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Geological notes and local details for 1:10 000 sheets: SP38SE (Coventry North-East). Part of 1:50 000 sheet 169 (Coventry). Technical Report WA/88/51 Sheet SP38SE. British Geological Survey Technical Report WA/88/51

D Mc C Bridge

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Bibliographic reference: Bridge, D Mc C 1988 Geological notes and local details for 1:10 000 sheets: SP38SE (Coventry North-East).  (Keyworth: British Geological Survey)

Author: D Mc C Bridge, Bsc. British Geological Survey Keyworth Nottingham NG12 5GG

Production of this report was supported by the Department of the Environment, but the views expressed in it are not necessarily those of the Department.

© Crown copyright 1988. Keyworth, British Geological Survey

Summary

The geology of Sheet SP38SE (Coventry North-East) is described with particular emphasis on significant borehole and shaft sections.

New information on the Cambrian basement is presented, and the stratigraphy of the concealed coalfield (Westphalian A to D), and of the succeeding Triassic rocks is reviewed.

Details are given of the glacial drift and later Quaternary deposits.

Attention is drawn to the mineral resources of the area with particular reference to coal, ironstone and brick clay. A schedule of boreholes is appended.

Introduction

This report describes the geology of 1:10 000 Sheet SP38SE (Coventry North-East), (Figure 1). The area was first surveyed geologically on the one-inch scale by H H Howell and was included in Old Series One-Inch Sheets 53 and 63, published in 1855. A survey at the six-inch scale was carried out by T Eastwood in 1914 and is included in Sheet 169 (Coventry), published in 1922. The accompanying explanatory memoir (Eastwood and others, 1923) provides useful information on the local geology.

The whole area was resurveyed on the 1:10 000 scale by D Mc C Bridge in 1987 under a research contract partly funded by the Department of the Environment. This report is one in a series covering Coventry and its environs; a more general account of the geology of the region is currently being prepared (Old, Bridge and Rees, in prep.). Palaeontological contributions to this report are by Dr A W A Rushton.

Copies of this report and uncoloured dyeline copies of the 1:10 000 maps may be purchased from the British Geological Survey, Keyworth.

Parallel reports covering adjoining 1:10 000 sheets are:

SP37NE Coventry South-East (Sumbler, 1985)

SP38SW Coventry North (Old, in prep.)

Geological sequence

Beds down to the Halesowen Formation are represented at outcrop or beneath drift. Lower beds are known from colliery workings and boreholes.

Cambrian

The pre-Carboniferous rocks beneath the area are of Cambrian age, and form part of the Stockingford Shale Group which is divided as follows:

Only two boreholes encountered Cambrian basement in the area covered by this report; one at Clifford Bridge [SP 3761 8079], the other at Sowe Common [SP 3750 8310]. In the Clifford Bridge Borehole 0.6 m of red-stained slightly micaceous shale were proved beneath 34.4 m of Trias. The lowest beds have yielded a sparse inarticulate brachiopod fauna, including two acrotretids, suggesting a stratigraphical level within the Merevale Shales. The faunal evidence is consistent with published results (Old and others, 1987, p.4), which indicate a basement of lower Tremadoc rocks in the subcrop to the north-east of the city.

The Sowe Common Borehole provided no samples and only an outline lithological log which has been re-interpreted as follows:

The shale sequence from 33 to 88 m is provisionally assigned to the Cambrian rather than the Carboniferous, as originally reported. The revised classification is based on information from Wyken Colliery which suggests that the base of the Coal Measures lies to the west of the borehole site. The 'granitic' rock encountered towards the base of the borehole may be a pre-Carboniferous lamprophyre, similar in type to the numerous sills which intrude the Stockingford Shales in the Nuneaton area

Upper Carboniferous (Westphalian A to D)

On the eastern limb of the Warwickshire Coalfield syncline, Upper Carboniferous (Westphalian) rocks rest directly on Cambrian basement. The Productive Coal Measures and Etruria Marl, concealed beneath Triassic cover, are known only from old borehole records and colliery workings; younger strata, from just above the base of the Halesowen Formation, underlie thin drift deposits in the west of the area.

The sequence consists mainly of mudstones, siltstones, sandstones and seatearths, and is coal-bearing in its lower part. The Productive Coal Measures and Halesowen Formation are formed of grey-coloured rocks, which contrast with the variegated grey, green and red-brown sediments of the Etruria Marl. Red measures characterise both the Keele Formation and Coventry Sandstone, with mudstones predominating in the former and sandstones in the latter. Due to a paucity of marine and non-marine fauna, particularly in the upper part of the Westphalian, a chronostratigraphic subdivision of the sequence is impractical, and the ages of the younger formations can only be estimated.

