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Geology of the Malmesbury District (explanation of one-inch geological sheet 251, New Series)

by R. Cave

Natural Environment Research Council. Institute of Geological Sciences. Memoir of The Geological Survey of Great Britain England and Wales

Geology of the Malmesbury District (Explanation of One-inch Geological Sheet 251, New Series)

By R. Cave with contributions by G. A. Kellaway, I. B. Paterson and F. B. A. Welch Palaeontology by I. E. Penn, A. W. A. Rushton and D. E. White Petrography by J. R. Hawkes and R. W. Sanderson Geophysics by R. B. Evans and A. J. Burley Water supply by M. Price Economic geology by E. M. Pyatt

London Her Majesty's Stationery Office 1977. © Crown copyright 1977. ISBN 0 11 881242 4*

The Institute of Geological Sciences was formed by the incorporation of the Geological Survey of Great Britain and the Museum of Practical Geology with Overseas Geological Surveys and is a constituent body of the Natural Environment Research Council. Institute of Geological Sciences, Exhibition Road, London SW7 2DE

Author R. Cave, B.Sc., PH.D. Contributors A. J. Burley, B.Sc., PH.D., R. B. Evans, M.SC., J. R. Hawkes, B.Sc., PH.D G. A. Kellaway, D.Sc., I. B. Paterson, B.Sc., I. E. Penn, B.Sc., PH.D., M. Price, M.Sc., E. M. Pyatt, B.A., A. W. A. Rushton, B.A., PH.D., R. W. Sanderson, B.Sc., F. B. A. Welch, B.Sc., PH.D., and D. E. White, M.SC., PH.D.

Preface

The district represented on the New Series One-inch Geological Sheet 251 (Malmesbury) was first geologically surveyed on the one-inch scale by W. T. Aveline, E. Hull, D. Williams, J. Phillips, A. C. Ramsay, H. W. Bristow and W. Saunders and the results published on Old Series One-inch Sheet 34 in 1857 and Sheet 35 in 1845. A revised edition of Sheet 34 was published in 1859, and revised editions of Sheet 35 in 1866 and 1872. The geology of the western part of the area was described in a memoir on the East Somerset and Bristol Coalfields published in 1876; the eastern part in 'Geology of parts of Wiltshire and Gloucestershire', published in 1858. A memoir entitled 'Wells and Springs of Gloucestershire' was written by L. Richardson and published in 1930.

The primary six-inch geological survey of the Malmesbury area was made in the years 1945–47 and 1957–62 by Dr F. B. A. Welch (District Geologist), Dr R. Cave, Mr I. B. Paterson and Dr G. A. Kellaway. The one-inch geological map was published in 1970. Lists of six-inch maps and the names of the surveyors are given on p. viii.

Of the Jurassic fossils, Miss B. Cox identified those from the Kellaways Clay, Dr H. C. Ivimey-Cook from the Lower and some of the Middle Jurassic and Dr I. E. Penn revised and extended this work on the Inferior Oolite and Great Oolite Series. The fossils from the Cambrian were identified by Dr A. W. A. Rushton and those from the Silurian by Dr D. E. White. We are grateful for the assistance of Dr M. G. Bassett and Dr L. R. M. Cocks in naming some Silurian brachiopods and of Dr M. K. Howarth, Prof. D. T. Donovan, Dr H. S. Torrens and Dr J. R. Senior in naming the ammonites; Mr M. J. Barker named some of the Bathonian gastropods.

The petrography of the Llandovery Traps was described by Mr R. W. Sanderson and that of some limestones of the Great Oolite Series by Dr J. R. Hawkes, both of the Petrographical Department. Geophysical investigations were described by Mr R. B. Evans and Dr A. J. Burley of the Geophysical Division, while an account of the Hydrogeology was contributed by Mr M. Price of the Hydrogeological Department.

We are grateful for the assistance of Dr M. L. K. Curtis of the City of Bristol Museum who provided valuable information from recent excavations within the district and to the North Wiltshire Water Board for borehole data. We wish to thank the landowners and tenants of the district for their co-operation in facilitating access to their properties.

The memoir was edited by Dr J. R. Earp and Miss E. M. Pyatt was responsible for the documentation of the text and the diagrams. The manuscript received final approval from Sir Kingsley Dunham, F.R.S., as Director before his retirement in December 1975.

A. W. Woodland DirectorInstitute of Geological Sciences Exhibition Road South Kensington London SW7 2DE 21st September 1977

(Front cover) The porch of Malmesbury Abbey. The abbey was built and restored with Box Ground Stone (Great Oolite) from Box, near Bath. (A12568).

(Back cover) Selection of fossils from the Malmesbury district comprising the following genera: Grammoceras, Pleydellia, Haugia, Cereithyris, Vaugonia, Rhactorhynchia, Plectothyris, Passaloteuthis, Clypeus, Stiphrothyris, Digonella, Palaeocylus, Pycnactis (3), Resserella (3), Meristina.

List of six-inch maps

Geological six-inch maps included wholly or in part in the Malmesbury (251) Sheet are listed below, together with the initials of the surveyors and the dates of survey. The surveyors were R. Cave, G. A. Kellaway, I. B. Paterson, P. Toghill, the late F. M. Trotter and F. B. A. Welch. The maps are available in manuscript form for public reference at the Institute of Geological Sciences, London and Leeds.

National Grid Maps: all within the 100 kilometre squares SO and ST.

Chapter 1 Introduction

THE district^‡1  represented on the One-inch New Series Malmesbury (251) Sheet lies athwart the Wiltshire–Gloucestershire boundary, the larger part of the south-east quadrant falling in Wiltshire. A physical division, and one with a geological basis, is effected by the Cotswold scarp, which is disposed south to north leaving one-third of the region to the west. This division underlies a number of aspects of the district which are dealt with as follows:

Physical features and drainage

West of the scarp is an undulatory low area. The surface of this rises to 300 ft (91 m) above sea level around Milbury Heath and north of Chipping Sodbury only. These eminences are part of two low ridges which follow the crop of the Carboniferous Limestone from Tytherington north-eastwards to just north of Cromhall, and from Chipping Sodbury through Wickwar to join the north end of the other ridge. These ridges control the drainage so that the direction of flow is almost entirely northwards or northwestwards into the Little Avon and thence into the Severn at Berkeley.

East of the scarp is a gently inclined plateau of higher ground. The slope approximates to the regional dip of the Mesozoic rocks, which is slight towards the south-east. The highest ground is thus in the north-west and near Symonds' Hall Farm [ST 790 960]^‡2  it rises to 816 ft (249 m) OD. The lowest is near Malmesbury, were the River Avon leaves the district, at about 210 ft (64 m) OD.

The greater part of the plateau area is drained south-eastwards, down dip, into the Bristol Avon which has one tributary rising at Crow Down Springs [ST 837 865], Sherston, and another at Wor Well [ST 901 939], Tetbury. A small portion of ground on the eastern margin of the region falls within the catchment of the River Thames, while the north-west fringe of the plateau is drained by obsequent tributaries of the Little Avon and Stroud Frome which have broken the scarp, cutting back deep valleys during late and post-Pleistocene times.

Economy

The economy of the western area differs from that of the plateau area in being more diversified and industrialised. Coal mining, although now defunct, helped in building up a society partly dependent on industry, and hematite mining in the locality of Frampton Cotterell assisted in this. Further impetus to the development of industry has arisen by virtue of the low relief of the area and its position between Bristol and the Midlands. The railway, the old main trunk road and the M5 Motorway serve this part of the district.

Stone quarrying in Carboniferous Limestone at Wickwar, Cromhall and Tytherington, and in Carboniferous quartzitic sandstone for road surfacing, at Cromhall; brick-making from Keuper Marl at Charfield, and celestine extraction from the base of the Keuper at a number of places are all indigenous industries still active. At one time very local use was made of Lower Lias limestone for building, while new light industries replaced the older industries such as coal mining and wool processing. The latter is still carried on in places, and at Cam there is some heavy engineering.

The plateau to the east is almost entirely agricultural. Sheep farming was once the main agricultural occupation and, attracted by the wool and water resources, a woollen industry grew in the valleys of the north of the district, for instance, around Nailsworth. This industry disappeared when sheep farming declined and has been replaced by small light industries producing furniture, bedding and plastics.

Geological sequence

The geological formations described in this memoir are as listed below:

Geological history

The only recorded events in the pre-Silurian history of the Malmesbury district are those of late Cambrian times when there was deposition of mud and silt in a fairly shallow sea inhabited by graptolites, trilobites and brachiopods. Some time during Ordovician and early Llandoverian times a period of earth movement followed by erosion created a major break in the stratigraphical sequence. In later Llandoverian times there was a renewed marine transgression and the sandy sediments that accumulated on the sea bed suggest that the water was shallower and closer to the shore than during the late Cambrian.

On two occasions during Llandoverian sedimentation lava flows spread as far as the western part of the Malmesbury district from a volcanic centre to the east or south-east.

As time progressed the water shallowed, and during the Wenlock epoch argillaceous sedimentation was interrupted periodically by sandy and calcareous phases. Within the main part of the Tortworth Inlier no Ludlow rocks have been observed, but west of the boundary fault passing between Tites Point, Berkeley and Buckover (the Berkeley Fault), Upper Ludlow rocks are flaggy, sandy, conglomeratic and calcareous. They were deposited in close proximity to an area undergoing some erosion, at least periodically, and it is likely that this area lay to the east of the Berkeley Fault. A major movement on the Berkeley Fault occurred before Upper Old Red Sandstone deposits spread across the district, for to the west of the fault, evidence from the adjoining Gloucester (234) district indicates that the Ludlow deposits were succeeded conformably by Lower Old Red Sandstone red muds and sands laid down in landlocked sheets of water under arid conditions. Well over 1000 ft (305 m) of these deposits accumulated west of the fault, but if they were laid down immediately to the east of it, they were eroded away following the sharp earth movements which occurred before late Devonian times. Arenaceous material that formed the Upper Old Redstone then invaded the district, consisting of coarse sands and pebbles which swept in from a more elevated terrain subjected to a higher rainfall.

The terrestrial basin of late Devonian times was invaded from the south by the sea at the beginning of the Carboniferous Period. To the south of the district the sediment consisted mainly of fine-grained lime mud and oolite–mud, with shell remains, indicating clear, rather shallow, warm waters while in the Malmesbury district, which lay nearer to the margin of this sea, terrigenous materials figure more prominently, and sand and dolomite-mud were common.

Arenaceous deposition continued into the Namurian epoch when earth movements interrupted the sedimentation. In Westphalian times deltaic swampy conditions with occasional marine incursions led to the formation of mudstones interbedded with coal seams and sandstones. The middle of this epoch saw a thick accumulation of sand over the whole district to form the Pennant Measures. Towards the end of Westphalian times red muds and sands began to accumulate in the basin and these heralded the onset of desert conditions which accompanied and followed the Armorican orogeny.

Strong earth movements ended Coal Measures deposition and initiated a period of very active erosion during Permian and early Triassic times. No deposits representative of these periods are known from the district, but erosion and weathering reduced the landscape and oxidized older rocks which suffered reddening, or even replacement by hematite, to a considerable depth. By Keuper times large flat areas, surrounded by a rather subdued relief, became the sites of extensive shallow lakes. Around the edges of these, scree and boulder deposits accumulated, while within them red muds with sand layers formed. As these sediments accumulated the landscape was buried and late in their history evaporation of these lakes caused sulphates to be deposited, including those of strontium and barium, to form the Celestine Bed^‡3 . Shortly after this a change in the conditions resulted in green sediments being deposited and the Tea Green Marl was thus laid down, burying the Red Marl before the close of the Triassic period.

Slight earth movements which terminated the Keuper deposition were followed by an extensive marine incursion. Little coarse terrigenous material entered this Rhaetic sea of the Malmesbury district and black pyritous muds formed under anaerobic underbottom conditions. Later these conditions were changed, possibly owing to the shallowing of the water, and deposits of pale grey calcareous muds followed. The algal deposit forming the famous Cotham Marble is a late-stage product of this period. Again slight earth movements interrupted sedimentation and in places resulted in. erosion. These also affected the early part of the Lias and in the southern part of the district shallow-water limestone formed. In the rest of the district the Lias sea-floor was muddy virtually from the start, but in the Middle and Upper Lias it became progressively siltier and sandier; and at the close a shallow-water argillaceous limestone with phosphatic patches was formed.

Deposition of the late Lias sands was followed by establishment over the district of a shallow warm sea in which a large thickness of carbonate material including ooliths and shell debris accumulated uniformly over a wide area. A marked hiatus in this deposition and much erosion occurred in Middle Inferior Oolite times, followed by deposition of the Upper Inferior Oolite.

The Fuller's Earth, Great Oolite and Forest Marble represent phases of occasionally interrupted sedimentation between a landmass which lay to the north-east and a marine basin which lay to the south. In the Malmesbury district the influence of the land lay in providing sand, silt and mud. Shallow clear warm water some distance offshore gave conditions for the formation of calcareous muds and the growth of banks of ooliths and shell fragments. In the deeper water to the south, marine clays accumulated. Initially it was this deeper water that held influence and in it formed the clays of the Lower Fuller's Earth. In the south of the district this influence remained dominant nearly to the end of the period of deposition of the Fuller's Earth, but northward the influence of progressive shallowing was felt and resulted in the formation of slightly silty calcareous muds and then oolites.

The oolite sand with shell fragments and some other calcareous debris which developed largely to the south of the district in shallow clear water gave rise to the Great Oolite, while to the north of it was an area of non-deposition or erosion. Great Oolite deposits have not been recognised with certainty in this northern area and there the influx of lower Forest Marble shelly oolitic material, which spread over the district, rests directly on the limestones of the Fuller's Earth. Later Forest Marble deposits represent shallow-water near-shore sediments, not far from estuarine conditions.

The Cornbrash in the main represents widespread and fairly uniform marine conditions and the succeeding Kellaways Clay and Oxford Clay are witness to a return to deeper water deposition.

Although younger Jurassic rocks and possible Cretaceous and Tertiary formations were laid down in the district, none now remains. Earth movements also affected the rocks during these times, however, producing some normal faults probably at the end of the Jurassic period and imparting the gentle south-eastward dip in the Tertiary era.

During the Pleistocene epoch the district escaped the extensive glaciation experienced in areas to the north, but severe frost and a locally formed cover of ice and snow did affect the rocks. During late and post-Pleistocene times meltwater carved deep valleys and deposited spreads of gravel and silt along the foot of the Cotswold scarp. Some of these valley slopes became unstable when the ground thawed and this resulted in a certain amount of landslipping.

Consolidation and lithification of sediment follows upon its deposition, but the age of a deposit does not necessarily reflect its degree of diagenesis. This depends largely on the nature of the sediment, the amount of water retained and the heat and pressures (depth of burial) to which it has been subjected.

Within the district probably none of the sediments exposed has suffered deep burial and with the exception of the igneous rocks heat has played a minor role in lithification. The most important factor appears to have been the nature of the sediment itself. Granular materials such as silica and carbonate sands, which are soluble in slightly acid water, appear to have consolidated and even lithified quickly. The silica sands, especially where grains are in contact creating pressure points, became bound by a silica gel, whilst carbonate silts and aggregates were cemented by secondary crystalline growth. The pH value and ionic content of contained water plays an important role in these processes. Some of the Jurassic limestones give indication of having lithified even before the succeeding bed was deposited, for bored limestone surfaces are preserved below and bored pebbles of these limestones are contained within the later sediments.

Muds on the other hand have suffered a lower order of lithification. Even the Lower Palaeozoic mudstones are relatively soft and Jurassic argillites are always represented at outcrop by clay. High pressure, expulsion of water and the drawing together of electrically charged clay mineral molecules are important in the lithification of muds and clays.

Chapter 2 Cambrian

Introduction

Lower Palaeozoic rocks are exposed within the district in the Tortworth Inlier, a tract of Cambrian (Tremadoc Series) and Silurian rocks bounded by outcrops of Old Red Sandstone and younger rocks between Milbury Heath and the Severn Estuary.

An important north–south fault lies between Buckover and Tites Point (Gloucester (234) Sheet). This fault is concealed beneath Triassic formations in the south, but in the north it separates off to the west an outcrop of the only undoubted Ludlow rocks of the Inlier.

Only the southern half of the Inlier is included within the Malmesbury district and this half has the form of an inverted letter Y. The base is north of Breadstone, and the arms divide near Falfield extending to Milbury Heath in the west and Chatfield in the east, and enclosing between them a synclinal area of Upper Old Red Sandstone and Carboniferous rocks. More very small inliers lie on the line of the southward extension of the eastern branch, along the Little Avon as far as Wickwar. The stem of this inverted Y consists largely of Tremadoc rocks and the arms are of Wenlock and Llandovery.

History of research

The great age of the sediments and associated igneous rocks of the Tortworth district of south Gloucestershire was early recognised through the researches of Buckland and Conybeare (1824), Weaver (1824) and Murchison (1839). All the rocks were initially classified as "Transition Series" or "Limestone" and later, following Murchison, as Silurian. Phillips (1848), in the course of a detailed survey of the Tortworth district, failed to detect the presence of rocks older than Silurian. Later work, by Woodward (1876) and Reed and Reynolds (1908a, b), concerned only the Silurian rocks of the southern part of the inlier. The presence of Tremadoc shales thus was not detected until Smith obtained fossils from the northern part of the Palaeozoic outcrop by means of which Stubblefield (in Smith and Stubblefield 1933) demonstrated a Lower Tremadoc age for the host sediments, the Breadstone Shales. Curtis (1955a) distinguished an upper more arenaceous division, the Micklewood Beds, characterised by numerous siliceous bands. In the present survey it has been possible, in spite of limited exposures, to map the two formations Breadstone Shales and Micklewood Beds with the aid of field brash and distinctive features.

In accordance with Geological Survey practice, the Tremadoc is classed here as Cambrian.

General account

Tremadoc strata, predominantly shale, occupy an elongate tract east of Berkeley. The outcrop is 2 miles (3.2 km) wide at Newport in the south but tapers towards Purton (Gloucester (234) Sheet), 5 miles (8 km) to the north. Approximately two-thirds of the outcrop falls within the limit of this district.

The broad structure of the Tremadoc rocks of the inlier shows a westerly dip of up to 80° in the north-east becoming southerly at about 30° in the south. A large fault bounds the Tremadoc on the west, throwing down Ludlow and Downtonian strata and, marginally to this on the east there seems to be a narrow belt of strata showing variable dips. The ground is poorly exposed and the structure of this strip is not known.

At Michael Wood, a small inlier of Tremadoc is separated from the main outcrop by a broad ridge of Triassic marls.

The oldest rocks thus occur in the north-east where highly unconformable Triassic rocks overstep them, whilst the youngest rocks occur in the south, plunging south-south-west with much lower dips at about 30° under unconformable Llandovery.

The total thickness of the exposed Tremadoc rocks cannot be established with any accuracy. Calculations based on the width of outcrop produce a figure of over 7000 ft (2100 m) but no reliance should be placed on this, for undetected repetition of strata by faulting could reduce it considerably.

The two divisions Micklewood Beds and Breadstone Shales have been adopted for use in the six-inch surveying of the district, but the definition of these divisions is not sharp and thus, where the succession is not otherwise apparent, the nature of the beds may not be sufficient to indicate to which formation the rocks belong. Although the Breadstone Shales are considered to be of lower Tremadoc age it has not been established that the complete thickness of the Micklewood Beds is of upper Tremadoc age. Certainly the fauna indicates that the major part of the formation is upper Tremadoc, but the lowest 200 to 300 ft (60–90 m) have yielded no fauna which sheds any light on their age.

Tremadoc Series

Breadstone Shales

The Breadstone Shales occupy the northern portion of the Tremadoc outcrop, between Breadstone and Newport, forming a low almost featureless tract of country. They consist of thinly bedded slightly micaceous shales, bluish grey when fresh, but weathering to khaki. Thin bands of micaceous siltstone or very fine-grained sandstone are common while thin bands of blue or green clay may be pulverised shale along planes of movement. Rarely, thin calcareous bands with cone-in-cone structures are developed. The shales are much jointed; both bedding and joint planes are characteristically coated with iron oxides. Folds, of small amplitude and having a westward or south-westward plunge, are apparent in most exposures of the shales. Exposures are uncommon as a result of the tendency for the shales to break down into heavy tenacious, yellowish brown clay, often to a depth of between 4 and 6 ft (1.2–1.8 m).

Smith and Stubblefield (1933) offered no estimate of the thickness of the Breadstone Shales but Curtis (1955a) suggested that it should be measured in thousands rather than hundreds of feet. If dip values obtained at the surface are reliable, 5000 ft (1525 m) may not be an exaggerated estimate of the thickness. The possibility of undetected faulting and folding should not, however, be excluded.

Palaeontology

The bulk of the fauna from the Breadstone Shales indicates a lower Tremadoc age (Dictyonema flabelliforme Zone). Most of the species were recorded from a shaft sunk at Breadstone House, just north of the present sheet, and include horny brachiopods, bellerophontoids, Beltella depressa, Micragnostus calvus, Niobella homfrayi smithi, Conophrys sp. and ostracod-like forms including Septadella jackmanae. In addition Dictyonema jiabelliforme s.l. and Borthaspis innotata? were collected from the northern part of the outcrop during the subsequent survey of the Gloucester (234) Sheet. A record by Curtis (1955a) of Clonograptus sp. at Mobley, near Berkeley, suggests that the C. tenellus Zone may also be present. A.W.A.R.

Micklewood Beds

The Micklewood Beds occur over about the southern one-third of the Tremadoc outcrop, roughly southward of Newport. The small inlier at Michael Wood is composed entirely of Micklewood Beds.

The formation is mainly argillaceous and consists of thinly bedded grey or bluish grey shales. The Micklewood Beds are distinguished from the Breadstone Shales by the occurrence of beds of micaceous siltstone and fine-grained sandstones which are locally thick enough to make quite prominent features, especially near Newport and in the Michael Wood area.

Curtis (1955a) considered the Micklewood Beds to be about 1200 ft (370 m) thick, but revised the figure to 1500 ft (460 m) in 1968. An estimate of the thickness based on the average dip and width of outcrop would, however, produce a figure nearer 2500 ft (760 m), but such a thickness seems so large that folding and faulting probably should be considered as having effected some repetition of the succession.

Palaeontology

The age of these beds is, in part at least, upper Tremadoc. Some layers are crowded with fragments of Lingulella sp. and yield occasional trilobite and ostracod fragments; Curtis (1968) has also collected Schmidtites cf. crassus, Angelina sedgwicki and Peltocare? olenoides. These new records enhance the North Welsh aspect of the Tremadoc fauna of this inlier, remarked upon by Stubblefield (in Smith and Stubblefield 1933, p. 374), whereas the resemblance to the fauna of the Shineton Shales of Shropshire is less marked. A.W.A.R.

Details

Breadstone Shales

Most of the section in the stream on the northern side of Bushy Grove lies within the Gloucester (234) Sheet but at its western end [SO 6962 0099], 1250 yd (1143 m) E30°N of Berkeley Station, olive-brown micaceous silty shales dip steeply to the west.

The shales are disturbed by minor folds with a steep south-westward plunge. A few specimens of Lingulella sp. and graptolithine scraps were found.

The stream rising near Standle Farm and flowing past Lorridge and Crawless farms cuts through the veneer of disintegrated shale at three places. North-north-east of Crawless Farm, approximately 100 ft (30 m) of soft, bluish grey shales with a few thin bands of blue clay are exposed in the stream bank [ST 700 998]. Thin beds of olive-green micaceous siltstone were also noted. The sediments dip to the south-west at 25° to 60° and are much disturbed by small steeply plunging folds. Upstream, at a locality [ST 703 994] 400 yd (366 m) E of Crawless Farm, greyish blue and green shales of similar aspect and structure are seen. Finally, 370 yd (338 m) E36°S of Lorridge Bridge there is a further exposure [ST 712 922] of shales as described above, with a very steep dip to the west. Here the shales lie only a few feet below the former Triassic land surface and are red and purple in colour.

Grey shale is exposed in a small stream [ST 695 989] 350 yd (320 m) NW of Coldelm Farm, dipping westward at about 45°. Bluish grey and greyish green shales were also exposed in a stream section [ST 701 981] due south of Heathfield Farm and again [ST 708 983] 700 yd (640 m) E of the farm, where the Tremadoc is reddened in patches. The shales dip very steeply almost due west.

Micklewood Beds

Newport–Woodford Green area

In 1964 a realignment of the A38 trunk road at Newport opened two sections in the Micklewood Beds. The most southerly continued for a distance of 800 ft (244 m) just south of Doverte Brook while the northerly was in the village itself covering a length of 600 ft (183 m) (Figure 3). The northernmost 300 to 350 ft (91–107 m) of strata in this section fall below the mapped base of the Micklewood Beds, and are considered as Breadstone Shales, but the lithological differences between the rocks of this part of the section and the rest are not striking. Most of the fauna collected from these sections, including specimens of Peltocare? olenoides and Angelina sedgwicki, is kept at the Bristol City Museum; additional specimens of Lingulella sp., a fragment of a trilobite and an ostracod are in the Geological Survey collections.

In the northerly section some 140 ft (43 m) of similar beds were exposed down to the horizon where Micklewood Beds were considered, during the mapping, to give way to Breadstone Shales. Below this a further 200 ft (61 m) of shales and mudstones of the same type occur, but containing fewer sandstone layers, particularly near the base. On the one-inch map these are shown as Breadstone Shales the junction with the Micklewood Beds having been mapped as coincident with a marked feature seen just west of Newport. This feature probably marks an outcrop of hard sandstone which divides beds with sandstone layers above from those devoid of sandstones below. The new cutting thus served to emphasise the imprecise nature of the division of the Tremadoc rocks of Tortworth into Micklewood Beds and Breadstone Shales.

The Micklewood Beds of the southerly section consist of some 300 ft (91 m) of shales with thin layers of fine-grained sandstone. The sandstones are commonly less than 6 in (15 cm) thick and rarely over l ft (30 cm) while many were seen to be lenticular. One bed was 3 ft (91 cm) thick. The colours of these rocks are grey and purplish red. R.C.

Flaggy micaceous sandstone dipping south-westwards was exposed in a shallow excavation [ST 6951 9630] 350 yd (320 m) N40°E of Woodfordgreen Farm. Six hundred yards (549 m) E24°N of the farm reddened sandstone with bluish grey mudstone was exposed.

Michael Wood area

In the Michael Wood Inlier, sandstone features describe an open synclinal fold plunging to the south. A group of shallow workings [ST 703 953] in sandstone at Michael Wood was overgrown and no exposure available. I.B.P.

In 1949 a borehole was sunk for water 120 yd (110 m) SSW from Michaelwood Lodge Farm [ST 7167 9455]; Dr M. L. K. Curtis examined the weathered remains of the Tremdoc portion of this core and kindly provided information regarding the strata penetrated.

The boring proved 120 ft (36 m) of Tremadoc under a cover of 80 ft (24 m) of Keuper Marl, the former being cored while the latter was percussion drilled. Most of the Tremadoc core was in a weathered condition or had been disturbed since the cessation of drilling; a lot had been lost. The lowest 20 ft (6 m) however were in good condition and Dr Curtis considered these to be fairly representative of the 120 ft (36 m) of Tremadoc penetrated. They consisted of dark and pale grey rather silty micaceous shales, somewhat siliceous in places, with bands of hard, pale grey, micaceous, fine-grained sandstone up to 11 in (28 cm) thick but commonly between 1 and 3 in (3–8 cm) thick. Many of these sandstone bands are lenticular. Dips vary from 4° to 17°.

The fauna collected from these beds includes Schmidtites cf. crassus, Lingulella cf. lepis, L. cf. davisii and L. spp.

In 1970 Mr R. J. Wyatt recorded details of the strata exposed in excavations for the M5 Motorway on the western side of Michael Wood. Together with evidence afforded by several trial boreholes, they established the presence of another small outcrop of Micklewood Beds. This, largely fault-bounded, is separated from the main body of the inlier to the east by Silurian beds disposed in a syncline having a steeply dipping and faulted western limb. Beds revealed in the cutting and boreholes mainly comprised reddish, mauvish grey and greenish grey, thinly bedded, micaceous, sometimes silty or sandy mudstones and shales; occasional thin bands of red or purple-stained, fine-grained, finely micaceous, hard, well-jointed, quartzose sandstone are present, generally up to about 1 ft (30 cm) in thickness, but sometimes reaching as much as 4 ft (122 m). A variety of minor sedimentary structures in these beds suggests deposition in shallow agitated waters, as adduced by Curtis (1968). Several specimens of Lingulella cf. lepis and one of L. cf. nicholsoni were collected from about 30 ft (9 m) below ground level in a borehole at the Damery road crossing [ST 6986 9501].

Dips are generally to the north-east and of the order of 60° to 70°, though vertical beds were recorded in some borehole cores. Quite intense small-scale folding and contortion were evident in the cutting and slickensided shear surfaces are not uncommon.

Charfield area

Fine-grained quartz sandstones and shales, occasionally with fragments of Lingulella sp. occur in ditches and trenches around Charfield Green. These include a roadside trench [ST 7270 9246] 150 yd (137 m) WSW of Watsome Bridge, trenches [ST 7266 9239] to [ST 7266 9230] in fields some 200 to 300 yd (183–274 m) SW of Watsome Bridge, and field ditches some 350 and 450 yd (320 and 411 m) due south of Watsome Bridge. Another small exposure occurs in the Little Avon River for a few hundred yards north and south of Bunsall Bridge [ST 727 898], where red shale and thin bands of pink grit are exposed beneath the Triassic rocks. R C.

Chapter 3 Silurian

Introduction

THE Silurian rocks crop out between Matford Bridge [ST 6832 9686] in the north and Horseshoe Farm [ST 6734 9025] in the south. To the south-east they extend to Charfield Green. Another small outcrop of Silurian rocks occurs from just east of Berkeley northward to Wanswell Court Farm [SO 6896 0104]. The oldest Silurian rocks are of late Upper Llandovery age. They rest unconformably on the Micklewood Beds of the Tremadoc and include two basaltic lavas referred to as traps. The Llandovery rocks pass upwards without apparent break into Wenlock rocks, but the discordant cover of newer rocks bounding the Inlier east of the Berkeley Fault has kept from sight any overlying rocks of undoubted Ludlow age. The highest Silurian rocks exposed east of the Berkeley Fault are probably those near Horseshoe Farm and though certain elements of their fauna could be interpreted as being of basal Ludlow age, the overall evidence of the fauna is in favour of a Wenlock age (Reed and Reynolds 1908b, p. 524).

The Silurian rocks outcropping west of the Berkeley Fault between Wanswell Court Farm and Long Bridge [ST 6879 9921], are of undoubted Ludlow age and are part of a fully developed Ludlow sequence overlain conformably by Lower Old Red Sandstone. The rocks are not naturally exposed within the confines of this district, but trenches dug on the boundary with the Gloucester (Sheet 234) district, near Wanswell Court Farm, revealed Ludlow rocks probably equivalent to the Bridgewood Beds of the standard section at Ludlow, Shropshire.

History of research

Little need be added to the account of the history of research on the Silurian rocks of the Tortworth Inlier given by Curtis (1955a, p. 71). The earliest account of them was by Weaver in 1824. Murchison (1834) mentioned the "Wenlock Rocks" and the Horderley and May Hill rocks' of Tortworth and later (1839, p. 454) he described in. more detail the Silurian rocks of Tortworth. He mentioned the rocks of Ludlow age which crop out at Tites Point to the north of the district, but was very cautious in ascribing a similar age to the rocks in the extreme south of the Inlier, near Horseshoe Farm, though he appreciated these to be the youngest in this part of the Inlier. He recognised rocks belonging to his earlier erected Wenlock formation (1839, p. 195) and also Llandovery rocks under the name Caradoc Sandstone, but he was convinced that the traps were intrusive (1839, p. 459).

J. Phillips (1848, p. 190) wrote a short account of the Tortworth district which presents the view that the traps were contemporaneously effusive, and his map depicts two beds of limestone in the Wenlock.

Lloyd Morgan and Reynolds (1901) and Reed and Reynolds (1908a, b) produced detailed stratigraphical accounts while Reynolds (1924), after investigating the traps, was of the opinion that the lower was intrusive. Whittard and Smith (1944) described small inliers of Lower Palaeozoic rocks near Wickwar and since then Curtis (1955a) produced a stratigraphical summary in which formational names for divisions of the Llandovery rocks were introduced. These names have been used subsequently by the Geological Survey and are embodied in the following classification which is employed in this account.

Llandovery Series

General stratigraphy

The Llandovery sediments are marine mudstones, siltstones and fine sandstones with some calcareous bands, in colours which include grey, purplish red and green. At the base is the Lower Trap of unproved age, for, so far as is known, no dateable Silurian sediments have been seen below it. It seems that this trap, and in its absence the Damery Beds, rest unconformably on Cambrian strata. There is little real doubt therefore that the Lower Trap is part of the Llandovery sequence.

The fauna of the Damery Beds includes Eocoelia curtisi and Costistricklandia lirata alpha, an association which according to Ziegler (1966, p. 540) is characteristic of Upper Llandovery C₅ beds. In contrast, above the Upper Trap, the Tortworth Beds contain Eocoelia sulcata and C. lirata lirata considered by Ziegler to indicate Upper Llandovery C₆ beds. Therefore there is every indication of a complete sequence between Llandovery and Wenlock approximately at the top of the Tortworth Beds, which, for practical purposes is taken at the base of the lowest of the beds of crinoidal limestone.

Thus in common with the shelf deposits of most of the Welsh Marches the Llandovery sediments of Tortworth belong to the Upper division. All of the exposed rocks lie east of the Berkeley Fault, which appears to have acted in a way analogous to the western boundary of the Malverns, and their age is restricted to the late Upper Llandovery. The Llandovery rocks which occur to the west lie too deep to have been investigated, but there the environment is analogous to that west of the western boundary of the Malvems, so that earlier Upper Llandovery rocks might exist, as they do at May Hill and Cowleigh Park, Malvern (Ziegler 1964, p. 468).

Ziegler (1965) produced an interesting analysis of the distribution of certain Llandovery marine communities and that of the Tortworth Llandovery he depicts (1965, fig. 3) as belonging to his Eocoelia community. There may be some oversimplification in this, for it is equally common to find Costistricklandia to the exclusion of Eocoelia. Ziegler did note this (1965, pp. 270–2) and at the same time drew attention to the sensitivity of these communities to changes in the sea depth. At Tortworth he maintained that it was possible to illustrate this in the changes wrought by the advent of the Upper Trap. It is assumed that the trap was a submarine lava and shallowed the sea by an amount equal to its thickness at any one place. At Woodford, Ziegler reported that a Costistricklandia community occurred below the trap which is only some 20 ft (6 m) thick and reappeared on top of it, the thickness of the trap being insufficient to displace it. Near Charfield to the south-east, he reported that a Costistricklandia community present prior to the lava flow, which here is some 210 ft (64 m) thick was replaced by an Eocoelia community above the flow. However, collecting by D. E. White at Woodford has revealed that while a Costistricklandia community does occur in the Damery Beds, this has given way to the shallower water community containing Pentameroides in the highest Damery Beds visible, just below the Upper Trap. Although we have no evidence regarding the fauna overlying the trap at Woodford, Ziegler's reported reappearance of Costistricklandia would read, according to his thesis, as evidence that deeper water followed the trap. This does not substantiate Ziegler's statement (1965, p. 270) that the Damery Beds show the sequence of depth controlled communities Eocoelia–Pentamerus–Stricklandia. Near Charfield, a Costistricklandia community occurs at Cullimore's Quarry [ST 7198 9269] in beds overlying a thick development of the Upper Trap, where a shallower water community might have been expected. Indeed Ziegler (1965, p. 272) implied that a Costistricklandia community occurs beneath the trap in this area too. It would thus seem that in the present state of knowledge precise applications of the thesis on 'communities' is not possible.

Lower Trap

This rock is an altered-olivine basalt lava that thins consistently from south-east to north-west along its outcrop from an estimated 110 ft (34 m) near Charfield Green to 80 ft (24 m) at Damery and only 30 ft (9 m) near Michael Wood [ST 702 951]. North-west of Michael Wood the Damery Beds rest directly on the Tremadoc indicating a complete absence of the Lower Trap. It could be inferred from this that the source of the basalt of the Lower Trap lay to the east or south-east, but there is no more direct evidence of its derivation.

Although quarried in the past as a source of roadstone, exposures are now few and no contact with the sedimentary rocks is visible. Red micaceous sandstone was reported to overlie the igneous rock just south-east of Charfield Green (Reed and Reynolds 1908b, p. 514) and lenses of red clay have been seen within it, which might indicate that some lateritic weathering of surfaces had taken place.

The petrography is discussed on p. 27.

Damery Beds

The Damery Beds consist of fine-grained sandstones, siltstones and mudstones. Limestone is very subordinate though some thin impure layers do occur and the sediments tend to be calcareous below the zone of weathering. The colour of the rocks is commonly dull red or green.

For detailed mapping purposes the Damery Beds between Charfield Green and Stone were divided into four units, each consisting largely of either sandstone or mudstone. Unfortunately the same subdivisions could not be established in the outcrop south-south-west of Falfield where the ground is poorly exposed, and it might be that the lithological units are impersistent.

The lowest subdivision is a sandstone some 120 to 180 ft (37–55 m) thick. Above this and producing a marked slack in the middle of the outcrop of the Damery Beds is an argillaceous subdivision about 120 ft (37 m) thick in the south-east, and about 100 ft (30 m) thick or less in the north-west. Another sandstone-with-mudstone subdivision follows, also seeming to alter in thickness a little along the outcrop. Near Charfield Green it is estimated to be 290 ft (88 m) thick, near Michael Wood [ST 701 948] about 200 ft (61 m) and near Woodford some 250 to 300 ft (76–91 m). Overlying this and beneath the Upper Trap is the fourth subdivision, mainly of mudstone. The higher mudstone is not everywhere recognisable, even along the Charfield Green to Woodford outcrop, and it is thin, being about 60 or 70 ft (18 or 21 m) thick, probably less in the Woodford area. R.C.

Palaeontology

The fauna collected from the Damery Beds is probably insufficient to analyse, but it suggests that Eocoelia curtisi has greater prominence in the lower two subdivisions, while Costistricklandia lirata alpha and Dalmanites weaveri are more characteristic of the upper two subdivisions.

Three of the Lower Silurian marine communities first described by Ziegler (1965) have been recorded from the Damery Beds. Of these, the Eocoelia community is well represented and the occurrence of a Costistricklandia community succeeded by a Pentamerus community close to the base of the Upper Trap has already been noted (p. 17).

The Eocoelia community is characterised by an abundance of Eocoelia curtisi, the other most persistent components being Streptelasma sp., Atrypa reticularis, Brachyprion arenacea, 'Camarotoechia' cf. nucula, Leptostrophia compressa, Mendacella (Dalejina?) cf. phiala, Strophochonetes sp., Tentaculites anglicus, Dalmanites weaveri and Encrinurus onniensis. Other fossils occasionally found in association with this assemblage include gastropods — Bucanella (Plectonotus) trilobata, Gyronema spp.,Liospira lenticularis — and bivalves, mainly pterinids.

A detailed statistical analysis of the composition of a typical Eocoelia community at approximately 50 ft (15 m) above the base of the Damery Beds, from an exposure near Damery Bridge [ST 7056 9428], has been described by Ziegler and others (1968, p. 9). At this locality the percentage composition of the community was calculated to be: E. curtisi 47, 'Camarotoechia' aff. decemplicata 17, Dalejina sp. 12, leptostrophid 5, rugose coral 2, and others 6.

The Costistricklandia community from an exposure of the Damery Beds near Woodford is characterised by the common occurrence of Costistricklandia lirata alpha. Subordinate components of the assemblage are mainly brachiopods including A. reticularis, B. arenacea, Cyrtia exporrecta and E. curtisi. Also present are Favosites sp., Streptelasma sp. and T. anglicus.

Overlying beds, just below the base of the Upper Trap, contain a Pentamerus community, including an abundance of Pentameroides sp. associated with Streptelasma sp. and Pholidostrophia sp.

From the vicinity of Damery Bridge, and therefore from an horizon likely to be within the Damery Beds, a graptolite was found which P. T. Warren considers to be an early form of Monograptus priodon. Curtis (1972, p. 16) records Monograptus marri also from these beds. D.E.W.

Upper Trap

This altered basaltic lava is thickest near Charfield Green where on average it is about 210 ft (64 m) thick. It thins slightly southwards over the short outcrop visible in that direction, and markedly north-westward, being about 100 ft (30 m) thick at Avening Green [ST 709 938], 60 to 70 ft (18–21 m) near Daniel's Wood [ST 699 943] and about 20 ft (6 m) at Woodford. In the area of Woodford the thickness is clearly uneven and diminishing north-westwards, in places there being no trace of it. There seems to be no doubt about its extrusive nature. It is amygdaloidal and it has been recorded that some exposures reveal irregular, thin deposits of limestone resting on the trap, and that this limestone is 'ashy' containing well-marked lapillae of the lava (Lloyd Morgan and Reynolds 1901, p. 271). Whether this deposit is a genuine tuff or merely the decomposition product of an irregular lava surface, it indicates that the igneous rock was a lava. The petrography of the Upper Trap is discussed on p. 27.

Tortworth Beds

The Tortworth Beds extend from the roof of the Upper Trap to a top taken arbitrarily at the incoming of crinoidal limestone. There is no certainty that in every one of the three separate areas where this has been done—namely Charfield Green, Falfield and Stone—the bottom limestone mapped is the same bed, but it seems likely that it is. Added difficulty stems from the fact that this bottom limestone can be seen to pass into sandstone in places, but from the map such a situation is fairly apparent.

Curtis (1955a), introducing the name Tortworth Beds, recorded their thickness as about 200 ft (61 m), but this is too low. The only outcrop where the top and bottom of the Tortworth Beds have been mapped is that from near Charfield to Woodford. Thicknesses of the Tortworth Beds are estimated to be about 350 ft (107 m) near St John's Church, Charfield Hill, 780 ft (238 m) at Avening Green and 940 ft (287 m) at Stone (Figure 4), (Figure 5). This thinning of the sequence towards the east suggests differential downwarp, hinging against a platform, or ridge, that formerly lay in that direction and is now under Mesozoic cover. The two lavas thicken towards, and possibly have effused from, the region of such a hinge. The presence of a large, positive magnetic anomaly there might be reflecting this source though it is probably mainly due to a rise in the basement rocks in continuity with the Malvern Hills. Within the Tortworth Beds four broad divisions into mudstone and sandstone were observed and an interpretation of their relationships is summarised (Figure 5).

In composition the rocks are fine-grained sandstones and mudstones. The sandstones are confined largely to the lower half of the Tortworth Beds, whilst the upper half is predominantly argillaceous, with subordinate sandstone beds. The mudstones tend to produce slight depressions in the topography between ridges representing the outcrops of the more resistant sandstone. These features facilitated the mapping.

Often present in the basal Tortworth Beds is a thin decalcified sandstone. This is characterised by the small coral Palaeocyclus porpita and was named the Palaeocyclus Band by Curtis (1955a, p. 5). This fossil is present above the Upper Trap in the outcrop north-west of Charfield Green, but it also occurs in the outcrop south of Falfield where, it seems likely, the Upper Trap is absent and only Tortworth Beds are exposed. R.C.

Palaeontology

The faunal assemblages of the Tortworth Beds indicate the presence of the Costistricklandia community and also, but less clearly defined, the Eocoelia community.

A Costistricklandia community is present in strata formerly exposed at Cullimore's Quarry immediately above the Upper Trap. In collections from these lowermost Tortworth Beds, C. lirata lirata is common, associated with other brachiopods including Eospirifer radiatus globosus, Cyphomena (Cyphomenoidea) wisgoriensis, L. compressa and Pentlandina tartana. Corals are also common, particularly Favosites multipora.

A probable example of an Eocoelia community is represented by a small collection of fossils from a temporary exposure of possibly high Tortworth Beds, near Falfield [ST 6823 9295]. This includes a small number of examples of Eocoelia sulcata in association with B. arenacea, L. compressa and Streptelasma?. Another possible example of this community is represented by a small collection from the lowermost Tortworth Beds of Daniel's Wood, which includes isolated examples of E. cf. sulcata together with F. multipora, Leptaena cf. purpurea, Cornulites serpularius, Dalmanites sp. and Encrinurus onniensis. P. porpita is common.

Curtis (1972, p. 16) records M. priodon associated with P. porpita, and a single example of M. marri which may also be from the Tortworth Beds. D.E.W.

Details

Whitfield–Falfield area

The Llandovery rocks in this area are very poorly exposed and occupy the core of an anticlinal structure, the western limb of which is concealed by Mesozoic rocks and truncated by a fault. No separation of the rocks into Damery Beds and Tortworth Beds has been effected, but north of Whitfield a rough division has been made into an area of mainly mudstone near Eastwood Park [ST 678 923], bordered on the east by an area of rocks which have greater proportions of sandstone and siltstone, and are possibly mostly younger. The more arenaceous beds produce slight ridges and a soil containing sandstone or siltstone fragments.

An exposure in the area of mainly mudstone—"Old Gravel Pit" on the six-inch map [ST 678 922] showed splintery red sandstone with some celestine as recorded by R. W. Pocock and yielded: Favosites cf. multipora, P. porpita—not abundant, crinoid columnals, 'Chonetes' sp., Leptostrophia compressa, Liospira sp. and Dalmanites weaveri—common.

Reed and Reynolds (1908b, p. 522) recorded a very similar fauna from this place (specimens from "South end of Eastwood Park" in Geological Survey collections), and the presence of P. porpita suggests a position in the basal Tortworth Beds.

Another exposure in the area of mainly mudstone was seen by R. W. Pocock who recorded fossiliferous Llandovery strata dipping north-westward at 60°, whilst earlier Reed and Reynolds (1908b, p. 522) had recorded reddish sandstone, some rather argillaceous and fissile, and some calcareous. They considered that certain red-stained material in Lord Ducie's collection came from here, including specimens of a species of Palaeocyclus and 50 yd (46 m) to the south R. W. Pocock recorded "Petalocrinus", two fossils not usually considered as contemporary. R.C.

The main road (A38) between Whitfield and Falfield passes approximately along the strike of the more arenaceous beds bordering the east of the area of mudstone, and a shallow trench dug alongside this road revealed in places the nature of these beds. The south end of the section was almost opposite Whitfield House [ST 6759 9159], while the north end [ST 6822 9301] was some 270 yd (247 m) S of St George's Church, Falfield. The trench was shallow and in many places penetrated only into weathered clay with sandstone fragments. Where exposed, the unweathered rocks consisted mainly of red, green and brown mudstone with various proportions of fine-grained sandstones and siltstones of similar colours, commonly in thin layers only inches thick. In places the mudstones are almost devoid of arenaceous 'ribs', for instance between 140 and 300 yd (128 and 274 m) NE of Whitfield House. In some other places the siltstone and sandstone ribs are more common, for instance towards the south end [ST 6759 9159] of the trench opposite and just north of Whitfield House, where a slight ridge is produced. Sandstones and siltstones are again common at the north end of the section. I.B.P., R.C.

At the south end of this section, opposite Whitfield House, a poor fauna was obtained which was characterised by Costistricklandia lirata lirata and included Atrypa reticularis. At the north end [ST 6823 9295], 350 yd (320 m) S of St George's Church, Falfield, the fauna included Streptelasma?, Brachyprion arenacea, Eocoelia sulcata, Leptostrophia compressa and Whitfieldella?. The identification of C. lirata lirata from the exposure at the south end and E. sulcata from the north end indicate positions within the Tortworth Beds. Dips recorded along the section by I. B. Paterson are very variable in direction and degree and the structure hereabouts may not be simple.

Between Whitfield village and Whitfield House [ST 6803 9178] (more northerly of the two houses of that name), there is much fragmentary surface debris and a ridge giving evidence of sandstone lying immediately below the bottom limestone of the Wenlock. Fragments from the roadside bank [ST 6751 9138] 250 yd (229 m) SSW of Whitfield House and just beneath the limestone have yielded Palaeocyclus porpita, Streptelasma sp. and Eocoelia sp. The finding of P. porpita apparently so close to the base of the Wenlock seems anomalous and has not been explained.

Limestone fragments found on the outcrop of the basal part of the bottom limestone, 170 yd (155 m) NE of Rifle Cottage [ST 6775 9133] have yielded "Stricklandia" to R. W. Pocock.

The anticlinal structure of this area continues north of Falfield towards Middle Mill [ST 6942 9514] but the rocks are almost unexposed. The only evidence of the age of the beds comes from fragments of sandstone in the soil which have yielded Llandovery fossils to R. W. Pocock. These were found in the neighbourhood of Horse Pool [ST 6842 9360], Falfield and on the ground between Middlemill Farm [ST 6932 9520] and Oldbrook Farm [ST 6930 9415], where there is also evidence for a north–south fault.

Reed and Reynolds (1908b, p. 531) recorded exposures in the side of a lane west of Falfield Farm [ST 6835 9367], and it was there that R. W. Pocock observed the beds to be vertical and to include red crystalline limestone. This type of rock would confirm the opinion of Reed and Reynolds that it is of Wenlock age and, although shown on the map as Llandovery, their diagnosis may be correct. Indeed a feature probably produced by a bed of sandy limestone or calcareous sandstone has been mapped northwards from this place and thence sharply round, turning back southwards to Falfield Farm, but no conclusive evidence as to the age of the limestone has yet been found, and the rocks here are obviously much folded and faulted. R.C.

Charfield Green to Stone

Lower Trap

The contact with the underlying Micklewood Beds was proved in sewer trenches at Charfield Green in 1960, but the trench had been filled in before it was noted. From material thrown out and from information supplied by workmen it would appear that some quite large masses of pink gypsum occurred near the base of the amygdaloidal basalt near the contact with the Tremadoc. Basalt was also proved in deep sewer trenches in the Avon Valley 200 to 300 yd (183–274 m) SE of the road at Charfield Mills. Here the contact with the Micklewood Beds was obscured by Drift deposits but the main part of the Lower Trap, consisting largely of amygdaloidal and massive basalt was well exposed. Some pockets of red clay were seen in the basalt. No exposures of the Lower Trap were seen between Charfield Green and Huntingford where the basalt emerges from beneath Mesozoic rocks. The trap can be traced from Huntingford to Whitehall Villa, Damery [ST 9598 9430], where the outcrop terminates against the Damery Fault. West of the fault the trap is seen in Damery Quarry whence it may be traced westwards and northwards to Michael Wood where it is last seen on the side of the Damery Fault in old quarries about 1000 yd (915 m) NW of Damery Bridge. From this point north-westwards to Woodfordgreen Farm the contact between the Silurian and the Tremadoc is faulted or partly concealed beneath Trias. Beyond Woodfordgreen Farm the Damery Beds rest unconformably on the Micklewood Beds and the Lower Trap appears to be missing.

Damery Beds

Nearly all the calcareous matter has been leached out of the weathered rocks, but in the trench [ST 7234 9283] at Charfield Mills, it could be seen that the lower part of the Damery Beds must contain a considerable amount of sandy and muddy limestone beneath the zone of decalcification. At Charfield and Damery the Damery Beds consist of fairly massive sandstone at the base, overlain by sandy mudstone with some impure limestone. O.A.K.

Long's Quarry, now disappeared but situated about 150 yd (137 m) N78°E of the railway station, Charfield Green, was within the basal arenaceous group of the Damery Beds. Fossils from here in the collections of the Institute of Geological Sciences include: Favosites multipora, Favosites sp.,crinoid columnals, Ptilodictya lanceolata, Atrypa reticularis, Brachyprion arenacea, 'Camarotoechia' nucula, Eocoelia curtisi—common, Mendacella cf. phiala, Mendacella sp., Stegerhynchus? weaveri, Strophochonetes sp., Bucanella?, Gyronema octavia multicarinatum, Gyronema sp., Loxonema sp., Actinoceras nummularium, Tentaculites anglicus—common, Dalmanites weaveri—common, Encrinurus onniensis—common, and Craspedobolbina (Mitrobeyrichia) clavata.

In 1961 a pipe trench was excavated between Watsome Bridge, Charfield Green and Charfield Mills and a short branch [ST 7237 9283] to [ST 7235 9281] from this revealed about 35 ft (11 m) of beds dipping south-westwards at about 20°. The beds consist of about 10 ft (3 m) of mainly bedded greenish siltstones followed by some 6 ft (2 m) of clay (weathered mudstone) containing layers of siltstone, with proportions of about 2 of clay to 1 of siltstone. This is overlain by some 4 ft (1.2 m) of hard flaggy reddish purple siltstone and finally visible were about 15 ft (5 m) of mainly clay (?weathered mudstone) with hard siltstone layers. The succession exposed here also represented part of the arenaceous lowest Damery Beds, and yielded : Streptelasma sp.,crinoid columnals, Brachyprion?, Eocoelia sp. and Encrinurus sp.

About 100 yd (91 m) to the north-west and immediately east of the road at Charfield Mills the main trench revealed micaceous mudstones, of the overlying mudstone group, with dips of 30° to 40°, between SSW and W and containing a small form of Favosites cf. multipora, crinoid columnals, Mendacella sp. and Stricklandia sp. R.C.

This group of mudstones gives rise to a well-marked slack in the Damery area where the outcrop may be traced from Huntingford to the Damery Fault at Damery; here it is displaced by the main fault and also by a subsidiary parallel fault. The valley of the Little Avon at Damery follows roughly the outcrop of the mudstone which forms a well-marked slack dominated on the south by sandstones in the upper part of the Damery Beds forming the steep river cliff extending through Iron Mill Grove [ST 703 942]. G.A.K.

From this steep bank of sandstones R. W. Pocock found fossils at two places. In the centre of the north side of the grove [ST 7029 9425] he collected Costistricklandia lirata alpha while at the north-west end [ST 7015 9434] crinoid columnals, Ptilodictya sp., 'Camarotoechia' sp. and Encrinurus sp. were obtained.

Near Damery Bridge, presumably in the laneside some 50 yd (46 m) S of the bridge (Reed and Reynolds 1908a, p. 33; 1908b, p. 517 and Lloyd Morgan and Reynolds 1901, p. 275) and thus probably from the mudstone group in the middle of the Damery Beds, which dip 30° SE, a rich fauna was collected by these authors. Large brachiopods Atrypa reticularis, Brachyprion arenacea and Leptostrophia compressa are prominent. Other fossils include Streptelasma sp.,crinoid columnals, 'Camarotoechia' nucula, Eocoelia curtisi, Howellella sp., Strophochonetes?, Eunema?, Liospira lenticularis, 'Pterinea' sp., Tentaculites anglicus, Dalmanites weaveri, Encrinurus onniensis and Warburgella cf. stokesii.

Close by, material from a septic tank pit [ST 7056 9428] in the same mudstone group, yielded crinoid columnals, Atrypa reticularis, 'Camarotoechia' nucula, Mendacella cf. lata and Dalmanites weaveri. Another temporary exposure [ST 7074 9428] revealed richly fossiliferous horizons with Atrypa reticularis, 'Camarotoechia' nucula and Eocoelia curtisi very common. The collection also includes Streptelasma sp.,crinoid columnals, Brachyprion arenacea, Mendacella cf. phiala, Strophochonetes aff. novascoticus, Bellerophon sp., Tentaculites anglicus and Dalmanites weaveri.

Beds belonging to the upper sandstone group of the Damery Beds were exposed near the stream at Avening Green (Reed and Reynolds 1908a, p. 32; 1908b, p. 517). In these Atrypa reticularis is very common and Costistricklandia lirata alpha well represented. Also present here are Favosites?, Periechocrinites?, bryozoa, Brachyprion arenacea, 'Camarotoechia' llandoveriana, 'C.' nucula, Howellella sp., Leptaena sp. and Encrinurus onniensis.

The upper sandstones of the Damery Beds crop out to the north-west of Crockley's Farm [ST 700 943] where they are very fossiliferous. They yielded a large fauna to Reed and Reynolds (1908a, p. 34; 1908b, p. 517). Fossils from this area presented to the Geological Survey by these authors include crinoid columnals, Atrypa reticularis, Leptaena cf. purpurea and Stricklandia sp. but their published lists show that much more material has been collected. R. W. Pocock recorded sandstone dipping at 40° WSW, 250 yd (229 m) N of Crockley's Farm, while a few yards to the west the dip is to the south-south-east. Between this outcrop and the river nearby, in the spur some 500 yd (457 m) WNW of Damery Bridge, is a well-defined change of slope marking the contrast between the sandstone and the underlying mudstone in the middle of the Damery Beds. R.C.

Hereabouts the continuity of the outcrop is again broken, by an east-west fault running from the south side of Michael Wood in the general direction of Lower Stone. North of the fault the mudstone slack can be traced northwards into Michael Wood, where the rocks appear to be folded into a syncline and are cut by several faults. Here the outcrop of the mudstone reflects this local folding and faulting. Further west in the area between Woodfordgreen Farm and the valley of the Little Avon near Matford Bridge there are indications of the presence of mudstone bands occupying a similar stratigraphical position.

The upper band of mudstone gives rise to clay soils and a slight slack between the upper sandstones and the trap in the Charfield Inlier and south of the Little Avon valley at Damery. This band is less well defined in the Damery area, but it can be traced from the Damery–Tortworth road past the south side of Iron Mill Grove to Crockley's Farm. The two outliers of Upper Trap seen at Fern Hill [ST 700 949] in the south-western part of Michael Wood are also separated from the outcrop of the upper sandstones of the Damery Beds by an area of stiff clay soil, which appears to mark the outcrop of the upper mudstones and helps to confirm the structural interpretation. West of the north-south fault bounding the Fern Hill outliers the outcrop of the upper mudstone follows a sinuous course along the east bank of the Little Avon to Middle Mill; it then curves north-eastwards and finally westwards in conformity with the outcrop of the Upper Trap between Middle Mill and Woodford. Beyond this point it can only be detected locally - as in the area extending westwards from Matford Lane, Woodford, to the alluvial flat of the Little Avon. G.A.K.

Roadworks in 1964 along the A38 at Woodford exposed a section of Llandovery rocks between the Matford Lane junction [ST 6902 9583], 180 yd (165 m) SW of the Baptist Chapel, and a point 235 yd (215 m) to the south-west [ST 6883 9576]. A plan and lithological description of the beds was made by Dr M. L. K. Curtis and with his permission the details have been incorporated into the Institute's manuscript records. Fossils recently collected are related to Dr Curtis' description as follows:

Upper Trap

The Upper Trap attains its greatest known thickness of about 250ft (76 m) adjacent to the railway cutting [ST 722 924] near Charfield Station. It has been traced north-westwards partly under Drift as far as Stone Mill. The most westerly outcrop may be on the west bank [ST 683 964] of the Little Avon valley 500 yd (457 m) SE of Lobthorn Bridge and about 900 yd (823 m) NNW of Stone Church where fragments of trap occur in the fields. Near Middle Mill it is thick and is exposed in the old quarry [ST 6964 9523] on the west bank of the Little Avon. This section, once famous, is now obliterated (Lloyd Morgan and Reynolds 1901, p. 278; Reynolds 1924, p. 108). Temporary exposures in the Upper Trap were seen during the survey, but well known localities, e.g. Cullimore's Quarry, Charfield, the old quarries on Avening Green and the quarries near Stone Mill have long been disused and are much overgrown or degraded. Thin ashy limestone occurs on top of the basalt at Charfield and Middle Mill as noted by Curtis (1955a, p. 6). G.A.K.

Tortworth Bed

South of Charfield Station the outcrop is very narrow; the beds are thin, and dip at up to 50° to the west. In this area too, differentiation into resistant beds with thin sandstones and softer mudstone beds was difficult. North of the main road to Charfield Hill, a lower and upper group of more sandy beds form broad ridges and are separable from mainly mudstone bands which weather into clay soils and produce hollows. By these features the four divisions were traced continuously northwards to Underwood Farm [ST 717 928] where the Tortworth Beds disappear under Triassic and Drift deposits.

