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Geology of the Fishguard district — a brief explanation of the geological map Sheet 210 Fishguard

C E Burt, J A Aspden, J R Davies, M Hall, D I Schofield, T H Sheppard, R A Waters, P R Wilby, and M Williams

Bibliographic reference: Burt, C E, Aspden, J A, Davies, J R, Hall, M, Schofield, D I, Sheppard, T H, Waters, R A, Wilby, P R, and Williams, M. 2012. Geology of the Fishguard district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 210 Fishguard (England and Wales).

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The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright and database rights 2012. Ordnance Survey licence number 100021290.

(Front cover) Cover Photograph Fishguard Harbour, looking seaward to the north-west. (Photographer: J I Rayner, (P604362)).

(Rear cover)

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

Notes

The word 'district' refers to the area covered by Geological Series Sheet 210. National grid references (NGR) are given in square brackets. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map.

Acknowledgements

This Sheet Explanation was edited by J R Davies, M Woods and J Thomas. Data provided by R E Bevins assisted in the interpretation of igneous rocks within the district. Petrographical descriptions are based on a contribution by J A McKervey. Regional geophysical maps were prepared by C P Royles. Palaeontological information was provided by A W A Rushton, S Howells, P Hodges and M Williams, utilising material collected by the BGS, the Countryside Council for Wales and the National Museum of Wales. Considerable reliance has been placed on published data and, in particular, on unpublished maps and manuscripts donated to BGS by W D Evans. The fossil collections of Evans were unavailable for re-examination and his taxonomic determinations where cited are given in quotes. The district was surveyed as part of the Central Wales Mapping 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.

Geology of the Fishguard district (summary from rear cover)

(Rear cover)

An explanation of sheet 210 (England and Wales) 1:50 000 series map.

This sheet explanation provides a short description of the geology of the Fishguard district, which lies south of Cardigan Bay and embraces large parts of the Pembrokeshire Coast National Park, including the picturesque coastline around Strumble Head and Newport, the wild uplands of the Preseli Hills, and the rolling tablelands to the south.

The bedrock of the district includes Precambrian rocks, which record some of the earliest geological events known from southern Britain, as well as volcanic and sedimentary rocks of Cambrian and Ordovician age, together spanning a period of time from over 587 to around 438 million years ago. A brief summary of each formation's physical characteristics and environment of deposition are presented, allowing the early evolution of western Wales to be described. A series of major fault lines traverse the district, which have a long history of movement that influenced the sedimentary environments of the Lower Palaeozoic. These movements culminated in an episode of regional deformation in which the rocks were complexly folded and 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 record at least two distinct phases of ice advance and retreat, with the last ice sheet withdrawing from the district around 11 500 years ago. Following the melting of the ice, fluvial sediments have been deposited by the rivers and streams of the district and a range of tidally influenced deposits and dune fields of blown sand have formed along the coast.

As well as summarising the traditional aspects of the geology, the new map ('Bedrock and Superficial Deposits') and this accompanying sheet explanation provide valuable information on the applied geological aspects of the district. These include mineral and water resources, potential geological hazards, engineering ground conditions and geological conservation, all of which are significant considerations for planning and development.

Chapter 1 Introduction

This sheet explanation provides a summary of the geology of the area covered by geological 1:50 000 Sheet 210 Fishguard. The map was published as a single combined bedrock and superficial deposits edition in 2010.

The greater part of the district lies within the county of Pembrokeshire, with a small area in the south-east within Carmarthenshire. It is bordered to the north by the picturesque coast of Cardigan Bay and includes the scenic uplands of the Mynydd Preseli with their distinctive rock tors or 'carns', which rise to a maximum height of 536 m OD at Foel Cwmcerwyn [SN 094 311]. The main rivers are the Afon Nyfer and Afon Gwaun, which drain the northern flanks of the Preselis into Cardigan Bay, and the Eastern and Western Afon Cleddau which, together with the Afon Syfynwi and Afon Taf, drain the southern flanks of the range into the Daugleddau estuary. Livestock-based agriculture remains a significant part of the local economy, although tourism is increasingly important for the principal towns of Fishguard and Newport. Both were important 19th Century ports, and the former is now a major terminal for passenger ferries to Eire. In 1952, the Pembrokeshire Coast National Park was established to conserve the county's spectacular scenery. About half of the district, including the coast and the Preseli Hills, lies within its limits.

The bedrock of the Fishguard district includes rocks of latest Precambrian (Neoproterozoic) age, which record some of the earliest geological events known from southern Britain, as well as Lower Palaeozoic rocks of Cambrian to Ordovician age (Figure 1). These have been affected by several periods of orogenic deformation and are mantled by Quaternary superficial deposits (drift) including Pleistocene glacial and periglacial sediments, more recent (Holocene) alluvial deposits located along the valleys of the major rivers and their tributaries, and coastal-zone deposits.

The rocks of the Fishguard district have been exploited by man since the Neolithic, when boulders of 'Bluestone' dolerite, from Carnmenyn (Carn Menyn) in the heart of the Preseli Hills, were transported to Salisbury Plain by the builders of Stonehenge. Later, from the 18th to the early 20th centuries, slate was worked for roofing, paving and building slabs throughout the district, with the quarries of the small village of Rosebush at the centre of this industry. The last of these workings, at Gilfach near Llangolman, closed in 1987.

Although the first account of the geology of the Fishguard district was arguably given by George Owen of Henllys in his 1603 Description of Pembrokeshire, the first geologist to visit the district was Murchison, who briefly discussed it in his Silurian System (1839). Initial geological surveys were undertaken by H T De la Beche, J Phillips, D H Williams, A C Ramsey, W T Aveline and J Rees (Junior) with the first map published by the Geological Survey as one-inch (old series) Sheet 40 in 1845, and revised by W T Aveline in 1857.

No further investigations were undertaken until Hicks (1877) who, having already made an extensive study of the Precambrian rocks west of the district, recognised their continuation into the area west of Wolf's Castle. Later, Marr and Roberts (1885) made the first palaeontological investigations, collecting Cambrian trilobites from rocks near Treffgarne Bridge. During the early years of the 20th century, the Geological Survey revised the geology of the South Pembrokeshire Coalfield, and Thomas and Jones (1912) extended this survey northward to include the Precambrian and Cambrian rocks of the Fishguard district. Later, Thomas and Cox (1924) made the first modern investigations of the Ordovician, examining the volcanic rocks of Roch, Treffgarne and Sealyham. Both published limited but detailed maps. At about the same time, some workers were attracted to the volcanic rocks in the northern part of the district, with outcrops at Fishguard investigated by Reed (1895), Elsden (1905), Cox (1930) and Thomas and Thomas (1956).

Evans (1945) made the first regional investigation of the sedimentary rocks, publishing a map of the Preseli Hills east of Rosebush and documenting his biostratigraphical observations; palaeoenvironmental models were later developed by Traynor (1988). For the most part, however, attention in the late 20th century focused on the volcanic and intrusive igneous rocks of the district. The geochemistry and petrology of these rocks has been described in a series of papers by Bevins and others (e.g. Bevins, 1982; Bevins et al., 1984, 1989), who also published detailed maps of coastal localities; Lowman and Bloxham (1981) provided the first modern regional map of volcanic rocks in the Preseli Hills. BGS work in the adjacent Cardigan district (Davies et al., 2003) included a reconnaissance study of the Caradoc succession exposed on the Cardigan Bay coast of the Fishguard district, with palaeonotological findings presented by Williams et al. (2003), Vandenbrouke et al. (2008) and Page et al. (2010). The diagenetic and early metamorphic history of some of the mudstones in the district has been appraised by Wilby et al. (2007a) and recent isotope and structural work has been presented by Schofield et al. (2008; 2009).

Although early workers made reference to the distribution of Quaternary (superficial) deposits in the course of mapping the bedrock, the earliest focused investigations were made by Charlesworth (1929), who attempted to reconstruct the patterns of glaciation along the Cardigan Bay coast. Later workers, including Bowen and Gregory (1965), Bowen (1967) and John (1970), investigated the glacial valleys south and west of Fishguard. More recently, the glaciation of the coastal tract has been reassessed by Etienne et al. (2006).

Chapter 2 Geological description

Neoproterozoic

Inliers of Neoproterozoic (upper Precambrian) rocks of Cryogenian to Edicarian age crop out in the south-west of the district around Brimaston and to the west of Welsh Hook (Figure 1). These rocks comprise volcanic and intrusive rocks of the Pebidian Supergroup (PN). The Pebidian outcrops are confined to the south of the Llanglydwen/Bronnant fault zone and its extensions into the St David's district that occupy a broadly anticlinal ridge (the 'Hayscastle Anticline') stretching westwards from Brimaston to the coast. Modern descriptions of these rocks are limited to the coastal exposures of the St David's district; the mapped distribution of Pebidian rocks in the Fishguard district follows that of Thomas and Jones (1912).

The Pebidian largely comprises a succession of basic lavas with subordinate acidic lavas, tuffs and intrusions. The basic rocks have a calc-alkaline geochemical signature indicative of subduction-related island-arc magmatism, whilst the acidic rocks have a geochemical signature indicative of a volcanic-arc granite affinity (Bevins and Horak, 2000). The extrusive rocks of the St David's area have been subdivided into the Lower Pebidian, which is dominantly composed of basic lavas; the succeeding Caerbwdy and Ramsey Sound groups, which largely comprise acidic tuffs; and the Rhosson and Ogofgolchfa groups, which are again dominated by basic lavas and tuffs (Bevins and Horak, 2000).

Much of the crop of the Pebidian in the Fishguard district comprises granitic intrusions and an Ediacaran date (see Intrusive rocks) from comparable rocks in the St David's district provides an upper age limit for the Pebidian succession. Undivided Pebidian Supergroup volcanic rocks into which these bodies were intruded crop out in a small area [SM 938 263] north-west of Upper Newton Farm. The rocks resemble those of the Ramsay Sound Group, but correlation with the coastal outcrop is uncertain. They comprise rhyolitic ashes, breccias, and thin lava flows, their mineral assemblages being dominated by alkali feldspar with very little quartz. Andesites which have been albitised by low-grade ocean-floor metamorphism (formerly known as 'keratophyres') are also present.

Cambrian

During the Cambrian the Fishguard district was the site of rift-related passive margin sedimentation and deposition of comparatively shallow shelf sandstones (Figure 1). Cambrian rocks crop out in the south-west of the district, in the area south and west of Welsh Hook, Wolf's Castle and Ford, where they flank the Neoproterozoic inliers (Figure 1). The British Cambrian is informally divided into the Comley, St David's and Merioneth Series, which respectively represent the Lower, Middle and Upper Cambrian (Cowie et al., 1972). Rocks representative of each division are found within the Fishguard district.

The Lower Cambrian rocks of the Caerfai Group rest unconformably on the Pebidian Supergroup and are conformably succeeded by the Middle Cambrian Solva Group. The latter is anomalously thin in the Fishguard district and it is probable that the contact with the succeeding Menevian Group is unconformable. The Upper Cambrian is represented by the Lingula Flags, which lie unconformably on middle Menevian rocks. Cambrian strata are poorly exposed in the Fishguard district and modern descriptions of the rock units are confined to coastal exposures in the St David's district. The mapped distribution again follows that of Thomas and Jones (1912).

The Caerfai Group is estimated to be up to 235 m thick in the Fishguard district and for the most part records deposition within wave base on a high-energy clastic shelf (cf. Crimes, 1970). The group includes the St Non's Bay Sandstone Formation and the overlying Caerfai Bay Shales and Caerbwdy Sandstone formations. The St Non's Bay Sandstone Formation (SNn) includes a basal conglomerate (cg) unit which is up to 50 m thick and contains clasts predominantly of vein quartz and Pebidian igneous rocks. The remaining part of the formation comprise around 100 m of greenish blue, micaceous and feldspathic sandstones. Beds are typically structureless, but some tabular cross-bedding has been reported from exposures south of Welsh Hook (Thomas and Jones, 1912). The succeeding Caerfai Bay Shales Formation (CaB) crops out in the vicinity of Welsh Hook. The 15 m thick unit comprises purple-red to maroon mudstones and numerous thin ash layers. These are the oldest fossiliferous rocks in the district and exposures in the area of Pen-y-Cwm and Crow-cwm Wood have yielded arthropod fossils (Thomas and Jones, 1912). Elsewhere, brachiopods and bradoriid arthropods have been found (Siveter and Williams, 1995).

Gradationally overlying the Caerfai Bay Shales, the Caerbwdy Sandstone Formation (Cbw), ranging up to 70 m in thickness, records the resumption of sand deposition at the top of the Caerfai Group. The formation crops out to the north and east of Welsh Hook and comprises purple-red or purple-grey, fine-grained, micaceous and feldspathic sandstones. It is thought to span the Comely– St David's (Lower–Middle Cambrian) boundary (Prigmore and Rushton, 1999).

Rocks of the succeeding undivided Solva Group (Sol) similarly record accumulation in shallow water above wave base. The group is not more than 30 m thick in the district where it crops out at Welsh Hook and in the vicinity of Wolf's Castle. Subdivisions present in the St David's area (Prigmore and Rushton, 1999), where the group is over 500 m thick, have not been recognised in the Fishguard district where it is represented by a succession of green, micaceous and quartzitic sandstones and pebbly sandstones. Jones (1940) thought the base of the Solva Group to be unconformable, but the presence of interbedded purple and green sandstones in the upper part of the underlying Caerbwdy Sandstone suggest a gradational contact (Thomas and Jones, 1912; Stead and Williams, 1971).

