Content and licensingview original scan buy a printed copy

Geology of the Builth Wells district — a brief explanation of the geological map Sheet 196 Builth Wells

D I Schofield, J R Davies, R A Waters, P R Wilby, M Williams, and D Wilson

Bibliographic reference Schofield, D I, Davies, J R, Waters, R A, Williams, M, and Wilson, D. 2004. Geology of the Builth Wells district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 196 Builth Wells (England and Wales).

© NERC 2004 All rights reserved Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail 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.

Keyworth, Nottingham: British Geological Survey, 2004. Printed in the UK for the British Geological Survey by B&B Press Ltd. Rotherham

(Front cover) Irfon valley, north of Abergwesyn [SN 846 536] (Photographer J R Davies; GS1278).

(Rear cover)

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 196 Builth Wells. National grid references are given in square brackets. Most of the district lies within the 100 km square SN and the letter prefix for grid references in this area are omitted. However, for localities, which lie within the adjacent 100 km square SO, the letter prefix is included. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map.

Acknowledgements

This Sheet Explanation was compiled by D I Schofield and R A Waters. Biostratigraphical information was supplied by M Williams, J A Zalasiewicz (University of Leicester) and A W A Rushton. (Natural History Museum). Information on low grade metamorphism was provided by S J Kemp and R J Merriman. P R N Hobbs contributed to the Potential geological hazards and Engineering ground conditions sections. The text was edited by A A Jackson; Figures were drawn by R J Demaine, P Lappage and G Tuggey. From 2000, the survey was funded partly by the Welsh Assembly Government.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2004.

Geology of the Builth Wells district (summary from rear cover)

An explanation of sheet 196 1:50 000 series map (England and Wales)

(Rear cover)

This Sheet Explanation contains a brief description of the geology of the Builth Wells district, which covers the south-eastern margin of the Cambrian Mountains, the steep escarpment of Mynydd Eppynt and the intervening belt of undulating ground occupied by Builth Wells, Llangammarch Wells and Llanwrtyd Wells. As such, the district provides a transect across the complex collage of geological formations which define the south-eastern margin of the Lower Palaeozoic Welsh Basin.

The solid geology comprises sedimentary and volcanic rocks of Ordovician, Silurian and Devonian age, which formed between approximately 470 and 390 million years ago. This survey has established new insights into the evolution of the south-eastern margin of the Welsh Basin and has significantly revised earlier studies. A brief summary of lithological characteristics and environments of deposition for each formation is presented, along with sedimentary architecture models, which illustrate how units relate to each other throughout the district.

The district was subject to a long history of fault movement that culminated in an episode of regional folding, cleavage formation and low-grade metamorphism that occurred during the latest Silurian and early Devonian. A brief history of these movements and outline description of the resulting structures is presented.

This survey also provides the first comprehensive study of the distribution and composition of Quaternary deposits of the district. These were deposited mainly during the last (Devensian) ice age, between approximately 20 000 and 14 000 years ago and form a patchy cover of unconsolidated superficial (drift) deposits. In the last 14 000 years, since the ice retreated, peat has accumulated in the upland areas and fluvial sediments have been deposited by the rivers Wye, Irfon, Ithon, Tywi and their tributaries.

As well as summarising the traditional aspects of the geology, the new maps (Solid and Solid and Drift) and this accompanying Sheet Explanation provide valuable information on the applied geological aspects of the district. These include mineral and water resources, potential geological hazards, engineering ground conditions and geological conservation, all of which are significant considerations for planning and development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the district covered by the geological 1:50 000 Series Sheet 196 Builth Wells. The map is published in two editions, Solid, and Solid and Drift, in 2005.

The district lies mainly in the county of Powys but also includes parts of Ceredigion and Carmarthenshire. The sparse population is concentrated in Builth Wells, Llangammarch Wells and Llanwrtyd Wells, situated in the broad belt of undulating low ground, that divides the district from north-east to south-west along the course of the River Irfon. To the north-west of this tract, lies the dissected upland plateau of the Cambrian Mountains. To the south-east is the steep escarpment of Mynydd Eppynt, which is cut at the eastern margin of the district by the deeply incised valley of the River Wye. Today, the local economy is based on agriculture and tourism. Major hard rock resources are currently worked for aggregate at Cribarth and Llanelwedd.

The geology of the district has a long history of research and was a key area in the development of Lower Palaeozoic stratigraphy. Influential British geologists who contributed to the early understanding of the area include Murchison (1839), who discussed the geology around Builth and Aberedw in The Silurian System. Measured sections were produced by De la Beche (1846), and advances in understanding the stratigraphy were made by Charles Lapworth (1880).

The bedrock exposed at surface in the district consists mostly of sedimentary and volcanic rocks, deposited at various times between about 470 and 390 million years ago, during the Ordovician, Silurian and Devonian periods (Figure 1). The sedimentary rocks include deep water and shallow marine deposits. The solid rocks are mantled by Quaternary sediments (drift), including Pleistocene glacial materials, deposited during the last (Devensian) major ice advance, some 20 000 years ago, as well as younger, mainly alluvial deposits, still accumulating today.

The district is situated along the southeastern margin of the Lower Palaeozoic Welsh Basin. The basin occupied an area of enhanced subsidence and deep water sedimentation that developed from Cambrian to late Silurian times. It is bordered to the south-east by the Midland Platform, a relatively stable area, characterised by shelf sedimentation and periodic emergence. The edge of the platform is broadly defined by a complex zone of north-west-trending faults and deformation, known as the Welsh Borderland Fault System (Woodcock and Gibbons, 1988), which comprises the Tywi and Pontesford lineaments and the Church Stretton Fault Zone. The Tywi Lineament is broadly coincident with the Tywi Anticline, cored by Ordovician rocks. In the district, the Pontesford Lineament is dominated by the Llandrindod-Pen-y-waun Fault Belt, which transects the Builth Volcanic Inlier. The Church Stretton Fault intersects the south-eastern corner of the district.

The oldest rocks in the district are preserved in the Builth and Llanwrtyd Ordovician volcanic inliers and include the products of acid, intermediate and basic volcanism linked to a widespread Llanvirn to Caradoc rifting event. During the late Ordovician and Silurian, a more rapidly subsiding basinal area, characterised by deep water sedimentation, became differentiated from a shelf to the south and east, where shallower water deposits accumulated. During this interval the divide between shelf and basin was broadly located along the south-eastern flank of the Tywi Lineament with fractures such as the Llanwrtyd and Garth faults influential at different times, and with much local interdigitation of shelf and basin facies. The successions in both settings record the effects of a series of marine transgressions and regressions, including widely recognised eustatic events, as well as periods when intra- and extra-basinal tectonism was the dominant control on sedimentation.

The shelf facies consists predominantly of mudstone deposited in deeper, distal settings, and sandstone and siltstone deposited in shallower, near-shore environments. Bioturbation is common, and fossils including brachiopods, trilobites and bivalves are locally abundant. Vertical alternations of deeper and shallower shelf facies are due to rises and falls in sea level; each transgressive deepening being followed by a basinward progradation of coarser facies producing a series of large-scale coarsening-upwards sequences. The earliest progradational shelf sequence is of Ashgill age and linked to the growth of an extensive ice cap on the contemporary super-continent of Gondwana and to the resulting fall in global sea level. Major progradations are also evident in the early and mid Llandovery, and in the late Wenlock, with a further two recognised in the early and late Ludlow. Capping some of these events are erosional non-sequences of varying magnitude and areal extent. Intervening transgressions resulted in the widespread deposition of fine-grained distal shelf deposits and, periodically, the south-eastward migration of basinal facies across the basin margin.

The basinal facies in the district are composed mainly of different varieties of turbidite, including conglomerate, sandstone and mudstone types (Plate 1), but hemipelagic mudstone deposited from suspension is locally important. Davies et al. (1997) recognised two distinct types of turbidite system in the Welsh Basin — slope apron and sandstone lobe, both of which are present in the district.

Slope apron systems comprise wedge-shaped accumulations of turbiditic and hemipelagic mudstone, ranging from tens to hundreds of metres in thickness, which thin westward from the shelf margin towards the basin centre and envelope lenticular bodies of coarse-grained turbidites. Slope apron deposition was active in the district throughout much of the late Ordovician to the early Silurian and was sensitive to the same sea-level movements that affected sedimentation on the adjacent shelf. Two categories of slope apron facies are distinguished on the basis of the type of hemipelagic mudstone present. In anoxic facies, the hemipelagic layers are dark grey, delicately laminated, pyrtic and preserve the remains of planktonic graptolites. The lack of burrowing and preservation of organic material testifies to deposition under stagnant, oxygen-depleted (anaerobic) bottom conditions. In the district, periods of anoxic facies deposition correlate with marine transgressions and may relate to the creation of a strongly stratified basin water column. In contrast, oxic facies, which contain pale, burrow-mottled hemipelagic mudstones and common phosphate nodules, record accumulation and early diagenesis beneath oxygenated bottom waters. These conditions prevailed during periods of marine regression and progradation on the shelf, when the effects of thermal stratification were reduced and bottom waters could be replenished with oxygen from the surface. Hence, anoxic slope apron facies were widespread during the postglacial, late Ashgill to Rhuddanian rise in sea level, and again during the late Aeronian (sedgwickii Biozone) transgression. Oxic facies dominated during the late Ashgill glacio-eustatic regression and again in the mid Aeronian and early Telychian (Davies et al, 1997).

Southerly derived sandstone lobe facies, of mid Telychian age, are confined to the western margin of the district, where they form the youngest part of the basinal sucession, and were deposited during a period of tectonic rejuvenation of sediment source areas and intra-basinal faulting (Davies et al., 1997).

The youngest strata in the district, the latest Silurian (Přídolí) and early Devonian red-bed sequence of Mynydd Eppynt, in conjunction with underlying Ludlow facies, accumulated in a rapidly subsiding foreland basin consequent to plate collision events in the Southern Uplands and English Lake District.

Subsequently, the rocks of the district suffered folding, faulting, uplift and erosion during the latest Silurian to mid-Devonian Acadian Orogeny.

There are no sedimentary rocks preserved in the district for the period between the late, early Devonian (about 390 million years ago) and the last glaciation in the Quaternary (about 20 000 years ago). During the Late Devensian (late Quaternary), glaciation was responsible for widespread erosion and deposition. Till and glaciofluvial deposits are widespread throughout the district, as are head deposits formed by solifluction under periglacial conditions. In the last 10 000 years, since the ice has retreated, peat has accumulated in upland areas and lake basins, and alluvial sediments have been deposited by the river systems.

Chapter 1 Geological description

Ordovician

The oldest rocks are of Llanvirn to Caradoc age and are confined to the two volcanic inliers at Builth Wells (Elles, 1940; Jones and Pugh, 1941, 1949) and Llanwrtyd Wells (Stamp and Wooldridge, 1923), which straddle the fractures that define the basin margin (Figure 1); (Figure 2). Basinal Ashgill rocks occupy much of the Tywi Anticline, and the core of the Rhiwnant Anticline. They also crop out to the east of the Garth Fault, interleaved with Ashgill shelf successions of the Garth and Crychan Forest areas.

Llanvirn to Caradoc volcanic inliers

The Builth Volcanic Inlier is surrounded by an unconformable cover of Silurian rocks. The succession (Figure 3) ranges from the early Llanvirn artus Biozone to the early Caradoc gracilis Biozone (Hughes, 1989). The architecture of the volcanic and sedimentary rocks is shown in (Figure 4).

The oldest unit, the Camnant Mudstones (Cte), crops out principally in the east of the inlier, but also occupies the core of the Gilwern Anticline. Thin sequences of basic lapilli tuff form mappable units that include the Gelli Tuff Member (Glh) exposed in the north-east of the district. The formation ranges from the artus into the murchisoni Biozone.

