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Geology of the Newcastle Emlyn district — a brief explanation of the geological map sheet 211 Newcastle Emlyn

P R Wilby, D I Schofield, D Wilson, J A Aspden, C E Burt, J R Davies, M Hall, N S Jones and J Venus

Bibliographic reference: Wilby, P R, Schofield, D I, Wilson, D, Aspden, J A, Burt, C E, Davies, J R, Hall, M, Jones, N S, and Venus, J. 2007. Geology of the Newcastle Emlyn district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 211 Newcastle Emlyn (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2007. © NERC 2007 All rights reserved.

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

Figures in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence No. 1000178997/2007.

(Front cover) The 14th century gatehouse of the castle [SN 3106 4074] at Newcastle Emlyn. (Photographer: P Witney; (P596680)).

(Rear cover)

(Geological succession) Geological succession in the Newcastle Emlyn district.

Notes

The word 'district' refers to the area of the geological 1:50 000 series sheet 211 (Newcastle Emlyn). National grid references are given in square brackets and all lie within the 100 km square SN. Letters in round brackets and lithostratigraphical names given in bold text are the same as those used on the geological map.

Acknowledgements

This sheet explanation was compiled by P R Wilby, D I Schofield and D Wilson using information provided by the co-authors. A W A Rushton (Natural History Museum, London), J A Zalasiewicz (Leicester University), J B Riding and M P A Howe provided biostratigraphical determinations; biostratigraphic collection was assisted by M G Bassett, J C W Cope and R M Owens (National Museums and Galleries of Wales). C P Royles produced the geophysical maps. Details of the pingo locality at Llanpumsaint were kindly provided by N Ross (Cardiff University). The text was edited by D Wilson (scientific) and the series editor is A A Jackson; figures were drawn by R J Demaine.

Mapping of the Afon Teifi catchment (Waters et al., 1997) was partly funded by Ceredigion, Pembrokeshire and Carmarthenshire, the former Dyfed county councils, and the Environment Agency (Wales). The survey of the remaining area, in 2004, was undertaken as part of the GeoCymru Project, supported by a grant from the Welsh Assembly Government. The BGS gratefully acknowledges the co-operation of landowners in allowing access to their lands, and the Forestry Commission for access to Brechfa Forest.

Geology of the Newcastle Emlyn district (summary from rear cover)

An explanation of sheet 211 (England and Wales) 1:50 000 series map

(Rear cover)

This sheet explanation provides a brief description of the geology of the Newcastle Emlyn district, which is defined in the north by the Afon Teifi valley and in the south by the Afon Tywi (Towy) valley; in the east it includes parts of Brechfa Forest.

The bedrock geology comprises sedimentary rocks of Ordovician and Silurian age, deposited between 470 and 430 million years ago on the southern margin of the Lower Palaeozoic Welsh Basin. The district provides a transect across the relatively shallow-water, shelf deposits of the Midland Platform and into the deep-water, predominately turbiditic deposits of the basin. Locally, thick tuffaceous units record contemporaneous volcanic eruptions, probably from centres beyond the district. A summary of the lithological characteristics and environment of deposition for each formation is given. Also described is the effect on sedimentation of changes in global sea level and of subsidence along the basin-bounding Welsh Borderland Fault System, here represented by the Cwm Cynnen Fault.

A brief history of the tectonic activity and the resultant structures is presented. The Acadian Orogeny is the main event represented in the district, and culminated in an episode of regional folding, cleavage formation and low-grade metamorphism. The evolution of the Quaternary landscape is outlined. Superficial (drift) deposits partially conceal the bedrock. These unconsolidated deposits originated mainly during the last major ice advance (late Devensian, approximately 20 000 to 15 000 years ago) but also include a substantial suite of periglacial solifluction deposits as well as postglacial alluvial sediments. Their composition and distribution is described, and they include substantial sand and gravel resources. Summary information for a range of applied geological issues that are pertinent to planning and development is included here—mineral and water resources, potential geohazards and engineering ground conditions. The district has a heritage of metal and slate mining. Lists of information sources and relevant references are also included.

Chapter 1 Introduction

This sheet explanation provides a summary of the geology of the district covered by the geological 1:50?000 series sheet?211 Newcastle Emlyn, published as a bedrock and superficial deposits edition in 2007.

The district lies mainly within the counties of Carmarthenshire and Pembrokeshire, but also includes a small part of Ceredigion north of the Afon (River) Teifi. It consists of an incised plateau which forms the watershed between northward-flowing tributaries of the Afon Teifi and southward-flowing tributaries of the Afon Tywi (Towy) and Afon Taf. The plateau rises towards Brechfa Forest in the east of the district, and reaches a maximum altitude of 367?m on Mynydd Llanllwni [SN 5016 3726]. The most important centres are the towns of Newcastle Emlyn and Llandysul; elsewhere the district supports a moderate to sparse population. Livestock farming is nowadays the mainstay of the local economy, but the district has an important mining heritage. In the mid 18th to the late 19th centuries it hosted one of the most productive silver-lead mines (Llanfyrnach) in south Wales, and from the mid 19th to early 20th centuries it was also an important centre for slate production. Slate continues to be supplied on a small scale from quarries at Glogue [SN 2165?3275].

The exposed bedrock consists of folded and faulted Ordovician and Silurian sedimentary rocks (Figure?1), deposited between 470 and 430 million years ago; tuffs form relatively minor components of the succession and the rocks are locally intruded by dolerites. In places, the bedrock is mantled by Quaternary superficial (drift) deposits. These include Pleistocene glacial and periglacial sediments, deposited during and immediately following the last (late Devensian) major ice advance around 20?000 years ago, as well as more recent (Holocene) deposits, mainly comprising alluvial materials that continue to accumulate within the river valleys.

During the Lower Palaeozoic the district was situated on the southern margin of the Welsh Basin, an area of enhanced subsidence characterised by predominantly deep-water turbiditic sedimentation. The basin developed on a fragment of the ancient supercontinent of Gondwana, known as Eastern Avalonia (Soper and Hutton, 1984), during the Caledonian orogenic cycle (Soper and Woodcock, 2003). To the east and south of the basin lay the Midland Platform, a relatively stable shallow marine shelf that was subject to periodic emergence. The platform margin was broadly defined by a complex zone of east- to north-east-trending faults, collectively termed the Welsh Borderland Fault System (Woodcock and Gibbons, 1988), a representative of which, the Cwm Cynnen Fault, transects the southern part of the district. Deposition in both the basin and platform areas was greatly influenced by a series of marine transgressions and regressions that were a response to eustatic fluctuations in sea-level and intra- and extra-basinal tectonics. Locally thick tuffaceous units within the lower part of the succession record contemporaneous volcanic eruptions, probably from centres outside of the district along the basin margin. Fossils, particularly graptolites, brachiopods and trilobites, are common within certain sequences, and allow dating of the succession and correlation with other areas (Fortey et al., 2000; Fortey, 2006).

The oldest exposed rocks within the district are of Llanvirn to Caradoc (Ordovician) age, and consist predominately of turbiditic and finely laminated pelagic mudstones, the latter representing 'rainout' of organic-rich, terrigenous mud from suspension. South-east of the Cwm Cynnen Fault, this basinal succession interdigitates with calcareous, bioclastic mudstones and sandstones that record deposition in a shallower, possibly distal shelf setting. During the late Ordovician and Silurian, the Welsh Basin underwent rapid subsidence, resulting in the widespread deposition of thick sequences of basinal turbidites. Two distinct types of turbidite depositional system occur within the basin (Davies et al., 1997). Slope apron systems, mostly of mudstone, characterise the Ashgill (Ordovician) to early Telychian (Silurian) sequence, whereas the mid to late Telychian sequence is dominated by a series of major sandstone lobe systems that introduced voluminous amounts of coarse sand; both of these types of depositional system have been recognised within the rocks of the district.

The slope apron systems comprise mainly wedge-shaped accumulations of sediment dominated by turbiditic and hemipelagic mudstone, which range from tens to hundreds of metres in thickness and typically thin from the basin margin towards its centre. Localised coarse-grained turbidite sandstones and conglomerates represent laterally restricted channel deposits that fed sediment to small sandstone lobes on the medial to distal parts of the aprons. Two types of slope apron mudstone facies have been recognised based on the nature of hemipelagic mudstone present. Anoxic facies are characterised by dark grey, finely laminated, pyritic and graptolitic hemipelagic mudstone. The lack of burrowing and the preservation of organic material in this facies is indicative of deposition within stagnant, anaerobic bottom conditions. A strong correlation exists between the deposition of such mudstones and marine transgressions, during which deep circulation of oxygen-bearing waters was prevented by the creation of a thermally stratified water column. In contrast, pale grey, burrow-mottled hemipelagic mudstones containing phosphate nodules record deposition and diagenesis beneath oxygenated bottom waters, when organisms were able to successfully colonise the sediment and destroy its original sedimentary fabric. Such conditions prevailed during regressive episodes, when the effects of thermal stratification were reduced and bottom waters were replenished with oxygen from surface levels.

