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Geological notes and local details for 1:10 000 Sheet SP37SW (Stoneleigh). British Geological Survey Technical Report WA/88/49

R. A. Old

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British Geological Survey. Technical Report WA/88/49 Sheet SO 37 SW Stoneleigh

Bibliographic reference: Old, R. A. 1988. Geological notes and local details for 1:10 000 Sheet SP37SW (Stoneleigh). British Geological Survey Technical Report WA/88/49.

Author: Old, R.A. BSc, PhD, 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, Nottingham British Geological Survey 1988

Summary

The geology of Sheet SP37SW (Stoneleigh) is described with emphasis on significant exposed sections and borehole logs.

The Cambrian and Lower Carboniferous formations are not represented at outcrop and are known only from a number of boreholes which are summarised in graphic form. The rocks at outcrop belong mainly to the Enville Group, with smaller outcrops of poorly exposed Triassic rocks. Details of the more important sections are given.

Details are given of the lithologies and thicknesses of the drift deposits and a diagram shows the elevation of the sub-drift topography.

Attention is drawn to the important coal reserves underlying the area and to other mineral resources. A schedule of boreholes is appended.

Introduction

This report describes the geology of 1:10 000 sheet SP37SW (Stoneleigh) (Figure 1). The area was first geologically surveyed on the one-inch scale by H.H. Howell and was included in Old Series One-Inch Sheet 53 published in 1855.

The primary 1:10 000 survey of the area was carried out in 1977–78; with minor revisions in 1987, by R.A. Old, and is included in 1:50 000 geological Sheet 184 (Warwick) published in 1984; an accompanying Memoir is also published (Old and others, 1987). The mapping and the production of this report were partly funded by the Department of the Environment.

Palaeontological contributions to this report are by Drs N.J. Riley and A.W.A. Rushton.

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

Parallel reports covering adjoining sheets are:

SP27SE Kenilworth (Old; 1988a)

SP37NW Coventry Central (Old, 1988b)

SP37SE Bubbenhall (Sumbler, 1988)

SP36NW Royal Leamington Spa (Ambrose; 1986)

Geological sequence

Beds down to the upper part of the Coventry Sandstone are represented at outcrop or beneath drift. Lower beds are known from boreholes.

Cambrian

Wherever proved the pre-Carboniferous rocks beneath the area are of Cambrian age; they are assigned to the Merevale Shales (Tremadoc Series).  Boreholes penetrating Cambrian rocks from which cores were obtained are tabulated below:

Because the small thicknesses penetrated do not contain any distinctive lithologies, no detailed correlation is possible between the boreholes.

The predominant lithology is dark grey mudstone, which in some boreholes becomes paler and green-grey for a few metres below the unconformity with the Westphalian. The top 30 cm of mudstone at Finham Brook are penetrated by roots from the Westphalian and sphaerosiderite occurring abundantly for over 5 m below the unconformity may have formed during the Westphalian. A few beds of sandstone, and siltstone, up to 0.5 m thick, are present in several of the boreholes and sandstone and siltstone laminae are common in the mudstones. The laminated beds display a variety of cross-bedding, soft-sediment deformation structures and penecontemporaneous microfaults.

Fossils from the Black Spinney, Crewe Farm and Stareton cores are diagnostic of the Dictyonema Flabelliforme Zone of Tremadian. The Black Spinney cores yielded Dictyonema flabelliforme flabelliforme and Lingulella sp.; those from Crewe Farm Lingulella sp., Linnarssonia belti and Dictyonema sp.; the fauna from Stareton included Lingulella nicholsoni, cf. Beyrichona triceps, Dictyonema flabelliforme, Tomaculum problematicum and a variety of burrows. At Ashow the fauna, indicating the slightly higher 'Transition Beds' zones between the. D. flabelliforme and Clonograptus tenellus, includes Sponge spicules (cruciform), Eurytreta sabrinae, Linnarssonia cf. belti, Lingulella nicholsoni, Shumardia curta and a variety of burrows.

Upper Carboniferous (Westphalian) and Lower Permian (Autunian)

The Westphalian beds rest directly on Cambrian strata; below the upper part of the Coventry Sandstone they do not crop out in the area, and are known only from boreholes (Figure 2). The sequence consists mainly of mudstones, siltstones, sandstones and seatearths. The lower part (Productive Coal Measures) and the Halesowen Formation are grey-coloured and coal-bearing; the intervening Etruria Marl Formation is variegated grey, green and red brown; the Keele Formation and the Enville Group consist almost entirely of red measures, predominantly argillaceous in the former and arenaceous in the latter. The only well identified Westphalian subdivision is that between Westphalian B and C marked by the Aegiranum Marine Band.

