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Geology of the Wolverhampton and Telford district—a brief explanation of the geological map Sheet 153 Wolverhampton

D McC Bridge and E Hough, abridged from the Sheet Description by R D Lake

Bibliographic reference: Bridge, D McC, and Hough, E. 2002. Geology of the Wolverhampton and Telford district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 153 (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2002.

© NERC 2003 All rights reserved.

Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract. Printed in the UK for the British Geological Survey by B&B Press Ltd Rotherham

(Front cover) Cover Photograph The world's first cast-iron bridge was built over the River Severn in 1779; it was one of the wonders of its day and still inspires visitors and artists. The bridge spans a gorge cut by meltwater during the Devensian glaciation. The steep sides of the gorge are potentially unstable, and slow downhill movement has affected buildings and the bridge itself. Conservation work on the bridge began as early as 1784, and was brought up to date in 1999 with work sponsored by English Heritage [SJ 6724 0340] (MN39748) (Photograph Caroline Adkin).

(Rear cover)

Notes

The word 'district' refers to the area covered by the geological 1:50 000 Series Sheet 153 Wolverhampton. National Grid references are given in square brackets; all lie within 100 km square SJ, unless otherwise stated; specific locations and boreholes are accompanied by their National Grid reference at their first mention within the text. Lithostratigraphical symbols shown in brackets in the text, for example [BmS], are those shown on the published map.

Acknowledgements

This Sheet Explanation was compiled by R D Lake from text written largely by D McC Bridge and E Hough. Information on the Telford area has been summarised from an existing memoir (Hamblin and Coppack, 1995). Additional geological contributions were provided by J N Carney (Precambrian) and by N J Riley (Dinantian). N J Smith contributed to the structural and concealed geology. The section on Applied Geology was compiled from contributions by C Cheney on water resources, A Foster on engineering geology and from other published BGS sources. The text was edited by A A Jackson. We acknowledge the help provided by the holders of data in permitting the transfer of these records to the National Geosciences Records Centre, BGS, Keyworth. We are especially grateful for the assistance provided by members of Wolverhampton Metropolitan Borough Council, Telford and Wrekin Council, the Coal Authority, Severn Trent and numerous civil engineering consultants. We also thank landowners, tenants and quarry companies for permitting access to their lands.

© Crown copyright reserved Ordnance Survey licence number GD272191/2002.

Geology of the Wolverhampton and Telford district (summary from the rear cover)

(Rear cover)

This Sheet Explanation provides an account of the geology of the district covered by Geological Sheet 153 Wolverhampton. The district extends from the edge of the Black Country conurbation in the south-east, to Telford in the west, and includes a large tract of rural Green Belt on the Staffordshire–Shropshire borders. The solid rocks at outcrop range from Precambrian to Triassic in age. The oldest rocks, pyroclastic tuffs, originated in an island-arc setting in south polar latitudes; the youngest are continental red-beds that formed in inland sabkha environments, when Britain lay just north of the Equator.

Although the geological record spans about 560 Ma, the modern landscape has been shaped largely by geological processes that operated during the last two million years. Successive glaciations have eroded the landscape, and the superficial deposits of till and glacial outwash that blanket most of the district are the product of a Late Devensian glaciation. The Ironbridge Gorge and other meltwater channels also date from this period. Since the ice retreated about 13 000 years ago, the postglacial history has been one of drainage development, valley incision and terrace aggradation.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 153 Wolverhampton. It is written for the professional user, and those who may have limited experience in the use of geological maps and may wish to be directed to further geological information. The district includes parts of Shropshire and Staffordshire, including the Metropolitan Borough of Wolverhampton as the main centre of population in the south-east. Telford, designated a New Town in 1968, is the major commercial and administrative centre in the west.

The relief is gently undulating, reaching a maximum elevation of 260 m above OD in the south-west. The main watershed divide of Central England passes in an irregular arc through the district and separates streams flowing northwards to the Trent (River Penk) from those draining southwards to the Severn (the Worfe and Smestow Brook). The Ironbridge Gorge, famous as the birthplace of the Industrial Revolution, takes the deeply incised River Severn through a former watershed divide. The geological sequence is dominated by the Permo–Triassic rocks of the Stafford Basin, which give rise to a subdued topography. Upper Carboniferous red beds and Coal Measures crop out on either side of the basin in the coalfields of South Staffordshire and Coalbrookdale. To the south of the Coalbrookdale Coalfield, Lower Palaeozoic rocks form the highest ground in the district. The Lilleshall Inlier is a prominent topographical feature located on the line of the Church Stretton Fault. Included within its core are rocks of Cambrian and Precambrian age. The entire district was glaciated in Quaternary times, and the products of this glaciation cover all but the highest escarpments.

The Precambrian volcaniclastic rocks of the Lilleshall Inlier are the most northerly exposures of the Uriconian Volcanic Group. Volcanic activity commenced around 566 Ma (Tucker and Pharaoh, 1991) when magmas of rhyolitic composition reached the surface and were erupted explosively during oceanic plate subduction on the northern margins of the southern hemisphere continent of Gondwana. Dismemberment of the volcanic arc followed, and the ensuing crustal movements gave rise to a block, known variously as Eastern Avalonia, the Midland Platform, or the Midlands Microcraton, which now forms the basement to central England.

Early Cambrian rocks preserve a record of marine transgression involving progressive submergence of the Avalonian landmass, commencing probably in latest Precambrian times. In the Lilleshall Inlier, the Comley Series (Lower Cambrian) is represented by shallow-water glauconitic sandstones. A break in deposition occurred during the St David's Series (Middle Cambrian) but sedimentation resumed during the Merioneth Series (Upper Cambrian) with deeper water mudstones. In late Ordovician times, uplift and inversion of the Midland Platform took place, causing the sea to retreat to the west. Sedimentation recommenced in the late Llandovery (about 427 Ma), probably as a result of a glacio-eustatic sea-level rise. As the shallow Silurian sea spread eastwards across the Midland Platform, intercalated carbonates and siliciclastic sediments were deposited. Final closure of the Iapetus Ocean in the late Silurian resulted in a change from marine through brackish to continental conditions, culminating in Prídolí times with the deposition of red beds. Caledonian (Acadian) earth movements produced gentle folding, uplift and prolonged erosion.

The Dinantian (Lower Carboniferous) period (360 to 320 Ma) was a time of major crustal extension in the British Isles, which saw the emergence of the Wales–Brabant High as a stable land barrier, bounded to the north by a series of small basins and intervening highs. Shallow-water carbonates and evaporites deposited on the Lilleshall High provide evidence of a mid-Courceyan (about 355 Ma) marine transgression on to the southern flanks of the Wales–Brabant landmass. Intra-Dinantian uplift followed before sedimentation resumed in late Asbian–Brigantian time with a succession of fluviatile sandstones, carbonates and contemporaneous extrusive lavas.

By late Dinantian times, as active extension was replaced by a pattern of more general thermal subsidence, an area of continuous sedimentation, the Pennine Basin, developed north of the land barrier. Thin representatives of the deltaic Millstone Grit may be present in the subsurface in the north of the district and elsewhere. Over a period of four million years, (from about 315 Ma), Coal Measures of Langsettian, Duckmantian and early Bolsovian age were laid down, predominantly in delta plain, lacustrine and swamp environments. Persistent cycles of deposition reflect changing subsidence rates and fluctuations in sea level. A typical cycle commences with a marine mudstone, passes up into a clastic terrigenous phase and is capped by a coal, with a seatearth palaeosol at its base. Under conditions of reduced subsidence, full cycles are rarely developed, however. Towards the basin margin, peat accumulation was sustained over long periods, resulting in the closely spaced or amalgamated coal seams that are so typical of the Coalbrookdale and Staffordshire coalfields.

During the latest Duckmantian to Bolsovian stages, the style of sedimentation changed as a consequence of episodic Variscan uplift in areas to the south and west of the coal basin. This led to the deposition of alluvial red beds of the Etruria Formation. A subsequent phase of igneous activity resulted in the intrusion of the Wednesfield dolerite and associated sills into the Westphalian strata. A further period of folding, uplift and erosion occurred, before conditions reverted to those more typical of the early Westphalian. The fluvial sandstones and lacustrine mudstones that characterise the Halesowen Formation point to a period of fairly slow subsidence, when the alluvial plain was crossed by large river-systems, probably fed from the south.

Towards the end of the Carboniferous Period renewed uplift within the Variscan foreland area resulted in rejuvenation of river systems flowing from rising uplands to the south of the Pennine Basin. Silts, sands and gravels carried northwards by these systems were deposited on broad, well-drained alluvial plains (Salop Formation). A change from a tropical to a more arid climate is indicated by the appearance in these red-bed sediments of well-developed calcretes.

Widespread subaerial erosion of Variscan highs occurred in early Permian times, when sedimentation was essentially confined to fault-bounded basins. Initially, mud-rich fan deposits, derived from an eroding hinterland to the south, were deposited across more rapidly subsiding areas of the basin (Clent Formation). Later in Permian times, continued extensional movement on faults in the basement led to the formation of the Stafford Basin, and the accumulation of wind-blown dune sands (Bridgnorth Sandstone).

Deposition within the Stafford Basin continued through the Triassic, when the region lay some 15º to 20º north of the equator. The earliest sediments, mainly conglomeratic and pebbly sandstones, were laid down by northerly flowing braided streams, sourced from the Armorican mountains in what is now the English Channel and Brittany. Later, the river system evolved into a more mature meandering system, linked on occasion to the sea; by middle Triassic times when the Mercia Mudstone was deposited, a more subdued landscape had evolved of broad floodplains and playa lakes. The absence of all younger solid formations from the district is due mainly to uplift and erosion during the Cainozoic.

The widespread glacigenic deposits of the district are attributed to a Late Devensian ice sheet that advanced into the district after 30 500 years BP. Local organic deposits have been found that date back to the preceding Ipswichian Stage. Evidence of an earlier glaciation has been reported from buried valleys to the south and east of the district and from beneath the Main Terrace of the River Severn (see for example Dawson, 1988). As yet, there is no unequivocal evidence for such deposits in this district. Postglacial erosion has extensively modified the landscape and some of the debris is incorporated in the younger Quaternary river terrace and alluvial deposits.

The broad structural subdivision of the district into tectonic areas (basins, blocks and horsts) is shown in (Figure 1).

Chapter 2 Geological description

Precambrian

Precambrian rocks, the Uriconian Volcanic Group [UV], crop out in the Lilleshall Inlier, where they form the prominent ridge of Lilleshall Hill. These predominantly acidic tuffites are best exposed in the crags along the eastern margin of the hill, extending northwards to just below the Memorial. The lower crags [SJ 729 156] consist of massive, pink- to grey-weathering, lithic-lapilli tuffs and breccias containing 20 to 30 per cent of pale grey or pink, glassy volcanic clasts, together with subordinate crystals of grey, glassy quartz. The matrix is grey to cream and coarse grained, with devitrified glass shards and pumice lapilli. The sequence is cleaved, near-vertical to steeply eastward dipping, and locally slightly overturned. Along the western parts of Lilleshall Hill, the dominant exposed lithology is a fine-grained pumice lapilli tuff without a cleavage structure.

Basic rocks are exposed in a former quarry near the south-western extremity of the hill [SJ 7275 1540]. They include dark green chloritic schists overlying mylonitised siliceous tuffs within a major shear zone dipping 40°N and, to the north, intrusive sheets of dark grey, fine- to medium-grained quartz microgabbro, which is possibly the unsheared equivalent of the chloritic schists. Uriconian rocks were proved at shallow depth in boreholes over an area to the north of the Boundary Fault (Hamblin and Coppack, 1995, p.7), and in the Leegomery– Kinley–Preston area beneath Upper Carboniferous beds. In the east of the district, the Heath Farm Borehole [SJ 9333 0926] penetrated about 180 m of mafic and felsic tuffs of Uriconian affinity.

