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Geology of the Cardigan and Dinas Island district — a brief explanation of the geological map — 1:50 000 Sheet 193 (including part of Sheet 210) Cardigan and Dinas Island

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

Bibliographic reference: 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 (including part of Sheet 210) Cardigan and Dinas Island England and Wales).

Keyworth, Nottingham: British Geological Survey, 2004. © NERC 2004. All rights reserved.

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

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

(Front cover) Cliffs in Dinas Island Formation, north of Ceibwr Bay [SN 107 458] (Photograph J R Davies; (GS1218))

(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 193 (including part of Sheet 210) Cardigan and Dinas Island. Symbols in round brackets are the lithostratigraphical symbols used on the map. National grid references (NGR) are given in square brackets. Most of the district lies within 100 km square SN except where indicated with the prefix SM.

Acknowledgements

ThisSheet Explanation was compiled by J R Davies and R A Waters. J R Davies wrote the Introduction and sections on the Ordovician, Structure, Quaternary and Applied geology. The section on metamorphism is based on a contribution provided by R J Merriman. Palaeontological information was provided by M Williams. The manuscript was edited by A A Jackson and figures were drawn by G Tuggey. BGS acknowledges the help of Professor M Hambrey and his colleagues in the Centre for Glaciology. University of Wales, Aberystwyth, with the interpretation of the Quaternary deposits of the district.

Surveying undertaken as part of the Afon Teifi Catchment Survey in 1995–97 was partly funded by Ceredigion, Pembrokeshire and the former Dyfed county councils, and by the Environment Agency. Surveying undertaken as part of the St Dogmaels Landslip investigation in 1994 was partly funded by Pembrokeshire County Council. The British Geological Survey gratefully acknowledges the co-operation of all the landowners in the district during the geological survey, in particular the operators of the sand and gravel workings at Penparc, Monington and Pantgwyn Mawr. Particular thanks are extended to Steve Hartley and the crew of the 'Sulaire' for their assistance with the mapping of the sea cliffs by boat.

Geology of the Cardigan and Dinas Island district (summary from rear cover)

(Rear cover)

This Sheet Explanation describes the geology of the area between Dinas Island in north Pembrokshire and the town of Cardigan in south Ceredigion, and summarises the results of the first detailed geological survey of the district. The spectacular coast between Dinas Head and Cemaes Head, where the cliffs stand up to 160 m above the sea, is part of the Pembrokeshire Coast National Park. Inland, the rolling landscape is incised by deep valleys and precipitous rock gorges.

The exposed bedrock of the district is composed exclusively of deformed Ordovician sedimentary rocks deposited between about 450 and 440 million years ago in a deep-water environment. The strata were deformed during the Acadian orogeny, around 400 million years ago, and show complex folding, cleavage and metamorphism. The Fishguard–Cardigan Fault Belt crosses the district; this has a long history of movement and played a major role during the accumulation of the Ordovician sediment and subsequently parts of it underwent major reversals of movement.

The solid rocks are mantled by a range of Quaternary glacial and postglacial sediments. Two episodes of glaciation are evident: an early pre-Late Devensian glaciation by Irish Sea ice was followed by the main Late Devensian glaciation, around 20 000 years ago. It was during the latter advance and recession of the ice that most of the glacial and periglacial deposits of the district were deposited. After the ice melted and since the postglacial sea level rose to its present-day level, around 5 000 years ago, a suite of alluvial, beach and blown sand deposits (Flandrian) has accumulated.

Extensive sand and gravel deposits are currently worked as a source of local aggregate. Abandoned slate quarries of the Cilgerran gorge and pits previously worked for brick clay in Cardigan bear testimony to an earlier extractive industry. The geological survey has also provided information on a range of applied earth science issues including mineral and water resources, conservation, and geohazards including land instability, gas emissions and flooding.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the area covered by geological 1:50 000 Series Sheet 193 Cardigan, but includes a small part (Dinas Island) of the adjacent Sheet 210 Fishguard. The map was published as a combined solid and drift edition in 2003.

The district lies in the counties of Ceredigion and Pembrokeshire and much of it lies within the Pembrokeshire Coast National Park. The main centre of population is Cardigan sited on the Afon Teifi. The spectacular coastal cliffs between Dinas Head and Cemaes Head attain heights of up 160 m above sea level. Inland, deep valleys and precipitous rock gorges incise a smooth, rolling landscape, which rises to 197 m above OD at Crugiau Cemaes [SN 126 417]. Today, the local economy is based on tourism and agriculture. Extensive sand and gravel deposits are currently worked at Penparc, Monington and Pantgwyn Mawr. The abandoned slate quarries of the Cilgerran gorge and pits previously worked for brick clay in Cardigan bear testimony to an earlier extractive industry.

The exposed bedrock of the district (Figure 1) is composed exclusively of deformed Ordovician, deep water sedimentary rocks deposited between about 450 and 440 million years ago, as part of the fill of the intracratonic, Lower Palaeozoic Welsh Basin. The contemporary shelf, situated on the Midland Platform, lay some 25 km to the south. The solid rocks are mantled by Quaternary sediments (drift), including Pleistocene glacial materials deposited during the last major ice advance some 20 000 years ago, as well as younger, mainly alluvial deposits still accumulating today.

The district is cut by several major powerful west-south-west–east-north-east-trending faults that appear to link with similar structures on the Strumble Head peninsula in the adjacent Fishguard district, some 10 km to the west (Figure 1). These faults, collectively termed the Fishguard–Cardigan Fault Belt, played a major role during the accumulation and subsequent deformation and metamorphism of the Ordovician succession. Lavas and tuffs of the early Ordovician, Fishguard Volcanic Group are exposed to the south and west in the adjacent Fishguard district, but also occur at depth in the Cardigan district. They accumulated mainly, some 465 million years ago, in a submarine, graben-like caldera, whose southern margin was defined by the Fishguard–Cardigan Fault Belt. Following the cessation of volcanicity, the Penyraber Mudstone was deposited along the southern flank of the graben. The younger Cwm-yr-Eglwys Mudstone and the Dinas Island Formation record turbidite deposition within and across the southern margins of the graben, the thickest and sandier sequences accumulating in the lowest parts of the trough. The influence of the fault belt waned during the deposition of the late Ordovician, turbiditic and mudstone-dominated Nantmel Mudstones Formation.

During the Acadian Orogeny, around 400 million years ago, the rocks of the district suffered complex folding, cleavage formation, regional metamorphism and uplift. Some strands of the Fishguard–Cardigan Fault Belt underwent major reversals of movement, so that their present displacements downthrow to the south.

A protracted period of erosion followed the Acadian inversion of the Welsh Basin. The nature of any Upper Palaeozoic or Mesozoic rocks subsequently deposited across the district is unknown. However, deep buried valleys providing evidence for more than one period of river incision and valley fill, record sea level movements associated with the pulsed late Cainozoic uplift of Wales and Pleistocene glaciations. Erratic pebbles, preserved in Pre-Late Devensian deposits, testify to an early glaciation of the district by an Irish Sea ice mass formed by glaciers emerging from the mountains of Scotland, northern England and North Wales. However, it was during the advance and recession of the Late Devensian, Irish Sea Ice Sheet, that occupied the district around 20 000 years ago, that the glacial and periglacial deposits of the district formed. Following the postglacial rise in sea level to its present day level, around 5000 years ago, a Flandrian suite of alluvial, beach and blown sand deposits has accumulated.