Productive Coal Measures (Westphalian A, B and ? C)

The Productive Coal Measures include all the Westphalian strata below the Etruria Marl. The boundary between Westphalian A and B is taken immediately above the Seven Feet Coal, the position where the Vanderbeckei (Seven Feet) Marine Band occurs in the northern part of the coalfield. The B/C boundary cannot be identified because the Aegiranum (Nuneaton) Marine Band, on which it is defined, has not been recorded in any of the shaft or borehole sections in the area. The nearest recorded occurrence is in the Binley Colliery workings on adjoining sheet SP37NE, where it lies at least 14 m above the Four Feet Coal (Mitchell, 1942; Sumbler, 1985).

Measures below the Thick Coal are estimated to be between 65 and 70 m thick, though no single shaft or borehole has penetrated their full thickness. The beds consist of grey mudstone, siltstone, seatearth and sandstone with thin coals and ironstones at several levels (Figure 2).

The shaft section at Hawkesbury Hall No.2 Ironstone Pit [SP 3653 8469] records ten seams of coal in 61 m of measures below the Thick Coal. The lowest named seam, the Yard, is represented by 1.98 m of coal, at a depth of 152.2 m.

The Seven Feet is the lowest seam that can be correlated southwards from the Hawkesbury workings, into Wyken Colliery. It varies in thickness from 0.6 to 1.37 m and occupies a position about 10 m below the base of the Thick Coal. The coals between the Yard and Seven Feet tend to be thin and impersistent.

Lying above the Seven Feet but separated from it by up to 6.6 m of fireclay and mudstone, is a thin seam (0.2 m at Wyken) known locally as the Fungus. It is regarded as the equivalent of the Smithy of the more northerly parts of the coalfield (Mitchell, 1942).

Ironstones occur irregularly throughout the sequence, chiefly as nodules, but more rarely in bands a few centimetres thick. The nodules vary in size but are commonly from 0.1 to 0.2 m in diameter (Strahan and others,1920). During the nineteenth century ironstone was produced at Grove Colliery [SP 365 847] and at Wyken Colliery [SP 367 833] from measures immediately below the Yard Coal (Black Ironstone) and from beds overlying the Fungus Coal (White Ironstone). The White Ironstone is the best known ironstone of the coalfield, and at Wyken consisted of nodules and thin bands extending over an interval of 1.4 m.

Reserves of the Thick Coal, the only seam to have been worked, are now virtually exhausted, and the seam has supported no active mining from collieries within the area since about 1938 when the Hawkesbury and Victoria pits closed. In the north the seam is composed of five leaves totalling up to 8.5 m, which are either in contact or separated by no more than dirt parting. In ascending order they are the Nine Feet (Slate), Ell, Ryder, Bare and Two Yard. When traced southwards from Hawkesbury, the Thick Coal splits; the dirt partings between the upper leaves thicken and the Nine Feet becomes separated from the Ell. The interval between the two lower leaves, which is 0.3 m at Hawkesbury, increases from 3.7 to 13.7 m through the Wyken workings, and reaches 15.5 m in the Binley No.1 borehole [SP 3745 8031].

The Thick Coal is not affected greatly by faulting, but the continuity of the seams is disrupted by washouts, the largest of which is some 350 m across and can be traced south-eastwards through the Hawkesbury workings until it breaks up just to the north of Wyken Colliery.

The beds between the Thick Coal and the base of the Etruria Marl are from 50 to 60 m thick, predominantly of grey mudstone and seatearth with an increasing proportion of sandstone towards the top (Figure 2). Several thin coals occur but are difficult to correlate; two more persistent seams, lying at 12.4 and 47.3 m above the Thick Coal in the Alexandra Pit, are taken to bethe Thin Rider and Four Feet, respectively.

Etruria Marl Formation (Westphalian C)

A sequence of mudstones, siltstones and sandstones, mainly greyish in colour but also variegated red, brown and yellow, occurs between the Productive Coal Measures and the overlying Halesowen Formation. The sequence lies beneath Triassic strata and the full thickness has been penetrated only in the Wyken

Shaft [SP 3667 8334] and Longford Borehole [SP 3624 8425] (Figure 3). The boundaries of the formation are rather indefinite but the base is taken at the lowest occurrence of red beds, and the top at the incoming of grey sandstones of the Halesowen Formation. As thus defined, the thickness of the formation varies from 48 m to 63 m, and its base is diachronous. Conglomeratic "espley" rocks, which characterise the Etruria Marl farther north, have not been recorded.