The earliest slope apron system is of Ashgill age, and relates to a global fall in sea level resulting from the development of an extensive ice-cap on the supercontinent of Gondwana. Emergence of the Midland Platform during this time led to an increase in the volume of coarser terrigenous material entering the basin. Concurrent instability in the slope apron, caused by rapid sedimentation, promoted widespread slumping and destratification of the sedimentary pile. The subsequent late Ordovician (late Hirnantian) global rise in sea level, which introduced anoxic bottom conditions to the Welsh Basin, marks the onset of the second slope apron depositional system that continued into mid-Aeronian times, when worldwide regression again took place (Johnson et al., 1991); a third system was initiated with a major eustatic marine transgression during the late Aeronian M. sedgwickii Biozone, shortly followed by regression into the early Telychian.

Major, southerly derived sandstone lobe systems of mid Telychian age record a period of coarse detrital input to the basin, concurrent with intra-basinal faulting and uplift and erosion of the hinterland. The introduction of these systems correlates with unconformity and mass wasting of sequences around the basin margin (e.g. Davies et al., 1997). The acme of these tectonic processes was reached during the Early Devonian (latest Emsian to earliest Eifelian) and resulted in the Acadian Orogeny (Soper and Woodcock, 1990), when the rocks of the Welsh Basin were folded, cleaved, faulted, uplifted and eroded. There are no strata preserved within the district for the period between the late Llandovery, about 430?million years ago, and the latter part of the Quaternary, about 470?000 years ago. However, the modern topography and drainage system is probably inherited from the Cainozoic (Tertiary) epoch, having been superimposed from a former Cretaceous cover (Brown, 1960). Remnants of former planation surfaces that developed during periods of fluvial stability in the Pliocene are preserved as high platforms, isolated by subsequent Pleistocene planation and river down-cutting (Bowen, 1964).

The Pleistocene was a time of global climate change that brought about a succession of ice ages in the British Isles. Most of the glacigenic deposits in the district correspond to the last (late Devensian) glaciation, which reached its acme around 20?000?BP (Bowen et al., 1999). At this time, an Irish Sea Glacier, fed by ice caps in Scotland, northern Ireland, northern England and north Wales, moved onshore, converging with Welsh ice moving south-westwards from an ice cap situated on the Cambrian Mountains. Deglaciation was completed by about 15?000?BP and the current drainage system was established during the Holocene. Peat is currently forming in upland areas and alluvial deposits continue to accumulate in many of the river valleys.

Chapter 2 Geological description

Ordovician

Ordovician rocks, ranging in age from early Llanvirn to latest Ashgill, crop out over most of the district (Figure?1). The lowermost Ordovician strata (Llanvirn to Caradoc age) (Figure?2) lie to south of the Cwm Cynnen Fault and occur within the hinge of the Foel Tyrch Anticline. Upper Ordovician (Ashgill) strata (Figure?3) extend over a broad swathe of country around the Central Wales Syncline and Teifi Anticlinorium.

Llanvirn to Caradoc succession

This succession displays complex variations in facies, thicknesses and sequences across the southern and western parts of the district. In the Foel Tyrch Anticline the lowest exposed part of the succession is occupied by the Aber Mawr Shale Formation; to the south of the Cwm Cynnen Fault, strata of equivalent age are represented by the Abergwilli Formation. These two units were included in the 'Bifidus beds' of early workers (Strahan et al., 1909). The Aber Mawr Shale Formation (AbM) is a turbiditic and pelagic basinal mudstone facies, broadly Abereiddian (early Llanvirn) in age, probably equivalent to the similarly named formation of north Pembrokeshire (Hughes et al., 1982; Fortey et al., 2000). It comprises over 250?m of relatively monotonous, dark blue-grey mudstone that is pyritic, poorly cleaved and locally graptolitic, and includes beds of hard felsic tuff and tuffite (Z), each up to 10?m thick (Plate?1), which represent the products of contemporary volcanism within the region. The Abergwilli Formation (Ab) comprises over 200?m of predominately medium grey, locally silty and micaceous, burrow-mottled turbidite mudstone with subordinate interbedded units of laminated and pyritic mudstone containing horizons with abundant graptolites. Sporadic laminae and beds of sandstone (generally less than 20?cm thick) and felsic tuff and tuffite (Z; up to 30?m thick) occur at intervals throughout the formation.

The Aber Mawr Shales and Abergwilli formations are conformably overlain by strata assigned to the newly defined Drefach Group (Dr), which ranges in age from the Llanvirn to latest Caradoc. The group records the prolonged accumulation of graptolite-bearing mud, predominately from suspension, in a deep-water, anoxic environment. In the south-east of the district it has been subdivided into a number of formations. Lithologies typical of these formations have been recognised in the west, although complex folding and faulting, together with a lack of biostratigraphical control, precludes its subdivision in this area.

The lowest division of the Drefach Group is the Felin-wen Formation (Fw) of Abereiddian age (Fortey et al., 2000). This has been informally known as the 'Murchisoni Beds' (e.g. Strahan et al., 1909) owing to the local abundance of the pendent didymograptid graptolite D. murchisoni e.g. found at [SN 4156?2162]. The formation comprises up to 80?m of dark grey, richly graptolitic and locally finely laminated pelagic mudstone and interbedded turbiditic mudstone. Thin beds of bioturbated, calcareous sandstone and limestone are locally present, together with thin, felsic air-fall tuffs that record continued volcanism along the basin margin at this time.

Volcanism continued into the early Llandeilian with the eruption and deposition of the Asaphus Ash Formation (AA) (Fortey et al., 2000). This unit conformably succeeds the Felin-wen Formation, and comprises up to 35?m of waterlaid felsic tuff, interbedded with ripple cross-laminated tuffaceous sandstone and dark grey mudstone. It is locally overlain by up to 15?m of blue-grey, buff-weathering mudstone and burrowed calcareous sandstone and limestone yielding a mixed graptolite and shelly faunal assemblage indicative of the teretiusculus Biozone e.g. track section at [SN 4920 2180]. These higher beds are a correlative of the Lan Flags of the adjoining Carmarthen district (Strahan et al., 1909), but are included here within the Asaphus Ash Formation.

The Asaphus Ash Formation is overlain by the Hendre Shales Formation (HS) of late Llanvirn to Caradoc age (Llandeilian to Aurelucian stages) (Fortey et al., 2000). The latter consists of up to 80?m of thinly interbedded, calcareous, silt-laminated turbiditic mudstone and hemipelagic mudstone, and includes scattered thin beds of limestone and calcareous sandstone. This unit is characteristically pervasively weathered and typically crops out as a soft, buff-coloured mudstone. It contains an abundant graptolite fauna, including a gracilis Biozone assemblage e.g. at [SN 3620?2207].

The uppermost division of the Drefach Group is the late Caradoc Mydrim Shales Formation (MS). It comprises about 180?m of black, pyritic, locally fissile, pelagic mudstone with abundant graptolites. The formation is of Caradoc to earliest Ashgill age (Fortey et al., 2000) and a clingani Biozone fauna was collected from an overgrown quarry [SN 3514?2204].

Ashgill slope apron succession

The Caradoc rocks are succeeded by a bioturbated muddy slope apron facies of Ashgill age, represented by the Nantmel Mudstones Formation (Ntm). It has a faulted contact with the Caradoc rocks in the district, but elsewhere the boundary appears conformable (Davies et al., 1997). The formation is widely developed across mid-Wales, marking an abrupt change from the anoxic environments that characterised deposition in the Llanvirn and Caradoc, to the oxic basinal conditions that prevailed throughout most of the Ashgill. This change may record the onset of polar cooling and deep thermohaline circulation that presaged the late Hirnantian (late Ordovician) glaciation (Armstrong and Coe, 1997).

The Nantmel Mudstones Formation includes strata previously termed the Glogue Slates (Evans, 1945) and comprises up to 950?m of pale to medium grey, burrow-mottled and colour-banded mudstone. Laminae and beds of siltstone and fine- to medium-grained sandstone (sa) are present in places. Locally, these contain dispersed shelly detritus indicative of derivation from a nearby shelf area. Distinctive white-weathering bands and dark, elongate phosphate nodules occur in the mudstones, recording early diagenetic oxidation fronts within the sediment pile (Smith, 1987a). Several units of dark grey, anoxic hemipelagic mudstone occur in the upper part of the formation (Davies et al., 2006b).

The Nantmel Mudstones Formation is conformably overlain by the Yr Allt Formation (YA), comprising up to 900?m of medium grey, silty and silt-laminated turbidite mudstone, locally interbedded with fine- to coarse-grained turbidite sandstone. Individual sandstones are commonly 1 to 2?m thick, occurring in packets that may exceed 10?m in thickness (sa). They include immature, quartz-cemented, pebble-grade conglomerates that are considered to have been deposited in submarine channels that fed small turbidite lobe systems on the slope apron. Bioturbation is only locally developed and is usually confined to the base of the sandstones. Slope failure of the prograding muddy wedge is recorded by interbedded units of slumped and destratified strata within parts of the formation. One laterally extensive unit of disturbed beds (db) that locally rests upon the underlying Nantmel Mudstones Formation comprises around 250?m of massive to poorly bedded, fine-grained sandstone with discontinuous beds, lenses and large, irregular masses of conglomerate, tens of metres in length.

The latest Ordovician rocks within the district form part of the subsequent slope apron system that developed as a result of sea-level rise following the end of the late Hirnantian glaciation. These deposits span the Ordovician–Silurian boundary and, as such, are here described with the earliest Silurian strata.

