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Geology of the Llangranog district — a brief explanation of the geological map sheet 194 Llangranog
J R Davies, T H Sheppard, R A Waters and D Wilson
Bibliographic reference: Davies, J R, Sheppard, T H, Waters, R A, and Wilson, D. 2006. Geology of the Llangranog district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 194 Llangranog (England and Wales).
Keyworth, Nottingham: British Geological Survey, 2006. © NERC 2006. 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 100017897/2006.
(Front cover) Sea stack sculpted from slumped beds of the Yr Allt Formation, Llangranog (Photograph P Witney; (P626105)).
(Rear cover)
(Geological succession) Summary of the geological succession in the district.
(Index map) Index to the 1:50 000 series maps of the British Geological Survey. The map below shows the sheet boundaries and numbers of the 1:50 000 series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.
Notes
The word 'district' refers to the area of the geological 1:50 000 series sheet 194 Llangranog. National grid references are given in square brackets; all lie within 100 km square SN.
Acknowledgements
M Howe, J A Zalasiewicz and M Williams have provided some new palaeontological interpretation utilising material collected by BGS and by staff of the National Museum and Galleries of Wales. Considerable reliance has been placed on published biostratigraphical data. BGS acknowledges the help of Professor Michael Hambrey and his colleagues in the Centre for Glaciology, University of Wales Aberystwyth, with the interpretation of the Quaternary deposits of the Teifi valley.
Surveying undertaken as part of the Afon Teifi Catchment Survey in 1995–97 was partly funded by Ceredigion, Carmarthenshire, Pembrokeshire and the former Dyfed county councils, and by the Environment Agency. The remainder of the district was surveyed as part of the GeoCymru Project, partly funded by a grant from the Welsh Assembly Government. The British Geological Survey gratefully acknowledges the co-operation of all the landowners in the district during the geological survey, in particular the operators of Cwrtnewydd quarry and the Crygyreryr sand and gravel workings at Talgarreg. 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.
Figures were produced in BGS Keyworth by RJ Demaine: page setting by A Hill: series editor is A A Jackson.
Geology of the Llangranog district (summary from rear cover)
An explanation of sheet 194 (England and Wales) 1:50 000 series map
(Rear cover)
This sheet explanation provides a short description of the geology of the Llangranog district, which borders Cardigan Bay and encompasses a rolling landscape situated between the river valleys of the Aeron in the north, and the Teifi in the south. Spectacular coastal cliffs expose a sequence of geological formations, which form part of the fill of the Lower Palaeozoic Welsh Basin.
The bedrock geology comprises a deep-water sedimentary succession that accumulated during the late Ordovician and early Silurian, between approximately 450 and 428 million years ago. A brief summary of each formation and interpretation of its environment of deposition are presented, allowing the evolution of this portion of the basin to be outlined.
A series of major fault lines traverse the district and have a long history of movement that culminated, during the latest Silurian and early Devonian, in an episode of regional deformation in which the rocks were folded, cleaved and underwent low-grade metamorphism. A brief assessment of these structural features is presented.
This survey also provides the first systematic record of the distribution and composition of Quaternary (superficial) deposits in the district. A complex suite of glacial and periglacial deposits date from the last ice age, between approximately 20 000 and 10 000 years ago. Following the melting of the ice, fluvial sediments have been deposited by the rivers Aeron and Teifi and their tributaries.
This sheet explanation provides a summary of the geology of the accompanying maps (superficial deposits and bedrock and superficial deposits). It also gives valuable information on the applied geology of the district, including mineral and water resources, potential geological hazards, engineering ground conditions and geological conservation, all of which are significant considerations for planning and development.
Chapter 1 Introduction
This sheet explanation provides a summary of the geology of the area covered by geological 1:50 000 series sheet 194 Llangranog. The map was published as a single combined bedrock and superficial deposits edition in 2006.
The district lies predominantly in the county of Ceredigion, bordering Cardigan Bay, with a small area, to the south of the Afon Teifi, in Carmarthenshire. The main centres of population are the ports of Newquay and Aberporth. The larger inland villages include Llanarth, Llechryd, Cwrtnewydd and Rhydowen. Spectacular coastal cliffs between the headlands of Allt Goch and Newquay Head are over 100 m above sea level. Inland, a rolling landscape rises to a high point of 324 m above OD at Rhos Ymryson [SN 460 500], which lies on the watershed between the river catchments of the Afon Teifi, to the south, and the Afon Aeron, in the north. This rural hinterland supports livestock-based agriculture, whereas the picturesque coastal tract, including sandy beaches and cliff top footpaths, is the focus for tourism. Lobster fishermen continue to ply their trade from Newquay harbour. Localised sand and gravel deposits are worked at several sites, notably at Talgarreg and Llechwedd-dderi. Ordovician rocks are quarried for aggregate at Cwrtnewydd. Elsewhere in the district, Ordovician and Silurian sandstones have been widely exploited as a source of local building stone.
The exposed bedrock of the district (Figure 1) comprises a complexly folded and faulted sedimentary succession of late Ordovician and early Silurian age formed between around 440 and 420 million years ago. The sequence records deposition beneath the deep marine waters which then occupied the rapidly subsiding central part of the Lower Palaeozoic Welsh Basin. The basin margin, and a shallow water shelf beyond, lay some 30 km to the south and east. Several major fault lines traverse the district and appear to have had a long history of movement, influencing sedimentation notably during the Silurian, and subsequently reactivated during the major orogenic deformation that affected the region some 400 million years ago. Folding and a regional cleavage testify to the intense compression suffered by the rocks during this event, caused by the collision of the Laurentian and Avalonian tectonic plates.
Mantling the bedrock are Quaternary deposits (drift), which include Pleistocene glacial and periglacial sediments, as well as more recent (Holocene) alluvial deposits located along the valleys of the major rivers and their tributaries. The glacial deposits of the district relate principally to the last major ice advance that affected the British Isles achieving its maximum extent some 20 000 years ago. During this glaciation, two separate ice masses invaded the district: the Irish Sea Glacier, which advanced southwards and eastwards from the coast, and the Welsh Ice Cap which advanced westwards and southwards from its upland source areas in the Cambrian Mountains. The district includes the deposits of a glacial lake, Llyn Teifi, formed in front of these advancing ice sheets, but the majority of the glacial and periglacial deposits were formed during and immediately following the melting of the ice. This process is thought to have been complete by about 14 500 years ago, since when the glacial deposits and landforms have been modified by the modern drainage systems and by the activities of man.
The previously remote nature of the district, allied to poor levels of inland exposure and the inaccessibility of many coastal cliffs, may account for the limited number of earlier investigations of the bedrock geology. The first detailed geological maps of central Wales by Jones (1912; 1938), which established the broad geological structure of the district, utilised unpublished observation made in the early part of the last century by the local amateur geologist D C Evans. Hendricks (1926) examined the coast and hinterland around Llangranog, which includes the boundary between the Ordovician and Silurian systems. A more detailed study of this same area was subsequently undertaken by Anketell (1963). Cliff exposures in the north of the district, in the Aberystwyth Grits, were studied by Wood and Smith (1959) as part of their seminal investigation of this turbidite sequence. Subsequently, sedimentary structures visible in strata previously thought immediately to underlie the Aberystwyth Grits at Graig Grogal attracted the attention of Anketell and Lovell (1976) and Smith and Anketell (1992). Sections in the district were also examined by McCann (1990) and Copus (1999) as part of broader sedimentological studies of the Welsh Basin succession. Papers assessing the geological structure include those by Fitches et al. (1986), Craig (1987) and by Anketell (1987).
In contrast to the bedrock, the Quaternary deposits and landforms of the district, notably those of the Teifi valley, have been a focus for numerous studies. In a detailed BGS assessment of the applied aspects of the Teifi catchment geology, Waters et al. (1997) provide a comprehensive review of this earlier work. Notable earlier accounts describing the glacial history, and the form and origin of the valley, include those by Charlesworth (1929), Jones (1965) and Lear (1986). Geophysical investigations of the valley fill were undertaken by Allen (1960), Francis (1964) and Nunn and Boztas (1977), and also formed an important part of the BGS study (see Waters et al., 1997). The results of more recent examinations of the glacial and preglacial features of the district and its adjoining areas are presented by Hambrey et al. (2001), Davies et al. (2003), Glasser et al. (2004) and Etienne et al. (2005a, b).
Bedrock facies and processes
Sediment avalanches and submarine slumping along the basin margin to the south-east have played a major role in the deposition of the sediments in this part of the Welsh Basin. Some units include large internally deformed slump-sheets (slumps), up to tens of metres thick, formed as intact masses of previously deposited sediment that slid downslope. In other cases, the sediment was deposited from muddy slurries (debris flows) in which pebbles and rafts of eroded material were carried in a cohesive, but fluid mud-matrix. Such debrites can range up to several metres in thickness.
However, the generally thinner and tabular sandstone and mudstone beds, which comprise the bulk of many of the rock formations in the district, record deposition from rapidly moving sediment flows known as turbidity currents. The resulting deposits, collectively known as turbidites, can vary greatly in thickness and internal composition and this is evident within the local succession (see Davies et al. 1997).
Commonly preserved between the various resedimented units (slumps, debrites and turbidites), which make up the bulk of the rock sequence, are very thin, but distinctive beds of mudstone deposited from suspension. These deposits, termed hemipelagites, record the slow rain of fine sediment on to the sea bed in between the more violent, but short-lived events marked by the other deposits. It follows that the character of these hemipelagites reflects more closely the environmental conditions that prevailed across the basin floor at the time they accumulated. Two types are recognised: a pyritic, dark grey, finely laminated variety, and a pale grey or green variety in which distinctive darker mottling picks out the shape of infilled burrows. The former laminated variety record accumulation beneath stagnant (anoxic) bottom waters when conditions were inimical to burrowing organisms. In contrast, the paler, mottled mudstones record periods when such creatures where able successfully to colonise the sea bed suggesting that the overlying water then contained more oxygen (oxic). Typically, many metres of strata and, in some cases, whole formations display only one of the two types of hemipelagite showing that either oxic or anoxic conditions were sustained across the basin floor for long periods of time. The laminated hemipelagic mudstone beds are additionally important because they commonly preserve the fossilised remains of now extinct, planktonic organisms known as graptolites. The form of the colonies these animals lived in underwent rapid evolutionary changes during the Ordovician and Silurian allowing the rocks in which they occur to be accurately dated according to a widely recognised succession of graptolite biozones (Figure 2).
Throughout the Ordovician and Silurian, contemporary movements in global sea level (eustasy) and tectonism combined to control the nature and distribution of facies within the Welsh Basin. Davies et al. (1997) recognised two distinct types of turbidite system within the succession of central Wales: slope-apron and sandstone lobe. In the former, eustasy was the dominant control, and tectonism in the latter. Both types are represented in the district.
Slope-apron systems comprise wedge-shaped accumulations of turbiditic and hemipelagic mudstone, ranging from tens to hundreds of metres in thickness, which thin from the basin margin towards the basin centre and envelope lenticular bodies of coarser grained turbidite facies. Slope-apron deposition was active in the district throughout much of the late Ordovician to the early Silurian and was sensitive to the same sea-level movements that affected sedimentation on the adjacent shelf. In the basin, the effect of these movements on basin circulation and organic productivity, are reflected in changes in bottom water oxicity, recorded by the nature of the hemipelgitic deposits. They also influenced the type and volume of resedimented deposits. During the late Ordovician, the supercontinent of Gondwana experienced an ice age. The build-up of snow and ice led to a fall in sea level evident in marine Ashgill sequences around the world (e.g. Armstrong and Coe, 1997). At the height of the glaciation, during the Hirnantian Stage, ancient sea level is thought to have fallen by as much as 100 m. Subsequently sea level rose, and latest Ordovician to earliest Silurian (Rhuddanian) marine sequences everywhere record deposition during this period of marine transgression and deepening water. Further, although less dramatic, global marine regressions and transgressions (Johnson et al., 1998) are also reflected in the Aeronian rocks of the district.
