fawr; in the Llynfi Valley it is 15 to 25 ft thick and is named the Lower Cockshot Rock. Immediately or close above lies a coal, which is usually thin but exceeds 2 ft in parts of the Llynfi and Ogmore valleys. The marine mudstone which forms the roof of this coal is known from several localities in the southern part of the district, and has been named the Hafod Heulog Marine Band (see revised six-inch maps of Pontypridd Sheet, 1957–60, e.g. Glam. 34 S.W.), after the farm 2 miles north-east of Kenfig Hill, near where it crops out. At Britannic Colliery the band is almost 11 ft thick, at Margam Park 7.5 ft, at Penllwyngwent it has thinned to 3 ft and at Llanharan it is less than 2 ft. The fauna contains little other than horny brachiopods and foraminifera, associated with Planolites ophthalmoides; Orbiculoidea up to half an inch across is particularly characteristic. The occurrence of Sphenothallus at Britannic provides an interesting comparison with the Haughton Marine Band of the Pennine province. The band deteriorates in the north of the district: only P. ophthalmoides was found at Mardy Colliery and no evidence of marine conditions at this horizon was forthcoming at Fernhill and Duffryn Rhondda collieries.

At Britannic Colliery another marine horizon, called the Britannic Marine Band, (see Glam. 27 S.W.) lies about 25 ft above the Hafod Heulog Band. It consists of 2 ft of foraminifera-bearing mudstone in the roof of a thin coal; the only other fossils noted were P. ophthalmoides and doubtful Lingula.At Penllwyngwent and Margam Park the equivalent horizon yields only Planolites,but at the head of the Rhondda Fawr L. mytilloides was found. The band is not developed at Llanharan, nor has it been recognized at Mardy and Duffryn Rhondda collieries. Its equivalent in Yorkshire appears to be the Sutton Marine Band, and poorly developed marine strata are known from this horizon in most of the Pennine coalfields.

In the Llynfi and Avan valleys an impersistent thin coal often lies in the measures between these two marine bands; above it another compact quartzite—the Upper Cockshot Rock—reaches a thickness of almost 50 ft at Caerau.

In the north and east of the district the Cefn Coed Marine Band occurs in the roof of the first coal above the Britannic horizon. This coal splits to the south of a line running from Glyncorrwg through Cwm-parc towards Porth, and thence south-westwards towards the South Crop. The two leaves may be as much as 30 ft apart; Naiadites sp.has been recorded from the lower leaf at Duffryn Rhondda and P.ophthalmoides at Britannic. Locally quartzitic sandstones are present beneath each of the leaves, and the many anomalies in this part of the sequence may be explained by washouts associated with these sandstones. In the south-east of the district the top coal may die out, the marine band resting directly on seatearth.

As befits a horizon which correlates with the Dukinfield and Mansfield marine bands of the Pennines, Skipsey's Marine Band of Scotland and the Aegir of the Continent, the Cefn Coed Marine Band (Ware 1930, p. 478) is readily identified throughout the district. In the south-eastern areas it carries only Lingula in grey micaceous shaly mudstone. Elsewhere dark blocky pyritic mudstones occur, particularly towards the middle of the band, and are often associated with lenses of ankeritic siltstone. These mudstones commonly carry a rich and varied, though dwarfed, fauna of horny and calcareous brachiopods (including Athyrids, Chonetids, Productids, and Spiriferids), lamellibranchs, nautiloids, goniatites including Anthracoceras aegiranum H. Schmidt, trilobites, ostracods and conodonts. At Aberbaiden on the South Crop (see p. 141) the richest fauna of any Coal Measures marine band in Britain was found during the present survey. The band is about 1.5 ft thick, and at least 80 species were identified from a 5.5-in layer in the middle (Ramsbottom 1952).

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

(Figure 24) shows the generalized section of these strata as well as the principal faunas.

The cyclothem above the Cefn Coed Marine Band is consistently one of the thickest in the Middle Coal Measures. It is 60 ft in the eastern areas but thickens westwards to 80 to 100 ft over much of the district and reaches almost 180 ft locally in the extreme south-west. The beds are generally argillaceous, with an abundance of ironstone pins: at Fernhill these were formerly called the Black Pins Mine-ground. Quartzitic sandstone in the upper part was mis-called the Cockshot Rock in the southern Rhondda. A sandstone in a similar position is generally present throughout the Avan Valley, and around Bryn reaches 40 ft in thickness : it continues to the west and south-west, and at Margam is 33 ft thick.

The succeeding coal is called the Caedavid in the Llynfi, Garw, and Ogmore valleys, where it is best developed, and where it was formerly worked extensively. Here it is about 3 ft thick, with two distinct partings. South-westwards from Maesteg and northwards from Caerau the upper parting thickens, generally to about 20 ft, though it reaches 50 ft at Margam. From Caerau the split appears to follow an eastward course to Tonypandy and Porth. To the north the lower coal is usually called the Gorllwyn and the upper one the Gorllwyn Rider. The former has been worked around Treherbert and Maerdy and is 2.5  ft thick. In the southeastern areas of the district only one thin coal is recorded in this general position. Shells are recorded from the seam at only one locality: at Glengarw Colliery abundant Anthraconaia adamsi is present in Caedavid spoil and is believed to come from one of the partings in the seam. Fish debris is recorded from cannelly mudstones in the roof of the Caedavid, and from the Gorllwyn Rider in the eastern parts of the district.

The cyclothem above is only 7 ft thick in the south-east, but elsewhere is generally 20 to 40 ft. Sandstones are common, especially towards the top: in the upper Rhondda they reach 20 ft, and in the Cynon Valley were referred to as the Big Rock.

The coal next above was named only in the Llynfi Valley, where it was called the Eighteen-Inch. It has never been worked commercially. It averages 6 to 15 in over much of the district, but it thickens to 24 in around Tonypandy and westwards to Maesteg. In the upper Rhondda a thin blackband forms a parting in the seam. At Gelli and Naval collieries abundant 'mussels' occur in the roof, and in the Llynfi a similar fauna (Evans and Simpson 1934, pp. 460–2) formerly believed to lie above the Two-and-a-Half Seam is now thought to occur above the Eighteen-Inch (p. 000). Species include variants of Anthraconaia hindi, A.cf. warei,Naiadites sp.cf. productus and N.cf. hindi (Trueman and Weir).

The Lower Pentre or Two-and-a-Half Seam lies 20 to 40 ft above, the intervening measures being usually composed of mudstones with bands of sandstone which become more numerous upwards; locally in the Llynfi Valley and around Bryn these beds thin to 2 ft. The coal varies from 18 to 30 in. In the Cynon and upper Rhondda valleys a 'stone' parting 2 to 3 in thick lies about 9 in above the base: southwards and westwards this disappears and no persistent partings occur over the rest of the district. The coal has been worked near Gelli, in the Llynfi and Garw valleys, and along its crop west of Bryn. In the east the roof is planty, but at Duffryn Rhondda an associated thin rider a short distance above has yielded A. hindi from its roof.

Over most of the district the Pentre or Upper Yard lies 20 to 40 ft higher, the intervening strata being almost wholly mudstone. The seam is one of the most extensively worked in this group of measures. It is thickest in the area between Wern-tarw and Meiros, where it is over 4 ft, and it is over 3 ft as far west as Margam. In the Llynfi, Garw, Ogmore and Rhondda Fawr valleys it was also worked extensively at a thickness varying from 32 to 36 in, but the section is locally marred by two 'stone' partings. To the north of Ferndale and Treorky the upper parting thickens rapidly to 15 ft and a similar deterioration in the seam around Bryn may represent the western continuation of the same split. A plant roof is usual on the lower split, and indeterminate shell fragments have been recorded from the upper split.

The cyclothem above the Pentre is very variable. In the east it consists of only 3 to 6 ft of seatearth and mudstone. Westwards and north-westwards it thickens to about 120 ft in the Maesteg area. In the upper Rhondda a sandstone appears in the roof of the Pentre and is nearly 30 ft thick at Pentre Colliery; it is persistent throughout the Avan Valley and it thickens southwards to 60 ft at DIffryn Rhondda. In the Llynfi Valley it is usually 30 to 40 ft thick and it reaches a maximum of 90 ft at Garth. Massive, false-bedded, and resembling pennant, it has been quarried around Maesteg. West of the town and around Bryn 30 ft of mudstones separate the top of the sandstone from the overlying Victoria Seam, and a thin coal is present at the base of these mudstones. This coal is associated with the Lower Blackband Ironstone, up to 15 in thick, which in the last century was mined under Mynydd Bach.

The succeeding coal is known as the Pentre Rider in the Rhondda Fawr, the Victoria in the Llynfi and Garw valleys and the Black towards Cwmavon. It is normally a clean coal varying from 7 in. in parts of the Cynon Valley to 39 in east of Maesteg, but over much of the district it averages 18 in. Wherever seen the immediate roof consists of marine mudstone. This Foraminifera Marine Band (so-named during this resurvey) is characterized by abundant foraminifera in its lowest part, associated with only rare Lingula and Edmondia; Planolites ophthalmoides is profuse in the upper part of the band and ranges for several feet upwards.

The three coals, Lower Pentre, Pentre and Pentre Rider, all coalesce to form one seam, about 8 ft thick, at Llanharan Colliery. This has been worked extensively in recent years at that colliery under the name Thick Seam, and its crop is lined by numerous bell-pits in the area to the south and east.

The cyclothem above the Pentre Rider is everywhere a thick one; it increases steadily from 50 to 60 ft in the Cynon Valley and around Pontypridd to about 100 ft at Maesteg and Margam. Locally even greater thicknesses are recorded, as at Llanharan where almost 130 ft of beds are present. The lower strata are usually argillaceous, but sandstones become important upwards, especially in the north and east. At Coedely and Werntarw these are 50 to 70 ft thick and comprise most of the cyclothem; more generally they are 20 to 30 ft. In the Margam Park Nos. 2 and 3 boreholes Myalina was found in striped beds about 12 ft below the base of the Abergorky, and this represents the only record within the district of the Five Roads Marine Band (Archer, A. A. in Eyles 1956, p. 34).

The next coal is the AbergorkyThis spelling is here preferred to that of 'Abergorkr used on the present one-inch geological map (1960, 1963), since it conforms with the usual rendering of the colliery of that name in the Rhondda Fawr Valley, after which the seam was named. of the Rhondda Fawr Valley, which equates with the Graig in the Cynon Valley. Around Treherbert and Gelli, where it has been worked on a small scale, it comprises two coals, the lower 12 in thick, separated from the upper, 21 to 24 in thick, by a foot or two of seatearth. Eastwards the parting dies out and in the Cynon Valley the Graig consists of 22 to 32 in of clean coal. Westwards from the Rhondda the two leaves split apart and are usually separated by 10 to 25 ft of sandy mudstone. In the Llynfi and Garw valleys the lower leaf appears to be the Albert, which is 18 in thick, and the upper leaf is thin and un-named. In the area east of Bryncethin and south of Forth, only one coal is present in this position, varying from 1.5 ft in the north to 2.25 ft in the south, and it is not certain whether this represents the full section or only the top coal. The roof of the top coal carries abundant Lioestheria cf. vinti,usually in dark shaly mudstone. This forms a useful marker band, for the fossils have been found wherever the seam-roof has been examined, except in the Cynon Valley where the dark mudstones are replaced by seatearth.

In the north-east at Mountain Ash a thin rider coal about 3 ft above the Graig carries the Lower Cwmgorse Marine Band. Westwards the separation increases to 30 to 50 ft in the Rhondda Fawr, and up to 60 ft in the south-west. Sandstones occur in the top of the cyclothem where it is thickest. The rider coal, which is nowhere named, is only a few inches thick in the north-east, but it thickens southwards to 21 in at Llanharan. To the west a second thin coal develops in the seatearth of the rider : it appears around Treherbert and carries a thin blackband in its roof. In the Llynfi and westwards to Cwmavon the lower coal is 6 to 9 in and the upper 12 to 15 in, and, between them, a 2-ft band of ironstone—the Upper Blackband Ironstone—was formerly extensively trenched and mined. Spoil from these workings has yielded abundant Geisina? subarcuata (Jones) associated with Naiadites daviesi Dix and Trueman and cf. Anthraconauta phillipsii.

The Lower Cwmgorse Marine Band (Trotter 1947, p. 92) cyclothem is 50 to 80 ft in the east and north-east of the district, 100 ft over much of the central area, and 125 ft in the south-west. In the eastern areas sandstones are prominent: in the Cynon Valley they are 30 to 60 ft thick in the upper part of the cyclothem; they figure largely at Fernhill, and at Llanharan 90 ft are present. Marine fossils can usually be found at the base, and, where the sequence is argillaceous, they may range through a considerable thickness: at Mountain Ash they have been found through 35 ft, and at Ton Pentre in the Rhondda Fawr to within 30 ft of the overlying Hafod. The fossils are not numerous, and only a few species are represented: most typical are Lingula, Dunbarella,Nuculids and Hollinella.

The overlying Hafod Seam is a single coal only in the east—in the Cynon Valley and the area south of Porth as far west as Werntarw. It has been worked, together with its fireclay, in the Cynon Valley as the Clay, and in the Hafod–Cilely area, and at Wemtarw, the thickness varying from 2 to 3 ft. The seam splits to the west into two, and locally, three coals, ranging over 20 to 40 ft, and to nearly 80 ft at Margam Park. The bottom coal is everywhere the thickest, and it has been worked as the Hafod at Aberbaiden on the South Crop. In the Maesteg–Bryn area it is again known as the Clay, reflecting the fact that it rests on a thick pale grey fireclay, and has been worked at several localities. In the northern Rhondda Fawr it carries in its roof a thin blackband yielding small crushed cf. A. phillipsii and Naiadites sp.together with ostracods including Carbonita humilis;a similar fauna was noted at Aberbaiden.

The Upper Cwmgorse Marine Band (Trotter 1947, p. 92) is carried in the roof of the Hafod or of its thin top split. In the eastern and south-eastern areas, where it varies from 2 to 8 ft thick, the fauna is poor : little other than Lingula, Orbiculoidea and a few Productids is found in a silty micaceous mudstone. In the Rhondda Fawr, where numerous surface exposures are known, it is about 5 ft thick and around Ton Pentre carries an abundant and varied fauna of lamellibranchs, nautiloids including Huanghoceras postcostatum (Bisat) and the goniatite Anthracoceras cambriense Bisat. In the Llynfi Valley, where only the basal part of the band is exposed, brachiopods are the commonest fossils. In the south-west of the area, and especially at Aberbaiden a marked thickening of the band takes place, and up to 42 ft of fossiliferous mudstones have been recorded. A large and varied fauna includes: Archaeocidarid spines; brachiopods including horny forms, Crurithyris and Productids; gastropods; lamellibranchs including Curvirimula,Myalina,Nuculids and Pectinids; nautiloids; goniatites including Anthracoceras cambriense and Homoceratoides kitchini Bisat; ostracods; conodonts; and fish debris.

Upper Coal Measures or Pennant Measures

Up to about 4000 ft of Upper Coal Measures are preserved within the district, though the sequence is everywhere incomplete. A distinctive feature is the presence within them of numerous thick sandstones, the Pennant of tradition, separated by relatively thin mudstones, each of which usually contains one or more relatively thin coals. These sandstones occupy the greater part of the surface of the district, and in particular form the high ground between the valleys. Few shafts have penetrated other than the lowest beds; the coals, although worked locally in former days on a varying scale, are not much exploited today. Surface exposures are usually confined to the sandstones, and large continuous sections are almost non-existent; faunal horizons are never distinctive. Knowledge of the formation as a whole is therefore largely based on the mapping of features, and the degree of precision attained in the elucidation of the Lower and Middle Coal Measures cannot be maintained. In particular the correlation of the individual leaves of coal within each mudstone 'slack' is rarely precise, though the identification of the 'slacks' themselves is generally reliable.

The characteristic massive pennant sedimentation was first established in the south-west of the district and gradually spread north-eastwards. On the western part of the South Crop it commenced with the Llynfi Rock, close above the Upper Cwmgorse Marine Band, and in this area about 700 ft of beds, consisting largely of pennant sandstone, had been deposited before similar conditions set in at the No. 2 Rhondda level in the Rhondda valleys, and a further 1100 ft had accumulated before the facies became firmly established in the extreme northeast, where massive and continuous pennant begins above the Brithdir. The lowest Pennant strata thus exhibit two contrasting lithologies : the one normally characteristic of the pennant litho-facies, the other scarcely different from that of the upper part of the Middle Coal Measures. Most of the problems of correlation in the past have been confined to the transitional belt between these two types of sedimentation.

A complication in Pennant stratigraphy arises from the theory that, over much of the district, the Swansea Beds of the coalfield west of Neath are largely or wholly cut out by unconformity beneath the Grovesend Beds (see Woodland and others 1957a, p. 9). This conclusion is based on the belief that the No. 3 Llantwit Seam of the south-eastern part of the district equates with the Mynyddislwyn of Monmouthshire, which is correlated on convincing faunal grounds with the Wernffraith or Swansea Four-Feet of the Swansea area. Acceptance of this view implies the absence in the Llantwit basin and in Monmouthshire of some 1500 ft of beds, including numerous cyclothems and coals, between the horizon of the Daren-ddu or Mountain and the Llantwit No. 3 or Mynyddislwyn: an unconformity appears to provide the most satisfactory explanation for this.

(a) Llynfi Beds

The Llynfi Beds have extensive outcrops along the lower slopes in the Cynon Valley around Aberaman, in the Rhondda Fach around Ferndale and Maerdy, in the Rhondda Fawr north of Pen-y-graig, in the upper reaches of the Ogwr Fawr, Garw and Llynfi, and north of the Moel Gilau Fault from Maesteg westwards to Cwmavon. Smaller inliers occur in the Ogwr Fach around Gilfach Goch and in the Ely Valley around Cilely. On the South Crop they can be traced continuously from east of Llantrisant to Margam Park. Traced from the northeast of the district to the south-west they show a thickening from little more than 250 ft around Mountain Ash and Merthyr Vale to over 700 ft in the area north and north-east of Margam ((Figure 8)A). As already indicated, this thickening is to a large extent related to a marked south-westwards increase in the development of sandstones.

The measures from the Upper Cwmgorse Marine Band to the Tormynydd are about 120 ft thick at Cwmavon, a thickness which is maintained from here northeastwards to the upper Rhondda Fawr. Eastwards and south-eastwards, they thin to some 80 ft in the central parts of the district, and to barely 60 ft around Merthyr Vale. A similar easterly attenuation to about 60 ft is shown when the measures are traced from Margam Park to Bryncethin, but the trend is reversed farther east along the South Crop, and from Wern-tarw to Llantrisant they expand to 110 or 120 ft. They are everywhere essentially argillaceous; any sandstones present are usually thin and impersistent. Two or three closely associated coals are distinguishable over most of the district. In the Cwmavon–Maesteg area, where these coals are best developed, the lowest, called the White, is succeeded in turn by the Jonah and the Tormynydd. All have been worked sporadically along their crops westwards from Maesteg, the White comprising 3 to 4.5 ft of dirty coal, though the Jonah and the Tormynydd rarely exceed 2 ft. Away from this area the seams thin, and precise correlation of the individual leaves is not easy. Around Maesteg the White yields a characteristic fauna of Anthraconaia aff. stobbsi,Naiadites daviesi and ostracods preserved in ferruginous mudstone ; shelly blackband is also present above the lowest coal of the group at Treherbert and Ferndale, and in the Taff Valley the Blackband Coal appears to be at the same horizon.

The sandstone directly overlying the Tormynydd throughout the central and south-western part of the district has been called the Llynfi Rock (see First Edition, p. 30); it is the lowest massive pennant-type sandstone in the succession. Locally between the Llynfi and Garw valleys, where it is immediately overlain by the No. 3 Rhondda Seam, its uppermost few feet are quartzitic and ganisterlike. Around Margam Park, Bryn, and Maesteg, and in the Garw and Ogmore valleys, the Llynfi Rock is 150 to 180 ft thick, but it thins steadily away from these areas. In the extreme north-west at Glyncastle it is still 90 ft, but eastwards at the head of the Rhondda Fawr it is rarely more than 15 ft. Farther south a similar eastwards thinning is apparent. In the Rhondda Fawr it is 30 to 50 ft thick over much of the west side of the valley, but it is virtually absent in the Ogwr Fach and Ely valleys. In the Rhondda Fach it still maintains a thickness of 24 to 30 ft in the area south of Tylorstown, but it cannot be identified with certainty in the Cynon Valley (except possibly to the north-west of Aberaman) or in the Merthyr Vale area. Along the South Crop at least 75 ft remain at Bryncethin, but at Wern-tarw and to the east it is absent and presumably replaced by mudstones.

Lying close above the Llynfi Rock, the No. 3 Rhondda is the most valuable seam in this part of the sequence. It is well developed throughout the southern half of the district but it deteriorates northwards along a line from Pont-y-gwaith to Pentre, and thence through Ogmore Vale, Blaengarw and Garth to Margam Park with such abruptness as to suggest that it is affected by a regional washout. South of this line it has been worked extensively from many collieries, with a thickness varying from about 3 to 4.5 ft. In the Ely Valley, at Gilfach Goch and Werntarw up to 1 ft of cannel lies in its immediate roof.

The measures between the No. 3 and No. 2 Rhondda are extremely variable and, in view of the paucity of detailed sections available for study, are difficult to correlate with certainty from one end of the district to the other.

The No. 3 Rhondda throughout much of the southern areas is overlain by sandstone up to 50 ft thick; this dies out northwards towards the heads of the Ogmore, Garw, and Llynfi valleys, and beneath the Avan–Neath watershed; and in the Ogwr Fach and Lower Rhondda areas the worked seam had a silty mudstone roof. In the mudstones overlying this sandstone there is a thin coal, unnamed except around Aberbaiden on the South Crop, where it is called the Rock Fach.

Above the Rock Fach, sandstones up to 150 ft or more in thickness are present at the western end of the South Crop and around Cwm Gwineu. These thin out northwards in much the same areas as the underlying sandstone, and to the east the lower portion is replaced by mudstones with several thin coals. A still higher sandstone is upwards of 100 ft thick in the Llynfi and parts of the Garw and Ogmore valleys, and this apparently extends into the upper Rhondda where it is still up to 50 ft thick around Treherbert.

In the area from Mynydd Pen-hydd to Cwmavon, the Rock Fawr and Rock Fach together with their associated mudstones die out abruptly, and about 400 ft of sandstones lie above the Tormynydd, extending upwards to close below the Wernpistyll.

In the north and north-east the beds equivalent to these arenaceous measures above the No. 3 Rhondda are essentially argillaceous, but they include a number of sandstones, up to about 15 ft thick, which are probably impersistent, for they cannot be traced from one shaft to another, and their relationships with the major sandstones of the south-western areas are obscure. Coal development is also erratic, and little precise correlation can be achieved. Concurrently with the disappearance of the massive sandstones, the measures thin to less than 150 ft in the extreme eastern areas.

The highest Llynfi Beds are everywhere essentially argillaceous. They include in the lower Avan Valley two thin coals, the Wernpistyll and Wernpistyll Rider, both of which have been locally mined on a small scale; these lie respectively about 80 to 135 ft and 30 to 50 ft below the top of the group. Mudstones with thin sandstones and striped beds separate these coals. To the east thin coals occupy similar positions over much of the area but their exact correlation is not certain, and over much of the South Crop only one coal is developed.

The Llynfi Beds as a whole are almost devoid of marker horizons. Apart from the shelly blackband associated with the White and its probable equivalents, the only fauna recorded from an identifiable horizon is that from the roof of the Wernpistyll Rider. The species here identified are restricted to Anthraconauta philhpsii and Carbonita sp.

(b) Rhondda Beds

The Rhondda Beds have the most extensive outcrops of any of the Coal Measures groups. They form most of the high ground of the plateau in the central areas of the district, as well as a strip half a mile wide along the South Crop widening out in the west around the end of the Bettws syncline. The beds thicken south-westwards from less than 600 ft in the Taff Valley to more than 1100 ft on the South Crop between Coytrahen and Margam Park (Figure 8). This reflects a marked increase in the development of pennant sandstones : in the Taff and Cynon valleys the middle and upper parts of the group are largely argillaceous, but elsewhere sandstones are dominant and make up to about 90 per cent of the sequence.

At the base the No. 2 Rhondda is the most extensively worked of all the Pennant coals within the district. It is a single seam only in the central, northeastern and south-eastern areas, where it varies from about 2 to 2.5 ft in the Aberaman and Merthyr Vale areas, to over 6 ft at Llantrisant, though it more usually averages 3 to 4 ft. The seam has a variable section, and in places consists of three or more distinct leaves. One of the partings, generally 1 to 2 ft from the top, develops into a true split, and over large areas two distinct seams are present. Two separate areas of split coals are recognized. One extends through the lower Rhondda Fawr from Pontypridd towards Porth, and includes much of the Rhondda Fach and the Cynon Valley south of Mountain Ash. The bottom coal, known in the Rhondda valleys as the No. 2 Rhondda, is up to 3 ft thick, while the upper or Fforest Fach, 3 ft thick in the Pontypridd area, thins northwards and in the Rhondda Fach appears to die out as a result of pene-contemporaneous erosion. Two seams are again present west of a line traceable northwards from Heol-y-cyw, through Ogmore Vale and Mynydd Blaenafan, and thence westwards through Cymmer (Avan) towards the Neath Valley. Here, the lower coal, known variously as the No. 2 Rhondda, Wernddu, Rock Fawr or Malthouse, is generally about 3 to 4 ft thick. A local split develops in this seam in the Avan Valley west of Duffryn, the lower leaf being known as the Wernddu Rider. The upper coal, known generally as the Field Vein, is about 2 ft thick, and south of Cwmgwyneu Colliery is separated from the Wernddu by as much as 120 ft.

In most of the district massive pennant sandstones succeed the No. 2 Rhondda, though locally, as in the Ogmore Vale area, 40 ft of planty mudstones intervene. Over wide areas the lowest few feet of these sandstones consist of quartz conglomerate containing 'pebbles' and 'rafts' of coal and rolled pellets of clay-ironstone. Where fully developed, throughout most of the district to the south and west of the Rhondda Fawr, the sandstones continue the sequence up to the mudstones beneath the No. 1 Rhondda, broken only by a few thin and impersistent bands of silty mudstone. They are thinnest, about 150 ft, in the extreme south-east at Rhiwsaeson, but they thicken rapidly, when traced westwards, to about 500 ft along the western part of the South Crop.

In much of the lower Rhondda Fawr area, extending from Pontypridd to Clydach Vale and Ystrad Rhondda, the upper part of the sandstones is replaced, in part or wholly, by mudstones (see (Figure 25)). Within these, the Daren Rhestyn coal, a dirty seam up to about 2 ft in thickness, is developed. North-east of Ystrad this coal dies out, and the associated mudstones are again replaced by sandstones in the Rhondda Fach. In the Cynon and Taff valleys a thin mudstone and coal horizon is traceable within the upper part of the sandstones overlying the No. 2 Rhondda, and this may be the equivalent of the Daren Rhestyn.

Similar lateral changes in lithology take place within the upper Rhondda Fawr, in the Corrwg Valley and in the Glyncastle area as far west as Melin Court Brook. Mudstones beneath the No. 1 Rhondda here reach 130 ft thick. A coal, once worked around Glyncorrwg and near Blaenrhondda, lies between 15 and 50 ft above the base of these mudstones, and is considered likely to equate with the Daren Rhestyn; both in the Rhondda Fawr and Corrwg it has locally a ferruginous mudstone roof carrying Anthraconauta phillipsii and ostracods. A thin smut or seatearth usually lies 15 ft above, and between Glyncorrwg and Cymmer two or three closely associated thin coals occur in this position. Distinctive, though thin, quartzitic sandstones are developed above, and in places below, this composite subsidiary coal horizon, separating it from the Daren Rhestyn and the No. 1 Rhondda.

