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Geology of the Lampeter district — a brief explanation of the geological map sheet 195 Lampeter

J R Davies, D I Schofield, T H Sheppard, R A Waters, M Williams and D Wilson

Bibliographic reference: Davies, J R, Schofield, D I, Sheppard, T H, Waters, R A, Williams, M and Wilson D. 2006. Geology of the Lampeter district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 195 Lampeter (England and Wales).

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

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved. Ordnance Survey licence number 100017897/2006.

(Front cover) Valley of the Pysgotwr Fawr [SN 7585 4875] (Photographer D I Schofield; (P606652))

(Rear cover)

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

Notes

The word 'district' refers to the area of the geological 1:50 000 series sheet 195 Lampeter. National grid references (NGR) are given in square brackets. The whole of the district lies within 100 km square SN and the letter prefix for grid references in this area are omitted. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map.

Acknowledgements

This sheet explanation was compiled by J R Davies and R A Waters. J R Davies and D Wilson wrote the Introduction, Geological description and Applied geology based on contributions by D I Schofield, T H Sheppard and R A Waters. Palaeontological information was provided by M Williams.

Surveying undertaken in the south-west of the district as part of the Afon Teifi Catchment Survey in 1995–97 was partly funded by Ceredigion, Carmarthenshire, Pembrokeshire and the former Dyfed county councils, and by the Environment Agency (Wales). The remainder of the district was surveyed as part of the Applied Geological Mapping in Central Wales Project, partly funded by a grant from the Welsh Assembly Government. The British Geological Survey gratefully acknowledges the co-operation of all the landowners in the district during the geological survey. Particular thanks are extended to the Environment Agency (Wales) and Celtic Water Management Limited for providing borehole and geological information from the Aeron valley.

Geology of the Lampeter district. An explanation of sheet 195 (England and Wales) 1:50 000 series map

This sheet explanation contains a brief description of the geology of the Lampeter district, which includes the central portion of the Cambrian Mountains in the east and the rolling country between the Aeron and upper Teifi valleys in the west. Thus, the district provides a transect across the complex collage of geological formations that define the fill of the Lower Palaeozoic Welsh Basin.

The solid geology comprises sedimentary rocks of Ordovician and Silurian age, which formed between approximately 450 and 428 million years ago. This survey has established new insights into the evolution of the turbiditic fill 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 long history of fault movement in the district culminated in an episode of regional folding, cleavage formation and low-grade metamorphism that occurred during the latest Silurian and early Devonian. A brief account of these movements and the resulting structures is presented.

The Quaternary sediments of the district were deposited mainly during the last ice age, between approximately 20 000 and 10 000 years ago and form a patchy cover. In the last 10 000 years since the ice retreated, peat has accumulated in the upland areas and fluvial sediments have been deposited by the rivers Aeron, Teifi and Tywi and their tributaries.

As well as summarising the traditional aspects of the geology, the new maps 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 geology depicted on BGS 1:50 000 series sheet 195 Lampeter, published in 2006 as a Bedrock and Superficial Deposits edition. The district lies in the counties of Ceredigion and Carmarthenshire.

Occupying much of the eastern part of the district are the deeply dissected uplands of the Cambrian Mountains. Separating this higher ground from a rolling landscape of lower relief in the west is the broad valley of the Afon Teifi, and to the north-west is the valley of the of the Afon Aeron. Streams draining the east of the district form part of the catchment Afon Tywi (Towi on older maps) and supply the Llyn Brianne Reservoir. The upland reaches of the district, amongst the most remote in southern Britain, support sheep grazing and extensive forestry plantations; lowland areas are utilised for beef and dairy-based agriculture.

The bedrock has been exploited widely in past as a source of local building materials, but the only working quarry today is at Ty Hywel [SN 598 447], east of Cwmann. Sand and gravel deposits of the Teifi valley are worked commercially at Abercoed [SN 667 579] and near Pant [SN 659 566]. Parts of the district lie within the Central Wales Mining Field and include the sites of several former lead and zinc mines, notably the workings at Nantymwyn where mining operations ceased in 1932.

Bedrock of the district (Figure 1) comprises a folded and faulted sedimentary succession of late Ordovician to early Silurian age, deposited around 440 to 415 million years ago. The 5000 m thick, sequence, comprises sedimentary facies of exclusively deep-water aspect, and forms part of the fill of the Lower Palaeozoic Welsh Basin. Contemporary, shallower water 'shelf' facies and the intervening margin to the basin are located just to the east of the district (Schofield et al., 2004; Barclay et al., 2005). Displacements on the major faults that cross the district, folds and the regional cleavage relate principally to the late Caledonian Acadian Orogeny. This widespread episode of compressive, plate tectonic deformation affected the rocks of the district some 400 million years ago. However, marked thickness and facies variations across some of the main fractures also testify to earlier, synsedimentary periods of movement on these structures.

Quaternary deposits (drift) that mantle the bedrock include Pleistocene glacial and periglacial sediments, as well as more recent (Holocene) largely alluvial deposits. During the Pleistocene, the high cwms of the Cambrian Mountains were the source for Welsh glacier ice, which covered much of the district some 20 000 years ago. As it melted, the material deposited and moulded by the ice was reworked by glacial melt waters and by periglacial processes, the latter giving rise to a distinct suite of deposits and landforms. Melting of the ice is thought to have been complete by about 14 500 years ago, though periglacial conditions persisted intermittently until around 10 000 years ago. Following the retreat of the ice, its deposits and landforms were further modified as the modern drainage systems of the district evolved, while in upland areas extensive spreads of hill peat accumulated.

The remote nature of the district is reflected in the limited number of earlier investigations of the bedrock geology. Jones (1912, 1938) was the first to establish the broad geological structure of the region. The Silurian succession in the east of the district attracted the attention of the palaeontologist and stratigrapher K A Davies (1933; see also Davies and Platt, 1933), and subsequently of Mackie and Smallwood (1987). Studies by Long (1966), Smith (1987a, b; 1988) and Clayton (1992) focused on the sedimentology of the late Llandovery strata, which crop out extensively in the centre of the district. Smith (1987c) and Woodcock (1990) also examined the structural aspects of these rocks. Wilson et al. (1992) provided a new depositional and structural model for the Aberystwyth Grits Group. Investigations of the mineralisation and past mining within the district include those by Hall (1971), Bick (1974) and Ball and Nutt (1976). Relevant regional accounts of the geology of central Wales are included in Woodcock et al. (1996), Davies et al. (1997) and Smith (2004).

The Quaternary deposits and landforms of the district have been examined by Jones (1924, 1965), Watson (1970) and, as part of an earlier BGS study, by Waters et al. (1997) who also provide a detailed assessment of the applied geology of the Teifi catchment.

Bedrock facies and processes

Many of the rocks of the district display features that show they were deposited as the result of underwater slumping and sediment avalanches along the adjacent basin margin. The resulting deposits, comprising slumps, debrites and turbidites, vary greatly in thickness and internal composition (see Davies et al. 1997).

Commonly preserved between the various resedimented units are very thin but distinctive beds of mudstone, termed hemipelagites, which record the slow rain of fine sediment on to the sea bed in between the more violent, but short-lived events marked by the other deposits. The character of these hemipelagites reflects more closely the environmental conditions that prevailed across the basin floor at the time they accumulated. Two types are recognised: a pyritic, dark grey, finely laminated variety which records accumulation beneath stagnant (anoxic) bottom waters; and a pale grey or green, burrow-mottled variety which records periods when creatures were able to colonise the sea bed suggesting that the overlying water then contained more oxygen (oxic). Typically, many metres of strata and, in some cases, whole formations display only one of the two types of hemipelagite showing that either oxic or anoxic conditions were sustained across the basin floor for long periods of time. The laminated hemipelagic mudstone beds are additionally important because they commonly preserve the fossilised remains of now extinct, planktonic animals known as graptolites. The form of the colonies these animals lived in underwent rapid evolutionary changes during the Ordovician and Silurian allowing the rocks in which they occur to be accurately dated according to a widely recognised succession of graptolite biozones and subzones (Figure 2).

Chapter 2 Geological description

Ordovician

Ordovician rocks of Ashgill age crop out in the south-east of the district along the western flank of the Tywi Anticline. They also occur at the southern margin of the district in the core of the Cothi Anticline, east of Pumsaint, and also along the western margin in the core of the Llanwenog Anticline, north-west of Llanwnnen.

The oldest rocks occur in the west and south-east, and are facies of the Bryn Nicol Formation, a 300 m-thick unit that crops out more fully in the Builth Wells district to the east. It comprises the laterally interfingering Coed Ifan Facies (CIF) and Pen Derlwyn Facies (PD). In the former, thin- to medium-bedded turbidite sandstones and mudstones are interbedded with burrow-mottled hemipelagites. The same lithologies are present in the Pen Derlwyn Facies, but this also includes thick beds of coarse-grained turbidite sandstone, pebbly mudstone and shelly conglomerate. The latter contain a mid-Ashgill fauna. The Bryn Nicol Formation as a whole, records deposition within a fault-bounded trough initiated during a period of mid-Ashgill tectonism (Schofield et al., 2004).

The succeeding Yr Allt Formation (YA) is up to 1200 m thick, and, where undisturbed by slumping, comprises unbioturbated, dark grey, silt-laminated turbidite mudstone with local packets of fine-grained turbidite sandstone. The absence of burrowing in this division has been ascribed to the very high rate of sedimentation that accompanied a major glacio-eustatic marine regression during the late Ashgill Hirnantian stage (Davies et al., 1997). This is also reflected in the widespread presence of slumped and destratified strata, disturbed beds (db), throughout the formation, the product of frequent slope failure along the front of a rapidly prograding sediment wedge. Such disturbed horizons comprise most of the Tywi Anticline Yr Allt Formation sequence in the district.

Throughout the Welsh Basin, rocks of latest Ordovician (late Hirnantian) age that record the rise in sea level and transgression which followed the late Ordovician glaciation are more naturally included with the formations that followed and are thus described with the Silurian rocks here.