From near St John's Church, the top mudstone division occupies a well-marked hollow lying immediately to the east of the bottom limestone of the Wenlock Series and its outcrop follows the west side of Poolfield Farm [ST 719 924]. Opposite Poolfield Farm, the bank of the lane revealed red and green clay with thin layers of sandstone.

The now defunct Cullimore's Quarry [ST 7198 9269] 500 yd (457 m) N40°W of the level-crossing at Charfield Green exposed the basal part of the Tortworth Beds (Reed and Reynolds 19086, p. 514). Fossils in the Institute's collections include Favosites gothlandicus, F. multipora, Heliolites cf. interstinctus, Monotrypa sp.,decalcified bryozoa, Atrypa reticularis, Coolinia pecten, Costistricklandia lirata lirata, Cyphomena (Cyphomenoidea) wisgoriensis, Eospirifer radiatus globosus, Hesperorthis?, Leptaena sp., Leptostrophia (Leptostrophia) compressa, Pentlandina tartana, 'Cyrtoceras' sp. and Crassiproetus? curtisi. Corals and brachiopods are abundant and Curtis (1972, p. 13) reported the presence of Palaeocyclus porpita.

Mentioned by Reed and Reynolds (1908a, p. 35) is an exposure by a little pond a short distance north-west of "Pool Farm". If this is a reference to Poolfield Farm, the exposure is very near the top of the Tortworth Beds. Fossils from here include Palaeocyclus sp. and its occurrence at such a high horizon in the Tortworth Beds would match its seemingly high position at Whitfield (p. 21), and would indicate that the fossil is not restricted to the basal layers of the Tortworth Beds.

The outcrop of the Tortworth Beds immediately south of Avening Green, is much wider, reflecting the lower angle of dip and the north-westward thickening of the deposits. From here the outcrop curves westwards around the nose of a southward pitching syncline ending in disturbed ground in Daniel's Wood where the dips are steep to the south-east against a major, approximately north–south, fault of pre-Quartz Conglomerate age.

The two sandstone divisions of the Tortworth Beds are less distinctly separated by the lower mudstone in this area, so together, these sandstones produce a broad rather featureless tract of ground between Avening Green and Daniel's Wood. On this ground, sandstone fragments are common in a sandy clay soil. The lower mudstone band is just discernible, causing a hollow some 60 yd (55 m) across in the middle of the tract from a position 200 yd (183 m) S of Avening Green west-north-westwards as far as the Tortworth to Damery Lane. Beyond the lane the feature becomes indefinite and the mudstone is not mapped to Daniel's Wood where the lower two-thirds of the Tortworth Beds appear to form a unified group of sandy rocks.

The top mudstone of the Tortworth Beds, however, makes a strong depression beneath the Wenlock from 100 yd (91 m) W of Little Tortworth Copse through Old Court Farm [ST 701 936] and thence west-north-westwards for a further 500 yd (457 m). There it is displaced southward by a subsidiary north–south fault, being terminated near Little Daniel's Wood by the major north–south fault just mentioned.

Palaeocyclus porpita has been recorded from the sandstones above the Upper Trap in Daniel's Wood and is included in the Institute's collections labelled 'West of Crockley's Farm'. This is probably a part of the collection referred to by Reed and Reynolds (1908b, p. 519), while a part of Reed and Reynolds' own collection from Daniel's Wood, south-west of Daniel's Bridge, Tortworth, is also in the Institute's collections and includes: Favosites multipora, P. porpita, Periechocrinites?, A. reticularis, Eocoelia cf. sulcata, Leptaena cf. purpurea, Cornulites serpularius, Dalmanites sp. and Encrinurus sp.

The basal Tortworth Beds are exposed again in the old quarry [ST 6955 9523] at Middle Mill. The quarry is largely in the Upper Trap, but as at Cullimore's Quarry, overlying beds are also exposed. These have been described by Reed and Reynolds (1908b, p. 519) and show close similarities with the sequence seen at Cullimore's Quarry. Some of the collection made by Reed and Reynolds, and presumably relating to their published account (op. cit., 1908b, p. 519), is housed in the Institute and includes: Favosites cf. gothlandicus, F. cf. hisingeri, Palaeocyclus?, A. reticularis, C. lirata lirata, Pentarnerus?, Platystrophia? and 'Orthoceras' sp. From the "tuff" at the top of the north side of Middlemill Quarry, Streptelasma crassiseptum? Smith was also collected by R. W. Pocock.

North-westwards, the outcrop of Tortworth Beds passes through Woodford Farm [ST 6943 9541] to Woodford, where the beds are displaced southwards by a north–south fault. West of Woodford and Stone the complete thickness of the Tortworth Beds is represented at outcrop, though the upper part of the beds is repeated southwards by a strike fault. The outcrop is wide and appears to represent up to 1000 ft (305 m) of beds (Figure 4). The prominent slopes adjacent to the flood plain of the Little Avon River near Stone Mill, and sandstone debris at surface reveal that most of the lower part of the Tortworth Beds of this area consists of sandstone, which is exposed in an old quarry [ST 6838 9585] 570 yd (521 m) WNW of Stone Mill, and was seen again temporarily exposed about 150 yd (137 m) to the south-east. The upper part of the Tortworth Beds in this area has produced an even more subdued topography which is usually badly drained and is considered to be the result of mainly mudstone rocks. In the middle of their outcrop is a low, rather ill-defined, but persistent ridge strewn with sandstone fragments and considered to be the surface expression of a bed of sandstone. This feature lies between Norton House, Stone, and the ground northward of Westend House [ST 6814 9563]. It is repeated to the south by a strike fault and there extends from Catherine Villa [ST 6838 9496] to just south of Westend House. Further south the low ground is bounded by a marked rise caused by a limestone considered to be the bottom limestone of the Wenlock.

It is stressed that these rocks are very poorly exposed and topographical features subdued so that the conclusions based on these details are provisional. A temporary exposure in beds lying just below the supposed bottom limestone of the Wenlock however tends to support the stratigraphical conclusions. This exposure occurred 630 yd (576 m) S of Stone Church and 100 yd (91 m) SE of Catherine Villa and revealed sandstone and decalcified sandy limestone dipping at about 45° SE. The fauna collected here suggests that the age of the beds is Upper Llandovery and includes solitary corals resembling Streptelasma, crinoid columnals, A. reticularis, 'Camarotoechia' nucula, Leptostrophia compressa, Mendacella?, Poleumita?, 'Pterinea', Cornulites serpularius and Proetus sp.

Further south, along the outcrop of the bottom limestone of the Brinkrnarsh Beds near Whitfield House [ST 680 918], R. W. Pocock noted 'Llandovery fossils' on his field-slip so that the lowest part of this limestone might be of Llandovery age. He also collected Palaeocyclus porpita from the laneside [ST 6751 9138] just below the bottom limestone at Whitfield (p. 20). R.C.

Petrography of the Lower and Upper Traps

This account is based on the examination of 74 thin sections, 31 from the Lower Trap and 43 from the Upper Trap, some 38 of which are new. The work suggests that the two bands are essentially similar and that the original rock type was a contaminated felsic basalt containing both clinopyroxene and orthopyroxene, with some olivine.

Reynolds (1924) and Dunham (1946) have both described the lavas in detail. The main difference between the petrography of the two accounts lies in whether or not olivine was present in the Upper Trap. The probability that it was present, is suggested by an examination of pseudomorphs.

The new work makes it necessary to modify some of Reynolds' conclusions mainly concerning the identity of the Upper Trap. He lists features (1924, p. 108) which he claims strongly distinguish the Upper Trap from the Lower, as follows :

Some four generations of feldspars including minute needles, laths, short square crystals and large phenocrysts. The latter typically possess a spongy core rimmed with clear material.

Quartz-xenocrysts commonly surrounded by a reaction rim.

Considerable amounts of glassy material in the groundmass.

Patches of groundmass having a different grain (i.e. texture) from the remainder and probably of xenolithic origin.

To this list may be added his statement that there is a complete absence of olivine in the Upper Trap.

While the above features may be typical of most specimens from the Upper Trap they are not entirely exclusive to this horizon, nor invariably present as the following notes will demonstrate.

1. Feldspars

Excluding the large phenocrysts, the size and shape of the feldspars is related to the amount of glass present in the rock, as would be expected. Where much glass occurs the feldspars show most variation (E16432)^‡5 . In other cases where the material is less hyaline or almost holocrystalline the feldspars are more uniform and there is a marked similarity in texture and grain size to the Lower Trap, e.g. Upper Trap, (E29858)–(E29859) and Lower Trap (E29832), (E29836), (E29848), (E29850). In general the Upper Trap rocks are of smaller average grain size than the Lower Trap, as well as being less completely crystalline. In the Upper Trap rocks examined the grain size varied from about 0.09 mm up to about 0.3 mm, contrasting with the Lower Trap ranging from about 0.2 to 0.5 mm. Although the Lower Trap feldspars do not generally show marked differences in their size, i.e. they were formed by continuous crystallisation as a rule, there are exceptions as in (E16426) and (E16446) where small (about 0.06 mm) and larger (0.33 mm) generations may be discerned. The feldspars are variably albitised with a consequent development of sericite inclusions. Phenocrysts (Plate 2, fig. 3) of generally rounded shape, containing an inner zone marked by a ramifying network of glass with a clear outer rim, were present in all but one (E29858) of the specimens from the Upper Trap that were examined. One section (E29836) of a Lower Trap rock from a trench [ST 7243 9271] between Charfield Mills and Charfield Green, also contains similar phenocrysts.

Owing to the variable degree of alteration found in these rocks the original composition of the feldspars is uncertain—all variations between albite and labradorite have been recorded. Rutley (1876) suggested that some of the simply twinned phenocrysts were orthoclase. Further examination of the rock and section (E666) described by Rutley has shown that while many of the phenocrysts show only simple twinning, fine lamellar twinning is also present in some individuals. Treatment of a flattened surface on the hand specimen with sodium cobaltinitrite also shows that the phenocrysts are not potassic and that although the stain was taken up by some of the constituents of the rock, the potash appears to be confined to chlorite or the glass rather than contained in a feldspar. Dunham (1946) records that the fresh phenocrysts have a composition approximating Ab₄₀ An₆₀. In most rocks the phenocrysts are albitised (e.g. (E21388)) although labradorite has been noted in, for example, (E21389), (E21390), (E21391).

A similar range of compositions is found within the groundmass laths. The freshest rocks from Daniel's Wood (Upper Trap) and Charfield Green (Lower Trap) contain labradorite.

2. Quartz xenocrysts (Plate 2, fig. 4)

Rounded grains of quartz, generally with a reaction rim of clinopyroxene and glass when fresh, are confined to the Upper Trap. They form a very minor portion of the rock, and were noted in only twenty specimens, i.e. in a little less than half the Upper Trap samples. Most occurrences were in rocks from Woodford, (seven of eight) and at Daniel's Wood, (eight of ten). Other occurrences are in rocks from the outliers in Michael Wood and in one specimen from Charfield [ST 723 923] (E29870). It seems that the greatest concentration of quartz xenocrysts occurs in the north, but this may be because only a quarter of the specimens come from south of Daniel's Wood.

A little interstitial quartz is usually present in the rocks containing the xenocrysts and also in some others from both the Upper and Lower Traps. Possibly it results from material contaminating the magma rather than from secondary silicification.

Secondary introduction of quartz is represented by patches of chalcedonic silica. In the Lower Trap free silica in the form of interstitial granular quartz occurs in nine of the thirty-one specimens. As some pseudomorphs after a ferromagnesian mineral are preserved in quartz (e.g. (E29849), (E29852)) the silica in these rocks may be at least partly of late introduction.

3. Glassy mesostasis

The Upper Trap rocks generally contain a much higher proportion of brown glass than the Lower. Nearly all the Lower Trap rocks that have been examined possess a mesostasis which has been made opaque by the development of red oxides. This makes its identification very difficult. In the less iron-stained rocks vague interstitial patches of a brownish colour occur. In a number of cases these are now crystalline and exhibit an indefinite, but low birefringence (e.g. (E29836), (E29838), (E29847)). In some cases the interstitial material shows optical continuity with the feldspar laths (E29847). Rarely the material is isotropic (e.g. (E29834)). Two rocks ((E7662) and (E7667)) from Damery Quarry [ST 705 944], although having an opaque mesostasis, resemble some Upper Trap rocks in that there are two generations of feldspar. The larger dispersed laths are set in an opaque mesostasis containing small needle-like feldspars. Here it would appear that plentiful, although heavily iron-stained, glass is or was present.

Fine-grained glassy margins are not uncommon around amygdales in rocks (e.g. (E16420), (E29836), (E29837) and (E29845)) from the Charfield and Damery regions. The first (Plate 2, fig. 2) shows a double ring around the amygdales which are filled with chlorite and a little chalcedonic silica. The inner ring about 1.2 mm wide is composed of narrow laths of albite averaging about 0.09 mm in length mixed with skeletal crystals of iron oxide. Partly surrounding the whole is an intermittent rim about 1 mm thick showing a more finely grained variolitic texture. Patches of similar texture are to be seen scattered throughout the rock.

4. Xenolithic fragments

Xenoliths are found most commonly in the Upper Trap.

Type a. Patches of groundmass having different texture from the remainder are perhaps the most common type. Many of these 'xenolithic' patches may be due merely to autobrecciation of the rock when partly solidified, as they grade into the groundmass (e.g. in (E7657), (E21390), (E29876)). In other cases (e.g. (E29870) from a trench parallel to the approach road to Charfield Station [ST 723 923]) the 'xenolithic' patch is strongly marked, but appears to be of Upper Trap type. These fragments may be either more or less glassy than the groundmass. In section (E29873) from the upper part of the trap near Charfield Station [ST 722 923] the rock is cut by glass-rich veins and possibly represents autobrecciated material recemented by the fluid magma.

Type b. A specimen (E21697) from Avening Green [ST 710 938] contains an undoubted fragment with variolitic texture. Similar material, but probably not xenolithic, occurs in (E16424) from Middle Mill about [ST 694 951]. It has also been mentioned above forming a rim to an amygdale in one of the Lower Trap rocks (E16420).

It is doubtful if any of the xenolithic fragments in the Upper Trap are foreign to the rock as a whole. However in the Lower Trap rocks from Damery Quarry [ST 705 944] (e.g. (E7662), (E7667), (E16434)) fragments of iron-stained mudstone and siltstone, probably incorporated during extrusion, are common. From Charfield Green [ST 726 923] two Lower Trap rocks ((E29848) and (E29850)) contain distinctive fragments. In (E29848) a lozenge-shaped fragment of devitrified rhyolite, 2.7 mm long, was noted. It is composed predominantly of interlocking patches of quartz about 0.39 mm across with a rudely spherulitic structure. Included within the quartz patches are small, rounded, solitary or coalescing areas of clay-mica averaging 0.03 mm in diameter. Some chlorite and iron oxide is present. Very small (0.04 mm) clear, colourless crystals with refractive indices appreciably less than those of quartz, and low birefringence, occur. They show lozenge or wedge-shaped outlines and in one case a portion of a hexahedron was seen. Although the low birefringence and somewhat undulose extinction occur, one crystal appeared to show segmental twinning in four parts. These may be either sanidine or possibly tridymite.

The second xenolith, in (E29850), is of a fine-grained albitic lava. The fragment is 3 mm long and composed of albite laths and equant grains up to 1.05 mm long. The latter consist of a skeleton of rectangular albite intergrown with green chlorite. In each individual patch the feldspar is in optical continuity and untwinned. Small rectangular laths of albite penetrate quartz laminae, 0.18 mm across.

The texture of this rock is quite unlike that of the Tortworth Traps. The skeletal feldspars have a slight resemblance to some of the better preserved cores to the phenocrysts which are so noticeable in the Upper Trap. Scattered through the surrounding lava are patches of quartz averaging 0.5 mm in diameter. These show irregular, but sometimes curved, cracks and markedly undulose extinction. Their size approaches that of the quartz xenocrysts in the Upper Trap. However they lack reaction rims and are polycrystalline, contrasting with the xenocrysts. It would seem that they are late-stage cavity fillings. From the sparse evidence available it appears that the xenoliths in the Lower Trap contrast with those in the Upper by being of rocks foreign to the magma. Those in the Upper Trap are fragments from earlier extruded parts of the traps.

5. Mineralogical composition

The feldspars, probably originally labradorite, have been dealt with above in section 1. All the rocks examined were rich in feldspar.

Almost invariably the ferromagnesian content of the rocks is represented by pseudomorphs in chlorite or bastite. In a few cases the pseudomorphs have subsequently been replaced by silica. Rarely fresh clinopyroxene is found, especially in the rocks from near Daniel's Wood [ST 696 941] e.g. (E21391). Poikilitic intergrowths of feldspar in chlorite represent an original ophitic texture ((E16420) and (E29831) from the Lower Trap; (E7658), (E7668), (E16444) from the Upper Trap).

Orthopyroxene, identified as enstatite by Reynolds, is found in both horizons, frequently as subhedral microphenocrysts preserved in strongly pleochroic, from deep bluish green to yellow, bastite ((E16421), (E29844), (E29863), Lower Trap; (E21386), (E21389), (E21390), (E21697), Upper Trap).

Olivine, represented by chlorite and iron-oxide pseudomorphs often of typical six-sided shape, was thought by Reynolds to be restricted to the Lower Trap. However pseudomorphs after olivine have been noted in some Upper Trap rocks (e.g. (E666), (E678), (E29857) and (E29859)) and possibly others. The pseudo-morphs identified as being after olivine occur in rocks lacking the quartz xenocrysts and which are therefore presumed to have received little contamination by the addition of quartz.

6. Chemical composition

A number of rocks from both horizons have been analysed, either partially (Lloyd Morgan and Reynolds 1901; Reynolds 1924) or fully (Van de Kamp 1969).

Deduction of the original composition of the lavas from these figures must take into account the petrographical evidence of contamination by foreign matter and secondary alterations and additions. When allowance has been made for such materials and the total iron recalculated as FeO, the composition will be close to that of a basic rock type. (Table 1) gives Van de Kamp's (1969) uncorrected analyses.

Conclusions

1. The two Trap bands are of similar composition and are contaminated, enstatite-bearing basalts, with or without olivine.
2. Differences between the two horizons may be attributed to different cooling histories and to contamination of the magma prior to extrusion. R.W.S.

Wenlock Series, Brinkmarsh Beds

General stratigraphy

The Brinkmarsh Beds crop out mainly in the tract north-east of Buckover and Horseshoe Farm [ST 6734 9025] to Daniel's Wood [ST 696 941] where the strike swings eastward. There the outcrop is faulted and displaced to the south slightly but continues as far as Tortworth. A strip of Brinkmarsh Beds also occurs on the west side of the Charfield Green outcrop and another small area exposing Wenlock rocks lies to the south-west of Stone. A larger part of the ground between Falfield and Stone has at times been shown as being occupied by Wenlock strata, for instance Reed and Reynolds (1908b, p. 513). Exposures and other evidence are scant over this ground and so conclusions as to the age of the rocks there are rather subjective, but during the recent survey it was thought that a large part of the area is occupied by Llandovery rocks, Wenlock occurring only south-west of a line from Catherine Villa [ST 6838 9496] to Westend House [ST 6814 9563]. The name Brinkmarsh Beds was used first by Curtis (1972).

The Brinkmarsh Beds consist predominantly of mudstones with thin layers and beds of siltstone and fine-grained sandstone. The sandstones are calcareous and in places even the mudstones are calcareous containing also occasional layers of hard calcareous nodules. Certain layers are highly fossiliferous. Sedimentary structures are also in evidence, particularly in the fine-grained sandstones which show ripple markings, current bedding and drag marks. There are also curved bedding and small rounded masses within the sandstones, suggestive of contemporaneous disturbance of the sediments. The total thickness of the exposed Wenlock succession appears to approach 800 ft (244 m).

Within the Brinkmarsh Beds are three prominent beds of limestone. These limestones are discontinuous and lenticular, especially in the northerly outcrop, possibly due in part to faulting, but mainly it is thought, as a depositional condition. The limestones are associated with sandstone beds, into which they pass in places. This is particularly true of the bottom limestone, which in the Charfield Green area has been traced, passing into sandstone northwards. The limestones are usually crinoidal and very fossiliferous, but impure and argillaceous. They form quite prominent ridges in the topography and yield a limestone brash to the soil. These characteristics and the fact that considerable small-scale quarrying has taken place in the limestone, have facilitated mapping in an otherwise poorly exposed area. The beds of limestone are not thick, the thickest and most quarried being the bottom one, which probably attains 100 ft (30 m) in places. The thinnest, and most difficult to trace, especially in the north near Daniel's Wood, is the top limestone. At the Buckover road cutting this bed is 12 to 21 ft (3.7 to 6.4 m) thick. A few feet of mudstone overlying the bottom limestone is characterised by the solitary coral Pycnactis [Hallia]mitratus. Curtis (1955a, p. 6) named this mudstone the Pycnactis Band.

The junction of the Wenlock with the Llandovery has for convenience been taken at the base of the Brinkmarsh Beds. This junction appears to be conformable and transitional and has been located with approximation near Little Whitfield Farm [ST 6724 9132], where the bottom limestone of the Brink-marsh Beds contains a Wenlock fauna, although R. W. Pocock noted on his field slips the presence of Llandovery fossils in its basal part and a subjacent sandstone. Anomalous is the occurrence here of Palaeocyclus porpita just beneath the bottom limestone, for this fossil is usually considered as being associated with the basal Tortworth Beds. In the Chatfield Green area, beneath the bottom limestone, there is a thin mudstone and it is presumed to be from this that Reed and Reynolds (1908b, pp. 533 and 541) obtained a Wenlock-type fauna. The slight difference of facies from that at Whitfield, reveals that conditions varied locally before limestone deposition prevailed more generally over the area.

The highest Brinkmarsh Beds containing a fauna which is indeterminately Wenlock or Ludlow are exposed in the south of the Tortworth Inlier around Horseshoe Farm, where they are succeeded unconformably by the Upper Old Red Sandstone Quartz Conglomerate. From a slightly lower horizon at the Buckover road section [ST 677 907] high Wenlock fossils were obtained from strata less than 100 ft (30 m) above the top limestone. A fauna found in trenches just south of Little Daniel's Wood [ST 6946 9354] (Reed and Reynolds 1908b, p. 529) was also considered to come from high Wenlock strata.

West of the Berkeley Fault Wenlock rocks are not exposed, but the pre-Upper Old Red Sandstone is much more complete and a borehole would be expected to prove the whole Wenlock succession. R.C.

Palaeontology

The lowest of the three limestones in the Wenlock Series is characterised by an abundance of brachiopods including Amphistrophia (Amphistrophia) cf. euglyphoides, Brachyprion waltonii, 'Camarotoechia' diodonta, Resserella whitfieldensis and Sphaerirhynchia davidsoni. Above this limestone R. whitfieldensis is common in the mudstones of the Pycnactis Band. Because of a lack of exposures, no summary of the fauna of the middle and upper limestones or of the intervening beds can be given, but there is some evidence that Clorinda (or Antirhynchonella) sp. is common in the middle limestone.

Mudstones close above the top limestone yield a typical Wenlock fauna, including the brachiopods Meristina obtusa and Trigonirhynchia stricklandii. Higher beds at Buckover contain an undiagnostic fauna of either Wenlock or Ludlow age. The main elements of this fauna are listed by Curtis and Cave (1964, p. 432).

The highest strata exposed in the Millbury Heath area yield forms recorded elsewhere from both Wenlock and Ludlow rocks, including the brachiopods Amphistrophia funiculata, Howellella sp., M. obtusa and Skenidioides lewisii and the trilobite Dalmanites cf. aculeatus. D.E.W.

Details

Cromhall Lane to Falfield

The bottom limestone has been worked along its outcrop, but many of the quarries are now completely overgrown, so that from Falfield to Rifle Cottage [ST 6775 9133], it was mapped only by features and field brash. Near Rifle Cottage an old quarry [ST 6781 9133] exposed 10 ft (3.1 m) of red, thickly bedded, medium-grained, somewhat sandy limestone with a concentration into lenticles of crinoid and shell debris. The faunal assemblage from this quarry is characterised by Amphistrophia (Amphistrophia) cf. euglyphoides and Resserella whitfieldensis but the complete faunal list includes crinoid columnals, 'Serpulites' (Serpuloides?) perversus, bryozoa, 'Camarotoechia' nucula, Leptaena depressa, Sphaerirhynchia davidsoni, Cypricardinia sp., Tentaculites sp. and Calymene sp. Scattered fish scales were also recorded.

The description of Brinkmarsh Quarry made by Reed and Reynolds (1908b, p. 525) cannot be bettered in its present state, but sections are still visible there. At the northwest extremity of the quarry up to 12 ft (3.7 m) of limestone can be seen. This is mainly greyish green in colour, but red in places and of fine and medium grain. Fossil debris is abundant, mainly crinoid ossicles, brachiopods and bryozoa, generally occurring in lenticles. Thin lenticular beds of greyish green, micaceous, cross-bedded sandstone were also seen. The limestone here is much broken and veined by calcite. Near-vertical planes along which movement has occurred show much gouging and horizontal slickensiding. Fossils from this limestone include crinoid columnals, bryozoa, Amphistrophia (Amphistrophia) euglyphoides, 'Camarotoechia' sp. and Resserella whitfieldensis.

In the centre of the south face of the quarry [ST 6740 9125] the section is: soft greyish green mudstone 4 ft (1.22 m) on calcareous greyish green crinoidal sandstone 1 ft 8 in (0.51 m), soft, green, shaly mudstone 8 in (0.20 m), hard, olive-green, ?chloritic, calcareous sandstone 1 ft 2 in (0.36 m) over massive reddish limestone 3 ft (0.91 m). None of the veins of celestine seen by Reed and Reynolds were visible recently.

At the eastern end of the south face of the quarry more of the mudstone overlying the limestone is present at the top of the face, and the top part of the limestone is less sandy than at the centre of the face. The section seen consisted of 1 ft (0.30 m) of mudstone with some nodular limestone, overlying 5 ft (1.52 m) of mudstone, below which 1.5 ft (0.46 m) of rubbly limestone were visible. The faunal assemblage from the limestone is characterised by Brachyprion waltonii, Sphaerirhynchia davidsoni and 'Camarotoechia' diodonta. It also includes Rothpletzella gotlandica, Favorites hisingeri, bryozoa, Hoivellella sp., Whitfieldella sp. and Cypricardinia subplanulata. I.B.P., D.E.W., R.C.

Some distance to the west, a diversion of the main A38 road recently gave a series of exposures just south of Pool Farm, Whitfield. Parallel with and very close to westward of these exposures a north-east to south-west fault, downthrow west, was mapped. This fault appears to have displaced the outcrop of the bottom limestone from Little Whitfield Farm on the east, to Pool Farm to the west. The nearest road exposure to this fault is the most northerly, but the rocks exposed are unlike those of a Wenlock limestone, and it would seem that the exposure is east of the fault and stratigraphically below the bottom Wenlock limestone. At Pool Farm [ST 6728 9145] the limestone was red, sandy, rubbly-bedded and poorly fossiliferous. The exposure [ST 6727 9140] 50 yd (46 m) NE of Little Whitfield Farm showed: clay and soil 2 ft (0.61 m) on greyish green calcareous siltstone 3 in (8 cm), over at least 2 ft (0.61 m) of red and green clay. Fossils collected include crinoid columnals, bryozoa, Atrypa reticularis, Howellella sp. and Dalmanites sp.

The next exposure to the south [ST 6723 9136], 15 yd (14 m) N of Little Whitfield Farm revealed the base of the bottom limestone:

The following fossils amongst which rhynchonellids and Atrypa reticularis are common were collected from crinoidal limestone: Favosites sp.,crinoid columnals, bryozoa, Amphistrophia (Amphistrophia) cf. euglyphoides, 'Camarotoechia' cf. borealis, 'C.' nucula, Leptostrophia sp., Rhynchotreta cuneata, Sphaerirhynchia davidsoni, Striispirifer plicatellus, Loxonema sp., Acaste downingiae and Dalmanites caudatus.

For a further 50 yd (46 m) to the south-west the exposures revealed red and grey limestone with some shelly and crinoidal bands. Beyond, only red and green mudstones with thin siltstones of the overlying beds were revealed.

In the Whitfield area, a thin bed of mudstone overlies the bottom limestone from south of Little Whitfield Farm to just beyond Rifle Cottage and in Brinkmarsh Quarry the lower part of this mudstone was exposed at the top of the succession in the south face (p. 35). There, at the western end, this mudstone yielded: Phaulactis cf. angusta, Pycnactis mitratus—abundant, and Resserella whitfieldensis—abundant. At the eastern end the lowest 5 ft (1.52 m) of mudstone yielded a similar abundance of P. mitratus and R. whitfieldensis together with crinoid columnals, Brachyprion waltonii, 'Camarotoechia' nucula, Craniops implicata, Cypricardinia subplanulata, Tentaculites sp. and Proetus sp. The top 1 ft (0.30 m) of mudstone with nodular limestone yielded crinoid columnals, bryozoa, B. waltonii, Glassia sp., Howellella sp., Whitfieldella sp. and Loxonema sp.

Red and green mottled mudstones with a few thin layers of pale green calcareous sandstone were temporarily exposed in a shallow trench south-west of Little Whitfield Farm. This mudstone is replaced upwards and laterally by a sandstone sequence which, north of Rifle Cottage, comes to rest directly on the bottom limestone. Monotrypa crenulata was obtained from the base of this sandstone, in the road bank [ST 6757 9107] 100 yd (91 m) N of Brinkmarsh Farm. R. W. Pocock recorded fossiliferous sandstone dipping 46° SW in an exposure [ST 6819 9223] 300 yd (274 m) S of Mount Pleasant. Flaggy olive-green micaceous sandstone, 3 ft (0.91 m) thick was exposed in the roadside [ST 6755 9122] 400 yd (366 m) ESE of Little Whitfield Farm.

The sandstone is overlain by a thick series of shales with occasional thin ribs of sandstone. The only exposure was in a shallow ditch [ST 6819 9174] 150 yd (137 m) NW of Whitfield Farm, which penetrated red and green clay. The shaly group is overlain by a mainly sandstone group now recognised only by field brash and features. Reed and Reynolds (1908b, p. 527), however, record shale with red and green grit containing blades of celestine in pits north and north-east of Brinkmarsh Farm.

The middle limestone makes features from a position 400 yd (366 m) ESE of Mount Pleasant to Whitfield Farm. Some 200 yd (183 m) S of the farm, the limestone is displaced by an east–west fault, downthrow south. At Brinkmarsh Farm the middle limestone supports a strong feature with abundant blocks of pink, rubbly-bedded shelly, crinoidal limestone. Scrappy exposure of the limestone in the farmyard, showed a southerly dip of 20°.

From field debris east of Brinkmarsh Farm, Whitfield [ST 677 910], a poor fauna was collected, characterised by common Clorinda (or Antirhynchonella) sp. and including crinoid columnals, Atrypa?, 'Camarotoechia' sp., Eoplectodonta sp., Leptaena depressa and Striispirifer plicatellus.

The middle limestone is overlain by thick mudstones with a narrow outcrop from Whitfield Lodge to Brinkmarsh, the outcrop widening towards Buckover. The mudstones make a heavy clay soil and are rarely exposed. Fossils found as field brash between 450 and 500 yd (411 and 457 m) west of Brinkmarsh Lane Farm, include crinoid columnals, Clorinda (or Antirhynchonella) sp., Cyrtia exporrecta, Eoplectodonta duvalii, Glassia obovata, cf. Leangella segmentum, Whitfieldella sp. and Encrinurus sp.

The Buckover road-cutting (Curtis and Cave 1964, p. 433) exposed approximately 70 ft (21 m) of mainly purplish red mudstone with a few nodular lumps of limestone at several horizons (beds 1–5). Purplish mudstones were also exposed in a ditch 200 yd (183 m) SW of Brinkmarsh Lane Farm. South of this farm a thin bed of fine-grained olive-green sandstone was recognised by brash and feature.

The top limestone was exposed near Buckover Farm (Curtis and Cave 1964) as 12 ft (3.66 m) of purple and purplish grey limestone, argillaceous and silty in lumpy, irregular beds with thin clay partings. A band of purplish blue clayey mudstone occurred 2 ft (0.61 m) above the base. The limestone was traced eastwards to Horseshoe Farm by a feature, thence northwards almost to Whitfield Farm. Blocks of limestone containing Atrypa reticularis were obtained in a small excavation [ST 6695 9041] in Rudge Wood. Purplish, rubbly silty limestone formed an abundant brash on a steep bank 300 yd (274 m) SSE of Brinkmarsh Lane Farm.

The highest Wenlock sediments exposed in the Buckover road-cutting consisted of 47.5 ft (14.5 m) of mainly green-streaked purplish mudstone overlain by 53 ft (16 m) of mainly siltstone and fine-grained sandstone. I.B.P.

Additional to the faunal list given by Curtis and Cave (1964) Salopina conservatrix is present. The small chonetids are probably Strophochonetes?, whilst some large chonetids possess spines which rise from the hinge-line at high angles, also suggestive of Strophochonetes.

Siltstones and fine-grained sandstones were also exposed in the roadside [ST 6736 9021] at Horseshoe Farm and lie at a slightly higher horizon than the topmost Wenlock beds in the Buckover road-cutting. Reed and Reynolds (1908b) concluded that they were probably Wenlock strata of "Ludlow" facies with a fauna of Ludlow affinities. This fauna is similar to that in top beds of the Wenlock in the Buckover road-cutting and includes: crinoid columnals, bryozoa, Amphistrophia funiculata, Fardenia?, Howellella sp. [Delthyris elevatus of authors], Meristina obtusa, Pentamerus?, Skenidioides lewisii, Loxonema?, 'Perinea' sp., Tolmaia?, Tentaculites ornatus, Dalmanites cf. aculeatus and Beyrichia sp.

Falfield area

North of Falfield, exposures of rocks of Wenlock age (Reed and Reynolds 1908b, p. 531), are very rare and the structures appear to be complex (p. 21). East and north-east of Falfield the beds have a consistent easterly or south-easterly dip and make features. The bottom limestone makes a fairly persistent ridge from some 180 yd (165 m) E of Mount Pleasant northward along the western bank of the river, and north-eastward through Heneage Court to Skay's Grove [ST 689 937] and 200 yd (183 m) beyond. The outcrop is of uneven width and the thickness of the limestone seems irregular. Some 65 ft (20 m) of limestone were proved in a well [ST 6878 9364] at Heneage Court and, from width of outcrop and dip, the limestone here and at Skay's Grove is likely to attain a maximum thickness of 100 ft (31 m).

An exposure of this limestone in the Old Windmill Quarry [ST 6841 9298], 390 yd (357 m) SE of Falfield Church (Murchison 1839, p. 455) dips south-east. Fossils include: Coenites spp., Favosites gothlandicus, Phaulactis glevensis, Pycnactis mitratus, crinoid columnals, Monotrypa crenulata, Howellella sp. [Delthyris elevatus of authors], Howellella elegans, Sphaerirhynchia cf. davidsoni and Whitfieldella sp. The position from which P. mitratus was obtained is not known.

Another quarry in the limestone [ST 6897 9390] at the northern end of Skay's Grove shows dips of 12° to 25° to the east-south-east. R. W. Pocock recorded from top to base: "rubbly limestone 5 ft; 'Wallia'red and green shale 5 ft; sandy limestone and shale partings, well bedded, 7 ft" in this quarry, and his collection of fossils is very similar to that from the Old Windmill Quarry, including also Loxonema sp., Poleumita cf. discors and Acidaspis sp.

Slightly more of the succession seems to have been seen in the Skay's Grove quarry by Weaver (1824, p. 336) and he noted that a 5 ft (1.52 m) bed of 'marly slate clay etc.' was particularly rich in coralloid remains. This bed may be the source of P. mitratus which is abundant, together with Resserella whitfieldensis.

Some 220 yd (201 m) W of Oldbrook Farm [ST 693 941], the feature passing through Skay's Grove dies out before reaching the river. There is no sign of a limestone to the north-east until a small feature and old quarries reveal limestone with a dip south-east at S40°E, running from 200 yd (183 m) NE of Oldbrook Farm to the northern extremity of Daniel's Wood where the large NNE–SSW pre-Quartz Conglomerate fault truncates the outcrop.

A small area of coarse crinoidal limestone similar to the bottom limestone outcrops in Little Daniel's Wood [ST 694 937]. If it is the bottom limestone it is a narrow slice of strata caught up in a bifurcation of the NNE–SSW fault just mentioned. An exposure of steeply, west-north-westerly dipping limestone in the stream [ST 6949 9387], 30 yd (27 m) W of the south-western boundary of Daniel's Wood may well overlie the bottom limestone and contains an abundance of small brachiopods. Fossils include: Conularia sp., Favosites gothlandicus, Periechocrinites?, crinoid columnals, Atrypa reticularis, 'Camarotoechia' cf. dayi, Craniops implicata, Howellella sp., Nucleospira?, Sphaerirhynchia cf. davidsoni, 'Orthoceras' sp., Tentaculites ornatus, Acaste downingiae, Homalonotus sp. and Hemsiella maccoyiana.

Charfield Inlier

Pink and grey crinoidal limestone surface debris follows the outcrop of the bottom limestone from about 200 yd (183 m) S of Manor Farm [ST 7221 9196] to the north-north-west. Just south of Poolfield Farm was the quarry (recorded on R. W. Pocock's map at [ST 7192 9235]) described by Reed and Reynolds (1908b, p. 533), and which exposed 4.5 ft (1.37 m) of mottled pink and yellowish somewhat rubbly, limestone with marl partings. This quarry probably exposed the base of the bottom limestone. Some 30 yd (27 m) W of Poolfield Farm, the limestone is very thin and northward from here it is very sandy. It becomes slightly thicker again as far as Underwood Farm [ST 7172 9278] and east of Tortworth Old Court the limestone appears to be thicker and very sandy. The only fossil in the Geological Survey collections from the Wenlock rocks of this area is Meristina obtusa.

Wickwar Inliers

Mudstones, siltstones and limestones are exposed along the course of the Little Avon River at three places [ST 730 881], [ST 729 884] and [ST 726 890] near Wickwar. A description of these rocks and the fossils collected from them was given by Whittard and Smith (1944, p. 65). The juxtaposition of Wenlock and Downtonian in the most northerly of these is not explained, but it is unlikely that the contact is depositional. In view of the absence of lower Downtonian and Ludlow strata, a north-south fault seems probable. R.C.

Ludlow Series

The Ludlow outcrop is limited to an almost unexposed strip of country up to 200 yd (183 m) wide immediately adjacent to the west side of the Berkeley Fault, lying between Wanswell Court Farm [SO 6896 0104] and Long Bridge [ST 6879 9921]. There is possibly a small area also near Westend House [ST 6813 9564].

The rocks consist mainly of purplish grey and grey, hard mudstones with thin layers of nodular argillaceous limestone and also some hard siltstones. The dips are westerly, of the order of 25° to 40° so that the outcrop consists of about 300 ft (91 m) of beds in places.

The fauna obtained from trenches at the edge and just north of the district with the aid of Dr M. L. K. Curtis is typical of the Bringewood Beds of the Ludlow area. The actual junction with the Old Red Sandstone was not observed, but it is clear that the beds in contact with the Thornbury Beds of the Lower Old Red Sandstone are of Bringewood Beds age. None of the fragment beds nor conglomeratic limestones with abundant Dayia navicula which characterise the Whitcliffe Beds and Leintwardine Beds respectively at Tites Point were seen (Cave and White 1971).

It is also known from the Brookend Borehole situated 1 mile (1.6 km) to the north of the district that the complete Ludlow sequence is present and that the basal beds of the Downton Series succeed it conformably (Cave and White 1968, p. 75). Thus the absence of the higher part of the Ludlow Series from the outcrop on the northern margin of the Malmesbury district indicates that the contact with the Old Red Sandstone is probably a faulted one. R.C.

Details

Wanswell Court Farm area

A pipe trench was dug in 1962 from Holywell Spring, in Tremadoc rocks [SO 6900 0140] across Ludlow rocks and into Old Red Sandstone some 300 yd (274 m) to the south-west. About 20 yd (18 m) SE of Holywell Spring rather crushed Tremadoc mudstone was revealed, indicating that the large south to north Berkeley Fault passes very close to the west. Between positions 90 and 270 yd (82 and 247 m) SW of Holywell Spring, green and red calcareous silty mudstones were exposed dipping westward.

Lithologically the rocks are similar to those just south of Tites Point, below the conglomeratic limestones of the Leintwardine Beds with their abundant D. navicula. The fauna indicates a correlation with the Bringewood Beds of the standard section at Ludlow, Shropshire and includes: bryozoa, cf. Phaulactis angusta, Aegiria grayi, Atrypa reticularis, 'Camarotoechia' nucula, Coolinia pecten, Craniops implicata, Dalejina hybrida, Howellella?, Isorthis cf. orbicularis, Leptaena depressa, Leptostrophia filosa, Protochonetes ludloviensis, Salopina?, Shagamella ludloviensis, Sphaerirhynchia wilsoni, Bembexia?, Gyronema? and 'Orthoceras'—fragment. R.C., D.E.W.

Chapter 4 Old Red Sandstone

Introduction

The outcrop of the Old Red Sandstone is limited to some 5 square miles (13 sq km) in the north-west of the district. Rather low undulating country is formed by the marls (calcareous-dolomitic mudstones) of the Thornbury Beds in which beds of sandstone are numerous. These crop out around and to the south of Berkeley and also near Buckover. The Tintern Sandstone Group and Quartz Conglomerate Group produce a bold slope in a narrow outcrop between Milbury Heath, Tortworth and Wickwar, marginal to the synclinal Carboniferous rocks. In this latter area the Lower Old Red Sandstone Thornbury Beds are absent following pre-Upper Old Red Sandstone erosion, but just east of Wickwar they are seen again in a small exposure. Geophysical gravity-survey evidence, together with the Hamswell Borehole (Cave 1963, p. 35) suggest that they are thick further east under Mesozoic cover.

History of research

There has been little research on the Old Red Sandstone rocks of the district; their disposition and exposure do not lend themselves well to study, but a few notes have been published. Whittard and Smith (1944) published an account of Downtonian sediments exposed in the river bed east of Wickwar. A regional description of the Old Red Sandstone was given by Kellaway and Welch (1948) and this included a mention of the outcrop near Thornbury and the first use of the term Thornbury Beds. White (1946) recorded the occurrence of Phialaspis in the Thornbury Beds at Sharpness. In 1955 two summaries of the Old Red Sandstone around Bristol were published. The first, by Kellaway and Welch (1955a), deals only with the Upper Old Red Sandstone giving an account which is summarised in a second publication (1955b) produced for the Bristol meeting of the British Association. An account of the Lower Old Red Sandstone also part of this second publication, was written by Curtis (1955b). Neither account gives details, but both cover the Malmesbury district. A detailed account of the Old Red Sandstone was published by Welch and Trotter (1961) in their description of the geology of the country around Monmouth and Chepstow. Although this has great relevance to the Malmesbury district attention was not specifically given to it. Lately, a road cutting at Buckover, through most of the Upper Old Red Sandstone, was described by Curtis and Cave (1964).

Classification

A large part of the varied succession recognised in the Lower Old Red Sandstone of the Chepstow district just to the west (Welch and Trotter 1961, p. 28) has no application within the Malmesbury district, for the only beds seen within the Malmesbury district are rather monotonous reddish brown marls with sandstone, called by Kellaway and Welch (1948, p. 15) the Thornbury Beds. The base of the Old Red Sandstone is not exposed within the district, but it is known from immediately adjacent ground that the highest Ludlow strata are followed conformably by a basal Downtonian sequence very like that in many places in the Welsh Borderland. The Ludlow Bone Bed is overlain by Downton Castle Sandstone followed by the Thornbury Beds. The full succession is as follows:

The junction between the Upper Old Red Sandstone and the Lower Old Red Sandstone has not been observed, but it is presumed to be strongly unconformable everywhere, so that it may be assumed that strata have existed which would have increased the thickness of the Thornbury Beds. Even so a conservative estimate of 2000 ft (610 m) of Thornbury Beds crop out and these were correlated by Welch and Trotter (op. cit., p. 32) with the Raglan Marl Group. This correlation, though broadly correct, cannot be precise in that the upper limit of the Raglan Marl Group is marked by the highest 'Psammosteus' Limestone and this limestone has not been identified east of the River Severn. It remains uncertain whether the absence of the 'Psammosteus' Limestone is due to non-development here, in which case the Thornbury Beds equate with a succession extending into the St Maughan's Group, or whether it is absent because the Thornbury Beds underlie it. If the latter case is true, a considerable thickening of the Raglan Marl Group must take place east of Chepstow and the overstep of the Upper Old Red Sandstone is sharp and large. The fact that the 'Psammosteus' Limestone crops out just beyond the north-west corner of the district, a mile (1.6 km) W of Sharpness, dipping westward at about 12° favours the latter view, but a borehole at Hamswell [ST 7348 7088] (Cave 1963, p. 35) proved 960 ft (293 m) of sub-Triassic reddish brown and purple, unstratified mudstone with sub-vertical, ramifying rods of pale dolomite. Lithologically this material is like the Thornbury Beds, but the borehole terminated in strata identified as "Downtonian stage I.7" of Wickham King (1934, p. 527) which Mr Toombs of the British Museum (Natural History) considered would normally lie about 200 ft (61 m) below the 'Psammosteus' Limestone. No comparable limestone was recorded in the borehole and so it is possible that the Thornbury Beds represent both the Raglan Marl Group and part of the St Maughan's Group.

Conditions of deposition

Following the deposition of shallow-water marine sediments in the late Silurian period a change in conditions is marked by the Ludlow Bone Bed. A short period of brackish-water environment ensued in which the Downton Castle Sandstone was deposited. This was followed by the deposition of thousands of feet of reddish marl, representing continental-type conditions in presumably a large subsiding "cuvette". The amount of dolomitic material in the form of sub-vertical ramifying rods and the reddish colour of the marl indicate desiccation and oxidation, whilst periodic flushes of water carried sand into the area, which formed beds of sandstones within the marl. It is possible that this activity may have occurred rhythmically, producing cycles of sedimentation. Apart from intraformational conglomerates within sandstones, no beds of conglomerate have been seen.

After the large late-Caledonian earth movements which resulted in the sub-Upper Old Red Sandstone unconformity, the greatest effect of which was the uplift and erosion of a ridge or horst passing N–S through Tortworth, continental type conditions continued. Erosion and deposition were then of a much more energetic type and deposits consisted first of gravels then of sand. These appear to have been swept into position by periodic torrents and now constitute the Quartz Conglomerate and Tintern Sandstone groups respectively.

The long period of continental-type conditions was terminated by a marine transgression which gave rise to the Lower Limestone Shale Group of the Carboniferous. The change was not abrupt and, as the transgression advanced northwards, the change from continental-type sandstone to marine shales took place earlier in the south. This diachronous condition makes the boundary between the Old Red Sandstone and the Carboniferous difficult to fix and for practical mapping purposes it has been taken at the local base of the Lower Limestone Shale Group.

Lower Old Red Sandstone

Thornbury Beds

The Thornbury Beds at outcrop are estimated as being over 2000 ft (610 m) thick and consist largely of finely micaceous, silty marl with colours varying between red, reddish brown and purplish, but commonly showing pale green reduction spheres and streaks. Impersistent beds of purple and red, flaggy, highly micaceous sandstones occur throughout the succession. In the lower part of the Thornbury Beds these are thin and constitute only a small proportion of the whole, but higher in the succession they are thicker and more important.

The sandstone beds usually possess sharp bases and are of coarsest grain-size near the bottom. They become finer upwards showing close, even bedding and pass to siltstone and silty marl. Cyclic sedimentation, producing such sandstones with sharp erosional bases, and grading up into siltstones, is described by Allen and Tarlo (1963) from the Red Downtonian of the Welsh Borderland.

The sandstone beds are recognised by the rounded, ridge features they produce and the accompanying field brash, but individual beds rarely can be traced far. The strata dip westwards, steeply in the east, but gently close to the River Severn.

Rather irregular and ramifying, dolomitic concretions are common in the marls. They are pale or white, sub-vertical and often long. In places these become so numerous as to coalesce forming a very impure limestone. Their appearance is not unlike that of coral growths, but the material is of inorganic origin. The only fossils which have been recovered within the district came from a small exposure just east of Wickwar where Whittard and Smith (1944, p. 69) found Phialaspis, suggesting a correlation with Wickham King's "Downtonian stage I.8" or lower. Fish fragments obtained by White (1946, p. 213) from Sharpness, immediately north of the district, prove that the Thornbury Beds there belong to "Downtonian stage I.7" of Wickham King (1934).

Upper Old Red Sandstone

As in adjacent districts (Welch and Trotter 1961) the Upper Old Red Sandstone falls into two groups. The lower group, the Quartz Conglomerate Group, is mainly conglomerate and has an erosional, strongly unconformable base. The upper group is the Tintern Sandstone Group consisting predominantly of sandstone which as previously discussed has a top which is diachronous passing into the Carboniferous Lower Limestone Shale Group.

Quartz Conglomerate Group

Where exposed in a road cutting at Buckover (Curtis and Cave 1964) the Quartz Conglomerate Group fell clearly into two parts. The upper part was 32 ft (9.8 m) thick, constituting the main conglomerate and providing nearly all the evidence, such as the pronounced feature and exposures, upon which the mapping of the formation was based. There is however a lower part, the existence of which is not normally evident. This consists mainly of softer mudstones and siltstones with purplish, brown and green colours, very like the underlying Wenlock rocks and is almost wholly non-conglomeratic. These beds might exist over the whole area, but if so their thickness of about 15 ft (4.6 m) would make little difference to the mapped base of the Upper Old Red Sandstone. The age of these beds was proved by a thin sandstone, rich in fish remains, lying some 3 ft 11 in (1.2 m) above their base. This bed has been termed the Buckover Fish Bed and it is believed to extend at least as far distant as the Forest of Dean.

The upper part of the Quartz Conglomerate Group consists mainly of hard, yellowish green and purplish brown, cross-bedded pebbly sandstones with beds, up to 10 ft (3 m) thick, of conglomerate containing well-rounded pebbles, mainly of quartz, but also of jasper and green mudstone. Subordinate amounts of thinly bedded sandstone and silty mudstone are present locally (Figure 6).

Tintern Sandstone Group

The thickness of the Tintern Sandstone Group is approximately 400 ft (122 m). More than 300 ft (91 m) of this was exposed near Buckover (Curtis and Cave 1964) consisting largely of purplish brown, grey or green, flaggy or cross-bedded sandstone with subordinate amounts of red and green silty mudstones. Yellow or green cornstone occurs at several horizons, generally as layers of nodules or as continuous bands less than 1 ft (0.3 m) thick, but an exceptionally thick conglomeratic cornstone of variegated colour was exposed at Buckover 40 ft (12.2 m) above the base of the Tintern Sandstone Group (Figure 6).

Scattered occurrences of quartz pebbles within the sandstones have been recorded along the outcrop, which is marked generally by a belt of sandy loam. In the area of Milbury Heath old excavations reveal that the sandstone has been used, probably for walling. The nature of the formation is too variable normally to allow extensive quarrying for although hard siliceous sandstone layers do occur, usually they are replaced laterally by softer sandstones which are pebbly in places. Interest however has been expressed in working parts of the Group for roadstone.

Details

Lower Old Red Sandstone

Thornbury Beds

Lower Stone to Ham

The Thornbury Beds outcrop in a narrow tract from Lower Stone to Ham. To the west and south they are overlain unconformably by Triassic marls whilst to the east, from Westend House [ST 6812 9562]–[ST 6812 9562] to Ham they are faulted against Tremadoc and Llandovery strata.

The features, caused by sandstone beds which occur mainly in the upper part of the Thornbury Beds, indicate considerable folding and faulting. Exposures are rare, but red mudstone and siltstones with a few thin beds of flaggy sandstone were seen in places, mainly with SSE dips up to 25°, in a roadside ditch [ST 665 944] 400 to 800 yd (366–732 m) SW of Lower Stone. Red, rectangular jointed siltstone and marl with thin beds of red, micaceous flaggy sandstone were exposed in a stream [ST 6649 9414] 1100yd (1006 m) SW of Manor Farm. The sediments dip to the south at 10° to 25°.

Berkeley area

Thornbury Beds occupy an area of over 2 square miles (5 sq km) north and north-west of Berkeley. In the east they rest on Ludlow rocks whilst to the south Triassic marls overstep them.

Strata near the middle of the Thornbury Beds were exposed in Stock Lane [ST 6814 9922], Berkeley, 250 yd (229 m) WSW of the Town Hall. These consisted of much folded, thinly bedded, red micaceous sandstone. Similar rocks, also much disturbed by folding, were exposed [ST 6787 9934] 500 yd (457 m) W of the Town Hall.

Towards the top of the Thornbury Beds, where, like the basal parts, marl predominates, red siltstone was exposed [ST 6720 9947] in the north bank of the Berkeley Pill, 850 yd (777 m) N82°E of Hamfield Farm. The highest beds crop out on the River Severn foreshore at the confluence of the Little Avon and for 600 yd (549 m) northward. The sediment consists of green-spotted, red, silty marls containing occasional ribs of red, medium-grained, flaggy, cross-bedded sandstone. Folding and faulting occurs, but the general dip is westward at a low angle.

The basal parts of the formation, composed of marl, are exposed in a number of shallow excavations in the eastern part of the outcrop. Green-spotted, red micaceous, silty marl was seen in shallow trenches [ST 6870 9930] 350 yd (320 m) N25°E of Berkeley Castle and also [ST 6842 9942] 500 yd (457 m) N15°W of the Castle. Marl was also exposed in a trench [SO 6855 0094] 700 yd (640 m) N of Berkeley Station.

A roadwork [SO 6841 0068], 250 yd (229 m) NNW of Berkeley Station revealed the following section in slightly higher strata, where sandstones are more important:

Steeply inclined flaggy, purplish, very micaceous sandstone was cut by a trench [SO 6844 0073] at Tintock Wood, 200 yd (183 m) ESE of Abwell, while 100 yd (91 m) NE of Abwell the following section was exposed by the roadside [SO 6833 0094] :

The dip of these sediments is from 20° to 35° generally to the west.

Wickwar area

Highly micaceous, flaggy, green sandstones and dark red shales are exposed in the bed of the Little Avon River [ST 7263 8913], 150 yd (137 m) SSE of Wickwar Station (Whittard and Smith 1944, p. 69). Fragmentary fish remains occur amongst which Phialaspis has been recognised indicating a Downtonian age. I.B.P., R.C.

Upper Old Red Sandstone

Quartz Conglomerate Group

Apart from the loose blocks of sandy quartz conglomerate that occur commonly along the outcrop very little is seen of the Quartz Conglomerate Group. The best exposure occurred in the A38 road cutting [ST 6665 9066] 500 yd (457 m) NE of Buckover, and a bed-by-bed description was made by Curtis and Cave (1964, p. 437). The following is a summary of this description:

The basal bed 1 ft 9 in (0.53 m) thick is welded to the eroded surface of calcareous sandstones belonging to the Wenlock Series and the bottom 4 in (10 cm) contain flakes and pebbles probably of penecontemporaneous rocks together with occasional quartz pebbles. Adjacent to the contact with Wenlock rocks are larger fragments broken from the immediately underlying strata. The Upper Old Red Sandstone sediment has enveloped these and invades the partly disrupted layers and joints of the Wenlock rocks to a depth of at least 5 in (13 cm). Arborescent manganese staining also lines the joint subjacent to the contact (Plate 3).

Small quarries [ST 6929 9323], 170 yd (155 m) ESE of Brook Farm revealed 10 ft (3 m) of massive cross-bedded quartz conglomerate.

Tintern Sandstone Group

A line of old excavations for sandstone occurs on the south-east side of the road near Milbury Heath and one of the excavations [ST 6597 8973] exposes 6 ft (1.8 m) of yellow, cross-bedded, coarse sandstone with occasional pebbles overlying 5 ft (1.5 m) of grit with quartz pebbles. The beds dip SE at about 29°. Some 30 ft (9.1 m) of yellow sandstone were seen in a quarry [ST 6729 8965], 670 yd (613 m) S of Horseshoe Farm. The best exposure, of all but about the top quarter of the formation, was seen in the Buckover road cutting [ST 665 905] and again a bed-by-bed description was made by Curtis and Cave (1964, p. 435) to which the reader is referred. Some 328 ft (100 m) of sandstones and siltstones with thick marl beds and frequent layers or nodules of cornstone were exposed. The marls occur mainly in the top 120 ft (36.6 m) of beds and the sandstones are cross-bedded, occasionally containing a few pebbles. The colours of these rocks are mainly purplish brown and red, but green is common and some of the sandstones are yellow (Figure 6). R.C.

Chapter 5 Carboniferous

Introduction

Only in the south-western part of the district are Carboniferous rocks exposed at the surface. These form the north-eastern part of the now defunct Bristol Coalfield and comprise Dinantian (Carboniferous Limestone Series), Namurian Millstone Grit) and Westphalian (Coal Measures) strata. The oldest Dinantian rocks rest conformably on Upper Old Red Sandstone. The highest strata preserved in the basin are of Upper Coal Measures (upper Westphalian) age. The total thickness of the Carboniferous rocks exposed within the district is about 6500 ft (1981 m).

Subdivision of the rocks of the Carboniferous System of the Bristol Coalfield into Carboniferous Limestone, Millstone Grit and Coal Measures may be said to date effectively from the work of Buckland and Conybeare (1824). Later workers were mainly concerned to clarify or develop this classification though the correlation of the 'Millstone Grit' presented many problems, some of which have only recently been solved (p. 59).

In general the Namurian and Westphalian strata form a belt of low-lying terrain, the rocks being seldom exposed at the surface. The crop of the Carboniferous Limestone Series is, however, marked by a low ridge or rim bounding the Coalpit Heath basin. This ridge stretches from Tytherington to Cromhall and, curving round the coalfield, continues southwards to Wickwar and Chipping Sodbury. With the exception of a belt where the rocks are affected by N–S faulting at Tytherington, this horseshoe-shaped ridge forms an almost continuous arcuate boundary to the northern part of the Bristol Coalfield.

In contrast with the poorly exposed Namurian and Westphalian rocks which lie within the horseshoe, the Carboniferous Limestone is well exposed in numerous quarries and natural sections.

The principal divisions of the Carboniferous rocks represented on the Malmesbury Sheet are given below. It will be observed however that there are considerable differences between the classifications of the Coal Measures shown on the Bristol District Special Sheet (1962) and the one used on this sheet. These are due primarily to the abandonment of the older classification in which the terms Lowcr Coal Series, Pennant Grit or Pennant Series, and Upper Coal Series were used, and the substitution of a classification based on the use of major marine bands as described by Stubblefield and Trotter (1957). At the time of the primary six-inch survey of the northern part of the Bristol Coalfield the position of the principal marine bands was known only in a very limited area. Evidence obtained in more recent times has now made it possible to apply the modern classification to the coalfield as a whole.

History of research

In his account of the geology of the Tortworth Inlier and the northern part of the Bristol Coalfield, Weaver (1824) observed that 'the Carboniferous Limestone alternates with sandstones of which the first bed that appears is forty fathoms in thickness'. He goes on to say, 'I have traced it around the basin, from West-End near Wickwar on the east, into the Tytherington ridge on the west, and in the whole of its course its position is conformable to that of the limestone in which it is included. The same may be said of two or three other beds of sandstone, from 1 to 2.5 feet [0.3–0.7 m] thick which may be followed from the Tytherington ridge into the dell north of Cromhall Church; and analogous beds are occasionally exposed to observation on the eastern side of the basin'. Weaver also noticed the passage of oolitic limestone into sandstone and commented on the presence of plant remains in some of the rocks on the Cromhall ridge. In a small quarry at Cromhall Rectory he noted a bed of limestone intercalated in the Cromhall Sandstone containing 'casts of terebratulites and entrochites' and remarked that these limestones pass laterally into a porous sandstone 'bearing the impression of these remains'. This limestone band is separated from the top of the Hotwells Limestone by about 1200 ft (366 m) of grit, sandstone and shale and is situated in an area where the Cromhall Sandstone is typically developed.