Where best known in the lower Rhondda Fawr, the No. 1 Rhondda is 2 to 3 ft thick. In much of the district it is imperfectly known though its continuity is not in doubt, except locally west of Ton and Cwm-parc, to the north-east of Gilfach Goch, and again in the Avan Valley between Cymmer and Pont Rhyd-y-fen, where it appears to be affected by washouts. On the South Crop, north and north-east of Llanharan, the seam is thick and dirty, though over much of the west of the district it is thinner but cleaner. It has been worked on a moderate scale at numerous localities in the Rhondda Fawr, as well as on Moelcynhordy, and in Cwmcedfyw between the Garw and Llynfi valleys. To the north the seam varies from 12 to 18 in between Troedyrhiw and the head of the Rhondda Fawr, but westwards it improves to 2 to 2.5 ft at Glyncorrwg and in the Neath Valley. At numerous localities throughout the district the seam is immediately overlain by as much as several feet of striped ferruginous mudstone or blackband containing abundant A.phillipsii and ostracods, which provide a useful marker.

In much of the lower Rhondda valleys the No. 1 Rhondda is closely overlain by a thin coal. The two seams are separated by 40 ft in Clydach Vale, 20 ft on Mynydd y Cymmer, and by only a few feet on the western slopes of Mynydd y Glyn between Cymmer and Trebanog. It may be that the upper coal is a split from the No. 1 Rhondda, but it is significant that it is the lower of the two seams which here carries the blackband fauna. In the south and west the No. 1 Rhondda is followed closely, and in some places immediately, by massive pennant sandstones 100 to 150 ft thick, which locally have a conglomeratic base. Eastwards and north-eastwards these become thin and are not recorded east of a line from Ferndale to Ynys-y-bwl. They also die out locally on the South Crop north-west of Llanharan and in the Corrwg Valley.

In most of the district a thin coal, the No. 1 Rhondda Rider, lies above about 20 to 30 ft of mudstone overlying the last mentioned sandstones. Near Pont Rhyd-y-fen it has been worked as the Graig Isaf, about 2 ft thick, but elsewhere few details are known concerning its section; both it and the underlying mudstones are thinner in the Corrwg and upper Rhondda valleys. Usually only one seam is developed within the mudstone, but locally, as in the Llantrisant shafts, a second thin coal lies near the base of the 'slack', about 30 ft below the 15 to 20-in Rider coal. 'Mussels' are recorded from the roof of the seam at a few places: in the Ferndale area Anthraconauta phillipsii is accompanied by A.cf. tenuis,and this represents the lowest occurrence of the latter in the district.

Throughout most of the district the No. 1 Rhondda Rider is succeeded by thick massive sandstones. At Llangynwyd these are 550 ft thick, continuing upwards to within 20 or 30 ft of the Brithdir horizon. They are 200 to 400 ft thick in the Avan Valley, and 400 ft in the Ely Valley.

In the Porth area of the Rhondda Fawr and generally throughout the Rhondda Fach a 50-ft mudstone is developed in the lower part of these sandstones, and is separated from the No. 1 Rhondda Rider by up to 30 ft of sandstone. A thin and impersistent dirty coal lies at the top of the mudstone and is immediately overlain by the main mass of sandstones. These thin from about 250 ft at Ynyshir to less than 70 ft on the slopes above Blaenllechau, and in the Cynon and Taff valleys the corresponding beds are essentially argillaceous; they contain two thin coals, the roof of the lower locally yielding Anthraconaia pruvosti and Anthraconauta phillipsii associated with abundant Euestheria simoni Pruvost.

In general, the highest beds of the group consist of 30 to 50 ft of argillaceous rocks locally carrying one or more insignificant coals. In the Taff Valley, however, a sandstone, up to 90 ft thick, appears to be developed in this part of the sequence, but this dies out westwards and cannot be mapped in the Cynon Valley.

Blue and red mudstones, recorded towards the top of the group at Penrikyber Colliery, represent the last appearance westwards of the coloured Den Beds of Monmouthshire.

(c) Brithdir Beds

The Brithdir Beds occur in three main areas: in the north-east of the district they underlie much of the Taff, Cynon, and Clydach valleys, as well as capping the high ground of Cefn Gwyngul on the east side of the Rhondda Fach; in the north-west they are preserved in the Cefnmawr Syncline between the Avan and Neath valleys; and they have their most extensive development within the Llantwit–Tonyrefail–Bettws Syncline in the south. They vary in thickness from about 500 ft in the north and north-east to rather more than 800 ft to the west of Llangynwyd ((Figure 8)c). In contrast to the Rhondda and Llynfi beds the measures are everywhere dominantly arenaceous, and the coals are thin and unimportant.

The Brithdir Seam at the base is widely worked in Monmouthshire, but it has largely deteriorated before entering the present district, where its precise equivalent is not known with certainty over much of the ground, for although the associated mudstone 'slack' is readily identifiable it contains several thin and dirty coals. The uppermost of these is usually the best defined and, for reasons of convenience, is equated with the Brithdir. The section is everywhere dirty, and the maximum known thickness of workable coal from the few trials on it is but 26 in.

Away from the Taff Valley, where locally a planty mudstone roof overlies the seam, the Brithdir is succeeded by massive pennant sandstones, which thicken from about 130 ft in the north-east of the district to at least 250 ft in the southwest. Usually these contain only insignificant lenses of silty mudstone, but in the north-western areas a more widespread development occurs about 60 to 120 ft from the top. Near Blaen-gwynfi and Cymmer this is associated with a thin coal. In the Upper Corrwg and Neath valleys the apparent equivalent of the mudstone is 35 ft thick, and a 2-ft coal, the Graig, lies towards the base. The roof of the coal carries Anthraconauta phillipsii, A. tenuis and ostracods.

The Brithdir Rider is also thin and, over most of the district, dirty. It has only been worked sporadically and never on a commercial scale. In the Cynon and Rhondda Fach valleys it consists of 15 to 18 in of clean coal, but elsewhere it is split by bands of rashings and seatearth. Generally its roof is sandstone, locally conglomeratic, but in the Rhondda Fach up to 20 ft of mudstones overlie the seam, and both here and in the Cynon Valley the immediate roof consists of dark shale with A.phillipsii and A. tenuis.

The succeeding sandstones are among the thickest in the sequence. They are upwards of 400 ft thick in the east of the district, and nearly 500 ft to the south of Maesteg. Tilestones ' are widely developed towards the base in the north-east. Impersistent developments of thin mudstone, accompanied near the top by a thin coal, are known from widely separated parts of the district. Although these appear to vary in position, they probably all represent the same horizon, which may equate with the Glyngwilym seam of the north-west of the district. In the lower Rhondda the thin seam lies about 100 ft above the Brithdir Rider, in the Dimbath–Ogwr Fach area the figure is 200 ft, and south of Maesteg about 350 ft. The Glyngwilym Seam, west of the Avan, consists of up to 26 in of clean coal. Its roof carries fragmentary 'mussels' and ostracods. From Aber-cregan to Pont Rhyd-y-fen sandstones, up to 150 ft thick, closely overlie the seam, but these thin northwards and in the upper Cregan and Corrwg valleys and around Glyncastle only mudstones and siltstones separate it from the Wenallt some 30 to 70 ft above.

(d) Hughes Beds

The Hughes Beds are preserved in three broadly synclinal areas: around Abercynon, on the neighbouring high ground between the Taff, Cynon and Clydach valleys, and in the trough formed by the Daren-ddu and Llanwonno faults; in the Cefnmawr syncline; and along the axis of the Llantwit–Tonyrefail, Bettws Syncline from Llantwit Fardre to Llangynwyd. The group is completely preserved only on Cefnmawr, where it is about 800 ft thick. Elsewhere the original thickness cannot be determined since the highest beds are not present. Each area has its own seam nomenclature, the southern one possessing several variations.

At the base of the group a complex coal horizon is known variously in the east and south as the Cefn Glas, Pen-y-coedcae, Ty-du, Lower Glynogwr and Bettws Four-Feet. Throughout these areas it has been worked in a few places, with a section varying from about 2.5 ft near Abercynon to 4.5 to 5 ft at Tonyrefail, Llandyfodwg, Bettws and Llangynwyd. In the north-west several distinct seams are developed, presumably by splitting, the most important being the Wenallt at the base, and the Wenallt Rider some 15 ft above; both average 2 to 2.5 ft and are of considerable local importance. An abundance of large Anthraconauta tenuis,associated with A. phillipsii and ostracods, is known widely from the northeastern outcrops and again around Bettws, while a similar fauna is known from the roof of the Wenallt Rider near Aber-cregan.

Thick and massive sandstones closely overlie this coal horizon, and locally infill small channels eroded in the underlying rocks. Around Bettws and in the Ogwr Fach up to 20 ft of mudstones may intervene between the coal and the sandstones, and west of Ton-mawr these expand to about 75 ft with a thin coal, the Garlant, at the top. The sandstones are thickest in the west of the district, 200 to 250 ft being measured in the Pelena Valley, but they thin eastwards and south-eastwards to 130 ft in the Cynon Valley and barely more than 100 ft in the Ogwr Fach. In the last-named area a thin mudstone and coal is locally present about 35 ft above the base.

The overlying mudstones are 20 to 40 ft thick. At or near their top a coal is widely present throughout the district, though it appears to fail north-eastwards in the Taff Valley. It is called the Daren-ddu near Pontypridd, the Dirty at Tonyrefail, the Upper Glynogwr near Llandyfodwg, and the Mountain north-west of the Avan. It is generally about 2 to 3 ft thick, but contains several dirt partings which are liable to thicken and render the seam unworkable. Mining has been most extensive in the trough between Pontypridd and Llanwonno, and around Ton-mawr. A massive sandstone roof is generally present, though planty mudstones intervene in a few places.

Yet another thick group of sandstones follow; these are 200 to 250 ft around Ton-mawr and this thickness is maintained in the north-east. Around Ynys-y-bwl a lens of thin mudstone with an associated coal streak occurs about 150 ft above the base; similar developments occur 90 ft above the base at Tonyrefail, and 50 to 100 ft above to the north-east of Llandyfodwg. In the Llantwit Syncline only about 150 ft of beds separate the Daren-ddu from the No. 3 Llantwit and this thinning may be due in part at least to unconformity.

On Cefnmawr the highest beds of the group consist of about 250 ft of mudstones. Near their base is a thin coal, the equivalent of the Westernmoor Seam of Neath, and 120 ft higher a second coal presumably represents the Shenkin of that area. No representatives of these beds have been recognized with certainty elsewhere in the district, though they may be present in the trough formed by the Moel Gilau and Felin-Arw faults between Pont Rhyd-y-cyff and Llangeinor.

(e) Swansea Beds

On Cefnmawr the Graigola Vein, a clean coal of about 6 ft, conformably overlies the Hughes Beds, and is succeeded by 80 ft of massive pennant. The Swansea Beds may also be in part preserved within the Felin-Arw trough, though the exact relationships of the strata here are obscure. A seam, possibly to be equated with the Graigola, is nearly 8 ft thick, and is also overlain by massive sandstones. Some distance above this sandstone a thick double coal, the Bettws Nine-Feet, may correlate with the Little and Hard veins of Gnoll Colliery, Neath.

(f) Grovesend Beds

These beds are restricted to the area of the Llantwit Syncline between Llantrisant and Llantwit Fardre. Here over 500 ft of measures are present including five coals. Of these the principal seams are: the No. 3 Llantwit at the base, about 4 ft thick; the No. 2 Llantwit, a dirty coal with an overall section of about 7 ft, some 110 ft above; and the No. 1 Llantwit, rather more than 250 ft above the latter, and consisting of two leaves 2 to 15 ft apart, together totalling about 8 ft of coal. Above the No. 1 Llantwit rather more than 100 ft of measures are preserved, including 50 ft or so of massive sandstone in the roof of the coal. Apart from this sandstone the measures are dominantly argillaceous. Red fire-clays are recorded about 100 ft above the No. 2 Llantwit, and provide a point of similarity with the Grovesend Beds of the type area.

Only one faunal horizon is known, spoil from the No. 2 Llantwit yielding Anthraconauta cf. tenuis,Spirorbis sp.and ostracods. A.W.W., W.B.E.

References

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DAVIES, D. 1921. The ecology of the Westphalian and the lower part of the Staffordian Series of Clydach Vale and Gilfach Goch. Quart. J. Geol. Soc.,77, 30–74.

DAVIES, J. H. and TRUEMAN, A. E. 1927. A revision of the non-marine lamellibranchs of the Coal Measures, and a discussion of their zonal significance. Quart. J. Geol.Soc.,83, 210–59.

DIX, Emily. 1933. The succession of fossil plants in the Millstone Grit and the lower portion of the Coal Measures of the South Wales Coalfield. Palaeontographica,78, 158–202.

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

EAGAR, R. M. C. 1962. New Upper Carboniferous non-marine lamellibranchs. Palaeontology,5, 307–39.

EDWARDS, W. and STUBBLEFIELD, C. J. 1948. Marine bands and other faunal marker-horizons in relation to the sedimentary cycles of the Middle Coal Measures of Nottinghamshire and Derbyshire. Quart. J. Geol. Soc.,103, 209–60.

EVANS, W. H. and SIMPSON, B. 1934. The Coal Measures of the Maesteg District, South Wales. Proc. S.Wales Inst. Eng., 49, 447–75.

EYLES, V. A. 1956. In Sum.Prog. Geol. Surv.for 1955.

HEARD, A. 1922. The Petrology of the Pennant Series. Geol. Mag.,59, 83–92.

JENKINS, T. B. H. 1960. Non-marine lamellibranch assemblages from the Coal Measures (Upper Carboniferous) of Pembrokeshire, West Wales. Palaeontology,3, 104–23.

JONES, D. G. and OWEN, T. R. 1956. The rock succession and geological structure of the Pyrddin, Sychryd and Upper Cynon valleys, South Wales. Proc. Geol. Assoc.,67, 232–50.

JONES, S. H. 1934. The correlation of the coal seams in the country around Ammanford. Proc.S. Wales Inst. Eng., 49, 409–38.

KIDSTON, R. 1893–4. On the fossil flora of the South Wales Coalfield and the relationship of its strata to the Somerset and Bristol Coalfield. Trans. Roy. Soc.Edin.,37, 565–614.

KIDSTON, R. 1894. On the various divisions of the Carboniferous Rocks as determined by their fossil flora. Proc. Roy. Phys. Soc. Edin.,12, 183–257.

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. and COX, A. H. 1943. The Coal Measure sequence in the Taff Valley, and its correlation with the Rhondda Valley sequence. Proc. S. Wales Inst. Eng., 59, 189–304.

NORTH, F. J. and HOWARTH, W. E. 1928. On the occurrence of millerite and associated minerals in the Coal Measures of South Wales. Proc. S. Wales Inst. Eng., 44, 325–48.

RAMSBOTTOM, W. H. C. 1952. The fauna of the Cefn Coed Marine Band in the Coal Measures at Aberbaiden, near Tondu, Glamorgan. Bull. Geol. Surv. Gt. Brit., No. 4, 8–30.

ROBERTSON, T. 1927. The Geology of the South Wales Coalfield. Part II. Abergavenny. 2nd edit. Mem.Geol. Surv.

ROBERTSON, T. 1933. The geology of the South Wales Coalfield. Part V. The Country around Merthyr Tydfil. 2nd edit. Mem. Geol. Surv.

STRAHAN, A. 1907. The Geology of the South Wales Coalfield. Part VIII. The country around Swansea. Mem.Geol. Surv.

STRAHAN, A. , CANTRILL, T. C., DIXON, E. E. L. and THOMAS, H. H. 1907. The geology of the South Wales Coalfield. Part VII. The Country around Ammanford. Mem. Geol. Surv.

STRAHAN, A. , TIDDEMAN, R. H. and GIBSON, W. revised by GIBSON, W. and CANTRILL, T. C. 1917. The Geology of the South Wales Coalfield. Part IV. The Country around Pontypridd and Maesteg. 2nd edit. Mem. Geol. Surv.

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.

TROTTER, F. M. 1947. The Structure of the Coal Measures in the Pontardawe–Ammanford area, South Wales. Quart. J. Geol. Soc.,103, 89–133.

WARE, W. D. 1930. An account of the geology of the Cefn Coed sinkings. Proc. S. Wales Inst. Eng., 46, 453–501.

WARE, W. D. 1939. The Millstone Grit of Carmarthenshire. Proc. Geol. Assoc.,50, 168–204.

WOODLAND, A. W., EVANS, W. B. and STEPHENS, J. V. 1957a. 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.

WOODLAND, A. W., ARCHER, A. A. and EVANS, W. B. 1957b. 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.

Chapter 4 Lower Coal Measures: details

In the following account certain local terms, applied by the miners to particular lithologies, have been used for the sake of convenience, or have been quoted. The following list gives explanations of these terms:

• Balls: nodules of ironstone.
• Bast: cannel or cannelly mudstone.
• Bastard: applied usually to non-typical or intermediate lithologies; thus 'bastard clift', usually a strong sandy mudstone not sandy enough to be called 'rock'. Blackband: ironstone or ferruginous mudstone with an appreciable amount of carbonaceous matter.
• Brass: hard bands, lenses, or nodules rich in pyrite.
• Clift (or cliff): blocky mudstone or silty mudstone.
• Clod: soft mudstone forming a band in a coal seam; or that part of the roof which comes away with the coal during working.
• Dirt: any band other than coal within a seam.
• Engine coal: inferior coal.
• Mine: ironstone; mine ground is clift with much ironstone in bands or nodules. Pin: thin band of hard rock, usually ironstone.
• Rashes or rashings: eithersoft carbonaceous shale with streaks of coal orhighly 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 th. former definition and 'rashings' for the latter.
• Rider: a coal commonly thin and unworkable, and usually, but not always, overlying a coal referred to in the name; thus No. 1 Rhondda Rider—the seam above the No. 1 Rhondda.
• Rock: hard massive sandstone.
• Stone: a hard band usually within a coal seam.

To facilitate description of the details of stratigraphy in this and the following two chapters, the district has been divided into seven areas. These are depicted on (Figure 26), which also shows the positions of the major collieries; the grid-references of the collieries are given in the Appendix of colliery and borehole sections, p. 300 ff.

Much confusion exists over the rendering of Welsh place-names, particularly with regard to the use of hyphens in compound names. Throughout this memoir the representation used on the 6-in Geological Maps (published 1957–60) has, in general, been followed. In certain instances this has led to apparent anomalies, for the official style of a colliery carrying a locality name may differ from the name used on the map. Thus Avon Colliery lies in the Avan Valley; Llwynypia Colliery is located at Llwyn-y-pia; and Wemtarw Colliery is in the vicinity of Wem-tarw Farm.

A. Gastrioceras subcrenatunl Marine Band to Garw Seam

These measures crop out in a narrow strip, seldom more than half a mile wide, extending along the South Crop of the Coalfield from Kenfig Hill in the west, through Aberkenfig and along Cefn Hirgoed to Penprysg. Eastwards they plunge beneath glacial drift and pass into the Bridgend district to the south, where practically nothing is known concerning them.

In 1954 the Margam Park No. 1 Borehole [SS 8194 8578] penetrated these measures, and gave for the first time in this district a complete section of the strata. In addition, the No. 2 Borehole [SS 8111 8632] reached a depth of 282ft below the Garw. The full logs of both these boreholes below the Gellideg Seam have already been published (Woodland and others 1957b, pp. 44–49), and the following record of the strata from the Garw to the base of the formation in the No. 1 Borehole is abridged from this source.

The evidence provided by the mapping of the western portion of the South Crop suggests that most of the significant horizons persist throughout this limited area. The G.subcrenatum Marine Band is not known at crop. The thick sandstone, about 80 ft above the band in the above borehole (equivalent to the 'Farewell Rock' in the western parts of the North Crop), persists eastwards to beyond Penprysg; it has been quarried [SS 8865 8285] in the northern part of Ffwyl Wood, 1100 yd S. 38° W. of Aberkenfig Church, where thinly bedded fine-grained sandstones were formerly worked for flags and tiles, and on the central part of Cefn Hirgoed [944)828], 500 yd W. 30° N. of Ffos-felen. The 30 ft of flaggy sandstones exposed in the cutting on the Llynfi Valley Railway [SS 8985 8337], 150 yd E.N.E.of Wern-dew, Aberkenfig, are probably part of the same sandstone.

The shales and mudstones immediately above are poorly exposed. The marine band, with base 745 ft 3 in below the Garw in the above borehole, appears to be represented in the stream just east of Bryn-coch, at a point [SS 9114 8350] 400 yd W. 15° N. of Pistyllarian, where grey shales yielded P. ophthalmoides, L.mytilloides and Euphemites? through a thickness of 10 ft (see below). Productus (P.) carbonarius de Koninck from the spoil dug in the Mid-Glamorgan Water Board's reservoir [SS 939 829], 650 yd W. 30° N. of Ffos-felen, may also have come from this horizon.

A sandstone close below the Crows Foot Coal is very variable in its development. It is poorly represented in the Margam Park No. 1 Borehole, but on the west side of Cefn Cribwr and again on Cefn Hirgoed it is well developed, becoming quartzitic towards the east. A disused quarry [SS 842 827] near the Old Camp, 300 yd S.E. of Kenfig Hill Church shows the following section:

On Cefn Hirgoed the crop of what appears to be the same sandstone is marked by a line of crags of coarse whitish quartzitic grits containing scattered jasper pebbles, extending from the reservoir [SS 939 829], mentioned above, to the bare feature of Garn Fawr [SS 956 827], 700 yd E. by N. of Perth Celyn.

The Crows Foot Coal is reputed to be 24 to 32 in thick, but it has not been exposed for many years. A level [SS 8335 8307] by the old quarry on the east side of the Porthcawl Railway, 750 yd W.N.W. of Kenfig Hill Church, presumably won the coal, and an old pit on Cefn Cribwr [SS 8543 8262], 1130 yd W. 5° S. of Cefn Cross, is said to have struck it at about 60ft. A coal, 2 ft thick, exposed at the back of a quarry [SS 915 835], near Caehelyg, 800 yd S. 10° E. of Bryncethin Church, and worked from bell-pits nearby, is presumed to be the same seam. The 1-in coal, 661 ft 2 in below the Garw in the Margam Park No. 1 Borehole, is roofed by mudstone carrying L. mytilloides and fish debris; no evidence of this marine band has been found at crop.

The measures from the Crows Foot downwards to a horizon presumably not far above the base of the Coal Measures are exposed in the stream running S.S.E. from the small bridge [SS 9112 8353] on the south-east of Bryncoch, 800 yd S. 20° W. of Bryncethin Church:

The beds associated with the Cefn Cribbwr Marine Band are seen in a much overgrown quarry [SS 8812 8298], 1500 yd W. 30° S. of Aberkenfig Church. After excavation of the poorly exposed parts the following section was measured :

Certain of these beds were also formerly worked for brick-making from an old level [SS 8796 8317], 100 yd N.W. of Tycribwr, and much fossiliferous material from the Cefn Cribbwr Marine Band was found in the spoil. The following list combines the collections made from both spoil and quarry: Crurithyris?, L. mytilloides;Caneyella cf. multirugata, Dunbarella cf. elegans (Jackson), D. papyracea, Posidonia cf. gibsoni Salter, P. sp. nov.[right valve with prominent anterior lobe]; Orthocone nautiloid; Anthracoceras sp., Gastrioceras coronatum Foord and Crick, G. listeri, Homoceratoides divaricatus (Hind) ; Listracanthus sp.,Palaeoniscid scales and Rhadinichthys sp.

This marine band is also poorly exposed in the south-east corner of the quarry near Caehelyg referred to above. A thin bed of blue-grey shale, very disturbed, lies between 40 ft of Cefn Cribbwr Rock, which was quarried, and massive quartzitic sandstone. A few fragmentary and poorly preserved goniatites and L. mytilloides were obtained.

The Cefn Cribbwr Rock, which varies in thickness from about 100 to 120 ft, is the first major sandstone below the main coals, and throughout its outcrop from Kenfig Hill to Penprysg has weathered into a marked ridge. The steep northern slopes of Cefn Cribwr and Cefn Hirgoed are formed by the dip slope of this sandstone, the clearly defined top of which is marked by a continuous spring-line. The sandstone varies from being flaggy and false-bedded, with a few silty partings towards the top, to being locally massive and quartzitic, and it has been much quarried. Several quarries near Waterhall Junction, on the north side of Kenfig Hill, show 30 ft of blocky and false-bedded sandstones, and similar beds are exposed in the side of the road from Fountain to Cefn Cribwr. Several adjacent quarries [SS 891 834] about 350 yd S.W. of Aberkenfig Church show 30 ft of well-bedded fine-grained sandstone, and similar sections are seen in quarries near Caehelyg [SS 9152 8360], half a mile S. of Bryncethin. On the northern slope of Cefn Hirgoed shallow quarrying was almost continuous from near Derwengopa [SS 9308 8333] eastwards for about 1.25 miles. The sandstone was extensively worked at Penprysg, where quarries [SS 962 825] on the west side of the Heol-y-Cyw road at the northern end of the village show:

Few exposures exist of the 300 to 400 ft of mainly argillaceous strata between the Cefn Cribbwr Rock and the Garw. The Margam Marine Rand is known only at one locality [SS 9113 8365], in the stream bed on the east side of Bryn-coch. The full thickness is not exposed, but 5 ft of rusty weathering dark shales yielded : Dunbarella aff. elegans, Caneyella cf. multirugata (? sp. nov.),Posidonia cf. gibsoni;a coiled nautiloid (cf. Domatoceras) and Anthracoceras sp.

Several sandstones, mostly impersistent and commonly quartzitic, are developed in the uppermost part of the sequence. They form definite though comparatively small features, colonized by bracken, and they stand out sharply against the featureless wet rushy land formed by the argillaceous strata. This contrast in vegetation is particularly well seen on the southern reaches of Hirwaun Common. One of the sandstones, 40 to 50 ft thick, lies close beneath the Garw and its crop seems to be more or less continuous from Kenfig Hill to Penprysg.

Elsewhere in the district, apart from a few old underground boreholes, which penetrated a little way below the Garw, these measures are unknown. A.W.W.

B. Garw To Amman Rider

(Figure 27) shows generalized sections of these measures for each of the areas described below.

Cynon-Taff area

The Garw and the measures upwards to the Gellideg are known only from the records of boreholes made before the present resurvey. The Garw is thin and has never been worked: at Fforchaman CollieryThe National Grid-references of the major collieries are given in the Appendix of shaft and borehole sections, pp. 300 ff. it is recorded as being 18 in thick, at Lower Duffryn 14 in and at Lady Windsor 27 in. The measures between the Garw and the Gellideg are about 100 ft thick and consist mainly of mudstone with numerous ironstone bands and a few thin sandstones.

The Gellideg varies from about 2.5 ft (in the northern part of the Penrikyber Colliery area) to 3.5 ft, the thickening taking place, in general, in a westerly and south-westerly direction. Usually the seam consists of a single clean coal, but in the south-west of the area, at Lady Windsor Colliery, and in the north-west, at Fforchaman and Cwmneol collieries, a coal about 5 in thick is recorded near the base. No fauna is recorded.

In all collieries, except those of the Aberaman and Cwmaman areas, there is little separation between the Gellideg and the Five-Feet, and the two seams together form one workable unit. At Merthyr Vale and Deep Duffryn collieries only the Five-Feet portion of the section is worked at the face, but the full section is removed in the roadways; in parts of Penrikyber colliery the Gellideg together with the bottom coal of the Five-Feet is removed, and the upper coal of the Five-Feet is left as roof. In the Aberaman and Cwmaman areas the separation between the two seams is very variable, being as much as 50 ft in the north-west. In these areas a massive sandstone is developed above the Gellideg, and this is locally associated with washouts which can be traced south-westwards into the Ferndale area.

