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Geology of the Wolverhampton and Telford district. Sheet description of the British Geological Survey 1:50 000 Series Sheet 153 (England and Wales)

By D McC Bridge and E Hough

Bibliographical reference: 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).

Authors: D McC Bridge and E Hough. Contributors: A Barnett, J N Carney, C S Cheney, B C Coppack, A Forster, R J O Hamblin, R M W Musson, N J Riley, C P Royles, N J P Smith and G Warrington.

Keyworth, Nottingham: British Geological Survey 2002 © NERC 2002. All rights reserved ISBN 085272 3997

Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: GD 272191/2002. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

(Front cover) Retaining wall, Queensway, near Telford [SJ 700 105]. Geological section created in tiles from Lancashire (MN39536).

(Back cover)

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Acknowledgements

This Sheet Description was compiled and largely written 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); G Warrington contributed on Triassic biostratigraphy. N J Smith contributed to the chapter on Structure and Concealed Geology. The chapter 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 manuscript was edited by R D Lake.

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.

Notes

Geology of the Wolverhampton and Telford district—summary

This Sheet Description 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 modified the landscape, and the superficial deposits of till and glacial outwash that blanket much 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.

The district has a long industrial heritage dependent, until recently, on the mineral wealth of the Carboniferous rocks exposed in the South Staffordshire and Coalbrookdale coalfields. During the 18th and 19th centuries, settlements grew and prospered as industries were established, founded on a plentiful supply of coal, clay, ironstone and limestone. The relicts of these industries have left parts of the district with a legacy of difficult ground conditions, aspects of which are described in the applied section of this report. Today, mineral extraction is of less importance but brickclay, fireclay and some opencast coal are still produced, and the Triassic conglomerates are an important source of aggregate. The Permo-Triassic rocks of the Stafford Basin hold important groundwater resources from which large volumes of water are abstracted for public supply.

(Table 1) Geological succession of the Wolverhampton and Telford district.

Chapter 1 Introduction

This Sheet Description provides a summary of the geology of the district covered by the 1:50 000 Geological Series Sheet 153 (Wolverhampton), published as a Solid and Drift edition in 2001. A simplified map of the bedrock geology is shown in (Figure 1), and the geological succession is summarised in (Table 1). A brief summary of the geology is also provided by the Sheet Explanation that accompanies the published map. More detailed information can be found in the Technical Reports for the constituent 1:10 000 scale geological maps.

The district lies within the West Midlands Region and includes parts of the shire counties of Shropshire and Staffordshire (Figure 2). The Metropolitan Borough of Wolverhampton is the main centre of population in the south-east, while Telford, designated a New Town in 1968, is the major commercial and administrative centre in the west. The intervening rural tract is mainly Green Belt, but includes a number of small towns, including Penkridge, Breward and Codsall.

The relief is gently undulating, reaching a maximum elevation of 193 m above OD in the 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 River Worfe and Smestow Brook). In the south-west, the Ironbridge Gorge, famous as the birthplace of the Industrial Revolution, takes the deeply incised River Severn through a former (preglacial) watershed divide.

The geological sequence is dominated by the Permo-Triassic rocks of the Stafford Basin. These give rise to a subdued topography, punctuated by low ridges formed by harder sandstone and conglomerate beds. 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. In the north, 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 economy of the district has long been dominated by traditional heavy industries based on the ready availability of raw materials. Coal, ironstone and brickclay, produced from the exposed coalfields, provided the stimulus for the industrial revolution in the 18th and 19th centuries. The decline of this industrial base and the move to a more diverse economy (consumer electronics, precision engineering, food processing) has left a legacy of derelict land, often with difficult ground conditions. Mineral workings (aggregate, coal, brickclay, fireclay) continue to be important to the local economy, and groundwater resources within the Permo-Triassic aquifers are extensively exploited as a source of potable water.

Outline of geological history

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 surface and were erupted explosively during oceanic plate subduction on the northern margins of the southern hemisphere continent of Gondwana (Thorpe et al., 1984; Pharaoh et al., 1987a, b; Gibbons and Horák, 1996). The geochemical evidence is consistent with eruption in a fault-bounded, ensialic marginal basin. Radiometric studies indicate that magmatic activity lasted until at least 560 Ma, after which oblique subduction brought about dismemberment of the arc system and accretion of the constituent elements on to the Gondwanan continental margin. The resulting crustal block, known as Eastern Avalonia, became detached from Gondwana in the early Ordovician, and migrated northwards, eventually colliding with the continental masses of Laurentia and Baltica. Avalonian rocks now form the basement to central and southern England. Western representatives of this crustal block, exposed at Lilleshall and elsewhere along the Church Stretton Fault, form part of the Wrekin Terrane (Pharaoh et al., 1987b).

Early Cambrian rocks preserve a record of marine transgression involving progressive submergence of the Avalonian microcontinent. It is most likely that this transgression commenced in latest Precambrian times, following a phase of rifting that established the Midland Platform as a periodically emergent high, flanked to the west by the more rapidly subsiding Welsh Basin. The Iapetus Ocean lay to the north. Fragmentary evidence of this marine transgression is found in the Lilleshall Inlier, where the Early Cambrian (Comley Series) is represented by shallow-water glauconitic sandstones. A break in deposition occurred during the Mid Cambrian (St David’s Series) but sedimentation resumed during the Late Cambrian (Merioneth Series) with deeper water mudstones. During the Ordovician (about 490 to 435 Ma), the Avalonian microcontinent separated from Gondwana and drifted northwards from subpolar to temperate latitudes. Collision with Baltica (Scandinavia) in late Ordovician times resulted in uplift and inversion, causing the sea to retreat to the west.

Sedimentation recommenced in the late Llandovery (about 427 Ma), probably as a result of a glacioeustatic global sea-level rise. As the Silurian sea spread eastwards across the Midland Platform, cycles of intercalated carbonates and siliciclastic sediments were deposited in a series of transgressive-regressive cycles. Final closure of Iapetus Ocean in the late Silurian resulted in a change from marine through brackish to continental conditions, culminating in Prídolí times in the deposition of red beds. Caledonian (Acadian) earth movements produced gentle folding, uplift and prolonged erosion.

The period from the late Devonian through to the end of the Dinantian (360 to 320 Ma) was a time of major crustal extension when Britain occupied an equatorial position, separated from oceans to the south by the Variscan Mountains. As warm, tropical seas began to transgress northwards, the emergence of a persistent, low-lying hinterland stretching from Wales through the Midlands (the Wales-Brabant High) strongly influenced the depositional pattern. 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 — began to evolve 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 there are other unconfirmed borehole provings at Lilleshall, and at sites in the Coalbrookdale and South Staffordshire coalfields.

Over a period of four million years, (from about 315 Ma), Coal Measures of Langsettian, Duckmantian and early Bolsovian age (Westphalian A to C) were laid down, predominantly in delta plain, lacustrine and swamp environments. Persistent cycles of deposition reflect changing subsidence rates and worldwide fluctuations in eustatic 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 on the delta top over long periods, resulting, after burial and compaction, in the closely spaced or amalgamated coal seams, so typical of the Coalbrookdale and Staffordshire Coalfields.

During the latest Duckmantian to Bolsovian stages (Westphalian B-C), 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 regression of the coal swamps, and deposition of alluvial red beds of the Etruria Formation. A subsequent phase of igneous activity resulted in emplacement 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 (Westphalian D to Stephanian) 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 basement controlled north and north-east-trending faults, 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 15 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. With time, the river system evolved into a more mature meandering system, and by mid-Triassic times, when the Mercia Mudstone was deposited, a more subdued landscape of broad floodplains and playa lakes had evolved, linked on occasion to the sea. The absence of all younger solid formations from the district is due mainly to uplift and erosion during the Tertiary caused by Alpine collisions to the south.

The widespread glacigenic deposits of the district are attributed to a Late Devensian ice sheet that advanced into the district sometime after 30 500 years BP. Organic deposits found beneath Devensian till at Four Ashes contain a biota indicative of interglacial and interstadial conditions, dating back to the 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 the present district. Postglacial erosion has extensively modified the landscape and some of the debris is incorporated in the younger Quaternary river terrace and alluvial deposits.

History of research

The Wolverhampton Sheet was originally surveyed at a scale of six-inches to one mile (1:10 560) in 1915–1923. The accompanying memoir (Whitehead et al., 1928) provides local details but is now out of print. Telford and the surrounding area was remapped in 1970–72, and the Telford 1:25 000 Special Sheet was published in 1977. This work was undertaken to provide an up-to-date geological basis for the development of the New Town. The accompanying memoir (Hamblin and Coppack, 1995) deals mainly with descriptions of the Coal Measures, but remains the primary source of detailed information for the western part of the district. The resurvey of the remaining part of the district was completed in 1993–98.

The resurvey has made use of recent red-bed research (Besly and Cleal, 1995, 1997; Powell et al., 2000a) to define unambiguously subdivisions within the barren measures of Upper Carboniferous and Early Permian age. Insights into the structural evolution of the district are given by Waters et al. (1994) and a synthesis of the deep structure of the district forms part of a wider study of the West Midlands and Cheshire region (Plant et al., 1999). Groundwater management remains an important area of research, and the Permo-Triassic aquifer is the focus of a major study currently being undertaken by the Environment Agency (Entec, 1998). The Quaternary deposits have received scant attention since the investigations at Four Ashes more than 30 years ago (A V Morgan, 1973). More recent works of note include Hollis and Reed (1981) on the Pleistocene deposits of the Worfe catchment, and Hamblin (1986) on the glacigenic deposits of the Telford area.

Subdivision into tectonic areas

The main structural elements of the district are shown in (Figure 3). In the north-west, the Market Drayton Horst is an uplifted block, underpinned at shallow depth by Uriconian rocks, and bounded to the south-east by the Boundary Fault of the Coalbrookdale Coalfield. The Permo-Triassic Stafford Basin underlies the central part of the district. Structurally, it comprises a half-graben, controlled along much of its eastern margin by the Breward Fault. Towards the south of the basin, the importance of the Breward Fault declines, and control passes on to the Patshull-Pattingham Fault System; the latter is associated with a basement high (Codsall High). The Stafford Basin deepens to the west of this structure into the Worfield Sub-basin, and to the east into the Bratch Trough. The latter connects southwards to the Worcester Basin. The South Staffordshire Horst in the east includes part of the South Staffordshire Coalfield, where strata of Palaeozoic age crop out or lie at shallow depth. A small downthrown block at the western margin of this horst is termed the Heath Farm Block.

Chapter 2 Precambrian

Uriconian Volcanic Group [UV]

Precambrian rocks crop out in the Lilleshall Inlier, where they form the prominent ridge of Lilleshall Hill. The predominantly acidic tuffites exposed here are at least 120 m thick and were described first by Calloway (1879) and later, in more detail, by Whitehead et al. (1928), who suggested they consisted mainly of ‘albite rhyolites’ along with tuffs, breccias and possibly lava flows. The most recent review (Carney et al., 2000) has furnished additional details, summarised below.

The principal accessible exposures of Uriconian rocks are the crags along the eastern margin of the hill, extending northwards to just below the Memorial. The lower crags [SJ 7291 1554] 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 tuff shows bedding at 1 to 2 m thick scale, about 4 m above the base of the crags. These higher beds include a 2.2 m bed of white-weathering tuff showing sinuous millimetre-scale lamination. This may be the lithology described as a vitric tuff by Whitehead et al. (1928). The sequence is near-vertical to steeply east-dipping, and locally slightly overturned, with a highly pervasive cleavage, visible at millimetre to submillimetre scale.

Across the summit of the Lilleshall Hill [SJ 7286 1554] and in exposures and crags along the western parts of outcrop, the sequence is mainly in unbedded grey to pink tuffs, with a ‘silty’ surface texture, and with sporadic lithic lapilli tuff layers up to several centimetres thick. Apart from the latter, these tuffs appear uniformly fine grained, but polished rock slabs show they contain abundant pale grey or cream lapilli between 2 and 8 mm in size. In thin section most of the shadowy fragments are seen to consist of devitrified scoria, interspersed with small quartz and K-feldspar crystals, classifying the rock as a pumice lapilli tuff. The tuffs dip westwards, hereabouts, and no cleavage structure is visible.

Basic rocks are exposed in a former quarry near the south-western extremity of the hill [SJ 7275 1540]. In the south of the quarry, they occur as dark green chloritic schists overlying mylonitised siliceous tuffs within a major shear zone dipping 40° to the north. Farther north, the quarry exposes intrusive sheets of dark grey, fine to medium-grained, quartz microgabbro, which is possibly the unsheared equivalent of the chloritic schists.

Interpretation

The Uriconian sequence at Lilleshall Hill includes a range of pyroclastic rocks indicative of a highly explosive style of volcanism. The presence of K-feldspar crystals in association with quartz and absence of plagioclase, suggests that the magmas were probably potassic and of rhyolitic composition. Based on tenuous way-up evidence, the succession may be interpreted as part of a density graded sequence characterised by a concentration of lithic material towards the base. Although an origin by processes of pyroclastic flow is possible, there is no evidence for significant welding and the few horizons with sedimentary structures indicate deposition on a wet substrate.

Concealed Precambrian Basement

Borings made by the Shropshire Coal Company from about 1860 onwards encountered what appeared to be Uriconian rocks at depths of between 50 and 100 m in the Hadley area [SJ 673 130] to the north of the Boundary Fault (Hamblin and Coppack, 1995, p.7). Three BGS boreholes sunk in the Leegomery–Kinley–Preston [SJ 671 148] also proved Uriconian banded tuffs and rhyolites beneath Upper Carboniferous beds. In the east of the district, the Heath Farm Borehole penetrated about 180 m of mafic and felsic tuffs of Uriconian affinity.

Chapter 3 Cambrian

Rocks of Cambrian age crop out in the faulted core of the Lilleshall Inlier. The sequence is poorly exposed, but the Croft Borehole provided a partial section through the Comley and Merioneth series (Lower and Upper Cambrian), 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 sequence is attributed (Smith and Rushton, 1993) to syndepositional movements on the Church Stretton Fault System, causing development of a tilt-block high in the Lilleshall area in mid-Cambrian times. Down-dip from Lilleshall, a more complete sequence interpreted as a Cambro-Ordovician basin-fill, is evident on seismic profiles (Chapter 8).

Comley (Lower Cambrian)

The informally named Lower Comley Sandstone [LCmS] comprises a sequence of shallow-water, current-washed, glauconitic sandstones. At Lilleshall, the formation crops out in faulted contact with Uriconian rocks. Sporadic exposures along the roadside to the south-west of the village show sandstone beds dipping at 23 to 50° to the south-east. The uppermost part (12 m) of the formation was also proved in the Croft Borehole, drilled just outside the village (Rushton et al., 1988). The cored sequence 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, and bedding styles vary from finely laminated, or cross-bedded, to unbedded. 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 sandstones beneath this level could be older but the fossils recovered are nondiagnostic.

In the Comley type area near Caer Caradoc Hill [SO 4845 9647], the Lower Comley Sandstone is overlain by the Lower Comley Limestones, a thin (2 m), highly condensed sequence of phosphatic sandy limestones, divisable into five members, each lithologically and faunally distinct and separated by disconformities. In the Croft Borehole, this sequence is represented by a single 0.25 m bed of pale grey to black, recrystallised, phosphatic algal limestone. This has yielded shells including Lapworthella, together with sponge spicules, inarticulate brachiopod fragments and Hyolithellus? fragments. Rushton et al. (1988) suggested a correlation with the Lapworthella Limestone Member of the type area.

Merioneth (Upper Cambrian)

The informally named Dolgelly Beds [DB] are not exposed in the Lilleshall Inlier, but were proved by drilling. A reference section was provided by the Croft Borehole. This proved 53.75 m of black, micaceous shales resting unconformably on Comley Series strata. The sequence is distinctive for its repeated occurrence of parallel laminated or ripple cross-laminated siltstone interbeds. The source of this silt-grade material is not known, but Rushton et al. (1988) suggest derivation from an uplifted block within the Church Stretton Fault complex. 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). However, as neither of these formations show the characteristic siltstone interbeds noted above, they are not directly comparable, and the informal designation, Dolgelly Beds, adopted for the Telford area (Hamblin and Coppack, 1995), is retained here.

Chapter 4 Silurian

Silurian rocks crop out beneath Coal Measures in the Lincoln Hill area [SJ 670 040] of Coalbrookdale, and more extensively in the south-west around Willey. A conformable sequence ranging in age from Wenlock to Prídolí is present at outcrop; older (Llandovery) rocks, equivalent to the Purple Shales and Pentamerus Sandstone formations of the adjoining Shrewsbury district, are presumed to be present in the subsurface.

Classification

The classification of the Silurian rocks is shown in (Table 2). The subdivisions of the Llandovery Series are those used by Pocock et al. (1938) and Hamblin and Coppack (1995). 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 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 transitional beds within the Much Wenlock Limestone, thereby ensuring the integrity of this formation along its entire outcrop. 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. The approximate correspondence between the two schemes is shown in (Table 2). In the Přídolí Series, the name Raglan Mudstone Formation is used to give continuity with areas recently mapped to the south-west.

Depositional setting

The Llandovery, Wenlock and Ludlow rocks are mostly of shallow marine origin, comprising alternations of siliciclastic shelf deposits and shallow-water carbonates with variable amounts of terrigenous material. In mid-Llandovery times, an eastward marine transgression on to the western flanks of the Midland Platform deposited grits and finer grained sediments of the Pentamerus Sandstone and Purple Shales formations. These early Silurian deposits, of Aeronian and Telychian age, are not present at outcrop, but on the evidence of the Heath Farm Borehole are presumed to underlie much of the district. Continued deepening of the sea is indicated by the succeeding muddy and somewhat calcareous sediments of the Buildwas and Coalbrookdale formations. They are sparsely graptolitic but contain a rich shelly macrofauna. Interbedded thin bentonites are testimony to distant volcanic eruptions. Evidence of regression during the late Homerian is provided by the Much Wenlock Limestone Formation. The small reef bodies for which this unit is renowned grew in waters probably no more than 30 m deep (Scoffin, 1971). 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. Deepening of the sea is generally postulated at the beginning of Ludlow times, followed by a gradual shallowing as the shelly muds of the Lower Ludlow Shales were replaced by shelf carbonates of the Aymestry Limestone. The succeeding Upper Ludlow Shales indicate a return to siliciclastic shoaling conditions with a low diversity brachiopod and bivalve fauna, and evidence of storm sedimentation. At the base of the Přídolí Series lies the Ludlow Bone Bed. This denticle-rich unit occupies hollows in the uppermost surface of the Upper Ludlow Shales, and is the lowest member of the Downton Castle Sandstone Formation. It is generally regarded as a condensed, low stand, regressive lag deposit, which accumulated in a strandline or wave-winnowed offshore location before being reworked during a later transgressive event (Allen and Tarlo, 1963; Ainsworth et al., 1993). It is overlain by sandstones, siltstones and mudstones of offshore or intertidal facies, followed by sandstones of littoral marine or high-energy shelf facies (Allen, 1985; Ainsworth, 1991). The overlying Temeside Shales show sedimentological features typical of intertidal and subtidal environments subject to periods of prolonged exposure (Allen and Taro, 1963; White and Lawson, 1989). The red beds of the Raglan Mudstone Formation represent deposition on an alluvial coastal plain subject to marine tidal influence (White and Lawson, 1989).

Wenlock

The ‘Wenlock Shale’ of Murchison (1834) comprises two conformable formations, the Buildwas Formation and Coalbrookdale Formation (Table 2). The cover of glacial drift makes it difficult to map precisely the outcrop of these two subdivisions. However on the evidence of published maps (for example Bassett, 1989) and outcrop geometry, it is unlikely that the lower division, the Buildwas Formation, comes to crop. In the subcrop, an intervening limestone, correlated with the Barr Limestone Formation of the Dudley–Walsall area, was proved in the Heath Farm Borehole.

