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Geological notes and local details for 1:10 000 sheets: SP37SE (Bubbenhall). British Geological Survey Technical Report WA/88/50

M G Sumbler

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Bibliographic reference: Sumbler, M G 1988 Geological notes and local details for 1:10 000 sheets: SP37SE (Bubbenhall). (Keyworth: British Geological Survey)

Author: Sumbler, M G, M A 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 SP37SE (Bubbenhall) is described with emphasis on significant exposed sections and borehole logs.

The Cambrian and older Carboniferous formations are not represented at outcrop and are known only from a number of boreholes which are summarised in graphic form. The younger Carboniferous and Triassic formations which come to crop are either very poorly exposed or concealed beneath widespread drift deposits.

Details are given of the lithologies and thicknesses of the drift deposits, which include important sections in sand and gravel. Diagrams show the thickness of the drift and the elevation of the sub-drift topography.

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

Introduction

This account describes the geology of 1:10,000 sheet SP37SE (Bubbenhall) (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 53NW, published in 1855. A brief account of the geology, although with few specific references to the area, was given by Howell (1859). The area was surveyed on the 1:10 000 scale by M.G. Sumbler in 1977 and 1978, and minor amendments were made by R A Old in 1988. It is included in 1:50 000 Sheet 184 (Warwick) published in 1984; an accompanying memoir is also published (Old and others, 1987).

The mapping and production of this report were partly funded by the Department of the Environment.

Copies of this report and uncoloured dyeline copies of the 1:10 000 map may be purchased from the Survey's office at Keyworth, Nottingham.

Parallel reports covering adjoining 1:10 000 sheets are:-

SP 36 NE Offchurch (Ambrose 1986)

SP 37 NW Coventry Central (Old 1988)

SP 37 NE Southeast Coventry (Sumbler 1985a) SP 47 Rugby West (Sumbler 1985b)

A report on the sand and gravel resources of the area is given by Crofts (1982).

Geological sequence

Beds down to the Tile Hill Mudstone are represented at outcrop or beneath drift. Lower beds are known from boreholes and the Binley Colliery workings.

Cambrian

Merevale Shales

Boreholes have proved Cambrian rocks underlying the Westphalian of the Warwickshire Coalfield. The Ryton No. 1 borehole [SP 3731 7397] proved 4.4 m of grey-green, slightly micaceous fissile mudstone and sandy Mudstone dipping at 25 to 40, below 904.2 m. Ryton No. 5A [SP 3915 7430] penetrated c. 5.5 m of probable Cambrian rocks, including purple and dark grey fissile Mudstones, below c.283.5 m. Ryton No. 1 yielded Dictyonema flabelliforme s.l., 'Acrotreta' sabrinae, and Lingulella c.f. nicholsoni, indicating a lower Tremadoc horizon, (Bulman and Rushton 1973), and Dr A.W.A. Rushton reports that samples below 645 m from the Burnthurst (1980) borehole [SP 3919 7170] yielded Linnarssonia belti? and Clonograptus? also of Lower Tremadoc age.

These beds correspond in age and lithology with the Merevale Shales of the Nuneaton District (Taylor and Rushton 1971).

Carboniferous

With the exceptions of the Tile Hill Mudstone, which crops out west of Bubbenhall, Carboniferous rocks, all of Westphalian age, are known in this area only from boreholes and the Binley Colliery workings. The beds dip westwards, thus successively younger rocks incrop against the base of the Trias from west to east, and the basal Westphalian unconformity probably incrops not far east of the coal workings in the north-eastern part of the map. The logs of four coal exploration boreholes drilled near Ryton in the 1940's are given in the Appendix I. More details of these boreholes, together with graphic logs, are given by Sumbler (1985a) in an account relating to the whole of Binley Colliery and in Old and others (1987). Graphic logs of recent British Coal boreholes are shown in (Figure 2). The classification of the Warwickshire succession is shown in (Table 1).

Westphalian A

Strata of Westphalian A age consist mainly of cyclic sequences, ideally comprising grey mudstone (rarely marine), siltstone, sandstone, seatearth and coal, in ascending order. The Westphalian A/B boundary is defined by the Vanderbeckei (Seven Feet) Marine Band (Ramsbottom and others 1978). This marine band is absent in this area, and the A/B, junction is uncertain but by comparison with those areas to the north where the marine band is present, the boundary is unlikely to be more than about 10 m below the base of the Thick Coal. In Ryton No.5a, Westphalian A is c. 75 m thick, and in Ryton No. 1 probably 60 m. Further south it thins to as little as 45 m at Weston Fields (Figure 2). The bulk of the strata are mudstones and seat clays and the sandstones occurring in the basal beds to the north (Sumbler 1985a) are not well developed. A coal which may be the Stumpy of the more northern part of the coalfield was proved in the Ryton Nos. 1, 5 and 5a bores, about 17 m above the base of the Westphalian. The Bench coal may be represented by a seam 9 to 11 m higher (Appendix). In Ryton No 5a, coals at 237.4 m and 232.0 m may represent the Double and Deep Rider group of coals, but in Ryton No. 1, much sandstone occurs at this horizon, and it is possible that the coals are washed out.

Coals thought to represent the Seven Feet and Thin are the lowest seams that can be widely correlated in the Binley area, although their identification is uncertain due to the absence of the Vanderbeckei Marine Band. They have been named solely on the basis of their position beneath the Thick Coal group of seams. Although present in the Ryton No. 4 borehole, they were not proved in this area, probably due to poor sample recovery in the older boreholes or because, they have died out. The measures between the coals consist largely of seat clay.

Westphalian B

Westphalian B strata, c 75 m thick in Ryton No. 1, constitute the highest normally productive measures of the Warwickshire Coalfield including the Thick Coal group of seams. Only the Nine Feet and Two Yard seams have been mined in this area; working ceased when Binley Colliery [SP 3795 7728] closed in 1963. The strata are cyclic, as in Westphalian A.

