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Geology of the Swansea district: a brief explanation of the geological map Sheet 247 Swansea

By W J Barclay

Bibliographic reference: Barclay, W J. 2011. Geology of the Swansea district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 247 Swansea (England and Wales).

British Geological Survey

By W J Barclay

Keyworth, Nottingham: British Geological Survey

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Notes

The word ‘district’ refers to the area of Sheet 247 Swansea. National Grid references are given in square brackets and all lie within 100 km squares SS and SN.

Acknowledgements

This Sheet Explanation was written and compiled by W J Barclay and edited by J R Davies, A A Jackson and J E Thomas. The figures were drawn by P Lappage and I Longhurst; pagesetting was by A R Minks and A J Hill.

The grid used on figures is the National Grid taken from Ordnance Survey mapping.

Summary of geology

The Swansea district extends from east Gower and Llanelli in the west to Port Talbot and Neath in the east, the city of Swansea occupying the central part. Pennant sandstones of the Warwickshire Group underlie most of the district, Coal Measures occupy the drift-covered coastal area from Swansea Bay eastwards and limestones of the Pembroke Limestone Group crop out in the Gower. Upper Old Red Sandstone quartz pebble conglomerates of Late Devonian age forming the core of the Cefn Bryn Anticline in Gower are the oldest exposed rocks in the district. The youngest strata are Triassic mudstones present in a small drift-covered outcrop in the south-east.

Coal was the mainstay of the district’s economy from the 16th century until the 1940s, but with the rapid decline of the coal industry, particularly accentuated in recent years, there is no current deep or shallow mining in the district, and only minor opencast mining activity. The tinplate works at Swansea and Llanelli and the steelworks at Port Talbot are the only remnants of a former intensive mineral smelting industry in the district. British Petroleum’s Llandarcy refinery was closed in 1998. Light industry and the service sector provide most employment today, but equally important is the growing tourism industry, with Swansea the gateway to the fine beaches of the Gower peninsula.

The district is one of the most important for Quaternary research in the United Kingdom, the deposits of the caves and raised beaches of Gower providing a record of the Pleistocene and its climatic oscillations, perhaps as far back as the Anglian glaciation 478 000 years ago. In the Late Devensian, the district was invaded by valley glaciers from the north and, in the coastal areas, by Irish Sea ice from the west. Man’s industrial activities have left a wide range of artificial deposits, including colliery spoil and smelting waste as well as a legacy of land contamination in the former industrial areas.

Summary of the geological succession in the district. (P945695)

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 247 Swansea, published as revised Bedrock Geology and Superficial Deposits editions in 2011. A more detailed account of the geology is provided by the Sheet Description (Barclay, in press).

The Swansea district extends from Gower and Llanelli in the west to Port Talbot and Neath in the east, the city of Swansea occupying much of the central part. Carboniferous rocks underlie the entire district, except for some Late Devonian conglomerates in the core of the Cefn Bryn Anticline in Gower. Late Carboniferous (Pennsylvanian) rocks of the Pennant sandstone-dominated Warwickshire Group are the most widespread. Steeply dipping, folded and faulted measures of the South Wales Coal Measures Group and Namurian Marros Group occupy the lower coastal area, except for the Mississippian limestones of the Pembroke Limestone and Avon groups of the Gower (Figure 1).

Coal was the mainstay of the district’s economy from the 16th century until the 1940s, with Swansea, Llanelli and Port Talbot exporting coal produced locally and from elsewhere in the coalfield. Following the rapid late 20th century decline of the coal industry, there is no mining in the district, except for one small opencast operation. Mineral smelting became an important industry in the 18th century, using local coal. Copper, tin, lead and zinc ores were imported from Cornwall and around the world. Today, the tinplate works at Swansea and Llanelli and the steelworks at Port Talbot are the only remnants of this industry. Light industry and the service sector provide most employment today, but equally important is the growing leisure and tourism industry, with Swansea the gateway to the fine beaches of the Gower peninsula.

History of survey and research

The district was originally surveyed at the scale of one inch to one mile (1:63 360) as Old Series Sheet 37, and published about 1844. The survey of the coalfield area was largely the private work of W E Logan, incorporated into horizontal and vertical sections (published in about 1850) and the first memoir of the Geological Survey (De La Beche, 1846). The first geological survey at six-inch scale (1:10 560) was carried out from 1896 to 1902 by A Strahan, R H Tiddeman, W Gibson and S B Wilkinson, followed by publication of the 1:63 360 scale Solid and Drift map in 1907 and an explanatory memoir (Strahan, 1907a); 1:10 560 component maps of the coalfield area were also published. The district was resurveyed at 1:10 560 scale by J V Stephens, W B Evans, R A Downing and A A Archer from 1946 to 1954. The maps of the resurvey were published at 1:10 560 and 1:63 360 scales, the latter as a Drift edition (1972) and Solid edition (1973). The Solid edition was reprinted at 1:50 000 scale in 1977. The Sheet Description (Barclay, in press) incorporates the data acquired during the 1946–54 survey and provides an update in the light of minor revisions carried out in the course of preparation of the new edition of the 1:50 000 map by W J Barclay, J R Davies, D I Schofield, H Sheppard and C N Waters between 2003 and 2007.

The Carboniferous limestones, and raised beach and cave deposits of the Gower have long attracted geological attention. Studies of the limestones and structure of the Gower were published by T N George (e.g. 1939, 1978). Many sedimentological studies of the limestones have been carried out (e.g. Wu, 1982; Beus, 1984; Simpson, 1985, 1987; Wright, 1986, 1987a; Ramsay, 1987, 1989, 1991; Faulkner, 1988; Tucker and Wright, 1990). In addition to the early work of the Geological Survey (Strahan, 1907a; Strahan and Pollard, 1915), published details of the Coal Measures around Swansea include the work of Jordan (e.g. 1910), Jones (1935) and Carey Jones (1953, 1957). Kelling (1964, 1968) studied the sedimentology of the Pennant Sandstone Formation at Briton Ferry (Bluck and Kelling, 1963; Owen, 1971; Cleal and Thomas, 1996). Work on the Quaternary deposits was summarised by Campbell and Bowen (1989) and Bowen (2005). This includes T N George’s work on the raised beaches, cave deposits, head and glacial deposits of Gower (e.g. George, 1932). These provide a record of climatic and sea level change that perhaps extends back to over 400 000 years ago (Bowen, 1999). Adams et al. (2004) describe the Mississippian sections of the Gower coast that are Sites of Special Scientific Interest (SSSI).

Chapter 2 Geological description

Devonian

The Devonian Period is represented at outcrop by the Upper Devonian (Upper Old Red Sandstone Group) Pennard Conglomerate Formation which forms a narrow, partly fault-bounded inlier in the core of the Cefn Bryn Anticline (Figure 1). A small outcrop of Lower Devonian sandstones of the Brownstones Formation is conjectured to be present on the western margin of the district. The outcrop of the Pennard Conglomerate extends south-eastwards across southern Gower, and is much affected by north-east-trending cross-faults and some strike thrusting. The exposed thickness of the formation is estimated to be about 100 m and its principal rock types are quartz conglomerate and quartz arenite. The quartz conglomerate is composed mainly of white vein-quartz pebbles, with a few quartzite and volcanic pebbles and more common jasper pebbles, set in a matrix of medium-grained, quartz arenite. Impregnation by hematite to varying degrees gives colours ranging from pale buff to red-brown and purplish red. Lower red and upper white units (Strahan, 1907a; the Red Conglomerate and White Conglomerate respectively of Allen, 1965, 1974) were thought by Strahan (1907b) to have no stratigraphical significance. The quartz arenite is dull purple-grey with sporadic quartz pebbles. Lenticular interbeds of bright red mudstone and very fine-grained sandstone occur throughout. The top of the formation is not exposed in the district, but there is an apparently conformable junction with the overlying mudstones of the Tournaisian Avon Group exposed [SS 471 914] about 3.2km west of the district boundary (Strahan, 1907b). Brachiopods occur in the uppermost beds (George et al., 1976). The formation is comparable with the northerly sourced, high-energy, braided river gravel deposits at this level elsewhere in the Anglo-Welsh Basin (e.g. Allen, 1965). The gradation into the marine mudstones of the Avon Group records a marine transgression and cessation of continental deposition at the start of the Carboniferous Period.

Carboniferous

Adoption in the UK of the terms Mississippian and Pennsylvanian as formal subsystems of the Carboniferous Period (Waters et al., 2009) has rendered the former subdivisions of Dinantian and Silesian redundant. However, pending the recognition of the new international stages for the Carboniferous, the extant European nomenclature of Tournaisian, Visean, Namurian, Westphalian and Stephanian is retained. The Mississippian/Pennsylvanian boundary in the district, and in the UK, is located within Namurian strata at a level within the Chokierian Substage.

Tournaisian and Visean

Rocks of Tournaisian and Visean age, traditionally referred to the Carboniferous Limestone and now to the Avon and Pembroke Limestone groups, crop out in east Gower in the south-west of the district, on the limbs of the Cefn Bryn Anticline and the southern limb of the Oxwich Bay Syncline. They form most of the coastline from Oxwich Point to Mumbles Head (Front cover) and are magnificently exposed in the sea cliffs and foreshore. The succession is about 900m thick and represents carbonate deposition on a southward prograding carbonate ramp that by late Visean times had evolved into a low gradient carbonate platform (Figure 2; Wright, 1986, 1987a; Wilson et al., 1990).

Avon Group

The Avon Group (Waters et al., 2009; formerly Lower Limestone Shales; equivalent to the Cefn Bryn Shale of George, 1939a) is about 150 m thick and consists of green-grey mudstones, olive to brown siltstones, very fine-grained sandstones, ooidal and red, crinoidal limestones and, in the upper part, thin, black, impure limestones interbedded with mudstone (Dixon and Vaughan, 1912). Quartz-rich, fissile, silty and locally gritty rottenstones also occur. Abundant fossils include brachiopods, bivalves, gastropods, ostracods and crinoid ossicles. However, there are no known exposures, the rocks being seen only in temporary excavations, and the subdivisions mapped in adjacent districts (e.g. Wilson et al., 1990) have not been recognised. Mud deposition below wave base in an outer ramp environment is envisaged, with the coarser beds probably deposited on shoals or during storms (Burchette, 1987; Tucker and Wright, 1990).

Pembroke Limestone Group

Black Rock Limestone Subgroup

The Black Rock Limestone Subgroup (formerly the Penmaen Burrows Limestone) succeeds the Avon Group conformably. It is 230 m thick and consists of well-bedded, ooidal, bioclastic and crinoidal limestones that are dolomitised to varying degrees. Its three-fold division into the Barry Harbour Limestone, Brofiscin Oolite and Friars Point Limestone formations replaces the earlier nomenclature of George et al. (1976). The Barry Harbour Limestone Formation (formerly Shipway Limestone) consists of medium to dark grey, well-bedded limestones (Wright, 1986; Faulkner, 1988). Lenses and nodules of chert occur in the lower part, and the formation becomes more fossiliferous upwards. A 5 m-thick dark grey, ooid-limestone with some crinoid debris about 50 m from the base is overlain by 32 m of dark grey, bedded limestones with zaphrentid corals (Fasciculophyllum and Zaphrentis delanouei), brachiopods (including ‘Camarotoechia’ mitcheldeanensis and Unispirifer tornacensis), bivalves, bryozoa and crinoid debris. The overlying Brofiscin Oolite Formation comprises a 3 to 4 m-thick unit of cross-bedded ooid-limestone with crinoid debris. Caninioid corals (Caninophyllum patulum patulum and Caninia cornucopiae) are absent in the beds below, but appear above the Brofiscin Oolite in the Friars Point Limestone Formation (formerly Tears Point Limestone) which comprises about 80 m of bedded, crinoidal limestones, dolomitised to varying degrees. The strongly dolomitised upper levels of the formation are equivalent to the Langland Dolomite and Laminosa Dolomite of earlier workers and contain the brachiopods Tylothyris laminosa and Delepinia destineri.

Gully Oolite Formation

The Gully Oolite Formation (also known as the Caswell Bay Oolite) is a massive, pale grey, white-weathering ooid-limestone 44 to 49 m thick. A minor palaeokarstic surface with rhizocretions lies 10 m from the top of the formation (e.g. Adams et al., 2004). Layers of crinoid debris are present in the lower part, but decrease in abundance upwards. There are a few layers of small fossils, but larger fossils are rare and the formation is poorly fossiliferous overall. Bellerophontid gastropods, schuchertellid brachiopods and the corals Syringopera cf. reticulata, Michelinia megastoma, Paleosmilia murchisoni and Koninckophyllum praecursor occur sporadically and ammonoids (goniatites) have been recorded in the topmost bed (George and Howell, 1939). The top of the formation is a prominent and widespread disconformity marked by a pot-holed, palaeokarstic surface veneered by a calcrete palaeosol (Wright, 1982; 1986; Adams et al., 2004). Ramsay (1987) interpreted the facies and their associations as the deposits of ooidal sand waves within ebb tidal deltas and beaches along a microtidal barrier island coast affected by hurricanes and storms. The mid-formation palaeokarst separates two units interpreted as shallowing-upwards, prograding shoreface–foreshore packages (Searl, 1989; Burchette et al., 1990; Adams et al., 2004).

Caswell Bay Mudstone Formation

The Caswell Bay Mudstone Formation is a distinctive marker unit of 3 to 7.5 m of thinly bedded calcitic and dolomitic mudstones and micritic limestones (George, 1978; Ramsay, 1987). The basal bed is a calcrete (Heatherslade Geosol of Wright, 1987b), and beds of algal laminite and oncoid limestone also occur. The rocks are pale grey to greenish grey, buff, brown and yellow, locally with some red staining. They are poorly fossiliferous, apart from a few beds of crinoidal limestone which are crowded with fossil debris. Calcisphaera is the most abundant fossil, ostracods and foraminifera occur sporadically, and a few bivalves are recorded, but most of the beds are not fossiliferous. Dixon and Vaughan (1912) recognised that these rocks formed in shallow water environments and referred to them as ‘lagoon phase’. They are interpreted as shallow-water, peritidal deposits formed in a tidal flat lagoon complex behind a beach barrier in a humid climate, with abundant evidence of subaerial exposure in the form of desiccation cracks (Riding and Wright, 1981; Wright, 1986 ; Ramsay, 1987). Minor amounts of gypsum pseudomorphs and evaporite dissolution breccias suggest occasional more arid, sabkha-type environments.

