Content and licensingview original scan buy a printed copy

Geology of the Talgarth district — a brief explanation of the geological map Sheet 214 Talgarth

W J Barclay and P R Wilby

Bibliographic reference: Barclay, W J, and Wilby, P R. 2003. Geology of the Talgarth district — a brief explanation of the geological map.Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 214 Talgarth (England and Wales).

Keyworth, Nottingham: British Geological Survey © NERC 2003. All rights reserved.

Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence number GD272191/2003.

(Front cover) View of the Black Mountains looking north-east from near Talybont-on-Usk. The southern end of Llangorse Lake and the Usk Valley lie in the middle distance. [SO 085 232] (Photograph Graham Bell).

(Rear cover)

(Geological succession) Geological succession in the Talgarth district.

Notes

The word 'district' refers to the area of Sheet 214 Talgarth. National Grid references are given in square brackets and all lie within 100 km square SO.

Acknowledgements

This Sheet Explanation was compiled by W J Barclay and P R Wilby, and edited by A A Jackson. The geological map was compiled mainly by P R Wilby, using ImageStation software on scanned air photography, with assistance from W J Barclay and D Wilson. Digital photogrammetry was carried out by D Tragheim and E A O'Connor. A limited amount of field checking was carried out by P R Wilby. The authors thank D Hawley, University of Wales, and R B Williams, Talgarth, for their collaboration and for providing details of key sections.

Geology of the Talgarth district (summary from rear cover)

(Rear cover)

Most of the Talgarth district lies within the Brecon Beacons National Park, the eastern boundary of which follows Offa's Dyke. The park area is one of outstanding natural beauty, encompassing the Black Mountains and parts of the Wye and Usk valleys. The economy of the district depends largely on farming, and tourism continues to grow in importance.

The oldest rocks of the district are Silurian sandstones and mudstones of the Wye valley, and Carboniferous rocks cap Pen-Cerrig-calch. The remainder of the district is underlain by rocks of Old Red Sandstone facies, ranging in age from late Silurian to late Devonian/early Carboniferous. The Black Mountains are carved from fluvial sandstones of the Brownstones and Senni formations. Interbeds of mudstone between the sandstones produce the stepped valley sides on which the geology is magnificently displayed. The Quartz Conglomerate Group is present as an outlier on Pen-Cerrig-calch. At the top of this hill, limestones and mudstones of the Carboniferous Limestone are overlain by quartzites ('Millstone Grit') of Namurian age.

During the Quaternary, valley glaciers occupied the major valleys including the Usk and the Wye valleys, and small, cirque glaciers may have been present during a later cold period. The glaciers produced a range of glaciogenic deposits, including tills, glaciofluvial outwash gravels, glaciolacustrine clays and silts, and hummocky ice contact moraines. Llangorse Lake is a fine example of a glacial lake: it overflowed into the Usk valley by two channels. Following the retreat of the glaciers, many of the oversteepened valley sides suffered landslipping, and some landslips remain active today.

The Neath and Swansea Valley disturbances are major north-east-trending faults that cross the district; they are ancient basement fractures that were reactivated at intervals throughout geological history.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Sheet 214 Talgarth, published as a solid and drift edition in 2002.

The district lies in the counties of Powys, Herefordshire and Monmouthshire, much of it within the Brecon Beacons National Park. The park is one of outstanding natural beauty, encompassing the Black Mountains and parts of the Wye, Usk, Monnow and Golden valleys (Plate 1). Hill farming, with some mixed farming in the lower ground to the east and south, is the backbone of the local economy. However, tourism and outdoor recreational activities continue to grow in importance. The beauty of the hills attracts pony trekkers, walkers and climbers, and Llangorse Lake is an outdoor pursuits and sailing centre.

All the exposed rocks of the district are sedimentary; the oldest strata are Silurian sandstones and mudstones in the Wye valley in the north-west. Apart from an outlier of Carboniferous rocks capping Pen-Cerrig-calch [SO 2170 2236], the remainder of the district is underlain by rocks of continental (Old Red Sandstone) facies, which range in age from the late Silurian (Pridoli) to late Devonian/early Carboniferous (Farlovian/ Tournaisian). The Black Mountains reach a maximum height of 810 m on the summit of Waun Fach [SO 2156 2997]; they are carved from fluvial sandstones of the Lower Devonian Brownstones (red-brown) and Senni (green and purple) formations. Quartzitic sandstones of the Upper Devonian Quartz Conglomerate Group are present as an outlier on Pen-Cerrig-calch. Interbeds of mudstone between the sandstones produce the stepped valley sides, on which the geology is magnificently displayed, and the sequence dips gently southwards. The Pen-Cerrig-calch outlier presents an attenuated version of the Carboniferous strata seen in thicker development on the margins of the South Wales Coalfield to the south. Limestones and fissile mudstones of the Carboniferous Limestone overlie the Quartz Conglomerate Group and are overlain by quartzites ('Millstone Grit') of Namurian age, which form the summit of the hill. The Neath Disturbance crosses the south-east margin of the district, and the Swansea Valley Disturbance crosses the north-west. These major north-east-trending faults can be traced from south Wales to the Welsh Borderland and are ancient basement fractures that were reactivated at intervals throughout geological history.

During the Quaternary, the district was occupied by valley glaciers during the Late Devensian (Dimlington) glaciation. The most powerful were in the Usk and Wye valleys, but the other major valleys were also occupied by glaciers. Small, high cirque glaciers may have been present during a later cold period (the Loch Lomond Stadial) from 11 000 to 10 000 years ago. The glaciers produced a range of glaciogenic deposits, including spreads of till, hummocky ice contact moraines, lacustrine silt and clay and glaciofluvial outwash. The study of these deposits has led to a detailed understanding of the advance and retreat of the ice (Lewis, 1970a). Llangorse Lake is a fine example of a glacial lake, which overflowed into the Usk valley via two channels. Following the retreat of the glaciers, many of the over-steepened valley sides suffered landslipping, and some of these landslips, such as the one at Cwmyoy [SO 3000 2364] in the Vale of Ewyas, remain active today.