The measures below the Thick Coal, 1.8 m thick in Ryton No. 4, consist of seat clays and mudstones, with dark mudstone towards the base. At their base is the Vanderbeckei Marine Band horizon although no evidence of marine beds has been found in this part of the coalfield.

Thick Coal Group

In this area the component seams of the Thick Coal are separated over a considerable vertical interval, and constitute parts of the Splitting, Split and Recombining zones defined by British Coal (Old and others, 1987). Details are only available for the older boreholes.

The Smithy is usually about 0.6 m in thickness, but reaches 1.37 m in Ryton No. 5 borehole. Between the Smithy and High Main coals are 3.5 to 4.5 m of seat clays and mudstones commonly with ironstone nodules.

A coal termed the 'Nine Feet' is the lowest seam that has been worked from Binley Colliery. In the workings at Ryton-on-Dunmore it comprises a lower leaf about 1 m thick, and an upper leaf averaging 1.7 m, corresponding respectively with the High Main and true Nine Feet further north (Cope and Jones, 1970). The seat clay parting between the leaves is from 0.02 to 0.05 m thick in workings just north of the area, but increases southwards, reaching 0.8 m in Ryton No. 5 borehole. For this reason, only the upper leaf has been worked at Ryton.

The measures between the Nine Feet and the Ell are about 20 m thick in Ryton No. 5 and 5a bores, and perhaps 25 m in Ryton No. 4. They consist of mudstone and siltstone, often with a development of sandstone. Small south-south east trending washouts affect the Nine Feet in the workings at Ryton. The Ell coal was proved in the Ryton No. 5 and 5a boreholes, in which it was 1.37 and 0.91 m thick respectively. In these bores, the measures between the Ell and Ryder coals consist of 0.15 to 0.30 m of seat clay.

The Ryder and Bare together form a composite seam; the Ryder averages about 1.4 m in thickness, the Bare about 0.6 m. The coals are commonly separated by a parting of a few centimetres of seat clay, which reaches 0.46 m in Ryton No. 5a borehole. In Ryton No. 5a, the interval between the Bare and Two Yard consists of 7.6 m of seat clay and mudstone.

The Two Yard Coal is the highest seam to have been worked from Binley Colliery, averaging about 1.6 m in thickness. In the Binley area, the Thin Rider Coal, recognised by its mussel-bearing roof is usually 10 to 12 m above the Two Yard. However, in the Ryton No. 4 borehole, mussels occur in the shale roof of the Two Yard, suggesting that the Thin Rider has joined the Two Yard in this area.

The measures between the Two Yard and Four Feet coals are 27 m thick in the Ryton No. 4, bore consisting mainly of seat clays and shaley mudstones. At Weston Fields they yielded Anthracosia atra/fulva, A. ex gr. atra, Carbonita sp., spirorbis sp. and fish debris.

The Four Feet averages 1.8 m of coal commonly split by a seat clay band just above the middle of the seam. At Weston Fields the roof of the Four Feet yielded A. ex gr. atra. The measures between the Four Feet and the Aegiranum Marine Band horizon are about 20 m thick in Ryton Nos 1 and 4 bores, and include much sandstone. At Weston Fields they included a coal correlated with the Half Yard. There is usually a thin coal (up to 0.3 m) immediately beneath the Aegiranum Marine Band.

Westphalian C

The base of the Westphalian C stage is defined by the Aegiranum (Nuneaton) Marine Band, which is the highest and most widespread marine horizon in the Westphalian of the Warwickshire Coalfield. The full thickness of Westphalian C is about 62.5 m in the Ryton No. 1 borehole although in the absence of cores the upper limit is rather indefinite.

Measures below the Etruria Marl Formation

These beds are 32 m thick in the Ryton No. 1 bore and probably about 44 m in Ryton No. 4. The Aegiranum Marine Band was proved in Ryton No. 1, but no fossils were collected; in Ryton No. 4 borehole, its position is only inferred. It was also proved in the more recent borehole (Figure 2) and at Weston Fields yielded Lingula mytilloides and Orbiculoidea sp. Above the dark grey mudstone of the Marine Band, the measures consist mainly of grey mudstones and seat earths, with some sandstones and thin coals (rarely up to 0.3 m).

Etruria Marl Formation

In the northern part of the Warwickshire coalfield the Etruria Marl consists of brown and red variegated "marls" with lenticular beds of sandstone (Eastwood and others 1923; Mitchell 1942). In the southern part of the coalfield the formation is by comparison poorly developed; it is represented by mudstones, siltstones and sandstones, mainly coloured grey, but always variegated red and brown, occurring between typical grey Productive Coal Measures and the Halesowen formation. Many of the sandstones are coarse and gritty, but the conglomeratic "espley" rocks, which characterise the Etruria Marl farther north, are rare. The formation is about 30 m thick, although the limits are rather indefinite.

Westphalian D

Halesowen Formation

The Halesowen formation, is dominated by thick beds of coarse grey, feldspathic sandstone, interbedded with grey mudstones, siltstones, seatearths and rare coals. Regionally in the Warwickshire Coalfield there is an unconformity at the base of the formation (Mitchell, 1942, p 13, Old and others, 1987) which eventually cuts out the whole of Westphalian A, B and C south of this area. There is little evidence of this in the Bubbenhall area, however, and the base of the Halesowen is indefinite due partly to the poor development of the Etruria Marl. At Weston Fields a coal near the base of the formation is correlated with the Milton which occurs widely at this horizon in south Warwickshire and north Oxfordshire.

Only the lower part of the Halewsowen formation has been cored, and the upper part is known only from chipping samples and down-hole geophysical logs. The upper limit of the formation is therefore, uncertain, but the probable thickness is about 125 m.

