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Geology of the country around Thame. Memoir for 1:50 000 geological sheet 237 (England and Wales)

By A Horton M G Sumbler B M Cox K Ambrose

Bibliographical reference: Horton, A, Sumbler, M G, Cox, B M, and Ambrose K. 1995. Geology of the country around Thame. Memoir of the British Geological Survey, Sheet 237 (England and Wales).

• Authors
• A Horton M G Sumbler B M Cox K Ambrose
• Contributors
• Stratigraphy A J M Barron H C Ivimey-Cook R D Lake M D A Samuel C J Wood M A Woods R J Wyatt
• Basement structure and geophysics J M Allsop
• Engineering geology A Forster
• Hydrogeology C S Cheney
• Petrography (Whitchurch Sand Formation) P Allen A Parker

British Geological Survey

London: HMSO 1995. ISBN 0 11 884515 2. © NERC copyright 1995. First published 1995. Printed in the UK for HMSO. Dd301318 C8 9/95 566 59226

• Authors
• M G Sumbler, MA, CGeol, FGS B M Cox, BSc, PhD, CGeol, FGS K Ambrose, BSc British Geological Survey, Keyworth
• A Horton, BSc, CGeol, FGS formerly British Geological Survey
• Contributors
• A J M Barron, BSc, CGeol, FGS A Forster, BSc, CGeol, FGS R D Lake, MA M D A Samuel, BSc M A Woods, BSc British Geological Survey, Keyworth
• J M Allsop, BSc, CGeol, FGS, MIScT H C Ivimey-Cook, BSc, PhD R J Wyatt, MBE formerly British Geological Survey
• C S Cheney, MSc, CGeol, FGS British Geological Survey, Wallingford
• C J Wood, BSc, CGeol, FGS Scops Geological Services, Croydon
• P Allen, BSc, MA, PhD, Hon DSc, FRS A Parker, BA, PhD, FGS Postgraduate Research Institute for Sedimentology, The University, Reading

(Front cover) Cover photograph Brill Windmill [SP 6519 1415], a famous local landmark, is a reminder of days gone by. The hummocks and hollows in the foreground are all that remain of another vanished industry; they result from centuries of quarrying of Portland and Purbeck strata for building stone, lime and sand (A15366).

(Rear cover)

Other publications of the Survey dealing with this and adjoining districts

Books

• Memoirs
• Geology of the country around Leighton Buzzard, Sheet 220, 1994
• Geology of the country around Chipping Norton, Sheet 218, 1987
• Geology of the country around Witney, Sheet 236, 1946
• Geology of the country around Aylesbury, Sheet 238, 1922
• Geology of the country around Henley-on-Thames and Wallingford, Sheet 254, 1908
• Geology of the country around Beaconsfield, Sheet 255, 1922
• Geology of the country around Oxford, Special Oxford Sheet, 1926
• Mineral Assessment Reports
• No. 28 The sand and gravel resources of the country around Eynsham, Oxfordshire, Sheet SP40 and part of SP41, 1977
• No. 38 The sand and gravel resources of the country around Abingdon, Oxfordshire, Parts of sheets SU49, 59 and SP40, 50, 1978
• No. 81 The sand and gravel resources of the country around Dorchester and Watlington, Oxfordshire, Sheet SU69 and part SU59. 1981
• Technical Reports (see also Appendix 2)
• WA/90/50 A preliminary study of potential resources of sand and gravel in Buckinghamshire north of the Chilterns, 1990
• WN/91/14 The engineering geology of the area around Thame, Oxfordshire.

Maps

• 1:2 500 000
• Sub-Pleistocene geology of the British Islands and adjacent continental shelf, 1979
• 1:1 584 000
• Geological map of the British Islands, 1969
• Smooth aeromagnetic map of Great Britain and Northern Ireland, 1970
• Tectonic Map of Great Britain and Northern Ireland, 1966
• 1:1 000 000
• Geology of the United Kingdom, Ireland and the adjacent continental shelf (South), 1991
• Pre-Permian geology of the United Kingdom (South), 1985
• 1:625 000
• Aeromagnetic map of Great Britain and Northern Ireland, South, 1965
• Bouguer anomaly map of the British Isles, South, 1986
• Geology map of the United Kingdom (Solid Geology), South, 1979
• Hydrogeological map of England and Wales, 1977
• Quaternary map of the United Kingdom, South, 1977
• 1:250 000
• Chilterns, Aeromagnetic anomaly, 1981
• Chilterns, Bouguer Gravity anomaly, 1983
• Chilterns, Solid Geology, 1991
• 1:100 000
• Hydrogeological Map No. 7, South West Chilterns, 1978
• Hydrogeological Map No. 14, Between Cambridge and Maidenhead, 1984]
• 1:50 000 or 1:63 360
• Abingdon (Sheet 253) Solid, 1971
• Abingdon (Sheet 253) Drift, 1971
• Aylesbury (Sheet 238) Solid with Drift, 1990 (Reprint of 1923 edition)
• Beaconsfield (Sheet 255) Solid with Drift, 1974
• Chipping Norton (Sheet 218) Solid with Drift, 1968
• Henley (Sheet 254) Solid with Drift, 1980
• Leighton Buzzard (Sheet 220) Solid with Drift, 1992
• Oxford (Special Sheet; parts of 236, 237, 253, 254), 1908
• Witney (Sheet 236) Solid with Drift, 1982

Preface

This memoir describes the geology of the district covered by 1:50 000 Sheet 237, which extends from the eastern suburbs of the city of Oxford to the western outskirts of Aylesbury, county town of Buckinghamshire. The district is largely agricultural but there is some light industry in the larger villages. Many of the rural population commute to Oxford, Aylesbury, Bicester or to London, using the rail links via Oxford, Haddenham and Aylesbury, or the M40 motorway. The solid geology ranges from Middle Jurassic to Upper Cretaceous, and produces a topography of clay vales separated by hills including, in the south-eastern corner of the district, part of the Chilterns.

At present, chalk is quarried for cement manufacture in the southeastern part of the district, and Jurassic limestones are worked spasmodically for aggregate in the north-west. However, in the past, sand (for building and glass), clay (for brickmaking) and limestone (for building stone and lime) were worked extensively; many of the old college buildings in Oxford are built of local Jurassic limestones. The proximity of this seat of learning, as well as relatively easy access from London, has meant that the local geology, with its lithological variety and rich fossil faunas, has attracted considerable interest in the past; as a result, the area is a classic one for Mesozoic stratigraphy with an extensive literature. This memoir integrates this earlier work with the large amount of new data gleaned from the recent detailed survey of the district. It is designed to be read in conjunction with the 1:50 000 scale geological map.

The demand for subsurface sources of water prompted the Thames Water Authority (now PLC) to request and contribute to the cost of the survey of the north-western part of the district.

The memoir will be of value to the scientific community in giving a concise synthesis of the geological development of the district, to the agricultural community and to those involved in planning, extractive industries, waste disposal and civil engineering, in outlining the distribution and characteristics of the formations depicted on the map.

Peter J Cook, DSc Director. British Geological Survey Kingsley Dunham Centre Keyworth, Nottingham NG12 5GG. 30 December 1994

Acknowledgements

In this memoir, Chapter Two was written by Mr A Horton with assistance from Mr M G Sumbler, Mr K Ambrose, Dr H C Ivimey-Cook and Mr R J Wyatt, and Chapter Three by Messrs Horton and Wyatt, with assistance from Mr Sumbler. Chapter Four was written jointly by Mr Ambrose, Dr B M Cox and Mr Horton, Chapter Five by Mr Horton and Dr Cox, and Chapter Six by Mr Sumbler and Dr Cox. Chapters Seven, Eight and Nine were written by Mr Sumbler, with assistance from Dr Cox (Chapter Seven) and Mr C J Wood (chapters Eight and Nine). Chapter Ten was written mainly by Mr Horton with assistance from Mr Sumbler (river terrace correlation). In Chapter Eleven, the account of basement structure and geophysics was written by Mrs J M Allsop, with assistance from Messrs Ambrose, Horton and Sumbler, and in Chapter Twelve, the account of the Hydrogeology by Mr C S Cheney. Chapter Thirteen was written by Mr A Forster. Appendix 4 was written by Professor P Allen and Dr A Parker, and Appendix 5 by Mr Ambrose. Other parts of the Memoir were written jointly by Mr Horton and Mr Sumbler. Technical reports by the authors, and those by Mr A J M Barron, Mr R D Lake, Mrs M D A Samuel and Mr R J Wyatt (see Appendix 2) have been freely used in the account, and, except in a few special instances, are not specifically referred to in the text. Prof. J H Callomon (University College London), Dr H G Owen (formerly Natural History Museum, London) and Mr H P Powell (University Museum, Oxford) made available their unpublished data. Mr Powell also provided access to the fossil collections and archives in his care. The memoir was compiled by Mr Sumbler, with assistance from Dr Cox and edited by Drs R A Bazley and Audrey A Jackson. The geological survey of the district was in part supported by the Thames Water Authority.

Notes

Throughout the memoir, the word 'district' refers to the area covered by the 1:50 000 Geological Sheet 237 (Thame). National Grid references are given in square brackets; all lie within 100 m grid square SP unless otherwise stated. Author citations for fossil species are listed on p. 160. Enquiries concerning the availability of geological data for the district should be addressed to the Manager, National Geosciences Records Centre, Keyworth.

Geology of the country around Thame—summary

In the past, the rocks described in this memoir have been a prolific source of building stone and can be seen in the fabric of many local buildings, including the Oxford colleges. Other natural resources for the construction industry have been worked in the district and the need for these continues. The memoir gives the geological background information that planners and civil engineers require for the future, if problems such as waste disposal, water quality and foundation stability are to be addressed.

The district extends from the eastern suburbs of Oxford to the western outskirts of Aylesbury, with the ancient market town of Thame lying midway between the two. Between the towns, the area is one of pleasant countryside, with a topography which closely reflects the outcrop geology of Jurassic and Cretaceous rocks. Broad vales, such as the Vale of Aylesbury and the valley of the River Thame, are developed on the outcrop of soft mudstones. They are separated by attractive hills capped by harder strata, which include the famous Corallian rocks of the area near Oxford, and the most extensive outcrops of the Portland Formation in the United Kingdom.

In the late nineteenth and early twentieth century, numerous small quarries and pits attracted the interest of both amateur and professional geologists. The great lithological variety and rich fossil faunas made it a classic area for students of Mesozoic stratigraphy. This memoir integrates some of the large body of existing literature with new information gathered during the recent detailed survey of the district. It indicates where more site-specific information exists in the Survey's extensive archives and is supplemented by a fully coloured 1:50 000 scale geological map, which is published separately.

(Front cover) Cover photograph. Brill Windmill [SP 6519 1415], a famous local landmark, is a reminder of days gone by. The hummocks and hollows in the foreground are all that remain of another vanished industry; they result from centuries of quarrying of Portland and Purbeck strata for building stone, lime and sand (A 15366).

(Geological succession) Geological succession in the Thame district

Chapter 1 Introduction

The memoir describes the geology of the Thame district, which is covered by 1:50 000 Geological Sheet 237. The west and south-western part of the district lies in Oxfordshire and includes both the eastern suburbs of the city of Oxford and the ancient market town of Thame. The remainder lies in Buckinghamshire and includes much of Aylesbury (the county town) and Princes Risborough. Outside these main conurbations, the district is mainly rural, with many small and picturesque villages.

The rocks which crop out in the district are of Jurassic and Cretaceous age (Figure 1). They comprise alternating units of more and less resistant strata which, in general, dip very gently (approximately half to one degree) towards the south-east. The physiography of the district (Figure 2) is dominated by clay lowland, lying mostly at less than 80 m above OD. This is divided into two northeastward-trending vales separated by a range of hills capped by the limestones and sands of the Corallian, Portland, Purbeck and Whitchurch Sand formations, which cross the district diagonally from near Oxford to Aylesbury. North-west of the hills, the vale of the River Ray is formed by the Kellaways, Oxford Clay and West Walton formations. To the south-east, the vale of the River Thame is cut mainly into the mudstones of the Ampthill Clay, Kimmeridge Clay and Gault formations. The latter forms the so-called Vale of Aylesbury, bounded to the south by the steep Chalk escarpment of the Chiltern Hills, which reach 244 m above OD [SP 773 008], near Chinnor. Both the Ray and Thame are tributaries of the River Thames which flows across the extreme south-west corner of the district.

Outline of geological history

The oldest rocks proved within the district are marine mudstones of Cambrian–Ordovician age, which may be several kilometres in thickness. Overlying Silurian lavas and tuffs probably occur at depth in the southern half of the district. These basement rocks were folded and uplifted during the Caledonian orogeny. Subsequently, red beds and fluvial sediments of Late Devonian age, proved in the Noke Hill Borehole [SP 5386 1285], were laid down and probably underlie much of the district.

Early Variscan uplift and erosion preceded the deposition of Carboniferous strata, represented locally by Westphalian fluvial and deltaic sedimentary rocks (Upper Coal Measures), which are now restricted to the westernmost part of the district. Folding and uplift during the Variscan orogeny (late Carboniferous to Permian) produced a platform of Palaeozoic rocks, which remained tectonically stable throughout the Mesozoic. Known as the London Platform, this block influenced sedimentation throughout the Triassic, Jurassic and early Cretaceous periods. During Permian and early Triassic times, it suffered severe erosion, such that the Cambrian–Ordovician basement was once more exposed in the northern and eastern part of the district.

Overall, the Mesozoic rocks record a progressive onlap onto the London Platform, although this was interrupted by several phases of regression and erosion. The earliest Mesozoic sedimentary rocks represented locally are late Triassic continental deposits (Mercia Mudstone Group), present in the north-western corner of the district. Marine transgression in late Triassic (Rhaetian) times resulted in the deposition of shallow-water sediments (Twyford Beds), which overlapped the earlier Triassic beds.

The regional distribution of Lower Jurassic rocks suggests a regression in earliest Jurassic times, with the sea retreating well to the north-west of the district. A renewed transgression occurred in late Sinemurian or early Pliensbachian times, and muds and carbonates (Lias Group) were deposited. In late Pliensbachian times, shallowing of the sea led to deposition of higher-energy facies, which constitute the Middle Lias, but mud deposition resumed in the Toarcian. It is likely that the Lias Group originally covered the whole district. However, regression associated with uplift of the London Platform in late Toarcian or early Aalenian times, was followed by subaerial erosion. This stripped the Lias from the south-eastern part of the district, exposing the Palaeozoic basement rocks once more.

A further transgression occurred in later Aalenian times, with the deposition of mainly shallow-marine sediments (Inferior and Great Oolite groups) continuing into the earliest Callovian. The marginal situation of the district on the London Platform is reflected in the great variety of sediments and environments represented in these groups. Although carbonates predominate, estuarine sediments with seatearths developed at times, particularly in the north-east.

Later Jurassic sedimentation was dominated by quiet water conditions, and a great thickness of muddy sediments (Ancholme Group) was deposited. Phases of somewhat higher energy occurred in the early Callovian and Oxfordian, with the deposition of sandy and silty units (Kellaways Sand and West Walton Formation). In the south-western part of the district, the sands and carbonates of the Corallian Formation indicate the presence of a local structural high in Oxfordian times, and may suggest proximity to land. Deposition of muds resumed in the late Oxfordian; the Ampthill Clay and Kimmeridge Clay were deposited throughout the district.

However, during the late Kimmeridgian, fine-grained sands were again swept into the area.

Shallow-water conditions prevailed in the last part of the Jurassic Period, when the sands and carbonates of the Portland and Purbeck formations were deposited. At times, lagoonal and emergent conditions existed.

During the early Cretaceous, the London Platform was affected by major 'late Cimmerian' uplift. Much of the region became land, on which predominantly fluvial sediments (Whitchurch Sand Formation) were laid down. Aptian times saw the onset of a marine transgression which, in phases, completely inundated the London Platform. The earliest deposits are coarse sands (Lower Greensand), deposited by strong tidal currents on top of the submerged land surface. During the Albian, a return to quiet-water conditions was marked by deposition of thick muds (Gault Formation). In the late Albian, a gradual increase in input of silt and fine sand (Upper Greensand) occurred.

After a minor phase of erosion, the marine transgression resumed, and the Chalk Group was deposited in progressively deeper waters. At first, clay material was swept into the environment, but as shorelines receded, sediments composed almost entirely of biogenic carbonate were deposited.

At the close of the Mesozoic Era, earth movements (Laramide) once more led to regional uplift, and the creation of land on which Palaeogene sediments (probably including locally the Reading Formation) were deposited, but these have long since been eroded and no representatives remain within the district.

The Quaternary history of the district is one of progressive erosion and fluvial downcutting. The earliest deposits, the Clay-with-flints, developed as a result of weathering of the Palaeogene deposits and Chalk. The oldest fluvial deposit, the Princes Risborough Sand and Gravel, was laid down by a south-eastward-flowing river. However, a major change of drainage resulted in the truncation of this and associated streams by the ancestral River Thame. The later part of the Quaternary was characterised by extreme fluctuations in climate. During the cold phases, rapid weathering and erosion occurred. A number of such erosive phases, interspersed with periods of aggradation are recorded by the terrace suite of the Thame. During one of the coldest phases (the Anglian Stage), an icesheet reached the north-eastern extremity of the district and deposited chalk-rich tills. These glacial deposits may have contributed much of the material of the later river terrace deposits.

By the end of the last (Devensian) cold stage, the landscape was essentially as we see it today, but erosion continues to the present. It is most dramatically evident in the landslips which are still sporadically active on some of the steeper clay slopes of the area. The products of this erosion are seen as the silty and clayey alluvium which occurs within the valleys of all the larger streams and rivers of the district. Other deposits, which are still accumulating at some localities, include peat and tufa associated with springs issuing from calcareous strata.

History of research

The Thame district has been the subject of geological research from the earliest days of the science, both because of the great variety of the geology, and because of its proximity to centres of learning at Oxford and London. One of the earliest geological accounts is by Plot (1676). Much early interest related to dinosaur remains; Baker (1753), for example, described fossil bones from brick-pits at the base of Shotover Hill. Parts of the district were described in accounts by Conybeare and Phillips (1822), Greenhough (1819), and Phillips (1855; 1858; 1860a,b; 1871). Major contributions were also made by W H Fitton, J F Blake, W H Hudleston, A M Davies and S S Buckman. More recently, the outstanding research of W J Arkell greatly enhanced the understanding of all aspects of the local geology, particularly of the Jurassic.

Maps at a scale of one-inch to the mile (1:63 360) covering the district (Old Series Sheets 7, 13, 45 and 46) were published by the Geological Survey between 1859 and 1865, and parts of the district are dealt with in corresponding memoirs (Hull and Whitaker, 1861; Whitaker, 1864; Green, 1864). Woodward (1894; 1895) described many sections in the Jurassic rocks of the district, and in the last years of the nineteenth century, the Cretaceous rocks were examined by A J Jukes-Browne (see Jukes-Browne and Hill, 1900; 1903; 1904). Parts of the eastern margin of the district were resurveyed (by Jukes-Browne and A C G Cameron) on the six-inch (1:10 560) scale around the turn of the century for a drift edition of Old Series Sheet 46 which, however, was never published. At about the same time, much of the western part of the district was surveyed at 1:10 560 scale and published as the One-inch Special Oxford Sheet in 1908. A descriptive memoir was published in the same year (Pocock, 1908), and a revised and enlarged edition some years later (Pringle, 1926). Borehole and well records were listed by Tiddeman (1910) for Oxfordshire, and Whitaker (1921) for Buckinghamshire.

The entire Thame district was resurveyed on the 1:10 000 scale between 1986 and 1990. The component maps are listed in Appendix 1, and relevant BGS reports in Appendix 2. A list of other BGS publications pertinent to the area can be found on p. iv.

Chapter 2 Concealed rocks

Palaeozoic

The Palaeozoic 'basement' of the region has been proved in many boreholes, mostly drilled for oil, gas and coal exploration, or, in a few cases, by BGS for strati-graphical purposes. Only two lie within the district. The location of boreholes and the geology of the basement surface is indicated in (Figure 3). A review of the more important boreholes is given by Molyneux (1991).

The oldest rocks proved beneath the district are of early Ordovician (Tremadoc) age, proved directly beneath Jurassic beds in the Westcott No. 2 Borehole [SP 7096 1649]. Geophysical evidence suggests that Cambrian and Ordovician strata may be up to 3 km in total thickness. In other parts of the district, Silurian, Devonian and Carboniferous strata are inferred to occur. In the north-west, the upper surface of this Palaeozoic 'basement' lies perhaps less than 100 m below the surface in places, but it gradually increases in depth to the south-east (Figure 3).

The predominant colour of most of the Palaeozoic succession is grey. However, the uppermost levels, below the Mesozoic cover, are mottled with purplish, reddish and ochreous browns, which result from subaerial weathering and oxidation dating from the Permian and Triassic periods. Pale and greenish grey mottling, which particularly affects the more porous beds, may result from secondary reduction, perhaps in part associated with late Triassic and early Jurassic transgressions.

Ordovician

The oldest strata known in the district were proved in the Westcott No. 2 Borehole between 163.7 m and terminal depth at 173.7 m, and in the adjacent No. 1 borehole. The strata comprise pale greenish grey micaceous mudstones, silty mudstones and highly micaceous sandstones with some purplish mottling. The beds dip at about 40°, and have yielded brachiopods (an acrotretid and Lingulella cf. lepis), and the trace fossil Tomaculum?, which indicate a Tremadocian age (Bulman and Rushton, 1973). The beds are of similar age and lithology to the Shineton Shales of Shropshire.

Tremadoc strata were also proved beneath Lias in several boreholes a few kilometres to the north of the district. The thickest succession was proved in the Calvert East Borehole [SP 6903 2457], which penetrated 291 m of steeply dipping fissile mudstones, with two thin sills of olivine basalt (Davies and Pringle, 1913). The mudstones yielded a few fossils including graptolites of the Clonograptus tenellus Biozone (Bulman and Rushton, 1973). The nearby Calvert West Borehole [SP 6870 2458], Marsh Gibbon No. 1 Borehole [SP 6481 2374] and the Twyford (1–4) boreholes [SP 6802 2569]; [SP 6760 2650]; [SP 6859 2659]; [SP 6697 2651], also proved Tremadocian strata (Bulman and Rushton, 1973).

In the Tring Borehole [SP 9121 1036], to the east of the district, pale greenish grey silty and micaceous mudstones, dipping at about 25°, were proved beneath the base of the Jurassic at 276.2 m depth. The lithologies closely resemble those from the Westcott boreholes, and Dr S G Molyneux (BGS) reports the presence of acritarchs proving a Tremadoc age, although they have previously been regarded as Devonian in some regional studies (e.g. British Geological Survey, 1985; Whittaker, 1985).

Silurian

Silurian strata have been proved in the Bicester [SP 5878 2081] and Little Missenden [SU 9009 9818] boreholes just outside the district (Figure 3). The beds, of shallow-marine origin, are assumed to rest unconformably on the Tremadoc.

In the Bicester Borehole, 44.5 m of interbedded sandstone and mudstone, of probable Late Llandovery age, overlay 128 m of highly altered basaltic and andesitic lavas and tuffs, for which an early Silurian age has been inferred (Pharaoh et al., 1991). The Little Missenden Borehole proved 20 m of grey limestones and siltstones with sandstone beds (Strahan, 1916). The fauna, including brachiopods, trilobites and bivalves suggest a late to post-'Downtonian' age (Straw, 1933). Mortimer and Chaloner (1972) and Chaloner and Richardson (1977) regarded the strata as early Devonian, but ostracods suggest correlation with the late Přídolí (Siveter, 1989), i.e. latest Silurian.

Devonian

Devonian strata including both shallow-marine and continental (Old Red Sandstone) facies are widespread in the region. Although in many cases their precise age is unproven, the strata in the vicinity of the district are mainly Upper Devonian. These strata are typically sub-horizontal or gently dipping and rest unconformably on Lower Devonian strata (Foster et al., 1989). Emsian continental sandstones and mudstones were proved in the Apley Barn Borehole [SP 3438 1066], 20 km west of the district, (Poole, 1969; Chaloner and Richardson, 1977), and these beds may be represented in the southern part of the district, overlying late Silurian strata such as were proved in the Little Missenden Borehole.

In the Steeple Aston Borehole [SP 4687 2586], 191.9 m of Devonian strata were proved below 775.9 m depth (Poole, 1977). The lowest 28.01 m of the succession (Hoperoft Halt Group) consist of reddish purple, green and grey, calcareous, silty mudstones with calcrete nodules and a few thin fine-grained sandstone beds. The overlying beds (Holt Farm Group) comprise shallow-water marine mudstones, limestones and sandstones, overlain by fluvial or deltaic sandstones and conglomer ates, with dark grey marine shales with thin limestones at the top. Evidence from miospores, acritarchs and conodonts suggests a latest Frasnian or early Famennian age for these beds. (i.e. Upper Devonian); fish remains from the Holt Farm Group are probably of Frasnian age.

Within the district, the Noke Hill Borehole [SP 5386 1285] proved 132 m of subhorizontal, greyish green and reddish to purplish brown silty mudstones, siltstones and thinly laminated sandstones, beneath the base of the Triassic Penarth Group at 115.8 m (Falcon and Kent, 1960). The uppermost beds may have included Triassic Mercia Mudstone (see below), but the bulk of the succession was undoubtedly Devonian, yielding the fish Bothriolepis and Holoptychius. The strata between 145.9 m and 200.8 m contained sporadic Lingula, suggesting a near-shore marine environment for this part of the succession, though many of the strata are probably of fluvial origin. Butler (1981) favoured a Famennian age.

The Northbrook Borehole [SP 4995 2246], 4 km northwest of the district proved 58 m of Devonian strata, comprising grey and purplish grey marine mudstones, silt-stones and sandstones, overlain by green and reddish brown siltstones and mudstones with fish and lingulids (Butler, 1981). Stratigraphically, this succession lies between those proved in the Steeple Aston and Noke Hill boreholes. Correlative strata may be represented by the interval 168.5 to 341.1 m in the Bicester Borehole; the succession is dominated by horizontally bedded, purplish brown mudstones and greenish grey, fine-grained sandstones, but also includes minor units of marine limestone.

Just south of the district, 55 m of red beds were proved beneath the base of the Jurassic at 275.7 m in the Chalgrove Borehole [SU 6565 9620]. They comprise horizontally bedded, reddish brown mudstones and silty mudstones with greenish grey reduction spots, and minor beds of greenish grey sandstone and siltstone with mudflake conglomerates. Much of the mudstone succession shows well-developed lamination. Desiccation cracks occur at many levels. Lingula and ?Cypricardella suggest a marine influence at some levels, but the bulk of the strata are more typical of an arid fluvial or playa regime. The basal beds (below about 325 m) yielded a few fish fragments. These include part of a lungfish toothplate, probably of a dipterid, suggesting a mid or late Devonian age (P Forey, British Museum (Natural History), written communication, 1992), and broad equivalence to part of the succession in the Noke Hill Borehole. A possibility that the upper part of the Chalgrove succession is of Triassic age, is discussed further below.

Carboniferous

The district lies on the eastern margin of the north-west-trending Oxfordshire Coalfield Syncline, in which Carboniferous rocks are preserved beneath the Mesozoic cover.

To the south of the district, Dinantian (Lower Carboniferous) strata were proved above the Devonian in the Aston Tirrold Borehole [SU 5581 8720]. The rocks consist of grey, argillaceous, bioclastic limestones interbedded with silty mudstones (Foster et al., 1989). These beds are not present in other boreholes in the region, and are probably overstepped by the Silesian (Upper Carboniferous) to the south of the district (Figure 3).

No Namurian rocks have been recorded in the region, and the Silesian is represented only by Westphalian strata ('Coal Measures'), proved in boreholes to the west of the district, which were drilled to determine the extent of the Oxfordshire Coalfield.

Langsettian (Westphalian A) and Duckmantian (Westphalian B) strata (Lower and Middle Coal Measures) are restricted in distribution, locally being proved only in the Aston Tirrold Borehole, where they comprise 100.3 m of siltstones and mudstones with thin coals, and intercalated basic volcanic and intrusive rocks (Foster et al., 1989).

Bolsovian (Westphalian C) to Westphalian D strata (Upper Coal Measures) are more widespread. They rest unconformably upon the earlier beds and overlap them to rest directly on Devonian strata. The most complete sequence (959.4 m) was proved in the Apley Barn Borehole near the centre of the Oxfordshire Coalfield Syncline (Poole, 1969). The sequence thins eastwards (Foster et al., 1989), as a result of the erosion which followed the Variscan folding, and it is probably absent over most of the district. However, the basal member of the succession, the Arenaceous Coal 'Group', is likely to be represented in the western part. It consists of coarse sandstones interbedded with minor mudstones, seatearths and coals.

Mesozoic

Triassic

Variscan folding in late Carboniferous and Permian times formed the synclinal basin of the Oxfordshire Coalfield and uplifted the region to its east to form the London Platform, which remained a stable structural high throughout the Mesozoic. Permian and early Triassic rocks are unknown on the platform and it is probable that it formed a land area which suffered severe erosion during this period. During later Triassic times, the subsiding Worcester Basin to the west of the platform was gradually infilled with sediment, mainly of terrestrial facies. The succession is divided, in ascending order, into the Sherwood Sandstone Group, the Mercia Mudstone Group and the Penarth Group. From the centre of the basin, these units thin to the east and pinch out against the Palaeozoic rocks of the London Platform. The Triassic is probably represented only in the northwestern part of the district, where the succession is likely to be very thin (Figure 4).

The Sherwood Sandstone Group is represented by the feather-edge of the Bromsgrove Sandstone Formation, probably present in the extreme north-western corner of the district. The formation, proved in boreholes to the north-west, comprises pebbly and gritty sandstones, finer-grained sandstones and silty mudstones, and is probably mainly of fluvial origin (Horton et al., 1987).

The Mercia Mudstone Group consists mostly of reddish brown, silty, generally blocky mudstones with thin beds of greenish grey siltstone and sandstone. It accumulated in an arid environment; the blocky mudstones formed from wind-blown dust on an inland sabkha, and the green beds were deposited in ephemeral lakes or by sheet floods. It is present in the north-western part of the district, where it overlaps the Bromsgrove Sandstone to rest on lithologically similar Devonian strata, from which it can be difficult to differentiate.

Smooth, reddish brown mudstone with greenish grey mottles from between 162.46 and 168.55 m depth in the Bicester Borehole are tentatively assigned to the Mercia Mudstone Group. They are similar in lithology to the underlying Devonian, but differ somewhat in colour.

In the Chalgrove Borehole, 55 m of red beds were proved below the base of the Jurassic. Although the lowest beds are Devonian (see above), Dr B Owens (BGS) reports the presence of Carboniferous miospores in several samples from the upper beds, about 47 m thick. The beds are unlikely to be Carboniferous, as red beds of this age are not known elsewhere in the region. It is, however, possible that the miospores could have been reworked into Triassic sediments, and that the Mercia Mudstone Group rests on the Devonian at this site, although no stratigraphical break was noted in the cores.

Grey-green siltstone and 'chocolate marl' below 115.8 m depth in the Noke Hill Borehole, have been tentatively equated with the Blue Anchor Formation and underlying 'Keuper Marl' at the top of the Mercia Mudstone (Falcon and Kent, 1960). The Blue Anchor Formation, which is cut out beneath the Penarth Group to the north-west of the district (Horton et al., 1987), is unlikely to occur at this site, although the presence of thin Mercia Mudstone is probable; greenish units are common at many levels. The thickness of Mercia Mudstone is uncertain.

The Penarth Group is a marine sequence which forms the youngest (Rhaetian) part of the Triassic succession. The Westbury Formation at the base and the overlying Cotham Formation may be present in the extreme north-west of the district (Horton et al., 1987, fig.11); they are dominated by mudstones and limestones. The overlying Twyford Beds represent a marginal facies of siltstones and locally conglomeratic sandstones, (Horton et al., 1987). These strata rest unconformably on the underlying beds, overlapping the earlier Penarth Group strata and probably also the Mercia Mudstone (Figure 4). However, they are unlikely to extend far into the district, as they are in turn truncated by an erosion surface at the base of the succeeding Lias Group (Jurassic).

In the Noke Hill Borehole (Falcon and Kent, 1960), 5.5 m of pale greenish grey siltstone and white, calcareous, pyritic sandstone between the base of the Lias and the probable Mercia Mudstone may be assigned to the Twyford Beds. The Twyford Beds were probably penetrated by the Bicester Borehole, but no cores were recovered. In the Twyford No. 2 Borehole, 9.3 m of Twyford Beds were proved.

Jurassic

Lias Group

A major transgression, in early Hettangian times, began a process of discontinuous onlap, so that progressively younger strata extend south-eastwards onto the Palaeozoic rocks of the London Platform (Donovan et al., 1979). The sediments deposited comprise the Lias Group, which probably originally extended across the entire district. However, a regression in late Toarcian or Aalenian times led to erosion of the Lias from the central parts of the London Platform, and it is probably absent from the south-eastern part of the district (Figure 5), where it is overlapped by Middle Jurassic strata.

The only boreholes to prove the Lias within the district are those at Westcott and Noke Hill, with thicknesses of 39.5 m and 67 m respectively. Only the former provided cores from which detailed stratigraphy can be determined, but boreholes in adjoining districts have enabled the regional distribution and thickness variation to be established (Figure 5).

Traditionally, the Lias Group has been subdivided into three lithostratigraphical units, the Lower, Middle and Upper Lias, which are of Hettangian to Lower Pliensbachian, Upper Pliensbachian, and Toarcian age respectively (Figure 6).

Lower Lias

The Lower Lias consists largely of marine mudstones with sporadic thin limestones. Within the district, the latter are developed principally at the base of the succession. Hettangian and Lower Sinemurian strata are known to the north and north-west of the district (Horton et al. 1987), but local sedimentation probably did not begin until later. Nearly 50 m of Upper Sinemurian strata were proved in the Steeple Aston Borehole (Poole, 1977) and equivalent beds were also present in several other boreholes to the north and west (Horton et al., 1987), and in the Harwell No. 3 Borehole [SU 4680 8644] to the south-west (Gallois and Worssam, 1983). It is probable that a small area of Upper Sinemurian beds is present at depth in the north-western corner of the district. These beds may include representatives of the Echioceras raricostatum Zone, of which the Echioceras rancostatoides Subzone was proved between 133.5 m and the base of the Lias at 135.2 m depth in the Calvert East Borehole.

Throughout most of the district, only Lower Pliensbachian rocks are represented in the Lower Lias. In the Noke Hill Borehole, they comprise 67 m of Shelly limestones, overlain by blue clays with ironstone nodules, between 43.3 m and 110.3 m depth (Falcon and Kent, 1960). Although no cores were taken, comparison with other boreholes suggests that the limestones are of Uptonia jamesoni–Tragophylloceras ibex zone age, and the overlying strata of ibex–Prodactylioceras davoei Zone age.

In the Westcott No. 2 Borehole, the Lower Lias is 39.5 m thick. The basal 4.4 m comprise pale grey, sparry and shell-fragmental limestones with some limonitic peloids and, at the base, small quartz and quartzite pebbles. These beds contain abundant crinoid debris and a fauna of bivalves and brachiopods including Rimirhynchia, Spiriferina and Tropiorhynchia. The overlying 2 m of strata comprise very silty to finely sandy mudstones, succeeded by grey mudstones and silty, micaceous, moderately fissile mudstones with abundant fossils. The lowest part of the limestones (below 161.5 m) may be of jamesoni Zone age, but the greater part of the succession belongs to the ibex Zone. The presence of Tropidoceras aff. ellipticum at 159.6 m and Tragophylloceras undulatum at 161.5 m indicate the oldest, Tropidoceras masseanum, Subzone. Tropidoceras cf. acteon recorded by Spath (BGS archives) from 157.06 m, gives the highest evidence for this Subzone. Overlying beds (up to 156.6 m depth) contain some aegoceratid and liparoceratid fragments, and may possibly belong to the Acanthopleuroceras valdani Subzone. Beds above yielded Beaniceras (between 131.6 and 156.66 m), and are attributed to the Beaniceras luridum Subzone at the top of the ibex Zone. The taxa present include Beaniceras dundryi at 133.6 and 139.2 m, Androgynoceras cf. hybridiforme at 140.60 m and Beaniceras crassum var. costatum at 144.14 m; the latter indicates the Crassum Zonule in the middle of the luridum Subzone (Phelps, 1985). Other faunas of these mudstones are dominated by a variety of bivalves. R V Melville also identified frequent Balanocrinus subteroides ossicles, as well as Eodiadema? and Ophiolepis at 132.74 m, belemnites, gastropods and some crustacean fragments; the fauna is similar to that from corresponding beds in the Chalgrove Borehole (see below). The topmost 7 m of Lias mudstones yielded no ammonites, but the few other macrofossils present are consistent with an ibex Zone age. The succeeding davoei Zone was not proved. The uppermost 3 m are greenish grey and varicoloured, with scattered grains of sphaerosiderite, resulting from weathering prior to deposition of the overlying Middle Jurassic strata.

In the Chalgrove Borehole, the Lias is represented by 7 m of beds belonging to the jamesoni and ibex zones. Immediately to the west of the district, the Wytham Borehole [SP 4857 0687] proved over 45 m of 'clunch' with ironstone (Blake and Whitaker, 1902), and the nearby Oxford City Brewery Borehole [SP 5081 0608] penetrated about 5 m of Lias mudstone without bottoming it (Woodward, 1893); these successions are probably also in beds of Pliensbachian age.

Middle Lias

At outcrop to the north-west of the district, the Middle Lias (Upper Pliensbachian), comprises a lower unit of silts and clays, and an upper unit of ferruginous limestones known as the Marlstone Rock. Neither was present in the Westcott or Noke Hill boreholes, but logs of boreholes around the northern and north-western margins of the district suggest that they may extend into the north-western corner.

In the Steeple Aston Borehole, the lower unit comprises silts and silty mudstones with thin siltstones, in some cases ferruginous and richly fossiliferous. The Calvert 1/76 Borehole [SP 6888 2462] p‘roved the topmost 0.33 m of the Marlstone Rock, before terminating at 70.41 m depth. The strata consist of ferruginous, chamositic, sandy, oolitic, shell fragmental limestone.

Upper Lias

A marine regression, resulting in some erosion, was followed by renewed transgression in early Toarcian times. Upper Lias limestones and mudstones probably overlapped the Middle Lias south-eastwards, to rest directly on Lower Lias across the whole district. However, following uplift of the London Platform in late Toarcian times, much of the succession was eroded away, so that Upper Lias is absent from the district except possibly in the extreme north-west. Prior to renewed sedimentation in Aalenian times (see below) the exposed Lias and Palaeozoic beds were weathered and rootlet beds developed. These altered beds, which have been proved in many boreholes, are commonly difficult to distinguish from seatearths within the overlying Middle Jurassic succession.

In the Calvert 1/76 Borehole, Upper Lias, belonging mainly to the Lower Toarcian Harpoceras falciferum Zone, was proved between 67.03 and 70.08 m depth. The basal 0.05 m, a silty mudstone rich in fish and shell debris, is overlain by 0.28 m of pale olive-brown and green limestones with much organic debris. These units are probably thin representatives of the Fish Beds and Inconstant Cephalopod Bed of Northamptonshire (Howarth, 1978), which together constitute the Fish Beds Member. Cleviceras cf. elegans at 69.49 m indicates the Cleviceras exaratum Subzone. The overlying beds comprise 1.45 m of alternating limestones, siltstones and mudstones, with a falciferumSubzone fauna, and sparse phosphatic ooliths, such as occur in the Cephalopod Limestones Member of the south-east Midlands (Horton et al., 1980). Above a minor conglomeratic layer at 68.42 m, are finely silty mudstones with some brown, possibly phosphatic ooliths (below 68.30 m) and a Dactylioceras. The topmost 1 m of strata, a seatearth, comprises pale ochreous-grey, micaceous siltstones and brown-grey sphaerosideritic mudstones with listric shear surfaces, and rootlets extending down from the overlying Middle Jurassic beds.

Inferior Oolite Group

The Inferior Oolite Group (Aalenian to Bajocian) is typically developed in the Cotswolds to the west of the district, where it comprises a succession, mainly of marine limestones, up to 100 m or more in thickness. In the East Midlands to the north-east, sediments were deposited in much shallower water, in which lagoonal, brackish and possibly fluvial deposits were laid down. The district was situated between these two sedimentary provinces, close to the shoreline on the London Platform, and consequently, the local Inferior Oolite succession is thin and incomplete, and of mixed facies. It is probably present at depth in the western part of the district, where it rests disconformably on the Lias. It is probably absent elsewhere, being overstepped by the Great Oolite. Typical successions from the district and surrounding areas are 1 shown in (Figure 7).

At several sites, a thin unit tentatively assigned to the Scissum Beds (Lower Inferior Oolite; Aalenian) has been proved at the base. In the Harwell No. 3 Borehole, the strata consist of pale grey, slightly sandy 'ironshot' limestone, 0.89 m thick (Gallois and Worssam, 1983). Similar beds may have been penetrated in the Oxford City Brewery Borehole. In the Chalgrove Borehole, the unit consists of 2.45 m of thinly laminated, bioturbated sandstones, calcareous sandstones and oolitic, shell-fragmental sparry limestones. Thin beds of conglomeratic sandstone with intraformational pebbles occur, and the basal bed contains pebbles of Lias cementstone. 'Pale green grey sandy limestones' overlying the Lias in the Noke Hill borehole is again suggestive of Scissum Beds.

In the extreme west of the district, overlying beds include coarse-grained, peloidal limestones of the Clypeus Grit, (Upper Inferior Oolite; Bajocian). The Harwell No. 3 Borehole proved it to be 6.8 m thick (Gallois and Worssam, 1983). In the Shipton Quarry Borehole [SP 4762 1718], the Clypeus Grit comprises 7.5 m of shelly and pisolitic micrite (ironshot in the lower 5.8 m), resting directly on Upper Lias. Specimens in the BGS collection suggest that the formation is also present in the Oxford City Brewery Borehole.

In parts of the district, the Scissum Beds or older strata are overlain by nonmarine mudstones, siltstones and silty sandstones, which include beds of seatearth character. These beds, though here referred to the Grantham Formation (i.e. Inferior Oolite of Aalenian age; (Figure 7)), may represent the Rutland Formation (Great Oolite Group; Chapter Three), parts of which are lithologically similar. The strata have not yet yielded age-diagnostic fossils which could resolve this uncertainty.

Beds provisionally attributed to the Grantham Formation were proved in the Chalgrove, Calvert 1/76 and (possibly) Noke Hill boreholes. At Chalgrove, they comprise 3.80 m of pale grey, locally lilac or khaki-tinted sandstones, overlain by lilac to chocolate-tinted, pale grey siltstone with silty sandy wisps and clay partings. Rootlets occur throughout, and sphaerosideritic seat-earth lithologies are present at some levels. Beds of similar lithology occur in the Calvert 1/76 Borehole where they rest on Upper Lias. In the Noke Hill borehole, the Grantham Formation may be represented by mixed calcareous sandstones, silts and silty mudstones from depths of about 34 to 42 m.

Chapter 3 Jurassic: Great Oolite Group

The Great Oolite Group, Bathonian to earliest Callovian in age, locally comprises (in ascending order) the Chipping Norton Limestone, Sharp's Hill, Taynton Limestone, Rutland, White Limestone, Forest Marble and Cornbrash formations (Figure 7). It is inferred to be present at depth throughout the whole district, but it crops out only in the north-west (Figure 1), in the core of the Charlton Anticline and Noke Pericline, and also in the extreme north-western corner. The oldest beds at outcrop belong to the White Limestone Formation, but the underlying Rutland and Taynton Limestone formations have been exposed in the floor of Woodeaton Quarry [SP 533 123]. Away from the outcrop, there is little information available from within the district; only the Westcott No. 2 Borehole [SP 7096 1649] provided a complete cored succession. However, outcrop and boreholes in surrounding districts provide much supplementary data.

During deposition of the Great Oolite, the district lay on the margin of the London Platform, between two sedimentary provinces, as had been the case during the deposition of the Inferior Oolite. Seaward, to the west and north-west, marine carbonates and calcareous muds accumulated, as typically developed in the Cotswolds. To the north-east, lagoonal, brackish and possibly fluvial deposits were laid down. In this district, the facies variation is seen within the basal part of the Great Oolite Group, whereas the higher formations (the White Limestone, Forest Marble and Cornbrash) show uniform facies throughout the district. The thickness of the Great Oolite Group reduces across the district from over 40 m in the west and south-west, to perhaps less than 25 m in the north-east (Figure 7). The strata rest disconformably on the Inferior Oolite, overstepping it eastwards to rest on Lias over much of the district, and on Palaeozoic rocks in the south-east.

Chipping Norton Limestone Formation

The Chipping Norton Limestone was proved in the Ship-ton Quarry Borehole [SP 4762 1718], just west of the district, where it is 6.1 m thick. The basal bed comprises 1.4 m of dark grey shelly mudstone, and is succeeded by about 4.7 m of uniform, moderately well-sorted, oolitic grainstone. The formation may persist in the subsurface into the western margin of the district.

Sharp's Hill Formation

The Sharp's Hill Formation comprises those beds between the Chipping Norton Limestone and the Taynton Limestone formations (McKerrow and Kennedy, 1973), and includes beds equivalent to the newly recognised Charlbury Formation of Boneham and Wyatt (1993). The formation consists largely of greenish grey, slightly silty, weakly calcareous mudstones with subordinate marls and laminated siltstones and mudstones. Thin shelly limestones are also present, and rootlet-bearing mudstones and seatearths occur. Siltstones and grey calcareous sandstones are common, particularly at the base. A facies change is evident across the district, from entirely marine in the south-west, for example in the Chalgrove Borehole [SU 6565 9620], to coastal marsh, lagoonal or brackish-water facies in the north-east, as in the Calvert 1/76 Borehole [SP 6888 2462]). The formation was absent in the Westcott No. 2 and Tring [SP 9121 1036] boreholes, being overstepped by younger formations, and in the Harwell No. 3 Borehole [SU 4680 8644], is represented by marine mudstones of the laterally equivalent Fuller's Earth (Figure 7).

Details

In the Chalgrove Borehole, the Sharp's Hill Formation, 2.27 m thick, consists of pale grey sands and calcareous sandstones, overlain by medium grey, silty and sandy marls and limestones with scattered shells and shell debris. The rocks are extensively bioturbated.

In the Oxford City Brewery Borehole [SP 5081 0608] (Blake, in Tiddeman, 1910), a sequence of grey to black mudstones, green sandy marls and shelly and oolitic limestones between 115.1 m and 123.8 m depth is assigned to the Sharp's Hill Formation. Specimens in the BGS collection include medium grey, sandy, bioclastic limestone with Praeexogyra hebridica, pale greenish grey, shelly, silty marl, pale grey mudstone with scattered shells and fibrous calcite 'beef, dark grey, sandy, silty mudstone with bivalve ghosts and pale grey weakly cemented siltstones. Specimens from 121.3 m consist of purplish brown and dark grey silty mudstone with much plant debris, and those from 122.53 m are pale greenish grey, micritic limestone with shrinkage cracks and brecciation, and purplish red speckling, suggestive of contemporaneous weathering.

In the M40 Borehole 231 [SP 5715 1718], 4.07 m of Sharp's Hill Formation was proved, but not bottomed. It comprises olive to olive-grey mudstones and silty mudstones with silty laminae, with abundant oysters at some levels. The topmost 1.76 m includes micritic, shell-fragmental limestone with abundant wisps and thin partings of mudstone, and may correspond with the Charlbury Formation of Boneham and Wyatt (1993), which is better developed in the Shipton Quarry borehole (Figure 7).

In the Calvert 1/76 Borehole, the Sharp's Hill Formation (1.9 m thick) comprises 0.76 m of greenish grey sandy silt-stone with scattered bivalves and some rootlets, overlain by a thin siltstone rich in plant debris. This is succeeded nonsequentially by a shelly sandstone, which passes upward into limestone and siltstone. The uppermost beds, silty mudstones and siltstones, represent a second seatearth horizon, truncated by the erosional base of the overlying Taynton Limestone.

Beds corresponding to the Charlbury Formation may have been cut out by the overstepping Taynton Limestone in this borehole.

Taynton Limestone Formation

The Taynton Limestone Formation is probably present throughout all except the most easterly parts of the district (Figure 7). It consists typically of massive, cross-bedded, shell-fragmental, oolitic limestones (grainstones) with a small proportion of iiiicritic limestones (packstones), marls and clays, and is up to at least 6 m in thickness. The topmost 0.6 m was formerly exposed in the floor of Woodeaton Quarry [SP 5341 1230] (Palmer, 1973).

Details

In M40 Borehole 231, the Taynton Limestone comprises 2.44 m of pale buff to mid grey oolitic and bioclastic limestone with mudstone partings and thin beds of calcareous siltstone and silty mudstone.

In the Westcott No. 2 Borehole, two specimens from 122.99 and 123.50 m depth are white oolites of typical Taynton Stone lithology. Samples from the underlying beds (to the top of the Lias at 124.16 m) are somewhat similar oolites, although the grains are set in a brown sideritic or limonitic micritic matrix; they probably represent an unusual marginal facies of the formation. A fauna from this level included serpulids, Chlamys (Radulopecten) vagans, Praeexogyra hebridica and echinoderm debris.

Rutland Formation and Hampen Marly Formation

In its type area [SP 060 204] near Cheltenham, the Hampen Marly Formation comprises the marine clays and limestones between the Taynton Limestone and White Limestone. Similar beds were proved in the Harwell No. 3 and Chalgrove boreholes. Traced northwards and eastwards, brackish and 'estuarine' facies with rootlet beds appear (Palmer, 1979; Horton et al., 1987; (Plate 1)). With increasing dominance of the nonmarine facies, the formation passes into the Rutland Formation (formerly Upper Estuarine Series; Bradshaw, 1978); the latter is developed over most of the district (Figure 8). The interdigitation of facies takes place over a large area, however, and the demarcation between the two formations is therefore arbitrary (see (Figure 8), inset). Thus, in the past the term Hampen Manly Formation (or Beds) has been applied locally (e.g. Arkell, 1947a; Palmer, 1979) to beds regarded as Rutland Formation in this account.