High Tor Limestone Formation

The High Tor Limestone Formation comprises about 120 m of grey to pale grey, well-bedded limestones with a faunal assemblage of crinoids, corals and brachiopods. Pericyclid ammonoids Muensteroceras corpulentum, M. euryomphalum, M. pseudaganides and Merocanites cf. compressus occur 42 m above the base of the formation in a grey, crinoidal limestone between Brandy Cove and Caswell Bay (George and Ponsford, 1935). Dark grey, ‘lagoon phase’, thinly bedded limestones with gastropods (Bellerophon and Euomphalus) occur 18 m above the base of the formation and at its top. Some chert nodules and stringers are present near the top. Most of the formation consists of 0.2 to 0.7 m-thick beds of extensively burrowed, crinoidal packstones and grainstones with bedding planes that are either impersistent erosion surfaces (hardgrounds) or scoured surfaces at the base of normally graded beds (Ramsay, 1987). Also present are wavy-bedded, crinoidal packstones and grainstones with lenticular partings of dolomitic mudstone. These beds have scoured bases and many grade from coarse crinoidal grainstone to finer packstone. Large caniniid corals and linoproductids are scattered along some bedding planes and the beds are pervaded by burrows (Zoophycus and Thalassinoides). The gastropod beds are foraminiferal wackestones interbedded with dolomitic mudstones and there are also beds of thin, trough cross-bedded and planar-laminated bioclastic packstones and grainstones. The junction with the underlying Caswell Bay Mudstone Formation is an erosion surface attributed to marine transgression and migration of barrier, shoreface and tidal channel deposits across the tidal flat complex (Ramsay, 1987; Adams et al., 2004). The facies were interpreted by Ramsay (1987) as the deposits within a barrier–lagoon–inlet complex that were subjected to sporadic subaerial exposure and palaeosol formation. Beus (1984) recognised three faunal associations:

a bellerophontoid association indicating onshore or very near-shore, low-energy, restricted environments, perhaps with abnormally high or low salinity
a caniniod association suggesting shallow, normal marine waters with mild to moderate wave and current action
a Rhipidomella association suggesting a subtidal environment below wave base.

Simpson (1985) interpreted layering in the basal 20 m of the succession as pseudo-bedding defined by stylolites formed during burial diagenesis.

Hunts Bay Oolite Subgroup

The Hunts Bay Oolite Subgroup (Plate 1) comprises mainly ooid-limestones and ooid-bioclastic limestones, the latter characterised by the presence of the Holkerian brachiopods Davidsonina carbonaria, Linoproductus corrugatohemisphericus and Composita ficoidea. Ooid-grainstones occupy most of the subgroup (Cornelly Oolite Formation of Wilson et al., 1990), with a thin peritidal (‘lagoon phase’) unit at the top (Stormy Limestone Formation of Wilson et al., 1990). Most of the formation is interpreted by Ramsay (1987) as the deposits of a deeper-water, low-energy offshore environment affected by storm events, with ooid sand waves prograding southwards when sedimentation rates exceeded subsidence rates.

Oxwich Head Limestone Formation

The Oxwich Head Limestone Formation consists of thickly bedded, fine- to coarse-grained, recrystallised limestones that were originally skeletal bioclastic, peloidal and ooidal packstones and grainstones. It is about 180 m thick at its type locality on the south side of Oxwich Bay. Characteristic features are pale and dark grey colour mottling and ‘pseudobreccias’ (Dixon and Vaughan, 1912), interpreted as bioturbation effects modified by diagenetic processes. Also typical is a crude cyclicity in which thick-bedded to massive bioclastic, peloidal and ooidal limestones are arranged in coarsening- and shallowing-upwards cycles truncated by irregular, pot-holed, palaeokarstic bedding planes (Plate 2). Medium- to coarse-grained grainstones are the commonest lithology, with finer-grained peloidal grainstones/packstones and more matrix-rich types typical of the lower parts of the cycles. The palaeokarstic surfaces are veneered by mottled green, grey and red clay palaeosols, capped at two levels by thin coals. The limestones are richly fossiliferous, with abundant solitary corals, large productid brachiopods and foraminifera. The formation is interpreted as the product of deposition of bioturbated, muddy lime sands on a shallow carbonate platform subjected to repeated sea-level oscillations and subaerial exposure (Wilson et al., 1990). The clay palaeosols are typical of degraded late Mississippian basaltic volcanic ash falls (Adams et al., 2004).

Oystermouth Formation

The Oystermouth Formation (formerly Upper Limestone Shales) consists of about 60 m of interbedded black, impure limestones and calcareous and silicified, fissile mudstones; these weather locally to decalcified rottenstone and pale grey, white and yellow clays, respectively. The limestones are argillaceous, locally crinoidal, commonly cherty packstones with some layers rich in sponge spicules (Ramsay, 1989, 1991; Adams et al., 2004). The fissile mudstones contain a rich brachiopod fauna and solitary rugose corals. The ammonoids Neoglyphioceras sp. and Sudeticeras sp. are recorded from the Kenfig Borehole [SS 8055 8167] close to the east of the district (e.g. Wilson et al., 1990). The formation is interpreted as the product of offshore deposition with increased terrigenous siliciclastic input in deepening waters at the end of the Visean (Ramsay, 1991; Adams et al., 2004).

Namurian

Strata of Namurian age, formerly ascribed to the Millstone Grit (Series), are named the Marros Group; the name Millstone Grit Group is now confined to the northerly derived, fluviodeltaic litharenites of the Pennines and adjoining areas (Waters et al., 2009). The outcrop of the Marros Group occupies a narrow belt crossing the Gower peninsula from West Cross on Swansea Bay to the Loughor estuary near Llanrhidian. In addition, the group forms the core of two small plunging anticlines at Newton and Oxwich Burrows. To the east of Swansea Bay, there are exposures near Aberkenfig in the Pontypridd district (Woodland and Evans, 1964), and Namurian rocks have been proved in boreholes at Kenfig Pool in the Bridgend district (Wilson et al., 1990).

Between 700 and 850 m of strata represent the thickest Namurian succession in south Wales. They are predominantly argillaceous (Bishopston Mudstone Formation; Cleal and Thomas, 1996; Waters et al., 2009), with a thin radiolarian chert phase (Aberkenfig Formation) at the base and fluvial sandstones (Llanelen Sandstones Member) 60 m below their top. The mudstones contain numerous ammonoid-bearing beds (marine bands) that are used to correlate the succession (Figure 3). Apart from the highest beds, the sediments are of basinal facies, and the fauna is entirely of deep-water ammonoid/pectinoid facies (Ramsbottom, 1971).

The ammonoid classification of the Namurian established in the Pennines is valid for the South Wales Coalfield (e.g. Jones, 1974). Ramsbottom (1978) subdivided the succession into mesothems, or major cycles of transgression and regression (Figure 3), the marine bands representing transgressive events caused by eustatic sea-level rise. He viewed the Gower Namurian succession as one of the most complete in South Wales, but noted that the lowest faunal horizons of several mesothems are probably absent. Thus, for example, Cravenoceras edalensis is apparently absent at the base of N2, the lowest Nuculoceras nuculum horizon is absent at the base of N3, Hudsonoceras proteum is absent at the base of N5 and Bilinguites gracilis is absent at the base of N9. Though this pattern may in part explain the absence of most E1 ammonoid taxa from the lowest levels of the Namurian succession, those parts of the Marros Group (Aberkenfig Formation and lowest Bishopston Mudstone Formation) that underlie the Eumorphoceras cowlingense Marine Band on Barland Common are considered Pendleian in age (Ramsbottom, 1971, 1978; Jones, 1974). The Cancelloceras cancellatum Marine Band is unexposed and has not yet been proved in Gower. The age of steeply dipping, coarsening-upwards, palaeosol-capped cycles exposed on the foreshore at West Cross in Swansea Bay [SS 6157 8945] remains in doubt. Dix (1931) recorded Gastrioceras cumbriense, Dimorphoceras sp., Pterinopecten sp., Lingula mytilloides, Edmondia arcuata and Sanguinolites sp. in dark micaceous shales overlying a tough sandstone. The fauna suggests that the horizon is the Cancelloceras cumbriense Marine Band, although Dix commented that the lithologies of the exposed sequence closely resemble parts of the basal Lower Coal Measures. Stephens (MS map Glamorgan 23SE and published sheet SS68NW) recorded only Lingula and bivalves, and placed the horizon in the Lower Coal Measures. George (2001) considered most of the beds to be of Namurian, late Yeadonian (G1b) age. He suggested that they provide evidence for a positive, low-relief feature in Swansea Bay during the late Namurian and early Westphalian (‘Swansea Bay High’), which may have formed as a result of reactivation of the Swansea Valley and Neath disturbances and contemporaneous movement on east–west faults.

Different ages have also been proposed for the Llanelen Sandstones Member. Dix (1931) suggested an early Westphalian age, Stephens (BGS maps) placed it directly below the Cancelloceras cumbriense Marine Band, thereby suggesting an early Yeadonian (G1a) age (cf. Ramsbottom et al., 1978, fig. 4, section D). George (2001) provides sedimentological support for this.

Aberkenfig Formation

The Aberkenfig Formation consists of 35–40 m of dark grey to drab grey, fissile, and bivalve-bearing mudstones and interbedded thin, lenticular, banded cherts containing radiolaria. The fauna of the Oystermouth Formation ceases abruptly at the base of the Aberkenfig Formation. A possible non-sequence at this level develops into a more pronounced unconformity east of the district and elsewhere around the coalfield (e.g. Wilson et al., 1990). The presence of bivalves including Leiopteria longinostris is consistent with a Pendleian age.

Bishopston Mudstone Formation

The Bishopston Mudstone Formation (Cleal and Thomas, 1996; Waters et al., 2009) comprises between 700 and 850 m of mainly dark grey mudstones with some ironstone nodules and a few bullions, as well as a few thin (5 to 25 cm) beds of fine-grained sandstone. At least twenty ammonoid-bearing marine bands, ranging in age from E2 to R2, are present in the stream on Barland Common (Ramsbottom, 1971; Owen, 1971; Cleal and Thomas, 1996). The Llanelen Sandstones Member comprises up to about 150 m of large-scale cross-bedded, fluvial quartzitic sandstones.

Westphalian to Stephanian

The Westphalian–Stephanian rocks, formerly referred entirely to the Coal Measures (see previous editions of Swansea map and Figure 4) are now subdivided into the South Wales Coal Measures Group (hereafter referred to as the Coal Measures) and the Pennant Sandstone Formation and Grovesend Formation of the Warwickshire Group (Waters et al., 2009). The revised classification retains the subdivision of the succession by marker marine bands established by Stubblefield and Trotter (1957) and the subdivision by coals of the Pennant Sandstone established by Woodland et al. (1957a). However, the Coal Measures Group and its constituent informal formations (Lower, Middle and Upper Coal Measures) are now confined to the grey, coal-bearing strata and given the epithet South Wales to distinguish them from those of the Pennine Basin (Pennine Coal Measures Group). The Warwickshire Group consists predominantly of red beds in the Pennine Basin, but contains a green, southerly derived Pennant sandstone facies (Halesowen Formation; Powell et al., 2000). Thus the Pennant Sandstone of the South Wales Coalfield and other basins along the Variscan front to the east (Forest of Dean, Bristol, Oxfordshire and Kent) is accorded formation status within the Warwickshire Group. It extends upwards from the base of Pennant sandstone development, close above the Cambriense Marine Band. Its constituent, sandstone-dominated, coal-defined units (Llynfi, Rhondda, Brithdir, Hughes and Swansea) were termed ‘Beds’ by Woodland et al. (1957a) and are now given member status, in accordance with the classification given by Cleal and Thomas (1996). The overlying Grovesend Beds of Woodland et al. (1957a) are given separate formation status, as they are predominantly argillaceous, although they also contain Pennant-type sandstones.

The Westphalian rocks occupy 260 km² of the 360 km² of the land area of the Swansea district. The greater part of the outcrop is occupied by the Pennant Sandstone and Grovesend formations. The Lower, Middle and Upper Coal Measures outcrop in a narrow steeply dipping belt across the northern part of Gower and on the east side of Swansea Bay, where thrust faulting in the Margam Thrust Belt causes repetition of the succession.

South Wales Coal Measures Group

The thickest development of the Coal Measures in the South Wales Coalfield is represented by about 1000 m of grey, argillaceous, coal-bearing strata. The base of the group is placed at the base of the Subcrenatum Marine Band, and it is subdivided into Lower, Middle and Upper formations by marker marine bands. The base of the Vanderbeckei (Amman) Marine Band is the junction between the Lower Coal Measures and Middle Coal Measures, and the top of the Cambriense (Upper Cwmgorse) Marine Band is the junction of the Middle and Upper Coal Measures (Figure 5). The beds formerly ascribed to the Upper Coal Measures are now largely ascribed to the Pennant Sandstone and Grovesend formations, except for the argillaceous strata between the top of the Cambriense Marine Band and the base of the Pennant sandstone facies, which constitute the revised Upper Coal Measures.

The group consists predominantly of grey mudstones and siltstones, commonly arranged in coarsening-upwards, coal-capped, fluviolacustrine cycles (cyclothems) ranging from 6 to 30 m in thickness (e.g. Woodland and Evans, 1964; Fielding, 1984). Four main facies are recognised (e.g. Hartley, 1993):

isolated channel-fills comprising thick (up to 20 m) sandstones or mudstone/siltstone/sandstone sequences
coarsening- or fining-upwards parallel-laminated mudstones, lenticular-bedded and cross-bedded siltstones and extensive, ripple-laminated, fine-grained sheet sandstones
extensive parallel-laminated, pale and dark grey mudstones
thick, laterally extensive, basin-wide coals with underlying rootlet-bearing siltstone and mudstone palaeosols (‘seatearths’). Thinner coals and palaeosols are also abundant.