Survey history

The district was originally surveyed on a scale of one inch to one mile by H T de la Beche, J Phillips, D H Williams, A C Ramsey, W T Aveline and J Rees (junior), and the results published in 1845 and 1850 as part of [Old Series] sheets 40 and 58. A small area around St Dogmaels was surveyed at 1:10 000 scale by C J N Fletcher in 1994 as part of the St Dogmaels landslip investigation. Subsequently, between 1995 to 1997, mapping at 1:10 000 scale of the eastern part of the district was undertaken as part of the Afon Teifi Catchment Survey (Waters et al., 1997). The remainder of the district was surveyed between 1997 and 1999 by J R Davies, R A Waters, P R Wilby and D Wilson.

During this final phase of mapping some of the geological lines and nomenclature shown on the earlier Afon Teifi Catchment Survey maps were revised and amended.

Chapter 2 Geological description

Ordovician

Ordovician rocks, ranging in age from the late Caradoc to early Ashgill, crop out beneath the whole of the district. The Caradoc rocks comprise the Cwm-yr-Eglwys Mudstone and Dinas Island formations, which are in part laterally equivalent. Ashgill rocks comprise the Nantmel Mudstones Formation. Older Ordovician divisions, thought to occur at depth beneath the district (and shown on the cross-sections), include the early Caradoc Penyraber Mudstone Formation and the underlying Fishguard Volcanic Group of Llanvirn age. The relationships between these older Ordovician units and the strata present in the district can be seen in the Fishguard district to the west (Figure 1).

Abundant fossil graptolites within some rock divisions allow dating and correlation of the succession (Figure 2); (Plate 1). A detailed assessment of the graptolite faunas collected during this survey is given by Williams et al. (2003).

Fishguard Volcanic Group (FVG)

The Fishguard Volcanic Group comprises a succession of predominantly subaqueous, acid ashflow tuffs, amygdaloidal rhyolites and acidic and basaltic pillow lavas. It thickens dramatically northwards across the Pen Caer and Goodwick faults of the Strumble Head peninsula (Thomas and Thomas, 1955), from less than 250 m in the south to 1800 m in the north. Graptolites from the uppermost part of the group, south of the Goodwick Fault, are of the mid Llanvirn murchisoni Biozone (Lowman and Bloxam, 1981).

Penyraber Mudstone Formation (PeA)

The formation consists predominantly of grey mudstone; it rests disconformably on the Fishguard Volcanic Group, south of the Goodwick Fault. The lowest part of the formation comprises the Castle Point Member (CaP), a 50 m-thick sequence of shelly and bioturbated sandstone seen at the type locality, Castle Point [SM 962 378]. In the succeeding Saddle Point Member (SaP), grey silty mudstone contains scattered angular blocks, some in excess of a metre in diameter, of sandstone and volcanic rock. Rafts, several metres long, of fissile, black mudstone, as well as slump-folded units, are also present throughout the 150 m-thick sequence. Graptolites from a black mudstone raft at Aber Howel [SM 990 385], suggest the multidens Biozone. The presence of Nemograptus gracilis, previously reported from this locality by Lowman and Bloxam (1981), has not been confirmed. The remainder of the Penyraber Mudstone Formation, possibly as much as 500 m of strata, comprises fissile, pale to medium grey, colour-banded, silty mudstone with sparse burrow mottles. Scattered laminae and sparse thin beds of calcareous, brown weathering siltstone and fine-grained sandstone contain fine shell debris, and are locally disrupted by simple, tube-like burrows. Thin elongate nodules of white-weathering phosphate are widespread. Thin beds of dark grey, laminated hemipelagic mudstone are sparsely developed, and have yielded no graptolites. This upper part of the formation includes part of the former Parrog Shales of McCann (1992). Reconnaissance mapping in the Fishguard district suggests that the outcrop of the Penyraber Mudstone Formation is confined to the north of the Aber Richard Fault and fails to extend farther east than Nevern [SN 083 400].

The abrupt thickening of the Fishguard Volcanic Group north of the Pen Caer and Goodwick faults on Strumble Head suggests that it accumulated within an active fault-bounded trough or graben (Kokelaar, 1988). The relationships of the group, together with the volume of erupted material, are in keeping with the rapid infilling of a collapsed, Llanvirn caldera sited along, and at least partly defined by, the Fishguard–Cardigan Fault Belt. The association of the thickest volcanic pile with the fault belt, suggests that it was this fracture zone, which provided the principal conduit for magma to propagate to the surface. It follows, moreover, that the subsequent caldera and its fill may have had a much greater lateral extent than present-day outcrops suggest, extending beneath the Cardigan district.

Following the cessation of volcanic activity, uplift along the southern edge of the Fishguard–Cardigan Fault Belt led to local emergence and erosion of the volcanic sequence. The sandstones of the Castle Point Member record a marine transgression across the resulting disconformity. The blocks of sandstone and volcanic rock, contained within massive, muddy debrites and slumps in the overlying Saddle Point Member, testify to the shedding and sliding of material from an up-slope, unburied, but not necessarily emergent tract of the Fishguard Volcanic sequence. An upfaulted scarp along the southern edge of the Fishguard–Cardigan Fault Belt is the likely source. The erosional event affecting the top of the Fishguard Volcanic Group may equate with the formation of the 'sub-gracilis' or mid Ordovician unconformity in parts of north Wales (Howells and Smith, 1997). There, strata overlying the unconformity locally comprise a sequence of mega-debrites (mélange) and associated disturbed beds (Smith et al., 1995). The comparable Saddle Point Member may mark this same episode of instability and mass wasting. The succeeding part of the Penyraber Mudstone Formation records the cessation of mass wasting and the accumulation of turbiditic silty mud in a moderately deep, weakly oxic (dysaerobic) setting. However, these deposits contrasted markedly with the coeval, black graptolitic mud deposited farther south (Figure 1) (Evans, 1945), suggesting that the southern edge of the Fishguard–Cardigan Fault Belt continued to act as an important Caradoc facies divide.

Cwm-yr-Eglwys Mudstone Formation (CyE)

This comprises the upper part of the former Parrog Shales of McCann (1992). It consists of thinly interbedded dark grey, silty turbidite mudstone with locally abundant laminae and thin beds of siltstone and fine-grained sandstone, and pyritic, laminated hemipelagic mudstone. The turbidites embrace a range of Bouma (1962) and fine-grained turbidite types described by Stow and Piper (1984). A single packet of massive, medium to thin-bedded turbidite sandstones, exposed south of Gethsemane, is included in the formation. The junction with the underlying Penyraber Mudstone Formation is not exposed in the district, but occurs just to the south in the Fishguard district.