Cambrian

Poorly exposed rocks of Cambrian age crop out in the faulted core of the Lilleshall Inlier. The Croft Borehole [SJ 7284 1505] provided a partial section through Comley and Merioneth strata, which can be correlated with the more widely studied sequences of the Comley and Wrekin areas (Rushton et al., 1988). The absence of St David's Series (Middle Cambrian) strata from the Lilleshall sequence is attributed (Smith and Rushton, 1993) to syndepositional movements on the Church Stretton Fault System. Down-dip from Lilleshall, a more complete sequence is probably present. The Lower Comley Sandstone [LCmS] comprises a sequence of shallow-water, glauconitic sandstones, seen at Lilleshall along the roadside to the south-west of the village. The uppermost part (12 m) of the formation was also proved in the nearby Croft Borehole; it comprises green, pink, purple and grey, glauconitic sandstone with highly micaceous bands, and a carbonate matrix. Grey, structureless beds of chert are present towards the base. The occurrence 6 m below the top of the formation of the small shelly fossil, Microdictyon, led Rushton et al. (1988) to suggest a correlation with the Strenuella Limestone (Ac4) division of the Lower Comley Limestones (Cobbold, 1921; Cobbold and Pocock, 1934). The Lower Comley Limestones are not exposed but in the Croft Borehole, this sequence is represented by a single 0.25 m-bed of pale grey to black, recrystallised, phosphatic algal limestone. Rushton et al. (1988) suggested a correlation with the Lapworthella Limestone Member of the type area, near Caer Caradoc Hill [SO 4845 9647]. Strata of Merioneth age, the informally named Dolgelly Beds [DB], are not exposed in the Lilleshall Inlier, but a section was provided by the Croft Borehole. This proved 53.8 m of black, micaceous shales with distinctive siltstone interbeds resting unconformably on Comley Series strata. Brachiopods and olenid trilobites are concentrated in thin bands, which are separated by greater thicknesses of sparsely fossiliferous mudstone. The sequence spans the zones of Parabolina spinulosa, Leptoplastus and Protopeltura praecursor. In the Comley area, these zones are represented by the Orusia Shales (below) and by the Bentleyford Shales (above).

Silurian

Rocks ranging in age from Wenlock to Prídolí crop out beneath Coal Measures in the Lincoln Hill area [SJ 670 040] of Coalbrookdale, and more extensively in the south-west around Willey. Llandovery rocks are recorded in the subsurface. The classification and lithologies of the Silurian rocks are shown in (Figure 2). The subdivisions of the Llandovery Series are those used by Pocock et al. (1938) and Hamblin and Coppack (1995) in the adjoining Shrewsbury district. The nomenclature of the Wenlock Series broadly follows the recommendations of Bassett et al. (1975). However, the term Tickwood Beds is no longer used for the sequence of siltstones and nodular limestones, 15 to 25 m thick, which is transitional between the siltstone-dominated Coalbrookdale Formation, below, and the predominantly limestone lithofacies of the Much Wenlock Limestone, above. Lateral facies variations within this part of the sequence have led to misuse of the term, and it is more appropriate to include these beds within the Much Wenlock Limestone. The flaggy, shelly limestones (20 m thick) of the Benthall Beds (Hamblin and Coppack, 1995) form a locally distinct lithofacies, laterally equivalent to the reef facies of the Much Wenlock Limestone Formation. The traditional broad lithostratigraphical subdivision of the Ludlow Series is retained in preference to the more modern nomenclature which depends on both lithostratigraphical and biostratigraphical criteria. In the Přídolí Series, the name Raglan Mudstone Formation is introduced to give continuity with areas recently mapped to the south-west.

The Pentamerus Sandstone and Purple Shales formations are presumed to underlie most of the district. Interbedded thin bentonites in the graptolitic and shelly Buildwas and Coalbrookdale [Cbrd] formations are testimony to distant volcanic eruptions. The Much Wenlock Limestone Formation [WeL] contains small reef bodies which grew in waters probably no more than 30 m deep. Corals, stromatoporoids and bryozoans provided the organic framework of the reefs, while reworked bioclastic detritus and carbonate-rich clays accumulated in inter-reef and off-reef environments. The thickly-bedded, middle part of the Benthall Beds [BeB], 12 to 15 m thick, was extensively worked underground and in deep quarries (Pocock et al., 1938, p.116) at Lincoln Hill [SJ 6720 0415]. Varying depths of sea are reflected by the Ludlow formations of the Lower Ludlow Shales [LLu], the Aymestry Limestone [AL] and the Upper Ludlow Shales [ULu]. The correlation with the standard Ludlow stratotype (Figure 2) is based largely on information from the Dean Borehole [SJ 6799 0006]. Within the first formation a limestone, correlated with the Barr Limestone Formation of the Dudley– Walsall area, was proved in the Heath Farm Borehole. The top 25 m of the formation in the Dean Borehole contain a prolific shelly fauna, dominated by large strophomenids (Hamblin and Coppack, 1995). At the base of the Prídolí Series lies the Ludlow Bone Bed, (0.08 to 0.2 m thick) a denticle-rich unit occupying hollows in the beds beneath. This is a condensed, low stand, regressive lag deposit that was reworked during a later transgressive event. It is overlain by the Downton Castle Sandstone Formation [DCS] of mixed facies. The basal olive-grey siltstones of the Downton Castle Sandstone, up to 2.5 m thick, contain a typical Downtonian fauna of poorly preserved bivalves, ostracods, and brachiopods. The succeeding sandstones also yield scattered fossils including mainly lingulid brachiopods, mollusca and plant remains. A lenticular bone bed, up 0.07 m thick occurs about 3 m above the base of the formation. This horizon may be the lateral equivalent of the Downton Bone Bed of Downton Castle (Ellis and Slater, 1906, p.209). The overlying Temeside Shales [TSh] contain lingulid brachiopods, vertebrate detritus and plant remains (Whitehead et al., 1928, p.25–26) and reflect intertidal and subtidal environments subject to periods of prolonged exposure. The red beds of the Raglan Mudstone Formation [Rg] represent deposition on an alluvial coastal plain subject to marine tidal influence (White and Lawson, 1989).

Carboniferous

Dinantian

These strata crop out in the Lilleshall Inlier, and have been proved at depth beneath the northern part of the Coalbrookdale Coalfield. The rocks are now not well exposed but limestone was extensively quarried and mined in the past. The writings of Murchison (1839) and Prestwich (1840) were the only reliable sources of information, but recent extensive drilling at Lilleshall has provided new information. Formal names are introduced to replace the six informal 'groups' recognised by Whitehead et al. (1928). A composite section of the sequence is included (Figure 3).

The earliest deposits are red, brown and green, terrigenous sandstones, breccias and conglomerates, the Village Farm Formation [Vlg] (this is 'Group 1' of Whitehead et al., 1928), which are inferred to be of late Devonian–Courceyan age. The contained angular clasts are of sandstone, quartzite, granophyre and banded rhyolite as well as crinoidal chert and dolomite, indicating erosion of local Precambrian and Palaeozoic source rocks. Beds in the uppermost 3 m are transitional with the overlying formation; they include calcareous sandstones with millet-seed grains, and thin supratidal pedogenic carbonates. In several boreholes, the top of the formation is defined by a fissured surface, which may represent an unconformity (Ove Arup and Partners, 1987).

The succeeding carbonates, Jackie Parr Limestone [JKP], represent a condensed transgressive sequence. At Lilleshall, this sequence is divisible into five lithofacies (L1–5, (Figure 3)). A period of intra-Dinantian uplift preceded resumed deposition in the Late Holkerian or Asbian, first with fluvial clastics (Lydebrook Sandstone), and then with a further carbonate build-up (Sylvan Limestone). Borehole provings at Lilleshall show the Lydebrook Sandstone [LyS] to comprise fine- to medium-grained sandstone with common well-rounded 'millet-seed' grains (Whitehead et al., 1928) and some siltstone, typically grey, purple or pink in colour, with some mudstone interbeds. It decreases in thickness to 9 m towards the Boundary Fault, indicating possible synsedimentary movement on this structure during deposition. A maximum thickness of 37 m is recorded locally in the Pitchcroft Mine [SJ 7392 1720]. The base of the formation is severely erosional with overstep on the underlying strata. Channel-fill facies predominate, particularly where downcutting is greatest. Blocky siltstones and more rarely mudstones show evidence of pedogenesis, with rootlets and colour mottling.

The Sylvan Limestone Formation [Syl] comprises nodular or rubbly limestone which is strongly calcretised and with multiple exposure surfaces. Crinoids, corals, brachiopods (Gigantoproductus) and other shell debris are scattered throughout the sequence (Whitehead et al., 1928). A borehole at New House Farm (Lilleshall L18: [SJ 7355 1640]) provides a complete section through the formation, and includes two thin basalt horizons, which are considered to be attenuated leaves of the Little Wenlock Basalt [LWB]. In the adjoining Shrewsbury district, this olivine basalt occupies a stratigraphical position a few metres above the Lydebrook Sandstone. In this district, the flow has a restricted outcrop between Doseley and Horsehay and is exposed in Doseley Quarry [SJ 6750 0680]; it is up to 60 m thick. In a review of the main exposures, Pocock et al. (1938) described highly vesicular and slaggy lavas, with a bole below the apparently unaltered overlying sedimentary rock. Pillow-structures noted in some exposures indicate deposition in a submarine environment.

Namurian

There is no outcrop which can be unequivocally assigned to the Namurian Stage, but rocks resembling the Millstone Grit have been reported from a number of localities. In the Lilleshall Inlier, the Sylvan Limestone is overlain by a sequence of grey-green, fine- to coarse-grained sandstones of uncertain affinity. The beds appear to be dominantly fluvial, but much of the primary sedimentology has been overprinted by pedogenesis. Presently, such beds are included with the Coal Measures, pending further work.

Westphalian: Coal Measures

In the South Staffordshire Coalfield and its concealed extension (the Cannock Coalfield), up to 400 m of Coal Measures rest unconformably on basement rocks of Silurian age (Figure 4). A sequence of comparable thickness is preserved in the Coalbrookdale Coalfield (Figure 5). Seven marine bands are known within the Coal Measures, of which the identities of the lowest three (?Subcrenatum, Listeri and Amaliae) are uncertain as they have yielded only poor faunas. The Vanderbeckei Marine Band marks the boundary between the Lower and Middle Coal Measures, and is also the boundary between the Langsettian (Westphalian A) and Duckmantian (Westphalian B) stages. The boundary between the Duckmantian and Bolsovian (Westphalian C) stages is taken at the regionally important Aegiranum Marine Band. In most areas, this is normally the highest marine band found, however in the more fully preserved northern parts of the Cannock Chase Coalfield, the Edmondia and Shafton (Silvester's Bridge) marine bands are also present locally. In both coalfields there is a diachronous upwards passage at about the level of the Aegiranum Marine Band from Middle Coal Measures into a red bed-dominated lithofacies.

South Staffordshire Coalfield

The exposed coalfield underlies eastern parts of Wolverhampton, and is bounded to the west by the Western Boundary Fault. The Coal Measures are preserved in a broad south-east-plunging syncline from which the higher strata have been removed by erosion. North of the Bentley faults, in the Cannock Chase sector of the coalfield, the measures are wholly concealed by younger strata. The measures, which comprise mudstones with subordinate sandstones, siltstones and coals, show a gradual thickening northwards into the Pennine Basin. A delicate balance between subsidence and sedimentation is reflected in the variation in thickness of the coal seams and interseam thickness on either side of the Bentley faults. The area south of the faults experienced relatively little subsidence, which resulted in a relatively thin overall sequence and the amalgamation of a number of seams to produce thicker seams such as the Staffordshire Thick Coal. North of the faults, more rapid subsidence led to a thickening of the clastic deposits separating individual coals, and the introduction of new coals due to seam splitting. In the exposed coalfield, eight coal seams are known, of which the Staffordshire Thick Coal (up to 10.7 m) is the highest, thickest and most persistent. Seam splits affect the Fireclay, Bottom and Thick Coal. Notable thicknesses of sandstone occur below the Mealy Grey and above the New Mine Coal. Of these, the New Mine Rock, is the most widely mapped, varying between 12 and 30 m in thickness. Ironstones, in the form of sideritic nodules or thin beds, occur sporadically, but are best developed beneath the Mealy Grey Coal (Blue Flats Ironstone), above the Stinking Coal (Pennystone), below the Rubble Coal (New Mine) and below the Thick Coal (Gubbin).

Dolerite [D] sills have been proved at depth in many boreholes, locally over 30 m thick. These are thought to emanate from a steepsided intrusion or stock in the Wednesfield area [SJ 950 005]. One of the thickest sills intrudes the sequence below the Fireclay Coal at New Cross Farm [SO 940 995], from where it transgresses to successively lower levels southwards. The sills were probably emplaced in Bolsovian times beneath a shallow Westphalian cover.

Concealed (Cannock Chase) Coalfield

About 365 m of Coal Measures are preserved at Hollybank [SJ 9661 0341], increasing to over 400 m (unbottomed) in the Lodgerail Borehole [SJ 9459 1538] in the north of the district. A much condensed sequence (less than 200 m) overlies a basement high, aligned approximately north–south through Brinsford. This structure appears to have been active throughout Coal Measures times, resulting in the accumulation of thin coals and clastic units, which are difficult to correlate with the standard sequence.

Coalbrookdale Coalfield

The Coal Measures of the Coalbrookdale Coalfield crop out widely between the Lightmoor and Boundary faults, and south of the Broseley Fault. These two areas represent the south-western limits of the Donnington and Madeley synclines, which plunge north-eastwards. A third more southerly syncline, the Coalport Syncline, preserves measures, which have a more limited outcrop at Caughley. The variation in the Coal Measures is illustrated by two cross-sections (Figure 5), the more easterly of which illustrates the marked southerly thinning of the Lower Coal Measures in the Madeley Syncline.

The lowest strata are conglomerates and sandstones succeeded by fireclays, shales and mudstones, which contain a Lingula band. The last is the Bottom Marine Band of Wills (1948, p.49), which is a possible local equivalent of the Subcrenatum Marine Band. These measures are succeeded by a massive sandstone known as the Farewell Rock, up to 29 m thick, and the Lancashire Ladies Coal. This coal generally rests on a fireclay or the Farewell Rock, but directly overlies Silurian strata at Madeley Meadow Pit [SJ 6900 0400] in the south-west. Above the Little Flint Rock, sandstones become less dominant, and the remainder of the Lower Coal Measures is characterised by fireclays with numerous workable coals. Above the Best Coal is a sequence of fireclays, with subordinate coals and sandstones. The number of coals varies from two, in areas north and west of Dawley, to six in the north-east of the Madeley Syncline, with resultant confusion of nomenclature. Rapid variations in the number of coals and the thickness of the measures, and the presence of lenticular coarse-grained sandstones, imply conditions of depositional instability not seen elsewhere in the coalfield. The Two Foot Coal is a single seam of almost constant thickness in the north but southwards and eastwards it splits into as many as five seams referred to as the Ganey Coals. The succeeding Clunch (or Viger or Sill) Coal underlies a sequence of fireclays up to 13.5 m thick, and known as the Upper Clunch Clays. However, to the south-east these become arenaceous, forming the Viger Coal Rock.

In the Middle Coal Measures, sandstones are less significant, and workable fireclays are absent. Apart from the Big Flint Coal, the thick valuable coals lie in two well-defined groups, referred to as the Top and Fungous Coal groups after the highest coal in each (Figure 5). The latter includes the Maltby Marine Band. Ironstones in the mudstones have been worked at ten horizons and three marine bands are known. The strata between the Vanderbeckei Marine Band and the Top Coal thicken north-eastwards, from less than 25 m to more than 50 m. Measures higher in the sequence evidently thicken in the same direction, but the variations are more difficult to quantify, due to the diachronous incoming of red beds in the north-east, and the removal of grey measures in the south-west beneath the sub-Etruria unconformity. Washouts affect the continuity of the coal seams in at least three areas. The most significant predates the Gur Coal and this traces a north-east-trending channel, from Sheriffhales to Dawley. At its south-western end, it cuts down to the level of the Yard Coal, implying contemporaneous uplift at the basin margins, probably involving movement on the Lightmoor Fault (Hamblin and Coppack, 1995). A second, parallel washout, in the Madeley Syncline, predates the Double Coal, and cuts down to the level of the Vanderbeckei Marine Band at Broseley, in the south-west. A third affects strata down to the Gur Coal, north-west of the Lightmoor Fault.

Westphalian To Early Permian: Warwickshire Group

This group of barren measures is dominated by red beds in which faunas are sparse, and biostratigraphical control is poor. The age determinations are best estimates, based on studies of plant macroflora, miospore assemblages, nonmarine bivalves and pelycosaur remains.

The Etruria Formation [Et] consists of a rather ill-defined group of mudstones, sandstones and breccia-conglomerates, variegated in shades of grey, red, purple and yellow. Their appearance in the sequence provides the first widespread evidence of Variscan uplift in source areas to the south and west of the coal basin. The genesis of these deposits was discussed by Besly and Turner (1983), and further overviews were given by Besly (1988), Besly and Fielding (1989), and Besly and Cleal (1997).

The formation crops out widely, and is proved in boreholes on the Heath Farm Block. The junction with the underlying Coal Measures is diachronous in the east of the district, but is marked by an unconformity in the west. The top, also defined locally by an unconformity, is taken at the incoming of predominantly grey beds (Halesowen Formation) (Plate 3). In areas where the Halesowen Formation is also in red-bed facies, this junction may be difficult to identify, but is defined unambiguously on the basis of sandstone composition, assemblages of detrital clay minerals, and geophysical log response (Besly and Cleal, 1997; Pearce et al., 1999). Much of the formation is probably of Bolsovian age, the transition to red beds occurring at horizons ranging from 10 to 40 m above the Aegiranum Marine Band.

The sequence consists of upwards-fining units of reddish purple, silty mudstone capped by palaeosols. Evidence of pedogenic alteration is ubiquitous and is recognised by loss of bedding, and distinctive patterns of colour mottling associated with oxidised rootlet beds. Sandstones and breccias, known as 'espleys', constitute a variable but locally significant part of the sequence; they are typically crudely bedded.

Around the Coalbrookdale Coalfield, the formation varies in thickness from less than 10 m to over 90 m. Synsedimentary movements on the major faults strongly influenced sedimentation. In the south of the coalfield, the formation is unconformable on Lower and Middle Coal Measures but as the formation thickens into the basin, the basal unconformity becomes steadily less marked, until it dies out. Beneath the unconformity a reddened weathering profile is commonly developed, making it difficult to separate primary red beds from those resulting from secondary reddening. Lithological variations within the formation were documented by Hamblin and Coppack (1995).

Blockley's Brickpit [SJ 683 117] provides the type section for the area, exposing some 34 m of strata below the junction with the overlying Halesowen Formation. Red and variegated mudstones and silty mudstones are interbedded with sheet sandstone units and channel bodies.

West of the Western Boundary Fault, the Etruria Formation is concealed by younger strata. The formation is thickest, at least 129 m, close to that fault. Farther west, the formation is thickest where preserved in the axes of major synclines, and it thins over the crests of anticlines, owing to denudation beneath the sub-Halesowen unconformity. Over the Brinsford High, the basal Halesowen unconformity cuts down to levels below the Aegiranum Marine Band, and the Etruria Formation is entirely cut out. The distribution of the formation beneath the deeper parts of the Stafford Basin is not known, but it is apparently not present in the Stretton Borehole [SJ 8756 1020].

The Halesowen Formation [Ha] records a partial return to Coal Measures type sedimentation, in which grey measures predominate over red beds. The principal outcrops are around the Coalbrookdale Coalfield, where the base is marked by the widely developed Main Sulphur Coal. Comparative sections (Figure 6) show that the formation generally maintains a fairly constant thickness of between 120 and 150 m. The formation is present at depth beneath the Stafford Basin, and is also proved in boreholes on the Market Drayton Horst, where it rests directly on Uriconian basement.

In the Coalbrookdale area, the formation comprises roughly equal proportions of sandstone and mudstone. It includes 11 coals, all thin and pyritic, and several beds of Spirorbis limestone of lacustrine origin. Major, named sand bodies are distributed at the base of the unit, and more persistently through the mid-parts of the succession, where multistorey stacks 20 to 40 m thick occur. Individual bodies are sheet-like in form and can be traced for several kilometres. The Thick Rock is the best example, reaching 38.1 m at Kemberton Pit [SJ 7129 0556]. It consists of thickly bedded, medium- to coarse-grained, and locally conglomeratic sandstone, with much carbonaceous material, comminuted coal and scattered plant stems. The sandstones are a characteristic green colour when fresh, due to an abundance of rock fragments rich in muscovite, biotite and chlorite. Individual sandstone bodies fine upwards and are capped by well-bedded to blocky mudstones. These too, are commonly micaceous and dominantly greyish green, but red and purple mottling occurs throughout the formation, and is particularly prevelent towards the top. The upper formational boundary is gradational and is taken at the first appearance of red-brown sandstones having the compositional characteristics of the Salop Formation.

In the concealed sequence of the Heath Farm Block, the Halesowen Formation rests unconformably on older strata (Whitehead et al., 1928, p.165; Mitchell, 1945, p.14; Hoare, 1959; Barnsley, 1964; Besly and Cleal, 1997). The effects of the unconformity are demonstrated most convincingly in the Hilton area, where the underlying Coal Measures and Etruria Formation are strongly folded, and the base of the formation rests on an eroded surface, ranging from 198 m above the Aegiranum Marine Band to 46 m below it. Secondary reddening of rocks beneath the unconformity is recorded in some boreholes, with staining penetrating to depths of several metres.

As in Coalbrookdale, there are major sand bodies in the lower and middle parts of the sequence, but towards the top of the sequence, there is a prominent argillaceous unit. This corresponds to the Dark Slade Member of other areas, and forms a distinctive facies, 40 to 50 m thick, comprising interbedded laminated mudstones, with caliche nodules, thin nonmarine limestones and rare sandstones. It is characterised geophysically by one or more high gamma-ray radiation peaks and is an important lithostratigraphical marker (Besly and Cleal, 1997). As the Halesowen Formation is traced eastwards into the Wood Lane– Cheslyn Hay area and to the north, the whole sequence passes laterally into red-bed facies.

The Salop Formation [Sal], of assumed Westphalian D–Stephanian age, is a red-bed sequence, conformable on the Halesowen Formation, and overlain unconformably by the Permian Clent Formation (Figure 6). The formation was defined in the West Midlands (Powell et al., 2000) to remove inconsistencies in nomenclature arising from miscorrelation of the lower mudstone-dominated part of the formation. The Salop Formation includes the former 'Keele Formation' of some areas (now Alveley Member) in its lower part, and the former Enville Formation (now Enville Member), in its upper part. The formation has an estimated maximum thickness in the east of the district of 290 m (proved in the Four Ashes Borehole [SJ 9153 0954]). In Coalbrookdale, a similar thickness of strata is preserved, although wide variations are reported, which Hamblin and Coppack (1995) partly ascribed to a northward and eastward thickening of the measures, and partly to the fact that the base of the red-bed facies transgresses diachronously down-sequence.

The Alveley Member [Alv] consists of red-brown and purple, calcareous mudstone with beds of red-brown, fine-grained sandstone, intraformational mud-flake conglomerates and caliche nodule layers. Thin, discontinuous beds of Spirorbis limestone are developed locally. The base of the member is gradational with the underlying Halesowen Formation but is taken at the upward change from micaceous sandstone of Pennant-type, to sandstone in which detrital carbonate material is the most distinctive constituent (Besly and Cleal, 1997). This transition coincides approximately with a change in sediment colour from grey and purplish grey to bright orange-red. Along its outcrop from Hugh's Bridge near Lilleshall, to the Severn Gorge, west of Sutton Maddock, the base of the unit is taken beneath a persistent sandstone, known as the Brookside Rock. As the sequence is traced east and north beneath younger cover rocks, red measures occur at progressively lower levels in the sequence, and the true base of the formation is difficult to locate.

The Enville Member [En] encompasses the sequence of sandstone and conglomeratic sandstone beds that dominate the higher parts of the formation. The sandstones are red-brown, quartzose, pebbly in part, and include clasts of Carboniferous limestone, chert, Silurian sandstone, and quartzite (Plate 1). The base of the member is taken beneath the first distinctive pebbly sandstone with extrabasinal clasts. The upper boundary, with the Clent Formation, is not exposed in the district but, in the subcrop, is taken at the point on geophysical logs where there is an upwards change from a sandstone- to a mudstone-dominated sequence. However, the recognition of Clent Formation lithologies depends on the identification of diagnostic volcaniclastic breccias (see below). In the subcrop of the Heath Farm Block and the Stafford Basin, the Alveley Member is up to 100 m thick, and the Enville Member up to 200 m.

In its type area of the Clent Hills, the Clent Formation [Cle] marks a major change in provenance and lithofacies. Proximal breccias characterised by Uriconian and volcaniclastic clasts are distinctive, and these are demonstrably unconformable on the underlying beds at the basin margins (Powell et al., 2000). As the formation is traced northwards (basinward), the proportion of breccias decreases with a concomitant increase in the proportion of mudstone and thin beds of sandstone. This more distal facies has been mapped to the south-west of Wolverhampton, and the formation is assumed to extend northwards into this district, underlying the unconformable Kidderminster Formation in the hanging wall of the Western Boundary Fault. In central Wolverhampton, the Fallings Park Borehole [SJ 9266 0024] proved some 91 m of 'marl' with subordinate sandstone, and similar lithologies were recorded in the 112 m-deep Oxley (Wolverhampton) Gasworks Borehole [SJ 9163 0050]. Both boreholes are thought to have been sunk mainly in the Clent Formation.

Immediately to the west of the Bushbury Fault, the Clent Formation is absent below the sub-Triassic unconformity. However, the regional westward dip brings the formation to incrop farther west in the Heath Farm Block. The only definite proving is in the Four Ashes Borehole, which is thought to have penetrated 21.6 m of sparsely sampled mudstones. Other more tentative determinations occur in boreholes close to, and just north of, the northern boundary of the district (Bangley [SJ 9445 1398], Lodgerail [SJ 9459 1538], Teddesley [SJ 9421 1701] and Ashflats [SJ 9177 1936]). Gamma-ray logs show a mudstone-dominated unit, ranging in thickness from 38 m (Lodgerail) to about 100 m (Ashflats), unconformably overlain by Triassic strata and apparently unconformably downcutting in a north-easterly direction through sandstones of the Salop Formation (Enville Member).

Late Permian and Triassic

The central part of the district between the exposed coalfields is underlain by up to 700 m of strata in the Stafford Basin. In the north-west of the district on the Market Drayton Horst, only the lower part of the sequence is present. The form of the basin is illustrated by (Figure 7). The aeolian Bridgnorth Sandstone (Warrington et al., 1980) crops out in a relatively narrow belt on the western margins of the Stafford Basin, where it rests unconformably on, or in fault contact with, older rocks. It dips eastwards beneath younger strata towards the centre of the basin. The thickness of the formation is highly variable due to irregularities on the pre-existing topographical surface, and to syndepositional movements on major basin-bounding faults. However, the general trend is one of southward thickening into two sub-basins that lie on either side of the Codsall High. On the Market Drayton Horst, boreholes record up to 80 m of beds. In the other western outcrops the formation thickens southwards from about 25 m at Woodcote Hill to about 130 m at Grindle. Thicknesses adjacent to the Pattingham–Patshull Fault may well exceed 300 m, although no boreholes have penetrated to the base. East of the Breward Fault, the formation is thin or absent from the north and central part of the Heath Farm Block, but thickens southwards into Bratch Trough (Sheet 167 Dudley). There are no unequivocal provings east of the Bushbury Fault.

Lithologically, the sandstone is dull red-brown, fine to medium grained and pebble free. Grains are well rounded, many resembling 'millet seed', and are weakly cemented by a thin layer of iron oxide. Large-scale, high-angle (dune) cross-stratification is characteristic (Plate 2a) and, in many sections, it is the dominant bedform. Planar bedding is much less common and this represents interdune dry sandsheets. Quartz is the dominant constituent with minor feldspar and volcanic grains. Calcite concretions observed in cored samples may relate to rootlet activity and the development of poor soils.

Sections include those at Woodcote Hill [SJ 7645 1471] in basal beds (6.2 m), Brimstree Hill road-cutting [SJ 7513 0580] with the junction between Bridgnorth Sandstone and Kidderminster Formation, and Apley Park [SO 7290 9681].

The fluvial Sherwood Sandstone Group comprises, in ascending order, the Kidderminster, Wildmoor Sandstone and Bromsgrove Sandstone formations. Of these, the Kidderminster Formation (Warrington et al., 1980) known formerly as the Bunter Pebble Beds, occurs on both sides of the Stafford Basin where it gives rise to prominent hills or escarpments. It consists of texturally mature, pebble/cobble conglomerates, medium- to coarse-grained sandstones and sparse, thin, mudstone beds, and ranges in thickness from 50 to 160 m. The formation is unconformable on the Bridgnorth Sandstone west of the Breward Fault, and on Upper Carboniferous (Salop Formation) rocks over most of the Heath Farm Block. The junction with the overlying Wildmoor Sandstone Formation is transitional, and difficult to recognise, but is drawn conventionally at the point above which the sequence becomes pebble free. Over much of the central part of the basin, thicknesses range from 100 to 150 m. These decrease to around 50 m in the north-west and north-east. In a narrow graben (the Westward–Hayward Trough) to the east of Hilton Main Fault (Figure 1) on the margins of the exposed South Staffordshire Coalfield, 226 m of Triassic strata are preserved; the greater part of this is thought to be Kidderminster Formation.

In much of the basin, the sequence commences with a unit mainly of conglomerates, 40 to 60 m thick. These contain pebbles of 'liver-coloured' and grey-brown quartzite, vein-quartz and rarer siliceous types including chert, rhyolite, and tuff. The pebbles display pitting and siliceous rims at points of contact, and are commonly bound by a carbonate cement. The beds show weak horizontal stratification and planar- or trough cross-stratification. Interfingering sandstones, pebbly sandstones and red-brown, micaceous mudstones form a minor part of the sequence. Aeolian grains reworked from the Bridgnorth Sandstone are present in beds up to 8 m above the base of the formation. This unit, which is locally sparsely developed, is overlain, in places sharply, by a sandstone containing pebbles that are dispersed or form discrete lenses or channel lags. The sandstones are typically red-brown, flecked white, fine to coarse grained and slightly micaceous. Primary structures include planar cross-stratification and multiple upward-fining packages.

Exposures include those at Saredon Hill Quarry [SJ 946 080] (Plate 2b), Stanton Hill Wood [SJ 7635 0780], Essington Quarry [SJ 944 038], Hatton Grange [SJ 7626 0378] and Old Forge Bridge, Grindleforge [SJ 7533 0323]. The name Wildmoor Sandstone [WrS] was introduced (Warrington et al., 1980) for beds formerly termed Upper Mottled Sandstone. The formation crops out around the margins of the Stafford Basin, where it gives rise to a subdued topography. The maximum thicknesses are inferred to occur in the south of the district in two areas identified by Bouguer gravity data. Boreholes drilled over the gravity anomaly to the west of the Pattingham Fault proved 240 m of strata at Stableford Pumping Station [SO 764 981]; a second anomaly, coincident with the northern limit of the Bratch Trough, may contain a comparable thickness. The formation thins towards the outcrop in the west (to around 50 m) and is overstepped by the Bromsgrove Sandstone with sharp unconformity in the extreme north-east corner of the district.

The formation is dominated by weakly cemented, micaceous, silty, fine-grained sandstone. The intense foxy red colour of the sandstone is distinctive, and derives from iron oxide coatings on the sand grains. Rare pebbly stringers occur low in the sequence, where they are associated with coarser sandstones. Red-brown and grey-green mudstones are present in beds less than 30 cm thick, but exceptionally reach a few metres towards the top of the sequence. Bedforms comprise low-angle, trough cross-bedding, low-angle planar bedding and ripple cross-lamination. Palaeocurrent directions indicate a unimodal palaeoflow towards the north-west.

Exposures include those at Tong Forge river-cliff, with the overlying Bromsgrove Sandstone Formation [SJ 7843 0826], Ryton disused quarry [SJ 7612 0256], The Rock (Tettenhall) [SJ 889 001] and Folley river-cliff [SO 7662 9813].

The Bromsgove Sandstone Formation [BmS], formerly known as the 'Lower Keuper Sandstone' (Warrington et al., 1980), thickens eastwards from 50 m at outcrop in the north-west to between 100 and 155 m adjacent to the Breward Fault. To the north of this district, the formation oversteps the Kidderminster Formation, and a comparable unconformable relationship probably exists hereabouts. This break may be equated with Hardegsen Disconformity (Warrington, 1970) and represents a period of uplift and erosion in late Early Triassic times. Hull (1869) divided the sequence into the 'Basement Beds', 'Building Stones' and 'Waterstones', which have been formalised elsewhere as the Burcot, Finstall and Sugarbrook members, respectively (Old et al., 1991). The two lowermost divisions can be readily identified on geophysical logs and in cored boreholes but they are not sufficiently distinctive to be mapped. The uppermost division is now included as the basal unit of the Mercia Mudstone Group (see below).

The Bromsgrove Sandstone consists of red-brown, predominantly fine- to medium-grained sandstone, with subordinate mudstone. Mica is abundant on bedding planes and feldspar is a common constituent; the beds are generally calcite cemented. Upward-fining cycles in the lower part of the formation typically commence with an erosional, polymict, pebbly sandstone or conglomerate. This passes upwards into cross-bedded sandstone, which, in turn grades up into silty, very fine-grained sandstone, with small-scale cross-bedding or ripple marks. Individual cycles are typically around 4 m thick. The conglomerates contain extraformational clasts (quartz and quartzite), intraformational mudstone rip-up clasts and beds of reworked caliche. Siltstones and mudstones form only a minor constituent. Higher in the sequence (Hull's 'Building Stones' division), fining-upward cycles persist but the sandstones become generally finer grained and the mudstones thicker, and more numerous, locally forming mappable units.

The Bromsgrove Sandstone has yielded vertebrate trace fossils and crustacea in the district and, elsewhere (Old et al., 1991), a rich and diverse flora and fauna of Mid Triassic (Anisian) age.

Sections include those at Great Chadwell Quarry [SJ 7952 1471], Weston Park Quarry [SJ 807 094], Beckbury road cutting [SJ 7662 0146], Stafford Road cutting, Wolverhampton [SJ 9134 0014] to [SJ 9129 0042] and Badger Dingle river-cliff [SO 7631 9911].

The Mercia Mudstone Group [MMG], formerly the 'Keuper Marl' (Warrington et al., 1980), has a broad crop that expands northwards from Wrotesley, following the axis of the Stafford Basin, and is bounded to the east by the Breward Fault. It is largely concealed beneath a thin mantle of glacial drift. The base of this red-bed group is gradational and is taken where mudstone with siltstone becomes dominant in the sequence. The thickest proving in the district is 173.8 m, recorded in the Hurst Farm (Ivetsey Bank) Borehole [SJ 8364 1158]. A comparable thickness (172 m) was proved in the Stretton Borehole [SJ 8756 1020]. Towards the northern margin of the district, the basin deepens and over250 m of beds occur in the district to the north. Fossils are rare and are mainly palynomorphs, but regional evidence shows that the group ranges in age from Anisian to possibly Carnian.

Although the Mercia Mudstone Group is shown undivided on the published map, three lithofacies can be identified at outcrop:

1. micaceous, interlaminated mudstone, siltstone and sandstone, 40 to 70 m thick; this corresponds in part to the 'Waterstones' of Hull (1869)

2. interlaminated mudstone and siltstone, interbedded with or overlying (i)

3. blocky, structureless mudstone and siltstone, is the most common lithofacies and is found in the higher parts of the succession. Grey-green reduction spots occur throughout, and interbeds, up to 6 cm thick, of dolomitic siltstone or very fine sandstone (skerry). Secondary gypsum veins are present in unweathered material. Pseudomorphs after halite are common on the underside of skerry siltstones

Although thick saliferous beds have not been recorded within the district, the Stafford Halite Member proved in the adjoining area (Arup Geotechnics, 1990) may be present as a thin fourth lithofacies. The superficial (drift) cover precludes a more formal subdivision of the Mercia Mudstone Group in this district. However, on a regional scale, lithofacies (i) is an approximate correlative of the Tarporley Siltstone Formation; other lithofacies fall broadly within the Eldersfield Mudstone Formation.

Quaternary

Superficial (drift) deposits cover most of the district, except for the higher ground. They consist predominantly of broad spreads of till and glaciofluvial outwash laid down during the last glaciation. The important Devensian Stage stratotype site at Four Ashes [SJ 915 083] lies within the district, and is the only site where Quaternary deposits predating the glaciation have been found. During the Late Devensian Substage, an ice sheet, originating in the west of Scotland and Cumbria, advanced into the Shropshire–Staffordshire lowlands from the Irish Sea Basin. At its maximum extent, the ice margin lay just to the south of the district, the mapped limit running from just south of Bridgnorth, eastwards towards Trysull and then north-eastwards through the southern suburbs of Wolverhampton. The district can be divided into five glacigenic domains (see I–V on the inset on the published map), each characterised by a sediment-landform association which adds a third dimension to the map. The glacial history of the district has been reconstructed by a number of workers (A V Morgan, 1973; Hamblin, 1986). The generally accepted view (see review in Hamblin, 1986) is that the preglacial watershed was breached only at Ironbridge during the Devensian glacial maximum. The glaciolacustrine deposits found towards the base of the drift in the north-west of the district probably formed in a proglacial lake impounded between the advancing ice and the higher ground of the Lilleshall–Telford watershed. These deposits were overridden as the ice-sheet advanced over the watershed and expanded southwards, reaching its maximum limit, probably around 17 000 years BP. On deglaciation, the ice withdrew northwards leaving behind patchy deposits of glaciofluvial sand and gravel (kames). It is likely that these were deposited at the margins of the receding ice sheet, by streams flowing in subglacial or englacial conduits. Larger kames may mark still-stand positions, when the ice sheet was compressed against the topographically higher ground of the main watershed. The Ironbridge Gorge may have developed during one such still-stand, excavated by meltwaters flowing marginally to the stationary decaying ice. Alternatively, it may have been cut by overflow from a small proglacial lake (Lake Buildwas) after the ice had retreated to the west of the watershed. Both are credible possibilities and it is reasonable to assume that a channel cut at the ice-margin could have been adopted and deepened by proglacial overflow at a later stage.

While Lake Buildwas was forming to the west of the watershed, a second proglacial lake, Lake Newport, developed farther to the north (Wills, 1924). This lake may have overflowed initially through the Oakengates gap and, later, drained into the Church Eaton Brook. Whether this lake ever connected with Lake Buildwas is uncertain but the restriction of glaciolacustrine sediments to the area around Lilleshall indicates that it may well have existed as separate body of water. During the final stages of deglaciation, the postglacial drainage pattern evolved, through processes that involved river capture and drainage diversion.

Widespread evidence of patterned ground on the surface of the till sheet to the north and north-west of Wolverhampton indicates that periglacial conditions were established following deglaciation. During the 13 000 years since the ice retreated, silts, sands and gravels, derived in part from the glacigenic deposits, have been reworked and incorporated in alluvium and river terrace deposits. The Four Ashes Sand and Gravel was formerly exposed in aggregate workings that stretched for some 2 km along the north side of Saredon Brook between Hatherton Junction and the A449. The gravels have largely been worked out and their presumed top is only visible in a small preserved face within a restored landfill site [SJ 9148 0829]. A V Morgan (1973), A Morgan (1973) and Andrew and West (1977) studied the deposit, which is up 4.6 m thick and composed largely of pebbles of 'Bunter' quartzite together with rare exotic clasts of flint, tuff, rhyolite and andesite. The bedding is complex with minor breaks and erosional channels; numerous organic lenses occur at several levels, including at the base of the deposit in shallow channels cut in Wildmoor Sandstone bedrock. The upper erosional surface of the deposit falls westwards from about 106.7 m at Hatherton Junction to 97.5 m OD at the A449. Intraformational wedge casts have been observed at least at two levels. The sedimentary structures are consistent with deposition in a braided stream environment; most material was derived from nearby outcrops of the Kidderminster Formation.

The organic lenses yielded radiocarbon ages ranging from the Ipswichian Interglacial to the Middle Devensian. Material from two basal channels has yielded macrofossils and palynological evidence indicative of the Ipswichian Interglacial and Chelford Interstadial.

Till mantles much of the district forming a widespread sheet, typically 1 to 3 m thick, but increasing locally to between 12 and 17 m beneath parts of Telford, and central Wolverhampton. On the higher ground and interfluves it has a patchy distribution. More varied and generally thicker glacigenic sequences are found mainly to the north-west of the Lilleshall–Telford watershed, and within buried channels: here they commonly include a basal till, separated from an upper till unit by waterlaid sediments.

The predominant matrix is a reddish brown, silty, clayey sand with a uniform clay mineral assemblage of illite, corrensite, kaolinite and chlorite. A sandier till was recorded locally (see below). The clasts are well rounded to subangular, and consist mainly of quartz and quartzite derived from Kidderminster Formation (Hollis and Reed, 1981; A V Morgan, 1973). The other clasts comprise Carboniferous sandstone and siltstone, Uriconian igneous rocks and clasts derived from farther afield (Welsh greywacke, Lake District slate and volcanic rocks). Large granitoid boulders, some in excess of a cubic metre, are scattered throughout the area, the maximum concentration occurring close to the postulated limit of ice advance (A V Morgan, 1973). Some of the more striking lithologies can be matched to outcrops in Eskdale and Ennerdale in the Lake District and to Dalbeattie in Scotland, providing clear evidence of the north-westerly derivation of the ice-stream. Pleistocene marine shells, which include Turritella, found in the Badger and Ackleton areas, indicate derivation from the floor of the Irish Sea Basin.

Sandy Till is confined mostly to the east-facing dip-slopes of the Kidderminster Formation and parts of the less-well featured Wildmoor Sandstone Formation. Identified solely by augering, it comprises silt, sand and pebbles with rare sandy clay patches.

Associated with the Irish Sea till there are spreads of glaciofluvial outwash, termed Glaciofluvial deposits, undifferentiated on the published map. These occur in a variety of settings and include ice-contact deposits, valley sandar and valley train deposits. A line of hummocky mounds and ridges of sand and gravel (kames) extends from Codsall Wood north-westwards through Boscobel, to Blymhill and then along the headwater valley of Back Brook to Orslow. The complex forms part of a belt of pebbly sands referred to as the 'Newport Esker Chain' (Whitehead et al., 1928). True eskers, represented by narrow sinuous ridges, are only recognised at the extreme northern end of the complex.

Spreads of flat-lying or slightly hummocky, sand and gravel, locally associated with till, cover the floor of the valley of the River Worfe. The base of these outwash sandar falls steadily southwards from about 95 m above OD at Crackley Bank to 45 m at the district boundary. The sands and gravels are up to 6 m thick, and, in the upper Worfe, are composed mainly of locally derived material. The Glaciofluvial sheet deposits (valley-trains) and glaciofluvial terraces depicted on the published map include sands and gravels that show a variably developed terrace form, notably along the River Penk and its tributaries. The thickest deposits lie in valleys drained by Church Eaton Brook and Whiston Brook, and form part of an outwash train that can be traced back to a col on the watershed at Gnosall in the Stafford district. The deposits comprise pebbles of 'Bunter' quartz and quartzite and assorted igneous clasts in a medium-sand matrix.

Glaciolacustrine deposits have been proved at surface and in boreholes at a number of sites in the district. The most widespread deposits are purple-brown, laminated clays and lacustrine rhythmites (alternating laminae of clay and sand) found both above and beneath the upper till north of the Lilleshall–Telford watershed. As noted earlier, these are thought to be mainly proglacial in origin. Ponding and lacustrine accumulation in this low-lying area is likely to have continued after deglaciation and some of the deposits may be of Flandrian age.

Glaciolacustrine deposits are also commonly encountered in the thick drift sequences of palaeovalleys and as smaller isolated spreads. Several of the former (buried channels), filled with thick glacigenic sediments, have been recorded within the district (see inset on published map). Those cutting the Lilleshall–Telford watershed are the best defined, and include the 20 km-long Lightmoor Channel (Hamblin, 1986; Hollis and Reed, 1981). The River Penk flows in an overdeepened, drift-filled valley, north of Penkridge (A V Morgan (1973). Deep drift-filled depressions also underlie two minor left-bank tributaries of Saredon Brook, one flowing to the east of Saredon Hill, the other joining the main river at Deepmore Farm. In the south-east, the Moxley Channel underlies north-east Wolverhampton.

The Lightmoor and Moxley channels are known to have irregular, deeply scoured, longitudinal profiles which descend in places to below the level of any drainage outlet. This suggests that they were cut by subglacial flow of meltwater under hydrostatic pressure. The deeper channels are filled by laterally variable sequences of till, glaciofluvial sand and gravel and glaciolacustrine clay. The bulk of these sediments are thought to be Late Devensian in age. However, the discovery of fossiliferous lake basin sediments of early Hoxnian age in a channel at Trysull only 3 km south of the district (A V Morgan, 1973) raises the possibility that some of the channels have a longer and more complex history. Details of the individual channels, their sediments and possible development history are given elsewhere (Hamblin, 1986; Hollis and Reed, 1981; A V Morgan, 1973).

A series of west- and north-west-trending meltwater channels originate on the flanks of Cannock Chase. They vary from shallow, flat-bottomed troughs to steep-sided depressions and are presently either dry or contain only minor streams. Deposits of sand and gravel, preserved in the floors of the channels, are generally no more than a metre or two thick, and range from spreads up to 200 m across, to narrow channel-fills. Head is a slope deposit formed mainly by processes of solifluction and gelifluction, and derives from parent materials upslope under conditions of alternate seasonal freeze and thaw. It is generally poorly structured and may contain relict shear surfaces. Although not widely mapped, thin head and colluvium (hillwash) are likely to be extensive. Head is thickest and most widespread in the Nedge and Mad Brook valleys, where up to 4.3 m are recorded.

Peat deposits are present within some of the alluvial tracts and former drainage lines exploited by meltwaters during deglaciation. The meltwater channel south-west of Pillaton Hall Farm [SJ 938 132] is floored by peat, up to 3 m thick. A sample gave a radiocarbon date of 11 660 ± 250 years BP (A V Morgan, 1973). A peat moor [SJ 925 112] occupying a hollow in a glacial meltwater channel, lies to the south of Rodbaston Hall. The deposit was first studied by Shotton and Strachan (1959) and dates of 10 300 ± 170 and 10 670 ± 130 years BP were reported (A V Morgan, 1973). At nearby Penkridge, organic material collected from fluvial gravels exposed in a sewerage pipeline [SJ 9244 1435] to [SJ 9235 1434] has given an age of 11 660 ± 250 years BP (A V Morgan, 1973). Beetles extracted from these three sites were described by Ashworth (1969). Peat-filled hollows mapped in the floor of Back Brook probably originated as kettleholes: one of the best examples is at Wyndford Mill [SJ 801 148].

Downstream of the Ironbridge Gorge, Wills (1924, 1938) recognised and named two suites of River Terrace Deposits (sands and gravels); a third lower terrace was mapped by the Geological Survey. More recently, Maddy et al. (1995) have formalised the nomenclature of the Severn terraces, giving member status to the individual terraces and naming the suite the Severn Valley Formation. Nomenclature of the stratigraphical units present in the Apley Park section of the river is listed in (Figure 8).

The Holt Heath Member is represented by two small patches of sand and gravel lying at about 76 to 82 m above OD. The deposits interdigitate with Devensian till, and contain a range of exotic clasts, showing the bulk of the material originated as outwash from the Devensian ice sheet. Upper and lower facets of the Second Terrace are recognised, with surfaces, respectively, 48 and 56 m above OD. The First Terrace forms an extensive area of sand and gravel contiguous with, and about 4 m above, the present floodplain.

The terraces of the River Worfe are graded to those of the River Severn (Wills, 1924; Hamblin and Coppack, 1995), though the exact correlation is speculative, and a formal relationship between the two systems has yet to be established. Flat-topped deposits of sand and gravel (Higher Terraces: undifferentiated ) occur along the southern Worfe catchment. The base of these terrace remnants ranges from 65 to 78 m above OD, indicating a possible correlation with the Holt Heath Member (Hamblin and Coppack, 1995).

Second Terrace deposits are mainly found in the south of the district (SO79NE) at about 55 m above OD.

The First Terrace lies typically 1 to 3 m above the floodplain.

Most of the major streams and rivers have floodplains underlain by alluvium , consisting of silt and clay overlying coarser beds of sand and gravel. Along the River Penk, north of Penkridge, the alluvial silts are typically about 2.5 m thick, and the underlying gravels about 8 m, though this latter figure may include some glaciofluvial outwash.

Landslips affect both sides of the Ironbridge Gorge, and extend into the deeply incised tributary valleys. Unstable or potentially unstable slope deposits extend along the length of the Gorge and the mapped distribution is a conservative estimate. Shallow-seated downhill movement has affected the village of Ironbridge over a long period, resulting in damage to buildings and to the Iron Bridge itself. The Halesowen Formation is particularly susceptible to degradation in this area. An active translational and rotational slip stretches from Jackfield to the Wilds, and includes the famous and active Jackfield slip described by Henkel and Skempton (1954) and Skempton (1964). This slip forms part of an older slipped complex, which extends 600 m back from the river.

Excavated (or backfilled) ground exists where the natural ground surface has been removed and the void partly or wholly backfilled with man-made deposits. In the Coalbrookdale Coalfield, areas of excavated ground are associated with opencast coal and brickclay operations. Elsewhere, there are also a number of large pits connected with former aggregate workings (Saredon Brook, Essington, and Saredon Hill).

Made ground is mapped where material generally in excess of 1.5 m thickness has been deposited on the original ground surface. The most extensive deposits occur in the exposed coalfields, where colliery spoil is the predominant fill material. Thicknesses of between 4 and 10 m are common over large areas of north-east Wolverhampton, exceptionally reaching 20 m. Old spoil heaps that have been levelled seldom form topographical features and the extent of such material is only known from borehole and trial pit data.

Concealed geology and structure

Beneath the Stafford Basin, the seismic evidence indicates a thick Cambrian succession overlying possible Longmyndian rocks and the Uriconian Volcanic Group. East of the Breward Fault, the Silurian rests unconformably on Uriconian rocks. Hence, the precursor of the Breward Fault was probably initiated during early Palaeozoic times as a growth fault, and defines the eastern boundary of a Cambro-Ordovician basin. Much of the Silurian subcrop is represented by strata of Ludlovian age, folded along north-east-trending (Caledonoid) axes. This is the dominant structural trend throughout the district, and is most evident in the Palaeozoic rocks around the Coalbrookdale Coalfield. Most of the faults at surface show normal throw, but many have a complex history, and throw has been reversed during successive tectonic episodes. The Boundary Fault lies on the structural line of the Church Stretton Fault and has an early compressional history (see below). The presence of Uriconian basement at shallow depths (less than 50 m) in the hanging wall, but at depths of over 1500 m in the footwall, suggests that the lineament suffered strong reverse movement, probably during the Acadian tectonic event. Other significant Caledonoid structures in the Coalbrookdale Coalfield are the Ketley, Lightmoor and

Limestone faults, for which late Dinantian to intra-Westphalian normal and reverse movements can be demonstrated (Hamblin and Coppack, 1995, p.102). Faults with north-west and west-north-west (Charnoid) trends are common in the Coalbrookdale Coalfield, but apart from the Broseley and Brewers Oak faults, none is of any great length, and most appear to be accommodation structures. The Breward Fault defines the eastern margin of the Stafford Basin and separates a thick Permo–Triassic basin-fill sequence from the thinner Permo–Triassic cover on the Heath Farm Block. The fault is characterised by a westward hade and associated antithetic faults dip east into it at depth. The displacement of the Triassic strata on the northern part of the Breward Fault is at least 170 m. However, this diminishes southwards as control is transferred onto the Pattingham– Patshull fault system through a plexus of smaller faults on the Codsall High. The bounding structure to the South Staffordshire Horst is the Western Boundary Fault, which trends north-eastwards through the suburbs of Wolverhampton, and juxtaposes the Wednesfield dolerite against red beds of the Clent–Salop formations. The downthrow to the west is estimated at about 600 m. The subparallel Bushbury Fault marks the western limit of workings in the concealed coalfield; in the south, it displaces the Triassic strata down to the north-west by between 125 and 200 m. Northwards the throw reduces to between 50 and 75 m. The Hilton Main and Essington Church faults define a graben, the Westcroft–Hayward Trough that is broadest in the south (about 3 km) but narrows to around 400 m west of the former Mid-Cannock colliery. The Hilton Main Fault hades at about 45° and throws down to the south-east by about 100 m. Downthrow to the north-west on the Essington Church Fault is up to 200 m.

Structural history

Deformation in the Uriconian rocks of the Lilleshall Inlier is localised in the footwall of the Boundary Fault, where bedding is near-vertical or locally overturned, there is a pervasive cleavage. This deformation could be Acadian (Siluro-Devonian), but it is similar in style to that affecting the Longmyndian and Uriconian rocks of the Church Stretton area which has been dated as latest Precambrian or early Cambrian.

Fold axes in the exposed Silurian rocks of the district generally trend north-eastwards; the folding is Caledonian in age, postdating the Downtonian Temeside Shales but pre-dating the Dinantian Lydebrook Sandstone. Between Devonian and early Permian times, the Midlands formed part of a foreland basin that lay to the north of the Variscan thrust front. The tectonic cycle within the Variscan foreland commenced in latest Devonian or earliest Dinantian with a phase of rifting and extension. Differential subsidence on tilt blocks and the close proximity of shorelines influenced sedimentation. The area remained as a structural high (the London–Brabant High) transgressed in early Dinantian times and later uplifted, before subsidence and submergence during the Holkerian. During the Namurian, a period of uplift led to erosion of Dinantian and earlier strata. By late Namurian times, the regime of extension-induced rifting had been replaced by one of slow subsidence (or thermal sag) leading to onlap of Coal Measures strata on the flanks of the high. Variations in seam thickness and seam splits signify tectonic instability, and many of the Caledonoid faults show evidence of synsedimentary movements. Compression reached a peak in the south-west of the district in early to mid-Westphalian times, resulting in folding of the Lower and Middle Coal Measures along north-east-trending axes. The major structures have a Caledonoid trend and plunge north-eastwards.

In the South Staffordshire Coalfield, the main deformation phase followed deposition of the Etruria Formation, thus giving rise to the unconformity beneath the Halesowen Formation and resulting in localised tight folding within the underlying Westphalian strata. Near the Bushbury Fault, an anticline complementary to the Moseley Syncline may have developed in its western (hanging) wall in an area of condensed Coal Measures termed the Brinsford Axis (Barnsley, 1964). Westphalian igneous activity was restricted to the intrusion of the Wednesfield Dolerite and associated sills. The occurrence of these and similar igneous intrusions in close proximity to major lineaments has been noted elsewhere in the Midlands (Waters et al., 1994), and is taken as evidence that intrusion occurred during a Bolsovian phase of extensional faulting.

The tectonic regime during deposition of the Halesowen and Salop formations is not well understood, but appears to have been one of regional subsidence, coupled with periods of local tectonically induced uplift (Waters et al., 1994). The presence of Lower Carboniferous limestone debris in the Enville Member, and the apparent unconformable relationship of the Clent Formation on older beds are evidence of pre-Clent uplift. A further period of uplift, tilting and erosion occurred before the deposition of the Bridgnorth Sandstone.

Permo–Triassic extension reactivated major basin-bounding faults resulting in substantial thickness variations between sediments in the hanging and footwall blocks. Growth on the Breward Fault occurred during the deposition of the Bridgnorth Sandstone, Kidderminster Formation and Wildmoor Sandstone. The more uniform thickness of the Bromsgrove Sandstone, and its apparent unconformable relationship in places on a thin Wildmoor Sandstone may be evidence for intra-Triassic movements responsible for the Hardegsen Unconformity. All the major north-east-trending faults show post-Triassic normal movement.

Chapter 3 Applied geology

The key geoscience constraints likely to influence land use, development and conservation within the district are listed in (Figure 9). More detailed guidance on dealing with natural and man-made hazards is published by the former Department of the Environment Transport and the Regions in the form of Planning Policy Guidance notes (PPGs), Mineral Planning Guidance notes (MPGs) and also as research projects. Ellison et al. (1997) review the main sources of geoscience information.

Ground conditions

Slope instability

Over most of the district, undisturbed natural slopes have generally attained a considerable degree of stability and present little hazard to development provided water is not introduced to the slope, and they are not undercut by excavation or natural features such as rivers. An exception is the Ironbridge Gorge, which has experienced a number of catastrophic failures in historic times. The listricated mudstones of the Coal Measures, Etruria Formation and Halesowen Formation and the mudstone-dominated Temeside Shales are all prone to failure on steep slopes, particularly where they are overlain by water-bearing sandstones. Factors contributing to slope failure along the gorge were documented (Henkel and Skempton, 1954; Skempton, 1964; Gostelow et al., 1991). Culshaw (1973) used a zoning technique to distinguish areas of 'high', 'medium' and 'low' risk. This type of approach accepts that development is possible in areas of 'low' risk providing precautions are taken to maintain the integrity of the slopes.

Man-made land instability

In the exposed coalfields, the principal concerns relate to ground instability caused by the collapse of shallow coal or ironstone workings. Dewatering of old workings may cause subsidence by removing hydraulic support; conversely flooding may cause instability by increasing pore water pressure in the adjacent strata. Collapse of shaft fill, linings or cappings may also result in surface subsidence. Records of shafts and abandoned mines are lodged with the Coal Authority, who should be consulted prior to development in former coalfield areas.

In the Coalbrookdale Coalfield, clay mining has also led to serious subsidence (Hamblin et al., 1989), as for example at the Red Church, Broseley. Subsidence has also been caused through underground mining of limestone, notably in the Lilleshall area (Ove Arup and partners, 1987).

Throughout the exposed coalfields, variable thicknesses of poorly compacted colliery spoil are widespread, introducing the possibility of severe differential settlement. Colliery spoil may contain iron sulphides (such as iron pyrites) that are prone to oxidise and produce sulphate-rich, acidic leachates. Geotechnical information on the engineering characteristics of the outcropping rock formations and superficial deposits is summarised elsewhere (Bridge and Hough, 2002).

Natural contamination

The radioactive decay of uranium, which is found in small quantities in all soils and rocks, produces radon, a colourless, odourless, radioactive gas. Radon disperses quickly in the open air but it may accumulate in poorly ventilated buildings and mines where it is a potential health hazard. BR211 (1999) provides revised guidance on protective measures for new dwellings, and defines the geographical areas where radon protection is necessary. Moderate to very high levels of radon may occur on some Carboniferous and Silurian formations in the west and south-east of the district and, in these areas, basic or full protection may be necessary depending on the local site geology. Specific advice on these areas may be obtained from the BGS.

Other potential gas hazards are associated with the build-up of methane and carbon dioxide (Appleton et al., 1995); emissions are associated mainly with coal and peat deposits. Methane is only freely released from coal either in the vicinity of geological disturbances, such as faults, or as a result of degassing of adsorbed gases as the coal is fractured during mining. The risk of gas emission at surface may increase if groundwater levels rise. Much of the carbon dioxide derived from coal mines is formed by the oxidation of coal through biological processes. Methane is potentially explosive whereas carbon dioxide is toxic in high concentrations. Both gases may act as asphyxiants and cause vegetation die back.

Industrial contamination

Soil sampling results (Bridge et al., 1997) showed that for the Wolverhampton Metropolitan area, the distribution of heavy metals in the soil can be linked to past and present industrial processes. Levels above the baseline typically occur in the long-established industrial areas. Landfill sites may contain organic matter which may biodegrade to form methane and carbon dioxide. The nature of landfill gas characteristics was discussed by Williams and Aitkenhead (1991) and Hooker and Bannon (1993). Reviews of landfill sites have been carried out by all Local Authorities within the district. Leachate migration may occur where rain-water or groundwater percolates through waste and becomes enriched in soluble components including inorganic, organic and microbial components. The resultant leachate may permeate into surface water and groundwater.

Impact of rising groundwater

The closure of much of the heavy industry, particularly in the urban areas, has led to reduced groundwater extraction and a consequent rise or 'rebound' of groundwater levels (Bridge et al., 1997). The closure of the coal mines during the second half of the last century and the cessation of dewatering has also contributed to a rapid rise in groundwater levels. Currently, levels are now believed to be at, or close to, those that existed prior to mining operations.

Hydrogeology

The Permo–Triassic sandstones of the Stafford Basin form part of one of the most heavily exploited aquifers in the country. Although there have been problems in recent years with over-abstraction and enhanced nitrate levels, this aquifer remains an important groundwater resource, and continues to supply large quantities of water for public and industrial use to the surrounding conurbations. The Sherwood Sandstone Group and Bridgnorth Sandstone are the most vulnerable to pollution (Environment Agency, 1997).

For the purposes of groundwater modelling, the aquifer is commonly considered to act as a single hydrogeological unit. In practice, cemented pebbly horizons and mudstone interbeds act as confining layers effectively partitioning the aquifer, and creating head differences across formation boundaries and within individual formations. Pumping tests indicate transmissivity ranges from 2 to over 5000 m²/day, with a mean of about 150 m²/day. Equivalent bulk hydraulic conductivity values are several orders of magnitude larger than intergranular hydraulic conductivity values obtained from cores, indicating a considerable contribution from fracture flow (Allen et al., 1997). Borehole yields are normally high, with some large-diameter production boreholes yielding 160 litres/second. Groundwater quality in the Permo– Triassic aquifer is generally good with total dissolved solids ranging between 300 and 600 mg/l. Most groundwaters are of a calcium-bicarbonate type but there is local evidence for more saline waters suggesting that up-coning of deeper saline waters may occur, possibly enhanced by flow along major adjoining faults. Nitrate concentrations are variable.

A number of boreholes abstract water from the Upper Carboniferous formations for industrial use, mainly from the Enville Member, and small supplies of groundwater for agricultural use are present within the Mercia Mudstone, where flow is dependent on fractures in sandstone beds. The wells that draw water from the superficial deposits penetrate the fluvioglacial gravels. This groundwater is highly vulnerable to contamination. The high proportion of effluent and urban run-off has led to many water courses being polluted.

Mineral resources

Deep coal mining in the district ceased in 1993, and the limited opencast activity in the Coalbrookdale Coalfield is confined to the area south of the River Severn, where seams in the Lower Coal Measures crop out beneath thin overburden.

The down-dip continuation of the Coalbrookdale and South Staffordshire coalfields beneath the Stafford Basin may have some coalbed methane potential. In the 1950s, the methane drainage system from Granville Colliery was connected to Wellington Gas Works. In the South Staffordshire Coalfield (Littleton area), a mean methane level of 3.3 m³/ton was quoted (Creedy, 1991). The overall prospectivity of the coalfields in relation to others in the region was reviewed by Glover et al. (1993).

The Etruria Formation is the principal brick clay resource in the district and is currently worked in three pits in the Coalbrookdale Coalfield: at New Hadley, Donnington and Caughley (Plate 3). The Lower Coal Measures of the Coalbrookdale Coalfield are an important source of fireclay. Beds of economic interest occur between the Little Flint and New Mine Coals.

The main resources of sand and gravel are in the Newport Esker Chain, in the Worfe valley, and in the buried valleys in the west of the district. However, rapid variations in thickness, high sand to gravel ratio, and compositional variability are the factors that reduce their potential value. Conglomeratic beds within the lower part of the Kidderminster Formation have been worked in quarries at Essington, Saredon Hill and Manor Farm. Although reserves at these sites are exhausted, the eastern outcrop remains an important aggregate resource, with deposits in excess of 60 m thick.

Information sources

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

Other geological information held by the British Geological Survey include borehole records, fossils, rock samples, thin sections, hydrological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Data Index system available in BGS libraries and on the web site (see back cover for addresses). BGS catalogue of geological maps and books is available, on request.

Maps

• 1:25 000
• Telford Special Sheet (Solid & Drift, 1978)
• 1:10 000
• The maps covering the 1:50 000 Series Sheet 153 Wolverhampton, that were surveyed in 1970–98 are not published, but copies can be purchased from BGS or may be consulted in BGS libraries and at the London Information Office.
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• Geochemical atlases
• 1:250 000
• Regional geochemistry of Wales and part of west-central England: stream water; stream sediments and soils (two volumes), 1999.
• Geochemical Survey Programme data are also available in digital form.
• Hydrogeological maps
• 1:100 000
• Groundwater vulnerability of South Staffordshire and East Shropshire (Sheet 22), 1997 (Solihull: Environment Agency).

Reports

Geological reports are available for a number of the 1:10 000 geological sheets relevant to the district. They may be purchased from BGS or consulted at BGS and other libraries.

Geology and land-use planning

North-west Wolverhampton is covered by a BGS Technical Report and accompanying set of thematic geological maps dealing with land use planning and development (Powell et al., 1992).

Mineral resources

The district is covered by two reports and accompanying 1:10 000 maps in the series Mineral Resource Information for Development Plans: Shropshire, 1998; Staffordshire, 1995.

Further information on mineral resources is available from the BGS Minerals Group, Keyworth.

MINGOL is a GIS-based minerals information system, from which hard copy and digital products tailored to individual client's requirements can be obtained.

Biostratigraphy

There is a collection of unpublished biostratigraphical reports; details are available from the Biostratigraphy Group, Keyworth. Macrofossils and micropalaeontological samples collected from the district are held at BGS, Keyworth.

BGS collections

Boreholes and shafts

BGS archives include records for logs of about 14 000 boreholes, samples and core from the boreholes, plans of underground mines for minerals (other than coal), and gravity and aeromagnetic data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data is available for the north and central parts of the district. Geophysical logs are available for the coal and hydrocarbon exploration boreholes.

Hydrogeology data are held at BGS Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB. Tel 01491 838800. Fax 01491 692345.

A Petrological collection database is maintained by the Mineralogy and Petrology Group at BGS Keyworth.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to current copyright legislation. A fuller account of the geology and bibliography is given in Bridge and Hough (2002).

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to current copyright legislation. A fuller account of the geology and bibliography is given in Bridge and Hough (2002).

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, Macdonald, A M, Wagstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. Allen, D J, Bloomfield, J P, and Robinson, V K (editors). British Geological Survey Technical Report WD/97/34; Environment Agency R&D Publication, No. 8. (Keyworth, Nottingham: British Geological Survey.)

andrew, R, and West, R G. 1977. Pollen analysis from Four Ashes, Worcs. In Early and Middle Devensian flora and vegetation. Philosophical Transactions of the Royal Society of London, Series B, Vol. 280, 242–246.

Appleton, J D, Hooker, P J, and Smith, N J P. 1995. Methane, carbon dioxide and oil seeps from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/1.

Arup Geotechnics. 1990. Review of mininginstability in Great Britain. Case study report, Stafford Brine Pumping (Staffordshire). Vol. 3/vi. Report to the Department of the Environment. (London: H MS O.)

Ashworth, A C. 1969. The Late Quaternary coleopterous faunas from Rodbaston Hall, Staffordshire and Red Moss, Lancashire. Unpublished PhD thesis, University of Birmingham.

Barnsley, G B. 1964. The stratigraphy and structure of the Cannock Chase Coalfield. Unpublished PhD thesis, University of London.

Bassett, M G, Cocks, L R M, Holland , C H, Rickards, R B, and Warren, P T. 1975. The type Wenlock Series. Report of the Institute of Geological Sciences, No. 75/13, 1–19.

Besly, B M. 1988. Palaeogeographic implications of late Westphalian to early Permian red‑beds, Central England. 200–221 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of Northwest Europe. Besly B. M, and Kelling G (editors). (Glasgow and London: Blackie.)

Besly, B M, and Cleal, C J. 1997. Upper Carboniferous stratigraphy of the West Midlands (U K) revised in the light of borehole geophysical logs and detrital compositional suites. Geological Journal, Vol. 32, 85–118.

Besly, B M, and Fielding, C R. 1989. Palaeosols in Westphalian coal-bearing and red-bed sequences, Central and Northern England. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 70, 303–30.

Besly, B M, and Turner, P. 1983. Origin ofred-beds in a moist tropical climate (Etruria Formation). 131–147 in Residual deposits. Wilson, R CL (editor). Special Publication of the Geological Society of London, No. 11.

B R211. 1999. Radon: guidance on protectivemeasures for new buildings. C RC Ltd.

Bridge, D McC, and Hough, E. 2002. Geology of the Wolverhampton and Telford district. Sheet Description of the British Geological Survey, 1:50 000 Series Sheet 153 (England and Wales).

Bridge, D McC, Brown, M J, and Hooker, P J. 1997. Wolverhampton urban environmental survey: an integrated geoscientific case study. British Geological Survey Technical Report, WE/95/49.

Cobbold, E S. 1921. The Cambrian horizons of Comley (Shropshire) and their brachiopoda, pteropoda, gasteropoda, etc. Quarterly Journal of the Geological Society of London, Vol. 76, 325–386.

Cobbold, E S, and Pocock, R W. 1934. The Cambrian area of Rushton (Shropshire). Philosophical Transactions of the Royal Society of London, Series B, Vol. 223, 305–409.

Cope, J C W, Ingham, J K, and Rawson, P F (editors). 1992. Atlas of palaeogeography and lithofacies. Memoir of the Geological Society of London, No. 13.

Creedy, D P. 1991. An introduction to geological aspects of methane occurrence and control in British deep mines. Quarterly Journal of Engineering Geology, Vol. 24, 209–220.

Culshaw, M G. 1973. A stability assessment of the north slope of the Ironbridge Gorge. British Geological Survey Technical Report, WN/73/1.

Dawson, M. 1988. Diamict deposits of pre-Late Devensian glacial age underlying the Severn Main Terrace at Stourport, Worcestershire: their origins and stratigraphic implications. Proceedings of the Geologists' Association, Vol. 99, 125–132.

Ellis, G L, and Slater, I L. 1906. The highest Silurian rocks of the Ludlow district. Quarterly Journal of the Geological Society of London, Vol. 62, 195–222.

Ellison, R A, Arrick, A, Strange, P J, and Hennessey, C. 1997. Earth science information in support of major development initiatives. British Geological Survey Technical Report, WA/97/84.

Environment Agency. 1997. Policy and practice for the protection of groundwater. Groundwater Vulnerability 1:100 000 Map Series Sheet 22, South Staffordshire and East Shropshire. (London: Stationery Office.)

Glover, B W, Holloway, S, and Young, S R. 1993. An evaluation of coalbed methane potential in Britain. British Geological Survey Technical Report, WA/93/24.

Gostelow, T P, Hamblin, R J O, Harris, D I, and Hight, D W. 1991. The influence of late and postglacial slope development on the engineering geology of Wenlock Shale, near Ironbridge, Salop. 349–359 in Quaternary Engineering Geology. Forster, A, Culshaw, M G, Cripps, J C, Little, J A, and Moon, C F (editors). Special Publication of the Geological Society Engineering Geology, No. 7.

Hamblin, R J O. 1986. The Pleistocene sequence of the Telford district. Proceedings of the Geologists' Association, Vol. 97, 365–377.

Hamblin, R J O, and Coppack, B C. 1995. Geology of Telford and the Coalbrookdale Coalfield. Memoir of the British Geological Survey, parts of sheets 152 and 153 (England and Wales).

Hamblin, R J O, Brown, I J, and Ellwood, J. 1989. Mineral resources of the Coalbrookdale Coalfield — basis of the Industrial Revolution. Mercian Geologist, Vol. 12, 9–27.

Henkel, D J, and Skempton, A W. 1954. A landslide at Jackfield, Shropshire, in a heavily over-consolidated clay. Geotechnique, Vol. 5, No. 2, 131–137.

Hoare, R H. 1959. Red beds in the coal measures of the West Midlands. Transactions of the Institute of Mining Engineers, Vol. 119, 185–198.

Hollis, J M, and Reed, A H. 1981. The Pleistocene deposits of the southern Worfe catchment. Proceedings of the Geologists' Association, Vol. 92, 59–74.

Hooker, P J, and Bannon, M P. 1993. Methane: its occurrence and hazards in construction. Construction Industry Research and Information Association (C IR IA), Report No. 130, 137pp.

Hull, E. 1869. The Triassic and Permian rocks of the Midland counties of England. Memoir of the Geological Survey of Great Britain.

Maddy, D, Green, C P, Lewis, S G, and Bowen, D Q. 1995. Pleistocene geology of the Lower Severn Valley, U K. Quaternary Science Reviews, Vol. 14, 209–222.

Mitchell, G H. 1945. The geology of the northern part of the South Staffordshire Coalfield in new series one-inch sheets 140, 153, 154. Geological Survey Wartime Pamphlet, No. 43.

Morgan, A. 1973. Late Pleistocene environ-mental changes indicated by fossil insect faunas of the English Midlands. Boreas, Vol. 2, 173–212.

Morgan, A V. 1973. The Pleistocene geology of the area north and west of Wolverhampton, Staffordshire, England. Philosophical Transactions of the Royal Society, Series B, Vol. 265, 233–297.

Murchison, R I. 1839. The Silurian System, founded on geological researches in the counties of Salop, Hereford, Radnor, Montgomeryshire, Carmarthen, Brecon, Pembroke, Monmouth, Gloucester, Worcester and Stafford: with descriptions of the coalfields and the overlying formations. (London: John Murray.)

Old, R A, Hamblin, R J O, Ambrose, K, and Warrington, G. 1991. Geology of the country around Redditch. Memoir of the British Geological Survey, Sheet 183 (England and Wales).

Ove Arup and Partners. 1987. Limestone mines in the Wrekin area. Lilleshall Report. Phase 2. Report to the Department of the Environment, Shropshire County Council, Wrekin District Council and Bridgnorth District Council.

Pearce, T J, Besly, B M, Wray, D S, and Wright, D K. 1999. Chemostratigraphy: a method to improve interwell correlation in barren sequences — a case study using onshore Duckmantian/Stephanian sequences (West Midlands, U K). Sedimentary Geology, Vol. 124, 197–220.

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).

Powell, J H, Chisholm, J I, Bridge, D McC,Rees, J G, Glover, B W, and Besly, B M. 2000. Stratigraphical framework for Westphalian to Early Permian red-bed successions of the Pennine Basin. British Geological Survey Research Report, RR/00/01.

Powell, J H, Glover, B W, and Waters, C N. 1992. A geological background for planning and development in the 'Black Country'. British Geological Survey Technical Report, WA/92/33.

Prestwich, J. 1840. On the geology of Coalbrook Dale. Transactions of the Geological Society of London, Series 2, Vol. 5, 413–495.

Rushton, A W A, Hamblin, R J O, and Strong, G E. 1988. The Croft Borehole in the Lilleshall Inlier of north Shropshire. Report of the British Geological Survey, Vol. 19, No. 3.

Shotton, F W, and Strachan, I. 1959. The investigation of a peat moor at Rodbaston, Penkridge, Staffordshire. Quarterly Journal of the Geological Society of London. Vol. 115, 1–15.

Skempton, A W. 1964. Long term stability of clay slopes. Geotechnique, Vol. 14, No. 2, 77–102.

Smith, N J P, and Rushton, A W A. 1993. Cambrian and Ordovician stratigraphy related to structure and seismic profiles in the western part of the English Midlands. Geological Magazine, Vol. 130, 665–671.

Tucker, R D, and Pharaoh, T C. 1991. U-Pb zircon ages for late Precambrian igneous rocks in southern Britain. Journal of the Geological Society of London, Vol. 148, 435–443.

Warrington, G. 1970. The stratigraphy and palaeontology of the 'Keuper' Series of the central Midlands of England. Quarterly Journal of the Geological Society of London, Vol. 126, 183–223.

Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, K E, Robinson, P L, Shotton, F W, and Taylor, F M. 1980. A correlation of Triassic rocks in the British Isles. Geological Society of London Special Report, No. 13.

Waters, C N, Glover, B W, and Powell, J H. 1994. Structural synthesis of S Staffordshire U K: implications for the Variscan evolution of the Pennine Basin. Journal of the Geological Society of London, Vol. 151, 697–713.

White, D E, and Lawson, J D. 1989. The Prídolí Series in the Welsh Borderland and south-central Wales. 131–141 in A global standard for the Silurian System. Holland, C H, and Bassett, M G (editors). National Museum of Wales Geological Series, No. 9.

Whitehead, T H, Robertson, T, Pocock, R W, and Dixon, E E L. 1928. The country between Wolverhampton and Oakengates. Memoir of the Geological Survey of Great Britain, Sheet 153 (England and Wales).

Williams, G M, and Aitkenhead, N. 1991. Lessons from Loscoe: the uncontrolled migration of landfill gas. Quarterly Journal of Engineering Geology, Vol. 24, 191–207.

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

Wills, L J. 1938. The Pleistocene development of the Severn from Bridgnorth to the sea. Quarterly Journal of the Geological Society of London, Vol. 94, 161–242.

Wills, L J. 1948. The palaeogeography of the Midlands. (Liverpool: Liverpool University Press.)

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Summary of geology and main structures of the district.

(Figure 2) Classification of Silurian strata.

(Figure 3) Dinantian sequence at Lilleshall.

(Figure 4) Coal Measures: selected sections from the South Staffordshire and Coalbrookdale coalfields.

(Figure 5) Coal Meaures: selected sections from the Coalbrookdale coalfield. For geological key, see (Figure 4).

(Figure 6) Halesowen and Salop formations: selected sections showing lithological variation and gamma-ray signatures.er formational boundary is gradational and is taken at the first appearance of red-brown sandstones having the compositional characteristics of the Salop Formation.

(Figure 7) Structure contours (metres OD) on the base of the Permo-Triassic Bridgnorth Sandstone — Kidderminster Formation.

(Figure 8) Terrace nomenclature of the River Severn.

(Figure 9) Constraints on development.

Plates

(Plate 1a) Channel bedforms in the Enville Member of the Salop Formation, M54 motorway cutting [SJ 738 090] (GS955).

(Plate 1b) Imbricate conglomerate in channel base (GS956).

(Plate 2a) Aeolian dune bedding in the Bridgnorth Sandstone, Brimstone Hill [SJ 7512 0582] (GS953).

(Plate 2b) Conglomerate in the Kidderminster Formation, Saredon Hill Quarry [SJ 945 080] (GS952).

(Plate 3) Etruria–Halesowen formation junction, Caughley Brickpit [SO 697 998] (GS949).

(Front cover) Cover Photograph The world's first cast-iron bridge was built over the River Severn in 1779; it was one of the wonders of its day and still inspires visitors and artists. The bridge spans a gorge cut by meltwater during the Devensian glaciation. The steep sides of the gorge are potentially unstable, and slow downhill movement has affected buildings and the bridge itself. Conservation work on the bridge began as early as 1784, and was brought up to date in 1999 with work sponsored by English Heritage [SJ 6724 0340] (MN39748) (Photograph Caroline Adkin).

(Rear cover)

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

Figures

(Figure 8) Terrace nomenclature of the River Severn

(Figure 9) Constraints on development