In addition, bedded sideritic siltstones (‘ironstones’) punctuate the strata. The cycles generally commence with parallel-laminated mudstones deposited in freshwater lakes, and coarsen upwards into siltstones and fine-grained sandstones. Shallowing of the marine/lacustrine waters initiated luxuriant plant colonisation and coal-forming peat mires. Only a selection of the resulting coal seams are shown on the map and described in this account; a fuller account is provided in the accompanying Sheet Description (Barclay, in press). Alternative names for the seams used locally within the district and in adjoining areas are given in brackets. Associated palaeosols commonly contain sideritic nodules, and quartzitic varieties (ganisters) also occur. Sandstones are relatively minor, fine-grained quartz arenites. They occur as thin, sheet-like bodies that form part of the coarsening-upwards cycles. However, coarser, lenticular sandstone bodies locally interrupting the cycles have sharp erosional bases, and lenses with ironstone, mudstone and coal clasts in their lower parts. These are fluvial sandstone bodies infilling channels incised into the underlying coal, forming the ‘washouts’ of mining terminology. There is also in the district a unique record of cross-bedded and mega-rippled quartzitic sandstones at three levels between the Six-Feet and Two-Feet-Nine coals at Eagle Brick Pit [SS 791 925]. These cap coarsening-upwards cycles, indicate transport from the south-south-east, and are interpreted as tidal flat or tidal channel deposits on the margins of a northward-prograding fluviodeltaic complex (Thomas, 1967; Kelling, 1971, 1974). Rare marine incursions produced shallow-water deposits with marine fossils — the ‘marine bands’. Most of these occur at the bases of cycles and can be correlated throughout the UK and European coalfields, showing that they were the product of periodic, eustatic sea-level rise. The standard nomenclature for these marine bands provided by Ramsbottom et al. (1978) is used in this account with the regional names formerly used in South Wales (e.g. Jones, 1935; Leitch et al, 1958; Woodland et al. 1957b; Woodland and Evans, 1964) shown in brackets. Most of the marine bands contain only a brackish, shallow-water fauna dominated by the horny brachiopod Lingula mytilloides and the trace fossil Planolites ophthalmoides, but the Aegiranum (Cefn Coed) Marine Band contains the richest benthonic fauna known in Britain (Ramsbottom, 1952; Woodland and Evans, 1964). Plant remains occur as comminuted shreds and well-preserved impressions, the latter providing another broad biozonal classification of the succession (Dix, 1934; Cleal and Thomas, 1996).

The freshwater lakes and swamps of the Westphalian favoured the growth and evolution of nonmarine bivalves (‘mussels’), which occur mainly at the bases of the fluviolacustrine cycles, immediately above the coal seams. They are commonest in the Coal Measures Group, with numbers and species being restricted in the Pennant Sandstone and Grovesend formations. They belong to the genus Anthracosiidae, and the work of Trueman at Swansea (e.g. Davies and Trueman, 1927) led to the establishment of a biozonal classification in south Wales that was later modified and applied throughout the country (e.g. George, 1974). However, this was largely superseded by the use of marine bands, which allows more detailed, high-resolution correlation and identification of the Langsettian–Duckmantian (Westphalian A to B) and Duckmantian–Bolsovian (Westphalian B to C) boundaries. Figure 5 shows the principal marine bands present in the Westphalian succession. Of these, four have been found at outcrop, the others being proved underground and in boreholes or collected from colliery spoil. The Bolsovian–Asturian (Westphalian D) boundary is defined by plant species and is less easy to pinpoint. It lies in the Rhondda Member (Cleal, 1974, 1978; Cleal and Thomas, 1996), as confirmed by an early Asturian flora in the overlying Brithdir Member at Aberdulais Falls [SS 772 995] (Thomas and Cleal, 2001).

The Lower Coal Measures crop out across Gower in a narrow, largely drift-covered east–west strip, but the lack of exposure and absence of complete borehole or shaft sections precludes comprehensive description. South-east of Swansea Bay, the lowest beds at outcrop lie between the Garw and Gellideg seams, but are almost entirely concealed by superficial deposits (drift). North of the Moel Gilau Fault, the beds occupy small areas in anticlines in the footwall of the fault. To the south of the fault, measures from below the Gellideg to the Vanderbeckei Marine Band crop out in three fault-bounded blocks in the Margam Thrust Belt, but the ground is drift-covered.

The Subcrenatum Marine Band is not proved in the district, but has been proved at Margam to the east (Woodland et al., 1957b; Woodland and Evans, 1964) and Cynheidre to the north-west (Archer, 1968). In Gower, strata from the Subcrenatum Marine Band up to the Lynch include 15 to 23 m of sandstone in the middle. The Lynch is 0.76 m thick west of the Ilston Fault and has not been identified to the east. The measures between the Lynch and the Farm include several thin coals and three marine horizons. A Lingula bed about 30 m above the Lynch may be the Springwood (M1) Marine Band of the north crop (Leitch et al.,1958; Ramsbottom et al., 1978). The Lynch Rider is a 0.25 m-thick, dirty coal resting on a sandstone 18 m thick. A marine band in the roof of a thin coal 15 m above contains Lingula and bivalves and may be the Honley Marine (M2) Band. The Listeri (Wernffrwd) Marine Band, 18 m higher, contains Gastrioceras listeri and is correlated with the Cefn Cribbwr Marine Band of Margam (Woodland et al., 1957b; Woodland and Evans, 1964, p.35), and the M3 Marine Band of the north crop (Leitch et al.,1958). A quartzite above the Listeri Marine Band is at least 18 m thick and may correlate with the Cefn Cribbwr Rock of Margam (Woodland and Evans, 1964). The Farm is about 0.9 m thick at the western end of its outcrop. A Lingula bed in its roof is probably the Meadow Farm Marine Band (Ramsbottom et al.,1978) equivalent to the M4 Marine Band of the north crop, and the Margam Marine Band (Woodland and Evans, 1964).

The mudstone-dominated strata above the Farm Vein contain a few thin coals, several sandstones and many layers of ironstone nodules, some of which have been worked. The mudstones contain a number of mussel bands attributed to the Carbonicola communis Zone. The New Lynch is the lowest seam to have been worked consistently throughout Gower. It is generally 1.8 m thick, with two thin partings, but tectonism has produced much variation, and it is locally reduced to 0.6 m. Hard, black mudstone with fossil fish debris, chiefly scales of Rhizodopsis, forms the roof of the seam in this area and is prominent in spoil from the workings. Between the New Lynch and the Vanderbeckei Marine Band are about 120 m of measures that include a group of workable coals. The lowest is the Yard (0.9 to 1.2 m) about 44 m above the New Lynch. The Four-Feet (1.24 to 1.37 m) lies about 15 m above the Yard. Three to six coals in the 27 to 36 m of measures between the Four-Feet and the Big (Gordon, in Strahan, 1907a) probably correlate with the Five-Feet and Seven-Feet coal groups east of Swansea Bay (Woodland and Evans, 1964). The Big (or Six-Feet) (1.68 to 1.83 m) is one of the best house coals in the area and was much worked. The Amman Rider is 25 m higher, and is a thin (0.15 m) dirty coal with roof mudstones containing Lingula (the Vanderbeckei Marine Band).

East of Swansea Bay, there is little exposure within the district, but the Margam boreholes (Woodland et al.,1957b; Woodland and Evans, 1964) close to the south-east provide details of the entire succession of the Lower Coal Measures (Figure 5). About 335 m of measures below the Garw are known only from Margam Borehole No. 1. The Subcrenatum Marine Band is 15 m thick and comprises two ammonoid-bearing phases. About 132 m higher, a 7.6 m-thick marine band is split by a nonmarine phase and is correlated with the Springwood (M1) Marine Band of the north crop of the coalfield. A thin overlying coal (Crows Foot) has a 0.48 m Lingula bed in its roof correlated with the Honley (M2) Marine Band. The succeeding cycle is 56 m thick and has the Listeri Marine Band at its base. It is 6.3 m thick and contains in its central part a layer rich in Gastrioceras listeri. Above the marine band 37 m of quartzitic sandstones (Cefn Cribbwr Rock) fine upwards through 10 m of siltstones and silty mudstones which are capped by a seatearth and thin coal. In the roof of the coal, 2.4 m of mudstones with Planolites and plant debris are overlain by 6.4 m of marine mudstones, the Meadow Farm (Margam) Marine Band. The succeeding 91 m of measures up to the Garw are predominantly argillaceous, with abundant ironstones in their lower part and thin, impersistent sandstones in the topmost portion. Several dark grey canneloid mudstone layers contain fish fragments, and one of them is probably the correlative of the Amaliae (M5) Marine Band. Mussels occur abundantly at several levels, with Curvirimula occurring at lower levels and species of Carbonicola in the higher part.

The Garw (or Cribbwr Fach) is recorded as 0.71 m to 0.96 m thick in old colliery records within the district, but boreholes and workings to the east show the coal to range between 0.46 m and 0.66 m (Woodland and Evans, 1964). The overlying measures are predominantly argillaceous and include numerous ironstones (formerly worked at Cwmavon) and a thin coal in their upper part. The Gellideg (or Cribbwr Fawr) lies about 52 m above the Garw and is 1.2 to 2.5 m thick. The beds between the Gellideg and Five-Feet comprise 55 to 62 m of mainly argillaceous strata with thin coals and some sandstones. Records of the Five-Feet Group (or Five Quarters or Upper Five-Feet) show a variable section, locally comprising five leaves totalling 2.88 m of coal. Ostracods and mussels occur in its roof. The Seven-Feet Group comprises several widely separated coals of varying thickness. The best are the ?Middle Seven-Feet (about 0.78 m in two leaves) and the Upper Seven-Feet (1.35 m in three leaves) about 11.9 m above. The Yard (about 1.2 m) lies about 8 m above the Upper Seven-Feet and has burrowed mudstones with Planolites montanus, Gyrochorte carbonaria and Cochlinchus kochi in its roof. Predominantly argillaceous strata between the Yard and the Vanderbeckei Marine Band include several thin coals with some ironstones and quartzitic sandstones. The Vanderbeckei Marine Band lies in the roof of a thin (0.19 m) coal.

The Middle Coal Measures are about 460 m thick at Cwmavon and Margam, probably more in Gower, and comprise the lower part of the Anthraconaia modiolaris and all of the Anthracosia similis–Anthraconaia pulchra nonmarine bivalve zones. They outcrop in a narrow belt across Gower from the Loughor estuary to Swansea Bay, continuing to the east from Baglan to the district boundary just north of Cwmavon. They are repeated south of the Moel Gilau Fault and occupy much of the coastal strip, but are covered by superficial deposits.

Gelli-orllwyn Slant [SS 5310 9507] provides the only detailed section of the measures between the Vanderbeckei and Aegiranum marine bands in Gower (Jones, 1935). The Vanderbeckei (Amman) Marine Band has not been found at outcrop in the district, but has been proved in a borehole at Morfa in the south-east of the district. In Gelli-orllwyn Slant, 0.91 m of mudstone with abundant Lingula and some Orbiculoidea form the roof of a coal and dirt seam. A rich mussel fauna occurs in the roof of a coal, blackband and cannel seam 2.44 m above the marine band. Two coals, both 0.3 m thick, lie 5.18 m and 10.36 m above, and a third, 0.46 m thick, underlies the seatearth of the Frog Lane, which lies 17.37 m above the Vanderbeckei Marine Band.

The Frog Lane is about 1.2 m thick and has small Anthracomya (allied to A. pulchra) in its roof beds. It probably correlates with the Lower Four-Feet of Cwmavon, which is the equivalent of the Lower Nine-Feet of Pontypridd. The Fiery is a thick, dirty coal about 18 m above the Frog Lane. About half of the 57 m of measures between the Fiery and Lower Rock are sandstones, with an unnamed 0.6 m coal 26 m above the former. The Lower Rock is 0.66 m thick and has a 3 m sandstone roof, succeeded by 15.2 m of measures up to the Dirty. This seam consists of two leaves, 0.43 m and 0.15 m thick, separated by 0.1 m of strata. About 27 m of measures between the Dirty and Crofty veins include three cannel beds, the highest yielding Anthracomya aff. dolabrata and A. cf. adamsi. The Crofty is about 0.9 m thick and has a roof containing the mussels Anthracomya cf. pulchra Hind, Carbonicola cf. aquilina (J de C Sow.), C. sp. and Naiadites aff. obliqua, as well as Leaia sp. and a rich flora. In the succeeding 21 m of measures up to the Trigloin, the White Rock about 10 m above the Crofty is 0.91 m thick and is overlain 12.2 m of sandstone. The Trigloin is variable in thickness and section, but has a normal thickness of up to 1.2 m.

About 27 m of measures separate the Trigloin and the Soapy, the main leaf of which is 0.96 m thick. Fossils from its roof include Carbonicola acutella Wright, C. aff. duponti Hind and Anthracomya sp. A two-leaf seam (coal 0.56 m; fireclay 1.67 m; coal 0.61 m) about 15 m above the Soapy has an abundant fauna of ‘small Carbonicola allied to C. acutella’ in its roof (Jones, 1935). A thin coal about 30 m above the Soapy has a marine mudstone roof containing abundant Lingula and some Orbiculoidea and ‘Euestheria’, correlated with the Trimsaran (Mole) Marine Band of the Gwendraeth valley (Archer, 1968). The mudstones are about 6.4 m thick, become carbonaceous upwards and contain numerous fish remains. Measures (29 m thick) separating the base of the marine band from the base of the Aegiranum Marine Band include at their mid point a 9 m-thick, hard, fine-grained, siliceous sandstone. The Aegiranum (Cefn Coed) Marine Band rests on a 0.3 m-thick coal and comprises 3.6 m of mudstones with a rich fauna including crinoids, corals, ammonoids, brachipods, bivalves, ostracods and fish fragments. A 0.13 m-thick, pyritous coal lies about 30 m above the marine band. The roof of a seam worked at Blue Anchor Colliery [SS 5495 9452] as the Golden 45 m above contains the foraminifera Agathammina sp. and Ammonema sp. and is correlated with the Edmondia (Foraminifera) Marine Band. The coal is thus the correlative of the Carway Fawr of the Gwendraeth valley (Archer, 1968) and the Black of Cwmavon. The Voylart lies about 97 m above and is about 0.7 m thick. The Cambriense (Upper Cwmgorse) Marine Band is about 30 m above the Voylart.