The overlying Builth Volcanic Group (formerly Builth Volcanic Formation) is Llanvirn in age, and formed during a period of widespread volcanism in Wales that also includes the volcanic sequences at Llanwrtyd, and in west Wales near Fishguard and on Cadair Idris. Both the chemistry of the erupted materials and plate tectonic reconstruction suggest that these volcanic centres developed in an ensialic, marginal basin setting (e.g. Bevins et al., 1984; Kokelaar et al., 1984). The nomenclature for the Builth Volcanic Group used in this account replaces that of Davies et al. (1997). The group lies entirely within the murchisoni Biozone, and is divided into six formations, which are described from base to top.

The basal division, the Llandrindod Tuff Formation (LdT), extends from the eastern flanks of the Carneddau, in the south of the inlier, northwards to Gilwern Farm and Caregwiber Bank [SO 083 594], where it crops out along both limbs of the Gilwern Anticline. It comprises acid ash flow tuff with relict glass shards and pumice fragments, and sharply overlies the Camnant Mudstones.

The overlying Gilwern Volcanic Formation (Gwn) consists mainly of massive and graded, basic lapilli tuff south of the Wern-to Fault, and also north of the Howey Fault, but in the intervening central tract, extensively reworked, brachiopod-bearing, planar- and cross-bedded tuff and tuffaceous sandstone predominate. Fining-upwards sequences of mudstone with graded tuff beds cap the formation in the north and in the south, but are absent in the central tract. The northern mudstone sequence, which expands northwards to over 140 m in thickness in the adjacent Rhayader district, includes strata laterally equivalent to both the Carneddau and Llanelwedd Volcanic formations farther south. These lateral facies variations suggest the influence of syndepositional faulting. However, the sea floor relief created by numerous dacite lava domes extruded within the central tract, some possibly as much as a 100 m high, also appears to have had an important control on facies distribution.

The Carneddau Volcanic Formation (Cdu) is present only in the south of the inlier, extending from east of Llanelwedd quarries [SO 0750 5685] to the east of Cwmamliw. An acid ash-flow tuff, locally up to 35 m thick, forms the lower part of the formation to the south of the Wern-to Fault, but is only locally preserved to the north. The abundance of feldspar crystals in the basic lapilli tuff and tuff, distinguishes them from those of the underlying Gilwern Volcanic Formation. On the Carneddau, the pronounced and rapid thickness variations (5 to 80 m) reflect the underlying topography, created by the earlier dacite domes. Younger, flow-banded dacite lava domes overlie the crystal tuffs of the Carneddau Volcanic Formation at Caer fawr [SO 057 532] and at Caer Einion [SO 065 530], where pink staining and the highly amygaloidal nature of the top of the lavas suggests emergence and subaerial weathing.

The basaltic and andesitic lavas and breccias of the Llanelwedd Volcanic Formation (Ldd) (Plate 2) are also restricted to the south of the inlier; its main outcrop extends from the Llanelwedd quarries northwards to Maesgwyn and Upper House, where it onlaps the Carneddau Volcanic Formation to rest on the Gilwern Volcanic Formation. Shoreline erosion of the emergent basaltic and andesitic lava pile formed by the Llanelwedd Volcanic Formation created an irregular unconformity, now overlain by the Newmead Sandstone Formation (Nwd). The unconformity displays features interpreted as sea cliffs, stacks and wavecut platforms (Jones and Pugh, 1949). The Newmead Sandstone Formation is restricted to the south of the Cwmamliw Fault. Abrupt thickness changes across faults, notably the Carneddau and Newmead faults, suggest that these actively influenced contemporaneous erosion and deposition.

The Cwm-amliw Tuff Formation (Cat) is restricted to the north-west of the inlier, dying out abruptly to the south of Cwmamliw Farm. North of Maesgwyn [SO 0630 5645], it rests on mudstone of the Gilwern Volcanic Formation but farther south it rests on the Llanelwedd Volcanic Formation although a thin mudstone sequence intervenes locally. Fragments of acid ash-flow tuff, locally present at the top of the Newmead Sandstone, near Newmead Farm [SO 0566 5381], point to the two formations being partly equivalent, and provide evidence that the Cwm-amliw Tuff may have been reworked in the shallower settings that prevailed across southern parts of the inlier. The tuff marks the final phase of the Llanvirn volcanic activity at the Builth centre.

The Llanfawr Mudstones Formation (LrM) crops out along the western side of the inlier, abruptly succeeding the Newmead Sandstone in the south and the Cwm-amliw Tuff in the north. Rich trilobite and graptolite assemblages confirm the presence of the Llanvirn murchisoni and teretiusculus biozones and the early Caradoc gracilis biozone. Within the district, Caradoc strata are confined to a tract along the south-western edge of the inlier, between the Wye valley and Pencerrig Lake [SO 045 541]; to the north Caradoc strata are overstepped by Silurian rocks (Institute of Geological Sciences, 1977). Intrusive igneous bodies, principally of dolerite, are common throughout the inlier, but occur mainly within the Camnant and Llanfawr mudstones. Most of the igneous rocks are lenticular, sill-like bodies ranging up to 100 m in thickness and up to 1.5 km in length; dykes up to 80 m thick and 700 m in length are also present, notably to the east of Tan-lan [SO 0569 5476]. Jones and Pugh (1946, 1948a, b) interpreted dolerites in the Llanfawr Mudstones, south of Pencerrig Lake, as complex laccolithic intusions with associated feeder dykes and plugs. Country rocks adjacent to dolerites are baked for up to 5 m from the contact. The dolerites represent high-level intrusions into only partially lithified sediments. Those emplaced into the Camnant Mudstones are probably related to the same phase of magmatism as the overlying Builth Volcanic Group; those in the Llanfawr Mudstones, which include bodies intruded into rocks of gracilis Biozone age, relate to a subsequent magmatic episode that was, at least in part, of Caradoc age.

The oldest rocks of the Llanwrtyd Volcanic Inlier (Figure 2) are those of the Llanwrtyd Volcanic Formation (Lla), a succession of subaqueous acid ash-flow tuff interbedded with dark grey mudstone, tuffaceous siltstone, sandstone, conglomerate and debrite, with extrusive and intrusive basalt. Two thin units of ooidal ironstone are also present. The formation is over 600 m thick but the base is not exposed. The tuffaceous sediments were deposited largely from turbidity currents and debris-flows that originated on the unstable slopes of a nearby submarine volcano or volcanic island. Two prominent acid ash flow tuffs occur in the middle of the sequence, the late Llanvirn Kilsby Tuff Member (KiT) and the early Caradoc Nant Cerdin Tuff Member (NCT). The observed parts of the formation range in age from the late Llanvirn, (teretiusculus Biozone) to the early Caradoc (gracilis Biozone). Both zones are identified at the surface and in the Gilfach Farm No. 1 Borehole [SN 8759 4922] (Cave and Rushton, 1996). The uppermost part of the formation has not been dated, and may possibly range into the multidens Biozone.

The St Cynllo's Church Formation (SCC) is dominated by mudstone and marks the waning of volcanicity along the eastern margin of the Welsh Basin, as rifting gave way to regional thermal subsidence. The formation rests sharply on the Llanwrtyd Volcanic Formation, and comprises two members. The lower, Pistyll Gwyn Member (PiM) is 150 m thick, and is composed of mudstone that is dark grey, variably sandy, micaceous, poorly bedded and blocky. In places it is slumped or debritic in origin. Packets of thin- to thick-bedded turbidite sandstone are developed locally. The upper Sugar Loaf Member (SLM) is 250 m thick, and consists of finely cleaved, fissile, black, pyritic hemipelagic mudstone with abundant graptolites. To the east of the Garth Fault, where exposure is poor, the formation has not been subdivided. The St Cynllo's Church Formation is Caradoc in age. The gracilis/multidens biozonal boundary has not been identified, but graptolites at the clingani Biozone have been recovered from close to the base of the Sugar Loaf Member.

Ashgill basinal succession

The Ashgill succession that rests conformably on the St Cynllo's Church Formation marks a change to predominantly oxygenated bottom conditions, which prevailed over most of the Welsh Basin until the onset of the latest Ordovician postglacial marine transgression. The architecture of the basinal sequence is shown in (Figure 2). The early Ashgill (Pusgillian to Rawtheyan) Nantmel Mudstones Formation (Ntm) (up to 1500 m thick) comprise a slope apron facies, consisting of grey, turbiditic and hemipelagic burrow-mottled mudstone. Colour banding, phosphate nodules, and thin beds and laminae of sandstone are present locally. Up to three units of dark grey, anoxic facies mudstone are present in the upper part of the formation; in the Rhayader district to the north (Sheet 179) these mudstones have yielded graptolites that suggest the anceps Biozone. Small-scale lenticular units of coarse clastic deposits occur locally in the slope apron mudstones; these consist of thick-bedded, turbiditic, coarse-grained sandstone, pebbly mudstone and conglomerate, and include the Doldowlod Conglomerate Formation and the Bryn Nicol Formation. The Doldowlod Conglomerate Formation (Dd), (up to 300 m thick) locally contains abundant igneous clasts, as well as coral and shell fragments derived from the shelf to the east of the Tywi Lineament. The conglomerate is confined to an area west of the Nant-y-fedw Fault; flute casts on the base of some beds indicate a transport direction from the north-east, parallel to the fault, suggesting that local fault movements were an important control on sedimentation.

In the south-west of the district, the upper part of the Nantmel Mudstones pass laterally into a major sandstone-dominated sequence, the Bryn Nicol Formation (up to 300 m thick). The formation is broadly lenticular in outcrop, and consists of two main turbiditic pulses that were derived from a north-north-easterly direction. It lies adjacent to, and on the western side of, the Llanwrtyd Fault, and is thickest adjacent to the fault. The Bryn Nicol Formation, like the Doldowlod Conglomerate, is interpreted as one of a series of sandy and conglomeratic bodies that were deposited in, and confined by fault-bounded troughs, during a period of mid-Ashgill tectonism that affected the basin margin (Figure 5). The formation includes the Coed Ifan Facies (CIF), comprising thin- to medium-bedded sandstones, and the Pen Derlwyn Facies (PD) that also incorporates thick beds of massive, coarse-grained sandstone turbidite and debritic units of pebbly mudstone and shelly conglomerate containing a rich resedimented mid-Ashgill fauna. Interbedded mudstones in both facies include laminated hemipelagic units that are the lateral correlatives of those within the Nantmel Mudstones. The distribution of lithologies suggests that the Coed Ifan Facies represents a fringing facies deposited adjacent to sites receiving the high-concentration, coarse-grained turbidites of the Pen Derlwyn Facies.

Units of disturbed beds within the Nantmel Mudstones indicate intermittent, localised slope instability; at Llwyncus [SN 931 504] pebbly mudstones (debrites) contain blocks of calcareous mudstone with a derived, diverse, mid-Ashgill (Cautleyan to early Rawtheyan) shelly fauna.

The deposition of the early Hirnantian Yr Allt Formation (YA) marks a change in sedimentation that occurred as a result of the late Ordovician glacio-eustatic regression. The formation is up to 1200 m thick, and generally comprises an oxic slope apron facies of silt-laminated turbiditic mudstone, locally interbedded with packets of fine-grained turbidite sandstone. Bioturbation is sparsely developed, probably due to the increased rate of sediment supply from the adjacent shelf areas as sea level fell. Frequent slope failure of this prograding muddy wedge is recorded in the units of slumped and destratified strata (disturbed beds), which are a common feature of the formation and the dominant facies in the south-west. Lenticular and more persistent units of coarse-grained turbiditic sandstones and conglomerates (Plate 1) are thought to have been deposited in small submarine channels and as small lobes within the slope apron mudstones. West of the Glanalders Fault, a sequence of laterally persistent coarse-grained sandstone, in the upper part of the formation, record deposition from a series of high-concentration turbidity flows that were probably impounded within a trough adjacent to the fault.