Sandstone lobe systems comprise thick, rapidly deposited accumulations of tectonically generated sandstone and mudstone, which can exceed 2000 m in thickness. Deposits of this type dominate the Telychian sequence of the district and record the marked increase in volume and grade of detritus being supplied to the basin at this time. This, in turn, was a response to the uplift of southern source areas and a consequence of contemporary plate collision events (Woodcock et al., 1996; Davies et al., 1997). In the basin, the influence of synsedimentary faulting, allied to rapid rates of deposition, largely overrode the effects of sea level change on the nature and distribution of the deposits.
Chapter 2 Geological description
Ordovician
The oldest exposed bedrock unit, the early to mid Ashgill Nantmel Mudstones Formation (Ntm), crops out extensively in the south-eastern parts of the district where it may exceed 1300 m in thickness. Although not seen in the district, the lowermost part of the formation and its contact with underlying Caradoc rocks is exposed in the adjacent Cardigan area (Davies et al., 2003). The Nantmel Mudstones consist predominantly of medium to pale grey, variably burrow-mottled and diffusely colour-banded mudstones. Thin beds and laminae of siltstone and fine-grained sandstone up to 15 mm thick are common locally. These lithologies represent varieties of fine-grained turbidite (Stow and Piper, 1984), thinly interbedded with burrow-mottled hemipelagic mudstones. White-weathering phosphate nodules are common.
Sandstone-rich units (sa) are present at two levels in the local Nantmel Formation sequence. The lower occurs principally in the Cardigan area (Davies et al., 2003); only the uppermost part of this unit crops out at the western margin [SN 220 485] of the Llangranog district. A second, 60 m thick sandstone-bearing unit located within the upper part of the formation is recognised in the Troedyraur area. Both these units consist predominantly of Bouma-type turbidite sandstone/mudstone couplets in which the sandstones range 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 from the lower unit suggest that the turbidity currents flowed towards the north-east.
Also, in the upper 100 m of the formation are at least three separate units, each several metres thick, in which turbidite mudstones are thinly interbedded with dark grey, laminated hemipelagic mudstones. These rocks are well exposed in the cliffs along Traeth Penbryn [SN 285 520] (Figure 3) where they contain graptolites, preserved in pyrite, indicative of the anceps Biozone. Here the top of the uppermost of these anoxic units is around 25 m below the top of the formation. Together, these intervals appear to equate with the 'Red Vein' of the Cader Idris district (Pratt et al., 1995) and with comparable units also recognised in the Rhayader and Builth Wells areas (Waters et al., 1992; Schofield et al., 2004).
The base of the Nantmel Mudstones Formation is widely taken in central Wales to approximate to the Caradoc–Ashgill boundary (Fortey et al., 2000). It records 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 muds, 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. The sandstone divisions record intervals during which sandy turbidite facies migrated into the area. The anoxic units in the upper part of the formation may record a widespread deepening event.
The succeeding, early Hirnantian Yr Allt Formation (YA) crops out throughout the southern part of the district thinning eastwards from over 600 m in the Teifi valley to less than 500 m at the coast. Undisturbed parts of the formation comprise thin-bedded, dark grey, silty and silt-laminated, turbidite mudstone (Plate 1). Thin, commonly lenticular beds of fine-grained, buff-weathering turbidite sandstone, up to several centimetres thick, are widespread. However, much of the Yr Allt Formation sequence present in the district displays features consistent with synsedimentary slumping and disturbance (Plate 2). Where exposures permit, notably in the coastal cliffs, discrete packets of slump-folded strata, tens of metres thick, are visible, each resting on a basal slide plane, which commonly truncates bedding in the underlying strata. These slump units typically display complex internal folding, disrupted by numerous internal slide planes. However, bedding in many of these masses of slumped strata has commonly been destroyed, and they comprise largely homogenous silty mudstone, cross-cut by a network of listric fracture surfaces; relict siltstone laminae and thin sandstone beds are preserved in isolated pods of less disturbed material. In these completely disrupted units, the regional tectonic cleavage, normally a regular closely spaced planar fabric, is replaced by a distinctive, highly irregular anatomising fabric.
Also present in the Yr Allt Formation are locally mappable packets composed of thick to very thick beds of massive sandstone (sa). Such sandstone packets can exceed 100 m in thickness, but many appear to be laterally discontinuous and discordant to regional bedding trends. Examples of such sandstone units, exposed on Ynys-Lochtyn [SN 315 555] and in Cwrtnewydd quarry [SN 492 485], display complex internal slide planes, giant sandstone pillows and mudstone diapirs (Plate 3). In contrast, the sandstone unit occurring at the top of the formation at Bwlch-y-fadfa [SN 438 491] comprises a regularly bedded sequence of thick- to medium-bedded, cream and buff weathering sandstones, which can be traced laterally for over 4 km. Many of the thick sandstone beds in these various packets are completely structureless, but others display convolute lamination and pillowing, proving evidence of forced dewatering associated with rapid deposition.
Also common in the Yr Allt Formation are thick, parallel-sided, debrite beds, comprising massive silty mudstone with 'balls' of sandstone and bedded rafts of the undisturbed silt-laminated mudstone (Plate 1). Such beds range in thickness from less than a metre, within sequences of undisturbed strata, up to several metres, possibly as amalgamated units. In the sequence exposed at the coast, undisturbed mudstones in the upper part of the Yr Allt Formation are noticeably smoother and less silty than those which make up the bulk of the formation (c.f. Schofield et al., 2004). In contrast to the underlying Nantmel Mudstones, evidence of bioturbation in the Yr Allt Formation is sparsely developed, being confined to the soles of thin sandstone beds, and hemipelagic deposits can seldom be recognised between the turbiditic units.
The deposition of the Yr Allt Formation marks an abrupt change in Welsh basin sedimentation that occurred as a result of the late Ordovician glacio-eustatic regression. The significantly reduced levels of bioturbation compared with the preceding Nantmel Mudstones, allied to the absence of hemipelagic deposits, is likely to reflect a marked increase in the volume and rate of sediment supply to the basin as sea level fell and adjacent shelf areas were exposed and eroded. Despite this, the formation can still be viewed as recording oxic slope-apron sedimentation. Frequent slope failure along the front of a rapidly prograding wedge of sediment is recorded in the units of slumped and destratified strata and is again consistent with very high rates of sedimentation along the basin margin during the glacial maximum. However, in the Builth Wells district (Schofield et al., 2004), there is also evidence of active tectonism during this same period suggesting that at least some of the slumping may relate to seismic activity associated with movement on basin bounding faults. The isolated and laterally discontinuous units of thick-bedded turbidite sandstone may record deposition in submarine channels, or, as perhaps in the Bwlch-y-fadfa example, as small sandy lobes sited on the slope-apron surface. However, the geometry of many of these sandstone packets, which are discordant to regional outcrop trends, suggests they too may have been affected by slumping and downslope movement and the exposures at Cwrtnewydd quarry are consistent with such an interpretation. The finer grained mudstones present in the upper part of the formation may record deposition during the stillstand that preceded the onset of deglaciation and the accompanying end-Ordovician to early Silurian marine transgression.
Throughout the Welsh Basin, rocks of latest Ordovician (late Hirnantian) age that record the rise in sea level which followed the late Ordovician glaciation are more naturally included with the formations of Silurian age, and are therefore described together.
Silurian
Within the latest Ordovician and early Silurian sequence of the Llangranog district two separate phases of deposition can be recognised. An earlier, easterly sourced, mudstone-dominated slope-apron phase active between the late Ashgill and early Telychian, and a later Telychian phase. During the latter, increased volume of sediment being supplied to the basin, intrabasinal faulting and the entry of sediment sourced from the south of the basin were all consequences of contemporary plate tectonic events.
Late Ashgill to early Telychian slope-apron phase
The slope-apron mudstone succession comprises the Cwmere Formation and the overlying Claerwen Group. Intercalated with these mudstone-dominated units are a series of more sandy and localised divisions included in the Rhyddlan and Allt Goch Sandstone formations and the Telychian Devil's Bridge Formation.
Abruptly overlying the Yr Allt Formation, the Cwmere Formation (CeF) consists predominantly of thinly interbedded grey, turbiditic and dark grey, laminated hemipelagic mudstones; locally the laminated mudstones contain abundant graptolites. Thin beds and laminae of siltstone and sandstone are present at the base of many of the turbidite mudstones. Where interleaving sandstone-bearing formations are absent, the formation may range up to 200 m in thickness, but throughout much of the district it is much thinner. For example along the coast north of Llangranog the Cwmere Formation is 60 to 70 m thick underlying the Allt Goch Formation (Figure 4), and is little more than 50 m where it underlies the Rhyddlan Formation in the Teifi valley, although greater thicknesses are present in the intervening ground. At the base of the formation is a thin but distinctive unit of burrow-mottled, pale grey mudstone with pyritic nodules, the Mottled Mudstone Member (Figure 4). The member is recognised throughout the Welsh Basin, but in this district, where it is only two to three metres thick, it has not been mapped. It contains late Hirnantian, persculptus Biozone graptolites. The overlying parts of the formation yield faunas that span the Ordovician–Silurian boundary. From the thin coastal sequence, Hendricks (1926), Ankatell (1963) and Jones (unpublished field data) recovered assemblages indicative of all the Rhuddanian biozones including cyphus Biozone assemblages from immediately below the Allt Goch Sandstone. Where free of sandstone beds, upper parts of the formation range into the earliest Aeronian triangulatus Biozone.
The Mottled Mudstone Member at the base of the Cwmere Formation appears to record oxic slope-apron deposition at the very onset of the marine transgression that followed the late Ordovician glaciation. Above, the abrupt introduction of anoxic bottom conditions, which is recorded by the presence of laminated hemipelagic mudstones throughout the remainder of the Cwmere Formation, was a response to the rapid deepening that followed, and is believed to have promoted both the creation of a strongly stratified water column in the basin as well as increased levels of organic production along the newly drowned basin margin. These processes combined to reduce the levels of dissolved oxygen present in basinal bottom waters.
The overlying Claerwen Group (Cla) comprises a sequence of predominantly thinly interbedded, pale grey and green, commonly colour-banded turbidite mudstones and burrow-mottled hemipelagites. Siltstone and fine-grained sandstone laminae at the base of each turbidite mudstone unit are a pervasive feature of the group in this district. However, in the vicinity of the various sandstone-rich units, with which the group interdigitates, such laminae are particularly abundant and can range up to a centimetre in thickness, displaying ripple lenses and cross-lamination. Thin fine-grained turbidite sandstone beds, up to 5 cm thick, are also common in these peripheral settings. As in the Cwmere Formation, intertonguing sandstone-rich sequences also account for marked variations in the thickness of the Claerwen Group. In the east of the district near Castell Moeddyn [SN 485 520], the group is 125 m thick where it underlies the Devil's Bridge Formation (see below). In contrast, in the west of the district, in the Pontgarreg area [SN 335 540], where the Devil's Bridge Formation has died out laterally, the group is up to 500 m thick.
Punctuating the largely oxic Claerwen Group succession are packets of strata in which the turbidites are interbedded with laminated hemipelagic mudstones, and which record the temporary return to anoxic bottom conditions. Graptolites from the Claerwen Group come exclusively from these anoxic intervals. They have yielded faunas generally indicative of the late Aeronian and early Telychian, with some better preserved assemblages allowing local recognition of the late Aeronian convolutus and sedgwickii biozones, as well as the basal Telychian turriculatus s.l. Biozone.
The oxic bottom conditions in evidence throughout much of the lower part of the Claerwen Group were the result of a widely recognised (and hence probably eustatic), mid Aeronian marine regression. This destroyed the water column stratification established during deposition of the Cwmere Formation allowing oxygenated surface waters to mix with bottom layers, and encouraging colonisation of the slope-apron surface by soft bodied burrowing organisms. The periodic return of stagnant bottom water conditions evidenced by the packets of anoxic strata record subsequent deepening episodes, of which the sedgwickii Biozone event appears to have been of global significance.