The Rutland Formation of the district is much thinner than in the type area, and comprises only the upper part of the type succession, lower parts being represented by the Taynton Limestone and Sharp's Hill formations.

The formation is dominated by marls and clays, with subordinate limestones, but is very variable in detailed lithology (Figure 8). Its most characteristic feature is its cyclic nature; locally, three main regressive rhythms can be recognised. Where fully developed, each rhythm consists of marine mudstones, marls or limestones at the base and a rootlet bed at the top, Which is truncated by the marine unit of the succeeding rhythm. The basal part of the lowest rhythm (corresponding with the Wellingborough Rhythm of Bradshaw, 1978), is the Taynton Limestone Formation (see above). This grades eastwards into lower energy, mainly micritic limestones interbedded with oyster-rich marls and mudstones at the base of the Rutland Formation. These in turn pass upwards into nonmarine marls and clays. The marine element of the higher rhythms(the Cranford and Finedon rhythms of Bradshaw, 1978) is less well developed, and the upper part of the formation is dominated by greenish clays with plant debris and rootlets (see Details).

The fauna of the Rutland Formation is dominated by bivalves such as Astarte, Corbula, Cuspidaria, Eomiodon, Isocyprina, Modiolus, Neomiodon, Pholadomya, Placunopsis, Plagiostoma, Praeexogyra, Protocardia, Pteroperna, and Trigonia, and includes both fully marine forms and those able to adapt to estuarine conditions.

Details

In 1991, the following section, was visible below the White Limestone Formation in Woodeaton Quarry [SP 5328 1235] (Plate 1):

The section totals about 3.6 m in thickness; Palmer (1973) gave the total thickness of the 'Hampen Marly Formation' as 3.7 m, including a thin marl at the top which is here classified with the White Limestone Formation. He listed a fauna mainly of bivalves from the formation, and also recorded vertebrae of Cetiosaurus, probably from Bed 4 of the section above. Rootlets are best developed in beds 4 and 8 (Plate 1) and are sharply truncated by the shelly marine sediments of the overlying beds; these non-sequences mark the termination of upward-shallowing rhythms (Figure 8).

In the M40 Borehole 231, the Rutland Formation is 4.1 m thick. The basal bed, a 0.19 m thick, muddy, bioturbated, sparsely oolitic limestone, is overlain by dark olive-grey, shelly, peloidal mudstone 0.18 m thick. These beds contain a marine fauna including Kallirhynchia cf. vagans, Liostrea and Modiolus. They are succeeded by olive bioturbated mudstone with silt partings and shells, including brackish-water bivalves such as Corbula and Cuspidaria (1.12 m); a siltstone bed (0.16 m); and a pale green silty mudstone seatearth, with rootlets at the top (0.80 m). This succession (with the underlying Taynton Limestone), forms a single shallowing-upward rhythm. The overlying beds, 1.62 m thick, comprise interbedded dark olive-grey silty mudstones and pale olive silts, commonly with lignite fragments; two of the shelly silt beds contain bivalves including Falcimytilus spp., Modiolus imbricatus and Trigonia sp.

In the Westcott No. 2 Borehole, the Rutland Formation is present between 110.6 m and about 122.6 m depth. The beds below about 114.9 m (included in the Taynton Limestone of Sumbler, 1988), comprise dark grey, sandy limestones and mudstones overlain by pale grey shell debris-rich, bioturbated, marly and micritic limestones with clay wisps and abundant rhynchonellids and oysters. Overlying beds comprise greenish grey, silty mudstones with scattered bivalves and shell pavements, finely interlaminated with silty sand partings and greenish grey, silty marls and marly limestones.

Largely or entirely marine sequences, classified as Hampen Marly Formation, were proved in the Shipton Quarry Borehole, the Harwell No. 3 Borehole (Gallois and Worssam, 1983), and in the Chalgrove Borehole. In the latter, the succession comprises 11.13 m of interbedded limestones, marls, siltstones and mudstones. The limestones are generally silty and marly, and include shelly and oolitic sparry and micritic types. The siltstones commonly contain partings and lenses of fine-grained sandstone. The mudstones and marls include laminated silt bands and shell pavements; oyster lumachelles occur in both limestone and mudstone lithologies.

White Limestone Formation

The White Limestone Formation, up to 16.6 m thick, is present throughout the district (Figure 7). It is dominated by white, cream and buff limestones (wackestones), with peloids (mainly faecal pellets) and shell debris set in a micrite matrix. There are minor interbeds of grey marly limestone and mudstone, but higher-energy packstones and grainstones are relatively rare. Shells occur throughout the sequence and most beds are strongly bioturbated.

The formation is divisible, in ascending order, into the Shipton, Ardley and Bladon members ((Figure 9); Palmer, 1979; Sumbler, 1984), which are thought to correspond with three main upward-shallowing sedimentary cycles. The cycles are analogous to those in the underlying Rutland Formation, but are developed in wholly marine facies. Several regionally persistent hard-grounds, each indicative of a sedimentary hiatus, can be recognised within the formation; each is characterised by particular species of the gastropod Aphanoptyxis (Barker, 1976). Two such hardgrounds define the top of the Shipton and Ardley Members (Sumbler, 1984), although locally they have been recognised only in boreholes. The Shipton and Ardley members are dominated by micritic limestones, and are difficult to separate at outcrop. The Bladon Member, however, can be distinguished throughout the district by the distinctive lithologies of the Fimbriata–Waltoni Bed at the base (Figure 9).

The formation crops out only in a restricted area on the western margin of the district, where it forms two out at the summit of Noke Hill [SP 534 124]; [SP 538 128] which coincide with the culmination of the Noke Pericline. The whole formation is exposed in Woodeaton Quarry [SP 533 123] (Plate 2) and most of the formation was, until recently, visible in Merton Quarry [SP 571 170] (Plate 3), now flooded. It was continuously cored in several site investigation boreholes drilled for the M40 motorway (Figure 9), and in the Westcott No. 2 Borehole.

The dominance of micritic, pelletal limestones indicates deposition in a low-energy environment where burrowing organisms were dominant. The Bladon Member contains seatearths and plant-rich mudstones; the former are associated with calcareous nodules of caliche type, suggesting subaerial exposure; it is probable that the member passes laterally north-eastwards into shallow-water lagoon facies of the Blisworth Clay. Algal laminae and fenestrae ('birds-eyes'), indicating periodic exposure, have also been recorded at other levels in the upper part of the formation (Palmer and Jenkyns, 1975; Sumbler, 1984).

The White Limestone yields an abundant fauna dominated by bivalves such as Plagiostoma and Pholadomya. Brachiopods (notably Epithyris oxonica) and gastropods (particularly nerineids) are abundant at some levels, and corals occur sporadically.

Details

Woodeaton Quarry

A fine section in Woodeaton Quarry [SP 533 123] (Plate 2) revealed the complete succession between the Rutland and Forest Marble formations:

Precise correlation of the section given above with the thinner succession recorded by Palmer (1973) is uncertain, but it seems probable that his section lacks the beds of the Bladon Member (locally cut out by the Forest Marble), and also Beds 10 to 13 (due to minor faulting). Other sections of the quarry are given by Bradshaw (1978) and Cripps (1987).

In the quarry, the Shipton Member (4.75 m thick), consists mainly of bioturbated, micritic and finely detrital limestones in which bivalves are the commonest fossils; brachiopods and corals occur at certain levels. The uppermost bed (Bed 9) probably correlates with the Aphanoptyxis excavata Bed (Barker, 1976; Sumbler, 1984), although the eponymous gastropod has not been recorded at Woodeaton.

The Ardley Member totals 6.05 m in thickness. The slightly sandy limestone with shell casts (Bed 11) is the so-called 'Roach Bed' (Palmer, 1979; Sumbler, 1984). Two beds are particularly fossiliferous. The lower (Bed 14) is the Modiolus–Epithyris Bed of Palmer (1973). It passes laterally into limestone with scattered Digonella digonoides which was erroneously considered to overlie the Modiolus–Epithyris Bed (Bed 14) by Palmer (1973). The character and stratigraphical position of the very fossiliferous, 'double' limestone Bed 16 suggests that it may equate with the A. ardleyensis hardground bed recorded elsewhere (Sumbler, 1984). The occurrence of Digonella digonoides in Beds 14 and 16 suggests correlation with the so-called 'Ornithella Beds' of the Cirencester district (Richardson, 1911) and the Digonoides Beds of the East Midlands (Torrens, 1967; Cripps, 1987). At this locality, there is no hard-ground at the top of the Ardley Member to compare with the A. bladonensis Bed which occurs widely elsewhere in Oxfordshire (Sumbler, 1984).

The Bladon Member, up to 1.32 m thick, is exposed in the north-west corner of the quarry only; in the south-west face it is cut out by the erosion surface at the base of the Forest Marble Formation, which there rests on Bed 14. The basal clays and marls (beds 22 to 24) comprise the Fimbriata-Waltoni Bed, of which the abundance of plant material is characteristic. The overlying limestone Bed 25, preserved only along a short section of the face, represents the Upper Epithyris Bed (Sumbler, 1984), the bulk of which is apparently cut out at this locality.

Merton Quarry

Excellent sections of the upper part of the White Limestone were visible during 1988–91 in Merton Quarry [SP 571 170] (Plate 3), which was excavated to provide fill material for construction of the M40 Motorway. Nearby cored site investigation boreholes proved the total thickness of the formation to vary from 13.1 m to 16.3 m. This variation is principally due to the channelling at the base of the overlying Forest Marble Formation. Detailed logs of the quarry and boreholes are held by BGS; a composite section is given below:

The Fimbriata-Waltoni Bed (Beds 13 to 16) varies from 1.6 to 3.85 m in thickness and is dominated by clays. The overlying Upper Epithyris Bed shows considerable variation in thickness and is entirely absent in places. The upper surface, beneath the Forest Marble, is highly irregular due to erosion, and locally exhibits burrows filled with shell debris.

Westcott No. 2 Borehole

In the Westcott No. 2 Borehole, the White Limestone is 12.34 m thick. The lower 11.5 m (the Shipton and Ardley members) comprises pale grey to white shell-fragmental and peloidal wackestones. The overlying Bladon Member comprises 0.5 m of grey mudstones packed with shell and plant debris including large lignite fragments (the Fimbriata-Waltoni Bed) overlain by 0.3 m of burrow-mottled, finely shell-fragmental recrystallised wackestone (the Upper Epithyris Bed).

Forest Marble Formation

The Forest Marble, first named by William Smith, comprises the beds between the White Limestone and the Cornbrash (see Sumbler, 1984). The formation crops out at Noke Hill and, to the north-east, at Oddington and Charlton-on-Otmoor, in the Noke Pericline and along the Charlton Anticline. It is present at depth throughout the district. Cored boreholes in the outcrop have proved thicknesses of up to 5.6 m, but the formation thins to the east, being only 3.3 m thick in the Westcott No. 2 Borehole, and 0.6 m at Tring.

The formation is made up of channelled units, such that the succession varies greatly from place to place (Figure 9). Generally, the lower part is dominated by limestones (Plate 2) and (Plate 3), most typically cross-bedded, flaggy-weathering, shell-fragmental and oolitic grainstones, but packstones and wackestones also occur. Comminuted oyster debris, small mud clasts and plant detritus are also characteristic of these beds. Generally, the upper part of the formation is dominated by mudstones, which range from greenish grey, smooth-textured, blocky, calcareous types, to laminated mudstones with silt, shell debris- and oolith-rich lenticles. Calcareous sandstones occur sporadically within the mudstones; they are generally very fine-grained and thinly bedded, commonly with gentle ripple cross-lamination.

The base of the formation rests on an eroded and channelled surface of the White Limestone; locally, derived limestone clasts occur in the basal beds. The irregularity of this junction is largely responsible for the thickness variation of both the White Limestone and Forest Marble (Figure 9). Many similar, though less pronounced channelled non-sequences occur within the formation.

The fauna of the Forest Marble is dominated by bivalves, mostly types adapted to a shallow marine, high energy environment. In addition, fish, crustacean and echinoderm debris occurs.

Details

In Woodeaton Quarry, up to 3.5 m of Forest Marble are exposed. It comprises 1.2 m of grey and brown mottled clay with calcareous race nodules, underlain by 2.3 m of fawn, cross-stratified shell-fragmental oolite (Plate 2), with angular fragments of white micrite at the base, presumably derived from the underlying White Limestone. The base of the formation is deeply channelled, and locally cuts down up to 4.7 m into the White Limestone.

The whole formation, comprising up to 4.57 m of limestones with marl interbeds, was formerly exposed in Merton Quarry [SP 571 170] (Plate 3). The lower 2 m is dominated by pale grey, blue-hearted, shell-fragmental and oolitic grainstones, but in places shell-fragmental micrites (wackestones and packstones), marls and mudstones occur at the base. The overlying unit comprises limestones interbedded with marls and mudstones. The limestones are predominantly shell-fragmental and oolitic grainstones, often showing grain-size banding. Packstones and wackestones also occur. The uppermost bed of the formation comprises pale grey micritic limestone, passing laterally into marl, with dark grey, clayey, shell-debris-filled burrows extending down from the overlying Cornbrash.

In the Westcott No. 2 Borehole, the Forest Marble is 3.3 m thick and consists mainly of pale greenish grey, blocky, slightly silty mudstone with sporadic layers of shell debris, specks of carbonaceous plant material and lenticles of sandy limestone.

It contains a fauna of bivalves and, at some levels, ostracods, the latter suggesting an estuarine influence.

Cornbrash Formation

The Cornbrash crops out in the extreme north-western corner of the district, and as inliers along the Charlton Anticline and Noke Pericline. It is also present at depth throughout the district. It varies from about 2.5 to 3.8 m in thickness, and rests with marked disconformity upon the Forest Marble. The upper boundary with the Kellaways Formation is transitional.

The Cornbrash has been subdivided into Lower and Upper parts on the basis of lithology and fauna, the boundary corresponding with that between the Bathonian and Callovian stages. Traditionally, it has been assumed that only Lower Cornbrash is represented within the Thame district, as for example at Stratton Audley, 6 km to the north (Douglas and Arkell, 1932). However, Upper Cornbrash faunas have been recorded at Woodstock and Shipton-on-Cherwell just to the west (Woodward, 1894; Page, 1989), and it is possible that thin Upper Cornbrash may occur locally. Work near Buckingham (Cox, Hopson and Sumbler, 1991), showed that there is little lithological distinction between the two divisions.

The Cornbrash consists almost entirely of bluish grey, medium- to fine-grained, shell-fragmental wackestones and packstones, which weather to an olive or brown colour (Plate 3). The rock is characteristically intensely bioturbated and bedding structures are poorly preserved; as a result, the Cornbrash typically weathers to rubble. The shell fragments are generally darker than the matrix, and are commonly micritised, many with pyritic margins, indicating repeated reworking of the sediment. Micritic peloids and ooliths occur rarely, and very small pebbles and intraclasts occur at some levels, particularly at the base.

The following fossils have been collected from M40 borehole cores and field brash: Cererithyris intermedia, C. cf. magnifica, Obovothyris obovata, Camptonectes sp.,Chlamys (Radulopecten) vagans, Entolium sp., Homomya sp., Limatula cerealis, Liostrea sp., Meleagrinella echinata, Placunopsis socialis, Pleuromya cf. alduini, P. uniformis, Protocardia sp., Pseudolimea? and the echinoid Nucleolites sp. The zonal ammonite Clydoniceras (Clydoniceras) discus (uppermost Bathonian) has been collected from the former Islip Quarry [SP 523 139], immediately to the west of the district (Pocock, 1908). Both the Cererithyris intermedia and Ornithella [Obovothyris] obovata brachiopod zones are present locally; Arkell (1944b) suggested that the greater part of the Cornbrash here lies within the younger obovata Zone.

Details

There were formerly many quarries along the line of the Charlton Anticline. One ?[SP 559 156], on the western outskirts of Charlton, exposed up to 1.6 m of beds, and yielded many fossils including Clydoniceras sp. (Douglas and Arkell, 1932). More recently, the Cornbrash was exposed in Merton Quarry, where it rests on an eroded surface of Forest Marble (Plate 3). The following section was recorded at the south-western corner c.[SP 5686 1685]:

The Cornbrash was proved in several site investigation boreholes (Figure 9); M40 Borehole 259 [SP 5955 1563] and 216 [SP 5826 1658] proved the complete sequence, 2.19 and 3.43 m thick respectively. In the Westcott No. 2 Borehole, it was 2.6 m thick.

Chapter Jurassic: Ancholme Group (Part 1)

The term Ancholme [Clay] Group was first introduced in Lincolnshire and included all the mudstones of Callovian–Kimmeridgian age, together with the intervening sandstone units (Gaunt, Fletcher and Wood, 1992). Because of the extensive drift cover and poor exposure there, it was not possible to recognise at outcrop the individual mudstone formations known elsewhere in eastern England, although boreholes confirmed their presence. In the relatively drift-free Oxford–Aylesbury district, it has been possible to map the Oxford Clay, West Walton, Ampthill Clay and Kimmeridge Clay formations separately and, in some cases, to trace members within them.

In the eastern part of the district, the Ancholme Group forms a unit, predominantly of mudstones, more than 150 m in thickness. In the west, however, the non-mudstone Corallian Formation divides the group into two parts. The lower part (Kellaways, Oxford Clay and West Walton formations) are described in this chapter; the upper part (Ampthill Clay and Kimmeridge Clay formations) is described in Chapter Six.

Kellaways Formation

The Kellaways Formation (formerly Kellaways Beds) comprises the Kellaways Clay (= Cayton Clay of Page, 1989) and Kellaways Sand members, which have been mapped separately. On earlier maps of the district, the Kellaways Formation was included in the Oxford Clay. The formation crops out in two areas in the north-western part of the district: firstly, in the extreme north-west, and, secondly, in association with the Noke Pericline and Charlton Anticline structures (Figure 1). At outcrop, it is generally 5.5 to 6 m in thickness. The formation is probably present at depth throughout the district, but it may thin to the east and south-east; it is only 3.12 m thick in the Tring Borehole [SP 9121 1036], 10 km beyond the eastern margin of the district. Details of the Kellaways Formation in temporary exposures at Kidlington [SP 49 14], some 4 km west of the district, were recorded by Callomon (1955).

Kellaways Clay Member

The Kellaways Clay consists of dark grey, smooth-textured, slightly silty, fissile mudstone. Pyritic pins and patches on bedding planes occur throughout, and rare, small, pale grey cementstone (argillaceous micritic limestone) nodules are also present. A coarse shell-debris marl, up to 0.4 m thick, marks the base of the Kellaways Clay throughout the district; the marl passes rapidly upward, with a decrease in shell debris, to typical mudstone. In places, it rests abruptly on the underlying Cornbrash, but elsewhere the junction is less clear. The top of the member is taken at the base of the first arenaceous bed of the Kellaways Sand. In places, a minor non-sequence with sand-filled burrows occurs at this junction, but elsewhere sandy mudstones may be present at the base of the Kellaways Sand and the boundary may be difficult to define. Because of these difficulties, the thickness of the Kellaways Clay recorded in boreholes varies considerably, from 1.9 m to at least 4.2 m, but compensatory thickness variation appears to occur in the Kellaways Sand, and the total thickness of the Kellaways Formation is relatively constant.

The Kellaways Clay is poorly fossiliferous. Ammonites from adjacent areas suggest that it lies within the Kamptus Subzone of the Herveyi Zone (Callomon, 1955; Page, 1989). The fauna consists largely of immature bivalves; mature individuals are uncommon, but include Gryphaea.

Kellaways Sand Member

The Kellaways Sand Member consists mainly of fine-grained quartz sands, coarse silts and silty mudstones. At outcrop, they weather to pale brown to buff silty sand, giving rise to a brown loamy soil. The coarser-grained beds may be patchily cemented to form doggers. They often show fine lamination, composed of alternations of clean sand with darker, more silty and clayey bands. The beds are commonly intensely bioturbated. Shell debris is present, commonly in lenses associated with thick-shelled bivalves, particularly Gryphaea, and belemnites. Other bivalves present include Anisocardia, Catinula, Oxytoma, Myophorella and Pleuromya. Ammonites are uncommon. The member is thought to lie within the Koenigi and Calloviense zones (Page, 1989).

The base of the member is drawn at the first appearance of clean sand and the upper junction at the top of the highest clean sand or sandstone. In the cored Otmoor A, B and C boreholes [SP 5681 1361]; [SP 5670 1326]; [SP 5708 1331], the thickness varies from 3.0 to 3.1 m, whilst in nearby M40 site investigation boreholes, the thicknesses recorded show variations from 1.7 to 4.8 m; the true thickness probably lies in the range 2 to 3.5 m.

Oxford Clay Formation

The Oxford Clay forms an area of predominantly low relief in the north-western part of the district. The formation is divided into the Lower, Middle and Upper Oxford Clay, recently renamed the Peterborough, Stewartby and Weymouth members respectively (Cox et al., 1993), which have been mapped separately. The Lower Oxford Clay has the most extensive outcrop as it is present on both sides of the Charlton Anticline. By contrast, the outcrop of the Upper Oxford Clay is relatively narrow, and, in the south-west where it forms the lower slopes of the Corallian escarpment, much of it is obscured by landslipping.

Within the district, geophysical (mainly gamma-ray) logs are available from a number of boreholes which penetrated the Oxford Clay, and there is sufficient control from those that were cored to enable them to be used for correlation and classification of uncored sequences ((Figure 10); Appendix 5).

The basal junction of the Oxford Clay, resting on the sand of the underlying Kellaways Formation, is generally sharp, although thin beds of fine-grained sand or silt occur in the basal part and, in some boreholes, the boundary is apparently transitional.

The thickness of the formation ranges from 65 to 75 m. The Westcott No. 2 Borehole [SP 7096 1649] proved a total thickness of 72 m and, just south of the district, the Chalgrove Borehole [SU 6565 9620] proved 66.27 m. To the north-east, in the Leighton Buzzard district (Shephard-Thorn et al., 1994), the formation maintains a thickness of about 70 m, typical of areas marginal to the London Platform. The Oxford Clay thickens gradually westward to 100 m in the adjoining Witney district.

At outcrop, the Oxford Clay is generally weathered to a depth of 2 to 3 m, and locally as much as 5 or 6 m. This weathered zone is characterised by selenite (gypsum) derived from the breakdown of pyrite and calcite. Near the surface, the selenite occurs as fine sand-grade crystals, coating fissures. At greater depth, well-formed crystals occur, reaching a maximum size of several centimetres at a depth of about 3 m. Race (small calcium carbonate nodules) may be common in the uppermost 1 to 2 m. All three members of the Oxford Clay form a pale grey subsoil, making distinction between them difficult at outcrop. However, the bituminous mudstones of the Lower Oxford Clay may persist close to the ground surface.

The formation is richly fossiliferous (Plate 4) and (Plate 5); Martill and Hudson, - 1991) but usually only the most robust fossils, in particular belemnites and the oyster Gryphaea, survive the weathering process and are found at outcrop. These can be useful stratigraphical indicators: Cylindroteuthis ((Plate 4):6; mainly Kellaways Formation–Lower Oxford Clay), Hibolithes ((Plate 4):8; mainly Middle–Upper Oxford Clay), Gryphaea dilobotes (Kellaways Formation–Lower Oxford Clay), G. lituola ((Plate 4):2; Middle Oxford Clay) and G. dilatata ((Plate 5):5; Upper Oxford Clay–Ampthill Clay); G. dilatata in the Upper Oxford Clay are generally smaller and thinner shelled than those in the overlying West Walton and Ampthill Clay formations. The distinctive serpulid Genicularia vertebralis ((Plate 4):3) is a useful marker for the Lower Oxford Clay and the basal part of the Middle Oxford Clay, and may be recovered from auger samples.

The Oxford Clay is entirely marine and was deposited in an offshore, quiet water environment. Rhythmic sedimentation is evident in each of the Lower, Middle and Upper Oxford Clay members, and longer-term changes in sediment supply, lime deposition and biological productivity resulted in the different characters of the three members.

The Oxford Clay spans most of the Callovian Stage and Lower Oxfordian Substage, the standard zonations of which are based on ammonites following Callomon (1964) and Callomon and Sykes (1980) (Figure 10).

Lower Oxford Clay (Peterborough Member)

The Lower Oxford Clay consists of interbedded greenish grey, slightly blocky mudstones and brownish grey, fissile, bituminous, shelly mudstones. Grey septarian cementstone (argillaceous micritic limestone) nodules occur at several levels and some form locally persistent nodule beds (see below). The member ranges in thickness from about 25 to 28 m.

The macrofauna is rich and varied, with bivalves, particularly nuculoids, being most abundant (Duff, 1978). Also present are ammonites (most commonly Kosmoceras), gastropods, belemnites, scaphopods, brachiopods, echinoderms, crustacea, serpulids, fish and reptile remains, as well as fossil wood. Fish debris may be common in the paler blocky mudstones which have a relatively poor invertebrate fauna. Much of the latter is preserved as crushed aragonitic shells. Shell beds and plasters, some pyritic, are common; these indicate pauses in sedimentation (Duff, 1975).

The Lower Oxford Clay is the best documented part of the succession, having been extensively worked for brick manufacture in the Midlands, where its organic carbon content is high enough (up to 6.1 per cent by volume at Calvert (Duff, 1975)) to enable partial self-firing (Callomon, 1968; see Chapter Twelve). The London Brick Company's Calvert pit [SP 67 23] (closed in 1991) lies about 5 km north of the district, and is a valuable reference section.

Five beds of cementstone nodules have been mapped in the district (Figure 10). The lower four have been given local names, in ascending order, the Merton, Arncott, Wendlebury and Blackthorn nodule beds. They form discontinuous, low scarp features and gentle dip slopes which commonly show cementstone debris; the Blackthorn Nodule Bed generally forms the most pronounced feature. Locally in the Ray valley, they can be traced beneath alluvium by gentle features. In the extreme north-west, they are concealed beneath a thin covering of remanie terrace deposits and they have not been located on the steeper slopes flanking the Noke Pericline. The nodules consist of grey, hard, splintery, cementstones which become noticeably pyritic towards their margins. They are typically septarian, being cut by a network of calcite veins, and vary in size up to 1 m in diameter. At outcrop, they fracture along the calcite veins to form angular blocks which readily weather to subrounded fragments, commonly with a pale grey, limonite-stained surface. The contained fossils are uncrushed, indicating that nodule growth started before compaction of the sediment. At Calvert, Hudson (1978) noted that nodules are commonly developed at or near to shell beds which provided a source of lime to sustain diagenetic nodule growth; they typically make up 5 to 10 per cent by volume of their host bed.

The Merton Nodule Bed, 2 to 3 m above the base of the Oxford Clay, is correlated with Callomon's (1955) Bed llb at Kidlington and lies within, and probably near the base of, the Medea Subzone. An impersistent nodule bed is locally developed south-east of Merton, between the Merton and Arncott nodule beds; it is correlated with Bed 12 at Kidlington (Callomon, 1955).

The Arncott Nodule Bed, some 4 to 5 m above the base of the formation, is correlated with Callomon's (1968) Bed ld at Calvert and Bed 16 at Kidlington (Callomon, 1955); this is approximately the position of the Medea–Jason subzonal boundary in the Jason Zone, and the nodule bed is closely associated with the basal Jason Subzone shell bed (Duff, 1974).

The Wendlebury Nodule Bed, 9 to 10 m above the base of the Oxford Clay, has not been observed in borehole cores but is deduced, from geophysical logs, to occur in the middle part of the Obductum Subzone, probably associated with a comparatively pale, Lingularich mudstone which has been observed in a number of sections at this level.

The Blackthorn Nodule Bed, about 14 m above the base of the Oxford Clay, is correlated with Bed 6 of Callomon's (1968) section at Calvert. This lies at the top of the Obductum Subzone. Within the district, it has yielded kosmoceras fragments, including ?K. obductum and K. ex gr. obductum, together with Dentalium, bivalve fragments and gastropods.

The youngest nodule horizon, the composite Acutistriatum Band–Comptoni Bed, rarely forms a feature, but is traceable over much of southern England. Within the district, it lies about 18 m above the base of the Oxford Clay. The basal part comprises the Comptoni Bed which, strictly speaking, is restricted to a pyritic nuculoid shell bed with the eponymous ammonite Binatisphinctes comptoni ((Plate 4):5) but it is convenient to include the underlying nuculoid-rich mudstones, which show an upward increase in shells and shell debris (Horton et al., 1974). In the district, nodule development is rare in the Comptoni Bed. The overlying Acutistriatum Band marks the base of the Athleta Zone. It comprises pale grey, distinctively fissile mudstones which weather to brownish grey. It contains cementstone nodules which may be septarian, as at Calvert, but more generally are not; they have a distinctive pale grey weathering cortex. The mudstones and nodules contain only crushed fossils, indicating that nodule growth started after compaction of the sediment. Hudson (1978) estimated that, at Calvert, nodules make up 20 per cent of the Acutistriatum Band and noted that they are less pyritic and less densely cracked than those at other levels. The few surface occurrences show debris of pale grey, brown weathering, non-septarian, silty cementstone with Kosmoceras ex gr. phaeinum, K (Spinikosmoceras) cf. acutistriatum and common Bositra buchii ((Plate 4):9) (see Details).

Middle Oxford Clay (Stewartby Member)

The Middle Oxford Clay consists predominantly of pale to medium grey, smooth to slightly silty, calcareous, poorly fossiliferous, blocky mudstones. Interbeds of pale, greenish grey, silty mudstones packed with immature shells of the bivalve Bositra buchii occur throughout. Gryphaea lituola ((Plate 4):2) may be common, particularly in the upper 10 m or so, which are generally more silty and include thin, very silty, nodular limestones or calcareous siltstones (Figure 10). The most important of these is the Lamberti Limestone Bed at the top of the member (see below). These more indurated lithologies and Gryphaea-rich beds give rise to a number of minor, discontinuous topographic features. Gryphaea are common on the dip slopes in arable fields but the calcareous beds tend to disintegrate rapidly through weathering. Cored sequences and gamma-ray logs from boreholes in the Pans Hill area [SP 62 15] show at least three persistent silty limestones which have been mapped locally.

The member is 20 to 26 m thick within the district. It is lithologically distinct from the underlying darker, bituminous Lower Oxford Clay but the boundary is transitional, with alternations of pale greenish grey mudstones and brownish grey mudstones separated by inter-burrowed horizons. In boreholes, the base is taken at the top of the highest bituminous mudstone. The position of the mapped boundary is necessarily approximate because of deep weathering and the frequent cover of superficial deposits.

Apart from beds rich in Gryphaea or Bositra, the fauna of the Middle Oxford Clay is generally sparse, though diverse. Small uncrushed pyritic ammonite casts, including zonally diagnostic Kosmoceras and Quenstedtoceras, occur sporadically throughout; bivalves, gastropods, belemnites and brachiopods are most common amongst the other macrofauna. Occurrences over a short vertical range of the solitary cup (or button) coral Trochocyathus magnevillianus (Plate-4:1) provide a useful faunal marker horizon (Trochocyathus Band) in the middle part of the member. This has been recognised in the Leighton Buzzard district to the north-east (Shephard-Thorn et al., 1994), in the Chalgrove Borehole to the south and at Calvert, where it occurs within Bed 13c of Callomon (1968). It lies near the boundary of the Proniae and Spinosum subzones, probably in the uppermost part of the former.

The highest 12 m of the Middle Oxford Clay were formerly exposed in Woodham Brickpit [SP 709 185] ((Plate 6); Arkell, 1939, 1947a; Rutten, 1956; Callomon, 1957, 1968; Hudson and Palframan, 1969; Palframan, 1966). The pit was the type locality for the Lamberti Limestone and several Upper Callovian subzones (Callomon and Sykes, 1980; see Details), but is now infilled. Middle Oxford Clay was also worked for brickmaking in the Summer-town pit [SP 5056 0864], 2.5 km west of the district.

Lamberti Limestone Bed

The Lamberti Limestone is typically a distinctive pale grey, cream-weathering, richly fossiliferous, soft, silty limestone but locally it may be a silty, calcareous mudstone. Apart from a diverse ammonite fauna ((Plate 4):10), it has yielded abundant bivalves, notably Oxytoma inequivalve and pectinids, belemnites and gastropods. In the former Woodham Brickpit, the bed was 0.3 m thick. It has been proved in several boreholes and trial pits, mostly in the Pans Hill area, where the proven thicknesses range from 0.18 to 0.72 m.

The bed is probably continuous across the district but its outcrop is difficult to locate. Due to weathering, the only evidence of its existence may be the presence of a pale grey silty clay with coarse shell debris, particularly corrugated fragments of pectinid bivalves. It produces a moderately distinct signature on gamma-ray logs ((Figure 10); Appendix 5) and, in several areas, these have been used to assist in locating the bed in shallow percussion boreholes. Typical shelly, silty limestone fragments have been found in a number of areas.

Upper Oxford Clay (Weymouth Member)

The Upper Oxford Clay consists of pale grey, poorly fossiliferous, smooth-textured mudstones and slightly silty mudstones. Subordinate harder calcareous siltstones and, rarely, thin silty limestones are present. In places, there are thin beds of dark grey mudstone. The member differs from the Middle Oxford Clay in being rather less silty, and calcareous, and Bositra-rich beds are generally absent. A weak rhythmic pattern of sedimentation can be recognised, in which the base of each rhythm is marked by a very thin, darker grey, slightly silty, carbonaceous mudstone which infills burrows in the underlying bed; the fine plant material, and also shell detritus, at the rhythm base decreases rapidly upwards.

The member averages 23 m in thickness, with a range of 20 to 25 m. At Pans Hill [SP 620 150], where several boreholes proved a complete sequence through the Upper Oxford Clay, the thickness is 20.4 to 21.6 m. In the Westcott No. 2 Borehole, a gamma-ray log indicated a thickness of 19.6 m.

The fauna consists mostly of bivalves, together with ammonites, mainly pyritised cardioceratids and, in the lower part, oppeliids ((Plate 5):8 to 11), belemnites and gastropods. The dark grey mudstones contain only crushed, aragonitic fossils. Oyster-rich beds and silty limestones locally produce topographical features. The Gryphaea, traditionally assigned to the species G. dilatata, show a progressive upward change from small, fairly tightly incurved forms (transitional to G. lituola) in the lower part of the member, to larger, broader, more open forms in the upper part. The member spans the Mariae Zone and part of the. Cordatum Zone. The junction with the overlying West Walton Formation lies within the Costicardia Subzone. The latter was exposed in the former brickpit at Horton-cum-Studley [SP 606 118] (Arkell, 1947a; see Details). The Upper Oxford Clay was worked in several brickpits, now obscured, within the eastern suburbs of Oxford.

Four to five metres above the base of the Upper Oxford Clay, a brownish grey calcareous bed, the Pans Hill Siltstone, locally forms a mappable feature. It lies at or near the base of a pale brownish grey mudstone, the Pans Hill Mudstone, which was first identified in M40 motorway site investigation boreholes and trial pits on Pans Hill. The type section is the M40 Borehole 128 [SP 6135 1389]. The Pang Hill Mudstone ranges in thickness from 3 to 4.8 m, and the Pans Hill Siltstone ranges from 0.21 to 0.46 m in thickness. Both are poorly fossiliferous but fine shell debris is scattered throughout and commonly concentrated in burrow linings; linear pyritic traces probably represent small burrows. The Pans Hill Siltstone and Mudstone occur in the upper part of the Scarburgense Subzone in the Mariae Zone, although both the base and the top of the mudstone are gradational, and it may continue up into the lowest part of the overlying Praecordatum Subzone (Figure 10).

The distinctive gamma-ray signature of the Pans Hill Siltstone has been recognised in M40 motorway boreholes and in the Westcott No. 2 Borehole ((Figure 10); Appendix 5). The 'hardened clay at 7 to 8 feet' [c.2.25 m] above the base of Bed A at Woodham Brick Pit (Arkell, 1939; see Details) was probably the Pans Hill Silt-stone but its characteristic colour may not have been distinguishable in the weathered zone. The same bed is present at 86.10 to 86.28 m in the Henwood Farm Borehole [SP 4783 0337] at Cumnor to the west of the district. To the east, it may correlate with Bed 17 of the Millbrook section [TL 005 390], near Ampthill (Wyatt et al., 1988).

Details

Elsfield

An exposure [SP 5315 0965] in a drainage ditch close to the Bayswater Brook proved very pale grey, silty clay with belemnites and numerous shell fragments, including small but easily recognisable corrugated fragments of pectinid bivalve. The latter could be traced to the north; it is assumed that this horizon is the weathered Lamberti Limestone.

Ot Moor–Noke

The Lower Oxford Clay (27 m thick) and basal Middle Oxford Clay were continuously cored in the Otmoor A, B and C boreholes in the 1950s; these proved a full subzonal sequence from the ?Proniae to Enodatum subzones resting on the Kellaways Formation. All three boreholes proved the Acutistriatum Band–Comptoni Bed, the shell bed(s) at the Grossouvrei–Obductum subzonal boundary with which the Blackthorn Nodule Bed is usually associated, 2 to 3 m of Lingula-rich mudstone in the Obductum Subzone and partially pyritised shell beds with oysters at the base of the Jason and in the Medea Subzone. The Merton Nodule Bed was recovered from the latter Subzone at 28.20 m depth in Otmoor B.

South-east of Oddington, fossiliferous specimens of the Acutistriatum Band and the Blackthorn Nodule Bed were recovered from drainage ditches [SP 5635 1449] and [SP 5627 1424] respectively.

Horton-cum-Studley

To the west and south of Horton Farm [SP 5902 1208], Hortoncum-Studley, the Lamberti Limestone gives rise to a low feature and can be traced from fossiliferous silty limestone debris in the fields. Similar debris occurs to the east of West Hill Farm [SP 5882 1277] and the knoll immediately to the north of the farm may result from the outcrop of calcareous siltstones in the Upper Oxford Clay, debris of which had been dug from a pit [SP 5880 1280].

According to Arkell (1936; 1947a), the brickyard [SP 606 118] at Horton-cum-Studley was worked until about 1930 and exposed about 3 m of pale grey 'soapy' clays with abundant large Gryphaea dilatata and at least two layers of ammonites preserved as 'brown limonitic and ochreous casts'. These included perisphinctids, Peltoceras and oppeliids but mainly Cardioceras from amongst which Buckman (1922; 1925; 1926) figured several new species. These include Cardioceras costicardia ((Plate 5):12) which gives its name to the Costicardia Subzone to which the clays of the brickyard must largely belong.

M40 motorway–Blackthorn

During the 1980s, a large amount of data became available from site investigation boreholes, trial pits, excavations and cuttings, connected with the construction of the Wendlebury [SP 56 19] to Waterstock [SP 63 05] section of the M40 motorway. Many cored boreholes through the Oxford Clay have been described, collected and geophysically logged. Within the framework of the members and marker beds, the gamma-ray logs have provided an effective tool for classification and correlation (Appendix 5) and little detailed biostratigraphy has been undertaken.

The Upper Oxford Clay was the least well known part of the sequence and M40 boreholes 128 and 129 [SP 6175 1426] near Pans Hill were examined in detail. Although a full, Lower Oxfordian, subzonal sequence was established, the Bukowskii Subzone was not satisfactorily identified and the characteristic preservation of ammonites in the Costicardia Subzone (see Horton-cum-Studley, above) was not observed in the borehole cores, although one was recovered from the motorway excavations [SP 6195 1303]. There may therefore be some scope for adjustment to subzonal boundaries within the Cordatum Zone when more sections are examined. At present, subzonal thicknesses and characteristics in the Upper Oxford Clay are as follows. The Scarburgense Subzone is about 8 m thick and pyritised oppeliids are common up to the level of the Pans Hill Silt-stone (Figure 10); the siltstone is overlain by brown mudstones, the Pans Hill Mudstone. The Praecordatum Subzone is about 7 m thick and contains small pyritised Peltoceras and Hibolithes, both rather common in the upper part. The Bukowskii Subzone, about 4 m thick, contains poorly preserved but common Cardioceras. The Costicardia Subzone (part only), about 2 m thick, includes the highest pyritised ammonite nucleus. The greater part of the Costicardia Subzone, with Cardioceras (Plasmatoceras), is represented by the 'transition beds' associated with, and included in, the overlying West Walton Formation. Upper Oxford Clay ammonites, probably indicative of the Costicardia Subzone, were collected from a culvert excavation [SP 6193 1303]; specimens indicative of the Mariae Zone were recovered from the debris from trial pits dug to about 4.5 m deep c.[SP 6145 1444].

The Lamberti Limestone (0.24 to 0.72 m thick) was recorded in many of the M40 boreholes (Appendix 5) but the presence of the Trochocyathus Band, lower down in the Middle Oxford Clay, has yet to be confirmed. A brownish-tinted, grey mudstone occurs in the uppermost part of the Middle Oxford Clay, generally within 1 m of the base of the Lamberti Limestone. It is smooth textured, like the Pans Hill Mudstone, but has a paler hue. Fossiliferous Lower Oxford Clay was collected from trial pits [SP 5568 1878]; [SP 5523 1905]; [SP 5532 1915]; [SP 5534 1916], the latter two yielding Kosmoceras suggestive of the Jason Zone.

East of the motorway, pieces of cementstone with Kosmoceras fragments from the nodule beds in the Lower Oxford Clay were collected from field brash [SP 5855 1633]; [SP 591 177]; [SP 596 172]. Similar pieces were recovered from a ditch [SP 6045 1788] near Arncott and from dredgings [SP 6314 1971] from the bed of the River Ray, near Blackthorn, just north of the district; they yielded Kosmoceras indicative of the Obductum Subzone. West of Arncott, fragments of cementstone (?Acutistriatum Band) with Bositra buchii were collected from field brash [SP 5981 1688] and south of Arncott, specimens of Gryphaea lituola were recovered from excavation debris [SP 6133 1675]. A nearby auger sample [SP 6174 1700] yielded a pyritised Kosmoceras indicative of the Middle Oxford Clay, and Gryphaea characteristic of the latter member were collected from a shell bed in a ditch [SP 6160 1536]. Typical, fossiliferous Lamberti Limestone was collected from nearby ditch debris [SP 617 154]; [SP 6168 1532].

Piddington–Woodham–Quainton

The Gallows Bridge Borehole [SP 6677 1959] proved the lithological transition between the Lower and Middle Oxford Clay. Farther south, Gryphaea indicative of the Middle Oxford Clay were collected from a pond [SP 6729 1929] and ditch [SP 6753 1901] at Tetchwick Farm. Both Middle and Upper Oxford Clay with the intervening Lamberti Limestone were formerly exposed in the railway cutting between Piddington and Ludgershall (the Ludgershall cutting of Davies, 1916) (BGS collection). Fauna indicative of slightly younger Upper Oxford Clay (topmost Mariae Zone) has been collected from the railway cutting [SP 6636 1654] to the south-east, and old but imprecisely located ammonite collections from the railway east of Wotton Underwood [SP 68 16] indicate the topmost part of the Oxford Clay (Costicardia Subzone).

Characteristically ammonitiferous Lamberti Limestone has been collected from ditches or ditch debris at several localities [SP 6578 1734]; [SP 6672 1788]; [SP 6688 1772]; [SP 6867 1884] between the Ludgershall cutting (see above) and the brickpit at Woodham (Plate 6) which formerly exposed the following sequence (bed notation follows Arkell, 1939 and Callomon, 1968):

North of Woodham, specimens of Lamberti Limestone were recovered from ditch debris [SP 7051 1893] and a nearby shell and auger borehole [SP 7032 1942] at Oving Hill Farm just outside the district proved Upper Oxford Clay with the Pans Hill Siltstone at about 13 m depth. The Lamberti Limestone was also recovered from debris from an excavation [SP 7094 1940] and, just north of the district, from a ditch [SP 7132 2002] near Binwell Farm. Further east, Woodward's (1895) description of the Quainton Road brickyard [SP 7322 1920] suggests that the Upper Oxford Clay was formerly worked there, and, just north of the district, debris from a drainage trench in the railway cutting [SP 7307 1952] to [SP 7329 1930] indicated Upper Oxford Clay of the Cordatum Zone.

The Westcott No. 2 Borehole proved the Oxford Clay succession in the nearby subcrop; coring commenced at 64.3 m depth (see also Appendix 5):

West Walton Formation

The term West Walton Beds (now Formation) was introduced for the sequence of alternating calcareous mudstones and silty mudstones, with cementstone or siltstone doggers, which occurs between the Oxford Clay and Ampthill Clay in Fenland (Gallois and Cox, 1977; Cox and Gallois, 1979). Limestone facies, which occur locally at this stratigraphical level in southern Fenland, were included as members within the formation. Throughout the East Midlands and in the eastern half of this district, the formation remains predominantly argillaceous but, in the west, the upper part of the formation is replaced by calcareous (including coralline) and arenaceous beds; these constitute the separate Corallian Formation (see Chapter Five). Thus, in the west of the district, the West Walton Formation is equivalent only to the lower part of the formation in its type area (Figure 11).

The formation crops out continuously across the district from the eastern suburbs of Oxford to north of Waddesdon (Figure 1). In previous accounts and maps of the district, it is not differentiated from the Oxford Clay. In the western half of the district, the outcrop forms the upper part of the steep slope capped by the Corallian Formation. In the eastern part of the district, the West Walton Formation forms a wide bench-like feature above the lower ground of the Upper Oxford Clay.

At outcrop, the formation gives rise to a stiff, mainly grey, clay soil but, where fresh it is seen to comprise rhythmic alternations of dark grey, silty mudstone passing up into pale grey mudstone. The dark mudstones contain much shell debris, finely divided plant material and well-developed Chondrites. The contact with the underlying paler mudstones is sharp, but intense interburrowing is common, piping dark grey, carbonaceous mudstone down into the underlying beds. Thin calcareous siltstones also occur. Gamma-ray logs of the West Walton Formation show a much greater fluctuation in radioactivity than in the underlying Oxford Clay; several distinct peaks and troughs occur and are valuable for correlation of uncored sequences (Appendix 5).

The base of the formation is commonly mapped at a change of slope, from gently sloping (Upper Oxford Clay) to more steeply inclined (West Walton Formation) ground; augering shows that this coincides with a change from pale grey and fawn clays, to dark grey, brown and ochreous clays often with race pellets. In the Waddesdon area, the base is mapped at the base of a particularly dark mudstone which may correlate with Bed 2 (or Beds 2 and 3) of the type Fenland sequence (Gallois and Cox, 1977; Cox and Gallois, 1979) (hereinafter referred to in the form WWF2, WWF3 etc.). In borehole sections further west, particularly those drilled for the M40 motorway site investigation, the base of the formation is drawn beneath the lowest dark grey, silty mudstone within the lithological transition from pale grey, smooth-textured, poorly fossiliferous mudstones typical of the underlying Upper Oxford Clay. This basal bed is commonly very silty, highly carbonaceous and bioturbated, with abundant shells and shell debris, and with an interburrowed horizon at the base. The very thin,, dark grey mudstones which occur in the topmost part of the Upper Oxford Clay contain less plant detritus and are less silty.

Where overlain by the Corallian Formation, the top of the West Walton Formation is generally clearly defined by a major lithological change. In the Brill and Ludgershall areas, the Corallian Formation becomes thinner and interdigitates with the West Walton Formation (Figure 11); the small thickness of the latter which overlies the Corallian Formation has been mapped with the Ampthill Clay.

Within the district, the formation ranges in thickness from 20 m in the south-west, to 15 m in the north-east. A complete sequence, 19.31 m thick, was proved in M40 Borehole 278 [SP 6224 0539] near Wheatley, and 19.5 m was proved in the Chalgrove Borehole, just south of the district. In the Brill No. 1 Borehole [SP 6570 1412], the main part of the West Walton Formation, beneath the Corallian Formation, is 15.39 m thick, and a further 2.73 m overlie the latter (Figure 11).

At outcrop, the most common fossil to be found is Gryphaea dilatata ((Plate 5):5) and Gryphaea-rich beds may produce slight topographic features. The calcitic shells are typically larger and thicker than those in the Upper Oxford Clay, and often bored and encrusted with serpulids and foraminifera. Intensely bored Gryphaea, sometimes reduced to small relics with borings on all surfaces ((Plate 5):6), are characteristic of the formation. Lopha and Nanogyra also occur sporadically. Where fresh, the dark, silty mudstones are often richly fossiliferous with cardioceratid and, less commonly, perisphinctid ammonites, bivalves, and coarse shell debris, all preserved in pinkish white aragonite; pyrite casts also occur. The cardioceratid ammonites are dominated by the very finely ribbed Cardioceras (Plasmatoceras) ((Plate 5):13). This taxon occurs preferentially in clays and silts at the expense of Cardioceras (Subvertebriceras) which predominates in more arenaceous facies (Sykes and Callomon, 1979). Both the Cordatum and Vertebrate subzones are believed to be represented within the West Walton Formation of the district (Figure 11) but since the typical ammonite assemblage of the latter Subzone is largely characterised by C. (Subvertebriceras), together with C. (Vertebriceras), differentiation of the two subzones is not straightforward and further work on the ammonite faunas is needed.

Despite the limited amount of data from the eastern half of the district, correlatives of some of the 16 numbered beds of the standard Fenland sequence (Gallois and Cox, 1977; Cox and Gallois, 1979) have been tentatively identified (Figure 11). In the western part of the district, the formation represents beds only up to ?WWF6 and is proportionately much thicker than in the type area, with a higher proportion of dark, silty, plant-rich mudstones; a direct bed for bed correlation with the type Fenland sequence has not yet been achieved.

A particularly dark, silty and highly micaceous mudstone with abundant ammonites and bivalves, including Gryphaea, Pinna and trigoniids, has been recognised in the M40 site investigation boreholes and is believed to form a marker horizon, the 'Black Bed', in the middle of the formation. It gives rise to a distinctive low count gamma-ray signature (Appendix 5). However, thinner but lithologically similar beds occur elsewhere in the formation and further work is needed to confirm its validity as a reliable and persistent marker.