In the northern part of the area the Five-Feet Seam consists of two leaves, each 24 to 30 in thick. In the Fforchaman and Lower Duffryn colliery areas the separation between the two coals may be 4 ft or more, but south-eastwards this lessens rapidly, and south-east of a line drawn from Merthyr Vale pits through Navigation Colliery, the seam consists of a single main coal about 4 ft thick. In the Fforchaman–Cwmaman area a thin coal is developed locally between the two main leaves, while a further thin coal is nearly always present at the base of the section. The roof of the seam consists usually of well-bedded blue-grey mudstone containing sporadic Curvirimula;a sandstone roof occurs locally in Fforchaman Colliery.

The measures between the Five-Feet and the Seven-Feet are very variable, both in thickness and lithology. In general, the thickness varies from 70 ft in the south at Penrikyber, to little more than 10 ft in parts of the Fforchaman area. However, in the north, there is considerable variation even within single colliery areas: 12 to 60 ft at Fforchaman, 15 to 80 ft at Cwmneol, and from 25 to 60 ft at Aberaman. In these northern collieries a thin coal, rarely more than 12 in thick, occurs locally 2.5 in to 30 ft below the Seven-Feet; this coal appears to die out southwards. The strata consist predominantly of mudstones with much ironstone, but sandstone, locally up to 30 ft thick, is extensively developed below the thin coal in the northern collieries. This sandstone dies out southwards and eastwards into the Merthyr Vale and Deep Duffryn areas, but it reappears in some strength in the Penrikyber area.

The Seven-Feet Seam in Merthyr Vale, Navigation, Deep Duffryn, Lower Duffryn, Aberaman and the southern part of Cwmneol collieries is 5 to 5.5 ft thick and consists of three leaves with well-defined partings. The middle coal is consistently the thickest, being 24 to 36 in, while both the top and bottom leaves vary from 12 to 20 in. To the west and north-west, at Fforchaman and Cwmaman, the partings thicken, and the top coal, in particular, becomes separate. Over much of Fforchaman the middle and lower coals are worked together, but in the western parts of the old Cwmaman taking the splitting is so complete that only the middle coal was worked. Southwards from Mountain Ash the bottom leaf splits away and at Penrikyber Colliery the top and middle coals are worked together. The roof of the upper coal consists of grey mudstone, usually with small ironstone nodules. Fossils are rare, but poorly preserved 'mussels' including Anthraconaia curtata and Naiadites sp.have been obtained from Aberaman, Deep Duffryn, Merthyr Vale and Penrikyber collieries.

The strata between the top coal of the Seven-Feet and the Yard vary in thickness from 40 ft at Deep Duffryn to less than 2 ft in the northern parts of the Fforchaman and old Cwmaman takings. Southwards from Deep Duffryn they thin to about 16 ft in Lady Windsor Colliery. The beds normally consist of grey mudstones with numerous ribs and nodules of ironstone, and the Yard seatearth is usually well developed being as much as 12 ft in thickness. Where the cyclothem is thickest a thin sandstone is locally developed towards the middle.

The Yard Seam (locally called the No. 2 Yard) varies from 29 to 38 in, and is thinnest in the north and east of the area. Throughout the collieries of the Cwmaman–Aberaman area a coal up to 8 in thick lies 2 to 12 in below the main coal, but appears to die out southwards and eastwards. In parts of Merthyr Vale and Penrikyber collieries a few inches of rashings occur at the top of the seam. In the northern parts of Fforchaman and Cwmneol the top of the Seven-Feet is close enough to be included in the seam section, while in the southern parts of Aberaman, Cwmneol, and Fforchaman and the western part of Deep Duffryn, the Amman Rider is so close that the two coals have occasionally been worked together.

The roof of the Yard normally consists of blocky grey mudstone, usually containing scattered solid 'mussels' through a thickness of 1 to 3 ft. In the Merthyr Vale and Navigation areas the fauna is made up of variants of Anthracosia regularis associated with Naiadites aff. triangularis (J. de C. Sowerby), while at Deep Duffryn, Aberaman and Fforchaman these accompany large numbers of variants of Carbonicola venusta,together with N. subtruncatus.At Deep Duffryn Lepidodendron ophiurus and Sigillariostrobus sp.are also recorded.

The Yard to Amman Rider cyclothem is essentially argillaceous. It varies from a few inches of seatearth in the central parts of the area (see above) to 20 to 30 ft in the north and east, and from 10 to 15 ft at Lady Windsor. The seatearth beneath the Amman Rider is well developed, especially in the north where it may make up more than half the thickness of the cyclothem.

The Amman Rider (locally termed the Bute Rider) consists normally of two leaves, usually 5 to 10 ft apart, but separated by as little as 2 in. in the north (e.g. at Cwmaman). The top coal varies from 12 to 22 in and the bottom coal from 4 to 8 in. A.W.W.

Rhondda Fach area

The Garw has been proved only in Standard No. 2 Shaft and in a staple pit 300 yd W.S.W. of Mardy No. 3 Pit. In each case 21 in of clean coal was proved and the seam has nowhere been worked. The measures separating it from the Gellideg are 80 ft thick at Standard and 105 ft at Mardy; they consist of chill with numerous ironstone bands, 3 to 7 in thick; several sandstones, up to 10 ft, occur in the upper part.

The Gellideg varies from slightly less than 2 ft in the northern part of the Mardy area to rather more than 4.5 ft at National and Standard. It consists generally of a single clean coal, but in Mardy and National collieries a thin coal, usually less than 6 in, is present near the base, in the latter area often associated with bast. A well-defined washout crosses the northern portion of the Ferndale area from N.N.E. to S.S.W. Over most of the Ferndale ground the proximity of the Five-Feet gobs above has prevented the exploitation of the seam, and only small patches have been worked at National. At Standard Colliery it was formerly worked 5 to 14 ft beneath the Five-Feet. Northwards from Ferndale into the Mardy area the separation from the Five-Feet increases to about 40 ft, much of this thickness being made up of sandstone.

The Five-Feet is largely worked out in all the Ferndale collieries, as it is also in the National and Standard takings. Over most of this area the amount of separation between the two main leaves of the seam rarely exceeds 4 in, and this falls to nothing in the south-eastern parts of the National and Standard workings. In the Mardy area the parting thickens and is known to reach nearly 5 ft in the No. 3 Pit. The top coal varies from 28 to 39 in and the bottom coal from 22 to 36 in, the top coal being almost invariably the thicker. In the Mardy area a thin coal is present close below the top coal, while a thin leaf is developed in the base of the seam at Standard, National, parts of Ferndale No. 6 and at both Mardy pits.

The measures between the Five-Feet and the Seven-Feet range from 20 to 90 ft, being, in general, thinnest in the north at Mardy, and thickest in the south at Femdale Nos. 8 and 9, and National. Over much of the Mardy and Ferndale Nos. 1 and 5 takings a massive sandstone, up to 30 ft thick, forms the roof of the Five-Feet, but southwards this splits up and its place is taken by mudstone with ironstone bands and nodules. The sandstone also dies out eastwards towards the Cynon Valley. A dirty coal, up to 11 in thick, usually lies above the sandstone at about 20 to 50 ft above the Five-Feet, but is not everywhere persistent.

The three coals, which in the Cynon Valley make up the full Seven-Feet Seam, are subject in this area to much splitting. They are always clearly distinct and are generally well separated. The Upper Seven-Feet varies from about 12 to 20 in; the Middle Seven-Feet (called in the Ferndale area the Two-Feet-Eight) 28 to 39 in; and the Lower Seven-Feet 10 to 20 in. The separation between the Lower and Middle coals varies from 4 to 20 ft, increasing fairly regularly in an east to west direction; that between the Middle and Upper coals from 1 in to 20 ft, being thinnest at Standard and National and in the north-east of the Ferndale area, and increasing to the west and south-west. The Middle and Upper Seven-Feet coals were worked extensively as a single seam at National and Standard, where it was called the No. 2 Seam. Over the rest of the area only the Middle Seven-Feet has been worked. Seatearth and mudstone with sporadic roots make up most of the ground between the split seams, but locally mudstone with ironstone occurs. At Mardy No. 1 Pit the mudstone roof of the Middle Seven-Feet yielded Cyclopteris sp.,Neuropteris heterophylla Brongniart and N. obliques (Brongniart).

Over most of the ground the Upper Seven-Feet and Yard are separated by only 3 to 5 ft of seatearth. In the eastern parts of Ferndale and at National the separation increases, and up to 30 ft of mudstone with ironstone bands may occur. At Standard Colliery the Upper and Middle Seven-Feet and Yard coals are all close together.

The Yard Seam normally consists of a main coal 28 to 44 in thick, with a thin coal, 4 to 8 in, near the base. The seam is thinnest in the south and south-east of the area, at Ferndale No. 9 Pit and at National.

Over most of the Ferndale Nos. 1 and 5 and Nos. 6 and 7 areas the Yard and Amman Rider are separated by 2 to 6 ft of seatearth, and in parts of No. 5 Pit by as little as 5 in. Northwards into Mardy the Amman Rider splits away, and up to 40 ft of ground is developed between the two seams. Here the roof of the Yard consists of smooth grey mudstone with Anthracosia regularis and Carbonicola venusta,followed upwards by strong planty mudstone and striped beds. Southwards into Ferndale Nos. 8 and 9, the western parts of National and at Standard, 10 to 25 ft of strata intervene between the two seams, and the typical Yard fauna is no longer found. In Ferndale No. 9 the roof consists of fine-grained sandstone or strong silty mudstone with plants: Alethopteris decurrens (Artis), A.lonchitica, Annularia radiata Brongniart, Asterophyllites sp., Bothrodendron minutifolium (Boulay), Calamites sp.,Cyclopteris trichomanoides Sternberg, Mariopteris muricata (Schlotheim), Neuropteris heterophylla,cf. Renaultia gracilis (Brongniart), Sphenopteris trigonophylla Behrend and Sigillaria ovata Sauveur. Large tree-boles, up to 2 ft in diameter, in position of growth, extend through the beds immediately above the coal, in some instances as far as the seam above.

The Amman Rider is everywhere a split seam: a top coal, 18 to 26 in, is underlain by a thin leaf, 6 to 18 in; in a few places a further few inches of coal is recorded below. A.W.W.

Northern Rhondda Fawr area

The Garw has not been worked within the area. It proved to be 18 in thick in a trial pit at Fernhill Colliery, 27 in. in an underground borehole at Park Colliery, and 12 in. in another at Gelli. At Fernhill it lay 122 ft below the Gellideg, the intervening measures being mainly argillaceous, with numerous bands and nodules of ironstone and a few thin beds of sandstone. The log of a recent borehole, which reached 114 ft below the Gellideg, is as follows:

Other boreholes at Fernhill proved similar sequences : in one a thin seatearth was noted 18 ft below the Gellideg, and in another P. ophthalmoides was obtained from silty mudstone at about the same horizon. At Tydraw Colliery the shaft proved a 3-in coal in the same position, and at Park what is presumably the same coal also carried P. ophthalmoides.At Fernhill Neuropteris pseudogigantea Potonie and N. obliqua were obtained just below this horizon.

At Park Colliery the Garw has been proved 117 ft below the Gellideg; a recent borehole passed through two horizons of thick calcite-shelled 'mussels' between the two coals; only the upper band appears to be present at Fernhill.

The Gellideg has been worked extensively in the southern part of the area and on a smaller scale in the north. A major washout, running from north-east to south-west through Tynybedw Colliery and Park shafts, affects the seam, and between the latter colliery and the Maindy workings has been proved to be 600 yd wide. To the south of this washout the seam is 4 to 5 ft thick, in general thinning towards the north-east. At Pentre Colliery a thin stone parting is developed about 3 in above the floor of the seam, and in the Maindy and Eastern areas there is a distinct top leaf of 3 to 8 in. To the north-west of the area the seam thins rapidly, and north of the washout the thickest recorded section is 39 in at Park, though it is more usually 27 to 33 in. In this northern area a 'stone' parting, 1 to 3 in thick, lies 30 in above the floor, and is presumably the same parting as that noted at Pentre. Another major washout, running north-west to south-east, forms the western limit of the workings from Maindy and Eastern, and continues south-eastwards into the Cambrian area and beyond.

The measures between the Gellideg and the Five-Feet vary considerably both in thickness and lithology. In the south-east at Pentre, Gelli, and Eastern collieries, the two seams are separated by less than 10 ft of mudstone and mudstone-seatearth. Westwards and northwards the measures thicken, largely through the incoming of sandstone in the roof of the Gellideg. West of Park they are 60 to 80 ft thick and consist largely of sandstone, which dies out north of Treherbert. At Tydraw the separation is less than 50 ft and sandstones are absent. In the Fernhill area boreholes show that there is a progressive westwards change in lithology from sandstones, first to striped beds, and then to mudstones; several boreholes show Curvirimula sp.associated with plant remains and attached Spirorbis sp.extending upwards for about 15 ft above the Gellideg. The upper part of the cyclothem has also yielded abundant plants including Alethopteris decurrens, ?Annularia radiata, Mariopteris daviesi Kidston, Neuropteris microphylla Brongniart, N. obliqua, N.cf. schlehani,N. tenuifolia (Schlotheim) and Sphenophyllum cuneifolium (Sternberg).

The Five-Feet has been worked on a large scale from most of the collieries of the area. Typically the seam consists of two principal leaves, the lower usually 21 to 27 in thick, and the upper 30 to 36 in. Over much of the area there is also a middle coal, about 6 in, and a bottom coal of 3 to 6 in. Locally, around Treorky, another thin coal of about 6 in joins the section from above, and may be the coal often recorded between the Five-Feet and the Seven-Feet in the area to the north-east. At Abergorky Colliery the section of the Five-Feet is further complicated by the close approach of the Lower Seven-Feet, and the so-called No. 5 seam of the colliery comprises in different districts, the combined Lower and Upper Five-Feet coals, the Upper Five-Feet alone, and the combined Upper Five-Feet and Lower Seven-Feet. The parting between the Upper and Lower Five-Feet leaves is less than 12 in in the south of the area, but thickens northwards and as much as 8 ft is recorded at Lady Margaret, Ynysfeio, the northern part of Park, and in much of Tydraw. Boreholes at Fernhill suggest that the separation there is again reduced to a foot or so.

Around Abergorky the Five-Feet and Seven-Feet are as little as 2 ft apart, but more generally the two seams are separated by about 20 ft of argillaceous beds. The roof of the Five-Feet is normally a silty mudstone containing poorly preserved plants, though in places sandstones and striped beds may occur. At Gelli and Bodringallt the shafts record a thin composite coal between the Five-Feet and the Lower Seven-Feet.

Throughout the area the Seven-Feet is split into three distinct seams. The Lower Seven-Feet thickens westwards across the area from 18 to 27 in. It was formerly unnamed except at Tydraw, where it was worked on a small scale as the Five-Feet Rider or Upper Five-Feet. Both here and at Park and Dare collieries a thin parting may be developed near the middle of the seam. Characteristically the immediate roof contains 0.25 in band of cannel, overlain by a few inches of slickensided mudstone with plant fragments; above this the mudstone is darker and more fissile, but has yielded only worm burrows and tracks, including Planolites montanus at Park. It passes upwards into grey silty mudstones with ironstone bands, again containing wormy beds near the base, and these continue for 10 to 30 ft to the Middle Seven-Feet. This seam has been worked under a variety of names: at Abergorky it was known as the No. 4 Seam; at Pentre, Tydraw, and Tynybedw as the Lower Seven-Feet; and at Park and Dare as the Phils Seam. Its overall section thins from almost 4 ft in the south of Dare to about 27 in at Tydraw. The section includes a main top coal of 30 in and a bottom coal which thins rapidly from 10 in to 3 in when traced north-westwards from Maindy, and which, at Tynybedw and Tydraw, carries an inch or so of cannel coal at the top. The two leaves are separated by mudstones up to 3 ft thick. Locally, as at Dare Colliery, a thin cannelly mudstone lies in the roof of the seam and it is probably from this that Moore and Cox (1943, p. 233) record Naiadites cf. triangularis.The seam is affected by washouts locally; at Abergorky they run north to south, and at Park north-east to south-west. At Fernhill Colliery the Middle and Upper Seven-Feet coals are almost united to the north of the shafts, but elsewhere in the area the two seams are distinct and separated by up to 50 ft of silty mudstones with a few thin bands of sandstone. The Upper Seven-Feet is normally 10 to 12 in thick. Where recorded, it is everywhere closely associated with the overlying Yard and at Gelli, Eastern, Tydraw, and parts of Park forms a bottom coal to that seam. Over much of the area from Ton Pentre to Treherbert it is not recorded and is presumably absent or very thin.

The Yard has been worked from most of the collieries. At Ynysfeio it was called the No. 2 Yard; at Tydraw, Tynybedw, and Pentre the Seven-Feet; at Abergorky, and Lady Margaret the No. 3 Seam; and at Park and Dare the Three-Feet-Ten. It maintains a constant section of 27 to 36 in of top coal, separated by a few inches of 'stone' from a bottom leaf of 3 to 9 in. At Park and Dare up to half an inch of cannel occurs at the top of the seam.

The roof of the seam is normally a dark mudstone up to 2 ft thick containing abundant ironstone bands and nodules and many 'mussels'. This is followed by paler sandy mudstone with ironstones and sporadic shells. Variants of Anthracosia regularis and Carbonicola venusta associated with rarer cf. C. oslancis Wright have been collected at all the collieries where the seam was adequately exposed; in addition Anthracosphaerium cycloquadratum (Wright) and Euestheria sp.were recorded at Park. At the top of the cyclothem fragmentary plants, including Mariopteris daviesi, Neuropteris gigantea, N.heterophylla and Sphenophyllum cuneifolium occur at Fernhill.

The Amman Rider coal lies some 20 to 60 ft above the Yard. The two coals are closest along a line running east and west through Treherbert; here the intervening measures consist mainly of mudstone, and the Rider is usually a double coal, the section at Abergorky being: top coal 20 in, rashings 2 in, coal 6 in. Both to the north and south of this line the measures thicken, siltstones and sandstones develop, and the Rider splits into two distinct seams, 1 ft 6 in apart at Tynybedw, 3 ft 3 in at Pentre, and 20 ft at Dare. Southwards the lower coal thickens to 14 in and the upper thins to about 12 in. Northwards from Treherbert the splitting of the Amman Rider is accompanied by further deterioration of the lower coal, which at Fernhill is represented only by a seatearth 20 ft below the upper coal. W.B.E.

Southern Rhondda area

The measures below the Gellideg have not been seen during the present survey. A trial pit at Great Western Colliety and the Upcast Shaft at Coedely Colliery were sunk to depths of 201 ft 3 in and 299 ft 3 in respectively below the Gellideg; both sections were generally similar, that at Coedely reading as follows:

At Great Western the trial pit reached nearly 100 ft below the Garw, the section of which was: top coal 10 in, rashings and coal 32 in, clod 6 in, coal 1 in. The Garw Seam has also been proved by boreholes at Britannic, Cilely, and Cwm collieries. At Britannic it is recorded as 26 in of clean coal lying 190 to 200 ft below the Five-Feet (the Gellideg being absent in washout). At Cilely 24 in of coal were proved 103 ft below the Gellideg and at Cwm 14 in of coal were penetrated 110 ft below the same seam.

The strata between the Garw and Gellideg are predominantly argillaceous, but several sandstones, up to 12 ft thick, occur at intervals at Britannic, Great Western, Cwm, and Coedely, while at Cymmer Colliery over 40 ft of 'rock' are recorded close below the Gellideg. Except at Cymmer, where 6 in of coal occur about 11 ft below the Gellideg, there is no record in this area of the thin coal which elsewhere characterizes the upper part of the measures between the Garw and the Gellideg.

The Gellideg has been worked extensively only at Cambrian, Llwynypia and Naval collieries (at all of which it was known as the Lower Five-Feet), at Cymmer Colliery (where it was called the Prince of Wales Seam), and at Cilely; in addition a few small patches have been worked beneath the gobs of the Five-Feet at Lewis Merthyr. Where not affected by washouts the seam varies from 27 in at Cwm and Maritime collieries to 5 to 5.5 ft at Cilely, Naval (including Ely). Llwynypia, and Cambrian. In the Naval and Ely areas, 12 in to 2 ft of bast occur at the base, while thin coals are developed at the top of the seam in parts of Cambrian and near the base over parts of Cymmer, Lewis Merthyr, and Great Western. The extensive washout, already noted on the west side of Eastern and Maindy continues south-eastwards into this area (see (Figure 11)). It crosses the Cambrian area in the neighbourhood of the shafts and continues into Britannic Colliery. Despite several widely spaced attempts to locate the seam at this last-named colliery it has not yet been proved, and it would seem that the washout embraces virtually the whole of this taking. The seam has not been proved at Coedely and it is possible that this colliery is also affected by the same washout.

At Lewis Merthyr, Great Western, Maritime and the greater part of Cwm collieries, the Gellideg and Five-Feet seams are close together, the separation being usually less than 2 ft. Northwards and westwards, however, the two seams split apart rapidly: in Naval and Ely pits 10 to 12 ft of strata, largely seatearth, separate the coals and these expand to 50 ft or more in the Cambrian area. Virtually the whole of the thickening is due to the development of massive sandstone, which forms the roof of the Gellideg over much of the western part of the area. Turbidity currents, associated with the influx of this sandstone, were presumably responsible for the extensive washouts in the Gellideg already described.

The Five-Feet everywhere consists of one main coal, the parting which separates the two principal leaves to the north having apparently died away through the northern margins of Cambrian and Llwynypia. This main coal varies from about 3.75 to 6.5 ft thick, being thinnest in the south-east in the Cwm taking, and thickening fairly steadily westwards to 4.5 ft at Great Western, Lewis Merthyr, and Cymmer, to 5 to 5.5 ft at Coedely, Cilely, Britannic, Naval, and Llwynypia, being thickest on the west side of the Cambrian workings. Over much of Llwynypia, Naval, and Britannic this single clean coal makes up the whole of the seam, but a thin coal, 4 to 9 in, is developed at the base, underlying 3 to 12 in of rashings, in parts of Cambrian, Cymmer, and Lewis Merthyr. Locally in Lewis Merthyr and Great Western, where the seam is very close to the Gellideg, a further few inches of coal occur immediately below. Over most of Lewis Merthyr, a bast, 4 to 10 in thick, is developed near the top of the seam, and passes locally into clod or rashings. The latter are particularly noticeable at Cwm, where 15 to 19 in of coal and rashings overlie the main coal of the seam.

The strata between the Five-Feet and the Lower Seven-Feet consist mainly of mudstones and silty mudstones with ironstone nodules, particularly in the upper part; a sandstone, usually thin but reaching 25 ft in the Ely area, occurs at about the middle. Over most of the area there is rapid variation of thickness from about 30 to 90 ft. The greatest thicknesses are recorded in a zone extending from Naval Colliery through parts of Cymmer, Lewis Merthyr, and Great Western to the north-eastern part of the Cwm taking, and there is a progressive thinning to the west and south. No fossils have been noted during the present survey from the roof of the Five-Feet, but Curvirimula subovata was collected from the roof of a 4-in dirty rider to the seam, which is locally developed in parts of Cambrian and Naval collieries. At Great Western Colliery Annularia radiata, Asterophyllites equisetiformis (Schlothehn), Neuropteris heterophylla and Sphenophyllum cuneifoliwn were collected from about 25 to 30 ft above the Five-Feet, while some 40 ft higher Cyclopteris trichomanoides occurred associated with N. heterophylla.

The Seven-Feet group of coals shows considerable variation. The Lower Seven-Feet coal, which has nowhere been worked in this area, ranges in thickness up to 21 in. It is thickest in the west, at Cambrian and in parts of Britannic; it thins eastwards and is usually less than 12 in at Cymmer and Lewis Merthyr, and appears to die out altogether in parts of Great Western, Maritime, and Cwm. The separation from the Middle Seven-Feet varies from 5 to 35 ft, being least in western parts of Lewis Merthyr and greatest at Britannic.

The Middle and Upper Seven-Feet coals form one extensively worked seam, locally called the Middle Five-Feet, at Cymmer, Lewis Merthyr, Great Western, Maritime, and Cwm collieries. At Cymmer and Lewis Merthyr the seam thickness averages about 3.5 ft, the upper leaf being 12 to 18 in, and the lower leaf 24 to 30 in. The parting dies out eastwards into Great Western and Cwm, and the single coal shows a marked thinning in the extreme south-east of the area, being only 20 to 28 in over most of Cwm.

Westwards into Cilely, Britannic, and Cambrian the Upper Seven-Feet leaf splits away and becomes a bottom coal to the Yard Seam above. In these collieries the Middle Seven-Feet (locally termed the Yard) shows a distinct westwards thickening and reaches 38 in. in the western parts of Britannic and Cambrian.

The amount of separation of the Upper Seven-Feet from the Yard is small over most of the area, but it reaches 10 to 30 ft in portions of the Lewis Merthyr and Great Western workings. The only fossils recorded are some poorly preserved Naiadites sp.in the roof of the upper leaf at Lewis Merthyr.

The Yard Seam has been known by various names: Bute at Cambrian, Llwynypia, Naval, and Britannic; Upper Five-Feet at Cymmer, Lewis Merthyr, Great Western, Maritime, and Cwm; Jubilee at Cilely, and Four-Feet at Coedely. The main coal has a thickness of 3.5 to 5.5 ft; it is thinnest in the north, in parts of Cambrian, Llwynypia, Naval and the northern parts of Lewis Merthyr and Great Western, and thickest in the south at Cwm and Coedely. Over the north-western half of the area a thin coal, 9 to 12 in, is typically developed at the base, separated from the main coal by a few inches of hard stone. Except in the south-west, and in the northern parts of Lewis Merthyr and Great Western, the Upper Seven-Feet Seam lies close beneath the Yard, and in some areas in the west, e.g. at Cambrian, is so close that it may be regarded as forming part of the seam. At Lewis Merthyr and Great Western a thin coal occurs at the top of the seam, separated from the main leaf by 2 to 12 in of rashings or bast; this thin coal appears to develop by splitting.

The strata overlying the Yard usually consist of planty mudstone with cannelly mudstone forming the roof in a number of places. At Great Western Colliery the following plants are recorded: Calamites carinatus Sternberg, Carpolithus sp. nov., Mariopteris sp.,Neuropteris gigantea and N. heterophylla.

In the eastern part of the Cwm area the separation between the Yard and the Amman Rider is less than 1 ft. North-westwards, however, this increases to 20 to 30 ft in the Great Western–Lewis Merthyr area, 45 to 60 ft at Cymmer and Cilely, 50 to 90 ft in the Naval–Llwynypia area, and thins again to about 40 ft at Cambrian. Except where thin and mainly seatearth, the strata consist of mudstones with ironstone nodules, though a few thin sandstone layers are developed locally. A dirty and impersistent coal sometimes occurs about 10 to 25 ft below the Amman Rider and may represent a split from that seam; this has been noted at Cambrian, Naval, Cymmer, Cilely, and Great Western, while a prominant band of rashings occurs in this position in the Naval and Ely shaft sections.

The Amman Rider coal itself is everywhere persistent and varies from 14 to 24 in. in thickness. A.W.W.