Halesowen Formation (Wesphalian D)

The Halesowen Formation comprises an estimated 115 m of grey sandstone with subordinate mudstone, seatearth and rare thin coals. Throughout the Warwickshire Coalfield there is an unconformity at the base of the formation (Mitchell, 1942; Old and others, 1987) but its presence cannot be demonstrated in the present area. The base of the formation is a convenient, though arbitrary, position for the base of Westphalian D.

Sandstones predominate in the lower part of the sequence (Figure 3); they are associated locally with thin, impersistent coals. A single 0.6 m seam recorded in the Wyken Shaft about 6 m above the base may be correlated with the Milton Coal which is developed widely in the area to the south. A bed of Spirorbis limestone, recorded in the Exhall Colliery Shaft to the north of Hawkesbury, forms a useful marker horizon at about 34 m below the top of the formation. Its conjectural position, calculated from mining information, is shown on the accompanying 1:10 000 map. The outcrop of the Halesowen Formation is drift covered and no exposures have been recorded.

Keele Formation

The Keele Formation mainly comprises an argillaceous sequence of red mudstones, with subordinate sandstones and rare thin beds of Spirorbis limestone (Figure 3). The base of the sequence is ill-defined and there is frequently a passage through an alternation of red and grey rocks into the Halesowen Formation below. The boundary with the overlying Enville Group is also transitional but is taken at the level where sandstones again become dominant.

The formation has an estimated thickness of about 250 m, of which the uppermost 164 m was proved in the Little Heath No.4 Borehole [SP 3525 8270] (Figure 3). The borehole record shows that about 70 percent of the cored sequence consisted of mudstone; the remainder comprised mostly red, or sometimes grey-coloured, thinly bedded sandstone, commonly with bands of mudstone pellets. More massive, coarse-grained sandstones displaying cross-bedding were also recorded, including a 7 m thick basal unit. Spirorbis limestones were noted at two levels, 29 and 86 m below the probable top of the formation. The upper bed consisted of 0.2 m of dark grey, fine-grained limestone with vertical borings; the lower one was a red and grey limestone, irregularly veined with purplish red marl. The beds probably correlate with two limestones found in the Courtaulds No.5 bore [SP 3554 8235].

Some of the shallow boreholes drilled on the line of the M6 motorway, to the north-west of Victoria Farm [SP 358 840] penetrated the lower part of the Keele Formation. Most proved mainly red mudstone, except for two boreholes at the Longford Road bridge [SP 8013 8467] which passed through sandstone up to 8.6 m thick.

Exposures of the Keele Formation are rare. Red mudstone was noted by Eastwood in the bed of the River Sowe, near Aldermans Green [SP 357 833], and during the present survey mudstone was augered in the valley side to the north and south of Foxford School [SP 354 838], [SP 354 842]. Sandstone was only seen as flaggy surface brash in the bank of the Coventry Canal, 300 m north-west of Grange Farm [SP 3554 8449].

Enville Group

Coventry Sandstone Formation (Westphalian D–Stephanian?)

The Coventry Sandstone Formation has a maximum thickness of 235 m in this area, but much of it is concealed beneath Trias; only the lower 137 m is known in any detail. The age of the formation is thought to be Westphalian D or Stephanian, though no unequivocal faunal evidence has yet been found. Sandstones, although not everywhere dominant over mudstones, make up a large part of the sequence, being predominent in the lower 60 m, and distinguish it from the underlying Keele Formation.

The upper 137.8 m of beds, cored in the Morris Motor Borehole [SP 3528 8106], are assigned to the Coventry Sandstone Formation. They consist of almost equal proportions of red sandstone and mudstone and include a 7.3 m bed of conglomerate 86.6 m above the base. Beds in the lowest part of the Coventry Sandstone were also penetrated in the Little Heath No.4 bore [SP 3525 8270].

Conglomeratic sandstone crops out in a tributary of the River Sowe in Foleshill [SP 3503 8198]. The exposure consists of a single bed, 0.2 m thick, of very hard, grey, quartzose sandstone, containing rounded pebbles of chert and limestone up to 3 cm in diameter. An exposure 15 m further downstream comprises thinly bedded, grey-green sandstone with intercalations of red mudstone. Variable and conflicting dips may indicate the presence of a fault betweenthe two outcrops, but this cannot be confirmed from the limited exposure. The conglomeratic sandstone may be the lateral equivalent of the conglomerate bed recorded in the Morris Motors Borehole.

