Geology of the South Wales Coalfield special memoir The Upper Carboniferous and later formations of the Gwendraeth Valley and adjoining areas in parts of the Carmarthen (229), Ammanford (230) and Worms Head (246) sheets

By A. A. Archer. Palaeontology by M. A. Calver, M. Mitchell, and W. H. C. Ramsbottom

Bibliographical reference: Archer, A. Geology of the South Wales Coalfield special memoir The Upper Carboniferous and later formations of the Gwendraeth Valley and adjoining areas in parts of the Carmarthen (229), Ammanford (230) and Worms Head (246) sheets. London HMSO, 1968.

Natural Environment Research Council. Institute of Geological Sciences

Memoirs of the Geological Survey of Great Britain England And Wales

Geology of the South Wales Coalfield special memoir The Upper Carboniferous and later formations of the Gwendraeth Valley and adjoining areas in parts of the Carmarthen (229), Ammanford (230) and Worms Head (246) sheets

A. A. Archer, B.Sc., A.M.I.M.M.

Palaeontology by M. A. Calver, M.A., M. Mitchell, M.A. and W. H. C. Ramsbottom, M.A., Ph.D.

London Her Majesty's Stationery Office 1968. © Crown copyright 1968. Published by Her Majesty's Stationery Office.

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

Preface

This district was first surveyed, on the one-inch scale, over 100 years ago, largely by Sir W. E. Logan. The original six-inch survey was carried out by E. E. L. Dixon and T. C. Cantrill, under the direction of Sir A. Strahan, from 1901-4. The resurvey of the South Wales Coalfield, commenced after the last war in the Pontypridd and Swansea areas, was extended in 1951 when Mr. A. A. Archer started work on the western part: the mapping, carried out under the supervision of Dr. V. A. Eyles and later of Dr. F. B. A. Welch, was completed in 1956. National Grid six-inch-to-mile geological maps of the district are published and new colour-printed editions of the Carmarthen (229) Sheet are now issued. The National Coal Board's programme of deep boreholes in the Cynheidre area was completed in 1963 and this account includes information it yielded: Cynheidre Shaft No. 4, completed in 1964, and only about 50 yd from Shaft No. 3, is included in Appendix I only.

As six-inch geological maps of the Carmarthen (229) Sheet were available, it was decided to restrict the resurvey of that sheet to the area occupied by Coal Measures: later the area was extended to include most of the Millstone Grit. The Coal Measures on the coastal strip west of Llanelly on the Worms Head (246) Sheet were also resurveyed. While the mapping was proceeding the National Coal Board were exploring the Cynheidre area with a view to sinking a new, very large colliery, which, when completed, was connected with Great Mountain No. 3 Shaft sunk in 1939-41 on the adjoining Ammanford (230) Sheet.

As only parts of the Carmarthen and West Gower sheets have been resurveyed, to avoid splitting the description of the Coal Measures of these adjacent areas, and because the detailed knowledge of the succession and structure proved in Cynheidre Colliery and the associated deep boreholes is best described in one account, this Memoir takes the form of describing the Upper Carboniferous rocks of parts of three one-inch sheets, although only a small area of Sheet 230, Mynydd Sylen (shown by a dotted line on (Figure 1)), is included.

Throughout the work there has been close co-operation between Mr. Archer and the staff of the Palaeontology Department, especially with Mr. M. A. Calver and Mr. M. Mitchell who identified the Coal Measures fossils, other than plants (identified by Dr. R. Crookall), and Dr. W. H. C. Ramsbottom who determined the Millstone Grit fossils. The fossils were collected by the author, with the considerable assistance of Mr. T. R. Hawkins, Dr. B. Kelk and Mr. I. H. S. Hall on the Cynheidre boreholes: Mr. R. J. Wyatt and Mr. D. E. White collected from some of the opencast coal sites. Dr. R. Dearnley, Dr. J. R. Hawkes and Mr. R. K. Harrison have examined lithological specimens, and the former has contributed an account of the facies variation and petrology which is included in Chapter III. The memoir has been edited and seen through the press by Dr. J. R. Earp.

At all times the work was greatly assisted by the enthusiasm of the N.C.B. officials concerned : by the Area Chief Surveyor, Mr. E. S. Rees, and the Group Surveyors, Mr. G. O. Thomas and Mr. K. C. Treharne and their staff, and by the Planning Department at Cynheidre Colliery, particularly Mr. P. S. Rose who was responsible for recording the complex details of the underground drivages. Mr. H. F. Adams, then Div. Coal Survey Officer, has contributed a short account of the properties and economics of the coals. Mr. R. H. Price joined the South-Western Division of the N.C.B. in 1957, and has been responsible for much of the detailed correlation and interpretation of structures in Cynheidre Colliery from that time. The Cynheidre, and the later Pentremawr boreholes were examined jointly by Survey and N.C.B. geologists, Mr. Price being assisted by Mr. J. Malthus. The role played by Mr. Price in elucidating the details of the structure proved underground in Cynheidre is acknowledged in Chapter 7, the relevant parts of which were written in collaboration with him. Friendly interest was shown by many other N.C.B. officials, both deep-mine and opencast, who, realizing the important part played by this area in the pioneer work on Coal Measures stratigraphy at Swansea University, under the leadership of the late Sir Arthur E. Trueman, went out of their way to help.

K. C. Dunham. Director. Institute of Geological Sciences, Geological Survey Office, Exhibition Road, South Kensington, London SW7. 29th December, 1967

List of six-inch maps

The National Grid six-inch maps in the following list include all the area described in this account: all have been published (uncoloured). The six-inch 'county' maps on which the results of the original survey were published are shown in brackets: all are in Carmarthenshire. Except for part of SN 50 SW, mapped by Mr. J. V. Stephens, all were surveyed by the author.

• SN 30 SE Tywyn Burrows (53 S.W.; 57 N.W.)
• SN 40 NW Kidwelly-Mynydd-y-Gareg (46 S.W., S.E.; 53 N.W., N.E., S.W., S.E.)
• SN 40 NE Cynheidre-Carway (46 S.E.; 47 S.W.; 53 N.E., S.E.; 54 N.W., S.W.)
• SN 40 SW Pembrey-Burry Port (53 S.W., S.E.; 57 N.W., N.E.)
• SN 40 SE Trimsaran-Llanelli (53 S.E.; 54 S.W.; 57 N.E.; 58 N.W.)
• SN 41 SE Pontyberem-Llangendeirne (46 N.E., S.E.; 47 N.W., S.W.)
• SN 50 NW Mynydd Sylen (47 S.W., S.E.; 54 N.W., N.E., S.W., S.E.)
• SN 50 SW Lliedi Valley-Llanelli (54 S.W., S.E.; 58 N.W., N.E.)
• The following map is available in manuscript: SS 49 NW Pembrey Burrows-Whiteford Point (Carm. 57 N.W., N.E., S.E.; Glam. 21 N.E.)

Chapter 1 Introduction

Geographical setting and industrial background

The district described in this Memoir is the westernmost part of the South Wales Coalfield, the Coal Measures of Pembrokeshire being considered a separate coalfield, not only because Carmarthen Bay intervenes, but also because it is not certain that Coal Measures are continuous from one to the other.

In contrast to much of the rest of the Coalfield, this district rises above 900 ft only on Mynydd Sylen (933 ft), and as a consequence the greater part is given to mixed farming, only a small area being covered by rough moorland vegetation, the critical height being about 800 ft. The Carboniferous Limestone and Basal Grit of the Millstone Grit form a prominent scarp feature, rising to 861 ft on Mynydd Llangendeirne, which separates the strike valleys of the rivers Gwendraeth fach (little) and Gwendraeth fawr (big). Locally the 'Gwendraeth Valley' is taken to be that of the Gwendraeth fawr. The Gwendraeth fach breaks the scarp near Kidwelly, where it is cut by a complex fault system, and the two rivers share a broad estuary west of Kidwelly. The pennant-type sandstones or 'pennant' of the Upper Coal Measures form the high ground between the Gwendraeth fawr valley and the Loughor estuary (or Burry River) which separates the prominent Coal Measures dip slopes behind Burry Port and Llanelli from the older rocks on the northern margin of the Gower Peninsula. Viewed from the hill to the north-west of Kidwelly, the pennant scarp is conspicuously broken by the upper part of the Morlais Valley, following a prominent westerly trending disturbance. This pennant upland ends abruptly at Pembrey, to the west of which there is a substantial tract of alluvium and blown sand, Tywyn (or Towyn) and Pembrey Burrows.

The economy of this district was once very largely dependant on coal and, to a much lesser extent, ironstone mining with subsidiary farming, although these activities were accompanied round its margin by the tin-plate industry at Llanelli and Kidwelly, smelting at Burry Port and the manufacture of refractory bricks at Kidwelly. The coal industry is now concentrated in two collieries, but the importance of anthracite is demonstrated by one of these, Cynheidre, being one of the largest projects undertaken by the National Coal Board. A considerable tonnage of coal has been won from opencast mines and prospecting is continuing. The tradition of miners running small farms continues in some cases and silica rock quarrying and refractory brick-making are still important industries, but the smelting and tin-plate industries are dead in this area, the latter largely being concentrated in large plants east of Llanelli. As a result of the changes in the industrial scene the docks at Llanelli, Burry Port, Pembrey and Kidwelly are now disused.

Geological sequence

The formations present in this district and shown on the maps, are as follows:

Solid Formations

Geological history

At the end of a period during which very little terrigenous material was being transported to the sea, when the Carboniferous Limestone was deposited, there was gentle folding and uplift of the neighbouring northerly landmass. In the district described this movement is marked by an unconformity at the base of the Millstone Grit, and by the deposition of a thick series of highly quartzitic sands derived from the weathering of the earlier mature landscape, now consolidated to form the Basal Grit. Rhythmic movements in the sedimentary basin, subsidence being followed by sedimentation until the water was very shallow, are evidenced in the Basal Grit and are clearly seen in the Millstone Grit Shales and in the Coal Measures. The period when the water was shallowest is marked by the presence of the roots of plants which were able to establish themselves; often during Coal Measures times sufficient vegetation grew for the accumulated debris to form a coal seam after its relatively rapid submergence, burial and compaction in an anaerobic environment.

The water which covered the district during at least the initial period of submergence was marine in Millstone Grit times. In this district the transition from the conditions of deposition of the Millstone Grit to those of the Coal Measures is gradual and the typical Coal Measures environment, in which the submerging sediments were covered by less saline ('non-marine') water, was not regularly established until several hundred feet of Coal Measures sediments had been deposited. During Lower Coal Measures times the periods of very shallow water stability, and plant growth, became longer, and the thicker coal seams, representing a substantial thickness of decaying vegetation, were formed. The Middle Coal Measures were deposited under similar conditions, although at intervals marine incursions still occurred, but above the highest, marked by the presence of the Upper Cwmgorse Marine Band at the top of the Middle Coal Measures, sand, hitherto not commonly abundant in the sediments deposited, containing a substantial proportion of material other than quartz, entered the district later to form the pennant-type sandstone, a subgreywacke, characteristic of, and forming a large part of, the Upper Coal Measures. The 6600 ft of Coal Measures strata and the underlying 800 ft of Millstone Grit Series are a corn-formable sequence, there being no evidence of intra-Upper Carboniferous movement. By analogy with the area only about 6 miles to the east of the district described it is probable that at least another 2000 ft of Coal Measures were deposited hereabouts, although not preserved.

The Palaeozoic rocks were folded and faulted during the Hercynian orogeny at the end of Carboniferous times, when all the main elements of the present structure were formed. There is no direct evidence of the greater part of the later geological history of this district. From evidence gathered elsewhere in South Wales it is likely that the Hercynian mountains were eroded under desert conditions in Permian and Triassic times and possibly submerged and covered by later marine Mesozoic sediments, which were removed during a later period of uplift. The present drainage pattern was initiated after regional tilting, and the essential elements of the present topography were formed before the onset of glaciation during the Pleistocene Period, when the whole district was covered by ice. Since the final retreat of the ice, and the consequent rise in sea level, substantial areas of alluvium and blown sand have been deposited, a process which is actively continuing today.

Chapter 2 Millstone Grit Series

General

Distribution and limits

The outcrop of the basal sandstone of the Millstone Grit Series gives rise to the ridges of Mynydd-y-Gareg, Mynydd Llangendeirne and Mynydd Cerig, which form the watershed between the Gwendraeth Fach and Gwendraeth Fawr valleys, and the long bare dip slopes of quartzite are prominent features of the landscape. The thin shales between the beds of quartzite, and the overlying shales with thin beds of sandstone, are generally poorly exposed. On the western margin of the Carmarthen (229) Sheet, the Basal Grit forms an extensive dip slope terminating in good exposures at Ragwen Point.

At the time of the original six-inch survey, a threefold lithological division of the Millstone Grit was adopted, the 'Basal Grit', the 'Shales' and the 'Farewell Rock', the top of the last-mentioned being taken as the top of the Millstone Grit Series. Without consideration of the palaeontological evidence, it is not surprising that sandstones at many horizons were identified as the 'Farewell Rock'.

Since the original survey, the results of research by Bisat and others in the north of England have been applied by Evans and Jones (1929) and Ware (1939) to the area now described. The base of the Millstone Grit of South Wales is still taken at the base of the Basal Grit, and it has been internationally accepted that the top of the Namurian, which equates with the Millstone Grit, should be taken at the base of the Gastrioceras subcrenatum Marine Band. Leitch, Owen and Jones (1958, p. 462) have demonstrated that the term 'Farewell Rock' no longer has any stratigraphical significance, and here it is sufficient to state that in the western part of South Wales, sandstones ranging from the Cwm Berem Sandstone, a short distance below the Rhasfach Vein (Ware 1939, pp. 195-7) and at the top of the Anthraconaia lenisulcata Zone, to a sandstone below the G. subcrenatum Marine Band (Jenkins 1962, pp. 71, 87), have been called 'Farewell Rock'. There is no evidence in this area that any of these sandstones is diachronous. Typically the 'Farewell Rock' of Carmarthenshire, as defined at the time of the original survey, occurs near the top of the G. subcrenatum cyclothem, but west of the Tumble Fault the name was applied to what is now known as the Cwm Berem Sandstone. The Rhasfach Vein, which was taken as the base of the Coal Measures where no 'Farewell Rock' was recognizable (Strahan and others 1909, p. 86), is about 700 ft above the top of the Millstone Grit.

Relationship to underlying beds

As the present work was confined to the Upper Carboniferous beds, the Millstone Grit being mapped only in order to establish the position of the base of the Coal Measures, little new evidence on the relationship between the Millstone Grit and older beds has been obtained. Since the publication of the Carmarthen (229) Sheet Memoir (Strahan and others, 1909) this relationship has been studied by several workers, and their conclusions will be summarized.

No reference is made in that memoir to an unconformity at the base of the Millstone Grit, but doubts were expressed about the age of the Tenlan Quartzite', the name applied to small areas of quartzite resting on Lower Old Red Sandstone which were found about 0.75 mile west, 1 mile N.N.E. and 2.5 miles N.E. of Kidwelly, which have not been resurveyed. Their flora suggested that the 'Penlan Quartzite' "cannot be older than the Upper Old Red Sandstone", and although its field relations were described as "extremely obscure" it was classified as Upper Old Red Sandstone (Strahan and others 1909, pp. 63, 68-70). Subsequent work led Dixon and Pringle (1927, pp. 123-6), both of whom were concerned with the first description of the occurrences, to conclude that the 'Penlan Quartzite' is of Millstone Grit age. The possibilities that the anomalous position of these quartzite outliers could be explained by low-angle thrust faulting, or by solution subsidence through the Carboniferous Limestone, were dismissed, and Dixon and Pringle concluded that the unconformity at the base of the Millstone Grit, first noted by Jones (1907, p. 53) in the Haverfordwest district is responsible for rapid overstep of the Carboniferous Limestone by the Millstone Grit in the Kidwelly area. The line on which the inferred marked uplift occurred, subsequently called the 'Penlan Axis' by Pringle and George (1937, p. 76), appears to be parallel with the west-south-westerly Llandyfaelog and Kidwelly disturbances which are here approximately parallel to the Towy Anticline, and may be compared with the Vale of Neath Disturbance, which has a similar trend and was also active in Carboniferous time (Owen 1954, p. 353). George (1927, pp. 51, 63) has described overstep of the Carboniferous Limestone by the Millstone Grit to the north-east, the Upper Limestone Shales, which are present at the south-west end of Mynydd-y-Gareg, being overstepped by the Basal Grit near Four Roads, whereas at Meinciau most of the Dibunophyllum Zone is missing. Fossils collected during the resurvey from spoil from a well [SN 5010 1384] National Grid References in this memoir lie within the 100 km square SN, unless otherwise indicated.60 yd south-east of Gam Bwll, on Mynydd Cerig, include Buxtonia sp.  nov. [with coarse costae] and Undaria? sp.  nov. in addition to Lingula sp., Linoproductus sp. and Orthotetid fragments which suggest that the beds penetrated, which include 'rottenstone', shale and chert, are of very high Lower Carboniferous (Viséan) age, presumably the Upper Limestone Shales, which are known to be present near Maes-Dulais (Strahan and others 1907, p. 79) about 0.33 mile to the north-east. The same species of Buxtonia was collected from the tip of an old lead mine on Mynydd-y-Gareg (Strahan and others 1907, pp. 79-80). There is no positive evidence of the presence of Upper Limestone Shales in the valley of the R. Gwendraeth Fach between the river and the main Carmarthen–Kidwelly road: as this outcrop is in places less than mile from 'Penlan Quartzite' it is likely that much of the Carboniferous Limestone has been overstepped.

It may be noted that the Carboniferous Limestone is unconformable on the Old Red Sandstone in the same area, the Brownstones being overstepped on Mynydd-y-Gareg. At Kidwelly the Lower Limestone Shales rest unconformably on the Senni Beds, and west of the River Towy similar beds rest on the Red Marls (Strahan and others 1907, p. 4).

Faunal phases and cyclothems

The cyclic sedimentation characteristic of the Upper Carboniferous is reflected in the Millstone Grit shales by a gradual change in each cyclothem from dark grey mudstones with a relatively rich fauna, generally including goniatites, through similar, but normally lighter coloured, beds with a restricted fauna, to silty beds. Thin sandstones are developed near the top of some of the cyclothems, and less commonly the top is marked by the presence of a rootlet bed or seat-earth. In the Basal Grit the variation in coarseness of the sediments follows the same pattern (Robertson 1932, p. 54), but as the argillaceous rocks are thin the variation is best seen in the arenaceous beds: most of the interbedded mudstones have yielded marine fossils in this district and rootlet beds and thin coal streaks are locally present.

The greater part of the argillaceous rocks in every cyclothem contains at least scattered marine fossils, and in this district no non-marine shells are known in the Millstone Grit. The coarser beds are normally devoid of animal fossils and although the presence of a seatearth indicates very shallow water, or emergence, examples are known in the Upper Carboniferous (e.g. the top of the Lower Cwmgorse Marine Band cyclothem) where a seatearth was established directly above marine beds. Notwithstanding that, from the evidence available, it is likely that most, if not all, of the Millstone Grit was deposited in a marine environment, it is convenient for descriptive purpose to continue the use of the term 'Marine Band' to describe that part of the cyclothem with the richest and most varied fauna, or the 'most marine' part of an essentially marine sequence.

Ramsbottom, Rhys and Smith (1962, p. 114) have described in detail the faunal phases in part of the basin facies of the Millstone Grit in Derbyshire. The littoral environment in which the Millstone Grit Shales of the district here described were deposited is reflected not only by greater lithological variation but in the faunal phases. Thus although the phases defined in Derbyshire:

• 1.      fish phase
• 2.      Planolites phase
• 3.      Lingula phase
• 4.      mollusc spat phase
• 5.      Anthracoceras or Dimorphoceras (A. or D.) phase
• 6.      thicker-shelled goniatite phase ('goniatite phase').

with the exception of the first, can be recognized in general terms, there are significant variations. Planolites, especially P. ophthalmoides Jessen, is normally abundant only in phase 2, but scattered specimens are found in phases 3 and 4, and more rarely in 5. Lingula, virtually confined to the Lingula phase in Derbyshire, has been found in all except the Planolites phase. As in Derbyshire mollusc spat are locally present in the Anthracoceras or Dimorphoceras phase and A. or D. may be associated with the thicker-shelled goniatite phase.

The different environment is also reflected in the presence of two faunal assemblages which are not present in Derbyshire. One of these may be called a lamellibranch phase, characterized by the presence of scattered Nuculid lamellibranchs (present only as spat in Derbyshire) in places accompanied by Planolites, Lingula, gastropods and mollusc spat, and more rarely by A. or D. The mudstone in which this phase is found is locally rather siltier than in the A. or D. and thicker-shelled goniatite phases. A brachiopod phase, typically containing calcareous brachiopods, is developed in two cyclothems, the best example being closely associated with the goniatite phase of the Gastrioceras cumbriense cyclothem, where Chonetids, Productids and Spiriferids are present in a thin limestone. The faunal phases in Derbyshire tend to occur in an order determined by increasing salinity and, perhaps, increasing depth of water, clearly reflecting the cyclic deposition of the sediments (Ramsbottom and others 1962, p. 114). This is also true of the same phases in this area, and the brachiopod phase is clearly associated with the 'most marine' part of cyclothems in which it occurs.

The position of the lamellibranch phase in the sequence is more variable, it having been found between A. or D. and spat, Lingula and A. or D., Lingula and barren beds, A. or D. and Lingula phases, or associated with Planolites phases, in the regressive part of a cyclothem (i.e. above the 'most marine' beds); it has also been found below the 'most marine' beds, occurring between barren beds and brachiopod phase, Lingula and brachiopod, and Lingula and A. or D. phases. This variability in the position of the lamellibranch phase (Figure 2) suggests that it may reflect environmental factors other than salinity or depth.

The presence at two horizons of relatively rich faunas, including Crurithyris sp.,which may be regarded as modified brachiopod phases, when considered in conjunction with the general sequence of faunal phases and the lithological variations (Figure 2), (Figure 3), justifies the identification of two cyclothems which do not contain goniatites (Archer 1965, p. 147 and (Figure 2)).

The G. cumbriense cyclothem (p. 7) is almost ideal, in the sense that the faunal phases indicate a general decrease in salinity as sedimentation proceeded after an initial relatively rapid subsidence, which caused submergence of the land surface represented by the seatearth at the top of the preceding cyclothem. Optimum conditions for the establishment of a brachiopod fauna and deposition of a limestone were achieved after nearly 6 ft of silty mudstone, with a lamellibranch faunal phase, had been deposited. No sandstone is present in the barren phase at the top of this cyclothem in the borehole, although thick sandstone occurs at this horizon in the extreme west of Carmarthenshire (Jenkins 1962, p. 71; Archer 1965, p. 147).

Basal Grit

The Basal Grit consists of a series of massively-bedded orthoquartzites with thin intercalations of shale, mudstone and finely laminated siltstone, locally with rootlets and thin coal partings. The beds of orthoquartzite are commonly very fine grained at the base, becoming coarser upwards, the top foot or so of some of the beds containing rounded quartz pebbles. Immediately overlying some of the beds of shale, and near joints and faults, the rock is locally friable, apparently due to the leaching of the siliceous cement, a process which has been much more extensive east of the area described, where 'silica sand' derived from the Basal Grit is of commercial importance.

The contact between the Millstone Grit and the Carboniferous Limestone was nowhere seen, and the only exposure of beds which are assumed to be equivalent to the Plastic Clay Beds, at the base of the Basal Grit in the Ammanford area, is on Mynydd Cerig, where about 7 ft of clay and very fine-grained siliceous material were seen. The process by which these rocks were derived from chert has been described by H. H. Thomas (Strahan and others 1907, p. 76) and confirmed by Ware (1939, p. 188) who also recorded "Posidonomya membranaceous (McCoy)" and "Leioptera longirostris Hind" [sic] (which are also reported from E₂ beds in Gower, Dix 1931, p. 533) in shale associated with the Plastic Clay near Llandybie. The pronounced hollow which is presumably due to the presence of the Plastic Clay and soft 'rottenstone' beds at the top of the Carboniferous Limestone, dies out towards the south-west, and it is unlikely that there is any representative of the Plastic Clay Beds west of the Crwbin–Pontyberem road.

The best exposures of the Basal Grit are in a series of quarries, some disused, on Mynydd-y-Gareg on which the generalized section (Figure 3) is largely based.

The lowest goniatite-bearing shale has yielded Homoceras subglobosum Bisat, indicating a basal H age, but although the lowest shale, probably about 30 ft above the base of the Millstone Grit, only contains horny brachiopods in this area, probably the same shale, that is the first below the H. subglobosum horizon, thickens considerably in the east and yielded Nuculoceras nuculum Bisat of E₂ age in a trial borehole [SN 5986 1585] near Llandybie [Sheet 230]. If this correlation is sound, the base of the Millstone Grit on Mynydd-y-Gareg is also of high E, age. The fact that the H. subglobosum bed has been found in several quarries over a distance of about 0.75 mile on Mynydd-y-Gareg, all about the same distance above the base of the Basal Grit, and that the same bed occurs in a similar position in a borehole [SN 4824 1308] on Mynydd Llangendeirne, emphasizes that although the Millstone Grit is unconformable on the Carboniferous Limestone, there is little, if any, overlap within the Millstone Grit in this area.

The beds containing H. subglobosum on Mynydd-y-Gareg are believed to be the same as those from which George (1927, p. 53) collected Hudsonoceras  aff. proteus (Brown) and Evans and Jones (1929, p. 171) recorded "H.  aff. diadema (Beyrich)" and "H.  aff. striolatum (Phillips)". The next shale 'slack' is thin and unfossiliferous, but Lingula mytilloides J. Sowerby was found at approximately this horizon in a borehole [SN 5007 1363] on Mynydd Cerig. In the same borehole, 1.5 in of coal and dark grey carbonaceous mudstone with thin coal streaks, overlain by about 1 ft 6 in of dark grey silty mudstone with plant debris, were recovered about 41 ft below the Lingula bed (see p. 17).

The succeeding marine horizon on Mynydd-y-Gareg contains poorly preserved, fragmentary goniatites which include Homoceratoides cf. prereticulatus Bisat and Reticuloceras sp.;this low R, horizon is probably represented in a borehole [SN 4996 1360] on Mynydd Cerig by shale with Homoceras henkei H. Schmidt.

The strata, including 'rottenstone', exposed in a cutting [SN 4360 0864] on Mynydd-y-Gareg are abundantly fossiliferous in parts, the fauna including Homoceras henkei, Homoceratoides mutabilis Bisat and Hudson, Reticuloceras cf. circumplicatile (Foord) and R.  aff. pulchellum (Foord), indicating the R. circumplicatile Zone (Ramsbottom and others 1962, p. 125) near the base of R1. This exposure is isolated by a fault and drift cover from the other sections in this area, but the probability that this is the highest marine horizon present within the Basal Grit is increased by the presence of Reticuloceras of the R. circumplicatile group and Homoceras sp.  [juv.] in the highest goniatite-bearing shale in the Basal Grit on Mynydd Llangendeirne, which is exposed in a disused quarry [SN 4821 1292] in which fossiliferous 'rottenstone' debris was also found.

The thickness of the Basal Grit decreases from about 400 ft on Mynydd Cerig to about 300 ft on Mynydd-y-Gareg, the decrease being due primarily to attenuation. At Ragwen Point the thickness is estimated to be about 90 ft (Cantrill and others 1916, p. 105): the reduction here, and in Pembrokeshire is due to overlap (Archer 1965, p. 149).

Shales

Although the greater part of the Millstone Grit, the 'Shales' division of the old classification, is poorly exposed, an underground borehole drilled for the National Coal Board in 1953-4 from the Trichwart workings near the bottom of Shaft No. 3, Cynheidre Colliery, proved an apparently complete succession down to the top of the Basal Grit. The appropriate part of (Figure 3) is based on this information.

In addition to the cyclothems containing the four goniatite-bearing horizons described by Ware (1939, p. 174) two other cyclothems can be recognized.

The Basal Grit is overlain by mudstone with Reticuloceras superbilingue Bisat and Donetzoceras sigma (Wright). It is clear that the top of the Basal Grit remains at the same stratigraphical horizon along most of the north crop of the coalfield, and the 'Twelve-Foot Sandstone' of the Merthyr Tydfil district which is believed to be present above the R. superbilingue beds east of Meinciau, is remarkably persistent (Robertson 1932, p1. iv; Ware 1939, fig. 11). Although neither R. superbilingue nor D. sigma were found in the Cynheidre Borehole, mudstone with Anthracoceras sp. ,above the quartzite at the bottom of the hole and about 80 ft below a thin sandstone, is probably at this horizon. The absence of diagnostic fossils may be explained by either non-recovery of the cores of the 'goniatite phase', or to a facies change, only the Anthracoceras or Dimorphoceras phase' being present. This cyclothem is clearly transitional between the Basal Grit and the overlying Shales.

Above the Twelve-Foot Sandstone in the borehole are more mudstones with Anthracoceras sp.  abundant in one bed, accompanied by a richer fauna than in the underlying cyclothem, but with no 'goniatite phase'. This is believed to be the persistent Anthracoceras band recorded by Ware in the River Twrch and elsewhere. Although Ware (1939, p. 177) recorded Gastrioceras cancellatum? Bisat in this band, it is considered to be of Upper Reticuloceras age (R₂) for the typical Lower Gastriocerasage (GO fauna lies above it : species of Gastrioceras are known elsewhere in the upper part of R₂. The strata between the Anthracoceras and G. cancellatum marine bands are about twice as thick as those in the two succeeding cyclothems, and a division into two is suggested by the presence of a relatively rich faunal phase, overlying barren silty mudstone, at a depth of 1099 ft in the borehole (Figure 2).

The presence of the G. cancellatum Zone, i.e. the lower part of G₁, is shown by a band with a rich fauna including Agastrioceras carinatum (Frech) and G. crencellatum Bisat, which has been found in the borehole on Mynydd Llangendeirne, where a thin limestone is present near the base, and on Mynyddy-Gareg. Between this horizon and the G. cumbriense Marine Band another well-defined cyclothem is present in the borehole, the fauna at the base being dominated by lamellibranchs, although Crurithyris sp.  is also present.

The G. cumbriense Marine Band, which in the borehole includes a thin limestone, also contains a rich fauna including Anthracoceras sp. , Homoceratoides  aff.divaricatus (Hind), G. cumbriense Bisat and G. crenulatum Bisat.

The Shales are about 450 ft thick.

Details

Basal Grit

Ragwen Point

The small area of Millstone Grit which crops out near the western margin of Sheet 229 has not been resurveyed, but additional non-diagnostic fossils have been collected at Ragwen Point. The section is as follows:

Kidwelly

Quartzite was encountered in sewer trenches along the main Kidwelly–Carmarthen road (A 484) between Wesley Terrace and Broomhill. A well dug for Kidwelly Borough Council 560 yd north-east of Kidwelly Castle [SN 4140 0728] was said to have proved: Drift 9 ft on shale with Lingula ? and Promytilus sp. 2 ft; quartzite 14.5 ft.

Mynydd-y-Gareg

Disused quarry [SN 4302 0833]: orthoquartzite c. 25 ft; no section c. 3 ft; orthoquartzite, coarse-grained, passing to coarse-grained darker grey sandstone towards base 4.5 ft.

Eastern end [SN 433 083] of Stephen's New Quarry (disused):

Pinnularia capillacea (Lindley and Hutton) and Lepidodendron sp. [decorticated, 'Knorria' condition] were collected from loose material in this quarry.

Stephen's 'Smart's' Quarry [SN 435 085] :

Cutting at entrance to Ty-Fry Quarry [SN 4365 0858] :

Fossils collected from the 'rottenstone', beds 13-15, include Hyalostelia cf. smithii, Paraconularia sp.,crinoid columnals, Chonetes sp.  s.1., Crurithyris sp.  nov., Eomarginifera sp.  nov., L. mytilloides, Productus carbonarius de Koninck, Schizophoria cf. hudsoni, Spiriferellina sp., Glabrocingulum sp., Naticopsis sp.,Platyceras sp., 'Pseudamussium' sp., Sanguinolites sp.,Orthocone nautiloid, Homoceras sp., Homoceratoides sp., Reticuloceras sp.

In an old trial cutting 70 yd to the north-west [SN 4361 0864] similar beds are exposed, and the details of beds 7 to 12 and 16 to 19 are as follows:

Part of bed 15 consists of hard siliceous 'rottenstone' with very irregular iron staining, with Eomarginifera sp.  nov. and Chonetes sp. in top 6 in.

Caneyella sp.  [juv],cf. Posidonia obliquata (Brown), Homoceras henkei and Reticuloceras  aff. pulchellum were also collected from this locality.

Clay has been worked from a small mine [SN 4344 0873] in the quarry north-west of Ty-Fry; material from the tip of this mine contained Homoceras subglobosum. Probably the same horizon is present in a disused quarry 425 yd to the north-east [SN 4378 0890] :

Large disused quarry 800 yd W.S.W. of crossroads at Four Roads [SN 440 092] :

No Millstone Grit fossils were seen in the group of disused quarries north-west of Four Roads: in the badly overgrown, most westerly quarry of this group [SN 4449 0965], orthoquartzite, soft towards the base, is exposed in close proximity to grey crinoidal limestone of D₂ Zone (George 1927, p. 50).

Half a mile south-west of Meinciau the continuous bare dip slopes of the Basal Grit on Mynydd-y-Gareg are interrupted by a fault trending approximately north-south, but the amplitude of sharp folds associated with the fault, especially on the downthrow side, is such that the net effect on the outcrop of the Basal Grit across the structure is very small.

Disused quarry [SN 4528 1019] 75 yd west of above fault: orthoquartzite 10 ft; on flaggy quartzite and striped quartzitic siltstone 2 ft to 3 ft; silty shale 0 to 3 in; orthoquartzite 3 ft.

Mynydd Llangendeirne

Between Meinciau and the Pontyberem–Crwbin road, the Basal Grit is badly exposed, most of the outcrop being drift covered. Near this road the beds are faulted; slickensides are seen in an outcrop of quartzite east of the road [SN 4811 1277] and the beds in a quarry 200 yd to the N.N.E. [SN 4821 1292] are disturbed (Ware 1939, p. 193). The section is:

The higher of the two beds of shale exposed in this quarry is also exposed 200 yd to the north-east [SN 4836 1305]: orthoquartzite 10 ft; on dark grey shale, sheared 5 in; grey silty micaceous mudstone with ?Lingula fragment and Euphemites sp. 2 in; very silty mudstone 1 in; dark grey quartzitic sandstone 1 ft 6 in; orthoquartzite 3 ft.

Shale debris by the north side of the Crwbin–Pontyberem road [SN 4797 1277] contained: Posidonia cf. obliquata, Homoceras? and Reticuloceras sp.,suggesting a R₁ age. The only other detailed section known on Mynydd Llangendeime is a trial borehole [SN 4824 1308], which proved:

Mynydd Cerig

Part of the Basal Grit has been proved in trial boreholes; the deepest [SN 5007 1363] proved:

In another borehole 130 yd to the south-west [SN 4996 1360], the beds which yielded Homoceras ? in the above borehole contain H. henkei, and the shale associated with the thin coal, which was also present in this borehole, contained a doubtful Palaeoniscid scale. A thickness of about 48 ft of quartzite was proved below the probable H. subglobosum horizon in a third borehole [SN 5004 1375].

The thin coal recovered from one of these boreholes [SN 5007 1363] was submitted to the Coal Survey Laboratory of the National Coal Board at Cardiff for analysis, as this is the lowest known coal seam in this area, lying about 1550 ft below the lowest worked seam, the Pumpquart Vein, and is therefore of interest in any study of the devolatilization of the coals in the South Wales Coalfield. Mr. H. F. Adams, the Chief Coal Survey Officer reported that the 2-in diameter core consisted of "0.5 in mainly dull coal of canneloid appearance with prominent thin (one-tenth in) nodules of spherulitic pyrite, some thin bright vitrainous banding and occasional flakes of ankerite. The coal merges into hard carbonaceous mudstone containing some thin coal streaks and plant debris". Forty-five grammes of clean coal were obtained by floating the crushed core at a specific gravity of 1.60, and the percentage analysis of this fraction is as follows :

Mr. Adams comments that this unusually low volatile matter has nowhere been seen in exposed seams in South Wales, although comparable values were obtained for the lowest seams in Abernant Borehole No. 6 (Sheet 230).

Shales

Kidwelly

Excavations for the foundations of the railway bridge across the River Gwendraeth Fach at Kidwelly proved 'shale' below alluvium and gravel. Specimens from tip material collected by E. E. L. Dixon (Strahan and others 1909, p. 151) on the east side of the river [SN 3995 0660] include grey mudstone with mollusc spat, similar to material from other Millstone Grit localities.

Planolites ophthalmoides and Productus sp. have been collected from the bluff below Kidwelly Castle [SN 409 070].

Mynydd-y-Gareg

Caneyella sp. [juv.], Donetzoceras sigma, Gastrioceras sp. [juv.] and fish fragments were obtained from a very poor exposure immediately overlying the Basal Grit, 200 yd south-south-east of Greenhill [SN 4491 0972]. Lingula sp. and Gastrioceras cf. crencellatum are present in the G. cancellatum Marine Band exposed in a small stream 200 yd east of Four Roads crossroads [SN 4495 0936].

Mynydd Llangendeirne

In one of the small headwaters of Aberlledle, 1050 yd southeast of the Quarry Arms, Crwbin, the Gastrioceras cancellatum Marine Band is exposed [SN 4811 1232] :

Crinoid debris, Crurithyris sp., Orbiculoidea sp., Productus carbonarius, Bucanopsis sp.,Caneyella sp., Huanghoceras sp., Agastrioceras carinatum and Gastrioceras cf. crencellatum were collected.

In another small stream 180 yd to the south-east [SN 4826 1224] the Gastrioceras cumbriense Marine Band is exposed : grey mudstone containing crinoid debris, Crurithyris sp.,Martinia sp., Productus sp., Schizophoria sp.,Aviculopecten  aff. losseni (von Koenen), Aviculopecten sp. [smooth, except ears], Palaeoneilo sp.,a coiled nautiloid, Gastrioceras crenulatum, G. cumbriense, Homoceratoides  aff. divaricatus and an ostracod. A short distance below the marine band dark grey fine-grained quartzitic sandstone with rootlets is also exposed.

Between Mynydd Llangendeirne and Mynydd Cerig, the Reticuloceras superbilingue Marine Band is exposed [SN 4932 1320] in a small stream, one of the head waters of the River Berem, about 1275 yd north-north-east of Banc-ffos-felen school, the fauna collected consisting of Caneyella cf. rugata (Jackson), ?Donetzoceras sigma, Gastrioceras sp.  nov. and R. superbilingue. This band is below a sandstone which is probably the "Twelve Foot Sandstone" of the Merthyr Tydfil district (Robertson 1932, p. 50). The strata between this horizon and the Gastrioceras subcrenatum Marine Band [SN 4991 1289] are poorly exposed, the only fossils collected being Planolites ophthalmoides and Lingula mytilloides [SN 4997 1302], the latter probably being from the Gastrioceras cumbriense cyclothem.

Cynheidre No. 3 Shaft Borehole

Site details of this borehole are given in Appendix I (p. 189). The average core recovery was 89 per cent, but was only 75 per cent below 1210 ft: difficulty was experienced in drilling the very hard quartzite at the top of the Basal Grit and the hole was abandoned at 1311 ft 6 in. The sequence of cores was incorrectly laid out in places, but the error, which appeared to be due to lengths, and sometimes individual pieces, of core being reversed, was never greater than about 10 ft: every effort was made to re-arrange the cores before they were examined. No evidence of faulting was seen, although polished and slickensided bedding planes were not uncommon: the dip varied from about 12° at the top of the Millstone Grit to about 4° near the bottom of the hole. The section of the Millstone Grit (Figure 2) is as follows:

References

ARCHER, A. A. 1965. Notes on the Millstone Grit of the North Crop of the Pembrokeshire Coalfield. Proc. Geol. Assoc., 76, 137-50.

CANTRILL, T. C., DIXON, E. E. L., THOMAS, H. H. and JONES, O. T. 1916. The Geology of the South Wales Coalfield. Part XII. The Country around Milford. Mem. Geol. Surv.

DIX, EMILY, 1931. The Millstone Grit of Gower. Geol. Mag., 68, 529-43.

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. O. 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.

JENKINS, T. B. H. 1962. The sequence and correlation of the Coal Measures of Pembrokeshire. Quart. J. Geol. Soc., 118, 65-101.

JONES, O. T. 1907. in Sum. Prog. Geol. Surv. for 1906,53.

LEITCH, D., OWEN, T. R. and JONES, D. G. 1958. The basal Coal Measures of the South Wales Coalfield from Llandebie to Brynmawr. Quart. J. Geol. Soc., 113, 461-83.

OWEN, T. R. 1954. The structure of the Neath disturbance between Bryniau Gleision and Glynneath, South Wales. Quart. J. Geol. Soc., 109, 333-65.

PRINGLE, J. and GEORGE, T. N. 1937. British Regional Geology: South Wales. 1st edit. Geol. Surv. Gt. Brit.

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

ROBERTSON, T. 1932. The Geology of the South Wales Coalfield. Part V. The Country around Merthyr Tydfil. 2nd edit. Mem. Geol. Surv.

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

STRAHAN, A., CANTRILL, T. C. and DIXON, E. E. L.and THOMAS, H. H. 1909. The Geology of the South Wales Coalfield. Part X. The Country around Carmarthen. Mem. Geol. Surv. WARE, W. D. 1939. The Millstone Grit of Carmarthenshire. Proc. Geol. Assoc., 50, 168-204.

Chapter 3 Coal Measures: general

In subsequent chapters, general accounts of each subdivision of the Lower and Middle Coal Measures, accompanied by a generalized vertical section, are immediately followed by detailed descriptions of the stratigraphy. The subdivisions selected, although necessarily somewhat arbitrary, are generally related to variations in the rocks: the Lower Coal Measures are divided at the Rhasfach Vein, below which the strata are essentially transitional from the Millstone Grit while those above include important coal seams. The lowest two subdivisions of the Middle Coal Measures, which are divided at the Ddugaled Vein because over a substantial part of the district strata at or near this horizon have been faulted out, contain most of the remaining anthracite seams. The overlying Middle Coal Measures strata, that is above the Graigog Vein, subdivided at the Carway Fach Vein, contain only one extensively worked seam; marine strata are relatively common between the Graigog and Carway Fach veins. The Upper Coal (or Pennant) Measures are subdivided on the basis of the most recent classification of these beds.

Classification

The classification proposed during the survey of other parts of the coalfield (Woodland, Evans and Stephens, 1957; Woodland and Evans 1964, pp. 27-8) is applied to this district. The widespread occurrence of readily recognizable marine bands establishes beyond doubt the identity of the Lower and Middle Coal Measures. Although the ground between the type areas of the new divisions of the Upper Coal Measures and the area here described had not been resurveyed, the identification of at least the general horizons of the No. 2 Rhondda, Brithdir, Hughes and Swansea Three Feet seams may be regarded as satisfactorily established. As in the type areas, the thicknesses of the new divisions are similar (p. 3).

Cyclothems

The repetition of a series of more or less similar lithological units, which is a characteristic feature of coal-bearing Carboniferous strata, forms the basis for detailed study of the Coal Measures. The lithological variation within each of the cyclothems reflects bathymetric changes during sedimentation more clearly in the Coal Measures than in much of the Millstone Grit, where the cyclothems are more readily identified by the succession of faunal phases, some of the members of the lithological series commonly being absent.

After a relatively long period of stability when the surface of the sediments was at or very near sea level and plant growth continued, subsidence occurred and the vegetation was killed. It is debatable where the base of a cyclothem should be drawn in the rhythmically repeated series of strata, but as it seems likely that the longest period of stability was at the time of formation of the coal seams, which are also the most important and readily recognizable rocks, it is appropriate to start with the coal. If the base is taken at the top of the coal the overlying fauna is separated from the seam it accompanies, whereas if it is taken at the base of the coal the seam is isolated from its seatearth. As coal is not always present, either through non-deposition or non-preservation, or through subsequent removal by local erosion or tectonically, in this account the top of the rootlet phase or seatearth is taken as the base of the cyclothem.

The maximum rate of subsidence during each cyclothem was probably during the initial submergence, the first two or three feet of sediment characteristically being argillaceous and containing the most abundant fauna in the cyclothem, commonly accompanied by the remains of drifted plant fragments in the lowest part. The presence of the 'most marine' faunal phase near the base of each Millstone Grit cyclothem, suggesting maximum salinity and therefore probably greatest depth, has already been described (p. 8). The relatively infrequent marine incursions in Lower and Middle Coal Measures times probably occurred when downwarping coincided with an eustatic rise in sea-level, at least when the marine bands of widespread occurrence throughout western Europe were deposited.

Typically the fauna in the argillaceous strata at the base of Coal Measures cyclothems consists of non-marine lamellibranchs (mussels), although accompanied in many cases by fish remains and, more rarely, by 'Estheria' (Lioestheria and Palaeolimnadiopsis) and ostracods. Much less commonly the sediments overlying the coal were deposited in a near-marine or marine environment. The former is indicated by the presence of Lingula, locally accompanied by foraminifera. In some cyclothems this phase is overlain by a more varied marine fauna including corals (rare), crinoid debris, echinoids, Productid and other calcareous brachiopods, lamellibranchs including Dunbarella, Edmondia, Myalina and many other genera, gastropods, and ostracods: nautiloids and goniatites, including Anthracoceras, are rarely present. Lithologically, some of the marine horizons are distinctive, although other than by the abundance of pyrite in some, it is not easy to convey the distinction in appearance between these rocks and some of the non-marine roof strata. Others of these bands appear to be at least macroscopically identical to other Coal Measures sediments, and range from dark grey carbonaceous mudstone to medium or light grey silty mudstone or striped beds. In some cyclothems the marine phases are either underlain or overlain by mussel-bearing strata.

Markings usually ascribed to 'worms', including Cochlichnus [Belorhaphe] kochi (Ludwig), Gyrochorte carbonaria Schleicher and Planolites spp. are present in most cyclothems, accompanying or overlying the non-marine faunas. P. ophthalmoides Jessen is normally associated only with cyclothems containing a marine or near-marine phase; Lioestheria is locally present overlying these phases in some cyclothems. Small circular markings (1.0-2.0 mm diameter), with sharply defined outer margin, are referred to as Planolites sp. (see Woodland, Archer and Evans 1957, p. 55); these structures are more characteristic of non-marine facies.

Typically the greater part of all Coal Measures cyclothems consists of sediments becoming progressively coarser grained upwards, increasing silt content giving rise to slightly silty mudstone, silty mudstone, very silty mudstone, siltstone, striped beds and sandstone. The finer-grained rocks in this series are more readily distinguished macroscopically than microscopically, particularly when a continuous sequence of cores is being examined. Although it is not easy to maintain consistency in the description of these rocks, both from day to day and from geologist to geologist the attempt is fully justified when sequences in a structurally complex area are being interpreted, as the thickness of the different members of the series are commonly very different from cyclothem to cyclothem, although broadly similar or subject to systematic variation within one cyclothem. Plant remains are present sporadically in most cyclothems, although in the coarser grained rocks they are usually present as corn-minuted fragments, or as thin coaly partings in some striped beds and sandstones. The highest rocks in most cyclothems, that is those which subsequently formed the seatearth which underlies the coal seam at the base of the overlying cyclothem, are usually relatively fine-grained, sandstone- or siltstone- seatearths being rare.

In subsequent chapters the Coal Measures are described in terms of the individual cyclothems. This is not to imply that the number of cyclothems remains constant or that it is possible to correlate the strata by counting cyclothems, even in the absence of structural complexity. There are numerous examples of the development of additional cyclothems, either by splitting, or by the development of an additional rootlet phase which may be heralded in other sections by other evidence of bathymetric, or at least facies, change.

Correlation

The soundest foundation for the identification of not only the principal coal seams but of all the strata, in order, for example, to interpret the structure, is to consider the whole cyclothem, since the sum of the faunal and lithological phases is commonly characteristic, whereas any one factor may be common to many cyclothems.

At the same time the reliability of the characteristics which enable a cyclothem to be recognized, and which are present over relatively large areas (for example, some are present wherever accessible in the district described and over much of the North Crop of the Coalfield at least as far east as the Neath Valley), may be considered to be of three orders. The distinction between the orders is arbitrary.

First order characteristics, or markers, include all the marine bands, although several containing only restricted faunas must be examined in detail before their identity can be established. Well-preserved assemblages of mussels, particularly when their preservation and matrix are considered, should also be included in this category, e.g. the faunas on the Stinking, Braslyd, Hwch and Penny Pieces veins. The well-preserved mussels immediately overlying, and commonly intercalated with, the Amman Marine Band are characteristic of that cyclothem.

In the second order may be included the characteristic preservation of fossils at some horizons, even if the fossils are not readily identifiable. For example, the roof of the Graigog Vein invariably contains poorly preserved 'ghost-like' mussels generally in rather micaceous, silty mudstone (the correlation being confirmed by the presence of distinctive 'solid' mussels in the overlying Graigog Rider cyclothem). Were it not that the coals are commonly deformed, the sections of many of the seams (for example, the Big and Soap veins would be included in this category), may be so included when it is reasonably certain that they are undisturbed. Similarly the Ddugaled cyclothem is usually recognizable by the presence of a thin bed of cannel in its roof, and the identification may be confirmed by the presence of mussels preserved as glossy films in its roof.

Third order characteristics can be of great value when combined in one cyclothem; on their own, reinforced by comparable characteristics in adjacent or nearby cyclothems; or when their approximate position in the sequence has been clearly narrowed down by higher order markers. Among the many examples which may be quoted are the presence of Planolites montanus R. Richter a short distance above the base of the Braslyd Rider cyclothem, the abundance of Gyrochorte carbonaria in the cyclothems between those containing the two Cwmgorse marine bands, the blackband ironstone towards the base of the Llynfi Beds and the presence of mussels, not of themselves diagnostic of horizon, near the middle of the Ddugaled cyclothem, or even the absence or great rarity of mussels in the roof of the Braslyd Fach Vein. Similarly the Big Vein cyclothem is unusually thick for a Middle Coal Measures cyclothem, it commonly contains sandstone at or near its base, well-preserved plant remains are usually present towards its top and it seldom yields mussels, and therefore it is usually readily recognized by the combination of these four third order characteristics.

Where continuous sections were accessible and could be accurately recorded it has proved possible by combining all the evidence to correlate firmly most of the cyclothems in the Lower and Middle Coal Measures. Other than in the Llynfi Beds, this cannot be said of the Upper Coal Measures, because of the paucity of detailed information.

Seam nomenclature

Although a diversity of names had been used for the anthracite seams in this district during the earlier periods of mining, only a limited number of synonyms has been in use in the last fifty or so years. Despite the demonstration by Dix (1928) that the main seams worked in the Trimsaran area corresponded with the main seams worked north of the Trimsaran Disturbance, local names remained in use in that area. Elsewhere in the district difficulties of correlation and misidentification of seams had led to considerable confusion and doubt, notably in the naming of all the seams worked in Carway Colliery and the higher seams worked in the collieries between Carway and Pontyberem.

During the resurvey the nomenclature applied to the seams in Great Mountain (now Cynheidre) No. 3 Shaft was adopted as the standard nomenclature throughout the district.

Facies variations and petrology

The boreholes and shafts sunk in connection with Cynheidre Colliery provided an opportunity to collect fresh lithological specimens and to study in detail facies variations. The area on which this account is based is about 5 square miles in the form of a 3.5 mile long north-east to south-west strip.

In (Figure 4) the comparative vertical sections of part of the succession have been corrected for dip and, as far as possible, for major thrusts; they are projected on to a N.W.–S.E. line of section through the group and have been much simplified to show only the sandstones, siltstones and marine bands.

Petrology

The major arenaceous beds may be divided on a petrographical basis into those of mainly pennant-type lithology above the Upper Cwmgorse Marine Band and those of impure ortho- or protoquartzite types which are generally more characteristic of the Middle and Lower Coal Measures.

Sandstones of the Middle Coal Measures (e.g. (E29563), (E29568), (E29581), (E30059), (E30071), (E30075))Specimen numbers refer to specimens in the sliced rock collection of the Petrographic Department. are generally impure, fine-medium grained protoquartzites consisting of subangular or rounded quartz, quartzite, microcrystalline silica, not very abundant feldspar, and occasional mudstone or siderite mudstone detrital fragments. A small amount of interstitial clay mineral, silica and more rarely minor carbonate cement occurs. Accessory heavy minerals include tourmaline, sphene, zircon, garnet and opaque oxides. Although lithic fragments occur including low grade metamorphic rocks (quartz schists and quartz-muscovite schists) they are rather rare. Siderite and carbonaceous fragments are common often as scattered dusty aggregates, but are less abundant than in the siltstones. These protoquartzites are generally easily distinguishable from the almost pure orthoquartzite of the Millstone Grit.

Siltstones (e.g. (E29573), (E29566), (E30079), (E30092)) consist mainly of subangular quartz grains with rare lithic fragments similar to those in the coarse grained protoquartzites, with a small amount of interstitial clay minerals (kaolinite, illite and minor chlorite). Carbonaceous fragments are abundant and the proportion of siderite is variable grading up to rocks which may be classified as sideritic siltstones ((E29575), (E30081), (E30092)) and which may carry sphaerosiderite (e.g. (E30084)). The siderite may occur as small grains or rhombs ((E29557), (E30085)) or as indistinct patches (E30084). Many of the siltstones are banded or striped ('striped beds', e.g. (E22757), (E29558), (E30076)) and consist of alternating bands of siderite-rich and siderite-poor quartzose siltstones with some interstitial clay minerals and scattered carbonaceous fragments. The dark siderite-rich bands are composed dominantly of fragments of siderite mudstone and siderite grains and rhombs and are closely associated with carbonaceous fragments. There is apparently little or no difference in the clay mineral content of the light and dark layers (e.g. (E29958)).

Siderite (E30080) and sphaerosideritic (E30057) mudstones and silty mudstones (E30597) and sideritic siltstones (E29575) make up a continuously variable series with differing mineral content and grain size.

Sphaerosideritic mudstones and silty mudstones (E29579) are relatively uncommon and the sphaerosiderite content is widely variable. With decreasing sphaerosiderite they grade into normal kaolinite-illite mudstones, which occur as relatively thin bands or as nodules interbedded with normal mudstones, sandstones and seatearths and are composed of clay minerals (dominantly kaolinite and illite) with minor chlorite and small angular quartz grains. Clay mineral patches are occasionally found and there is a transition to sideritic mudstones ((E29566), (E29569)) with increase in the amount of siderite in proportion to the silt and clay mineral content; rocks occur which are almost entirely composed of siderite (E29564).

Siderite mudstones in which the siderite occurs as grains or rhombs and not as spherulites are relatively common ((E29566), (E29569)).

Mudstones (e.g. (E23824), (E30080), (E30088), (E30596)) and silty mudstones make up a large proportion of the borehole sections. Kaolinite is usually the dominant clay mineral present, with minor illite and scattered siderite grains and dusty carbonaceous material. Banding of the mudstones becomes more evident with increase in the silt component although relatively homogeneous types are common ((E29572), (E30351), (E30356)).

Above the Upper Cwmgorse Marine Band the arenaceous rocks of the Pennant (Upper Coal) Measures have a characteristic lithology. They are subgreywackes of Pettijohn's classification (1957) and have generally similar characters within the region. They are similar also to the Pennant-type sandstones throughout the coalfield and compare well with pennant-type sandstones for instance from Maesteg (E21737) and Skewen (E19742) in Glamorgan, and Abercarn (E17475) in Monmouthshire, and are closely similar also to the Bristol pennant sandstone (e.g. (E24486), (E24752), (E24756)). Similar types are again seen in some of the Coal Measure subgreywackes as far away as the Kent Coalfield (e.g. at Chislet Colliery, (E25030), (E25031), (E25032)).