Ogmore-Garw-Llynfi area

The strata below the Gellideg are known only from a few underground boreholes drilled before the present survey. At Wyndham Colliery two holes proved the Garw, 13 in and 20 in thick, 125 ft below the Gellideg, the intervening strata being described as light and dark clift with a few thin rock bands. One of the holes penetrated a further 150 ft through light and dark chill ending in 6.5 ft of 'rock'. A similar section was proved at Ffaldau Colliery, 24 in of coal lying 125 ft 5 in below the Gellideg. At Caerau Colliery a borehole reached 183 ft 3 in below the Gellideg: the Garw, consisting of 12 in of coal underlain by a further 18 in of coal and rashings, is here separated from the Gellideg by 124 ft 9 in of clift, containing a few sandy beds towards the middle and numerous ironstone bands in the upper part, passing into 6 ft 3 in of seatearth at the top. The lowest beds recorded are 50 ft of clift with a few ironstone nodules. A hole at Coegnant Colliery was drilled to a depth of 167 ft below the Gellideg; though no Garw Coal was proved, the sequence was similar to that at Caerau. At St. John's Colliery two boreholes record a coal, 6 to 7 in thick, about 14 ft below the Gellideg.

At the time of survey the Gellideg had been worked only at Wyndham, Ffaldau, Garw, St. John's, and Coegnant collieries, though it had been proved by boreholes at Western, International, and Caerau. At Western several boreholes to the east of the Glyncorrwg Fault have recorded varying thicknesses up to 2 ft 4 in, and it seems possible that the seam here is still affected by the major washout, the eastern margin of which was proved at Maindy, Eastern, and Cambrian (see pp. 87, 89). Over the rest of the area the seam averages 4.5 to 5 ft of clean coal. It is thickest in the south, at Ffaldau and St. John's, where it reaches 5.5 ft, and thins northwards to about 4 ft in the northern parts of Garw.

A shelly roof to the Gellideg is recorded only at St. John's and in the south of the Coegnant workings. Here some 3 ft of mudstone overlie the coal and abundant 'mussels' occur towards the top; these latter range up for about 1 ft into silty mudstones containing sandstone bands. The fossils occur as well-preserved internal moulds, and forms allied to Carbonicola communis make up the bulk of the assemblage. Species

identified include: variants of C. communis, C. aff. cristagalli, C.cf. martini and C.cf. rhomboidalis Hind. Elsewhere the mudstone roof is not developed. In the Ogmore and Garw valleys the coal is succeeded by strong striped siltstones or fine-grained sandstones, and in the north of the area strong sandstones form the roof. In the northern part of the Coegnant taking, washouts and roof-rolls associated with this sandstone are common, trending a few degrees north of west. Similar washout conditions have recently been encountered at the northern end of the Wyndham workings, and it is interesting to note that both these isolated areas of seam failure are in line with the extensive washout extending north-eastwards from Park and Dare through Ferndale to the Cwmaman–Aberaman area (see pp. 82, 84).

Throughout the area west of the Glyncorrwg Fault about 40 to 60 ft of strata intervene between the Gellideg and the lower coal of the Five-Feet group. Much of this ground normally consists of strong silty mudstone with massive sandy and silty beds in the lower part.

In the extreme east of the area the Five-Feet has a complex section, not unlike that in the south-west part of the Park workings: thus in the neighbourhood of the Wyndham shafts it consists of top coal 52 in, clod 9 in, coal 11 in, clod 3 in, coal 24 in, rashings 3 in, coal 6 in, a thin parting being sometimes developed about 3 to 4 in from the top of the main coal. Westwards from this area a major split develops between the top coal, or Upper Five-Feet, and the lower coals, at presumably the same horizon as that previously described in the northern parts of the Rhondda and Cynon valleys (see pp. 82, 87). The Upper Five-Feet Coal, usually called the Five-Feet in the Garw Valley and the Lower Five-Feet in the Maesteg area, varies from 4 ft in the north of the area to 6k ft in the south-west, e.g. at St. John's, which implies a considerable thickening south-westwards of the top coal of the Five-Feet of the Rhondda Valley. A thin coal of 6 to 9 in usually occurs within 1 to 2 ft of the base. The separation of this from the Lower Five-Feet increases rapidly west and south-west of Wyndham and reaches 40 to 45 ft at Ffaldau and in the Maesteg area, the strata consisting mainly of strong silty mudstones. The Lower Five-Feet is itself rather complex in the Garw Valley and the following sections have been recorded:

In the Maesteg area the top and middle coals of these sections appear to be represented by 18 to 27 in of coal, which at Coegnant has a sandstone roof. The bottom coal thickens somewhat and splits away: thus, at Coegnant, it is 19 in thick with two thin partings and lies about 15 to 25 ft below the main part of the Lower Five-Feet.

Through virtually the whole of this area the roof of the Five-Feet consists of mudstone, usually somewhat soft and weak. No fauna has been collected during the present survey, but Evans and Simpson (1934, p. 450) record Naiadites sp.and fragmentary shells in the Maesteg area. The measures upwards to the base of the lowest coal of the Seven-Feet group consist of 12 to 20 ft of strong silty mudstones passing into seatearth.

Everywhere within the area the Seven-Feet group of seams comprises three distinct coals, covering in all some 27 to 100 ft of strata. These coals represent the same three splits as those described in the previous areas. The Lower Seven-Feet (locally called the Upper Five-Feet or Little Seam) shows a distinct thickening when traced from the east into this area, and a parting is developed a little below the middle of the seam. This parting makes its appearance between Park, Cambrian, and Britannic collieries on the one hand and Western on the other, and is present in all the collieries. At Wyndham and Garw this dirt band may reach as much as 8 in, but elsewhere it seldom exceeds 1.5 in and may be less than 0.5 in. In total thickness the coal averages 2.75 to 3.25 ft, the top leaf being 19 to 24 in and the lower about 14 to 18 in. The seam has been worked on a small scale in the Llynfi Valley and at Garw Colliery. The roof invariably consists of mudstone, which in the western part of the area may be dark, soft and slickensided. This passes upwards into strong mudstones and silty mudstones, with sandstone bands developed towards the top of the cyclothem. These beds vary from 20 to 50 ft, being thinnest in the east of the area and thickest in the collieries of the Llynfi Valley.

The Middle Seven-Feet also becomes more complex when traced westwards from the Rhondda Valley area: at least two, and possibly three, new partings develop and thicken as the seam is traced towards Maesteg. The following representative sections illustrate this:

The single coal of the Rhondda Valleys has already developed partings near the top and bottom in the south-western parts of Park Colliery; both these partings are well marked in Western, Garw, and Ffaldau, though in Wyndham the upper appears to have disappeared again. The lowest of the resulting three coals thickens westwards, though it always remains subordinate to the leaf above. The detailed correlation of the seam between the Garw and Llynfi valleys is obscure; it would seem that the top leaf either dies out or rejoins the middle coal, while the lowest coal again splits near its base. Except at Western Colliery the seam has not been worked in this area. The partings developed in the coal consist normally of clod or seatearth while the roof of the uppermost coal is usually silty mudstone with plants and plant debris.

The separation between the Middle and Upper Seven-Feet seams is very variable: at Caerau it appears to be but 3 ft; in the Garw Valley it is about 12 to 15 ft but this increases eastwards to about 35 to 40 ft at Wyndham and Western, and westwards to 50 to 65 ft in the southern collieries of the Maesteg area. The strata consist mainly of mudstone and silty mudstone with numerous ironstone bands, and in the Maesteg area subordinate beds of sandstone appear.

The Upper Seven-Feet varies from 9 to 15 in and normally consists of a single coal. In the eastern parts of the area, at Western and Wyndham, the coal lies close beneath the Yard Seam, the separation being as little as 1 in. Elsewhere the two seams have split away from one another. In the Garw Valley 15 to 20 ft of mudstone and mudstone-seatearth separate the two seams, while in the Llynfi Valley area this is reduced to 7 to 15 ft.

The Yard (locally known as the Three-Feet-Ten at Western, the Seven-Feet at Wyndham and in the Garw Valley, the No. 8 Seam at St. John's, Caerau, and Coegnant, and the New Seam at Garth Merthyr) everywhere consists of two coals. The upper and main coal is 27 to 41 in thick, being thickest in the east of the area, while the lower coal is 8 to 21 in, the higher figure being again recorded in the Ogmore Valley. The parting between the two coals is usually a hard 'stone', varying in thickness from 2 in to about 12 in. Locally, as in Ffaldau and Garw collieries, the parting consists of clod or rashings. In the Maesteg area a 0.5-in band of cannel lies in the immediate roof of the seam.

The roof of the Yard everywhere consists of grey mudstone with thin ironstone bands and it contains the usual characteristic 'mussel' fauna, the principal members of which are variants of _Anthracosia regularis and Carbonicola venusta.In addition the following species have also been identified from most of the collieries: A.sp.cf. aquilina,cf. C. oslancis, Anthracosphaerium sp.and Naiadites spp.belonging to the productus/quadratus group. At Ffaldau and Garw collieries Belinurus bellulus (Konig) has also been recorded.

The measures between the Yard and the Amman Rider vary from about 60 to 100 ft, being, in general, thinnest in the Ogmore Valley collieries and thickest in those of the Maesteg area. A thin coal of 6 to 12 in seems to be everywhere developed about midway between the Yard and the Amman Rider, and this may represent a split from the Amman Rider, as previously noted in the Rhondda valleys (pp. 86, 91). The strata intervening between these coals are made up mainly of mudstone and silty mudstone with numerous thin bands and nodules of ironstone in the lower cyclothem. Locally, as at Ffaldau and in parts of the Llynfi Valley, ribs of sandstone are developed in the higher cyclothem, while at Ffaldau a prominant band of 'white rock' is recorded not far below the thin coal.

The Amman Rider is about 22 to 30 in thick. In the Ogmore and Garw valleys there is usually a thin parting near the top. In the Maesteg area the coal is again composite, consisting normally of three leaves up to 10 in thick separated by partings of 1 to 2 in. It appears likely that the lower leaf splits away when the seam is traced westwards into the Bryn area. A. W. W., W. B. E.

Avan Valley

At the time of survey these measures had been proved only at Bryn and Avon collieries, and were accessible at only the former.

The Garw is known only at Bryn Colliery; it is 27 in thick and lies 190 ft below the Gellideg horizon. The intervening measures are predominantly argillaceous, but thin sandstones occur and bands of ironstone are numerous. At Cwmavon (in the Swansea district, one-inch Sheet 247) several of these ironstones were formerly worked, and some of these workings appear to have extended eastwards into the present district. As in other areas where these ironstones were worked, the individual productive layers were named by the old miners: the lowest horizon, the Ram's Head Mine, lies about 100 ft below the Gellideg, the Yellow Mine some 25 ft higher, and the Black Mine and Spotted Pins are associated with an 11-in coal lying 40 ft below the Gellideg.

The Gellideg was worked at Bryn as the Lower Vein; it varies from 4 ft in the north of the taking to about 6 ft in the south. In the northern parts of the colliery it has a sandstone roof which in places has cut out part or all of the seam. The general trend of such wash-outs and 'rolls' varies between west to east and south-west to north-east. In the south of the taking the seam is reported to have a mudstone roof containing abundant 'mussels', presumably the same fauna as that known in the Llynfi Valley (see p. 91).

The main part of the Lower Five-Feet lies between 25 and 70 ft above the Gellideg at Bryn, the intervening measures being mainly sandy mudstones. It has been called the Yard and is said to be up to 3 ft thick, although where seen during the present survey it comprised barely 12 in of dirty coal. Some 10 to 30 ft higher there is another 12 in of coal, of sulphurous appearance, as in the Llynfi Valley.

The Upper Five-Feet is about 120 ft above the Gellideg at Bryn, where it has been extensively worked under the name Middle Vein. It varies in thickness between 4.5 and 6 ft, while further thin coal horizons lie about 4 ft and 10 ft beneath it.

At Avon Colliery the 'K' Seam was formerly worked with the following section: top coal 28 in, mudstone 1 to 2 in, coal 4 in, mudstone 1 to 2 in, coal (with a thin parting locally towards the base) 18 in. This section is similar to that of the central parts of the Rhondda Fawr and it may represent the whole of the Five-Feet.

Both at Bryn and Avon the measures between the Five-Feet and Lower Seven-Feet usually comprise some 30 ft of unfossiliferous silty mudstones containing numerous thin ironstone bands. In the west of the Bryn taking, however, they are reduced to only a few feet and at Cwmavon are measurable in inches.

The three main coals of the Seven-Feet group, where proved, comprise separate split seams, and are likely to remain split in those northern parts of the area yet unexplored. The Lower Seven-Feet has been worked on a small scale at Bryn as the Little Vein, a coal 2.5 to 3 ft thick. At Avon its equivalent appears to be the 'H' Seam, which is recorded as having a top coal of 19 in, rashings 11 in, and bottom coal 9 in. The roof at Bryn is slickensided mudstone yielding only worm-tracks. Some 30 to 45 ft of sandy mudstones separate it from the Middle Seven-Feet or No. 6 Seam of Bryn. This seam here comprises three leaves: top coal 18 to 24 in, seatearth 12 in, middle coal 24 in, seatearth 5 ft, rashings 6 to 12 in, bottom coal 12 in. The roof is a barren silty mudstone which continues upwards through 30 to 35 ft to the No. 5 Seam, the local equivalent of the Upper Seven-Feet, which is about 2 ft thick. At Avon the Middle Seven-Feet appears to separate into two seams some 36 ft apart: the lower, seam G, is 26 in thick, with a 4-in rider close below; the upper is 20 in thick. The Upper Seven-Feet is here a thin rider coal close below the 'F' Seam. The roof, where seen at Bryn, is a yellow-weathering mudstone with fragmentary plants; it is overlain by silty mudstones which continue to the seatearth of the Yard, the two seams being 8 ft apart at Avon and 15 to 20 ft at Bryn.

The Yard has been called the No. 4 Seam at Bryn and the 'F' Seam at Avon. At Bryn it is about 3 ft thick with a thin parting about 9 in from the base and up to 1 in of cannel in its roof. At Avon it is about 2.75 ft thick. Its roof, at Bryn, consists of dark mudstone with ironstone bands, the lowest foot of which yielded an abundant fauna of Anthracosia regularis,and variants of Carbonicola venusta and C. oslancis,together with Planolites montanus.

The sequence continues at Bryn through 70 to 75 ft of mudstones and sandy mudstones to the Amman Rider. Two 10-in coals occur; one about 30 ft and the other about 15 ft below the top. Where seen, the Rider coal was badly exposed in disturbed ground; a single coal 10 in thick was measured, but the seam has also been recorded as two thin leaves, 3 to 4 in thick, separated by 18 in of seatearth. At Avon also the coal was seen only in disturbed conditions and its measured thickness of 18 in is unreliable. W.B.E.

South Crop

West of the Ogmore Valley

The strata from the Garw to the Gellideg are known in detail only from the recently drilled Margam Park boreholes. This portion of Nos. 1 and 2 boreholes has been described elsewhere (Woodland and others 1957b, pp. 45, 48). No. 3 Borehole [SS 8161 8730], situated 1900 yd E. 35° N. of Margam Church, showed the following section:

The Garw or Cribbwr Fach Seam was formerly worked from bell-pits near its crop in the vicinity of the Porthcawl Branch railway between Kenfig Hill and Cefn Junction [SS 8590 8340]: farther east several drift mines worked the coal on a small scale, as for example, Bankershill Slant [SS 8699 8323], 1070 yd W. 19° S. of Park Slip Colliery. The dark roof shales with fish-remains, noted in the above borehole, weather at surface to brittle paper-shales, and these are a characteristic feature of the spoil from the bell-pits. The seam is said to vary from 2 ft 3 in to 3 ft of clean coal.

As on the eastern parts of the North Crop (Robertson 1927, pp. 63–7; 1933, pp. 100–2) the strata between the Garw and the Gellideg (or Cribbwr Fawr) on the western part of the South Crop contain important developments of ironstone. The ironstone layers, which vary up to 15 in. in thickness, were formerly worked extensively from open patchworks in a strip south of Nantiorweth-goch from Cwm-ffos westwards towards Kenfig Hill. They were also mined at several of the collieries in the same area, for example, Bryndu Colliery; while an old slip-mine [SS 8487 8338] 370 yd W.S.W. of the Old Cefn Slip Colliery was driven for the sole purpose of working the ironstones. At Bryndu Colliery the following section, which names the principal productive ironstone horizons, is recorded:

The Garw Pins are reported as being the principal veins, and at Bryndu were taken when drawing back after the workings of the Garw coal. Most of the patch-workings are much degraded and grown over, and few exposures except of sandstones and strong silty mudstones are now to be seen. An old quarry [SS 8526 8337], 1450 yd E. 22° N. of Kenfig Hill Church shows:

Apart from the ironstones obtained during the working of the quarry the associated mudstones were utilized to produce a good red brick.

The seams from the Gellideg upwards to the Yard have been worked fairly extensively in the old collieries between Pyle and Bryncethin. However, only at the western end of this tract, at Newlands Colliery, can the measures now be seen and a complete picture of the succession made out. All the other old drift mines and pits have long since been abandoned ; in these, except for that portion of the sequence between the Gellideg and Five-Feet seams, the measures are subject to a great deal of disturbance, and since the only information now available consists of unreliable records of old cross-measures roadways, in which no attempt was made to unravel the structure or to record the true sections of the seams, exact correlation is seldom possible. In consequence, it is not thought worth while to reproduce the sections given in the plate facing p. 56 of the Second Edition of this memoir. Reliable sections of the measures on the South Crop between the Newlands–Cribbwr Fawr–Bryndu area in the west and Llanharan Colliery some ten miles to the east do not exist. Considerable expansion, accompanied by seam-splitting, occurs in this portion of the sequence from Llanharan to Newlands and little is known about the exact nature of the variations involved.

These measures crop out in a narrow east-west strip, varying from 150 to nearly 300 yd wide, extending from the neighbourhood of Bryndu Colliery to Evanstown, where they pass beneath the alluvial deposits of the Ogmore River. In the area between Old Cefn and Park Slip the surface is marked by numerous bell-pits and by extensive patchworks. In this western tract the strata between the Gellideg and Upper Five-Feet are relatively competent and the records in this portion of the sequence are generally much more reliable than those for the rocks above.

The crop of the Gellideg or Cribbwr Fawr between Kenfig Hill and Aberkenfig can be followed fairly easily, for numerous bell-pits were formerly sunk to win the seam, while, in addition, a number of adits or slips were driven on it, as for example Old Cefn Slip [SS 8515 8350], Jenkin's Slip [SS 8537 8346], Wain Arw Slip [SS 8570 8342], Cwm-ffos [SS 8704 8337] and New Cribbwr [SS 8894 8370]. The seam was worked extensively at Cribbwr Fawr, Bryndu, Cefn, and Park collieries, at all of which it appears to have been relatively undisturbed. At Park Slip the lowest workings, in the so-called New Seam, are believed to have been in the Gellideg, a thrust-fault separating the two sets of workings. The seam consists everywhere in this area of a clean coal ; it is thickest at Newlands Colliery, and the eastward thinning is indicated by the following sections: Newlands 9 ft 3 in to 8 ft 10 in, Cribbwr Fawr 8 ft to 7 ft, Bryndu 6 ft 6 in to 6 ft, Cefn 6 ft 4 in to 5 ft 6 in, and Park Slip 5 ft 6 in to 4 ft 6 in. At Ynysawdre the seam appears to be about 5 ft, while at the old Barrow Pit, Bryncethin, a seam called the Cribbwr, but by no means certainly the Gellideg, was said to be 3 ft 8 in. in thickness. In the Cefn area a thin bast, up to 4 in thick, is recorded at the top of the seam.

Over most of the area the roof of the Gellideg Seam consists of rather weak dark shaly mudstone, which in the Margam Park boreholes yielded the following fauna: Anthraconaia cf. fugax Eagar, Anthracosphaerium sp., Carbonicola cf. communis, C.cf. martini, C.cf. pseudorobusta, Curvirimula cf. subovata and Naiadites sp.In parts of the Park Slip area this mudstone is cut out by massive sandstone (which normally lies a little way above the shell-bed) and this forms a massive roof to the seam.

The strata from the Gellideg to the Lower Five-Feet (or Five-Quarter) Seam vary in thickness from 30 ft in the Park Slip area to 70 ft at Newlands. They consist of strong mudstones or silty mudstones, with bands of sandstone, particularly in the middle of the cyclothem. In the Margam Park No. 2 Borehole there is an apparently abnormal development of Curvirimula-bearing mudstones as shown by the following section:

The Lower Five-Feet (or Five-Quarter) consists everywhere of a top coal of 42 to 48 in and a bottom coal of 10 to 24 in separated by about 6 in to 3 ft of clod or fireclay. The coal usually has a strong roof and floor, and, in consequence, the main haulage slips at Cefn and Park collieries were driven in this seam. During 1924 Park Slip was reopened and exploratory work carried out in the seam in the neighbourhood of the main slant. The section proved to be very irregular, and this, associated with the massive sandstone roof, is suggestive of wash-out conditions.

The Upper Five-Feet (or Nine-Feet of the South Crop) usually lies about 50 to 70 ft above the Lower Five-Feet. The intervening measures are strong and consist mainly of massive mudstone with sandstone, and include a coal, locally called the Danllyd or Fiery, about 2 ft thick, in a position some 6 to 20 ft below the Upper Five-Feet. This coal appears to correlate with a thin coal near the base of the top coal of the Five-Feet in the north-east parts of the district. The Upper Five-Feet itself is always thick, but the available sections are now so few and variable that little can be said concerning its lateral behaviour. The following sections have been noted:

The measures from the Upper Five-Feet to the Lower Seven-Feet, which are essentially argillaceous in character, show the usual east to west thickening, being about 60 ft at Park Slip and Cefn, and about 100 ft in the Cribbwr Fawr–Newlands area. At Newlands Colliery a 9-in coal, 19 ft below the Lower Seven-Feet, carries a roof of grey shaly mudstone with Carbonicola aff. oslancis and Naiadites flexuosus Dix and Trueman overlying a 10-in band of dark cannelly shale.

The Lower Seven-Feet (or Slatog Fawr) has a section similar to that in the Garw and Ogmore valleys. Recorded sections are reliable only in the Bryndu–Newlands area, where a top coal, 24 to 30 in thick, is separated from a bottom coal of 12 to 15 in by 0.5 to 8 in of clod or rashings. At Newlands the roof consists of weak mudstone carrying plant debris including Asterophyllites charaeformis (Sternberg), Diplotmema furcatum (Brongniart), Mariopteris muricata and ?Neuropteris obliqua.

The distance from the Lower Seven-Feet to the Middle Seven-Feet seems to be very variable, though few of the records are dependable. At Park Colliery the seams cannot be identified with certainty; at Cefn the two seams are said to be 90 ft apart, while at Bryndu only 22 ft appears to separate them; in Newlands and Cribbwr Fawr figures of 65 ft and 90 ft respectively seem reliable. The strata between the two seams are essentially argillaceous, and at Newlands a thin coal occurs about 10 ft below the Middle Seven-Feet.

The Middle Seven-Feet (or Slatog Fach) is a composite seam which, because of the large amount of dirt developed within it, has rarely been worked. At Newlands the following section was measured: roof of strong barren mudstone, carbonaceous shale with streaks of coal 2 in, coal 29 in, coal and dirt in bands 7 in, seatearth 3 to 20 in, coal 16 in, seatearth 4 in, coal with thin parting 4 in. At Cribbwr Fawr the top coal was 34 to 35 in, while at Bryndu a section reads: coal 30 in, clod 15 in, coal 15 in.

The ground between the Middle Seven-Feet and the Yard consists of rather silty mudstone with sandstone bands and varies from about 10 ft at the Park Slip opencast site to about 55 ft at Newlands. A thin coal, usually found a short distance below the Yard, is presumed to represent the Upper Seven-Feet.

The Yard (or Six-Feet of the South Crop) consists of 4 to 4.5 ft of clean coal at Newlands and Cribbwr Fawr, and it appears to thicken eastwards, 6 ft being recorded at Cefn and 7 ft at the Ironworks Colliery nearby. A roof of smooth grey mudstone with ironstone nodules yielded scattered solid 'mussels' at Newlands: Anthracosia regularis,Carbonicola venusta, C.cf. oslancis together with squashed Naiadites sp.intermediate between productus and quadrates.

At Park Slip opencast site the Amman Rider Seam was separated from the presumed Yard by only 1 ft of seatearth. Elsewhere, however, a fully developed cyclothem intervenes between the two seams. At Bryndu 87 ft of clift with a prominent sandstone band in the middle passes upwards into a thick seatearth immediately beneath the lower split of the Amman.

The Amman Rider is usually split into two distinct coals, both of which are composite and variable, separated by 15 to 20 ft of strata. The upper seam, known locally as the Wythien Fach or Little Seam, normally consists of two or three leaves having an overall thickness of 2 to 3 ft.

East of the Ogmore Valley

On Hirwaun Common the crop of the Garw can be followed by means of a double line of closely spaced bell-pits which extends for over a mile from about 100 yd N.E. of Heol-y-llan [SS 9374 8332] to about 500 yd S. of Caeaucerrig [SS 9529 8359]. The seam, about 2 ft 3 in thick and containing much disseminated pyrite, was exposed by the Opencast Executive in a trench [SS 9184 8371] near the western end of Hirwaun Common, about 750 yd S.E. of Bryncethin Church, where the characteristic roof of dark blue-grey shale with fish remains was observed. The measures exposed by the trench were badly disturbed and it was impossible to determine the true succession of strata between the Garw and Gellideg seams. However, three distinct bands of dark slightly silty shale and mudstone, carrying the thick calcite shells typical of these beds, were seen. The lowest, composed of very thinly bedded black shale, occurred about 70 to 80 ft above the Garw and carried: Spirorbis sp., Carbonicola cf. centralis (J. de C. Sowerby), C.aff. communis,fragmentary Curvirimula sp.;Geisina arcuata and fish remains. A dark silty mudstone, some 40 ft higher, contained Carbonicola aff. communis and Curvirimula cf. candela,while 20 to 30 ft above lay a dark micaceous silty mudstone with Spirorbis sp., Carbonicola pseudorobusta, C.cf. rhindi (Brown), Curvirimula subovata, Naiadites cf. flexuosus, Geisina arcuata and fish remains.

At Llanharan a coal, 9 in thick, was encountered about 60 ft below the Cribbwr (Gellideg and Five-Feet) Seam in the Nos. 2 and 3 horizons main cross-measures drifts in the south-east sector of the colliery. This coal, which presumably correlates with the horizon of the 2-in cannel in the Margam Park No. 3 Borehole (see above), carries a much-slickensided roof of grey mudstone with C.cf. subovata and fish remains. A prominent Shelly bed 20 ft higher marks the highest of the "pseudorobusta' bands: variants of Carbonicola communis, C. martini and C. pseudorobusta well preserved in dark shaly mudstone, and this is overlain by grey mudstone containing Spirorbis sp., C.rhindi, Geisina arcuata and fish remains.

At Ynysawdre the correlation of the coals above the Five-Feet is doubtful. The following section from the Gellideg to the supposed Amman horizon was recorded in the North Drift from pit-bottom:

The marked thinning of the measures between the Gellideg and Upper Five-Feet foreshadows the coalescence of the coals to form a single seam at Llanharan and in the area to the east. These lower coals were undoubtedly proved at Bryncethin (Barrow) Pits, but the sections which survive are not capable of interpretation. The seams must crop out on the southern reaches of Hirwaun Common, but no natural exposures exist and during the various stages of opencast activity in this area the strata exposed were always so highly disturbed that no true stratigraphical sections could be made.