East of Swansea Bay, the principal worked coals of the Middle Coal Measures are the Six-Feet (Big), Upper Four-Feet, Two-Feet-Nine (Finery), Upper Cockshot, Golden, Golden Rider, Black, Cwmmawr, Cwmbyr and Clay. All the coals are less than 1 m thick, except the Six-Feet, which is 2 m thick, and the ?Upper Four-Feet, which is 1.4 m thick at Baglan Hall Colliery [SS 7455 9277]. The Bute (Balance Pit) lies about 12 m above the Vanderbeckei Marine Band. It is about 0.6 m thick and has a rich mussel fauna in its roof that includes Anthraconaia williamsoni, Anthracosphaerium exiguum and A. Turgidum (Evans, 1971). The Lower Four-Feet (Lower Nine-Feet of the Pontypridd and Maesteg area) (1.4 m) lies about 12 m above the Bute, with two thin seams intervening. The overlying 18 m of measures include a 6 m sandstone near the mid point with a coal smut some distance below. The Upper Five-Feet (Upper Nine-Feet of Pontypridd and Maesteg) is a split seam comprising 0.92 to 1.17 m of coal in one to three leaves. The Coal and Mine (Red of Pontypridd and Maesteg) is 0.6 to 0.7 m thick, with ironstone nodules and layers in its roof. Some 26 to 34 m of measures between the Coal and Mine and the Big contain two thin seams, the Lower Clay and the Little. The former (probably the Caerau of the Pontypridd and Maesteg area) is 0.51 m thick. The Little is 0.2 to 0.38 m thick and is overlain by the seatearth of the Big. The Big (Six-Feet of Pontypridd and Maesteg) varies in thickness because of tectonic thinning and repetition, but has a normal thickness of about 2.7 m.

Measures (19.8 to 24.4 m thick) between the Big and Upper Four-Feet include the Ball Mine, 3 to 4.6 m above the former, which was worked for ironstone in the Cwmavon area. The Upper Four-Feet comprises a group of seatearths, coals and carbonaceous shale, the topmost coal of which is 1.1 to 1.5 m thick. The Sulphury lies 9 to 12 m above the Upper Four-Feet and is 0.9 m thick. The Finery (Two-Feet-Nine of Pontypridd and Maesteg) lies about 4.6 m above the Sulphury. It is 0.5 to 1.4 m thick and has a sandstone roof in the Cwmavon area. Above the Finery, 18 to 27 m of measures comprise mudstones with a few layers of ironstone nodules and two sandstones less than 6 m thick, above which is the Balling (0.37 m thick). The Lower Cockshot (Silver) lies about 6 m above the Balling and is 0.3 m thick. Between it and the Upper Cockshot, 97 m of strata include three fine-grained quartzites, the Lower, Middle and Upper Cockshot rocks, and several thin coals. The Lower Cockshot Rock is 4.3 m thick and lies 3 m above the Lower Cockshot coal, the Middle Cockshot Rock (4.9 m thick) lies 18 m above the coal, and the Upper Cockshot Rock (1.5 m thick) lies 94 m above. The Lower and Middle Cockshot rocks persist westwards but the Upper Cockshot Rock appears to wedge out westwards under Cwmavon.

The Aegiranum Marine Band is thought to lie in the roof measures of a thin coal about 30 m below the Upper Cockshot, but there are no records or exposures in this area. The Upper Cockshot is a multilayer seam, the highest coal being the thickest (0.23 m). The upper split of the Caedavid is a 0.45 m coal 5.8 to 7.9 m above the Upper Cockshot. The Golden lies 17 to 20 m above the Upper Cockshot and is 0.46 m thick. The Golden Rider, lying 9 m above the Golden, is 0.3 to 0.5 m thick. Two coals are present in the 24 m of measures between the Golden Rider and Black seams. The lower (Upper Yard) lies 3.66 m above the Golden Rider and comprises 0.4 m of coal and an underlying 1 m dirty coal with partings. The upper coal 9.75 m higher consists of two or more leaves, the top coal (0.2 m) being thickest. The Black Victoria of Pontypridd and Maesteg (0.35–0.63 m) lies 7.6 to 9.1 m above and carries the Edmondia (Foraminifera) Marine Band in its roof. Between 27 and 30 m of fissile mudstones with layers of ironstone nodules and one or two thin sandstones separate the Black and Cwmmawr seams.

The Cwmmawr (0.5 to 0.6 m) is separated by 7.6 m of measures from the overlying Cwmbyr, with a thin (0.3 m), intervening coal 2.1 m below the latter. The Cwmbyr has a variable section, ranging from a single 0.5 m seam at Baglan to a split two-leaf seam (coal 0.05 m, parting 0.10 m; coal 0.46 m) at Ynysafon to a three-leaf seam (coal 0.25 m; parting 0.63 m; coal 0.1 m; parting 0.3 m; coal 0.08 m) at Cwmavon. About 18 m of measures, mainly mudstones, separate the Cwmbyr from a thin coal and ironstone (coal 0.2 m; seatearth 0.91 m; coal 0.02 m), which probably is the equivalent of the Upper Blackband Coal and Ironstone of the area to the east (Woodland and Evans, 1964). The Shafton (Lower Cwmgorse) Marine Band should overlie the coal, in 10.7 m of mudstones with ironstone layers and nodules, but is unproved in the district; 36 m of beds separate the coal from the Clay seam which ranges from 0.5 to 0.8 m thick at Tylerfedwen and Ynysafon, respectively, and is a split seam (coal 0.15 m; parting 0.05 m; coal 0.25 m; parting 0.08 m; coal 0.3 m) at Cwmmawr. Some 18 to 21 m of measures separate the Clay from the Clay Rider. This is 0.46 m thick at Ynysafon and 0.58 m, including a 0.02 m parting 0.13 m from the base, at Cwmmawr. The roof measures of the Clay Rider contain the Cambriense (Upper Cwm- gorse) Marine Band to the east (Woodland and Evans, 1964), but the horizon is unproved in this area.

As recently redefined by Waters et al. (2009), the Upper Coal Measures comprise the predominantly argillaceous strata between the Cambriense (Upper Cwmgorse) Marine Band and the base of the first prominent Pennant sandstone marking the base of the Llynfi Member of the Pennant Sandstone Formation. These are 36 m thick in the Cwmavon area. The lowest beds are 15 to 18 m of mudstones, which become silty upwards and contain a few thin sandstones at the top. These are succeeded by a group of thin coals, the White and Jonah seams. The White consists of two or three leaves totalling 0.6 to 1.3 m of coal. The most recent workings (1954–1960) at Seaview Colliery [SS 7639 9321] proved coal 0.61 m; claystone 0.46 m; coal 0.76 m.The Jonah is 3.5 m above the White and about 0.5 m thick. About 15 m of mudstone that is silty in the lower half separate the Jonah from the Tormynydd. This is 0.24 to 0.66 m thick and was worked until 1956 at Tylerfedwen Colliery [SS 7688 9320], where it consists ofcoal 0.56 m; fireclay 0.15 m; coal 0.13 m. In its roof is the Llynfi Rock marking the base of the Llynfi Member of the Pennant Sandstone Formation. The Tormynydd is also recognised to the west of the Neath Valley Disturbance. A yet higher seam, the Rock Fach of Pontypridd and Maesteg underlies the Pennant Sandstone Formation in the south-east of the district.

Warwickshire Group

Pennant Sandstone Formation

The Pennant Sandstone Formation is characterised by grey-green, immature, feldspathic, lithic arenites, mostly arranged in thick, sheet-like units of very large-scale, cross-bedded and horizontally bedded, channellised sandbodies (Plate 3), (Plate 4). They are medium to coarse grained and typically arranged in fining-upwards units. Plant fragments commonly line foreset and set boundaries (e.g. Jones and Hartley, 1993). Many of the sandstones are underlain by conglomerates, which form the base of the fining-upwards cycles and rest on erosion surfaces. Clast- and matrix-supported types occur, with clasts including siderite nodules, coalified plant material (including common logs), rare rounded coal pebbles and rounded quartz pebbles. Some conglomerate bodies consist entirely of coalified log casts (Jones and Hartley, 1993). About 10 per cent of the formation consists of thin, but laterally extensive beds of mudstone and siltstone, with minor fine-grained sandstones. These beds contain coal seams and underlying seatearth palaeosols (mainly at their tops); most of the coals are sufficiently thick to have been worked. The most widespread seams provide a framework to subdivide the Pennant Sandstone Formation (Woodland et al, 1957a). These subdivisions, formerly refered to as ‘Beds’, are now recognised, in ascending order, as the Llynfi, Rhondda, Brithdir, Hughes and Swansea members (Cleal and Thomas, 1996). The Pennant Sandstone Formation is interpreted as the deposits of an alluvial braidplain system in which the sandstones were deposited within a network of rapidly shifting and avulsing channels (Jones and Hartley, 1993) and were derived from the rising Variscan nappes to the south. The finer lithologies are interpreted as floodplain facies, with the siltstones and fine-grained sandstones representing flood deposits, the mudstones deposited from suspension in lakes and the coals formed as peat in floodplain lakes and mires. Plant remains are the commonest fossil, and allow a late Bolsovian to Asturian (Westphalian D) age to be assigned to the Pennant Sandstone Formation, the base of the latter stage occurring within the Rhondda Member (Cleal, 1974, 1978; Cleal and Thomas, 1996; Thomas and Cleal, 2001). Nonmarine bivalve (‘mussel’) faunas occur in the mudstone roofs of some coals. Fossil insect wings have been recovered from above the Swansea Five Feet at Pentre Colliery, Landore (Bolton, 1934).

Llynfi Member

The Llynfi Member is about 160 m thick to the east of the Neath Valley Disturbance, but expands to around 200 m to the west. It consists mainly of Pennant sandstones (Llynfi Rock). The principal coals are the Wernpistyll and Wernpistyll Rider at Cwmavon and the Gleilyd and Clement in north Gower. The Rhondda Member is about 300 m thick and rests on the Wernddu (No. 2 Rhondda) in the east of the district; in the west its basal sandstones overlie measures with a group of thin coals including the Rock and the Jonah seams of north Gower. Other principal coals are the No. 1 Rhondda and No. 1 Rhondda Rider. The Brithdir Member is 200 m thick and contains two commonly worked thin coals, the Brithdir Rider and the Glyngwilym. The Brithdir at the base of the member is about 0.5 m thick in the extreme east of the district, but thins and fails westwards. Argillaceous beds at the base of the Hughes Member include the Hughes, Bodwr and Garlant (Slatog) seams. The Swansea Two-Feet lies in the upper part of the member. The Swansea Member is about 250 m thick. The principal coals are the Swansea Three-Feet, Swansea Six-Feet and Swansea Five-Feet.

Grovesend Formation

The Grovesend Formation is confined to synclinal areas in the Gorseinon–Llanelli area, the highest strata lying between the Grovesend and Gorseinon faults. It contrasts with the underlying Pennant Sandstone Formation in consisting predominantly of argillaceous sediments of floodplain/lacustrine facies, with minor amounts of Pennant sandstone. Red beds occur locally (Archer, 1965). About 400 m of strata are present, in which the principal coals are the Swansea Four-Feet, Loughor Little, Penyscallen, Drews (Little Bryncoch), and the Gelli Group and Grovesend Group of seams. Characteristic features are the floods of the branchiopod crustacean Leaia and the nonmarine bivalve Anthraconauta tenuis, which occur in the roof mudstones of the coals. Insect wings are also recorded (Bolton, 1934).

The Grovesend Formation is now considered to be of late Westphalian (Asturian) to early Stephanian (Cantabrian) age on the basis of its plant macrofossils (Cleal in Ramsbottom et al., 1978; Cleal, 1997; Cleal and Thomas, 1996).

Triassic

Inversion of the South Wales Basin during the culmination of the Variscan Orogeny resulted in erosion throughout the Permian and early Triassic. The Triassic rocks are those of the Mercia Mudstone Group, the deposition and distribution of which was dependent on the local contemporaneous topography. There is only a small outcrop in the extreme south-east of the district, faulted down against Coal Measures to its north and hidden by a cover of aeolian sand and underlying glacial deposits. Boreholes close to the east of the district proved red mudstones with dolomitic conglomerate layers in their lower part underlying 20 m of Quaternary glaciofluvial gravels and resting unconformably on Coal Measures mudstones. The conglomerates are part of a suite of marginal alluvial fan and sheet flood facies of the Mercia Mudstone Group sourced from nearby Mississippian outcrops. The red mudstones were the deposits of playa lakes, with low lake levels represented by evaporitic beds (Wilson et al., 1990).

Quaternary deposits

Quaternary deposits cover all but the higher parts of the district. They include the deposits of the caves and raised beaches of Gower, which provide a record of the Pleistocene and its climatic oscillations, perhaps as far back as the Anglian glaciation 478 000 years ago (Figure 6; Campbell and Bowen, 1989; Bridges, 1997; Bowen, 1999, 2005). Late Devensian (Dimlington Stadial) glacial tills are widespread, the rivers Tawe and Neath occupying glacially incised palaeovalleys filled by thick glacial, glaciofluvial, glaciolacustrine and alluvial deposits. Hummocky gravelly deposits are the lateral, end and retreat moraines of the Late Devensian glaciers. Extensive spreads of Holocene deposits laid down from around 10 000 years ago to the present occupy river valleys and the coastal area, and include beach, tidal flat and estuarine deposits, aeolian sand dunes and submerged peats.