In thin section the pennant-type sandstones consist of about 60 per cent of detrital quartz grains together with a lesser amount of lithic fragments, while the clay mineral matrix makes up only about 10 per cent of the rock. Minor amounts of detrital feldspar and muscovite are present together with an accessory heavy mineral assemblage including tourmaline, zircon, rutile, chlorite and garnet. In almost all specimens a minor proportion of chemical cement is present usually consisting of carbonates (mainly calcite) and secondary silica. The detrital quartz fragments are dominantly sub-angular in shape and may be either clear and only slightly strained, or partly cloudy with minute inclusions and showing well-developed undulose extinction typical of quartz derived from a metamorphic area. Occasional fragments of vein quartz occur.

The dominant lithic fragments vary slightly in different specimens and either quartzites or argillaceous fragments may predominate. The most common quartzite type consists of a fine-grained sutured aggregate of strained quartz with which may be associated minor accessory iron oxides or sericite. A second, rarer, type is a sheared cataclastic and foliated quartzite or quartz-schist with drawn-out thin quartz plates, typical of high grade metamorphic terrains, and similar to the quartz-schists found in the Gwna Beds of Anglesey. Wallis (1927, p. 778) regarded the presence of similar "fragments and pebbles of colourless nemoblastic quartz mozaics in the Old Red Sandstone" as the "most important similarity between the Bristol Old Red Sandstone and Mona Complex of Anglesey". It would seem likely that these fragments in the Old Red Sandstone and Pennant Measures may have a common origin. A third variety of quartzite or quartz schist in the pennant-type subgreywackes contains chlorite or muscovite ((E32703), (E32721), (E32705)). The presence of these distinctive muscovite-quartz schists is another indication that the source rocks were similar to those now exposed in the Mona Complex of Anglesey; the detrital fragments resemble very closely Mona Complex specimens ((E9934), (E9993), (E6107)) of muscovite and chlorite schists. Fragments of micro-crystalline silica are common in most specimens. Often a zoned structure is present, probably due to leaching of iron oxides.

Siltstone or shale fragments are common in almost all the specimens of these subgreywackes (e.g. (E32715), (E32722), (E32698), (E32701)). Mineralogically some of these siltstone fragments are similar to Lower and Middle Coal Measures siltstones and may thus be of local derivation. The detrital siltstones may be sideritic but only minor siderite occurs throughout the remainder of the subgreywackes. This probably indicates that some sideritization of the underlying rocks had taken place before the deposition of the Pennant Measures. Heard (1922, p. 90) suggested that much of the sideritic constituent of the cement in some Pennant specimens may have been due to replacement of calcium carbonate, but considered that the scattered siderite granules in the Glamorgan 'Pennant Series' may have been derived from the 'Lower Coal Series'.

The detrital feldspars include albite, microcline and sporadic fragments of perthite, but the feldspar is only present in minor amounts and is commonly altered and dusty with inclusions. Muscovite is a common detrital mineral but it only forms a minor proportion of the rock. There is apparently no appreciable change in the amount of either feldspar or micas throughout the Pennant Measures. Occasional flakes of biotite occur, but these in general are subordinate to the muscovite. Chlorite flakes are present in small amounts. No evidence has been found to support the commonly used description of pennant-type sandstone as a feldspathic sandstone (e.g. Strahan and others 1907, p. 93; Robertson 1927, pp. 51, 97).

The matrix of these rocks forms only a small percentage, usually about 5-10 per cent, but occasionally a rather larger proportion is present and the rock then grades into a more typical greywacke. The matrix consists of a fine-grained mixture of sericite, chlorite, quartz and clay minerals, usually too fine to be easily resolved under the microscope. Detrital tourmaline, rutile, garnet and zircon together with occasional fragments of organic material occur in most specimens.

The general characteristics of the pennant-type subgreywackes are the scarcity of detrital feldspar and the relative abundance of Ethic fragments, quartz and chert, and the minor amount of clay mineral matrix. Analyses of pennant sandstones from the Bristol area (Wethered, 1882) and from Pontypridd correspond chemically to the subgreywacke types of Pettijohn (1957, p. 319), confirming the petrographical classification.

Varieties of a rarer rock type composed essentially of ankerite and dolomite occur at a few horizons in the Lower, Middle and Upper Coal Measures. These unusual Coal Measures rocks are fine-grained and massive and occur as nodules or thin beds, consisting of finely divided and recrystallised ankerite-dolomite with a minor proportion of clay mineral and scattered carbonaceous fragments. They have been described recently by Hawkins (1967).

Facies variations

In the Cynheidre area the petrologically distinct characters of the arenaceous beds above and below the Upper Cwmgorse Marine Band may be correlated with the markedly different directions of thickening of the beds above and below that horizon (Figure 4). The total thickness of the sandstones between the Cefn Coed and the Upper Cwmgorse marine bands increases northwestwards from about 20 feet to about 150 feet in a distance of about 1 miles. A similar north-westwardly thickening occurs in the sandstone near the top of the Lower Cwmgorse Marine Band cyclothem; this sandstone is up to about 50 feet thick to the north-west but thins to the south-east and apparently splits into a number of thin beds separated by shales, siltstones and thin coals.

Above the Upper Cwmgorse Marine Band thickening of the sandstones in the Cynheidre area takes place in the opposite direction, that is to the south-east. Sandstone 'C' of pennant-type (i.e. the first above the Upper Cwmgorse Marine Band; (Figure 18)) is about 85 feet thick in the south-east but when traced to the north-west it rapidly thins and splits. It is absent in four of the most northwesterly boreholes, a change of facies being indicated by the occurrence at this horizon of shales, seatearths, coals and thin siltstone beds. The next higher sandstone (B') shows a similar thickening to the south-east and interfingering with shales, silts and thin coals to the north-west. This facies transition occurs further to the north-west than that associated with sandstone 'C'.

To the south-east (e.g. in Borehole 4/3) practically the whole succession between the Upper Cwmgorse Marine Band and sandstone 'A' consists of sandstone with only thin shales and a few coals, but to the north-west (Bore-holes 3/1, 4/1 and 5/1) sandstones form only a minor proportion of the total thickness of the succession, the number of coal seams being greatly increased.

Pennant-type sandstone contitutes one member of a typical subgreywackeshale association ('Molasse') (see Pettijohn, 1957, p. 618) formed under a general environment which must have been essentially that of a coastal floodplain or delta, with perhaps local exposure to sub-aerial conditions. The relative abundance of metamorphic quartzites and quartz-schists resembling the Mona Complex of Anglesey, together with the similarity of these detrital fragments with those in the Old Red Sandstone from Shropshire to Cardiff and the Bristol area seem to indicate a derivation of a large part of the Pennant Measures from a northerly direction; it may in part represent a reworking of the now eroded Old Red Sandstone to the north. Some locally derived material may be represented by the shale fragments, due to uplift and erosion of already consolidated Coal Measures shales near the margin of the area of deposition, to the north of the district described in this Memoir. R.D.

'Outbursts' and devolatilization

The term 'outbursts' is applied in this district to the sudden emission of a large volume of methane accompanied by the ejection of often large quantities of mainly finely divided coal in colliery workings. This specialized problem was not investigated during the resurvey, nor was the cause of devolatilization, to which it may be related since in Great Britain 'outbursts' of this type are virtually restricted to the anthracite area of the South Wales Coalfield. There is probably a relationship between this phenomenon and relatively local structural anomalies, and this restriction may be related to the larger scale compressional structures in the area, although 'outbursts' are not characteristic of the overthrust area on the south crop of the coalfield.

The rank of the coal seems is discussed in Chapter 9. The origin of anthracite has been discussed recently by Trotter (1949, 1950, 1954) and Jones (1949, 1951). Trotter (1954, pp. 288-92) has indicated the relationship between the isovols in coal and the temperature gradient due to the intrusion of an igneous mass to which the mineral zones of the North Pennines are related. Trotter relates the progressive devolatilization of the South Wales coals to their distance from an inclined plane, probably a thrust, beneath the coalfield, and dismisses the suggestion that it might also be related to a concealed intrusion (1954, pp. 292-5). It may be noted that there is mineralization in this area, for example, nickeliferous linnaeite has been identified in a thin veinlet in the Trimsaran Borehole by Mr. F. W. Dunning, and non-ferrous ores have been mined on Mynydd-y-Gareg and further north near the River Towy (Strahan and others 1909, pp. 152-5). There is evidence that the mineralization elsewhere in Wales (Archer 1959), as in Devon and Cornwall to the south, is post-Carboniferous. It is tempting to suggest that the devolatilization in South Wales is due at least in part to thermal metamorphism connected with this mineralization, although there is no geophysical evidence to indicate the presence of a granite mass underlying the coalfield.

References

ARCHER, A. A. 1959. The distribution of non-ferrous ores in the Lower Palaeozoic Rocks of North Wales. In The Future of Non-Ferrous Mining in Great Britain and Ireland—A Symposium. 259-76. London Inst. Min and Metall.

DIX, EMILY. 1928. The Coal Measures of the Gwendraeth Valley and Adjoining Areas. Proc. S. Wales Inst. Eng., 34, 423-510.

HAWKINS, T. R. W. 1967. Ankerite/dolomite—and siderite-bearing rocks from the Coal Measures of South Carmarthenshire, South Wales. Bull. Geol. Surv. Gt. Brit. In press.

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

JONES, O. T. 1949. Hilt's Law and the Volatile Content of Coal Seams. Geol. Mag. 86, 303-12, 346-64.

JONES, O. T. 1951. The distribution of coal volatiles in the South Wales Coalfield and its probable significance. Quart. J. Geol. Soc., 107, 51-83.

PETTIJOHN, F. J. 1957. Sedimentary Rocks. 2nd edit. New York.

ROBERTSON, T. 1927. The Geology of the South Wales Coalfield, Part II. Abergavenny 2nd edit. 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., CANTRILL, T. C., DIXON, E. E. L. 1909. The Geology of the South Wales Coalfield. Part X. The Country around Carmarthen. Mem. Geol. Surv.

TROTTER, F. M. 1949. The devolatilisation of coal seams in South Wales. Quart. J. Geol. Soc., 104, 387-437.

TROTTER, F. M. 1950. The devolatilisation equation for South Wales Coals. Geol. Mag., 87, 196-206.

TROTTER, F. M. 1954. The genesis of the High Rank Coals. Proc. Yorks. Geol. Soc., 29, 267-303.

WALLIS, F. S. 1927. Old Red Sandstone of the Bristol District. Quart. J. Geol. Soc., 83, 760-87.

WETHERED, E. B. 1882. On the composition of the Pennant Grits in contact with and at a distance from carbonaceous deposits. J. Chem. Soc., 41, 79-84.

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

WOODLAND, A. W., 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. Palaeontology by M. A. Calver. Bull. Geol. Surv. Gt. Brit., No. 13,39-60.

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

Chapter 4 Lower Coal Measures

Gastrioceras subcrenatum Marine Band To Rhasfach Vein: general

The strata in this part of the Lower Coal Measures crop out on the northwestern slopes of the valley of the Gwendraeth fawr, the slopes often being nearly parallel to the dip and much obscured by drift deposits; apart from a few exposures of sandstone, the rocks are seldom seen except in stream sections, the most extensive of which are in Cwm Berem, Aberlledle and Cwm Clydach. The only complete section examined was provided by the underground borehole near Cynheidre Colliery Shaft 3, which proved an apparently unfaulted section through these beds (Figure 5).

These lowest Coal Measures are clearly transitional between the essentially marine deposits of the Millstone Grit and the normally 'non-marine' beds of the greater part of the Coal Measures. Each cyclothem below the Cwm Berem sandstone contains a marine phase with the exception of the dominantly silty, plant-bearing cyclothems overlying a seatearth a short distance above the 'Farewell Rock'. The transitional nature of these strata is emphasized by the absence of workable coal seams, although seatearths occur much more commonly than in the Millstone Grit; in some cases these are accompanied by coal streaks and thin seams, for example the thin coal on which trial levels have been driven, probably for fireclay, in Cwm Clydach and Cwm Teilo.

As an account of the basal Coal Measures in the Cynheidre No. 3 Shaft underground borehole, including a discussion of the correlation of these beds with other parts of the South Wales Coalfield, has been published (Woodland and others 1957) and a summary of the log is given on p. 36, only the more important beds will be described here.

The Gastrioceras subcrenatum Marine Band is well developed in this district, being over 80 ft thick in the Cynheidre Borehole, where the lowest 32 ft contain sporadic lamellibranchs such as Sanguinolites and Schizodus, accompanied by Planolites ophthalmoides and Lingula; Curvirimula sp. was found 7 ft from the base. These are overlain by about 2 ft of massive silty mudstone and striped beds with lamellibranchs and Orthotetid fragments which underlie over 40 ft of mudstone, silty in parts, with goniatites, predominantly G. subcrenatum (Frech) which is abundant in the bottom 7 in; other fossils in this phase include Dunbarella sp., 'Chiton' sp. and mollusc spat. The presence of this horizon in Cwm Berem, where it is exposed to a thickness of more than 30 ft, was first recorded by Ware (1939, p. 196), who also described other Lower Coal Measures marine bands in this section. About 40 ft of silty mudstone with thin ribs of sandstone near the top, and sporadic Planolites, occur between the marine band and the overlying 'Farewell Rock'; the confusion which surrounds the use of this latter term has already been discussed (p. 5). In this account and on the maps the name is retained for convenience of description, and is applied to the sandstone near the top of the G. subcrenatum cyclothem. In this district the 'Farewell Rock' consists of striped beds interbedded with thin bands of quartzitic sandstone overlying more massive sandstone which is probably never more than about 30 ft thick. The presence of contorted bedding in part of the 'Farewell Rock', seen in the borehole section, has also been recorded at this horizon in Pembrokeshire (Jenkins 1962, p. 95; Kuenen 1949, p. 373).

Plant remains are abundant in the less silty parts of the 50 ft or so of beds overlying a seatearth above the 'Farewell Rock'. These are believed to be at the same horizon as the Nant Llech Plant Beds (Dix 1933, pp. 164-6).

Above these beds are two marine bands about 20 ft apart, separated by thin beds of quartzitic sandstone underlain by mudstone with rootlets which extend down into Lingula-bearing slightly silty mudstone of the lower band. These marine bands, which are here regarded as separate because a rootlet phase is present between them, are correlated with the M1 Marine Band of Leitch and others (1958, p. 466), which is characterized by the presence within it of the 'One-Foot Sandstone'. These two marine bands are here named M 1(a) and M1(b). The fauna from both comprises Lingula, Productids, lamellibranchs including Pectinids, and coiled nautiloids; in Cwm Berem the higher band, M 1 (b), also contains Acanthopecten sp.  nov. which is believed to be the same species as that found at the same horizon in Margam Park No. 1 Borehole (Woodland and others 1957, p. 47); Ware (1939, p. 197) records "Conularia sp. " at this higher horizon.

A very thin marine band at the base of the overlying cyclothem, containing Lingula and overlain by mudstone with Planolites ophthalmoides, is known in this district only in the Cynheidre underground borehole, the relevant beds being nowhere exposed at the surface. This marine band is correlated with the band overlying the Crow's Foot Coal in the Margam area (Woodland and Evans 1964, p. 35) which contains a similarly restricted fauna, and with the M2 Marine Band, although Robertson (1933, p. 99) and Leitch and others (1958, p. 469) record a richer fauna, including Productids, elsewhere on the North Crop.

The succeeding marine band, the Cefn Cribbwr Marine Band of the South Crop (Woodland and Evans 1964, p. 35), where mudstone containing a rich fauna including Gastrioceras listeri (J. Sowerby) is overlain by a thick sandstone, has been found only at Cynheidre. In the underground borehole dark grey mudstone, silty in parts and containing sporadic pyrite granules, yielded Lingula mytilloides and fish remains, and is overlain by thin quartzitic sandstone; fossils collected from Cynheidre Shaft 3 included a turreted gastropod (Donaldina?) and cf. Geisina, which were believed to have come from this horizon. The M3 Marine Band of Leitch and others (1958, p. 465) sometimes contains a more varied fauna. The thin sandstone above this marine band is overlain by strata characterized by the presence of abundant foraminifera, accompanied by Lingula and P. ophthalmoides, believed to be equivalent to the M4 Marine Band, This band, the Margam Marine Band of Woodland and Evans (1964, p. 36). has been found in Cwm Berem, Cynheidre Shaft No. 3 and underground borehole, Aberlledle and Cwm Clydach.

The M5 Marine Band of Leitch and others is present in Cwm Berem, where it was recorded by Ware (1939, p. 196), in Cynheidre Shaft No. 3 and underground borehole, Cwm Clydach and other localities in the area. At some localities Lingula is particularly abundant in some beds, and is the commonest fossil in all the localities, usually accompanied by fish remains and sporadic Orbiculoidea: abundant foraminifera are present in this marine band in Cwm Clydach, where it is underlain by a thin coal; elsewhere the top of the underlying cyclothem is marked by a ganister. This marine band occurs at the base of a series of rocks over 250 ft thick, which in most of the area are devoid of rootlets, and therefore, using this Coal Measures criterion, may be considered as a single abnormally thick cyclothem. A comparable cyclothem is present in the Pennine coalfields (Woodland and others 1957, fig. 2), and other parts of the South Wales Coalfield (Woodland and Evans 1964, p. 37). Although there is a general coarsening of the grain size from marine mudstone at the base to sandstone near the top in the three sections where these strata were best seen (Figure 5), this series of rocks departs considerably from an ideal Lower Coal Measures cyclothem. For example, in the Cynheidre underground borehole a rootlet phase is present immediately below the sandstone, another thin marine band is present in places, and fish remains were found about 70 to 80 ft above the M5 Marine Band in the borehole and in Cwm Clydach. The thin marine band occurs about 120 ft above the base of the cyclothem in Cwm Berem and has been found in a stream section about 1 miles south-west of Cwm Berem; it is probably present in the borehole (Woodland and others 1957, p. 51). Because this marine band is over 100 ft above M5 and the intervening strata have yielded only fish remains (at one horizon), tracks and burrows, it is regarded as a separate marine band, here named the Cwm Berem Marine Band. The fauna in this band is restricted to Campylites?, Lingula and fish remains; pyritized strap-like markings ('fucoids') have also been seen. Anthraconaia sp. and Curvirimula sp. were collected between the horizon of the Cwm Berem Marine Band and the rootlet bed in the borehole. Thus in Millstone Grit terms, this thick cyclothem could be divided into at least three cyclothems. The sandstone near the top of this cyclothem, which is about 60 ft thick, in two beds, in Cwm Berem, is the Cwm Berem Sandstone, first named by Dr. F. M. Trotter on 6-inch map Carmarthenshire 48 N.E. (1946).

A thin cyclothem immediately underlying the Rhasfach has a non-marine fauna near its base, the mussels being overlain by plant-bearing mudstone.

The total thickness of Coal Measures below the Rhasfach is about 675 ft.

Gastrioceras subcrenatum Marine Band To Rhasfach Vein: details

Cynheidre No. 3 Shaft Borehole

This borehole started from the Lower Trichwart Vein; an account of the Coal Measures below the Pumpquart Vein has been published (Woodland and others 1957), and a summary of the log of the Millstone Grit is given above, (p. 19). Site details are in Appendix I. The following record of the basal Coal Measures is abbreviated from the published account.

The record of this borehole may be regarded as typical for the lowest Coal Measures in the western part of the north crop of the coalfield; the only section in the district here described in which a fuller development of any of the marine bands is known is Cwm Berem, where the details of the Cwm Berem Marine Band can be studied.

Cynheidre No. 3 Shaft

In 1955 this shaft (Appendix I) was deepened from the Pumpquart at 1794 ft, to 2281 ft, that is to 1735 ft below Ordnance Datum. As a detailed record had been obtained from the borehole, which was sited only about 100 yd from the shaft, the section in the shaft was not examined in great detail. As in the borehole, the Cwm Berem Sandstone was found to be thin, the total thickness of sandstone recorded being only about 17 ft, although this was accompanied by a considerable thickness of siltstone and striped beds. The Cwm Berem Marine Band was not noted, but the M5 Marine Band, containing abundant L. mytilloides, is present at 2189 ft, and Lingula, fish scales, P. ophthalmoides and abundant foraminfera at 2223 ft are probably from M4. A specimen containing several Lingula, reputed to be from 2213 ft, may have been misplaced from M5. The presence of a turreted gastropod (Donaldina?) and cf. Geisina with L. mytilloides at 2235 ft is unusual, as the M3 Marine Band in the borehole yielded only Lingula and fish remains.

Cwm Berem

The section exposed in Cwm Berem, which joins the Gwendraeth fawr at Pontyberem, is incomplete above the Cwm Berem Sandstone, but upstream from there the section is as follows:

Ware has recorded (1939, p. 196) "Anthraconauta minima (Ludwig)" in rocks "somewhat similar to certain marine shales" underlying the Cwm Berem Sandstone: these appear to be at, or near, the horizon of the Cwm Berem Marine Band.

Cwm Berem to Aberlledle

In this area there are few exposures. A disused quarry in the 'Farewell Rock' [SN 4905 1230], 300 yd E.N.E. of Banc-Ffos-Felen School, exposes 14 ft of quartzitic sandstone, which include a mass of weathered, brown, ferruginous sandstone: the base of one bed of sandstone fills a channel about 3 ft deep in the underlying bed. Dark grey, fissile, micaceous mudstone with L. mytilloides exposed in a ditch [SN 4918 1213], 370 yd E.N.E. of Banc-Ffos-Felen Post Office, a short distance above weathered shale with plant debris, a 4-in coal seam, mudstone with coal streaks and a seatearth, is probably the base of the M1(a) Marine Band, the plants being at the horizon of the Nant Llech Plant Beds. Dark grey micaceous mudstone with L. mytilloides [abundant] and fish remains including Elonichthys sp.,Palaeoniscid scales and an Acanthodian spine, above dark grey slightly silty mudstone with pyrite tubes and small concretions, exposed for several yards [SN 4885 1171] near the head of the stream in which faulted Cwm Berem Sandstone is exposed near Gwely-Harvey farm [SN 4926 1135], is thought to be the MS Marine Band. The Cwm Berem Marine Band, sporadic L. mytilloides in dark grey shale, is exposed [SN 4863 1099] 100 yd S.W. of Cwm-gro-loew farm, to the east of which the Cwm Berem Sandstone forms a prominent dip-slope feature: above this sandstone, in the same stream, Curvirimula subovata (Dewar) was collected [SN 4873 1080].

The G. subcrenatum Marine Band, containing L. mytilloides, Nuculid lamellibranchs, G. subcrenatum and G. sp.  nov. [lirae widely spread] is exposed in a ditch [SN 4661 1062] 500 yd E. by S. of Meinciau cross-roads, and the overlying 'Farewell Rock' forms a prominent feature from here to Wenallt farm, where it is displaced by a small fault.

Aberlledle

East of Wenallt the dip slope of the 'Farewell Rock' overlooks Aberlledle, a stream which rises on Millstone Grit and cuts obliquely across the basal Coal Measures; it runs on boulder clay for much of its course and the sequence is further obscured by landslips and faulting, but the following fossiliferous sections, both in the main and tributary streams, are exposed.

Aberlledle [SN 4745 1023], 700 yd north of Red Lion, Pont-Henry:

Small tributary stream [SN 4731 1041] 380 yd N.E. of Maensant farm:

Very small stream [SN 4722 1032], 250 yd E.N.E. of Maensant farm:

The Cwm Berem Sandstone is exposed in Aberlledle, 350 yd north of the Red Lion, Pont-Henry, where 25 ft of quartzitic sandstone are cut by a fault en echelon with the Pont-Henry fault, and in a tributary stream to the west where 21 ft of fine-grained quartzitic sandstone, overlain by mudstone, silty mudstone and striped beds 20 ft thick, have been quarried [SN 4700 0978], [SN 4693 0992] and form a low waterfall. Dark grey mudstone with fish scales is exposed [SN 4715 0972] above the striped beds, in the top of which there are rootlets; higher in the same stream [SN 4669 1042], slickensided mudstone with rootlets and plant fragments, possibly near the horizon of the rant Llech Plant Beds, is cut by an overthrust, probably small. Two small faults were seen in Aberlledle; one [SN 4767 1070] dips east at about 60°, and the other [SN 4755 1039] curves from a dip of 40° east to almost parallel with the bedding.

Aberlledle to Cwm Clydach

The boulder clay cover is particularly widespread in this area. The 'Farewell Rock' has been quarried south-west of the Meinciau road, near Maen-sant-uchaf, and the Cwm Berem Sandstone has been worked [SN 4658 0961] east of the same road.

Cwm Clydach

The following descending sequence (Figure 5) is exposed upstream in Cwm Clydach, a tributary of the Gwendraeth fawr at Pont-newydd:

The M4 Marine Band is remarkable for the abundance of foraminifera; M5 is almost 20 ft above the base of the cyclothem, if this is taken at the thin coal seam; the 4-in bed of sandstone at c. 188 ft 8 in from the top of the section probably corresponds to the similar sandstone recorded at 207 ft 4 inches in Cwm Berem, which is presumably the 'piped grit' of Leitch and others (1958, p. 470). In 1944 a level [SN 4421 0790] for fireclay was driven on the thin coal for a short distance in the west bank of the stream; on the abandonment plan the coal is recorded as 6 in thick. The Cwm Berem Marine Band was not found; the shale with fish remains at 112 ft is probably present in both Cwm Berem and the Cynheidre underground borehole (Figure 5).

From the proximity of these marine bands to the Basal Grit of the Millstone Grit and to the presumed position of the 'Farewell Rock', it is likely that there is a strike fault, possibly a lag, below the marine bands.

Cwm Teilo

In Cwm Teilo, half a mile south-west of Cwm Clydach, a slickensided coal 6 in thick, overlain by striped beds, in the mouth of an old level [SN 4358 0734], is cut by a normal fault dipping north-west at 38°, with a throw of about 4 ft. This level is a short distance above the 'Farewell Rock' which is poorly exposed but probably thin, and the small fault may be associated with the larger structure postulated in Cwm Clydach. Further down Cwm Teilo, shale with fish remains, on a 7-in coal smut, is poorly exposed: the Cwm Berem Sandstone has been worked in two small quarries [SN 4384 0708], now overgrown, 150 yd N.E. of Morfa-bach, the debris including flaggy sandstone and striped beds.

South of the Trimsaran Disturbance

The only exposures in this area of the basal Coal Measures occur north-west of Llandyry. Fine-grained quartzitic sandstone, exposed up to 20 ft in a series of quarries in a prominent scarp feature about 600 yd long, is probably the Cwm Berem Sandstone. Similar sandstone, folded and faulted, is exposed 300 yd north-west of Morfa farm [SN 427 053], surrounded by alluvium, and may be at the same horizon; fine-grained quartzitic sandstone is also exposed in a well in another hillock at Ty'r Banc [SN 426 055], 300 yd to the north. The latter sandstone may be the 'Farewell Rock', or the Cwm Berem Sandstone repeated by strike faulting, but for the purpose of indicating the position of the base of the Coal Measures on the map it has been assumed that it is immediately underlain by the Gastrioceras subcrenatum Marine Band.

Samborn's Trial Pit [SN 4325 0537], which was started a short distance below the Cwm Berem Sandstone, was sunk for 120 ft and a further 36 ft was bored; no coal was found.

Rhasfach Vein to Amman Rider Vein: general

The transitional beds between the dominantly marine Millstone Grit and the non-marine Coal Measures (p. 33) give way approximately at the horizon of the Rhasfach Vein to strata deposited in what may be regarded as a normal Coal Measures environment.

The measures between the Rhasfach and Pumpquart veins were very imperfectly known until recently, their outcrop being on the lower slopes of the Gwendraeth valley where they are heavily obscured by drift. The Rhasfach, which is known as the Bryn Vein near Ammanford and is correlated with the Garw in the eastern part of the coalfield, attains its greatest recorded thickness in this district near Pontyates, where it is up to 22 in thick, although it averages rather less. Beds ('pins') of ironstone above the Rhasfach, as well as the coal, have been extensively worked near the crop for about mile either side of Rhasfach Farm [SN 4627 0884], north-west of Pontyates. The seatearth has also been worked as fireclay. Iron-ore was smelted at Furnace [SN 4736 0924], west of Pont-Henry, and at Pontyberem (Strahan and others 1909, p. 155).

Recently some of the boreholes in the Cynheidre area have been drilled to the Rhasfach, and the strata between this seam and the Pumpquart are now known in detail. Four cyclothems can be recognized (Figure 6), all but the highest being characterized by the presence of large thick-shelled mussels, typically of the Carbonicola communis Davies and Trueman/pseudorobusta Trueman group, although in this area the 'pseudorobusta bands', beds with abundant shells preserved in calcite occurring elsewhere in South Wales (Woodland and others 1957, p. 55), are uncommon. Curvirimula, characteristic of the C. communis Zone, and Geisina arcuata (Bean) are commonly present in two of the cyclothems; Planolites spp. are found at certain horizons, and Cochlichnus kochi is usually present. This readily recognizable and distinctive faunal assemblage is of practical importance, as its presence in boreholes and drivages indicates a position below the lowest workable coal seam.

The upper part of the Lower Coal Measures contains two important coal seams which have been extensively mined in the area described. East of the Coalbrook Fault the lower of these, the Pumpquart ('Five-quarters') Vein, is a first class high rank anthracite (volatile content less than 6 per cent), with low ash, sulphur and arsenic values. This seam has been worked as the Pumpquart as far east as the Amman Valley, where it is known as the Lower Vein; it is correlated with the Lower Gellideg of the standard succession in the eastern part of the South Wales Coalfield. It is sometimes distinguished as the Lower Pumpquart, as in some collieries outside the district here described the Gras Uchaf Vein is called the Middle Pumpquart and the Green Vein is known as the Upper Pumpquart. The roof of the Pumpquart is usually strong, consisting of sandstone over much of the area worked in Glynhebog Colliery; the only fossil recorded, apart from plants, is Carbonicola?

About 1300 yd from the outcrop, and close to the Coalbrook Fault, a small area of thick coal, consisting of combined Pumpquart and the overlying Trichwart has been worked in both Glynhebog and Pentremawr collieries. Westwards the thick seam splits again, but as this split occurs lower than that to the east, the bottom leaf, less than 21 in thick and known as the Low (or Little) Vein, is too thin to be worked economically. The name 'Pumpquart' is retained for the workable upper leaf, although slightly more of its coal is equivalent to the Trichwart than to the Pumpquart east of the area where these two seams are combined. To the south the two lines of splitting cross, and in part of the Cynheidre area the Trichwart and Pumpquart seams are represented by three thin seams (Figure 8). Although the positions of the lines of splitting cannot be mapped accurately, it is apparent that they are not related to the Coalbrook Fault, locally a commonly held view. The average thickness of the Pumpquart on both sides of the small area of thick combined seams is about 36 to 39 in. No identifiable fossils have been collected from the Low Vein.

The Trichwart or Triquart ('Three-quarters) Vein has been worked to a very limited extent in Glynhebog Colliery, where it is only about 24 in thick. The roof, which continues as the roof of the Pumpquart west of the Coalbrook Fault, consists of mudstone from which Cochlichnus kochi, cf. Planolites sp. [small], Anthraconaia ?, Carbonicola cristagalli Wright, Geisina arcuata, Leaia sp.,a Belinurid fragment and fish remains have been collected. The ash and sulphur contents of the Trichwart, although low, are higher than in the Pumpquart east of the Coalbrook Fault. It follows that the Pumpquart Vein west of this fault is not of such high quality.

The Stinking Vein, so named because of its high sulphur and ash content, averages about 19 in thick. The mudstone overlying this seam usually contains a very distinctive fauna characterized by the presence of Carbonicola spp. of the oslancis Wright and cristagalli groups, Curvirimula spp., Naiadites spp., Geisina arcuata and fish remains. As fossils are generally uncommon in the 200 ft or so of strata below the Amman Marine Band, the presence of this fauna in the roof of the Stinking Vein is of great value in correlation; for example, any doubt about the correlation of the Pumpquart Vein of Pentremawr and Glynhebog collieries was removed by the presence of the Stinking Vein fauna in these collieries.

The detailed correlation of the strata between the Stinking and Gwendraeth veins, in the absence of diagnostic fossils, has proved difficult or impossible, even in the Cynheidre area where these rocks have been accessible. There is considerable variation in the succession, even where allowance has been made for the faulting present in these beds. A seam with a very variable, and in places complex, section appears to persist about 90 ft above the Stinking Vein, at least near the outcrop, in the Pontyberem–Pont-Henry area; this seam was named the Stinking Rider during the re-survey. Mussels have been found in the roof of this seam only in the two most south-easterly boreholes in the Cynheidre area. Elsewhere in that area this seam is not invariably identifiable, and more than one seam over 24 in thick is locally present. Bastard seatearth, usually containing sphaerosiderite, is commonly present between the Stinking and Gwendraeth seams, particularly above the Stinking Rider; it is in places present at more than one horizon. Except in parts of Glynhebog Colliery, sandstone is subordinate to argillaceous strata. A mussel band containing Anthraconaia spp., Carbonicola venusta Davies and Trueman and Naiadites subtruncatus (Brown) is locally present about 20 ft below the Gwendraeth Vein.

The other seam of economic importance in these measures is the Gwendraeth Vein, which is correlated with the Yard of the standard Aberdare succession. The most extensive workings in this seam in the Gwendraeth Valley were in Pentremawr and Ponthenry collieries. The thickness of this seam averages about 34 to 36 in, and it is characterized by the presence of about 6 in of anthracite cannel which usually occurs about 6 in from the top of the seam. The fauna above this seam includes Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, cf. P. sp. [small], Spirorbis sp., Anthraconaia  aff. modiolaris (J. de C. Sowerby), Anthracosia sp.  (regularis (Trueman) group), Carbonicola sp., Naiadites sp. (intermediate between productus Brown sp. and quadratus J. de C. Sowerby sp. ), N. sp. [large] and Leaia sp. In part of the Cynheidre area (Figure 10) a thick sandstone has been proved at the horizon of the Gwendraeth Vein.

The top of the Lower Coal Measures is taken at the base of the Amman Marine Band; for practical purposes this corresponds to the top of the Amman Rider seam, which is normally about 40 ft above the Gwendraeth Vein. It is usually thin, seldom exceeding 9 in, and has a high sulphur content. In much of the Cynheidre area another thin coal seam is present about 10 to 15 ft below the Amman Rider, off which it is presumably a split; this thin seam usually has mussels, including Anthracosia regularis, in its roof.

Rhasfach Vein to Amman Rider Vein: details

East of the Coalbrook Fault

Between the Lletty Wilws and Coalbrook faults the Pump-quart Vein has been very extensively worked in Glynhebog Colliery and in older workings from Maesmawr Pit [SN 5130 1187]. The main access was Glynhebog Slant [SN 5166 1221] but the workings were connected by a cross-measures drift (the 'roofing hole') to Coalbrook Colliery slant in the Green Vein [SN 5131 1141]; in 1939–41 a new shaft, now called Cynheidre No. 3 [SN 5260 1063], was sunk from the surface to the Trichwart Vein, with connections to the Pumpquart workings.

In Glynhebog Colliery the Pumpquart varies from about 34 to 45 in thick, the average section being about 39 in. A thin 'dirt' parting near the middle of the seam has been recorded in some districts, and in places the bottom 6 in is said to be of relatively inferior quality. The roof is usually sandstone or very silty mudstone and striped beds, although a 'clod' is sometimes present. Carbonicola? [solid] is the only fossil collected from the seam in this area.

The Trichwart Vein, which normally lies about 30 ft above the Pumpquart and is about 18 to 27 in thick, has been worked only to a very limited extent: the ash, sulphur, and arsenic content is higher than in the Pumpquart, and a thin bed of pyrite is present in places about 10 in from the top. Carbonicola cristagalli has been collected from poorly exposed mudstone roof measures, but Dix has recorded (1928, pp. 456–7) "large fragments of Carbonicola sp., C. cf. aquilina (J. de C. Sow., sens. lat.), C. cf. obtusa Hind, Anthracomya sp., A. sp.  nov., Leaia tricarinata Meek and Worthen forma minima Pruv. and ?Rhizodopsis sauroides Wills.", in addition to plants from this horizon.

A small area of a thick seam, believed to be the combined Pumpquart and Trichwart veins, has been worked in the 10 West District [SN 507 103] of Glynhebog Colliery, west of the Coalbrook Fault. This is described in the account of the Pontyberem to Pont-Henry area (p. 50).

The Stinking Vein, sometimes called the Yellow Vein, lies about 20 ft above the Trichwart; its high ash content and thin section, about 18 to 20 in, make it unworkable. The distinctive fauna in its roof consists of cf. Planolites sp.,[small], Carbonicola cf. cristagalli [jiv.], C. aff. oslancis, C. pseudorobusta?, Curvirimula candela (Dewar), C. cf. trapeziforma (Dewar), Naiadites flexuosus Dix and Trueman, Geisina arcuata, Rhabdoderma sp.,and has been collected in Larry's Cross-measure Drift [SN 521 108] where the section is:

A complex seam about 70 to 90 ft above the Stinking Vein in this area has been named the Stinking Rider. This seam is recorded as 25 in thick, with high sulphur and ash content, overlying 1 ft 2 in of 'rashings', in Cynheidre No. 3 Shaft; coal 20 in, parting 0.5 in, coal 4 in, coal and rashing 10 in, in the 'roofing hole', and coal 10 in, seatearth with ironstone nodules 2 ft, coal 17 in, in a disturbed section in Larry's Cross-measure.

Three cyclothems with very thin coals or shale with coal streaks at their bases, were proved between the Stinking Rider and the Gwendraeth Vein in Cynheidre No. 3 Shaft. The strata between the Stinking and Gwendraeth veins are generally more arenaceous than those between the Rhasfach and the Stinking veins, and consist, apart from the coal horizons, of silty mudstone and striped beds with thin beds of sandstone. The only fossils recorded are Neuropteris heterophylla Brongniart, Sphenopteris laurenti Andrae and Sphenophyllum cuneifolium from about 25 ft above the Stinking Rider in No. 3 Shaft. N. heterophylla also occurs about 25 ft below this seam.

The Gwendraeth Vein has been worked on a very limited scale from the 5 West cross-measures Drift in Glynhebog Colliery [SN 5188 1120], where the following section was recorded:

In the 'roofing hole' the Gwendraeth Vein is recorded as 34 in thick, and the section in No. 3 Shaft is: inferior coal 6 in, cannel 6 in, coal 22 in. Dix and Pringle (1929, p. 97) recorded Belinurus bellulus Koenig from the roof of the Gwendraeth Vein.

The Amman Rider, consisting of 10 in of inferior coal, is present 35 ft above the Gwendraeth Vein in the 'roofing hole'.

Pontyberem to Pont-Henry

An old trial level [SN 5086 1074] 1050 yd north-east of the bridge at Pontyberem was driven N.20°W. for about 120 yd, and is said to have proved the Rhasfach Vein, but no workable ironstone.

The Rhasfach Vein and the ironstones above it have been worked north of Pontyberem. Shale and mudstone on the tip of an old descending drift [SN 5030 1150] contains Curvirimula fragments and Carbonicola cf. martini Trueman and Weir [long anterior end]. 'Rashes' overlain by dark grey shale in Cwm Berem [SN 5003 1151] are probably at the Rhasfach horizon, and this coal may have been exposed near Cilcarw-fawr [SN 4824 1053] in the excavation for a water-wheel.

Fossils collected from the tip from an old shaft about 60 yd west of the Pont-Henry Fault north-west of Pont-Henry [SN 4750 0969], said to have been sunk for ironstone above the Pumpquart (Strahan and others 1909, p. 92), include Carbonicola cf. pseudorobusta, C. cf. communis and Curvirimula candela, an assemblage which indicates an horizon nearer to the Rhasfach. This horizon is also suggested by the presence of Carbonicola cf. robusta (J. de C. Sowerby), Curvirimula sp.,and fish remains including Rhabdoderma sp. on a landslip in Aberlledle [SN 4739 0985].

The Low Vein, sometimes known locally as the Little Vein, has been proved in two underground pits in Pentremawr Colliery; in one [SN 4974 0981] 19 to 21 in of coal, with a sandstone roof, was proved 33 ft below the Pumpquart Vein, where an abortive attempt was made to work the seam. In the other [SN 4911 0870] the following section was recorded:

In Ponthenry Colliery the Pumpquart and the Low are about 22 ft apart : the average section of the Low Vein, which was not worked, is recorded as 15 in, overlain by 'rock'. The Low was worked in Pont-Henry opencast site, where the average thickness was about 18 in and the 20 to 25 ft of strata between it and the Pumpquart consisted mostly of mudstone.

Gwendraeth (Pontyberem Old) Colliery is one of the oldest in this district of which any plans are preserved. Two shafts, Chasm [SN 5041 1134] and Pumpquart [SN 5070 1107] pits, were sunk 84 ft and 450 ft respectively to the Pumpquart, over 100 years ago: the latter pit was referred to as the 'Gwendraeth Pit' by Strahan and others (1907, p. 137). Fairley (1868, fig. 19B and pp. 22, 34) provides evidence that the thick coal of the combined Pumpquart and Trichwart seams was worked, as well as the two component coals to the east of the line of splitting. The workings are limited to the east by the Coalbrook Fault, but there was a connection to Maesmawr Pit.

Between Pontyberem and Pont-Henry the Pumpquart, which from here to the west consists of the combined Trichwart and the top split of the Pumpquart, has been largely worked out down to about 1000 ft below O.D.

Capel–Ifan Colliery [SN 4930 1058] worked the Pumpquart Vein from a slant which started on the only drift-free part of the outcrop of the seam between the Coalbrook and Pont-Henry faults. This colliery was succeeded by Pentremawr Colliery, in which descending drifts start in the Stinking Rider (sometimes locally called the 'Trichwart'). A typical section of the Pumpquart is as follows: mudstone with plant fragments, ironstone nodules at base on mudstone, lenticular ironstone at base, 7 in; fissile mudstone with cf. Planolites sp.,[small], Carbonicola cf. cristagalli, and a Belinurid fragment, mussels common about 1 ft 1 in above the coal, 1.5 ft; slightly silty mudstone with plant debris, 4 in; COAL, 6 in; 'rashes', 2 in; COAL 33 in.

The bottom coal is often thinner, and in a place where it is 26 in thick, a parting has been recorded 1 ft from the base.

The Coalbrook Fault, with a downthrow east of 90 ft, was proved in the 10 West District of Glynhebog Colliery. About 70 yd east of the fault a 36 in seam was proved 6 ft above the Pumpquart, the distance between the seams was reduced to 1 ft 10 in of seatearth in proximity to the main component of the fault and to the west of the fault a small area [SN 507 103] of a seam with the following section was worked: coal 9.5 in on parting 0.5 in, coal 16 in, 'rashes' 1.5 in, coal 34 in, 'rashes' 9 in, coal 24.5 in. It is probable that the top two coals of this section correspond to the Trichwart Vein of Glynhebog, the 0.5-in parting being in the position of the pyrite recorded in that seam, while the bottom two coals are the Pumpquart of Glynhebog, the 9 in 'rashes' being developed at the horizon of the thin parting sometimes present in Glynhebog.

More recently this thick seam has been mined in Pentremawr Colliery, the average thickness being about 69 in. In Pentremawr the Low Vein has been proved to be the bottom split of this thick coal, a section near the split being coal 30 in on a parting 2 ft and coal 12 in. The two splits of the thick coal are shown diagrammatically in (Figure 8). The average total thickness of the Pumpquart and Trichwart veins in Glynhebog is about 61 in, and the average total of the Low and Pumpquart veins in Pentremawr is 59 in, but the different combination of sections, together with the thickening of the bottom leaf of the Trichwart, have resulted in the quality of the Pumpquart of Pentremawr Colliery being inferior to that of Glynhebog. As the Glynhebog Trichwart contributes most coal to the seam known as the Pumpquart Vein west of the Coalbrook Fault, the former name might be more appropriate, but the latter has been retained because of long-standing custom in the area.

A similar section was worked in Ponthenry Colliery, in which the main access was a gently descending drift [SN 4801 0974] driven north-westwards to intersect the seam. An important factor in limiting the workings in this seam in Ponthenry Colliery was the prevalence of 'outbursts' (Roblings 1926).

The Pumpquart Vein worked in Pont-Henry opencast site averaged about 39 in thick; sandstone up to about 12 ft thick was generally present in this cyclothem.

In addition to the fauna listed above from the Pumpquart in Pentremawr Colliery, Dix (1928, p. 458) recorded the following fossils from Pentremawr and Ponthenry collieries "Carbonicola cf. dawsoni (Brown), C. cf. ovalis (Martin), C. cf. obtusa Hind, C. cf. aquilina (J. de Sow., sens. lat.), C. sp.,fragments of large shells, Anthracomya minima Ludwig, A. sp.  nov., Naiadites triangularis (J. de C. Sow.), N. cf. triangularis and Rhizodopsis sauroides Wills".

The Stinking Vein, also known locally as the Little Vein, has the following section in the Main Slant of Pentremawr Colliery, where it lies about 30 ft above the Pumpquart:

The following fossils were identified: Carbonicola cf. browni Trueman and Weir, C. cf. communis, C. cristagalli ?, C.  aff. oslancis, Curvirimula candela, C. cf. trapeziforma, Naiadites flexuosus, Geisina arcuata [common], fish remains including Rhabdoderma sp. and Rhizodopsis sp. The Stinking Vein is exposed in air bridges across the main slant where thicknesses of 23, 16 and 12 in of coal have been recorded. In Ponthenry Colliery, the Stinking Vein, there called the 'Trichwart', is said to be 15 in thick, with a sandstone roof, and to be about 30 ft above the Pumpquart: in Pont-Henry opencast site a very thin Stinking Vein, the same height above the Pumpquart, was sometimes exposed in the high-wall.

In the return airway of Pentremawr Colliery the Stinking Rider was seen imperfectly exposed in disturbed sections: silty mudstone with plant debris on coal 15 to 29 in; silty mudstone with plant debris on coal 8 in, silty mudstone about 2 ft, coal 27 in, seatearth, in another section.

A staple pit [SN 4967 0998] in Pentremawr proved a distance of 114 ft between the Stinking Rider and Pumpquart seams; the distance between the Stinking and Stinking Rider seams averages about 90 ft in both Ponthenry and Pentremawr collieries. In Ponthenry the Stinking Rider is present as 33 in of coal and 'rashes'.

The strata between the Stinking Rider and Gwendraeth seams have not been proved in an undisturbed succession in Pentremawr Colliery, but in a drift from the Pumpquart to the Gwendraeth in Ponthenry, three cyclothems have been recorded.

The most extensive workings in the Gwendraeth Vein in the Gwendraeth Valley are in Pentremawr and Ponthenry collieries. The Gwendraeth Vein has been worked from three slants in Pentremawr, No. 1 Slant [SN 4940 1043] No. 2 [SN 4935 1042], and Phil Skym [SN 5003 1085]. The following section was recorded in No. 1 Slant: mudstone, slightly silty in parts, cf. Planolites sp. [small] and mussel fragments including Naiadites sp. [juv.], seen to 2.5 ft; 'rashes', 1 to 3 in; carbonaceous mudstone with coal streaks, 0 to 1 in; coal, 5 in; cannel, 5.5 in; coal, seen to 18 in.

A cross-measures drift from the Pumpquart Vein at the southern limit of the workings in Pentremawr Colliery proved the Gwendraeth Vein [SN 5037 0864] to consist of coal 9.5 in, on cannel 5 in, coal 33.5 in, 'rashes' 6 in. The cannel, which averages about 5 inches in thickness, is found in places to be present at the top of the coal; the total thickness of the seam, including cannel, is recorded as 38 in on the working plan, but it probably averages about 34 in. In Ponthenry Colliery the average thickness is recorded as 33 in. From the Gwendraeth Vein of Pentremawr and Ponthenry Collieries, Dix (1928, p. 463) recorded "Carbonicola cf. venusta Davies and Trueman, C. cf. turgida (Brown), C. cf. fusca Davies and Trueman, C. sp.,crushed, C. sp.,small and stout Anthracomya cf. modiolaris (J. de C. Sow.), A. cf. wardi Salter (non Hind), A. cf. insignis Davies and Trueman, Naiadites cf. triangularis (J. de C. Sow.), N. producta (Brown) and N. modiolaris (J. de C. Sow.)". This seam is approximately 225 ft above the Pumpquart in this area.

The Amman Rider seam is about 30 ft above the Gwendraeth, and is thin, no record giving more than 11 in of coal.

Pont-Henry to Carway

The Rhasfach Vein and the overlying ironstones have been very extensively worked between Pont-Henry and Pont-Newydd by opencast mining, bell-pits and levels. The plans of four small mines are preserved: at Rhasfach Colliery [SN 4677 0918] 18 to 20 in of coal is underlain by 4 to 6 in of 'rashes' and 3 ft of fireclay, no section is recorded of the coal at Tanerdy (or Tynherdy) Colliery [SN 4592 0830], but at Cwmbach Colliery [SN 4523 0765], last worked in 1943, the seam is 18 to 22 in thick, and the following details have also been recorded (Strahan and others 1909, p. 100): "Shale with three pins of ironstone up to 4 in thick and also scattered balls, 7 ft; shale, 7 ft; Rhasfach Vein, 1.75ft; Fireclay, the upper half worked for bricks and for lining furnaces, 7 ft". No details are preserved of the section proved at Glyn Abbey Colliery [SN 4468 0751].

Two shafts were sunk to the Rhasfach between Pontyates and Llwyn-yr-hebog Farm, one of which has been referred to as Llwyn-hebog Pit (Strahan and others 1909, p. 102) but they are also known as Llwyn-y-Wiwer Colliery [SN 4661 0850]; the extent of the workings is not known. Two sections, which include details of the beds of ironstone above the Rhasfach, which is 20 in thick, were recorded by Logan (see Appendix I). Logan believed the distance between the Pumpquart and Rhasfach veins to be 204 ft 9 in according to one section, and 218 ft 3 in according to the other, but if the 'coal shale culm' in one section is the Low Vein, as appears likely, then the Pumpquart and Rhasfach are probably about 190 ft apart.

Two seatearths, one overlain by a coal smut, and a 4-in coal [SN 4715 0913], probably the Low Vein, are exposed between the Rhasfach and Pumpquart seams in the small stream which passes Rhas Cottage. A collection of 'solid' shells from the tips between Rhas Cottage and the Red Lion, Pont-Henry [SN 4743 0968] represents a community of Carbonicola  aff. pseudorobusta, although most of the specimens are smaller than typical C. pseudorobusta. A similar fauna was collected from the tip from an old level [SN 4666 0870] 190 yd S.S.W. of St. Mary's Church, Pontyates, 15 yd to the south-east of which mudstone with thin beds of ironstone is exposed. A bed of ironstone 8 in thick, in strata folded in close proximity to a small fault, is exposed near Tyn-y-cwm [SN 4605 0854].

Carbonicola cf. pseudorobusta [small] was collected from the tip of Glyn Abbey Colliery [SN 4468 0751]. The northerly trending fault which displaces the Millstone Grit near Blaen-y-bryn [SN 4535 1008] may continue south to the River Gwendraeth, sharply folded strata, no longer visible, being recorded (Strahan and others 1909, p. 101) 50 yd west of Cwm-bach [SN 4516 0775].

The Low and Pumpquart veins have been worked in Gwendraeth opencast coal site, which extended from 100 yd south-west of Furnace [SN 4736 0924] to 400 yd south-east of St. Mary's Church, Pontyates. This site was worked after the six-inch maps were prepared for publication, and the worked-out areas are not shown. The Low Vein was proved to be about 18 in to 21 in thick, some 25 to 30 ft below the Pumpquart; sandstone is commonly present between these two seams. The Pumpquart was similar to that proved in Ponthenry Colliery, a typical section being coal 3 in on 'rashes' 4 in and coal 36 in. In a weathered section in a trial pit [SN 4706 0882], the bottom coal consisted of two leaves, with a clean parting between them: the top leaf had a substantially higher ash content than the bottom leaf, and they probably represent the lower part of the Trichwart and the upper part of the Pumpquart of Glynhebog Colliery respectively. No identifiable fossils were found in the dark grey shale in the immediate roof. The Stinking Vein, about 15 in thick, was proved about 30 to 35 ft above the Pumpquart. The Pumpquart has been worked near the crop in Glan-Gwendraeth (or Pont-Yates) Colliery [SN 4694 0849], the workings extending for about 0.75 mile along the strike; the pit is said to be 132 ft deep to the Pumpquart. Carbonicola communis? has been collected from the tip of Jenkin's Slant [SN 4690 0876] which is connected, in the Pumpquart, to Glan-Gwendraeth Shaft. Although not shown on the plans, an old section also shows a connection in the Pumpquart between Glan-Gwendraeth and Llwyn-y-Wiwer collieries, and there is evidence that a limited area of Low Vein was also worked.

The Pumpquart Vein has been worked below boulder clay cover in shallow pits between Llwyn-y-Wiwer Colliery and Tanerdy [SN 4598 0798], and there is evidence of attempts to work the Low Vein. Very little coal is recorded as having been worked from Pumpquart (or Pum-cwart) Colliery, a group of old pits near Tanerdy. The northerly trending fault east of Tanerdy was proved in this colliery, the throw perhaps being 48 ft (Fairley 1868, p. 22).

The average section of the Pumpquart recorded on the Abandonment Plan (dated 1928) of Plasbach Colliery [SN 4684 0795] is coal 8 in on 'clod' 2 in and coal 40 in, but the bottom coal may be too thick to be representative. The Stinking Vein (called the 'Lower Trichwart'), 20 in thick, was proved 23 ft 6 in above the Pumpquart in one section, and 35 ft elsewhere in this colliery. In Plasbach Colliery the strata from the Pumpquart Vein to a short distance above a seam recorded as the Braslyd were proved in a cross-measure drift at the south-eastern limit of the workings [SN 4762 0753]; details of this section are given in Appendix I. Two seams with complex sections are recorded 120 ft and 188 ft above the Pumpquart, and numerous beds of Tweelay' are also recorded: no detailed section of the Upcast Pit [SN 4684 0795] is known, but a seam called the 'Trichwart', in the Stinking Rider position, was proved about 140 ft above the Pumpquart in a cross-measure drift [SN 4682 0789] near the shafts.

The Gwendraeth Vein has been worked in Caepontbren [SN 4722 0830] and Plasbach collieries, where the average thickness is probably about 36 in; it is said to have 3 in of cannel in its roof. In the cross-measures drift already referred to [SN 4762 0753], where the Gwendraeth Vein is probably 300 ft above the Pumpquart, the section is recorded as: coal 1 in, on cannel 5 in, coal 40 in, soft coal 2 in, coal 7 in. Another seam, 50 ft higher, with the section: coal 1 in, on cannel 6 in, coal 9 in, soft coal 1 in, coal 5 in, pyrite ('brass') 2 in, coal 25 in was thought to be the Braslyd Vein, but the section suggests that this may also be the Gwendraeth Vein, repeated by a thrust. An easterly trending fault was proved at the southern limit of the 'workings in Caepontbren Colliery [SN 4745 0757], and disturbed ground with a similar trend was proved under Pontyates School [SN 4735 0823].

Carway to the Trimsaran Disturbance

No information is available about the strata between the Rhasfach and Pumpquart veins between Pont-newydd and the Trimsaran Disturbance.