Silurian

The Silurian succession of the district is divided into two broad architectural elements, following Davies et al. (1997):

a late Hirnantian to early Telychian, mudstone-dominated slope-apron, which was supplied predominantly with sediment from the east, and displays facies changes in step with contemporary sea-level changes
a set of mid to late Telychian sandstone-lobes that received sediment from the south and was strongly influenced by intra-basinal tectonism

The late Hirnantian to early Telychian slope-apron sequence crops out in both the east and west of the district separated by a broad tract occupied by the younger, southerly sourced Telychian rocks that are associated with the faulted core of the Central Wales Syncline. It is convenient to describe the slope-apron sequences on either side of this syncline separately.

Slope-apron sequence east of the Central Wales Syncline

Succeeding the Hirnantian, glacio-eustatic low-stand facies of the Tywi Anticline are the Cwmere Formation and overlying Claerwen Group. These predominantly mudstone divisions envelope more laterally restricted sequences of coarser grained turbidite facies including the Nant Brianne Formation, as well as several un-named sandstone and conglomerate bodies (Figure 3). The complex outcrop pattern of these divisions is due to the intersection of a series of major folds and the deeply incised topography in the south-east of the district.

The Cwmere Formation (CeF) comprises a sequence, ranging up to 360 m in thickness, of thinly interbedded turbiditic and laminated hemipelagic mudstones with scattered very thin beds and laminae of turbidite siltstone and sandstone. At the base of the formation is the Mottled Mudstone Member (MMb), a pale grey, burrow-mottled facies, locally up to 10 m thick, which sharply overlies the Yr Allt Formation. It contains latest Hirnantian, persculptus Biozone graptolites. Remaining parts of the formation range through the Rhuddanian into the earliest Aeronian (magnus Biozone). The Mottled Mudstone Member was deposited at the onset of the major marine transgression that followed the end of the late Ordovician glaciation. The remainder of the formation records the subsequent rise in sea level, the creation of a strongly stratified water column and imposition of anoxic bottom conditions throughout much of the Welsh Basin.

The Claerwen Group is a predominantly oxic slope-apron mudstone sequence, and is subdivided into the Derwenlas Formation and overlying Rhayader Mudstones Formation. The 350 m-thick, Derwenlas Formation (DlF) comprises a colour-banded sequence of thin-bedded pale grey and green turbidite mudstones interbedded with burrow-mottled hemipelagites. Thin siltstone laminae are common at the base of the turbidite units, which display features typical of deposition from low-concentration turbidity currents (Stow and Piper, 1984). Thin units of anoxic facies mudstone present in the upper part of the Derwenlas Formation yield late Aeronian convolutus Biozone graptolites (lhc). The oxic bottom conditions that prevailed during the accumulation of much of the formation are equated with a widely recognised, and hence probably eustatic, mid Aeronian marine regression.

The overlying Rhayader Mudstones Formation (Rhs), ranging up to 500 m in thickness, consists predominantly of greenish grey, burrow-mottled turbidite and hemipelagic mudstone. However, thin sequences with laminated hemipelagites are also present and the base of the formation is defined by one such sequence, the Monograptus sedgwickii Shales Member (lhs). This widely recognised unit is about 10 m thick. It correlates with a transgressive event evident in adjacent shelf sequences to the east and also much further afield, so that this too can be viewed as eustatic. In the vicinity of the Llyn Brianne Reservoir, the whole of the local Rhayader Mudstones sequence passes laterally into the sandstone-bearing Nant Brianne Formation (see below). Parts of the Rhayader Mudstones that succeed the Nant Brianne Formation in the east of the district contain abundant, very thin beds and laminae of turbidite siltstone and sandstone, and comprise a separately mapped silt-laminated facies (sl) of early to mid turriculatus Biozone age.

South of Llyn Brianne dam, sequences in which turbiditic and hemipelagic mudstones are interbedded with abundant thin beds of turbidite sandstone (sa), replace the lower part of the Cwmere Formation and also straddle a level equivalent to the Cwmere–Derwenlas formation contact. The Cwmere Formation crops out at the eastern margin of the district in the vicinity of Ystradffin [SN 788 466] where up to 150 m of sandstone-rich facies with laminated hemipelagites succeed the Mottled Mudstone Member. To the south-east, this unit passes rapidly into the enveloping Cwmere Formation mudstones. The second sandstone-rich sequence, exposed in crags [SN 774 458] south of the Afon Tywi, is up to 200 m thick but thins rapidly both to the north and south. Parallel- and cross-laminated, tabular and lenticular, turbidite sandstones range up to 0.1 m in thickness and locally comprise up to 60 per cent of exposed sections. The presence of laminated hemipelagites in the lower half of this sequence demonstrates a lateral equivalence to the Cwmere Formation. In contrast, burrowed hemipelagites show the upper part to be coeval with the enveloping Derwenlas Formation. The lower, anoxic levels are restricted to the south of Dinas [SN 780 465], but the upper, oxic portion is more widespread and forms the whole of the sandstone-rich sequence on and to the north of this prominent rock buttress and along both limbs of the Doethie Anticline.

On the north-west flank of Mynydd Mallaen, a 120 m-thick sequence of thick-bedded turbidite conglomerate (cg) crops out along the limbs of the Cothi Anticline forming the prominent crags of Creigiau Ladis and Crugiau Merched [SN 722 455]. This conglomerate body is enclosed within the Cwmere Formation and compares closely in both facies and stratigraphical setting with parts of the Caban Conglomerate Formation of the Rhayader district (Davies et al., 1997). South-west of Crugiau Merched, the conglomeratic sequence thins and wedges out. To the north-east, a separate sequence of sandstone-rich facies (sa) within the Derwenlas Formation crops out in the vicinity of Pont Rhyd-felin [SN 730 465].

The Nant Brianne Formation (NBr) crops out between the Llyn Brianne Reservoir and the Pysgotwr valley on both limbs of the Doethie Anticline and in the core of the Cilgwyn Syncline. Ranging up to 410 m in thickness, it consists predominantly of thin to medium-bedded turbidite sandstones and mudstones (Plate 1). However, around Llyn Brianne dam [SN 793 483] a basal sequence, which includes thick-bedded and conglomeratic, turbidite sandstones (sa) and associated disturbed beds (db), occupies a kilometre-wide belt confined on its eastern side by the Llyn Brianne Fault. These strata yield late Aeronian convolutus Biozone graptolites and are coeval with strata on the east side of the fault included in the Derwenlas Formation. To the north-east, in the Builth Wells district, and also to the south-west of this belt, the diachronous base of the Nant Brianne Formation rises to above the level of the Monograptus sedgwickii Shales. Here lower parts of the formation contain early turriculatus Biozone graptolites and equate with the Rhayader Mudstones. Upper parts of the formation range into the utilis subzone. The Nant Brianne Formation equates with the lower part of the Hafdre Formation of Mackie and Smallwood (1987) (upper parts of the Hafdre Formation are here included in the Doethie and Glanyrafon formations).

The Nant Brianne Formation, together with the various un-named conglomerate and sandstone-rich sequences, locate the positions of a series of narrow belts within which coarser grade sediment was deposited and routed to the outer parts of the slope-apron system (Figure 3) (cf. Davies and Waters, 1995). The marked lateral expansion of the sequence coeval with the lower part of the Derwenlas Formation coincided with the marine regression, which introduced oxic bottom conditions across the slope-apron (see above). The late Aeronian phase of deposition of the Nant Brianne Formation appears to have been confined by syndepositional movement on the Llyn Brianne Fault, which also led to localised slumping. Subsequently, during the Telychian, the corridor along which the Nant Brianne sediments were transported, and to which the silt-laminated facies of the Rhayader Mudstones appears as fringing or overbank facies, may have acted as one of the sediment-supply routes for the Devil's Bridge Formation in the west of the district (see below).

Slope-apron sequence west of the Central Wales Syncline

The same Hirnantian to Telychian mudstone divisions as occur in the east are also recognised to the west of the Central Wales Syncline — the Cwmere Formation and Claerwen Group. In this part of the district the associated sandstone-rich facies are included in the Rhyddlan and Devil's Bridge formations.

In outcrops to the north-west of Llanwnnen, the Mottled Mudstone Member, reduced to just 2 m in thickness and hence not depicted on the 1:50 000 map, underlies the predominantly anoxic facies of the Cwmere Formation. Here, and in the core of the Teifi Anticlinorium west of Llanybydder, much of the upper part of the Cwmere Formation has been replaced laterally by the Rhyddlan Formation (Rhy), in which abundant thin turbidite sandstones are interbedded with turbidite mudstones and laminated hemipelagites. In the Teifi valley, the formation ranges up to 250 m in thickness and has replaced all but the lowermost 50 m of the Cwmere Formation. The turbiditic lithologies of the Rhyddlan Formation are arranged in sandstone-mudstone couplets (Bouma, 1962) with the sandstones displaying internal sedimentary structures typical of such units including parallel- and cross-lamination. Though the sandstone beds are typically less than 0.1 m thick, they exceptionally range up to 1 m in thickness, and locally comprise up to 80 per cent of the formation. In these latter, sandstone-dominated parts of the Rhyddlan Formation, hemipelagic mudstones are rarely discernable between the individual turbidite units. Graptolites indicative of the early Rhuddanian atavus Biozone have been obtained from the lower part of the formation in the adjacent Llangranog district (Davies et al., in press).

The Rhyddlan Formation records turbidite deposition on a lobe-like body located on the outer part of the contemporary slope-apron surface, which accumulated sand-grade sediment from atavus through to magnus biozone times. Deposition here was coeval with some of the conglomeratic and sandstone-rich sequences in the east, though the orientation of depositional tracts suggests that these are unlikely to have formed part of the Rhyddlan Formation supply route (Figure 3).

In the overlying Claerwen Group, the recognition of the anoxic Monograptus sedgwickii Shales Member (lhs) allows this western sequence also to be divided into the Derwenlas Formation (DlF) and the overlying Rhayader Mudstones Formation (Rhs). Both formations comprise predominantly oxic slope-apron facies identical with those in the east and reflecting the same eustatic influences. Anoxic levels yielding convolutus Biozone graptolites are again present in the upper part of the Derwenlas Formation sequence, here 120 m thick. As in the east, the Rhayader Mudstones exhibit lateral variations in thickness commensurate with the distribution of contiguous sandstone-bearing facies, here included in the Devil's Bridge Formation. Locally, within a north-west-trending belt of anomalously thin Rhayader Mudstones, centred on Llanwnnen [SN 520 470], the base of the overlying Devil's Bridge Formation is separated from the Monograptus sedgwickii Shales by less than a metre of oxic Rhayader Mudstones Formation (Figure 3). To the north and south, this base rises and sequences of Rhayader Mudstones along the limbs of the Teifi Anticlinorium increase to around 200 m in the north of the district, east of Tregaron, and to more than 300 m in the south.