Keele Formation

The formation has not been cored in this area and is known only from chipping samples. To the north it consists dominantly of red-brown mudstones with subordinate sandstones, and a few thin Spirorbis limestone beds (Eastwood and others 1923). The base is marked by a gradual change from the predominantly grey beds of the Halesowen Formation. The full thickness (apparently, 247 m) of the Keele formation was proved in the Ryton No. 1 borehole.

Enville Group

Coventry Sandstone Formation

The full thickness of the formation has been penetrated only in the Rock Farm [SP 3644 7428] and Weston Fields [SP 3622 7153] British Coal boreholes; down-hole geophysical logs indicate that the formation is c.300 to 310 m thick. It consists of sandstones, often coarse, interbedded with mudstones. Both sandstones and mudstones are dominantly red-brown in colour, but subordinate green grey patches and beds occur. Sandstones make up a large part of the formation, and this distinguishes it from the underlying Keele and overlying Tile Hill Mudstone formations, which are more argillaceous. Conglomeratic beds, and mudflake breccias are fairly common in the sandstone units.

In the Lodge Farm water borehole at Bagington [SP 3640 7410], 64.8 m of Coventry Sandstone was cored below the base of the Tile Hill Mudstone at 89.9 m (Figure 3). Thick beds of red and occasionally grey sandstone make up 70% of the sequence, and thinner beds of red marl about 30%.

Tile Hill Mudstone Formation

The basal beds of the Tile Hill Mudstone formation immediately overlying the Coventry sandstone. Formation were proved in the Lodge Farm borehole; 19.5 m of 'red marl' beneath the Trias, including 4.7 m of interbedded red sandstone and mudstone are assigned to this formation.

In the Avon valley West of Bubbenhall, the upper part of the Tile Hill Mudstone formation crops out on the upthrow side of the south-east trending Bubbenhall Fault. South of Bubbenhall Bridge, four sewerage boreholes proved mudstones beneath the First Terrace gravels; the deepest [SP 3503 7249] proved "hard red and green marl" to 6.1 m, below 1.5 m of gravel. Debris from trenches 0.75 km south of the bridge [SP 3523 7234], [SP 3535 7526] and [SP 3524 7230] proved mudstone beneath the terrace deposits. The river bluff west of Bubbenhall Bridge is made up of tough red mudstones and siltstones, with some thin sandstone beds. North-east of the bridge [SP 3522 7301] a roadside exposure showed 2 m of red, medium to coarse grained gritty sandstone. Dips varied from 15 to the south, to 10 to the north.

The gamma log of the Weston Fields Borehole [SP 3632 7153] records approximately 198 m of Tile Hill Mudstone, to a depth of 256 m, consisting mainly of mudstone but with sandstones up to 18.5 m thick. The Rock Farm Borehole [SP 3644 7428] proved the basal 24 m of the Formation to 100 m.

Triassic

Bromsgrove Sandstone Formation

The Bromsgrove sandstone rests on Carboniferous or Cambrian rocks. It consists of about 25 m to 40 m of cross-bedded sandstones interbedded with mudstones which form up to 40% of its total thickness. The sandstones are grey to buff, with a gypsum or calcite cement when fresh, but weathering produces a soft buff or brown rock. They are generally well sorted, containing fine to medium grade quartz and feldspar, commonly with abundant mica concentrated on bedding planes and cross-laminae. The mudstone units are generally 1 to 2 m thick and of a dark red-brown colour, in some cases mottled green-grey. They are usually fairly massive and may be smooth, silty or sandy. The major sandstone beds invariably have sharp, erosive bases, and commonly contain layers of the intraformational mudstone-flake conglomerate. Many of the sandstones grade upwards into mudstone, forming fining-upwards cycles which suggest a fluvial environment. As would be expected from this model, individual sandstone and mudstone units are lenticular in form.

Bromsgrove sandstone crops out on the south side of the Avon Valley south-west of Bubbenhall. It has been quarried at several sites east of Waverley Farm [SP 3530 7177], [SP 3535 7168] and [SP 3531 7141]. All these are ploughed over, but are marked by hollows with debris of soft, brown, fine to medium grained sandstone, with darker brown ferruginous spots. Two lenticular beds of mudstone near Waverley Farm give rise to damp clayey ground. The base of the upper 2 m bed, was exposed near Waverley Farm [SP 3527 7134], where 0.4 m of red and grey-grey mottled mudstone rested on sandstone. At 'Fairies Hole', a small man-made cave 0.5 km southwest of Bubbenhall Church [SP 3561 7228], 2 m of massive, level-bedded, medium to fine grained pink-brown sandstone was seen. An old quarry 0.25 km to the east-northeast [SP 3586 7231] showed 5 m of massive, pale pink-brown sandstone, with cross-bedding indicating derivation from the south and west. The outcrop is cut by the southeast trending Bubbenhall Fault which downthrows the formation roughly 20 m to the northeast. On the downthrow side, the Bromsgrove Sandstone crops out in the river bluff north of Bubbenhall Church, beneath basal Mercia Mudstone. A borehole 30 m north-east of the Church [SP 3600 7262] proved 1.8 m of soft green sandstone, beneath Mercia Mudstone.

West of Bubbenhall Bridge [SP 3508 7297] 0.4 m of pale buff fine-grained micaceous sandstone was exposed, close above Tile Hill Mudstone.

About 1 km north of Bubbenhall, Bromsgrove sandstone was formerly worked for building stone in Rock Spinney, but the quarries are now largely overgrown. A section [SP 3620 7357] showed 2.0 m of massive sandstone, with large load casts or erosional scours at the base, resting on 0.5 m of red sandy mudstone. Just to the north, the Bromsgrove sandstone is faulted against Mercia Mudstone by the northwest trending Princethorpe fault which here has a throw of about 45 m.