Eastwards of Penprysg (at the eastern end of Hirwaun Common) the seams pass beneath an extensive cover of glacial deposits and it is only at Werntarw and Llanharan collieries that they can be studied again. The Llanharan sequence shows marked attenuation together with considerable re-uniting of the coals accompanied by reduction of the total coal thickness. The following section was taken in the No. 1 Conway cross-measures drift:

The coals of the Upper and Lower Five-Feet of the ground to the west have joined to form a single seam, whereas the Gellideg lies very close below. These two coals together were formerly known as the Cribbwr or No. 12 Seam. The Seven-Feet group of coals have deteriorated considerably; the 2 to 2.5-ft of coal (No. 11 Seam), which occur some 1 ft to 30 ft below the Yard (No. 10), probably represent the combined Upper and Middle Seven-Feet, while the Lower Seven-Feet is developed only locally in the colliery. The Yard is 5 to 6 ft thick and frequently has a strong sandstone roof; under these conditions the seam is subject to wash-outs.

At Werntarw Colliery, about 2.5 miles W.N.W. of Llanharan, the Yard (originally called the Six-Feet) has been worked fairly extensively; 5 to 5.5 ft thick, it is separated from the Amman Rider by 44 ft of mudstone with ironstone bands and nodules. The Rider consists of a clean coal 23 in thick. Nothing is known with certainty of the sequence below the Yard.

There is no further evidence concerning these lower coals on the Sheet to the east of Llanharan, but sections are recorded on the abandonment plans of the old South Cambria Colliery, on the south-west margin of the Newport district. Here the Cribbwr Seam is reported to be a clean coal 5 ft 6 in thick, representing presumably both the Five-Feet and the Gellideg together. The Brass Seam above (with a section given as: coal 6 ft, clod 3 to 8 in, coal 18 to 24 in) presumably represents the Yard together with the Upper and Middle Seven-Feet. These sections, therefore, show further attenuation of the measures, a process which continues when these strata are traced farther eastwards to Nantgarw and beyond. A.W.W.

References

EVANS, W. and SIMPSON, B. 1934. The Coal Measures of the Maesteg District, South Wales. Proc. S.Wales Inst. Eng., 49, 447–75.

MOORE, L. R. and Cox, A. H. 1943. The geological sequence in the Taff Valley, and its correlation with the Rhondda Valley sequence. Proc. S. Wales Inst. Eng., 59, 189–304.

ROBERTSON, T. 1927. The Geology of the South Wales Coalfield. Part II. Abergavenny. 2nd edit. Mem.Geol. Surv.

ROBERTSON, T. 1933. The Geology of the South Wales Coalfield. Part V. The Country around Merthyr Tydfil. 2nd edit. Mem. Geol. Surv.

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.

Chapter 5 Middle Coal Measures: details

A. Amman Marine Band to Cefn Coed Marine Band

(Figure 28) shows generalized sections of these measures for each of the areas described in the following text.

Cynon-Taff area

The measures between the Amman Rider and the Bute are thinnest in the Penrikyber and Lady Windsor colliery areas, where they vary from 12 to 16 ft; over most of the rest of the area they range from 20 to 40 ft. The greatest thicknesses occur at Fforchaman and Cwmneol, where 64 and 58 ft are recorded, but even here as little as 20 ft may separate the two seams locally. The Amman Marine Band is present almost everywhere, and its failure in the Navigation Colliery North Shaft is presumably due to wash-out at the base of a 40-ft sandstone. The band is up to 4.5 ft thick, and usually consists of blue-grey blocky mudstone with abundant pyritic granules, Tucoids', and nodules in the lower part, and blue-grey micaceous silty shale with ironstones in the upper part. The pyritic mudstone carries small Lingula mytilloides,associated with rare Orbiculoidea sp., Donaldina?, Dunbarella sp.and Nuculana sp.,and the silty shale is rich in larger specimens of Lingula.The marine strata are directly followed by smooth mudstone which everywhere contains poorly preserved 'mussels' including Anthraconaia cf. williamsoni,Anthracosia cf. concinna, A.aff. ovum, A.cf. phrygiana, A.cf. regularis and Naiadites quadratus; Spirorbis sp.has also been recorded. The rest of the cyclothem is mainly argillaceous, though one or more sandstone bands commonly occur in the middle part.

The Bute is fairly constant in character throughout most of the area: two coals, each 21 to 30 in thick, are separated by a well-developed clod or seatearth. The seam has been worked at Lady Windsor and the collieries of the Aberaman–Cwmaman area, where the clod is less than 2 ft. Elsewhere the clod thickens and the individual coals are not considered worth working; at Merthyr Vale, the separation is as much as 15 ft. Little is known about the seam at Penrikyber Colliery, where only one coal appears to be represented in the shafts, and it is possible that wash-out conditions affect the seam here as they do at Aberaman and Cwmneol collieries, where the top coal is locally absent.

At Merthyr Vale the bottom coal carries a fauna of small solid 'mussels', including Anthracosphaerium exiguum and Naiadites aff. subtruncatus,in blocky grey mudstone. Few fossils are found in the roof of the top coal: at Fforchaman poorly preserved squashed shells, including Anthraconaia aff. williamsoni,occur in a thin layer immediately above the coal, and N.cf. quadratus has also been found.

The strata between the Bute and the Nine-Feet consist of mudstone and silty mudstone with numerous ironstone bands and nodules, with, locally, a few thin sandstones in the middle. The thickness ranges between about 10 and 80 ft, and the variation is very rapid and local: thus in the Aberaman–Cwmaman area figures of 17 to 50 ft are recorded; at Deep Duffryn 18 to 75 ft; at Penrikyber 55 to 80 ft; at Lady Windsor only 20 ft separate the two seams.

In much of the area, and more particularly in the north, the Nine-Feet is very disturbed. The disturbance is largely confined to the plane of the seam and is commonly associated with the development of thick structural rashings, especially in the roof. The coal varies in thickness from 4 ft to 15 ft or more, and is so mixed with dirt and rashings that the original section is difficult to make out. North of Mountain Ash a parting, locally associated with cannel, occurs about 3.5 to 4 ft from the top of the coal, and locally, as in Deep Duffryn and Merthyr Vale, may thicken to 12 ft. In the Cwm Cynon, Penrikyber and Lady Windsor areas, a parting appears in the bottom coal, while at the same time the top separation thins considerably. This lower split develops westwards and south-westwards to produce the distinct Upper Nine-Feet and the Lower Nine-Feet seams. The following sections illustrate the variations.

At Merthyr Vale and the collieries in the south of the Cynon Valley the roof of the Nine-Feet consists of dark carbonaceous shale with abundant 'mussels': Anthraconaia aff.salteri, Anthracosia aff. beaniana, A. sp.cf. disjuncta,and variants of the group represented by the species A. aquilina,A. ovum, A. phrygiana and A. retrotracta.At Aberaman the Shelly roof fails, and in the strong clift above the disturbed coal the following plants were found: Asterophyllites equisitiformis, Calamites suckowi Brongniart, Diplotmema furcatum,Neuropteris heterophylla, Palaeostachya sp., Pinnularia capillacea Lindley and Hutton, Sphenophyllum cuneifolium and Sphenopteris trigonophylla.

Between the Nine-Feet and the Six-Feet the measures are irregular in character. This is mainly due to the variable development of the individual coals of the Red Vein group. In the Cwmaman area these are concentrated into a single compound seam, usually with three distinct leaves, e.g. coal 15 in, fireclay 31 in, coal 27 in, clod 3 in, coal 22 in. Eastwards and south-eastwards into the Aberaman and Mountain Ash areas the coals split apart and thin markedly. Thus at Lower Duffryn Colliery four coals, each 4 to 8 in thick, are recorded through 25 ft of strata; at Merthyr Vale three coals, 3 to 12 in thick, cover 44) to 50 ft. At Navigation Colliery no coals are present in the shafts, though a thick fireclay horizon 40 ft or so above the Nine-Feet may indicate the general position of the group. At Penrikyber and Cwm Cynon two thin coals associated with much seatearth occur between the Nine-Feet and the Six-Feet. Towards Lady Windsor the coals improve in thickness, for at that colliery 21 in of coal are recorded 74 ft above the Nine-Feet, as well as a coal 12 in thick 27 ft still higher and near the base of the Six-Feet.

Between the Nine-Feet and the Red Vein group there are usually some 40 to 70 ft of strata, mainly mudstones with ironstones together with a few intercalated sandstones in the upper half of the cyclothem. The measures associated with the Red Vein coals and extending to the Six-Feet are normally wholly argillaceous with a great deal of seatearth. In the Aberaman–Cwmaman area, where the Red Vein forms a distinct seam, 10 to 50 ft of measures separate it from the Six-Feet, the thickness increasing from east to west, while in the Mountain Ash area the corresponding strata measure 30 to 45 ft.

The only fossils associated with the Red Vein group in this area were found at Cwm Cynon and Penrikyber collieries. At both localities Anthraconaia cf. curtata and Naiadites cf. productus occur 6 to 24 in above a thin coal about 50 ft above the Nine-Feet.

In the south and east of the area the Six-Feet is a compound seam consisting of three main coals:

Locally, as at Merthyr Vale, the middle coal is associated with bast, and a further thin coal occurs between it and the top coal. Northwards and westwards from Deep Duffryn Colliery the top coal splits away and becomes the Upper Six-Feet (No. 1 Yard); the lower coal—the Lower Six-Feet—is similar to the middle and lower coals of the above sections, with the difference that its upper leaf thins considerably northwards:

Where the two seams are apart the separating measures, which may be as much as 70 ft thick in the extreme north-west of Cwmaman, consist of mudstones with ironstones, and with some local sandy developments in the middle part.

The Lower Six-Feet, where it develops a mudstone roof, as at Aberaman and Fforchaman, usually yields Lioestheria striata associated with Naiadites obliquus Dix and Trueman. At Merthyr Vale, Penrikyber and Lady Windsor, the roof of the Six-Feet contains abundant 'mussels', including Anthracosia cf. aquilina, A.aff. atra, A.cf. lateralis, A. sp.cf. phrygiana, A. planitumida,and Naiadites cf. alatus Trueman and Weir. Lepidodendron ophiurus, Neuropteris heterophylla and Trigonocarpus parkinsoni Brongniart have also been recorded.

About 12 to 55 ft separate the Six-Feet (or Upper Six-Feet) from the Four-Feet. The seams are closest together in the Aberaman-Cwmaman area, while the greatest separations occur in parts of Deep Duffryn and in the Penrikyber-Lady Windsor area. The strata are made up mainly of mudstones with ironstones, but sandy intercalations are noted in Deep Duffryn and Navigation.

In the Fforchaman–Aberaman–Deep Duffryn–Merthyr Vale region the Four-Feet consists of 5.5 to 6 ft of clean coal. Formerly it was worked extensively but is now largely exhausted. In other parts of the area three partings develop, giving the four-leaf section which becomes characteristic of the seam in the Rhondda Fach Valley. In the extreme north-west, in the old Cwmaman workings, both upper and lower partings are present, giving a section: coal 19 in, clod 2 in, coal 48 in, clod 10 in, coal 10 in. South of Deep Duffryn in the Cynon Valley only the lower parting appears, e.g. at Cwm Cynon: coal 66 in, clod 4 in, coal 16 in. In the extreme south of the area all three partings are present at Lady Windsor: coal 15 in, stone 1 in, coal 15 in, stone 1 in, coal 23 in, rashings 3 in, coal 19 in.

The roof of the Four-Feet normally consists of blocky rather weak mudstone with ironstone nodules, and non-marine lamellibranchs often occur in the bottom 1.25 ft. These are particularly abundant at Cwmneol and Lady Windsor and include: Anthraconaia sp. nov.(cf. curtata), A. sp.(cf. lanceolata),Anthracosia aff. atra, Anthracosphaerium cf. exiguum,A.cf. propinquum, A. sp.intermediate between propinquum and turgidum, Naiadites obliquus,and N. sp.cf. alatus. Spirorbis sp.and the plants Diplotmema sturi Gothan, and Lepidodendron rimosum Sternberg have also been recorded during the present survey.

20 to 50 ft of strata normally separate the Four-Feet and Two-Feet-Nine, though thicknesses of 70 ft occur at Cwmneol and Merthyr Vale. The two seams are closest together in the Mountain Ash–Aberaman area, where the measures are wholly argillaceous; where the separation is markedly thicker, sandstones appear in the middle and lower portion of the cyclothem.

Northwards and westwards of Deep Duffryn Colliery the Two-Feet-Nine forms a single seam, 24 to 34 in thick, with one or two thin coals at or near the base. Typical sections are:

To the south and east of Deep Duffryn the seam section is complicated by the fact that the compound Three Coals, which is clearly discrete in the ground to the north, here rests close upon the Two-Feet-Nine: at Cwm Cynon the section reads: coal 19 in, fireclay 27 in, coal 8.5 in, fireclay with streaks of coal 6 in, coal 5 in, fireclay 7 in, coal 13 in, rashings and fireclay 24 in, coal (Two-Feet-Nine) 30 in; and at Penrikyber: coal 20 in, fireclay 26 in, coal 10 in, fireclay 27 in, coal 14 in, fireclay 33 in, coal (Two-Feet-Nine) 20 in.

Only in this last described area have the measures above the Two-Feet-Nine been seen during the present survey. Elsewhere they are known only from shaft records, from which correlation of the various horizons must be inferred. North and north-west of Deep Duffryn the Three Coals forms a distinct though compound and inferior seam, lying about 30 to 50 ft above the Two-Feet-Nine. It consists usually of three coals varying from 3 to 10 in thick separated by varying amounts of fireclay or rashings; locally as at Deep Duffryn and Cwmneol five coals may be present.

Fossils have been collected from the roof of the Three Coals only at Cwm Cynon Colliery. The following forms, concentrated in a thin layer 2.25 ft above the top coal, have been identified: Anthraconaia sp. nov.cf. cymbula,Anthracosia aff. atra, Anthracosphaerium ct. affine,A. sp.cf. exiguum,variants of A.propinquum,together with Carbonita hutnilis.

The strata between the Three Coals and the presumed position of the Cefn Coed Marine Band total about 85 to 95 ft in thickness. They vary a good deal in character, but much sandstone and seatearth are usually present. A coal about 3 in thick appears to underlie the Cefn Coed Marine Band in most cases, but this is not always recorded in the shaft logs, e.g. at Penrikyber and Lady Windsor, where the marine band may rest directly on a seatearth. In the Abercwmboi shaft a 2-in coal, 33 ft above the Three Coals, may carry the Hafod Heulog Marine Band, and a cannel and blackband horizon 14 ft still higher may represent the position of the Britannic Marine Band. At Cwmneol these two horizons are 63 and 76 ft respectively above the Three Coals. At Lady Windsor no coals are recorded between the Three Coals equivalent and the Gorllwyn horizon, seatearths carrying the marine strata having presumably passed unnoticed during the sinking of the shafts. A.W.W.

Rhondda Fach area

The Amman Marine Band has yielded only Lingula mytilloides in this area during the present survey, though Orbiculoidea sp.and Productus sp.were recorded by Moore and Cox (1943, p. 226) at National Colliery. The marine strata vary from about 3 to 6 ft in thickness and are succeeded by mudstone containing 'ghost'-like 'mussels'. At Mardy Colliery the following section was measured: striped silty mudstone with plant debris and 'ghosts' of stunted Anthracosia spp. at base, 5 ft 6 in, underlain by 3 ft 2 in of grey silty mudstone with abundant L. mytilloides.The rest of the cyclothem varies in thickness from 15 to 50 ft, the higher figure being recorded at Ferndale No. 5. The strata are mainly mudstones with ironstone bands and nodules, though in the upper part a few sandy bands are present in the lower Ferndale collieries and in the National–Standard area.

The Bute Seam (called the Nine-Feet at National and Standard) consists everywhere of two coals separated by a clod 1 to 17 in thick: the top coal is 22 to 30 in thick in the Mardy and Ferndale areas, and 27 to 36 in thick at National and Standard; the bottom coal is 20 to 28 inches in the former area and 20 to 36 inches in the latter. Few fossils are recorded from the roof of the seam; poorly preserved crushed forms include Naiadites aff. quadrates in the Ferndale Nos. 1 and 5 areas.

At Standard Colliery, the Bute and the Lower Nine-Feet, separated by only 2 to 3 ft of seatearth, were sometimes worked together. Traced northwards the measures between them expand progressively; at Ferndale they average 20 to 30 ft, and over much of the Mardy area 40 to 60 ft. The strata are mainly argillaceous, though a little sandstone occurs at Mardy and National. In parts of Mardy a thin coal and rashings appear in the middle.

The Nine-Feet comprises a single seam at Mardy, where it is thick and often highly disturbed; a section in No. 3 Pit reads: coal 37 in, coal and rashing 13 in, coal 16 in, bast and rashings 9 in, coal 42 in, stone 2 in, coal 29 in. Near the southern margin of the taking the seam splits and southwards from this point the two resulting seams (the Upper and Lower Nine-Feet) progressively diverge. The separating mudstones are 15 to 25 ft thick in the northern Ferndale Pits, 40 ft at Ferndale Nos. 8 and 9, 60 ft at National and 70 ft at Standard.

The Lower Nine-Feet (formerly known as the No. 1 Bute at Ferndale, and the Red Vein at National and Standard) consists of a clean coal 3 ft thick in the northern parts of Ferndale, but to the south partings are developed in the top and bottom of the seam, e.g. at Ferndale No. 7: coal 3 in, clod 9 in, coal 30 in, rashings 10 in, coal 9 in; and at National: coal 2.5 in, dirt 4.5 in, coal 36 in, fireclay and rashings 23 in, coal 2.5 in, rashings 1 in, coal 4 in. No fauna has been recorded from the roof.

The Upper Nine-Feet (Six-Feet of National and Standard) is subject to much internal disturbance and has not been worked to any extent. At Ferndale a small area was worked under the mistaken impression that it was the Six-Feet. In normal section the seam everywhere appears to consist of two coals, separated in the Ferndale area by 10 to 12 in of bast, and in the National-Standard area by 1 to 5 in of rashings. The two leaves vary in thickness: at Ferndale the top coal is 30 to 48 in, and the bottom coal 42 to 60 in; in National and Standard collieries the top coal is 36 to 60 in and the bottom coal 26 to 32 in.

At Mardy Colliery (No. 3 Pit) only plants were found in the roof of the Nine-Feet: Neuropteris gigantea,N. heterophylla, Sphenophyllum cuneifolium.At the Ferndale collieries and at National, black carbonaceous, highly slickensided shales form the roof of the Upper Nine-Feet Seam and these yield : Anthraconaia aff. salteri, Anthracosia aff. beaniana, A.cf. ovum,and A. phrygiana.These shales are succeeded upwards by blocky grey mudstone yielding small solid shells including Anthracosia aff. nitida, A.cf. angulata,and problematical forms which may belong to either Anthraconaia or Anthracosia.At Ferndale No. 9 Pit the supposed egg-case Vetacapsula? was also recorded.

The measures between the Upper Nine-Feet and the Lower Six-Feet are variable. In the northern and eastern parts of the Mardy area the Red Vein forms a single though inferior seam, e.g. coal 6 in, fireclay 17 in, coal 19 in, stone 0.5 in, coal 12.5 in. Southwards the seam splits into its component coals and at National the following section of the ground between the Upper Nine-Feet and the Lower Six-Feet was measured:

The three thin coals in this section below the Six-Feet appear to correspond to the Red Vein at Mardy. In most of the southern part of the Ferndale area the coals are still split, but in the north, at Ferndale Nos. 2 and 4, a seam with section: coal 10 to 12 in, clod 1 to 7 in, coal 18 to 24 in occurs 25 ft below a thin coal, which in turn lies 10 ft below the Lower Six-Feet. In the Mardy area upwards of 85 ft of mudstone with some sandstone bands separate the Nine-Feet from the lowest coal of the Red Vein group, but this decreases to the south as shown in the above section. The distance between the top of the Red Vein group and the Lower Six-Feet grows from less than 1 ft at National to 12 ft or so at Mardy.

Apart from the fossils listed above, Anthraconaia cf. wardi, Naiadites cf. productus and Euestheria sp.were obtained from the roof of the second thin coal below the Lower Six-Feet, and 'solid' Naiadites including N. aff. productus and N. sp.intermediate between productus and subtruncatus,from the first rider coal above the Upper Nine-Feet at Ferndale No. 1 Pit.

The Lower Six-Feet consists of a main coal 3.5 to 4 ft thick with a thin coal, 7 to 18 in, close below. When traced into the north of Mardy and north-eastwards into the Cwmaman ground the bottom coal appears to die out. In the south of the area, at National and Standard, a further thin coal near the base is part of the Red Vein group: top coal 55 in, clod 4 in, coal 18 in, clod 5.5 in, coal 15 in.

In Mardy No. 1 Pit the roof of the Lower Six-Feet yielded Lioestheria sp.from the immediate roof, while at Mardy No. 3 the following plants were obtained : Eupecopteris volkmanni (Sauveur), Neuropteris gigantea, Myriophyllites gracilis Artis, Sphenophyllum cuneifolium, ?Sphenopteris striata (Gothan).

The measures between the Lower Six-Feet and the Upper Six-Feet are mainly argillaceous, though some sandy beds occur in the upper part of the cyclothem in the Ferndale area. They vary in thickness from about 15 ft at Standard to about 60 ft in the Mardy area.

The Upper Six-Feet consists everywhere of a single clean coal, 22 to 26 in thick; only at Mardy Colliery have fossils been recorded from its roof: these include Anthracosia cf. faba',Naiadites sp.and cf. Planolites ophthalmoides [2 mm.].

Some 20 to 40 ft of mudstones with much ironstone and sporadic thin bands of sandstone usually intervene between the Upper Six-Feet and Four-Feet seams. They are thinnest in the north at Mardy, while in parts of Ferndale Nos. 8 and 9 pits and at National they may reach 70 ft.

Throughout the entire area the Four-Feet, which was called the Two-Feet-Nine at National and Standard, is remarkably constant in character. Typically it consists of four coals, between which the partings tend to become more prominent in the south of the area. In the north the two middle coals coalesce. Typical sections are:

The roof of the Four-Feet is usually made up of friable blocky mudstone with numerous ironstone nodules, which yields an abundant fauna: Anthraconaia sp. nov.(cf. curtata),Anthracosia cf. acutella, A. aff. aquilina, A. aff. concinna, A.cf. planitumida, A. sp. nov.(figured Trueman and Weir, pl. 18, fig. 5), Anthracosphaerium cf. exiguum, A. sp.intermediate between exiguum and affine,cf. A. propinquum.

Throughout the Mardy–Ferndale area the Two-Feet-Nine Seam lies about 20 to 30 ft above the Four-Feet, the intervening strata consisting almost wholly of mudstone.

The Two-Feet-Nine has been worked throughout the Ferndale area, where it consists of a main coal 3.5 to 4 ft thick with one or two thin coals at or near the base. Northwards the main coal appears to split, giving in the Mardy area the following section: top coal 20 in, soft and dark shale 55 in, coal with brass 22 in, fireclay 30 in, coal 17 in. In places the separation between the top and middle coals is even greater. A mudstone roof to the middle coal has yielded: ?Anthraconaia librata, A.cf. rubida, Anthracosia cf. aquilina,A. atra, A. lateralis and A. aff. simulans.Southwards from Ferndale into the National and Standard takings there is a sharp deterioration of the section, not more than 22 to 24 in of coal being present in these areas. No fauna was recorded from the roof of the Two-Feet-Nine.

As in the Taff–Cynon area, the strata between the Two-Feet-Nine and Cefn Coed Marine Band are nowhere visible in the Rhondda Fach. In the north of the Mardy area the Three Coals forms a distinct compound horizon some 38 ft above the Two-Feet-Nine: coal 22 in, fireclay 58 in, coal 10 in, shale and fireclay 28 in, coal 8 in, rashings 6 in, coal 10 in. A coal 13 in thick 27 ft above may carry the Hafod Heulog Marine Band; a 7-in coal 54 ft above the Three Coals may mark the horizon of the Britannic Marine Band, while the Cefn Coed Marine Band presumably occurs above a 2-in coal some 51 ft 9 in still higher. These three horizons were seen at isolated places in the new cross-measures roadways driven at Mardy Colliery. Lingula mytilloides and 'fucoids' were recorded from the Cefn Coed Marine Band; the Hafod Heulog Band was presumably represented by 'sulphury' mudstones, 1 in thick, succeeded by dark mudstone with abundant ironstone nodules and sporadic Planolites ophthalmoides;while the intermediate coal carried a sandstone roof, stained yellow at its base.

The Three Coals horizon splits when traced southwards, and at Ferndale Nos. 8 and 9 four coals occur over a thickness of 64 ft: (top) coal 14 in, measures 8 ft 10 in, coal 7 in, measures 1 ft 7 in, coal 8 in, measures 21 ft 4 in, coal 10 in, measures 27 ft 11 in, coal 15 in. The top coal of the section may carry the Hafod Heulog Marine Band, and the assumed position of the Cefn Coed Marine Band is 90 ft higher. The sections at National and Standard indicate that the Three Coals group of coals has deteriorated still further in the south of the area. A.W.W.

Northern Rhondda Fawr area

The Amman Marine Band has been identified at Fernhill, Lady Margaret, and Dare collieries. It is about 1 to 2 ft thick and contains abundant Lingula mytilloides.At Tydraw and certain parts of Fernhill and Dare the marine strata fail, and planty mudstones with abundant macrospores and a few fish scales rest directly on the coal : this failure is related to wash-out conditions in the underlying coal. Immediately above the marine band abundant 'ghostly' non-marine shells are everywhere present in dark mudstone: Anthracosia cf. aquilina,A.cf. concinna, A.cf. ovum, A. sp.intermediate between ovum and phrygiana, A.regularis and Naiadites sp.The rest of the cyclothem normally consists of 15 to 30 ft of mudstones; at Pentre about 50 ft of beds are present, the thickening corresponding to the incoming of sandstones. A similar thickening occurs north of Fernhill associated with the development of 35 ft of sandstone.

The Bute has been worked extensively from most of the collieries. At Fernhill it was known as the New Seam; at Bute, Lady Margaret, Ynysfeio, and Rhondda Merthyr as the Seven-Feet; at Tydraw, Pentre, Tynybedw, and Gelli as the Yard; at Park, Dare, Eastern, and Maindy as the Brunts; and at Abergorky as the No. 2 Seam. Throughout much of the area the coal is divided into two leaves by a thin mudstone; this parting dies out westwards from Fernhill, the thickness of the seam decreasing from 4.5 ft to under 3 ft. Elsewhere the overall thickness varies from about 4 to 4.5 ft. The immediate roof consists of about 8 in of dark, well-bedded mudstone containing many 'mussels'; this is succeeded by paler silty mudstones which yield only rare shells; locally, as at Dare, these rest directly on the coal. The fauna includes variants of Anthraconaia modiolaris (J. de C. Sowerby), A. williamsoni, Anthracosphaerium affine, A.bellum, A. exiguum, A. fuscum (Davies and Trueman), A. turgidum and Naiadites sp.; Spirorbis sp.is also recorded.

The Bute and Nine-Feet are about 35 ft apart in the Ynysfeio area and this interval increases both northwards and south-westwards to about 60 ft. In the extreme south-east, at Gelli, the two seams close to within 3 to 15 ft. For the most part the beds are argillaceous, but in the north a well-developed sandstone up to 20 ft is present. Sandstones are also present in the south of the area, and locally, as at Park, are known to cut out the Bute. A 1- to 2-in coal is sometimes found about 10 ft below the Nine-Feet.

The Nine-Feet splits southwards within the valley, the line of split running between Fernhill and Tydraw collieries, and the separation increases to about 60 ft at Eastern and Gelli. The full Nine-Feet is found only at Fernhill where it comprises 8 to 9 ft of coal with a 6 to 12 in cannelly parting about 3 to 3.5 ft from the top. In the workings north of the shafts the immediate roof of the seam is silty mudstone containing a few plant fragments; above this an abundant fauna of large crushed 'mussels' is found in about 3 ft of dark cannelly mudstone containing numerous ironstone bands. The 'mussels' include the usual Nine-Feet assemblage: Anthracosia beaniana, A.cf. ovum, A.aff. phrygiana,A. sp.intermediate between phrygiana and aquilina and A. sp.intermediate between phrygiana and ovum.Paler grey mudstones above yield only worm tracks.