There are few other outcrops of the Coventry Sandstone. A low feature in the valley floor 0.75 km to the north of Foleshill exposes 1 m of (?) cross-bedded sandstone. In the adjacent cemetery [SP 351 828], sandstone blocks and disaggregated rock sand have been observed in newly dug graves. The highest exposed beds in the sequence are displayed in the banks of the Coventry Canal [SP 3503 8088] near Courthouse Green, where red mudstones can be seen just below the waterline.

Triassic

Sherwood Sandstone Group / Bromsgrove Sandstone Formation

The Sherwood Sandstone Group is represented in the area by the Bromsgrove Sandstone (formerly Keuper Sandstone). The formation rests with marked unconformity on Cambrian or Carboniferous rocks and comprises some 25 to 38 m of current-bedded sandstone interbedded with mudstone. The sandstones, when fresh, are mainly grey in colour, but weather to a soft buff or brown rock. Mica occurs as disseminated flakes and as concentrations on bedding planes and cross-laminae. The sandstones are generally well-sorted and feldspathic. The formation shows well developed cyclic sedimentation, particularly in its lower part. Each complete cycle comprises a basal sandstone unit which fines upwards into siltstone and then mudstone. The thicker sandstones commonly contain small pebbles or mudstone flakes, and usually rest on sharp, erosive bases. The intervening mudstones range from a few centimetres to more than 4m in thickness. The deposits are of fluvial origin; the sandstones probably accumulated in migrating river channels and the mudstones as overbank flood deposits. The resulting bedforms tend to be lenticular and laterally impersistent.

The Bromsgrove Sandstone has been encountered in many shallow site investigation drill holes and in several deeper boreholes sunk for water or coal. In the Clifford Bridge Borehole [SP 3761 8079], the formation is 29 m thick and rests unconformably on Merevale Shales (Figure 4). The basal mudstone, with polished listric surfaces, is overlain by a cyclical sequence of alternating sandstones and mudstones with a notable coarse pebbly sandstone at the base. The upper part of the sequence consists of tabular cross-bedded and parallel laminated sandstones, separated by red silty sandstones and thin mudstones. About 30 per cent of the sequence comprises mudstone, with more in the lower part than the upper, some of the mudstones being veined with gypsum. The Bromsgrove Sandstone in the Binley No.1 borehole [SP 3745 8031] is probably of similar thickness (?25 m), but there is some doubt about the classification of the strata.

The formation crops out beneath drift along the Sowe valley, but exposure is poor. The best section is in a roadside cutting at the northern end of Bell Green Road [SP 3574 8217] where there is a 2.8 m high cliff in soft buff and red streaked sandstone. The beds are strongly cross-bedded with erosive bases and contain quartz pebbles and yellow sandy marl pellets.

A stream section [SP 3625 8303], 150 m south of Wyken Pool, shows 1 m of red sand (weathered sandstone), separated by a clay parting from 1.3 m of flat-lying, cross-bedded, buff-coloured sandstone. Close by, in the same stream [SP 3625 8301] up to 1.8 m of sandstone containing mudstone clasts is displayed.

Mercia Mudstone Group

The outcrop of the Mercia Mudstone (formerly Keuper Marl) extends across the eastern half of the area but is largely drift covered. The strata thicken towards the south where a maximum of about 25 m of beds can be expected; over the central part of the outcrop the thickness is much less, probably between 5 and 10 m.

The Mercia Mudstone consists predominantly of blocky red mudstones with a few bands of green siltstone and fine-grained sandstone. The base is gradational but, where drift-free, has been mapped below a conspicuous green siltstone. This bed has been proved by augering in the school playing-field in Wyken [SP 3681 8044] and in the floor of the valley to the east of Potter's Green [SP 382 829]. The position of the base of the Mercia Mudstone beneath the thicker drift deposits in the centre and north of the area is conjectural.

Beds in the lower part of the sequence were proved in site investigation drill holes for the Coventry Eastern Bypass. A borehole [SP 3933 8196] just to the south of the M6/M69 interchange proved 17.2 m of red mudstone, with gypsum veining, resting on Bromsgrove Sandstone. Exposures of Mercia Mudstone are seen in a tributary of the Sowe, 250 m to the north of High Bridge [SP 3882 8090].

Structure

Little is known of the detailed structure of the Cambrian basement. However, faunal evidence from deep boreholes indicates that the Tremadoc rocks are affected by open folds that trend north-eastwards across the area. A relatively low dip (about 15) was obtained from the Merevale Shales in the Clifford Bridge Borehole.

Intra-Carboniferous movements are responsible for the unconformity at the base of the Halesowen Formation.

Folding in post-Carboniferous - pre-Triassic times produced a shallow southward-plunging syncline, in which the coal reserves of the Warwickshire Coalfield are preserved. Dips on the eastern limb of the syncline are mostly in the range 10 to 15 but the beds steepen to almost 30 towards the incrop. Folding of the Carboniferous predates the Trias, which truncates the syncline, so that progressively older beds incrop north-eastwards against the Triassic unconformity.

The main effect of post-Triassic movements has been to impose a gentle south-eastwards dip of 1 to 2 on the Bromsgrove Sandstone and Mercia Mudstone. The Triassic crop is faulted along part of its western margin by a north-northeastward trending fracture which throws down about 10 m to the east.

Quaternary

Glacial Drift

Thick spreads of glacial drift cover much of the area, and form part of a layered sequence first recognised by Shotton (1953) between Rugby, Coventry and Leamington. The nomenclature proposed by Shotton and later modified by Sumbler (1983), has been adopted in this report. Conventionally, the glacial deposits have been attributed to the Wolstonian Glaciation (Shotton and West, 1969), but the stratigraphic basis of this glaciation is now considered questionable, and the deposits are regarded by many workers as Anglian in age (Sumbler, 1983; Bowen and others, 1986).

The glacial drift was deposited on an undulating topography which has partly been exhumed by post-glacial erosion along the valleys of the River Sowe and its tributaries. The drift is thickest on the high plateau south of Barnacle, where it exceeds 20 m; farther to the west, the deposits become thinner and slightly patchy where the rockhead elevation rises above 85 m 0.D. (Figure 5) and (Figure 6).

Two drift-filled channels have been identified in the Hawkesbury area (Figure 5); one follows the line of the modern valley southwards through Hawkesbury to the site of the old Coventry Power Station [SP 363 842], where it swings east south-eastwards and continues as a shallow depression beneath the M6 motorway; the other underlies the valley to the west of Foxford School [SP 354 843]. Both channels are filled with upwards of 15 m of drift, comprising till on a basal sand and gravel unit.

Baginton Sand and Gravel

The oldest proven drift is the Baginton Sand and Gravel, which was deposited directly on Triassic bedrock by streams draining northeastwards along the 'proto-Soar' valley (Shotton, 1953). The deposit is mainly confined to a small area south-east of Walsgrave Hill, where it crops out beneath till and river terrace deposits. Here it is approximately 6 m thick and comprises mainly medium- to fine-grained sand with some 'Bunter' pebbles.

At the M6/M69 interchange, sand and gravel, probably representing the Baginton Sand and Gravel, has been proved beneath till in two site investigation boreholes [SP 3937 8228], [SP 3930 8226].  The deposit is up to 6.7 m thick and appears-to infill a channel no more than 100 m across.

Glacial Sand and Gravel (undivided)

Several small outliers of sand and gravel occur along the western edge of the area, between Foleshill [SP 351 821] and Foxford [SP 353 836]. None of the deposits is well-exposed, nevertheless they appear to form a separate and lithologically distinct group at an elevation some 10 m higher than the Baginton Sand and Gravel. Trial pits in the grounds of Foxford School [SP 3548 8402] proved 1.9 m of poorly sorted cobble gravel (not bottomed), beneath a thin till cover. The gravel comprised blocks, up to 0.5 m across, of quartzite, sandstone and shale. Similar poorly sorted material has been recorded from the cemetery off Windmill Road [SP 3502 8280]. Farther to the north, the sand and gravel is concealed beneath thicker till deposits but can be traced in boreholes; one at the Coventry Canal/M6 crossing [SP 3543 8439] proved 5.1 m of sand and gravel, and another at Hawkesbury Lane Station [SP 3556 8489] passed from till into 8.8 m of sand and gravel.

Other occurrences of sand and gravel, at a slightly lower elevation, crop out beneath till along the valley sides, south of Potter's Green [SP 374 820] and are recorded in site investigation drill holes for the Manor Farm Estate [SP 369 811]. Sands up to 1.3 m thick, also occur within the till sheet around Henley Green, but they are not mappable through the urban area.

The various deposits referred to above are believed to have been laid down beneath ice or close to the ice margin. The occurrence of shale blocks in the Foxford area implies an input of material from the north, probably from the exposed coalfield.

Till (undivided)

A sheet of till between 7 and 12 m thick covers most of the area; in the south-east it rests with sharp contact on Baginton Sand and Gravel; elsewhere it rests on Triassic or Carboniferous bedrock, sometimes with an intervening basal sand or gravel. The commonest lithology is a reddish brown, stony clay of Triassic derivation, comparable with the Thrussington Till (Rice, 1968). It contains blocks of grey and green sandstone (skerry), reddish-brown siltstone, pebbles of 'Bunter' quartzite and coal fragments. Rarer erratics include of diorite, ironstone and Carboniferous Limestone.

Interleaved with this reddish-brown till are local additions of predominantly brown till containing chalk and flints. The latter facies, defined by augering, comes to crop in a narrow lobe which extends from the M6/M69 interchange southwestwards to Walsgrave. Its boundary (Figure 7) has been drawn to encompass all surface records of chalk-bearing till. In at least one motorway borehole [SP 3894 8237], brown chalky till was recorded at the base of the drift sequence, implying that ice carrying chalk-rich material advanced into the area at an early stage.

Sections in the till are rare and tend to be temporary. Foundation trenches at a site in Walsgrave [SP 382 816] showed about 3 m of dark brown till containing chalk and whole flint nodules, interbedded with bands of red 'Thrussington-type' till. A shallow gravel-filled channel, 4 m wide, traversed the site from north to south.

In the upper part of the till sheet, there is an upward change from red stony till to stoneless clay as the contact with the overlying Wolston Clay is approached. Where the transition occurs over several metres, there are extensive outcrops of passage beds, notably in the areas of Wood End, Wyken and to the east of the Coventry Eastern Bypass (Figure 7).

A temporary exposure at Brookfield Farm [SP 381 832], typical of the passage beds, showed 0.5 m of red silty and stony clay, with thinly interbedded seams of brown stoneless clay. Similar lithologies were seen in site works off Deedmore Road [SP 368 826] and Dutton Road [SP 369 830].

While it is possible to show the general distribution of the passage beds, they do not form mappable units, and are classified with till on the accompanying 1:10 000 scale map.

Wolston Clay

The Wolston Clay forms the prominent feature of Walsgrave Hill and crops out on the higher ground in the north-east; there are also several small outliers. The base of the deposit rises from an elevation of about 76 m in the south-east to over 99 m in the north. The dominant lithology is a chocolate brown and grey, mottled, smooth clay; it is usually calcareous and commonly contains small white race nodules. In section it may appear structureless or display a fine lamination, which has been interpreted as varying (Shotton, 1976a, p.248). The formation is about 17 m thick at Walsgrave Hill and decreases to 13 m north-west of Ansty.

A 10 m borrow pit was dug on the flank of Walsgrave Hill [SP 391 805] in 1987 to provide embankment fill for the Coventry Eastern Bypass. Beds at the base of the pit comprised brecciated, structureless clay with some pods of flow(?) till. Towards the top, silt and fine-sand layers, displaying flaser bedding, were recorded.

The close association between the Wolston Clay and the underlying till suggests that during Wolston Clay times deposition occurred in close proximity to ice, possibly in transient glacial lakes. The passage beds at the base of the sequence provide the first evidence of ponding, when till and lake clays were deposited contemporaneously, possibly beneath floating ice or at the ice margin.

Ochreous, poorly sorted, flinty gravels are piped into the upper surface of the Wolston Clay on Walsgrave Hill [SP 3918 8043]. These very localised deposits are thought to be the only representatives in the area of the Dunsmore Gravel (Old and others, 1987, p.57).

Wolston Sand and Gravel

The Wolston Sand and Gravel crops out beneath on outlier of Oadby Till in the northeast of the area. It forms a sheet between 1 and 4 m thick, which maintains a fairly constant elevation of about 105 m AOD. The typical lithology is a fine- to medium-grained silty sand, commonly interbedded with grey sticky clay. Thin chalky gravels are recorded locally. The deposit rests with a sharp base on the underlying Wolston Clay, and is interpreted as glacial outwash from the Oadby Till ice.

A section formerly seen at Barnacle Hall Brickpit [SP 3850 8381] was:

Another site in Ansty village [SP 399 834] shows:

The maximum recorded thickness of the deposit was 4.2 m proved in a site investigation borehole for the M69 motorway [SP 3920 8437].

Because the sands are underlain by impervious clays, they form a local aquifer, and this has posed a particular problem with regard to pipe-laying operations around the village of Barnacle.

Oadby Till

The Wolston Sand and Gravel is capped by a 9 m thick sheet of olive brown and grey till containing chalk, flint and Jurassic erratics. The till, first described from the Leicester area (Rice, 1968), has a clay matrix derived largely from Jurassic mudstones and is typically mottled brown and grey with powdery streaks of chalk. Thin sands are present locally within the till.

The contact between the Oadby Till and the underlying Wolston Sand and Gravel is exposed at Barnacle Hall Brickpit [SP 384 838], at an elevation of about 106 m AOD.

River terrace deposits

Fluviatile spreads of sand and gravel, probably representing four terrace levels, occur on both sides of the Sowe valley. All the terraces are of similar composition and are differentiated solely on the basis of height. They are composed dominantly of flint, 'Bunter' quartzite and Carboniferous sandstone pebbles.

Fourth Terrace Deposits form gravelly flats about 15 m above the floodplain of the River Sowe, on the south-east flank of Walsgrave Hill. Most of the deposits are conspicuously flinty and form low features overlapping onto till or Baginton Sand and Gravel. A thin patch of clayey pebbly sand capping the col [SP 399 800] to the north of Hill Fields Farm is also designated as Fourth Terrace, though it is somewhat higher than the surrounding terrace deposits.

Third Terrace Deposits have only been mapped in the Wyken area [SP 374 806] which is now built-up. The terrace flat lies about 8 m above the River Sowe floodplain and corresponds to ground formerly mapped by Eastwood as sand and gravel. The back of the terrace is indistinct, and thin gravels have been recorded in the school playing fields [SP 372 804] beyond the mapped feature.

Second Terrace Deposits are proved as small patches of gravel lying between 3 and 6 m above the floodplain of the Sowe. One small gravel pit [SP 363 811], now filled-in, showed 1.5 m of sandy gravel with 'Bunter' pebbles and rare flints. Site investigation boreholes for the shopping precinct at Bell Green [SP 360 821], proved up to 3.6 m of fine- to coarse-gravel, with some cobbles.

First Terrace Deposits form flats about 1. 5m above the alluvium downstream from Bell Green. The effects of urban development, coupled with landfill, have largely obscured the true form of these deposits.

Alluvium

Alluvial deposits occur in all of the major valleys. The alluvium of the Sowe has been proved in boreholes to consist of a loamy clay above a basal gravel; and to be no more than 6 m thick.

To combat the affects of mining subsidence, and to prevent flooding, the river has been straightened, and large tracts of the floodplain have been built-up artificially. Major landfill schemes have been completed at Alderman's Green [SP 357 833] and are continuing at Wyken Croft [SP 370 809].

Along the main tributary of the Sowe, which flows to the south-east of Walsgrave, there is a fairly wide alluvial tract which rises imperceptably away from the river. Although it is reported to flood across its entire width, there may locally be First Terrace Deposits, particularly on the right bank, which have not been distinguished.

Peat

Peat up to 1 m thick has formed in the valley draining into Combe Pool.

Economic geology

Brick clay

Clay for brick and tile-making was formerly dug at several sites in the area (Table 1). Two of the pits were located in drift (Wolston Clay), one in Keele Formation mudstones, and a fourth in mudstones within the Bromsgrove Sandstone. The favoured formation to the north of this area, the Etruria Marl, is covered by several metres of till and has never been worked.

Fireclays beneath the Thick Coal were formerly exploited in the underground workings at Wyken Colliery.

Coal

The concealed part of the Warwickshire Coalfield has a long and chequered history of mining for which there is documentary evidence dating from the 1570s (Grant, 1982). All of the recoverable reserves in the Thick Coal, which was initially worked close to its incrop from shafts at Hawkesbury, Victoria, Wyken and Craven. Towards the later part of the nineteenth century, new shafts were sunk on these sites and roadways were driven down-dip to allow extraction of reserves from deeper parts of the coalfield. Production continued into the early 1900s until the pits became uneconomic and were eventually abandoned. Wyken Colliery was the first to close in 1914, followed by Craven Colliery in 1928. Production at the Exhall Colliery (which included Hawkesbury and Victoria, continued until 1938. Undermining finally ceased in 1971 when the last recoverable reserves were extracted from beneath Foleshill in headings driven from the Coventry Colliery.

Isopachytes and nomenclature for the Thick Coal seam have been published by British Coal (formerly National Coal Board) (NCB, 1957); additional information is given in the Westphalian section of this report.

Ironstone

The production of ironstone from the Warwickshire Coalfield during the late nineteenth century has been reviewed by Strahan and others (1920). Output reached a maximum between 1874 and 1875 when 100,000 tons was produced; by the turn of the century this figure had reduced to 92 tons.

In the present area, production was centred on the White Ironstone measures which were worked at Hawkesbury (Grove) and Wyken collieries. There are no details of production levels from these mines or indeed of the composition of the iron-ore. The stratigraphy of the ironstone-bearing measures has been reviewed earlier.

Water supply

Boreholes sunk to provide water for industrial use are listed below (Table 2). The principal aquifers are the sandstones of the Enville Group and Halesowen Formation.

References

BOWEN, D.Q., ROSE, J., McCABE, A.M. and SUTHERLAND, D.G., 1986. Correlation of Quaternary Glaciations in England, Ireland, Scotland and Wales.  In: Quaternary Glaciations in the Northern Hemisphere. Quat. Sci. Reviews Vol.5, 299–329.

EASTWOOD, T., GIBSON, W., CANTRILL, T.C. and WHITEHEAD, T.H. 1923. The geology of the country around Coventry, Mem. Geol. Surv. G.B.

GRANT, E.G. 1982. Changing Perspectives in the Warwickshire Coalfield. In Field and Forest, SLATER, T.R. and JARVIS, P.J. (editors).

MITCHELL, G.H. 1942. The Geology of the Warwickshire Coalfield.  Geol. Surv. Wartime Pamphlet, No.25, 42pp.

NATIONAL COAL BOARD, 1957. Warwickshire Coalfield Seam Maps, N.C.B. London.

OLD, R.A. (in prep.) Geological notes and local details for 1:10 000 Sheets: SP38SW (Coventry North), Keyworth: British Geological Survey.

OLD, R.A., BRIDGE, D.Mc.C. and REES, J.G. (in prep). The geology of the Coventry area.

OLD, R.A., SUMBLER, M.G. and AMBROSE, K. Geology of the country around Warwick. Mem. Br. Geol. Surv., Sheet 184 (England and Wales).

RICE, R.J. 1968. The Quaternary deposits of Central Leicestershire. Philos. Trans. R. Soc. London, Series B, Vol.262, 459–509.

RICHARDSON, L. 1928. Wells and springs of Warwickshire. Mem. Geol. Surv. G.B.

SHOTTON, F.W. 1953. The Pleistocene deposits of the area between Coventry, Rugby and Leamington, and their bearing on the topographic development of the Midlands. Philos. Trans. R. Soc. London, Series B, Vol.237, 209–260.

SHOTTON, F.W. 1976. Amplification of the Wolstonian Stage of the British Pleistocene. Geol. Mag., Vol.113, 241–250.

SHOTTON, F.W. and WEST, R.G. 1969. Stratigraphic table of the British Quaternary. 155–157 in Recommendations on a stratigraphic usage. Proc. Geol. Soc. London, No.1656.

STRAHAN, A., GIBSON, W. and SHERLOCK, R.L. 1920. Special Report on the Mineral Resources of Great Britain, Vol.13 - Iron-ores, Mem. Geol. Surv. G.B.

SUMBLER M.G. 1983. A new look at the type Wolstonian glacial deposits of Central England, Proc. Geol. Assoc., Vo1.94, 23–31.

1985. Geological notes and local details for 1:10 000 Sheets: SP37NE (South-east Coventry). Keyworth: British Geological Survey.

Appendix Schedule of boreholes on sheet SP38SE

C as a suffix to the borehole number denotes information held on a 'Commercial in Confidence' basis.

* in the depth column denotes a site for which more than one record exists.

Figures, plates and tables

Figures

For figures, plates and tables see PDF

(Figure 1) Area of report relative to area of whole contract shown with bold outline

(Figure 2) Comparative sections of the Productive Coal Measures

(Figure 3) Comparative secti0ons in the Upper Coal Measures (Westphalian C and D)

(Figure 4) The Bromsgrove Sandstone in the Clifford Bridge Borehole

(Figure 5) Sub-drive rockhead elevation

(Figure 6) Thickness of drift

(Figure 7) Distribution of glacial drift

Plates

(Plate 2B) Volcaniclastic (?) mudstone with root stems: Etruria Formation, Weston Hill Farm Borehole.

(Plate 2B) Red pebbly diamict (Thrussington Till), Weston Hill Farm Borehole.

Tables

(Table 1) Location of former brick pits

Tables

(Table 1) Location of former brick pits

(Table 2) Water-borings and wells