In the east of the district, much of the upper part of the Cwmere Formation has been replaced laterally by the Rhyddlan Formation (Rhy) in which abundant thin turbidite sandstones are interbedded with turbidite mudstones and laminated hemipelagites. The formation is thickest in the east, where, in the Teifi valley, it ranges up to 250 m in thickness and has replaced all but the lowermost 50 m of the Cwmere Formation. However, it thins rapidly westwards and little more than 50 m is present near Bwlch-y-fadfa [SN 450 497] where it succeeds a commensurately much thicker Cwmere Formation sequence. The formation has not been recognised west of this area. The turbiditic lithologies of the Rhyddlan Formation are arranged in Bouma-style sandstone/mudstone couplets with the sandstones displaying internal sedimentary structures typical of such units including parallel- and cross-lamination. Although the sandstone beds are typically less than 0.1 m thick, they exceptionally range up to 1 m in thickness. The sandstone content too is highly variable ranging from as little as 10 per cent in areas of lateral passage into the Cwmere Formation, to locally as high as 80 per cent. In these latter, sandstone-dominated parts of the Rhyddlan Formation, hemipelagic mudstones are rarely discernable between the individual turbidite units. Lower parts of the thicker eastern sequence, which is exposed in the type section of the formation in the rock gorge [SN 495 431] on the Afon Teifi near Rhyddlan (also spelt Rhuddlan on some maps), have yielded graptolites indicative of the early Rhuddanian atavus Biozone.
In the west of the district, between the coast and Capel Cynon [SN 385 495], upper parts of the Cwmere Formation and lower levels of the Claerwen Group are replaced laterally by a separate sandstone-rich division, the Allt Goch Sandstone Formation (AlG). This unit, initially recognised by Hendricks (1926) and subsequently named by Anketell (1963, 1987), underlies the prominent headland of Pendinaslochtyn [SN 315 548] (Figure 4), north of Llangranog, where its base lies some 60 to 70 m above the base of the local Cwmere Formation sequence. The cliffs to the east, in the vicinity of Traeth y Gaerglwyd [SN 323 550], provide a section through the whole of the local Allt Goch Formation sequence, estimated to be around 130 m thick. Here, the lower half of the formation comprises turbidite sandstone/mudstone couplets intercalated with laminated hemipelagites, a facies closely comparable to the Rhyddlan Formation farther east, and in which the presence of laminated hemipelagites demonstrates a lateral equivalence to the Cwmere Formation. However, in the upper part of the Allt Goch Formation, the bases of the turbidite sandstone beds preserve casts of trace fossils and overlie pale, burrow-mottled hemipelagic mudstones. Clearly this portion of the formation is the lateral equivalent of the lower part of the Claerwen Group. Inland, along the Hofnant valley [SN 325 520], the formation has increased to around 150 m in thickness, but from this point it thins north-eastwards. The lower, anoxic part of the formation can be traced as far as the vicinity of Brynhoffnant [SN 340 510], but only the oxic upper portion is present in the Capel Cynon area [SN 385 495] farther east. Here, the Allt Goch Formation sequence is reduced to around 70 m in thickness and, in Pwll-y-gravel [SN 386 501], the base of the formation lies 8.5 m above the local Cwmere Formation–Claerwen Group contact. The formation is absent in the Plwmp area and has not been recognised south of the Bronnant Fault. Graptolites from the lower, anoxic part of the formation indicate an early Aeronian triangulatus Biozone age, whereas faunas recovered from rare laminated hemipelagites within the overlying predominantly oxic portion are of the magnus Biozone (Hendricks, 1926).
The Rhyddlan and Allt Goch formations record deposition from mixed sand and mud-laden turbidity currents which flowed across the contemporary slope-apron surface. The distribution of the two formations suggests they represent two separate lobe-like bodies, which were supplied with sediment from the south-east. The Rhyddlan lobe was sited in the east of the district; it received sediment and gained topographic relief from atavus through to triangulatus biozone times. The position and limits of the Allt Goch lobe, in the west of the district, suggest its location was influenced by the topography of the earlier Rhyddlan Formation feature. The earliest Aeronian (triangulatus Biozone) part of the Allt Goch Formation demonstrates that some sandy turbidity currents were already being diverted, and were depositing to the side of the earlier Rhyddlan Formation lobe even as the latter continued to receive sediment. The marked lateral expansion of the oxic portions of Allt Goch lobe coincided with the final abandonment of the Rhyddlan lobe as a site of sand deposition, but also with the marine regression, which introduced oxic bottom conditions across the slope-apron surface (see above). Both events may have contributed to the increase in sediment being supplied to the Allt Goch lobe.
Telychian phase
During the Telychian Stage there were some major changes in the style and distribution of turbidite facies, both within the district and throughout the Welsh Basin. Plate tectonic events were responsible for huge increases in the volume and grade of sediment being supplied to the basin, increasingly from the south. They also initiated movements on intrabasinal faults, which led to marked lateral variations in thickness and facies. In the district, such changes are evident across the Aber Richard Fault and, to the south, across the important Bronnant Fault Zone (Davies et al., 1997). The distribution of the Devil's Bridge, Borth Mudstones and Erwan Fach formations illustrate a complex transition from the earlier slope-apron phase of deposition into the sandstone-lobe sequence of the Aberystwyth Grits Group.
In the eastern part of the district, south of the Aber Richard Fault, the Claerwen Group is gradationally overlain by the Devil's Bridge Formation (DBF), a sequence of thinly interbedded turbidite sandstones and mudstones. The formation exhibits marked lateral variations in thickness commensurate with those in the underlying division. The attenuated Claerwen Group sequence of the Castell Moeddyn area [SN 485 520] underlies 550 m of the Devil's Bridge Formation. Traced south-westwards, the base of this thick local sequence rises and, in the Talgarreg area, the lower part of Devil's Bridge Formation exhibit a lateral passage into the adjacent Claerwen Group. The formation is not recognised as a separately mappable unit north of the Aber Richard Fault.
Lower and upper parts of the Devil's Bridge Formation differ in character. In the lower part, thin lenticular and cross-laminated turbidite sandstones are interbedded with very thin beds and laminae of siltstone and pale grey and green colour-banded mudstone. In contrast, upper portions of the formation comprise well developed, rhythmically bedded, sandstone-mudstone couplets in which a thin basal sandstone is overlain by a typically thicker unit of structureless grey mudstone. The dominant type of hemipelagite throughout the formation is the burrow-mottled variety, but intervals with laminated hemipelagites are also present. Sandstones in the lower part of the formation are typically 10 to 50 mm thick, but widely scattered thicker beds ranging up to 0.3 m in thickness also occur. At the base of the Devil's Bridge succession around Castell Moeddyn is a sandstone-dominated sequence (sa), exposed in Rhyd-y-cwrt quarry [SN 496 52]. At this locality, 'beds' of sandstone are over 1 m thick, but are composite in nature due to the amalgamation of a number of thinner turbidite sandstone units. Here, and elsewhere in the lower part of the formation, the sandstone content of the formation locally approaches 80 per cent, although in the bulk of exposures it normally comprises around 30 to 40 per cent of the sequence. In the Bouma turbidite couplets, which dominate upper parts of the Devil's Bridge Formation, parallel- and cross-laminated sandstones ranging up to 0.1 m in thickness underlie grey mudstone units, which can be up to 0.3 m thick. The total sandstone content rarely exceeds 30 per cent and falls to as low as 10 per cent in sequences gradational with the overlying Borth Mudstone Formation.
The thick, Castell Moeddyn sequence closely overlies an anoxic level in the Claerwen Group, which contains a probable sedgwickii Biozone assemblage. Graptolites recovered from the Devil's Bridge Formation are all indicative of the turriculatus s.l. Biozone. Palaeocurrent measurements suggest turbidity currents supplying the formation flowed towards the north-west.
The Devil's Bridge Formation crops out widely in the Llanilar and Lampeter districts to the north and east, where Davies et al. (1997; 2006) recognised two phases of deposition: an 'early pathway' phase, and a 'blanket' phase. The Castell Moeddyn sequence, with its basal, sandstone-dominated packet, appears to be the westward continuation of an 'early pathway' sequence of lowermost turriculatus s.l. Biozone age recognised in the Lampeter district. It records deposition of the lower Devil's Bridge Formation facies within a north-west-trending corridor that was supplied by sandy turbidity currents at a time when Claerwen Group slope-apron mudstones were still accumulating in adjacent areas. The southern margin of this corridor is defined by the lateral passage between the two divisions noted in the Talgarreg area (see above). Upper parts of the Devil's Bridge Formation record more widespread sand deposition as part of the 'blanket' phase, and relate to the overall increase in sediment being supplied to the basin in response to tectonic uplift of source areas. Whilst much of this sediment continued to be derived from the east (Morton et al., 1992), upper parts of the formation gradational with the Borth Mudstones may, additionally, record input of sand grade detritus from new sources located to the south of the basin.
The Borth Mudstones Formation (BMF) comprises 300 m of thin to medium-bedded turbidite mudstones with widely scattered thin turbidite sandstones. Between individual turbidite units, thin beds of hemipelagic mudstone are mainly of the pale grey, burrow-mottled variety. Darker, laminated hemipelagites are present in subordinate packets of strata. In the district, the formation crops out on both sides of the Bronnant Fault Zone, but has not been recognised to the north of the Aber Richard Fault. Sandstones in the Borth Mudstones, where present, occur at the base of sandstone-mudstone couplets, comparable with those in the upper part of the local Devil's Bridge Formation. Close to the contact with the latter division, and also with the succeeding Aberystwyth Grits Group, parallel- and cross-laminated sandstones may range up to 0.1 m in thickness. Below the contact with the Aberystwyth Grits, some of these sandstones are coarse-grained and feldspathic. In these gradational settings, both at the base and top of the Borth Mudstones, packets of strata in which the sandstone content can exceed 10 per cent have been noted, but throughout the remainder of the formation sandstones (rarely exceeding 30 mm in thickness) comprise less than 10 per cent of sequences dominated by tabular beds of medium grey, structureless mudstone. The latter, though normally less than 0.15 m, can locally exceed half a metre in thickness. Graptolite assemblages from the Borth Mudstone Formation in the adjacent Llanilar district indicate a lower turriculatus s.l. Biozone age.
The Borth Mudstones have been interpreted as a muddy fringing facies associated with the Aberystwyth Grits sandstone lobe turbidite system (Davies et al., 1997). Geochemical analyses of both turbiditic and hemipelagic mudstones reveal close links between the two divisions consistent with sediment supply from a common source located to the south of the basin rather than to the east (Ball et al., 1992). As a fringing facies, the Borth Mudstones accumulated beyond the limits of Aberystwyth Grits turbidite sand deposition, both distally, as in its type area (Cave and Hains, 1986), and to the side, as in this district.
Along the coastal tract, north of the Aber Richard Fault, the mudstones of the Claerwen Group pass upwards into the Erwan Fach Formation (Erw), a sequence of thinly interbedded turbidite sandstones and mudstones with burrow-mottled hemipelagites. This sandstone-rich division is equivalent to the Wig Sandstones of Anketell (1963), but was subsequently renamed the Grogal Sandstones by Anketell and Lovell (1976) in the belief that rocks exposed at Craig Grogal [SN 373 594] belonged to this same stratigraphical interval. The present survey has shown this not to be the case and that the sequence at Craig Grogal, previously thought to underlie the Aberystwyth Grits, is a packet of strata contained within this latter division (see below). The turbidite sandstones of the Erwan Fach Formation range up to 0.1 m in thickness with the thicker beds more common in the upper part. These thicker beds are tabular in form and display sedimentary structures typical of Bouma turbidites including basal groove and flute casts and internal sequences of parallel- and cross-lamination. The more abundant thinner sandstones tend to be markedly lenticular in form displaying current ripples and cross-lamination both of which suggest that the depositing turbidity currents flowed towards the north-east. Sandstones locally comprise up to 50 per cent of the formation. Thinly interbedded pale greenish grey mudstone beds display silt-lamination and colour-banding and occur with burrow-mottled hemipelagites that are closely comparable with the underlying Claerwen Group facies. The Erwan Fach Formation ranges up to 120 m in thickness in the cliffs between Trwyn Crou [SN 335 555] and Cwmtydu [SN 355 575], but this sequence thins rapidly inland, and the formation has not been recognised either to the east or south of exposures in Nant Fothau [SN 3635 5365]. No graptolites have been recovered from the Erwan Fach Formation, but in its type area its base lies 35 m above an anoxic level in the Claerwen Group which contains a low turriculatus s.l. Biozone fauna.