The attenuated Solva Group succession of the Fishguard district is unconformably overlain by rocks assigned to the Menevian Group (Men) undivided. The local 130 m thick succession records a marine transgression followed by a progressive deepening. A basal conglomerate (cg), up to 30 m thick, is exposed along the Western Cleddau and in an old railway cutting [SM 948 268] immediately south of Musland Farm. The thick-bedded, greenish yellow pebbly sandstones and conglomerates are coarser than the lower Menevian sandstones exposed on the coast of the St David's district. The succeeding 100 m-thick sequence of greenish to dark blue, well-cleaved, pyritic mudstones (the 'Ford Beds' of Thomas and Jones, 1912), which crop out to the west and north-west of Wolf's Castle, probably equate with the middle part of the Menevian Group in the St David's district (Prigmore and Rushton, 1999). Thin tuffaceous beds record ash falls from distant volcanic eruptions.

Rocks representative of the upper Menevian Group are not found in the Fishguard district and it is presumed that they were removed by erosion prior to the deposition of the Upper Cambrian (Merioneth Series) Lingula Flags Formation (LF). Overlying its transgressive base, the latter comprises a succession at least 200 m thick of coarse- to fine-grained, dark olive green, thin-bedded and flaggy sandstones and blue-grey silty mudstones that records deposition predominantly below wave base on a clastic shelf. It crops out widely in the western part of the district in the Treffgarne area, near Welsh Hook, in a broad belt between Scleddau and Llanychaer, and along the Gwaun valley. In addition to the eponymous lingulid brachiopods (Lingulella davisii), trilobites indicative of the Upper Cambrian have been found in exposures near Treffgarne (Marr and Roberts, 1885).

Ordovician

Rocks of Ordovician age dominate the geology of the Fishguard district (Figure 1) and are described according to the standard chronostratigraphical and graptolite biozonal schemes for the British Ordovician (Figure 2). Active tectonic control on sedimentation and volcanism are manifested as marked lateral differences in facies, thickness and sedimentary succession across major faults, but the possibility that these contrasting successions may reflect the juxtaposing of once-distant stratigraphies by postdepositional strike-slip fault movements is also possible (cf. Davies et al., 2003). The earliest preserved Ordovician rocks include the upper Tremadoc Treffgarne Volcanic Formation and the lower to middle Arenig (Moridunian to Whitlandian) Triffleton Group. Abrupt facies changes in the late Arenig to early Llanvirn see upper parts of the anoxic Penmaen Dewi Formation replaced in central parts of the district by the burrowed and oxic Foel Tyrch Formation, shown by a crop confined between the Bronnant and the Llanglydwen faults. During the latest Fennian and earliest Abereiddian, renewed extrusive activity may have seen the exploitation of both faults as risers for the magmas of the Sealyham Volcanic Formation. South of the Llanglydwen Fault, in a condensed upper Arenig to lower Llanvirn succession, the Penmaen Dewi Formation is overlain by the oxic Ysguborwen Formation. Along the southern margin of the district, in an expanded succession present to the south of the Penffordd Fault, the Penmaen Dewi Formation is overlain by a Fennian to Abereiddian succession which includes the fossiliferous Pontyfenni and Llanfallteg formations.

The succeeding Llanvirn and Caradoc rocks of the district exhibit similarly complex facies changes associated with continuing activity on several of these faults, and also the major Aber Richard and Newport Sands faults, both components of the Fishguard–Cardigan Fault Belt (Davies et al., 2003; Williams et al., 2003). The Abereiddian Abergwilli Formation is confined to the south of the Llanglydwen Fault, whereas throughout the remainder of the district the basal Abereiddian is represented by the Aber Mawr Formation. The mid Abereiddian magmatism caused the eruption of the Fishguard Volcanic Group and emplacement of its associated suite of intrusive rocks centred on a graben-like caldera developed along the Cardigan– Fishguard Fault Zone. The group is confined to the north of the Bronnant Fault. North of the Aber Richard Fault, the Fishguard Volcanic Group is overlain unconformably by the lower Caradoc Penyraber Mudstone Formation, a weakly oxic (dysaerobic) facies whereas the succeeding graptolitic Cwm-yr-Eglyws Mudstone Formation was the product of late Caradoc anoxic deposition. To the north of the Newport Sands Fault, coeval late Caradoc anoxic facies include the sandstone-dominated Dinas Island Formation. Between the Bronnant and Aber Richard faults the Fishguard Volcanic Group is unconformably overlain by the Drefach Group, a succession of richly graptolitic black mudstones which spans the whole of late Llanvirn and Caradoc time. South of the Bronnant Fault, the Drefach Group includes trilobite-bearing calcareous mudstones.

Across the district, the rapid upward transition from the various Caradoc graptolitic successions into the pale, burrow-mottled mudstones of the Nantmel Mudstones Formation is associated with the base of the Ashgill Series (Williams et al., 2003, but see Vandenbroucke et al., 2008). These youngest rocks in the district appear not to display significant facies or thickness changes in the Fishguard and adjacent districts, suggesting that during their deposition faulting had ceased as an active tectonic control on sedimentation. However, current outcrop patterns clearly testify to substantial movements along the major faults during subsequent tectonic episodes (see below).

Upper Tremadoc to Arenig rocks

The oldest Ordovician rocks of the district, belonging to the Treffgarne Volcanic Formation (TFV), are exposed in Treffgarne Quarries [SM 958 239] and in a series offaulted inliers to the south. The formation is at least 230 m thick in its type area; the 'Llandeloy Ashes' of Williams (1933) may be its lateral correlative in the St David's distict (Traynor, 1988; BGS, 1992). The age of the Treffgarne Volcanic Formation is uncertain. Geochemical similarities suggest an affinity with the Rhobell Volcanic Group of Snowdonia (Bevins et al., 1984) and Traynor (1988) has observed fragments of Treffgarne-type volcanics in the overlying Moridunian rocks. Both lines of evidence support a late Tremadoc age, and magnetostratigraphical work by Trench et al. (1992) appears to confirm that the formation lies close to the Tremadoc–Arenig boundary. It comprises an assemblage of andesitic lavas and epiclastic tuffs (Thomas and Cox, 1924; Traynor, 1988). The andesites are plagioclase-phyric with most of the phenocrysts being albitised; clinopyroxene phenocrysts are replaced by chlorite (Bevins et al., 1984). Geochemically, Bevins et al. (1984) considered the rocks to be transitional between tholeiitic and calc-alkaline island-arc magmatism.

The unconformity at the top of the Treffgarne Volcanic Formation and which oversteps onto Cambrian rocks throughout most of the district, represents the widespread sub-Arenig unconformity of Wales, the product of a widely felt episode of orogenic uplift (e.g. Howells et al., 1997; Schofield et al., 2008). The succeeding rocks record shallow, clastic-shelf sedimentation associated with a basal Arenig transgression. The current classification of these strata is unsatisfactory and they are here included in the newly erected Triffleton Group (Tri). They include rocks referred to as 'Brunel Beds' by Thomas and Cox (1924), but also strata they included in their 'Tetragraptus Shales'. The main crop, centred on Golden Hill, includes the Brunel railway cutting exposures [SM 969 247], but further outcrops are located at Wolf's Castle, north of Welsh Hook, in the vicinity of Letterston and Little Newcastle and within a faulted tract associated with the Gwaun valley. The succession is at least 150 m thick and comprises green-grey, thin-bedded, ripple cross-laminated to hummocky cross-stratified sandstones, interbedded with silty mudstones. Units of rhyolite lava (ZR) are present in the upper part of the group in the Golden Hill and Wolf's Castle areas.

On Ramsey Island and across the St David's district, identical basal Arenig (Moridunian) facies are known as the Ogof Hen Formation (Kokelaar et al., 1985; BGS, 1992; Fortey and Rushton, 2000). The lowest part of the section exposed in the Brunel cuttings on the east side of Treffgarne Gorge has yielded a fauna including bivalves of the genus Glyptarca, which is also found in the Ogof Hen Formation on Ramsay Island (Fortey and Owens, 1987). However, at the nearby Triffleton Quarry [SM 977 242], in strata which they considered equivalent to the upper part of the section exposed in the Brunel cuttings, Fortey and Owens (1987) reported trilobites indicative of the lowermost Whitlandian. Rocks of this age on Ramsay Island are assigned to the Abercastle Formation, which lies unconformably on the Ogof Hen Formation (Owens, 1999). Fortey and Owens (1987) included the succession at Triffleton within their 'Blaencediw Formation', a unit of early Whitlandian age in the Haverfordwest district. They also considered other parts of the Brunel Beds in the vicinity of Triffleton Quarry to equate to the middle Whitlandian 'Henllan Member' of their 'Colomendy Formation'. Clearly the 'Brunel Beds' cannot be considered simple correlatives of the Ogof Hen Formation, but in the absence of good biostratigraphical controls it has not yet proved possible to differentiate the Moridunian and Whitlandian components of the unit throughout its various outcrops. The Triffleton Group therefore includes the Moridunian Ogof Hen Formation as well as lower to middle Whitlandian strata equivalent to the Abercastle, Blaencediw and (part of) the Colomendy formations. South-west of Trecwn, arenaceous strata near Fronrhydd Farm [SM 959 316], considered by Lowman (1977) to be part of his 'Llanfair Beds', have also been included in the Triffleton Group.

In the area around Treffgarne, the Roch Volcanic Formation (RRF), which is at least 240 m thick, occupies a fault-bounded crop between Little Trefgarn Mountain and Leweston Mountain and is well exposed at sites including Poll Carn, Maiden Castle and both Great and Little Treffgarne Rocks. West of the district, the basal part of the formation comprises a series of laharic breccias and brecciated rhyolites (Brown et al., 1987), which are overlain by yellowish grey, white weathering silicified, 'flinty' rhyolites and rhyolitic breccias (ZR). These rocks comprise the lowest units of the formation exposed in the Fishguard district, and typically contain small, recrystallised quartz and pseudomorphed feldspar phenocrysts in a quartz-dominant matrix (Brown et al., 1987). In some places there is evidence of flow banding, whilst at Poll Carn, Maiden Castle and Great Treffgarne Rocks, veins of intrusive breccia form prominent networks (Brown et al., 1987). The passage into the overlying succession of interleaved tuffs, tuffaceous siltstones and mudstones and thinly interbedded quartizitic sandstones and mudstones, the 'Nant-y-Coy Beds' of Thomas and Cox (1924), is observed in the railway cutting [SM 957 252] below Little Treffgarne Rocks.

Early workers believed the Roch Volcanic Formation to be Precambrian in age, but the discovery of a trilobite fragment, possibly part of a juvenile Olenid (Brown et al., 1987), suggests the formation cannot be older than Merioneth in age. Brown et al. (1987) suggest that the volcanic rocks of the Roch Volcanic Formation are the correlatives of rhyolite lavas in the upper parts of the Triffleton Group, and hence Whitlandian in age.

The Triffleton Group is conformably overlain by, and probably in its upper part laterally transitional into, the Penmaen Dewi Shales Formation (PeD). This unit records quiescent deposition in a deeper-water, outer-shelf setting formed in response to continued Arenig sea-level rise. The formation crops out in the south of the district in a broad tract between Spittal and Login, being well exposed around the Llys-y-Fran reservoir. A further belt to the north of the Bronnant Fault extends east-north-eastwards from Henry's Moat to Llethr. Faulted outcrops are present on Mynydd Castlebythe, in a broad arc north of Letterson and along the Gwaun valley. It is also conjectured to crop out in the core of the Mynydd Cilciffeth Anticline. The rocks comprise at least 1200 m of undifferentiated sooty black, mudstones and silty mudstones of Whitlandian to Fennian age. They include strata equivalent to the 'Wolfscastle Shales' of Thomas and Cox (1924) and the 'Sealyham Shales' of Evans (1945), but early workers most commonly refer to these strata as the 'Tetragraptus Shales'. Evans (unpublished data) records the graptolites 'Diplograptus sp', 'Tetragraptus bigsbyi' and a trilobite, 'Aeglina bindosa 'from exposures in the vicinity of Alltypistyll [SN 115 250]. His records of 'Diplograptus hirundo', 'Diplograptus extensus' and 'Diplograptus nitidus' from close to the base of the overlying the Sealyham Volcanic Formation south of Carnmenyn, but north of the Bronnant Fault, appear to confirm a late Fennian age for the upper part of this northern succession. During the course of the survey a specimen of the long-ranging Arenig graptolite genus Callograptus was found in Cronllwyn Quarry [SM 985 352], Llanychaer.

The presence of orange-weathering acidic tuffs (Z) in Cwm Gwaun, both to the west [SN 012 344] and east [SN 039 339] of Ponfaen, provide evidence of volcanism in the lower part of the Penmaen Dewi Shales Formation, perhaps related to activity at Roch. Rhyolite lavas (ZR) in the upper part of the formation crop out at Llanerch [SN 055 354] and at Farthings Hook [SN 049 273] relate to a subsequent volcanic episode.