The pre-Devensian deposits were referred to the Llandewi Formation and the Pennard Formation by Bowen (1999). The Llandewi Formation comprises a ridge of weathered glaciofluvial sands and gravels just to the west of the district, that extends from Paviland Manor in Gower eastwards. It was interpreted by Bowen (1969, 1981, 1999, 2005) as a degraded end-moraine, but till occurs south-east of the ridge. The ridge lies south of the supposed Late Devensian limit, as envisaged by Campbell and Bowen (1989) and contains Namurian quartzite and other northerly derived Welsh erratics, as well as erratics of Irish Sea provenance. It was speculated to be the product of the Anglian glaciation. However, the distinction between supposed pre-Devensian and Devensian tills is unclear and unproven and they have not been differentiated on the map.

The Pennard Formation comprises a diverse assemblage of raised beach deposits and cave deposits of the Gower coast (too small to show at the scale of the map), all derived from local outcrops (Strahan, 1907a; George, 1932; Bowen and Sykes, 1988; Campbell and Bowen, 1989; Bridges, 1997) and assigned member status by Bowen (1999). The chronostratigraphy of the succession (Figure 6) is based on amino-acid ratios in shells from the interglacial raised beach deposits (Bowen, 1999; but see McCarroll (2002) for a critique of this method). The Minchin Hole Member is attributed to Oxygen Isotope (OI) Stage 7 on the basis of its amino-chronology and consists of 2 m of raised and cemented intertidal beach sand with marine shells on bedrock at 9.9 m OD in Minchin (or Mitchin) Hole Cave [SS 5553 8686]. The High Tor Member also has its stratotype in Minchin Hole Cave and comprises up to 20 m of head deposits, with layers of red, bone-bearing, ‘cave-earth’ and includes separate units of Wolstonian and Devensian age. The Hunts Bay Member is a cemented storm beach gravel at its type locality [SS 562 868]–[SS 566 865] consisting predominantly of Carboniferous limestone pebbles. It is attributed to OI Stage 5e on the basis of amino- chronological and U-series correlation with the ‘lower cave earth’ at Bacon Hole Cave [SS 559 868], which contains a warm-temperate mammalian fauna of the Ipswichian Interglacial. The member includes the Patella beach and Neritoides beach of George (1932). On the east side of Langland Bay [SS 613 871], the member consists of aeolian sand above raised beach gravel. The Bacon Hole Member consists of Early Devensian head deposits at Bacon Hole Cave. Stratified deposits with stalagmite layers dip into the cave and slope deposits outside it include the ‘upper cave earth’ (e.g. Sutcliffe et al., 1987) also with a mammalian fauna. The Rotherslade Member is a head deposit which overlies the Hunts Bay Member at its type locality [SS 613 871] east of Langland Bay and is in turn overlain by Late Devensian stratified, gravelly glacial deposits.

Most of the glacial deposits of the district are attributed to the Late Devensian (Dimlington Stadial) glaciation (Brecknockshire Formation of Bowen, 1999), from about 26 000 to 15 000 years ago (e.g. Campbell and Bowen, 1989). Ice accumulated on the Brecon Beacons and the mountains of central Wales and fed valley glaciers, which flowed southwards through the South Wales Coalfield valleys and cols (e.g. Jansson and Glasser, 2005). These ice streams excavated deep rock basins in the Tawe (Swansea) and Neath valleys (e.g. Strahan, 1907a, Anderson, 1968; 1974; Al-Saadi and Brooks, 1973; Anderson and Owen, 1979; Culver and Bull, 1979) before coalescing to form a piedmont lobe in Swansea Bay. This terminated in a line extending westwards through Gower to Rhossili Bay (Bowen, 1970; Campbell and Bowen, 1989). Estimates of thickness of ice cover at the maximum stage of glaciation range from 200 m (Jansson and Glasser, 2005) to over 300 m (Bowen, 1980; Campbell and Bowen, 1989). The deposits contain only Old Red Sandstone and Carboniferous erratics from the South Wales Coalfield and its margins. The highest parts of the interfluves are generally devoid of till, but the presence of erratics and glacial striae indicates the passage of ice during the maximum stage of the Late Devensian glaciation.

The valleys of the rivers Tawe and Neath are underlain by a series of flat-bottomed, U-shaped, interconnected rock basins excavated by glaciers and filled by up to 60 m of deposits (Codrington, 1898; Jones, 1942; Anderson, 1968, 1974; Al-Saadi and Brooks, 1973). The Tawe valley basins are scoured to 60 m below OD, the lowermost bounded by a lip at the entrance to Swansea Bay. In contrast, the Neath valley system is open at its seaward margin and comprises a system of multiple buried channels. A deeper palaeochannel from Neath Abbey to Swansea Bay is filled mainly with till, suggesting that the Neath switched course during deglaciation when its preglacial channel became blocked by sediment or stranded ice. Mounds of sand and gravel at Landore and Glais in the Tawe valley and at Tonna in the Neath valley may mark halt stages in the retreat of the glaciers up-valley during the Late Devensian (Strahan, 1907a; Jones, 1942). These moraines dammed the valleys to produce deep glacial lakes and glaciolacustrine deposition. Similar buried glaciolacustrine deposits occur offshore in Swansea Bay (Culver and Bull, 1979). Between about 12 600 and 11 400 years ago, after the retreat of the Late Devensian glaciers, a period of cold climatic conditions (Loch Lomond Stadial) introduced periglacial processes across the district (Campbell and Bowen, 1989) and promoted solifluction and landsliding. During the Holocene, freshwater peat formed in waterlogged hollows including former kettle holes. Rising sea level drowned earlier coastal deposits, altered the coastal zone and led to a range of tidal, beach and aeolian deposition.

Till forms a thin blanket in upland areas and thicker accumulations in topographic depressions and valley sides and bottoms. It is a brown-grey, stiff to hard, silty, sandy diamicton of clays with boulders, cobbles and pebbles. Stony, clayey sands and gravelly clays also occur, and the deposits become more gravelly southwards (Strahan, 1907a). Retreat moraines are distinguished as hummocky glacial deposits and comprise ill-sorted, clay-rich sands and gravels. Glaciolacustrine deposits form much of the infill of the rock basins of the Tawe and Neath valleys, with up to 25 m of these overlying till in the former. The deposits are dark grey, laminated silt and clay with lenses and layers of sand. Glaciofluvial ice contact deposits form sheeted and hummocky kame terraces and kettled mounds. The deposits range from poorly sorted, clayey sands and gravels to moderately well-sorted pebble and cobble gravels. Sand and gravel mounds and ridges on the interfluves above Morriston [SS 658 998] and north of Cwmavan [SS 776 923] are elongated in the direction of ice transport (Strahan, 1907a), consistent with kame-like ice-contact deposition. The largest and thickest (up to 30 m) spread is in the Olfcha–Sketty–Uplands area of Swansea, which has a moundy, morainic form and a heterogeneous, but predominantly gravelly nature. Glaciofluvial sheet deposits are the remnants of gravel outwash plains (sandar) that formed during deglaciation (Jones, 1942).

Alluvium comprises silt, clay, sand and gravel. The most extensive deposits are in the Afan, Neath, Tawe, Lliw and Loughor valleys. At Llansamlet [SS 687 975] it occupies breached kettle holes in the adjacent glaciofluvial deposits. Tidal flat deposits, marshy organic-rich muds, occupy broad regions of the Loughor estuary, and the rivers Neath and Tawe are tidal for some distance upstream of their mouths, where estuarine clays interdigitate with riverine alluvium. Alluvial fan deposits are sands and gravels occurring where tributaries join their parent stream, or, in the case of the largest in the district, where the Afan debouches into the coastal alluvial belt at Aberavon.

Thick spreads of solifluction and colluvial deposits (head) are present below Pennant sandstone scarps in the Tawe, Neath and Afan valleys. Crymlyn Bog [SS 695 950] is the largest peat deposit, covering an area of about 3 km². Rising sea level in the early Holocene submerged former coastlines; the resulting older coastal zone deposits comprise soft, blue clays and silts with thin beds of freshwater peat. Several such peat horizons have been proved in boreholes up to 18 m below Ordnance Datum in the coastal zone, demonstrating lowered sea levels and subsequent Holocene sea-level rise. A widespread peat in the lower parts of the Swansea and Neath valleys and coastal zone lies between 2 m above and 2 m below OD, increasing in depth towards the south. Offshore a lower peat lies 16 to 20 m below OD. Modern beach erosion periodically exposes these early Holocene deposits including peats with in situ tree stumps recognised previously as ‘submerged forest’. Modern beach and intertidal deposits (Plate 5) including storm beach deposits veneer these earlier sediments around Swansea Bay, Oxwich Bay and the Loughor estuary. Well-sorted, dark grey to brown and locally shelly aeolian sands (blown sand) are widespread in the foreshore areas of the district and can range up to about 6 m in thickness (Plate 6).

Landslides are present to a minor extent in the Lliw, Tawe, Neath and Afan valleys, the greatest concentration being on the slopes above Port Talbot and Margam (Conway et al., 1980). All are shallow translational slide/debris flows, with minor rotation and backtilting in one at Craig Tir-isaf [SN 782 014]. All are in argillaceous horizons and/or superficial deposits, with failure caused by seepages at the bases of Pennant sandstone scarps, or along fault planes, as in one on Mynydd Dinas [SS 763 907].

Swansea’s long history of industrialisation and urbanisation, and the associated restructuring of the local landscape, is reflected in the district’s extensive and complex array of man-made deposits and excavations. This artificially modified ground is subdivided into made ground, worked ground, infilled ground, landscaped ground and disturbed ground. As part of the recent resurvey a detailed investigation has been undertaken of the impacts of human interaction on the landscape of the lower Swansea and Neath valleys (Waters et al., 2006). Made ground includes waste from the former mineral smelting, chemical and petrochemical industries, as well as from the tinplate works of Llanelli and the steelworks at Port Talbot. In the valleys, the deposits are predominantly the legacy of coal mining and iron smelting. Worked ground largely comprises former opencast coal sites, but there are also some former brickpits and gravel workings; and these excavations are shown as infilled ground where they have been fully or partially back-filled. Areas of landscaped ground, where remodelling of the original land surface has been so extensive that areas of cut and fill cannot be distinguished, include large tracts of coastal Port Talbot and Llanelli, central Swansea and also the site of Swansea Airport [SS 570 915]. Small areas of ill-defined mineral workings, for example [SS 6381 9846] near Llangyfelach, are depicted as disturbed ground.

Structure

The district lies on the southern limb of the South Wales Coalfield, which is a broadly asymmetric, east–west-trending syncline. The coalfield is the remaining part of a basin, interpreted as a peripheral foreland basin that formed in response to the approaching Variscan Orogen to the south (Kelling, 1988). Various models have been proposed for the structure of the coalfield, which underwent shortening of 20 to 30 per cent. In the absence of evidence for major detachments (Jones, 1991), Frodsham and Gayer (1997) proposed a model invoking internal shortening along coal seams and other bed-parallel detachments (‘easy-slip’ deformation), which collectively accommodated the Variscan compressive stresses. Superimposed on the broad synclinal structure of the coalfield are a number of smaller east–west-trending synclines and anticlines, the frequency of which increases in the south-west of the district. This culminates in vertical beds, tight folding and over-thrusting in the Mississippian limestones of Gower and in the Margam Thrust Belt in the south-east, where Variscan compressive stresses were greatest. Figure 8 shows the principal structures of the district. The numerous faults can be assigned to five main elements.

The Swansea (or Tawe) Valley Fault and the Neath Disturbance are north-east- or north-north-east-trending (Caledonoid) faults that formed part of the Lower Palaeozoic Welsh Basin-margin (Welsh Borderland) fault system and were reactivated as strike-slip/compressional faults during the Variscan Orogeny.
The Moel Gilau Fault is a roughly east–west extensional Variscan fault in the south-east. It throws down to the south up to about 1200 m and may have been reactivated in the Mesozoic.
A suite of major Variscan cross-faults spaced at 1 to 2 km intervals. These are truncated or displaced by the Neath Disturbance; to its south-east, the faults have a north-north-west trend. To its north-west, they are generally north–south, but with north-west-trending splays locally.
Major east–west-trending Variscan strike thrusts.
A widespread suite of minor compressional faults and dislocations in the Lower and Middle Coal Measures result from the incompetence of these argillaceous measures during Variscan compression.

Folding in the district is broadly of east–west trend, or, as in the Gower, of west-north-west trend. The principal structure is the asymmetric syncline of the South Wales Coalfield, compressed in the district to the extent that both northern and southern limbs are represented. Steep northerly dips on the southern limb in the Swansea–Port Talbot area reduce northwards to the central, synclinal area, with gentle southerly dips on the northern limb on the northern margins of the district. The main coalfield synclinal axis is named the Llanelli Syncline west of the Loughor estuary, and the Pont Lliw Syncline to the east. Superimposed on it in the west are a series of minor synclines and intervening anticlines. To the south of the Pont Lliw Syncline are two synclines (Gorseinon and Gowerton) of similar east–west trend, with intervening anticlines. West of the Loughor estuary (and the Plas Isaf Fault) the Tir Bacas Anticline lies south of the Llanelli Syncline and the Carnarfon Fault. East of the Neath Disturbance, the Cefnmawr Syncline is the main synclinal axis of the coalfield (Woodland and Evans, 1964). The Cefn Bryn Anticline forms the main and northernmost element of a series of tight, east-south-east-trending folds in Gower (George, 1940). It lies in the hanging wall of the north-verging Cefn Bryn Thrust, to the south of which are the Oxwich Bay Syncline and Oxwich Point Anticline. The Cefn Bryn Anticline and Cefn Bryn Thrust are displaced sinistrally by the north-north-east-trending Lunnon Fault. This joins the Pwll-du Fault, a north-west-trending structure that extends from the Loughor estuary to the Gower coast and also has a sinistral strike-slip displacement. To its east, the Bishopston Thrust is the southerly of two east-south-east-trending thrusts to the south of which are numerous folds and thrusts, of which the Langland Anticline is the main anticlinal axis in Swansea Bay. To its north are the Oystermouth Syncline and Coltshill Anticline. These structures are displaced dextrally by the north-trending Caswell Bay Fault, to the west of which are the Hareslade Syncline and Backingstone Anticline, which lie between the Pwll-du and Heatherslade East faults. To the west of the former, they are named the Pwll-du Head Syncline and Heatherslade Anticline respectively.