Ashgill shelf succession

South and east of the Llanwrtyd Fault, the basinal Nantmel Mudstones interdigitate with the Tridwr Formation (Tri), a mid to outer shelf facies of mudstone with subordinate thin locally shelly, storm-generated sandstones, the thickest development (up to 1400 m) of which occurs in Crychan Forest. The successive intercalations of Nantmel Mudstones within the Tridwr Formation record a period of basin deepening, with the progressive south-eastward retreat of the shelf facies and the establishment of deepwater sedimentation across the area, which probably reached its acme during the late Rawtheyan. It is not clear whether this deepening was the result of eustatic sea level rise or intra-Ashgill tectonism, but it may have contributed to the anoxicity that led to the deposition of laminated hemipelagic units in the upper part of the Nantmel Mudstones.

In the Garth and Crychan Forest areas (Figure 6), the Nantmel Mudstones pass upwards and laterally eastwards into the thoroughly bioturbated, sandy mudstone and muddy sandstone of the Cribarth Formation (Cri), a progradational division that contains shelly fossils of Rawtheyan age. Locally at the top of the formation, the Penrhiwmoch Member (Prm) is distinguished as a thin unit with unburrowed siltstone and sandstone laminae. It marks a transition into the overlying Hirnantian sequence in which pronounced and rapid lateral facies changes reflect the influence of the late Ashgill glacio-eustatic regression. The basinal unbioturbated Yr Allt Formation of the Tywi Anticline passes eastwards into the Ciliau Formation (CF) comprising weakly burrowed mudstone and calcareous sandstone that is ooidal in places. Slumped and destratified disturbed beds (db), included in the Yr Allt Formation, are associated with this facies transition in both areas. In the Garth area, the mapped relationships between the two formations appear to pick out at least two separate progradational events, and also suggest that abrupt facies and thickness changes were accommodated by syndepositional normal faulting.

It has been estimated that, at its maximum, the late Ashgill glaciation caused global sea level to fall by as much 100 m. A pronounced disconformity at the base of the postglacial, Cwm Clyd Sandstone (p.16) records the resulting widespread emergence and associated erosion of the local Hirnantian succession. However, in the west of the Garth and Crychan Forest areas, a sequence of lenticular, wave-ripple cross-laminated sandstones, the Cwmcringlyn Formation (CgF), records deposition during this lowstand period and allows the local limits of the regression to be charted. In marked contrast, much of the earlier Ashgill and Caradoc succession deposited to the east of the Llandrindod–Pen-y-waun Fault Belt was stripped away. In the intervening Crychan Forest area, the pattern of overstep at the base of the Cwm Clyd Sandstone (Figure 2) clearly testifies to the active nature of the Crychan Fault Belt, either during or immediately prior to this episode. The reduced levels of bioturbation displayed within the Hirnantian succession is in marked contrast to both the underlying and overlying sequences. It may reflect very high rates of sedimentation associated with the glacio-eustatic regression and the consequential rapid and deep erosion of, still unconsolidated, earlier sediments, at a time when there was no land vegetation cover. Throughout the Welsh Basin, rocks of late Hirnantian age record the rise in sea level following the end of the Late Ordovician glaciation. This sea level rise reached its acme in the early Silurian (Rhuddanian), and for this reason latest Hirnantian facies normally form part of, or are contigious with, formations that are predominantly Silurian in age. It is therefore convenient to describe them with the Silurian rocks.

Silurian

Deep-water basinal rocks of Llandovery age crop out in three areas: to the north-west of the Tywi Anticline (Davies, 1926, 1928, 1933; Mackie and Smallwood, 1987), as synclinal and fault-bounded outliers along the axis of this anticline and adjacent to the Garth Fault. In contrast, the extensive Silurian succession which underlies much of the eastern part of the district, east of the Garth Fault, comprises shallower, ramp and shelf facies of Llandovery, Wenlock and Ludlow age. These are succeeded in the south of the district by Prídolí rocks of continental aspect (Old Red Sandstone). The Dulas Valley–Rock Park Fault can be used to subdivide this broad swathe of Silurian strata into a central and an eastern sequence. The former includes the Llandovery rocks of Garth (Andrew, 1925; Jones, 1947; Williams and Wright, 1981) and Crychan Forest, the latter comprising the northern part of the Llandovery Series international stratotype (Jones, 1949; Cocks et al., 1984). The eastern sequence flanks the Builth Volcanic Inlier and includes the Wenlock, Ludlow and Prídolí rocks of Mynydd Eppynt and the Wye valley (Elles, 1900; Straw, 1937, 1953).

Basinal succession

The lower part of this succession (late Hirnantian to early Telychian) (Figure 2); (Figure 7) comprises a mudstone-dominated slope apron sequence (Cwmere and Tycwtta Mudstones formations, and Claerwen Group), which overlies the Ashgill slope apron deposits of the Tywi and Rhiwnant anticlines and interfingers with coeval shelf sediments adjacent to the Garth Fault. The slope apron mudstones are punctuated by three, small-scale, easterly derived lenticular units of coarse-grained turbidites (Allt-y-clych Conglomerate Member, Caban Conglomerate Formation and Nant Brianne Formation).

The Cwmere Formation (CeF), at the base of the slope apron sequence west of the Garth Fault, comprises anoxic mudstone with scattered, very thin turbidite siltstone and sandstone. The Mottled Mudstone Member (MMb) is a thin pale grey unit of oxic facies, and is present at the base of the formation where it sharply overlies the dark grey mudstone of the Yr Allt Formation. The member contains late Hirnantian, persculptus Biozone graptolites, while the rest of the formation ranges from the late Hirnantian through the Rhuddanian into the earliest Aeronian. The coeval Tycwtta Mudstones Formation (Tyc) crops out on the eastern side of the Tywi Anticline, and represents a mixed oxic/anoxic facies deposited on the proximal part of the slope apron; it is up to 600 m thick and includes the Allt-y-clych Conglomerate Member (AcC), which is up to 65 m thick. The anoxic bottom conditions that prevailed during deposition of the Cwmere and Tycwtta mudstones formations resulted from the marked rise in sea level following the end of the late Ordovician glaciation.

The overlying Claerwen Group comprises predominantly oxic facies mudstone and is subdivided into the Derwenlas Formation and overlying Rhayader Mudstones Formation. A few thin units of anoxic facies mudstone punctuate the upper part of the Derwenlas Formation (DlF) and are of late Aeronian convolutus Biozone age. The oxic bottom conditions in evidence throughout much of the formation are the result of a mid Aeronian marine regression.

The overlying Rhayader Mudstones Formation (Rhs) consists predominantly of greenish grey, diffusely burrow-mottled mudstone with scattered thin units of anoxic facies mudstone. The base of the formation is defined by one such unit, the M. sedgwickii Shales (lhs), reflecting a widespread transgressive event. Abundant, very thin beds and laminae of turbidite siltstone and sandstone in the upper part of the formation comprise a silt laminated facies of early to mid turriculatus Biozone s.l. age. They may represent a fringing or overbank facies derived from the Nant Brianne Formation (p.15).

The coarse-grained turbidites of the Caban Conglomerate Formation are confined to the Rhiwnant Anticline and Gwesyn Syncline. Four mappable facies are recognised (Davies and Waters, 1994). The latest Hirnantian, Cerig Gwynion Grits facies (CyG) forms the south-western edge of an easterly sourced sandy lobe, the greater part of which is situated to the north-east in the adjacent Rhayader district. The facies is fringed by the thin-bedded Dyffryn Flags facies (Dfn). Three sequences of Caban Conglomerate facies (Cbn) are developed. They are numbered following the scheme used in the adjacent Rhayader district, where the facies is well developed and five sequences are present. In the Builth district, the oldest, thickest and most widespread, the second sequence, occupies a 6 km-wide channel, the north-eastern margin of which is just to the north of the district. Confined between levees of Dyffryn Flags facies and coeval slope apron mud and fringing facies, the second sequence occupied a channel along which high-concentration turbidity currents were funnelled to the more distal parts of the slope apron during the mid to late Rhuddanian. The other two sequences of Caban Conglomerate facies (fourth and fifth sequences) are locally present immediately below and above the M. sedgwickii Shales, respectively. The fifth sequence is overlain by the Gafallt Shales facies (Ga).

In the district, the Nant Brianne Formation (NBr) crops out in the hinge of the Doethie Anticline, and comprises thin to medium bedded turbidite sandstone and mudstone. It is defined around Llyn Brianne Dam [SN 793 483], to the west of the district, where the oldest part of the formation, a sequence of late Aeronian (convolutus Biozone), thick bedded and conglomeratic, turbidite sandstone and disturbed beds occupy a kilometre-wide belt. The base of the formation in the district is diachronous lying just above the M. sedgwickii Shales in the west, but within the early turriculatus Biozone s.l. (runcinatus or gemmatus Subzone) on the eastern shore of Llyn Brianne; the top may range locally into the utilis Subzone. The Nant Brianne Formation occupies the eastern edge of a confined corridor in the early Telychian slope apron mudstone (Rhayader Mudstones), along which sandy turbidites were transported to the more distal parts of the slope apron. It is similar in facies and age to the Devils Bridge Formation, a large-scale, sandy turbidite system that blanketed much of the distal slope apron in the Aberystwyth and Llanilar districts in the early Telychian. It is likely that the Nant Brianne Formation represents part of the feeder system for the Devils Bridge Formation (Davies et al. 1997). Slope apron deposition ended in the early Telychian when large-scale, southerly sourced, sandstone lobe turbidite systems entered the basin in response to tectonic uplift that is thought to mark the onset of Acadian orogenesis. This is reflected by the appearance of mud-dominated lobe-fringing facies of the Caerau Mudstones, and the overlying sand-dominated turbidite formations of the Cwmystwyth Grits Group.

The Caerau Mudstones (CaM) overlie the silt-laminated facies of the Rhayader Mudstones. They crop out on the northern limbs of the Rhiwnant and Doethie anticlines, and in the intervening broad, open synform. They are of late turriculatus Biozone s.l. age. The base is defined by the appearance of thick units of anoxic facies mudstone, which comprises approximately 50 per cent of the formation.

Only the lower part of the Cwmystwyth Grits Group is present in the district, where it is represented by the Rhuddnant Grits Formation, the laterally equivalent Doethie Formation and the overlying Glanyrafon Formation. The base of the group is defined by the first appearance of medium to thick beds of turbidite sandstone. In the Rhuddnant Grits these comprise tabular beds with a high mud content (high-matrix sandstones) deposited by dilute slurry-like debris flows (Davies et al., 1997). In the Doethie Formation, which replaces the Rhuddnant Grits southwards, the thicker beds comprise cleaner, commonly amalgamated sandstones with convolute lamination. The overlying Glanyrafon Formation (Glr) comprises thinly interbedded turbidite sandstone and mudstone, devoid of thick sandstone beds. The base of the Cwmystwyth Grits Group is diachronous, being late turriculatus Biozone s.l. (carnicus Subzone) in age in the Carreg yr Fran area [SN 808 588] and within the crispus Biozone in the Hadfre area [SN 802 545]. This reflects the fact that this sandstone lobe system sidelaps to the south-east (Davies et al., 1997).

Coeval with the deposition of the Telychian sandstone lobe systems, mud-dominated, oxic slope apron facies was deposited across the Tywi Anticline. This is the Dolgau Mudstones (Dgu), which are exposed on the western flank of the Twyi Anticline adjacent to the Nant y Fedw Fault and rest disconformably on the Ashgill Yr Allt Formation. They were deposited following a period of tectonically induced, mass-wasting along the Tywi Lineament in the early Telychian, and are of crispus Biozone age.

Shelf and ramp succession

West Of Dulas Valley Fault

In the Garth area and Crychan Forest, latest Hirnantian facies record the rise in sea level which followed the Hirnantian glacio-eustatic regression. They rest conformably on regressive earlier Hirnantian strata in the north-west, but a transgressive sandstone, the Cwm Clyd Sandstone (CCy), oversteps south-eastwards to rest unconformably on pre-Hirnantian strata (Figure 2); (Figure 8). The succeeding mudstone of the Garth House Formation (GHF) is overlain by latest Hirnantian to earliest Silurian strata. The fossiliferous sandy mudstone of the Bronydd Formation (BrF) coarsens upwards into muddy sandstone of the Crychan Formation (CcF). Mixed shelly and graptolite assemblages show these span much of the Rhuddanian (Figure 2). In the Garth area these divisions pass rapidly northwards into coeval mudstone of the Chwefri Formation (ChF) and these pass in turn into the basinal Tycwtta Mudstones Formation (p.14). The overall pattern of facies and thickness change in this area is consistent with basinward progradation of coarser, shallower facies across a rapidly subsiding sector of the basin margin, following the latest Hirnantian postglacial deepening (p.16).