The Erwan Fach Formation marks the first significant entry into the Welsh Basin of sand-carrying turbidity currents derived from the south, during the Telychian Stage. It was precursor to the Aberystwyth Grits turbidite system (see below), but differs in that the sandy turbidites were deposited on a slope-apron surface that was still actively accumulating thin-bedded, colour banded and burrowed, oxic mudstones. The formation wedges out to the south-east, consistent with sidelap of the southerly supplied sandy facies against the pre-existing slope-apron surface (Figure 5).
The succeeding Aberystwyth Grits Group (AG) occupies much of the northern part of the district. Its relationships to underlying formations differs markedly across the main fault blocks. Along the coast, the base of the group exposed at Cwmtydu [SN 3565 5757], is taken at the first appearance above the thin-bedded Erwan Fach Formation facies of very thick beds of coarse-grained sandstone with abundant green mudstone rip-up clasts (Plate 4). To the south, between the Aber Richard and Bronnant Faults, the base of the Aberystwyth Grits has a quite different motif. Here the thin turbidite sandstones that occur at the base of sandstone-mudstone couplets gradually increase in thickness and abundance as part of a gradational passage from the underlying Borth Mudstones Formation. The group is not recognised south of the Bronnant Fault Zone. The top of the Aberystwyth Grits is not seen in the district, but the local sequence may exceed 1000 m in thickness. It includes strata which are the lateral equivalent of both the Borth Mudstones and parts of the Devil's Bridge Formation.
The Aberystwyth Grits Group comprises a sequence of interbedded, tabular turbidite sandstones and mudstones with subordinate hemipelagitic mudstones that include both laminated and borrow-mottled varieties. Within the group, two laterally intergradational formations are recognised — the Mynydd Bach Formation and the Trefechan Formation (Wilson et al, 1992; Davies et al, 1997). Though the map shows the Mynydd Bach Formation as the dominant division within the district, coastal sections suggest a more complex facies architecture which the current survey, allied to poor levels of inland exposure, has failed to elucidate. In the Mynydd Bach Formation (MBa) the sandstone beds vary greatly in thickness and internal structure. Massive, coarse-grained sandstone beds, up to several metres thick, are commonly composite in character and may display internal scours with a fill of sand and impersistent granule-rich layers. Dewatering structures are common in these beds which record deposition from surging, high concentration, sand-charged flows (Lowe, 1982). Medium to thick beds of very muddy (high-matrix) and feldspathic sandstone, some with abundant contorted rip-up clasts, are gradational with debrites, and record deposition from highly fluid, slurry-like flows (Wood and Smith, 1959; Clayton, 1994; Davies et al., 1997). These thicker varieties of sandstone commonly occur concentrated throughout packets of strata, up to tens of metres thick, separated by thinner-bedded sequences. In the latter, the turbiditic lithologies are commonly arranged as sandstone-mudstone couplets and include complete Bouma-type (1962) turbidite sequences. In such couplets, well developed flute or groove casts are commonly preserved on the base of the sandstone. The sandstone portion of the couplet comprises a lower, normally graded but otherwise structureless division, a middle parallel-laminated division and an upper division that is either cross-laminated or convolute-laminated. Thin siltstone laminae are commonly present at the base of the overlying, but otherwise structureless mudstone. Such couplets record uninterrupted deposition from a single, decelerating sand and mud-laden turbidity current. However, such complete sequences of Bouma-type divisions are uncommon. In many of the couplets the middle laminated portion may be missing so that a graded sandstone bed, which may be up to a metre thick, is directly overlain by mudstone. More often, however, it is lower parts of the Bouma sequence which are missing so that the couplets comprise a thin, parallel and/or cross-laminated sandstone bed, typically less than 0.10 m thick, overlain by mudstone. The mudstone portion of such couplets is identical to that in the Borth Mudstone Formation, and in transitional sequences with the latter it can range up to 0.5 m in thickness. Here, and within packages of strata up to tens of metres thick elsewhere in the group, the proportion of sandstone can drop to under 10 per cent. However, throughout most of the group up to 40 per cent sandstone is typical, and packets comprising over 90 per cent occur in the lower part of the sequence on the coast. Commonly preserved on the base of many of the sandstone beds in the Aberystwyth Grits are superb trace fossil assemblages (Crimes and Crossley, 1991). In the district, the mapped outcrop of the Trefechan Formation (TrF) is confined to a small area north of the Aeron valley. In this division of the Aberystwyth Grits Group the thicker sandstones that characterise the Mynydd Bach Formation are absent.
Palaeocurrent measurements from sole structures and cross-lamination typically demonstrate that the turbidity currents supplying the Aberystwyth Grits flowed from a south-westerly quadrant. However, at Graig Grogal [SN 373 594], in strata which they believed to underlie the Aberytswyth Grits, Anketell and Lovell (1976) were the first to recognise sandstones with conflicting palaeocurrent evidence. In these, the sole structures are consistent with flow from the south-west, whereas cross-lamination in the upper parts of many of the beds suggests flow was from the south-east. They suggested that this rotation recorded the reworking of sands, previously deposited from turbidity currents, by ambient oceanic currents flowing parallel to the basin margin, so called contour currents. In a subsequent re-interpretation, Smith and Anketell (1992) suggested the rotation was due to the deflection of turbidity currents by a north-westward facing, intra-basinal slope.
The present survey of the coastal exposures in the Aberystwyth Grits has shown that the sequence containing the aberrant palaeoflow indicators at Graig Grogal is a mudstone-dominated packet within the group. Although it does not contain any of the thicker sandstones that characterise much of the adjacent Aberystwyth Grits sequence, it is underlain by some 200 m of strata which do, and which succeed the true base of the group exposed at Cwmtydu (Plate 4) and near Erwan Fach. Moreover, when examined in detail, the sandstone-mudstone couplets which make up much of the sequence at Graig Grogal, including decimetre-scale beds of structureless mudstone, do not resemble facies of the Erwan Fach Formation.
In the district, graptolite assemblages indicative of the lower turriculatus s.l. Biozone have been recovered from the Aberystwyth Grits on Newquay Head. In adjacent districts, the group has yielded a range of younger turriculatus Biozone assemblages. These demonstrate its lateral equivalence to upper parts of the Devil's Bridge Formation and the Borth Mudstones Formation present to the south of the Aber Richard Fault and the Bronant Fault Zone (Cave and Hains, 1986; Loydell, 1991; Davies et al., 1997).
Viewed regionally, the Aberystwyth Grits Group represents the first of a series of large-scale, southerly sourced, mixed sand-mud turbidite systems which invaded and migrated across the Welsh Basin during the late Llandovery (Telychian Stage) (Davies et al., 1997). Collectively, these systems record massive erosion of rejuvenated source areas lying to the south of the basin and this is partly evidenced by a marked unconformity at this stratigraphical level in Pembrokeshire (Hillier, 2002). The huge volumes of detritus supplied to the basin during this interval are consistent with a major fluvial point source and with sediment being supplied to the basin via an associated delta (Hillier, 2002). Repeated failures of the delta front, some triggered by seismic events others by storms or in response to high rates of sedimentation and oversteepening, generated the range of debrites and turbidites evident within the Aberystwyth Grits. The group itself displays all the features of a sandstone-lobe turbidite system (Mutti and Normark, 1987). The packets with thicker turbidite sandstones set within sequences of thinner bedded turbidites suggest that deposition from fast moving, high-concentration flows was confined to a series of elongate, lobe-like constructional features, but that these lobes were repeatedly abandoned as deposition switched to adjacent sites. In contrast, slower moving and lower concentration turbidity currents spread more widely across the system surface depositing Bouma turbidite couplets on the lobes, but also to the side and in front of these features.
At the coast, the base of the Aberystwyth Grits Group records the marked change in the style of turbidite entering the basin. The deposition of slope-apron mudstone facies, which had accumulated alongside southerly sourced turbidites within the Erwan Fach Formation, was brought to an abrupt end as erosive, high-concentration flows began to deposit massive sand beds, rich in rip-up clasts of the underlying facies. Inland, gradational contacts between the Aberystwyth Grits and underlying Borth Mudstones are consistent with the subsequent eastwards expansion of the Aberystwyth Grits turbidite system across its muddy fringing facies. In detail, the distribution of the two divisions, and also the Devil's Bridge Formation, suggests that the major faults influenced deposition (Figure 5). The packets of thick sandstone beds which characterise the lower part of the coastal sequence north of Cwmtydu, appear to have been confined within a downfaulted trough located along the north side of the Aber Richard Fault and it was perhaps flows deflected off the margin of this structure which deposited the sandstones with aberrant palaeocurrent features seen at Graig Grogal. The uplifted region to the south served initially to limit the westwards spread of the Devil's Bridge Formation, but it was later blanketed by southerly sourced mud (Borth Mudstones) deposited from low-concentration flows able to spread beyond the limits of the trough to the north. Subsequently, Aberystwyth Grits facies overstepped the Aber Richard Fault, but its eastwards spread was then halted by a topographic feature created by the Bronnant Fault, and this it failed to breach (Wilson et al., 1992).
Structure
The principal deformation to affect the district was that caused by the late Caledonian, Acadian Orogeny resulting from 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 to mid Devonian. The complex folding, spectacularly displayed in the coastal cliff sections (Plate 5), is evidence of the intense compression the rocks were subjected to during this event, which also imposed a strong cleavage fabric seen in all the strata in the district as part of their associated low-grade metamorphism.
However, many of the major east-north-east-tending faults that traverse the district have a longer history displaying evidence of movement during sedimentation. Fractures in the west of the district, which form part of the Cardigan–Fishguard Fault Belt, were active in Llanvirn and Caradoc times and influenced the distribution of sedimentary and volcanic facies in adjacent districts to the south-west (Davies et al., 2003; Williams et al., 2003). The Aber Richard Fault and Bronnant Fault Zone have links with this same fracture zone and were possibly also active during the mid Ordovician, but it was movements on these faults during the Telychian Stage which controlled deposition of the Aberystwyth Grits Group and its coeval formations. At this time, a synsedimentary sense of down-throw to the north is indicated for both structures.
These pre-existing faults were reactivated during the Acadian orogeny, and many probably experienced major reversals of movement. At the same time, the rocks were deformed into a series of predominantly east-north-east-trending folds, though sigmoidal, north-east-trending folds are also present, notably between the Bronnant and Aber Richard faults. The 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 (Plate 5). Interlayered sandstone-mudstones sequences, such as the Aberystwyth Grits Group, exhibit complex disharmonic fold patterns. Such folds can vary over short distances from open, upright and symmetrical to closed, inclined and asymmetrical, commonly displaying steep, vertical and locally overturned limbs. In contrast, mudstone-dominated facies, such as the Nantmel Mudstones and Cwmere formations, and the Claerwen Group, commonly display more regular shallow, upright open folds (Figure 4). Fold axes within the district plunge predominantly to the east-north-east or north-east, but periclines with opposing plunge directions are also common.
Cleavage fabric varies from a penetrative, slaty and pressure solution variety developed within mudstones to a spaced and anastomising fracture cleavage developed in some argillaceous sandstones. Slumped and disturbed levels in the Yr Allt Formation are characterised by a highly irregular cleavage. The fabric is broadly axial planar with the major fold trends of the district, but locally displays transecting relationships with minor fold hinges.