The Swansea (or Tawe) Valley Fault (Weaver, 1975; Owen and Weaver, 1983) and the Neath Disturbance (Owen, 1954) reach Swansea Bay at the south-west end of their lengthy courses across the South Wales Coalfield and the Welsh Borderland. They are interpreted as steep, long-lived basement faults that were intermittently active from the Neoproterozoic (Owen and Weaver, 1983; Frodsham and Gayer, 1997). The displacement of the Coal Measures along both faults includes elements of sinistral strike-slip and dip-slip, suggesting partially oblique Variscan reactivation (Woodland and Evans, 1964; Frodsham and Gayer, 1997). The Neath Disturbance truncates the cross-faults, suggesting movement that postdates them, although it is possible that it exerted influence and compartmentalised the cross-faulting. The throw of the Tawe Valley Fault is estimated to be about 230 m down east to the south of Clydach. That of the Neath Disturbance is estimated to range from 70 m near the eastern boundary of the district to almost 100 m near Neath.

The Moel Gilau Fault extends westwards from the Pontypridd and Maesteg district (Moel Gilau–Ty’n-y-ant system) to Swansea Bay (Woodland and Evans, 1964; Jones, 1991; Frodsham and Gayer, 1997). It crops out along a roughly east–west line south of Cwmavon, down-faulting strata of the Pennant Sandstone Formation to the south against Lower and Middle Coal Measures to the north. A tight footwall anticline is present on the north side. Vertical displacement on the fault is up to 1140 m at Cwmavon, decreasing westwards to about 820 m at Baglan. A sinistral displacement of about 400 m has been recorded to the east of the district (Woodland and Evans, 1964). It remains unclear as to why such a major east–west extensional structure should have developed in a Variscan compressive regime, and there is no evidence that it was a contemporaneous growth fault affecting sedimentation (cf. Jones, 1989). Since it truncates or displaces the cross-faults, which postdate the folding, its main movement must have been late in the Variscan Orogeny, perhaps as compressive stresses were relaxed. Mesozoic extensional reactivation may also have taken place. The Carnarfon Fault in the north-west of the district, and the unnamed fault in the south-east with Triassic rocks in its hanging wall may be a similar structures.

The general trend of the cross faults is north–south. They are part of a suite of faults that cross the South Wales Coalfield and are predominantly extensional although strike- slip movement is demonstrable in some in the Gower. Woodland and Evans (1964) noted differential folding on either side of some of these faults in the Pontypridd and Maesteg district. Cole et al. (1991) suggested pre-Variscan and even post-Variscan extension on these faults, as well as syn-Variscan extension. The maximum throw in the district (about 463 m) appears to belong to the Rhydding Fault; the Duffryn Fault to its east throws about 160 m and the Gardeners Fault to its west throws up to about 290 m.

Variscan east-south-east-trending strike thrusts in Gower are north-directed and occur in association with folding of the Namurian, Mississippian and underlying Devonian rocks. Thrusts in the south-east are in the Middle and Lower Coal Measures and lie at the western end of the Margam Thrust Belt, a zone of steep dips, high-angle, northerly dipping, south-verging thrusts and lag faults (Woodland and Evans, 1964). The vergence of these structures is opposite to that of the main north-directed Variscan shortening, and they are therefore backthrusts. The zone is well known to the east of the district through deep mining, exploratory boreholes, opencast mining and seismic profiling. The Newlands Thrust is the southerly of two south-verging thrusts mapped in the adjoining Pontypridd and Maesteg district (Woodland and Evans, 1964). Both are apparently offset dextrally by the north-west-trending Morfa Fault in the south-east of the district. The Margam Thrust Belt continues westwards across Swansea Bay into north Gower, and is represented by the Bishopston Thrust and parallel thrusts to the north, but lack of exposure precludes a detailed picture.

Minor compressional structures are common in the mudstone-dominated successions of the Lower and Middle Coal Measures. Those formerly exposed in Eagle Brick Pit [SS 791 925] on the eastern margin of the district lie between the Six-Feet and Two-Feet-Nine coals (Woodland and Evans, 1964, plate VI; Kelling, 1971). Other examples include the Pen-y-Gaer Thrust in the Llanelly Syncline (Strahan, 1907a) and an overlap of 10 m in the Penclawdd Vein in the Rhydydefaid Colliery main slant [SS 6060 9270]. The Pen-y-Gaer structure is a north-directed thrust with an overlap of 18 to 27 m.

Chapter 3 Applied geology

Geological factors have a crucial role in land-use planning and development. They include the presence of mineral resources which may be sterilised by building development, and their former working which has left a legacy of mining subsidence and contaminated land, both of which require appropriate mitigation measures prior to building development. Geological hazards present a public health risk and require costly remediation. The waste produced and soil contamination caused by the traditional metal smelting industries in the coastal zone of the Llanelli and Swansea–Port Talbot areas are examples. Engineering ground conditions are entirely dependent on the underlying superficial deposits and/or solid bedrock. Conservation sites, including Sites of Special Scientific Interest (SSSI), play an important part in planning considerations, and include the Gower, designated as an Area of Outstanding Natural Beauty (AONB). A study of the use of earth science information in planning development in the Swansea–Llanelli area was carried out by Department of the Environment/Welsh Office (1997). A study focussing on contaminated land in the Swansea–Port Talbot area was carried out by the British Geological Survey 2005 (Waters et al. 2006).

Mineral resources

The coals of the district were included in a survey by Strahan and Pollard (1915) and fireclay workings were described in a Geological Survey memoir (1920). George (1939b) and Bowen (1980) gave accounts of the resources of the Swansea and Llanelli districts respectively. Coal was the mainstay of the district’s economy from the 16th century until the 1960s, with Swansea, Llanelli and Neath–Port Talbot exporting coal produced locally and from elsewhere in the coalfield. Strahan (1907a) noted that in the early 1900s the population of the district was almost wholly engaged in mining and shipping coal, or in the numerous tinplate works, foundries and copper works that were established within easy reach of the coal supply and of the ports. Copper and tin were initially imported from Cornwall, but later from around the world, with the ships that transported coal returning with ore. The coals were extensively worked from adits and a few deep mines, and more recently in a few small opencast sites. The bulk of the workings were north of Swansea, with less working of the structurally complex Lower and Middle Coal Measures of north Gower. The coals range in rank from medium-volatile to anthracitic (George, 1939b; Adams, 1967). The anthracites occur in a narrow zone extending south-east from Brynlliw Colliery [SN 5964 0100] in the north-west of the district. The seams elsewhere are bituminous, low-volatile coking steam coals with variable caking properties, except for narrow zones of medium-volatile, prime coking coals in north Gower and south-east of Gorseinon.

Fireclay

Fireclay was formerly quarried and mined by pillar-and-stall methods (Geological Survey, 1920). The fireclays of the Graigola (Llwyndu Colliery [SN 7108 0155] and New Wernddu Colliery [SN 7328 0072]) and Golden (Glynea [SS 5483 9915]) seams were then being worked, as well as a clay 10 to 12 m below the Swansea Five-Feet at Graig Brickworks, Morriston [SS 6670 9715].

Common clay

Common clay was widely dug for brick making, mainly from the argillaceous beds of the Pennant Sandstone Formation, but also from the Lower and Middle Coal Measures. Alluvial clay was also dug south of Llanelli at Machynys [SS 5119 9829] and at Morfa e.g. [SS 5100 9848]; [SS 5063 9898]; [SS 5200 9880]; [SS 5080 9912].

Sandstones

The sandstones of the Pennant Sandstone Formation were formerly quarried for building stone, roadstone, aggregate, walling and paving stone. The only working sandstone quarry in the district is Gilfach Quarry [SS 754 998], where sandstone in the Hughes Member is crushed for aggregate and high PSV roadstone. Former quarries include Hillhouse Quarries, Cockett [SS 629 940], one at Townhill [SS 6437 9345], Maesmwlyn Quarry, Skewen [SS 7275 9810] and Cwmrhydyceirw Quarry [SS 665 993].

Limestones

The limestones of Gower were formerly quarried for building stone, lime, and roadstone, but mainly for aggregate. The principal disused quarries are Ilston [SS 555 906], (Oxwich Head Limestone), Barland Quarry [SS 575 895] (Hunts Bay Oolite), Coltshill Quarries (Oxwich Head Limestone), Llethrid Quarry [SS 531 911] (Oxwich Head Limestone) and Oystermouth Quarry [SS 6148 8836] (Oystermouth Formation).

Sand and gravel

The sand and gravel resources of the district were described by the Department of the Environment/Welsh Office (1992). The resources in the Tawe (Swansea) valley were worked at Llansamlet [SS 689 992] in 1992. The resources comprise alluvial and glaciofluvial kame deposits in the main valleys and extensive beach, foreshore and blown dune sand deposits. However, much of the resource is sterilised by development, or in the case of the coastal deposits of Gower, unlikely to be worked in an Area of Outstanding Natural Beauty. There are no current workings, with the most recent operation working blown sand in 1998 at Port Talbot Docks [SS 755 885]. British Steel PLC quarried similar sands at Margam Burrows [SS 776 869] in 1994.

Metalliferous mineral occurrences

Metalliferous mineral occurrences are minor and restricted to hematite and galena and an abundance of calcite. Hematite was dug from a north–south-trending hematite–calcite vein up to 1.4 m wide at The Cut, Mumbles Hill and mined from an adit [SS 6256 8717]. Calcite-hematite veins are also present on the west side of Limeslade Bay, on the coast east of Langland Bay [SS 6143 8706], where there are old trials, on the coast south-south-east of Bacon Hole [SS 5656 8662], and on the coast at Heatherslade [SS 5507 8717]. In addition to those veins with hematite, calcite veining is ubiquitous in the Mississippian limestones, with almost every small fault and many joints lined with it. One at Newton Cliff [SS 6020 8697] is 1.2 to 1.5 m wide; one along the Southend Fault on the west side of Limeslade Bay [SS 6229 8694] is 3.1 m wide; in Bracelet Bay, one vein is 2.3 m wide and another is 3 to 3.35 m wide, the latter splaying out across the intertidal area into a 14 m-wide zone of veinlets. A galena vein was worked between Brandy Cove and Hareslade. The vein is on a fault that extends to the Bishopston valley, where there is a small disused adit [SS 5842 8862], but the absence of spoil suggests that it was little worked.

Water resources

The district’s water supply comes largely from reservoirs outside the district, including the Cray Reservoir south of Sennybridge and Llyn Brianne reservoir in the Builth Wells district. Although classified as a major aquifer, the Mississippian limestones are heavily faulted and folded, and a cover of glacial deposits impedes karstification and recharge (Allen et al., 1997). The sandstones of the Pennant Sandstone Formation constitute the main aquifer of the district. Although perched and confined by argillaceous beds to produce a complex multilayered aquifer, fissuring caused by mining subsidence and the flow in the mine workings has produced hydraulic continuity (Jones et al., 2000). Fissuring of the sandstones has also led to increased recharge at the surface and increased storage capacity and transmissivity. The plethora of faults in the district adds further complexity to the hydrogeological regime. Wells in the superficial deposits are liable to contamination.

Geological hazards

The contamination of land, migration of leachates and potential pollution of groundwater are problems in the former heavily industrialised areas around the Loughor estuary at Llanelli and in the Swansea–Port Talbot area. Inland, areas of made ground, largely of colliery and ironworks spoil, may contain toxic residues (Waters et al., 2006). Former coal mining presents a legacy of mining subsidence, the possibility of which may still exist as old roof supports collapse. Gas emissions include naturally occurring methane and carbon dioxide, methane produced by decomposition of household waste in landfill sites, and radon. The Namurian and Westphalian rocks have a moderate susceptibility to the emission of methane and carbon dioxide. Radon occurs naturally as a product of the radioactive decay of uranium, which is present in small quantities in all rocks and soils, but in greater amount in the argillaceous rocks of the Carboniferous. It presents a health risk where it accumulates in poorly ventilated spaces in buildings.

Slope instability in the form of landslides is common on the steeper valley sides of the district. Although most existing landslides date from the melting of Late Devensian ice, reactivation may occur where the toe areas are excavated or where pore water pressures are increased by changed groundwater movement. One, cut by the A4107 at Cwmavon [SS 780 915] was active in 1980. This is a translational failure of sandy superficial debris overlying sandstones above the Brithdir Coal, probably accompanied by bedding plane slipping of the steeply dipping (26o to 30o) sandstones. Thick accumulations of head are present locally below Pennant sandstone escarpments. Flood risk is present on river floodplains (shown by the extent of riverine alluvium), on low terraces bordering the floodplains and in the low-lying coastal zone. Seismic hazard is present in the district. Apart from minor events triggered by the collapse of disused coal workings, earthquakes have occurred historically in Swansea in 1727, 1775, 1832, 1868 and 1906 — a periodicity that suggests that another is overdue. The 1906 event was felt over a wide area of southern Britain and was one of the most damaging of the 20th century in the British Isles. Twenty cartloads of debris from collapsed walls and chimneys were reported to have been removed from the centre of Swansea. The deep-seated Swansea (Tawe) Valley Fault and Neath Disturbance present potential focuses of deep-seated earth movement. Davison (1907) postulated that the epicentre of the 1906 event was a southerly dipping, east-north-east-trending fault to the north of Swansea, but no such fault exists (Strahan, in discussion of Davison, 1907). The epicentre of an earthquake in 1930 was east of Neath.