The Devil's Bridge Formation (DBF), comprising thinly interbedded turbidite sandstones and mudstones, crops out extensively within the western half of the district. It displays thickness changes consistent with those in the underlying Rhayader Mudstones, but which are compounded, in the north-west of the district, by a lateral passage into the Borth Mudstones in the vicinity of the Bronnant Fault. The attenuated Rhayader Mudstones sequence of the Llanwnnen area underlies a thicker (600 m) Devil's Bridge Formation, although this thins to the north-west where the upper part is replaced by coeval Borth Mudstones facies. Outside this belt, at the northern edge of the district, the Devil's Bridge Formation is reduced to around 450 m in thickness and it is less than 300 m thick in the south, east of Aber Giâr [SN 515 407].

Lower and upper parts of the Devil's Bridge Formation differ in character. In the lower part, thin lenticular and cross-laminated turbidite sandstones are interbedded with very thin beds and laminae of siltstone and pale grey and green colour-banded mudstone. In contrast, upper portions of the formation comprise well developed sandstone-mudstone couplets in which a thin basal sandstone is overlain by a typically thicker unit of structureless grey mudstone. The dominant type of hemipelagite throughout the formation is the burrow-mottled variety, but intervals with laminated hemipelagites are also present. Sandstones in the lower part of the formation are typically 10 to 50 mm thick, but widely scattered thicker beds ranging up to 0.3 m in thickness also occur. In the Bouma turbidite couplets, which dominate upper parts of the Devil's Bridge Formation, parallel- and cross-laminated sandstones, ranging from 1 to 10 cm thick, underlie grey mudstone units up to 30 cm thick. The total sandstone content of the formation locally exceeds 30 per cent, but falls to 10 per cent in sequences gradational with contiguous mudstone divisions. Graptolites from the Devil's Bridge Formation are all indicative of the turriculatus s.l. Biozone. Palaeocurrent measurements suggest that the turbidity currents supplying the formation flowed towards the north-west.

The lower part of the expanded Devil's Bridge Formation near Llanwnnen, appears to represent an 'early pathway' sequence (Figure 3) comparable but separate from that recognised in the Llanilar district by Davies et al. (1997). It records deposition, during the early turriculatus s.l. Biozone (runcinatus to gemmatus subzones), within a north-west-trending corridor being supplied by sandy turbidity currents at a time when adjacent areas to the north and south were still accumulating Claerwen Group slope-apron mudstones. Upper parts of the Devil's Bridge Formation, of gemmatus to utilis subzone age, record more widespread sand deposition, the 'blanket phase' of Davies et al. (1997), and relate to the overall increase in sediment being supplied to the basin in response to tectonic uplift of the source areas. This phase of deposition was coeval with Borth Mudstones and Aberystwyth Grits deposition in the vicinity and to the west of Bronnant Fault Zone.

Telychian sandstone-lobe systems

Slope-apron mudstone deposition was ended in the early Telychian when a series of large-scale, southerly sourced, sandstone lobe turbidite systems (sensu Mutti and Normark, 1987) invaded the basin (Figure 4), a response to tectonic uplift and contemporary plate collision events. These sandstone-lobe systems first entered in the west of the basin (and the district), before migrating eastwards. Intra-basinal faults, notably the Bronnant Fault and fractures associated with the Central Wales Syncline, strongly influenced the pattern and timing of this facies migration. The earliest and most westerly sandstone-lobe system is represented by the Aberystwyth Grits Group, to which the Borth Mudstones Formation is a fringing facies. Younger, more easterly systems are included in the Cwmystwyth Grits Group, with its mud-dominated fringing facies represented by the Blaen Myherin Mudstones and Caerau Mudstones formations. Collectively, these systems record massive erosion of rejuvenated source areas lying to the south of the basin and this is partly evidenced by a marked unconformity at this stratigraphical level in Pembrokeshire (Hillier, 2002). In the basin, in marked contrast to the earlier slope-apron phase of deposition, the influence of synsedimentary faulting, allied to rapid rates of deposition, largely obliterate the effects of sea level change on the nature and distribution of the deposits.

Aberystwyth Grits Group and fringing facies

The Borth Mudstones Formation (BMF) comprises thin to medium-bedded turbidite mudstones with widely scattered thin turbidite sandstones. Thin beds of either burrow-mottled or laminated hemipelagic mudstone occur between the individual turbidite units. The formation crops out in the north-west of the district, on both sides of the Bronnant Fault Zone. North of the fault zone, an estimated 300 m underlie the Aberystwyth Grits Group; south of the fault zone a thickness of at least 600 m is present. Sandstones in the Borth Mudstones, where present, occur at the bases of sandstone-mudstone couplets comparable with those in the upper part of the local Devil's Bridge Formation and in the Aberystwyth Grits. In general, however, sandstone beds comprise less than 10 per cent of the formation, which is dominated by tabular beds of medium grey, structureless mudstone ranging up to 0.5 m in thickness. Graptolites from the Borth Mudstones Formation in the adjacent Llanilar district indicate a lower turriculatus s.l. Biozone age (renaudi to lower utilis subzones).

The Borth Mudstones are interpreted as a muddy fringing facies associated with the Aberystwyth Grits sandstone lobe turbidite system (Davies et al., 1997). Geochemical analyses suggest a close link between the two divisions consistent with derivation from a common source to the south of the basin (Ball et al., 1992).

The outcrop of the Aberystwyth Grits Group is confined to the north-west corner of the district, north of the Bronnant Fault Zone. The group comprises two laterally interdigitating divisions: the Mynydd Bach Formation and the Trefechan Formation. At the base of the group, thick sandstones overlie a transitional sequence with the Borth Mudstones in which the sandstones of sandstone-mudstone couplets gradually increase in thickness and abundance upwards. The top of the Aberystwyth Grits is not seen in the district. The local sequence preserved between the Bronnant and Mynydd Bach faults may exceed 2000 m in thickness (Wilson et al., 1992), although this has been contested (Loydell et al., 1997).

The Mynydd Bach Formation (MBa) comprises a sequence of interbedded, tabular turbidite sandstones and mudstones. The sandstone beds vary greatly in thickness and internal structure. Medium to very thick (up to 2 m), graded beds of muddy (high-matrix) and feldspathic sandstone, some with abundant contorted rip-up clasts, are diagnostic of the formation and record deposition from highly fluid, slurry-like flows (Wood and Smith, 1959; Clayton, 1994; Davies et al., 1997). These thicker sandstones commonly occur concentrated in packets of strata tens of metres thick, in which the sandstone content can approach 90 per cent. Between the thicker sandstones, and between the packages in which they occur, are intervals of thinner bedded Bouma turbidites. The thicker sandstone beds are absent from the Trefechan Formation (Trf). In the latter, the turbiditic lithologies are arranged in sandstone-mudstone couplets. Individual couplets comprise a thin basal sandstone bed (up to 0.4 m), displaying parallel-, cross- or convolute-lamination, overlain by a mudstone unit that can range up 0.5 m in thickness. Flute or groove casts and a wide variety of trace fossils are commonly preserved at the base of the sandstones. Locally, the proportion of sandstone in the Trefechan Formation can be less than 10 per cent, but values ranging between 30 and 60 per cent are typical. Subordinate hemipelagic mudstones, including both laminated and burrow-mottled varieties, are commonly present between the turbidite couplets.

Palaeocurrent measurements from sole structures and cross-lamination show that the turbidity currents supplying the Aberystwyth Grits Group flowed towards the north-north-east, parallel to the trend of the Bronnant Fault. No graptolites have been recovered from the Aberystwyth Grits in this district, but in adjacent areas the group has yielded a range of turriculatus Biozone assemblages. These demonstrate its lateral equivalence both to upper parts of the Devil's Bridge Formation and the Borth Mudstones Formation present to the south of the Bronnant Fault Zone (Cave and Hains, 1986; Loydell, 1991; Davies et al., 1997).

Gradational contacts between the Aberystwyth Grits and underlying Borth Mudstones are consistent with the eastwards expansion of the Aberystwyth Grits turbidite system across its muddy fringing facies. The packets with thicker turbidite sandstones in the Mynydd Bach Formation record sand deposition by fast moving, high-concentration flows, on a series of lobe-like constructional features. In contrast, slower moving and lower concentration turbidity currents spread more widely across the system surface depositing Bouma turbidite couplets on these lobes, but also to the side and in front of these features where they accumulated as sequences of the Trefechan Formation. The distribution of the two formations suggests that many of the lobe-building flows were deposited within a down-faulted graben created by synsedimentary movements on the Bronnant and Mynydd Bach faults. The uplifted footwall region to the south of the Bronnant Fault served, initially to limit the westward spread of the Devil's Bridge Formation, but it was later blanketed by southerly sourced mud (Borth Mudstones) deposited from low-concentration flows able to spread beyond the limits of the trough to the west.

Cwmystwyth Grits Group and fringing facies

The Blaen Myherin Mudstones Formation (BMM) crops out beneath the Cwmystwyth Grits Group to the east of Tregaron and in a faulted outlier north of Llanio [SN 640 590]. A small inlier of the formation is also present near Carreg Hirfaen [SN 627 463], close to the faulted core of the Central Wales Syncline. The Blaen Myherin Mudstones range up to 200 m in thickness. Lithologically, the formation compares closely with the Borth Mudstones and it also gradationally succeeds the Devil's Bridge Formation. Some laminated hemipelagites in the Blaen Myherin Mudstones contain graptolites of the upper utilis and johnsonae subzones and the formation is therefore younger than any part of the Borth Mudstones farther west. It records southerly sourced turbidite mud deposition beyond the initial limits of Cwmystwyth Grits Group sand deposition.