An examination of the sense of fold asymmetry and cleavage facing directions reveals the presence of a series of vergence divides traversing the district associated with the major fault zones. South-eastwards verging belts in which folds display steep to overturned south-eastwards facing limbs and cleavage fabric dips predominantly to the north-west are located north of the Cardigan–Fishguard Fault Belt in the coastal tract west of Aberporth, and also to the south of the Aber Richard Fault and in the Sarnau area. This is also the dominant sense of vergence throughout the eastern part of district, south of the Bronnant Fault, in common with much of the Welsh Basin succession farther east. North-westwards verging belts displaying steep to overturned north-west-facing fold limbs and a cleavage that dips predominantly to the south-east occupy ground to the south of the Cardigan–Fishguard Fault Belt and to the north of the Bronnant Fault. This sense of vergence prevails throughout much of the outcrop of the Aberystwyth Grits Group in the north of the district. The vergence belts recognised during the survey present a more complex pattern than envisaged by Craig (1987), but his broad conclusions that they represent zones juxtaposed within a broad and complex belt of transpressive deformation, located above a system of deep basement fractures, remains tenable. In this context, many of the large fractures present in the district are also likely to have experienced further movement during Variscan, and possibly Alpine tectonic episodes (see Craig, 1987).
Geophysical data for the district, in the form of the Bouguer gravity anomaly map (Figure 6), reveals a north-westward increasing gradient towards a linear gravity high sited beneath the coastal tract. The trend is north–north-east, parallel with the structural grain of the district as well as the course of the major fault belts that traverse it. The westwards increase in gravity anomaly values is thought, in part, to reflect the increase in thickness of sedimentary rocks towards the centre of the Welsh Basin, but the presence, at depth, of dense basement rocks is also suspected (Davies et al., 1997).
Quaternary
The Quaternary (superficial) deposits of the Llangranog district comprise sediments deposited by, or derived from, Devensian glaciers, together with later alluvial, colluvial and coastal deposits of Holocene age. The district has a long history of research, and detailed investigations have most recently been undertaken by Hambrey et al. (2001), Glasser et al. (2004), Etienne (2004) and Etienne et al. (2005a, b). Broadly, the history of the district reflects four distinct periods of sedimentary activity. Firstly, the bedrock geomorphology was fashioned by fluvial erosion in the late Neogene (Tertiary) and early Pleistocene. This was followed by one or more episodes of pre-late Devensian glaciation, which caused extensive geomorphological modification of the coastal tract of Cardigan Bay. The last major ice advance to affect the district, in the late Devensian, resulted in the widespread deposition of glacigenic material. As the climate ameliorated at the end of the period, fluvial systems were re-established, and the postglacial landscape was modified during the Holocene.
Late Neogene and early Pleistocene landscape formation
Uplift of the area that is now west Wales began in the early Cainozoic, in response to the rise of the North Atlantic Mantle Plume beneath the continental crust of eastern Greenland. Associated rifting in the north-east Atlantic prompted the uplift of Wales, Ireland, Scotland and much of western England as a single landmass. West Wales was progressively denuded throughout this period, with the erosion of successive subaerial peneplains (Brown, 1960); the subdued hilltops of the Llangranog district represent remnants of the youngest such surface, on which the modern rivers of the district became established in the late Palaeogene to Neogene (Dobson and Whittington, 1987). These rivers formed as tributaries of the Irish Sea River System, a major fluvial network draining the Irish Sea basin (Gibbard and Lewin, 2003; Etienne, 2004).
In the present district, a major watershed trends roughly south-west to north-east from Blaenporth to Cilian Aeron (Figure 7). To the south-east the Arberth, Hirwaun, Ceri, Cerdin, Cletwr and Cledlyn Cynllo rivers drain southward into the Afon Teifi, the principal river of west Wales. To the north-west of this watershed, the smaller rivers of Nant Howni, Afon Saith, Afon Soden, Afon Felen, Afon Gido and Afon Drwi, drain directly into Cardigan Bay. West of Cilian Aeron, the watershed is breached by the Afon Mydr, the principal tributary of the Afon Aeron, which also crosses the watershed to flow north-westward across the Llangranog district.
The tributary valleys of the Afon Teifi catchment are cut in bedrock with a meandering form, and show relatively little evidence of later modification by glacial erosion (cf. Etienne, 2004; Glasser et al., 2004). The Teifi itself shows evidence of avulsion, meander loop abandonment, and the incision of new courses in response to falling sea-level during at least two periods of forced regression associated with early Pleistocene glaciations (Jones, 1965; Waters et al., 1997; Hambrey et al., 2001; Davies et al., 2003). Three such abandoned meander loops, now concealed beneath later cover, are preserved in the area around Cenarth, while a fourth lies south-east of Llechryd (Hambrey et al, 2001).
North of the Blaenporth–Neuadd lwyd watershed, the river valleys are short, deeply incised into bedrock, and have a subdued meandering morphology with few tributaries. Similar valleys occur along the coastal tract of west Wales, having been interpreted in the Fishguard area as glacial chute channels (Bowen and Gregory, 1965) and in the Cardigan area as fluvial channels, possibly of pre-Pleistocene age and tributary to the Irish Sea River System (Etienne, 2004). It is likely that such valleys would have been exploited by subglacial streams, and their morphology is more consistent with a fluvial origin.
That the Aeron crosses the Blaenporth– Cilian Aeron watershed is significant, because it suggests that the valley developed in response to a derangement of drainage after the establishment of the regional pattern in the late Palaeogene to Neogene. Although the Mydr now drains north-eastward into the Aeron, the watershed between it and the Cletwr lies in a deep, dry valley, and it seems probable that the formation of the Aeron led to the capture of the Cletwr's headwaters and the establishment of the Mydr. Although the timing of these events is uncertain, it is possible that they represent a response to sea-level fall in the early Pleistocene.
Pre-Late Devensian glaciation
Global cooling, beginning in the mid Cainozoic and intensifying towards the end of that period, resulted in the onset of an Ice Age in Europe at the end of the Pliocene. Ice accumulated in northern Britain, and, as climate grew colder in the Pleistocene, ice caps in Scotland, northern England, Ireland and Wales coalesced to form the British and Irish Ice Sheet (Bowen et al., 2002). Several periods of growth and decay of this ice sheet was driven by climatic change in response to variation in global insolation. In western Wales, two such periods of ice advance have been identified: the Last Glacial Maximum, which occurred in the late Devensian (about 20 000 years), and an earlier glaciation of uncertain age (Bowen, 1973; Etienne, 2004). Several authors have recognised geomorphological elements that they have attributed to this pre-late Devensian glaciation (e.g. Mitchell, 1960; John, 1970; Bowen, 1971; Campbell and Bowen, 1989), and in all cases, these features have been ascribed to the advance of an ice mass known as the Irish Sea Glacier, which evacuated ice via the Irish Sea, impinging on the coastal fringe of west Wales as it progressed southwards.
Some authors (e.g. Hambrey et al., 2001; Davies et al., 2003; Glasser et al., 2004; Etienne, 2004) have attributed a series of meltwater channels in the Cardigan area to the pre-Late Devensian advance of the Irish Sea Glacier. In the Llangranog district, several valleys are linked across watersheds by cols and thus probably also represent meltwater channels. These include the spectacular landform of the Hawen and Fothau valleys, which form a single channel linking Cwmtydu with Llangranog.
The timing of pre-late Devensian glaciation in west Wales remains uncertain. Bowen et al. (2002) suggest a glaciation in the earlier part of the late Pleistocene (i.e. early or mid-Devensian), on the basis of amino-acid geochronology from sites elsewhere in Wales. However, the reliability of this technique has been disputed by McCarroll (2002). Hambrey et al. (2001), Glasser et al. (2004) and Etienne (2004) suggest that ferricretes which line some of the Cardigan channels reflect iron cementation during a period of climatic amelioration, and that any earlier glaciation therefore, may predate the Ipswichian interglacial.
Late Devensian glaciation
Early in the Late Devensian, the Irish Sea Glacier advanced into the Llangranog area once again, at the same time as ice from an expanding Welsh Ice Cap spread into the district from the north-east. Glacigenic deposits are preserved in all major valleys, with the exception of the Cynllo and its catchment, suggesting that the two Late-Devensian ice masses merged and together covered most of the district (Figure 7)a. Glaciogenic deposits derived from the Irish Sea Glacier are characterised by the presence of far travelled erratic clasts of granite, and other igneous lithologies, Carboniferous limestone, and red Permo-Triassic sandstone, which can be matched to various source areas in North Wales, the English Lake District and Scotland. Marine shells dredged from the floor of the Irish Sea are also common. Deposits containing such exotic materials are widespread throughout the north and west of the district. Welsh Ice Cap deposits contain clasts derived exclusively from the local Ordovician and Silurian bedrocks as well as those of the adjacent Cambrian Mountains. Such deposits occupy the south-east of the district. However, sited between these areas of contrasting deposit provenance is a linear north-east-trending belt, stretching from Cenarth in the south to Ciliau Aeron in the north, containing deposits of uncertain or mixed derivation which serves to define a zone along which the two advancing ice fronts merged and interacted. Principal amongst the deposits of this belt are the clays and silts of glacial lakes impounded between the two separate ice masses, and which by inference received sediment from both. The most extensive and well studied of these lakes was sited in the Teifi valley.
Deposits of mixed or uncertain provenance
As it advanced along the coastal tract of West Wales, the Irish Sea Glacier dammed the mouth of the Teifi valley at Cardigan (Charlesworth, 1929; Bowen and Gregory, 1965; Jones, 1965; Lear, 1986; Etienne, 2004) (Figure 7)a. A glacial lake, Llyn Teifi, formed as the catchment ponded, possibly extending upstream to beyond Llandysul (Waters et al., 1997). Geophysical profiles (Heaven et al., 1998) have thus shown that the Teifi valley is underlain by thick successions of glaciolacustrine deposits (Wilby, 1998; Fletcher and Siddle, 1998; Hambrey et al., 2001; Davies et al., 2003; Etienne, 2004), which are also found at surface outside the trunk valley where Holocene deposits are not present, for example in the abandoned meanders north and south of Cenarth, and the lower reaches of the Cynllo valley around Henllan. Exposures are poor, but similar deposits described from boreholes in the Cardigan district (Wilby, 1998; Etienne, 2004) comprise clays with silt laminae and laminated silts ('laminite' facies), together with alternating interbeds of clay and silt ('rhythmite' facies). Etienne (2004) considered the laminites to be deposited partly by settling from suspension, and partly by turbidity flows, while the rhythmite facies may represent either turbidity flows or seasonal varves. Dropstones in both facies indicate rain-out from icebergs (Davies et al., 2003) released into the lake by calving of the Irish Sea Glacier in the mouth of the Teifi valley, where the depth of water associated with Llyn Teifi probably forced the glacier to decouple from its bed (Wilby, 1998; Harris et al., 2005).
Etienne et al. (2005b) have suggested that the advancing Irish Sea Glacier dammed not only the Teifi catchment, but also that of the coastal tract between Aberporth and Newquay, the waters of 'glacial lake Aberporth' then being progressively displaced inland by ice-advance until they spilled over a col in the Blaenporth–Cilian Aeron watershed (the 'Rhosygadair channel' of Etienne et al. (2005b), ultimately draining into Llyn Teifi via the Arberth valley. Though present in the Aeron valley (Davies et al, 2006), the present survey has found no evidence of glacial lake deposition in the Aberporth area where ice-contact deposits appear to rest directly on rock head. Ice contact deposits of uncertain provenance, present within the zone of confluence of the Irish Sea and Welsh Ice masses, occupy the headwaters of the Cletwr valley and are present in the Aeron valley and include diamicton (Till) and clast-supported clayey gravel (Hummocky Glacial Deposits).
Deposits of the Irish Sea Glacier
As the Last Glacial Maximum approached, Irish Sea ice advanced inland towards a maximum limit just south of the district, overriding the lower Teifi valley in the Cardigan area (Hambrey et al., 2001; Davies et al., 2003), and, although perhaps substantially impeded by hills such as Gernos Mountain where thick weathering profiles suggest minimal ice cover, ultimately impinging eastward at least as far east as Talgarreg (Figure 7)b. Deposits of overconsolidated diamicton, weathering a chocolate-brown colour and with a calcareous clay matrix, blanket valley floors north and west of this limit. Both clast-poor and clast-rich examples of these deposits are well seen in landslide-affected coastal sections east of New Quay (Plate 6). These materials are interpreted as subglacial tills deposited by the Irish Sea Glacier as it advanced towards its maximum limits; their calcareous character is attributable to finely comminuted shell material reworked from marine sediments in Cardigan Bay. Locally, such tills are overlain or replaced by heterogeneous glacial deposits, poorly sorted, clast-rich, matrix-supported gravel diamictons. These materials are thought to represent proglacial debris, reworked either at the margins of the advancing Irish Sea Glacier by subglacial processes (Etienne, 2004) or by mass movement on slopes shortly after deglaciation (Harris, 1998).
Localised deposits of cross- to ripple-bedded sands, and thin silts, interpreted as glaciofluvial deposits, are thought to have been deposited by meltwater issuing from the Irish Sea Glacier during recession (Hambrey et al., 2001; Davies et al., 2003).
Glacial lake deposits separate from those of Llyn Teifi are encountered in the Cletwr valley at Pant-y-defaid [SN 433 445], and in boreholes south of Talgarreg (information from Neil Ross, Cardiff University). In the nearby Crygyreryr gravel pit [SN 420 503] steeply dipping foresets of gravel and sand (Plate 7) are interpreted as glaciofluvial deltaic deposits. Similar deposits form an isolated mound [SN 4620 5205] north-west of Ffynenrhys. These deltaic bodies prograded into temporary meltwater lakes formed in the upper reaches of the Teifi's tributaries as the Irish Sea Glacier retreated from the coastal tract of west Wales (Figure 7)c. Undifferentiated glacial deposits, comprising an interbedded mixture of laminated clay, silt, sand and gravel, and diamicton, underlie an area [SN 220 460] near the western margin of the district north of Llechryd. Such deposits are widespread in the adjacent Cardigan district where they are thought to record deposition in ephemeral lakes formed in the Teifi valley during ice retreat (Waters et al, 1997).
Deposits of the Welsh Ice Cap
Welsh ice advanced into the Llangranog district from the high ground of the Cambrian uplands to the north-east (Waters et al., 1997). The oldest deposit derived from it is till, which comprises clasts of local Palaeozoic bedrock set in a noncalcareous silty clay matrix. Welsh till is found lining valley floors in the eastern part of the district as far north as Mydroilyn, but none has been found west of the Ythan valley, a tributary of the Afon Cerdin. In contrast, outwash deposits derived from Welsh ice are found in the Teifi valley as far downstream as Cwm Cou near Newcastle Emlyn (Figure 7)a. Comprising well sorted and stratified sand and gravel, they are divisible into undifferentiated glaciofluvial deposits, thought to possibly represent deltaic bodies that prograded into Llyn Teifi (Waters et al., 1997) and glaciofluvial sheet deposits, thought to represent the remnants of proglacial outwash sheets or braidplains developed during recession (Waters et al., 1997). In the Teifi valley upstream from Llandysul and in tributaries including the Cerdin and Cletwr, mound-like bodies and benches of sand and gravel, locally including clayey gravels and diamictons, are interpreted as glaciofluvial ice-contact deposits that accumulated at the margins of retreating valley glaciers.
The wide distribution of till indicates that for the most part the Welsh Ice Cap advanced into the Llangranog district as a single sheet (Figure 7)a, although Waters et al. (1997) have suggested that fast-flowing ice in the Teifi valley might have formed a valley glacier at its head. The absence of glacigenic deposits in the Cynllo catchment, however, makes it clear that the Welsh Ice Cap did not advance west of Croes-lan (Figure 7)b. Here, periglacial frost-shattering of bedrock produced angular mudstone 'chips' which form extensive deposits of head gravel. Welsh ice deposits are also absent north of a line between Capel Cynon and Cilian Aeron. Immediately south of Mackwith [SN 438 396], glaciolacustrine clay attributed to Llyn Teifi is overlain by Welsh till indicating a westward advance of Welsh ice after lake formation. The presence of valley-filling ice contact deposits in the Teifi upstream of Llanfihangel-ar-Arth, which are largely absent outside the main valley, suggests that Welsh ice remained in the Teifi as a valley glacier long after it had retreated from the rest of the Llangranog district. No glaciolacustrine deposits are known to overlie these gravels, indicating that Irish ice had left much of the district and that Llyn Teifi had drained before the withdrawal of the Teifi glacier. However, as glaciodeltaic deposits of Irish ice origin are found in tributary valleys that were presumably dammed by ice and/or sediments in the main valley (Figure 7)c, and the Welsh ice must have retreated from the remainder of the district before the final withdrawal of the Irish Sea Glacier from the coastal tract.
Complex microtopography
A unique geomorphological feature of west Wales is the development of 'complex microtopography', comprising clustered or superimposed arcuate ridges, partly or wholly enclosing basins, in valley floors (Harris et al., 2005). Complex microtopography is particularly well developed in the Llangranog district, being found in the upper parts of the Hirwaun, Ceri, Cletwr, Cledlyn and Granell catchments, as well as in the upper reaches of the Afon Drwyi (Watson, 1972; Watson and Watson, 1974). These features have long been thought to be of periglacial origin, being originally described by Pissart (1963) and Watson (1971) as pingos, and later re-interpreted by Pissart and Gangloff (1984) and Gurney (1995) as mineral palsas. As supposed periglacial features, they have been used to infer late-Devensian ice-limits (Watson, 1972) and Boulton and Caban (1995) and Boulton et al. (1996) have related them to the expulsion of meltwater through permafrost by a subadjacent glacier.
Investigations of these features, by means of trial pits and trenches in the Hirwaun and Cledlyn valleys, have been undertaken on behalf of BGS by Cardiff University (Harris et al., 2005). In both cases the ridges have been found to be composed of over-consolidated diamicton interpreted as till (Harris et al., 2005). Clast fabric analyses indicate that the deposits have not suffered subsequent periglacial modification. The preferred interpretation of Harris et al. (2005) is that the Hirwaun features represent De Geer moraines, structures formed at or near the grounding line of a glacier which terminates in a body of water (Blake, 2000), while the Cledlyn features represent an allied, as yet indeterminate, subglacial form. Because complex microtopographies would presumably have been destroyed if ice had overridden them, these features are thought to have formed during deglaciation.
Postglacial deposits and landscape adjustment
Following the withdrawal of Irish Sea and Welsh ice from the Llangranog district, the Teifi and other rivers re-established their courses. The rivers cut through the newly deposited glacigenic materials, flushing large volumes of sediment from their catchments. A notable feature of the postglacial landscape is the abandonment of parts of the preglacial river course and the cutting of deep rock gorges by the Afon Teifi and its tributaries (Hambrey et al., 2001; Davies et al., 2003). The finest example is seen at Cenarth, and the incision of these gorges may reflect a preference for stable bank conditions; similar features are seen in the Ceri and in the Cerdin valleys. The gorge at Neaudd lwyd [SN 478 589] through which the Mydr enters the Aeron may also have been cut at this time. Other forms of landscape adjustment occurred in the immediate postglacial period; silty deposits of head represent the products of solifluction and slopewash, while it is likely that many of the landslides found in the Teifi valley also formed during this period. As climate continued to ameliorate, the modern fluvial system began to evolve, with alluvium and river terrace deposits forming in the river valleys of the district, while alluvial fan deposits formed where tributaries entered the larger valleys. As postglacial sea level rose beach deposits formed along the shoreline, and in areas of impeded drainage, such as kettleholes, lacustrine deposits and peat began to form, and continue to do so at the present day.
Chapter 3 Applied geology
Some geological knowledge is essential for efficient planning and development on both regional and site-specific scales. Consideration of earth science issues early in the planning process can help ensure that site and development are compatible, and that appropriate mitigation measures are taken prior to development. The exploitation of geological resources may conflict with other land-use, and the care of the natural environment. Potential geological hazards may present a public health risk or require costly remediation. Engineering ground conditions and designated sites of geological conservation strongly influence the location and design of any new development.
An account of the earth science factors critical to effective land-use planning within the Teifi catchment is provided by Waters et al. (1997).
Mineral resources
Mineral resources comprise natural concentrations of minerals or bodies of rock that are of potential economic interest.
Hard-rock for aggregate is currently extracted from one quarry in the district, at Cwrtnewydd [SN 492 485] where a sandstone-dominated unit within the Yr Allt Formation is worked. Some of the numerous abandoned quarries in the Aberystwyth Grits Group were probably worked for aggregate. Other formations (for example the Erwan Fach Formation, and parts of the Nantmel Mudstones, Allt Goch Formation and Claerwen Group) are potential aggregate sources, but the pyrite-bearing anoxic levels in these and throughout the Cwmere and Rhyddlan formations would be generally unsuitable for use as an aggregate or fill, because, on weathering, they may give rise to sulphate attack and cause heave.
Building stone for local use was obtained from numerous small quarries throughout the district. Use of the tabular sandstone beds of the Aberystwyth Grits Group is evident in buildings throughout the northern part of the district. Sandstone bodies in the Yr Allt Formation were a favoured source of stone in the south, and one such sandstone, at Bwlch-y-fadfa [SN 438 491], is currently worked for ornamental stone.
Sand and gravel is not widely exploited in the district. Small commercial pits working glaciofluvial deposits operate at Talgarreg [SN 421 503] (Plate 7), Llechwedd-dderi-uchaf [SN 502 509] and north of Llandysul [SN 408 427]. Other localised deposits are worked by individual farms, for example on Crug Moel [SN 462 521] and at Castell Cendwr [SN 477 580]. Potential resources exist in the Aeron valley, and also along the Teifi valley where there are extensive glaciofluvial ice-contact deposits, although the heterogeneous nature and variable clay content of these materials may limit their commercial value. More localised glaciofluvial sheet and river terrace deposits may offer a better resource, but the former may contain large boulders up to a metre in diameter. The quality of these potential resources may be further limited by the relatively high proportion of weak, local mudstone clasts they are likely to contain.
The thick and extensive local sequences of glaciolacustrine clay present within the Teifi valley were previously worked for brick clay at sites west of the district in Cardigan. However, stones present in the deposits may render them unsuitable as a modern resource for this purpose. However, the impermeable and plastic nature of the clays suggests they may have potential as a lining material for landfill sites. Clay extracted from a site [SN 304 406], north of Newcastle Emlyn, has been used to line an oil refinery slurry lagoon at Milford Haven.
Water resources
The principal water resource of the district is surface water. High annual rainfall over mid Wales supplies the catchments and tributaries of the Teifi and Aeron rivers. Mains water is extracted directly from the Afon Teifi at Llechryd.
Groundwater is not abstracted for public supply, however private supply boreholes and springs provide water to many farms. Although the Lower Palaeozoic rocks of Central Wales were not thought to be a significant aquifer, recent studies in the region have shown that modest groundwater supplies can be obtained from fractured and weathered solid rocks in the near surface zone and from permeable superficial deposits (Merrin, 1999; Robins et al., 2000). Natural springs, commonly located either at drift-solid contacts or where gravelly drift rests on impermeable clay deposits are still utilised as a source of groundwater by many individual farms.
Potential geological hazards
Areas of worked or made ground presents a significant pollution potential for the migration of toxic leachate may lead to contamination of local surface or groundwater resources and alluvial sediments. Problems may be associated with poorly lined landfill sites, agricultural waste-disposal sites and active or former industrial sites such as sewage works, gravel pits and quarries. Runoff during heavy rainfall, or due to poorly planned remediation of such sites may also allow soluble contaminants or contaminative sediment to enter surface waters.
The Afon Teifi catchment drains parts of the Central Wales Mining Field and water draining from the underground metal mine workings and through waste tips poses a pollution threat for the lower reaches of the river. Any future remedial works or renewed exploitation of these mines and tips must ensure that soluble contaminants and toxic mine sediment is not released into surface waters of the catchment. Moreover, alluvial deposits of the lower Teifi valley are likely to contain layers rich in mine waste, including particles of lead and zinc ore, dating from the main period of mine working during the mid 19th century. The risk and consequences of disturbing these contaminated layers should be a consideration when planning excavations in these deposits, for example for sand and gravel extraction or construction purposes.
River and stream floodplains within the district, notably those of the Afon Teifi and Afon Aeron, are susceptible to regular flooding. An indication of those areas that are prone to regular flooding is given by the extent of active floodplain deposits, shown on the map face as alluvium. However, low river terraces may also be susceptible to flooding, and other areas may also suffer inundation during anomalously large floods, or due to blocked drains or culverts. Areas regarded as at risk of flooding are also shown on Environment Agency maps.
Within the district, gas emissions represent a hazard in areas associated with the accumulation of methane or radon. Methane is likely to be generated by decomposition of material in landfill sites and unconsolidated organic-rich deposits such as peat. It is toxic, an asphyxiant, and explosive in high concentrations. Methane is less dense than air, is capable of migrating through permeable strata, and accumulating in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions are generally unlikely to pose a significant hazard in the district, the risk can be further mitigated through correct design of landfill and developments in the vicinity of 'at risk' sites. Radon is a naturally occurring ionising gas produced by radioactive decay of uranium-bearing minerals, which although present in small quantities in all natural rocks and soils occur in higher concentrations in certain igneous rocks and black mudstones. Radon released from rocks and soils is normally quickly dissipated within the atmosphere and does not present a hazard. However, where it is allowed to accumulate in poorly ventilated spaces it can give rise to an elevated risk of cancer of the respiratory tract. In areas at risk of radon accumulation, protective measures for new developments and remediation of existing homes and workplaces should be carried out. Advice about radon and its associated health risks can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire, OX11 0RQ. The limited existing data suggests that radon potential in the district, in common with much of west central Wales, is generally high with between three and ten per cent of properties likely to exceed the UK 'Action Level' of 200 becquerels per cubic metre.
Slope instability, typically manifested as landslides, is locally present in the district. The main trigger is the erosion of the toe by water courses or by the sea, the latter giving rise to the extensive landslides affecting the till cliffs of Newquay Bay [SN 395 594] and Little Quay Bay [SN 411 597] (Plate 8). Most of the landslides identified in the district are in superficial deposits, with the largest sited along the tributary valleys of the Afon Teifi in the west of the district. Bedrock landslides affect coastal cliffs. Although most existing landslides in the district are located in rural areas, development pressures for housing or improved infrastructure increase the possibility of encountering them or creating the conditions where land may become unstable. The most effective strategy for dealing with unstable ground relies on recognition of problem areas in advance so that suitable preventative or remedial measures can be employed. Waters et al. (1997) provide a detailed assessment of the factors responsible for landslides in the Teifi valley.
Engineering ground conditions
Knowledge of ground conditions is a primary consideration for identifying land suitable for development, and underpins cost-effective design. Engineering ground conditions vary depending on the physical and chemical properties of the local materials, topography, behaviour of groundwater and surface water, and the nature of past and present human activity. The most significant development problems likely to be encountered in the district are due to the variability of natural superficial deposits, weathering of solid rocks and landslides. These can be effectively dealt with by obtaining adequate information, including properly focussed site investigation, to confirm the properties of individual sites.
The solid rocks normally have high bearing capacities, except in the weathered zone. Weathering of the pyritic mudstones in the Cwmere, Rhyddlan and the lower part of the Allt Goch formations, and also in parts of the Claerwen Group, may cause heave and sulphate attack in concrete.
Glaciolacustrine deposits and peat have low bearing capacities and can give rise to moderate settlement. Till and hummocky glacial deposits have moderate bearing capacity but are highly variable. Head, head gravel, alluvial fan deposits and alluvium, all have low to moderate bearing capacities with moderate to high settlement. Although glaciofluvial and river terrace deposits embrace a variety of foundation conditions, they generally have high bearing capacities. Artificial ground is highly variable and may include contaminated land requiring remediation.
Geological conservation
The geological heritage of the district forms a resource for tourism, education and scientific research and is thus a key issue in planning and development. Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). These are statutory designated conservation sites that have some protection under the Wildlife and Countryside Act 1981. Further information on the extent and designation of SSSIs and other Regionally Important Geological Sites (RIGS) can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd, LL57 2LQ. Special features of interest being considered for notification as SSSIs are described in the Geological Conservation Review series, published by the Nature Conservancy Council.
Information sources
Further geological information held by the British Geological Survey relevant to the district is listed below. It includes memoirs, reports, published and unpublished maps, documentary and material collections. Enquiries concerning geological data and geological advice for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.
Searches of indexes to some of the collections can be made on the Geoscience Data Index system available at BGS web site. Maps, books and other publications are listed in the BGS Catalogue of geological maps and books, available on request or may be viewed online. Maps and other publications can be purchased online, at BGS offices or the BGS information centre in the Natural History Museum, Earth Galleries, London (see back cover for addresses).
BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS web site or at: British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.
Maps
Original 1:63 360 geological maps are not available for purchase, however copies of these maps can be consulted at the BGS library, Keyworth. Unpublished 1:25 000 scale geological surveys, listed below, are available for public consultation in the BGS libraries in Edinburgh and Keyworth, and the London Information Office in the Natural History Museum, London; coloured print-on-demand copies are available for purchase.
Results of the Geochemical Baseline Survey of the Environment (G-BASE) are published in atlas form. The geochemical data, with location and site information, are available as hard copy for sale or in digital form under licensing agreement. The coloured geochemical atlas is also available in digital form (on CD-ROM or floppy disk) under licensing agreement. BGS also offers a client-based service for interactive GIS interrogation of G-BASE data.
Groundwater vulnerability maps are published by the Environment Agency from data commissioned from The Soil Survey and Land Research Centre and BGS; the maps are also available from The Stationery Office (020 7873 0011).
- Geological maps
- 1:625 000
- Solid geology map UK, south sheet, 2001
- United Kingdom, south sheet, Quaternary geology, 1977
- 1:250 000
- Geological map of Wales, Solid geology, 1994
- Sheet 52N 04W Mid-Wales and Marches, solid geology, 1990
- 1:63 360
- [Old series] Sheet 40, 1847
- [Old series] Sheet 41, 1857
- [Old series] Sheet 57SE, 1848
- [Old series] Sheet 58, 1850
- 1:50 000
- Sheet 194, Llangranog, Bedrock and Superficial Deposits, England and Wales, 2006
- 1:25 000
- The component 1:25 000 scale maps of the Llangranog district, based on BGS surveying, are listed below, along with the surveyors' initials and dates of survey. The surveyors were J R Davies, R A Waters, T H Sheppard, J K Prigmore and D Wilson; in addition, D G Woodall and J A Zalasiewicz surveyed a narrow strip along the northern edge of the district in 1990 and 1992, respectively. Initials followed by * indicate that the work was part of the Afon Teifi Catchment Survey, between 1995 and 1997.
| Map number | Surveyor | Date |
| SN 24 | JRD*, JKP*, THS, RAW* and DW* | 1995–1997, 2004 |
| SN 25 | THS and DW* | 1997, 2004 |
| SN 34 | JRD*, THS, DW* | 1995–1997, 2004 |
| SN 36 | JAZ | 1992 |
| SN 44 | JRD*, RAW and DW* | 1995–1997, 2004 |
| SN 45 | JRD and RAW | 2004 |
| SN 46 | JAZ and JRD | 1992, 2004 |
| SN 54 | JRD* and RAW | 1995–1997,2004 |
| SN 55 | JRD, RAW and DW | 2003, 2004 |
| SN 56 | DGW | 1990 |
- Digital geological map data
- In addition to the printed publications, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets are:
- DigMapGB-625 (1:625 000 scale)
- DigMapGB-250 (1:250 000 scale)
- DigMapGB-50 (1:50 000 scale)
- DigMapGB-25 (1:25 000 scale)
- DigMapGB-10 (1:10 000 scale)
- The current availability of these can be checked on the BGS web site.
- Geophysical maps
- 1:500 000
- Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997.
- Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998.
- Geochemical atlases
- 1:250 000
- Wales and part of west-central England: Streamwater, 2000
- Wales and part of west-central England: Stream sediments and soils, 2001
- Hydrogeological maps
- 1:625 000
- Hydrogeological Map of England and Wales, 1997
- Groundwater vulnerability maps
- 1:100 000
- Sheet 27 Groundwater Vulnerability of Dyfed, 1998
Books and reports
Books, reports and papers are listed in References. Most of these are available for consultation at BGS and other public libraries. Details of BGS Technical Reports and other internal BGS reports, on aspects of the geology and biostratigraphy are also available through the enquiry service.
Documentary collections
Documentary collections include records of boreholes and site investigations carried out within the district. These are available for consultation at the BGS, Keyworth. Copies of most records can be purchased through the sales desk. Further information about documentary material can be obtained through the BGS enquiry service. Index information, including site references, is held in digital format and can be viewed through the Geoscience Data Index, available on the BGS web site.
Material collections
Material collections from the district are available for inspection at BGS, Keyworth, and include petrological hand specimens, thin sections and fossils. Index data for petrological specimens is listed in the Britrocks database that can be searched through the Geoscience Data Index. Further information about material collections can also be obtained through the enquiry service.
References
British Geological Survey holds most of the references listed below, and copies may be obtained via the library service subject to copyright legislation (contact libuser@bgs.ac.uk for details). The library catalogue is available at: http://geolib.bgs.ac.uk
Allen, A. 1960. Seismic refraction investigations of the pre-glacial valley of the River Teifi near Cardigan. Geological Magazine, Vol. 97, 276–282.
Anketell, J M. 1963. The geology of the Llangranog district, southwest Cardiganshire. Unpublished PhD thesis (Queen's University of Belfast).
Anketell, J M. 1987. On the geological succession and structure of south-central Wales. Geological Journal, Vol. 22, 155–165.
Anketell, J M, and Lovell, J P B. 1976. Upper Llandoverian Grogal Sandstone and Aberystwyth Grits in the New Quay area, central Wales. A possible upwards transition from contourites into turbidites. Geological Journal, Vol. 11, 101–108.
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.
Ball, T K, Davies, J R, Waters, R A, and Zalasiewicz, J A. 1992. Geochemical discrimination of Silurian mudstones according to depositional process and provenance within the southern Welsh Basin. Geological Magazine, Vol. 129, 567–572.
Blake, K P. 2000. Common origin for De Geer moraines of variable composition in Raudvassdalen, northern Norway. Journal of Quaternary Science, Vol. 15, 633–644.
Boulton, G S, and Caban, P E. 1995. Groundwater flow beneath ice sheets: part II — its impact on glacier tectonic structures and moraine formation. Quaternary Science Reviews, Vol. 14, 563–587.
Boulton, G S, Caban, P E, van Gijssel, K, Leijnse, A, Punkari, M, and van Weert, F H A. 1996. The impact of glaciation on the groundwater regime of northwest Europe. Global and Planetary Change, Vol. 12, 397–413.
Bouma, A H. 1962. Sedimentology of some flysch deposits. (Amsterdam: Elsevier.)
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. 1973. The Pleistocene succession of the Irish Sea. Proceedings of the Geologists' Association, Vol. 84, 249–72.
Bowen, D Q, and Gregory, K J. 1965. A glacial drainage system near Fishguard, Pembrokeshire. Proceedings of the Geologists' Association, Vol. 74, 275–81.
Bowen, D Q, Phillips, F M, McCabe, A M, Knutz, P C, and Sykes, G A. 2002. New data for the last glacial maximum in Great Britain and Ireland. Quaternary Science Reviews, Vol. 21, 89–101.
Brown, E H. 1960. The relief and drainage of Wales. (Cardiff: University of Wales Press.)
Campbell, S, and Bowen, D Q. 1989. The Quaternary of Wales. Geological Conservation Review Series. No. A4.1. (Peterborough: Nature Conservancy Council.)
Cave, R, and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey, sheet 163 (England and Wales). (London: HMSO for the British Geological Survey.)
Charlesworth, J K. 1929. The south Wales end moraine. Quarterly Journal of the Geological Society of London, Vol. 85, 335–58.
Clayton, C. 1994. Contrasting sediment gravity flow processes in the late Llandovery Rhuddnant Grits turbidite system, Welsh Basin. Geological Journal, Vol. 29, 167–181.
Copus, J M J. 1999. Sedimentology and facies architecture of the early Silurian slope-apron turbidite systems of the Welsh Basin. Unpublished PhD thesis (University of Cambridge).
Craig, J. 1987. The structure of the Llangranog lineament, west Wales: a Caledonian transpression zone. Geological Journal (Thematic Issue), Vol. 22, 167–181.
Crimes, T P, and Crossley, J D. 1991. A diverse ichnofauna from Silurian flysch of the Aberystwyth Grits Formation, Wales. Geological Journal, Vol. 26, 27–64.
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, 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 (including part of sheet 210) Cardigan and Dinas Island (England and Wales).
Davies, J R, Schofield, D I, Sheppard, T H, Waters, R A, Williams, M, and Wilson, D. 2006. Geology of the Lampeter district — a brief explanation of the geological map. Sheet Explanation of the British geological Survey. 1:50 000 sheet 195 Lampeter (England and Wales).
Dobson, M R, and Whittington, R J. 1987. The geology of Cardigan Bay. Proceedings of the Geologists' Association, Vol. 98, 331–353.
Etienne, J L. 2004. Quaternary glacigenic sedimentation along the Welsh margin of the Irish Sea Basin. Unpublished PhD Thesis (Aberystwyth: University of Wales).
Etienne, J L, Hambrey, M J, Glasser, N F, and Jansson, K N. 2005. West Wales. in The glaciations of Wales and adjacent areas. Lewis, C A, and Richards, A (editors). (Hereford: Logaston Press.)
Etienne, J L, and seven others. 2006. Palaeoenvironmental interpretation of an ice-contact glacial lake succession: an example from the late Devensian of south-west Wales, UK. Quaternary Science Reviews, Vol. 25, 739–762.
Fitches, W R, Cave, R, Craig, J, and Maltman, A J. 1986. Detatchment structures in the Lower Palaeozoic Welsh Basin. Journal of Stuctural Geology, Vol. 8, 607–620.
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, 389–99.
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.
Glasser, N F, Etienne, J L, Hambrey, M J, Davies, J R, Waters, R A, and Wilby, P R. 2004.Geomorphological and sedimentary evidence for multiple glaciations in the lower Afon Teifi region, Wales, UK. Boreas, Vol. 33, 224–237.
Gurney, S D. 1995. A reassessment of the relict Pleistocene 'pingos' of west Wales: hydraulic pingos or mineral palsas? Quaternary Newsletter, Vol. 77, 6–16.
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.
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, 455–82.
Harris, C. 1998. The micromorphology of paraglacial and periglacial slope deposits: a case study from Morfa Bychan, west Wales, UK. Journal of Quaternary Science, Vol. 13, 73–84.
Harris, C, Ross, N, and Sheppard, T H. 2005. Geological mapping solutions for Quaternary ground-ice systems: Final Report, April 2005.
Heaven, R E, Raines, M G, and Napier, B. 1998. A geophysical study to determine the thickness and nature of glacial deposits in the buried valleys of Afon Teifi between Cardigan and Lampeter. British Geological Survey Technical Report, WK/98/13.
Hendricks, E M. 1926. The Bala succession in the Llangranog district (south Cardiganshire). Geological Magazine, Vol. 63, 121–139.
Hillier, R D. 2002. Depositional environment and sequence architecture of the Silurian Coralliferous Group, southern Pembrokeshire, UK. Geological Journal, Vol. 37, 247–268.
John, B S. 1970. Pembrokeshire. 229–265 in The glaciations of Wales and adjoining regions. Lewis, C A (editor). (London: Longman.)
Johnson, M E, Rong, J-Y, and Kershaw, S. 1998. Calibrating Silurian eustasy against the erosion and burial of coastal topography. 3–13 in Silurian cyclces: linkages of dynamic stratigraphy with atmospheric, oceanic and tectonic changes (James Hall Centennial Volume). Landing, E, and Johnson, M E (editors). New York State Museum Bulletin, No. 491.
Jones, O T. 1912. The geological structure of central Wales and the adjoining regions. Quarterly Journal of the Geological Society of London, Vol. 68, 328–344.
Jones, O T. 1938. On the evolution of a geosyncline (Anniversary address). Proceedings of the Geological Society of London, Vol. 94, 60–110.
Jones, O T. 1965. The glacial and post-glacial history of the lower Teifi valley. Quarterly Journal of the Geological Society of London, Vol. 121, 247–281.
Lear, D L. 1986. The Quaternary deposits of the lower Teifi valley. Unpublished PhD thesis (Aberystwyth: University of Wales.)
Lowe, D R. 1982. Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology, Vol. 52, 279–297.
Loydell, D K. 1991. The biostratigraphy and formational relationships of the upper Aeronian and lower Telychian (Llandovery, Silurian) formations of western mid-Wales. Geological Journal, Vol. 26, 209–244.
McCann, T. 1990. Palaeoenvironmental evolution of an Ordovician–Silurian deep-marine sedimentary succession in the Welsh Basin. Unpublished PhD thesis (University of Leicester).
McCarroll, D. 2002. Amino-acid geochronology and the British Pleistocene: secure stratigraphical framework or a case of circular reasoning? Journal of Quaternary Science, Vol. 17, 647–652.
Merrin, P D. 1999. Report on the drilling and installation of exploratory boreholes, Afon Teifi valley, west Wales. British Geological Survey Hydrogeology Series, Technical Report, No. WD/99/24.
Mitchell, G F. 1960. The Pleistocene history of the Irish Sea. British Association for the Advancement of Science, Vol. 17, 313–325.
Morton, A C, Davies, J R, and Waters, R A. 1992. Heavy minerals as a guide to turbidite provenance in the Lower Palaeozoic Southern Welsh Basin: a pilot study. Geological Magazine, Vol. 129, 573–580.
Mutti, E, and Normark, W. 1987. Comparing modern and ancient turbidite systems. 1–38 in Marine clastic sedimentology: concepts and case studies. Leggett, J K, and Zuffa, G G (editors). (London: Graham and Trotman.)
Nunn, K R, and Boztas, M. 1977. Shallow seismic reflection profiling on land using a controlled source. Geoexploration, Vol. 15, 87–97.
Pissart, A. 1963. Les traces de pingos du Payes de Galles (Grande Bretagne) et du Plateau des Hautes Fagnes (Belgigue). Zeitschrift für Geomorphologie, Vol. 7, 381–392.
Pissart, A, and Gangloff, P. 1984. Les palses minerals et organiques de le vallee de l'Aveneau, près de Kuujjuaq, Québec Subartique. Géographie physique et Quaternaire, Vol. 38, 217–228.
Pratt, W T, Woodhall, D G, and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, sheet 149 (England and Wales). (London: HMSO for British Geology)
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.
Schofield, D I, Davies, J R, Waters, R A, Williams, M, and Wilson, D. 2004. Geology of the Builth Wells district — a brief explanation of the geological map. Sheet Explanation of the British geological Survey. 1:50 000 Sheet 196 Builth Wells (England and Wales). (Keyworth, Nottingham: British Geological Survey.)
Smith, R D A, and Anketell, J M. 1992. Welsh Basin 'contourites' reinterpreted as fine-grained turbidites: the Grogal Sandstones. Geological Magazine, Vol. 129, 609–614.
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.
Waters, R A, Davies, J R, Fletcher, C J N, and Wilson, D. 1992. Discussion of 'A late Ordovician/early Silurian non-depositional slope and perched basin along the Tywi Anticline, Mid Wales' by D M D James. Geological Journal, Vol. 27, 285–294.
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, No.WA/97/35.
Watson, E. 1971. Remains of pingos in Wales and the Isle of Man. Geological Journal, Vol. 7, 381–392.
Watson, E. 1972. Pingos of Cardiganshire and the latest ice limit. Nature, Vol. 236, 343–344.
Watson, E, and Watson, S. 1974. Remains of pingos in the Cletwr basin, south-west Wales. Geografiska Annaler, Vol. 56A, 213–225.
Wilby, P R. 1998. The Quaternary sequence of the buried valley of the Teifi near Cardigan: a sedimentological investigation of three cored boreholes. British Geological Survey Technical Report, No.WA/98/33C.
Williams, M, Davies, J R, Waters, R A, Rushton, W A, and Wilby, P. 2003. Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptoloites from the Cardigan area, south-west Wales. Geological Magazine, Vol. 140, 549–571.
Wilson, D, Davies, J R, Waters, R A, and Zalasiewicz, J A. 1992. A fault-controlled depositional model for the Aberystwyth Grits turbidite system. Geological Magazine, Vol. 129, 595–607.
Wood, A, and Smith, A J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). Quarterly Journal of the Geological Society of London, Vol. 114 (for 1958), 163–195.
Woodcock, N H, Butler, A J, Davies, J R, and Waters, R A. 1996. Sequence stratigraphical analysis of late Ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment. 197–208 in Sequence stratigraphy in British Geology. Hesselbo, S P, and Parkinson, D N (editors). Geological Society of London Special Publication, No. 103.
Index to the 1:50 000 series maps of the British Geological Survey
The map below shows the sheet boundaries and numbers of the 1:50 000 series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately.
(Index map)
Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.
Figures and plates
Figures
(Figure 1) Simplified solid geology of the district.
(Figure 2) Chronostratigaphical and biostratigraphical subdivisions of the Ordovician and Silurian succession of the Llangranog district. (Caradoc stages omitted).
(Figure 3) Sketch of cliffs in the upper part of the Nantmel Mudstones Formation (Ntm) at Traeth Penbryn [SN 292 525] showing units (numbered) of anoxic facies mudstones and the contact with the overlying Yr Allt Formation (YA).
(Figure 4) Sketch of the Cwmere Formation and adjacent units exposed in cliffs below Pendiaslochtyn [SN 315 550].
(Figure 5) Schematic sections illustrating the transition from slope-apron to sandstone lobe deposition in the north-west of the district.
(Figure 6) Bouguer gravity anomaly map in milligals (mGal) calculated against the Geodetic Reference System 1967, referred to the National Gravity Reference Net, 1973. Contour interval 1 mGal.
(Figure 7) Form of Welsh and Irish Sea ice masses during the late Devensian advance into the region leading to the formation of the ice-dammed Llyn Teifi.
Plates
(Plate 1) Yr Allt Formation thin-bedded silty mudstone with debrite bed (about 3 m thick) near base of cliff displaying 'balls' and disrupted beds of sandstone, Carreg y Nodwydd [SN 297 533] (P62996).
(Plate 2) Slump-folded strata, Yr Allt Formation, Llangranog [SN 3106 5431. Rucksack is about 35 cms high (P626997).
(Plate 3) Giant sandstone pillows and mudstone diapirs, Yr Allt Formation, Ynys-Lochtyn [SN 315 555]. The headland is about 40 m high (P62998).
(Plate 4) Thick sandstone beds at base (by rucksack) of Aberystwyth Grits Group overlying the Erwan Fach Formation, Cumtydu [SN 356 576] (P626999).
(Plate 5) Synclinal fold in interbedded sandstone and mudstone, Aberystwyth Grits Group, cliffs [SN 364 586] north of Castell Bach (P627000).
(Plate 6) Diamicton with strongly aligned clasts, Irish Sea Till, Little Quay Bay, [SN 411 597]. Hammer is 25 cm long (P627001).
(Plate 7) Large-scale cross-bedding, glaciofluvial deltaic deposits, Crygyreryr Sand and Gravel Pit [SN 421 503], near Talgareg. The cutting is about 24 m high (P627002).
(Plate 8) Recent landslides in Irish Sea Till, Little Quay Bay [SN 411 597]. The cliff is about 25 m high (P627003).
(Front cover) Sea stack sculpted from slumped beds of the Yr Allt Formation, Llangranog (Photograph P Witney; (P626105)).
(Rear cover)
(Geological succession) Summary of the geological succession in the district.
(Index map) Index to the 1:50 000 series maps of the British Geological Survey.
