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Geology of the Llandovery district — a brief explanation of the geological map Sheet 212 Llandovery

D I Schofield, J R Davies, N S Jones, A B Leslie, R A Waters, M Williams, D Wilson, J Venus and R D Hillier

Bibliographic reference: Schofield, D I, Davies, J R, Jones, N S, Leslie, A B, Waters, R A, Williams, M, Wilson, D, Venus, J, and Hillier, R D. 2009. Geology of the Llandovery district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 212 Llandovery (England and Wales).

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(Front cover) Plateau Beds Formation, Tyle du [SN 780, 220] (Photographer: J R Davies; (P680309))

(Rear cover)

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 212 (Llandovery). National Grid references are given in square brackets and all lie within the 100 km square SN. Letters in round brackets and lithostratigraphical names given in bold text are the same as those used on the geological map.

Acknowledgements

This Sheet Explanation was compiled by D I Schofield, J R Davies and J Venus using information provided by the co-authors. M Williams and S G Molyneux provided biostratigraphical determinations. The text was edited by D Wilson (scientific) and J E Thomas (copy); figures were drawn by R J Demaine; page setting by A J Hill. The northern and eastern margins of the district were surveyed by J R Davies, D I Schofield, R A Waters and D Wilson in 1996 and between 2002 and 2004. The remainder of the district was surveyed in 2005 by J R Davies, N S Jones, A B Leslie, D I Schofield, J Venus, R A Waters and D Wilson as part of the GeoCymru Project, supported by a grant from the Welsh Assembly Government. The BGS gratefully acknowledges the co-operation of landowners in allowing access to their land, and the Forestry Commission for access to Brechfa Forest. R D Hillier and R A Waters contributed as Honorary Research Associates of the Geology Department, National Museum of Wales, Cardiff.

Geology of the Llandovery district (summary from rear cover)

An explanation of Sheet 212 (England And Wales) 1:50 000 Series map

(Rear cover)

This Sheet Explanation contains a brief description of the geology of the Llandovery district which includes rolling hills and uplands making up the southern margin of the Cambrian Mountains and the north-west facing approaches to the Black Mountains south of the broad floodplain of the Afon Tywi. As such, it provides a transect across a spectrum of geological formations that define the Lower Palaeozoic Welsh Basin, its transition into the adjacent shelfal area of the Midland Platform and continental Old Red Sandstone and overlying Carboniferous Limestone.

The bedrock geology predominantly comprises sedimentary rocks of Ordovician, Silurian, Devonian and Carboniferous age that were deposited at various times between around 470 and 330 million years ago. This survey has provided new insights into the dynamic relationships between the basin and platform areas and encompasses the classic type areas of both the Llandeilean Stage and Llandovery Series of the Silurian, the latter having been the subject of significant reappraisal. A brief summary of the lithological characteristics and environments of deposition for each unit is presented, along with a description of the relationships of these units within the district.

A long history of fault movement and localised folding in the district, recorded along the basin margin, culminated in an episode of regional folding, cleavage formation and low-grade metamorphism that took place during the latest Silurian and Early Devonian. A brief account of these movements and description of the resulting structures is presented.

Quaternary superficial deposits were mainly deposited during the last ice age, between approximately 26 000 and 14 500 years ago as a patchy veneer across the district. From around 10 000 years ago, fluvial deposits have accumulated in the valley of the Afon Tywi and its tributaries, and localised deposits of peat have accumulated in upland areas.

As well as summarising bedrock and superficial geology, this Sheet Explanation provides valuable information on the applied geological aspects of the district. These include mineral and water resources, potential geological hazards, engineering ground conditions and geological conservation, all of which are significant considerations for planning and development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology depicted on BGS 1:50 000 Series Sheet 212 Llandovery, published in 2008 as a Bedrock and Superficial Deposits edition.

The district lies in the county of Carmarthenshire with the main centres of population, Llandovery and Llandeilo, lying within the valley of the Afon (River) Tywi. To the north-west of the Tywi valley, the district comprises rolling hills and moorland that form the southern margin of the Cambrian Mountains, rising to a height of 408 m on Mynydd Llanybyther [SN 535 395]. The south-east of the district lies within the Brecon Beacons National Park, its north-west facing escarpments forming the flanks of the Black Mountains, with the highest point in the district, at 585 m AOD, on Carreg Yr Ogof [SN 777 214]. Sheep grazing and forestry plantation are widespread throughout much of the upland area, with rearing of dairy and beef cattle common on the lower ground. Bedrock has been widely exploited as a source of building stone in the past, notably the thick-bedded sandstone deposits exposed in the inactive quarry at Pen y Dinas [SN 627 356] which were used for construction of the Llyn Brianne dam. The gold mine at Dolaucothi [SN 626 335] in the north-east of the district, has a history of exploitation, probably from pre-Roman times, with the last phase of extraction ceasing in 1938. Shafts and adits in the vicinity of Cwm Brân [SN 755 280] include those of the Casara lead and silver mine.

The bedrock exposed at the surface within the district comprises a succession of Ordovician, Silurian, Devonian and Carboniferous sedimentary rocks deposited at various times between about 470 and 330 million years ago. Much of the district is underlain by a thick succession of Late Ordovician and Silurian deep-water turbiditic strata that form part of the fill of the Welsh Basin, an area of tectonic subsidence that persisted throughout much of the Lower Palaeozoic. Contemporaneous sediments of shallower water 'shelfal' aspect that occur in the south-east of the district were deposited on the margins of the basin, where it adjoined the Midland Platform, a relatively tectonically stable area that lay to the south-east. The basin margin was a tectonically complex region, where facies distributions were strongly influenced by movement along a series of north-east-trending faults that formed part of the Welsh Borderlands Fault System (Woodcock and Gibbons, 1988). During the late Silurian, the onset of a major, collisional plate-tectonic episode resulted in an influx of coarse sediment into the basin, which was rapidly infilled and subsequently uplifted. Uplift and shallowing led to the accumulation of nearshore, coastal plain and fluvial sediments of the Lower Old Red Sandstone that are preserved in the south-east of the district. Plate collision, during the late Early Devonian is marked by folding, cleavage formation and low-grade metamorphism throughout much of the district. The culmination of this event, termed the Acadian Orogeny (Soper and Hutton, 1984), is recorded in Wales by a major unconformity and the absence of Middle Devonian sediments. In the south-east of the district, the Lower Old Red Sandstone is unconformably overlain by post-orogenic continental red beds of the Late Devonian, Upper Old Red Sandstone. The youngest rocks of the district are predominantly limestones of early Carboniferous age that record renewed subsidence and marine transgression at the onset of the Variscan orogenic cycle. These were subject to late Carboniferous folding and faulting during the subsequent collisional phase of this tectonic episode.

The principal structural elements of the district are shown in (Figure 1). Shelf sequences, including those of the Llandovery World Stratotype, mainly lie within and immediately to the west of the Myddfai Steep Belt, a plexus of faults and steeply dipping strata that forms part of the Pontesford Lineament (Woodcock, 1984), a component of the Welsh Borderland Fault System (Woodcock and Gibbons, 1988). Further west, Ordovician strata lie within the Tywi Lineament (= Tywi Anticline; Jones, 1912), a complex fold and fault structure with a long history of movement, that broadly defines the transition between shelf and basin (George, 1963; Davies et al., 1997). The youngest Silurian strata in the extreme north-west of the district crop out within another long-lived structure, the Central Wales Lineament (Smith, 1987; the Central Wales Syncline of Jones, 1912) that exercised a major control on turbidite sedimentation during the late Silurian (Smith, 1987; Davies et al., 1997).

Much of the Ordovician and Silurian basinal succession comprises sandstones and mudstones deposited from rapidly moving submarine flows known as turbidity currents. Individual flow units are stacked in sequences (sandstone- or mudstone-dominated) that are commonly hundreds of metres in thickness, and each unit is usually capped by a thin hemipelagic mudstone which records the slow rain out of suspended, organic-rich material from the water column between each event. Of the latter, two distinctive facies have been recognised: oxic and anoxic. Anoxic hemipelagic mudstones are typically black, organic-rich, laminated and pyritic, and thinly interlayered with dark grey turbidite mudstone. The laminated hemipelagic beds often contain the fossilised remains of now extinct planktonic animals, known as graptolites. Preservation of this facies is favoured by low oxygen concentrations at the seabed–sediment interface, which inhibited burrowing organisms. In contrast, the oxic facies comprises pale grey and green thinly interbedded turbiditic and hemipelagic mudstones which commonly display burrow-mottling. This facies formed beneath bottom waters in which higher concentrations of oxygen allowed colonisation of the seabed by burrowing organisms and led to chemical changes in the sediment itself.

Anoxic facies deposits are thought to have formed during periods when a stratified water column inhibited circulation of oxygen to the sea floor. These conditions are associated with marine transgressions when sea level was rising. Conversely, oxic facies conditions are thought to reflect the unrestricted circulation of oxygenated sea-water associated with periods of falling sea level (marine regressions) (Davies et al., 1997). Thus, the alternation between anoxic and oxic facies sediments in the basinal succession reflects fluctuations in sea level. These effects are also recorded in the adjacent shelf successions where marine regressions resulted in emergence, erosion and the development of unconformities, whilst marine transgressions caused inundation and the expansion of deeper water facies belts.

Many of these transgressive and regressive events are recognised internationally, and were clearly the product of global (eustatic) sea-level movements. Principal among these was a major regression caused by a well-documented late Ashgill glacial episode, which caused global sea level to fall by as much as 100 m (Brenchley et al., 2006). Early Llandovery sequences in the district record the subsequent pulsed, post-glacial, sea-level rise. However, other facies changes are unique to the Welsh Basin and its margins, and these appear to record the influence of more localised tectonic effects.

Quaternary superficial deposits form a patchy mantle over the bedrock, and include Pleistocene glacial and periglacial sediments, Holocene fluvial sediments and peat. Around 26 000 years ago much of the district was covered in ice from glaciers that originated in the uplands of the Cambrian Mountains and the Brecon Beacons. These dramatically altered the landscape, scouring out and over-deepening river valleys and depositing till and ice contact deposits along the valley bottoms and sides. During melting of the glaciers, material eroded and deposited by the ice was reworked by glacial meltwaters and freeze–thaw periglacial processes to form many of the distinctive landforms preserved today. Deglaciation was complete by about 14 500 years ago, although cold, periglacial conditions persisted until about 10 000 years ago. Since then the present-day drainage system has established itself, further modifying the glacial geomorphology and forming wide spreads of alluvial deposits in the river valleys. Also, peat has developed locally in poorly drained upland parts of the district.

There has been a long history of geological research within the district, which was recognised from an early date as preserving some of the key stratigraphical sections of the British Lower Palaeozoic. The 'Llandeilo Flags' of the Tywi Valley were first identified by Murchison (1839). However, it was the six-inch survey of the Ammanford district to the south by Strahan et al. (1907) that defined the lithostratigraphical succession that was subsequently adopted by Williams (1953) in his seminal account of the geology of the Llandeilo area. The Ashgill to Llandovery shelf succession of the Llandovery region is well exposed and locally highly fossiliferous and, for these reasons, it has also been the focus for much research. Jones (1925; 1949), Williams (1951), Woollands (1970), Cocks et al. (1970; 1984), and references therein provide details of the rich shelly and sparse graptolitic faunas these strata contain. This work led to the selection of the upper part of the sequence as the global standard for the Llandovery Series (Cocks et al., 1984; Cocks, 1989). Two of the Llandovery stages, the Aeronian and Telychian, also have their international stratotypes in the area, although only the latter is located in the district.

Chapter 2 Geological description

Ordovician

Ordovician rocks crop out in a broad north-east-trending belt across the centre of the district (Figure 1). The central part of this crop, comprising rocks of Caradoc to Ashgill age, occupies the complex core of the Tywi Lineament. In the south of the district, an older Llanvirn succession crops out within the Garth–Llanwrtyd Fault Belt and the associated Carn Goch Anticline. The pre-Ashgill rocks within, and to the south of, the Tywi Lineament are predominantly of shelfal aspect, but embrace a range of relatively shallow to deep-water facies. The former characterised by a shelly fauna of brachiopods and sighted trilobites, the latter by graptolites and rare, sightless trilobites. The thick Ashgill basinal succession of the Tywi Lineament was accommodated by subsidence within the basin; to the north-west it passes up into basinal Llandovery rocks, but to the south-east, rapid lateral and vertical facies changes into shallower, shelfal deposits of both Ashgill and Llandovery age occur. In the context of such complexity, it is convenient for descriptive purposes to group the older Ordovician (Llanvirn to Caradoc) sequence with the early Ashgill Sholeshook Limestone, and deal with the basinal Ashgill succession separately. Ashgill shelf facies are described with their contiguous Llandovery facies.