The outcrop of the Pumpquart Vein passes under the alluvium of the River Gwendraeth a short distance west of the mouth of Pumpquart Colliery adit, but is repeated twice by overthrusts on the south side of the river in the area near Bryn Forest Inn [SN 4500 0697]. The position of the north-easterly trending fault near Pont-newydd, shown under the alluvium and gravel on the map, is inferred from the position of the outcrop of the Pumpquart and Rhasfach veins north of the river relative to the Pumpquart Vein proved in Hansard's Slope No. 1 described below; the fault is presumed to be an overthrust which dies out to the north-east, as there is no evidence of a fault between the workings in Glan-Gwendraeth and Plasbach collieries. The exploration in this area by Hansard was in progress during the original six-inch survey, and the following details are derived from the Carmarthen Memoir and E. E. L. Dixon's MS notebooks. Berkenshaw's Slope is older.

Hansard's Slope No. 1 [SN 4508 0700] was driven for 100 yd on a seam overlain by mudstone with three beds of ironstone, believed to be the Pumpquart Vein because of its similarity with that seen in Plasbach Colliery, and because a well [SN 4501 0699] proved a coal seam 20 in thick and overlain by 6 to 7 ft of sandstone, believed to be the Low Vein, 36 ft below this coal. The seam dips southwards at 16° to 18° for about 70 yd, flattens for 30 yd and then the dip increases sharply adjacent to a fault: beyond the fault a further 110 yd of drivage proved gently folded but unrecognizable strata.

Brynfforest Colliery, also known as Hansard's Slope No. 3 [SN 4481 0665], was another attempt to work the Pumpquart Vein, which was followed down from the surface for nearly 300 yd. Although the section of the seam recorded on the Abandonment Plan is coal 21 in on 'holing' 10 in and coal 42 in, the coal was highly disturbed, being slickensided and cut by small thrust faults or 'overlaps' and proved unworkable. The details of the structure proved in this colliery, including an older drainage adit, are shown in (Figure 9). The thrust is presumably the same as that proved in Hansard's Slope No. 1. Mussels on the tip from an old shaft [SN 4476 0657] include Carbonicola?; in another old shaft [SN 4496 0665] the Low Vein, 16 to 20 in thick, was proved 30 ft below the Pumpquart and fine-grained sandstone, about 7 ft thick, between these seams is said to have been worked in the shaft.

The identity of the seam worked at Berkenshaw's Colliery [SN 4541 0790] has, until recently, been in doubt (Strahan and others 1909, p. 105); since the six-inch resurvey was completed the Low and Pumpquart veins have been worked on Lambert's opencast coal site [SN 455 071], in which it was proved that the coal shown as the Pumpquart on the published six-inch sheet SN 40 NE, is the Stinking Vein. The section proved in this site was as follows:

Dix (1928, p. 484) recorded "Carbonicola sp. ", similar to specimens seen above the Lower Trichwart higher up the Gwendraeth Valley, from the tip of Berkenshaw's Colliery; old workings were found in the Pumpquart in the opencast site and it is possible that this colliery worked the Pumpquart Vein, although it is clear that the slant was driven in the Stinking Vein.

On the south side of the river between Berkenshaw's and Plasbach collieries, the only record of the Pumpquart Vein is at the end [SN 4603 0670] of the 'Wembley' Cross-measures Drift in Carway Colliery. At 545 ft below O.D., a seam with the section: coal 9 in on 'rashes' 6 in, coal 50 in and 'rashes' 6 in, thought to be the Pumpquart, was followed for about 30 yd along the strike; another seam, 16 in thick, 29 ft 3 in higher, was correlated with the Stinking Vein. These correlations made at the time the heading was driven and based on 'traditional' methods, that is thickness comparisons and the nature and section of the coal seams, are rendered highly speculative by the very disturbed nature of the strata proved in this area. The 'Pumpquart' is over 700 ft below the Big Vein.

The section exposed in the Fforest-fach Level [SN 4579 0732], which was driven in a generally southerly direction, was examined by Logan and is the basis for the section published in 'Vertical Sections', Sheet 4, Geol. Surv. Gt. Brit., but it is impossible to reconcile the published summary (Strahan and others 1909, p. 113) with Logan's MS. notes, or identify the pits. It is possible that the seam 27 to 35 in thick near the entrance to the Level, underlain by seatearth on sandstone, and overlain by mudstone with ironstone nodules, is the Pumpquart, and that the 'Brasslwyd' or 'Bot-y-ci', 16 in thick, 16 ft above this seam and said to be sulphurous, is the Stinking Vein. The strata are so highly disturbed that it is impossible to interpret the sequence proved in the rest of this section.

To the south-east of Brynfforest the identity of the seams proved near Glassberry and Ty'r-eithin is uncertain. The following section is said to have been proved in a trial shaft [SN 4429 0635] : boulder clay, 12 ft; measures, with three beds of ironstone 22.5 in, 3 in and 4 in thick near base, 60 ft; shale with ironstone nodules, 9 ft; coal 20 in on fireclay. A few old workings or trials south-west of Glassberry may be on the same seam, which is more likely to be at, or near, the horizon of the Pumpquart than the Rhasfach Vein (Strahan and others 1909, p. 101). The seam 144 in thick, under 36 ft of boulder clay, found in a pit [SN 4458 0625] near Ty'r-eithin, is said to have been followed for 90 yd down dip in a slant [SN 4454 0622]. The average thickness is 42 in and it is overlain by a 'stinking vein' or 'Drawllyd'.

A shallow anticline is exposed in a ditch [SN 4435 0635] south-east of Glassberry. There is no trace of an old quarry, said to be south [SN 4461 0653] of Glassberry, in the sandstone above the Low Vein, which was used for the construction of Kidwelly Quay.

In the 'Wembley' Cross-measures Drift in Carway Colliery, the highly disturbed section between the Braslyd and Pumpquart veins (p. 54) includes three seams with complex sections estimated to be 141, 173 and 268 ft above the Pumpquart; the highest was thought to be the Gwendraeth Vein. North of Carway a seam worked at the crop for about 600 yd is probably the Gwendraeth Vein, but this area is now obscured by the tip from Carway Colliery; unsuccessful attempts to mine the seam were made in Lambert's [SN 4616 0736] and Capt. Smith's [SN 4621 0739] collieries. The thrust proved in the Brynfforest area probably extends nearly to Plasbach, and accounts for the displacement of the seams between Lambert's and Plasbach collieries.

The only other seam which may possibly be identified as the Gwendraeth Vein is that worked in Hansard's Slope No. 2 [SN 4522 0662]. The coal was followed for about 200 yd at a dip of about 14°, most of this drivage being in old workings drained by an old adit found 150 yd from the entrance; the coal was found to be folded and the thickness variable. The seam was faulted at 200 yd, beyond which a cross-measures drift proved disturbed strata including thin coal seams; according to one account the beds dip W.N.W. at 65° at the end of the drift. From the position of this seam relative to the Pumpquart Vein of Brynfforest Colliery and the Big Vein of Carway opencast site, it is likely that it is the Gwendraeth Vein, although at one time it was thought to be the Gras Uchaf Vein.

The position of the Amman Marine Band throughout the area from Ponthenry to the Trimsaran Disturbance is conjectured.

South of the Trimsaran Disturbance

The beds between the Cwm Berem Sandstone and the Low Vein are very poorly exposed in Afon Morlais, near Moat Farm [SN 4325 0524]; a short distance to the west the Coal Measures are obscured by alluvium. An old drift close to Moat Farm is said to have been driven to a coal 6 in thick, possibly the Rhasfach; dark grey shale is exposed at the mouth. Ironstone and coal were got in very limited crop workings on the opposite side of the river [SN 4329 0532].

Sandstone quarried [SN 4360 0532] 470 yd north-east of Llandyry Church is probably the sandstone between the Low and Pumpquart veins. Only 3 ft of quartzitic sandstone, overlain by 9 ft of silty mudstone with impersistent beds of ironstone, is exposed, but Logan (MS.) recorded 12 ft of sandstone underlain by 2 ft of coal. The same sandstone was quarried [SN 4337 0514] south-east of Moat Farm; the seam overlain by sandstone worked in an old slant [SN 4318 0503] south-west of Moat Farm is probably the Low Vein.

The Pumpquart Vein, 39 in thick and overlain by mudstone with three beds of ironstone, is said to have been proved at a depth of 36 ft in a pit [SN 4368 0535] 540 yd northeast of Llandyry church. The same seam was found at 144 ft in Old Llandyry Pit [SN 4371 0528], and may have been worked at outcrop [SN 4352 0520], as the Bwt-y-ci or Cwt-y-ci Vein, 300 yd north-east of the church. Dix (1928, p. 485) collected "Carbonicola cf. ovalis (Martin), C. cf. communis Davies and Trueman and C. cf. aquilina (J. de C. Sow.)" from the tip of Old Llandyry Pit, which confirmed that the Pumpquart had been worked.

The identity of the seams proved in the old adit level [SN 4367 0526], part of which was described as "the long-abandoned Cil-rhedyn Colliery" in 1909 (Strahan and others, p. 122) and in Llandyry Slant [SN 4373 0521], abandoned in 1898, is in doubt (Figure 9). Logan based part of Vertical Sections, Sheet 1, Geol. Surv. Gt. Brit. on the strata proved in Cil-rhedyn Level and Cil-rhedyn Pit, which is probably another name for Old Llandyry Pit: he called the Pumpquart the 'Gwendraeth' Vein, and recorded a level cross-measures drift 120 ft long from this coal in the pit to the Low Vein, giving a vertical separation of 57 ft, which is abnormally high.

Logan recorded a seam, known as the Hwch, 37 yd from the mouth of Cil-rhedyn Level, with the section: coal 1 ft on underclay 1 ft 6 in, coal 4 ft (varying from 1 in to 15 ft), underclay 14 ft and coal 1 ft 4 in, with a high sulphur content; Logan also recorded that the three coals, which he estimated lie 156 ft above the Pumpquart, came together. This seam, which was correlated later with the Gwendraeth Vein (Strahan and others 1909, p. 124), may be the Stinking Rider.

About 0.75 mile east of Llandyry Slant, Carbonicola sp.  (cristagalli group) was collected by Dixon on the north bank of the River Morlais [SN 4484 0517], suggesting that the beds there are near the C. communis–A. modiolaris zonal boundary. This locality is near an exposure of mudstone with thin beds of ironstone separated by a small gap in the section from seatearth, in which the beds dip at 50° to 75°. The similarly highly inclined, folded and faulted strata, involving coal 'smuts', intermittently exposed in the banks of the River Morlais from Llandyry to a point about 200 yd east of Culla House [SN 4512 0529], and in the road cutting by Culla House, are probably all in the Lower Coal Measures, although no other fossils have been found. Local names, including Upper and Lower Cwm Cathan and Garwed, have been applied to seams outcropping in this very disturbed area.

The Trimsaran Borehole [SN 4483 0416] was stopped 61 ft below the Amman Marine Band. The presence of Anthracosia regularis (accompanied by Gyrochorte carbonaria, cf. Planolites sp. [small] and Naiadites sp. ) above a 9 in coal seam 38 ft below this marine band suggests that this seam is probably between the Amman Rider and Gwendraeth veins (Figure 6). Anthraconaia sp.  (modiolaris/curtata (Brown) group) was found about 13 ft below this thin coal; the borehole may have stopped a very short distance above the Gwendraeth Vein.

Although the structure and succession in the heavily boulder clay-masked area between Trimsaran and the marine alluvium is largely unknown, the complexity of the structure is indicated by the presence of Curvirimula sp. and ?Geisina arcuata in dark grey shale overlying silty mudstone with rootlets, and overlain by striped beds, in a road cutting [SN 4391 0442] 0.25 mile south-east of Llandyry church; the presence of Curvirimula suggests a horizon not higher than the Stinking Vein. Mudstone with a thin bed of ironstone is exposed in the mouth of an old level [SN 4385 0442] 60 yd west of the road cutting.

Cynheidre area

In addition to the underground borehole near Cynheidre No. 3 Shaft, from which details of the Coal Measures have been published (Woodland and others 1957), several deep surface boreholes, Gwendraeth Valley No. 2 and Cynheidre Nos. 3/1, 3/2, 4/1, 4/2, 5/1 and 6/1, have been drilled below the Pumpquart, and were stopped a short distance above or below the Rhasfach: Cynheidre No. 3 Shaft (p. 39) was deepened to below the Rhasfach. The locations of these boreholes, and abridged logs, are given in Appendix I.

The abbreviated log of the bottom of Cynheidre Borehole No. 3/1 [SN 5017 0734] is as follows:

The details of the four cyclothems between the Rhasfach and Pumpquart veins in the Cynheidre area (Figure 6) can be summarized as follows.

The Rhasfach Vein, which is consistently thin, the maximum thickness recorded being 3 in, is overlain by dark grey silty mudstone with plant debris and occasional fish fragments, followed by dark grey mudstone with Anthraconaia sp., Carbonicola sp.  (martinilcommunis group), C.  aff. pseudorobusta, Curvirimula cf. candela, C. cf. subovata, Naiadites sp.,Geisina arcuata, and fish remains including Rhabdoderma sp. and Rhadinichthys sp. The overlying lighter grey mudstone, silty in parts, has yielded Spirorbis sp., Anthraconaia sp.,Carbonicola polmontensis (Brown), C. sp.  (pseudorobusta group), Naiadites flexuosus, and fish remains. The top of this cyclothem is marked by mudstone with rootlets, rather than a fully developed seatearth, overlying sandstone.

The succeeding cyclothem is similar, although the fauna is restricted; Spirorbis, Curvirimula, Naiadites, and ostracods have not been recorded, but Carbonicola sp.  (?pseudorobusta group transitional to cristagalli) and Anthraconaia sp. [juv.] are present. Sandstone is usually present towards the top, which is marked by a rootlet phase.

The third cyclothem in this group in this area, like the Rhasfach cyclothem, approaches the ideal for the Lower Coal Measures; dark grey mudstone, with Carbonicola  aff. pseudorobusta, Curvirimula spp. including C. candela ?, Geisina arcuata, and fish remains including Rhadinichthys sp.,is followed by lighter grey mudstone, with Spirorbis sp., Carbonicola  aff. pseudorobusta, Curvirimula trapeziforma, and N. flexuosus? which is overlain by sandstone and seatearth. A Belinurid ? fragment is recorded in one borehole; Anthraconaia, present in the other three cyclothems, is present in only one borehole, in which A. sp. [small, elongate] and A. sp. [juv.] were found.

The fourth cyclothem, overlying a coal up to 6 in thick, has yielded Anthraconaia sp., Carbonicola cf. antiqua (Brown), C. communis, C. declivis?, C. cf. rhomboidalis, Naiadites  aff. flexuosus and fish remains, including Elonichthys sp., Rhabdoderma cf. elegans, Rhadinichthys sp. and Rhizodopsis?. Gyrochorte carbonaria is rarely present, Carbonicola of the pseudorobusta group, present in the underlying three cyclothems, has not been recorded and mussels are generally less common in this cyclothem. Ostracods and Spirorbis have not been seen, and Curvirimula cf. candela has been recorded in only one borehole. Cochlichnus kochi and Planolites spp. including P. sp. [large, 12 to 15 mm], cf. P. sp. [small, 1 to 2 nun], and P. sp. [2.5 to 6.0 mm] are relatively common in all four cyclothems, except that C. kochi is rare in the Rhasfach cyclothem.

The deep boreholes from the surface have shown that in the western part of the Cynheidre take the succession is the same as that in Pentremawr Colliery, but further east the Trichwart and Pumpquart veins are present as three thin seams, the two lines of splitting proved either side of the combined Trichwart and Pumpquart seams of part of Glynhebog and Pentremawr collieries having crossed (Figure 8). Cf. Planolites sp. and Geisina arcuata were collected from the roof of the Pumpquart, 57 in thick, in underground borehole No. 1A [SN 4970 0785] south of Pentremawr Colliery; the Low Vein, 16 in thick, was proved 17 ft below the Pumpquart. No fossils have been found on the Low Vein and the fauna on the upper split of the Pumpquart is limited to worm tracks and burrows, including Cochlichnus kochi and Planolites spp.;both of these seams are directly overlain by sandstone in places.

Planolites spp.,particularly cf. P. sp. [small], C. kochi and other 'wormy' markings are also found on the Trichwart Vein, accompanied by mussel fragments including Anthraconaia?, Carbonicola sp.  [juv], Curvirimula sp. [one record], Geisina arcuata, Leaia sp.,and plant and fish fragments including Rhabdoderma? and Rhizodopsis sp. In Cynheidre Colliery, two cross-measures drifts on the 490 and 560 horizonsThe development of Cynheidre Colliery is based on underground drivages at four levels or horizons. These are referred to as the 490, 560, 660 and 760 horizons, the figures being their depths in yards in Shaft No. 3 (they were originally known as the 910, 1150, 1450 and 1750 horizons, based on their height below Ordnance Datum, in feet). Subsequently in this account the horizon is indicated by the appropriate figure in brackets after the name of the drivage., from the main slant of Glynhebog Colliery (now known as Cynheidre Slant) towards Shaft No. 3, have proved the succession between the Pumpquart Vein and the Amman Rider. On the 490 horizon the section of the Trichwart is coal 27 in, on coal and 'rashes' 10 in, and on the 560 horizon it is coal 6.5 in, on dark grey carbonaceous mudstone 1 in, coal 19 in, 'rashes' 6 in, overlain at a distance of about 7 ft by mudstone with Curvirimula sp. and Palaeoniscid scales.

In the same drifts the Stinking Vein is 20 in thick on the 490 horizon and 31 to 35 in on the lower horizon, with high ash content in the bottom part and high sulphur content throughout. In the Cynheidre area this seam is seldom more than 15 in thick. The characteristic fauna found on the Stinking Vein in the boreholes includes Cochlichnus kochi, Gyrochorte carbonaria, Planolites spp., Spirorbis sp.,Anthraconaia sp. [one specimen], Carbonicola cristagalli, C.  aff. oslancis, C. rhomboidalis, Curvirimula candela, C. subovata, C. trapeziforma, Naiadites flexuosus, Geisina arcuata, Rhabdoderma sp. and Rhadinichthys ?. Fossils are often most abundant about 10 ft above the seam. The following details from Cynheidre Borehole No. 3/1 are typical:

The Stinking Rider Vein has a similar section in both the drifts referred to above, coal 5 to 9 in being separated from a bottom coal 21 to 27 in thick on 'rashes', seat-earth and silty mudstone. Detailed correlation of the strata between the Stinking and Gwendraeth veins in the drivages, underground and surface boreholes elsewhere in the Cynheidre area has proved impossible; after eliminating structural complication where possible, there remains considerable variation between neighbouring sections, and stratigraphically significant faunal evidence is almost entirely absent. A seam in the Stinking Rider position, that is approximately half way between the Stinking and Gwendraeth seams, is often present, but is very variable in thickness and in places has a complex section; Cochlichnus kochi, ?Gyrochorte carbonaria, Planolites sp.,and Naiadites sp. are present at this horizon in boreholes Nos. 4/3, 5/2 and 6/3 (the most south-easterly sections proved). In some sections there is more than one seam thicker than 24 in in these strata; e.g., in borehole No. 6/3 an inferior coal with a total thickness of 42 in was proved 25 ft above the Stinking Vein. Bastard seatearth, often accompanied by sphaerosiderite, is usually present, commonly at more than one horizon, especially between the Stinking Rider and Gwendraeth seams. Beds of sandstone, if present between the Stinking and Gwendraeth seams, are usually thin (except in boreholes Nos. 3/1 and 3/2; see below). Plant fragments in these beds include: Calamites undulatus Sternberg, Cyclopteris sp., Mariopteris nervosa (Brongniart), Neuropteris gigantea, ?N. heterophylla, N. obliqua, Pinnularia capillacea Lindley and Hutton. Cochlichnus kochi, Planolites spp. and Anthraconaia ? [one record] also occur in these strata, in which the only other fossiliferous horizon is about 20 ft below the Gwendraeth Vein. This latter horizon has been found in Gwendraeth Valley Borehole No. 2 and the two most easterly boreholes, Nos. 6/1 and 6/3: the fauna includes Spirorbis sp.,Anthraconaia spp. of the A. modiolaris group, Carbonicola venusta [as figured by Trueman and Weir 1947, pl. xi, figs. 19–40], Naiadites sp.  (productuslquadratus group), Geisina arcuata [in Nos. 6/3], 'Estheria' sp. [in No. 6/3, common] and Rhizodopsis sp. The same horizon, about 12 ft above the higher of two thin seams between the Stinking Rider and Gwendraeth in one of the cross-measures drifts (490) from Cynheidre Slant, contains Anthraconaia ? and Naiadites subtruncatus; mussels are also present above silty mudstone with rootlets, 16 ft below the Gwendraeth in Cross-measures No. 5 (560).

The Gwendraeth Vein has been proved in several cross-measures drifts as well as in underground and surface boreholes. In the eastern part of the Cynheidre area this seam was intersected twice by the drift on the 490 horizon from the Slant to Shaft No. 3; west of the Lletty Wilws Fault highly slickensided mudstone with Anthraconaia sp.  (?modiolaris group), ?Carbonicola venusta, Naiadites sp. intermediate between productus and quadratus [cf. Melville 1947, fig. 13e] and N. sp. [large] rests on 39 in of coal, the immediate roof consisting of 1 to 2 in of dark grey carbonaceous mudstone; east of the same fault, mudstone rests on coal 10.5 in, cannel 29 in and coal 16 to 18 in. Nearby, in the drivage to Cross-measures No. 5, another intersection of disturbed Gwendraeth Vein proved coal 7 to 11 in on cannel 5 in and coal 8 in; in Cross-measures No. 5 (490) a more typical section was proved, consisting of coal 3 to 5 in, cannel 7 to 8 in on coal 15 in, while in Cross-measures No. 5 on the 560 horizon a highly complex, disturbed section was found.

West of the Coalbrook Fault (Figure 10) the Gwendraeth Vein is cut out by the Pentremawr Lag Fault in Shafts Nos. 1 and 2, underground borehole No. 1A, Cross-measures No. 7 (660) and Borehole No. 4/2; it is disturbed because of its close proximity to this fault in the Main Lateral (660), but may be present immediately below the fault in Borehole 4/1. In Cross-measures No. 7 (560) what is probably a typical undisturbed section, overlain by mudstone with Anthraconaia  aff. modiolaris, was proved [SN 4841 0807] : coal 5 to 6 in on cannel 2 to 3 in and coal 30 to 31 in. A similar section, coal 5 in, cannel 4 in, shale 2 in on coal 30 in and inferior coal 6 in, was found in Cross-measures No. 4 (660). The discovery of thick sandstone at the horizon of the Gwendraeth Vein in Boreholes Nos. 3/1 and 3/2 was unexpected, as sandstone is absent or present only in very thin beds in all the nearby boreholes and drivages shown in (Figure 10). The absence of the Gwendraeth Vein in Borehole No. 3/1 is interpreted as due to the sandstone filling a 'washout' in the seam; the following is an extract from the log of Borehole No. 3/2:

These details, particularly the absence of a rootlet phase beneath the coals, suggest that the coal was formed from drifted material which is likely to have been derived from the Gwendraeth Vein. Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], Spirorbis sp., Anthraconaia sp.  (curtata group), Anthracosia sp.  (regularis group), Naiadites sp.  (productus group), Leaia sp. and Vetacapsula ? have also been found on the Gwendraeth Vein in the Cynheidre area.

Over much of the Cynheidre area a thin coal seam is present about 10 to 15 ft below the Amman Rider; it is probably a split off that seam, which is usually less than 9 in thick. In places the roof of the lower coal contains mussels, including Anthraconaia sp.  (modiolaris group), Anthracosia regularis, Naiadites sp.  (productus/quadratus group) and N. sp. [juv.];there are isolated records of Geisina arcuata and a fragment of a Belinurid telson.

References

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TRUEMAN, A. E. and WEIR, J. 1947. A monograph of British Carboniferous non-marine Lamellibranchia. Pt. 3, 45–71. Palaeont. Soc.

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

WOODLAND, A. W. and EVANS, W. B. 1964. The Geology of the South Wales Coalfield. Pt. IV. The Country around Pontypridd and Maesteg. 3rd 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 coal horizon in South Wales. Palaeontology by M. A. Calver. Bull. Geol. Surv. Gt. Brit., No. 13, 39–60.

Chapter 5 Middle Coal Measures

Amman Rider to Ddugaled Vein : general

Although the lowest part of the Middle Coal Measures contains several seams which have been mined in this district, these strata were accessible only in Glynhebog, Pentremawr and Cynheidre collieries during the resurvey. As a result of the examination of the strata in these collieries, and in the many boreholes drilled in connexion with the development of Cynheidre and Pentremawr, a detailed knowledge of each cyclothem has accumulated. Outside this eastern part of the district described, these measures have been seen only in the faulted succession proved in the Trimsaran Borehole; surface exposures are of negligible importance. However, the examination of higher strata in Carway and Trimsaran collieries, and in opencast coal mines in that area, together with a study of the plans of abandoned workings, has made it possible to confirm almost all the correlations made by Dix (1928) west of Pont-Henry, but the structure and succession of the Middle Coal Measures below the Ddugaled Vein in the Plasbach and Llandyry areas remains uncertain, as it is in much of the northern part of the Carway area.

Assessment of the average thickness of the seams and of the distances between them is complicated not only by incompetent structures but also by the presence of the Pentremawr Lag Fault, which affects these beds, especially between the Gras Uchaf and Ddugaled veins, in much of the area where they are accessible.

A thick development of the Amman Marine Band has been proved in the Cynheidre, Pentremawr and Trimsaran areas, and it is reasonable to suppose that it is present throughout the district. The very thin Amman Rider coal is overlain by about 6 ft of dark grey mudstone, often micaceous, in which Lingula mytilloides is common, and foraminifera are present in many places. This phase is overlain by about 4 or 5 ft of mudstone with a dominantly molluscan fauna, Dunbarella sp. being common; one or more so-called 'non-marine intercalations', usually marked by the occurrence of Anthracosia spp.,are characteristically present near the top of this phase. The marine band is overlain by lighter grey mudstone, 3 to 5 ft thick, often silty and/or micaceous in part, containing abundant mussels preserved as shiny films and 'semi-solid' shells, the characteristic fossils being Anthracosia spp. of the ovum Trueman and Weir, and phrygiana (Wright) groups. The cyclothem is composed essentially of argillaceous rocks, except in part of the Cynheidre area where sandstone up to 15 ft thick has been proved near the top.

The Braslyd Fach Vein, which is never more than 18 in thick, has the average section: coal 10 in on a variable parting and coal 3 inches in the eastern part of the area. The silty mudstone and striped beds with thin beds of sandstone between this seam and the Braslyd Vein rarely contain fossils other than plant debris: Cochlichnus kochi, Gyrochorte carbonaria Schleicher and Anthracosia ovum? [one specimen] have been identified.

The Braslyd Vein, worked in Pentremawr, Ponthenry, Carway and Cynheidre collieries, and to a limited extent elsewhere, has a two coal section in these collieries, the top coal, about 7 in thick, resting on a parting up to 3 in thick and a bottom coal about 22 in thick. The top coal probably thickens to about 12 inches in the Carway area. Two distinct faunal and lithological facies are commonly present above the Braslyd Vein; the lowest few inches of the roof consist of dark grey carbonaceous mudstone with pyrite lenses containing a fauna restricted to flattened impressions, locally pyritized, of Naiadites spp. and rare Anthraconaia spp. This facies is overlain by about 1 ft of grey mudstone with 'solid' mussels, typically Anthracosphaerium spp. including affine (Davies and Trueman), exiguum (Davies and Trueman) and turgidum (Brown), accompanied by Anthracosia spp. of the aquilina, ovum, and nitida (Davies and Trueman) groups. Cochlichnus kochi, Gyrochorte carbonaria, Spirorbis sp., Leaia (Hemicycloleaia) minima Pruvost, Lioestheria striata Pruvost (non Muenster), Belinurus sp. and fish fragments have also been collected from the roof of the Braslyd Vein, but are not typical of this horizon. The overlying striped beds and silty mudstone locally form the immediate roof of the coal, but it is difficult to establish whether this is due to original variation in sedimentation or subsequent structural movement of the less competent beds. Plant fragments are relatively common in these silty strata, in which beds of sandstone up to 10 ft thick have been proved. A characteristic feature of the beds between the Braslyd and Braslyd Rider seams is the presence of a seatearth, sometimes containing sphaerosiderite, about 10 ft below the Braslyd Rider seatearth.

A thin but persistent coal seam next above the Braslyd Vein was named the Braslyd Rider Vein during the resurvey. Mussels occur sporadically up to 30 ft above the coal, in the less silty parts of a succession consisting of beds of mudstone and silty mudstone; striped beds and thin beds of sandstone are sometimes present near the top of the cyclothem. The fauna includes Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, Anthraconaia sp. [rare], Anthracosia beaniana King, A. cf. ovum, ?A. regularis, A. sp. intermediate aquilina/ovum, A. spp.  (disjuncta Trueman and Weir group), ?Anthracosphaerium affine, A. exiguum, A. cf. turgidum, Naiadites quadratus, N. sp. intermediate productus/ quadratus, Carbonita sp. [rare], Leaia (Hemicycloleaia) minima, Lioestheria striata Pruvost (non Muenster), Palaeolimnadiopsis pruvosti Raymond [rare], Belinurus sp. and fish fragments [uncommon] including Vetacapsula cooperi Mackie. The presence of Planolites montanus associated with thin beds of ironstone is typical of this cyclothem.

The Gras Isaf Vein, although persistent, is normally very thin, the average thickness varying from 5 to 7 in. Over much of the area the measures between this seam and the overlying Gras Uchaf Vein consist of silty mudstone, striped beds and sandstone (usually in thin beds, but up to 12 ft thick in parts of Cynheidre Colliery). In this cyclothem fossils, other than plant fragments, are uncommon, even when the immediate roof of the seam consists of mudstone, as in Pentremawr Colliery and parts of Cynheidre, where Spirorbis sp., Anthraconaia modiolaris, A. sp. cf. williamsoni [juv.], Anthracosia aquilina, A. spp. of the ovum/nitida group, Anthracosphaerium acne?, A. cf. turgidum and Naiadites cf. productus were found. Cochlichnus kochi, cf. Planolites sp. [small] and Estheria' sp.,although recorded, are rare.

The Gras Uchaf Vein was worked in Coalbrook and Gwendraeth collieries, and to a very limited extent in Ponthenry Colliery (where it was highly disturbed) and elsewhere. It is now being worked in Pentremawr and Cynheidre collieries. The average thickness of the coal varies from about 27 to 32 in, the thicker coal tending to be present west of the Coalbrook Fault. Typically, where undisturbed, the lowest 2 or 3 ft of the roof consist of fossiliferous dark grey mudstone, the bottom few inches being dark grey carbonaceous mudstone, sometimes with a canneloid appearance on joint faces, with mussels, ostracods and fish fragments: these incompetent beds are often, at least in part, sheared to form a 'clod' above the coal. The mussel fauna, typically preserved as glossy films, includes ?Anthraconaia salteri (Leitch), Anthracosia beaniana, A. cf. nitida, A. cf. ovum, A. cf. phrygiana, Anthracosphaerium exiguum, Naiadites cf. quadratus, N. sp. intermediate productus/quadrates. Carbonita cf. humilis (Jones and Kirkby) and fish fragments, including Platysomus parvulus? Williamson, Rhabdoderma sp., Rhadinichthys sp. and Rhizodopsis sp. are characteristically present. Cochlichnus kochi [uncommon], Spirorbis sp. and 'Estheria' sp. [one specimen] have also been found. Striped beds and thin beds of sandstone are present in many places towards the top of the cyclothem.

Although the Stanllyd Vein in Pentremawr and Ponthenry collieries is probably about 36 in thick, attempts to work it were unsuccessful because it is affected by the Pentremawr Lag Fault, which may extend as far west as Plasbach. A fruitless attempt to work the Stanllyd was also made in Carway Colliery, where the seam is probably about 24 in thick, but it occurs in very disturbed strata. The strata between the Stanllyd and Hwch seams are normally silty mudstone or coarser-grained sediments, and fossils, other than plant fragments, which are relatively abundant, are rare; Anthraconaia sp. (cf. lanceolata Hind sp. ), Anthracosia sp.  (phrygiana group) and Naiadites cf. productus [internal mould] have been found.

In the eastern part of the area described the Stanllyd Vein is combined with the Hwch Vein, the combined coal commonly being very thick (Figure 12). When these seams are together the Stanllyd is sometimes identifiable as a relatively thick, clean coal at the base of a complex series of thin coals (mostly inferior), seatearths and 'rashes'. West of the line of split in the Cynheidre area, the Hwch is usually a relatively clean coal about 20 in thick, the top commonly being pyritized, the thin coals disappearing a short distance west of this line of split; this coal is also recognizable at the top of some of the complex sections. There is evidence from some of the boreholes in the Cynheidre area that a split develops within the Hwch Vein. The section of the Hwch Vein is further complicated by the incompetent nature of the complex seam and the adjacent strata, the combination of depositional and structural variations giving rise to an extraordinarily wide range of coal sections. In Carway Colliery the top coal of the Hwch Vein, 20 in thick, overlies thin coals interbedded with 'rashes'; three thin seams between these and Stanllyd Vein are probably part of the '11wch complex'. It is probable that a similar sequence was proved in Gwendraeth Valley Borehole No. 2, in which there was much sheared and faulted ground at the appropriate positions. In Trimsaran Colliery, although now inaccessible, it is likely that the 'Two-Feet Vein' is equivalent to at least the top of the Hwch Vein.

Normally the strata between the Hwch and Ddugaled veins are predominantly argillaceous, striped beds and sandstone being either thin or absent, and silty mudstone less common than in most Middle Coal Measures cyclothems. Mussels, preserved as shiny films, are usually common or abundant up to at least 10 ft above the seam, and much higher in several sections. The typical 'Hwch fauna' includes Cochlichnus kochi, cf. Planolites sp. [small], Spirorbis sp., Anthraconaia spp. including A. sp.  nov. [elongate], Anthracosia cf. beaniana, A. spp. of the nitida group, A. sp. intermediate ovum/phrygiana, A. sp.  nov. [arched dorsum], Naiadites cf. productus, N. sp.  nov. [cf. Dix 1928, figs. 13e and 13g], N. sp. intermediate productus/quadratus, Lioestheria striata Pruvost (non Muenster), and Palaeolimnadiopsis pruvosti. Gyrochorte carbonaria and cf. Planolites montanus, although recorded, are not common, and fish fragments are rare. Belinurus carwayensis Dix and Pringle has also been found in Carway and Cynheidre collieries.

Amman Rider to Ddugaled Vein: details

East of the Coalbrook Fault

The Amman Marine Band has been recorded in the 5 West Cross-measures drift [SN 5188 1117] in Glynhebog Colliery, in the 'roofing hole' [SN 5159 1085] from Glynhebog to Coalbrook Slant, and in Cynheidre Shaft No. 3 [SN 5260 1063]; fossils collected include Glomospira sp., Lingula mytilloides, Bucanopsis? Dunbarella sp.,orthocone nautiloid ? and Hollinella cf. bassleri (Knight). The band is overlain by mudstone with Anthracosia cf. aquilina and A. cf. ovum.

The Braslyd Fach is 24 ft below the Braslyd in the 'roofing hole' where it consists of coal 12 in on a 0.5-in parting and coal 3 in, and it is 10 in thick, 13 ft 6 in below the Braslyd in Shaft No. 3. This seam is usually overlain by silty mudstone and coarser beds, but in the 'roofing hole' 1 in of mudstone is present in the immediate roof.

Apart from some of the very old workings from Gwendraeth Colliery which extend east of the Coalbrook Fault, the Braslyd Vein has not been worked in this area. In the 5 West Cross-measures [SN 5188 1115] mudstone with solid mussels and ironstone nodules overlies 5 in of dark grey ferruginous, carbonaceous mudstone with mussel impressions, in part pyritized, on coal 28 in with a parting 6 in from the top; similarly preserved mussels are present above the Braslyd, 27 in thick, in the 'roofing hole' [SN 5160 1085]. The fossils collected include Lepidostrobus sp., Spirorbis sp.,Anthraconaia curtata, A. robertsoni (Brown), Anthracosphaerium affine, A. exiguum, A. cf. turgidum and Naiadites cf. productus. A similar fauna, but including Leaia sp.,was found above the Braslyd, 28 in thick, in Shaft No. 3; in this shaft and in the 'roofing hole' a bed of sandstone about 10 ft thick overlies the fossiliferous mudstone.

The Braslyd Rider is about 25 ft above the Braslyd Vein in the 'roofing hole' [SN 5161 1085], where the section is as follows:

This coal seam, 12 in thick, lies about 50 ft above the Braslyd in Shaft No. 3, and has the section: sandstone, on dark grey carbonaceous mudstone 6 in, coal 5 in, 'rashing' 3.5 in, coal 3.5 inches in the 5 West Cross-measures.

The Graf Isaf Vein is also thin in this area, consisting of coal 7 in on coal and 'rashing' 12 in, overlain by mudstone with an indeterminate shell fragment, in the 'roofing hole'. The section is similar, but thinner, in Shaft No. 3.

A relatively small area of the Gras Uchaf Vein was worked in Coalbrook Colliery, where the thickness of the seam was recorded as 27 to 33 in; it is 33 in thick in Shaft No. 3. In the 'roofing hole' this seam is probably combined with the Stanllyd and Hwch seams in a mass of disturbed coal, soft shale and 'rashing' 40 ft thick, on 6 in of 'rashing' and coal 64 in, which was lined to prevent further falls and is inaccessible. Carbonita sp. was collected from the roof of the Gras Uchaf in Shaft No. 3.

An unsuccessful attempt was made in Coalbrook Colliery to work the Stanllyd Vein, where the section proved is coal 24 in, 'rashing' with coal streaks 12 in on coal 36 to 144 in and 'rashing' 1 ft 6 in to 9 ft. Anthracosia sp., Naiadites sp. and 'Estheria' sp. [large] were found above the Hwch Vein in Shaft No. 3, where the Stanllyd is combined with the Hwch.

The Hwch Vein has not been worked in this area.

Pontyberem to Pont-Henry

A thick development of the Amman Marine Band, first described in this area by Jones (1934, p. 416), was examined in the 'Brussels' Cross-measures Drift, which has been driven to the Soap Vein [SN 5001 0954] from the Gwendraeth Vein [SN 4976 0973] in No. 1 Slant [mouth at [SN 4939 1043] of Pentremawr Colliery. The faunal sequence is as follows :

Distance above coal

Three thin beds of ironstone occur 1 ft 2 in, 8 ft 10 in and 9 ft 8 in above the Amman Rider seam, which is 11 in thick.

The Braslyd Fach, lying about 20 ft below the Braslyd, is also a thin seam in Pentremawr and Ponthenry collieries. In the 'Brussels' Cross-measure, 9 in of coal are overlain by silty mudstone, striped in part, with rare indeterminate mussel fragments.

The Braslyd Vein was worked in the old Gwendraeth (or Pontyberem Old) Colliery, in which two cross-measures drifts were driven S.S.E. from Pumpquart Pit [SN 5070 1107], one from the Pumpquart Vein near pit bottom, and the other from an inset in the Gwendraeth Vein. The higher horizon of this mine was connected with another shaft, South Pit [SN 5086 1082], in the Ddugaled Vein. A summary of the shaft sections is given in Appendix I.

The Braslyd Vein has been worked extensively in Pentremawr Colliery, where the average thickness shown on the Abandonment Plan is 27 in of coal on 3 in of 'rashes'. In the only place wherethe roof was accessible during the resurvey, it consisted of fine-grained quartzitic sandstone, striped beds and very silty mudstone, unfossiliferous except for plant debris, but Dix recorded the typical Braslyd fauna from Pentremawr and Ponthenry collieries (1928, pp. 463–5); she also recorded "Leaia tricarinata f. minima Pruvost" and Belinurus sp. (see also Dix and Pringle 1929, p. 100) in Pentremawr. In Ponthenry Colliery the section: top coal 10 in on bottom coal 19 in was worked. Nearer Pontyberem the Braslyd Vein has been worked from Old Pentremawr Colliery [SN 5003 1085] and another slant 140 yd to the E.N.E. [SN 5015 1089]; these old workings were drained by two adit levels [SN 4992 1095], [SN 5006 1102].

Leaia (Hemicycloleaia) minima ? and Naiadites cf. quadratus were found in mudstone about 12 ft above the Braslyd Rider, which is not more than about 10 in thick, in the 'Brussels' drift in Pentremawr Colliery.

The Gras Isaf Vein, generally less than 6 in thick, is less than 30 ft below the Gras Uchaf Vein; the abbreviated details of the intervening strata, as proved in Pentremawr

Borehole No. 3 [SN 5083 1035] are given in Appendix I. A limited fauna has been collected from the roof of this seam in Pentremawr Colliery, including Spirorbis sp.,cf. Anthraconaia pukhella Broadhurst, A. sp. cf. williamsoni [juv.] Anthracosia cf. ovum [juv.] and Naiadites cf. productus.

The Gras Uchaf Vein has been worked on both sides of the Coalbrook Fault from both Gwendraeth and Pentremawr collieries. In Pentremawr, coal averaging about 32 in is overlain by 'clod' up to about 18 in thick; the seam was proved in Pentremawr boreholes Nos. 2 [SN 5077 1022] and 3. The lowest 2 to 3 ft of the roof consists of dark grey mudstone with Spirorbis sp., Anthracosia sp.  (ovum group), Naiadites quadratus?, accompanied in the bottom few inches in darker grey, carbonaceous mudstone, by Carbonita humilis. This immediate roof is commonly sheared, the resultant 'rashing' constituting the 'clod'. A very small area of Gras Uchaf was worked from an old slant, also known as 'Pentremawr Colliery' [SN 5008 1079], abandoned in 1892; the main drift followed the seam, known as the 'Ddugaled', for about 425 yd down dip.

The Gras Uchaf was worked to a very limited extent in Ponthenry Colliery, where it was found to be highly disturbed, presumably because of the close proximity above it of the Pentremawr Lag Fault. The average thickness is recorded as 48 in. Dix (1928, p. 466) recorded mussels from the roof similar to those above this seam in the Cross Hands area.

The Stanllyd and Hwch veins were closely associated in Gwendraeth Colliery, and were not worked, but in old Pentremawr Colliery, abandoned in 1875, probably the combined Stanllyd and Hwch veins were worked as the 'Big Vein', from a slant [SN 5022 1082] which was driven for 300 yd down dip; in the present Pentremawr Colliery an attempt to work a seam [SN 499 098] thought to be the Stanllyd, said to be 42 in thick, was unsuccessful. The only mussel collected from the Stanllyd Vein, the roof of which is usually silty mudstone with plant fragments, is Naiadites cf. productus [internal mould]. The lower of two thick seams proved in the Gras Cross-measure Drift in Pentremawr Colliery is probably the Hwch; according to one account (Strahan and others 1909, p. 93) this seam [SN 4953 1019] consists of coal 18 in on 'rashes' 15 ft and coal 12 in. This seam is about 28 ft above the Stanllyd which was said to have the section: coal 41 in, parting 9 in on coal 30 in (or a single coal 32 in thick, according to another account). The Stanllyd is 17 ft 6 in above the Gras Uchaf. The Stanllyd and Hwch seams were proved very close together in Pentremawr Boreholes No. 2 and 3, the section between the Stanllvd and Ddugaled seams in No. 2 being as follows:

Pentremawr Borehole No. 4 [SN 5075 1037] was stopped a short distance below the Hwch Vein, which had a comparable section to that in No. 2. Other fossils collected include Cochlichnus kochi, cf. Planolites sp. [small], Spirorbis sp.,Anthracosia sp. (?ovum/nitida group), N. sp.  (productus group) [slight posterior wing], and Rhabdoderma?.

A very small area [SN 483 092] of coal known as the 'Big Vein', said to be 10 ft thick, was worked in Ponthenry Colliery, but the measures were highly disturbed. This seam is near the conjectured position of the Pentremawr Lag Fault and may be either the Hwch (possibly combined with the Stanllyd) below the fault, or the Big Vein above it. This is probably the seam "varying up to 18 ft of good coal at Ponthenry" which Dix (1928, p. 467) stated was worked as the Stanllyd; she recorded plants in shale "from near" the seam.

Pont-Henry to the Trimsaran Disturbance

Other than in poor exposures in a stream section [SN 4585 0701] near Fforest Fach Farm, none of the strata between the Amman Marine Band and the Ddugaled Vein were accessible during the resurvey.

The Amman Marine Band has not been recorded. A seam with the section: coal 6 in on 'rashing' 1 in, coal 10 in, 'rashing' 2 in and coal 2 in, about 20 ft below the Braslyd Vein in the 'Wembley' Cross-measures Drift in Carway Colliery, is probably the Braslyd Fach.

The Braslyd Vein was worked to a very limited extent in Caepontbren Colliery, abandoned in 1897; the average dip is said to be S.E. at 12°, and the section to consist of: coal 16 in on pyrite 3 to 5 in and coal 12 in. According to one account the workings were abandoned because of difficulty in controlling the water. There is no record of this seam being worked in Plasbach Colliery, although it is recorded as 36 in thick in the Downcast Pit [SN 4690 0788] : a seam called the 'Braslyd' in the cross-measures drift at the southern limit of this colliery may be the Gwendraeth Vein (p. 53), although there are some similarities with the Braslyd of Carway Colliery.

The seam worked at the outcrop N.W. of Fforest-uchaf Farm is probably the Braslyd; 48 in of coal with 3 in of pyrite in the middle was recorded in the mouth of an old slant [SN 4630 0732]. Three seams 10, 24 and 10 in thick were proved in a cross-measures drift above the Gwendraeth Vein in Lambert's Colliery [SN 4616 0736], one of which may be the Braslyd.

In the 'Wembley' Drift in Carway Colliery the Braslyd Vein [SN 4626 0639], about 135 ft below the Stanllyd, was worked; a typical section is: coal 12 in on 'rashing' 3 in, coal 3 in, pyrite 0 to 1 in, coal 22 in. This seam is presumably that referred to as 'Seam I' by Dix (1928, p. 481); although Dix suggested that mussels collected from the roof of the underlying 'Seam J' were typical of the Braslyd Vein, the fauna described on 'Seam I' is not incompatible with the correlation of this seam with the Braslyd.

The Braslyd Rider is probably 'Seam H' of Dix, 7 in thick and about 45 ft above the Braslyd. The presence of a seatearth about 10 ft below the Braslyd Rider, recorded in the 'Wembley' drift, has been noted elsewhere in the district (Figure 12).

The Gras Isaf remains thin in this area, thicknesses of 5.5 in and 7 in being recorded in Carway Colliery. It is said to have been found below the made ground at the top of the Downcast Pit of Plasbach Colliery.

The seam worked in Woodbridge Colliery ?[SN 4736 0839], 400 yd N.E. of Pontyates Station, is said to be the Gras Uchaf Vein, but no details are preserved. The old crop workings extending from 120 yd S.E. of Pontyates for about 300 yd to the S.W. are probably on the same seam. Plasbach Slant [SN 4691 0786] is in line with these crop workings; conflicting accounts of the workings exist on a plan and three sections and in the Memoir (Strahan and others 1909, pp. 108–10). The thickness of the seam on which the slant (Figure 9)a is driven, said to be the Gras Uchaf Vein, is shown on the abandonment plan as 25 and 38 in in different parts of the very small area worked. The distance between this seam and the underlying workings in the Pumpquart Vein is about 470 ft which is not inconsistent with its correlation with the Gras Uchaf, but as E.–W. disturbances, presumably thrust faults or asymmetrical folds, were proved in the workings this basis for correlation is highly unreliable. A cross-measures drift was driven from the bottom of the slant, beyond an E.–W. disturbance, and an attempt was made to work a thick seam shown on the plan as the 'Big or Stanllyd' seam; this seam, which is believed to be the Big Vein, and others proved beyond folded measures to the S.E. of the very limited workings in it, are described below (pp. 90–92). A seam said to be the Stanllyd was proved 75 ft above the Gras Uchaf of the slant, in the Gras or Upper Pit (Appendix I).

In the 'Wembley' Drift [SN 4627 0662], Carway Colliery, the Gras Uchaf Vein consists of coal 28 in on 'rashes' 2 in, coal 4 in, 'rashes' 2 in and coal 5 in; it was not worked. The Gras Uchaf lies about 20 ft below the Stanllyd Vein; Dix (1928, p. 480) found "Naiadites? cf. quadrata, N. producta (Brown) and some doubtful fish fragments" in the roof.

The seam worked to a very limited extent in the Gras Slant (Lambert's, Fforestuchaf, or New Carway Colliery: (Figure 9)b) [SN 4610 0715], at one time thought to be the Gras Uchaf (Strahan and others 1909, p. 112), is now believed to be the Ddugaled Vein.

An unsuccessful attempt was made to work the Stanllyd Vein in Carway Colliery, where, although the typical thickness was about 24 in, in places the seam is only 18 in thick. Dix (1928, p. 480) recorded " ?Carbonicola sp., Anthracomya sp., Naiadites sp. " from "blue, well-laminated shale" above this seam, and plant remains in the overlying "dull grey shale". The roof of the Stanllyd Vein in Carway Colliery is the type locality for Belinurus pustulosus Dix and Pringle.

Three thin coal seams about 20 ft above the Stanllyd in Carway Colliery (Figure 12) and a complex seam, consisting of coal 20 in on coal and 'rashing' 60 in, coal 1.5 in, 'rashing' 1 ft 6 in and coal 5 in, about 40 ft above the Stanllyd, may be regarded as equivalent to the Hwch Vein. Dix (1928, p. 479) recorded "numerous crushed shells", including "Carbonicola cf. aquilina (J. de C. Sow.), Naiadites producta (Brown), N. cf. triangularis (J. de C. Sow.) and N. sp. " from the roof of the highest of these coals, which is the type locality for Belinurus carwayensis. Although Dix stated that "thick sandstones and sandy shales occur", the detailed log of the strata, made when the 'Wembley' Drift was driven in about 1928, records only argillaceous rocks between the Hwch and Ddugaled seams.

South of the Trimsaran Disturbance

As explained above (p. 64), nothing is known with certainty about the structure and succession in the lowest part of the Middle Coal Measures in this area.

In the Trimsaran Borehole the Amman Marine Band is about 9 ft 6 in thick and is overlain by mudstone with mussels, but it proved impossible to establish satisfactorily the identity of the seams between the Amman and Graigog marine bands. It is possible that the Braslyd seams was represented at 681 ft (Appendix I, p. 200), that is about 65 ft above the Amman Marine Band: at this depth 11 in of core was lost, the underlying rock was silty seatearth with pyritous ironstone nodules and the overlying 11 in of mudstone, below quartzitic sandstone, were highly sheared. The Braslyd Fach Vein may be represented by 11 in of inferior coal at 707 ft, from the roof of which poorly preserved mussels were collected.

In Waun-hir Colliery (or Waun-hir Slant, Trimsaran Colliery) [SN 4578 0500], two small areas were worked in a seam known locally as the Two-Feet (or 'Double Two-Feet'), with the section, according to the Abandonment Plan: coal 18 in on pyrite 1 to 3 in and coal 3 to 6 in; a more complex section is recorded elsewhere. Dix (1928, pp. 489–90, fig. 12) recorded "Carbonicola cf. nitida Davies and Trueman, C. cf. aquilina (J. de C. Sow.), C. transversa (Brown), C. similis (Brown), C. sp., Anthracomya cf. rubida Davies and Trueman, Naiadites cf. modiolaris (J. de C. Sow.) N. cf. triangularis (J. de C. Sow.), N. sp.  and Rhizodopsis sp. " from "very fine-grained, well-laminated shale" in the roof. This description of the roof and its fauna, and its position about 30 ft below the Yard Vein, believed to be at, or near, the horizon of the Ddugaled, is consistent with the correlation of this seam with the Hwch Vein, possibly combined with the Stanllyd. A cross-measures drift, the North Drift, was driven against the dip from the Yard Vein and proved the underlying 400 ft of strata. There is no record of any faults in this drift, although it is very unlikely that none was present in view of the structures proved in the workings in this colliery: very tentative correlation of some of the seams is included in the log (Appendix I). Local names, including Gregog Fach, Waunhir, Vangog, Cwm Cathan, Culla and Garwed veins were applied to some of the seams proved.

An old borehole, said to be 200 yd south-west of Caedean Slant, Trimsaran Colliery [SN 4535 0494], is said to have proved the 'Two-Feet', 30 in thick, 46 ft below the 'Yard' Vein, and coal 27 in at 98 ft, coal 29 in at 176 ft, coal 3 in at 197 ft, coal 6 in at 331 ft and coal 9 in at 339 ft below the 'Yard'. It is possible that the first unnamed seam proved is the Gras Uchaf Vein, and the second the Braslyd Vein. In the same area the 'Two-Feet' Vein and underlying beds were proved in the New Inn Level, the mouth [SN 4454 0503] of which is probably in the Lower Coal Measures: a summary of Logan's MS. account is reproduced in Appendix I. Again no faults were recorded and it is unlikely that none were encountered: Logan (Vertical Section No. 1) records that the 'strata are very much disturbed by an anticlinal line' at the mouth of the Pen-y-groes (i.e New Inn) Level, and the fact that parts of the level are 'arched' suggests rock weakened by faulting. Although the correlation of the 'Two-Feet' and the overlying seams in the Level is probably sound, as it is likely that the workings in these seams in Trimsaran Colliery were connected to the level, it is impossible to correlate the underlying strata.

The seams proved above the ? Stinking Rider in Cil-rhedyn Level (p. 57) and in Llandyry Slant are probably in the lower part of the Middle Coal Measures. 'No. 1' seam: coal 12 in on dirt 18 in, coal 9 in and 'rubbish' 4 in, proved in both the level and slant ((Figure 9)d) and identified as the 'Lower Graces' by Logan, was correlated later with the Braslyd Vein. Although Dix (1928, p. 486) found no typical Braslyd Vein mussels on the tip, she regarded it as "fairly certain" that this seam was worked. 'No. 2' seam, the 'Upper Graces' of Logan, was correlated with the Gras Isaf by Strahan and others (1909, p. 124) but its section, consisting of coal 15 in on 'rubbish' 3 in and coal 24 in, is much thicker than is known for this seam anywhere in this district. Between Nos. 2 and 3 seams in the level "what appears to have been an overfold leaning to the north, and having the effect of a southerly upthrow" has been noted: the line of this structure is shown on the map as a fault, further evidence for its existence being the disturbed strata exposed in the banks of the River Morlais and in the small stream about 200 yd east of Llandyry House [SN 4340 0496]. The 'No. 3' seam in the level, "correlated with some doubt with the Gras Uchaf" (Strahan and others 1909, p. 129), is shown as the "Ddugaled ?" on the Abandonment Plan, on which its thickness is recorded as 48 in. 'No. 5' seam in the slant, which is also 48 in thick, is said to be the same seam: the average dip is recorded as S.38°E. at 8° in the very limited area of this seam worked. No details are known about 'No. 4' seam, but 'No. 6' consists of 51 in of coal. A recent National Coal Board trial pit [SN 4384 0502], almost above the workings in 'No. 3' seam in Llandyry Slant, proved coal 28.5 in on 'rashing' 3.5 in and inferior coal 22 in at a depth of 11 ft: this may be 'No. 6' seam.