Engineering ground conditions

Engineering ground conditions depend on the physical and chemical properties of the soils and rocks, groundwater and surface water movement, and the nature of past and present human activity. For engineering purposes, soils include the superficial deposits, which can be excavated by digging. Ground conditions requiring special foundation design include compressible soft superficial deposits, such as alluvial clay and peat, and areas of landslide deposits and head, as well as those areas at risk of mining subsidence.

Geological conservation

Geological conservation in the district is partly enshrined in the designation of the Gower as an Area of Outstanding Natural Beauty. Geological localities deemed to be of national importance are protected as Sites of Special Scientific Interest, of which there are 14 in the district. Six are in the Mississippian limestones of Gower (Three Cliffs Bay, Pwlldu Head, Caswell Bay, Bracelet Bay, Ilston Quarry and Oystermouth Old Quarry; Adams et al., 2004), one is in the Namurian (Barland Common, Bishopston; Cleal and Thomas, 1996), one is in the Grovesend Formation; Penllergaer railway cutting; (Cleal and Thomas, 1996) and six are in the caves and Quaternary deposits of Gower (Rotherslade (Langland Bay), Hunts Bay, Bosco’s Den, Bacon Hole Cave, Minchin Hole Cave and Cat Hole Cave; Campbell and Bowen, 1989).

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Swansea district and adjacent areas are listed here.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

Geological maps

1:1 500 000
Tectonic map of Britain, Ireland and adjacent areas (1996)
1:1 000 000
Industrial mineral resources map of Britain (1996)
Pre-Permian geology of the United Kingdom (South) (1985)
1:625 000
Bedrock geology of the United Kingdom: South Map (2007)
Quaternary geology: South sheet (1977)
1:250 000
Mid Wales and the Welsh Marche, Solid Geology (1990)
Adjoining 1:50 000 maps
Ammanford 230
Worms Head 246
Pontypridd and Maesteg 248
Old Series one-inch map
Sheet 37 c.1844
1:10 000 and 1:10 000 maps
See index map on margin of the 1:50 000 map
Copies of maps from these and earlier large-scale surveys are available for reference in the BGS Libraries at Keyworth and Edinburgh, and at the BGS London Office in the Natural History Museum Earth Galleries, South Kensington. Copies for purchase are produced on a print-on-demand basis and are available from the BGS Sales Desk.

Digital geological map data

In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.

Geophysical maps

1:1 500 000
Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, (1996)
Colour shaded relief magnetic map of Britain, Ireland and adjacent areas (1996)
1:625 000
Gravity anomaly map of the UK: South sheet (2007)
Magnetic anomaly map of the UK: South sheet (2007)
1:250 000
Bouguer gravity anomaly map of the British Isles, south sheet (1996)
Aeromagnetic map of Great Britain, south sheet (1965)

Geochemical atlases

1:250 000
Regional geochemistry of Wales and part of west central England (2000)
Regional geochemistry of Wales and part of west central England, streamwater (2000)
Geochemical maps
1:625 000
Methane, carbon dioxide and oil susceptibility, Great Britain, south sheet, (1995)
Radon potential based on solid geology, Great Britain, south sheet (1995)
Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb, and Zn), Great Britain, south sheet (1995)

Mineral maps

1:1 000 000
Industrial mineral resources of Britain, 1996
Metalliferous mineral resources of Britain, (1996)

Hydrogeological maps

1:625 000
England and Wales (1977)
1:100 000
Groundwater vulnerability map

Books

British Regional Geology Guide
Wales (2007)
Memoir
South Wales Coalfield, Part VIII. The country around Swansea, Sheet 247 (1907)
Sheet description
Geology of the Swansea district, in press
Reports
Technical reports relevant to the district, including biostratigraphical reports, may be consulted at the BGS library or purchased from the BGS Sales Desk.

Documentary records collections

Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.
Borehole and trial pit records
Borehole records for the district are catalogued in the NGDC at BGS Keyworth. Index information, which includes site references, names and depths for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at BGS Keyworth.
Hydrogeological data
Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.
Geophysical data
These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data from coal and hydrocarbon exploration programmes is available for the north of the district. Indexes can be consulted on the BGS website.
BGS Lexicon of named rock units
Definitions of the stratigraphic units shown on BGS maps, including those named on Sheet 247 (Swansea) are held in the BGS Stratigraphic Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.
BGS photographs
The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collection can be viewed at BGS libraries at Keyworth and Edinburgh, and on the BGS website. Copies of the photographs can be purchased from the BGS.

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.
BGS addresses
The addresses of the main offices are given on the back cover.
BGS Hydrogeology Enquiry Service
British Geological Survey, Maclean Building , Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB. Telephone 01491 838800; fax 01491 692345.

References

British Geological Survey holds most of the references listed below, and copies may be obtained via the library service subject to copyright legislation (contact libuser@bgs.ac.uk for details). The library catalogue is available at: http://geolib.bgs.ac.uk

Adams, A E, Wright, V P, and Cossey, P J. 2004. South Wales–Mendip shelf. 395–475 in British Lower Carboniferous stratigraphy. Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A, and Wright, V P. Geological Conservation Review Series, No. 29. (Peterborough: Joint Nature Conservation Committee.)

Adams, H F. 1967. The seams of the South Wales Coalfield. Monograph of the Institution of Mining Engineers.

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, Macdonald, A M, Wagstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication, 8.

Allen, J R L. 1965. Upper Old Red Sandstone (Farlovian) palaeogeography in south Wales and the Welsh Borderland. Journal of Sedimentary Petrology, Vol. 35, 167–195.

Allen, J R L. 1974. The Devonian rocks of Wales and the Welsh Borderland. 47–84 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.)

Al-Saadi, R, and Brooks, M. 1973. A geo- physical study of Pleistocene buried valleys in the Lower Swansea Valley, Vale of Neath and Swansea Bay. Proceedings of the Geologists’ Association. Vol. 84, 135–153.

Anderson, J G C. 1968. The concealed rock-surface and overlying deposits of the Severn Valley and estuaries from Upton to Neath. Proceedings of the South Wales Institute of Engineers, Vol. 83, 27–47.

Anderson, J G C. 1974. The buried channels, rock-floors and rock-basins and overlying deposits, of the south Wales valleys from Wye to Neath. Proceedings of the South Wales Institute of Engineers, Vol. 88, 1–25.

Anderson, J G C, and Owen, T R. 1979. The late Quaternary history of the Neath and Afan valleys, south Wales. Proceedings of the Geologists’ Association. Vol. 90, 203–211.

Archer, A A. 1965. Red beds in the Upper Coal Measures of the western part of the South Wales Coalfeld. Bulletin of the Geological Survey of Great Britain, No. 23, 57–64.

Archer, A A. 1968. Geology of the South Wales Coalfield. The Upper Carboniferous and later formations of the Gwendraeth valley and adjoining areas. Special memoir of the Geological Survey. (London: HMSO.)

Barclay, W J. In press. Geology of the Swansea district. Sheet Description of the British Geological Survey, Sheet 247 (England and Wales).

Beus, S S. 1984. Fossil associations in the High Tor Limestone (Lower Carboniferous) of south Wales. Journal of Palaeontology, Vol. 58, 651–667.

Bluck, B J, and Kelling, G. 1963. Channels from the Upper Carboniferous Coal Measures of south Wales. Sedimentology, Vol. 2, 29–53.

Bolton, H. 1934. New forms from the insect fauna of the British Coal Measures. Quarterly Journal of the Geological Society of London, Vol. 90, 277–301.

Bowen, D Q. 1970. South-east and central south Wales. 197–227 in The glaciations of Wales and adjoining regions. Lewis, C A (editor). (London: Longman.)

Bowen, D Q. 1980. The Llanelli landscape: the geology and geomorphology of the country around Llanelli. (Llanelli Borough Council).

Bowen, D Q. 1981. The ‘South Wales end-moraine’: fifty years after. 60–67 in The Quaternary of Britain. Neale, J, and Flenley, J (editors). (Oxford: Pergamon Press.)

Bowen, D Q. 1999. Wales. 79–90 in A revised correlation of the Quaternary deposits in the British Isles. Bowen, D Q (editor). Geological Society of London Special Report, No. 23.

Bowen, D Q. 2005. South Wales. 145–162 in The glaciations of Wales and adjacent areas. Lewis, C A, and Richards, A E (editors). (Almeley: Logaston Press.)

Bowen, D Q, and Sykes, G A. 1988. Correlation of marine events and glaciations in the north-east Atlantic margin. Philisophical Transactions of the Royal Society, Vol. B318, 619–635.

Bridges, E M. 1997. Classic landforms of the Gower coast (Second edition). (Sheffield: Geographical Association in conjunction with the British Geomorphological Research Group.)

Burchette, T P. 1987. Carbonate-barrier shorelines during the basal Carboniferous transgression: the Lower Limestone Shales Group, south Wales and western England. 309–329 in European Dinantian environments. Miller, J, Adams, A E, and Wright, V P (editors). (Chichester: John Wiley.)

Burchette, T P, Wright, V P, and Faulkner, T J. 1990. Oolitic sandbody depositional models and geometries, Mississipian of south-west Britain: implications for exploration in carbonate ramp settings. Sedimentary Geology, Vol. 68, 87–115.

Campbell, S, and Bowen, D Q. 1989. Quaternary of Wales. Geological Conservation Review Series A4.1. (Peterborough: Nature Conservancy Council.)

Carey Jones, S. 1954. The seam splitting of the Graigola Seam and its relationship to the Swansea Six Feet and Three Feet seams. Proceedings of the South Wales Institute of Engineers, Vol. 68, 104–115.

Carey Jones, S. 1957. The northward attenuation of the coal seams in the Swansea district. Transactions of the Institution of Mining Engineers, Vol. 116, 445–453.

Cleal, C J. 1974. The recognition of the base of the Westphalian Stage in Britain. Geological Magazine, Vol. 121, 125–129.

Cleal, C J. 1978. Floral biostratigraphy of the upper Silesian Pennant Measures of south Wales. Geological Journal, Vol. 13, 165–194.

Cleal, C J. 1997. The palaeobotany of the upper Westphalian and Stephanian of southern Britain and its geological significance. Review of Palaeobotany and Palynology, Vol. 95, 227–253.

Cleal, C J, and Thomas, B A. 1996. British Upper Carboniferous stratigraphy. Geological Conservation Review Series, No. 11.(London: Chapman and Hall.)

Codrington, T. 1898. On some submerged valleys in South Wales, Devon and Cornwall. Quarterly Journal of the Geological Society of London, Vol. 54, 251–278

Cole, J E, Miliorizos, M, Frodsham, K, Gayer, R A, Gillespie, P A, Hartley, A J, and White, S C. 1991. Variscan structures in the opencast coal sites of the South Wales Coalfield. Proceedings of the Ussher Society, Vol. 7, 375–379.

Conway, B W, Forster, A, Northmore, K J, and Duncan, S V. 1980. South Wales Coalfield landslip survey. Volume 2. Institute of Geological Sciences Report, No. EG80/4.

Culver, S J, and Bull, P A. 1979. Late Pleistocene rock basins in south Wales. Geological Journal, Vol. 14, 107–116.

Davies, J H, and Trueman, A E. 1927. A revision of the nonmarine lamellibranchs of the Coal Measures. Quarterly Journal of the Geological Society of London, Vol. 83, 210–259.

De La Beche, H. 1846. On the formation of the rocks of south Wales and south-western England. Memoir of the Geological Survey of Great Britain, No. 1, 1–296.

Department Of The Environment/Welsh Office. 1992. An appraisal of the land-based sand and gravel resources of south Wales. Engineering Geology Unit, Department of Earth Sciences, University of Liverpool.

Department Of The Environment/Welsh Office. 1997. Swansea–Llanelli earth science information for planning and development. Ove Arup & Partners, Serial No. 96/3179.

Dix, E. 1931. The Millstone Grit of Gower. Geological Magazine, Vol. 68, 529–43.

Dix, E. 1934. The sequence of floras in the Upper Carboniferous with special reference to south Wales. Transactions of the Royal Society of Edinburgh, Vol. 57, 789–281.

Dixon, E E L, and Vaughan, A. 1912. The Carboniferous succession in Gower (Glamorgan) with notes on its fauna and conditions of deposition. Quarterly Journal of the Geological Society of London, Vol. 67, 477–571.

Faulkner, T J. 1988. The Shipway Limestone of Gower: sedimentation on a storm-dominated early Carboniferous ramp. Geological Journal, Vol. 25, 85–100.

Fielding, C R. 1984. A coal depositional model for the Durham Coal Measures of north-east England. Journal of the Geological Society of London, Vol. 141, 919–931.

Frodsham, K, and Gayer, R A. 1997. Variscan compressional structures within the main productive coal-bearing strata of south Wales. Journal of the Geological Society of London, Vol. 154, 195–208.

Geological Survey. 1920. Refractory materials: fireclays. Memoirs of the Geological Survey, Special Reports on the Mineral Resources of Great Britain, Vol. 14.

George, G T. 2001. Late Yeadonian (Upper Sandstone Group) incised valley supply and depositional systems in the south Wales peripheral foreland basin: implications for the evolution of the Culm Basin and for the Silesian hydrocarbon plays of onshore and offshore UK. Marine and Petroleum Geology, Vol. 18, 671–705.

George, T N. 1932. The Quaternary beaches of Gower. Proceedings of the Geologists’ Association of London, Vol. 43, 291–324.

George, T N. 1939a. The Cefn Bryn Shales of Gower. Geological Magazine, Vol. 76, 1–6.

George, T N. 1939b. The geology, physical features and natural resources of the Swansea district. Social and economic survey of Swansea and district Pamphlet, No. 1. (University College of Swansea.)

George, T N. 1940. The structure of Gower. Quarterly Journal of the Geological Society of London, Vol. 96, 131–198.