The Lodge Farm Borehole (Figure 3) penetrated the full thickness (26 m) of Bromsgrove sandstone, consisting mainly of grey sandstone with beds of 'red marl'.

Mercia Mudstore Group

The Mercia Mudstone group comprises the red mudstones formerly known as the "Keuper Marl", and the overlying green mudstones, of the Blue Anchor formation. The red mudstones are of the order 170 m thick, and the Blue Anchor formation about 7 m.

Red Mudstones (Keuper Marl)

These beds consist predominantly of blocky, red-brown mudstone, with a thin unit of green-grey sandstone and mudstone, the Arden sandstone, about 120–130 m above the base.

The basal beds commonly consist of green-grey silty clay, probably corresponding with the Waterstones facies proved in the B.G.S. Home Farm borehole [SP 4317 7309] [Sumbler 1980, 1985b]. They crop out southwest of Bubbenhall [SP 354 714] to [SP 359 719] where augering proved green-grey silty clay just above Bromsgrove Sandstone. In the B.G.S. Bubbenhall No. 2 borehole [SP 3578 7167] 0.35 m of pale green-grey, sandy mudstone with ochreous patches was proved beneath 1.9 m of predominantly red-brown mudstone. Another borehole close by [SP 3575 7170] proved 0.61 m of 'soft blue sandy clay' resting on Bromsgrove sandstone at 3.0 m and in a borehole 0.3 km southwest of Bubbenhall Church 0.76 m of 'stiff green clay' was proved beneath drift resting on Bromsgrove sandstone at 3.4 m [SP 3617 7230].

On the northeast (downthrow) side of the Bubbenhall Fault 30 m northeast of the Church, a bore proved 1.4 m of hard red marl above 0.46 m of 'stiff green clay' on Bromsgrove sandstone at 3.5 m. A borehole at Rock Spinney [SP 3610 7357] proved 0.55 m of 'hard green-grey and pale red-mottled, slightly shaly, silty clay', resting on Bromsgrove sandstone at 1.8 m. A nearby section at the top of an old quarry [SP 3613 7354] showed 1.4 m of green-brown mottled silty mudstone with sandstone lenticles, resting on Bromsgrove sandstone.

Southeast and east of Bubbenhall, a large number of sand and gravel trial bores penetrated Mercia Mudstone beneath Baginton Sand and Gravel, proving red clay or marl. Ditches in the bottom of the Ryton Wood gravel pit [SP 3721 7240] exposed 2.5 m of red blocky mudstone with some silty layers, and with small green-grey reduction spots.

In a ditch 1 km west of Ryton Police College [SP 3715 7320] 1.5 m of red brecciated crumbly mudstone with irregular green blotches was exposed in a zone 3 m wide, either side of which the mudstones were more coherent. This may mark the outcrop of the Princethorpe fault. On the northeast (downthrow) side of the fault, up to 5.8 m of red silty mudstone with some beds of green grey mottled mudstone and green siltstone were proved by a number of boreholes north of the Rock Farm Sewage Works [SP 361 739]. The Lodge Farm borehole [SP 3640 7410] proved 44.6 m of Mercia Mudstone, consisting mainly of 'red marl with green fish eyes', with 1.7 m of grey sandy marl, overlying 3.0 m of red and grey mottled marl at the base, corresponding with the lower part of the 'Waterstones' (see above).

Southeast of Ryton on Dunsmore, exposures and borehole records indicate the outcrop of the Arden Sandstone, mainly beneath drift. Reddish-purple mudstone associated with rarer green mudstone and siltstone was dug in the bottom of the Manor Farm gravel pit [SP 3928 7367] (c.f. the distinctive top beds of the Arden Sandstone in the Home Farm Borehole, Sumbler 1980). The nearby Ryton No. 12 (1958) borehole [SP 3922 7386] proved 1.5 m of 'red-brown, lilac and grey marl and silty marl' overlying 4.6 m of 'green-grey marl and silty marl' at 12.2 m, again suggestive of Arden Sandstone, although the depths and thicknesses may be unreliable. Ryton No. 6 proved 3.0 m of 'grey sandy marl' beneath gravel at 10.7 m, and Ryton No. 7 proved 3.7 m of green 'marl' and grey sandstone bands at 15.5 m beneath gravel and red marl. Southwest of Manor Farm [SP 386 733 to SP 389 738] a large number of sand and gravel trial bores proved green clay beneath drift. The Ryton No. 5 and 5a boreholes proved traces of 'grey marl' and 'light grey sandstone' immediately beneath drift gravel. In the road cutting just to the south [SP 3914 7418] green-grey silty clay was augered just below the base of the drift.

A bore southeast of The Grange [SP 3983 7383] proved 'green marly clay' to 1.8 m below drift, and gravel trial bores and exposures to the east (SP47; Sumbler 1985b) suggest the presence of Arden Sandstone displaced south from the outcrops described above, by a north-south fault downthrowing c. 20 m to the west.

Blue Anchor Formation

The Blue Anchor formation probably crops out beneath drift on the northeast side of the Princethorpe fault, 3.5 km east of Bubbenhall. In the area to the east (Sumbler 1985b) it consists of 5 to 7 m of pale, green-grey massive, silty dolomitic mudstone.

Penarth Group

Formerly known as the Rhaetic, the Penarth group is thought to be represented beneath drift on the northeast side of the Princethorpe fault, near Bull and Butcher Farm [SP 396 719]. It comprises the Westbury formation, Cotham Member, and Langport Member in ascending order. In the adjacent area (Sumbler 1985b), the Westbury formation consists of c 5 m of dark-grey to black fissile mudstones, and the Cotham Member 12 m of pale-green grey blocky mudstone. The Langport Member, formerly known as the White Lias, comprises about 2 m of white porcellanous limestone.