The Lower Nine-Feet, usually called the Bute, but at Abergorky the No. 1 Seam, has been worked from several collieries particularly on the east side of the valley. The section varies little: a top coal 22 to 30 in thick is separated from a bottom coal 4 to 10 in by 1 to 10 in of clod or rashings. In places in the north-east a 2-in coal is recorded in the top of the seam. Only plant debris has been observed in the silty mudstone roof.

The Upper Nine-Feet, extensively worked under the name Nine-Feet, maintains a section of 5.5 to 6.5 ft throughout the area. In general, it comprises a double coal: the top leaf 3.5 to 4.5 ft thick is separated from the lower leaf 1 to 1.5 ft thick by about 12 in of cannel, which often forms the floor of the worked section. On the western side of the valley another thin parting is often present 3 to 6 in below the roof. At Tydraw and Dare the roof is a planty mudstone.

The 70 to 130 ft of measures between the Upper Nine-Feet and the Lower Six-Feet are poorly exposed and the mining records show several anomalies. At Fernhill only one major coal—called the Red Vein—is recorded; this has a complex and variable section: a bottom coal of 12 to 15 in is overlain by up to three thin coals varying between 2 and 10 in. In the shaft a further thin coal lies between this seam and the Nine-Feet. At Tydraw a 2-in coal about 20 ft above the Nine-Feet carries a cannelly mudstone roof containing shells similar to those in the Nine-Feet roof at Fernhill: forms collected include Anthraconaia cf. robertsoni (Brown), Anthracosia cf. phrygiana and A. sp.intermediate between phrygiana and ovum.It is possible that this thin coal represents a split from the top of the Nine-Feet. Some 40 ft above, the 'Red Vein' consists of a top coal 3 to 12 in thick and a bottom coal 30 in thick; its roof consists of planty mudstone. Similar conditions exist further south. The thin coal carrying the Nine-Feet fauna is up to 22 ft above the Upper Nine-Feet at Park Colliery, and the Red Vein is about 30 ft below the Lower Six-Feet. At both Park and Dare the lower coal of the Red Vein often has a distinct parting near the middle, while the top coal splits away and may be several feet above the rest of the seam. At Tynybedw and Pentre this upper coal is near the floor of the Lower Six-Feet and farther south it forms a bottom coal to that seam.

The Lower Six-Feet, worked extensively as the Six-Feet, is made up of a main coal 4 ft thick and a bottom leaf, which is up to 22 in. in the south but thins to only 5 in at Rhondda Merthyr. At Park, Dare, Pentre and Tynybedw the bottom coal lies close beneath the main coal, but northwards it splits away and lies a few feet below. At Park and Dare a thin coal 2 to 6 in thick, and in part cannel, lies in the roof of the seam; this everywhere carries mudstone with abundant Lioestheria striata and Naiadites sp.,and northwards at Tydraw and Glenrhondda lies 3 ft above the Lower Six-Feet.

The Upper Six-Feet, 2 to 3 ft of clean coal, is everywhere distinct and has been worked on a small scale in the north of the area as the Upper Yard or No. 1 Yard. At Dare it is up to 60 ft above the Lower Six-Feet, but northwards this decreases, and to the north and north-west of Fernhill the two seams unite to form a composite Six-Feet. Throughout the area its roof carries a fauna of comparatively small Anthracosia including A. atra, A.cf. planitumida and A. sp.cf. phrygiana,together with a few Naiadites obliquus preserved as films; associated with the 'mussels' is a small form of cf. Planolites ophthalmoides.Above, mudstones and sandy mudstones continue for 25 to 40 ft to the Four-Feet.

The Four-Feet forms a single complex seam in the north and east of the area. At Fernhill a typical section reads: top coal 12 in, shale 1 in, coal 10 in, clod 12 in, coal 26 in, plane parting, coal 26 in, shale 12 in, coal 12 in. This is similar to the typical four-leaf section of the Rhondda Fach with the top coal of that area having split into two leaves. A similar section is recorded at Gelli, where this top coal appears to suffer further splitting into three distinct leaves: top coal 6 in, stone 2 in, coal 6 in, parting I in, coal 6 in, clod 2 in, coal 22 in, bast and fireclay 12 in, coal 25 in, rashings 12 in, coal 12 in. Traced northwards from Fernhill the two top leaves of the above section split away and have been proved to lie 5 ft above the rest of the seam. In the central and south-western parts of the area the complex top coal splits even farther away, while in the extreme south-west the complete splitting of the seam in Cwm Clydach and the Ogmore Valley is foreshadowed at Eastern Colliery. The following sections illustrate this behaviour:

At Fernhill the roof of the combined seam is a barren sandy mudstone. At Glenrhondda the uppermost coal, split 40 to 50 ft above the rest of the seam, carries plant fragments and film-like impressions of Naiadites sp.in its immediate roof, overlain by sandy mudstones containing Anthracosphaerium cf. exiguum.Nowhere else was the roof of the top coal accessible. Planty mudstones form the roof of the split lower seam.

The Two-Feet-Nine usually lies 15 to 30 ft above the top coal of the Four-Feet. South of Treorky it consists of a main coal 4 to 5.5 ft thick with an underlying coal 12 to 27 in, sometimes in two leaves, up to several feet below. In the Park workings a parting develops about 18 in above the base of the main coal and this appears to increase to a true split in the collieries between Lady Margaret and Fernhill. In this latter area the top split has been worked sporadically with a section varying from 24 to 4 ft. The following sections illustrate the variation of the seam.

The roof of the lower split of the main coal has been examined at Fernhill and Glenrhondda. A fauna, poorly preserved in irony mudstone, includes: Spirorbis sp.; ?Anthraconaia librata, Anthracosia atra, A.cf. planitumida,Naiadites productus and scales of Rhizodopsis sp.At Park Colliery an abundant fauna was obtained 2 to 3.5 ft above the main coal: Gyrochorte carbonaria Schleicher, Planolites montanus; Anthraconaia cf. cymbula,Anthracosia cf. acutella, A.cf. atra, A.cf. concinna, A.cf. planitumida, A. aff. simulans and Naiadites cf. obliquus.Small poorly preserved 'mussels' ranged upwards for several feet. At Fernhill a sparse fauna included Anthraconaia cf. rubida, Anthracosia cf. planitumida and Naiadites sp.

The succeeding measures to the Cefn Coed Marine Band are 140 to 200 ft thick. At Gelli Colliery the Three Coals horizon is represented by a complex group of four coals occurring about 25 ft above the Two-Feet-Nine: top coal 20 in, fireclay 55 in, coal 14 in, fireclay 38 in, coal 12 in, clod 12 in, coal I I in. Elsewhere in the area these seams split completely. At Maindy, for example, five thin coals occur over 54 ft. These are succeeded by three or four thin coals which presumably carry marine strata at two or three levels. The most complete section of these strata was examined at Fernhill. The coals of the Three Coals group comprise four thin leaves spread through about 45 ft and lying about 55 ft above the Two-Feet-Nine. The supposed Hafod Heulog horizon has yielded no fossils, but the Britannic Marine Band occurs on a 7- to 9-in coal about 40 ft below the Cefn Coed Band, and has yielded abundant Planolites ophthalmoides and rare Lingula mytilloides.The Cefn Coed Marine Band is about 2 to 3 ft thick at Fernhill and has yielded: macrospores; crinoid columnals, Archaeocidaris sp.; Chonetes cf. granulifer Owen, C.cf. skipseyi Currie, C. (Tornquistia) diminutus Demanet, Crurithyris carbonaria (Hind), Orbiculoidea cf. nitida, Productus (P.) carbonarius, P. (Dictyoclostus) craigmarkensis Muir Wood, cf. P. (Levipustula) rimberti Waterlot, Rhipidomella cf. carbonaria (Swallow); Straparollus sp.; Nuculana?, Posidonia?;Ditomopyge sp.Lenses of 'milk' occur within the band and the richest fauna is closely associated with these.

Locally sandstones are developed below the horizons of both the Hafod Heulog and Britannic bands; these thicken towards Park, where a third sandstone occurs close below the Cefn Coed. W.B.E.

Southern Rhondda area

The beds between the Amman Rider and the Bute vary in thickness between 13 and 40 ft. They are thickest in the central part of the area, over 30 ft being recorded at Britannic, Naval, Cymmer and Lewis Merthyr collieries; and they are thinnest at Llwynypia and Coedely. Mudstones generally predominate, but thin sandstones occur in the middle where the cyclothem is thickest. The Amman Marine Band consists of 10 in to 3 ft of mudstone, more or less silty and micaceous, everywhere overlain by paler smoother mudstone with the usual 'ghostly' non-marine lamellibranchs. Lingula mytilloides is ubiquitous in the marine band, and at Lewis Merthyr and Great Western collieries was the only fossil recorded. At Coedely Orbiculoidea cf. nitida was also found, while at Naval, Cambrian, and Maritime pyritized sponge spicules were abundant. In the south, at Coedely and Cwm collieries, more varied faunas occur: in addition to the forms mentioned above the following forms were collected: Ammonema ?; Emnwnsia parasitica (Phillips); Sphenothallus?, Conularia sp.;crinoid columnals; polyzoa ; Productus (Levipustula) piscariae Waterlot; Pleuroplax attheyi (Barkas). Among the 'mussels', dwarfed Anthracosia predominate; they include A.sp.cf. concinna, A. sp.intermediate between concinna and planitumida,A. cf. ovum and Naiadites sp.,associated with Spirorbis sp.

Over most of the area the Bute and Lower Nine-Feet are close together, forming one seam, called the Nine-Feet at Llwynypia, Naval, Cymmer, and Cilely collieries, and the Two-Feet-Nine at Coedely. In the western parts of Cambrian and Britannic, and again in the northern and eastern parts of Lewis Merthyr and Great Western, and in the eastern parts of Cwm, the two seams split apart. Where separate, the Lower Nine-Feet was known as the Red Vein, and the Bute as the Nine-Feet. Typical sections of the combined seam are as follows:

In parts of Lewis Merthyr and Great Western collieries the separation between the two seams may reach 30 ft, while at Cwm 15 to 20 ft is locally recorded. Typical sections are:

The Bute is usually a double coal, but thin leaves are locally developed in the middle of the seam (e.g. at Naval) and, more frequently, at the base. The Lower Nine-Feet consists of one main coal in the eastern part of the area, but this splits westwards into Cambrian and Britannic. In the northern parts of Lewis Merthyr a thin coal occurs at the top of the seam. On the west side of Cambrian Colliery an airbridge driven 30 ft above the Bute struck the Lower Nine-Feet with the following section: roof, dark grey mudstone with thin ironstone bands and nodules and sporadic coal streaks 8 in, cannelly shale with coal streaks and a layer of cannel 4.5 in, coal 1 in, black cannelly shale 1 in, inferior coal 2 in, cannel, shaly at top, 11.75 in, coal 5 in, 'brass' 0 to 1.5 in, coal 13 in, rashings 1 in, coal 18 in, hard seatearth floor. The presence of cannel in the top of the coal is characteristic of the Cambrian area.

Only plants have been recorded from these two seams. At Lewis Merthyr the roof of the Bute yielded: Alethopteris decurrens, Asterophyllites charaeformis, A. equisetiformis,A. grandis (Sternberg), ?Bothrodendron sp., Calamites sp., Diplotmema furcatum, Lepidophyllum morrisianum Lesquereux, Mariopteris muricata, Myriophyllites gracilis, Neuropteris heterophylla,Pinnularia capillacea, Trigonocarpus parkinsoni.At Ty Mawr No. 1 Pit (Great Western Colliery) the following species were obtained from the roof of the Lower Nine-Feet: Alethopteris decurrens, Boulaya fertilis Renault, Calamites undulatus Sternberg, Lepidodendron sp.,Lonchopteris bricei Brongniart, L. eschweileriana Brongniart, Lycopodites sp. nov., Mariopteris muricata, Myriophyllites gracilis, Neuropteris gigantea, N. heterophylla, N.obliqua, Pinnularia capillacea, Sphenophyllum cuneifolium,Trigonocarpus parkinsoni, Urnatopteris tenella (Brongniart).

In the north-eastern portion of the Lewis Merthyr taking the separation between the Lower and Upper Nine-Feet seams may be as little as 3 ft; this marks the beginning of the split in the Nine-Feet Seam from the direction of Lady Windsor Colliery, where the Nine-Feet coals form a single seam (see p. 106). Southwards and westwards the split develops rapidly, and over most of the area 50 to 70 ft, and locally, as at Llwynypia and Coedely, 100 ft or more of strata may be present between the two coals. Mudstones with ironstone bands and nodules make up the bulk of the cyclothem, but thin sandy beds are present in the middle or lower part in the Naval and Cambrian areas, while several beds of sandstone up to 25 ft thick occur at Coedely.

Except at Coedely Colliery the Upper Nine-Feet Seam was known throughout the area as the Six-Feet. At Great Western and Lewis Merthyr its section is similar to that described for the Rhondda Fach area: top coal 39 to 51 in, parting 1 in, bottom coal 18 to 36 in. Traced southwards to the southern parts of Lewis Merthyr, a second parting locally develops in the bottom coal: coal 48 in, clod 1 in, coal 18 in, clod 1 in, coal 12 in. In the Maritime and Cwm areas the partings die out and the seam consists of a clean coal 39 to 67 in thick, averaging about 54 in. In Cambrian and Llwynypia the parting between the two main coals often consists of bast. A thin leaf, 4 to 6 in thick, at the base of the seam is a constant character throughout the collieries west of Porth. On the south-west side of Naval, in Cambrian, and Britannic a further parting appears in the upper part of the top coal:

At Coedely the seam was formerly regarded as the bottom coal of the Three Coals horizon. Its section (coal 15 in, fireclay 32 in, coal 36 to 44 in) indicates that the main parting has died out, as in Cwm, and the upper leaf may be one of the Red Vein group coals.

At Llwynypia, Naval, and Cambrian collieries a massive sandstone, 25 to 50 ft thick, overlies the Upper Nine-Feet, and in the last-named area is associated with wash-outs in the underlying seam. Elsewhere the roof of the coal consists of blocky grey mudstone, usually carrying small solid 'mussels'. These are particularly well represented at Lewis Merthyr, Cwm, and Coedely collieries, and the following species have been identified: cf. Anthraconaia pulchella, A.pumila (Salter), Anthracosia cf. angulata, A.nitida, A. sp. nov.aff. nitida, Naiadites cf. productus.At Ty Mawr No. 1 Pit an abundant fauna of solid Naiadites was collected; species include: N.cf. productus, N.subtruncatus and N. sp. nov.(figured Trueman and Weir, pl. 31, figs. 44, 45), associated with Anthraconaia cf. curtata.In strong mudstones 7 to 10 ft above the coal at Cilely Colliery the following plants were collected: Asterophyllites equisetiformis, Asterotheca miltoni (Antis), Calamites undulates, Diplotmema furcatum,Lepidodendron sp., Lonchopteris rugosa Brongniart, Mariopteris nervosa, Neuropteris heterophylla, Sphenophyllum cuneifolium,S. myriophyllum Crepin.

The Red Vein group of coals consists of a well-defined and fairly consistent Caerau Seam, lying everywhere close to the floor of the Six-Feet, together with one or more thin coals not far below. The distance from the Upper Nine-Feet to the Caerau Seam varies from 10 to 15 ft at Coedely and 30 to 40 ft at Cwm in the south of the area, to 110 to 120 ft at Naval and Llwynypia in the north. Apart from the thick sandstone developed at the base in the Llwynypia, Naval, and Cambrian areas, these measures consist of mudstones with ironstones together with varying amounts of sandstone. In the Nantgwyn Hard Heading at Naval Colliery the following section was measured:

A similar section occurs in the shafts at Cymmer and Lewis Merthyr. At Great Western only one coal, 16 to 23 in, is recorded, about 11 ft below the Caerau, and at Cwm Colliery 8 to 9 in of coal occur in a similar position. At Cilely Colliery, where 70 to 80 ft of ground lie between the Upper Nine-Feet and the Six-Feet-Caerau group, a 7- to 8-in band of coal and bast occurs in the middle; the mudstone roof of this coal yielded Anthracosia sp. nov.aff. nitida, A.cf.angulata and Naiadites sp.cf. productus at 6 to 18 in above the seam.

The Caerau portion of the Red Vein group normally lies close beneath the Six-Feet. Only on the east side of Cwm Colliery is there sufficiert ground between the two coals to allow the lower coal (called the Four-Feet Bottom Coal) to be worked, and even here the separation is little more than 3.5 ft. The Caerau consists of two leaves, with an overall thickness of 2.25 to 2.5 ft in the north and east and 3.5 to 4.5 ft in the west and south. The clod or rashings parting varies from a mere film to 2 ft. Representative sections are given with those of the Six-Feet below.

The Six-Feet (formerly known as the Four-Feet at all the collieries of this area except Llwynypia and Cambrian) splits into its component Upper and Lower coals. The line of split passes through Naval Colliery, thence southwards through the western part of Cilely before turning westwards. In the eastern workings of Cwm Colliery the parting between the two coals dies out. To the south-east of the line of split the Six-Feet has fairly constant characters: it consists of a top coal 15 to 22 in thick (the coal which forms the Upper Six-Feet), a main coal of 42 to 54 in and a bottom coal of 12 to 18 in, which does not seem to have a separate existence in the east and south of the area. The following sections of the combined Six-Feet and Caerau seams illustrate the variation:

Northwards and westwards the mudstones separating the Lower and Upper Six-Feet increase from 10 ft over much of the Naval area, to 30 ft at Llwynypia, 30 to 45 ft at Cambrian and 15 to 20 ft at Britannic.

Typical sections of the Lower Six-Feet together with the associated Caerau Vein in the areas of splitting are as follows:

In the western and southern parts of Naval Colliery and in parts of Llwynypia a persistent 'brass' development a few inches thick occurs 12 to 18 in from the top of the Lower Six-Feet Seam, while in Cambrian Colliery 7 ft of clean coal have been recorded locally.

The separated Upper Six-Feet usually consists of 20 to 28 in of clean coal, and is thinnest in the west of the area. In parts of Cambrian a thin clod is locally recorded about 5 in from the bottom of the seam.

No fossils have been recorded from the roof of the Lower Six-Feet in this area, but the roof of the Upper Six-Feet (or Six-Feet where the two coals are united) carries the characteristic 'mussel' fauna. The shells occur as solids or semi-solids in ironstone-shale (blackband) close above the coal, though some specimens occur in the grey mudstone above. Variants of the following species make up the bulk of the fauna collected: Anthracosia acutella, A. aquilina,A. sp. nov.cf. aquilinoides, A. atra, A.caledonica Trueman and Weir, A. concinna, A.elliptica, A. sp.cf. phrygiana,A. planitumida, A. sp.intermediate between atra and lateralis, A. sp.intermediate between atra and barkeri and Naiadites sp.; Carbonita humilis is also associated.

The measures between the top coal of the Six-Feet and the lowest coal of the Four- Feet group vary from about 25 ft to about 90 ft. They are thinnest in the Cambrian area and thickest at Cwm, Cilely, and Britannic; elsewhere thicknesses of 50 to 65 ft are common. The strata consist mainly of mudstones and silty mudstones with ironstone bands and nodules, but thin sandstones are developed in the middle of the cyclothem at Naval and Great Western and, near the top, at Cymmer.

In the north of the Llwynypia taking the Four-Feet forms a single complex seam composed of four principal coals, as in the Rhondda Fach area (see p. 111): top coal 12 in, stone 1 in, coal 3 in, rashing 4 in, coal 28 in, 'bunker' 48 in, coal 24 in, rashing 15 in, coal 12 in. Southwards into the Naval and Dinas colliery areas a split develops in the middle of the section, giving two distinct seams, here called the Upper and Lower Four-Feet. At the main Naval shafts the separation is 10 to 10.5 ft, and in Ely pit the section reads: Upper Four-Feet (called Lower Two-Feet-Nine), coal 5 in, rashings 9 in, coal 5 in, rashings 8 in, coal 20 in; separation 12 ft; Lower Four-Feet, coal 10 in, rashings 3 in, coal 22 in, fireclay 30 in, coal 24 in. At the collieries of the lower Rhondda, where the combined seam carried the name of Two-Feet-Nine, a similar behaviour is to be observed. Over much of the northern part of Lewis Merthyr and Great Western a single four-leaf section prevails, e.g. coal 13 in, fireclay 20 in, coal 29 in, fireclay 19 in, coal 26 in, rashings 8 in, coal 13 in; but locally the top coal splits away by as much as 30 ft. In Cwm and Coedely collieries two distinct seams are developed by the same splitting as that described at Naval Colliery:

At Cilely Colliery the Lower Four-Feet has the typical two-leaf section, but the Upper Four-Feet has split into its component parts. The following section was measured on the main roadway just west of the shafts:

Westwards from Llwynypia, Naval and Cilely, complete splitting of the various leaves takes place and the seam is represented by a series of single coals spread through a considerable thickness of strata. At Cambrian Colliery the section in No. 2 Shaft reads (from top): coal 11 in, shale 7 ft 6 in, coal 15 in, clift and shale 17 ft 2 in, coal 5 in, fireclay and clift 16 ft 5 in, coal 17 in. At Britannic the details are unreliable owing to structural complication; the section in the South Pit is given as: coal and rashes 5 in, measures 4 ft, coal and rashes 20 in, measures 15 ft 7 in, coal 12 in, measures 10 ft 7 in, coal 13 in, measures 25 ft 3 in, coal 13 in.

At Cilely, Coedely, and Cwm the immediate grey mudstone roof of the Lower Four-Feet carries a poor fauna of non-marine lamellibranchs, including Anthraconaia obscura, A.cf. subcentralis and Naiadites sp.At Naval, Cilely, and Cwm collieries the typical Four-Feet forms are abundant in the roof of the Upper Four-Feet: Anthraconaia cf. cymbula, A.cf. librata,A. rubida, A. sp. nov.(cf. curtata),Anthracosia cf. atra, A. concinna, A. aff,fulva, A.cf. lateralis, A. planitumida,A. simulans, Anthracosia sp.intermediate between aquilina and lateralis, A. sp. intermediate between atra and fulva, A. sp.intermediate between atra and planitumida,Anthracosphaerium cf. exiguum, A. aff. propinquum,A. sp.intermediate between propinquum and turgidum,Naiadites obliquus.At Lewis Merthyr, where the top coal splits away, a poor fauna of Anthracosia aff. atra, Naiadites cf. alatus and N.cf. obliquus occurs.

The measures between the Four-Feet group and the Two-Feet-Nine are thinnest in the east and south-east of the area, where they are as little as 10 ft in Cwm and Great Western, and in the north, being 20 to 30 ft thick in the Naval–Llwynypia area; they thicken south-westwards and westwards to 40 ft at Coedely, 45 to 50 ft at Cambrian, and 65 to 70 ft at Ely. They consist mainly of mudstones with ironstone bands, but some sandstone is recorded in the thicker sections.

The Two-Feet-Nine is a workable seam only at Llwynypia, Cambrian, and Coedely. At Llwynypia the general section is: coal 46 to 51 in, fireclay 2 to 3 ft, bottom coal 3 to 8 in. Southwards and south-westwards it thins rapidly and in the collieries of the Porth–Pontypridd–Cwm area the main coal is seldom more than 27 in thick and the seam is un-named. The coal has been extensively worked at Cambrian Colliery where the thickness varies from 5.5 ft in the extreme west to 34 in about 0.5 mile E.S.E. of the shafts. In this latter area a thin rider coal, 7 to 8 in thick, is separated from the main coal by 3 to 4 ft of seatearth. In the neighbourhood of the shafts at Britannic Colliery the seam is 22 in thick and the thin rider occurs about 5 ft above. It is not known whether or not this rider coal is a split from the main coal, but it is significant that the fauna carried in its roof (see below) is very different from that immediately overlying the Two-Feet-Nine in the adjoining Ogmore Valley and on the South Crop. At Coedely Colliery the seam (formerly known as the Pentre) has been extensively worked: over most of the taking the section consists of 50 to 54 in of clean coal, but in the north it splits into two leaves, each of which thins quickly. On the east side, bordering on Cwm, the seam again thins rapidly to some 18 to 24 in of coal.

At Coedely a few shells occur above the normal seam, including Anthraconaia sp. nov.(cf. Hind, pl. xix, fig. 21) and Anthracosia cf. atra,but plants are more abundant: Annularia radiata, Calamites sp., Mariopteris nervosa, Neuropteris gigantea, N. heterophylla, N.obliqua, Sphenophyllum cuneifolium.At Naval Colliery Leaia sp.was found associated with Naiadites sp.

The thin rider coal close above the Two-Feet-Nine at Cambrian and Britannic carries a distinctive non-marine fauna in dark blackband : in the immediate roof Anthracosia atra, A.aff. barkeri, A.aff. fulva and Naiadites sp.were collected, and from 3 ft above Anthracosia cf. atra,A.aff. barkeri, A. aff. fulva, A. sp.intermediate between atra and barkeri and A. sp.intermediate between atra and fulva.

The strata between the Two-Feet-Nine and the Cefn Coed Marine Band thicken markedly when traced from east to west : at Cwm Colliery they are about 60 ft, and 77 ft is recorded in a trial pit between the Llanwonno and Daren-ddu faults on the east side of Great Western Colliery; in the west thicknesses of 130 to 150 ft are indicated at

Llwynypia, Naval, Cambrian, and Britannic collieries. The presumed positions of the marine bands and their correlation in these shafts is shown in (Plate 4). In the north and west of the area all the coals of the Three Coals group appear to be represented. At Llwynypia the following section occurs some 25 ft above the Two-Feet-Nine: coal 24 in, clift 5 ft, coal 10 in, clift 2.5- ft, coal 6 in, rashings 4 in, coal 4 in; a sandstone, 25 ft thick, occurs above. Similar sections are recorded at Cambrian. At Naval the Three Coals consists of three coals, 9 to 21 in thick, spread over 25 ft of strata and associated with much sandstone. South-eastwards the coals deteriorate and tend to die out.

The marine bands of the Hafod Heulog-Cefn Coed group have been proved only at Naval, Britannic, and Cwm collieries. The Cefn Coed Marine Band, together with a second marine horizon beneath, was examined in the Nantgwyn Hard Heading at Naval Colliery, and the following section measured:

Because of major faulting immediately above and below this part of the section it is not possible to relate the sequence exactly to the general succession; the upper marine horizon is certainly the Cefn Coed, and it is likely that the lower one is the Britannic. At Britannic Colliery all three marine bands are developed. The following extract from the No. 4 Borehole sunk in 1952 is given in detail since it provides the type section for the Britannic Marine Band:

The No. 3 Borehole succession was similar, and Sphenothallus sp.was additionally recorded from the Hafod Heulog Band.

Eastwards the Britannic Marine Band deteriorates, and at Cwm seems to have died out, only one cyclothem being recognized between the Hafod Heulog and Cefn Coed bands. A.W.W.

Ogmore-Garw-Llynfi area

The Amman Marine Band has been examined at Wyndham, Garw, Ffaldau, St. John's, and Coegnant collieries. Lingula mytilloides was virtually the only fossil found: usually it occurs in the immediate roof of the Amman Rider, but the shells may be scattered through nearly 3 ft, as at St. John's. The marine strata are succeeded by 2.5 to 5 ft of grey mudstones carrying the usual small 'mussels' with 'ghostly' preservation, including Anthraconaia?, Anthracosia cf. concinna,A. aff. ovum, A. sp.of ovum/aquilina group and A.cf. regularis.At Coegnant Spirorbis sp.was attached to some of the shells.