There are crop workings on two seams 100 and 180 yd S.E. of Llandyry House; the former was mentioned by Logan as "supposed to be one of the Graces", and the latter was probably also worked in Gent's Pit [SN 4353 0476]. The 'upper work' at Cilrhedyn Colliery was said to be largely on the "Stanllid Vein" (Logan) which was worked from a pit identified by Strahan and others (1909, p. 125) as Burgage Pit [SN 4415 0522] : the seam was present at a depth of 144 ft, the total depth being 216 ft.

The complexity of the structure in this area, indicated by the diverse directions and amounts of dip shown on the map, has been confirmed by recent prospecting by the N.C.B. Opencast Executive. Under these structural conditions, and in the absence of accessible, fresh sections, no further comment can usefully be made on the correlation.

Cynheidre Area. The Amman Marine Band has been proved many times in the drivages and boreholes. The details in Gwendraeth Valley Borehole No. 2 are as follows:

Other fossils from the Amman Marine Band and associated measures in this area include: Ammodiscus sp., Glomospira sp.,sponge spicules, Cochlichnus kochi, Orbiculoidea cf. nitida, Anthracosia ovum, A. sp. intermediate between ovum and phrygiana, A. cf. regularis, Edmondia cf. goldfussi de Koninck, cf. Geisina sp., Hollinella cf. bassleri, Paraparchites sp., Belinurus sp. [associated with Anthracosia sp. in the intercalations at top of marine band]. The Amman Rider seam is typically about 6 in thick, and only exceptionally is thicker than 9 in. Usually the strata between the Amman Rider and the Braslyd Fach Vein are predominantly argillaceous, but in the drivage (490) from Cynheidre Slant to Shaft 3, 15 ft of sandstone were proved above the marine beds and 4 ft 6 in of sandstone were proved in this position in Shaft 3, but in the next cross-measure drift to the west, No. 5, only 2 ft 2 in of sandstone are present.

The Braslyd Fach Vein has been proved with thicknesses from less than 12 in up to 18 in: in the western part of the area, on Cross-measures No. 7, this seam has the section: coal 7 to 11 in, on 'rashes' or carbonaceous shale 1 to 7/ in, and coal 4 in. The strata between this seam and the Braslyd Vein usually consist of silty mudstone and striped beds with thin beds of sandstone. Fossils, other than plant debris, are rare; apart from Cochlichnus kochi and Gyrochorte carbonaria, present in several of the boreholes, Anthracosia ovum? was found in Cross-measures No. 5.

Typically the Braslyd Vein has a two-coal section, the average thickness of the top coal being about 7 in and that of the bottom coal about 24 in, the parting being about 3 in thick, but in the workings off Cross-measures No. 5 the total thickness is only about 28 in; the sulphur content, less than 1 per cent, is low. The abbreviated log of the Braslyd cyclothem in Borehole No. 4/2 [SN 5119 0761] is as follows:

This section is representative of these strata in this area, except that the seatearth immediately below the Braslyd Rider is typically separated by mudstone with rootlets from another seatearth, sometimes containing sphaerosiderite, about 10 ft below. Individual beds of sandstone are rarely thicker than 4 ft, although nearly 10 ft of sandstone is present immediately above the Braslyd Vein position in the most southwesterly borehole, Gwendraeth Valley No. 2 [SN 4900 0601].

In the Cynheidre area the dark grey, sometimes fissile, mudstone in the immediate roof of the Braslyd Vein has yielded flattened impressions of Anthraconaia sp.  (modiolaris group) and Naiadites sp.,as well as Elonichthys sp.,Rhizodopsis sp. and Strepsodus sp.;the overlying mudstone usually contains 'solid' mussels, including Anthracosia cf. aquilina, cf. A. nitida, A. ovum, Anthracosphaerium affine, A. exiguum and A. turgidum. The two distinct facies are not always present, and sometimes the arenaceous beds extend down to the coal. Cochlichnus kochi, Spirorbis sp., Leaia (Hemicycloleaia) minima, Lioestheria striata Pruvost (non Muenster) and Belinurus sp. have also been found, less commonly; there is one record of Carbonita?. Thin beds of ironstone are usually present; plant fragments found towards the top of the cyclothem include Calamites sp., Mariopteris nervosa and Neuropteris gigantea. Spirorbis sp. [on plant fragment] and Anthraconaia cf. subcentralis (Salter) were collected on a seam in disturbed ground, believed to be the Braslyd, in Cross-measure No. 7 (660) [SN 4918 0786].

The Main Lateral (560) was driven at or very near to the horizon of the Braslyd Rider for over 800 yd, and the characteristic fauna above this seam has been collected from most of the boreholes, as well as elsewhere underground. The typical 'pattern' of the strata overlying this seam is seen in the following abbreviated extract from the log of Gwendraeth Valley Borehole No. 2:

The Braslyd Rider is normally about 11 in thick. In addition to the fossils listed above, the following have been found in the Cynheidre area: Cochlichnus kochi, Anthraconaia sp.  (williamsoni group), Anthracosia cf. ovum [juv.], A. sp. intermediate aquilina/ovum, A. sp.  (?disjuncta/lateralis Brown sp. ), ?Anthracosphaerium affine, A. exiguum, A. cf. turgidum, Naiadites quadratus, N. sp.  (productus group), Lioestheria striata Pruvost (non Muenster) [one with circular markings, ?egg cases], Belinurus sp.,fish remains including Vetacapsula cooperi; Planolites montanus is characteristically present, but ostracods are rarely found.

The Gras Isaf Vein is consistently very thin, the average thickness being about 5 in. The strata between this seam and the Gras Uchaf Vein about 10 ft to 30 ft above, consist mainly of silty mudstone and striped beds with thin beds of sandstone normally up to about 4 ft thick, although the immediate roof of the Gras Isaf in the Main Lateral (560) consists of more than 12 ft of sandstone with thin partings of striped beds. Commonly the only fossils present in this cyclothem are plant fragments, including Calamites suckowi, Diplotmema sp., Pinnularia capillacea and megaspores, but in some places, less rarely in the western part of the area, where the roof is mudstone, other fossils have been found, as in the following section in the Main Lateral (660):

Anthracosphaerium affine ? and A. cf. turgidum were collected from the roof of this seam in Cross-measures No. 3 (660); a poorly preserved fossil, accompanied by cf. Planolites sp. [small] from Shaft 1 may be 'Estheria'. Cochlichnus kochi was found in Borehole 6/1.

Thicknesses varying from 24 to nearly 84 in have been proved for the Gras Uchaf Vein in the drivages and boreholes, but the average section is probably about 27 to 30 in in the eastern part of the area, and possibly slightly thicker in the western part. In the Main Lateral (660) the section is as follows:

The whole cyclothem is 26 ft thick in the nearby Shaft 2. In addition, Cochlichnus kochi [uncommon], Anthracosia beaniana, A. cf. nitida, A. cf. ovum, Naiadites sp. intermediate productus/quadratus, N. sp.  nov. [see Dix 1928, fig. 13e], Carbonita cf. humilis, 'Estheria' sp. [Shaft 1 only], Elonichthys sp. and Platysomus parvulus have been collected. Asterophyllites equisetiformis (Sternberg), Calamites sp., Lepidophloios laricinus (Sternberg), Neuropteris gigantea and N. heterophylla were present towards the top of the cyclothem in Gwendraeth Valley Borehole No. 2.

In none of the drivages in Cynheidre Colliery has the Stanllyd Vein been proved as a separate, undisturbed seam. In Cross-measures No. 5 (490) the lowest 50 in, largely clean coal, of a very thick seam, is probably the Stanllyd, and this probably applies to the analogous thick seam on the 560 horizon; the bulk of the thick coal consists of the Hwch and Big veins. On Cross-measures No. 3 (660) the lower part of a disturbed seam, the roof of which contains the 'Hwch fauna', may be the Stanllyd, although this seam is apparently unrepresented in the nearby Underground Borehole 1. A seam containing pyrite concretions in disturbed ground above the Gras Uchaf in Cross-measures No. 7 may be the Stanllyd Spirorbis sp., Anthraconaia sp. and ?Naiadites quadratus [large] were collected close to this seam. In boreholes Nos. 4/1, 4/3 and 6/3 the Stanllyd is probably immediately below the Hwch Vein, while in boreholes Nos. 3/1, 3/2, 4/2, and in shafts Nos. 1 and 2, the distance between these seams varies from 10 to 40 ft. A seam 94 in thick in Borehole No. 5/1 is probably the Stanllyd; in the other boreholes this seam varies from less than 24 to 58 in. In boreholes Nos. 4/1, 3/2 and Gwendraeth Valley No. 2, and in Shaft No. 2, a two-coal section was proved, the top coal, 3 to 7 in thick, being separated from 10 to 34 in of coal by a parting which is thickest, 2 ft 4 in, in the most south-westerly section, Gwendraeth Valley No. 2.

No single section can be regarded as representative of the strata between the Stanllyd and Hwch veins, which normally consist of silty mudstone and striped beds with thin beds of sandstone. The only mussels identified are Anthraconaia sp. (cf. lanceolata) and Anthracosia sp.  (phrygiana group), but plant fragments are often common; Alethopteris lonchitica, Asterophyllites equisetiformis, Calamites suckowi, Carpolithus sp., Cordaites principally (Germar), Cyclopteris sp., Cyperites bicarinatus Lindley and Hutton, Neuropteris heterophylla, Sigillaria tessellata, Sphenophyllum cuneifolium, Stigmaria sp. and megaspores have been identified from disturbed ground associated with the Stanllyd Vein in Gwendraeth Valley Borehole No. 2.

The Hwch Vein has been proved to be extraordinarily variable in the Cynheidre area. In Shafts Nos. 1 and 2 and the Main Laterals (560 and 660) this seam consists of a single coal about 20 in thick, sometimes with about 2 in of pyritized coal at the top, overlain by mudstone with a characteristic fauna which includes Spirorbis sp.,cf. Planolites sp. [small], cf. Anthraconaia pulchella, A. sp. cf. salteri, A. sp.  nov. [elongate], Anthracosia cf. beaniana, A. cf. nitida, A. cf. ovum [including short form], cf. A. regularis, A. sp.  (nitida/ovum group) [small], A. sp. intermediate ovum/phrygiana, A. sp.  nov. [arched dorsum], Naiadites cf. productus, N. sp. intermediate productus/ quadratus, N. sp.  nov. [cf. Dix 1928, figs. 13e-g], 'Estheria' (Lioestheria ?) sp.,Palaeolimnadiopsis pruvosti, Belinurus carwayensis, Euproops sp.,and Palaeoxyris sp. The belief that the very thick sheared seam proved on the 490 horizon (and, by inference, probably the thick seam on the 560 horizon) in Cross-measures No. 5 includes up to about 9 ft of Hwch and Stanllyd coal, as well as about 9 ft of Big Vein, was confirmed by the presence near the middle of the coal of a wedge of mudstone which yielded Anthracosia sp.  nov. [arched dorsum] and Naiadites productus, the preservation of the fossils being that typical of the 'Hwch fauna'. The juxtaposition of the Hwch and Big veins is explained by the presence between them of the Pentremawr Lag Fault.

In the deep boreholes it is particularly difficult to distinguish between depositional variation in the Hwch Vein and subsequent structural complication. The variability of the section proved is illustrated by the presence of only 7 in of coal in Borehole No. 5/2, while in Borehole No. 6/3, less than a mile away, 34 ft of coal (probably including the Stanllyd at the base) were proved in 50 ft of strata, which includes a continuous section of 10 ft of roof strata. In general it appears that in the area where the Stanllyd and Hwch are combined the Hwch consists of a thick complex sequence of thin beds of coal, carbonaceous shale and 'rashes' with a relatively thick (e.g. 16 inches in Borehole No. 4/1) clean coal at the top (cf. Pentremawr boreholes Nos. 2 and 3 and Cynheidre Shaft No. 3); to the west of the split between the Stanllyd and Hwch the top coal persists (as proved in Shafts Nos. 1 and 2 and the nearby drivages) and the underlying complex section is absent. The possibility of a split occurring within the Hwch Vein is suggested by the following details of the section proved in Borehole No. 4/3:

Comparison of the strata at 2701 ft 10 in, 2716 ft 4 in and 2720 ft 4 in suggests that at least the coals at 2703 ft 6 in and 2716 ft are not repetitions of the same seam. The strata proved between the Hwch and Ddugaled in this borehole are too arenaceous to be typical of this cyclothem in this area, but the presence of mussels and 'Estheria' in strata up to more than 10 ft above the seam is characteristic. The Ddugaled is usually underlain by seatearth; in this borehole it has evidently been faulted out, but elsewhere (e.g. in Cynheidre Shafts) as much as 35 ft of sheared seatearth have been found. Fossils collected from this cyclothem in this area, other than those listed above, include Anthracosia cf. phrygiana [short form], A. sp.  (?ovum/lateralis group), Naiadites productus, Lioestheria striata Pruvost (non Muenster), and 'Estheria' sp. [fragment showing circular markings, ?egg cases]. Scattered Cochlichnus kochi are usually present, and, less commonly, Gyrochorte carbonaria and cf. Planolites montanus have been recorded.

Ddugaled Vein to Graigog Vein: general

Whereas the correlation of the seams between the Rhasfach and Ddugaled veins was established satisfactorily, with minor exceptions, before the resurvey was commenced, the correlation of the seams in the Middle Coal Measures above the Hwch Vein had for long been in doubt. The principal controversy was centred on the correlation of the 'Big' Vein, the naming of the seams above and below having been considerably influenced by this correlation. The name Big Vein is here applied to the horizon of the Big Vein of Great Mountain, Emlyn and Cross Hands collieries, to the east of the district described: further east this seam, which typically has a three-coal section, splits : the upper leaf is known as the Rock Vein, and the lower as the Little Vein, in Wernos and Pantyffynnon collieries, near Ammanford. Dix (1928, pp. 476, 503) correlated the 'Big' veins of Carway and Trimsaran with the Big Vein, rather than with the Stanllyd Vein, as had been suggested tentatively for the Trimsaran seam by Strahan and others (1909, pp. 111, 134), but Dix believed the Soap Vein in the 'Mafeking' Cross-measures Drift in Pentremawr Colliery to be the Big Vein (1928, p. 470).

Locally the view was widely held that the Big Vein was not present as a thick seam west of the Coalbrook Fault, and that where a thick seam is present it is at the Stanllyd/Hwch horizon. This opinion was based, in part, on the fact that in Coalbrook Colliery workings in the Big Vein did not extend far to the west of the main slant, from which it might be concluded that the seam had thinned. There is some evidence that in at least part of the Coalbrook area the distance between the Green and Big veins decreases, and as the Green Vein thins to the west, the top and bottom coals of the Soap ('Green') Vein of Ponthenry and Pentremawr collieries were correlated with the Big and Green veins respectively. Further, a thick seam lies only some 30 to 50 ft above the Gras Uchaf Vein in most of the workings in Pentremawr Colliery, which led to its being mined as the 'Big (Hwch)' Vein. The evidence obtained during the resurvey, particularly from several new cross-measures drifts, Cynheidre Shafts Nos. 1 and 2 and the cores of boreholes drilled in the Pentremawr and Cynheidre areas, has confirmed Dix's correlation of the Carway and Trimsaran Big Veins, and established that the 'Big (Hwch)' is also the Big Vein. The anomalous distance between the Gras Uchaf and Big veins in Pentremawr Colliery is explained by the existence of the Pentremawr Lag Fault between them; approach towards this structure probably explains the difficulties encountered in the Big Vein on the west side of Coal-brook Slant. The evidence on which these conclusions are based is given below (pp. 86–87).

The Ddugaled Vein has been worked in Gwendraeth, Carway, Trimsaran (as the 'Yard') and Cynheidre collieries, but in none were the workings very extensive: the seam is about 30 to 36 in thick, and is overlain by 1 to 3 in of cannel, except in Trimsaran Colliery, where none has been recorded. A thin bed of dark grey mudstone, carbonaceous in part, normally forms the immediate roof of the coal, and has yielded Anthracosia cf. ovum, A. sp. ( ?intermediate nitida/ovum), A. sp. cf. phrygiana [short form, tilt not marked], Naiadites sp.  nov. [see Dix 1928, fig. 13e], N. sp.  (productus group) [no posterior wing] preserved as shiny films, the N. spp.  being characteristic of this horizon. 'Estheria' sp. [not common] and Rhabdoderma sp. have also been found. This mussel phase is overlain by up to about 30 ft of silty beds which in places rest directly on the coal; sandstone is present in the Trimsaran area, and probably in the southeasterly part of the Cynheidre area. Overlying the silty beds a second mussel phase, with 'semi-solid' shells in dark grey mudstone, contains Anthraconaia? [elongate], Anthracosia cf. ovum, A. cf. phrygiana, A. spp.  intermediate aquilina/ ovum, ovum/beaniana and phrygiana/nitida, Naiadites sp.  nov. [see Dix 1928, fig. 13e], N. cf. angustus Trueman and Weir, Lioestheria sp. and fish remains. Cochlichnus kochi is usually present in this cyclothem, Gyrochorte carbonaria is rare, and there is one record each of Planolites montanus and an ostracod.

A thin coal seam, rarely as much as 6 in thick, lying about 35 ft above the Ddugaled Vein in Carway Colliery, was named the Ddugaled Rider during the resurvey. Where the coal is absent, the base of the cyclothem can be recognized by the presence of the seatearth at the top of the Ddugaled cyclothem; on the south-eastern side of the Cynheidre area, where this rootlet phase fails, the cyclothem can still be recognized by the persistent mussel fauna at its base. Anthraconaia  aff. robertsoni, A. sp.  (pulchella group), A. sp.  (wardi/williamsoni group), Anthracosia cf. aquilina, A. cf. beaniana, A. cf. caledonica Trueman and Weir, A. cf. ovum, A.  aff. phrygiana, Naiadites cf. productus, and N. sp.  (alatus Trueman and Weir group) have been found, ranging up to 10 ft from the base of the cyclothem; Anthracosia, commonly preserved as 'solid' shells, dominates the assemblage. Other fossils collected include Lioestheria striata Pruvost (non Muenster), Palaeolimnadiopsis pruvosti, Belinurus cf. carwayensis and fish remains. Cochlichnus kochi is not uncommon, Planolites montanus has been recorded, but cf. P. sp. [small] is rare and Gyrochorte carbonaria has not been recorded. The sediments become coarser upwards and quartzitic sandstone is commonly present near the top of the cyclothem.

The Green (Wyrdd) Vein has been mined only in Coalbrook and Gwendraeth collieries, where it is about 36 in thick. In most of the area described the seam is probably less than 24 in thick and less than 20 ft below the Big Vein, where it has often suffered incompetent deformation. In the Carway and Trimsaran districts the Green Vein thins further and is probably less than 18 in thick; to the south and east of the Cynheidre area the distance between the Green and Big veins increases. Most of the cyclothem is composed of silty mudstone with thin beds of sandstone in some places. Fossils, other than plant fragments, are rarely present, but Cochlichnus kochi [rare], cf. Planolites sp. [small, rare], Gyrochorte carbonaria [one record], Anthraconaia sp.  (?pumila (Salter) group), Anthracosia? Naiadites sp. [juv., oblique], and 'Estheria' sp. have been identified in three or four of the Cynheidre boreholes.

The Big Vein has been worked in Coalbrook, Gwendraeth, Pentremawr, Carway, Trimsaran and Cynheidre collieries, and in Carway opencast site. Unsuccessful attempts have been made to work this seam elsewhere in the area, for example, in Plasbach Colliery. Owing to the structural variations in thickness of the seam, incompetent deformation having given rise to variations from 0 (in so-called 'washouts') to at least 40 ft, it is very difficult to estimate its original thickness, but the evidence suggests that not less than 84 in of coal were present originally at this horizon throughout the area. The Big Vein cyclothem is distinctive in that it is substantially thicker than any other in the main productive group of seams, being more than 100 ft in much of the area. Normally sandstone is present, but its thickness is very variable; argillaceous rocks, commonly containing well-preserved plant remains, are present towards the top of the cyclothem, and in some places have also been found in the immediate roof. Fossils other than plants are rare but cf. Anthraconaia pulchella, Anthracosia ovum [juv.],Naiadites sp.  [juv.], 'Estheria' ?, Palaeolimnadiopsis pruvosti and Fayolia cf. crenulata Moysey have been found and cf. Planolites sp. [small] and Gyrochorte carbonaria were seen in the most easterly of the Cynheidre boreholes. Details of the flora are given in the detailed accounts which follow.

In many of the sections seen a seatearth or distinct rootlet phase is present about 20 ft below the Penny Pieces Vein. This phase often contains sphaerosiderite and is a bastard seatearth in places. In some of the Cynheidre boreholes this seatearth is overlain by mudstone with plant fragments.

The names 'Penny Pieces' and 'Soap' were first applied to coal seams in the Gwendraeth Valley during the sinking of Cynheidre (formerly Great Mountain) Shaft No. 3. It is probable that these two seams are only in part equivalent to the Penny Pieces and Soap veins in their type locality, near Ystalyfera (near the eastern margin of the Ammanford Sheet), and their detailed correlation must await the completion of the resurvey of the intervening ground. The Soap Vein of Cynheidre is known to split to the east, the bottom split being in close proximity to the 'Penny Pieces' Vein of Wernos and Pantyffynnon collieries, near Ammanford, the top split continuing as the Soap Vein of that area. The presence of disc-shaped ironstone nodules (Penny Pieces' ironstones) on the seam of that name at both Cynheidre and Ystalyfera, despite the fact that the tops of these seams at Cynheidre and Wernos appear to be at different horizons, should be noted. Notwithstanding this uncertainty about the detailed correlation, the broad equivalence of these seams is apparent, and the seams named the 'Penny Pieces' and 'Soap' in Cynheidre Shaft No. 3 are so named in the area here described.

The Penny Pieces Vein has a two-coal section throughout the area, the few records where this is not so probably being of disturbed sections. The thickness of the top coal is normally between 7 and 9 in, while the bottom coal thickens westwards from about 5 to 7 inches in the Pentremawr–Cynheidre districts to at least 12 in at Carway and Trimsaran, where there are several records of the bottom coal being 18 to 20 in thick. A thin bed of cannel was found in the roof in Gwendraeth Valley Borehole No. 2. This seam was known as the King's Vein in Trimsaran Colliery and as the 'Ddugaled' in Ponthenry Colliery, and is probably the 'Lower Felen' of some accounts, but has nowhere been worked. Typically there are two distinct lithological and faunal facies present in the roof. Where both are present, the immediate roof consists of a thin bed of dark grey, carbonaceous, fissile mudstone, sometimes with a canneloid appearance on joint faces, from which cf. Planolites sp. [small], Spirorbis sp. [attached to shells], Anthracosia atra (Trueman), A. cf. elliptica (Chernyshev non Wright), A.  aff. fulva (Davies and Trueman), A. cf. lateralis, A. planitumida, A. spp.   (atra/ barkeri Leitch), Naiadites obliquus Dix and Trueman, N. cf. product us, and fish remains, have been collected, the mussels usually being preserved as shiny films This facies is overlain by lighter grey mudstone which in places contains disc-shaped ironstone nodules; the following mussels have been found, commonly preserved 'solid' or 'semi-solid': Anthraconaia cf. cymbula (Wright) [juv.], cf. A. glotae (Weir and Leitch), A. sp.  nov. cf. obscura (Davies and Trueman), A. rubida (Davies and Trueman) [juv.], Anthracosia atra, A. cf. aquilinoides, A. cf. caledonica [short form], A. cf. fulva, A. cf. lateralis, A. planitumida, A. cf. subrecta Trueman and Weir, A. sp.  (atralbarkeri group), Naiadites  aff. alatus [posterior wing not strongly developed], and N. sp. [not winged posteriorly]. In addition Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp., Spirorbis sp. [attached to shells], Belinurus sp.,and fish remains have been found in this facies. Other fossils collected from the Penny Pieces cyclothem include Planolites montanus [one specimen], Anthracosphaerium  aff. gibbosum (Hind) and 'Estheria' sp. (one only). The Penny Pieces cyclothem is relatively thin, the thickness averaging about 30 ft; it is composed essentially of argillaceous rocks, except in part of the Cynheidre area where quartzitic sandstone, up to 10 ft thick, has been proved near the roof of the coal.

A separate rootlet phase is present about 15 ft below the Soap Vein and its seatearth in the south-western part of the Cynheidre area; like the similar phase below the Penny Pieces Vein, it is in some places overlain by shale with plant remains.

As a consequence of the misidentification of the Big Vein, the nomenclature of the Soap Vein in the area described is confused. In Capel Bach opencast site, above the workings of Pentremawr Colliery, it was called the 'Upper Soap', and in that colliery the seam now known to be the Soap Vein was thought to be the Green Vein (Strahan and others 1909, p. 95) or the Big Vein (Dix 1928, p. 470) and, more recently, has been called the 'Felen' (Yellow) Vein; it was known as the 'Green' Vein in Ponthenry and Caepontbren collieries, although in this district it had also been called the 'Upper Felen' (or 'Upper Felin') Vein. In Carway Colliery it was worked as the Telen' Vein, and in Trimsaran Colliery as the 'Green' Vein, although Dix (1928, p. 507) had suggested that this seam is near the horizon of the Soap Vein of Ystalyfera.

The Soap Vein has a characteristic section, a top coal which varies between about 24 and 36 in being separated from a bottom coal about 8 to 12 in thick, by 12 to 24 in of 'rashes' or seatearth; the top coal tends to be thickest in the western part of the area, that is at Carway and Trimsaran. The roof of the seam, and the remaining strata up to the Graigog Vein, are variable; in much of the Pentremawr and Cynheidre districts sandstone, up to about 15 ft thick, is present, usually in the lower half of the cyclothem and forming the immediate roof in places. In these districts fossils, other than scattered plant debris, are uncommon. Sandstone exposed in stream sections near Plasbach is probably at this horizon, although it was correlated with the sandstone above the Big Vein (Strahan and others 1909, pp. 111, 117). Further west, at Carway and Trimsaran, the cyclothem is composed essentially of argillaceous rooks, and the roof of the seam consists of mudstone, from which Anthraconaia obscura, A. cf. rubida, A. sp.  nov. cf. rubida [carinate], A. sp.  (pumila/subcentralis group), Anthracosia cf. aquilinoides [juv.], Naiadites cf. productus [Spirorbis attached], and 'Estheria' sp. [not common] have been collected. Elsewhere Gyrochorte carbonaria [in one Cynheidre borehole only], Anthracosia planitumida, A. cf. simulans Trueman and Weir, A.  aff. simulans [elongate] and Naiadites  aff.alatus have been found. The distance between the Soap and Graigog seams increases from about 30 ft at Trimsaran and Carway to about 60 ft in the Pentremawr and Cynheidre areas; in the easternmost part of the Cynheidre area this distance is about 70 ft.

Ddugaled Vein to Graigog Vein: details

East of the Coalbrook Fault

The first 260 yds of the main slant [SN 5132 1142] of Coal-brook Colliery were driven from just below the Ddugaled Vein to the Green Vein: near to the mouth of the slant the following section of the Ddugaled was exposed: grey mudstone on dark grey carbonaceous mudstone 1 in, cannel 1 in, coal 30 in. Anthracosia sp. cf. phrygiana [short form; tilt not marked] and Naiadites sp.  Nov. [see Dix 1928, fig. 13e], preserved as shiny films, were collected, the Naiadites being much more abundant than the Anthracosia. In the 'roofing hole' between Coalbrook and Glynhebog collieries this seam is poorly exposed, but the section is recorded as: 'inferior coal' (? cannel) 2 in, on coal 11 in, dirt 3 in and coal 19 to 28 in, and in Shaft 3, cannel 5 in is said to overlie dirt 12 in and coal 24 in. The only workings in the Ddugaled east of the Coalbrook Fault were from Gwendraeth Colliery (p. 50).

The Ddugaled Rider Vein, about 55 ft above the Ddugaled, is 6 in thick in Coalbrook Slant, where it is overlain by mudstone with Anthraconaia  aff. robertsoni [elongate], Anthracosia cf. beaniana, A. cf. phrygiana, A. sp. intermediate ovum/ aquilina and A. sp. intermediate ovum/phrygiana, preserved as slightly crushed 'solid' shells. In the 'roofing hole' and Shaft 3 only 'rashes' and coal streaks are recorded at this horizon. The strata in both the Ddugaled and Ddugaled Rider cyclothems consist predominantly of silty mudstone, with thin more arenaceous beds.

The Green Vein was worked in Coalbrook Colliery, where the thickness is said to have varied between 36 and 39 in: at the end of the cross-measures drift the section is: slightly silty mudstone on dark grey mudstone 5 in, carbonaceous mudstone with coal streaks 2 ft 7 in, coal 42 in. At the top of the 'roofing hole', which is connected to the main slant, the Green Vein is 32 in thick, and in Shaft 3 this seam is adjacent to the Big Vein in disturbed ground but its thickness is recorded as 31 in. The Green Vein is normally about 25 ft above the Ddugaled Rider and about the same distance below the Big Vein.

The Big Vein is said to have varied from 108 to 144 inches in thickness where it was worked in Coalbrook Colliery. This seam is in disturbed ground in Cynheidre Shaft No. 3; the roof is said to be 'very broken', the seam is said to vary from 84 in to 120 in and the distance between it and the Green Vein to vary between 8 in and 12 ft. Half-inch of coal, on seatearth, is recorded about 40 ft above the Big Vein; there is no other record of a seam in this position in the area described. The strata between this very thin coal and the Penny Pieces Vein are largely arenaceous, although the thickest single bed of sandstone recorded is only 7 ft.

The Penny Pieces Vein, about 130 ft above the Big Vein, consists of coal 2 in on seatearth 1 ft and coal 3 in in Shaft No. 3. The roof, consisting of grey mudstone with small disc-shaped ironstone concretions, contains Spirorbis sp., Anthraconaia rubida [juv.] Anthracosia cf. aquilinoides[juv.] figured Trueman and Weir 1951, pl. 18, fig. 12], A.  aff. planitumida.

The Soap Vein, separated from the Penny Pieces by 20 ft of argillaceous rocks, has the abnormally thick section, coal 46 in on seatearth 2 ft 4 in and coal 12 in, in Shaft No. 3. The 57 ft of strata between the Soap and Graigog veins consist of sandstone, (14 ft is recorded, 3 ft above the soap) siltstone and silty mudstone, with a thin bed of mudstone at the base which yielded Spirorbis sp., Anthracosia planitumida, A. cf. simulans and Naiadites  aff. alatus [posterior wing not marked] preserved as flattened impressions, and associated with plant debris.

Pontyberem to Pont-Henry

In the greater part of this area most of the strata between the Hwch and Big veins is missing due to the presence of the Pentremawr Lag Fault, but south and south-east of Pontyberem, where this fault lies above the Big Vein at and near the outcrops, the full succession was proved in Gwendraeth Colliery and Pentremawr boreholes Nos. 2, 3 and 4.

The Ddugaled Vein was worked on both sides of the Coalbrook Fault in Gwendraeth Colliery, the South Pit [SN 5085 1080] having been sunk to this seam. No record is preserved of the thickness of the seam in the old workings, but 48 in of coal is recorded in the shaft section (Fairley 1868, p. 21); other old records show the thickness of the Ddugaled in this district to be between 34 and 42 in. The details of the Ddugaled cyclothem proved in Pentremawr Borehole No. 4 are as follows:

The cyclothem was only 59 ft thick in Borehole No. 2; Anthracosia cf. ovum [juv.], N. sp.  nov. [as Dix 1928, fig. 13e], 'Estheria' sp.,Rhabdoderma?, Rhadinichthys sp.,and Rhizodopsis sp. have also been found.

The Ddugaled Rider seam was not present in any of the Pentremawr boreholes, but the thickness of the cyclothem was found to be relatively constant, varying from 22 to 29 ft, the strata becoming coarser grained upwards; thin beds of sandstone were proved in No. 2. Mussels, including Anthracosia cf. nitida and A. sp.  (ovum group), were present near the base of the cyclothem in boreholes Nos. 3 and 4.

A very small area of the Green Vein was worked in Gwendraeth Colliery, where it was said to be 36 in thick and to lie about 10 ft below the Big Vein. The Green Vein was found to be less than 30 in thick in Pentremawr boreholes Nos. 3 and 4, and to be only 2 ft 2 in below the presumed Big Vein in No. 3.

The thickness of the Big Vein in Gwendraeth Colliery is recorded as 108 inches in South Pit, and 36 to 84 inches in the workings according to other accounts, but only a relatively small area was mined. The identity of this seam was established by Pentremawr boreholes Nos. 3 and 4, the old workings being recognizable at the Big Vein horizon in No. 4. In No. 3 the Big Vein cyclothem was proved to be about 150 ft thick; sandstone, in beds up to about 5 ft thick, sometimes with mudstone, ironstone and coal pebbles, occurs near the base; 'slump' structures are sometimes evident. The greater part of the strata consists of silty mudstone, with ironstone nodules and thin ironstone beds, containing plant fragments. Dark grey mudstone above the Big Vein in Borehole No. 3 contains Anthracosia cf. ovum and Naiadites?, and in the same borehole a very thin bed of bastard seatearth was proved 47 ft below the Penny Pieces Vein.

The identity of a thick seam in Pentremawr Colliery has long been the subject of doubt and speculation; this seam, the higher [SN 4957 1011] of two thick seams proved in the Gras Cross-measures Drift (p. 70), and very doubtfully correlated with the Ddugaled Vein by Strahan and others (1909, pp. 93–5), has been worked as the 'Big (Hwch)', as it lies only some 30 to 50 ft above the Gras Uchaf Vein in most of the colliery. The thickness of the 'Big (Hwch)' varies between wide limits, as much as 30 or 40 ft of coal being present in places, while in so-called 'washouts' the seam is absent or very thin, so that it is difficult to establish its original thickness, which may have been about 96 in. The quality of the coal is also variable, the variation, like the thickness variation, being largely, if not entirely, due to structural factors. Thus, in some parts of Pentremawr Colliery the seam consists of clean coal, whereas in others the coal is mixed with 'rashes' derived from the roof or floor and mixed with the coal during the process of incompetent deformation. Nothwithstanding these variations, this seam is more closely comparable with the Big Vein than with the Hwch Vein, as seen, for example, in Pentremawr Borehole No. 2. As it was believed, following Strahan and others (1909, pp. 95, 98–9) and Dix (1928, pp. 468–70), that the Green and Big veins lie above the 'Big (Hwch)', several attempts have been made to prove these seams by cross-measures drifts and boreholes. In two of these, the underground borehole in the Penhill', District [SN 5025 0934] and the more recent extension of the 'Brussels' cross-measures to the 'Felen' (i.e. Soap) Vein [SN 4996 0956], the Penny Pieces Vein was found about 175 ft above the 'Big (Hwch)' Vein, but in Pentremawr Borehole No. 1 [SN 5037 0983], in the same district, this distance was proved to be only 130 ft. The thickness of sandstone present towards the base of the cyclothem is very variable, over 80 ft being present in the 'Penhill' Borehole and 22 ft, in four beds, in the 'Brussels' Cross-Measures. While the variation in the thickness of sandstone is an original sedimentary feature, the variation in the distance between the seams is believed to be due to low-angle thrust faulting, as shown on the map and in (Figure 24). Plant fragments are commonly present in the top half of the cyclothem. Although this cyclothem is variable in thickness and lithology, in general it is closely comparable with the Big Vein, rather than the Hwch, cyclothem (Figure 12) and (Figure 14). Pentremawr Borehole No. 2 confirmed the correlation with the Big Vein, primarily because it proved the existence of the Ddugaled and Hwch veins in the area: it followed from this correlation that a lag fault, named the Pentremawr Lag Fault, was present, cutting out most of the strata between the Gras Uchaf and Big veins over much of the Pentremawr Colliery area. Sheared and slickensided strata are commonly, although not always, present between these seams, and no major plane of movement has been identified. The sandstone above the Big Vein has been quarried [SN 4940 1012] 100 yd S.W. of Capel-Ifan Farm, and forms the prominent ridge on which St. John's Church stands; the break in this ridge about 400 yd N.E. of the church is due to the Pentremawr Lag Fault intersecting the sandstone.

Nothing is known positively about the Big Vein in Ponthenry Colliery; the very small area worked in a seam known as the 'Big Vein', may have been in the Big Vein, rather than at the Stanllyd/Hwch horizon (p. 71).

The Penny Pieces is consistently thin but usually readily recognizable by its distinctive two-coal section and its fauna: a typical section is: coal 7 in on a parting 5 in and coal 5 in. In the 'Brussels' cross-measures drift in Pentremawr Colliery the section is as follows:

Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], Anthracosia spp.  of the acutellalconcinna group, Naiadites sp. [juv.] and fish scales have also been recorded from the roof of the Penny Pieces Vein. A thin seam [SN 4993 0943] 152 ft above the Big Vein in an older cross-measures drift ('Mafeking') in Pentremawr Colliery is believed to be the Penny Pieces, although Dix (1928, p. 470) thought this seam to be the Green Vein. The strata between the Penny Pieces and Soap veins, which average about 40 ft thick, are essentially argillaceous, with rare thin beds of sandstone, usually towards the top, except at the crop, N.E. of Pont-Henry, where field evidence suggests the presence of sandstone in or near the immediate roof of the seam.

The seam was seen in a trench 100 yd N. of Bola-haul [SN 4934 1002]. In Ponthenry Colliery the Penny Pieces, with the section: coal 12 in on, 'rashes' 12 in, coal 12 in, and shown as the 13dugaled' on some sections (p. 83), was proved some 30 ft below the Soap (or 'Green') Vein.

The Soap Vein has been proved in Pentremawr Colliery (during attempts to find the Big Vein, when the Big (Hwch) was thought to be the Hwch) and in the Pentremawr boreholes. The 'Brussels' cross-measures drift has been extended recently with the intention of developing this seam, which is unworked underground, although a short level was driven in it [SN 4965 0992] at the end of the Gras cross-measures drift, where it was thought to be the Green Vein (Strahan and others 1909, p. 95). As is the case with other seams in this area, the sections of the seam proved show considerable variation. In Capel Bach opencast site [SN 495 098], (part of Tyn-y-cwm site, worked since the resurvey was completed), the average section: top coal 24 in, parting 12 in, bottom coal 10 in, may be regarded as typical for this area, although in Pentremawr boreholes Nos. 1 and 2, and in the 'Brussels' and 'Mafeking' drifts, the top coal averages about 30 in. Much of the Soap Vein cyclothem consists of arenaceous rocks, sandstone, up to 15 ft thick, sometimes forming the immediate roof of the seam, and the only fossils commonly present are plant fragments. In borehole No. 3, where the roof consists of silty mudstone, Gyrochorte carbonaria and Anthraconaia sp. [small, elongate] were present; in Borehole No. 1, Anthracosia sp.  (ovum group) and Naiadites sp. [juv.] were found in mudstone about 20 ft above the coal. The thickness of the cyclothem varies from 45 to 65 ft the average being about 60 ft. The low-angle faulting which affects the strata between the Big and Penny Pieces veins in the Pentremawr area, also affects the Soap Vein. Between the Pontyberem to Pont-Henry road and Ponthenry Colliery the Soap has been worked at outcrop and from several old shafts.

The seam worked as the 'Green' (or 'Stepney') Vein in Ponthenry Colliery is now correlated with the Soap Vein, rather than the Big and/or Green veins, as believed locally. The reasons for this conclusion may be summarized as follows:

1. Dix (1928, pp. 468–9), although apparently accepting the correlation of this seam with the Green Vein of the upper Gwendraeth Valley noted that "the fauna and flora of this seam are not identical with those of the Green Vein of Cross Hands".

2. Although the strata associated with this seam are no longer accessible, the following section is exposed in the mouth of an old trial level [SN 4840 0954] on the first coal below the 'Green' Vein, 275 yd N.E. of the 'Green' Vein Slant [SN 4820 0940] of Ponthenry Colliery: sandstone on mudstone with Anthracosia cf. caledonica [short form], A. cf. lateralis, A. planitumida and Naiadites sp. [not winged posteriorly] 1 ft 9 in, coal 8 in, carbonaceous mudstone with coal streaks 8.5 in, coal 6 in. This fauna suggests an horizon near the Penny Pieces, and the seam section is closely comparable with that seam in this area.

3. The section of the 'Green' Vein, recorded on the Abandonment Plan as: coal 30 in on 'rashes' 3 in, 'hard bottom' 15 in and coal 8 in (coal 32 in on 'rashes' 16 in and coal 8 in according to another account), is closely comparable with that of the Soap Vein in this district.

4. It is impossible to draw any conclusions about correlation from the distances between the 'Green' Vein and known stratigraphical horizons, all evidence based on thickness of strata being suspect in this area, for structural reasons; for example, the distance to the Gwendraeth Vein varies from 350 ft (less than the normal distance from Gwendraeth to Green) near the crop to 600 ft (more than the normal distance from Gwendraeth to Soap) at the southern limit of the 'Green' Vein workings. It may be noted, however, that the Trimsaran Marine Band is exposed in Afon Hafren [SN 4839 0907], 220 ft above the 'Green' Vein; this is the average distance between this marine band and the Soap Vein in this area.

The variation in the distance between the Soap and Gwendraeth veins, as well as the absence of the Big and Green veins, is explained by the presence of low-angle faults, including the Pentremawr Lag Fault.

Nothing is known of the strata between the Soap and Graigog veins in the Pont-Henry area.

Pont-Henry to the Trimsaran Disturbance

Very little is known of the details of the measures between the Ddugaled and Graigog veins between the Pont-Henry Fault and Carway, and nothing is known of the measures between the Ddugaled and Big veins in the heavily boulder clay masked ground between Carway and Trimsaran. In the whole of this area surface exposures are poor; apart from exposures (together with several old pits and drifts) in the valley [SN 4675 0752] S.S.E. of Ynys-fawr, faulted strata are exposed [SN 4713 0786] in the small stream 400 yd S.S.E. of Pontyates Station. The beds, including a coal smut, folded into an asymmetrical syncline with a vertical southern limb, exposed in the small stream [SN 4539 0540] 1450 yd S.S.W. of Carway School may be below the Graigog Vein.

The Ddugaled Vein is known with certainty in the Carway area. It was worked to a very limited extent in the 'Gras' Slant (p. 72), abandoned in 1903, where the section is recorded as: cannel 2 in on coal 36 in (Figure 9(b)), and more extensively in Carway Colliery, where the section is: cannel 2 to 3 in on coal 33 to 36 in. The Ddugaled cyclothem is about 35 ft thick in Carway Colliery, a typical section being as follows:

The Ddugaled Rider, known in this area only m Carway Colliery, is a very thin seam, a typical section being: mudstone with ironstone nodules on ironstone, 2 in; darker grey mudstone, Anthracosia cf. beaniana, A. cf. caledonica, A. cf. ovum, A.  aff.phrygiana, A. sp. intermediate ovum/aquilina, Naiadites sp.,1 ft 4 in; ironstone, 1 in; on coal, 5 in. This cyclothem, which sometimes includes a thin bed of sandstone, is about 35 ft thick.

A coal seam 15 to 17 in thick between the Ddugaled Rider and Big veins in Carway Colliery, is probably the Green Vein. The roof consists of silty mudstone with abundant plant fragments, and this lithology continues to the top of the cyclothem which is about 30 to 40 ft thick.

A seam said to have consisted largely of 'culm' (Strahan and others 1909, p. 108) was extensively dug at outcrop at Pontyates between the Llanelli and Carway roads [SN 4726 0819] to [SN 4701 0794]. Since the resurvey was completed opencast prospecting has proved this seam to be thick, friable in part and cut by several overthrusts. The drilling also proved that sandstone up to 60 ft thick, although normally not more than 30 ft thick, is present over most of the area prospected, and that the distance between this seam and next above is about 100 ft: this evidence suggests correlation of the thick seam with the Big Vein, rather than with the Stanllyd/Hwch. As the distance between this thick coal and the workings in the Pumpquart Vein varies between about 500 and 600 ft, correlation of this coal with the Big Vein makes it necessary to postulate the presence of a lag fault to account for the missing strata. This fault may be a continuation of the Pentremawr Lag Fault which is shown on the map between the Stanllyd and Penny Pieces seams about I mile to the E.N.E. A thick coal ('Big' or 'Stanllyd') proved in the cross-measures drift in Plasbach Slant ((Figure 9)(a); p. 72) is also believed to be the Big Vein. In the drift the section of the seam is: coal c. 20 ft on 'stone' 9 in, coal 15 in, 'stone' 12 in, coal 36 in, according to one account, but the average thickness is said to be 54 in (Strahan and others 1909, p. 110); this seam is about 550 ft above the Pumpquart Vein. The roof is said to consist of 'dark shale, 4 ft thick, with scattered ironstone nodules' overlain by 'fine-grained micaceous sandstone with little shale, 80–90 feet thick', although another old section shows the immediate roof as 'rock' (i.e. sandstone). Fossils collected from the tip (Strahan and others 1909, p. 109), but said to have come from this seam, include plant fragments, Anthracosia spp.  [including 'solid' forms], and Naiadites sp. [film-like preservation], but the fossils are too poorly preserved to be of diagnostic value; the presence of "Posidoniella?" is not confirmed. Although the only evidence on which the correlation of this seam can be based is derived from conflicting accounts, the thickness of the coal, as well as of the overlying barren strata which include sandstone, and the sections of the overlying seams (p. 91) all point to this seam being the Big Vein, rather than the Stanllyd/Hwch.

The Big Vein workings in Carway Colliery were no longer accessible at the time of the resurvey, but the average section is said to be: top coal 15 to 20 in, middle coal 34 to 42 in, bottom coal 36 to 45 in (sometimes with 6 in of pyrite, 12 in from the base). In Carway opencast site [SN 455 060] the thickness of the seam was very variable, as much as 30 or 40 ft being present locally. The strata between the Big and Penny Pieces veins, which vary from about 85 to 105 ft apart, consist predominantly of silty mudstone with impersistent beds of sandstone up to at least 10 ft thick. Plant fragments, including Alethopteris decurrens (Artis), A. decurrens var. gracillima Boulay, Calamites undulatus, Mariopteris dernoncourti Zeiller, M. muricata (Schlotheim), Neuropteris gigantea and N. heterophylla are usually present. In places the immediate roof is mudstone, from which Palaeolimnadiopsis pruvosti was collected in the opencast site, and Dix (1928, p. 474) obtained "Carbonicola cf. fulva Davies and Trueman, C. cf. aquilina (J. de C. Sow.), Anthracomya sp., Naiadites subtruncata (Brown), N. triangularis (J. de C. Sow.)" from Carway Colliery. The correlation of this seam has also long been in doubt; Strahan and others (1909, p. 111) favoured correlation with the Stanllyd, and Dix (1928, p. 476) with the Big Vein. The evidence obtained during the resurvey has established beyond doubt that the latter correlation is correct; this evidence may be summarized as follows:

1.      The Ddugaled and Ddugaled Rider seams occur in the appropriate positions below the Big Vein, apparently in an undisturbed succession.

2.      The quality and section of the Big Vein of Carway are comparable with the Big Vein of the upper part of the Gwendraeth Valley, rather than with the Stanllyd/Hwch seam.

3.      The details of the Big Vein cyclothem are comparable with the Big Vein cyclothem of Cynheidre Colliery and elsewhere, rather than with the Hwch cyclothem. For example, characteristically the Hwch Vein is overlain by mudstone with a distinctive fauna : this is not present at Carway.

4.      The Penny Pieces and Soap seams, both readily recognizable, are present in the appropriate positions above the Big Vein. The Graigog Vein has also been identified in Carway Colliery and in a nearby opencast coal site [SN 460 063].

At Pontyates, in the area [SN 472 080] beneath the Carway and Llanelli roads, a seam believed to be the Penny Pieces, with a two-coal section, has been proved in opencast coal trial boreholes. Although the distance between this seam and the Big is variable, due to the presence of incompetent structures, it is not less than 100 ft. Strata, including thin coal seams, possibly the Penny Pieces, Soap and Graigog veins, are exposed in a nearby stream section [SN 4713 0786], where they are cut by several faults. Anthracosia? [elongate] and fish remains were collected above the higher of two of these seams and Naiadites  aff. angustus [not strongly winged] on the lower.

The Penny Pieces Vein, and the strata between it and the Soap Vein, have been seen in several cross-measures drifts and boreholes in Carway Colliery, and in Carway opencast site. The seam is thin, although slightly thicker than in the Pentremawr and Cynheidre areas; as in those areas, it has a two-coal section, but at Carway the bottom coal is usually thicker than the top coal, a typical section being: coal 8 in, 'rashes' 5 in, coal 12 in. Recent opencast prospecting [SN 463 068] is said to have proved the bottom leaf to have an average thickness of 18 in. The immediate roof normally consists of up to 3 in of dark grey carbonaceous mudstone with a canneloid appearance on joint faces, which has yielded Anthracosia sp.,Rhadinichthys sp. and a Palaeoniscid scale. This facies is overlain by dark grey, very finely micaceous, fissile mudstone, from which the following have been obtained: Anthraconaia? [juv.], Anthracosia  aff.atra, A. cf. elliptica, A. cf. fulva, A. cf. planitumida, Naiadites sp. and Elonichthys sp.,which passes upwards to grey mudstone. The latter facies has yielded 'semisolid' Anthracosia atra and A. planitumida as well as 'solid' A. cf. lateralis. Naiadites obliquus? has also been recorded, probably from the Penny Pieces horizon, in one of the underground boreholes. The remainder of the cyclothem, which is about 50 ft thick, becomes siltier towards the top; a 3 to 9 in bed of quartzitic sandstone has been seen about 20 ft above the seam in two of the sections.

It is possible to deduce from the Abandonment Plans that the workings in the 'Green' Vein in Ponthenry and Caepontbren collieries are probably at the same level, and as the section of this seam in Caepontbren Colliery: coal 32 in on shale and 'clad' 20 in and coal 9 in is almost identical to that of the Soap (i.e. 'Green') Vein in Ponthenry, the Caepontbren 'Green' is similarly correlated with the Soap. Recently a large diameter borehole [SN 4840 0772] has been drilled from the surface as part of a scheme to de-water the 'Green' Vein workings in Caepontbren Colliery, mouth of slant at [SN 4748 0828]. As the original borehole was not cored, electrical (spontaneous potential) and gamma logs were made; as a result of the interpretation of these logs and the driller's record, and of the determination of fossils from the lower part of the borehole, part of which was subsequently cored, it is possible to advance the following interpretation :

The fauna at 1140 ft is possibly the Penny Pieces horizon; a fault is postulated between this horizon and the old workings as it is most unlikely that the Penny Pieces Vein was worked.

In Plasbach Colliery, the cross-measures drift from the bottom of the slant ((Figure 9)(a) was driven beyond the presumed Big Vein (p. 90) through a syncline and an anticline, the strike of the vertical middle limb being N.75°E. On the southern limb of the anticline a seam with the section coal 29 in on 'rashes' 5 in coal 7 in (shown as the 'Green' Vein, with the section: coal 34 in on 'clod' 7 in, coal 8 in, on one section) was proved, but was found to die out when followed for a short distance. About 20 ft of 'sound ground' (Strahan and others 1909, p. 109) separated the seam from another with a similar section: coal 29 to 35 in on 'stone' 2 in and coal 4 in (or coal 21 in), which was proved to continue with a relatively uniform section, but was not worked. The drift was continued in disturbed ground for about 33 yd where a seam (the 'Big' Vein) was proved, consisting of 6 to 7 ft of coal and shale, which thinned to 21 in when followed down dip for about 30 yd. According to one section a fourth seam (the 'Felen' 13 in thick, was proved near the end of the drift. The so-called 'Big' Vein is about 160 ft above the presumed Big Vein, about 175 ft below the Mole Marine Band and about 475 ft below the Carway Fawr; these distances are not inconsistent with this seam being at or near the horizon of the Soap Vein. The sections of the 'Green' Vein, which is about 100 ft above the Big Vein, and of the first seam above it in the drift are similar to each other and to the Soap Vein of Ponthenry and Carway collieries. On the conflicting evidence available it is impossible to correlate these seams with any certainty, but it is almost certain that at least one of them is the Soap Vein.

The stream 450 yd S.S.E. of Ynys-fawr follows the strike of a fine-grained quartzitic sandstone for about 200 yd [SN 468 074]; this sandstone, which is about 15 ft thick, is probably between the Soap and Graigog veins.

A very small area of Soap Vein has been worked in Carway Colliery, where it was known as the Telen' Vein. The section of the seam is similar to that proved in Ponthenry and Caepontbren collieries; the top coal varies from 12 to 50 in, the normal section probably being about 36 in, the seatearth parting averages about 2 ft, although it is sometimes thicker, and the bottom coal is more constant at about 10 in. The immediate roof is usually mudstone, silty in parts, from which Spirorbis sp., Anthraconaia spp.  [including pumilalsubcentralis group], Anthracosia cf. aquilinoides [juv.], Naiadites sp. and 'Estheria' sp. [not common] have been collected. The remaining beds between the Soap and Graigog veins, all argillaceous, have only been seen in one cross-measures drift, where the seams were 24 ft apart; this cyclothem was proved to be 20 to 35 ft thick during recent opencast prospecting operations up dip from the deep mine workings. A very small tonnage of Soap Vein was worked in Carway opencast site [SN 4521 0548].

South of the Trimsaran Disturbance

With one exception noted below, the strata between the Ddugaled and Graigog veins were inaccessible both at the surface and in Trimsaran Colliery at the time of the resurvey, but the beds from the Big Vein to above the Graigog Rider were subsequently exposed in Trimsaran Wood opencast site [SN 460 050]. The stratigraphy of the appropriate part of the Trimsaran Borehole could not be interpreted because of structural complications, and between Syddyn Farm [SN 4444 0436] and Carmarthen Bay the Coal Measures are masked by thick drift deposits, and remain unknown.

The seam worked as the 'Yard' or 'Three-Feet' Vein in Trimsaran Colliery is believed to be the Ddugaled Vein; its average thickness is about 36 in, but the presence of cannel at the top of the seam has not been recorded. Like the Ddugaled Vein of Carway Colliery, it lies about 100 ft below the Big Vein. Anthracosia cf. aquilina, A. cf. ovum, A. cf. phrygiana [short form], and Naiadites sp.  nov. [see Dix 1928, figs. 13e-g] were collected about 3 ft 6 in above this seam a short distance down Caedean Slant [SN 4535 0495]; 11 ft of sandstone is recorded 5 ft above the coal in one section, but Dix (1928, p. 490) reported that this sandstone is in places present immediately above the seam. A trial pit [SN 4450 0447], 140 yd N.E. of Syddyn Farm, proved a seam 24 to 37 in thick, believed to be the Ddugaled, at a depth of 15 ft.

Seatearths recorded 54 and 17 ft below the Big Vein in Trimsaran Colliery possibly mark the positions of the Ddugaled Rider and Green veins respectively; in the New Inn Level the former may be represented by 2 in of coal and the latter by 10 in of 'coal and carbonaceous shale'.

The identity of the Big Vein of Trimsaran Colliery, like that of Carway, has been in doubt. The resurvey has confirmed Dix's correlation of this seam with the Big Vein of Tumble (1928, pp. 501–3). The section of the Big Vein, although very variable, averages: coal 12 to 18 in on coal 39 to 42 in and coal 36 to 45 in (sometimes with pyrite nodules). This section is closely comparable with the Big Vein elsewhere in the Gwendraeth Valley. The roof normally consists of silty mudstone and striped beds with beds of sandstone up to about 30 ft thick: apart from one specimen each of Anthracosia ovum [juv.] and Fayolia cf. crenulata, the only fossils found in the roof are plants, including Alethopteris decurrens, A. lonchitica, Calamites sp., Cordaites principalis, Lepidodendron aculeatum Sternberg, Lepidostrobus triangularis (Zeiller), Mariopteris muricata, Neuropteris gigantea, N. heterophylla, N. obliqua, N. tenuifolia (Schlotheim), Sigillaria boblayi Brongniart, Sphenopteris striata Gothan and Stigmaria ficoides Sternberg var. rugosa Heer. The thickness of the cyclothem averages about 80 ft. The Big Vein has been worked in the Caedean, Waunhir and Top [SN 4666 0526], slants of Trimsaran Colliery, and in Trimsaran Wood opencast site; part of the workings in Trimsaran Colliery were drained by the St. Leger Level [SN 4524 0521]. Between Trimsaran and Syddyn Farm this seam has been worked near the crop but in a trial pit [SN 4450 0442] it was found to be thin.