South of the Newport Sands Fault, the Cwm-yr-Eglwys Mudstone is estimated to be 250 to 300 m in thickness. Graptolites within this tract prove that the formation ranges from the clingani Biozone to the linearis Biozone. North of the fault, the formation is entirely of clingani Biozone age, and exhibits a complex and rapid lateral passage into the Dinas Island Formation. The sequence on Dinas Island has only yielded graptolites of the caudatus Subzone (Plate 1), but at Newport Sands [SN 053 408], the local Cwm-yr-Eglwys Mudstone sequence ranges into the higher, morrisi Subzone. In both sections the formation is overlain by the Dinas Island Formation.

Dinas Island Formation (DI)

This sandstone-dominated late Caradoc formation is spectacularly exposed (Plate 2); (Plate 4) in the cliffs between Dinas Head and Poppit Sands (James, 1976; 1997; McCann 1992). Confined to the north of the Newport Sands Fault, the Dinas Island Formation occupies an extensive tract between Newport Sands and Cardigan, but its type section is on Dinas Island. It includes the former Newport, Ceibwr and Poppit formations of McCann (1992). The current survey has demonstrated lateral equivalence of the Newport and Poppit formations. The Ceibwr Formation is broadly equivalent to the Carreg Bica Mudstone Member of this account.

The Dinas Island Formation comprises a succession (in excess of 1300 m thick) of interbedded turbidite sandstone and mudstone, with subordinate mass-flow conglomerates, pebbly mudstones (debrites) and pyritic, laminated hemipelagic mudstones. The sequence is punctuated by several units, up to 100 m thick, of interbedded turbiditic and hemipelagic mudstone, with subordinate turbidite sandstones. The base of the formation is marked by the appearance of thick- to medium-bedded sandstones above the mudstones of the Cwm-yr-Eglwys Mudstone.

Two main groups of turbidite are recognised, coarse-grained types (Lowe, 1982) and Bouma (1962) types. The coarse-grained types comprise medium- to very thick-bedded conglomerate/sandstone couplets, massive sandstones, locally displaying dish structures, and high-matrix sandstones. The Bouma types comprise thin- to medium-bedded sandstone/mudstone couplets and mudstone-dominated turbidites.

The dominant sandstone-bearing parts of the formation appear to exhibit little internal organisation (Plate 2); (Plate 4). Local thinning- and thickening-upwards sequences occur, but throughout much of the succession, the distribution both of the various types of turbidite and of bed thicknesses appears to be largely random. Local correlation within the sandstone-dominated parts of the formation is aided by the presence of 'bundles' of thick, massive, commonly amalgamated, turbidite sandstone beds (Figure 3). Individual sandstones within such bundles can exceed 8 m in thickness. Again, however, the distribution of these sandstone bundles appears random. Within the formation, the geometry of the beds is generally tabular, but in the cliffs lateral wedging out of sandstone beds is widely observed, particularly some of the thicker beds associated with the sandstone bundles. Channel features, on a variety of scales, are also common, ranging from shallow, flat-bottomed scours a few centimetres deep and 2 to 3 m across, to larger, more obvious, concave-upwards channel forms over a metre deep and tens of metres across (Plate 3). Conglomeratic units, rich in volcanogenic pebbles, mudstone rip-up clasts and locally shell debris have been recorded throughout the formation.

The units of interbedded turbidite and laminated hemipelagic mudstone that punctuate the formation range up to 100 m in thickness (Plate 2). Many have been given informal names, and some can be traced laterally for some distance in coastal cliff sections (Figure 3). In general, however, it has not been possible to correlate the various mudstone units between the principal cliff sections, suggesting that many are laterally impersistent. An exception is the Cwm Degwel Mudstone (CwD) Member, which is everywhere present at the top of the formation. The Carreg Bica Mudstone Member (CBi), the thickest and oldest mudstone unit in the formation, contains abundant slumped and disturbed strata as well as tabular high-matrix sandstones. The formation thickens northwards from 200 m on the north of the Newport Sands Fault to 550 m north of the Pen Cafnau Syncline [SN 050 418]. This thickening is partly achieved by a rapid lateral transition from the Cwm-yr-Eglwys Mudstone into the Dinas Island Formation in the vicinity of the Newport Sands Fault. North of the Ogof Cadno Fault the formation is thought to exceed 1300 m, but the base is not seen.

North of the Ogof Cadno Fault, the lowest exposed part of the formation lies probably within the caudatus Subzone; graptolites of this age have been recovered from the lower part of the Carreg Bica Mudstone. The caudatus/morrisi subzonal boundary occurs in the middle of this member. In contrast, to the south of the Carreg Bica Fault, the base of the formation is within the morrisi Subzone. The presence of a probable Normalograptus tubuliferous in an assemblage obtained from the Cwm Degwel Mudstone, north of Pen-y-bal [SN 048 417], suggests the top of the formation may range into the linearis Biozone.

The distribution and relationships of the Cwm-yr-Eglwys Mudstone and the Dinas Island formations suggest that the Fishguard–Cardigan Fault Belt strongly influenced sedimentation and thickness. The dramatic thickening of the Dinas Island Formation to the north of the Ogof Cadno Fault, and the bounding nature of the Newport Sands Fault, indicate that they operated in concert to confine and accommodate this sand-dominated turbidite formation in a linear, tectonic trough. The latter was probably inherited from the earlier Fishguard Volcanic Group caldera. Features which indicate that deposition was close to the turbidite source area include: the presence of abundant channel and scour features, shelly and rip-up clast-rich conglomerates, thick to very thick and commonly amalgamated massive sandstone beds, mudstone debrites, and common Ta to Tb Bouma intervals. The abundance of volcanoclastic material throughout the formation suggests that much of the sediment was ultimately derived from the nearby Fishguard Volcanic Group, probably not directly, but recycled via shallower water accumulations of volcanic detritus. Palaeocurrent data from the formation is ambiguous. Most flute marks and cross-lamination suggest that turbidity currents flowed from the west-south-west along the axis of the trough. However, channel features, which are consistently orientated north-north-west–south-south-east, testify to lateral supply either from the north or from the south. The present-day distribution of Caradoc facies (Figure 3), in providing no obvious sediment supply route from the south, suggests derivation was largely from the speculated northern margin of the Caradoc trough (Figure 4). Alternatively, the current juxtaposition of facies may be the product of strike-slip movement along the Fishguard–Cardigan Fault Belt and this may have displaced points of southerly sediment in-put which were of equal or greater importance than those to the north.

The system probably comprised a strongly confined, linear belt of sandy lobes and channels, in which the zones of active deposition were constantly switching. Local fining-upwards sequences may represent the fills of more stable and longer lived channels, whilst local thickening-upwards sequences possibly record the growth of the small lobes these channels supplied. The mudstone units were deposited during periods when sand deposition was locally abandoned due to channel and lobe switching.

The Cwm-yr-Eglwys Mudstone was deposited by low-concentration turbidity currents that carried mud, silt and fine sand, and as hemipelagic mud, in a setting from which the coarser facies of the Dinas Island Formation were excluded. Initially the Ogof Cadno Fault and subsequently the Newport Sands Fault were influential in achieving this separation (Figure 4).