The rest of the cyclothem consists of 20 to 55 ft of mudstones and silty mudstones, which in general thicken from east to west. In the south-west, at St. John's and Celtic, a rider coal, 6 to 8 in thick, lies about 10 ft below the Bute.

The Bute has been extensively worked; it was named the Yard at Penllwyngwent, Wyndham, and Ffaldau, the Brunt's at Western, Garw, and International, the Lower New at Caerau, Coegnant, and St. John's, the Five-Feet at Oakwood and Celtic and the Lower Three-Feet at Duffryn Madoc. Throughout the Ogmore and Garw valleys the seam has two leaves: top coal 16 to 24 in, stone or clod 0 to 2 in, bottom coal 30 to 42 in. In the Llynfi Valley the seam consists of a single coal varying from 3 ft in the north to 5 ft in the south, though the thin parting is present locally. At Coegnant and in the Garw Valley 3 to 8 in of inferior coal occur in the base of the seam, 3 to 12 in of rashings intervening, and this may be the thin coal recorded 10 ft below the seam in the south-west of the area. Typical sections are:

The roof of the Bute carries its distinctive fauna throughout the three valleys. At St. John's the lowest 5 ft of the roof are dark mudstones, increasingly cannelly towards the base and containing numerous ironstone bands. The basal layers abound in crushed shells, but in the upper part 'solids' are also present. The succeeding silty mudstones carry abundant solid forms. At other collieries the cannelly mudstone with crushed shells is thinner: 3 in at Ffaldau, 12 in at Garw and 4 in at Coegnant. At Caerau about 12 in of silty mudstone with a 'solid' fauna may intervene between the cannelly mudstone and the top of the coal. The crushed shells belong to the same species as the 'solids' and include variants of Anthraconaia sp.(of modiolaris group), Anthracosphaerium affine, A.bellum, A. exiguum, A. turgidum, Naiadites quadratus and N. sp. nov.cf. modiolaris Hind (non J. de C. Sowerby).

The Bute and the Lower Nine-Feet are mostly 40 to 50 ft apart, though thicknesses up to 80 ft are recorded in parts of Penllwyngwent and Wyndham, and in the north of the Llynfi Valley they may be somewhat closer. The strata consist mainly of mudstones and silty mudstones, with a few thin impersistent sandstones developed locally.

The Lower Nine-Feet, formerly known as the Bute Seam throughout the Ogmore and Garw valleys, the Upper New Seam at Caerau, Coegnant, and St. John's, and the Lower Four-Feet at Oakwood, Celtic, and Dufftyn Madoc, has also been extensively worked. Typically the seam has a two-coal section with a separation between the leaves varying from 0 to 3 in. The top coal varies from 29 to 39 in. in the Ogmore Valley and in the north of the Garw and Llynfi areas, to 42 to 48 in. in the area south of Maesteg. At Western a cannel, up to 17 in thick, occurs at the top of the coal. Locally in Wyndham and Ffaldau collieries a parting develops 14 to 18 in from the bottom of the leaf, while in the Llynfi area thin and impersistent partings occur near the top, and pyrite lenses about 15 in above the base. The bottom coal is 34 in thick at Western, but thins westwards to 17 in and less in the Garw Valley, and to less than 12 in. in the north-west of the Caerau taking; in the Llynfi Valley the coal again thickens southwards to more than 2 ft in places in St. John's Colliery, where up to 15 in of cannel may form the top of the leaf. At Garw Colliery and south of the anticline in the Maesteg area there are no records of partings, the seam consisting of 40 to 63 in of clean coal. The following sections illustrate the variation:

The immediate roof of the Lower Nine-Feet is usually a carbonaceous mudstone which passes upwards into smooth shaly dark mudstone. It has yielded no definite fossils. At Penllwyngwent about 40 ft of strong mudstones separate the Lower and Upper Nine-Feet seams and in the Garw Valley these thicken to about 75 ft. In the Llynfi Valley the strata become more arenaceous, and at Coegnant up to 30 ft of sandstones occur in the middle of the cyclothem.

The Upper Nine-Feet has always been known as Nine-Feet except in the Llynfi Valley; at Caerau, Coegnant, and St. John's it was called the Harvey, at Duffryn Madoc the Upper Four-Feet, and it appears to be the seam known as the Two-Feet-Ten or Lower Two-Feet-Nine at Celtic and Oakwood. In the east of the area the seam consists of a main coal 4.5 to 5.5 ft thick with an inferior or 'engine' coal lying close above. The main coal frequently has an impersistent 'brass' layer near the top and a thin stone parting about 1.5 to 2 ft above the base. At Western the latter parting is replaced by 6 to 12 in of cannel; and cannel is also present at the top of the main coal section in many places. Traced westwards the main coal thins to about 3.75 ft in the Maesteg area. The position of the thin 'engine' coal at the top of the seam is variable, being sometimes separated by as little as 5 in of cannel from the main coal, as in Wyndham, or lying several feet above, as in parts of the Llynfi Valley. Locally a bottom coal, 3 to 6 in, has been recorded, separated from the base of the main seam by 3 to 12 in of clod or rashings. The following sections illustrate the variations exhibited by the seam in full section:

At Penllwyngwent typical species of the 'Red Vein fauna' occur about 9 in above the seam. Many variants of the Anthracosia nitida series have been recognized, including Anthracosia angulata;also present are shells which approach Anthraconaia pulchella.At Western Colliery many shells occur above thin coal streaks 3 ft above the main coal: the fauna, which includes cf. Anthraconaia pulchella, Anthracosia cf. nitida, Naiadites aff. productus,N.cf. subtruncatus,is noteworthy for the abundance of well-preserved 'solid' Naiadites.This fauna has been followed eastwards and is the same as that recorded on the bottom coal of the Red Vein group in the Rhondda Fach.

In the Ogmore and Garw valleys the Caerau lies 80 to 120 ft above the Upper Nine-Feet, the mainly argillaceous strata thickening westwards. Within these, one thin coal is usually recorded, lying, in general, 20 to 30 ft below the Caerau. This coal evidently represents some part of the lower portion of the Red Vein of the Aberaman area; it is normally less than 12 in thick, but at International Colliery the shaft shows: top coal 18 in, fireclay 4.25 ft, coal 15 in. Immediately above this coal at Western occur 18 in of smooth dark carbonaceous, ferruginous mudstone with the typical Nine-Feet fauna' : A. sp.cf. beaniana, A.cf. ovum, A. phrygiana,A. aff. retrotracta and Naiadites sp.intermediate between productus and quadratus.This is succeeded 3 to 4.5 ft higher by a good example of the small 'Red Vein fauna' comprising variants of cf. Anthraconaia pulchella, Anthracosia aff nitida and Naiadites sp.

The measures between the Upper Nine-Feet and the Caerau are not well known in the Llynfi Valley. Within the 70 to 100 ft of strata one seam is usually developed, lying 30 to 40 ft below the Caerau. At Coegnant Colliery it is about 13 in thick and carries a thin cannelly mudstone in its roof. Grey mudstone above yielded crushed 'mussels' of the Anthracosia phrygiana group at about 2 to 3 ft above the coal, and this was overlain by small 'solids' of 'Red Vein' type. Farther to the south-east the coal is 25 in thick with 7 in of cannelly mudstone in two bands in its roof; it is 26 in thick at Celtic Colliery and 33 in at Duffryn Madoc. Various old records in the valley show other coals between the Upper Nine-Feet and the Caerau, but the presence of these is believed to be the result of structual repetition.

The Caerau has been worked extensively in those areas where the separation from the overlying Lower Six-Feet is sufficient to permit adequate roof control. The seam, known as the Red Vein in the Ogmore and Garw valleys and the Seven-Feet in the Maesteg area, correlates with the upper part of the Red Vein of the Aberaman–Cwmaman area, though the details are obscure. It consists typically of two leaves separated by an inch or so of rashings, as shown by the following sections:

At Wyndham Colliery 3 to 6 in of cannel occur 5 to 8 in from the top of the seam, while at Coegnant a further thin coal is recorded 2 to 3 ft above. The Caerau is the lowest seam known to crop out in the Llynfi Valley, and was formerly worked in 'patches' in the area to the north-east of Maesteg Railway Station [SS 8540 9157].

Around the shafts at Wyndham Colliery as little as 6 in of seatearth separate the Caerau from the Lower Six-Feet; at Ffaldau the separation varies from 2 to 30 ft. Westwards and north-westwards the two seams are 15 to 30 ft apart and the intervening measures are made up largely of seatearth or mudstone with roots and ironstone nodules. At Coegnant the immediate roof of the Caerau consists of seatearth which yielded a few 'mussels', identified as the internal moulds of Carbonicola?,and appear to represent a unique occurrence of this genus at so high a level. In the Llynfi Valley a thin dirty seam is usually recorded between the Caerau and Lower Six-Feet. At Coegnant this has a complicated section totalling 2.5 ft; at St. John's 7 in of coal lies within 2 ft of the base of the Lower Six-Feet.

In the Ogmore and Garw valleys the Lower and Upper Six-Feet seams are 50 to 70 ft apart. The Lower Six-Feet has been virtually exhausted at all collieries except Penllwyngwent. The section varies little: 4 to 5.5 ft of top coal is separated from a bottom coal of 16 in to 2.5 ft by up to 2 in of dirt. In many parts of Wyndham and Ffaldau the seam has been recorded as 5.5 to 7 ft of clean coal, but a smooth plane parting appears to be always present 1.5 to 2 ft from the base. At Penllwyngwent up to 12 in of cannel occur in the top of the seam, and lenticular 'brass' is locally present in the middle of the top coal.

At Western Colliery Lioestheria sp.,associated with Naiadites sp.and Planolites montanus,has been collected from the 8 in of grey mudstone immediately overlying the Lower Six-Feet, the first-named being more common in the bottom 2 in. The strata up to the Upper Six-Feet consist mainly of strong mudstone with ironstones, and sandstone bands are recorded at Ffaldau and International and, near the top of the cyclothem, at Western. At the last-named colliery cannelly blackband shale lying about mid-way between the Lower and Upper Six-Feet seams yielded: Spirorbis sp.[on Naiadites], Anthraconaia sp.of librata/oblonga group, N. productus and Carbonita humilis.

The Upper Six-Feet (normally an un-named seam) has not been worked in any of the collieries of the Ogmore and Garw area; it consists everywhere of a clean coal varying in thickness from 20 to 26 in. The roof carries the typical Six-Feet fauna and the following species have been identified: Anthracosia cf. aquilina, A.cf. atra, A. sp.intermediate between atra and laterally,A. sp.intermediate between acutella and concinna,A. sp.cf. ovum, Anthracosphaerium aff. propinquum and Naiadites sp.

The Lower and Upper Six-Feet seams, locally called the Six-Feet and Red Vein respectively, exist as two separate seams in the east of the Llynfi Valley area, but to the west they unite to form a single seam. The line of union follows the valley bottom, and to the west the full section, about 9 ft, has been worked in parts of Duffryn Madoc and Coegnant. This composite seam was formerly known as the Nine-Feet or Furnace Vein. Although the Lower Six-Feet has been worked extensively in the eastern part of the area, few working sections have been preserved and the seam is now only rarely accessible. In general it appears to be about 6 ft thick, but it is commonly much affected by structures due to incompetence, which give the seam a variable section. A few records show a top coal of about 5 in, and this may well represent the thin coal at or near the top of the seam which elsewhere carries Lioestheria.This conclusion is supported by the presence at St. John's Colliery of an inch or so of cannel between the thin coal and the main part of the seam, an association similar to that in the Treorky area (see p. 113). The roof of the Lower Six-Feet has been seen at only a few points: at St. John's 'slips' in the immediate top of the seam frequently cut out the true roof. At Coegnant the mudstone roof has yielded Anthracosia cf. aquilina, A. planitumida and Naiadites cf. obliquus.At Caerau the roof carries fragmentary plant remains, suggesting that the thin coal with its fauna may have split away from the main seam.

The Lower and Upper Six-Feet seams are separated by up to 60 ft of strong silty mudstones. The Upper Six-Feet has been worked on a small scale in most of the collieries in the valley and it has been examined at Caerau, Coegnant, and St. John's. At Caerau Colliery it consists of clean coal 26 to 40 in thick, while at Coegnant it is about 29 in. At all localities a rich fauna, well-preserved in ferruginous mudstone in the south of the area, and in a more cannelly one in the north, has been obtained. Among the forms identified are variants of: Anthraconaia confusa ? (Trueman), Anthracosia atra, A. aquilina,A. caledonica, A. concinna, A. fulva,A. lateralis, A. planitumida and Naiadites sp.(?alatus group).

Over most of the Ogmore and Garw area some 30 to 40 ft of strata usually intervene between the Upper Six-Feet and the lowest coal of the Four-Feet group, though in parts of Wyndham and Ffaldau they may be reduced to as little as 11 ft. These measures consist of mudstones with a few sandy layers; thin ironstone layers are frequent in the lower part of the cyclothem and large nodules in the upper part.

In the Ogmore Valley four coals spread over 60 to 80 ft represent the Four-Feet group of coals. At Western the section is (from top): coal 7 in, rashes 2.5 ft, coal 7 in, fireclay and rashes 5 ft, coal 2 in, strong mudstone with balls of mine 24 ft 11 in, coal 21 in, mudstone and fireclay 22 ft 11 in, coal 14 in; and at Wyndham: rashes with coal bands 6 ft 3 in, fireclay, clift and rock 42 ft 11 in, coal 18 in, mine ground 14 ft 8 in, coal 11 in, fireclay and rock 22 ft 5 in, coal 13 in. These sections are similar to those recorded at Cambrian and Britannic collieries (see p. 120), and the four coal horizons presumably correlate with the individual leaves of the typical Four-Feet section in the Cynon and Rhondda Fach valleys. The top coal of the group at Western Colliery yielded the typical Four-Feet fauna: Anthraconaia cf. rubida, A. sp. nov.(cf. curtata), Anthracosia cf. acutella, A.cf. aquilina,A. sp.intermediate between aquilina and lateralis,A. sp.intermediate between atra and barkeri,Anthracosphaerium cf. exiguum, A. aff. propinquum and Naiadites obliquus.

In the northern part of the Garw Valley all the coals of the Four-Feet group are much closer together; the following section of the ground between the Lower Six-Feet and the Two-Feet-Nine measured in a cross-measures roadway at Garw Colliery shows that the several coals are spread over only 16 ft 11 in of strata:

At International Colliery the Four-Feet coals are still closer together and the group is represented in the shaft by a single compound dirty seam 94 ft above the Upper Six-Feet: top coal 9 in, clod 2 in, coal 12 in, clod 46 in, coal 13 in, clay 13 in, coal 3 in.

In the Llynfi Valley the main part of the Four-Feet group lies about 90 to 135 ft above the Upper Six-Feet; here again it has a very variable behaviour. In the north, at Caerau Colliery, a single compound seam has a section similar to that at International : top coal 6 in, bast 2 in, coal 9 in, bast 1 in, coal 15 in, bast 1 in, coal 21 in. At Coegnant a parting of about 3 ft develops within the seam and foreshadows the splitting southwards into two distinct seams, formerly known as the Lower Six-Feet (lower split) and Truro (upper split), and which represent the Lower and Upper Four-Feet seams respectively. The individual coals are thicker than in the areas to the north and east, the shaft record showing the following sections: Lower Four-Feet, top coal 36 in, clod 9 in, coal 27 in, rashes 3 in, coal 4 in; and the Upper Four-Feet, top coal 13.5 in, clod 3 in, coal 15.5 in, rashings and coal 30.5 in. Two further thin coals, each about 6 in, are recorded about 10 and 20 ft below the Lower Four-Feet, and these apparently equate with two thin streaks of coal developed within the seatearth of the main seam at Duffryn Rhondda Colliery to the north. At St. John's Colliery the position is obscure. The only details available are those of the shaft sections, where thrusting may be present. At Duffryn Madoc and Gin collieries the Lower and Upper Four-Feet are reputed to be 6 ft apart, and at Bryn Rhyg, 1200 yd east of Maesteg Station, they were worked separately, the former being said to have been 6 ft thick and the latter 3 ft. At Oakwood and Celtic where both seams were worked they are 30 ft apart: the lower seam is said to be 4 to 6 ft thick with an 18-in parting about 2 ft from the base; and the upper seam 3 ft thick in three leaves.

The roof of the Upper Four-Feet at Coegnant and the highest coal of the group at St. John's yielded the diagnostic fauna, including variants of Anthraconaia sp. nov.(cf. curtata), Anthracosia planitumida, Anthracosphaerium exiguum and A. propinquum.The lowest few inches of the roof mudstones contained crushed poorly preserved shells referred to Anthracosia? and Naiadites obliquus.

The strata from this horizon up to the Two-Feet-Nine vary in the Ogmore and Garw valleys from 15 to 40 ft, being thickest in the northern part of the Garw. In the Llynfi Valley they range from 25 ft in the south to 50 ft in the north. Mudstones make up the bulk, but sandstones occur towards the top in the thicker sections.

The Two-Feet-Nine is renowned for its quality throughout the Ogmore and Garw valleys, and it has been worked extensively. The seam varies little: a top coal, 4 to 5 ft, is separated from a bottom coal of 1 to 11 ft by 6 in to 3.5 ft of seatearth. Variable amounts of cannel up to 12 in are usually present in the top of the seam, and the bottom coal has a thin stone parting in the middle in parts of Western and Wyndham. An abundant 'mussel' fauna, which occurs everywhere about 3 to 5 ft above the top of the seam, is made up of variants of the following species: Anthraconaia cymbula, A. sp. nov.cf. lanceolata,A. sp. nov.cf. wardi, Anthracosia acutella, A.aquilina, A. sp.intermediate between acutella and concinna,A. atra, A. sp.intermediate between atra and planitumida,A. concinna, A. planitumida, A. simulans,Anthracosphaerium propinquum, Naiadites obliquus. Planolites montanus preserved in nodular ironstone is a constant associate of these forms, while at Ffaldau Colliery 'Estheria' was also found.

In the Llynfi Valley the Two-Feet-Nine lies 25 to 50 ft above the Four-Feet, the distance increasing northwards. In the south the intervening strata are mainly mudstones containing numerous ironstone bands and nodules, but as they thicken they become more sandy. The Two-Feet-Nine has been worked at all collieries except Caerau. Throughout most of the area the main part of the seam is about 5 ft thick with a further coal, 1 to 2 ft thick, lying about 2 to 5 ft below, and it may be this lower leaf that crops out [SS 8552 9187], 300 yd W.S.W. of Maesteg Deep Colliery. Over much of the area a plane parting, which locally thickens to an inch or so, is developed 9 to 18 in from the top of the main coal. In the north-west of the valley another parting develops about 2 ft above the floor of the main coal; this thickens rapidly and has limited the working of the seam, hardly any being extracted to the north-west of a line joining. Coegnant and Caerau shafts. The roof of the seam has been examined at Coegnant and St. John's, where it consists of highly slickensided grey mudstone containing worm tracks and sporadic shells, including Anthracosia sp.,Anthracosphaerium cf. exiguum and Naiadites obliquus?.Spoil at Maesteg Deep Colliery yielded a well-preserved and varied fauna of solid shells, all of small size: Anthraconaia librata, A.cf. obscura,Anthracosia aff. acutella, A.cf. aquilina.A. concinna, A. aff. lateralls, A.cf. simulans,Anthracosphaerium cf. affine, A. exiguum, A.cf. propinquum, A.cf. turgidum.There are certain differences from the normal Two-Feet-Nine fauna, notably in the occurrence of the several species of Anthracosphaerium.In this connexion it should be noted that a small amount of a seam variously described as the 'Four-Feet' or 'Seven-Feet' has also been worked from this colliery; it seems likely that the various species of Anthracosphaerium came from this horizon, which presumably equates with the upper part of the Four-Feet.

In general the measures from the Two-Feet-Nine to the Cefn Coed Marine Band vary from about 200 to 250 ft in the Ogmore and Garw valleys. Except for an isolated occurrence of the Cefn Coed band, brought in by faulting, at Glengarw Colliery, these measures can be examined only at Penllwyngwent Colliery, where they were penetrated by a cross-measures drift from the No. 3 Rhondda to the Two-Feet-Nine through the Aber Fault. The strata in this old roadway are much obscured, but both the Cefn Coed and Hafod Heulog marine bands yielded fossils and the horizon of the Britannic Marine Band was identified. The Hafod Heulog Marine Band occurs as a band about 3 ft thick above a thin coal some 120 ft above the Two-Feet-Nine, and yielded foraminifera (including Glomospira sp.), Lingula mytilloides and large Orbiculoidea cf. nitida from dark blue shale. The horizon of the Britannic Marine Band occurs above an 8-in coal 45 ft higher in the sequence, and although it yielded no marine fossils other than doubtful foraminifera, the lithology was that of a typical marine sediment; furthermore Planolites ophthalmoides was obtained some little way above the seam. The Cefn Coed Marine Band is well developed and occurs above a 3-in coal about 100 ft above the Hafod Heulog horizon and 100 ft below the Caedavid (Gorllwyn). The marine strata are about 2 ft thick, consisting of smooth dark mudstone at top and bottom, with Lingula mytilloides and indeterminate gastropods, and about 6 in of blocky mudstone in the middle carrying the following varied fauna: crinoid columnals ; Archaeocidaris aff. acanthifera Trautschold ; ?Crurithyris carbonaria, Chonetes (Tornquistia) diminutus, L. mytilloides; Straparollus sp.,turreted gastropods; Aviculopecten sp., Dunbarella cf. macgregori (Currie), Nuculana cf. acuta,cf. 'Nucula' luciniformis Phillips, ?Nuculopsis gibbosa, Posidonia sulcata (Hind); Orthocone nautiloid, Metacoceras?, Anthracoceras cf. aegiranum;Hollinella sp.

At Wyndham and Western collieries the shaft sections show a complex coal horizon about 50 ft above the Two-Feet-Nine which is clearly the equivalent of the Three Coals of the Aberaman area. In Wyndham No. 2 Shaft the section of this coal reads: top coal 9 in, clod 44 in, coal 9 in, clod 22 in, coal 2 in; the Hafod Heulog Marine Band presumably lies on a coal some 50 ft higher in the sequence and the Cefn Coed Marine Band on a 1.5-in coal 90 to 100 ft still higher. Between these two latter horizons occur two further coals, one of which presumably carries the Britannic Marine Band. The sequence is similar in the Garw Valley: at Ffaldau, for example, the Three Coals horizon, separated from the Two-Feet-Nine by 75 ft of strata, consists of: top coal 10 in, fireclay and clift with balls of ironstone 11 ft, coal 9 in, clift and fireclay 3 ft 9 in, coal 9 in, and the presumed positions of the Hafod Heulog and Cefn Coed marine bands lie 42 ft and 154 ft above respectively.

At Glengarw Colliery the Cefn Coed Marine Band is exposed in a drivage through a fault to the Caedavid workings. It occurs above a 1-in coal about 100 ft below the Caedavid, and the following fauna has been collected: Chonetes (Tornquistia) diminutus,Productid fragments, Orbiculoidea cf. nitida, L.mytilloides; Glabrocingulum sp.,Nuculid lamellibranchs, Palaeoneilo ?,cf. Scaldia fragilis de Koninck; Metacoceras sp.,Anthracoceras cf. aegiranum; Hollinella sp.

This part of the sequence between the Two-Feet-Nine and the Cefn Coed Marine Band in these areas contains a good deal of hard quartzitic sandstone in beds up to about 30 ft thick, those associated with the marine bands corresponding in position with the Cockshot Rocks of the Maesteg area.

In the Llynfi Valley the measures from the Two-Feet-Nine to the Cefn Coed Marine Band crop out in the area around Maesteg and certain of the shafts have been sunk through them. The measures thicken from north to south, from about 225 ft at Caerau to upwards of 300 ft at Celtic, Oakwood, and Duffryn Madoc, though the strata at these latter collieries appear to be highly disturbed. The sequence is similar to that in the Garw and Ogmore valleys, except that the quartzitic sandstones are more prominent, and clay-ironstone bands attain a local importance. About 50 ft above the Two-Feet-Nine occurs a thin coal, which at Maesteg Deep comprises 13 in of coal underlain by 4 in of inferior coal and 5 in of cannel. A short distance above, a coal, about 18 in thick and having a roof of dark cannelly shale with fish scales and spores, appears to be the seam formerly referred to as the Fireclay. A coal said to be 2 ft thick at Maesteg, but only a few inches at Caerau, lies some 30 ft higher in the sequence; this has been called the Coal Tar seam at Maesteg, though it does not appear to be the same as that so called at Oakwood (see below).

The Lower Cockshot Rock, developed within the overlying cyclothem, is 15 to 25 ft thick. The massive quartzitic sandstone lies immediately above the Coal Tar at Coegnant and Caerau while 20 to 30 ft of mudstones intervene to the south. Near Maesteg Deep Colliery its base is sharp and rests on a strongly eroded surface. Quarries are located [SS 8463 9195], [SS 8550 9210] to the north-west and north-east of Maesteg.

A short distance above the sandstone is the seam referred to at Oakwood and Celtic as the Coal Tar or Upper Two-Feet-Nine. At Oakwood two leaves, each 11 in thick, are separated by 9 in of dirt. Comparison with the closely similar sequences at Penllwyngwent and Britannic suggest that the roof of this seam should carry the Hafod Heulog Marine Band. The coal seems to deteriorate rapidly to the north, being insignificant at Caerau and not recorded at Coegnant.

The Upper Cockshot Rock overlies a 6-in coal some 40 ft higher in the sequence. The sandstone is quartzitic on the east side of the valley, though not on the west. It is thickest, over 50 ft, in the Caerau shafts, but it thins southwards to about 30 ft. It has been quarried [SS 8470 9207], [SS 8548 9226] in the Duffryn Madoc area, while in Maesteg itself it is exposed along Garn Road [SS 8565 9096].

The Britannic Marine Band is assumed to lie in the roof of a 3- to 8-in coal almost immediately above the Upper Cockshot. At Oakwood an 8-in coal 35 ft above is thought to be the lower split of the Cefn Coed Marine Band coal, the upper split lying 25 to 40 ft still higher. Sandstone developed to the south between the two coals has been wrongly termed the Upper Cockshot at Oakwood.

The Cefn Coed Marine Band overlies a coal usually about 3 in thick. The band itself has been seen at only one locality—the stream [SS 8575 9328], 200 yd E. of Coegnant Colliery—where it rests directly on seatearth. The lower part consists of 15 in of calcareous mudstone containing an abundant and varied fauna, and this is overlain by 3 ft of smooth dark mudstone with sporadic Lingula.The following forms have been identified from the calcareous mudstone: Zaphrentites postuma (S. Smith); Archaeocidaris sp.;crinoid columnals; Chonetes (Chonetinella) sp., Chonetes (Tornquistia) diminutus, Crurithyris carbonaria, L. mytilloides, Productus (Dictyoclostus) craigmarkensis; Straparollus (S.) sp. nov.,turreted gastropods; cf. Nuculopsis gibbosa, Pernopecten carboniferus,Posidonia cf. sulcata, Schizodus carbonarius (J.de C. Sowerby); orthocone nautiloid; Anthracoceras cf. aegiranum,Gastrioceras sp.; Hollinella cf. bassleri, Paraparchites sp.; Hindeodella sp.,platformed conodonts; fish debris. A.W.W., W.B.E.

Avan Valley and Bryn area

These measures crop out in a strip about half a mile wide on the north side of the Moel Gilau Fault in the neighbourhood of the village of Bryn. Surface exposures, however, are poor and provide little information additional to that obtained by mining.