Thus in 1824 it was already known that the upper part of the Mountain Limestone of the Bristol Coalfield passes laterally into an arenaceous (Cromhall Sandstone) facies. When the official geological survey was carried out on a scale of one-inch to one mile in 1845, the sandstone bands in the Carboniferous Limestone were mapped by H. W. Bristow. These were not shown on the published one-inch Geological Survey map (Sheet 19) however, and in later years their existence appears to have been largely forgotten (Kellaway and Welch 1955a, p. 4).

This encouraged the popular belief that the application of zonal palaeontology was responsible for the discovery of the contemporaneity of the Upper Cromhall Sandstone and the higher Viséan limestones in the Bristol Coalfield. Weaver however was the first to recognise that the upper part of the Mountain Limestone passes laterally into sandstone in the northern part of the Bristol Coalfield and his discovery, later supported and clarified by the mapping of Bristow, was made half a century before the concepts of zonal palaeontology were applied to the classification of these rocks.

One of the most important papers relating to the Dinantian stratigraphy of the northern part of the Bristol Coalfield was provided by Lloyd Morgan (1889). Using evidence from the Tytherington–Grovesend railway cuttings and tunnel, Lloyd Morgan employed a classification based on that of Wethered (1888). This in turn derives largely from Buckland and Conybeare (1824) and De la Beche (1846). Lloyd Morgan identified many well-known formations in these cuttings, e.g. the Bryozoa Bed, Black Rock Limestone and Gully Oolite of the Avon Gorge. He described the 'Mitcheldeania-Beds' and 'Upper or Lithostrotion Limestone' the former being the Clifton Down Mudstone and the latter the Clifton Down Limestone of the present Geological Survey classification. By using this classification Lloyd Morgan was able to show that several of the divisions of the Carboniferous (Mountain) Limestone are thinner at Tytherington than they are in the Avon Gorge. He also observed that the bituminous limestones on either side of the Firestone at Tytherington (a highly siliceous sandstone band) are equivalent to those seen in the 'bituminous beds' of Great Quarry in the Avon Gorge.

When, in 1905, Vaughan defined the coral–brachiopod zones of the Avonian, he published brief descriptions of the Carboniferous Limestone of Chipping Sodbury, Cromhall, Wickwar and Tytherington. This paper marked a new approach to Carboniferous stratigraphical palaeontology and much of the work was of great value. Unfortunately, as we can now see, it was linked with the suppression of formational names or their replacement by zonal terms which were supposed to be based on palaeontological criteria. The effect of these changes has been discussed elsewhere (Kellaway and Welch 1955a). On the whole however the adverse effects were less marked in the northern part of the Bristol Coalfield than in the south and palaeontological evidence has been of considerable assistance in establishing the correlation of the fossiliferous limestones of the Avon Gorge, Tytherington and Chipping Sodbury.

Vaughan's work led to a great increase in interest in the palaeontology of the Carboniferous rocks and his zonal classification was applied in a series of papers, notably those of Wallis (1924) and Tuck (1926). These conclusions were summarised on a map of the Bristol District published by Reynolds (1937). The crops of the Dinantian rocks shown in this publication are generally in agreement with those of the Old Series Geological Survey maps but differ considerably from those of the later New Series editions.

Following primary six-inch mapping on the Chepstow (250), Malmesbury (251), Bristol (264), Bath (265), Wells (280) and Frome (281) sheets a new lithostratigraphical classification of the Dinantian rocks was proposed by Kellaway and Welch (1955a). Many of the formations named in this paper are seen in the Avon Gorge at Bristol. In the Malmesbury district, however, thick Viséan sandstones, and extensive tracts of strongly dolomitised Tournaisian limestones are present.

With regard to the lower boundary of the Carboniferous Limestone Series (Dinantian) the problem of definition has attracted relatively little attention. For present purposes it is taken at the contact of the Tintern Sandstone Group (Sheet 250) and Lower Limestone Shale, though it is known that a 'Carboniferous' brachiopod and fish fauna was in being at the time of the deposition of the Tintern Sandstone Group (Welch and Trotter 1961, p. 51; Kellaway and Welch 1955a, p. 7). Other correlation problems between the Avonian of the South-western Province and the Dinantian of Belgium led Vaughan (1905) to propose Clevedonian and Kidwellian as local alternatives to Tournaisian and Viséan. In practice the Tournaisian–Viséan boundary has been generally taken at the non-sequential contact of the Clifton Down Mudstone and the Gully Oolite in the northern part of the Bristol Coalfield. This, however, does not coincide with the change from a Tournaisian to a Viséan fauna (Vaughan 1905; Kellaway and Welch 1955a), and the Clevedonian–Kidwellian boundary cannot be identified in the Mendips. The base of the Gully Oolite (or the associated pale grey crinoidal limestone ='Sub-Oolite' or Sub-oolite Bed) is therefore taken as the bottom of the Clifton Down Group in the Bristol Coalfield. In effect it can now be shown that on faunal evidence this boundary is comparable with the base of the C₂ Zone as defined in northern England (Mitchell 1972) and may, for all practical purposes, be taken as the base of the Viséan.

The contact of the Black Rock and Clifton Down groups therefore coincides with the Tournaisian–Viséan boundary. In the northern part of the Bristol Coalfield, however, the upper part of the Black Rock Group is thin or absent and a considerable stratigraphical hiatus is now thought to exist in the dolomitised rocks at the contact of the Black Rock (Tournaisian) and Clifton Down (Viséan) groups.

With regard to the problem of defining the Dinantian–Namurian boundary, the situation is now much more satisfactory. Recognition by Stanley Smith (1942) of a late Viséan (P₂) fauna in the Tanhouse Limestone at Yate and the identification of the overlying Tanhouse Chert with the basal Namurian chert of Winford (Kellaway and Welch 1955a) has enabled a satisfactory correlation to be made with the succession at Bristol (Kellaway 1967). Thus the position of the boundary between the Carboniferous Limestone Series (Dinantian) and the Quartzitic Sandstone Group (Namurian) at Yate, Bristol and Winford may be confidently equated with the P₂–E₁ boundary in South Wales and the Pennines.

Rock types encountered in the Carboniferous Limestone of Chipping Sodbury and Wick have been described by Murray and Wright in 1971.

General account

So far as the principal thickness changes are concerned the table given below was prepared by Dr F. B. A. Welch. It shows that in general the Tournaisian and Viséan rocks are thinner in the west than in the east of the Bristol Coalfield, a trend which is also shown by the Namurian sediments. The thickness changes in the major groups are most strongly marked in the older Tournaisian rocks (Lower Limestone Shale Group) and in the younger Viséan rocks (Hotwells Group) and are minimal in the Black Rock Group. So far as individual formations are concerned the pattern is somewhat different, for example the Clifton Down Limestone is about 240 ft (73 m) thick in the west and 450 ft (137 m) in the east. This however is affected by facies changes, as is the development of the Upper Cromhall Sandstone which is 150 ft (155 m) thick in the west and 800 ft (244 m) thick on the eastern side of the coalfield. For convenience of description the Middle Cromhall Sandstone has been included with the Hotwells Group though it may be equivalent in part to the uppermost limestones of the Clifton Down Group as developed at Bristol. The general changes in thickness are summarised below.

Tournaisian

Lower Limestone Shale Group

Although it is predominantly a shale formation, the lower part of the Lower Limestone Shale Group consists of some 60 ft (18 m) of crinoidal limestone. This is developed on the south side of Milbury Heath and extends through the Tortworth area to Wickwar. In the Milbury Heath area part of this basal limestone is of the red 'Bryozoa Bed' type. The shale part of the formation is rarely exposed, though, from the railway cutting between Tytherington and Grovesend (on Chepstow (250) Sheet), Wallis (1924, p. 60) recorded a considerable fauna including, at the top, 'Cleistopora' aff. geometrica (= Vaughania vetus Smyth).

Black Rock Group

Throughout the area the Black Rock Group as a whole shows little variation in thickness, though its components, the Black Rock Limestone and Black Rock Dolomite, display related variations in thickness, depending upon the extent to which dolomite has replaced limestone. In the area north-east of Tytherington almost total replacement of limestone by dolomite has taken place.

The Black Rock Limestone is a massive, dark grey, crinoidal limestone, slightly shaly and more thinly bedded at the base. Chert appears to be absent. The Black Rock Dolomite is a blue or purplish grey dolomite in which practically all traces of fossils have been destroyed. In the eastern part of the large quarry (Grovesend Quarry) [ST 658 888] some 300 ft (91 m) of Black Rock Limestone are overlain by about 100 ft (30 m) of Black Rock Dolomite, but on Baden Hill, north of Tytherington, dolomite replaces much of the limestone. Farther north, apart from a small area of limestone at the base of the group in Priest Wood, the whole of the Black Rock Group is represented by dolomite as far north as Bloody Acre, south of Tortworth Court. There, two bands of limestone appear within the dolomite. The upper band has only a limited lateral extent, but the lower may be traced through Woodend and Hammerly Down to a point a little north of Wickwar.

In the Wickwar area the Black Rock Group is formed by two bands of dolomite with a median limestone band. Between Wickwar and Chipping Sodbury, however, the lower band of dolomite gradually passes into limestone. Thus the sequence at Chipping Sodbury resembles that of the Avon Gorge where massive Black Rock Limestone is overlain by a single band of Black Rock Dolomite.

The fauna of the Black Rock Limestone in the Tytherington–Wickwar area is closely related to that of the type section in the Avon Gorge. Caninioid corals are however exceedingly rare, though Tuck (1926, p. 243) and Wallis (1924, p. 62) have recorded 'Caninia cylindrica Scouler' from the C Zone. Mr M. Mitchell has carried out an examination of all the known caninioid material from the Bristol area however, and this suggests that the forms previously recorded as C. cylindrica are likely to be Caninophyllum patulunz. The remaining fossils listed by Tuck under the heading 'Z1' and 'Z2' are mainly brachiopods such as Productus cf. semireticulatus, Spirifer tornacensis, Leptaena analoga, Rhipidomella michelini and Schizophoria resupinata. Corals are represented by zaphrentoids and Syringopora. The Bristol fauna is in all essentials similar to that of the Tytherington area (Wallis 1924) where the crinoidal and dolomitised limestones of the Black Rock Group may, like those of the Avon Gorge, include only the lower and part of the middle division of the Black Rock Limestone of the Mendips (Mitchell 1972, p. 159). G.A.K., F.B.A.W.

Viséan

Clifton Down Group

The Gully Oolite usually consists of greyish white, current-bedded oolite. The rock was renamed Caninia-Oolite by Vaughan (1905) on the grounds that it occurs in the Caninia-Zone, but it is usually unfossiliferous and caninioid corals are virtually unknown in it. From the railway tunnel west of Tytherington around the outcrop to the Wickwar region the lower part of the Gully Oolite is formed by a grey, rather coarsely crinoidal, highly fossiliferous limestone, termed by Vaughan (1905, p. 221) the 'Sub-Oolite' (=Sub-oolite Bed of this account). It is comparable in character with the lower part of the Crease Limestone in the Forest of Dean, and with the Vallis Limestone of the Mendips. The Suboolite Bed contains numerous papillionaceous chonetoids and orthotetoids, and may in places reach a thickness of 70 ft (21 m). It contrasts strongly with the almost unfossiliferous oolite, 140 ft (43 m) thick, which overlies it.

The Clifton Down Mudstone comprises 130 to 140 ft (40–43 m) of splintery, greyish white calcite and dolomite mudstone with some subordinate shale. In some bands calcareous algae (Girvanella, Ortonella) are numerous. Being of little commercial value other than as rough the rocks are seldom exposed in quarry workings.

At the top of the Clifton Down Mudstone there is a marked change in lithology, the calcite mudstone facies being replaced by sandstone with subordinate bands of mudstone and shale. This facies is known as the Lower Cromhall Sandstone (Kellaway and Welch 1955a). Locally the base of the Lower Cromhall Sandstone is conglomeratic and includes fragments of the subjacent mudstone. The sandstone, which reaches a maximum thickness of 140 ft (43 m) at Cromhall, thins gradually southwards, being 60 ft (18 m) at Tytherington and only 20 ft (6 m) thick at Chipping Sodbury. It is seldom well exposed, but its crop can be identified from the fact that it is the only water-bearing formation in a dry limestone tract, small farms and ponds having been established on it.

The overlying Clifton Down Limestone shows a wide range of lithological variations. In the Tytherington area, in the west, it comprises alternations of thick white oolite and dark grey, massive, compact limestones with occasional 6-in (15-cm) thick shale bands particularly near the base. At a number of horizons there are abundant masses of Lithostrotion and Syringopora; Composita is common. Some 150 ft (46 m) above the base of the formation two coarse, calcareous sandstone bands, each 3 to 4 ft (0.9–1.2 m) thick, occur locally as at Tytherington. The shale bands, though thin, present difficulties in quarrying. The removal of a supporting 'toe' of a gently dipping limestone mass overlying one such band resulted in a huge rock-slide in Camp Quarry, Tytherington.

The same lithological succession continues around the syncline to Wickwar, south of which a number of changes take place, notably the development of shales. Concretionary or stromatolitic limestones, algal limestones and bands of calcareous grit are present as intercalations in the massive limestones.This type of lithology has been described by Tuck (1926) at the Bury Hill and Yate Rocks quarries. At Chipping Sodbury the eastern back slope of the great quarries north of the town shows this 'lagoon facies' lithology to advantage. In this area Davidsonina carbonaria is more abundant than elsewhere.

The Middle Cromhall Sandstone (Kellaway and Welch 1955a) may be contemporaneous with the Concretionary Beds of the Avon Gorge and should therefore be included with the Clifton Down Group. Locally, however, the Middle and Upper Cromhall Sandstones are in contact with one another and under these circumstances the thick mass of shale and sandstone above the Clifton Down Limestone is virtually indivisible and has been classified with the Hotwells Group.

Hotwells Group

While the lower part of the Hotwells Group is represented locally by the Hotwells Limestone by far the greater part is formed by the combined Middle and Upper Cromhall sandstones. Mapping of the group shows that it is largely a mass of sandstone with lenticles of limestone near the base and one or two calcareous bands in the upper part: the Tanhouse Limestone at the top provides a particularly useful marker horizon (Smith 1942).

The limestones are usually oolitic and crinoidal and contain clisiophylloid corals typical of the group. The Middle Cromhall Sandstone, which is taken to mark the top of the Clifton Down Group, may occur either as a single band of sandstone, as between Cromhall and Chipping Sodbury, or as two bands separated by limestone, as around Tytherington and Itchington. Between Cromhall and Tytherington the Middle and Upper Cromhall sandstones appear to have coalesced to form a single sandstone. The Upper Cromhall Sandstone is often very hard and quartzitic and, near Cromhall, is worked for road-dressing and aggregate. F.B.A.W.

Details

The Carboniferous Limestone is completely exposed in the western and northwestern part of its outcrop. The rocks are therefore described in order of occurrence commencing with Milbury Heath and ending at Chipping Sodbury.

Tytherington–Tortworth Court area

On the south side of Milbury Heath the basal crinoidal limestone of the Lower Limestone Shale crops out in a dip slope at the foot of which a depression indicates the outcrop of the overlying shale. From Priest Wood to the lake at the west side of Harris's Wood the basal limestone is concealed by Dolomitic Conglomerate, but from the lake to Tortworth Court it is seen in a number of natural exposures dipping at about 30° to the east-south-east. In an old quarry [ST 6994 9252] 650 yd (594 m) E of Tortworth Court some 20 ft (61 m) of crinoidal limestone dip at 10° to the south.

North-eastwards from Baden Hill the Black Rock Group is almost entirely represented by dolomite of which 400 ft (122 m) was exposed in a quarry [ST 6980 9188] 850 yd (777 m) SE of Tortworth Court. At an horizon 100 ft (30 m) above the base of the Group a band of shelly crinoidal limestone develops within the dolomite, being first observable at Bloody Acre; this gradually expands eastwards into a bed some 150 ft (46 m) thick running through Woodend.

The Gully Oolite was well exposed in Grovesend Quarry [ST 6608 8871] on the north side of the railway west of Tytherington. The beds here dip at 22° to the south-east and there is a good development of the basal crinoidal limestone (Sub-oolite Bed) containing chonetoids and Schellwienella. There are overgrown quarries in Gully Oolite some 300 yd (274 m) N of Tytherington Church. The largest exposures, however, are in the Poor End region, north-east of Cromhall. At Poor End a quarry [ST 6965 9132] shows 50 ft (15.2 m) of strongly jointed oolite dipping at 16° S, similar rocks being exposed in the Ley Hill quarries [ST 6982 9138] 200 yd (183 m) to the east-north-east.

In this region the coarse crinoidal Sub-oolite Bed is thick, though the rock is not exposed in any of the quarries.

The fullest exposure of the Clifton Down Mudstone is in the side of the railway cutting [ST 6630 8851] east of the tunnel mouth 650 yd (594 m) WNW of Tytherington Church. Here white calcite mudstone with shale bands dips at 200 to the south-southeast and contains abundant Girvanella. Between Baden Hill and the north end of Priest Wood the outcrop of the Clifton Down Mudstone is cut out by strike thrust-faulting which brings Lower Cromhall Sandstone into contact with Gully Oolite. At the top of Ley Hill quarries (see above) a few feet of Clifton Down Mudstone is exposed.

The Lower Cromhall Sandstone is exposed in the Sodom–Bibstone area where it is about 60 ft (18.3 m) thick. An exposure [ST 6903 9094] 140 yd (128 m) W of Sodom Mill showed 1.5 ft (0.5 m) of Clifton Down Limestone overlying 12 ft (3.7 m) of coarse Lower Cromhall Sandstone. East of the Mill the hard sandstone gives rise to the prominent Wick's Hill and to the relatively high ground immediately north of Bibstone. Here in a natural exposure [ST 6986 9111] 850 yd (777 m) ENE of Sodom Mill a thin limestone band is developed locally within the grit.

The Clifton Down Limestone is typically exposed in two large disused quarries at Tytherington, Camp Quarry [ST 6650 8830] 440 yd (402 m) SW of the church and Church

Quarry [ST 6700 8845] 200 yd (183 m) to the north-east. The formation is exposed for a distance of about 1100 yd (1006 m) north-eastwards of Church Quarry in a series of natural outcrops, crags and small quarries in the steep slope known as Tytherington Hill. In Camp Quarry some 150 ft (45.7 m) of oolite with Composita alternates with massive limestone containing Lithostrotion and dip at 28° to 30° in a south-southeasterly direction. Near the base thin shale bands occur, and along one of these a landslip down the bedding plane of an enormous mass of overlying limestone ended quarrying activities. A very similar section is seen in Church Quarry, a detailed section of which is given by Wallis (1924, pp. 67–8). Here, as well as on the hillside to the north-east, a band of calcareous sandstone (Firestone) may be seen. In an old quarry [ST 6851 9040] on the north side of Priest Wood, splintery limestone full of Lithostrotion dips at 42° to the east-south-east. In the Cromhall area crags of oolite, dipping at 22° to the south, are exposed on either side of the river for a distance of 50 to 150 yd (46–137 m) S of Sodom Mill.

Some 1050 yd (960 m) SW of Tytherington Church a small tongue of Carboniferous Limestone projects into the Triassic terrain. Here the Middle Cromhall Sandstone is represented by two bands of sandstone 40 and 60 ft (12.2 and 18.3 m) thick separated by some 75 ft (22.9 m) of rather coarse, oolitic-crinoidal limestone. Overlying the uppermost sandstone band, Hotwells Limestone dips at 30° in a south-easterly direction: the limestone is again exposed in an inlier 500 yd (457 m) to the north-east. At Tytherington, however, the limestone parting the two sandstones, as well as the Hotwells Limestone appear to have been replaced by sandstone for, over a distance of 2 miles (3.2 km) in a north-easterly direction, the Clifton Down Limestone is apparently succeeded by sandstone without any limestone intercalations and it is not until the Cromhall region is reached that these reappear. Here the Middle Cromhall Sandstone is 90 ft (27.4 m) thick and separated from the Upper Cromhall Sandstone by some 100 ft (30 m) of oolitic limestone. The Middle Cromhall Sandstone is a fine-grained brown sandstone which can be seen 470 yd (430 m) WNW of Cromhall Church dipping at 28° to the south-east; east of the Iron Acton Fault its broad outcrop extends almost to Talbot's End. The overlying oolite crops out in riverside crags 200 yd (183 m) SW of Cromhall Church and continues to Talbot's End, reappearing through the Trias in a low ridge to the south-east of this locality where it dips 18°–22° in a general south-westerly direction.

At an horizon 125 ft (38.1 m) above the base of the Upper Cromhall Sandstone is a lenticular band of limestone up to 4 ft (12 m) thick, containing Lithostrotion pauciradiale. This can be observed in the large disused 'quartzite' quarry [ST 6875 9020] 625 yd (572 m) SW of Cromhall Church where a 2 ft (0.6 m) band of iron-stained coral-bearing limestone is interbedded with the quartzitic sandstone and dips at 32° to the south-east. On the opposite side of the river the working quarries [ST 6900 9015] of the Cromhall Quartzite Company show a succession of yellowish brown quartzitic sandstone bands with reddish purple shale partings. The dip is 30°–40° in a general southerly direction. In the top of the most southerly quarry face [ST 6892 9013] a 4 ft (1.2 m) band of oolitic-crinoidal limestone with Lithostrotion pauciradiale is developed, but this does not appear to extend far into the broad sandstone outcrop to the east.

Some 520 ft (159 m) above the base of the Upper Cromhall Sandstone is a small but important band of sandy limestone which appears to correlate with the Tanhouse Limestone of the Yate area. About 1360 yd (1244 m) SW of Cromhall Church is a small steep bank [ST 6852 8949] formed by the outcrop of brown decalcified limestone dipping at 48° to the south-east, and crowded with impressions of Buxtonia scabricula. The same band has been observed on the east side of the Iron Acton Fault, in the side of the ravine [ST 6878 8976], and 780 yd (713 m) farther to the east in the cellar of the rectory, recorded by Weaver (1824, pp. 344–5).

Tortworth–Wickwar–Chipping Sodbury

The basal limestone of the Lower Limestone Shale occupies an almost unbroken outcrop from near Tortworth Court to Wickwar, but the overlying shales are rarely exposed and form a belt of low relief: between Charfield and Southwood Farm they are concealed by overstepping Rhaetic. Small exposures of reddish crinoidal limestone occur at Tafarn Bach [ST 7135 9191]. Some 210 yd (192 m) SSW of Newhouse Farm [ST 7205 9017] thinly bedded decalcified limestone rests on brown sandstone dipping at 25° to the west-south-west. South of Southwood Farm the basal limestone gives rise to a low north–south ridge situated about 150 yd (137 m) W of Wickwar Church.

The band of black crinoidal Black Rock Limestone parting an upper from a lower mass of Black Rock Dolomite runs east-south-east from Woodend and is indifferently exposed on Hammerley Down. The largest exposures are the two disused Oldbarn quarries, one [ST 7200 8890] 380 yd (347 m) W of Wickwar Church, the other [ST 7200 8880] 400 yd (366 m) WSW of the church. In both, the Black Rock Limestone dips at 38° to the west-south-west and underlies Black Rock Dolomite: in the former 80 ft (24.4 m) of dark crinoidal limestone with abundant zaphrentoids can be seen; in the latter a greater thickness of dolomite is present. Tuck (1926, p. 244) here records a band containing fish teeth, and the occurrence of 'Caninia cylindrica' at an horizon 10 ft (3 m) below the top of the Black Rock Limestone.

Pale grey Black Rock Dolomite is the principal formation exposed in the large Slickstones Quarry [ST 7043 9152] 1670 yd (1527 m) WNW of Charfield Church, dipping at 16° to the south. In an old quarry [ST 7089 9149] 550 yd (503 m) to the east 20 ft (6.1 m) of massive grey dolomite dip at 14° to the south-west. At Chipping Sodbury Black Rock Dolomite is seen in the side of a water-filled quarry [ST 7281 8241] 150 yd (137 m) NE of the church, dipping at 42° to the west-south-west.

The Sub-oolite Bed 20 ft (6.1 m) and Gully Oolite 20 ft (6.1m) are exposed above the Black Rock Dolomite in the south face of Slickstones Quarry (see above). The oolite has, in the past, been worked in a number of shallow pits in the woods mile (0.8 km) to the south-east. Some 1100 yd (1006 m) NW of Wickwar Church the Suboolite Bed and Gully Oolite can be seen dipping at 36° to the west-south-west in two old quarries [ST 7179 8972] and [ST 7172 8974] on the east and west sides of the road respectively. The same formations are to been in an old quarry [ST 7190 8893] 500 yd (457 m) WNW of the church. Immediately south of the bridge over the ravine leading westwards to Yate Rocks, on the east side of the A432 road, an old quarry [ST 7250 8477] shows Gully Oolite resting directly upon Black Rock Dolomite without the intervening Sub-oolite Bed. A small exposure of Gully Oolite beneath 3 ft (0.9 m) of Rhaetic clay is to be seen [ST 7272 8263] 370 yd (338 m) N of Chipping Sodbury Church.

Lower Cromhall Sandstone is exposed in the eastern (back) face of the large Churchwood Quarry [ST 7140 8992] 1500 yd (1372 m) NW of Wickwar Church. The section seen below the Clifton Down Limestone is: pinkish calcareous grit weathering white, irregular stained top 5 ft (1.5 m) on clay parting 0.5 in (2 cm), brown sandstone 2 ft 4 in (0.7 m), red sandy clay 2 in (5 cm), reddish white, coarse sandstone 5 ft (1.5 m), purplish green shale 3 ft 6 in (1.1 m) dark silty clay 1 ft 4 in (0.4 m) over hard grey grit seen to 3 ft (0.9 m).

In the Yate Rocks area a natural exposure [ST 7238 8480] shows the basal part of the grit to be conglomeratic incorporating pieces of the underlying Clifton Down Mudstone.

Clifton Down Limestone comprising oolite and dark compact limestone with two prominent shale bands near the base is well exposed dipping at 20°–30° to the west in the large Churchwood Quarry (see above). In a large overgrown quarry [ST 7170 8886] 750 yd (686 m) W of Wickwar Church some 150 ft (45.7 m) of Clifton Down Limestone dip at 36° to the west-south-west. The upper part consists of massive limestone with abundant Lithostrotion, the lower part contains more oolite and shaly partings. The succession measured by Tuck (1926, p. 244) recorded a thin grit band and a band of algal limestone foreshadowing the development of this lithology farther south at Bury Hill and Yate Rocks. In an old quarry [ST 7180 8696] 1 mile (0.4 km) NNW of Hillhouse Farm Davidsonina carbonaria is particularly abundant. A measured section of the now abandoned and flooded quarry [ST 7200 8545] at Bury Hill has been given by Tuck (1926, pp. 245–6) where some 253 ft (77 m) of beds were formerly visible, a noticeable feature being the development of shales and algal limestones denoting lagoon facies conditions of deposition. A somewhat similar succession in the lower part of the Clifton Down Limestone was described by Tuck (1926, pp. 246–7) at the abandoned Yate Rocks Quarry [ST 7220 8477] where some 233 ft (71 m) of beds are exposed dipping at 30° to the west-south-west. The most extensive exposures of Clifton Down Limestone are in the quarries extending for nearly 1 mile (1.1 km) along the Wickwar road north of Chipping Sodbury. These are known, from north to south, as Limeridge and Barnhill quarries and in each the beds dip at 40° to the west. A certain amount of small-scale faulting is seen parallel to the bedding and on some of the fault planes blende occurs. Whilst most of the rocks comprise dark, fine-grained and oolitic limestone, one of the most striking features is the great weathered dip slope forming the eastern side of Barnhill Quarry formed of algal limestone, calcite mudstone, shale bands, and oolite.

Rocks of the Hotwells Group occur in the inlier [ST 712 869] 0.75 mile (1.2 km) SE of Talbots End. Here brown and white Middle Cromhall Sandstone is overlain by coarse crinoidal oolite containing Lithostrotion and Dibunophyllum. This sandstone is intermittently exposed southwards through The Cliffs and West End to the vicinity of the railway tunnel. Over this stretch the Middle Cromhall Sandstone contains a thin median parting of limestone (not shown on the one-inch map) which continues southwards from Hillhouse Farm through Bury Hill and Yate Rocks where it is visible in the top of the quarries (mentioned above). Towards Chipping Sodbury, north of The Ridge, the upper sandstone band appears to die out; the lower band continues south across the entrance road into the Chipping Sodbury quarries (Tuck 1926, p. 248, but now much obscured) and forms the slight hill at the west end of Chipping Sodbury main street where it is exposed in a low cutting at the junction of Rounceval Street and Silverhill Road. Hotwells Limestone overlain by Upper Cromhall Sandstone is exposed beneath Triassic rocks in the western approach cutting to Wickwar tunnel. In an old overgrown quarry [ST 7175 8596] 550 yd (503 m) E of Yate Court, white oolitic limestone containing Linoprotonia sp. hemisphaerica group dips at 36° to the west-south-west, and similar limestone is exposed in two small inliers immediately to the north. Old quarries to the west of Bury Hill and Yate Rocks show white oolitic limestone, the outcrop of which can be followed southwards to The Ridge.

The main outcrop of the Upper Cromhall Sandstone occupies a broad flat tract west of Bury Hill, and between Goose Green and Chipping Sodbury. The top of the formation is indicated by a thin partly decalcified limestone (Tanhouse Limestone) which has been traced almost continuously from Hall End through Yate Court to some 0.25 mile (0.4 km) W of Yate Rocks. The best exposure is described by Stanley Smith (1942, pp. 335–6) near Tanhouse Farm. A tract of Keuper Marl and Drift deposits conceals the crop of the Tanhouse Limestone at Goose Green, but crinoidal limestone was exposed during excavations for a housing estate north of the County Infirmary [ST 719 825] at Chipping Sodbury. F.B.A.W.

Millstone Grit Series (Namurian)

General account

Above the Tanhouse Limestone at Yate lie many hundreds of feet of strata, mainly barren sandstone and mudstone, largely devoid of coal but otherwise similar to the basal Coal Measures with which they were at first included. Thus, on the One-inch Bristol District Special Sheet published in 1962 the Millstone Grit appears to be very thin in the area north of Yate and Chipping Sodbury. On the sheet to which this memoir relates however the Millstone Grit is shown with a much wider crop, which includes strata formerly classified as Coal Measures. This change in the stratigraphical classification resulted from the location of the crop of the Crofts End Marine Band in two shallow boreholes at Broad Lane, Yate (Kellaway 1970). Prior to this discovery the Crofts End Marine Band was thought to lie below the Yate Hard Vein (p. 64). As soon as it was established that the crop of the Crofts End Marine Band lies some 200 ft (61 m) E of the crop of the Yate Little Vein at Broad Lane, Yate, the position of the Winterbourne Marine Band and the approximate position of the base of the Lower Coal Measures could be estimated, using the evidence of the Yate Deep and Westerleigh boreholes (pp. 62–64). This showed that the crop of the Namurian strata is much wider in the Yate–Cromhall area than was formerly suspected, and that the thickness of the formation had therefore been underestimated.

Subsequently a borehole was drilled in 1968 by the Institute at Limekilns Lane [ST 7065 8587], near Rangeworthy with the object of establishing the succession in the Millstone Grit. Unfortunately the Ashton Vale Marine Band was not proved by this borehole, either because the crop of the marine band lies a short distance to the west of the site of the boring, or, because it is missing in this area. Of the two possibilities the former is thought to be more likely. However from several independent lines of evidence it may be estimated that the thickness of the Millstone Grit in the Yate area is probably of the order of 1000 ft (305 m). The strata consist mainly of barren mudstone and quartzitic sandstone with some poorly preserved plants, though a marine band with Lingula was proved at a depth of 463 ft (141.4 m) in the Limekilns Lane Borehole (p. 312). This band may correlate with marine strata proved in Bristol at the top of the lower division of the Millstone Grit (Moore 1941; Kellaway 1967). With the exception of the fauna of the Lingula band very few fossils were found in the Limekilns Lane Borehole other than Productus carbonarius, 674 to 678 ft (205.4–206.6 m), poorly preserved plant remains, and some acritarchs which proved to be derived Tremadoc forms of no direct stratigraphical significance. For technical reasons it was not possible to continue the borehole to prove the basal Namurian cherts.

Surface exposures of the Millstone Grit strata are small and poor, being mainly confined to the bands of quartzitic sandstone. Some of the sandstone bands form subdued yet fairly continuous sandstone ridges but the intervening mudstones and shales are seldom exposed.

Details

The Namurian rocks are best known in the Yate area and their stratigraphy is doubtful or uncertain in the Tytherington–Cromhall area. The description therefore starts with Yate and Chipping Sodbury and proceeds northwards to Cromhall.

Tanhouse Cherts and basal sandstones

South of the County Infirmary at Chipping Sodbury the basal cherts and sandstones of the Quartzitic Sandstone Group are largely concealed by Triassic rocks or alluvium. North of the Infirmary, however, chert and sandstone are exposed in a low ridge running in a northerly direction to a point on the road from Goose Green to Yate Rocks, about 360 yd (329 m) SW of Wellstead Farm. In this area the rocks have a WSW dip [ST 715 842] of about 36°. North of the road from Goose Green to Yate Rocks the crop of the sandstones and chert is concealed by Keuper Marl, but the rocks reappear about 600 yd (549 m) NW of Wellstead Farm and the crop extends thence as far as Hall End. Some 700 yd (640 m) S of Yate Court lies Tanhouse Farm [ST 714 853]. Narrow abandoned workings in the Tanhouse Limestone extend for some 300 yd (274 m) to the south of the Farm, the basal Namurian chert being present locally at the top of the Tanhouse Limestone. Opposite the farm on the south side of the road, brittle splintery yellowish pink chert is seen to dip at 27° in a west-southwesterly direction.

At the northern end of the crop sandy cherts interbedded with mudstone rest on softer partly decalcified Tanhouse Limestone. About 150 yd (137 m) N of Yate Court the beds dip at 35° to the west. They form a low ridge between Yate Court and Hall End and fragments of splintery chert are strewn on the dip slope.

Beds above the Tanhouse Chert

South of Chipping Sodbury the Quartzitic Sandstone Group is concealed by Keuper and Jurassic rocks. The first exposures are seen near Stanshawe's Court [ST 7145 8190] where two well-marked bands of hard quartzite sandstone with a NNE–SSW strike have a westerly dip of about 40°. In this area the Celestine Bed forms the base of the Keuper Marl and secondary concentrations of celestine are present on the eroded surface of the Quartzitic Sandstone Group. Nodular masses of celestine have been recovered from the weathered clays and shales of the Namurian rocks and veins and strings of the mineral extend into joints and fissures in the sandstones. Several old quarries for quartzitic sandstone could formerly be seen near Stanshawe's Court notably in the upper sandstone band about 547 yd (500 m) W of Raysfield Farm and a somewhat larger working [ST 7165 8230] 328 yd (300 m) SW of Chipping Sodbury County Infirmary. Here the crops of the sandstone bands are displaced in an easterly direction by two faults. North of the principal fault the upper sandstone band can be traced almost continuously to Goosegreen Farm [ST 7113 8375]. The lower band can be followed to Brickhouse Farm [ST 7140 8325] where it passes beneath Keuper Marl and has not been recognised further north.

Little is seen of the upper part of the Quartzitic Sandstone Group in the area between Goose Green and Tanhouse Lane, the contact with the Lower Coal Measures being mainly concealed by Keuper or Drift deposits. Where the upper part of the Quartzitic Sandstone Group and the basal Coal Measures crop out in the flat terrain between Engine Common and Hall End, the featureless ground consists of clay or sandy clay with small residual areas of celestine-bearing Keuper Marl giving little or no indication either of the stratigraphy or structure of the underlying Carboniferous rocks. One prominent quartzitic sandstone band crosses Tanhouse Lane about 109 yd (100 m) E of Leechpool Farm [ST 7090 8518]. This may correlate with the upper sandstone seen near Stanshawe's Court.

North of Tanhouse Lane the upper part of the Quartzitic Sandstone Group and the contact with the Coal Measures lies somewhere in the vicinity of Limekilns Lane.

A borehole put down in 1968 by the Institute at the Celestine Works [ST 7066 8589] failed to prove the Ashton Vale Marine Band but established the succession in the main mass of the Quartzitic Sandstone Group. A summary of the borehole log is given on p. 307. It is now considered likely that the marine horizon proved at a depth of 450 ft 11 in to 463 ft 1 in (140.5 m-141.2 m) may correlate with the Lingula-Bed proved at the base of the upper division of the Quartzitic Sandstone Group at Bristol (Kellaway 1967). No marine strata comparable with the Ashton Vale Marine Band were found, but on palynological evidence Dr B. Owens has concluded that the base of the Westphalian may lie at about 200 ft (60.96 m).

North of Limekilns Lane Borehole little is seen of the quartzitic sandstones which form thin intercalations in barren mudstone and siltstone. At Hall End sandstone and purplish grits are exposed locally in the valley of the Ladden Brook about 437 yd (400 rn) SW of the hamlet of Hall End. Here the rocks have an easterly dip of about 30–32°. The lower part of the Quartzitic Sandstone Group between Yate Court and Hall End consists of about 150 ft (46 m) of mudstone resting on the Tanhouse Chert. The mudstone is succeeded by a sandstone which can be traced northwards to Hall End. Beyond this point the general succession in the area extending northwards to Barbers Court Farm and Cromhall Common is obscure. Much of the ground has very low relief and the Carboniferous rocks are obscured by celestine-bearing Keuper Marl or Drift deposits. Celestine has been worked extensively in this area both in Keuper Marl and on the surface of the Quartzitic Sandstone Group. Some of these excavations have disclosed the presence of quartzitic and pebbly sandstone interbedded with mudstone and shale.

At some point between Hall End and Heath End, Cromhall, in an area where the Palaeozoic rocks are concealed by Keuper Marl and Drift deposits, the Tanhouse Chert disappears, and at Heath End the base of the Quartzitic Sandstone Group is taken above a thin band of decalcified calcareous sandstone with Buxtonia sp. Neither the base nor the top of the Quartzitic Sandstone Group is exposed in this area and most of the formation consists of mudstone in which, however, there are some substantial beds of quartzitic sandstone. About 25 ft (7.6 m) of quartzitic sandstone was formerly exposed in a trench in the Cromhall–Rangeworthy road about 33 yd (30 m) N of Cole's Bridge [ST 6974 8953]. Further west the proportion of sandstone increases as the rocks are traced westwards towards Tapwell Bridge [ST 6873 8945]. The position of the Ashton Vale Marine Band is not known in this area but extensive old workings on the Cromhall Vein in Cromhall Common may mark the presence of a seam in the Quartzitic Sandstone Group. Another coal, the Tapwell Bridge Vein, is present in the mudstones overlying the basal sandstone of the Namurian. On the published six-inch maps (ST 68 NE, ST 78 NW) the crop of the Tapwell Bridge Vein is shown extending from Heath End to Tytherington being displaced by the Iron Acton Fault at Tapwell Bridge. The measures lying between the Tapwell Bridge and Cromhall veins consist mainly of barren quartzitic sandstone and mudstone weathering to give a clay soil. The rocks are known to be faulted in this area (Anstie 1873). A few small exposures have been seen, the best being in an old quarry [ST 6926 8967] near Cromhall Rectory when purplish quartzitic sandstone dips to the SSE at about 20°. These measures are now thought to lie mainly within the Quartzitic Sandstone Group.

West of the Iron Acton Fault the crop of the Tapwell Bridge Vein, which is now thought to be roughly comparable in age with the Raysfield Seam of Yate, can be traced by a belt of old workings extending from the Iron Acton Fault at Tapwell Bridge in a south-westerly direction towards Newhouse Farm, Tytherington. According to the 1871 Coal Commission plans the workings continue to a point about 164 yd (150 m) E of Mill Farm [ST 6742 8803]. Further south the Tapwell Bridge Vein is concealed by Keuper Marl and Dolomitic Conglomerate.

In view of the close proximity of the Tapwell Bridge Vein to the Tanhouse Limestone and the apparent absence of the Tanhouse Chert in the Cromhall–Tytherington area [ST 68 NE] it would appear that attenuation of the lower division of the Quartzitic Sandstone Group is perhaps accompanied by an increasing degree of overstepping of the lower by the upper division of the Group in a westerly direction. F.B.A.W., G.A.K.

Coal Measures (Westphalian)

Classification

Three major divisions of the Coal Measures were recognised by the miners and early geologists working in the Bristol Coalfield. These divisions reflect major changes in the lithology and are also related to the productivity of the measures in terms of the presence or absence of workable coals. Thus the Lower Coal Series and the Upper Coal Series are essentially composed of productive coal-bearing mudstones with subordinate sandstone, while the Pennant Grit or Pennant Series is a thick, generally unproductive mass of sandstone or barren sandy mudstone in which few coal seams occur.

The substitution of a classification based primarily on the presence of marine bands was for many years an unattainable ideal in the Bristol and Somerset coalfields. Indeed at the time of the production of the first edition of the Bristol District Special Sheet (1962) there was no alternative to the older system of classification. Since the discovery of the principal marine bands in the Bristol Coalfield and the identification of the crop of the Crofts End Marine Band at Yate (p. 63) it has been possible to apply the generally accepted classification in the form of a combined Lower and Middle Coal Measures and an Upper Coal Measures. Equally important is the new classification of the Upper Coal Measures now introduced for the first time in an official memoir. This involves the separation of the Upper Coal Measures into major groups and the classification of these groups into two sets of measures, the Pennant Measures and Supra Pennant Measures. The division between the Pennant and the Supra-Pennant Measures is for the present somewhat arbitrary but it may serve as a basis for comparison with the upper Westphalian succession in South Wales and thus assist in correlating the Upper Coal Measures in the two coalfields.

The Pennant Measures comprise two major groups of strata, the Downend Group at the base and the Mangotsfield Group at the top. The Downend Group extends from the Winterbourne Marine Band to the base of the Mangotsfield Seams, the Mangotsfield Group from the Mangotsfield Seams to the base of the High Vein of Coalpit Heath (Kellaway 1970, fig. 4). In effect therefore the Downend and Mangotsfield groups cover the greater part of the massive sandstone and conglomerate which constitute the Pennant Grit or Pennant Series of the older classifications.

The Supra-Pennant Measures (not to be confused with the Supra-Pennant Group of the Forest of Dean) embrace all the Upper Coal Measures rocks which used to constitute the Farrington and Barren Red groups. In Somerset the Radstock and Publow groups are also included, but these may be unrepresented in Gloucestershire where post-Westphalian erosion has removed the youngest Westphalian rocks.

Working of coal in the northern part of the Bristol Coalfield is known to have been in progress in medieval times. The most extensive workings on Sheet 251 are those in the central part of the Coalpit Heath basin where Upper Coal Measures coals were exploited, and in the eastern part of the coal basin between Chipping Sodbury and Rangeworthy where coals of the Middle Coal Measures were formerly mined. Comparatively little working has taken place on the north-western margin of the Bristol Coalfield where the seams of the Lower and Middle Coal Measures are thought to be in a thin and deteriorated condition. Only the salient points of the stratigraphy of the Coal Measures are discussed below, a fuller account is in preparation in a larger memoir dealing with the Bristol District. Some of the details are also available in the published works referred to below.

General account

Lower And Middle Coal Measures

The maximum thickness of the Lower and Middle Coal Measures is thought not to exceed about 1300 ft (396 m). This figure is uncertain, for complex stratigraphical relationships make it difficult to compare thicknesses in different areas. Little or nothing is known of the Lower and Middle Coal Measures in the ground east of Tytherington. Between Cromhall and Yate however there is sufficient evidence to enable the main outlines of the stratigraphy to be given. Many of the collieries in this area were abandoned by the end of the 19th century. In consequence there is little information about the coal seams that they worked, and even less about the stratigraphy or palaeontology of the enclosing measures. The best known area lies between Yate and Rangeworthy where numerous old shafts and tips attest the activities of the miners in times past.

Here, in the deep Yate Borehole [ST 6975 8252], described first by Pringle (1921) and later by Moore and Trueman (1937) two marine bands were proved. These have now been correlated with the Crofts End and Winterbourne marine bands, the latter being the equivalent of the Upper Cwmgorse Marine Band of the South Wales Coalfield (Kellaway and Welch 1955a, p. 21). The Crofts End Marine Band, first described by Moore and Trueman (1937) from Crofts End brick pit, Bristol, was shown to be the equivalent of the Cefn Coed Marine Band of South Wales and the Mansfield Marine Band of the Midlands.

Owing to the structural complexity of the Coal Measures and the absence of good exposures it was not possible at the time of the primary six-inch survey to identify the position of the crop of either marine band in the Yate area or to determine with any degree of accuracy their relationship to the seams worked in the Yate collieries. The discovery of the Crofts End Marine Band at Yate has been mentioned (p. 62) and this enabled the stratigraphical position of the Yate coal seams to be determined. It is now known that the main worked seams at Yate occupy the same position as the principal coal seams of the Middle Coal Measures formerly worked at Kingswood and Harry Stoke near Bristol (Kellaway 1971, fig. 3).

Two other marine bands which are known in the Bristol area, the Harry Stoke Marine Band (Kellaway and Welch 1955a) and the Ashton Vale Marine Band, have not yet been proved in this district so it is impossible to define accurately the limits of the Lower Coal Measures. The Lower and Middle Coal Measures have therefore to be grouped together.

Despite the lack of information about the precise position of the crop of the Ashton Vale Marine Band, the approximate position of the base of the Coal Measures can be roughly estimated by using data derived from adjacent areas and comparing this with local information. In general it would appear that there are few coal seams in the Lower Coal Measures and that any coals that are present are likely to be either impersistent or too thin and dirty to be of economic value. The coals of the Middle Coal Measures appear to be of better quality and thickness. Some of them, notably the Yate Hard, had a good reputation and seem to have been comparable in quality with the principal seams worked at Kingswood and east Bristol.

Upper Coal Measures

The central area of the Coalpit Heath basin is formed of Upper Coal Measures in which there are a number of coal seams which have been extensively worked. The two principal divisions, viz. the Pennant Measures and Supra-Pennant Measures differ in character, the former being dominantly composed of sandstone and sandy mudstone, the latter of mudstone and shale with subordinate sandstone. The general classification is given on p. 49.

Comparison of vertical sequences in the Upper Coal Measures of the Kingswood, Downend, Frampton Cotterell area with the sequence between Westerleigh and Rangeworthy suggests that the Downend Group which is over 2000 ft (609 m) thick in north-east Bristol, may be about 400 ft (122 m) thick near Yate, and about 200 ft (61 m) at Rangeworthy. On the other hand the Mangotsfield Group, averaging about 1500 ft (457 m), shows very little change, either in thickness or facies in the Bristol Coalfield. The shale 'slack' which is thought to mark the position of the Mangotsfield seams is well marked in the outcrop east of Yate though it is difficult to map in the northern part of the coalfield between Wickwar and Rangeworthy. Above the Mangotsfield seams however the succession in the Upper Coal Measures is fairly constant. This is also true of the productive measures proved at Coalpit Heath, the principal seams of the Supra-Pennant Measures having been worked almost uninterruptedly over the central part of the Coalpit Heath basin.

Downend Group

Though thin, the Downend Group is of some stratigraphical significance. In the area between Yate and Rangeworthy it consists of mudstones at the base and thick beds of conglomerate with abundant vein quartz pebbles. The base of the conglomerate is marked by an erosion surface and the conglomerates are clearly transgressive since the Winterbourne Marine Band is missing at Westerleigh Common (Kellaway 1971, fig. 3). Locally, in the area between Rangeworthy and Broad Lane, Yate, the conglomerates are missing or pass laterally into sub-greywacke of Pennant type. All these rocks are overlain by a thin but extensive bed of shale which marks the probable position of the Mangotsfield seams and is taken as the base of the Mangotsfield Group.

Mangotsfield Group

The lower part of the Mangotsfield Group, amounting to about two-thirds of the total thickness is composed of sub-greywacke or Pennant Grit with bands of sandy and silty mudstone. The upper part consists of beds of Pennant-type sandstone intercalated with thick bands of shale, some containing traces of thin dirty coal seams. No workable coals are known, however, the lowest workable coal in the Upper Coal Measures of the Coalpit Heath basin being the High Vein. This coal is regarded as marking the base of the Supra-Pennant Measures.

Supra-Pennant Measures

Three, locally four, seams were worked extensively in the Coalpit Heath basin up to the time of the closure of Coalpit Heath colliery in 1949. These coals have, to all intents and purposes, been exhausted. The lowest seam, deceptively known as the High Vein, has a roof with abundant Leaia and non-marine bivalves in Coalpit Heath colliery, though the same seam yields only plants in the adjacent Parkfield Pit where it is represented by the Hollybush and Great veins (Moore and Trueman 1937, p. 232). The Top Vein of Parkfield (Hollybush of Coalpit Heath) and the Hard Vein are the two other seams generally worked in the Coalpit Heath basin.

Above the Hard Vein there are no workable coals known in the Coalpit Heath basin. The overlying measures, which are at least 400 ft (122 m) thick, consist mainly of red mudstone with thin sandstone bands. These measures are not unlike the Barren Red Group of Somerset but the thickness of red measures in the Coalpit Heath basin is greater than in the adjacent parts of the Somerset Coalfield and it is uncertain how much of the Somerset succession is represented.

No details of the Coal Measures or the mine workings are given here as these are to appear in a separate memoir on the Bristol and Somerset Coalfield. G.A.K.

Chapter 6 Triassic

General account

The Triassic rocks of the district are continental deposits burying a preexisting landscape of moderate relief, much of which is now exhumed. No deposits of "Bunter" age have been recognised and the Keuper rocks consist in part of coarse conglomerates, breccias and some sandstone, but mainly of soft mudstone ("Marl"), and they rest with sharp unconformity on Palaeozoic rocks. An influential factor in their deposition was the synclinal disposition of the outcrop of Carboniferous Limestone after the Armorican folding and subsequent erosion. This created a broadly parabolic narrow outcrop of Carboniferous Limestone etched into sharp relief over softer Carboniferous rocks outcropping within the parabola, and Lower Old Red Sandstone and Lower Palaeozoic rocks on its flanks.

The earliest Triassic deposition took place as an encroachment from the south and south-west into the open end of the parabola, and from the northwest and the east on to its external flanks. A reconstruction of the Triassic base-contours (Figure 7) reveals that along the eastern side of the coalfield, or central area, there were probably shallow elongated depressions with little or no drainage egress. It is in the region of these depressions that the present workings of celestine developed.

The Triassic desert landscape was buried progressively during the later part of the Keuper, but it was the Rhaetic sea which finally spread across the eminences of Carboniferous Limestone to submerge the whole area.

To the north of Tortworth an attenuation of the beds in the Triassic outcrop is apparent which clearly does not stem from sharp irregularities in the Triassic land surface, for here the Triassic rocks overlie uniform Tremadoc mudstones and there is no banking of deposits against steep slopes. Consequently there is no Dolomitic Conglomerate here, only marl. This attenuation seems to affect not only the Keuper Marl, but the Tea Green Marl and even the Rhaetic and its cause would appear to relate to an area of slower subsidence that lay northward of Tortworth. Here, little and sometimes no deposition took place during the Keuper.

The following subdivisions were mapped within the district:

Keuper

Dolomitic Conglomerate

This is a deposit of very restricted occurrence, being confined to the steeper slopes of the former Triassic landscape and particularly the small hollows on these slopes. It closely follows the outcrop of the Carboniferous Limestone and, being derived largely from this, is composed of cemented scree debris, mainly coarse, angular and ill-sorted lumps of Carboniferous Limestone. Apart from being cemented, this deposit has undergone considerable dolomitisation under the desert conditions which prevailed during the Triassic period.

In places the Dolomitic Conglomerate rests on or near the Quartz Conglomerate outcrop. Distinction between these two conglomerates can be made on the grounds of pebbles contained within them. The Dolomitic Conglomerate contains, or is composed of, locally-derived largely limestone pebbles or boulders which are commonly subangular and usually in a marly dolomitic matrix which weathers yellow. The Quartz Conglomerate on the other hand is composed of well-rounded, smaller siliceous pebbles in a sandstone matrix.

Where present, the Dolomitic Conglomerate occurs usually as the basal deposit of the Triassic, but its accumulation ceased earlier in some places than in others as the landscape was progressively buried. In many places it is succeeded by Keuper Marl, and in others by Tea Green Marl suggesting a late-stage formation, but there are places where it may never have been covered by any other Keuper deposits at all, as is suggested by some of the occurrences overlain directly by Rhaetic deposits. It is nowhere thick, probably never exceeding 30 ft (9 m).

Keuper Marl (Red Marl)

The Keuper Marl, forming the greater part of the Keuper, is composed of dull red, feebly calcareous mudstone. Locally, beds of siltstone are present in the lower part and thin bands of green marl become progressively more abundant towards the top. Largely because of the irregular relief of the Triassic land-surface the thickness of the Keuper Marl is very variable.

South of Yate the Keuper Marl has a thickness of some 70 ft (21 m) and in the south-west, near Latteridge it is about 60 ft (18 m) thick. From here there is continued northward thinning of the deposits as they become restricted to progressively higher horizons in the Keuper, so that west of Wickwar thicknesses are usually less than 30 ft (9 m). Eastwards of Wickwar the Keuper Marl thickens rapidly and reaches about 100 ft (30 m) within a mile (1.6 km). West of Falfield it is also thick, exceeding 100 ft (30 m). North of Tortworth the Keuper Marl remains thin, being some 35 to 40 ft (11–12 m) at Michael Wood, and near Stinchcombe the thickness is only about 25 ft (8 rn).

Near the top of the Keuper Marl in the Bristol area (Sheet 264) there is an horizon of celestine-rich marl. The celestine (celestite) is secondary, but penecontemporaneous, and reveals that conditions at this late stage in the Keuper became very saliferous. The mineral is associated with some gypsum and it has been suggested that the gypsum deposits at a similar position elsewhere were coeval (Kellaway and Welch 1948, p. 47). The minerals occur as nodular masses and discontinuous layers within a few feet of marl. They form a useful marker horizon which has been referred to as the Celestine Bed (Kellaway 1960).

Around Chipping Sodbury this bed has been mapped and lies some 30 ft (9 m) below the Tea Green Marl. As far south as Dundry (Sheet 264) it was found at 29 ft 1 in (8.86 m) below the Tea Green Marl, and it is considered that an interval of 30 ft (9 m) between the Celestine Bed and the base of the Tea Green Marl is generally applicable over the area of Triassic outcrops of the Malmesbury district. In a study of celestine now in progress in the area of Yate^‡7 , it seems that this interval is locally greater there. Treated thus, as a marker horizon, the bed reveals the nature of the Keuper transgression, for, northward of Chipping Sodbury, the red marl beneath the Celestine Bed becomes thinner until the latter rests in places, particularly between Yate and Wickwar, on or near the surface of the Palaeozoic rocks. Clearly in other places, for example at Wickwar and nearby to the north-west, Keuper deposition had not commenced when the Celestine Bed was forming and there the Keuper Marl seems to be less than 30 ft (9 m) thick—being representative only of portions above the Celestine Bed. Since the area of deposition was extending during the formation of the Keuper Marl, the beds are transgressive and thus there are places where, at the time of formation of the Celestine Bed, the evaporites were laid directly upon the surface of Palaeozoic rocks. Such contact with very saline waters may explain some of the occurrences of celestine in the older rocks, as near Buckover and Falfield in the Silurian rocks (Weaver 1824, p. 337), but especially in Carboniferous rocks between Yate and Cowship Farm [ST 701 887] 1.5 miles (2.4 km) W of Wickwar. This was the area of most intense celestine deposition and, as indicated by the sub-Triassic contours, the most landlocked (Figure 7).

Celestine has been observed at other levels in the Triassic and Rhaetic, but it is usually of a local nature and some is possibly a result of redistribution.

Tea Green Marl

The Tea Green Marl is composed of firm, pale green, calcareous mudstone locally containing rounded grains of quartz. The formation usually succeeds the red Keuper Marl and the junction is normally abrupt, but in some places the red marls herald the green by being spotted and streaked with green.

Like the Keuper Marl, the thickness of the Tea Green Marl is variable for two reasons. First, there is the effect of the irregular surface of pre-Triassic rocks, the highest parts of which were still exposed during the deposition of the Tea Green Marl. The transgressive Tea Green Marl thus overlaps on to these older rocks in places, and thins out against them with the development of conglomerates locally. This effect is seen adjacent to the Carboniferous Limestone outcrops mainly between Chipping Sodbury and Wickwar and around Tytherington. As well as this, the Tea Green Marl is affected by an attenuation over a 'structure' passing approximately beneath Berkeley Road. Thus, while the Tea Green Marl is commonly 12 to 15 ft (3.7–4.6 m) thick, in the north near this 'structure' it is much reduced. Only 10 ft (3 m) are present near Berkeley Road South Junction [ST 712 984] and, at the north edge of the district near Breadstone, there are probably less than 5 ft (1.5 m), but possible minor pre-Rhaetic erosion may partly account for the latter.

It is unlikely that the colour of the Tea Green Marl is a product of the reduction of the top part of the Keuper Marl by conditions which prevailed in the Rhaetic. The changes of thickness of the Tea Green Marl are too regular to support the idea, and the predictable vertical interval between it and the Celestine Bed indicates a primary sedimentary genesis. Thus it is held that the Tea Green Marl was formed during a short period of change between the continental oxidising conditions under which the red Keuper Marl was formed, and the more marine reducing conditions of Rhaetic sedimentation.

Where wide enough, the outcrop of the Tea Green Marl may be revealed as a bench above the Keuper Marl slope and this is taken to be a reflection of the firmer, more calcareous nature of the Tea Green Marl.

Rhaetic

The Rhaetic is thin and is composed of argillaceous rocks and thin limestones. The former are mainly marine mudstones and shales which weather to sticky soft clays. These rocks are poorly exposed in the district so that subdividing them for the purpose of the present survey was impracticable. Most of the boundaries were proved by augering; the basal Bone Bed providing an easily recognisable horizon.

The Rhaetic beds are almost 19 ft (5.8 m) thick at Chipping Sodbury (Reynolds and Vaughan 1904, pp. 195–7), but locally, where they overstep on to the Carboniferous, they become thin over ridges on the sub-Rhaetic surface and in one area, about 1 mile (1.6 km) N of Chipping Sodbury, the Rhaetic appears to be absent due to overstep or overlap by the Lias. North of Tortworth, like the underlying Keuper formations, the Rhaetic becomes more attentuated. It is about 15 ft (4.6 m) thick near Stinchcombe according to Richardson (1904b, p. 533) and, at the extreme north of the district, its thickness is possibly less than 10 ft (3 m). There are two formations, the Westbury Beds and the Cotham Beds.

The Westbury Beds which comprise the greater part of the Rhaetic, consist mainly of almost black shales with thin beds of sandstone and limestone containing Chlamys valoniensis. The base of the Westbury Beds is marked by a fossiliferous sandstone composed of well-rounded grains of quartz. The sandstone is bluish grey, calcareous and pyritic when fresh, but is generally decalcified and weathered to a rusty-yellow. It is commonly 6 to 8 in (15–20 cm) thick, though locally up to 3 ft (0.9 m). It contains the representative of the Rhaetic Bone Bed with cubes of pyrite and a few fish teeth and scales. In places over the Carboniferous Limestone the Bone Bed may be absent or represented by a film of scales adhering to the subjacent rock.

The Cotham Beds consist of greyish green shales, mudstones and pale yellow marls with thin pale limestones. At the top of these beds is the Cotham Marble, usually a hard splintery calcite mudstone with arborescent markings. The thickness of the Cotham Beds is about 6 ft (1.8 m) or less within the district.

The Cotham Beds give rise to land instabilities on slopes and could present a hazard to engineering works.