The sandy mudstones of the Trefawr Formation (TrF) in the Garth area overlie the Crychan Formation and record a transgressive deepening in the late Rhuddanian, cyphus Biozone. The subsequent mid-Aeronian, eustatic fall in sea level influenced facies throughout the Welsh Basin. On and to the north of Comin-y-garth [SN 982 547], the green Ty Gwylim Sandstone Member (TyG), at the top of the Trefawr Formation, records deposition during this shoaling event, but to the south the top of the formation is marked by an erosional nonsequence increasing in magnitude southwards. Resting on this erosion surface, is the distinctive Tyncoed Sandstone Member (TyS), a transgressive quartzite containing abundant disarticulated, pentamerid brachiopod valves (Plate 3), at the base of the late Aeronian Comin Coch Formation (CoC). The overlying olive-green, pebble and granule-rich mudstone has yielded rare graptolites of the sedgwickii Biozone (Figure 2). The mudstone coarsens upwards into pebbly, shallower water sandstone of the Ty-mawr Formation (Tym), which crops out between Garth Bank [SN 945 505] and Hafod-yr-ancr [SN 980 534]. The burrow-mottled mudstone of the Cerig Formation (Cer) abruptly succeeds the Ty-mawr Formation in the south of the Garth area, and indicates a deepening event at the base of the Telychian succession. However, neither the Comin Coch nor the Ty-mawr formations are recognised north of Comin-y-garth, where an expanded Cerig Formation succeeds the Trefawr Formation conformably. Within this northern sequence, a distinctive unit of red mudstone is well exposed in the River Wye [SO 0068 5745] and volcanic tuff beds (bentonite) occur locally, for example at Cilfodeg [SN 9950 5595]. Rare graptolite assemblages from the upper part of the formation confirm the presence of the late Llandovery griestoniensis, crenulata and spiralis biozones. The overlying Dolfawr Mudstones (Dol) comprises a transitional division which spans the Llandovery–Wenlock boundary, and consists of strata deposited in both oxic and anoxic conditions. It is overlain by the pervasively laminated Builth Mudstones (BMd), deposited in an anoxic environment; these are the youngest strata present west of the Dulas Valley Fault and range into the mid-Wenlock rigidus Biozone.

East Of Dulas Valley Fault

The sequence ranges in age from mid Llandovery to Devonian (Figure 9); (Figure 10); early Llandovery (Rhuddanian and early Aeronian) strata are absent and an attenuated Cerig Formation rests unconformably on the Ordovician rocks of the Builth Volcanic Inlier. The Trecoed Sandstone Member (Trs) at the base contains both late Aeronian and early Telychian brachiopods. Calcareous, green, shelly mudstone, locally present in the Cerig Formation include the 'Acidaspis Limestone' (Jones, 1947), the probable correlative of the Dolfawr Mudstones farther west.

At the eastern edge of the district, on Aberedw Hill, the Builth Mudstones (BMd) span the Wenlock Series and range into the lower Ludlow scanicus Biozone. However, traced westwards and southwards, the contact with the overlying Llangammarch Formation (Llg) is strongly diachronous, so that to the south of Llangammarch, siltstone-striped mudstone of the Llangammarch Formation occupy the whole of the Wenlock. Thin units of slumped and destratified strata (disturbed beds) are present in the Builth Mudstones. A new assessment of graptolite biozones of the formation (Zalasiewicz and Williams, 1999) has shown that units regarded as separate slump sheets by Jones (1947) are faulted parts of a single, widespread horizon, now named the Caer-beris Member (Cbs). In its type section [SO 0310 5061], the Caer-beris Member rests on burrow-mottled Builth Mudstones of upper Wenlock nassa-ludensis Biozone age. However, to the west it rests on earlier lundgreni Biozone, laminated mudstone, and south of Rhosforlo [SN 9815 5100], it overlies Llangammarch Formation mudstones of similar age. In the south of the district, the Llangammarch Formation underlies and passes southwards into the late Wenlock Tirabad Formation (Tir) in which partially bioturbated mudstone, siltstone and sandstone indicate a shallower depositional setting. The distribution of Wenlock facies suggests an upward shoaling and northwards progradation, accompanied by slumping (Caer-beris Member). This pattern is consistent with the widely recognised Homerian regression or still-stand (Johnson et al., 1991).

Slumped and destratified mudstones (disturbed beds) dominate much of the Irfon Formation (IrF) in the central and eastern parts of the district, but lying between these and the older Caer-beris Member is a widely recognised, undisturbed sequence of nilssoni Biozone age, providing evidence for an earliest Ludlow deepening event. In contrast in the south of the district, the Irfon Formation is dominated by undisturbed facies, and this facies also overlies the main slumped sequence in the north. The base of the overlying Cwm Graig ddu Formation (CGd) is diachronous, younging northward. It consists of sparsely burrowed, very thinly bedded mudstone, siltstone and sandstone that form the lower part of a coarsening-upwards sequence, which is capped by bioturbated muddy siltstone and sandstone of the Aberedw Formation (AbF). The Aberedw Formation is probably latest leintwardinensis Biozone in age, and represents a Ludfordian shallowing event. Evidence for a subsequent deepening is provided by the siltstone-laminated mudstone at the base of the Fibua Formation (Fib), which sharply overlies the Aberedw Formation and contains graptolites of the Bohemograptus proliferation interval. The deepening was short-lived, and a further shoaling and progradation is recorded by the mudstone, siltstone and sandstone in the upper part of the Fibua Formation and the overlying Cae'r mynach Formation (Car). The Fibua Formation is restricted to the south of the district. Elsewhere the Cae'r mynach Formation overlies the Aberedw Formation.

The basal Přídolí Tilestones Formation (Til) rests sharply, but conformably, on the Cae'r mynach Formation and forms the local base of the Old Red Sandstone. The Tilestones Formation consists of laminated and low-angle cross-bedded, yellow-weathering sandstone, interpreted as a northward prograding barrier spit. The green calcretised siltstone of the Temeside Mudstone Formation (TSh) was deposited behind this barrier. The barrier sands are absent in the east of the district, where the Temeside Mudstone Formation rests conformably on the Cae'r mynach Formation.

The Raglan Mudstone Formation (Rg) is a red bed sequence, deposited on a broad mud-dominated alluvial plain. The formation forms featured moorland on Mynydd Eppynt; the scarps are composed of sandstone units or immature calcrete profiles in siltstone (Plate 4). Sandstones are most abundant in the upper half of the formation, but in the easternmost part of the district the basal 200 m contains abundant red and green sandstones. The Townsend Tuff Bed (TWT) occurs 260 m below the top of the formation and is 1.75 m thick. It comprises three airfall tuffs each separated by porcellaneous green mudstone. The top of the lowest porcellaneous mudstone exhibits distinctive faecal pellet and burrow casts. The Bishop's Frome Limestone (BFL) that forms a prominent mature calcrete at the top of the formation elsewhere in south Wales is thin and locally absent in the district.

Devonian

The base of the Devonian in south Wales is thought to lie within the uppermost part of the Raglan Mudstone Formation.

The St Maughans Formation (SMg) forms well featured ground on Mynydd Eppynt. It consists of intraformational conglomerate, sandstone and mudstone in stacked fining-pwards cycles of fluvial origin. Both calcrete and sandstone are more abundant than in the underlying Raglan Mudstone Formation.

Structure and metamorphism

The structure of the Builth district is characterised by widespread folding, faulting and cleavage development, which formed largely during the Caledonian orogenic cycle (Cambrian to mid Devonian). During this period, the south-eastern margin of the Welsh Basin was subject to cycles of subsidence and inversion. The structural evolution culminated in latest Silurian to mid Devonian times with an episode of regional deformation, the Acadian Orogeny, which resulted from the terminal collision of the palaeocontinental plates of Avalonia and Laurentia.

Pre-Acadian tectonism in the district is represented by syndepositional fault movements mainly along the north-east- to south-west-trending faults, which run through the centre of the district (Figure 1). The volcanic centres at Builth and Llanwrtyd were probably located along active faults along the basin margin and record a period of regional crustal extension and rifting during the mid Ordovician (Kokelaar, 1988).

The earliest evidence for such movements is observed in the Llanvirn rocks of the Builth Volcanic Inlier. Several periods of syndepositional fault activity can be demonstrated by the rapid thickness and facies changes in the Gilwern Volcanic Formation and Newmead Sandstone across east–west-orientated faults, which also influenced the form of the unconformity at the base of the Newmead Sandstone.

In the basin sequence, the coarse clastic units of the Bryn Nicol and Doldowlod Conglomerate formations, confined within the mid Ashgill slope apron succession of the Tywi Anticline, provide evidence of contemporary movements on the Llanwrtyd and Nant y fedw faults, respectively. Intra-Ashgill movements along the basin margin are demonstrated in Crychan Forest by the overstep beneath the transgressive Cwm Clyd Sandstone. The degree of overstep increases markedly across individual strands of the Crychan Fault Belt. In the Garth area, the dramatic basinwards thickening of the late Hirnantian regressive sequence (Yr Allt and Ciliau formations) across a series of east–west faults, may record this same period of syndepositional fault activity.

Early folds within the Builth Volcanic Inlier, truncated by the unconformity at the base of the Cerig Formation may have been imposed at any time between the Caradoc and the latest Aeronian, though it is likely that they are also partly intra-Ashgill in age. This deformation was probably associated with fault movements along the Pontesford Lineament. Evidence of intra-Telychian movements are widespread in the Welsh Basin. They are manifested along the Tywi Anticline by a period of submarine mass-wasting in which up to a kilometre of late Ashgill and early Llandovery strata were locally removed. In the adjacent Rhayader district, the degree of overstep beneath the Dolgau Mudstones disconformity changes across fault strands of the lineament, demonstrating that the trigger for the mass-wasting was tectonic (Davies et al., 1997). On the shelf, the Builth Volcanic Inlier was sustained as a high during the Telychian by movements on fault strands of the Pontesford Lineament, in particular the Llandrindod–Pen-y-waun Fault Belt.

Slumping during the Wenlock and early Ludlow, in mid to distal ramp settings, suggests oversteepened slopes resulting nfrom penecontemporaneous fault activity along the Pontesford Lineament.

Subsequent movements are attributed to the Acadian Orogeny, during which uplift of the entire basin occurred, strata were folded and cleaved, faults were reactivated and new ones were propagated.

Acadian folds are developed in several orders of magnitude across the Welsh Basin. The largest, first order, structure is the Tywi Anticline, which is complementary to the Central Wales Syncline of the Llanilar and Rhayader districts. In detail, it comprises a number of juxtaposed lower order antiforms that have their corresponding synforms excised along faults parallel to the fold axes. Axial planes dip steeply to the north-west. However, the distribution of Ordovician strata within the hinge-zone of the Tywi Anticline owes as much to pre-Acadian faulting and subsequent submarine mass-wasting, as it does to Acadian compression.

Second order folds preserved within the district are the en échelon Rhiwnant and Doethie anticlines to the north-west of the Tywi Anticline. These are generally open structures, with axial planes that dip steeply towards the north-west. They are separated by the Gwesyn Syncline.

Lower order folds are commonly exposed on the north-west-facing limbs of second order structures. They are generally open to tight with steeply north-west-dipping axial planes, shallow plunging axes and verge toward the south-east. Those that occur in the broad synclines are locally neutral to divergent. Lower order folds are strongly controlled by lithology, and are most widespread in the interbedded sandstone and mudstone sequences.