The deposition of the Cwm Degwel Mudstone, at the top of the formation, marked the end of coarse-grade Caradoc turbidite deposition and the onset of blanket mudstone sedimentation. It probably records the cessation of contemporary fault activity. The presence of laminated hemipelagic mudstones throughout this member, and throughout both the Dinas Island and Cwm-yr-Eglwys formations as a whole, point to a setting in which anoxic bottom water conditions were sustained.

Nantmel Mudstones Formation (Ntm)

The formation crops out extensively beneath the southern and eastern parts of the district on either side of the Fishguard–Cardigan Fault Belt. It is up to 1300 m thick, but the top is not seen in the district. It comprises medium to pale grey, variably burrow-mottled and diffusely colour-banded mudstone. Thin beds and laminae of siltstone and fine-grained sandstone up to 15 mm thick are common in the lower half of the formation (Plate 4). Where they are absent, especially in the upper half of the formation the mudstones are massive with bedding planes spaced 1 to 2 m apart. These lithologies represent varieties of fine-grained turbidites (Stow and Piper, 1984) that are thinly interbedded with bioturbated, hemipelagic mudstones. White-weathering phosphate nodules are common.

A sandstone-rich unit (sa''), 300 to 400 m thick, is present some 100 to150 m above the base of the formation. It comprises Bouma-type turbidite sandstone/mudstone couplets in which the sandstones are up to 0.2 m in thickness. Thin bedded, colour-banded and burrow-mottled turbiditic and hemipelagic mudstones, similar to the rest of the formation, are also present. Palaeocurrent data suggests that they were deposited from turbidity currents flowing towards the north-east.

The base of the formation is widely taken in central Wales to approximate to the Caradoc/Ashgill boundary (Fortey and Rushton, 2000). In marked contrast to the earlier Caradoc sequence, the Fishguard–Cardigan Fault Belt failed to exercise a significant influence on the distribution of Ashgill facies.

The Nantmel Mudstones record an abrupt and widespread change from the dark, graptolitic, anoxic lithologies, which had prevailed during much of the Caradoc, into bioturbated oxic facies. It has been suggested that this change was due to the onset of polar cooling as a precursor to the late Ashgill glaciation and the initiation of a deep thermohaline circulation (Armstrong and Coe, 1997). The blanket deposition of burrowed turbidite mud, which now prevailed across the district, in common with much of the basin, records the growth of an extensive wedge (slope apron) of fine-grained sediment, built out from the basin margin (Davies et al., 1997). The sandstone division records an interval during which sandy turbidite facies migrated into the area.

Structure

The principal deformation to affect the Cardigan district was that related to the late Caledonian, Acadian Orogeny, consequent to the closure of the Iapetus Ocean and the oblique collision of the Avalonian and Laurentian plates. In Wales, this event reached its acme during the early Devonian. The complex folding imposed on the rock succession during this period is spectacularly displayed in the coastal cliff sections (Figure 3); (Plate 5).

The Fishguard–Cardigan Fault Belt had a significant pre-Acadian history of movement, exercising a powerful influence on the development of Ordovician facies within and beyond the district. The sense of movement on its component fractures during this period was to downthrow to the north. The current sense of displacement, however, on both the Ogof Cadno and Newport Sands faults, is a downthrow to the south, the former by possibly as much as 800 m. This is thought principally to record their reactivation as reverse faults in response to Acadian compression. The Ceibwr Bay Fault, in contrast, retains a northward sense of downthrow of at least 600 m. There is little doubt that the fault belt, in representing a line of crustal weakness, is likely to have experienced further movement during Variscan, and possibly Alpine tectonic episodes.

Acadian deformation imposed east-north-east-trending folds and a regional axial planar cleavage. Larger scale folds are picked out by the distribution of the formations on the geological map, but most sections are dominated by smaller scale folds with wavelengths and amplitudes ranging up to hundreds of metres. Interlayered sandstone-mudstones sequences, such as the Dinas Island Formation and the sandstone facies of the Nantmel Formation, exhibit complex disharmonic fold patterns with associated faulting (Figure 3); (Plate 5). Such folds can vary over short distances from open, upright and symmetrical to closed, inclined and asymmetrical, commonly displaying steep, vertical and locally overturned southern limbs. Associated mudstone sequences vary in thickness dramatically, thickening into hinge zones, but thinning along steep fold limbs. In contrast, the mudstone-dominant facies, such as the Cwm-yr-Eglwys Mudstone and Nantmel Mudstone formations, commonly display more regular shallow, upright open folds (Plate 4). Fold axes within the district plunge predominantly to the east-north-east, but periclines with opposing plunge directions are also present.

Cleavage fabrics vary from penetrative, slaty and pressure solution varieties developed within mudstones, to a spaced and anastomising fracture cleavage developed in some argillaceous sandstones. A later crenulation cleavage is present locally.

Metamorphism

The illite crystallinity pattern in the Cardigan district shows that the region to the north of the Newport Sands Fault is generally of lower metamorphic grade than the region to the south. This suggests a link between grade and lithology, with the sandstone-dominated crop of the Dinas Island Formation at lower grade than the mudstone-dominated tract to the south, which is now effectively a slate belt. The metamorphism probably developed in two phases. Early burial metamorphism occurred when this part of the Welsh Basin was still a site of active sedimentation and contained an accumulated overburden several kilometers thick. Stratigraphical proximity to a major volcanic centre, south-west of the area, suggests that a higher than normal geothermal gradient may have generated burial temperatures in the range 200° to 250°C. A later phase of metamorphism can be related to Acadian basin compression and transpressive deformation along the Fishguard–Cardigan Fault Belt. During this phase, a slaty cleavage was developed in mudstone lithologies, and strain-generated recrystallisation of clay minerals in these rocks resulted in the highest grades found in the district.

Quaternary

The Quaternary (Drift) deposits of the district consist mainly of materials deposited during the last (Late Devensian) glaciation. However, locally cemented, pre-Late Devensian sediments are patchily preserved, while post-glacial, alluvial and colluvial deposits have accumulated since the retreat of the Late Devensian ice sheet. A detailed account of the Quaternary deposits and landforms investigated during the current survey is provided by Hambrey et al. (2001).

Pre-Late Devensian landscape

The form of much of the district relates to the evolution of the Teifi valley, the broad form of which, was the product of late Tertiary uplift and erosion (Brown, 1960; Dobson and Whittington, 1987). However, during this period and the Quaternary, the Afon Teifi did not maintain and incise a single course, for deeply incised, older courses of the river record two or more periods of bedrock erosion, each followed by sediment infill and river diversion (Jones, 1965). In places, the modern river follows these earlier courses, where it is flanked today by broad flood plains. In contrast, elsewhere it flows through steep rock gorges (probably of post-late Devensian age), as at Cilgerran (Plate 6) leaving segments of its earlier courses abandoned and in places concealed beneath a thick fill of glacial drift. Geophysical profiles across these preglacial courses of the Afon Teifi have been undertaken by Allen (1960), Francis (1964) and Nunn and Boztas (1977) but were run extensively as part of the recent survey (Waters et al., 1997). Boreholes drilled on these profiles (Hambrey et al., 2001) have demonstrated that the ancient courses of the Teifi extend, near Cardigan, to depths of up to 40 m below OD. Beneath the Teifi estuary, geophysical investigations undertaken on behalf of BGS by R J Whittington (University of Wales Aberystwyth) have shown the rock floor of the valley to descend to at least 55 m below OD. Such deep incision relates to the substantial falls in sea level which accompanied periods of Pleistocene glaciation. Although the Late Devensian sea level lowstand may account for one such period of down-cutting, the cross-section profile of the valley testifies to incision in response to at least one earlier glacial event, and it is possible that the main periods of incision were entirely pre-Late Devensian. The tributary valleys of the Afon Teifi exhibit comparable styles of incision, abandonment and infill.