The details of the Penny Pieces Vein cyclothem in Trimsaran Wood opencast site are as follows:

In Trimsaran Colliery, where this seam was known as the King's Vein, the section is recorded as : coal 9 in on parting 3 in and coal 12 in.

The seam worked as the 'Green' Vein in Trimsaran Colliery and Trimsaran Wood opencast site is the Soap Vein; the average section in the colliery is recorded as: top coal 39 in on a bottom coal 12 in, but in the opencast site a more typical Soap Vein section was seen, the details of the cyclothem, which averages about 35 ft thick, being as follows:

The thickness of the top coal was proved to be at least 45 in and the bottom coal 18 inches in part of this site. Anthracosia sp. cf. aquilinoides and A.  aff. simulans [elongate] were also collected. West of Trimsaran the Soap Vein has been worked at the crop, and in a trial pit [SN 4455 0440] near Syddyn Farm 36 in of coal was proved at 22 ft; a small area of disturbed coal, up to at least 96 in thick, believed to be the Soap Vein, was worked in Syddyn opencast site [SN 442 041].

Cynheidre area

The Ddugaled Vein, although missing due to the presence of the Pentremawr Lag Fault in part of this area, has been proved in several cross-measure drifts and boreholes, and has been worked from Cross-measures No. 3. The characteristic thin bed of cannel, up to about 1 in thick, is usually present at the top of undisturbed sections. The average thickness of the coal is about 32 in, although substantially thicker sections including up to 8 in of cannel have also been recorded. In Gwendraeth Valley Borehole No. 2, the Ddugaled cyclothem was repeated by a fault with a vertical throw of 85 ft: the abbreviated details of the higher repeat, with additional fossils found in the lower, are as follows:

In the Cynheidre area the thickness of this cyclothem averages about 45 ft. In addition to the Naiadites sp.  nov., preserved as glossy films, which is characteristic of this horizon, Anthracosia sp. ( ?intermediate nitida/ovum), Naiadites sp.  (productus group), 'Estheria' sp.,and Rhabdoderma sp. have been found in the thin bed of dark grey mudstone, carbonaceous in part, which normally forms the immediate roof of the seam. The higher mussel phase is commonly present, and in addition to the fossils recorded in Gwendraeth Valley Borehole No. 2, Anthracosia cf. ovum, A. cf. phrygiana, A. sp.  (?beaniana), Naiadites cf. angustus, N. sp.  nov. [see Dix 1928, fig. 13e], N. sp.  (?productus), Lioestheria sp.,Platysomid and Palaeoniscid scales have been collected; the mussels in this phase are often preserved as 'semi-solid' shells. Scattered Cochlichnus kochi are usually present, but Gyrochorte carbonaria is rare. The two mussel phases are separated by silty mudstone, striped beds and siltstone; thin beds of quartzitic sandstone are present in Borehole No. 4/3 [SN 5137 0686], while in Borehole No. 6/3 [SN 5334 0879] about 40 ft of sandstone with thin beds of silty mudstone, some sheared, are present above a complex sequence of coals, with cannel associated, believed to be the Ddugaled Vein repeated by several thrust faults. In Borehole No. 5/2 [SN 5193 0810] about 35 ft of sandstone are present 15 ft above a seam which is possibly the Ddugaled; in neither No. 6/3 nor No. 5/2 is there sandstone above what are believed to be lower repeats of the seam. If these correlations are sound, the presence of thin sandstone in Borehole No. 4/3 and thicker sandstone above thrust faults in boreholes Nos. 5/2 and 6/3, that is in the three most southeasterly boreholes (Plate 1), suggests that sandstone may be more widespread in this cyclothem in this direction, the sandstone having been thrust from a southerly direction in Nos. 5/2 and 6/3.

The Ddugaled Rider cyclothem, which averages about 30 ft thick, is normally recognizable in the Cynheidre area, although the Ddugaled Rider seam is usually only about 4 in thick. In Borehole No. 5/2 this seam is probably represented by coal streaks on a thin seatearth; in Borehole No. 4/3 the base of the cyclothem is marked only by rare rootlets in the top of the Ddugaled cyclothem, while in the third borehole on the south-eastern side of the Cynheidre area, No. 6/3, no rootlet phase is present but the base is recognizable by the presence of mudstone with the appropriate fauna overlying silty mudstones with abundant plant fragments at the top of the Ddugaled cyclothem. The section recorded in Shaft No. 2 is as follows:

Other fossils collected include Lepidodendron ophiurus, Lepidostrobus sp., Neuropteris heterophylla, Anthraconaia sp.  (pulchella group), Anthracosia  aff.aquilina, A. cf. caledonica, A. cf. phrygiana, A. sp. intermediate aquilina/phrygiana, A. sp. intermediate phrygianalovum, Naiadites productus? [large], N. sp.  nov. [see Dix 1928, fig. 13e], Elonichthys?, Rhabdoderma sp. and Strepsodus sp. Quartzitic sandstone, about 10–15 ft thick, is commonly present near the top of the cyclothem.

The thickness of the Green Vein is probably about 20 in. In Shafts Nos. 1 and 2, the underground drivages and in those deep boreholes where it can be recognized, with the four exceptions noted below, this seam is less than 15 ft below the Big Vein. The intervening strata consist mainly of seatearth and mudstone, usually silty, with rootlets; beds of sandstone up to 4 ft thick are sometimes present, and dark grey mudstone, sheared, is present in the immediate roof of the coal in some sections. The distance between the Green and Big veins increases to the south and east, 17, 36, 54 and 22 ft of strata being present in Cynheidre boreholes Nos. 4/3, 5/2, 6/3 and Gwendraeth Valley No. 2, respectively. The abbreviated log of the latter borehole is as follows:

This section is a typical in that fossils, other than plant remains, are generally rare in this cyclothem; apart from fragments in two boreholes, the only other record of mussels is in Borehole No. 5/2. Cochlichnus kochi and cf. Planolites sp. [small] although rare, have been recorded, and there is one record of Gyrochorte carbonaria. The greater thickness of the cyclothem in Borehole No. 6/3 is partly due to the presence of more thin beds of sandstone and striped beds in the lower part than is usual; the thickness of the seam in this borehole, 42 in, is also unusually great.

The original or 'normal' thickness of the Big Vein is impossible to establish accurately, but a conservative estimate would be 7 ft; thicker sections have been proved both in the workings in Cynheidre Colliery and in the boreholes. The seam has been seen to consist of three leaves in some sections, and pyrite is sometimes present. The Big Vein cyclothem is thick, averaging about 150 ft, and consists mainly of arenaceous strata, but it tends to be more argillaceous towards the top; mudstone and silty mudstone are sometimes present in the immediate roof of the seam. The thickness of the sandstone is very variable, as indicated diagramatically in (Figure 13): the greatest thickness proved was in Borehole No. 6/1 and Gwendraeth Valley No. 2, in which about 100 ft were present. The rapid lateral variation is illustrated by the presence of over 70 ft of sandstone in Shaft No. 2 and only 6 ft in the Main Lateral (560) 380 yd to the N.N.E. Plants collected from the roof of the seam include Alethopteris lonchitica [common], Asterophyllites equisetiformis, Calamites sp.,Mariopteris nervosa and Neuropteris heterophylla. Jones (1934, p. 423) noted that A. lonchitica was abundant above the Rock Vein of Wemos Colliery. Calamites sp., Cordaites sp., Mariopteris sp.,Neuropteris sp., Pinnularia sp. and Sigillaria sp. have been collected from silty mudstone towards the top of the cyclothem, where plant debris is locally common. Fossils, other than plant fragments, are uncommon, but cf. Anthraconaia pulchella, Anthracosia sp.  (ovum group), Naiadites sp.  (productus group), N. sp. [juv.] and 'Estheria' ? have been found, associated with plant debris, in the roof, and Gyrochorte carbonaria and cf. Planolites sp. [small] were found near the top of the cyclothem in Borehole No. 6/3.

In most of the sections proved in the Cynheidre area a separate seatearth or rootlet phase, overlain by mudstone with plant fragments, is present about 20 ft below the Penny Pieces Vein. Sphaerosiderite is commonly present in this seatearth, which is a bastard seatearth in parts of the area.

In Cross-measures No. 5 (490) the section of the Penny Pieces Vein is: coal 7 to 12 in on parting 5 in and coal 6 to 7 in, which may be taken as representative for the Cynheidre area; in Gwendraeth Valley Borehole No. 2, 1.75 in of cannel was proved in the immediate roof of this seam. Two distinct facies are often present in the roof; the lower, consisting of a thin bed of dark grey, carbonaceous, fissile mudstone, has yielded cf. Planolites sp. [small], Spirorbis sp. [attached to shells], Anthracosia  aff. atra, A. cf. fulva, A. cf. lateralis, A. planitumida, A. sp.  (atralacutella group), Naiadites cf. productus, N. sp. [slight posterior wing] and fish remains including Elonichthys?, Rhabdoderma?, Rhadinichthys sp.,and indeterminate Palaeoniscid and Platysomid scales. This facies is overlain by smooth grey mudstone with Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], Spirorbis sp. [attached to shells] Anthraconaia cf. cymbula [juv.],?A. rubida, Anthracosia cf. aquilinoides, A. sp.  (acutella/concinna group), A. planitumida, Naiadites  aff. alatus, Belinurus sp.,Platysomid squamation and a fish jaw. Other fossils collected in this cyclothem include Planolites montanus [rare], P. sp. [3.0 mm], Anthracosphaerium  aff. gibbosum, Naiadites cf. obliquus, 'Estheria' sp. [rare] and Belinurus sp. [long telson]. The average thickness of the cyclothem is about 25 ft, the strata being mainly argillaceous; round, flattened ironstone nodules are locally present in the lower part. Quartzitic sandstone, up to 10 ft thick, has been proved in boreholes Nos. 3/1 and 3/2, occurring only 4 in above the coal in the former. In the south-western part of the area, i.e. in boreholes Nos. 3/1, 3/2, 4/1, 4/3 and Gwendraeth Valley No. 2, a separate seatearth or rootlet bed is present about 15 ft below the Soap Vein and in the latter borehole dark grey shale above this bed contained Cordaites principalis, Lepidostrobus sp., Neuropteris heterophylla, N. obliqua, and Sphenophyllum cuneifolium; Cochlichnus kochi has also been found at this horizon.

A typical section of the Soap Vein in the Cynheidre area is : coal 28 in on parting 10 in, coal 8 in, although notably greater thicknesses have been proved, e.g. 94 inches in Gwendraeth Valley Borehole No. 1 and 96 inches in No. 6/1. That these thick sections are anomalous and due to structural factors, is strongly suggested by the sections seen in Cross-measures No. 5 (490) where the first intersection of this seam proved: coal 22 to 24 in on parting 9 to 11 in, coal 9 to 10 in, and the disturbed section, where the seam was repeated 120 yd inbye by a thrust fault, consisted of coal and 'rashes' varying from 36 to 141 in thick, but included at least 44 in of clean coal in parts. The roof usually is mudstone or silty mudstone with plant debris, but sandstone, usually present in the cyclothem, sometimes forms the immediate roof, as in Cross-measures No. 5. The beds of sandstone are generally not more than 5 to 10 ft thick. The only fossils seen, other than plant fragments, are Gyrochorte carbonaria (in Borehole No. 4/2 only), Anthraconaia?, Anthracosia sp., Naiadites? with Spirorbis sp. attached and Rhadinichthys sp. The average thickness of the cyclothem is about 50 ft, increasing to 70 ft in boreholes Nos. 5/1, 5/2 and 6/3, on the eastern side of the area.

Graigog Vein to Carway Fach Vein: general

Surface exposures of these strata, although generally better than in the lower part of the Middle Coal Measures, are poor and discontinuous. However, recent boreholes, and to a lesser extent colliery drivages, have provided much evidence not available at the time of the original six-inch geological survey, or to Dix.

The first seam above the Soap Vein south of the Trimsaran Disturbance was there known and worked as the Graigog Vein. This name has now been extended throughout the area described, although with minor exceptions it has nowhere else been worked. In the Trimsaran area this seam is about 39 in thick, including partings, but to the east and north-east the seam thins, the thickness in the Pentremawr area being not more than about 10 in; it normally has very high ash and sulphur contents. Throughout the area the roof consists of mudstone, usually micaceous and often silty, in which mussels with a characteristic 'ghostlike' preservation, including Anthraconaia cf. librata, Anthracosia cf. acutella, A. atra, A. cf. caledonica, A. cf. planitumida, A. cf. simulans, A. sp. (cf. aquilinoides) [juv.], Anthracosphaerium? and Naiadites alatus, are present. The mussels are commonly accompanied by fish remains including Elonichthys sp., Rhadinichthys wardi (Ward), Rhizodopsis sp. and Palaeoniscid and Platysomid ? scales. In the Trimsaran area a marine band containing Lingula mytilloides and Palaeoniscid scales is present between the coal and the mussel phase; this is the Graigog Marine Band (Archer 1959, p. 27), which is probably the Two-Foot Marine Band of the Pennine coalfields, but is unknown elsewhere in South Wales. Sandstone, usually thin but up to about 12 ft thick in places, is commonly present near the top of the cyclothem, the thickness of which varies from about 35 to 50 ft.

The coal succeeding the Graigog Vein, named the Graigog Rider during the resurvey, is not more than 12 in thick in the Trimsaran area, and like the Graigog Vein thins to the east and is very thin in the eastern part of the Cynheidre and Pontyberem areas. Although the coal is unimportant, it is a valuable marker horizon as its roof normally contains a distinctive mussel fauna, including Anthraconaia cf. librata and Anthracosia spp.  of the acutella and atra groups, many preserved as small 'solid' shells, possibly dwarfed or juvenile. Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, 'Estheria' sp. and fish remains including Rhizodopsis sp. and Palaeoniscid scales have also been recorded. The overall faunal evidence suggests that this might be the horizon of the Clown Marine Band of the Pennine coalfields, although no marine fossils have been found in South Wales, The distance between the Graigog Rider and Mole veins (or Mole Vein position) is about 65 to 70 ft in the Trimsaran and Cynheidre areas, increasing northwards to about 90 ft in the Pontyberem district. In the south-western part of the Cynheidre area, in the Pentremawr boreholes and at Trimsaran, this part of the succession is interrupted by a bastard seatearth or rootlet phase, only overlain by coal in one of the Cynheidre boreholes. Sandstone is present a short distance below this shallow-water or emergent phase in the Trimsaran area, and the only fossil seen above it is a Belinurid. In some sections quartzitic sandstone is present a short distance below the Mole Vein; this is the Lower Cockshot Rock of the Llynfi Valley (Woodland and Evans 1964, p. 59).

The Mole Vein was known by that name in Trimsaran Colliery where it is thickest, although it only exceeds 24 in exceptionally. It is very thin or absent in much of the Cynheidre area; when the seam is absent the base of the cyclothem is taken at the top of a bastard seatearth or seatearth with sphaerosiderite. Except in three of the Pentremawr boreholes, where the Mole Vein is present it is overlain by a marine band characterized by the presence of horny brachiopods, with Lingula more abundant than Orbiculoidea, in dark grey, micaceous, in places silty, mudstone. The horny brachiopod phase is overlain by mudstone with Lioestheria vinti (Kirkby), which in some sections extends up into the overlying rootlet phase; L. vinti is accompanied by rare mussels, some pyritized, including Anthracosia? and fish scales. This is the Mole Marine Band (Archer in Welch 1957, p. 30), which is thought to be the same horizon as the Hafod Heulog Marine Band in the eastern part of the South Wales Coalfield (Woodland and Evans 1964, p. 59), and is probably equivalent to the Haughton Marine Band of the Pennine province. This marine band is exposed in stream sections near Pontyates, and was exposed in Top Colliery cross-measures drift in Trimsaran Colliery (the type section, now inaccessible).

Where the Mole Marine Band is present there is a thin coal on a seatearth (in the Trimsaran area) or a bastard seatearth (in the Cynheidre area) about 35 to 45 ft above the Mole Vein. This seatearth is overlain by mudstone with a distinctive, although not abundant, mussel fauna which includes cf. Anthraconaia rubida, Anthracosia cf. acutella, A. sp.  nov.cf. acutella, A. cf. planitumida, Curvirimula sp. and Naiadites sp.;Planolites montanus and Lioestheria sp. have also been recorded. The shell referred to A. sp.  nov.cf. acutella is similar to those figured by Trueman and Weir (1951, pl. 18, figs. 33, 34) from a similar horizon in Cefn Coed shafts (Sheet 231); there is some doubt about the identity of the shell listed as cf. Anthraconaia rubida, and it is possible that it should more correctly be referred to Anthracosia. The record of Curvirimula sp. near the top of the Lower similis-pulchra Zone can be matched in the Pennine coalfields (Calver 1956, p. 35), where it occurs between the Haughton and Sutton marine bands.

Where the Mole Marine Band is absent there is only one cyclothem between the Mole and Trimsaran marine bands, and this contains a mussel fauna similar to that described above, including Anthracosia cf. acutella, A. sp.  nov.cf. acutella, A. cf. simulans, Naiadites angustus, N.  aff. alatus [slight posterior wing], N. sp.  (productus group). Some of the shells are pyritized, inviting comparison with the rare mussels found associated with Lioestheria vinti above the Mole Marine Band. Typically Planolites montanus is present, and Lioestheria sp. has been found infrequently.

Where the Mole Marine Band is present and there are two cyclothems between its base and the Trimsaran Marine Band, the strata between the marine bands thicken slightly from about 70 ft in the Cynheidre area to about 80 ft at Trimsaran. Where the Mole Marine Band is absent the cyclothem between the Mole Vein position and Trimsaran Marine Band is about 45 ft thick. Quartzitic sandstone is commonly present a short distance below the Trimsaran Marine Band; this is the Upper Cockshot Rock of the Lynfi and Avan valleys (Woodland and Evans 1964, p. 59).

The coal seam at the base of the Trimsaran Marine Band cyclothem is nowhere more than 6 in thick, and in the five boreholes in the Cynheidre area in which the Mole Marine Band is present this coal is absent. The presence of a marine band above the first coal below the Drap Vein in Trimsaran Colliery was noted by Dix (1928, pp. 496–7), who suggested its correlation with a marine horizon is the Cwmgorse boreholes (near Gwaun-cae-Gurwen, Sheet 230) now known to be the Mole Marine band. The name Trimsaran Marine Band was adopted (Archer in Welch 1957, p. 30) during the resurvey; this band is now known to be present in parts of the eastern half of the South Wales Coalfield, where it is known as the Britannic Marine Band (Woodland and Evans 1964, p. 59), and it is correlated with the Sutton Marine Band of Yorkshire. The dark grey mudstone at the base of the marine band contains coalified plant debris, Lingula mytilloides and Orbiculoidea cf. nitida [generally more abundant than Lingula]. The overlying rather lighter grey mudstone commonly contains horny brachiopods near the base, but is characterized by the presence of Planolites ophthalmoides, commonly abundant and well preserved, locally accompanied by foraminifera. Fish scales have also been recorded, but Dix's record (1928, p. 496) of "Euestheria sp. " in this marine band has not been confirmed in spite of extensive collecting in the Trimsaran area and elsewhere. Goniatites with 'ghost-like' preservation have been collected from several of the Cynheidre boreholes. Quartzitic sandstone, up to nearly 30 ft thick, is present over much of the area. This cyclothem is about 40 to 50 ft thick.

The Drap Vein, like the Graigog and Mole veins, was first named in the Trimsaran area, where it has been worked to a limited extent. It is thickest in this area and has a variable section, but a representative section is: top coal about 7 in separated by seatearth from a bottom coal about 19 in thick; elsewhere the Drap Vein is generally less than 12 in thick. It commonly has a high ash content. Fossils are not common in the roof, although in places they range some distance above the coal; Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, Anthraconaia sp. [juv.], cf. A. rubida [similar to the mussel found below the Trimsaran Marine Band], A. or Anthracosphaerium sp. [small], Anthracosia sp.  cf. ovum, Naiadites cf. obliquus, N. sp. [slight posterior wing], 'Estheria' sp. [not common] and Belinurus sp. have been found. Quartzitic sandstone is normally present in this cyclothem, the maximum thickness recorded being about 40 ft, but it is very thin or absent in the Pentremawr district. It should be noted that the most persistent developments of quartzitic sandstone in the district here described are above the Trimsaran Marine Band, whereas in the western part of the Pontypridd district (Sheet 248) the Cockshot Rocks are below this horizon.

In parts of the eastern half of the district there is a bastard seatearth, seat-earth, or mudstone with rootlets, exceptionally overlain by a very thin coal seam, between the Drap Vein and the base of the Cefn Coed Marine Band cyclothem. Fossils are not common above this terrestrial phase, but Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], and mussel fragments have been seen; an ostracod and 'Estheria' sp. have also been recorded. Sphaerosiderite is commonly present in the seatearth below the Cefn Coed Marine Band; more rarely this marine band is underlain by bastard seatearth. The thickness of the strata between this horizon and the Drap Vein averages about 50 ft.

The Cefn Coed Marine Band, which occurs widely in north-west Europe, is well developed and is characteristically divisible into three parts. The lowest contains horny brachiopods, not uncommonly accompanied by goniatites including Anthracoceras aegiranum H. Schmidt; rarely, foraminifera and fish remains occur in these basal layers. The middle part of the band is represented by the most varied fauna, including corals, crinoid debris, echinoids and calcareous brachiopods, locally preserved in calcite; these fossils occur in a distinctive lithology containing small pyrite concretions and commonly associated with an ankeritic rock, known as 'cank' (Dunham in Edwards and Stubblefield 1948, pp. 251–2). This rich marine phase is overlain by mudstone which commonly contains horny brachiopods near the base, and Lioestheria vinti, abundant in some beds, extending as far as 60 ft above the base of the cyclothem. Worm tracks, including Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], and, more rarely, mussels, have been found in these L. vinti beds, which in parts of the area are overlain by quartzitic sandstone. A higher L. vinti phase, in which there is one record of cf. Anthraconaia adamsii (Salter), is commonly present about 100 ft above the base of the cyclothem, which, averaging about 125 ft, is exceptionally thick; the only cyclothems of comparable thickness in the Middle Coal Measures are those with the Big Vein and Lower Cwmgorse Marine Band at their bases.

Graigog Vein to Carway Fach Vein: details

Pontyberem to Carway

The only complete records of the succession between these seams in this area were obtained from the Pentremawr boreholes; Cynheidre Shaft No. 3, proved all but about 70 ft of strata between the Cefn Coed and Trimsaran marine bands which were faulted out. An abridged log of Pentremawr Borehole No. 3 is given in Appendix I: the only significant variation between the Pentremawr boreholes is that in Borehole No. 2 a coal 25 in thick was proved between the Drap Vein and the Cefn Coed Marine Band. The beds between the Graigog and Carway Fach seams were penetrated by the de-watering borehole [SN 4840 0772] to the Soap Vein in Caepontbren Colliery, an interpretation of which has been given on p. 91. Apart from these sections few details are known of the succession.

In Cynheidre Shaft No. 3 the Graigog Vein was found to be only 11 in thick; the cyclothem is 25 ft thick, the upper half of the strata being described as 'bastard rock (rough sandstone)'. In the Pentremawr area the Graigog Vein has been proved in surface and underground boreholes and in an old trial pit; the normal thickness is probably not more than 10 in. In this area the cyclothem is about 35 ft thick, and consists of silty mudstone with striped beds and thin beds of sandstone near the top; the seatearth at the top of the cyclothem contains sphaerosiderite in some sections. Cochlichnus kochi, cf. Planolites sp. [small], Anthracosia cf. atra [small], A. sp.  (acutella group) and fish remains including Elonichthys sp., Rhabdoderma sp.,cf. Rhadinichthys wardi, Rhizodopsis sp.,and a Platysomid? scale have been collected. S.E. of Pont-Henry an old drift [SN 4835 0935] was probably driven on the Graigog Vein, and the very old workings, primarily for fireclay, at Ynys-hafren Colliery [SN 479 098] on a seam known as the 'Upper Felen', were probably at the same horizon. In a short 'roofing-hole' above the Soap Vein in Carway Colliery the section of the Graigog Vein was proved to be: coal 24 in on silty mudstone 2 in, coal 33 in, mudstone with ironstone nodules 0 to 6 ft 10 in and coal 2 in, the quality of the coal being inferior; silty mudstone in the roof yielded indeterminate mussels with a 'ghostly' preservation and fish scales including Elonichthys sp., Rhizodopsis sp. and a Platysomid. A small area of Graigog Vein was worked in an opencast site [SN 460 063] 350 yd south of Carway School, where the average thickness was about 24 in; Anthracosia atra was obtained from the roof.

The Graigog Rider Vein is 6 in thick in Cynheidre Shaft No. 3, and was thin in the opencast site at Carway and the Pentremawr boreholes, in which the distance between this seam and the Mole Vein is about 90 ft, and a bastard seatearth, not overlain by coal, is present about 50 ft above the Graigog Rider. In Borehole No. 1 there is quartzitic sandstone between this bastard seatearth and the Mole Vein and in all four of the Pentremawr boreholes the seatearth immediately below the Mole Vein is a bastard seatearth in part. Gyrochorte carbonaria, Planolites montanus, P. sp. [2.5 to 5.00 mm], cf. P. sp. [small] and 'solid' mussels, including Anthracosia cf. acutella, A. cf. atra [small], A. cf. concinna and A. cf. fulva [small] have been collected.

The following section is exposed [SN 4704 0758] in the small stream which runs a short distance west of Plasbach Colliery: grey mudstone, silty in parts, with ironstone nodules, 13 ft; on Mole Marine Band: dark grey, micaceous, fissile mudstone with Lingula mytilloides and a Palaeoniscid scale, 4 ft; coal, Mole Vein, 1 ft 8 in; carbonaceous shale with coal streaks, and 'rashes', 4 in; seatearth with ironstone nodules, about 9 ft; mudstone, about 8 ft; striped beds, bedding contorted, 1 ft 6 in; dark grey mudstone with Rhizodopsis sp. [scale], 2 in; striped beds, ferruginous, 1 ft. A marine band exposed in a small stream 240 yd to the S.W. [SN 4688 0744], from which ? burrows [2.5 mm diameter, slightly oblique to bedding], Lingula sp. and Orbiculoidea cf. nitida were obtained, is also believed to be the Mole Marine Band. This marine band is absent in the Pentremawr boreholes, in which a coal seam not more than 9 in thick, present in three of the boreholes, is overlain by mudstone with G. carbonaria, cf. Planolites sp. [small], mussels, some pyritized, including Anthracosia sp.  (ovum group), Naiadites sp.  [juv.] (obliquus group), Lioestheria sp. and an ostracod (in No. 4); sandstone is characteristically present near the top of the cyclothem, which is about 45 ft thick. The Mole Vein is 16 in thick in Cynheidre Shaft No. 3, where the only fossils recorded are plant fragments; in this shaft a seatearth is present 45 ft above the Mole Vein and 20 ft below the Trimsaran Marine Band.

The Trimsaran Marine Band is exposed in Mon Hafren 970 yd S.S.W. of St. John's Church, Pontyberem, and this stream runs at or near this horizon from 120 yd N.E. to 370 yd S.W. of Pont Cwm-bwchle [SN 4831 0900] on the Cynheidre to Pont-Henry road, a typical section being:

High in this cyclothem quartzitic sandstone with impersistent thin beds of mudstone and siltstone, 15 ft thick but wedging out downstream, is exposed in the stream and was worked in small old quarries on the north bank [SN 4811 0890]. The Trimsaran Marine Band was readily recognizable in the Pentremawr boreholes, where the cyclothem was proved to be about 40 ft thick; the coal seam at the base is not more than 6 in thick, sandstone was present in only one of them, and sphaerosiderite was seen in the seatearth at the top in two of the others.

The Drap Vein is persistently thin in the Pentremawr boreholes, the greatest thickness being 7 in; Cochlichnus kochi, G. carbonaria, Planolites montanus, Anthraconaia sp. [juv] Anthracosia sp.  [juv.] (ovum group) and Naiadites sp.  [juv.] (obliquus group) were found in the roof. In Borehole No. 2, 25 in of coal, on seatearth, were recovered 32 ft above the Drap Vein, but the presence of sheared and quartz-veined mudstone in its roof suggests that this is an abnormal section. In all four boreholes a thin coal, seatearth or bastard seatearth, overlain by mudstone with C. kochi, G. carbonaria, P. montanus, P. sp. [small] and ostracods is present between the Drap Vein and the Cefn Coed Marine Band. In two of the boreholes the seatearth immediately below the Cefn Coed Marine Band is a bastard seatearth in part. It should be noted that in the Pentremawr area, in contrast to most of the area described, there is very little quartzitic sandstone between the Trimsaran and Cefn Coed marine bands.

The rich marine phase at the base of the Cefn Coed Marine band cyclothem in Pentremawr Borehole No. 1 is overlain by about 50 ft of mudstone with Lioestheria sp.,the top half of the cyclothem consisting mainly of silty mudstone and striped beds, with one very thin bed of quartzitic sandstone; the Lioestheria phase recognized near the top of this cyclothem elsewhere is represented by mudstone, silty in parts, about 13 ft from the top; cf. Anthraconaia adamsii was collected a short distance below this phase. Dark grey mudstone with Lioestheria sp. exposed in Mon Hafren [SN 4922 0932], [SN 4940 0936] S.S.W. of St. John's Church, Pontyberem, and in a stream [SN 4808 0849] 500 yd S.E. of Ynys-Hafren Farm, is probably in the Cefn Coed Marine Band cyclothem.

South of the Trimsaran Disturbance

The only deep mine in which the Graigog Vein has been worked was Trimsaran Colliery, where the main access was Top Colliery Slant [SN 4665 0527]; the thickness of the seam is recorded as about 39 in, 3 in partings being present 12 in from the top or bottom according to different accounts. The quality of the coal is reported to be inferior. In the Trimsaran Borehole Anthracosia spp.  ['ghostly' preservation], Naiadites alatus? and Rhizodopsis sp. were found about 6 ft above a marine phase with Lingula mytilloides and Palaeoniscid scales, overlying a coal streak believed to be at the Graigog horizon. During subsequent prospecting for, and working of Trimsaran Wood opencast site this correlation was confirmed, a typical section [SN 4634 0507] being:

In other sections the upper half of the cyclothem, which is normally thicker than 33 ft, is more arenaceous than in this section. The Graigog Vein was found to have been worked relatively extensively near the crop, despite its poor quality; there are also old crop workings west of Trimsaran [SN 4482 0447].

The Graigog Rider is not more than 12 in thick. The typical fauna, including Gyrochorte carbonaria, Planolites montanus, Anthraconaia cf. librata, Anthracosia atra, A. cf. atra, A.  aff. atra, A. sp. intermediate atra/barkeri, A. planitumida? and indeterminate fish scales, was found in the Trimsaran Borehole, Trimsaran Wood opencast site and in the cross-measures drift from the Graigog Vein in Top Colliery Slant to the Carway Fawr Vein; in the borehole the mussels ranged up sporadically to c. 20 ft above the seam, at which height 'Estheria' was also found. Quartzitic sandstone is overlain by a rootlet phase about 50 ft above the Graigog Rider and 20 ft below the Mole Vein.

The Mole Vein had the section: coal 16.5 in on shale 0.5 in and coal 2 inches in the Trimsaran Borehole, and varies from 11 to 26 in thick in the old and new Trimsaran brickpits and in Top Colliery cross-measures drift. In this drift the section of the cyclothem is as folows:

In the Trimsaran Borehole the lowest 2 ft 2 in of the roof was found to contain pyrite concretions: in both sections the overlying mudstone contained pyritized structures provisionally identified as 'pyritized burrows', oblique to the bedding. A spine of Listracanthus cf. wardi Woodward was found in the marine band in the borehole. The Mole Vein, 11 to 14 in thick, is the lowest seam exposed in Trimsaran Brickpit [SN 465 050]; L. mytilloides has been obtained from the overlying dark grey, yellow weathering, fissile mudstone. The Trimsaran Marine Band is probably exposed near the top of the brickpit, but was highly weathered where accessible, the strata between the Mole and Trimsaran marine bands, which include a thin coal seam, being largely inaccessible.

The Mole and Trimsaran marine bands are about 80 ft apart in this area, and the cyclothem below the latter has 9 in of coal at its base in the cross-measures drift, and 11 inches in the borehole. Mussels, including cf. Anthraconaia rubida, Anthracosia cf. acutella, A. sp. nov. cf. acutella, Curvirimula sp.,and Naiadites sp. associated with Gyrochorte carbonaria, have been collected from grey mudstone, slightly silty in parts, the mussels being most common between 3 and 10 ft above the seam. In the borehole striped beds and a 5-ft bed of quartzitic sandstone were proved near the top of the cyclothem, but this sandstone is not present in the old Trimsaran Brickpit [SN 467 051].

The coal underlying the Trimsaran Marine Band is very thin; the immediate roof consists of medium-dark grey, slightly silty micaceous mudstone, fissile towards the base, with coalified plants, L. mytilloides [not common] and Orbiculoidea cf. nitida. This facies is overlain by grey mudstone with Planolites ophthalmoides [abundant in some beds, and ranging up to about 20 ft above the coal] and L. mytilloides and foraminifera near the base. Fish scales have been collected, but are rare; Dix's record (1928, p. 496) of "Euestheria sp. " at this horizon is anomalous. Quartzitic sandstone up to 13 ft thick, including partings, is present near the top of the cyclothem in the borehole, the cross-measures drift and the old brickpit.

The Drap Vein has been worked to a very limited extent in Trimsaran Wood opencast site [SN 468 051] and in Drap (or 'Trimsaran Upper') Colliery [SN 4591 0483] and an attempt was made to work this seam off Top Colliery Slant cross-measures drift. The section of the seam is very variable in this small area; typically it consists of two coals, the upper 7 to 18 in thick, the lower 18 to 26 in, separated by a parting 9 to 24 in thick. The ash content is relatively high. The section in Top Colliery Slant cross-measures drift was as follows : coal, 9.5 in on seatearth, sandy, yellow weathering, 1 ft 9 in; quartzitic sandstone, 10 ft 6 in; mudstone, yellow weathering, disturbed, 1 ft 2 in; silty mudstone and mudstone, with rootlets, 4 ft 2 in; quartzitic sandstone, 1 ft 7 in; seatearth, 2 ft 9 in; carbonaceous shale with coal streaks, 5 in; seatearth, 1 ft 2 in; on the Drap Vein, 2 ft (coal, 11 in on parting, 9 in; on coal (presumably disturbed) 4 in). The presence of thick beds of quartzitic sandstone, which sometimes form the immediate roof of the seam, is characteristic of this cyclothem in this district; this sandstone has been quarried [SN 4590 0480] near Drap Colliery. West of Trimsaran there are a few old crop workings on the Drap Vein as far as Bryn Hwthan [SN 4411 0395]., The only fossils seen were plants, including Sigillaria sp.

The drilling of the Trimsaran Borehole started 130 ft above the thin coal seam at the base of the Cefn Coed Marine Band cyclothem: below 3.75 ft of drift and about 3 ft of rock which was not cored, unfossiliferous very silty mudstone and silty mudstone was proved to a depth of 60 ft 8 in. The underlying 60 ft of mudstone, mostly slightly darker than the overlying strata, contained scattered Lioestheria vinti throughout; rare fish scales and one specimen of cf. Planolites sp. [small] were also found.

About 7 ft dark grey, smooth, fissile mudstone, slightly micaceous in parts towards the base, with Lioestheria vinti and rare foraminifera? and Lingula ?overlay the marine band, which had two distinct phases, the higher, 1 ft 2 in thick, consisting of blocky grey mudstone with scattered small pyrite concretions and abundant fossils, some preserved in calcite, overlying a lower phase of 2 ft 10 in of dark grey mudstone, micaceous and fissile in part, with Lingula, Orbiculoidea and a spine of Listracanthus. Fossils collected from the rich faunal phase in the Top Colliery cross-measures drift include Zaphrentites postuma (S. Smith), Archaeocidaris  aff. acanthifera Trautschold, crinoid columnals, Chonetinella cf. flemingi (Norwood and Pratten) crassiradiata (Dunbar and Condra), Lingula sp. [juv.], Orbiculoidea cf. nitida, Productus cf. carbonarius de Koninck, Rugosochonetes? skipseyi (Currie), Tornquistia diminuta (Demanet), Straparollus (S.) sp. nov., Politoceras politum (Shumard), Hollinella cf. bassleri, and Rhizodopsis sp. [scale]. The total thickness of this cyclothem in the cross-measures drift is about 140 ft; 2 ft of sandstone are present near the top.

Cynheidre area

The thickest sections proved of the Graigog Vein were in Cynheidre Shafts Nos. 1 and 2, 22 in and 25 in respectively; apart from boreholes Nos. 3/1 (15 in), 3/2 (19 in), possibly Gwendraeth Valley No. 1C (18 in ?, but only 6 in in No. 1B) and a disturbed section in No. 6/1, this seam has nowhere been proved in the boreholes to be more than 10 in thick. In the only drivage to intersect this seam, Cross-measures No. 5 (490), the thickness varied from 6 to 12 in. The abridged log of this cyclothem in Gwendraeth Valley Borehole No. 2 is as follows:

This section may be taken as typical for this area: in some of the sections fine-grained quartzitic sandstone (up to 12 ft thick in Borehole No. 5/1) was present near the top of the cyclothem. Although beds of ironstone are usually present, the bed at 1794 ft 11 in is abnormally thick. The mussels have the typical 'ghost-like' preservation characteristic of this horizon, but the thickness of barren strata in the roof of the seam is unusually great. The thickness of the cyclothem varies from about 35 ft to 50 ft. Other fossils found include Anthracosia cf. aquilinoides, A. cf. fulva, Anthracosphaerium?, Naiadites alatus, Rhadinichthys sp., Rhizodopsis sp.,and indeterminate scales of Palaeoniscids and Platysomid ?. Cochlichnus kochi, Gyrochorte carbonaria, and Planolites sp. [small] are commonly present. An unusual fish scale found in Gwendraeth Valley Borehole No. 2 was also found at this horizon in Cynheidre Shaft No. 1 and in Wemos Shaft (near Ammanford, Sheet 230).

There is some evidence that the Graigog Rider Vein is thicker in the western part of the Cynheidre area, where a thickness of 10 in was found in Shafts Nos. 1 and 2 and Borehole No. 3/1; elsewhere, including Cross-measures No. 5 (490) where this was probably the highest coal seam encountered in all the drivages, it is very thin. The typical fauna, ranging up to as much as about 20 ft above the seam and consisting of small 'solid' mussels, including Anthracosia atra, A. cf. acutella, A. sp. intermediate atra/barkeri, ?A. cf. simulans, and Naiadites sp. was found in most of the sections. Gyrochorte carbonaria, Cochlichnus kochi and Planolites sp. [2.5 to 7.0 mm diam.] are usually present; 'Estheria' and fish remains, including Rhizodopsis sp. and Palaeoniscid scales, are not uncommon, and Planolites montanus has also been recorded.

In the S.W. part of the Cynheidre area, that is in boreholes Nos. 3/1, 3/2, 4/3 and Gwendraeth Valley No. 2, and in Shafts Nos. 1 and 2, a bastard seatearth or rootlet phase is present between the Graigog Rider and the Mole Vein position. The only fossil recorded at this horizon is a Belinurid found in Gwendraeth Valley Borehole No. 2. A bastard seatearth, or a seatearth containing sphaerosiderite, is present immediately below the Mole Vein position in the sections where this seam is absent; sandstone is rarely present a short distance below this seatearth. The average distance between the Graigog Rider and the Mole Vein is about 65 ft irrespective of the presence or absence of an intervening terrestrial phase.

The Mole Vein has been proved only in boreholes Nos. 3/2 (coal and dirt 14 in), 4/3 (coal and dirt 16 in), 6/1 (10 in), 6/3 (16 in) and Gwendraeth Valley No. 2 (1 in) on the south and east sides of the Cynheidre area. The Mole Marine Band is present only in the same boreholes. The marine band consists of up to about 12 ft of dark grey micaceous mudstone, slightly silty in parts, with scattered pyrite concretions and pyrite-filled burrows, Lingula mytilloides, Orbiculoidea cf. nitida and fish scales including Rhabdoderma sp. and Rhadinichthys sp. This marine phase is overlain by up to about 13 ft of lighter grey mudstone with Lioestheria vinti extending up into a rootlet phase. In some sections there are about 6 ft of unfossiliferous silty mudstone between the marine and Lioestheria phases; the latter phase has yielded poorly preserved mussels, some pyritized, including Anthracosia?, and fish scales including Rhadinichthys sp. In the five boreholes in which the Mole Vein and Marine Band were found, the strata between the Mole and Trimsaran marine bands, which are about 70 ft apart, are divided into two cyclothems by a bastard seatearth, or seatearth with sphaerosiderite, about 35 ft above the Mole Vein : this seatearth is not seen in the other nine borehole and shaft sections in the area. The higher of these two cyclothems has yielded Calamites schutzeiformis Kidston and Jongmans, Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, cf. P. sp. [small], cf. Anthraconaia rubida, Anthracosia sp.  (concinna/acutella group), A. cf. planitumida, Curvirimula sp. [juv.], Naiadites cf. angustus, N. sp.  (obliquus group), Lioestheria sp.,and a fragment of a Belinurid telson. The succession in Gwendraeth Valley Borehole No. 2 is unique in that there is a 1-in coal at the base of this upper cyclothem, which is separated from the Mole Vein cyclothem by 8 ft 8 in of 'normal' seatearth and mudstone with rootlets, other plant fragments and Lioestheria sp.

In those sections where the Mole Vein and Marine Band are absent, the one cyclothem between the Mole Vein position and the Trimsaran Marine Band is about 50 ft thick. Fossils, including Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, cf. P. sp. [small], Anthracosia cf. acutella, A. sp.  nov. cf. acutella [cf. Trueman and Weir 1951, pl. 18, figs. 33, 34], A. cf. simulans, Curvirimula sp., Naiadites angustus, N.  aff. alatus [slight posterior wing], N. sp.  (productus group), Lioestheria sp. [in No. 3/1 and No. 3/2], ostracods [in No. 5/2; pyritized, crushed], a Belinurid [one specimen], and fish fragments including a Palaeoniscid scale, normally occur sporadically up to about 25 ft above the base of the cyclothem. P. montanus is recorded in most of the sections and is one of the characteristics of these strata. In nearly all these sections some of the mussels are pyritized, occurring in some of them in dark grey ferruginous mudstone (cf. 'blackband' ironstone). Fine-grained quartzitic sandstone is commonly present near the top of the cyclothem, the maximum recorded thickness being about 17 ft in Shaft No. 2 and Borehole No. 3/1, in the western part of the area. No seatearth is present below the Trimsaran Marine Band in Borehole No. 4/3 and the top of the cyclothem is taken at a sharp junction between silty mudstone with plant debris and an overlying 3-in bed of canneloid shale (with Rhabdoderma sp. ) at the base of the marine band. Mudstone with rootlets, rather than a seatearth, is present at this horizon in Borehole No. 6/3, in which it is also overlain by a thin bed of canneloid shale, and in Gwendraeth Valley Borehole No. 1.

Where a coal is present at the base of the Trimsaran Marine Band cyclothem, it is nowhere thicker than 5 in; the only boreholes in which no coal was found are the five in which the Mole Vein is present (excluding Borehole No. 5/2 where there was a fault at this horizon); in Borehole No. 3/2 this seam is represented by 3 in of cannel underlain by seatearth. Typically the Trimsaran Marine Band in the Cynheidre area consists of about 2 to 3 ft of dark grey finely micaceous mudstone with pyrite concretions, and coalified plant debris (in the basal few inches). Orbiculoidea cf. nitida is the commonest fossil in this phase, but Lingula mytilloides is normally present, in places more abundant near the base; goniatites, with a characteristic 'ghost-like' preservation, have been found in seven of the boreholes, and fish remains, including Rhadinichthys sp. and Palaeoniscid scales, are not rare. This phase is overlain by 10 to 15 ft of grey mudstone with Planolites ophthalmoides [well preserved and abundant in some beds]; Lingula sp. [locally in 'nests' of several specimens together] and foraminifera, including Glomospira sp.,are distributed very sporadically in the Planolites phase. In Gwendraeth Valley Borehole No. 2 there is an unique record of Naiadites sp. [small] between the Orbiculoidea and Planolites phases. Thin beds of fine-grained quartzitic sandstone are present in some sections, but in boreholes Nos. 3/1 and 3/2 sandstones 28 and 24 ft thick respectively replace most of the Planolites phase. The thickness of the cyclothem is unusually constant, the range in 13 sections, not corrected for dip, being 43 to 54 ft.

In over half of the sections in the Cynheidre area there is only one cyclothem, with the Drap Vein at its base, between the Trimsaran and Cefn Coed marine band cyclothems, but in boreholes Nos. 3/1, 3/2, 4/2, 4/3, 5/1 and Gwendraeth Valley No. 1B (but not recorded in the adjacent 1C) there is a rootlet phase between the Drap Vein and the seatearth below the Cefn Coed Marine Band. The Drap Vein is thin in the Cynheidre area, the thickest sections being in Gwendraeth Valley No. 1B (12 in) and No. 2 (15 in). Fossils are usually not common in the grey mudstone, silty in part, above the Drap Vein, although in some sections they extend up to as much as 25 ft above the coal: the fossils collected include Cochlichnus kochi, Gyrochorte carbonaria, Planolites montanus, cf. Anthraconaia rubida, A. sp. [juv], A. or Anthracosphaerium sp. [small], Anthracosia sp. cf. ovum, Naiadites cf. obliquus, N. sp. [slight posterior wing], 'Estheria' sp. [not common] and Belinurus sp. [long telson]. Fine-grained quartzitic sandstone commonly constitutes an important part of the total thickness of this cyclothem; for example, 40 ft are present in Shaft No. 1 and 36 ft in Borehole No. 6/1, in both cases the sandstone being interbedded with striped beds. In the six boreholes in which the Drap Vein cyclothem is interrupted, the position of the extra rootlet phase varies from the immediate roof of the Drap Vein, in No. 5/1, to 41 ft above that seam in Gwendraeth Valley No. 1B where seatearth is overlain by 6 in of coal. The strata above this rootlet phase seldom contain fossils other than plant debris, but mussel fragments were seen in Borehole No. 5/1, and G. carbonaria, cf. Planolites sp. [small] and 'Estheria' sp. in Borehole No. 4/3. The distance between the Drap Vein and the base of the Cefn Coed cyclothem averages a little less than 50 ft.

Normally the base of the Cefn Coed Marine Band cyclothem is marked by a thin coal seam. The marine band has been found in all the sections in the Cynheidre area; the basal part consists typically of about 2 ft of dark grey finely micaceous mudstone with horny brachiopods, foraminifera (in some sections only) and goniatites (in several sections), Planolites sp. and fish remains being rare. This phase is overlain by the characteristic very fossiliferous phase which varies from only 1 in to about 3 ft thick; typically it is about 9 in thick. Closely associated with this phase in most sections there is a bed of 'milk' up to 8 in thick, which is poorly fossiliferous. The overlying dark grey micaceous mudstone commonly contains horny brachiopods up to about 1 ft above its base, above which Lioestheria vinti becomes the principal fossil in mudstone becoming lighter grey and slightly silty in parts. L. vinti is generally present, scattered, but abundant on some bedding planes, up to about 30 ft and more sporadically as far as about 60 ft above the base of the cyclothem. Cochlichnus kochi, Gyrochorte carbonaria and cf. Planolites sp. [small], are usually present in these strata which also contain rare mussels in four of the boreholes on the S.E. side of the area (Gwendraeth Valley No. 2; Cynheidre Nos. 3/2, 4/3 and 6/3). Quartzitic sandstone up to about 25 ft thick, including partings, is usually present below another L. vinti phase about 90 to 100 ft above the base of the cyclothem; these L. vinti beds yielded mussels in Borehole No. 4/3. Further thin beds of sandstone are sometimes present near the top of the cyclothem. The total thickness of the cyclothem is about 125 ft.

Carway Fach Vein to top of Upper Cwmgorse Marine Band: general

The Carway Fach Vein is not more than about 21 in thick from Pontyberem to Trimsaran and in the Cynheidre area, but there is some evidence of this seam thickening to about 30 in west of Trimsaran. Although the overlying strata are poorly fossiliferous, Lioestheria vinti is usually present, and there are records of worm tracks including Cochlichnus kochi, Gyrochorte carbonaria and cf. Planolites sp. [small]; there is one record of Anthraconaia sp.  (?pulchra (Hind) group).

The only Middle Coal Measures seam above the Soap Vein which has been worked at all extensively in this area is the Carway Fawr. The correlation of the Waunffynnonau seam of the Trimsaran area with the Carway Fawr, suggested by Strahan and others (1909, p. 134) and Dix (1928, p. 504) has been confirmed, but the tentative correlation of this seam with the Red Vein (Strahan and others 1907, pp. 92, 134; Jordan 1910, p. 42), questioned by Dix (1928, pp. 471, 504), has been proved untenable. The Carway Fawr is now known to split to the east and is represented by two thin seams lying over 300 ft below the Red Vein in the Ammanford area (Sheet 230). East of Pontyberem the Carway Fawr is less than 24 in thick, and includes a parting in Cynheidre Shaft No. 3, but from Gwendraeth Colliery, between Pontyates and Carway, to Trimsaran it is 36 to 45 in thick, and in the Cynheidre area it is about 35 in thick. Foraminifera were found in the roof of this seam in Gwendraeth Valley Borehole No. 1B by Mr. J. V. Stephens, and have subsequently been found elsewhere: these beds have been named the Carway Fawr Marine Band (Archer 1959, p. 27). This band is known nowhere else in Great Britain and its distribution in the area here described is irregular; foraminifera are the only marine fossils found. Other than foraminifera, where these are present, and worm tracks including C. kochi, G. carbonaria and Planolites spp.,fossils are not common above the Carway Fawr, but there are isolated records of Anthraconaia adamsii, A. pulchra, A. cf. warei (Dix and Trueman), Naiadites cf. hindi Trueman and Weir, 'Estheria' sp., Leaia sp.,ostracods, a Belinurid, and fish remains. Pennant-like sandstone, up to about 30 ft thick, is sometimes present in this cyclothem, and forms the immediate roof of the coal in parts of the Trimsaran area.

Four other cyclothems can be recognized between the Carway Fawr Vein and the Foraminifera Marine Band. Large mussels, Anthraconaia spp. of the adamsii group, have been found at three horizons in addition to the two described above. In the Pontypridd area (Sheet 248) a similar fauna is present at the three horizons in the equivalent beds (Woodland and Evans 1964, pp. 26, 62). Other fossils found in these strata include C. kochi, G. carbonaria, Naiadites sp. [juv.], and, more rarely, Leaia sp., Lioestheria vinti, ostracods and fish remains. Sandstone is commonly present but the only thick bed proved was in Cynheidre Borehole No. 5/1, where over 80 ft of sandstone was proved in one cyclothem, although there was none in Borehole No. 4/1 and only 12.5 ft in Borehole No. 5/2, the two nearest boreholes, less than mile away.

The Foraminifera Marine Band (Woodland and Evans 1964, p. 62) probably occurs throughout the area. A non-marine phase indicated by the presence of Naiadites melvillei Trueman and Weir commonly occurs between the thin coal at the base of the cyclothem and the marine band, in which foraminifera are usually abundant. Lingula mytilloides is not common and there is one record of a Nuculid lamellibranch. Geisina subarcuata (Jones) is present a short distance above the basal foraminifera phase, accompanied by Planolites ophthalmoides which is also seen higher in the cyclothem in some sections. There are records of other marine fossils near the middle of the cyclothem in a few of the Cynheidre sections.

The Five Roads Marine Band was first found in a stream section near the Five Roads to Pontyates road and its stratigraphical position was confirmed in Gwendraeth Valley Borehole No. 2 (Archer in Eyles 1952, p. 26; Archer in Eyles 1956, p. 34). Although a rootlet phase is not always present below the Five Roads Marine Band, the potential development of a separate cyclothem is usually apparent from the increase in grain size of the sediments above the Foraminifera Marine Band, and the recognition of a separate marine horizon is further justified by the distance between them, which averages about 60 ft. This marine band has a distinctive fauna, dominated by abundant lamellibranchs; the assemblage includes Bucanopsis sp., Euphemites anthracinus (Weir), Anthraconaia   aff. pruvosti (Weir and Leitch non Chernyshev), cf. Edmondia goldfussi, E.   aff. transversa, Myalina compressa, Geisina subarcuata [abundant near the top], Hollinella sp. and fish remains. Naiadites sp. has rarely been found in the top of the band. In the easternmost part of the area described quartzitic sandstone is present at this horizon, and a separate cyclothem cannot be recognized.

The two cyclothems which are present between the top of the Five Roads Marine Band cyclothem and the Red Vein are dominantly argillaceous but poorly fossiliferous.

The Red Vein has been mined extensively in the Ammanford area but splits into two coals south-east of that town (Strahan and others 1907, p. 116). In the Gwendraeth Valley the seam lying about 80 ft below the Lower Cwmgorse Marine Band, and worked to a very limited extent as the Cwm Cobbler or Golden Vein, is believed to be the bottom split of the Red Vein, which is thicker than the top split; it is shown on the maps as the Red Vein. This seam is probably not more than 18 in thick, except in the Trimsaran area where it is said to thicken to 24 in. The fauna above this seam is characterized by the presence of Lioestheria vinti, which extends up to 20 ft above the coal; apart from G. carbonaria, other fossils are uncommon, but include C. kochi, Planolites sp., Anthraconaia sp. [juv., one record], Naiadites sp.  [juv.] Geisina subarcuata [in three of the boreholes in the eastern part of the Cynheidre area] and fish remains.

The top split of the Red Vein is thin; its roof contains a fauna similar to that of the underlying coal, except that L. vinti is not present at all places, and ostracods have not been seen. In the most easterly section seen, Cynheidre Borehole No. 6/3, apparently undisturbed, L. vinti and ostracods were collected above coal streaks on a seatearth between this coal and the Red Vein.

The Lower Cwmgorse Marine Band is well developed and the cyclothem which includes it about 120 ft thick. The main marine phase is dominated by Dunbarella macgregori (Currie), abundant in some beds, but accompanied by other marine lamellibranchs, in addition to Campylites sp.,sponge spicules, horny brachiopods, gastropods, nautiloid fragments and ostracods. Between this phase and the immediate roof of the very thin coal at the base of the cyclothem, mudstone with pyrite concretions, and which usually yields Planolites ophthalmoides and sporadic marine fossils sometimes including foraminifera, rests on a non-marine phase which contains Lioestheria vinti and Geisina subarcuata, exceptionally accompanied by Naiadites cf. daviesi Dix and Trueman. The strata above the main marine phase are commonly characterized by the presence of cf. Tomaculum sp. and scattered marine lamellibranchs. Quartzitic sandstone is usually present near the top of the cyclothem.

