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Geology of the Welshpool district — brief explanation of the geological map Sheet 151 Welshpool

R Cave

Bibliographic reference: Cave, R. 2008. Geology of the Welshpool district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 151 Welshpool (England and Wales).

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(Front cover) Front cover Powis Castle (Photograph P Witney; P624288).

(Rear cover)

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 151 Welshpool. National grid references are given in square brackets. Most of the district lies within the 100 km square SJ and the letter prefix for grid references in this area are omitted. However, for localities that lie within the adjacent 100 km square SO, the letter prefix is included. Symbols in brackets after lithostratigraphical names are the same as those used on the geological map.

Acknowledgements

This sheet explanation was written by R Cave, Honorary Departmental Fellow, Institute of Geography and Earth Sciences, University of Wales Aberystwyth (UWA), as part of a collaborative project between BGS and the UWA to produce the 1:50 000 Provisional Series geological map Sheet 151 under NERC contract GA/98E/25. The contract was supervised by R A Waters (BGS), who compiled and edited the Sheet Explanation. M Williams and S G Molyneux (BGS) identified Silurian graptolites and palynomorphs respectively and R Cave identified shelly fossils. R Evans provided geophysical interpretations. The text was copy edited by S G Molyneux; figures were drawn by Ian Longhurst, and page-setting was done by A Hill.

The British Geological Survey gratefully acknowledges the co-operation of all the landowners in the district during the geological survey. The geological survey of the Shelve Mining Field was funded by the former Department of Industry.

Geology of the Welshpool district (summary from the rear cover)

(Rear cover)

An explanation of sheet 151 (England and Wales) 1:50 000 series map

Historically, geology has been an important part of the economy of the Welshpool area: coal and vein minerals have been mined, and quarrying and brick manufacture were carried out.

Late Precambrian igneous rocks and fluvial metasediments crop out in the south-east, and were deformed during the late Precambrian Cadomian orogeny and are now faulted against younger rocks. Cambrian strata are absent. During the Ordovician and Silurian, distinct sequences were deposited to the east and west of the Severn valley, and the rocks record the history of development of the eastern margin of the Lower Palaeozoic Welsh Basin. To the east, the Ordovician Shelve Sub-basin accommodated 5000 m of marine sedimentary and volcanic rocks, and this succession was inverted—folded, uplifted and eroded—near the end of the Ordovician. To the west of the Severn, deposition continued more or less continuously through the late Ordovician in the main part of the Welsh Basin.

At the end of the Ordovician, the palaeo-shoreline retreated westwards, attributed to a drop in sea level caused by glaciation in the southern hemisphere. Later the shoreline migrated eastward, and this transgression is marked by lower Silurian shoreface sandstones and conglomerates. The Severn valley marks the south-eastern limit of this transgression of the early Silurian, and also the approximate line of transition between basin and shelf successions of the later Silurian. Marine sedimentation continued until the gradual infill of the basin in the late Silurian (Pridoli), when shallow marine, brackish and finally terrestrial environments developed.

No rocks of Devonian age are present. The sedimentary record recommences with coal-bearing, late Carboniferous strata. The overlying Permian and Triassic rocks were deposited in desert conditions following the Variscan orogeny.

Quaternary superficial deposits of the last (Devensian) glaciation and the Holocene include till and glaciofluvial deposits, which are widespread, and glaciolacustrine deposits occur in many eastern valleys. Head was later deposited under periglacial conditions. In the last 10 000 years, peat has accumulated in lake basins and alluvial sediments have been deposited by the river systems.

This sheet explanation provides a summary of the geology of the Welshpool district and also a synopsis of aspects of applied geology pertinent to planning and development, including mineral and water resources, potential geological hazards, engineering ground conditions and conservation.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology depicted on the geological 1:50 000 Series Sheet 151, Welshpool.

The district is essentially rural, embracing the towns of Llanfair Caereinion in the west, Westbury in the east, Berriew in the south and Llandrinio in the north, and it lies athwart the north–south border between England and Wales. The main rivers are the Severn, which crosses the district from south to north through Welshpool, and its tributary the Vyrnwy, which rises to the west. The highest parts of the district are formed by Long Mountain, at Beacon Ring (408 m), the Shelve area, at the Stiperstones (502 m), and the Breidden Hills, at Moel-y-Golfa (404 m). The rest of the district is dissected hill country.

The economy of the district is centred on agriculture, mainly grazing. In latter years, light industries have been encouraged, especially around Welshpool. Westbury in the east at one time thrived on coal mining, and Minsterley farther south was the centre of the West Shropshire (Shelve) Mining Field, where lead, copper, zinc and barite were worked in the nineteenth and early twentieth centuries. Many small quarries on the Ordovician igneous rocks once provided bulk minerals, but Criggion Quarry in the Breidden Hills is the only one still active. Bricks were produced until recently from Silurian mudstone at Buttington.

Early investigations in the Welshpool area include those by Boyd Dawkins (1869) and Bickerton Morgan (1885, 1891a, b), and Watts (e.g. 1885, 1886) described the geology of the Breidden Hills and Long Mountain. Spasmodic studies of more localised areas continued throughout the last century, notably those of Wade (1911), Wedd (1932), King (1928), Whittington (1938), Palmer (1970) and Cave and Price (1978). A field guide by Cave and Dixon, mainly on the Ordovician rocks of Welshpool and the Breidden Hills, was published in 1993. Graptolite biostratigraphy of upper Llandovery sequences in Buttington Brick Pit and the River Banwy was described by Loydell and Cave (1993, 1996), and Mullins (2000) described a chitinozoan lineage from the uppermost Llandovery and lowest Wenlock beds of the River Banwy section. The western part of the Hanwood Coalfield and the overlying Permo-Trias rocks were described by Pocock et al. (1938), and the mining operations in the West Shropshire (Shelve) Mining Field were described by Dines (1958) and Adams (1962; 1968). Unpublished work resides in PhD theses by Cave (1955), Palmer (1972) and Dixon (1991).

Geological history

The bedrock of the district comprises sedimentary, volcanic and intrusive igneous rocks, deposited and emplaced at various times in the Precambrian, Ordovician, Silurian, Carboniferous, Permian and Triassic periods (Figure 1). It is mantled by Quaternary superficial deposits of the last ice age (Devensian) and the last 10 000 years.

The oldest bedrock to crop out in the district comprises late Precambrian volcanic and sedimentary rocks deposited 570 to 560 Ma ago. They belong to the stable Midland Platform, which was a part of Avalonia, a crustal block on the northern margin of the ancient continent of Gondwana. Volcanicity related to subduction, accretion of volcanic arcs, and the formation of a narrow sedimentary basin account for a succession of late Precambrian extrusive and intrusive igneous rocks and a thick sequence of fluvial sandstones and mudstones. Today, only a small part of the Avalonian crust is exposed within the district. It occurs in the extreme south-east, where it forms the north-west flank of the Long Mynd. The sequence was folded and faulted, probably in the late Precambrian Cadomian orogeny.

Avalonia split off from Gondwana in the Early Ordovician, and drifted northwards towards the continent of Laurentia during the remaining Ordovician and early Silurian. The oceanic crust of the intervening Iapetus Ocean was subducted beneath Avalonia. During the early Palaeozoic, a linear north-north-east-orientated, downfaulted crustal warp lay to the west of the Midland Platform near the northern margin of Avalonia; this is referred to as the Welsh Basin.

The western margin of the Midland Platform was defined at different times by one of three north-north-east-trending parallel and spaced faults zones:

Severn Valley Fault Belt, which may extend south-south-westward into the Tywi Lineament
Pontesford–Linley Fault
Church Stretton Fault Zone, which lies beyond the south-east margin of the district (Figure 1)

These comprise the Welsh Borderland Fault System (Woodcock and Gibbons, 1988), which influenced the inception and evolution of the Welsh Basin. During the Ordovician and Silurian (495–417 Ma), the Welshpool district was situated along the south-east margin of the Welsh Basin. The basin acted as a trap for marine sediment and volcanic products until its infill and demise in the late Silurian.

During the Ordovician, a tract between the Pontesford–Linley and Severn Valley faults developed rather differently from the main basin to the west. Beginning in the Arenig, the tract subsided to accumulate 5000 m of marine sedimentary and volcanic rocks before the end of the Caradoc. Then, near the end of the Ordovician, this basin was tectonically inverted, and, unlike the deposits of the main basin to the west, those of the Shelve tract were folded and eroded, and are overlain unconformably by Llandovery strata. For such reasons, this component of the Welsh Basin is referred to as the Shelve Sub-basin and the tectonic episode as the Shelveian Event.

Following the deposition of a shore-zone facies, much of the sediment preserved in the Shelve Sub-basin was laid down as mud in poorly oxygenated conditions, but early in the Caradoc an event of wide significance brought an incursion of clean shelly sand, after which the sediments were more oxygenated. This event was coeval with the early Caradoc marine transgression of the Midland Platform.

Another significant event that occurred later in the Caradoc (post-Woolstonian) was the cessation of volcanism in the main basin. This is associated with the end of southward subduction of the Iapetus oceanic plate beneath Avalonia as it drifted northwards to collide with Laurentia. The response to this in north Wales was the development of a bathymetric high upon which there was initially no deposition. The resultant hiatus (of at least Marshbrookian and Actonian duration) was succeeded by deposition of black anoxic, mostly graptolitic mud that ubiquitously covered north Wales in Onnian times. By then, the basin had evolved, more-or-less, into a single entity, which, during the Ashgill and Silurian, was a simple, fault-bounded depocentre. However, this does not mean that sedimentation took place invariably in deep water, especially over north Wales. There, for most of the Caradoc, Ashgill and early Llandovery, mud, sand and volcanic products accumulated, with trilobite and brachiopod faunas of mid to outer shelf and shallower environments. At the same time, deep water lay over western mid Wales where graptolitic mud and turbidites accumulated. Thus, while the eastern margin of the broader subsiding basin still lay along the Welsh borders, a narrow steep slope down into deep water must have existed just to the west of this district, in the area of Lake Vyrnwy. Water over the shelly platform area in the district may in fact have been quite shallow for the Late Ordovician (Hirnantian) glaciation of the Gondwanan continent, which depressed sea levels by about 100 m (Brenchley, 1988), was sufficient to cause the shoreline to recede westwards from somewhere near the line of the Severn at Welshpool to Lake Vyrnwy. The maximum water depth of the late Hirnantian and mid Llandovery over the platform (including the district) must have therefore been less than 100 m.

The main facies divide of the Silurian lies roughly along the line of the Severn valley in this district. The marine transgression did not cross this to flood the Midland Platform until the mid Llandovery (late Aeronian). In the late Llandovery (Telychian) barren and graptolitic muds were deposited to the west of the Severn line, and a trilobite– brachiopod mud facies lay to the east. In the succeeding Wenlock, when the marine environment became largely dysaerobic (oxygen depleted), the Severn line lay at the top of a slope that declined westward towards the basin (around Lake Vyrnwy). An apron of laminated hemipelagites and fine-grained turbidites accumulated on this slope, and the Montgomery Trough lay to the west. This trough funnelled coarse turbidites from south to north. To the east of the Severn line, the slope-apron facies merged into a facies of hemipelagite and silt. Deposition within the Welsh Basin probably ended in the Pridoli, for although there are no rocks of Pridoli age in the basinal area west of the Severn to prove this, the succession east of the Severn indicates an evolution to shallow marine, brackish and ultimately terrestrial depositional environments.

As a result of the final collision of Avalonia and Laurentia, the Lower Palaeozoic rocks of the district were folded, faulted, uplifted and eventually eroded in the latest Silurian to mid Devonian during the Acadian orogeny.

The district probably remained land throughout the Devonian for no deposits of Devonian age are present. The sedimentary record in the district recommences with coal-bearing, fluvial strata of late Carboniferous (305 Ma) age, deposited when equatorial floodplain forest encroached from the north-east. The end Carboniferous Variscan orogeny, with its main fold-belt some 120 km to the south, tilted the Carboniferous rocks so that there is an unconformity at the base of the overlying Permo-Triassic succession, albeit with only a small angular difference in dip between the Carboniferous and Permian rocks. The latter were deposited under desert conditions in which erosion and deposition were contemporaneous. With increasing maturity of the landscape in the Early Triassic (about 248 Ma) there was a reduction in topographical relief, and the deposition of desert sediments continued; these probably overlap and unconformably overlie the Permian. These strata form the youngest bedrock of the district. They are succeeded only by the Quaternary superficial deposits of the last (Devensian) glaciation, 30 000 to 10 000 years ago, and the subsequent Holocene. During the late Devensian, glaciation was responsible for widespread erosion and deposition. Till and glaciofluvial deposits are widespread and glaciolacustrine deposits occur in many eastern valleys. Subsequently, head was deposited under periglacial conditions. In the last 10 000 years, peat has accumulated in lake basins and alluvial sediments have been deposited by the river systems.

Chapter 2 Geological description

Precambrian

In the south-east corner of the district, Precambrian rocks occur as a very small part of a larger outcrop, The Long Mynd, which lies mainly to the south-east of the district. They comprise parts of the Uriconian Group, of about 650 Ma, and the Bayston–Oakswood Formation, part of the Wentnor Group (Longmynd Supergroup), of about 600 to 575 Ma. On the Long Mynd, the Uriconian Group is at least 575 m thick, and the Bayston-Oakswood Formation some 900 to 1500 m.