The Amman Marine Band has been seen only at Bryn Colliery. Here Lingula mytilloides was obtained from 2 in of slickensided mudstone in the roof of a thin coal between the Nos. 3 and 4 seams. Above this several feet of mudstone yielded 'mussels', including Anthracosia cf. ovum and A. sp.cf. aquilinoides,with the 'ghostly' preservation typical of the horizon. At Avon Colliery a similar fauna occurs about 8 in to 3 ft above an 18-in coal which appears to be the first below the Bute: Anthracosia cf. beaniana, A.cf. ovum and Naiadites sp.have been identified. The mudstone in the immediate roof of the coal was highly disturbed and no marine fossils were found, despite the favourable lithology.

The Bute, 30 to 40 ft above, has been worked at Glyncorrwg, Duffryn Rhondda, and Bryn, while the workings from Glyncastle extend into the district. The seam has been named the Peacock at Glyncastle and Glyncorrwg, Seam 'E' at Avon, New Seam at Duffryn Rhondda, and No. 3 Seam at Bryn. Over most of the area it consists of a single clean coal, about 3 ft thick, but at Bryn a 2-in leaf occurs up to 12 in above the top of the main coal, and 6 to 18 in of inferior coal and rashings are present in the floor. At Duffryn Rhondda, Glyncastle, and Glyncorrwg the roof to about 6 in above the coal is planty and 'mussels' are abundant in the overlying 1 to 2 ft. At Avon and Bryn up to 18 in of cannelly mudstone containing many crushed shells are overlain by grey silty mudstone with ironstone bands and solid 'mussels'. The same species are found in all localities: Anthraconaia williamsoni, A.aff. insignis (Davies and Trueman), Anthracosphaerium affine, A. exiguum, A. fuscum, A.turgidum and Naiadites sp. nov.cf. modiolaris.

The Bute is succeeded by 30 ft of strata, mainly mudstone with up to 10 ft of sandstone, locally unconformable, in the middle. In places a thin coal is present midway in the cyclothem.

The Nine-Feet splits southwards between Glyncorrwg and Glyncastle in the north and Avon and Duffryn Rhondda in the south. To the north the seam averages 9 ft in thickness and it has been worked extensively at Glyncorrwg. At Glyncastle the roof consists of black cannelly mudstone with an abundant fauna, including Anthracosia beaniana,A.aff. ovum, A. sp.intermediate between phrygiana and ovum, A. sp.cf. phrygiana.At Glyncorrwg the immediate roof is planty.

To the south the Lower Nine-Feet is called the Seven-Feet at Duffryn Rhondda, the No. 2 Seam at Bryn, the Bute or 'D' Seam at Avon and the Lower Four-Feet at Eryl, south of Mynydd Bychan. It has been worked on a limited scale at all these collieries and it crops at Eryl. Characteristically the seam consists of an upper coal, 3.25 to 4 ft thick, and a bottom leaf, about 1 ft thick. At Avon, Duffryn Rhondda, Cwmavon, and Eryl the two leaves are 6 to 24 in apart, but the separation thickens south-westwards and is 2 to 10 ft at Bryn. Here a thin parting develops 4 in from the top of the seam, and a band of pyrite is frequently found 12 in from the base of the upper leaf. No fauna has been reported from the roof, which locally cuts down into the coal at Duffryn Rhondda and Bryn.

The separation of the Lower and Upper Nine-Feet increases southwards to 45 ft at Duffryn Rhondda, 70 ft at Avon, and from 70 to 100 ft at Cwmavon. Sandy mudstones and siltstones with a few thin sandstones make up the bulk of these strata.

The Upper Nine-Feet has been worked at outcrop from levels near Eryl under the name of Five-Feet, and attempts to work it have been made at Avon and Duffryn Rhondda. Few sections are reliable: it appears to vary from 2.5 to 4 ft thick with thin leaves present in floor and roof locally. At Nantewlaeth it is probably the coal called the New Seam, and was reported to be about 3.5 ft thick. At Duffryn Rhondda the roof yields a sparse 'Red Vein fauna' : Anthraconaia aff. pulchella and Naiadites cf. productus; 'Estheria' sp.was also noted.

The measures from the Nine-Feet to Six-Feet are among the least well-known. In Glyncorrwg the two seams are 100 to 120 ft apart, and two coals have been proved in the intervening measures. The 'Red Vein', equivalent to the Caerau of the Llynfi and Garw areas, lies some 80 ft above the Nine-Feet and has a section: top coal 7 in thin parting, coal 27 to 30 in. About 10 to 15 ft below the Six-Feet a 6- to 9-in coal, with a mudstone roof containing plants and fragmentary shells, appears to equate with the bottom leaf of the Lower Six-Feet in the Rhondda Fawr. At Avon Colliery this coal, about 12 in thick, lies 1 to 2 ft below the Six-Feet. The 'Red Vein', 20 to 30 ft below, has been worked on a small scale: top coal 10 in, clod 4 in, coal 32 in. A further dirty coal, 16 in thick, here lies 20 to 25 ft below the 'Red Vein' and 35 ft above the Nine-Feet. The roof of this coal carries small solid 'mussels' 10 to 16 in above the roof: a typical 'Red Vein fauna' includes Anthraconaia sp. nov.cf. pulchella and Anthracosia cf. nitida.A few hundred yards away the same coal carries a cannelly mudstone with ironstone bands and 'wormy' layers. A crushed fauna of characteristic 'Nine-Feet' type includes such forms as Anthracosia aff. heaniana, A. sp.intermediate between lateralis and ovum, A.aff. phrygiana and A. sp.intermediate between phrygiana and aquilina.

At Duffryn Rhondda the Upper Nine-Feet and Six-Feet are 150 ft apart, and only one coal has been proved between them. This is called the 'Red Vein', and is 24 to 27 in thick with an inch of cannel in its roof, and lies 30 to 40 ft above the Nine-Feet. Anthracosia cf. phrygiana and A. sp.cf. ovum were collected from the roof; 20 ft of mudstones overlain by 60 to 80 ft of sandstone and siltstone follow. It is possible that both here, and at Nantewlaeth, the 'Red Vein' (Caerau) of Avon is washed out by these sandstones.

At Bryn this portion of the sequence is much obscured by structural complications. In the workings a coal has been proved 5 to 15 ft below the Six-Feet. This 'No. 2 Seam', about 3 ft thick and disturbed, appears to be the Caerau Seam of the Llynfi area and the 'Red Vein' of Avon.

The sequence near Cwmavon is anomalous. The distance between the Nine-Feet (Five-Feet) and the Six-Feet (Big) is variously given as being as little as 108 ft and as much as 220 ft, but 175 ft is most often quoted. Three coals are recorded. The uppermost, called the Little Vein, is 15 in thick and lies 10 to 20 ft below the Six-Feet. About 30 ft lower is the Clay Seam, which varies from 2 to 2.5 ft and appears to be equivalent to the Caerau. Some 40 ft below, a seam called the Coal and Mine is 2.5 to 3.5 ft thick and contains several bands of ironstone in its roof.

Except at Avon Colliery the Six-Feet forms a single composite seam. It consists of two main coals, with locally a third thin leaf developed in the middle by splitting of the bottom coal. The following sections are typical:

At Glyncorrwg the immediate roof consists of 3 in of slickensided mudstone; above this 4 ft of dark mudstone carries crushed and solid 'mussels' associated with abundant ironstone nodules; forms include Anthracosia cf. aquilina, A.cf. faba' (Wright), A. planitumida and Naiadites cf. alatus associated with a small form of Planolites cf. ophthalmoides.At Glyncastle a coal up to 6 in thick lies about 4 ft above the top of the seam; the mudstone roof carries plant debris, Naiadites? and Lioestheria sp.The last-named form is characteristically found above the Eighteen-Feet in the Neath Valley.

No reliable sections are known in the area between Bryn and Cwmavon, for the seam here is subject to intense disturbance. It is recorded as being between 5 and 40 ft in thickness, but 8 to 9 ft is probably normal. The roof consists of the ferruginous black-band mudstone distinctive of the seam along the South Crop. Spoil from Bryngymos Colliery [SS 8051 9204] contains well-preserved Anthracosia atra, A.cf. fulva and A. aff. lateralis.At Eagle Brick-pit [SS 7919 9253] and in bell-pit spoil from the adjacent Patch Mawr [SS 7934 9225] the following fauna was collected: Anthraconaia sp.(of subcentralisl pumila group), Anthracosia cf. lateralis,A. sp.cf. phrygiana, Naiadites cf. alatus, N.cf. obliquus and Carbonita cf. humilis.

At Avon Colliery the top coal splits away to form the Upper Six-Feet or Yard; 27 in thick, it lies about 40 ft above the Lower Six-Feet. Its roof is similar to that of the Six-Feet at Glyncorrwg. The fauna includes cf. Planolites ophthalmoides (2 mm. diameter), Anthracosia aff. lateralis, A. sp.cf. planitumida;'wormy' beds are also common. The Lower Six-Feet, 4 ft thick, carries a rider about 5 ft above the seam: it consists of two 1-in bands of coal separated by 5 in of cannel, succeeded by dark mudstone containing Naiadites sp.and abundant Lioestheria sp.

The Four-Feet at Avon is a single composite seam comprising up to 15 ft of coal, rashings and clay partings, and lying some 25 ft above the Upper Six-Feet. At Duffryn Rhondda, where the seam has been worked on a small scale, the section is cleaner and includes four leaves: top coal 8 to 12 in, rashings 4 in, coal 12 to 20 in, clod 2 in, coal 5 in, clod 6 to 22 in, coal (with parting near the middle) 48 in. Two further thin coals, 2 in and 5 in, occur at 9 ft and 17 ft respectively below this composite seam. At Bryn the seam was formerly worked in the south-east of the taking, but no reliable sections exist: in the shaft was recorded 7.5 ft of coal with a thin parting 27 in from the top; a rider seam of 9 to 12 in lies up to 6 ft above. Old crop workings near the Eagle Brickworks suggest that the seam is in two leaves about 10 ft apart, but no details are available. The seam was worked at Nantewlaeth with a section: top coal 8 in, parting 3 in, coal 3 to 4.5 ft.

At Nantewlaeth and Avon the typical 'Four-Feet fauna' was collected, including Anthraconaia sp. nov.(cf. curtata) and Anthracosphaerium aff. propinquum.At Duffryn Rhondda and Bryn similar forms occur together with Anthracosia lateralis, A. planitumida, A.cf. simulans and Anthracosphaerium cf. exiguum;the immediate roof contains Naiadites obliquus preserved as films. At Eagle Brick-pit the typical 'solid' fauna occurs, but at Glyncastle only the crushed fauna was obtained.

The Two-Feet-Nine was formerly worked at Bryn and Cwmavon as the Finery. The seam thins westwards across Bryn from 4.5 to 1.5 ft. This thinning is associated with the oncoming of a sandstone roof, which can be seen at Eagle Brick-pit overlying 2 ft of coal. Some 10 ft lower is the Sulphury Seam, an inferior coal 18 in thick. At Avon the Two-Feet-Nine has been worked south-east of the shafts where it has a section: top coal 36 in, clod 10 in, coal 19 in. At Duffryn Rhondda the whole seam appears to be represented by 15 in of coal, at Nantewlaeth by 9 in of 'rashings' and at Glyncorrwg possibly by 12 in of coal. Only at Avon has the seam yielded an abundant fauna—a typical Two-Feet-Nine assemblage—which includes: Anthracosia acutella, A. atra, A. aff. concinna,A.cf. planitumida and Naiadites obliquus.

The Two-Feet-Nine crops out on the south side of Nant Cwm-farteg, dipping south, and has been worked from old levels adjoining the ruined Farteg-fach farm [SS 8215 9181]. It crops again in the disused Bryn brickyard [SS 8266 9181], on the north side of the stream, dipping to the north-north-east at 15° on the northern limb of the Maesteg Anticline.

The sequence continues upwards in mudstone about 70 ft thick to two thin coals, the Balling and the Silver. These are about 20 ft apart and probably equate with the Fireclay and Coal Tar seams of Maesteg. A few feet above the Silver, a persistent quartzitic sandstone, presumably the Lower Cockshot Rock of Maesteg, has been worked on a small scale south of the Bryn-Maesteg road [SS 8230 9208], while it makes a prominent feature on Mynydd Bychan [SS 796 923]. Above this the strata are poorly exposed. The Upper Cockshot Rock of Maesteg is seen east of Bryn [SS 8251 9213] but dies out near the village. Other impersistent sandstones occur in this part of the sequence, one of which has been quarried [SS 8091 9212] near Bryngyrnos Colliery. The measures above the Two-Feet-Nine were penetrated in all the shafts of the Avan Valley collieries. The group as a whole varies from about 200 ft at Glyncorrwg to 225 ft at Nantewlaeth. Three thin coals appear to be developed between the Two-Feet-Nine and the Lower Cockshot Rock. The latter does not persist as far as the northern parts of the valley, but the Upper Cockshot is well developed, being up to 45 ft thick at Nantewlaeth. Of the marine bands only the Cefn Coed has been seen—at Duffryn Rhondda Colliery, where it rests on 3 in of coal, the first below the Caedavid Seam. Marine fossils were found throughout 18 in of mudstone, the most abundant fauna occurring in pyritic mudstone, 8 in thick, near the base of the band. The following forms have been identified: crinoid columnals; Chonetes (C.) cf. skipseyi, Lingula mytilloides;cf. Nuculana attenuata, Pleurophorella?,Pectinid fragments; Straparollus (S.) sp. nov.; Anthracoceras sp.; Hollinella cf. bassleri and fish debris. The top 10 in of the band carried only L.mytilloides and Orbiculoidea cf. nitida.Lenses of 'cank' occur in the pyritic mudstone. A specimen of this is described by Dr. K. C. Dunham as follows: 'A black compact finely crystalline chalybite-siltstone composed of interlocked well-developed rhombs of chalybite, probably containing some CaMg (CO₃)₂ in solid solution. The carbonate rhombs average 0.05 mm. long and are very even-grained. Irregular small streaks of pyrite are present and there is a sparse matrix of carbonaceous and clay material'. W.R.E.

South Crop

The strata between the Amman and Cefn Coed marine bands crop out along the southern margin of the district in a strip varying in width from about 200 yd near Rhiwsaeson to about 1.75 miles in the Margam–Pyle area, where the measures are not only very much thicker but are also repeated several times by thrusts. Natural sections in this mainly argillaceous sequence are rare and knowledge of the stratigraphy is derived almost entirely from present day workings at the widely separated collieries in the extreme west and centre of the tract, from open-cast works to the west of the Ogmore Valley, and from deep exploratory boreholes in the Margam area. As is also usual with the beds of the upper part of the Lower Coal Measures these measures are everywhere much affected by compressional structures, and continuous undisturbed sections are almost non-existent. Many of the coals were worked, to some extent at least, at all the old collieries along the Crop. The records of these collieries include many sections of cross-measures roadways, all of which show such disturbance, both of the general strata and of the individual coals, that in no instance can satisfactory correlations be made.

West of the Ogmore Valley

In the extreme west the measures are concealed beneath morainic drift up to 100 ft thick, and the outcrops can be inferred only by calculation from mining and borehole information. The great width of the outcrop, more than 3000 yd, between Water Street and Margam Abbey is explained by the existence of major thrusting which apparently repeats the crop of the upper part of the sequence at least three times. In this area the southern limit of the exposed Coal Measures is formed by the down-faulted outcrop of the Keuper deposits. The British Industrial Solvents No. 1 Borehole [SS 8002 8344], 1 mile 1100 yd W. 15° N. of Pyle Church, after penetrating about 70 ft of gravelly drift, entered the Middle Coal Measures just above the Upper Nine-Feet; No. 2 Borehole [SS 7996 8316] some 300 yd S. 12° W. of No. 1, proved 65 ft of gravel and 161 ft of Keuper Marl before entering Coal Measures. It is probable, therefore, that in the area between Pyle and the coast the lower beds of the Middle Coal Measures as well as the Lower Coal Measures 'crop' beneath the Trias to the south of the boundary fault.

Newlands and Cribbwr Fawr collieries have worked extensively the main coals of this group to the south of the Newlands Thrust. Newlands Slant [SS 8104 8349] and Cribbwr Fawr No. 1 Slant [SS 8226 8342] were both driven on the Upper Nine-Feet (South Fawr) Seam, while the No. 2 Slant at Cribbwr Fawr descended along the Lower Nine-Feet (Ail). The only other seam located from the surface in this tract is the Red Vein (North Fawr) on the sub-drift crop of which a small working, Coalbrook Slant [SS 8260 8353], 950 yd N. of Pyle Church, was formerly located. It is inferred that the coals from the Lower Nine-Feet to the Two-Feet-Nine crop out in a belt of country 500 to 700 yd wide situated, in general, to the north of a line drawn through Newlands and Cribbwr Fawr collieries. Although Cribbwr Fawr workings are now inaccessible the records show the sequence and coals to be very similar to those at Newlands which have been examined in detail during the present survey.

The Amman Marine Band is 3.5 ft thick at Newlands and yielded the following fauna: pyritized sponge spicules; pentagonal crinoid columnals; Lingula mytilloides, Spirifer pennystonensis T. N. George; Planolites ophthalmoides,pyritized 'worm burrows' ; and fish fragments. The band proved to be 9 ft 2 in thick in the Margam Park No. 1 Borehole and provided the following additional forms: foraminifera including cf. Hyperammina and Rectocornuspira ? and small gastropods.

Some 40 to 60 ft of argillaceous strata separate the Amman Marine Band from the Bute (or Drydydd) Seam, which is normally 5 to 5.75 ft thick with an impersistent layer of 'brass' about 10 to 12 in from the top. The roof is highly characteristic over large areas, the following being typical:

About 30 to 70 ft above the Bute a coal, 6 to 12 in thick, is separated from the lower Nine-Feet by 12 to 20 ft of mudstone and seatearth. Scattered shells, including Anthraconaia williamsoni and N.cf. product us,are recorded between 4 and 12 in above this coal.

The Lower Nine-Feet (or Ail) is characteristically a dirty seam consisting of three variable leaves : top coal 10 to 16 in, rashings and fireclay 0.5 to 34 in, coal 12 to 14 in, stone and rashings 0.5 to 17 in, coal 12 to 22 in. The roof has always proved to be barren of animal fossils.

Lying some 40 to 90 ft higher in the sequence, the Upper Nine-Feet (or South Fawr) was the principal seam worked at both Cribbwr Fawr and Newlands. In normal section the seam varies from about 10 to 13 ft and contains several distinct though thin partings; it is however subject to much structural variation, being in places as much as 30 ft, when it is soft and difficult to work. Under these conditions large quantities of coal were left behind and gob fires in the old workings were not infrequent. It is difficult to reconcile this great thickness of coal with the 3 to 5 ft which is normal in the nearest part of the main coalfield to the north. In the North Drift exploration through the Newlands Thrust a seam was proved in a tophole about 200 yd north of the disturbance which is almost certainly the Upper Nine-Feet: the section, top coal with 'brass' 22 in, rashings 1.5 in, coal 14 in, rashings 0.5 in, and coal 26 in, is markedly different from that of the workings and is much more like that of the Maesteg and Garw valleys.

The true roof of the Upper Nine-Feet is a massive blocky mudstone, which immediately above the coal carries a sparse 'Red Vein fauna' with cf. Anthraconaia pulchella,Anthracosia cf. nitida and Naiadites sp.cf. productus.

The thickness of measures between the Upper Nine-Feet and the Red Vein is very variable, being as little as 50 ft in the North Drift at Newlands and as much as 126 ft in the Margam Park No. 2 Borehole. Strong mudstones with a few sandstone layers in the lower part make up the bulk of these strata and the Red Vein is usually underlain by a thick seatearth.

At Cribbwr Fawr and in much of the Newlands area the Red Vein (North Fawr) consists of two leaves: top coal 18 to 30 in, rashings 6 to 12 in, bottom coal 46 to 52 in. Locally, however, as in the No. 2 Scheme district at Newlands, the top coal splits away from the main coal, from which it may be separated by upwards of 20 ft of strong mudstone with ironstone, a hard fine-grained sandstone often being present at the base. Under these conditions the top coal is markedly thinner, 6 to 16 in being recorded in the No. 2 Scheme. A sparse fauna, consisting of typical Red Vein forms, occurs above the top coal and includes Anthracosia cf. angulata and A.cf. nitida.

The Red Vein is separated from the Caerau Vein (Caegarw) by about 30 ft of mudstone with sandstone bands. The Caerau is the topmost seam proved with certainty at Newlands and its section of 5 to 6 ft of top coal with irregular layers of coal and rashings up to 8 ft thick in the floor, is typical of the relatively small areas so far worked. The roof consists of about 12 ft of soft mudstone with poorly preserved planty debris, followed upwards by strong grey silty mudstone with good plants, among which the following have been identified: Alethopteris lonchitica,Lepidodendron obovatum Sternberg, L. ophiurus, Mariopteris nervosa, Neuropteris gigantea, N. obliqua, N.pseudogigantea and Sphenopteris sp.

On the north side of the Newlands Thrust several of the coals of this group were formerly worked at Bryndu Colliery, about one mile north-east of Pyle Church. The site of this colliery was presumably determined by the extensive bell-pitting of coal which in early days took place in the rectangle of ground between Bryndu Slip and the road from Waterhall Junction to Pentre and Aberbaiden, that is, close to Bryn-du Fawr Farm [SS 8340 8400]. In this area a window of solid rocks, overlain only by head deposits, appears from beneath glacial drift and includes all seams from the Yard to the Two-Feet-Nine. The crops of the Upper Nine-Feet, Red Vein and Caerau, in particular, are marked by lines of closely spaced bell-pits, and just to the north of Bryn-du Fawr spoil from crop workings yields blackband fragments carrying 'mussels' typical of the Six-Feet Seam. Bryndu Slip itself was driven on the Upper Nine-Feet. Westwards from Bryndu the crops of the coals of this group presumably lie beneath thick drift in a band several hundred yards wide extending through Caegarw [SS 8220 8375] and Eglwys-Nynnid [SS 8030 8470].

The few records still available of the coals at Bryndu Colliery do not seem to be wholly reliable. A seam, presumably the Bute, is given as 50 in thick, and the section of the Lower Nine-Feet (Ail) appears to be: top coal 14 in, parting 1 in, coal 11 in, clod 4 in, coal 20 in. The various sections given for the Upper Nine-Feet (South Fawr) indicate that the coal was frequently in a disturbed condition; according to one authority it was generally about 14 ft in thickness.

Between Bryndu and Cribbwr Fawr collieries the Newlands Thrust takes on a markedly west-north-west to east-south-east trend and abuts against the boundary fault in the Kenfig Hill area before running out of the Coal Measures beneath the Trias. The eastwards continuation of the crop from the Bryndu area through the ground north of Mill Pit [SS 8396 8367] and Park Slip [SS 8792 8352], therefore, constitutes the southernmost outcrop of these measures in this part of the coalfield. The coals of this group do not appear to have been worked to any great extent at Mill Pit, Cefn or Park collieries, presumably because of structural difficulties. However, in the area between Mill Pit in the west and Evanstown in the east the ground was extensively worked over to a depth of 20 to 30 ft in search of ironstone, which, in the mid-nineteenth century, was smelted at the Cefn Iron Works, just over a half-a-mile northwest of Cefn Cross. This patch-working for ironstone was associated with bell-pitting for coal on a large scale.

Before the recent extensive opencast mining the shales of the Amman Marine Band were to be seen as spoil from shallow workings at several points along the crop between Cefn and Park collieries: they consisted of dark blue micaceous paper-shales showing a tendency to yellow 'sulphury' weathering, and carrying abundant Lingula mytilloides.These were observed at the following localities: [SS 8656 8351] 300 yd S. by W. of Ysgubor-y-parc Farm; [SS 8722 8358] 800 yd W. by N. of Park Colliery; and [SS 8766 8368] 350 yd W. 30° N. of Park Colliery.

The characteristic blackband of the Six-Feet was observed in the roof of the seam proved in trial pit No. 4 of the Kenfig Opencast Site, sunk to a depth of 36 ft at a point [SS 8505 8410] 1750 yd N. 40° E. of Kenfig Hill Church, and from the spoil of old bell-pit workings [SS 8576 8393] 650 yd S.S.E. of Aberbaiden Farm. This seam was exposed in a small area near the plane of the Kenfig Thrust in the Kenfig Opencast Production Site [SS 848 841] where variants of the following forms were collected from the roof: Anthracosia atra, A. aquilina,A. aquilinoides, A. sp.cf. elliptica and Naiadites sp.At all these localities the Six-Feet exists as a single though usually disturbed seam apparently about 6 ft thick. On the Cefn Opencast Site about one mile to the east, a trial-cut approximately 400 yd north of Cefn House [SS 8607 8385] showed the seam split into its usual two component parts separated by some 10 to 12 ft of seatearth. The Lower Six-Feet was about 3 to 4 ft thick; the Upper Six-Feet, 2 ft in thickness, carried a blackband fauna: Anthracosia atra, A.cf. aquilinoides, A. sp.intermediate between atra and barkeri, Naiadites cf. angustus Trueman and Weir and N. obliquus.

An exploration trench approx. [SS 864 836] on the Cefn Opencast Site, about 200 yd S.S.W. of Ysgubor-y-parc, on the west of Ffordd-y-gyfraith, exposed the measures from the Amman Rider to the presumed Caerau Vein. The beds above the Nine-Feet were very disturbed (see (Figure 43)), but the following section gives a probable interpretation of the sequence:

The crop of the Two-Feet-Nine has been proved by drilling followed by opencast working almost continuously from a point [SS 8167 8500] 300 yd S.S.W. of Hirwaun Farm as far east as [SS 8803 8414] 200 yd S. by E. of Park Farm, a distance of 4 miles. The Upper and Lower Four-Feet seams below were also proved over much of this ground, the crops lying 50 to 100 yd south of that of the Two-Feet-Nine. The Opencast Production Sites were known (reading from W. to E.) as East Lodge, Bryndu West, Kenfig, Cefn, and Park Farm.

Measurements made at the Kenfig and Cefn Opencast sites give a good section from the Lower Four-Feet to some distance above the Two-Feet-Nine, measures which were remarkably constant in character over the entire length of opencast working.

A similar section was proved in the Oaklands Drift [SS 8499 8413] which was sunk on ground immediately adjoining the Kenfig Opencast Site to the east. This trial followed the Lower Four-Feet from outcrop until it was lost in disturbed ground some 250 ft from the surface. After passing through the Kenfig Thrust the Caerau Vein was located in a disturbed condition and a level cross-measures roadway driven to the Two-Feet-Nine. The section below the Lower Four-Feet was as follows:

Throughout this area the Two-Feet-Nine Rider or Lantern Seam is about 28 in thick and it was worked to some extent during opencast operations. It normally lies about 70 to 100 ft above the Two-Feet-Nine and the roof carries 'mussels' similar to those above the Two-Feet-Nine. The Two-Feet-Nine was intensely disturbed throughout the length of the opencast workings, being particularly subject to 'overlapping'. Thicknesses of coal varied from 0 to 30 ft or more, and nowhere was a section observed which could be regarded as normal (see (Figure 44), p. 261). It is thought, however, that the true thickness of the coal is about 5 ft. The Upper Four-Feet also was always subject to much disturbance, the coal often proving to be thinner than normal due to small scale lag-faulting. An extensive wash-out in the Lower Four-Feet was proved in the Oaklands Workings.

Immediately to the west of Evanstown the measures from the Amman Rider to the lower part of the Four-Feet group of coals have been quarried on a large scale for the making of bricks by the Tondu Brick Works Co., Ltd. The quarry [SS 890 840] covers an area of about 12 acres and has been excavated to a depth of about 80 ft. The following section was measured in April 1957.