The first coal seam above the Lower Cwmgorse Marine Band, the Lower Welsh Vein (Trotter 1947, p. 92; Welsh Vein of earlier authors, e.g. Davies and others 1928) is probably not more than about 15 in thick over most of the area, perhaps thickening to about 20 in near Trimsaran, where it has been mined at outcrop as the Mountain Vein. Cochlichnus kochi and Gyrochorte carbonaria [common], locally accompanied by poorly preserved mussels, Geisina subarcuata and fish remains, have been found above this seam. A similar fauna, but with Gyrochorte carbonaria and Geisina subarcuata more abundant, occurs in the highest complete cyclothem in the Middle Coal Measures.

The Upper Cwmgorse Marine Band is readily recognizable. The principal marine phase, characteristic of this horizon, contains sponge spicules, crinoid columnals, horny brachiopods and Productids, lamellibranchs, gastropods, nautiloids, goniatites including Anthracoceras cambriense Bisat and Politoceras kitchini (Bisat), ostracods and fish remains. These beds are overlain by mudstone with a more restricted marine fauna, including Planolites ophthalmoides and Campylites sp.,which gives way to a non-marine phase from which Cochlichnus kochi, Gyrochorte carbonaria, Curvirimula sp., Naiadites melvillei, Geisina subarcuata, 'Estheria' sp. and fish scales have been collected. The top of the Middle Coal Measures is taken at the top of this marine band (Stubblefield and Trotter 1957, pp. 2–3); although fully justified for the practical purpose of portraying the boundary between Middle and Upper Coal Measures on the maps, the division in the district here described is somewhat arbitrary when any specific section is considered in detail.

Carway Fach Vein to top of Upper Cwmgorse Marine Band: details

Pontyberem to Carway

The Carway Fach Vein was proved to be thin, about 15 to 21 in, and to be about 50 ft below the Carway Fawr Vein, in Cynheidre Shaft No. 3, Pentremawr Borehole No. 1, and during prospecting on Tyn-y-cwm [SN 494 092] and Hafren [485 085, 487 088] opencast sites. This seam has been worked to a very limited extent; the main slant of Gwendraeth Colliery [SN 4724 0731], about 0.75 mile N.E. of Carway, was driven in the Carway Fach for 65 yd and a small amount of this seam was also worked. Nearer Carway a trial borehole [SN 4661 0610] proved this seam to be 15 in thick. In Pentremawr Borehole No. 1 thin beds of quartzitic sandstone, with contorted bedding in part, were proved about 20 ft above the seam and sporadic Gyrochorte carbonaria and Lioestheria vinti were collected from the lower part of the cyclothem.

The Carway Fawr Vein in Cynheidre No. 3 Shaft has the section: coal 14 in on 'rashes' 8 in and coal 6 in, the roof being recorded as silty mudstone with plant fragments. The outcrop of the Carway Fawr between the Lletty Wilws and Coalbrook faults has been prospected for opencast coal, but the seam was found to average less than 24 in thick; analyses of thicker, and probably disturbed, sections showed a high ash content. A recent attempt to work this seam in a small mine near Marchogllwyn Farm [SN 5063 1004], west of the Coalbrook Fault, was unsuccessful. The Carway Fawr Vein, 14 in thick, at a depth of 23 ft, is overlain by the Carway Fawr Marine Band, with foraminifera, in Pentremawr Borehole No. 1. Between this borehole and the Cynheidre to Pontyates road, this seam was worked in the old Ty'n-y-cwm [SN 5005 0956], Ty'n-y-wern [SN 4898 0898] and Ffou [SN 4818 0812] collieries, and, since the resurvey was completed, in Ty'n-y-cwm and Hafren opencast sites. In these sites the thickness of the coal varied considerably, but was assessed at 24 in, except in the western part of Ty'n-y-cwm, where it was assessed at 45 in; in view of the structural complication proved on these sites it is clear that this was a structural, not depositional, variation. Thin beds of sandstone constituted an unimportant part of the overlying strata, which were essentially argillaceous. Detailed examination of the grey mudstone in the roof demonstrated that the distribution of the Carway Fawr Marine Band is very patchy. In the eastern site at Hafren [SN 487 088] foraminifera were abundant throughout the lowest 4 ft 10 in of the roof of the seam and were also present, associated with plant debris, up to 3 ft above the same seam where repeated by an overthrust at the S.W. end of the site: a specimen from 3 ft above the seam contains foraminifera on one side and Anthraconaia sp.  (warei group) on the other. Foraminifera, again associated with plant debris, were also present in an isolated exposure at the S.W. end of Ty'n-y-cwm site, but elsewhere on this site, and in the whole of the western site at Hafren [SN 485 085] no foraminifera were found after washing continuous samples collected up to 7 ft above the seam. No transition from marine to non-marine strata was exposed at the time the sites were examined; there is no macroscopic difference between the mudstones. In Ffou Colliery, Strahan and others (1909, p. 118) recorded a parting in the Carway Fawr Vein.

South-west of the Cynheidre to Pontyates road strata a short distance above the Carway Fawr Vein are exposed in Cwm Cobbler (a stream not so named on current maps), and an adjacent quarry [SN 4787 0763]; 18 ft of sandstone with ferruginous concretions are separated by an unexposed gap in the section of about 8 ft from a further 1 ft 3 in of sandstone which rests on mudstone 3 or 4 ft above the seam position. 550 yd to the S.W. striped beds and the same sandstone, 20 ft thick, are exposed in another stream [SN 4744 0739]. The Carway Fawr Vein was worked extensively in Gwendraeth Colliery [SN 4724 0731], where its thickness was said to be 39 to 45 in. Dix (1928, pp. 471–2) found plant remains accompanied by "Naiadites cf. triangularis (J. de C. Sow.), Anthracomya sp.  nov., Beyrichia sp. and a fish scale and tooth" in the roof of this seam in this colliery. Nearer Carway there are crop workings on both sides of Ty-mawr Farm [SN 4685 0640] and opencast prospecting proved the immediate roof to be sandstone in part of the area. South of a sharp fold, possibly faulted in part, the Carway Fawr has been mined on Morlais opencast site [SN 460 057] where the average thickness of the coal was assessed at 45 in. Grey mudstone 2 to 6 in above the seam contained foraminifera including Ammonema sp.;sandstone up to at least 30 ft thick was proved not less than about 15 ft above the seam.

In addition to the relatively good stream section in the Ffou Valley, not so named on the maps, but which runs roughly parallel to, and a short distance N.E. of, the Cynheidre to Pontyates road, there are poor sections of the strata between the Carway Fawr Vein and the Upper Cwmgorse Marine Band in several small streams, tributaries to the Hafren and Gwendraeth.

Naiadites sp. [jay.] was collected from dark grey mudstone between the Carway Fawr and Red veins exposed [SN 4793 0757] in Cwm Cobbler. The Foraminifera Marine Band is exposed in the Ffou Valley [SN 4850 0788], where it contains Ammonema sp. and Glomospira sp.,and fish remains. The Five Roads Marine Band is well exposed [SN 4772 0723] under the lower of several very small waterfalls in a stream immediately north of the minor road from Five Roads to Pontyates: 4 ft 10 in of mudstone, silty in parts, yielded Bucanopsis sp., Euphemites?, Anthraconaia   aff. pruvosti (Weir and Leitch non Chernyshev), Aviculopecten sp.,cf. Edmondia goldfussi, E.   aff.transversa, Myalina compressa, Hollinella sp. and Rhabdoderma sp. [scale]. This marine band contains a similar fauna where exposed in the Ffou Valley [SN 4853 0782] and in the roof of an old trial level [SN 4950 0864] near a small stream 1400 yd to the N.E.: at the latter locality Naiadites sp. [including juv.] and Geisina subarcuata [common] were also collected.

The Red (Cwm Cobbler or William Edmond's Little) Vein, has been worked near the crop on both sides of the Cynheidre to Pontyates road. The only surviving plans are of very limited workings in Tygwyn Colliery, in the Ffou Valley [SN 4857 0772], abandoned in 1930, where the seam thickness is recorded as 18 in. This is in accord with the thickness of 15 to 18 in quoted by Strahan and others (1909, p. 120). Naiadites sp. [juv.] and Lioestheria vinti were collected from faulted mudstone a short distance above the Red Vein position in the Ffou Valley [SN 4865 0775] and a similar fauna was also found in a comparable position in Cwm Cobbler, where also there are old drifts on this seam. The outcrop can be followed for about 4 mile to the S.W., where a spring [SN 4746 0675] is said to mark the site of an old level.

The Lower Cwmgorse Marine Band is exposed [SN 5288 1061] in a stream about 180 yd E. of Cynheidre Shaft 3, in two streams [SN 5085 0961], [SN 5065 0944] about 1.667 mile to the S.W., and in the Ffou Valley [SN 4888 0767]. Fine-grained quartzitic sandstone near the top of this cyclothem is exposed to 30 ft at the last two localities, in other stream sections and in a cutting on the Llanelly and Mynydd Mawr Railway [SN 5144 1014]; this sandstone has been quarried [SN 4826 0744] 0.5 mile W.N.W. of Cynheidre crossroads. Dunbarella cf. macgregori and D.   aff.macgregori [7 ribs in left anterior ear] have been found in the marine band.

Mudstone with Geisina subarcuata, overlying a thin coal smut in a ditch [SN 5089 0450], about 1 mile S.E. of Pontyberem, probably lies between the Lower Welsh Vein and the Upper Cwmgorse Marine Band.

The Upper Cwmgorse Marine Band is exposed S.E. of Pontyberem in two stream sections [SN 5222 1027], [SN 5070 0932] near the Llanelly and Mynydd Mawr railway, and in a cutting [SN 5143 1003] on that railway. The section exposed in the first stream, which includes part of the cyclothem below the marine band, is as follows:

Only Lingula mytilloides was collected from the other exposures.

South of the Trimsaran Disturbance

Little is known of the Carway Fach Vein in this district, the only section seen being in the Top Colliery cross-measures drift, where this seam is 12 in thick, the bottom 3 in being inferior coal. The immediate roof is a bed of mudstone 10 in thick, the remaining 55 ft of the cyclothem consisting of silty mudstone. Nothing is known of the Carway Fach between this drift and Waun-Baglam [SN 4424 0381], nearly 1 mile S.W. of Trimsaran: from Waun-Baglam to 300 yd W.S.W. of Bryndias Farm [SN 4355 0340] there are old workings on two seams. No record is known of the thickness of the upper of these seams, but opencast prospecting near Waun-Baglam suggests that the lower seam is at least 30 in thick, and the same seam was worked (as the Waunffynnonau) in Bryndias Colliery, where its thickness is recorded on the Abandonment Plan as 44 in. Strahan and others (1909, p. 130) also identified this lower seam as the Waunffynnonau, but did not comment on the upper seam although it is shown on their map. Neither of the seams, nor their immediate roofs were exposed during the resurvey. In the absence of any evidence of a split in the Carway Fawr Vein, or that the normally very thin seam next above the Carway Fawr has thickened, it is as likely that the Carway Fach has thickened to the south-west, and this interpretation is shown on the map.

The Carway Fawr (Waunffynnonau) Vein, about 36 to 42 in thick, has been worked more extensively than any other seam in the Trimsaran district, from Trimsaran colliery slants [SN 4580 0456], [SN 4597 0470] and the Top Colliery cross-measures drift. In much of the eastern part of the workings the roof was sandstone, while in the western part mudstone, up to at least 5 ft thick, was commonly present. A small area of this seam was worked in Trimsaran Wood opencast site [SN 471 050], where its thickness averaged about 42 in, and mudstone was usually present between the coal and the overlying sandstone. Further west small areas of this seam were worked in Caenant [SN 4559 0446] and Star [SN 4526 0435] collieries; in the latter Fairley (c. 1868, p. 6) records the thickness as 39 to 42 in. A disturbed section consisting of fissile mudstone 7 ft, on coal 30 in, below 5 ft of boulder clay, was exposed at the mouth of a trial slant [SN 4501 0420] 300 yd S.W. of Star Colliery. Strahan and others (1909, p. 133) described a trial shaft [SN 4204 0325] nearly 1 mile west of Bryndias Farm, which proved a seam 24 to 48 in or 33 to 60 in thick, according to different accounts, at a depth of 120 ft; this seam "was supposed to be the Waun-ffynnonau Vein". E. E. L. Dixon (MS.) noted a borehole [SN 4207 0314], 125 yd S.S.W. of this trial shaft, which is said to have proved the same seam at 204 ft and in 1953 an opencast prospecting borehole [SN 4203 0329] was drilled 50 yd N.N.W. of the shaft and proved 42 in of coal at a depth of 113 ft. It is possible that both boreholes and the trial shaft proved the Carway Fawr Vein.

The pennant-like sandstone overlying the Carway Fawr Vein is exposed in the stream [SN 4720 0507] 150 yd N. of Cae-ffaldre Farm, where the strike swings to a northerly direction, a reflection of the proximity of the Trimsaran Disturbance. Cross-bedded sandstone is also exposed in two old quarries, 250 yd S.E. [SN 4681 0483] and 600 yd S.W. [SN 4610 0470] of Plas Trimsaran: in the stream adjacent to the former quarry, silty mudstone and mudstone with ironstone nodules are exposed between the seam and the sandstone. In the latter quarry 23 ft of sandstone, flaggy especially towards the top, include large masses of weathered ferruginous sandstone. Sandstone overlying the higher of the two seams worked near Bryndias Farm is poorly exposed in an old quarry [SN 4355 0345], in which Dixon (MS.) recorded a fault hading S.60°E. at 45°. There is some evidence (Strahan and others 1909, p. 131) that this sandstone thins under the largely drift covered area between Trimsaran and Waun-Baglam.

The strata between the Carway Fawr Vein and the Upper Cwmgorse Marine Band are very poorly exposed in this area. A seam about 400 ft above the Carway Fawr has been worked extensively at its outcrop as the Golden (Lower Mountain or William's) Vein. As this distance is closely comparable with the distance between the Carway Fawr and Red veins, and its thickness (reported to be 24 in by Strahan and others 1909, p. 131) is similar, this seam is correlated with the Red Vein. The only palaeontological support for this correlation is the doubtful identification of poorly preserved specimens collected from an old tip [SN 454 042] south of Trimsaran village, as 'Estherid ? and Hollinella? or Geisina?; the presence of 'Estheria' and Geisina would be consistent with this correlation.

For similar reasons the Mountain (or Upper Mountain) Vein, about 250 ft above the Golden Vein, is correlated with the Lower Welsh Vein. This seam has also been extensively worked at outcrop, and to a limited extent down dip, for example in Pen-y-mynydd [SN 4576 0414] and Waun-y-clyn [SN 4540 0399] collieries. In the latter the seam is 19 to 21 in thick and mudstone from the roof yielded megaspores, plant fragments including Lepidodendron sp.,Palaeolimnadiopsis pruvosti and fish remains including a Palaeoniscid scale and Rhabdoderma sp.

Cynheidre area

The thickness of the Carway Fach Vein varies from only 5 in (in two sections) to 19 in (in four sections), and 21 inches in one borehole. The fauna in the mudstone overlying this seam includes Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], Anthraconaia sp.  (?pulchra group) [one specimen] and Lioestheria vinti; the latter fossil is characteristic of this horizon, although never abundant. There is one record of Rhadinichthys sp. This cyclothem is about 40 ft thick.

The stratigraphy of the 250 ft or so of strata between the Carway Fawr Vein and the Foraminifera Marine Band has been found to be complex, with considerable variations within the Cynheidre area. The essential details of these variations are shown diagramatically in (Figure 17). Between the Carway Fawr and the seatearth immediately below the Foraminifera Marine Band there are four terrestial phases which appear to be persistent, so that five cyclothems can always be recognized.

The average thickness of the Carway Fawr Vein is 35 in, if the anomalous thicknesses, i.e. below 24 in or above 42 in, in four sections are excluded. The Carway Fawr Marine Band was found in five of the boreholes where it consisted of mudstone, not so dark grey as in most marine bands, with foraminifera, including Agathamminoides sp., Ammodiscus sp., Glomospira sp. and Glomospirella sp.,usually accompanied by plant debris. In boreholes Nos. 4/2, 5/1 and 5/2 the marine band is in the immediate roof of the seam, and is less than 2 ft thick; in Gwendraeth Valley No. 1 foraminifera are present, abundantly in some beds, up to about 8 ft above the coal, while in Borehole No. 3/2 foraminifera were found 10 ft above the coal. Above the marine band, or above the coal where the marine band is not present, Cochlichnus kochi, Gyrochorte carbonaria, cf. Planolites sp. [small], Anthraconaia pulchra, Naiadites cf. hindi, 'Estheria' sp. [rare], Leaia sp. [rare], ostracods [rare], a Belinurid and fish fragments, including Rhabdoderma sp. and Rhadinichthys sp.,have been found, up to about 30 ft above the seam. Anthraconaia adamsii was collected about 5 ft above the coal in Borehole No. 4/3. Sandstone, rarely more than 10 ft thick, is present near the top of the cyclothem in some sections. The average thickness of the cyclothem is about 60 ft, the only significant exception being in Borehole No. 5/1, where it is less than 30 ft.

Anthraconaia adamsii, A. hindi and A. cf. warei have been found either above an impersistent seatearth near the top of the Carway Fawr cyclothem or at the base of the overlying cyclothem, in boreholes Nos. 4/1, 4/2, 4/3 and Gwendraeth Valley No. 2; Naiadites sp. [juv.], ostracods and fish scales, also present at this horizon, occur more widely. This cyclothem, in which thin beds of quartzitic sandstone are not uncommon, is about 60 ft thick, except in Borehole No. 5/1 where over 80 ft of sandstone was found, the thickness of the cyclothem being over 120 ft.

The base of the next cyclothem is marked by a complex series of very thin coal seams in the western part of the area, and A. cf. adamsii was found on the lowest of this group in Shaft No. 2; Lioestheria vinti was found, probably at the same horizon, in Borehole No. 3/1. This cyclothem is poorly fossiliferous.

The two cyclothems immediately below the Foraminifera Marine Band are rather thinner than those below, their combined thickness being about 60 ft; the boundary between them is sometimes a bastard seatearth. Naiadites sp.  [juv.] is commonly present, and Anthraconaia spp. of the adamsii/hindi group [cf. Hind 1895, pl. xiii, fig. 8] were found in the lower of these cyclothems in boreholes No. 5/1 and Gwendraeth Valley No. 1; ostracods have also been recorded at this horizon. There is one record each of Naiadites sp., 'Estheria' ?, Leaia sp. and ostracods in the higher cyclothem.

The distance between the Foraminifera and Five Roads marine bands averages about 60 ft; in some of the sections in the eastern half of the area a rootlet phase, yielding ostracods and marine lamellibranchs in one borehole, is present below the latter marine band and in several other sections sandstone is present near this horizon, so that two separate cyclothems can be recognized, each with a marine band at the base. Over much of the Cynheidre area the lowest 2 or 3 ft of the Foraminifera Marine Band cyclothem contains plant fragments, characteristically including Lepidostrobus sp. and megaspores with Naiadites melvillei, Elonichthys sp. and indeterminate fish fragments, in grey mudstone. The marine phase, typically dominated by abundant foraminifera including Agathamminoides sp.,Ammonema sp., Glomospira sp. and Glomospirella sp.,but not uncommonly accompanied by scattered Lingula mytilloides, and fish remains including Rhadinichthys sp.,is present in dark grey mudstone: there is one record of a Nuculid lamellibranch near the top of this phase, which is up to about 5 ft thick. These beds are overlain by a few feet of grey mudstone with Planolites ophthalmoides [sometimes abundant], cf. P. sp. [small] [uncommon] and Geisina subarcuata. P. ophthalmoides occurs sporadically in most of the overlying part of the cyclothem, but is commonly more abundant in some beds. Other fossils are rare; there is one record each of an Eurypterid fragment (about 5 ft above the marine band) and of Cochlichnus kochi. Graded-bedded silty mudstone was recorded in this part of the cyclothem in some of the western sections. In three boreholes on the S.E. side of the area, Nos. 4/3, 6/1 and 6/3, rare scattered marine fossils, namely foraminifera including Glomospira sp.,a pyritized gastropod (Bellerophontid ?) and several fragmentary or juvenile Myalina, have been found 25 to 45 ft above the base of the cyclothem.

The Five Roads Marine Band has been recognized in all except the most easterly boreholes. The characteristic faunal phase, in which the lamellibranchs are usually abundant, is up to about 3 ft 6 in thick, and includes Bucanopsis sp., Euphemites anthracinus, Anthraconaia  aff. pruvosti (Weir and Leitch non Chernyshev), Edmondia cf. goldfussi, E.  aff. transversa, Myalina compressa, ostracods, including Geisina subarcuata and Hollinella sp.,and fish remains including Elonichthys sp., Rhabdoderma sp., Rhizodopsis sp. and indeterminate Palaeoniscid scales. Up to about 1 ft of barren strata often separate this bottom phase from about 6 in of strata with Geisina subarcuata [abundant] and fish remains including Rhabdoderma sp., Rhizodopsis sp.,Palaeoniscid and Platysomid scales, rarely accompanied by A.  aff. pruvosti and Naiadites sp. Pyrite is generally common in these beds as small concretions, replacing fossils and, above the G. subarcuata beds, as unusual small truncated cone shaped concretions. All these strata consist of dark grey micaceous mudstone, usually silty and sometimes striped; this lithology not uncommonly extends above and below the fossiliferous strata. As the area where a rootlet phase is present below this marine band is approached, the lamellibranch phase dies out. In Borehole No. 6/1 over 20 ft of quartzitic sandstone, with intercalations of silty mudstone with sandstone wisps, in two beds, extends from the top of the Foraminifera Marine Band cyclothem to near the top of the Five Roads Marine Band cyclothem, which is unrecognizable in both this borehole and in Borehole No. 6/3. Sphaerosiderite is usually present in the complex seatearth at the top of the Five Roads Marine Band cyclothem, and bastard seatearth has been recorded in several of the sections.

Two cyclothems, sometimes with very thin coal seams at their bases but in places divided by a bastard seatearth, can be recognized between the top of the Five Roads Marine Band cyclothem and the Red Vein. These strata, which are about 50 ft thick, normally contain Cochlichnus kochi and Gyrochorte carbonaria but otherwise are poorly fossiliferous; mussels, including Anthraconaia pulchra and Naiadites sp. [juv.], have been collected, and there is one record of a Protorthopterous wing. Quartzitic sandstone, up to 10 ft thick in Shafts Nos. 1 and 2, is present in some places a short distance below the Red Vein.

In the Cynheidre area the Red Vein is only about 13 in thick. The characteristic fossil in its roof is Lioestheria vinti, which ranges up as far as about 20 ft above the coal. Gyrochorte carbonaria is usually present and Cochlichnus kochi, Planolites sp.,Anthraconaia ? [juv.; one record], Naiadites sp.  [juv.],Geisina subarcuata [in three of the eastern boreholes and Shaft No. 1] and fish remains including cf. Rhabdoderma sp.,cf. Rhadinichthys sp.,and Vetacapsula sp. have also been found. The cyclothem is about 35 ft thick and is interrupted 13 ft from the top by carbonaceous shale with coal streaks on seatearth and overlain by dark grey mudstone with G. carbonaria, L. vinti and ostracods, in Borehole No. 6/3, the most easterly section in the area.

The top split of the Red Vein is usually thinner than the bottom split, but the cyclothems are of about the same thickness. A similar fauna is present, including C. kochi, G. carbonaria, cf. Planolites sp. [small], Anthraconaia sp.  [juv.], Anthraconauta or Naiadites?, N. hindi ?, N. sp. [juv.],L. vinti, and Rhadinichthys sp.,but L. vinti does not dominate the fauna and is not everywhere present.

In most of the sections there are two seatearths, about 10 ft apart, both overlain by very thin coal seams, below the Lower Cwmgorse Marine Band cyclothem. There is only one record of fossils above the lower seatearth, i.e. L. vinti and Rhadinichthys sp. in Borehole No. 4/3; the possibility cannot be excluded that a short length of core was misplaced, or that the bottom of the overlying cyclothem was repeated by faulting. Below the marine band there is usually a non-marine phase consisting of about 1 ft of medium dark grey mudstone, carbonaceous and fissile at the base in some sections, with small pyrite 'blebs', plant fragments, Carbonita sp., Geisina subarcuata, L. vinti and fish remains including a tooth of Diplodus sp.,and scales of Palaeoniscids, Platysomids and Rhadinichthys sp. : Spirorbis sp., Naiadites cf. daviesi and N. sp. [slight posterior wing] were found in this phase in the most westerly borehole, Gwendraeth Valley No. 2. The lowest marine strata consist of 3 to 4.5 ft of mudstone, usually rather lighter grey than above, with a particularly smooth texture and characterized by the presence of much pyrite, some in the form of infilled burrows; in this phase Planolites ophthalmoides is accompanied by foraminifera [uncommon) including Glomospira sp. and Glomospirella sp., Lingula mytilloides [rare], Dunbarella macgregori [not common, often fragmentary and partly pyritized], Hollinella cf. bassleri [rare] and fish remains including Listracanthus sp. [spine]. The main marine phase, consisting of dark grey mudstone, usually finely micaceous, fissile in parts, sometimes with a curious 'sooty' appearance and generally about 4 to 5 ft thick, is characterized by the presence of Dunbarella macgregori, commonly abundant towards the base. Other fossils recorded include carbonized plant filaments, Spirorbis sp. [attached to Dunbarella fragment], sponge spicules [pyritized], Campylites sp., Lingula mytilloides, Orbiculoidea cf. nitida, Euphemites?, Pleurotomariid gastropods, turreted gastropods [pyritized], Myalina cf. compressa, Posidonia sulcata [Hind], cf. Coelogasteroceras dubium (Bisat), coiled nautiloid indeterminate, Hollinella cf. bassleri, Lioestheria sp. [one record, near top], Hindeodella sp. and fish remains including Palaeoniscid scales. The overlying grey mudstone, varying from 10 to 12 ft thick, usually contains cf. Tomaculum sp. and cf. Planolites sp. [small], accompanied towards the base by Campylites sp. and sporadic specimens of P. ophthalmoides, L. mytilloides, Dunbarella sp., Palaeoneilo sp.  nov. 1 [cf. Demanet 1943, pl. iii, figs. 13, 16 and ?17], P. sp.  nov. 2, Polidevcia acuta (J. de C. Sowerby) or attenuata (Fleming) and rare ostracods. The sediments in the greater part of the cyclothem are coarser, and quartzitic sandstone is usually present near the top. The thickness of this sandstone is variable, even between adjacent boreholes; for example, compared with the thickest development proved in the Cynheidre area, 50 ft, with a conglomeratic base, in Borehole No. 6/3, only 11 ft were found in Borehole No. 6/1 1100 yd away. This cyclothem is interrupted in two of the boreholes by the presence of a thin coal or bastard seatearth about 10 ft below the top; in most of the other sections sandstone is present at this horizon. The total thickness of the cyclothem is about 120 ft.

The thickness of the Lower Welsh Vein varies from 8 to 24 in, the average being about 15 in. This cyclothem, which is about 50 ft thick, characteristically contains Cochlichnus kochi and common or abundant Gyrochorte carbonaria: plant debris, cf. Planolites sp. [small], Anthraconauta or Naiadites?, N. sp.  [juv.] 'Estheria' sp., Geisina subarcuata [present in 3 boreholes] and fish remains (not common) have also been collected. An insect wing was found in Gwendraeth Valley Borehole No. 2; Ware (1930, p. 465) recorded an insect wing at this horizon in Cefn Coed Shafts (Sheet 231). Sandstone has been proved in only one section.

The fauna above the seatearth, sometimes overlain by a very thin coal seam approximately midway between the Lower Welsh Vein and the Upper Cwmgurse Marine Band, is similar to that above the Lower Welsh Vein, but Gyrochorte carbonaria is more abundant and is nearly everywhere accompanied by Geisina subarcuata, which is often common. There is one record each of Anthraconaia?, and cf. Anthraconauta phillipsii (Williamson): plant fossils include Cyperites bicarinatus, Lepidodendron rimosum Sternberg, Lepidostrobus anthemis. Sandstone is rarely present.

The coal seam at the base of the Upper Cwmgorse Marine Band cyclothem is normally about 10 in thick, and is overlain by about 1 in of dark grey mudstone with abundant pyrite and coalified plant debris. This is succeeded by the marine band, the greater part of which typically consists of about 5 to 7 ft of dark grey finely micaceous mudstone with pyrite concretions. The fauna in the lower, and generally slightly silty, part of this mudstone consists largely of brachiopods including cf. Dictyoclostus craigmarkensis (Muir-Wood), Lingula mytilloides, Orbiculoidea cf. nitida, Linoproductus sp.,and Orthotetid fragments, accompanied by relatively rare molluscs, such as Euphemites sp., Dunbarella sp.,Solenomya?, Pectinids, and coiled nautiloids including cf. Coelogasteroceras dubium: Campylites sp.,crinoid columnals and Rhadinichthys sp. have also been found. The higher part usually contains more abundant lamellibranchs, including Anthraconeilo sp. nov. [cf. Demanet 1943, pl. III, figs. 14, 19], Edmondia?, cf. Wucula' luciniformis Phillips, Palaeoneilo sp.  nov., Polidevcia attenuata (Fleming), Yoldia glabra Demanet pars, non Beede and Rogers, Gen. et sp.  nov., cf. Solenomorpha parallela (Hind), and Pectinid fragments: other fossils collected from this part of the band include sponge spicules [pyritized], crinoid columnals, Crurithyris cf. carbonaria (Hind), L. mytilloides, Productid fragments, Coleolus?, Platyconcha cf. hindi Longstaff, Anthracoceras cambriense Bisat,  Politoceras kitchini, nautiloids, including Huanghoceras postcostatum (Bisat), cf. Carbonita sp., Hollinella cf. bassleri, cf. Tomaculum sp.,small ovoid bodies (?faecal pellets) and fish remains including Rhabdoderma sp. The highest marine phase, about 3 ft thick, in which Planolites ophthalmoides [not common], Campylites sp.,crinoid columnals, L. mytilloides, O. cf. nitida, scattered marine lamellibranchs including Palaeoneilo sp.  nov. 1 [cf. Demanet 1943, pl. III, fig. 16] and P. sp. nov.2 and fish remains are present, passes up to a non-marine phase containing Cochlichnus kochi [uncommon], Gyrochorte carbonaria, Curvirimula sp., Naiadites melvillei, N. sp.  [juv], Geisina subarcuata, 'Estheria' sp. and fish remains including scales of Palaeoniscids, Platysomids and Rhizodopsis sp. This non-marine phase, which usually includes darker grey mudstone, is commonly 2 or 3 ft thick. Other fossils found in this marine band in Cynheidre Shafts 1 and 2, but which it is not possible to assign to a particular part of the band with certainty, include Campylites stubblefieldi (Schmidt and Teichmueller), cf. Linoproductus cora Demanet (non d'Orbigny), Leptoptygma sp.,cf. Portlockiella sp.,cf. Streptacis sp., Aviculopecten sp., 'Limatulina' ?, Palaeolima sp.  nov. (cf. boltoni Demanet), ?Posidonia sulcata.

References

ARCHER, A. A. 1959. In Sum. Prog. Geol. Surv. for 1958.

CALVER, M. A. 1956. Die stratigraphische Verbreitung der nicht-marinen Muscheln in den penninischen Kohlenfeldern Englands. Z. deutsch. geol. Ges., 107, 26–39.

DAVIES, D. FARR, DIX, EMILY and TRUEMAN, A. E. 1928. Boreholes in Cwmgorse Valley, Proc. S. Wales Inst. Eng., 44, 37–136.

DEMANET, F. 1943. Les horizons marins du Westphalien de la Belgique et leur faunes. Mem. Mus. Roy. Hist. Nat. Belg. No. 101.

DIX, EMILY. 1928. The Coal Measures of the Gwendraeth Valley and Adjoining Areas. Proc. S. Wales Inst. Eng., 44, 423–510.

DIX, EMILY. and PRINGLE, J. 1929. On the fossil Xiphosura from the South Wales Coal- field with a note on the myriapod Euphoberia. Sum. Prog. Geol. Surv. for 1928, Pt. 2, 90–114.

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.

EYLES, V. A. 1952. In Sum. Prog. Geol. Surv. for 1951.

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

FAIRLEY, W. 1868. Practical Observations on the South Wales Coal Field. London.

HIND, W. 1895. A monograph on Carbonicola, Anthracomya and Naiadites. Pt. 2, 81–170. Palaeont. Soc.

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

JORDAN, H. K. 1910. The South Wales Coalfield Pt. 2. Proc. S. Wales Inst. Eng., 27, 172–249.

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

STRAHAN, A., CANTRILL, T. C., DIXON, E. E. L.1909. The Geology of the South Wales Coalfield. Pt. X. The Country around Carmarthen. 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.

TRUEMAN, A. E. and WEIR, J. 1951: 1956. A monograph of British non-marine Lamellibranchia. Pt. 5, 101–52; Pt. 9, 243–71. Palaeont. Soc.

WARE, W. D. 1930. An account of the geology of the Cefn Coed sinkings. Proc. S. Wales Inst. Eng., 46, 453–94.

WELCH, F. B. A. 1957. In Sum. Prog. Geol. Surv. for 1956.

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

Chapter 6 Pennant Measures

Llynfi Beds: general

The definition of the base of the Pennant Measures (or Upper Coal Measures) at the top of Upper Cwmgorse Marine Band (Woodland and others 1957) is fully justified in this district as can be seen if the generalized section of the Llynfi Beds (Figure 18) is compared with the sections of the underlying strata. In the Llynfi Beds, thick beds of pennant-type sandstone are developed at three horizons which, for convenience of description, are identified as sandstones 'A', 'B' and 'C'; the lowest, 'C' is impersistent and is usually quartzitic towards the base. The Lower Pinchin Vein, and the approximate horizons of the Upper Welsh and Upper Pinchin veins (Trotter 1947, pp. 92–3) of the Ammanford area can be recognized, but the only coals of economic importance in these beds are the Lower Brondini Vein and the Lower Caerhedyn and Cenrhos veins of the Burry Port area. Both the Lower and Upper Brondini veins were so named by Jordan (1915, pp. 53, 60–1), although much earlier they had been identified as the Lower and Upper 'Brondine' seams by Logan (Vertical Sections No. 3).

The detailed resurvey of this district and of the western part of the adjacent Ammanford (230) Sheet, has confirmed the approximate correspondence of the Brondini veins with the No. 2 Rhondda (Strahan and others 1909, p. 136), although this was disputed by Jordan (1910, pp. 218–9; 1915, p. 65). Precise correlation could not be achieved. As the base of the Rhondda Beds is defined at the No. 2 Rhondda Seam and its equivalents, the division in this area must be arbitrary; where coal seams, for example the Upper Brondini and Cenrhos veins, are present over a considerable area the base of the Rhondda Beds has been drawn at them, but elsewhere the division has been placed at the base of the sandstone which everywhere overlies these seams, though not always directly. For convenience of description the strata up to this sandstone are described here, although some are in the Rhondda Beds.

It is clear that the Cenrhos Vein of the Burry Port area is at or near the horizon of one of the Brondini veins, although its precise correlation has proved impossible. Hitherto the Gwscwm Vein, some 700 ft above the Cenrhos Vein, has been correlated with the 'Brondini' Vein (Strahan 1907, pp. 19–20). Although Jordan's estimate of the distance between the Cenrhos and Gwscwm veins, 753 ft (1915, p. 79), is substantially more accurate than Strahan's, 558 ft, he agreed with the correlation of the Gwscwm and Brondini veins (1915, p. 74). The correlation now suggested is based largely on the presence of at least 500 ft of sandstone between the Cenrhos and Gwscwm seams: no remotely comparable sandstone is known in the Llynfi Beds of this area, but the thickness of sandstone, over 300 ft, overlying the Brondini veins on Mynydd Sylen is of the right order and is known to decrease still farther to the east. Direct determination of the distance between, say, the Carway Fawr and the Cenrhos veins is not possible in the area where the latter is best known and was most extensively worked, north of Burry Port, but the simplest interpretation based on extrapolation of the lower seam under the boulder clay covered area on Mynydd Pembrey, suggests that the Cenrhos is at a stratigraphical position not lower than the Brondini veins. This interpretation is confirmed by the distance between the Carway Fawr and the presumed position of the Cenrhos Vein near Ffald Farm, about 1 mile S.E. of Trimsaran village. The only palaeontological evidence bearing on this correlation is the presence above the Lower Caerhedyn Vein, about 180 ft below the Cenrhos Vein, of Anthraconauta phillipsii (Williamson) and A.  aff.phillipsii (tending towards A. tenuis Davies and Trueman sp. ). The occurrence of the latter form suggests that the Cenrhos Vein may be above, rather than below, the horizon of the Brondini veins. It seems that the Lower Caerhedyn is probably at, or near, the horizon of the Upper Pinchin.

The succession in the Llynfi Beds is complex, but an attempt to summarize the detailed variations is shown in (Figure 18). Bastard seatearths occur at several horizons; a persistent bed of blackband ironstone about 160 ft above the Upper Cwmgorse Marine Band proved to be a valuable marker horizon. Fossils are not abundant and the fauna is restricted to relatively few forms, of which Anthraconauta phillipsii and variants are the commonest: Anthraconaia stobbsi (Dix and Trueman) was recorded towards the top of the Upper Cwmgorse Marine Band cyclothem in several of the Cynheidre boreholes and A. sp.  (pringlei (Dix and Trueman) group) was found on the Upper Brondini Vein. Euestheria spp. including E. simoni (Pruvost), have been recorded from five cyclothems including the Upper Brondini, but are nowhere common, and ostracods have been recorded from a similar number of horizons. Gyrochorte carbonaria and cf. Planolites sp. [small] are present sporadically throughout, but Cochlichnus kochi has rarely been recorded higher than the Upper Welsh Vein.

The Llynfi Beds are approximately 700 ft thick.

Llynfi Beds: details

In the Cynheidre boreholes the Upper Cwmgorse Marine Band passes upwards to a non-marine phase (pp. 119–120); Anthraconaia sp.  nov. cf. cymbula (Wright) was found about 10 ft above this phase in boreholes Nos. 4/3, 5/2 and 6/3, on the S.E. side of the Cynheidre area. In boreholes Nos. 3/1, 4/1, 5/1, 6/1 and 6/3, which are on a north-easterly trending line, A. stobbsi or A. cf. stobbsi were found in dark grey silty mudstone about 50 ft above the marine band, both the preservation of the fossils and the lithology being similar to that of the Five Roads Marine Band. In some of the boreholes Geisina subarcuata is also present either in or near these beds, and there are single records of A. sp.  nov. cf. cymbula and Naiadites melvillei. In the most northerly boreholes, Nos. 5/1, 6/1 and 6/3, scattered Gyrochorte carbonaria and mussels, including Anthraconaia sp. [juv.] and N. daviesi are present between the marine band and the A. stobbsi beds; in No. 5/1, A. stobbsi [pyritized] is present at a second horizon, nearer the Lower Pinchin Vein.

Sandstone 'C' is thin or absent between the Lletty Wilws Fault and the Cynheidre to Pontyates road: in the stream section [SN 5222 1027] near Bryngwenyn Farm (p. 114), it is only 3.5 ft thick and this horizon is probably represented by 20 ft of siltstone and striped beds exposed in a cutting [SN 5016 0895] on the Lianelly and Mynydd Mawr Railway and by a very thin bed of sandstone exposed in a small stream [SN 4980 0850] about 650 yd to the S.W. Between the Pontyates road and the Trimsaran Disturbance a strong feature [SN 484 073] suggests a thicker development of sandstone, although it is nowhere exposed. The variability in thickness of sandstone 'C' was proved in the Cynheidre boreholes ((Figure 4)): in the sections on the N.W. side of the area it was found to be absent, but it was present in all the boreholes beyond a line to the S.E. of boreholes Nos. 3/1, 4/1 and 5/1, reaching a maximum thickness in the most southeasterly borehole, No. 4/3. In this borehole it replaces much of the strata between the Upper Cwmgorse Marine Band and sandstone 'B', including the Lower Pinchin Vein. In the Trimsaran area this sandstone is probably absent or very thin.

The strata between the Lower Pinchin Vein and the base of sandstone 'B' are exposed in the stream near Bryngwenyn, in which all the coal seams are thin, although a seam believed to be the Lower Pinchin has been worked to a limited extent nearby; 164 in of coal are exposed in the mouth of an old drift [SN 5155 0992]. In this stream section [SN 5222 1027], another small stream [SN 5128 0972] and the spoil from an old trial [SN 5109 0956], ?Anthraconauta phillipsii [juv.] Carbonita cf. humilis and C. pungens (Jones and Kirkby) have been collected from blackband ironstone below pennant-like sandstone. To the S.W., carbonaceous shale with coal streaks and 'rashes' up to about 10 ft thick, immediately below sandstone 'B', are exposed in a stream section [SN 5076 0914] near Maen-gwyn Farm, near Ffynnon-ymenyn Farm [SN 5023 0883], [SN 4994 0838], in a stream section [SN 4957 0788] nearer Cynheidre and near Danyquarry [SN 4784 0633] and Cilferi-isaf [SN 4784 0543] farms; at some of these localities attempts have been made to work this seam. In the stream near Maen-gwyn an attempt was made to mine the underlying blackband ironstone (Strahan and others 1907, p. 138). The details of these strata as proved in the Cynheidre boreholes are summarized in (Figure 18). The Lower Pinchin Vein was found to be very thin in all the sections except boreholes Nos. 4/1 and 5/1, the two nearest the area where it has been worked at the crop; in Borehole No. 4/1 the Lower Pinchin consisted of 15 in of relatively clean coal. In several of the sections a bastard seatearth is present between this coal and a persistent bed of blackband ironstone which occurs about halfway between the Lower Pinchin and the base of sandstone 'B'. The seam immediately underlying this sandstone was found to have a complex, variable and dirty section in most of the boreholes. ?Anthraconauta cf. phillipsii, Carbonita sp.  (humilis group), 'Estheria' sp. [fragments], and fish scales including Palaeoniscid scales, Rhabdoderma? and Rhadinichthys? were found associated with the blackband ironstone. Otherwise, apart from scattered Cochlichnus kochi, Gyrochorte carbonaria and cf. Planolites sp. [small], the strata between the top of the Upper Cwmgorse Marine Band cyclothem and sandstone 'B' are poorly fossiliferous, although Anthraconauta phillipsii, Euestheria simoni, 'Estheria' sp.,ostracods, Rhabdoderma sp.,Rhizodopsis sp. and a fish spine have been found.

Sandstone 'B' forms a prominent feature along most of its outcrop and it is exposed in most of the last mentioned localities (to about 40 ft at [SN 5023 0883]). It has been quarried between the Five Roads to Pontyates road and Danyquarry Farm. In a large quarry [SN 482 069] near the road, up to about 30 ft of pennant-type sandstone are exposed; at the S.W. end of the quarry the beds are folded about axes trending E.15°S; these folds are believed to be due to penecontemporaneous deformation; the cross-bedding dips N.24°E. at 17°, and there are two prominent sets of joints trending E.20°S. and S.30°W. In another quarry [SN 4803 0653] the cross-bedding dips between N. and N.20°E. at about 12°, and the joints trend E.15° to 20°S. and S.3° to 8°W.; about 20 ft of sandstone are exposed. In the Cynheidre boreholes sandstone 'B' was proved to be more than 40 ft thick in all the unfaulted sections; it normally has a conglomeratic base. Sandstone 'B' has been quarried S. and S.W. of Trimsaran: about 30 ft of cross-bedded sandstone are exposed in an old quarry [SN 443 037], the lower part being flaggy.

The strata between sandstones 'A' and 'B' form a 'slack', partly drift filled, between scarps formed by the sandstones, and are very poorly exposed. There are trials on a coal seam at Brynbanal-fawr Farm [SN 5171 0987]; this is probably the first coal above sandstone 'B' (Figure 18). In most of the Cynheidre sections three cyclothems can be recognized between sandstones 'B' and 'A'. Coal is present at the base of the lowest cyclothem in all the sections except Gwendraeth Valley Borehole No. 2; it is usually thin, but 24 in or more was proved in boreholes Nos. 5/1, 5/2 (with a parting) and No. 6/3, on the N.E. side of the area (No. 6/1 was not cored above the Lower Pinchin Vein). The first coal below sandstone 'A', immediately below that sandstone in some sections, and believed to be at or near the horizon of the Upper Welsh Vein, is at least 24 in thick in boreholes Nos. 4/2, 5/1, 5/2 and Gwendraeth Valley No. 1B, in the central part of the Cynheidre area; it is of inferior quality. Fossils present, although not commonly, in the roof of this seam include Sphenophyllum cunelfolium, Gyrochorte carbonaria, cf. Planolites sp. [small], A. phillipsii, ostracods including cf. Geisina sp.,and fish scales including cf. Rhadinichthys sp.  Lepidodendron ophiurus, Mariopteris nervosa, Neuropteris tenuifolia, Cochlichnus kochi, G. carbonaria, cf. P. sp. [small], A. phalipsii, fish scales and a fragment of an insect wing have been found elsewhere in the strata between these sandstones.

Sandstone 'A' is exposed near Brynbanal-fawr, in the cutting [SN 5108 0936] for the disused mineral railway from Cae-glas Colliery and in a small quarry [SN 4811 0636] E. of Danyquarry. This sandstone was found in all the Cynheidre boreholes, the minimum thickness proved being over 70 ft, including thin striped beds. Conglomeratic beds are not uncommon in the borehole sections; a conglomerate is exposed [SN 4339 0288] S.W. of Trimsaran.

Little is known of the strata between sandstone 'A' and the Lower Brondini Vein at the outcrop, which is largely obscured by drift. These beds, and the underlying sandstone, are intersected by the Llannon and Trimsaran disturbances in the vicinity of Five Roads and Horeb: there are exposures, mainly of sandstone, in Mon Lliedi between Horeb and the Upper Lliedi Reservoir, and quartz-veined and faulted sandstone was exposed in a trench [SN 4874 0523] 300 yd W.S.W. of Five Roads. South of the disturbances, a prominent scarp feature [SN 496 047] S.E. of Five Roads is probably due to sandstone 'A'. A complex succession was proved between the top of sandstone 'A' and the Lower Brondini Vein in the six Cynheidre boreholes which were cored through all or most of this part of the Llynfi Beds: the details are summarized in (Figure 18). The thickest coal seam, approximately at the horizon of the Upper Pinchin Vein, was nowhere proved to be thicker than 1.5 in. The fauna in these beds is limited to C. kochi, G. carbonaria, cf. P. sp. [small], Anthraconauta phalipsii, A.  aff. phillipsii, Euestheria simoni, ostracods and fish fragments including Rhabdoderma sp. Sphaerosiderite is commonly present in the seatearth underlying the Lower Brondini Vein.

Between the Lletty Wilws and Coalbrook faults the Lower Brondini Vein has been worked extensively at outcrop, and to a limited extent in depth in Penllwyn [SN 5246 0901] and Caeglas [SN 5130 0908] collieries. The average section in Penllwyn Colliery is: coal 18 in on 'rashes' 4 to 10 in and coal 10 to 12 in, and in Caeglas Colliery it is recorded as: coal 15 in on 'clod' 2 to 4 in and coal 12 to 14 in; Anthraconaia? (possibly A.  aff. pruvosti Chernyshev sp. ) was collected from the tip of Penllwyn Colliery. In the mouth of an old drift [SN 5155 0909] 2.5 ft of mudstone are exposed between the seam and the overlying sandstone. On Cynheidre Hill [SN 503 078], where there was thought to be an outlier of the Lower Brondini Vein (Strahan and others 1909, p. 136), the outcrop of this seam is now known to be bounded to the S.E. by the Cynheidre Thrust. An opencast prospecting pit [SN 4995 0773] in this area proved the section: top coal 35 in, parting 3 in, coal 17 in. On the outlier on Brondini Hill about 1 mile to the S.W. the section in a trial pit [SN 4870 0659] was: top coal 13 in, parting 49 in, coal 16 in, and Logan recorded the section: coal 18 in on shale 8 in, coal 15 in. The parting is said to be thicker on the south side of Brondini Hill. It is said that "the coal has been largely cleared out" under Cynheidre and Brondini hills (Strahan and others 1909, p. 136): less evidence of old workings was seen on the former hill. The immediate roof of the Lower Brondini Vein is sandstone over parts of the Brondini and Cynheidre Hill areas, e.g. in the mouth of an old drift [SN 4923 0698] 400 yd S.E. of Cynheidre crossroads; 15 ft of cross-bedded sandstone, flaggy in part, are exposed a few feet above the seam in an old quarry [SN 4933 0678] 250 yd away. On the faulted western flank of Mynydd Sylen there are crop workings on this seam, as well as relatively extensive workings in Sylen Colliery [SN 5082 0833], where the section recorded on the Abandonment Plan is: coal 18 in on 'stone' 3 in, coal 15 in, although at the time the colliery was working the section was said to be: coal 12 in on 'stone' 6 in, coal 12 in (Strahan and others 1907, p. 160). Some 12 ft of silty mudstone with ironstone nodules are exposed in the entrance to an old drift [SN 5132 0548] c. 1100 yd S.S.E. of this colliery. In the four Cynheidre boreholes from which cores of the Lower Brondini were recovered, the section varied from coal 20 in on seatearth 5 in and coal 7 in to a single coal about 39 in thick; in all cases the coal was clean. The only fossils seen in the roof of the seam were G. carbonaria, cf. Planolites sp. [small], Anthraconaia? [juv.], and an ostracod. In the much older Tack Borehole (Jordan 1915, p. 61), drilled a few yards north of the Llannon Disturbance [SN 5205 0674], the Lower Brondini has the section: coal 16 in on 'shale' 5 in, coal 16 in.

Nothing is known of the Upper Brondini Vein (also known as the 'Cap Vein', Strahan and others 1909, p. 136) between the Lletty Wilws Fault and Cynheidre Hill, where two trial pits [SN 5040 0804], [SN 4998 0765] proved: coal 12 in on shale 3 ft 10 in, coal 18 in and : coal 10 in on parting 2 ft 8 in, coal 3 in, parting 6 in, coal 12 in, respectively. On Brondini Hill the section in a trial pit [SN 4874 0637] was recorded as: coal 4 in on parting 2 ft 7 in, 'coal and shale mixture' 10 in, coal 15 in. Mudstone overlying 12 in of coal with well-developed cone-in-cone structure was exposed [SN 4926 0691] in a field about 100 yd S.S.W. of Graig Farm, and a double 'smut' was seen in the mouth of an old level [SN 4928 0641] 400 yd N.N.E. of Brondini-fawr Farm: Logan recorded the section: coal 8 in on shale and underclay 4 ft, coal 18 in. About 30 ft of strata above this seam, mainly argillaceous, are exposed in an old quarry [SN 4928 0672] 700 yd N.N.E. of Brondini-fawr Farm; a coal 'smut' is present above the quarry face. Coal 18 in thick, said to have been found in an air shaft [probably [SN 5075 0639] at Sylen Colliery, is presumably the Upper Brondini Vein, and the 'smut' of this seam is exposed at the side of the track [SN 5138 0608] to Maes-llydan Farm, south of the Llannon Disturbance, where there are crop workings. In Cynheidre boreholes Nos. 3/1 and 4/1 this seam is 15.5 in and 10 in thick respectively, and in faulted beds in No. 4/2 it had the section: coal 4 in on seatearth 27 in, coal 18 in. In No. 4/2 coal streaks immediately overlain by massive sandstone were present about 45 ft above the Upper Brondini Vein. G. carbonaria, cf. P. sp., Anthraconaia sp.  (pringlei group; cf. Dix and Trueman 1931, p1. xvii, fig. 17), Anthraconauta phillipsii, and Euestheria sp. were collected from the roof of the Upper Brondini. Two coals, the upper 11 in and the lower 14 in thick, separated by nearly 6 ft of seatearth, were proved about 50 ft above the Lower Brondini Vein in the Tack borehole; they were identified as the Upper Brondini Vein by Jordan.

The Lower Caerhedyn Vein was mined in Cwm Capel Colliery [SN 4520 0209], N.E. of Burry Port; the Abandonment Plan records the section: 'shale' 1 ft 6 in on coal 8 to 15 in, shale 1 in, coal 24 to 48 in, 'shale' 3 to 12 in, coal 18 to 21 in; even the thinner values for the coal thicknesses are rather thicker than those given by other accounts, one of which records: coal 6 in on parting, coal 15 in, 'clod' 9 in, coal 15 in. Anthraconauta phillipsii and A.  aff. phillipsii (tending to A. tenuis) were collected from the debris from a trial pit [SN 4450 0265] on this seam, near Ty-newydd Farm. Sandstone between the Lower Caerhedyn and Cenrhos veins is exposed in a lane [SN 4511 0291] 550 yd N.E. of that farm, and was probably worked in an old quarry [SN 4555 0318] now grassed over. No record is known of the strata proved in the level cross-measures drift in Cwm Capel Colliery, which was started above the Cenrhos Vein and continued to about 60 ft below the Lower Caerhedyn Vein, but Jordan (1915, pp. 79–80) records another seam, the 'Upper Coed-Rhyal' (or U. Caerhedyn), 21 ft above the Lower Coed-Rhyal (or L. Caerhedyn), with a section similar to those given above for the latter, while his section of the Lower Coed-Rhyal also consists of three coals, although each leaf is much thinner. In the drift the Lower Caerhedyn and Cenrhos veins are about 180 ft apart; elsewhere in the colliery this distance varies from 175 to 190 ft. Logan (Vertical Sections No. 1) records several thin coals between the Upper Caerhedyn and Cenrhos veins.

In Coed Rhyal, about a mile farther west, the Lower and Upper Coed Rhyal veins, probably corresponding to the Lower and Upper Caerhedyn veins, are said to have been worked. In Ffrwd Colliery a relatively recent (1906) attempt to work the Lower Coed Rhyal by re-opening an old level [SN 4302 0283], in which it was 33 to 36 in thick including a 'clod', was unsuccessful: beyond a fault a seam only 6 in thick was proved in disturbed ground. The Upper Coed Rhyal Vein was said to be too thin to be workable.

The Cenrhos Vein has been mined extensively in Cwm Capel and Ffrwd [SN 4290 0273] collieries, and to a lesser extent in Rhiwlas Colliery [SN 4564 0289] and Penllwyn Slant [SN 4362 0230]; Cape (or Lloyd's) Level [SN 4294 0271] connected Penllwyn Slant with Ffrwd Colliery, part of which was drained by the Caban Water Level [SN 4260 0276]. The Cenrhos has been worked extensively at the crop near Ty-Newydd Farm and in Coed Rhyal. The seam section is complex; over much of the area it consists of four coals, typical sections being: coal 11 to 13 in on 'clod' 1 to 3 in, coal 3 to 8 in, 'shale' 0.5 to 2 in, coal 11 to 15 in, 'parting' 0 to 3 in, coal 11 to 14 in. Near the southern limit of the workings in Cwm -Capel Colliery the two middle coals combine to form a single 24 in leaf. In Rhiwlas Colliery the section of the seam worked as the Cenrhos Vein is recorded as: pyrite 5 in on coal 4 in, 'clod' 2 in, coal 5 in, 'clod' 1 in, coal 14 in. The field evidence confirms the identification of this seam as the Cenrhos, although the plans of Cwm Capel and Rhiwlas colliers show the workings overlapping, those in the Cenrhos Vein from Cwm Capel being nearly 100 ft below those in Rhiwlas. Although the possibility of low-angle faulting cannot be eliminated entirely, it is likely that there is an error in one, or both, plans, and that the deterioration in the seam is due to 'washing-out' by the sandstone which, as far as is known, immediately overlies the Cenrhos Vein throughout this area.