Structure

Little is known of the structure of the Cambrian, as it does not reach outcrop in the area. In boreholes, it is usually gently dipping, although dips up to 45 have been recorded. 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 fold movements in the Cambrian Rocks presumably date from the Caledonian orogeny.

The Carboniferous rocks lie on the eastern flank of the Warwickshire Coalfield, which has the form of a broad syncline, plunging gently to the south. Folding of the Carboniferous predated deposition of the Trias, which truncates the syncline, so that progressively older beds incrop eastwards against the Triassic unconformity. The incrop of the basal Carboniferous unconformity occurs in the eastern part of the area. The regional dip of the Carboniferous rocks is about 3–4 to the southwest, but dips steepen rapidly towards the incrop. This dip slackens again to the east in Binley Colliery workings, where the beds are affected by a belt of NNW–SSE folding 600 to 800 m across (Old and others 1987, Fig.3). East of this fold belt, the eastward rise of the beds at 15 to 30 resumes, and the coals are assumed to incrop just east of the workings.

The steepening dip of the Carboniferous as the incrop is approached suggests relative post-Carboniferous uplift by folding or faulting of the block of Cambrian basement uplifted block were removed rocks. The minor folding faulting in the underlying Coalfield. to the east. The Carboniferous beds on the by erosion prior to deposition of the Triassic in the colliery workings may be related to Cambrian or to the fold or fault bounding the Coalfield.

The Triassic beds dips to the south east at 1 to 2. A number of faults have been mapped in the Trias, often on the basis borehole evidence. The fault offsetting the Arden Sandstone east of Ryton-on-Dunsmore (see above) may reflect structures in the underlying basement; its position approximates to the eastern limit of the Carboniferous. Although its throw is in the opposite sense, it probably joins with the Princethorpe fault to the south, which geophysics suggests coincides with the Carboniferous incrop.

Quanternary

Glacial drift

The glacial deposits of the area were first described by Shotton (1953). They form an orderly layered sequence which has been designated the stratotype of the "Wolstonian" Stage of the British Quaternary (Shotton and West 1969). The nomenclature follows Sumbler (1983) and is modified after Shotton (1953). (Figure 4) and (Figure 5) show the thickness of the drift deposits and the elevation of the sub-drift (rockhead) surface respectively.

Shotton (1953, p.213–214), recognised 'Bubbenhall Clay' (supposed glacial deposits) apparently underlying Baginton Sand and Gravel at two localities regarding it as evidence of a separate older glaciation. In an augered traverse of Waverley Wood, [SP 358 708], Shotton (1953, p.213 and fig. 10) found 'stoneless red silty clay...largely consisting of reworked Keuper Marl' (i.e. 'Bubbenhall Clay') in a valley bottom and at the break of slope at the base of the Baginton Sand and Gravel outcrop. A nearby section [SP 357 718] (Shotton, op. cit.) showed 1.2 m of red sandy clay with scattered Bunter pebbles resting on Bromsgrove sandstone. However, the BGS Waverley Wood No. 2 [SP 3566 7147] and Bubbenhall No. 2 [SP 3578 7168] boreholes, drilled on the main outcrop of 'Bubbenhall Clay' shown by Shotton (1953. fig. 9) each proved only thin head deposits resting on Mercia Mudstone. The IGS Bubbenhall No. 1 borehole [SP 3583 7152] proved Baginton Sand and Gravel resting directly on Mercia Mudstone. Shotton's figured section shows that 'Bubbenhall Clay' was found only beyond the outcrop of the Baginton Sand and Gravel, and that where the Sand and Gravel was bottomed by augering it was found to rest upon Mercia Mudstone. Possibly then, the material described by Shotton is weathered Mercia Mudstone, or head derived from the Mercia Mudstone and nearby glacial drift deposits.

The Baginton Sand and Gravel is a fluvial deposit consisting of a basal gravel facies overlain by a sand facies (respectively the Baginton-Lillington Gravel and Baginton Sand of Shotton (1953)). Near the margins of the deposit, the basal gravel may be absent, having been overlapped by the sand. The maximum thickness of the deposit is about 10 m in the Bubbenhall and Ryton areas, where the gravel and sand are of about equal thickness.

The limits of the deposit indicate that it was laid down in a broad north-north-east draining valley, the 'Proto-Soar' (Shotton, 1953), bordered by the higher ground of the Enville Group on the west, and by the rising ground of the upper part of the Mercia Mudstone group on the east. Deposits near Princethorpe occur in a south-westerly draining tributary valley.

The gravel is generally clean and well sorted, the pebbles consisting almost entirely of well rounded Bunter quartzite pebbles. A pebble count on material from Ryton Wood Pit [SP 374 726] gave the following results:-

In addition Shotton (1953 p. 214, 215) recorded 1% of "brown chert-like flint" possibly derived from Irish Cretaceous rocks, and also one specimen of Lake District andesite. The sand is generally medium to find grained, with many lenses of Bunter quartzite gravel, and coal fragments. Both sand and gravel facies are generally strongly cross-bedded. The dips of foresets are rarely consistent in any one locality, but in the pits at Ryton Wood, and Manor Farm they generally indicate a flow from between west and south.

The Baginton Sand and Gravel has been extensively worked southwest of Ryton-on-Dunsmore. A section at Ryton Wood pit [3764 7261] showed:-

Both the sand and gravel are worked at Manor Farm Pit, 1 km south-south-east of Ryton-on-Dunsmore [SP 392 737]. A composite section showed:-

In the south face of the pit 0.5 km northwest of Ryton-on-Dunsmore [SP 3819 7485] up to 3 m of Baginton Gravel was exposed, consisting of coarse brown Bunter quartzite gravel with seams of coarse sand.

At Waverley Wood pit [SP 365 713] the Mercia Mudstone surface undulates and the base of the Baginton Sand and Gravel is preserved in a channel about 270 m across. A composite section of the whole pit is as follows:

In the writer's view the basal silts immediately predate the Baginton Sand and Gravel and were deposited in a small flood plain lake bounded by Mercia Mudstone. However, Professor F W Shotton (personal communication) considers that they are separated from the Baginton Sand and Gravel by an unconformity. The site is currently being studied intensively by a group of researchers at Birmingham University, Coventry Polytechnic and the British Museum (Natural History), because the basal silt and the sand and gravel are fossiliferous. Both have yielded mammalian remains and the silts insects and pollen. The base of the gravels has yielded several hand-axes. The results of this research are not available at the time of writing.

The general distribution of the Thrussington Till is similar to that of the Baginton Sand and Gravel, although the latter is locally overlapped by the Till, which then rests directly on Mercia Mudstone. The Till is a tough red-brown clay containing erratics predominantly of greengrey sandstone and siltstone with subordinate red mudstone, Bunter quartzite pebbles, Carboniferous sandstone and small fragments of coal. The green sandstones and siltstones commonly occur as large blocks often clearly striated, and, together with the red mudstone pebbles and red-brown clay matrix are evidently largely derived from the Mercia Mudstone, but may include some Bromsgrove Sandstone and Enville Group rocks. Rarer erratics include Leicestershire diorites, Westphalian ironstone and Carboniferous limestones. The Till is quite sandy in parts possibly due to incorporation of Baginton Sand and Gravel, which may also have contributed some of the Bunter quartzites and coal.

The Thrussington Till forms a sheet typically 3 to 5 m thick, and probably not exceeding 6 m anywhere in this area. The contact with the underlying Baginton Sand and Gravel is flat lying, and in exposures is usually sharp and flat, with little or no disturbance of the Sand and Gravel. Near Weston Fields Farm, however, [SP 369 709], [SP 365 702] undulations in the base of the Till cut out the Baginton Sand and Gravel.

The B.G.S. Burnthhurst Farm borehole [SP 3880 7158] (Sumbler 1981) proved the upper 1.7 m of Thrussington Till, beneath Wolston Clay, consisting of crumbly red-brown clay with many Triassic sandstone and siltstone and Bunter quartzite pebbles, and a few coal specks.

The Till is well exposed in several pits, where it forms the overburden above Baginton Sand and Gravel. At the south end of Ryton Wood Pit [SP 3746 7210] 3.5 m of red-brown clay with Triassic sandstone and siltstone, and Bunter quartzite pebbles was seen, the basal 0.2 m being very sandy. The junction with the underlying Baginton Sand was very sharp, but undulating. The Till graded upwards into Wolston Clay, through becoming darker and greyish-brown, and by a reduction in the size and quantity of the clasts. At the north end of the pit [SP 3789 7271] 4 m of Till with large blocks of Triassic sandstone and siltstone and a few pods of brown sand, was exposed beneath Wolston Clay. The junction with the underlying Baginton Sand, exposed over a considerable distance, was flat-lying. In a section 250 m to the south-west [SP 3764 7260] the Till had thinned to 2.2 m.

In Manor Farm Pit [SP 3933 7363] 3 m of Till was exposed above Baginton Sand. Just to the southwest of the pit [SP 389 735] mapping and gravel trial bores indicate a depression in the base of the Till, similar to those described above.

The Wolston Clay is typically an almost stoneless clay or silty clay with numerous silt laminae, but locally contains bodies of till. It is primarily a still-water deposit laid down in glacial lakes and ponds. The clays are usually grey-brown when fresh, but weather to a reddish colour. They are usually somewhat calcareous, particularly in the upper part of the sequence. The Wolston Clay reaches a maximum thickness of about 18 m in the area south of Ryton, where it is split into Lower and Upper divisions by the Wolston Sand and Gravel.

Debris from an excavation 0.3 km northeast of Hill Farm, Wappenbury [SP 3830 7051] consisted of grey-brown clay with scattered pebbles including Bunter quartzite, flint and rare chalk, Lias limestone and derived Gryphaea.

A cutting on the Fosse Way 0.7 km southwest of Princethorpe [SP 3988 7021] exposed 2.5 m of till-like Wolston Clay, consisting of stiff red-brown clay with abundant Triassic siltstone, rare chalk, and a Leicestershire diorite boulder, resting on 1 m of brown silty clay and silt.

In the B.G.S. Burnthhurst Farm borehole (Sumbler 1981), the Lower Wolston Clay was 12.6 m thick and consisted mainly of smooth brown grey plastic clay with laminae of pale brown silt, together with some more uniform silty clay layers and a few beds of fine sand in the upper part. The lowest 2 m contained rare pebbles of red Triassic mudstone and green-grey siltstone and sandstone, and graded downwards into Thrussington Till. The uppermost 2.5 m below the Wolston Sand and Gravel contained many Bunter quartzite and Triassic siltstone pebbles. The Upper Wolston Clay was 3.2 m thick, and again consisted of laminated clay, in part layered with silt, but was more blue-grey in colour than the Lower Wolston Clay. The clay contained rare Bunter quartzite and Triassic siltstone pebbles, and in the basal 0.3 m much chalk.

About 1 m of smooth, brown Lower Wolston Clay, grading down into Thrussington Till was seen at several points in the overburden of the Ryton Wood Pit [SP 3747 7210], [SP 3673 7260] and [SP 3789 7271].

In Ryton Wood a borehole [SP 3756 7225] proved 4.9 m of reddish-brown Upper Wolston Clay, above Wolston Sand and Gravel. A second bore 0.4 km to the northeast [SP 3784 7250] proved 6.4 m of Upper Wolston Clay, above Wolston. Sand and Gravel. The Lower Wolston Clay ("brown clay") was respectively 4.7 and 2.1 m thick, above c.3.4 and 6.4 of Thrussington Till ("red clay"). Shotton (1953, fig. 10) figures an augered section through Ryton Wood, suggesting that the combined thickness of the Lower Wolston Clay and Thrussington Till is about 7 m which is confirmed by the above boreholes.

The Wolston Sand and Gravel forms an almost continuous outcrop, close to the 91 m contour, southeast of Ryton-on-Dunsmore. It consists of up to 3 m of red, fine grained sand and silt with many lenses of plastic Wolston Clay and a thin basal layer of small pebbles. It locally gives rise to strong groundwater seepage. Unusually broad outcrops in Weston Wood [SP 357 702] and Waverley Wood [SP 358 710] occur where it forms hilltop outliers. An augered section through these outcrops (Shotton 1953, fig. 10) showed the sand to be c.3 m thick.

The Wolston Sand and Gravel in the B.G.S. Burnthurst Farm borehole (Sumbler 1981) consisted of 1.0 m of very fine-grained red sand, slightly clayey in parts, with some Bunter quartzite and green Triassic siltstone pebbles in the basal 0.3 m.

Shotton's (1953, fig. 10) augered section across Ryton Wood proved the Wolston Sand and Gravel to be about 3 m thick, including in the southeast,bands of plastic clay in the upper part. The boreholes in Ryton Wood mentioned above proved 0.8 m of 'orange brown fine sand', and 0.6 m of "clay and sand" on 1.8 m of 'brown sand' respectively.

The Dunsmore Gravel is the uppermost member of the glacial drift sequence. It is a sheet like deposit rarely exceeding 5 m in thickness which forms a plateau on the high ground southeast of Ryton. It is a brown, commonly ochreous, poorly sorted, sandy, clayey gravel containing lenses of sand and is generally particularly clayey in the lower part. Constituent pebbles are mainly subangular flints and rounded Bunter quartzites. Cobbles of Carboniferous sandstone also occurs, and Jurassic ironstone is usually present. Locally where unweathered at depth, the gravel contains some Jurassic limestone and chalk, but the upper layers are invariably leached and decalcified. There is usually an ochreous layer of iron enrichment just below the ground surface, and a layer of hardpan consisting of gravel cemented by purple-black limonite is commonly developed. The base of the Dunsmore Gravel is locally channelled into the underlying Wolston Clay. In the Burnthurst Farm borehole (Sumbler, 1981), the Dunsmore Gravel was 1.5 m thick. Crofts (1982) lists several boreholes proving up to 3 m of Dunsmore Gravel, and gives a number of pebble counts.

River deposits

Gravel deposits, corresponding with terraces 1, 2 and 4 of Shotton (1953) are developed in the Avon valley and small spreads of gravel and loam are present along the Leam tributaries in the Princethorpe area. They are largely derived from older drift deposits and younger terraces may consist in part of material reworked from older terraces. All the terraces are thus compositionally similar and are differentiated solely on the basis of height. Like the Dunsmore Gravel they are composed dominantly of flint and Bunter quartzite pebbles, but are generally better sorted and less clayey.

Fourth Terrace gravels form broad flats along the Avon, around 15 m above the modern flood-plain. Their distribution shows that the fourth terrace floodplain was up to 2 km across. The fourth terrace may be early Devensian (Shotton, 1968) or Wolstonian (Sumbler 1983) in age.

Sections in old gravel pits southwest of Ryton-on-Dunsmore show that the fourth terrace gravels are thin, and the bulk of the workings are in Baginton Sand and Gravel. Northwest of Ryton Lodge [SP 375 744] up to 1.5 m of flinty terrace gravel rests on Baginton Sand. A section 400 m northeast of Ryton Lodge [SP 3814 7425] showed 1.2 m of brown sand with a few flint and Bunter quartzite pebbles, above Baginton Sand.

Extensive spreads of Second Terrace gravels occur along the Avon. These deposits commonly slope gently downwards towards the river; possibly the second terrace bench was destroyed during later downcutting. The deposits extend up to c.8 m above the modern floodplain, and commonly extend below the level of the first terrace. The second terrace is of mid-Devensian age (Shotton 1968).

Gravels up to 5 m thick were exposed in the pit 0.5 km northwest of Ryton-on-Dunsmore [SP 382 750]. The base of the gravels is gently undulating and falls gradually northwards (towards the river). At the southern limit of the second terrace channel, the gravels are banked steeply against Baginton Sand and Gravel and Mercia Mudstone. A composite section [SP 3824 7494] to [SP 3815 7500] showed:-

The peaty deposit is restricted to the margin of the channel and dies out rapidly northwards, but occurs over the whole 150 m width of the pit. A similar deposit on the north side of the river (SP37NE) is described by Shotton (1968).

The First Terrace is well developed along the Avon and borders the flood-plain on one side of the river or the other almost continuously. In a number of places the terrace is divided into two levels, la and lb. Terrace lb corresponds in height with the undivided first terrace, and terrace la is somewhat lower, usually 1 m or so above the modern alluvium. The deposits, rarely exceeding 3 to 4 m in thickness consist of loamy sand and gravel, commonly very clayey near the surface. The gravel consists dominantly of Bunter quartzite and flint pebbles, like the older terraces, but is finer in overall grade.

There is no direct evidence for the age of the first terrace deposits in this area, but judging from the supposed ages of the second terrace, and the alluvium, they are presumably late Devensian, or early Flandrian.

A number of boreholes penetrate the first terrace south of Bubbenhall Bridge. One [SP 3525 7238] proved 4 m of brown sandy clayey gravel and another [SP 3501 7251] proved 1.7 m of similar deposits.

Alluvium forming the Avon flood plain consists of an upper layer of loamy clay underlain by a basal lag deposit of gravel. There is no direct evidence of the age of the alluvium, but it is presumed to be wholly Flandrian

Industrial minerals

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 and Structure sections 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

Most of the larger deposits of sand and gravel have at some time been worked, with the exception of the Dunsmore Gravel, which is too clayey for most purposes. The more recent workings have been concentrated in the Bubbenhall and Ryton areas, where the main source is the Baginton Sand and Gravel. The Sand is normally extracted in a separate operation to the underlying Gravel, to minimize the need for grading the product. Large areas of sand and gravel have been worked out, although at the time of survey, pits still operated at Waverley Wood Farm [SP 363 716], Ryton Wood [SP 379 728], Manor Farm [SP 392 737] and northwest of Ryton [SP 382 747]. In 1988 the only pit operating was at Waverley Wood. Southwest of Ryton [SP 380 742] fourth terrace gravels have been worked in conjunction with the underlying Baginton Sand and Gravel, and first and second terraces have been worked near Ryton [SP 382 750].

Although much of the outcrop of the Baginton Sand and Gravel is either worked out, or built over, large areas of potentially workable sand and gravel remain, mainly in the Bubbenhall area. Although thin, some of the large spreads of terrace gravel may be of economic value.

References

AMBROSE, K. 1986. Geological notes and local details for 1:10 000 sheets: SP36NE (Offchurch). (Keyworth: British Geological Survey).

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

BULMAN, O M B, and RUSHTON, A W A. 1973. Tremadocian faunas from boreholes in central England. Bull. Geol. Surv. G.B., No. 43, pp 1–40.

COPE, K G and JONES, A R L. 1970. The Warwickshire Thick Coal and its Mining Environment. Compte Rendu de Congres Intern. Strat. Geol. Carbonif. Sheffield 1967. pp 585–598.

CROFTS, R G. 1982. The sand and gravel resources of the country between Coventry and Rugby, Warwickshire: description of 1:25,000 sheets SP47 and part of SP37. Miner. Asses. Rep. Inst. Geol. Sci. No.125.

EASTWOOD, T, GIBSON, W, CANTRILL, T C and WHITEHEAD, T H. 1923. The geology of the country around Coventry. Mem. Geol. Surv. 149 pp.

HOWELL, H H. 1859. The geology of the Warwickshire Coalfield and the Permian rocks and Trias of the surrounding district. Mem. Geol. Surv. 57 pp.

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

NATIONAL COAL BOARD. 1985. The South Warwickshire Prospect: A consultation paper. (NCB: South Midlands Area).

OLD, R A. 1988. Geological notes and local details for 1:10 000 sheets: SP37NW (Coventry Central). (Keyworth: British Geological Survey).

OLD, R A., SUMBLER, M G and AMBROSE, K. 1987. Geology of the country around Warwick. Mem. Br. Geol. Surv. (Sheet 184).

RAMSBOTTOM, W H C, CALVER, M A, EAGER, R M C, HUDSON, F, HOLLIDAY, D W, STUBBLEFIELD, C J and WILSON, R B. 1978. A correlation of Silesian Rocks in the British Isles. Geol. Soc. London Special Report, No. 10, 82 pp.

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. Phil. Trans. Roy. Soc. Series B, Vol. 237, pp 209–260.

SHOTTON, F W. 1968 The Pleistocene succession around Brandon, Warwickshire. Phil. Trans. Roy. Soc. Series B, Vol. 254, pp 387–400.

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

STUBBLEFIELD, C J AND TROTTER, F M. 1957. Divisions of. the Coal Measures on Geological Survey maps of England and Wales. Bull. Geol. Surv. G.B. No. 13, pp 1–5.

SUMBLER, M G. 1980. Home Farm Borehole pp 4–6 in I.G.S. boreholes, 1978 Rep. Inst. Geol. Sci., No. 79/12.

SUMBLER, M G. 1981. Burnthurst Farm Borehole, p 2. in I.G.S. Boreholes 1979. Rep. Inst. Geol. Sci., No. 80/11.

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

SUMBLER, M G. 1985a. Geological notes and local details for 1:10,000 sheets: SP 37NE (Southeast Coventry). (Keyworth:British Geological Survey.

SUMBLER, M G. 1985b. Geological notes and local details for 1:10,000 sheets: SP4714E, SW, SE (Rugby West). (Keyworth: British Geological Survey).

TAYLOR, K and RUSHTON, A W A. 1971. The pre-Westphalian geology of the Warwickshire Coalfields. Bull.Geol. Surv. G.B., No. 35, 152 pp.

TRUEMAN, A E and WEIR, J. 1946. A monograph of British Carboniferous non-marine Lamellibranchia. Palaentogr. Soc. Monograph.

Appendix I Logs of selected boreholes

The numbers are those of the BGS 1:10,000 record system, in which they are prefixed SP 37 SE. Depths and thicknesses are given in metres

2. Ryton No. 5 (1948) [SP 3916 7431] SL+ 80.6

3. Ryton No. 5a (1949) [SP 3915 7430] SL+ 80.5

32. Ryton No. 1 (1946) [SP 3731 7397] SL+ 70.2

30. Ryton No. 4 (1948) [SP 3872 7491] SL+ 67.1

Appendix II Schedule of boreholes on Sheet SP37SE

Detailed logs of non-confidential boreholes may be examined at the BGS National Geosciences Data Centre, Keyworth, by prior appointment and on payment of the current fee.

c. Denotes confidential- records details of which may only be released to a third party by permission of the original client.

Figures and tables

For figures 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) Vertical section of the Lodge Farm Borehole

(Figure 4) Thickness of drift deposits

(Figure 5) Elevation of sub-drift (rockhead) surface

Tables

(Table 1) Classification of the Westphalian of the Warwickshire Coalfield (in part based on Ramsbottom et al 1978 Plates 1 to 3)