Silurian

The latest Ordovician rocks, together with Silurian strata of Llandovery age, crop out within the Central Wales Syncline in the central and north-eastern part of the district (Figure?1). Easterly derived, latest Hirnantian to early Telychian mudstone slope apron facies crop out on the limbs of the syncline, surrounding a core of southerly derived, mid to late Telychian, sand-dominated turbidite lobe facies.

Late Hirnantian to early Telychian slope apron succession

The lowest part of the deep-water, slope apron succession is represented by the Cwmere Formation (CeF), which spans the Ordovician–Silurian boundary and extends into the early Aeronian (Davies et al., 1997). It comprises an anoxic facies sequence of up to 295?m of dark grey, turbidite mudstone and finely laminated, hemipelagic mudstone with scattered thin siltstones and thin beds of fine-grained, turbidite sandstone. At its base is the Mottled Mudstone Member (MMb), which sharply overlies the Ordovician Yr Allt Formation, and consists of up to 15?m of pale grey, burrowed, oxic turbidite mudstone. It includes a thin unit of anoxic hemipelagic mudstone that elsewhere has yielded a late Hirnantian persculptus Biozone fauna. The Mottled Mudstone Member broadly correlates with the onset of the marine transgression that followed glaciation at the end of the late Hirnantian. It was deposited prior to the creation of a deep, thermally stratified water column and the widespread development of anoxic sequences across of the basin.

North-west of the Craig Twrch Fault, the Cwmere Formation passes into the laterally equivalent Rhyddlan Formation (Rhy). This formation is of Rhuddanian to Aeronian age (Davies et al., 2006b) and represents a sandstone turbidite system that developed towards the outer margins of the contemporary (Cwmere Formation) slope apron. It comprises up to 315?m of thinly interbedded turbiditic sandstone and mudstone. The sandstones are generally pale grey, fine- to medium-grained, and form about 50 per cent of the formation. They usually occur in beds less than 0.1?m thick, although in places they range up to 1.5?m thick. The turbidite mudstones are dark grey and are commonly capped by a laminated hemipelagic mudstone indicative of the prevailing anoxicity within the basin at this time. Anoxic mudstones of the Cwmere Formation continued to be deposited within the district following the termination of the Rhyddlan Formation turbidite system.

The Claerwen Group (Cla) sharply overlies the Cwmere Formation. It comprises up to 580?m of thinly interbedded, pale grey-green colour-banded turbiditic mudstone and hemipelagic mudstone with subordinate thin siltstones and sandstones. The bases of individual turbidite mudstone units commonly include thin (1?mm or less) silt laminae. Both the turbiditic and hemipelagic mudstones are burrow mottled, and horizons of diagenetic phosphate nodules occur at intervals, indicating that the succession was deposited beneath predominantly oxygenated bottom waters during the prolonged mid-Aeronian regression (Johnson et al., 1991). Units of dark grey, laminated hemipelagic mudstone, rarely more than 2 to 3?m thick, record brief periods of anoxia within the succession, but are not separately mapped. The most important of these is the M. sedgwickii Shales, in places up to 20?m thick, which relates to a late Aeronian eustatic transgression (Johnson et al., 1991) and correlates with a major sequence boundary in adjacent platform areas (Woodcock et al., 1996).

In the north-east of the district, the muddy slope apron deposits of the Claerwen Group are diachronously overlain by the Devil's Bridge Formation (DBF), an early Telychian (turriculatus s.l. Biozone) division comprising up to 400?m of rhythmically interbedded turbidite sandstone and mudstone. The sandstones are fine- to medium-grained, rarely more than 2 to 3?cm thick, and commonly preserve trace fossils at their bases. Rarely, they also preserve flute casts that suggest a derivation direction from the south-east. The interbedded mudstones are generally thicker, pale to medium grey and generally burrow mottled, indicative of deposition in an oxic environment. The architecture of the Devil's Bridge Formation is complex. The earliest parts of the formation appear to have been deposited in a laterally restricted corridor, coeval with mudstone deposition elsewhere on the slope apron, whereas subsequent deposition was more widespread, possibly involving multiple sediment sources (Davies et al., 1997). It is likely that this later phase was associated with the tectonism and rejuvenation of source areas that affected the Welsh Basin during the Telychian (Soper and Woodcock, 1990). The Craig Twrch Fault may have been active within the district at this time, since the formation is absent to the south-east of the structure. Here, on the south-east limb of the Central Wales Syncline, the Claerwen Group is overlain by the Glanyrafon Formation (see below).

Middle Telychian sandstone lobe succession

During the middle Telychian, slope apron deposition was arrested by the incoming of southerly derived, sandstone-dominated, turbidite lobe systems, represented in the district by the Cwmystwyth Grits Group. Deposition at this time was driven by tectonic uplift to the south of the basin and was strongly controlled by intrabasinal faulting. Within the district, the Craig Twrch Fault was an important structure which probably gave rise to a topographic depression that partly confined sequences of thick, high-density turbidite flows.

The Blaen Myherin Mudstones Formation (BMM), which overlies the Devil's Bridge Formation on the north-west side of the Craig Twrch Fault has, in adjacent districts, yielded graptolites of mid Telychian age (Davies et al., 2006b). It is a mixed oxic and anoxic facies succession consisting of up to 120?m of grey, thin to medium bedded alternations of turbiditic and hemipelagic mudstone. These were deposited from southerly sourced, low-density turbidity currents, and mainly represent a distal muddy fringe facies to the sandstone-dominated Cwmystwyth Grits Group.

The Cwmystwyth Grits Group is represented within the district by the Glanyrafon Formation and the Rhuddnant Grits Formation. These formations form the more proximal parts of a single mid to late Telychian sandstone lobe turbidite system (Davies et al., 1997). The Glanyrafon Formation (Glr) consists of more than 1850?m of rhythmically interbedded turbidite sandstone and bioturbated mudstone, with subordinate laminated, hemipelagic mudstone. The sandstones form 10 to 50 per cent of the succession, in beds that rarely exceed 5?cm, but may locally reach 20?cm in thickness. The Glanyrafon Formation crops out mainly on the south-eastern limb of the Central Wales Syncline, where it directly overlies mudstones of the Claerwen Group. It represents a sandstone lobe fringe facies, deposited as unconfined low-density turbidite flows that were able to spread beyond the confines of the topographic depression created by the hangingwall of the Craig Twrch Fault. The Rhuddnant Grits Formation (Rdd), which succeeds the Glanyrafon Formation south-east of the fault, is up to 520?m thick and is marked by the appearance of subordinate, but locally abundant, thick beds of structureless, mud-rich, feldspathic sandstone ('high-matrix sandstones'). These are interpreted as the deposits of sediment gravity flows, varying between slurry-like debris flows and high-concentration turbidity currents (Davies et al., 1997). They range up to 1.5?m in thickness and usually occur in bundles separated by packets of thin, rhythmically bedded Bouma sandstones and mudstones identical to those of the Glanyrafon Formation. North of the Craig Twrch Fault, the Glanyrafon Formation is absent and the Rhuddnant Grits Formation directly overlies the Blaen Myherin Mudstones Formation. In this area, it is entirely represented by the Llyn Teifi Member (LyT), a basal division in excess of 650?m thick, characterised by a greater abundance of thick (1 to 2?m), commonly amalgamated beds of coarse-grained, high-matrix sandstone. The distribution of this high-density turbidite facies strongly suggests that its deposition was preferentially directed along, and confined to, the hangingwall side of the Craig Twrch Fault (Smith, 1987b, 1987c, 2004; Davies et al., 1997).

Structure

The structure of the district (Figure?1) is largely attributable to events within the Caledonian orogenic cycle (Cambrian to Middle Devonian). During this period, the Welsh Basin was subjected to phases of subsidence and inversion, culminating with an episode of latest Silurian to Early Devonian regional deformation termed the Acadian Orogeny, that was caused by the terminal collision of the palaeocontinents of Avalonia and Laurentia.

The most important structural feature of the district is the Central Wales Lineament (Smith, 1987c), consisting of a plexus of anastomosing, north-east-trending faults coincident with a major, first order fold, the Central Wales Syncline. The syncline is complementary to the Teifi Anticlinorium of the Lampeter district (Davies et al., 2006a), of which the Foel Tyrch Anticline in the north-west of the Newcastle Emlyn district is a component part. The Cwm Cynnen Fault cuts across the hinge of the Central Wales Syncline in the south of the district and represents the northernmost expression of the basin-bounding Welsh Borderlands Fault System.

Evidence for pre-Acadian deformation has largely been overprinted by subsequent tectonism, but is demonstrated by sedimentary facies variations across the Craig Twrch Fault that forms part of the Central Wales Lineament. Uplift along this fault, probably related to the onset of terminal collision in the Telychian (Soper and Woodcock, 1990), created barriers that confined the depositional pathways of the prograding Telychian sandstone lobe turbidite systems.

Evidence in the Llanilar and Lampeter districts (Davies et al., 1997; 2006a) suggests that the Bronnant Fault, which traverses the north-west of the district, acted in a similar manner, while the Llanglydwen Fault, which extends into the adjoining Fishguard district, may have been an important control on the distribution of earlier Arenig deposition (BGS, work in progress).