To the east of Rhiwlas Colliery, and south of the Trimsaran Disturbance, very little is known of the Cenrhos and Caerhedyn horizons. At Gelli-fawr Farm [SN 4845 0405], 1500 yd S.S.W. of Five Roads, a pronounced scarp feature is probably due to the sandstone at the base of the Rhondda Beds; silty mudstone is exposed nearby. East of the Stradey Fault there is also a well-developed feature, and the section: pennant type sandstone on coal 2 in, soft clay 11 in and coal 6 in is exposed in the mouth of an old level [SN 5022 0438] near Gwaun-crychydd Farm, 1 mile S.E. of Five Roads. It is likely that the absence of workings on the Cenrhos Vein for such a considerable distance is not entirely due to much of this area being drift covered, but is due also to further thinning of the coal to the east of Rhiwlas.

Under the large area of blown sand and alluvium west and south-west of Ffrwd Colliery little is known about the Cenrhos Vein, but several seams, the thickest recorded as 57 in (Appendix I), were proved at a depth of about 1100 ft near the bottom of Ashburnham Borehole No. 6 [SN 4082 0038]; these seams may be at the top of the Llynfi Beds, as, more doubtfully, may be the seams 36 and 32 in thick at depths of 167 and 211 ft respectively in Ashburnham Borehole No. 2 [SN 3917 0088].

Rhondda Beds: general

The Rhondda Beds are well known in two distinct areas, on and near Mynydd Sylen and in the Burry Port-Pembrey area, north and south of the Trimsaran Disturbance respectively. They are also present in the neighbourhood of Tre-Fanau, between the Lliedi Valley (Sheet 230) and the Dulais stream which flows southwards from near Five Roads.

The position of the base of the Rhondda Beds has been discussed in the general account of the Llynfi Beds. It proved impossible to identify the precise equivalent of the Brithdir Seam in this district, but from a general consideration of the succession (Figure 19) it is likely that the Goodig (or Clay) Vein is near this horizon: the top of the Rhondda Beds is taken at the base of the sandstone overlying the argillaceous strata associated with this seam.

The 'Cenrhos Rock', the sandstone at the base of the Rhondda Beds, is conspicuously thick, not less than 500 ft, in the Burry Port area: it is over 300 ft thick on Mynydd Sylen, where shales, sometimes including a coal seam at the top, are present up to about 200 ft above the Lower Brondini Vein in part of the area. The 'Cenrhos Rock' has been quarried extensively near Burry Port.

At least four seams, the Rhengog Fawr, Rhengog Fach, Gwscwm and Gwscwm Rider, are present in a group of argillaceous strata, about 200 ft thick, above the 'Cenrhos Rock' in the Burry Port area. Only the Gwscwm and Rhengog Fawr veins have been mined extensively: both have complex sections but the former usually includes at least 30 in of coal and the latter slightly less. Anthraconauta phillipsii has been found associated with the Gwscwm. Although these strata are of comparable thickness on Mynydd Sylen and three seams have been worked at the crop, the absence of more extensive workings, and unsuccessful opencast prospecting, suggest that the seams are here thinner.

The Goodig, or Clay, Vein at, or very near, the top of the Rhondda Beds, and about 350 ft above the Gwscwm Vein, is thin, but the fireclay associated with it has been mined relatively extensively.

The Rhondda Beds are about 850 to 1100 ft thick.

Rhondda Beds : details

North of the Trimsaran Disturbance

Only one of the Cynheidre boreholes, Gwendraeth Valley No. 1, penetrated all the sandstone above the Brondini veins, which forms the broad expanse of Mynydd Sylen. It proved the thickness, corrected for dip, to be over 300 ft, and if the overlying sandstone, siltstone and striped beds, with thin beds of mudstone, seen only as chippings, are included, the thickness is about 350 ft. A bed of dark grey mudstone about 3 ft thick, with mussel fragments, 90 ft above the base of the sandstone in this borehole, thickens in a northerly direction: although the core was lost, in Cynheidre Borehole No. 5/2 coal may have been present above a rootlet phase underlain by about 30 ft of silty mudstone, about 75 ft above the base of the sandstone. Disturbed silty mudstone, mudstone, coal and seatearth, believed to be at this horizon are exposed [SN 5116 0837] close to the Cynheidre Thrust in the small stream 300 yd E.N.E. of Blaen-Lliedi Farm. At the outcrop on the northern scarp of Mynydd Sylen these beds thicken further, and a coal seam 20 to 22 in thick (Strahan and others 1907, p. 159) has been worked near Pen-llwyn-uchaf Farm [SN 5251 0884] and about 400 yd to the N.E. This seam is some 200 ft above the Lower Brondini Vein. The narrowing of the outcrop of the overlying sandstone towards the Lletty Wilws Fault is due partly to this argillaceous development and a decrease in thickness of the remaining sandstone, and partly to the structure.

Three coal seams have been worked on the steep south-eastern side of the 'Pantyr-yn brook', which joins the River Morlais at Pen-llwyn-fach [SN 5293 0882]: no details are known of the seams. The highest is about 650 ft above the Lower Brondini Vein and is correlated with the Gwscwm Vein, the two lower seams corresponding, at least approximately, with the Rhengog Fawr and Rhengog Fach veins. The thickness of strata in this 'slack' is nearly 200 ft. In view of the synclinal structure of this area, it is more probable that the coal seams exposed in disturbed strata in the E. bank of the River Morlais [SN 5310 0753], 700 yd W.N.W. of Pont Morlais, are near the Gwscwm horizon rather than higher (Strahan and others 1907, p. 161); one of these seams has been worked to very limited extent.

South of the Trimsaran Disturbance

Near Burry Port the 'Cenrhos Rock', the sandstone above the Cenrhos Vein, is exposed in old quarries near Ty-gwyn [SN 4648 0312], in Nant Dyfatty [SN 456 025], near Cwm Capel Colliery [SN 451 021] where about 100 ft of sandstone with thin, impersistent beds of mudstone are exposed, and near Gwscwm [SN 4392 0167] where about 40 ft of massive sandstone are overlain by 10 ft of flaggy sandstone and underlain by 6 ft of conglomerate with ironstone, mudstone, coal and quartz pebbles. This sandstone increases in thickness from about 500 ft N.E. of Burry Port to about 575 ft at Pembrey.

The strata which include the Gwscwm Vein and the seams associated with it give rise to a very prominent scarp feature in Cwm Ferman, but are poorly exposed except in a brickpit. Old accounts, including Logan's (Vertical Sections No. 1), confirm that these strata are predominantly argillaceous, with some thin beds of sandstone between the Rhengog Fach and Gwscwm veins.

The Rhengog Fawr Vein is about 20 ft above the Cenrhos Rock in Cwm Ferman, but west of Achddu it is very near, or resting on, that sandstone. It has been mined to a limited extent in New Lodge Colliery [SN 4594 0119] where it is said to have the section: coal 9 in on 'dirt' 1 in, coal 7 in, 'rubbish' 5 in, coal 6 in, but in a nearby small mine, Carmel Colliery [SN 4559 0203], coal 30 in on parting 6 to 9 in, coal 6 to 9 in were seen. Further west the Rhengog Fawr was worked more extensively from the Rhengog Fawr Slant [SN 4394 0159] of Gwscwm Colliery, where the complex section: coal 8 in on 'dirt' 4 in, coal 6 in, 'dirt' 5 in, coal 6 in, 'dirt' 3 in, coal 7 in was recorded, although a later account shows a single coal, 36 in thick. Strata including 10 in of coal are exposed in a small stream [SN 4273 0237] 100 yd S.E. of Coed Farm. Coed Level [SN 4266 0241] followed the Rhengog Fawr for about 350 yd, and this seam was proved in a cross-measures drift in Hay's Level [SN 4237 0250]. According to one account the section proved in Craiglon Pit [SN 4187 0210] is: coal 10 in on 'strong chill and sandstone intermixed' 69 ft, coal and 'clod' 28 in, sandstone 12 ft. This is similar to Strahan's record (1907, p. 29), which includes the information that a borehole drilled from the bottom of the pit for 123 ft proved no coal. The section in Winde Trial Pit [SN 4195 0201] is reported to be: 'supposed Little Vein on measures 48 ft, supposed Gwscwm, measures (the bottom 18 ft drilled) 102 ft. From consideration of the succession proved in these two pits, in Ashburnham (or Elkington's) Level [SN 4200 0197] and by the mapping of the western extremity of Mynydd Pembrey, it is probable that the upper seam in Craiglon Pit and the lower in Winde Pit are the Rhengog Fach, and that the seam worked as the Gwscwm Vein in Cae-Gwyn [SN 4219 0237] and Craiglon [SN 4197 0222] collieries is probably the Rhengog Fawr. The sections in these collieries are recorded as 18 in and 9 to 48 in, average 24 in, respectively.

The Rhengog Fach Vein is about 50 ft above the Rhengog Fawr. It has been worked at the crop at the N.E. end of Cwm Ferman where it lies a short distance below sandstone; the tips contain abundant Cordaites sp. This seam is exposed in a brickpit further down Cwm Ferman [SN 4570 0206], as follows: head, about 3 ft; on silty mudstone, slightly striped, about 20 ft; carbonaceous shale with coal streaks, 4 in; coal, 2 in; seatearth with a few coal streaks, 3 ft; coal, 5 in; carbonaceous shale with coal streaks, 8 in; coal, 14 in; carbonaceous shale with coal streaks, 8 in; silty seatearth, 7 in; silty mudstone with rootlets and small ironstone nodules, 2 ft; on silty mudstone, mostly slightly striped, 18 ft. An attempt was made to work this seam in Achddu Colliery [SN 4496 0160], where it consisted of: coal 4 in on 'stone and clod' 9 in, coal 13 in; a larger area was worked in Gwscwm Colliery, further west, with the section: coal 6 in on 'clod' 8 in, coal 14 in. The Rhengog Fach was intersected in the Hay's Level cross-measures drift and Logan (MS.) recorded the thickness in a cross-measures drift in 'Coed-y-Marchog' Colliery as 14 in (Coed-y-Marchog is at the western end of Mynydd Pembrey).

The Gwscwm Vein has been worked extensively underground. Typical sections in the collieries where it was mined are as follows. New Lodge: coal 36 in with partings 4 and 2 in thick (or coal 6 in on parting 6 in, coal 18 in according to another account); Pemberton [SN 4515 0174] (very limited workings): coal 1 in on 'shale' 1 in, coal 8 in, 'shale' 3 in, coal 24 in; Achddu: coal 9 in on parting 2 in, coal 22 in; Gwscwm (or Pembrey) [SN 4402 0138]: coal 8 in on coal 32 in. Logan (MS.) recorded: coal 8 in on coal 12 in at Coed-y-Marchog. Although mining is said to have stopped in coal only 13 in thick in the N.E. part of New Lodge workings [SN 4645 0205], this seam was worked extensively at the crop in Cwm Ferman. The distance between the Gwscwm and Rhengog Fach veins is about 120 ft at New Lodge, 90 ft at Achddu, and 120 ft at Gwscwm, but to the N.W. it decreases to 75 ft in an underground shaft [SN 4328 0172] and in Hay's Level, and to about 50 ft at Craiglon.

West of the Moreb Fault the Gwscwm Vein is overlain by argillaceous beds; Strahan (1907, pp. 25–6) recorded 6 ft of 'shale' between the Gwscwm Vein and the overlying massive sandstone in the mouth of New Lodge upcast slant [SN 4560 0198], but only 300 yd to the west this 'shale' is probably about 30 ft thick where worked in an old brickpit [SN 4535 0185]. During the resurvey about 10 ft of sandstone, with thin, impersistent beds of ironstone conglomerate and an impersistent conglomerate with abundant coal pebbles and coalified plant debris up to 1 ft 6 in thick, overlying 6 ft of silty mudstone, were exposed in this brickpit. Mudstone with Anthraconauta phillipsii [small] was collected from the tip [SN 4500 0175] of Achddu Colliery; the workings were almost entirely confined to the Gwscwm Vein.

In the air pit [SN 4495 0149] of Achddu Colliery, a thin seam the Gwscwm Rider (Strahan and others 1903, p. 181) was found above the Gwscwm, and is present 45 to 50 ft above that seam from there westwards. A limited area of this seam was mined (as the 'Clay Seam') in Gwscwm Colliery; one of the slants [SN 4359 0155] was later re-opened, and sandstone on 8 to 10 in of coal is exposed in its mouth. The fireclay below this seam, said to be 7 to 9 ft thick, was worked. The section in an old quarry [SN 4203 0192] at Craiglon is: flaggy sandstone, 5 ft; on coarse striped beds and flaggy sandstone with coal pebbles and streaks, variable, say 1 ft 6 in; massive sandstone with mudstone and ironstone pebbles, especially near base, 8 ft 9 in; shale, about 3 ft 9 in; coal, dirty, Gwscwm Rider, about 5 in; seatearth, 9 in; sandstone and striped beds, 7 ft on sandstone. Sandstone is also present between the Gwscwm and the Gwscwm Rider in Stanley's Pit [SN 4328 0126], part of Gwscwm Colliery, in which the latter has the section: coal 12 in on 'shale, etc.' 1 ft 6 in, coal 12 in.

The Gwscwm seams were almost certainly penetrated by Ashburnham Borehole No. 6, and the seam 37 in thick at a depth of 534 ft may be identified, tentatively, as the Gwscwm Vein. In Ashburnham Borehole No. 2 it is possible that one of the seams at either 167 ft or 211 ft may be the Gwscwm, rather than the Cenrhos Vein (p. 127).

East of the Moreb Fault, almost as far as the Trimsaran road, the position of the Gwscwm shales is marked by a scarp feature, although less imposing than that in Cwm Ferman. There is some evidence of attempts to find coal, and a coal smut and mudstone are exposed in the yard of Tre-partridge Farm [SN 4682 0302] (no longer named on the maps). Up to 1 ft of sandstone on 1 to 7 in of seatearth and a further 10 ft of sandstone with beds of silty mudstone near the top, are exposed in a small disused quarry 50 yd N.E. of that farm. Trial boreholes through boulder clay on the N.E. side of the Trimsaran road, from Parc-Wilws Farm [SN 4705 0327] to over 400 yd to the S.E., proved continuous sandstone: it is concluded that the whole of the Gwscwm slack, nearly 200 ft thick only 0.5 mile to the S.W., has been replaced by sandstone.

Apart from a few exposures of sandstone in Cwm Mawr, the Rhondda Beds are almost entirely drift covered as far east as the main Llanelly to Carmarthen road, east of the Stradey Fault. The 'Cenrhos Rock', possibly about 500 ft thick, is overlain by about 50 ft of softer strata which form a depression at Tre-fanau-uchaf Farm. An attempt was made to mine a seam with the section: 'shale' on coal 18 in, 'dirt' 1 ft, coal 6 inches in Tre-fanau Colliery [SN 5000 0401]; this seam, probably one of the Rhengog veins, was followed down dip for about 80 yd, the average dip being 25°. About 50 ft of 'shale', with a thin bed of sandstone and 12 in of coal, were proved at a depth of 540 ft in Castle Colliery Borehole No. 4 [SN 4993 0365], which continued to 938 ft in sandstone with thin 'shale' partings.

The sandstone between the Gwscwm and Goodig 'slacks' is about 300 ft thick N.E. of Burry Port, decreasing to about 250 ft at the western end of Mynydd Pembrey. It has been quarried in Cwm Mawr [SN 4775 0292], where conglomeratic beds are exposed; in Cwm Ferman [SN 4645 0245], where 4 ft of sandstone rest on up to 5 ft of striped beds, with contorted bedding in part, and a further 12 ft of sandstone; at Achddu, where about 45 ft of cross-bedded sandstone are exposed in the larger [SN 451 014] of two quarries, and near Gwscwm [SN 4392 0137]. This sandstone forms the crags at the western end of Mynydd Pembrey. It is overlain by a group of softer beds, about 70 ft thick in New Lodge Colliery, and probably 50 ft thick at Craiglon: these beds include several thin coal seams, the highest of which is the Goodig (or Clay) Vein. The fireclay associated with this seam was worked both opencast and in levels, including Goodig Level [SN 4796 0274] (also known as Cwmmawr Colliery), on the south side of Cwm Mawr, where the coal is 12 in thick but is overlain by 2 ft 6 in of fireclay and underlain by 2 ft 6 in of 'rubbish' on a further 3 ft of fireclay: the upper fireclay has a sandstone roof. Two feet of mudstone on 20 in of coal, 7 in of carbonaceous shale with coal streaks and 2 ft of seatearth were exposed [SN 4607 0175] near Llety'rychen-fach Farm, and 60 yd to the S.W. 6 ft of silty mudstone with rootlets are exposed in the mouth of a level. A further 190 yd S.W. is the upcast slant from the Goodig workings in New Lodge Colliery, where 14 in of coal (overlain by 15 to 18 in of inferior coal according to one account) overlie 3 ft of fireclay, which was mined. In a trial hole [SN 4564 0161] 350 yd W.S.W. of Llety'r-ychen-fawr Farm a lower seam, 12 in thick and overlain by 6 ft of mudstone, was exposed. Under Burry Port, argillaceous beds, including an inferior coal seam, were proved from 174 to 274 ft in a water borehole [SN 4488 0106] for Elliot's Metal Co. (Appendix I, p. 189). In 1864 the Copper Works Shaft [SN 4491 0028] was sunk to a depth of 457 ft, and intersected three coal seams in argillaceous rocks between 273 ft and 381 ft (Appendix I, p. 178); the lowest seam was the thickest, 24 in, and may correspond to the lowest seam, 10 in thick, in the New Lodge cross-measures drift (Strahan 1907, p. 27), although it has also been correlated with the Goodig Vein (Strahan and others 1903, p. 182). The dip in the shaft is northwards at about 3°.

A coal seam which has been tried at the crop [SN 4260 0209] at the faulted western end of Mynydd Pembrey is about 300 to 350 ft above the Gwscwm Vein, and is believed to be the Goodig Vein. Mudstone in this 'slack' is exposed in a recent landslip scar [SN 4217 0202] and about 30 ft of sandstone, conglomeratic and with coalified plant debris in parts, overlies a thin bed of shale on 0 to 5 in of coal, seatearth and silty mudstone with rootlets, in two quarries in the toe of the landslip [SN 4220 0190], [SN 4210 0190].

East of the Stradey Fault, the argillaceous rocks associated with the Goodig Vein were penetrated from 123 to 217 ft in Castle Colliery Borehole No. 4, and included 6 in of coal; a seam with the section: sandstone on coal 4 in, 'shale' 6 in, coal 4 in at 367 ft in Castle Colliery Borehole No. 5 [SN 4991 0331] may be the Goodig Vein. About 250 yd S. of Tre-fanau-uchaf Farm the outcrop of these strata is marked by a scarp feature which is better developed near Berllan-dywyll Farm (Sheet 230) about mile to the east.

Brithdir Beds: general

The Hughes Vein is now taken as the boundary between the Brithdir and Hughes Beds, and between the Lower and Upper Pennant Measures (Woodland and others 1957, p. 11). The correlation of the Pwll Big Vein with the Hughes Vein, suggested by Strahan (1907, p. 20) and Jordan (1915, p. 77), has been confirmed; like most of the correlation of the Pennant Measures in the South Wales Coalfield this is based on mapping and general consideration of the succession (Figure 19).

The Brithdir Beds are well known west of the Stradey Fault, which runs a short distance west of Llanelli, but their details are imperfectly known east of that fault.

The Cwmmawr (or Rees) Vein, at or near the horizon of the Brithdir Rider Vein, lies at the top of a thin group of argillaceous beds which overlie about 300 ft of sandstone at the base of the Brithdir Beds. It was mined extensively between the Stradey Fault and the Moreb Fault, a downthrow east, beyond which it is unworked in a limited, drift-covered area as far west as Burry Port. The Cwmmawr Vein consists of an upper leaf which thins to the south from about 22 in to 12 in, and a bottom leaf of inferior coal about 9 in thick which is not recorded in the most southerly workings; the parting thickens from about 9 in to 24 in.

A further 300 ft or so of sandstone separate the Cwmmawr Vein from argillaceous strata which contain the Cilmaenllwyd (Weasel or Shenkin) Vein, an unimportant seam not more than 18 in thick which has been worked to a very limited extent at crop.

The thickness of the highest sandstone in the Brithdir Beds varies from 150 to 225 ft; it is overlain by about 25 ft of shale immediately underlying the Hughes Vein.

The Brithdir Beds are about 900 ft thick.

Brithdir Beds: details

The sandstone between the Goodig and Cwmmawr veins is very poorly exposed, except in crags in Cwm Mawr [SN 4830 0246]. Old quarries [SN 4660 0153], [SN 4579 0127] east of Burry Port are overgrown, but 10 ft of sandstone, flaggy in parts, are exposed behind Goodig House [SN 4565 0138]. This sandstone is about 275 to 300 ft thick. The hollow between the presumed Goodig and Cwmmawr vein horizons at Pant-y-llydufawr Farm [SN 5015 0364] is interpreted as due to argillaceous rocks about 100 ft above the Goodig, although none are exposed. These 'shales' werenot present in Castle Colliery Borehole No. 5, although a coal seam 3 or 9 in thick, according to different accounts, in Castle Colliery cross-measures drift (p. 135) may possibly be at this horizon. In New Lodge Colliery cross-measures drift, 30 ft of 'shale' were recorded 130 ft above the Goodig Vein.

The succeeding shale slack, about 30 ft thick, contains only one coal seam, the Cwmmawr Vein. A considerable area of this seam has been mined between the Moreb and Stradey faults in Cwmmawr [SN 4835 0241], Brynmawr (possibly including Pant and Barclay drifts between Coldblow and Pant Farm [SN 4763 0222], New Pool [SN 4676 0132] and Crown [SN 4701 0105] collieries. Typically this seam has a two-coal section, the thickness of the top coal being 22 to 23 in near the outcrop, but thinning to 18 to 22 in in the S.E. part of Crown Colliery workings [SN 483 012] and still further to 12 inches in the southernmost section recorded [SN 4765 0075]; the thickness of the parting averages about 9 in near the crop but thickens to 11 to 24 inches in the S.E.: the bottom coal, said to be of inferior quality (Strahan 1907, p. 24), is thickest, 9 in, in the west, but thins to 6 in towards Cwm Mawr, is only 1 to 4 in thick in the S.E. part of the workings, and is missing in the most southerly section. A thin bed of shale is present at some places between the Cwmmawr Vein and the overlying sandstone. In the mouth of Cwmmawr Colliery the immediate roof consists of 2 to 3 ft of conglomerate with much coalified plant debris, and in another level 70 yd to the N.W. the seam is overlain by 1 ft of shale under sandstone. East of the Stradey Fault the Cwmmawr Vein crops out near Pant-y-llydu-fach Farm [SN 5012 0338] and attempts have been made to mine it, although it is said to be only 12 in thick: there are more extensive crop workings on this seam about 0.5 mile to the east (Sheet 230).

The 'No. 6' Seam, with a sandstone roof, in Castle Colliery cross-measures drift is probably the Cwmmawr Vein: it consists of 16 in of coal and 'dirt' according to one account, or of coal 10 to 12 in according to another. A seam about 18 to 21 ft lower in the sequence is also thin. Although the probable horizon of the Cwmmawr Vein was penetrated near the bottom of both the Stradey [SN 5016 0130] and Castle Colliery No. 1 [SN 5028 0166] boreholes, neither proved the seam.

The sandstone above the Cwmmawr Vein is exposed in old quarries [SN 4837 0220], [SN 4849 0210] by the side of the Trimsaran to Llanelli road, and in Mon Dulais in an old quarry [SN 4887 0229] and in the stream nearby. It is about 340 ft thick west of the Stradey Fault but rather less at the outcrop to the east.

The Cilmaenllwyd Vein occurs in a shale 'slack' about 50 ft thick; it is near the middle of these beds in Crown Colliery shafts, but is probably nearer the top at the crop. This seam is recorded as 18 in thick in Crown Colliery; in Castle Colliery drift it is an unimportant seam; in Castle Colliery Borehole No. 1 there are three seams in this slack, the thickest, recorded as 9 in, being the lowest, near the middle of the 'shales' and in the Stradey Borehole there is 31 in of 'coaly shale' at the top of the slack and 15 in of coal near the middle. It has been mined to a very limited extent at the crop on both sides of the Stradey Fault.

Apart from the 20 or 30 ft of argillaceous rocks immediately below the Hughes Vein, the remainder of the Brithdir Beds consist of sandstone, 150 to 225 ft thick. This sandstone is exposed in the track from Pant-Hywel Farm [SN 4832 0171] down to Dulais Mill, and in the mouth of an old trial level [SN 4878 0178] 300 yd upstream from the Mill.

Hughes Beds: general

The Hughes Vein is thin on the north crop west of the Gardener's Fault, which crosses the Swansea Valley near Clydach (Strahan and others 1907, pp. 148, 170; Jones 1957, p. 32), but is of workable thickness west of the Stradey Fault, and workings in this seam, known locally as the Pwll Big Vein, extend beneath the sea. It consists of at least two leaves, the upper thinner than the lower, the parting being up to 4 ft thick.

The Pwll Little Vein, about 100 ft above the Hughes Vein, is thin, but has been mined together with the associated fireclay.

The massive sandstone overlying the Pwll Little Vein has been quarried: it is succeeded by a group of argillaceous strata, up to about 200 ft thick which includes several coal seams. Three of these, the Cille No. 1, Cille No. 2 and Cille No. 3 veins, have been mined, but only Cille No. 2 has been worked extensively, although it is less than 18 in thick. These seams have not been proved west of the Stradey Fault, although probably present.

The overlying sandstone, some 200 ft thick, gives rise to the prominent slopes north of Stradey Park and is quarried. The strata between this sandstone and the Swansea Three Feet Vein are essentially argillaceous and are present below low, drift covered ground. A thin seam near the base of these beds is probably the Lower Maesmelyn of the Neath area. The Swansea Two Feet Vein, known locally as the Bushy Vein, is present only to the east of the Stradey Fault and has been worked out over most of the area it occupies, on both sides of the Llanelli Syncline: it is probably 24 in thick.

The Hughes Beds are about 875 ft thick.

Hughes Beds : details

In the last century the Hughes (or Pwll Big) Vein was mined from Old Pwll (or Pool) Colliery [SN 4753 0094] under the sea, where the position of the Llanelli Syncline was established, and in Stradey Colliery [SN 4871 0120]. At a later date some of the pillars in Stradey Colliery were mined from Pant Hywel Colliery [SN 4861 0164]. Between the Moreb and Stradey faults it is likely that the Hughes Vein has been worked out at least down to adit level. There are several records of the seam section in this area; according to some there are two leaves, the upper about 15 to 20 in, separated by up to 4 ft of strata from a lower leaf 24 to 31 in thick, while other records show a middle leaf up to 15 in thick. A very small area of Hughes Vein ('No. 4' Seam) was worked off the cross-measures drift in Castle Colliery, where the total thickness of coal was 14.5 inches in 4 leaves, the thickness of the partings totalling 5 in, or 20 in of coal in 5 leaves according to another account. In the Stradey Borehole, drilled near the end of the last century, the section was recorded as: coal 26 in on 'shale' 2 ft 3 in, coal 12 in, 'broken shale' 2 ft 10 in, coal 28 in, but in Castle Colliery Boreholes Nos. 1, 2 [SN 5000 0113] and 3 [SN 5044 0086] drilled in 1922–3 in an attempt to prove reserves in the Hughes Vein, this seam was either very thin (in No. 1) or consisted of as many as eleven leaves (in No. 2), the total thickness of coal being 73 in and of partings 50 in, although the lowest 41 in of the section contained only four partings totalling 6 in; the top coal in No. 3 Borehole was 27 in thick. In at least part of the area the Hughes Vein is overlain by sandstone, either in the immediate roof or a short distance above, but most of the 100 ft or so of strata between this seam and the Pwll Little Vein are argillaceous.

The Pwll Little (Clay or Black Rock Clay) Vein was mined in Stradey Colliery, and more recently both the coal and the underlying fireclay have been worked in Pwll No. 1 Level [SN 4829 0098], at one time known as Pant-y-yard Colliery, and another level ('Pwll Colliery') [SN 4765 0090], 700 yd to the west. The section of this seam, according to different accounts, varies from: coal 8 in on 'soft clod' 1.5 in, coal 5 in, to a total thickness of 22.5 in, including 4 partings, the thickest 1.5 in; the fireclay is said to be 6 ft thick according to the latest record. The coal has a sandstone roof. The Pwll Little Vein has been worked at the crop both at Pwll, and to a lesser extent east of the Stradey Fault. In Castle Colliery drift this seam is said to be 19.5 in thick, including three partings with a total thickness of 5 in, and similar, although thinner, sections were proved in Castle Colliery Nos. 1, 2 and 3, and the Stradey boreholes.

The succeeding massive sandstone, about 135 to 185 ft thick, gives rise to the prominent hill behind Pwll, and an equally well-defined feature east of the Stradey Fault. It has been quarried at Pwll [SN 4803 0114], [SN 4835 0115] and is exposed [SN 4869 0121] near the mouth of Stradey Colliery and in a large disused quarry [SN 5035 0230] near Tre-beddrod Farm. In a small quarry by the side of the main Llanelli to Carmarthen road [SN 4970 0240] two sets of joint planes are prominent, strike joints trending E.10°N. dip N. at 78°, and almost vertical dip joints trend S.10°E.

The overlying 'shales' are thicker, about 145 to 200 ft than in any underlying slack in the Brithdir and Hughes Beds. In the borehole sections up to six seams are recorded, but only two, the Cille No. 2 Vein and, to a lesser extent, the Cille No. 3 Vein, have been mined in Castle (Cille, Furnace or Sandy) Colliery. The Cille No. 3 Vein, which has the section: coal 5 to 7 in on partings 2 to 3 in, coal 11 to 14 in is about 50 to 60 ft above the base of the 'slack'. The Cille No. 2 is about 50 ft above the Cille No. 3, and has been mined almost to the outcrop 300 yd W.S.W. of Bwlch-yfedwen Farm [SN 4938 0227] from Castle Colliery: it was also worked in Llanfair Colliery [SN 5002 0214]. The Cille No. 2 Vein is 15 in thick and is said to have a sandstone roof, although this is shown on none of the borehole sections. The Cille No. 1 Vein is at the top of the 'slack', and has been worked at the crop above the main road; according to the borehole records it is 7 to 14 in thick.

The sandstone above the Cale veins is about 210 ft thick, and is quarried in Furnace Quarry [SN 503 017] where about 90 ft of cross-bedded sandstone are exposed, flaggy near the top and massive below. The coal seam, which is said to have been found at the north end of the quarry, is presumably the Cille No. 1, which was present at a shallow depth in Castle Colliery Borehole No. 1, drilled from the floor of this quarry (then known as Mercer's Quarry).

The highest Hughes Beds are dominantly argillaceous and are drift covered under the low ground at Stradey Park. The only complete section of the strata between the Swansea Three Feet and the sandstone above the Cille veins is that proved in Castle Colliery Borehole No. 3, in which these beds were about 260 ft thick. A seam with the section: coal 12 in on 'shale' 5 in, coal 3 in, found within 10 ft of the base of these beds, may be at the horizon of the Lower Maesmelyn Vein of the Neath area (Sheet 247).

The Swansea Two Feet (Bushy) Vein was mined extensively during the 19th century in Old Castle Colliery [SN 4996 0010], where a fault touched at the N.W. edge of the workings may be the Stradey Fault; this seam was proved in Caebad [SN 4998 0067], Cae-main (or -maen) [SN 5025 0048] and Cae'r-elm [SN 4996 0084] pits and in an unnamed pit [SN 4998 0096]. The only record known of the thickness of the Swansea Two Feet is in Old Castle and Caebad pits, in both of which it was said to be 24 in; Strahan (1903, p. 178) gave the thickness as 34 in, in one coal. A coal seam was found during the excavations for Llanelli North Dock (Strahan 1903, p. 178; 1907, p. 21), but the doubt about its identity is increased by discrepancies between these accounts, Strahan's MS. notes and the MS. and published 6 inch map Carmarthen 58 N.W.: the seam, with the section: coal 30 in on 'clay' parting, coal 18 in, was probably the 'Bushy' Vein, and is so shown on the recently published 6 inch map, SN 40 SE. Sandstone, in beds up to about 20 ft thick, is present between the Swansea Two and Three Feet veins, which are about 110 ft apart.

Swansea Beds: general

About 370 ft of Swansea Beds are present in the core of the Llanelli Syncline. The Swansea Three Feet (or Golden) Vein, at the base, and the Lower (Fiery) and Upper (Rosy) Six Feet veins were largely mined out in the 19th century. The Swansea Beds are predominantly argillaceous, but they contain several thin beds of sandstone. These beds are nowhere exposed.

Swansea Beds: details

The Swansea Three Feet (Golden) Vein was mined extensively in Old Castle Colliery, the western limit probably being the Stradey Fault; apart from Caebad and Cae-main pits the Swansea Three Feet was also reached in Erw Pit [SN 5031 0012] and there was an air pit [SN 4906 0092] south of Stradey Castle. The thickness of the coal is shown as 24 in on two shaft sections: sandstone is commonly present between the Swansea Three Feet and the overlying seam, which are about 110 ft apart.

In the district here described the Lower Swansea Six Feet (Fiery) Vein has also largely been worked out, from Old Castle and Cae-main pits. In the section of the shafts at the former colliery (Appendix I, p. 192), the thickness of this seam is recorded as 36 in, and sandstone is present in the 56 ft of strata between this coal and the Upper Swansea Six Feet.

The Upper Swansea Six Feet (Rosy) Vein is also said to be 36 in thick, and has been worked out.

As Old Castle shafts are in the middle of the Llanelli Syncline, they penetrated the highest strata in this area. It can be assumed that the 'walling' of the top 80 ft indicates the presence of drift deposits to approximately that depth; the remaining 200 ft of strata above the Swansea Six Feet include beds of sandstone up to about 17 ft thick, and seatearths.

References

DIX, EMILY and TRUEMAN, A. E. 1931. Some non-marine lamellibranchs from the upper part of the Coal Measures. Quart. J. Geol. Soc., 87, 180–211.

JONES, S. CAREY. 1957. The Northward Attenuation of the Coal Seams in the Swansea District. Proc. S. Wales Inst. Eng., 72, 28–39.

JORDAN, H. K. 1910. The South Wales Coalfield Pt. 2. Proc. S. Wales Inst. Eng., 27, 172–249.

JORDAN, H. K. 1915. The South Wales Coalfield Pt. 3. Proc. S. Wales Inst. Eng., 31, 49–135.

STRAHAN, A. 1907. The Geology of the South Wales Coalfield. Pt. IX. West Gower and the Country around Pembrey. Mem. Geol. Surv.

STRAHAN, A. CANTRILL, T. C. and DIXON, E. E. L. 1903. The Coal Measures of Llanelly, Llannon and Cross Hands in the South Wales Coalfield. Sum. Prog. Geol. Surv., for 1902, 170–91.

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

STRAHAN, A. CANTRILL, T. C. and DIXON, E. E. L. and THOMAS, H. H. 1909. The Geology of the South Wales Coalfield. Pt. X. The Country around Carmarthen. Mem. Geol. Surv.

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

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

Chapter 7 Structure

Intra-Carboniferous movement

The only evidence of intra-Carboniferous tectonic movement is at the base of the Millstone Grit: above the break due to the Sudetic Phase of folding, between the Carboniferous Limestone Series and the Millstone Grit Series, there is a conformable succession of Millstone Grit, Lower, Middle and Upper Coal Measures rocks. On the main outcrop, although there is little or no overlap within the Millstone Grit, the E, Stage and part of E₂ are missing, and the basal beds of the Millstone Grit overstep most of the Dibunophyllum Zone of the Carboniferous Limestone (p. 6). A spectacular development of the unconformity has been described near Kidwelly, where the Millstone Grit oversteps on to Lower Old Red Sandstone, the line of uplift, the Penlan Axis, probably having a Caledonoid trend, like the Vale of Neath Disturbance which also was active in Carboniferous times (p. 6): an alternative structural interpretation is suggested below (p. 142). At Ragwen Point this unconformity is less pronounced than at Penlan, and is more appropriately considered with the north crop of the Pembrokeshire Coalfield (Jones 1907, p. 53; Archer 1965, pp. 137–50).

The possibility of some of the structures in the Coal Measures having resulted from intra-Carboniferous movement is discussed below under the heading of incompetent structures.

Post-Carboniferous movements

As this area contains some of the highest grade anthracite in the world, intensive efforts to mine it have been made; as these efforts have been dominated by the structural complexity, much attention has been paid to it by the miners. This has resulted in the amassing of much detailed structural information, both on the working plans and in numerous detailed sections; in this account only the broad outlines, supported by appropriate examples, will be given.

In Pembrokeshire, the influence on the Hercynian structures of the close proximity of the Pre-Cambrian and Lower Palaeozoic rocks of St. George's Land to the area in which the Carboniferous rocks of South Wales were deposited, is expressed by the greater complexity of the structures there than elsewhere in South Wales. A distinction has been drawn (Strahan and others 1914, pp. 206–13; Cantrill and others 1916, p. 140) between two sets of disturbances, both characterized by overfolding and thrusting with northerly downthrows; one set, the 'Northern Type', is parallel to the Caledonoid trend, that is south of west, and the other, the 'Southern Type', strikes a little north of west, the Armoricanoid trend. As there was no evidence of one set displacing the other, these structures were not assigned to two different orogenic episodes (Strahan and others 1914, p. 207). The boundary between the two structural zones in which these occur is relatively sharp (1914, p. 209); in the course of preparation of the Tectonic Map of Great Britain, Dunning (1964, p. 99) concluded that the division between the Hercynides (or Variscides) and Caledonides should be drawn at the boundary.

In the main Coalfield, including the area here described (Plate 2), the distinction between structures with Armoricanoid and Caledonoid trends is sufficiently marked to have led Strahan to suggest that all the major Caledonoid structures were of mid-Tertiary age (1902, pp. 214–6, 220–1). Trotter (1947, pp. 117–21, 122–3) recognized the two trends, and agreed that the Tawe Valley Disturbance (the Cribarth Disturbance of Strahan), a sinistral transcurrent fault, might have come into existance in Tertiary times, but believed that the other post-Carboniferous Caledonoid structures in the western part of the coalfield were due to the presence to the north of older rocks with that trend, against which the younger rocks were driven. Later (in Owen 1954, p. 363) he made no distinction between the two structural trends. Owen (1954, p. 357) suggested that the Caledonoid trend of the intra-Carboniferous and Hercynian Neath Disturbance may be due to structures in the underlying sub-Devonian rocks. Although all the major Caledonoid structures have been grouped together by several authors (Strahan 1902, pp. 214–6; Strahan and others 1914, p. 206; George and Jones in Trotter 1947, pp. 126, 128), the doubt about the age of some need not imply doubt about all, as it is likely that there are two distinct types of Caledonoid structure, the mechanics of their formation, and hence their age, being different. The axis of least strain was vertical at the time the overfold and thrust structures (the 'Northern Type' of Pembrokeshire, the Llandyfaelog-Careg Cennen Disturbance, the 'compression belts' of Trotter, etc.) were formed, but the vertical axis was that of mean stress at the time of the transcurrent movement along nearly vertical planes which is an essential part of the Neath (Owen 1954, pp. 34850) and Tawe Valley disturbances. Analogy with areas where Mesozoic rocks are preserved suggests that at least some of the movement on the latter is likely to be later than Hercynian. Large scale, mainly dextral, transcurrent Tertiary faulting has been described in Devon and Cornwall (Dearman 1964).

It is believed that in the district here described the structures, including both Caledonoid and Armoricanoid compression structures, were initiated and largely formed at substantially the same time, that is during the Hercynian orogeny. As in analogous areas there may have been further development of the structure during the Alpine orogeny, including faulting and gentle folding, not necessarily exactly coincident with earlier structures (Strahan and Cantrill 1904, pp. 90–3; Trotter 1947, pp. 122–3; Woodland and Evans 1964, p. 273). The structural complexity of this area and of adjoining parts of the Ammanford Sheet is probably due, at least in part, to the convergence of the two structural trends (cf. De la Beche 1846, pp. 223–4); as in Pembrokeshire the boundary between the Hercynides and Caledonides is relatively sharp, and may be drawn at the Trimsaran Disturbance.

The structures of the Gwendraeth Valley and adjoining areas may be classified under the following headings: 1. folds, 2. low-angle thrust and lag faults, 3. incompetent structures, and 4. cross-faults.

Although all these structures occur in both structural areas, no example has been found of a cross-fault crossing the Trimsaran Disturbance the division between the two (cf. the 'Main Thrust' of Trotter 1947, p. 122).

The use of the term 'Disturbance', adopted by previous workers in this area to describe some of the larger, complex structures, is continued as they are commonly a combination of folding and faulting; as low-angle faults are characteristic the disturbances are described under that heading (2).

Folds

Almost the whole area is on the northern limb of the main South Wales 'syncline' which in fact, to the south and south-east of this district, consists of several folds (Strahan 1907a, pp. 32–4 and Plate). One of these folds, the Llanelli Syncline, is the most important fold present in the district described; it trends E.–W. from Llanelli almost to Burry Port, but swings to the north of west when traced to Pembrey. The regional strike on the north crop swings from approximately E.–W. to the east of Ammanford to N.E.S.W. between Pontyberem and Kidwelly, and is about W.30°S. in the Trim-saran area. In the southern part of the district the strike is controlled by the Llanelli Syncline which, eastwards from Burry Port, plunges to the east. At Burry Port the plunge is about 5°; for about 0.75 mile to the W.N.W. the syncline is horizontal, but in the most westerly workings in which it has been proved the plunge is about 12° to the E.S.E.

The regional dip is nowhere very steep. The average proved in workings in the Pumpquart and Carway Fawr veins, both in more competent strata than the other seams worked, varies between about 11° and 16° from Glynhebog Colliery, east of Pontyberem, to Gwendraeth Colliery, east of Carway, but in the workings on the west side of the latter colliery, and in parts of Carway Colliery (in the Big Vein), the dip is nearer 20°. South of the Trimsaran Disturbance the dip proved in relatively extensive workings in the Carway Fawr Vein is about 24°. On the north side of the Llanelli Syncline the dip proved in seams in the competent Pennant Measures increases from about 13° north of Llanelli and near the outcrop between the Stradey and Moreb faults, to between 18° and 21° (locally slightly higher) below Pwll, Burry Port and westwards towards Pembrey, before flattening near the axis of the syncline. Although there are only limited workings on the south side of the syncline, it appears to be symmetrical near Burry Port, but under Llanelli the southern limb, probably dipping at about 20°, has a steeper dip than the northern limb.

The structure beneath the wide expanse of drift deposits west of Kidwelly, Trimsaran and Pembrey, and below the sea in Carmarthen Bay (Cantrill and Dixon 1904, p. 171), remains unknown. The regional strike of the Old Red Sandstone continues in a south-westerly direction as far as the estuary of the River Taf: west of Pendine the Carboniferous rocks at the eastern end of the Pembrokeshire coalfield strike in a north-westerly direction. To the south, in the Gower Peninsula, the strike has an Armoricanoid trend, that is W.N.W., although at the western end, in the competent Brownstones and Conglomerates on Rhossili Down, the strike is north-south, and the axis of the Llanmadoc Hill Anticline is curved (George 1940, p. 171), consonant with the variations in strike between Burry Port and Pembrey. Trueman and George (in George 1940, pp. 196–7) have discussed the possibility of an interruption under Carmarthen Bay of the 'armoricanoid' structures by 'caledonoid' influences. The effect of the easterly plunge of the Llanelli Syncline and of most of the folds in Gower may be compensated by the similar plunge of the folds in Pembrokeshire (Dixon 1921, p. 176). Thus, although the general line of the synclinal depression of the Main South Wales Coalfield almost certainly continues beneath Carmarthen Bay to reappear in Pembrokeshire (Pringle and George 1964, p. 6), it is by no means certain that Coal Measures continue (Martin 1806, p. 343; Ramsay 1846, p. 302), as there is evidence that the Pembrokeshire 'syncline' may close to the east in the Carboniferous rocks, and the convergence of the north and south crops of the main South Wales Coalfield may continue so as to close that area to the west. However, it is possible that the combined effects of the Armoricanoid trend of the structures south of the front, and of the upthrow south of thrusts proved on both sides of Carmarthen Bay, are sufficient to have allowed some Upper Carboniferous strata to have been preserved, so that the main and Pembrokeshire coalfields are separated only by relatively recent denudation (Strahan and others 1909, p. 1). It is clear that there is a closure in the Pennant Measures, as the highest recognizable carboniferous horizon in Pembrokeshire, except near the west coast, is the Picton Point (Cefn Coed) Marine Band (Jenkins 1962, p. 93). Between Kidwelly and Pembrey, the structure is probably as complex as in the nearest areas where these strata can be studied, and the positions of the Gastrioceras subcrenatum and Upper Cwmgorse marine bands, at the base of the Lower Coal Measures and Pennant Measures respectively, shown on the maps are purely hypothetical.

For the reasons explained above, the folds associated with the disturbances are described with those structures. There is no evidence of westerly plunging Caledonoid folds, such as have been described further east (Trotter 1947, p. 122; Owen 1954, p. 336).

Low-angle thrusts and lag faults

(a) North of the Trimsaran Disturbance

The structure of this area is dominated by the Llandyfaelog, or Gwendraeth Valley (Strahan 1903, p. 47), or Llanstephan (Strahan 1904, p. 45), Kidwelly and Llannon disturbances, all of which have a Caledonoid trend (Plate 2). The north-easterly continuation of the Llandyfaelog Disturbance, the Careg Cennen, or Cennen, Disturbance is the 'Main Thrust' of Trotter, who believed it to be of fundamental importance both structurally and in relation to the devolatilization of the coal seams (1949, pp. 393–7). The principal fracture in this Disturbance was originally depicted as a steeply inclined reverse fault (Strahan and others 1907, figs. 10 and 11; 1909, figs. 13 and 14) although thrust faults are associated with it (Strahan 1903, p. 46; 1904, pp. 46–7). Trotter (1949, p. 393) claimed that at Careg Cennen Castle the main fault is a thrust with a hade of about 45°, and suggested that the hade increases to the south. The ground traversed by the Llandyfaelog Disturbance has not been resurveyed and no further evidence is available, but it is now shown as a thrust fault on the One-inch map. Its westerly continuation shown on the map is conjectured: as this structure is considered to be the same as the 'Northern Type' in Pembrokeshire, it may, like them, meet the structures of the 'Southern Type' of the Amroth area, rather than die away before they meet as previously suggested (Strahan and others 1909, p. 140).

With one exception the group of faults constituting the Kidwelly Disturbance downthrow to the south-east (Strahan and others 1909, pp. 140–1); the only fault with a northerly downthrow is the north-westernmost, which is shown throwing Millstone Grit (Tenlan Quartzite') on the north against Senni Beds. Cantrill and Dixon (1904, p. 170) recorded compressional structures believed to be associated with this disturbance on Mynydd-y-Gareg. Although the area has not been re-examined, in view of their structural setting it may be speculated that the faults downthrowing south-east are lags, that the down-throw north is a thrust and that the anomalous position of the Penlan Quartzite' may be due to lag faults cutting out the Carboniferous Limestone, rather than to intra-Carboniferous uplift on the parallel Penlan Axis'.

The western end of the Llannon Disturbance, the third major Caledonoid structure, forms the southern limit of the Cynheidre area; it is joined by the Pont Morlais Fault (Sheet 230), a major Armoricanoid structure, near the eastern limit of the district described. The course of the main dislocation is marked by two valleys, the watershed between which is near Nant-garedig Farm [SN 5178 0660]. The identification of the Brondini veins in the Tack Borehole at the level at which they might be anticipated by extrapolation of the Lower Brondini from Sylen Colliery, indicates that the main fracture is south of the borehole (Jordan 1915, p. 61). To the west the outcrop of the fault is entirely obscured by drift deposits, but it is believed to join the Trimsaran Disturbance, and in the absence of conflicting evidence the approximate position shown on earlier maps has been adopted: strong springs [SN 4944 0565], [SN 5003 0581] are probably associated with it. North of this fracture there is a gentle syncline, plunging N.E., between the Lletty Wilws and Coalbrook faults, the dip of the southern limb being greater than the northern. West of the Coalbrook Fault the syncline is barely perceptible in Sylen Colliery, where the strike is a few degrees E. of N., but the almost horizontal dip of the competent strata in Gwendraeth Valley Borehole No. 2, the very low southerly dip in the outlier on Brondini Hill and the northerly dip near Cilferi-isaf Farm [SN 4784 0543] indicate a gentle syncline in that area. South of the fault the Brondini veins in the Coalpit Hall inner [SN 529 067] are exposed in the core of a faulted anticline, the northern limb of which is overturned in places: to the west a more gentle anticline affects the outcrop of the Brondini veins near Maes-llydan Farm [SN 514 063]. In the area described the effect of the Llannon Disturbance, combined with the Pont Morlais Fault, is a northerly downthrow; the vertical displacement of the fracture is about 200 ft near Capel Sylen [SN 5105 0650]. At the surface, in competent strata, the Llannon Disturbance is an asymmetrical fold, the middle limb of which is faulted; although there is no direct evidence on the inclination of the main fracture, it is believed to be a thrust fault (Strahan and others 1903, p. 184). In the incompetent main coal-bearing strata at depth it is likely to have a complex structure similar to that of the Trimsaran Disturbance.

In addition to the major Caledonoid structures north of the Trimsaran Disturbance, there are numerous other structures with this trend. Between Pontyberem and Pont-Henry the structure of the main productive group of seams is dominated by the Pentremawr Lag Fault. The distance between the Gras Uchaf Vein and the next seam above, the 'Big (Hwch)', has long been known to be very variable (Jordan 1910, p. 211) and the correlation of the latter seam presented several difficulties (Strahan and others 1909, pp. 93–5), but as no seam was known to be present between them in Pentremawr Colliery for a distance, down dip, of over 0.75 mile, the presence of a fault was not suspected. When it was established that the Big (Hwch) is the Big Vein, it followed (p. 57) that there is a lag fault between, and approximately parallel to, the seams over the whole of this area (Archer 1957, p. 30): it was subsequently found to be present in the Cynheidre area (p. 62). A lag fault is also believed to be present in the Pontyates–Plasbach area, and it is possible that this is a continuation of the Pentremawr Lag Fault further to the south-west than is shown on the maps. The presence of a lag fault is the preferred explanation of anomalies in the basal Lower Coal Measures succession in Cwm Clydach, east of Kidwelly (p. 43): these lags are en echelon with the Kidwelly Disturbance, which, as suggested above, may consist of several lags. No substantial thrusts are known between these faults and the underlying Llandyfaelog Disturbance, but several are known between them and the Trimsaran and Llannon disturbances.

In Pentremawr Borehole No. 1 the Soap Vein and about 100 ft of strata were found to be repeated: no corresponding fault is known in the underlying workings in Pentremawr Colliery, and the repetition is clearly due to a thrust; in Tyn-y-cwm and Hafren opencast sites, repeated thrusting of the Carway Fawr Vein was proved (p. 113). N.W. and W. of Carway a thrust which repeats the Pumpquart Vein was proved in Brynfforest Colliery (Figure 9) and the fault postulated to account for the positions of the Pumpquart and Rhasfach veins north of the River Gwendraeth relative to the Pumpquart Vein to the south, is also likely to be a thrust. The thrust in Brynfforest Colliery is co-linear with the thrusts proved in the opencast sites on the Carway Fawr, and may be part of the same structure; in the intervening ground it is apparent from the records of Plasbach Colliery that the structure is complex and may include this fault. Near Carway, the Carway Fawr Vein is sharply folded, the middle limb being very steeply inclined and possibly faulted in part; an easterly trending fault with a downthrow north near Danyquarry Farm may be part of the same structure.

The structure in the Cynheidre area has been proved by mapping, opencast coal prospecting, deep boreholes and, more recently, by the development of Cynheidre Colliery, to be more complex than had been thought. The most important structural elements include a thrust, the Cynheidre Thrust, with an anticline, itself cut by two thrusts and a lag, on the southern, upthrow side, and a thrust or asymmetrical fold between the Coalbrook and Lletty Wilws faults which at the surface cuts high Llynfi Beds: the Pentremawr Lag Fault is present in the main group of seams. Of these structures, only the anticline had previously been recognized (Strahan and others 1907, p. 160); it follows from the fact that this fold is associated with, and above, the southerly inclined Cynheidre Thrust, that it is not present in the vertically underlying Lower and Middle Coal Measures, as might have been expected. East of the Coalbrook Fault, the structure of the relatively large area of Mynydd Sylen occupied by the 'Cenrhos Rock' remains uncertain. The drift-filled broad depression to the W.S.W. of Ystodwen-uchaf Farm [SN 5165 0855] probably marks the position of the continuation of the Cynheidre Thrust, the presence of which is suggested by the marked easterly convergence of the outcrops of the Brondini and Gwscwm veins, probably only partly explicable by attenuation of the strata and easterly increase in dip (p. 129). The presence of horizontal strata in a small disused quarry 500 yd W.S.W. of Ystodwen-uchaf is consistent with this interpretation, and later evidence afforded by the deep boreholes Gwendraeth Valley No. 1, Cynheidre Nos. 5/1 and 5/2 has increased the probability that a thrust is present (Plate 1): any offset of the thrust at the Coalbrook Fault can only be small.

The strike of the lower part of the Llynfi Beds between the Lletty Wilws and Coalbrook faults follows the regional direction, whereas the strike of the Brondini veins is approximately east-west. The thickness of strata between the Lower Brondini and the Lower Pumpquart Vein in Glynhebog Colliery is about 2500 ft at the mouth of Caeglas Colliery [SN 5130 0904] but increases to about 2700 ft 0.5 mile to the east; these thicknesses are affected by the Pentremawr Lag Fault, but the vertical displacement by this fault is probably similar in both. The normal separation is about 2550 ft, and the easterly increase in thickness indicates the presence of a structure throwing down to the north; this is compatible with the evidence obtained from exposures in small streams and in a temporary trench in the area between the outcrop of the Lower Brondini, Capel Bethel [SN 5178 0963] and the main road (B 4306). Most of the area is drift covered and the surface position of the structure is uncertain, but it is shown on Nat. Grid Sheet SN 50 NW some 300 yd S.E. of the position shown on the old map, Carmarthenshire 47 S.E.; as the sandstone at Bont-fawr [SN 5218 0984] may be sandstone 'A' (p. 125) repeated, another 'possible fault', corresponding in part with that shown on the earlier map, is also shown. No comparable structures are present in the Lower Pumpquart Vein, which has been worked extensively up to the outcrop, so that they are assumed to be either southerly dipping thrusts, as suggested by Jordan (1915, pp. 69–71), or asymmetrical folds with similarly inclined axial planes. Neither structure was present in Cynheidre Borehole No. 5/1, but the area to the east, where the throw of the larger structure is likely to be greatest, remains unproved. This structure is en echelon with the Cynheidre Thrust, giving rise to the striking 'S-shaped' configuration of the outcrops between the Upper Cwmgorse Marine Band and the Gwscwm Vein.

In Carway Colliery a relatively large fault, the 'Gower's Overthrust', was proved in the Big Vein. The trend of this fault is anomalous; its intersection with the Big Vein trends about N.10°E., and, as the strike of the Big Vein is about N.20°E., the strike of the fault lies between these directions. The vertical displacement of the Big Vein is about 75 ft. About 150 yd up dip the Penny Pieces Vein is repeated twice in a cross-measures drift between the Big and Soap veins: if the thrusts which cause this repetition are part of the Gower's structure, its inclination is 25° that is 10° steeper than the dip of the strata. The anomalous direction of this thrust suggests that it may have been formed at an early stage, with a Caledonoid trend, and was folded with the strata, the strike of which is also abnormal and probably due to the close proximity of the Trimsaran Disturbance.