Llanvirn to Caradoc succession

The oldest unit in the district, the Abergwilli Formation (Ab), exceeds 520 m in thickness and was formerly known as Didymograptus bifidus Beds after the abundance of characteristic tuning-fork shaped graptolites at certain horizons (Strahan et al., 1909). It principally crops out in the core of a complex, folded and fault-bounded structure, the Carn Goch Anticline, which lies south-east of the Afon Tywi, between the Afon Sawde near Llangadog and Llandeilo Bridge [SN 713 263] – [SN 628 220]. The formation is of lower Llanvirn, Abereiddian age (c.f. Williams, 1953), and comprises brown-weathering, black mudstone interlayered with units of micaceous, dark grey, burrow-mottled mudstone and laminated hemipelagic mudstone in the upper part of the succession. The mudstones were deposited in a distal shelf setting under dysaerobic conditions, but where periodic oxygenation allowed communities of burrowing benthos to become established. The formation also contains scattered thin to thick, structureless beds of fine-grained sandstone, fine to coarse-grained tuff and tuffaceous sandstone containing abundant angular clasts of feldspar and mafic minerals (Williams, 1953).

The Abergwilli Formation passes into, and is also overlain by the Ffairfâch Grit Formation (FG), a sandstone-dominated succession of Abereiddian to Llandeilian age (Williams, 1953; Bergström et al., 1987). The greatest thickness of this unit (up to 130 m) is exposed within a faulted anticlinal fold closure in the Carn Goch area [SN 690 243], whereas to the south-west, around Dinefwr Park [SN 614 225] and Aberglasney [SN 583 222], a much thinner sequence (up to 15 m) is preserved.

The formation was originally included within the 'Llandeilo Flags', one of the major divisions of the Silurian System of Murchison (1839), subsequently divided into a lower 'Ffairfâch Grit' and upper 'Llandeilo Flags and Limestones' of the Ordovician 'Llandeilo Series' (Strahan et al., 1907). A detailed account of the stratigraphy of the Llandeilo area published by Williams (1953) showed the Ffairfâch Grit to comprise five informal units, later regarded as formations within a 'Ffairfâch Group' (Williams et al., 1981). Although the stratigraphy of Williams (1953) can be recognised locally (e.g. Rushton et al., 1999), it has proved unsustainable elsewhere in the district as a result of rapid lateral facies changes, and has been regarded as a formation during the present study.

The Ffairfâch Grit Formation comprises pale grey, thin- to thick-bedded, medium- to coarse-grained arkosic sandstone and pebbly sandstone with local, crude normal grading and poorly developed cross-stratification. The formation also includes massive, structureless, medium to coarse-grained, arkosic sandstone with irregular, probably diagenetic, 'pseudobedding' which imparts a flaggy appearance to the rock (Plate 1). The grits were probably deposited as turbidites or hyperconcentrated flow deposits, and contain a sparse reworked trilobite and brachiopod fauna (Williams et al., 1981) indicating that they were sourced from an adjacent shelf setting that may also have derived feldspar from a locally emergent volcanic edifice. However, the Ffairfâch Grit is contiguous both vertically and laterally with shelf facies (Abergwilli Formation and Llandeilo Flags Formation), and this stratigraphical context suggests that the formation represents an intrashelf accumulation of resedimented material, possibly confined within a fault generated hollow.

The Coed Duon Volcanic Formation comprises 60 m of green, fine-grained felsic tuff (Z) with abundant mafic crystal and rhyolitic lithic clasts, overlain by around 15 m of white weathering, fine-grained, pink rhyolite (R^R). It is a lateral correlative of the Ffairfâch Grit, representing the product of eruptions from a local volcanic centre located around Coed Duon [SB 710 255], active briefly during the period of upper Ffairfâch Grit Formation deposition.

The succeeding Llandeilo Flags Formation (LIF) records marine regression and deposition in a more proximal shelf or shoreface environment during the Llandeilean (late Llanvirn) and earliest Caradoc. The formation has been estimated at around 850 m (Williams, 1953) or 716 m (Wilcox and Lockley, 1981) in thickness from the partial type section in the Cennen Valley. In Dinefwr Park, it is up to 240 m thick and is conformably overlain by mudstones of the Drefach Group. South of the Afon Tywi, the Llandeilo Flags Formation occupies the flanks of a complexly faulted anticline, along which it overlies both the Ffairfâch Grit and Coed Duon Volcanic formations; in this area the formation is unconformably overlain by strata of Llandovery age.

Much of the Llandeilo Flags Formation comprises buff-weathering, grey, fine- to medium-grained, hummocky cross-stratified sandstone, with interbedded limestone, and wave rippled and bioturbated silty sandstone and siltstone. These lithologies are interpreted as having been deposited in a storm influenced environment. The sandstone and limestone beds are either sheet-like or lenticular, varying in thickness from a few centimetres to up to about 40 cm, commonly with sharp, erosive bases locally marked by lags of brachiopod and trilobite fragments and mudstone clasts. Wave-ripple cross-lamination, formed during fair weather conditions, is a common feature of the finer-grained sandstones, but is locally disrupted by bioturbation in the upper parts of beds. At some localities (e.g. Aberglasney Gardens [SN 582 221]), beds of matrix-supported conglomerate, deposited from dilute debris flows, and associated lenses and discontinuous beds of hummocky crossstratified sandstone with strongly scoured bases, reveal the influence of occasional larger storm events.

Towards the eastern part of its crop, the formation comprises diffusely bioturbated, thinly interbedded, calcareous mudstone, siltstone and limestone. In this area, storm event beds are not well recognised in the upper part of the formation (e.g. between Pontbren Araeth [SN 659 236] and the hinge of the Carn Goch Anticline [SN 712 264]–[SN 714 260]), suggesting that deposition took place in a more distal setting.

Around Dinefwr Park and Aberglasney, the Llandeilo Flags are overlain by the Drefach Group (Dr), a sequence of proximal and distal shelf mudstones of Llandeilean age that possibly ranges into the earliest Ashgill (Zalasiewicz et al., 1995). In the adjacent Carmarthen and Newcastle Emlyn districts (Strahan et al., 1909; Wilby et al., 2007) the Drefach Group overlies the Abergwilli Formation and is in part coeval with the Ffairfâch Grit and Llandeilo Flags formations. The lowest division of the group preserved in the district is the Hendre Shales Formation (HS) comprising approximately 100 m of silt-laminated mudstones with laminated and burrowed hemipelagic mudstone and scattered thin beds of calcareous sandstone and limestone. The uppermost division is the Caradoc to earliest Ashgill Mydrim Shales Formation (MS) which is up to approximately 200 m in thickness, but thins to around 35 m at Dinefwr Park. It is a distinctive division, deposited in a low energy, disaerobic offshore setting (Fortey, 2006), comprising fissile, black, pyritic hemipelagic mudstones containing abundant graptolite fragments and rare, deep-water, bottom-dwelling trilobites. The mudstones are locally interlayered with thin felsic tuff beds, the distal products of widespread Caradoc volcanic eruptions in the Welsh Basin (Kokelaar et al., 1984).

In the south of the district, the Mydrim Limestone Formation (ML) lies between the Mydrim Shales and underlying Hendre Shales but is absent elsewhere in the district. It comprises about 15 m of flaggy, thin-bedded calcareous sandstones and limestones deposited under shallow marine conditions that locally prevailed in this area at this time.

The Sholeshook Limestone Formation (SL) in places overlies the Mydrim Shales Formation and comprises a discontinuous unit of early Ashgill age that records the renewed influence of shallow marine conditions (Zalasiewicz et al., 1995). The principal exposures of the formation are at Crûg Farm [SN 626 230] and Birdshill Quarry [SN 601 231]. At these localities the formation ranges up to 65 m in thickness and comprises pale grey, thin to thickly bedded, coarsely crystalline limestone rich in echinoderm and brachiopod debris.

Ashgill basinal succession

Basinal Ashgill rocks underlie much of the central part of the district, along the southernmost extension of the Tywi Lineament. The basinal succession rests conformably upon the Mydrim Shales and, in the south of the district, on localised developments of the Sholeshook Limestone. It records a change to predominantly oxic bottom conditions at the beginning of the Ashgill that prevailed throughout much of the Welsh Basin until the onset of the latest Ordovician postglacial marine transgression. The lower Ashgill Nantmel Mudstones Formation (Ntm) comprises a slope apron succession, around 1300 m in thickness, of pale to medium grey, thin turbidite mudstones, interbedded with burrowed hemipelagic mudstones. Several units of dark grey, anoxic, laminated hemipelagic mudstones interbedded with turbidite mudstones occur in the upper part of the formation and scattered lenticular beds of thin- to thick-bedded sandstone turbidites (sa) are preserved near the top. Where exposed in the Tywi valley, the upper part of Nantmel Mudstones commonly comprise destratified, 'disturbed beds' (db), which pass into shelfal strata of late Ashgill age (see below). An extensive lens of coarse clastic turbidite deposits, the Taliaris Formation (Tal), crops out in the central part of the district, in a series of major folds within the upper part of the Nantmel Mudstones succession. This formation ranges up to 370 m in thickness and comprises thin to medium-bedded, fine- to coarse-grained turbidite sandstones and debritic units of pebbly mudstone and shelly conglomerate (cg) (Plate 2). The formation lies adjacent to a splay of the Llanwrtyd Fault and is interpreted as one of a series of coarse clastic bodies (which includes the Bryn Nicol Formation and Doldowlod Conglomerate Formation of the adjacent Builth Wells district (Schofield et al., 2004), that were deposited in small fault-controlled depressions during a period of localised mid Ashgill tectonism.

The upper Ashgill Yr Allt Formation (YA) records the onset of the widespread late Ordovician (Hirnantian) glacioeustatic regression, which was marked by increased sediment supply from emergent shelfal areas in the south-east of the district. The formation comprises more than 1250 m of typically silty oxic slope apron turbidite mudstones, with locally interbedded siltstone laminae. Units of thin- to thick-bedded, fine-grained turbidite sandstone (sa) are scattered throughout the Yr Allt Formation. The most prominent of these, comprising up to 190 m of thickly bedded sandstones, crops out in a disused quarry at Pen y Dinas near Llansawel [SN 627 356] in the core of the Cothi Anticline, and was first described by Drew and Slater (1910) in their early study of the district. Very low levels of bioturbation in the Yr Allt Formation might reflect the increased rates of sediment supply. Increased sedimentation rates also led to rapid slope build-up, instability and failure, giving rise to units of slumped and destratified mudstone and sandstone, particularly in the lower part of the formation. Such units of 'disturbed beds' (db) have only been distinguished south-east of the Garth–Llanwtyd Fault Belt, where they form part of a transition into contemporary shelfal facies of the Ciliau Formation (see below). In the Tywi Lineament, the uppermost part of the Yr Allt Formation comprises a distinctive sequence of noticeably finer-grained, faintly laminated, dark grey mudstones.

Silurian

Late Hirnantian to early Telychian slope-apron succession

Deep-water basinal rocks of late Hirnantian to Llandovery age crop out in the north-west part of the district (Figure 1). The lower part of the succession is predominantly an easterly-derived, mudstone-dominated slope apron sequence that overlies the Yr Allt Formation and passes up into a southerly-derived sandstone lobe succession that commenced during the middle Telychian (Davies et al. 1997).

The Cwmere Formation (CeF) records deposition during a period of postglacial marine transgression. It comprises an anoxic facies up to 400 m thick, of turbidite mudstones and interbedded laminated hemipelagic mudstones, scattered thin siltstones and thin beds of fine grained turbidite sandstone. At the base of the formation is the Mottled Mudstone Member (MMb) (around 10 m thick), a unit of strongly burrowed pale grey, oxic turbidite mudstones and burrowed hemipelagic mudstones, which overlies the dark grey mudstones at the top of the Yr Allt Formation. Elsewhere in the region, the member has yielded late Hirnantian, persculptus Biozone graptolites, with the overlying anoxic mudstones ranging into the early Aeronian (Davies et al. 1997). Two units of thickly bedded to massive conglomerate (cg), each ranging up to around 85 m in thickness, are interbedded with the anoxic mudstones of the Cwmere Formation. The lower, and more extensive of these bodies, occurs at the base of the formation and crops out in a series of fold closures around Banc Bwlchdrebannau [SN 718 371]. This division is synonymous with the Bwlch Trebanau Conglomerate of Sedgwick (1854) who described a shelly fauna from the unit of late Ashgill to early Llandovery age, indicating derivation from an adjacent shelf area. These units make up part of a series of discrete sandstone and conglomerate bodies within this area, the youngest of which is mid Llandovery in age. Together they define a corridor in the contemporary slope apron (Cwmere Formation and Claerwen Group), across which coarse clastic turbidites were routed to the more distal parts of the basin.

The Claerwen Group (Cla) comprises up to 660 m of thin-bedded, pale greenish grey, oxic turbidite mudstones and burrowed hemipelagic mudstones that record deposition on the slope apron during a period dominated by marine regression which lasted until middle Telychian times (Davies et al. 1997). Subordinate thin beds of dark grey, anoxic mudstone that occur throughout the succession indicate brief transgressions that were either of local extent or glacioeustatically driven. One such anoxic unit, the Monograptus sedgwickii Shales, which has been recorded in adjacent districts (e.g. Schofield et al., 2004), correlates with a worldwide marine transgression (Johnson et al. 1991) and equates with a major sequence boundary in the shelf areas to the south-east. It allows the Claerwen Group elsewhere to be divided into the Derwenlas and overlying Rhayader Mudstones formations.

In the north-east of the crop, the lower part of the Claerwen Group contains several major lenses of thickly-bedded coarse sandstone and conglomerate (cg) lying within the upper part of the aforementioned coarse clastic corridor. The most prominent of these is exposed on the hilltops at Pigyn Shon Nicholas [SN 667 353], where thick beds of turbidite sandstone immediately above the base of the Claerwen Group are interbedded with thin, discontinuous beds of burrow-mottled mudstones.

The Devil's Bridge Formation (DBF), which diachronously succeeds the Claerwen Group, crops out in the extreme north-west of the district, on the north-western side of the Central Wales Lineament. The formation consists of up to 300 m of thinly interbedded turbidite sandstones and mudstones of early Telychian age (turriculatus s.l. Biozone), with rare flute casts that indicate derivation from the south-east. The earliest parts of the formation appear to have been restricted to a narrow corridor north of the district, whereas subsequent deposition was more widespread (Davies et al., 1997); this probably reflects rejuvenation of source areas during Telychian tectonism (Soper and Woodcock, 1990).

Telychian sandstone-lobe succession

During the mid Telychian, the accumulation of muddy slope-apron turbidite sequences gave way to a series of southerly-derived, sandstone-dominated, turbidite lobe systems that developed in response to contemporary tectonic uplift of source areas south of the basin. In mid Wales these deposits are represented by the Cwmystwyth Grits Group and its muddy fringing facies, the Blaen Myherin Mudstones and Caerau Mudstones formations that crop out west and east of the Central Wales Lineament respectively.

In the district, the muddy fringing facies are confined to the Caerau Mudstone Formation (CaM), which crops out east of the Central Wales Lineament between the Claerwen Group and overlying Cwmystwth Grits Group. The Caerau Mudstone Formation however, is restricted to the north-east of the district, for in more proximal settings to the south-west, the Cwmystwth Grits Group rests directly on the Claerwen Group. The formation comprises up to 450 m of alternations of thick- and thin-bedded oxic and anoxic turbidite mudstones and hemipelagites containing a late turriculatus Biozone (s.l.) age fauna (Davies et al., 2007). The base of the Caerau Mudstones is taken at the incoming of medium- to thick-bedded turbidite mudstones with laminated hemipelagites. Although the Teifi Fault downthrows the Cwmystwyth Grits Group against the Devil's Bridge Formation in the area to the north-west of the Central Wales Lineament, it is postulated that muddy fringing facies (Blaen Myherin Mudstone Formation) are absent at subcrop in this relatively proximal part of the system.

Only the lower part of the Cwmystwyth Grits Group crops out within the district, where it comprises two distinctive, interdigitating facies, the Glanyrafon Formation (Glr) and Rhuddnant Grits Formation (Rdd). Higher parts of the group (Pysgotwr Grits Formation; Davies, 1933) and lateral equivalents of the Rhuddnant Grits (Doethie Formation; Schofield et al., 2004) occur in adjacent districts. Both formations consist of thinly interbedded laminated and cross-laminated turbidite sandstones and mudstones, deposited from southerly sourced turbidite lobes, but the Rhuddnant Grits Formation, greater than 900 m thick, additionally includes packets of medium to thick, tabular beds of massive, feldspathic muddy sandstone (high-matrix sandstone). The latter are interpreted as deposits of fast moving, sediment gravity flows that varied between slurry-like debris flows and high-concentration turbidity currents (Davies et al., 1997). Within the Central Wales Lineament, the lower part of the Rhuddnant Grits Formation includes the Llyn Teifi Member (LyT). The member is around 600 m thick and characterised by the presence of abundant medium to very thick beds of coarse-grained, high-matrix sandstone, commonly amalgamated, and locally graded with granules and small pebbles at the bases of some beds. South-east of the Craig Twrch Fault, the Rhuddnant Grits Formation crops out between tongues of the Glanyrafon Formation, with which it laterally interdigitates. East of the main Rhuddnant Grits outcrop an expanded sequence of Glanyrafon Formation, overlying the Claerwen Group, ranges up to 800 m in thickness.

Mid Ashgill to Ludlow shelf succession

To the south of the Tywi Lineament, basinal Ashgill and Llandovery rocks pass into their coeval shelf facies, an assemblage of shelly and locally graptolitic sandstones and mudstones. The outcrop of these rocks, located to the east of Llandovery town, includes the eponymous international type area for the lowest Silurian series (Figure 1). The Afon Sawdde to the south of Llangadog reveals exposures in a separate outcrop of Llandovery rocks. The succeeding Wenlock and Ludlow sequence occupies a steeply dipping belt of strata, locally known as the Myddfai Steep Belt, which traverses the district from the Myddfai area in the north-east, to the vicinity of Ffairfâch in the south. The shelf succession is characterised by rapid lateral changes in facies and thickness, consistent with its accumulation along the rapidly subsiding fault-controlled south-eastern margin of the Lower Palaeozoic Welsh Basin. The vertical succession of lithostratigraphical divisions, however, strongly reflects the influence of contemporary movements in sea level, and associated climatic and oceanographic effects many of which, including the end Ordovcian glaciation, appear to have been of global significance. Deposition during the latest Hirnantian and Llandovery records the pulsed nature of the subsequent glacioeustatic transgression that followed glaciation (Davies et al., 2009). Further movements in marine base level are recorded by the three major progradational sequences revealed in Wenlock and Ludlow facies.

Ashgill to Llandovery

The long history of research on the shelfal rocks of the Llandovery area is reflected in the various, and sometimes contradictory stratigraphical schemes previously presented for the region. The current BGS survey of the Llandovery sheet, following that of the adjacent Builth Wells and Brecon districts (Schofield et al., 2004; Barclay et al., 2005), has provided an opportunity to critically assess and rationalise the previously used nomenclature for the whole of the Llandovery type area. Because of the international relevance of the sequence, existing names have been retained wherever possible, but the detailed nature of the BGS mapping, allied to new fossil discoveries, has necessitated significant changes to some divisions. The relationship between the new BGS scheme and previous nomenclature is shown in (Figure 2). Shelly faunas, particularly in the late Llandovery rocks, have also allowed the recognition of brachiopod 'communities' that are believed to record differences in water depth during deposition (Ziegler, 1965; Ziegler et al., 1968a and b). Cocks et al., (1984) interpreted the presence of the Clorinda, Sticklandia and Pentamerus communities to infer changes from deeper to shallower conditions. However, the shallowest communities (Eocoelia and Lingula) are not reported from the local succession.

Within the Llandovery outcrop marked lateral facies changes are observed. These are most profound in the south of the area, in the tract of ground between Cilgwyn Wood [SN 753 297] and Mandinam [SN 736 281], and it is convenient to separate the descriptions of this sequence and the relationships of this region from the rest, but to include them with those seen in the Afon Sawdde near Rhyd-y-saint [SN 715 259].

At the base of the mid Ashgill to Ludlow shelf succession, the upper parts of the basinal Nantmel Formation pass upwards and laterally southwards into the sandy mudstones of the Cribarth Formation (Cri). This unit is interpreted as a progradational division, up to 300 m thick, of thoroughly bioturbated, sandy mudstones with shelly fossils of Rawtheyan age. A unit of pebble and granule-rich sandstone and conglomerate (cg) is locally present near the base of the formation. In divisions above the Cribarth Formation, rapid lateral changes in facies record the influence of the Hirnantian glacioeustatic regression. The Yr Allt Formation of the Ashgill basinal succession passes south-eastwards into the Ciliau Formation (CF), a sequence of weakly burrowed, silty mudstones and calcareous sandstones ranging up to 400 m in thickness. Several levels of slumped and destratified disturbed beds (db) are associated with this facies transition. In detail the mapped relationships between the two formations appear to highlight at least two separate progradational events. The lower event, exposed in the vicinity of both Ystradwalter [SN 785 360] and Glasallt-fawr [SN 735 302], is capped by a thin packet of thick-bedded, locally pebbly sandstone (sa) with dewatering features. At the top of the upper event is the more widely recognised Cwmcringlyn Formation (CgF), a 60 m thick sequence of thin-bedded, lenticular and wave-ripple cross-laminated sandstones, well exposed below Llandovery Castle [SN 767 342], which records deposition during the glacial maximum and the time of greatest sea-level fall.

Evidence of widespread emergence and associated erosion during this period is preserved south-east of Glasallt-fawr. Beneath a disconformity which increases in magnitude eastwards, the divisions of the local Hirnantian sequence are successively lost so that, in the Cilgwyn area [SN 745 300], rocks deposited during the postglacial transgression (described below) come to rest on the Cribarth Formation and, just 1.5 km to the south, near Lletyrhyddod [SN 745 285], they overlie the pebbly sandstone unit recognised in the lower part of this formation. Moreover, abrupt changes in the level of the disconformity across some of the faults suggest that these structures existed and were active during the Hirnantian. Comparable relationships were observed in the adjacent Builth Wells district to the north (Schofield et al., 2004).

The basal division of the overlying late Hirnantian glacioeustatic transgressive sequence is represented by the Glassalltfawr Sandstone Formation (Gll). This formation comprises thick-bedded, planar-, wave-rippled and cross-laminated, locally conglomeratic sandstone, 23 m thick in its type area. It dies out to the north and east. The succeeding Garth House Formation (GHF) comprises a sequence up to 85 m thick, of thinly interbedded smooth grey mudstones and wave-rippled sandstones. Both these divisions were included by and Woodcock and Smallwood (1987) in the upper part of their regressive Scrach Formation, but the transgressive nature of these rocks is clear and necessitates abandonment of the earlier name ((Figure 2); Davies et al., 2009). Units of disturbed beds (db) are present at several levels in the Garth House Formation, and thicker, locally mappable examples occur towards the middle and at the top. The close association of these disturbed strata with wave-rippled sandstones is indicative of shallow water conditions, and suggests that the former may be a product of in situ, seismically triggered liquefaction and dewatering rather than down-slope slumping. Most bedding surfaces in the Garth House Formation display an array of delicate trace fossils, but overall levels of bioturbation are extremely low. This contrasts markedly with succeeding facies included in the Chwefri and Goleugoed formations.

The Chwefri Formation (ChF' and ChF'') comprises a sequence, ranging up to 1 km in thickness, of grey, colour-banded and burrow-mottled silty mudstones with common siltstone laminae and scattered thin sandstone beds. A distinctive unit of thoroughly bioturbated sandy mudstones and muddy sandstones with scattered coarse sand grains, granules and shells, the Ystradwalter Member (Yst), is locally present at its base. The strata immediately above this member contain persculptus Biozone graptolites (Davies et al., 2009). In the south, the Chwefri Formation passes laterally into the Goleugoed Formation (Gol) which comprises bioturbated and shelly muddy sandstones with common thin, only partially burrowed, tabular sandstone beds. From a maximum of 550 m in its type area [SN 750 305], this sandy division thins south-westwards towards the termination of its crop at Mandinam [SN 736 281]. The abrupt transition from the sparsely burrowed facies of the Garth House Formation into the succeeding bioturbated and locally fossiliferous units (Chwefri and Goleugoed formations) marks a fundamental, widespread event; a similar facies change also occurs in equivalent basinal sequences (Davies et al. 1997).

The Chwefri and Goleugoed formations, together with the succeeding Llandovery divisions, record a series of progradational events in which mud-prone facies pass both laterally and vertically upwards into more sand-prone units. Typically, changes in the level of bioturbation record upward gradations from thinner-bedded and weakly disrupted facies in which separate beds and laminae of sandstone, siltstone and mudstone are preserved, into thicker-bedded facies in which primary sand, silt and mud layers have been thoroughly mixed together to produce homogenous sandy mudstones or muddy sandstones. The progradational sequences which make up the Llandovery succession have previously been viewed as deltaic in origin (Woollands, 1970; Cocks et al., 1984), even though sedimentary structures to support this are noticeably lacking. Moreover, the fossiliferous nature of the strata and the varying levels of benthic activity therein are more consistent with shelfal deposition. The most bioturbated facies compare with those recorded in the 'transition zone', located between the coastal shoreface and distal shelf in modern shelf seas (Reineck and Singh, 1975). Along low-energy coastlines the upper limit of this zone can occur in water depths as shallow as 2 m, while its lower limit can extend to depths of 30 m. A scarcity of tractional bedforms within the sand-prone Llandovery facies indicates that true shoreface facies are poorly represented. Therefore, the pro-gradational sequences within the district appear to record the repeated advance of transition zone facies across deeper, more distal and less bioturbated shelf facies.

In this context, the Chwefri Formation represents a distal shelf facies. The intertonguing sand-prone facies die out northwards towards an area centred on Dolau Gwynion [SN 795 355] where the thickest, uninterrupted Chwefri Formation sequence is developed. This area, therefore, is considered to mark the site of deepest and most distal deposition within the type Llandovery area, and was also a site of sea-bed instability, as recorded by the presence of extensive units of slumped and disturbed strata (db). In this area also, a thin sequence of sandy facies (Trefawr Formation, see below) separates an approximately 800 m thick lower portion of the Chwefri Formation (ChF') from a thinner (140 m) upper leaf (ChF'').

Within the Llandovery area, the distribution of the Chwefri and Goleugoed formations records an initial major progradational event, identical to that observed within the Chwefri, Bronydd and Crychan formations in the Brecon and Builth Wells districts to the north-east (Schofield et al., 2004; Barclay et al., 2005). Dating in the latter areas show that this event spanned the Ordovician–Silurian boundary, lasting from persculptus to early cyphus Biozone times (Cocks et al., 1984). This progradational sequence developed during the period of rising post-glacial sea level, and must therefore record high levels of sediment supply, and perhaps tectonic rejuvenation of clastic source areas (Davies et al., 1997). In the district, there followed a progressive south-westwards contraction of the sandy facies, prior to a renewed advance that was initiated during the upper part of the cyphus Biozone and persisted into convolutus Biozone times. This latter progradation coincided with a late Aeronian eustatic regression, and is represented by a lateral and vertical passage from the Chwefri Formation into the friable, intensely bioturbated sandy mudstones of the Trefawr Formation (Trf), a division which ranges up to 250 m in thickness. This in turn passes into the tough, burrow-mottled muddy sandstones which form the lower leaves of the Cefngarreg Sandstone Formation (Ceg). Both these divisions were previously recognised further north (Cocks et al., 1984; Barclay et al., 2005), but are newly applied to the Llandovery district (Figure 2), the Cefngarreg Sandstone broadly equating with the Rhydings Formation of Cocks et al. (1984). However, these two formations can be shown to comprise intergradational and interleaving facies — one dominated by tough, burrow-mottled, muddy sandstones, and the other by green, sandy mudstones. The latter facies dominates only in the type area of the Rhydings Formation (Rdg) (between Allt Rhydings [SN 782 334] and the Cefn Cerig Road [SN 775 327]) where it forms a sequence up to 150 m thick; the name is now restricted to the area of outcrop of this sequence. The feature-forming sandstone facies is included in the separate Cefngarreg Sandstone Formation, which ranges up to 375 m in thickness.

South of the Cefn Cerig Road, two thin sequences of green, sandy mudstone with thin beds and laminae of sandstone included in the newly defined Rhydings Formation, form well-defined features (slacks) and sub-divide the upper part of the Cefngarreg Sandstone sequence into separate units. Each of these appears to record a separate progradation of sandy over more muddy facies. The sandy facies, when traced to the north, persists as discrete leaves within an expanded Rhydings Formation crop. It is these units that were formerly selected for quarrying in Allt Rhydings. The sandstones die out to the north, and it is the equivalent levels of the Rhydings Formation which pass laterally into the upper Chwefri Formation (ChF'', see above), north of Allt Glynrhoddod [SN 790 342]. Cocks et al. (1984) regarded the whole of their Rhydings Formation as of sedgwickii Biozone age, but it is now suspected that it also includes strata belonging to the older convolutus Biozone. It is also unclear if, or how, these events correlate with comparable divisions in the Cefngarreg Formation type area to the north-east (Barclay et al., 2005). What does seem clear, however, is that the uppermost leaf of the Cefngarreg Sandstone in the adjacent Brecon district is equivalent to strata ascribed to the Wormwood Formation (Wow) by Cocks et al. (1984). This highly fossiliferous division is up to 100 m thick and, in addition to brachiopods, contains both solitary and colonial corals, bryozoa, trilobites, orthocones and bivalves. The presence of thinly interbedded silty and sandy mudstones and tabular shelly sandstone beds makes it a lithologically distinctive formation, but it also contains thick units of burrow-mottled sandstone that are otherwise identical to the main Cefngarreg Sandstone facies. Fine-grained facies at the base of the Wormwood Formation record an initial deepening prior to yet another progradation of sand-prone facies. Stricklandia overlain by Pentamerus brachiopod community assemlages appear to confirm a 'progressive shallowing' (Cocks et al., 1984), although the presence of several minor thickening- and coarsening-upwards sequences suggests a more complex event. North of Gwernfelin [SN 793 345], the Wormwood Formation passes into a unit of disturbed beds (db) representing a major slump horizon, contiguous with that recognised in the Brecon district (Barclay et al., 2005). Evidence for further synsedimentary disturbance at this level is also seen at Llwynwormwood [SN 769 316], where sandy mudstones with blocks of sandstone and shelly limestone record the filling of a shallow channel or slump scar by debrites.

The Wormwood Formation is abruptly overlain by the green, burrow-mottled mudstones with widely scattered thin sandstone beds of the Cerig Formation (Cer). This transition marks a further deepening, reflected in the presence of Clorinda community brachiopod assemblages. The Cerig Formation is estimated to be 750 m thick in the adjacent Brecon district (Barclay et al., 2005), but the local crop is disrupted by strike-parallel faulting. A sequence with abundant interbedded sandstones, the Mwmffri Sandstone Member (MfS), can be traced from the eastern edge of the district southwards to Penlantelych [SN 787 333], but here it is faulted out and from this point southwards it is likely that much of the Cerig Formation sequence has been lost. Evidence of synsedimentary disturbance within the formation is widespread.

In the south of the main Llandovery crop, and in the Afon Sawdde, marked changes in the Llandovery stratigraphy are apparent. In Cilgwyn Wood [SN 745 295] the thin sequence of Trefawr Formation mudstone which underlies the Cefngarreg Sandstone further north, pinches out so that the sandstone unit comes to rest directly on the underlying Goleugoed Formation. To the south of this point, both formations undergo a progressive, but dramatic attenuation. The sequence is much affected by faulting, but it is clear that at some point between Cilgwyn Wood and the southern limit of the main Llandovery outcrop at Mandinam [SN 736 281], the Cefngarreg Sandstone (or one or more levels within it) begins to overstep the underlying beds. It thins concomitantly beneath a strongly transgressive Wormwood Formation, which ultimately comes to rest upon the Garth House Formation and underlying Rawtheyan facies. These relationships are revealed in exposures in the vicinity of Mandinam, where the Wormwood Formation can be observed to overstep first the Cefngarreg Sandstone and then the Goleugoed Formation. The strongly unconformable nature of the contact provides clear evidence that the earlier Llandovery strata had suffered uplift, and probably fault-related deformation, prior to their erosion and subsequent overstep.

Further evidence of overstep is seen in a tributary of the Afon Sawdde [SN 714 259] near Rhyd-y-saint, 3 km to the south-west. Here, a thin (2–3 m) Wormwood Formation sequence, comprising sandy mudstones and muddy sandstones with scattered granules, small pebbles and pentamerid brachiopod valves, rests on the mid Ordovician Llandeilo Flags. At this locality the unconformity is of compound origin, representing the product of two or more phases of intra-Aeronian erosion prior to the Wormwood transgression, but also intra-Hirnantian denudadation during the glacioeustatic low stand. Ziegler (1966) reports Eocoelia hemispherica from this locality. At Mandinam and in the Afon Sawdde, the local Wormwood Formation sequences are succeeded by green Cerig Formation facies. At Mandinam, faulting also disrupts the succession, but contacts with overlying Wenlock rocks appear conformable. The Afon Sawdde provides a largely intact section through the local Cerig Formation, but includes an overlying sequence, up to 20 m thick, of dark grey laminated hemipelagic mudstones interbedded with burrow-mottled facies, thin shelly sandstones and limestones, which is included in the Dolfawr Mudstones Formation (Dol). In the Builth Wells district this latter formation straddles the Llandovery–Wenlock boundary and, as in this area, intervenes between the Cerig and Builth Mudstones formations (Schofield et al., 2004).

Wenlock to Ludlow

This sequence of rocks is exposed in the gorge section of the Afon Sawdde, and has been the focus of considerable past research (Straw, 1929; Williams, 1953; Potter and Price, 1965; Bassett, 1974; Hurst et al., 1978; David Siveter, 2000; Derek Siveter, 2000). The succession within the Myddfai Steep Belt includes earliest Pridoli rocks, and comprises three major progradational sequences (Schofield et al., 2004; Barclay et al., 2005). The first spans the Wenlock, the second occupies the early Ludlow, and the third spans the late Ludlow to earliest Pridoli. Evidence of transgressive deepening is present at the base of each sequence. These progradational events are revealed as south-west (proximal) to north-east (distal) facies changes along the outcrop, although the actual direction of progradation was possibly oblique to this belt. The shoreline is assumed to have lain to the south of the district. The earliest Pridoli facies, which cap the third progradation, are historically included in, and here described with, the Lower Old Red Sandstone (see below).

The Wenlock progradation is represented by progressive vertical and lateral facies changes from the deeper water muddier facies of the Builth Mudstones Formation into the successively shallower and increasingly sandy Tirabad, Sawdde Sandstone and Ffinnant Sandstone formations. The vertical succession of facies describes an overall, coarsening- and thickening-upwards; however, the upper, sandstone-dominated formations display additional, smaller scale, thickening- and coarsening-upwards units. These were the product of second order progradations and intervening deepening episodes superimposed on the regional shoaling event. Wenlock strata are only observed resting conformably on the Dolfawr Mudstones in the Afon Sawdde [SN 7174 2563]. To the north-east, within the Llandrindod–Pen-y-waun Fault Belt, the contact with Llandovery rocks is marked either by a fault or sedimentary slide. West of Mandinam the basal strata, of early to mid Wenlock age, comprise about 190 m of fissile, brown-weathering, laminated anoxic hemipelagites known as the Builth Mudstones Formation (BMd). The widespread introduction of anoxic bottom waters in the Welsh Basin, and elsewhere, during the early Wenlock was a response either to a deepening event or to changes in climate and marine circulation (Davies et al., 1997). However, the presence of micro-burrows and scattered phosphate nodules, together with the lack of abundant graptolite faunas, suggests that the mudstones in the district were deposited under bottom conditions that were not as anoxic as in other areas where graptolites are abundant (e.g. the Builth Wells district; Schofield et al., 2004). Units of slumped and destratified strata and thin debrites are locally present in the formation. Packets of bioturbated mudstone with beds of decalcified limestone and a brachiopod and trilobite fauna are present in a faulted outlier of Builth Mudstones near Bethlehem [SN 689 258]. These shallower and more oxic facies suggest that, during the early to mid Wenlock, a local high existed in this area. To the north-east of Mandinam, sequences of Builth Mudstones juxtaposed with the Cerig Formation (e.g. at Gwernfelen [SN 793 335]), yield mid Wenlock graptolites, demonstrating that, in this area, the contact between the two formations is either a fault or sedimentary slide. Elsewhere the Cerig Formation is overlain by a thick sequence (about 250 m) of disturbed beds (db) which include slumped Builth Mudstones and, additionally, buff-weathering, silt-laminated, silty mudstones, derived from adjacent facies belts that include the Llangammarch and Tirabad formations (the former is a Wenlock division recognised in the Builth Wells and Brecon districts; Schofield et al., 2004; Barclay et al., 2005).

The Tirabad Formation (Tir) was deposited in a distal shelf to ramp setting, and comprises grey mudstones with thin siltstone and sandstone beds and laminae that increase in abundance and thickness upwards. The formation is characterised by vertical and horizontal burrows that also become more common upwards. Some slumping is evident in the lowermost part of the formation. Brachiopod and crinoid debris is locally present, commonly as thin lags. Rare graptolites, collected from the Afon Sawdde (Hurst et al., 1978) suggest that the base of the formation is mid Wenlock in age (riccartonensis Biozone or younger). West of Mandinam, the Tirabad Formation overlies the Builth Mudstones with a gradational contact, whereas to the east it overlies the thick sequence of intervening disturbed beds. The presence of slumped Llangammarch Formation in the sequence of disturbed beds in the east, and its absence in the west, may possibly be due to the juxtapositioning of different successions by strike-slip faulting along the Llandrindod–Pen-y-waun Fault Belt. As a result of faulting there is no complete section through the Tirabad Formation in the district, but at least 510 m are present in the east and at least 130 m in the south-west.

The Sawdde Sandstone Formation (Saw) is characterised by well-jointed, tabular fine-grained sandstones 5 cm to 60 cm thick, interbedded with thinly bedded bioturbated mudstones, siltstones and sandstones. The percentage of tabular sandstone beds increases upwards, reaching 80 per cent in the upper part where, in the south-east of the district, a few thickening upwards bundles are locally developed. Internally these sandstones are parallel or cross-laminated, the thicker ones exhibiting hummocky cross-stratification. Shelly lags are commonly present at the bases of the tabular sandstones. Rare graptolites reported by Hurst et al. (1978) from the upper part of the formation are referable to the mid or late Wenlock dubius to lundgreni biozones. The formation represents a storm-generated mid to proximal shelf facies that prograded over the more distal Tirabad Formation. The formation thins shoreward from 570 m in the north-east to 115 m at the southern edge of the district.

In the north-east of the district, the Sawdde Sandstone is overlain by the distinctive limestone-bearing Halfway Farm Formation, but west of Rhiwiau [SN 749 269] it passes up into the Ffinnant Sandstone Formation (Ff). The latter unit, up to 160 m thick, comprises thin-bedded, wave-rippled and bioturbated sandstones, siltstones and mudstones, punctuated by several thickening-upwards packets of medium- to thick- bedded, fine-grained sandstones, up to 12 m thick. Hummocky cross-stratification and low angle cross-bedding are the dominant structures in the sandstone packets, but unidirectional cross-bedding and some channelling is present in the uppermost ones. Most of the packets record a progradation of shoreface deposits across more muddy offshore strata. In the Afon Sawdde the thickest packet occurs just below the middle of the formation and comprises a 12 m-thick quartzitic sandstone. It is sharply overlain by 3 m of silt and sand-striped mudstones. Some of the highest packets with cross-bedding and channelling may represent tidal channels or mouth bars associated with an estuarine setting. A few thin, argillaceous limestones are also present in this upper part of the formation, while at the very top a 0.4 m-thick bed of grey bioclastic limestone is sharply overlain by a distinctive 0.3 m-thick bioturbated, red ironstone. The latter contains haematitic ooids as well as bored limestone pebbles derived from the underlying bed. Erosion surfaces are present at the base and top of the ironstone. Within the district, the Halfway Farm Formation (Haf) is up to 95 m thick and comprises thinly bedded, commonly bioturbated, greenish-grey shelly mudstones with thin beds of siltstone and sandstone, and argillaceous and sandy limestone. A prominent limestone (lst), up to 9 m thick, comprising thick-bedded crinoidal packstone and some grainstone, is present at the base of the formation at Pwll-calch [SN 768 285]. In its type area, the formation has been considered to record a slight deepening at the end of the Wenlock, prior to a major deepening event at the beginning of the Ludlow (Barclay et al., 2005). Correlation of the Halfway Farm Formation with the Ffinnant Sandstone is uncertain. The limestone beds in the upper part of the latter suggest a lateral equivalence and it is this interpretation which the map depicts, but it is also possible that strata equivalent to the Halfway Farm Formation are absent in the south-east and that, in this area, the interval is represented by the disconformities associated with the ironstone at the top of the Ffinnant Sandstone.

The early Ludlow progradation begins with the marine Hafod Fawr Formation which is overlain by the coarser and more proximal facies of the Mynydd Myddfai Sandstone Formation. The acme of the progradation is represented by the continental deposits of the Trichrug Formation that caps the sequence. The Hafod Fawr Formation (Hod) is characterised by the appearance of tabular, fine-grained sandstone beds ranging from 5 to 40 cm in thickness. The sandstones increase in abundance and thickness upwards and exhibit parallel- and cross-lamination; some thicker beds display hummocky cross-stratification. They are interbedded with thin-bedded mudstones, siltstones and sandstones affected by varying degrees of bioturbation. The formation thins from 700 m in the east to 325 m in the south-west. In the east it rests on the Halfway Farm Formation and south-west of Rhiwiau [SN 749 269] it succeeds the Ffinnant Sandstone. The base of the formation in the Afon Sawdde comprises a 15 cm conglomeratic limestone with phosphate clasts, which overlies the ironstone at the top of the Ffinnant Sandstone, and represents a transgressive lag preserved at the base of the earliest Ludlow deepening. South-west of Rhiwe Farm [SN 739 256], the Cwar Glas Member (CwG), a unit of medium- to thick-bedded sandstones, up to 66 m thick, with local packets of thin-bedded mudstones, siltstones and sandstones, is developed at the top of the formation. The medium- to thick-bedded sandstones are mainly greenish grey but purple colours become dominant west of the Afon Sawdde. They are predominantly fine to medium grained and exhibit low angle and hummocky cross-stratification. The thickening and coarsening-upwards sequence of the Hafod Fawr Formation records shallowing on a storm influenced shelf. The tabular sandstones record individual major storm events whereas the medium- to thick-bedded sandstones of the Cwar Glas Member represent shoreface deposits.

The overlying Mynydd Myddfai Sandstone Formation (MyS) thickens from 29 m in the east to 70 m in the south-west. It mainly comprises medium- to thick-bedded, variably pebbly, coarse-grained, purplish-grey to greyish-purple quartzites and quartzitic sandstones displaying low-angle cross-bedding, and subordinate conglomerates occurring as pebble stringers, thin beds and lenses.

The formation was predominantly deposited in a mouth bar delta, distributary channel deposits being confined to the upper part of the formation in the south-west of the district. The contact with the overlying continental Trichrûg Formation (Tug) is sharp in the Afon Sawdde but becomes gradational when traced south-west. The formation comprises red-brown gritty, argillaceous sandstone debrites and very subordinate siltstones and mudstones, with scattered thin to medium tabular beds of medium to coarse-grained greyish-purple quartzite. Bioturbation, including Skolithos 'pipes', is abundant, suggesting deposition in very shallow water bodies in a wetland environment (Davies et al., 2008), fed by debris flow-dominated alluvial fans. The presence of calcretes, ferricretes (Plate 3) and desiccation cracks demonstrates periodic subaerial exposure. The only marine shelly fauna is restricted to three thin units of grey and grey-green mudstone and sandstone that have yielded Lingula and Orbiculoidea (Straw, 1929) suggesting that the shallow water wetland was periodically inundated by the sea. The thickest development, of 170 m occurs 2 km west of the Afon Sawdde, and the formation thins eastwards to 16 m and south-westwards to 100 m.

A major transgression in the late Ludlow re-introduced the marine shelf facies of the Cae'r mynach Formation (Car) across the district. The formation comprises a sequence of thinly interbedded, bioturbated mudstones, sandstones and siltstones, punctuated by thin, storm-generated, tabular sandstones. Shelly lags are common at the base of many of these tabular sandstones. The formation thins from 230 m in the east to 44 m in the south-west. At its base, up to 12 m of low angle and hummocky cross-stratified quartzites and conglomeratic quartzites, the Cribyn Du Member (CrD), record the southward passage of the shoreface during the transgression. The thickening and coarsening upwards sequence of the Cae'r mynach Formation represents the lower part of the late Ludlow to earliest Pridoli progradation. The upper part comprises the Tilestones and Pontar-llechau formations of Pridoli age.

Devonian and Carboniferous

Pridoli to Early Devonian — the Lower Old Red Sandstone

Rocks of this age occupy the south-eastern portion of the district (Figure 1), and are well exposed in the Sawdde River gorge, where the sandstones, siltstones and mudstones of the late Ludlow Cae'r mynach Formation are gradationally overlain by the Tilestones Formation (Til). This contact is conventionally taken as the base of the Old Red Sandstone and of the Pridoli Stage, and is now regarded as conformable (Barclay et al., 2005), contrary to previous interpretations (e.g. Potter and Price, 1965). The Tilestones are highly micaceous, laminated and low-angle cross-bedded, yellow-weathering sandstones which vary from around 38 to 47 m in thickness. The formation records the northward progradation of upper shoreface and mouth bar facies across the underlying deeper marine deposits. The sandstones yield a restricted marine fauna including the inarticulate brachiopod Lingula, Modiolopsis bivalves, gastropods and orthocones, and display Skolithos vertical escape traces (Almond et al., 1993). The formation has been extensively quarried and lines of degraded pits and spoil banks characterise its linear outcrop.

In the Afon Sawdde, the overlying Pont ar Llechau Formation (Pol) is up to 43 m thick and comprises red-brown, bioturbated, gritty argillaceous sandstones and siltstones. Thin packets of green mudstones with thin sandstones are locally present. The formation is only present to the south-west of Rhiwe Farm [SN 737 250] and is interpreted as the deposits of a wetland alluvial fan that received periodic marine incursions. The succeeding Raglan Mudstone Formation (Rg) records the onset of fan deposition on a fluvially dominated coastal plain environment, subjected to periodic subaerial exposure. Red siltstones and mudstones alternate with subordinate feature-forming sandstones and calcrete profiles throughout an approximately 700 m thick sequence. The formation contains rare bivalves and some of the earliest land plant remains as well as the trace fossil Beaconites. The distinctive 2.9 m-thick Townsend Tuff Bed (TwT), recorded in the Afon Sawdde [SN 732 238] around 600 m above the base of the formation, comprises three airfall tuffs. The top of the lowest tuff exhibits distinctive faecal pellet and burrow casts. Further tuff beds, including the Picard Bay Tuff Bed, have also been recognised in this vicinity (Almond et al., 1993). Calcrete levels are most abundant in the upper half of the formation, but in the absence of the Bishop's Frome Limestone, a prominent mature calcrete developed at the top of the formation elsewhere in South Wales, the top is gradational with the succeeding St Maughans Formation (SMg). The latter is 500 m thick and comprises a series of fining-upwards cycles in which fluvial sandstones overlie intraformational conglomerate beds and are in turn overlain by red siltstones and mudstones that are commonly calcretised. The sandstones and conglomerates were deposited in laterally migrating meandering stream channels; the siltstones and mudstones on the adjacent floodplain.

The Senni Formation (Sen) is up to 240 m thick and predominantly comprises green and green-grey, cross-bedded, micaceous sandstones with subordinate thin beds of red-brown and green siltstone, and mudstone with local calcrete profiles and desiccation cracks. Intraformational conglomerates composed of mudstone and siltstone rip-up clasts and reworked calcrete nodules overlie common erosion surfaces. The formation records sand deposition by braided streams with the finer-grained lithologies interpreted as floodplain lake and channel abandonment facies (Loeffler and Thomas, 1980). The preservation of locally abundant vascular plant remains, and the predominantly green colour, is in marked contrast to underlying and overlying red-bed facies, reflecting the prevalence of reducing diagenetic conditions, consistent with elevated water tables.

The red-brown micaceous sandstones that characterise the Brownstones Formation (Brs) include interbeds of red-brown muddy siltstone and silty mudstone. Within the 300 m-thick sequence, the sandstones typically form laterally extensive, tabular sheets constructed of stacked-trough and planar cross-bedded sets. Intraformational mudstone, siltstone and calcrete clasts are common at the bases of the fining-upwards sandstone units, which Tunbridge (1981) interpreted as deposited by a braided network of low-sinuosity streams during flash-floods. Cross-bedding throughout much of the formation suggests that streams flowed from the south to south-west, but in the upper half of the formation on Tyle Du [SN 782 225], conglomerates rich in vein quartz and volcanic pebbles compare with those derived from an easterly source recognised by Tunbridge (1980) in the adjacent Brecon district.

Upper Old Red Sandstone and Carboniferous Limestone

Middle Devonian strata are missing from the Llandovery district, as across much of South Wales. Their absence is the result of regional uplift and erosion caused by the Acadian orogenic event (see below). Thus, in the south of the sheet, on Tyle Du, rocks of Late Devonian age (Upper Old Red Sandstone) unconformably overlie the Brownstones Formation. The Plateau Beds Formation (PlB) is the lowest division of the Upper Old Red Sandstone. Red-brown and purple pebbly sandstones and conglomerates dominate in the lower part of this approximately 15 m-thick sequence (see over), whereas finer-grained, ripple-marked, red sandstones are more prevalent towards the top. Cross-stratification within the conglomerates indicates that current flow was from the north, the sequence probably representing a series of braided stream deposits. A marine or possibly tidal setting has been suggested for the upper sandstone facies (Lovell, 1978) which contain fish and marine fossils, including brachiopods and bivalves, indicating a Frasnian to Famennian age (Taylor and Thomas, 1975; Barclay et al., 1988). The view of Allen (1974) that the succeeding Grey Grits Formation (GG) rests unconformably on the Plateau Beds has been endorsed by Barclay (1989). Lovell (1978) interpreted these distinctive grey-weathering quartzitic sandstones and conglomerates as an alluvial fan facies sourced from the north. Less than 5 m of Grey Grits are exposed at Carn Gigfran [SN 779 219], but they provide evidence of extensive dewatering and liquefaction of this unit. The Grey Grits are regarded as the equivalent of the Wern Watkin Formation, the basal division of the Quartz Conglomerate Group in the Abergavenny area (Barclay, 1989). However, strata equivalent to the upper parts of this group are absent from the Upper Old Red Sandstone successions of the Llandovery, Merthyr Tydfil and Ammanford districts where the contact between the Grey Grits and the succeeding Carboniferous succession appears to record a further erosional discontinuity.

The Carboniferous outcrop at Garreg yr Ogof [SN 777 215] comprises divisions of the Carboniferous Limestone of Tournaisian–Visean (Mississippian) age. At the base of the sequence a poorly exposed and attenuated Cwmyniscoy Mudstone Formation (CcM) consists of less than 9 m of grey marine mudstones with thin beds of limestone. These rocks form part of the Lower Limestone Shale Group, and elsewhere have yielded Courceyan fossils (Barclay, 1989; Waters and Lawrence, 1987). Another non-sequence separates these strata from the overlying Dowlais Limestone Formation (DoL) (formerly the Cil-yr-ychen Limestone) (Plate 4) that contains fossils diagnostic of the much younger Holkerian Substage, including the distinctive brachiopod Davidsonina carbonaria. In this area the formation is around 90 m thick, and its base is marked by a thin, but distinctive sequence of dolomitised, orange-weathering, sandy limestones with scattered quartz and lithic pebbles, stromatolitic limestone and a conglomerate of angular limestone clasts. The varied suite of limestones which characterises the remainder of the formation includes dark grey, fine-grained, skeletal and peloidal wackestones, packstones and grainstones with beds rich in algal pisoliths, articulated brachiopods and turreted gastropods, or coquinas of brachiopod valves; porcellaneous limestones (calcite mudstones) also occur locally. About 7 m above the base of the formation there is a widely recognised coral bed composed of radiating colonies up to a metre across of Siphonodendron (Lithostrotion) martini (George et al., 1976). These varied facies were deposited within an extensive shallow lagoon during a slow, but prolonged, marine transgression; sheltering barriers lay to the south (Wilson et al., 1988; 1990). At the base of the succeeding Asbian strata, the distinctive and widespread Honeycombed Sandstone Member (HS) is overlain by well-sorted, ooidal grainstones of the Penderyn Oolite Member (POo). Both members are divisions of the Oxwich Head Limestone Formation, a name now used in place of the Penwllt, Mynydd-y-Gareg or Llandyfan limestones of earlier accounts (Waters et al., in press).

Structure

During the Early Palaeozoic, the Welsh Basin and adjacent Midland Platform formed part of the continent of Eastern Avalonia that was founded on sialic continental basement of Neoproterozoic age (e.g. Gibbons and Horák, 1996). Early Ordovician development of the basin was instigated adjacent to a zone of oceanic subduction beneath the northern edge of the Avalonian palaeoplate. The subsequent evolution of the basin resulted from convergence of this plate with the northern supercontinent of Laurentia (part of modern day North America) and the contraction of the intervening Iapetus Ocean. Prior to its collision with Laurentia, Avalonia collided with the Baltica palaeoplate and this event may explain widespread fault reactivation along the margins of the Welsh Basin during the Late Ordovician (the so-called Shelvian Orogeny of Toghill, 1992). Later subsidence was probably instigated by the onset of collision with Laurentia when crustal loading of the Avalonian foreland, in the Southern Uplands of Scotland and English Lake District during the Silurian and Early Devonian, gave rise to renewed basin development (Soper and Woodcock, 1990; King, 1994). The history of many of the structures within the district can be traced back to the Ordovician and Silurian development of the basin. In particular, major faults that were active during episodes of basin growth are thought to have controlled the distribution and geometry of some of the sedimentary units. However, the regional structural style was imposed during late Early to Mid Devonian (Emsian to Eifelian) during a phase of regional uplift and deformation known as the Acadian Orogeny (Soper and Hutton, 1984). Debate continues as to the causes of this event; although traditionally viewed as the result of continued shortening between Laurentia and Avalonia, more recent studies suggest that impingement of the Armorican palaeoplate to the south may have had a significant influence (Woodcock et al., 2007). During the early Carboniferous, subsidence and marine transgression was controlled by a renewed cycle of oceanic subduction and continental convergence, this time located to the south of Avalonia. This culminated in the Variscan Orogeny when Avalonia was accreted onto the main continental mass of the supercontinent of Pangaea (Franke, 1989). The south-east part of the district is located on the very northern limit of Variscan orogenic deformation and only suffered relatively weak folding and fault movement, although earlier structures were probably reactivated at this time.

The main structural features of the district are summarised in (Figure 1). The most significant of these are long-lived zones of movement that largely control the distribution of later Acadian deformation. In the south-east of the district, the Welsh Borderland Fault System forms a plexus of roughly strike-parallel faults that are broadly coincident with the boundary between basinal and shelfal deposits. This fault system has been the focus of repeated reactivation, recognised in the movement histories of the ramifying Crychan and Llandrindod–Pen-y-waun fault belts. The northern margin of this system is roughly defined by the Garth–Llanwrtyd Fault Belt, while the Dulas Valley Fault forms its southern limit. These elements coalesce into a narrower Dynefor Fault, postulated to run beneath the course of the river Tywi at Llandeilo, contiguous with similarly oriented structures that transect the Ammanford and Carmarthen districts to the south and south-west, and include a prominent splay; the Sawdde Fault that runs along the northern margin of the Mydffi Steep Belt. The most southerly structure of the Welsh Borderland Fault System, the Church Stretton Fault, cuts across the south-east corner of the district to link with the Carreg Cennen Disturbance of the Ammanford district. Pre-Acadian displacements along the Welsh Borderland Fault System, gave rise to local emergence, with local tilting and folding of strata and the development of unconformities.

Evidence of pre-Wenlock deformation along the Welsh Borderland Fault System was first described by Williams (1953), who recognised abrupt changes in the facing direction of Ordovician strata beneath overstepping Wenlock strata in the type area of the former Llandeilo Series. To the north, in the type Llandovery area, the unconformity between transgressive late Hirnantian facies and underlying Rawtheyan strata was undoubtedly the product of deep erosion during the Hirnatian glacioeustatic lowstand. However, the angular nature of the contact, and abrupt changes in the level of erosion across faults also provide clear evidence of a period of mid to late Ashgill tectonic activity, either immediately prior to, or during this event. The angular overstep of earlier Llandovery divisions by the Aeronian Wormwood Formation suggests a subsequent, intra-Llandovery period of tectonism. Thus, the relationships in the south of the district described by Williams (1953) are likely to be composite in origin, i.e. the product of more than one phase of deformation, uplift and erosion.

The broad north-east-trending belt of Ordovician basinal strata that crosses the centre of the district coincides with the south-western continuation of the Tywi Lineament, a plexus of folds, faults and differing sedimentary facies first recognised by Jones (1912). Although the disposition of this structurally complex zone is largely the result of Telychian uplift and mass wasting (Davies et al., 1997), its component structures reveal an earlier history of movement. The earliest of these events within the district, occurred during the mid to late Ashgill, Shelvian Orogeny (Toghill, 1992) and initiated structures such as the Abergwesyn Fault which controlled the deposition of confined coarse clastic turbidite bodies (Schofield et al., 2004), including the Taliaris Formation of the Llandovery district. Intra-Telychian fault movements and mass-wasting along the Tywi Lineament, previously recognised in the Rhayader and Builth districts (Davies et al., 1997; Schofield et al., 2004), occurred in response to east-ward-propagating tectonism that resulted in a major change in the pattern of sedimentation within the Welsh Basin and is thought to record the onset of Avalonia and Laurentia plate collision (Davies et al., 1997; Soper and Woodcock, 1990). Within the Silurian sequences north-west of the Tywi Lineament, this event is recorded in major sedimentary thickness and facies variations across the Craig Twrch Fault, which forms part of the Central Wales Lineament, demonstrating the importance of the structure in controlling the distribution of the southerly sourced turbidite systems.

As collision progressed, crustal loading in the region of the Iapetus suture zone, located beneath southern Scotland, led to the formation of a southward migrating foreland basin (King, 1994). In Wales, regional subsidence associated with this feature accommodated the thick late Silurian to Devonian succession present in the south-east of the district which ended with the final Mid Devonian inversion and shortening of the Welsh Basin. The associated Acadian uplift and erosion is marked by the regional unconformity between the Lower and Upper Old Red Sandstone sequences.

During these events, plate vectors are thought to have been oblique to one another (Soper and Hutton, 1984), and led to the propagation of a unique arrangement of structures attributed to transpressive strain, in which movement results from a combination of shortening and lateral displacement (Harland, 1971). In the Welsh Basin the shortening component is preserved as widespread folding and cleavage formation; while the lateral component is preserved by variable, small angles of obliquity between the cleavage surfaces and fold axes (termed cleavage transaction) that imply passive rotation of fold axes in relation to cleavage during deformation, together with proposed strike-slip displacement along selected fold limbs and major strike-parallel faults (Woodcock, 1990).

The major first order fold structures appear to have nucleated in zones of complex facies variation where older basement structures influenced the distribution of major facies. In the district, these are represented by a major first-order syncline developed within the Central Wales Lineament (often referred to as the Central Wales Syncline), which nucleated on the thicker Rhuddnant Grits sequence. To the south-east, the corresponding antiformal structure within the Tywi Lineament (often referred to as the Tywi Anticline is not clearly marked, as it impinges on faults within the Welsh Borderland Fault System. Second order folds are widely developed within the district. These are typically close, shallowly plunging periclinal folds withwavelengths in the order of 5 km that verge south-eastwards and have steeply north-west-dipping axial surfaces.

The closely spaced cleavage that is present throughout much of the central and north-western parts of the district, is generally steeply inclined toward the north-west. Its development was accompanied by very low-grade metamorphism across the region. The latter has been calibrated against growth of the clay mineral illite that occurred as a result of both diagenetic and metamorphic reactions during extensional basin subsidence, associated sedimentary burial and subsequent tectonic thickening during the Acadian event (Merriman, 2006). Isotopic ages of around 400 Ma have been yielded by small crystals of mica within the plane of the cleavage and have been used to infer an Early Devonian (Emsian) age for deformation (Sherlock et al., 2003).

The present fault pattern is largely the result of Acadian transpression, reactivation of earlier, syndepositional structures and propagation of new ones such as the Pontarsais Fault that appears to have no control on sedimentary architecture but across which considerable thicknesses of strata are locally excised (Wilby et al., 2007). It is dominated by north-east-trending, strike-parallel faults, locally linked by short, east-north-east-trending structures to form an anastomosing array. Most of the faults record variable vertical offsets, although it is inferred that they also underwent strike-slip movements. Fold and fault patterns in the south of the Llandovery stratotype, in the vicinity of Mandinam [SN 740 280], are consistent with dextral transcurrent displacement along the prominent set of curvilinear fractures present in this region. The current alignment of Llandovery facies belts suggest such movements may have exceeded 3 km, although the amount and type of any Variscan reactivation is unknown.

The south-eastern margin of the pervasive Acadian deformation is represented by the Myddfai Steep Belt. This fault-dissected zone is coincident with the continuation of the Welsh Borderland Fault System and forms the steep limb of a regional, south-eastward facing monoclinal syncline, across which Acadian folds and cleavage largely disappear. Within the steep belt, cleavage becomes packeted and more sporadic, and is generally absent south-east of the Sawdde Fault.

Evidence of post-Acadian tectonism is confined to the south-eastern part of the district where strata are displaced by an array of predominantly north-north-west- to north-trending faults. These have been interpreted as the terminations of oblique-reverse faults that dissect Carboniferous rocks in the South Wales Coalfield (Roberts, 1972). Although these were active during the Variscan Orogeny, the possibility that they were initiated during earlier phases of movement along the Welsh Borderland Fault System cannot be ruled out. Likewise, the east-north-east-trending Carreg Cennen Disturbance of the Ammanford district, a zone of faulting which affects Carboniferous rocks (Strahan et al., 1907) has been interpreted as a major thrust fault at the leading edge of Variscan deformation (Shackleton et al., 1982), but its link to the long-lived Church Stretton Fault suggests these displacements result from local re-activation of the underlying pre-Variscan structure.

Mineralisation

Lead and zinc mineralisation

The district lies at the south-eastern margin of the Central Wales Mining Field, a region extensively exploited for lead- (mainly galena) and zinc- (mainly sphalerite) bearing minerals during the mid-nineteenth century. Detailed accounts of the historical and geological aspects of the mining field are provided by Ball and Nutt (1976) and Davies et al. (1997 and references therein). The main area of mineralisation and working in the district was sited to the south-west of Myddfai, near Cwm Brân [SN 755 280]. This area occupies a unique setting within the mining field as the mines in this area, including the productive Casara, or Great Welsh Lead-Silver Mine, worked mineral veins associated with north-west–south-east trending faults which cross-cut steeply dipping, shelfal Wenlock rocks. Workings in veins that cut Llandovery shelf sequences in the vicinity of Penlan-Telych [SN 7831 3344] exploited a similarly orientated mineralised fracture. Other, less productive mines worked mineral veins within basinal Ordovician and early Silurian rocks along the western limb of the Tywi Lineament, notably in the vicinity of the conglomerates at Banc Bwlchdrebannad [SN 720 370], and at Penegarreg [SN 626 335].

Various hypotheses have been proposed for the source of the sulphide mineralising fluids in mid Wales. In the absence of evidence for a magmatic hydrothermal source, an origin which invokes leaching of metals from the Lower Palaeozoic sediments during Acadian dewatering and metamorphism has been proposed, and is supported by geochemical evidence (Davies et al., 1997).

Gold mineralisation

The gold-bearing strata exploited at the Dolaucothi gold mine [SN 663 403], near Pumsaint, lie in the upper part of the Ordovician Yr Allt Formation. The nature of the mineralisation, and its origins, has been assessed in detail by Annels and Roberts (1989) on which this account is based. It takes several forms, but appears to be localised along north-east-trending folds and faults formed during the Acadian deformation of the district. The Romans worked a sigmoidal, 6 m-thick, quartz 'saddle reef' in which the gold was associated with pyrite. Later workings exploited quartz veins associated with local faults and thrusts, including a north–south-trending argentiferous galena and sphalerite-bearing cross-fault. Gold is also present, in relatively high concentrations, in pyritised zones within the host mudstones, although there is little evidence of these ever having been worked.

The gold normally occurs as tiny, sulphide-locked gains ranging up to 200 μm in size included within, or attached to, crystals of pyrite or arsenopyrite. It is also recorded infilling microfractures in these minerals and is associated with complex intergrowths of chalcopyrite, sphalerite and galena. The principal gangue minerals are quartz, manganese-rich ankerite and hydromuscovite.

Annels and Roberts (1989) suggested that much the Dolaucothi gold and other metals were leached from deep-seated igneous or volcanic source rocks by hot, highly pressured fluids during the early stages of Acadian deformation and metamorphism. Subsequent fracturing allowed these fluids to migrate to higher crustal levels where they precipitated their dissolved mineral load in low pressure zones associated with fold hinges and faults, and also promoted localised pyritisation of the host mudstones. Late in the deformation, the gold was locally remobilised and incorporated in cross-cutting lead and zinc sulphide-bearing veins otherwise comparable with those elsewhere in the Central Wales Mining Field.

Quaternary

During the Pleistocene, global climatic changes brought about a series of ice ages that affected much of the British Isles. During this period, the action of glaciers was the main erosional and depositional agent responsible for modifying the landscape to its present form.

Prior to the Pleistocene glaciations, the landscape of the district was largely moulded by river downcutting of a westward sloping coastal plain during Cenozoic (Tertiary) regional uplift (Dobson and Whittington, 1987). The presence of older, now abandoned or captured, sections of the main river valleys, illustrate the pre-Pleistocene drainage pattern of the district.

The subsequent glaciation not only altered the earlier drainage pattern but also led to oversteepening and overdeepening of the valleys and deposition of glacial material in valleys and hollows. Following glaciation, periglacial processes modified many of the surface glacial deposits. Subsequently, modern alluvial sediments and peat were deposited during the Holocene. Although evidence for more than one episode of glaciation is preserved in south Wales, landforms and deposits in the district appear to relate exclusively to the last major ice advance during the Late Devensian which lasted from around 26 000 to 14 500 BP (Campbell and Bowen, 1989). During this glaciation, ice accumulated in the Cambrian Mountains to form a Central Wales Ice Sheet. At its greatest extent, ice probably covered the whole of the district, with thicker accumulations in the Cothi, Dulais, Bran and Tywi valleys, the latter merging with an ice mass flowing north-west from the Brecon Beacons.

The main deposit of the Late Devensian glaciation is till. This comprises an ill-sorted material varying from gravelly, sandy, silty clay to sandy, clayey gravel with individual clasts ranging up to boulder size. Till comprises material eroded, transported and deposited directly from the ice sheet. Two types have been recognised in the district, one containing clasts of Old Red Sandstone in a commonly red, sandy matrix. This variant is thought to be derived from the Brecon Beacons ice mass and is restricted to the ground south-east of the Tywi valley. The second facies is generally blue-grey in colour and comprises stiff gravelly sandy clay with only Lower Palaeozoic clasts derived from the Central Wales Ice Sheet. Till forms a patchy veneer over much of the district but is more thickly developed in the Dulais and Tywi valleys where it ranges up to 15 m thick. Downslope movement during later periglacial episodes has locally imparted slope-parallel clast fabrics.

During deglaciation, ice withdrew from the lowland interfluves leaving a series of constructional landforms (moraines) that record deposition in front and along the sides of these valley glaciers. The landforms are represented by glaciofluvial ice-contact deposits and hummocky glacial deposits. The former comprise poorly sorted sand and gravel and clayey gravel, which probably represent degraded kame terraces and occur as irregular terrace-like features on the lower slopes of valleys, such as in the upper reaches of the Afon Tywi. Hummocky glacial deposits are characterised by irregular mounds that are more heterolithic and include bedded sands and gravels, clay-bound gravels and local lenses of till. These are interpreted as retreat or push moraines formed in front of valley glaciers either during pauses in retreat or temporary re-advances. These deposits are widespread in the tract of low ground around the confluence of the Tywi, Bran, Sawdde and Gwydderig rivers, and have occluded the broad glaciated valley at Talley [SN 639 330], forcing the river Cothi to deviate to its present, incised, south-westerly course.

As the valley glaciers melted and retreated back toward the uplands of the Cambrian mountains, large volumes of water were released which redistributed glacial deposits as spreads of sand and gravel in the valley bottoms. Subsequent river incision has left these glaciofluvial sheet deposits as a series of high terraces above the present river level.

Immediately following ice retreat, wide-spread seasonal freezing and thawing took place under tundra-like periglacial conditions that persisted until about 10 000 years ago. These conditions modified many of the glacial landforms, leading to shattering of bedrock and solifluction, the downslope movement of material by frost creep or saturated flow, to form deposits of head. These comprise highly variable, gravelly, sandy, silty clay and mudstone screes made up of stratified, angular mudstone clasts in a silty clay matrix. In the district, head deposits are ubiquitous, but have only been recorded where they have significant thickness and a topographic expression; hence much of the soliflucted till has not been distinguished from its parent material.

During the Holocene, the modern drainage pattern was established. The main drainage feature of the district is the Afon Tywi, which was initially established as a broad braid plain. However, following repeated river down-cutting, in response to post-glacial regrading and isostatic readjustment, the former braid plain deposits are now preserved as areas of elevated river terrace deposits comprising stratified sand and gravel. Gently sloping spreads of alluvial fan deposits, also comprising stratified sand and gravel, accumulated where steeper graded tributaries emerged into the main Tywi valley. These also show evidence of subsequent incision and are dissected by the recent river deposits of alluvium. The latter underlies the modern flood plains and represents the area of modern river deposition, typically comprising silt and organic-rich clay with beds and lenses of sand and gravel.

Small areas of lacustrine alluvium, comprising organic-rich silt and clay, have accumulated in lakes and ponds formed within enclosed hollows on poorly drained ground, such as the hummocky moraine at Talley. As vegetation was re-established under more temperate climatic conditions, deposits of peat accumulated in areas of restricted drainage (Plate 5). Scattered small landslides are present, particularly where deposits of till on oversteepenend valley sides have been undercut by meandering rivers and streams; many remain intermittently active at the present day.

During the Holocene, the landscape has been modified by human activity. Although the district has not been significantly industrialised, artificial (man-made) geology of limited extent can be found. This is typically associated with quarrying and mining activities and comprise excavated areas of worked ground, deposits of made ground, made up of tipped material or quarry spoil of variable composition and thickness, and worked and made ground where former quarries have been back-filled with overburden or waste material. Made ground of limited thickness is likely to be encountered underlying most settlements.

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, and that appropriate mitigation measures are taken prior to development. Exploitation of natural mineral resources frequently conflicts with agricultural land use, pre-existing developments and the environment. Potential geological hazards may present a public health risk or require costly remediation. Engineering ground conditions and designated sites of geological conservation strongly influence the location and design of any new development.

Mineral resources

The mineral and aggregate resources of the Llandovery district have traditionally provided a significant input to the local economy. The main resources are metalliferous mineral deposits formed in areas of lead, zinc and gold mineralisation described above. Lead with silver, zinc and copper were extracted from a total of nine mines in the district. The earliest recorded workings are those of the Casara mine [SN 753 275], which was active from the 18th century and closed in 1865. The most recent documented workings in the Llandovery area are from Penegarreg [SN 626 335], which continued production until 1892 (Hall, 1971).

Gold mineralisation has a long history of exploitation from the Dolaucothi mine [SN 663 402]. More recently, additional exploration was carried out in the district, such as at Llanfynydd, [SN 554 281] where reports of gold in the Afon Sannon resulted in a trial level being sunk in 1892–1893 (Hall, 1971).

The Dolaucothi gold mine itself is believed to have Iron Age origins, but was first developed industrially by the Romans (Annels and Burnham, 1986) (Plate 6). Although active at several periods, the most significant extension of the mine took place between 1872 and 1938. Production levels peaked in 1938 but mining was halted due to the outbreak of war in 1939. The mine was finally abandoned in 1940 and allowed to flood, although sporadic exploration work in the Cothi Anticline has been undertaken since, both commercially and by the University of Wales for undergraduate teaching. In 1941–1943 it was bequeathed to the National Trust and is now open to the public for underground tours of the old workings.

Commercial aggregate extraction in the district is not widespread, although thickly bedded quartzitic sandstones of the Yr Allt Formation were worked until recently at Pen y Dinas quarry [SN 627 356], approximately 1 km south-east of Llansawel. Aggregate from this quarry was used for local building stone, road construction, gabions and river bank protection and as a source of silica from the crushed rock. Other potential hard-rock resources include sandstone and conglomerate of the Taliaris, Yr Allt, Goleugoed and Cefngarreg Sandstone formations and the Cwmystwyth Grits Group. The Cribarth Formation is worked as high-quality road stone in the adjacent Builth Wells district and may also have similar potential in this district. Extraction of building stone for local use was once widespread across the area. Buildings in the north and centre of the district tend to be constructed of Ordovician and Silurian sandstone, while those in the south-east of the district are often constructed of shelfal sandstones including the Llandeilo Flags, Glasallt Fawr Sandstone Formation and Cwar Glas Member of the Hafod Fawr Formation. The thin, flaggy sandstones of the Tilestones Formation were formerly worked for roofing tiles. Thinly bedded mudstones of the Yr Allt and Nantmel formations and parts of the Claerwen Group have been worked locally for slate.

There are numerous small sand and gravel workings within the area, although these have not generally been extracted on a commercial basis. Potential resources include hummocky glacial deposits, glaciofluvial ice-contact deposits, alluvial fans, river terraces and glaciofluvial sheet deposits of the Afon Tywi and its tributaries. The heterogeneous nature of hummocky glacial deposits and variable clay content of ice-contact deposits suggests that they would be of poor to intermediate quality. The glaciofluvial sheet deposits, alluvial fan and river terrace deposits may be of better quality, but the former could contain large boulders up to a metre in diameter. The quality of these resources may be further limited by the relatively high proportion of weak, locally derived mudstone clasts.

Peat, although not widespread in the district, was probably formerly cut as a local source of fuel.

Water resources

Due to high annual rainfall, relatively impermeable bedrock and elevated run-off, much of the water resources within the area are derived from surface water. This is extracted for local residential and agricultural use, and is abstracted for public supply from the Afon Tywi near Llandeilo. The flow of the Afon Tywi is controlled by release of water from the Llyn Brianne reservoir. This is managed just south of the district at Nantgaredig, timed with a major abstraction point supplying much of South Wales.

Groundwater is only abstracted for public supply form a single borehole in the Tywi valley at Llandovery. However, boreholes for private supply and naturally occurring springs provide water for isolated farms. Although the Lower Palaeozoic rocks of mid Wales have not previously been thought to behave as high quality aquifers, recent studies have shown that viable amounts of water can be abstracted from boreholes in both the bedrock and superficial deposits (Robins et al., 2000).

Potential geological hazards

Low-lying parts of the district are prone to flooding during high river-flow conditions. An indication of areas likely to be susceptible is given by the extent of active floodplain deposits, shown as alluvium on the map. The towns of Llandovery and Llanwrda have previously been subject to flooding, although Llandovery now has a flood-defence scheme in place. During more extreme flow conditions, river terraces and head-filled valleys may also be subject to flooding. For more information contact the Environment Agency Wales, Cambria House, 29 Newport Road, Cardiff, CF24 0TP.

Slope instability, particularly in the form of landslides, is locally present across the district. The majority of landslides identified during the present survey occur in superficial deposits, where the main trigger has been erosion of the toe of the deposit by rivers and streams. Although most existing landslides are located in rural areas, altering slope dynamics and structures during new developments may trigger landslides in urban areas. The most effective strategy for dealing with unstable ground relies on recognition of the problem areas in advance so that suitable preventative or remedial measures can be employed.

Areas of foundered ground (Plate 7) have been identified in the south-west of the district within the crop of the Dowlais Limestone. They comprise collapsed sink-holes, up to around 30 m in diameter, that represent roof failures of underlying shallow caves that have been partially infilled with collapse debris and washed-in soil. Karst areas such as this present a significant hazard, both from new collapses occurring, possibly initiated by new development, and instability of infill material, typically in response to changing groundwater levels.

Migration of leachate from the underground mine workings and materials used in ore processing, such as mercury used in gold recovery around Dolaucothi mine (Annels and Burnham, 1986), presents a significant pollution potential, which may lead to contamination of ground and surface water. Toxic minerals may also be present in spoil heaps and surface workings, where heavy rainfall or poorly planned remediation could cause mobilisation. Prior to any development of former mine sites, environmental remediation programmes should carried out by a qualified body.

Natural gas emissions are unlikely to be a major hazard in the Llandovery district, although methane can also be generated by the decomposition of material in landfill sites and unconsolidated organic rich superficial deposits such as peat. It is less dense than air and can migrate through permeable strata, or cracks and fissures and accumulate in poorly ventilated cavities such as basements, foundations and excavations. Methane is an asphyxiant and is toxic; it is also explosive in high concentrations. The risk may be mediated by implementing correct site design to national guidelines to prevent gas migration through the ground into properties.

Radon is a naturally occurring gas generated through the radioactive decay of uranium, which is present, in small quantities, in most rocks. It does not present a risk if allowed to dissipate into the atmosphere; however if allowed to accumulate in poorly ventilated spaces it may cause an elevated risk of cancer of the respiratory tract. In areas at risk of radon accumulation, protective measures should be undertaken on new developments, and remedial work on pre-existing housing. Advice and information on Radon can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire, OX11 0RQ.

Engineering ground conditions

Knowledge of ground conditions is a primary consideration for identifying land suitable for development, and underpins cost-effective design. Engineering ground conditions vary depending on the physical and chemical properties of the local materials, topography, behaviour of groundwater and surface water, and the nature of past and present human activity. The most significant development problems likely to be encountered in the district are due to the variability of natural superficial deposits, weathering of solid rocks and landslides. These can be effectively dealt with by obtaining adequate information, including properly focused site investigation, to confirm the properties of individual sites.

The majority of the solid rocks have a high bearing capacity, except in the weathered zone. Pyritic mudstones of the Cwmere Formation and at certain horizons within the Claerwen Group may cause heave and promote sulphate concrete attack.

Lacustrine deposits and peat have low bearing capacities and can give rise to moderate settlement. Till and hummocky glacial deposits have moderate bearing capacity but are highly variable. Head, 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

Geodiversity considerations are becoming an increasingly important part of the planning process and not only include natural geodiversity based on the intrinsic geology and landscape of an area, but also cultural geodiversity based on in situ man-made features with geological significance and geological resources such as museum collections, literature and maps. Natural geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). Non-statutory designated conservation sites are Regionally Important Geological Sites (RIGS) and Locally Important Geological Sites (LIGS), however none of these are defined in the district at present. Further information on the extent and designation of SSSIs, RIGS and LIGS 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.

The Llandovery district has a rich geological heritage. Important SSSIs reflecting this include the Dolaucothi gold mines, Fairfâch road cuttings [SN 627 211] and sections in Dynefor Park [SN 615 224] that make up the type areas of Ffairfâch Grits and Llandeilo Flags respectively. Important sections in the type area of the Llandovery Series are located at Cilgwyn, in the Ydw Valley [SN 748 299] and along the Cefn Cerig road [SN 775 323].

Information sources

Further geological information held by the British Geological Survey relevant to the district and adjoining areas is listed below. Searches of indexes to some of the collections can be made on the Geoscience Data Index (GDI) system, available online at www.bgs.ac.uk. The BGS Catalogue of geological maps, books and data is available on request or may be viewed on the website. Maps and other publications may be purchased online (www.geologyshop.com) or through the BGS sales desks, and can be consulted at the BGS libraries (see (Rear cover) for addresses).

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre (NGRC), BGS, Keyworth. Geological advice should be sought from the Chief Geologist, Geology and Landscape, Wales, BGS, Cardiff. The BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS website or at: Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

Publications

Original 1:63 360 scale geological maps are out of print, but can be provided as facsimiles or can be consulted at the BGS library, Keyworth. Unpublished 1:25 000 scale geological maps, listed below, are available on request. Groundwater vulnerability maps are published by the Environment Agency from data commissioned from BGS and The Soil Survey and Land Research Centre, and are available from BGS sales desks and The Stationary Office (020 7873 0011).

Many BGS products and data are available in digital form under licensing agreement, details of which are available from the Intellectual Property Rights Manager, BGS, Keyworth. Digital datasets include those covering geochemistry, geophysics, geohazards, hydrogeology, borehole logs and mapping and allow the information to be used in GIS applications.

Geological maps

The district was originally surveyed at the scale of 1:63 360 by H T de la Beche, W E Logan and J Phillips and published as part of [Old Series] Sheet 41 in 1845, revised in 1857 with additional linework by W T Aveline. A small area along the southern edge of the district was surveyed at the 1:10 560 scale during mapping of the adjacent Ammanford district (England and Wales Sheet 230) by T C Cantrill and H H Thomas between 1901 and 1905 and H C Squirrel and J R Earp between 1965 and 1966. The district was resurveyed at the 1:20 000 scale by J R Davies, N S Jones, A B Leslie, D I Schofield, J Venus, R A Waters and D Wilson in 2005 as part of the GeoCymru Project, supported by a grant from the Welsh Assembly Government. A thin strip along the northern margin of the district was surveyed at the 1:10 000 scale by J R Davies, D I Schofield, R A Waters and D Wilson in 1996 and between 2002 and 2004.

1:1 500 000
Tectonic map of Britain, Ireland and adjacent areas, 1996.
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, 1994.
Sheet 51N 06W Lundy, Solid Geology, 1983.
1:50 000
Sheet 212, Llandovery, Bedrock and Superficial Deposits, England and Wales, 2008.
1:25 000
The component BGS maps of the district at this scale are listed below, along with the surveyors' initials and the dates of survey.

Map number	Surveyor	Date
SN 52	NSJ, ABL	2005
SN 53	DW	2005
SN 54	JRD, THS, RAW	1996, 2003
SN 62	ABL, DIS, RAW	2005
SN 63	DIS, JV	2005
SN 64	JRD, DIS, THS, DW	2003, 2004
SN 72	JRD, DIS, RAW	2005
SN 73	JRD, RAW	2005
SN 74	DIS, RAW, DW	2000, 2002, 2003

Geophysical maps
1:1 500 000
Earthquakes 1980–1999, British Isles and adjacent areas, 2002.
1:1 000 000
Gravity Anomaly Map, Southern Britain: 48ºN–54ºN, 6ºW–0ºE. 1998.
Magnetic Anomaly Map, Southern Britain: 48ºN–54ºN, 6ºW–0ºE. 2004.
1:625 000
Gravity anomaly map of the UK: South Sheet (2007).
Magnetic anomaly map of the UK: South Sheet (2007).
Hydrogeological maps
1:625 000
Sheet 1: England and Wales, 1977.
Groundwater vulnerability maps
1:100 000
Sheet 27: Dyfed, 1990.
Sheet 34: Pembroke, 1990.
Geochemical atlases
1:250 000
Stream waters in Wales, 2000.
Stream sediment and soil: Wales, 2000.

Books, reports and other publications

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

Documentary collections

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

Material collections

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

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation. BGS Library catalogue can be searched online at: http://geolib.bgs.ac.uk

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

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

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

Figures and plates

Figures

(Figure 1) Simplified bedrock geology, incorporating the principal geological structures of the district. Inset box shows position of regional structural lineaments. AF Abergwesyn Fault; CA Cothi Anticline; CFB Crychan Fault Belt; CGA Carn Goch Anticline; CSF Church Stretton Fault; CTF Craig Twrch Fault; CWL Central Wales Lineament; DF Dynefor Fault; DVF Dulas Valley Fault; GLF Garth–Llanwrtyd Fault Belt; LPFB Llandrindod–Pen-y-Waun Fault Belt; MSB Myddfai Steep Belt; PF Pontarsais Fault; SF Sawdde Fault; TF Teifi Fault.

(Figure 2) Late Ashgill to early Wenlock succession of the type Llandovery area as surveyed by BGS, compared with earlier schemes (Abbreviations: BMd Builth Mudstones Formation; CCy Cwm Clyd Sandstone Formation; Ceg Cefngarreg Sandstone Formation; CF Ciliau Formation; CgF Cwmcringlyn Formation; ChF' and ChF'' Chwefri Formation, lower and upper tongues; Cri Cribarth Formation; db slumped and disturbed strata; GHF Garth House Formation; Gll Glasalltfawr Sandstone Formation; md mudstone; MfS Mwmffri Sandstone Formation; Ntm Nantmel Mudstones Formation; Rdg Rhydings Formation; sa sandstone; TrF Trefawr Formation; Tyc Tycwtta Mudstones Formation; YA Yr Allt Formation; Yst Ystradwalter Member).

Plates

(Plate 1) Homogeneous arkosic sandstone, Ffairfâch Grit Formation, Carn Goch [SN 688 240] (P680306).

(Plate 2) Thickly bedded turbidite sandstone and mudstone debrites of the Taliaris Formation, hillside outcrops near Maes-y-Castell [SN 262 227] (P680307).

(Plate 3) Trichrûg Formation, Sawdde Gorge [SN 7275 2465] (P708660). Bioturbated, argillaceous, gritty debrites and siltstones. Ferricrete in the centre is concentrated on vertical Skolithos burrows.

(Plate 4) Quarried crags in the Dowlais Limestone Formation, Garreg yr Ogof [SN 777 215] (P680309).

(Plate 5) Peat bog, Mawnog Tyle Du [SN 778 217] (P680309), with exposures of Grey Grits Formation to the north (left) and Dowlais Limestone Formation to the south (right).

(Plate 6) Roman adit, Dolaucothi gold mine [SN 663 402] (P680311), Pumsaint, note wider top to excavation to allow ease of passage for miners shouldering ore baskets.

(Plate 7) Foundered ground and associated sinkholes caused by dissolution of underlying Dowlais Limestone Formation, Waun Bwdel [SN 782 214] (P680312).

(Front cover) Plateau Beds Formation, Tyle du [SN 780, 220] (PhotographerJ R Davies; (P680309))

(Rear cover)

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