History of research

Murchison (1839) and Symonds (1872) were among early workers who visited and published data on the area. The district was systematically mapped at one inch to one mile between 1845 and 1857 and published as [Old Series] Sheets 42NE, 42SE and parts of 42SNW and SW. George (1927) described the Carboniferous rocks of Pen-Cerrig-calch. Croft (1953) discovered fossil fish and plant localities in the Black Mountains and sketch-mapped the base of the Senni Formation, but his work is largely unpublished, except for details of some of the localities (White, 1950). Spore assemblages of the Senni Formation are described by Hassan (1982). The district is included in a regional review by Allen (1974), and in studies of drainage and landscape development by Clarke (1935), O T Jones (1934), R O Jones (1931, 1939), Thomas (1959), Weaver (1975) and George (1942, 1961, 1974, 1980). The Quaternary glaciation and deposits of the district are described by Lewis (1966, 1970a, b). Descriptions of some Old Red Sandstone fossil sites are given by Hawley (1989), Turner et al. (1995) and Dineley (1999).

Chapter 2 Geological description

The geological succession present at outcrop in the district is shown inside the front cover. Most of the district is underlain by sedimentary rocks of the Lower Devonian Old Red Sandstone; Silurian shallow marine rocks outcrop in the Wye valley in the north-west, and Upper Old Red Sandstone and Carboniferous rocks outcrop on Pen-Cerrig-calch. Glacial deposits (consisting mainly of till), glaciofluvial gravels, solifluction deposits (head) and alluvial deposits infill the valleys.

Silurian

The Silurian System is represented by marine rocks of the Ludlow Series and by the predominantly terrestrial red beds of the lowest part of the Old Red Sandstone (Pridoli Series).

The former are present at outcrop only in a small area in the Wye valley in the north-west of the district. The latter, comprising the Temeside Mudstone Formation and most of the Raglan Mudstone Formation, outcrop around the lower flanks of the Black Mountains to the north of Talgarth [SO 1550 3400] and in the Golden Valley to the east. The position of the Silurian– Devonian boundary has not been determined within the red mudstones of the Raglan Mudstone Formation because of a lack of fossils, but is believed, from miospore evidence elsewhere, to lie in the upper part, below the Bishop's Frome (Psammosteus) Limestone.

Strata of Ludlow age (Ludlow strata, undifferentiated) are the oldest exposed rocks within the district. They outcrop in a small, partly fault-bounded inlier in the Wye valley to the west of Boughrood [SO 1250 3950]. These strata were not examined at outcrop in the district, but their character is well known from studies in the Builth Wells district immediately to the north (Straw, 1937; Wilby, 2000). They consist of hard, medium grey and brown, fine- to medium-grained, hummocky cross-bedded sandstones and interbedded argillaceous beds. The sandstones are generally less than 0.3 m thick, have sharp bases, many with concentrations of shell debris, and are commonly stacked one on top of another. The argillaceous strata form thinner interbeds of laminated mudstone and bioturbated mudstone and siltstone that contain abundant evidence of soft sediment disturbance. The strata are typical of storm-wave generated beds deposited in a shallow (15 to 50 m depth) marine shelf environment. The overlying Temeside Mudstone Formation consists predominately of green-grey siltstone and contains abundant, invariably rotted calcrete glaebules. Where fresh, it is hard and splintery but softens considerably where weathered. The formation generally appears massive at outcrop, but bedding can be discerned locally by the presence of thin sandstones or calcrete-rich horizons. It is believed to be approximately 30 to 50 m thick and is well exposed on either side of a glacial meltwater channel [SO 1046 4016]. It is interpreted as a sand-starved lagoonal or tidal flat deposit that experienced frequent and prolonged periods of emergence (Wilby, 2000).

The Raglan Mudstone Formation is the lowest formation of the Old Red Sandstone facies. It forms the hills on either side of the Wye valley and also outcrops in the complex fault zone of the Golden Valley. Although not examined within the district, its character is well known from studies in adjacent areas (e.g. Barclay, 1989; Brandon, 1989). It comprises up to about 900 m of red mudstone and siltstone, much of which show evidence of varying degrees of pedogenic alteration. There is a broad, cyclical pattern of pedogenesis comprising the development of soil carbonate (calcrete), from vague calcareous mottling to small, then increasingly large nodules (glaebules). Colour mottling is common, ranging from pale green reduction spots and patches to purple, green and red mottling in the more pedogenically altered horizons. Locally, the calcrete nodules increase in size and coalesce to produce mature, massive, rubbly limestone calcretes. These are particularly common in the uppermost part of the formation, with the Bishop's Frome (Psammosteus) Limestone Member the thickest and best developed. This is a regional marker present throughout south Wales and the Welsh Borderland, forming a resistant and easily mapped horizon. It is exposed at Pwll-y-Wrach [SO 1695 3258] in the River Enig, where it is 2.5 m thick, and shows the characteristic upward development from small to large nodules, coalescing to a compact limestone in the top 1 m (Hawley, 1989; Turner et al., 1995). Elsewhere (e.g. in Felindre Brook [SO 1934 3548]), two well developed closely spaced limestones are present (Plate 2). Sandstones occur sporadically in the Raglan Mudstone Formation. A number of them are well exposed in an excellent section in Felindre Brook [SO 1870 3640] upstream of Velindre. They are generally less than 2 m thick, grey-brown, red-brown and purple in colour, fine to medium grained and have a variable mica content. The Townsend Tuff Bed is the thickest and most extensive of a series of air-fall tuffs that occur throughout south Wales and the Welsh Borderland (Allen and Williams, 1981). It lies about 50 m below the Bishop's Frome Limestone Member, and is exposed at several localities in the district, including Cusop Dingle [SO 250 400], where 1 m of tuff is overlain by intercalated tuff and tuffaceous sandstone (Hawley in Barclay et al., 2002). The Raglan Mudstone Formation is interpreted as the deposits of a coastal alluvial floodplain, subjected to frequent periodic soil-forming episodes and occasional marine incursions. The sandstones are the deposits of shallow, ephemeral streams and sheet flood events. The Bishop's Frome Limestone Member and its correlatives represent a major, regional depositional hiatus, when mature soil carbonate profiles, each perhaps representing periods of up to 10 000 years (Allen, 1986), formed on a regional interfluvial pediment.

Devonian

Lower Devonian – Lower Old Red Sandstone

The St Maughans Formation occupies the lower and middle slopes of the Black Mountains, and is particularly well exposed in the numerous streams on the scarp between Cwm Cwnstab [SO 2020 3290] and Rhiw y Fan [SO 2180 3438]. It consists mainly of a stacked succession of fining-upwards sandstone-siltstone-mudstone cycles. Basal sandstones and intraformational conglomerates overlie erosion surfaces cut in the underlying mudstones at the top of the previous cycle. These are interpreted as channel deposits, which fine upwards through siltstone into thick floodplain mudstones, many of which are calcretised. The sandstones are fine to medium grained, planar and trough cross-bedded, and range from red-brown to purple and green. Some contain fish fragments and the green varieties contain sporadic plant fragments, mainly concentrated in the basal few centimetres. The intraformational conglomerates are characteristic of the St Maughans Formation; they are up to 1 m thick, purple, red-brown and green, and consist mainly of calcrete clasts, with lesser amounts of siltstone and sandstone clasts, in a calcareous sandstone cement. Fish fragments are common in the conglomerates. An intraformational conglomerate (0.3 m thick) 1 m above the base of the formation at Pwll-y-Wrach [SO 1695 3258] yielded a microvertebrate fauna of heterostracan and cephalaspid fragments, thelodont scales including Turinia pagei, acanthodian remains including Nostolepis arctica, chondrichthyan scales and placoderm plates (Turner et al., 1995; Vergoossen, 1999). Skolithos burrows recorded by Turner et al. in the mudstone underlying the conglomerate were interpreted as indicating marginal marine coastal plain deposits. Wayne Herbert quarry [SO 335 320] higher in the formation is a classic vertebrate locality (Dineley, 1999 and references therein). It has yielded a rich fauna of rare, complete fossil cephalaspid, pteraspid and acanthodian fishes, as well as arthropod remains. The complete specimens were recovered from a 5 cm-thick, green siltstone lens, with disarticulated remains occurring in the overlying sandstone. Only a small outcrop of weathered sandstone is now exposed at the locality.

Towards the top of the formation, two closely spaced, mature calcrete limestones are named the Ffynnon Limestones (Croft, 1952); (Plate 3). These are correlated with the Ruperra and Pontypool limestones of Pontypool (Squirrell and Downing, 1969) and the Abdon Limestones of the Clee Hills (Allen, 1974; Ball et al., 1961), and represent episodes of prolonged geomorphic stability, nondeposition and carbonate soil formation. The Senni Formation (formerly Senni Beds) is characterised by its green colour, and is dominated by channelised sandstones. It is particularly known for its Early Devonian vascular plant remains. Symonds (1872) was the first to examine the beds in the Black Mountains. Croft (1953) provided some details of the formation, although much of his data remains as unpublished notes in the archives of the British Museum. He noted that the base of the formation is gradational with the underlying 'Red Marls' (now the St Maughans Formation).

Allen (1974) noted that the position of the base of the formation is unclear, and that according to Ball et al. (1961), Croft placed it informally a little below the lower of the two Ffynnon Limestones. Allen (1974) further noted that these limestones occur in red and purple beds, and that the base of the Senni Formation would be best placed above the limestones, where the colour changes from red to green. Hassan (1982) included the area in a palynological study, but much of his data also remains unpublished. He placed the base of the formation at the base of a grey, thick-bedded, fine-to medium-grained, 2.5 m-thick sandstone which forms a marked feature. On the accompanying map, the base of the Senni Formation is placed at the top of the most persistent calcrete, above which the succession is sandstone-dominated (Plate 3); (Plate 4). The Senni Formation is about 150 to 200 m thick and consists mainly of cross-bedded, channelised sandstones (Allen, 1974). The sandstones are green, grey-green, purple and purplish green. The basal part of the formation is mainly purplish, as in the Abergavenny district to the south (Barclay, 1989), with green-grey beds generally appearing higher. The sandstones range from very fine to medium grained, with coarser, pebbly varieties appearing at higher levels. They comprise tabular sheets, constructed from lenticular, cross-bedded, channelised packages with inter-erosional surfaces. Red-brown mudstones and siltstones interbedded with the sandstones produce the stepped, terrace-like features on the valley and lower mountain sides. The sandstones are variably calcareous, and calcrete clasts occur in the base of the sandstone bodies, along with other intraformational mudstone and siltstone clasts. The sandstone bodies generally fine upwards from the conglomeratic base, and the tops are commonly truncated by scour or erosion surfaces. The argillaceous interbeds contain calcrete nodules, with more mature massive and rubbly calcretes seen locally. Desiccation cracks occur in places. Overall, the Senni Formation is interpreted as the deposits of low-sinuosity, seasonally flowing streams, attributed to Allen's (1979) medial fluvial facies of sandy, braided streams (Loefffler and Thomas, 1980). The finer grained lithologies are interpreted as floodplain lake, crevasse splay and channel abandonment facies. High water-table conditions are invoked for the preservation of the plant remains and the predominantly green colour of the formation, which is due to the presence of chloritic mica.

Apart from fossil plant fragments and miospores, no animal fossils have been found in the district. However, the so-called Breconian index fossil fish Rhinopteraspis dunensis (cornubica) was discovered by W N Croft at Primrose Hill Quarry, Crick-howell [SO 2070 2000] immediately to the south (Barclay, 1989 and references therein). This fossil is also the index for the Rhinopteraspis dunensis Zone in the Artois–Ardenne region of northern France and Belgium (e.g. Blieck et al., 1995). Croft (1953) noted that the sandstones contain distinctive, problematical structures ('pepper pot' structures, W H Ball, personal communication), but no work has been done on these. They may be burrow traces, trace fossils having been noted elsewhere in the formation.

The Brownstones Formation consists mainly of deep red-brown sandstones, with red-brown mudstone–siltstone interbeds. It caps much of the high ground of the Black Mountains and its outcrop is characterised by well featured slopes. The formation is about 200 m thick, and is truncated by an unconformity at the base of the overlying Quartz Conglomerate Group. It has not been studied in detail in the district, but sedimentological studies were carried out by Tunbridge (1981) in the Brecon Beacons, and Barclay (1989) provided a description of the formation in the Abergavenny district to the south. There appears to be little, if any, facies change between the Senni Formation and the Brownstones, other than colour. The sandstones of the Brownstones Formation are mainly arranged in laterally extensive, tabular sheets of trough and planar cross-bedded multistorey sandbodies. They are deep red-brown, purple-brown and pinkish, calcareous and micaceous, and range from fine- to coarse-grained, with gravelly, pebbly beds present locally. Intraformational mudstone, siltstone and calcrete clasts are common at the bases of the sandstone units, which generally show a fining-upwards motif. Tunbridge (1981) interpreted similar rocks in the Brecon Beacons as low-sinuosity, flash-flood, channel deposits merging downslope into muddy flood-basin deposits. The interbedded mudstones and siltstones have been interpreted as floodplain muds and silts, deposited from suspension in lakes or slow-moving water bodies, but an aeolian origin, or deposition from bedload as pedogenic mud aggregates (e.g. Ékes, 1993) may also be possible. The Brownstones Formation of this and adjoining districts has yet to yield animal fossils, although plant and rootlet fragments were recorded in the area to the south-west (Barclay et al., 1988).

Upper Devonian – Upper Old Red Sandstone

The Quartz Conglomerate Group unconformably overlies the Brownstones Formation. It crops out on Pen-Cerrig-calch, and in a small, landslipped outlier capping Table Mountain [SO 226 208]. It consists of green-grey, locally pebbly, quartzitic sandstones, and is about 60 m thick. The best exposure is in the backscar of a landslip at The Daren [SO 2135 2125] (Front cover), where about 20 m of sandstones are exposed (Allen, 1965; Barclay, 1978). The sandstones are mainly pale grey, green and buff, and occur as tabular, internally cross-bedded units, with a few thin impersistent red-brown siltstone interbeds. A thicker (2 m) siltstone lies below these sandstones, below which are 4 m of gravelly, red-brown and green sandstones with fish fragments (including cf. Bothriolepis sp.) at the base. Murchison (1839) recorded Holoptychius from these beds (Robertson, 1927). These gravelly red-brown sandstones, together with the underlying red-brown, bioturbated siltstones (which contain a few fish fragments) may be a remnant of the Plateau Beds. Brown, coarse-grained sandstones typical of the Brownstones Formation lie below, with a green, decalcified, leached sandstone at the base of the section. About 10 m of quartzites cap Table Mountain [SO 226 208]. Murchison (1839) thought that they belonged to the Millstone Grit. Robertson (1927, pp.14, 15) correctly attributed them to the Quartz Conglomerate Group, but considered that they owe their position about 80 m below the main outcrop of the group on Pen-Cerrig-calch to the north because of faulting. They are, however, part of a landslipped mass (Barclay, 1989).

The grey-green, quartzitic sandstones resemble the Wern Watkin Formation, the basal formation of the Quartz Conglomerate Group of the Abergavenny district, which, in turn, is correlated with the Grey Grits Formation of the Merthyr Tydfil district (Allen, 1965; Lovell, 1978; Barclay, 1989). They are interpreted as the deposits of shallow, possibly ephemeral, sandy, braided, southward-flowing streams of an alluvial fan complex.

Carboniferous

Carboniferous rocks form the outlier that caps Pen-Cerrig-calch (Murchison, 1839; George, 1928). About 20 m of Namurian quartzitic sandstones form the summit of the hill, unconformably overlying about 45 m of Dinantian strata (Carboniferous Limestone).

The Carboniferous Limestone, described in detail by George (1928), comprises a thin, attenuated succession in comparison to that of the north crop of the South Wales Coalfield to the south of the Usk valley in the Abergavenny district. A thin basal limestone is overlain by a mudstone-dominated sequence that forms a pronounced 'slack' feature around the hill and an outcrop peppered with sink holes. Together about 3 to 4.5 m thick, these beds are overlain by 4.5 to 6 m of grey, coarse-grained, fossiliferous grainstone (the Crinoidal Limestone of George). All these beds are correlated with the Lower Limestone Shale Group of the South Wales Coalfield. The basal limestone may correlate with the Castell Coch Limestone Formation, the higher beds with the Cwmyniscoy Mudstone Formation (Burchette, 1981; Lovell, 1988; Barclay, 1989). The basal limestones are thinly bedded, impure and micritic, with grey and buff fissile mudstone interbeds. George (1928) also noted 'irregularly bedded earthy limestones' and that some beds are reddish, and some are conglomeratic, with flat mudstone clasts. Rounded quartz grains up to about 5 mm are common. A restricted fauna comprising the brachiopod Orbiculoidea nitida (Phillips), bivalves (Modiola sp.), ostracods, the worm Spirorbis sp. and fish (Psephodus sp.) was recorded by George, the ostracods locally being concentrated into coquinas. Referred to the Modiola phase of the Lower Limestone Shales by George, these beds are typical of the shallow-water, peritidal or 'lagoon phase' rocks of the basal Tournaisian. The Crinoidal Limestone is richly fossiliferous; the fauna is dominated by spiriferid brachiopods. Other brachiopods, notably productids and Macropotamorhynchus [= 'Camarotoechia'], are also common and Syringothyris is well represented. George lists the fauna as comprising the brachiopods Avonia bassus (Vaughan), Productus sp. nov. (= Productus. cf. martini (Sowerby) sensu Vaughan), Chonetes sp., Schellwienella cf. crenistria (Phillips), Spirifer [Unispirifer] tornacensis de Koninck, Spirifer spp., Syringothyris principalis North, Syringothyriscuspidata (Martin) mut. cyrtorhyncha North and Camarotoechia [Macropota-morhynchus] aff. mitcheldeanensis Vaughan, with bivalves (Modiola spp.), gastropods (Naticopsis spp.), trilobites (Phillipsia sp.), bryozoa and fish (Psephodus). George noted the presence of a rottenstone near, or at, the top of the Crinoidal Limestone; this is very similar to one in the Lower Limestone Shales, seen as loose blocks above old workings on the southern side of the hill. In this unit, he identified the brachiopods Avonia bassus, Productus sp., Schellwienella cf. crenistria, Syringothyris cuspidata and Camarotoechia [Macropota-morhynchus] aff. mitcheldeanensis, and the bivalves Modiola spp. and cf. Parallelodon sp.

Burchette (1981, 1987) referred the basal interbedded limestones and mudstones to the Mitcheldean Member. The basal limestones of the Castell Coch Limestone Formation farther south are absent here, with a non-sequence between the lowermost beds and the underlying Quartz Conglomerate Group. Burchette (1981, 1987) shows a ravinement (marine erosion) surface truncating these beds, and interprets the overlying Crinoidal Limestone of George as a carbonate shoal, deposited in the shallow, lagoonal waters.

The topmost bed of the Carboniferous Limestone, the Yellow Oolite of George, is 10 to 12 m thick; it is a grainstone which is buff, ooidal and with massive to flaggy bedding. Burchette (1981, 1987) refers it to the Main Limestone (of the South Wales Coalfield).

Some of the ooids are stained with red iron-oxide. Shelly layers yielded the following to George: the brachiopods Productus sp. nov. (= P. cf. martini), Schellwienella cf. crenistria, Syringothyris cuspidata (Martin) mut. cyrtorhyncha and Camarotoechia [Macropotamorhynchus] aff. mitcheldeanensis, bivalves including Myalina sp. and Modiola spp., and the bryozoan Fenestella sp. The age and correlation of this bed are uncertain. George (1928) considered it to be an unusual facies within the Lower Limestone Shale Group and suggested that the fauna indicates a late Cleistopora Zone (K2) (= early Courceyan) age. Burchette's (1981, 1987) correlation of it as part of the Main Limestone implies that it is a remnant of the Abercriban Oolite (=Blackrock Limestone Group) of the Abergavenny district, which is of Courceyan to Chadian age. If this is correct, it is not clear with which, if any, of the ooidal formations of the Abercriban Oolite it correlates, and it is shown as Abercriban Oolite (undivided) on the accompanying map.

The summit of Pen-Cerrig-calch comprises about 20 m of Namurian pebbly quartzitic sandstones and quartz conglomerates attributed to the Basal Grit of the Millstone Grit. These rest unconformably on the Carboniferous Limestone and are correlated with the quartz arenites forming the basal part of the Namurian succession on the North Crop of the South Wales Coalfield. Over 200 m of Carboniferous Limestone beds occur on the north crop of the South Wales Coalfield to the south; by comparison the 45 m of beds here represent considerable pre-Namurian erosion at this unconformity, notwithstanding the amount of internal thinning of the Carboniferous Limestone and the hiatus at its base. The outcrop of the sandstones is much affected by landslip and cambering, and is largely covered by scree. The sandstones are unfossiliferous, apart from some plant remains that include a lepidodendroid noted by George (1928). It is likely that, as noted by George, they are similar in age to the basal quartzites of the north-east outcrop of the coalfield, which are late Marsdenian (R2) to Yeadonian (G1) in age (Jones and Owen, 1967; Barclay, 1989).

Quaternary

The Quaternary deposits of the district consist mainly of glacial deposits formed during the advance and retreat of glaciers in the main valleys during the late Devensian. Spreads of alluvium also occur in the valleys, and the slopes of the Honddu valley are extensively landslipped. The Quaternary deposits have not been examined in detail in the district during this study, but their character is well documented in adjacent districts and data are available from published sources (e.g. Lewis, 1970a, b), unpublished research (e.g. Williams, 1968) and site investigations. These sources have been supplemented by available borehole data. Glacial deposits (undifferentiated) occupy much of the low ground and many of the valleys in the district. Although the maximum altitude at which substantial spreads of these deposits have been mapped is between 323 and 354 m above OD [SO 1726 3004]; [SO 1608 2832], glacial erratics occur at 380 m [SO 240 387], suggesting that ice was more widespread. The deposits are lithologically variable tills, and consist of pebbles and cobbles in a stiff, grey, fawn and red-brown, silty sandy clay or clayey silty sand. Glaciofluvial sand and gravel, and locally Head, are commonly interbedded, the former having been quarried locally e.g. [SO 3640 3880]. Much of the tills contain exotic clasts, but those in the Honddu and upper Monnow valleys contain only locally derived material (Clarke, 1936; Lewis, 1970a, b).

Typically, the deposits are characterised by sporadically marshy, irregular or smooth, concave slopes and undulating, gently sloping ground. Thickness ranges from about 1 m (where it is not generally mapped) to over 10 m in some of the valleys. Up to 10 m are exposed in Felindre Brook [SO 1932 3549], and 11 m have been drilled at Bronllys. The distribution of the deposits, and the presence or absence of exotic clasts in them, has provided evidence of the degree to which the district was penetrated by glaciers, and of the relative importance of locally derived and far-travelled ice. Ice derived from the Usk and Wye valley glaciers covered most of the low ground, but, except for the cols at the head of the Rhiangoll valley, was unable to override the high, north-west facing scarp of the Black Mountains. Instead, the valleys of these mountains were occupied by locally sourced ice (Clarke, 1936; Lewis, 1970a, b).

Large spreads of glaciofluvial deposits occur in the Wye valley to the north of Llyswen, and in the Rhiangoll valley where it widens south of Cwmdu [SO 1800 2370]. Smaller areas occur west of Bronllys [SO 1280 3480], north-west of Pencelli [SO 2580 0890] and in the Honddu valley west of Llanvihangel Crucorney [SO 3200 2070]. Typically, the deposits form flat, or very gently sloping, dry ground. In the Wye valley, they comprise a series of steep-fronted terraces, each several metres high (Pocock, 1940) above the modern flood plain. Most of the deposits are dissected by the modern rivers and are locally overlain by alluvial fan deposits. Based on exposures just to the north of the district and on details given by Pocock (1940), the deposits in the Wye valley comprise poorly sorted, locally slightly clayey or silty, cobble pebble gravels, cobble pebble sands, sandy gravels and medium grained sands. Boulders up to several metres in diameter also occur. The deposits may be over 10 m thick and locally include tills and glaciolacustrine silts and clays. In the Rhiangoll valley, over 10 m of sandy gravel with interbedded tills and clayey sandy gravels have been proved in boreholes. Most of the deposits were formed as outwash from the melting glaciers. Those near Pencelli were considered by Williams (1968) to belong to a delta system that prograded into glacial Lake Talybont, and to extend beneath the alluvium of Nant Menasgin.

Hummocky ice contact deposits occupy parts of three valleys — the Wye north of Glasbury [SO 1740 4000] and to the south-east of Llyswen [SO 1420 3760], the Usk between Talybont and Llanddetty Hall [SO 1244 2056], and the dry valley south-east of Bwlch [SO 1530 2140]. Characteristically, they form irregular, hummocky ground with numerous kettle holes. The deposits at Llanddetty are a halt moraine of the Usk glacier (Williams, 1968), the others formed as ice-marginal kames and melt-out deposits. The deposits are highly variable in character and are likely to exhibit vertical and lateral changes. They are dominated by interbedded sand and gravel, clay, and sandy pebbly clay till, but may contain minor amounts of silt and peat. In the Usk valley, the deposits consist of 33 m of sand and gravel at Talybont.

Glaciolacustrine deposits have been identified in three areas — near Bryndu [SO 1254 3610], between Pandy [SO 3340 2210] and Pontyspig [SO 2844 2100], and in the vicinity of Llangorse Lake, which is the remnant of a larger glacial lake. This drained into the Usk valley by overflow channels at Pennorth [SO 1123 2600] and west of Bwlch [SO 1425 2250]. The deposits are characterised by flat, or gently sloping, commonly boggy ground and consist of interbedded clay, silty clay and clayey sand. Clay is exposed in the overflow channel at Pennorth, and 8 m of sand and clay on thin gravel have been drilled on Llangorse Common [SO 127 275]. Substantial thicknesses of clays have been proved at Ashford [SO 1240 2184]; these were deposited in glacial Lake Talybont, which formed behind the Llandetty moraine.

River terrace deposits form flat-topped features up to a few metres above the present floodplain and consist principally of sand and gravel (Pocock, 1940). They have been identified in the valleys of the Wye, Usk, Honddu, Monnow, and Dore. Two river terraces in the Wye valley recognised by Williams (1968) are not distinguished on the geological map.

Lacustrine deposits occur in abandoned loops of the River Wye and its lowest terrace, and in kettle holes in the glacial deposits. A fine example of a kettle hole in hummocky ice contact deposits occurs in the grounds of Buckland House [SO 1295 2167] in the Usk valley. The deposits are likely to consist of interlaminated sand, silt and clay, and may include some peat.

Alluvial fans develop where tributary rivers emerge from the confines of their valley and deposit bedload at the confluence with the main river. They occur in all the major river valleys of the district and typically form smooth, gently sloping, cone-shaped features, which are dissected by the modern river channel. Three fans also developed on the eastern side of Llangorse Lake. Large fans at Three Cocks [SO 1720 3770], Talgarth, Llangorse [SO 1320 2750] and Talybont-on-Usk [SO 1160 2300] each cover over 0.5 km2. With the possible exception of some of the smaller fans and the one at Talybont-on-Usk, most are largely inactive, having mainly formed following the retreat of the glaciers. Boreholes indicate that they consist predominantly of variably clayey sand and gravel and silty, sandy clay with angular and rounded clasts. At least 4 m of these deposits occur at Talgarth and 10.5 m at Felinfach [SO 0920 3320].

Alluvium occurs in the most important rivers of the district; the widest spreads are those of the Wye, Usk, Monnow and Dore (Golden Valley). Typically, the alluvium forms flat or very gently sloping ground. It is laterally variable and consists of interbedded clay, silt, sand and gravel with varying proportions of fine-grained material. Stringers of pebbles are common within the finer units, indicating channel migration or periodic increases in flow regime, and a basal gravel lag deposit is common. In addition, the alluvium is likely to contain thin lenses of peat and lacustrine deposits. Boreholes in the Usk valley prove up to 7.5 m of alluvium overlying bedrock. Accumulations of peat and organic silt occur over much of the higher ground of the Black Mountains (McCaw, 1936), but are generally too thin or discontinuous to be mapped. Thicker peats occur locally in slacks or in flat ground at several places e.g. [SO 2045 3260]; [SO 2170 2970].

Slope deposits, collectively referred to as Head, accumulated under periglacial conditions and are preserved in depressions e.g. [SO 1145 3715], at the base of slopes e.g. [SO 2545 3672] and in the bottom and sides of many of the valleys in the district, such as that of the River Monnow [SO 3220 3040]. The deposits are variable, depending on their upslope source, but generally consist of clayey silt or sand with abundant locally derived angular clasts and some rounded glacial erratic pebbles and cobbles.

Landslips are extremely common at or just below the St Maughans Formation/ Senni Formation boundary. They are also common at a higher level, particularly on the northern side of Cwm Coedycerrig at the southern edge of the district. Two of the largest landslips are at Cwmyoy [SO 3000 2350] (about 1.25 km2) and Capel-y-ffin [SO 2430 3170] (2 km2). They and many of the other large landslips are composites of several mainly translational failures. They form striking landforms consisting of pinnacles of rock and hummocky ground separated by a fissure up to 100 m wide from a near-vertical backscar tens of metres high. Table Mountain [SO 2254 2068] is part of a large landslipped mass which, based on the correlation of the quartzites of the Quartz Conglomerate Group that cap it with the main outcrop on Pen-Cerrig-calch, has a vertical sense of displacement of about 80 m (Barclay, 1989). In addition to landslips in the solid rocks, several small slips in superficial deposits are also present e.g. [SO 2040 3590]. Most of these have formed on steep valley sides where the drift is thin and is being undercut. They are probably common throughout the district, but are too small to depict at 1:50 000 scale. Most of the landslips were probably initiated under periglacial conditions, as a consequence of the glacially oversteepened valleys. Those at the boundary of the St Maughans Formation and Senni Formation are associated with an active spring-line. Except for the Cwmyoy slip, which is still active, the present state of activity of the landslips is not known, but erosion of toes and freshness of landforms suggest that many have experienced recent movement.

Structure

The area lies mainly between two major north-east-trending Caledonoid structures, the Swansea (or Tawe) Valley Disturbance (Weaver, 1975) in the north-west and the Neath Disturbance in the south-east (Barclay, 1989). The structure of much of the district in the intervening ground is generally simple, with a gentle southerly regional dip. However, there appears to be considerable north-south faulting in the west affecting the outcrop of the Raglan Mudstone and St Maughans formations. There are also large north-west-trending faults in the north-east of the district, which continue into the adjoining Ross-on-Wye district (Barclay and Smith, 2002) and cause the repetition of the Bishop's Frome Limestone Member.

A suggestion by Weaver (1975) that renewed movement took place on the Swansea Valley Disturbance in the late Neogene was discounted by George (1980). Although there are no seismic data available for the district, the nature of the Neath Disturbance at depth is known to the east (Butler et al., 1997), where it was a growth fault in the Llandovery, controlling rifting and sedimentation. The present displacement is down to the north by about 200 m near Crickhowell (Owen, 1954), where it is locally named the Coedycerrig Fault, and about 150 m in the Gwryne Fawr valley (Barclay, 1989).

Chapter 3 Applied geology

Mineral resources

There are some industrial mineral resources in the district, but little commercial extraction. There are substantial amounts of sand and gravel in the Wye valley and Rhiangoll valley [SO 1750 2200] (Williams, 1968) and small-scale extraction has taken place at Talgarth, Bronllys [SO 1400 3520] and Stockley Hill [SO 3700 3880]. An assessment of the sand and gravel resources was carried out by the Department of the Environment (1992). Sandstones were formerly quarried for local building needs and mudstones in the St Maughans Formation were dug, presumably for brickmaking. A welcome recent development is the opening of small quarries ('delves') to supply local demand for building stone and roofing tiles. Tredomen Quarry [SO 1173 3041] is currently working sandstone in the St Maughans Formation for building stone. Coed Major Quarry [SO 257 372] (Plate 5), also in the St Maughans Formation, and worked since 1995, is currently supplying roofing tiles for the reroofing of Dore Abbey, as is Pennsylvani Quarry [SO 288 377]. The latter has been working since November 2001, replacing Grigland Quarry, which lies to the south. The Carboniferous Limestone was formerly dug south-west of the summit of Pen Cerrig-calch, probably for lime making. The Bishop's Frome Limestone Member was similarly quarried for lime burning at several localities, particularly in the Golden Valley.

Metalliferous minerals were formerly worked to a very minor extent. A lead mine worked a vein at Gospel Pass [SO 2365 3515] in the late 18th century (Hall, 1971). Galena can be seen in the stream today (D Hawley, personal communication, 2001). There was a former copper mine at Trawscoed [SO 085 343] comprising a large pit and an adit driven south-westwards from the river (Murchison, 1839). Copper mineralisation is also recorded on Lord Hereford's Knob [SO 2252 3505]. A disused 'coal' adit [SO 252 402] referred to by Murchison in his 1932/4 notebook was probably sited on coalified plant remains in the Senni Formation.

Water resources

The water supply of the district formerly came from the numerous wells and springs in the Old Red Sandstone but is now largely obtained from reservoirs and the major rivers. The hilly terrain and high rainfall result in abundant surface water in rivers and lakes, and there are reservoirs at Talybont, just to the south-west of the distict and in Gwryne Fawr [SO 230 308] in the Black Mountains. Groundwater from springs and wells continues to be used locally. These yield variable, but generally small amounts of water from sandstones and massive calcretes. The Brownstones and Senni formations together comprise the largest potential aquifer in the district, but groundwater is compartmentalised by mudstone beds and the potential is limited by low to moderate porosity and variable permeability. The larger spreads of gravels in the district are generally in hydraulic continuity with the River Wye and may have limited aquifer potential, but are vulnerable to surface contamination.

Geology and planning

Much of the district lies within the Brecon Beacons National Park. It is an area of outstanding natural beauty, encompassing the Black Mountains, and its status as a national park provides protection from large-scale or unsympathetic quarrying operations. The district is included in reports on mineral resources and their economic potential in relation to planning issues (Bloodworth et al., 1999; Highley et al., 1997).

The Old Red Sandstone rocks and the sand and gravel deposits of the district generally present good foundation conditions below the surface weathered zone. However, soft, weakened ground may occur in mudstone outcrops below springs. Similarly, superficial head deposits and landslips need careful investigation and appropriate foundation design. The extensive alluvial areas, comprising interbedded compressible clay, silt and sand, as well as peat lenses, provide poor foundation conditions, and are subject to flooding. Alluvial sands may be thixotropic, and running sands may also be encountered in the glacial deposits. There are no current waste-burial sites in the district, but development of former sites requires consideration of unstable foundation conditions and the possibility of methane generation from the decomposition of domestic waste. There is also potential for aquifer pollution from former or proposed disposal sites in sand and gravel.

Information sources

Further geological information held by the British Geological Survey relevant to the Talgarth district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Regional Geologist, UK South, BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries and on the BGS web site. At the present time (2002) some data sets are limited and not all are complete. The indexes which are available are listed below:

Maps

Books

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series 214 Talgarth Sheet, are held in the Lexicon database. This is available on BGS web site. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, and details of aquifer protection policy and former licensed landfill sites are held by the Environment Agency.

Earth science conservation sites

Information on the Sites of Special Scientific Interest present within the Talgarth district is held by English Nature, Headquarters and Eastern Region, Northminster House, Peterborough, PE1 1UA.

Addresses for data sources

BGS Hydrogeology enquiry service; wells, springs and water borehole records.

British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

See also (Rear cover).

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

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.)

Allen, J R L, and Williams, B P J. 1981. Sedimentology and stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh borders. Journal of the Geological Society, Vol. 136, 361–366.

Ball, W H, Dineley, D L, and White, E I. 1961. The Old Red Sandstone of Brown Clee Hill and the adjacent area. Bulletin of the British Museum (Natural History), Geology, London, Vol. A5, 177–310.

Barclay, W J. 1978. Abergavenny (232) Sheet. Devonian. 30 in Annual Report for 1977. Institute of Geological Sciences. (London: Institute of Geological Sciences.)

Barclay, W J. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Third edition. Memoir of the British Geological Survey, Sheet 232 (England and Wales).

Barclay, W J, and Smith, N J P. 2002. Geology of the country between Hereford and Ross-on-Wye. Sheet Explanation of the British Geological Survey, Sheet 215 (Ross-on-Wye).

Barclay, W J, Taylor, K, and Thomas, L P. 1988. Geology of the South Wales Coalfield, Part V, the country around Merthyr Tydfil. Third edition. Memoir of the British Geological Survey, Sheet 231 (England and Wales).

Barclay, W J, Browne, M A E, McMillan, A A, Pickett, E A, Stone, P, and Wilby, P R. in press. Old Red Sandstone rocks of Great Britain. Geological Conservation Review Series. (Peterborough: Joint Nature Conservation Committee.)

Blieck, A, Goujet, D, Janvier, P, and Meilliez, F 1995. Revised Upper Silurian–Lower Devonian ichthyostratigraphy of northern France and southern Belgium (Artois–Ardenne). Bulletin du Muséum National d'Histoire Naturelle, Paris, 4e Série, Vol. 17, Section C, 447–459.

Bloodworth, A J, Cameron, D G, Harrison, D J, Highley, D E, Holloway, S, and Warrington, G. 1999. Mineral resource information for development plans: Phase One Herefordshire and Worcestershire: resources and constraints. British Geological Survey Technical Report, WF/99/4.

Brandon, A. 1989. Geology of the country between Hereford and Leominster. Memoir of the British Geological Survey, Sheet 198 (England and Wales).

Burchette, T P. 1981. Lower Limestone Shales. 13–27 in A field guide to the Carboniferous Limestone around Abergavenny. Wright, V P, Raven, M, and Burchette, T P (editors). (Cardiff: Department of Geology, University of Wales.)

Burchette, T P. 1987. Carbonate-barriershorelines during the basal Carboniferoustransgression: the Lower Limestone Shale Group, South Wales and western England. 239–263 in European Dinantian Environments. Miller, J, Adams, A E, and Wright, V P (editors). (Chichester: John Wiley & Sons.)

Butler, A J, Woodcock, N H, and Stewart, D M. 1997. The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland. Journal of the Geological Society of London, Vol. 154, 209–223.

Clarke, B B. 1936. The post-Cretaceousgeomorphology of the Black Mountains. Proceedings of the Birmingham Natural History and Philosophical Society, Vol. 16, 157–172.

Croft, W N. 1953. Breconian: a stage name of the Old Red Sandstone. Geological Magazine, Vol. 90, 429–432.

Department ofthe Environment. 1992. An appraisal of the land based sand and gravel resources of South Wales. Engineering Geology Unit, Department of Earth Sciences, University of Liverpool.

Dineley, D L. 1999. Early Devonian fossil fish sites of the Welsh Borders. 107–144 in Fossil fishes of Great Britain. Dineley, D L, and Metcalf, S J (editors). Geological Conservation Review Series. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.)

Ékes, C. 1993. Bedload-transported pedogenic mud aggregates in the Lower Old Red Sandstone in southwest Wales. Journal of the Geological Society of London, Vol. 150, 469–471.

George, T N. 1928. The Carboniferous outlier at Pen-Cerrig-calch. Geological Magazine, Vol. 65, 162–168.

George, T N. 1942. The development of the Towy and upper Usk drainage pattern. Quarterly Journal of the Geological Society of London, Vol. 98, 89–137.

George, T N. 1961. The Welsh landscape. Science Progress, Vol. 49, 249–264.

George, T N. 1974. The Cenozoic evolution of Wales. 341–371 in The Upper Palaeozoic andpost-Palaeozoic rocks of Wales. Owen, T R (editor). (Cardiff: University of Wales Press.)

George, T N. 1980. Landform and structure in the terrain of the Tawe and Neath Disturbances in South Wales. Proceedings of the Geologists' Association, Vol. 91, 155-168.

Gradstein, F M, and Ogg, J. 1996. A Phanerozoic time scale. Episodes, Vol. 19, 3-5.

Hall, G W. 1971. Metal mines of southern Wales. (Westbury-on-Severn: G W Hall.)

Hassan, A M. 1982. Palynology, stratigraphy and provenance of the Lower Old Red Sandstone of the Brecon Beacons (Powys) and Black Mountains (Gwent and Powys), South Wales. PhD thesis, University of London.

Hawley, D. 1989. The geology of Pwll-y-wrach. The Breconshire Naturalist, No. 49, 3-6

Highley, D E, Cameron, D G, And Linley, K A. 1997. Mineral resource information for development plans Phase One South Wales: resources and constraints. British Geological Survey Technical Report, WF/97/10.

Jones, O T. 1934. The upper Towy drainage system. Quarterly Journal of the Geological Society of London, Vol. 80, 568-609.

Jones, R O. 1931. The development of the Tawe drainage. Proceedings of the Geologists' Association, Vol. 42, 305-321.

Jones, R O. 1939. The evolution of the Neath-Tawe drainage system, South Wales. Proceedings of the Geologists' Association, Vol. 50, 530-566.

Lewis, C A. 1966. The Breconshire end-moraine. Nature, Vol. 212, 1559-1561.

Lewis, C A. 1970a. The Upper Wye and Usk regions. 147-173 in The glaciations of Wales and adjoining regions. LEWIS, C A (editor). (Harlow: Longman.)

Lewis, C A. 1970b. The glaciations of the Brecknock Beacons, Wales. Brycheiniog, Vol. 14, 97-120.

Loeffler, E J, And Thomas, R G. 1980. A new pteraspid ostracoderm from the Devonian Semi Beds Formation of South Wales and its stratigraphical significance. Palaeontology, Vol. 23, 287-96.

Lovell, R. 1978. The sedimentology and stratigraphy of the Upper Old Red Sandstone and Lower Limestone Shales of the South Wales Coalfield. PhD thesis, University of Bristol (unpublished).

Mccaw, L S. 1936. The Black Mountains. A physical, agricultural and geographical survey, 1932-1936. Unpublished MA thesis, University of Manchester.

Murchison, R I. 1839. The Silurian System. (London: J Murray.)

Pocock, T I. 1940. Glacial drift and river terraces of the Herefordshire Wye. Z. Gletscherkd, Vol. 27, 98-117.

Robertson, T. 1927. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Second Edition. Memoir of the Geological Survey of Great Britain, Sheet 232 (England and Wales).

Straw, S H. 1937. The higher Ludlovian rocks of the Builth district. Quarterly Journal of the Geological Society of London, Vol. 93, 15-29.

Symonds, W S. 1872. Record of the rocks. (London: J Murray.)

Thomas, T M. 1959. The geomorphology of Brecknock. Brycheiniog, Vol. 3, 55-136.

Tunbridge, I P. 1981. Old Red Sandstone sedimentation - an example from the Brownstones (highest Lower Old Red Sandstone) of south central Wales. Geological Journal, Vol. 16, 111-124.

Turner, S, Vergoossen, J M J, And Williams, R B. 1995. Early Devonian microvertebrates from Pwll-y-wrach, Talgarth, South Wales. Geobios, Vol. 19, 377-382.

Vergoossen, J M J. 1999. Silurian-Devonian microfossils of Acanthodii and Chondrichthyes (Pisces) from the Welsh Borderland /South Wales. Modern Geology, Vol. 24, 23-90.

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.

White, E I. 1950. The vertebrate faunas of the Lower Old Red Sandstone of the Welsh Borders. Bulletin of the British Museum (Natural History), Vol. Al, 57-67.

Wilby, P R. 2000. Geological field notes for SO 04NE, SO 04SE, SO 04SW, SN 94SE and park of SN 94NE and SN 94SW. British Geological Survey Technical Report, IR/00/12.

Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately.

(Index map)

Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Figures and plates

Figures

(Figure 1) Old Red Sandstone (Siluro-Devonian) rocks in the district.

Plates

(Plate 1) Landsat image of the district. The Black Mountains occupy the central area, with the Wye valley in the north-west and Llangorse Lake in the south-west. The Neath Disturbance crosses the south-east corner (Winter false colour composite Landsat TM image displayed in band combination 4, 5, 7).

(Plate 2) Bishop's Frome Limestone, Felindre Brook [SO 1934 3458]. Two closely spaced massive calcretes form the waterfalls (GS1194).

(Plate 3) Topmost St Maughans Formation and basal Senni Formation exposed in a gully [SO 208 336], Pen Rhos-Dirion. Mudstones with thin sandstone sheets are capped by a white-weathering calcrete, the topmost of the Ffynnon Limestones. Basal sandstones of the Senni Formation lie above, forming the bold scarp feature (GS1195).

(Plate 4) Hay Bluff [SO 245 367] viewed from the north-west. The Senni Formation caps the summit of the hill, two lines of old diggings in the Ffynnon Limestones are visible in the topmost St Maughans Formation (GS1196).

(Plate 5) Coed Major Quarry [SO 257 372] working sandstones of the St Maughans Formation for roofing tiles (GS1197).

(Front cover) View of the Black Mountains looking north-east from near Talybont-on-Usk. The southern end of Llangorse Lake and the Usk Valley lie in the middle distance. [SO 085 232] (Photograph Graham Bell).

(Rear cover)

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey.

(Geological succession) Geological succession in the Talgarth area.