Details

Keuper

West side of coalfield

A road-cutting on the top of Latteridge Hill [ST 660 850] showed 12 ft (3.7 m) of Red Marl overlain by 12 ft (3.7 m) of Tea Green Marl. To the north and south of Latteridge overgrown marl pits are seen. Fragments of celestine occur on the surface and a trial for celestine was made some 70 years ago on the southern outskirts of the hamlet: the mineralised layers, however, proved too thin to work. A deep ditch [ST 671 864] 1.25 miles (2 km) SSE of Tytherington Church exposed 5 ft (1.5 m) of red marls overlain by 8 to 18 in (20–46 cm) of irregularly-developed greenish calcareous sandstone with small celestine nodules. Above this was 2 ft (0.6 m) of dark red marl overlain by a 6 in (15 cm) band of celestine nodules. The section was capped by 18 in (46 cm) of red manly soil.

In the southern approach-cutting to Wickwar Tunnel red marls with hard bands can be seen resting on the planed-off edges of the Carboniferous Limestone (Plate 5).

East side of coalfield

Along the edge of the limestone ridge the conglomeratic facies of the Trias is well shown in a number of quarries. One of the most instructive examples is on the west side of the great Chipping Sodbury Quarry at a point [ST 724 827] 600 yd (549 m) NW of Chipping Sodbury Church. Here quarrying has cut through an ancient Trias-filled valley showing, beneath thin soil and tip: Tea Green Marl, sandy green marl with conglomerate lenses 5 ft (1.52 m) on green conglomerate rock 8 in (20 cm), red marls with soft yellow dolomite 2 ft 6 in (0.76 m), fine-grained dolomite rock with limestone chips 1 ft (30 cm), irregular lenses of red marl up to 1 ft (30 cm) over buff dolomite rock with limestone fragments seen 10 ft (3.05 m). F.B.A.W.

A plateau extends north-eastwards from Grove Farm [ST 704 801] which in the south marks the outcrop of the Celestine Bed, and this has been mapped beyond Sergeant's Farm [ST 710 807] nearly to Stanshawe's Court [ST 715 819] where it abuts against Carboniferous rocks. Old celestine workings are common on this plateau and northward the workings persist in the Carboniferous rocks as far as The Rectory [ST 7143 8309], Yate. Further north the Keuper thickens again slightly, and celestine was worked within the basal Keuper Marl around Brickhouse Farm [ST 714 832]. Between here and Cowship Farm [ST 710 887] were the principal celestine workings (Sherlock and Hollingworth 1938), and here it was worked in the adjacent Carboniferous rocks as well as the Trias. R.C.

In the Trias within fissures in the Carboniferous Limestone of Slickstones Quarry [ST 705 915] 1670 yd (1527 m) WNW of Charfield Church, reptilian remains, including Glevosaurus hudsoni, have been found (Robinson 1973) (Plate 4). F.B.A.W.

Wickwar to Charfield area

Small amounts of celestine occur east of the Carboniferous Limestone outcrop along the west bank of the Little Avon north of Bishops Hill Wood [ST 733 874]. They follow an horizon about 30 ft (9 m) below the Tea Green Marl that can be seen in the sides of lanes and streams descending to the Little Avon. Traces of old workings occur in fields [ST 732 879] some 0.75 mile (1.2 km) SE of Wickwar Church. Nearby [ST 733 883] at Sturt Farm, Whittard and Smith (1944, p. 66) recorded beds with celestine, the base of which is nearly 38 ft (11.6 m) below the Tea Green Marl. Their upper limit was not given but it seems certain that they constitute the Celestine Bed. The total thickness of the Keuper here is 52 ft (16 m), 13 ft (4 m) being accounted for by the Tea Green Marl. Whittard and Smith also gave an account of the dolomitic nature of the basement beds of the Keuper in the Little Avon below Loanda [ST 730 888], and in the bank of the same river [ST 7272 8984] at Bunsall Bridge, a section through the basal beds shows:

At Little Bristol [ST 725 908] the mapping revealed that the thickness of the Tea Green Marl might be as much as 20 ft (6 m). Further north, in the brickpit [ST 714 926] at the south end of Tortworth Copse a section revealed: Tea Green Marl 5 ft (1.52 m) on red and green marl with some gypsum at base 30 ft (9.14 m), red marl with celestine 2 ft (0.61 m), over red marl seen for 4 ft (1.22 m).

At the north end of Michael Wood, a hill of Keuper Marl is capped by a plateau of Tea Green Marl and a small outlier of Rhaetic. The total thickness of the Keuper here is about 40 ft (12 m). For further details of the Celestine Bed see Sherlock and Hollingworth 1938, pp. 83–7. R.C.

At Eastwood Park, Triassic marls rest unconformably on Palaeozoic strata forming the lower part of a steep-sided outlier separated from the Whitcliff Park outlier by a deep valley at Lower Stone. The Keuper is 150 ft (46 m) thick in the north but thins rapidly south-eastwards on to a swell in the pre-Triassic land surface. The Tea Green Marl is from 12 to 15 ft (3.7–4.6 m) thick and near Morton Farm [ST 661 915] supports a broad shelf. A small stream flowing north from Stump's Wood [ST 669 921] exposes red marl at intervals as far as Black Covert. Near Morton Farm, an old marl pit [ST 6635 9178] shows Tea Green Marl resting on green spotted red marl.

Red marl was formerly worked in a series of pits on the slopes east of Whitcliff Park but the workings are now much overgrown. Three feet (0.91 m) of Tea Green Marl resting on red marl was however exposed in a pit [ST 6615 9532] 900 yd (823 m) S40°W of Hystfield Farm.

A steep-sided outlier of Triassic rocks extends from Newpark Farm [ST 6635 9497] to Blackhall [ST 673 981]. The Keuper Marl measures 140 ft (43 m) in thickness in the south decreasing to 90 ft (27 m) in the north. The Tea Green Marl is consistently about 12 ft (3.7 m) thick. Some 800 yd (732 m) E10°S of Bluegates Farm a shallow pit [ST 6711 9789] shows the following section: red marl with occasional green spots 3 ft (0.91 m) on greyish green closely laminated shale 9 in (23 cm) over soft red (green-spotted) siltstone 3 ft (0.91 m). Between Michaelwood Farm and Hogsdown Farm [ST 710 972] the Keuper Marl is about 50 ft (15 rn) thick and is exposed in numerous shallow pits. Up to 20 ft (6 m) of red marl with pale green spots and streaks are exposed in a railway-cutting [ST 7135 9560] south of Wick Bridge. A group of excavations [ST 709 987] near Kittsgreen on an east-west ridge of Keuper Marl showed up to 10 ft (3 m) of firm green-spotted red marl, while the railway-cutting at Berkeley Road South Junction afforded a somewhat overgrown section of 20 ft (6 m) of marl. At this place the Keuper Marl is 30 ft (9 m) thick. The Tea Green Marl is exposed to its full thickness of 10 ft (3 m) in a stream section at Standle Farm [ST 7202 9865] where it is somewhat sandy and indurated. North of Baynhamcourt Farm [ST 717 975] the marl raises a westward-facing scarp, striking NNW–SSE. At Lorridge Brake [ST 7179 9973], 10 ft (3 m) of red marl were exposed resting on red faintly micaceous siltstone. At Green Farm, the Keuper Marl, exposed in an old pit [SO 7127 0096], is approximately 20 ft (6 m) thick and is overlain by 5 ft (1.5 m) of Tea Green Marl. I.B.P.

Rhaetic

The best section through Rhaetic rocks occurs at the southern margin of the district in the Chipping Sodbury railway-cuttings. It is now rather overgrown, but was described by Reynolds and Vaughan (1904, pp. 195–7). A summary of this section [ST 7314 8160], 100 yd (91 m) E of Kingrove Lane Bridge (Sheet 265) is as follows :

R.C.

In the large quarry outside the north-west corner of Chipping Sodbury Cemetery the Bone Bed [ST 726 825] can be seen attached to the bored surface of the Carboniferous Limestone, while 400 yd (366 m) to the north the total thickness of the Rhaetic was recorded as 14 ft (4.3 m). It was in this area of the quarry that, during boring operations, the Rhaetic was observed to be affected by a series of reversed faults related to bedding-plane slip in the Carboniferous described by S. H. Reynolds (1938). The evidence has now been entirely destroyed. From a position above the quarry due east of The Ridge [ST 723 830] to a point east of Yate House [ST 721 840], there does not appear to be any trace of the Rhaetic, which is either cut out by a fault or has been overstepped or overlapped by Lower Lias. Further north the Rhaetic reappears and crops out as a band extending past Hampstead Farm [ST 728 842] and the high ground north of Yate Rocks, to a point a little south of Wickwar. In the middle of this outcrop, the Cromhall Sandstone projects as a long ridge through the clay.

In the outlier of Rhaetic west of Wickwar and overlying the railway tunnel, the Bone Bed, 9 in (23 cm) thick, was exposed in the bank of a pond [ST 718 883] 950 yd (869 m) SW of Wickwar Church. F.B.A.W.

Little Avon Valley to Breadstone

The Rhaetic has a relatively narrow outcrop on the east side of the Little Avon.

In a trench (p. 84), dark bluish grey clay of the Westbury Beds with some grey calcilutite bands near the base, were observed at localities between places [ST 7228 8724] and [ST 7230 8720]; also at [ST 7254 8689]. Fossils recorded from these places include: Actaeonina sp., Chlamys sp., Eotrapezium sp., Protocardia rhaetica, Rhaetavicula contorta, Tutcheria cloacina, fish fragments. Pale grey calcareous silty clay and some buff manly limestone of the Cotham Beds were also encountered at places, e.g. [ST 7235 8709], [ST 7268 8675]. R.C.

A temporary section was afforded by a trial hole [ST 731 883] 1100 yd (1006 m) SE of Wickwar Church, from which Whittard and Smith (1944, p. 66) recorded 5 ft 3 in (1.6 m) of black shales with the Bone-Bed at the base. F.B.A.W.

Traces of green and pale cream-coloured clays of the Cotham Beds were recorded at many places along the outcrop north of Chipping Sodbury, and the Cotham Marble, broken by penecontemporaneous brecciation (Crazy Cotham Marble) was exposed in the stream [ST 7319 8508], about a 0.67 mile (1 km) E of Yate Rocks. Cotham Marble was also exposed 6 to 9 in (15–23 cm) thick [ST 7456 8774], on the east bank of the Little Avon River, and it is exposed again [ST 7413 8882] near Chase Hill. This last exposure was also recorded by Richardson (1904b, pp. 534–5).

At 270 yd (247 m) SW of Southend Farm [ST 7262 9045] an exposure in the base of the Rhaetic showed blue clay with a 2 in (5 cm) band of impersistent hard, blue limestone containing fish remains resting on Tea Green Marl. The only noteworthy exposures north of here were those about 200 yd (183 m) E of Standle Farm [ST 722 986] which were recorded by Richardson (op. cit., pp. 532–3). From these he produced a composite section giving the total Rhaetic thickness as 14 ft 8 in (4.5 m). Gyrolepis alberti and fish remains were obtained. Richardson suggested the figure may be inaccurate and in fact it looks a little excessive. To the north the outcrop becomes very narrow and insignificant, and with the rapid diminution of the underlying Tea Green Marl as well, the Rhaetic comes to rest on red Keuper Marl less than a mile (1.6 km) N of the district. R.C.

Chapter 7 Jurassic: Lias

General account

Introduction

The Jurassic rocks of the Malmesbury district are of marine origin and consist predominantly of limestones, silts and clays. Sands also occur in parts of the succession, notably at the top of the Lias and locally in the upper part of the Forest Marble. Epeirogenic movements occurred frequently leaving erosional or non-depositional gaps in the rock succession.

In broad terms the Jurassic rocks fall into a repetitive pattern of clay and limestone. Apart from the White and Blue Lias, which can be considered as a limestone phase following the argillaceous deposits of the Rhaetic, the Lias is largely a clay and silt formation and is followed by Inferior Oolite limestones. Above is the clay of the Fuller's Earth, capped by limestones of the Upper Fuller's Earth and Great Oolite. The basal beds of the Forest Marble continue this limestone phase and are overlain by the largely argillaceous rocks of the higher parts of the Forest Marble. The succeeding Cornbrash, though comparatively thin is a limestone phase of wide occurrence in England, but this is followed in turn by the youngest Jurassic formations represented in the district, the Kellaways Clay and the Oxford Clay.

In portraying the Jurassic rocks in this way it is not implied that the repetitions of clay and limestone all represent cycles in the sedimentation of a single continuing depositional basin, nor that any two clays or limestones can be compared and considered to represent similar stages in the sedimentation of their respective depositional environments, though this may be true in certain cases. Some limestones such as the Lower Inferior Oolite and the Great Oolite are obviously similar and accumulated under similar conditions, but these limestones seem very different from others such as the Marlstone Rock Bed, the Cross Hands Rock and the Cornbrash.

During the survey, the Lias rocks were divided into seven lithological units. These formations can be grouped to coincide approximately with the three divisions Upper, Middle and Lower Lias, though the Dyrham Silts are known to lie partly within the top of the Lower Lias.

The classification of the Lias is :

Lower Lias

Though largely a clay formation, in the south of the district two sequences of rubbly limestone occur at and near the base. The lower of these constitutes the undivided White and Blue Lias, for in the Malmesbury district it has been impossible to map the White Lias separately from the Blue, a practice which thus differs from that adopted during six-inch mapping of some areas to the south. The higher limestone sequence lies about 45 ft (13.7 m) above the lower and is a northward extension of the upper part of the Blue Lias. For the major part of the district a two-fold lithological division of the Lower Lias has therefore been adopted, comprising the White and Blue Lias below and the Lower Lias Clay above. Only in a limited area in the extreme south does the higher Blue Lias limestone slightly complicate this classification, for northward it becomes lenticular and soon vanishes.

Exposures in the Lower Lias are rare and few fossils have been found; application of any zonal classification is thus limited to small parts of the succession, but to enable comparison to be made there follows a summary of the Lower Lias ammonite zones and subzones of this district according to Dean, Donovan and Howarth (1961, p. 441).

Apart from the limestones of the White and Blue Lias and some higher layers, which form small dip and scarp features, the Lower Lias occupies a tract of low, rather featureless and badly drained land, following the foot of the Cotswold scarp. This outcrop, about 2 miles (3.2 km) wide in the south narrows to as little as 0.5 mile (805 m) near Stinchcombe in the north.

It has been calculated from the map that the thickness of the complete Lower Lias near Chipping Sodbury, in the south, is about 220 ft (67 m), while further north, near Hawkesbury the similarly derived figure is in excess of 400 ft (122 m). However, in the extreme north of the district, at Stinchcombe, the outcrop of the Lower Lias has become very narrow and the thickness appears to be only some 200 ft (61 m) again (Figure 8). This thinning may be due to the westward deviation of the outcrop here thus bringing it closer to the Malvern Line. Similar eastward thickening, as though into a basin of deposition, can be detected in the Keuper. Further north, in the Vale of Gloucester, there is a marked increase in the thickness of clay, accompanied by great expansion of the outcrop.

White and Blue Lias

To the south of the district, around Bath, the White Lias and Blue Lias limestones are easily separable and have a combined thickness of about 45 ft (14 m). At Chipping Sodbury the White Lias is only 2 ft (0.6 m) thick and is not separable on the map from the overlying limestone, their combined thickness being only 7 ft (2.1 m). There is evidence from nearby excavations for stone at Westerleigh rail sidings (Sheet 265), which were inspected by K. J. Ackermann and H. C. Ivimey-Cook, that the Blue Lias limestone belongs to a zone no higher than that of Psiloceras planorbis.

The higher "Blue Lias" limestone is separated from the lower by about 45 ft (14 m) of argillaceous beds near Chipping Sodbury and is up to 17 ft (5.2 m) thick. This thins away just south of Little Sodbury by passage into Lower Lias Clay and is not seen northwards until beyond the district, just east of Purton. Evidence, again from the Westerleigh sidings, suggests that this higher limestone belongs mainly to the Schlotheimia angulata Zone, both the underlying Alsatites liasicus Zone and the overlying Arietites bucklandi Zone being mainly clay. Further south the Blue Lias thickens towards Keynsham (Sheet 265) where the bucklandi Zone is represented by Blue Lias limestones.

Northward the White and Blue Lias also become thinner and with the disappearance of the higher limestone it is likely that over most of the district the Blue Lias does not extend above the planorbis Zone.

The White Lias consists of very pale cream marl and nodular argillaceous limestone and porcellanous calcite mudstone. It is present in the Chipping Sodbury railway cutting near the station (Sheet 265), but north of this it is rarely exposed and its position has been inferred from the presence of soil brash to as far north as Lorridge Brake [ST 718 995]. Further north, the White Lias is absent and Blue Lias rests on the Cotham Beds, possibly reflecting a period of erosion prior to the deposition of the Blue Lias (Donovan 1947, p. 182).

White Lias has been recognised on the outlier at Whitcliff Park near Berkeley, but not on the Falfield outlier where it may be absent.

The lower Blue Lias limestone is present throughout the area though the thickness diminishes northward, barely passing into the Gloucester (Sheet 234) district before failing completely. It consists of a thin series of bluish grey shelly limestones with intervening beds of grey shale and crops out on a low escarpment between Chipping Sodbury and Breadstone. It also supports a dip slope platform locally 400 yd (366 m) wide and this feature together with the soil brash makes the beds readily distinguishable from the overlying Lower Lias Clay.

Lower Lias Clay

The Lower Lias Clay forms an outcrop up to 2 miles (3.2 km) wide passing from east of Chipping Sodbury through Kingswood (near Wotton-under-Edge) to Stinchcombe and Berkeley Road. It forms low, poorly drained ground rising gradually eastward to a scarp formed by the Marlstone Rock Bed. Locally the limestone ribs of the lower portion make impersistent features, especially in the south. As far as can be established the thickness of the Lower Lias Clay is about 200 ft (61 m) at the north and south of the district but possibly as much as 400 ft (122 m) at the centre near Alderley.

The limestones of the White and Blue Lias are succeeded by argillaceous beds in which a broad distinction can be made between the lower and upper portions. The lower portion consists of blue and grey calcareous shales in which thin ribs of bluish grey, fine-grained, argillaceous limestone are common. The top part of this lower portion belongs to the bucklandi Zone, the middle part to the angulata Zone and the bottom part to the liasicus Zone. It is apparent, therefore, that the upper limestone of the Blue Lias present just to the south, passes northward into the middle and probably upper portions of this lower subdivision of the Lower Lias Clay. It can also be seen (Figure 8) that this change takes place as the whole of the Lower Lias Clay thickens.

The upper subdivision of the Lower Lias Clay, which constitutes very approximately one-third of the whole, is devoid of the limestone 'ribs' and consists of rather smooth, stiff, grey clay which, when weathered, may be pale grey and orange-coloured. This subdivision contains a semicostatum Zone fauna in the bottom half and there is evidence for the raricostatum to T. ibex zones near the top. The ibex Zone occurs very close to the base of the Dyrham Silts just south of Hawkesbury, so, if the davoei Zone exists there, it is probably represented in the Dyrham Silts.

At the top the Lower Lias Clay shows a passage up into silt and silty clay which constitute the Dyrham Silts. These are largely Middle Lias, but records of Androgynoceras from the Dyrham Silts, near Cam, in the north part of the district and from near the middle of the Dyrham Silts at Alderley and Hillsley indicate that they range down into the Lower Lias. Androgynoceras and Oistoceras of the Prodactylioceras davoei Zone, occur just below the Dyrham Silts at Stonehouse Brick Pit [SO 810 053], about 4 miles (6.4 km) NE of Cam and were found in the equivalent bed of siltstone in Tuffley's Quarry, Robins Wood Hill (Ackermann MS, IGS File; Howarth 1958, p. x; Palmer 1971), but within Dyrham Silts.

Middle Lias

In the South Cotswolds two formations comprise the Middle Lias each readily recognised on lithological grounds. The lower is the Dyrham Silts (Stubblefield 1963, p. 9) which consist of micaceous silts and silty clays (Paterson 1963, p. 33) and the upper, which in the south is impersistent, is the Marlstone Rock Bed—a ferruginous, and in its lower parts, very sandy limestone, containing a rich brachiopod, belemnite and bivalve fauna. It is commonly decalcified almost to a rusty sand.

The base of the Dyrham Silts is ill-defined, for the Lower Lias Clay forming flat ground gives way imperceptibly to silt which forms the rise to the prominent topographical shelf formed by the Marlstone Rock Bed.

The junction between Lower Lias and Middle Lias occurs about the middle of the Dyrham Silts in places and it appears that the junction of the Dyrham Silts with the underlying Lower Lias Clay is transgressive upwards to the north. The mapped boundary between Lower Lias Clay and Dyrham Silts was taken at the change of slope between wet flat clay ground of 'Lower Lias Clay' and the steep incline on mottled silt and it is marked by water seepages or springs in many places.

The palaeontological classification of the Middle Lias into zones and subzones used in this account is that of Dean, Donovan and Howarth (1961) shown in the table below:

Numerous ammonite zones have been established in the Lower Lias, but only two, the margaritatus and spinatum zones have been erected for the Middle Lias. Fossils diagnostic only of the higher zone are recorded currently from the Marlstone Rock Bed of the district, though older works (e.g. Witchell 1882, p. 18) imply that the lowest parts of the Marlstone Rock Bed, now poorly exposed, belong to the margaritatus Zone; for example as seen in the quarry at the foot of Stinchcombe Hill.

A large part of the Dyrham Silts falls within the margaritatus Zone.

Dyrham Silts

In the south the formation consists of approximately 70 ft (21 m) of fine micaceous and argillaceous silt, grey when fresh, weathering pale buff with orange and rusty mottling at the surface. Northward the thickness increases to about 100 ft (30 m) near Hawkesbury, 120 ft (37 m) near Wotton-under-Edge and approaches 150 ft (46 m) in the extreme north. If this increase in thickness is at the expense of the Lower Lias Clay then clearly the base of the Dyrham Silts is diachronous, ageing northwards or north-westwards and in the Gloucester district there is evidence of this (Ackermann MS, IGS File). There is, however, also evidence that the Dyrham Silts north-east of Dursley become interbedded with clay, the basal layers actually passing into clay of Lower Lias age and suggesting just the opposite trend, north-eastwards. It seems possible therefore, that these facies changes do not take place in a south to north direction but, as in the Upper Lias, in a west to east direction.

The outcrop occupies the foot slope of the main Cotswold scarp below the 'step' caused by the Marlstone Rock Bed where this is present. Since in the south the Marlstone Rock Bed is impersistent, the step is absent, and the junction of the Dyrham Silts with the overlying Cotteswold Sands is commonly as difficult to map as the junction with the underlying Lower Lias Clay. By means of an auger the change from orange mottled, buff silt to the cleaner, yellow sand of the Cotteswold Sands could usually be detected within a few yards. Exposures and fossils are rare, and, traced northwards, the formation alters little in composition until east of Stinchcombe Hill, though north of Hawkesbury and around Hillsley, two thin silty limestone bands occur in the Dyrham Silts, just above and below the middle of the formation. These bands form two very distinct features on the slope and may represent thin limestone of wide occurrence at this horizon.

East of Stinchcombe Hill, on the lower slopes of the Cam–Uley valley, which is deeply incised into the Cotswold escarpment, intercalations of silty clay have been distinguished and traced north-eastwards along the crop to Coaley Peak, where the total thickness of the Dyrham Silts, including the clay intercalations is about 150 ft (46 m). The clay-silt alternation manifests itself locally by terracing of the scarp, but in most places the boundaries between outcrops of clay and silt have been determined by augering. In the area containing Dursley, Uley and Coaley Peak the local stratigraphy may be illustrated diagrammatically (Table 2, p. 88).

Sections in the Dyrham Silts, in Dulkin Brook [ST 759 997] and in a small stream [ST 771 989] flowing south from Uleyfield Farm to join the River Cam, show well-developed cyclic sedimentation, in which considerable variation in the constitution of the cycles is observed (Duff and Walton 1962). In some cycles almost unfossiliferous, poorly stratified, grey silty clay is succeeded by micaceous silt, which becomes increasingly calcareous, fossiliferous and indurated towards the top. The siltstone is overlain by a hard, often richly fossiliferous sandy limestone, abruptly succeeded by clay of the next cycle (Hallam 1964a, b). The shelly limestone is often absent, the cycles then being capped by a hard calcareous siltstone. In other examples, the basal clay may be thin or absent. There is a tendency for cycles dominated by silt to fall into groups, forming mappable divisions. Three such cycles are recorded in the silts of Dulkin Brook while the converse obtains in the clay of Ashmead. Mapping reveals lateral change of facies, but exposures are insufficient to trace the lateral variation of individual cycles.

Marlstone Rock Bed (and Junction Bed)

The Marlstone Rock Red is generally a rusty brown, greyish blue when fresh, ferruginous shelly limestone, very sandy in lower part. It everywhere contains an abundant fauna including molluscs, brachiopods and much crinoid debris. Ammonites are rare.

The conspicuous pebble bed at the base of the Marlstone Rock Bed in the Midlands (Edmonds. Poole and Wilson 1965, p. 38) has not been observed in the district. In the Midlands this base is sharp and erosive and appears to coincide with the base of the Pleuroceras spinatum Zone, thus confining the formation to this zone. If, as seems likely, the sandy beds mapped as the lower part of the Marlstone Rock Bed in the Malmesbury district belong to the Amaltheus margaritatus Zone then the formation extends to lower horizons than it does in the Midlands.

A small patch of limestone [ST 755 817] has been mapped near the southern limit of the district and produces a small platform on the Lias scarp at Old Sodbury. In part it yields an Upper Lias fauna (Reynolds and Vaughan 1902, p. 732) and because of this it should be classified as Junction Bed, a formation commonly developed to the south. Northwards, other small and very localised features occur on the Cotswold scarp between Dyrham Silts and Cotteswold Sands and these yield lumps of ferruginous sandy limestone typical of the Marlstone Rock Bed. Fragments of pale grey, argillaceous limestone are not uncommon on the down-dip slopes of these shelf-like features, but an Upper Lias age for them has not been established and it has been expedient to map all as Marlstone Rock Bed. North of Hawkesbury these features become larger and debris of ferruginous sandy limestone more abundant, but the Marlstone Rock Bed is not well or continuously developed until north of Wotton-under-Edge. Decalcification of the sandy limestone has in places produced a deep sandy soil on dip-slope surfaces.

The Marlstone Rock Bed outcrop has been well favoured as a site for settlement. The formation, even in the sporadically developed state, produces a marked break of slope and a pronounced spring line: Horton, Hawkesbury, Hillsley, Wortley and Wotton-under-Edge are all communities which have grown on the Marlstone Rock Bed outcrop.

In the north, at Stinchcombe Hill and Dursley, the Marlstone Rock Bed attains its maximum thickness of about 20 to 22 ft (6.1–6.7 m) and supports a broad shelf-like feature below the main Cotswold escarpment. North-eastwards from Cam to Coaley Peak, the Marlstone Rock Bed is somewhat attenuated, considerably decalcified and seldom raises a prominent feature. Similar characteristics are noted eastwards towards Uley in the valley of the River Cam.

Upper Lias

Between Chipping Sodbury and Wotton-under-Edge the Upper Lias can be divided into: 1. Cotteswold Sands and 2. Cephalopod Bed, though the latter division is too thin to appear on any maps. To the north and north-east clay enters at the base and a three-fold division can be made: 1. Upper Lias Clay, 2. Cotteswold Sands, 3. Cephalopod Bed.

At Old Sodbury a thin argillaceous limestone occurs at the very base of the Cotteswold Sands, approximately in the position assumed by the basal Upper Lias Clay further north. This thin limestone, together with the Middle Lias Marlstone Rock Bed on which it rests, constitutes a small outcrop of the Junction Bed. The outcrop at Old Sodbury is the only one to have been recorded in the Malmesbury district and is too small to show separately.

North-eastwards from the Cotswold scarp the sand of the Cotteswold Sands gives place from the base upwards to an argillaceous silt. This condition has been observed as far south-west as Ozleworth Bottom. The north-eastward increase in argillaceous and silty matter in the Upper Lias is accompanied by a thickening of the formation. From 150 to 160 ft (45.7–48.8 m) in the south where the Cotteswold Sands account for virtually the whole thickness of the Upper Lias, the mainly sand formation increases to nearly 200 ft (61 m) near Hawkesbury. At Wotton-under-Edge the lower part of the Cotteswold Sands has become silt and, at the base the Upper Lias Clay is developed, though immediately to the north-west, near North Nibley the silt and clay are absent again. The total thickness of the Upper Lias is here over 200 ft (61 m) and it continues to increase northward until, at Coaley Peak, it is about 250 ft (76 m).

Eastwards away from the Cotswold scarp the Upper Lias is unfortunately not exposed to its base, so that the complete thickness in the valley regions of the north-east has not been established. In these valleys, and also along the Cotswold scarp, just to the north of the district, where the scarp turns north-east into the Frome Valley, the argillaceous silty nature of the lower part of the Cotteswold Sands has been depicted on the maps, though no local formational name has been devised for it. Being much more argillaceous than the Cotteswold Sands, these silts and the Upper Lias Clay have been treated for mapping purposes as one unit. This practice accords with that adopted by previous workers (e.g. Hull 1857, p. 23; Kellaway and Welch 1948, p. 46) who considered all the clays and silty beds of the Upper Lias, above the Marlstone Rock Bed, as Upper Lias Clay (=Upper Lias Shale of Hull). When well exposed, the paler clays in which thin argillaceous limestone bands are common immediately overlying the Marlstone Rock Bed are, however, clearly different from the siltier beds which laterally replace the lower parts of the Cotteswold Sands. In the description of details this distinction has been preserved where possible. A study of the sedimentation in the Upper Lias from Cheltenham to Bridport was conducted by Davies (1969).

Fossils are not common in the Upper Lias except in the Cephalopod Bed and the Upper Lias Clay, but the stratigraphy of the Upper Lias can be referred to the ammonite zonal scheme modified after Dean, Donovan and Howarth (1961, p. 441) and quoted below:

Upper Lias Clay

The Upper Lias Clay consists of rather pale, bluish grey, somewhat silty clay with numerous thin layers, generally nodular, of finely-grained argillaceous limestone. It has been detected as far south as Alderley and has been observed at Wotton-under-Edge. In Waterley Bottom and at Stancombe Park [ST 737 974] the clay is somewhat attenuated (Woodward 1893), while at Dursley it is about 15 ft (4.6 m) thick.

In both clay and limestone an abundant ammonite fauna is preserved indicative of the falciferum Subzone. No representative of the exaratum Subzone has been found.

At Millend, 1 mile (1.6 km) S of Dursley, a bed of hard, brown-weathered, manly ferruginous limestone, with abundant ammonites filled with argillaceous, compact limestone, caps the clay. It also contains ferruginous ooliths, may be chamositic, and is 8 to 10 in (20–25 cm) thick at Millend, but thins to 3 to 5 in (8–13 cm) of largely decalcified limestone at Dursley. In places, 2 or 3 ft (0.6–0.9 m) of brown marls and limestone occur at the base of the clay.

Cotteswold Sands

The Cotteswold Sands consist of very fine-grained, silty, micaceous sand (Boswell 1924). They are grey and slightly calcareous when fresh, but weather rapidly to a pale yellow colour. Bedding appears to be poorly developed, the sand looking generally massive and very uniform. Blue-hearted calcareous sandstone, sometimes as continuous beds, but more often as lines of doggers, is developed at several levels. Fossils are rare in the sands, but may be moderately plentiful in the sandstones. Rapid continuous sedimentation is indicated and there is little evidence of winnowing, waves or strong currents.

The thickness of the Cotteswold Sands in the south is about 150 ft (46 m) and this increases northwards to nearly 200 ft (61 m) at Hawkesbury, over 200 ft (61 m) around Dursley, and nearer 250 ft (76 m) at Coaley Peak. Eastward from the Cotswold Scarp, the thickness diminishes rapidly, being 100 ft (30 m) in the Newmarket valley 0.67 mile (610 m) W of Newmarket near Nailsworth, and less than 100 ft (30 m) 0.5 mile (457 m) E of Nailsworth. In these regions the diminution of thickness of Cotteswold Sands is presumably accounted for by the greatly increased thickness of silt and clay beneath.

It is reasonable to postulate that the present position of the Cotswold scarp here approximately marks the position of maximum sand development in the Upper Lias, though no ground control exists to the west of the scarp, apart from Dundry Hill to the south-west. Since this part of the Cotswold scarp lies only slightly displaced from the southern extension of the Malvern Line, the latter has been associated with a supposed ribbon-like development of sand and the opinion expressed that uplift on this line during the Upper Lias gave rise to the deposition of sand, while clay was deposited in the deeper lateral regions (Kellaway and Welch 1948, p. 65). Boswell (1924, p. 261) suggested a derivation of the sand from the south. In the south of the district deposition of this sand commenced probably slightly earlier than it did in the north and S. S. Buckman (1889, pp. 444, 446) demonstrated that it continued longer in the south.

Cephalopod Bed

The Cephalopod Bed, as far as is known, underlies the Inferior Oolite everywhere in the district. It consists of a thin series of yellow and brown marls and manly limestones. It is commonly very shelly, containing large thick bivalve shells, ammonites and belemnites and abundant small dark brown limonitic pellets (Richardson 1910, p. 81). The pellets are in places concentrated into 'nests' and in one bed are so abundant, flattened and ellipsoidal, like linseeds, that the name 'Linseed Bed' has been applied to it (S. S. Buckman 1889, p. 444; Richardson 1910, p. 107).

The Cephalopod Bed varies considerably in thickness (Figure 9). It is thick in the region of the Cotswold scarp as far north as Stinchcombe, very thin in the areas east of this belt, but showing signs of becoming thicker again in the northeast. Ammonites are common and diagnostic of several subzones. Exposures of the complete thickness are not numerous and many of these have been recorded by previous geologists, such as S. S. Buckman (1889), Woodward (1893) and Richardson (1910) who provided more complete descriptions than can be made today.

The lowest beds of the Inferior Oolite are represented by a hard arenaceous limestone, sometimes conglomeratic at the base and containing yellow limonitic pellets or ooliths. This rock is of similar appearance to the Cephalopod Bed and is always present except where the Cephalopod Bed is extremely thin.

At Little Sodbury, Richardson (1910, p. 94) following Buckman (1889, p. 446), recorded the Cephalopod Bed as 10 ft (3 m) thick, belonging to the dispansum to aalensis subzones inclusive. Northwards the bed has been traced by surface fragments and small exposures to Hawkesbury where 9 ft 2 in (2.8 m) of beds were recorded by Richardson (1910, p. 99). At Wotton-under-Edge he recorded (pp. 105–6) 13 ft 6 in (4.1 m) of beds, spanning the striatulum to aalensis subzones inclusive. At Nibley Knoll 13 ft 6 in (4.1 m) of strata belonging to the dispansum to moorei subzones inclusive were recorded by Buckman (1889, p. 445), but further north near Dursley the Cephalopod Bed is only 4 ft 3 in (1.3 m) thick. North-eastwards the thickness diminishes further until at Coaley Wood [ST 786 997] up to 3 ft 1 in (0.9 m) of beds represent the striatulum to moorei subzones inclusive. Opinions on the thickness here have varied. Buckman (1889, p. 444) regarded it as 2 ft 3 in (0.7 m) while Richardson (1910, p. 115–6) quoted 2 ft 6 in (0.8 m). The Geological Survey borehole at Sherston proved 6 in (15 cm) of Cephalopod Bed referable to the moorei Subzone. No Cephalopod Bed was recorded by Richardson (1919) in the Shipton Moyne No. 3 Borehole, though examination of fragments from the No. 4 Borehole indicate that 10 in (25 cm) of strata were referable to the striatulum Subzone. Some 1 ft 6 in (0.5 m) belonging to the same subzone are recorded in the Tetbury No. 4 Borehole (Appendix 1).

Details

Lower Lias

White and Blue Lias

Chipping Sodbury to Charfield

The following section was observed in the railway cutting between 400 yd (366 m) W and about 500 yd (457 m) E of Chipping Sodbury Station (Sheet 265) [ST 7367 7815]: Nodular limestone and marl, seen to 17 ft (5.2 m) on shales with Plagiostoma gigantea, 36 to 40 ft (11–12.2 m), Blue Lias limestones 5 ft (1.5 m) on White Lias 2 ft (0.6 m) over Cotham Beds. This description corresponds to that given in greater detail by Reynolds and Vaughan (1902, pp. 719–22).

Dr M. L. K. Curtis has supplied details and fossils from a trench between places [ST 7228 8724] 1300 yd (1189 m) W66°N from Birdbush Farm, Wickwar and [ST 7636 8602] 300 yd (274 m) W40°N from Upper Chalkley Farm, near Hawkesbury. Here White Lias including some pale grey calcilutite, deeply bored, with ferruginous infillings, and with a rich bivalve fauna was encountered and the basal beds of the Blue Lias including rough gritty limestones with fine shell fragments and coarse shelly limestones occurred at places between [ST 7301 8646] and [ST 7313 8635]; [ST 7320 8628] and [ST 7334 8616]; also possibly between [ST 7336 8614] and [ST 7350 8608]. The fauna of the White Lias included an indeterminate simple coral, Diademopsis spines, Astarte sp., Cardinia cf. hennocquii, C. sp., Dimyopsis sp., "Gervillia" sp.,Liostrea sp., Meleagrinella decussata, Modiolus hillanoides, M. laevis, Pholadomya?, Pteromya aff. crowcombeia, P. tatei and ostracods. The Blue Lias fauna was also dominated by bivalves including Camptonectes?, Cardinia sp., Gervillia cf. exilis, Liostrea hisingeri, Modiolus laevis, M. cf. minimus, Parallelodon hettangiensis, Plagiostoma sp.,Protocardia phillipiana, Pteromya tatei and fish remains.

Other beds of the Blue Lias were seen between [ST 7351 8608] and [ST 7352 8607]. They consisted of grey clay with layers of grey limestone up to 3 in (8 cm) thick containing abundant oysters. Fossils include: crinoid ossicles, Gryphaea sp.,Liostrea hisingeri, Plagiostoma gigantea and Pseudolimea sp.Donovan (1947, p. 183) described a section in about 3 ft (0.9 m) of the basal part of the Blue Lias in Saltmoors Ditch [ST 7418 8881], 850 yd (777 m) W of Inglestone Farm. Thin layers of bluish grey, light grey and dark grey limestone were separated by shale and contained Pteromya tatei. The White Lias beneath is of irregular thickness and absent in places, the Blue Lias then resting on Cotham Marble. R.C.

White Lias is known near Standle Farm where a stream section [ST 7218 9863] exposes 2 to 3 ft (0.6–0.9 m) of white marl with thin nodular bands of pale fine-grained limestone.

Pale clay containing small calcareous lumps (race) was augered on the outlying Lias outcrop in Whitcliff Park south of Berkeley. A similar outlier of Lias 1 mile (1.6 km) W of Falfield shows no evidence of White Lias.

Charfield to Breadstone

Between Hunting Brake [ST 725 935] and Green Farm [SO 714 009] Blue Lias supports a well-marked dipslope with abundant fragments of bluish grey fine-grained often shelly limestone. North of Standle Farm [ST 722 986] the outcrop varies in width between 100 and 200 yd (91–183 m), but to the south may in places exceed 500 yd (457 m). Shallow workings west of Bushstreet Farm [ST 7192 9560], at Lorridge Brake [ST 7195 9948], and [SO 7135 0095] north-east of Green Farm, although now overgrown, suggest that at least 5 ft (1.5 m) of limestone was available. In a stream section [ST 7227 9728] 200 yd (183 m) SE of Holt's Farm the following strata were exposed: thin blue grey limestone beds with shale partings, 2 ft 6 in (0.8 m) on bluish grey clay, 1 ft (0.3 m), blue fine-grained limestone with thin shale partings, 2 ft 6 in (0.8 m), blue shale, 4 ft (1.2 m), over thin limestone ribs with shale partings, 3 ft (0.9 m).

Whitcliff Park

A small outlier of White and Blue Lias is preserved on the plateau in Whitcliff Park, south of Berkeley. In a shallow pond 70 yd (64 m) S of Park House 2 ft (0.6 m) of bluish grey clay with a 4-in (10-cm) bed of fine-grained shelly limestone was exposed.

Falfield Outlier

A shallow trench [ST 6618 9222] near Pound House showed pale blue clay with nodular shelly limestone. Elsewhere the Blue Lias is not seen but is represented by an abundant field brash of slabby shelly limestone. I.B.P.

Lower Lias Clay

Chipping Sodbury to Wotton-under-Edge

Exposures of the lower part of the Lower Lias in this tract are few. The trench mentioned previously (p. 84), which crossed the entire outcrop of the Lower Lias Clay yielded ammonites, as well as numerous bivalves. In the western part of the trench, from west to east, strata of the liasicus and angulata zones, consisting of interbedded clays and limestones, the latter slabby, nodular, generally about 6 in (15 cm) thick, but with some layers up to 1 ft (0.3 m) yielded at [ST 7385 8601] Schlotheimia (Waehneroceras) sp. and at [ST 7394 8601], [ST 7396 8601] and [ST 7403 8601], Schlotheimia sp. In the middle part of the trench, strata of the angulata Zone consisting of alternations of grey clay and layers of slabby and nodular grey limestone up to 6 in (15 cm) thick yielded at [ST 7470 8601] and [ST 7478 8601], Schlotheimia sp.; and at [ST 7485 8601], Schlotheimia (s.s.) sp. Similar lithologies occur in the bucklandi Zone and yielded Coroniceras aff. rotiforme at [ST 7490 8601], indicating that subzone, Coroniceras sp. [juv.] at [ST 7494 8600], and Vermiceras scylla at [ST 7503 8601], [ST 7520 8601] and [ST 7522 8601]. In the eastern part of the trench strata of the semicostatum Zone consisted of pale grey clay with very occasional layers of nodular limestone. At [ST 7578 8601] these yielded Arnioceras bodleyi and Euagassiceras sp. at [ST 7611 8602]. Further east similar lithologies but of the raricostatum-ibex zones yielded Epideroceras? at [ST 7630 8602], Platypleuroceras or Acanthopleuroceras sp.,at [ST 7631 8602], Lytoceras fimbriatum at [ST 7635 8602], Liparoceras cheltiense at [ST 7635 8602]. The proximity of the semicostatum Zone [ST 7571 8601] and the raricostatum Zone [ST 7611 8602] suggests a fault between them.

Clay with layers 2 to 6 in (5–15 cm) thick of argillaceous limestone were reported at Tileysgreen Farm, Horton, and in the stream [ST 7551 8621] 380 yd (347 m) SW of Cat House, Hawkesbury, strata exposed were: loam and silt, 2 to 3 ft (0.6–0.9 m) on grey clay, 2 to 3 ft (0.6–0.9 m), grey argillaceous limestone layer, 6 to 9 in (15–23 cm) over more clay with layers of limestone below water.

A collection of fossils obtained from trenches dug across Inglestone Common, near Hawkesbury was recorded by Donovan (1947, pp. 183–5). From these the presence of the planorbis, bucklandi, semicostatum, turneri, obtusum, oxynotum, raricostatum and ibex zones was inferred, but the juxtaposition of the semicostatum and ibex zones is anomalous and was explained by hitherto unsuspected faulting in the Lower Lias Clay (op. cit., p. 186). The apparent presence of the obtusum Zone at the base of the Dyrham Silts, near Coldchange Cottage (op. cit., p. 185) may require a similar explanation.

In the stream [ST 7558 9011], 300 yd (274 m) NE of Lower Withymoor the section is: band of bluish grey argillaceous limestone, 9 in (23 cm), on dark grey clay, about 10 in (25 cm), grey argillaceous limestone, 6 in (15 cm), dark grey clay, 4 in (10 cm), thin limestone layer, dark grey clay, 2 ft 0 in (0.6 m), grey argillaceous limestone, 5 to 7 in (13–18 cm), dark grey shaly clay, 1 ft 2 in (0.4 m), impersistent argillaceous limestone, up to 4 in (10 cm) over dark grey clay, 1 ft 6 in (0.5 m).

The upper, purely clay, portion of the Lower Lias Clay has even fewer exposures and has been traced by means of ditch debris and auger.

North of Wotton-under-Edge

Natural exposure of the Lower Lias Clay is rare but the clay was proved in augering. Thin ribs of limestone are common in the lower part and support persistent features, though limestone could not be detected by use of the auger to a depth of 4 ft (1.2 m) at least.

A trench between [ST 7190 9912] 1100 yd (1006 m) E30°S from Lorridge Farm and the northern edge of the district [SO 7335 0095] 850 yd (777 m) N75°E from the Leathern Bottle exposed Lower Lias clays and limestones and was examined by M. L. K. Curtis. In the south-western part of the trench interbedded grey clay and limestone (Blue Lias) overlying grey to white clay [ST 7197 9920] 1300 yd (1189 m) S38°W of Clingre House yielded Psiloceras planorbis. Further north-east, grey clay with limestone nodules (Lower Lias Clay) at [SO 7274 0036] 240 yd (219 m) due N of Clingre House yielded Acanthopleuroceras sp. and Passaloteuthis? indicating the valdani Subzone. Grey clay with limestone nodules (Lower Lias Clay) at [SO 7282 0044] 330 yd (302 m) N17°E of Clingre House yielded Pleuromya costata and Tragophylloceras loscombi, and grey clay with limestone nodules (Lower Lias Clay) [SO 7285 0046] N20°E of Clingre House yielded P. costata and Androgynoceras sp. indicating the lower part of the davoei Zone.

There is a significant lack of evidence of beds intermediate between the planorbis and ibex zones over the short distance between the first and second localities above and this would accord with the presence of the fault as mapped in that area downthrowing to the north.

A temporary exposure in Lower Lias Clay at the reservoir [SO 7285 0065] at Leathern Bottle, 580 yd (530 m) N10°E from Clingre House was also examined by M.L.K. Curtis and the fauna included: Isocrinus basaltiformis, Balanocrinus subteroides, Astarte sp., Parainoceramus ventricosus, Pseudopecten sp.,Ptychomphalus expansa, Androgynoceras heterogenes Spath (1938, pl. 13, figs, 6a, b, non Young & Bird), Androgynoceras maculatum, Regoceras (Beaniceras) luridum, Liparoceras naptonense, Tragophylloceras carinatum, Tragophylloceras loscombi, Tragophylloceras sp., Hostiles cf. stonebarroensis, Homaloteuthis raphael, Homaloteuthis?, Passaloteuthis apicicurvata, Passaloteuthis sp. cf. auricipitis, Pseudohastites sp. cf. longissimus auct. non Miller.

Horizon: Tragophylloceras ibex Zone, luridum Subzone, also basal part of the maculatum Subzone of the davoei Zone.

The presence of the davoei Zone still some distance from the base of the Dyrham Silts appears to contrast with the situation further south near Hawkesbury (p. 88) and further east near Cam (p. 89), where the zone occurs in the bottom half of the Dyrham Silts. Either the davoei Zone is very thick, spanning the top of the Lower Lias Clay and the bottom of the Dyrham Silts, or, near Stinchcombe, the base of the Dyrham Silts occurs higher in the sequence.

Another temporary exposure examined by M. L. K. Curtis situated [SO 7200 0011] 120 yd (110 m) W of the hotel, Berkeley Road, yielded Arnioceras semicostatum, probably indicative of that zone. Its proximity to the outcrop of the White and Blue Lias leaves little room for beds belonging to the bucklandi, angulata and liasicus zones and suggests that these beds might be thin at that place. This could explain the lesser prominence of limestones in the Lower Lias Clay of the north of the district compared with further south. R.C.

Middle Lias

Dyrham Silts

Old Sodbury to North Nibley

Some 270 yd (247 m) NW of Hawkesbury Church an exposure [ST 7662 8709] revealed: brown, shaly mudstone, about 2 ft (0.6 m) on ferruginous rottenstone, 1 to 2 ft (0.3–0.6 m), hard, sandy silt, 2 to 3 ft (0.6–0.9 m) over argillaceous limestone, seen. A small excavation [ST 7651 8872] 150 yd (137 m) N of Lovattswood Farm showed the upper of the two prominent silty limestone bands noticed in this area. Here a few inches of brown calcareous mudstone rest on 2 ft (0.6 m) of bluish brown silty limestone in beds up to 1 ft (0.3 m) thick, over hard greyish brown mottled silt. These beds belong to the capricornus Subzone and yield: Tetrarhynchia cf. tetrahedra, Ceratomya petricosa, Modiolus scalprum, Pleuromya costata, Protocardia truncata, Unicardium cardioides, Androgynoceras capricornus and A. lataecosta.

This siltstone could well be the same as the similar bed, 10 ft (3 m) thick, seen in Tuffley's Quarry [SO 8360 4490], Robinswood Hill where it occupies a similar position in the Dyrham Silts (Ackermann MS, IGS File; Howarth 1958, p. x). It could also correlate with the 5 ft (1.5 m) siltstone seen in Stonehouse Brick Pit [SO 8097 0536] (Ackermann MS, IGS File; Howarth 1958, p. x) which falls there within the top of the Lower Lias Clay (see also Palmer 1971). Some 700 yd (640 m) N of this exposure a bomb crater [ST 7642 8935] is surrounded by very large lumps of grey silty limestone containing rhynchonelloids and pectenids. The limestone weathers brown and adhering to the lumps is brown micaceous mudstone. This debris is of the same appearance as the limestone in the previous exposure and also that at the base of the next exposure [ST 7665 8982], which occurs in the laneside 320 yd (293 m) NW of Hillsley Church, showing: greyish brown micaceous silt with ochreous pellets, seen 11 ft (3.4 m) on brown micaceous and ferruginous mudstone which becomes grey, hard, silty limestone at the base, 3 ft (0.9 m).

The laneside bank [ST 7671 9112] 380 yd (347 m) NNW of Alderley Church exposes 12 ft (3.7 m) of micaceous argillaceous silt beneath the Marlstone Rock Bed and in another laneside exposure [ST 7604 9214] 6 ft (1.8 m) of similar silt in a similar position beneath the Marlstone Rock Bed occur 400 yd (366 m) E of Leys Farm. M. L. K. Curtis collected fossils from ferruginous silt in a water pipe trench [ST 7669 9128] 100 yd (91 m) ENE of Broad Bridge, Alderley, constructed in 1952. These include: Ceratomya petricosa, Grammatodon insons, Gryphaea gigantea, Mactromya arenacea, Modiolus scalprum, Oxytoma inequivalve, Pleuromya costata, Pronoella intermedia, Protocardia truncata, Androgynoceras sp. (group of A. capricornus), A. (Oistoceras) sp. These beds lie in the davoei Zone at the very top of the Lower Lias.

North Nibley to Dursley

Owing to the gradational nature of the change in lithology from Lower Lias Clay, the base of the Dyrham Silts is imprecise, so that estimates of thickness of the formation lack consistency. It seems however that at Nibley Green the thickness is about 110 ft (33.5 m) increasing northwards to about 130 ft (39.6 m) at Taits Hill. Clean sections in the silt are rare owing to their unconsolidated nature at surface, but 2 to 3 ft (0.6–0.9 m) of pale grey, soft silt were exposed 400 yd (366 m) 287° from St Martins Church, North Nibley. Debris from a shallow trench [ST 7416 9664] 400 yd (366 m) due west of Nibley Mill included ferruginous sandstone containing poorly preserved bivalves including Grammatodon sp., Parallelodon buckmani and Pseudopecten sp. R.C.

West of Stinchcombe Hill the Dyrham Silts consist only of soft micaceous silt, pale grey when fresh, but weathering rapidly to yellow or brown.

Dursley–Coaley Peak–Uley

In this area, the Dyrham Silts have been subdivided into a sequence of alternating beds of silts and silty clays (Table 2) and considerable lateral variation is displayed.

West of Fieldlane Farm a lens of blue clay 10 ft (3 m) thick is present 30 ft (9.1 m) above the base of the Dyrham Silts. The clay is marked by a feature and is easily recognised with the auger. Two hundred yards (183 m) W of Cam Mills blue clay was augered in a comparable stratigraphical position and traced half a mile (0.8 km) to the south-south-east until it passed beneath the recent alluvium of the River Cam. The clay of Dulkin Brook is underlain by silt, also moderately well exposed in Dulkin Brook to a thickness of approximately 45 ft (13.7 m) and comprising three minor clay-silt alternations, each culminating in a silty fossiliferous limestone. The section is:

Of the overlying clay of Dulkin Brook, only the lowest 4 ft (1.2 m) are exposed, consisting mainly of bluish grey silty clay with a few thin, fossiliferous silty beds. Androgynoceras capricornus is diagnostic of the middle subzone of the davoei Zone, thus placing this silt [ST 7645 9985] 900 yd (823 m) N57°E from St Georges Church, Cam, near the top of the Lower Lias. No zonally significant fossils were found in the clay. Other fossils found in the silts between [ST 7619 9978] and [ST 7639 9977] include: Camptonectes mundus, Cucullaea sp., Hippopodium ponderosum, Modiolas scalprum, M. subcancellatus, Oxytoma inequivalve, Parallelodon buckmani, Protocardia truncata, Pseudopecten cf. acuticostata and P. aequivalvis?.

The silt and clay of Dulkin Brook can be traced south through Cam, until near Dursley they pass beneath the alluvium of the River Cam. Evidence from augering is in places supplemented by features.

East of Dursley, in the Uley valley, the lower portion of the Dyrham Silts is exposed only in a faulted block south of Rockstones [ST 780 978]. A bed of yellow-weathering silt, approximately 40 ft (122 m) thick, is correlated with the silt of Dulkin Brook and is overlain by 20 ft (6.1 m) of soft blue clay. From Dulkin Brook through Upthorpe to Greenstreet, the silt maintains a thickness of 45 ft (13.7 m). At Upthorpe the top of the silt is composed of soft, fine-grained, ferruginous sandstone which supports a platform more than 100 yd (91 m) wide. At Greenstreet an impersistent lens of bluish grey clay probably represents a local exaggeration of the clay portion of a minor sedimentary cycle (see p. 80). East of Greenstreet, the silt corresponding with that in Dulkin Brook thins rapidly and cannot be distinguished east of Ashmead House [SO 7745 0023].

From Dulkin Brook to Ashmead House, the clay of Dulkin Brook maintains an almost uniform thickness of 25 to 30 ft (76–9.1 m). South-east of Greenstreet a thin lens of silt is developed within it and raises a faint feature. East of Ashmead House, with the failure of the silt of Dulkin Brook the clay cannot be distinguished from the Lower Lias Clay.

Near Ashmead House the clay of Dulkin Brook is overlain by silt followed by clay, then an upper silt and an upper clay. West of Cam the upper silt can no longer be recognised, while near Coaley Peak it is the lower clay that fails (Table 2).

At Cam, overlying silt and the clay are both 40 ft (12.2 m) thick. West of the River Cam at Everlands the silt is 65 ft (19.8 m) thick while the clay above is reduced to 25 ft (7.6 m). The clay is further reduced in thickness northwards, and, west of Cam Mills, it can no longer be recognised. South of Cam, the silt remains thick and is divided by an impersistent bed of clay. In the Uley Valley, the upper part of the silt is well exposed in a stream section [ST 7697 9866] 150 yd (137 m) NW of Coldharbour Farm: bluish grey, faintly micaceous, flaggy siltstone, 1 ft (0.3 m), on soft, buff-weathered, micaceous silt, 4 ft (1.2 m), richly shelly, bluish grey, silty limestone with irregular base and top, 8 to 15 in (20–38 cm), over dark blue, slightly micaceous, shaly silt, seen 10 ft (3 m). The lower silt of Ashmead is approximately 40 ft (12.2 m) thick at Upthorne, but thins gradually eastwards to Tickshill where a small stream [SO 7793 0004] showed : hard, lenticular bed of dark blue, micaceous, calcareous siltstone, 1 to 2 ft (0.3–0.6 m), on soft micaceous siltstone with occasional lamellibranchs, 3 ft (0.9 m), lenticular bed of hard blue calcareous siltstone, 6 to 12 in (15–30 cm), over blue micaceous silt, 1 ft 6 in (0.5 m). The clay of Ashmead is first recognised west of Cam Mills. Eastwards, near Ashmead Green is 45 ft (13.7 m) thick and is divided into upper and lower divisions by a lens of silt, the upper silt, which persists almost to Coaley Peak where it unites with the lower silt. The lower clay of Ashmead thins eastward and finally disappears while the upper clay persists with a thickness of approximately 30 ft (9.1 m). No exposures of the clays and silts of Ashmead were available in this ground.

Southward from Cam the combined upper and lower clay of Ashmead is readily traced on both sides of the Cam Valley, through Dursley, then eastwards as far as Uley. Throughout this area it has a thickness of 30 to 35 ft (9.1–10.7 m). Two sections are available in a small stream near Coldharbour Farm.

The first [ST 7693 9880] 150 yd (137 m) NNW of the farm shows: soft blue shale, 2 ft (0.6 m), on rusty decalcified sparsely shelly siltstone, 4 in (10 cm), brown slightly micaceous siltstone, 3 ft 6 in (1.1 m), brown fine-grained calcareous siltstone, 4 in (10 cm). The second section [ST 7689 9877] is a few feet lower in the succession and shows: rusty, micaceous siltstone with belemnites, pectenids, etc., 4 in (10 cm), on buff weathered soft micaceous siltstone, 1 ft (30 cm), bluish grey siltstone closely laminated with clay, 9 in (23 cm), mainly blue clay with wisps of silt, 6 in (15 cm), grey limestone nodules, 4 in (10 cm), dark grey micaceous silt passing down into clay, 4 ft (1.2 m). The silt of Highfields, the highest division of the Dyrham Silts is thin but the most widespread. It underlies the Marlstone Rock Bed at Cam and Dursley and can be traced, as a continuous bed 10 to 15 ft (3–4.6 m) thick to Uley and Coaley Peak. This silt was, however, exposed only in a temporary excavation [ST 7336 9827] 220 yd (201 m) NNE of St Marks Church, Dursley, and consisted of 18 ft (5.5 m) of greyish green, buff-weathering ferruginous black micaceous silt with a sparse bivalve fauna. I.B.P.

Marlstone Rock Red (and Junction Bed)

Horton to Wotton-under-Edge

The most southerly outcrop of the Marlstone Rock Bed was mapped across Horton Hall, from a location 250 yd (229 m) S of Upper Widdenhall Farm, Horton, to about 200 yd (183 m) NNW of the farm. It forms a narrow plateau on which fragments of brown limestone with ferruginous ooliths are common and grey, hard, argillaceous limestone fragments, possibly of Upper Lias age, also occur. Another, though broader, plateau occurs for some 400 yd (366 m) SW of Hawkesbury Church and this yielded similar fragments.

A long outcrop, broadening to form small plateaux in places, has been traced north from Lovattswood Farm almost to Hillsley Mill [ST 7703 9051] and it is likely that the Marlstone Rock Bed is more than 5 ft (1.5 m) thick in places. Prof. D. T. Donovan observed 8 ft (2.4 m) of Marlstone Rock Bed in an excavation [ST 7728 9146] 860 yd (786 m) NNE of St Kenhelms Church, Alderley. Some 3 ft (0.9 m) of nobbly dark greyish brown limestone with pectenids were visible 610 yd (558 m) SE of Hillsley Mill.

The grey argillaceous limestone fragments encountered on the dip slopes of some of the Marlstone Rock Bed features have not been recorded north of Alderley, though between Alderley and Wotton-under-Edge the outcrop is very narrow.

Three feet (0.9 m) of brown limestones were seen in the laneside 400 yd (366 m) E of Leys Farm, near Wotton. In the Potter's Pound district of Wotton-under-Edge a very disturbed section of Marlstone Rock Bed was revealed in a road cutting [ST 762 933] near the Full Moon Inn and 330 yd (302 m) SE of St Mary's Church. The structure is not fully understood, but is thought likely to have resulted from a late or post-Pleistocene valley-bottom disturbance. The beds dip ESE up to 60° (inverted) in one place. Some 16 ft 6 in (5 m) of ferruginous sandy limestones are exposed with Upper Lias Clay, all inverted, as follows:

Most of Wotton-under-Edge is sited on a quite extensive gently eastward to northeastward dipping limestone plateau formed by the Marlstone Rock Bed (Plate 6).

Shallow trenches in this area have revealed brown rubbly limestone. R.C.