Steep sided, deeply incised valleys, rising up and over low cols are common in the district. Those crossing the southern interfluves have been interpreted as glacial lake overflow channels or subglacial channels (e.g. Bowen and Gregory, 1965; John, 1970; Bowen, 1967, 1971). Examples are the Cippyn [SN 140 480], Moylgrove [SN 116 446] and Llantood [SN 155 419] channels. Though undoubtedly utilised and modified by Late Devensian glacial process, at least some of these features are suspected to date from one or more, earlier glacial episodes.

Pre-Late Devensian deposits

At several localities, principally along the main coastal section, cemented breccias and gravels occur as thin veneers clinging to rock platforms and steep cliff faces; they also infill fissures and line gullies cut in rock (Plate 7). The breccias consist almost exclusively of sandstone, mudstone and vein quartz clasts derived from the local bedrock. Significantly, however, the ill-sorted boulder, cobble and pebble gravels also include exotic volcanic and granite clasts. All the clasts are set in a red ferruginous or, locally, black manganese oxide matrix.

Reworked blocks and pebbles of the cemented material are commonly present in overlying Late Devensian glacial deposits, demonstrating that the cemented deposits are of pre-Late Devensian age. They are interpreted as local regolith, scree and alluvial deposits. The presence of igneous clasts in the gravel records the availability of erratics for inclusion within these deposits and, hence, provides evidence of an earlier (pre-Late Devensian) advance of Irish Sea ice into the district.

Similar cemented sands and gravels overlying an extensive subhorizontal rock platform at Poppit Sands [SN 145 489] have been interpreted as raised beach deposits and the platform as wave-cut in origin (John, 1971; Bowen, 1977; Campbell and Bowen, 1989). Bowen and Lear (1982) have suggested that the platform was cut during the Ipswichian sea level highstand, around 125 000 years ago, on the basis of foraminifera from the cemented deposits. However, Hambrey et al. (2001) suggest the platform, together with a matching one on the northern side of the estuary at Gwbert [SN 160 498] (Plate 4), may be the product of glacial scour, and that the overlying deposits are fluvial or glaciofluvial in origin, with the foraminifera being derived from older deposits. The cemented deposits at Poppit, in keeping with those elsewhere, are likely to be pre-Late Devensian in age, but whether they and the underlying platform, date from the Ipswichian Interglacial remains in doubt.

Late Devensian deposits

The Late Devensian deposits relate to the advance and recession of an Irish Sea Ice Sheet. This ice mass was supplied by glaciers flowing out from the Highlands of Scotland, the Lake District and north Wales. During its southwards advance it overrode the Welsh coast so that, at its maximum extent, some 20 000 years ago, as well as occupying much of the present Irish Sea Basin, it also covered much of the coastal tract of south-west Wales. Although the Preseli Hills appear to have limited the inland advance of the ice sheet, it certainly covered the entire district. The subsequent down wasting and retreat of the ice sheet from the district was probably completed by 15 000 years ago.

The glacial deposits associated with the Irish Sea Ice Sheet principally include proglacial lake clay, till, glaciofluvial sand and gravel, and a range of heterogenous and heterolithic glacial deposits. Head gravel and talus formed under the periglacial conditions which preceded ice advance and followed its retreat. All the deposits derived from the Irish Sea Ice Sheet are characterised by the presence of erratics derived from northern source areas, and from the floor of the Irish Sea basin. These include Scottish granites, Lake District and Snowdonia volcanic rocks, Carboniferous limestones, red Permo-Triassic sandstones, Cretaceous flints and 'Tertiary' lignite. The deposits also contain shell fragments eroded by the ice from the floor of the Irish Sea.

Occupying the older, deeply incised portions of the Teifi valley, and preserved in valleys farther south at Moylgrove, Tregamman and Pontgarreg are thick sequences of structureless and silt-laminated (varved) clay (Plate 8) and laminated silt, interpreted as glaciolacustrine deposits. In the Teifi valley (Figure 5), where they were proved in boreholes drilled at 10 cm Llwynpiod [SN 1768 4764], Pen-y-bryn [SN 1761 4285], St Dogmaels [SN 1585 4549] and Penparc [SN 2012 4844], these clay and silt sequences exceed 75 m in thickness and extend to over 37 m below OD (Fletcher and Siddle, 1998; Hambrey et al., 2001). They record deposition within an extensive proglacial lake, Llyn Teifi, created by the damming of the Teifi estuary by the Irish Sea ice mass. However, it is now recognised that this lake formed during the initial advance of the Irish Sea ice into the region and not, as suggested previously (Charlesworth, 1929; Jones, 1965; Bowen and Lear, 1982), during the withdrawal of the Devensian ice mass (Figure 6). Thin units of unconsolidated, angular and rounded gravel, 2 to 3 m thick, which intervene between the clay sequences and bedrock, represent the regolith, head and fluvial deposits, which occupied the valley prior to drowning. Dropstones and thin units of diamicton within the clays record the inclusion of material shed from icebergs floating in the lake. As the lake level rose pre-existing channels in the southern watershed were exploited as spillways. The threshold of the lowest and most westerly, the Cippyn channel [SN 140 480], is less than 100 m above OD, but as the ice mass pushed inland, this was occluded and two higher features, the northern [SN 153 416] and southern [SN 151 411] Llantood channels (about 120 m OD) were utilised. Llyn Teifi meltwater drained via these channels into the separate ice-dammed lakes impounded within the valleys at Moylgrove and Tregamman.

A widespread brown-weathering, stiff, stony clay (diamicton) interpreted as a basal till rests locally on the lake deposits within the abandoned segments of the Teifi valley, and on striated bedrock on the higher ground both to the south and north, and at the mouth of the Teifi estuary. Both the distribution of this deposit, and deformation structures within underlying lake clays, provide compelling evidence that, as the Irish Sea Ice Sheet continued its southwards advance, it displaced the lake and overrode the deposits of Llyn Teifi (Figure 6).

Along the coastal tract, the till sheet is either overlain, or replaced by a blanket of highly variable, but predominantly gravelly material termed heterogenous glacial deposits (informally referred to as rubble drift.) This comprises an intergradational suite of ill-sorted clay- and silt-bound gravels, clast-rich diamictons and unstratified sand and gravel. Where these materials rest on bedrock they grade downwards through angular head gravel into regolith.