George, T N. 1974. Fossil molluscs, and molluscs in stratigraphy: the work of A E Trueman. 1–30 in The Upper and post-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.)

George, T N. 1978. Mid Dinantian (Chadian) limestones in Gower. Philisophical Transactions of the Royal Society, Vol. B282, 411–462.

George, T N, and Howell, E J. 1939. Goniatites from the Caninia Oolite of Gower. Annals and Magazine of Natural History, 11th series, Vol. 4, 545–561.

George, T N, and Ponsford, D R A. 1935. Mid Avonian goniatites from Gower. Annals and Magazine of Natural History, 10thSeries, Vol. 16, 354–370.

George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Rambsbottom, W H C, Sevast-opulo, G D, and Wilson, R B. 1976. A correlation of the Dinantian rocks in the British Isles. Geological Society of London, Special Report, No. 7.

Hartley, A J. 1993. Silesian sedimentation in south-west Britain: sedimentary responses to the developing Variscan Orogeny. 159–196 in Rhenohercynian and Subvariscan foldbelts. Gayer, R A, Greiling, G O, and Vogel, A K (editors). (Braunschweig: Vieweg Publishing Earth Evolution Series.)

Jansson, K N, and Glasser, N F. 2005. Palaeoglaciology of the Welsh sector of the British–Irish Ice Sheet. Journal of the Geological Society of London, Vol. 162, 25–37.

Jones, D G. 1974. The Namurian Series in south Wales. 117–132 in The Upper Palaeozoic and Post-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales.)

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, Macdonald, A M, Coleby, L M, Talbot, J C, Mckenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/04. Environment Agency R&D Publication No. 68.

Jones, J A. 1989. The influence of contem-poraneous tectonic activity on Westphalian sedimentation in the South Wales Coalfield. 243–253 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). Yorkshire Geological Society Occasional Publication, No. 6.

Jones, J A. 1991. A mountain front model for the Variscan deformation of the South Wales Coalfield. Journal of the Geological Society of London, Vol. 148, 881–891.

Jones, J A, and Hartley, A J. 1993. Reservoir characteristics of a braid-plain depositional system: the Upper Carboniferous Pennant Sandstone of south Wales. 143–156 in Characterisation of fluvial and aeolian reservoirs.North, C P, and Prosser, D J (editors). Geological Society of London Special Publication, No. 73.

Jones, O T. 1942. The buried channel of the Tawe valley near Ynystawe, Glamorganshire. Quarterly Journal of the Geological Society of London, Vol. 98, 61–88.

Jones, S H. 1935. The Lower Coal Series of Gower. Proceedings of the South Wales Institute of Engineers, Vol. 50, 317–336.

Jordan, H K. 1910. The South Wales Coal Field. Sections and Notes. Proceedings of the South Wales Institute of Engineers, Vol. 27, 172–254.

Kelling, G. 1964. Sediment transport in part of the Lower Pennant Measures of south Wales. 177–184 in Developments in sedimentology. Vol. 1. Deltaic and shallow marine deposits. Van Straaten, L M J U (editor). (Amsterdam: Elsevier.)

Kelling, G. 1968. Patterns of sedimentation in Rhondda Beds of south Wales. Bulletin of the American Association of Petroleum Geologists, Vol. 52, 2369–2386.

Kelling, G. 1971. Upper Carboniferous sedimentation in the central part of the South Wales Coalfield. 85–95 in Geological excursions in South Wales and the Forest of Dean. Bassett, D A, and Bassett, M G (editors). (Cardiff: Geologists’ Association, South Wales Group.)

Kelling, G. 1974. Upper Carboniferous sedimentation in south Wales. 185–224 in The Upper Palaeozoic and post-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.)

Kelling, G. 1988. Silesian sedimentation and tectonics in the South Wales basin: a brief review. 38–42 in Sedimentation in a synorogenic basin complex, the Upper Carboniferous of northern Europe. Besley, B, and Kelling, G (editors). (Glasgow: Blackie.)

Leitch, D, Owen, T R, and Jones, D G. 1958. The basal Coal Measures of the South Wales Coalfield from Llandybie to Brynmawr. Quarterly Journal of the Geological Society of London, Vol. 113, 461–486.

McCarroll, D. 2002. Amino-acid geochronology and the British Pleistocene: secure stratigraphical framework or a case of circular reasoning. Journal of Quaternary Science, Vol. 17, 647–651.

Ogg, J G, Ogg, G, and Gradstein, F M. 2008. The Concise Geologic Time Scale. (Cambridge: Cambridge University Press.)

Owen, T R. 1954. The structure of the Neath Disturbance between Bryniau Gleision and Glynneath, south Wales. Quarterly Journal of the Geological Society of London, Vol. 109, 333–365.

Owen, T R. 1971. The Gower Peninsula. 125–134 in Geological excursions in south Wales and the Forest of Dean. Bassett, D A, and Bassett, M G. (editors). (Cardiff: Geologists’ Association, South Wales Group.)

Owen, T R, and Weaver, J D. 1983. The structure of the main South Wales Coalfield and its margins. 74–87 in The Variscan Fold Belt in the British Isles. Hancock, P L (editor). (Bristol: Hilger.)

Powell, J H, Chisholm, J I, Bridge, D M, Rees, J G, Glover, B W, and Besley, B M. 2000. Stratigraphical framework for Westphalian to early Permian red-bed successions of the Pennine Basin. British Geological Survey Technical Report, WA/99/01.

Ramsay, A T S. 1987. Depositional environments of Dinantian limestones in Gower. 265–308 in European Dinantian environments. Miller, J, Adams, A E, and Wright, V P (editors). (Chichester: John Wiley & Sons Ltd.)

Ramsay, A T S. 1989. Tectonics and sedimen-tation of late Dinantian limestones in south Wales. 225–241 in The role of tectonics in Devonian and Carboniferous sedimentation in the British Isles. Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). Yorkshire Geological Society Occasional Publication, No. 6.

Ramsay, A T S. 1991. Sedimentation and tectonics in the Dinantian limestones of south Wales. 485–511 in Sedimentation, tectonics and eustacy: sea-level changes at active margins. Macdonald, D I M (editor). International Association of Sedimentologists Special Publication, No. 12. (Oxford: Blackwell Scientific Publications.)

Ramsbottom, W H C. 1952. The fauna of the Cefn Coed Marine Band in the Coal Measures at Aberbaiden, near Tondu, Glamorgan. Bulletin of the Geological Survey of Great Britain, No. 4, 8–30.

Ramsbottom, W H C. 1971. South Wales Excursion 2, Day 2, Gower peninsula, Namurian succession. 1675–1676 in Compte Rendu, Sixième Congrès International de Stratigraphie et de Géologie du Carbonifère, Sheffield 1967, Volume IV.

Ramsbottom, W H C. 1978. Namurian mesothems in South Wales and northern France. Journal of the Geological Society of London. Vol. 135, 307–312.

Ramsbottom, W H C, Calver, M A, Eagar, R M C, Hodson, F, Holliday, D W, Stubblefield, C J, and Wilson, R B. 1978. A correlation of Silesian rocks in the British Isles. Special Report of the Geological Society, London, No. 10.

Riding, R, and Wright, V P. 1981. Palaeosols and tidal flat/lagoon sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. Journal of Sedimentary Petrology, Vol. 51, 1323–1339.

Searl, A. 1989. Diagenesis of the Gully Oolite (Lower Carboniferous), south Wales. Geological Journal, Vol. 24, 275–293.

Simpson, J. 1985. Stylolite-controlled layering in an homogeneous limestone: pseudo-bedding produced by burial diagenesis. Sedimentology, Vol. 32, 495–505.

Simpson, J. 1987. Mud-dominated storm deposits from a Lower Carboniferous ramp. Geological Journal, Vol. 22, 191–205.

Strahan, A. 1907a. The geology of the South Wales Coalfield, Part VIII. The country around Swansea. Memoir of the Geological Survey, Sheet 247 (England and Wales).

Strahan, A. 1907b. The geology of the South Wales Coalfield, Part IX. West Gower and the country around Pembrey. Memoir of the Geological Survey, Sheet 246 (England and Wales).

Strahan, A, and Pollard, W. 1915. The coals of south Wales. Memoir of the Geological Survey of England and Wales. (London: HMSO.)

Stubblefield, C J, and Trotter, F M. 1957. Divisions of the Coal Measures on Geological Survey maps of England and Wales. Bulletin of the Geological Survey of Great Britain, No. 13, 1–5.

Sutcliffe, A J, Currant, A P, and Stringer C B. 1987. Evidence of sea-level change from coastal caves with raised beach deposits, terrestrial faunas and dated stalagmites. Progress in Oceanography, Vol. 18, 243–271.

Thomas, B A, and Cleal, C J. 2001. A new early Westphalian D flora from Aberdulais Falls, south Wales. Proceedings of the Geologists’ Association, Vol. 112, 373–377.

Thomas, L P. 1967. A sedimentary study of the sandstones between the horizons of the Four Feet Coal and the Gorllwyn Coal of the Middle Coal Measures of the South Wales Coalfield. Unpublished PhD thesis, University of Wales.

Tucker, M E, and Wright, V P. 1990. Carbonate sedimentology. (Oxford: Blackwell Scientific Publications.)

Waters, C N, Price, S J, Marchant, A P, Davies, J, Brown, S E, Tye, A M, Hawkins, M P, Fiorini, E, Fleming, C, Schofield, D I, Barclay, W J, and Garcia-Bajo, M. 2006. A background to urban geoscience studies in the Swansea–Port Talbot area. British Geological Survey Internal Report, IR/05/073.

Waters, C N, Barclay, W J, Davies, J R, and Waters, R A. 2009. Stratigraphical framework for Carboniferous successions of southern Great Britain. British Geological Survey Research Report, RR/09/01.

Weaver, J D. 1975. The structure of the Swansea Valley Disturbance between Clydach and Hay-on-Wye, south Wales. Geological Journal, Vol. 10, 75–86.

Wilson, D, Davies, J R, Fletcher, C J, and Smith, M P. 1990. Geology of the South Wales Coalfield, Part VI. The country around Bridgend. Memoir of the British Geological Survey, Sheets 261 & 262 (England and Wales).

Woodland, A W, and Evans, W B. 1964. The geology of the South Wales Coalfield. Part IV. The country around Pontypridd and Maesteg. Third edition. Memoir of the Geological Survey of Great Britain, Sheet 248 (England and Wales).

Woodland, A W, Evans, W B, and Stephens, J V. 1957a. Classification of the Coal Measures of south Wales with special reference to the Upper Coal Measures. Bulletin of the Geological Survey of Great Britain, No. 13, 6–13.

Woodland, A. W, Archer, A A, and Stephens, J V. 1957b. Recent boreholes into the Lower Coal Measures below the Gellideg–Lower Pumpquart horizon in south Wales. Bulletin of the Geological Survey of Great Britain, No. 13, 39–60.

Wright, V P. 1982. The recognition and interpretation of palaeokarsts: two examples from the Lower Carboniferous of South Wales.Journal of Sedimentary Petrology, Vol. 52, 83–94.

Wright, V P. 1986. Facies sequences on a carbonate ramp: the Carboniferous Limestone of south Wales. Sedimentology, Vol. 33, 221–241.

Wright, V P. 1987a. The evolution of the early Carboniferous Limestone province in south-west Britain. Geological Magazine, Vol. 124, 477–480.

Wright, V P. 1987b. The ecology of two early Carboniferous palaeosols. 345–358 in European Dinantian environments. Miller, J, Adams, A E, and Wright, V P (editors). (Chichester: John Wiley & Son.)

Wu, Xian-Tao. 1982. Storm-generated depositional types and associated trace fossils in Lower Carboniferous shallow marine carbonates of Three Cliffs Bay and Ogmore-by-sea. Palaeontology, Palaeoecology and Palaeoclimatology, Vol. 39, 187–202.

Figures and plates

(Figure 1) Simplified map of the bedrock geology of the Swansea district.

(Figure 2) Generalised section of the Mississippian rocks with interpretation of their depositional environments (partly after Tucker and Wright, 1990).

(Figure 3) Classification of the Namurian strata in the UK.

(Figure 4) Classification of the Westphalian–Stephanian rocks of the Swansea district.

(Figure 5) Generalised vertical section of the Westphalian–Stephanian succession in the Swansea district.

(Figure 6) Quaternary deposits of the Swansea district. Based on Campbell and Bowen (1989) and Bowen (1999).

(Figure 7) Sketch map showing distribution of superficial deposits in the Swansea district.

(Figure 8) Principal structures of the district.

(P945695) Summary of the geological succession in the district.

Plates

(Plate 1) Steeply dipping limestones of the Hunts Bay Oolite Subgroup, Great Tor [SS 530876]. View looking west from Threecliff Bay (P663719).

(Plate 2) Prominent bedding plane in limestone of the Oxwich Head Limestone Formation at Oxwich Head [SS 2512 1855] showing karstic weathering along joints. (P66384).

(Plate 3) Pennant Sandstone Formation exposed in road cuttings at the M4–A48 interchange, Briton Ferry [SS 730 943] viewed from the west. The beds dip about 40˚ to the north (P726170). (W J Barclay).

(Plate 4) Quarry in the Pennant Sandstone Formation (Hughes Member) at Cwm-bach, near Neath [SS 759 991]. The sandstone includes low angle cross-bedding and a channel incision surface towards the top (P593438).

(Plate 5) Tidal flat deposits at Crymlyn Burrows [SS 717 931] looking east towards Baglan (P593405).

(Plate 6) Aeolian sand dunes, Baglan [SS 732 910] looking towards the Baglan Energy Park Power Plant (P593560).

(Front cover) Mumbles Head, Gower viewed from the west. Limestones of the Hunts Bay Oolite Subgroup crop out in the foreground and on the hill to the north of the lighthouse, which sits on downfaulted limestones of the Oxwich Head Limestone Formation (Photograph G Bell; P595527).