Wotton-under-Edge to Stinchcombe

Near Southend, old quarries [ST 743 953] indicate that the Marlstone Rock Bed is at least 10 ft (3 m) thick. At North Nibley its outcrop is broad and strongly cambered. Quarries at Hunt's Court Farm [ST 7415 9624] and near Northfield House [ST 7392 9617] show up to 7 ft (2.1 m) of ferruginous fossiliferous limestone, often rubbly and broken.

At Millend, near the Corn Mill, a temporary section [ST 7519 9642] revealed beneath Upper Lias Clay: blue-hearted, sandy, oolitic limestone with ammonites, 4 to 10 in (10–25 cm) on brown, ferruginous, sandy, shelly limestone, 14 ft (4.3 m). It is considered that the full thickness of the Marlstone Rock Bed was seen.

Between North Nibley and Stinchcombe the Marlstone Rock Bed supports a shelflike feature 100 to 200 yd (91–183 m) wide near the foot of the Cotswold escarpment and is exposed in a number of quarries. Some 10 ft (3 m) are visible in Stancombe Park [ST 7387 9752] and a similar thickness of ferruginous sandy limestone with abundant fossils was exposed in an old quarry [ST 7317 9900], 400 yd (366 m) W of Street Farm.

Stinchcombe to Cooley Peak

A series of quarries near The Quarry shows the Marlstone Rock Bed in its most typical development. Up to 12 ft (3.7 m) are exposed of rubbly or thickly bedded sandy ferruginous limestone, brown weathered and generally much decalcified. Fauna is abundant, crinoid ossicles, belemnites and pectinids being most characteristic. Terebratulids are also abundant, while Tetrarhynchia tetrahedra are commonly concentrated in lenses. The old quarry [ST 735 995] about 600 yd (549 m) E7°S of Clingre Farm yielded the following fossils from the spinatum Zone: Isocrinus?,Lobothyris punctata, Gibbirhynchia cf. gibbosa, G. micra, Homoeorhynchia acuta, Quadratirhynchia sp.,Tetrarhynchia tetrahedra, Camptonectes sp., Entolium liasianum, Gryphaea sp., Lopha sp., Plagiostoma sp., Pseudopecten cf. aequivalvis, Pleuroceras sp. and Acrocoelites sp.

The Marlstone is estimated to be slightly in excess of 20 ft (6.1 m) here — the greatest thickness recorded within the district. North and east of The Quarry, the Marlstone Rock Bed supports an extensive platform, terminated to the north by an east-west fault and severely cambered on its eastern side. Broken and rubbly Marlstone Rock Bed was exposed by excavations [ST 745 999] 500 yd (457 m) N of Tilsdown House. On the northern, downthrown, side of the fault a small block of much cambered Marlstone Rock Bed is preserved south-east of Fieldlane Farm [SO 7370 0075].

A temporary section [ST 7536 9827] in Dursley, 440 yd (402 m) E15°S of the railway station in Dursley revealed 11 ft 10 in (3.6 m) of brown ferruginous sandstone belonging to the spinatum Zone and containing Tetrarhynchia tetrahedra and Pleuroceras spinatum.

Between Dursley and Uley the Marlstone Rock Bed becomes thinner, is largely decalcified and has been little worked. Up to 6 ft (1.8 m) were exposed in an old quarry [ST 7702 9844] 260 yd (238 m) E72°S of Coldharbour Farm and 8 ft (2.4 m) were exposed in a road cutting [ST 7850 9793] at Marsh Farm near Uley.

At Downhouse Farm 7 ft (2.1 tn) of ferruginous, fossiliferous, sandy limestone are exposed in a small quarry [ST 7639 9916]. Eastwards to Coaley Peak the Marlstone Rock Bed thins and is generally rubbly and decalcified. Exposure is limited, but the outcrop is readily traceable by the features produced and the field brash it supports. I.B.P.

Upper Lias

Upper Lias Clay

Old Sodbury to Wotton-under Edge

Although the Upper Lias Clay is not thick enough to map along the Cotswold edge south of Wotton-under-Edge, it has been detected as far south as Alderley. Prof. Donovan (personal communication 1962) observed clay with Harpoceras and Hildoceras in excavations some 850 yd (777 m) NNE of Alderley Church.

From the Fountain, beneath Tor Hill, clay has been augered above the Marlstone Rock Bed outcrop northwards and westwards around Wotton-under-Edge.

The Zone and Subzone of H. falciferum were proved in the roadside section [ST 7619 9325] near the Full Moon Inn (p. 90). Some 10 ft (3 m) of clay were proved in an excavation for a petrol station in Bear Street, while higher, about 100 yd (91 m) to the north of Gloucester Street an old brickyard [ST 7553 9352] exposed some 25 ft (7.6 m) of silty clay. In this area the lower part of the Upper Lias is becoming very argillaceous and can be mapped separately from the Cotteswold Sands. R.C.

Wotton-under-Edge to Dursley

Excavations 100 yd (91 m) SE of Millend [ST 7520 9642] exposed the full thickness of the Upper Lias Clay: very hard, blue-hearted, brown-weathering, crinoidal limestone, richly fossiliferous with dactylioceratid ammonites filled with fine-grained argillaceous limestone and Pseudogibbirhynchia jurensis, 8 to 9 in (20–23 cm), on silty clay, bluish grey, buff-weathering, with 10 beds 1 to 3 in (3–8 cm) thick, of bluish grey, fine-grained, argillaceous limestone, richly fossiliferous yielding: Dactylioceras sp., Harpoceras falciferum and Hildaites murleyi, 7 ft (2.1 m), blue clay, sparsely fossiliferous, 3 ft (0.9 m), over blue sandy oolitic limestone with irregular top and base, welded to underlying Marlstone Rock Bed - a few ammonites, 4 to 10 in (10–25 cm). The thin crinoidal limestone at the top of the section (specimen PN 42) is equivalent to a similar, but thinner limestone near Dursley. The section of the Upper Lias Clay illustrated by Woodward (1893, p. 264) is probably that in an overgrown quarry 200 yd (183 m) N35°E of Stancombe Park [ST 7390 9261].

The Upper Lias Clay crops out on a gentle slope at the base of a steep scarp of Cotteswold Sands, and, in the northern part of Sheet 251, is readily mapped by features and an abundant field brash composed of fragments of fossiliferous argillaceous limestone. Natural exposure is rare, but excavation 30 yd (27 m) E of the Rectory, 400 yd (366 m) WNW of Dursley Church, gave the following section [ST 7536 9831]:

The following fossils from the Upper Lias Clay indicate horizons in or above the falciferum Zone: Dactylioceras sp., H. falciferum, Hildaites subserpentinus. A specimen (PN 255) of the ammonitiferous marl bed is comparable with that found at the temporary section at Millend (p. 96). A similar series of alternating clay beds and thin, fossiliferous bluish grey, argillaceous limestones was temporarily exposed to a thickness of 5 ft (1.5 m), 130 yd (119 m) N40°E of St Marks Church, Dursley [ST 7617 9788]. I.B.P.

Wotton-under-Edge to Nailsworth

The incoming of argillaceous silty material noted northwards from Wotton-under-Edge also takes place eastwards from the Cotswold scarp there, and micaceous argillaceous silt has been augered along Ozleworth Bottom to where the beds dip beneath the valley floor at the northward bend below Ozleworth. In this part of the valley the argillaceous silt throws out copious springs from the base of the Cotteswold Sands. Similar grey micaceous silt has been traced eastward along Tiley Bottom to nearly a mile (1.6 km) E of Coombe. Again springs occur at the base of the Cotteswold Sands.

The silt has been mapped along the bottom of the Newmarket valley eastwards from High Wood, and 18 ft (5.5 m) of dark grey silt and sandy silt were seen in the stream bank [ST 8308 9965] 0.5 mile (0.8 km) W of Newmarket.

Micaceous, mottled silt occurs in the lower part of the valley slopes through Nails-worth, northwards to Inchbrook and eastward for a distance of about half a mile (0.8 km). There are few exposures and, though some are afforded by the Midland Railway cuttings, even these are largely covered by grass and waste ballast. About 50 ft (15.2 m), perhaps more, of this micaceous mottled silt outcrop in the valley sides and it is estimated that some 40 ft (12.2 m) of micaceous very fine silt occur in the bank [ST 8533 9947] below the playing fields in east Nailsworth.

A temporary section at the Post Office, Nailsworth, which lies on an outcrop low in these argillaceous silts yielded Hildoceras semipolitum, indicating the bifrons Zone. Also from Nailsworth came the following fossils in the Lycett Collection: Catacoeloceras dumortieri, C. confectum, C. crassum, Collins mucronata, Haugia compressa, H. grandis ("Longfords nr. Nailsworth"), H. ogerieni, Pseudolioceras dumortieri, P. bicarinatum, Lytoceras cornucopia. These fossils indicate the variabilis Zone and possibly included specimens from the top of the bifrons Zone; whilst most of the material could have come from the argillaceous silts low in the Upper Lias, a specimen from "Longfords" (?Longford's Mills) would be from the Cotteswold Sands. R.C.

Cotteswold Sands and Cephalopod Bed

Old Sodbury to Wotton-under Edge

Sand appears commonly along the outcrop in rabbit burrows and lane banks, and most extensively from just beneath the Inferior Oolite, where the slope is steepest. The outcrop forms warm, well-drained ground which, apart from the gradient, is easily cultivated. To overcome the gradient disadvantage the slopes have been terraced in places and traces of these remain to the present, though the chief use to which the ground is now put is for grazing cattle.

Cart tracks which ascend the scarp slope or valley slopes are usually incised into the Cotteswold Sands, and at the top, the Cephalopod Bed. The best sections are found in these tracks.

The old roadway up the Cotswold scarp 530 yd (485 m) SSE of Little Sodbury Church reveals in its banks the following section [ST 7597 8381] in Cotteswold Sands of the striatulum Subzone: earthy silt 3 in (8 cm) on fawn-grey sandy limestone with rusty flecks marking weathered shell fragments, 8.25 in (21 cm), brown, sandy silt with belemnites and Pseudotrapezium?, 5.75 in (15 cm), grey (with patches weathered fawn), fine silty or argillaceous limestone, with swellings of veined limestone at base, 3 in (8 cm), brown, argillaceous and sandy silt with Cucullaea sp.,and Grammoceras cf. striatulum, 11.5 in (29 cm), brown, silty limestone with G. thouarsense, 3 in (8 cm), parting of brown silt with belemnites and ammonites, G. thouarsense from the base, 0.5 in (1 cm), massive, hard bed of brown silty limestone with Esericeras fascigera, G. thouarsense, from top 6 in and G. audax and Grammoceras sp. from the base, 14.25 in (36 cm), greyish brown, silty sand with shell and remains of Grammoceras sp.,7.75 in (20 cm), pale grey, fine sandy limestone with G. audax from the middle and G. sp. from the base, 4.25 in (10 cm), brown, hard sand, almost calcareous sandstone in places, e.g. 10 in (25 cm) from top, 15 in (38 cm) seen. Dip-and-fault structure affects the scarp in this area.

Another exposure in the road cutting [ST 7626 8326] 570 yd (521 m) NNE of the former shows 6 ft (1.8 m) of silty sand and marly clay with thin layers of argillaceous silty limestone.

Some 600 yd (549 m) NNE of Horton Rectory a small exposure [ST 7675 8478] about 18 ft (5.5 m) below brown rather hard and fractured Inferior Oolite limestone shows: rather fissile limestone with coarse limonitic pellets, 1 ft (30 cm), hard, fawn limestone, containing nests of limonitic pellets which are otherwise sparsely scattered, 8 in (20 cm), over soft rubbly limestone with limonitic pellets seen for 4 in (10 cm). There is then a gap in the section of about 2 ft (0.6 m), below which lies the Cephalopod Bed consisting of greyish brown, soft limestone with linseed-like limonite pellets seen for 6 in (15 cm). Some 1020 yd (933 m) N by W of Starveall the basal parts of the Cephalopod Bed are visible [ST 7873 8800] as follows: greyish brown, ochreous and rubbly, ironshot limestone, ?bored, containing gryphaeids, a large nautiloid and a trigoniid, 1 ft (30 cm), on brown sandy silt.

A rather poor exposure of the Cephalopod Bed occurs in a small waterfall and gorge [ST 7964 8936] 1060 yd (969 m) ENE of Lower Kilcott and shows below the Inferior Oolite: brown limestone with ferruginous pellets and manly layers. The limonitic pellets are concentrated into nests along some layers, visible about 2 ft (0.6 m) on soft marl 6 in (15 cm), over brown ferruginous shelly limestone with ferruginous pellets, seen for 6 in (15 cm).

A small outcrop of the Cephalopod Bed occurs in the lane [ST 7698 8889] 120 yd (110 m) NNE of Splatts Barn (p. 117) and has yielded Phlyseogrammoceras? of the levesquei Zone.

A well-exposed, but incomplete section of the Cephalopod Bed was observed in the side of the track [ST 7718 8822] 650 yd (594 m) SSE of Splatt's Barn. The section below the Inferior Oolite is: rubbly parting, 1 in (3 cm), brown, rubbly and marly, thin-bedded limestone with abundant small limonitic pellets and Goniomya sp., Pleuromya costata, Protocardia?, Pleydellia venustula, indicating the aalensis Subzone, seen about 6 ft (1.8 m). About 12 ft (3.7 m) of the underlying strata were obscured over brown, hard, speckled, Shelly limestone in the lane bank which yielded Esericeras?, G. thouarsense, ?Pseudogrammoceras cotteswoldiae, about 6 ft (1.8 m). Phlyseogrammoceras? was also obtained from this exposure indicating that the dispansum Subzone is probably present.

About 180 yd (165 m) due S of Lower Kilcott a section at the side of a track shows about 30 ft (9.1 m) of rather argillaceous, buff sand with grey hard, blocky, micaceous, sandy silt visible at the base. Above these beds are 40 ft (12.2 m) of yellowish brown, finely micaceous sand, the section ending some 30 ft (9.1 m) below the Inferior Oolite.

At Alderley Mill [ST 7759 9025] 1000 yd (914 m) SE of Alderley Church 14 ft (4.3 m) of pale brown, finely micaceous, silty sand were seen. Here, two 9 in (23 cm) layers of buff, silty limestone occur 5 and 10 ft (1.5 and 3 m) respectively from the base of the section. Yellowish, fine sand occurs in a number of small exposures in and about Alderley; for instance in the side of the lane 200 yd (183 m) NE of the church; at the side of the Tresham road 360 yd (329 m) ENE of the church, and up to 10 ft (3 m) were seen in the bank 220 to 330 yd (201–302 m) NNE of the church. Another track, where it turns north [ST 7687 9175], 1020 yd (933 m) due N of Alderley Church, reveals sand containing two silty limestone bands. Above this follow some 40 more feet (12.2 m) of sand. Similar sand is seen in Cuckoo Lane where it ascends Tor Hill from the south-west. Here about 3.5 ft (11 m) of Cephalopod Bed with overlying sandy limestone of the scissum Beds are visible [ST 7647 9241], 900 yd (823 m) ENE of Leys Farm, Wortley.

There are three sections in parts of the Cephalopod Bed on the north side of Ozleworth Bottom.

The first is at the side of a cart track [ST 784 915] 930 yd (850 m) WNW of Tresham Church showing: yellow-buff marly limestone with fine shell fragments and limonitic pellets, about 2 ft (0.6 m), on brown rubbly clay with dark brown limestone containing limonitic pellets like linseeds, seen for 6 in (15 cm).

The second is [ST 788 918] 820 yd (750 m) NW of Tresham Church where the section beneath basal Inferior Oolite is:

The basal stratum of the Cephalopod Bed here belongs to the striatulum Subzone, Bed 3 belongs to the fallaciosum Subzone, Beds 6 and 7 to the dispansum Subzone and Bed 8 to the aalensis Subzone.

The third section [ST 792 923] is 150 yd (137 m) SW of Holwell Farm:

The faunas of Beds 2 and 3 indicate the G. striatulum Subzone, that of Bed 3 indicates the P. fallaciosum Subzone while that of Bed 4 indicates both to be present.

Eastwards, in the Newmarket valley an exposure in the stream gully [ST 8240 9660] 900 yd (823 m) NNE of Upper Lutheridge Farm, shows 10 ft (3 m) of buff, hard, fine sand. Sand is also visible in the laneside banks [ST 8523 9927] 400 yd (366 m) SSE of Nailsworth parish church, while nearby, augering detected thin clay partings in the sand. R.C.

Wotton-under-Edge to Stinchcombe Hill

Ten feet (3 m) of Cotteswold Sands with 2 ft (0.6 m) of calcareous sandstone at the base were exposed in the excavation [ST 7520 9642] 100 yd (82 m) SE of Millend.

Catacoeloceras cf. confectum was found in a calcareous dogger within the Cotteswold Sands of a roadside exposure [ST 7426 9902] 1400 yd (1280 m) N82°E from Stinchcombe Church. The horizon is high in the Cotteswold Sands and suggests either a high bifrons Zone or variabilis Zone age.

Exposures of yellow fine-grained micaceous sand were available in the road north from Stinchcombe Hill House [ST 7421 9870]. At one point 350 yd (320 m) N10°E of the house doggers of calcareous sandstone with belemnites and ammonites cropped out approximately 100 ft (30 m) below the top of the sands. The Cephalopod Bed is seen 300 yd (274 m) due west of Little Park Lodge [ST 7470 9775] where the 'Linseed Bed' rests on yellow sand with beds of calcareous sandstone.

Stinchcombe Hill to Owlpen

Numerous sections of the Cotteswold Sands and Cephalopod Bed are available on the narrow ridge connecting Stinchcombe Hill to the main escarpment.

A section [ST 7596 9683] 500 yd (457 m) S of Woodmancote showed hard oolitic limestone of the scissum Beds resting on:

A similar succession is exposed in a track [ST 7597 9690] 500 yd (457 m) S5°E of Woodmancote.

The fauna of the 1 ft 1 in (0.3 m) shaly marl is a mixed fattna from the thouarsense Zone and base of the levesquei Zone. Both subzones of the thouarsense Zone are probably present. Alternatively it is possible that the two specimens of dispansum Subzone species have come down from a higher level so that the overlying two beds are also of the thouarsense Zone. I.B.P., R.C.

South of Breakheart Hill, the Cephalopod Bed is again exposed while 200 yd (183 m) due south of Woodmancote the following section [ST 7588 9649] of the Cotteswold Sands was seen: yellow, micaceous sand, 2 ft (0.61 m), on yellow, fine-grained, shelly, sandy limestone, 12 in (0.3 m), yellow, micaceous sand, 4 in (10 cm), yellow, blue-hearted, calcareous sandstone, 6 in (15 cm), over yellow, micaceous sand. This sequence lies approximately 100 ft (30 m) below the top of the Lias. Eastwards towards Ridge Farm, the upper hill slopes are largely concealed by scree and exposures of the Cotteswold Sands and Cephalopod Bed are less common. The Cephalopod Bed with a rich fauna of ammonites was seen, however, 600 yd (549 m) W of Ridge Farm [ST 7789 9677]. The higher hill slopes are generally concealed by gravel scree and the Upper Lias is rarely seen. The following section [ST 7944 9740], 250 yd (229 m) W35°N of Lampern House, of the Cephalopod Bed, 1 ft 11 in (0.6 m) thick, was observed beneath basal Inferior Oolite: soft, bluish grey marl with abundant ellipsoidal, orange-brown, limonite pellets, 8 in (20 cm), on blue fine-grained hard argillaceous limestone with Ceratomya bajociana, Propeamussium laeviradiatum and Grammoceras sp. indicating the thouarsense Zone, 5 in (13 cm), soft iron-shot manly limestone, 1 in (3 cm), very hard fine-grained bluish grey shelly limestone with some limonite pellets, 9 in (23 cm), over fine-grained micaceous Cotteswold Sands, seen for 5 ft 6 in (1.7 m).

The top bed yielded Ftircirhynchia cotteswoldiae, Homoeorhynchia cynocephala, Myophorella sp., Neocrassina lurida, Opis (Trigonopis) trigonalis, Protocardia cf. buckmani, Dumortieria costula — a loose specimen, D. pseudoradiosa, Grammoceras thouarsense, and Phlyseogrammoceras sp. These fossils indicate the presence of both subzones of the thouarsense Zone and the subzones of the overlying levesquei Zone possibly up to and including the moorei Subzone. Additional fossils from this locality include Neocrassina lurida, Trigonia costata, Catuloceras cf. dumortieri which indicate the moorei-aalensis subzones and therefore suggest even higher horizons of the Cephalopod Bed than did the fossils precisely located in the top bed. A certain amount of superficial disturbance of the slope here is evident, but this cannot affect the supposition that the top bed contains the highest Upper Lias ammonite subzones in a very condensed sequence, with the possibility that the overlying bed is not scissum Beds but belongs also to the aalensis Subzone. None of the Upper Lias ammonite subzones is absent as a result of sub-Inferior Oolite erosion. I.B.P., R.C., H.C.I.-C.

Cam to Nympsfield

The section of the Cephalopod Bed at Cam Long Down [ST 7716 9948] (Richardson 1910, p. 114) is largely overgrown but shows: hard, creamy, shelly pellet limestone with ellipsoidal grains of brown limonite, 1 ft 4 in (0.4 m), on soft, cream-coloured, oolitic and shelly limestone with elongated limonitic pellets, 2 ft 2 in (0.7 m), hard, buff-coloured, argillaceous limestone with orange limonitic pellets, 1 ft 6 in (0.5 m). Between Cam Long Down and Peaked Down a sand-pit [ST 7691 9929] shows 30 ft (9.1 m) of typical Cotteswold Sands.

A complete section [ST 7863 9947] of the Cephalopod Bed is exposed in Coaley Wood 70 yd (64 m) W of Crawley Barns and shows minor differences from that recorded by Richardson (1910, pp. 115–6).

Evidence for the moorei Subzone and possibly the aalensis Subzone comes from a trackside exposure [ST 7938 9981], 550 yd (503 m) NW of Dingle Farm, Uley, where Dumortieria moorei, D. pseudoradiosa, D. sp. and Pleydellia? were found.

The Frocester Hill section [SO 7941 0077] of the Cephalopod Bed (Richardson 1910, p. 119) is now incomplete, showing only the following beneath fine-grained sandy limestone of the scissum Beds: hard, buff-coloured marl-ironstone, attached to base of the overlying bed, 4 in (10 cm), on soft, buff-coloured marl with brown limonite pellets, 10 in (25 cm), over yellowish marl with numerous limonite pellets, 7 in (18 cm). The Cotteswold Sands are seen in a roadside exposure [SO 7938 0084] 1000 yd (914 m) N27°E of Hetty Pegler's Tump and contain a 12 in (30 cm) bed of orange, sandy, crinoidal limestone with occasional ammonites. This bed is approximately 60 ft (18.3 m) below the top of the sand.

Kingscote to Nailsworth

The Cephalopod Bed is extremely thin in this area and records of exposures are included with the details of the Inferior Oolite (p. 123). The Cotteswold Sands, too, are thin and very poorly exposed. Most of the Upper Lias is composed of argillaceous silt in this area and sand is restricted to the top 30 ft (9.1 m) or so. I.B.P., R.C.

Chapter 8 Jurassic: Inferior Oolite

History of research

Much of the early research into the Inferior Oolite was carried out on the better exposed and more complete succession between Stroud and Cheltenham. From this work stems the classification now employed for the Inferior Oolite of the Malmesbury and adjacent districts.

Murchison (1834) gave one of the earliest accounts of the geology of the mid-Cotswolds in his book 'Outline of the Geology of the neighbourhood of Cheltenham'. An augmented and revised edition was published in 1845 with the assistance of J. Buckman and Strickland. Following these works were many detailed and descriptive accounts of the Inferior Oolite by such as Brodie (1850), Wright (1856,1860), Lycett (1857), Hull (1857), J. Buckman (1858), Holl (1863), Witchell (1880,1882a, 1886a) and Woodward (1894). Some of these accounts employed a broad two-fold division of the Inferior Oolite into an upper group 'the Ragstones' (e.g. Hull 1857), and a lower comprising mainly oolite formations. Woodward (1894) went further, subdividing his Ragstones division into Upper and Lower though on different lines to subsequent workers.

The basis of the modern classification was propounded in two papers by S. S. Buckman (1897, 1901), in the latter of which the terms Lower, Middle and Upper are employed in the sense which has been adopted for the general classification of the Inferior Oolite of the Cotswolds. Buckman drew attention to two major breaks in the succession which were utilized as the basis of the three-fold division, each division being further subdivided lithologically.

Following S. S. Buckman, Richardson in many papers presented to the Geological Society of London and the Cotteswold Naturalists' Field Club between 1907 and 1921 provided detailed accounts of the Inferior Oolite, while Arkell (1933) collated results and provided an extensive bibliography.

General account

The Inferior Oolite makes a continuous outcrop along the Cotswold scarp from Old Sodbury to Coaley Peak. It is this formation more than any other which is responsible for the existence of the scarp, and in most places is the cap rock to the major scarp feature. The formation also crops out in the sides of the deep valleys in the north of the district, around Nailsworth and Horsley, while further information has been obtained from a number of boreholes in the centre and east of the district.

Within the confines of the Malmesbury district only Lower Inferior Oolite and Upper Inferior Oolite occur at outcrop. No Middle Inferior Oolite is known for certainty, but being so close to areas where it is present, all the aspects of the Inferior Oolite as it occurs in the Malmesbury district cannot be considered properly without reference to the Middle division as known from the adjacent ground.

Lithologically the three divisions of the Inferior Oolite are dissimilar, for, although all are largely made up of limestone formations, they differ in type. The Lower is composed mainly of oolith and shell-detrital material, which accumulated in shallow warm seas. The Middle, while incorporating some oolite, is made up mostly of sandy or silty, sparsely oolitic limestones, with thick bivalve shells and their fragments predominating in its fauna. The Upper consists of argillaceous and shell-fragmental limestone at the base, a more compact fine-grained limestone middle portion, while oolite reappears at the top. In the Upper Inferior Oolite brachiopods feature much more prominently in the fauna than in either the Middle or the bulk of the Lower.

Each of the divisions Lower, Middle and Upper represents a separate phase in the sedimentary history of the Inferior Oolite, and the thicknesses of each, taken in conjunction with the relationship of one with another, have been expounded upon in the past by many, following the initial work of S. S. Buckman (1897), e.g. Kellaway and Welch 1948 (see Cave and Penn 1972). In most places there are, separating these three divisions, two indurated, planed, bored and oyster-encrusted surfaces. These surfaces represent comparatively long pauses in sedimentation, possibly preceded by considerable erosion. Thus, following the deposition of the Upper Freestone/Oolite Marl there was widespread uplift with some erosion and when deposition was renewed, in Middle Inferior Oolite times, it would seem that it was more active along certain belts than others. These belts were disposed roughly NNW–SSE in the north and possibly were due to differential warping or faulting of the basement, which allowed greater accumulation in the downwarp than on the relatively positive ridges separating them and with the gradual expansion of these basins each bed overlapped the preceding one. The close of the Middle Inferior Oolite sedimentation was brought about by another uplift and erosion which removed the Middle Inferior Oolite from the areas which had already possessed slight positive tendencies during deposition. Although no undoubted Middle Inferior Oolite remained within the sheet boundaries following this event, it is clear (Figure 10) that the remnant Lower Inferior Oolite is thickest along a NNE–SSW belt through Sherston, and this could be the continuation of the Painswick 'Syncline' (Kellaway and Welch 1948, p. 64).

The second bored surface is associated with this latter 'pause' in deposition and when the Upper Inferior Oolite transgression renewed the depositional environment, it did so over a very wide area, the deposits having an overstepping relationship to older formations. The ensuing deposits were correspondingly uniform, though it has been noted, just beyond the northern margin of the district, that, where the Middle Inferior Oolite is thickest, so the lowest member, at least of the Upper Inferior Oolite, thickens. Hence it is possible that the processes which controlled the sedimentation and warping of the Middle Inferior Oolite persisted weakly into the Upper.

Regarding total thicknesses of the Inferior Oolite the Lower Inferior Oolite is the most variable. The Upper Inferior Oolite is more uniform, being between 30 ft (9 m) in the south and 45 ft (14 m) in the north. Total thicknesses therefore range from some 40 ft (12 m) in the south to an estimated 140 to 150 ft (43–46 m) in the north and 136 ft (41 m) in the east at Tetbury (Richardson 1915a).

The Inferior Oolite corresponds approximately to the Bajocian Stage, the ammonite taxonomy of which is at present undergoing intensive revision and which may lead to a different classification from that in current use. The issue is complicated in the Malmesbury district by the rarity of ammonites although over the years sufficient have been found both here and in adjacent areas to enable the mapped formations to be assigned approximate zonal positions.

Lower Inferior Oolite

Adjacent to the northern margin of the area the Lower Inferior Oolite is complete and can be subdivided as follows:

Over most of the Malmesbury district this classification is irrelevant, for the major part of the succession is absent in the south-west half of the district. This absence is due mainly, and maybe wholly, to the overstep of the Upper Inferior Oolite.

The Oolite Marl and Upper Freestone are restricted to the extreme northeast, while the Lower Freestone extends a little further south-west to a line approximately through Horsley towards Shipton Moyne and then eastwards. The Pea Grit is absent west of a line approximately from Owlpen to Sherston and this may be due to overstep or to lateral passage into a facies indistinguishable from Lower Limestone. In the absence of the Pea Grit the Lower Inferior Oolite is classified as 'Undifferentiated'.

Except where Oolite Marl and other marly beds constitute the top layer, the surface of the Lower Inferior Oolite is planar, oyster-encrusted and penetrated by abundant very slender, straight, sub-vertical borings infilled with marly brown oolite and descending to a depth of some 8 in (20 cm). Where the uppermost horizons consist of white oolite the top inch or two (3–5 cm) are commonly hardened and discoloured brown.

The Geological Survey borehole at Carriers' Farm, Sherston proved that the Lower Inferior Oolite is slightly thicker there than either immediately to the east or west (Figure 10) indicating that the Painswick 'Syncline' (S. S. Buckman 1901) extends south at least as far as Sherston. This means either there was greater deposition there during the Lower Inferior Oolite, or that less erosion preceded the Upper Inferior Oolite transgression. Erosional effects are certainly great, as witnessed by the overstepping of Lower Inferior Oolite by the Upper (see (Figure 10)) but either case is an argument for the Painswick 'Syncline' (S. S. Buckman 1901) having extended southwards (Kellaway and Welch 1948, fig. 22).

From a thickness of about 10 ft (3 m) at the southern margin of the district, near Old Sodbury, the Lower Inferior Oolite thickens northward and Richardson (1910, p. 99) recorded 35 ft 10 in (10.9 m) at Hawkesbury, while just north of Hawkesbury, it is 41 ft (12.5 m). The outcrop around Wotton-under-Edge occurs on very steep slopes and has an apparent thickness of 80 to 90 ft (24–27 m), some 30 to 40 ft (9–12 m) in excess of that recorded in earlier accounts (Richardson 1910, pp. 104–5 and 115). This apparent thickness results from the cambered condition of the Inferior Oolite. Further north, near Coaley Peak, the thickness is nearer 100 ft (30 m). In the extreme north, near Nails-worth the thickness is estimated at over 100 ft (30 m) and it is 92 ft (28 m) in the Tetbury Waterworks Borehole No. 4 (p. 305).

A study of the Lower Inferior Oolite proved in boreholes from west to east across the line of the main pre-Upper Inferior Oolite downwarp reveals that the thicknesses of the constituent beds may not vary proportionately with the whole, indicating that the process was irregular.

The Inferior Oolite follows the Lias without any major biostratigraphic break in the west of the district where the lowest Bajocian zone succeeds the uppermost Toarcian and is succeeded by representatives of all Lower Bajocian zones except the youngest, Graphoceras concavum Zone (Table 3). In the concealed ground of the east of the district however, the scissum Beds are thin, possibly because of lateral passage into Lower Limestone and, if this is the correct explanation, then the zonal status of these Lower Bajocian formations may change across the district. The uppermost Lower Bajocian is cut out by the unconformity at the base of the Upper Trigonia Grit which oversteps on successively older Bajocian subzones within the Ludwigia murchisonae Zone and, at Old Sodbury Tunnel just to the south, comes to lie on the basal Bajocian Zone of Leioceras opalinum (Reynolds and Vaughan 1902, p. 734).

scissum Beds (Richardson 1904c)

The scissum Beds have been named Sandy Limestone (Witchell 1886a), Sandy ferruginous Limestone (S. S. Buckman 1889), and Sandy Ferruginous Beds (S. S. Buckman 1901). All these terms are more descriptive than scissum Beds but are poorly established in literature and Richardson's term is adopted here.

In the main, the scissum Beds consist of cream, buff and orange-coloured sandy limestone and calcareous sandstone, massive in places, but generally thickly and unevenly bedded. Locally, a basal conglomerate is developed, composed of pebbles of fine-grained oolitic limestone contained in a matrix of fine-grained bioclastic limestone with a few ferruginous ooliths and rounded quartz grains. Beds of shelly limestone, slightly oolitic or with ferruginous pellets may alternate with the sandy limestones which are generally poorly fossiliferous.

So far as is known the scissum Beds are present everywhere in the district, though the upper part may interdigitate with the Lower Limestone. Thus while 7 to 9 ft (2–3 m) of fine-grained sandy limestones were seen near Hawkesbury in the west, in the Sherston Borehole there were some 14.5 ft (4.4 m) of rather fine-grained oolitic and shelly sandy limestones, but palaeontological evidence suggests that only the bottom 2 ft 1 in (0.63 m) belong to the scissum Beds. Above this Propeamussium cf. laeviradiatum is common. In the Tetbury Borehole (No. 4) (Appendix 1) only about 6 in (15 cm) are similarly described.

The general tendency is for the scissum Beds to thin eastwards, being thickest and well developed along the main outcrop between Old Sodbury and Frocester. Apart from being very thin in the Tetbury and Shipton Moyne boreholes (Richardson 1915a, b; 1919) they are less than 2 ft (0.6 m) thick around Horsley and Nailsworth, places where the Upper Lias Cephalopod Bed is also thin (Figure 9).

Lower Limestone (Witchell 1886a, p. 267)

At the southern limit of the district the Lower Inferior Oolite is extremely thin, and, apart from the scissum Beds, it is there indivisible consisting of cream-coloured, or pale brown, massive, shell-detrital oolite freestone. Probably this is the representative only of the Lower Limestone subdivision of further north. The thicknesses increase from about 12 ft (4 m) near Old Sodbury in the south, to 30 ft (9 m) or just over at Hawkesbury (see also Richardson 1910, p. 99). At Sherston the lowest part is sandy like the underlying scissum Beds.

At Tor Hill, near Wotton-under-Edge the vertical interval between the scissum Beds and the Upper Inferior Oolite is some 80 ft (24 m) but this is probably because the spur on which this outcrop is situated has been affected by camber and minor landslipping. Richardson (1910, p. 104) recorded only about 40 ft (12 m) at Wotton-under-Edge, but here and northwards he separated Lower Limestone from Lower Freestone along a flat oyster-covered surface, overlain by a few inches of limestone rubble seen in many of the quarries around Dursley. This rubble he considered to represent the Pea Grit and called it the Pea Grit Horizon (or Equivalent) though the fossils listed in the past from it are undiagnostic. Around Dursley therefore, this descriptive account recognises only undifferentiated Lower Inferior Oolite, believed to be possibly all Lower Limestone (Figure 11).

In a number of places, e.g. between Dursley and Nailsworth, rather large quartz grains have been observed in the Lower Limestone.

Pea Grit (Murchison 1834a)

Typically, the Pea Grit is developed in the district to the north, for example at Leckhampton Hill (Sheet 234), 12 miles (19 km) to the north-east. There it is a rather rubbly limestone containing flattened pisoliths of diameter up to half an inch (1.5 cm) and an abundant and varied fauna.

In the north of the Malmesbury district the Pea Grit consists of rubbly, marly pisolitic limestones interbedded with marl or even grey clay partings which are usually pisolitic. Further south, in the borehole at Shipton Moyne there appears, from the available records and selected samples, to be less argillaceous matter, so that the Pea Grit might be passing southwards into a shell-detrital oolite with pisoliths, not unlike the Lower Limestone. This passage would possibly explain why, despite there being 46 ft (14 m) of Lower Inferior Oolite, no Pea Grit was recorded in the Sherston (Carriers' Farm) Borehole.

The maximum recorded thickness of the Pea Grit within the district comes from the Tetbury Waterworks Boreholes and is about 37 ft (11 m). At outcrop it is about 10 ft (3 m) at Nympsfield and Horsley, and about 19 ft (6 m) at Avening. In other boreholes to the south and east it was 22 ft (6.7 m) at Shipton Moyne (Richardson 1919, p. 157, see also Appendix 1) and 35 ft (10.7 m) at Kemble (Sheet 252) but there resting on Lias (Richardson 1913, p. 188). Kemble lies just north-east of the district, but the record serves to show that the pisolite does not everywhere separate two oolite limestone formations. These figures illustrate the problem of the Carriers' Farm, Sherston Borehole where the Pea Grit appears to be absent. In an area where the Lower Limestone is relatively thick it is possible that the erosion prior to the Tesulian' transgression has removed the Pea Grit there. Otherwise it must be represented in the top part of the strata classified on appearance as Lower Limestone.

Lower Freestone (Hull 1857)

The Lower Freestone consists of well-sorted, pale cream-coloured, fine- and medium-grained oolitic limestone, in which the ooliths are commonly contained in sparry calcite cement. Shell debris is present in variable amounts, both as nuclei of the ooliths and concentrated in thin wisps which impart a lamination to an otherwise massive oolite. Cross-stratification may be present, occurring on all scales to units several feet thick.

The top surface of the Lower Freestone is normally very distinct, flat and oyster-encrusted and can easily be identified when the overlying Oolite Marl is present.

The formation is restricted by the overstep of the Upper Trigonia Grit to the area north and east of a line through Nailsworth and Shipton Moyne. Where it crops out around and east of Nailsworth it has been mined for building stone, levels having been driven in from the steep sides of the valleys.

No precise complete thicknesses of the Lower Freestone have been ascertained, for at outcrop it is not completely exposed, and in boreholes is only partially preserved beneath the overstepping Upper Trigonia Grit. At Tetbury Waterworks (No. 4) Borehole (Appendix 1) the thickness is 19 ft (5.8 m), but pisoliths were recorded from near the bottom, so this part may belong to the Pea Grit.

Just north of the district, at Pen Wood [SO 8226 0235] about 38 ft (12 m) were seen, probably overlain by Upper Trigonia Grit. No Oolite Marl was seen there, but the 38 ft (12 m) must be an almost complete thickness. Between Nailsworth and Avening the formation is entire, probably between 30 and 40 ft (9 and 12 m) thick. The figure of about 70 ft (21 m) given for the thickness at Selsley Common, 2.5 miles (4 km) NNW of Nailsworth (Witchell 1887a, pl. 6, p. 102; Richardson 1910, pl. xxi) may therefore be exaggerated. Complete exposure does not exist there and the succession has been derived compositely from various quarries; so, in view of the extremely cambered and probably landslipped nature of the hill, it is not surprising that thicknesses have been overestimated.

Oolite Marl and Upper Freestone (Buckman, J. 1842; Hull 1857)

The Oolite Marl is a cream-coloured, oolitic, soft, finely grained limestone. It weathers quickly and is rubbly, containing pink and brown pisolitic pellets and abundant whole shells of which the most characteristic is the brachiopod Plectothyris fimbria. The formation also contains thin layers of brown clay.

Undoubted occurrences of these beds have been observed only in the Avening Valley east of Nailsworth. The maximum thickness seen is 11 ft (3.5 m) of Oolite Marl about 1 mile (1.6 km) NW of Avening. Near Ball's Green, 1 mile (1.6 km) E of Nailsworth, some 6 ft (1.8 m) of Oolite Marl are overlain by 2 ft 9 in (0.84 m) of fawn to cream-coloured oolite and the latter is considered to represent the Upper Freestone which is merely an oolite facies of the Oolite Marl. It commonly occurs at the top of the marl facies, hence the rough two-fold division. The thicknesses and lateral development of the oolite are variable and, where the Upper Inferior Oolite rests directly on Oolite Marl, it is not certain whether the absence of the Upper Freestone is due to overstep or to lateral change. The combined thicknesses of oolite and marl indicate that the greatest development occurs somewhere just east of Avening. Here then is the likely 'axis' of the Painswick 'Syncline' which would therefore run from the neighbourhood of Sherston, through the Avening–Cherington area and beyond.

Middle Inferior Oolite

No deposits of the Middle Inferior Oolite have been seen in the Malmesbury district, but, noting the likelihood that the axis of the Painswick 'Syncline' lies approximately along a line through Sherston to Frampton Mansell, it is reasonable to suggest that a thin development occurs at the north end of this line under Aston Down (Field Barn) [SO 912 003]. The substance of this would only be proved in a borehole and the deposits, if like the Middle Inferior Oolite on adjacent ground, would probably consist of no more than a foot or two (0.3–0.6 m) of rubbly brownish (if weathered), coralline, silty or sandy limestones, containing small ferruginous particles, typical of the lower part of the Lower Trigonia Grit.

Upper Inferior Oolite

Like the Lower, the Upper Inferior Oolite of the district is depicted on the maps in one colour only and no subdivisions have been shown, even on six-inch maps. In quarries and boreholes, however, four subdivisions have been recognised, which largely follow Richardson (1910, et. seq.) as follows:

It has been pointed out that within the district the Upper Inferior Oolite thickens from south to north, but the west to east variations are also revealing, for they show that a belt of thicker Upper Inferior Oolite passes north-northeastwards through the eastern central portion of the district. This is a belt which has been referred to earlier, as lying along the projection southward of the Painswick 'Syncline'. In the neighbourhood of Old Sodbury Richardson (1910) recorded a thickness of 29 ft (8.8 m), and at Hawkesbury about 25 ft (7.6 m). The Sherston Borehole, lying on the line of the Painswick 'Syncline' proved some 41 ft (12.5 m) of Upper Inferior Oolite and following this line north-eastwards other comparable thicknesses have been recorded at Tetbury, 45 ft 2 in (13.8 m) (Richardson 1915a) and at Hillsome Farm [ST 903 943] near Tetbury, about 44 ft (13.4 m). East of the line the thicknesses are less, for at Shipton Moyne, Richardson (1919) recorded 31 ft (9.5 m) (see also Shipton Moyne Borehole No. 5A, Appendix 1), and at Kemble, 6 miles (9.7 km) E by NE of Tetbury, he found (1913) 32.5 ft (9.9 m).

These figures reinforce the view already drawn from the variations of thickness of the Lower Inferior Oolite, that the basement movements which influenced sedimentation during Middle Inferior Oolite times, extended their influence well to the south of the Gloucester district, and it can be inferred that they continued to operate, if weakly, during the deposition of the Upper Inferior Oolite. A recent borehole at Swainswick some 9 miles (13 km) south of the district, revealed 50 ft (15.2 m) of Upper Inferior Oolite limestones. This unusually large thickness occurs on the line of the same north-north-eastward trending tract.

The break at the base of the Upper Inferior Oolite is considerable, excluding representative beds of the uppermost Lower Bajocian, the whole of the Middle Bajocian and basal Upper Bajocian Strenoceras subfurcatum Zone (Table 3). All the succeeding Bajocian subzones may be represented as are, with the exception of the basal Parkinsonia (Gonolkites) convergens Subzone, those of the succeeding Bathonian Zigzagiceras zigzag Zone. It is within this last Zone, in the Oxycerites yeovilensis Subzone, that the base of the overlying Great Oolite "Series" occurs. The full magnitude of the break at the base of the Bathonian is not known for certain since the rarity of ammonites in that part of the sequence does not preclude collection failure. Likewise the significance of the postulated non-sequence at the base of the Clypeus Grit and the position in the sequence of this district, equal to the Dundry Freestone (Richardson 1910), are uncertain.

Upper Trigonia Grit

The Upper Trigonia Grit is the most distinctive member of the Upper Inferior Oolite. It consists of between 3.5 and 7.5 ft (11 and 2.3 m) of non-oolitic grey shell and shell-fragmental limestone. It is well bedded into rather ragged layers, which may be about 1 to 1.5 ft (30–46 cm) thick and separated by thin partings of brown rubbly marl. The fauna is rich and characteristic in its abundance of trigoniids, rhynchonelloids and terebratuloids but ammonites are rare.

Though not consistently true, the thickness variations of the Upper Trigonia Grit tend to reflect those of the complete Upper Inferior Oolite. At Sherston and Tetbury, the thicknesses are 6 and 7.5 ft (1.8 and 2.3 m) respectively, while at Hawkesbury and Shipton Moyne they are 4 ft 10 in (Richardson 1919) and 3.5 ft (1.5 and 1.1 m) respectively. Along the northern limit of the district, near Nailsworth and at Kemble, the thickness is about 6 ft (1.8 m). Unfortunately, no figure is available in the Cherington area where a thickening might be expected.

Clypeus Grit (Hull 1857)

Clypeus Grit Beds

The Clypeus Grit Beds are pale brown to fawn at outcrop, consisting of poorly bedded fine-grained limestone with a moderately prolific brachiopod fauna. Clypeus has not been found in the recent survey, but Acanthothiris spinosa and Stiphrothyris tumida are common. A distinctive feature of these beds is the liberal scatter of yellow limonitic pellets or 'ooliths' throughout the rock. Since the limestone is slightly silty it has an appearance not unlike the scissum Beds. The thicknesses are not as well recorded as those of the Upper Trigonia Grit for the formation has not been sought as building stone and is exposed only as a result of the exploitation of Upper Trigonia Grit. Furthermore the junction with the overlying oolite is not always sharp, so that measurements may not always be made from the same upper limit. In such cases the measured thicknesses tend to include some fawn oolitic limestone not as pure as that of the overlying oolite. Figures so obtained amount to 8 to 9 ft (2.4–2.7 m) in the Horton and Hawkesbury areas, 6 ft 5 in (1.96 m) at Sherston, about 10 ft (3 m) at Tetbury, and 5 ft 3 in (1.6 m) near Nailsworth.

White Oolite Beds

The oolite above the Clypeus Grit Beds appears to correspond with that designated as Clypeus Grit by Hull (1857, p. 46), but later placed above the Clypeus Grit by Witchell (1880, p. 119). It has not been depicted separately on the maps except in rare circumstances where it forms part of an Inferior Oolite dip slope beneath a Fuller's Earth scarp and there it was possible to show its outcrop on the six-inch maps.

The White Oolite Beds have a thickness which varies between 10 ft 4 in (3.15 m) at Hawkesbury, 23 ft (7 m) at Sherston, and 21 ft 7 in (6.6 m) at Tetbury, but lack of exposure makes it difficult to study the formation comprehensively. In weathered exposures and in field brash, the oolite is white and shell detritus is not abundant. In a less weathered state the colour is buff or fawn. The ooliths are distinct and rather large and the rock characteristically contains whole terebratuloid shells, which, when broken, are commonly partially filled with crystalline calcite or are hollow. The oolite has a blocky fracture and will break irrespective of the shell surfaces.

Rubbly Beds (Richardson 1907a)

The Rubbly Beds are the top few feet of the Inferior Oolite. Lying immediately below the Lower Fuller's Earth Clay, they are beds which have not been observed in the district at surface. They appear in borehole cores and about 6 ft (1.8 m) were recorded at Sherston, 4 ft 6 in (1.4 m) at Shipton Moyne (Richardson 1919), and 5 ft 4 in (1.6 m) at Tetbury. They consist of brown rubbly argillaceous limestone containing ooliths and some shell detritus. Whole brachiopods are common in some layers. Where seen, as in the core of the Sherston Borehole, no sharp change of lithology marks the junction with the overlying Fuller's Earth and the impression gained is of a passage between the formations. A sharp base may be seen in boreholes and at Sherston pebbles were associated with this so that perhaps grounds do exist for linking the Rubbly Beds with the Fuller's Earth.

Palaeontology

The Inferior Oolite fauna has been tabulated stratigraphically and ecologically. The variability in samples comprising these data has been checked statistically and the result indicates that the present palaeontological survey is little more than a reconnaisance. Stratigraphical and ecological conclusions are tentative, and only emphasised where statistically significant results were obtained.

The zonal palaeontology is even less firmly based since the critical ammonites are so few that published records, often of poorly localised material, must be used. A search for relevant material has been made in a number of museums and the accession numbers of all those discovered is given (C=British Museum (Natural History), B=Bristol University Museum, BCb=Bristol City Museum, IGS & GSM=IGS collections, MCZ= Museum of Comparative Zoology, Harvard, USA, R=Reading University, SMJ=Sedgwick Museum, Cambridge).

The lower part of the scissum Beds (the opaliniforme Bed of Richardson (1910, p. 81) has yielded: Leioceras opalinum at the Sodbury Tunnel to the south, B 3028, B 3032–5 (Reynolds and Vaughan 1902, p. 734); at Ozleworth, B 3032a; at Wotton-under-Edge (Ager 1955, p. 357) though this appears to have been later determined as Leioceras sp. (Ager 1969, p. B33); L. opalinum at Nibley Knoll (Buckman 1888, pp. 40, 46) and at Coaley Wood, SMJ 6426 (Buckman 1888, p. 45, pl. 13 figs. 4, 5) where L. opalinum, C 6254, R 3049 have also been found but from an uncertain position within the scissum Beds. The upper part of the scissum Beds has yielded Tmetoceras scissum at Wotton-under-Edge (Ager 1955, p. 357) and Leioceras comptum bifidatum of Contini (1969, p. 17), MCZ 2506 from Stinchcombe (Buckman 1899, pp. xxxviii–xxxix and suppl. pl. 8, figs. 10–12). To the north of the area these beds have yielded Tmetoceras scissum at "Long Wood" (Richardson 1910, p. 82) and at Pen Wood (Buckman 1892, p. 275) where it, GSM 16266, has been recorded (Buckman 1899, p. clxxi; Richardson 1910, p. 122) as T. regleyi which is considered (Westermann 1964, p. 430) to be a synonym of T. scissum. L. comptum bifidatum (Leioceras striatum Buckman 1899, p. xlii ; Richardson 1910 p. 122) also came from these beds at Pen Wood.

The dominance of L. opalinum in the lower part of the scissum Beds and its replacement higher by Tmetoceras scissum associated with L. comptum bifidatum suggest the normal ammonite zonal sequence where the basal Bajocian opalinum Zone is followed by the scissum Zone (Table 3).

The Lower Limestone (undifferentiated) has yielded: Ancolioceras cf. sub-striatum, GSM 114894, from the Cross Hands area to the south of the district; Ludwigia cf. austera, GSM 27820, at Wotton-under-Edge and L. (Pseudographoceras) subtuberculata, GSM 25593, (Contini 1969, p. 58) from Nibley. The Cross Hands specimen indicates the basal Ancolioceras opalinoides Subzone of the Ludwigia murchisonae Zone but the other two specimens from the thicker beds to the north suggest a higher position in the sequence, although their precise horizon is unknown.

The Pea Grit has yielded Ludwigia sp.,GSM 114895 from Nailsworth and an indeterminate graphoceratid, IGS Ba 3303 from 346 ft (105.46 m) in the Shipton Moyne Borehole No. 5A (p. 296) and may belong to the Ludwigia murchisonae Subzone. No ammonites have been found in the Lower Freestone and the Oolite Marl/Upper Freestone in this area but where the beds are more fully developed to the north they appear to be within the murchisonae Subzone and the Brasilia bradfordensis Subzone of the murchisonae Zone respectively (Arkell 1956, p. 34).

The Upper Trigonia Grit has yielded Parkinsonia cf. rarecostata, C 9171, at Long Wood which agrees with fossils found to the north and south and indicates that the formation belongs to the Garantiana garantiana Zone (Buckman 1893, p. 513; 1895, pp. 394–5). Two other ammonite species are recorded but their precise stratigraphical position is uncertain. They are a septate Prorsisphinctes meseres, B 3403, from the "garantiana Zone" of Nibley Knoll and Parkinsonia parkinsoni, B 16084, from the "Upper Trigonia Grit" of Wooton Hill. The matrices of both suggest, however, that they may have come from overlying beds.

The basal bed of the Clypeus Grit Beds, the Upper Coral Bed of Richardson, yielded two ammonites (unfortunately not so far traced) which he considered indicated the presence of the Strigoceras truelli Subzone, the basal subzone of the Parkinsonia parkinsoni Zone. The overlying Clypeus Grit Beds have yielded Parkinsonia (Gonolkites) bomfordi, R 3086, from the Workhouse Quarry at Stroud Hill and it is inferred from the locations that they have yielded Parkinsonia sp.,R 3084 from the "Ragstones" at Tresham (Richardson 1910, p. 101) and GSM 25217 from the "Upper Inferior Oolite" of Rodborough Hill. The White Oolite Beds have yielded Zigzagiceras (Procerozigzag) crassizigzag, C 10063 at Stroud (Torrens 1968, p. 74) as well as an indeterminate perisphinctid, R 3099, from Stinchcombe Hill (Richardson 1910, p. 110). The first is a Bathonian, Zigzagiceras zigzag Zone fossil and is confined to the Morphoceras macrescens Subzone of the type section for this zone (Sturani 1966, pl. 1, p. 49) and may indicate the presence of that subzone at Stroud (Torrens 1968, p. 74 and fig. 1). The succeeding Rubbly Beds of the district have not yielded any ammonites, but to the south two ammonites, apparently from the Rubbly Beds at Dodington Ash, have been found (Fry 1951, p. 200; Torrens 1968, p. 74). These proved difficult to identify since the proceritid, BCb 4699, is wholly septate and the other BCb 4700, has zigzagicerated ribbing but is not matched by anything in the literature (H. S. Torrens personal communication 1971). The Zigzagiceras zigzag Zone is certainly indicated here and, if these Rubbly Beds are equivalent to those described at the Doulting Railway Cutting south of Bath, then they must belong to the Oxycerites yeovilensis Subzone (Richardson 1907a, p. 390; Torrens 1969b, p. B18).

Since the overlying basal Fuller's Earth has apparently yielded Procerites fullonicus, GSM 52180, at Kingscote and perhaps also to the south of the district (Richardson 1910, p. 79) then the Inferior Oolite/Great Oolite "Series" boundary must lie within the Oxycerites yeovilensis Subzone.

scissum Beds

The scissum Beds are poorly fossiliferous and contain a dominantly bivalve fauna of which Propeamussium cf. laeviradiatum is the most common species. The majority of species were probably epifaunal, sessile bivalves being particularly common.

Undifferentiated Lower Inferior Oolite (see Lower Limestone, p. 105)

Compared to the underlying scissum Beds, this part of the Lower Inferior Oolite yields an increased number of species and some new corals and echinoderms appear. P. cf. laeviradiatum is the commonest shell and occurs here in significantly greater proportions than in the scissum Beds. It predominates near the base of the sequence as seen in the Sherston Borehole and at Wotton-under-Edge (Richardson 1910, p. 97). It has not been found in abundance immediately underlying true Pea Grit (in contrast to Richardson 1910, pp. 97 and 101) but has rather been found at Nibley Knoll [ST 745 957], though sparingly, above his "Pea Grit horizon".

Corals, though occurring at the "Pea Grit horizon" at Stinchcombe [ST 74920 97881], also occur at various levels below the Upper Trigonia Grit and are not of great stratigraphical value. The stratigraphical importance of Eolepas aalensis, in the Tetbury Water Works Borehole, has already been commented on (Pringle 1928, p. 189). Epifaunal and free-swimming species again dominate, though there is a significant tendency towards a decrease in the proportion of sessile bivalves.

Pea Grit

The fauna of the Pea Grit is characterised by the greater abundance of brachiopod species and a slight increase in the number of bivalve species when compared with underlying beds. The brachiopod fauna is closely allied to that of the Oolite Marl in which are corresponding, perhaps conspecific, forms to those occurring here (see Buckman 1901, p. 244). Plectothyris fimbria [subfimbriate], Plectoidothyris plicata and Lophrothyris aff. withingtonensis are commonest.

The gastropods derive from one area and are probably from the same beds as those which Witchell (1887b) and Hudleston (1888, pp. 59–61) made famous. Although there is some increase in the number of infaunal species, epifaunal and free-swimming elements predominate. The fall-off in the proportion of sessile bivalves in the Lower Limestone is reflected in the Pea Grit where the epifaunal proportions are maintained by the brachiopod faunas.

Lower Freestone

Only shell fragments and poorly preserved specimens have been obtained.

Oolite Marl/Lower Freestone

The Oolite Marl is marked by the dominance of epifaunal brachiopods. In many cases this brachiopod fauna is similar at species level to that of the Pea Grit. The distinctiveness of the fauna is thus in the relative decline of the other elements. Plectothyris fimbria, Plectoidothyris polyplecta and Globirhynchia subobsoleta are the commonest forms.

Upper Trigonia Grit

Within the Inferior Oolite of this area the Upper Trigonia Grit yields the most abundant and varied fauna. It is mainly a bivalve/brachiopod fauna with the former predominating. The rhynchonellides, Acanthothiris spinosa, Rhactorhynchia subtetrahedra, R. hampenensis, R. cf. turgidula and the terebratulide Stiphrothyris tumida are the commonest brachiopods, while Entolium corneolum, Oxytoma inequivalve, Gryphaea sublobata, Trigonia costata and various 'species' of Gervillella are the commonest bivalves.

Again the proportions of the major groups do not differ significantly from that of earlier Inferior Oolite formations excepting the Oolite Marl. Epifauna dominates infauna although there is a marked but not significant rise in the proportion of infauna especially among the inactive burrowing bivalves.

Clypeus Grit

Clypeus Grit Beds

The fauna of the Clypeus Grit Beds is similar but less abundant and varied than that of the Upper Trigonia Grit. Corals at the base (Richardson's Upper Coral Bed) are significantly fewer than those in the Lower Inferior Oolite. Bivalves again dominate over brachiopods. Of the latter, A. spinosa and S. tumida and of the former, O. inequivalve and E. corneolum are the commonest species although none of these occurs in proportions that are significantly different from its occurrence in the underlying beds. Epifauna dominates infauna and free-swimmers, but more markedly than in the Upper Trigonia Grit.

White Oolite Beds and Rubbly Beds

Less is known of the fauna of these beds which contain elements of the earlier Upper Inferior Oolite faunas. S. tumida is the commonest fossil but does not occur in proportions significantly different from its earlier occurrences.

The degree of dominance of epifauna over infauna is very variable as a result of the small size of the samples.

Palaeoecology

The gradual changes in lithology involving sandy, oolitic and pisolitic limestones and marls invite comparison with the animal communities reconnoitred in similar habitats on the Great Bahama Bank (Newell and others 1959). Although the Inferior Oolite material is very scanty and therefore precise equivalence of animal community and habitat correspondingly difficult to prove, certain similar tendencies are indicated. Thus the dominance of bivalves and brachiopods suggests, in the main, 'level' or 'sediment' bottom habitats. The dominance of the epifauna suggests that these were stable bottom habitats although the shell-fragment freestones suggest an environment akin to an unstable sand habitat (Newell and others 1959, p. 217; Ager 1969, p. B31). A tendency towards a muddy sand habitat (Newell and others 1959, p. 222–4) is suggested by the restriction of fauna begun in the Pea Grit and emphasised in the Oolite Marl.

The environmental regime does not appear to have been radically different in the Upper Inferior Oolite although the faunal diversity at its base may reflect colonisation of rocky bottom habitats associated with the transgression at the base of the Upper Inferior Oolite. The upward diminution of this fauna is presumably related to the development of oolitic sands of the White Oolite Beds. I.E.P.

Details

Old Sodbury to Hawkesbury Upton

Between Old Sodbury and Horton the Inferior Oolite forms a flat dip slope up to 700 yd (640 m) wide. It is estimated that its total thickness near Old Sodbury is about 30 ft (9 m). The formation supports a thin reddish brown loam with abundant fragments of oolite and compact bioclastic limestone.