Upper Arenig (upper Fennian) to lower Llanvirn rocks north of the Bronnant Fault

North of the Bronnant Fault, tuffs and tuffaceous mudstones in the upper part of the Penmaen Dewi Shales Formation signal a transition into the succeeding Sealyham Volcanic Formation (ShV) (Evans, 1945). This marks the onset of a period of extrusive magmatism during the latest Fennian, as lavas and tuffs sourced from an eruptive centre (or centres) located to the west of the district spread eastwards. The faulted outcrop of the Sealyham Volcanic Formation straddles the central part of the district (Figure 1), extending from its type area to Llethr in the east; it is also conjectured to crop out on the limbs of the Mynydd Cilciffeth Anticline. The formation is at least 200 m thick in the western part of the district, thinning eastwards.

Evans (1945) and Bevins et al. (1992) provide the only detailed accounts of these volcanic rocks. Parts of the formation exposed near Bryn Gomer [SM 958 270] and at Garn Turne Rocks [SM 979 272] comprise green-grey, pillowed, andesitic or dacitic hyaloclastites (Bevins et al., 1992). Similar hyaloclastic andesites occur in fault-bounded inliers at Rinaston and Ambleston (Thomas and Cox, 1924; Bevins et al., 1992). In the Sealyham area itself the formation is dominated by trachytic tuffs which crop out between Sealyham Farm and the Sealyham Quarries, and include a unit of tuffaceous mudstone (md). A U–Pb single-crystal Zircon date of 471.7 ± 1.5 Ma obtained for the volcanic rocks (Evans and Boulton, 2007) is consistent with a latest Fennian to earliest Llanvirn age. Porphyritic trachytic lavas and subordinate tuffs are the only representatives of the formation throughout the remainder of the district; hyaloclastic parts of the formation therefore appear confined to the area around Garn Turne Rocks. Eastern outcrops include units of epiclastic breccia as seen in crags [SN 062 305] on Banc Du near New Inn (Plate 1).

In thin section, the trachytic rocks display a pilotaxitic texture formed during cooling and crystallisation. The effects of later alteration, which has led to the formation of serecite, carbonate, quartz and, exceptionally, amphibole or epidote, are widespread. Geochemical investigations undertaken by Bevins et al. (1992) confirm a calc-alkaline chemistry, indicating that, like the Treffgarne Volcanic Formation, they are the products of island-arc magmatism.

Upper Arenig to lower Llanvirn rocks south of the Bronnant Fault

In contrast to upper Arenig strata north of the Bronnant Fault, equivalent facies to the south record deposition in a setting where more oxygenated sea-bed conditions prevailed. In a tract between the Bronnant and Llanglydwen faults, the Penmaen Dewi Formation is succeeded by the Foel Tyrch Formation (FT). This unit is at least 400 m thick and occupies a broad ridge of high ground extending from Maenclochog in the west to Pentre Galar in the east. The rocks principally comprise dark to medium blue-grey, well-cleaved mudstones with indistinct wispy lamination and local burrow-mottling. Poorly cleaved, burrowed, tuffaceous, silty and sandy mudstones, with beds of normally graded crystal and dust tuffs e.g. [SN 1770 3012], become prevalent in upper parts of the formation. On Foel Dyrch e.g. [SN 1520 2964], normally graded, fine- to medium-grained, decimetre-scale sandstones occur, and a quarry [SN 171 313] north of Bryngwyn exposes several metres of yellow-weathering acid tuff (Z). In many areas the formation displays the effects of thermal alteration associated with numerous dolerite intrusions, with the mudstones locally bleached and spotted e.g. [SN 1138 2880] or altered to slates. From roadside quarries to the east of Carn Wen, Evans (unpublished data) reported a Fennian fossil assemblage of 'Diplograptus hirundo' and trilobites, including 'Aeglina bindosa', 'Calymene cambriensis' and 'Ogygia cf. selwyni'. He recorded additional taxa including 'Diplograptus nitidus', 'Trigonograptus ensiformis', 'Caryocaris sp.' and 'Lingula' from a quarry [SN 1814 2096] south-south-east of Pentre Gala. From a higher level in the formation, a single specimen of the graptolite (Eoglyptograptus dentatus) recovered from a roadside quarry [SN 1770 3005] indicates a latest Arenig (Fennian) to earliest Llanvirn age. An autobrecciated andesitic lava ('keratophyre') exposed in a quarry [SN 183 313] north of Pentre Gala, overlies and intrudes the Foel Tyrch Formation; together with associated tuffs, these rocks have been included in the Sealyham Volcanic Formation (Evans, unpublished data).

Between the Llanglydwen and Penfordd Faults, oxic late Arenig deposition is recorded by the silt-prone facies of the Ysguborwen Formation (YSG), cropping out in the ground south-west of Efailwen and to the west of Crosshands where it is exposed in excavations e.g. [SN 1922 2221], [SN 1814 2287] and crags e.g. [SN 1819 2294] around Ysguborwen Farm. The base of the formation is taken at the incoming of colour banding and/or burrowing above the monotonous black mudstones of the Penmaen Dewi Shales Formation, and the bulk of the 40 m thick unit consists of medium to dark grey, pale weathering, burrow-mottled, silty mudstones with abundant siltstone laminae and widely scattered, thin (4–5 cm), fine-grained sandstone beds. Like the Foel Tyrch Formation, the Ysguborwen Formation may span the Arenig–Llanvirn boundary, but, in contrast, it records the accumulation of a much thinner succession in a shallower, silt-rich setting, perhaps above an up-faulted region to the south.

It is likely that the uppermost Arenig succession to the south of the Penfordd Fault represents the fill of another Fennian graben or half-graben. Overlying the local Penmaen Dewi Shales Formation succession are Fennian rocks assigned to the trilobite-bearing Pontfenni Formation (Pni) of Fortey and Owens (1987). This formation, which is up to 220 m thick, crops out in a broad tract of ground to the east and west of Llandissilio, and principally comprises anoxic, dark grey to black, slightly micaceous and silty mudstones, with a typically irregular but well-developed and closely spaced cleavage. Locally, small (<5 cm diameter), black silicate nodules are common e.g. [SN 1062 2241]. In its upper part, the formation becomes finely silt-laminated, as seen in numerous exposures e.g. [SN 1165 2162] along a stream to the north of Pencnwc West. The presence of an atheloptic fauna of small-eyed and blind trilobites led Fortey and Owens (1987) to suggest a water depth of 300 m or more, consistent with deposition within a down-faulted trough. The outcrop of the conformably succeeding Llanfallteg Formation (LLf) (Fortey and Owens, 1987) is restricted to the southern margin of the district around Llandissilio, Penfordd and Upper Haythog. For the most part, the formation comprises light green and grey, sporadically burrow-mottled oxic mudstones together with subordinate tuffaceous mudstones and locally quarried tuffs (Z) e.g. [SN 0364 2260]. Characteristically, the mudstones of the Llanfallteg Formation contain abundant, dispersed graptolite and trilobite debris. The faunal evidence indicates that the formation spans the Arenig– Llanvirn boundary (Fortey and Owens, 1987; Kennedy, 1989), suggesting that its tuff units may relate to volcanism at the Sealyham centre (see above). The top of the formation is nowhere seen, but at least 200 m are present in the district.

Lower Llanvirn (Abereiddian) rocks north of the Llanglydwen Fault

In the district to the north of the Llanglydwen Fault, the Arenig palaeotopography is draped by lower Abereiddian mudstones of the Aber Mawr Shale Formation (AbM), which, following a regional deepening episode in the early Llanvirn, record deposition in an anoxic outer shelf or basinal setting. The formation, equivalent to the 'Bifidus Beds' of early workers, crops out widely in the northern part of the district, in the uplands of the Preseli Hills east of Carnmenyn and extending westwards to Waun Mawn and the flanks of the Mynydd Cilciffeth Anticline. It is seen in the old slate quarries of Rosebush [SN 079 301]. The formation is also found north of the Gwaun valley on the flanks of Mynydd Carnigli, extending westward to Fishguard, and in faulted crops in the vicinity of Letterston. In central parts of the district the Aber Mawr Shale Formation overlies the Sealyham Volcanic Formation; further north it succeeds the Penmaen Dewi Shales Formation. Its thickness varies from as much as 1300 m to the north of the Bronnant Fault to no more than 400 m to the south. Evans (1945) suggested that this variability reflected an unconformable relationship with the succeeding Fishguard Volcanic Group, but evidence for this is lacking and the influence of faulting and of an earlier bathymetry appears to have been significant. The rocks comprise locally silty mudstones which vary from grey-black in the lower part to green-black in the upper part. Tuffaceous mudstones in the upper part of the formation, for example south-east of Carn Fawr [SN 013 366], may signal the onset of volcanism that culminated with the subsequent eruption of the Fishguard Volcanic Group, and the formation is extensively intruded by the dolerite sills associated with this magmatic episode.

The top of the Aber Mawr Formation may lie close to, or span, the boundary between the early and late Abereiddian. Graptolites from the topmost beds in the Gwaun valley, at Aber Bach [SM 9600 3727], immediately below the base of the Fishguard Volcanic Group, include Acrograptus cf. nicholsoni and Diplograptus artus (Owens, 1999), indicative of the lower Abereiddian artus (formerly bifidus) Biozone, whilst, from exposures around Tower Hill [SM 9620 3700], Kennedy (1989) recovered faunas which he assigned to the younger murchisoni Biozone. In the ground south of the Bronnant Fault, where the Fishguard Volcanic Group is absent, Evans (unpublished data) recovered a rich trilobite fauna together with 'Diplograptus bifidus' (now Diplograptus artus) and 'Diplograptus protobifidus' from exposures south of Carn Wen. A series of prominent acid tuffs and tuffaceous sandstones (Z) present in the upper part of the formation are well exposed in the Afon Gafel north of Glandwr. If these volcaniclastic units relate to the magmatism at Fishguard, the Aber Mawr Formation of this eastern region also ranges into the upper Abereiddian, though Evans continues to record 'Diplograptus bifidus' (artus) from localities e.g. [SN 2030 3168] in this part of the succession.

North of the Bronnant Fault, the Aber Mawr Formation is succeeded by the Fishguard Volcanic Group (FVG), which crops out between Strumble Head as far east as Craig-yr-Hwch, forming the northern flanks of Mynydd Preseli. In the cliffs north of Goodwick and in Fishguard Harbour (Plate 2), Bevins and Roach (1979) (following Cox, 1930) recognised three subdivisions: the lower Porth Maen Melyn Volcanic Formation, the overlying Strumble Head Volcanic Formation and the upper Goodwick Volcanic Formation. East of Dinas Cross these divisions can no longer be distinguished and remaining parts of the group are shown as undivided. Similarly, in the St David's district to the west, the group passes into a succession of undivided tuffs known locally as the Llanrian Volcanic Formation. The Fishguard Volcanic Group is thickest in the Goodwick area where around 1800 m of volcanic facies were ponded within a graben or caldera located to the north of the Strumble Head Fault, with the Cardigan–Fishgard Fault Zone providing a conduit for magma extrusion (cf. Kokelaar, 1988; Davies et al., 2003). Abrupt thickness changes are also consistent with active faulting in the Brynberian area. The group is broadly considered to be late Abereiddian in age, confirmed by the discovery of the zonal graptolite Diplograptus murchisoni in ashy mudstones near the top of the Goodwick Volcanic Formation during the construction of the Fishguard bypass [SM 947 371], and at Castle Point ((Plate 2); Lowman and Bloxam, 1981; Williams et al., 2003). Evans (unpublished data) recovered the same taxon together with 'Diplograptus cf. suecicus', trilobites and 'Lingula', from a quarry [SN 1725 3355] on the north side of Craig-yr-Hwch.

The Porth Maen Melyn Formation (PMV) crops out between Manorwen Hill and Fishguard Harbour and eastwards as far as Cwm Mawr. The rocks comprise rhyodacite lavas and acidic tuffs, and have a calc-alkaline signature comparable with that of the older island-arc rocks found around Treffgarne and Sealyham (cf. Bevins et al., 1991). In contrast, the succeeding Strumble Head Volcanic Formation (SHV) comprises pillowed basaltic lavas with a tholeiitic affinity (Bevins, 1982). Over 1300 m of these rocks are present to the north of the Strumble Head Fault, but no more than a 100 m are present to the south. The succeeding Goodwick Volcanic Formation (GdV), which crops out around Dinas Cross, Fishguard, and Goodwick, comprises rhyolitic lavas and tuffs which Bevins et al. (1991) considered to be derived from a similar (basic) parent magma by crystal fractionation. On Strumble Head it includes basaltic tuffs (Z) and tuffacous sandstones (sa).

The undifferentiated flow-banded rhyolitic lavas and welded or partly welded rhyolitic tuffs which represent the group east of Dinas Cross were again considered by Bevins et al. (1991) to be fractionated from a basic magma and comparable to the acidic tuffs of the St David's district (Lowman and Bloxham, 1981). These rocks, well exposed at Carn Ffoi [SN 049 379], also include units of tuffaceous mudstone (Z) both near the base, to the south-west of Brynberian, and at the top of the group. Taken as a whole, the rocks of the Fishguard Volcanic Group were considered by Bevins et al. (1992) to be typical products of back arc (marginal basin) magmatism.

Lower Llanvirn rocks south of the Llanglydwen Fault

South of the Llanglydwen Fault, the Abergwilli Formation (Abg), which crops out over a wide tract between Llandilo and Crosshands, gradationally succeeds the upper Arenig–lower Llanvirn rocks of the Ysguborwen Formation. Silty mudstones with siltstone laminae present at the base of the 400 m-thick Abergwilli Formation are succeeded by pale burrow mottled mudstones and silty mudstones interleaved with units of dark grey, anoxic hemipelagic mudstone; tuffs (Z) and micaceous sandstones are present in the upper part of the formation (Wilby et al., 2007b). The Abergwilli Formation, like the Aber Mawr Shale Formation, was included in the 'Bifidus Beds' by early workers, however, the Abergwilli Formation records early Llanvirn accumulation in a shallower setting where oxic and anoxic bottom conditions alternated. The lower Abereiddian graptolite Diplograptus artus is abundant in the anoxic levels and was seen by Evans (unpublished data) in roadside exposures to the west e.g. [SN 1690 2665] of Llanglydwen.

Upper Llanvirn to Caradoc rocks south of the Aber Richard Fault

South of the Aber Richard Fault, the lower to middle Llanvirn rocks are overlain by those of the Drefach Group (Dre). The group, which comprises at least 200 m of black, pyritic mudstones, crops out on the eastern margin of the district between Cefn-y-Pant and Llanglydwen in the south and Felindre Farchog in the north.

North of the Bronnant Fault, the Drefach Group overlies the Fishguard Volcanic Group, and a 15 cm thick conglomerate bed with abundant mudstone rip-up clasts present to the west of Pencnwc Bach [SN 122 375] has been cited as evidence for a basal unconformity (Evans, 1945). From the overlying mudstones, Evans recorded an upper Abereiddian murchisoni Biozone graptolite fauna, and from exposures in the farmyard to the east [SN 124 375], Williams et al. (2003) obtained a Llandeilian teritiusculus Biozone assemblage. Graptolites suggestive of the lower Caradoc gracilis Biozone are present in a road section [SM 097 387] near Felindre Farchog, and faunas of the succeeding multidens and clingani biozones have been proved in railway cuttings [SN 184 338] to [SN 182 295] at Crymych (Williams et al., 2003). The smooth mudstones of the group are typically finely cleaved and internal structures are difficult to discern, but in some sections, notably at Crymych [SN 182 338], a delicate and pervasive hemipelagic lamination is displayed. Graptolites are common throughout and rare thin tuff and tuffaceous mudstone beds are present locally, as in farmyard exposures [SN 127 368] near Crosswell. The mapping of this area has failed to confirm the presence of facies similar to the Hendre Shales of the Carmarthen district as suggested by Evans (1945). The Drefach Group here records the prolonged accumulation (from suspension) of graptolite-preserving black muds beneath anoxic bottom waters in a restricted distal shelf environment (Williams et al., 2003). Aside from the absence of a local equivalent of the Castell Limestone, the upper Llanvirn to Caradoc succession north of the Bronnant Fault compares closely with that at Abereiddi on the north Pembrokeshire coast (Owens, 1999; Fortey et al., 2000).

South of the Bronnant Fault, the Drefach Group overlies the mudstones of the Aber Mawr and Abergwilli formations. In this region, black, pyritic mudstones again predominate, but in contrast to the north, the succession here includes subordinate units of distinctive soot-black, but yellow-weathering, calcareous mudstone closely comparable to the Hendre Shales of adjacent areas (Fortey et al., 2000; Wilby et al., 2007b), and which contain a comparable mixed trilobite and graptolite faunal assemblage. Sections in these strata, in which trinucleid trilobites are locally abundant, occur at Frowen [SN 190 245] and to the south of Llanboidy [SN 214 227] and in the adjacent Newcastle Emlyn district. Limited exposure, and complex folding and faulting precluded the separation of these units, which are thought to lie within the middle part of the group. Their presence, in contrast to the area to the north, indicates periods and areas of shallower, calcareous mud accumulation and trilobite colonisation of the sea floor.

One of the components of the Drefach Group, the Asaphus Ash Formation (AAF), can be distinguished throughout much of the area south of the Llanglydwen Fault. It comprises interbedded felsic tuff and dark grey mudstone, locally overlain by buff weathering, burrowed mudstones, bioturbated calcareous sandstones and thin limestones (Wilby et al., 2007b). The base of the Asaphus Ash is thought to approximate to the base of the Llandeilo stage (Fortey et al, 2000). Tuffs (Z) are also present in the underlying parts of the Drefach Group in this region.

As to the north, graptolite assemblages from the lower part of the Drefach Group, for example from a quarry [SN 198 251] near Cefn-y-pant, confirm a murchisoni Biozone age. Calcareous levels exposed in a nearby quarry [SN 198 248] contain a Llandeilian (teretiusculus Biozone) trilobite fauna. In the upper part of the group, graptolite fragments from black mudstone exposures [SN 197 262] at Llwyn-llwyd are suggestive of the middle Caradoc multidens Biozone.

Upper Llanvirn to Caradoc rocks north of the Aber Richard Fault

In marked contrast to the graptolitic shelf mudstones south of the Aber Richard Fault, the rocks which overlie the Fishguard Volcanic Group to the north of the fault represent the fill of a trough or graben formed in response to active tectonism along the Fishguard–Cardigan Fault Belt. The volcanic rocks are immediately succeeded by the Penyraber Mudstone Formation (PeA), which occupies the coastal tract between the Goodwick Syncline in the west, and Nevern in the east. The formation, which embraces strata included by McCann (1992) in his 'Parrog Shales', is estimated to be around 700 m thick, with much of its upper part comprising finely cleaved pale to medium grey mudstones. However, in its type area around Fishguard and Goodwick, the presence of distinctive facies in the lower part of the formation allow the basal Castle Point and succeeding Saddle Point members to be distinguished (Davies et al., 2003; Williams et al., 2003).

The base of the Castle Point Member (CaP) at its type locality [SM 961 377] (Plate 2) comprises cross-stratified calcareous shelly sandstone with pebbles of volcanic rock, mudstone, oncoliths, and ferruginous ooliths, resting on an irregular erosion surface at the top of the Fishguard Volcanic Group (Plate 2). In the overlying 150 m-thick fining-upwards succession exposed on the Castle Point headland and in quarries south of Goodwick e.g. [SM 941 371], mudstone-clast conglomerates are interbedded with partially bioturbated sandstones, siltstones and locally burrow-mottled silty mudstones. On the headland [SM 957 377] west of Castle Point, this bedded succession is abruptly overlain by the Saddle Point Member (SaP), a chaotic assemblage of debrites and slumped strata. The clasts in the debrites are set in a grey silty mudstone matrix, and include boulders of various Fishguard Volcanic Group lithologies, many over a metre in diameter, as well as thin rafts, several metres in length, of black mudstone and argillaceous limestone (Plate 3). Steeply plunging, isoclinal slump-fold closures within units of thinly-interbedded mudstone and lenticular sandstone provide further evidence of syndepositional mass movement. An estimated 125 m of the Saddle Point Member are present at its type section, where its top is not exposed.

Upper parts of the Penyraber Mudstone Formation are exposed in the coastal cliffs west of Newport (Plate 4) and in a former brick pit [SM 940 376] in the core of the Goodwick Syncline. They predominantly comprise medium and pale grey, colour-banded and sparsely burrow-mottled mudstone, with common laminae and scattered thin, lenticular beds of brown-weathering siltstone and fine-grained sandstone. Thin beds and nodules of black, but white weathering, phosphatised mudstone are common; also present are rare thin beds of laminated hemipelagic mudstone. In addition, the current survey has also shown that debrite units, ranging from a few centimetres to several metres in thickness, are widespread in this upper part of the formation, notably in the cliff sections west of Pwll Cwn [SN 002 395]. Though the clasts in these units are predominantly of black mudstone and phosphatic material, they also include small blocks of volcanic rock and recrystallised limestone. These findings suggest that the debrite horizon exposed at Aber Hywel (or Howel) [SM 990 385], previously regarded by Williams et al. (2003) as part of the Saddle Point Member, may lie at a higher level in the Penyraber Mudstone. Williams et al. (2003) recovered a graptolite assemblage suggestive of the middle Caradocian multidens Biozone from a black mudstone raft in the Aber Hywel debrite. The presence of the lower Caradocian form Nemograptus gracilis, previously reported from this locality by Lowman and Bloxam (1981) remains unconfirmed. Abundant Diplograptus (Mesograptus) multidens from the former brick pit exposures in the Goodwick Syncline (Directory of British Fossil Localities, 1954, p.187) and Glyptograptus teretiusculus Biozone assemblages reported by Kennedy (1989, p.8) were possibly derived from similar rafts. The similarity of limestone blocks in the Saddle Point Member to the Llandeilian Castell Limestone of the Abereiddi section (Owens, 1999) suggests an age no older than the early Caradoc for this part of the formation, and the disconformity beneath the Castle Point Member has been equated with the basal Caradoc (sub-gracilis) unconformity of North Wales (Davies et al., 2003).

Taken as a whole, the facies of the Penyraber Mudstone represent a progressive deepening and marine transgression following a period of uplift and erosion of the previously erupted Fishguard Volcanic Group. The debrites and slumps of the Saddle Point Member evidence a period of sustained fault-induced instability which promoted the shedding and sliding of material, probably from up-faulted scarps along the line of the Aber Richard Fault to the south. Seismically triggered debris flows continued periodically to interrupt the accumulation of fine-grained silt and mud turbidites which comprise much of the upper part of the Penyraber Mudstone Formation, throughout which burrow-mottling and diagenetic phosphate nodules provide evidence of oxygenated bottom conditions.

The Penyraber Mudstone is succeeded by the Cwm-yr-Eglwys Mudstone Formation (CyE), which embraces the upper part of McCann's (1992) Parrog Shales, and predominantly comprises dark grey, pyritic, and locally graptolitic mudstones. The formation's sharp contact with the paler, largely unfossiliferous rocks of the Penyraber Mudstone is seen in cliffs south of Pwll Cwn [SN 002 395]. The formation is well exposed in cliffs to the west and east of Cwm-yr-Eglwys (Plate 4) and on Dinas Island, as well as further east at Newport Sands and in the vicinity of Felindre Farchog [SN 100 390]. Throughout the Cwm-yr-Eglwys Mudstone, turbiditic lithologies, including silty mudstones with abundant laminae and thin beds of siltstone and fine sandstone, are thinly interbedded with finely laminated, locally graptolite-bearing, hemipelagic mudstones.

South of the Newport Sands Fault, the Cwm-yr-Eglwys Mudstone is estimated to range up to 300 m in thickness. Within this tract, where the formation is overlain directly by Ashgillian rocks, it has yielded graptolites of the upper Caradocian clingani Biozone and, close to its top, of the uppermost Caradoc to lowermost Ashgillian linearis Biozone (Williams et al., 2003). In contrast, to the north of the Newport Sands Fault, the upper part of the formation passes laterally into a northward-expanding succession of the sandstone-rich Dinas Island Formation (Davies et al., 2003). At Newport Sands, the surviving Cwm-yr-Eglwys Mudstone succession still ranges into the upper part of the clingani Biozone (morrisi Subzone), but on Dinas Island it only yields graptolites of the older caudatus Subzone. A fossil bivalved arthropod preserving the impressions of its soft tissue is reported from the Cwm-yr-Eglwys Formation by Page et al. (2010).

The base of the Dinas Island Formation (DI) is exposed in west-facing cliffs at its type locality [SN 0001 4039] and at Newport Sands [SN 054 408] (Figure 1). The rocks comprise a succession of interbedded turbidite sandstones and mudstones, with subordinate mass-flow conglomerates, pebbly mudstone debrites and laminated hemipelagic mudstones. The formation includes many of the turbidite types recognised in central Wales (e.g. Davies et al., 1997). Medium- to very thick-bedded conglomerate/sandstone couplets, massive sandstones locally displaying dish structures, as well as a range of distinctive, high-matrix sandstones, all display the features of coarse-grained turbidites described by Lowe (1982) and Clayton (1992). Thinner- bedded, sandstone/mudstone couplets and mudstone-dominated turbidites are included in the spectrum of Bouma (1962) turbidites. The sharp, erosive bases of the various turbidite sandstone types display a range of sole markings including load, flute and groove casts, as well as rare trace fossils. Localised channelling and lateral wedging of thick sandstone beds is also observed. Palaeocurrent measurements obtained from the Dinas Island Formation, principally from sole structures and channel forms, indicate that the sediment supplying turbidity currents flowed predominantly from a westerly quadrant, but with a significant number of readings, including some channel orientations, recording flow from the north-west or south-east.

Punctuating the sandstone-dominated bulk of the Dinas Island Formation are packets of strata, tens of metres thick, composed of interbedded turbidite and laminated hemipelagic mudstones. These are seen on Dinas Island where several have been named, for example the Trwyn Pendalfa (TrP), Aber Pensidian (AbP) and Pwll Glas (PwG) mudstones. A further unit, the Godir Tudur Mudstone (GTu) crops out in the cliffs north of Newport Sands. The highest such mudstone, occupying the cores of synclines at Dinas Head [SN 004 413] and [SN 009 412], may equate with the Cwm Degwel Mudstone Member (CwD) present at the top of the formation in the Cardigan district. The Dinas Island Formation succession exposed on Dinas Island itself is 550 m thick, and graptolites recovered from the Aber Pensidian Mudstone here are probably of the upper Caradoc morrisi Subzone (Williams et al., 2003).

The distribution and relationships of the Cwm-yr-Eglwys Mudstone and the Dinas Island Formation have been described in detail by Davies et al. (2003) and Williams et al. (2003), and suggest strongly that the major fractures of the Fishguard–Cardigan Fault Belt actively influenced sedimentation and thickness. Thus, the sand-carrying turbidites of the Dinas Island Formation were confined within a trough sited to the north of the Newport Sands Fault, along which they flowed from west to east, whilst the finer sediments of the Cwm-yr-Eglwys Mudstone were deposited along its flanks. The presence of laminated hemipelagic mudstones throughout the Dinas Island and Cwm-yr-Eglwys formations testifies to a period, spanning much of the clingani and linearis Biozones, during which anoxic bottom water conditions were sustained.

Upper Ordovician (Ashgill) rocks

The Caradoc rocks of the district are everywhere succeeded by the Ashgill Nantmel Mudstones Formation (Ntm). This widely recognised mudstone-dominated formation (e.g. Davies et al., 1997, 2003) is the youngest bedrock division in the district, embracing strata included by Evans (1945) in his 'Glogue Slates' and 'Freni Fawr Beds'. Although over a kilometre of Nantmel Mudstones is exposed within the district (Figure 1), this represents only the lower two thirds of the formation, which predominantly comprises medium to pale grey, colour-banded, burrow-mottled mudstones with locally abundant, very thin, beds and laminae of siltstone and fine sandstone. These lithologies represent a spectrum of silt-laminated and silt-free mudstone turbidites and burrowed hemipelagic mudstones. Distinctive, white-weathering bands and elongate nodules of phosphate are the products of early diagenesis associated with oxidation fronts once present within the sediment (Smith, 1987). The Nantmel Mudstones Formation records an abrupt and widespread change from the anoxic sea-bed conditions which had prevailed during much of the Caradoc, into a bioturbated facies indicative of oxygenated bottom waters. This sudden change was possibly a response to a lowering of sea level, though a change in oceanic circulation in response to polar cooling has also been suggested (Armstrong and Coe, 1997; Page et al., 2007).

Some 100–150 m above the base of the formation, both the thickness and abundance of turbidite sandstones increases and a mappable, 300 to 400 m thick sand-prone unit (sa) can be distinguished. This division comprises turbidite sandstone/mudstone couplets of Bouma type, but with colour-banded and burrowed hemipelagic mudstones typical of the rest of the formation interbedded throughout. Palaeocurrent data obtained from this sand-rich division suggest that depositing turbidity currents flowed towards the north–east (Davies et al., 2003). In cuttings e.g. [SN 142 386] at Eglwyswrw, the sandstone beds range up to 30 cm in thickness, and are locally shelly, indicating that the sands were derived from shelf areas to the south. These strata appear to equate with the now poorly exposed 'Freni Fawr Beds' of Evans (1945), estimated to be around 300 m thick, and in which shell-bearing sandstones are also recorded. No graptolites have been recovered from the Nantmel Formation in the district, but the base of the formation is widely taken in central Wales to approximate to the Cardoc–Ashgill boundary (Pratt et al., 1995; Fortey et al., 2000; but see Vandenbroucke et al., 2008). In contrast to the preceding Llanvirn and Caradoc rocks of the district, the Ashgill succession displays no marked changes in thickness or facies across major faults, suggesting that active tectonism no longer influenced sedimentation.

Intrusive rocks

Intrusive rocks hosted by the Pebidian crop out in the area around Brimaston. South of the village and in the area of the Nant-y-Coy stream, the Pebidian is intruded by Granitic rock (G) with abundant alkali feldspar, whereas the broad tract of country north and east of the village is underlain by porphyritic quartz-feldspar granite (qfG) in which the intergrowth of quartz and feldspar crystals to produce a granophyritic texture (Plate 5) suggests an affinity with the St David's Granophyre. Pratchett and Jocelyn (1979) report a likely Ediacaran U– Pb Zircon age of 587 (+25/−14) Ma for the latter intrusion.

Intrusive rocks of Ordovician age are widespread in a belt trending through the central sector of the district between Fishguard and Foel Tyrch. Comparatively few are known south of the Langlydwen Fault. The outcrop of these bodies is based largely on Evans (1945 and unpublished data). The intrusions are found only in strata of Arenig or Llanvirn age, indicating that emplacement was largely coeval with Early Ordovician magmatism and was not, as supposed by Evans (1945), related to Acadian (late Caledonian) orogenesis.

Along most of its crop, the stratigraphy below the Fishguard Volcanic Group, principally the Aber Mawr Formation, but other units as well, has been intruded by a series of dolerite and gabbro sills (D). The main intrusive complex can be traced from Carn Fawr on Mynydd Dinas to Carnedd Meibion Owen near Cilgwyn, and east along the main range of the Preseli Hills to Moel Drygarn. It affects a 2 km thickness of strata at its greatest development around Mynydd Carningli. The dolerites were derived from a tholeiitic parent magma and were probably intruded as high-level sills largely coeval with the later phases of Fishguard Volcanic Group magmatism (Bevins et al., 1989). On the southern flanks of the Preseli Hills around Carn Menyn, and on the northern side of the mountains [SN 135 336] near Carnalw, are outcrops of columnar-jointed dolerite. These rocks, in which aggregates of quartz and feldspar crystals up to 2 cm in diameter impart a 'spotted' appearance, are the famous 'Bluestones' of Stonehenge.

Microtonalite (FT) intrusions at Mynydd Carningli [SN 063 373] (Plate 6), on the southern flank [SN 035 313] of Mynydd Morvil and at Dinas Cross, intrude the Fishguard Volcanic Group and underlying stratigraphy (Aber Mawr Formation). Their origin is uncertain, but these rocks may have been formed by later, post-volcanic partial melting of the sub-Ordovician basement (Bevins et al., 1989). Microgranodiorite (FGd) intrusions have been identified at Craig llwyd [SM 993 324] and north-east of Tygrug [SM 993 331]; the former intrudes a dolerite body hosted by the Aber Mawr Formation and the latter is thought to intrude, and therefore to post-date, the Sealyham Volcanic Formation. These bodies are thought to be derived from calc-alkaline magmas (Bevins et al., 1992) possibly associated with the emplacement of the Porth Maen Melyn Formation, during the early stages of Fishguard Volcanic Group magmatism.

Structure and metamorphism

The major faults, many with a history of synsedimentary movement, and major fold axes shown on (Figure 1) reflect the district's prolonged structural history and the influence of dramatic changes in global plate configurations. During Neoproterozoic times, the Eastern Avalonia rocks of the district formed as a direct result of subduction and magmatism along the active margin of the supercontinent of Gondwana. Fragmentation of Gondwana during Cambrian times led to opening of the Iapetus and Rheic oceans along the northern and southern margins of Eastern Avalonia respectively, and the tectonic processes that led to the final reassembly of the supercontinent of Pangaea during Late Palaeozoic times are largely responsible for the basin subsidence, magmatism and structural development that characterise the geology of the district.

As the Iapetus Ocean contracted, the Welsh Basin underwent periods of inversion, uplift and deformation, culminating in the Early to Mid Devonian Acadian Orogeny (Emsian–Eifelian; Soper and Hutton, 1984). This event is traditionally viewed as the result of terminal collision between Eastern Avalonia and the palaeoplate of Laurentia, although recent studies suggest that impingement of the palaeoplate of Armorica to the south during closure of the Rheic ocean may have had a significant influence (Woodcock et al., 2007).

The earliest fault movements recorded in the district relate to a short-lived, but discrete episode of uplift and erosion during Tremadoc times that is of similar age to deformation preserved in north Wales and on Anglesey (e.g. Schofield et al., 2008). During renewed Mid Ordovician basin subsidence, evidence for active tectonic control on sedimentation is provided by facies and thickness changes across major faults from latest Arenig times. Of these, the east–west-trending Strumble Head Fault and Newport Sands Fault along with the Aber Richard Fault make up part of the Fishguard–Cardigan Fault Belt that was critical in controlling Llanvirn to Caradoc sedimentation in the district (Davies et al., 2003). In the centre of the district, significant late Arenig facies changes occur across the east-north-east-trending Bronnant Fault, a structure known to have strongly influenced Silurian deposition to the north-east (Davies et al., 1997). In the south of the district, the east-west-trending Llanglydwen Fault and Penfordd Fault are the local components of the Welsh Borderland Fault System, a plexus of faults that experienced a long history of movements and which define the southern margin of the Lower Palaeozoic Welsh Basin. Deeper water basinal deposits characterise successions to the north whereas shallower shelfal deposition was largely restricted to the south. The northern edge of this fault zone and its extension into the adjacent St David's area form the northern limit to Neoproterozic outcrop in south-west Wales and of a region of geophysically shallow basement rocks (see below).

The overall structural architecture of the district results from more pervasive deformation during the Acadian Orogeny. At that time, oblique plate convergence (Soper and Hutton, 1984) gave rise to a specific arrangement of structures attributed to transpressive strain in which deformation was partitioned into both shortening and transcurrent movements (Harland, 1971). Typically within the Welsh Basin, the shortening component is preserved as widespread near-upright folds and steeply inclined cleavages. Evidence of transcurrent movement is provided by small angles of obliquity between the cleavage planes and fold axial traces (cleavage transection). These imply passive rotation of fold axes in relation to cleavage during deformation, together with strike-slip displacement along some of the major strike-parallel faults (Woodcock et al., 1988). Steep cleavages and upright folds dominate throughout most of the Fishguard district, but south of the Penfordd Fault, within the Welsh Borderland Fault System, an anomalous gently inclined cleavage is widely developed, exposed for example in crags at Rhydwilym [SN 1133 2563] to [SN 1137 2522] (Plate 7) (faults in black, quartz veins in blue) (adapted from Schofield et al., 2009)." data-name="images/P1004446.jpg">(Figure 3); (Plate 7). This is interpreted as recording the localised horizontal translation of strata (thrusting or inversion) during the Acadian Orogeny, resulting from the buttressing of the basinal succession against a significant basement high (Schofield et al., 2009).

The effects of low-grade metamorphism developed across the district during the Acadian Orogeny can be measured by the growth of crystals of the clay minerals illite and monazite in response to sedimentary burial, tectonic thickening and cleavage formation (Merriman, 2006; Wilby et al., 2007a). Precambrian to Llanvirn strata typically preserve relatively high-grade (epizone) illite crystalinities, reflecting relatively deep burial and the development of a penetrative cleavage. Lower grade (anchizone) crystalinities, reflecting a lesser degree of burial and deformation, characterise Caradoc and Ashgill mudrocks in the south. An isotopic age of around 400 Ma yielded by small crystals of mica that grew within the plane of cleavage has been used to infer an Early Devonian (Emsian) age for the deformation (Sherlock et al., 2003).

During the late Carboniferous, much of south-west Wales, south of the Fishguard district was strongly deformed during the Variscan Orogeny that resulted from final closure of the Rheic Ocean. Although the Fishguard district is historically thought to lie beyond the limit of pervasive Variscan deformation (e.g. Hancock et al., 1983), localised folding of the Acadian cleavage and propagation of south-west-directed thrust faults within the Welsh Borderland Fault System of the district has been inferred to record the reactivation of the fault belt during Variscan shortening (Schofield et al., 2009).

Regional geophyiscs

Interpretations of the regional structure of the Fishguard district are aided by aeromagnetic anomaly (Figure 4) and Bouguer gravity anomaly (Figure 5) maps. The aeromagnetic anomaly map is dominated by a substantial west-south-west trending high in the southern part of the district, together with a series of highs in the area of Mynydd Preseli. The southern high, known as the 'Haverfordwest anomaly', is thought to indicate the presence of Neoproterozoic rocks rising to a depth of 3 km (Norton et al., 2000). The outcrops of these rocks in the area around Brimaston display a similar magnetic response (Figure 4), but because the Haverfordwest anomaly appears unrelated to the rocks at outcrop, there is likely a significant discordance at depth (Norton et al., 2000). Seismic investigations by Brooks et al. (1983) support the interpretation of the anomaly as a basement high, that may have been fundamental in controlling the location and style of structural development in the district (Schofield et al., 2009). In comparison to the Haverfordwest anomaly, magnetic highs in the northern part of the district are thought to be associated with the basic lavas of the Fishguard Volcanic Group and their allied dolerite intrusions (Norton et al., 2000).

The Bouguer gravity anomaly map (Figure 5) shows a significant west-south-west-trending high in the northern part of the district, together with a gravity low in the central part and a second high in the southern part of the district. In the south-west of the district, the gravity low is associated with low-density acidic intrusions of Neoproterozoic age (Norton et al, 2000); its continuation to the north-east probably indicates the presence of these rocks, at depth, in the cores of Felindre–Mynydd Cilcifeth anticlines. McDonald et al. (1992) noted the association of gravity highs with major fault zones in the Welsh Basin, and the strong response in the north-west of the district probably represents the high-density sedimentary fill of grabens associated with the Fishguard–Cardigan Fault Belt. Similarly, the gravity high in the southern part of the district may record a thickened high-density sedimentary succession bounded by the Bronnant and Penfordd faults (Figure 1).

Quaternary

Throughout much of Quaternary times (2.58 Ma to present), the northern uplands of the British Isles supported an ice mass known as the British and Irish Ice-Sheet (Bowen et al., 2002). The Quaternary (superficial) deposits of the district were deposited during distinct phases of landscape evolution associated with the waxing and waning of this ice mass. This followed the initiation of the drainage system in the Oligocene and a prolonged period of erosion and deep, tropical weathering in the late Pliocene and early Pleistocene.

Pre-late Devensian landscape evolution and glaciation

In common with much of Wales, the landscape of the district was initiated through uplift and erosion in the Pliocene and early Pleistocene. In the nearby Llangranog district, deep bedrock weathering profiles (saprolites) have been identified (Davies et al., 2006), indicating a prolonged period of tropical weathering. Battieau-Queney (1984) identified many similar saprolite profiles in the Fishguard district, and suggested that the prominent 'carns' of the district (Plate 6), formed of igneous rock, are the weathered and eroded remnants of Pliocene inselbergs (steep-sided, isolated hills) formed under a savannah climate. In the early Pleistocene, marine erosion of the land below about 220 m OD created a series of now degraded coastal plateaux (Brown, 1960; Bowen, 2005).

Global cooling in the mid Pleistocene saw sea levels fall and ice begin to accumulate in upland Britain. On more than one occasion ice moving down the Irish Sea Basin overrode the coastline of western Wales, crossed the Preseli Hills and flowed into Carmarthen Bay, impinging eastward into Gower and the Vale of Glamorgan (Bowen, 2005). Erratics, possibly from the Fishguard district, have been reported in the Cardiff area (Waters and Lawrence, 1987) and, at its maximum, the ice may have reached as far east as Somerset (Gilbertson and Hawkins, 1978). Although Kellaway (1971) has suggested that the Stonehenge 'Bluestones' were carried by this ice from Carnmenyn to Salisbury Plain, there is no evidence that ice ever reached Wiltshire (Green, 1973). Moreover, fragments of Bluestone from Stonehenge have yielded cosmogenic CI36 dates of 14 ± 1.9 ka (reported in Bowen, 2005). This indicates that they first became exposed to the atmosphere on Carnmenyn only in the latest Devensian, long after the disappearance of any ice that could have transported them eastwards, and very probably after the final deglaciation of the district.

Each glacial advance reworked the landscape and deposits formed by the preceding glaciations, however, the Preseli Hills appear to have formed an effective barrier to the final, Devensian, advance of ice into the district. Deposits to the south, although highly dissected, weathered and substantially remobilised by Devensian periglacial process, are likely to include the products of earlier, more extensive glacial periods, possibly the Anglian (450 000 to 300 000 years ago) and/or Wolstonian (352 000 to 132 000 years ago) glaciations (Bowen, 2005). These earlier materials, principally mapped as head regolith and included in the Penfro Formation by Bowen (1999), are widespread in the uplands of the Preseli Hills and upper parts of tributary valleys to the south where they comprise clayey diamicton with cobble to metre-scale boulders of local volcanic and sedimentary rock as well as erratic clasts derived from northern Britain and the Irish Sea Basin. The majority of clasts are angular, but some are rounded and abraded. Locally younger head and scree materials, the product of gelifluction and slopewash during the late Devensian, veneer and rework these earlier deposits.

In the Eastern Cleddau area, a small incised body of hummocky glacial deposits north of Alltypityll [SM 113 250], similar to materials found in the adjacent Newcastle Emlyn district, is also considered a remnant of an earlier ice advance (Wilby et al., 2007b). Benches and mounds of glaciofluvial deposits present in the upper part of the Eastern Cleddau catchment and glaciofluvial sheet deposits in both the Eastern Cleddau and Taf valleys may comprise the denuded remnants of formerly more extensive kame deposits and outwash sheets dating from this 'Penfro' glaciation.

Perhaps the most spectacular legacy of ealier glaciations is the series of wide, box-shaped valleys which form an anastamosing network in the area between Fishguard and Wolf's Castle, of which the Gwaun valley, up to 500 m wide and in places over 70 m deep, is perhaps the most striking example. Many of these valleys are dry, whilst others host small misfit streams; all have flat, commonly rocky floors and few have any substantial tributaries. Known collectively as the Gwaun–Jordanstown system (John, 1970), they are thought to have been cut by meltwaters flowing under hydrostatic pressure beneath a pre-Devensian ice mass (Bowen and Gregory, 1965; Bowen, 1971). However, it seems likely that they were also exploited at glacial lake spillways and for sub- and proglacial drainage during the late Devensian (Figure 6) as suggested by Charlesworth (1929) and endorsed by Etienne et al. (2006).

Late Devensian glaciation

During the late Devensian, the British and Irish Ice-Sheet again expanded southward from its northern source areas, reaching its maximum extent some 22 500 years ago. At this time, ice again impinged into the coastal tract of western Wales from the Irish Sea, and also from the upland interior of mid Wales. Much recent work has been undertaken on the deposits of this glaciation, by the BGS in collaboration with both Aberystwyth and Cardiff universities, and the limits of the Irish Sea Glacier are now well established (e.g. Etienne et al., 2006). To the north of the Fishguard district, it impinged several kilometres inland, flowing up coastal valleys such as the Aeron and Teifi; but in the present district it failed to surmount the Preseli Hills, being deflected along the lower flanks of Carningli and Mynydd Dinas, and penetrated inland only to the west of this topographical barrier ((Figure 6)c). The deposits of the late Devensian Irish Sea Glacier have been grouped as the St Asaph Formation by Bowen (1999).

North of the district in the Teifi valley, thick successions of glaciolacustrine deposits, comprising clays, silts and silty clays, represent the deposits of a large, proglacial lake ('Llyn Teifi') which formed in the river's lower reaches as the valley mouth was dammed by advancing Irish Sea ice (Hambrey et al., 2001; Davies et al., 2003, 2006) ((Figure 6)a). Waters from this lake spilled southward through cols on the eastern periphery of the Fishguard district, for example at Blaenffos, near Boncath and at Llantood, north of Eglwysywrw (Charlesworth, 1929; Jones, 1965; Hambrey et al., 2001; Davies et al., 2003). These waters are thought to have fed a series of proglacial lakes in the Fishguard district, formed as the coastal valleys here were also dammed and impeded by the advancing ice mass. The discovery of glacial lake deposits confirms that such lakes, as modelled by Etienne et al. (2006), existed in the Nevern valley ('Llyn Nevern') and in the Goodwick valley ('Llyn Manorwen'), and utilised the Gwaun–Jordanstown System of channels for drainage before being overridden by ice ((Figure 6)b).

The principal deposits of the Irish Sea Glacier are subglacial till, comprising clayey diamicton with clasts derived from the local Lower Palaeozoic rocks and erratics of limestone, igneous and metamorphic rock derived from diverse sources in northern Britain and the Irish Sea Basin. Till deposits are widely distributed along the flanks of the Preseli Hills south of Brynberian, throughout the Nevern valley, and along the coastal plateau north of Mynydd Carnigli; patchy deposits are preserved in the area around Fishguard, the lower Gwaun valley, and south of Goodwick. Till deposits to the north of the Preseli Hills are overlain or replaced by heterogeneous glacial deposits comprising poorly sorted, clast-rich, matrix-supported gravel diamictons, thought to represent proglacial debris. A ridge of such hetregeneous debris immediately east of Llanerch [SN 058 355] may mark the terminal position of a small glacier in the upper reaches of the Gwaun valley.

Initial climatic amelioration, coincident with a significant global sea-level rise around 19 000 years ago (Yokoyama et al., 2000), saw the rapid collapse of the Irish Sea Glacier in west Wales, perhaps at a rising tidewater margin in the south-central Celtic Sea (e.g. Scourse and Furze, 2001). In the present district, outwash sands and gravels, in the form of Devensian glaciofluvial sheet deposits, were deposited in front of the retreating ice margin (Plate 8). Extensive spreads in the Western Cleddau and the Anghof valley, for example at Stradland Farm [SM 995 265], were sourced from the western tongue of the Devensian ice-sheet. Erratic clasts, including gneissic rocks from northern Britain, for example at Morvil Farm [SN 037 307], Puncheston, confirm an Irish Sea provenance. In the north-east of the district ice-contact glacio-fluvial deposits were formed as accumulations of sand and gravel buttressed against the retreating ice. Degraded sand and gravel landforms of uncertain origin in this area are shown as undifferentiated glaciofluvial deposits.

Periglacial and postglacial deposits and landforms

Though much of the area was unglaciated even during the Devensian glacial maximum, it was subject to severe peri-glacial conditions. The Pliocene inselbergs, already much eroded during earlier Pleistocene glaciations, underwent spectacular frost shattering to produce the rubbly carns of the district, the most impressive of which is Mynydd Carnigli south of Newport (Plate 6). The debris produced by frost-shattering of exposed bedrock accumulated as deposits of head gravel and talus. Periglacial conditions continued to prevail throughout the district during and immediately following the withdrawal of ice from the district, and associated solifluction and slope wash contributed to accumulation of silty head deposits along many small tributary valleys. Continued climatic amelioration during the latest Devensian saw the final withdrawal of ice from the Fishguard district around 14 000 years ago. Postglacial sea levels continued to rise throughout the early Holocene, only establishing the modern coastline around 5000 years ago. Raised tidal flat deposits comprising organic-rich clays and silts found at Newport and impounded behind barrier storm beaches in a cove [SN 999 393] facing Fishguard Bay represent the earliest, now abandoned coastal zone deposits. They predate the modern beach, storm beach, tidal river and salt marsh deposits (Plate 8). Inland rivers have deposited extensive alluvial tracts including abandoned and incised river terrace deposits and the alluvium of the modern floodplains. Alluvial fan deposits have formed where tributary streams meet larger rivers. Thin peat and peaty soils are widespread in the upland reaches of the district, but thicker and mappable peat accumulations are confined to areas of impeded drainage and ponding as at Wauncledau [SN 163 320]. Lacustrine deposits have accumulated in enclosed hollows in glacial deposits, possibly former kettleholes, as to the north of Newport [SN 064 3999].

Artificially modified ground

Made ground comprises areas where substantial thicknesses of material, including mine waste, spoil and refuse, have been deposited as well as road and railway embankments. Those areas shown on the map are principally the spoil tips of former slate and aggregate quarries notably at Rosebush [SN 079 301], Gilfach [SN 128 268] and Treffgarne [SM 958 238], but also include the dam of the Llys-y-fran Reservoir and the coastal installations of Goodwick harbour.

Chapter 3 Applied geology

Earth science factors have a significant influence on the activities of man and as such are major considerations for land-use planning and development. Consideration of earth science issues early in the planning process can help ensure that site and development are compatible, that local resources are not damaged or contaminated, and that any appropriate mitigation measures are taken prior to development. 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.

Mineral resources and construction materials

Evidence of limited metalliferous mineralisation is provided by a small number of old workings or trials for both base and precious metals, for instance at Llanglydwen [SN 179 268], Fronlas [SN 166 340] and Treffgarne [SM 961 236], where it is said that a gold-bearing vein was encountered during the construction of the railway line. A stream-sediment survey of the Preseli Hills by the BGS Mineral Reconnaissance Programme (Cameron et al., 1984) suggested some potential for metal enrichment in the mudrocks of the Drefach Group, where they overlie the Fishguard Volcanic Group, but there is limited potential for epithermal gold mineralisation in the Roch Volcanic Formation (Brown et al., 1987) and Sealyham Volcanic Formation (Colman et al., 1995). There may also be shear-zone mineralisation along major fractures such as the Penfordd Fault (Norton et al., 2000). Immediately east of the district, the Llanfyrnach mines were a major 19th Century producer of silver and lead.

In comparison to the paucity of metal workings, the rocks of the Fishguard district have long been exploited for building materials, particularly roofing slate and slab, around which a substantial industry grew up in the 18th and 19th centuries. The workings divided into the 'blue slate' and 'green slate' quarries; almost all profitable 'blue slate' quarries of the district, such as Rosebush, Sealyham and Summerton, exploited the well-cleaved rocks of the Penmaen Dewi and Aber Mawr formations, whilst the 'green' or 'Whitland Abbey' slate quarries of the Eastern Cleddau, including Dandderwen, Llangolman and Tyrch, worked the contemporaneous but dissimilar Foel Tyrch Formation. At the brief peak of the Pembrokeshire slate trade in the early 1870s, Rosebush produced some 5000 tonnes annually, mostly of roofing slate. However, the blue slate fell out of fashion in the late 19th Century, and most of these quarries closed. Although individual sites produced less material, green slate remained in production into the middle part of the 20th century. Slates from Waun Isaf [SM 1460 2936] were used in the Houses of Parliament, and the last active quarry at Gilfach, near Llangolman, closed in 1987, and is now the site of a popular slate workshop and heritage centre. More information on the history of slate working in the Fishguard district can be found in Richard (1998).

The Treffgarne Volcanic Formation was once extensively worked at Treffgarne quarries [SM 958 238] for roadstone, but these were abandoned in the early 1980s. Parts of the Fishguard Volcanic Group have been similarly worked, particularly in the area around Dinas Cross. Recently worked dolerite intrusions in the Foel Tyrch Formation occur at Carn Wen [SN 1166 287] and to the north-east at [SN 176 291], the latter omitted from the published map (but see (Figure 1)). A quarry at Plascwrt [SN 118 274] works the Foel Tyrch Formation 'slates' as a decorative stone (Cameron et al., 2005). Sand and gravel resources are scarce within the district; there are no active pits and most materials used in construction works are imported from elsewhere.

Water resources

The majority of the district derives water from a public mains supply, largely fed by abstraction of surface water from the major rivers, but many farms also abstract groundwater from private boreholes and springs for purposes such as spray irrigation. Throughout the district, as in much of mid Wales (Edmunds et al., 1998), groundwater flow is shallow, and limited to the weathered regolith, although there is probably some flow through fractures in fresh bedrock at depth (Robins et al., 2000). In areas of volcanic rock, springs may form where large fractures reach the surface, although most springs in the district are found at the contact between bedrock and superficial deposits. The unweathered bedrocks lack significant primary porosity; superficial deposits, where present, may locally include moderately permeable sands and gravels.

The agricultural nature of the district means that its major rivers can potentially contribute to faecal-indicator organism loading of coastal waters during the bathing season (e.g. Crowther et al., 2003). The beach at Cwm-yr-Eglwys holds a Green Coast Award in recognition of its excellent water quality and environmental management practices.

Potential geological hazards

The distribution of floodplain deposits (alluvium), as shown on the 1:50 000 scale geological map, provides an indication of those areas of the district which are prone to river flooding. 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. Periods of enhanced catchment discharge may increase the pollution potential of ground and surface waters, as leachate, soluble contaminant or contaminative sediments from poorly lined landfill sites, agricultural waste-disposal sites and active or former industrial sites (e.g. sewage works, pits and quarries) may enter surface waters, groundwaters or alluvial sediments. Storm and tidal flooding affecting low-standing areas of coast in Fishguard and Newport bays has been significant in the past (Plate 8), and Fishguard Lower Town is at high risk as spring tides frequently overtop the habour wall, car park and areas adjacent to the Gwaun.

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 organic 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 unlikely to pose a significant hazard in the district, the risk can be further mitigated through correct design of landfills 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, though present in small quantities in all natural rocks and soils, occur in higher concentrations in certain igneous rocks and black mudstones. Radon released from rocks and soils is normally quickly dissipated within the atmosphere and does not present a hazard. However, where it is allowed to accumulate in poorly ventilated spaces it can give rise to an elevated risk of cancers of the respiratory tract. The district is at risk of radon accumulation, and protective measures for new developments and remediation of existing homes and work-places may be necessary. Building regulations in place at the time of writing require a minimum level of radon protection measures to be undertaken across most of the district, and full protection measures in the region of the igneous rocks that crop out in the Fishguard and Treffgarne areas. Advice about radon and its associated health risks can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire, OX11 ORQ.

No major landslides have been identified in the district, although small areas of slippage are locally present on valley slopes e.g. [SM 173 233] and rock falls are widespread on the coastal cliffs. The most effective strategy for dealing with such unstable ground relies on recognition of problem areas in advance so that suitable preventative or remedial measures can be employed.

Engineering ground conditions

Knowledge of ground conditions is a primary consideration for identifying land suitable for development, and underpins cost-effective design. Engineering ground conditions vary depending on the physical and chemical properties of the local materials, topography, behaviour of groundwater and surface water, and the nature of past and present human activity. The most significant development problems likely to be encountered in the district are due to the variability of natural superficial deposits, and the weathering of solid rocks. 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 where saprolites or weathered profiles are present; this may be a potential problem along the crop of the igneous rocks. Some pyritic parts of the Drefach Group may be susceptible to heave or sulphate attack of concrete. Till and heterogeneous glacial deposits have moderate bearing capacity but are highly variable, particularly in the case of the older till, where large boulders of dolerite with a high bearing capacity may be found set in a matrix more prone to settlement. Head, head gravel, alluvial fan deposits and alluvium all have low to moderate bearing capacities with moderate to high settlements. Although glaciofluvial and river terrace deposits embrace a variety of foundation conditions, they generally have high bearing capacities. Artificial ground is highly variable and may include contaminated land requiring remediation.

Geological conservation

Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). These are statutory designated conservation sites which have some protection under the Wildlife and Countryside Act 1981. Further information on the extent and designation of SSSIs and locally designated Regionally Important Geodoversity Sites (RIGS) can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Rd., Bangor, Gwynedd LL57 2LQ. Special features of interest, being considered for notification as SSSIs, are described in the Geological Conservation Review (GCR) series, published by the Nature Conservancy Council.

Information sources

Sources of geological information held by the British Geological Survey relevant to the Fishguard district are listed below.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues. Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Enquiries concerning geological data for the district should be addressed to the BGS Enquiry Service at Keyworth. Geological advice for this area should be sought from the Chief Geologist (Wales), BGS, Cardiff. The addresses of the BGS offices are given on the back cover and at the end of this section.

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, South Kensington. Coloured print-on-demand copies are available for purchase from the BGS sales desk.

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

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

Geological maps
1:625 000
Bedrock geology of the United Kingdom: South Map (2007)
Quaternary geology: South Sheet (1977)
1:250 000
Geological Map of Wales, Solid geology, 1994 Sheet 52N 06W Cardigan Bay, Solid geology, 1982
1:63 360
Sheet 52N 06W Cardigan Bay including part of Waterford, Quaternary Geology, 1990
Sheet 5 IN 06W Lundy, Solid Geology, 1983 [Old Series] Sheet 40, 1847
1:50 000
Sheet 210, Fishguard, Bedrock and Superficial Deposits, England and Wales, 2010.
1:25 000
The component 1:25 000 scale maps of the Fishguard district, based on BGS surveying, are listed below, along with the surveyors' initials and dates of survey. The surveyors were J A Aspden, C E Burt, J R Davies, M R Hall, T H Sheppard and P R Wilby; in addition, J R Davies, P R Wilby and R A Waters surveyed small parts of SN 04 and SM 94 in 1995–1999 as part of the survey of 1:50 000 Sheet 193 (Cardigan and Dinas Island).

Map number	Surveyor	Date
SM 94	JRD, PRW, RAW*, CEB	1995–1999*, 2005
SM 93	CEB, JRD, THS	2005
SM 92	JAA	2005
SN 04	JRD, PRW, RAW	1995–1999
SN 03	THS, JRD	2005
SN 02	MRH	2005
SN 14	JRD	2005
SN 13	JRD, THS	2005
SN 12	PRW	2005

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 website.
Geophysical maps
1:500 000
Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas
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
England and Wales, 1997
1:100 000
Groundwater Vulnerability of Dyfed (Sheet 27), 1998

Books and reports

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

Documentary records collections

Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.

Borehole and trial pit records

Borehole records for the district are catalogued in the NGDC at BGS Keyworth. Index information, which includes site references, names and depths for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at BGS Keyworth.

Hydogeological data

Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.

Geophysical data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth.

BGS Lexicon of named rock units

Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 210 Fishguard, are held in the BGS Stratigraphical Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collection can be viewed at BGS libraries at Keyworth and Edinburgh, and on the BGS website (GeoScenic). Copies of the photographs can be purchased from BGS.

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.

References

Most of the references listed here can be consulted at the BGS Library, Keyworth. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.

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

Battieau-Queney, Y. 1984. The pre-glacial evolution of Wales. Earth Surface Processes and Landforms, Vol. 9, 229–252.

Bevins, R E . 1982. Petrology and geochemistry of the Fishguard Volcanic Complex, Wales. Geological Journal, Vol. 17, 1–21.

Bevins, R E, and Horak, J M. 2000. St David's Peninsula. 131–136 in Precambrian rocks of England and Wales. Carney, J N, Horak, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, Bevins, R E, Cope, J C W, and Ford, T D (editors). Geological Conservation Review Series, No. 20. (Peterborough: Joint Nature Conservation Committee.)

Bevins, R E, and Roach, R A. 1979. Early Ordovician volcanism in Dyfed, S W Wales. 603–609 in The Caledonides of the British Isles — reviewed. Harris, A L, Holland, C H, and Leake, B E (editors). Special Publication of the Geological Society of London, No. 8.

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

Bevins, R E, Lees, G J, and Roach, R A. 1989. Ordovician intrusions of the Strumble Head– Mynydd Preseli region, Wales: lateral extensions of the Fishguard Volcanic Complex. Journal of the Geological Society of London, Vol. 146, 113–123.

Bevins, R E, Lees, G J, and Roach, R A. 1991. Ordovician bimodal volcanism in S W Wales: geochemical evidence for petrogenesis of the silicic rocks. Journal of the Geological Society of London, Vol. 148, 719–729.

Bevins, R E, Lees, G J, and Roach, R A. 1992. Petrogenesis of Ordovician igneous rocks in the southern part of the Welsh Basin. Geological Magazine, Vol. 129, 615–624.

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

Bowen, D Q. 1967. On the supposed ice-dammed lakes of south Wales. Transactions of the Cardiff Scientific and Field Naturalists' Society, Vol. 93, 4–17.

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

Bowen, D Q. 1999. Wales. 79–90 in A revised correlation of Quaternary deposits in the British Isles. Bowen, D Q (editor). Special Publication of the Geological Society of London, No. 23.

Bowen, D Q. 2005. South Wales. 145–162 in The glaciations of Wales and adjacent areas. Lewis, C A, and Richard, A E (editors). (Hereford: Logaston Press.)

Bowen, D Q, and Gregory, K J. 1965. A glacial drainage system near Fishguard, Pembrokeshire. Proceedings of the Geologists' Association, Vol. 74, 275–281.

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.

British Geological Survey. 1992. St David's. England and Wales Sheet 209, Solid and drift (Provisional). 1:50 000. (Southampton: Ordnance Survey for British Geological Survey.)

Brooks, M E, Mechie, J, and Llewellyn, D J. 1983. Geophysical investigations in the Variscides of south-west Britain. 186–197 in The Variscan fold belt in the British Isles. P LHancock (editor). (Bristol: Adam Hilger Ltd.)

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

Brown, M J, Allen, P M, Cooper, D C, Cameron, D G, Pease, S F, Beddoe-Stevens, B, and Evans, A D. 1987. Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales. Mineral Reconnaissance Report, No. 86 (Keyworth, Nottingham: British Geological Survey.)

Cameron, D G, Cooper, D C, Allen, P M, and Haslam, H W. 1984. A geochemical drainage survey of the Preseli Hills, south-west Dyfed, Wales. Mineral Reconnaissance Programme Report No. 72. (Keyworth, Nottingham: British Geological Survey.)

Cameron, D G, Bartlett, E L, Highley, D E, Lott, G K, and Hill, A J. 2005. Directory of mines and quarries, 2005. (Keyworth, Nottingham: British Geological Survey.)

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

Clayton, C. 1992. The sedimentology of a con-fined turbidite system in the early Silurian Welsh Basin. Unpublished PhD thesis, University of Cambridge.

Colman, T B, Norton, G E, Chacksfield, B C, Cooper, D C, and Cornwell, J D. 1995. Exploration for volcanogenic mineralisation in south-west Wales. Mineral Reconnaissance Report, No. 37 (Keyworth, Nottingham: British Geological Survey.)

Cowie, J W, Rushton, A W A, and Stubblefield, C J. 1972. A correlation of Cambrian rocks in the British Isles. Special Report of the Geological Society of London, No. 2.

Cox, A H. 1930. Preliminary note on the geological structure of Pen Caer and Strumble Head, Pembrokeshire. Proceedings of the Geologists' Association, Vol. 41, 274–289.

Crimes, T P. 1970. Afacies analysis of the Cambrian of Wales. Palaeogeography, Palaeo-climatology, Palaeoecology, Vol. 7, 113–170.

Crowther, J, Wyer, M D, Bradford, M, Kay, D, and Francis, C A. 2003. Modelling faecal indicator concentrations in large rural catchments using land use and topographic data. Journal of Applied Microbiology, Vol. 94, 962–973.

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

Davies, J R, Waters, R A, Wilby, P R, Williams, M, and Wilson, D. 2003. Geology of the Cardigan and Dinas Island district. Sheet Explanation of the British Geological Survey, Sheet 193 (England and Wales).

Davies, J R, Sheppard, T H, Waters, R A, and Wilson, D. 2006. Geology of the Llangranog district. Sheet Explanation of the British Geological Survey, Sheet 194 (England and Wales).

Directory of British Fossil Localities. 1954. (London: Palaeontographical Society.)

Edmunds, W M, Robins, N S, and Shand, P. 1998. The saline waters of Llandrindod and Builth, Central Wales. Journal of the Geological Society of London, Vol. 155, 62–87.

Elsden, J V. 1905. The igneous rocks occurring between St David's Head and Strumble Head. Quarterly Journal of the Geological Society of London, Vol. 61, 579–607.

Etienne, J L, Jansson, K N, Hambrey, M J, Glasser, N F, Davies, J R, Waters, R A, and Wilby, P R. 2006. Glacigenic sedimentation and palaeoenvironmental reconstruction of an ice-contact glacial lake succession: an example from the late Pleistocene of south-west Wales, U K. Quaternary Science Reviews, Vol. 25, 739–762.

Evans, J A, and Boulton, N. 2007. U–Pb ages for the Ordovician Sealyham Volcanic Unit 2, and the Nant Y Coy granite, S W Wales. N ER C Isotope Geosciences Laboratory Report Series, No. 217.

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

Fortey, R A, and Owens, R M. 1987. The Arenig Series in south Wales. Bulletin of the British Museum (Natural History), Vol. 41, 69–307.

Fortey, R A, and Rushton, A W A. 2000. South Wales. 13–17 in A revised correlation of Ord-ovician rocks in the British Isles. Fortey, R A, Harper, D A T, Ingham, J K, Owen, A W, Parkes, M A, Rushton, A W A, and Woodcock, N H (editors). Geological Society of London Special Report, No. 24.

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.

Gilbertson, D D, and Hawkins, A B. 1978. The Pleistocene succession at Kenn, Somerset. Bulletin of the Geological Survey of Great Britain, No. 66, 44.

Green, C P. 1973. Pleistocene river gravels and the Stonehenge Problem. Nature, Vol. 243, 214–216.

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

Hancock, P L, Dunne, W M, and Tringham, M E. 1983. Variscan deformation in south-west Wales. 186–197 in The Variscan fold belt in the British Isles. Hancock, P L (editor). (Bristol: Adam Hilger Ltd.)

Harland, W B. 1971. Tectonic transpression in Caledonian Spitsbergen. Geological Magazine, Vol. 108, 27–42.

Hicks, H. 1877. On the Precambrian (Dimetian and Pebidian) rocks of St Davids. Journal of the Geological Society of London, Vol. 33, 229–241.

Howells, M F, Smith, M, Rushton, A W A, Moly-neux, S G, Tunnicliff, S P, Colman, T B, and Carruthers, R M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales).

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

Jones, O T. 1940. Some Lower Palaeozoic contacts in Pembrokeshire. Geological Magazine, Vol. 77, 405–409.

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

Kellaway, G A. 1971. Glaciation and the stones of Stonehenge. Nature, Vol. 232, 30–35.

Kennedy, R J. 1989. Ordovician (Llanvirn) trilobites from South West Wales. Monograph of the Palaeontographical Society, London.

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

Kokelaar, B P, Bevins, R E, and Roach, R A. 1985. Submarine silicic volcanism and associated sedimentary and tectonic processes, Ramsey Island, S W Wales. Journal of the Geological Society of London, Vol. 142, 591–613.

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

Lowman, R D W. 1977. The geology, petrology and geochemistry of an area of Lower Palaeozoic rocks east of Fishguard, northern Pembrokeshire. Unpublished PhD Thesis, University of Wales (Swansea).

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

Marr, J E, and Roberts, T. 1885. The Lower Palaeozoic rocks of the neighbourhood of Haver-fordwest. Quarterly Journal of the Geological Society of London, Vol. 41, 476–491.

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

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

Merriman, R J. 2006. Clay mineral assemblages in British Lower Palaeozoic mudrocks. Clay Minerals, Vol. 41, 473–512.

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

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

Owens, R M. 1999. Arenig to Ashgill in South Wales. 127–179 in British Cambrian to Ordovician Stratigraphy. Rushton, A W A, Owen, A W, Owens, R M, and Prigmore, J K (editors). Geological Conservation Review Series. No. 18. (Peterborough: Joint Nature Conservation Committee.)

Page, A A, alasiewicz, J A, Williams M, and Popov, L E. 2007. Were transgressive black shales a negative feedback modulating glacio-eustasy during the Early Palaeozoic Icehouse? 123–156 in Deep time perspectives on climate change: marrying the signal from computer models and biological proxies. Williams, M, Haywood, A M, Gregory, F J, and Schmidt, N (editors). The Micropalaeontological Society Special Publications. (London: The Geological Society.)

Page, A A, Wilby, P R, Williams, M, Vannier, J M C, Davies, J R, Waters, R A, and Zalasiewicz, J A. 2010. Soft-part preservation in a bivalved arthropod from the Late Ordovician of Wales [electronic version]. Geological magazine, Vol. 147, 242–252.

Pratchett, P J, and Jocelyn, J. 1979. U–Pb Zircon ages for late Precambrian igneous rocks in south Wales. Journal of the Geological Society of London, Vol. 136, 13–19.

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).

Prigmore, J K, and Rushton, A W A. 1999. Cambrian of South Wales: St David's area. 51–66 in British Cambrian to Ordovician stratigraphy. Rushton, A W A, Owen, A W, Owens, R M, and Prigmore, J K (editors). Geological Conservation Review Series, No. 18. (Peterborough: Joint Nature Conservation Committee.)

Reed, F R C. 1895. The geology of the country around Fishguard, Pembrokeshire. Quarterly Journal of the Geological Society of London, Vol. 51, 149–194.

Richard, A J. 1998. The slate quarries of Pem-brokeshire. (Llanrwst: Gwasg Carreg Gwalch.)

Robins, N S, Shand, P, and Merrin, P D. 2000. Shallow groundwater in drift and Lower Palaeo-zoic 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 DR (editors). Special Publication of the Geological Society of London, No. 182.

Schofield, D I, Evans, J A, Millar I L, Wilby, P R, and Aspden, J A. 2008. New U–Pb and Rb–Sr constraints on pre-Acadian tectonism in North Wales. Journal of the Geological Society of London, Vol. 165, 891–894.

Schofield, D I, Aspden, J A, Kemp, S J, Merriman, R J, and Wilby, P R. 2009. Basement controls on Acadian thrusting and fault reactivation along the southern margin of the Welsh Basin. Geological Journal, Vol. 44, 526–536.

Scourse, J D, and Furze, M F A. 2001. A critical review of of the glaciomarine model for Irish sea deglaciation: evidence from southern Britain, the Celtic shelf and adjacent continental slope. Journal of Quaternary Science, Vol. 16, 419–434.

Sherlock, S C, Kelley, S P, Zalasiewicz, J A, Schofield, D I, Evans, J A, Merriman, R J, and Kemp, S J. 2003. Precise dating of low temperature deformation: strain-fringe dating by Ar/Ar lasserprobe. Geology, Vol. 31, 219–222.

Siveter, D J, and Williams, M. 1995. An early Cambrian assignment for the Caerfai Group of South Wales. Journal of the Geological Society of London, Vol. 152, 221–224.

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

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

Stead, J T G, and Williams, B J P. 1971. The Cambrian rocks of north Pembrokeshire. 180–198 in Geological excursions in South Wales and the Forest of Dean. Bassett, D A, and Bassett, M G (editors). (Cardiff: Geologists' Association South Wales Group.)

Thomas, G E, and Thomas, T M. 1956. The volcanic rocks of the area between Fishguard and Strumble Head, Pembrokeshire. Quarterly Journal of the Geological Society of London, Vol. 112, 291–314.

Thomas, H H, and Cox, A H. 1924. The volcanic series of Treffgarne, Roch and Ambleston (Pembrokeshire). Quarterly Journal of the Geo-logical Society of London, Vol. 80, 520–548.

Thomas, H H, and Jones, O T. 1912. On the Precambrian and Cambrian rocks of Brawdy, Hayscastle and Brimaston (Pembrokeshire). Quarterly Journal of the Geological Society of London, Vol. 68, 374–401.

Traynor, J J. 1988. The Arenig in South Wales: sedimentary and volcanic processes during the initiation of a marginal basin. Geological Journal, Vol. 23, 275–292.

Trench, A, Torsvik, T H, Dentith, M C, Walder-haug, H, and Traynor, J J. 1992. Ahigh southerly palaeolatitude for southern Britain in Early Ordovician times; palaeomagnetic data from the Treffgarne Volcanic Formation, S W Wales. Geophysical Journal International, Vol. 108, 89–100.

Vandenbroucke, T, Williams, M, Zalasiewicz, J A, Davies, J R, and Waters, R A. 2008. Integrated Upper Ordovician graptolite–chitinozoan biostratigraphy of the Cardigan and Whitland areas, south-west Wales. Geological Magazine, Vol. 145, 199–214.

Waters, R A, and Lawrence, D J D. 1987. Geology of the South Wales Coalfield, Part 3, the country around Cardiff. Memoir of the British Geological Survey, Sheet 263 (England and Wales).

Wilby, P R, Page, A A, Zalasiewicz, J A, Milo-dowski, A E, Williams, M, and Evans, J A. 2007a. Syntectonic monazite in low-grade mudsrocks: a potential geochronometer for cleavage formation? Journal of the Geological Society of London, Vol. 164, 53–56.

Wilby, P R, Schofield, D I, Wilson, D, Aspden, J A, Burt, C E, Davies, J R, Hall, M R, Jones, N S, and Venus, J. 2007b. Geology of the Newcastle Emlyn district. Sheet Explanation of the British Geological Survey, Sheet 211 (England and Wales).

Williams, M, Davies, J R, Waters, R A, Rushton, A W A, and Wilby, P R. 2003. Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, south-west Wales. Geological Magazine, Vol. 140, 549–571.

Williams, T G. 1933. The Precambrian and Lower Palaeozoic rocks of the eastern end of the St David's Precambrian area, Pembrokeshire. Quarterly Journal of the Geological Society of London, Vol. 145, 901–914.

Woodcock, N H, Awan, M A, Johnson, T E, Mackie, A H, and Smith, R DA. 1988. Acadian tectonics of Wales during Avalonia/Laurentia convergence. Tectonics, Vol. 3, 483–495.

Woodcock, N H, Soper, N J, and Strachan, R A. 2007. A Rheic cause for the Acadian deformation in Europe. Journal of the Geological Society of London, Vol. 164, 1023–1036.

Yokoyama, Y, Lambeck, K, De Deckker, P, Johnston, P, and Fifield, L K. 2000. Timing of the last glacial maximum from observed sea-level minima. Nature, Vol. 406, 713–716.

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.

Figures and plates

Figures

(Figure 1) Simplified bedrock geology of the district.

(Figure 2) Ordovician chronostratigraphy and graptolite biozones referred to in the text (based on Fortey et al., 2000).

(Plate 7) (faults in black, quartz veins in blue) (adapted from Schofield et al., 2009)." data-name="images/P1004446.jpg">(Figure 3) Structural traverse through crags [SN 1137 2522] in the Penmaen Dewi Formation exposed near Rhydwilym showing the location of (Plate 7) (faults in black, quartz veins in blue) (adapted from Schofield et al., 2009).

(Figure 4) Aeromagnetic anomaly map of the Fishguard district and surrounding areas. Based on data in the BGS National Aeromagnetic Databank. Total field magnetic anomalies in nanotesla (nT) relative to a local variant of IGRF90. The anomalies are shown as a colour shaded relief presentation using the BGS COLMAP Package. The shaded topographical effect has been created using an imaginary light source, located to the north–west. Contour interval 10nT. Flown at a mean terrain clearance of 305 m on north–south flight lines 2 km apart with east–west tie lines 10 km apart. The inset frame indicates the extent of the Fishguard district.

(Figure 5) Bouguer gravity anomaly map of the Fishguard district and surrounding areas. Based on data in the BGS National Gravity Databank. Bouguer gravity anomalies in milligals (mGal) calculated against the Geodetic Reference System 1967, referred to the National Gravity Reference Net, 1973. Variable Bouguer reduction density. Station distribution approximately 1 per 1.3 km². The anomalies are shown as a colour shaded relief presentation using the BGS COLMAP Package. The shaded topographical effect has been created using an imaginary light source, located to the north-west. Contour interval 1mGal (1 mGal = 1 x 10⁻⁵ m/s²). The inset frame indicates the extent of the Fishguard district.

(Figure 6) Late Devensian proglacial lake a and drainage development, adapted from Etienne et al. (2006). a) Formation of Llyn Teifi. b) Formation of Llyn Nevern and Llyn Manorwen. c) Maximum advance of late Devensian ice into the district. BC Blaenffos Channel; CC Cippyn Channel; GC Gwaun Channel; G-JS Gwaum– Jordanstown System; LC Llantood Channel.

Plates

(Plate 1) Sealyham Volcanic Formation epiclastic breccia (front) faulted against Aber Mawr Formation mudstone, Banc Du, New Inn (P781474).

(Plate 2) Cliffs in the Fishguard Volcanic Group viewed from the old harbour quay looking north to Castle Point. The unconformity between the Llanvirn volcanic rocks and the overlying Castle Point Member (Penyraber Mudstone Formation) is exposed below the headland (P780586).

(Plate 3) Blocks of volcanic and sedimentary rock in the Saddle Point Member (Penyraber Mudstone Formation), cliffs south of Goodwick breakwater (P780584).

(Plate 4) Cliffs looking east from Trwyn Isaac exposing the Penyraber Mudstone and Cwm-yr-Eglwys Mudstone formations (P780587).

(Plate 5) Pebidian Supergroup alkali-feldspar granite, Brimaston (P781475).

(Plate 6) Frost-shattered blocks of microtonalite, Mynydd Carningli (P781476).

(Plate 7) Shallowly inclined bedding (S0) and cleavage (S1) seen south of the Penfordd Fault, Rhydwilym (P781477).

(Plate 8) Cwm-yr-Eglwys beach and ruined church destroyed during the 'Royal Charter' Storm in 1859. Eroded glaciofluvial sheet deposits underlie the graveyard (P780585).

(Front cover) Cover Photograph Fishguard Harbour, looking seaward to the north-west. (Photographer: J I Rayner, P604362).

(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.