The south-east of the district is largely unaffected by Acadian folding, although immediately adjacent to the Dulas Valley Fault, strata are steeply dipping and define a monoclinal synform shallowing toward the south-east.

A closely spaced to penetrative cleavage is pervasively developed in the mudstone-dominated divisions to the north-west of the Tywi Anticline and is ubiquitously steeply dipping toward the north-west. It is commonly present within the Ordovician strata of the Tywi Anticline, but disappears over a broad zone to the east, and is normally absent from strata east of the Dulas Valley Fault.

Acadian faults include the array of anastomosing north-east to south-west-trending structures preserved within the centre of the district. Though many of these had an earlier history of movement, they were undoubtedly reactivated during the Acadian event. Typically, they show variable vertical offsets, but locally strike-slip displacements can be inferred. The Church Stretton Fault Zone is known from evidence outside the district to have had a long history of movement. It exhibits a southerly downthrow in the district. Approximately north to south-trending faults, with small normal displacement, are present in the north-west and south-east of the district. These cross-cut Acadian folds and cleavage, but are considered to have been initiated during the latter part of the event.

Selective reactivation of faults within the district is likely to have occurred during subsequent Variscan (late Carboniferous to early Permian) and Alpine (Cainozoic) orogenic deformations.

The pattern of low-grade metamorphism in the district is indicated by the Kubler Index (Figure 11) and reflects the growth of illite in response to sedimentary burial, tectonic thickening and accommodation of strain through cleavage formation (Kemp and Merriman, 2002). Lowest grade, late diagenetic zone assemblages, reflecting burial to a depth of around 4 to 5.5 km, are confined to the Tywi Anticline and the late Silurian to Early Devonian strata. Highest grade, high-anchizone or epizone assemblages, reflecting burial to a depth of around 6 to 8 km, are confined to the Ashgill to early Telychian strata around the Rhiwnant Anticline, where thicker, basinal sedimentary sequences were deposited and penetrative slaty cleavages formed during deformation.

Quaternary

During the Pleistocene, global climatic change brought about a succession of ice ages that affected much of the British Isles. Throughout this period the action of glaciers was the main erosional and depositional agent responsible for modifying the landscape into its present form.

Prior to the Pleistocene glaciations, the landscape of the district is thought to have comprised a series of platforms at different levels, locally preserved today within the Cambrian Mountains (Brown, 1960; Potts, 1968). These represent former planation surfaces that became isolated as a result of river down-cutting during Cainozoic (Tertiary) regional uplift (Dobson and Whittington, 1987).

Ice has modified this landscape by changing drainage patterns, over-steepening and over-deepening valleys and depositing glacial material in valleys and hollows. Following glaciation, periglacial processes modified many of the surface glacial deposits. Subsequently, modern alluvial sediments and peat were deposited during the Holocene.

The landforms and glacial deposits preserved in the district were produced by the Late Devensian glaciation, which is thought to have reached its acme around 20 000 BP (Campbell and Bowen, 1989). Deglaciation of the district was largely complete by 14 000 BP. During the glaciation, ice accumulated in the Cambrian Mountains forming the Central Wales Ice Sheet. In the district, it is thought to have flowed southward and accumulated in the topographic depression formed by the Irfon and Ithon valleys. From this area, ice may have escaped south-westward across the col at the Sugar Loaf into the Tywi valley, eastward through the low ground south-east of Builth Wells into the Wye valley and across Mynydd Eppynt. In the latter area, it may have merged with an ice mass flowing north-west from the Brecon Beacons (Dwerryhouse and Miller, 1930).

The main deposit of the Late Devensian glaciation is Till (glacial diamiction). It is most widespread in the broad depression occupied by the Irfon and Ithon river systems. The presence of till across the whole district suggests that the region was entirely covered by ice during glaciation. Till varies from gravelly, sandy, silty clay to sandy, clayey gravel. Clasts range up to boulder size, and two types are recognised. Red till contains clasts of Old Red Sandstone and commonly has a sandy matrix (Plate 5). It is restricted to Mynydd Eppynt and the area around Tirabad. Blue-grey till comprises stiff gravelly sandy clay and contains only Lower Palaeozoic clasts.

Throughout the district, tills have locally been modified by downslope movement under periglacial conditions. This has led to lowered slope angles and imparted a slope-parallel clast fabric to some tills. Solifluction terraces in till are also common, where it has accumulated in valley floors and has been incised by rivers.

During deglaciation, Glaciofluvial Icecontact Deposits and Hummocky Glacial Deposits were deposited principally in the Wye, Irfon and Ithon valleys adjacent to the retreating ice. The ice-contact deposits comprise sand and gravel and clayey gravel, which occur as irregular terrace-like features on the lower slopes of valleys. They probably represent degraded kame terraces. Hummocky glacial deposits occur as large mounds and irregular ridges of kamiform or morainic origin. They are more heterolithic and include bedded sands and gravels, clay-bound gravels and local lenses of till. As the ice withdrew, Glaciofluvial Sheet Deposits were deposited in the Wye valley and its tributaries as spreads of sand and gravel along the valley bottom. Subsequent postglacial incision has left them as a series of high terraces above the present river. Glaciolacustrine Deposits occur west of Cilmery and comprise clay and silt deposited in a small drift-dammed glacial lakes. They also occur beneath peat in numerous large kettleholes in the north-east of the district.

Immediately following ice retreat, seasonal freezing and thawing were widespread under periglacial conditions. Solifluction, the downslope movement of material by frost creep or saturated flow, reworked earlier glacial deposits to form Head, comprising gravelly sandy clay. Head Gravel (nivation scree) comprises stratified gravel composed of angular mudstone fragments with a sparse silty clay matrix. It is common in steep-sided, craggy valleys such as the Irfon, north of Abergwesyn (Front Cover). In the district, head deposits have only been recorded where they are of a significant thickness and have a topographic expression. Talus (Scree) deposits are present on many of the steeper valley sides, particularly in the Cambrian Mountains; they commonly grade into head gravel and are not distinguished from it.

The modern drainage pattern was established during the Holocene. Initially, the main river valleys were occupied by broad braid plains characterised by a series of major trunk channels, large bars and eyots. Following repeated river down-cutting in response to postglacial regrading and isostatic readjustment, these complex braidplain surfaces are now preserved as River Terraces, of which up to two levels are recognised in the Wye, Ithon and Irfon valleys. Today these rivers and their tributaries follow an entrenched, meandering course, locally flowing through shallow gorges incised in rock, and flanked by only narrow and discontinuous tracts of Alluvium. A ?Neolithic standing stone on the first terrace of the Wye near Newbridge suggests that, here, the earlier braided system had been abandoned prior to about 4000 years ago. Alluvial Fans developed where steeper graded tributaries emerged from their channels to intersect main river valleys. Small areas of Lacustrine Alluvium, some of which overlie earlier glaciolacustrine deposits, developed in lakes, and ponds formed within enclosed hollows or kettleholes. As vegetation was re-established under more temperate climactic conditions, Peat deposits accumulated in areas of restricted drainage in the upland areas of the Cambrian Mountains and Mynydd Eppynt and elsewhere in kettleholes. Scattered Landslips are present, particularly where thick deposits of till are present on oversteepened valley sides and have been undercut by meandering rivers and streams.

During the Holocene, the landscape has been modified by human activity. Although the region has not been significantly industrialised, artificial (man-made) deposits of limited extent can be found. Made Ground composed of tipped material of variable composition and thickness occurs as quarry spoil, and the castle mound and former gas works in Builth Wells. Made ground of limited thickness and extent is likely to be encountered underlying most settlements.

Chapter 2 Applied geology

Earth science factors have a significant influence on land-use, planning and development. Consideration of earth science issues early in the planning process can help ensure that site and development are compatible, and that appropriate mitigation measures are taken prior to development. Exploitation of natural geological resources frequently conflicts with agricultural landuse, pre-existing developments and the environment. Potential geological hazards may present a public health risk or require costly remediation. Engineering ground conditions and designated sites of geological conservation strongly influence the location and design of any new development.

Applied aspects of the geology of the district are also dealt with via a set of thematic maps prepared for the Welsh Assembly Government and designed to inform land-use planners of relevant geological considerations.

Mineral resources

The main historical resources of the region are metalliferous minerals of the Central Wales Mining Field (Ball and Nutt, 1976; Hall, 1971). This comprises an array of fault-controlled mineralised veins hosted by Ashgill and Llandovery strata, west of the Dulas Valley Fault. The main period of extraction was from 1850 to 1870, but the last mine ceased operating in 1927. Six mines operated in the district, producing lead, copper and zinc ores. A single iron ore mine was also worked at Cwm Dyfnant [SN 9138 4214], where pyrite was extracted. The potentially polluting legacy of metalliferous mining in the district is an important planning consideration.

Hard-rock for aggregate is currently extracted from two quarries in the district. At Cribarth quarry [SN 9550 5210] high quality roadstone is produced from the Cribarth Formation and the Cwm Clyd Sandstone. At Llanelwedd quarry [SO 0550 5220], in the Builth Volcanic Inlier, aggregate is produced from basalt, andesite and tuff. Potential hard rock resources include the thick bedded sandstones and conglomerates of the Cerig Gwynion Grits facies which have been extracted commercially in the Rhayader district, the Caban Conglomerate facies, the Doldowlod Conglomerate, Bryn Nicol and Yr Allt formations. Other potential resources include the tuffs and dolerites in the Llanwrtyd Volcanic Inlier. Mudstone from the St Cynllo's Church, Cwmere and Builth Mudstones formations and parts of the Nantmel Mudstones, Rhayader Mudstones, Caerau Mudstones and Caban Conglomerate formations are pyritic. On weathering these may cause heave and concrete attack, and are generally unsuitable as aggregate or fill.

Building stone for local use was supplied by numerous small quarries throughout the district. Buildings in the centre and north-west of the district tend to be constructed of Ordovician and Llandovery sandstone and mudstone. In marked contrast, buildings in the south-east of the district are commonly constructed with flags of Llangammarch, Irfon and Cwm Craig Ddu formations. The thin flaggy sandstones of the Tilestones Formation were formerly worked for roofing tiles. Thinly bedded mudstones of the Yr Allt, Nantmel and Derwenlas formations have been worked locally for roofing slate. The slate mine at Penygeulen [SN 9050 5378] is the only one at which underground workings were developed (Richards, 1991).

Sand and gravel is exploited from a few scattered pits in the Wye valley. The principal resources are hummocky glacial deposits, glaciofluvial deposits and the river terraces of the Wye, Ithon and Irfon valleys. The heterogeneous nature of the hummocky glacial deposits and the variable clay content of the glaciofluvial ice contact deposits suggests that they would be of poor to intermediate quality. The glaciofluvial sheet deposits and river terrace deposits are of better quality, but the former do contain large boulders up to a metre in diameter. The quality of these resources may be further limited by the relatively high proportion of weak, local mudstone clasts.

Peat occurs in kettleholes and is widespread in the upland areas. It has been used as fuel in the past.

Water resources

The principal water resource of the district is provided from surface water. High annual rainfall over the Cambrian Mountains combined with narrow, deeply incised glaciated valleys provides an ideal catchment area. Within the district, Llyn Brianne reservoir, completed in 1973, provides much of the public water supply for the Swansea district.

Groundwater is not abstracted for public supply. However, private supply boreholes and springs provide water for isolated farms, and recent studies in the region have shown that modest supplies can be obtained from groundwater in fractured and weathered solid rocks in the near surface zone and from permeable superficial deposits (Robins et al., 2000).

Natural mineral waters were developed in the early 19th century for therapeutic purposes, in the 'spa towns' of Builth Wells, Llangammarch Wells, and Llanwrtyd Wells. Although this contributed significantly to the development of the region, usage has declined through the 20th century. The springs and wells are of moderately saline, iron-rich, sulphurous and barium-rich compositions, and are thought to be sourced from the host Ordovician and Silurian strata by deep circulation of meteoric water through the fracture system of the Tywi Anticline and Pontesford Lineament (Edmunds et al., 1998).

Potential geological hazards

Migration of leachate from the underground workings of former metalliferous mines presents a significant pollution potential and may lead to contamination of local surface or groundwater resources and alluvial sediments. Similarly, runoff during heavy rainfall, or due to poorly planned remediation or exploitation of mine tips, may also allow soluble contaminants or contaminative sediment to enter surface waters. Contamination of groundwater by toxic leachate from areas of worked or made ground is also a potential hazard. These include poorly lined landfills, agricultural waste-disposal sites and active or former industrial sites such as sewage works, gasworks, gravel pits, quarries and railway sidings.

River and stream floodplains within the district are susceptible to regular flooding. Of particular risk are the Wye, Irfon and Ithon. An indication of those areas which are prone to regular flooding is given by the extent of active floodplain deposits, shown as alluvium on the map (Sheet 196 Builth Wells). However, river terraces, notably parts of the complex first terrace of the river Wye, between Newbridge and Builth Wells, are also susceptible to flooding. The extent of flooding in the Wye valley is partly regulated by the Elan Valley and Claerwen reservoirs in the adjacent Rhayader district. Areas lying outside the mapped limits of alluvium and river terrace deposits may also be at risk during anomalously large river floods, or to flooding caused by blocked drains and culverts. Areas regarded as at risk of flooding are also shown on Environment Agency maps. Within the district, gas emissions represent a hazard in areas associated with the accumulation of methane or radon. Methane is likely to be generated by decomposition of material in landfill sites (e.g. to the south of Rhosforlo [SN 981 506]) and unconsolidated organic-rich deposits such as peat. It is toxic, an asphyxiant and explosive in high concentrations. Methane is less dense than air, is capable of migrating through permeable strata, and accumulating in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions represent a significant hazard, risk can be mitigated through the correct design of landfill and developments in 'at risk' areas. Radon is a naturally occurring ionising gas produced by radioactive decay of uranium, which is present in small quantities in all natural rocks and soils. Radon may also accumulate in poorly ventilated spaces and gives rise to an elevated risk of cancer of the respiratory tract. In areas at risk of radon accumulation, protective measures for new developments and remediation on existing homes and workplaces should be carried out. Advice about radon and its associated health risks can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire, OX11 0RQ.

Slope instability, typically manifested as landslip, is locally present in the district. About 50 landslips were identified during the survey and most are in drift, especially till. The main trigger is toe erosion by water courses. Although most existing landslips in the district are located in rural areas, development pressures for housing or improved infrastructure increase the possibility of encountering them, or creating the conditions where land may become unstable. The most effective strategy for dealing with unstable ground relies on recognition of problem areas in advance so that suitable preventative or remedial measures can be employed.

Engineering ground conditions

Knowledge of ground conditions is a primary consideration for identifying land suitable for development, and underpins cost-effective design. Engineering ground conditions vary depending on the physical and chemical properties of the local materials, topography, behaviour of groundwater and surface water, and the nature of past and present human activity. The most significant development problems likely to be encountered in the district are due to the variability of natural superficial deposits, weathering of solid rocks and landslips. These can be effectively dealt with by obtaining adequate information, including properly focussed site investigation, to confirm the properties of individual sites.

The solid rocks have high bearing capacities, except in the weathered zone. Pyritic mudstones, notably in the St Cynllo's Church, Cwmere, Caerau Mudstones and Builth Mudstones formations may cause heave and concrete attack.

Glaciolacustrine Deposits and Peat have low bearing capacities and can give rise to moderate settlements. Till and Hummocky Glacial Deposits have moderate bearing capacity but are highly variable. Head, Head Gravel, Alluvial Fan Deposits and Alluvium, all have low to moderate bearing capacities with moderate to high settlements. Although glaciofluvial and River Terrace Deposits embrace a variety of foundation conditions, they generally have high bearing capacities. Artificial ground is highly variable and may include contaminated land requiring remediation.

Geological conservation

The geological heritage of the district forms a resource for tourism, education and scientific research, and is also a key issue in planning and development. Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). These are statutory designated conservation sites which have some protection under the Wildlife and Countryside Act 1981. Within the district there are 14 SSSIs, including sites designated for palaeontological, structural or stratigraphical importance. Further information on the extent and designation of SSSIs and RIGS can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd LL57 2LQ. Special features of interest, being considered for notification as SSSIs, are described in the Geological Conservation Review Series, published by the Nature Conservancy Council.

Information sources

Further geological information held by the British Geological Survey relevant to the district is listed below. In addition to published books and reports (see References) this includes published and unpublished maps, photographs, documentary and material collections.

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Regional Geologist, Integrated Geological Surveys (South), BGS, Keyworth.

Searches of indexes to some of the BGS collections can be made on the Geoscience Data Index system available online. Maps, books and other publications are listed in the BGS Catalogue of geological maps and books and digital data in the volume Britain beneath our feet; both are available on request or may be viewed online. Maps and other publications can be purchased through the BGS sales desk or online. (See back cover for addresses.) BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS web site or at: British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

Maps

The district was originally surveyed on a scale of one inch to one mile by W T Aveline, H T de la Beche, T E James, W E Logan, J Phillips, A C Ramsay and J Rees (junior), and the results published in 1845, 1848 and 1850, as part of [Old Series] sheets 41, 42, 56 and 57. A small area along the northern boundary was surveyed at the 1:10 000 scale by C J N Fletcher, R A Waters, D Wilson, D G Woodhall and J A Zalasiewicz in 1987–8 as part of the survey of the 1:50 000 Sheet 179 Rhayader. The remainder was surveyed at the 1:10 000 scale by J R Davies, C J N Fletcher, W T Pratt, D I Schofield, R A Waters, D G Woodhall, P R Wilby and D Wilson at intervals between 1993 and 2001.

The original 1:63 360 geological maps are not available for purchase. Copies can be consulted at the BGS library, Keyworth. Unpublished 1:25 000 scale geological maps, listed below, are available for public consultation in the BGS libraries in Edinburgh and Keyworth, and the London Information Office in the Natural History Museum, South Kensington. Coloured print-on-demand copies are available for purchase from the BGS sales desk.

Results of the Geochemical Baseline Survey of the Environment (G-BASE) are published in atlas form. The geochemical data, with location and site information, are available as hard copy for sale or in digital form under licensing agreement. The coloured geochemical atlas is also available in digital form (on CD-ROM or floppy disk) under licensing agreement. BGS also offers a client-based service for interactive GIS interrogation of G-BASE data.

Groundwater vulnerability maps are published by the Environment Agency from data commissioned from The Soil Survey and Land Research Centre and BGS, and are also available from The Stationery Office (020 7873 0011).

Map Surveyor † Date
SO 04 RAW, DIS, PRW 1995, 1999–2000
SO 05 RAW, DGW, JRD, DIS 1988, 1993, 1995, 1999–2000
SN 95 CJNF, DW, JRD, WTP, RAW, DIS 1988, 1993–1996, 2000–2001
SN 94 JRD, DIS, RAW, PRW 995, 1999–2000
SN 85 JAZ, CJNF, DW, WTP, DIS 1986–1988, 1993–1994, 1996,1998–2000
SN 84 WTP, DIS, RAW, DW, JRD 1994, 1997–2000
SN 75 (part of) CJNF, DIS 1987, 1998
SN 74 (part of) DW, DIS 1998–2000
† see page 31

Books and reports

References

Most of the references listed below are held in the Library of the British Geological Survey, Keyworth, Nottingham. Copies of the references may be purchased from the Library, subject to current copyright legislation. Bibliographical services and catalogue search facilities are available on line at: geolib.bgs.ac.uk

Most of the references listed below are held in the Library of the British Geological Survey, Keyworth, Nottingham. Copies of the references may be purchased from the Library, subject to current copyright legislation. Bibliographical services and catalogue search facilities areavailable on line at: geolib.bgs.ac.uk

Andrew, G. 1925. Llandovery rocks of Garth (Breconshire). Quarterly Journal of the Geological Society of London, Vol. 81, 389–406.

Ball, T K, and Nutt, M J C. 1976. Preliminary mineral reconaissance of Central Wales. Report of the Institute of Geological Sciences, No. 75/14.

Bevins, R E, Kokelaar, B P, and Dunkley, P N. 1984. Petrology and geochemistry of Lower to Middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition. Proceedings of the Geologists' Association, Vol. 95, 337–347.

Brown, E H. 1960. Relief and drainage of Wales. (Cardiff: University of Wales Press.)

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

Cave, R, and Rushton, A W A. 1996. Llandeilo (Ordovician) Series in the core of the Tywi anticline, Llanwrtyd, Powys, U K. Geological Journal, Vol. 31, 47–60.

Cocks, L R M, Woodcock, N H, Rickards, R B, Temple, J T, and Lane, P D. 1984. Llandovery Series of the type area. Bulletin of the British Museum (Natural History). Geology series, Vol. 38, 131–82.

Davies, J R, and Waters, R A. 1994. The Caban Conglomerate and Ystrad Meurig Gritsformations — nested channels and lobe-switching on the mud-dominated latest Ashgill to Llandovery slope-apron of the Welsh Basin, Wales, U K. 184–193 in Atlas of deep waterenvironments; architectural style in turbidite systems. Pickering, K T, Hiscott, R N, Kenyon, N H, Ricci Lucchi, R, and Smith, R D A (editors). (London: Chapman and Hall.)

Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G, and Zalasiewicz, J A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).

Davies, K A. 1926. Geology of the country between Drygarn and Abergwesyn (Breconshire). Quarterly Journal of the Geological Society of London, Vol. 82, 436–64.

Davies, K A. 1928. Contributions to the geology of central Wales. I. Notes on the geology of the southern portion of central Wales. II. The geology of the country between Rhayader (Radnorshire) and Abergwesyn (Breconshire). Proceedings of the Geologists' Association, Vol. 39, 157–68.

Davies, K A. 1933. Geology of the country between Abergwesyn (Breconshire) and Pumpsaint (Carmarthenshire). Quarterly Journal of the Geological Society of London, Vol. 89, 172–201.

De la Beche, H T. 1846. On the formation of the rocks of south Wales and south western England. Memoir of the Geological Survey of Great Britain, Vol. 1.

Dobson, M R, and Whittington, R J. 1987. Geology of Cardigan Bay. Proceedings of the Geologists' Association, Vol. 98, 331–53.

Dwerryhouse, A R, and Miller, A A. 1930. Glaciation of the Clun Forest, Radnor Forest, and some adjoining districts. Quarterly Journal of the Geological Society of London, Vol. 86, 96–126.

Edmunds, W M, Robins, N S, and Shand, P. 1998. The saline waters of Llandrindod and Builth, Central Wales. Journal of the Geological Society of London, Vol. 155, 627–37.

Elles, G L. 1940. Stratigraphy and the faunal succession in the Ordovician rocks of the Builth–Llandrindod Inlier, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 95, 385–445.

Elles, G L. 1900. Zonal classification of the Wenlock Shales of the Welsh Borderland. Quarterly Journal of the Geological Society of London, Vol. 53, 370–414.

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

Hughes, R A. 1989. Llandeilo and Caradoc graptolites of the Builth and Shelve inliers. Monograph of the Palaeontological Society, London, No. 577.

Institute of Geological Sciences. 1977. Llandrindod Wells Ordovician Inlier. Solid. Parts of 1:25 000 sheets S O 05, 06, 15 and 16. Classical areas of British geology. (Southampton: Ordnance Survey for Institute of Geological Sciences.)

Johnson, M E, Kaljo, D, and Rong, J-Y. 1991. Silurian eustacy. In The Murchison Symposium: proceedings of an international conference on the Silurian System, Bassett, M G, Lane, P D, and Edwards, D (editors). Special Papers in Palaeontology, Vol. 44, 145–163.

Jones, O T. 1947. Geology of the Silurian rocks west and south of the Carneddau range, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 103, 1–36.

Jones, O T. 1949. Geology of the Llandovery district. Part II: the northern area. Quarterly Journal of the Geological Society of London, Vol. 105, 43–63.

Jones, O T, and Pugh, W J. 1941. Ordovician rocks of the Builth district. A preliminary account. Geological Magazine, Vol. 78, 185–191.

Jones, O T, and Pugh, W J. 1946. The complex intrusions of Welsfield, near Builth Wells, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 102, 157–188.

Jones, O T, and Pugh, W J. 1948a. A multilayered dolerite complex of laccolithic form near Llandrindod Wells, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 104, 43–70.

Jones, O T, and Pugh, W J. 1948b. The form and distribution of dolerite masses in the Builth–Llandrindod Inlier, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 104, 71–98.

Jones, O T, and Pugh, W J. 1949. An early Ordovician shoreline in Radnorshire, near Builth Wells. Quarterly Journal of the Geological Society of London, Vol. 105, 65–99.

Kokelaar, B P, Howells, M F, Bevins, R E, Roach, R A, and Dunkley, P N. 1984. Ordovician marginal basin of Wales. 245–269 in Marginal basin geology, volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins. Kokelaar, B P, and Howells, M F (editors). Geological Society of London Special Publication, No. 16.

Kemp, S J, and Merriman, R J. 2002. Meta-morphism of the Lower Palaeozoic rocks of the Builth Wells district, Wales, 1:50k Sheet 196. British Geological Survey Internal Report, I R/02/025.

Kokelaar, P. 1988. Tectonic controls on Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society of London, Vol. 145, 759–775.

Lapworth, C. 1880. On the geological distribution of the Rhabdophora. Annals and Magazine of Natural History (Series 5). Vol. 5, 45–62.

Mackie, A H, and Smallwood, S D. 1987. A revised stratigraphy of the Abergwesy–Pumpsaint area, mid-Wales. Geological Journal, Vol. 22, 45–60.

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

Potts, A S. 1968. Glacial and periglacialgeomorphology of central Wales. Unpublished PhD thesis, University of Wales (Swansea).

Richards, A J. 1991. A gazeteer of the Welsh slate industry. (Llanrwst: Gwasg Carreg Gwalch.)

Robins, N S, Shand, P, and Merrin, P D. 2000. Shallow groundwater in drift and Lower Palaeozoic bedrock: the Afon Teifi valley in west Wales. 123–131 in Groundwater in the Celtic regions: studies in hard rock and Quaternary hydrogeology. Robins, N S, and Misstear, B D R. (editors). Geological Society of London, Special Publication, No. 182.

Stamp, L D A, and Wooldridge, S W. 1923. The igneous and associated rocks of Llanwrtyd (Brecon). Quarterly Journal of the Geological Society of London, Vol. 79, 16–46.

Straw, S H. 1953. Silurian succession at Cwm Craig Ddu (Breconshire). Liverpool and Manchester Geological Journal, Vol. 1, 208–19.

Straw, S H. 1937. Higher Ludlovian rocks of the Builth district. Quarterly Journal of the Geological Society of London, Vol. 93, 406–56.

Williams, A, and Wright, A D. 1981. The Ordovician–Silurian boundary in the Garth area of southwest Powys, Wales. Geological Journal, Vol. 16, 1–39.

Woodcock, N H, and Gibbons, W. 1988. Is the Welsh Borderland Fault System a terrane boundary? Journal of the Geological Society of London, Vol. 145, 915–923.

Zalasiewicz, J, and Williams, M. 1999. Graptolite biozonation of the Wenlock Series (Silurian) of the Builth Wells district, central Wales. Geological Magazine, Vol. 136, 263–283.

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.

(Index map)

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.

(Index map)

Figures and plates

Figures

(Figure 1) Simplified solid geology and main structural features of the district. AgF Abergwesyn Fault; CaF Cwm-amliw Fault; CDF Cwm Dyfnant Fault; CF Crychan Fault; CMF Cwm Mawr Fault; CSF Church Stretton Fault; DA Doethie Anticline; DVF Dulas Valley Fault; ES Eppynt Syncline; GA Gilwern Anticline; GF Glanalders Fault; GaF Garth Fault; GoF Goytre Fault; GS Gwesyn Syncline; HF Howey Fault; LwF Llanwrtyd Fault; NFF Nant y Fedw Fault; RA Rhiwnant Anticline; RGF Rhiw Graidd Fault; RPF Rock Park Fault; WtF Wern-to Fault

(Figure 2) Chronostratigraphical architecture of the Ordovician and Llandovery succession between the Tywi Anticline and the Builth Volcanic Inlier AcC Allt-y-clych Conglomerate Member; BNF Bryn Nicol Formation; BVG Builth Volcanic Group; CgF Cwmcringlyn Formation; db disturbed beds; Dd Doldowlod Conglomerate Formation; LrM Llanfawr Mudstones Formation; PiM Pistyllgwyn Member; SLM Sugar Loaf Member of the St Cynllo's Church Formation. Cross-hatched ornament indicates missing strata. Abbreviations adjacent to fossil symbols refer to localities within the Tywi Anticline (TA), Llanwrtyd Volcanic Inlier (L), Garth (G) and Crychan Forest (CF) areas, and the Builth Volcanic Inlier (B).

(Figure 3) Ordovician succession of the Builth Volcanic Inlier.

(Figure 4) Schematic volcanic and sedimentary architecture of the southern part of the Builth Volcanic Inlier.

(Figure 5) Cartoon illustrating the depositional setting of the coarse-grained turbidite systems of the Ashgill Bryn Nicol and Doldowlod Conglomerate formations.

(Figure 6) Late Ashgill shelf succession

(Figure 7) Silurian (including the Ordovician, late Hirnantian) basinal succession

(Figure 8) Llandovery shelf succession, west of the Dulas Valley Fault (see also (Figure 2))

(Figure 9) Chronostratigraphical architecture of the Wenlock to Lower Devonian shelf/ramp succession. For key to symbols see (Figure 2). sa unit with tabular shelly sandstone beds; bmm burrow-mottled mudstones; db disturbed beds

(Figure 10) Mid Silurian to Devonian shelf/ramp succession, east of the Dulas Valley Fault (see (Figure 9) for lateral relationships of formations).

(Figure 11) Metamorphic map of the district showing metapelitic zones. CSF Church Stretton Fault

Plates

(Plate 1) Unit of thick-bedded turbidite sandstones in the Yr Allt Formation, Craig Alltwinau [SN 8586 4915], Irfon valley (GS1279).

(Plate 2) Basaltic boulder tuff-breccia (hyaloclastite), Llanelwedd Volcanic Formation, Carneddau [SO 0706 5543], Builth Inlier. (GS1280).

(Plate 3) Sandstone block with abundant pentamerid brachiopod valves, Tyncoed Sandstone Member, Comin Coch Formation, Comin-y-garth [SN 9840 5520]. (P580547).

(Plate 4) Scarp features in Raglan Mudstone Formation, south-west of Drovers Arms [SN 9810 4415], Mynydd Eppynt. (GS1282).

(Plate 5) Red till, with abundant Old Red Sandstone clasts, Ysgir Fawr [SN 9755 4095], Mynydd Eppynt. (GS1283).

(Front cover) Irfon valley, north of Abergwesyn [SN 846 536] (Photographer J R Davies; GS1278).

(Rear cover)

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

Figures

(Figure 3) Ordovician succession of the Builth Volcanic Inlier

Lithostratigraphy Lithostratigraphy Lithology Depositional environment
Llanfawr Mudstones Formation (up to 200 m in the district) dark grey mudstone, subordinate thin beds of fine-grained sandstone, siltstone and tuff Low energy, dysaerobic, offshore setting with thin sandstones and siltstones introduced by storm-generated density currents. Thin tuff beds deposited from intermittent eruptions at distant volcanic vents
BUILTH VOLCANIC GROUP Cwm-amliw Tuff Formation (up to 35 m) fine-grained acid ash-flow tuff Subaqueously deposited, non-welded, pyroclastic flow (ignimbrite), the product of a large, explosive eruption from an unknown centre. Reworking of the tuff in an up-faulted, shallow-water or emergent setting, south of Cwmamliw, may account for the local presence of acid volcanic clasts at the top of the Newmead Sandstone
Newmead Sandstone Formation (up to 65 m, thins northwards) tuffaceous sandstone and conglomerate Beach and shoreface sediments
Local unconformity
Llanelwedd Volcanic Formation (up to 250 m, thins northwards) feldsparphyric, locally amygdaloidal, basalt lavas and intrusions with subordinate lapilli-tuffs and tuffs, passing northwards into basaltic tuff-breccias (hyloclastites) (35 m); andesite lavas at top in south A series of effusive, basaltic eruptions, possibly form the same centre as the Carneddau Formation, supplied subaerial, basalt lava flows from the south, prior to a final andesitic eruption. North of the Newmead Fault, an apron of hyloclastites formed where lavas entered the sea and was progressively over-ridden by later lava flows. Dacite domes of the Gilwern Volcanic Formation continued to affect the northward advance of southerly sourced volcanic facies
Carneddau Volcanic Formation (up to 80 m, thins northwards) massive and graded, crystal-rich, basic lapilli-tuff and tuff, common reworked tuffs in upper part; acid ash-flow tuff (up to 35 m) locally at base, and dacite intrusions/lavas (RD) locally at top An initial, violent, silicic eruption, at a vent probably to the south of the inlier (subaqueously deposited, basal ash-flow tuff), preceded a prolonged phase of explosive basaltic volcanism which supplied crystal-rich, pyroclastic debrites (massive tuffs) and turbidites (graded tuffs). Reworking of the tuffs is more prevalent in the upper part of the formation, suggesting upward shallowing, consistent with the evidence that the overlying, dacite lava domes became emergent
Gilwern Volcanic Formation (up to 300 m) massive or graded, basic lapilli-tuff passing into fossilferous, reworked tuff and tuffaceous sandstone; sequences of interbedded graded tuffs and mudstones (md) in upper part; dacite intrusions/lavas (RD) Amalgamated, pyroclastic debris flows (massive tuffs) and turbidites (graded tuffs) emplaced during an inital series of large, explosive eruptions at a centre to the south or west of the inlier.

Contemporaneous reworking of these deposits and local colonisation by shelly benthos in shallower water conditions occurred north of the Cwm-amliw Fault. Interbedded mudstone and tuff sequences record the subsequent decline in volcanic activity. Dacite bodies occur as high-level intrusions and extrusive lava-domes
Llandrindod Tuff Formation (up to 80 m, thins southwards) acid ash-flow tuff; reworked tuff and cross-bedded, brachiopod-bearing, tuffaceous sandstone (up to 5 m) locally at top Large volume, largely non-welded, subaqueously deposited pyroclastic flow (ignimbrite) generated by one or more violent silicic eruptions from a vent to the north of the district. The abrupt shallowing caused by emplacement of the flow allowed shallow-water reworking and brachiopod colonisation of its upper levels, and may have resulted in local emergence
Camnant Mudstones Formation (over 900 m) dark to medium grey mudstone with subordinate micaceous sandstone and basic lapilli-tuff, including Gelli Tuff Member (up to 50 m) Low energy, dysaerobic, offshore setting with thin sandstones introduced by storm-generated density currents. Associated lapilli-tuffs were emplaced as pyroclastic debris flows initiated by explosive volcanic eruptions from nearby vents

(Figure 6) Late Ashgill shelf succession

Lithostratigraphy Lithology Depositional environment
Cwmcringlyn Formation (up to 35 m) lenticular, symmetrical ripple cross-laminated sandstone interbedded with subordinate dark grey silty mudstone Prograding upper shoreface facies deposited within fair weather wave-base during the acme of the Hirnantian glacioeustatic regression. At the point of maximum lowstand, much of the area east of the Garth Fault was emergent and subjected to subaerial erosion
Ciliau Formation (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) glacio-eustatic marine regression. Shelly fossils belong to the distinctive, cool water, Hirnantia association. Evidence of syndepositional faulting
Cribarth Formation (up to 400 m) very thickly bedded, dark grey, sandy and locally shelly mudstone and muddy sandstone; includes unit with siltstone and sandstone laminae (Penrhiwmoch Member; up to 30 m) at top Offshore sandy mud facies in which storm deposited sand layers have been thoroughly mixed with the intervening mud by pervasive bioturbation