North of the "Hill Fort" [ST 761 826] the Inferior Oolite can be mapped as two subunits, Lower and Upper. The Lower Inferior Oolite outcrop is marked by a soil containing fawn sandy limestone fragments, probably derived from the scissum Beds, while the Upper supports a soil with more ragged lumps of limestone often containing oyster shell remains and trigoniids, from the Upper Trigonia Grit as well as oolitic limestones from above. The junction between the Upper and Lower is marked by debris of brown, strongly bored, oolitic limestone. This junction is visible in a small excavation [ST 7635 8288] 400 yd (366 m) E of Manor House, Little Sodbury, where 6 in to 1 ft (15–30 cm) of shell-fragmental brownish limestone containing abundant trigoniids (Upper Trigonia Grit) overlie 5 ft (1.5 m) of cream-coloured shell-detrital, oolitic limestone with a planed and bored top surface.

These beds can be seen again immediately west of the road [ST 7654 8382], 1000 yd (914 m) NE of Little Sodbury, where trigoniid shells of the Upper Trigonia Grit are bored and serpulid encrusted and the bored top surface of the Lower Inferior Oolite is covered with oysters. Oolites are present at the top of the Lower Inferior Oolite indicating that the scissum Beds are here unaffected by the Upper Inferior Oolite overstep.

In the old quarry [ST 7663 8397], 300 yd (274 m) S of Horton Rectory (Richardson 1910, pp. 92–7) the Lower Inferior Oolite is not exposed. An E–W fault, down north, passes through this quarry. Exposed to the north of the fault are strata of the White Oolite Beds (=Anabacia Limestone of Richardson 1907a, p. 388; 1910, pp. 86–7) seen 11 ft (3.4 m) on brown marl, 1 in (3 cm), rather hard brown limestone and coral limestone with Isastraea sp. and Chlamys cf. articulata, 2 ft 8 in (0.81 m), rubbly manly limestone with Lithophaga cf. fabella, 4 to 6 in (10–15 cm) brown limestone with brown specks, hard and unevenly bedded, about 5 ft 6 in (1.7 m) over ochreous rubbly limestone seen about 1 ft (30 cm). To the south of the fault about 8 ft (2.5 m) of thinly bedded brown, hard limestone of the Clypeus Grit Beds, rubbly at the base, were exposed. Lopha marshii was obtained.

In the side of the old track [ST 7685 8475] 400 yd (366 m) SE of Hortoncourt Farm, Lower Fuller's Earth Clay overlies the Rubbly Beds: fawn, marly, rubbly limestone 1 to 2 ft (0.3–0.6 m) on fawn to brown hard limestone 2 ft (0.6 m) over White Oolite Beds: white oolite, seen 6 ft (1.8 m). At the top of the slope 100 yd (91 m) to the west a small excavation exposes the Lower Inferior Oolite, scissum Beds: fawn to brown crystalline limestone 2 ft (0.6 m) on brown marl and rubbly limestone, 2 in (5 cm), brown sandy limestone with some shell detritus, 2 ft (0.6 m) over the Cephalopod Bed. Some 18 ft (5.5 m) vertically below, in a small exposure within a possibly superficially displaced mass, the scissum Beds consist of fawn limestone, rather hard and fissile, with yellow pellets scattered or collected within nests, resting on the Cephalopod Bed.

At the west edge of Broad Hill [ST 7672 8639] which is slightly cambered, a small excavation revealed 4.5 ft (1.4 m) of Upper Trigonia Grit resting on a bored surface of Lower Inferior Oolite, white oolite.

Richardson (1910, p. 99) described a composite section through the Inferior Oolite of Hawkesbury Upton largely based on the quarry [ST 7711 8727] 270 yd (247 m) WSW of Home Farm. The following beds are still visible; the highest occurring at the east end of the quarry:

Hawkesbury Upton to Wotton-under-Edge

The Hawkesbury Monument Quarry [ST 7700 8765] is now largely overgrown, but the bored oyster-encrusted surface of the Lower Inferior Oolite is well exposed. This quarry was described by Richardson (1906, p. 195; 1910, p. 100).

Quarries at Hawkesbury Knoll above the road in Chandler's Cliff [ST 7694 8837] reveal a section in undifferentiated Lower Inferior Oolite: flaggy, oolitic limestone, 2 ft (0.6 m), on rubbly limestone parting about 6 in (15 cm) over fawn to cream-coloured oolitic limestones, mainly massive, but rubbly in top 3 ft (0.9 m), 14 ft (4.3 m).

An old excavation [ST 7735 8826] at Folly Brake 700 yd (640 m) SE of Splatt's Barn exposed Clypeus Grit Beds: brown compact rather brittle limestone, 6 in (15 cm), on Upper Trigonia Grit; brown rubbly Shelly limestone containing trigoniids, 3.5 ft (1.1 m), over Lower Inferior Oolite (undifferentiated); brown hard oolitic limestone with flat bored top 1 ft (0.3 m).

A trackside exposure [ST 7719 8823] 630 yd (576 m) E76°S of Splatt's Barn, supplemented by estimates of thickness from the adjacent hillslope gave the following section of the Lower Inferior Oolite.

In the lane [ST 7701 8883] alongside Splatt's Barn sporadic exposures provide a section through most of the Inferior Oolite and show: Clypeus Grit and Upper Trigonia Grit: oolite, visible, 1 ft (0.30 m), unexposed, about 6 ft (1.8 m), brown and greyish brown hard compact limestone, trigoniids near base, about 12 ft (3.7 m). Lower Inferior Oolite, Undifferentiated : fawn and brown oolitic limestones, about 30 ft (9 m). Scissum Beds: brown, hard limestone and yellowish brown sandy limestone with Rhynchonelloidea subangulata and Propeamussium cf. laeviradiatum about 11 ft (3.4 m). Upper Lias, Cephalopod Bed: ochreous and ironshot rubbly argillaceous limestone, seen about 8 ft (2.4 m).

This section yields a value of about 41 ft (12.5 m) for the thickness of the Lower Inferior Oolite which is greater, though a little further north, than Richardson's estimate for the thickness at Hawkesbury.

In the wood [ST 7756 8948], 750 yd (686 m) E of St Giles's Church, Hillsley, 16 ft (4.9 m) of fawn-brown oolites resting on sandy limestone (scissum Beds) probably at least 9 ft (2.7 m) thick are exposed in an old quarry.

Around the ramifying valleys near Lower Kilcott the oolites of the White Oolite Beds are distinct enough to map separately at the 6-in scale and, near Claypit Wood [ST 783 886], the thickness of the underlying Clypeus Grit Beds is about 14 ft (4.2 m). A quarry [ST 7953 8909] 900 yd (823 m) E of Lower Kilcott shows:

Some 4 ft (1.2 m) of brown hard semi-crystalline sandy limestone (scissum Beds) were seen in a stream [ST 7964 8936], resting on Upper Lias Cephalopod Bed.

A quarry [ST 7782 9073] in Upper Inferior Oolite near Winner Hill, about 1050 yd (960 m) E of Alderley Church was described by Richardson (1907b, p. 83) and his account is more comprehensive than any obtainable now. However the topmost 6 ft (1.8 m) of beds now visible consist of cream-coloured to white oolite (White Oolite Beds) resting on a surface, which is fairly flat but pocked by a number of hollows, of harder, compact, blocky limestone, more sparsely oolitic (Clypeus Grit Beds). In the cart track [ST 7881 9187], 810 yd (741 m) NNW from Tresham Church, part of the scissum Beds consisting of 2.5 ft (0.8 m) of grey and brown hard silty limestone containing small yellowish brown?ooliths rests on 11 ft 3 in (3.4 m) of Upper Lias Cephalopod Bed.

The spurs of higher ground directed to the south-south-east and overlooking Ozleworth Bottom are small dipslopes supporting Upper Inferior Oolite. From these and especially that [ST 785 926] between Clay's Wood and Barnhill Covert, a rough succession can be worked out from the surface features.

The quarry [ST 7657 9256] at Tor Hill, 0.5 mile (800 m) SE of Wotton-under-Edge has been mentioned by Richardson (1907b, p. 83; 1910, p.101). The beds now visible are: Upper Trigonia Grit; limestone, shell-detrital, brown and rubbly seen 4 ft (1.2 m) on Lower Inferior Oolite; oolite freestone, buff, rather shelly and massive 26 ft (7.9 m), oolite, brown and soft, 4 ft (1.2 m), oolite, buff, rather shelly and more massive, seen 6 ft (1.8 m). Underlying unexposed Inferior Oolite is estimated to be about 32 ft (9.8 m) thick indicating the thickness of the Lower Inferior Oolite to be about 86 ft (26.2 m), which is clearly excessive. However, 36 ft (11 m) are exposed, and probably the rest of the old quarry face was in limestone, so that a thickness of at least 54 ft (16.5 m) is likely and the Cephalopod Bed could be superficially displaced.

Wotton-under-Edge to Owlpen and Stinchcombe

A 0.5 mile (800 m) ENE of Wotton-under-Edge are two small-quarries on Listway. One [ST 7696 9356] exposes 2 ft (0.6 m) of brown and ochreous limestone (Clypeus Grit Beds), with brown speckles (ooliths or granules), separated by 4 ft (1.2 m) of unexposed strata from 5 ft (1.5 m) of Upper Trigonia Grit; limestone, shell-detrital, hard and rubbly with trigoniids, resting on a bored, flat surface of oolite.

The fauna of the top 4 ft (1.2 m) of the Upper Trigonia Grit includes Aulacothyris sp., Acanthothiris spinosa, Rhactoryhnchia hampenensis, R. subtetrahedra, R. cf. turgidula, Stiphrothyris sp., Anisocardia aff. loweana, oysters and trigoniids. That of the bottom 1 ft (0.3 m) includes Acanthothiris?, Gervillella cf. scarburgense, Modiolus? sp.,Orthotrigonia gemmata, Oxytoma inequivalve, Vaugonia producta. Clypeus Grit; Thecosmilia sp., Entolium corneolum, Homomya aff. gibbosa.

The other quarry [ST 7688 9354] continues the sequence 10 ft (3 m) below the bored surface exposing oolite, fawn, manly and rather broken, containing pentagonal crinoid ossicles and 1 in (3 cm) brown clay 2 ft (0.6 m) above base, 13 ft (4 m), on oolite, buff-fawn and hard with very flat top, seen for 1.5 ft (0.5 m). The manly oolite had an appearance similar to the Oolite Marl but no fimbriate terebratuloids were observed. It remains a possibility that these 13 ft (4 m) of beds might be a southerly representative of the Pea Grit thus making the underlying oolite Lower Limestone.

Fossils collected from the Lower Inferior Oolite include Camptonectes laminatus? and Propeamussium cf. laeviradiatum.

The quarries of Wotton Hill [ST 754 939] reveal less now than in the days of Richardson (1910, pp. 104–5). The thickness he ascribed to the Upper Trigonia Grit suggests unusual attenuation in this area.

At Nibley Knoll, 2 miles (3.2 km) NW of Wotton-under-Edge a large quarry [ST 7450 9570] 175 yd (160 m) NE of the monument was described by Richardson (1910, p. 107); only the Lower Inferior Oolite is still well exposed and the reader is referred to Richardson's description for details of the beds. It is to be noted that the recorded thickness of the Upper Trigonia Grit is again low, barely 3 ft (0.9 m) including Richard-son's Upper Coral Bed. Fossils collected during the recent survey, using Richardson's bed numbers, include: Lower Inferior Oolite, Bed 3 ; Oxytoma sp., Propeamussium cf. laeviradiatum. Bed 5; Rhynchonelloidea subangulata, P. cf. laeviradiatum. Upper Trigonia Grit-Bed 6; Acanthothiris spinosa, Aulacothyris sp.,Rhactorhynchia cf. brevis, R. subtetrahedra, R. cf. turgidula, Stiphrothyris tumida [juv.], Zeilleria cf. waltoni, Ctenostreon sp.,Oxytoma inequivalve, Pleuromya cf. uniformis. Bed 7, (Richardson's Upper Coral Bed) Clypeus hugi, A. spinosa, Sphaeroidothyris?, Anisocardia sp.,Limatula gibbosa, Plagiostoma sp., Quenstedtia? sp., Clypeus Grit, Bed 8 ; Sarcinella socialis, A. spinosa, Camptonectes? sp., Modiolus sp.,O. inequivalve, Pinna?, Quenstedtia sp. R.C.

An old quarry [ST 7582 9657] 760 yd (695 m) ENE of Millend disclosed, low in the Lower Inferior Oolite 3 ft (0.9 m) of flaggy, well sorted fine- to medium-grained oolitic limestone with little shell debris but a few well-rounded grains of quartz, on 8 ft (2.4 m) of well-bedded medium- to coarse-grained, cream-coloured oolitic limestone with well-rounded quartz grains, up to 3 mm in diameter, quite common.

The large quarry on Breakheart Hill [ST 7560 9675] exposed: Clypeus Grit Beds; limestone, roughly bedded, fine-grained bioclastic, with a few small ooliths, sparsely fossiliferous, 4 ft (1.2 m), on?Upper Trigonia Grit; fine-grained cream-coloured shelly limestone, 1.5 ft (0.5 m), Upper Trigonia Grit; rubbly bed of shelly, bioclastic limestone, 5 ft (1.5 m), limestone, hard, shelly, orange-coloured, 11 in (28 cm) over cream-coloured Lower Inferior Oolite limestone with hardened, bored and oyster-encrusted top surface, 3 ft (0.9 m). During 1961, extensive areas of the bored surface were bared in quarrying activities. Fauna from the Upper Trigonia Grit includes Aulacothyris mandelslohi (Oppel), Rhactorhynchia cf. turgidula and S. tumida. Numerous small quarries occur between here and Ridge Farm and reveal the Upper Trigonia Grit to vary from 2.5 to 4.5 ft (0.8–1.4 m). A quarry [ST 7653 9683] above Dursley Wood, exposes: Upper Trigonia Grit; fine-grained shelly limestone with trigoniids 4 ft (1.2 m). Lower Inferior Oolite, undifferentiated; cross-bedded oolite with bored top surface seen 20 ft (6.1 m).

Another quarry [ST 7697 9702] nearby exposes: Clypeus Grit Beds; rubbly-bedded, cream-coloured, shelly limestone with small ferruginous pellets, 2.5 ft (0.8 m), on Upper Trigonia Grit; hard, unevenly bedded, orange and buff-coloured limestone, 2.5 ft (0.8 m), very hard yellow, shelly limestone with Trigonia, 1 ft (0.3 m) over cream-coloured Lower Inferior Oolite with bored and hardened top surface, seen 4 ft (1.2 m).

At the head of Nunnery Lane (Richardson 1910, p. 109) a quarry [ST 7553 9710] exposes Clypeus Grit Beds; orange-coloured, fine-grained, sparsely fossiliferous, bioclastic limestone, 3 ft (0.9 m) on Upper Trigonia Grit; hard shelly limestone, with some ooliths, 5 in (13 cm), soft rubbly bed of shelly, bioclastic limestone, 6 in (15 cm), very hard abundantly fossiliferous bed of shelly limestone, 1 ft (0.3 m), over cream-coloured Lower Inferior Oolite with bored, hardened, oyster-covered surface 6 ft (1.8 m).

Some distance below this, in the northern quarry [ST 7554 9711] the sequence continues downwards: medium-grained oolite limestone 24ft (0.8 m), on bed of oolitic limestone with irregularly shaped ferruginous pellets, 8 in (20 cm), cream-coloured medium-grained oolitic limestone, 9 ft (2.7 m), bed of oolitic limestone with coral fragments (=?Richardson's Pea Grit Equivalent) 10 in (25 cm), conglomerate of iron-stained oolitic limestone pebbles (=?Richardson's Pea Grit Equivalent), 2 in (5 cm), cream-coloured oolitic limestone, 2 ft (0.6 m), parting of oolite pebbles, 2 in (5 cm), over cream-coloured, medium-grained, oolitic limestone, 2 ft (0.6 m).

A complete section through the scissum Beds was visible at the side of a track [ST 7597 9689], 1200 yd (1097 m) S85°W of Folly Wood consisting of about 15 ft (4.6 m) of hard, rather sandy limestone, thinly and irregularly bedded, containing brown ferruginous pellets in the lower part.

An old quarry [ST 7679 9666], 1820 yd (1664 m) W of Ridge Farm showed 6 ft (1.8 m) of sparsely shelly, yellow, sandy, fine-grained limestone. Similar limestones with orange-coloured ferruginous pellets were exposed in a track [ST 7763 9701], 970 yd (887 m) W17°N of Ridge Farm.

The lower parts of the Inferior Oolite were quarried in Hermitage Wood, Dursley, where 25 ft (7.6 m) of beds have been exploited and 15 ft (4.6 m) of cream-coloured, coarse oolite are still visible. Other old workings occur 300 to 400 yd (274–366 m) to the SSE.

The promontory of which Stinchcombe Hill forms the main part is surrounded by a number of exposures, some of which have been described in detail previously. Stancombe Quarry [ST 7441 9787] 850 yd (777 m) S84°E of Drakestone Point is now largely overgrown so that reference is made to the details supplied by Richardson (1910, p. 112) who composed a comprehensive section of the Inferior Oolite of this hill.

Fossils collected from the quarry include: Lower Inferior Oolite, top 2.5 ft (0.8 m); Thamnasteria terquemei, Thecosmilia wrighti?, Pedina rotata, rhynchonelloid indet., terebratuloid indet., pectinid indet. Upper Inferior Oolite, Upper Trigonia Grit; serpulids indet., A. spinosa?, Aulacothyris carinata, R. subtetrahedra, S. tumida, Stiphrothyris juv., arcid indet., Coelopis sp., Cucullaea sp., Ctenostreon pectin:forme?, Gervillella sp., Gryphaea sublobata, Liostrea eduliformis?, Myophorella sp.,oysters indet., Trigonia costata, trigoniids indet. Clypeus Grit Beds; A. spinosa, Stiphrothyris sp.,bivalves indet.

In the large quarry [ST 7490 9790] 1350 yd (1234 m) E of Drakestone Point there are:

No boring of the sub-Upper Inferior Oolite surface was seen and it is considered that no more than 6 ft (1.8 m) of Lower Inferior Oolite remains, unexposed, above the scissum Beds.

A small quarry [ST 7464 9840] 1150 yd (1052 m) ENE from Drakestone Point exposes: Upper Trigonia Grit, cream-coloured abundantly fossiliferous bioclastic limestone, 1 ft (0.3 m) on rubbly iron-stained shelly limestone, 6 in (15 cm), very fossiliferous orange-coloured shelly limestone with a hardened top 1 ft 4 in (0.4 m), resting on a bored surface of lower Inferior Oolite.

Yellow sandy limestone of the scissum Beds was revealed in a hill-slope exposure [ST 7440 9775] 860 yd (786 m) ESE of Drakestone Point.

In a quarry [ST 7380 9875] 900 yd (823 m) E of St Cyr's Church, Stinchcombe, the following Lower Inferior Oolite was exposed: cream-coloured, flaggy, cross-bedded, medium-grained oolite, 8 in (20 cm), on a layer varying from 0.5 in (13 mm) of pebbles to a more continuous oolitic limestone up to 4 in (10 cm) thick, but totally absent in places (=Pea Grit Horizon of Richardson), thickly bedded, cream-coloured, medium-grained oolite with small amounts of shell detritus, 5 ft 4 in (1.6 m), rubbly, iron-stained soft oolite, 8 in (20 cm), over hard, medium-grained, pale cream-coloured oolite, with a level top surface and with brown irregular lumps up to 3 mm across, 4 ft (1.2 m) seen. A similar Lower Inferior Oolite succession is exposed in the quarry [ST 7427 9883] 1430 yd (1308 m) E of Stinchcombe Church: flaggy, cross-bedded, cream-coloured oolite of medium-grain size, 8 ft (2.4 m), on rubbly iron-stained coarse oolite with pebbles of oolite and coral fragments (=?Pea Grit Horizon of Richardson) 8 in (20 cm), over massive medium-grained cream-coloured oolite 3 ft (0.9 m) seen.

Some 100 yd (91 m) to the SE [ST 7421 9878] lower beds are exposed in another old quarry: flaggy cross-bedded oolitic limestone 15 ft (4.6 m), on scissum Beds; massive, yellow, fine- to medium-grained, sandy limestone, 10 ft (3 m).

About 8 ft (2.4 m) of yellow calcareous sandstone of the scissum Beds are also exposed [ST 7423 9882] in a track lower on the slope between the latter two localities and the total thickness of the scissum Beds here is estimated at between 15 and 18 ft (4.6 and 5.5 m).

A quarry [ST 7923 9704] 500 yd (457 m) W27°S of Lampern House afforded the following details of the Lower Inferior Oolite: cream-coloured, medium-grained flaggy and well-jointed oolitic limestone with some shell detritus, 4 ft (1.2 m), on orange-coloured medium-grained massive oolitic limestone, 11 ft (3.4 m), over very pale, cream-coloured, fine-grained, oolitic limestone much jointed and cross-bedded, 15 ft (4.6 m). An old quarry [ST 7946 9742] 270 yd (247 m) NW of Lampern House exposed in the upper part of the Lower Inferior Oolite 18 ft (5.5 m) of white oolites.

About 100 yd (91 m) to the N, the top surface of the Lower Inferior Oolite, which can be seen in a small quarry, is extensively bored and overlain by at least 4 ft (1.2 m) of Upper Trigonia Grit. Similarly, 4 ft (1.2 m) of Upper Trigonia Grit overlie Lower Inferior Oolite at the side of the track [ST 7979 9743] 260 yd (238 m) NE of Lampern House.

A large quarry [ST 7961 9740] 150 yd (137 m) N of Lampern House shows almost 60 ft (18.3 m) of pale cream-coloured, fine-grained, well sorted, oolitic limestone, in places cross-bedded. There is little variation throughout and no evidence of the Pea Grit Equivalent of Richardson was seen. I.B.P., R.C.

At Owlpen, the valley ends abruptly eastwards against the steep scarp rising to Owlpen House [ST 8085 9815]. The Lower Inferior Oolite in this scarp is estimated to be about 75 ft (23 m) thick and measurements nearby on another slope [ST 801 977] suggest a greater thickness, nearer 90 ft (27 m).

An exposure [ST 8066 9838], 320 yd (293 m) NW of Owlpen House, in the side of the bridle path, near the top of the Lower Inferior Oolite reveals: brown, rubbly marl 1.5 ft (0.5 m), on pale grey to white soft marly oolitic limestone with Eryma bedelta seen 6 ft (1.8 m). The latter limestone has an appearance similar to Oolite Marl, but no diagnostic fossils were found. It seems possible that this could represent an appearance of the 'Pea Grit' beneath the overstepping Upper Trigonia Grit. If this be so then it is unlikely that Richardson's Pea Grit Equivalent in the Stinchcombe area truly represents that horizon. It is estimated that there are some 50 ft (15 m) or more of oolites and limestones below these beds at Owlpen. About 110 yd (101 m) downslope to the WNW an exposure on the slope revealed 3 ft (0.9 m) of brown, hard, sandy limestone of the scissum Beds, resting on Upper Lias Cephalopod Bed.

Owlpen to Nympsfield

Nothing which might be considered to be the Pea Grit Equivalent as recognised by Richardson was found in this area. R.C.

On Uley Bury the thickness of the Inferior Oolite is estimated at about 85 ft (26 m). A series of shallow workings here [ST 789 989], 100 yd (91 m) S of Crawley exposed yellow sandy limestone of the scissum Beds. Similar beds were exposed at the side of a track [ST 7953 9997], 600 yd (549 m) N25°W of Dingle Farm, also on Cam Long Down [ST 7710 9945] and to a thickness of 3 ft (0.9 m) in an old quarry [ST 7772 9852] on Downham Hill. Also on Cam Long Down a quarry [ST 7558 9945] revealed 10 ft (3 m) of unevenly bedded cream-coloured oolitic limestone of fine grain in the lower part, but coarser upwards. Similar limestone is preserved as a cap to Downham Hill and 5 ft (1.5 m) are exposed in an old quarry [ST 7753 9840].

A large quarry [ST 7867 9949] in Coaley Wood, and 600 yd (549 m) WNW of Crawley shows about 55 ft (17 m) of pale cream massive oolitic limestone in a largely inaccessible quarry face. The limestone is not readily subdivided, yet Richardson (1910, p. 115) did indicate that the lowest 20 to 25 ft (6.1–7.6 m) are to be regarded as Lower Limestone, referring an overlying 6-in (15-cm) bed of rubble and marl to the Pea Grit "horizon". The Upper Trigonia Grit, composed of 5 ft (1.5 m) of orange-coloured very shelly limestone, rests on this Lower Inferior Oolite which has the characteristic bored and flat top surface.

The large quarry at Frocester Hill, 0.5 mile (805 m) W of Nympsfield exposes much of the Lower Inferior Oolite in two portions. The upper parts can be seen [SO 7945 0070] as follows:

A Pea Grit "horizon" could not be recognised within this sequence although the base of the section was estimated to be some 46 ft (14 m) below the Upper Trigonia Grit.

At the north end of the quarry [SO 7941 0079] the scissum Beds were exposed as follows:

A very good exposure [SO 7938 0112] of the scissum Beds occurs 360 yd (329 m) N of the previous section. Some 18 ft (5.5 m) of cream to orange-coloured oolitic limestone with much shell detritus overlie the scissum Beds, which consist of:

The gaps in the section might represent softer layers of calcareous sand which have weathered away.

Kingscote to Nailsworth

A bank [ST 8326 9716] 1010 yd (924 m) NNE of Hazlecote Farm exposes the basal beds of the Inferior Oolite resting on Lias. Some 3 to 4 ft (0.9–1.2 m) of flaggy, clean, shell-detrital oolite rest on the scissum Beds which are brown, hard, compact, silty limestone with Sarcinella socialis, Montlivaltia sp.,Kallirhynchia sp., Camptonectes?, Entolium sp., Inoperna plicata, Isognomon?, Liostrea, Modiolus imbricatus, Myophorella sp., Neocrassina sp.,Plagiostoma sp., Pleuromya?, Pronoella?, Propeamussium cf. laeviradiatum, Pseudopecten sp., Pteroperna plana, about 1 ft 4 in (0.4 m). Upper Lias Cephalopod Bed; brown, hard, compact limestone with ochreous patches and brown pellets, top surface flat and bored 1 in (3 cm). Brown rubbly clay, with S. socialis, Gervillella sp., Gresslya abducta, Inoperna plicata, Liostrea sp.,Myophorella sp., Neocrassina sp., Plagiostoma sp.,Propeamussium sp., Pseudogrammoceras sp.,probably of the P. fallaciosum Subzone, 3 to 6 in (8–15 cm). Bed of grey, hard, silty limestone divided in middle by puckered layer 0 to 1.5 in (0–4 cm) thick of brown clay, with Homoeorhynchia cynocephala, Entolium corneolum, of the ?G. thouarsense Zone, about 10 in (25 cm). Cotteswold Sands, brown sand seen 10 ft (3 m). A loose specimen of Grammoceras audax was found, suggesting that the G. striatulum Subzone is present, presumably in beds 1 or 2.

Around Horsley are a number of old quarries which reveal a rather manly and sometimes pisolitic development of limestone within the Lower Inferior Oolite. This is probably the southward extension of the Pea Grit. One such exposure occurs [ST 8388 9790] 170 yd (155 m) SE of St Martins Church, Horsley:?Head; rock rubble 3 ft (0.9 m), on?Pea Grit; nodular, rubbly, manly, fawn and brown, oolitic limestone with pellets or pisoliths, 3 ft (0.9 m), clay parting 1 in (3 cm), slabby, yellow to fawn chalky oolite, 4 ft (1.2 m), more massive oolite with echinoid fragments, 3 ft (0.9 m), irregular layer of brown manly clay with large brown ooliths 3 to 6 in (8–15 cm), rather soft brownish blocky oolite 1 ft 2 in (0.4 m), brown clay 1 in (3 cm), over?Lower Limestone; very massive white oolite with very smooth flat bored top, seen 7 ft (2.1 m).

A similar set of beds was seen in the quarry [ST 8414 9824] 450 yd (411 m) NE of St Martins Church, Horsley:?Pea Grit; fawn-buff rubbly, oolite with ferruginous pellets or occasionally pisoliths, parts very shelly, 7 ft (2.1 m), on brownish rather rubbly oolite and cream-coloured, compact fine-grained limestone (chalky), 11 in (28 cm), brown rubbly marl 2 to 5 in (5–13 cm), brownish marly oolite about 1 ft 2 in (0.4 m), ?marl 2 in (5 cm), ?Lower Limestone; shelly and shelly oolitic limestone, often less coarse than the beds below and more fissile 1 ft 8 in (0.5 m), brownish, hard, coarsely shell oolitic limestone with brown ochreous pellets, patches or pebbles of oolite near base which is uneven, 1 ft (0.3 m), oolite rather massive, pale, with ochreous flecks and pellets; finely cross-bedded, 5 ft (1.5 m); brown clay, 0.5 in (12 mm), oolite, rather fissile, cream-coloured, with ochreous specks or pellets 5 ft 2 in (1.6 m), fawn to brown oolite, massive, with ochreous specks, seen, 7 ft 2 in (2.2 m).

Further evidence for Pea Grit hereabouts was found in the 10 ft (3 m) bank [ST 8391 9826], 300 yd (274 m) NE of St Martins Church, Horsley, which exposed fawn, hard, compact, pisolitic limestone high in the Lower Inferior Oolite. This marly pisolitic facies in the Lower Inferior Oolite around Horsley could be developing northwards beneath the overstepping Upper Inferior Oolite.

In this same area a number of small exposures give additional evidence that there is a thin development only of sandy limestones (scissum Beds) at the base of the Lower Inferior Oolite, for example

1. [ST 8379 9780], 250 yd (229 m) S of Horsley Church:?Lower Limestone; shelly, oolitic limestone and oolite, 6 ft (1.8 m), on scissum Beds; brown, hard, sandy limestone with flat top, 1 ft 1 in (0.33 m), over?Upper Lias; rubbly and clayey grey silt, 1 ft 2 in (0.36 m), passing to brown sandy rock.
2. [ST 8424 9819] 530 yd (485 m) ENE from Horsley Church:?Lower Limestone; broken, slabby, shelly limestone, 1.5 ft (0.5 m), broken shelly oolitic limestone and shelly oolite, about 1 ft 4 in (0.4 m), scissum Beds; brown hard, sandy limestone with a few shells, 8 in (20 cm), over Upper Lias, ?Cephalopod Bed; patchy grey, micaceous, fine-grained limestone with pockets of small linseed-like pellets, 3 in (8 cm), Cotteswold Sands; brown sand seen 2 ft (0.6 m).
3. [ST 8413 9827], 490 yd (448 m) NE of Horsley Church:?Lower Limestone; very fissile fawn-buff oolite and shelly oolite with brown specks, 6 ft (1.8 m), on hard shelly oolitic limestone, rather sandy and with ochreous pellets, 4.5 ft (1.4 m), scissum Beds ; brown, very hard, sandy limestone, about 1 ft 8 in (0.5 m), over Upper Lias, Cephalopod Bed; khaki, argillaceous limestone with dark brown pellets, about 6 in (15 cm). Cotteswold Sands; sand seen.
4. [ST 8419 9830], 530 yd (485 m) NE of Horsley Church:?Lower Limestone; broken shelly oolite, 2 ft (0.6 m), on scissum Beds; buff to brown, hard, rather broken, silty to sandy limestone containing lamellibranch remains and brown specks, top surface flat, 1 ft 8 in (0.5 m), over Upper Lias, Cephalopod Bed; rubbly marl with nodules of grey silty limestone and ferruginous hard lumps containing dark brown linseed-like pellets, ammonites including Grammoceras sp. and belemnites, 1 ft (0.3 m), ?Cotteswold Sands; grey and brown hard silty limestone with pebbles of similar limestone, irregular base with penetrations into sand below, 6 in (15 cm), Cotteswold Sands; sand, seen 3 ft (0.9 m).

Further north the manly pisolitic development in the Lower Inferior Oolite is seen again in the old quarry [ST 8451 9883] 980 yd (896 m) E of Wallow Green and once more it rests on a conspicuous flat and bored surface of oolitic freestone, taken to be the Lower Limestone. The section is:?Pea Grit; cream-coloured to fawn rather broken oolite seen about 6 ft (1.8 m), on ochreous, marly, rubbly limestone and shelly oolite with flat pisoliths, 2 ft 8 in (0.8 m), over Lower Limestone; white oolitic freestone with flat bored surface seen 2 ft (0.6 m).

In this area no evidence for the Upper Freestone or Oolite Marl has been seen and there are many adits which give good exposure of the top few feet of the Lower Inferior Oolite which have been a source of building stone. These mine adits occur at, north and east of Rockness Hill [ST 844 989] and the sought-after stone in these mines was usually Lower Freestone. The absence of mines south and west of Rockness Hill thus gives good indication that the Lower Freestone does not occur there and no oolites which can be undoubtedly attributed to the Lower Freestone have been observed in those directions. The roof bed to most adits is the Upper Trigonia Grit and in consequence this formation is fairly well exposed in the area and is about 6 ft (1.8 m) thick.

The old quarry and adit [ST 8474 9860] 200 yd (183 m) SSE of Harleywood exposed: White Oolite Beds; cream-coloured broken oolite seen 3 ft (0.9 m), on buff rubbly limestone with scattered, brown?ooliths, very rubbly layer, with terebratuloids 1 ft (0.3 m) above base, 5 ft 5 in (1.7 m), rubbly marl 3 in (8 cm), oolite, 1 ft 0.5 in (0.3 m), Clypeus Grit Beds; rubbly, lumpy limestone with terebratuloids, 1.5 ft (0.5 m), buff to brown, compact limestone with brown specks, 2 ft 7 in (0.8 m), brown to buff, hard, knobbly, compact limestone with brown specks and rhynchonelloids, 2 ft 1 in (0.6 m), Upper Trigonia Grit; pale greyish brown, hard shelly and rubbly limestone in layers 9 in to 2.5 ft (23 cm-0.8 m) thick, 6 ft 1 in (1.9 m), over Lower Freestone; white oolite with flat deeply bored surface, seen.

Another old quarry and adit [ST 8457 9920], 300 yd (274 m) NW of Milbottom showed : White Oolite Beds; white oolite seen 5 ft (1.5 m) on Clypeus Grit Beds; buff, hard, lumpy limestone with brown speckles about 7 ft (2.1 m), Upper Trigonia Grit, ragged layers of coarse shelly limestone 6 ft (1.8 m), Lower Freestone; white oolite with flat oyster-covered surface and narrow vertical borings penetrating to a depth of 8 in (20 cm), seen.

Nailsworth to Nympsfield

A number of small quarries exist in this area particularly in the Woodchester Valley which straddles the northern limit of the map. These show the Upper Trigonia Grit to be 6 to 6.5 ft (1.8–2 m) thick and also indicate that the sandy development at the base of the Inferior Oolite is thicker than near Horsley.

An old quarry [ST 8397 9976], at Newmarket, partly affected by landslip, revealed :

A small quarry [ST 8323 9975] 1040 yd (951 m) ESE of Tinkley Farm showed: Clypeus Grit Beds; buff to brown hard rather rubbly limestone, seen 4 ft (1.2 m), on Upper Trigonia Grit; rubbly hard shelly limestone, 6 ft 6 in (2 m), over?Lower Freestone; white oolite freestone with flat bored top surface, seen 5 ft (1.5 m). A further quarry [SO 8411 0005] 620 yd (567 m) SSE of Windsoredge, exposes similar beds, displaced by differential superficial movements along joints: Clypeus Grit Beds; brown rubbly limestone, seen 3 ft (0.9 m), on Upper Trigonia Grit; raggedly bedded shelly limestone, 6 ft (1.8 m), over Lower Freestone; white oolite freestone with a flat bored top surface, seen 25 ft (7.6 m).

Much lower beds are seen in an old excavation, [SO 8277 0081] 710 yd (649 m) NNE of Tinkley Farm:?Pea Grit; broken rubbly oolite containing brown pellets or pisoliths with shell fragments at their centres, seen 6 ft (1.8 m), on cream-coloured oolite with brown pisoliths, 3 ft 6 in (1.1 m), over?Lower Limestone; white oolite, seen, 6 ft (1.8 m).

A rather greater thickness of Upper Trigonia Grit appears near the northern limit of the district in an old quarry [SO 8319 0110] 1290 yd (1180 m) NE of Tinkley Farm: Clypeus Grit Beds; brown blocky limestone, seen 6 ft (1.8 m), on Upper Trigonia Grit; raggedly bedded shelly limestone, 7 ft 4 in (2.2 m), over Lower Freestone; white oolite freestone with a flat, very bored top surface, seen 15 ft (4–6 m). This quarry also displays evidence of superficial movements along joints.

Beds probably representing the Pea Grit were seen in the old quarry [SO 8030 0153] 1420 yd (1298 m) N of Nympsfield Church:

Nailsworth to Avening

The valley between Nailsworth and Avening is deep, penetrating into the Lias, so that the complete thickness of the Inferior Oolite crops out in its sides. Again the Lower Freestone has been mined to provide building stone, so that exposure of the Upper Trigonia Grit and the top beds of the Lower Inferior Oolite is good. Noteworthy is the first appearance of the Oolite Marl/Upper Freestone. Also well exposed was the Pea Grit.

An old mine adit occurs [ST 8503 9900] just to the east of Tetbury Lane, 260 yd (238 m) E of Milbottom giving a section which is repeated in greater detail 60 yd (55 m) to the SE: Clypeus Grit Beds; brown, rubbly, nodular, limestone with terebratulids, seen 4 ft (1.2 m), on Upper Trigonia Grit; brown limestone with trigoniids and other large bivalves and rhynchonelloids in the bottom 1 ft 6 in (0.5 m) of the bed, 5 ft 3 in (1.6 m), very thin marl parting, over Lower Freestone; white oolite freestone, mined and with flat deeply bored surface, seen 10 ft (3 m). A temporary section [ST 8688 9908] 570 yd (521 m) SE of Balls Green was exposed during road-widening. The following section was measured in the bank above the road.

Other material obtained from various levels in the Pea Grit includes: Aulacothyris cf. mandelslohi, Kallirhynchia?, Plectoidothyris polyplecta, Rhynchonelloidea sp., Stroudithyris aff. pisolithica, Eopecten tuberculosus, 'Lucina' cf. bellona, Plagiostoma cf. channoni, P. cf. crickleyensis, Pronoella cf. beneckei, Pseudolimea cf. interstincta, Tancredia sp., Ludwigia sp.

Higher beds are exposed in a small quarry and adit [ST 8738 9880] 1030 yd (942 m) N50°E of Brandhouse Farm.

On the north side of the valley there are many more exposures, again mainly in the higher beds because of past mining activity. However, scars on the hillslope [ST 8759 9962] 1160 yd (1061 m) W of Balls Green, probably resulting from old instability in the valley side, exposes the lower beds of the Lower Inferior Oolite:?Pea Grit oolite with irregular fracture and with layers of coarsely pisolitic limestone which is brown and manly, especially in basal 3 ft (0.9 m), seen about 16 ft (4.9 m), on?Lower Limestone; oolite freestone with friable, weathered surfaces, coarsely shelly, especially in lower parts, top surface sharp and flat, seen 22 ft (6.7 m). Beneath this limestone the beds are obscured for about 20 ft (61 m), but a few yards to the west, near the roadside, sandy limestone ( Tscissum Beds) is exposed.

Some 100 yd (91 m) ENE from here old quarries [ST 8566 9966] reveal: Clypeus Grit Beds; buff to pink, hard, rather broken limestone, compact, non-oolitic at least 6 ft (1.8 m), on Upper Trigonia Grit; buff rubbly shelly limestone, with Acanthothiris spinosa, Stiphrothyris tumida and oysters, a large bivalve at the very base 6 ft 6 in (2 m), over?Lower Freestone; massive oolite freestone, top surface uneven and in places very bored, a well-marked parting 3 ft (0.9 m) from top reveals a very flat smooth surface, seen 13 ft (4 m).

Similar Upper Trigonia Grit and?Lower Freestone are exposed in the mine entrance [ST 8593 9961] 280 yd (256 m) to the east, and here the top 6 in (15 cm) of the mined freestone is hardened and brown.

A further similar exposure occurs [ST 8645 9958] in and near a mine entrance 390 yd (357 m), S29°E of Box House, Minchinhampton and 200 yd (183 m) W13°N of Balls Green :

It is to be noted that the Oolite Marl here has yielded additionally Curtirhynchia cf. oolitica, Plectothyris fimbria [subfimbriate], Globirhynchia buckmani and Stroudithyris aff. pisolithica which are more commonly associated with the Pea Grit and also Globirhynchia subobsoleta, Kallirhynchia aff. acutaplicata, K. aff. distendens, Lophrothyris sp. [juv.], P. fimbria.

At Balls Green [ST 8685 9954] about 8 ft (2.4 m) of Oolite Marl/Upper Freestone is exposed in an old mine entrance: fine-grained oolite with irregular fracture, about 2 ft (0.6 m), on yellow and brown marl with rubbly white limestone and fimbriate terebratuloids 9 in (23 cm), cream-coloured, very rubbly marl and limestone, oolitic with a fine-grained matrix, 5 ft 8 in (1.7 m), over Lower Freestone; massive cream-white oolite freestone in beds up to 3 ft (0.9 m) thick, top surface very flat, seen 13 ft (4 m). R.C.

Chapter 9 Jurassic: Great Oolite "Series"

Introduction

The Great Oolite "Series" comprises the Fuller's Earth, Great Oolite, Forest Marble and the Cornbrash. During the mapping it was not possible to separate the two parts of the Cornbrash, and for convenience the Lower Cornbrash is described with the Upper Cornbrash in the next chapter. All these formations are represented in the district and each comprises usually more than one well-marked cycle of sedimentation. Every cycle is represented in the north-east by a 'shelf' limestone or non-sequence. They pass south and west into oolite, then to fine-grained argillaceous limestones interbedded with clay before finally passing into calcareous clay of the basin area.

The position of the lateral passage between corresponding lithological facies of successive cycles shifts southwards as the limestones/oolites progressively invade the clay 'basin' area. In the Malmesbury district the Lower Fuller's Earth is wholly of the clay phase. In the Upper Fuller's Earth the transition from oolite to argillaceous limestone and clay occurs mainly to the south-west of centre, whilst in subsequent cycles of deposition, those of the Great Oolite and above, oolites pass to clay well south of the district.

Within each cycle a number of minor repetitive rhythms is evident, while the end of a cycle is usually marked by a depositional pause and represented by indurated or porcellanous limestones with top surfaces which are sharp, flat, bored and oyster-encrusted. In the clay 'basin' facies it seems that these cycles are also often well marked, but there they are represented at base by thin, though persistent, non-oolitic limestones of the smithi Limestone type (Green and Donovan 1969).

The oolitic facies of different cycles are not easily differentiated out of context. The shell-detrital oolites are of similar appearance wherever they occur and in any case they are variable within each cycle. With the exception of the ammonites their macrofaunas are equally unhelpful, for they appear to be facies controlled and since the Great Oolite "Series" was deposited in a comparatively short period they are all very similar, whether they be from Fuller's Earth, Great Oolite or Forest Marble oolites.

The Great Oolite "Series" corresponds approximately to the Bathonian Stage the foundations of whose zonal stratigraphy were laid by Arkell (1951–9). More material has been discovered since then and the north-west European Bathonian ammonite sequence established with increased certainty largely as a result of the work by Westermann (1958) and Hahn (1968) in Germany, a number of workers in France whose results are summarised (Mouterde and others 1971) and in Britain by Torrens (1965, 1974). Controversy only remains about the limits, status and nomenclature of the various zones and subzones.

In the Malmesbury area few ammonites have been found but by considering adjacent ground it is possible to assign most of the mapped units to their approximate zonal position (Table 4).

Fuller's Earth

History of research

Amongst the earliest stratigraphical works that deal with the Fuller's Earth of the Malmesbury district are those by Ramsay, Aveline and Hull (1858), J. Buckman (1858) and Woodward (1894). An early account of the palaeontology of the oolites of Upper Fuller's Earth equivalence at Minchinhampton was given by Morris and Lycett (1851–5) and supplemented by Lycett (1863). These works, together with the revision by Cox and Arkell (1948–50) also give valuable stratigraphical data.

Lycett (1857) and Witchell (1882b, 1886) described the geology of various northern parts of the district and it is interesting to note that Witchell (1868) in his paper 'On the denudation of the Cotteswolds' appreciated the significance and mechanism of landslip, so prevalent in the steep valley sides.

A new approach to the stratigraphical problems was made by S. S. Buckman (1901) who concluded from the fossil evidence that the Fuller's Earth Rock of Somerset was included in the base of the Minchinhampton Beds. Arkell (1933) dissented from this view but it was substantiated later (Arkell and Donovan 1952), and is upheld in this account.

Between 1914 and 1921 Richardson provided detailed descriptions of many boreholes in the district and these provide data about the Great Oolite "Series" in areas where it is otherwise mostly obscured. The work of Arkell and Donovan (1952), previously mentioned, is the most recent comprehensive account of the Fuller's Earth of the district. Later accounts of specific sections include those by Torrens (1968) on the M4 Motorway cutting at Tormarton and Curtis (in preparation) on the Gas Council Trench at Dyrham.

General account

The upper and lower limits of the Fuller's Earth were formulated by William Smith in 1799, while working in the area around Bath. He distinguished the beds between the Inferior Oolite and Great Oolite limestones, as Fuller's Earth, for it was from this mainly argillaceous formation that fulling clays were, and still are, obtained.

North of Bath on about the latitude of the southern margin of the Malmesbury district, the thick clays which constitute the Fuller's Earth to the south, show signs of radical changes in lithology. The main changes affect the Upper Fuller's Earth which, northwards across the district, develops into a dominantly limestone formation. This change is accompanied by a great overall increase in the thickness of the Fuller's Earth (Figure 12), so that it is some 100 ft (30 m) thick in the south, while in the north it approaches 200 ft (61 m) thick. No commercial fuller's earth has been found at the top of the Upper Fuller's Earth within the district. Its distribution at this horizon is centred between Bath and the eastern end of the Mendips and in an initial study of it R. J. Merriman^‡8 , has revealed glass shards and silt grade feldspars indicative of a trachytic volcanic origin. The location of the volcanism responsible is not known but a possible position would be in the Western Approaches, or alternatively the North Sea. Although of poor grade montmorillonitic clays occur also in the Lower Fuller's Earth at the north of the district and the origin of these too might be partially volcanic.

For descriptive purposes the Fuller's Earth is divisible into:

2. Upper Fuller's Earth—clays with some limestone in the south, and limestone with some clay in the north.

1. Lower Fuller's Earth

b. Cross Hands Rock—limestone

a. Lower Fuller's Earth Clay.

Lower Fuller's Earth

Lower Fuller's Earth Clay

Where it enters the district north of Cross Hands, the thickness of the Lower Fuller's Earth Clay is estimated to be only 30 ft (9 m). Northwards and north-eastwards it thickens so that at Hawkesbury it is about 50 ft (15 m), at Sherston it is about 70 ft (21 m), while in the Shipton Moyne and Tetbury areas it is up to 90 ft (27 m) thick.

This clay is slightly silty with some laminae which are hard, silty and calcareous. The top 2 to 3 ft (0.6–0.9 m) are usually sparsely fossiliferous, but in beds below, some 6ft (1.8 m) thick in the south and 15 ft (4.5 m) in the north, shells of Praeexogyra acuminata are so abundant as to form a 'marker band' at times called "acuminata"; Beds. In places the shells are cemented into an oyster limestone up to 2 ft (0.6 m) thick and parts of this have been mapped as "acuminata" Limestone. The rest of the fauna of the Lower Fuller's Earth Clay includes various species of Kallirhynchia, Rhynchonelloidella, Wattonithyris and ?Procerites, while Bositra buchi occurs characteristically at the very top and near the middle of the formation and Catinula knorri at the base.

Associated with the horizons at the top of the Lower Fuller's Earth Clay rich in P. acuminata is an abundance of dark blue ferruginous pisoliths. They have at their centres large chips of shell and are thus commonly discoidal or of irregular rounded shapes. Such pisoliths are abundant also in the Cross Hands Rock in places and the dark pellicle may be a growth of algal origin.

The Lower Fuller's Earth Clay has been the cause of much landslipping in the steep valleys around Kilcott and Ozleworth, and where there is overburden of Upper Fuller's Earth limestones and Forest Marble these formations have also been involved in the slips. The weakness of the clay may partially result from water-seepage and lubrication along thin silty limestone laminae.

A formation mainly of thin-bedded, sandy, stinking limestones in the Gloucester (Sheet 234) district at the top of the Lower Fuller's Earth Clay, and subjacent to the group of limestones which dominate the Upper Fuller's Earth is named the Througham Tilestones. This formation is seen in the Malmesbury district only at Cherington. Near the bottom of a steep, mainly limestone, valley side it underlies limestone of a Cross Hands Rock aspect and overlies clay forming the valley bottom.

Cross Hands Rock

The Cross Hands Rock of the Malmesbury district and its partner of slightly higher horizon further south, the Fuller's Earth Rock, form a group of limestones which divide the Fuller's Earth of Bath, into upper and lower portions. Like the Lower Fuller's Earth Clay, this median limestone partition is very persistent across the district, so the Cross Hands Rock and the Lower Fuller's Earth Clay are here considered together. Depositionally the Cross Hands Rock indeed may represent the top of a Lower Fuller's Earth cycle of sedimentation and the hardgrounds in what has been mapped as Cross Hands Rock in the north of the district could be construed as evidence. The occurrence of Morrisiceras morrisi within this Cross Hands Rock (p. 162), however, raises the possibility that part of it includes horizons which elsewhere have been found within the Fuller's Earth Rock.

However, in the south of the district at least, the Fuller's Earth Rock is not developed as a limestone. Its horizons at Cross Hands are present within a clay fades containing ornithellids, apparently at the commencement of a new cycle of sedimentation extending up into the Upper Fuller's Earth.

Where it enters the district, just north of Cross Hands (Sheet 265), the Cross Hands Rock is not strongly developed. Its position is marked on the clay scarp by yellow, rubbly marl and patches of harder limestone rubble. A small feature is apparent which swells in places to form a local shelf on the scarp. Even where the feature is negligible the horizon has a marked effect on the colour of growing graminaceous crops, for instance, young growing corn is more lush and of a darker green colour than on the rest of the scarp. There is little or no limestone in this part of the area representative of the Fuller's Earth Rock of Bath, those horizons are represented in the basal Hawkesbury Clay.

Near Horton the Cross Hands Rock becomes more prominent and produces a marked step in the Fuller's Earth, mainly clay scarp. This feature becomes stronger northward and around Clay Hill, Hawkesbury Upton, it produces a very distinct plateau. Its thickness there is difficult to assess, as pointed out by Arkell and Donovan (1952, p. 235), but 15 to 20 ft (5–6 m) might be rather excessive. Further north the outcrop passes into an area of steep valleys and landslip. There the Cross Hands Rock has been detected only as loose limestone blocks in the landslip, and on the rare unslipped spurs where its topographical feature and brash are quite obvious.

The outcrop emerges from the area of landslip near Kingscote and there its thickness has increased to possibly more than 16 ft (4.9 m). Around and to the north of Kingscote, erosion has removed much of the Forest Marble and Upper Fuller's Earth limestones, so that the lower parts of the Fuller's Earth crop out on the plateau. Under these circumstances the effects of landslip are absent and the Cross Hands Rock outcrop is well displayed.

To this point the lithology of the Cross Hands Rock has not changed drastically. At Cross Hands and Grickstone, in the weathered state, it is pale fawn, compact fairly fine-grained limestone in which there are pale yellow-weathered soft limonitic pellets and a little dark ferruginous finely-fragmented shell debris. There are also small pieces of coral and chips of echinoids. Northwards from there the Rock becomes a little harder and more compact until near Kingscote much of it is almost porcellanous. Where the top surface is displayed it is flat, oyster-encrusted and bored. Parts are here charged with large dark brown ferruginous pellets or ooliths. Further north the changes are more marked. South-east of Nympsfield another bored surface occurs 5 ft (1.5 m) below the top one. These bored surfaces and the associated porcellanous limestone below each may prove to represent widespread depositional pauses of importance. Also in this area layers of oolite occur within the Cross Hands Rock. North-east of Nympsfield this oolite dominates the lower part of the Cross Hands Rock so that it was possible to map a binary division. The lower part is a white to cream-coloured shell-detrital oolite containing oysters of the P. acuminata type, while the upper part remains largely compact, almost porcellanous, fawn limestone with ferruginous flecks and pellets, though oolitic material is present as well. Some parts of the Cross Hands Rock contain large ferruginous pisoliths, ferruginised small gastropods and shell fragments. The total thickness around Park Farm, Nympsfield, is estimated at some 25 ft (8 m). It seems that here there is some extension of the limestone downwards, to a position which further south is occupied by clay. It is possible that this could be approximately the position of the Taynton Stone of Oxfordshire, but here no sandy tilestones are developed below to equate with the Stonesfield Slate.

Further to the north-east, the Hawkesbury Clay, which separates the Cross Hands Rock from the overlying oolitic limestones, itself develops into a limestone and once east of the Nailsworth–Stroud valley, at Pinfarthing, only argillaceous limestone and marl occur above limestones of a Cross Hands Rock type in the top of the slipscar and below the main oolite limestones mass of the Upper Fuller's Earth. The Cross Hands Rock has thus virtually become part of the limestones sequence of the Upper Fuller's Earth and in this condition it passes into the southern part of the Gloucester district.

Away from the outcrop, in the east of the district, the Cross Hands Rock has been proved in boreholes between Sherston and Tetbury. In these boreholes it is separated from the Upper Fuller's Earth limestones by clay—the Hawkesbury Clay. The thicknesses are about 10 ft (3.1 m) at Sherston, 11 to 12 ft (3.4–3.7 m) at Shipton Moyne (Richardson 1919) and 15 ft (4.6 m) at Tetbury. In a borehole at Milbourne, about 1 mile (1.6 km) E of Malmesbury, P. acuminata occurred in clay beneath the main limestones of the Fuller's Earth and it is probable that there, as in the extreme north-east of the district, the Cross Hands Rock is united with the limestones of the Upper Fuller's Earth and difficult to distinguish.

The unweathered nature of rock in boreholes presents the Cross Hands Rock in an aspect different from that at outcrop. It appears as a compact, fine-grained, argillaceous limestone with dark grey pellets. These pellets look like pisoliths or ooliths and consist of a thick rather ferruginous carbonate coating centred usually on small fragments of shell.

Upper Fuller's Earth

The rapidity and variety of the facies changes to which the Upper Fuller's Earth is subject make effective classification difficult (Figure 13). The simplest scheme, though not strictly suited to the rocks found in the extreme south, is:

• 2. Limestones sequence
• Coppice Limestone^‡9 
• Athelstan Oolite
• Tresham Rock
• 1. Hawkesbury Clay

Hawkesbury Clay

This clay first appears to the south of the Malmesbury district, in an area where it overlies the Fuller's Earth Rock. At Cross Hands on the southern margin of the district the Hawkesbury Clay does not overlie the Fuller's Earth Rock and the base of the clay has yielded ornithellids. These indicate the presence there of horizons which at Bath lie within the Fuller's Earth Rock. This formation consists usually of stiff clay; less silty than the clay of the Lower Fuller's Earth and rather smooth and sticky, and at the surface it is brown to khaki. In the south the thickness is about 25 ft (8 m), while at Hawkesbury Upton and Upper Kilcott it is some 35 ft (11 m), perhaps a little more. Near Kingscote the thickness diminishes to 28 ft (8.5 m) and around Nympsfield to 14 to 16 ft (4.3–4.9 m). North-eastwards and eastwards from this area the Hawkesbury Clay is progressively more marly and, at Pinfarthing, has passed into argillaceous limestone and marl partings.

East of the outcrop similar changes in lithology occur; in boreholes at Sherston, Shipton Moyne and Tetbury, the clay has a thickness of 23 ft (7 m), 20 ft (6.1 m) and 13.5 ft (4.1 m) respectively, but the Sherston Borehole core revealed that this thinning north-eastwards was accompanied by an influx of calcareous and oolitic matter, much as at outcrop eastwards from Kingscote. Thus the Hawkesbury Clay passes north-eastwards into limestone.

Limestones sequence

In the extreme south, between Cross Hands and Grickstone this sequence is neither strongly developed nor is it a unit. Thin vestigial clay beds separate individual limestone layers, but do not persist northwards, limestone finally replacing most of the Upper Fuller's Earth clay of the Bath and Pennsylvania areas (Lansdown Clay of Arkell and Donovan 1952). Only the upper Lansdown Clay extends any distance north-eastwards and a thin representative occurs above the Athelstan Oolite in the Sherston Borehole (Figure 27).

The Upper Fuller's Earth succession south of Grickstone is thus:

• Clay (upper Lansdown Clay)
• Athelstan Oolite
• Clay (lower Lansdown Clay)
• Tresham Rock
• Hawkesbury Clay

It is overlain by the Grickstone Beds of the Great Oolite and underlain by the Cross Hands Rock. North of Grickstone the succession is as on p. 137 and can conveniently be described in three parts; the south and north parts of the main scarp and areas east of the scarp.

Grickstone Farm to Boxwell

Tresham Rock

Just north of Grickstone Farm [ST 777 830] the Tresham Rock (Arkell and Donovan 1952), little more than 10 ft (3 m) thick, consists of a pale grey compact calcite siltstone or calcite mudstone containing incipient ooliths. There is a considerable content of brown argillaceous matter which becomes evident when the limestone is dissolved in acid. It is poorly fossiliferous yielding a few turbinate gastropods and lamellibranchs. Of these Nerinella sp. nov.which occurs near the top may prove the most significant.

Exposures of the rock are small and uncommon. The stone is unsuitable for building on account of its poor weathering qualities and has been little quarried.

As far north as Hawkesbury it creates a broad south-east dipping plateau overlooking the westward-facing clay scarp. This plateau is as much as mile (1.2 km) wide in parts with generally thin reddish-brown soil containing much brash of the underlying rock.

Locally in the south thin pockets or lenses of clay separate the Tresham Rock from the overlying Athelstan Oolite, and in places blister-like masses of coral occur on the top of the Tresham Rock.

On the south side of Hammouth Hill, 1 mile (1.6 km) S of Tresham, a layer of white oolite is developed within the Tresham Rock and northwards this thickens and joins with the base of the Athelstan Oolite. North of Tresham the Tresham Rock becomes increasingly more coarsely detrital, containing small fragments of shell together with oolitic material. No persistent pure oolite has been recorded at the base of the Tresham Rock so that it has not been possible to map the Hens' Cliff Oolite of Arkell and Donovan (1952, p. 236). Where the basal parts of the limestone have been seen, near Ozleworth, slabby shelly limestone, sparsely oolitic but containing claystone pebbles, occurs. A rather brown, shell-detrital, sparsely oolitic limestone is common above this, on the spur of high ground to the west of Ozleworth. The remaining succession there, probably representing part of the Athelstan Oolite too, contains three layers of oolite which have been traced northwards to Symonds' Hall Farm and mark the onset of a different facies.

Northwards from Grickstone the Tresham Rock increases in thickness, but this can be only approximated since there is no complete exposure of the formation, whilst north of Hawkesbury the outcrop is normally affected by camber above steep valley slopes. Near Upper Kilcott [ST 794 883] the total thickness of Athelstan Oolite and Tresham Rock is about 60 ft (18 m), whilst near Boxwell it is probably approaching 70 ft (21 m). Of these the Tresham Rock constitutes about half.

Athelstan Oolite

This oolite is usually white, of rather fine texture, with fine-grained well-sorted, commonly millet-seed ooliths. In many places it is not cross-bedded. Fragmented shell detritus occurs in bands or streaks but is not dominant. Weathering qualities are very poor, though in two quarries near Leighterton the oolite has been worked, presumably for wall-stones.

The size of the constituent ooliths is larger in the north than in the south, and the white colour and even grain size make the rock distinct from oolites of the overlying formations, or even from the Great Oolite in the south of the district. It is with the latter ('Bath Stone and Bath Lower Rags' of Arkell and Donovan 1952, p. 235) that the Athelstan Oolite has formerly been equated.