Inversion of the basin during the Acadian Orogeny reactivated pre-existing faults and instigated new ones, such as the Pontarsais Fault. The latter occupies the southern flank of the Central Wales Lineament and downthrows north-westward, excising much of the Cwmere Formation and locally juxtaposing the upper part of the Yr Allt Formation against rocks of the Claerwen Group. In the south-east of the district, the Abergwesyn Fault, which joins the Cwm Cynnen Fault near Bronwydd Arms, represents the south-west extension of a long-lived structure that extends from the north-western flank of the Tywi Anticline in the Builth Wells district (British Geological Survey, 2005). The Cwm Cynnen Fault, although not revealing any significant displacement within the district, juxtaposes Llanvirn basinal sequences against more proximal, basin-marginal rocks to the south-east. It was thus an important control on sedimentation at an early stage in the development of the basin, although it does not appear to have been active during the Caradoc or Ashgill as a complete sequence of these rocks is exposed in the south-east of the district.

Folds are developed on a variety of scales. The largest, the Central Wales Syncline, plunges gently towards the north-east and comprises a series of lower order structures that are most strongly developed in the sandstone-rich lithologies. In general, the lower order folds are moderate to tight, and have axial surfaces steeply inclined towards the north-west. Locally, their south-east-facing limbs are overturned. Lower order structures are largely absent from the mudstone divisions to the south of the Pontarsais Fault; however, those in the Llanvirn to Caradoc succession, to the south-east of the Abergwesyn and Cwm Cynnen faults, also plunge gently towards the north-east and verge south-eastwards.

A closely spaced, approximately axial planar cleavage, inclined steeply towards the north-west, occurs throughout much of the district, notably in the finer grained mudstone lithologies. South of the Cwm Cynnen Fault it is developed only in discrete domains. A younger spaced fabric locally crenulates this cleavage, and dips moderately to steeply north-westward. It is most widespread in mudstone units within the hinge zone of the Central Wales Syncline, e.g. at Cynwyl Elfed [SN3736?2704] to [SN 3748?2678], and most likely developed in response to renewed late to post-Acadian tectonic shortening.

Evidence for post-Acadian fault movement is confined to the southern part of the district where Middle Ordovician strata, south of the Cwm Cynnen Fault, are displaced by a prominent set of north-north-west- to north-trending faults. The latter represent the northern terminations of oblique-reverse faults that dissect Carboniferous rocks in the South Wales Coalfield (Roberts, 1972) and are largely attributable to movements during the Variscan Orogeny. Additionally, the axial traces of Acadian folds in the vicinity of Llanboidy have been displaced by a west-north-west-trending array of faults. These two fault sets reflect dextrally transpressive tectonics at the extreme northern margin of the orogenic belt (e.g. Gayer et al., 1998).

Regional geophysics

Interpretations of the regional structure are aided by colour-shaded Bouguer gravity anomaly and aeromagnetic maps (Figure?4); (Figure?5).

The gravity anomaly pattern shows a general westward and north-westward increase in values, consistent with an increase in thickness of relatively high density, Lower Palaeozoic mudstones (typically 2.80 to 2.85?Mg/m3) towards the centre of the Welsh Basin. The particularly strong response in the far north-west of the map coincides with the thick sedimentary fill of grabens within the Fishguard–Cardigan Fault Belt in the adjacent Cardigan district (see Davies et al., 2003). The general trend is overprinted by a series of linear, east-north-east- and north-east-oriented anomalies that reflect the juxtaposition of rocks of different densities along major mapped structures within the district. An anomalous gravity high in the south-west may correspond to an area of high density basement rocks or to concealed igneous rocks; volcanics and dolerite intrusions are exposed in the adjoining Fishguard district (BGS, work in progress).

The magnetic anomaly pattern shows a substantial increase in values along the southern margin of the district, reflecting a decrease in the thickness of non-magnetic sedimentary rocks over more magnetic basement at the arcuate edge of the Midland Platform. The pronounced magnetic high in the south-west corner of the map is a component of the so-called 'Haverfordwest anomaly', which is thought to indicate the presence of Precambrian rocks rising to a depth of 3?km (Norton et al., 2000). A low magnitude, north-east-oriented, positive anomaly in the north-west of the district broadly coincides with the Teifi Anticlinorium, and suggests that magnetic basement may lie at shallower depths within the core of the structure. The northern margin of the anomaly is broadly congruent with the trend of the Bronnant Fault.

Quaternary

Global climate change during the Quaternary (2?Ma–present) led to a succession of glaciations that affected much of the British Isles. These were accompanied by a drop in sea level and by pronounced river incision; in contrast, during warm stages sea level rose and marine planation surfaces developed (Bowen, 2005). Ice advanced into the Newcastle Emlyn district on at least two occasions. Most of the glacigenic deposits are assigned to the (last) late Devensian glaciation, which reached its acme about 20?000 to 18?000 years ago (Campbell and Bowen, 1989). However, scattered, degraded glacigenic deposits in the centre and the west of the district may be ascribed to an earlier (?Anglian) glaciation. Areas that remained ice free were subjected to intense periglacial weathering, resulting in the accumulation of soliflucted deposits (Head). Final retreat of ice from the district was completed by about 15?000 years ago and, with climate amelioration, alluvial deposits began to accumulate in the river systems during the Holocene.

Pre-late Devensian landscape evolution

The modern drainage system was probably initiated in the Oligocene from rivers that developed on a former Cretaceous and Tertiary cover (Brown, 1960; Gibbard and Lewin, 2003). Platforms occupying the high ground in the centre of the district represent remnants of former fluvial planation surfaces that formed during periods of stability in the Pliocene (Bowen, 1964). It has been suggested that these platforms became isolated by a series of marine planation events in the early Pleistocene, with much of the land below about 220?m OD in the region being planed off (Brown, 1960). At least two phases of pronounced river incision occurred within the Teifi valley during the later Pleistocene in response to glacio-eustatic falls in sea level (Jones, 1965; Davies et al., 2003; Glasser et al., 2004). These produced a series of fluvially cut benches and meanders incised into bedrock, the latter now locally abandoned and filled with later deposits, as in the vicinity of Waungilwen [SN 3420 3920], Drefach [SN 3620 3880] and Pentre-cwrt [SN 3840?3850]. Geophysical profiles indicate that rockhead may lie at depths of up to 45?m beneath the surface in the abandoned section near Mackwith [SN 4320 3980] (Carruthers et al., 1997).

Pre-late Devensian deposits

Highly weathered superficial deposits of mixed or uncertain age and provenance, with notably subdued morphologies, occur on the interfluves in the centre of the district and form isolated bodies in the south-west. They comprise thin spreads of poorly drained, silty gravel, tentatively interpreted as a remanié till (till, undifferentiated), and small masses of sand and gravel (glaciofluvial deposits, undifferentiated). These deposits have not been examined in detail, but they are presumed to be the products of a pre-late Devensian glaciation based on their position outside of the modelled late Devensian limit (see Waters et al., 1997; Bowen, 2005). They may be equivalent to deposits that have been assigned to the mid-Pleistocene 'Penfro Formation' in the adjacent Fishguard district (BGS, work in progress), a unit of possible Anglian age (Bowen, 2005).

Late Devensian landscape evolution

Most glacigenic materials and landforms in the district are attributed to the late Devensian glaciation, detailed accounts of which are given by Waters et al. (1997), Hambrey et al. (2001) and Etienne et al. (2006) for the north of the district. They were derived from two coeval ice masses (Figure 6): an Irish Sea Glacier, sourced from northern Britain and Ireland, and a Welsh Ice Cap, centred on the Cambrian Mountains. Welsh ice advanced into the area first and spread westwards into the Teifi valley. Ice-contact deposits derived from it extend as far down valley as Llandysul [SN 4280 4070]; the valley upstream of this point was extensively modified and overdeepended by scour at the base of the ice, resulting in a broadly U-shaped profile. Erratics in the south of the district show that another major Welsh ice stream entered the district via the Tywi valley (the 'Towy Glacier' of Pringle and George, 1937), fed by glaciers from north of Mynydd Epynt and from Mynydd Du. At its maximum extent, it reached as far west as the Dewi Fawr valley and terminated 6?km to the south of the district in the vicinity of St Clears (Strahan et al., 1909). In the west, Irish Sea ice covered most of the coastal tract of south-west Wales (Bowen, 2005) and occupied the Teifi valley as far upstream as Pentrecagal [SN 3385 4030]. It failed to surmount the Preseli Mountains but, as it impinged on the coastline, it dammed the Teifi estuary and impounded a large proglacial lake (Llyn Teifi) in the lower reaches of the valley (Fletcher and Siddle, 1998; Hambrey et al., 2001). The Irish Sea ice coalesced with Welsh ice immediately to the north of the Teifi valley, but in the valley itself, the two ice masses remained separated by lake water (Waters et al., 1997); calving into the lake may have acted to control the position of the Welsh ice in the valley (Etienne et al., 2006). The area beyond the two ice fronts, in the south-west of the district, remained largely ice free during the late Devensian and was subject to protracted periglacial weathering.

Llyn Teifi's level was probably controlled by a series of pre-existing channels situated at varying heights along the southern watershed (Charlesworth, 1929; Jones, 1965; Waters et al., 1997; Hambrey et al., 2001; Glasser et al., 2004; Etienne et al., 2006). As the ice advanced inland, these were sequentially occluded and the displaced water mass was forced to overspill at progressively higher altitudes. One of the highest postulated overflows, that at Pedran [SN 2570?3245], lies in the west of the district at an altitude of about 190?m?OD. Large volumes of sediment were debouched into the lake from the Welsh ice and created substantial deltas, remnants of which occur at Llandysul, Pentre-cwrt and near Henllan (Jones, 1965). Ponding also occurred in the Gwili valley and its tributaries in the vicinity of Llanpumsaint, though it is not clear whether this resulted in the development of a single substantial lake or several smaller ones.