They record the reworking and mixing of earlier head and weathered solid material with glacial sediment, at the margin of the ice as it advanced inland. Constructional landforms composed of this heterogeneous material locally flank the sides of valleys to the north and south of Penparc, and the south side of the Teifi estuary, and are interpreted as degraded kame terraces.

In the east of the district, both to the north and south of the Afon Teifi, earlier lake clay and till are overlain by a complex assemblage of sediments included together in the single category, glacial deposits undifferentiated. In these deposits, silt is interbedded with subordinate stony clay (diamictons), stoneless clay, sand and gravel. Enclosed depressions in the surface of the deposit, notably in the region of Cilbronau [SN 205 453], represent kettleholes marking the former position of buried masses of ice. The deposits themselves reflect the complex range of processes and settings associated with the withdrawal of the Irish Sea Ice Sheet. Much of the deposition may have been in ephemeral recessional lakes, but also included is material derived directly from down wasting ice masses, as well as meltwater streams.

Mound-like bodies of glaciofluvial deposits, comprising stratified sand and gravel, rest locally on all the previously described materials, as well as on bedrock (Plate 9). Detailed studies have been made of the deposits exposed in the Cardiganshire Sand and Gravel Pit [SN 202 485] at Penparc (Banc-y-Warren of the literature) (Mitchell, 1972; Helm and Roberts, 1975; Allen, 1982; Worsley, 1984) and at Trefigin quarry, Monington (Owen, 1997). At Penparc, the cross-stratified gravels contain clasts of sand that were evidently frozen when transported. The associated ripple-drift cross-laminated sands are affected by numerous small-scale conjugate faults. Hambrey et al. (2001), reject the earlier interpretation that these deposits are lake delta and supraglacial in origin, and view the Penparc deposit, in common with many of the other sand and gravel mounds, as a buildup of meltwater sediment that was confined between lobes of the receding ice sheet. As the supporting buttresses of ice melted, the pile of glaciofluvial material collapsed around the edges, leaving the mass of sediment as an upstanding topographical feature (Figure 6).

Head gravel and talus, composed principally of angular mudstone 'chips' and other local bedrock fragments, are widespread particularly on the crop of the Nantmel Mudstones Formation. They flank areas of solid exposure and line the floors of valleys cut in rock; they fringe and interfinger with the proglacial lake clay sequences of the Teifi valley, but also abut and overlie the younger glacial deposits. These gravels represent accumulations of frost-shattered rock debris formed under tundra-like periglacial conditions. Those associated with the Llyn Teifi clays were deposited as fan-like bodies around the margins of the expanding, ice-dammed lake. Other SW Monington Pantgwyn Mawr deposits were generated during the freeze-thaw conditions which accompanied the recession of the ice, and also later during the period of the Loch Lomond Re-advance.

Postglacial deposits and evolution of the modern landscape

Following the withdrawal of the Irish Sea ice dam, the Afon Teifi and other rivers of the district, began to establish their modern courses, cutting into and eroding the newly deposited glacial materials which filled their valleys. Solifluction and slope wash processes contributed to local accumulations of head and it is likely that many of the larger landslips in drift were initiated at this time. At the outset of this period of incision, global sea level, though rising, was still much lower than the present level and the effects of isostatic rebound were also important. At intervals along its length, the Afon Teifi cut through the new glacial sediments to flow on bedrock and then proceeded to cut the deep rock gorges which are such a feature of the river today. In so doing, it abandoned extensive segments of its preglacial valley leaving some of these partially excavated, but others still buried and concealed beneath their Late Devensian glacial fill.

As sea level attained its present-day position, around 5000 years ago, the lower Teifi valley was drowned as far as the Cilgerran gorge. The estuary has subsequently silted-up with tidal flat and saltmarsh deposits, and a spit of beach deposits (Poppit Sands) has grown across the river mouth. Inland, floodplains of alluvium have accumulated in the valleys of the Teifi catchment and other local rivers and streams, and alluvial fan deposits have built up where some tributary streams emerge into larger river valleys. Local accumulations of lacustrine deposits and peat have formed in kettle holes and other enclosed depressions, and at sites of impeded drainage. Ridges of storm beach gravel line many of the small coves in the cliffs between Dinas Island and Cemaes Head. Dune complexes of blown sand developed on the western sides of Newport Bay and the Teifi estuary, where there was optimum exposure to the prevailing westerly winds.

Artificially modified ground

Worked ground is shown on the 1:50 000 map where the ground surface has been excavated for minerals, and has not been back-filled. Only the larger sites are shown and these relate to sand and gravel extraction, notably to the west of Penparc, where the active Cardiganshire Sand and Gravel Pit [SN 2020 4847] currently extends over 8 ha. Made ground indicates areas where the ground surface is underlain by material (over 1 m in thickness) that has been deposited as a result of human activity. This can include areas where pre-existing pits, quarries and other forms of excavation have been partially or wholly back-filled, as well as areas where the deposition of materials has been used to elevate the ground surface, for example on railway and road embankments and for flood prevention.

Areas where the original ground surface has been extensively remodelled, but where it is impossible or impracticable to distinguish areas of excavation from areas of fill (made ground) are distinguished as landscaped ground.

Chapter 3 Applied geology

Earth science factors have a significant influence on the activities of man and 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 fit for purpose, that local resources are not sterilised or contaminated, and, where necessary, that appropriate mitigation measures are taken prior to construction. A detailed assessment of the earth science factors relevant to local planning and development in the region was undertaken as part of the Afon Teifi Catchment Survey (Waters et al., 1997). There are considerable sand and gravel resources in the district, and locally the diverse drift deposits give rise to problematical ground conditions and landslips. Other important factors are water resources, gas and radon emissions and the risk of flooding.

Mineral resources and construction materials

The main construction material available in the district is sand and gravel. Glaciofluvial deposits, are currently worked at the Cardiganshire Sand and Gravel Pit [SN 2018 4844], Trefigin Quarry [SN 1375 4350], and Pantgwyn Mawr Quarry [SN 1230 4235] (Plate 9). The Cardiganshire Sand and Gravel Pit works a 50 m-thick sand-dominant sequence, while the other two work gravel-dominated sequences. The materials are used locally for building sand, concrete aggregate and fill. The resource potential of the sand and gravel deposits of the district has been assessed by Crimes and Lucas (1992).

The upper, siltstone-poor part of the local Nantmel Mudstones succession has been worked in the past for roofing slate and flagstones in the Cilgerran gorge [SN 193 446], from where they were transported down river to Cardigan. Building stone for local use has been obtained in the past from numerous small quarries in all the solid rock divisions, but notably from the Nantmel Mudstones. Clays in the glaciolacustine deposits have been worked in the past as brick clay at two sites [SN 1790 4652] and [SN 1832 4659] in Cardigan.

Water resources

Although the district is largely on mains supply, many farms and some local businesses use groundwater from springs and private boreholes. Springs are commonly located either at drift-solid contacts, or where gravelly (high permeability) deposits rest on clayey (low permeability) drift deposits. A powerful spring [SN 1798 4911], south-west of Ferwig, previously supplied water to Cardigan. All the solid rocks in the district have potential as moderately permeable aquifers and many of the private supply boreholes are either wholly or partly in solid rock. Their aquifer potential stems from the presence of fracture porosity in the near-surface zone, especially in the vicinity of faults and particularly in weathered zones (Robins et al., 2000). Moderately permeable drift aquifers include most of the deposits that contain sand and gravel, notably glaciofluvial deposits, heterogeneous glacial deposits, head gravel, blown sand and alluvium.

Foundation conditions

Foundation conditions vary widely in the district. Few problems are normally encountered with the solid rocks but some of the drift deposits can give rise to difficult ground conditions on certain sites. The limited geotechnical data available for the district has been assessed as part of the Afon Teifi Catchment Survey (Waters, et al., 1997).

The solid rocks have high bearing capacities, except in the weathered zone. Pyritic mudstones in the Dinas Island and Cwm-yr-Eglwys Mudstone formations may cause heave and concrete attack due to sulphate content.

The clays of the glaciolacustrine deposits have a low bearing capacity and can give rise to moderate settlement. Long term instability may be encountered in excavations. Till, heterogeneous and undifferentiated glacial deposits, head, head gravel, alluvium and alluvial fan deposits, all have low to moderate bearing capacities with moderate to high settlement. Peat, lacustrine deposits, tidal flat and saltmarsh deposits have low bearing capacities and may give rise to high settlement. Although, glaciofluvial, beach and blown sand deposits may embrace variable foundation conditions, they generally have high bearing capacities. Artificial ground is highly variable and may include contaminated land and give rise to polluted groundwater.

Slope stability

Landslips are common throughout the district, mainly in drift. Those in solid, are largely limited to the coast south of Cemaes Head. The main drift deposits involved in landslips are glaciolacustrine deposits (clay), till and heterogeneous glacial deposits. The predisposing factors responsible for the slips are the nature of the drift materials, the hydrogeology of the site and the slope angle. The main trigger for landslips in the district is erosion of the toe of the landslip by watercourses. As a result, many of the landslips occur in valleys adjacent to watercourses. Many of the landslips have long histories, probably dating back to the retreat of the late Devensian ice sheet. A case in point is the St Dogmaels Landslip [SN 160 455] (Plate 10), which is an old, complex slip involving till, and glaciolacustrine deposits (clay and silt). It became reactivated in 1994, causing damage to property in the village. The focus of a detailed investigation (Maddison et al., 1994; Maddison, 2000), it is now stabilised as a result of remedial works. The largest area of landslip in the district, some of which is active, occupies the steep coastal slope [SN 146 489], west of Poppit Sands. It is developed in heterogeneous glacial deposits. The coastal landslips in solid rock, south of Cemaes Head, have formed predominantly where bedding planes dip seawards out of the cliffs. The constant erosion of the cliffs stimulates movement and leads, periodically, to catastrophic failures of the cliff face.

Gas emissions

Methane is commonly generated by decomposition of domestic and industrial refuse in landfill sites. It is toxic, an asphyxiant and explosive in high concentrations. Methane is less dense than air and is capable of migrating through fractures in rocks and permeable drift deposits. It may accumulate in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions may represent a significant hazard at particular sites, risk can be mitigated by appropriate design measures.

Natural radon emissions

Radon is a naturally occurring ionising gas, produced by the progressive decay of uranium, which though present in small quantities in all natural rocks and soils, occurs in slightly higher concentrations in some igneous rocks and black mudstones. Radon released from rocks and soils is normally quickly diluted in the atmosphere and does not present a hazard. However, radon that enters poorly ventilated spaces, such as basements can be potentially hazardous and give rise to an elevated risk of serious illness including lung cancer.

Within the district, radon potential is generally high. Between 3 and 10 per cent of properties are likely to exceed the UK 'Action Level' of 200 bequerels per cubic metre.

Flooding

River and stream floodplains throughout the district are susceptible to regular flooding. Particularly prone is the floodplain of the Afon Teifi at Llechryd and at its confluence with the Afon Piliau, east of Cardigan. The damaging effects of past flooding events along the lower reaches of the Afon Mwldan, which enters the Teifi at Cardigan, have led to the construction of a flood alleviation tunnel, designed to capture and divert flood water into the Teifi below the town. On the geological map, the extent of recent floodplain deposits, shown as alluvium, provide an indication of those areas prone to regular flooding.

Groundwater pollution

Pollution of groundwater is a potential hazard in areas of worked or made ground, which may contain toxic residues. These include poorly lined landfill sites, agricultural slurry pits and active or former industrial sites such as sewage works, gasworks, gravel pits, quarries and railway sidings.

Geological conservation

Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). These are statutory designated conservation sites which have some protection under the Wildlife and Countryside Act 1981. Within the district there are three SSSIs all designated for their Quaternary features. Further information on the extent and designation of SSSIs can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road., Bangor, Gwynedd LL57 2LQ. Special features of interest being considered for notification as SSSIs are described in the Geological Conservation Review series, published by the Joint Nature Conservancy Committee.

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

Searches of indexes to some of the BGS collections can be made on the Geoscience Data Index system available in BGS libraries and on the web site. BGS Catalogue of geological maps and books is available on request. Published maps can be purchased through the BGS sales desk. (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. Teleph one 01491 838800. Fax 01491 692345.

Maps

Books and reports

Books, reports and papers are listed in the Selected bibliography. 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, borehole core and fossils. Index data for petrological specimens is listed in the BRITROCKS database that can be searched through the BGS Geoscience Data Index (see back cover for web address. Further information about material collections can also be obtained through the enquiry service.

References

Most of the references listed below are held in the libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright legislation.

Allen, A. 1960. Seismic refraction investigations of the pre-glacial valley of the River Teifi near Cardigan. Geological Magazine, Vol. 97, 276–282.

Allen, J R L. 1982. Late Pleistocene (Devensian) glaciofluvial outwash at Banc-y-Warren, near Cardigan (West Wales). Geological Journal, Vol. 17, 31–47.

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.

Bouma, A H. 1962. Sedimentology of some flysch deposits — a graphic approach to facies interpretation. (Amsterdam/New York: Elsevier.)

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. 1971. The Pleistocene succession and related landforms in north Pembrokeshire and south Cardiganshire. 260–266 in Geological Excursions in South Wales and the Forest of Dean. Bassett, D A, and Bassett, M G (editors). (Cardiff: Geologists' Association.)

Bowen, D Q. 1977. The coast of Wales. 223–256 in The Quaternary History of the Irish Sea. Kidson, C, and Tooley, M J (editors). Geological Journal Special Issue, No. 7. (Liverpool: Seel House Press.)

Bowen, D Q, and Gregory, K J. 1965. A glacial drainage system near Fishguard, Pembrokeshire. Proceedings of the Geologists' Association, Vol. 74, 275–282.

Bowen, D Q, and Lear, D L. 1982. The Quaternary geology of the lower Teifi Valley. 297–302 in Geological Excursions in Dyfed, south-west Wales. Bassett, M G (editor). (Cardiff: National Museum of Wales.)

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

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

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

Crimes, T P, and Lucas, G. 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 (Welsh Office), P EC D 7/1/382.

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).

Dobson, M R, and Whittington, R J. 1987. The geology of Cardigan Bay. 331–353 in Geology and sediments of off-shore Wales and adjacent areas. Bassett, M G (editor). Proceedings of the Geologists' Association, Vol. 98.

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, Harper, D A T. Ingham, J K, Owen, A W, Parkes, M A, Rushton, A W A, and Woodcock, N H. 2000. A revised correlation of Ordovician rocks in the British Isles. Geological Society of London Special Report, No. 24.

Francis, T J G. 1964. A seismic refraction section of the pre-glacial Teifi Valley near Cenarth. Geological Magazine, Vol. 101, 108–112.

Hambrey, M J, Davies, J R, Glasser, N F, Waters, R A, Dowdeswell, J A, Wilby, P, 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, 455–482.

Helm, D G, and Roberts, B. 1975. A re-interpretation of the origin of sands and gravels around Banc-y-Warren, near Cardigan, west Wales. Geological Journal, Vol. 10, 131–146.

Howells, M F, and Smith, M. 1997. The geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales).

James, D M D. 1976. Caradoc turbidites at Poppit Sands (Pembrokeshire), Wales. Geological Magazine, Vol. 112, 295–304.

James, D M D. 1997. Llanvirn-Llandovery activity on the Llangrannog lineament in southwest Ceredigion. Mercian Geologist, Vol. 14, 68–78.

John, B S. 1970. Pembrokeshire. 229–265 in The glaciations of Wales and adjoining regions. Lewis, C A (editor). (London: Longman).

John, B S. 1971. Glaciation and the west Wales landscape. Nature in Wales, Vol. 12, 138–155.

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.

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

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.

Lowman, R D W, and Bloxham, T W. 1981. The petrology of the Lower Palaeozoic Fishguard Volcanic Group and associated rocks, east of Fishguard, north Pembrokeshire (Dyfed), South Wales. Journal of the Geological Society of London, Vol. 138, 47–68.

Maddison, J D. 2000. St Dogmaels landslide: deep drainage by wells. 106–108 in Landslides and landslide management in south Wales. Siddle, H J, Bromhead, E N, and Bassett, M G (editors). National Museum and Galleries of Wales Geological Series, No. 18.

Maddison, J D, Siddle, H J, and Stephenson, B. 1994. St Dogmaels Landslide: report on investigation. (Cardiff: Sir William Halcrow and Partners Ltd.)

McCann, T. 1992. The stratigraphy of the Ordovician rocks around Cardigan, Wales. Proceedings of the Yorkshire Geological Society, Vol. 49, 57–65.

Mitchell, G F. 1972. The Pleistocene history of the Irish Sea: second approximation. Scientific Proceedings of the Royal Dublin Society, Series A, Vol. 4, 181–199.

Nunn, K R, and Boztas, M. 1977. Shallow seismic reflection profiling on land using a controlled source. Geoexploration, Vol. 15, 87–97.

Owen, G. 1997. Origin of an esker-like ridge — erosion or channel-fill? Sedimentology of the Monington 'Esker' in southwest Wales. Quaternary Science Reviews, Vol. 16, 675–684.

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

Smith, M, Rushton, A W A, and Howells, M F. 1995. New litho- and biostratigraphic evidence for a mid-Ordovician hiatus in southern central Snowdonia, North Wales. Geological Journal, Vol. 30, 145–156.

Stow, D A V, and Piper, D J W. 1984. Deep-water fine-grained sediments: facies models. 611–646 in Fine-grained sediments: Deep-water processes and facies. Stow, D A V, and Piper, D J W, (editors). Geological Society of London, Special Publication, No. 15.

Thomas, G E, and Thomas, T M. 1956. The volcanic rocks between the area between Fishguard and Strumble Head, Pembrokeshire. Quarterly Journal of the Geological Society of London, Vol. 112, 291–311.

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.

Williams, M, Davies, J R, Waters, R A, Rushton, A W A, and Wilby, P. 2003. Graptolite biostratigraphy and palaeoecology of the Caradoc (Upper Ordovician) succession of the Cardigan area, south-west Wales. Geological Magazine, Vol. 140.

Worsley, P. 1984. Banc-y-Warren. 68–76 in Wales: Gower, Preseli, Fforest Fawr. Bowen, D Q, and Henry, A (editors). (Cambridge: Quaternary Research Association Field Guide.)

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.

Figures and plates

Figures

(Figure 1) Simplified geology map of the district and adjoining areas showing the main strands (named) of the Fishguard–Cardigan Fault Belt.

(Figure 2) Ordovician graptolite biozones referred to in the text.

(Figure 3) Sketch of cliff section in the Dinas Island Formation (based on photographs with no adjustment for distortion or perspective effects).

(Figure 4) Depositional model for the Caradoc succession of the district and adjoining areas (relationships and facies of the northern side of the graben are speculative).

(Figure 5) Summary logs of the principal boreholes through the glacial succession of the lower Teifi valley showing relative heights and relationships of the main glacial units (modified after Hambrey et al., 2001).

(Figure 6) Schematic cross-sections illustrating the events and deposits associated with the Late Devensian glaciation of the lower Teifi valley and adjacent areas (modified after Hambrey et al., 2001).

Plates

(Plate 1) Graptolite-strewn bedding plane, Cwm-yr-Eglwys Mudstone Formation, Dinas Island [SN 0024 4025] (GS1219).

(Plate 2) Syncline in thick- and thin-bedded sandstone turbidites overlying the Aber Pensidian Mudstone, Dinas Island Formation, Dinas Island [SN 0007 4076].

(Plate 3) Debrite-lined channel overlain by thick turbidite sandstone, Dinas Island Formation, Poppit Sands [SN 1442 4897]. Hammer is 36 cm long.

(Plate 4) Open anticline in Nantmel Mudstones Formation, cliffs south of Gwbert [SN 1622 4925]. A horizontal rock platform cut into the cliff can be seen above the person (GS1221).

(Plate 5) Acadian folding and faulting, Dinas Island Formation, cliffs west of Cemaes Head [SN 1290 4960]. Height of cliffs is approximately 115 m (GS1222).

(Plate 6) Cilgerran gorge, looking north from Cilgerran castle [SN 1950 4313] (GS1223).

(Plate 7) Pre-Late Devensian cemented gravel resting on a bedrock platform, Ceibwr Bay [SN 1084 4580]. Note book is 18 cm long (GS1224).

(Plate 8) Silt-laminated clay (glaciolacustrine deposits), St Dogmaels Landslide Investigation Borehole number SDL4, 35.0–35.45 m below surface level [SN 1597 4547] (GS1225).

(Plate 9) Glaciofluvial sand and gravel, Pantgwyn Mawr Quarry (GS1226).

(Plate 10) Fresh slip scars formed during the reactivation of the St Dogmaels landslip in February 1994 [SN 4540 1580], note apron of made ground (centre right) resting on upper part of slip. (Photograph courtesy Dyfed Powys Police; GS1227).

(Front cover) Cliffs in Dinas Island Formation, north of Ceibwr Bay [SN 107 458] (Photograph J R Davies; (GS1218))

(Rear cover)

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

(Index map)