(b) South of the Trimsaran Disturbance

The ground intersected by the Trimsaran Disturbance near the eastern margin of the area described is poorly exposed: the main fracture is thought to be present below a wet, drift-filled depression between the upper end of the Upper Lliedi Reservoir (Sheet 230) and the Carmarthen–Llanelli road near Iroshill Farm [SN 493 048]. South of this hollow the strata dip regularly to the south, although the dip is apparently higher near the fault: a syncline is present to the north, the strata, mainly sandstone, exposed by the River Lliedi having a northerly dip. Subsidiary thrusts are exposed in the Lliedi valley and a small tributary stream to the west (p. 155). Westwards towards the crop of the Carway Fawr Vein, the position of the main fracture below boulder clay-obscured ground is conjectured. The strati-graphical position of the strata between the Llannon and Trimsaran disturbances in the Five Roads–Horeb area is unknown, but the faulted sandstone mentioned above, and in temporary sections W.S.W. of Five Roads (p. 125), is probably high in the Llynfi Beds, so that the vertical displacement on the main fracture, like that on the Llannon Disturbance near Sylen, is not large. The Carway Fawr has been mined opencast about 150 yd north and about 250 yd south of the Disturbance, and part of the ground between was proved during prospecting for these sites; the horizontal displacement of this seam is only about 100 yd. Much of the effect of this Disturbance is here seen as large but relatively shallow folds on both sides of the fault.

A fault, or disturbed strata, was encountered on the N.E. side of the Big, Soap and Graigog workings in Trimsaran Colliery; drivages [SN 472 451] were made beyond this fault from the Big Vein (Strahan and others 1909, pp. 128–9). The seam proved, but not successfully worked, north of the fault is probably the Soap Vein, so that the throw of this fault, known to be a thrust from its relative positions in other seams, is not large. The southernmost workings in Carway Colliery, in the Big Vein, from about 200 yd S.W. of Tycanol Farm [SN 4720 4586] to 500 yd N.E. of Genf r-wydd Farm [SN 4601 5083], encountered easterly and north-easterly trending thrusts and belts of disturbed ground: the zone of highly disturbed strata between the Big Vein workings in Carway and Trimsaran collieries is not less than about 600 yd wide. If the uniform dips proved in the Big Vein in Carway and Trimsaran collieries are extrapolated, the total diplacement normal to dip is about 1175 ft (Figure 20). Other direct evidence of the Trimsaran Disturbance is found about a quarter of a mile further west, where intensely folded and faulted strata are exposed by the River. Morlais for about mile near Culla House [SN 451 053] (p. 57). Farther west along the Morlais, although the dips are high in places, the beds are relatively undisturbed, suggesting that the Disturbance is some little distance away. North of Culla House, the Big Vein was worked extensively in Carway opencast site; only two faults are shown on the map, but in fact over much of the area the seam was repeatedly folded and faulted, both types of structure throwing down to the north. (Plate 4).

Thus, the conception of the Trimsaran Disturbance in the Trimsaran area envisaged by earlier workers, that "there is no single line of fracture, but a belt of plication and overthrusting. The lines of fault on the map, therefore, are only diagrammatic indications of the position of disturbed belts" (Strahan and others 1909, p. 133), has been confirmed by subsequent deep mine and opencast workings. It is suggested that the structures known to exist in the incompetent productive group of coal seams in the Trimsaran–Carway area are present in the same strata at depth to the east, where the manifestation of both the Trimsaran and Llannon disturbances in the competent Upper Coal Measures and upper part of the Middle Coal Measures is gentle folding apparently accompanied by relatively clean-cut thrust faults. Evidence in support of this suggestion was obtained from Gwendraeth Valley Borehole No. 2, drilled some 500 yd N. of the Llannon Disturbance; the distance between the Cefn Coed Marine Band and the Pumpquart Vein was found to be nearly 1600 ft, compared with a normal separation of about 1100 ft, the increase being due to repetition at several horizons. In this borehole the strata above the Cefn Coed Marine Band were almost undisturbed (Figure 20).

Apart from the thrust and lag faults proved in Trimsaran Colliery, which are considered as incompetent structures and described below, little is known of the details of the structure in the Lower Coal Measures and lower part of the Middle Coal Measures south of the Trimsaran Disturbance, which for this reason are shown west of Trimsaran as undivided 'Middle and Lower Coal Measures' on the maps. In the Trimsaran Borehole [SN 4483 0416], the Graigog and Amman marine bands were found to be less than 300 ft apart, but the identity of the coal seams between the marine bands is in doubt; as the seams between the 'Two-Feet' and Graigog probably crop out in normal succession to the north of the borehole, a thrust fault, inclined at a lower angle than the strata, is shown on the map to explain the absence of some 200 ft of strata in the borehole. It is possible that the throw of the lag fault proved in the Big and Yard (Ddugaled) veins in Trimsaran Colliery (Plate 3) increases towards its outcrop, intersecting the outcrop of these seams in the drift covered area [SN 451 046] N.W. of Trimsaran School, although underground its throw decreases in that direction: if the fault in the borehole is a lag, rather than a thrust, it must dip at the same angle as the seams between the borehole and the outcrop to the north. Further west the structure is even more uncertain; most of the area is drift covered, but field evidence and the limited documentation of old attempts to work coal, strongly suggest that the structure is complex (pp. 57, 74). Although Ashburnham boreholes Nos. 1 [SN 4098 0289], 3 [SN 4032 0366] and 4 [SN 3906 0302] were drilled into the Lower or Middle Coal Measures below the large expanse of blown sand and marine alluvium west of the main railway line, in the absence of palaeontological evidence the sequences (Appendix I) and hence structures, proved cannot be interpreted: the unfamiliar sequences suggest structural complexity. The structures in the Coal Measures on the western side of Carmarthen Bay, between Amroth and Saundersfoot, are complex and it is probable that similar Armoricanoid thrust and fold structures are present between Trimsaran and the sea. 'Probable' and 'hypothetical' faults shown on older maps (Strahan and others 1909, p. 132) have been omitted, since the uncertainty is so great. The boundary between the low-lying, largely drift-covered area and the higher, relatively drift-free, ground, runs in a south-westerly direction from Trimsaran, and is marked by a steep scarp feature, oblique to the strike between Bryndias [SN 4355 0345] and Coed [SN 4265 0245] farms, but formed by the Gwscwm group of seams and the overlying sandstone beyond Coed. This feature may be a fault line scarp but the change of dip from south-easterly to south-westerly along part of this Caledonoid line is not necessarily supporting evidence as suggested by Strahan and others (1909, p. 132), as a similar change is also evident in the Upper Coal Measures N.E. of Pembrey, there not associated with obvious faulting.

The only other known fault with an Armoricanoid trend is on the southern limb of the Llanelli Syncline: this is the continuation of a fault which runs to the south of Pen-y-fan quarry [513 994] (Sheet 247), itself believed to be the westerly continuation of the Carnarfon Fault (Strahan and others 1903, pp. 177–8). The Carnarfon Fault is said to consist of at least five step faults, all downthrowing south, the total throw being 420 ft (Strahan 1907b, p. 95); an overthrust is known on the northern limb of the Llanelli Syncline in that area, and it is not impossible that the Carnarfon Fault may be connected with a complementary structure on the southern limb.

Incompetent structures

Mining of the main group of anthracite seams, in the upper part of the Lower Coal Measures and lower part of the Middle Coal Measures, is largely dominated by the presence in them of numerous relatively small-scale structures. Such structures, briefly mentioned by Trotter (1947, p. 99) and concisely described in the eastern part of the coalfield (Woodland and Evans 1964, pp. 258–63), are much more abundant in these beds than in those above and below. The contrasting response to stress of different parts of the Coal Measures is due to variation in competence, clearly largely determined by the abundance and thickness of coal seams and their unbedded, commonly soft, seatearths. This is strikingly demonstrated in Pembrokeshire, where the main productive beds are highly contorted, while the underlying dominantly argillaceous Millstone Grit Shales are almost undisturbed (Archer 1965, p. 138): similarly the Millstone Grit Shales, basal Coal Measures and upper part of the Middle Coal Measures of Carmarthenshire consist mainly of argillaceous rocks, the proportion of sandstone and striped beds being similar to that in the main productive group of strata, yet they seldom contain these structures except sometimes in close association with coal seams. This contrast in deformation can be demonstrated by considering the dips recorded in deep boreholes; in (Figure 21) the contrast is clearly marked, the dip above the Soap Vein being relatively uniform, except in proximity to faults, but very variable below. The contrast in the form of major disturbances in competent and incompetent strata has been described above in the account of the Trimsaran Disturbance. The four main types of incompetent deformation are as follows :

1. The thickness of individual seams is commonly subject to considerable variation, so that in extreme cases it is impossible to determine their original thickness. For example, the Big Vein in Pentremawr Colliery is known to vary from 0 to at least 40 ft thick; current mining practice is to take out the top six or seven feet, not uncommonly leaving coal in the floor so that the full thickness is not known, although shown to be substantial in some districts when it is necessary to 'cut bottom' to maintain main haulage roads which are subject to 'pooking' (the floor rising due to stresses created about the excavation). Areas where the roof and floor of a coal seam are in contact, or separated by 'rashes' (see below) are usually described as 'washouts' by the miners, although examination of several such areas showed them to be of structural, rather than depositional, origin. Another extreme example of the variability in thickness of seams has been given in the account of the Hwch Vein in the Cynheidre area, an extraordinary thickness of coal being recorded in some of the deep boreholes (p. 78); on the other hand, the Big Vein in Cynheidre Borehole No. 4/1 had a poor section consisting of: coal 1 in on 'rashes' 3 in and coal 23 in, whereas a nearby drivage in this seam proved: coal 72 to 84 in on 'rashes' 2 to 22 in and coal 0 to 48 in, a 'normal' section for this area (Figure 22).

The coal has yielded by plastic deformation, as well as by shearing, as the coal in abnormal sections is only locally comminuted. As a consequence of the susceptibility of the coal to 'flow', the information on seam thicknesses obtained in boreholes, even where core recovery is 100 per cent, can, at best, only be treated as a general indication of the presence of a seam and of its quality: to a lesser extent this principle applies to shafts and cross-measures drifts. The thickness and structure, and hence the workability, of at least some of the seams cannot be firmly established until a substantial length has been exposed. Although generalizations must be approached with great caution, possibly the Gras Uchaf, Stanllyd/Hwch, Big and Soap veins are subject to greater variability than the other seams, the variation in the Gras Uchaf perhaps being due to its proximity to the Pentremawr Lag Fault in the principal areas where it is known.

2. Closely related to variations in coal thickness is shearing of the seatearth and immediate roof, particularly likely where the latter consists of dark grey mudstone and where mudstone is overlain, at a short distance, by arenaceous rocks. A result of such shearing is the formation of a bed of weak rock ('clod') which commonly has to be mined with the coal, and is more or less persistent above some of the seams. Shearing of the roof, coal and floor, producing an intermingled mass of sheared mudstone, coal and seatearth in small fragments bounded by listric surfaces, is not uncommon; the mixture is known locally as 'rashes', its thickness often being very variable.

3. Thrust and lag faults present in one seam and not in adjacent seams above and below are regarded as incompetent structures, although the distinction between those with a larger throw and the low-angled faults described above is arbitrary, since at least some of the latter reflect the relatively incompetent nature of the Lower and Middle Coal Measures as a whole and probably did not continue into the Upper Coal Measures. In general three sets of these low-angled faults can be distinguished, their trends being approximately E.–W., commonest in Carway and Ponthenry collieries, but also present in the Ddugaled Vein in Trimsaran Colliery; N.W.–S.E., present in most of the area except doubtfully in Carway Colliery; N.E.–S.W., only recorded commonly in the workings in the Ddugaled, Big and Soap vein. The plans of Pumpquart Vein workings show thrust structures with these trends to be uncommon, but some of the relatively common N.–S. faults in this seam in Glynhebog and Pentremawr collieries are recorded as 'overlaps'.

The Ddugaled Vein and the strata associated with it are particularly susceptible to repetition by thrust faulting, as proved in cross-measures drifts in Cynheidre Colliery (Figure 23), and the deep boreholes in that area: out of 10 holes it was cut out by the Pentremawr Lag Fault in one, but was present twice or more in six of the others, in one borehole being present six times in 70 ft of core. In Cynheidre, and in Carway Colliery, where the Ddugaled was successfully mined for a distance of about 30 yd above and below a thrust fault, this seam and the overlying strata, but not everywhere the seatearth, are remarkably undisturbed even in close proximity to thrust faults, suggesting that the competence of these beds, including the coal ('Ddugaled' maybe translated as 'black and hard'), is such that the strain was normally relieved by faulting rather than by plastic deformation.

Faults with a displacement up to 50 or 60 ft in one seam are not uncommonly absent in seams worked not more than 100 ft above or below (Plate 3). Opencast coal sites have provided opportunities to study the accommodation of these structures in the intervening measures, the displacement along the fault plane being replaced by slip along numerous bedding planes and by local thickening of the least competent strata.

Carway opencast site provided striking proof that at least some thrust faults were formed at an early stage in the tectonic history, as suggested for the Gower's thrust (p. 144). The Big Vein was seen to be doubled by a thrust plane which was parallel to the seam for about 75 yd, but which cut the overlying strata at an oblique angle: the doubled seam was asymmetrically folded, the height of the nearly vertical limb being about 20 to 30 ft.

Although many of these faults have a small throw, they have an important bearing on the deep-mining of the seams, particularly by modern mechanized methods.

4. Small scale folds are commonly present: the smallest give rise to local variations in the thickness of a coal seam when present in either roof or floor. The larger, but still relatively small folds affecting a greater thickness of strata are often asymmetrical and are replaced, at least in part, by thrust faults: typically they are disharmonic. The folds, known locally as 'rolls', generally have the same trends as the low-angled faults, as do the zones of disturbed coal and 'washouts'.

Although the examples of incompetent structures given are largely drawn from the weakest part of the cyclothems, i.e. the seatearths, coals and immediate roofs, in the more intensely disturbed areas (e.g. as exposed by the River Morlais near Trimsaran) they are present throughout the strata.

The combined effect of these structures, characteristically not continuous from one seam to another, is that the inter-seam distances are very variable (Trotter 1947, p. 96), and as a corollary, even where the structure of one seam is known in detail from extensive workings, it is impossible to predict other than the generalized structure even in adjacent seams (Figure 20) and (Plate 3). The presence of these structures in one seam but not others led some authors to suggest that they were due to intra-Coal Measures folding (e.g. Robertson 1932, pp. 193–9). Although it is difficult, in view of the complexity of the later structures, to prove that there was no intra-Coal Measures folding, other than the gentle differential downwarping which gave rise, for example to split coal seams, the absence of evidence of unconformities and the relative scarcity of these small scale structures in the Millstone Grit and Lower Coal Measures up to and including the Pumpquart Vein renders this explanation unlikely, unless there was intra-formational movement, itself unlikely in the generally tranquil environment of Coal Measures sedimentation.

Cross-faults

Except near the Llanelli Syncline, steeply inclined faults trending in northerly or northerly-westerly directions are relatively unimportant. Farther east, in the Ammanford area, tear faults along which movement in a generally northerly direction took place during the main period of compression, and an independent system of tension faults, provisionally thought to be post-Triassic, have been recognized (Trotter 1947, pp. 93–4). The throw of most of the faults which cross the Llanelli Syncline, none of which extends as far north as the Trimsaran Disturbance, increases towards the axis of the syncline, but apart possibly from the Stradey Fault, they cause little, if any, displacement of the axis. Although there is no evidence in this area, the throw of other faults in this series (p. 153) reaches a maximum near the axis of the syncline (Strahan 1907b, p. 97), strongly suggesting that the faults and the fold were formed contemporaneously. Woodland and Evans (1964, pp. 252–8), having reviewed the evidence for the existence of two independent but parallel sets of dip faults in other parts of the coalfield, concluded that in the Pontypridd district they were all formed in close association with the Armoricanoid structures; following these authors the term cross-fault is adopted.

The widely believed suggestion that at least some of the cross-faults were active during the deposition of the Coal Measures, and therefore affected the thickness of the coal seams (e.g. Fairley 1868, p. 22) or that lines of seam-splitting coincide with these faults, is entirely unsupported by recent work.

North of the Trimsaran Disturbance, the most important cross-faults are the Lletty Wilws and Coalbrook faults, both of which, arbitrarily, were taken in part as the boundary of the area described. The former has been proved to consist of a complex series of fractures in the Pumpquart Vein, with a net downthrow west. The latter has an easterly downthrow which, in the Pump-quart Vein, increases from about 30 ft near the outcrop to 90 ft 0.75 mile to the south; the throw decreases again farther south where, in Sylen Colliery, it has a throw of 27 ft in the Lower Brondini, but it was not recognized in Cynheidre Borehole No. 4/3 and may have died out in the beds below the Lower Brondini. A series of small faults en echelon to the Coalbrook Fault was proved in the Pumpquart Vein. The Cynheidre Thrust, or an asymmetrical fold replacing it, if present east of the Coalbrook Fault, can only be displaced a small distance by it.

The maximum displacement of the Syddyn Fault, which throws down west, occurs north of the River Gwendraeth, where it is exposed near Syddyn Farm [SN 4860 1062]; in Pentremawr and Ponthenry collieries its throw was not proved, but it is likely to be less than 50 ft, and it was not found in Cynheidre Colliery. The Pont-Henry Fault has a downthrow of 50 ft to the east near the outcrop of the Pumpquart Vein, but dies out to the south, probably in the Soap Vein workings in Caepontbren Slant; en echelon fractures were proved in Pont-henry Colliery. The fact that the Pentremawr Lag Fault is displaced by the Coalbrook, Syddyn and Pont-Henry faults, apparently by the same amount as the seams at the outcrops, suggests that at least some of the movement of these faults was later than movement on the lag, but is not conclusive evidence since this movement could have continued on both sides of the cross-faults after these were initiated. The relative displacement by the Coalbrook Fault of the lag fault and the seams at depth in Cynheidre Colliery has not been proved.

Small cross-faults with both easterly and westerly downthrows were proved near the eastern limit of workings from Caepontbren Pit, but have not been recognized at the surface. Further west, steeply inclined strata are exposed very near a westerly downthrow which was proved in Pumpquart Colliery (p. 53), but the displacement of the Gwendraeth Vein between Carway and Plasbach Colliery is thought to be due to a Caledonoid thrust rather than to the continuation of this cross-fault, the Fforest Fault', south of the River Gwendraeth as shown on earlier maps (with an easterly downthrow). Other grounds for continuing this fault (Strahan and others 1909, pp. 106, 110, 120) have been proved invalid; the 'Big Vein' of Carway has now been shown to be the Big rather than the Stanllyd Vein, so that there is no obvious displacement of this seam, and the overlap of about 150 yd in workings in the Big and Carway Fawr veins disproves the existence of a steeply inclined fault further south. A fault touched, but not proved, at the western limit of the workings in the latter seam may be an abnormally trending (cf. the Gower's Thrust) lag fault, not shown on the maps but concealed by boulder clay west of Blaen Carway [SN 4722 0675].

Several relatively small cross-faults displace the Millstone Grit; one of these gives rise to a prominent gap in the escarpment 1000 yd W.S.W. of Meinciau, followed by the Kidwelly to Meinciau road.

South of the Trimsaran Disturbance the Llanelli Syncline is cut by several cross-faults, the three largest of which, the Stradey, Moreb and Stanley Bar faults, are the continuation of a series of easterly downthrows (Strahan and others 1903, pp. 173–5) which extends westwards from Pontardulais (Sheet 230).

Two westerly downthrows, the Nine Fathom and Twenty-eight Fathom faults were proved in Old Castle Colliery, Llanelli, presumably with throws of 54 ft and 168 ft; a belt of disturbed ground about 200 yd wide in the Cille No. 2 Vein, across which there is little or no net throw, may correspond to the former fault, but there is no evidence for the northward continuation of the latter. Where the Stradey Fault displaces the outcrop of the Hughes and Pwll Little veins its throw is about 600 ft; the Moreb Fault throws the Cwmmawr Vein against the Goodig Vein, about 340 ft below, and the westerly downthrow of the Garreg Lwyd Fault increases southwards from 0 to 100 ft in the Gwscwm Vein in a distance of about 500 yd, and is probably about 200 ft at the synclinal axis. The next fault to the west, in which a pronounced swing of strike has been proved, has an easterly downthrow of about 50 ft under the main line railway. The throw of the Stanley Bar Fault increases rapidly; it does not affect the outcrop of the Cenrhos Vein but the depth to the Goodig Vein in the Copper Works Shaft shows it to throw the Gwscwm Vein about 270 ft down east near the axis of the syncline, below Burry Port. To the west several cross-faults which do not extend southwards into the workings in Gwscwm Colliery were proved in the Cenrhos Vein and at the outcrop of the Gwscwm Vein, but conversely the West Bar Fault, a westerly downthrow of about 30 ft in those workings, is not recorded in the main slant of Ffrwd Colliery in the Cenrhos Vein. In Gwscwm Colliery there is a small easterly downthrow en echelon with the West Bar Fault.

The structure of the western end of Mynydd Pembrey is complex, there being several cross-faults west of the West Bar Fault; all the available records suggest that efforts to find workable coal failed, except for the workings believed to be in the Rhengog Fawr Vein at Craiglon Colliery, which extended for about 300 yd westwards under the blown sand without encountering any substantial fault.

Details

Only a few examples are presented in detail to illustrate the structures. Most of the detailed information has been derived from coal workings, both deep-mine and opencast, either by examination of the workings, plans or other records. Diagrams based on old sections are shown in (Figure 9); the recent, very detailed sections on a scale of 1 to 250 on which (Figure 22) and (Figure 23) are based, were prepared by National Coal Board staff in the Planning Department of Cynheidre Colliery, guided by Mr. R. H. Price, then Div. Geologist to the South-Western Division of the N.C.B.: (Plate 5A) is reproduced by courtesy of the National Coal Board.

Low-angle thrust and lag faults

River Morlais, near Coed-cyw Mill: disturbed and faulted strata close to the Llannon Disturbance and Lletty Wilws Fault include anticlines plunging to the N.E. and S.W. [SN 5315 0751].

Near Ty'r Bryn Farm [SN 5270 0672], between the Coalpit Hall inlier and the Llannon Disturbance: sandstone dips N.W. at 55° in the farmyard; in 1902, tightly folded sandstone, inverted in places, was exposed in the lane 200 to 250 N.E. of the farm.

Tributary to the River Lliedi, 0.75 mile E.S.E. of Five Roads; two small disused quarries [SN 5014 0508]; [SN 5011 0550] and adjacent stream section [SN 4999 0508]: Pennant-type sandstone, dip generally northerly, is intersected by a thrust fault; the gouge, 1 to 5 ft thick, consists of fine sand with coal streaks and angular blocks of sandstone.

The thrust dips at about 15°, to the E. of S. Downstream the dip of the sandstone is low and variable; gentle folds trend E.20°S.: further downstream and in a nearby quarry [SN 5024 0505], sandstone is cut by another fault, probably parallel to, and below, the first. In the Lliedi valley, sheared sandstone, siltstone, silty mudstone, seatearth ? and coal streaks, 6 in to 3 ft thick, are exposed in the N.W. corner of a disused quarry [SN 5035 0505] and in the bed of the river [SN 5029 0517]: this gouge appears to dip with the strata at a low angle in a northerly direction.

Small thrust faults are exposed in many of the sections in the Lower and Middle Coal Measures, e.g. in a small stream [SN 4963 0935] N.W. of T'yn-y-cwm Farm, where strata near the Carway Fach Vein are cut by a thrust dipping in a southerly direction at 36°.

Details of the Pentremawr Lag Fault are illustrated in (Figure 22) and (Figure 23) and (Plate 1). In Cross-measures No. 4 on the 560 Horizon (Figure 22) of Cynheidre Colliery there is a fault between the Gras Uchaf and Big veins, as had been deduced in the Pentremawr area; 300 ft below, on the 660 Horizon, the Ddugaled and Ddugaled Rider veins, absent at the outcrop, were present beyond disturbed ground above the Gras Uchaf. The position of the Big Vein between these horizons was established by the Big Vein drivage, providing conclusive evidence that the Pentremawr Lag Fault continues into the Cynheidre area, and that to the south its inclination relative to that of the strata increases. In boreholes further south it was proved at lower stratigraphical horizons, and it was probably below the bottom of boreholes 3/2 and 4/2, although because its vertical displacement tends to decrease as it cuts down through the strata (Plate 1), it is not so readily recognizable. In those areas where it is parallel to the seams, and most of the strata between the Gras Uchaf and Big Veins, normally about 200 ft apart, are missing, the apparent south-easterly translation along the fault plane is more than 0.75 mile, as shown, for example, by the 'want' in the Ddugaled Vein measured in that direction. Although partly explicable as due to movement oblique to the dip of the fault, the variation in the translation along the fault is probably due to accommodation structures in the incompetent parts of the strata overlying the fault (Figure 24). There is some evidence of greater complexity above the lag, e.g. the constant repetition of the Ddugaled Vein commonly found in the Cynheidre area, where this seam is everywhere above this fault. Variations in vertical displacement can be similarly explained. Near the outcrops S. and S.E. of Pontyberem, where the lag fault cuts across the strata above the Big Vein, much less detail is known, but the evidence from Pentremawr boreholes Nos. 2, 3 and 4 (Appendix I) indicates that the vertical displacement, and probably the movement along the plane, is also small in that area.

The Pentremawr Lag Fault is a relatively simple structure on Cross-measures No. 4 (560 horizon) but is more complex on the 660 horizon; in other areas it consists of multiple shear planes (Figure 23).

The thrust faults proved in the Carway Fawr Vein S.E. of Pont-Henry were not proved in the workings of Ponthenry Colliery (Plate 3), but probably caused the repetition of about 200 ft of strata in the de-watering borehole into Caepontbren Colliery workings (p. 91); if so, their average dip is about 10° greater than the strata.

Incompetent structures

N. of Trimsaran, in the River Morlais from about 400 yd E. to about 300 yd W.S.W. of Culla House [SN 4511 0529], discontinuous sections of disturbed strata associated with the Trimsaran Disturbance are exposed. The structures present include asymmetrical folds, e.g. a syncline with the northern limb dipping S. at 40°, the southern limb vertical [SN 4538 0540]; an anticline [SN 4527 0528], northern limb dipping at 36°, southern limb vertical, the fold axis striking E.–W., and thrust faults [SN 4504 0515], [SN 4526 0531], associated with generally high dips.

(Figure 22) and (Figure 23) and (Plate 4) and (Plate 5) illustrate the four main type of incompetent structures proved in coal workings, as follows:

1. The deformation and variation in thickness of the seams, conspicuous in the Big Vein, but in some degree common to all, are recorded. In parts of the Cynheidre area the Green Vein and its seatearth are very close below the Big Vein, so that the original combined thickness of coal and seatearth was substantial, making these beds particularly susceptible to deformation of the type shown : this may be contrasted with the relatively uniform thickness of the Ddugaled Vein. The sections illustrate the difficulty in estimating the thickness of the seams, and hence the reserves, even when they are exposed in cross-measures drifts.

2. Involvement of the immediate floor and roof of the Big Vein in the 'flow' structures is demonstrated in both (Figure 22) and (Figure 23), below the Gras Uchaf in the former, and below the Gras Isaf in (Plate 5A): the distortion illustrated in that Plate is an extreme example of the incompetent structures found throughout the area described, and not restricted to Cynheidre Colliery. The normal section of this seam, only a few inches thick, is seen in the top right-hand corner of the Plate.

3. Numerous relatively small thrust faults are shown, generally inclined in a southerly direction at about 20° to 30° to the bedding, the biggest (Figure 23) throwing the Hwch Vein against the Ddugaled Rider, normally about 80 ft above. In one of the sections (Figure 23) the Ddugaled and Ddugaled Rider are present three times in a distance of about 200 yd. Small lag faults cut the Big Vein in (Figure 22) and reduce the distance between the 'middle' Ddugaled and Ddugaled Rider in (Figure 23).

Faulting on a very much smaller scale is shown in (Plate 5B). The faults displace dark grey, finely micaceous mudstone and a relatively competent pyrite-impregnated bed 0.15 in thick, part of the Trimsaran Marine Band in Cynheidre Borehole 3/2. The fractures did not extend into the beds above and below, the shortening of the faulted bed being accommodated by bedding plane slip, as shown by highly polished planes at the top and bottom of the specimen. The pattern of the faults is similar to that believed to affect at least the competent rocks in part of the Cynheidre area (Plate 1), although the presence of a complementary set of faults is unusual.

4. Small scale folds associated with thrust faults are present in the Hwch and Ddugaled veins in (Figure 23). Larger-scale asymmetrical folds, with an amplitude of about 25 ft, are shown in Plate 4: such folds were typical of the structure of the Big Vein as mined in Carway opencast site. Although proof is lacking, as the underlying seams have not been worked, these are probably disharmonic folds. The fold in (Plate 4B) is cut by a thrust fault near the top left-hand corner of the photograph: the amplitude of the fold in the Big Vein, partly obscured by talus; is less than in the overlying interbedded sandstone and silty mudstone, which are cut by another thrust fault in the left centre.

An unusual example of incompetent folding was proved in Cynheidre Borehole No. 4/3, in which there is an overfold in the Amman Marine Band and the Gwendraeth Vein (Appendix I); the Gwendraeth Vein is absent in the faulted middle, inverted, limb of the fold.

On the section shown in (Figure 22) the distance between the Ddugaled and Big veins varies from about 110 ft to 140 ft over the short distance, about 100 yd, where both were proved: the 'normal' i.e. undisturbed, thickness of the intervening strata is probably about 90 ft in the Cynheidre area.

References

ARCHER, A. A. 1957. In Sum. Prog. Geol. Surv. for 1956.

ARCHER, A. A. 1965. Notes on the Millstone Grit of the North Crop of the Pembrokeshire Coalfield. Proc. Geol. Assoc., 76, 137–50.

CANTRILL, T. C. and DIXON, E. E. L. 1904. The Coal Measures of the Valley of the Gwendraeth-fawr in South Wales. Sum. Prog. Geol. Surv. for 1903, 162–71.

CANTRILL, T. C. and DIXON, E. E. L. THOMAS, H. H. and JONES, O. T. 1916. The Geology of the South Wales Coalfield. Pt. XII. The Country around Milford. Mem. Geol. Surv.

DEARMAN, W. R. 1964. Wrench-faulting in Cornwall and South Devon. Proc. Geol. Assoc., 74, 265–87.

DE LA BECHE, Sir HENRY. 1846. On the formation of the Rocks of South Wales and South Western England. Mem. Geol. Surv., 1, 1–296.

DIXON, E. E. L. 1921. The Geology of the South Wales Coalfield. Pt. XIII. The Country around Pembroke and Tenby. Mem. Geol. Surv.

DUNNING, F. W. 1964. The British Isles. Tectonics of Europe, Moscow, 87–103.

FAIRLEY, W. 1868. Practical Observations on the South Wales Coalfield. London.

GEORGE, T. N. 1940. The Structure of Gower. Quart. J. Geol. Soc., 96, 131–98.

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JONES, O. T. 1907. In Sum. Prog. Geol. Surv. for 1906.

JORDAN, H. K. 1910. The South Wales Coalfield Pt. II. Proc. S. Wales Inst. Eng., 27, 172–249.

JORDAN, H. K. 1915. The South Wales Coalfield Pt. III. Proc. S. Wales Inst. Eng., 31, 49–135.

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STRAHAN, A. 1904. In Sum. Prog. Geol. Surv. for 1903.

STRAHAN, A. 1907a. The Geology of the South Wales Coalfield. Pt. IX. West Gower and the Country around Pembrey. Mem. Geol. Surv.

STRAHAN, A.1907b. The Geology of the South Wales Coalfield. Pt. VIII. The Country around Swansea. Mem. Geol. Surv.

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Chapter 8 Pleistocene and Recent

Glacial

The deposits of Boulder Clay and Glacial Sand and Gravel in this district are directly connected with the widespread cover of ice which was present at intervals during the Pleistocene Period over much of Great Britain. The common occurrence of erratic boulders even on the highest ground demonstrates that the whole area was covered by ice. The erratics all have a Welsh provenance, no evidence for the presence of Irish Sea ice, which was present in South Pembrokeshire and Gower, having been found. Although some authors (e.g. Griffiths 1939, Mitchell 1962) have ascribed deposits in the South Wales Coalfield to more than one glaciation, in the district described, as in the Pontypridd area (Woodland and Evans 1964, p. 275), there is no evidence that any of the glacial deposits belong to more than one period. From the degree of dissection of the deposits it is likely that this was during the last, Newer Drift, glaciation.

The distinction between 'Boulder Clay', that is till or ground moraine as deposited by the ice and not significantly modified later, and 'Glacial Sand and Gravel' is clear in most of the area; the difference between this area and eastern parts of the coalfield, where there is a complete transition between the two deposits (Woodland and Evans 1964, p. 277), probably reflects the contrast between the wide valleys of the former area and the deep narrow eastern valleys where glacial meltwater, responsible for much of the resorting, was narrowly confined.

Much of the area occupied by the Millstone Grit Shales and the Lower and Middle Coal Measures is mantled by till, which extends on to the Llynfi and Rhondda beds around Cynheidre, the upper part of the Morlais valley and Five Roads, whence continuous deposits of till are present from this watershed down the Cwm-mawr, Dulais and upper Lliedi valleys to the Llanelli area.

Over most of the area the till consists of a tenacious bluish grey clay with striated boulders, ranging up to many feet across, predominantly of material derived from the Coal Measures, but accompanied by farther travelled rocks, commonly including the tough quartzite of the Basal Grit, but with Carboniferous Limestone (seldom seen in natural sections), Old Red Sandstone and Lower Palaeozoic material also present. Variation in both the matrix and the boulder content confirm that the ice moved obliquely, in a southwesterly direction, across the Gwendraeth fawr valley (Strahan and others 1909, pp. 143–4). Near the watershed between the Gwendraeth fawr and fach valleys, for example near Meinciau [SN 4623 1091], [SN 4598 1066] and near the top of Aberlledle [SN 4799 1214], till overlying Millstone Grit is commonly red, mottled red and grey in places, with less clay in the matrix; similar till is locally present substantially farther from the outcrop of the Old Red Sandstone, for example south of Trimsaran where up to about 5 ft of mottled red, yellow and brown till rests [SN 4510 0408] on the bluish grey till typical of this area. Near Kidwelly and on the northern slopes of the valley the proportion of Upper Carboniferous material is higher, but in places the matrix is brown and sandy. Although the change is not so well marked as in the adjoining areas to the east, the passage of the ice over the Pennant Measures is reflected in the till by the increasing proportion of sand in the matrix and of boulders of pennant-type sandstone, so that at the southern edge of the area described, for example, under Llanelli [SN 5010 0013], the till is arenaceous and not always readily distinguishable from Glacial Gravel unless well exposed. This variation in the composition of the till is not, however, consistent: sections [SN 5013 0563], [SN 5001 0557], [SN 5034 0507] in and near the Lliedi Valley E. and S.E. of Horeb show stiff sandy till interbedded with lenses of sand and gravel, while in the same district up to about 20 ft of stiff bluish grey till, slightly sandy in parts, the boulders dominantly of pennant-type sandstone but with Millstone Grit and Old Red Sandstone also present, are exposed in a brick-pit [SN 499 054]; a mile farther south, in Afon Dulais [SN 4895 0351], interbedded stiff bluish grey till and sandy till are exposed.

The delimitation of the till deposits sometimes proved difficult, especially where the 'solid' rocks are dominantly argillaceous, giving rise to similar poorly drained soils (reflected by similar floras) and to impersistent features. The criterion was adopted that an area was mapped as 'solid' if the features were believed to be due to 'solid' rocks, even if drift deposits up to 2 or 3 ft thick were known to be present in the vicinity. The thickness of the till is very variable. On Carway opencast site it was about 50 ft thick over much of the area: a trial borehole [SN 4942 0718] in the col between Cynheidre and Brondini hills was drilled to 100 ft without proving the total thickness. In both natural and artificial sections steep slopes in the typical bluish grey till are relatively stable, but when disturbed and exposed to water and frost, as in temporary mounds formed during the excavation of an opencast coal site, its stability is very considerably reduced and it has a strong tendency to flow.

Morainic mounds of sand and gravel, deposited across the valleys by meltwater from the snout of the glaciers during a pause in their recession, are present in the Gwendraeth fach valley about 1.5 miles above Kidwelly and in the valley of the Gwendraeth fawr 0.25 mile east of Pontyberem (on the eastern margin of the area described), at Pont-Henry and at Pont-newydd, 1.5 miles downstream of Pontyates. These moraines are poorly exposed, but probably consist of torrent-bedded, poorly sorted material as they do in adjoining areas. Outwash gravel related to these moraines is present as discontinuous terrace deposits in both the valleys: a section [SN 4986 1093] near Pontyberem consists of coarse gravel composed of Coal Measures sandstone and siltstone, quartzite and Old Red Sandstone rocks, 6 ft thick, on 22 ft of sand, some beds clayey and others with abundant coal detritus. In another section, near Kidwelly, 6 ft of gravel, including large boulders, rest on light brown stiff till [SN 4172 0727].

Other deposits of glacial sand and gravel include spreads from Meinciau almost to Crwbin, near or on the watershed between the two Gwendraeth valleys, which rest on red till in places [SN 4688 1228], [SN 4728 1269] and include some clay in the matrix; these deposits may in part be gravelly till, or till partially resorted by streams marginal to the retreating glaciers. Small dry mounds of sand and gravel, generally poorly exposed, are present between Cynheidre and Llanelli; one of these near Five Roads [SN 4905 0531] has been quarried to a very limited extent. A similar mound south of Trimsaran was proved by the Trimsaran Borehole [SN 4483 0416] to be at least 40 ft thick. The small patches of gravel between Burry Port and Llanelli are also poorly exposed: the most westerly [SN 436 006], a mound aligned to the N.N.W., surrounded, and partly overlain, by blown sand, consists largely of local material, the larger pennant-type sandstone pebbles being subangular and the smaller rounded, but containing some quartzite and Old Red Sandstone; at one time this deposit was quarried.

Gravity accumulations

On the lower parts of all but the gentlest slopes there are accumulations of material which has been transported downslope, largely under periglacial conditions when solifluxion processes were widespread. As these accumulations, which are included in the term 'Head', are so widespread and are not always readily distinguishable from till, which is commonly incorporated in the deposits, they have been mapped separately only at the foot of the long sandstone dip-slope rising northwards from Stradey Park, Llanelli; there the upper limit shown on the maps is arbitrary. Examples of exposures of Head, up to 15 ft thick, include one of the mouths of Ffrwd Colliery [SN 4302 0282] and in an excavation [SN 4287 0162], on the scarp and dip slopes respectively of Mynydd Pembrey, and in an air shaft [SN 5077 0636] to Sylen Colliery. In these sections the Head consists mainly of pennant-type sandstone, commonly subangular or angular, but sometimes rounded near the base, accompanied by a few erratics, in a brown clayey sand or sandy clay matrix; the deposits are commonly roughly stratified. Head up to 12 ft thick, consisting of angular limestone debris including large boulders, in a matrix of reddish brown clay, is exposed [SN 4151 0803] in a cutting on the main road nearly a mile N.N.E. of Kidwelly.

Mass movement of material as landslips is not common, although several small examples are shown on the maps. Of greatest interest, perhaps, is a landslip at the western end of Mynydd Pembrey [SN 421 019], in which a mass of sandstone some 200 yd in diameter and over 30 ft thick has slipped over a thin coal seam and associated argillaceous strata: the presence of an inch of gouge in the latter, about 10 ft below the seam, indicates that they have also moved. This slip is said to have been induced in 1934 by blasting in the quarry, still worked intermittently, at the foot of the dip slope; the dip is about 23°.

River terraces, alluvium and alluvial fans

Small discontinuous patches of gravel are present above the level of the modern alluvium of the southward draining Cwm-mawr, Dulais and Lliedi streams; an exposure in the mouth of an old trial level [SN 5006 0547] near Horeb showed 4 ft of terrace sand and gravel on 10 ft of stiff brown till, and 6 ft of coarse gravel was seen in a small old pit on the right bank of Afon Cwm-mawr [SN 4775 0346]. A small area of a higher terrace has been mapped above Afon Dulais [SN 4875 0270]. These deposits, mapped as river terraces, were probably formed during the retreat of the ice up these valleys, their present elevation being due to late- and post-glacial downcutting of the streams, which here, as elsewhere in South Wales (Woodland and Evans 1964, p. 280) have excavated narrow channels through the glacial deposits.

The River Gwendraeth fawr is flanked by a broad stretch of alluvium except where its post-Glacial course has been constricted by morainic deposits; the alluvium broadens out below Pont Spwdwr ('Spudder Bridge') [SN 4340 0586] and merges into the very wide expanse of marine alluvium which forms the Kidwelly and Pembrey 'Flats'. Shells collected from a temporary excavation adjacent to Pont Spwdwr, up to 3 ft deep in sand and sandy clay, include Cardium edule Untie, Donax vittatus (da Costa), Macoma balthica (Lint*, Mytilus edulis Lint* Scrobicularia plana (da Costa) and Buccinum undatum Linne: this assemblage indicates estuarine conditions, and early maps of the area show a broad estuary at the mouth of the Gwendraeth Fawr extending as far east as this bridge. These recent shells, with their original lustre and colouration, may be contrasted with a similarly preserved assemblage collected from an excavation on the site of Carmarthen Bay Power Station [SN 4493 0017], the section being:

C. edule, D. vittatus, Ensis siliqua (Linne), Littorina littorea (Linne), Macoma balthica, Mactra corallina Linne, Mytilus edulis, Ostrea sp., Tellina tenuis da Costa and Venus striatula (da Costa), indicating a marine middle to lower shore environment, were found at about 10 ft below O.D.; a femur of Cervus sp.,and a molar tooth of C. elaphus Linne were also found on this site.

A number of trial boreholes on the 'Flats' are shown on the Six-inch maps; they all proved the marine alluvium to consist essentially of fine to medium-grained sand with a few thin beds of silty sand and clay. These deposits are underlain in some of the boreholes by a bed of gravel up to 11 ft thick; less commonly till is present between the gravel and the 'solid' rocks. The thickness of the drift deposits is shown by these, and the Ashburnham, boreholes to increase to the west, the isopachytes being approximately parallel to the landward margin of the alluvium, the line of Mynydd Pembrey continuing as a gentle ridge westwards below the alluvium. In the most westerly boreholes solid rocks are at about 100 ft below O.D.

A borehole by Pont Spwdwr proved sand to about 3 ft below O.D. (i.e. about 20 ft below the surface), but near Pontyberem, nearly 5 miles upstream, piles driven at a sewage works [SN 4939 1080] probably proved about 35 ft of alluvium, the rockhead being at about 58 ft above O.D.

At Kidwelly, near the mouth of the River Gwendraeth fach, an excavation for a sewage pump [SN 4052 0676] proved 2 to 3 ft of made ground on 3 to 10 ft of alluvium, composed mainly of brown clay but with red clay near the base on the east side. Bluish grey clay, with roots and containing vivianite, was present up to 1 ft 9 in thick on the N. side only; the alluvium is underlain by up to 22 ft of stiff reddish brown, slightly sandy, till.

The alluvium of the smaller, steeper, streams is generally much coarser grained, typically consisting of coarse gravel; for example, in Afon Cwmmawr [SN 4787 0379], gravel and sand up to about 14 ft thick are exposed.

The reclaimed lower parts of the Gwendraeth fawr alluvium, particularly the area [SN 414 054] which is slightly lower than the surrounding alluvium, and shown as marsh on the maps, are subject to flooding when the river flow is backed up by high spring tides.

Alluvial fans are present at the confluences of the tributary streams with the Gwendraeth fawr; an excavation in one of them [SN 4938 1086] near Pontyberem showed it to consist of gravel, with thin beds of sand, at least 12 ft thick. A much larger, but poorly exposed, sand and gravel fan has been formed by the combined Cwm-mawr and Dulais streams where these reach the marine alluvium at Stradey Park, on the western outskirts of Llanelli; this fan adjoins another deposited by the River Lliedi before it was diverted to the south in late-glacial times. The boundary between the two fans is indefinite, suggesting that the greater part of both were deposited late-glacially, presumably during the period of relatively rapid downcutting by the streams.

Peat

Deposits of peat are relatively uncommon in this area. Several small patches are present in the col between Brondini and Cynheidre hills, and there is a larger area [SN 486 047] S.W. of Five Roads. A flat-floored hollow [SN 491 071], on the S. side of Cynheidre cross-roads, surrounded by till except for a narrow outlet to the Lliedi valley on the east, has a covering of peat; this was probably a small glacial lake, damned by a remnant of the ice in the col.

Although peat interbedded with marine alluvium is present elsewhere in South Wales (Godwin 1940), it has not been recorded under Kidwelly and Pembrey 'Flats', but the presence of 'Submerged Forest' deposits nearby is suggested by fragments of peat washed up on the shore. Excavations at Burry Port [SN 4442 0076], [SN 4396 0064] have proved thin blown sand on peat, said to be 11 ft thick at the latter locality, and 2 ft 6 in of peat were proved below 27 ft of made ground and alluvium in a trial pit [SS 4976 9933] near Llanelli docks.

Blown sand

The western part of the area is covered by a large expanse of blown sand, Tywyn (or Towyn) and Pembrey Burrows. The dunes, which rise to over 50 ft above O.D., are generally aligned parallel to the present shoreline; away from the shoreline the high dunes give way successively to low dunes and a stretch of almost flat blown sand which grades imperceptibly to the marine alluvium. Much of the area of blown sand has been stabilized by forestry planting and marram grass, but aeolian movement since the original geological survey is shown by the obliteration of ponds and marshy ground which were present on Tywyn Burrows. The seaward form of the area is being substantially modified; at the northern end, near Towyn Point and for about 3 miles to the S.S.E., military 'pillboxes' built during the 1939–45 war are now as much as 60 yd from highwater mark, visibility from them being restricted or blocked by sand dunes, while there is active marine erosion of the dunes where the direction of the shore swings about [SN 392 007] to the S.E., confirmed by the 'pillboxes' in that area being undermined and collapsing on to the beach.

Farther to the S.E., and extending almost as far east as Pembrey Harbour, is an area of blown sand, almost enclosing salt marsh to the north, which is actively growing to the south and east. Owing to these relatively rapid changes in the distribution of blown sand and alluvium and of the position of the high water mark, coverage by aerial photographs was arranged in 1954, and the mapped boundaries are partly based on these.

References

GODWIN, H. 1940. A Boreal Transgression of the sea in Swansea Bay. New Phyt., 39, 308–21.

GRIFFITHS, J. C. 1939. The Mineralogy of the Glacial Deposits of the Region between the Rivers Neath and Towy, South Wales. Proc. Geol. Assoc., 50, 433–62.

MITCHELL, G. F. 1962. Summer Field Meeting in Wales and Ireland. Proc. Geol. Assoc., 73, 197–213.

STRAHAN, A., CANTRILL, T. C., DIXON, E. E. L. and THOMAS, H. H. 1909. The Geology of the South Wales Coalfield. Pt. X. The Country around Carmarthen. Mem. Geol. Surv.

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

Chapter 9 Economic geology

Coal

The following account of the coal is contributed by H. F. Adams, Esq., Head of the South Wales Geological Outstation, National Coal Board.

Coal mining history

Coal mining has been practised within the district for at least 400 years, and the difference in character between the coals of the Gwendraeth Valley and of Llanelli was recognized as early as the sixteenth century. In the first half of the present century many small- or medium-sized collieries were in production. They fall into two groups. In the first group, which includes some of the larger collieries, were those in the Gwendraeth Valley mining the main productive group of seams in the Lower and Middle Coal Measures. Most of these were slants driven down-dip from the outcrops. The second group comprises those mining seams of the Upper Coal Measures in or near to the coastal belt between Llanelli and Pembrey; these usually were slants or cross-measures drifts into the hillsides, but some were driven to win coal beneath the Burry Estuary. Since the sinking of the new shafts at Cynheidre Colliery underground mining in the Gwendraeth Valley is now concentrated at this one large modern colliery and the older colliery at Pentremawr. In recent years large scale opencast mining has been undertaken in the Gwendraeth Valley. Coal is no longer produced from the upper seams of the coastal belt.

Variation of rank and related properties

Coal from the main seams of the Lower and Middle Coal Measures of the Gwendraeth Valley have a well-established reputation as anthracite of high rank and quality. The seams of the Upper Coal Measures of the coastal belt yield low-volatile steam coals. From the detailed Coal Survey examination of the anthracite seams and a more limited examination of the upper seams of the coastal region it is known that the district contains seams ranging in coal rank from anthracite of the highest rank to medium-volatile coals of about 20 per cent volatile matter, but there is reason to believe that a much wider range is present in the district. High-volatile coal (volatile matter over 32 per cent) occurs in the Penlan Seam immediately to the east of the district. The Ashburnham boreholes drilled over 50 years ago to the west of Pembrey indicated the existence of seams ranging in volatile matter from under 10 per cent to over 30 per cent in a distance of about 2 miles from north to south. The available information does not readily lend itself to the detailed mapping of seam properties that has been achieved in other districts but it is clear that the coal seams of this district probably exhibit a range of volatile matter from under 5 to over 30 per cent, and that the change of rank in some localities is very rapid.

The seams of the main productive group, i.e. Pumpquart Vein to Soap Vein, in the Gwendraeth Valley yield anthracite throughout the region of their exploitation. At Cynheidre and Pentremawr collieries these seams yield anthracite of the highest class (N.C.B. coal rank code 101) with a volatile matter of 4 to 6 per cent on the dry, mineral matter free basis. The principal seams have low ash, 21 to 5 per cent, and contain 0.5 to 1.5 per cent of sulphur; other seams occur with higher ash and sulphur. The anthracite formerly mined at collieries farther to the south-west was of rather lower rank. Anthracite of lower rank occurs also in the Carway Fawr and other seams above the Soap Vein in the Gwendraeth Valley.

The upper seams of the coastal belt are low-volatile steam coals usually with weak to medium caking properties but probably varying from very weakly to strongly caking. There is a wide variation of ash and sulphur, and some of the seams have very high sulphur content. No reliable analyses are available for the Gwscwm, which is one of the more important of these upper seams and has been extensively worked.

Utilization of coal mined in the district

The anthracites of the Gwendraeth Valley are in strong demand as premium smokeless fuels for domestic use in slow combustion appliances including heat-storage cookers and central heating boilers. They are used also for glasshouse heating and for malting and hop drying. A recent development is the use of cleaned anthracite fines as a main constituent of briquetted domestic fuel made by the mild heat treatment process. Apart from their main use as fuel these anthracites of very high carbon content have special uses, such as filtration media in water purification and as raw material for the manufacture of electrodes. The coals formerly worked from upper seams in the coastal belt had a general use in local industries as well as for house coal.

Iron ore

Clay-ironstone ('mine'), that is siderite mudstone, in beds ('pins') and nodules ('balls'), was formerly worked, the principal horizons being in the strata above the Rhasfach Vein. Details of this once important but now defunct industry are given by Joseph (1880) and Strahan and others (1920).

Quartzite

The Basal Grit of the Millstone Grit is an important source of quartzite for refractory uses; it is being quarried on Mynydd-y-Gareg for the manufacture of silica bricks at Kidwelly, and on Mynydd Cerig, on the eastern margin of the area described, for use in the north of England. Details of the stratigraphy of the Basal Grit have been given above: the following analysis of a specimen from New Quarry, Mynydd-y-Gareg [SN 431 081] (Thomas and and others 1920, p. 22), is typical:

Shale interbedded with the quartzite has been mined on Mynydd-y-Gareg [SN 4344 0874], [SN 4457 0951]; this material is ground with soft quartzite or silica sand to make the cement used to bond the silica bricks in the furnaces.

Fireclay

In the Lower and Middle Coal Measures the seatearths of the Rhasfach and Graigog ('Upper Felen') veins were formerly worked as fireclay, and the Carway Fawr is said to rest on thick fireclay in places (Strahan and others 1909, pp. 100, 118, 119). More recently fireclay has been mined below the Goodig and Pwll Little veins in the Pennant Measures : details are given in the account of the stratigraphy.

Building bricks

The beds between the Mole and Trimsaran marine bands have been extensively quarried near Trimsaran [SN 465 050], [SN 467 051] for use in the local brickworks, the coal and harder sandstones being rejected. Near Horeb boulder clay, after removal of the larger boulders, has been used for the manufacture of bricks [SN 499 054]. Near Burry Port the argillaceous rocks associated with the Gwscwm group of coal seams have been quarried [SN 4535 0185], [SN 4570 0206].

Sandstone

Many of the sandstones in the Coal Measures have been quarried, mostly for limited local use as building or paving stone or, crushed, as aggregate. The only quarry working, Furnace Quarry [SN 503 017], at the time of the resurvey had a capacity of 100 tons per day of 1-in aggregate.

Lead and copper ores

Small amounts of lead and copper ores have been mined on Mynydd-y-Gareg (Strahan and others 1909, p. 155); although largely in the Carboniferous Limestone, one of the lodes extends into the basal beds of the Millstone Grit.

Material on the tip from an old shaft [SN 4318 0840] includes quartzite veined with quartz carrying traces of copper minerals.

Sand and gravel

Sand and gravel has been dug on a very limited scale for purely local use: none is now worked.

Underground water supply

Apart from wells and springs still in use on some farms, the only underground water used is obtained by Kidwelly Corporation from a shallow well [SN 4139 0730] in the Millstone Grit Shales, less than 20 yd from the edge of the Gwendraeth fach alluvium. On a test, 9500 gallons per hour were obtained from a depth of 23 ft.

References

JOSEPH, T. 1880. Observations on the Clayband-ironstone or "Mine" of the northern outcrop of the South Wales Coalfield. Proc. S. Wales Inst. Eng., 12, 255–84.

STRAHAN, A., CANTRILL, T. C., DIXON, E. E. L. and THOMAS, H. H. 1909. The Geology of the South Wales Coalfield. Pt. X. The Country around Carmarthen. Mem. Geol. Sum

STRAHAN, A.,GIBSON, W., CANTRILL, T. C., SHERLOCK, R. L. and DEWEY, H. 1920. Iron ores. Pre-Carboniferous and Carboniferous bedded ores of England and Wales. Mem. Geol. Surv., MM. Resources, 13.

THOMAS, H. H., HALLIMOND, A. F. and RADLEY, E. G. 1920. Refractory materials: Ganister and silica-rock; sand for open-hearth steel furnaces; dolomite. Petrography and Chemistry. Mem. Geol. Sum, Min. Resources, 16.

Appendix 1 Records of principal colliery and borehole sections

Ashburnham No. 1 Borehole

Height above O.D. c. 15 ft. 1-in 229. 6-in SN 40 SW; Carms. 53 S.E. Site 450 yd W.N.W. of Pen-y-bedd Farm. National Grid Ref. [SN 4098 0290]. Drilled 1908–9.

Ashburnham No. 2 Borehole

Height above O.D. c. 15 ft. 1-in 246. 6-in SN 30 SE; Carms. 57 N.W. Site 3100 yd S.W. of Pen-y-bedd Farm. National Grid Ref. [SN 3917 0089]. Drilled 1909.

Ashburnham No. 3 Borehole

Height above O.D. c. 15 ft. 1-in 229. 6-in SN 40 SW; Carms. 53 S.W. Site 1700 yd N.W. of Pen-y-bedd Farm. National Grid Ref. [SN 4007 0366]. Drilled 1910.