(Figure 7) Silurian (including the Ordovician, late Hirnantian) basinal succession

Lithostratigraphy Lithology Depositional environment
CWMYSTWYTH GRITS GROUP Glanyrafon Formation (up to 200 m+) thinly interbedded turbidite sandstone and mudstone with bioturbated and laminated hemipelagic mudstone Deposition from low-concentration turbidity currents on the fringes of sandstone lobes
Rhuddnant Grits Formation (up to 240 m) thinly interbedded turbidite sandstone and mudstone and hemipelagic mudstone with scattered medium to thick beds of high-matrix turbidite sandstone Sandstone lobe facies, comprising medium to thick sandstones deposited from high-concentration turbidity currents and dilute debris flows; separated by thin-bedded interlobe facies deposited by low-concentration turbidity currents
Doethie Formation (up to 100 m) thinly interbedded turbidite sandstone and mudstone with packets of medium to thick turbiditic sandstone, commonly amalgamated and displaying dewatering structures
Caerau Mudstones (Formation) (up to 300 m) interbedded oxic and anoxic facies mudstone with scattered thin turbidite siltstone and sandstone Deposition from low-concentration turbidity currents at the sides and front of sandstone lobes onlapping the slope apron from the west
Nant Brianne Formation (up to 150 m) thinly interbedded turbidite sandstone and oxic facies mudstone Deposition from low-concentration turbidity currents in a broad but confined channel on the slope apron
CABAN CONGLOMERATE FORMATION CLAERWEN GROUP Rhayader Mudstones (Formation) (170–450 m) greenish grey, diffusely burrow-mottled turbiditic and hemipelagic mudstone; including silt laminated facies (up to 380 m) and M. sedgwickii Shales (20 m) at base Deposition from very low-concentration turbidity currents and hemipelagic 'rain'. Mainly oxic bottom conditions. Slope apron setting. M. sedgwickii Shales record the brief introduction of anoxic bottom conditions in response to the late Aeronian transgression
Derwenlas Formation (200 m) colour-banded and burrow-mottled turbiditic and hemipelagic mudstone; scattered thin turbidite siltstones and sandstones Deposition from very low-concentration turbidity currents turbidity currents and hemipelagic 'rain'. Mainly oxic bottom conditions. Slope apron setting
Cwmere Formation (350–400 m) medium grey turbidite mudstone, thinly interbedded with dark grey, laminated hemipelagites; scattered thin turbidite siltstones and sandstones. Mottled Mudstone Member (up to 10 m) comprising pale grey, burrow-mottled hemipelagic and turbiditic mudstone at base Deposition from low-concentration turbidity currents and hemipelagic 'rain'. Mainly anoxic bottom conditions in response to postglacial sea level rise. Slope apron setting
CABAN CONGLOMERATE FORMATION (up to 280 m, occuring as lenticular bodies in the Cwmere Formation and Claerwen Group) Gafallt Shales facies: (0–50 m), oxic facies mudstone with thin turbidite sandstone. Caban Conglomerate facies: second (0–15 m), fourth (0–10 m)and fifth (0–15 m) sequences of thick-bedded turbidite conglomerates and sandstones. Dyffryn Flags facies: (0–50 m) thinly interbedded turbidite sandstone and anoxic facies mudstone. Cerig Gwynion Grits facies: (0–70 m) thick bedded turbidite sandstone Caban Conglomerate facies deposited by high-concentration turbidity currents in broad channels, confined by levees of fringing facies and slope apron muds. Down-current, the channels fed small-scale, sandy turbidite lobes (e.g. Cerig Gwynion Grits facies). Lobes and channels fringed by less sandy facies (Dyffryn Flags and Gafallt Shales facies)

(Figure 8) Llandovery shelf succession, west of the Dulas Valley Fault (see also (Figure 2))

Lithostratigraphy Lithology Depositional environment
Dolfawr Mudstones (Formation) (up to 75 m) interbedded burrow-mottled and laminated (hemipelagic) mudstone, with scattered shells and thin sandstones Alternation of oxic and anoxic bottom conditions in a deep offshore setting, shelly detritus reworked from the Builth High
Cerig Formation (15 to 250 m) olive-green and grey, locally red and purple, burrow-mottled mudstone, with scattered sandstone beds and rare debrites. Coarse-grained sandstone (Trecoed Sandstone Member; up to 7.5 m) at the base of the attenuated eastern sequence; shelly, calcareous mudstone present locally in this area In the west, the accumulation of bioturbated offshore mud, devoid of indigenous shelly fossils, records deepening brought about by a marine transgression initially in the late Aeronian and subsequently at the base of the Telychian. In the east, tectonic uplift on the Builth High delayed the onset of mud deposition. Here the condensed sequence of sandy near-shore facies (Trecoed Sandstone Member), coeval with the Comin Coch, Ty-mawr Sandstone and basal Cerig formations was deposited in a shallow-water setting sustained throughout the late Aeronian and early Telychian. In contrast to those in the west, the offshore mud subsequently deposited across the Builth High remained within the colonising depths of shelly benthos
Ty-mawr Formation (up to 38 m) fine to coarse-grained sandstone with lenses and thin beds of granule conglomerate Progradation of shoreface sand and gravel facies during the late Aeronian (?eustatic) regression
Comin Coch Formation (up to 75 m) shelly, olive-green sandy mudstone with dispersed pebbles and granules; quartzitic sandstone (Tyncoed Sandstone Member, up to 5 m) at base. Passes laterally into Cerig Formation Beach/upper shoreface sands (Tyncoed Sandstone Member) were driven south-eastwards across the underlying subaerially eroded surface during the onset of the late Aeronian eustatic transgression. Mud with thoroughly bioturbated pebble and granule-rich, coarse sand was deposited in an offshore environment established by the transgression
Trefawr Formation (up to 220 m) thick-bedded green-grey sandy mudstone; green burrow-mottled sandstone (Ty Gwylim Sandstone Member, up to 25 m) at top in north Following a deepening and a widespread return to sandy mud deposition, a muddy sand facies (Ty Gwilym Member) was reintroduced during the mid Aeronian eustatic marine regression. At the culmination of this event, much of the south-west of Comin y Garth was exposed to subaerial erosion
Crychan Formation (up to 550 m) shelly and micaceous, muddy sandstone with scattered pebbles and granules, with thin to medium, tabular sandstone beds Mid shelf/ramp muddy sand facies in which some storm sheet sands are preserved
Bronydd Formation (up to 220 m) thick bedded, greenish grey, sandy mudstone North-westward progradation of both offshore sandy mud facies in which storm-introduced sandy layers have been thoroughly bioturbated into the intervening mud
Chwefri Formation (up to 800 m) unfossiliferous, colour banded, pale to medium greenish grey, silty mudstone, locally burrow-mottled, with scattered siltstone laminae and thin sandstone beds Distal shelf/upper slope facies deposited principally from suspension (hemipelagic) under oxic bottom water conditions
Garth House Formation (50–75 m) unfossiliferous, smooth, dark grey mudstone with beds of lenticular sandstone, decreasing in abundance and thickness upwards Rapid transition from upper and lower shoreface facies with wave-reworked lenticular sands into offshore mud-dominated facies. The sequence records deepening brought about by the continuing postglacial transgression
Cwm Clŷd Sandstone (up to 30 m) symmetrical ripple cross-laminated sandstone with abundant Skolithos trace fossils and mudstone lenses; locally conglomeratic Transgressive, beach and upper shoreface facies driven eastwards across the underlying subaerially eroded surface during the late Hirnantian, postglacial eustatic rise in sea level

Figure 10 Mid Silurian to Devonian shelf/ramp succession, east of the Dulas Valley Fault (see Figure 9 for lateral relationships of formations).

Lithostratigraphy Lithology Depositional environment
St Maughans Formation (400 m+) red micaceous silty mudstone/ siltstone with abundant thick cross-bedded sandstones up to 7 m thick, commonly with basal intraformational conglomerates. Abundant immature calcrete profiles The sandstones and conglomerates represent channelised deposits of meandering rivers, while the mudstone/siltstone and calcretes record overbank deposition and periodic subaerial exposure
Raglan Mudstone Formation (1160 m) red micaceous silty mudstone/siltstone with subordinate thin sandstones. Abundant immature calcrete profiles and scattered units of cross-bedded sandstone up to 3 m thick. The top of a prominent calcrete, the Bishop's Frome Limestone (0 to 3 m thick) defines the top of the formation Alluvial plain with sand deposited in the channels of meandering rivers. Calcretes record periodic subaerial exposure
Temeside Mudstone Formation (40–80 m) green-grey, massive muddy siltstone with scattered calcrete nodules. Some thin grey sandstones in lower part Lagoonal/coastal tidal flats subject to periodic subaerial exposure and soil forming processes, including calcretisation
Tilestones Formation (up to 7 m) thick-bedded, grey to green micaceous, fine-grained, laminated and low-angle cross-bedded sandstone Shoreface and barrier sands
Cae'r mynach Formation (65–130 m) grey thin-bedded calcareous mudstone, siltstone and fine-grained sandstone with variably abundant medium-bedded sandstone. Burrowing limited to thinner beds Progradation of mid to proximal shelf facies characterised by sublittoral sheet sands. Fair weather reworking in uppermost part
Fibua Formation (up to 150 m) coarsening upwards sequence of grey calcareous mudstone with very thin beds and laminae of siltstone and very fine-grained sandstone. Scattered burrows Basal mudstone-prone sequence represents a return to mid to proximal ramp facies in response to a

late Ludforian deepening. Rest of sequence records a transition from proximal ramp to shelf facies resulting from further progradation
Aberedw Formation (55–170 m) grey thin- to medium-bedded calcareous muddy siltstone and fine-grained sandstone. Intensively burrowed and variably shelly Acme of progradation and shallowing of shelf facies characterised by storm-generated sand and silt.

Abundant infauna
Cwm Graig ddu Formation (230–580 m) coarsening upwards sequence of grey calcareous mudstone with variably abundant very thin beds and laminae of siltstone and very fine-grained sandstone. Scattered burrows 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 Formation (up to 1050 m) thinly bedded, grey mudstone with thin beds and laminae of siltstone, and of laminated hemipelagite; thick sequences of slump-folded and destratified units with pebbly mudstone debrites (disturbed beds), present in the north Renewed deepening at the onset of Ludlow times is marked by the reintroduction of mid-ramp facies. While these continued to accumulate in the south, slumping resumed in the north
Tirabad Formation (up to 500 m) grey mudstone with variably abundant thin beds and laminae of fine-grained sandstone and siltstone. Scattered burrows. Some thicker bedded shelly sandstones in uppermost part Progradation of proximal ramp to shelf facies in the south-west of the district comprising low-concentration turbidites and storm driven event beds. Oxic bottom conditions. Coarsening upwards sequence represents response to Homerian regression
Llangammarch Formation (up to 650 m) thinly bedded, grey mudstone with sparse to abundant thin beds and laminae of siltstone, and of laminated hemipelagite; capped in the north by pebbly mudstone debrites and slump-folded units of the widespreaed Caer-beris Member (up to 250 m) Progradation of mid ramp facies comprising thin, silt-based event beds emplaced by low-concentration density currents (?storm induced). Laminated hemipelagites record anoxic bottom conditions. The onset of widespread slumping marked by the Caerberis Member coincided with the widely recognised Homerian marine regression or still-stand.
Builth Mudstones (Formation) (200–320 m) blue-grey, pervasively laminated (hemipelagic), variably calcareous graptolitic mudstone with scattered shelly fossils and debris; thin units of slumped and destratified strata (disturbed beds) and muddy limestone; rare units with burrow-mottling 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 marinecirculation. Local disturbed units record contemporary, possibly fault-induced slumping