In a number of places, particularly in the south of the district, meltwater channels were cut into the underlying bedrock by subglacial streams. These are preserved as dry, narrow, generally box-shaped channels that traverse present-day watersheds, and include examples of probable subglacial chutes e.g. [SN 3470?2180] as well as ones with characteristically humped longitudinal profiles e.g. [SN 4320?2260]. The majority are oriented north-east–south-west (Figure?6), parallel to the subglacial water pressure gradient and presumably that of the ice flow direction (cf. Shreve, 1972).

Deglaciation of the district was probably accomplished by a combination of in situ downwasting and valley glacier marginal recession, the ice retreating first from the interfluves and surviving longest in the valleys where it was thickest (Bowen, 1967). Substantial volumes of material were released by the wasting ice and were locally deposited as lateral benches (kames) by meltwaters. Where the glaciers paused during retreat, bodies of ice-contact materials were deposited, as at Llandysul [SN 4280?4070], Glanrhydypysgod [SN 4690?4035] and Bronwydd Arms [SN 4155?2440]. Some of these structures impounded meltwaters (Waters et al., 1997), but were eventually breached by rising lake levels.

Late Devensian deposits

Deposits derived from the Welsh and Irish Sea ice masses can be distinguished on the basis of palynomorph associations (Riding, 2004) and clast composition. Those derived from the Welsh Ice Cap contain exclusively 'indigenous' clasts, dominated by Lower Palaeozoic mudstones and sandstones, whereas those derived from the Irish Sea Glacier in addition contain northerly sourced 'exotic' erratics. These include Scottish granites, Lake District igneous rocks, Carboniferous limestone, Permo-Triassic sandstone, Cretaceous flint and shell fragments eroded from the floor of the Irish Sea Basin (Jones, 1965; Lear, 1986; Davies et al., 2003).

Older fluvial deposits, overlying bedrock in a borehole [SN 3110?4036] near Newcastle Emlyn, comprise unconsolidated angular gravels up to 3?m thick. They are possibly of late Devensian age, having occupied the valley prior to drowning by Llyn Teifi (Hambrey et al., 2001; Etienne et al., 2006). They are overlain by thick sequences of glaciolacustrine Deposits, consisting of grey or chocolate-brown clay, silt-laminated clay and clayey silt, which constitute much of the fill of the Teifi valley (Hambrey et al., 2001; Etienne et al., 2006) and its preglacial tributaries. Up to 20?m have been proved in a borehole [SN 3087?4042] near Newcastle Emlyn, and a greater thickness may be present in the abandoned segment of the valley nearby (Carruthers et al., 1997). In other parts of the Teifi valley, at least as far up valley as Mackwith [SN 4365?4015], similar deposits, also attributable to Llyn Teifi, lie concealed beneath later glacigenic and fluvial deposits. Dropstones and thin units of diamicton represent rain-out from ice-bergs (Hambrey et al., 2001; Etienne et al., 2006).

Till deposits locally overlie glaciolacustrine clays in the Teifi and Gwili valleys and form a discontinuous blanket over the interfluves. They comprise diamictons, ranging from stiff, matrix-supported, gravelly sandy clay, to compact, ill-sorted, clast-supported, clayey gravel. Till (Welsh) is typically blue-grey and is confined to the eastern half of the district, its westernmost occurrence in the Teifi valley being near Llandysul [SN 4282?4038]. Locally, it may attain substantial thicknesses; up to 35?m are thought to occupy the abandoned preglacial meander-loop of the Teifi near Mackwith [SN 4320?3980] (Carruthers et al., 1997). Till (Irish) is confined to the headwaters of streams within the Teifi catchment west of Llandyfriog [SN 3300?4140]. It is not well exposed in the district, but it has been studied in detail in the adjacent Cardigan district, where it locally overlies striated bedrock surfaces and sheared glaciolacustrine sequences (Davies et al., 2003). It is typically chocolate-brown (grey or yellow weathering) and contains a varied clast assemblage, as well as small, irregular, calcium carbonate nodules in weathered profiles. At Abercych [SN 2525?4035], Heterogeneous Glacial Deposits (Irish Sea) form an undulating, kamiform feature comprised of clayey and silty gravels. They are believed to record proglacial reworking of head and regolith at the Irish Sea ice margin (Waters et al., 1997).

Irregular mounds of hummocky glacial deposits have been identified from aerial photographs in the south-east of the district. They have not been examined in detail, but are dominated by ill-sorted clay- and silt-bound gravel, as near Bedw Farm [SN 4364?2764]. They are interpreted as having been generated by the Welsh ice during deglaciation, and include bodies that probably formed during still-stands (or oscillations) of the ice front, as well as by down-wasting stagnant ice. Near Bronwydd Arms, they form a well defined halt moraine [SN 4160?2444], pinned by a bedrock spur at the confluence of the modern Afon Gwili and Nant Brechfa.

Glaciofluvial deposits form extensive spreads in the Teifi valley and occur as isolated bodies elsewhere in the district. They comprise mostly accumulations of sand and gravel that were deposited by meltwater streams emanating from both the Welsh and Irish Sea ice masses during advance and retreat (Waters et al., 1997). Glaciofluvial deposits, undifferentiated (Welsh) form a series of degraded benches and irregular spreads between Newcastle Emlyn and Llanfair [SN 4340?4065], and also occur in the Tyweli valley from Pencader [SN 4450?3570] northwards. They generally comprise well sorted, stratified sand and pebble cobble gravel, and are well exposed in a gravel pit [SN 3905?3908] at Pentre-cwrt. The majority of these deposits lie to the west of the known limits of the Welsh ice advance and, as such, are considered to represent remnants of outwash deposits derived from it, or possibly deltas that entered Llyn Teifi. Glaciofluvial Ice-Contact Deposits (Welsh) comprise similar materials (Plate?2), but additionally include poorly sorted, structureless, clayey gravels and subordinate gravelly clays with clasts ranging from pebbles to boulders. They lie within the former limits of the Welsh ice, forming mounds and gently sloping benches along the sides of the Teifi valley to the east of Llanfihangel-yr-arth [SN 4560?3980], and also a substantial bench [SN 4035?2805] high on the side of the Gwili valley. These deposits accumulated predominately as kames at the margins of melting valley glaciers, but locally they include landforms that cross valley floors, such as that near Glanrhydypysgod [SN 4710?4030], which is interpreted as a moraine formed in front of the valley glacier during a pause in its retreat (Waters et al., 1997). Glaciofluvial Sheet Deposits (Welsh) are represented by suites of well sorted, stratified sand and gravel that form high terraces between Llandysul and Newcastle Emlyn in the Teifi valley, and are well exposed in a 5?m section [SN 3538?3970] in Nant Bargod near Drefach. They also form a well marked terrace [SN 4275?2160] above the Afon Gwili on the northern outskirts of Carmarthen. Such deposits are interpreted as the remnants of late glacial deltas or outwash braidplains (sandar) which, prior to dissection, would have occupied much of each valley floor. Glaciofluvial Ice-Contact Deposits (Irish) occur only as two isolated bodies in the north-west of the district, situated at moderately high levels on the eastern side of the Afon Cych valley. They comprise poorly organised sand and structureless, poorly sorted, pebble cobble gravel, the latter including large angular blocks of locally derived mudstone. Both bodies represent the remnants of kames that were deposited into spaces created during the retreat of the Irish Sea Glacier from this area; one of the bodies [SN 2700?3865] is closely associated with a branching network of meltwater channels.

Head, undifferentiated consists of accumulations of soliflucted material deposited under periglacial conditions. It is widespread in the west, which remained ice free during the late Devensian and was thus subjected to freeze-thaw activity for a longer period, though it occurs sporadically throughout the entire district. It has only been mapped where it attains a significant thickness or exhibits a marked topography, such as at the base of steep slopes and in narrow valleys. Head lithologies depend on the nature of the up-slope source material. Stratified, moderately well sorted and clast-supported gravels and silty gravels, composed principally of angular granule and pebble-grade mudstone fragments, are widespread on the crop of the Nantmel Mudstones and Yr Allt formations, and may attain thicknesses of tens of metres. Elsewhere, silty and sandy, variably gravelly clay and clayey silty gravel dominate.

Post-late Devensian evolution of the modern landscape

Following withdrawal of ice from the district, the rivers began a phase of rapid down-cutting and erosion; global sea level, though rising, was substantially lower than at present forcing the rivers to regrade to a lower base-level. Solifluction and slope wash processes continued under periglacial conditions, contributing to accumulations of head. Locally, under permafrost conditions and where hydrological regimes were favourable, substantial masses of ground ice formed; well developed pingos (Harris et al., 2005) occur in the vicinity of Helfa Hall [SN 4180?2772] (Plate?3), and ice-wedge casts are present elsewhere (Plate?2). It is likely that many of the larger landslides in superficial deposits were initiated at this time, when support from ice was lost, the water table rose and undercutting accelerated. The preglacial drainage was re-established in the west of the district which had remained largely ice free. Elsewhere, the drainage pattern was superimposed through thick glacigenic deposits, locally causing rivers to be diverted from their former courses. At intervals along the Afon Teifi, the river exits its broad, preglacial valley to flow through narrow, deep rock gorges (Figure?6), which transect formerly concealed bedrock spurs (Hambrey et al., 2001; Glasser et al., 2004). In so doing, it has abandoned extensive lengths of the valley, leaving some sections wholly concealed beneath thick late Devensian sediments. Obstruction of the preglacial course of the Afon Gwili by a glacigenic complex at Pontarsais [SN 4360?2780] has forced a reversal in this river's drainage and the incision of a diversionary gorge westwards from Llanpumsaint (Figure?6).