Productive Coal Measures

The Productive Coal Measures include all the Westphalian strata below the Etruria Marl. Because the Vanderbeckei Marine Band is not developed in this area the position of the Westphalian A/B boundary is uncertain, but by comparison with those areas of the Warwickshire Coalfield where the marine band has been proved, the boundary is unlikely to be more than about 10 m below the base of the Thick Coal (Cope and Jones, 1970).

The beds below the Thick Coal are between 35 and 6 0 m thick (Figure 2). They consist of grey mudstone, siltstone, seatearth and sandstone with impersistent coals up to 0.5 m thick. Breccio-conglomerates up to 1 m thick, with clasts of Cambrian mudstone and quartz pebbles, occur at the unconformity with the Cambrian at Crewe Farm and Wainbody Wood.

Details of the Thick Coal, the only seam of economic importance in the area, are confidential. The area lies within the zone of Splitting Thick Coal (National Coal Board 1985, Old and others 1987) and the maximum thickness of coal proved, excluding partings, was 4.5 m at Wainbody Wood.

The beds between the top of the Thick coal and the base of the Etruria Marl are from 60 to 70 m thick. They are predominantly grey mudstone with siltstone, seatearth and sandstone in lesser proportions. Non-marine bivalves occur in the roof of the Thick Coal and sporadically in the beds between the Thick Coal and the Half Yard.

The impersistent thin coal about 10 m above the Thick Coal is probably the Four Feet. The Half Yard is persistent but somewhat variable in thickness.

The Aegisanum Marine Band, which is underlain by a thin persistent coal, consists of about 1 m of black or dark grey mudstone with marine brachiopods. At Crewe Farm the Marine Band was not recorded and may have been 'washed out' by sandstone. Lingula mytilloides and Orbiculoidea sp. occured in the Marine Band at Ashow, and Serpuloides sp. and conodont debris additionally at Black Spinney.

Etruria Marl Formation

The base of the Etruria Marl is marked by the appearance of brightly coloured beds; in places the change is gradational so that the formational boundary is somewhat imprecise. The top of the formation is more clearly defined at the base of thick grey Halesowen sandstone, overlying coloured mudstone. The variation in thickness of 35 to 55 m of the formation is due to the combined effects of a diachronous junction with the Productive Coal Measures and possibly an unconformity at the base of the Halesowen Formation.

The argillaceous beds of the formation are characterised by a variable development of colours in shades of brown, red, purple and green-grey. Some beds are variegated, some show colours blotched over a grey background and others are predominantly of one colour. Intervals of grey beds, indistinguishable from Productive Coal Measures also occur.

Some of the sandstones in the Etruria Marl are of 'espley' facies, containing breccio-conglomerate bands with angular to rounded clasts of exotic lithologies such as Cambrian mudstone, vein quartz and igneous rocks, as well as pebbles of Westphalian type.

Halesowen Formation

The Halesowen Formation comprises 100 to 150 m of pale grey sandstone with subordinate grey mudstone, siltstone and seatearth and a few thin coals. The top of the formation has not been cored and is taken by British Coal to be at a persistent high gamma peak in borehole gamma logs which occurs at or close to the junction with the predominantly red-brown Keele Formation beds above. The base is a convenient, although arbitrary, base for Westphalian D.

The sandstones commonly have erosive or load casted bases with thin intra-formational breccias or conglomerates. They may occur in fining-upwards units with a variety of cross-bedding structures, or they may be massively bedded. Many sandstones are micaceous and a calcareous cement is typical. Coaly laminae and debris are abundant.

The Milton seam (Figure 2) persists throughout the area and attains a maximum thickness of 1.6 m at Crewe Farm. The roof of the seam is commonly a mudstone containing non-marine bivalves and at Ashow, abundant Spirorbis.

The British Coal log of the Wainbody Wood Borehole records Lingula, Orbiculoidea and ostracods immediately above a fault some 9 m above the Milton seam. Specimens have not been preserved from these mudstones.