The Coalbrookdale Formation [Cbrd] consists of a monotonous sequence, about 160 m thick, of olive-grey to dark blue-grey silty mudstones. Thin calcareous siltstones and nodular limestone horizons occur sporadically, and there are numerous thin partings of bentonitic clay. The top of the formation is transitional with the overlying Much Wenlock Limestone, and is defined at the incoming of bands of coalesced limestone nodules which appear some 15 to 25 m below the main limestone development. Within the type area, the formation has a stratigraphical range from mid-Sheinwoodian to late Homerian.

Key localities

The exposures originally described in Coalbrookdale railway cutting are now much overgrown, but beds in the upper part of the sequence crop out in Loamhole Dingle, just to the west of the district boundary. The largest exposure [SJ 6641 0555] reveals 15 m of blue-grey mudstone with beds of bentonitic clay up to 5 cm thick.

The succeeding Much Wenlock Limestone Formation [WeL] crops out at Lincoln Hill [SJ 670 040], but within a few hundred metres passes beneath the unconformable cover of Coal Measures. In the Wenlock type area, the formation is the uppermost lithostratigraphical division of the Wenlock Series (Bassett, 1974) and belongs to the ludensis biozone of the Homerian Stage. The formation is renowned for its diverse and generally well-preserved fauna. Palaeontological studies have focused mainly on brachiopods (Hurst, 1975), corals (Abbott, 1976; Powell, 1980) and stromatoporoids (Powell, 1980).

The basal part of the Much Wenlock Limestone comprises 15 to 25 m of grey shaly mudstones, with bands of discrete limestone nodules, and coalesced nodular beds. This basal unit corresponds to the Tickwood Beds of Pocock et al. (1938) and the Farley Member of the Coalbrookdale Formation (Bassett et al., 1975). The outcrop forms part of the scarp slope of the Much Wenlock Limestone feature. Pocock et al. (1938, p.11) found the fauna of these beds generally rather sparse. However, in contrast to the Coalbrookdale Formation, large benthic brachiopods are present suggesting a shallower water environment than that which prevailed previously. Locally, Gothograptus nassa is common, the basal part of the G. nassa Zone evidently straddling the boundary with the Coalbrookdale Formation. The upper limit of the zone is uncertain, but there is evidence that to the south of the district a horizon 0.9 m below the top of the ‘Tickwood Beds’ belongs to the overlying M. ludensis Zone (Bassett et al., 1975, p.8).

The main carbonate build-up comprises some 20 m of flaggy, shelly, crystalline limestone, interbedded with impersistent siltstones towards the base and top. The name Benthall Beds [Beb] was introduced (Hamblin et al., 1995) to distinguish this lithofacies from the reef-bearing nodular limestones and siltstones, which are mapped as a separate lithofacies in the adjoining district. At Lincoln Hill [SJ 6720 0415] the Benthall Beds were extensively worked underground and in deep quarries (Pocock et al., 1938, p.116). A middle unit, 12 to 15 m thick, of relatively pure, thickly bedded limestone separates upper and lower sequences of flaggy and concretionary beds. In the roof of the mined middle beds, a waxy, pale grey, bentonitic clay up to 0.1 m thick is present.

The highest beds of Wenlock age consist of 4 to 5 m of flaggy to thinly bedded crinoidal limestone. These overlie the Benthall Beds just to the west of the district.

Ludlow

Rocks of Ludlow age crop out in the south-west of the district between the Dean Corner and Willey faults [SJ 675 000], and in separate inliers in the valleys of Dean Brook [SO 695 990] and Linley Brook [SO 685 980]. Generally, the rocks have a gentle south-eastward dip, but low-amplitude folding along a north-easterly trend gives rise to local variations. The Ludlow Series is represented by three divisions: the Lower Ludlow Shales, the Aymestry Limestone and the Upper Ludlow Shales. The correlation of these mappable divisions with the standard Ludlow stratotype (Table 2) is based largely on information from the Dean Borehole, drilled 1.5 km south of Broseley.

Apart from an outcrop in the floor of the Severn Gorge at Ironbridge, the main proving of Lower Ludlow Shales [LLu] is in the Dean Borehole, which penetrated the top 44.27 m of the formation. The log records medium grey, argillaceous, calcareous, flaggy siltstones with shelly limestone beds and nodules, which gradually become less calcareous and more silty down-sequence. The top 25 m contain a prolific shelly fauna, dominated by large strophomenids, and are correlated with the Lower Bringewood Formation; the remainder of the sequence is probably equivalent to the Upper Elton Formation; brachiopods are less numerous than in the higher beds, while bivalves are well represented. A full faunal list is given by Hamblin and Coppack (1995, p.18).

The Aymestry Limestone Formation [AL] crops out in an inlier in the valley of Dean Brook, and along the axis of a north-east-trending anticline between Willey and The Dean, where it forms a prominent ridge. The lower and middle parts of the formation consist of calcareous siltstones and nodular limestones; the upper part comprises massive to flaggy crystalline limestones. Apart from the Dean Borehole, which proved Aymestry Limestone from a depth of 17.5 to 55.5 m, the most complete section was found in a small tributary that enters Dean Brook from the north [SO 699 992] (Whitehead et al., 1928). This exposed some 24 to 30 m of fossiliferous, calcareous mudstone and nodular limestone, commencing at an estimated 6 m above the base of the formation. The section is overgrown and degraded and has not been re-examined since the original survey. Whitehead et al. (1928) provide a comprehensive faunal listing.

Other sections are noted by Hamblin and Coppack (1995).

Key localities

The Upper Ludlow Shales [ULu] consist of about 50 m of olive-grey calcareous siltstone and mudstone. Limestone nodules and layers are common in the lower part, particularly towards the base in the transitional zone with the Aymestry Limestone. The basal 8 m have a fauna comparable to that occuring at the top of the Aymestry Limestone. The higher beds contain an abundant but restricted shallow-water fauna, in which brachiopods are well represented. The sequence of faunas can be matched with those of the Upper Leintwardine, Lower Whitcliffe and Upper Whitcliffe formations of the type area (Lawson and White, 1989). The Dean Borehole proved Upper Ludlow Shales to a depth of 17.52 m. The beds below 11.38 m are correlated with the Upper Leintwardine Formation, and those above this depth are considered to equate with the Lower Whitcliffe Formation.

Key localities

The principal exposures are in Dean Brook.

Přídolí

The Přídolí Series comprises the rocks of Old Red Sandstone facies, traditionally referred to as the Downtonian. Its base is placed at the base of the Downton Castle Sandstone Formation, the junction marked locally by the Ludlow Bone Bed Member. The classification of the Prídolí Series is shown in (Table 2). The principal outcrops are in Linley Brook, along Dean Brook, and on the flanks of the Willey-Dean Anticline.

The base of the Downton Castle Sandstone Formation [DCS] is exposed in Linley Brook [SO 6893 9816] (Bradfield and Tucker, 1986) and was formerly seen in an excavation in the base of an old quarry at Willey Park Hall [SO 6731 9912] (White and Coppack, 1977). A composite section is given in (Figure 4). In both sections, the basal member of the formation, the Ludlow Bone Bed, is marked by a thin siltstone (0.08–0.20 m), containing fish remains (mainly thelodont denticles) and small black horny brachiopod and annelid fragments. Scattered burrows extend up to 50 mm into the underlying Upper Ludlow Shales, those at Willey commencing immediately beneath the lowest denticle layer. The basal member is in sharp contact with the underlying Ludlow Series, but there is no evidence to suggest a major break in sedimentation.

Overlying the Ludlow Bone Bed are the basal siltstones of the Downton Castle Sandstone, 0.97 to 2.5 m thick, commonly olive-grey, finely cross-laminated, and containing a typical Downtonian fauna of poorly preserved bivalves, ostracods, and brachiopods. Fish scales and spines are present in small numbers locally at the top of this unit. The siltstones are succeeded by thinly interbedded siltstones and sandstones, and a higher unit entirely of sandstone, in all totalling some 6.5 m. The sandstones are mostly yellow, fine grained, micaceous, ripple-laminated or cross-bedded, with scattered fossils including mainly lingulid brachiopods, mollusc and plant remains. A lenticular bone bed, up to 0.07 m thick, occurs about 3 m above the base of the formation at both localities. This horizon may be the lateral equivalent of the Downton Bone Bed of Downton Castle (Ellis and Slater, 1906, p.209).

The Temeside Shales Formation [TSh] is conformable on the Downton Castle Sandstone Formation, and is represented by a sequence, about 27 m thick, of olive-grey mudstones and siltstones with subsidiary, richly micaceous sandstones. A gully on the right-bank of Linley Brook yielded lingulid brachiopods, vertebrate detritus and plant remains (Whitehead et al., 1928, p.25–26).

Key localities

The Raglan Mudstone Formation [Rg] succeeds the Temeside Shales, covering much of the higher ground between Willey and Linley. The rocks are chiefly red mudstones in which there are few good sections. Several persistent sandstone beds form hill-cappings. On the evidence of soil brash here, the dominant lithology is greenish grey and black spotted, flaggy, micaceous sandstone. Outcrops of sandstone in the adjoining Shrewsbury district (Sheet 152) have yielded Lingula, ostracods and fish fragments (Whitehead et al., 1928, p.27–28).

Chapter 5 Carboniferous and Lower Permian

Dinantian

Strata of Dinantian age 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, until recently, the only reliable sources of information. However, during the last decade, remedial work to counter mining subsidence in Lilleshall village has involved extensive drilling and provided an opportunity to re-examine parts of the sequence in more detail. The results reported here are based on logging and interpretations of selected cored boreholes, summarised by Riley (1998). 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 5).

The earliest deposits are terrigenous sandstones and conglomerates, which are inferred to be of Late Devonian-Courceyan age. The succeeding carbonates (Jackie Parr Limestone) represent a condensed sequence deposited when Tournaisian seas transgressed on to the margins of the Wales-Brabant landmass. The absence of deposits attributable to the Chadian and Arundian stages signifies a period of intra-Dinantian uplift. Sedimentation resumed in the late Holkerian or Asbian, first with fluvial and alluvial-plain siliciclastics (Lydebrook Sandstone), and then with a further carbonate build-up (Sylvan Limestone).

The Village Farm Formation [Vlg], equivalent to ‘Group 1’ of Whitehead et al. (1928), comprises some 23 m of beds of predominantly continental facies. Two site investigation boreholes (Lilleshall L6, L17) penetrated the upper part of the unit; a third (Lilleshall L8) proved a full sequence, terminating in the underlying Lower Comley Sandstone.

Much of the formation consists of polymictic conglomerates, breccias and sandstones, forming upward fining units, 1 to 2 m thick. The breccias and conglomerates contain angular clasts up to 100 mm in diameter of sandstone, quartzite, granophyre and banded rhyolite as well as crinoidal chert and dolomite, indicating erosion of local Precambrian and Palaeozoic source rocks. The sandstones range in colour from red, through pale brown to green, with coarser varieties showing poor sorting and composition approaching greywackes. Beds in the uppermost 3 m are transitional with the overlying formation; they include calcareous sandstones with milletseed grains, and thin supratidal pedogenic carbonates. Intertidal influence is suggested by possible Diplocraterion burrows. In several boreholes, the top of the formation is defined by a fissured surface, which has been interpreted as an unconformity (Ove Arup and Partners, 1987). Whether this represents a significant hiatus is uncertain. If there is an upwards transition into the overlying Jackie Parr Limestone, as appears from the log of the Lilleshall L6 Borehole, then it is likely that the upper part of the formation is also early Tournaisian. However, a Late Devonian age cannot be discounted, particularly for the lower and middle clastic intervals.

The Jackie Parr Limestone Formation [JKP] is the lower of two marine-dominated intervals. The formation has a maximum thickness of 20 m, which reduces progressively from north to south due to differential overstep by the overlying Lydebrook Sandstone. The sequence is divisible into five lithofacies, L1–5, (Figure 5) which can be correlated throughout the Lilleshall area with the aid of downhole geophysical logs. Detrital carbonates occur at two levels; the lower unit, known colloquially as the ‘Red Limestone’ (L2), was formerly mined at Limekiln Lane [SJ 7330 1590] and Willmoor Lane [SJ 7351 1579]. It comprises 6 m of grey and pink, nodular limestone (packstone/grainstone). The upper unit (L4) is a coarse, dolomitised grainstone with abundant rolled and worn crinoid, brachiopod and bryozoan hash. This latter unit is probably the ‘Grey Limestone’ of Murchison. Units L1, L3 and L5 are predominantly clastic sediments but there are also localised evaporites at the base of the formation.

The association of lithofacies is indicative of a spectrum of sub-environments ranging from turbulent shallow marine to high intertidal/supratidal conditions. Evidence for the early Tournaisian age of this formation was reviewed by Mitchell and Reynolds (1981). Reynold’s conodont data suggest an assemblage in the Siphonodella Conodont Biozone. According to Mitchell, the macrofauna lies in the Vaughnia vetus Biozone.

The overlying Lydebrook Sandstone Formation [LyS] marks a return to clastic sedimentation. Borehole provings recorded a fairly uniform sequence, 20 to 23 m thick, of fine to medium-grained sandstone and siltstone, typically grey, purple or pink in colour, with some mudstone interbeds. The formation shows a decrease in thickness to 9 m towards the Boundary Fault, indicating possible synsedimentary movement on this structure during deposition. A maximum thickness of 37 m was recorded locally in the Pitchcroft Mine [SJ 7392 1720], just to the north of the district.

The base of the formation is severely erosional with overstep on to the underlying strata (see above). Channel-fill facies predominate, particularly where downcutting is greatest. Individual depositional units are organised into 1 to 3 m-thick, upward-fining packages. These commence with a thin basal pebble lag of quartz pebbles and mudstone rip-up clasts. Sandstone is the dominant lithology and this contains a high proportion of well-rounded ‘millet-seed’ grains (Whitehead et al., 1928). Internal structures are dominated by trough and planar cross-stratification, with ripple lamination present in the siltier upper parts of some channels. Plant debris is also preserved in these finely, laminated beds. Blocky siltstones and more rarely mudstones cap some units or form thicker aggradational sequences. These show evidence of pedogenesis, with rootlets, colour mottling and perturbation structures. The sequence is interpreted as a series of vertically and laterally stacked fluvial channels, interspersed with alluvial plain palaeosols.

On the basis of casts of bivalves and brachiopods recovered from an outcrop of the Lydebrook Sandstone in Wenlock Wood (3km west of the district boundary), Bracewell (1925) concluded that the Lydebrook Sandstone could be assigned an Asbian age: Dibunophyllum Zone (D1 subzone). More recent palynological studies from exposures in the Ironbridge Gorge area (Turner et al., 1995) yielded spores indicative of a late Dinantian (Asbian-Brigantian age).

Key locality

In the Lilleshall Inlier, the Lydebrook Sandstone is overlain by the Sylvan Limestone Formation [Syl], a 20 m-thick sequence of nodular or rubbly limestone, which is only seen completely in one borehole (Lilleshall L18). The interval includes two thin basalt horizons, which are considered to be attenuated leaves of the Little Wenlock Basalt (see below).

In cored section, the limestones are nearly all strongly calcretised, and with multiple exposure surfaces. No primary sedimentary structures are preserved, and much of the original fabric has been replaced by fine-grained neomorphic calcite and/or dolomite. Crinoids, corals, brachiopods (Gigantoproductus) and other shell debris are scattered throughout the sequence. Whitehead et al. (1928) listed the fauna. Thin clays with calcrete nodules and fine-grained sandstones form a minor part of the sequence.

The age of the Sylvan Limestone is poorly constrained but spans the Asbian and Brigantian stages: beds below the lower basalt at Lilleshall definitely include strata of late Asbian age on account of the presence of the brachiopod Davidsoninia septosa (Lilleshall L18 Borehole). Evidence for a Brigantian age is based entirely on historical records of Lonsdaleia floriformis (Murchison, 1839, p.107–9). The Brigantian/Asbian boundary is drawn below the Little Wenlock Basalt in its type area, and is taken here, for present purposes, at the base of the lower basalt leaf at Lilleshall.

Deposition of marine carbonates was interrupted in Asbian times by a volcanic episode that led to the extrusion of basaltic lava, Little Wenlock Basalt [LWB], across parts of the carbonate platform. In the adjoining Shrewsbury district, the basalt occupies a stratigraphical position a few metres above the Lydebrook Sandstone, a position maintained throughout much of its outcrop. In this district, the basalt has a restricted outcrop between Horsehay and Doseley. In a review of the main exposures, Pocock et al. (1938) noted that the basalt was extrusive, comprising highly vesicular and slaggy lavas, and with a bole below the apparently unaltered overlying sediment. Pillow-structures noted in some exposures indicate a local shallow submarine origin. Pocock et al. (1938) classified the rock as a ‘microporphyritic olivine basalt in which small well-formed phenocrysts of olivine occur in a matrix of slender labradorite laths, small granules and prisms of augite, and some finely divided magnetite’. As noted earlier, basalts believed to be contemporaneous with the Little Wenlock Basalt occur in two leaves in a borehole at Lilleshall (Lilleshall L18).

Key locality

Namurian

Although there are no outcrops which can be unequivocally assigned to the Namurian Stage, rocks resembling the Millstone Grit have been reported from a number of localities. In the Lilleshall Inlier, the Sylvan Limestone is overlain unconformably by a sequence of grey-green, fine to coarse-grained sandstones of uncertain affinity. Up to 12 m of beds are exposed in the limestone quarries to the north of the village [SJ 7356 1656], and a comparable thickness of strata was proved beneath superficial deposits in boreholes drilled down-dip. The beds appear to be dominantly fluvial in origin but much of the primary sedimentology has been overprinted by pedogenesis. In the first survey of the district, these strata were assigned to the Millstone Grit but in later surveys, they were regarded as the basal beds of the Lower Coal Measures, based on the presence of bivalves which were attributed to the species Carbonicola. Re-examination of these specimens has failed to confirm their identification due to poor preservation. Rather than erect a new lithological subdivision, the inclusion of these beds with the Coal Measures is accepted in the interim, pending further work.

Key localities

Other key localities for Namurian strata:

Westphalian to Lower Permian

Coalfields in the east and west of the district contain rocks ranging from Upper Carboniferous to Lower Permian. In the South Staffordshire Coalfield and its concealed northern extension (the Cannock Chase Coalfield), up to 400 m of Coal Measures and 450 m of barren measures rest unconformably on Silurian rocks. A sequence of comparable thickness is preserved in the Coalbrookdale Coalfield. 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 (Figure 6). 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 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 in which faunas are sparse, and biostratigraphical control is poor. The ages assigned to this latter group of sedimentary rocks are best estimates, based on studies of plant macroflora, miospore assemblages, nonmarine bivalves and pelycosaur (reptilian) remains (Waters et al., 1995).

The Upper Carboniferous succession was deposited during the latter stages of the Variscan orogeny. These coincided with a gradual change in climate from humid to semi-arid conditions, when coal-forming back-swamp deposition gave way to red-bed accumulation. Tectonic instability resulted in intermittent sedimentation with localised uplift, folding and erosion.

Coal Measures

The Lower and Middle Coal Measures were deposited in a poorly drained, delta-plain environment close to the northern margins of the Wales-Brabant High (Guion et al., 1995). The succession consists predominantly of grey mudstone, with subordinate sandstone. The mudstone is typically laminated or pedogenically altered to seatearth with root traces, sphaerosiderite and sideritic ironstone nodules common throughout. Sandstone beds occur most commonly towards the base of the sequence, where they are coarse and pebbly. Coal seams comprise only a small part (13 to 17 per cent) of the total thickness of the sequence, but many of these have been worked extensively. The geology of the southern part of the South Staffordshire Coalfield was described by Whitehead and Eastwood (1927); details of the northern part, also known as the Cannock Chase Coalfield, were reported by Barnsley (1964) and Mitchell (1945). Hamblin and Coppack (1995) have provided a full account of the Coalbrookdale Coalfield.