As sea level continued to rise, eventually attaining its present-day position around 5000 years ago, the rivers were again forced to regrade and, in their lower reaches, they developed broad floodplains.

Postglacial deposits

Remnants of broad braidplains that developed at the start of the Holocene, following the complete withdrawal of ice from the district, are preserved as river terrace deposits. These comprise cross-bedded sand and pebble cobble gravel, and intermittently flank the modern floodplains of several rivers. They form a series of well defined terraces, each recording a separate episode of incision in response to postglacial regrading and isostatic readjustment. Three terraces have been mapped in the Afon Teifi valley (numbered sequentially upwards from youngest to oldest), but they may not correlate directly with each other throughout the length of the river because of variations in the rates of aggradation and incision on either side of rock gorges.

Low-angle cones of alluvial fan deposits have developed where tributaries intersect main valleys and emerge from the confines of their channels. They consist of intercalated silty sandy clay and variably clayey sand and gravel. The majority of fans were probably most active in the immediate postglacial period, when water and sediment loads were greater than at present. Most are contiguous with the modern floodplain, suggesting that they are still active, but some are graded to river terraces e.g. [SN 2693 2325]; [SN 4574?4000], implying that they are largely abandoned.

Tracts of alluvium have been deposited by all of the major rivers and their principal tributaries within the district. Typically, this deposit forms very gently sloping ground consisting of a laterally variable suite of laminated clays, silts and cross-bedded coarse-grained sands and pebble cobble gravels. The coarser grained units represent former river channel deposits and, though typically unconsolidated, are locally cemented by iron and manganese oxides (hardpans), as in riverbank sections [SN 4672?4018] near Glanrhydypysgod. The finer grained units represent overbank deposits.

Accumulations of lacustrine deposits, consisting of organic-rich silt and clay, occupy the sites of former and extant ponds and lakes such as kettleholes (e.g. [SN 4672?4073]) and oxbows (e.g. [SN 4820?2150]; [SN 4654?4049]). Locally, peat has accumulated on waterlogged upland areas underlain by bedrock or till, and in poorly drained valleys, such as that occupied by Nant Corrwg [SN 4550?2770]. Over 4?m have been proved in pingo basins [SN 4202?2764] near Llanpumsaint (Harris et al., 2005).

Landslide deposits comprise masses of material that have undergone downslope movement due to slope failure. They are most prevalent on the lower slopes of steep valleys cut in glaciolacustrine deposits, glaciofluvial deposits, head and till (Waters et al., 1997), but have also developed on bedrock slopes. The majority are the result of relatively shallow earth and debris slides, involving a range of complex movements including components of translation, rotation and flow (Varnes, 1978).

Artificially modified ground

Worked ground indicates areas of man-made excavation, created either for engineering purposes or in the pursuit of mineral resources. Only the largest areas are shown and these relate to sites of bedrock extraction, notably in sandstones [SN 4077 2578] and [SN 3929?2577] near Pentre Morgan. Made ground is where the natural surface has been artificially raised, normally for engineering purposes or through the tipping of mine waste. The composition of such ground varies greatly, depending on its purpose and on the source of the material used. Landscaped ground indicates areas where the original ground surface has been extensively remodelled by man, and includes industrial estates, playing fields and ancient earthworks. Such ground is likely to be composed of material derived from within the site. Disturbed ground is used for areas of ill-defined surface working, where it is impractical or impossible to differentiate between excavation, fill and spoil [SN 3396?3999].

Chapter 3 Applied geology

Earth science factors are significant in land-use planning and have been assessed in detail for the north of the district (Waters et al., 1997). Their early consideration can help to ensure that developments are compatible with ground conditions and that resources are not sterilised. Exploitation of mineral resources may conflict with demands from agriculture, urban development and the environment. Water resources are vulnerable to contamination and geological hazards may require costly remediation or represent potential threats to public health. Engineering ground conditions and designated conservation sites may influence the design and the location of new developments.

Mineral resources

Parts of the district are included in two reports on mineral resources and their economic potential in relation to planning issues (Crimes et al., 1992; Highley et al., 1997). Currently, only two bedrock quarries are operating commercially in the district. Bedrock aggregate is being produced from sandstones at the base of the Yr Allt Formation in Foelfach Quarry [SN 3930?2575], and roofing slates and flagstones are being worked from the Nantmel Mudstones Formation in Glogue Quarry [SN 2188?3284]. Slates were produced on a larger scale in the district during the mid 19th to early 20th century (North, 1946), with additional centres at Llwynpiod [SN 4330?2294] and Capel Gwyn [SN 4651?2240], the latter working the Felin-wen Formation. Building stone has been obtained on a small scale for local use from most of the solid rock divisions in the district, notably from the Asaphus Ash, the Nantmel Mudstones, and the Yr Allt formations. Sandstones in the disturbed beds near the base of the Yr Allt Formation have been extensively worked [SN 4080?2580]; [SN 3904?2576] in the vicinity of Pentre Morgan.

Lead, silver and zinc were worked from the mid 18th to the late 19th century in the vicinity of Llanfyrnach [SN 2242?3170]; galena has also been recovered from levels [SN 2632?2821] driven near Cwm. The mines at Llanfyrnach were one of the most productive in south Wales and produced over 15?000 tons of ore concentrate (Hall, 1971). Reconnaissance geochemical surveys suggest that additional base metal mineralisation may still be present in this area (Cameron et al., 1984). The district's potential as a gold prospect has been examined by Cooper et al. (2000) and the economic potential of nodular monazite (rare earth element phosphate) has been assessed by Smith et al. (1994).

Sand and gravel deposits may be locally significant, but have previously only been exploited on a small scale. The largest reserves lie within the Teifi valley, where many are protected by environmental designations or have been sterilised by development. Glaciofluvial deposits represent the most important potential resource, but vary considerably in quality and volume. In general, they are moderately well to poorly graded with a low percentage of fines. However, the ice contact deposits in particular contain considerable amounts of interbedded unwanted material, including clay-rich facies, deleterious rock types and oversized clasts. Glaciofluvial deposits were formerly worked in Aber-Arad [SN 3184?4062] and are being exploited on a small scale for local use elsewhere e.g. [SN 2688?3868]; [SN 2964?3807]. River terrace deposits are likely to include good to intermediate quality resources. Limitations to their potential are similar to those for glaciofluvial deposits, but may additionally include hydrological problems associated with extraction close to rivers. They have been worked [SN 3507?4038] on a small scale near Pont Bargod. Hummocky glacial deposits and alluvial fan deposits constitute additional potential resources, but they are likely to be of variable quality and display marked lateral variations in grading and thickness. A poor quality deposit, including interbedded tills, has been worked for local use [SN 4364?2764]. Glaciolacustrine clay was formerly dug [SN 2793?4066]; [SN 3396?3999] as a bulking agent for mixing with coal washings to produce a cheap, low-grade fuel.

Water resources

The district relies largely on surface water resources (Environment Agency, 2004, 2005), but many farms and some local businesses abstract small quantities of groundwater from private supply boreholes. Glaciofluvial and head deposits, river terraces and modern alluvial deposits constitute the main aquifers within the district (Waters et al., 1997). In the Teifi valley, granular deposits beneath the glaciolacustrine and till sequences may also offer significant aquifer potential (Robins et al., 2000). The primary porosity of the bedrock is very low; groundwater flow and storage is predominately within joints and fault-related fractures (Jones et al., 2000). The chemistry of the groundwater within the district varies and, in the Afon Teifi catchment, is strongly influenced by residence time, variations in flowpath and the composition of the superficial deposits through which it has passed (Robins et al., 2000). A chalybeate well [SN 3540?3796] is located at Felindre.

Potential geological hazards

The current distribution of landslides in the district gives an indication as to the potential susceptibility of various units to failure, and where future instability might be anticipated. The majority occur on agricultural land remote from potential development sites. Once movement has occurred, the strength of the landslipped material is greatly reduced, increasing the risk of further movement if conditions at the site change. Subsequent movement may be triggered by natural processes, such as heavy rainfall and river erosion, or by anthropogenic interference, such as excavations in the toe region, modification of the drainage and loading. The current state of activity is not known for all of the landslides, but recent activity in some is suggested by fresh back-scars. The thick sequences of glaciolacustrine clay preserved within the abandoned segments of the Teifi valley are particularly prone to landsliding where undercut by river or stream erosion, for example at Newcastle Emlyn.