Keele Formation

The Keele Formation has not been cored in this area and is known mainly from chipping samples and geophysical borehole logs. In the district to the north it consists predominantly of red-brown mudstones with subordinate sandstones; and a few thin beds of Spirorbis limestone (Eastwood and others, 1923). From the few cores available in other areas there appears to be a gradual transition from the predominantly grey Halesowen Formation to the red measures of the Keele. The geophysical logs from the boreholes show a remarkably high gamma radiation over a narrow zone at about this level; and this has been taken as the base of the Keele Formation; although it may not correspond precisely with the lithological boundary: the geophysical correlation of the sandstone beds in the overlying beds seems to confirm that this is a widely persistent marker. The formation is distinguished from the overlying Coventry Sandstone formation by its more argillaceous nature and its top is thus rather indefinite. As thus defined the formation is about 255 to 295 m in thickness.

Enville Group

The Enville Group of the Warwickshire Coalfield was first defined as comprising all the pre-Triassic strata that succeed the Keele Formation (Eastwood and others, 1923, p.77). This report follows the modified of Shotton (1929, p.169) which includes all the pre-Triassic rocks above the Tile Hill Beds of the earlier writers.

Almost the whole area is underlain by rocks of the Enville Group. The predominant dip is southerly and the beds at crop include, from north to south, part of the Coventry Sandstone; the upper part of the Tile Hill Mudstone, the Kenilworth Sandstone and the lower part of the Ashow Formation. The total thickness of the Group at crop and proved in boreholes is about 780 m.

The base of the Permian has been placed by the Geological Society at the base of the 'Kenilworth Breccia Group' (Smith and others; 1974; Ramsbottom and others, 1978). This conclusion is founded on the supposed correlation of the Kenilworth and Clent Brecci (Shotton, 1929 , p.201), and on the assumption that the unconformity at the base of the latter marks the base of the Permian. Due to the thin and impersistent nature of the breccias in the Kenilworth Sandstone, the base of Shotton's 'Kenilworth Breccia Group' can be recognised only locally and does not provide a satisfactory position for the boundary between two systems. In contrast, the base of the Kenilworth Sandstone Formation (as defined below) can be traced over wide areas and is easily recognisable in downhole geophysical logs. It is consequently taken here as a convenient, if arbitrary, base to the Permian in the light of present day knowledge.

The occurrance at Kenilworth, probably in the Kenilworth Sandstone; of the pelycosaurs Sphenacodon brittanicus and Haptodus grandis (Paton, 1974) and the amphibian Dascyceps bucklandi (Paton; 1975) lends support to the Permian age for the upper part of the Enville Group. All these species are confined to Autunian strata. A jaw-bone of the pelycosaur Ophiacodon; discovered near the top of the Coventry Sandstone '3/4 mile northwest of Coventry' (Murchison and Strickland 1837, p.347); is assigned a late-Stephanian to early-Autunian age by Paton (1974). Less well attested are the views of Haubold and Sergeant (1973; p.908) and Haubold and Katzung (1975, p.118), based on scanty evidence from reptile footprints, that the base of the Permian should be placed at the base of the Enville Group.

There is no chronologically significant fossil evidence for the age of the Enville Group below the top of the Coventry Sandstone; but the general concensus is that it is uppermost Carboniferous, Westphalian D or Stephanian. Dix (1935) reviewed the floral evidence and placed the whole of the Enville Group in the Permian, but no significant plant fossils occur below the Tile Hill Mudstones. Three specimens of 'Strophalosia' from the Enville Group figured by Howell (1859, p.32) have been reidentified as the gymnosperm seeds Cariocarpon reniforme and C.ottonis (Cox, 1953).

The Geological Society's proposed schemes of correlation for the Permian (Smith and others; 1974) and the Silesian (Ramsbottom and others, 1978) have not clarified the position. In the former, the Ashow, Kenilworth and Gibbet Hill subdivisions, are considered to be groups (following Shotton, 1929; p.170): in the latter the Gibbet Hill and lower named subdivisions are thought of as members of an Enville Formation.

Coventry Sandstone Formation

Based on the interpretation of geophysical borehole logs, the Coventry Sandstone increases in thickness from about 260 m in the west to about 330 m in the east. No cores have been taken but borehole chippings are mainly of red-brown sandstone with subordinate mudstone.

A small outcrop of beds near the top of the Coventry Sandstone occurs north of the Princethorpe Fault. Small exposures of red-brown sandstone are found along the south bank of the River Sowe at Coventry Golf Course [SP 334 749] to [SP 336 747].

Tile Hill Mudstone Formation

The Tile Hill Mudstone totals about 265 m in thickness of which only the upper half comes to crop in the area. The full thickness of the formation has been proved in a number of coal exploration boreholes, none of which was cored at this level. The formation is generally more argillaceous than those above and below, but includes sandstones which tend to be impersistent. Both the mudstones and the sandstones are predominantly red-brown and commonly include grey-green reduction spots or 'fish-eyes'.

At Finham Sewage Works [SP 3375 7408] temporary excavations revealed up to 5 m of soft, red-brown, thin bedded sandstone; nearby [SP 3359 7411] 4 m of the same sandstone is exposed in the banks of Finham Brook. A 5 cm sandstone within mudstone; exposed in a ditch on St. Martin's Road [SP 3317 7402]; showed ripple marks with steeper sides facing SSE and cross-bedding dipping NNW.

A thin section (E51559) of sandstone from the bank of the River Sowe at Baginton [SP 3405 7602] consists mainly of angular grains of quartz; quartzite and argillaceous rocks, with a few of altered andesite or rhyolite. The small percentage of other mineral grains includes muscovite; biotite; microcline, albite and amphibole. The cement is calcite in large plates.

Kenilworth Sandstone Formation

The whole of this formation; about 95 m thick, crops out in this area. Its base-is-marked-by the incoming of thick, massive sandstones which give rise to a prominent northeast-facing escarpment. The top of the formation is taken at the base of the thick mudstones of the Ashow Formation. The formation includes the 'Kenilworth Breccia' and 'Gibbet Hill' groups of Shotton (1929).

Red, massive, and commonly soft sandstones form the bulk of the formation, with subordinate thin lensing mudstones. Near the base the impersistent Gibbet Hill Conglomerate marks the site of a fluvial channel, and thin lenses of breccia occur beneath some sandstone beds, probably as channel lag deposits. There are few localities where any appreciable thickness of sandstone is exposed. In 1978, however, a pipe-trench was excavated across virtually the whole width of the Kenilworth Sandstone outcrop between Gibbet Hill [SP 307 748] and Crewe Farm [SP 313 720]. This confirmed that most of the sandstones are rather soft; at least to depths of 2–3 m, and include a few very hard, flaggy, calcareous, lustre-mottled bands. Red-brown mudstones with green spots were also encountered, varying from blocky to thinly bedded. A number of small exposures show mudstone clasts in sandstone bands. A disused sandstone quarry on Motslow Hill [SP 3294 7233] shows the following section:

Thin sections of Kenilworth Sandstone from Motslow Hill (E51562) and Stoneleigh (E51561) show that these rocks are polylithic arenites. The grains are predominantly angular or sub-angular and moderately well sorted.

Quartz; quartzite and micaceous siltstone are the most important constituents and mudstone fragments are common. Altered rhyolite or andesite is also present together with rare mica schist. The carbonate cement forms a fine mosaic in (E51561) and large plates in (E51562).

The type locality of the Gibbet Hill conglomerate lies just within the area to the north [SP 3405 7521] and was first described by Shotton (1929, pp.173–175). He estimated that the conglomerate occurs approximately 60 m above the base of the 'Gibbet Hill Group'; but it is in fact only about 25 m.

An excavation at Cryfield Grange [SP 3005 7471] showed 1 m of bedded conglomerate with well rounded pebbles of dark grey and brown quartzite; red-brown chert and a few of white quartz, together with clasts of red-brown mudstone. The sandy matrix had a lustre-mottled carbonate cement. Similar conglomerate appears as loose blocks in a field at Wainbody Wood Farm [SP 3145 7435]. This latter occurrence was considered to be 'Kenilworth Breccia' by Shotton (1929; p.177, loc.d), but the conglomerate contains almost entirely rounded pebbles, unlike typical 'Kenilworth Breccia', and it lies close to the base of the Kenilworth Sandstone; well below the level of the breccia at Kenilworth.

The outcrop of the conglomerate could not be traced east of a fault at Wainbody Wood Farm; but the bed is said to have been found in excavations at New Era Farm [SP 3173 7414]. Two conglomerate beds 0.3 and 0.6 m thick were encountered at depths of 4.6 and 5.5 m respectively in a borehole at Kingswood Farm [SP 3200 7325].

Lensing breccias occur within the Kenilworth Sandstone at a number of horizons, but can be used for stratigraphical correlation only very locally and lack the regional significance ascribed to them by Shotton (1929). The following downwards sequence was measured in a trench west of Westley Bridge [SP 3136 7367]: conglomerate with flattened pebbles of mudstone; siltstone and ironstone and rare quartzite and red chert 0.2 m; very hard calcareous sandstone with partings of mudstone clasts and a few pebbles 0.2 m; breccia with quartzite and chert pebbles and clasts 0.1 m. This occurrence is about 30 m above the base of the formation, and so is very near to the horizon of the Gibbet Hill conglomerate. A breccia recorded by Shotton (1929, p.177, loc.c) in 'the bank of a pool 1000 yards (305 m) south of Gibbet Hill cross-roads' may be at, or a little above, this horizon; and is well below the level of Shotton's 'lower breccia' at Kenilworth.

Ashow Formation

The Ashow Formation is equivalent to the Ashow Group of Shotton (1929), and comprises all the Enville Group above the Kenilworth Sandstone. It is predominantly argillaceous; but contains several thick sandstones. The total thickness of the formation is about 170 m, the lowest 115 m of which crops out in this area. The base of the formation is marked by the incoming of a sequence of mudstones 50 to 65 m thick, divided in places by a sandstone up to 15 m thick. These are the 'Whitemoor Marls' and 'Whitemoor Sandstone' named by Richardson and Fleet (1926, pp.297–298) after the former Whitemoor Brickworks, Kenilworth [SP 295 717]. The mudstones, temporarily exposed to a depth of 8 m in a trench at Victoria Spinney [SP 306 721], were red-brown with green reduction spots and included silty beds with small-scale cross bedding. Reduction spots from Windy Arbour [SP 304 721] are described by Mykura and Hampton (1984). Slightly lower in the sequence a trench near Crewe Farm, [SP 310 717], proved the presence of thin intercalations of sandstone.

The major sandstone above these mudstones is approximately 60 m thick, and forms the long dip-slope on which Ashow is built. An almost complete traverse of the outcrop was provided by a trench west of Ashow. The sandstone is soft, flaggy, cross-laminated and deeply weathered, with a few hard calcareous bands, micaceous bedding planes and thin mudstone bands. A thin section of this sandstone from Ashow (E51563) consists of about 70% of well sorted angular quartz grains, with quartzites and mudstones forming the bulk of the remainder. Among the mineral grains muscovite and plagioclase are common and biotite rare.

Triassic

Bromsgrove Sandstone Formation

The Bromsgrove Sandstone overlies the Enville Group unconformably in the east of the area, commonly forming a prominent feature. Exposures are poor and the detailed succession is unknown, but in general the formation consists of about 20 m of pale buff or grey-brown; soft, micaceous sandstone with minor red-brown mudstone. The only exposure of note is in a small quarry 200 m north of Chantry Heath Cottages [SP 3451 7290] where the section is:

Mercia Mudstone Group

Two small outcrops of Mercia Mudstone occur. The first, in the extreme northeast of the area is entirely drift covered and has only been confirmed in the adjacent areas. In the extreme southeast the base of the Mercia Mudstone consists locally of green mudstone and siltstone about 2 m thick, overlain by red-brown mudstone. There are no exposures.

Structure

A synthesis of the structure of the Warwick district is given by Old and others (1987). This area forms part of the Coventry Horst.

Pre-Westphalian structures

Viewed in their regional setting (Old and others 1987, Fig.2) the Cambrian rocks lie on the NW limb of a broad NE–SW syncline, which has been identified by plotting the faunal zones proved in boreholes. The varied dips proved in the boreholes listed earlier are not necessarily in accord with this structure and the folding of these beds is evidently more complicated in detail. The structure, in the Cambrian rocks presumably date mainly from the Caledonian orogeny.

Post-Westphalian–Pre-Triassic structures

Following the deposition of the Enville Group the Carboniferous rocks were gently folded (Old and others 1987; fig.19) and this area lies on the eastern limb of an open syncline plunging gently southwards.

Post-Triassic structures

Little structural information is available for the Triassic rocks; but they appear to be tilted gently south-eastwards. Most of the faults have a predominantly NE–SW trend and small throws; and are probably mainly of post-Triassic age.

Quaternary

The older Quaternary deposits are the product of a single glacial stage, the Wolstonian, and lie close to the type area to the east where Shotton (1953) identified a stratigraphic sequence within these deposits. The nomenclature used in this report is that of Sumbler (1983). Only much dissected remnants of more extensive glacial deposits are preserved (Figure 3).

Baginton Sand and Gravel

The Baginton Sand and Gravel is the oldest unit in the glacial sequence and was deposited by a river; the 'proto-Soar', flowing towards the northeast (Shotton 1953). Old and others (1987) summarise the faunal evidence from the Sand and Gravel and conclude that it points to a cold steppe environment. In this area only fragmentary evidence is found for the major tributary along the course of the present day Finham Brook; postulated by Tomlinson (1935; p.449). Shotton (1953) named the Baginton facies of the 'Baginton-Lillington Gravel' after sand and gravel workings at Baginton Hill (SP 346 749] which have since been back-filled. He divided the deposits into an upper sand (Baginton Sand) and a lower gravel (Baginton Gravel). The section given by Shotton (1929, p.212) reads:

The Baginton Sand can be traced around Baginton itself where it rarely exceeds 2 m in thickness. It gives rise to a noticeably sandy, and virtually pebble-free, soil. Shotton (1953) mapped the Baginton Sand southwards from Baginton to Chantry Heath Wood, but it could not be confirmed during the present survey. All the fields in that area are extremely pebbly and augering nearly always proved gravel.

The Baginton Gravel consists almost entirely of well rounded pebbles of quartzite and quartz, presumably derived from the Triassic 'Bunter' Pebble Beds, with a little locally derived material. In contrast to the river terrace deposits the gravel is normally flint-free (Shotton 1953 p.215), but a mound of gravel 450 m north of Baginton Church [SP 3438 7429] yields abundant angular flints. Flints were also seen in fields 300 m northeast of Chantry Heath Wood, and in 5 m of gravel exposed by the side of the A444 at Motslow Hill [SP 3319 7239].

Between the River Sowe and Knowle Hill [SP 300 726] Baginton Sand and Gravel occurs in small irregular patches, probably the eroded remnants of a once continuous spread. At King's Hill a gravel-filled valley falls gradually south-westwards. A borehole towards the upstream end of the channel [SP 3270 7501] proved 9 m of sand and gravel. These gravels were included by Shotton (1953 p.228) in the fourth terrace, but they are at too high an elevation (around 76 m OD) and contain only a few flints. A similarly orientated gravel-filled channel falling gently northeast is preserved 500 m northwest of Westley Bridge. Excavations for a gas main [SP 3132 7396] revealed 2 m of sandy and silty mainly 'Bunter' gravel with a few flints. Large sandstone and subordinate limestone boulders occurred towards the contact with the underlying Kenilworth Sandstone. Old pits proving up to 5 m of 'Bunter' gravel occur at the golf course east of Knowle Hill [SP 355 726].

Till (Boulder Clay)

Two small outcrops of till occur both contain Chalk erratics and are comparable to the Oadby Till. The till at Knowle Hill oversteps the Baginton Sand and Gravel and occupies lower ground. Very chalky till with a few flints was seen overlying sand and gravel in a trench 300 m north-northeast of Kingswood Farm [SP 3140 7333].

Wolston Clay

Smooth, grey brown, stoneless clay overlies Baginton Sand and Gravel at King's Hill.  It is correlated with the Wolston Clay of the area to the east, but it is uncertain whether the upper or lower subdivision is present here.

River Terrace Deposits

Flights of river terrace deposits are well developed along the rivers Avon and Sowe and to a lesser extent along Finham Brook. All post-date the glacial deposits are of broadly similar lithology, consisting of gravelly sands and silt. The pebbles are mainly 'Bunter' quartzite and flints.

The Fourth Terrace deposits always form isolated outcrops, in contrast with those of the lower terraces which are always contiguous. Typically, at Ashow and Stareton for example, they occur as wide spreads with base levels at c.69 m OD about 9 to 16 m above the top of the alluvium. These spreads are evidently dissected remnants of a single extensive gravel surface. The gravels are very flinty, resembling closely those of the Dunsmore Gravel (Sumbler, 1983) from which they were probably derived.

A low ridge of flinty gravel, at about 76 m OD north of Ashow [314 708], lies above the general level of the Fourth Terrace hereabouts. Small spreads of Fourth Terrace along Finham Brook also have a base at around 76 m OD.

Second Terrace deposits occur mainly along the rivers Avon and Sowe where the terrace flats lie up to 5 to 6 m above the alluvium. The terrace may be subdivided in places into levels 2a and 2b separated by a low rise.

At Stoneleigh [SP 3311 7275] excavations revealed 0.6 m of bedded orange-brown sand and red-brown silty sand overlying at least 0.8 m of bedded sand and gravel with angular flints and rare bone fragments. Excavations at Finham Sewage Works [SP 3375 7407] exposed flinty gravel with sandy and clayey bands. The gravel thinned downslope from 2.0 m to 1.1 m and had a very uneven base.

An alluvial flat with slightly higher marginal gravel spreads; probably of Second Terrace age, extends more than 1 km southeast from Stareton [SP 3417 7135] to the A445. There is a marked feature at the back of the First Terrace at Stareton and the stream flowing over the second terrace is incised. Towards the R. Avon the terrace deposits are gravelly, but become increasingly muddy when traced upstream.

The First Terrace, in places subdivided into la and lb levels; forms a clearly defined flat attaining a height of up to 2 m above the flood plain. It rarely exceeds 150 m in width but above Cloud Bridge [SP 3438 7235] it extends for more than 700 m from the flood plain.

Sections in the First Terrace deposits are largely confined to river banks where the flood plain is absent. A section 400 m upstream from Cloud Bridge [SP 3478 7245] reveals 1.8 m of very flintly gravel, imbricated in parts. Abundant sandstone fragments occur towards the contact with the underlying Kenilworth Sandstone. First Terrace deposits in the bank of the R. Sowe at Baginton [SP 3412 7441] comprise 0.7 m sandy alluvium, on 0.3 m coarse brown sand, on 0.6 m bedded sand and gravel.

Alluvium

The larger streams and rivers have continuous alluvial flood plains. The alluvium probably does not exceed a few metres in thickness and small exposures of rock are common in river banks. The flood plain attains a width, in excess of 300 m at Stoneleigh Abbey [SP 318 710] but is normally no more than 100 m wide. Most exposures of alluvium reveal a red-brown silty loam grading downwards into sand. Lenticular beds of gravel are common in the lower parts of the alluvium.

Economic geology

Building stone

Sandstone from the Enville Group has been widely quarried on a small scale for building as it can provide a good quality, durable freestone.

Coal

The area lies at the southern end of the South Warwickshire Prospect; a coalfield completely concealed by younger rocks. Details of the coal-bearing strata are given in the Westphalian section of this report. All of the recoverable reserves are in the Thick Coal and isopachytes and nomenclature for this seam have been published by British Coal (National Coal Board 1985; British Coal Corporation, 1987). The coal would provide excellent domestic, industrial and power station fuels.

Sand and gravel

The Baginton Sand and Gravel was formerly quarried extensively around Baginton, and the majority of the outcrops of this deposit have small, old workings. The area with the greatest potential as a resource lies within the boundary of Coventry (Baginton) Airport.

Small areas of Fourth Terrace have been worked at Baginton in combination with Baginton Sand and Gravel. No significant workings occur in the river terrace deposits elsewhere and their resource potential is unknown.

References

AMBROSE, K. 1986. Geological notes and local details for 1:10 000 sheets: SP36NW (Royal Leamington Spa). (Keyworth: British Geological Survey).

BRITISH COAL CORPORATION, 1987. Proposed colliery at Hawkhurst Moor. (British Coal: Central Area).

COPE, K.G. and JONES, A.R.L. 1970. The Warwickshire Thick Coal and its mining environment. C.R. 6e, Congr. Int. Stratigr. Geol. Carbonif., Sheffield 1967, Vol.II 585–598.

COX, H.M.M. 1953. The fossil plants of the Permian beds of England. Unpublished Ph.D4 thesis, University of Cambridge.

DIX, E. 1935. Note on the flora of the highest 'Coal Measures' of Warwickshire. Geol. Mag., Vol. 72, 555–557.

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

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Appendix Schedule of boreholes for SP37SW

Figures

For images see PDF

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

(Figure 2) Comparative Westphalian successions in cored boreholes

(Figure 3) Rockhead elevation at the base of the drift deposits