South of Starveall the Athelstan Oolite is overlain by Great Oolite and separated from it by a thin parting of marl and greyish green clay. Under these circumstances the top of the Athelstan Oolite is slightly oyster-encrusted and pitted by small mollusc borings. There is little or no hardening of the top. North of Starveall the Athelstan Oolite is commonly overlain by the hard and porcellanous Coppice Limestone and usually one appears to pass gradationally into the other. There are places however where a thin greyish green clay layer also separates these two formations and the top of the Athelstan Oolite again shows no recrystallization. There are yet other places, north of Starveall, where there is no Coppice Limestone and where Forest Marble directly overlies the Athelstan Oolite. The top of the Athelstan Oolite is then usually hardened and brown and its surface flat, bored and oyster-shell covered. The thickness of the Athelstan Oolite too increases northwards, from less than 10 ft (3 m) in the south, to some 30 ft (9 m) or more near Boxwell.

From Boxwell northwards

In general the Tresham Rock and Athelstan Oolite continue to show greater coarseness of detritus and oolite content northwards and, north and west of Ozleworth and Bagpath, their composition alters so much and is so variable both vertically and laterally that the two divisions lose definition. Combined they form a group some 60 to 70 ft (18–21 m) thick. The problem of applying a local name to this development has not been faced as it would have such a limited use. Between Ozleworth and Hunters' Hall Farm thick layers of oolite are interbedded with buff, compact limestones and brown detrital limestones. Locally between Horsley and Nympsfield the top 15 to 20 ft (4–6 m) consist of cream-coloured compact finely oolitic limestone beneath which are 40 to 50 ft (12–15 m) of rather fissile weathering, buff, sparsely oolitic limestones and shell-detrital oolitic limestones. It is in this area that the diagram of total Fuller's Earth thicknesses (Figure 12) shows local north-westward thinning and it may be that some fundamental influence on sedimentation here came from that quarter.

Further north and north-east around Nympsfield only the basal parts of the group crop out and these consist of flaggy, grey, brown and bluish, ill-sorted shell-detrital, slightly oolitic limestone.

At Minchinhampton the limestones are represented by thick shell-detrital oolites which have been exploited for building purposes and to a small extent still are (Lycett 1857, p. 90). It is impossible to be sure of the equivalents of the Athelstan Oolite and Tresham Rock within the body of these Minchinhampton shelly oolites, for change of facies has resulted in a radically different sequence, but they must be represented in their upper part. The total thickness of the limestones (i.e. between the Lower Fuller's Earth Clay and the Coppice Limestone) is claimed by Morris and Lycett (1851, p. 4) to be 60 ft (18 m), using measurements from well-sinkings. Their more general geological remarks (p. 1) giving a total thickness of 120 ft (37 m) to the different beds of the "Great Oolite" are rather suspect, for the account overlooks the landslip-disrupted valley sides and the cambered plateau edges. Their descriptive account of the oolites exposed at the time in quarries, together with that of Lycett (1857, pp. 93–7), is important, for there are no such exposures available today. However, the sequence they tabulated on the information obtained from these quarries is probably applicable only over a very small area. This is remarked upon by Lycett (1857, p. 93) and Woodward (1894, p. 278) and no doubt is the reason why precise correlation of beds across the space of Minchinhampton Common is difficult. A possible correlation would seem to be that the Athelstan Oolite and Tresham Rock are represented in the rocks above the "Scroff", this being a manly bed containing "Lima cardiiformis and Ostrea hebridica" (Cox and Arkell 1948, p. xvii; Channon 1950, p. 251). The "Scroff" would then represent approximately the position of the Hawkesbury Clay and the oolites below as seen in Crane Quarry; beds C and D of Lycett (1857, p. 93) would equate with the Cross Hands Rock, including subjacent oolite. In this respect the "Ostrea acuminata" recorded by Lycett (p. 95) would compare with the small acuminata oyster which occurs in the lower, oolite portion of the Cross Hands Rock at Park Farm, Nympsfield.

East of the Cotswold scarp

The remaining outcrops are restricted to a few valley sides. Exposures are not common, occurring mainly in inliers around Tetbury and near Kemble (Sheet 252), in the north-east.

At these places a limestone facies dominates the Upper Fuller's Earth, just as it does near Minchinhampton, but a number of boreholes between the Cotswold scarp and the eastern margin of the district help to clarify the facies changes that take place across the intervening ground. At Sherston 10 ft 7 in (3.23 m) of Athelstan Oolite directly overlie 41 ft 11.5 in (12.79 m) of Tresham Rock making a total of 52 ft 6.5 in (16.02 m). At Shipton Moyne the combined thickness is probably 75 ft 5 in (23 m), but near Malmesbury a borehole at Milbourne penetrated a combined thickness of about 50 ft (15 m). Further north at Veizey's Quarry, Tetbury, it has increased to about 70 ft (21 m)—comparable with the Shipton Moyne thickness. In the extreme north-east of the district the limestones crop out in the valley sides at Cherington and the total thickness is estimated at about 100 ft (30 m).

In these areas the limestones are normally rather thick-bedded, massive, compact and commonly very oolitic and the name Athelstan Oolite is applied to the whole. Their colour is medium grey when unweathered in borehole cores, but pale yellow otherwise. Their appearance here is thus similar to that of the White Limestone, the upper parts of which are the Oxfordshire equivalent of the Athelstan Oolite and Tresham Rock.

In places the top portion of the limestones is a white oolite almost millet-seed, and thus is more directly comparable with the Athelstan Oolite in the main outcrop. Such an oolite has been observed near Culkerton but there, oval or sub-circular lumps and 'pellets' of white porcellanous limestone are common in it near the top. These pellets may be algal or stromatoporoid in origin though some show no traces of an organic structure. Similar lumps occur in the Coppice Limestone, for instance east of Burden Court.

Coppice Limestone

The Coppice Limestone is a very distinctive, though thin formation which has been found only north of a line joining Hawkesbury and Tetbury. Despite this it crops out over large areas and is conspicuous by the abundant white, hard, angular lumps of limestone it produces in a reddish brown soil.

The formation overlies the oolitic limestones of the Upper Fuller's Earth and is considered conveniently as the topmost limestone of this sequence. It is, however, the one limestone not traceable into the Fuller's Earth clay sequence to the south. The type exposure is in a small quarry in a coppice 1 mile (1.6 km) SSE of Tresham (p. 173). The maximum observed thickness is 5 ft (1.5 m), but in places it is absent so that the Athelstan Oolite is the top limestone. Commonly it consists of a very hard fawn or cream-coloured, porcellanous calcite mudstone, usually with wide ramifying voids. It is a rock-type which occurs near Daglingworth, Gloucestershire and the name Dagham Stone taken from this locality (Richardson 1933, p. 48) would have made a good lithological term were it not for the loose usage made of it. Rocks only broadly comparable, and which recur a number of times in the succession of north-east Gloucestershire, have been given this name.

In places the Coppice Limestone is oolitic and less porcellanous, and it has been observed to pass down into pure oolites. In the type locality and elsewhere it is separated from the underlying oolite by a thin khaki clay. This, and the fact that it contains a fauna distinctively composed of bivalves and gastropods indicate primary origin; presumably it was deposited in a warm very shallow lagoonal area of the sea, representing the final stage of the depositional environment of the underlying oolites. In other places it contains small hard limestone and oolite pebbles, while other rounded pebble-like pellets seem to be organic, possibly algal or stromatoporoid growths. Such a development has been seen between Tresham and Leighterton and also north of Tetbury. The top surface is always very hard, flat, oyster-encrusted, and bored.

A fauna is difficult to extract on account of the hard splintery nature of the rock, but elements of this fauna suggest a correlation with the beds containing Aphanoptyxis bladonensis of the upper parts of the White Limestone of Oxfordshire (Arkell 1931, pp. 573–80).

The disappearance northwards of the Great Oolite at Starveall, not far south of the southern extremity of the Coppice Limestone, probably led Arkell and Donovan (1952, p. 236) to equate it with the basal Forest Marble 'Reef Bed', or Petty France Limestone. The two deposits are however lithologically and faunally distinguishable and quite separate. It is stressed however that the possibility of the Coppice Limestone being partly representative of a depositional phase which further south was in the form of a shell and oolite bank (i.e. the Great Oolite) cannot be overlooked, and its distinct separation from the underlying Athelstan Oolite in places may favour this view.

Zonal palaeontology

As shown previously (p. 113), the base of the Fuller's Earth most probably lies within the Oxycerites yeovilensis Subzone of the Zigzagiceras zigzag Zone. The succeeding beds contain, just south of Bath, the sole known British representative, Asphinctites recinctus of the A. tenuiplicatus Subzone (Torrens 1974, p. 585; Hahn 1968, p. 17) and it is possible that Procerites cf. subcongenor from the Tetbury No. 4 borehole (Arkell 1958, p. 197) came from this subzone.

The overlying Lower Fuller's Earth clays are poorly exposed and the boundary between the Lower and Middle Bathonian correspondingly uncertain. At Cross Hands, just to the south, Wagnericeras fortecostatum (Torrens 1969a, p. 73) and various proceritids (Torrens 1968, p. 431) from the M4 Motorway cutting near Tormarton, indicate that the acuminata Beds underlying the Cross Hands Rock belong to the Procerites progracilis Zone.

The Cross Hands Rock, as mapped on the Malmesbury sheet, has not yielded any ammonites but in the M4 cutting to the south it has yielded Tulites sp. (Torrens 1968, p. 431) indicating the presence of the Tulites subcontractus Zone while to the north, at Woodchester Park Farm (see p. 162 and Torrens 1969a, p. 70), Morrisiceras morrisi and Lycetticeras sp.,indicating the succeeding Morrisiceras morrisi Zone, have been found. Thus it is likely that the Cross Hands Rock, as mapped, includes correlatives of the younger Fuller's Earth Rock of the Bath area and, indeed, in limestone succeeding the Cross Hands Rock at Tormarton, Tulites sp. and Lycetticeras comma have been found (Torrens 1968, p. 430). This being so it is likely that the loose specimen of L. comma from Kilcott (Arkell 1954, p. 125) came from the Cross Hands Rock of the Malmesbury area.

The Hawkesbury Clay of the district has yielded no ammonites, but to the south it has been proved to lie within the 'Prohecticoceras retrocostatum' Zone (Torrens 1969a, p. 68).

The Tresham Rock lies consistently in the 'Prohecticoceras retrocostatum' Zone (Torrens 1969a, p. 68). Indeed the holotype of Bullatimorphites bullatimorphus which is characteristic of this zone comes from the Tresham Rock (but not from the locality in Arkell (1954, p. 110) see p. 175).

The Lansdown Clay, and its Athelstan Oolite equivalent lie in the 'retrocostatum' Zone since Choffatia homeomorpha and the holotype of Wagnericeras bathonicum have been found in equivalent beds to the south (Torrens 1969b, p. B22), but the status of the overlying beds, the Coppice Limestone in the north, the Great Oolite in the south, is not at present clear (Table 4).

The characteristic terebratulide of the Athelstan Oolite is Avonothyris sp. nov. A, while Avonothyris sp. nov. B occurs in a marl which in places overlies the Athelstan Oolite but underlies the Coppice Limestone. The latter species occurs again commonly in the succeeding basal Forest Marble. Similarly the gastropod fauna of the Coppice Limestone includes Bactroptyxis bacillus and Cossmannea sp. nov. which are not common in the Fuller's Earth or equivalent strata. However they are found in the basal beds of the overlying Forest Marble and B. bacillus occurs abundantly in the Great Oolite, a few metres below the Acton Turville Beds, south-east of Tormarton. These affinities suggest a closer relationship between the Coppice Limestone and the Great Oolite than with the Fuller's Earth and thus the Coppice Limestone is more likely to belong to the higher Upper Bathonian Oxycerites aspidoides Zone than to the 'retrocostatum' Zone. I.E.P.

Great Oolite

History of research

William Smith coined the name Great Oolite and used it in his notes of about 1800 (Phillips 1844, pp. 59–60). It appeared in a number of publications by himself or friends between 1811 and 1815 (e.g. Warner 1811) and denoted the strata, mainly oolites, resting on the clays of the Fuller's Earth and beneath the Forest Marble.

Lonsdale (1832, p. 251) subdivided these oolites creating a tripartite classification into Upper rags, Fine freestones and Lower rags. His Upper rags are herein considered as belonging to the Forest Marble. Ramsay, Aveline and Hull (1858) wrote an account of the Great Oolite of an area which included the Malmesbury district, but related the Great Oolite in the south to older rocks of similar lithology in the north.

Woodward (1894, p. 248) used the term Great Oolite Series to embrace all formations from the Fuller's Earth to Cornbrash inclusive, including the Great Oolite Limestone. The constituents of this Great Oolite Limestone were 'Great Oolite, and Forest Marble and Bradford Clay'. His general classification of the Great Oolite combined into a Lower Division, Lonsdale's Fine freestones and Lower rags, whilst Lonsdale's Upper rags constituted an Upper Division. This classification was similar to that adopted by Ramsay, Aveline and Hull earlier (1858), and unfortunately, like Hull (1857) he misapplied it in the northern areas. There his Upper Division included 'Kemble Beds' (=Great Oolite and probably basal Forest Marble) and White Limestone (of Upper Fuller's Earth equivalence), while his Lower Division included Stonesfield Slate and the oolites of Minchinhampton which are of Lower and Upper Fuller's Earth equivalence respectively.

This misapprehension that the Great Oolite of Bath was the same formation as the thick oolites in the north was no doubt based upon their close lithological similarities together with the apparently identical stratigraphical position overlying some 100 ft (30 m) of Fuller's Earth clays and beneath Forest Marble. Arkell and Donovan too (1952, p. 235) considered the oolites of the Boxwell and Starveall area to be the continuation of the Bath Stone and Bath Lower Rags.

The most recent account of the type-area, at Bath, was published in 1969 (Green and Donovan) and in that account the Upper Rags are included within the Great Oolite.

General account

At Corsham (Sheet 265) the thickness of the Great Oolite exceeds 100 ft, but north of Castle Combe it diminishes rapidly so that at the southern margin of the Malmesbury district, near Acton Turville, it is only about 50 ft. This diminution of thickness and eventual failure takes place northwards and north-east wards (Figure 13) and (Figure 14). Because there is continuity of outcrop northwards from the Bath area, the Great Oolite is easily recognised and mapped. Thus it has been traced from near Cross Hands (Sheet 265), past Grickstone Farm [ST 777 830] to Starveall; north of here it is missing, at least in places, and the Forest Marble rests on Athelstan Oolite and Coppice Limestone.

It is difficult to conceive of this disappearance of the Great Oolite as due purely to a northward erosional overstep of the Forest Marble, for, were that the case, the Forest Marble could be expected to continue to transgress on to lower horizons northwards beyond Starveall. This is not so and depositional reasons for the observed changes have to be considered. The vicinity of Starveall may represent the northern limit of the Great Oolite basin or bank where open marine deposition of shell-detrital oolite took place. North of this its position seems to be represented by a non-sequence, but possibly occupied partially by the shallow lagoonal type of calcite mudstone of the Coppice Limestone though the evidence at present is inconclusive. Lateral transition between Coppice Limestone and Grickstone Beds (the basal beds of the Great Oolite) has not been observed. Secondly, there are places to the north of Starveall, though randomly disposed, where Forest Marble rests directly on Athelstan Oolite without any intervening Coppice Limestone.

Since the oolites are devoid of diagnostic fossils, the presence of other outcrops of Great Oolite, not in direct continuity with the main Cotswold scarp outcrop, can only be inferred on the basis that they lie between Forest Marble with its basal Bradford Clay fauna together with sporadic reef limestones, and Upper Fuller's Earth clay (or limestone equivalent). On this basis Great Oolite has been mapped in the Sherston Inlier between Luckington and Sherston, and in thin lenticular patches north of Starveall and Tetbury. By definition such rocks would appear to be Great Oolite. They could be part of the basal Forest Marble, but this would require the recognition of a Forest Marble formation below the Reef Bed and strata with a Bradford Clay fauna. Such a formation has not been observed in the south of the area.

The base of the Great Oolite appears to be sharp and there is evidence nearby, from Tormarton (Sheet 265), that it is erosive for it contains pebbles of underlying limestones. The Passage Beds of Reynolds and Vaughan (1902, p. 738) largely represent the lateral transition from clays of the Upper Fuller's Earth to the south into Athelstan Oolite and Tresham Rock and not a vertical passage into Great Oolite.

A feature of the Great Oolite is its bored, brown and hardened top surface which is flat and oyster-encrusted in parts, and on which rests the Forest Marble. Although a similar surface has been observed within the Great Oolite south of the district (p. 178–9) and while other bored surfaces occur within oolites of the basal Forest Marble (including Acton Turville Beds), they are not so perfect, sharp or persistent laterally. Nor do they separate brownish argillaceous ill-sorted oolite (or shelly limestone), containing whole brachiopod shell material, from an underlying distinctly different, cleaner and whiter oolite or shell-detrital oolitic limestone. The boring molluscs seem to have preferred a firm surface of pure oolite to an incoherent shelly or cross-bedded oolite and thus the bored surface is displayed best in the south-west where such oolites are well developed at the top of the Great Oolite. The majority of the borings on this surface are mainly shallow and fairly broad, while longer narrow borings are associated with the lower surface seen in adjacent ground to the south.

In the Bath area the Twinhoe Beds are a prominent member of the Great Oolite (Green and Donovan 1969). It is presumed that they lose their distinctiveness over a short distance, by change of facies, becoming almost inseparable from the rest of the oolites of the succession and thus making the Great Oolite less clearly subdivisible. In this respect it is noteworthy that about 1 mile (1.6 km) SE of Tormarton, and some 15 to 20 ft (5–6 m) below the bored top of the Great Oolite, the second bored surface mentioned above has been recorded. This surface is overlain by limestones with 'Epithyris' and it seems probable that it is the same surface as is exposed further south, near Castle Combe, at some 33 ft (10 m) below the top of the Great Oolite. There it is overlain by several feet of marly, compact, coralline and pisolitic limestones and these are considered to represent the Twinhoe Beds (p. 179) with the oolite below being Combe Down Oolite (Figure 13) and oolite above being Bath Oolite.

Towards the district from the south this Combe Down Oolite maintains a fairly constant thickness, while it is the Bath Oolite that suffers the consistent diminution described above (Figure 13). Thus within the district the Great Oolite probably consists mainly of the equivalent of the Combe Down Oolite.

Main outcrop

In the south the northward thinning of the Great Oolite is still far from complete and two divisions of the formation are readily mappable. These divisions are: (i) Grickstone Beds, 0 to 15 ft (0–5 m) thick, and (ii) cream-coloured oolite, 0 to about 30 ft (0–9 m).

The Grickstone Beds consists of flaggy shell-detrital oolitic limestones and are commonly cross-bedded. Shell-detrital oolites at the base of the Great Oolite have been noted over a large area around and to the north of Bath and on approaching the Malmesbury district it was possible to distinguish these rocks on maps. Their outcrop enters the district in the south near Lygrove Farm [ST 778 816] where they have an estimated thickness of about 15 ft (5 m), and they have been traced beneath the cream-coloured oolite as far as Petty France. North of this the overlying oolite is absent and the thickness of the Grickstone Beds diminishes under the Forest Marble until they too are absent north of Starveall.

The cream-coloured oolites consist of a massive shell-detrital oolite freestone. At the southern margin of the district they are much thinner than further south, but have been the source of building stone, and disused mines in the formation are reminders of this. The nearest of these mines are Brookman's Quarry, Tormarton, some mile (0.8 km) S of the district, and Eyles's Quarry [ST 772 806] almost at the southern margin of the district, near Cross Hands. Where the formation enters the district it is some 30 ft (9 m) thick, but diminution persists northwards until nothing of it exists at Petty France.

Inliers near Sherston

For 1.5 miles (2.4 km) along the valley sides between Luckington and Sherston, cream-coloured and pale brown shell-detrital oolites crop out beneath brown, rather ferruginous limestones with argillaceous partings of the basal Forest Marble. These rocks are cross-bedded and up to 22 ft (6.7 m) of beds are exposed in the small quarries formerly worked for building stone at Luckington. Similar rocks are still worked for building stone near Sherston and near Knockdown 1.5 miles (2.4 km) NW of Sherston, where they outcrop at the north end of a narrow inlier which follows the bottom of a dry valley. Proof of the Great Oolite age of these rocks has not been obtained, but the presence of underlying clay and overlying limestones with a Bradford Clay fauna establish that they correlate, in relative position, with the Great Oolite, while their lithology and absence of diagnostic, or even well-preserved fauna, are equally compatible with the Great Oolite. The top surface of these limestones, where observed, is bored and oyster-encrusted.

In the Carriers' Farm Borehole, Sherston, [ST 8474 8542] there are 44 ft 8 in (13.61 m) of shell-detrital oolites lying below strata which almost certainly are basal Forest Marble. Within these 44 ft 8 in (13.61 m) of beds there is a unique 3 in (8 cm) layer of hardened brown limestone with a flat top surface showing mollusc borings up to 1 in (2.5 cm) deep. Beneath this surface there remain only 18 ft 1 in (5.51 m) of the oolites and this thickness is taken to represent the Great Oolite, while the overlying 26 ft 7 in (8.10 m) are, in the absence of any other evidence, considered to belong to the basal Forest Marble. The uncertainty about the base of the Forest Marble in this core may be due to the fact that diagnostic differences in, say, colour, texture, hardness, are readily detectable at surface largely owing to the selective influence of weathering. This influence is missing from the borehole core and in these circumstances one oolite can look very like another. A bored and hardened surface which may be distinctively brown at outcrop may be so unobtrusive in the borehole core as to be missed.

In Widley's Gorse Quarry [ST 847 848] some 36 ft (11 m) of similar oolites have been quarried. These had a rather spectacular, bored, oyster-encrusted planar top surface, overlain by a thin clay containing digonellids. The bottom of the quarry is waterlogged and clay is thought to be present immediately below. All the beds seen below the bored surface have been classified as Great Oolite, but the thickness is so much greater than that below the bored surface in the Carriers' Farm Borehole that there is some doubt about this. The possibility exists that the bored surface seen in the quarry is within basal Forest Marble strata and that a lower bored surface or level diagnostic of the top of the Great Oolite may be present in some part of the succession now obscured. The fossil record from these beds, however, does not favour a Forest Marble age.

An unusual occurrence in the Great Oolite of this inlier is a development of about 2 ft (0.6 m) of sandy limestone, seen in the quarries near Luckington, up to 8 ft (2.4 m) below the base of the Forest Marble. This development was not observed in the Carriers' Farm Borehole and must be of a local nature.

A series of boreholes will be drilled around Malmesbury and the results will assist in a more definitive correlation of the oolites in that area.

Outcrops north of a line, Didmarton to Tetbury

In the north-east quadrant of the district it is common to find a thin development of shell-detrital oolite between the Forest Marble and the Athelstan Oolite or Coppice Limestone and these rocks are described as Great Oolite. By definition (Woodward 1894, p. 250) this Great Oolite could be termed Kemble Beds, but the term has been used so loosely, even by Woodward himself, commonly to include much Forest Marble and even beds as low as Athelstan Oolite, that it is not adopted here. The beds crop out as thin and discontinuous developments, mainly of cream to yellow oolite, heavily charged with shell detritus, but little whole-shell material. Nothing of the brachiopod fauna found in shell-detrital oolites and limestones at the base of the Forest Marble has been found in them, and they are clearly demarcated from the overlying limestones, which do contain this fauna, by a planar, commonly bored and oyster-covered surface. In most places a thin marl layer immediately overlies this Great Oolite and in a number of places this marl contains abundant Rhactorhynchia obsoleta (Davidson non Sowerby). At other places this same marl layer directly overlies Athelstan Oolite and Coppice Limestone.

The maximum observed thickness of the Great Oolite in this area is about 13 ft (4 m), for example near Avening, Hazelton Manor Farm [ST 929 983] and Culkerton. These beds have not been seen west and north of a rough arc from Starveall through Leighterton, Tiltups End (Horsley), just north of Avening to Sapperton, while south and east of this line the formation is discontinuous. The formation is absent for instance over areas near Beverstone, Tetbury, Culkerton and Tarlton Down. This discontinuous mode of occurrence is not unlike that of the Signet Beds (Worssam and Bisson 1961) which also constitute the lower part of what have been termed Kemble Beds.

The uneven and sporadic development of the Great Oolite north of Tetbury could be depositional. Shallow banks of oolith and shell sand forming on a winnowed surface of lithified earlier limestones might produce such a configuration. R.C.

Zonal position of the Great Oolite

The Twinhoe Beds of the Great Oolite of the Bath area yield the only British ammonite fauna definitely ascribable to the Oxycerites aspidoides Zone (Torrens 1974, p. 593). The subjacent Combe Down Oolite and the overlying Bath Oolite have yielded no ammonites and their zonal positions are uncertain. In the Malmesbury district the zonal position of the Great Oolite is complicated by lateral lithological change, thinning and even disappearance of beds northwards. The Grickstone Beds and overlying oolites are considered to pass southwards into Combe Down Oolite (p. 146) therefore they may belong either to the 'retrocostatum' Zone or the aspidoides Zone. The same can be said of the sporadic developments of?Great Oolite overlying the Coppice Limestone in the north of the district though this higher stratal position makes it unlikely that they belong to a zone lower than the aspidoides Zone. I.E.P.

Forest Marble

History of research

The name Forest Marble, 'marble' being a misnomer, arises from the Wychwood Forest in Oxfordshire where shelly, oolitic limestone has been quarried for ornamental and building work. It was adopted and defined by William Smith in 1799 and 1815. Hull (1857, p. 63) employed the term in the same sense and, although based on an area to the north of this district, he gave a very good general description of the formation. Ramsay, Aveline and Hull (1858) gave an account of the Forest Marble in parts of Wiltshire and Gloucestershire which included "all the strata between the white limestone and the Cornbrash". Lycett (1857) discussed the Forest Marble of the north-eastern parts of the district, and Witchell (1882b, p. 82) also described the Forest Marble of an area which included the north-east part of the district. He considered the Bradford Clay there as merely a bed of clay belonging to the Forest Marble. Witchell (1886b) wrote an account of the Forest Marble and Upper Beds of the Great Oolite between Nailsworth and Wotton-under-Edge and in 1894 came a more comprehensive description by Woodward, who included the Bradford Clay in the Forest Marble (pp. 338, 340). He looked upon the Bradford Clay as the local basement bed of the Forest Marble (p. 352).

In general, however, the Forest Marble of the Malmesbury district has not attracted great interest. Reynolds and Vaughan (1902) described the sections opened in the cuttings for the Wootton Bassett to South Wales railway, while S. S. Buckman in his Type Ammonites (1924, p. 28) also discussed some of the basal Forest Marble exposed along this line, coining the name Acton Turville Beds.

Passing reference was made by Douglas and Arkell (1928) to the Forest Marble immediately subjacent to the Cornbrash around and to the south of Malmesbury, while Arkell (1933) described the Forest Marble more extensively over the district. He made a case for the abandonment of the name (p. 286) on the grounds that the 'Forest Marble' quarried in the days of William Smith cannot be localised with precision and might even have been obtained from beds below those with a Bradford Clay fauna. In place of Forest Marble Arkell proposed the term Wychwood Beds, to include strata from the top of the beds containing the Bradford Clay fauna to the base of the Cornbrash. In practice the lower limit of his Wychwood Beds would be difficult to employ as a formation boundary and Arkell was probably aware of this, for, after excluding the 'Bradford Beds' from his definition of Wychwood Beds, he made no provision for them elsewhere. In contrast, the retention of the base of the Forest Marble at the lower limit of strata with the Bradford Clay brachiopod fauna has been a practical mapping proposition over the Malmesbury district and beyond.

General account

The only generality, noted by some previous workers, is that the Forest Marble is very variable in nature both vertically and laterally. Beds are commonly lenticular and embrace a range of rock types including blue and brown sandy, shelly limestones; fine-grained, fissile, sandy limestones; clays, sands, shell-detrital oolites and compact reef-type limestones. Wood and bone fragments, ripple marks, abundant oolith and shell material, clay galls and other pebbles of limestone and cross-bedding are all common and indicate that the sediments accumulated in a shallow near-shore marine environment.

The thickness of the Forest Marble is about 92 ft (28 m) in the south, at Alderton. Some 76 ft (23 m) of strata were seen in the Shipton Moyne No. 2 Borehole which commenced at an estimated 10 ft (3 m) below the Cornbrash; and in the Tetbury No. 4 Borehole 48 ft (14.6 m) of Forest Marble were encountered below what is thought to be the upper limestone. A total thickness here of about 68 ft (21 m) would thus be a reasonable estimate. North-east of Tetbury, where the Forest Marble becomes dominantly limestone, there is no Cornbrash to prove a total thickness, but the impression gained from the recent survey is that the thinning continues north-eastward. East of Malmesbury, the Milboume Borehole penetrated some 113 ft (34.4 m) of strata considered to be Forest Marble. This is a greater thickness than encountered elsewhere in the district.

In the south the lowest part, possibly up to one-third of the Forest Marble is almost entirely yellow or brownish limestone. In places it is argillaceous limestone, but usually it is a shell-detrital oolite. Much of this part of the formation may contain elements of the Bradford Clay fauna and, in the south, these beds are known as the Acton Turville Beds, a distinction made on the grounds that they contain the Bradford Clay fauna within an oolite lithology (S. S. Buckman 1924, p. 28). They are the extension northwards of the Upper Rags which Green and Donovan (1969) place at the top of the Great Ooolite. In the central and north-eastern areas of the district similar beds occur which in places have been considered as belonging to the Kemble Beds (Woodward 1894, p. 250). In all these areas sporadic development of approximately plano-convex masses of compact, white, reef-limestones occur at the very base.

The Bradford Clay of the type section (Woodward 1894, pp. 353–4) may be merely one of a number of laterally impersistent clay beds within the basal part of the Forest Marble (Green and Donovan 1969, p. 24). Any one of these clay beds is likely to yield the fauna considered diagnostic of the Bradford Clay and such is the clay present in the roadside bank beneath the old Great Western Railway Tetbury Road Station described by Woodward (1894, p. 363). This clay lies at the base of the Forest Marble and within beds which could be termed locally 'Kemble Beds'.

The rest of the Forest Marble consists of clay, with fissile, shell-fragmental, sandy and oolitic limestones with some sand layers. The clays dominate the succession in the south and are usually calcareous containing small white calcareous concretions referred to as "race". Around Shipton Moyne limestone becomes more dominant and further north-east and east a persistent bed of limestone has been mapped near the top.

The following table summarises the succession:

Zonal stratigraphy of the Forest Marble and Cornbrash

As stated (p. 153) the base of the redefined Acton Turville Beds is mapped northwards into the base of the Forest Marble confirming Buckman's (1924, p. 28) recognition of their equivalence to beds at Tetbury Road Station in which Clydoniceras hollandi was found. Mapping in the north of the area has shown that they are equivalent to the beds containing the 'digonoides Beds' fauna at Hill-some Farm (pp. 201–3) and those beds from which this fauna is thought to come at Tiltups End (p. 196) while to the south the Acton Turville Beds are considered to pass into the Upper Rags of the Bath area. Thus the discus Zone (and perhaps the hollandi Subzone) appears to form a distinctive horizon over a very wide area in the Cotswolds and contains the distinctive fauna of Bradford Clay aspect.

The succeeding Forest Marble clays and Lower Cornbrash yielded no ammonites though the latter has long been known to lie in the discus Subzone (Torrens 1969b, pp. 66–7) while the Upper Cornbrash has yielded (p. 217) ammonite fragments indicative of the basal Callovian Macrocephalites macrocephalus Zone. I.E.P., R.C.

Basal Limestones

The term Acton Turville Beds is used as a local term for the formation (see (p. 152) around and to the south of Acton Turville, while the 'Reef Bed', consisting of small limestone reefs and the associated apron deposits, is a basal member of the formation.

The Reef Bed consists of shell-detrital oolite in which there are lenticular masses of compact, rather earthy calcite mudstone (Patch Reefs) containing a fauna of large brachiopods and thick-shelled bivalves such as epithyrids and limids, together with corals and bryozoans.

The presence of the Reef Bed is most obvious where patch reefs themselves are exposed, or where their distinctive lithology is seen as surface brash. The lithology is so distinctive as to have given rise to the name Petty France Limestones (Arkell and Donovan 1952, p. 236). This name was used also for the nearby Coppice Limestone which produces similar brash but is of lower horizon. The name has therefore not been employed here. Where the Reefs are absent and the Reef Bed consists only of shell-detrital oolite its existence probably goes unrecognised and, as its thickness varies greatly over short distances, it is reasonable to expect non-development in some areas. The reefs, or attendant debris have been observed sporadically from near West Kington (Sheet 265), to near Kemble (Sheet 252).

The maximum observed thickness of the Reef Bed is about 10 ft (3 m). The top is regular and flat, capped by a layer of marl up to 2 ft (0.6 m) thick. The patch reefs, when seen in median section are usually piano-convex lenticles rising from the flat surface of the underlying Great Oolite. Normally they fill the thickness of the Reef Bed and are truncated by its top surface. Their margins are commonly in sharp contact with the shell-detrital oolites and the surface is often bored. Borings are common internally too, presumably produced in early growth stages of the reef. In places the margins may be ill-defined, either transitional into or inter-digitating with the shell-detrital oolite. Such is the case with the asymmetrical reef in the railway cutting near Kemble (Figure 18). Reef limestone 'suspended' within the shell-detrital oolite, above the base of the Reef Bed, was also seen in places and may represent protruberances from a lenticular reef only tangentially intersected by the exposure face. The Reef Bed as a whole is normally separated from the underlying Great Oolite, with its bored and hardened surface, by a thin marl parting. No exposure has been seen where the Reef Bed follows above Athelstan Oolite or Coppice Limestone although surface debris in juxtaposition with the outcrop of these formations has been noted and suggests that this relationship must occur.

Where a reef is exposed medially, with a lenticular shape, the surrounding shell-detrital oolite dips away from the centre of the reef, presumably quaquaversally as an apron (Figure 18) and (Figure 19).

The marl overlying the Reef Bed has yielded a fauna characteristic of the Bradford Clay and in places the Reefs yield a similar fauna. The Reef Bed is therefore considered as part of the Forest Marble.

The name Acton Turville Beds (Buckman 1924, p. 28) was given to the beds at the east end of the Sodbury Railway Tunnel, the exposures of which were described in 1902 by Reynolds and Vaughan. They were diagnosed as 'Great Oolite' beds of Bradford Clay facies, which implies that they are oolites not dissimilar from the Great Oolite, but possessing a Bradford Clay fauna (Welch 1959, p. 26). In places this is so, but compact, brown, sandy limestones, argillaceous limestone, marl layers, and, at the base, reef limestone are also common. The term may in fact have been built upon a misconception of Reynolds and Vaughan's original description (p. 180). Within the fauna preserved rhynchonelloids are common while Digonella digona and Dictyothyris coarctata are present. In the mapping of these beds their upper limit has been drawn at the base of the lowest mappable clay. However, their variable nature must inevitably mean that the horizon of this boundary is not constant.

The Acton Turville Beds have a maximum thickness of about 25 ft (7.6 m), so that Reynolds and Vaughan's 46 ft (14 m) of Upper Great Oolite (p. 144) (=Buckman's Acton Turville Beds) would seem to include some genuine Great Oolite. The Sodbury Tunnel railway-cutting is now largely obscured by the masonry of retaining walls and tunnel entrance. However, about 29 ft (8.8 m) of oolites and marls are visible above the tunnel portal, and the bottom 6 ft (1.8 m) of these could belong to the Great Oolite. In any case the mapping nearby does not substantiate a thickness as large as 46 ft (14 m) for these beds and the Geological Survey usage does not correspond with Buckman's definition.

North-east of Badminton these rocks become more shell-detrital, slightly less argillaceous and probably thicker, though no complete section is available. This change is not sharply defined nor is it pronounced, but, on the one-inch map, the northward termination of the Acton Turville Beds has been arbitrarily depicted as occurring in the neighbourhood of Dunkirk [ST 789 833].

East of Badminton, Forest Marble Basal Limestones are seen in the valley system around Luckington and Sherston, but the Acton Turville Beds appear to be thin near Luckington, consisting of some 15 ft (4.6 m) of layered, brown, shell-detrital oolitic and hard, compact limestones with partings of marl. The thickness of these beds is reduced to some 10 ft (3 m) between Luckington and Sherston, but around and to the north and east of Sherston a rather thicker development of shell-detrital oolitic limestones containing less argillaceous material appears at the base of the Forest Marble. Acton Turville Beds have been terminated on the one-inch map, eastwards of about Carriers' Farm just west of Sherston.

To the north-east and east of Sherston the shell-detrital oolites and oolitic limestones have been recorded only in boreholes at Shipton Moyne and near Malmesbury. Near the latter place they reach a thickness of about 34 ft (10.4 m).

North and north-east of Dunkirk, the outcrop of the Basal Limestones is wide; large areas of the gently south-eastward dipping slope around Didmarton, Leighterton and Calcot Farm [ST 8400 9500] are formed by them. Digonella digona is fairly common in the brash of shell-detrital oolitic limestone, but Dictyothyris coarctata was found again only in the north-east corner of the district. Lumps of ragged oystery limestone in this soil mark the base of the Forest Marble in places, while in others, lumpy brash of compact white 'Lima' and Tpithyris' limestone occurs at this horizon, representing the reef limestone of the Reef Bed.

In places, in both exposure and surface debris, a yellowish marl has been observed at the bottom of the Basal Limestones. This contains an abundance of a large Rhactorhynchia obsoleta, noted also by Arkell and Donovan (1952, p. 241) and other Bradford Clay fossils. It was taken to mark the base of the Forest Marble, for instance near Boxwell, Tiltups End and Avening, but R. obsoleta is present commonly in the overlying limestones too.

The outcrop of the Basal Limestones extends north-eastwards to the northeast margins of the district. North of Tetbury the limestones are more tiley, brown and shell-detrital containing Rhactorhynchia obsoleta, Digonella digona and Avonothyris sp. nov. B.Some beds are very sandy and the soils express this condition in deep sandy loarns. Conglomeratic layers also occur at the very base in places, such as near Beverston, Tetbury Upton and Avening. The pebbles forming these conglomerates are very flat and irregular disc-shaped fragments or flakes of limestone and calcite mudstone, while bone and teeth also occur.

The thickness of the limestone in the north-east of the district is irregular being at least 25 ft (7.6 m) in places. The description of the Tetbury Waterworks Borehole No. 3 (Richardson 1915a, p. 58) indicates 21 ft 3 in (6.48 m) of Forest Marble limestone, but specimens from, and the log of Borehole No. 4 (Appendix 1) on the same site, drilled in 1928, are more indicative of some 48 ft (14.63 m) of limestones from the base of the Forest Marble to about the horizon of the sandy Upper Limestone. Some irregularity in thickness of the Basal Limestones may be accounted for by deposition in the form of banks of shell debris, ooliths and sand, with the Acton Turville Beds representing contemporaneous sediments which accumulated more evenly a little further off-shore to the south-west.

Forest Marble excluding Basal Limestones

Above the Basal Limestones, the Forest Marble succession shows a change from dominantly clay in the south to dominantly limestone in the north-east. Accompanying the lithological change is a reduction north-eastwards of thickness, perhaps reflecting an approach to 'shore'.

At Alderton, in the south, the clay, with thin limestones, is some 58 ft (17.7 m) thick; between Sherston and Shipton Moyne, limestones become more abundant as impersistent or lenticular layers without stratigraphical regularity. North of Malmesbury and around Tetbury a limestone some 10 to 15 ft (3–4.6 m) below the Cornbrash and about 10 to 15 ft (3–4.6 m) thick is laterally persistent and was mapped. The limestone is separated from the Basal Limestones below by about 30 ft (9 m) or more of clay which in the north-east of the district becomes thin and lenticular.

The clays are calcareous, particularly subjacent to the Cornbrash where greenish clay commonly contains race, which consists of small concretionary lumps of calcium carbonate. Brown and orange are common colours in weathered clay, though, when unweathered as in boreholes, they are normally grey. In places the clays are also sandy, while a common feature is "risely" clay—a rather silty clay containing paper-thin laminae of calcareous siltstone or silty limestone. On freshly exposed surfaces of "risely" clay the laminae weather out as thin flakes.

The limestones too are commonly sandy, especially towards the top of the Forest Marble, and in places they appear to pass laterally into beds of fine sand. Whether or not this sand is merely an expression of near-surface decalcification is not known. Other limestones are oolitic and shell-detrital or shell-fragmental, commonly grey or blue, differing from the yellowish shell-detrital oolites encountered at the base of the Forest Marble. R.C.

Petrography

Ninety limestone specimens have been examined, but as they are not part of a systematic areal or stratigraphical collection, the following notes are of a descriptive nature only. The terminology used is broadly that suggested by Folk (1959) for limestones.

Througham Tilestones

In a solitary example (E35152), terrigenous material (chiefly silt-grade, sub-angular and subrounded quartz, with accessory muscovite) forms between 30 and 40 per cent of a fine sand-grade, shell-detrital sparry limestone.

Cross Hands Rock

Available specimens suggest that this formation is essentially a biomicritic limestone (E35109), (E35111), (E35115)–(E35116), (E35132). However, although fossil debris (often bearing micritic coatings) is common, it is usually accompanied by rolled intraclasts, chiefly of micritic limestone. Allochemical constituents range from as little as 20 to as much as 85 per cent of the rocks by volume. The average grain-size of this material is either of fine to medium sand grade, or very poorly sorted, ranging from fragments of fine sand size up to broken shells several millimetres in length. Most of the rocks contain traces of, or between 1 and 3 per cent silt grade, terrigenous quartz. Scattered granules of limonite are common and are associated with allochemical constituents rather than the micritic matrix.

Among the samples examined are two that have sparry calcite matrices (E35128), (E35129), and another poorly sorted sparry type (E35113) which contains abundant pellets alongside intraclasts of coarse sand or gravel.

Tresham Rock and equivalent limestones

Most of the samples collected from the Tresham Rock are intrasparites, some containing traces, of between 1 and 2 per cent of silt grade terrigenous quartz (E35135), (E31583), (E31584), (E31585), (E31587), (E31588), (E31589), (E31590), (E31593), (E31594), (E31595), (E31597), (E31599), (E31600), (E31601). They are characterised by the presence of fine sand grade, rolled, micritic intraclasts and by micrite-coated fossil debris in the fine and medium sand grades (with a few gravel-sized fragments) set in fine-grained sparry calcite matrix. Rare ooliths were noted in certain specimens e.g. (E35135), (E31599), while numerous intraclasts ranging up to medium sand grade bear concentric oolitic overgrowths. Matrix calcite constitutes between 10 and 35 per cent of the rocks, but mainly in the range of 20 to 30 per cent. Micritic patches occur in the matrices of some specimens (E31584), (E31587), (E31588) and there are rare examples where sparry calcite gives way entirely to micrite e.g. (E31589).

The only important variation seen in the Tresham Rock samples is where fossil debris substitutes for intraclastic material as the dominant allochemical constituent, yielding types grading into biosparite (E31591), (E31586). Locally sparry matrix gives way to micrite (E31602), (E31603), (E35136).

Many of the Tresham Rock samples contain grains of limonite and scattered, transported flakes of chloriteichamosite were noted in three specimens overlying Upper Fuller's Earth clay (E31589), (E31602), (E31603).

In the north-western part of the district, around Hunters' Hall and Kingscote, limestones which are equivalents of the Tresham Rock show appreciable variation in the nature and proportions of allochemical constituents. The rocks from this horizon have two features in common; the matrices are generally composed of sparry calcite, while included material is typically in the fine to medium sand range. Pelsparites are the commonest type among the collected samples e.g. (E35131), (E35142), (E35146). The pellets are accompanied by variable quantities of rolled micritic intraclasts, some fossil debris and 1 to 3 per cent silt grade, terrigenous quartz. Sparry matrix calcite accounts for about 10 per cent of the rocks by volume. A certain amount of micritic calcite occurs in the matrix of one of the specimens (E35151).

Other allochemical constituent types represented are intrasparite (E35143), (E35145), fossiliferous (bio) intrasparite (E35130), biosparite (E35149) and oosparite (E35144). Most contain between 15 and 30 per cent spar matrix and minor quantities of silt grade, terrigenous quartz.

The majority of specimens contain scattered granules of limonite which are associated mainly with the allochemical constituents.

Athelstan Oolite

Available samples can be grouped into two main types. The first, oösparite, is dominated by fine or medium sand grade ooliths, fossil debris and intraclasts occurring in comparatively small quantities (e.g. (E35094), (E35097), (E35141). Sparry matrix calcite accounts for 10 to 20 per cent of the rocks by volume. Rare grains of terrigenous silt grade quartz were seen only in one specimen (E35141).

The second and apparently more common rock type is intrasparite. Micrite-coated, rolled intraclasts (typically micritic limestone or oömicrite) predominate over ooliths and fossil debris (also commonly micrite-coated) (E35099), (E35124). In several specimens of this type ooliths are comparatively abundant e.g. (E31609), (E35123), suggesting that within the formation there is complete gradation between intrasparite and oösparite. Allochemical constituents in the intraclastic rocks are poorly sorted, their grain size ranging from the fine to coarse sand grades, although no ooliths are larger than the medium sand-grade. One specimen contains intraclasts up to 7 or 8 mm long (E35147). The sparry calcite matrix is more abundant than in the oosparites and ranges from about 20 to 35 per cent. Micritic patches are locally present (E35123). As in the case of the oösparites, terrigenous quartz is rare, grains of silt grade being noted only in one rock (E35139).

Granular limonite occurs in many samples chiefly in allochemical constituents. Limonitic staining is also a common feature particularly in the micritic overgrowths on fossil and intraclastic material.

Coppice Limestone

This formation is characterised by micrites (E35100), (E35102) containing scattered ooliths and fossil debris. Veins of sparry calcite occur in some (E35100), (E35101), (E35140). In specimens from a locality near Leighterton, micritic rock merges with material rich in ooliths of fine sand grade (E35103), (E35104). The latter specimens contain a little silt grade terrigenous quartz. Other minor variants in the Coppice Limestone include micrites rich in finely comminuted fossil debris (E31582), and poorly sorted sparry types containing fossil debris and intraclasts ranging in grade from fine to coarse sand (E31580)–(E31581). Some samples contain a little granular limonite, but iron oxides seem less common than in the underlying limestones.

Great Oolite

Scattered samples of the top few inches of the Great Oolite may loosely be described as oolitic limestones (E35091), (E35092), (E35098), (E35114), (E35119), (E35138). However, ooliths are not generally the dominant allochemical constituent. They are accompanied by varying proportions of rolled, micrite-coated intraclasts (fossiliferous and oolitic micrite limestones) and by rolled, micrite-coated fossil debris (mainly shells). Matrix calcite accounts for 25 to 30 per cent of the rocks examined and although sparry, it is fine-grained. Micritic matrix occurs in two specimens (E35098), (E35138) suggesting that the top of the Great Oolite is not always particularly well sorted.

Tufa-like growths adhere to the top surface of the formation and a specimen from one (E35120) consists of micrite, a little sparry calcite that may represent recrystallised fossil fragments, and patchily distributed grains of terrigenous, silt-grade quartz.

Terrigenous, silt-grade quartz forms approximately 20 per cent of a specimen representative of a characteristic bed in the Sherston Inlier. This consists mainly of an assortment of fossil fragments and micritic intraclasts set in a matrix of sparry calcite and irregular patchy micrite. Small crystals of siderite, commonly showing rhombohedral form, are scattered sparsely through the rock (E35093).

Compared with other limestones of the Great Oolite "Series" these rocks contain only minor quantities of granular limonite.

Forest Marble Basal Limestones

At this level the Forest Marble is characterised by poorly-sorted biomicritic limestones e.g. (E34725), (E31575), (E35137) in which shell and coral debris may be accompanied by intraclasts of micrite and oomicrite. Locally the matrix contains sparry calcite patches. Patches of poorly fossiliferous micritic limestone are also present (E35118). Other samples are virtually conglomerates, with coarse fossil debris, micrite, biomicrite and oiimicrite intraclasts and scattered ooliths set in a sparse, sparry matrix (E35117). Granular limonite is more common than in the underlying Great Oolite specimens and it occurs in both allochemical constituents and the matrix.

Limonite staining

The majority of the specimens are buff in colour owing to the presence of limonite smears in the interstices between, and on the faces of, individual crystals. Micritic developments around fossil and intraclastic allochemical constituents are commonly favoured sites for the iron oxides. This may be a reflection of the greater surface area presented by calcite in this kind of crystalline development.

Limonite (goethite) also occurs as scattered grains or aggregates of grains, sometimes in the matrix, but more commonly associated with allochemical constituents. It seems likely that this material results from the oxidation of pyrite previously developed in the Great Oolite "Series" limestones during deposition or diagenesis, even though fresh pyrite was noted in only one specimen (E35107) from just below the Cross Hands Rock. The oxidation presumably stems mainly from groundwater movements connected with the evolution of the present erosion surface, the dispersed limonite 'staining' representing a partial redistribution of oxidation products. Some of the limonite may have been derived from siderite rather than pyrite, but the former mineral seems to be rare in the Great Oolite "Series" limestones. Specimen (E35093) from the sandy bed in the Great Oolite of the Sherston Inlier is the only example containing appreciable amounts of the carbonate. J.R.H.

Details

Fuller's Earth

Lower Fuller's Earth Clay

Cross Hands to Hawkesbury

This formation forms the lower part of the scarp slope rising along the east side of the Inferior Oolite dipslope. Its upper limit is marked by a small feature produced by the Cross Hands Rock. In the clay just below this feature Praeexogyra acuminata is abundant and was collected from about 16 ft (4.9 m) below the Cross Hands Rock in a temporary excavation [ST 7720 8300] 560 yd (512 m) W of Grickstone Farm. Clypeus sinuatus was obtained about 10 ft (3 m) higher. Similarly in field brash just below the Cross Hands Rock [ST 7718 8354], 1600 yd (1463 m) 78° from Little Sodbury Church where Wattonithyris tutcheri was also obtained.

From the base of the Lower Fuller's Earth Clay [ST 7685 8539], 490 yd (448 m) 38° from Horton Church, Kallirhynchia deliciosa was obtained, whilst clay with marly limestone brash [ST 7698 8540] 70 yd (64 m) to the east yielded the following fauna: Kallirhynchia deliciosa common, K. superba, Rhynchonelloidella cf. wattonensis, Wattonithyris fullonica, W. cf. midfordensis common, Anisocardia? sp., Chlamys (Radulopecten) vagans, Meleagrinella echinata, Pleuromya sp., Praeexogyra acuminata common. Nearby [ST 7696 8422], D. T. Donovan collected^‡10  Kallirhynchia superba immature, Wattonithyris? - broad sulcate form and abundant P. acuminata. Near the base of the formation [ST 7705 8668] 360 yd (329 m) 136° from St Mary's Church, Hawkesbury, Wattonithyris fullonica and Procerites sp. were obtained. In many places in the area of Hawkesbury debris from one or more thin limestones at a few feet below the Cross Hands Rock is visible in banks and fields.

Hawkesbury to Kingscote

The outcrop of Lower Fuller's Earth follows the intricate pattern of valley slopes over this area and is largely affected by landslip. From a landslip [ST 7930 9219] 250 yd (229 m) S of Holwell Farm, Tresham, D. T. Donovan collected Chlamys (Radulopecten) vagans and P. acuminata, while 50 yd (46 m) to the N [ST 7930 9227] he obtained: an echinoid, Acanthothiris midfordensis, Kallirhynchiaorbis, Wattonithyris parva and Chlamys (Radulopecten) hemicostata. Debris of the P. acuminata-rich limestone and clay from the top of the Lower Fuller's Earth Clay becomes increasingly common northwards, mainly in landslip. At the ends of the long, inter-valley spurs the Lower Fuller's Earth Clay is usually undisturbed and about 1 mile (1.6 km) SW of Tresham its thickness was estimated at about 75 ft (23 m). Other similar small areas of undisturbed clay occur to the north, e.g. 800 yd (732 m) ESE of Ozleworth and sporadically along the north side of Ozleworth Bottom around Wotton-under-Edge and on the south side of Waterley Bottom. In the outcrop [ST 794 800] near Lampern House an acuminata Limestone composed largely of P. acuminata detritus can be mapped within the clay and about 10 ft (3 m) below the Cross Hands Rock.

A silage pit [ST 795 934], 120 yd (110 m) NE of St Nicholas Church, Ozleworth, revealed the junction between Fuller's Earth and Inferior Oolite: clay 3 ft (0.9 m) on rubbly marl, ?Fuller's Earth, with Wattonithyris midfordensis?, Procerites sp. [juv.] and Gresslya peregrina 1 ft (0.3 m), over Inferior Oolite consisting of 1 ft (0.3 m) of rubbly limestone.

Kingscote to Avening area and Nympsfield

In the valley slopes from Kingscote to Avening the Lower Fuller's Earth Clay outcrop is landslipped, but north of Kingscote the gradients are gentler and landslipping less extensive. Here, in the unslipped clay it is almost everywhere possible to map the acuminata Limestone. It is much more distinctly developed and lies some 10 to 15 ft (3–4.6 m) below the Cross Hands Rock, for example on the ridge east and west of Park Farm [SO 811 011]. Estimates of thickness of Lower Fuller's Earth Clay made where it outcrops at the ends of these 'ridges' are very low, e.g. about 20 ft (6.1 m) [ST 820 994] 800 yd (732 m) E of Field Farm, where the acuminata Limestone occurs in the middle. Internal erosion of the clay and camber in the overlying limestone is considered to have caused these anomalies. Where the formation appears to be complete its thickness is in the order of 85 ft (25.9 m), e.g. [ST 801 981] about 500 yd (457 m) N of Owlpen and [ST 837 991] at Wallow Green.

There are few exposures in which the acuminata Limestone is undisturbed.'One in a gully [ST 8139 9913] 350 yd (320 m) SE of Field Farm shows: compact limestone composed largely of P. acuminata detritus, 1 ft 2 in (0.36 m); resting on yellow marly clay with P. acuminata. From the clay with abundant P. acuminata, at the top of the formation on Bown Hill, Woodchester, W. J. Arkell collected Kallirhynchia superba group, Wattonithyris fullonica.

Cross Hands Rock

Cross Hands to Kingscote

The type-section of the Cross Hands Rock lies 600 yd (549 m) S of the Malmesbury district, at the side of the Acton Turville road at Cross Hands. It was described by Richardson (1936, pp. 279–80) and Arkell and Donovan (1952, p. 238). Its thickness is recorded as 9 ft 1 in (2.77 m), but Richardson's comments on the ornithellids collected by Dr Kellaway need qualifying, for the latter states that these fossils were not found in situ and could have rolled from clay higher in the bank; nor were they found in abundance.

During the construction of electricity pylon No. XL 135, [ST 7681 8207] 1100 yd (1006 m) 29° from Cross Hands, white marl and white argillaceous compact limestone was dug up and yielded: Wattonithyris cf. fullonica, Ornithella bathonica, Meleagrinella echinata, Pholadomya lirata, Praeexogyra acuminata.

Northwards the outcrop is marked by a small step in the Fuller's Earth clay slope. Along this outcrop is surface debris of pale, argillaceous limestone, in places containing small, brown, ferruginous flecks, which often have small chips of shell at their centres, e.g. [ST 7709 8385], 180 yd (165 m) E of Crowshall Barn. From a temporary exposure in such rock [ST 7715 8577], 550 yd (503 m) 98° from Upper Chalkley Farm, D. T. Donovan collected: Rhynchonelloidella wattonensis, Kallirhynchia superba, K. orbis, Wattonithyris cf. fullonica, Wattonithyris cf. midfordensis, Wattonithyris sp. [juv.], Rugitela sp. Camptonectes laminatus and Meleagrinella echinata. Limestone debris, probably from the Cross Hands Rock occurs on the underlying clay slope [ST 7736 8798] 1160 yd (1061 m) 25° from St Mary's Church, Hawkesbury. Here D. T. Donovan collected : a regular echinoid, Kallirhynchia expansa, K. superba, Wattonithyris cf. fullonica, W. cf. midfordensis, W. cf. nunneyensis, terebratuloids [immature], small ornithellids [juv.], Chlamys (Radulopecten) vagans, Praeexogyra acuminata [wide var.], common, and Pseudolimea duplicata.

At the south-west and north-east ends of Clay Hill the Cross Hands Rock produces minor plateaux and is revealed, probably the bottom part, in a small crag [ST 7782 8787] 200 yd (183 m) W of Starveall. It consists of brownish, marly, calcite siltstone containing shell-detritus and small ferruginous particles. A similar small plateau on the end of the spur 1400 yd (1280 m) SW of Tresham reveals surface debris of a compact shell-detrital limestone. The shell detritus is of a fine-grain and in a 'contorted' condition resembling that in the Cornbrash.

Little was seen of the Cross Hands Rock north of Tresham, the Fuller's Earth being much landslipped but its outcrop can be detected 300 yd (274 m) N of and 800 yd (732 m) W of Ozleworth and immediately north and south of Workham Bushes [ST 772 928], where it consists of compact, marly, calcite siltstone containing a scatter of small, pale brown pellets.

A small valley-side excavation [ST 7865 9549] 600 yd (549 m) SW of Symonds' Hall Farm revealed 13 ft (4 m) of Cross Hands Rock showing: limestone, creamy, fine-grained sparsely shelly, 2.5 ft (0.8 m), on shale, creamy and silty, 2 ft (0.6 m), limestone, creamy, fine-grained, with shell fragments and crinoid columnals, 1.5 ft (0.5 m), clay, bluish, grey, silty, 1 ft (0.3 m), limestone, hard, creamy and shelly, 1.5 ft (0.5 m), limestone, soft, rubbly, fine-grained and shelly, 2 ft (0.6 m) over limestone, hard, buff, fine-grained and shelly, seen for 2.5 ft (0.8 m).

On the spur around Symonds' Hall Farm [ST 789 960] the outcrop of Cross Hands Rock is mainly undisturbed yielding fragments of limestone of the type described, but east of Ridge Farm [ST 785 967] the width of the outcrop increases as the formation becomes thicker. A thin layer of clay occurs within the formation there and parts of the limestone are more shelly than seen further south. From about 200 yd (183 m) E of Ridge Farm W. J. Arkell obtained, probably from surface brash: Wattonithyris fullonica and Sphaeroidothyris pentagonalis.

Kingscote to Nympsfield and Avening

North of Kingscote is a plateau area where Fuller's Earth clays are exposed in shallow depressions instead of on the steep valley slopes. In this region the Cross Hands Rock, together with the beds above and immediately below, are undisturbed and their outcrops are distinct. It is at once obvious that here the Cross Hands Rock is thicker than it is near Hawkesbury and in the top part there are two phases of predominantly hard, semi-porcellanous, splintery limestone which breaks into small rectangular lumps. The rock is ironshot, containing ferruginised, finely fragmental shell debris. The top surface of the Cross Hands Rock is bored and oyster-covered while 5 ft (1.5 m) lower is another similar surface capping a second compact splintery limestone.

Some 900 yd (823 m) NW of Kingscote Church a small spur feature capped by Cross Hands Rock reveals surface debris of compact and ironshot calcite siltstone containing abundant small turbinate gastropods. About 200 yd (183 m) NW of Binley Farm, at the end of another small spur, a natural exposure of the Cross Hands Rock [ST 8161 9740] revealed Hawkesbury Clay on:

A number of records of the Cross Hands Rock in this area were made by Arkell and Donovan (1952, p. 239), but none of the sources of their collections in the British Museum (Natural History) can be matched precisely with this exposure.

Cross Hands Rock is revealed again [ST 8145 9917] 340 yd (311 m) ESE of Nympsfield Farm, where a landslip scar produces debris of compact, pale creamy limestone such as is common to the south, but also reveals a 2 in (5 cm) band of yellow-buff "pisolite". This "pisolite" is an aggregate of mainly small fragments of echinoid?spines. The fragments are of uniform size, rounded and possess thin calcareous pellicles.