The Uriconian Group (U) comprises a complex of extrusive felsic and mafic rocks, generally highly altered and commonly silicified. It is poorly exposed in the district, but immediately to the south, in the Montgomery district, the contiguous outcrop comprises basalts, rhyolites, undifferentiated volcanic rocks and sandstones, and is intruded by microgranite. The group is bounded to the west by the Pontesford–Linley Fault, and to the east by Longmyndian strata.

The overlying Bayston-Oakswood Formation (YWO) is composed of massive to poorly bedded, locally flaggy, purple sandstones with units of conglomerate up to 10 m thick. The formation was deposited by braided streams on an alluvial plain. The relationship with the Uriconian Group is uncertain but thought by Pauley (1991) to be faulted, possibly on a splay of the Pontesford–Linley Fault

Ordovician

The Ordovician rocks crop out as two distinct sequences, east and west of the Severn valley (Figure 2). The eastern sequence is exposed in the Shelve area and between Forden and the Breidden Hills.

In the Shelve area, there is a nearly complete sequence from low in the Tremadoc Series to the middle of the Caradoc Series. The outcrops at Forden and in the Breidden Hills correlate with the top part of this sequence. The western sequence ranges from mid Caradoc, with comparatively minor breaks, to the top of the Ashgill Series. In the east, around Welshpool, and everywhere east of the River Severn, this sequence has been truncated at the erosive local base of the Silurian strata.

East of the Severn valley

The Ordovician rocks of the Shelve area (British Geological Survey, 1991) form a discrete outcrop of marine sedimentary and igneous rocks, described in detail as part of the adjacent Montgomery district (Cave and Hains, 2001) (Figure 2); (Figure 3). The oldest rocks, comprising the Shineton Shale Formation (SnSh), are silty mudstones with thin sandstones of Tremadoc age. Only the upper part of the formation is exposed, as the unit is bounded to the east by the Pontesford–Linley Fault. The coarsening-upward sequence records a shallowing marine environment, possibly culminating in brief emergence and erosion.

Deposition of the overlying Arenig and Llanvirn strata commenced with a shoreface environment in the east of the district, and continued through a long period of marine deepening into an anoxic basinal environment. The basal shoreface deposits, the Arenig Stiperstones Quartzite Formation (StQ), form a striking ridge of crags. The formation comprises a hard, pale grey, quartz arenite, massive in the lower part but well bedded (beds 0.1 to 0.5 m thick) in the upper part. A slight disconformity may be present at the base. The overlying, mid to late Arenig Mytton Flags Formation (My) consists of flaser and lenticular bedded, thin, pale grey, intensively burrowed sandstones, commonly less than 10 mm thick, and darker grey mudstones. The percentage of mudstone increases upwards. Packets of strata that have been mapped as sandstone (sa) are mostly 20 to 40 m thick; they weather out from the rest and probably reflect a lower mud content rather than coarser sand. The formation records a nearshore shelf environment becoming deeper with time.

By Llanvirn times, with continued deepening, dysaerobic bottom conditions were established, and graptolites and a sparse trilobitic benthos are preserved in anoxic mudstones of the Hope Shale Formation (Hop). The Hope Shale was deposited on the mid to outer shelf, and the fining-upwards trend continues The formation is dominated by darkish grey mudstone with siltstone laminae. It is a thick formation that includes the local and largely submarine Hyssington Volcanic and Stapeley Volcanic members. The two members occur as separate, laterally impersistent wedges and lenses. The Hyssington Volcanic Member (HyS) is present in the lower part of the formation to the east of the Shelve Anticline, and the Stapeley Volcanic Member (SyV) occurs in the upper part of the formation, largely to the west. The Hyssington Volcanic Member consists principally of felsic vitroclastic crystal and lithic tuffs and tuffites. The Stapeley Volcanic Member, though of similar rock types, has a more mafic composition (andesitic and basaltic). Units of andesitic and basaltic tuffs and hyloclastites (Z^B), rhyolitic tuffs (Z^R), volcaniclastic sandstone and wackes (Z^Z), monomict conglomerates (cg) and andesite and basalt lava (B) are also present in the Stapeley Volcanic Member.

The late Llanvirn Weston Flags Formation (Wsn) is made up of fine-grained, planar bedded, flaggy, micaceous sandstone showing ripples and delicate cross-lamination, with interbeds of siltstone and mudstone. Locally, it is intensively burrowed (Skolithos), and it also contains a low-diversity shelly fauna indicating a change in the nature of the basinal water mass, or perhaps a marine shallowing. Several, more massive, tuffitic sandstones (sa) occur as individual beds up to 20 m thick, or as packets of beds up to 100 m thick.

The overlying Betton Shale, Meadowtown and Rorrington Shale formations pass gradationally from one into the next in upward sequence. They consist largely of soft, dark grey, pyritous mudstone deposited under dysaerobic bottom conditions. The mudstones of the Betton Shale Formation (Bet) are silty and micaceous, and in the south contain a thin basaltic tuff (Z^B). The Meadowtown Formation (Mtn) is distinguishable by its many impersistent beds of shelly calcareous sandstone, siltstone and limestone. The sandstones (sa) are commonly burrowed, demonstrating that oxygenated bottom conditions periodically prevailed. Dysaerobic bottom conditions were re-established, as reflected in the Rorrington Shale Formation (Rrr), which is dominated by rusty weathering, laminated hemipelagite, with a fauna of graptolites and orthocones. The formation contains the Llanvirn–Caradoc boundary, Nemagraptus gracilis occurring in the upper part.

Although there is a thin passage into the overlying Spy Wood Sandstone Formation (SpW), the latter marks a pronounced change in the sequence and probably a fundamental change in the depositional regime. The formation makes a narrow outcrop of well-bedded sandstone, which is pale to medium grey, medium to fine grained, calcareous and quartzose. It contains an early Caradoc (Costonian) shallow water shelly fauna, with Nemagraptus gracilis occuring about in the middle of the formation. Individual beds are planar, increasing upwards in thickness in the basal 5 m to a maximum of 0.2 m, but thinning upwards above this. Many are parallel-laminated, micaceous and fissile, weathering into flags about 2 cm thick. The formation resulted from a series of storm-generated currents that carried comparatively clean sand into a low-energy euxinic muddy environment of the Shelve Sub-basin, contrasting with the earlier Llanvirn volcaniclastic sands. At the same time, a marine transgression inundated large parts of the Midland Platform east of the Pontesford–Linley Fault. It is concluded that shore-face detritus produced during the transgression prograded into the deeper water of the sub-basin to form the Spy Wood Sandstone. After deposition of the Spy Wood Sandstone no more laminated hemipelagic muds were deposited in the Shelve area.

The remainder of the Shelve succession comprises three, early to mid Caradoc shale formations, the Aldress Shale (AdSh), the Hagley Shale (HgSh) and the Whittery Shale (WhSh), separated by two volcaniclastic units, the lower Hagley Volcanic Formation (Hgy) and the upper Whittery Volcanic Formation (Wty). Graptolites are preserved but are sparse, and there is little shelly benthos. Nevertheless, the shale formations and the Hagley Volcanic Formation all contain Soudleyan faunas, and the lower part of the Aldress Shale Formation is inferred to be Harnagian, although no Harnagian faunas have been proved in the Shelve area. The shale formations comprise soft, medium-grey to olive grey-green, micaceous mudstone. Thin, generally medium grey, fine-grained sandstones are also present. In the Aldress Shale, sandstones are sparse, relatively thin and lenticular. The thicker sandstones (sa), very few of which reach a thickness of several metres, are massive with reworked shell fragments. In the Hagley Shale, most of the sandstones are micaceous, up to 6 cm thick and up to 0.15 m apart. Cross-lamination and parallel lamination are common. Packets of thicker sandstones (sa) are present in the north, and include a quarried volcaniclastic sandstone [SJ 2856 0087] with interbedded mudstones. The sandstones in the Whittery Shale are similar to those in the Hagley Shale, and there are a few mappable tuffitic sandstones (sa) in the north of the outcrop. An impersistent unit of felsic tuff breccia (Z^R) is present in the northern part of the Hagley Shale outcrop.

The Hagley and Whittery volcanic formations are similar, formed mainly of brindled pale green to khaki, massive, volcanogenic, feldspathic sandstones, containing the remains of a shallow-water benthos. The sandstones were derived from nearby volcanic centres that were largely submarine, and were deposited mainly by high density turbidity currents. A spectacular volcanic breccia of dacitic and andesitic clasts in the Whittery Volcanic Formation is localised around Rockabank [SO 2801 9978]. These Soudleyan volcanic centres failed to deliver any of their products to the contemporaneous sequences of the Forden or Breidden Hills areas, or to the probably coeval Stone House Shale at Welshpool.

In the Breidden Hills area (Figure 2); (Figure 4), the early Caradoc (probably Costonian to Soudleyan) Stone House Shale Formation (StH) forms the bulk of the sedimentary sequence, and consists of soft, crumbly mudstone with graptolites, very similar to that of the Aldress Shale in the Shelve area. Sandstones are mainly thin and unobtrusive. Up to 500 m are present, but the base is not seen. At the base of the exposed sequence, cropping out in the River Severn [SJ 333 155], is a 'packet' of white and black sandstones interbedded with grey mudstone and known as the 'Black Grit' (Dixon, 1990). It is possible that this belongs to the gracilis Biozone (Cave and Hains, 2001) and thus would equate with the Costonian, Spy Wood Sandstone of the Shelve area. The rest of the Stone House Shale belongs to the multidens Biozone. A further packet of sandstones (sa) [SJ 3120 1472] occurs to the north of Bulthy. A white, very fine-grained felsic tuff (Z^R), probably only a metre or two thick, is present in the middle of the formation.

In the uppermost part of the formation, the Middletown Quarry Member (MiQ) (Middletown Formation of Dixon, 1990) comprises bright green, chloritised, rhyolitic tuffs and breccias containing white bentonitic material, and pebbly mudstones (debrites). The member is the product of a very localised, explosive volcanic centre. It has a maximum thickness of 60 m in Middletown Quarry [SJ 2990 1289], with two beds of mudstone in the top third, but it thins to a feather edge within 400 m along strike to the north-east. High in the quarry (and the member) is a listric growth-fault (Plate 1). Downthrow is directed eastward, and sedimentary units thicken abruptly on the downthrow side and are back-tilted into the slide-plane. Such a structure typifies contemporaneous collapse of sediment accumulating on a slope, and may indicate either an unstable cone or the inward collapse of a vent. A debris-flow entraining a tabulate coral (Cave and Dixon, 1993) must signify transportation from shallow water, perhaps volcanically warmed. At Trewern, 1.5 km to the south-west, the member is exposed in a small quarry [SJ 2831 1159].

The overlying Bulthy Formation (Bu) comprises a coarse conglomerate of rounded clasts of andesite. It rests upon and around the partly intrusive Moel-y-Golfa Andesite [SJ 292 124], and here it is 300 m thick. It thins north-eastwards (5 m thick near Braggington [SJ 3331 1462]) and becomes finer grained with beds of coarse feldspathic turbiditic sandstone and mudstone. The clasts were derived during the brief partial erosion of the Moel-y-Golfa Andesite (p.14), but there is no sign of this erosive event in the sequence west of the Severn valley.

The highest formation in the Breidden Hills is the Hill Farm Formation (HiF) of Soudleyan age, which consists largely of medium grey, bioturbated, silty mudstone with Broeggerolithus broeggeri. Thin beds (up to 15 cm) of storm-generated sheet sandstone occur, especially near Hill Farm [SJ 3207 1431] where they are concentrated into a sandstone-rich packet (sa). The formation crops out only in the east where the Silurian overstep is smaller. Up to 340 m are exposed.

The mid Caradoc (Soudleyan) Forden Mudstone Formation (FMF) crops out in the Forden area and northwards along the western edge of the Long Mountain, where at Buttington, it passes laterally into the Stone House Shale. The formation consists mainly of medium-grey, shaly mudstone with moderately abundant thin beds (up to 16 cm) of parallel and cross-laminated, fine-grained sandstone. Neither the top nor bottom is seen, but about 480 m of strata are estimated to be present. Andesitic conglomerates (cg) with B. broeggeri occur within the formation, seemingly as three discrete ellipsoidal masses up to 200 m across. They are identical with the conglomerate of the Bulthy Formation and that at Castle Hill [SO 222 968], Montgomery (Cave and Hains, 2001). A packet of sandstone (sa) is also present.

West of the Severn valley

West of the Severn valley (Figure 2); (Figure 5), the lowest beds exposed belong to the top part of the Stone House Shale Formation (StH) (Trelydan Shale of Cave and Dixon, 1993) and occupy the core of the Guilsfield Anticline. About 500 m are exposed. In the uppermost part, the Middle House Member (MDH) (50 m thick) is a grey-buff, blocky silty mudstone, which provides a passage up into the overlying Pwll-y-glo Formation and may correlate with the isolated exposure [SJ 221 080] of rather blocky mudstone (the 'Trilobite Dingle Shale' of Wade, 1911 and Cave, 1957) at Welshpool. The Middle House Member and Trilobite Dingle Shale contain a benthic fauna dominated by trilobites, with Broeggerolithus constrictus and Salterolithus caractaci and the graptolite Diplograptus foliaceus indicating a late Harnagian or Soudleyan age. (B. constrictus has been placed in synonomy with B. broeggeri by Bowdler-Hicks et al., 2000).

The Pwll-y-glo Formation (Pyg) consists of 235 m of grey to buff, bioturbated silty micaceous mudstone with abundant thin (typically about 0.1 m), hard, fine-grained, storm-generated sandstones. With increasing amounts of sandstone, it passes up into the upper Caradoc Gaer Fawr Formation (GFF), in which the siltstones and sandstones become thicker bedded while the micaceous silty mudstone becomes very subordinate. The formation is 200–360 m thick and contains an increasing upwards abundance and variety of brachiopods and trilobites, both indigenous and as current-winnowed fragments. Scattered crinoid fragments are also present, as are extensive rippled surfaces. The Gaer Fawr Formation reflects the generally fairly shallow shelf environment that had evolved across the Berwyn Hills to the north of the district at this time (Brenchley, 1978).

In the higher parts of the formation, the sandstones are calcareous with shallow water shell detritus, and contain small igneous clasts, shale lapilli and coarse feldspar sand, reflecting concurrent volcanism, probably in the Berwyn Hills and elsewhere in north Wales. Near Guilsfield, at least 12 m of calcareous mudstone of Woolstonian age caps the Longvillian sandstones, followed by about 115 m of thickly bedded calcareous sandstones with mudstone partings. The fauna in these latter sandstones includes the trilobite Estoniops alifrons, which characterises the Woolstonian Substage. These highest calcareous sandstones are the lateral equivalent of the Pen-y-garnedd Limestone of the Oswestry district (British Geological Survey, 2000), to the north. There, the Pwll-y-glo and Gaer Fawr formations have not been separated and the sequence is known as the Allt-tair-fynnon Formation.

Overlying these beds are the euxinic shales of the Nod Glas Formation (Nog). Over a wide area of the Berwyn Hills, to the north of the district, this formation overlies Woolstonian rocks, leaving a considerable non-sequence (Marshbrookian to early Onnian) between them, but no disconformity (Cave, 1965). It consists of up to 16 m of mainly soft, very fine-grained, black, pyritous shale with shelly (Onnian) and graptolitic (morrisi Subzone) faunas. A thin (less than 2 m thick) basal bed (or beds) of nodular phosphatic mudstone on black phosphatic limestone and comprises the Pen-y-garnedd Phosphorite Member.

The Ashgill Dolhir Formation (Dolh), incorporating the Trawscoed Mudstone of Cave and Price (1978), overlies the Nod Glas with a small and poorly quantified non-sequence (Cave and Price, 1978). It is 200 to 350 m thick and comprises massive, grey to buff, bioturbated silty mudstone with thin, silty, storm-generated sandstones. It contains a fairly sparse, small brachiopod and trilobite benthos of Cautleyan to Rawtheyan age and was deposited under low-energy, oxygenated conditions on the mid shelf. In places west of Meifod (p.13), a thin Rawtheyan limestone occurs at the top. The onset of the Late Ordovician Gondwanan glaciation in the early Hirnantian was accompanied by a glacioeustatic fall in sea level in the Welsh Basin (Brenchley, 1988). In this district, the recently deposited sediments were subjected to erosion and a karstic surface with clints developed on the Rawtheyan limestone near Meifod (Brenchley, 1993).

Throughout the Welsh Basin, rocks of latest Ordovician (late Hirnantian) age record the rise in sea level that followed the end of the Late Ordovician glaciation. This sea-level rise reached its acme in the early Silurian, and for this reason, latest Hirnantian facies normally form part of, or are contiguous with, formations that are predominantly of Silurian age. It is therefore convenient to describe them with the Silurian rocks.

Intrusive igneous rocks

Intrusions, mainly of microgabbro and andesite, occur as sills, dykes and possibly stocks in a number of Caradoc and older formations. They are of Late Ordovician age, probably Soudleyan.

Microgabbro (D) (dolerite) intrusions range in composition from alkali-olivine to tholeiitic, and comprise calcic plagioclase (up to 65% and with the composition of labradorite), clinopyroxene (25–30%) and ilmenite. In the Shelve area, a large intrusion [SO 3385 9908] into the Mytton Flags is of nodular appearance; the nodules, up to 10 mm in diameter, are highly altered plagioclase with large poikilocrysts of clinopyroxene. In higher formations to the west, a large sill-like body (possibly comprising four individual bodies) of similar microgabbro occurs at Stapeley Hill. The central parts of this body are gabbroic. A number of smaller sills intrude the Aldress Shale between Rorrington and Wotherton [SJ 2935 0039]. A swarm of dykes with a trend about 080º occurs between Estell [SJ 357 041] and Lower Wood [SJ 309 024].

In the Breidden Hills, microgabbro occurs in a complex of outcrops that probably comprise a multiple intrusion into the Stone House Shale, and occur little or no higher stratigraphically. It is typically a grey-green, largely fine-grained rock that is characterised by abundant white flecks of albitised plagioclase. Textural varieties range into gabbroic. Petrographically it is an albitised, olivine (up to 10%) microgabbro with 60 per cent plagioclase (oligoclase– andesine), a composition that suggests soda metasomatism. In form, the main mass in Breidden Hill has been thought of as a laccolith (Watts, 1886) with a thickness of at least 260 m in and around Criggion Quarry [SJ 291 143], but drilling has failed to reach the base. Therefore, this mass could be a discordant stock (Dixon, 1991). Associated with it, on its south-east side, are the intrusive masses of similar microgabbro that form higher ground, for example Old Mills Hill [SJ 288 130], New Pieces [SJ 295 137] and Brimford Wood [SJ 305 143]. They form linear, essentially concordant outcrops that become thinner breaking up laterally. Mindful of the regional dip (50º to 60ºSE), it is clear that these microgabbros lie immediately above the main 'stock' and are partially separated from it and from one another by 'leaves' of Stone House Shale. Thus the whole complex simulates a 'cedar tree' of feeders and anastomosing sills. Where exposed [for example [SJ 2936 1378]; [SJ 3009 1414]; [SJ 3001 1360], especially within the sill complex, the top and bottom microgabbro/sediment contacts comprise curved, almost pillowed surfaces, with sediment enveloping and invading the igneous rock. This indicates magma intruding wet sediment. Above the main intrusion, the stratigraphically higher leaves of the microgabbro become progressively more vesicular, the effect of upward decrease of the sediment overburden as the sediment/marine interface was approached and perhaps breached. Thus these microgabbros can be dated as probably mid Soudleyan.

At Lower Wood [SJ 3089 0264], at the northern limit of the Shelve Ordovician outcrop, a small intrusion has been quarried for roadstone. It consists of andesite (A) that is massive, pale greenish blue, trachytic and contains phenocrysts of plagioclase (andesine) up to 3 mm in length. A similar, smaller intrusion occurs nearby [SJ 3061 0211]. In the Breidden Hills, at Moel-y-Golfa [SJ 292 124], there is a large discordant intrusion of pyroxene andesite (Dixon, 1990). The rock is well jointed, massive and over 100 m thick, and the outcrop is about 1 km long. It is a grey-green aphanitic rock with microlites of plagioclase and a liberal scatter of tabular plagioclase phenocrysts, typically zoned. On its western side, the intrusion has an autobrecciated, peperitic and pillowed carapace (the marginal facies of Dixon). It is overlain by the Bulthy Formation, which is composed of rounded clasts of the underlying andesite. Dixon (1990) viewed the Bulthy Formation partly as a host sediment that also received andesite clasts as a result of late-stage gravitational collapse of the fluid-rich, pillowed and brecciated carapace of the intrusion. It is obvious that this intrusion, or nearby co-magmatic intrusions, were unroofed and eroded to contribute to the bulk of the Bulthy Formation, thus proving the intrusion to be of Soudleyan age.

The Standard Quarry [SJ 2183 0767] in Welshpool exposes part of a large body of trachyte (T), thought to intrude Caradoc mudstone. The irregularly shaped outcrop is some 800 m long and 300 m wide. The shape suggests that the body is discordant, but being situated in the Severn Valley Fault Belt, its shape may not reflect its original form. The age of the intrusion is further constrained by the fact that it is overlain unconformably by the early Llandovery Powis Castle Conglomerate. The contact is visible only in the quarry, where the conglomerate, composed of clasts of the trachyte up to boulder size, rests upon what is probably a very uneven surface of the intrusion. Well-developed columnar joints plunge at about 25º west-south-west. The trachyte is fine-grained and pale grey-green in colour. It consists uniformly of flow-aligned, small idiomorphic feldspars (albite-oligoclase) in a chloritic groundmass. The total absence of porphyritic feldspar makes a notable visual difference from the andesites east of the Severn valley. Another characteristic is the almost millimetre-scale 'voids' partially infilled by radiating growths of chlorite, possibly after pyroxene.

Silurian

The description of the Silurian rocks is also divided into east and west of the Severn valley, but largely for different reasons. Again the sequence on one side is more complete than that of the other, but in this case the youngest beds are present to the east, in the Long Mountain (Figure 4); (Figure 6). However, there are major differences between the Silurian rocks of the two sides, stemming from their environment of deposition. Where it crosses the district the Severn valley approximates to a divide between the Welsh Basin in the west and the Midland Platform on the east, so that the main differences are gradational and are expressed in a variety of sedimentary facies that merge one into the other.

West of the Severn valley

During the Silurian, the area west of the Severn valley was situated on the basin-ward side in deeper water (Figure 5), and in general accumulated a greater thickness of clastic marine sediment.

The Late Ordovician (late Hirnantian) postglacial rise in sea level (p.13), assisted by tectonic subsidance after the Shelveian event continued into the Silurian (Rhuddanian) and a marine transgression spread eastward over the recently eroded Ashgill and older rocks. In the west, the Graig-wen Sandstone Formation (GwS) was the earliest deposit laid down as a shore-face sand, probably in the late Hirnantian (persculptus Biozone). This is a thin, mature, but impersistent sandstone (12 m thick at Graig-wen [SJ 1025 0937]) that overlies the palaeokarstic surface capping the Rawtheyan limestone at the top of the Dolyhir Formation, and infilling the grykes. The sandstone is present south-west of Meifod but has not been recorded to the north-east. However, to the north-west, in the Dinas Mawddwy district, it is represented by the 'crassa Sandstone' of King (1923).

As the shoreface migrated eastward, diachronous sands and gravels, the Powis Castle Conglomerate Formation (PCC) of Rhuddanian and probably Aeronian age were deposited in the area around Welshpool. These transgressive basal deposits overstep south-eastwards onto successively older Ashgill and Caradoc formations, but occur only west of the Severn Valley Fault Belt, between Berriew [SJ 188 002] and Brook House [SJ 224 145]. The Powis Castle Conglomerate is mainly massive or poorly bedded. It is a heterogeneous deposit, derived in some places from local rocks and in others from a distant terrain to the east beyond the Severn Valley Fault Belt. From Welshpool, where it is an ill-sorted rudite of trachyte, it is replaced southwestwards by a more mature, well-sorted conglomerate of rounded quartz, volcanic and sedimentary clasts at Powis Castle [SJ 2170 0652]. Farther south-west [SJ 2027 0422], the formation is a coarse quartzose sandstone with rounded clasts of quartz and mudstone. At Rose Hill [SJ 1877 0020], below the Laundry Mudstone, there are about 5 m of well-bedded, thick conglomeratic sandstones with clasts of quartz, igneous rocks and mudstone; part of the sequence appears to be overturned. Beneath this are 1.5 m of conglomeratic 'melange'. It seems likely that the early Llandovery shoreline along the western side of the Severn valley was at least influenced by the Severn Valley Fault Belt. North-westward from Welshpool, the composition of the conglomerate is equally varied; at Ceunant [SJ 2188 0829] it includes beds of sandy limestone.

Although not prolific, the fauna is indicative of shallow water. It includes brachiopods, corals, bryozoa and a few trilobites, which suggest that the eastward transgressive Llandovery shoreline had reached the Severn valley in late Rhuddanian times. In places, the basal part of the overlying Laundry Mudstone Formation (LdM) comprises less than 10 m of thickly bedded, dark grey, micaceous sandstones (sa) with mudstone partings. The rest of the formation is grey-buff mudstone in which thin, storm-generated sandstones are common. It contains Rhuddanian brachiopods (Cave 1955), but its top may range into the Aeronian, for the overlying Tarannon Mudstone Formation is of Telychian age on both sides of the Severn valley, without any evidence for a hiatus below. The Laundry Mudstone Formation was deposited in a shallow shelf environment.

The Severn valley marked the most south-easterly position of the early Silurian shoreline until the early Telychian. Then it became the approximate position of the upper slope of the Welsh Basin, marking transitions between shelf and basinal facies until the early Ludlow.

The Telychian Tarannon Mudstone Formation (Tar) is a widely distributed and generally unfossiliferous, bioturbated, soft mudstone, commonly pale grey-green and maroon in colour. In the River Banwy [SJ 132 104] it is 122 m thick. In a stream [SJ 2418 1198] at Varchoel Hall, contacts between the maroon and green mudstones are sharp. The former are homogeneous whereas the latter are faintly silt laminated. The mudstones were deposited under oxic bottom conditions. Thin layers of dark grey, graptolitic, hemipelagic mudstone are also present, recording the first, albeit brief, episodes of Silurian marine-bottom anoxicity across the district. Distant volcanism provided ash that is preserved as numerous clay bentonites within the succession. The formation is absent in an area around Welshpool, between Guilsfield and Leighton, but replaced by up to 5 m of pale greenish, soft shelly mudstone, the Ty-brith Mudstone Member (Tyb), which is only exposed sporadically and too thin to map. This possibly condensed sequence was deposited on isolated shoals of the sea bed, which were colonised by a deep-water shelly fauna that included Dicoelosia and Meifodia, amongst other small brachiopods. Surrounded by Tarannon Mudstone, the bathymetric high was probably tectonically induced.

At the beginning of the Wenlock, there is evidence of widespread dysaerobic marine deposition, and the development of the north–south-orientated Montgomery Trough in the west of the district. The lower part of the Nant-ysgollon Mudstone Formation (NyG) is dominantly of graptolitic, laminated, dark grey, anoxic hemipelagite that was deposited on the slope-apron. Slumped and debritic units are present locally (Plate 2). At the base of the formation the Banwy Member (Ban) overlies the Tarannon Mudstone Formation (Figure 6) forming passage beds that comprise interbedded grey, tough, silty, bioturbated mudstone and graptolitic, laminated, hemipelagic mudstone. The member is 36 m thick in the River Banwy [SJ 1339 1025] near Mathrafal. It contains the base of the Wenlock Series. The Nant-ysgollon Mudstone Formation is early to mid Wenlock in age but in the extreme west only early Wenlock strata are present.

In the west, near Llanfair Caereinion, the feather edge of the Penstrowed Grits Formation (PdG) (Denbigh Grits of the adjacent Bala district), tongues into the lower part of the Nant-ysgollon Mudstone. The Penstrowed Grits consist of thick beds of coarse greywacke turbidite and high-matrix sandstone with subordinate mudstone. These southerly sourced, sandy turbidites were funnelled northwards along the Montgomery Trough. In the north-west of the district, the formation tongues into the Gregynog Mudstone Member of the Nantglyn Flags Formation. Here it comprises disturbed beds (db), predominantly composed of slumped thick-bedded sandstones. The Penstrowed Grits are of early to mid Wenlock age.

Over most of the western part of the district, the Nant-ysgollon Mudstone Formation is overlain by the Nantglyn Flags Formation. The Gregynog Mudstone (GYM), the lowest member of the Nantglyn Flags Formation (NgF), is confined to the south-west and north-west of the district. It consists of mass-flow and slumped anoxic mudstone and, where bedding remains intact, partly of turbidite mudstone–sandstone couplets. These deposits result from destabilisation of the palaeoslope on the eastern margin of the Montgomery Trough, thereby creating a mix of anoxic debris flows and slope-apron deposits. Eastwards, up-slope, this facies passes laterally into anoxic slope-apron mudstone of the Nant-ysgollon Mudstone Formation of mid Wenlock age.

The Mottled Mudstone Member (MMu) of the late Wenlock to early Ludlow Nantglyn Flags Formation was deposited across all these middle Wenlock facies (Figure 6). The member is a pale grey, commonly fawn-coloured calcareous mudstone with characteristic flecks of rusty goethite. It is very bioturbated, with an impoverished fauna of small brachiopods and the graptolite Gothograptus nassa. Bedding is poorly developed or massive, and the member usually forms one 'bed' less than 2 m thick, as at Pentre'r beirdd [SJ 1905 1423] (Plate 3), but in places appears as two leaves, for example west of Big Forest [SJ 153 103]. In many other places it is absent or has been involved in slumping. Interbedded calcareous bedded mudstone is present in places and contains G. nassa and Pristiograptus pseudodubius of the nassa Biozone. Immediately above the Mottled Mudstone at Pentre'r beirdd [SJ 1904 1429] there is a 2.1 m-thick bed of vitroclastic tuff.

The remainder of the Nantglyn Flags Formation above the Mottled Mudstone Member consists of thinly bedded units, less than 10 cm thick, each comprising a basal thin turbidite sandstone overlain by grey homogenous turbidite mudstone and capped by dark grey, laminated, anoxic hemipelagite. These turbiditic rhythmites are probably slope-apron deposits, which upwards and north-eastwards consist predominantly of anoxic hemipelagite and are thus similar to the Nant-ysgollon Mudstone below. Such is the similarity of these formations that there is doubt as to which is present in the widely drift-covered, structurally complex area between the Severn valley and Dyffryn Meifod, where the crucial Mottled Mudstone Member is impersistent. In such circumstances, the two formations are shown as undivided (NyG/NgF) on the map. Disturbed beds (db) in the Nantglyn Flags comprise slumped, chaotically bedded mudstone with incorporated masses of sandstone and, near the base of the formation, pale grey mottled mudstone. In places, the latter has been derived from the Mottled Mudstone Member, thus explaining the absence of the member in some outcrops. On Yr Allt [SJ 245 101] the topmost beds of the Nantglyn Flags (nilssoni Biozone) have a fissility similar to that of the coeval Gyfenni Wood Shale Formation, east of the Severn valley (p.20).

In the slightly later Ludlow (scanicus or tumescens Biozone) sediment was carried into the region from the south-east. Each incursion of sand was followed by silty mud simulating the turbidite rhythm. These sediments spread westward well beyond the Severn valley, seemingly undiminished or confined by lateral bathymetric slopes. They comprise the mid Ludlow Bailey Hill Formation (BAI), which is the highest formation west of the Severn valley, occurring as synclinal outliers on some of the highest ground. The formation consists of sandstone and mudstone. In parts, the sandstones are dominant. The mudstone is silty, grey-buff and homogenous. Thin anoxic laminated hemipelagites do occur, in such a way as to indicate that the sandstones and mudstones are related couplets, possibly initiated by storms or by slumping to the south-east. In some places there is a basinal conglomerate, the Bryn-rorin Conglomerate (cg), at or near the base. The clasts are well rounded and commonly of Nantglyn Flags lithologies. Within the formation, units of disturbed beds (db) are also present.

East of the Severn valley

It was not until the mid Llandovery (late Aeronian) that the sea advanced east of the Severn valley, and then rapidly inundated an area extending many miles eastwards, towards Church Stretton (Figure 6). In the Welshpool district, specifically around Long Mountain, the oldest Silurian deposit is the Cefn Formation (Cfn) (Figure 4). Resting unconformably on Ordovician strata, the Cefn Formation has a basal conglomeratic sandstone, a shoreface deposit about a metre thick containing Pentamerus and probably of latest Aeronian age. The rest of the formation falls within the late Llandovery (Telychian: turriculatus Biozone s.l.) and consists of an alternation of grey silty mudstone and thin beds of hard, cross- and wavy-bedded sandstone, some containing laminae of sand-sized, black, bitumen-coated grains. The sandstones decrease in thickness and abundance upwards and were the product of a shallow, but deepening, shelf sea characterised by storm-generated sheet sandstones, which in the lower part are up to 15 cm thick. Their waning in the upper part of the formation reflects progression to a more tranquil sea; a 10 cm bed of shelly silty mudstone with bryozoa in growth position near the top of the formation in Buttington Brick Pit [SJ 267 102] attests to this. The formation is broadly similar to the Laundry Mudstone Formation to the west, but to the east, in the Shelve area, it passes into sandstone-dominated shoreline and nearshore deposits of the Pentamerus Sandstone Formation (PeS), which contains an abundant shelly fauna, including the brachiopod Pentamerus.

The Tarannon Mudstone Formation (Tar) overlies the Cefn Formation east of the Severn valley, for example at Buttington. Within 5 km north-east of Buttington, however, the graptolitic Tarannon Mudstone passes into a shelly, dominantly maroon to purple silty mudstone, the Purple Shales Formation (PuS). This formation then dominates the Telychian deposits of the shelf south-eastwards to Church Stretton. From the end of turriculatus Biozone s.l. times, mud deposition of Tarannon Mudstones/ Purple Shales facies dominated during the rest of the Telychian of the district.

As on the west side of the Severn valley, the base of the Wenlock lies within the Banwy Member of the Nant-ysgollon Mudstone Formation; the member represents a transition from anoxic to oxic conditions. In the east, it is 9 m thick and is overlain by the Trewern Brook Mudstone Formation (TBM) (formerly the Bromleysmill Shale Formation of the adjacent Montgomery district; Cave and Hains, 2001). This comprises 450 to 475 m of rather dark grey, silty, graptolitic, hemipelagic mudstone, which is much diluted with fine terrigenous silt and mud, probably from nepheloid plumes. It is the equivalent of the Nant-ysgollon Mudstone and Nantglyn Flags formations, but being high on the eastern slope of the basin, it was largely above the effects of turbidity currents yet largely uninfluenced by storms. The Mottled Mudstone Member (MMu) of the Nantglyn Flags Formation tongues into the upper part of the Trewern Brook Mudstone in the west. Farther east, however, most of the upper part of the Trewern Brook Mudstone on Long Mountain is replaced by medium grey to buff, burrowed silty mudstone with a shelly fauna, similar to the Mottled Mudstone. This comprises the Aston Mudstone Formation (AST) of late Wenlock age.

Deepening, and renewed deposition of graptolitic mudstones, the Gyfenni Wood Shale Formation (GFS), took place during the early Ludlow (nilssoni to scanicus biozones). The mudstones overlie the Trewern Brook Mudstone and Aston Mudstone formations, and comprise anoxic, very fissile, graptolitic, silty mudstone with abundant thin, fine-grained sandstones. They are absent from part of the south-west flank of Long Mountain, together with the top part of the Trewern Brook Mudstone. The missing beds probably slid away downslope penecontemporaneously to be reconstituted with the basal part of the Bailey Hill Formation (BAI). These are shown as disturbed beds (db) in the south-eastern outcrop [SJ 280 033]. The Bailey Hill Formation, succeeds the Gyfenni Wood Shale west of a line from Brockton [SJ 319 045] to Trewern [SJ 281 113], with sandstones up to 70 cm thick in the south-west (Plate 4). The highest part of the Bailey Hill Formation, the Cwm-yr-hob Member (CYH), contains more mudstone and is more fissile with thin, fine-grained sandstones interbedded with laminated hemipelagic shales that have yielded graptolites of the leintwardinensis Biozone and Bohemograptus proliferation interval; the member is 70 to 100 m thick.

Eastwards, the sandstones of the Bailey Hill Formation thin and are replaced by scattered thin siltstone laminae in tough silty mudstone. This lateral facies forms a separate formation, the Irfon Formation (IrF) (the Striped Flags of Kirk, 1951), which is also overlain by the Cwm yr Hob Member. In the extreme east [SJ 3475 0830] and low in the Irfon Formation are some slumped mudstones containing a scatter of boulders and concretions of allochthonous limestones and shells (Palmer, 1972).

Deposition in the district during the Wenlock and most of the Ludlow was almost wholly dysaerobic except in the extreme east. In late Ludlow times, however, marine conditions became totally oxygenated as the wider Welsh Basin became shallower and was gradually infilled. Initially, low-energy distal-shelf conditions prevailed, and the Knucklas Castle Formation (KCB) was deposited. This formation comprises rather massive, bioturbated, greenish grey, very argillaceous siltstone, with wispy laminae of pale grey siltstone and fine-grained sandstone that increase in abundance upwards. It contains a few brachiopods but no graptolites. The overlying Cefn Einion Formation (CEF) is also a grey-buff, silty mudstone, very bioturbated but with a significant shelly content and scattered fine-grained, storm-generated sheet sandstones up to 10 cm thick. It represents a shelf environment nearer to shore, with incursions of storm-generated sand and a profilic shelly benthos. The top few metres of the formation incorporate calcareous siltstones with coquinas of shelly debris that include Turbocheilus helicites, a gastropod that is characteristic of this marine to brackish transition facies across the Welsh Borderland.

A marked change introduces the Pridoli Series. The basal Tilestones Formation (Til) consist of very evenly and thinly flaggy micaceous sandstone, which weathers to a yellow colour. The Tilestones preserve a segment of the sandy shoreface facies deposited in the short period of transition between the fully marine Ludlow and the coastal silty mud flat environment with periodic marine incursions of the overlying Temeside Mudstone Formation (TSh). The latter comprises a massive to irregularly flaggy argillaceous siltstone with small calcrete nodules. Above it lies the red mudstone of the Raglan Mudstone Formation (Rg), which contains impersistent beds of red or green channel-fill, cross-bedded sandstone. Calcrete nodules are again present in the mudstone. The formation was deposited on a broad alluvial plain that was subject to alternate flooding and exposure to pedogenic processes.

Carboniferous

The Acadian (Early Devonian) orogeny initiated a period of uplift and subaerial erosion that incised into the older folded rocks, and removed any later Silurian and earlier Devonian rocks. Later Devonian and early Carboniferous rocks are not preserved but may never have been deposited. In the late Carboniferous (Westphalian D), the district formed part of the Wales-Belgium landmass, onto which fluvial systems encroached from the north and east.

Carboniferous rocks, forming part of the Warwickshire Group, are limited to the Westphalian D and possibly early Stephanian strata of the western part of the Hanwood Coalfield, described in detail by Pocock et al. (1938). At the base is the upper part of the Etruria Formation (Et) (formerly the Ruabon Marl Formation), which rests unconformably on Ordovician and Silurian strata and consists of about 7 m of red and purple calcareous mudstone. Two or three freshwater limestones, one with the gastropod Spirorbis, occur near the top of the formation. Where it overlies the Raglan Mudstone Formation, the base is difficult to identify. The Etruria Formation mudstones were deposited on a well-drained alluvial plain fed by low-sinuosity ephemeral streams. They are overlain by about 120 to 160 m of greenish white and grey, cross-bedded sandstones and subordinate grey mudstone, correlated with the Halesowen Formation (Ha) (formerly Coed-yr-allt Formation). The sandstones (sa), 10 to 20 m thick, form two discontinuous ridges consisting of packets of sandstone-rich strata. Five coal seams are present: the three thickest are the Thin (Th), the Yard (Y) and the Half-Yard (HY), each underlain by a seatearth or mudstone with seatearths. The Halesowen Formation reflects a change to a poorly drained alluvial plain with more active streams. The Salop Formation (Sal) (formerly Erbistock Formation) at the top of the Carboniferous sequence is over 500 m thick, and is marked by the abrupt appearance of 12 m of red and purple and locally green mottled mudstone, followed by soft weathering feldspathic and pebbly sandstones of green, brown and purple hues, with subordinate purple mudstones. Individual sandstones (sa) are up to 11 m thick, and, as in the Halesowen Formation, form ridges as though concentrated in bundles 45 to 75 m thick. The formation is Westphalian D in age, but may range into the earliest Stephanian. It reflects a return to a well-drained alluvial plain with active streams.

Permo-Trias

The Permo-Triassic rocks of the district were deposited under desert conditions, following Variscan faulting, folding and uplift and subsequent erosion. The Permian Alberbury Breccia Formation (AB), tentatively placed in the Warwickshire Group, is strongly unconformable on the Salop Formation, and infilled irregularities in the palaeo-landscape. It is thus of variable thickness, ranging up to about 110 m. It is composed of large (up to 30 cm diameter) to small, angular to subrounded fragments of Carboniferous limestones and sandstones, some quartzitic, in a red calcareous sandstone matrix. It was probably formed by flash floods that deposited a series of alluvial fans in the valleys and lower ground. The overlying Kinnerton Sandstone Formation (KnS) (formerly Lower Mottled Sandstone Formation) is a bright red, soft sandstone, commonly cross-bedded and pebbly only at the base. It is regarded as being mainly aeolian in origin. The base is not exposed in the district, but as it overlaps the Alberbury Breccia and is banked against Precambrian rocks to the east, it is inferred to be unconformable. The Chester Pebble Beds Formation (CPB), above, comprises dull red, coarse-grained sandstones, commonly with pebbles of quartz and quartzite, both scattered and in lenses. The formation may mark the base of the Triassic but its outcrop is concealed by superficial deposits.

Structure and metamorphism

Folding

The district has been affected by several tectonic events in the last 600 Ma, the earliest being evident in the Precambrian rocks. These were folded during the Cadomian Orogeny (about 550 Ma) into a large, asymmetrical, north-north-east trending syncline with an overturned western limb. The axis lies 1.5 km to the south-east of the district.

Localised folding occurred at the end of the Caradoc, mainly affecting rocks just to the west, in the Bala district. It was mild, but produced an unconformity at the base of the Dolhir Formation. Folding and uplift recurred again at about the end of the Ordovician Period, the Shelveian Event, with greatest effect in the terrain adjacent to and east of the Severn valley. For instance, the Ordovician rocks of Shelve were folded into the broad north-north-east-trending Llan Syncline and complementary Shelve Anticline, folds that do not continue up into the overlying unconformable Silurian (Llandovery) strata. West of the Severn valley, the differential in folding between Ordovician and Silurian rocks is much smaller. For example, at Guilsfield the Powis Castle Conglomerate oversteps a syncline of Ordovician rocks as old as Soudleyan but approaching Meifod from the east the differential disappears and folds in the Ordovician rocks match those in the Silurian; only a nonsequence remains and that is attributable to the end-Ordovician (Hirnantian) temporary drop in sea level. The folding must therefore have occurred between the late Hirnantian and the very early Rhuddanian. Sedimentation was renewed in this area by the eastward transgression of the Llandovery sea.

The most pervasive folding of the district occurred during the late Early Devonian Acadian Orogeny. West of the Severn valley the main folds are broad anticlines and synclines that plunge gently off the Berwyn Dome to the north-west of the district. They include the horst-like Guilsfield Anticline, faulted along each limb. The folds trend north-north-east and are asymmetrical, with steeper dips to the east-south-east. They tend to tighten south-eastwards towards Welshpool, where they merge into the plexus of the Severn Valley Fault Belt. East of the Severn valley the district is dominated by the very open Long Mountain Syncline. This too has a north-north-east trend and, like all the Acadian folds of the district, swings east-north-east in the north. The syncline affects all strata from the Ordovician of the Breidden Hills up to and including beds of Pridoli age. In the Breidden Hills, the strike swings dextrally (clockwise) into the east-north-east direction noted above. Thus, the alignment of the palaeomagnetism of the igneous rocks here has a 12º dextral difference from that of the igneous rocks of the Shelve area, and a physical rotation of the rocks of the Breidden Hills has been suggested (Piper, 1995).

Acadian cleavage is weak and developed only locally, mainly in the mudrocks west of the Severn valley. It is strongest in the north-west, where dips are commonly below 60º to the north-west towards the Berwyn Dome, swinging dextrally with the trend of the folds (cf. Shackleton, 1953).

The uplift and the erosion that followed the Acadian orgeny resulted in the unconformity at the base of the late Carboniferous sequence, which oversteps across the Ordovician and Silurian rocks of the Long Mountain Syncline. There is no basal conglomerate, but the rocks below are strongly reddened.

The end-Carboniferous Variscan Orogeny, with its fold-belt some 120 km to the south, gave rise to the unconformity at the base of the Permo-Triassic sequence. Above the unconformity, the sporadically developed basal conglomerate, the Alberbury Breccia, contains clasts from sub-Etruria formations that are now missing from the district. Here, however, there is only a small angular difference in dip between the Carboniferous and Permian rocks.

Faults

The Pontesford–Linley Fault throws Precambrian against Ordovician strata in the Shelve area. Clearly it was active before the late Llandovery for it does not affect the onlapping Silurian rocks.

The Severn valley, particularly its western side, coincides with a north-north-east-trending plexus of normal faults, the Severn Valley Fault Belt. Many downthrow to the east, others to the west; transcurrent movement may have occurred, but is not obvious. Several faults splay south-westward from this plexus between Welshpool and Berriew and pass distally into Silurian rocks. The major Guilsfield Fault, downthrowing to the west, may also be a south-westward splay of the Severn Valley Fault Belt, but its point of divergence from the latter, somewhere near Arddleen [SJ 260 160], is concealed by thick drift. However, this is also the postulated point of intersection of the post-Acadian, north-east-trending Wem Fault when projected from the margin of the Permo-Triassic Cheshire Basin. Thus it is conceivable that the Wem Fault crosses the Severn Valley Fault Belt to continue south-westward as the Guilsfield Fault. The Guilsfield Fault has itself several south-westward splays. They are of small displacement and show mainly where they cross the broad Ordovician outcrop between Burgedin [SJ 239 145] and Groes-lwyd [SJ 211 113]. A fault similar to the Guilsfield Fault may be concealed beneath Dyffryn Meifod, and what appear to be west-south-west splays from it are most obvious where they too cross the Ordovician outcrop.

The age of these faults is not fully established. The Severn Valley Fault Belt at Welshpool is associated with heavily hematised wall rocks for example [SJ 2209 0770], and thus may have terminated upwards at an early Permo-Triassic weathering surface. It displaces strata as young as Ludlow so movement may have been Acadian. However, one fault, the Llyswen splay [SJ 2248 1350], seems to terminate the outcrop of the Caradoc Gaer Fawr Formation, while the Llandovery Powis Castle Conglomerate passes across the fault with little displacement. If correct, this implies pre-late Rhuddanian movement. Thus some faults may have been initiated at the time of the end-Ordovician folding and reactivated during the Acadian orogeny. The Severn Valley Fault Belt may fall into this category in that, during Rhuddanian times, it separated deposition in the north-west from erosion to the south-east.

There was less faulting to the east of the Severn valley, the most obvious faults being those in Carboniferous and Permo-Triassic strata. Most of these post-Acadian normal faults trend between north and east-north-east, producing horsts and graben.

Geophysics

The Bouguer gravity anomaly and aeromagnetic anomaly data for the district and surrounding area are displayed in (Figure 7a), (Figure 7b) as contoured maps, with shaded relief to enhance the main features. Carruthers et al. (1992) have described these datasets and the constraints they impose on the structures in Central Wales.

The Bouguer gravity anomaly data (Figure 7a) indicate structures with density contrasts at depth extending south-westward from the Permo-Triassic Cheshire Basin (see Gale et al., 1984) into the Lower Palaeozoic rocks of the district. A major gravity gradient to the north divides southward at Arddleen on the northern edge of the district, and thence two branches continue southward and south-westward, respectively, fading at the southern limit of the district. A comparison of the anomaly map with the simplified geological map reveals a close coincidence of the anomalies with the Severn Valley Fault Belt and the Guilsfield Fault, respectively, implying that these are deep-seated fractures. Some small, short-wavelength gravity lows have been described in Cave and Hains (2001), for example G11d on their figure 37 where the Aylesford Brook [SJ 270 006] joins the Camlad Valley, near Marton. Others, for example G11c, occur in the Severn valley [SJ 243 082], about 4 km north-east of Welshpool. These small anomalies indicate channels infilled with thick Quaternary deposits in the river valleys.

The aeromagnetic anomaly data (Figure 7b) reveal a large and extensive anomaly centred on the Long Mountain Syncline, which suggests an association of deep basement structure with Lower Palaeozoic sedimentation. Close to the southern margin of the district, short wavelength magnetic anomalies are associated with the Disgwlfa microgabbro and a specific magnetic layer within the Corndon Hill microgabbro (see Cave and Hains, 2001). An anomaly of about 50 nT from the original 1950s aeromagnetic surveys is associated with the Breidden Hills, and this anomaly has also been confirmed on the south-western edge of a more recent high-resolution survey of central England (Peart et al., 2003). Outcrops of microgabbro at Criggion Quarry [SJ 287 142] and Crewgreen Quarry [SJ 3185 1575] on the northern edge of the Breidden Hills have a significantly higher magnetic susceptibility than the other rock types measured in the district. These magnetic microgabbros occupy the lowest exposed parts of the intrusion and they could explain the aeromagnetic anomalies detected around the Breidden Hills.

Piper (1995) discovered that the Breidden microgabbro and indeed similar microgabbros in the Shelve Area possessed dual magnetic polarity, and this he related to a reversal of the Earth's magnetic poles during primary magmatic differentiation of the cooling magma. The same differentiation of heavy iron-rich minerals might also have been the cause of the distinctive magnetic signature of the basal parts of the intrusion. Piper also revealed that the declination of the magnetic remnance in the Breidden microgabbros is offset in a clockwise direction relative to that in similar microgabbros in the Shelve area. The difference is the same (probably about 12º) as the difference between the trends of the fold axes of the two areas and Piper considered that it might be the effect of subsequent differential block rotations.

The recent earthquake activity of the region around Welshpool is described in Cave and Hains (2001), including the nearby earthquakes at Bishop's Castle in 1990 of magnitude 5.1 ML, and Newtown in 1994 of magnitude 3.1 ML.

Quaternary

Quaternary deposits that predate the last glaciation (Devensian) have not been identified in this district. Devensian deposits (Figure 8) include irregular spreads of till (boulder clay) over hills and slopes and in valleys. This consists of a mix of grey silt, clay and ill-sorted rock-clasts, and since most of the ice that entered the district came from mid Wales, the clasts are mainly of greywacke, mudstone and a small proportion of quartz. In areas east of Long Mountain, erratics of microgabbro from the Breidden Hills are common and indicate the passage of ice from the north, not from Wales. However, the matrix of these tills is not obviously red or sticky like the tills of the Irish Sea ice sheet in Shropshire and Cheshire. This area may have been at the junction between the Welsh and Irish Sea ice sheets. Field brash also includes clasts of Alberbury Breccia, which likewise might indicate derivation from the north, but the possibility that farmers might have spread this material for its content of lime cannot be discounted. The evidence for ice having overridden the Shelve area is equivocal (Cave and Hains 2001). Deposits of brown, stony, silty clay do occur across the high ground of the Shelve area, but it is possible that many of these deposits may have formed by solifluction under periglacial conditions, perhaps incorporating till. Drumlins are hills of various sizes that formed as comminuted rock debris in the base of the ice sheet. They are oval in plan, with long axes of up to one kilometre aligned with the local direction of ice flow; in the Severn valley and around Guilsfield, this was to the north-east, but approaching the Tanat/Vyrnwy valley was east-north-east. The height of the drumlins (up to 50 m) probably equates with the thickness of the till.

Valley-bottom till with hummocky topography, which initially blocked the valley bottom drainage, for example across the northern end of Dyffryn Meifod, has been classified as hummocky glacial deposits. (morainic drift). It was released from the icefront, probably at a pause in its final retreat. As a result, it contains less clay and is more clast-rich than normal till, and in part is water sorted. Transitory drainage blockage by glacial deposits and other agents, such as the glacier of a main valley blocking the exit of a side-valley already free of ice, ponded melt-water to form proglacial lakes. In these, glaciolacustrine deposits of silty clay, commonly laminated, accumulated, together with, and usually interbedded with, gravel introduced by rivers from the valley sides (glaciofluvial deposits). Several valleys hold such clay, for example between Castle Caereinion and Llanfair Caereinion (Lake Caereinion), but the largest extent of glaciolacustrine silty clay was deposited in Lake Trederwen [SJ 2688 1592], a possible proglacial lake and remnant of the larger Lake Lapworth that occupied the northern end of the Severn valley (Wills, 1924; Pocock et al., 1938).

Glaciofluvial deposits undivided comprise gravel, sand and silt that were carried by floodwaters from melting ice and then deposited downstream. However, it is commonly possible to characterise such deposits by their morphology and relationship with topography. Thus, glaciofluvial sheet deposits are those deposited from braided rivers, for example the Vyrnwy and Tanat where they emerged from valley confinement near Llandysilio [SJ 269 193] onto a broad flat area. Across this, the braided rivers spread their load as a sheet. It may even have been deltaic distally should the area have been flooded, for example west-south-west of Llandrinio [SJ 295 170]. Some valleys too may be wide enough to produce sheet deposits when their valley-bottoms become filled with river detritus, for example at the confluence of the rivers Severn and Rhiw [SJ 202 015]. However, it is usual for such deposits to be modified once the meltwaters wane and the braided rivers mature into single channels. They then become erosive, incising their channels downwards into glaciofluvial deposits, and laterally to form new and lower floodplains. Earlier glaciofluvial floodplains are thus abandoned and their remnants left as benches [SJ 215 046] along the valley sides. If sufficiently expansive, some may still qualify as glaciofluvial sheet deposits. Glaciofluvial fan deposits occur usually on the lower parts of valley sides [e.g. 208 032] where tributary streams drop their loads of detritus on meeting the gentler incline of the major valley. Their surfaces grade imperceptibly at the distal edges into the surface of the glaciofluvial deposits of the major valley, not into the surface of the alluvial floodplain, and thus indicate a pre- or early Holocene age.

As the proglacial lakes drained, or filled with sediment, they become the sites of swamps and reed beds, so that thin beds of peat formed upon (e.g. Marton Pool [SJ 295 027]) and in places within the lake deposits.

The draining of the proglacially formed lakes continued well into the Holocene and was assisted latterly by human settlement. Marton Pool is an extant example of such a lake. Together with peat, a layer of white diatomaceous earth formed here and in Lake Caereinion [SJ 1751 0528], which is now drained. It is composed of diatoms (microscopic, unicellular, silica-secreting aquatic plants) that lived in cold conditions.

Alluvium is the product of the present-day, mature river systems, in which single-channel rivers have become incised below the thalweg of their melt-water, braided predecessors. They migrate laterally within a narrow re-established floodplain and thus the alluvium of these is mostly silt and reworked gravel, the latter forming ribbons and lenses within the former. Thicknesses are varied, but there are up to 6 m of alluvium in the banks [SJ 2758 1295] of the River Severn near Criggion, where it overlies glaciolacustrine clay. Alluvial fan deposits are developed where modern, steeper graded tributaries emerge from their channels to intersect the main river valleys. River terrace

deposits are slightly older (late Holocene) flood plains that are now at a slightly higher level due to down-cutting in response to postglacial regrading and isostatic readjustment. Abandoned alluvium covers a floodplain that has recently been put beyond the reach of flooding. The sole occurrence is in the Vyrnwy valley [SJ 115 120], just below Pontrobert, and probably resulted from the flow being regulated by the construction of the Vyrnwy dam. Talus and head are post-Devensian, gravitational slope deposits of weathered rock. Head possesses a fine matrix of silt or clay and was mobilised by saturation with water.

Talus is a scree of ill-sorted rock fragments that accumulated at the base of a very steep slope below crags of bedrock. Lacustrine deposits are the products of Holocene temperate lakes, generally of silt and clay with some peat. The flanks of some drumlins are so steep as to have become unstable, perhaps during permafrost thaw or during the Loch Lomond Stadial. This once unstable ground has been mapped as Landslip. Landslips in bedrock are rare, the only record [SJ 1782 0965] being on Nantglyn Flags, caused perhaps by a thin bed of bentonite.

Chapter 3 Applied geology

Earth science factors have a significant influence on 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 geological 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

Although there are minor occurrences of baryte across the district, for example near Bulthy in the Breidden Hills, the extraction of metalliferous minerals (lead, zinc, copper and barium sulphate) took place mainly in Ordovician rocks within a four mile radius of Shelve. Galena, baryte and sphalerite, together with calcite, occur in steeply dipping to vertical veins up to 5 m wide and aligned south-east to north-west. Mining took place in Roman times, then in the 12th and 13th century, but mainly in the late 19th and early 20th centuries. The mineralisation occurred in depth zones so that lead ores are found below zinc and the latter below barium (Dines, 1958), and it appears that some control on ore enrichment was exerted by the adjacent country rock (Dines, 1958). For instance, the 'hard' sandy lower part of the Mytton Flags is a preferred host to lead-zinc-rich lodes, possibly because the overlying Hope Shales acted as a seal against upward-migrating, mineral-bearing fluids.

Coal from the western part of the Hanwood Coalfield came from three seams in the Halesowen Formation. These are the Thin, up to 0.5 m thick, the Yard up to 1 m thick (rarely 2 m), and the Half Yard, up to 0.5 m thick. Of these, the best coal is in the Thin, and this was the most exploited. The other seams are recorded as being of poor quality and pyritous (Pocock et al., 1938). Exploitation began some years before 1839 and continued until the late 1940s.

Currently the main quarry for hardrock is at Criggion [SJ 290 143] in the Breidden Hills microgabbro. It produces large amounts of aggregate for roadstone, rail ballast, and building work. No other quarry for such products is active within the district, but there are many old quarries, now disused. Many of the igneous intrusions in the Breidden Hills and the Shelve area were also quarried for example at Moel-y-Golfa (andesite) [SJ 2875 1195], Trewern (microgabbro) [SJ 278 117] and Lower Wood (andesite) [SJ 3089 0264]. At Middletown [SJ 299 129], the Middletown Quarry Member (rhyolitic tuffs and breccias) has been quarried, but the stone is not of very durable quality. The sedimentary rocks have provided little stone of good quality, although many small roadside quarries attest to their one time use as road 'metal'. Mudstones from the Nod Glas, Rorrington Shale and Nant-ysgollon Mudstone formations are particularly pyritic. Others, including the Trewern Brook Mudstone and some shales below the Rorrington Shale, are less so. On weathering, aggregate from these units may cause heave and concrete attack, and is generally unsuitable.

The Standard Quarry [SJ 218 077] at Welshpool produced a great deal of building stone (trachyte), which was used across a wide area, especially on the Powis Estate. The last use for the rhyolitic tuffs and breccias of the Middletown Quarry Member was as an ornamental garden stone on account of the bright green chloritised pumice. Some building stone for use in the immediate vicinity has been derived from quarries in the sandier formations, for example the Gaer Fawr Formation on Moel y Main [SJ 1775 1580], and the Bailey Hill Formation near Trefnant Hall [SJ 181 038] and at Pentre Quarry [SJ 249 059], near Leighton. The Powis Castle Conglomerate was also quarried for building stone, for example at Cloddiau [SJ 204 091] and for use on site at Powis Castle [SJ 216 063].

For many years, the mudstones of the Tarannon Mudstone Formation were quarried at Buttington [SJ 267 100] for brick manufacture, but production has recently ceased. Currently, the rock is transported to the English Midlands for use in the ceramics industry. Before the mid 19th century, the clay in low-lying agricultural areas was used almost on site to make bricks for many farm buildings. Such clay was derived from till, possibly by elutriation during postglacial freeze-thaw, which left the clasts behind on adjacent slopes. Glaciolacustrine clay has also been exploited. There are excavations at Pool Quay [SJ 2590 1260], but whether this was used for bricks or, equally likely, for puddling the adjacent Montgomery Canal, has not been ascertained. The Tilestones Formation has been locally worked in the past for roofing tiles (Plate 5).

In neither the Severn nor the Vyrnwy valleys has there been significant sand and gravel extraction from the glacial deposits. Currently there are no working pits.

Water resources

There are no major abstraction points within the district. The Palaeozoic rocks are of low porosity and only weakly permeable. However, they may be regarded as a minor aquifer in the near surface zone, where fracture porosity is higher due to weathering. Many privately owned boreholes do yield sufficient water for the average small farm, although others have proved to be failures. Water from such boreholes, especially those in Silurian formations such as the Bailey Hill, is rather hard, having leached calcium carbonate from the rocks. The Permo-Triassic Kinnerton Sandstone and Chester Pebble Beds formations are major aquifers in the English Midlands, but have not been exploited in the district.

Glaciofluvial and alluvial gravels associated with the Severn and Vyrnwy valleys are minor aquifers, and in many places they are in hydraulic continuity with the parent rivers. In such circumstances they present potential sources of water. The local presence of glaciolacustrine clay beneath the alluvium would act as an aquiclude.

Potential geological hazards

Migration of leachate from the underground workings of former metalliferous mines presents a significant pollution potential, and may lead to contamination of local surface or groundwater resources and alluvial sediments. Similarly, run-off during heavy rainfall, or due to poorly planned remediation or exploitation of mine tips, may allow soluble contaminants or contaminative sediment to enter surface waters. Contamination of groundwater by toxic leachate from areas of worked or made ground is also a potential hazard. Such areas include poorly lined landfills, agricultural waste-disposal sites and active or former industrial sites such as sewage works, gasworks, gravel pits, quarries and railway sidings.

River and stream floodplains within the district are susceptible to regular flooding. At particular risk are the Severn and the Vyrnwy around Meifod and Llandysilio. An indication of those areas that are prone to regular flooding is given by the extent of active floodplain deposits, shown as alluvium on the map. Areas lying outside the limit of alluvium may also be at risk during anomalously large river floods, or at risk to flooding caused by blocked drains and culverts. Areas regarded as being at risk of flooding are also shown on Environment Agency maps.

Within the district, gas emissions represent a hazard in areas associated with the accumulation of methane or radon. Methane is likely to be generated by decomposition of material in landfill sites and unconsolidated organic-rich deposits such as peat. It is toxic, an asphyxiant, and explosive in high concentrations. Methane is less dense than air, and is capable of migrating through permeable strata and accumulating in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions represent a significant hazard, risk can be mitigated through correct design of landfills and developments in 'at risk' areas. Radon is a naturally occurring, ionising gas produced by radioactive decay of uranium, which is present in small quantities in most rocks and soils. Radon may also accumulate in poorly ventilated spaces and gives rise to an elevated risk of cancer of the respiratory tract. In areas at risk of radon accumulation, protective measures should be provided for new developments, and remediation work should be carried out in existing homes and workplaces. Advice about radon and its associated health risks can be obtained from the National Radiological Protection Board, Chilton, Didcot, Oxfordshire OX11 0RQ.

Slope instability, typically revealed by landslip, is locally present in the district. Ten landslips were identified during the survey and nine are in superficial deposits, especially till. Their main trigger was probably climate change from freeze to thaw at the end of the Devensian. The flanks of many drumlins, for instance at Trawscoed Farm [SJ 2130 1262], north-west of Guilsfield, and Dysserth [SJ 2071 0516], south-south-west of Welshpool, have been the locus of landslipping. Although most existing landslips in the district are located in rural areas, development pressures for housing or improved infrastructure increase the possibility that building or engineering work will encounter them, or will create the conditions under which land may become unstable. The most effective strategy for dealing with landslips relies on recognition of problem areas in advance so that suitable preventative or remedial measures can be employed.

Subsidence is to be anticipated in areas of metalliferous and especially coal mining. The Shelve area contains many underground workings and surface entries, but they are mainly concentrated along the mineral veins. Along and down dip of the outcrop of the Halesowen Formation in the Hanwood Coalfield, from Coedway to Minsterley, there are scattered small shafts (many not recorded), which together with comparatively shallow workings, pose a notable hazard that requires careful site-investigation.

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 landslips. These can be effectively dealt with by first obtaining adequate information, including properly focussed site investigation, to confirm the properties of individual sites.

Geotechnical details of rock strengths in the district are sparse. Most of the bedrock generally has high bearing capacities, except in the weathered zone. Thin (less than 15 cm) beds of white swelling clay (bentonites), particularly in the Silurian rocks, create weaknesses, especially where they dip concordantly with the topographical slope. The Carboniferous Etruria Formation comprises calcareous red mudstones that are soft, particularly when wet. Similar mudstones occur at intervals between the sandstones of the Salop Formation. Pyritic mudstones in the Nod Glas, Rorrington Shale and, Nant-ysgollon Mudstone formations may cause heave and concrete attack. Glaciolacustrine deposits and peat have low bearing capacities and can give rise to

moderate settlement. Although the distribution of glaciolacustrine varved clay is mainly restricted at surface to the outer edges of the Severn alluvium, it is present beneath that alluvium at least north of Welshpool. Peat deposits are thin and restricted to the sites of former proglacial lakes, where they only add to the inherent weaknesses of such sites. Till and hummocky glacial deposits have moderate bearing capacity but are highly variable. The behaviour of till is erratic, especially after disturbance by landslip or human activity. Its poor internal drainage can shift unpredictably when the deposit is disturbed; wet till is a weak rock. Head, alluvial fan deposits and alluvium all have low to moderate bearing capacities. Although glaciofluvial deposits embrace a variety of foundation conditions, they generally have high bearing capacities. Artificial ground (made ground) is highly variable and may include contaminated land (e.g. metalliferous mine waste) requiring remediation.

Geological conservation

The geological heritage of the district forms a resource for tourism, education and scientific research, and is also a key issue in planning and development. Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSIs). These are statutory designated conservation sites that have some protection under the Wildlife and Countryside Act 1981. Within the district, there are four SSSIs.

Non-statutory designated conservation sites are Regionally Important Geological Sites (RIGS), and listing of these for the district is now in hand. Further information on the extent and designation of SSSIs and RIGS can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd, LL57 2LQ. Special features of interest, being considered for notification as SSSIs, are described in the Geological Conservation Review (GCR) series, published by the Nature Conservancy Council.

Information sources

Further geological information held by the British Geological Survey relevant to the district is listed below. It includes memoirs, reports, published and unpublished maps, documentary and material collections.

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Searches of indexes to some of BGS collections can be made on the Geoscience Data Index system available online. Maps, books and other publications are listed in the BGS Catalogue of geological maps and books and digital data in the volume Britain beneath our feet; both are available on request or may be viewed online. Maps, books and other publications can be purchased through the BGS sales desk or online. (See back cover for addresses).

The BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS website or at: British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

Maps

The district was originally surveyed on a scale of one inch to one mile by W T Aveline, H W Bristow, E Hull and A C Ramsay, and the results published in 1850 and 1855 as part of [Old Series] sheets 60 NE and 60 SE. The easternmost parts of the district, mainly the western part of the Hanwood Coalfield and the northern part of the Shelve Ordovician area, were resurveyed at the 1:10 560 scale by D A Wray and C B Wedd in 1915, 1921, 1926 and 1928, as part of the survey of the one-inch Sheet 152 Shrewsbury. The Shelve Ordovician area was resurveyed on the 1:10 000 scale by R L Langford and B D T Lynas in 1982–4 as part of the survey of the 1:25 000 Shelve Ordovician Inlier Special Sheet (British Geological Survey, 1991), and the southernmost part of the district was resurveyed on the 1:10 000 scale by R Cave in 1986–8 as part of the survey of the 1:50 000 Sheet 165 Montgomery. The remainder was surveyed by rapid and reconnaissance mapping by R Cave (University of Wales, Aberystwyth) in 1998–02 and compiled at the 1:25 000 scale.

The original 1:63 360 geological maps are not available for purchase. Copies of these maps can be consulted at the BGS library, Keyworth. Unpublished 1:10 560, 1:10 000 and 1:25 000 scale geological maps, listed below, are available for public consultation in the BGS libraries in Edinburgh and Keyworth, and in the London Information Office in the Natural History Museum, South Kensington. Print on demand maps (1:25 000 scale) or uncoloured photocopies (1:10 000 scale) are available for purchase from the BGS sales desk.

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

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

Geological maps

1:625 000
Solid geology map UK, South Sheet, 2001 United Kingdom, South Sheet, Quaternary geology, 1977
1:250 000
Geological map of Wales, Solid geology, 1994
Sheet 52N 04W Mid-Wales and Marches, Solid geology, 1990
1:63 360
[Old Series] Sheet 60 NE
1:50 000
Sheet 151, Welshpool, Bedrock and Superficial deposits, England and Wales, 2007
1:25 000
The Shelve Ordovician Inlier and adjacent areas. Parts of SO 29 and 39, SJ 20 and 30. 1991

The component 1:25 000 scale maps of the Welshpool district, based on University of Wales, Aberystwyth, and BGS surveying, are listed below, along with the surveyor's initials and dates of survey.

Map number	Surveyor	Date
SJ 00	RC	1999, 2002
SJ 01	RC	1998
SJ 10	RC	1999, 2002
SJ 11	RC	1998–9, 2002
SJ 20	RC	2000, 2002
SJ 21	RC	1998–2000, 2002
SJ 30	RC	2000–2001
SJ 31	RC	1999–2001

1:10 000

The component 1:10 000 scale maps of the southern margin and Shelve area of the Welshpool district, based on BGS surveying, are listed below, along with the surveyors' initials and dates of survey.

Map number	Surveyors	Date
SO 09 NE	RC	1988
SO 19 NW	RC	1988
SO 19 NE	RC	1988
SO 20 SE	BDTL	1984
SO 29 NW	RC	1986
SO 29 NE	BDTL	1982
SO 30 SW	BDTL	1984
SO 30 SE	BDTL	1984
SO 39 NW	BDTL, RLL	1983
SO 39 NE	BDTL, RLL	1983–4
SO 29 NW	RC	1986
SO 29 NE	BDTL	1982
SO 30 SW	BDTL	1984
SO 30 SE	BDTL	1984
SO 39 NW	BDTL, RLL	1983
SO 39 NE	BDTL, RLL	1983–4

1:10 560
The component 1:10 560 scale County series maps of the Hanwood Coalfield area, based on BGS surveying, are listed below, along with the surveyors' initials and dates of survey.

Map number	Surveyors	Date
Shropshire 26 SE	DAW	1915
Shropshire 27 SW	DAW	1914–5
Shropshire 32 NE	CBW, DAW	1915, 1921
Shropshire 32 SE	CBW	1921
Shropshire 33 NW	CBW, DAW	1915, 1921, 1926
Shropshire 33 SW	CBW	1921
Shropshire 40 NW	CBW	1921, 1926, 1928

Digital geological map data
In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the intellectual property rights manager at BGS Keyworth. The current availability can be checked on the BGS website; the main datasets are:

DiGMapGB-625 (1:625 000 scale)
DiGMapGB-250 (1:250 000 scale)
DiGMapGB-50 (1:50 000 scale)
DiGMapGB-10 (1:10 000 and 1:25 000 scales)

Geophysical maps
1:500 000
Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997. Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998.
Geochemical atlases
1:250 000
Wales and part of west-central England: Stream water, 1999
Wales and part of west-central England: Stream sediments and soils, 2000
Hydrogeological maps
1:625 000
Hydrogeological Map of England and Wales, 1997
Groundwater vulnerability maps
1:100 000
Sheet 21 Groundwater Vulnerability of West Shropshire, 1995

Books and reports

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

Documentary collections

Documentary collections include records of boreholes and site investigations carried out within the district. These are available for consultation at BGS, Keyworth. Copies of most records can be purchased through the sales desk. Further information about documentary material can be obtained through the BGS enquiry service. Index information, including site references, is held in digital format and can be viewed through the Geoscience Data Index, available on the BGS website.

Material collections

Material collections from the district are available for inspection at BGS, Keyworth, and include petrological hand specimens, thin sections and fossils. Some of the Lower Palaeozoic shelly faunas have been deposited in the Sedgwick Museum, Cambridge. Index data for petrological specimens is listed in the BRITROCKS database that can be searched through the Geoscience Data Index. Further information about material collections can also be obtained through the BGS enquiry service.

References

British Geological Survey holds most of the references listed below, and copies may be obtained via the library service subject to copyright legislation (contact libuser@bgs.ac.uk for details). The library catalogue is available at: http://geolib.bgs.ac.uk

Adams, D R. 1962. Survey of the south Shropshire lead mining area. Shropshire Mining Club, Account No. 2.

Adams, D R. 1968. Survey of the south Shropshire lead mining area. Shropshire Mining Club, First Supplement to Account No. 2.

Bickerton Morgan, J. 1885. Local geological notes. 1) Northern Montgomeryshire. Montgomeryshire Collections, Vol. 18, 149–154.

Bickerton Morgan, J. 1891a. On the strata forming the base of the Silurian in north-east Montgomeryshire. Report of the British Association for 1890, Leeds, 816.

Bickerton Morgan, J. 1891b. The strataforming the base of the Silurian in north-east Montgomeryshire. Montgomeryshire Collections, Vol. 25, 359.

Bowdler-Hicks, A, Ingham, J K, and Owen, A W.2002. The taxonomy and stratigraphicalsignificance of the Anglo-Welsh Cryptolithinae (Trinucleidae, Trilobita). Palaeontology, Vol. 45, 1075–1105.

Boyd Dawkins, W. 1869. The geology of Powisland. Montgomeryshire Collections, Vol. 2, 434–442.

Brenchley, P J. 1978. The Caradocian rocks of the north and west Berwyn Hills. Geological Journal, Vol. 13, 137–64.

Brenchley, P J. 1988. Environmental changes close to the Ordovician–Silurian boundary. 377–385 in A global analysis of the Ordovician–Silurian boundary. Cocks, L R M, and Rickards, R B (editors). Bulletin of the British Museum (Natural History), Geology Series, Vol. 43.

Brenchley, P J. 1993. The Ordovician of the south Berwyn Hills. 39–50 in Geological excursions in Powys, Central Wales. woodcock, N H, and Bassett, M G (editors). (Cardiff: University of Wales Press, National Museum of Wales.)

British Geological Survey. 1991. The Shelve Ordovician Inlier and adjacent areas. 1:25 000 Classical areas of British Geology Series. Parts of Sheets S O 29, 39, S J 20 and 30. (Solid and Drift edition.) (Keyworth, Nottingham: British Geological Survey).

British Geological Survey. 2000. Oswestry.1:50 000 Geological Sheet. 137 (England and Wales), Solid and Drift Geology. (Keyworth, Nottingham: British Geological Survey).

Carruthers, R M, Fletcher, C J N, Mcdonald, A J W, and Evans, R B. 1992. Some constraints on the form of the Welsh Basin from regional gravity and aeromagnetic data, with particular reference to Central Wales. Geological Magazine Vol. 129, 515–522.

Cave, R. 1955. The stratigraphy of the Welshpool area (Montgomeryshire). Unpublished PhD thesis, University of Cambridge.

Cave, R. 1957. Salterolithus caractaci (Murchison) from Caradoc strata near Welshpool. Geological Magazine, Vol. 94, 281–90.

Cave, R. 1965. The Nod Glas sediments of Caradoc age in North Wales. Geological Journal, Vol. 4, 279–298.

Cave, R, and Dixon, R J. 1993. The Ordovician and Silurian of the Welshpool area. 51–84 in Geological excursions in Powys, Central Wales. Woodcock, N H, and Bassett, M G (editors). (Cardiff: University of Wales Press, National Museum of Wales.)

Cave, R, and Hains, B A. 2001. The geology of the district around Montgomery, including the Ordovician rocks of the Shelve area. Memoir of the British Geological Survey, Sheet 165 and part of 151 (England and Wales).

Cave, R, and Loydell, D K. 1998. Wenlock volcanism in the Welsh Basin. Geological Journal, Vol. 33, 107–120.

Cave, R, and Price, D. 1978. The Ashgill Series near Welshpool, North Wales. Geological Magazine, Vol. 115, 183–194.

Dines, H G. 1958. The West Shropshire Mining Region. Bulletin of the Geological Survey of Great Britain, No. 14, 1–43.

Dixon, R J. 1990. The Moel-y-Golfa andesite: An Ordovician (Caradoc) intrusion intounconsolidated conglomerate sediments, Breidden Hills Inlier, Welsh Borderland.Geological Journal, Vol. 25, 35–46.

Dixon, R J. 1991. The Ordovician (Caradoc) igneous and sedimentary rocks of the Breidden Hills, Shelve and Forden Inliers, Welsh Borderlands: rift-related volcaniclasticsedimentation in a back-arc tectonic setting. Unpublished PhD thesis, University of Wales.

Gale, I N, Evans, C J, Evans, R B, Smith, I F, Houghton, M T, and BurgessS, W G. 1984. The Permo-Triassic aquifers of the Cheshire and West Lancashire Basins: British Geological Survey, Investigation of the Geothermal Potential of the U K. British Geological Survey Geothermal Energy Research Programme, Energy Resources Report, WJ/G E/84/17.

King, W B R. 1923. The Upper Ordovician rocks of the south-western Berwyn Hills. Quarterly Journal of the Geological Society of London, Vol. 79, 671–702.

King, W B R. 1928. The geology of the district around Meifod (Montgomeryshire). Quarterly Journal of the Geological Society of London, Vol. 84, 671–702.

Kirk, N H. 1951. The Silurian and Downtonian rocks of the anticlinal disturbance of Breconshire and Radnorshire; Pont Faen to Presteigne. Proceedings of the Geological Society of London, 1474, 72–74.

Loydell, D K, and Cave, R. 1993. The Telychian (Upper Llandovery) stratigraphy of Buttington Brick Pit, Wales. Newsletters on Stratigraphy, Vol. 22, 91–103.

Loydell, D K, and Cave, R. 1996. The Llandovery–Wenlock boundary and related stratigraphy in eastern mid Wales with special reference to the Banwy River section. Newsletters on Stratigraphy, Vol. 34, 39–64.

Mullins, G L. 2000. A chitinozoan morphological lineage and its importance in Lower Silurian stratigraphy. Palaeontology, Vol. 43, 359–373.

Palmer, D. 1970. A stratigraphical synopsis of the Long Mountain, Montgomeryshire and Shropshire. Proceedings of the Geological Society of London, Vol. 1660, 341–346.

Palmer, D. 1972. The geology of the Long Mountain, Montgomeryshire and Shropshire. Unpublished PhD thesis, Trinity College, Dublin.

Pauley, J C. 1991. A revision of thestratigraphy of the Longmyndian Supergroup, Welsh Borderland and of its relationship to the Uriconian volcanic complex. Geological Journal, Vol. 26, 167–183.

Peart, R J, Cuss, R J, Beamish, D, and Jones, D G. 2003. The high resolution airborne resource and environmental survey (Phase 1) (HiRes-1): background, data processing, dissemination and future prospects. British Geological Survey Internal Report, I R/03/112.

Piper, J D A. 1995. Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motions of eastern Avalonia and regional rotations. Geological Magazine, Vol. 132, 65–80.

Pocock, R W, Whitehead, T H, Wedd, C B, and Robertson, T. 1938. Shrewsbury district, including the Hanwood Coalfield. Memoir of the Geological Survey of Great Britain, Sheet 152 (England and Wales).

Shackleton, R M. 1953. The structuralevolution of North Wales. Geological Journal, Vol. 1, 261–297.

Wade, A. 1911. The Llandovery and associated rocks of northeastern Montgomeryshire. Quarterly Journal of the Geological Society of London, Vol. 67, 415–459.

Watts, W W. 1885. On the igneous and associated rocks of the Breidden Hills in east Montgomeryshire and west Shropshire. Quarterly Journal of the Geological Society of London, Vol. 41, 532–546.

Watts,W W. 1886. The Corndon laccolites. Report of the British Association, Birmingham, 670–671.

Wedd, C B. 1932. Notes on the Ordovician rocks of Bausley, Montgomeryshire. Summary of Progress, Geological Survey of Great Britain, Part 2, 49–55.

Whittington, H B. 1938. The geology of the district around Llansantffraid-ym-Mechain, Montgomeryshire. Quarterly Journal of the Geological Society of London, Vol. 94, 423–457.

Wills, L J. 1924. The development of the Severn valley in the neighbourhood of Ironbridge and Bridgnorth. Quarterly Journal of the Geological Society of London, Vol. 80, 274–314.

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

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

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

(Index map)

The area described in this sheet explanation is indicated by a solid block.

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Simplified bedrock geology.

(Figure 2) Ordovician stratigraphy.

(Figure 3) Stratigraphy of the Shelve area.

(Figure 4) Ordovician and Silurian stratigraphy east of the Severn valley.

(Figure 5) Ordovician and Silurian stratigraphy west of the Severn valley.

(Figure 6) Silurian stratigraphical architecture.

(Figure 7a) Colour shaded relief Bouguer gravity anomaly map in milligals (mGals) calculated against the Geodetic Reference System 1967, referred

(Figure 7b) Colour shaded relief magnetic anomaly map. Total field magnetic anomalies in nanotesla (nT) relative to a local variant of IGRF90. Contour interval 10 nT.

(Figure 8) Quaternary deposits.

Plates

(Plate 1) Listric growth fault (in red) in tuffs and breccia of the Middletown Quarry Member, Middletown Quarry [SJ 2990 1289], Breidden Hills (P613388).

(Plate 2) Disturbed beds (slump or debrite) in upper part of Nant-ysgollon Mudstone Formation, road section [SJ 1905 1423] near Pentre'r beirdd (P613389).

(Plate 3) Massive mudstones of the Mottled Mudstone Member overlying the Nant-ysgollon Mudstone Formation, road section [SJ 1905 1423] near Pentre'r beirdd (P613390).

(Plate 4) Thick-bedded sandstones of Bailey Hill Formation, Pentre Quarry [SJ 2487 0590], Leighton (P613391).

(Plate 5) Roof tiles from Tilestones Formation, Hampton Hall [SJ 310 056], Worthen (P613392).

(Front cover) Front cover Powis Castle (Photograph P Witney; P624288).

(Rear cover)

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

Figures

(Figure 3) Stratigraphy of the Shelve area

Age		Formation Thickness	Lithology
SILURIAN	LLANDOVERY	Purple Shales 50 to 250 m	Purple and maroon, and partly green mudstone with a diverse shelly fauna
SILURIAN	LLANDOVERY	Pentamerus Sandstone 0 to 112 m	Well bedded, fine- to medium-grained, shelly calcareous sandstone
Unconformity
ORDOVICIAN	CARADOC	Whittery Shale up to 810 m	Olive grey micaceous mudstone with thin fine-grained sandstone
		Whittery Volcanic 40 to 160 m	Volcaniclastic breccia, mudstone, tuffite and feldspathic sandstone
		Hagley Shale 300 to 400 m	Olive grey micaceous mudstone with thin fine-grained sandstone
		Hagley Volcanic 25 to 175 m	Massive, khaki, feldspathic sandstone and some conglomerate
		Aldress Shale 350 to 560 m	Olive grey micaceous mudstone with sparse thin sandstone beds
		Spy Wood Sandstone 40 to 100 m	Pale grey, well-bedded, fine-grained quartzose sandstone
		Rorrington Shale 250 to 500 m	Dark grey, anoxic graptolitic mudstone
	LLANVIRN	Meadowtown 160 to 550 m	Flaggy bioclastic sandstone and dark grey mudstone
		Betton Shale 50 to 200 m	Dark grey mudstone with basaltic tuff and hyaloclastite
		Weston Flags 425 to 610 m	Flaggy bioturbated sandstone and siltstone
		Hope Shale up to 1450 m	Dark grey mudstone with felsic and mafic volcanic rocks
	ARENIG	Mytton Flags 735 to 875 m	Flaggy bioturbated sandstone and siltstone
	ARENIG	Stiperstones Quartzite 120 to 325 m	Grey to white, massive quartzose sandstone
	TREMADOC	Shineton Shale 925 m Seen	Silty mudstone and thin siltstone

(Figure 4) Ordovician and Silurian stratigraphy east of the Severn valley

Age		Formation	Lithology
SILURIAN	PRIDOLI	Raglan Mudstone Formation up to 80 m Seen	Red and green mudstone with calcrete nodules and impersistent beds of sandstone
		Temeside Mudstone Formation 60 to 70 m	Grey-green argillaceous, massive, irregular flaggy siltstone, with calcrete nodules
		Tilestones Formation c. 14 m	Fissile, yellow, fine-grained micaceous sandstone
	LUDLOW	Cefn Einion Formation (190 to 200 m)	Bioturbated shelly silty mudstone, with thin interbeds of sandstone
		Knucklas Castle Formation (170 to 180 m)	Bioturbated greenish grey silty mudstone with fissile siltstone partings
		Bailey Hill Formation up to 560 m	Silty mudstone and thin-bedded (< 70 cm), fine-grained sandstone. Non-bioturbated; slumped near base. Thin fissile sandstone and graptolitic mudstone in uppermost part (Cwm yr Hob Member)
		Irfon Formation up to 220 m	Silty mudstone with siltstone laminae or partings of fine sandstone. Minor slump sheets. Non bioturbated
		Gyfenni Wood Shale Formation up to 110 m	Thinly bedded fissile sandstones and graptolitic shales
	WENLOCK	Aston Mudstone Formation up to 130 m	Shelly, bioturbated, buff, silty mudstone
		Trewern Brook Mudstone Formation 450 to 475 m	Fissile, grey, graptolitic silty mudstone, crudely laminated, (hemipelagic) in parts. Mottled Mudstone Member of Nantglyn Flags Formation developed in middle of unit but is subsumed eastward into Aston Mudstone Formation
		Nant-Ysgollon Mudstone Formation 9 m	Banwy Member: interbedded laminated and burrowed hemipelagite
	LLANDOVERY	Tarannon Mudstone Formation 70 to 80 m(= Purple Shales Formation, up to 90 m, farther east; see Fig. 3)	Grey-green and maroon mudstones with scattered thin bands of black graptolitic mudstone and white bentonite. Green shelly mudstone (Ty-brith Mudstone Member) locally at Leighton
	LLANDOVERY	Cefn Formation (70 to 80 m) (= Pentamerus Sandstone Formation, 0 to 112 m, farther east; see Fig. 3)	Mudstone, with sandstone beds thinning upwards
Unconformity
ORDOVICIAN	CARADOC	Hill Farm Formation up to 340 m	Silty mudstone and thin sandstone beds
		Bulthy Formation 5 to 300 m	Mass-flow conglomerate of rounded clasts of andesite. Turbiditic sandstones in the east
		Stone House Shale Formation 650 m +	Friable grey graptolitic mudstone. up to 60 m of acid tuff and breccia in uppermost part (Middletown Quarry Member)
		Forden Mudstone Formation 480 m +	Grey, micaceous shale with fine-grained sandstone beds more than 12 cm thick. Top and bottom not exposed. Large bodies of conglomerate with andesite clasts

(Figure 5) Ordovician and Silurian stratigraphy west of the Severn valley.

Age		Formation/member thickness	Lithology
SILURIAN	LUDLOW	Bailey Hill Formation seen up to 350 m	Medium to thin-bedded sandstone and interbeds of silty mudstone. Intraformational conglomerate sporadically at base (Bryn-rorin Conglomerate)
		Nantglyn Flags Formation 250 to 490 m	Thin-bedded mudstone–sandstone rhythmites (turbidites). Some slump beds and mass-flow intra-formational conglomerates. Fissile shaly siltstone at top. Calcareous shelly bioturbated silty mudstone (up to 20 m) (Mottled Mudstone Mbr) in middle. 200 m + thick unit of mudstone, rhythmites and pebbly mass-flow deposits in lower part in west (Gregynog Mudstone Member)
	WENLOCK
		Penstrowed Grits Formation (Denbigh Grits) up to 30 m	Coarse turbidite sandstone and mudstone, locally disturbed
		Nant-Ysgollon Mudstone Formation 160 to 200 m Banwy Mbr c. 36 m	Dark grey, laminated, silty mudstone (hemipelagite). Some thin sandstone beds, slump beds and mass-flow synbasinal conglomerate. Interbedded laminated hemipelagite and burrowed siltstone at base (Banwy Member)
	LLANDOVERY
		Tarannon Mudstone Formation up to 122 m Ty-Brith Mudstone Member very thin, up to 5 m	Grey-green and maroon mudstone with scattered thin bands of black graptolitic mudstone and white bentonite. Passes into green shelly mudstone (Ty-brith Mudstone Member) in south-east.
		Laundry Mudstone Formation 50 to 170 m	Grey silty mudstone with frequent thin, fine sandstone beds. Fissile grey siltstone at base
		Powis Castle Conglomerate Formation up to 30 m	Laterally variable granule to boulder conglomerate; local and exotic clasts
ORDOVICIAN	ASHGILL	Graig-Wen Sandstone Formation up to 12 m	Massive, quartzose, fine-grained sandstone
		Unconformity
		Dolhir Formation 200 to 350 m	Grey, partly silty mudstone. Sporadic bodies of limestone at top
	CARADOC	Nod Glas Formation up to 16 m	Soft, black, mudstone; phosphatic nodules near, and limestone at base
		Gaer Fawr Formation 200 to 360 m	Coarsening upwards sequence of siltstone and sandstone; calcareous, argillaceous interval near top.
		Pwll-Y-Glo Formation up to 235 m	Silty mudstone with interbedded, thin, fine-grained sandstone beds
		Stone House Shale Formation 500 m +	Friable grey, graptolitic mudstone. Top 50 m is more blocky (Middle House Member)

(Figure 8) Quaternary deposits

HOLOCENE	FLANDRIAN	Deposit	Thickness	Morphology	Composition
		Landslip	Various	Steep, concave and hummocky slopes of till and rarely bedrock	As for till except where in bedrock
		Alluvium	Up to 6 m	Flat land adjacent to rivers and subject to flooding	Silt with horizontal ribbons of sand and gravel up to cobble grade
		Abandoned alluvium	Unestablished	Recently abandoned as flood plain due to changed river flow	As alluvium but no longer subject to flooding, e.g. Vyrnwy below Pontrobert, probably as a result of Vyrnwy dam
		Peat and diatomaceous earth	Up to 1 m and up to 0.2 m	Flat low land, wet to boggy, 'a' usually overlies 'b' where present	Biogenic remains; a) of vegetation — carbonaceous b) of diatoms — siliceous
		Head	Up to 6 m	On low land, commonly with gentle slope	Silty clay with rock fragments, commonly angular
		Talus	Unestablished	Commonly at base of steep rock slopes as a drape, or infilling hollows	Angular fragments of local rocks accumulated from downslope creep
		Glaciolacustrine deposits	Various, but unestablished. Probably up to c.5 m	Flat low land, poorly drained. Normally concealed under other drift deposits	Silty clay, medium grey, very thinly laminated by silt varves. Local interbeds of sand and fine gravel
		Glaciofluvial sheet deposits	Various, but unestablished. Probably up to 5 m	Flat and level shoulders overlooking flood-plains	Coarse gravel with sand and silt
		Glaciofluvial fan deposits	Various, but unestablished. Probably up to 5 m	Low-gradient, flat low land grading imperceptibly into glaciofluvial sheet and glaciolacustrine deposits	As for glaciofluvial sheet deposits. Glaciofluvial fan deposits may pass into either glaciolacustrine deposits or glaciofluvial sheet deposits
		Glaciofluvial deposits undivided	Various and unestablished	Non-specific	As for glaciofluvial sheet deposits
		Hummocky glacial deposits	May be <10 m	Hummocky, cultivatable land. May block valley drainage apart from in incised spillway	Similar to till, but more open texture and less clay. May contain water-sorted material, e.g. sand and gravel
		Till	Various, possibly up to 50 m in drumlins, much less (<5 m) in other hilly areas	Not distinctive, except in drumlins, other than by poor drainage and irregular downslope gullying on hill sides	Ill-sorted clasts to boulder size, generally rounded, some with parallel grooves on smoothered faces, in a stiff grey-brown silt and silty clay matrix