Low-lying areas adjacent to rivers are potentially at risk from flooding. An indication of those areas that are most susceptible is given by the extent of alluvium on the map, and by Environment Agency flood risk maps. Low river terraces, head-filled valleys and alluvial fans may be inundated during exceptional weather events. Areas shown as peat are likely to be perennially waterlogged. Landfill sites and abandoned mine workings represent potential point sources of water pollution, as well as areas of subsidence and land contamination. There are currently six licensed or redundant landfill sites within the district, and approximately twenty recorded mine shafts and trials, the precise positions of some of which are uncertain (Hall, 1971; Foster-Smith, 1981). The vulnerability of any aquifer to pollution is controlled by the physiochemical properties of its constituent materials; remediation is difficult, prolonged and expensive. Elevated levels of metal, particular lead and zinc, occur downstream of the mines at Llanfyrnach (Cameron et al., 1984) and locally elsewhere (Robins et al., 2000).

Gas emissions represent potential hazards where they build up in basements, foundations and excavations. Methane is highly explosive and carbon dioxide is toxic and asphyxiating at high levels. The bedrock geology of the district has a low susceptibility to emissions of these gases (Appleton et al., 1995), but they may be generated from domestic refuse in landfill sites and from organic-rich superficial deposits such as peat. The risk can be mitigated by correctly venting landfill and adversely affected buildings. Radon is a naturally occurring ionising gas produced by the radioactive decay of uranium and radium, which are present in small quantities in all natural rocks and soils. It may accumulate in poorly vented spaces, increasing the risk of respiratory cancers. The potential for radon emission generally ranges from moderate to high in the district (Appleton and Ball, 1995). Site-specific reports assessing the potential risk at a given location, and the level of remedial measures required for new developments, can be obtained from the BGS Enquiries Office, Keyworth.

Engineering ground conditions

Ground conditions are influenced by the physical and chemical properties of the bedrock and superficial deposits, the topography, the surface and groundwater regimes, and the nature of past and present human activity. The bedrock in the district generally has a high bearing capacity and provides good foundation conditions, except in the weathered zone and in the vicinity of major fractures. Calcareous units within the Drefach Group may be susceptible to dissolution and a reduction in rock strength. Pyritic mudstones, such as those of the Drefach Group and Cwmere Formation, are generally unsuitable for fill, as weathering of the pyrite may cause sulphate attack of concrete.

The engineering properties of the superficial deposits are more variable; they commonly exhibit considerable variations in thickness, and materials of very different properties may be closely associated. Modern alluvial deposits, peat and lacustrine deposits have low bearing capacities, high compressibilities and high groundwater tables, giving rise to uneven and excessive total settlements. Head deposits usually offer poor foundation conditions due to their variable thickness, the possible presence of soft, compressible zones, and their generally low shear strength. Glaciofluvial and river terrace deposits generally present good foundation conditions, but running sands may be encountered in excavations that extend below the water table. Hummocky glacial deposits are typically heterogeneous and are potentially susceptible to differential settlement. Tills may exhibit high consolidation settlements where they have been softened adjacent to water-bearing sand lenses. Glaciolacustrine deposits have variable bearing capacities and compressibilities, medium to high shrink/swell potentials, and are susceptible to instability on slopes.

Artificial deposits cover only a small proportion of the district, but are typically highly variable in composition and thickness. They therefore possess very different geotechnical properties and commonly present difficult ground conditions. Landslides are areas of known instability and, wherever possible, should not be developed. If this is not feasible, it is essential that they, and the surrounding land, are properly evaluated and effectively remedied.

Geological conservation

The geological heritage of the district forms a resource for tourism, education and scientific research, and is also an issue in planning and development. Information on the location and designation of important geological sites within the region 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 and adjoining areas is listed below. Searches of indexes to some of the collections can be made on the Geoscience Data Index (GDI) system, available online at www.bgs.ac.uk. BGS Catalogue of geological maps, books and datais available on request or may be viewed on the website. Maps and other publications may be purchased online (www.geologyshop.com) or through the BGS sales desks, and can be consulted at the BGS libraries (see back cover for addresses). Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre (NGRC), BGS, Keyworth. BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS website or at: Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

Publications

Original 1:63?360 scale geological maps are out of print, but can be provided as facsimiles or can be consulted at the BGS library, Keyworth. Unpublished 1:25?000 scale geological maps, listed below, are available on request. Groundwater vulnerability maps are published by the Environment Agency from data commissioned from BGS and The Soil Survey and Land Research Centre, and are available from BGS sales desks and The Stationery Office (020 7873 0011). Many BGS products and data are available in digital form under licensing agreement, details of which are available from the Intellectual Property Rights Manager, BGS, Keyworth. Digital datasets include those covering geochemistry, geophysics, geohazards, hydrogeology, borehole logs and mapping and allow the information to be used in GIS applications.

Geological maps

The district was originally surveyed at the scale of 1:63?360 by W?T?Aveline, H?T de la Beche, W?E?Logan, J?Phillips, A?C Ramsay, J?Rees (junior) and D?H?Williams, and published as part of [Old Series] sheets 40 and 41 in 1845 and 1857, respectively. A small area along the southern margin was remapped at the 1:10?560 scale by H H Thomas between 1902 and 1906 as part of the survey of the 1:50?000 scale geological sheet 229 Carmarthen. The Teifi valley and its tributaries below the 120?m OD contour were resurveyed at the scale of 1:10?000 by J?R?Davies, J?K?Prigmore, R?A?Waters and D?Wilson in 1995–6 as part of the Afon Teifi Catchment Project, co-funded by BGS, the former Dyfed County councils and the Environment Agency (Wales). Remaining parts of the district were surveyed at the 1:20?000 scale by J?A?Aspden, C?E?Burt, M?Hall, R?J?O?Hamblin, N?S?Jones, D?I?Schofield, T?H?Sheppard, J?Venus, P?R Wilby and D?Wilson in 2004 as part of the GeoCymru Project, supported by a grant from the Welsh Assembly Government.

Map number Surveyor Date
SN 22 (part) PRW, MH 2004
SN 23 JKP, DW 1995–6
CEB, MH 2004
SN 24 JKP, DW, RAW 1995–6
SN 32 (part) DIS 2004
SN 33 JKP 1995–6
RJOH, JV 2004
SN 34 JKP, DW 1995–6
SN 42 (part) JAA, NSJ, THS 2004
SN 43 DW, JRD 1996
DW, CEB 2004
SN 44 JRD, RAW, DW 1996
SN 52 (part) JAA, NSJ 2004
SN 53 DW 2004
SN 54 DW, RAW 2004

Books

Books, reports and other select publications relevant to the district are listed in the references.

Documentary collections

Records of boreholes and site investigations pertaining to the district are available for consultation at NGRC in BGS, Keyworth. Index information, including site references, is held in digital format and can be viewed through the GDI, available on the BGS website.

Material collections

Material collections from the district are available for inspection at BGS, Keyworth, and include petrological hand specimens, thin sections and fossils. Index data for petrological specimens and for fossils is listed in the BRITROCKS and Palaeosaurus databases, respectively. These may be searched through the GDI on the BGS website.

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

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

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

Armstrong, H A, and Coe, A L. 1997. Deep-sea sediments record the geophysiology of the late Ordovician glaciation. Journal of the Geological Society of London, Vol. 154, 929-934.

Bowen, D Q. 1964. Contributions to the geomorphology of Central South Wales. Unpublished Ph.D thesis, University College, London.

Bowen, D Q. 1967. On the supposed ice-dammed lakes of South Wales. Transactions of the Cardiff Naturalists Society, Vol. 93, 4-17.

Bowen, D Q, Cameron, T D J, and Holmes, R. 1999. A revised correlation of Quaternary deposits in the British Isles. Geological Society of London Special Report, No. 23.

Bowen, D Q. 2005. South Wales. 145-164 in The glaciations of Wales and adjacent areas. Lewis, C A and Richards, A E (editors). (Logaston Press, UK.)

British Geological Survey. 2005. Builth Wells. England and Wales, Sheet 196. Solid and Drift Geology. 1:50 000 Series (Keyworth, Nottingham: British Geological Survey).

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

Cameron, D G, Cooper, D C, Allen, P M, and Haslam, H W. 1984. A Geochemical drainage survey of the Preseli Hills, south-west Dyfed, Wales. British Geological Survey, Mineral Reconnaissance Programme Report,WF/MR/84/72.

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

Carruthers, R M, Chacksfield, B C, and Heaven, R E. 1997. A geophysical investigation to determine the thickness and nature of glacial deposits in the buried valleys of the Afon Teifi between Cardigan and Llanybydder. British Geological Survey Technical Report, WK/97/05.

Cooper, D C, Rollin, K E, Colman, T B, Davies, J R, and Wilson, D. 2000. Potential for mesothermal gold and VMS deposits in the Lower Palaeozoic Welsh Basin. British Geological Survey Research Report, RR/00/009.

Charlesworth, J K. 1929. The south Wales end-moraine. Quaterly Journal of the Geological Society of London, Vol. 85, 335-358.

Crimes, T P, Thomas, G S P, and Hunt, N. 1992. An appraisal of the land-based sand and gravel resources of South Wales. University of Liverpool technical report for the Department of the Environment.

Davies, J R, and five others. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets178 and 179 (England and Wales). (London: HMSO for the British Geological Survey).

Davies, J R, Waters, R A, Wilby, P R, Williams, M, and Wilson, D. 2003. Geology of the Cardigan and Dinas Island district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 193 Cardigan and Dinas Island (England and Wales).

Davies, J R, Scohfield, D I, Sheppard, T H, Waters, R A, Williams, M, and Wilson, D. 2006a. 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).

Davies, J R, Sheppard, T H, Waters, R A, and Wilson, D. 2006b. The geology of the Llangranog district – a brief explanation of geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 194 Llangranog (England and Wales).

Etienne, J L, Jansson, K N, Glasser, N F, Hambrey, M J, Davies, J R, Waters, R A, Maltman, A J, and Wilby, P R. 2006. Palaeoenvironmental interpretation of an ice-contact glacial lake succession: an example from the late Devensian of southwest Wales, UK. Quaternary Science Reviews, Vol. 25, 739-762.

Environment Agency. 2004. Teifi catchment abstraction management plan. (Cardiff: Environment Agency).

Environment Agency. 2005. The Tywi, Taf and Gwendraeth catchment abstraction management plan. (Cardiff: Environment Agency).

Evans, W D. 1945. The geology of the Prescelly Hills, north Pembrokeshire. Quarterly Journal of the Geological Society of London, Vol. 101, 89-110.

Fletcher, C J N, and Siddle, H J. 1998. Development of glacial Llyn Teifi, west Wales: evidence for lake-level fluctuations at the margins of the Irish Sea ice sheet. Journal of the Geological Society of London, Vol. 155, 389-399.

Fortey, R A. 2006. A new deep-water Upper Ordovician (Caradocian) trilobite fauna from south-west Wales. Geological Journal, Vol. 41, 243-253.

Fortey, R A, and six others. 2000. A revised correlation of Ordovician rocks in the British Isles. Geological Society of London Special Report, No.24.

Foster-Smith, J R. 1981. The non-ferrous metal mines of the south Wales area. British Mining, Vol. 18.

Gayer, R A, Hathaway, T, and Nemcok, M. 1998. Transpressionally driven rotation in the external orogenic zones of the western Carpathians and the SW British Variscides. 253-266 in Continental transpressional and transtensional tectonics. Holdsworth, R E, Strachan, R A, and Dewey, J F (editors). Geological Society of London Special Publication, No.135.

Gibbard, P L, and Lewin, J. 2003. The history of the major rivers of southern Britain during the Tertiary. Journal of the Geological Society of London, Vol. 160, 829-845.

Glasser, N F, Etienne, J L, Hambrey, M J, Davies, J R, Waters, R A, and Wilby, P R. 2004. Glacial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol. 33, 224-237.

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

Hambrey, M J, Davies, J R, Glasser, N F, Waters, R A, Dowdeswell, J A, Wilby, P R, Wilson, D, and Etienne, J L. 2001. Devensian glacigenic sedimentation and landscape evolution in the Cardigan area of southwest Wales. Journal of Quaternary Science, Vol. 16 (5), 455-482.

Harris, C, Ross, N, and Sheppard, T H. 2005. Geological mapping solutions for Quaternary ground-ice systems; Final report March 2005. (Cardiff: University of Wales.)

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

Hughes, C P, Jenkins, C J, and Rickards, R B. 1982. Abereiddi Bay and the adjacent coast. 51-63 in Geological excursions in Dyfed, south-west Wales. Bassett, M G (editor). (Cardiff: National Museum of Wales).

Johnson, M E, Kaljo, D, and Rong, J-Y. 1991. Silurian eustasy. 145-163 in The Murchison Symposium: proceedings of an international conference on the Silurian System. Bassett, M G, Lane, P D, and Edwards, D (editors). Special Papers in Palaeontology, No 44 (London: The Palaeontological Association).

Jones, H K, and 23 others. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication 68.

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.

Lear, D L. 1986. The Quaternary deposits of the lower Teifi Valley. Unpublished Ph.D thesis, University of Wales.

North, F J. 1946. The slates of Wales. (Third edition). (Cardiff: National Museum of Wales.)

Norton, G E, Cooper, D C, Davies, J R, and Cornwell, J D. 2000. Evidence for gold mineralization in the Lower Palaeozoic and Precambrian rocks of south-west Wales. British Geological Survey Research Report, RR/00/10.

Pringle, J, and George, T N. 1937. British regional geology series, south Wales. (London: HMSO.)

Riding, J B. 2004. A palynological investigation of seventeen till samples from Wales. British Geological Survey Internal Report, IR/04/182.

Roberts, J C. 1972. Minor thrust faults in the Avonian of the north crop of the South Wales Coalfield. Geological Journal, Vol. 8, 23-28.

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.

Sheppard, T H, Wilby, P R, Davies, J R, Aspden, J A, Burt C E, and Hall, M R. in preparation. The geology of the Fishguard district - a brief explanation of geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 195 Lampeter (England and Wales).

Shreve, R L. 1972. Movement of water in glaciers. Journal of Glaciology, Vol. 11, 205-214.

Smith, R D A. 1987a. Early diagenetic phosphate cements in a turbidite basin. 141-156 in Diagenesis of sedimentary sequences. Marshall, J D (editor). Geological Society of London Special Publication No.36.

Smith, R D A. 1987b. The Griestoniensis Zone Turbidite System, Welsh Basin. 89-107 in Marine clastic sedimentology. Leggett, J K, and Zuffa, G G (editors). (London: Graham and Trotman.)

Smith, R D A. 1987c. Structure and deformation history of the Central Wales Synclinorium, northeast Dyfed: evidence for a long-lived basement structure. Geological Journal (Thematic Issue), Vol. 22, 183-198.

Smith, R D A. 2004. Turbidite systems influenced by structurally induced topography in the mulit-sourced Welsh Basin. 209-228 in Confined turbidite systems. Lomas, S A, and Joseph, P (editors). Geological Society of London Special Publication, No.222.

Smith, R T, Cooper, D C, and Bland, D J. 1994. The occurrence and economic potential of nodular monazite in south-central Wales. British Geological Survey Technical Report, WF/94/1 (BGS Mineral Reconnaissance Programme Report 130).

Soper, N J, and Hutton, D H W. 1984. Late Caledonian sinistral displacements in Britain: implications for a three-plate collision model. Tectonics, Vol. 3, 781-794.

Soper, N J, and Woodcock, N H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, Vol. 127, 527-542.

Soper, N J, and Woodcock, N H. 2003. The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension? Geological Magazine, Vol. 140, 627-647.

Strahan, A, Cantrill, T C, Dixon, E E L, and Thomas, H H. 1909. The geology of the South Wales Coalfield. Part X. The country around Carmarthen. Memoir of the Geological Survey of Great Britain. (London: H.M.S.O.).

Varnes, D J. 1978. Slope movement types and processes. 11-33 in Landslides: analysis and control. (Special Report 176). Schuster, R L, and Krizek, R J (editors). (Washington D C: National Academy of Sciences.)

Waters, R A, Davies, J R, Wilson, D, and Prigmore, J K. 1997. A geological background for planning and development in the Afon Teifi catchment. British Geological Survey Technical Report, WA/97/35.

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

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

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

(Index map)

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

Figures and plates

Figures

(Figure 1) Simplified bedrock geology and structure map. AF?Abergwesyn Fault; BF?Bronnant Fault; CF?Craig Twrch Fault; CCF?Cwm Cynnen Fault; CWS?Central Wales Syncline; FTA?Foel Twrch Anticline; LF?Llanglydwen Fault; PF?Pontarsais Fault; TA?Teifi Anticlinorium

(Figure 2) Middle Ordovician succession: Llanvirn to Caradoc.

(Figure 3) Late Ordovician to Silurian succession: Ashgill to Telychian.

(Figure 4) Colour-shaded relief Bouguer gravity anomaly map of the Newcastle Emlyn district and surrounding area; coastline shown as thin blue line. Contours are given at 1 milligal (1?×?10−5?m/s2) intervals and have been calculated using a variable Bouguer reduction density.Based on data in the BGS National Gravity Databank; station distribution approximately 1 per 1.3?km2.

(Figure 5) Colour-shaded relief total field aeromagnetic anomaly map of the Newcastle Emlyn district and surrounding area; coastline shown as thin blue line. Contours are given at 10 nanotesla intervals relative to IGRF90. Based on data in the BGS National Aeromagnetic Databank, collected as part of the Hi-Res 1 survey (1998). Flown at a mean terrain clearance of 305 m on N–S flight lines 2 km apart, with E–W tie-lines 10 km apart. Although the data were subject to manual deculturing, some anomalies of non-geological origin may remain.

(Figure 6) Quaternary landscape evolution.

Plates

(Plate 1) Photomicrograph of felsic tuff, Aber Mawr Shales Formation. A very fine-gained, foliated, sericitic groundmass contains subangular to subrounded porphyroclasts of quartz, feldspar and altered rock fragments (P640677).

(Plate 2) Casts of ice-wedges developed in Glaciofluvial Ice Contact Deposits [SN 4490 2746] under periglacial conditions along fissures probably initially created by the melting of an ice buttress (Photograph N Ross; P625660).

(Plate 3) Oblique aerial photograph of pingos, looking east towards Helfa Hall [SN 4180?2772] in centre-left ground (CET 61, © Cambridge University Collection of Air Photographs).

(Front cover) The 14th century gatehouse of the castle [SN 3106 4074] at Newcastle Emlyn. (PhotographerP Witney; (P596680)).

(Rear cover)

(Geological succession) Geological succession in the Newcastle Emlyn district.

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