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.

The Coal Measures in the South Staffordshire Coalfield 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 succession 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 development of more coals partly due to seam splitting (Figure 6).

Shaft records from the exposed coalfield mostly date from the 19th century and provide few lithological details. The succession is illustrated by reference to the Ettingshall Lodge Colliery and Chillington shafts (Figure 7). The former provides the most complete section, penetrating 90 m of Lower Coal Measures [LCM], and a further 46 m of Middle Coal Measures [MCM]. The strata consist predominantly of grey mudstone, which typically shows root traces and a listric texture. Eight coal seams are known, of which the Thick Coal (up to 10.7 m) is the thickest and most persistent. Seam splits affect the Thick, Fireclay and Bottom coals.

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 developed, varying between 12 and 30 m in thickness. This includes siltstone and sandy mudstone lithologies and is commonly coarse-grained and pebbly at the base.

Ironstones, in the form of sideritic nodules or thin beds, occur sporadically, but are best developed beneath the Mealy Grey Coal (Blue Flats and Diamonds Ironstone), above the Stinking Coal (Pennystone), below the Rubble Coal (New Mine) and below the Thick Coal (Gubbin).

Dolerite [D] sills, proved at depth in many boreholes, are thought to emanate from a steep-sided intrusion or stock in the Wednesfield area [SJ 950 005]. One of the thickest sills (up to 36.6 m) intrudes the sequence below the Fireclay Coal at New Cross Farm opencast site [SO 940 995], from where it transgresses to successively lower levels southwards. It lies just above the Bottom Coal at Bowmans Harbour [SO 939 994], and is found between the Bottom and Mealy Grey coals at Priestfield No. 78 pit [SO 6915 9995]. Although the sills have been dated radiometrically (Kirton, 1984), there are discrepancies between the radiometric dates and the deduced stratigraphical ages. However, the concensus is that the bodies were emplaced in Bolsovian times beneath a shallow Westphalian cover.

Cannock Chase Coalfield

The western (concealed) part of the Cannock Chase Coalfield was formerly worked from collieries at Hilton Main and Littleton. Information from these workings and the associated shafts, together with exploratory borings carried out from 1947 onwards has greatly improved knowledge of this area. Selected sections (Figure 7) show the main elements of the stratigraphy and additional details, summarised from Barnsley (1964), are given in (Table 3). About 365 m of Coal Measures are preserved at Holly Bank, increasing to over 400 m (unbottomed) in the Lodgerail Borehole 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 the deposition of the Coal Measures, resulting in the accumulation of thin coals and clastic units, which are difficult to correlate with the standard sequence. The Hilton Main Fold belt (Chapter 8) also shows interseam variations, which may relate to tectonic movements in advance of the main late Bolsovian deformation.

Coalbrookdale Coalfield

The Coal Measures of the Coalbrookdale Coalfield crop out widely between the Lightmoor and Boundary Faults, and south of the Broseley Fault (Figure 1). 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 8) and additional details are summarised in (Table 4).

Lower Coal Measures strata below the Vanderbeckei Marine Band are thickest in the north-east of the coalfield, typically reaching 100 m. A thicker sequence proved in Lilleshall No. 6 Borehole (120.7 m, unbottomed) may have penetrated the highest beds of the Millstone Grit (see p.11). South-westwards, the Lower Coal Measures thin to 37.5 m at Madeley Meadow Pit. This thinning takes place almost entirely within the lower, arenaceous part of the sequence and is caused partly by basin margin onlap and by thinning of individual beds in the condensed sequence. There are few coals in the lower part of the Lower Coal Measures, and only the Little Flint coal has been worked. Higher in the sequence, sandstones become less dominant, and the remainder of the Lower Coal Measures is characterised by fireclays with several worked coals.

The Middle Coal Measures thicken north-eastwards but the detailed variations are 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. The greatest overall thickness occurs in the Childpit Lane Borehole (154.2 m). The base of the sequence is defined by the Vanderbeckei (Pennystone) Marine Band. This is the thickest marine band in the coalfield and the only one to contain a significant fauna apart from Lingula. The marine band occupies the lower 1 to 2 m of the Pennystone Ironstone, so named for its small, flat, round nodules. The Pennystone Ironstone lies at the base of the most complete cyclothem in the productive measures, being succeeded by nonmarine mudstone, sandstone (Big Flint Rock), seatearth and the Big Flint Coal. The last was widely worked throughout the coalfield as a single 1.1 to 1.6 m seam, but it splits north-eastwards. Apart from the Big Flint Coal, the thick valuable coals lie in two distinct groups, referred to as the Top and Fungous Coal groups after the highest coal in each.

Washouts affect the continuity of the coal seams in at least three areas. The most significant predates the Gur Coal. It follows a north-east-south-west trend, from Sheriffhales [SJ 760 120] to Dawley [SJ 684 078], and relates to a channel system wholly contained between the Limestone and Lightmoor faults. 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.

Etruria Formation [Et]

The Etruria Formation consists of a rather ill-defined group of mudstones, sandstones and breccio-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 and marks the transition from the poorly drained, alluvial backswamp conditions of the Coal Measures to the free-draining regime of an elevated floodplain. 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 around the Coalbrookdale Coalfield, and is proved in boreholes on the Heath Farm Block (Figure 9). The junction with the underlying Coal Measures is diachronous in the east of the district, but over wide areas of the exposed Coalbrookdale Coalfield, is marked by an unconformity. The top, also marked locally by an unconformity, is taken at the incoming of the predominantly grey measures of the Halesowen Formation. In areas where the Halesowen Formation is also in red-bed facies, this junction may be difficult to identify, but is recognised unambiguously on the basis of sandstone composition, assemblages of detrital clay minerals, and geophysical log response (Besly and Cleal, 1997; Pearce et al., 1999). Sandstones of the Etruria Formation are rich in immature lithic grains, mainly comprising altered basic volcanic material, whereas those of the Halesowen Formation contain an abundance of low-grade metasedimentary grains (chlorite, biotite and muscovite). 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. Where this marine band is unproven, the lowest strata may possibly be of Duckmantian age.

The bulk of the sequence consists of upwards-fining units of reddish purple, silty mudstones 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 in which the root traces have been replaced by hematite and goethite. The thickness of individual profiles and the intensity of pedogenic alteration provide an indication of soil maturity, and form a basis for classification (Besly and Fielding, 1989). Sandstones and breccias constitute a variable but locally significant part of the sequence. They are typically coarse grained, and crudely bedded, and are known colloquially as ‘espleys’.

Coalbrookdale

Around the Coalbrookdale Coalfield, the formation varies in thickness from less than 10 m to over 90 m. The isopachyte map (Figure 9) shows a series of north-east-trending troughs with intervening ridges, which parallel the dominant structural grain. The evidence indicates that 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. In the Madeley Wood boreholes, the reddening extends 14.3 m below the level of the unconformity.

Lithological variations within the Etruria Formation of the Coalbrookdale area were documented by Hamblin and Coppack (1995). Their north-south section (fig. 14) shows that channel-fill ‘espley’ deposits of coarse-grained sandstone and fine-grained breccia are concentrated to the south and west of the coalfield where the formation is thinnest, and where locally derived debris is associated with overstep of the Etruria Formation on to Lower Carboniferous, Silurian and Precambrian rocks.

Key locality

South Staffordshire Coalfield, Heath Farm block and Stafford basin subcrop

In the exposed coalfield, the Etruria Formation crops out below drift against the Western Boundary Fault [SO 924 975]. On the adjoining Heath Farm Block, the formation is concealed by younger strata and its distribution is known only from shaft records and coal exploration boreholes. The thickest sequences are preserved in the axes of major synclines, with thinning over the crests of anticlines, owing to denudation beneath the sub-Halesowen unconformity (Figure 9). For example, in the Holly Bank Colliery Staple Pit, located close to the axis of the Windmill Syncline, at least 129 m were reported. This contrasts with provings over the Brinsford Anticline, where the basal Halesowen unconformity cuts down to levels below the Aegiranum Marine Band, and the Etruria Formation is entirely cut out. Farther north, thicknesses of about 70 m were recorded in the Lodgerail and Bangley boreholes. Locally, on the Heath Farm Block, the formation is divisible into an upper sandstone-rich, and a lower sandstone-poor unit, as seen in the Saredon Hill Borehole (Besly and Cleal, 1997). The distribution of the formation beneath the Stafford basin is not known, but it is apparently absent in the Stretton Borehole.

Halesowen Formation [Ha]

The Halesowen Formation 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. Palynological samples from this horizon have been dated as late Bolsovian (Owens, in Hamblin and Coppack, 1998, p.38), although this conflicts with the early Westphalian D age ascribed by Butterworth and Smith (1976) to a suite of samples from about this horizon. Macrofloral determinations on selected borehole material support a Westphalian D age but give consistently younger ages than those indicated by palynological studies (Besly and Cleal, 1999).

Comparative sections (Figure 10) 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 (Leegomery House Farm, Kinley Farm and Lodge Farm), where it rests directly on Uriconian basement, with no evidence of an intervening Etruria Formation.

Coalbrookdale

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, 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 and silty mudstones. These, too, are commonly micaceous and dominantly greyish green, but red and purple mottling occurs throughout the formation, and is particularly prevalent towards the top. The upper formational boundary is gradational and is taken at the incoming of red-brown sandstones having the compositional characteristics of the Salop Formation (p.17). Exposures, many of them temporary, are listed by Hamblin and Coppack (1995).

Heath Farm block and Stafford basin subcrop

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 [SJ 942 042], 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 (Figure 7). 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 gammaray 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.

Depositional environment

Petrographical and heavy mineral studies indicate that the source rocks for the Halesowen Formation were rich in mica-schist, with garnet and chloritoid the most significant heavy minerals (Hallsworth, 1992a, b). The Cornubian-Armorican Highlands, located far to the south of the Wales-Brabant Massif are considered the most likely source of this detritus. Deposition is thought to have occurred on a poorly drained alluvial plain, subject initially to conditions of high flux and low aggradation (Glover and Powell, 1996; Besly, 1988). These conditions resulted in a lack of accommodation space that allowed laterally persistent sand bodies to accumulate over wide areas. The association of carbonates and thin, laterally impersistent coals indicate that the water table was sufficiently high at times to allow local colonisation by plants and the establishment of peaty mires on the floodplain. During the latter stages of deposition, the association of colour-mottled palaeosols, laminated mudstones and fresh-water carbonates indicates a change from a fluvial-dominated system to one of mainly lacustrine facies.

Salop Formation [Sal]

The Salop Formation, 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 10). The formation was defined in the West Midlands (Powell et al., 2000a) 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). 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 redbed facies transgresses diachronously down-sequence.

Alveley Member [Alv]

This lower subdivision of the Salop Formation consists of red brown and purple, calcareous mudstones 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 sandstones in which detrital carbonate material is the most distinctive constituent (Besly and Cleal, 1997). This transition coincides approximately with a change in dominant sediment colour from grey and purplish grey to bright orange-red.

The outcrop of the Alveley Member forms a north-south belt of low ground from Hugh’s Bridge near Lilleshall, to the Severn Gorge, west of Sutton Maddock. The beds are poorly exposed, owing to their low relief and an extensive cover of drift. Along its outcrop, the base of the unit is taken beneath a persistent sandstone, known informally 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. It may be that much of the sequence originally reported as ‘Keele Formation’ (i.e. Alveley equivalent) is in fact reddened Halesowen facies. The greatest thickness of strata reported above the Brookside Rock is 158.5 m in Lilleshall No. 6 Borehole, but, generally, thicknesses are around half that figure.

Enville Member [En]

The Enville Member 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. 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 fairly arbitrarily 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).

The lower sandstones in the sequence form a west facing scarp which runs southwards from Lilleshall Hall to Halesfield; higher sandstones form scarp features farther to the east. In the centre of the outcrop, west of Shifnal, six persistent major sandstones can be distinguished, of which the second and fifth include the most conglomeratic beds.

Key locality

Heath Farm block and Stafford basin subcrops

Coal exploration boreholes were mostly open holed through the Salop Formation but geophysical logs show the overall variation in proportions of sandstone to mudstone; Stretton Borehole: (Figure 10). The Alveley Member is up to 100 m thick, and the Enville Member up to 200 m. Surface exposures are limited to a small cutting along Holly Bush Lane [SJ 9610 0600], where poorly exposed sandstones are tentatively classified as Alveley Member.

Depositional environment

The mud-dominated Alveley Member formed under slow rates of sedimentation on a semi-arid alluvial plain. Well developed caliche palaeosols and reworked caliche nodules provide evidence of an increasingly arid climate, with high rates of evapotranspiration. Sandstones are likely to have been deposited during flash floods or from high sinuosity fluvial systems (Glover and Powell, 1996). The Spirorbis limestones are considered to represent lensoid units rather than single continuous beds. This reflects their formation in localised shallow lakes resulting in their lateral impersistence and low preservation potential. The sand dominated Enville Member records renewed fluvial dominated sedimentation with material sourced from local sources, mainly from uplifted areas on the Wales-Brabant Massif (Pearce et al., 1999).

Clent Formation [Cle]

In its type area of the Clent Hills [SO 935 807] to the south of the district, the Clent Formation marks a major change in provenance and lithofacies from the underlying Salop Formation. Proximal breccias characterised by Uriconian volcaniclastic clasts form a distinctive lithofacies, and the formation is demonstrably unconformable on Upper Carboniferous strata (Whitehead and Pocock, 1947, p.94). As the formation is traced northwards (basin ward) from the Clent Hills, 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, (Powell, 1991), where it forms a subdued topography typified by broad slopes with only minor features formed by the coarser beds. The formation is assumed to extend northwards into the present district, but its distribution is poorly known because of the difficulty of distinguishing Clent lithologies from those of the underlying Salop Formation, particularly in the areas of thick drift cover. Consequently in the hanging-wall side (west) of the Western Boundary Fault, the two formations are shown undivided.

In central Wolverhampton, the Fallings Park Borehole proved some 91.4 m of ‘marl’ with subordinate sandstone, and similar lithologies were recorded in the 112 m deep Oxley (Wolverhampton Gasworks) boring. 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, where it was recorded in the Four Ashes Borehole. This penetrated the base of the Triassic at 179.7 m, then continued in a sequence of mudstones (sparsely sampled), before entering undoubted Enville Member conglomerates. It is difficult to define the base of the Clent Formation unequivocally in this borehole because of irregular sampling, but it is tentatively taken at 201.2 m, giving a total thickness for the formation, hereabouts, of 21.6 m.

Other provings in which the Clent Formation has been tentatively identified form a set close to, and just north of, the northern boundary of the district (Bangley, Lodgerail, Teddesley and Ashflats boreholes). 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 downcutting in a north-easterly direction through sandstones of the Salop Formation (Enville Member). The boreholes were not cored through this interval, but if the assignation of these beds to the Clent Formation is correct, the correlation suggests that an angular unconformity is present at the base of the formation in the north-east of the district.

The Clent Formation is interpreted as the distal facies of a large-scale alluvial fan system, which was deposited under semi-arid conditions. Breccias and conglomerates represent more proximal elements derived from eroding uplands of Neoproterozoic and volcanic rocks to the south and west.

Chapter 6 Late Permian and Triassic

The central part of the district between the Coalbrookdale and South Staffordshire coalfields is underlain by up to 700 m of Late Permian and Triassic rocks forming the sedimentary fill to the Stafford Basin. Structure contours drawn on the base of the lowermost formation (Bridgnorth Sandstone or Kidderminster Formation) show the form of the basin (Figure 11a), and downhole geophysical logs for the Stretton Borehole illustrate the signatures of the constituent formations (Figure 12).

Bridgnorth Sandstone Formation [BnS]

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 (Figure 11b). The implication from the isopachytes is that synsedimentary growth occurred on the Pattingham-Patshull Fault System during deposition of this formation. On the Market Drayton Horst, boreholes record up to 80 m of beds. In the other western outcrops, to the east of the Coalbrookdale Coalfield, the formation thickens southwards from about 25 m at Woodcote Hill to about 130 m at Grindle. Thicknesses adjacent the Pattingham-Patshull Fault System 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 the Bratch Trough. 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, often resembling ‘millet seed’ and are weakly cemented by a thin layer of iron oxide (Shotton, 1937). Large-scale, dune-bedding is characteristic (Plate 3), with solitary sets displaying planar cross-stratification and grouped sets, trough cross-bedding (Karpeta, 1990). Sets range up to 20 m high and 60 to 100 m across, with foreset dips of between 25 and 30°. Individual foresets show normal to reverse grading in units up to 10 cm thick, and less commonly, a diffuse bimodal grain-size lamination on a scale of a few millimetres. Coarser laminae are thought to represent sandflow deposits formed by material avalanching down the steep lee sides of migrating dunes; finer laminae are attributed to grainfall processes involving fall-out of material carried in suspension. Planar bedded units make up a small proportion (12 per cent) of the Hatton Grange core, and are interpreted as interdune dry sandsheets. Quartz is the dominant constituent (95 per cent) with minor feldspar and volcanics grains (Hough, 1997). Calcite concretions observed in cored samples may relate to rootlet activity, and indicate the development of poor soils (Hough and Barnett, 1998).

Key localities

Depositional environment

Shotton (1937) interpreted the Bridgnorth Sandstone as the product of the repeated westward migration of a system of simple barchan palaeodunes. In a refinement of this model, Karpeta (1990) working on the outcrops around Bridgnorth, concluded that three recurrent facies were identifiable; isolated large-scale transverse dunes (or draa), barchanoid draa and bimodal sand sheets with isolated dome dunes. His study confirmed that draa orientation was controlled by a prevailing easterly wind, but he also recognised the importance of medium-term fluctuations from a more northerly direction, affecting smaller scale dune bedforms.

Sherwood sandstone group

This name was introduced (Warrington et al., 1980) for the sequence of predominantly continental red beds that makes up the lower part of the British Triassic succession. The group comprises, in upwards sequence, the Kidderminster Formation, the Wildmoor Sandstone Formation and the Bromsgrove Sandstone Formation.

The Kidderminster Formation [Kdm] (Warrington et al., 1980) known formerly as the Bunter Pebble Beds, crops out on both sides of the Stafford Basin half-graben. 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. Outcrops on the western limb give rise to one or more parallel escarpments that are best developed in the northern part of the crop (north of Lizard Hill), and south of Stockton. Over the intervening ground, and particularly around the Sands [SJ 754 047], the escarpment is much subdued, and disrupted by faulting. Evidence from boreholes drilled down-dip to the east suggests that this may be due to the relatively poor development of a basal conglomeratic unit, hereabouts. In the eastern crop, the basal conglomeratic beds form a series of prominent hills stretching from Hatherton in the north, through Saredon Hill, to Bushbury Hill on the northern outskirts of Wolverhampton. 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, particularly in uncored boreholes, but is drawn conventionally at the point above which the sequence becomes pebble free.

Borehole provings are too sparse too produce a detailed isopachyte map but thicknesses for the better constrained sites are given in (Figure 11c). 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. A narrow graben, the Westward-Hayward Trough (Figure 17), on the margins of the exposed South Staffordshire Coalfield preserves a Triassic sequence, 226 m thick, the greater part of which is believed to be Kidderminster Formation.

In many parts of the basin the fill commences with a unit composed dominantly of clast and matrix-supported 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 conglomerate beds show a range of internal structures including poorly developed horizontal stratification and planar or trough cross-stratification. Interfingering sandstones, pebbly sandstones and red-brown micaceous mudstones form a minor part of the sequence (less than 30 per cent at Saredon Hill). Aeolian grains reworked from the Bridgnorth Sandstone are present in beds up to 8 m above the base of the formation (Hough and Barnett, 1998). On geophysical logs, the basal conglomeratic unit is readily distinguished by its uniformly low gamma-ray values and high sonic velocities (about 75 ms/ft). It is overlain, in places sharply, by a sandstone unit in which pebbles 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. Although this twofold subdivision is recognisable at selected sites and in geophysical log profiles, the distribution and lateral continuity of the individual units is poorly known, and neither is mappable on a regional basis. As noted above, the basal beds in the western outcrops are relatively poor in conglomerates. One possible explanation is that the sequence onlapped westwards and the beds are younger than those proved in the axis of the basin.

The conglomerates are interpreted as having formed as mid-channel and longitudinal bars in a fluvial braided stream environment; the sandstones and pebbly sandstones represent deposition as subaqueous dunes and sand waves (Steel and Thompson, 1983). Regional palaeogeographical constructions (Wills, 1948; Fitch et al., 1966; Audley-Charles, 1970; Warrington and Ivimey-Cook, 1992) envisage deposition occurring in a northward-flowing, braided-river system sourced mainly from the Variscan mountains in the Wessex Channel area between England and France. The initial influx of coarse conglomeratic material was probably confined tectonically to the eastern margins of the developing half-graben, where the facies is thickest and syndepositional movement on major bounding faults can be demonstrated. The ensuing reduction in pebble size and volume, together with the increase in the proportion of sandstone to conglomerate, suggest a reduction in stream flow through time, possibly reflecting decreasing relief in the source areas.

No fossils are known from the formation in the district. However, occurrences of a crustacean (Euestheria) in the adjoining Lichfield district (Cantrill, 1913; Barrow et al., 1919) reflect a contemporary fauna. To the south, in the Droitwich district, Wills and Sarjeant (1970) recorded trace fossils, including Permichnium, of possible insect origin, and vertebrate tracks; the latter were restudied by Sarjeant (1996) but King and Benton (1996) considered these structures to be artefacts of inorganic origin. Warrington et al. (1980) assigned an Early Triassic age to the formation.

Key localities

Saredon Hill Quarry [SJ 946 080] (Plate 4a) and (Plate 4b) The quarry exposes an upwards passage from cemented pebbleconglomerates, through pebbly sandstones to pebble-free sandstones. The south-west corner of the quarry (at the time of inspection in 1996) shows pebble-conglomerates interbedded with sandstones in the proportions 70:30. The sandstones are up to 2 m thick, and show low-angle planar cross-bedding, with pebble ‘stringers’ on reactivation surfaces. The conglomerates vary from clast to matrix supported, are bimodal, locally imbricated, and display vague cross-stratification.

The name Wildmoor Sandstone Formation [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 half-graben, where it gives rise to a subdued topography, and, in drift free areas, red sandy soils. The maximum thicknesses are inferred to occur in the south of the district in two areas most clearly identified by Bouguer gravity data (see 1:50 000 sheet inset map). Boreholes drilled over the gravity anomaly to the west of the Pattingham Fault (Figure 11c) proved 240 m of strata at Stableford Pumping Station; a comparable thicknesss may be present beneath a second anomaly, coincident with the northern limit of the Bratch Trough. The formation thins towards the outcrop in the west (to around 50 m) and is overstepped by the Bromsgrove Sandstone in the extreme north-east corner of the district.

The formation is dominated by very 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. The beds are generally pebble-free but pebbly stringers occur locally, low in the sequence, where they are associated with coarser sandstones. Red-brown and grey-green mudstones are present as thin beds (up to 30 cm thick), exceptionally reaching a few metres towards the top of the sequence. Bedforms comprise low-angle, trough cross-bedding, low angle planar bedding and ripple cross-lamination; thin (0.5 to 1 m) co-sets are typical. Palaeocurrent directions derived from both trough and planar cross-bedding indicate a unimodal palaeoflow towards the north-west (Hassan, 1964). The base of the formation, as already noted, is transitional on the Kidderminster Formation; the upper boundary, in contrast, is sharp and erosional beneath the unconformable Bromsgrove Sandstone Formation. The formation was formerly dug for use as moulding sand, because the uniform grain size and poorly cemented nature made it particularly suited to this use.

The environment of deposition has been interpreted by different authors as subaerial (Whitehead et al., 1928), shallow lacustrine (Hains and Horton, 1969) and distal braid-plain (Powell, 1991). The persistent low-angle cross-bedding implies deposition in a fluvial environment. However, the low proportion of mudstone argues against a high-sinuosity river system. The absence of pebbles and well-formed channels, and the high incidence of planar parallel bedding are consistent with distal braidplain or low sinuosity fluvial conditions. A fluvial input to a shallow lake (sandy sabkha environment) is probably the most likely setting. The reduction in coarser clastic sediment supply, as compared to the Kidderminster Formation, may be explained by lower transport gradients into the basin.

Key localities

The Bromsgrove Sandstone Formation [BmS], formerly known as the ‘Lower Keuper Sandstone’, crops out around the margins of the Stafford Basin, and has been proved below the Mercia Mudstone in a few deep boreholes near the axis of the basin. The formation thickens eastwards from 50 m at outcrop in the north-west to between 100 and 155 m adjacent to the Breward Fault.

The base of the Bromsgrove Sandstone is generally sharp and around most of the outcrop the lowest beds give rise to a scarp feature. To the north of the district, the formation oversteps the Kidderminster Formation, and a similar unconformable relationship is thought to prevail more widely elsewhere in the district. This break may be equated with the Hardegsen Disconformity (Trusheim, 1963; 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, because their identification relies heavily on sedimentary structures. The uppermost division, is now included as the basal unit of the Mercia Mudstone Group. The boundary of the Bromsgrove Sandstone with the overlying Mercia Mudstone Group is gradational and is taken beneath the first thick mudstone at the base of a predominantly mudstone-dominated sequence. 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 markedly 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 extra-formational clasts (quartz and quartzite), intraformational mudstone rip-up clasts and beds of reworked caliche in the form of calcareous and dolomitic concretions. 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 overall reduction in grain-size, coupled with the greater proportion of overbank deposits, suggests a meandering rather than braided stream setting (Warrington, 1968, 1970; Selley, 1985).

Fossils recorded from the formation in the district comprise the crustacean Euestheria from a quarry about [SJ 870 030] near Codsall (Whitehead et al., 1928) and vertebrate tracks (Lister, 1860) from a quarry about [SJ 897 077] in the highest beds of the formation near Brewood. The latter site was referred to as ‘Chillington’ by Thompson (1902) and by Woodward (1902), who illustrated some specimens. The tracks include rhynchosauroid types and Chirotherium (Beasley, 1906). In districts farther south, particularly around Bromsgrove and Warwick, the formation has yielded diverse floral and faunal associations of Anisian (early Mid Triassic) age. These include spores, pollen and macrofloral remains, invertebrates including Euestheria and scorpions, and remains of vertebrates, including fish, amphibians and reptiles (Old et al., 1991; Benton et al., 1994).

The sandstones were derived from a complex source area of igneous, metamorphic and sedimentary rocks to the south (Ali and Turner, 1982). The Bromsgrove Sandstone was deposited within a gently subsiding basin, in a semiarid fluvial environment (Wills, 1970). The major pebbly sandstones, commoner in the lower part, formed in low sinuosity, braided channels (Warrington, 1970). The fine-grained sandstone and mudstone units more prevalent within the upper part were deposited in channels of a higher sinuosity and lower flow, and have been interpreted as overbank and channel abandonment facies (Warrington, 1970). The presence of lungfish suggests that rivers on the floodplain were prone to seasonal drought but there was intermittent marine flooding from a northern sea (Warrington and Ivimey-Cook, 1992), which enabled the incursion of fish, bivalves and semi-aquatic reptiles.

Key localities

Mercia Mudstone Group [MMG]

This group has a broad crop that expands northwards from Wrotesley, following the axis of the Stafford Basin, and bounded to the east by the Breward Fault. It is largely concealed beneath a thin mantle of glacial drift, and although pits survive in drift-free areas, good exposures are rare. The base of the group is conformable on the Bromsgrove Sandstone. The junction is usually transitional and may be diachronous, but is placed at the base of the sequence where siltstones and mudstones predominate over sandstones. Depending on the nature of the terrain and degree of drift cover, identification of the base may be difficult, and in some drift-covered areas may be fairly arbitrary. The thickest total proving in the district is 173.8 m, recorded in the Hurst Farm Borehole. A comparable thickness (172 m) was proved in the Stretton Borehole, 4 km to the east. Towards the northern margin of the district, the basin deepens and beds higher in the succession are preserved: a thickness of 255 m was recorded in the Ashflats Borehole, 3 km north of the district.

Although the Mercia Mudstone Group is shown undivided on the published map, three lithofacies can be identified at outcrop, and a fourth may be present at depth.

Lithofacies 1 Interlaminated mudstone, siltstone and sandstone

This facies is developed in the lowest part of the group and corresponds, in part, to the unit known formerly as the ‘Waterstones’ (Hull, 1869). It consists of alternations of red brown mudstone, siltstone and thin, or very thinly bedded, very fine-grained sandstone. Highly micaceous bedding planes are characteristic. In places, the sandstones thicken sufficiently (2 to 5 m) to form mappable features, as at Orslow and farther south at Weston Park and Shackerley. This facies is about 40 m thick at outcrop, but on wireline log evidence thickens to around 70 m in the axis of the basin.

Lithofacies 2 Laminated mudstone-siltstone facies

Red-brown, and more rarely, green mudstone occurs finely interlaminated with buff siltstone to form a distinctive facies found overlying, or interbedded with Lithofacies 1. Such beds have been recorded mostly in auger holes, particularly north of Weston-under-Lizard, but the typical lithology is exposed in a small pit [SJ 8192 1512], 1.3 km west of Orslow. In the Hurst Farm Borehole, two units described as ‘striped’ marls (120.1 to 134.1 m; 143.6 to 146.0 m) may also be in this facies.

Lithofacies 3 Blocky mudstone-siltstone facies

This is found in the higher parts of the succession and is the most common lithofacies. The beds are red or less commonly green, with a blocky structure and little superficial evidence of primary sedimentary structures. Grey-green reduction spots occur throughout and are common at some levels. Secondary gypsum veins were recorded in boreholes below the zone of groundwater leaching (usually 20 to 30 m depth). The log of the Hurst Farm Borehole, records gypsum to a depth of 124.6 m. occuring mainly in veins up to 4 cm thick. Interbedded with the mudstones are thin units, typically 2 to 6 cm thick, of grey, greyish green and pinkish grey, dolomitic siltstone or very fine sandstone (skerry). In areas free of drift, these more resistant lithologies form low features. Surface brash from skerry outcrops 800 m south of Marston village [SJ 8312 1341] comprises pinkish grey, finely laminated and ripple laminated dolomitic siltstone. Pseudomorphs after halite are common on the underside of skerry siltstones. More marked topographical features developed to the north of the A5 trunk road [SJ 840 114]; [SJ 823 112] may represent thicker sandstone beds.

Lithofacies 4 Saliferous beds

Although significant saliferous beds have not been recorded within the district, the Stafford Halite Member proved in the adjoining area (Arup Geotechnics, 1990) may extend at depth southwards into this district. The Ashflats Borehole sunk within the outcrop just to the north of the district encountered highly saline artesian flow from a horizon about 35 m below ground surface. A single thin seam of halite was also reported from a depth of 63.4 m in the Hurst Farm Borehole. The southern limit of the main halite accumulation lies between these two provings but on current information cannot be located more precisely.

Geophysical correlation

The gamma-ray log of the Stretton Borehole (Figure 12) shows the broad lithological character of the Mercia Mudstone: a distinction can be drawn between the spiky log signature of the sandstone-bearing lower unit (mainly Lithofacies 1) and the finely serrated signature of the mudstone-dominated upper part (Lithofacies 3). This distinction was recognised in the north of the Stafford Basin (Rees and Wilson, 1998), where the lower unit was formalised as the Maer Formation. Under new proposals for rationalising nomenclature throughout the Triassic Basins in England and Wales, the term Maer Formation will be dropped in favour of Tarporley Siltstone Formation. The upper mudstone-dominated facies becomes part of the Eldersfield Mudstone Formation, a term defined first in the Worcester district (Barclay et al., 1997).

Age of the Mercia Mudstone Group

The Mercia Mudstone Group ranges in age from late Anisian to possibly Carnian. Fossils are rare and comprise only trace fossils, from a pit, around [SJ 839 045] near Codsall Wood (Whitehead et al., 1928), and palynomorphs recovered during this survey. Sparse miospore assemblages recovered from the Codsall Borehole indicate that beds in the lower part of the sequence (between 33.53 to 57.91 m depth) are early Mid Triassic (Anisian) in age (Warrington, 1995). Higher strata are assigned a Ladinian to Carnian age (late Mid Triassic to early Late Triassic), based on miospore determinations on material from the Coton Fields No 7 Borehole [SJ 9275 2425] on the adjoining Stafford Sheet (Warrington, 1998). The occurrence in that borehole of Ovalipollis pseudoalatus and Duplicisporites spp. in samples taken above the level of the Stafford Halite not only confirms the Ladinian to Carnian age of these higher strata but provides the first positive palynological evidence for correlation between the Stafford Halite, the Wilkesley Halite in the Cheshire Basin and the Droitwich Halite in the Worcester Basin.

Depositional environments

The red-bed sequences of the Mercia Mudstone were mainly deposited in a low-relief, continental basin dominated by inland sabkhas or playa lakes. Recent models (see for example Arthurton, 1980; Talbot et al., 1994) emphasise the polygenetic nature of the sediments and illustrate by reference to modern analogues the likely interplay between fluvial, lacustrine and aeolian processes. The finest sediment probably accumulated as loess, transported by the prevailing south-westerly trade winds, and was deposited in shallow lakes, or trapped on moist surfaces, or by vegetation. Ephemeral streams flowing across low gradient floodplains may also have supplied quantities of fine sediment; some of this may have been transported as sand-size mud aggregates, derived from the erosion of floodplain soils (Rust and Nanson, 1989; Talbot et al., 1994). The majority of thin skerries are probably the product of sheet floods. Widespread evaporitic horizons are not recorded in the district but the occurrence of salt pseudomorphs indicates the presence of ephemeral pools with increased salinities, formed possibly as a result of continental run-off or from marine influxes (Taylor, 1983). One such influx can be inferred from the presence in the Codsall Borehole of tasmanitid algae, recovered from a depth of 45.72 m. Fibrous gypsum veins (common in the Hurst Farm Borehole) formed after lithification, but may derive from connate brines.

Chapter 7 Quaternary

Quaternary superficial (drift) deposits cover most of the district, except for the higher ground (Figure 13). They consist predominantly of broad spreads of till and glaciofluvial outwash laid down during the Late Devensian 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 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.

Preglacial 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 deposit has now largely been worked out and its presumed top is only visible in a small preserved face within a restored landfill site [SJ 9148 0829]. A detailed account of the deposit was given by AV Morgan (1973); biota studies by A Morgan (1973) and Andrew and West (1977) provided information on the palaeoenvironments.

Sections recorded by AV Morgan (1973) show the deposit is up to 4.6 m thick, and composed largely of pebbles of ‘Bunter’ quartzite together with rare exotic fragments of flint, tuff, rhyolite and andesite. The bedding is complex with minor breaks and erosional channels, and includes current and graded-bedded sequences. Organic lenses occur at different stratigraphical levels throughout the deposit and more rarely are found at the base of the deposit in shallow channels cut in the Wildmoor Sandstone bedrock. The upper erosional surface of the deposit falls westwards from about 106.7 m at Hatherton Junction to 97.5 m above 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.

Radiocarbon dating of the many organic lenses found within this deposit suggests accumulation began during the Ipswichian Interglacial and continued spasmodically during the Early and Mid Devensian. Organic material from two basal channels has yielded macrofossils and palynological evidence indicative of both interglacial and interstadial conditions. Comparison with other sites suggests these are the Ipswichian Interglacial and Chelford Interstadial. The first cold episode recorded at Four Ashes took place prior to 43 000 years BP, almost certainly after the Chelford Interstadial. Insect evidence points to a sparsely vegetated landscape subject to a continental climate of very cold winters and cool summers. This was followed by a brief period of climatic amelioration between 42 500 and 38 500 years BP before the climate deteriorated once more, and periglacial conditions ensued. Late Devensian ice advanced over the Four Ashes Gravel some time after 30 500 years BP, that being the youngest date obtained.

Glacigenic deposits

During the Late Devensian Substage, an ice-sheet, originating from centres in the west of Scotland and the Lake District, 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 resulting glacigenic sediments cover much of the district and comprise till, sand and gravel outwash and glaciolacustrine deposits.

Radiocarbon dates obtained from organic deposits found in gravels beneath the till at Four Ashes and preserved on its surface at Stafford indicate that ice advanced into the region sometime after 30 500 ± 400 years BP, and had retreated by 13 490 ± 375 years BP (A V Morgan, 1973). 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.

The district can be divided into five glacigenic domains, each characterised by a sediment-landform association which adds a third dimension to the essentially two dimensional geological map. The domains, together with their typical sediment associations are identified in (Figure 13) and stylised sections, based on motorway borehole logs, are illustrated in (Figure 14).

Till mantles much of the district forming a widespread sheet (Figure 13), 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, resting directly on bedrock or on impersistent spreads of glaciofluvial outwash. More varied and generally thicker glacigenic sequences are found mainly to the north-west of Coalbrookdale Coalfield, and within buried channels: they commonly include a basal till, separated by water laid sediments from an upper till unit. No genetic subdivision of the tills has been attempted but lodgement and basal melt out processes have been invoked (Hamblin, 1986; Hollis and Reed, 1981) to explain the characteristics of tills seen in exposed sections. Supraglacial melt out tills characterised by a hummocky topography are not widespread.

The predominant till lithology is a reddish brown material, with a stiff clayey consistency and with wellrounded to sub-angular pebbles, cobbles and boulders including exotic clasts, representing material incorporated into the ice-sheet during its south-eastward advance into the region. The coarse material (over 2 mm) comprises about 10 per cent of the bulk of the till, and consists of a mix of locally derived and far-travelled material. Published pebble counts (Hollis and Reed, 1981; AV Morgan, 1973) show that quartz and quartzite pebbles derived from the Kidderminster Formation make up the highest proportion, the remainder comprising 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 (AV 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 (Lister, 1862), found in the till sheet in the Badger and Ackleton areas, indicate some incorporation by the ice sheet of material from the floor of the Irish Sea Basin.

A shell-and-auger drilling programme undertaken in Wolverhampton has shown the remarkable consistency of the till, both in respect of its particle size and clay mineralogy (Kemp and Mitchell, 1994). Most samples are ‘silty clay sands’, having a uniform clay mineral assemblage of illite, corrensite, kaolinite and chlorite.

Sandy Till is confined mostly to the east-facing dip-slopes of the Kidderminster Formation and to 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. (Glaciofluvial deposits, undifferentiated). 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 [SJ 845 051] north-north-westwards through Boscobel to Blymhill, and then along the headwater valley of Back Brook [SJ 785 165]. The complex forms part of a belt of glaciofluvial deposits referred to as the ‘Newport Esker Chain’ (Whitehead et al., 1928). The deposits range in size from large mounds up to 15 m high, irregular in outline but having a pronounced linearity, to small mounds of low relief, commonly circular in plan. True eskers, represented by narrow sinuous ridges, are rare and only recognised at the extreme northern end of the complex.

In the south, field relationships indicate that the deposits overlie till at heights of up to 150 m above OD. North-westwards, the elevation of the deposits reduces, reaching a minimum of about 90 m OD along Back Brook where the deposits rest directly on bedrock. Contours drawn on the base of the deposits show some evidence of downcutting and channel incision, but in many cases the sands and gravels appear to drape the underlying substrate. Auger holes indicate that the principal lithology is an orange, red and buff, fine to coarse-grained sand, commonly with pebbles of quartz and quartzite. Other isolated mounds of sand and gravel with a similar morphology but lying beyond the main ‘esker’ belt have been mapped at Westonunder-Lizard, Church Eaton and Little Onn.

The constructional form and over-steepened flanks of many of the mounds led Dixon (in Whitehead et al., 1928, p.176) to suggest that the whole complex was deposited by two or more closely related subglacial drainage systems. He maintained that the larger, irregular shaped areas of sand and gravel, such as occur south-west of Bishop’s Wood [SJ 830 085], were deposited during temporary recessional halts where the streams emerged from beneath the active ice margin. By projecting the trend of these marginal features, with other geomorphological evidence (for example notches formed by meltwater spillways), Dixon was able to postulate a series of subparallel, east-north-east-west-south-west orientated still stands. Although other workers have argued that the field evidence for Dixon’s reconstruction is inconclusive, there is an undoubted alignment of some of the largest kames along the trend indicated by Dixon. Whether or not the interpretion of a retreat, involving nine separate stages, is justified is debatable.

Spreads of flat-lying or slightly hummocky sand and gravel, locally associated with till (Worfe valley outwash sandar), cover the floor of the valley of the River Worfe (Figure 13). The base of the deposit falls steadily southwards from about 95 m above OD at Crackley Bank to 45 m at the district boundary. Boreholes (SJ80NW/1), (SJ80NW/2), (SJ80NW/3), (SJ80NW/4), and (SJ80NW/5) show that the deposits locally underlie till, but there is also evidence in sections and from auger holes that thin till layers occur within the outwash. The sands and gravels form deposits up to 6 m thick, and, in the upper Worfe, are composed mainly of locally derived material, sourced from the nearby Triassic outcrops; they include some exotic clasts of Scottish and Welsh origin. Below the junction with Mad Brook, a more varied exotic suite is evident (Wills, 1924), indicating the importance of this tributary as an outwash feeder. Dixon in Wills (1924) subdivided the deposits of the upper Worfe into three outwash fans, referable to recessional positions of the ice-front. This resurvey has shed no light on the basis for such a subdivision.

Glaciofluvial sheet deposits of sand and gravel (Whiston-Penk valley trains) showing a variably developed terrace form, border the River Penk and its tributaries (Figure 13). The thickest occurrences lie in valleys drained by the Church Eaton Brook and Whiston Brook and form part of an outwash train which can be traced back to a col on the watershed at Gnosall (in the Stafford district to the north). The deposits comprise pebbles of ‘Bunter’ quartz and quartzite and assorted igneous clasts in a matrix of medium-grained sand. Between Codsall and Penkridge, the glaciofluvial deposits are confined to narrow strips bordering the recent alluvium. Below the confluence with Church Eaton Brook, the tract of glaciofluvial outwash expands to almost a kilometre in width, supporting the contention (Whitehead et al., 1928; A V Morgan, 1973) that much of the fluvioglacial input to the Penk system derived from meltwaters flowing down Whiston Brook, rather than from the higher reaches of the Penk.

Glaciolacustrine deposits have been proved at surface and in boreholes at a number of sites in the district (Figure 13). 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. These are thought to be mainly proglacial in origin, and may have formed when meltwaters became impounded between the ice margin and the higher ground of the main watershed during advance and retreat stages. 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 filling palaeovalleys (see below). Smaller spreads found by augering on the till plateau have not been delineated.

Key localities

Glacigenic channels

Several buried channels, filled with thick glacigenic sediments, have been recorded within the district (Figure 13). Those cutting the Lilleshall–Telford watershed are the best defined and include the 20 km-long Lightmoor Channel (Hamblin, 1986; Hollis and Read, 1981).

Closely spaced boreholes in the urban areas of Telford and Wolverhampton have enabled the subdrift form of some of the channels to be resolved with reasonable certainty. In the case of the Lightmoor and Moxley channels, irregular, deeply scoured, longitudinal profiles can be demonstrated. These descend in places to below the level of any drainage outlet and 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 boundary (A V Morgan, 1973), raises the possibility that some of the channels have a longer and more complex history, dating from the pre-Devensian.

Details of the individual channels, their sediments and possible development history are given elsewhere (Hamblin, 1986; Hollis and Read, 1981; A, V Morgan, 1973) and only brief details are summarised here.

The deposits in the northern part of the Lightmoor Channel comprise an upper till, overlying sands and gravels and clay-silt rhythmites, and locally a lower till. This succession persists southwards to near Astol [SJ 7395 0025] where resistivity and gravity surveys (Atitullah and Freeman, 1973) indicate about 36 m of drift, consisting mainly of sand and gravel, down to 37 m above OD. Near Bailey’s Corner [SO 735 990] the channel swings south-eastwards following the modern valley, which is underlain by deposits of grey, stoneless silt and clay. At Stableford, the channel system connects with a meltwater spillway cut in Wildmoor Sandstone. According to Hollis and Read (1981), the buried channel continues south-eastwards for a further 7 km to beyond Sandford in the Dudley district. The association of sediments in the southern section of the channel was interpreted by Hollis and Read in terms of two ice advances, with an intervening retreat phase during which subaerial lacustrine sedimentation predominated. In reviewing the evidence from the northern part of the channel, Hamblin (1986) concluded that the channel complex could have been cut and filled within one advance and retreat phase. Worsley (1991) pointed to the similarities between the Lightmoor Channel and the Seisdon-Stourbridge channel, 5 km to the east, which also trends north-west to south-east, and extends well to the south of the accepted Devensian limit. The occurrence in this channel of lake-basin sediments correlated with the early Hoxnian Interglacial (A V Morgan, 1974), led Worsley to speculate that the Lightmoor Channel may also be a much older feature, dating from an earlier glaciation. No organic deposits have yet been found to substantiate this view.

The northern part of the Oakengates Channel is steep sided and filled with at least 17.5 m of sands and gravels and soft clays, locally overlain and underlain by up to 5 m of till. The channel cuts through the Lilleshall–Telford watershed, its base falling northwards from 118 m [SJ 6992 0980] to 84.7 m above OD [SJ 6936 1165]. Farther south, near Stafford Park [SJ 703 082], more than 12.6 m of clean, well-sorted, current-bedded sand and pale yellow, laminated clay, overlain by till, are probably deposits of the same southward-draining channel. This may continue south along the line of the Mad Brook to join the Lightmoor Channel, but evidence is lacking.

Sands, silts and gravels of the Shifnal Channel system, up to 6.4 m thick, fill channels beneath till in the upper part of Westland Brook valley [SJ 733 098] and in the Wesley Brook valley downstream from Castle Farm [SJ 725 093]. Farther south-east at Shifnal, the channels are marked by an outcrop of glaciofluvial sand more than 1 km wide. South of the town, a main channel can be recognised west of Shifnal Manor, but terminates before Ryton in the Worfe valley.

Part of the north-west-trending Moxley Channel underlies north-east Wolverhampton. The channel has no surface expression but is filled to a depth of 36 m with sand, gravel, till and glaciolacustrine clays.

The River Penk, north of Penkridge, flows in a drift filled valley excavated to a depth of 20 m below the alluvial flat (at 60 m above OD). A rockhead map of the valley was figured by AV Morgan (1973). Deep drift-filled depressions also underlie two minor left-bank tributaries of Saredon Brook, one flowing to the east of Saredon Hill [SJ 930 070], the other to the west [SJ 958 080].

Four 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 [as at 955 145], to narrow channel-fills only a few tens of metres across. Two examples, cited by A V Morgan (1973), run approximately parallel to each other, one passing to the north of Pillaton Farm [SJ 945 134], the second, some 600 m to the south. Records from motorway boreholes show that the latter channel passes into a buried valley complex of sands, gravels, and laminated clays referred to in the primary survey as the ‘Penkridge esker’. This has a rounded, moundy top and bears the hallmarks of an ice contact feature. Its line may be continued north-westwards across the River Penk by a series of elongate sand bodies mapped around The Whitta moors [SJ 905 155].

Glacial history

The glacial history of the district has been reconstructed by a number of workers (Wills, 1924; A V Morgan, 1973; Hollis and Reed, 1981; Hamblin, 1986). The precise configuration of the preglacial landscape and its drainage systems remains conjectural (see review in Hamblin, 1986, p.374). However, the generally accepted view is that the preglacial watershed was only breached at Ironbridge during the Devensian glacial maximum. Before this time, the upper Severn flowed out northwards to the Irish Sea, and the Dean Brook formed the headwaters of that part of the Severn which now flows south through Apley to Bridgnorth.

During the Late Devensian glaciation, ice advancing down the Irish Sea Basin expanded into the Shropshire-Staffordshire borderlands, laying down a discontinuous sheet of basal lodgement till. The glaciolacustrine deposits found towards the base of the drift in the north-west of the district are believed to have 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). These were probably deposited at the margins of the receding ice sheet by streams flowing in subglacial or englacial conduits. Larger kames, orientated at right-angles to this trend, such as those between Blymhill Common and Brineton, and around Lynn, 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 (Clayton, 1977). 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 a separate body of water. During the final stages of deglaciation, the postglacial drainage pattern evolved: the Tweedale Brook captured the upper part of the Lightmoor Channel drainage, and the Moat Brook probably captured the headwaters of the Smestow system diverting them northwards in to the River Penk.

Periglacial deposits

Ice-wedge casts and patterned ground have been widely recognised on the till sheet to the north of Wolverhampton (Morgan, 1971). Further evidence for the presence of ground ice is suggested by a pingo-like feature, floored by peat, in the valley south of Blymhill Grange [SJ 812 117].

Head is a slope deposit formed mainly by processes of solifluction and gelifluction, and results from the downslope movement of superficial deposits or weathered bedrock, usually in periglacial conditions of alternate seasonal freeze and thaw. It is generally a poorly structured deposit, comprising unsorted debris of the local bedrock or superficial deposits, and may contain relict shear surfaces, which have implications for slope stability and foundation design. Although not widely mapped, thin deposits of head and colluvium (hillwash) are likely to be present on the lower slopes of the more striking topographical features, and as fill in the smaller valleys. Head is thickest and most widespread in the Nedge and Mad Brook valleys, where up to 4.3 m are recorded. In the Mad Brook valley [SJ 710 057], head has flowed over earlier glacial deposits, causing the stream to divert to its present position.

Postglacial deposits

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 from the thickest part of the deposit, taken from a depth of 2.85 to 2.90 m gave a radiocarbon date of 11 660 ± 250 years BP (A V Morgan, 1973).

A peat moor, also occupying a hollow in a glacial meltwater channel [SJ 925 112], lies to the south of Rodbaston Hall. The deposit was first studied by Shotton and Strachan (1959) who published an account of its stratigraphy, fauna and flora. Radiocarbon 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 sewage pipeline [SJ 92446 14354] to [SJ 92353 14341] gave an age of 11 660 ± 250 years BP (A V Morgan, 1973).

Beetles extracted from these three sites were described by Ashworth (1969). At Pillaton Hall and Penkridge, the beetle evidence points to a climatic amelioration at around 11 500 years BP thought to correspond to the Windermere Interstadial. The beetle faunas obtained from deposits at Rodbaston dated to between 10 700 and 10 300 years BP are indicative of a colder climate, probably representing the Loch Lomond Stadial (about. 11 000–10 000 years BP).

An extensive area of peat floors a shallow, north-trending valley above Lower Snowdon [SJ 785 020]. A recently dug reservoir showed the deposit to be at least 2 m thick.

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]. The Bog centred around [SO 753 976] is another larger peat-based depression, also interpreted as a kettlehole (Whitehead et al., 1928). More recent and extensive peat deposits, interbedded with alluvial silts and clays, occupy a low lying tract in the north-west corner of the district between Kynnersley and Preston - an area known as the Weald Moors. Boreholes show intercalations of soft brown and grey silty sand, silty clay with bands of peat and thicker peat accumulations.

River terrace deposits

River Severn

Downstream of the Ironbridge Gorge, Wills (1924, 1938) recognised and named two terraces; 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. The stratigraphical units present in the Apley Park section of the river are shown in (Table 5).

Third (Holt Heath) Terrace

The Holt Heath Member is represented by two small patches of sand and gravel lying beneath a level at approximately 76 to 82 m above OD. The deposits interdigitate with Devensian till, and contain a range of exotic fragments, supporting the widely held view that the bulk of the Holt Heath material originated as outwash from the Devensian ice sheet.

Second (Worcester) Terrace

Deposits correlated with the Worcester Terrace underlie Apley House [SO 712 982]. Lower and upper facets are recognised, with surfaces at 48 and 56 m above OD, respectively.

First Terrace

In Apley Park, the First Terrace forms an extensive area of sand and gravel contiguous with, and about 4 m above, the present floodplain. The terrace is tentatively correlated with the Power House Member.

River Worfe

Terraces occur along the valley of the Worfe, downstream from Cosford, and also along Mad Brook and Hilton Brook. The terraces of the 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.

Higher Terraces (undifferentiated)

Flat-topped deposits of sand and gravel 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 of the Severn Valley Formation (Hamblin and Coppack, 1995).

Second Terrace

Second Terrace deposits are mainly found in the south of the district (SO79NE) and consist of orange-brown sand and pebbly sand. The deposits are best developed between Hilton and Wyken [SO 769 954] where they form an extensive flat attaining a maximum height of 55 m above OD. Immediately north-west of Hilton, the Second Terrace can be subdivided into lower and upper facets. The lower facet occurs up to 50 m above OD, the upper to 59 m OD. Similar upper and lower divisions were identified in the Worcester Terrace of the River Severn by Whitehead and Pocock (1947).

First Terrace

The First Terrace is best developed and probably thickest towards the south of the district, between Stableford [SO 759 987] and Worfield [SO 759 957], where it is up to 650 m wide and about 4 m thick. The deposits consist of orange-brown sand and pebbly sand. North of Stableford, the terrace is typically 1 to 3 m above the alluvium and up to 200 m wide. The upper surface of the terrace is slightly undulating.

Alluvium

Most of the major streams and rivers in the district are flanked by alluvial floodplain deposits, consisting of silt and clay overlying coarser beds of sand and gravel. The most extensive spreads are found along the River Penk, and its two main tributaries, Whiston Brook and Saredon Brook. Motty Meadows [SJ 835 130] in the headwaters of Whiston Brook, is a fine example of a flood meadow, and is preserved as a nature reserve, under the control of English Nature. Auger profiles through the upper part of this alluvial sequence show thin grey silty clay, or peat at surface, overlying 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; along the smaller tributaries, thicknesses of 2 to 3 m are more likely. The alluvial deposits of the Weald Moors areas have been referred to earlier.

Landslips

Landslips affect both sides of the Ironbridge Gorge, and extend into the deeply incised tributary valleys. A recent survey has indicated that unstable or potentially unstable slope deposits extend along the length of the gorge and the mapped distribution of landslips is very much a conservative estimate of the problem.

Shallow-seated downhill movement in the surface layers of made ground and weathered Coal Measures has affected the village of Ironbridge over a long period, resulting in deformation and cracking of buildings on the steep slopes above the Severn and of the Iron Bridge itself. The first recorded catastrophic failure occurred in 1969, when fill material forming the flat playground behind a school [SJ 6745 0351] became waterlogged and slipped; this loaded the weathered Coal Measures on the steep slope below and caused a rotational slip which extended down to the Ironbridge-Madeley road. Farther downstream, at Lloyds Coppice, a large area of the Halesowen Formation is actively moving as a series of rotational slips and surface slides. The Halesowen Formation is particularly susceptible to degradation in this area because it includes more silty strata and less sandstone than elsewhere. Areas of slip occur on either side of Lee Dingle [SJ 692 037] both have clear back scarps but their lower limits are not easily defined. On the north side of the Severn, at Coalport a further large area of slipping [SJ 698 025] comprises a series of shallow structures wholly contained within the Halesowen Formation. A larger area of active translational and rotational slips stretches from Jackfield to the Wilds, and includes the famous Jackfield slip [SJ 688 028] described by Henkel and Skempton (1954) and Skempton (1964). This failed in 1951–52 causing damage to houses, railway and mains services. This slip forms part of a much older slipped complex, which extends 600 m back from the river. Continuing movement is evident from the recent fissuring seen in the pasture, north of Woodhouse Farm.

Away from the River Severn there are active shallow seated movements on the steep slopes [SO 893 983] overlooking Linley Brook in mudstone of the Temeside Shales Formation.

Artificial deposits

The land surface, particularly in the more heavily urbanised industrial areas, has been extensively modified by man.

Excavated (or infilled) 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. There are also a number of large pits connected with former aggregate workings (Saredon Brook, Essington, and Saredon Hill). In Wolverhampton, the redevelopment of the Bowmans Harbour site [SO 938 993] involved excavation to a depth of over 30 m to stabilise the site, allow removal of remaining coal reserves, and to enable domestic landfill to be relocated within a permanent till lined repository.

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. At the turn of the 19th century, much of this material was dumped haphazardly in waste mounds at the pitheads, leading to local waterlogging. As urbanisation proceeded the material was levelled, so that nowadays it seldom forms topographical features and its extent is only known from borehole and trial pit data. In the long-established industrial areas, colliery spoil is frequently mixed with other fill types, often of different generations. Such deposits include foundry sand, building rubble, and variable quantities of metal, timber, glass, plastic and other miscellaneous by-products of the paint, metal finishing and electrical industries.

Chapter 8 Concealed geology and structure

Concealed geology

Precambrian to Tremadoc

No boreholes have penetrated the pre-Silurian rocks beneath the Stafford Basin but the seismic evidence indicates a thickening of the Cambrian succession down dip from Lilleshall. This is shown by a reflector-free zone interpreted as St David’s to Merioneth Series (Middle to Upper Cambrian) and Tremadoc shales (Figure 15). A deeper reflector-free zone is tentatively attributed to latest Precambrian (Longmyndian) rocks, and a high amplitude reflector marks the top of the Uriconian Volcanic Group. East of the Breward Fault, St David’s Series to Tremadoc rocks are absent, and the Silurian rests unconformably on Uriconian rocks. Hence it is probable that the precursor of the Breward Fault was initiated during early Palaeozoic times as a growth fault, and defines the eastern boundary of a Cambrian-Ordovician basin.

Silurian

Beneath the Carboniferous unconformity, Silurian rocks are known from several boreholes in the Coalbrookdale and South Staffordshire coalfields. In the Stafford Basin, a deep oil exploration borehole (Codsall) penetrated 255 m of Silurian strata of Wenlock and Ludlow ages. This information can be combined with King’s (1921) map of the South Staffordshire Coalfield and seismic data to provide a pre-Carboniferous subcrop map (Figure 16). Prídolí rocks are recognised at the bottom of a number of shafts in the South Staffordshire Coalfield but are not seen clearly on the seismic profiles. Much of the Silurian subcrop is represented by strata of Ludlovian age. The Silurian rocks have been folded, probably along Charnian trends in the north but the main strike of the rocks from the subcrop map is north-east-south-west.

Faulting

The surface fault pattern reflects the interaction in the cover rocks of three distinct trends inherited from the accretionary terranes that constitute the Proterozoic basement of the Midlands Microcraton (Figure 17). The Caledonoid (northeast) trend is dominant throughout the district, and is most evident in the Palaeozoic rocks exposed in and around the Coalbrookdale Coalfield. Faults with north-west and westnorth-west (Charnoid) trends are common in the Coalbrookdale Coalfield but apart from the Broseley and Brewer’s Oak faults, none is of any great length, and most appear to be accommodation structures. The Malvernian (north-trending) grain reflects the suture between two terranes, considered to have fused in Proterozoic times (Carney et al., 2000). It is reflected in some of the stuctures in the Pattingham-Patshull fault system, and in faults in the more northerly parts of Stafford Basin.

Most of the Caledonoid structures are normal faults, in terms of their youngest phase of movement, but many show reversal of throw 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).

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; associated antithetic faults dip east into it at depth. It gives rise to a prominent escarpment south of Water Eaton [SJ 900 107] but in the more subdued ground to the south of Penkridge [SJ 919 132] its surface position was only proved by augering. North of the town, its position is based on seismic evidence. The displacement of the Triassic strata on the northern part of the Breward Fault is at least 170 m, based on the thickness of Mercia Mudstone proved in the Stretton Borehole. However, the displacement diminishes southwards as control is transferred on to the Pattingham-Patshull fault system through a plexus of smaller faults on the Codsall High.

The bounding structure to the South Staffordshire Horst (Figure 3) 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 formation. The downthrow to the west is estimated at about 600 m. Soil gas measurements of radon levels have been used successfully in the Nordley Hill area [SJ 944 009] to pin-point the position of the Western Boundary Fault beneath thin glacial cover. 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. North of northing 05 the throw reduces to between 50 and 75 m. The surface position of the fault is only well constrained between Dunstall and Whetstone Green where it was encountered in a water well (SJ90SW/6); northwards its position through drift-covered ground is more speculative. An underground heading from Littleton Colliery encountered a fault with a throw of 41 m, presumed to be the Bushbury Fault, to the west of Yew Tree Cottages [SJ 957 140], beyond which it may pass northwards into the Teddesley Park Fault. The Hilton Main and Essington Church faults (the latter on the district boundary) 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 runs parallel to the Bushbury Fault, trending north-eastwards in the south but more northerly in the north. It hades at about 45º and throws down to the south-east by about 100 m. The throw on the Essington Church Fault is up to 200 m, down to the north-west. A number of faults, most with small throws, run oblique to, or perpendicular to, the major faults noted above.

Structural history

The structural evolution of the district has been determined by combining the results of surface mapping with data from shafts and boreholes, and from geophysical surveys. Although much of the district is underlain by Mesozoic rocks, structurally controlled highs expose Lower Palaeozoic and older basement, allowing a partial reconstruction of the deformation history of the district from Precambrian times.

Precambrian (Avalonian/Cadomian)

Deformation in the Uriconian rocks of the Lilleshall Inlier is localised in the footwall of the Boundary Fault. Bedding attitudes in the pyroclastic rocks adjacent to the fault steepen to become near-vertical or locally overturned. Coincident with this eastern near-vertical zone, there is a highly pervasive cleavage, visible at a millimetre to submillimetre scale. These structures indicate that the compression of the sequence was localised along the Boundary Fault. This deformation could be as young as Acadian (Siluro-Devonian), but it is similar in style to that affecting the Longmyndian and Uriconian rocks of the Church Stretton district (Pauley, 1991), which has been dated to the latest Precambrian. The cleavage formation could therefore be related to one of the major late Precambrian terrane accretion events along the Welsh Borderlands Fault System (see for example Gibbons and Horák, 1996). Further complexity is indicated by the discrete shear zone, trending across the southern part of the outcrop (Whitehead et al., 1928), which shows asymmetric drag folds indicative of a phase of normal faulting.

Caledonian (Acadian) deformation

Fold axes in the exposed Silurian rocks in the south-west of the district generally trend north-eastwards; the folding is Caledonian in age, postdating the Raglan Mudstone Formation but predating the Dinantian Lydebrook Sandstone. Between the Willey and Deancorner faults, strong Caledonian folding in the Ludlow rocks extends beneath, but does not affect the Coal Measures. Folding, probably along Charnian trends, affects the Silurian rocks in the subcrop in the north-east of the district (Figure 16).

Carboniferous to Early Permian deformation

Between Devonian and early Permian times, the Midlands formed part of a foreland basin which 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. Shallow-water carbonates and siliciclastic deposits, proved in the Lilleshall boreholes, indicate that this area remained a structural 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 progressive onlap of Coal Measures strata on to the flanks of the Brabant Massif. Variations in seam thickness and seam splits signify tectonic instability and many of the Caledonoid faults show synsedimentary movements. Intra-Westphalian compressive movements 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 are the Muxton, Stirchley, Hem and Sutton Maddock anticlines, and the complementary Donnington, Madeley and Coalport synclines. All have a Caledonoid trend and plunge north eastwards. The Donnington Syncline reflects the structure in the underlying Dinantian strata, but it is a very gentle and poorly defined structure dying out both to the north-east and south-west, and is confused by minor folds. The Stirchley Anticline is asymmetrical with a steeper eastern limb, as may be the Hem Anticline. The crests of these anticlines run close to the lines of the Limestone and Madeley faults and both anticlines decline in amplitude northwards. The Madeley Syncline apparently bifurcates to the south-west as does the Hem Anticline, resulting in relatively complex folding. Dips are generally less than 6º. The deformation predates the Etruria Formation but postdates the Fungous Group of coals which incrop beneath the sub-Etruria unconformity (Symon Unconformity) in the Donnington and Madeley synclines.

In the South Staffordshire Coalfield, deformation followed deposition of the Etruria Formation, resulting in localised tight folding of the Westphalian strata, and development of an angular unconformity beneath the Halesowen Formation. Folds active during this deformation phase include a group of north-north-east trending structures forming the Hilton Main Fold Belt. The Windmill Syncline is a close asymmetrical fold at the southern end of the Westcroft-Haywood Trough. Its eastern limb is heavily faulted and its western limb turns sharply into the adjoining Hilton Anticline. The latter has flanking dips on its eastern and western limbs of 45º and 18º, respectively, and the crest of the structure is disrupted by the Hilton Main Fault. Farther west, the Moseley Syncline is another asymmetrical fold, with a more steeply dipping eastern limb. All three structures plunge towards the south-south-west at between 5º and 10º. Evidence from workings close to the Bushbury Fault indicate that an anticline complementary to the Moseley Syncline may have developed in the western (hanging) wall of the Bushbury Fault in an area of condensed Coal Measures termed the Brinsford Axis (Barnsley, 1964). The Huntington Syncline encountered in workings from Mid-Cannock and Littleton Collieries may be an extension of the Moseley Syncline or an en echelon fold. This structure can be traced northwards to just beyond the Littleton Shafts before dying out.

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.

Latest Carboniferous to Permian deformation

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.

Post-Permian movements

Northwest-southeast-directed 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.

Explanation of geophysical inset maps

Regional Bouguer gravity and magnetic anomaly data are included as inset maps on the 1:50 000 scale Sheet 153 Wolverhampton. These have been compiled from information held in the national gravity and aeromagnetic databanks. The Bouguer gravity anomaly map reflects variations in rock density. Highs occur where higher density rocks (Precambrian and Lower Palaeozoic basement) come close to the surface, and ‘lows’ where lower density rocks (Permo-Triassic sandstones, Coal Measures) are thickest. The magnetic anomalies are primarily a function of the varying magnetic properties, and depth below surface, of the basement rocks. The South Staffordshire Horst is associated with a large gravity high, and a partly coincident magnetic high, seen to the east of the district margin. This may indicate that a body (or bodies) of higher density, magnetic material, such as Uriconian volcanic rocks, lies at comparatively shallow depths. In the south-west of the district, a broad gravity high corresponds to the Silurian outcrop. This anomaly declines north-eastwards towards Shifnal, where the higher density Silurian rocks dip below thickening, lower density Coal Measures. Uriconian and associated Precambrian metamorphic rocks within the Church Stretton Fault system give rise to a positive Bouguer gravity anomaly which strikes north-eastwards towards the Lilleshall Inlier. The aeromagnetic response is more pronounced, with high amplitude anomalies and steep gradients indicating substantial fault control along this Uriconian axis. The Staffordshire Basin, with up to 600 m of low density Permo-Triassic rocks, is seen as a broad gravity anomaly low. The basin is bisected by a north-east-trending positive anomaly, corresponding to the Codsall High. On this horst, a weaker positive feature is delineated by the Brewer’s Oak and Coppice Green faults. In the northern part of the district, resolution of the Stafford Basin by the Bouguer gravity map is less clear. This is due to an increasing contribution to the gravity anomaly low by Carboniferous rocks, with densities close to those of the overlying Permo-Triassic sequence.

Earthquakes

Geological faults in this area are of ancient origin and are currently mainly inactive. There has been little record of seismic activity in the area. The largest event (and the only historical one) was the earthquake of 21 April 1805, which had a magnitude of 3.2 ML on the Richter Local Magnitude scale and probably had an epicentre south of Stafford, but the location is only approximate on the limited data available. Recent instrumental monitoring by BGS in the last thirty years has detected only six small events in this district, none of them larger than 2.1 ML. These all occurred in the area east of Telford, south of Newport, and west of Albrighton. This is near the eastern margin of the exposed Coalbrookdale Coalfield, but given that they occurred some time after mining ceased, and are not close to the main mined area, a connection with mining activity is doubtful.

Chapter 9 Applied geology

The key geoscience constraints likely to influence land use, development and conservation within the district are listed in (Table 6). 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 (for example Thompson et al., 1998). A review of the main sources of geoscience information was given by Ellison et al. (1997).

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 by Henkel and Skempton (1954), Skempton (1964), and Gostelow et al. (1991). In an unpublished report, Culshaw (1973) used a zoning technique to distinguish areas of ‘high’, ‘medium’ and ‘low’ risk. This type of approach is important because it 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. A review of mining instability in the UK was provided by Arup Geotechnics (1992). 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. A report on site investigations carried out at Lilleshall was given by Ove Arup (1987).

Throughout the exposed coalfields, variable thicknesses of poorly compacted colliery spoil are widespread, and this may suffer severe differential settlement. Colliery spoil may contain iron sulphides (such as iron pyrites) that are prone to oxidise and produce sulphate-rich, acidic leachates. These may be harmful to concrete present in foundations or buried services, thus requiring the use of sulphate-resistant cement. This oxidation process may also result in expansion and differential heaving of foundations constructed upon such deposits.

Geotechnical information on the engineering characteristics of the outcropping rock formations and superficial deposits is summarised in (Table 7) and (Table 8). Four main issues are considered:

Other potential constraints that need to be considered are the geological structure, slope stability, the presence of undermining and the depth and degree of weathering.

Contamination

Natural contamination

Radon is a colourless, odourless, naturally occurring radioactive gas produced by the radioactive decay of uranium, which is found in small quantities in all soils and rocks. Radon released from rocks and soils disperses quickly in the open air, but it may accumulate in poorly ventilated buildings and mines where it is a potential health hazard. In order to limit the risk to individuals, the Government has adopted an Action Level for radon in homes of 200 becquerels per cubic metre (Bq m-3). The Government advises householders that, where the radon level exceeds this level, measures should be taken to reduce the concentration. BR 211 (1999) provides revised guidance on protective measures for new dwellings, and defines the geographical areas where radon protection is necessary. In this district, radon emissions measured on the Permo-Triassic bedrock are low to moderate and no protection is needed. Moderate to very high levels 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 British Geological Survey.

Other potential gas hazards associated with the build-up of methane and carbon dioxide are discussed fully by Appleton et al. (1995). Methane and carbon dioxide 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 ground water 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

Contamination of the natural environment is associated, in most cases, with human activity. The BGS through its Geochemical Baseline Survey is providing a database on the occurrence and distribution of a wide range of Potentially Harmful Elements (PHEs) in both urban and rural settings. The data provide a useful baseline guide to the distribution of PHEs but should not be used to assess individual plots of land as contaminated or uncontaminated. 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. This pattern may be observed in most former industrial areas.

Landfill gas

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

This 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 depending on factors such as the permeability of the substrate adjacent to the site. Historically, landfill operations in the UK practised a policy of ‘dilute and disperse’ within unconfined sites. Such a method could potentially lead to a reduction of water quality and consequently, recent landfill sites have tended to be engineered containment sites with treatment of leachates.

Leachates may be a particular problem at old landfill sites in which tipping was uncontrolled and little attempt was made to prevent pollution migration. Studies undertaken during the redevelopment of the Bowmans Harbour site [SO 937 995] in Wolverhampton showed that where the site is underlain by till, leachate migration is retarded; much of the pollution is restricted to the uppermost metre of the till body. Because of its effectiveness in retardation of the permeation of contaminants, the local till has been used at this site to line an engineered repository for domestic waste (Plate 7).

Flood risk

In the urban areas, localised flooding is a problem mainly in relation to culverted water-courses, which may overflow after heavy rain. The problem is exacerbated by new development, which increases the area of impermeable surfaces, leads to reduced infiltration, rapid run-off, and increased discharge to streams. In the Telford area, the flood risk is reduced by directing overflow to flood meadows. Low-lying ground along the Severn Gorge floods under conditions of high river flow, notably on the south bank in the Jackfield area.

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). This trend is illustrated in Wolverhampton by the Wolverhampton Gas Works boring (Figure 18a), which since monitoring commenced in 1973, has shown a recovery of 19 m over a 24 year period, with only a slight decline between 1989 and 1993, possibly in response to the 1988 drought. The closure of the coal mines during the second half of the last century and the cessation of dewatering have 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. A relative lowering of the land surface by subsidence over the mined area could result in localised flooding. Areas at risk include the low-lying areas close to the main river valleys where the water table is within 5 m of the surface.

Water resources

Surface water

The district straddles the surface water divide between the rivers Severn and Trent. The headwaters of the River Worfe and Smestow Brook drain southwards into the Severn/Stour network, while the Penk flows northwards into the Trent. Minor headwater streams of the River Tern drain the extreme north-west corner of the district. The surface water drainage pattern has been modified by the construction of a canal system which includes parts of the Staffordshire and Worcester, Wyrley and Essington, and Shropshire Union canals. The average annual rainfall ranges from 650 to 750 mm over much of the district. It is lowest near the northern and southern margins, and rises to a maximum of over 800 mm in the west over the high ground to the south of Telford. Mean annual evaporation is of the order of 500 mm.

Summer flow in the upper reaches of the River Worfe is extremely low and the river receives no base flow from the underlying aquifer (see next section); winter flow reaches 20 megalitres (Ml) per day. Smestow Brook, which has its origins in Wolverhampton, receives a large component of its flow from sewage treatment works discharges, and from storm-water run-off. Summer base flow is maintained largely through sewage works discharges. Winter base flow is comparable with that of the Worfe. The River Penk drains a comparable catchment area to the Worfe but has a higher and flashier base flow suggesting that run-off from the Mercia Mudstone or till-covered slopes is a significant factor.

The high proportion of effluent and urban run-off has led to many water courses being polluted. The Staffordshire and Worcestershire Canal receives discharges from the Barnhurst, Coven Heath and Gailey sewage treatment works and the quality of the canal water reflects this. The River Penk and Smestow Brook are similarly affected.

Groundwater

Permo-Triassic rocks

The Permo-Triassic sandstones of the Stafford Basin form part of one of the most heavily exploited aquifers in the country, with a history of abstraction dating back to the 1860s. Although there have been problems in recent years with over-abstraction and enhanced nitrate levels, the aquifer remains an important groundwater resource, and continues to supply large quantities of water for public and industrial use to the surrounding conurbations. The Environment Agency is currently funding a major review of the aquifer to establish a water management policy that will ensure the sustainability of this important resource (Entec, 1998).

There are 13 public supply abstraction sites licensed in the Worfe catchment and four in the Penk/Smestow catchment. All are operated either by Severn Trent Water or South Staffordshire Waterworks Company. Most of the water abstracted is supplied to Wolverhampton and Telford for public use. Abstraction for industrial purposes is restricted to the Wolverhampton area but has reduced as heavy industry has declined. In contrast, demand in rural areas has increased across the predominantly arable farmland of the Shropshire and Staffordshire borders where spray irrigation is used increasingly during summer months. (Table 9) summarises groundwater abstraction and usage based on data provided by the Environment Agency.

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 (Fletcher, 1989). (Table 10) summarises the hydrogeological properties of the individual lithological units.

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, such as those at Stableford, yielding 160 litres per sec (l/s). Other public supply sites in the Worfe catchment (Grindleforge, Neachley, Lizard Mill and Cosford) typically produce between 100 and 115 l/s.

Groundwater levels

Regional contour plots (for example Entec, 1998) show that groundwater levels are generally a subdued reflection of surface topography. Levels are high beneath watershed divides and beneath the topographically higher ground of the surrounding Carboniferous coal basins where recharge is greatest. Gradients are generally flat within the basin but steepen rapidly towards the basin margins, and against major faults. The north-south-trending Pattingham Fault is associated with a spring line and has a significant head difference across it of 21 to 27 m (Fletcher, 1994). A similar situation exists across the Bushbury Fault, where groundwater levels are 30 m lower on the basin ward side of the fault (Allen et al., 1997). Impermeable faults to the west and east of Bishton Farm Borehole, near Albrighton were thought to account for unexpectedly low test yields in this area; similarly, the analysis of test pumping at Nurton Hill Farm indicated the presence of an isolated fault-bounded block (Fletcher, 1989).

Long-term abstraction has resulted in drawdown and gradual lowering of the water table in parts of the Permo-Triassic basin. The impact is most apparent in the upper Worfe valley where the water table now lies below the natural stream level. When first constructed, public supply boreholes in the valley were artesian. The decline in levels is illustrated by the hydrograph for the Lizard Wood No 1 Borehole (Figure 18b). Similar trends are shown in hydrographs of other observation boreholes in the Worfe valley. Many borehole hydrographs for the southern and eastern parts of the district show a seasonal water level variation of up to 2 or 3 m, but often less than 1 m, with no discernible long-term trend.

Water level responses to periods of drought tend to be muted although recharge events in 1976, 1992 and 1996 are observable on the hydrograph at Nurton Hill Farm (Figure 18c).

Hydrochemistry

The 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, as illustrated by the majority of analyses in (Table 11). There is evidence for more saline waters at Copley, Stableford, Somerford and Cosford, suggesting that up-coning of deeper saline waters may occur locally, possibly enhanced by flow along major adjacent faults.

Nitrate concentrations are variable but are high in the upper Worfe, suggesting a significant agricultural input in this area (Entec, 1998).

Other strata

There are no public supply boreholes outside the Permo-Triassic outcrop but a number of boreholes abstract water from the Upper Carboniferous formations for industrial use. The majority draw from the Salop Formation (Enville Member), with only a few penetrating the Middle Coal Measures. Yields are rarely reported but in the Coalbrookdale area they appear to be in the range 0.6 to 1.9 l/s for very variable amounts of drawdown. In the South Staffordshire Coalfield, boreholes of 150 to 250 mm diameter generally yield 0.6 to about 6 l/s, with specific capacities almost invariably less than 1 l/s/m. The highest reported yield was obtained from a 250 mm-diameter borehole penetrating the Enville Member at Park Lane, where a yield of 12.6 l/s was obtained for a drawdown of about 16 m.

Shallow groundwater in the Coal Measures and the succeeding red measures is generally of good quality and of calcium bicarbonate type, but chloride and sulphate concentrations are greater than in Triassic sandstone groundwaters. Few full analyses are available but that from Springfield Brewery (Table 11) is reasonably typical, although the chloride and sulphate concentrations may be slightly elevated. Minewaters tend to be rich in sulphate and iron due to the oxidation and dissolution of pyrites. Elevated metal and chlorinated solvent concentrations, sometimes found in Coal Measures groundwater in the south-east of the district, reflect industrial contamination (Bridge et al., 1997).

Small supplies of groundwater are present within the Mercia Mudstone, where flow is dependent on fractures in sandstone beds. Such sources are of considerable importance to the agricultural community in some central and northern parts of the district. The majority of boreholes are of narrow diameter (130 to 150 mm) and generally less than 60 m deep and there are some shallow large-diameter wells. Yields generally range from 0.2 to 1.3 l/s, with an exceptionally high yield of 5 l/s from a 180 m-deep borehole at Langley. Drawdowns are highly variable and specific capacity values only occasionally exceed 0.2 l/s/m. Failure to penetrate a productive sandstone horizon is likely to result in a very low yield. The thin sandstone horizons have small crops and recharge is consequently limited. Yields may decline with time as storage is depleted by pumping.

Little information exists on the chemistry of Mercia Mudstone groundwater in the district but it is generally potable although very hard, with sulphate concentrations often in excess of 100 mg/l.

Superficial deposits

The majority of wells drawing water from the superficial deposits utilise the fluvioglacial gravels along the valley of the River Penk and its tributary, the Church Eaton Brook. A group of boreholes also abstract from the glaciofluvial sands and gravels in the Wednesfield area, in the extreme south-east of the district. Yields commonly range up to about 1 l/s in the north, from boreholes up to 12 m deep and 150 mm in diameter. The boreholes in the Wednesfield area are larger, up to 380 mm in diameter and penetrate to a depth of about 15 m. Only two yields are recorded in this area, 2.3 and 0.8 l/s for drawdowns of 12.4 and 9.5 m, respectively. The limited data available suggest that the groundwater is potable, with chloride concentration of less than 40 mg/l, but is highly vulnerable to contamination.

Groundwater vulnerability

The main source of information on the potential for groundwater pollution is a series of Groundwater Vulnerability Maps published at 1:100 000 scale by the Environment Agency. Using a combination of geological and soil information, they give a broad indication of areas where diffuse or point-source pollutants may have a significant detrimental impact on groundwater quality. The major vulnerable aquifers in the district are the sandstones and conglomerates of the Sherwood Sandstone Group and Bridgnorth Sandstone, which support large abstractions for public supply. The distribution of these aquifers is indicated on Groundwater Vulnerability Map Sheet 22 (Environment Agency, 1997).

Mineral resources

The distribution of mineral resources of current or potential interest in the district is set out in two reports in the series Mineral Resource Information for Development Plans (Bloodworth et al., 1998; Highley and Cameron, 1995), which cover Shropshire and Staffordshire, respectively. The mineral resources of the Coalbrookdale Coalfield are also reviewed in a paper by Hamblin et al. (1989). The following sections concentrate mainly on developments that have occurred since these reports were published.

Opencast Coal

Opencast activity in the Coalbrookdale Coalfield is confined to the area south of the River Severn, where seams in the Lower Coal Measures (Little Flint to New Mine coals) crop out beneath thin overburden. The coals are thin, mostly bituminous and dull, and with a high ash content. At Caughley, the only site still operational in late 1999, there is an associated production of fireclay. Opencast coal production in Shropshire has shown a decline in the last five years, with output falling from around 250 000 tonnes/annum to under 75 000 tonnes. Accessible resources are now largely depleted and the potential for future developments is limited.

In Wolverhampton, a number of small opencast operations have been carried out as part of a programme of brownfield-site regeneration; the largest of these was the redevelopment of the former Bowmans Harbour site [SO 937 995].

Deep mined coal

Coal together with associated ironstone and fireclay seams have been mined in the exposed South Staffordshire coalfield since medieval times. Most of the named seams have been worked, apart from the Upper Sulphur Coal, which is thin and of poor quality. To the north of the Bentley Faults most of the seams down to the level of the Deep Coal have apparently been worked. South of the Bentley Faults, the most extensive workings are in the Thick Coal, New Mine Coal and Fireclay Coal seams, where the ‘pillar and stall’ method of extraction was favoured for the thicker seams, and the longwall method for the thinner coals and ironstones. Workings are comparatively shallow, extending from close below surface in the Bowmans Harbour area, to a maximum depth of around 60 m above OD.

Underground mining of coal ceased in the Coalbrookdale Coalfield (at the Granville Colliery) in 1979, and in the concealed Cannock Chase Coalfield (at the Littleton Colliery) in 1993. The coals are ranked as dominantly high-volatile, very weakly caking to weakly caking types (Wandless, 1960). Seismic profiles and deep drilling have demonstrated the continuity of the Coal Measures beneath the Stafford Basin, and some are condensed over basement highs (p.12). The measures reach their maximum depth of about 1000 m below OD close to the northern boundary of the district.

Coalbed methane and mine gas

The down-dip continuations 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 1973, some 115 m3/minute of methane was being obtained from boreholes driven upwards from the Double Coal (Hamblin et al., 1989). Statistics on gas content of the South Staffordshire Coalfield (Littleton area) were summarised by Creedy (1991); a mean methane level of 3.3 m3/ton was quoted, based on 119 samples from a mean depth of 775 m. The overall prospectivity of the coalfields in relation to others in the region was reviewed by Glover et al. (1993).

Hydrocarbons

Two deep wells were drilled by Shell UK in 1984 to test the oil and gas prospectivity of potential plays beneath the Stafford Basin. The Codsall Borehole was drilled on the flanks of a post-Silurian-pre-Carboniferous anticline, between the Breward Fault and several antithetic faults; it yielded only a tarry residual oil show in basal Coal Measures. The Heath Farm Borehole, located on a high to the east of the Breward Fault, was drilled to investigate a possible play in Dinantian limestone. This too proved dry.

In the Coalbrookdale Coalfield, bitumen is widely recorded as impregnations in sandstones of Langsettian to Westphalian D age. The Coalport Tar Tunnel [SJ 6948 0254] is the most celebrated of these occurrences, at its peak yielding 4500 l/week (Prestwich, 1840, p.438). The material is a heavy asphaltic oil of fairly low sulphur content and negligible wax content.

Clay and fireclay

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 [SJ 684 118], Donnington [SJ 710 114] and Caughley [SO 697 998]. The New Hadley operation, operated by Blockley Brick, manufactures facing bricks and pavers on-site; the raw material from the other sites is transported to other brick factories in the Midlands. Much of the Etruria outcrop in the Coalbrookdale Coalfield has been sterilised by urban development. Workings on the edge of the South Staffordshire Coalfield mainly lie outside the district but, recently, permissions have been granted for new workings close to the M6 motorway.

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, and are currently worked at the Caughley site.

Sand and gravel

Glaciofluvial sand and gravel

Deposits of glaciofluvial sand and gravel, although widely mapped, have only supported minor workings in the past. Rapid variations in thickness, high sand-to-gravel ratio, and compositional variability are factors that reduce their value as a potential resource. The main occurrences are in the Newport Esker Chain, in the Worfe valley, and in the buried valleys in the west of the district. Glaciofluvial sheet deposits are identified as a separate category on the map; their deposition is inferred to have occurred beyond the ice margin, thus raising the possibility of a deposit better sorted by running water, with a reduced content of fines. Spreads occur in the lower part of Whiston Brook and along the River Penk, but their potential is unknown.

Four Ashes Sand and Gravel

Preglacial sands and gravels within a palaeovalley beneath Devensian till were formerly worked along Saredon Brook. Any accessible reserves are presumed to be exhausted.

River Terrace Deposits

Fragmentary terrace deposits of sand and gravel occur in the upper Worfe valley. It is unlikely that the volumes present would be sufficient to be considered a viable resource. The terrace deposits of the River Severn are not well developed in the Ironbridge Gorge area, but those downstream in the Dudley district have supported workings in the past.

Bedrock deposits

Conglomeratic beds within the lower part of the Kidderminster Formation have been worked in quarries at Essington [SJ 943 041], Saredon Hill [SJ 945 079] and latterly, at Manor Farm [SJ 949 035], in the eastern outcrop. Although reserves at these sites are exhausted, the eastern outcrop remains an important aggregate resource, with deposits in excess of 60 m thick.

There are no current workings on the western outcrop, although sand and gravel was formerly extracted at the Burlington Gravel Pit [SJ 718 1228]; [SJ 708 1184]. Extraction ceased when adequate yields could only be achieved by blasting.

Moulding sand

The Wildmoor Sandstone was formerly worked as a source of moulding sand in the Compton [SO 88 98] and Gorsebrook Road [SJ 910 004] areas of Wolverhampton but production has now ceased. It was also dug at Ruckley Grange [SJ 773 070], from where it was transported by rail to Coalbrookdale (Bridge, 1998).

Building stone

Most of the thicker sandstones within the Triassic and Carboniferous sequences have been used as local sources of building stone, mainly on a small scale. The large number of old workings in the Bromsgrove Sandstone testifies to its former importance. It has been used extensively in the Penkridge area (for example, Penkridge Church), where it was locally termed the ‘Penkridge Stone’ (Whitehead et al., 1928). A quarry at Park Lane [SJ 888 067] was the probable source for many of the bridges over the Shropshire Union Canal. Further main quarries in the formation, all currently disused, are at Great Chatwell [SJ 795 148], Weston Park [SJ 806 094], Tong [SJ 798 075], Cosford Brook [SJ 782 046], and Badger Dingle [SO 763 991].

Conservation

English Nature protect a network of Sites of Special Scientific Interest (SSSIs), of important research and educational value. The designated SSSIs with a strong geological interest include Lincoln Hill [SJ 668 038], Four Ashes Pit [SJ 915 083] and New Hadley Brickpit [SJ 684 118]. The district also includes the World Heritage Site of the Severn Gorge. There is also a national nature reserve at Mottey Meadows. The locations of these and other National Nature Reserve Sites are indicated on Map 6 West Midlands, published at 1:200 000 scale by English Nature.

Information sources

Further geological information held by the British Geological Survey relevant to the Wolverhampton district is listed below. It includes published material in the form of maps, memoirs and reports and unpublished material, including maps, reports and other sources of data held by BGS in a number of collections. Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries or through the Web site http://www.bgs.ac.uk/.

Maps

Geology maps

Geophysical maps

Contamination

Hydrogeological maps

Books

British Regional Geology

Memoirs

Technical reports

Technical reports relevant to the district are arranged below by topic. Most are not widely available, but may be purchased from BGS or consulted at BGS and other libraries.

Geology

(Table 12) shows the reference number for the technical reports covering the geology of individual or combined 1:10 000 scale geological sheets.

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

Biostratigraphy

There is a collection of British Geological Survey internal biostratigraphical reports, details of which are available from the Biostratigraphy Group, Keyworth.

Sedimentology

Knox (1996) provided information on heavy minerals from the Upper Carboniferous of the Four Ashes Borehole.

Documentary collections

Boreholes and shafts

Borehole and shaft data for the district are catalogued in the BGS archives (National Geosciences Records Centre) at Keyworth on individual 1:10 000 scale sheets. For the Wolverhampton district the collection consists of the sites and logs of about 14 000 boreholes, for which index information has been digitised. For further information consult the BGS Web site at http://www.bgs.ac.uk/ or contact: The Manager, National Geosciences Records Centre, British Geological Survey, Keyworth, Nottingham NG12 5GG. Boreholes mentioned in the text and selected boreholes in, and adjacent to, the district are listed alphabetically in (Table 13), together with their National Grid Reference and BGS registered number.

Mine plans

BGS maintains a collection of plans of underground mines for minerals other than coal, mostly for sandstone and fireclay.

Geophysics

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. The profiles are from surveys by Shell and British Coal. Geophysical logs are available for the coal and hydrocarbon exploration boreholes.

Hydrogeology

Data on water boreholes, wells and springs and aquifer properties are held in the BGS (Hydrogeology Group) database at Wallingford.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheet 153 Wolverhampton are held in the Lexicon database. Information on the database can be obtained from the Lexicon Manager at BGS Keyworth. The database can also be consulted on the BGS Web site: http://www.bgs.ac.uk.

Material collections

Palaeontological collection

Macrofossils and micro-palaeontological samples collected from the district are held at BGS Keyworth. Enquiries concerning all the macrofossil material should be directed to the Curator, Biostratigraphy Collections, BGS Keyworth.

Petrological collections

Hand specimens and thin sections are held in the England and Wales Sliced Rocks collection at BGS Keyworth. A collection database is maintained by the Mineralogy and Petrology Group at BGS Keyworth. The Group Manager should be contacted for further information, including methods of accessing the database. Charges and conditions of access to the collection are available on request from BGS Keyworth.

Borehole core collection

Samples and core from a small number of boreholes in the Wolverhampton district are held by the National Geosciences Records Centre, BGS, Keyworth

BGS (Geological Survey) photographs

Copies of photographs used in this report are deposited for reference in the BGS Library, Keyworth. Colour or black and white prints and transparencies can be supplied at a fixed tariff.

Other relevant collections

Coal abandonment plans

Coal abandonment plans are held by The Coal Authority (for address see below).

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and extent of wash lands and licensed landfill sites are held by the Environment Agency.

Earth science conservation sites

Information on the Sites of Special Scientific Interest present within the Wolverhampton district is held by English Nature.

Addresses for data sources

References

Most of the references listed below are held in the libraries of the British Geological Survey at Murchison House, Edinburgh and at Keyworth, Nottingham. Copies of the references can be purchased from the Keyworth office subject to the current copyright legislation.

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ALI, A D, and TURNER, P. 1982. Authigenic K-Feldspar in the Bromsgrove Sandstone Formation (Triassic) of Central England. Journal of Sedimentary Petrology, Vol. 52, 187–197.

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APPLETON, J D, and BALL, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and relevance to planning in Great Britain. British Geological Survey Technical Report, WP/95/2.

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BENTON, M J, WARRINGTON, G, NEWELL, A J, and SPENCER, P S. 1994. A review of the British Middle Triassic tetrapod assemblages. 131–160 in In the Shadow of the Dinosaurs, Fraser, N C, and Sues, H -D (editor) [Cambridge: Cambridge University Press.]

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. 1995. Lithostratigraphy and macrofloral biostratigraphy of the Late Carboniferous of the Midlands and Oxfordshire coalfields. British Geological Survey Technical Report, WH/99/81R.

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

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BESLY, B M, and TURNER, P. 1983. Origin of red-beds in a moist tropical climate (Etruria Formation). 131–147 in Residual Deposits. Wilson, R C L (editor). Special Publication of the Geological Society of London, No. 11.

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BR211. 1999. Radon: guidance on protective measures for new buildings. CRC Ltd.

BRACEWELL, S. 1925. Notes on the Lower Carboniferous rocks of the Wrekin district. Proceedings of the Geologists’ Association, Vol. 36. 389–393.

BRADFIELD, K E S, and TUCKER, E V. 1986. A section in Linley Brook (Shropshire) showing a previously unrecorded junction between the Ludlow and Prídolí Series. Geological Journal, Vol. 21, 375–380.

BRASIER, M D. 1980. The Lower Cambrian transgression and glauconite-phosphate facies in western Europe. Journal of the Geological Society of London, Vol. 137, 695–703.

BRIDGE, D MCC. 1997. Geology of the district between Wolverhampton and Penkridge (SJ90NW, SJ90SW, SJ91SW and parts of SJ90NE, SE, SJ91NW, NE, SE). British Geological Survey Technical Report, WA/96/82.

BRIDGE, D MCC. 1997. Geology of the Shifnal, Blymhill and Bishop’s Wood areas. British Geological Survey Technical Report, WA/97/26.

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CLAYTON, K M. 1977. River terraces. 153–167 in British Quaternary studies: recent advances SHOTTON, F W (editor). (Oxford: Clarendon Press.)

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.

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.

EDMUNDS, W M, and MORGAN-JONES, M. 1976. The geochemistry of groundwaters in the British Triassic Sandstones: the Wolverhampton-east Shropshire area. Quarterly Journal of Engineering Geology, Vol. 9, 73–101.

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.

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Figures, plates and tables

Figures

(Figure 1) Solid geology of the Wolverhampton district.

(Figure 2) Topography and physical features of the district.

(Figure 3) Structure map of the district and surrounding area.

(Figure 4) Composite section of the Upper Ludlow Shales and Prídolí rocks exposed in Linley Brook [SO 6845 9805] to [SO 6860 9807] and the old quarry at Willey Park Hall [SO 6731 9912] based partly on Bradfield and Tucker (1986) and White and Coppack (1977).

(Figure 5) Dinantian sequence at Lilleshall.

(Figure 6) Coal Measures: selected sections from the South Staffordshire and Coalbrookdale coalfields.

(Figure 7) Coal Measures: selected sections from the Cannock Chase Coalfield.

(Figure 8) Coal Measures: selected sections from the Coalbrookdale Coalfield. See (Figure 6) for key.

(Figure 9) Isopach map of the Etruria Formation (based partly on Hamblin and Coppack, 1995).

(Figure 10) Halesowen and Salop formations: selected sections showing lithological variation and gamma-ray signatures.

(Figure 11a) Structure contours and thickness data for the Permo-Triassic rocks of the Stafford Basin. Contours (metres OD) on the base of the Bridgnorth Sandstone-Kidderminister Formation

(Figure 11b) Structure contours and thickness data for the Permo-Triassic rocks of the Stafford Basin. Isopachs of preserved thickness (in metres) of the Bridgnorth Sandstone Formation

(Figure 11c) Structure contours and thickness data for the Permo-Triassic rocks of the Stafford Basin. Thickness of the Kidderminister Formation (in black) and the Wildmoor Sandstone Formation (in red). Underlined figures indicate that the base was proved

(Figure 12) Geophysical logs (natural gamma-ray and sonic) of the Permo-Triassic succession in the Stretton Borehole.

(Figure 13) Glacigenic landform associations.

(Figure 14) Schematic cross-section of glacigenic landform associations.

(Figure 15) Lower Palaeozoic and Precambrian stratigraphy beneath the Stafford Basin based on seismic interpretation.

(Figure 16) Sub-Carboniferous map of the major units.

(Figure 17) Structure map of the district.

(Figure 18a) Groundwater hydrograph. Wolverhampton Gasworks

(Figure 18b) Groundwater hydrograph. Lizard Wood No.1

(Figure 18c) Groundwater hydrograph. Nurton Hill Farm

Plates

(Front cover) Retaining wall, Queensway, near Telford [SJ 700 105]. Geological section created in tiles from Lancashire (MN39536).

(Plate 1) Etruria Formation-Halesowen Formation junction, Caughley Brickpit [SO 697 998] (GS949).

(Plate 2a) Enville Member, Salop Formation, M54 motorway cutting [SJ 738 090]. Channel bedforms (GS955).

(Plate 2b) Enville Member, Salop Formation, M54 motorway cutting [SJ 738 090]. Imbricate conglomerate in channel base (GS956).

(Plate 3) Aeolian dune bedding in the Bridgnorth Sandstone, Brimstree Hill [SJ 7512 0582] (GS953).

(Plate 4a) Kidderminister Formation: Saredon Hill Quarry [SJ 945 080]. Well cemented pebble conglomerate and interbedded pebbly sandstones (GS951).

(Plate 4b) Kidderminister Formation: Saredon Hill Quarry [SJ 945 080]. Bedform detail (GS952).

(Plate 5) Wildmoor Sandstone, The Rock, Wolverhampton [SJ 8895 0010] (GS950).

(Plate 6) The Bromsgrove Sandstone overlying the Wildmoor Sandstone (beneath the overhang) Badger Dingle [SO 7631 9911] (GS954).

(Plate 7) Remediation of Bowmans Harbour Site, Wolverhampton [SO 935 995]: coal extraction and ground stabilisation in the foreground and preparation of till-lined landfill repository in the background (GS957).

(Back cover)

Tables

(Table 1) Geological succession of the Wolverhampton and Telford district.

(Table 2) Classification of Silurian strata. †Not present at outcrop; inferred from seismic profiles or geophysical logs of deep (uncored) boreholes.

(Table 3) Seam and interseam variation in the Cannock Chase Coalfield.

(Table 4) Seam and interseam variation in the Coalbrookdale Coalfield.

(Table 5) Nomenclature used for the Severn terraces in the Apley Park section.

(Table 6) Constraints on development.

(Table 7) Engineering geology of the bedrock formations. ‡ Manual of contract documents for highway works, 1991. § Pettifer and Fookes, 1994. * BS 5930, 1999

(Table 8) Engineering geology of the superficial deposits. † Soils trafficability, 1959. ‡ Manual of contract documents for highway works, 1991. § Pettifer and Fookes, 1994. * BS 5930, 1999

(Table 9) Licensed abstraction data for the Wolverhampton district (derived from data provided by the Environment Agency, Midlands Region). Bracketed numbers indicate the number of licences in any particular category

(Table 10) Hydrogeological properties of the Permo-Triassic sandstone formations

(Table 11) Typical chemical analyses of groundwaters in the district.

(Table 12) Component 1:10 000 scale maps, technical reports, and survey details of Sheet 153 Wolverhampton. * denotes availability in digital format

(Table 13) Boreholes and shafts referred to in the text. * denotes availability in digital format

Tables

(Table 3) Seam and interseam variation in the Cannock Chase Coalfield

Named horizon (coals in bold)

Details

Middle Coal Measures

Mudstones, with inferior named coals of uncertain correlation; variable thickness beneath diachronous base of Etruria Formation

Shafton (Sylvester’s Bridge) Marine Band

Proved in Saredon Hill Borehole

Top Robins

1.8–2.5 m with dirt partings; 1.3 m thick around Hilton Main

Edmondia Marine Band

Up to 30 m, mainly mudstone and seatearth with several thin coals

Proved in Saredon Hill, Plantation and Four Ashes boreholes; thins towards the south

Bottom Robins

2–2.5 m in the western parts of the coalfield; thins to 0.7 m over Brinsford axis

Mainly dark grey mudstone with a tonstein (Hilton Main workings)

Wyrley Yard

1–1.5 m at Hilton Main but thins northwards

Aegiranum Marine Band

Predominantly grey mudstones 5–15 m in Huntington Syncline

Fully marine fauna recorded in Hilton Main Colliery (Mitchell, 1945)

About 40 m in the Hilton Main Fold Belt with up to 5 thin irregularly developed coals and sheet sandstones towards top; thins across Brinsford Axis

Brooch

0.8–1.3 m; good quality coal

Maltby (Sub-Brooch) MB

Occurs 3–4 m below Brooch Coal; thin but persists throughout coalfield

24 m of beds in Huntington Syncline

Benches

Two leaves (0.3 and 0.7 m) in Hilton area; leaves combine northwards

15–17 m of silty mudstones passing up into siltstones and sandstones

Eight Feet

2 m, locally increasing to 2.5 m; 2 or more thick dirt partings around Ashmore Park

Mudstone-seatearth lithologies with some thin coals (as at Calf Heath); up to 22 m

Old Park

2 leaves (1.5, 1.0 m) at Hilton Main separated by 10–12 m of dark mudstone and seatearth; single seam (0.7–0.9 m) elsewhere

Up to 55 m of measures including 4 coals which die out laterally

Heathen

0.7 m but washed out locally

Vanderbeckei Marine Band

Dark grey mudstones with abundant nonmarine bivalves

2.7 m reducing to 0.3 m over Brinsford Axis: fauna includes Lingula mytiloides and foraminifera, together with sponge spicules, fish remains and conodonts

Lower Coal Measures

Stinking

Split into 2 leaves, 0.2, 0.5 m thick. Both washed out locally in the vicinity of Calf Heath by a thick sequence of stacked channel sandstones

10–17 m of measures comprising dark grey mudstone, passing up into siltstone and sandstone

Yard

0.7–1 m in Hilton Main area, thinning to 0.7 m at Calf Heath

9–14 m of measures. Sandstones developed locally and irregularly

Bass

Less than 0.4 m and may be split; the Bass Rider lies some 5 m above the main seam

Sandstones, pale and coarse-grained in Plantation and Calf Heath boreholes

New Mine

Split locally into 3 leaves; middle leaf (1.3 m) is best developed in a limited area to the east of Hilton Main

Dark grey mudstone in roof of Shallow; overlying beds (seatearths and mudstones ) are thickest 15 m around Hilton Main but thin on to the Brinsford Axis and northwards

Shallow

One of the thickest and most important seams in the district (2.7 m in Holly Bank No. 5 Pit)

Thin clastic unit in the area of the Brinsford Axis; farther north up to 20 m of mudstone and seatearth intervenes

Deep

1.2–1.5 m ; Deep and Shallow coals unite in the area to the north of the Hilton Main Shafts

34 m of beds including 2 thin coals

Mealy Grey

0.9–1.2 m

(Table 4) Seam and interseam variation in the Coalbrookdale Coalfield

Named horizon (coals in bold)

Details

Middle Coal Measures

Aegiranum Marine Band

Up to 40 m of grey measures in north-east of coalfield (Childpit Lane Borehole), and including 5 coals beneath diachronous base of Etruria Formation

Dark grey mudstone with Lingula, productids, bivalves and fish remains (Lilleshall No. 2 Borehole, New Hadley, Wombridge Water Engine Pit)

8.3–24.6 m comprising: Fungous Coal Rock (grey-buff sandstone which locally washes out Fungous to Deep Coals); Chance Pennystone Ironstone (4.6 m); Chance Pennystone Rock (0.6–1.6 m), and up to three thin coals

Fungous

0.1–1.3 m: good quality coal, widely worked, no partings, commonly washed out

Maltby (Blackstone) MB

Dark, fissile, micaceous mudstone with well preserved Lingula and fish remains; persistent throughout the area; overlain by grey mudstones with local ironstone nodules (Ragged Robins Ironstone)

Blackstone

0.2–0.6 m: not worked, Blackstone Ironstone below

Deep

0.7–1.9 m: not widely worked

Gur

0.7–0.8 m: not worked, locally joined with Deep

16–40 m of measures including two workable ironstones (Ballstone and Brickmeasure), separated by black shales and locally a thin poor coal; over a wide area this sequence is cut out by a major sandstone washout up to 30 m thick

Top

0.6–2.1 m: good quality coal, widely worked, no partings

Grey to black fireclays and mudstones, locally with ironstones and and a basal black shale; a thin coal (Foot) occurs locally in an area from Madeley to Ketley

Three Quarter Double

0.2–2.8 m: little worked; ganister (Double Coal Rock) below

Mudstones including Yellowstone Ironstone, normally 1–3 m; washout aligned along axis of Madeley Syncline

Yard

0.6–1.4 m: good quality, consistent thickness, widely worked

Mudstones including Blue Flat Ironstone; up to 11 m but variable and largely absent south of the River Severn; White Flat Coal (0–0.3 m) present locally

Big Flint

0.3–1.8 m: two thin leaves in Madeley Syncline, otherwise widely worked

Vanderbeckei (Pennystone) Marine Band

Big Flint Rock (4–9 m on average), widest ranging sandstone in the middle part of the Coal Measures

Mudstones including Pennystone Ironstone. Pennystone Marine Band at base up to 6 m thick with Lingula and rare spirifers

Lower Coal Measures

New Mine

0.3–2.1 m: sulphurous, commonly parted, widely worked

Viger Coal Rock (up to 4.9 m) in Madeley Syncline, otherwise fireclays

Clunch (or Sill) Viger of Madeley Syncline

0.3–1.4 m: worked extensively only where thick; elsewhere fireclays above and below are mined in preference

Fireclays (typically 1–3 m), widely worked

Two Foot, Ganey of Madeley Syncline

0–1.2 m: good quality coking coal, widely worked; represesented by two or three coals in the Madeley Syncline (Main or Lower Ganey, Little Ganey, Upper Ganey or Two Foot)

Sandy fireclays and lenticular sandstone (Best Coal Rock)

Best, Randle, Clod

0.1–2 m, 0.1–1.2 m, 0.1–1.5 m: good coals, widely worked, separated by thin clay partings

Sandstone (Little Flint Coal Rock), white, fine-grained, massive, 5–12 m thick

Little Flint

0.2–1.2 m: generally the lowest worked coal, widely mined, sandstone roof and sandy fireclay floor

?Amaliae (Crawstone Marine Band)

Sandstone (Little Flint Rock) 0.9–4.6 m

Crawstone Ironstone

Crawstone

0–0.3 m: not worked

Sandstone (Crawstone Flint) 2.7–4.7 m , pale grey to brown, pebbly in lower part

Lancashire Ladies

Rarely worked; a single seam (0.1–0.5) throughout most of the coalfield; two coals recorded north-east of Granville

?Listeri (Lancashire Ladies) Marine Band

Orbiculoidea and Lingula found in grey mudstone above Lower Lancashire Ladies Coal (Lilleshall No. 7A)

Sandstones and grits (Farewell Rock)

(Table 6) Constraints on development

Constraint

Area of concern

SLOPE INSTABILITY. Landslip, slope failure, rockfall

Ironbridge Gorge, Linley Brook

MAN-MADE LAND INSTABILITY

Exposed coalfields

Subsidence over abandoned mine workings

Former underground limestone workings (Lilleshall)

Collapse of abandoned mineshafts

Lincoln Hill

Subsidence caused by consolidation of uncompacted fill

Former clay workings

Subsidence caused by consolidation of organic materials within landfill sites

Landfill sites

HEAVE AND SETTLEMENT

Generally not a problem on natural substrates; some settlement associated with peat; Coal Measures may include shrink-swell clay horizons; expansion and heave a potential problem on some types of made ground, for example colliery spoil

NATURAL CONTAMINATION. Radon, methane, carbon dioxide

Potential for high levels of radon on Carboniferous limestones, moderate levels on coalfields, and generally low levels on Permo-Triassic rocks. Surface emissions of methane recorded at Halesfield Colliery [SJ 710 040]

Oil seeps

Tarpatch Dingle [SJ 690 030], Coalport [SJ 694 027], Coalport Tar Tunnel [SJ 690 020], Caughley Opencast [SJ 690 000]

INDUSTRIAL CONTAMINATION

Present in former industrial areas

FLOOD RISK

Low-lying ground adjacent to the River Severn, Local flooding related to culverted streams in built-up areas. Rising groundwater levels in Wolverhampton

NATURAL RESOURCES

Water

Aquifer protection; over-abstraction (Worfe valley), enhanced nitrate levels

Minerals

Sterilisation of potential resources; inadequate information on distribution and composition of industrial minerals, particularly brick clay and aggregate resources

(Table 10) Hydrogeological properties of the Permo-Triassic sandstone formations

Component formation

Hydraulic conductivity

Porosity %

Comment

Bromsgrove Sandstone

2.4 x 10−4 to 16 m/d

8–38

Sandstones display marked anisotropy

Wildmoor Sandstone

3 x 10−4 to 10 m/d

20–30

Sandstones relatively isotropic; primary water transmission is intergranular

Kidderminster Formation

4.6 x 10−6 to 18 m/d

4–30

Lowest hydraulic conductivities are associated with better cemented horizons

Bridgnorth Sandstone

2.5 x 10−4 to 9.4 m/d

17–34

Poorly cemented, highly permeable; intergranular hydraulic conductivity, grainsize dependent

(Table 13) Boreholes and shafts referred to in the text. * denotes availability in digital format

1:10 000 sheet number

Borehole number

Borehole reference

Name

Grid Reference

SJ91NW

15

(SJ91NW/15)

Ashflats

[SJ 9177 1935]

SJ90NW

6

(SJ90NW/6)

Aspley

[SJ 9217 0736]

SJ91SW

41

(SJ91SW/41)

Bangley

[SJ 9444 1398]

SJ70SE

4

(SJ70SE/4)

Beckbury P.S. No 1

[SJ 5720 0152]

Bishton Farm

[SJ 8030 0170]

SJ90NW

4

(SJ90NW/4)

Brinsford

[SJ 9258 0614]

SO99NW

36

(SO99NW/36)

Butler’s No 2 (Springfield Brewery)

[SO 9192 9939]

SJ90NW

7

(SJ90NW/7)

Calf Heath

[SJ 9466 0889]

SJ71NE

6

(SJ71NE/6)

Childpit Lane

[SJ 7560 1519]

SO99NW

27

(SO99NW/27)

Chillington

[SO 9309 9828]

SJ80NW

69

(SJ80NW/69)

Codsall

[SJ 8333 0537]

SO89NW

2

(SO89NW/2)

Copley Borehole

[SO 8080 9862]

SJ70SE

2

(SJ70SE/2)

Cosford P.S. No 2

[SJ 7807 0461]

SJ90SW

10

(SJ90SW/10)

Courtauld’s No 1

[SJ 9026 0024]

SJ90SW

111

(SJ90SW/111)

Courtauld’s No 2

[SJ 9049 0031]

SJ71NW

16

(SJ71NW/16)

Croft

[SJ 7284 1505]

SJ60SE

619

(SJ60SE/619)

Dean

[SJ 6799 0006]

SJ60NE

1337–1338

(SJ60NE/1337), (SJ60NE/1338)

Deepfield Pit

[SJ 6828 0638]

SJ80SE

16

(SJ80SE/16)

Dunstall Hall Works

[SJ 8998 0032]

SJ80SE

17

(SJ80SE/17)

Dunstall Hall Works

[SJ 8981 0030]

SO99NW

686

(SO99NW/686)

Ettingshall Lodge

[SO 9330 9650]

SJ90SW

109

(SJ90SW/109)

Fallings Park

[SJ 9266 0024]

SJ90NW

10

(SJ90NW/10)

Far Laches

[SJ 9360 0657]

SJ90NW

8

(SJ90NW/8)

Four Ashes

[SJ 9153 0954]

SJ90SW

107

(SJ90SW/107)

Goodyear Tyre Co.

[SJ 9122 0162]

SJ71SW

10

(SJ71SW/10)

Granville Colliery No 2

[SJ 7254 1205]

SJ90SW

8

(SJ90SW/8)

Great Western Railway

[SJ 9044 0128]

SJ70SE

1

(SJ70SE/1)

Grindleforge

[SJ 7524 0348]

SJ70SE

6

(SJ70SE/6)

Hatton Grange

[SJ 7608 0425]

SJ90NW

49

(SJ90NW/49)

Heath Farm

[SJ 9332 0925]

SJ71SE

7

(SJ71SE/7)

Hilton Bank, No 2

[SJ 7663 1256]

SJ90SW

3

(SJ90SW/3)

Hilton Main No 1

[SJ 9410 0436]

SJ90SW

4

(SJ90SW/4)

Hilton Main No 2

[SJ 9403 0434]

SJ90NW

179

(SJ90NW/179)

Hilton Park Colliery No 1

[SJ 9334 0502]

SJ90SE

19

(SJ90SE/19)

Holly Bank Colliery No 5 Pit

[SJ 9661 0341]

SJ90SE

1

(SJ90SE/1)

Holly Bank, Staple Pit

[SJ 9551 0437]

SJ81SW

4

(SJ81SW/4)

Hurst Farm

[SJ 8364 1158]

SJ70NW

209

(SJ70NW/209)

Kemberton Pit

[SJ 7129 0556]

SJ61SE

24

(SJ61SE/24)

Kinley Farm

[SJ 6716 1478]

Langley

[SJ 8458 0448]

SJ61SE

25

(SJ61SE/25)

Leegomery House Farm

[SJ 6638 1268]

SJ71NW

31

(SJ71NW/31)

Lilleshall L2

[SJ 7329 1602]

SJ71NW

33

(SJ71NW/33)

Lilleshall L6

[SJ 7358 1572]

SJ71NW

34

(SJ71NW/34)

Lilleshall L8

[SJ 7348 1569]

SJ71NW

63

(SJ71NW/63)

Lilleshall L17

[SJ 7331 1648]

SJ71NW

64

(SJ71NW/64)

Lilleshall L18

[SJ 7354 1639]

SJ71SE

2

(SJ71SE/2)

Lilleshall No 2

[SJ 7564 1429]

SJ71SE

4

(SJ71SE/4)

Lilleshall No 5

[SJ 7688 1095]

SJ71SE

1

(SJ71SE/1)

Lilleshall No 6

[SJ 7770 1395]

SJ91SE

8

(SJ91SE/8)

Littleton Colliery No 3

[SJ 9718 1289]

SJ70NE

73

(SJ70NE/73)

Lizard Mill No 1

[SJ 7851 0949]

SJ70NE

74

(SJ70NE/74)

Lizard Mill No 2

[SJ 7866 0952]

SJ70NE

(SJ70NE/)

Lizard Wood No 1

[SJ 7799 0934]

SJ61SE

26

(SJ61SE/26)

Lodge Farm, Trench

[SJ 6887 1297]

SJ91NW

19

(SJ91NW/19)

Lodgerail

[SJ 9459 1538]9459 1538]

SJ60SE

6–8

(SJ60SE/6), (SJ60SE/7), (SJ60SE/8)

Madeley Meadow Pit

[SJ 6900 0400]

SJ70NW

16

(SJ70NW/16)

Madeley Wood No 1

[SJ 7387 0876]

SJ70NW

12

(SJ70NW/12)

Madeley Wood No 2

[SJ 7311 0878

SJ70NE

1

(SJ70NE/1)

Madeley Wood No 5

[SJ 7500 0628]

SJ90NW

3

(SJ90NW/3)

Moat Farm

[SJ 9355 0720]

SJ90NW

106

(SJ90NW/106)

Motorway borehole

[SJ 9288 0733]

SJ90NW

123

(SJ90NW/123)

Motorway borehole

[SJ 9262 0926]

SJ90NW

124

(SJ90NW/124)

Motorway borehole

[SJ 9253 0982]

SJ90NW

125

(SJ90NW/125)

Motorway borehole

[SJ 9256 0962]

SJ90NW

126

(SJ90NW/126)

Motorway borehole

[SJ 9259 0947]

SJ90NW

183

(SJ90NW/183)

Motorway borehole

[SJ 9427 0879]

SJ91SW

187

(SJ91SW/187)

Motorway borehole

[SJ 9329 1367]

SJ90NE

242

(SJ90NE/242)

Motorway borehole

[SJ 9559 0729]

SJ80NW

1

(SJ80NW/1)

Motorway Borehole No 77

[SJ 8010 0674]

SJ80NW

2

(SJ80NW/2)

Motorway Borehole No 78

[SJ 8031 0669]

SJ80NW

3

(SJ80NW/3)

Motorway Borehole No 79

[SJ 8064 0659]

SJ80NW

4

(SJ80NW/4)

Motorway Borehole No 79b

[SJ 8086 0638]

SJ80NW

5

(SJ80NW/5)

Motorway Borehole No 80

[SJ 8082 0651]

SJ70NE

4

(SJ70NE/4)

Neachley No 2

[SJ 7837 0667]

SO89NW

6

(SO89NW/6)

Nurton Hill Farm

[SO 8318 9974]

SJ90NW

2

(SJ90NW/2)

Orchard Farm

[SJ 9459 0682]

SJ90SW

8

(SJ90SW/8)

Oxley Siding

[SJ 9044 0129]

SJ90SW

109

(SJ90SW/109)

Park Lane, Fallings Park

[SJ 9267 0021]

SJ90NE

93

(SJ90NE/93)

Plantation

[SJ 9547 0760]

So99NW

746

(So99NW/746)

Priestfield No 78 Pit

[SO 9441 9745]

SJ90NW

1

(SJ90NW/1)

Saredon Hill

[SJ 9498 0801]

SJ90NW

9

(SJ90NW/9)

Shareshill

[SJ 9388 0062]9

SJ70NE

70

(SJ70NE/70)

Shifnal

[SJ 7614 0777]

SJ80NE

1

(SJ80NE/1)

Somerford Pumping Station

[SJ 8964 0933]

SO79NE

10

(SO79NE/10)

Stableford No 1

[SO 764 981]

SJ81SE

13

(SJ81SE/13)

Stretton

[SJ 8756 1020]

SJ80SE

295

(SJ80SE/295)

Tettenhall Pumping Station

[SJ 8837 0009]

SJ90SW

106

(SJ90SW/106)

Whetstone Lane

[SJ 9174 0268]

SJ90SW

2

(SJ90SW/2)

Whitgreaves Wood

[SJ 9384 0487]

SJ90SW

333

(SJ90SW/333)

Wolverhampton Gas Works

[SJ 9163 0050]

SJ61SE

613

(SJ61SE/613)

Wombridge Water Engine Pit

[SJ 6920 1212]

SJ90NE

96

(SJ90NE/96)

Wood Lane

[SJ 9647 0848]

SJ60NW

7

(SJ60NW/7)

Wrekin Buildings

[SJ 3623 1087]