North of here the nature of the Cross Hands Rock changes fundamentally. An exposure [ST 8136 9954] 250 yd (229 m) NE of Field Farm, Nympsfield, shows Hawkesbury Clay on: rather rubbly, chalky, ironshot limestone, some coarsely oolitic and shelly; top surface hard, extensively bored and oyster-encrusted, 5 ft (1.5 m), over hard compact limestone with a bored and oyster-encrusted top surface seen for 6 ft (1.8 m).

The two bored surfaces are very distinct and the semi-porcellanous limestones may represent two calcareous phases in a sedimentation cycle, each followed by sedimentation pauses which produce the small non-sequences. A fauna collected from the bottom bed includes: Anisocardia caudata, A.rloweana, Ceratomya concentrica, Ceratomyopsis undulata, Liostrea sp., 'Lucina'?, pectinoid indet., Pholadomya ovalis, Pleuromya alduini, Protocardia sp.,Pseudolimea sp., Pseudotrapezium cordiforme, Quenstedtia bathonica. From the top bed the following were obtained: crinoid columnals, Thamnasteria sp.,Liostrea sp., Meleagrinella sp., Plagiostoma bynei, P. subcardiiformis.

Dr F. W. Anderson reported that thin sections of the bottom bed showed that it contained ostracods and bivalve shell fragments, foraminifera, bryozoa, echinoid, coral and sponge debris, fragment of bone, micrite pellets, some angular quartz grains and some authigenic chalcedony. The matrix is a medium-grained clear calcite mosaic, coarser in some areas, with irregular patches and fragments of micrite, some of which is flocculent. Some of the micrite appears to be algal and there are borings of endolithic algae. The rock appears to be an agglomeration of shell and other detritus with fragments of broken algal crust formed as a shallow-water deposit with the calcite mosaic as an infilling of voids.

At the top of a landslip scar [ST 815 973] 300 yd (274 m) W of Binley Farm the Cross Hands Rock yielded: Flabellirhynchia sp. nov.fairly common, Tubithyris globata, Anisocardia truncata, Anisocardia (Antiquicyprina) davidsoni, Eopecten sp., Lithophaga fabella, 'Lucina' bellona, Pholadomya lirata, Plagiostoma minchinhamptonensis, Trigonia sp., Nerinella sp. nov., an indeterminable naticoid.

Another exposure [ST 808 997] also at the back of a landslip, 720 yd (658 m) to the WNW and 800 yd (732 m) SE of Nympsfield Church reveals:

A further collection of material made by W. J. Arkell, from "N.W. of Binley Farm N. of Kingscote", includes: abundant Wattonithyris cf. fullonica and Tubithyris sp. There is no precise location to these and at least in part they may have come from the underlying Lower Fuller's Earth Clay (cf. Arkell and Donovan 1952, p. 239).

A significant point of this section is the development of shelly oolite within the more typical compact chalky and ironshot limestones. This has not been observed further south, but develops north-eastwards apparently mainly from the lower part of the Cross Hands Rock and the exposure near Field Farm illustrates a stage in this transition.

The exposure [SO 8105 0094] at Park Farm illustrates the passage at a more complete stage and, according to Channon (1950 p. 259), shelly oolites have here developed within the upper part of the Cross Hands Rock as well. The following section largely confirms this.

There may be about 10 ft (3 m) more of the lowest visible bed beneath the exposure before clay is encountered and this would bring the thickness of the Cross Hands Rock to about 26 ft (7.9 m). The top surface of 'Bed 6', being distinct, reveals small displacements across joints which have opened to become small gulls, the results of movements relating to the nearby valley side. Channon quotes a number of fossils obtained from this quarry, the most important of which is Morrisiceras morrisi from Bed 7. P. acuminata present in abundance in the lowest bed is also significant for it implies that limestone deposition was taking place earlier here than to the south. In places around Lynch Knoll, about 0.5 mile (0.8 km) to the east, surface debris shows parts of the Cross Hands Rock to be a manly, fine-grained calcite siltstone, containing a profusion of closely packed, very small, turbinate gastropod shells, partially replaced by iron-rich carbonates. There are a few rounded fragments of echinoid spines and the rock clearly originated under conditions similar to that seen 340 yd (311 m) ESE of Nympsfield Farm, where the detrital material is all echinoidal.

Similar Cross Hands Rock occurs as an outlier on Bown Hill, Woodchester. This is the most northerly occurrence of the formation and an exposure [SO 8240 0179] 80 yd (73 m) E of the Longbarrow exhibits: brown, rubbly marl, 2 ft (0.6 m), on pale buff, tily oolite, containing some fine shell detritus and breaking into small sub-rectangular fragments, 1 ft 8 in (0.5 m), buff, rubbly, compact limestone and oolitic limestone with a few bivalves, 2 ft (0.6 m), passing down into buff, rubbly, compact limestone with a sprinkling of ironshot at bottom, seen for 2 ft (0.6 m).

Unfortunately the formation constitutes the cap rock of the hill so that its topmost beds are probably not present and the thickness cannot be attested. The lower parts of the formation are not exposed in the quarry face but mapping the outlier reveals that, as near Nympsfield, they consist of cream flaggy shelly oolite and shelly detrital limestone.

Eastwards from Nympsfield and Owlpen, along the sides and particularly at the ends of faulted and landslipped spurs directed towards Nailsworth and Horsley, the Cross Hands Rock can still be mapped. Small exposures and debris of compact, ironshot white limestone are common. Some 2 ft (0.6 m) of splintery, ironshot limestone can be seen resting on clay in the roadside 160 yd (146 m) NW of Horsley Church, and a pit dug 150 yd (137 m) S of Fooks, Wallow Green, revealed: broken, white, ironshot limestone 2 ft (0.6 m), on brown and grey clay with a few P. acuminata, 4 ft (1.2 m). Similar limestone is visible in the lane 550 yd (503 m) WNW of Fooks.

Likewise on the east side of the Horsley to Nailsworth valley, small exposures or fragments of Cross Hands Rock can be seen, as in the pond opposite Haylane Farm, Tiltups End, and again, at the time of the survey, 150 yd (137 m) westwards down the lane towards Horsley, in bungalow footings. The most easterly evidence of this outcrop occurs as field brash 150 yd (137 m) NE of Windsorash, 0.67 mile (1.1 km) S of Nailsworth. Further east towards Avening the slopes are landslipped and the normal position of the Cross Hands Rock is obscured. Around Horsley very little oolite was observed, and this contrasts with the formation as seen around Nympsfield. It suggests that the limit of oolith formation or influx lay to the north of Horsley.

Over the part of the district so far discussed the Cross Hands Rock crops out as a limestone formation with clay both above and below. However, on the western edge of Minchinhampton Common at Pinfarthing (Arkell and Donovan 1952, p. 242), only interbeded silty marls and brown silty limestones seem to separate it from the thick shelly oolites above. Towards the base of these "Pinfarthing beds" compact, white and ironshot limestone and compact, brown, fine, shell-detrital limestone typical of Cross Hands Rock occurs, while shelly oolite, such as is seen near Nympsfield, is also present even lower and overlying clay with P. acuminata. Cambering has affected these beds, but it does not seem essential to invoke a fault to explain their position in relation to the other limestones of the Common (Cox and Arkell 1948, p. xvii). The "Pinfarthing beds" could pass into the "Minchinhampton Shelly Beds" and "Weatherstone" and perhaps the "Scroff". Such an arrangement would confine the known occurrences of Morrisiceras from this district to beds approximately of Cross Hands Rock equivalence (Cox and Arkell 1948, p. xvii) without involving the "Planking". The dearth and uneven distribution of exposure now handicap investigation of the oolites of Minchinhampton Common and quarries which provided the basis for earlier detailed accounts are now largely obscured. However records in Lycett (1848, pp. 184–7) and in Morris and Lycett (1851, pp. 3–5) of a rather impersistent, not abundantly oolitic, compact, mainly non-shelly, limestone near Minchinhampton as lateral replacements of the "Weatherstones" and lying above the clay, or sandy limestone resembling "Stonesfield Slate", would seem to refer to approximate equivalents of the Cross Hands Rock. The "Scroff" might represent the horizon of the Hawkesbury Clay.

In an exposure [ST 9014 9879] near the valley bottom at Cherington, rocks at the base of the main 90 ft (27 m) of limestone forming the valley sides, show: pale fawn-grey, compact limestone containing scattered ironshot or ferruginous ooliths, 2 ft 9 in (0.8 m), on grey, hard, sandy limestone, 8 in (20 cm), over brown or dun-coloured, finely micaceous, very fissile, fine, sandy limestone, seen above spring for 1 ft 3 in (0.4 m).

There seems no reason to doubt that the white, slightly ironshot limestone is the Cross Hands Rock, the point of interest being the presence beneath it of fissile dun-coloured, sandy limestone. This has the appearance of the Througham Tilestones of the Gloucester district (=Stonesfield Slate?) and provides the only exposure where rocks identical with the Cross Hands Rock overlie these sandy limestones. Some 400 yd (366 m) to the west of this exposure, clay with abundant P. acuminata is exposed a little lower in the succession.

Hawkesbury Clay

Details of this formation are almost completely lacking owing to the absence of exposure. Where landslip is absent the outcrop is mapped using the scarp feature it produces above the Cross Hands Rock, by augering and by the traces of sticky orange to brown clay seen in roadside banks, etc.

A collection of fossils in the British Museum (Natural History) was made by D. T. Donovan from a rabbit warren [ST 7998 9558] 0.67 mile (1.1 km) ESE of Symonds' Hall Farm. This position appears to be on the outcrop of the Hawkesbury Clay below the base of the overlying limestones and the surface debris might contain fossils from both limestone and clay. They include: Rhactorhynchia sp.,Wattonithyris cf. fullonica, W. cf. midfordensis, and Palaeonucula waltoni.

An excavation [ST 8046 9628] at Ashel Barn revealed: basal Tresham Rock, bedded brown argillaceous limestone about 4 ft (12 m) on Hawkesbury Clay, reported 5 to 6 ft (1.5–1.8 m). Debris from the excavation contained brown sandy and silty clay with an indeterminate gastropod and Praeexogyra cf. acuminata. It would seem that these came from the Hawkesbury Clay

In many places north of Kingscote the outcrop is undisturbed by landslipping and it can be traced around a number of small hills Its thickness steadily decreases northwards and north-eastwards. It is about 28 ft (8.5 m) thick 800 yd (732 m) WNW of Binley Farm; about 20 ft (6.1 m) at Field Farm, and 400 yd (366 m) to the south; about 14 ft (4.3 m), 300 yd (274 m) SE of Park Farm and about 16 ft (4.9 m) a further 600 yd (549 m) to the south-east.

Eastwards from here it is even thinner and the clay becomes interbedded with soft marl and possibly thin limestone layers. No exposure was seen, but these changes are easily detected by the use of an auger in the area of Fooks [ST 840 988] and Wallow Green [ST 837 989] and again between Bartonend House [ST 847 981] and 100 yd (91.4 m) NE of Windsorash [ST 851 986]. In the base of the landslip scar between Pinfarthing and Amberley there is white ironshot limestone, representing the Cross Hands Rock. The succession above consists of at least 18 ft (5.5 m) of buff and white, fine-grained, silty limestones, commonly with long slender vertical burrows, and with marl and soft, shaly limestone partings. It is considered that these beds represent the Hawkesbury Clay.

East of the main outcrop Hawkesbury Clay has been detected in the Carriers' Farm, Sherston, Shipton Moyne and Tetbury boreholes (Figure 27 and Appendix 1). Its thickness becomes less north-eastwards and the section in Carriers' Farm Borehole confirms what was detected at outcrop, that the formation becomes very calcareous in this direction and is invaded by ooliths.

Limestones sequence

Grickstone to Boxwell

Tresham Rock

Between Grickstone and Hawkesbury Upton this formation produces a broad dipslope and rather thin soils in which fawn to white compact limestone abounds. The limestone is oolitic and shelly in places, but is predominantly a compact, fine-grained elastic limestone. These are mostly non-oolitic calcitic grains, but a few, up to 0.07 mm in diameter do show a single thin calcareous pellicle. There is a scatter of small quartz grains and the matrix is of finely crystalline calcite.

In the extreme south, around and to the south of Grickstone Farm, gastropods are common at or very near the top of the Tresham Rock, as seen in a temporary excavation [ST 7697 8142] in which hard, blue, splintery limestone with gastropods was overlain by fine-grained Athelstan Oolite and also in surface debris [ST 7703 8171], [ST 7718 8231] and [ST 7724 8241]. Just north of Grickstone Farm the outcrop expands and the gastropod limestone occurs commonly in field brash, e.g. at locality [ST 7753 8326] where Aulacothyris sp. and Nerinella sp. nov.were collected; at locality [ST 7753 8328] where Nerinella sp. nov., Liostrea sp. were collected. Pholadomya deltoidea was obtained from field brash at locality [ST 7730 8411], 1150 yd (1052 m) 98° from Upper Widdenhall Farm, Horton, while D. T. Donovan obtained the following from field brash at the very top of the Tresham Rock outcrop [ST 7794 8529] 250 yd (229 m) N of the west end of Bodkin Hazel Wood: Rhactorhynchia obsoleta, Avonothyris sp. nov. A.

The base of the Tresham Rock forms a capping outlier to Clay Hill, Hawkesbury, and it is from here that the fauna listed by Arkell and Donovan (1952, p. 235) came.

A section within the upper two-thirds of the Tresham Rock was revealed in the track-side [ST 7950 8865] 210 yd (192 m) NNE of Upper Kilcott, just north of the old quarry: brown to fawn, rather soft, impure 'chalky' oolite, 7 ft (21 m), on brown, hard, compact limestone with scattered white ooliths about 1 ft (0.3 m), fawn to brown, less compact, rather rough-textured, oolitic limestones 3 ft (0.9 m), brown to cream, hard, compact limestone 7 ft (21 m), brown, hard, compact limestone and shaly, fine-grained limestone 1 to 2 ft (0.3–0.6 m) over cream-coloured, hard, compact limestone with some shell detritus in the top part, seen for 6 ft (1.8 m). Interbedded developments of oolite, of which the top part of the above section is one, appear more commonly north of here. Around the south end of Hammouth Hill one oolite layer can be mapped thickening northwards and apparently joining with the overlying Athelstan Oolite.

Immediately north-east of here, near Twizzle Well [ST 800 905], the Tresham Rock crops out in a slope at the head of a steep-sided valley. In this position there is little camber in the limestone and a moderately accurate thickness measurement is possible. Some 20 ft (61 m) occur in the valley side and about 10 ft (3 m) more above, giving about 30 ft (91 m) for the complete Tresham Rock. This is similar to the estimate obtained 1.5 miles (2.4 km) due north from the outcrop [ST 801 924] near Boxwell Farm. About 80 yd (73 m) S of Furlongs Farm, Tresham, 8 ft (2.4 m) of cross-bedded oolite with marl partings occur in a tilted condition above landslip. In its present position this must rest near the base of the Tresham Rock and may be the Hens' Cliff Oolite of Arkell and Donovan (1952, p. 236).

Westwards and west-south-west from Tresham the spur of high ground is capped by the Tresham Rock which, from most of its thickness, produces surface debris of buff compact limestone. Near the end of the spur [ST 7834 9075] the basal portion produces a brash of slabby, rather shelly limestone containing ooliths. In thin section this is a elastic limestone, different samples revealing various grain sizes, of shell and bryozoan detritus, small rounded fragments of fine-grained limestones, some with oolitic rims, shale fragments, occasional?derived ooliths and quartz grains. From here D. T. Donovan obtained: Ceratomya concentrica, Homomya gibbosa, 'Lucina' sp., Pholadomya sp. and Protocardia buckmani. From locality [ST 7868 9086] near Furlongs Brake, he also obtained Holectypus sp. About 500 yd (457 m) N of Tresham is the arcuate slip scar of Hens' Cliff. The base of the northern part of this scar is draped in a scree of rock rubble at least 4 ft (1.2 m) thick, composed of blocks of oolite and other limestones. About 30 yd (27 m) to the S, near the top, two crags revealed: buff, compact, fine-grained limestone containing microscopic ooliths, or carbonate silt showing thin oolitic coatings, 2 ft (0.6 m), above an unexposed interval of 4 ft (1.2 m), on rather fissile, compact limestone, similar to that above, with shelly layers containing ooliths, 3 ft (0.9 m).

At Boxwell, a quarry by the road junction 480 yd (439 m) ESE of Boxwell Church [ST 8156 9251] shows 3.5 ft (1.1 m) of fine-grained, white Athelstan Oolite on cream-coloured to brown compact limestone with open vertical fracturing due to slight camber. Clusters of upward-branching tube-like voids, moulds of Peronidella pistilliformis', rise from bedding surfaces, mainly in top 7 ft (21 m) and particularly at 5 ft 2 in (1.6 m) below top, seen for 15 ft (4.6 m). In the Athelstan Oolite most ooliths possess large elastic cores having rather thin pellicles while some clasts are poorly rounded with no pellicle. The matrix is finely crystalline calcite. A sample from the top part of the Tresham Rock revealed a non-oolitic rock consisting of fine-grained, calcareous sand or silt, containing slightly larger quartz grains and small fragments of shell. It also contains Rhactorhynchia sp., Chlamys (Radulopecten) hemicostata fairly common, and Pseudolimea aff. scabrella. Similar 'sponge' remains have been observed at other localities, e.g. in oolites at Simmonds Quarry [ST 8624 0144], Burleigh, Minchinhampton (Sheet 234).

Athelstan Oolite

The most southerly development of Athelstan Oolite is near Cross Hands, just north of the main road (B 4040) and some 550 yd (503 m) due E of the Cross Hands crossroads [ST 765 811]. It was observed as surface debris and here Lansdown Clay separates it from the Great Oolite above and from the Tresham Rock below. The lower clay is thin and was proved by augering, but the overlying clay is still thick enough to produce a small feature. The oolite is white and fine-grained.

It was next seen in the surface debris of the higher parts of the fields north of Grickstone Farm and, in the angle between Hall Lane and the Bath Road, a quarry [ST 7761 8405] exposed 10 ft (3 m) of thin-bedded cream-coloured pure oolite.

Although no clay stratum separates them north of this quarry the Athelstan Oolite is distinguishable from the overlying Great Oolite by the fineness of the surface fragments it produces and the greater purity of the oolite. The line of separation is marked [ST 7760 8454] by sporadic lumps from coral blisters containing Isastraea sp. and other, recrystallised, corals which possibly grew on the top surface of the Athelstan Oolite. In the quarry [ST 7785 8480], 50 yd (46 m) from the south-west corner of Bodkin Hazel Wood (p. 182) a few inches of oolite of fine and even grain size are still visible in the base of the face. The top shows a very brown rind and the surface is flat and prolifically oyster-encrusted. Insufficient of this surface was seen to detect extensive boring, but the observations suggest that a depositional hiatus separates the Athelstan Oolite from the Great Oolite here.

D. T. Donovan collected fossils from the surface brash, but probably all from the Athelstan Oolite, at locality [ST 7987 8910], Rhactorhynchia obsoleta and Epithyris sp.,at locality [ST 7762 8362], R. obsoleta?, Avonothyris sp. nov. A and Epithyris?, at locality [ST 7994 8905], Rhactorhynchia obsoleta, terebratulide indet., Avonothyris sp. nov. A.

A quarry [ST 7857 8640] 50 yd (46 m) SE of the Barley Mow public house exposed: thin-bedded, cream-coloured to brown oolite 2.5 ft (0.8 m) on rather coarse, shell-detrital oolitic limestone, 2 in (5 cm), over thinly cross-bedded, cream-coloured oolite 4 ft (1.2 rri).

Surface debris in the fields of this area reveal that an increased proportion of shell detritus is present within the Athelstan Oolite, and D. T. Donovan obtained from this debris, Rhactorhynchia obsoleta? and Epithyris sp. [ST 7870 8631].

The basal layers of the formation crop out immediately to the north-east of Hawkesbury Upton and a soft, biscuity oolite or oolitic limestone predominates. Some 6 ft (1.8 m) of these beds containing pockets of sparsely oolitic limestone and shelly oolite at the bottom are exposed in the banks of the lane [ST 7824 8724].

The top part of the formation can be observed in the Starveall quarries. In the north-east quarry [ST 7972 8784] (p. 182) about 1 ft 8 in (0.5 m) is exposed (Bed 1). The top 1 in (3 cm) is a thin coffee-coloured crust the surface of which is flat and pitted by mollusc borings. The colour fades down through the next 2 in (5 cm) into cream-coloured pure oolite containing Liostrea sp. and Pseudolimea sp. The surface is overlain (Bed 2) by about 10.5 in (26 cm) of grey and brown, smooth, laminar clay, containing Belemnopsis sp.,Liostrea sp. and Pseudolimea sp. and with some friable marly oolite towards the base. In the south-west quarry [ST 7964 8778] beneath 2 to 3 in (5–8 cm) of laminar grey and brown clay the Athelstan Oolite consists of: white to cream-coloured oolite, some shell-detrital and cross-bedded, upper surface slightly hardened and brown, showing, rounded depressions about 1 in (3 cm) diameter and thickness varying from 3 ft (0.9 m) in the south to 1 ft 8 in (0.5 m) in the north, on regular layer of resistant shell-detrital limestone 2 to 4 in (5–10 cm) over white to cream-coloured oolite seen

4 ft (1.2 m). The Athelstan Oolite is not strongly cross-bedded. A quarry [ST 7921 9001] at Field Barn (south), Tresham, revealed 3 ft (0.9 m) of Athelstan Oolite, rather coarse at the top. The top surface of the oolite, seen mainly as surface debris around the rim of the quarry and in adjacent fields, is pitted by mollusc borings and covered with oyster shells. Here it is overlain by Forest Marble.

A similar relationship occurs in the quarry [ST 7965 9090] 110 yd (101 m) SSE of Burdon Court, Tresham, where the top 3 ft (0.9 m) of Athelstan Oolite are exposed as a fine-grained white oolite, hardened and brown at the top, the surface of which is bored and flat and overlain by a marl with a Forest Marble fauna (p. 195). Although the Coppice Limestone does not intervene between the formations in the quarry, there is ample evidence of it close by (p. 174). A quarry [ST 8004 9075] on the south side of the lane 250 yd (229 m) W of Field Barn (north), Tresham, exposes 20 ft (6.1 m) of Athelstan Oolite (p. 174). The bottom 10 ft (3 m) are a more massive oolite than the top 10 ft (3 m), harder and with a higher proportion of shell detritus and the basal few feet produce large blocks. The top 10 ft (3 m) are composed of white, rather soft oolite breaking freely into small rectangular fragments. The uppermost 3 to 6 in (8–15 cm) are hardened and brown, and the surface is pitted by small borings. This surface is visible only in the northern half of the eastern face and is overlain by about 2 ft (0.6 m) of Forest Marble shell-detrital, oolitic limestone yielding digonellids and Rhactorhynchia obsoleta. The southern part of the face shows only horizons below this surface, but fissures, penetrating down about 1 ft 6 in (0.5 m) into the oolite, are filled with rather ochreous, compact, fine-grained limestone containing rhynchonellids which is probably an infilling of Forest Marble. Some 1 ft 6 in to 2 ft (0.5–0.6 m) below the top of this face there is at least one small area of cream-coloured, porcellanous limestone. It is surrounded by and merges into the oolite and is the nearest resemblance to Coppice Limestone observed in the quarry and it is distinct from the brown, hardened top of the Athelstan Oolite. Nearby, in Hamgreen Covert, a quarry [ST 8112 8073] is partially filled but still exposes 15 ft (4.6 m) of fine-grained rather compact thin-bedded oolite. Traces of Coppice Limestone occur in the soil above the north side of the quarry, Shakespeare Quarry, a further 700 yd (640 m) NNE shows 2 ft (0.6 m) of brown shelly limestone (Forest Marble) resting on about 13 ft (4 m) of cross-bedded cream to fawn and white oolite, but the top surface of the Athelstan Oolite does not appear to be bored. The Athelstan Oolite is exposed again at Boxwell (p. 165) and a calculation based on the map suggests that its total thickness nearby is about 30 ft (9.1 m).

North of Boxwell to Kingscote

The valley 300 yd (274 m) NW of Bowldown Farm is incised deeply enough to expose Athelstan Oolite and Tresham Rock in an old quarry [ST 8382 9292]. Some 4 ft (1.2 m) of Athelstan Oolite were seen consisting of thinly cross-bedded white oolite and brown biscuity limestone, overlying 2 ft (0.6 m) of Tresham Rock which is a white compact limestone.

North and west of Ozleworth is a long SW–NE aligned spur. Between the end of the spur and the summit [ST 795 951] (774 ft (236 m) above OD on the one-inch map) three outcrops of oolite were encountered. These form slight scarp features and are interbedded with buff, fine-grained compact limestones, mainly sparsely oolitic, often buff and fissile, especially in the lowest member on the end of the spur, where fine shell detritus is common. The mainly non-oolitic limestone is considered to approximately equate with the Tresham Rock, having within it the lowest two oolite layers. The third and thickest layer of oolite, at Goose Green [ST 791 946] is probably an approximate representative of the Athelstan Oolite, for a little higher at the summit (774 ft above OD) is debris of more oolite and fragments of porcellanous limestone like the Coppice Limestone. Between here and the main oolite of Goose Green is debris from a thin layer of compact buff and creamy fine-grained sparsely oolitic limestone which is perhaps the same as the better developed one at the top of the same group of limestones further north around Woodleaze Farm [ST 816 977].

The plateau top of this spur, north-west of Ozleworth, is bounded by an almost continuous landslip scar therein giving good exposure of the limestones of the lower parts of the group; lateral variants probably of the lower parts of the Tresham Rock.

Adjacent to the drive [ST 7881 9374], 130 yd (119 m) NW of Fernley Farm, the following succession, near the base of the limestones is exposed: fine-grained, fissile, tily, fawn to brown compact limestone, some layers oolitic and some of biscuity texture containing calcitic (?echinoid) fragments 9 ft (2.7 m) above an unexposed interval 12 ft (3.7 m) on fine-grained fawn to brown hard compact seemingly non-oolitic limestone, with some growth-like ramifying ochreous voids, seen for 6 ft (1.8 m).

Some 220 yd (201 m) to the SSW [ST 7871 9355] a similar exposure reveals: tily and flaggy buff to brown fine-grained limestone, seen for 6 ft (1.8 m) on flaggy hard and crystalline brown partially oolitic limestone and fine-grained compact limestone with ochreous sub-vertical growth-like ramifying voids near base, seen for 6 ft (1.8 m).

About 500 yd (457 m) W of Newark Farm further exposures [ST 7764 9320] show 10 ft (3 m) of fissile, fawn to pale grey, mainly non-oolitic, compact limestone with fine shell detritus and shelly oolitic layers.

Some 400 yd (366 m) to the SW, on the end of the spur, a small excavation [ST 7745 9255], almost at the base of the limestones, revealed 3 ft (0.9 m) of brown, hard, 'spangly' (?echinoid detritus) shelly limestone, slightly oolitic. Similarly, the basal layers of the limestone, immediately above Hawkesbury Clay, 700 yd (640 m) to the NNW [ST 7712 9312], consist of brown, slabby, shelly, slightly oolitic limestone and contain flakes or pebbles of clay.

A quarry [ST 7773 9338] 1400 yd (1280 rn) SW of Field Barn, Ozleworth and not so close to the base of the limestones revealed: buff to brown, fine-grained, tily limestone, mostly non-oolitic, 4 ft (1.2 m), fissile, cream-coloured, rather compact limestone with fine-grained ooliths and shelly oolite, 2 ft (0.6 m), buff to brown, fissile, oolitic limestone about 6 ft (1.8 m), more massive, brown to buff fine-grained compact limestone mostly oolitic, seen 6 ft (1.8 m).

The top limestone bed is mainly fine-grained, composed of rounded, derived limestone fragments, fine shell debris and a scatter of quartz grains in a calcite matrix. The underlying limestones are similar, but the rounded clasts are more commonly coated with at least one calcareous pellicle.

In the quarry [ST 7910 9465] at Goose Green at the north end are: limestone rubble and soil 3 ft (0.9 m), on thin-bedded, almost fissile, cream-coloured biscuity oolite, possibly cross-bedded, 14 ft (4.3 m).

At the south end of the quarry the oolite is shell-detrital in parts. A SSW-trending gull, 1 ft (0.3 m) wide occurs in the oolite, so that the 2° dip of strata to the NNW might be camber.

From the summit (774 ft (236 m) above OD) of the Ozleworth spur northwards and north-westwards to the spur directed south-westwards from Symonds' Hall Farm, a descending order of beds crops out and field brash reveals the same or a similar set of oolite layers within buff or orange-coloured, fine-grained, detrital limestones, usually oolitic. A quarry [ST 7743 9537] towards the end of the spur exposes limestones near the base of the group: buff-coloured fine-grained elastic limestone, 3 ft (0.9 m), on orange-coloured, medium-grade, shelly, oolitic limestone, seen for 2 ft (0.6 m).

There is another quarry [ST 7845 9624] in 5 ft (1.5 m) of similar limestones at about the same horizon, while a further quarry, now infilled [ST 7941 9646], 740 yd (677 m) NE of Symonds' Hall Farm and stratigraphically higher than the previous two revealed: rather fissile, buff to grey, compact, argillaceous limestone with ooliths, 3 ft (0.9 al), on softer marly limestone, 1 in (3 cm), over buff, detrital, oolitic limestone, semi-crystalline and finely cross-bedded, rather fissile when weathered and the basal part less oolitic (with marly parting at 1 ft 8 in (0.5 m)), 5 ft (1.5 m).

Microscopically the basal bed is largely a elastic limestone composed of non-coated small fragments of shell, bryozoa and other fine-grained limestones including oolites with a calcite matrix. The top bed is largely a very fine-grained, elastic limestone with small shell fragments, small rounded fragments of fine-grained limestones and fine quartz sand in a calcite matrix. A few grains possess a thin 'oolitic' pellicle. Other small roadside quarries west-north-west and north-west of Ashel Barn reveal similar limestones at about the middle of the group of limestones.

An old quarry [ST 8002 9577] 1050 yd (960 m) 238° from Ashel Barn exposes about 4 ft (1.2 m) of grey to white, hard, compact finely oolitic, and finely sandy limestone. A 2 in (5 cm) rubbly marl occurs in the middle of the limestone, while laterally the latter passes into buff to brown, hard, fine-grained oolitic limestone, semi-porcellanous, white oolitic limestone and white porcellanous limestone. D. T. Donovan collected Avonothyris sp. nov. A from here and the Coppice Limestone crops out in the field immediately above the quarry face.

Further east, as seen in the old roadside quarry [ST 8214 9523] 1120 yd (1024 m) SSE of Kingscote Church, the top few feet of the Limestones sequence consist mainly of a pure oolite referable to the Athelstan Oolite, in the same way as oolite to the south and east is. There is no sign of terrigenous and other detrital material, which is present in the limestones to the west:

Kingscote to Minchinhampton

Around and to the north of Kingscote the binary division of the limestone into Athelstan Oolite and Tresham Rock remains inoperative. In a quarry [ST 8136 9792] 400 yd (366 m) WNW of Woodleaze Farm, the horizon of the Athelstan Oolite is occupied by compact, cream-coloured, finely oolitic limestone 10 ft (3 m) of which are exposed, overlying buff, hard and massive-bedded oolitic limestone, 4 ft (1.2 m) of which are visible and which yielded Eryma bedelta.

The underlying beds consist of buff, compact, fine-grained, shell-detrital limestones, sparsely oolitic limestone and shell-detrital oolitic limestone which, when weathered, are rather fissile or thinly bedded. The bottom 22 ft (6.7 m) are visible in crags [ST 8176 9855] 280 yd (256 m) S of Woodleaze Farm. The lower parts of this limestone formation crop out also in outliers north-east of Woodcock Farm, Owlpen, around Field Farm [ST 8087 9950] and Park Farm, Nympsfield [SO 8122 0081] and on the nearby Lynch Knoll [SO 8155 0043]. These places reveal flaggy, grey, brown and bluish, ill-sorted shell-detrital, slightly oolitic limestones.

Tetbury, Cherington and Rodmarton areas

Near Tetbury Cottage Hospital a roadside quarry [ST 8937 9287] shows:

The?Coppice Limestone here yielded: echinoid spines, Epithyris sp.,Anisocardia truncata, Astarte sp., Ceratomya concentrica, Limatula sp.,' Lucina' bellona, Parallelodon hirsonensis, Plagiostoma sp., Protocardia stricklandi, Bactroptyxis cf. bacillus. The underlying marl yielded: cidarid spine, Avonothyris sp. nov. B, Anisocardia truncata, Ceratomya concentrica, Chlamys (Radulopecten) hemicostata, Meleagrinella?, Pholadomya lirata, Placunopsis socialis, Plagiostoma sp., Protocardia?, Dicroloma sp., Pleurotomaria sp.,wood fragments. The Athelstan Oolite yielded : Astarte oolitharium, Entolium?, Plagiostoma sp.,Protocardia?.

The north end of the Tetbury railway station cutting [ST 8937 9326] shows:

Comparison of this section with that near Tetbury Cottage Hospital shows that Bed 10 of the latter almost certainly is Bed 4 of the former. Whether it should be considered as the top of the Athelstan Oolite or base of the Coppice Limestone is not clear, but if the Lithophaga is significant then the top of the Athelstan Oolite is better placed at the top of Bed 3.

Woodward (1894, p. 275) records this succession as a composition from a number of exposures. In it he classified Athelstan Oolite, Coppice Limestone and even higher beds as 'White Limestone', but he noted and collected the characteristic Avonothyris sp. nov. B from the distinct bed at the horizon of Bed 3, which enables the separation of the immediately overlying strata into a discrete Coppice Limestone unit.

A comparable section occurs in Veizey's Quarry [ST 8811 9435] 1 mile (1.6 km) NW of Tetbury:

Woodward (1894, p. 276) described these beds, but he classified the strata of Beds 2 to 4 under "Great Oolite (Kemble Beds)".

Coppice Limestone

Starveall, Leighterton and Boxwell area

In this area the Coppice Limestone crops out as a sporadic development between the Athelstan Oolite and, mainly, the Forest Marble, the latter yielding Digonella digona at the base.

The most southerly undoubted outcrop of the limestone is revealed as field brash some 250 yd (229 m) E of Nan Tow's Tump, near Oldbury on the Hill. Brash of mollusc-bored porcellanous limestone occurs at places for 1 mile (1.6 km) or so to the ENE and a small exposure [ST 8231 8974] 1 mile (1.6 km) NNE of Oldbury shows 3 ft (0.9 m) of porcellanous limestone with a flat bored and oyster-encrusted top surface, overlain by 1 to 2 ft (0.3–0.6 m) of khaki clay and Reef Bed limestone of the Forest Marble. Some 300 yd (274 m) to the NW [ST 8214 8992] abundant fragments of this limestone occur in the field brash and have been used in the construction of walls. It is fawn to pink, very hard and splintery with an almost conchoidal fracture. In some fragments there are small lumps of compact limestone (algal or stromatoporid growths) and ooliths with interstitial brown calcite. Empty moulds left by gastropods are common and other fragments have yielded: borings, nerinellid indet., Astarte?, Fimbria?, 'Lucina' bellona. Nearby [ST 822 899] 1250 yd (1143 m) 195° from St Andrews Church, Leighterton, H. C. Ivimey-Cook collected surface debris yielding: Bakevellia?, Fimbria cf. neptuni common, Isognomon (Mytiloperna)?, Mactromya?, Globularia formosa, borings indet.

It is in this area that a clear indication of the relationships between the Coppice Limestone and the overlying beds would be most valuable in deducing whether part or all of it represents the Great Oolite of further south, or whether it is a formation representing totally subjacent strata. Unfortunately good exposures are rare.

In the quarry [ST 8510 8964] at Westonbirt (p. 192), 1 ft 4 in (0.4 m) of hard, brown, porcellanous limestone is overlain by up to 18 ft (5.5 m) of Great Oolite. The porcellanous limestone here is not typical of the Coppice Limestone to the north and its fauna includes: Chomatoseris sp., Stylina sp.,compound coral indet., Plagioecia sauvagei, serpulids indet., cidarid spines indet., echinoid fragments indet., Avonothyris sp. nov. Bcommon, Epithyris oxonica, terebratuloids indet., very common, Arcomytilus sp.,Camptonectes sp., Chlamys (Radulopecten) vagans, Isognomon sp.,Liostrea sp. fairly common, Modiolus sp., Plagiostoma subcardiiformis.

The presence of Avonothyris sp. nov. Bsuggests there is no correlation with the rock of similar appearance and rather problematic horizon, exposed at Crow Down Springs (p. 190), but does relate it to the Coppice Limestone of Tetbury (pp. 170–71).

A little further north, more typical Coppice Limestone was seen in small exposures in the valley slopes, for instance on the county boundary [ST 8433 9011] where it yielded Gresslya?, Isocyprina?, and Trigonia and also 1320 yd (1207 m) to the W [ST 8312 9004]. On the circumference of Bowldown Wood, a quarry [ST 8464 9170] revealed up to 3 ft (0.9 m) of white to brown porcellanous limestone with an uneven surface showing depressions of up to 1 ft (0.3 m) infilled with a yellow sandy marl. The overlying oolites here were mapped as possibly Great Oolite.

The type section of the formation is in a small quarry [ST 8080 9059], alongside the coppice in the northerly angle between the main A46 Bath–Stroud road and the lane to Tresham. It revealed:

In the quarry [ST 8004 9075] 500 yd (457 m) ESE of Burdon Court, Tresham, the Forest Marble limestone rests directly on the bored surface of Athelstan Oolite, no porcellanous limestone intervening, and in another quarry alongside Burdon Court this condition is paralleled. However, it is obvious that both north and south of Burdon Court the Coppice Limestone is well developed and D. T. Donovan (personal communication 1963) observed the following sequence in a silage pit [ST 7964 9081] adjacent to the sharp bend in the lane 200 yd (183 m) SSE of Burdon Court.

The last few exposures described provide factors essential to any consideration of the position and affinities of the Coppice Limestone. The top few feet of the Athelstan Oolite seen in the quarry 500 yd (457 m) ESE of Burdon Court have been discussed (p. 167), certain aspects of the oolite suggesting a possible lateral passage into Coppice Limestone. On the other hand the exposures at Crapwell Coppice and in the silage pit show a clear separation of the porcellanous formation from the underlying oolite.

Some 0.75 mile (1.2 km) NE of Burdon Court abundant slabs and lumps of hard porcellanous limestone occur as field brash and wall-stones [ST 8053 9156]. The limestone of many slabs has a conglomeratic appearance, containing small lumps of porcellanous limestone and it is also oolitic. At least some of the rounded pebble-like lumps are algal or stromatoporoid growths, while gastropods and lamellibranchs are common. The fauna also includes: echinoid fragments, Montlivaltia?, Sarcinella socialis, Astarte?, Costigervillia crassicosta, Liostrea hebridica, Lithophaga fabella fairly common, 'Lucina' sp., Trigonia pullus, Vaugonia?, Bactroptyxis bacillus, Cylindrobullina sp., Cossmannea sp. nov.common, nerineid, Trochus?.

At Boxwell the formation crops out in the bank [ST 8148 9238] at the side of the lane leading north-east from Boxwell Farm to Boxwell Court and 2 to 3 ft (0.6–0.9 m) of a flesh-coloured, hard, compact porcellanous oolite are visible. Fossils from here included Anisocardia?, Astarte?, trigoniid indet., Pleurotomaria cf. composita, borings indet.

Boxwell to Tiltups End

Splintery porcellanous limestone occurs extensively as surface debris on the plateau around Lasborough and Bagpath. Further east between Bowl-down Farm [ST 839 926] and Calcot Farm [ST 840 950] the formation crops out in the slopes of small valley depressions.

Coppice Limestone is well exposed in the roadside quarry [ST 8214 9523] 1120 yd (1024 m) SE of Kingscote Church (p. 169). It appears to pass down into Athelstan Oolite as part of a bed 4 ft 9 in (1.45 m) thick. The thickness of the porcellanous limestone at the top varies from 6 in to 3 ft (0–15–0.91 m) and fossils from it include: bryozoan indet. fairly common, Lithophaga?, Plagiostoma bynei, Cosmannea sp. nov.fairly common, Fibula sp. The oolite of the bed contains algal or coral patches and Thamnasteria sp.

An old quarry (p. 197) on the north side of the road junction [ST 821 984] 720 yd (658 m) 37° from Woodleaze Farm exposes porcellanous Coppice Limestone overlying Athelstan Oolite as follows: cream-coloured to buff, flinty, porcellanous limestone, bored to a depth of 6 in (15 cm) and with a flat, oyster-encrusted surface, 1 ft (0.3 m), passing down to a fine-grained white compact limestone, chalky and showing traces of horizontal bedding (Figure 15), 4 ft (1.2 m), over similar limestone slightly oolitic and biscuity seen, 1 ft (0.3 m).

Fossils obtained from the porcellanous limestone include: coral fragments, echinoid indet., pentacrinoid indet., Limopsis?, Liostrea sp., Lithophaga sp.,nerinellid indet., while the underlying chalky limestone yielded: rhynchonelloids, terebratuloids, gastropod indet., Protocardia? and an ammonite indet.

In the old quarry [ST 8440 9690] on the east side of the main road 290 yd (265 m) SSW of the cross-roads at Tiltups End, 2 ft (0.6 m) of hard porcellanous oolite with a brown bored top passes down into white Athelstan Oolite, 2 ft (0.6 m) of which were visible. Witchell (1886, p. 267) saw 7 ft (2.1 m) of these limestones. His "upper stratum of the Limestone" contained large numbers of nerinellid gastropods and is clearly the Coppice Limestone with "Terebratula maxillata, and Lima sp.", corals and bryozoans also being common.

The quarry [ST 8456 9723] at Tiltups End (p. 196) has been the subject of considerable attention. Witchell (1886, p. 267), as part of a larger study of the "White Limestone", which includes the Coppice Limestone, noted 4 ft (1.2 m) of white and chalky limestones, some very hard and containing numerous fossils in a highly crystalline state. These include: Bactroptyxis bacillus, Cossmannea sp. nov.and Fibuloptyxis witchelli. The ammonite Bullatimorphites bullatimorphus reported to have come from here is labelled "Tiltups Inn" (Lycett 1863; Cox and Arkell 1950, p. 52; Torrens 1967, p. 88, 1969a, p. 68) and is embedded in a compact micro-oolitic limestone. The Tresham Rock is probably the source of the specimen. Also reported from the quarry are brachiopods which prove to be Epithyris sp. and Kutchithyris?, the latter having possibly given rise to the misidentification Ornithella sp.= Digonella of the literature. If digonellid brachiopods were found in this "White Limestone" they would be the only specimens known from the Coppice Limestone of the district. Digonella ["Ornithella digona"]is present locally (Arkell and Donovan 1952, p. 241), together with other Bradfordian fossils in the base of the Forest Marble which in fact occurs in the same quarry face. Only 1 ft (0.3 m) of porcellanous limestone is now visible at the base of the quarry face and this has a flat, bored surface. It is overlain by a thin layer of brown, marly clay, about 1 in (3 cm) thick where visible, which is devoid of fossils. Witchell (p. 267) also remarked on this clay as being barren, despite the abundance of nerineids below, but he appears to have been influenced by a report, or a miscorrelation, that in the next quarry, 290 yd (265 m) to the south [ST 8440 9690] a layer, which must overlie the Coppice Limestone, was highly fossiliferous containing nerineids.

Tetbury, Minchinhampton and Rodmarton areas

Limestones probably representing a slight variant of the Coppice Limestone are visible near the top of the quarry face [ST 8937 9287] opposite Tetbury Cottage Hospital and in the Tetbury railway station cutting. Fossils obtained are listed with the exposure descriptions (pp. 170–72). Woodward (1894, p. 275) recorded the Coppice Limestone in the cutting as 4 ft (1.2 m) of hard white compact oolitic limestone ("like Dagham stone") with 1 ft 6 in (0.5 m) of soft limestone and marl below. He collected Avonothyris sp. nov. B, Ceratomya concentrica fairly common, and Bomomya gibbosa probably from this marl.

Porcellanous limestone occurs in the base of the excavation for a garage [ST 8948 9340] 400 yd (366 m) NNE of Tetbury Station, but it is absent at Veizey's Quarry [ST 8811 9435] (p. 171) and Picketpiece Quarry [ST 8815 9722] (p. 200). Abundant debris of fawn to cream-coloured, porcellanous limestone occurs in the walls 860 yd (786 m) to the SSW [ST 8769 9649]. This is undoubtedly Coppice Limestone, obtained from nearby outcrops and contains: echinoid spine, Anisocardia caudata, Liostrea hebridica, Modiolus imbricatus?, Pholadomya?, Plagiostoma cf. minchinhamptonensis, Bactroptyxis bacillus very common, Cossmannea sp. nov., Nerinella sp. Near an old laneside quarry [ST 8900 9768], 930 yd (850 m) N of Star Farm camber towards the valley affects the outcrops. This quarry reveals some dip-and-fault structure and exposes 6 ft (1.8 m) of thin-bedded shelly?Great Oolite overlying: buff rubbly hard limestone with Anisocardia truncata, Isocyprina depressiuscula, Mactromya sp. and a nerineid, 0 to 3 in (0–8 cm), on buff, porcellanous limestone containing in the lower part pellet-like lumps and gastropods seen 2 ft 6 in (0.8 m). Nearby [ST 8813 9754] 'Lucina' bellona was obtained from a loose block.

Immediately east of Minchinhampton the outcrop of the Coppice Limestone produces more porcellanous limestone field brash containing many gastropods and bivalves. In an exposure at the side of an old shallow excavation [SO 8809 0100] 950yd (869 m) W of Crackstone, hard porcellanous limestone is seen below a 2 in (5 cm) marl band at the base of the Forest Marble. Overlying is 1 ft (0.3 m) of yellow-buff, thin-bedded, shelly limestone and 3 ft (0.9 m) of surface debris of similar limestone.

Eastwards across Aston Down the formation continues to crop out as a dip surface, producing ragged lumps of bored porcellanous limestone, often oolitic. The Long Stone [ST 8835 9991], standing just north of Gatcombe Park, is of Coppice Limestone and the formation is exposed in places, for instance, in the small quarry [ST 8843 9863] opposite Hill House 800 yd (732 m) N of Avening (see p. 201). Some 0.75 mile (1.2 km) to the north and 400 yd (366 m) E of the Long Stone another quarry [ST 8869 9985] shows:

It is not uncommon in the north-eastern parts of the district and adjacent ground, for the top foot or so of the Athelstan Oolite or Coppice Limestone to yield Nucleolites cf. woodwardi and in places north of Culkerton, the pure porcellanous limestone of the Coppice Limestone is less well differentiated from the top of the Athelstan Oolite or less well developed. Instead the top few feet of oolite are hard, cemented and white or buff, containing lumps of?algal or?stromatoporid limestone and seeming to represent a transition between Athelstan Oolite and Coppice Limestone; for example the quarry [ST 9358 9590] 400 yd (366 m) ENE of Manor Farm, Culkerton, and also the following four quarry exposures. The first [ST 9041 9916] lies 350 yd (320 m) E of Aston Farm.

The second [ST 9267 0074] lies 350 yd (320 m) E of Lowsmoor Farm and is a composite description of various faces in the quarry:

The third [ST 9364 9898] lies 1650 yd (1509 m) ESE of Lowsmoor Farm:

The fourth [SO 9350 0136] is 1350 yd (1234 m) NE of Churnhill Barn

Great Oolite and Forest Marble

Castle Combe area (Sheet 265)

The following two natural exposures reveal information which might correlate with the Twinhoe Beds and Combe Down Oolite of the Bath area (Green and Donovan 1969), and possibly explain why these formations have not been distinguished in the Malmesbury district.

A succession of weathered crags [ST 8432 7714] is present 200 yd (183 m) ESE of Castle Combe Church. The top exposure is here estimated to be some 18 ft (5.5 m) below the base of the Acton Turville Beds.

Similar beds occur to the north in crags [ST 8322 7912] at Coulter's Mill, The Gib, from which the following section has been compiled. The highest part of this section lies some 20 ft (6.1 m) below the Acton Turville Beds.

Further outcrops of shell-detrital oolite occur lower in the slope, but the surface of Bed 4 of the latter section probably correlates with the top of Bed 1 in the former and a northward diminution of thickness from about 32 ft (9.8 m) to about 23 ft (7 m) has taken place in the overlying beds of the Great Oolite. The distance between the two exposures is 1.33 miles (2.1 km), so that a rate of attenuation of approximately 7 ft per mile could be operating and if maintained it would mean that the Great Oolite would be absent about 8 miles (13 km) N of Nettleton (Nettleton Borehole, p. 294). Furthermore, south of Bath, the Twinhoe Beds commonly overlie Combe Down Oolite which has a distinct and bored surface and a recorded thickness of up to 60 ft (18.3 m) (Green and Donovan 1969). It is not unlikely that the bored and oyster-encrusted surface of the oolites in these sections represents the top of the Combe Down Oolite and the overlying marly, coralliferous, detrital limestones with ferruginous pisoliths are the representatives of the Twinhoe Beds. The latter are losing their characteristics northwards and eventually attenuate or are overlapped by the Forest Marble.

Another important section south of the district is that at the eastern end [ST 7928 8123] of the Sodbury railway tunnel. Now visible above the portal are:

Eastward over about 200 yd (183 tn) the horizon of Bed 5 is replaced by brown rubbly and marly limestone, partially coralline, eventually becoming largely a pale, rather compact chalky limestone of reef lithology, with D. digona. In terms of the section by Reynolds and Vaughan (1902, p. 144) Bed 1 is probably their Bed D, obscured horizons (Bed 2) probably represent at least E, while Beds 3 to 7 inclusive represent most of F. The thicknesses given by Reynolds and Vaughan are generalised to apply to the whole section, but the correspondence of their total to that of the present description is close, 21 ft (6.4 m) as opposed to just under 23 ft (7 tn). The 50 ft (152 m) quoted by Buckmann (1924, p. 28) includes Reynold's and Vaughan's Bed D, which has the appearance of the Great Oolite. The statement that it is abounding in the fossils of the Bradford Clay is an exaggeration (Arkell 1933, p. 271). Interpreted strictly, Buckman's term Acton Turville Beds probably includes the top 25 ft (7.6 m) of the Great Oolite, which as far north as here might represent a low part of the Bath succession.

Cross Hands to Starveall

The most northerly stone mines of the Great Oolite occur in the upper division at two quarries between 1 and 1.25 miles (1.6 and 2 km) N of Tormarton.

Brookman's Quarry [ST 7740 7992] still shows:

At Eyles's Quarry [ST 7727 8066] only 6 ft (1.8 m) of bedded oolite freestone now remain visible beneath oolites and shell-detrital oolite of the Forest Marble (Acton Turville Beds).

The Grickstone Beds are marked at outcrop by surface debris of coarse, shell-detrital limestone and at Grickstone Farm [ST 7777 8296] they are exposed in the farmyard to the east.

An overgrown excavation at the south-west corner of Grickstone Farm yard exposes a 1 to 1.5 ft (0.3–0.5 m) thick bed of cross-bedded, coarsely shell-detrital limestone, overlying some 4 ft (1.2 m) of brown, thinly cross-bedded oolites. The rock of the shelly bed is identical with that of the Grickstone standing nearby and contains: echinoid fragments, serpulids, Chlamys, 'Corbula', Liostrea hebridica, Placunopsis socialis, Pseudolimea.

At American Barn [ST 7909 8423] the junction of the basal Forest Marble limestone with the Great Oolite is exposed in a small quarry. Here a small development of the reef limestone appears to be present within a 'pothole' in the top of the Great Oolite.

The main development of the reef limestone is enclosed in a chamber some 3 ft (0.9 m) deep and 3 ft (0.9 m) across, lying beneath the hardened brown top surface of the Great Oolite. The clay above passes over the top with a slight change of dip on the east side. Fossils in the top bed include: Rhactorhynchia obsoleta, Avonothyris cf. corrugata?, Digonella digona.

About 1100 yd (1006 m) to the WNW lumpy surface debris of this compact oyster limestone covers a large area near the outcrop of the base of the Forest Marble. Between Seven Mile Plantation [ST 7860 8454] and Bodkin Hazel Wood [ST 7794 8485] at Petty France, this debris has yielded : Isastraea limitata?, Stylina sp., Rhactorhynchia obsoleta very common, Avonothyris cf. plicatina, Epithyris oxonica, terebratuloids, Arcomytilus sp.,Chlamys sp., Liostrea?, Lithophaga fabella, Nanogyra.

This is the Petty France White Limestone of Arkell and Donovan (1952, p. 235), but is not the same as the hard porcellanous Coppice Limestone which occurs further north, near Leighterton. The base of the Forest Marble lies across the hamlet of Petty France and surface debris of the Basal Limestones, including the Reef Bed, is common in the vicinity. The latter has yielded coral and echinoid fragments and Epithyris oxonica at a place [ST 7878 8622] 300 yd (274 m) 320° from Dunkirk crossroads, and Stylina? and Cererithyris aff. intermedia at a place [ST 7829 8584] 670 yd (613 m) to the WSW. These two occurrences of the Reef Bed reveal the presence of a small WSW-trending fault with downthrow to the north.

The quarry [ST 7785 8480] 50 yd (46 m) from the south-west corner of Bodkin Hazel Wood exposes most of the Grickstone Beds, showing 7 ft (2.1 m) of thinly cross-bedded shell-detrital limestone, shell-detrital oolitic limestone and some marly oolite, on crumbly marl and clay, 1 in (3 cm), over Athelstan Oolite (p. 166).

The total thickness of the Grickstone Beds here is about 10 ft (3 m) and the nearby outcrop of the base of the Forest Marble leaves room only for a very few feet of the cream-coloured oolite (?Combe Down Oolite) overlying the Grickstone Beds.

A roadside quarry [ST 7991 8658] opposite the junction of the Sherston Lane with the Tetbury Road at Dunkirk shows: shell-detrital oolitic limestones 3 ft (0.9 m), passing to cream-coloured shell-detrital oolite 2 ft (0.6 m), passing to fawn or brown soft, fissile, marly oolite 1 ft (0.3 m), on brown clay 2 in (5 cm) over 2 ft (0.6 m) of white Athelstan Oolite. Only 6 ft (1.8 m) of the Great Oolite remain at Starveall quarry [ST 7972 8784] and these belong to the Grickstone Beds as indicated in the following description of the whole section.

In the south-west quarry [ST 7964 8778] 6 ft (1.8 m) of Grickstone Beds were visible, consisting of shell-detrital, oolitic limestone which is cross-bedded and hard with parts having a compact, fine-grained matrix.

Between Dunkirk, Didmarton and Saddlewood Farm [ST 8117 8983] the Basal Limestones crop out extensively, producing debris of flaggy, brown, shell-detrital, oolitic limestones and lumps of compact, fine-grained limestone of the Reef Bed containing detritus of large oyster-shells. Lophoid and other oysters are common together with Isastraea sp., Thamnasteria sp.,Digonella digona, D. digonoides and Pseudolimea duplicata.

Alderton to Knockdown (Sherston Inlier)

Deep cuttings [ST 8445 8243] through strata from near the base of the Cornbrash into the Basal Limestones have been made at each end of the Alderton railway tunnel. The better of these is that at the west end and the description below is taken from three separate positions.

During the survey it was thought that the oolites exposed at the base of this section were the Great Oolite. The more recent discovery of D. coarctata suggests that they might belong to the top of the Basal Limestones of the Forest Marble (Acton Turville Beds), meaning that the Great Oolite could lie some 15 to 20 ft (4.57–6.10 m) below. Fossils collected include: Bed 12. Stylina solida?, Camptonectes annulatus, 'Corbula' buckmani?, Gervillella sp., Liostrea hebridica common, Placunopsis socialis. Beds 8 (part) and 9. Camptonectes laminatus, Dacryomya lachryma?, Liostrea hebridica?, Modiolus imbricatus, Placunopsis socialis, Trigonia pullus, wood fragments common. Bed 8. Ceriocava corymbosa, Obovothyris sp., Liostrea sp., Lopha gregarea, Nanogyra nana fairly common, Palaeonucula waltoni. Bed 7. Liostrea hebridica. Bed 5. Cladophyllia sp., Acanthothiris?, Rhactorhynchia obsoleta, Epithyris sp., Eudesia?, Camptonectes sp.,'Corbula' buckmani, Liostrea sp. common, Meleagrinella echinata?, Dacryomya sp., Placunopsis socialis common, Pseudolimea duplicata, Procerithium sp. Bed 4. Cidaris bradfordensis?, Mesenteripora davidsoni, Kallirhynchia sp. fairly common, Epithyris oxonica, Obovothyris?, Chlamys (Radulopecten) sp., Liostrea hebridica common, Placunopsis socialis?, Procerithium?. Bed 3. Coral fragments, pentacrinoids indet., bryozoans indet. very common, rhynchonelloids indet. very common, Kallirhynchia sp.,terebratuloids indet. very common, Eudesia cardium?, Arcomytilus asper, Camptonectes laminatus?, Entolium sp., Limopsis minima, Liostrea hebridica, Nanogyra sp., Oxytoma costatum, Placunopsis socialis fairly common, Pseudolimea duplicata fairly common, Trigonia sp. [juv.], Procerithium minchinhamptonensis, wood fragments, Bed 1. Cladophyllia sp., Stylina sp., Sarcinella socialis common, pentacrinoids indet., Berenicia sp., Cava subcompressa, Ceriocava corymbosa common, Reptomultisparsa microstoma, Dictyothyris coarctata, Epithyris?, Eudesia cardium, Astarte oolitharium, A. squamula, Barbatia pratti, Camptonectes laminatus, C. (Camptochlamys) rosimon, Corbula' buckmani common, 'Corbula' cf. coriniensis, Dacryomya lachryma, Gervillella monotis, Limopsis?, Liostrea sp., 'Lucina' sp., Musculus lycetti fairly common, Oxytoma costaturn, Placunopsis socialis very common, Plagiostoma cardiiformis, Protocardia stricklandi, Pseudolimea duplicata very common, P. aff. scabrella, Tancredia cf. truncata, Trigonia pullus, Cylindrites turriculatus, Exelissa sp.,nerineid, Procerithium minchinhamptonensis, Procerithium sp. common.

At the south-west extremity of the 'inlier' a quarry [ST 8173 8317] 1190 yd (1088 m) ENE of Badminton Church exposes: 3 ft (0.9 m) of manly clay with siltstone flakes, overlying the top 10 ft (3 m) of Acton Turville Beds consisting of fawn, thinly cross-bedded shell-detrital oolitic limestone.

In the top few inches of the limestone the following fossils were found: Dictyothyris coarctata, Digonella digonoides, Nanogyra nana, Plagiostoma sp.,Protocardia? sp. The rest of the limestones yielded bryozoa indet. fairly common, Kallirhynchia sp.,terebratulide indet. common, Digonella digona, Chlamys sp., Corbula' attenuata?, Liostrea sp.,Placunopsis fibrosa, P. socialis, Plagiostoma subcardiiformis, Protocardia sp.,Procerithium sp.,wood fragments fairly common. Between here and Ashbridge Cottage 1450 yd (1326 m) due E these Acton Turville Beds appear to crop out at the top of the shallow valley sides below which a 10 ft (3 m) thick, hard, fawn, marl occurs. The marl rests on more oolitic limestone belonging to the Acton Turville Beds and it seems that the upper division of limestones, traced north-east from here, passes into clay continuous with that above. Thus, around and to the north of Luckington, it would appear that only the lower limestone occurs and the Acton Turville Beds are correspondingly thinner. If the top of the Great Oolite has been correctly identified at the old quarry [ST 8354 8463] 550 yd (503 m) NNW of St Mary's Church, Luckington, the Acton Turville Beds are there only about 8 ft (2.4 m) thick.

A quarry [ST 8348 8308] 760 yd (695 m) W of Alderton Church shows:

Another quarry [ST 8348 8324] 190 yd (173.7 m) to the N shows:

On the opposite side of the valley a further 320 yd (293 m) N an old quarry [ST 8350 8357] exposes: