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Geology of the Brecon district— a brief explanation of the geological map sheet 213 Brecon

W J Barclay, J R Davies, A J Humpage, R A Waters, P R Wilby, M Williams and D Wilson

Bibliographic reference: Barclay, W J, Davies, J R, Humpage, A J, Waters, R A, Wilby, P R, Williams, M, and Wilson, D. 2005. Geology of the Brecon district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 213 Brecon (England and Wales).

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 permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, email ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) Summit of Pen y Fan [SO 042 215], Brecon Beacons, looking north-east from the summit of Corn Du. Resistant sandstones of the Plateau Beds Formation form the top of the mountain (Photographer Graham Bell; P577535).

(Rear cover)

(Geological succession) Geological succession in the Brecon district

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 213 Brecon. National Grid references are given in square brackets and lie within 100 km squares SO and SN. Letters in round brackets after lithostratigraphical names are those used on the geological map.

Acknowledgements

This Sheet Explanation was compiled and edited by W J Barclay using material provided by D Wilson and J R Davies (Ordovician), J R Davies, R A Waters and P R Wilby (marine Silurian) and W J Barclay, A J Humpage and P R Wilby (nonmarine Silurian and Devonian). All co-authors contributed to the sections on geological structure, Quaternary deposits and applied geology. C Royles supplied the geophysical data and M Williams (now British Antarctic Survey) provided biostratigraphical determinations of the Silurian rocks. D Headworth (Environment Agency, Wales) reviewed the section on water resources and H Bolton (Dwr Cymru Welsh Water) provided details of current water abstraction. The text was copy-edited by S G Molyneux; figures were produced by P Lappage and pagesetting was done by A Hill. The work was co-funded by the British Geological Survey, the Planning and Environment Protection divisions of the Welsh Assembly Government and the Environment Agency (Wales).

We gratefully acknowledge the cooperation of landowners and farmers in allowing access to their lands and of the Commander of Army Training Estates, Wales for access to the Sennybridge Training Area.

Geology of the Brecon district (summary from rear cover)

An explanation of sheet 213 1:50 000 series map (England and Wales)

(Rear cover)

This Sheet Explanation gives a brief description of the geology of the area that includes the upper Usk valley, part of Mynydd Eppynt, and the dissected ground between there and Llandovery to its north-west. The district also includes parts of the Carmarthen Fans, Fforest Fawr and the Brecon Beacons, encompassing some of the most outstanding mountain scenery of southern Britain.

The oldest exposed rocks are basinal mudrocks of Late Ordovician (Ashgill Series) age in the north-west; the youngest are Late Devonian red terrestrial sandstones on the summits of the mountains in the south. The district lay on the south-east margin of the Lower Palaeozoic Welsh Basin, where part of the Welsh Borderland Fault System, here represented by the Llandrindod–Pen-y-waun Fault Belt of the Pontesford Lineament, controlled subsidence and sedimentation. A complex pattern of basin-margin and shelf facies rocks was further complicated by later folding and faulting. Detailed mapping has elucidated the depositional environments of these rocks, and revealed the effects of glacio-eustatic sea level changes that occurred throughout the Late Ordovician and Silurian. Continental amalgamation and uplift produced the Late Silurian (Přídolí)–Devonian continental Old Red Sandstone rocks. These are affected by north-east–trending faults belonging to the Carreg Cennen–Church Stretton and Swansea Valley fault systems, also part of the Welsh Borderland Fault System. Uplift attributed to the Acadian Orogeny in the late Early Devonian resulted in a regional unconformity that separates the Late Devonian (Upper Old Red Sandstone) rocks from the Early Devonian (Lower Old Red Sandstone) succession.

Valley glaciers occupied the district during the Late Devensian (Dimlington) glaciation. The most powerful was the Usk valley glacier, fed by series of smaller tributary glaciers. Small, high cirque glaciers were present during the later Loch Lomond Stadial. The glaciers produced a range of glaciogenic deposits, including tills, outwash gravels and moraines.

This booklet also describes aspects of the applied geology of the district pertinent to planning and development, including mineral and water resources, potential geological hazards, engineering ground conditions and conservation.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the Geological 1:50 000 Sheet 213 Brecon, published as a Bedrock Geology and Superficial Deposits edition in 2005.

The district lies mainly in the county of Powys, with a small part in the west in Carmarthenshire. Much of the southern part lies in the Brecon Beacons National Park, with the summits of the Brecon Beacons, Fforest Fawr and Mynydd Du lying along the southern margin. The north-facing scarps and glacial cwms of these mountains provide some of the most dramatic scenery in southern Britain, as well as its highest point in Pen y Fan at 886 m. A large area of Mynydd Eppynt north of Sennybridge is owned by the Ministry of Defence and used as a military training area (SENTA). Hill farming provides the main contribution to the district's economy, but tourism and outdoor leisure activities continue to grow in importance. Brecon is the main population centre, strategically placed at the confluence of the Usk and Honddu rivers. It dates back to pre-Roman times, when an Iron Age fort was established on Pen-y-crug; its cathedral and castle are Norman. In addition to being a market town and tourist centre, Brecon is home to the Royal Regiment of Wales.

All the exposed rocks of the district (Figure 1) are sedimentary. The oldest are mudstones of Ashgill age, occurring in the north-west in a folded and faulted belt of Ordovician and Silurian marine strata. Most of the district is underlain by red beds of Late Silurian to Early Devonian age — the Lower Old Red Sandstone. The youngest rocks are red sandstones of the Late Devonian (Upper Old Red Sandstone) Plateau Beds Formation, which rest unconformably on the Lower Old Red Sandstone and form the highest points of the mountains. North-east trending faults belonging to the Welsh Borderland Fault System controlled deposition in the Lower Palaeozoic. Later fault movements and folding are attributed to the late Early Devonian Acadian Orogeny. Variscan (end-Carboniferous) tectonics imparted southerly dips to the Old Red Sandstone strata, which lie on the northern limb of the South Wales Coalfield syncline.

The exposed rocks reveal the history of the infilling of the Welsh Basin from the late Ordovician onwards. Glacioeustatic sea level changes in the late Ordovician resulted in major changes in basin architecture and facies, with regression during the late Ashgill glaciation and sea level rise during deglaciation in the latest Ashgill (Hirnantian). Sea level changes in the Wenlock and Ludlow produced three major transgressive-regressive cycles of deposition. Collision of Avalonia with the Laurentian margin in the Late Silurian resulted in shoaling of the Welsh Basin, and the inception of continental red-bed sedimentation on the south-east margin of the newly amalgamated Laurussian (Old Red Sandstone) continent. This was brought to an end by the Acadian Orogeny, the result of the transpressive collision of Gondwana-derived continental crust and the Laurussian margin. During this event, the Lower Palaeozoic basin-fills were subjected to low-grade metamorphism, inverted and eroded. Deposition recommenced in the Late Devonian, with continental and marginal marine sedimentation at the start of the Variscan orogenic cycle. The last main glaciation of the district was during the Late Devensian Dimlington Stadial, from about 27 500 to 13 500 years ago, the glaciers producing a range of landforms and glaciogenic deposits. Small, high cirque glaciers formed during a late cold period (Loch Lomond Stadial) from about 11 000 to 10 000 years ago.

Following the mapping of the rocks east of Llandovery by the Geological Survey in 1855–6, the lower part of the Silurian System was named the Llandovery Series. This type area was mapped by Jones (1925; 1949) and its brachiopod faunas were described by Williams (1951). More recently, Cocks et al. (1984) erected a formal lithostratigraphy and defined the three stages recognised globally in the series.

Higher in the succession, the Ludlow rocks, and particularly their transition into the Old Red Sandstone, have received much attention. Croft (1953) erected the term Breconian as a local chronostratigraphical (stage) name for the Senni and Brownstones formations of the Lower Old Red Sandstone. The Přídolí and Devonian rocks of the district have yielded important flora and vertebrate fauna, and fine examples of arthropod tracks.

The Late Devensian deposits and landforms, particularly the glacial cirques and moraines of the Brecon Beacons, have long attracted interest. More recent studies include those of Shakesby (2002) and Shakesby and Matthews (1993, 1996), who refer to previous work, including relevant PhD theses.

Chapter 2 Geological description

Ordovician

Rocks of the early Ashgill Series in the north-west of the district form part of the Tywi Anticline. Their exposed thickness is up to 1600 m. They comprise part of a 2500 m thick succession of turbiditic rocks (Figure 2) and (Figure 3) that is present throughout the Welsh Basin, and they indicate the establishment of predominantly oxygenated seafloor conditions following prolonged basin anoxicity during the mid-Ordovician (Llanvirn to Caradoc). The Nantmel Mudstones Formation (Ntm) (Cautleyan to Rawtheyan) (Figure 2) passes south-eastwards into the Tridwr Formation (Tri). This formation is thickest (up to 1150 m) in the Crychan Forest area, east of the Crychan Fault Belt. Shelly fossils and graptolites (Cocks et al., 1984) confirm a late Rawtheyan (anceps Biozone) age. Ashgill facies of shallower water aspect underlie the Silurian succession of the Crychan Forest area, where the upper parts of the Nantmel Mudstones pass upwards and laterally eastwards into the Cribarth Formation (Cri), a progradational unit up to 400 m thick that has also yielded Rawtheyan shelly fossils (Williams and Wright, 1981).

Pronounced lateral facies changes in the overlying late Ashgill sequence record the (Hirnantian) glacio-eustatic marine regression (Figure 2) and (Figure 3). The absence of bioturbation in the early Hirnantian Yr Allt Formation (YA) has been ascribed to the very high rate of sedimentation that accompanied the regression (Davies et al., 1997). This is also reflected in widespread slumped and destratified strata (disturbed beds; db), the products of frequent slope failure along the front of the rapidly prograding mud wedge. The disturbed beds merge in the upper part of the formation, and comprise most of the succession north-west of the Afon Brân. The Yr Allt Formation passes northwards and eastwards into the Ciliau Formation (CF), which comprises a coeval shelf facies.

The late Ashgill glaciation caused a global sea level fall of up to 100 m (Brenchley et al., 1994). The Cwmcringlyn Formation (CgF) records shoreface deposition during the lowstand. This formation, equivalent to the lower part of the former Scrach Formation (Cocks et al., 1984; Woodcock and Smallwood, 1987), yields diagnostic Hirnantian shelly fossils.

Much of the earlier Ashgill succession deposited across and to the east of the Crychan Fault Belt was exposed and eroded during the regression, the marked overstep at the base of the transgressive Cwm Clyd Sandstone Formation demonstrating movement of the fault belt, either during or immediately prior to this period of emergence. Across the western part of the fault belt, the sandstone rests on different remnants of the Hirnantian regressive sequence. To the east [SN 847 396], a coarse conglomerate containing rounded clasts of amygdaloidal volcanic rock, dolerite and decalcified, shelly sandstone at the base of the Cwm Clyd Sandstone rests with angular unconformity on interbedded mudstones and sandstones of the Tridwr Formation. The unconformity provides evidence of tectonic tilting prior to overstep by the Cwm Clyd Sandstone, possibly during an intra-Ashgill episode of fault-controlled tectonism (Schofield et al., 2004).

Silurian

Rocks of latest Hirnantian age, deposited during the sea level rise at the end of the Late Ordovician glaciation, are included in formations that are predominantly of Silurian age. Highly fossiliferous rocks of Llandovery age and predominantly of marine shelf facies (Figure 3) and (Figure 4) crop out in the Crychan Forest area, where they form part of the global stratotype of the series (Cocks et al., 1984). Marine rocks of the Wenlock and Ludlow series underlie the prominent north-west facing escarpments of Mynydd Bwlch-y-Groes and Mynydd Bach Trecastell; Ludlow rocks crop out in an inlier along the Church Stretton Fault Belt in the north-east. The Ludlow strata (Potter and Price, 1965) are succeeded by a Přídolí to Lower Devonian succession that initially comprises shallow marine strata overlain by continental red beds (Old Red Sandstone). The Silurian–Devonian boundary lies within the red beds (Raglan Mudstone Formation), and is tentatively placed just below the Bishop's Frome Limestone Member.

The marine Silurian strata exhibit complex lateral facies changes, many occurring across major faults or fault belts. The Llandrindod–Pen-y-waun Fault Belt, in particular, appears to separate successions of very different facies and thickness. The contrasts in facies across this and other major fracture belts are attributed to synsedimentary activity, but the current distribution of facies may be due, at least partly, to postdepositional strike-slip displacements.

West of the Llandrindod–Pen-y-waun Fault Belt

In the north-west of the district, where transgressive latest Hirnantian facies are conformable on earlier regressive strata, the Cwmcringlyn Formation is directly overlain by the Garth House Formation (GHF) (Figure 4), which records the transition from shoreface to off-shore deposition during postglacial sea level rise. Across and to the south-east of the Crychan Fault and its splays, the formation overlies the Cwm Clyd Sandstone Formation (Ccy), a transgressive beach and nearshore deposit that rests unconformably on pre-Hirnantian strata (see above).

The Silurian–Ordovician boundary probably lies in the lower part of the Bronydd Formation (BrF) (Cocks et al., 1984), a mid-shelf facies that coarsens upwards and laterally into the more proximal Crychan Formation (CcF). Shelly and graptolite faunas show that these formations span much of the Lower Llandovery (Rhuddanian Stage). Along the western side of the Crychan Fault Belt, their thicknesses and facies change markedly. Less bioturbated units, in which some thin sandstone beds remain intact, form part of a lateral passage from the Bronydd Formation into distal shelf mudstones of the Chwefri Formation (ChF). Lower beds of the Chwefri Formation pass into the basinal Tycwtta Mudstones Formation (Tyc), in which turbidite mudstones are thinly interbedded with dark grey, laminated hemipelagic mudstones (Davies et al., 1997; Schofield et al., 2004). The overall pattern of postglacial facies and thickness change indicates a rapid deepening event, followed by basinward progradation of coarser, shallower facies across a rapidly subsiding sector of the basin margin.

Overlying the Crychan Formation, the Trefawr Formation (TrF) records transgressive deepening in the late Rhuddanian cyphus Biozone. The type section of the formation in Crychan Forest includes the global stratotype for the Aeronian Stage of the Llandovery Series, its base taken at the incoming of triangulatus Biozone graptolites in the Trefawr forestry track section [SN 8383 3953] (Cocks et al., 1984). The subsequent mid-Aeronian eustatic fall in sea level influenced sedimentation throughout the Welsh Basin. In the Crychan Forest area, the Cefngarreg Sandstone Formation (Ceg) records the progradation of sandy, inner shelf facies during the regression. Two widespread, thin, green, burrow-mottled mudstone units (md) may record late Aeronian deepening events. The formation appears to include strata that are laterally equivalent to the Rhydings and Wormwood formations (Cocks et al., 1984) of the southern part of the Llandovery type area.

Distal shelf mudstones of the Cerig Formation (Cer) appear to succeed the Cefngarreg Sandstone Formation abruptly along the flanks of the Babel Anticline and in the core of the Cefngarreg Syncline. However, the northern margin of a synsedimentary slide and slump complex (db) on the western limb of the syncline is exposed [SN 817 375] east of Glyn-môch. This complex progressively truncates the undisturbed sandstones of the Cefngarreg Formation, its base resting on the Chwefri Formation near Nant-y-gollen [SN 802 365]. It comprises bedded, discordantly folded masses of the Cefngarreg Formation tens of metres across, rafts of smooth green mudstone, beds of disturbed, sandy mudstone with pillow structures, and smaller blocks and lenses of sandstone. Along much of its outcrop, to the west of Mwmffri and extending to the western edge of the district, the top of the disturbed sequence appears to be faulted against the overlying Cerig Formation. However, adjacent to its discordant northern margin in the Glyn-môch section, disturbed, smooth, grey mudstones with abundant listric surfaces are overlain by the normal green and burrowed mudstones of the Cerig Formation. Cocks et al. (1984) attributed the onset of deposition of the Cerig Formation in the Llandovery area to a deepening event during the early part of the turriculatus Biozone. The slump complex records a major slope failure on the shelf margin around the time of this transgressive event, perhaps seismically triggered. Thin sandstones are common throughout the Cerig Formation of the Mwmffri area, where it is over 700 m thick. A prominent, feature-forming, 50 m thick succession of interbedded sandstones and mudstones (the Mwmffri Sandstone Member; MfS) forms the crest of Mwmffri hill, and appears to be displaced by strike faults so that it crops out on the prominent ridge to the east. The tabular sandstones are up to 0.2 m thick, commonly have trace fossils and flute and groove casts on their sharp bases, and were possibly deposited by storm-generated turbidity currents.

The Builth Mudstones Formation (BMd) (Figure 6) has yielded mid Wenlock (dubius or rigidus Biozone) graptolites at Gwernfelen [SN 7930 3352] and in the valley of the Afon Gwydderig [SN 8035 3488]. The presence of basal Wenlock (centrifugus Biozone) graptolites in the base of the formation 5 km west of the district (Cocks et al., 1984) suggests that the lower boundary of the formation is faulted and not disconformable (cf. Jones, 1925). The succeeding 500 m of predominantly slumped and destratified, silt-striped mudstones (disturbed beds; db) are included in the Llangammarch Formation (see below). Slump folding, slide planes, micro-faulting and all stages of destratification occur, with structureless mudstones being particularly common. Scattered packets of undisturbed strata commonly contain thin phosphate lenses and beds. The disturbed beds have yielded rare mid Wenlock graptolites ascribed to the dubius to lundgreni biozones.

East of the Llandrindod–Pen-y-waun Fault Belt

The late Hirnantian to Llandovery succession north of Pen-y-waun, on the west side of Castell Craigyrwyddon [SN 846 366], comprises attenuated sequences of the Garth House Formation (up to 70 m), Bronydd Formation (up to 40 m) and Crychan Formation (up to 100 m). They are preserved beneath sandstones of the Derwyddon Formation (Ddd), which thins from 50 m in the south to 25 m in the north. The base of the latter overlies a marked unconformity (Figure 3), at which the underlying formations are rapidly overstepped northwards, until it rests directly on the mid Ashgill Tridwr Formation east of Cefn-blewog [SN 852 379]. There, the unconformity is undoubtedly composite in origin, owing much to late Ashgill erosion (see above), but the overstep of Rhuddanian and Hirnantian strata farther south is consistent with the marine transgression of a surface that was subaerially exposed and eroded during the mid Aeronian sea level fall. Deposition of the Derwyddon Formation may therefore record the late Aeronian sea level rise in the sedgwickii Biozone (Davies et al., 1997; Schofield et al., 2004). Contemporaneous uplift along the Llandrindod–Pen-y-waun Fault Belt may also have sustained shallow water depths and erosion. In the Craigyrwyddon area, upper parts of the Derwyddon Formation pass laterally southwards into brown-mottled sandstones of an attenuated Cefngarreg Sandstone Formation. Cocks et al. (1984) regarded the Derwyddon Formation as a shallow water equivalent of the Rhydings and Wormwood formations of the southern Llandovery area. By inference, it should also therefore equate with some or all of the Cefngarreg Formation of this account. However, the BGS survey suggests that it is likely to equate with only the upper part of the Cefngarreg Formation farther west (Figure 3). Deposition of the Cerig Formation, which abruptly overlies the Derwyddon Formation in the north and the Cefngarreg Formation in the south, occurred during a deepening event initiated in the turriculatus s.l. Biozone.

The Wenlock and Ludlow succession of Mynydd Bach Trecastell and Mynydd Bwylch-y-groes, east of the Llandrindod–Pen-y-waun Fault Belt, comprises three major progradational sequences (Figure 5) and (Figure 6). Evidence of transgressive deepening is present at the base of each sequence. The progradational events are characterised by south-west (proximal) to north-east (distal) facies changes along the linear Wenlock–Ludlow outcrop known as the Myddfai Steep Belt, the actual direction of progradation probably being oblique to the belt.

Wenlock progradation

This event is marked by progressive lateral and vertical? facies changes from the deeper water, muddier facies of the Tirabad and Llangammarch formations in the north, into the shallower-water Sawdde Sandstone Formation in the south. The progradational sequence is capped by the distinctive, limestone-bearing Halfway Farm Formation, which may record a gradual deepening during the latest Wenlock.

The Llangammarch Formation (Llg) (Figure 6) occupies the lower part of the Wenlock Series, ranging up to the rigidus Biozone. It is 170 m thick at the northern edge of the district, increasing south-westwards to 470 m. It comprises grey, silt-striped mudstones in which siltstone laminae increase in abundance upwards, but generally make up less than ten per cent of the formation. Sparse, dark grey, 1 to 5 mm thick, laminated, hemipelagic mudstone layers yield graptolites. Slumped and destratified mudstones are locally present, and disturbed beds (db) occupy all but the basal 70 m of the formation west of the easternmost splay of the Llandrindod–Pen-y-waun Fault Belt.

A coarsening-upwards sequence above the Llangammarch Formation comprises the Tirabad and Sawdde Sandstone formations. Siltstone and sandstone comprise 15 to 50 per cent of the mud-prone Tirabad Formation (Tir). The formation thickens from 150 m in the south-west to over 740 m in the north-east, where the uppermost part is faulted out. It is characterised by small horizontal and vertical burrows, the burrowing ranging from scattered burrows to intense bioturbation. The latter results in irregular beds of muddy sandstone up to 3 cm thick. Lenses and thin beds of black phosphate are locally present, and some sandy burrow-fills are also phosphatised. The only fossils comprise scattered brachiopod and crinoid fragments. The Tirabad Formation comprises a more proximal ramp facies than the Llangamarch Formation (Figure 6).

The proportion of sandstone in the Sawdde Sandstone Formation (Saw) (Figure 6) ranges from 20 to 80 per cent, generally increasing upwards. The characteristic tabular sandstones are mainly parallel-laminated, with burrows confined largely to their upper parts. The thinly interbedded sandstones, siltstones and mudstones are variably burrowed; homogenised units of muddy sandstone up to 1 m thick occur locally. The sandstones and siltstones are laminated and cross-laminated, generally 1 to 3 cm thick (and locally up to 5 cm). Sandstone beds up to 1 m thick, and comprising mainly sheet sandstones, are locally present in the upper half of the formation. Reworked brachiopod and crinoid debris commonly occurs as lags at the base of tabular sheet sandstones, and within the thinner sandstones and siltstones. Deposited above storm wave base, the formation is a mid-shelf facies wedge that prograded north-eastwards over the Tirabad Formation, thinning from 660 m in the south-west to 120 m at Clawdd Brythonig [SN 8625 3683] in the north-east, where its feather edge is faulted out. The coarsening-upwards sequence of the Tirabad and Sawdde Sandstone formations was deposited during the late Wenlock (Homerian) regression or stillstand (Hurst et al., 1978).

The Sawdde Sandstone is sharply overlain by the Halfway Farm Formation (Haf). A slight deepening appears to have halted the progradation of Sawdde Sandstone facies and introduced an offshore setting. The mudstones are bioturbated, with dark burrow mottles or sand-filled burrows, and contain a patchy but generally rich fauna of brachiopods, bivalves, trilobites and crinoid debris. A 13 m thick limestone in the middle of the formation, and an 8 m thick limestone near the top, consist mainly of thin, irregular to nodular bedded, dark grey, argillaceous limestone, with laminae and thin beds of calcareous mudstone. Individual limestone beds are up to 15 cm thick and locally weather to rottenstone. The lower limestone unit contains 4 m of dark, fine-grained skeletal packstone in 30 cm thick beds. The age of the formation is poorly constrained, but the presence of beyrichiacean ostracods resembling forms of Undipila described from the late Wenlock (Siveter, 1980) suggests a similar age.

Early Ludlow progradation

The Irfon and Cwm Graig Ddu formations comprise the deeper, distal facies of the early Ludlow progradation, and coarsen upwards and south-westwards into the more proximal Hafod Fawr and Aberedw formations. Near the western edge of the district, the sandstones of the Mynydd Myddfai Sandstone and Trichrug formations are the most proximal facies (Figure 5) and (Figure 6).

The Halfway Farm Formation is sharply overlain by the Irfon Formation (Irf) in the north-east and the Cwm Graig Ddu Formation (CGd) in the south-west (Figure 5) and (Figure 6). Both formations have yielded graptolites of the lower Ludlow nilssoni or scanicus Biozone, and the Cwm Graig Ddu Formation contains sporadic vertical burrows that increase in abundance upwards. From Clawdd Brythonig north-eastwards, the Cwm Graig Ddu Formation overlies the Irfon Formation, thickening to 425 m at the northern edge of the district. Its upper part becomes increasingly sandy, with abundant siltstone and sandstone laminae (reaching up to 30 per cent of the formation), as well as increasingly bioturbated. The formation contains upper Ludlow leintwardinensis Biozone graptolites in the adjacent Builth Wells district (Schofield et al., 2004). The change in facies from the Halfway Farm Formation to the Irfon Formation, deposited under anoxic bottom conditions, indicates the onset of the early Ludlow transgression. The Cwm Graig Ddu Formation comprises a more proximal facies deposited under oxic bottom conditions, and its sandier upper part records the onset of mid Ludlow progradation, which continued with the overlying Hafod Fawr Formation.

The Hafod Fawr Formation (Hod) (Figure 6) is restricted to the south-west of Fibua [SN 8940 3890], from where it thickens south-westwards (Figure 5); the most proximal facies are in the south-west. It is capped by the Aberedw Formation in the north-east and the Mynydd Mydffai Sandstone and Trichrug formations south-west of Y Pigwn [SN 8280 3123]. To the north-east, it passes laterally into all but the lowest part of the Cwm Graig Ddu Formation. There is considerable lateral variation from the feather-edge in the north-east to the more proximal part in the south-west, and the proportion of sandstone increases upwards from 30 to 85 per cent. Burrowing is widespread, but only locally intense enough to produce thin units of muddy sandstone. The sandstones and siltstones are parallel and cross-laminated, and range from 1 to 5 cm, increasing in thickness upwards. The sheet sandstones are mainly restricted to the lower two thirds of the formation and are rarely more than 30 cm thick, but some up to 80 cm thick occur in the south-west. Between the valley of the Afon Gwydderig [SN 8390 3245] and Gam Rhiw [SN 8545 3435], the uppermost part of the formation is characterised by thin (1 to 5 cm) sandstones with shaly partings and locally strong bioturbation. Shelly fossils, predominantly brachiopods and crinoids, occur throughout, mainly as lags in the sandstones. The beds in the uppermost part of the formation may have been locally subjected to fair weather reworking.

The Aberedw Formation (AbF) (Figure 6) gradationally overlies the Hafod Fawr Formation north-east of Y Pigwn and the Cwm Graig Ddu Formation north-east of Fibua. Bioturbated muddy sandstone resulted from the homogenisation of thin sands with mud laminae, and occurs in units 0.1 to 1 m thick with irregular wavy partings. Less bioturbated sandstones are up to 7 cm thick. Sandstone comprises 75 to 95 per cent of the formation.

South-west of Y Pigwn, the Hafod Fawr Formation is gradationally overlain by the quartzitic sandstones and quartzites of the Mynydd Myddfai Sandstone Formation (MyS). Small gastropods, crinoid ossicles and calcrete and red mudstone clasts are present in the sandstones. In the south-west, a thin unit of interbedded red sandstone, muddy conglomerate and green mudstone with a bentonite occurs in the middle of the formation, and is interpreted as a back barrier unit within a shoreface and barrier sand succession.

South-west of Mynydd Myddfai, the Mynydd Myddfai Sandstone is overlain sharply by the Trichrug Formation (Tug). This was deposited in a wide lagoon behind the Mynydd Myddfai Sandstone barrier. The red colour of the mudstones and the presence of calcrete clasts in the barrier sands suggest that the lagoonal sediments were subject to periodic emergence and carbonate soil formation. The thin quartzites may have been washover sands derived from the barrier.

Late Ludlow to early Přídolí progradation

The regressive units that cap the early Ludlow progradation are sharply overlain by transgressive late Ludlow facies at the base of the upwards-coarsening Fibua Formation in the north-east and the Cae'r mynach Formation in the south-west. The deposition of fully marine facies across the district was ended by the advance of barrier facies of the Tilestones Formation.

The sharp base of the Fibua Formation (Fib) is defined by a 20 cm thick conglomerate or gritty mudstone containing clasts of red mudstone and pink calcrete as well as quartz pebbles and granules. The lowermost 10 m of mudstones near Fibua [SN 8940 3870] have only sparse siltstone laminae, are unburrowed, and contain millimetre thick, laminated hemipelagites that yield rare graptolites (Bohemograptus), thought to indicate the Bohemograptus proliferation interval. Up-sequence, the siltstone fraction gradually increases to 40 per cent, with individual laminae up to 5 mm thick. Sporadic centimetre-thick sandstones are also present. The sequence immediately above the basal transgressive conglomerate records a rapid deepening, with deposition of silt-based beds probably generated by storms. The remainder of the formation represents progradation of near-shore facies.

The Cae'r mynach Formation (Car) rests disconformably on the Trichrug Formation, and has a sharp contact with the Aberedw Formation and a gradational contact with the Fibua Formation. It thickens from 155 m in the north-east to 260 m in the south-west. Thicker beds of sandstone, generally up to 30 cm thick, are either prominent, parallel-sided, well-jointed sheet sandstones or units of amalgamated thinner sandstones. Sheet sandstones up to 1 m thick occur locally at the top of the sequence in the transition into the overlying Tilestones Formation. Beds of muddy, intensely bioturbated sandstone up to 20 cm thick are also common. Sandstone content increases upwards from 40 to 80 per cent. At the base of the formation on the western edge of the district, a medium to coarse grained, quartzitic sandstone, up to 4 m thick and containing granule-size clasts of calcrete, overlies the Trichrug Formation. Deposited in a shoreface environment, the sandstone records the late Ludlow transgression. Much of the formation was deposited in a mid to proximal shelf setting as the transgression continued, the sandstones being mainly storm-generated.

The base of the Tilestones Formation (Til) (the Long Quarry Beds of Potter and Price, 1965) is traditionally taken as the local base of the Přídolí Series, but is likely to be diachronous. The formation records the progradation of a shoreface and barrier sand body, and the end of fully marine environments in the district. On stratigraphical, thickness and palaeontological grounds, the formation has previously been regarded as disconformable on the underlying Ludlow sequence, overstepping westwards onto lower strata (Potter and Price, 1965). However, the junction between the Tilestones and the Cae'r mynach formations appears to be gradational, and the base of the Tilestones in Capel Horeb Quarry (Plate 1) has been placed at a level higher than the supposed disconformity (cf. Edwards and Richardson, 1978; Almond et al., 1993; Lane, 2000).

Undifferentiated Ludlow Series strata in the north-east

The north-east part of the district is structurally complex, with steeply dipping strata cut by numerous large fractures of the Church Stretton Fault Belt. It has not been mapped in detail and is depicted on the map as Ludlow strata, undivided (Lu). The succession appears to be broadly similar to the Ludlow sequence of the Builth Wells district to the north (Schofield et al., 2004). The lowest exposed strata crop out in two areas [SO 0372 3808] and [SO 0336 3700], adjacent to large faults, and compare with the silt-striped mudstones of the Irfon Formation. Higher silt-striped mudstones with small burrows, well exposed in Llaneglwys Wood e.g. [SO 0612 3934] and on both sides of the Honddu valley north of Lower Chapel, are similar to the Cwm Graig Ddu Formation. They are gradationally overlain by hard, pale to medium blue-grey, calcareous, largely homogeneous, muddy, silty sandstones that are correlated with the Aberedw Formation. The sandstones separate along closely spaced (3–4 cm), irregular to wavy pseudobedding planes and contain dispersed shelly material. Thin, centimetre scale, buff, rippled, quartzose sheet sandstones and dark grey, burrow-mottled mudstones occur intermittently. The top of the unit is gradational and marked by an increase in the number of sandstones and more clearly defined bedding. The highest beds equate with the Cae'r mynach Formation and consist of interbedded hard, medium grey and brown, fine to medium grained sandstones up to 30 cm thick, commonly stacked on each other, and thinner beds of laminated, bioturbated mudstones and siltstones. The sandstones are sharp-based, slightly micaceous, locally quartz cemented, and have low-amplitude wavy or rippled upper surfaces. Shelly fossils are locally abundant. This unit is overlain by calcrete-bearing siltstones at the base of the Temeside Mudstone Formation, except in the west of the inlier, where isolated outcrops [SO 0118 3698] of micaceous sandstone containing some shell and calcrete fragments may represent a thin development of the Tilestones Formation.

Přídolí regressive facies above the Tilestones Formation

The Temeside Mudstone Formation (Tem) crops out in the north-east of the district in several fault-bounded outliers and along the northern part of the Mynydd Eppynt escarpment, where it can be traced as far south as Cwm Dwr. In some exposures, bedding is defined by calcrete, but the calcrete nodules are generally distributed at random and the formation appears massive. Green and brown, micaceous, locally cross-bedded sandstones up to 1.5 m thick at several localities e.g. [SO 0306 3562] may have been washover sands. The formation has not yet been recognised south of Cwm Dwr because the equivalent beds are red and cannot be distinguished from the overlying Raglan Mudstone Formation in poorly exposed ground.

The Raglan Mudstone Formation (Rg) crops out on Mynydd Eppynt and in the north-east of the district. A broadly cyclical pattern of pedogenesis is indicated in 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, although these appear to be less common than in the Builth Wells district to the north. There is little evidence for the widespread, mature calcrete development seen at the top of the formation elsewhere in south Wales and the Welsh Borderland (the Bishop's Frome Limestone Member; BFL), but it may be a calcrete in the cutting at Groesfford [SO 0766 2746], east of Brecon (D Hawley, personal communication, April 2003). Thick, calcrete-rich, intraformational conglomerates are abundant at that level, suggesting fluvial reworking of the member. The sandstones of the formation are typically sharp-based, cross-bedded, red, mauve, green and white, fine to medium grained and variably micaceous. The green and white sandstones, which are most abundant in the lower and middle parts of the formation, are commonly 2 to 3 m thick (and locally up to 6 m), harder (because of silica cement), more micaceous and more prominently cross-bedded. Almost all of the formation was proved in the Usk Reservoir tunnel (BGS archive notes), where sandstones occupy about 10 per cent of the succession, decreasing in abundance upwards from 14 per cent in the basal part to less than 6 per cent in the topmost part. They range in thickness between 0.15 and 11 m, with an average of 1.5 m, the thickest lying near the base of the formation. A 5.5 m thick zone of calcretes lies about 640 m up from the base. Lingula was collected from red-brown mudstones in tip material and a 4.3 m thick green mudstone lies about 380 m from the base. The sandstones are the deposits of seasonally flowing streams and sheet flood events.

Devonian

Lower Devonian

Much of the St Maughans Formation (SMg) (Figure 7) comprises fining-upwards alluvial cycles, in which basal sandstones and intraformational conglomerates overlie erosion surfaces cut in mudstones at the top of the underlying cycle. These are interpreted as meandering channel deposits, which fine upwards through siltstones into thick floodplain mudstones, many of which are calcretised. Mature calcretes occur locally e.g. [SN 9244 3933], but most calcrete occurs as reworked intraformational clasts in the conglomerates. Mudstone channel-fills also occur locally, for example in a disused quarry at Defynnog [SN 9270 2790]. The sandstones are typically up to 3 m thick, but locally reach 6 m e.g. [SN 9667 2704] and, exceptionally, 15 m, as at Pantymaes Quarry [SN 9139 2658]. They are fine to medium grained, planar and trough cross-bedded, and range from red-brown to purple, green and grey. Quartz pebble conglomerate occurs on Cefn Llechrid [SN 943 273] and pebbly beds occur locally elsewhere e.g. [SN 9106 3756] and [SO 0178 3148]. The intraformational conglomerates are generally up to 1 m thick, purple, red-brown and green, and comprise mainly calcrete clasts with lesser amounts of siltstone and sandstone in a calcareous sandstone matrix. Some contain extraformational quartz pebbles, and fish fragments occur sporadically. Fragments of Traquairaspis (Phialaspis) symondsi and Anglaspis cf. macculloughi have been recorded from Crwcas Wood Quarry [SO 0410 2758] (White, 1946, 1950b), and Pteraspis leathensis has been recorded from a small exposure [SO 0452 2740] east of Pen-y-lan Farm (White, 1950b). The mudstones/siltstones are predominantly red-brown, with some green beds and local leaching of the red beds to pale green.

The 15 m thick sandstone at Pantymaes Quarry is characterised by laterally extensive erosion surfaces (first-order bounding surfaces) that separate channel complexes deposited by large, permanent but flashy, braided river systems. The architecture of the complexes, in which erosion surfaces step consistently northwards, suggests pulsed strike-slip movement on the Church Stretton (Carreg Cennen) Fault (Owen and Hawley, 2000; Barclay, 2005a). Arthropod trackways (Diplichnites gouldi) in the quarry (Smith etal., 2003) are comparable to tracks at Crwcas Quarry near Brecon (Bassett and Owens, 1974). A 15 m thick red-brown mudstone above the sandstone complex, and separated from it by an erosion surface, has parallel-bedded siltstones and sandstones at its top and base. It is bioturbated and calcretised, and is interpreted as the floodplain deposits of a meandering stream system, the switch from the preceding braided systems also suggesting a tectonic trigger.

The sandstones of the Senni Formation (SB) range from very fine to medium grained, with coarser, pebbly varieties at higher levels. They mainly comprise tabular sheets constructed from lenticular, cross-bedded, channelised packages with internal erosional surfaces. Deformed cross-bedding is common, as are red-brown and grey-green mudstone and siltstone interbeds. The sandstones are variably calcareous, and calcrete clasts occur in the bases of sandstone bodies, along with intraformational mudstone and siltstone clasts. The sandbodies generally fine upwards from conglomeratic bases, and their tops are commonly truncated by scour or erosion surfaces. The argillaceous interbeds contain calcrete nodules, with more mature massive and rubbly calcretes present sporadically. Desiccation cracks are also seen locally. The formation is interpreted as the product of low-sinuosity, seasonally flowing, sandy, braided streams (Loeffler and Thomas, 1980). The finer grained lithologies are interpreted as floodplain lake, crevasse splay and channel abandonment facies.

The Senni Formation is known for its early Devonian vascular plant remains. The principal locality in the district is Allt Ddu [SO 027 242] (Edwards and Richardson, 2000, 2004). The presence of zosterophylls (rootless, leafless plants) at Allt Ddu extends their initial major diversification back to the late Gedinnian (=Lochkovian) (Edwards and Richardson, 2000). High water table conditions, perhaps in a wetter, more humid climate, are invoked to explain the preservation of the plant remains and the predominantly green lithologies of the formation.

Heol Senni Quarry [SN 9145 2210] provides a fine section of the formation, exposing about 40 m of grey-green sandstones with some grey mudstone interbeds (Edwards et al., 1978; Barclay, 2005b). It is the type locality of the fish Althaspis senniensis, known only from here (Loeffler and Thomas, 1980; Dineley, 1999), and a wide range of fossil plant remains and miospores has been recorded (Edwards et al., 1978). Protopteraspis gosseleti has been collected from Allt Ddu (Edwards and Richardson, 2004).

The Brownstones Formation (Brs), truncated by an unconformity at the base of the overlying Plateau Beds Formation, is magnificently exposed on the north-facing scarps of the Carmarthen Fans, Fforest Fawr and Brecon Beacons (Plate 2). The sandstones occur mainly 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. 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 the beds as low-sinuosity, flash-flood, channel deposits, merging downslope into muddy floodbasin 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 mud aggregates (e.g. Ékes, 1993) may also be possible, at least for some (e.g. Marriott and Wright, 2004).

In addition to the facies types described by Tunbridge (1981), pebbly sandstones and conglomerates occur in the west, around Llyn y Fan Fawr [SN 834 216] (Tunbridge, 1980). The clasts range from subrounded to angular and include acid volcanic rocks, lithic arenites and vein quartz. These gravels were derived from the east, in contrast to the south and south-east directed palaeocurrents that are present throughout most of the formation.

Upper Devonian — Upper Old Red Sandstone

The Plateau Beds Formation (PlB) unconformably overlies the Brownstones Formation; a slight angular discordance is visible on Bannau Sir Gaer and Fan Brycheiniog. The formation is generally harder than the Brownstones and forms resistant cappings to the Brecon Beacons and Carmarthenshire Fans. There are outliers on Bannau Sir Gaer, Fan Brycheiniog, Corn Du and Pen y Fan (Front cover) and (Plate 2), with exposures mainly confined to the tops of steep cliffs and scarps. To the south, the strata form gentle southerly dip slopes, giving the plateau appearance from which their name is derived. The strata are assigned to the Lower Plateau Beds of Hall et al. (1973) and parts of Divisions A and B of Lovell (1978). A maximum of about 14 m are present on Fan Brycheiniog (Lovell, 1978). Trough cross-bedded, pebbly and conglomeratic sandstones, interbedded with mudstones, form the lowermost 5.2 m of the formation on Bannau Sir Gaer and Fan Brycheiniog–Fan Foel (Lovell, 1978). At the latter locality, a 0.65 to 3.6 m thick, granule-rich, pebbly mudstone at the base of the formation contains mainly vein quartz pebbles, with some jasper and other igneous lithologies, and is interpreted as a mudflow. The overlying beds comprise laterally impersistent (channelised), trough cross-bedded, pebbly sandstone and conglomerate. A braided stream environment was favoured by Lovell (1978), with southerly directed palaeocurrents. Division B is best seen at Fan Brycheiniog, where about 6.5 m of thin to medium bedded, red-brown sandstones contain some coarser, internally cross-bedded and ripple-laminated units, with the cross-bedding indicating a north-east or north-north-east current direction. Lovell (1978) suggested an aeolian origin for these beds, but a tidal flat origin may be more likely (R D Hillier, personal communication, 2004).

Structure

Most of the fault movements and folding in the district are attributed to the Caledonian orogenic cycle, which culminated in the mid Devonian Acadian Orogeny, but the end Carboniferous Variscan Orogeny imparted the broadly southerly dips to the Old Red Sandstone strata and reactivated many of the Caledonian faults. Strata in the north-west and north-east are folded and dip very steeply; elsewhere, dips are more gentle. The dominant structures are the suites of north-east–trending faults (Figure 1). These include the Llandrindod–Pen-y-waun ('Pontesford Lineament'), Church Stretton and Swansea (Tawe) Valley fault systems, all part of the Welsh Borderland Fault System that straddled the south-east margin of the Welsh Basin (e.g. Woodcock and Gibbons, 1988).

Pre-Acadian syndepositional movements on the Llandrindod–Pen-y-waun, Crychan and Garth–Llanwrtyd fault belts resulted in a complex pattern of facies distribution. The earliest evidence for such movements is the transgressive overstep by the latest Ashgill Cwm Clyd Sandstone Formation across the Crychan Fault, basinward-directed faulting occurring during a mid Ashgill event (Schofield et al., 2004). The later unconformity at the base of the Derwyddon Formation is composite, resulting from erosion during the late Ashgill and the mid Aeronian lowstand, but uplift along the Llandrindod–Pen-y-waun Fault Belt may have sustained shallow water conditions and erosion to the east during the mid to late Aeronian. Earliest Telychian movement on the Crychan Fault Zone probably triggered the sedimentary slide/slump complex beneath the Cerig Formation on the western limb of the Cefngarreg Syncline, and the slumped and destratified beds of the Llangammarch Formation probably record movement on the Pen-y-waun Fault in the early–mid Wenlock.

The Acadian Orogeny produced folding, local cleavage and faulting, and also reactivated earlier structures. Folding and a locally well-developed cleavage are limited to the west of the Dulas Valley Fault, a component of the Llandrindod–Pen-y-waun Fault Belt. Cleavage is concentrated in the mudstone-dominated Ordovician and Llandovery formations, but occurs patchily elsewhere, including the Lower Old Red Sandstone. The north-east-trending folds in the north-west range from open to tight, their axial planes and cleavage dipping steeply to the north-west. The major folds are the Cefngarreg Syncline and the Babel Anticline. The Cefngarreg Syncline is cut by anastomosing splays of the Crychan Fault, and the Babel (or Noethgrug) Anticline by the Llandrindod–Pen-y-waun Fault Belt. Bedding typically dips at less than 40° on the north-western limb of the anticline, and steep, vertical and strongly overturned dips are present on its faulted south-eastern limb. This tract of steeply dipping and inverted strata, which extends for over 2 km across strike, is named the Myddfai Steep Belt. To its east, the dips decrease towards the core of the Pentre Bach Syncline. Acadian faulting mainly involved reactivation of the north-east–trending Llandrindod–Pen-y-waun, Crychan and Garth–Llanwrtyd fault belts. These show variable vertical offsets, but strike-slip movements can be inferred locally. Several north-west–trending cross faults are also present, most notably along the Myddfai Steep Belt. The major cross fault that transects the Cefngarreg Syncline–Babel Anticline downthrows to the south-west.

Movement on the Church Stretton Fault System was responsible for the anticline that brings Ludlow strata to the surface in the north-east of the district. Strands from this system appear to link with the Swansea (Tawe) Valley–Cribarth Fault Belt to the south, which extends north-eastwards to Brecon and beyond (Weaver, 1975). The Swansea Valley Fault forks to the south of the district, the more southerly component being the Cribarth Fault. It also forks within the district, the more southerly splay being named the Heol Senni Fault (Owen and Hawley, 2000). Beds of the Senni Formation are folded in an anticline in the hanging wall (north-west) of the Swansea Valley Fault in the Crai valley.

The gravity anomaly map of the district (Figure 8) shows the broad north-east trend of the Church Stretton Fault System. The anomaly pattern shows a general north-westwards increase in value into the Welsh Basin, consistent with increasing thickness of relatively high-density Lower Palaeozoic mudrocks. The lower values and complex pattern over the remainder of the district are due to the lower densities of the Old Red Sandstone and the different densities of basement rocks juxtaposed within the north-east–trending faults of the Church Stretton and Swansea Valley fault systems. The aeromagnetic anomaly pattern (Figure 9) shows a north-east–trending pattern consistent with the presence of the Church Stretton and Swansea Valley fault systems in the underlying Avalonian basement. Magnetic basement rocks in the centre and south-east of the district are separated by a magnetic low, suggesting the presence of non-magnetic sedimentary rocks, perhaps in a concealed basin bounded by the Swansea Valley Fault and a concealed fault to the south-east. The prolonged history of these faults may thus include Late Precambrian (Neoproterozoic) initiation, as proposed by Woodcock and Gibbons (1988), and later Lower Palaeozoic reactivation as basin-margin synsedimentary faults. Owen and Hawley (2000) suggested that Early Devonian dextral strike-slip movement on the Church Stretton Fault System might account for the facies patterns and channel migration in a sandstone complex in the St Maughans Formation at Pantymaes Quarry. Similarly, Acadian uplift on the Swansea Valley Fault (Tunbridge, 1980) or the Tywi Anticline (Tunbridge, 1986) may account for the Llyn y Fan fawr conglomerates in the Brownstones Formation. Sinistral transpressive reactivation of the Welsh Borderland Fault System resulted in north–south shortening during the Acadian Orogeny, although with modest displacements (Woodcock and Gibbons, 1988). The Church Stretton and Swansea Valley fault systems were also reactivated during the Variscan Orogeny (e.g. Owen and Weaver, 1983), and the former during Permo-Triassic rifting, but George (1980) discounted Weaver's (1975) suggestion that further movement took place on the Swansea Valley Fault System in the late Neogene.

Quaternary

The Pleistocene glacial deposits and landforms of the district are attributed mainly to the Late Devensian glaciation (Dimlington Stadial), which probably reached its acme in the district about 20 000 to 18 000 years ago (Campbell and Bowen, 1989; Shakesby, 2002). High-level cirque glaciers formed during the Loch Lomond Stadial from about 12 900 to 11 500 years ago (11 000 to 10 000 radiocarbon years). Concurrent periglacial conditions and subsequent rapid climate amelioration in the Holocene (Flandrian) produced a range of slope, alluvial and organic deposits.

Ice accumulated in the Carmarthen Fans, Fan Fawr and Brecon Beacons during the Late Devensian glaciation (Figure 10). North–flowing glaciers coalesced with and fed a major glacier in the Usk valley, and ice in the headwaters of the Tawe valley flowed southwards. To the north of the Usk valley, ice that accumulated in the Cambrian Mountains (Central Wales Ice Sheet) was deflected by the Mynydd Eppynt escarpment, one tongue flowing south-westwards and occupying the north-west of the district, the other flowing south-eastwards into the Wye valley. At its acme, it overtopped the Mynydd Eppynt escarpment and flowed southwards into the Nant Bran and Cilieni valleys, merging with the Usk glacier and bringing Lower Palaeozoic erratics to the Usk valley. Similarly, Central Wales Ice may have flowed eastwards through the cols on Mynydd Bach Trecastell, west of Trecastle. Deglaciation at the end of the Dimlington Stadial was punctuated by still-stands in the retreat of the Usk glacier and its tributaries, which produced a series of cross-valley moraines.

The small, high-level cirque glaciers of the Loch Lomond Stadial formed in north–facing, wind-sheltered locations. The glaciers left a range of glaciogenic deposits when they melted, including tills, hummocky moraines and glaciofluvial outwash gravels. The Loch Lomond cirque moraines retain their fresh depositional morphology, and those of the Dimlington Stadial also retain much of their original form. Deposits filling the dead ice hollow at Traeth Mawr [SN 968 258] preserve a sequence from the Dimlington Stadial to the Flandrian. Those in a former lake site in Cwm Cerrig-gleisiad [SN 965 220] are entirely postglacial (Shakesby, 2002).

The Dimlington Stadial deposits comprise tills laid down as the ice advanced, and a range of hummocky, kettled and sheeted till and glaciofluvial sands and gravels that were deposited as the glaciers melted and ice fronts retreated upstream. The irregular bedrock profile of the Usk valley floor, comprising rock basins and intervening rock bars, was caused by glacial scouring. Glacial lakes occupied some of the rock basins as ice retreated. Ellis-Gruffydd (1972) noted the complex pattern of the glacial deposits around Brecon, proposing that the Usk glacier split into three west of the town, the main stream following the preglacial course of the Usk through the Cradoc gap, a second tongue following the present valley, and a third flowing north-eastwards through the Penoyre gap (Figure 10).

Till is the most widespread glaciogenic deposit, reaching a maximum thickness of over 10 m. The Brecon Beacons till (Brecknockshire Formation of Bowen, 1999) is characterised by its red colour, derived from the Old Red Sandstone. It is a variable diamicton, ranging from clast-supported and gravel-rich to matrix-supported and clay-rich; its clasts are entirely of Old Red Sandstone, ranging from pebbles to large boulders of sandstone (mainly) and mudstone. The tills containing Lower Palaeozoic clasts north and west of the Usk valley (Elenid Formation of Bowen, 1999) are grey or pink, those overlying Ordovician and Llandovery rocks being grey, whereas pink colours predominate on the outcrops of the Wenlock and Ludlow rocks. North of the Usk valley, till is largely restricted to valleys and depressions. However, recent exposures on Mynydd Bwlch-y-Groes [SN 8685 3525]–[SN 8670 3488] revealed 2 to 3.5 m of red, very slightly clayey, sandy pebble gravel, on Raglan Mudstone Formation bedrock and preglacial head, or regolith up to 2.5 m thick. The clasts appear to be exclusively of Old Red Sandstone siltstone and mudstone, with some sandstone, and are predominantly subangular to subrounded, with angular boulders and cobbles common in the lower half of the till. The matrix comprises abundant granule-grade siltstone clasts, with sand and silt and a little clay. Ill-defined near-vertical zones of boulders and cobbles may be stone stripes or parts of freeze-thaw polygons.

Late Devensian Hummocky Glacial (morainic) Deposits are distinguished by their moundy morphology, the result of deposition at a static ice front or as meltout from down-wasting of stagnant ice. They range from clayey pebble-boulder gravel to sandy, pebble-boulder silt (Plate 3). Those forming cross-valley moraines mark still-stands in the retreat of the valley glaciers (Figure 10) and consist predominantly of till and gravel.

Glaciofluvial Deposits are sands and gravels, mainly of cobble and very coarse gravel-grade, formed as outwash from melting ice. They occur in the south-east and north-west of the district, where they include an isolated mound of sand and gravel [SN 859 379] on the west side of Nant Gwernol, and a fan shaped body at Cwm-sidan [SN 834 345]. Glaciofluvial ice-contact deposits in the Honddu and Usk valleys probably originated as kame terraces, formed as the valley glaciers melted. Their depositional form is now mostly degraded, or in the case of one in Cwm Rhiwiau [SO 08 40], extensively landslipped. They are stratified, silty sands and cobble to coarse gravel deposits, interbedded locally with clayey, sandy gravels, till, and laminated clay and silt. Glaciofluvial Sheet Deposits are thickest and most extensive in the Tarrell and Usk valleys, where they form a terrace with a surface up to 10 m above river level. Glaciofluvial Fan Deposits have incised fronts and consist predominantly of sand and cobble gravel with variable quantities of silt. Glaciolacustrine Deposits formed where drainage was dammed by a bedrock constriction, ice or glacial deposits. Over 6 m of silts are present in a former proglacial lake site in the Honddu valley [SO 036 330], and similar deposits occur as minor components in morainic and glaciofluvial ice-contact deposits. Up to 13.4 m of silts are preserved in a former ribbon lake site upstream of a rock bar at Glan Usk Farm [SO 077 278]. Glaciolacustrine sediments also accumulated in the cirque lakes during the Loch Lomond Stadial, as for example in Llyn y Fan Fach (Curry, 2002).

Small glaciers and permanent snowfields formed in the cirques of Mynydd Du, Fforest Fawr and the Brecon Beacons during the Loch Lomond Stadial (Robertson, 1988; Carr, 2001; Shakesby, 2002 and references therein), leaving moraines and boulder ridges on melting. Deposits assigned to this cold period are also present at Traeth Mawr. The cirque moraines are fresh, arcuate ridges of till, gravel and boulders. They enclose small lakes or peat flats, which have been radiocarbon dated at Craig Cerrig-gleisiad and Craig y fro (Walker, 1980, 1984). Shakesby (2002) provides details of these moraines and those at Llwyn y Fan Fawr, Llyn y Fan Fach (Strahan et al., 1904; repeated with minor amendments by Robertson, 1933), Heol Senni, Llyn Cwm Llwch (Reade, 1894; Campbell and Bowen, 1989 and references therein; Carr, 2001), Cwm Cynwyn, Cwm Oergwm and Cwm Cwarelli (Strahan et al., 1904).

Traeth Mawr [SN 968 258] is the northerly of two kettle holes on Mynydd Illtyd, formed by in situ melting of Late Devensian (Dimlington) stagnant ice. Pollen zonation of its organic sediments has been the subject of debate and the biostratigraphical record is difficult to interpret (Moore, in Lewis, 1970a; Ellis-Gruffydd, 1972; Walker, 1980, 1982a, 1984). Campbell and Bowen (1989) provide the most recent comprehensive account. The basal sediments comprise about 70 sand/silt/clay rhythmites, each probably representing annual varves deposited in a proglacial lake during melting of the ice sheet. The overlying organic muds appear to show two oscillations during the Windermere Interstadial (Pollen Zone II), not detected in similar deposits elsewhere in Wales. They are C¹⁴ dated at 11 600 ± 140 to 10 620 ± 100 BP, although the earlier date is 1000 years younger than the onset of the Loch Lomond Stadial elsewhere in Britain (Walker, 1980, 1982b); further dating is needed to resolve this anomaly (Campbell and Bowen, 1989). The end of the Loch Lomond Stadial is marked by a return to organic deposition at 9970 ± 115 BP at the start of the Holocene.

Craig Cerrig-gleisiad [SN 965 220] is an impressive glacial cirque, enhanced by landslipping and containing a range of morainic and landslip deposits (Shakesby and Mathews, 1996; Shakesby, 2002 and references therein). On the basis of radiocarbon dating of peat enclosed between inner and outer ridges, Bowen (1999) attributed the inner (younger) ridge to the Loch Lomond Stadial and named it the Graig Cerrig-gleisiad Member (of the Brecknockshire Formation). However, Shakesby (2002) proposes a more complex model in which post-Dimlington landslipping of the cirque backwall and reworking of the debris during the Loch Lomond Stadial by a small glacier produced a small morainic ridge on melting, behind which peat accumulated in a hollow.

Slope deposits, including colluvium and solifluction deposits, are collectively termed Head. They accumulated initially under periglacial conditions in depressions and on valley slopes. Thicknesses range up to 5 m e.g. [SN 9660 3843]; only the thicker and more extensive spreads have been mapped. Lithologies depend on the upslope bedrock or superficial deposit source, and range from silt to gravel, mainly with angular to subangular bedrock clasts. Head deposits along the eastern flanks of Noethgrug, in the north-west of the district, are stratified, loose, permeable gravels (Head Gravel), composed of angular mudstone and sandstone fragments derived from the Bronydd and Crychan formations. Talus accumulated in aprons and cones at the bases of the backwalls of the glacial cirques, as at Mynydd Du [SN 800 216], along with colluvium and debris flow deposits (Curry, 2002; Curry and Black, 2003).

The modern drainage system was established at the start of the Holocene, when the main rivers occupied wider braidplains than at present. Pulsed downcutting in response to postglacial regrading and isostatic readjustment resulted in the removal of much of these earlier deposits, except for remnants preserved as River Terrace Deposits. Two terrace levels are present in the Usk valley and its tributaries, 0.5–1 m and 2 m above the modern floodplain. The deposits are predominantly sandy, fining upwards from gravels to sandy silts, with coarse boulder gravels in the high valleys in the south of the district (e.g. Wenallt Fawr [SN 8000 2376]).

Alluvial fans occur at the confluences of tributary streams and their parent river, and are graded to the present floodplains. Alluvial Fan Deposits are mainly clay, silt and sand. Alluvium occurs in most of the streams and rivers of the district, ranging from clay, silt and sand to cobble gravel, with sandy silts commonly forming the uppermost floodplain layer. Lacustrine alluvium, peat and organic silts fill ponds and abandoned channels in river floodplains. Peat is widespread on much of the higher ground, with both blanket wasting peats and continuing peat formation in waterlogged hollows, as in the mires behind the Loch Lomond cirque moraines in Mynydd Du, Fforest Fawr and the Brecon Beacons. Rarely over 1 m thick, peat also occurs in the lower ground, in former lake sites e.g. [SN 875 297] and kettleholes e.g. [SO 0470 3230].

There are 85 recorded Landslips in the district, many of them occurring in over-steepened river cuttings in superficial deposits. Examples of large landslips in bedrock include one on the eastern slope of Castell Craigyrwyddon [SN 846 366] (Plate 4), where the steeply dipping Derwyddon Formation has slipped and toppled downslope, the slip comprising masses of rotated, partially disaggregated and fissured sandstone. Another on the eastern flanks of Fan Frynych [SN 975 235] was facilitated by the steep dip of strata into the valley. Landslips in superficial deposits include earthflows e.g. [SO 0440 3700], circular rotational failures e.g. [SN 833 292] and [SO 0805 4000], the former active, and a recent non-circular rotational failure at Fenni-Fach (Plate 3).

Artificially modified ground is of minor extent. Made ground occurs in road, rail and flood embankments, and in areas of human settlement, but only the large occurrences are shown on the map.

Chapter 3 Applied geology

Mineral resources

The district was included in a broad study of mineral resource information for planning authorities by Highley et al. (1997), and in an impact assessment of sand and gravel extraction by Thompson et al. (2002). Ball and Nutt (1976) noted a former trial for lead at Scrâch [SN 847 394].

Industrial minerals include hard-rock aggregate, building stone, brickclay, and sand and gravel. Of the hard-rock aggregate resources, the Cwm Clyd Sandstone Formation and the argillaceous sandstones of the Cribarth Formation have good PSVs (Polished Stone Values) in the Builth Wells district to the north, where they are worked for road aggregate. The tough, argillaceous sandstones of the Crychan and Cefngarreg formations are similar. The sandstones of the Derwyddon Formation are a potential hard rock resource, but outcrop width is a limiting factor. The sandstones of the Tilestones Formation have been worked for aggregate at Capel Horeb [SN 8460 3240] (Plate 1) and on the south side of Cwm Dwr [SN 8440 3230]. The most recent major hard-rock aggregate working was at Heol Senni Quarry in the Senni Formation, which operated until the late 1970s (Plate 5). Head Gravel has been quarried [SN 837 935] for aggregate for forestry tracks in the north-west of the district.

Exploitation of all the sandstones of the district for building stone was once widespread. The Tilestones Formation was worked for roofing tiles extensively. The harder silica-cemented sandstones of the Raglan Mudstone Formation were extensively quarried e.g. [SO 0200 3394], [SO 0404 3600] and [SO 0786 3668], as were sandstones of the St Maughans Formation e.g. [SN 9365 3785], [SN 9338 3368] and [SO 0140 3146]. The siltstones of the St Maughans Formation were formerly dug for brickclay at Pen-y-crug [SO 031 301].

Crimes et al. (1992) and the Symonds Group (2000) identified sand and gravel resources in the Usk valley. There are an estimated 23.3 million tonnes of potential resource, but constraints to exploitation include its location in the Brecon Beacons National Park, hydrological problems of extraction close to the river, the small size of individual prospects, and the distance from the main urban markets (Symonds Group Ltd, 2000; Thompson et al., 2002). The large alluvial fans and glaciofluvial sheet deposits have the greatest aggregate potential, but the Llanfaes fan [SO 040 285] has been largely sterilised by development. Most of the fans are dominated by cobble gravel and contain relatively little fine-grade material. The main glaciofluvial sheet deposits are in the Tarrell valley and along the south bank of the Usk between Llanfaes and Llanspyddid. The glaciofluvial ice-contact deposits and the hummocky moraines of the Honddu, Usk and Senni valleys contain sand and gravel, but the deposits are probably interbedded with tills and lacustrine clays, and of poor quality.

Water resources

The principal water resource of the district is its surface water. There are reservoirs in the Crai valley (Cray Reservoir) and in the upper Usk valley (Usk Reservoir), and Llyn y Fan Fach [SN 803 218] is a natural lake that was further impounded by a small dam. The Cray Reservoir was built to supply Swansea; the Usk Reservoir supplies the Usk valley, and the Tywi valley by a tunnel that emerges on the western flank of Mynydd Myddfai [SN 806 304].

The district is devoid of major bedrock groundwater supplies, but the sandstones and more mature carbonate palaeosols (calcretes) of the Lower Old Red Sandstone yield limited quantities from shallow depths. Modest amounts are also present in fractured and weathered near-surface zones in the older rocks (Jones et al., 2000). The primary porosity of the rocks is generally low, and groundwater flow is mainly in fractures. Groundwater in the Old Red Sandstone is compartmentalised by mudstone beds, giving a complex, multi-layered aquifer whose potential is further limited by low porosity and variable permeability. The effective saturated thickness is taken as 40 m, below which fractures are closed (Jones et al., 2000). Zones of fractured and cleaved mudrocks increase permeability locally, and the north-east–trending faults that cross the district are also likely to have an influence on groundwater movement.

Historically, groundwater from springs and wells supplied the district. These sources continue to be used, but yields are variable and generally low. Many farms with a mains supply use spring water for agricultural purposes. In places, up to 80 per cent of farms visited during the survey rely on private supplies, either exclusively or to supplement the mains supply. Most private supplies are from springs and shallow wells, with about 10 per cent coming from boreholes.

Brecon's water is abstracted from three boreholes [SO 0320 2885] in river terrace and glaciofluvial gravels. The water table is at a depth of 4 m and 5.77 Ml/d are currently licensed for abstraction (personal communication H Bolton, 2004). The water is treated near the boreholes and pumped to the St Davids and Pen-y-crug service reservoirs. A water treatment works at Portis [SN 870 276], which serves communities west of Brecon, is currently supplied by the Usk Reservoir. The gravels in the Usk valley are generally in hydraulic continuity with the river and their groundwater is vulnerable to surface contamination. However, the Brecon production boreholes show that there is little recharge of the gravel aquifer from the river, and that baseflow is largely unaffected by the abstraction.

Geological hazards

Water contamination by toxic leachate is a potential hazard from areas such as inadequately lined domestic landfill or agricultural waste-disposal sites, sewage works, and disused quarries and gravel pits. River and stream floodplains within the district are liable to flooding. An indication of the risk-prone areas is given by the extent of active floodplain deposits, shown as alluvium on the geological map, and also shown on the maps of the Environment Agency. Low river terraces, low-gradient alluvial fans and areas of Head may also be susceptible during periods of high rainfall. The River Usk presents the largest flood risk, and embankments and walls on both banks protect Brecon.

Gas emissions present a hazard where methane, carbon dioxide or radon accumulate (Appleton et al., 1995; Appleton and Ball, 1995). The bedrock geology of the district has a low susceptibility to methane emission, but the gas may be generated from domestic refuse in landfill sites and unconsolidated organic-rich deposits. It can migrate through permeable strata and accumulate in poorly ventilated basements, foundations and excavations. Risk can be mitigated by venting of landfill and buildings. Carbon dioxide is produced by the oxidisation of organic matter in landfill or peat, and high levels are toxic and asphyxiating. Radon is a naturally occurring ionising gas produced by the radioactive decay of uranium, which is present in small quantities in all rocks and soils. It may accumulate in poorly ventilated spaces, increasing the risk of cancer of the respiratory tract. Protective measures should be provided in new buildings and remediation measures carried out in existing buildings in areas of high radon emission. The potential for radon emission ranges from low (Upper Old Red Sandstone) to high (Ludlow rocks), with most of the district having moderate susceptibility (Appleton and Ball, 1995). Advice on radon and its associated health risks can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire, OX11 0RQ.

The potential for slope instability is widespread throughout the district, because of high rainfall, an abundance of seeps and spring lines, and incision of glacial deposits by rivers. Most active landslips are in rural or upland areas, but some e.g. [SO 033 335] threaten roads and properties. Numerous landslips in superficial deposits, such as that at Fenni-Fach [SO 015 286] (Plate 3), are currently or were recently active e.g. [SN 8860 3520] and [SO 0778 3500]. An active landslip in bedrock [SN 833 292] occurs where groundwater seepage has weakened an oversteepened slope. At Craig Cerrig-gleisiad [SN 964 200], a range of morainic and landslip features (e.g. Shakesby, 2002), including tension cracks and scarps above the backwall of the cirque, indicate continuing movement.

Engineering ground conditions

Ground conditions are of prime importance in the planning and foundation design of buildings and infrastructure such as roads and dams. Ground conditions are influenced by the physical and chemical properties of the bedrock and superficial deposits, topography, groundwater and surface water regimes, and human activity, past and present. The bedrocks of the district generally have high bearing capacity and good foundation conditions below the weathered zone, but all the mudrocks are weakened and soft below springs and seepages. The mudstones of the Garth House and Builth Mudstone formations should not be used as fill, as their pyrite content may cause sulphate attack on concrete.

Glaciofluvial and river terrace gravels encompass a range of foundation conditions, but generally have high bearing capacities. Head, head gravel, alluvial fan deposits and alluvium have low to moderate load-bearing strengths, with moderate to high settlement rates, and the alluvial areas provide poor foundation conditions. Head deposits require careful investigation and remediation measures to mitigate against reactivation or instigation of instability. Till and hummocky morainic deposits have moderate, but variable bearing capacities, and their heterogeneous nature may produce variable foundation conditions. Glaciolacustrine deposits and peat have low bearing capacities and carry a high settlement risk. Thixotropic, running sands may be encountered below the water table in the alluvial, glacial and glaciofluvial deposits. There are no current waste burial sites in the district, but developers of former sites should be aware of unstable foundation conditions and the possibility of methane generation.

Geological conservation

Geological localities of national importance are protected as Sites of Special Scientific Interest (SSSIs) under the Wildlife and Countryside Act (1981). There are 12 such sites in the district: one fossil fish site (Heol Senni Quarry; Dineley and Metcalf, 1999; Barclay, 2005b), four sites in the type Llandovery area (Scrâch Track, Trefawr Track, Cwm Coed Aeron and Cwm Glyn–Môch Track; Aldridge, 2000), one site in the late Silurian (Capel Horeb Quarry; Lane, 2000), one site in the Old Red Sandstone, proposed for its sedimentology and animal tracks (Pantymaes Quarry; Barclay, 2005a), and four Quaternary sites (Mynydd Du, Traeth Mawr, Cwm Llwch and Craig Cerrig-gleisiad; Campbell and Bowen, 1989). Further information on the SSSIs can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd, LL57 2LQ.

Further geological information held by the British Geological Survey relevant to the district and adjoining areas is listed below. Enquiries concerning geological data should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth, and the BGS Hydrogeology Enquiry Service can be contacted at: Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX0 8BB; Telephone 01491 838800

Searches of indexes to some of the collections can be made on the Geoscience Data Index system in BGS libraries and on the BGS website (http://www.bgs.ac.uk). The BGS Catalogue of geological maps and books may be downloaded from the BGS website, or is available on request.

Information sources

Maps

Geological maps
1:25 000 scale maps
SN72, 73 (small parts only), 82, 83, 92, 93; SO02, 03
1:10 000 maps
Manuscript field maps at this scale are available only for the north-west of the district (parts of SN 73NE and SE, and SN 83 NW, NE, SW and SE
Digital geological map data
In addition to the printed publications, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the intellectual property rights manager at BGS Keyworth
Geophysical maps
1:1 500 000
Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997
Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
Magnetic and Bouguer gravity anomaly maps are also available at 1:625 000 and 1:250 000 scales
Geochemical atlases and maps
1:250 000 atlases
Regional geochemistry of Wales and part of west–central England 2000
Regional geochemistry of Wales and part of west–central England, streamwater 2000
1:625 000 maps
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
Minerals maps
1:1 000 000
Industrial mineral resources map of Britain, 1996
Metalliferous mineral resources of Britain, 1996
Hydrogeology
1:625 000
Hydrogeological map of England and Wales, 1977
1:100 000
Sheet 28 Groundwater Vulnerability of Powys, 1996

Books

British Regional Geology
South Wales. Third edition, 1970

Boreholes

Borehole data for the district are catalogued in the BGS archives at Keyworth. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

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

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Almond, J, Williams, B P J, and Woodcock, N H. 1993. The Old Red Sandstone of the Brecon Beacons to Black Mountain area. 311–330 in Geological excursions in Powys, Central Wales. Woodcock, N H, and Bassett, M G (editors). Geologists' Association South Wales Group. (Cardiff: University of Wales Press.)

Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Appleton, J D, Hooker, P J, and Smith, N J. 1995. Methane, carbon dioxide and oil seeps from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/1.

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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. 1946. The genus Phialaspis and the 'Psammosteus Limestones'. Quarterly Journal of the Geological Society of London, Vol. 101, 207–242.

White, E I. 1950a. The vertebrate faunas of the Lower Old Red Sandstone of the Welsh Borders. Bulletin of the British Museum (Natural History), Vol. A1, 51–67.

White, E I. 1950b. Pteraspis leathensis White: a Dittonian zone-fossil. Bulletin of the British Museum (Natural History), Vol. A1, 69–90.

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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. Almost all BGS maps are available flat or folded and cased.

(Rear cover)

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.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Simplified geological sketch map of the district showing broad geological divisions and main structural features.

(Figure 2) Ashgill succession in the Brecon district.

(Figure 3) Correlation of the late Ordovician and early Silurian (Llandovery) successions in the Brecon district.

(Figure 4) Late Ashgill and Llandovery shelf succession of the Brecon district.

(Figure 5) Correlation of the Mid Silurian (Wenlock and Ludlow) successions of the Brecon district.

(Figure 6) Mid to Late Silurian succession in the Brecon district. See (Figure 5) for lateral relationships of the formations.

(Figure 7) Old Red Sandstone (Late Silurian–Devonian) rocks of the Brecon district.

(Figure 8) Colour-shaded 1:500 000 scale Bouguer gravity anomaly map of the Brecon district and surrounding area. Contour interval 1mGal (1 mGal = 1 3 10^-5m/s2). Based on data in the BGS National Gravity Databank. Station distribution approximately 1 per 1.3 km².

(Figure 9) Colour-shaded relief total field aeromagnetic anomaly map of the Brecon district and adjoining area. Scale 1:500 000. Contours are in nanotesla (nT), based on data in the BGS National Aeromagnetic Databank. Flown at a mean terrain clearance of 305 m on N–S flight lines 2 km apart with E–W tie-lines 10 km apart.

(Figure 10) Ice movements in the Brecon district during the Late Devensian Dimlington Stadial. Also shown are the moraines marking halt stages as the valley glaciers retreated during climatic amelioration. Glacial lakes formed at that time where meltwater was dammed by glacial deposits or ice. The later high-level cirque moraines formed during the Loch Lomond Stadial are also shown.

Plates

(Plate 1) Capel Horeb Quarry [SN 8450 3245] in steeply dipping Late Silurian strata. The Upper Ludlow Cae'rmynach Formation is overlain by the Lower Přídolí Tilestones Formation and succeeding Temeside Mudstone Formation. The junction between the highest Ludlow rocks (formerly known as the Upper Roman Camp Formation) and the Tilestones Formation has been traditionally placed at a supposed disconformity at a prominent bedding plane above the figure. However, re-examination of the section shows that the Tilestones Formation commences at a higher level (P593805).

(Plate 2) North face of Pen y Fan, Brecon Beacons: sheeted sandstones of the Brownstones Formation are unconformably capped by thicker bedded sandstones of the Plateau Beds Formation forming the topmost few metres of the mountain (A11105).

(Plate 3) Till exposed in backscar of a landslide (February 2002) at Fenni-Fach [SO 015 286]. The deposit forms a morainic mound and comprises a darker, more clay-rich lower part with a higher proportion of cobbles and boulders and an upper, more sandy part with fewer clasts. Note map case (30 cm) for scale (P593806).

(Plate 4) Landslip in the Derwyddon Formation on the east side of Castell Craigyrwyddon [SN 846 366]. View looking south-west across the valley, which follows the outcrop of the Dulas Valley fault (P593807).

(Plate 5) Heol Senni Quarry [SN 9145 2210], in the Senni Formation, was worked for aggregate until the late 1970s (P595978).

Figures

(Figure 2) Ashgill succession in the Brecon district

Formation	Lithology	Depositional environment/event
CWMCRINGLYN (up to 50 m)	Lenticular, symmetrical ripple cross-laminated sandstone beds interbedded with subordinate dark grey silty mudstone	Prograding upper shoreface facies within fair weather wave-base deposited during the acme of the Hirnantian glacioeustatic regression. At the point of maximum lowstand, much of the area east of the Garth Fault was exposed and suffered subaerial erosion
CILIAU† (up to 250 m)	Dark grey, weakly burrowed, silty mudstone with abundant laminae and thin beds of calcareous, locally shelly, siltstone and sandstone	Lower shoreface facies with partially bioturbated, storm sheet sand and silt beds; deposited during the early part of the late Ashgill (Hirnantian) glacioeustatic marine regression. Shelly fossils belong to the distinctive, cool water, Hirnantia association. Evidence of syndepositional faulting
YR ALLT (up to 300 m)	Silt-laminated turbiditic mudstone, locally interbedded with packets of fine-grained sandstone turbidites	Basin slope-apron oxic facies with silt and sand turbidites
CRIBARTH* (up to 400 m)	Very thickly bedded, dark grey, bioturbated sandy and locally shelly mudstone and muddy sandstone	Offshore sandy mud facies in which storm-deposited sand layers were thoroughly mixed with the intervening mud by pervasive bioturbation
TRIDWR* (up to 1150 m)	Grey mudstone with subordinate thin to medium bedded, locally shelly sandstone beds	Mud-dominated mid to outer shelf environment subject to sand influx during storm events
NANTMEL MUDSTONES (up to 700 m)	Grey, burrow-mottled mudstone with local colour banding, phosphate nodules and thin sandstone beds and laminae	Oxic, basin slope-apron facies with mud deposition from suspension (hemipelagites) and mud/sand-laden turbidity currents
* passes laterally into the Nantmel Mudstones Formation † passes laterally into the Yr Allt Formation (see (Figure 3))

(Figure 4) Late Ashgill and Llandovery shelf succession of the Brecon district

Formation	Lithology	Depositional environment/event
CERIG (750 m)	Grey to olive-green, burrow- mottled mudstone, with sporadic thin siltstone and fine-grained sandstone; includes a unit of thinly bedded tabular sandstones and mudstones (MWMFFRI SANDSTONE MEMBER)	Distal shelf mud deposition following a transgressive deepening event; storm-generated turbidity currents introduced the sandstone and siltstone
DERWYDDON† (25 to 75 m)	Thinly to medium-bedded sandstone, locally conglomeratic and locally rich in pentamerid shells east of the Llandrindod–Pen-y-waun Fault Belt	Nearshore, shallow-water shelf sands deposited during the late Aeronian eustatic marine transgression
CEFNGARREG SANDSTONE (up to 400 m)	Feature-forming, green, burrow-mottled sandstone with two widespread, thin, green, burrow-mottled mudstone units	Progradational inner shelf sands deposited as a result of a mid-Aeronian regressive event; periods of offshore mud deposition in response to the late Aeronian transgression. Disturbed beds record a major slope failure on the shelf margin
TREFAWR (up to 200 m)	Sandy, locally pebbly, grey-green, bioturbated, shelly mudstone	Mid shelf sandy mud deposition following a transgressive deepening event in the late Rhuddanian cyphus Biozone
CRYCHAN (up to 170 m)	Green, muddy, thick-bedded, thoroughly bioturbated, burrow-mottled, micaceous, locally pebbly sandstone	Inner shelf sand deposition
BRONYDD (up to 275 m)	Grey-green, sandy, thoroughly bioturbated mudstone	Mid shelf sandy mud deposition
CHWEFRI* (up to 750 m)	Unfossiliferous, colour banded, grey to greenish grey, silty mudstone, locally burrow-mottled, with thin siltstone laminae	Distal shelf/upper slope mud facies deposited principally from suspension (hemipelagic) under oxic bottom water conditions
GARTH HOUSE (up to 90 m)	Unfossiliferous, smooth, dark grey mudstone with abundant lenticular beds of cross-laminated sandstone	Rapid transition through upper and lower shoreface facies, with wave-reworked lenticular sand beds, into offshore mud-dominated facies. The formation, with an upward decrease in sandstone content, records deepening during the continuing postglacial transgression
CWM CLYD SANDSTONE (up to 10 m)	Symmetrical ripple cross-laminated sandstone with abundant Skolithos trace fossils and mudstone linsen; locally conglomeratic	Transgressive, beach and upper shore-face facies driven eastwards across the underlying subaerially eroded surface during the late Hirnantian, postglacial eustatic sea level rise
† passes laterally into the Cefngarreg Sandstone Formation * passes laterally into the Garth House, Bronydd, Crychan and Trefawr formations (see (Figure 3))

(Figure 6) Mid to Late Silurian succession in the Brecon district

Formation	Lithology	Depositional environment/event
RAGLAN MUDSTONE (1100 m)	Red-brown mudstone/siltstone with subordinate planar bedded and cross-bedded sandstone and abundant immature calcrete profiles; mature calcrete (Bishop's Frome Limestone Member) locally at top	Alluvial floodplain subject to occasional marine inundation; sands deposited in the channels of meandering rivers; calcretes record sub-aerial exposure and soil formation
TEMESIDE MUDSTONE (up to 50 m)	Dull green to green-grey, slightly micaceous, muddy siltstone with abundant small calcrete nodules; some thin, green, micaceous sandstone beds	Sand-starved lagoon or tidal flat subject to periodic subaerial exposure and carbonate soil formation, probably behind the Tilestones Formation barrier
TILESTONES (15 to 35 m)	Thin to medium-bedded, yellow-weathered, grey, purple and green, fine to medium grained, locally micaceous, parallel-laminated and low angle cross-bedded sandstone	Shoreface and barrier sands
CAE'R NWNACH (155–260 m)	Grey, thinly interbedded mudstone, siltstone and fine-grained sandstone with thicker sandstone beds; common bioturbation; quartzitic sandstone at base in west	Basal shoreface quartz sand deposited in response to the late Ludlow transgression; succeeded by mid to proximal shelf mud and silt deposition with mainly storm-generated sandstones; wave-generated structures in upper part suggest fair weather reworking in a nearshore environment
FIBUA (40–170 m)	Coarsening upwards sequence of grey, silt and sand-laminated mudstone, with hemipelagites in lowest 10 m; scattered burrows. Passes southwards into Cae'r mynach Formation	Offshore correlative of the lower part of Cae'r mynach Formation. Basal hemipelagites represent a return to mid to proximal ramp facies in response to the late Ludlow transgression; higher beds record a transition from proximal ramp to shelf facies resulting from renewed progradation
TRICHRUG (up to 20 m)	Thin to medium-bedded, red- brown, bioturbated and pedoturbated muddy sandstone and gritty mudstone; thin, purple quartzites	Back-barrier mud and sand formed behind the Mynydd Myddfai Sandstone and subjected to emergence and carbonate soil formation; quartzites may have been washover sands
MYNYDD MYDDFAI SANDSTONE (up to 30 m)	Pale grey, thick-bedded quartzitic sandstone and quartzite; medium to coarse-grained and locally conglomeratic with red mudstone and calcrete clasts	Shoreface and barrier sands, with back-barrier sediments present as a thin unit of mudstones and sandstones in mid-formation in the south-west, and as reworked calcrete and mudstone clasts
ABEREDW (up to 80 m)	Grey, thin to thick-bedded, calcareous, muddy siltstone and fine-grained sandstone; intensively burrowed and variably shelly	Acme of progradation and shallowing of shelf facies in north of district; characterised by storm-generated sands and silts; abundant infauna
HAFOD FAWR (up to 710 m)	Interbedded grey mudstone, siltstone and sandstone; prominent, parallel sided, well-jointed sheet sandstones; variably bioturbated	North-east–prograding mid to inner shelf facies with storm-generated sand sheets
CWM GRAIG DDU (70–95 m; thickening to 425 m at northern edge of district)	Coarsening upwards sequence of grey-brown, thinly bedded, calcareous mudstone; locally shelly, variably bioturbated, with sparse to abundant thin beds and laminae of siltstone (up to 5 mm) and sporadic very fine-grained sandstone (up to 30 mm)	Progradation of mid-ramp to shelf facies comprising low concentration turbidites and storm-driven event beds. Predominantly oxic bottom conditions. Coarsening upwards sequence represents onset of mid Ludlow shallowing
IRFON (up to 120 m)	Grey, thinly bedded mudstone with mm thick siltstone laminae and sparse mm thick laminated hemipelagic mudstone	Distal mid-ramp mud with storm-generated turbidite silt laminae deposited during early Ludlow deepening
HALFWAY FARM (up to 105 m)	Greenish grey, calcareous, shelly, bioturbated mudstone with sporadic thin beds (up to 50 mm) of siltstone and sandstone, and thin beds and thicker units of shelly argillaceous limestone	Muddy shelf with rich indigenous shelly fauna and storm generated clastic and carbonate sand sheets; formed in response to a slight deepening that halted the progradation of the Sawdde Sandstone Formation
SAWDDE SANDSTONE (up to 660 m)	Thinly interbedded grey mudstone, siltstone and sandstone with characteristic thicker (3–30 cm) tabular beds of fine-grained, well-jointed sandstone	Progradation of a mid shelf facies in response to the Homerian regression; tabular sandstones are sublittoral sheet sands that were deposited as storm event beds above storm wave base
TIRABAD (up to 740 m)	Grey mudstone with abundant thin beds (up to 15 mm) and laminae (up to 5 mm) of parallel-laminated, locally cross-laminated siltstone and fine grained sandstone; sporadic thicker beds (up to 30 mm) of tabular, well-jointed sandstone; characteristic burrows	Progradation of proximal ramp to shelf facies in response to Homerian regression; silt and sand-based event beds deposited by storm-induced, low concentration turbidity currents; burrowing suggests oxic bottom conditions
LLANGAMMARCH (up to 500 m)	Thinly bedded, grey mudstone, variably calcareous, with sparse to abundant thin beds and laminae of siltstone, and of laminated hemipelagic mudstone; thick sequences of slump-folded and destratified strata (disturbed beds)	Progradation of mid-ramp facies comprising thin, silt-based event beds emplaced by low-concentration density currents (?storminduced). Laminated hemipelagites record anoxic bottom conditions
BUILTH MUDSTONES (up to 260 m)	Blue-grey, pervasively laminated (hemipelagic), variably calcareous mudstone; units of slumped and destratified strata (disturbed beds)	The laminated hemipelagic mudstone comprises a distal offshore/ramp facies deposited principally from suspension, under anoxic (anaerobic) bottom conditions. The widespread, basal Wenlock introduction of anoxic bottom waters in the Welsh Basin, and elsewhere, was a response either to a deepening event, or to changes in climate and marine circulation. Disturbed units record contemporary, possibly fault-induced slumping