To the east of the Central Wales Syncline, various divisions of the Cwmystwyth Grits Group succeed, and pass laterally into, the Caerau Mudstones Formation (CaM) and these facies changes account for marked variations in thickness. The formation ranges up to 280 m in thickness north of the Llyn Brianne Reservoir where it overlies silt-laminated facies of the Rhayader Mudstones. It succeeds the normal facies of the latter formation along the limbs of the Clyngwyn Syncline, but thins dramatically from 450 m in the east to little more than 50 m on the western limb. The base of the Caerau Mudstones is taken at the base of a thick (tens of metres) sequence of thin- to medium-bedded turbidite mudstone with laminated hemipelagites. Throughout the formation, these anoxic laminated beds alternate with oxic sequences containing burrowed hemipelagites. In the lowermost part of the formation are packets of strata closely comparable to the underlying Claerwen Group slope-apron facies. In the district, the Caerau Mudstones are of late turriculatus Biozone age (proteus to carnicus subzones). This division also records southerly sourced turbidite mud deposition in front and to the east of Cwmystwyth Grits Group sand deposition.

The outcrop of the Cwmystwyth Grit Group occupies a broad tract across the centre of the district associated with the complex Central Wales Syncline and forming the high ground of the Cambrian Mountains. The group, the total thickness of which may exceed 3000 m, comprises four formations: the Rhuddnant Grits, Doethie Formation, Pysgotwr Grits and the Glanyrafon Formation. All four divisions comprise thinly interbedded turbidite sandstone and mudstone, but the Rhuddnant Grits and the Pysgotwr Grits are also characterised by the presence of packets of medium to thick, tabular beds of stuctureless muddy sandstone (high-matrix sandstone). In contrast, the thicker beds diagnostic of the Doethie Formation are composed of cleaner sandstone which typically display tractional lamination and dewatering structures. Parts of the group devoid of these packets of thicker sandstone comprise the Glanyrafon Formation, which interdigitates with all the other units. Palaeocurrent indicators demonstrate that all the divisions of the group were derived from the south, thus recording the advance, during the Telychian, of a major sandstone lobe turbidite system, the form of which was strongly influenced by intrabasinal faulting (Davies et al., 1997).

The Rhuddnant Grits Formation (Rdd) crops out on both limbs of the Central Wales Syncline. In the west, it includes a lower portion that is 1000 m thick, the Llyn Teifi Member (LyT), characterised by abundant medium to very thick, commonly amalgamated, beds of coarse-grained, feldspathic high-matrix sandstone. Granules and small pebbles have been noted at the base of some beds. For much of its crop in the district, the member is confined between the Teifi and the Carnau faults and its main outcrop ends south of Tre-Herbert, where these fractures converge. A smaller outcrop occurs at the southern margin of the district on the western flanks of Mynydd Llanybydder. In the upper part of the Rhuddnant Grits in the west, high-matrix sandstones, the majority under 0.5 m thick, occur in packets up to 50 m thick, separated by thinner intervals devoid of these beds. Throughout the formation, high-matrix sandstones range from very muddy varieties (commonly with contorted rip-up clasts that typically display the regional Acadian cleavage) to harder, less muddy, normally graded beds. These various sandstones were interpreted as the deposit of fast moving, sediment gravity flows, which varied between slurry-like debris flows and high-concentration turbidity currents (Davies et al., 1997). The base of the western sequence of the Rhuddnant Grits probably lies within the upper turriculatus Biozone (johnsonae Subzone) with the top dated as upper crispus Biozone (sartorius Subzone) in age.

Within the core and on the eastern limb of the Central Wales Syncline, the Rhuddnant Grits comprises a series of tongues separated by thick sequences of thinly interbedded turbidite sandstone and mudstone (Glanyrafon Formation). The lowest and most easterly of these tongues forms the base of the Cwmystwyth Grit Group in the north-east of the district where it succeeds Caerau Mudstones of uppermost turriculatus Biozone (carnicus Subzone) age. To the south, this basal Rhuddnant Grit sequence passes laterally into the Doethie Formation (Dot) as the high-matrix sandstone beds of the former give way to cleaner sandstones in which dish structures and convolute lamination record the dewatering of rapidly deposited turbidite sand (Plate 2). This division thickens southwards at the expense of the Caerau Mudstones so that in the Doethie and Pysgotwr valleys (Front cover) a 600 m-thick sequence rests on the Nant Brianne Formation. Upper parts of this sequence yield lower crispus Biozone graptolites. Small outliers of the Doethie Formation occur in the south of district along the axis of the Branddu Syncline, where loose blocks suggest the presence of conglomerate beds, and on the western limb of the Doethie Anticline, where a lateral passage south-eastwards into the Glanyrafon Formation is evident.

Separated from the Rhuddnant Grits and Doethie Formation by the Glanyrafon Formation, the younger Pysgotwr Grits Formation (Ptr) crops out in a series of fault slices along the fractured core of the Central Wales Syncline. The formation ranges up to 500 m in thickness. As in the Rhuddnant Grits, packets with high-matrix sandstone beds alternate with sequences of exclusively thin-bedded sandstones and mudstones. Many of the high-matrix sandstones are noticeably coarser grained with scattered granules, and the mudstones are typically green. Locally present in the formation are lenticular units, up to 10 m thick, of very thick-bedded turbidite conglomerate. Within these bodies, individual flow units ranging up to 4.5 m in thickness display the features described by Lowe (1982), typical of surging high-concentration, coarse-grained turbidites. Above a thin, inversely graded base, imbricate pebble-cobble conglomerates grade upwards into granule conglomerates and coarse sandstone, but within these are lenses and stringers of coarser conglomerate that mark the bases of a number of fining-upwards subsequences. Megaripple cross-stratification and dewatering structures occur locally in the sandstones. Smith (1987b) recognised that these conglomerate bodies (Plate 3) occupy giant scours, several hundreds of metres across, but which may extend for over 3 km along strike, in the direction of palaeoflow. The deepest parts of the scours are sited at the southern (up-stream) end of each body. Clasts in the conglomerates are predominantly well-rounded pebbles, but range up to boulders over a metre in diameter. They include abundant white-weathering rhyolite and ash-flow tuff, as well as granite, vein quartz and rare corals, in addition to locally derived rip-up clasts of sandstone and mudstone. In the adjacent Rhayader district, the base of the Pysgotwr Grits was shown to lie in the uppermost part of the crispus Biozone, but the bulk of the formation is of griestoniensis Biozone age (Davies, 1933; Smith, 1987b; Davies et al., 1997).

Thick sequences within the Cwmystwyth Grits Group that do not contain thicker sandstones are included in the Glanyrafon Formation (Glr). Along the western limb of the Central Wales Syncline, the formation forms a subordinate part of the group, occupying two discrete horizons. A lower tongue (Glr'), up to 200 m thick, separates the Rhuddnant Grits from the Pysgotwr Grits, and an upper tongue (Glr''), also up to 200 m thick, succeeds the Pysgotwr Grits to form the highest parts of the group. The lower tongue lies within the upper crispus Biozone (sartorius Subzone). The base of the upper tongue lies within the upper part of the griestoniensis Biozone and a horizon near the top, above a packet of thicker sandstones (sa) to the east of Llyn Berwyn, has yielded graptolites of the spiralis Biozone, the first record of this assemblage from the Central Wales Syncline.

More complex relationships are apparent within the faulted core of the syncline where further tongues of the Glanyrafon Formation are interleaved with units of Rhuddnant Grits. In contrast, along the eastern limb of the syncline, the Glanyrafon Formation dominates the Cwmystwyth Grit Group outcrop. Here a highly folded sequence, up to 500 m thick, succeeds attenuated sequences of the Rhuddnant Grits and Doethie Formation present in the north of the district. Farther south, along the limbs of the Branddu Syncline, the Glanyrafon Formation appears beneath the Doethie Formation increasing southwards to occupy much of the lower part of the group. Here the sub-Pysgotwr Grits sequence rests directly on the Rhayader Mudstones and may approach 750 m in thickness. Upper turriculatus Biozone (proteus Subzone) graptolites are present near the base. To the east, in the core of the Clyngwyn Syncline, the Glanyrafon Formation exhibits a rapid lateral passage south-eastwards into the Caerau Mudstones. The thin-bedded sandstone-mudstone couplets that make up the Glanyrafon Formation were deposited by relatively slow moving, low concentration, turbidity currents.

From the marked changes in thickness and facies, which occur across the faulted axis of the Central Wales Syncline, it is clear that this fracture belt influenced deposition of the Cwmystwyth Grits Group. To the north, the thick western sequences of Rhuddnant and Pysgotwr grits were deposited in an actively subsiding, half-graben located along the western side of the Claerwen Fault (Davies et al., 1997), and this passes southwards into the complex fault belt recognised in the Lampeter district. The topographic trough (half graben) created by these faults was the focus for deposition from high-concentration turbidity currents and debris flows as evidenced by the numerous packets of high-matrix sandstone. Deposition of the Llyn Teifi Member of the Rhuddnant Grits is thought to record a period when the frequency and volume of such flows was particularly high, but also a time when an uplifted ridge above the footwall of the fault was particularly effective at confining these types of flow to its western side. Subsequently, as this fault-induced topography became more subdued due to infilling, high-concentration flows were able to spread more widely to deposit the much thinner and younger eastern sequences of Rhuddnant Grits within a trough limited by the Tywi Lineament (Figure 4). Within this trough, the northwards (down current) transition from the Doethie Formation into the Rhuddnant Grits supports a suggestion (Davies et al., 1997) that the flows which deposited the high-matrix sandstones of the Rhuddnant Grits may have evolved from high-concentration, sandy turbidity currents into cohesive, debris flows as they entrained mud eroded from the basin floor.

The thick, slow moving, low-concentration turbidity currents, which deposited the ubiquitous thin-bedded sandstone-mudstone couplets present throughout the group, were less influenced by fault-induced topography. Thus, the distribution of the Glanyrafon Formation broadly illustrates those regions where such currents were able to deposit beyond the reach of the higher concentration flows. However, the discrete lower and upper tongues of the formation present along the western limb and core of the Central Wales Syncline appear to record periods when high-concentration flows were not entering the basin. It has been suggested that the high-matrix sandstone-bearing parts of the Cwmystwyth Grits Group record periods when seismic activity was promoting regular slumping along the southern basin margin (Smith, 1987b; Clayton, 1992). The western tongues of the Glanyrafon Formation may therefore record periods of tectonic quiescence. The abundant volcanic clasts in the conglomerate bodies of the Pysgotwr Grits can be matched with igneous rocks in Pembrokeshire likely to have been exposed during this interval. The giant scours that these bodies infill are thought to mark sites where dense, sediment-laden gravity flows emerged from confining channels on to the basin floor (Smith, 1987b).

Overlying the upper tongue of Glanyrafon Formation within the faulted core of the Central Wales Syncline, the Dolgau Mudstones (Dgu) comprise distinctive olive-green and locally red mudstone with siltstone laminae and widely scattered thin beds of sandstone. These are the youngest rocks exposed in the district and may exceed 150 m in thickness. No graptolites have been recovered from these burrow-mottled strata, but in the adjacent Rhayader district, they underlie strata of Wenlock age. The Dolgau Mudstones record a cessation of sandstone-lobe deposition and the resumption, during the latest Telychian, of oxic slope-apron mud accumulation, an event widely recognised throughout the Welsh Basin (Davies et al., 1997).

Structure

The Lampeter district lies within the Lower Palaeozoic Welsh Basin, a small transtensional sedimentary basin, founded on the intracratonic crust of the microcontinent of Eastern Avalonia (Soper and Hutton 1984; Pickering et al., 1988). Plate tectonic reconstructions indicate that Eastern Avalonia was formed from the breakup of the supercontinent of Gondwana during the early Ordovician. It migrated northwards during the later Ordovician impinging on the Laurentian continent in early to mid-Silurian times as the intervening Iapetus oceanic crust was consumed by subduction before finally closing in the late Silurian (Soper and Woodcock, 1990). Further collisions between this amalgamated continent (Laurussia) and rifted portions of Gondwana (Armorica, Iberia) took place during Early to Mid-Devonian times, before the final convergence of Laurussia and Gondwana, which resulted in the Variscan Orogeny at the end of the Carboniferous epoch.

The structural features of the Lampeter district are largely the result of tectonic movements during the Caledonian orogenic cycle (Cambrian to Early Devonian), during which the Welsh Basin underwent periods of subsidence, inversion and uplift. These movements culminated in the late Early Devonian (Emsian) regional deformation known as the Acadian Orogeny (McKerrow et al., 2000; Soper and Woodcock, 2003). The effects of Acadian deformation have largely overprinted the evidence of any earlier basin-forming tectonic episodes, but intra-Ordovician to Silurian movements are indicated by the changes in facies and thickness in the turbidite sequences adjacent to several of the major fault systems. The latest fault activity within the district, probably a late or post-Acadian event (Fletcher et al., 1993; Davies et al., 1997), produced a series of west-north-west-trending mineralised cross-faults.

The principal geological structures of the district are shown in (Figure 5). The district is crossed by a number of regional structural lineaments, represented by belts of complex folding and faulting, which have been active throughout much of the geological history of the Welsh Basin. In the north-west, the Glandyfi Lineament, broadly coincident with the Bronnant Fault and its splays, marks an important 'vergence divide' (Cave and Hains, 1986; Wilson et al., 1992). It is separated from the Central Wales Lineament by the complexly folded Teifi Anticlinorium in which a number of major second order anticlines and synclines have been recognised. The Central Wales Lineament, a broadly synclinal belt of anastomosing faults and tight folds, preserves the youngest strata observed within the district. Older Silurian and Ordovician strata to the south-east are brought to crop within a series of tight, second order folds extending north-eastwards from Mynydd Mallaen to Llyn Brianne. The Ordovician rocks in the extreme south-east of the district form part of the western flank of the Tywi Lineament that broadly coincides with the transition from shelf to basinal facies within the Welsh Basin and marks the south eastern limit of strong regional Acadian deformation.

The coarse clastic Bryn Nicol Formation accumulated in a small 'pull-apart' basin formed during a period of late Ordovician fault activity affecting the Tywi Lineament (Schofield et al., 2004). However, there is little to indicate that such movements continued into the early Silurian (Rhuddanian to Aeronian). The only evidence of syndepositional fault movement within the district during this period is provided by the Llyn Brianne Fault which, during the late Aeronian, influenced deposition of the Nant Brianne Formation.

By mid-Telychian times, the convergence of Avalonia and Laurentia resulted in localised uplift around the margins of the Welsh Basin, notably in Pembrokeshire (Hillier, 2002). The voluminous amount of sediment thus produced was supplied to the basin along fault-controlled pathways defined by the major lineaments (Soper and Woodcock 1990; Wilson, et al. 1992; Davies et al., 1997). In the Lampeter district and adjacent areas, the southerly derived turbidity flows of the Mynydd Bach Formation were impounded along the western side of the Glandyfi Lineament in a trough formed by the hanging wall of the Bronnant Fault Zone, allowing over 2000 m of coarse-grained sediment to accumulate during the gemmatus to utilis subzones interval. The focus of deposition appears to have switched to the Central Wales Lineament during the late turriculatus Biozone, where up to 1500 m of the southerly sourced Rhuddnant Grits preferentially accumulated against the Claerwen Fault and associated structures. The lineament continued to be a focus of deposition, at least intermittently, until the latest Telychian (griestoniensis Biozone), when the 500 m-thick sequence of Pysgotwr Grits was deposited.

By early Wenlock times, the main focus for southerly sourced sand deposition had moved to the east of the district, adjacent to the reactivated basin-bounding, Twyi Lineament (Davies et al., 1997). There was, therefore, a systematic eastward shift in the focus of sedimentation throughout the late Llandovery as successive fault-controlled sub-basins became choked with sediment. It is not clear why this occurred, although it probably resulted in part from successive strike-slip (or transtensional) reactivation of these intrabasinal faults as Eastern Avalonia docked obliquely and rotated anticlockwise against Laurentia (Soper and Woodcock, 1990; Woodcock and Strachan, 2000; Soper and Woodcock, 2003).

Most of the major lineaments were probably reactivated during the widespread transpressive Acadian deformation, which also gave rise to a weak to moderately strong regional cleavage and an array of north-east- to south-west-plunging periclinal folds the hinges of which are commonly cut by a series of north-east-trending strike faults. Folding occurs on several orders of magnitude. First order folds, which are typically gentle to open structures with wavelengths of 16 to 20 km, include the Teifi Anticlinorium and Central Wales Syncline, the latter apparently nucleating on the thick sandstone successions of Rhuddnant and Pysgotwr Grits that accumulated along the Central Wales Lineament (Smith, 1987c; Woodcock et al., 1988). Within these structures are a number of open to closed, second order anticlines and synclines (Figure 5) with wavelengths of around 5 km, with hinges that are commonly cut out by faults. Lower order folds have commonly nucleated on the limbs of the second order structures and, being strongly controlled by lithological layering, are most widespread in formations comprising interbedded sandstone and mudstone. Over most of the district, the second and lower orders of folds display a south-easterly vergence, with steep to overturned south-eastern limbs and steep north-westward inclined axial surfaces (Plate 4). In the north-west of the district however, they consistently verge north-westward. Their axial surfaces and associated cleavage dip south-eastwards at moderate angles, but steepen progressively towards the Bronnant Fault Zone, which is characterised by a belt of upright folds and near-vertical cleavage. The feature coincides with the eastern boundary of the Aberystwyth Grits Group, and marks the course of the Glandyfi Lineament (Cave and Hains, 1986; McDonald et al., 1992; Davies et al., 1997), a regionally important structure that extends northwards into the Cadair Idris district (Pratt et al., 1995) and south westwards into the Llangranog area (Davies et al., 1997).

Cleavage is variably developed throughout the district, but is generally weaker to the west of the Central Wales Lineament, and only sporadically developed in the Borth Mudstones Formation in the north-west. Over most of the district however, cleavage is more strongly developed within the mudstone-dominated lithologies, and is locally a penetrative fabric, although slumped and disturbed mudstones usually display only a rough, discontinuous structure; in sandstones, cleavage generally comprises spaced, locally anastomosing, solutional seams. The orientation of cleavage generally mirrors that of the fold axial surfaces, maintaining a steep north-westward inclination over most of the district, but fanning over the Glandyfi Lineament to dip south-eastwards in the north-west. However, it is not exactly axial planar to the folds, but transects their hinges in a clockwise sense at angles of up to 16º. Such transection is thought to reflect the sinistral transpressive nature of the Acadian Orogeny in Eastern Avalonia. (Woodcock et al., 1988; Woodcock, 1990; Davies et al., 1999).

The dominant fault trend throughout the district is north-eastwards. A subsidiary linked set of faults is orientated north to north-north-east, and both these groups are cut by the array of mineralised east-north-east-trending fractures that are largely confined to the area west of the Central Wales Lineament. Most of the faults reveal variable vertical offsets, although it is inferred that they underwent strike-slip displacements also. The history of fault movement within the district is complex, with several of the larger fractures revealing elements of syndepositional, late Acadian and possibly Variscan displacements. North to north-east-trending structures such as the Teifi and Mynydd Bach faults may represent antithetic fractures to the Bronnant and Claerwen fault systems that were active during deposition of the Aberystwyth and Cwmystwyth Grits groups (see above). Subsequently they, and other similarly orientated faults, were probably active components of a series of strike-slip duplexes that developed along the major lineaments during the Acadian deformation (Woodcock et al., 1988). These arrays of strike-faults commonly cut out fold limbs, thereby confirming that their subsequent movements are associated with the terminal phase of the Acadian Orogeny or are even later. They are rarely mineralised, the main exception being the Abergwesyn Fault in the south-east of the district. The east-north-east-trending set of cross faults that cut the dominant north-east array, between the Glandyfi and Central Wales lineaments, appear to represent one part of a conjugate fault set that hosts the bulk of the lead-zinc mineralisation of the Central Wales Mining Field. Isotopic evidence provided by the vein minerals indicates that fault movements along these lines took place during the late Early Devonian and, thus, are related to the climax of the Acadian Orogeny. A second set of isotopic dates from these veins infer a second significant period of fault displacement during the early Carboniferous (Fletcher et al., 1993; Davies et al., 1997).

Metamorphism

The low-grade metamorphism that affected the district is reflected in the growth of illite in response to sedimentary burial, tectonic thickening and accommodation of strain through cleavage formation during the Acadian orogenic event, and is usually measured by its Kubler Index (Robinson et al., 1990; Bevins et al., 1995). Unpublished BGS data for the district indicates that the rocks were metamorphosed to the low anchizone grade, although lower grade, late diagenetic zone assemblages, reflecting burial to a depth of around 4 to 5.5 km, are a feature of the Borth Mudstones Formation and certain parts of the Tywi Anticline where cleavage is poorly developed. High anchizone assemblages are confined to the Pysgotwr Grits and Dolgau Mudstones within the Central Wales Lineament, and small areas of high anchizonal strata are also found to the east of Llyn Brianne, in the core of the Cothi Anticline, and immediately west of Llanybydder, where cleavage is strongly developed. The reason for these elevated metamorphic areas is not simply a reflection of burial depth, as they affect some of the youngest Silurian strata within the district. Therefore, it is likely that a component of high strain associated with the Central Wales Lineament and other structures is responsible for their development.

Mineralisation

The district lies within the Central Wales Mining Field and mineral veins were widely exploited for lead and zinc sulphide minerals during the mid 19th century (Institute of Geological Sciences, 1973; Ball and Nutt, 1976).

Mineral veins are confined to the east of the district where they are located in Ordovician rocks forming the western flank of the Tywi Anticline and within outcrops of the Silurian Cwmere Formation and Claerwen and Cwmystwyth Grits groups. Many of the veins are developed along steeply dipping, west-north-west-trending, normal faults and joints. However, at Nantymwyn mine [SN 790 448], at Rhandirmwyn, lead ore was worked from a conjugate set of north-east- and north-trending veins along the Abergwesyn Fault and its splays developed in the Bryn Nicol and Yr Allt formations. North-east-trending veins in Silurian rocks were also worked at North Nantymwyn [SN 765 482], Rhydtalog [SN 794 534] and Brynambor [SN 745 509] mines. In general, the magnitude of the displacements across of veins is small. Many of the faults associated with the veins can be traced along strike for several kilometres, but only relatively short sections contain economic mineralisation.

The veins typically comprise a stockwork, locally up to 30 m wide, of mineralised breccias and lenses of sulphide minerals. However, only the highest grade sections, commonly less than a metre wide, were normally worked. The brecciation is thought to have resulted from several phases of in situfragmentation by mineralising fluids under extremely high pressures (Phillips, 1986). The most important minerals worked in the mining field were galena (PbS) and sphalerite (ZnS) with minor amounts of chalcopyrite (CuFeS₂). Lead was the principal metal produced, with silver as a by-product. Other sulphide minerals include crystalline and framboidal pyrite (FeS₂), and trace amounts of arsenopyrite (FeAsS) and cobaltite (CoAsS) (Raybould, 1974).

Various hypotheses have been proposed for the source of the sulphide mineralising fluids in mid-Wales. The absence of igneous intrusions at outcrop, and of geophysical evidence to suggest their presence at depth beneath the mining field, appears to discount previous suggestions of a magmatic hydrothermal source. A preferred model, supported by isotopic and fluid inclusion evidence, invokes leaching of metals from the Lower Palaeozoic sediments during dewatering and low-grade metamorphism associated with the Acadian Orogeny (Davies et al., 1997).

Geophysics

The regional Bouguer gravity anomaly map reveals a more or less steady decline from north-west to south-east across the Lampeter district (Figure 6). The trend of the contours mirrors the north-east–south-west structural grain of the region as well paralleling the major fracture belts which traverse it. The area of lowest anomaly values, in the south-eastern corner of the district, coincides with the outcrop of Ordovician rocks associated with the Tywi Anticline (or Lineament) and is consistent with a thinning of sedimentary fill of the Lower Palaeozoic Welsh Basin in the vicinity of this basin-bounding structural belt.

Quaternary

During the Pleistocene, climatic change brought about a succession of ice ages that affected much of the British Isles. It was largely the erosion and deposition by ice sheets, valley glaciers and glacial melt waters during this period which fashioned the landscape of the district into its present form.

Prior to the Pleistocene glaciation, the district is thought to have exhibited a series of platforms at different elevations (Brown, 1960; Potts, 1968), representing former marine planation surfaces that became isolated as a result of river down-cutting during Cainozoic (Tertiary) regional uplift (Dobson and Whittington, 1987). Remnants of these surfaces are still preserved within the Cambrian Mountains today. The presence of older, now abandoned, sections of the main river valleys, and of incised meanders, also testify to these periods of uplift (Waters et al., 1997).

Ice subsequently modified this landscape in the Pleistocene by changing drainage patterns, over-steepening and over-deepening valleys to give typical 'U'-shaped profiles (Plate 5) and depositing glacial material in valleys and hollows. Though the district was probably covered and moulded by ice on more than one occasion, the surface glacial deposits in the district appear to relate exclusively to the last major ice advance, the Late Devensian glaciation, which is thought to have lasted from around 26 000 to 14 468 BP (Campbell and Bowen, 1989). During this glacial episode, ice accumulated in the Cambrian Mountains spreading outwards as the Central Wales Ice Sheet. At its greatest extent, some 20 000 years ago, this ice mass probably covered the whole of the district, with thicker lobes located along the Aeron, Teifi, Tywi and Dulais valleys. Material eroded and carried by the ice was widely deposited from beneath the ice mass, as a thin till veneer over the bedrock, both during its advance and retreat. During its retreat, as ice withdrew from lowland interfluves, the thicker ice along the main valleys persisted as a series of separate glaciers. A suite of constructional landforms (moraines) preserved within these valleys record sediment accumulation in front and along the sides of these glaciers during ice retreat. Glacial melt waters, released in huge quantities by the down-wasting ice, reworked and redistributed the recently deposited glacial material and also deposited it in short-lived proglacial lakes that were impounded by morainic dams. Narrow rock gorges and wide alluvial tracts reflect the influence of these glacial landforms on the courses followed by the main rivers as they re-excavated their channels. Kettleholes mark the former sites of large masses of ice left buried within the glacial and reworked sediment during the deglaciation of the district.

As the ice withdrew, tundra-like periglacial processes, active periodically until about 10 000 years ago, modified many of the surface glacial deposits and produced a series of distinctive landforms and deposits. Seasonal freeze-thaw activity led to the shattering of bedrock and promoted solifluction on the slopes of the new glacial deposits. Deep weathering profiles, thick regolith, and the rarity of crags throughout much of the western and southern parts of the district, in lowland and upland areas alike, show these regions were deglaciated first, and exposed to periglacial conditions longest. In contrast, the well-featured and craggy landscape of the Cambrian Mountains in the north-east of the district, suggests that the ice cover survived there for much longer. In some lowland settings in the Afon Teifi catchment, periglacial conditions promoted the growth of masses of ice within the glacial sediment, which heaved up the ground surface into a series of dome-like features known as pingos (Watson and Watson, 1974) (though this has been contested by Harris et al. 2005). Following the melting of their ice cores, the locations of these features are now marked by deep, peat-filled hollows surrounded by distinctive, circular or irregular ramparts. Throughout this periglacial period and subsequently, during the Holocene, alluvial deposits and hill peat accumulated. In more recent time the landscape has been modified by man.

The most widespread glacial deposit, which accumulated beneath the ice, is a blue-grey Till that comprises stiff, ill sorted, gravelly sandy clay (diamicton). Clasts, locally ranging up to boulder size, are all composed of bedrock present within the Cambrian Mountains. Downslope movement during later periglacial episodes has locally imparted slope-parallel clast fabrics. Till deposits range up to several metres in thickness.

Glaciofluvial Ice-contact Deposits and Hummocky Glacial Deposits were deposited principally within the confines of major river valleys during deglaciation. The former consist predominantly of poorly sorted, but commonly stratified, sand and gravel, and also include lenses of clay-bound gravel, gavelly clay and laminated clay. Hummocky glacial deposits, generally composed of clay-rich, nonstratified gravels, give rise to more poorly drained ground, but can include lenses of better sorted and stratified sand and gravel. Thickness varies greatly, and both types of deposit range up to tens of metres. Both types of deposit form irregular, moundy or terrace-like features along the sides of the valleys, expanding locally to occupy much of the centre of these valleys. The lateral features represent degraded kame terraces deposited along the sides of down-wasting glaciers. The central masses are interpreted as retreat or push moraines formed at the front of these valley glaciers during either pauses in their retreat or temporary re-advances of the ice. In the Teifi valley, such moraines are located at Pencarreg [SN 535 455], Llanfair Clydogau [SN 626 515] and Abercarfan [SN 665 577]. Similar features are present in the Aeron valley at Llundain-fach [SN 560 565] and Hafod [SN 575 575], and in the Tywi valley at Penrhyn [SN 773 423]. Hollows in the surfaces of these features, some filled with water, others with lacustrine and peat deposits, represent kettleholes. Llyn Pencarreg, which is up to half a kilometre across, occupies the largest kettlehole in the district located in the Pencarreg moraine.

Melt-water streams emerging from the front of the retreating ice deposited extensive spreads of stratified sand and gravel (Glaciofluvial Sheet Deposits; (Plate 6)) along the bottoms of the main valleys and as fan-like bodies at the confluence with major tributaries. Examples of the latter in the Teifi valley near Lampeter may represent deltas build out into a former glacial lake (see below). Following postglacial incision, many of these sheet deposits now occur as a series of high terraces some in excess of 15 m above the modern river. Kettleholes are also common in the surfaces of some of these features.

In lakes, dammed by the retreat moraines, glaciolacustrine depositscomprising laminated clays and silts accumulated until out-flowing water breached the dams or cut gorge-like exits in the adjacent rock of the valley side. The Teifi and Aeron valleys each display evidence of a succession of these lakes, the deposits of which are now largely concealed beneath wide belts of more recent alluvium. The lake clays have been proved in several boreholes in both valleys. One such borehole [SN 526 569], near Ystrad Aeron in the Aeron valley, encountered 53 m of these deposits beneath 9.5 m of alluvium and alluvial fan deposits. In the Teifi valley, these clays occur close to the surface in pingo-affected ground near Garthenor [SN 635 560]. Glaciolacustrine deposits are also likely to be present at depth in many of the larger kettleholes.

In the wake of the retreating ice, seasonal freezing and thawing promoted downslope movement of glacially deposited material by frost creep or saturated flow to form head. In the district, head deposits have been recorded only where they are of significant thickness and have topographic expression; hence much soliflucted till present in the district has not been distinguished from its parent material. Head deposits comprise highly variable, gravelly, sandy and silty clay. Localised deposits of distinctive stratified gravels composed of angular mudstone fragments in a silty clay matrix are distinguished as Head Gravel and represent accumulations of rock fragments (nivation scree) produced by periglacial frost shattering.

The modern drainage pattern was established during the Holocene. Initially, the main river valleys were occupied by broad braid plains, but following repeated river down-cutting in response to postglacial regrading and isostatic readjustment, these former braid plain deposits are now preserved as a series of elevated river terrace deposits. These typically comprise stratified sand and gravel. Alluvial fan deposits, composed of similar sand and gravel, developed where steeper graded tributaries emerged from their channels into the main river valleys. These too show evidence of subsequent incision and many are now abandoned as sites of active sedimentation. The more recent river deposits, which are still prone to regular flooding, comprise the modern alluvium and are composed of silt and organic-rich clay with beds and lenses of sand and gravel. Small areas of lacustrine alluvium, some probably overlying earlier glaciolacustrine deposits, have accumulated in lakes and ponds formed within enclosed hollows including kettleholes and pingos. As vegetation was re-established under more temperate climatic conditions, Peat deposits accumulated in many of these same sites as well in areas of restricted drainage in the upland areas of the district as hill peat. Isolated landslides are located mainly where drift deposits have been undercut by meandering rivers and streams and many of these remain active today. A large bedrock landslide [SN 6925 4685] is situated in Blaen Rhisglog Plantation. Locally present along some of the steeper valley sides are accumulations of angular rock fragments (scree or talus).

The influence of man has been extensive. Massive deforestation took place in the Bronze Age around 4000 years ago and impacted on drainage and flooding. Though made ground composed of tipped material of variable composition and thickness occurs in most settlements, only the largest deposits are distinguished and for cartographic reasons, major road and railway embankments are not depicted on the 1:50 000 scale map.

Chapter 3 Applied geology

Earth science factors have a significant influence on human activity, and as such must be taken into account in 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 land-use, pre-existing development 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.

A detailed account of the earth science factors critical to effective land-use planning along the Teifi valley is provided by Waters et al. (1997).

Mineral resources

These are natural concentrations of minerals or bodies of rock that are of potential economic interest.

Parts of the district lie within the Central Wales Mining Field and metalliferous minerals, notably ores of lead and zinc, were widely exploited during the 19th century and they remain a potential resource (Ball and Nutt, 1976). The most important lead and zinc mines were at Llanfair Clydogau [SN 629 511], where silver was also obtained, and at Nantymwyn [SN 788 446] (Plate 7). Apart from the Pumpsaint gold mines in the adjacent Llandovery district, Nantymwyn was the most productive metal mine in southern Wales in the 18th and 19th centuries. Production ceased in 1932. The records for zinc production across the mining field appear to bear little relation to the amount of lead produced and, since the price of zinc was low during the main production period, it has been suggested that much zinc ore (sphalerite) may have been left unworked (Davies et al., 1997).

Hard-rock for aggregate is commercially extracted at Ty Hywel quarry [SN 598 447], which works a sequence of thick sandstones and interbedded mudstones in the Cwmystwyth Grits Group. Numerous disused quarries testify to the past exploitation of many of the other bedrock divisions, notably the Devil's Bridge Formation and Aberystwyth Grits Group, though many of these were probably worked as local sources of building stone. The presence of pyrite-bearing (anoxic) horizons in many of the mudstone divisions, but most notably in the Cwmere and Rhyddlan formations, mitigates against their commercial use as a bulk aggregate or fill. On weathering such material may give use to sulphate concrete attack and cause heave.

Sand and gravel is worked by individual farmers at several small pits in the Teifi valley with commercial operations at Abercoed [SN 667 579] and near Pant [SN 659 566]. Many of the pits are sited in glaciofluvial ice-contact deposits. Similar deposits in the Aeron and Tywi valleys also have resource potential, though the heterogeneous nature and variable clay content limits their commercial value. More localised glaciofluvial sheet and river terrace deposits may provide better quality material, but the former can contain large boulders up to 1 m in diameter. The quality of these potential resources may be further limited by the presence of locally weak mudstone clasts, including pyritic lithologies, and of clasts of lead and zinc sulphide-bearing vein material.

Peat has been worked in the past as a source of fuel, both in the upland areas and in some lowland basins.

Water resources

High annual rainfall means that the principal water resource of the district is surface water. The upper Tywi catchment supplies water to the Llyn Brianne Reservoir, the source of mains water for Swansea and Carmarthen. The reservoir is also utilised to generate hydroelectric power.

Groundwater is abstracted for public supply from the Olwen Borehole [SN 5817 4959] sited in glaciofluvial sheet deposits, and private supply boreholes and springs provide water to many individual farms. In the Aeron valley at Talsarn, groundwater from alluvial fan and glaciofluvial gravels is abstracted via boreholes and bottled as 'spring water' at Llanlyr [SN 543 555]. Although the Lower Palaeozoic rocks of central Wales were not thought to be a significant aquifer, recent studies in the region have shown that modest groundwater supplies can be obtained from fractured and weathered solid rocks in the near surface zone as well as from permeable superficial deposits (Robins et al., 2000; Merrin, 1999).

Potential geological hazards

Migration of toxic leachate from areas of worked or made ground presents a significant pollution potential, and may lead to contamination of local surface or groundwater resources and alluvial sediments. Potential sources of pollution may include the former metal mines and their waste tips, poorly lined landfill sites, agricultural waste-disposal sites and active or former industrial sites such as sewage works, gravel pits and quarries. Similarly, runoff during heavy rainfall, or due to poorly planned remediation of such sites may also allow soluble contaminants or contaminative sediment to enter surface waters.

Both the Teifi and the Tywi catchments drain parts of the Central Wales Mining Field and water draining from the underground metal mine workings and through waste tips poses a pollution threat. Any future remedial works or renewed exploitation of these mines and tips needs to ensure that soluble contaminants and toxic mine sediment is not released into surface waters of the catchment. Moreover, the alluvial deposits of these rivers and their tributaries are likely to contain layers rich in mine waste, including particles of lead and zinc ore, dating from the main period of mine working during the mid 19th century. The risk and consequences of disturbing these contaminated layers should be a consideration when planning excavations in these deposits, for example for sand and gravel extraction or construction purposes.

River and stream floodplains within the district are susceptible to regular flooding. An indication of those areas that are prone to regular flooding is given by the extent of active floodplain deposits, shown on the map face as alluvium. However, low river terraces may also be susceptible to flooding, and other areas may also suffer inundation during anomalously large floods or when drains and culverts are blocked. 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 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 may accumulate in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions are generally unlikely to pose a significant hazard in the district, the risk can be mitigated through correct design of landfill and development in the vicinity of the 'at risk' sites. Radon is a naturally occurring ionising gas produced by radioactive decay of uranium-bearing minerals, which though present in small quantities in all natural rocks and soils occur in higher concentrations in certain igneous rocks and black mudstones. Radon released from rocks and soils is normally quickly dissipated within the atmosphere and does not present a hazard. However, where it is allowed to accumulate in poorly ventilated spaces, it can give 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 of 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. The limited existing data suggests that radon potential in the district, in common with much of west central Wales, is generally high with between 3 and 10 per cent of properties likely to exceed the UK 'Action Level' of 200 becquerels per cubic metre.

Slope instability, typically manifested as landslides, is locally present in the district. The main trigger is toe erosion along water courses, as for example by the Afon Teifi [SN 535 462] to the north of Pencarreg. Most of the landslides identified in the district are in superficial deposits. A bedrock landslide affects east-facing slopes [SN 6925 4685] in the Glanyrafon Formation, in Blaen Rhisglog Plantation. Although most existing landslides 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 slope instability relies on recognition of problem areas in advance so that suitable preventative or remedial measures can be employed. Waters et al. (1997) provide a detailed assessment of the factors responsible for landslides in the Teifi valley.

Mine entrances and underground workings associated with the mineral veins worked within the district are likely to be highly unstable and their collapse may lead to local unstable ground conditions.

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 landslides. 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 normally have high bearing capacities, except in the weathered zone. Weathering of the pyritic mudstones in the Cwmere and Rhyddlan formations, and in coeval sandstone and conglomerate sequences, and also in parts of the Claerwen Group, may cause heave and promote sulphate concrete attack.

Glaciolacustrine deposits and peat have low bearing capacities and can give rise to moderate settlement. Till and hummocky glacial deposits have moderate bearing capacity but are highly variable. Head, head gravel, alluvial fan deposits and alluvium, all have a low to moderate bearing capacity. 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

Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). Nonstatutory designated conservation sites are Regionally Important Geological Sites (RIGS). Further information on SSSIs and RIGS can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd LL57 2LQ.

Information sources

Further geological information held by the British Geological Survey relevant to the district is listed below. It includes memoirs, reports, published and unpublished maps, documentary and material collections. Enquiries concerning geological data and advice for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Searches of indexes to some of the collections can be made on the Geoscience Data Index system available at BGS web site. Maps, books and other publications are listed in 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 online, at BGS offices or the BGS information centre in the Natural History Museum Earth Galleries, London (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

Original 1:63 360 geological maps are not available for purchase, however copies of these maps can be consulted at the BGS library, Keyworth. Unpublished 1:25 000 scale geological surveys, 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, London. Coloured print-on-demand copies are available for purchase.

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 (Telephone 020 7873 0011).

Geological maps
1:625 000
Solid geology map UK, south sheet, 2001 Quaternary geology, UK, south sheet, 1977
1:250 000
Geological map of Wales, solid geology, 1994
Sheet 52N 04W Mid-Wales and Marches, solid geology, 1990
1:63 360
[Old Series] Sheet 41, 1845 and 1857
[Old Series] Sheet 57SW, 1848
[Old Series] Sheet 57SE, 1848
1:50 000
Sheet 195 Lampeter, bedrock and superficial deposits, 2006
1:25 000
The component 1:25 000 scale maps of the Lampeter district, based on BGS surveying, are listed below, along with the surveyors initials and dates of survey. The surveyors were J A Aspden, J R Davies, D I Schofield, T H Sheppard, R A Waters and D Wilson; in addition, C J N Fletcher and D G Woodall surveyed a narrow strip along the northern edge of the district between 1987–1990. Surveyors whose initials are followed by* undertook some of the surveying as part of the Afon Teifi Catchment Survey between 1995 and 1997.
Digital geological map data
In addition to the printed publications noted above, many BGS maps are available in digital form. The data must be licensed for use. Licensing details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets form part of a series of DiGMapGB products that are available at a variety of scales. Current availability can be checked on the BGS web site.
Thematic maps
1:50 000 scale applied geology maps produced for parts of the Teifi valley as part of the Afon Teifi Catchment Survey Project:
Drift deposits, 1997
Solid geology, 1997
Mineral deposits and surface mineral workings, 1997
Engineering properties, 1997
Hydrogeology and flood limits, 1997
Landslips and slope steepness, 1997
The 1:50 000 applied geology maps for Sheet 195 Lampeter are listed below:
Superficial materials
Bedrock materials
Mineral resources
Engineering geology
Landslide susceptibility
Flood susceptibility
Geophysical maps
1:500 000
Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997.
Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998.
Geochemical atlases
1:250 000
Wales and part of west-central England: streamwater, 2000
Wales and part of west-central England: stream sediments and soils, 2001
Hydrogeological maps
1:625 000
Hydrogeological map of England and Wales, 1997
Groundwater vulnerability map
1:100 000
Sheet 27 Groundwater vulnerability of Dyfed, 1998

Books and reports

Books, reports and papers are listed in References. Most of these are available for consultation at BGS and other public libraries. Details of BGS Technical Reports and other internal BGS reports, on aspects of the geology and biostratigraphy are also available through the enquiry service.

Documentary collections

Documentary collections include records of boreholes and site investigations carried out within the district. These are available for consultation at the BGS, Keyworth. Copies of most records can be purchased through the sales desk. Further information about documentary material can be obtained through the BGS enquiry service. Index information, including site references, is held in digital format and can be viewed through the Geoscience Data Index, available on the BGS web site.

Material collections

Material collections from the district are available for inspection at the BGS, Keyworth, and include petrological hand specimens, thin sections and fossils. Further information about material collections can also be obtained through the enquiry service.

References

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

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

Ball, T K, Davies, J R, Waters, R A, and Zalasiewicz, J A. 1992. Geochemical discrimination of Silurian mudstones according to depositional process and provenance within the southern Welsh Basin. Geological Magazine, Vol. 129, 567–572.

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

Bevins, R E, Merriman, R J, Roberts, B, and Robinson, D. 1995. A field guide to the low-grade metamorphic rocks of the Lower Palaeozoic Welsh Basin. Technical Report, WG/95/4.

Bick, D E. 1979. The old metal mines of mid Wales Part 1: Cardiganshire south of Devil's Bridge. (Oxford: S & S Press.)

Bouma, A H. 1962. Sedimentology of some flysch deposits. (Amsterdam: Elsevier.)

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

Campbell, S, and Bowen, D Q. 1989. The Quaternary of Wales. Geological Conservation Review Series. No. A4.1. (Peterborough: Nature Conservancy Council.)

Cave, R, and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey, Sheet 163 (England and Wales). (London: HMSO.)

Clayton, C. 1992. The sedimentology of a confined turbidite system in the early Silurian Welsh Basin. Unpublished PhD Thesis, University of Cambridge.

Clayton, C. 1994. Contrasting sediment gravity flow processes in the late Llandovery Rhuddnant Grits turbidite system, Welsh Basin. Geological Journal, Vol. 29, 167–181.

Davies, J R, and Waters, R A. 1995. The Caban Conglomerate and Ystrad Meurig Grits Formation — nested channels and lobe switching on a mud-dominated latest Ashgill to Llandovery slope-apron, Welsh Basin, UK. 184–193 in Atlas of deep water environments: architectural style in turbidite systems. Pickering, K T, Hiscott, R N, Kenyon, N H, Ricci Lucci, F, 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). (London: HMSO for the British Geological Survey.)

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

Davies, K A, and Platt, J. 1933. The conglomerates and grits of the Bala and Valentian rocks of the district between Rhayader (Radnorshire) and Llansawel (Carmarthenshire). Quarterly Journal of the Geological Society of London, Vol. 89, 202–220.

Dobson, M R, and Whittington, R J. 1987. The geology of Cardigan Bay. Proceedings of the Geologists' Association, Vol. 98, 331–353.

Fletcher, C J N, Swainbank, I G, and Colman, T B. 1993. Metallogenic evolution in Wales: constraints from lead isotope modelling. Journal of the Geological Society of London, Vol. 150, 77–82.

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

Harris, C, Ross, N, and Sheppard, T H. 2005. Geological mapping solutions for Quaternary ground-ice systems. British Geological Survey University Collaboration Project Final Report, Cardiff University.

Hillier, R D. 2002. Depositional environment and sequence architecture of the Silurian Coralliferous Group, southern Pembrokeshire, UK. Geological Journal, Vol. 37, 247–268.

Institute of Geological Sciences. 1973. Central Wales Mining Field. 1:100 000 series. (Southampton: Ordnance Survey for Institute of Geological Sciences.)

Jones, O T. 1912. The geological structure of central Wales and the adjoining regions. Quarterly Journal of the Geological Society of London, Vol. 68, 328–344.

Jones, O T. 1924. The Upper Towy drainage system. Quarterly Journal of the Geological Society of London, Vol. 80, 568–609.

Jones, O T. 1938. On the evolution of a geosyncline (Anniversary address). Proceedings of the Geological Society of London, Vol. 94, 60–110.

Jones, O T. 1965. The glacial and post-glacial history of the lower Teifi valley. Quarterly Journal of the Geological Society of London, Vol. 121, 247–281.

Long, G H. 1966. Investigations into the sedimentation and sedimentary history of the Talerddig Grits (Upper Llandoverian) and their lateral equivalents in central Wales. Unpublished PhD Thesis, University College. London.

Lowe, D R. 1982. Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology, Vol. 52, 279–297.

Loydell, D K. 1991. The biostratigraphy and formational relationships of the upper Aeronian and lower Telychian (Llandovery, Silurian) formations of western mid-Wales. Geological Journal, Vol. 26, 209–244.

Mackie, A H, and Smallwood, S D. 1987. A revised stratigraphy of the Abergwesyn– Pumpsaint area, Mid-Wales. Geological Journal (Thematic Issue), Vol. 22, 45–60.

McDonald, A J W, Fletcher, C J N, Carruthers, R M, Wilson, D, and Evans, R B. 1992. Interpretation of the regional gravity and magnetic surveys of Wales using shaded relief and Euler deconvolution techniques. Geological Magazine, Vol. 129, 523–531.

McKerrow, W S, MacNiocaill, C, and Dewey, J F. 2000. The Caledonian Orogeny redefined. Journal of the Geological Society of London, Vol. 157, 1149–1154.

Merrin, P D. 1999. Report on the drilling and installation of exploratory boreholes, Afon Teifi valley, west Wales. British Geological Survey Technical Report, Hydrogeology Series,WD/99/24.

Mutti, E, and Normark, W R. 1987. Comparing modern and ancient turbidite systems. 1–38 in Marine clastic sedimentology: concepts and case studies. Leggett, J K, and Zuffa, G G (editors). (London: Graham and Trotman.)

Phillips, W J. 1986. Hydraulic fracturing effects in the formation of mineral deposits. Transactions of the Institution of Mining and Metallurgy: Section B, Vol. 95, B17–B24.

Pickering, K T, Bassett, M G, and Siveter, D J. 1988. Late-Ordovician–early Silurian destruction of the Iapetus Ocean: Newfoundland, British Isles and Scandinavia — a discussion. Transactons of the Royal Society of Edinburgh: Earth Sciences, Vol. 79, 361–382.

Potts, A S. 1968. The glacial and periglacial geomorphology of central Wales. Unpublished PhD. Thesis, Wales. Aberystwyth.

Pratt, W T, Woodhall, D G, and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149 (England and Wales). (London: HMSO.)

Raybould, J G. 1974. Ore textures, paragenesis and zoning in the lead-zinc veins of mid-Wales. Transactions of the Institution of Mining and Metallurgy: Section B, Vol. 83, B112–B119.

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.

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

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

(Index map)

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

Figures and plates

Figures

(Figure 1) Simplified bedrock geology of the district.

(Figure 2) Late Ordovician and Silurian graptolite zones.

(Figure 3) Late Hirnantian to early Telychian coarse-grained sediment pathways and depositional tracts. Lines shown for the 'early pathway' phase of the Devil's Bridge Formation delineate a central corridor where the underlying Rhayader Mudstones sequence is thinnest.

(Figure 4) Depositional model for the southerly derived sandstone lobe systems. Abbreviations: BMM Blaen Myherin Mudstones Formation; BMF Borth Mudstones Formation; CaM Caerau Mudstones Formation; Dot Doethie Formation; Glr Glanyrafon Formation; LyT Llyn Teifi Member; MBa Mynydd Bach Formation; Rdd Rhuddnant Grits Formation; TrF Trefechan Formation.

(Figure 5) Map of the principal geological structures in the district.

(Figure 6) Bouguer gravity anomaly map of the district and adjacent areas.

Plates

(Plate 1) Thinly interbedded turbidite sandstones and mudstones, Nant Brianne Formation, Craig Pysgotwr [SN 7575 4875] (P608890).

(Plate 2) Thick bed of turbidite sandstone displaying dewatering structures, Doethie Formation, Pysgotwr valley [SN 7530 4910] (P608891).

(Plate 3) Base of conglomerate-filled megascour, Pysgotwr Grits Formation, Craig Twrch [SN 6525 4810] (P609293).

(Plate 4) Anticline with overturned eastern limb, Rhuddnant Grits Formation (Llyn Teifi Member), Berwyn valley [SN 7235 4835] (P609294).

(Plate 5) Glaciated U-shaped valley of the Afon Berwyn [SN 7270 5785] looking west (P603672).

(Plate 6) Cross-bedded sand and gravel (glaciofluvial sheet deposits) overlain by river terrace gravels, Afon Tywi [SN 7660 4530] (P609295).

(Plate 7) Spoil tips and flotation mill, Nantymwyn mine [SN 790 448] (P608892).

(Front cover) Valley of the Pysgotwr Fawr [SN 7585 4875] (Photographer D I Schofield; (P606652))

(Rear cover)

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

(Index map) 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. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.