Details

South-east Oxford

Both the West Walton Formation and underlying Upper Oxford Clay were probably worked at the Cowley Marsh Pit [SP 5455 0485] which was sited on the outcrop of the West Walton Formation and overlying Temple Cowley Member. Fossils in the Oxford University Museum indicate the Cordatum Zone (Arkell, 1947a); according to Professor J H Callomon (written communication, January 1989), probably the Costicardia Sub-zone. The pits at Cowley Road [SP 5372 0517], St Clements [SP 5300 0585] and Marston Road [SP 5300 0745] (New Marston or Jack Straw's Pit) were all sited on the West Walton Formation outcrop or astride its boundary with the Upper Oxford Clay. Most of the ammonites preserved from the St Clements Pit are from the Upper Oxford Clay, 'and specimens labelled 'Cowley Fields Drainage' in the University Museum Collections again indicate the Costicardia Subzone (written communication, Professor J H Callomon, January 1989). There are no specimens available from the other pits.

Characteristic medium and dark grey, silty and very silty, plant-speckled mudstones, with Cardioceras including C. (Plasmatoceras) and several very large Gryphaea dilatata, were collected in 1989 by H P Powell (Oxford University Museum) from boreholes [SP 537 054] on the east side of Jesus College Playing Fields, Barracks Lane.

Holton and Wheatley

The formation was completely cored in two M40 site investigation boreholes associated with the proposed River Thame bridge east of Wheatley. Boreholes 24 [SP 6232 0540] (Figure 11) and 278 proved thicknesses of 17.46 m and 19.31 m respectively. The 'Black Bed' was recorded at 24.34 m and 23.95 m depth respectively.

Horton-cum-Studley, Arngrove and Boarstall

The former brickyard at Horton-cum-Studley was excavated in the lower part of the West Walton Formation and underlying Upper Oxford Clay. Between Arngrove and Boarstall, a large number of site investigation boreholes were drilled in the vicinity of the Pans Hill Cutting on the M40 motorway. The formation was completely cored in Borehole 129 (Figure 11) and Borehole 128 which proved thicknesses of 17.3 m and 16.04 m respectively. The 'Black Bed' occurs at 12.31 m and 12.66 m depth respectively. Gamma-ray logs from several boreholes indicate a formational thickness ranging from 16.9 m to 17.2 m (Appendix 5). The outcrop hereabouts is affected by landslip-ping (see Chapter 13), notably on the north-west-facing scarp slope to the south of Clue Hills Farm [SP 626 163]. Landslips also occur on the valley slopes near Corble Farm where a section exposing about 1.5 m of interbedded, burrow-mottled olive-grey and dark grey mudstones with weathered ferruginous nodules and Gryphaea dilatata was measured in a landslip scar [SP 6388 1615].

Brill

In the Brill No. 1 Borehole [SP 6570 1412], the main, and lower, part of the West Walton Formation occurs between 107.70 m and 123.09 m depth, and it is separated from the upper part (99.85 to 102.58 m depth) by a thin development of the Corallian Formation. The lower part comprises interburrowed mudstones with six indurated calcareous siltstones, 0.08 to 0.33 m thick, and a higher, 0.21 m thick, silty, argillaceous limestone. The upper part consists of pale and medium grey, silty, bioturbated mudstones with thin siltstones and limestones. The topmost bed is a 0.15 m-thick siltstone.

Ludgershall and Wotton Underwood

The West Walton Formation was exposed in cuttings during construction of the Great Western Railway, west and south of Ludgershall (Barrow, 1908; Davies, 1907b, 1910, in Arkell, 1927; Cox and Sumbler, 1989), and the now abandoned Great Central Railway, to the east and south of Wotton Underwood (Davies, 1907a). In 1986, a small exposure [SP 6675 1614], estimated to be 2 or 3 m above the base of the formation, was recorded at the bottom of the cutting by the railway bridge east of Poletrees Farm. It showed about 0.5 m of pale and medium grey, silty mudstones with burrow-mottling, specks of plant material, pyritised trails, pockets of shell debris and fossils including ammonites suggestive of the Cordatum Zone and Sub-zone. Farther south, the Rushbeds Wood railway tunnel is excavated in the upper part of the formation; Davies (1907b) recorded some highly fossiliferous clay brought from the tunnel including fragile specimens of Modiolus, Myophorella, pectinid and Cardioceras, as well as Gryphaea dilatata.

Just south-east of the Wood Siding roadbridge [SP 6733 1538], the formation is down-faulted to some depth [SP 6740 1529] but, farther south along the cutting, it reappears beneath the Oakley Member in the core of a gentle anticline [SP 677 149], yielding abundant broken valves of Gryphaea dilatata. The topmost 4 m or so of the formation were formerly exposed in this cutting (Dorton Cutting). Other fossil taxa from these railway cuttings in the BGS collections or cited in the literature include 'Astarte'', Camptonectes, Chlamys, Grammatodon, Lopha, Nanogyra, Oxytoma, Pinna, Pleuromya, Cardioceras spp. (particularly C. (Plasmatoceras) spp. but also Cardioceras s.s., C. (Scoticardioceras) and C. (Vertebriceras or Subvertebriceras), Peltoceras or Perisphinctes, Hibolithes and pentacrinoid columnals.

Waddesdon

The lower part of the formation was cored in the BGS Station Road Farm [SP 7450 1955] Borehole, just outside the district, and also in the Littleton Manor [SP 7346 1822] Borehole. Both proved a silty cementstone near the base of the formation and a layer rich in Gryphaea, serpulids and rarer Lopha, 4 to 5 m higher. A similar fossiliferous band occurs in the railway cutting [SP 7329 1931] near the bridge at Quainton Junction. Younger beds crop out in the old Great Central Railway cutting [SP 702 169], west of Westcott Airfield, from where Davies (1907a) recorded Chlamys, Gryphaea dilatata and Cardioceras 'vertebrale'

An excavation [SP 7777 1770] at Lower Farm, east of Waddesdon, reached the West Walton Formation. It yielded medium to pale grey mudstone and silty mudstone with some layers rich in fine shell debris, and a fauna including 'Astarte', Chlamys, Myophorella, Cardioceras ex gr. maltonense, Perisphinctes and common pentacrinoid columnals; this lithological and faunal assemblage suggests WWF8/9 of the standard Fenland sequence. The uppermost part of the formation, about 8 m thick, were penetrated in the Folly Farm Borehole [SP 7958 1904] (Figure 11). Silty cementstones at 23.59 m and 24.20 m depth probably belong to WWF15, and a cluster of Cardioceras tenuiserratum at 26.22 m confirms the Tenuiserratum Zone.

Chapter 5 Jurassic: Corallian Formation

The district lies at the north-eastern limit of the main development of the Corallian Formation/Group^‡1  in southern England. The characteristic lithofacies persists along the outcrop from Oxford to the Dorset coast, and extends into the subsurface. The formation corresponds with the 'Corallian beds' (Blake and Hudleston, 1877) which comprised the Lower Calcareous Grit, Coralline Oolite, Coral Rag and Upper Calcareous Grit.

Within the district, the formation reaches a maximum thickness of about 45 m in the south-west. North-eastwards from Wheatley, it thins dramatically and cannot be traced beyond Waddesdon (Figure 1). It rests on the West Walton Formation and is overlain by the Ampthill Clay Formation (see Chapter Six), except around Brill and Ludgershall, where it interdigitates with the West Walton Formation (see Chapter Four).

The lower part of the formation is predominantly arenaceous and is subdivided into the Temple Cowley, Arngrove Spiculite and Beckley Sand members (Figure 12); these constitute part of the loosely defined 'Lower Calcareous Grit' of the earlier geological maps. The upper part of the formation is largely calcareous and comprises the Wheatley Limestone (including Coral Rag), Littlemore and Oakley members. With the exception of the Temple Cowley Member, these names are formalised variants of previously used terms.

Shell and pebble beds occur within the lower, predominantly arenaceous members, but precise correlations between these beds are uncertain. A pebble bed at the base of the Wheatley and Littlemore members at a number of localities ((Figure 12), and Details) must indicate a fairly major hiatus within the Corallian Formation at this level.

The terms Berkshire Oolite Series and Osmington 0olite Series, which were used by Arkell (1927 et seq.) and Callomon (1960) for the local Corallian succession, are abandoned. The latter 'Series' included the strata of the Wheatley Limestone, Littlemore and Oakley members. Within the underlying arenaceous strata, new lithostratigraphical units and relationships have been recognised, and the term Berkshire Oolite Series, like the Lower Calcareous Grit, is no longer appropriate for this area.

Many parts of the Corallian Formation are richly fossiliferous (Plate 5). Corals are largely restricted to the Coral Rag facies of the Wheatley Limestone Member but gastropods, bivalves, ammonites and echinoids, are more widespread. The fauna and facies of the Corallian Formation of the Oxford area were the subject of W J Arkell's earliest published papers (Arkell, 1926a, b; 1927) which led later to monographs on the bivalves (Arkell, 1929–37) and ammonites (Arkell, 1935–48), as well as many other papers. The formation's facies and faunas have also prompted investigations into the relationship of fauna to substrate, and depositional environments (e.g. Ffirsich, 1976a, b; 1977), as well as analysis of the sequence in terms of depositional cycles and rhythmic sedimentation (Arkell, 1933, 1935; Wilson, 1968a, b; Talbot, 1973; Torrens and Wright, 1980).

Shallowing of the sea, noted during the deposition of the West Walton Formation (Chapter Four), continued with the deposition of the lower, arenaceous members of the Corallian Formation. The Temple Cowley Member was probably deposited in a subtidal to intertidal environment, and the Arngrove Spiculite Member accumulated in a sheltered marine environment, protected from the input of large quantities of terrigenous sediment. Progressive shallowing led to deposition of the Beckley Sand Member; Wilson (1968a) suggested that the quartz sand of the latter was derived from a source to the northwest of the present outcrop. Its absence from the Chalgrove Borehole [SU 6565 9620], about 4 km south of the district, is consistent with this idea, suggesting that only very small quantities of sand were derived from the proximal part of the London Platform.

The Wheatley Limestone Member developed in an open shelf environment on the western flank of the London Platform. The district lay at the very edge of the patchwork of small coral colonies, which formed the Coral Rag facies of this member. The 'reef environment was colonised by a rich variety of organisms whose remains were progressively reworked and now constitute the detrital limestone facies of this member. To the northeast, the limestones pass into marls and clays of the Oakley Member; these were deposited in deeper water. The Littlemore Member, to the south-west, is of similar facies but its depositional environment is uncertain; it may be lagoonal. Arkell (1947a, b) suggested that the thickening of the Wheatley Limestone from Headington towards Wheatley was related to movements along the Wheatley Fault Zone, which may also have controlled the limits of the Arngrove Spiculite, Temple Cowley, Beckley Sand and Wheatley Limestone members (see Chapter Eleven).

The Corallian Formation belongs to the Middle Oxfordian Substage. The zonation traditionally applied to these strata in southern England is based predominantly on perisphinctid ammonites; the sequences in the Oxford area have played a key role in its development (Arkell, 1936; Callomon, 1960, 1964). A more boreal zonation based on cardioceratid ammonites can also be applied in part to the Oxford area. From Wheatley eastwards to Brill, the Corallian Formation is relatively thin and its age is constrained by cardioceratid ammonite faunas in the Arngrove Spiculite Member (Densiplicatum Zone, Vertebrale Subzone; (Plate 5):7) and the Oakley Member (Tenuiserratum Zone, Tenuiserratum Subzone) (Figure 12). The latter zone/subzone also includes the basal beds of the Ampthill Clay and any intervening West Walton Formation. To the west, the Corallian Formation is thicker and more arenaceous, and ammonite distribution is patchy; perisphinctids commonly predominate and correlations are more difficult. Ammonites are generally rare and often poorly preserved in the Wheatley Limestone Member. In both the Wheatley Limestone Member and the laterally equivalent Littlemore Member, they are exclusively perisphinctid and were used by Callomon (1960) as the basis of the Parandieri Subzone (Figure 12). This, therefore, constrains the age of the youngest Corallian Formation in the western part of the district. The overlying Ampthill Clay (AmC36; Chapter Six) follows non-sequentially and therefore does not provide a tight age control on the Corallian Formation.

In most of the westerly areas, the basal unit of the formation is the Temple Cowley Member, but in the Beckley area, the Beckley Sand rests directly on the West Walton Formation. The mixed cardioceratid and perisphinctid faunas of the Beckley Sand, were used to define the Vertebrale Subzone of which the Oxford area was originally cited as type locality (Arkell, 1947a; Callomon, 1960, 1964); this has since been superseded by a section on the Isle of Skye (Sykes and Callomon, 1979). The Temple Cowley Member has not yielded ammonites, but must also belong to this Subzone because the upper part of the underlying West Walton Formation has yielded diagnostic Vertebrale Subzone ammonites (albeit mainly Cardioceras (Plasmatoceras); see Chapter Four). Thus, using either zonal scheme, the Corallian Formation of this district spans only three ammonite-based subzones (Figure 12).

Temple Cowley Member

The Temple Cowley Member is dominated by fine-grained silty sands and clayey silts. It was first recognised in temporary excavations [SP 541 041] and site investigation records at Between Towns Road, Temple Cowley; these indicated the presence of fine-grained, arenaceous sediments with varying proportions of clay and silt, and with thin, lime-cemented beds. The member overlies the West Walton Formation, and its base is drawn at the first appearance of medium- to coarse-grained sand. It passes laterally into the Arngrove Spiculite Member (Figure 12) and (Figure 13).

The member is restricted to the south-western extremity of the district; its northern limit [SP 5190 1029] is west of Elsfield where fine-grained, weakly cemented sandstones were recorded. It can be traced south and then eastwards along the valley of the Bayswater Brook where it passes beneath the Arngrove Spiculite Member. In Barton village [SP 5550 0785], the Arngrove Spiculite dies out and the Temple Cowley Member is consequently overlain by the Beckley Sand. It dies out eastwards in the sub- surface; it may be represented by a thin (less than 1 m) bed of silty sandstone in M40 motorway boreholes near Wheatley, for example Borehole 24 [SP 6232 0540], but was absent from excavations near Waterperry.

The designated type section of the Temple Cowley Member is in the Garsington No. 1 Borehole [SP 5726 0289], where its greatest thickness (11.91 m) within the district has been recorded. The member can be recognised in a number of other boreholes (Figure 11) and (Figure 13) and further afield in the Harwell No. 3 Borehole [SU 4680 8644] (between 215.6 and about 224 m depth) (Gallois and Worssam, 1983).

The thin lamination, ripple bedding and clay drapes within the component sands and sandstone indicate deposition in an environment with relatively low energy but fluctuating currents, capable of winnowing plant detritus into layers. Burrow-structures are relatively common. Shelly fossils are rare, and include Pinna and serpulids.

Details

Garsington No. 1 Borehole

The base of the member is drawn at 44.82 m, where fine-grained, silty sandstone rests abruptly on silty mudstone of the West Walton Formation. The upper boundary is less clearly defined with medium- to fine-grained, bioturbated sandstone passing up, at 32.91 m, into bioturbated sand of the Beckley Sand Member. The Temple Cowley Member is dominated by sandstones, commonly medium to pale grey and fine-grained. With an increasing proportion of Rhaxella spicules, they pass into spiculitic sandstones and, rarely, spiculite identical with the Arngrove Spiculite; this change is associated with development of pale grey to bluish grey colours. All the arenaceous beds show some lamination, either darker clay laminae or clay partings, or wispy bedding traces. Most beds are intensely bioturbated with cylindrical burrows up to 10 mm diameter and small-scale Chondrites mottling. Some beds are weakly calcite-cemented.

Littlemore

Debris of grey sand and sandstone dredged from the bed [SP 5436 0238] of the Littlemore Brook contained several fragments of Rhaxella-spiculite. Two specimens were examined using an electron microscope; one was a buff to pale grey, very porous lightweight siltstone. A E Milodowski (BGS) reports that it possesses extensive diagenetic porosity in the form of spherical or subspherical voids lined by opal-crystobalite lepispheres and globular silica growths. The voids may result from the dissolution of Rhaxella spicules. Interstitial material is intensely silicified clay with rhomb-like cavities which may result from dolomite dissolution (see Milodowski and Wilmot, 1983). The second specimen was a calcite-cemented quartz and shell-fragment siltstone which did not contain Rhaxella.

Arngrove Spiculite Member

The Arngrove Spiculite Member, formerly known as the Arngrove Stone (Davies, 1907a) has a distinct lithological character first noted by Buckland (in Phillips, 1855, p. 307). It was worked in several quarries [SP 613 137] near Arngrove Farm (now Old Arngrove) [SP 6105 1388] (Davies, 1907a). The Brill No. 1 Borehole [SP 6570 1412] and the M40 Borehole 010 [SP 6205 0544] are here designated as reference sections.

The Arngrove Spiculite is a pale grey sandstone or silt-stone with tiny bluish or milky white ellipsoidal grains; these are the spicules of the tetractinellid sponge Rhaxella perforata. The colours reflect the different degrees of recrystallisation which, in some cases, has led to complete solution of the spicules, leaving voids. Medium- to fine-grained quartz sand is also present; the proportion is variable, giving a transition from spiculite, with a siliceous cement, to spiculitic sandstone. Rarely, the latter has a calcareous cement. The spiculites develop a pale brown mottling on weathering and have a characteristic cinder-like texture ('pin-hole chert'). The uncemented sands contain varying proportions of fine material, are darker grey when fresh, and weather to pale ochreous brown colours. Relatively thin, flat bedding, with finer-grained wisps and traces of low-angle gentle cross-stratification are characteristic. Some of the beds contain wisps of silt, clay, or rarely shell debris. The base of the member is drawn at the first appearance of spiculitic lithologies.

The Arngrove Spiculite Member is present over an area extending from Barton [SP 533 085], north Oxford, to Beckley, and thence to Wheatley and Brill. The outcrop gives rise to escarpments at Beckley, Studley, Arngrove and Boarstall, which stand high above the Oxford Clay vale. The member generally rests upon the West Walton Formation. It dies out westward near Beckley, where it may have been mapped with the succeeding Beckley Sand. At Barton, it thins rapidly westwards and rests upon, and probably passes laterally into, the Temple Cowley Member. It thins eastwards (1.64 m thick in the Brill No. 1 Borehole) and is overstepped by the Oakley Member just north of Brill. It is at least 3.2 m thick in M40 Borehole 008A [SP 6214 0538] where it rests on thin Temple Cowley Member. It is absent in the south Oxford area and in the Chalgrove Borehole where it is replaced by its lateral equivalent, the Temple Cowley Member, which locally contains beds of spiculitic sandstone. The maximum recorded thickness is 4.2 m in M40 Borehole 280 [SP 6189 0632]. Boreholes and excavations at Arngrove proved at least 3.8 m, and the total thickness hereabouts may be as much as 5 m.

The Arngrove Spiculite Member is almost invariably bioturbated with large (up to 10 mm) sand and silty clay-filled burrows and pale grey Chondrites. Ammonites (mainly small cardioceratids) are common at some levels and are preserved as ghost-like impressions. Otherwise the macrofauna is almost exclusively bivalves, of which Davies (1907a, p. 43) recorded an extensive list. The fauna suggests a relatively stable substrate, and the abundance of sponge remains suggests a clear-water environment with only gentle, but relatively constant, currents introducing quartz grains.

Details

Brill No. 1 Borehole

The sequence proved in the Brill No. 1 Borehole, between 106.06 and 107.70 m, is designated as a reference section for the member:

Oakley–Studley–Boarstall

Davies (1907a) described a well section at Boarstall Road, Studley probably [SP 6030 1271] showing 2.7 m of Arngrove Spiculite Member overlying West Walton Formation.

During the recent survey, specimens of Cardioceras including C. (Subvertebriceras) densiplicatum and C. (S.) sowerbyi ((Plate 5):7) were collected at the edge [SP 6154 1390] of the disused Arngrove Farmhouse quarry, together with serpulids, Chlamys (Radulopecten) fibrosa, Myophorella sp., Nanogyra nana and Pleuromya alduini. Debris from excavations at Tower Farm Barn [SP 6250 1435] yielded a similar ammonite and bivalve fauna together with Discomiltha lirata, Modiolus bipartitus and Pholadomya sp.

The member was recorded in several M40 motorway trial pits. Trial pit 127 [SP 6148 1402] showed 2.98 m of spiculitic sandstone with thin interbeds of silts to sandy clay, overlying West Walton Formation. Trial pit 133 [SP 6138 1407] showed a similar section through the basal 1.77 m of the member. A temporary section in the foundations of the Pans Hill Overbridge [SP 6159 1400] on the motorway showed:

Similar beds were seen during the excavation of the Pans Hill [SP 616 141] and Honeyburge [SP 622 126] cuttings.

Waterstock–Worminghall

The member was proved in a number of site investigation boreholes and trial pits for the M40 motorway (Figure 11) and (Figure 13).

The sequence proved in M40 Borehole 010, between 10.20 and 13.68 m depth, is designated as a reference section:

The member was recorded in excavations in the floor of the M40 motorway cutting [SP 6179 0668] near the Waterperry over-bridge where 0.8 m of dark grey, silty sand with clay-filled burrows, rare spicules and scattered bivalves overlay 0.2 m of dark to mid grey, spiculitic sandstone resting on at least 0.3 m of sand. A similar section was recorded in the excavation for the overbridge [SP 617 064] by H P Powell (University Museum Oxford) who recorded bivalves (including 'myids') and Cardioceras from the sandstone, and bored and encrusted Gryphaea from the overlying sand. A sample from the sandstone was examined by Dr G K Lott (BGS) who reported many radially structured carbonate grains which probably replace original siliceous spicules.

Specimens from the member were also collected by H P Powell and the late J M Edmonds in 1966–67 from pylon excavations (e.g. Pylon 148 [SP 617 140]) west of Worminghall (University Museum Oxford collections).

Beckley Sand Member

The Beckley Sand Member occurs throughout the western part of the district and, in most areas, overlies the Arngrove Spiculite or Temple Cowley members, and is overlain by the Wheatley Limestone Member (or the equivalent Oakley or Littlemore members). The name Beckley Sand was introduced by Arkell (1942) for the arenaceous strata exposed at Benfield and Loxley's Quarry [SP 5665 1035] (Arkell, 1936).

The type area of the Beckley Sand, at Beckley village [SP 565 112], lies at the northern extremity of its outcrop. The member passes beneath the Thame alluvium south of Wheatley Bridge but it was proved in boreholes for the M40–River Thame overbridge [SP 622 054]. It reappears east of Holton [SP 605 064] but thins northwards to die out north-west of Waterperry [SP 622 077]. It has an extensive outcrop at, and south of, Elsfield [SP 540 100], and in the suburbs of Oxford (at Headington [SP 540 068], Cowley [SP 545 040], Blackbird Leys [SP 555 030] and Littlemore [SP 538 029] ) but passes beneath deposits of the Thames at Sandford-on-Thames [SP 533 014]. It was proved at depth in the Garsington No. 1 Borehole. Just outside the district, it was present in the Henwood Farm, Cum-nor Borehole, but absent in the Chalgrove Borehole.

There have been many exposures in the Beckley Sand, although most showed only a metre or so of strata. Consequently, information on the member is largely derived from boreholes. Those at Garsington and Cumnor proved 11.31 m and 11.15 m respectively. The maximum thickness of the member is probably around Beckley itself, where estimates from outcrop thicknesses are 15 to 20 m. An apparent thickness of 30 m, to the west of the village, is probably exaggerated by cambering.

The Beckley Sand Member consists of fine- to medium-grained quartz sands with minor shell debris and few shells. Calcite-cemented beds and doggers are present (Plate 7). Unweathered material is pale to medium grey but, near the surface, percolating groundwater will readily oxidise any pyrite and leach the shells to give brown to pale yellow sands, and sandstones. The proportion of sandstones varies, partly due to differential weathering, but generally (for example at Stanton St John [[SP 579 090] ) the thinnest sequences contain the highest proportion of sandstone beds. Doggers may survive at outcrop which is characterised by a reddish brown, loamy soil with sandstone debris. The hard sandstone bed known as the Worminghall Rock (Buckman, 1925a) is included herein in this member.

Shell debris is mainly disseminated, but in places is concentrated into thin beds, together with fine plant detritus. Structures within the major sand and sandstone beds are difficult to identify, but low-angle cross-stratification occurs at some levels. Thin lamination, including ripple lenses and wispy bedding, is commonly accentuated by clay partings. Rarely, thinly laminated, muddy sandstones occur in the lower part. The beds are intensely bioturbated. Small rounded pebbles of quartz, lydite and phosphate occur at some levels, together with larger pebbles of pale buff micrite and intraformational calcareous sandstones. Pyrite, possibly in association with clay, occurs as pellicles around some sand grains.

The base of the Beckley Sand is gradational. Where it overlies the Temple Cowley Member, the Beckley Sand is coarser grained and lacks the interlaminated sand, silt and clay beds which characterise the former. Its included sandstones are much thicker, more massively bedded, and are much less intensely bioturbated. Sponge spicules are absent so the Beckley Sand can be readily separated where it overlies the Arngrove Spiculite.

The member is rather poorly fossiliferous, the fauna tending to be restricted to the better cemented horizons as solution has removed most of the calcareous material from the uncemented sands. Large bivalves occur throughout, but most have been collected from the youngest strata, the so-called Shell-Beds or Shell-Pebble-Beds. Museum collections tend to be dominated by ammonites (particularly perisphinctids but also Cardioceras; see p. 37). Echinoids are also present. Within the district, the member appears to be younger in the east than in the west where it is also much thicker, and where correlatives of the younger development are shell beds and non-sequence (Figure 12). This may indicate some structural control on sedimentation along the line of the Wheatley Fault Zone (see Chapter Eleven).

The depositional environment is envisaged as one of relatively shallow water, periodically swept by currents. Pebbly horizons probably represent channel lag deposits. Coarseness of the sand tends to increase upward, probably reflecting more active reworking of sediment.

Details

Sandford, Littlemore and Cowley

South of Sandford, the Beckley Sand is up to 10 m thick. Callomon (1960, pp. 180–181) recorded the topmost 6.14 m, underlying the Littlemore Member (see p. 52), in a sewage trench running from the Old Dorchester Road [SP 533 020] to Sandford sewage works (Figure 14):

North-east of Sandford, soft clean sand was formerly dug from beneath a 0.4 m thick, hard sandstone in a 2.8 m-deep quarry [SP 5391 0203]. Cobbold (1880) described a section in this quarry which, beneath marl (Littlemore Member), showed 2.7 m of sand and limestone. Excavations [SP 5428 0187] at the Sandford sewage works showed the following section beneath Littlemore Member (based on Callomon, 1960, p. 182):

The following section, beneath the Littlemore Member in the Littlemore Railway Cutting [SP 5315 0278] to [SP 5280 0275], is based on Pringle (1926) and the ammonite records of Callomon (1960, p. 181).

The section at the cutting (Plate 7) and (Plate 8) was also recorded by Woodward (1895, p. 129) and Pringle (1926, pp. 52–53). The uppermost 2 m of the Beckley Sand are still visible [SP 5307 0277]. A temporary excavation [SP 5324 0278] to the east exposed 3.7 m of sand with sandstone doggers.

Large doggers of blue-hearted calcareous sandstone (up to 1.2 m diameter and 0.5 m thick) on the Recreation Ground [SP 5366 0295] were probably excavated from the sites of adjacent air-raid shelters. A temporary section [SP 5444 0285] at Sandy Lane West showed 3.85 m of brown sand with thin sandstone ribs. Evidence of calcareous sandstones within the Beckley Sand is provided by debris and shallow exposures throughout the Iffley and Rose Hill districts of Oxford; at least two cemented horizons crop out within the Rose Hill Cemetery [SP 537 039].

The Cowley Motor Works are largely built on the Beckley Sand outcrop. At the southern end of the site, an exposure [SP 5471 0358] showed 1.6 m of soft brown sand with a thin, fine-grained, calcareous sandstone with dark grey clay wisps. Another section [SP 5554 0372] showed 0.5 m of massive, grey, calcareous sandstone with minor shell debris and clay traces. Shallow sections [SP 5586 0420]; [SP 5588 0411], adjacent to the Roman Way, exposed a coarse-grained, shelly, shell-debris-rich calcareous sandstone near the top of the Beckley Sand. A borehole [SP 5572 0400] at the Works proved buff, medium-grained sand with sandstone beds to a depth of 13.5 m, resting on probable Temple Cowley Member.

The Beckley Sand was formerly exposed in several quarries within the area of the present Cowley Works. At these localities, it underlies Wheatley Limestone Member. In the Industrial School South Quarry [SP 5550 0405], the Beckley Sand succession (based on Arkell, 1936, p. 167), was as follows:

The Industrial School North Quarry [SP 5551 0435] showed 'Shell-cum-Pebble Bed', 0.3 m thick, resting on sand with a band of doggers (Arkell, 1927, p. 89).

The Horspath Road Quarry [SP 5565 0453] showed about 2 m of Beckley Sand (based on Arkell (1936, p. 168) and Pringle (1926, p. 54)):

The Beckley Sand at Brittleton Barn Quarry [SP 5582 0455], situated north of Horspath Road and east of the Roman Way, showed 1.65 m of shelly sand and calcareous sandstone (Woodward, 1895, pp. 131–132).

Several sections of Beckley Sand (beneath Wheatley Limestone) close to the Eastern Bypass and Horspath Road roundabout were described by Callomon (1960). A trench [SP 554 044] showed:

Another trench [SP 554 046], 200 m further north showed:

Callomon (1960) also recorded a section [SP 550 046] at the Horspath Road Roundabout on the Eastern By-Pass:

The section showed rapid vertical and lateral changes with bed 5 dying out, and beds 3 and 4 merging.

Arkell (1927, p. 90) described about 1.4 m of interbedded sands and hard limestones, including a shell-pebble bed, beneath Wheatley Limestone at a section ?[SP 5499 0500] near Cowley Barracks.

A section beneath the Littlemore Member at the Garsington Road electricity substation [SP 5614 0309] was recorded by Callomon (1960, p. 183):

Field debris [SP 5610 0300], 40 m to the south-west, includes shelly pebbly sandstone (bed 8). Similar shelly, sandy, pebbly limestone occurs at the top of the Beckley Sand near Gayden's Farm [SP 5610 0306], 250 m NNE of the substation. The pebble bed is classified with the Beckley Sand Member because of its arenaceous character.

Headington–Elsfzeld–Beckley–Stanton St John

The Beckley Sand outcrop forms the high ground around Headington [SP 54 07]. Shallow exposures generally show medium-grained sand and some show thin sandstone beds.

Beds of sandstone form features on the outcrop south-east of Elsfield. Sandstone doggers and massive sandstones give rise to strong spurs north of Folly Farm [SP 5560 1096], Beckley. These features have been accentuated by spring sapping and landslip.

At Beckley, large sandstone doggers occur in the gardens of several houses along the High Road. The member has been described from three pits to the south and south-east of the village where it underlies Wheatley Limestone. The Beckley Quarry [SP 5665 1035] of Messrs Benfield and Loxley was described and figured by Arkell (1936, pp. 171–172):

The ammonites from Bed 3 (some of which taxa also occur in Bed 1), based on Arkell's (1936) list, are: Cardioceras (Scoticardioceras) excavatum, C. (Vertebriceras) vertebrale, C. (V.) rachis, Euaspidoceras sp., Goliathiceras microtrypa, Perisphinctes (Arisphinctes) ariprepes, P. (A.) headingtonensis, P. (Kranaosphinctes) sp.,P. (Liosphinctes) cf. apolipon and Perisphinctes (P.) tumulosus, together with Paracenoceras hexagrmus. Callomon (1953, p. 85) noted that subsequent workings to the north revealed younger levels of Bed 8, from which he recorded: Cardioceras (Cawtoniceras) cawtonense, C. (Maltoniceras) cf. highworthense, C. (Sagitticeras) sp., C. (Scoticardioceras) excavatum, C. (Subvertebriceras) zenaidae, C. (Vertebriceras) vertebrale, Goliathiceras cf. gorgon, Perisphinctes (Arisphinctes) cotovui, P. (A.) pickeringius, P. (Kranaosphinctes) decurrens and P. (K) trifidus.

In the Horton Road Quarry [SP 5705 1040], some 300 m to the east, Callomon (1953, p. 86) noted Wheatley Limestone resting on 1.6 m of yellow sand with 'gritstone' layers. In the Wood-perry Road Quarry [SP 5695 1085], about 300 m to the north, he recorded (Callomon, 1953, p. 83) 2 m of sand with a 0.2 m sandy limestone, and impersistent doggers and layers of 'grit-stone', beneath Wheatley Limestone. The beds yielded Cardioceras (Scot.) excavatum, Perisphinctes (Arisphinctes) cotovui, P. (A.) cowleyensis, P. (A.) helenae, P. (A.) ingens, P. (A.) maximus, P. (A.) pickeringius, P. (A.) vorda, P. (Dichotomosphinctes) antecedens, P. (D.) magnouatius, P. (D.) aff. maltonensis, P. (D.) aff. ouatius, P. (D.) rotoides, P. (Kranaosphinctes) bullingdonensis, P. (K.) cymatophorus and P. (K.) aff. decurrens.

The member was exposed beneath the Wheatley Limestone in quarries south of Stanton St John. Green (1864, p. 44) described 3 m of yellow and brown sandy clay with irregular beds of concretionary limestone overlying 1.8 m of hard, dark blue limestone in a section possibly [SP 580 089] to the south-east of the village.

Holton, Wheatley and Worminghall

The highest beds of the Beckley Sand were exposed along the A40 [SP 5970 0614] at the Holloway Road overpass:

A seam of grey clay above calcareous sandstone crops out [SP 6085 0649] in the fields about 300 m east of Holton Church.

A temporary section [SP 6062 0641] nearer the Church showed higher beds consisting of brown sand (1 m thick) on grey bioturbated calcareous sandstone with shell debris to 0.3 m.

A temporary section [SP 6053 0575] near the old Worminghall Road, showed:

The late J M Edmonds (Oxford University Museum) recorded sections in the adjacent Holton (Worminghall) Underpass [SP 6056 0577] of the A40:

He described lower beds in the excavations for the west pier.

The Beckley Sand was recorded beneath the Oakley Member and above Arngrove Spiculite in the M40 motorway cutting [SP 6179 0668] near the Waterperry overbridge:

The south-easterly extent of the member is uncertain. Some 3.0 to 4.4 m of beds were proved in site investigation boreholes for the M40 overbridge [SP 622 054] across the River Thame. The succession comprises fine-grained, calcareous, bioturbated silty sand, sandy siltstone or silty mudstone, overlain by a silty micritic limestone with large perisphinctid ammonites and, at the top, dark to medium grey, medium-grained to fine-grained sand with much shell debris and mud-flake conglomerate horizons. The middle unit probably equates with the lowest bed of the previous section, the basal bed of the Beckley Sand which outcrops to the north, and is believed to be the Worminghall Rock.

Buckman (1925a, p. 54) introduced the term Worminghall Rock to describe a hard band proved in a well at Field Farm [SP 630 097], Worminghall. The sequence was as follows:

Assuming that the well was located near the farm buildings, which are sited on Oakley Member, the Rock could occur either at the base of that member (Sykes and Callomon, 1979, pp. 849–850; Wright, 1980, fig. 11a, p. 69) or at the top of the underlying Arngrove Spiculite (Figure 12). However, to the west of Worminghall, where material assumed to be from the Worminghall Rock was collected from pylon excavations in 1967 and from the Worminghall sewage works [SP 90 0800] in 1955 (University Museum Oxford), a thin representative of the Beckley Sand may intervene between the Arngrove Spiculite and Oakley members. A similar bed of sandstone with a fauna of ammonites, 'myid' bivalves, pleurotomariid gastropods and Gryphaea has been recovered further south from M40 motorway site investigation boreholes and excavations (see above). It lies at, or just above, the base of a thin (up to 3 m) development of Beckley Sand. On this basis, the Worminghall Rock is here considered to be part of the Beckley Sand Member. However, it is possible that, as this Member dies out rapidly northwards, its feather-edge, including the Worminghall Rock, could have been included in the Oakley Member during mapping.

A reassessment of the ammonite fauna of the Worminghall Rock suggests that it is older than the age given by Sykes and Callomon (1979, pp. 849–850), and Callomon in Wright (1980, p. 69) who 'securely dated' it as belonging to the Tenuiserraturn Subzone (cardioceratid zonation), believing that Buckman's genus and species 'Miticardioceras mite' (Buckman, 1923, pl. 375; 1925a, p. 54), based on a single specimen from the Worminghall Rock, was the macroconch partner of the zonally diagnostic Cardioceras tenuiserratum. The new records show that the ammonite fauna is almost exclusively perisphinctid, including Perisphinctes (Arisphinctes) cotovui, P. (A.) ingens, P. (A.) pickeringius, P. (Dichotomosphinctes) ouatius and P. (D.) rotoides. Only a single Cardioceras (C. (Subvertebriceras) zenaidae) has been collected during the recent survey and the collections at Oxford appear to be restricted to Perisphinctes. This assemblage strongly suggests the Antecedens Subzone of the predominantly perisphinctid zonation, and the broadly equivalent Maltonense Subzone of the cardioceratid zonation (Figure 12).

Wheatley Limestone Member (including Coral Rag)

The term Wheatley Limestone was introduced by Blake and Hudleston (1877), and formalised by Arkell (1927, et seq.), for a sequence of interbedded hard and soft limestones composed of shell detritus which pass laterally into the Coral Rag. The latter term was introduced by William Smith and subsequently used by Townsend (1813) to describe 'a hard and ragged stone which is a mass of coral'.

Both detrital limestones (Wheatley Limestone sensu Arkell) and Coral Rag occur within the district. On the 1:50 000 and 1:10 000 maps, the detrital and Coral Rag facies are depicted together as a single member which is restricted to the south-western part of the district (Figure 14). In the south, at Northfields Farm [SP 564 033], it rests on, and probably laterally replaces, the Littlemore Member. It can be traced northwards to Heading-ton and Cowley; north-eastwards, it increases in thickness towards the Wheatley Fault Zone and attains a maximum thickness of 26 m at Wheatley, where it is of detrital facies. The member has extensive outcrops in the Beckley–Holton area but dies out to the north-east and east, being replaced by the Oakley Member.

The detrital facies of the Wheatley Limestone Member (Plate 9) was described by Blake and Hudleston (1877) as the 'washings' of the Coral Rag. The characteristic lithology is a shell-fragmental grainstone (biosparite), with grain-size banding in places and generally showing flaggy weathering. These hard beds are interbedded with less well cemented, rubbly weathering marls rich in shell debris. Clay partings are rare, but a clay bed occurs at the base in the Beckley area. In contrast, the Coral Rag facies is a poorly cemented, rubbly limestone. Arkell (1947a) referred to both massive and branching corals forming nodular masses up to 1.5 m in diameter and with large amounts of marl and clay in the interstices.

Arkell (1947a) stressed the intimate relationship between the detrital and Coral Rag facies, with the former occurring both above and below the latter, and the two facies being laterally equivalent. He envisaged (Arkell, 1927, p. 128) a lateral passage from the coral rock through a reef-debris zone to a detrital limestone and, in the area west of Oxford, mapped 'coral reefs' separated by 'channels' infilled with detrital limestone (Arkell, 1936; 1947a). The present survey confirmed the local dominance or admixture of rock types, but most of the exposures studied by Arkell are now lost; debris from excavations and in gardens is a mixture of coralline, shelldebris-rich and sandy limestones. The corals occur as isolated masses or as debris in detrital limestones. Corals are particularly common in the vicinity of New Heading-ton [SP 553 072] and Sandhills [SP 567 077], and in the outliers to the north-west towards Elsfield [SP 545 103].

The coral colonies were small, low mounds, tens of metres in extent, rather than major patch reefs, and the corals were by no means the source of all the shell debris in the detrital facies. Coral debris decreases in abundance very rapidly with distance from the coral-rich areas, and the bulk of the detrital limestone is composed of mollusc and echinoderm debris. Between Beckley and Wheatley, the field debris has been described as oolitic. In fact, true ooliths are rare, and the appearance results from algal micritisation of the shell fragments (Wilson, 1968b).

The presence of large-scale cross-bedding in the member in quarries in east Oxford, and at Wheatley and Lye Hill as suggested by Arkell (1947a), cannot be substantiated, although smaller scale cross-stratification is certainly present.

Arkell (1947b) suggested that contemporaneous movement of faults may have influenced sedimentation, envisaging a shelf on which corals grew, limited to the north-east by the Wheatley Fault Zone with deeper water beyond. The rapid thickening of the Wheatley Limestone south-west of the fault zone, and its disappearance to the north-east, is indeed probably related to local subsidence associated with the fault zone, but there is no definite evidence of a sudden increase in water depth to the north-east, nor any evidence of a fault scarp in Corallian times.

The base of the Wheatley Limestone Member is taken at the first appearance of limestones with abundant shell debris. In many places, this horizon is marked by a pebble bed which includes oolitic and micritic limestone pebbles, as well as those of lydite and quartz. In the Wheatley Limestone, quartz sand in significant quantities is restricted to the basal bed and is probably derived by erosion from the underlying Beckley Sand.

Details

Cowley

In the Cowley area, the Wheatley Limestone comprises a mixture of coralline limestones and oyster-rich, rubbly bedded limestones. Field brash indicates the presence of slightly sandy, shell fragmental limestones and marls. The limestones include micritic types (packstones), similar to those in the Littlemore Member.

An old pit [SP 5605 0365] occurs south of the Littlemore–Wheatley railway, but more extensive quarries lay to the north, where shallow exposures along the Roman Way show pale grey, slightly silty and sandy limestones, sandy Shelly limestones, micritic limestones and marls. Corals occur singly and in beds.

The junction with the underlying Beckley Sand was formerly exposed in at least three places within the Cowley Works. In the Industrial School South Quarry [SP 5550 0405] (Arkell, 1936, p. 167), the base is drawn beneath a bed containing bored and serpulid-encrusted pebbles of oolitic limestone, and smaller quartz and lydite pebbles, which rest on a sandy marl with serpulids. Further north, in the Industrial School North Quarry [SP 5551 0435] (Arkell, 1927, p. 89), 2 m of Wheatley Limestone Member rest on an eroded surface of 'Shell-cum-Pebble Bed' which is here included in the Beckley Sand. To the north-east, Woodward (1895, pp. 131–132) described the section in the Brittleton Barn Quarry [SP 5582 0455] near Bullingdon Green, where Buckman (1925a, p. 51) later noted 3.66 m of Wheatley Limestone Moral Rag') resting on an eroded surface of an underlying shell bed. Arkell (1927, p. 89) described the latter as the 'Shell-cum-Pebble Bed' although pebbles were not specifically recorded here. The sections at all three quarries suggest the presence of a significant stratigraphic break beneath the Wheatley Limestone.

In the Horspath Road Quarry [SP 5565 0456] (Arkell, 1936, p. 168; Pringle, 1926, p. 54), there is no record of an erosional surface at the base of the member, but the lithological contrast with the underlying Beckley Sand is marked. Callomon (1960) described several sections in the vicinity of the Oxford Eastern By-Pass which runs about 200 m west of these old quarries. These showed up to 1.5 m of Coral Rag facies passing laterally into detrital limestones. There were also extensive quarries around the Horspath Road–Holloway cross-roads.

New Headington

Arkell (1927, p. 131) described two quarries–Wingfield Hospital (now Nuffield Orthopaedic Centre) South [SP 5480 0575] and North [SP 5469 0599]–which showed the basal beds of the member overlying Beckley Sand. Traces of the larger Windmill Quarry (Arkell, 1927) [SP 550 061], where the total thickness of Wheatley Limestone is estimated at 6.5 m, can still be recognised.

The Cross Roads Quarry [SP 550 065], at the junction of Old Road and Windmill Road, originally exposed 4.6 m of limestone resting on sand (Arkell, 1927). The corals, in situ at the southern end of the quarry, become increasingly broken towards the north-north-east and, at the northern end, there is only debris composed of small coral fragments. McKerrow and Kennedy (1973, p. 32) confirmed this lateral passage and also described well-bedded detrital limestones at the base of the pit. A section [SP 5505 0650] is still visible and shows up to 3 m of rubbly to massively bedded, pale grey, shell-fragmental limestone with abundant corals and bivalves (Plate 9).

Some 300 m to the north-east, the Vicarage Quarry [SP 5529 0665] proved the following sequence overlying Beckley Sand (based on Arkell (1927, p. 87)):

Woodward (1895, p. 131) described a quarry ('left of the road leading from Oxford to Shotover Hill') about [SP 554 065] showing 4.15 m of Wheatley Limestone beneath Ampthill Clay. A similar succession was seen in a temporary section [SP 5538 0642] close to the quarry.

Headington Quarry

The old village of Headington Quarry is built on the sites of old workings and spoil heaps which cover almost 20 hectares. Quarrying continued for at least six centuries (see Chapter 12). A temporary section [SP 5545 0717] in Pitts Road exposed the face of an old quarry:

Samples from a borehole, 10 m from this site, proved 10.1 m of beds as described above; an adjacent borehole, about 1.5 m lower, is reported to have entered sand (presumably Beckley Sand) at between 8 and 9 m depth.

The Magdalen or Workhouse Quarry [SP 5513 0718] was worked this century and Pringle (1926, pp. 56–57) recorded a section on which the following is based:

The section showed considerable lateral variation (Arkell, 1927, fig.12) which Pocock attributed to cross-bedding but Bed 2 (the basal bed of the Wheatley Limestone Member) was persistent throughout the quarry. The most significant feature is the lateral passage of beds into Tendle' which Arkell (1927) described as a mass of white porous comminuted calcareous fragments (possibly shell debris), and which Buckman (1925a, p. 50) called a 'whitish oolite'. A face [SP 5520 0708] is still visible at the southern end of the quarry, and exposes 3 m of slightly rubbly, shell-fragmental limestones.

A site investigation close to the Oxford Eastern By-Pass, and south of Kiln Lane, indicates that Wheatley Limestone had been worked beneath Ampthill Clay at the site of a former brick and tile works. Here, the deepest borehole [SP 5573 0687] proved 7.3 m of Wheatley Limestone consisting of variably cemented detrital limestone, with corals at some levels, and clay partings, resting on Beckley Sand. The A40 Headington Borehole 3 [SP 5557 0737] proved a basal conglomerate with highly rounded, bored pebbles, up to 20 mm diameter, of micrite, sandy limestone and calcareous sandstone, and smaller, less common pebbles of lydite and quartz. The maximum thickness of Wheatley Limestone is estimated to be 10 m hereabouts.

Beckley, Stanton St John and Holton

There are numerous small outcrops of Wheatley Limestone in the Wheatley Fault Zone (Figure 14); the member caps low hills formed of Beckley Sand. Field debris comprises a mixture of flaggy, shell-fragmental limestone, and coralline limestone. The undulating topography and the local abundance of corals may give the impression that the knolls represent in-situ coral colonies ('reefs'), but the corals appear to be most common in detrital limestones where they are associated with Nanogyra nana and echinoid spines.

The outcrop [SP 551 100] east of Elsfield comprises flaggy and oyster-rich, shell-fragmental, limestone debris, with rubbly, shelly oyster lumachelle containing rare corals. Large fragments of coral occur at the western limits. Coralline limestone, with a seam of grey clay at or near the base, forms the narrow ridge of Wadley Hill [SP 105 099], but debris on its north-westward extension consists of shelly and shell-detrital limestones.

The quarry [SP 5502 1059] north of the Elsfield Lane is largely backfilled but shows up to 1 m of interbedded flaggy shell-fragmental grainstone and porous, rubbly, partly friable pack-stone (biomicrite). Debris of the latter in the fields to the north-west includes shelly, shell-debris-rich lithologies with corals.

The outcrop [SP 557 106] north of Stow Wood consists of flaggy, detrital grainstones, sometimes shelly and with coral debris, interbedded with rubbly, shelly and rarely coralline micritic limestones and oyster-rich marl. A seam of silty clay crops out near the base [SP 5635 1063] parallel to the New Inn Road. In the outcrop [SP 570 105], south-east of Beckley, shell-fragmental grainstones are dominant. The basal beds of the Wheatley Limestone were exposed in Beckley Quarry [SP 5665 1035] (Arkell, 1936, pp. 171–172):

The Woodperry Road Quarry [SP 5695 1085] has now been largely infilled and built over. Callomon (1953, pp. 83–85) recorded a section showing over 5 m of Wheatley Limestone overlying Beckley Sand:

The highest beds are still exposed in the rockery gardens which incorporate the original quarry face. Wilson (1968b, p. 101, plate IIIC) illustrated a 'skeletal debris' (partly coralline) grainstone from this quarry, and McKerrow and Kennedy (1973, p. 33) recorded oolitic limestones. Callomon (1953, p. 86) 'described a similar section, showing about 6 m of Wheatley Limestone overlying Beckley Sand, at the Horton Road Quarry [SP 5705 1040]. Additional faunal records included Hyboclypeus wrighti in Bed 4, and Bourguetia saemanni, Natica arguta, Procerithium muricatum, Pseudomelania heddingtonensis, Chlamys (Radulopecten) fibrosa, Deltoideum delta, Gervillella aviculoides, Gryphaea dilatata, Isognomon promytiloides, I. subplana, Mactromya aceste, Pleuromya uniformis, Trigonia reticulata, Cardioceras (Cawtoniceras) cawtonense, Perisphinctes (Arisphinctes) kingstonensis, P. (A.) laevipickeringius, P. (A.) cf. maximus, P. (A.) pickeringius, P. (A.) plicatilis, P. (Kranaosphinctes) decurrens, Nucleolites scutatus and Plegiocidaris florigemma spines in Bed 2. The highest 1.8 m of beds are still exposed.

The Wheatley Limestone was formerly dug in several quarries south of Stanton St John. Blake and Hudleston (1877, p. 310) described one possibly [SP 578 088] which exposed some 7.3 m of alternating layers of 'hard doggery grey bands and marl'. Towards the base, the hard bands were closer together, thicker, and more crystalline, the lowest being a 'hard, blue, compact limestone [0.3 m] in thickness, with fragments of shells'. Corals were extremely scarce but echinoderms were abundant; these were associated with Perisphinctes, bivalves and gastropods. Only the highest beds are now exposed. The field debris hereabouts includes shell-fragmental limestone with superficial ooliths and rare coral fragments were noted in similar limestones in the fields south-east of Vent Farm, Forest Hill [SP 5877 0774].

Wheatley

The member is exposed in two large quarries on either side of the B4027, Islip Road. The more westerly quarry [SP 5915 0678] shows 3.6 m of massive detrital limestone. To the east is the Lye Hill Quarry where the following section, showing 22 m of shell-fragmental grainstones with minor marls, was recorded on the south-eastern face [SP 5927 0680];

The dip appears to be 8 to 10° in a direction slightly north of east, and the total thickness of beds visible on the existing face is estimated to be between 20 and 26 m. This is much greater than the thickness of Wheatley Limestone in other areas. The site lies within the Wheatley Fault Zone and it is likely that localised subsidence during deposition accounts for the unusual thickness of the succession here.

The lowest 4 m of the member, overlying Beckley Sand, were exposed in the A40 road cutting near the Wheatley–Holton overpass [SP 5970 0614] :

There were at least three quarries in Wheatley village. The most northerly [SP 594 061], in Westfield Road, still shows 1.5 m of coarse-grained detrital limestone dipping gently north-east. The second, Gaol House Quarry [SP 595 059], now forms part of the Recreation Ground; some 4.2 m of alternating hard and soft, shell-fragmental, oolitic limestone are exposed. The shell debris ranges from coarse- to medium-grained, with some grain-size banding. Nanogyra nana is common. To the west, part of the original quarry face, some 8 m high, forms the rear boundary of the gardens in a housing development. Pocock (BGS Archives) records the beds dipping N60°E. A site-investigation report suggests the presence of a quarry [SP 5968 0585] immediately north-west of Wheatley Church. A temporary section [SP 5962 0588] exposed 0.3 m of pale buff detrital limestone.

To the east, the limestone thins rapidly. Some 0.5 m of detrital limestone was exposed in foundations [SP 5993 0592] in Crown Road, but generally the field debris comprises rubbly weathering, oyster-rich lithologies transitional to those of the Oakley Member. In the most easterly outcrop [SP 6115 0436], near the River Thame, the field brash includes burrowed shell-fragmental and oolitic limestones.

Littlemore Member

The member corresponds with the Littlemore Clay Beds of Arkell (1927, pp. 140–142) who described alternating layers (0.08 to 0.6 m thick) of brownish black clay and white weathering argillaceous limestone. It ranges up to 12 m in thickness although, at outcrop, the maximum is estimated to be 8 m.

The Littlemore Member is restricted to the south-west of the district (Figure 14), cropping out around Sandford village, striking north-eastward towards Blackbird Leys, and dying out northwards. North of Gwydens Farm [SP 5622 0331], it is impossible to distinguish between the Littlemore and Wheatley Limestone members. An outlier of the Littlemore Member occurs at Rose Hill, underlying the Ampthill Clay, which caps the summit. The subcrop extends eastwards beyond the site of the Garsington No. 1 Borehole, and south-eastwards to Chalgrove.

The type locality is the Littlemore Railway Cutting [SP 531 028] (Plate 8), which was also described by Blake and Hudleston (1877), Cobbold (1880) and Woodward (1895). At the type locality, the member comprises at least 20 alternations of limestone and mudstone; the average bed thickness is 0.27 m (Plate 8). A new section [SP 5310 0277] at the Railway Cutting and a section [SP 5390 0155] on the new Dorchester Road at the Sandford Underpass both proved alternating marls and limestones, with mudstones (see Details). The mudstones are dark grey, with varying proportions of shell debris, and range from gritty beds with abundant detritus, to laminated beds with silty fine-grained detritus. The limestones are micritic (packstones and wackestones) with fine shell detritus. Fossils are scattered throughout the sequence, which is bioturbated in parts. Field debris suggests the presence of a high proportion of the oyster Nanogyra nana ((Plate 5):4), and oyster-rich marls and limestone, as in the equivalent Oakley Member.

The base of the member is drawn at the appearance of calcareous mudstone, marl and limestone lithologies; it is a difficult boundary to identify, particularly in old records. However, the member contains very little quartz sand, in contrast to the underlying Beckley Sand. Quartz of medium grain size occurs in the basal bed and also in a bed of sandy limestone at the top of the member in the Garsington Road area [SP 568 032]. South of Sandford, a shell-fragmental, oolitic grainstone forms the topmost bed. This can be traced to beyond the Sandford sewage works [SP 5429 0189], but is absentaround Blackbird Leys Farm [SP 555 023]; it reappears [SP 561 030] north of the Northfield Brook. At this northern limit, the bed is intermediate in lithology with that of the overlying Wheatley Limestone and probably passes laterally into the latter. This is the first indication of the northward transition of the entire Littlemore Member into the Wheatley Limestone Member (see Arkell, 1927). This transition from predominantly mudstone sequences to mixed sequences with increasing proportions of limestones, particularly shell-fragmental limestones, can be seen at outcrop. North of the Northfield Brook, the member contains a higher proportion of marls, and oyster-rich mudstones and limestones with abundant shell debris.

The Littlemore Member was deposited in a quiet marine, possibly lagoonal, environment where the precipitation of calcareous mud was concurrent with the ingress of clay and shell detritus. The presence of coral detritus, particularly in the lower beds, and ooliths with coarse shell debris in the upper beds, represent facies transitional to the higher energy Wheatley Limestone environments to the north and east. The member thins south-eastwards where the appearance of limonitic ooliths may indicate a more marginal environment.

Details

Littlemore–Sandford-on-Thames

The succession in the Littlemore Railway Cutting [SP 5315 0278] to [SP 5280 0275] (Plate 8), based on Pringle (1926) and the ammonite records of Callomon (1960, p. 181), is:

Both Cobbold (1880, fig. 1) and Pringle noted that the beds undulate gently and that there is an apparent dip. westwards towards the River Thames. In the cutting, the overall dip is 5° to just south of west. Pringle (1926) reported that, in 1867, T Codrington noted numerous small faults in the cutting, but suggested that the apparent dislocations resulted from the presence of masses of 'rudely stratified' limestone, covered with Cycloserpula intestinalis and Nanogyra nana, within otherwise evenly bedded strata. Arkell (1927, p. 143) described a similar phenomenon and interpreted it as a shoal or shell bank.

The basal beds, overlying Beckley Sand, were visible in a new graded section close to the railway [SP 5310 0277]:

The section is repeated about 100 m from the road bridge, probably due to a small fault.

A section in a pipe trench extending from the main Dorchester Road [SP 533 020] to the sewage processing plant [SP 5429 0189], 550 m to the west, showed about 6 m of thinly interbedded clays, marls, limestones and rarer sands overlying Beckley Sand (Callomon, 1960, p. 180–181).

A loose block, found in field debris [SP 5523 0210] south-west of Blackbird Leys Farm, consisted of a rounded mass of laminated algal limestone, 0.3 m in diameter, which may originally have formed a free-standing stromatolite.

Callomon (1960, p. 182) noted a section at the sewage works [SP 5429 0189], on which the following is based:

Sandford-on-Thames

The higher beds of the Littlemore Member were exposed in a shallow ditch [SP 5390 0155] adjacent to the sliproad at the Sandford Underpass, beneath the Henley Road.

Cobbold (1880) recorded equivalent and lower beds in a sewage trench [SP 5344 0187] extending up Sandford Hill. Beckley Sand at the base was overlain by 4.57 m of 'bluish grey marls', 3.81 m of 'very hard fine-grained limestone', and then 0.76 m of 'very sandy limestone'. These limestones, perhaps 4 m thick, were dug at Lower Farm [SP 5372 0056], and can be traced from thence to a point [SP 5499 0167], east of the Thames Water Sandford sewage treatment plant. Shell-detrital limestones dominate, although oyster limestones, and clay and marl beds are interbedded in places.

Cowley–Garsington

Callomon (1960, p. 183) described a section at the Cowley electricity substation [SP 5614 0309] which showed 0.15 m of dark brown, loamy clay with worn lumps of corals (Isastraea and Thamnasteria), resting on up to 0.30 m of cream, marly, nodular, rubbly limestone and passing down into grey, marly clay. Perisphinctes (Arisphinctes) cotovui, P. (A.) cf. parandifarmis, P. (Dichotomosphinctes) cf. antecedens, P. (D.) buckmani, P. (P.) cf. chlaroolithicus, P. (P.) cf. parandieri were recorded.

In the Garsington No. 1 Borehole, limestones comprise more than half the total thickness of the Littlemore Member, with substantially less marl and even fewer mudstones. Grain-supported, sparry limestones (grainstones) are more common than those with a micritic matrix (packstones) and occur from 10.31 to 15.31 m depth and, form 1.14 m, at 17.55 m. The shell-fragmental grainstones show traces of cross-stratification and clay partings. The marls contain an abundance of shell debris, and the mudstones commonly contain partings of shell debris. All lithologies are bioturbated; fossils are rare. Petrographic inspection shows that many of the shell fragments, which are dominated by bivalve debris with lesser coral debris, have micritised rims and some show superficial oolith coatings. A grainstone from 16.2 m depth includes coral, bivalve, echi noid spines, gastropod and serpulid remains, with serpulids encrusting some of the larger grains. Two specimens of packstone examined contain poorly sorted, heavily abraded shell fragments, including coral and bivalves, with some large serpulidencrusted grains, scattered in a matrix of fine-grained, crystalline, possibly calcite mud, which forms about 60 per cent of the rock.

This sequence differs from those described from Littlemore and Sandford because of the relatively high proportion of shell debris. The limestones are a mixture of grainstones, like those of the detrital facies of the Wheatley Limestone Member, and micritic limestones similar to those in the Littlemore Railway Cutting (see above); the latter are dominant in the lower part.

Oakley Member

The term Oakley Clay was applied by Buckman (1927) to clays and clayey marls, previously known as the Exogra nana beds/clay/zone' (Davies, 1907b; Barrow, 1908) which outcrop in the vicinity of Oakley [SP 639 118]. The name was amended to Oakley Beds (Arkell, 1933, p. 409; 1942; 1947a), and these were correlated with strata here classified as the Wheatley Limestone and Littlemore Clay members. The unit is now formalised as the Oakley Member.

The most westerly occurrence of the member is at Forest Hill [SP 587 080], where it rests on the Beckley Sand (Figure 14). It has been mapped as discontinuous outcrops to the east of Wheatley and has been proved in several site investigation boreholes drilled for the M40 motorway - River Thame crossing. From there, the outcrop is continuous north-eastwards to near Waddesdon. To the north-east, the Oakley Member becomes increasingly argillaceous and passes laterally into the upper part of the West Walton Formation (Figure 11). However, further afield, the facies reappears intermittently, for example near Ampthill in the adjoining Leighton Buzzard district (Shephard-Thorn et al., 1994). North-west of Waterperry, the Oakley Member overlaps the Beckley Sand to rest directly on the Arngrove Spiculite which is in turn overlapped to the north of Brill, so that the Oakley Member then rests directly on the West Walton Formation. The member is generally overlain by the Ampthill Clay (but see (Figure 11) and Chapter Six). The most extensive outcrops lie between Worminghall [SP 641 084] and the type area near Oakley [SP 639 118].

The member is thickest, about 5 m, in the central part of the outcrop near Worminghall; 4.2 m were proved in the Brill No. 1 Borehole.

The Oakley Member consists of interbedded pale grey marls, silty and argillaceous, micritic limestones, silt-stones, and silty to sandy mudstones. The base is drawn at the appearance of these lithologies, which contrast strongly with the underlying beds. Some beds are highly fossiliferous, and the outcrop is commonly marked by an abundance of the oyster Nanogyra nana, as both loose valves and cemented aggregates ((Plate 5):4). Serphlids are also common. The marls and clays are generally deeply weathered and the subsoil commonly contains small nodules of race. Generally, arenaceous beds occur at the base of the member, where material has been recycled from older formations. Rarely, the base is marked by a pebble horizon, for example, M40 Borehole 24 and the M40 cutting [SP 6179 0668]. Carbonate ooliths are generally rare, but have been recorded at two localities. However, ferruginous grains, generally limonitic ooliths, have been noted at many sites.

Although the Oakley Member is laterally equivalent to the Wheatley Limestone, the change in lithology appears to be abrupt. The member resembles the Littlemore Member but is less fossiliferous. The depositional environment was intermediate between those of the Wheatley Limestone and West Walton Formation.

Details

Forest Hill and Wheatley

At its most westerly outcrop [SP 5868 0808], the member consists of silty micritic limestones with oyster-marls and a thin bed of limonitic peloid biosparite. Lenticular micrites in shell-debrisrich clays occur in the A40 cutting [SP 6002 0586] and adjacent District Council Office grounds. The basal bed is a shell-debris marl which becomes increasingly sandy downwards.

The member was proved in several M40 site investigation boreholes where it is up to 2.1 m thick; only 0.95 m was proved in the most southerly (Borehole 15 [SP 6259 0498]), and it probably dies out hereabouts. In the boreholes, the sequence consists of interbedded, silty, micritic limestones, shell-fragmental micrites (wackestones and packstones) and thin shell-fragmental, oolitic grainstones, interbedded with marls and shell-debris mudstones. Small pebbles occur at the base in M40 Borehole 24, but the sequence becomes increasingly sandy downward in all cases. An exposure beneath Wheatley Limestone in the A40 road cutting near the Wheatley–Holton overpass [SP 5970 0614] showed 0.22 m of argillaceous, dark grey to purplish limestone passing laterally into a rubbly weathering, pale greenish grey, micritic, oyster limestone with patchy cementation. Oysters, coarse shell fragments with brown limonitic oolith grains, and serpulid masses are present. The base is ill defined and poorly exposed, but is possibly sandy.

The late J M Edmonds (Oxford University Museum) recorded an unspecified thickness of an oyster-rich lithology at the top of sections in the Holton (Worminghall) Underpass [SP 6056 0577] of the A40.

Waterperry–Warminghall–Oakley

The base of the member, overlying Beckley Sand, was recorded in a drainage ditch and at the top of the adjacent cutting [SP 6179 0668] on the M40 motorway near the Waterperry overbridge, where at least 1.0 m of pale cream, weathered marl, with abundant Nanogyra, scattered echinoids and an ochreous brown clay seam at the base, overlie 0.4 m of fawn-weathering, pale grey, medium- to fine-grained sandstone, with shell debris and rare small pebbles, and a brownish clayey marl band at the base.

The outcrop is characterised by an abundance of valves of Nanogyra nana, many in cemented masses, some enclosed in a marly limestone matrix. Serpulids are also common. Sandy beds mapped near the base of the member hereabouts probably include thin representatives of the Beckley Sand and the

Worminghall Rock (see above). Shell-fragmental grainstones with limonitic peloids occur locally. The estimated thickness in this area is 2 to 4 m. Debris from the Moat [SP 624 142] at Boarstall includes pale brown, marly, micritic limestones with serpulid- and oyster-rich marls and mudstones.

Brill–Ludgershall

The member crops out on the lower slopes of Muswell and Brill hills. Almost the complete thickness was exposed in a ditch [SP 637 155] south of Corble Farm:

The Brill No. 1 Borehole proved 4.2 m of Oakley Member and provides a reference section (Figure 13). Marls and limestones are the dominant lithologies with minor siltstones. Fine-grained shell debris and shells (mainly bivalves) occur throughout the sequence, and N. nana forms lumachelles. The beds are commonly bioturbated. A petrographic thin section of limestone from 103.5 m depth shows sparse, fine- to coarse-grained, ferruginous ooliths with scattered, coarse-grained, bioclastic debris in a finely crystalline carbonate matrix. Some shell fragments are bored and many are encrusted by serpulids and possibly foraminifera, or have a ferruginous micritic coating. A second thin section of a packstonetype limestone shows sparse bivalve and coral debris and ferruginous ooliths, with sporadic quartz grains, set in a finely crystalline carbonate cement.

Dorton

The Oakley Member was formerly exposed in the railway cutting between Dorton and Ashendon Junction [SP 693 134] (Figure 11). Davies (in Arkell, 1927 and BGS Archives) described the following sequence between the Ampthill Clay and West Walton Formation:

Waddesdon

Field evidence indicates the presence of up to 3 m of clays, silts and marls with impersistent limestones. Ferruginous ooliths occur in some beds and cemented clusters of Nanogyra are common. It is increasingly difficult to recognise the facies to the north-east of Westcott [SP 724 174].

Chapter 6 Jurassic: Ancholme Group (Part 2)

The upper part of the Ancholme Group (i.e. that part which overlies the Corallian Formation in the western part of the district) comprises the Ampthill Clay and Kimmeridge Clay formations. The lower part of the Group is described in Chapter Four.

Ampthill Clay Formation

The outcrop of the Ampthill Clay Formation extends across the district from Sandford-on-Thames in the south-west to beyond Waddesdon in the north-east (Figure 1). On early maps of the district (Old Series One-Inch Sheets 13, 45 and 46, published in the 1860s), the strata were included in the Kimmeridge Clay, but Seeley (1869) differentiated and finally named the Ampthill Clay as a separate formation beneath the Kimmeridge Clay, in particular in Buckinghamshire, Bedfordshire and Cambridgeshire. His Ampthill Clay was considered to be the lateral equivalent of the Corallian Formation but, as currently defined, the Ampthill Clay overlies the latter which correlates with the West Walton Formation (Figure 11).

The thickness of the Ampthill Clay varies greatly across the district (Figure 15). It reaches a maximum of about 23 m in the north-east, where the Corallian Formation is thin or absent. Traced south and south-westwards from Muswell Hill [SP 641 155], mapping suggests a gradual thinning of the Ampthill Clay but in the Wheatley–Oxford area, where the greatest development of the Corallian Formation occurs, there is a marked thinning to less than 5 m. This is due mainly to loss of the basal beds by overlap. There is also some evidence of minor thickness variation in the topmost beds of the formation, associated with the disconformable base of the overlying Kimmeridge Clay Formation.

Throughout most of the district, the base of the Ampthill Clay has been taken at the top of the underlying Corallian Formation, as the pronounced lithological change at this level (from clays above, to limestones and marls below) is readily mapped. However, data from the Brill No. 1 Borehole [SP 6570 1412] suggests that in the central part of the district, the basal part of the Ampthill Clay, as mapped, corresponds with the uppermost part of the West Walton Formation of the Fenland type area (see Chapter Four; (Figure 15)). North-east of Waddesdon, the Corallian Formation is absent and Ampthill Clay rests directly on the West Walton Formation. Here the base of the Ampthill Clay has been mapped principally on the basis of a change of gradient reflecting a more subtle lithological change, from mudstones above, to silty mudstones below, and is necessarily approximate.

The Ampthill Clay gives rise to a pale grey or fawn clay soil. Boreholes show that when unweathered, the formation consists of medium to dark grey mudstones and silty mudstones, and pale grey calcareous mudstones. These are believed to have been deposited in a shallow shelf sea, with the different mudstone types reflecting differences in water depth and distance from land (Gallois and Cox, 1977). A number of nodule beds occur, some of which have been mapped locally by means of the weak topographic features or brash which they produce. These nodules, commonly up to 0.25 m diameter, are composed of pale grey, smooth-textured cementstone, which develops a white, buff or yellow coating on weathering. Some are septarian, and are veined with colourless or brown calcite. Beds with black to bluish grey, phosphatic, angular to rounded clasts (rarely up to 40 mm diameter), occur at several levels, mainly in the higher part of the formation. Many of the larger clasts are internal moulds of bivalves or ammonite body-chambers. These beds are indicative of minor non-sequences, and suggest periods of higher-energy, possibly shallower conditions.

Boreholes and other sections show that the Ampthill Clay succession in the district is closely comparable to that in Fenland, where Gallois and Cox (1977) and Cox and Gallois (1979) divided it into 42 units based on gross lithology and macrofaunal associations; numbers prefixed by AmC in the following account refer to these units which are used as a standard for correlation.

The formation contains a fauna dominated by bivalves and ammonites; the latter are the basis of the standard zonation (Figure 15). However, due to weathering, the only fossils normally found at outcrop are belemnites (notably large pachyteuthids; (Plate 10):5) and oysters. Of the latter, Gryphaea dilatator ((Plate 5):5) is common in the lower part of the formation; at some levels it occurs in great abundance, locally making feature-forming beds. As in the West Walton Formation, the shells are commonly bored and heavily encrusted with epifauna. The flat oyster Deltoideum delta ((Plate 10):6) replaces Gryphaea in the upper part of the formation, and forms lumachelles, locally cemented, in the uppermost beds ((Plate 10):8). This sequence of fossil oysters is of great value in mapping.

In the middle part of the succession, a very shelly, impersistent cementstone, the Brill Serpulite Bed ((Plate 10):7), is characterised by abundant serpulids and bivalves, notably Isocyprina (Venericyprina) pellucida. It probably correlates with AmC34, in the Regulare Zone (Cox and Sumbler, 1989). Less shelly cementstone nodules in the Serratum Zone (?AmC17) have also been recorded at several localities.

Throughout much of the district, the topmost 2 to 3 m of the Ampthill Clay is characterised by pale calcareous mudstones and cementstones with an abundant fauna apparently dominated by oysters (D. delta, Nanogyra nana, Lopha), serpulids (Sarcinella) and the large, globose, asymmetric brachiopod Torquirhynchia inconstans ((Plate 10):2). Though tentatively assigned to AmC40–42 by Cox and Sumbler (1989), these strata are now known to include the Inconstans Bed (KC1; see below) which, at the Dorset type locality, marks the base of the Kimmeridge Clay and the Kimmeridgian Stage (Arkell, 1933; Cox and Sumbler, in press; (Figure 16)).

Details

Oxford–Wheatley

In 1989–90, the highest beds of the Ampthill Clay were exposed in cuttings and excavations for the M40 motorway near Waterstock [SP 63 05] and were also proved in nearby site-investigation boreholes. Borehole 15 [SP 6259 0498] indicates a total Ampthill Clay thickness of about 20 m. Borehole 007A [SP 6254 0490] proved the Inconstans Bed and its associated pale grey and Lapha-rich mudstones and cementstones; the latter were also proved in Borehole 009 [SP 6341 0429]. These distinctive beds are over 3 m thick in this area, the maximum recorded in the district (Figure 16). They were well exposed in excavations for the eastern pier of the A418 overbridge [SP 6257 0498], where a cementstone bed contains 'nests' of T. inconstans and yielded 'myid' bivalves in growth position, Ctenostreon, pleurotomariid gastropods and very rare Pictonia. Representatives of AmC37–39 and AmC36 were identified in Borehole 007A and were also exposed in shallow cuttings and drainage ditches. The base of AmC36 is marked by phosphatic nodules and an associated shell bed with oysters and other large bivalves. Below this, the sequence is known mainly from boreholes, but core losses make detailed classification uncertain. The Brill Serpulite Bed ((Plate 10):7) was recovered from a trial pit [SP 6233 0528] at shallow depth. The boundary of AmC22 on AmC21 was identified in Borehole 007A at 19.50 m depth where fissile, brownish grey, foraminifera-spotted mudstone (cf. oil shale) rested on pale grey mudstone with a striking interburrowed junction. This borehole terminated at 22.60 m depth, probably within AmC17.

Farther west, a much reduced thickness of Ampthill Clay is present in the Garsington No. 1 [SP 5726 0289] and Thornhill No. 2 [SP 5682 0699] boreholes. These proved similar sequences to that formerly exposed in the overburden of the Headington limestone quarries at the foot of Shotover Hill [SP 556 066] or[SP 562 072] (Phillips, 1871). At all three localities, only the upper part of the Ampthill Clay succession is represented, the lower and middle parts having been overlapped as the formation encroaches onto the limestones of the Corallian Formation (Figure 16). The non-sequence at the base is generally indicated by a phosphatic pebble bed with oysters and belemnites which represents the base of AmC36. The greater part (about 4 m) of the overlying mudstones are medium grey and typically include Amoeboceras, Ringsteadia, D. delta (in plasters), Oxytoma and serpulids; according to Arkell (1933), these beds (at Shotover) yielded the type specimens of D. delta and N. nana. The highest beds (up to 1 m thick) are pale grey mudstones with cement-stones, and include beds rich in Lopha, N. nana and echinoid spines. One of these cementstones is believed to be the Inconstans Bed as at Shotover, where T. inconstans was recorded in a septarian limestone band with marine reptile bones (Phillips, 1871). There are specimens of T. inconstans from both Shotover and Garsington in the University Museum Oxford. Material collected from field brash [SP 5370 0126] at Sandfordon-Thames also indicates the presence of these highest beds in the extreme south-west of the district.

Brill–Shabbington

The Brill No. 1 Borehole penetrated the complete Ampthill Clay succession between 79.60 and 99.85 m depth (Figure 15). The uppermost beds (79.60 to 82.25 m) are distinctive pale grey mudstones with a pair of cementstones, the lower of which (at 81.30 to 81.59 m) is probably the Inconstans Bed (Figure 16). Small core diameter and core loss made detailed classification of the underlying variably shelly mudstones difficult, and the classification of the borehole as shown in (Figure 15) is partly inferred. However, the presence of the following beds is confirmed: the serpulid-rich AmC34 (87.29 to 87.60 m), the brownish grey fissile mudstones of AmC22 on AmC21 (at 90.35 m) and the characteristic medium and pale grey, mainly smooth-textured mudstones with Amoeboceras and Perisphinctes of AmC17 (91.80 to 93.14 m).

The basal part of the Ampthill Clay was exposed during construction of the Great Western Railway cutting east of Rid's Hill [SP 673 152]. The section, recorded by Davies (in Arkell, 1927), was reassessed by Cox and Sumbler (1989). Approximately 12 m of Ampthill Clay were exposed on the southern (down-throw) side of a fault south-east of Wood Siding Bridge [SP 6741 1528]. The strata (beds 5–7 of Davies, in Arkell, 1927) comprised pale grey clay, with a cementstone (Bed 6) and a band of large G. dilatata, about 3 m and 6 m respectively above the base. The cementstone yielded fossils including Cardioceras cf. tenuiserratum, suggestive of the Tenuiserratum Zone, and comparison with the Brill No. 1 Borehole suggests correlation with AmC10. The Gryphaea bed probably marks the base of AmC13.

Higher beds of the Ampthill Clay were formerly exposed in Rid's Hill Brickpit [SP 664 151]. The section, originally described by Davies (1907a), was repeated with some modification by Pringle (1926) and Arkell (1947a), and reassessed by Cox and Sumbler (1989). In the brickpit, Bed 1 of Davies (1907a) is a cementstone, probably within AmC17. In the Brill No. 1 Borehole, the top of AmC17 was proved (at 91.80 m depth) 10.8 m above the base of the Ampthill Clay; thus the Rid's Hill and railway cutting sections probably overlap slightly. Bed 2 of the brickpit section comprised 4.3 m of black, shaly, selenitic and jarositic clays. In 1986, material from the lower part of Bed 2 was found at two sites within the pit. Grey clay from a pond [SP 6654 1520] yielded Deltoideum delta, a serpulid-encrusted oyster, and Myophorella. Further west, a small excavation [SP 6642 1517] showed 0.5 m of dark blue-grey, shaly mudstone with crystals of selenite up to 10 cm long. The fauna includes common belemnites, abundant Protocardia, and small tuniform gastropods, and is suggestive of AmC22 to 24.

The Brill Serpulite Bed was first described from Rid's Hill, but it was apparently only seen in situ by Pringle (1926) who placed it between Davies' beds 2 and 3. It was also noted in a ditch section [SP 6429 1611] near Muswell Hill Farm, north of Brill, during this survey. It probably correlates with a serpulid-rich udstone, proved at 87.60 m depth in the Brill No. 1 Borehole, ich lies 15 m above the local mapped base of the Ampthill Clay. 1986, a small section in the pit [SP 6655 1519] (probably within Be 3 of Davies, 1907a) showed 2 m of pale grey, sparsely shelly udstone, with some yellow jarositic staining, and small selenite crystals. The fauna, mainly preserved in brownish pink aragonite, included Grammatodon, oysters, Protocardia, Thracia, Procerithium?, and perisphinctid ammonites (cf. Microbiplices).

A pair of cementstones (Beds 4 and 6) separated by 0.6 m of pale grey calcareous clay (Bed 5) were assigned to the Mutabilis Zone by Cox and Sumbler (1989), but are now known to equate with beds at 80.60 m and 81.59 m depth in the Brill No. 1 Borehole, which lie within the Baylei Zone (Cox and Sumbler, in press; (Figure 16)). The lower cementstone is the Inconstans Bed, and was presumably the source of the T. inconstans recorded by Davies (1907b). Abundant N. nana, very shelly cementstone and a worn fragment of Ringsteadia or Pidonia recovered from ditch debris [SP 6394 1375] west of Brill, suggest the presence of the Inconstans Bed in this vicinity too. Cementstone debris from a pipeline [SP 673 079] to [SP 673 080] north-north-east of Shabbington may also be from these highest beds.

North-west of Shabbington, Amoeboceras ex gr. serratum in cementstone was collected from ditch debris [SP 6586 0855]; this probably comes from AmC17, the same level as the lowest stratum formerly exposed in Rid's Hill Brickpit. Unlocalised specimens of the Brill Serpulite Bed, together with solid whorl fragments of Amoeboceras and belemnites were collected in the past at Ickford [SP 65 07] (BGS collections). At Ickford, the Church Road Borehole [SP 6471 0726] proved the lowest beds of the Ampthill Clay (AmC1–6) at about 5 m depth. These strata are characterised by medium grey mudstone with Cardioceras, including crushed C. kokeni and C. tenuiserratum preserved in white and pinkish aragonite. A similar assemblage was recovered from a depth of about 5 m at Ickford sewage works pumping site [SP 648 074] (material in University Museum, Oxford).

Hartwell Borehole

The upper part of the Ampthill Clay (down to AmC17) was proved in the Hartwell Borehole [SP 7926 1223], between 64.98 m and terminal depth at 76.69 m ((Figure 15); Cox et al., 1994). Below 69.88 m, the sequence comprises mainly smooth-textured medium to pale grey, generally poorly fossiliferous mudstones, with pyrite trails and pins, though somewhat darker mudstones between 71.00 m and 72.68 m are more shelly, with bivalves (particularly Protocardia), belemnites and ammonites. A gritty, shell-debris-rich mudstone with phosphatic pebbles at 69.88 m depth marks a non-sequence at the base of AmC36, above which the mudstones are generally more shelly. Between 66.80 m and 69.88 m depth, they contain phosphatic patches, nodules and pebbles at several levels. At 69.50 m and 70.90 m depth, there are shell 'plasters' of D. delta. The topmost beds of the formation (64.98 m to 66.80 m depth) comprise very pale grey, calcareous mudstones with two smooth, pale grey cementstone bands; the upper cementstone is probably the Inconstans Bed (Figure 16).

Waddesdon-Folly Farm

North-east of Waddesdon, near Upper Blackgrove Farm [SP 773 186], the outcrop of the Ampthill Clay forms a shelf, which terminates in a pronounced south-facing feature above a steep slope. The change of gradient at the bottom of this slope has been mapped as the base of the formation. However, the lowest part of the formation thus defined may include a small thickness of beds which should strictly be included with the West Walton Formation (see above). Some confirmation of this was provided by an excavation at Lower Farm [SP 7777 1770]; though sited an estimated 3 m to 5 m above the mapped base of the Ampthill Clay, the fauna and lithological character of the material from the pit (about 5 m deep) was suggestive of the West Walton Formation.

The entire Ampthill Clay (3.31 to 22.45 m depth) was penetrated in the Folly Farm Borehole [SP 7958 1904] near Hardwick (Figure 15), although, because of extensive core loss, depths may not be entirely reliable. The cores recovered are mainly medium grey, smooth to slightly silty, moderately shelly mudstones with a fauna of bivalves and ammonites preserved in white or iridescent aragonite. The uppermost 1.2 m are pale greyish fawn in colour and include a pale fawn silty cementstone, 0.38 m thick at 4.05 m depth. Just to the east of the borehole, debris from land-drains [SP 7977 1906] to [SP 8000 1893] included fawn and very pale grey silt probably representing the weathered residue of this cementstone; this material yielded a few Lopha and Nanogyra. Abundant D. delta, probably within AmC36, occur at 7.10 m, and a shell bed with belemnites at 11.40 m may mark the base of AmC24. Three other cementstones occurred lower in the borehole at 13.80 m, 14.80 m and 17.95 m depth; the upper two of these may correspond with the lower two of the three mapped in the surrounding area; at least one probably lies within AmC17. An Amoeboceras ex gr. serratum at 13.40 m confirms the Serratum Zone. The base of the formation is marked by a downward change to less shelly, paler grey, calcareous silty mudstones and siltstones with fauna commonly preserved as 'ghosts'.

At several localities near Waddesdon, limestones and marls are developed at the top of the Ampthill Clay. This unit forms a broad bench to the south of the A41 road near Wormstone Farm [SP 752 164], and also occurs as a downfaulted outlier just north of the road. Here [SP 7532 1704], debris from excavations consisted of pale bluish grey clay with race and pieces of fawn to pale grey, crumbly cementstone yielding an abundant fauna including bivalves (arcid, Ctenostreon, D. delta, Lopha, N. nana, N cf. virgula, Pleuromya, Trigonia), clusters of small gastropods (Dicroloma?), brachiopods (T. inconstans ((Plate 10):2) and a terebratulid), ropes of tiny serpulid tubes (Sarcinella), pentacrinoid columnals and echinoid spines. Additional fossils collected nearer Wormstone Farm include Camptonectes, Chlamys, Oxytoma, Serpula and, locally, corals including Thamnasteria and thecosmiliids. Corals, preserved as greyish brown crystalline limestone, are particularly abundant 250 m north-east of the farm [SP 7512 1648]. Ammonites are rare, though two perisphinctid fragments were found.

The same beds form a strong feature between Waddesdon Church [SP 7403 1703] and Waddesdon Dairy [SP 7368 1693]. Pieces of shelly cementstone from graves in the churchyard yielded abundant T. inconstans, Lopha, D. delta and N. nana, some with encrusting foraminifera and serpulids. A similar, though less abundant fauna has been found at the same horizon near Common Leys Farm [SP 726 151], south-west of Waddesdon and, to the east near Berryfields Farm [SP 786 168], amongst stream dredgings [SP 7981 1755] and near Folly Farm [SP 8001 1908]. D. delta is common in 2 to 3 m of strata underlying these pale beds. Cemented lumachelles ((Plate 10):8) are developed locally, for example south-east of Briar Hill Farm [SP 7527 1695] and west of Fleet Marston Church [SP 7791 1598], and in places form shelflike features.

Kimmeridge Clay Formation

The outcrop of the Kimmeridge Clay Formation^‡2  crosses the district from Toot Baldon in the south-west, to Aylesbury in the north-east (Figure 1). Generally, it occupies hill slopes capped by the Portland Formation and is relatively narrow. However, in the Thame valley near Shabbington [SP 66 07], and in its upper reaches around Aylesbury, the outcrop widens considerably, forming rather featureless ground of low relief.

The base of the Kimmeridge Clay has been mapped at a pronounced change in soil colour (from pale Ampthill to dark Kimmeridge clays) which generally corresponds approximately with the change of gradient at the foot of the hill-slopes. Boreholes and sections show that this boundary corresponds with a non-sequence marked by a burrowed surface overlain by a bed of small black phosphatic pebbles. This phosphatic pebble bed lies at the base of the Cymodoce Zone (Lower Kimmeridgian), and cuts down to various levels within the topmost beds of the Ampthill Clay ((Figure 16); Cox and Sumbler, in press).

Estimates of the thickness of the Kimmeridge Clay from outcrop are unreliable because of extensive cambering of the overlying Portland Formation. However, boreholes and sections indicate 40 to 50 m of Kimmeridge Clay throughout most of the district; the thickest succession proved is 55.6 m in the Brill No. 1 Borehole (see Ruffell and Wignall, 1990 and Cox, Sumbler et al., 1991 for a discussion of thickness variation). In this district, the sequence is thin and incomplete in comparison with the type sequence in Dorset where it is over 500 m thick. The succession is highly condensed, with many minor non-sequences marked by burrowed horizons and phosphatisation. The sequence is most condensed in the Wheatley area, where it is only 30 m thick. This area coincides with a thick development of the Corallian Formation (Chapter 5), but was an area of reduced subsidence during late Oxfordian and Kimmeridgian times. Correlatives of the topmost zones of the Dorset sequence are absent in this district, probably due to early Portlandian erosion, and slight channelling below the Portland Formation may account for minor local thickness variations. However, the thinnest Kimmeridge Clay succession probably occurs between Great Milton [SP 63 03] and Thame [SP 71 06] where Cretaceous erosion has removed the upper part of the formation; for example at Albury [SP 656 052], mapping suggests that as little as 25 m of Kimmeridge Clay may be preserved.

Throughout the district, exposure is extremely poor; even those areas shown as drift free commonly have a thin cover of head and hill wash. Beneath this, the mudstones are weathered, often to a depth of several metres, forming medium to pale grey and fawn clays commonly containing crystals of selenite or specks and tiny nodules of race. However, the sequence is known from boreholes, notably Brill No. 1 and Hartwell. In addition, important sections were exposed at Aylesbury and on the M40 motorway near Waterstock [SP 63 05] during the course of the survey, and some details of former brickpits at Brill, Hartwell, Long Crendon, Shotover and Wheatley (Littleworth) are recorded in the literature.

Throughout southern Britain, the Kimmeridge Clay is a succession of marine strata, dominated by mudstones which, at various levels, may be calcareous, kerogen rich (bituminous mudstones and oil shales), silty or sandy; there are associated thin cementstones, siltstones and sandstones which occur as tabular beds or doggers. These lithologies occur in a succession of variable small-scale rhythms (Gallois and Cox, 1976; Cox and Gallois, 1979; 1981). For descriptive purposes, it is convenient to use the traditional subdivisions of Lower and Upper Kimmeridge Clay which correspond with the Lower and Upper Kimmeridgian substages of British chronostratigraphy. In general, no significant lithological change occurs at this level (although there is a minor non-sequence), and the junction between them is drawn largely on the basis of ammonite faunas (Aulacostephanus below, Pectinatites above; (Plate 11)).

Unweathered, the Kimmeridge Clay is highly fossiliferous and at most levels contains abundant ammonites (Plate 11), which are used as the basis of the standard zonation (Figure 17). Bivalves, other molluscan groups, echinoderms, brachiopods, serpulids and crustaceans are also present. At outcrop, fossils are generally rare due to weathering, although the oysters are relatively resistant to solution and often survive in the soil. Small thin-shelled ?Deltoideum occur sporadically in the lower part of the formation and the distinctive striate Nanogyra virgula ((Plate 10):1) appears a few metres above the base. Other fossils to be found at outcrop are Laevaptychus ((Plate 10):3–4) (the calcitic jaws or operculae of the ammonite Aspidoceras) and bones of marine vertebrates ((Plate 11):5). The latter were relatively common in the Kimmeridgian seas adjacent to the London Platform.

By combining the faunal markers within the framework of the ammonite-based zonation, the formation has been subdivided into a number of small units (Gallois and Cox, 1976; Cox and Gallois, 1979, 1981) which have proved to be a valuable aid to correlation within the district. In the following account, numbers prefixed by KC refer to these units.

Cementstone nodule bands also form useful markers, locally forming minor scarp features, which can be mapped. Though generally leached at outcrop, fragments occur sporadically in the- soil. Typically, they consist of grey, silty argillaceous limestone weathering to a brownish colour, and in some cases are weakly veined with calcite; they frequently contain scattered shells and shell fragments. Nodular cementstones, within KC30 (Eudoxus Zone), have been noted at several localities (see Details). KC30 is one of the most readily identifiable units of the Lower Kimmeridge Clay. As well as cementstones, its pale calcareous mudstones contain abundant N. virgula, which may occur in cemented masses to form the Virgula Limestone, and the Crussoliceras Band, a widespread ammonite marker bed. In addition, it contains a characteristic Eudoxus Zone ammonite fauna of Amoeboceras, Aspidoceras, Aulacostephanus, Laevaptychus and Sutneria. A second nodular cementstone, probably within KC40 (Wheatleyensis Zone), is also widespread. It is underlain by a shell bed (probably KC38) with pyritised lenses of N. virgula. Higher in the sequence, a third cementstone, known as the Wheatley Nodule Bed from its occurrence in the Littleworth (Wheatley) Brickpit [SP 589 055] (Pringle, 1926; Arkell, 1947a; see Details), yields an ammonite fauna, in particular Pectinatites (Virgatosphinctoides) woodwardi ((Plate 11):2), which has long been considered a classic assemblage of the Wheatleyensis Zone (Neaverson, 1924). Paradoxically, however, its lithology and relationship to other beds suggests correlation with KC44, which lies within the Hudlestoni Zone.

The Lower Lydite Bed is another useful marker band, recognised in boreholes and sections throughout the district, and more rarely at outcrop in field brash. This is a thin bed packed with phosphatised casts of bivalves and ammonite whorl-chambers, generally accompanied by sporadic small lydite^‡3  and quartz pebbles. Also known from the Swindon area to the south-west, it marks an erosional event at the base of the Pallasioides Zone. An exposure on the M40 motorway (see Details) suggests that the phosphatic clasts are fossils reworked from Pectinatus Zone sediments, and that phosphatisation occurred before the erosive event.

Sands and silts are widespread in the Upper Kimmeridge Clay of the south and central Midlands. The availability of this coarser material may indicate the proximity of land during Late Kimmeridgian times. Where possible within the district, these arenaceous units have been mapped separately from the rest of the Kimmeridge Clay. In the past, a variety of local names were used because stratigraphic relationships were uncertain, but it now appears that there are only two main arenaceous horizons, corresponding with the Elmhurst Silt and Hartwell Silt members of Oates (1991) (Figure 17).

The Elmhurst Silt, as developed at its type section (Watermead, Aylesbury; Oates, 1991) and in the Hartwell Borehole, comprises dark grey argillaceous silts or silty mudstones, 4 to 8 m thick. In the Hartwell Borehole, and as far west as Long Crendon and Brill, the Elmhurst Silt is split into two by a median unit of predominantly smooth mudstones. In the central and western parts of the district, clean, fine-grained sands are developed at the level of the Elmhurst Silt; in the upper part, these are often patchily cemented into large calcareous sandstone doggers locally containing sporadic lydite pebbles. These sands with doggers are particularly well developed in the south-western part of the district (Shotover and Wheatley) where, formerly, they have been termed the Shotover Grit Sands or Pectinatus (Neaverson, 1925a). Most of these pebbles are of Palaeozoic rocks, and were probably derived (directly or indirectly) from the land area of the London Platform.

Sandstone (Plate 12), and the underlying, uncemented, argillaceous silts or sands have been termed the Shotover Fine Sands or Pectinatus Sands (Buckman, 1922; Pringle, 1926; Arkell, 1947a). To avoid confusion with other beds previously known as Shotover Sand(s) (see Chapters Seven and Eight), these Kimmeridgian sands are herein referred to collectively as the Pectinatus Sand (Figure 17).

The Hartwell Silt (formerly known as the Hartwell Clay) and its equivalents are present at the top of the Kimmeridge Clay throughout most of the district (Figure 17). Named after the now obscured Hartwell Brick-pit [SP 804 125], it became well known for its abundant, diverse and well-preserved fauna ((Plate 11):3; see Oates, 1974). It has been assigned to strata ranging from Lower Kimmeridge Clay to Portland Formation (Blake, 1875; Hudleston, 1880; Woodward, 1895) but it was eventually accepted as part of the Upper Kimmeridge Clay (Neaverson, 1924, 1925b), and later correlated with the type succession of Dorset (Casey, 1967). Some authors have treated the Hartwell Silt as the lateral equivalent of the Swindon Clay of Wiltshire (e.g. Cope, 1980), where the base of the latter is marked by the Lower Lydite Bed (Arkell, 1933). However, in this district, the Hartwell Silt is seen to be younger than the Swindon Clay (Oates, 1991). The Swindon Clay consists predominantly of smooth mudstones, and overlies the Lower Lydite Bed. Between Brill and Aylesbury, the Hartwell Silt comprises argillaceous silts much like those of the Elmhurst Silt, and likewise contains scattered lydite pebbles in the uppermost part. The lowest part comprises silty mudstones; locally these may have been mapped with the underlying Swindon Clay. As with the Elmhurst Silt, clean, fine-grained sands take the place of the Hartwell Silt in the Wheatley area; these have been termed the Wheatley Sand (Buckman, 1922; Arkell, 1947a).

In the Thame–Long Crendon area, sands and silts up to 15 m thick are developed at the top of the Kimmeridge Clay (Figure 17). These beds, termed the Thame Sand (Buckman, 1922), probably include correlatives of both the Elmhurst Silt and Hartwell Silt which have coalesced due to local development of sands in the Swindon Clay.

Details

Shotover, Wheatley and Garsington

The Kimmeridge Clay crops out on the hillsides between Shotover, Wheatley and Garsington [SP 56 06] to [SP 60 03]. Estimates of thickness based on mapping suggest a total of 40 to 45 m at Shotover Hill [SP 560 064] and 30 to 35 m at Wheatley; these thicknesses are uncertain due to the problem of cambering and possible structural complexities close to the Wheatley Fault Zone.

At the former Shotover Brickworks [SP 5560 0627] (Figure 17), and adjoining sandpits [SP 561 066] (Phillips, 1871; Woodward, 1895; Pocock, 1908; Pringle, 1926; Arkell, 1933; 1947a), 3.0 m of bluish black clay with cementstone nodules were exposed; some of the nodules (those of the so-called Wheatley Nodule Bed; see below) yielded a Pectinatites fauna (Neaverson, 1925b). Overlying sands (1.2 m thick) are grey, loamy and fine grained (Shotover Fine Sands of Buckman, 1922) and are succeeded by buff, yellow and whitish coarse sands (4.9 m thick) (Shotover Grit Sands of Buckman, 1922). These coarse sands contain lydite pebbles, which are especially abundant towards the top, and large cemented doggers, many over 1 m in diameter (Plate 12); they have yielded an extensive fauna of bivalves (listed in Pringle, 1926) and ammonites characteristic of the Pectinatus Zone ((Plate 11):1), including the type specimen of Pectinatites (P.) pectinatus. Grey clays or sandy clays, the Swindon Clay, with the Lower Lydite Bed at the base, separate these sands from the overlying Portland Formation. Arkell (1947a) recorded a thickness of only 1.5 m for the Swindon Clay but the Portland Formation here is landslipped (Plate 12); mapping suggests a total thickness of 5 to 10 m of Swindon Clay (and overlying Hartwell Silt or Wheatley Sand, if present).

Sections recorded on the Oxford eastern by-pass partly fill the stratigraphic gap between the lowest strata recorded at the brickworks and the highest strata recorded in the adjacent part of the Headington limestone quarries (see above). Fissile mudstones with Aulacostephanus ex gr. eudoxus and N. virgula, and oil shale with Amoeboceras (Amoebites) and A. (Nannocardioceras), Aspidoceras, Protocardia and fish scales were collected [SP 5567 0637] in 1958 near the Shotover Bridge (material in University Museum Oxford); these indicate KC29, 31 or 32. During the recent survey, a trench section [SP 5561 0638] showed shelly mudstones with bivalves, ammonites including Rasenia evoluta trans. to Xenostephanus and R cf. moeschi, and Lingula (KC12–14). In a nearby stone pit [SP 5568 0657], dark grey, shelly mudstones with iridescent Rasenia spp. including plasters of very finely ribbed forms (?KC9), rest on medium and pale grey mudstones with 'raseniid' ammonites and bivalves. This fauna is typical of the beds immediately above the basal Cymodoce nodule bed (KC5 or 5/8) which marks the base of the Kimmeridge Clay in the district (Figure 17).

A similar Lower Kimmeridge Clay succession was penetrated in the Thornhill No. 2 Borehole, Risinghurst, which proved the basal 10.35 m of the Kimmeridge Clay (Figure 17). Within the weathered zone, very shelly mudstones with N. virgula (KC30) overlie oil shales with common Amoeboceras (Nannocardioceras) (KC29). As elsewhere in the district (see below), the base of the Eudoxus Zone (KC24; at 5.5 m depth) cuts down disconformably into pale, shelly mudstones with Aulacostephanus eulepidus (KC18) of the Mutabilis Zone. The base of the latter zone is marked by a pale shelly bed (KC15), with oysters including common N. virgula (at 7.8 m depth).

To the north-east, in the A40 site investigation Borehole 70 [SP 5721 0761], Lower Kimmeridge Clay was proved to at least 12.95 m depth; this includes fissile, shelly mudstones, and possibly oil shales, with Aulacostephanus and Sutneria to 8.15 m depth, N. virgula-rich beds at 8.35 m and 12.83 m, and Rasenia at 12.70 m and 12.94 m. Farther east, Pringle (1926, p. 63) reported clays belonging to the Cymodoce Zone resting on Corallian limestones in a deep trench east of Shotover Lodge [SP 5826 0711]. However, these clays, like those at Headington Quarry, are now believed to belong with the Upper Oxfordian Ampthill Clay.

At the now-obscured Littleworth (or Wheatley) Brickpit [SP 5895 0555] (Pringle, 1926; Arkell, 1933; 1942; 1947a; McKerrow and Kennedy, 1973), the lowest strata exposed belong to the Eudoxus Zone (Figure 17). Near the base of the section, Pringle's (1926) record of 'hard lumps of earthy limestone crowded with the shells of Exogyra [Nanogyra]virgula'indicates the presence of the Virgula Limestone, and specimens in the BGS collections of Aspidoceras in solid preservation suggest the presence of the Crussoliceras Band; both are within KC30. Higher in the succession, cementstone nodules yielded the well-preserved pectinatitid ammonites of the 'Wheatley Nodule Bed', described and figured by Neaverson (1925b). Most later authors assumed them to have come from the large cementstone nodules ('crackers') in Bed 2 of Pringle (1926), but work by Oates (1991) at Aylesbury shows that the ammonitiferous Wheatley Nodule Bed is characterised by smaller, less conspicuous septaria which lie a little higher in the succession, albeit still within Pringle's Bed 2. The large cementstones noted by Pringle and Arkell probably lie within KC40, and occur in all sections which have yielded the classic ammonitiferous Wheatley Nodule Bed, possibly as far west as Chawley, 5 km west of Oxford (Arkell, 1947a).

The Upper Kimmeridge Clay at Littleworth Brickpit also includes sandy units, in particular a highly fossiliferous sandstone (Pectinatus Sandstone of Arkell, 1947a). This contains a rich ammonite and bivalve fauna comparable with that from the large doggers in the sands at Shotover. Although the Lower Lydite Bed is not recorded as such, the overlying 3.7 m of clays (Swindon Clay) of the Pallasioides Zone included 'small brown [phosphatic] nodules containing fragments of 'Pavlovia'.Up to 3.7 m of buff to grey, sharp sands, the Wheatley Sand, lies between these clays and the overlying Portland Formation.

The presence of the Eudoxus Zone in the Garsington area is suggested by many specimens of Laevaptychus collected in the last century and now in the University Museum Oxford ((Plate 10):3–4). The Garsington No. 1 Borehole proved the base of the Kimmeridge Clay at 6.05 m depth, below about 2.5 m of Lower Kimmeridge Clay with Rasenia and N. virgula.

At Toot Baldon, south of Garsington, pieces of cementstone were recovered from stream debris [SP 5761 0117]; [SP 5760 0117]; [SP 5785 0062] and field brash about [SP 576 012]. At two of these localities [SP 5760 0117]; [SP 5785 0062], they are associated with N. virgula and Laevaptychus, and the beds are assigned to KC30; the latter locality also yielded weathered oil shale with plasters of Amoeboceras (Nannocardioceras), almost certainly belonging to KC31 or KC32. Between Toot Baldon and Sandford-on-Thames, fragments of Laevaptychus, indicative of the Eudoxus Zone or lowest Autissiodorensis Zone, were recovered from field brash [SP 5470 0083]; farther west [SP 5359 0114], silty cementstone with shell fragments, including N. virgula, and distinctive dark and pale burrow-mottling (?KC24) occur.

Waterstock

In 1989–90, a complete Kimmeridge Clay succession was exposed in temporary sections during construction of the M40 motorway near Waterstock (Figure 17). The maximum thickness of the Kimmeridge Clay proved in site investigation boreholes in this area is about 37 m. About 20 m of Upper Kimmeridge Clay was exposed in a cutting [SP 6274 0479] to [SP 6311 0448] near Holloway Farm (Cox, Horton and Sumbler, 1990). The Hartwell Silt and Wheatley Sand are absent, and the highest preserved beds are Swindon Clay which is unconformably overlain by Lower Greensand. Marker beds within the Upper Kimmeridge Clay include from below: a shell bed with pyritised lenses of N. virgula (?base KC38), a bed of large (0.75 m diameter) cementstone doggers (?KC40) with Pectinatites (Virgatosphinctoides) pseudoscruposus, the Wheatley Nodule Bed (dark grey, irregularly shaped (reniform) cementstone nodules up to 0.25 m in diameter and 0.08 m thick with Pectinatites (Virgatosphinctoides) woodwardi?), the Pectinatus Sand (7 m of silts and silty mudstones including, at the top, a 0.9 m-thick bed of sand with fossiliferous calcareous sandstone doggers), and the Lower Lydite Bed at the base of the Swindon Clay. The lower part of the Lower Lydite Bed contains an abundant black phosphatised fauna of bivalves (some in growth position) and ammonite whorl fragments (some with planed off top surfaces); the upper part of the bed contains scattered black phosphatic clasts.

The northern end of the cutting exposed the upper part of the Lower Kimmeridge Clay with the Crussoliceras Band and Virgula Limestone (both in KC30). Excavations for the east pier of the A418 overbridge [SP 6261 0496] exposed the basal Cymodoce Zone nodule bed at the base of the Kimmeridge Clay. Nearby, Professor J H Callomon (written communication, 1991) measured a detailed section through about 8 m of overlying shelly mudstones with Rasenia and Aulacostephanus.

The whole of the Lower Kimmeridge Clay (12.74 m thick) was cored in M40 Borehole 009 which proved a minor non-sequence at the boundary of the Lower and Upper Kimmeridge Clay (KC35 missing) at 35.56 m depth (Figure 17). Below this, the sequence included: Aulacostephanus autissiodorensis at 35.62 m, bituminous mudstone and oil shales rich in Amoeboceras (Nannocardioceras) (KC32), pale grey silty calcareous mudstones rich in N. virgula and with cementstone (KC30), an interburrowed silty shelly bed with N. virgula and Aulacostephanus eudoxus (KC24 at the base of the Eudoxus Zone) resting non-sequentially on pale and very pale grey calcareous mudstones with echinoid fragments and pentacrinoid columnals (KC18), dark grey smooth-textured shelly mudstones with iridescent Aulacostephanus eulepidus and A. linealis (KC16), mudstones with Rasenia fragments, a pale silty mudstone or silt-stone with scattered black phosphatic clasts, Rasenia, Camptonectes, oysters and Pholadomya acuticosta, marking the base of the Cymodoce Zone and the Kimmeridge Clay at 48.30 m.

In a trial pit [SP 6451 0563] east of Waterstock, mudstones rich in N. virgula were proved at 4.5 to 5.0 m depth overlying a cementstone (probably in KC30). South-west of the village [SP 6298 0523], a cementstone with N virgula and associated Laevaptychus is indicative of a similar stratigraphic level. Laevaptychus were also collected nearby [SP 6317 0525] and there are similar specimens in the BGS collections from a well at Tiddington [SP 65 05]; these almost certainly indicate the Eudoxus Zone.

Brill

The Brill No. 1 Borehole (Figure 17) proved a complete Kimmeridge Clay succession (55.6 m thick), but recovery was generally poor. The basal Cymodoce Zone nodule bed, marking the base of the Kimmeridge Clay, was penetrated at 79.52 to 79.60 m depth. Within the overlying Lower Kimmeridge Clay (total. thickness 15.85 m), distinctive horizons include: a pale very shelly bed with ammonite and bivalve fragments including Xenostephanus? and N. virgula (KC15), pale and very pale grey mudstones with echinoid spines (KC18), and a shelly siltstone with Aspidoceras and other ammonite and bivalve fragments (KC24, marking the base of the Eudoxus Zone). Specimens of both Aulacostephanus ex gr. eudoxus and Aspidoceras occur up to about 0.3 m below this level. The sequence appears to be more complete than at some other localities in the district where KC24 cuts down on to KC18 (Figure 17). Mudstones rich in N virgula (KC27–30) are overlain, at 67.67 m, by oil shales with plasters of Amoeboceras (Nannocardioceras) (KC31–KC32). KC33 and KC34 contain rhynchonellid brachiopods and Aulacostephanus. They are overlain non-sequentially (at 63.75 m depth) by shelly, in part bituminous, mudstones with Pectinatites (basal Upper Kimmeridge Clay).

A silty cementstone with poorly preserved bivalves at 60.50 m depth represents a dogger horizon (?KC40) which is widespread throughout the district at this level. Higher in the sequence, the Wheatley Nodule Bed was not recovered probably due to the relatively small core diameter. The boundary between the Holman's Bridge Shale and Watermead Clay members of Oates (1991) is at 58.00 m depth, where bituminous mudstones are overlain by smooth-textured, pale and medium grey mudstones. Specimens of Pectinatites are plentiful throughout these two members but are not determinable at species level, again because of the small core diameter. Between 54.04 m and 58.00 m, they are crushed but often partially in-filled with cream-coloured phosphate; there are occasional phosphatic nodules and a number of shell beds with Pectinatites and oysters.

The Elmhurst Silt Member (43.76 and 54.04 m depth) consists predominantly of argillaceous silts with a median unit (4.4 m thick) of moderately smooth mudstone. In the upper silty unit, 2.62 m thick, silts pass up into pale grey, fine-grained sand, in part cemented into calcareous sandstone which contains sporadic lydite and phosphate pebbles. The overlying Swindon Clay Member (37.6 m to 43.76 m depth) comprises smooth mudstones; a few small, well-rounded phosphatic pebbles at 43.76 m indicate the Lower Lydite Bed.

Above the Swindon Clay, the strata comprise grey to brownish grey, silty mudstones, argillaceous, bioturbated silts and very fine-grained sands which are lithologically indistinguishable from the Hartwell Silt of the Hartwell Borehole. The uppermost 0.5 m, below the base of the Portland Formation at about 24 m, contains abundant, rounded to subrounded lydite pebbles up to 20 mm diameter.

Kimmeridge Clay was formerly worked at the Rid's Hill Brickpit [SP 664 151] north-east of Brill. Originally described by Davies (1907a), and later by Pringle (1926) and Arkell (1947a), the section was reassessed by Cox and Sumbler (1989; in press). Comparison with the Brill No. 1 Borehole suggests that about 7 m of beds at the base of the Kimmeridge Clay are 'missing' in the brickpit section, probably having been cut out by a fault. The Kimmeridge Clay present (Davies' beds 7 to 9) probably ranges from the upper part of the Mutabilis Zone to the lower part of the Upper Kimmeridge Clay, and includes a band of septarian cementstones (near the top of Bed 7) probably representative of KC30. Above the old brickpit, two beds of fine-grained sand have been mapped around the upper slopes of Rids Hill. The uppermost bed, which forms the summit, contains common lydite pebbles (pace Davies, 1907a), and correlates with the sandstone immediately below the Lower Lydite Bed in the Brill No. 1 Borehole. The outcrop of the Lower Lydite Bed is indicated by phosphatic pebbles, including abraded ammonite body-chambers, which have been found in the soil at several localities on the slopes of Brill Hill.

At least eight other former brickpits at Brill are known (Buckinghamshire County Museum, 1980) but geological details exist for only one of these possibly [SP 6501 1422] on the north-western side of Brill Hill. This exposed the Hartwell Silt, and probably part of the underlying Swindon Clay, beneath Portland Formation. According to Woodward (1895) and Arkell (1947a), 0.9 m of brown and greenish loamy sand rested on 6.1 m of clay, sandy in the upper part, with iridescent 'Ammonites biplex'[Pavlovia] and other fossils (particularly bivalves; Green, 1864). At the top of the section, there was 0.3 m of 'brown clay with pebbles' overlain by glauconitic sands, marls and limestones of the Portland Formation. Arkell (1947a) regarded this pebbly clay as the Upper Lydite Bed at the base of the Portland Formation, but it probably belongs with the Kimmeridge Clay; lydites have been noted in the uppermost Kimmeridge Clay in the Brill No. 1 Borehole, and at outcrop at several localities (for example Muswell Hill [SP 6445 1587] and Upper Winchendon [SP 7457 1502]).

A ditch section [SP 6371 1550] to [SP 6371 1552] north-west of Muswell Hill yielded material from two cementstones in the Lower Kimmeridge Clay. The higher one (probably in KC30), was weakly septarian and included Aspidoceras, perisphinctid fragments, N. virgula and serpulids; the lower one (probably in KC24), showed burrow-mottling with shell fragments including arcids, Modiolus?, N. virgula and Sutneria?. Specimens of N. virgula in the BGS collections suggest the presence of similar levels of the Lower Kimmeridge Clay near Addingrove Farm [SP 664 112] between Brill and Long Crendon.

Thame, Long Crendon and Chearsley

Although no borehole penetrates the full thickness of the Kimmeridge Clay in this area, estimates from mapping suggest a total thickness of 40 to 50 m. Several cementstone nodule bands have been mapped within the Lower Kimmeridge Clay, notably between Long Crendon and Shabbington. Specimens recovered from field brash [SP 6792 1016]; [SP 6780 0961]; [SP 6782 0954] to [SP 6785 0944]; [SP 676 089]; [SP 6809 0833], pipeline debris [SP 6730 0773]; [SP 6728 0767]; [SP 6731 0643] and ditch debris [SP 6735 0755]; [SP 673 074] have been tentatively assigned to KC17, KC18, KC24 and KC30; material in the BGS collections suggests that a cementstone in KC30 was also recovered from graves in Shabbington churchyard.

Two units of greyish green, glauconitic clayey silt and fine-grained sand occur in the upper part of the formation. The lower unit, mapped north of the River Thame, is typically 1 to 2 m thick and is probably lenticular, being apparently absent in places south of Long Crendon. It is separated from the higher sand (the Thame Sand) by a variable thickness of silty mudstones, rarely up to 10 m thick. Comparison with other sections, combined with the lithological, palaeontological and mapping evidence, strongly suggests that the mudstones and the lower sand together correlate with the Elmhurst Silt (Figure 17); if so, the relationship of the Thame Sand to other sands in the district, which has been the subject of debate in the past (Arkell, 1933), becomes clearer.

The Thame Sand reaches its greatest thickness at Thame; a borehole [SP 7059 0519] at Bates Leys Farm, south-west of the town, proved brown, yellow, green and grey sands to a depth of 13.1 m (not 70 ft (21.3 m) as reported by Ballance (1963)). The total thickness of the Thame Sand hereabouts (overlain by Lower Greensand) is probably about 15 m. The driller's log records 3 m of sand and grit with 'small black stones' near the base, which may represent the Lower Lydite Bed; if this is so, then the greater part of the Thame Sand here is equivalent to the Hartwell Silt and Swindon Clay (Pallasioides Zone). However, a specimen of Paravirgatites'obtained from hard layers of sandstone in a former pit close to this locality (Buckman, 1925b, plate 603) suggests the upper part of the Pectinatus Zone (Cope, 1978), and that at least part of the Thame Sand correlates with the Pectinatus Sand. Large, calcareous sandstone doggers, comparable with those at Shotover (see above), reported by Fitton (1836) from the upper part of the sands at Barley Hill [SP 715 065] just north of Thame are also suggestive of the Pectinatus Sand, but if the correlation indicated in (Figure 17) is correct, they must be a local dogger development at a higher level.

The Thame Sand thins northwards from Thame, and at Long Crendon it is about 10 m thick (Fitton, 1836; Davies, 1899a). A borehole [SP 6956 0849] proved 11.3 m (not bottomed), but Buckman's (1925b, p. 36) claim of 24.4 m (80 ft) is implausible. Farther north, around Chilton [SP 68 11], it is only a few metres thick. At Long Crendon, Davies (1899a) (who included it in his lithological Portland') reported a bright green sand at its base and scattered 'lydite' pebbles in the overlying sands and clayey sands. Fitton (1836) had earlier recorded 'worn fragments of black flint' in the topmost 0.20 m of the 'greenish grey sand' (Thame Sand) underlying the Portland Formation. Poorly fossiliferous strata, immediately below the Thame Sand, were formerly exposed in a brickyard [SP 697 082] south of the village (Davies, 1899a). Buckman (1923) claimed these to be Hartwell 'Clay' (a large number of Hartwell Silt ammonites, purchased from Buckman and alleged to be from Long Crendon, are in the BGS collections), but this has been disputed (Arkell, 1933) and is almost certainly incorrect. Clays excavated in the floor of the pit yielded a pre-Pectinatus Zone Upper Kimmeridgian ammonite fauna, and Exogyra [Nanogyra]virgula clays' were recorded about 2 m below (Arkell, 1933). The latter, now estimated to be about 10 m below the base of the Thame Sand, must represent the lowest beds of the Upper Kimmeridge Clay, in which N. virgula can be quite common, and not the better known and richer occurrences in the Lower Kimmeridge Clay (for example KC27–30 in the Eudoxus Zone).

A section [SP 713 102] south-west of Chearsley village, recorded by H B Woodward (BGS Archive) during construction of the Great Western and Great Central Joint railway in 1904, is closely similar to that recorded further east at Hartwell and Aylesbury, although the Hartwell Silt has passed into sands. Beneath 'green glauconitic clay' and lydites' of the Portland Formation it showed:

The calcareous sandstones were also noted at Chearsley by Oates (1991), and described as 'typical Pectinatus Sandstone'.

The presence of 'bones' (of marine reptiles) and 'large septaria' (cementstones) in the clays near the base of the section relate well to the lowest part of the Upper Kimmeridge Clay of other sections (Figure 17). The fact that no large cementstone doggers were reported from the Long Crendon brickyard (see above) strongly suggests that the section there was exclusively above this widespread dogger horizon (?KC40).

Waddesdon, Stone, Hartwell and Aylesbury

The Hartwell Borehole (Cox et al., 1994) proved a complete thickness (47.03 m) of the Kimmeridge Clay Formation (Figure 17). The basal bed, at 64.98 m depth, consists of grey silty mudstone with angular phosphatic clasts including septate whorl fragments of ammonites; it rests, with a sharp, inter-burrowed junction, on pale, calcareous mudstones of the Ampthill Clay. It marks the base of the Cymodoce Zone and represents KC8 or KC5 and 8 combined. A few metres higher, another bed with phosphatic clasts marks the base of KC24 (at 60.68 m depth) which, as at many localities within the district, rests non-sequentially on KC18.

Between 47.20 m and 60.25 m (KC26–42), there are numerous thin beds of oil shale. The upper part of this unit (47.20 m to 51.82 m) equates with the Holman's Bridge Shale Member of Oates (1991), the base of which coincides with the base of the Upper Kimmeridge Clay (total thickness 33.87 m). Unlike other localities within the district (Figure 17), there is no significant non-sequence at this level. A shelly cementstone at 49.70 m depth is the only cementstone penetrated in the Kimmeridge Clay of the borehole, though several others have been found at outcrop in the vicinity. This cementstone almost certainly represents the widespread dogger horizon (?KC40) reported at this level elsewhere in the district (Figure 17). Between 43.65 m and 47.20 m depth, the Kimmeridge Clay comprises mainly smooth, medium grey mudstones corresponding to the Watermead Clay Member of Oates (1991).

Between 35.57 m and 43.65 m depth, the sequence comprises argillaceous silts and very silty mudstones, with a median unit of much smoother mudstones between 37.76 m and 41.20 m; the uppermost silts contain sporadic lydite pebbles. The interval represents the Elmhurst Silt Member. Lithologically, it is similar to the Hartwell Silt from which it is separated by 6.12 m of predominantly smooth mudstones of the Swindon Clay, with a basal bed containing phosphatic pebbles including rolled bivalve casts. This is the Lower Lydite Bed which has also been found at outcrop on Waddesdon Hill [SP 7535 1594], 5 km to the north-west, and in site investigation boreholes at Aylesbury (Oates, 1991).

The Hartwell Silt Member, forming the highest part of the formation in the borehole (17.95 to 29.45 m depth), consists of greenish grey, slightly argillaceous silts or very fine sands, with subordinate silty mudstones particularly in the lowest part. At outcrop in the Stone area, it is a soft, dull brownish grey silt, locally mottled pale grey and ochreous yellow, and is typically 5 to 7 m in thickness. Boreholes in central Aylesbury indicate a total thickness of about 8 m (Oates, 1974, 1991). Hereabouts, it weathers to a grey, fawn or khaki, silty or finely sandy clay, which can be seen, occasionally, in excavations on the slopes below the Portland Formation for example [SP 821 144].

The Hartwell Silt was formerly worked for brick-making at Hartwell, and also just outside the district at Aylesbury [SP 842 142], Bierton [SP 839 157] and Whitchurch [SP 807 205]. According to Woodward (1895), at least 3 m were exposed at Hartwell and Bierton, but there are no detailed sections recorded in the literature. The Hartwell Silt of the Aylesbury area is the stratotype of the Pallasioides Zone (Neaverson, 1924; 1925b; Cope, 1978).

In recent years, site investigation boreholes and excavations in Aylesbury, in particular that at Watermead [SP 820 134], have provided details of the Upper Kimmeridge Clay, which Oates (1991) divided into five members (Figure 17); these can be recognised (but not mapped) in much of the district. In ascending order, these are: Holman's Bridge Shale (5.2 m), Watermead Clay (about 6 m), Elmhurst Silt (4.5 m), Swindon Clay (7.4 m) and Hartwell Silt (about 8 m). Mapping indicates a thickness of about 14 m for the Lower Kimmeridge Clay at Aylesbury. These thicknesses are closely similar to those proved in the Hartwell Borehole (see above), 3 to 4.5 km to the southwest; minor thickness discrepancies are probably due to slight inconsistency in placing boundaries.

The lowest few metres of the sequence exposed at Water-mead, comprising grey mudstones with thin beds of oil shale, is Lower Kimmeridge Clay. A cementstone nodule band reported just below the lowest exposed beds (Oates, 1991) is probably within KC30, which has been found at outcrop at several localities in the north-eastern part of the district e.g. [SP 7547 1701]; [SP 7796 1494]; [SP 7810 1431]; [SP 7862 1552]; [SP 7865 1552]; [SP 7872 1551]; [SP 7889 1550]; [SP 7949 1800]; [SP 7957 1659]; [SP 7960 1787].

As well as cementstone (s) in KC30, others, probably relating to KC24, KC40 and KC44, can also be detected at outcrop in this area. Those in KC30 are weakly septarian, those in KC40 and KC44 (the Wheatley Nodule Bed) are more strong ly so; KC40 and KC44 contain crystals of white and honey-coloured calcite, respectively. The cementstones in KC24, KC30 and, possibly, KC40 have associated N. virgula. In addition, all have an assorted, fragmentary and largely indeterminate bivalve fauna, often including Protocardia. The ammonite fauna is exclusively 'perisphinctid' for KC40 and KC44 (Pectinatites spp.), commonly 'perisphinctid' for KC30 (Propectinatites? = Crussoliceras) but generally not 'perisphinctid' for KC24. Both KC24 (for example in field brash [SP 7446 1513] ) and KC30 may include, or be associated with, the more varied ammonite fauna of the Eudoxus Zone; loose specimens of Laevaptychus are also found in field brash for example [SP 8022 1883]. The KC40 cementstone was present in an excavation [SP 8012 1441] made in 1943 at Quarrendon; specimens (BGS collections) comprise silty cementstone and siltstone with common bivalves and some ammonites, including Pectinatites (V) wheatleyensis.

Evidence of lower horizons of the Kimmeridge Clay are found more rarely at outcrop but the basal nodule bed was recorded east of Waddesdon [SP 7558 1710] (also proved at 3.31 m depth in the Folly Farm Borehole), and shelly mudstones of KC16 and a cementstone with Aulacostephanus eulepidus, A. mutabilis and Lapha, suggestive of KC17, were collected from the debris from fence-post holes [SP 7345 1611] and [SP 7436 1607] respectively, south-west of the village.

Chapter 7 Jurassic: Portland Formation and Purbeck Formation

Portland Formation

The largest area of exposed Portland Formation in Great Britain crops out in this district. North of the River Thame, the formation occurs as outliers on hills, including those at Shotover [SP 565 064] and Brill [SP 657 139]. South of the river, the principal outcrops are around Great Haseley [SP 642 018] in the south, and between Thame and Aylesbury in the east (Figure 1). Between Milton Common [SP 653 033] and Thame, the formation is absent due to early Cretaceous erosion; here Lower Greensand or Gault rests directly on Kimmeridge Clay. Down-dip of the outcrop, boreholes indicate that the Portland Formation is cut out rapidly beneath the cover of Cretaceous rocks, but correlative strata are known in the Wessex–Weald Basin.

The greatest proven thickness of Portland Formation within the district is 12.79 m in the Hartwell Borehole [SP 7926 1223] near Aylesbury ((Figure 18); Cox et al., 1994); Arkell's (1944a; 1947a) claim of up to 13.7 m (45 ft) near Great Haseley, is probably an overestimate. Other sections from the literature suggest that the formation, where complete, averages 10 to 12 m. Thinner, incomplete successions occur locally (mainly in the south-western part of the district), where the formation is overlain unconformably by Cretaceous rocks. In the Dorset type area, the Portland Formation is up to 70 m thick (Townson, 1975). In this district, the formation is thinner, but it corresponds with only the middle part of the type succession (Wimbledon, 1980), and in terms of relative thickness, is not significantly condensed.

The Portland Formation is made up of marine limestones and calcareous sands. Although its limits had been correctly recognised by Hudleston (1880), the Portland 'Beds' of most other early workers and of previous geological maps of the district included sands which are now placed in the Kimmeridge Clay. These include the Portland Sands of Hull and Whitaker (1861) and Green (1864); the Shotover Sands of Blake (1880); and the Lower Portland Beds of Woodward (1895), Pocock (1908) and Davies (1899a).

The Portland Formation was formerly worked from many quarries (see Chapter Twelve), but virtually all are now obscured, and exposure is very poor. The strata are strongly affected by cambering, particularly in the outliers, and thickness estimates from outcrop are therefore unreliable; even the succession of beds can be difficult to establish in many areas. However, the formation is well represented in the literature, and this, together with data from recent mapping and boreholes, shows a marked variation in the Portland succession across the district.

The sequence established by Blake (1880) has formed the basis of all subsequent classifications of the Portland Formation of Oxfordshire and Buckinghamshire. Using the nomenclature of Arkell (1933) (also adopted by Wimbledon, 1980), the succession is:

Thicknesses relate to the Hartwell Borehole. Although this succession has come to be regarded as standard for Oxfordshire and Buckinghamshire (Arkell, 1947a; Wimbledon and Cope, 1978), only the lowest part is typically developed throughout the region, and the full succession is applicable only to Buckinghamshire, i.e. the central and north-eastern part of this district (Figure 18). The constituent units can be recognised in quarry sections and boreholes but, in mapping poorly exposed ground, the formation can be divided only into two parts. Following Arkell (1933), these are termed the Portland Sand Member (equivalent to the Glauconitic Beds) and the Portland Stone Member (equivalent to the Aylesbury Limestone, Crendon Sand and Creamy Limestone). These two members can generally be separated even in pastureland and urban areas, as the Portland Stone forms a minor scarp feature rising above the Portland Sand outcrop. The members correspond with the Lower and Upper Portland Beds respectively of Ballance (1960; 1963), who studied the central part of the district. The tripartite classifications of Arkell (1947a) and Bristow (1963, 1968), which combine the Glauconitic Beds and Aylesbury Limestone into one unit (Lower Portland Beds or Lower Limestone), but separate out the Crendon Sand (Middle Portland Beds of Arkell, 1947a), is inappropriate in the district.

South-westward from Haddenham [SP 740 085], the Aylesbury Limestone degenerates so that the Glauconitic Beds and the Crendon Sand coalesce (Figure 18). In the southwestern part of the district, the entire Portland Formation is dominated by sands, with sandy limestones developed sporadically throughout the sequence, and the formation is shown undivided on the 1:50 000 geological map. Limestones in the middle part of the succession have been equated with the Aylesbury (Rubbly) Limestone of the north-east, and those at the top have been equated with the Creamy Limestone (Arkell, 1933; 1944a; 1947a; Wimbledon, 1980). However, they lack the continuity of these units and the correlation remains uncertain.

The Portland Formation contains a rich fauna of large bivalves, notably Protocardia dissimilis and trigoniids, gastropods (Plate 13) and ammonites. The ammonites (see Frontispiece) are relatively common, and are typically extremely large (0.5 m or more in diameter); these giant specimens are often seen as curios in gardens, or as features built into walls (Plate 18). They are used in the standard zonation of the formation, but there are practical problems in its application due to their great size and weight, and also because of the generally poor preservation of the diagnostic inner whorls. S S Buckman, who for much of his life resided at Long Crendon, made a detailed study of the ammonite succession (e.g. Buckman, 1926). Much of this work, though still valid, has been rationalised by later authors, and the current standard zonation is based on the ammonite biozonation of Wimbledon and Cope (1978). The local succession is believed to span three Portlandian zones; from below, Glaucolithus, Okusensis and Kerberus (Wimbledon, 1980).

Throughout the district, the base of the Portland Formation is marked by the Upper Lydite Bed (the 'basal conglomerate of the Portlands'; Hudleston, 1880), which rests non-sequentially on Upper Kimmeridge Clay. Typically a few centimetres thick, it is highly lenticular and may be a metre or more in thickness at some localities but absent from others. It is characterised by abundant, small (5 mm to 10 mm), rounded pebbles of black or brownish grey chert (lydites sensu stricto; see footnote p. 63), white or yellowish quartz and quartzite, and rarer black phosphate; the phosphate includes fish teeth and casts of bivalves and ammonite whorl fragments derived from the topmost part of the underlying Kimmeridge Clay. The exotic pebbles may also come from this source (see Chapter Six), though ultimately originating from Palaeozoic rocks. The pebbles are set in a matrix which varies from sand, through calcareous sandstone, to cream-coloured micritic limestone; all of these lithologies generally contain a scatter of coarse-grained black to olive-green glauconite sand. The bed is typically very fossiliferous, yielding large bivalves and ammonites. The latter are mainly species of Glaucolithites characteristic of the Glaucolithus Zone (Wimbledon, 1980). The Upper Kimmeridgian Rotunda and Fittoni zones, and the basal Portlandian Albani Zone are absent beneath the Upper Lydite Bed, suggesting either non-deposition, or a considerable amount of erosion. Ammonites from these 'missing' zones, in particular Pavlovia, may occur as a derived element in the fauna.

The overlying and greater part of the Glauconitic Beds are dominated by greenish grey and brown, weakly cemented, calcareous, fine- to very fine-grained sandstones and siltstones. Medium to coarse, black to olive-green grains of glauconite are abundant, and lenses of dark green, almost pure glauconite sand occur locally immediately above the Upper Lydite Bed. These glauconite-rich beds are developed throughout almost all of the district, being absent only in the Toot Baldon area. At outcrop, they are generally decalcified and oxidised, and are seen as brownish yellow sands and silts. In the upper part of the succession, glauconite is generally sparse and where the Aylesbury Limestone is absent, the beds cannot be distinguished from the Crendon Sand. More-strongly cemented sandy limestones or calcareous sandstones are found locally at outcrop; in some areas, these make up a considerable part of the sequence. In some cases, they contain abundant serpulids and, less commonly, Nanogyra aff. virgrula.

The Aylesbury Limestone typically comprises pale grey to white, finely sandy limestones in the lower part, and less sandy, shell-fragmental and sparsely peloidal micritic limestones (wackestones) in the upper. The rocks are poorly bedded, probably due to intense bioturbation, and at outcrop weather to rubble ('Rubbly Beds' of Blake, 1880). The rock is commonly packed with bivalve moulds.

The Crendon Sand consists of grey sands and silts which weather to a brownish yellow colour; it contains little or no glauconite. Ballance (1960; 1963) and Bristow (1963; 1968) considered it to be a readily mappable horizon, but in many places they appear to have confused the Crendon Sand with the upper part of the Glauconitic Beds which is lithologically similar. Though sections indicate that it is present throughout the central and north-eastern part of the district, it can rarely be traced at outcrop, due to cambering of the overlying limestones. As mentioned above, the Crendon Sand combines with the Glauconitic Beds in the south-western part of the district, where together they comprise up to 9 m of brown and yellowish sands with bands and doggers of calcareous sandstone.

The basal part of the Creamy Limestone comprises finely sandy limestones, like those of the lower part of the Aylesbury Limestone, and the two units can only be distinguished because of the intervening Crendon Sand. The upward transition from the sands of the Glauconitic Beds to the Aylesbury Limestone, and from the Crendon Sand to the Creamy Limestone are essentially identical sedimentary cycles, each suggestive of shoreline regression. The upper part of the Creamy Limestone includes white to yellow, strongly burrowed micrite and peloidal micrite with large bivalves (mainly Protocardia and trigoniids) and giant ammonites (Titanites and Galbanites). Serpulid-rich limestones are developed locally, and fine-grained recrystallised limestones with 'Ostrea' expansa are common just beneath the base of the Purbeck Formation. At a few localities, limestone with abundant bivalve moulds and the 'Portland Screw' Aptyxiella portlandica ((Plate 13):3) occur at the top of the succession. Thin, probably impersistent, beds of 'oolite' in the upper part of the Portland Formation in the south-western part of the district may also represent the Creamy Limestone. The famous oolitic freestones of Dorset are slightly younger than the topmost Portland Formation of the Thame district (Wimbledon, 1980).

Details

Shotover Hill–Forest Hill

The Portland Formation crops out on the steep slopes of Shotover Hill, but it is difficult to locate in many places because of cambering and downwash from the overlying Whitchurch Sand, and in one area [SP 563 060], it may be absent due to overstep by the latter. The maximum thickness of the Portland Formation hereabouts is about 12 m (Pringle, 1926; Figure 18). The lower part comprises glauconitic sands and sandy limestones, with the basal Upper Lydite Bed developed locally. The middle and greater part of the succession comprises' pale grey or mustard-yellow, slightly glauconitic, fine- to medium-grained sands with bands of calcareous sandstone. Shell-fragmental or sparsely peloidal limestones, have been recorded at the top of the succession at several localities, but are absent in many others, probably due to Cretaceous erosion.

Numerous pits were dug in the past along the western and north-western slopes of Shotover Hill. A former brickpit at the north-western extremity probably [SP 562 066] (Pringle, 1926; Arkell, 1947a) apparently exposed the complete formation between the Whitchurch Sand and the Kimmeridge Clay:

The siliceous and ferruginous alteration of Beds 6 and 7 is discussed by Thomas (in Pocock, 1908; Pringle, 1926). This phenomenon also occurs a short distance to the south about [SP 560 063], where Strickland (in Fitton, 1836, p. 275) described the topmost bed of the Portland Formation (op. cit. Bed 4) as 'hard brown sandstone with a few Trigoniae'. Masses of siliceous limestone and cherry sandstone have also been observed in the upper part of the formation at the eastern end of Shotover Hill (for example at Shotover Orchards [SP 5802 0535]).

Debris of 'the Rubbly Limestones' was recorded from excavations for the reservoir ?[SP 575 058] on Shotover Hill by Arkell (1933). The material yielded large ammonites 'comparable with those from the Cockly Bed' (of Swindon (Okusensis Zone); Wimbledon, 1980).

Pocock (1908) assigned 'glauconitic limestones with Holcostephanus cf. pallasianus'beneath the Whitchurch Sand, at Forest Hill cemetery [SP 583 076] to the Portland Formation. However, the resurvey indicates that these beds lie within the Kimmeridge Clay. North-west of Forest Hill, the Portland Formation crops out in a downthrown block between the Wheatley and Shepherds Pit faults [SP 573 083]. The strata comprise fine-grained glauconitic limestones with ammonite fragments, with a basal, pebbly, glauconitic, micritic limestone (Upper Lydite Bed).

Wheatley–Garsington

As on Shotover Hill, sands make up the greater part the formation between Wheatley and Garsington. At the base, the Upper Lydite Bed comprises up to 1 m of pebbly, glauconitic, micritic limestone, marl or calcareous silt. It passes upwards into calcareous sands and sandy marls, 1 to 2 m thick, with scattered fine-grained glauconite grains. These beds are overlain by fine-grained pale or mustard-yellow sand, with thin calcareous sandstones. They are generally non-glauconitic, but shell debris occurs in some beds, and peloids and ooliths are a minor component. At a few localities, where the thickest successions are preserved, the topmost beds of the formation include limestones.

A well [SP 5885 0526] at Littleworth proved the formation to total 11.07 m in thickness (Pocock, 1908; (Figure 18)). The succession consists principally of sand, with beds of stone at the top and bottom. In the nearby Littleworth Claypit [SP 5898 0541], the basal part of the formation consists of 0.3 m of greenish grey, sandy, glauconitic limestone and calcareous sandstone with scattered shell debris and abundant bivalves and ammonites, overlying 0.03 m of greenish grey, clayey, highly glauconitic silt with scattered lydite pebbles. This rests on sands of the Kimmeridge Clay.

A well [SP 5943 0492] at Coombe House south of Wheatley proved 8.22 m of Portland Formation, which includes about 2 m of limestone (in part 'oolitic') at the top, beneath an irregular contact with the Whitchurch Sand (Pringle, 1926). Hull and Whitaker (1861) recorded a similar eroded surface between the Whitchurch Sand and Portland Formation in excavations ?[SP 586 036] north of Garsington. However, the 'irregular cavities and waves' noted by Fitton (1836, p. 277) in Portland limestones, where it is overlain by Purbeck Formation in quarries ?[SP 580 035] near Garsington, are probably the result of solution.

In 1988, the basal part of the Portland Formation was exposed in excavations [SP 5783 0261] at Garsington; the section showed 1 m of greenish grey, glauconitic sand and sandy limestone with lydite pebbles in the lower half, overlain by off-white micritic glauconitic limestone with scattered shells. Davies (1899a, p. 19) described the sequence in a small quarry hereabouts probably [SP 5784 0257], and a nearby section which exposed 4.6 m pale yellowish sand. The following section is still visible in the quarry:

In a trench at Scholarswell [SP 584 030], north of Garsington [SP 584 030], Davies (1899a) described the lower part of the Portland Formation, resting on sands of the Kimmeridge Clay ('Lower Portland' of Davies). The strata comprised 3.65 m of limestone, glauconitic and pebbly at the base, overlain by 2.7 m of sand with a stone band. In 1988, about 2 m of greenish white, fine-grained, well-sorted sand and calcareous sandstone were exposed in nearby foundation trenches [SP 5829 0396].

Excavations possibly around [SP 5829 0305] south of City Farm, exposed virtually the whole of the Portland Formation, which is only about 6 m thick hereabouts, and lies beneath the Whitchurch Sand (Pringle, 1926):

A quarry ?[SP 6036 0350] north-east of Cuddesdon, formerly showed 3.3 m of soft to hard calcareous sandstone or sandy limestone, with casts of trigoniids and 'Ostrea', beneath the Whitchurch Sand (Davies, 1899a, p. 28). At this or a nearby quarry, the strata include 'oolites' (Pocock, 1908), suggestive of the Creamy Limestone. The uppermost bed yielded Ampullospira ceres, Laevitrigonia gibbosa? and Lucina'portlandica (Pringle, 1926).

Toot Baldon

At Toot Baldon [SP 567 007], the Portland Formation is dominated by yellow to pale brown sands. They are largely decalcified near the surface, but small fragments of calcareous sandstone occur in the soil throughout the outcrop, and debris from excavations [SP 5589 0015, 5001 0053] suggest the presence of large 'doggers' at depth. In some cases, the sandstones contain shell debris, and sandy limestone with both shell debris and peloids was noted in a wall [SP 5692 0080]. Glauconite is rare. An excavation [SP 5658 0013] showed 1 m of pale grey fine-grained sand with darker silty sand partings and rare calcareous sandstone ribs. A grey pebbly sandstone at the base of the succession was exposed in a ditch [SP 5637 0067] west of the village; this differs from the Upper Lydite Bed elsewhere in the district in its lack of glauconite.

The fields to the south of Nineveh Farm [SP 5517 0031] contain small quartzite pebbles which are probably residual traces of Lower Greensand. Similar pebbles, associated with remanie drift gravel, cap the low hill to the east of the farm. Perhaps for this reason, the deposits were originally thought to be Lower Greensand (Old Series One-Inch Geological Sheet 13; Phillips, 1871; Davies, 1899a). However, the lithology and fauna indicate that the strata are Portland Formation (Pocock, 1908).

At Chiselhampton Hill [SP 595 007], the basal part of the Portland Formation is preserved as a small outlier, but is cut out by the basal Gault unconformity to the west. Here the Upper Lydite Bed is overlain by shelly, glauconitic, micritic limestone. Ammonites collected from field debris were mainly Glaucolithites, with a single Epivirgatites?. Small oysters (Nanogyra?) and Protocardia dissimilis were also present.

Great Milton–Great Haseley

The formation is present only south-west of the Great Milton Fault; to the north-east, it is absent due to Cretaceous erosion. The strata are gently folded, with minor faulting, and are probably up to 9 m thick (Figure 18). Arkell (1944a) estimated a somewhat greater total, and subdivided the formation into three units, although during this survey, the uppermost two could not be separately mapped (see above).

The formation is predominantly arenaceous and gives rise to brown loamy soils. The lower part comprises about 2 m of rubbly-weathering, fossiliferous glauconitic limestone, calcareous silt or marl, and includes the Upper Lydite Bed, locally up to 1 m thick at the base. This unit (= the Lower Portland Beds of Arkell, 1944a) is depicted on the 1:10 000 scale geological map of the area. It is overlain by pale to brownish yellow, fine- to medium-grained sands, with minor shell debris. Doggers and bands of pale grey to brown, glauconitic, sandy limestone occur locally. In the topmost part, sandy limestones, oolitic, shell-fragmental or serpulitic sandy limestones and, less commonly, micritic limestones are present. These uppermost beds (Upper Portland Beds of Arkell, 1944a) include strata formerly worked for building stone; Arkell (1947a) equated them with the Creamy Limestone. They are locally silicified where overlain by Whitchurch Sand.

North-west of Little Milton [SP 616 019], flaggy fine-grained sandstones occur at the top of the sequence. Lower beds comprise sandy, shelly, sometimes serpulitic limestones and rarely, sandstones with homoiolithic pebbles and, at a lower level, brownish yellow sands with shell debris and thin sandy limestones. Some of the latter are finely glauconitic. Arkell (1944a) noted a quarry ?[SP 6166 0140], north of Little Milton Church, showing 'a few feet' of 'oolitic' limestone and marl with sand below. In 1988, an excavation [SP 6173 0070] near Little Milton church exposed the Upper Lydite Bed resting on a channelled surface of Kimmeridge Clay. It comprised 0.9 m of pale grey micritic and glauconitic, rather rubbly and locally decalcified fossiliferous limestone, passing laterally and downwards into glauconitic marl. Lydite pebbles were scattered throughout, and also concentrated in lenses.

Around Little Haseley, the Portland Formation is much decalcified; a slurry pit [SP 6412 0038] at Court Farm was reported to have been dug 2 m into soft sand but near Pitchend Farm [SP 6249 0026], the soil contains pieces of serpulitic sandy limestone. In 1904, 4.5 m of sand with beds of stone were exposed south-west of the village [SP 6384 0021] (MS field map, BGS archive), and grey sparry sandstone debris occurs in the fields nearby. Flaggy, sandy and shell-fragmental limestones were formerly worked in a pit [SP 6482 0114] north-east of the village.

The Upper Lydite Bed occurs in the floor of the Haseley Brook [SP 6477 0015], [SP 6484 0021], south-east of Little Haseley. On the south side of the valley, Gault rests directly on the Portland Formation, and the highest part of the formation may have been removed by pre-Gault erosion. Here sandy limestones are overlain by serpulid and shell debris limestones.

Between Great Haseley and Great Milton, field brash includes sandy, shell-fragmental limestones and, higher in the succession, shell-fragmental and peloidal limestones. Immediately below the Whitchurch Sand, small patches of recrystallised limestone occur. There are many former pits hereabouts; a large pit within Great Haseley village [SP 6414 0197] was described by Fitton (1836, p. 276). Beneath Whitchurch Sand, it showed 3.3 m of Portland Formation, comprising sands and fossiliferous sandy limestones, with a further 3.6 m of sand below the floor of the pit. A quarry [SP 6461 0179] (now restored) north-east of the church, described by Davies (1899a; see also Pringle, 1926) exposed beds close to the top of the formation:

A quarry ?[SP 6320 0295] east of Great Milton, formerly showed about 3 m of Portland Formation beneath the Whitchurch Sand (Woodward 1895; Pocock, 1908; Pringle, 1926). The strata comprised sand and calcareous sandstone, 'cherry' in places, passing down into grey and greenish grey, sandy oolitic limestone with 'Ostrea', Isognomon and Laevitrigonia gibbosa.

In 1989, an excavation [SP 6251 0338] adjacent to the sewage works to the north-west of Great Milton, exposed the basal beds, comprising glauconitic manly limestone. Further northwest [SP 619 037], many former pits worked sandy, oolitic limestones which overlie brownish yellow sands with beds and doggers of sandy limestone.

Thame–Haddenham

There are extensive outcrops of Portland Formation between Thame and Haddenham [SP 740 086]. Here, the formation, divisible into Portland Sand and Portland Stone members, is up to 12 m thick where complete, but between Kingsey [SP 743 068] and Ford [SP 778 093], it is locally much thinner where the highest beds are absent beneath the Gault.

The Upper Lydite Bed is generally well developed, comprising up to 0.3 m of pebbly sand, clayey sand, silt or sandy limestone. The overlying beds comprise greenish grey, variably glauconitic, patchily cemented calcareous sandstones and silt-stones, though the beds are normally decalcified and oxidised near the surface. Around Thame, the upper part of this unit includes a patchy development of sandy, shelly and peloidal limestones. This may be a weak representative of the Aylesbury Limestone, but it cannot be mapped separately from the surrounding sands, and is thus included in the Portland Sand Member. However, at Haddenham and to the east, the Aylesbury Limestone is better developed as pale grey, rubbly, sandy (quartz) limestones, often packed with bivalve moulds, in particular large myids and Protocardia, and is included with the Portland Stone. Here, the overlying Crendon Sand comprises 1 to 1.5 m of orange to yellow, locally calcareous, fine-grained sand.

The Creamy Limestone includes shelly, micritic or crystalline limestones, locally sandy, or peloidal, which often contain large bivalves (notably Protocardia and trigoniids) and giant ammonites. The holotypes of Buckmans's taxa Behemoth megasthenes and Galbanites galbanus probably came from the Creamy Limestone north of the old Haddenham railway station [SP 735 082], and that of Briarites polymeles from Scotgrove c.[SP 714 076] (Buckman, 1921, 1922).

Up to 5 m of Portland Formation caps the higher ground [SP 699 048] east of Moreton. The lower part (Glauconitic Beds) comprises greyish buff, shelly, glauconitic sandy limestone, with the Upper Lydite Bed well developed at the base. Bivalves including ?Laevitrigonia gibbosa, Isocyprina pringlei, Modiolus hudlestoni ?, Mytilus suprajurensis, Sowerbya?, Nanogyra nana, Plicatula sp., Protocardia dissimilis and an ammonite fragment (Glaucolithites?) were collected from brash [SP 7084 0494]. Overlying strata (possibly equivalent to the Aylesbury Limestone), comprise off-white to very pale grey, nonglauconitic, rough-textured micritic limestones and cream-grey, shelly finely sparry limestone. Again, the beds are locally fossiliferous; brash [SP 7004 0486] yielded moulds of Pleurotomaria, Protocardia dissimilis, Camptonectes lamellosus, Pleuromya uniformis and ?Laevitrigonia gibbosa.

At 'Barley Hill' [SP 715 065], Fitton (1836, p. 282) recorded 2.8 m of glauconitic Portland Sand, containing calcareous, fossiliferous concretions and a basal, fossiliferous stone band. This rested on the Thame Sand of the Kimmeridge Clay; the Upper Lydite Bed was not recorded at this locality.

In 1987, an excavation [SP 7308 0854] at Haddenham railway station showed 1.0 m of dark brownish grey, sandy, shell-detrital limestone with serpulids, resting on 0.6 m of greenish grey, sandy marl and buff, sandy, shell-detrital limestone. Nearby, Jukes-Browne (MS field map; BGS archive) recorded the basal 4 to 5 m of the Portland Formation in the railway cutting [SP 728 088]; the beds comprised 1.8 m of fossiliferous stone, with a basal lydite bed, overlain by about 3 m of 'decomposed manly and clayey beds'. Further to the north-west c.[SP 7250 0915], Davies (1904) recorded 2.4 m of beds, mainly buff sand and glauconitic sand, with a basal lydite pebble bed, 0.23 m thick. The topmost sands included 'some rubbly limestone', probably indicating the base of the Aylesbury Limestone, as Portland Stone has been mapped at the top of the cutting here.

Ballance (1963) considered the Glauconitic Beds (including Upper Lydite Bed) to total less than 0.6 m in the railway cutting, apparently from his preferred interpretation of Davies' section. However, this is not borne out by BGS mapping, nor by Davies' (1899a) record of a ditch at Dadbrook Hill [SP 740 140], 2 km to the north east. The latter showed 3.7 m of glauconitic sand (Glauconitic Beds) with a basal lydite bed, overlain by 5.2 m of poorly exposed, mainly limestone beds. Further along the cutting to the south-east [SP 732 085], Ballance (1963) measured the total thickness of the Creamy Limestone as 3.96 m. The mid part of the section was obscured, but he described the topmost bed (beneath Purbeck marl) as 0.9 m of hard, grey, crystalline, limestone, with abundant fossils and shell debris, and the lowest 0.6 m as sparsely oolitic, slabby, shelly limestone. The latter rested on 1.2 m of Crendon Sand. The topmost 2.4 m of the Creamy Limestone was also exposed in an excavation [SP 7430 0845] in Haddenham where, beneath the Purbeck Formation, Ballance (1963) recorded:

Brill and Muswell Hill

The Portland Formation crops out at the top of the slopes around Brill Hill and Muswell Hill. Exposure is poor, and the strata are strongly affected by cambering, but it seems likely that where complete, the succession comprises 8 to 10 m of strata. The Portland Sand Member, about 4 m thick, consists predominantly of buff to greenish grey, fine-grained, calcareous, glauconitic sands, in places weakly cemented, with the conglomeratic Upper Lydite Bed, up to about 0.2 m thick, at the base. The Portland Stone is particularly poorly known. It is thin in places due to localised erosion beneath the Whitchurch Sand, and is commonly obscured by downwashed material. However, it probably reaches up to 6 m in thickness and includes a variety of micritic, shelly and shell-detrital limestones and sandy limestones. The Crendon Sand, if present, is probably thin.

The Brill No. 1 Borehole [SP 6570 1412] proved the formation between approximately 16.5 m and 24.0 m, but core recovery was very poor (about 45 per cent). On the basis of recovered samples, the Portland Sand (below about 18.3 m depth), comprised 4 to 5 m of brown, orange-brown and greenish brown, fine-grained, soft glauconitic sandstones, with a few lydite pebbles at the base. The overlying Portland Stone comprised 2.2 m of buff to grey limestones with scattered shell fragments and peloids, and shelly marls. Apparently, only the Aylesbury Limestone was represented; the attenuated succession is probably due to faulting or superficial structures.

The formation was formerly worked in many pits, all now degraded and obscured. At an unspecified locality, Mitchell (1834) noted 2.1 m of soft, white fossiliferous limestone (Portland Stone), overlying 2.1 m of sand and rubble with fossils (Portland Sand), which rested on sands of the Kimmeridge Clay. At a pit south-west of the village possibly [SP 650 136], Fitton (1836, p. 281) recorded 3.8 m of limestones beneath the Purbeck Formation. Nearby, these beds were underlain by about 3 m of stone and 'yellowish grey sand' (possibly the Crendon Sand), which rested on an unspecified thickness of glauconitic sand.

The lower part of the formation was formerly exposed above the Kimmeridge Clay in the Brill Brickyard probably [SP 6502 1422] and an adjacent quarry (Woodward, 1895; Pringle, 1926; Arkell, 1947a). The Portland Sand comprised about 3 m of glauconitic sands and sandy limestones with a basal Upper Lydite Bed. These strata yielded Camptonectes lamellosus, 'Ostrea', Protocardia dissimilis, trigoniids and ammonites. The overlying Portland Stone comprised 3.35 m of chalky (micritic), oolitic and shelly limestones and marls, with Lucina' portlandica, 'Ostrea' expansa, trigoniids and ammonites.

In 1986, part of the formation was poorly exposed in a landslip backscar on Muswell Hill [SP 6428 1581] to [SP 6445 1586]. Material from the Portland Sand comprised glauconitic sandstone and sandy limestone with Glaucolithites fragments and trigoniids. Debris from the overlying Portland Stone included micritic and sparry limestones with Camptonectes lamellosus, Lima rustica, oysters, Pleuromya uniformis, Protocardia dissimilis, serpulids and ammonites.

Wimbledon (1980) recorded an unusually thin Portland succession in a trench at Dorton Hill [SP 678 131] showing 0.3 to 0.9 m of Glauconitic Beds, 1.3 m of Aylesbury Limestone and 0.4 m of Crendon Sand. This apparent thinning may be due to cambering effects.

Long Crendon

The Portland Formation was formerly worked extensively at Long Crendon. Beneath the Purbeck Formation, Fitton (1836, p. 282) recorded 2.4 m of stone, marl and calcareous sandstone (Portland Stone) underlain by 6.3 m of glauconitic sand and rubbly sandy stone (Portland Sand). His section was based mainly on a quarry probably [SP 6944 0848] and the cutting on the adjacent road to Thame. A section recorded by Woodward (1895, p. 220) is similar, but a generalised account by Davies (1899a, p. 21) gives a somewhat greater formational thickness (12.2 m).

Buckman (1922; 1926) recorded a composite section totalling 10 m, based on a quarry at 'Barrel Hill' (probably that mentioned above [SP 6944 0848] ), and on quarries at the northwestern end of the village, probably including one [SP 6880 0922] in which about 1.2 m of rubbly, glauconitic sandstone (?Buck-man's beds 22–24) is still exposed. A summary of the section is given below (see also (Figure 18)). There may be a small gap or overlap between beds 11 and 12, which together comprise the 'type' Crendon Sand:

Elsewhere, estimates from mapping suggest that the Glauconitic Beds (Portland Sand) are generally thicker than in Buckman's section, comprising 3 to 6 m of buff to greenish grey, rubbly, fine-grained calcareous, glauconitic sandstone or sandy limestone. The Upper Lydite Bed, up to 0.2 m thick, varies from a well-cemented sandy limestone to soft sand and clay. In 1987, a temporary section [SP 6928 0851] exposed the basal part of the formation; it comprised about 1 m of pale grey to buff, rubbly, fine-grained, sandy limestone with a little glauconite, resting sharply on 0.6 m green to buff, fine-grained, highly glauconitic, calcareous sandstone with a basal layer of sandy, moderately glauconitic limestone with lydite pebbles.

The Portland Stone comprises interbedded grey to buff shell-detrital micritic and sandy limestones, which are commonly peloidal. Porcellanous limestones, micrites and oolites are developed in places, particularly at the top of the succession. The Crendon Sand has not been located at outcrop.

Chearsley-Upper Winchendon-Ashendon-Waddesdon

The Buckinghamshire Waterboard boreholes 41 [SP 7316 1298] and 44 [SP 7223 1246] between Chearsley and Upper Winchendon proved the formation to total 10 to 11 m in thickness. From these boreholes and mapping, the Portland Sand probably averages about 5 m in thickness, and comprises yellow to brown, medium-grained, manly sand and pale grey to cream, nodular, sandy limestones, with glauconite particularly abundant in the lower part. The Upper Lydite Bed is generally present, but seems to fail locally, notably around Upper Winchendon.

The Portland Stone consists of about 5 m to 8 m of variable limestones. The Crendon Sand was proved in the boreholes mentioned above, and was mapped at outcrop near Whaddonfield Farm [SP 719 118]; [SP 721 115]. It is present at Waddesdon Hill [SP 758 153] (Coney Hill of some authors), where Blake (1880, p. 216) observed a section in a quarry [SP 7585 1513], now ploughed over. From Blake's generalised account, the section apparently showed:

Bed numbers follow Blake (1880). Davies (1899a) recorded 2.2 m of Creamy Limestone beneath the Purbeck Formation at this quarry.

Blake (1880) also mentioned a cutting [SP 7283 1654] made during the construction of Waddesdon Manor on the top of Lodge Hill. The section, in the upper part of the formation, showed 3.65 m of 'rubbly beds' overlying 3.81 m of sand and shell-beds, the latter possibly including the Crendon Sand. These rested on glauconitic stone. Mapping suggests that the Portland Formation totals about 10 m in thickness here. In 1987, the lowest 2 m of the Portland Sand Member were exposed in the driveway cutting [SP 7339 1628]; the strata comprised soft, rubbly cream micrite and brown sandy limestone and sand, with scattered glauconite grains, and abundant lydites in the basal 0.4 m. On the adjoining Windmill Hill [SP 738 157]; [SP 740 159], lenticular beds of stiff, blue-black clays occur at the top of the Portland Sand; similar clays (Portland Clay) occur in the Portland Formation of Dorset (Arkell, 1933).

A section [SP 7283 1649] of Portland Stone in the grounds of the Manor showed 1 m of soft white micrite and marl overlying 1.2 m of rubbly, cream to brown limestone with scattered peloids and shell fragments and sparse bivalves. Partly silicified serpulid-rich limestones form a strong feature on Windmill Hill [SP 7334 1569], and hard, grey and white peloidal limestone with moulds of trigoniids and Aptyxiella ((Plate 12):3) occur just beneath the Purbeck nearby. Several large, artificially constructed rock 'outcrops' [SP 7312 1660]; [SP 7287 1656] in the Manor grounds incorporate large blocks of Portland Stone from the Isle of Portland in Dorset.

Stone-Hartwell

The Hartwell Borehole proved the Portland Formation to total 12.79 m in thickness ((Figure 18); Cox et al., 1994). The Portland Sand Member comprises 5.76 m of greenish grey and brown, fine-grained, weakly cemented, calcareous sandstones and siltstones, containing a scatter of coarser glauconite grains in the lower part. The Upper Lydite Bed at the base comprises 0.12 m of brownish grey siltstone with abundant pebbles, and ammonite and bivalve moulds. Its base is irregular with burrows and possible desiccation cracks extending down into the underlying Kimmeridge Clay (Hartwell Silt). The lowest 1.5 m of the Portland Sand, formerly exposed at Hartwell Brickpit [SP 805 105], comprises hard blue rock, with lydite pebbles at the base (Hudleston, 1880; Woodward, 1895, p. 225).

In the Hartwell Borehole, the Portland Stone is 7.03 m thick, which may be near the maximum for the neighbourhood, as estimates from outcrop seldom exceed 5 m. The Aylesbury Limestone (1.84 m thick) and the Creamy Limestone (2.25 m) are lithologically similar. Both show an upward progression from pale grey, uniform limestones containing a high proportion of quartz sand, to purer, strongly burrowed, shell-fragmental, micritic limestones and (in the Creamy Limestone) pure micrites (lime mudstones). The Crendon Sand was 2.28 m thick, the maximum known in the district. It consists of sands and silts very similar to those of the Portland Sand, though without macroscopic glauconite. It has been located by augering in a few places, and was seen in temporary sections at Stone [SP 7963 1215]; [SP 7840 1205].

Ballance (1963) gave a section of a temporary exposure at Westlington [SP 7630 1039], in which he claimed that the 'Glauconitic and Rubbly Beds' were reduced to 0.25 m of sandy limestone. The implied thinning of the lower part of the Portland Formation does not accord with BGS mapping which indicates that the overlying fine-grained calcareous sand (1.07 m) is also part of the Glauconitic Beds.

The Portland Stone Member was formerly exposed beneath the Purbeck Formation in the Bugle Pit [SP 794 121] at Hartwell. Some details are given by Phillips (1871) and Hudleston (1880, 1887), but the best section is that by Woodward (1891), which later formed the basis of his figured 'Section at Aylesbury' (Woodward, 1895), subsequently reproduced by Davies and Emary (1897), Davies (1912; 1934) and Arkell (1933; 1947a). Beneath the base of the Purbeck Formation, it showed:

In 1988, a section [SP 7839 1205] in a silage pit at Stonethorpe Farm showed:

Several other quarries formerly exposed the Portland Formation, notably Dinton Stone Pits [SP 7590 1140], Church Furlong Pit [SP 7973 1099] and St John's Lodge Quarry [SP 7876 1212] (Fitton, 1836; Smyth, 1864) but few details of the strata are available.

Aylesbury–Weedon

An outlier of the Portland Formation caps the hill [SP 818 138] on which the older, central part of Aylesbury is built. Records of sections show that the succession is much like that proved in the Hartwell Borehole, though overall slightly thinner. Several temporary sections were recorded in general terms by Hudleston (1880; 1887). Of the figured 'Section at Aylesbury' (Woodward, 1895), the greater part relates to the Bugle Pit, Stone (see above), and only the part below the Crendon Sand, to Aylesbury. These latter beds comprised 2.44 m of white, rubbly, fossiliferous limestone 'roach' (Aylesbury Limestone), overlying 3.05 m of yellow and greenish sand (1.8 to 2.4 m) with a basal lydite bed (Glauconitic Beds). In foundations at Walton Street [SP 8225 1330], Bristow (1963) recorded 1.3 m of Aylesbury Limestone, comprising laminated, white limestone with Protocardia and a basal layer of sandy roach. The underlying Glauconitic Beds comprised 3 m of mustard-yellow sands. The basal 0.76 m of the formation was formerly exposed at the Bierton Road Brickpit [SP 8220 1420]. It comprised 'tawny', decalcified sandy limestone with lydite pebbles at the base (Hudleston, 1880; 1887; Sherlock, 1922; Davies, 1899a).

In south-west Aylesbury, the former Walton Quarry [SP 8138 1205] now forms a grassy hollow in a playing field at Walton Court shopping precinct. Beneath the Purbeck Formation, Davies (1899a) recorded 2.2 m of 'chalky limestone' with a thin marl bed. Just beyond the eastern margin of the district, the whole formation was formerly visible in the Walton Railway cutting [SP 8242 1283] though here the top of the formation is truncated by the basal Gault unconformity. The section (Figure 18) is based on that recorded by A Strahan (MS, 1891, BGS archive), with additional data from Sherlock (1922; see also Woodward, 1895).

An outlier of Portland Formation, overlain unconformably by Gault, caps the hill at Weedon [SP 814 182] in the north-eastern part of the district. Approximately 5 m of beds are present, comprising khaki, orange-brown and yellow, fine-grained, commonly glauconitic sands and silt, and pale grey to buff sandy shelly limestones and sandy glauconitic limestones (Glauconitic Beds). The Upper Lydite Bed has been located at Wee-don Lodge Farm [SP 8186 1795].

Purbeck Formation

Within the district, the most extensive outcrops of the Purbeck Formation (Purbeck Limestone Formation of Wimbledon, 1980), occur between Haddenham [SP 740 087] and Stone [SP 782 121], and smaller outcrops occur on the hills to the north-west, such as those at Brill, Long Crendon and Waddesdon. Apart from minor remnants near Great Haseley, the formation is generally absent to the south-west of Thame (but see Details), and borehole evidence suggests that it is also absent beneath the Gault to the south-east. It is likely that the formation was originally laid down across the entire district, but it was removed in some areas by early Cretaceous erosion. Thus, at many localities, the formation is overstepped by Whitchurch Sand, but elsewhere (notably in the eastern part of the district), local overstep by both Lower Greensand and Gault can be demonstrated.

The Purbeck Formation comprises limestones, interbedded with marls, clays and sandy clays, and the outcrop is characterised by a brashy, dark brown clay soil. Up to 6 m of strata are preserved near Haddenham and Stone; a slightly thicker succession (about 6.6 m) recorded at Whitchurch [SP 805 225] just outside the district (Barker, 1966) is the maximum known in the region. The most typical rock type is a white micrite limestone (lime mudstone), generally massive, but in some cases showing algal laminae or stromatolitic structure ((Plate 13):6). These limestones are commonly brown mottled, and are intensely hard and splintery, due to recrystallisation. Bored bedding surfaces, indicating penecontemporaneous cementation, are common. Syneresis (shrinkage) cracks, fenestrae ('birds-eyes') and, more rarely, mudcracks and possible salt pseudomorphs also occur, and indicate periods of subaerial emergence. At some localities, sandy, peloidal or oolitic limestones have been recorded but, in general, such higher energy facies are rare.

Virtually all recorded sections through the formation indicate laminated marls and limestones in the lowest part. Such limestones at this level have traditionally been known as the Tendle' (Fitton, 1836; Morris, 1856; Jones, 1885), which typically comprises alternating laminae (up to 10 mm thick) of shell-fragmental, commonly sandy, ostracod-rich limestone (packstone to grainstone), and pure micrite (lime mudstone). The Tendle' locally up to 0.5 m thick, forms a useful marker at or just above the base of the formation.

As in the type area of Dorset, the junction with the underlying Portland Formation is gradational, and differentiation of the two formations depends on recognition of predominantly nonmarine lithofacies and fauna in the Purbeck, in contrast to the marine aspect of the Portland. Traditionally, therefore, the base of the formation has been drawn at the base of the Tendle' which is 'the first occurrence of a bed in which large molluscan macrofauna is absent, and fine regular lamination is well developed' (Radley, 1991). However, during this survey, it was found more practical to define the base of the formation at the top of the massive, marine limestones of the Portland Formation. Although these two horizons are coincident in many places, at others (including the Hartwell and Brill No. 1 boreholes), up to 0.5 m of shelly marl with 'Ostrea' expansa and other large bivalves, intervenes between the Portland Formation and the Tendle'. This 'Upper Shell Marl' was included in the Portland Formation by Bristow (1968) and Radley (1991).

There are few recorded sections in the upper part of the formation, but mapping shows that it is generally dominated by marls and clays, commonly greenish grey and plant rich where unweathered. This raises local difficulties in placing the top of the formation where the basal unit of the overlying Whitchurch Sand is also clay. However, an absence of calcareous material and an abundance of ironstone have been regarded as characteristic of the latter formation, although decalcification and development of secondary ironstone may have occurred locally in the topmost part of the Purbeck as a result of Cretaceous weathering.

The fauna and lithofacies of the Purbeck Formation indicate a wide range of salinities, from fresh to hypersaline and, with the evidence of periodic emergence, it seems likely that the strata were laid down in a very shallow, coastal or lagoonar environment. Fauna is generally sparse, though small bivalves (notably Modiolus) and gastropods (e.g. Valvata and Viviparus) occur in some beds, and fish and insect fragments are also quite common. Ostracods are abundant at some levels and are used in biozonation, since ammonites are absent. However, some workers doubt the correlative value of the ostracods, as they are known to be strongly influenced by salinity and sedimentary facies. Ostracods from the Purbeck Formation of the district and of the adjoining area (i.e. sections at Quainton, Whitchurch and Stewkley) are listed by Jones (1885), Chapman (1897; in Davies, 1899a), Merrett (1924) and Barker (1966). Although Merrett (1924) claimed that the species included Middle and Upper Purbeck (sensu Dorset) forms, Sylvester-Bradley (in Arkell and Sylvester-Bradley, 1942) discounted this, and Barker (1966) also recognised only Lower Purbeck types. On the basis of Barker's data, Anderson (MS, BGS archive) assigned the strata to his Quainton and Warren 'faunicycles', implying correlation with the lowest part of the Lower Purbeck of Dorset (Anderson, 1985, fig.5), i.e. the basal Lulworth Beds of Casey (1963). However, the conformable contact between the Creamy Limestone (Portland Formation; Kerberus Zone) and the Purbeck Formation suggests that the latter most probably equates with the uppermost Portland Formation of Dorset (which extends into the succeeding Anguiformis Zone), i.e. the onset of Purbeck facies sedimentation in the Thame district began earlier than in Dorset (Wimbledon, 1980).

Details

Shotover–Wheatley–Garsington

Former quarries and sections generally showed Whitchurch Sand resting directly on the Portland Formation. However, two sections (see below) show that remnants of Purbeck Formation occur locally, but it has proved impossible to delineate these outcrops because of obscuring wash from the overlying Whitchurch Sand.

Fitton (1836, p. 277) recorded a pit possibly [SP 5798 0349] near Garsington which showed 1.2 m of Purbeck Formation (Bed 3) beneath Whitchurch Sand. The former rested on an irregular surface of Portland limestone, probably due to recent differential solution. The Purbeck Formation comprised light greenish grey marls and limestone, the former containing fragments of bone and silicified wood in the upper part. The lower part of the succession was, in places, 'like the "Pendle" of the pits at Whitchurch', a soft fissile limestone. The limestones were in part oolitic, and contained small gastropods, bivalves and ostracods. A second pit 'about half a mile to the east' showed a similar succession.

At a quarry possibly [SP 5929 0461] at Combe Wood, south of Wheatley, Fitton (1836, p. 275) described the Purbeck Formation as a freshwater limestone, and in parts a grey and brownish, fine-grained, oolite, with abundant ?Viviparus.

Great Milton–Great Haseley

Newly recognised outliers of the Purbeck Formation (overlain by Whitchurch Sand) occur at Windmill Hill [SP 62 02] north of Little Milton, west of Little Haseley [SP 633 006], and between Great Milton and Great Haseley [SP 633 028] to [SP 641 023]. Up to 1.5 m of strata are present, comprising white, porcellanous micritic limestones, interbedded with marls. The relatively extensive outcrop on the south side of Windmill Hill may be partly a result of cambering.

Formerly, the formation must have been exposed in several stone pits between Great Milton and Great Haseley, although in the only published section [SP 6414 0197], Fitton (1836, p. 276) suggested that it was absent. During this survey, typical Purbeck lithologies were seen in a ditch [SP 6335 0051] to [SP 6335 0060] near Canker Lease, west of Little Haseley. At the southern end, the basal beds of the formation comprise 0.7 m of very dark grey, carbonaceous clay with shell debris, passing upwards into 0.35 m of grey marl containing micritic limestone masses with small gastropods. Continuing northwards, the succession is dominated by off-white to pale grey marls and clays, and pale grey and khaki-mottled micrite with small gastropods, but peloidal sparry limestone with shell debris and buff oolitic limestone also occur. In the stream bed nearby [SP 6343 0052], white to pale olive grey porcellanous limestone with gastropods is associated with marl and clay.

Thame–Haddenham

North-eastwards from Great Milton, the Purbeck Formation is absent as far as Thame, where Kitchin and Pringle (1922c) reported Purbeck limestone beneath Gault in a well to the east of the former railway station [SP 713 052]. The formation emerges from beneath Gault north of Towersey [SP 735 059] and near Kingsey [SP 741 063]. Here, the beds comprise pinkish grey, subporcellanous micrites, pale grey sandy micrites, possible algal laminites and peloidal micrites, together with marls and grey to green clays. Davies (1899a) recorded 1.4 m of marls and thin-bedded limestones above Portland limestones in a former pit [SP 735 059] on the Towersey outcrop. The section included a band of 'nodular calcareous chert' at the top. A marl contained many 'Paludina'[Viviparus], a fish tooth and ostracods.

A small outlier of Purbeck Formation in western Haddenham [SP 733 083] is traversed by the railway cutting, where Davies (1904) recorded marl with nodules of chert (as at Towersey), and thin, bituminous clayey beds with ostracods.

The Purbeck outcrop between Haddenham and Dinton is the largest of the district, and the formation reaches its greatest thickness of about 6 m near Gibraltar [SP 759 109]. South of the Ford Brook, the formation is probably largely absent due to pre-Gault erosion, though small areas crop out at Ford [SP 778 096] and Aston Sandford [SP 757 083]. Arable fields typically show a brash of white and brown mottled micrite, generally recrystallised and intensely hard and splintery, in a dark brown clay soil.

Ballance (1963) described a temporary exposure [SP 7430 0845] in Haddenham village, showing 1.77 m of Purbeck limestones between Portland Stone and the piped base of the Whitchurch Sand:

Davies (1899a) recorded a pit [SP 7402 1017] at King's Cross, north of Haddenham, showing 2.9 m of Purbeck Formation:

Bed 5 may infill a channel, as Davies reported 'slight traces of contemporaneous erosion' at its base, and another section of the pit (by Jukes-Browne in Davies, 1899a) showed that it thickens to 0.6 m.

Fitton (1836, p. 285) recorded the following (summarised) section at Dinton Quarries (probably those [SP 759 115], now restored, south of Spring Hill Farm (see Davies, 1899a) ):

Beds 3 to 5 were excluded from the Purbeck by Fitton, though the site (if correctly identified) lies well within the Purbeck outcrop, and similar strata are recorded from King's Cross (see above) within the Purbeck Formation. The basal bed (11) is the Pendle', which is particularly well developed in the fields [SP 762 099] south of Westlington, where it is approximately 0.5 m thick. South and west of Dinton Quarries [SP 751 113] to [SP 759 113], brash of yellowish brown ferruginous micrite, apparently within a succession of unaltered limestones, may be a result of penecontemporaneous weathering, as described at the Bugle Pit (see below).

Brill–Long Crendon

On the slopes of Brill and Muswell hills, the Purbeck Formation has a considerable outcrop characterised by pale grey clays, marls and debris of white micrite. A shelly, oyster-rich marl occurs at the base in many places. Apparent thicknesses of up to 8 m are estimated, but the beds are strongly affected by cambering, and the true stratigraphic thickness is probably much less.

The presence of Purbeck beds at Brill was first recognised by Brodie (1867), who noted ex situ blocks of limestone, with ostracods and fish debris. However, one of the sections described earlier by Fitton (1836, p. 280, locality C possibly [SP 6502 1366] ) probably included the Purbeck Formation, comprising 1.37 m of thinly stratified clay and sandy clay with plant debris and shell fragments. Phillips (1871) gave a generalised section based on quarries on Brill and Muswell hills. It indicates 3 m of Purbeck strata, comprising interbedded limestones, marls and clays. Pocock (1908) however, considered the maximum thickness was no more than 1.8 m, and recorded several sections; one [SP 646 145] showed about 1.1 m of Purbeck beds comprising white stone and marl overlain by a thin clay, beneath Whitchurch Sand. Several other exposures showed thinner Purbeck strata, including bright green clays and tough black to dark grey earthy clays.

In the Brill No. 1 Borehole, the formation is present between about 15 and 16.5 m depth. The small amount of core recovered comprises olive to dark grey clays, and (below about 16 m) grey manly clay with abundant shell debris, plant material and micrite clasts. Dr I P Wilkinson (BGS) obtained a 'Lower Purbeck' ostracod assemblage (i.e. Assemblage 1 of Anderson, 1985) from these marls. Some 2.7 m of clays and marls were penetrated in the Brill No. 3 Borehole [SP 6629 1402], and possibly as much as 4.4 m in Brill No. 2 [SP 6606 1403]; however, in the latter, there is some uncertainty over the level of the junction with the overlying Whitchurch Sand.

The Purbeck Formation has a discontinuous outcrop between Chilton [SP 688 115] and Long Crendon [SP 694 088], and a faulted outlier occurs near Notley Abbey [SP 713 094]. It comprises up to 2 m of pale grey, porcellanous limestones with subordinate pale grey clays. Its local absence (e.g. around Chilton Grounds [SP 694 112] ) is due to overstep by the Whitchurch Sand.

Several authors (notably Fitton, 1836; Blake, 1893; Davies, 1899a; Lamplugh, 1922) mention the presence of Purbeck beds at Long Crendon, most probably referring to the former Windmill Quarry [SP 693 093] ((Plate 16)). Here, Davies (1899a) recorded 1.8 m of Purbeck strata, mainly 'limestone with clay veins', underlain by 'pale clay with ostracods' (0.30 m) and a basal 'crumbly calcareous sandstone' (0.13 m). Blake (1893) mentioned 0.9 m of 'well-bedded limestones and marls of fine grain' with ostracods and fish debris, probably at this or a nearby pit.

Chearsley–Waddesdon

Thin remnants of the Purbeck Formation, up to 2 m thick, are preserved locally beneath the Whitchurch Sand on the hills between Chearsley [SP 718 106] and Waddesdon [SP 742 169]. The succession is dominated by white porcellanous limestones, with subordinate marls and clays. 'Pendle' lithologies have been noted at a few localities.

On Windmill Hill [SP 735 159], slabs of Pendle' occur at the western edge of the plantation [SP 7335 1587] and, to the north [SP 7334 1606], a small scrape showed 0.6 m of soft, white micrite with rare small bivalve moulds. In a degraded pit [SP 7380 1590] east of the plantation, debris of hard, white and brown mottled recrystallised micrite occurs. On the adjoining Lodge Hill, an exposure [SP 7283 1649] by the driveway of Waddesdon Manor showed about 1 m of soft white micritic limestone and marl overlying Portland Stone. Elsewhere, the outcrop of the formation (if present) would be extremely narrow due to the steepness of the hill slopes, and consequently has not been shown on the 1:50 000 scale geological map.

At Waddesdon (or Coney) Hill, the formation is largely obscured by wash from the overlying Whitchurch Sand, and the outcrop boundaries shown on the map are generalised. However typical Purbeck lithologies occur in the brash in a few places, including fissile, ostracod-rich 'Pendle' [SP 7572 1517] and white algal laminites [SP 7588 1552]. Davies (1899a) recorded a section in a former quarry [SP 7585 1513] showing about 1.8 m of Purbeck limestones, with beds of sand and marl, and Tendle' at the base, resting on Portland Stone.

Stone–Aylesbury

The Purbeck Formation has an extensive outcrop south-east of Stone [SP 793 120], where up to 6 m of strata are preserved, and between Bishopstone [SP 800 105] and Southcourt [SP 820 124] in the Aylesbury suburbs. Just beyond the eastern margin of the district, the formation is overstepped by Gault and is absent to the north-east. Similarly, west of Bishopstone [SP 785 100], it is absent in the block bounded by the Upton and Dinton faults.

A number of sections are recorded in the literature. Above the basal marls and Pendle', the succession is dominated by micrites and marls. Algal laminae and stromatolites can be seen in situ at the Bugle Pit, and stromatolites occur in the fields at Whaddon Hill [SP 787 133] and north-east of Bishop-stone [SP 809 110].

At the Bugle Pit, a small section [SP 7932 1205] adjacent to a formerly extensive quarry (now infilled) is preserved as a Site of Special Scientific Interest (SSSI) (Plate 14). The literature pertaining to this section and that of the adjacent quarry is considerable. The earliest records are by Phillips (1871) and Woodward (1891); the latter forming the basis of a 'Section at Aylesbury' (Woodward, 1895, p. 224), which is reproduced with minor modifications in several other publications (e.g. Arkell, 1947a). Other important accounts are by Merrett (1924), Barker (1966) and Radley (1991), the last two giving details of the fauna. In 1992, about 3 m of beds were exposed:

The ferruginous, cavernous nature of the limestone at the base of Bed 5 is probably the result of penecontemporaneous weathering, as it is overlain by relatively unaltered beds. Other sections of the Bugle Pit (notably those of Merrett, 1924; Barker, 1966, and Radley, 1991) record a further 0.5 m of underlying marls and clays, which rest on limestones of the Portland Formation. The basal Purbeck stratum is a dark shelly mudstone. It contains a varied marine fauna, dominated by 'Ostrea' expansa, and on this basis, Radley (1991) excluded it from the Purbeck Formation, defining the base of the latter at the base of the overlying 'pale calcareous and dark carbonaceous laminated mudstone', i.e. the 'Pendle'.

The Hartwell Borehole, sited 200 m north-west of the Bugle Pit SSSI, proved the base of the Purbeck Formation at 5.16 m depth, though only the basal 2.3 m was cored (Cox et al., 1994). The total thickness of the formation hereabouts is probably about 6 m. The succession proved was closely similar to that in the pit, though the 'Pendle' was somewhat better developed. Again, a unit of bioturbated, shelly marly clay of marine aspect underlies the 'Pendle', and forms the basal bed of the Purbeck.

St John's Lodge Quarry [SP 7876 1212], now built over, lay 0.5 km west of the Bugle Pit. The section, recorded by Smyth (1864) showed 3.73 m of Purbeck clays and limestones resting on Portland Stone. To the south-east, at Church Furlong Pit [SP 7973 1099], Fitton (1836, p. 287) described 4.4 m of Purbeck strata above the Portland Formation. The succession is comparable with that at the Bugle Pit and in the Hartwell Borehole; in each case, the lowest part of the formation is dominated by laminated clays, sandy clays and marls, and the upper part by white limestones, with an intermediate bed of hard sandy limestone. (NB the site [SP 8075 1090] given by Ballance (1960) is incorrect; see Smyth, 1864).

Davies (1899a) and Merrett (1924) recorded sections of Walton Quarry [SP 8138 1205], which formerly showed 0.83 m of Purbeck limestones above the Portland Formation.

Chapter 8 Lower Cretaceous

Whitchurch Sand Formation

Arenaceous deposits occur between the Purbeck Formation and the Gault throughout much of the district. On early geological maps (Old Series sheets 13, 45 and 46, published 1859–63), these were classified as Lower Greensand, although it was recognised that as well as typical marine Lower Greensand, they included nonmarine deposits, notably at Shotover Hill [SP 567 063] near Oxford (Strickland, 1836; Fitton 1836). These 'Shotover Sands' (or 'Shotover Ironsands') have traditionally been ascribed to the 'Wealden' in particular to the Hastings Beds (Prestwich, 1879; Pocock, 1908; Pringle, 1926, Arkell, 1947a).

Sands at Whitchurch [SP 796 210] and Quainton Hill [SP 748 221], just to the north of the district, were also included in the 'Shotover Sands' (Taylor, 1959) but, on the basis of their marine fauna, Casey and Bristow (1964) separated them as a distinct formation, which they named the 'Whitchurch Sands'. However, as the deposits at Shotover and Whitchurch are essentially identical in terms of lithology and petrology (Appendix 4), they are regarded as a single unit in the present account. This is here named the Whitchurch Sand Formation; although the term 'Shotover Sands' has historical precedence, it is rejected because it has also been used for the Portland Sand (Woodward, in Pringle, 1908) and for beds within the Kimmeridge Clay (Buckman, 1922).

The Whitchurch Sand rests unconformably on the Purbeck Formation overstepping it in many places to rest on the older Portland or Kimmeridge Clay formations. Originally, it probably extended across the entire district, but it was removed in many areas during a period of erosion which preceded the deposition of the Lower Greensand and Gault, and was further dissected during development of the present landscape. As a result, the formation occurs mainly as scattered outliers which cap hills such as those at Shotover and Brill [SP 665 138] where up to 20 m of beds are preserved. Elsewhere, the formation is generally absent due to overstep by the Lower Greensand and Gault, and only near Great Haseley [SP 642 019], Thame [SP 708 048], Long Crendon [SP 692 097] and Bishopstone [SP 804 104] is relatively thin Whitchurch Sand seen beneath these strata (Figure 19). Apart from a small deposit at Swindon (Casey and Bristow, 1964), the Whitchurch Sand is restricted to this district and its environs.

The Whitchurch Sand Formation is dominated by quartz-rich, commonly ferruginous sands. These are predominantly buff in colour, but range from white, through yellow, orange and red to dark brown, depending on the proportion and nature of the iron minerals present. The sands are generally unconsolidated, but siliceous concretions (Plate 18) occur at Stone. Seams and irregular masses of 'ironstone', cemented by purplish black brown-weathering limonite (i.e. hydrated iron oxide minerals, probably including goethite) are common (Plate 15). In many cases, subhorizontal bands of ironstone cut across bedding structures; these may be related to former groundwater levels. Ironstone is also common near the ground surface, locally forming a subsoil ironpan. Fragments are common in the soil on the outcrop; some are hollow 'boxstones' containing loose or partially cemented sand. Phillips (1871) recorded 'pisolitic or oolitic iron ore' and Jones (1878a) 'cavities left by the removal of oolitic grains' within ironstones at Shotover Hill. Oolitic ironstone is also known near Wheatley [SP 593 046] but such beds are very rare.

The sands of the formation are characteristically fine-to medium-grained; Taylor (1959) recorded a median diameter of 1/8 mm (fine to very fine sand). Although pebbly lenses have been recorded from a few localities, coarse material is generally rare; this characteristic has been used in mapping to separate the Whitchurch Sand from the coarser-grained Lower Greensand. Taylor (1959) showed that the heavy mineral suite of the Whitchurch Sand was similar to those from the sands of the Portland and Kimmeridge Clay formations, and concluded that the Whitchurch Sand was derived (in part at least) by reworking of these Jurassic rocks, although a common source area is an alternative possibility. He also pointed out that the heavy mineral assemblage of the sands differs from that of the Wealden beds of south-east England, which implies palaeogeographic isolation from the Weald Basin. Allen and Parker (Appendix 4) concur with these conclusions, and note the similarities of the Whitchurch Sand mineral suite to that of other late Jurassic and early Cretaceous sands on the northern flanks of the London Platform, for example the Spilsby Sandstone of the East Midlands.

Lenticles and thin seams of silt and clay are common throughout the formation, and beds of clay up to several metres thick occur, particularly in the lower part. Like the sands, these clays vary considerably in colour, but are most typically pale grey to white. They commonly contain nodules of purple limonitic ironstone, rarely with a paler sideritic core, and seams of yellow to reddish brown limonitic clay are quite common; such beds were formerly worked as a source of ochre (see Chapter Twelve). The clays are predominantly kaolinitic; 'Fuller's Earth' was recorded by early authors (Fitton, 1836), but this was doubted by Taylor (1959). Recent BGS work proved a smectite content of up to 35 per cent, but no true fuller's earths (see also Appendix 4).

Many authors have described fossil plants from the Whitchurch Sand, including a large tree trunk from Brill (Fitton, 1836); rootlets and seatearth-like textures have been noted in clays at several localities. The formation has also yielded a sparse fauna of nonmarine bivalves and gastropods, generally preserved as empty moulds in limonitic ironstone, principally from Shotover Hill but also from near Wheatley, Brill and Stone. The fauna (reassessed by Morter, 1984) is essentially freshwater, and is dominated by Neomiodon sublaevis [Cyclas medius auctt., Cyrena media auctt., and Neomiodon medius of Arkell (1947a)] ((Plate 13):4), Una spp. including 'Unio' stricklandii ((Plate 13):1), and Viviparus [Paludina] spp., including V. cf. antiquus, V. cariniferus, 'V.' ornata and V. subangulata. Casey and Bristow (1964) recorded a more diverse fauna with marine affinities from a thin bed within the Whitchurch Sand at Whitchurch. It comprised Corbula inflexa, Protocardia purbeckensis, Laevitrigonia gibbosa, Myrene spp., as well as other bivalves, gastropods and Serpula coacervata. P. purbeckensis was also recorded from Quainton Hill [SP 748 221]. Together, the fauna, flora and sedimentary facies of the deposits, suggest that the Whitchurch Sand was laid down in a predominantly fluvial environment (Taylor, 1954); the marine faunas from Whitchurch may indicate a more estuarine environment at that locality, or may relate to a temporary marine incursion.

From the earliest times, the age of the Whitchurch Sand Formation has been controversial. Its fauna was regarded as of little stratigraphic significance, but, from a consideration of its facies and stratigraphical position, it has traditionally been assumed to be of 'Wealden' age (i.e. Ryazanian to Barremian; Pocock, 1908). The unconformable relationship of the Whitchurch Sand Formation with the underlying strata suggests that a substantial period of erosion occurred prior to its deposition, and further erosion preceded deposition of the overlying Lower Greensand, of late Aptian age. Although this is consistent with a 'Wealden' age for the Whitchurch Sand, it does not rule out the possibility that the Whitchurch Sand could also be Aptian (Casey and Bristow, 1964). In this connection, it is worth noting the lithological resemblance of the Whitchurch Sand to the lower part of the Woburn Sands Formation (Lower Greensand) of the Leighton Buzzard district (Shephard-Thorn et al., 1994). Like the Whitchurch Sand, these strata are overlain unconformably by beds of younger Aptian age Clipper Woburn Sands'), which resemble the Lower Greensand of the Thame district.

From the discovery of Myrene and Laevitrigonia at Whitchurch, Casey and Bristow (1964) correlated the deposits there with the mid-Purbeck Cinder Bed of Dorset, which is taken to mark the base of the Ryazanian Stage (basal Boreal Cretaceous; Rawson et al., 1978). Morter (1984) deduced a slightly earlier age, corresponding with the uppermost Lulworth Formation of Dorset (latest Portlandian, Jurassic) for both the marine deposits of Whitchurch and the nonmarine 'Shotover Ironsands'. However, the stratigraphical significance of the bivalve-dominated faunas has been questioned (Wimbledon and Hunt, 1983); the vertical ranges of the relevant taxa are poorly known because of the lack of reference sections of appropriate facies, and to a large extent they may be controlled by sedimentary environment rather than age.

An ostracod fauna from the sands of Shotover Hill, originally described by Jones (1878a, b; 1885) has an important bearing on the age of these depositss. As reassessed by Anderson (1966; in Anderson et al., 1967), it comprises Cypridea aculeata, C. bispinosa, C. verrucosa, C. valdensis and Mantelliana phillipsiana. With the exception of C. valdensis (a long-ranging form) these species are typical of the Wealden Lower Tunbridge Wells Sand and Grinstead Clay (Valanginian); none are recorded from earlier Wealden or Purbeck strata (Anderson et al., 1967; Anderson, 1985). During this survey, clays from the base of the formation at Beachendon Hill [SP 7553 1329], Upper Winchendon, yielded the spore Impardecispora apiverrucata, regarded as typically Wealden by Couper (1958), and the foraminifer Pseudonodosaria cf. vulgata, known from the Speeton Clay (Valanginian to Barremian). Both forms are long-ranging taxa, but their presence lends some support to the ostracod data.

Clay mineralogy also provides evidence of the age of the Whitchurch Sand. Samples from the Brill No. 1 Borehole [SP 6570 1412] and elsewhere were investigated by Allen and Parker (Appendix 4) and Dr R W O'B Knox (BGS). All those from the Whitchurch Sand have a substantial kaolinite content, contrasting with those from the local Purbeck Formation in which it is absent. In southern England, kaolinite is very rare in the lower part of the Purbeck Group, but it appears as a minor constituent slightly above the level of the Cinder Bed of Dorset, with an upward increase at the base of the Wealden (Sladen and Batten, 1984; Hallam et al., 1991). Kaolinite is regarded as a product of weathering in acid soils in a warm climate with high precipitation (Ross and Kerr, 1931). Thus, the rarity of kaolinite in the lower part of the Purbeck Group is believed to be related to a period of aridity, and the incoming of kaolinite marks a climatic change, and can be assumed to be almost synchronous throughout southern England. On this basis, the Whitchurch Sand can be no older than the upper part of the Purbeck Group of Dorset (Ryazanian), and the relatively high proportion of kaolinite would be consistent with a later 'Wealden' age; in the marine Speeton Clay of North Yorkshire (Ryazanian to Barremian), the kaolinite:illite ratio 'peaks' in beds of Valanginian age (Knox, 1991).

Principally on the basis of clay mineralogy, Allen and Wimbledon (1991) suggested that the Whitchurch Sand 'ranges up to at least lower–Middle Purbeck (sensu Dorset)', but, combining the detrital petrography, Allen and Parker (Appendix 4) conclude that it may range up to an horizon 'somewhat younger than late Wadhurst Clay'. On the basis of all the available evidence, therefore, an early Cretaceous, probably Valanginian age is favoured here. The presence of nonmarine oolitic ironstones of similar type in both the Whitchurch Sand (see above) and the Wadhurst Clay (Taylor, 1992) is noteworthy, though the correlative significance of these rare lithologies is unknown.

Details

Shotover Hill

The Whitchurch Sand Formation forms an extensive outlier capping Shotover Hill [SP 570 060]. Soils suggest that silts and fine sands are common in the lower part of the succession, whilst coarser sands and ferruginous sandstones dominate the higher part. The beds were formerly worked for sand and ochre at several localities; a generalised section totalling 15 m (Pocock, 1908; Pringle, 1926), should be treated with caution because of the uncertain stratigraphic relationships of the pits.

The only extant exposure [SP 5654 0642] is at Monks Farm, Risinghurst which, in 1987 showed about 6 m of strata close to the base of the formation. Grey and buff sands with clay partings and some limonitic layers (Beds 1–9) are overlain by clays (Bed 10) which appear to infill a channel cut into the underlying sands. A composite section measured near the deepest part of the channel showed:

A block of ferruginous sandstone contains an impression of a fern pinnule, identified as Cladophlebis, a long-ranging morphotype (C R Hill, British Museum (Natural History), personal communication, 1988).

In a former pit at the western extremity of the outlier, Fitton (1836, p. 275) recorded 1.5 m of 'yellowish clay' with 'yellow ochre' at the base, above 0.9 m of 'sand and rubbly marl'. A nearby pit probably [SP 5607 0632] exposed 0.9 m of yellow sand overlying up to 2.4 m of pale grey to white clay and silt (Pocock, 1908). At the base of the silt, large 'concretions of purple ironstone' yielded external moulds of Viviparus and 'Unio', and plant remains. Jones (1878a) recorded ostracods, small Neomiodon, 'Unio', Viviparus and fish bones from loose blocks of ferruginous sandstone in this vicinity.

At a large pit [SP 5752 0598] (now obscured) at the eastern end of Shotover Hill, Arkell (1947a) recorded 4.6 m of 'rust-coloured ironsands and tabular and flaggy ironstone, passing down into yellow and orange sand'. Phillips (1871) also described a succession here; it comprised 3.7 m of ferruginous sandstone, overlying 0.9 m of pale blue laminated sandy clay, which in turn rested on about 5 m of yellow, brown and white sands with beds of pale clay and sandy clay.

Traces of workings 300 m to the south-east [SP 5770 0575] may mark the pit recorded by Plot (1676; see Pocock, 1908) and later by Conybeare and Phillips (1822), who described about 5 m of ferruginous grit and yellow and grey sand, underlain by beds of ochre and clay (no thickness given), on a further 12 m of variable beds which probably extend into the Portland Formation. Pocock (1908) recorded ironstone with 'Unio', Neomiodon and Viviparus amongst the spoil from these pits, and also further west [SP 5682 0584] near Westhill Farm. Arkell (1947a) noted several sections 'at the end of the avenue spur leading to Shotover House' [SP 561 064], which exposed up to 1.2 m of red, orange and white sands, and 'boxy ironstone' with lilac clay near the base. A pit [SP 5744 0549] 'south of Rough Barn' formerly showed varicolored sand with clay bands, and ironstone concretions which yielded 'casts' of freshwater shells and plant fragments (Pocock, 1908).

On Shotover Hill, Whitchurch Sand rests on the Portland Formation. However, on the nearby small outlier of Red Hill [SP 586 074], it rests directly on Kimmeridge Clay, suggesting some movement on the Wheatley Fault, which passes between the two hills, prior to deposition of the Whitchurch Sand (see Chapter Eleven). The beds appear to have been worked on the north-eastern slopes of Red Hill, where up to 15 m of sands with ironstone, and basal clays and silts, are estimated to be present.

Wheatley-Garsington

The most extensive outcrop of Whitchurch Sand in the district caps the hill between Wheatley and Garsington [SP 590 040]. As on Shotover Hill, the succession is dominated by sands with ironstone, but beds of grey clay and silt occur mainly in the lower part.

Near Wheatley Windmill, a pit [SP 589 053] (Plate 15) formerly showed 3.7 m of 'rubbly iron sandstone, impure limonite and yellow ochre', including a thin bed of 'iron sandstone' with Neomiodon and Viviparus, and another with plant remains (Prestwich, 1879).

In the roadside wall at Combe Wood [SP 593 046], Pocock (1908) described flaggy ironstone with 'casts' of 'Unio', Neomiodon and rarer Viviparus. The stone probably originated from a nearby pit possibly at [SP 5929 0461] which showed ferruginous sand overlying clay, resting on probable Purbeck beds (Fitton, 1836, p. 275). Several fossiliferous specimens from this locality (BGS Collection) are well-sorted, limonite-cemented, limonite oolith grainstones of medium sand grade ((Plate 13):1 and 13:4). Some of the ooliths show a concentric structure and quartz sand nuclei; in others only voids in the cement are left. Ironstone with fossil shells has also been recorded in field brash at two other localities [SP 5885 0430]; [SP 591 038].

In a pit possibly [SP 5798 0349] north of Garsington, Fitton (1836, p. 277) recorded 2.4 m of brown, ferruginous sand with clay seams, and a basal clay bed above the Purbeck Formation, and noted a similar sequence in a second pit to the east ?[SP 5875 0356].

Great Milton-Great Haseley

Up to 9 m of Whitchurch Sand Formation is present, preserved only to the south-west of the Great Milton Fault. The lower part of the succession is dominated by clays, silty clays and silty sands, and the upper part by sands and iron-cemented sandstones.

Pale grey silty clays, silts and sandy silts dominate the formation around Little Haseley [SP 641 006], but ferruginous sandstone occurs to the north of the village. A well [SP 6367 0069] proved 2.4 m of 'whitish clay' resting on 'grey sand' (BGS archive). These beds were exposed in a ditch to the south [SP 6350 0055], which showed 0.65 m of dull pale grey, medium grained, well-sorted sand resting on 0.35 m of khaki-tinted grey, plastic clay, resting with an uneven base on the Purbeck Formation.

At Great Haseley, Fitton (1836, p. 276) recorded a quarry probably [SP 6414 0197] showing Whitchurch Sand descending into a deep 'gull' in the underlying Portland Formation. The Whitchurch Sand comprised up to 1.8 m of red and brown loam and sand with ironstone fragments resting on 2.4 m of pale grey thinly bedded clay, with plant impressions and pods of red sand. Phillips (1871) described a similar succession, probably at the same quarry. South-east of Great Haseley [SP 550 012], pale grey silty clay is dominant, although there is an abundance of sandy ironstone in places.

A former stone pit [SP 6321 0295] at Great Milton, now part of the recreation ground, formerly showed about 4.3 m of Whitchurch Sand (Pocock, 1908). It comprised brown loam, overlying buff to white, cross-bedded sand with ferruginous layers and concretions and some bands of white and ochreous clay. A basal layer of lydite pebbles and ironstone rested on an eroded surface of Portland limestone.

Thame

Around Thame, the Whitchurch Sand is generally absent, although a remnant was formerly exposed beneath Gault in a brickpit at Park Meadow Farm [SP 709 048]. Davies (1899a) recorded 'purplish-grey and yellow sands with ironstone above and below' and 'soft white clay' at this locality, and Kitchin and Pringle (1922c) described 'fine white-weathering silty clays with a layer of large bluish grey nodules'. T Codrington (MS 1865; BGS archive) recorded up to 0.8 m of 'white chalky marl' resting on a 0.15 m band of 'iron grit', the former cut out westwards by the discordant base of the Gault. Codrington's record suggests that the Whitchurch Sand rests on Kimmeridge Clay here, but a fault at the northern margin of the pit juxtaposes Portland Stone against the Whitchurch Sand. The apparent lack of the Portland Formation on the southern (downthrow) side is suggestive of movement in the opposite sense, followed by erosion, prior to deposition of the Whitchurch Sand.

Brill-Muswell Hill

The Whitchurch Sand is about 18 m thick at Brill, and comprises ferruginous sands, with beds of clay mainly in the lower part (Phillips, 1871; Pocock, 1908). Generally, it rests on the Purbeck Formation, though the latter is absent locally, due either to overstep by the Whitchurch Sand, or to cambering.

The Brill No. 1 Borehole proved Whitchurch Sand to about 15 m depth. Most of the succession comprises buff, ochreous and brown very fine- to medium-grained sand with sporadic wisps of grey and brown clay, and a few layers of limonite-cemented sandstone. Probable burrows occur at some levels, and granules and pebbles (up to 7 mm) of quartz and lydite at 9.53 m depth. Below about 8 m, the succession includes several thin beds of pale grey, khaki and ochreous mottled mudstone with listric shear surfaces. Beds at 11.03 m and 11.74 m depth, contain probable rootlets. A mudstone bed at 13.4 m depth (1.14 m thick) contains much carbonaceous plant material in its upper part. Due to core loss, the base of the Whitchurch Sand was not recovered, but the gamma-ray log suggests that the basal part of the formation is sand.

The Whitchurch Sand was formerly worked at many sites for sand, clay and ochre, often in conjunction with the underlying limestones of the Portland and Purbeck formations. Several quarries were described by Fitton (1836, p. 280); the thickest succession comprised 7.8 m of grey and ferruginous sands with clay layers, resting on Portland Stone.' A second pit probably [SP 6511 1367], exposed 2.5 m of white and yellow sand with a coarse-grained 'carstone' at the base, resting on 0.5 m of 'yellow ochre'. Below, over 1.8 m of bluish grey clay contained a fossil tree, which was 12 m long and preserved in lignite with much pyrite. A nearby pit ?[SP 6502 1366] showed 1.5 m of grey sand resting upon 1.5 m of sheared clay with layers of 'yellow ochre' in the lower part.

A section at Brill Common [SP 6559 1438] apparently showed 0.3 m of pale grey clay with plant material and possible ostracods underlain by white and buff sands with ironstone (Pringle, 1926). However, the succession was much disturbed by slipping, and it may be that the clay belonged to the Purbeck Formation, which was worked in the lower part of the pit.

An estimated 15 m of Whitchurch Sand caps Muswell Hill north-west of Brill. A number of old pits occur, but all are overgrown. In the road cutting [SP 6385 1513], 0.8 m of very pale grey and ochreous, silty mudstone with seams of soft, poorly cemented ferruginous siltstone and fine sandstone was seen, resting on 0.2 m of ochreous, fine-grained, silty sandstone with clay lenses. In the same cutting, Blake (1893) described cross-bedded, ferruginous sands with occasional sandstone 'doggers' and bands of white 'marl' and recorded 'Unio' from a nodule. Pocock (1908) noted blocks of ironstone with Neomiodon and 'Unio' in the cutting and from ploughed fields on the southeast side of Muswell Hill.

Chilton–Long Crendon

Whitchurch Sand caps outliers at Dorton Hill [SP 680 128] and Chilton [SP 684 118]. It comprises up to about 8 m of ferruginous sands with subordinate clays, which generally rest on Purbeck strata but locally overstep onto Portland Stone. South of Chilton, the formation is dominated by orange-brown and pale grey, mottled clays with interbedded ferruginous sandstone. The overlying Gault cuts down to the south, so that north of Long Crendon [SP 691 092], the Whitchurch Sand is generally no more than 2 m thick and is locally absent.

The Whitchurch Sand was formerly exposed above Purbeck limestones in the Windmill Quarry [SP 693 093] (Plate 16). The formation here has been variously classified as 'Shotover Ironsands' or Lower Greensand by previous workers. Blake (1893) noted 0.6 m of ferruginous sandstone, but other workers recorded a sequence of clays with ironstone bands or nodules, varying from 0.46 m to 0.76 m in thickness (Fitton, 1836, p. 282; Woodward, 1895, p. 220; Davies, 1899a; Jukes-Browne and Hill, 1900, p. 277); Lamplugh, 1922.

Haddenham Low–Cuddington

A thin remnant of Whitchurch Sand was formerly exposed in the railway cutting west of Haddenham [SP 734 083]. Davies (1904) recorded 'reddish sands with ironstone' here, and Ballance (1963), recorded about 1 m of chocolate coloured, clayey sand with ironstone, passing up into sandy soil.

In the extensive outcrop at Haddenham Low [SP 750 100], up to 7 m of Whitchurch Sand are present. The basal beds, up to 5 m thick, are pale grey and ochreous-yellow clays with limonitic concretions and sand and silt seams. The clays were shown as Gault on Old Series Sheets 45 and 46, but their true identity was realised by Davies (1899a). They were formerly worked for brick-making in an extensive pit [SP 7525 1018], which is now flooded. Overlying beds consist of orange, yellow and white mottled fine- to medium-grained sand, and give rise to a reddish brown sandy loam with local brash. At the southern margin of the outcrop [SP 793 093], Ballance (1963) recorded casts of ? Viviparus in ironstone [SP 7512 0924]. In the smaller outlier at Cuddington [SP 745 114], the succession is similar.

Stone–Dinton

At Stone [SP 781 124], the Whitchurch Sand reaches 9 m in thickness. The beds were formerly exploited in many pits; all are now obscured, infilled or built over. Those recorded by Fitton (1836), Morris (1867) and Davies (1899a), showed white, yellow orange and brown varicoloured sands with seams of ironstone and clay. The lower part of the sequence was dominated by grey and white sand, once in demand for glass manufacture. These purer sands were generally unconsolidated but, from one pit [SP 7845 1230], Morris (1867) recorded large, mammillated siliceous concretions ('bowel-stones'), examples of which can be seen in the boundary wall of Hartwell Park [SP 7948 1210] (Plate 18).

At Eythrope Road Sand Pit [SP 779 126] (now infilled), Ballance (1960) recorded 3.65 m of grey, white, ochreous and orange clays with seams of limonitic ironstone and ferruginous sand, overlying a further 3.65 m of white, fine-grained, cross-bedded sand with patchy iron staining and cementation. These lower sands contained sporadic seams of 'black peaty material', grey silt and pebbly sand. Ballance (1963) claimed that the white glass sands at Stone were restricted to a river channel incised into the Purbeck and Portland formations. However, this survey showed that the south-western boundary of the outcrop, regarded as a channel margin by Ballance, is in fact a fault. Nevertheless, there is evidence of minor channelling at the base of the Whitchurch Sand to the north of the pit, where the base of the formation cuts through the Purbeck Formation to rest on Portland Stone.

Davies (1899a) found a loose block of sandstone with ? Viviparus at Castle's Pit, Stone [SP 7815 1240], and Bristow and Kirkaldy (1962) recorded a 'paludinoid gastropod' from the Eythrope Road pit [SP 779 126]. Morris (1867) described 'masses of ferruginous sandstone' with 'Unio' and Viviparus near the base of the sequence near Stone Hospital probably [SP 780 122] area, and 'many specimens of Endogenites erosa'[Tempskya, a cycad] near Stone Church [SP 784 123]. He also recorded blocks of brown sandstone with 'Unio', Neomiodon and Viviparus and plant traces within the Lower Greensand at Peverel Court [SP 796 118]. These were probably derived blocks of Whitchurch Sand.

About 5 of Whitchurch Sand cap the hilltop at Dinton Castle [SP 763 114], 2 km south-west of Stone. In 1991, several excavations [SP 764 115 area] showed white, yellow and buff well-sorted fine- to medium-grained sands with minor ironstone cementation. Yellow and white mottled clays occur at the top of the local succession, and a thin bed of greenish grey and ochreous mottled clay at the base.

Bishopstone

At Bishopstone [SP 803 103], the Whitchurch Sand Formation comprises pale grey and yellow-mottled clay, silty clay and sandy clay, probably up to 5 m thick. The clays contain small, purple-black, concentric nodules of limonite and some lenses of silt and sand. North-east of Bishopstone [SP 810 112], several pieces of silicified wood were found in the soil. South-west of the village [SP 800 103], loamy ferruginous sands overlie the clays and locally cut down to the base of the formation. A roadside section [SP 8038 1034] showed 0.9 m of ochreous clays with ironstone resting on Purbeck marl and limestone (Ballance, 1960). In 1988, a small section in the opposite bank showed a sharp, possibly erosive, contact between the two formations.

Within the outlier north-east of Bishopstone [SP 810 113], the Whitchurch Sand Formation is again dominated by clays. Mapping indicates the presence of local channels several metres deep, at the base of the formation, which cut down through the Purbeck into the Portland Stone.

Ashendon–Chearsley–Waddesdon–Upper Winchendon

On the hilltop at Ashendon [SP 704 141], up to 6 m of Whitchurch Sand are preserved, comprising alternating varicoloured sands and clays with subordinate beds of black, limonite-cemented ironstone; clays are particularly well developed at the base of the sequence. The formation rests on

Purbeck beds west of Ashendon [SP 700 141], but traced northeastwards through the village, the formation cuts downwards so that at Cuckoo Pens [SP 711 147] it rests on Portland Sand. At Lower Pollicot [SP 703 131], boreholes suggest up to 4.3 m of Whitchurch Sand, dominated by brown, grey and yellow clays, silts and sands, resting on the Portland Formation. North-west of Chearsley, the Buckinghamshire Water Board Borehole 12 [SP 7118 1142] proved 7.3 m of grey to brown sandy clays with a basal reddish brown sandstone.

On Lodge Hill [SP 732 167], Windmill Hill [SP 735 158] and Waddesdon Hill [SP 758 153], up to 8 m of Whitchurch Sand may be preserved. There are no exposures, but augering and field brash indicates a succession of varicoloured sands, locally cemented by purple-black limonite. Boxstones containing loose orange sand and concentric nodules of purple limonite occur locally. Lenses of white, yellow and brown ochreous clays up to 1.5 m thick occur at the base of the formation in all three outliers [SP 7294 1663]; [SP 7334 1612]; [SP 7576 1552]. On Windmill Hill, the Whitchurch Sand was formerly worked from two small pits [SP 7363 1580]; [SP 7363 1590]. On Waddesdon Hill, yellow calcareous sandstone forms a brash at one locality [SP 7560 1526].

At Upper Winchendon, Whitchurch Sand is preserved in a small graben [SP 745 142]. The beds comprise brown sand and grey to orange silty clay with ironstone, proved by the Buckinghamshire Waterboard Borehole 49 [SP 7451 1491], to be 3 m thick. Three small outcrops of Whitchurch Sand occur on the downfaulted outlier of Beachendon Hill [SP 756 133]. The strata are principally ochreous clays with ironstone. Debris from a slurry pit [SP 7553 1329] comprised very pale greenish grey soapy clay, with rare nodules of purplish black limonitic ironstone. Some of these, when broken, showed a pale olive-coloured, probably sideritic, interior.

Lower Greensand Formation

Isolated outcrops of Lower Greensand occur throughout the central part of the district. The most extensive lies between Great Milton [SP 630 027] and Tiddington [SP 650 050], where the deposits are up to 16 m in thickness. Smaller outcrops, with thinner successions, occur at Thame [SP 710 052], Aston Sandford [SP 759 084], Ford [SP 778 095]; [SP 783 101], Bishopstone [SP 808 105]; [SP 800 111] and Hartwell [SP 796 118] (Figure 19).

The Lower Greensand is the earliest deposit of a marine transgression which, in several phases, gradually overwhelmed the land that had been created during early Cretaceous earth movements, and on which the Whitchurch Sand had been deposited. The formation rests unconformably on underlying strata, including the Whitchurch Sand, Purbeck, Portland and uppermost Kimmeridge Clay formations. It is estimated that, locally, up to 20 m of strata were eroded prior to the deposition of the Lower Greensand. Originally, the formation probably extended throughout the district, but severe erosion prior to deposition of the Gault removed it from many areas. The formation has been preserved mainly within valleys or channels cut into the underlying beds, some of which were probably controlled by earlier Cretaceous faults, as at Great Milton, Thame, and Ford (see Chapter 11). Several such pockets of Lower Greensand have been penetrated by water wells beneath the Gault (Figure 19). The present patchy outcrop distribution is thus the combined result of this early erosion, and later dissection during development of the modern landscape.

On early geological maps of the district, the Lower Greensand was not distinguished from the beds now classified as Whitchurch Sand. However, rare fossiliferous deposits show that the Lower Greensand can be separated by its lithology. It comprises reddish brown, poorly to moderately well-sorted, coarse and very coarse-grained sands and pebbly sands, which contrast with the finer sands, silts and clays of the Whitchurch Sand Formation. Grain size typically averages around 0.5 to 2 mm, and the sands commonly contain subangular to well-rounded, polished granules and small pebbles (up to 10 mm) of yellow and white quartz and quartzite and grey to black chert (lydites), like those in the Upper Kimmeridge Clay and basal Portland Formation. The strata are mainly unconsolidated, though in places they are cemented by limonite into a pebbly gritstone (car-stone). This is purplish black with a metallic lustre where fresh, but weathers to a rusty brown colour.

Fossils are generally rare in the Lower Greensand of the district, but faunas including bivalves, gastropods and brachiopods have been recorded from near Hartwell [SP 796 118] and Thame and, just outside the district, at Quainton Hill [SP 746 218] (Casey and Bristow, 1964) and Culham [SU 511 949] (Pringle, 1926). None of the fossils are closely age-diagnostic, but a rudist-dominated fauna from Thame and Quainton Hill indicates an incursion of Tethyan faunas into the British province which, elsewhere in southern England, occurs mainly within the Parahoplites nutfieldiensis Subzone ((Figure 20); Owen, 1994), the stratigraphical equivalent of the P. cunningtoni Subzone (P. nutfieldiensis Zone) of Casey (1961). Lithologically, the Lower Greensand of the district is similar to strata of P. nutfieldiensis Subzone age, overlying the famous Sponge Gravels near Faring-don [SP 298 957], some 25 km to the south-west of Oxford (Casey, 1961). It also resembles parts of the 'Upper Woburn Sands' (Silver Sands) of Leighton Buzzard, Bedfordshire (Shephard-Thorn et al., 1994), which probably extend from the P. nutfieldiensis Subzone into the latest Aptian Hypacanthoplites jacobi Subzone.

The above dating of the local Lower Greensand suggests that the initial phases of early Aptian transgression, which flooded most of the Wealden basin, failed to reach this district. However, further transgression during P. nutfieldiensis Subzone times created a narrow seaway connecting the Wealden area with the Boreal Ocean to the north (Casey, 1961; Narayan, 1963). The Lower Greensand, deposited within this seaway, is preserved in a north-easterly trending belt of outliers between Wiltshire and Bedfordshire. The 'Upper Woburn Sands' have been interpreted as an offshore, subtidal sandwave deposit, and the deposits of this district are probably part of this same complex (Bridges, 1982), and were probably originally contiguous with the Folkestone Beds of south-east England.

Details

Great Milton–Tiddington

The Lower Greensand reaches a maximum thickness of about 16 m at Milton Common [SP 653 033], but is cut out beneath the Gault to the north-east near Tiddington [SP 667 047], and to the south-west near Great Milton [SP 635 030]. The formation straddles the Great Milton Fault, which shows evidence of movement, both before and after the deposition of the Lower Greensand (see Chapter Eleven).

Lower Greensand was exposed in cuttings up to 8 m deep during construction of the M40 motorway. The deposits seen were entirely unconsolidated, but site investigation boreholes indicate the presence of weakly cemented patches. Above an uneven surface of Kimmeridge Clay, the strata comprise medium- to coarse-grained sand with varying proportions of small pebbles up to 8 mm diameter. Throughout most of the succession bedding is massive, but within the basal 3 m, Hesselbo et al. (1990) recorded cross-stratified sets up to 1 m thick, with foresets dipping to the south-east. They also recorded clay drapes and burrows on some bedding surfaces.

Thame–Haddenham

Lower Greensand crops out on the southern outskirts of Thame [SP 710 051], near the former railway station. In 1991, a trench [SP 7114 0511] produced debris of brown, oxidised limonitic gritty and pebbly sandstone, and also pieces of less altered sandstone with a cream-coloured, ferroan calcite cement and a proportion of rounded limonitic grains including ooliths. One large block of well-cemented, calcareous, pebbly sandstone was also recovered; though essentially nonferruginous, it was otherwise similar to the other material. It contained small oyster shells and moulds of other bivalves, gastropods and possible corals, and has yielded nannofossils confirming a Cretaceous age.

In 1963, excavations at the British Oxygen Company site about 1 km to the south-east [SP 7215 0495], proved Lower Greensand at shallow depth beneath Gault. The Lower Greensand, probably a metre or two in thickness, rested unconformably partly on the Portland Formation, and partly on Kimmeridge Clay (C R Bristow, (BGS), personal communication, 1991). Specimens (BGS Collections) comprise brown-weathered, limonite-cemented, medium- to coarse-grained, cavernous sandstone with abundant rounded to subangular quartz and quartzite granules and pebbles. Many of the voids may result from solution of limestone clasts. The material contains abundant empty moulds of fossils, including gastropods and bivalves. The latter are dominated by monopleurid rudists, including Toucasia lonsdalei.

During construction of the railway cutting south of Thame in about 1865, T Codrington (MS, BGS archive) recorded Lower Greensand beneath Gault on the western (upthrow) side of a fault [SP 707 053], and noted about 3 m of ferruginous sand, with cemented pebbly bands at the top and bottom, rest ing on sands of the Kimmeridge Clay Formation. In the cutting 1.5 km_ farther west [SP 693 055], the Lower Greensand was reduced to a thin band of ferruginous pebbly sandstone. He also recorded Lower Greensand in a quarry near Kingsey, probably south of the village [SP 746 064] (see Ballance, 1963). Between the Gault and the Portland Stone, the section showed 1.2 m of coarse red sand 'full of teeth and scales of Lepidotus [Lepidotes] mantelli'.To the north of this locality, no evidence was found for the extensive outcrops of Lower Greensand between Tythrope Park [SP 732 070] and Aston Sandford [SP 755 073] shown by Ballance (1963, fig.7); this tract is now mapped as Gault.

Several water wells south-east of Thame prove that Lower Greensand is present locally beneath the Gault (Figure 19). The maximum thickness recorded is 6.4 m at Emmington [SP 7408 0243].

Aston Sandford Ford

Small areas of Lower Greensand at the margin of the Gault outcrop near Aston Sandford [SP 759 084] and Ford [SP 778 095]; [SP 783 101] infill shallow channels cut into the underlying Jurassic beds. Additional remnants of Lower Greensand may be present at depth beneath the Gault cover, for example at Pasture Farm, where water from a borehole [SP 7807 0668] is reported often to contain 'red gritty sand'.

At Aston Sandford, the Lower Greensand comprises 2.5 m of unconsolidated, coarse-grained, reddish brown, pebbly sand with small fragments of brown, limonitic ironstone at the base. At Ford, Ballance (1960) recorded brown pebbly sand at the roadside pond [SP 7989 0951]. Davies' (1.899a) record of pebbly ferruginous sands 'near Bridgefoot Farm' [SP 7761 0965] probably refers to this outcrop.

Bishopstone–Hartwell

Small outliers of Lower Greensand occur at Peverel Court [SP 796 118] and Curzeley Hill [SP 800 111], and a more extensive tract [SP 809 104] occurs at the margin of the Gault outcrop just east of Bishopstone. At each locality, the strata comprise reddish brown, coarse-grained, pebbly sands, locally with a layer of limonitic ironstone clasts at the base. At Curzeley Hill, Ballance (1960) proved 4.6 m by augering. The deposits are mainly unconsolidated, though rare pieces of black, pebbly carstone occur. The sands were formerly worked from several pits [SP 8115 1045]; [SP 8090 1050]; [SP 7993 1109]; [SP 7995 1102]; [SP 7955 1176].

In each outcrop, the Lower Greensand rests on an uneven, erosional surface. At Bishopstone [SP 8095 1072] and Curzeley Hill, it transgresses the Whitchurch Sand to rest on the underlying Purbeck Formation. At Peverel Court, the Lower Greensand channels down about 2 m below the top of adjacent Purbeck limestones in a degraded sand pit [SP 7955 1176], and Ballance (1960) proved a further 2.1 m of gritty sand beneath the floor of the pit. Thus the Lower Greensand infills a depression cut at least 4 m into the Jurassic beds here. At this pit, Morris (1867) recorded 1.8 m of coarse-grained, pebbly, ferruginous sands resting on an irregular surface of Portland Stone. The sands contained fossil moulds including 'Exogyra sinuata' [Aetostreon latissimum] as well as many other bivalves, brachiopods, foraminifera, corals, bryozoa and large pieces of coniferous wood. Woodward (1891) recorded 'casts of Lima, Ostrea, Pecten and Rhynchonella from gritty and ferruginous beds between Hartwell and Bishopstone'. These were in the Hartwell House collection of Dr Lee (BGS Palaeontological Collections Register F, p. 141), and although Ballance (1963) suggested that they may have come from Curzeley Hill, more probably they too were from the Peverel Court pit, which belonged to Lee.

Stone–Waddesdon

Davies (1899a) incorrectly grouped all of the sand deposits of the Haddenham Low and Stone area as 'Bishopstone Beds' (i.e. Lower Greensand); the majority of these deposits are now classified as Whitchurch Sand.

Smyth (1864, plate 4) recognised the presence of Lower Greensand at Peverel Court and Bishopstone (see above), and also indicated a large outcrop above the Whitchurch Sand north and west of Stone Church [SP 780 124 area], with a small outlier at Round Hill [SP 788 123] and it is possible that there may be remnants of Lower Greensand in ground now built over. For example, at Stone Farm Sand Pit [SP 7863 1242], Davies (1899a) recorded a band of hard, ferruginous, small-pebble conglomerate at the top of the section. An ancient trough in Stone churchyard [SP 7841 1222], carved out of a single block of pebbly carstone, may have come from such a local pit. Debris of brown and black, pebbly carstone in the soil on the highest parts of the Whitchurch Sand outliers at Haddenham Low [SP 751 104] and Dinton Castle [SP 762 113] may also be remnants of Lower Greensand. Similarly, head deposits on Windmill Hill [SP 7334 1612] near Waddesdon, include coarse pebbly sandstones derived from the densely wooded hilltop. Lower Greensand may also cap Lodge Hill, on which Waddesdon Manor is built; post-holes around the deer-pen [SP 7297 1646] proved coarse-grained, red sand.

Gault Formation

The outcrop of the Gault forms a tract of low relief, 4 to 5 km across, parallel to the Chalk escarpment, and small outliers cap hills south of Garsington [SP 589 001], between Long Crendon and Chilton [SP 692 0097] and at Weedon [SP 818 182] (Figure 1).

A period of erosion occurred between deposition of the Lower Greensand and Gault, so that the latter rests unconformably on underlying beds. Generally, the contact is of low relief but, in a few places, channels up to 10 m deep occur. Over considerable areas, Lower Greensand, Whitchurch Sand and Purbeck strata were removed, and even deeper downcutting, into the Kimmeridge Clay, occurred near Thame and north-eastwards from Aylesbury, just outside the district. During the transgression which followed this erosion, Gault clays were eventually deposited over the whole of the London Platform.

The Monks Risborough Pumping Station Borehole [SP 8142 0461] apparently proved 89.6 m of Gault, the maximum known in the district, although there is some uncertainty in the classification. However, partial thicknesses in boreholes at Emmington [SP 741 025] and Sydenham [SP 730 018] suggest totals of only 70 m, and near Tetsworth [SP 687 017] it may be as little as 60 m. This gradual south-westward thinning is in part due to lateral passage of the uppermost beds into Upper Greensand.

Large areas of the Gault outcrop are concealed by head and other drift deposits, and even those areas shown as drift-free on the geological map are commonly covered by a thin layer of hill-wash. Elsewhere, the outcrop is characterised by dark grey clayey soils, with an underlying subsoil of pale or medium grey clay, which commonly contains abundant 'race' (secondary nodules of calcium carbonate). In most areas, the base of the Gault is readily recognised due to the lithological contrast with the underlying strata. In localities where it rests on Kimmeridge Clay, the Gault can be distinguished by its paler colour and the presence of phosphatic fragments.

Most of the Gault succession is dominated by grey mudstones and silty mudstones; in general, the strata become paler and more calcareous upwards. Greyish buff, phosphatic nodules, grown in situ commonly around fossils or burrows, occur at many levels. Additionally, erosive nonsequences are commonly marked by bands of bluish black, commonly polished, phosphatic pebbles. Most of these are nodules eroded from underlying beds, which, during reworking, have undergone abrasion and further phosphatisation. They commonly include fossil fragments, which may relate to strata considerably older than the mudstones in which they lie. Thus, the distinction between autochthonous nodules and allochthonous pebbles is critical, although in many accounts of the Gault, both types are indiscriminately termed nodules or 'coprolites'.

The Gault contains a fauna dominated by bivalves and ammonites, but, due to weathering, fossils are generally rare on the outcrop, except for phosphatised material as described above, and sporadic small belemnites (in particular Neohibolites). The standard biozonation of the Gault is based on ammonites (Figure 20). Certain bivalves are also of stratigraphical value; for example the concentrically ribbed inoceramid Birostrina concentrica is replaced by the radially ribbed and sulcate form B. sulcata at the base of the Mortoniceras inflatum Zone, but reverts to the B. concentrica form at the base of the Hysteroceras varicosum Subzone. Aucellina, present at some horizons in the Callihoplites auritus Subzone, and common above the base of the Stoliczkaia dispar Zone, exhibits changes in shell microsculpture, enabling subdivision of the higher parts of the Gault (Morter and Wood, 1983).

Detailed stratigraphical study of the Gault was first undertaken by De Rance (1868) who divided the succession at Folkestone, Kent, into units based on combined lithological and faunal characters. This concept was extended by Price (1874) and later by Jukes-Browne and Hill (1900), whose thirteen 'beds' (I–XIII) still form the basis for subdivision and regional correlation. More recently, Gallois and Morter (1982) divided the Gault of East Anglia into nineteen beds. In all these schemes, many of the bed boundaries are defined by nonsequences which, in some cases, correspond with ammonite biozone (or subzone) boundaries. This is the case with the boundary between the two major lithostratigraphical subdivisions of Lower and Upper Gault, which coincides with the base of the M. inflatum Zone, the basal zone of the Upper Albian.

The earliest Gault strata of the region are the 'Junction Beds' of Leighton Buzzard (Owen, 1972; Shephard-Thorn et al., 1994). These are thin, condensed beds of very varied lithology (sand, clay, limestone and phosphatic nodule beds), with many internal nonsequences, and are of early Albian age (Leymeriella tardefurcata Zone and Douvilleiceras mammillatum Superzone). Probable remnants of these 'Junction Beds' occur at Long Crendon (where fossils indicate the Leymeriella regularis Sub-zone), and Great Milton.

Fossils from several localities show that generally, the basal Gault of the district belongs to the Middle Albian Hoplites dentatus Zone; this accounts for the greater part of the local Lower Gault, which is probably about 12 m thick near Thame. Within the upper part of the Lower

Gault, the Euhoplites loricatus Zone may be thinly represented near Thame and Haddenham, but there is no evidence of the succeeding Euhoplites lautus Zone; it is probably cut out beneath an erosive nonsequence which marks the base of the Upper Gault throughout southern England. The Lower Gault succession is dominated by dark grey mudstones, with streaks or laminae of glauconitic silt in the basal 2 to 3 m. In many places, the basal stratum is very sandy, with coarse sand grains and pebbles of quartz, presumably derived from the Lower Greensand, or more locally a layer of limonitic ironstone fragments, probably derived from the Whitchurch Sand.

The basal beds of the Upper Gault of the district belong to the Dipoloceras cristatum Subzone, but these beds are no more than a few metres thick. About 15 to 20 m above the base of the Gault, a well-developed bed of phosphatic pebbles is known from several localities in the eastern part of the district, notably near Ford [SP 790 094], where it was formerly worked for fertiliser manufacture (see Chapter 12). It contains derived fossils which, at Leighton Buzzard, are found in two separate beds, namely the Euhoplites inornatus Band at the base of the Hysteroceras orbignyi Subzone, and the varicosum pebble bed' at the base of the H. varicosum Subzone (see Owen, 1972). In part of the district, therefore, the erosion surface beneath the varicosum pebble bed has cut down to the level of the inornatus Band, removing the H. orbignyi Zone. However, near Tetsworth [SP 678 021], about 1 m of H. orbignyi Subzone strata have been recorded above the inornatus Band.

Due to lack of exposure, little detail is known of the overlying and greater part of the Gault. However, in the topmost few metres, augering reveals very pale, greenish grey, marly silts with seams of very fine-grained, greyish buff sand, and the boundary with the Upper Greensand is gradational. There is evidence of marked facies variation across the district at this level (Figure 21); near Tetsworth, the Upper Greensand extends down into the uppermost part of the M. inflatum Zone, brit at Long-wick [SP 783 056], only 10 km to the north-east, the upper 25 m or so of Gault lie within the succeeding Stoliczkaia dispar Zone. This rapid lateral facies and thickness change may imply tectonic control on sedimentation, and it is possible that structures such as the Wheatley Fault Zone, known to have moved in earlier Cretaceous times (see Chapter 11), remained active during the Upper Albian.

Further evidence of a north-eastward expansion of the uppermost Gault comes from Puttenham [SP 886 141], about 6 km east of Aylesbury, where a phosphatic nodule or pebble bed (or beds) occurs 45 to 50 m above the base of the formation (Jukes-Browne, 1875). Jukes-Browne and Hill (1900) considered the bed to lie at the base of the Upper Gault, but their fossil lists suggest that more probably, it lies at the base of the S. dispar Zone (Mortoniceras rostratum Subzone) or within the underlying C. auritus Subzone. If so, as the total thickness of the Gault is about 90 m hereabouts (see above), then Gault of S. dispar Zone age may be as much as 40 m thick (Figure 21).

Details

Great Milton

In 1990, Lower Gault was exposed during construction of the M40 Motorway cutting north of Chilworth Farm [SP 6350 0422] to [SP 6368 0417]. The basal 1 m of strata comprises medium bluish grey and ochreous mottled sandy clay, containing coarse sand and pebbles probably derived from the underlying Lower Greensand. The clay contains small pockets of dark green, highly glauconitic sand, and sporadic buff to dark grey, concentric phosphatic nodules. Overlying beds, about 4 m thick, comprise dark bluish grey, moderately fissile mudstone with some pyritic and glauconitic lenses, and phosphatic nodules. Fossils from between 2 and 5 m above the base of the Gault include buff-coloured, phosphatised Hoplites (H.) dentatus and ?H. (H.) spathi, indicative of the Hoplites spathi Subzone. Other fossils, mostly crushed and preserved as moulds with some pink, aragonitic shell material, include gastropods (cf. 'Solarium', cf. Trochus), bivalves (Birostrina concentrica, ?Nucula (Pectinucula), oysters) and belemnites (Neohibolites).

In the cutting to the north-west, the Lower Greensand contains several 'gulls' within which the Gault had been down-folded. A typical section [SP 6363 0439] showed:

Beds 3 to 5 resemble the basal Gault as described above, but the underlying Beds 1 and 2 are tentatively equated with the 'Junction Beds' of Leighton Buzzard. The purple colouration of Bed 2 is suggestive of subaerial weathering. In a similar section nearby [SP 6339 0428], rare, poorly preserved, indeterminate bivalves were present in Bed 1.

Sections exposed in 1972, during construction of the M40 near Tetsworth, were recorded by Dr H G Owen (British Museum, Natural History) who supplied the following information (Owen, personal communication, 1992). A large-diameter boring at Manor Farm bridge [SP 6782 0213] proved:

Excavations for a bridge 1.5 km to the south-east [SP 6907 0116] showed pale to mid-grey clay with scattered phosphatic nodules and large pyrite-infilled burrows. Shell fragments include Birostrina cf. concentrica, indicating the H. varicosum Subzone. Field relationships suggest that this horizon may be as little as 15 m below the base of the Upper Greensand, although there may be structural complications hereabouts.

Thame–Haddenham

Lower Gault was formerly worked for brickclay at several sites on the outskirts of Thame. At a pit 'south-east of Thame', Jukes-Browne and Hill (1900) recorded 6 m of 'dark grey micaceous clay containing a few scattered phosphatic nodules' and Birostrina concentrica. A borehole proved a further 6 m of 'dark, tough clay' below, resting on sand, possibly Lower Greensand. The site was possibly that near Towersey [SP 728 055], or more likely, the infilled Priest End brick pit [SP 689 054] (actually south-west of Thame), where, Spath (1943, p. 746) noted 'at least 40 ft' (12.2 m) of Lower Gault with common ammonites indicating the H. spathi Subzone (H. dentatus Zone) and possibly the succeeding Anahoplites internedius Subzone (Euhoplites loricatus Zone).

At another pit 'south of Thame', probably that [SP 707 048] near Thame Park, Jukes-Browne and Hill (1900) recorded 'Ammonites interruptus' [= Hoplites dentatus of Spath] and Belemnites [Neohibolites] minimus'. Davies (1899b) also listed foraminifera. The pit is now totally obscured, but sketches by T Codrington (MS, 1865, BGS archive), indicate Gault resting unconformably on the underlying strata, with a bed up to 0.6 m thick of 'rusty concretions' and lydites' at the base. Specimens of this material (BGS Palaeontological Collections) include fish teeth (Lepidotes).

Black phosphatic internal moulds of the ammonites Euhoplites aff. meandrinus and Dimorphoplites aff. niobe in the Buckinghamshire County Museum suggest the presence of a pebble bed of late E. loricatus Zone age within the district. It may correspond with a level in the upper part of Bed IV of the Folkestone succession, in the Euhoplites meandrinus Subzone (Owen, 1971, p. 65). The specimens are labelled 'Haddenham' but, as Haddenham [SP 741 087] is situated on the Portland outcrop, Owen (1971) suggested that the specimens may have come from Haddenham in Cambridgeshire [TL 46 75], which is on Gault.

Long Crendon

Three small outliers of Gault cap the hilltops between Chilton and Long Crendon [SP 694 111]; [SP 698 107]; [SP 693 097]. In the largest and southernmost outcrop, 25 m of Gault are preserved, of which the lowest beds were formerly exposed in quarries exploiting the underlying Portland and Purbeck limestones. In 1885, the Windmill Quarry [SP 693 093] (Plate 16) showed 3.5 m of 'tough grey clay, slightly micaceous and showing layers of darker and lighter grey' (Jukes-Browne and Hill, 1900) from which Davies (1899a) recorded Birostrina concentrica and Neohibolites minimus. Although no ammonites were found (Davies, 1899b), B. concentrica suggests Lower Gault (Owen, 1971), despite statements by Kitchin and Pringle (1921; 1922a 1922b) that the material was Upper Gault.

Beneath the Gault at this locality, Jukes-Browne and Hill (1900) recorded up to 0.46 m of brown, ferruginous, pebbly sandstone and, in places 'lumps of calcareous stone'. The latter is evidently a very local development as it was not mentioned by Fitton (1836), Blake (1893) or Woodward (1895), but was again encountered by Lamplugh (1922) in a trench, and may also be represented by the 'ironstone concretions containing calcite' recorded by Davies (1899a). Fossils from the bed are listed by Lamplugh (1922), who suggested correlation with the Shenley Limestone of Leighton Buzzard. This prompted an acrimonious response from Kitchin and Pringle (1922b); they suggested that the fossils had been obtained from an erratic in drift, because they believed the Leighton Buzzard sequence to be inverted, and the Shenley Limestone to be of Cenomanian age (Kitchin and Pringle, 1920). Casey (1961), however, reaffirmed the presence of Shenley Limestone at Long Crendon.

The fossils from the bed (BGS Collection) have been reassessed by Mr A A Morter (BGS). Mainly brachiopods, they include several specimens of Rectithyris shenleyensis and also Gemmarcula menardi var. pterygotos, Capillithyris diversa, Terebrirostra arduennensis, Burrirhynchia shenleyertsis, the bivalve Septifer cf. lineatus, an echinoid spine and serpulids. The fauna includes many forms found in the type Shenley Limestone and is probably likewise of L. regularis Subzone age.

Shell and auger boreholes, up to 11.4 m deep, drilled on the site of the underground reservoir [SP 691 098] 1 km north of Long Crendon, proved pale grey clays, becoming somewhat darker and including more silty mudstones in the lower part. Fossils indicate that most of the succession is Upper Gault. Strata assigned to the D. cristatum Subzone are 2 to 3 m thick and have yielded Birostrina sulcata, Entolium and Neohibolites. The overlying beds of the H. orbignyi Subzone are probably up to 5 m thick. They contain numerous B. sulcata (including B. sulcata subsulcata) and also Hysteroceras orbignyi, ?Beudanticeras, Euhoplites inornatus?, E. sublautus monacantha, Hamites, Neohibolites, Nucula (Pectinucula), Pinna, Dentalium and fish debris.

A phosphatic pebble bed, probably that at the base of the H. varicosum Subzone, is well developed in several of the boreholes, and has yielded an assortment of derived fossils. It has been traced around the hill top on the basis of a minor topographic feature and debris in the soil for example [SP 688 109], and apparently lies about 15 m above the base of the Gault. A second pebble bed, about 4 m higher in the succession, has been recognised in two of the boreholes. Common Birostrina concentrica 5 or 6 m below the varicosum pebble bed in one borehole, indicate Lower Gault which, therefore, can be no more than about 10 m thick at this locality.

Ford

At Ford, the base of the Gault is highly transgressive, cutting across Lower Greensand, Purbeck Formation and Portland Stone within a distance of a few hundred metres. East of Bridgefoot Farm [SP 781 097], the Gault forms ground of subdued relief, topographically lower than the outcrop of Portland Stone immediately to the west. This appears to be the combined result of channelling and tectonics (see Chapter 11).

About 18 m above the local base of the Gault, a band of phosphatic pebbles 'coprolites' was formerly worked as a fertiliser near Moreton Farm [SP 790 094], 1 km east of Ford. The pebbles, common in the soil hereabouts, are typically bluish grey to black and irregularly shaped; many are internal moulds of ammonite segments or bivalves. Most show evidence of abrasion, and many are bored or show signs of encrusting epifauna (for example, Atreta). Jukes-Browne and Hill (1900) gave a few details recorded in 1876; the main coprolite seam was 8 to 10 cm thick, but a second, apparently lenticular seam of smaller nodules occurred higher in the sequence. Hollis and Neaverson (1921) estimated that about 1 m of material had been worked. They listed foraminifera extracted from the residues, which consisted chiefly of quartz sand, together with grains of glauconite and limonite, ostracods, shell fragments, sponge spicules, echinoid spines and fish teeth.

The coprolite workings are now ploughed over, but fossils collected from the soil in 1989 include Neohibolites minimus, and reworked Birostrina sulcata and Hysteroceras orbignyi from the H. orbignyi Subzone. Additional fossils listed by Jukes-Browne and Hill (1900) include 'Ammonites cristatus' [Dipoloceras cristatum] which may indicate reworking from the underlying D. cristatum Subzone. Arkell (1947a) stated that the nodule bed lay at the base of the Upper Gault, but there is no evidence that the nodule bed rests directly on Lower Gault.

Longwick

On the basis of regional dip, samples from an excavation [SP 7833 0557] near Roundhill Farm, Longwick, (collected in 1966; BGS Palaeontological Collection), probably came from a level about 25 m below the base of the Upper Greensand. The material comprises pale grey clay, with some small phosphatic clasts, abundant shell debris and common Aucellina. These are mainly A. gryphaeoides with the smooth microsculpture (Morter and Wood, 1983) characteristic of specimens from Bed 17 of Gallois and Morter (1982), at the base of the S. dispar Zone.

Aylesbury

The basal 1.8 m of the Gault was formerly exposed in the Walton railway cutting [SP 8242 1283] 400 m beyond the margin of the district. It contained coprolites and quartzite pebbles at the base (A Strahan, MS field map Buckinghamshire 28SE, 1891). A short distance to the south [SP 824 124], a trench section showed the lowest 3.5 m of the Gault, with a shelly bed about 2 m above the base containing fossils (including Hoplites dentatus and Birostrina concentrica) indicating an horizon in the lower part of the H. spathi Subzone (Owen, 1971).

In 1988, debris of pale grey blocky mudstone and bluish grey shaly mudstone was excavated from drains [SP 8194 1128] 3 m deep at Stoke Farm. Some blocks include a thin mudstone bed containing black phosphatic pebbles, with a strikingly interburrowed base. Amongst the pebbles, reworked fossils include Birostrina sulcata and Hysteroceras, the former suggesting derivation from the H. orbignyi Subzone. A similar reworked phosphatic fauna from a trench at Stoke Mandeville Hospital [SP 828 117] contained Hysteroceras varicosum, Prohysteroceras (Goodhallites) goodhalli, abundant Birostrina sulcata and rare B. concentrica (Dr M J Oates (British Gas), personal communication, 1991). Comparable faunas from nearby trenches [SP 828 117]; [SP 827 117], and from a locality 1.4 km to the north-east [SP 832 130], are held at the Buckinghamshire County Museum.

Together, these collections indicate the presence of a composite pebble bed of H. varicosum Subzone age which, as near Ford (see above), probably rests nonsequentially on basal H. orbignyi, or possibly on uppermost D. cristatum Subzone strata. The bed is estimated to lie about 15 m above the local base of the Gault. Wright and Wright (1939) also described fossils from this horizon, collected from just outside the district. Their material was obtained from trench spoil at unspecified localities along the A41 from 0 to 3.2 km north-west of Aston Clinton [SP 880 120] (Wright and Wright, 1939, p. 115), and not from Aston Clinton itself as stated by Spath (1943) and others; field relationships indicate that strata there are much higher in the Gault.

Upper Greensand Formation

The Upper Greensand crops out as a narrow strip in the south-eastern part of the district, forming a prominent scarp and shelf at the foot of the main Chilterns escarpment. A few small outliers occur between Tetsworth [SP 687 017] and Sydenham [SP 730 018]. The formation consists predominantly of uniform, very fine-grained sandstones and siltstones, which give rise to pale grey, loamy soils with a patchy, but locally abundant brash. More clayey areas in some fields suggest the presence of thin beds of mudstone. The rock is pale bluish grey and calcite cemented when fresh, but on weathering it decalcifies and discolours so that most of the rock in field brash or from shallow excavations is soft and of a very pale buff or whitish colour, resembling chalk, particularly when dry. Some beds contain scattered fine flakes of mica, or, particularly near the top of the formation, minute dark grains of glauconite. The rock generally lacks obvious bedding structure, probably in part because of intensive bioturbation; in places, individual burrows may be seen as colour mottles or as three-dimensional casts. Rarely, poorly developed lamination is preserved, in some cases picked out by wisps of grey clay.

The basal junction of the Upper Greensand with the Gault is transitional (see above), and for mapping purposes, the base of the formation has been drawn at the concave break of slope at the foot of the scarp. This commonly coincides with a groundwater seepage line, and springs occur at a number of localities.

The average thickness of the Upper Greensand in the district is 10 m, but there is a pronounced north-eastward thinning along the outcrop. At Watlington [SP 6845 9375], 5 km south of the district, a borehole penetrated 20 m of Upper Greensand without reaching the Gault, and near Tetsworth [SP 698 005], on the southern margin of the district, it is about 16 m thick. To the east at Monks Risborough Pumping Station, it thins to only 6.7 m and this trend continues north-eastwards (Sherlock, 1922) with the Upper Greensand disappearing entirely within the Leighton Buzzard district (Shephard-Thorn et al., 1994). Thinning of the Upper Greensand is accompanied, locally at least, by a concomitant thickening of the Gault, suggesting lateral passage of the lower beds of the Upper Greensand into Gault facies (Figure 21); the faunal evidence supports this hypothesis (see below, and Gault above). In addition, the thinning of the Upper Greensand may be accentuated by erosive downcutting at the base of the Lower Chalk (Chapter Nine). Because of the time-equivalence of the Upper Greensand with parts of the Upper Gault elsewhere, Jukes-Browne and Hill (1900) introduced the term 'Selbornian' to include beds of Gault and Upper Greensand age. This term appears in a few Geological Survey memoirs (for example Sherlock, 1922), but was never widely adopted.

Fossils are generally rare in the Upper Greensand, probably due to dissolution of aragonite shells, although a few beds contain small white calcitic bivalves, for example Entolium and Aucellina. Brown phosphatic fragments, mainly pieces of arthropod exoskeleton, also occur sporadically. At certain horizons, moulds of ammonites can be found; ammonites indicative of both sub-zones of the S. dispar Zone have been collected near Chinnor, where fossils from the underlying Gault and overlying Glauconitic Marl show that the Upper Greensand of the district is entirely of S. dispar Zone age. However, the thicker Upper Greensand succession near Tetsworth extends down into the upper part of the C. auritus Subzone of the preceding M. inflatum Zone (Figure 20), (Figure 21).

Details

Tetsworth–Postcombe

Partial thicknesses proved in boreholes along the line of the M40 motorway south-east of Tetsworth suggest a total of at least 16 m of Upper Greensand. The following section, in the middle part of the formation, was recorded by Dr H G Owen in the motorway cutting [SP 7022 0015] to [SP 6070 0070] during its construction in 1972:

The lower part of Bed 1 yielded forms of Callihoplites, suggesting a late C. auritus Subzone age. The Shelly seam in Bed 3 contained large Mortoniceras rostratum and Callihoplites, indicating the M. rostratum Subzone. The layer of phosphatic nodules in Bed 1 may represent an erosional event during C. auritus Subzone times, which is widely known elsewhere (Owen, personal communication, 1992).

About 1 km to the south-east, just beyond the margin of the district, the uppermost part of the formation was exposed during construction of the Salt Lane underpass at Postcombe [SU 7075 9930]. Beneath the sharp base of the Glauconitic Marl (Chapter Nine), the section showed 3.06 m of fawn-grey to buff, poorly bedded sandstone and silty sandstone, mostly rather soft but with some harder bands. A 0.2 m-thick bed of tough, blocky sandstone 1.5 m below the top yielded a Mortoniceras perinflatum Subzone fauna. The underlying sandstone contained calcitic fossils, and is probably of M. rostratum Sub-zone age, and may equate with the upper part of Bed 3 of the section recorded above (Owen, personal communication, 1992). Material from these beds, collected in 1925 from roadside trenches in the vicinity [SU 709 995], is held in the BGS Collections. The fauna includes ammonites (Pleurohoplites cf. subvarians, Callihoplites aff. vraconensis, with some bivalves (notably Aucellina gryphaeoides) and echinoids (for example Holaster).

Both M. rostratum and M. perinflatum Subzones are represented, with the latter best represented.

Springs and seepage lines help to define the base of the Upper Greensand at Chalford [SP 7183 0084] and in valleys north of Postcombe [SP 7233 0008]; [SP 7104 0023]. Higher in the formation, a succession of minor features south of Copcourt and Chalford [SP 707 006] and [SP 716 004] areas] are formed by beds of fine-grained sandstone separated by softer beds. Poorly preserved moulds of bivalves including Aucellina ex gr. uerpmanni, A. ex gr. gryphaeoides and Syncyclonema sp., were collected from field brash north of Lower Farm, Postcombe [SP 7095 0027].

Chinnor–Meadle

The Upper Greensand forms a prominent scarp feature, particularly well displayed to the south of the Lower Icknield Way (B4009) between Pitch Green and Longwick. Several springs rise from the base of the formation, most notably at Henton [SP 7607 0199]; [SP 7672 0228], Bledlow [SP 7787 0250]; [SP 7805 0267] and Meadle e.g. [SP 8080 0592]. Typically, these springs lie at the head of steep-sided valleys cut back into the Upper Greensand scarp; these are probably the result of 'spring sapping'. The streams issuing from the springs are used for watercress cultivation at Bledlow [SP 7774 0263]; [SP 7802 0267] and there are abandoned cress beds elsewhere.

Fossils collected from field brash at a level 3 to 4 m above the local base of the Upper Greensand west of Upper Farm, Henton [SP 7592 0214], include a mould of the ammonite Anisoceras saussureanum, indicating the S. dispar Zone. Debris from a nearby electricity pylon [SP 7596 0219], from slightly higher in the succession, yielded moulds of Aucellina ex gr. uerpmanni, the ammonites Anahoplites (Leptohoplites) falcoides, Arrhaphoceras cf. precoupei, Idiohamites elegantulus elegantulus and fish debris. The fauna indicates the M. perinflatum Subzone. Jukes-Browne and Hill (1900) recorded 'Ammonites auritus (var. catillus) [possibly Callihoplites auritus] Am. planulatus and Avicula [Aucellina] gryphaeaides' from a well sunk through the Upper Greensand at Chinnor.

The Upper Greensand was formerly exposed in the railway cutting at Great Kimble [SP 8213 0607] to [SP 8234 0656], just beyond the eastern margin of the district. Here Sherlock (1922) recorded 4.8 m of flaggy calcareous sandstone interbedded with layers of sandy marl.

Chapter 9 Upper Cretaceous: Chalk Group

The Chalk Group crops out in the south-east of the district, forming the escarpment of the Chiltern Hills; about 165 m of strata are represented. As elsewhere in England, apart from the basal beds, the succession consists almost entirely of chalk. Diagenesis has produced a variety of fabrics and textures, but all chalks are essentially micrites (lime mudstones) made up predominantly of microscopic fragments and whole skeletal plates (coccoliths) of planktonic algae. Coarser material (silt to sand grade) is mostly comminuted shells of various organisms. The lower part of the sequence contains a significant component of terrigenous clay, silt and sand, but the purity of the chalks increases up sequence, indicating gradual recession of shorelines.

The apparent uniformity of the Chalk Group hindered early attempts at subdivision; the first schemes merely recognised a lower part without flints, and an upper part with flints. Later, Jukes-Browne and Hill (1903, 1904) introduced the division into Lower, Middle and Upper Chalk, based principally on the recognition of the Melbourn Rock (at the base of the Middle Chalk) and the Chalk Rock (at the base of the Upper Chalk). In south-eastern and eastern England, the Chalk Rock loses its distinctive character, and some modern lithostratigraphical schemes combine the Middle and Upper Chalk into a single formation (for example the White Chalk Formation of Sussex; Mortimore, 1986). However, in the Chilterns, and certainly within this district, the traditional tripartite scheme remains appropriate and forms the basis of the classification used in this account (Figure 22), in which the Lower Chalk, Middle Chalk and Upper Chalk are regarded as lithostratigraphical formations. The boundaries between these formations are close to the chronostratigraphical boundaries between the Cenomanian, Turonian and Coniacian stages respectively (Figure 22), although the latter are defined on the basis of faunas.

The fauna of the Chalk is diverse and indicative of fully marine conditions. However, recognisable fossils are often rare, in part due to early diagenetic solution of aragonitic shell material (such as that of ammonites, gastropods and most bivalves). Consequently, in much of the succession, the only fossils to be found are those with calcitic shells, for example brachiopods, inoceramid bivalves, oysters and echinoderms. The morphology of some of the benthonic fauna, for example the expanded Gryphaea-like left valves of some inoceramids, suggests adaptation to a sea floor with a very soft consistency. Hancock (1975) concluded that most chalks were deposited in depths of between 100 and 600 m, but some winnowed, shelly beds, and condensed beds (for example the Chalk Rock) suggest shallower conditions, or at least increased current activity. Epifauna characteristic of hard substrates is rare except at such horizons.

Global correlation within the Upper Cretaceous utilises a standard ammonite zonation which forms the basis of the internationally recognised scheme of chronostratigraphical stages and substages. However, as ammonites are rare in much of the English Chalk successions, a scheme of assemblage biozones has evolved, which utilises several macrofossil groups including brachiopods, bivalves, belemnites, crinoids and echinoids, as well as ammonites. This biostratigraphic scheme is unsatisfactory since many of the zonal boundaries have never been formally defined and, in most cases, the precise relationship with the standard ammonite zonation is uncertain, causing doubts over the placing of stage boundaries. In recent years, more emphasis has been placed on lithostratigraphy, with detailed correlation being achieved by the recognition of marker beds of distinctive lithological and faunal character which are thought to record essentially synchronous events.

Lower Chalk

The Lower Chalk is estimated to be 60 m thick within the district. Its outcrop forms the gentler lower slopes of the Chilterns escarpment on which Chinnor, Bledlow and Princes Risborough are built. It is entirely of Cenomanian age (Figure 22). In the district, the Lower Chalk can be subdivided into four units of member status, the Glauconitic Marl, Chalk Marl, Grey Chalk, and Plenus Marls, in ascending order. On the 1:50 000 scale map of the district, the thin Plenus Marls are included with the Grey Chalk.

Glauconitic Marl

A distinctive glauconite-bearing bed, 1 to 2 m thick, between the Upper Greensand and the Chalk Marl, can be traced across the district. In places, it gives rise to a markedly greenish loamy soil, and elsewhere it can be identified by augering. At depth, the strata vary from greenish brown, fine-grained, almost pure glauconite sand to creamy white or pale bluish green marl. The marl contains abundant glauconite grains, commonly concentrated in patches which probably represent burrow-fills. In a few places for example [SP 785 028], small brown phosphatic pebbles occur, and locally for example [SP 7675 0195], the bed is split by a layer of cream to yellow, nonglauconitic marl. The lithology of the material is similar to the Glauconitic Marl ('Chloritic Marl' of early authors) of south-east England, and the Cambridge Greensand of Bedfordshire and Cambridgeshire, which occupy an analogous stratigraphical position. However, the Cambridge Greensand characteristically rests nonsequentially on Upper Gault (Hart, 1973; Morter and Wood, 1983) and contains derived Upper Albian fossils, and so the term Glauconitic Marl is most appropriate in this district. As elsewhere, it probably rests nonsequentially on the underlying Upper Greensand, and it is possible that this relationship may, to a small extent, account for the north-eastward thinning of the latter.

Details

In 1972, a section exposed during construction of the M40 Salt Lane underpass [SU 7075 9930] at Postcombe, just south of the district, showed 1.04 m of Glauconitic Marl, comprising highly glauconitic loam with patches of buff marl and streaks and burrow-fills of glauconite and limonitic nodules. It rested with a sharp, slightly undulating base, on Upper Greensand. The topmost 0.38 m was more manly and less glauconitic, and the junction with the overlying Chalk Marl was gradational (H G Owen, personal communication, 1992).

In 1989, debris from two shallow excavations [SP 8213 0605]; [SP 8212 0614] in the railway cutting at Smoky Row, about 150 m beyond the eastern margin of the district, proved pale greenish grey, sandy manly clay with abundant glauconite grains, overlain by dark greenish grey, medium-grained to coarse-grained, clean, well-sorted sand, composed almost entirely of glauconite.

Chalk Marl

The beds between the Glauconitic Marl and the Totternhoe Stone are termed the Chalk Marl (Jukes-Browne and Hill, 1903, Rawson et al., 1978). In the absence of borehole data, a mean thickness of 35 m has been estimated from outcrop, but there are large variations in apparent thickness, from 25 m between Chinnor and Bledlow, up to nearly 40 m south of Chinnor. These variations may in part result from the erosive non-sequence below the Totternhoe Stone (see below).

The outcrop of the Chalk Marl, at the foot of the Chilterns scarp, is characterised by a pale to dark grey marl or clay soil which overlies a pale yellowish white or fawn silty marl subsoil. The basal metre or so of the succession commonly contains scattered grains of glauconite. The upper part of the Chalk Marl is exposed in Chinnor Quarry where it can be seen to comprise rhythmically interbedded grey to fawn marls and soft marly chalks. In thin section, these chalks are typically somewhat argillaceous lime mudstones with common bioclasts (mainly foraminifera tests and calcispheres), scattered coarse silt-grade quartz and commonly a small proportion of silt-grade glauconite.

Beds of strongly cemented chalk or limestone perhaps 1 to 2 m thick, occur at several levels, and locally give rise to minor scarp features with associated brash. They are important local aquifers within the relatively impermeable Chalk Marl and give rise to springs at Chinnor [SP 747 003], Wainhill [SP 7695 0161], Bledlow [SP 7732 0180]; [SP 7790 0217], Saunderton [SP 7915 0207]; [SP 7930 0149]; [SP 7985 0153], Princes Risborough [SP 8013 0296]; [SP 8024 0292] and Monks Risborough [SP 8130 0443]; [SP 8141 0461]. Previously, these springs were thought to mark the outcrop of a single bed, the Marl Rock or Risborough Rock (Jukes-Browne and Hill, 1903; Jukes-Browne and White, 1908).

Details

An excavation [SP 8209 0600], about 1 m deep in the railway cutting at Smoky Row, produced debris of fawn marl from very close to the base of the Chalk Marl. It yielded benthonic foraminifera indicative of early Cenomanian Zone 8 of Carter and Hart (1977) (UKB2 of Hart et al., 1989), which falls within the basal Cenomanian Neostlingoceras carcitanense Subzone of the Mantelliceras mantelli Zone.

A section recorded during construction of the M40 Salt Lane underpass [SU 7075 9930] at Postcombe, showed 6.53 m of pale, creamy, blocky chalk marl, resting with a gradational base on Glauconitic Marl (H G Owen, personal communica tion, 1992). Between 4.3 m and 4.7 m above the base, it contained concretions with large ammonites. The underlying strata were somewhat sandy; the lower part yielded fossils suggesting the N. carcitanense Subzone.

A bed of hard, grey, rubbly silty limestone about 6 m above the base of the Chalk Marl, crops out on the slopes of knolls [SP 7650 0175]; [SP 771 019] north and north-east of Lower Wainhill. It is richly fossiliferous, yielding rusty-brown moulds of bivalves (Chlamys sp., Entolium sp., Gryphaeostrea canaliculata, 'Inoceramus' crippsi, cf. Lima subovalis), ammonites (Hypoturrilites gravesianus, Schloenbachia varians) and rare brachiopods (Kingena sp., Tropeothyris sp.?), gastropods and sporadic partly silicified sponges (cf. Exanthensis labrosus). The assemblage indicates the M. mantelli Zone, and possibly the higher part of the N. carcitanense Subzone. This bed may equate with the Doolittle Limestone of the Totternhoe area (Shephard-Thorn et al., 1994).

Two closely spaced beds of flaggy silty limestone probably slightly higher in the sequence give rise to several springs west of Chinnor [SP 747 002]. A fauna similar to that described above was obtained from abundant brash here.

West of Lower Wainhill [SP 7626 0116], a band of hard, flaggy, pale greyish buff silty limestone about 15 m above the base of the formation has also yielded a few ammonites and bivalves including 'I.' crippsii, Hypoturrilites tuberculatus and S. varians. The same bed is probably represented by material in the University Museum, Oxford, which was collected from trenches in Chin-nor Quarry 'near the works' [SP 754 000] from a level deeper than now exposed; a preliminary faunal list (based on information supplied by Dr W J Kennedy) includes ' crippsi, Plicatula inflata, common Mantelliceras saxbii, Mariella lewesiensis, Scaphites obliquus and common S. varians (see also McKerrow and Kennedy, 1973). The bed, within the M. saxbii Subzone, is also known from sites in Sussex and the Isle of Wight, but is thought to be absent from many other localities due to down-cutting at the base of the Mantelliceras dixoni Zone.

Currently (1993), the uppermost 11 m or so of the Chalk Marl are exposed in Chinnor Quarry (Figure 23); a descriptive lithological section is given by Sumbler (1990b), and the biostratigraphy is discussed by Sumbler and Woods (1992). The Chalk Marl forms the floor of the working quarries 1 and 3 [SP 759 011]; [SU 758 998], respectively north-west and south-east of the Upper Icknield Way bridle path. The beds exposed in the quarry faces comprise pale fawn, manly, silty to finely sandy chalks alternating with thin bands of greyish brown marl; some beds contain elongate nodules of pyrite. The strata are moderately fossiliferous, containing fairly common inoceramids (' L' crippsi and Inoceramus virgatus) and other bivalves (principally Plagiostoma globosum and Plicatula inflata), brachiopods (Monticlarella? rectifrons) and rare ammonites (notably Schloenbachia varians).

Typically, the chalks are very soft and weak, and rapidly break down on weathering. However, two bands, respectively 5.3 and 2.2 m below the Totternhoe Stone, are patchily cemented and stand out slightly in the face (Figure 23). The lower lies about 1 m below a distinctive dark marl, and as such probably equates with the 'Dixoni Limestone' in the lower part of the M. dixoni Zone of the Dunstable area (Shephard-Thorn et al., 1994), and with a double limestone in the Folkestone succession (Gale, 1989, Bed M6). The upper bed has yielded the ammonite Turrilites scheuchzerianus and may correspond with a bed in the Southerham Grey pit, near Lewes, Sussex (Lake et al., 1987) which, though formerly thought to be of Mid Cenomanian age, is now included in the Lower Cenomanian. The underlying marl has yielded a fauna suggestive of the M. dixoni Zone (Sumbler and Woods, 1992).

Grey Chalk

The Grey Chalk includes those beds between the top of the Chalk Marl and the base of the Plenus Marls. The Totternhoe Stone is included as the basal bed (Sumbler and Woods, 1992).

The Grey Chalk consists predominantly of off-white, blocky, very slightly argillaceous chalks which, in contrast to the underlying beds, generally form a brash in arable fields. Typically, these brashy beds form a moderately steep slope, and are evidently more resistant to erosion than the underlying Chalk Marl. The whole of the Grey Chalk is exposed in Chinnor Quarry, where it is about 25 m thick (Figure 23). Thicknesses elsewhere are rather uncertain, but only 15 m is estimated east of Chinnor [SP 765 009], near Princes Risborough [SP 810 030], and at Butlers Cross, [SP 843 070], 2 km beyond the eastern margin of the district (Jukes-Browne and Hill, 1903). This apparent thinning may be due largely to local structural complications, but some variation in stratigraphical thickness is likely due to erosion surfaces at both the base and top of the Grey Chalk, and possibly at one or more additional levels within it (see below).

The Totternhoe Stone is poorly developed by comparison with its type area (some 30 km to the north-east [SP 985 220] ), where it is several metres thick and was worked as a freestone. In this district, it comprises 0.6 to 1 m of soft, brownish grey, sandy, fossiliferous chalk. It is little harder than the overlying Grey Chalk and does not generally form a mappable feature. However, near Princes Risborough for example [SP 806 027], it forms a pronounced step on the Chilterns scarp, suggesting local cementation, perhaps accompanied by slight thickening.

Thin sections show that the sand component of the Totternhoe Stone is largely carbonate shell debris (mainly inoceramid prisms), but it also includes about 10 per cent quartz sand, and also scattered grains of glauconite, the latter often quite coarse and conspicuous in hand-specimen. Characteristically, the Totternhoe Stone contains sporadic dark brown, lustrous, phosphatic shell fragments including fish and arthropod debris and, in the lower part of the bed, larger phosphatised fragmented internal moulds of fossils probably derived from older beds. Associated with the latter, irregular greenish brown, partially phosphatised and glauconitised pebbles of chalk up to 30 mm diameter are fairly common.

As at the type locality, the local Totternhoe Stone is probably of late A. rhotomagense Zone age (Turrilites acutus Subzone), with much of the fauna reworked from the preceding Turrilites costatus Subzone. In the type area, the base locally cuts down to strata of early M. mantelli Zone age, but in this district, erosion prior to the deposition of the Totternhoe Stone has not been so deep; in Chinnor Quarry it rests on late M. dixoni Zone strata (Figure 23).

The Grey Chalk above the Totternhoe Stone is overall less argillaceous than the Chalk Marl but, as seen in Chinnor Quarry, exhibits similar (though less pronounced) rhythms between more and less marly chalks. The lower part is somewhat more silty and argillaceous than the upper. Jukes-Browne and Hill (1903) recognised this change in Buckinghamshire and Oxfordshire, describing 'greyish' chalks below, changing to 'nearly white' above. At or near the junction between the two, they recorded 'a peculiar bed of rough grey chalk' or 'rag', similar in appearance to the Totternhoe Stone, but lying about 10 m above it. Such a bed, presumably marking a significant non-sequence, was recorded at Butler's Cross (see Details) and at localities further north-east (near Tring, Luton and Royston; Jukes-Browne and Hill, 1903), but no evidence of it has been found within the district.

From the data available, it appears that the interval between the 'rag' and the Plenus Marls is highly variable. This may be due to differential erosion at the base of the Plenus Marls (see below), but it is also possible that 'rag' is developed at more than one horizon. At Totternhoe [SP 980 223], a 'rag' occurs within the Acanthoceras jukesbrownei Zone (Shephard-Thorn et al., 1994), and a probably correlative bed is known from Hitchin. However, at some other localities, the 'rag' probably lies within the Calycoceras guerangeri Zone. Possible scour surfaces within the C. guerangeri Zone at Chinnor Quarry, may indicate such an event.

Details

Jukes-Browne and Hill (1903) recorded exposures of the Totternhoe Stone in a quarry at Parkfield, south of Princes Risborough approximately [SP 807 026] (where it contained Orbirhynchia mantelliana), in roadways near Culverton around [SP 807 021], Bledlow [SP 776 018], and near the former Chinnor Station [SP 7588 0043]. The bed is exposed in the lower part of the adjacent Chinnor Quarry 1 [SP 759 002], but is more easily examined in Quarry 3 [SU 758 998] which lies just beyond the southern boundary of the district (Sumbler and Woods, 1992). Here, the base of the Totternhoe Stone is sharp, with minor scours, up to 0.1 m deep, cut into the underlying marl. Locally, Thalassinoides burrows infilled with Totternhoe Stone sediment are seen in the topmost part of this bed (Kennedy, 1967). A fauna from the now restored Quarry 2 [SU 752 995], was listed by McKerrow and Kennedy (1973); phosphatised (probably derived) fossils comprise sponges, gastropods (Pleurotomaria), scaphopods (Dentalium) and ammonites (Schloenbachia, Acanthoceras, Cunningtoniceras), the latter indicating the A. rhotomagense Zone. Unphosphatised fossils, probably for the most part representing the indigenous fauna, include bivalves (Inoceramus, Pycnodonte, Plicatula), brachiopods (Orbirhynchia, Concinnithyris) and giant ammonites (Acanthoceras and Austiniceras), which again are indicative of the A. rhotomagense Zone.

Immediately above the Totternhoe Stone, the strata are rather silty and slightly more brownish in colour than the higher beds. In the lowest 12 m above the Totternhoe Stone, interbedded darker, greyish, somewhat manly beds of chalk (0.1 to 0.3 m thick) occur at 1 to 2 m intervals. These manly beds are commonly intensely bioturbated. Some of the chalks in this lower part contain elongate pyrite nodules up to 10 cm in length. Strata in this part of the succession were formerly worked in a quarry south of Bledlow [SP 7763 0168], where Jukes-Browne and Hill (1903) recorded 'tough greyish white blocky chalk with large ammonites' (Acanthoceras rhotomagense and A. sussexiense).

The overlying 9 m or so of beds at Chinnor are massive, off-white chalks. Subhorizontal shears at approximately 2 m intervals probably mark marly layers. A number of curved, channel-like surfaces in the topmost beds, most notably about 4 m below the Plenus Marls, may be erosional scours accentuated by shearing, suggesting a possible nonsequence at this level. There are several sheared bands of marly chalk in the uppermost 2 m of Grey Chalk.

Overall, the Grey Chalk is poorly fossiliferous, and the zonation indicated on (Figure 23) is provisional. From 3 to 7 m above the Totternhoe Stone, brachiopods including Concinnithyris subundata suggest a level high in the A. rhotomagense Zone. Loose blocks probably from the immediately overlying beds include a few probable 'Inoceramus' atlanticus, suggesting the lower part of the succeeding A. jukesbrownei Zone. A thin marly chalk about 9 m above the Totternhoe Stone, and some higher beds, yielded common Amphidonte, suggesting the C. guerangeri Zone.

The section formerly exposed at Butler's Cross (or Chalkshire) Quarry [SP 843 070], about 2 km beyond the eastern margin of the district, was described by Hill and Jukes-Browne (1886), and Jukes-Browne and Hill (1903). Approximately 5 m below the Plenus Marls, they recorded 0.6 m of hard greyish chalk ('rag') with scattered green (glauconite) grains and large nodules of hard yellowish (weakly phosphatised) chalk and smaller green-coated (glauconitised) nodules, some bored and with encrusting oysters. It yielded Ostrea vesicularis', 'Rhynchonella', Terebratula 'semiglobosa', 'Inoceramus'and abundant fish teeth. Its level in relation to the Plenus Marls invites correlation with the possible erosion surface recorded in the C. guerangeri Zone of Chinnor Quarry (see above), but the presence of pycnodonteine oysters and terebratulid brachiopods (possibly Ornatothyris sulcifera) suggests that the 'rag' here may represent the earlier A. jukesbrownei Zone event (see above), and that its proximity to the Plenus Marls is due to local down-cutting at their base.

Plenus Marls

In a number of places along the chalk scarp, a slight concavity in slope profile below the step-like feature of the Melbourn Rock indicates the presence of about 1 m of soft beds. These are the Plenus Marls (the Belemnite Beds of Jukes-Browne and Hill, 1903) named from the presence of the belemnite Actinocamax plenus which, although not abundant, is characteristic of the higher part.

The Plenus Marls are present throughout most of the Anglo-Paris basin. They were described by Jefferies (1962, 1963), who recognised a standard succession of beds (1 to 8) distinguished by lithological and/or faunal characteristics. The base of the Plenus Marls rests on a regional erosion surface; the apparent thinness of the Grey Chalk at Butler's Cross (see above) may be a consequence of downcutting at this level. There are other disconformities within the member, notably beneath Bed 4, with others of less widespread extent at the base of Beds 2, 6 and 8. The 'Actinocamax plenus Zone' (or 'Subzone') was formerly regarded as of earliest Turonian age, but the Plenus Marls are now considered to lie within the Metoicoceras geslinianum Zone (Figure 22).

Details

At the now obscured Butler's Cross Quarry [SP 843 070], Jefferies (1963) indicated that the sequence, just over 1 m thick, was incomplete, with Bed 3, containing pebbles at its base, resting directly on an irregular, burrowed surface of Grey Chalk. A section recorded by Jukes-Browne and Hill (1903, fig.42) apparently shows Bed 3 wedging out southwards against a rising surface of Grey Chalk, so that Bed 4 locally rests on the latter.

At the time of Jefferies' work, the Plenus Marls were not exposed at Chinnor Quarry, but currently (1993) they are well displayed in the north-eastern face of Quarry 3 [SP 760 000], where they average 1 m in thickness (Plate 17). All of Jefferies' eight standard beds can be recognised (McKerrow and Kennedy, 1973; Sumbler and Woods, 1992), as indicated in the following section:

Beds 1, 3 and 5 are blocky chalks little different from those of the underlying Grey Chalk, but the darker interburrowed marls and manly chalk of Beds 6 to 8 form a striking colour band in the quarry face, which is displaced by several small faults. Undulating erosion surfaces are apparent beneath beds 1 and 4.

Middle Chalk

The Middle Chalk forms the steepest slopes of the Chilterns escarpment, with gradients of more than 30° being found in many places. It comprises mainly pure white, firm chalks and nodular chalks, and is estimated to be 65 m thick. In arable fields, the Middle Chalk typically forms a brash of small angular fragments contrasting with the larger blockier fragments of the underlying Grey Chalk. There is no evidence of flint in the Middle Chalk of this district, except for impersistent bands in the uppermost part, although to the north-east, sporadic flints appear quite close to the base of the Middle Chalk (Shephard-Thorn et al., 1994). The Middle Chalk is almost entirely of Turonian age, except for the very lowest beds, which are Cenomanian.

The base of the Middle Chalk is marked by the Melbourn Rock, named from a locality in Cambridgeshire [TL 38 44] (Hill and Jukes-Browne, 1886). It comprises about 4 m of pure white, intensely hard, porcellanous, nodular chalk, which generally forms a slight bench on the slopes, and locally forms prominent spurs, notably south of Saunderton [SP 790 009]; [SP 797 007]; [SP 802 013]. However, in areas of less mature landscape with steeper slopes, for example near Chinnor [SP 766 007] and east of Princes Risborough [SP 819 044], the outcrop can be difficult to locate.

Details

The Melbourn Rock is poorly exposed at several localities in trackways, for example in the floor of the Upper Icknield Way bridle path [SP 8181 0400] at Princes Risborough.

The basal 13 m of the Middle Chalk can be seen in Chinnor Quarry 3 [SP 760 000] ((Figure 23); Sumbler and Woods, 1992). The Melbourn Rock, 4.5 m thick, rests sharply on the Plenus Marls (Plate 17). It is a nodular chalk with the bedding picked out by numerous wisps and irregular partings ('flasers') of greenish brown marl, which ramify through the rock and envelope the chalk nodules ('griotte' structure). The marl partings commonly have stylolitic or polished listric contacts with the adjacent chalk nodules. Additionally, several more or less continuous beds of greenish marl occur; these pinch and swell along the face and are rarely up to 0.05 m thick. Such 'griotte' chalks probably originated by early nodule formation within a softer chalk matrix; subsequent compaction, accompanied by pressure solution of the matrix, concentrated the insoluble clay residue into thin seams (Kennedy and Garrison, 1975).

At Chinnor Quarry the Melbourn Rock has yielded Mytiloides labiatus, Orbirhynchia cuvieri and rare ammonites (Mammites, Austiniceras, Sciponoceras)(McKerrow and Kennedy, 1973). Recent BGS collecting yielded Sciponoceras and Inoceramus ex gr. pictus (typically Cenomanian) from the lower part, and a fauna with Orbirhynchia cuvieri and Mytiloides (typically Turonian) from the upper (Sumbler and Woods, 1992). On this evidence, the Cenomanian–Turonian Stage boundary must be placed about 1.65 m above the base of the Melbourn Rock.

The top of the Melbourn Rock is gradational. Less well-developed 'griotte' chalks occur in the immediately overlying beds, and less compacted nodular chalks are common above.

Fossils are moderately common; some beds contain abundant large fragments of Mytiloides, and sporadic Orbirhynchia and echinoids (Conulus) also occur.

The uppermost 8 m of Middle Chalk, belonging to the Terebratulina lata Zone, are exposed in the Aston Rowant cutting on the M40 motorway [SP 730 965], about 3.5 km south of the district (Sumbler, 1990b). The beds consist of pure white, massive, granular chalks, with some weakly nodular horizons. They contain scattered shell fragments but are otherwise poorly fossiliferous. Two thin seams of greenish grey marl occur about 1.25 and 1.0 m below the top. The upper seam, 0.10 m thick, is the Fognam Marl, which can be recognised throughout much of the Chilterns (Bromley and Gale, 1982); in Wiltshire, it lies within the Chalk Rock complex (Upper Chalk). The Fognam Marl may correlate with the Glynde Marl of the south coast (Wray and Gale, 1993), which marks the base of the Lewes Chalk Member (Mortimore, 1986).

The uppermost 10 m of Middle Chalk are bared in the steep slope below Whiteleaf Cross [SP 8215 0399], a landmark cut into the scarp face just east of Princes Risborough, just beyond the margin of the district. Exposure is poor, but the chalk here appears to contain scattered flints.

Upper Chalk

The Upper Chalk forms the highest part of the Chilterns escarpment. Up to 40 m of strata are represented within the district (at Chinnor Hill [SP 768 002] and Loosely Row [SP 819 010] ), but to the south (Sheet 254), the total thickness preserved beneath Tertiary beds is about 75 m. The Upper Chalk succession comprises white, well-bedded, massive and nodular chalks which characteristically contain nodules and bands of flint. Most of the outcrop is given over to woodland, scrub or pasture, but in the rare arable fields, it produces a poor soil made up predominantly of angular fragments of white to pale grey flint with some pieces of white chalk.

The base of the Upper Chalk is defined by the Chalk Rock, which comprises about 1 m of very hard, porcellanous, nodular chalk, with scattered grains of glauconite, becoming more abundant in the topmost part. It generally forms a slight step on the scarp face, and its outcrop is commonly marked by small diggings, where it was formerly worked for roadmetal.

The Chalk Rock, present between Hertfordshire and Dorset, contains a number of hardgrounds indicative of temporary breaks in sedimentation during which cementation of the sea floor took place. These surfaces are often highly irregular as a result of cementation around Thalassinoides burrow systems, and are characteristically mottled green and brown due to glauconite and phosphate mineralisation. Some are overlain by bored pebbles of similarly mineralised chalk (Kennedy and Garrison, 1975). Bromley and Gale (1982) recognised at least seven separate hardgrounds, although only the upper few are represented within the Chalk Rock of the district. The top surface of the Chalk Rock is probably a composite hardground, invariably surmounted by a pebble bed. This has yielded a fauna indicative of the latest Turonian Subprionocyclus neptuni Zone.

Another hard chalk bed, the Top Rock, about 4 m higher in the sequence, has been located in a number of places where exposed in pathways. Its top surface, is developed as a hardground, mineralised with glauconite and phosphate, but in contrast to the Chalk Rock, the matrix contains no glauconite grains.

The basal beds of the Upper Chalk in the district are flint free; the lowest flints occur just below the Top Rock and they are abundant above it. Two main types occur, both consisting of dark grey to black chalcedony with a white cortex. Nodular flints occur isolated or as bands; in many cases the nodules are so closely spaced as to form an almost continuous bed. They are typically eccentrically shaped with knobs and fingers probably resulting from growth around Thalassinoides and other burrows. Some are hollow and filled with powdery chalk having formed around sponges. Tabular flints form laterally extensive sheets, up to several centimetres thick, generally lying approximately parallel to bedding surfaces, or rarely filling high-angle shear planes.

The formation of flints probably occurred soon after deposition of the surrounding chalk, at some depth within the sediment column. The general absence of compaction structures around the flints, and the inclusion of fossils in some, suggests a passive replacement of a largely compacted chalk matrix. Commonly, a flow-like deformation fabric can be seen in tabular flints involved in shears, implying an initial plastic consistency. The silica is generally thought to be of biogenic origin, having been derived by dissolution of sponge spicules etc. (Clayton, 1986).

Due to poor exposure, the stratigraphy of the Upper Chalk in the Chilterns is poorly known. The basal part below the Top Rock is placed in the Sternotaxis plana [Holaster planus] Zone, and regarded as of late Turonian age. The base of the succeeding Micraster cortestudinarium Zone is drawn at the base of the Top Rock, which is generally taken to mark the base of the Coniacian Stage, (Rawson et al., 1978). The Upper Chalk of the district probably extends into the lowest part of the Micraster coranguinum Zone, and no post-Coniacian beds are represented.

Details

Approximately 40 m of Upper Chalk are exposed in the M40 Aston Rowant cutting [SU 732 965] to the south of the district. The Chalk Rock comprises hard, nodular chalk, 1.3 m thick. The base is gradational, but the top is a sharply defined, mineralised hardground (the Hitch Wood Hardground); other major mineralised hardground surfaces occur at 0.6 and 1.0 m above the base (the Fognam Farm and Leigh Hill hard-grounds; Bromley and Gale, 1982). The upper 1 m of the Chalk Rock contains scattered glauconite grains, which are particularly coarse and abundant near the top. The 4.4 m of strata between the Chalk Rock and the Top Rock consist of white nodular chalks with sporadic Echinocorys and Micraster, and with flint nodules in the uppermost part. The Top Rock comprises 0.5 m of intensely hard, nodular, shell-fragmental, off-white to yellowish brown chalk, surmounted by an uneven, greenish, glauconitised hardground.

Above the Top Rock, the beds consist mainly of firm, nodular chalk. Sporadic flint nodules occur immediately above the Top Rock, but the first major nodule band is about 2.5 m higher. Above this, courses of flint nodules occur at 0.2 to 1 m vertical intervals. About 20 m above the base of the Upper Chalk, a bed of fairly hard, massive, slightly yellowish chalk, 0.6 m thick forms a prominent marker band in the cutting. Above this level, the chalk is generally more massive and uniform than the nodular chalks below, but contains flints as in the underlying beds. Two thin bands of greenish grey marl occur at 39 m and 44 m above the base of the Upper Chalk in the highest, inaccessible part of the cutting faces. They probably equate with the paired Shoreham (East Cliff) Marls of Sussex (Mortimore, 1986), the upper of which lies at the base of the M. coranguinum Zone.

Jukes-Browne and Hill (1904) figured a section of a quarry by the road on Chinnor Hill [SU 7614 9968], just beyond the southern margin of the district. It showed the upper 0.6 m of the Chalk Rock and about 8 m of overlying beds (see below). Jukes-Browne and White (1908) described the Chalk Rock at this locality as 'hard yellowish limestone with a layer of green-coated nodules at the top and many green and black grains'.

Intensely hard brownish limestone with glauconite grains marks the outcrop of the Chalk Rock where it crosses pathways through Chinnor and Bledlow Woods [SP 7642 0018]; [SP 7654 0049]; [SP 7740 0106]; [SP 7753 0020], in old quarries west of Highlands [SP 7673 0075] and north of Keepers House [SP 7799 0063], and in the arable field south of Highlands [SP 7700 0062]. The bed is poorly exposed in the tracks west of The Bluff [SP 7624 0040] and north of Highlands [SP 7685 0088], and in the bared surface of the Whiteleaf Cross [SP 8215 0399]. The outcrop has also been traced around the scrub-covered outlier of Lodge Hill [SP 792 002] on the basis of material thrown out from animal burrows. Here, the bed occurs topographically lower than in the main outcrop to the west and east, suggesting structural complications. In a former quarry 'about a mile east of Princes Risborough', probably that on Cop Hill [SP 8204 0337], Jukes-Browne and Hill (1904) recorded at least 1.8 m of Chalk Rock, with layers of 'green-coated nodules' at the top and 0.96 m below the top.

The Top Rock has been located in a few places along pathways through the woods [SP 7643 0015]; [SP 7683 0083]; [SP 7733 0105]; [SP 7752 0014]; [SP 7900 0021] and in an arable field [SP 7694 0061]. At Chinnor Hill, Jukes-Browne and White (1908) recorded it as 0.3 m of 'hard, yellow rock, upper surface well defined', 4.3 m above the Chalk Rock.

Chapter 10 Quaternary

The Quaternary (Drift) deposits of the district range in age from the earliest Pleistocene to the present day. The most ancient, the Clay-with-flints, relate to a late Neogene topography very different from the present landscape. After formation of the Clay-with-flints, considerable dissection of the landscape occurred before deposition of the Princes Risborough Sand and Gravel by a south-eastward-flowing river with headwaters probably well to the north of the district. At a later stage, the topography and drainage was totally changed, with the development of the River Thame, flowing south-westwards to the Thames.

The district was affected by several cold periods during the Pleistocene, and glacier ice entered the district during at least one of these. Erratics of northern origin in the oldest river terrace deposits may have originated from ancient glaciers, and a later ice sheet laid down till and associated glaciofluvial sand and gravel in the north-eastern part of the district. These latter deposits are generally considered to belong to the Anglian Stage. Subsequent accumulation of river terrace and head deposits continued until the end of the last (Devensian) cold stage. During the Flandrian Stage, of the last 10 000 years, alluvium, calcareous tufa and peat have been deposited.

Clay-With-Flints

Clay-with-flints is widespread on plateaux and interfluves in the Chalk country of southern England. Its distribution suggests that it is an ancient deposit which once blanketed the entire Upper Chalk surface, before erosional dissection. Catt (1986) concluded that the underlying Chalk surface is essentially coincident with that left by early Palaeogene marine erosion. Clay-with-flints is principally a Chalk solution residue enriched by illuvial clay from overlying or adjacent Palaeogene, Neogene or Quaternary deposits (Loveday, 1962; Catt, 1986). However, deposits mapped as Clay-with-flints may include other materials, such as the Brickearth of Whitaker (in Hull and Whitaker, 1861), or the Plateau Drift of Love-day (1962), which include a more substantial component derived from the Palaeogene Reading Beds, locally with an admixture of early Quaternary river deposits. There seems to be little evidence for the glacial component suggested by some authors.

Within the district, the principal outcrops of Clay-with-flints cap the Upper Chalk at Bledlow Great Wood [SP 769 002] and The Cop [SP 773 008]. The deposits are restricted to the highest ground (more than 240 m OD), and are probably nowhere more than 2 or 3 m thick. They form a red and ochreous clay or sandy clay soil with abundant, white to yellowish brown, subangular flints, commonly up to 15 cm diameter. The material has been pitted in a number of places; at one locality [SP 7653 0002], 1.5 m of reddish brown flinty clay was exposed. Many pebbles show black, manganiferous staining. In sections at Chinnor Hill, immediately to the south of the district, the Clay-with-flints contains pebbles and unworn nodules of flint, as well as 'ironstone' fragments (Jukes-Browne and White, 1908). The basal surface is uneven, with 'pinnacles of rotted chalk'.

Small outcrops on Lodge Hill [SP 794 001] are similar in lithology to those to the west. However, they occur at a lower level (about 200 m above OD) and may possibly be somewhat younger head deposits.

Princes Risborough Sand and Gravel

This term is introduced to describe sands and gravels rich in chalk and flint which crop out within the dry gap in the Chilterns escarpment, known as the Princes Risborough Gap (Figure 24). This lies at the head of the valley of the River Wye, a tributary of the Thames which flows south-eastwards through High Wycombe. On the floor of the gap, the deposits occur at about 134 m above OD, but extend up to over 160 m above OD on the slopes. The low level deposits are bedded and probably of fluvial origin, and some of the higher-level deposits could also represent degraded fluvial sediments, but were more probably formed by solifluction.

Because of its topographical position, the Princes Risborough Sand and Gravel must be considerably younger than the Clay-with-flints. It was laid down by an ancestral River Wye before its headwaters were captured by the River Thame, another tributary of the Thames, and must predate all of the terrace deposits of the Thame. Because of this relationship, Sumbler (1995) suggested that the Princes Risborough Sand and Gravel may equate broadly with the Beaconsfield and Gerrards Cross sands and gravels of the River Thames. Similar deposits of chalk and flint gravel occur in other gaps in the Chilterns, at Wendover, Tring and Dagnall, to the east (Avery, 1964; Sherlock, 1922; Barrow and Green, 1921); they also occur in the Compton Gap in the Berkshire Downs, to the west of the Goring Gap, an analogous gap still occupied by the River Thames (Figure 24). All these deposits were laid down by rivers which, like the present Thames, flowed south-eastwards through the Chalk escarpment. They have been equated with the Wallingford Fan Gravel, which occurs on a bench on the Chalk escarpment in the Henley-on-Thames district (Horton et al., 1981).

At the surface, the Princes Risborough Sand and Gravel is invariably decalcified to a reddish brown sandy loam with abundant flint pebbles. These are mostly angular, probably as a result of postdepositional frost-shattering, but smaller, rounded, flint pebbles are present. Traces of chalk occur locally. A section [SP 8042 0086] in the railway cutting south of Saunderton Tunnel shows up to 2.4 m of stratified flint-rich gravel resting, in sequence, on reddish brown sand, chalk-flint gravel and Chalk. Sections to the south show between 2.5 and 5.1 m of variably decalcified chalk-flint gravel. At one point [SP 8062 0020], 2.5 m of beds comprise flint-rich gravel on small-pebble, chalk gravel with a chalk 'flour' matrix, and seams of chalky sand. A ploughed surface [SP 8050 0075] showed an abundance of rounded flints, few angular flints and rare quartzite and sandstone pebbles. A large block of Hertfordshire Puddingstone (1.0 x 0.6 x 0.4 m) occurring nearby was probably derived from the Clay-with-flints.

In the adjoining Henley-on-Thames district, probable deposits of Princes Risborough Sand and Gravel in the valley to the north of Bledlow Ridge [SU 800 975] and in the parallel Saunderton Lee valley, are shown as Claywith-flints on 1:50 000 Geological Sheet 254.

Glacial Deposits

Small outcrops of till and glaciofluvial sand and gravel occur in the north-eastern part of the district. These deposits are probably of Anglian age (i.e. broadly coeval with the Lowestoft Till of East Anglia), and mark the local limit of Anglian ice advance ((Figure 24); Sumbler, 1995). In addition, rounded 'Bunter' pebbles of quartz, quartzite and other exotic lithologies derived from the Triassic Kidderminster Formation of the West Midlands, are scattered over much of the high ground in the north and west of the district. They are probably residues from former deposits of Northern Drift, patches of which occur near Oxford and to the north-west; these may include pre-Anglian glacial deposits, but are now more generally thought to be fluvial deposits related to the early Thames (Hey, 1986).

Till

Thin deposits of till (boulder clay) cap low hills near Putlowes Farm [SP 779 153] and near Folly Farm [SP 794 177]; [SP 795 185] to the north-west of Aylesbury. They are probably remnants of a formerly more continuous sheet, as small pockets of till occur elsewhere, for example northeast of Berryfields Farm [SP 785 165]; [SP 791 168]. The deposits are rarely more than 1.5 m thick and are deeply weathered. Typically, they consist of brown, locally gravelly clay with abundant flint and 'Bunter' quartz and quartzite pebbles. When fresh, the till is grey in colour and contains grains and small pebbles of chalk. In places, particularly near the base, it is composed of reworked Kimmeridge Clay bedrock, comprising sheared and deformed mudstone with few erratics.

Till is absent elsewhere in the district, but an extensive sheet of lithologically similar, and presumably contemporaneous till, covers much of the area to the north and north-east.

Glaciofluvial Sand and Gravel

Deposits of brown, stony, sandy clay and clayey, sandy gravel cap the interfluves between the streams in the upper reaches of the River Thame basin north and west of Quarrendon [SP 793 193]; [SP 805 176]; [SP 808 157]; [SP 779 147]. They give rise to a brown, loamy soil with abundant flint and 'Bunter' quartz and quartzite pebbles, and minor quantities of ironstone and limestone. Generally, the pebbles range from 10 to 100 mm diameter; the smallest include chalk, and limestone from the Portland Formation. The deposits occur at much the same topographic level as the tills described above, contain a similar pebble suite, and are presumably genetically related. They have poorly defined terraced forms and may be contemporaneous with Fourth Terrace deposits of the River Thame downstream (see below; (Figure 25); Sumbler, 1995).

River Terrace Deposits

The most extensive river terrace deposits are associated with the River Thame, but the smaller River Ray has deposited sands and gravels in the north-west of the district. The deposits generally underlie flat-topped terraces, which can be grouped according to their level above the modern river floodplain. Seven groups have been recognised, numbered from 7 (the highest) to 1 (the lowest), each representing successive stages in the history of the river valley. In general, it is likely that the deposits beneath a terrace are broadly coeval with the associated terrace, although in some cases a terrace may be developed on earlier deposits. The longitudinal profiles of the terrace surfaces are shown in (Figure 25). Each of the terraces increases in height relative to the flood-plain when traced downstream. The nomenclature, correlation and chronology of the terrace deposits is more fully discussed by Sumbler (1995).

River Thame

Seventh (Three Pigeons) Terrace Deposits

The Seventh Terrace deposits are the oldest fluvial deposits of the district which are related to the modern drainage system of the River Thame. They cap the Three Pigeons plateau at Milton Common [SP 653 035], and smaller outliers occur to the east [SP 679 037]. The highest point on the Three Pigeons plateau lies at 104 m above OD, with most of the area between 100 and 102 m above OD, i.e. about 45 m above the Thame floodplain.

The deposits consist of sandy, clayey gravel with abundant flints and minor 'Bunter' pebbles. The maximum proved thickness was 4.9 m recorded in an A40 site-investigation borehole [SP 6502 0340]. Blake (1899) noted between 3 and 3.9 m in well sections around the Three Pigeons Inn, and at Heath Farm [SP 647 034], a maximum of 3 m of sand and gravel are present. Up to 3 m of sand and gravel are present in the two small outcrops which cap Lobbersdown Hill [SP 680 037].

Pringle (1926) recorded well-bedded sand and gravel in a pit ?[SP 6515 0350] north-west of the Three Pigeons Inn. Flint pebbles, both fresh and weathered forms, dominated, but 'Bunter' pebbles were also present. In the former pits at Milton Pools [SP 655 030], Arkell (1947a) noted 3 m of partly cross-bedded, flint gravel and flinty sand. The upper part of the sequence was cryoturbated. Here recent shallow exposures proved a small proportion of 'Bunter' pebbles.

The altitude of the Seventh Terrace deposits suggests that they are considerably older then the Anglian glacial deposits of the district (Figure 25). Arkell (1947a) suggested correlation with the Hanborough (Fourth) Terrace of the River Thames but, from their altitude, the Thame Seventh Terrace deposits are very much older than these too. Most probably, they should be assigned to the Northern Drift (Pocock, 1908; Pringle, 1926), which includes deposits of various ages, related to the early River Thames and its tributaries (Hey, 1986).

Sixth (Tiddington) Terrace Deposits

Small outcrops of sand and gravel around Tiddington [SP 650 051] and downstream, lying between about 26 and 39 m above the floodplain, have been grouped as Sixth Terrace deposits, although the large variation in altitude suggests that they may represent more than one period of aggradation or bench cutting. The outcrops all comprise sand with flint pebbles and lesser proportions of 'Bunter' pebbles. Thicknesses are generally less than 2 m. The highest deposits cap the hill [SP 660 046] east of Fernhill Wood, south-east of Albury, and lie at about 90 m above OD. Gravels at roughly the same height also occur on the southern margin of the district [SP 591 003]. The level of these surfaces with respect to the Thame floodplain suggests correlation with a terrace surface at Sugworth [SP 5152 0143], on the Thames west of Oxford, thought to postdate channel deposits of Cromerian age (Shotton et al., 1980).

Fifth (Chilworth) Terrace Deposits

The Fifth Terrace is represented at Holloway Farm [SP 627 044] and Chilworth House [SP 631 048]; its surface lies at about 78 m above OD, i.e. approximately 22 m above the Thame Floodplain. Sections at Chilworth House [SP 6310 0479] show 0.3 m of soil resting on 1.4 m of reddish brown, poorly sorted, clayey sand and gravel (hoggin), overlying more than 2.1 m of pebbly sand. To the west [SP 6333 0480], at least 2.7 m of gravel are present, and although the base is not exposed in these sections, Kimmeridge Clay occurs within 1.1 m of the surface between the sites, suggesting that the base of the terrace deposits is channelled.

Projection of the terrace profile downstream suggests correlation with the Fourth, or Hanborough Terrace of the River Thames.

Fourth (Blackditch) Terrace Deposits

The Fourth Terrace has been recognised between Thame and Wheatley. It occurs approximately 15 to 16 m above the floodplain, suggesting correlation with the Third or Wolvercote Terrace of the Thames. Extrapolation of the profile upstream suggests correlation with the Anglian glaciofluvial deposits of the north-eastern part of the district, with important chronological implications (Sumbler, 1995). At Thame, extensive Fourth Terrace deposits occur in the south-east part of the town [SP 718 051]. To the east [SP 729 050], comparable gravelly deposits are classified as Older Head (see below), and it seems likely that the Fourth Terrace floodplain was originally flanked by contemporaneous head deposits throughout much of its length.

At many localities, most notably Thame, North Weston [SP 684 060] and Waterstock [SP 643 657], sand and gravel deposits form a continuous slope between the Fourth Terrace level and that of the Third Terrace (see below). Such deposits are indicated as Third–Fourth Terrace deposits undivided on the 1:50 000 geological map of the district. At Thame, for example, the main part of the town [SP 703 061] lies on the Third Terrace but the surface rises imperceptibly southwards and eastwards to the Fourth Terrace level.

Fourth Terrace deposits were exposed in a disused pit [SP 6798 0587] at North Weston, which showed about 1 m of poorly sorted clayey gravel with chalk and flint pebbles, up to 10 cm diameter. The nearby Abbey Farm Borehole [SP 6809 0573], proved 1.3 m of yellow to orange-brown and grey-mottled, clayey, sandy gravel with flint quartzite, ironstone and chert pebbles.

Third (Shabbington) Terrace Deposits

A fairly well-defined Third Terrace can be traced from the southern margin of the district, at Little Milton [SP 613 011] to Thame [SP 706 058], with two isolated outcrops at Starveall [SP 763 125]. Downstream, the terrace surface increases in height above the floodplain, from 7 m at Starveall to 10 or 11 m near Little Milton. From its altitude, correlation with the Second, or Summertown–Radley Terrace of the River Thames seems likely. Recent work suggests that this is of 'Wolstonian' age.

The deposits west and north-west of Little Milton are very sandy, but fine 'pea-gravel' was exposed in ditches in several places. Coarse gravel appears to be uncommon. The site investigation for the M40 Motorway east of the Wheatley Bridge proved up to 2.5 m of sand and gravel [SP 6198 0506], although one trench [SP 6236 0508] revealed 4 m of deposits without reaching the base. The sequence was described as fine to medium sand with sub-rounded to subangular gravel. A seam of pale orange-brown weathering, sandy, silty calcareous clay was recorded in two boreholes. A comparable bed of calcareous clayey silt with race, infilling a 1 m-deep channel beneath gravel was exposed [SP 6240 0512] during the excavation of the M40 motorway.

The extensive terrace east of Waterstock [SP 643 657] lies 9 to 10 m above the alluvium, but it rises gradually at its southern margin onto older Fourth Terrace deposits or contemporaneous head gravel deposits. A borehole is reported to have proved 3.4 m of reddish brown, sandy, flinty gravel [SP 6452 0563]. At Waterperry [SP 627 064], the Third Terrace outcrop is characterised by brown clayey sand with scattered patches of coarse flint gravel.

The extensive outcrop of Third Terrace deposits south of Shabbington [SP 664 064] is up to 3 m thick and comprises flint pebbles with minor quantities of quartzite, ironstone and shell fragments. Most of the pebbles are less than 10 mm diameter, but range up to 40 mm.

Codrington (1864) recorded a section through Third Terrace deposits in the now disused railway cutting [SP 674 054] 0.6 km south west of North Weston. It showed 0.6 m of coarse gravel with angular flints (about 50 per cent), quartz, 'hornstone', ironstone, chalk, 'Tertiary pebbles' and large blocks of Sarsen sandstone. This was overlain by reddish yellow sand with a few small flints and rare chalk pebbles, passing laterally into blue sandy clay and stiff blue clay with chalky seams. This sequence passed up into sandy clay with angular flints and other pebbles. Thicknesses were not given for the upper beds, but Codrington's illustration (fig.2) suggests that they were at least 1 m thick. The lower two beds were fossiliferous, yielding freshwater and terrestrial molluscs 'Ancylus fluviatilis, Anodon (= Anodonta sp.), Cyclas (= Sphaerium sp.), Helix, Limnaea, Pisidium, Planorbis, ? Unio (= Potomida) litoralis [recte littoralis]'), and abundant mammalian remains. Codrington listed 'elephant', 'rhinoceros', 'horse', 'ox', 'deer' and a 'small carnivore'. Comparison with the list by Dawkins (1869) suggests the presence of forms such as mammoth and woolly rhinoceros, indicating cold-stage conditions. An additional record by Dawkins (1869) lists hippopotamus from this site indicating temperate conditions (Ipswichian Interglacial).

Second (Ickford) Terrace Deposits

Sediments assigned to the Second Terrace have been recognised only downstream of Shabbington [SP 665 057]. They give rise to poorly defined terrace features extending from 2 to 4 m, and exceptionally to 7 m, above the alluvium. They are generally contiguous with the First Terrace deposits and difficulty was encountered in separating the two. In places, an unbroken slope extends from the edge of the alluvium to the back of the outcrop of the Second Terrace deposits. The Second Terrace appears to correspond with a higher facet (lb) of the composite First (Floodplain) Terrace of the Thames (Corser, 1978).

A temporary section [SP 6078 0132] west of Warren Barn, Little Milton, showed 1.1 m of small gravel on grey, clayey sand. South-west of Wheatley Bridge, the deposits include sand and small gravel with beds of coarser flint gravel. Boreholes drilled along the line of the A40 show that the deposit consists primarily of medium to fine, clayey sands with some gravel, and minor silty sands and clays. This agrees with the 1.8 m of flinty sand (on black clay) recorded by Arkell (MS, BGS archives) [SP 6174 0503].

Two small outcrops [SP 587 059]; [SP 588 059] of gravelly sand occur in the valley above Wheatley. Lydite pebbles and ironstone fragments are dominant and are associated with fragments of silicified limestone, all of which have probably been derived from nearby Shotover Hill.

First (Quarrendon) Terrace Deposits

First Terrace deposits are extensive and well preserved along the Thame, and they also occur in many tributary valleys. Typically, they are associated with a well-defined terrace, which is 1.5 to 2 m above the alluvium throughout most of the district. However, in many places the terrace grades almost imperceptibly upward from the flood-plain to levels higher than 2 m and merges with the Second Terrace (see above) or the surface developed on Head deposits. Two distinct benches (1a and 1b) are associated with the First (Floodplain or Northmoor) Terrace deposits of the River Thames (Corser, 1978), and it is likely that the First Terrace of the Thame corresponds with the lower (la) facet.

The Thame First Terrace deposits consist of oxidised, reddish brown, loamy sands. They infill a broad channel into which the modern river is incised; in many places, the deposits extend beneath the alluvium. The base of the First Terrace deposits is generally relatively planar but, locally, thicknesses may increase markedly, possibly due to the presence of buried channels.

River Ray

Traces of three groups of terrace deposits have been recognised, but only those of the First Terrace have extensive outcrops. The gravels contain clasts mainly of local rock types, with few erratics such as flint. Thus, there is little evidence of Anglian meltwater discharge into the Ray basin, which is probably mainly of post-Anglian origin.

Cryoturbated pockets of Third Terrace deposits occur in the extreme north-west corner of the district, but no mappable deposits remain. The relict terrace surface is 7 to 8 m above the alluvium and probably equates with the Second (Summerton–Radley) Terrace of the Thames. A trial pit [SP 5529 1916] north-east of Weston Park Farm, exposed a 2.3 m-deep gravel-filled, ice-wedge cast in Oxford Clay. Adjacent pits showed from 0 to 1.7 m of gravel with an uneven cryoturbated base. The gravel is commonly clayey, with small clasts of local Jurassic limestone pebbles (mostly Cornbrash), a few flints, quartzite and Jurassic ironstone.

Two outcrops assigned to the Second Terrace have been mapped, to the west and north-west of Oddington [SP 541 147]; [SP 548 157], and remnant pockets occur [SP 157 165] north-west of Charlton-on-Otmoor. Lithologically, they resemble the older Third Terrace deposits but include pebbles of cementstone, worn belemnites and Gryphaea, all derived from the Lower Oxford Clay.

First Terrace Deposits of the Ray can be divided into two sets on the basis of lithology. They are thought to have been laid down by two streams, the Northern Ray, which flowed on the north side of the Charlton Anticline, and the Eastern Ray, equivalent to the modern River Ray, which drained the ground to the south (Ambrose and Horton, 1991). The Northern Ray deposits, to the north of Merton, crop out as low mounds surrounded by alluvium. In the north, up to 1.5 m of small-clast limestone gravel (derived from Middle Jurassic rocks) passes southwards into sand, and finally into pebbly silty clay and silt.

The deposits of the Eastern Ray occur around the edges of Ot Moor [SP 57 14], and consist of up to 1.5 m of reddish to orange-brown clayey sands. Pebbles are rare, and consist of flint, quartz and quartzite; locally derived Upper Jurassic fossils also occur. Middle Jurassic limestones are absent, indicating that at this stage the Northern Ray did not drain into Ot Moor. The deposits on the south side of Ot Moor were laid down by a northward-flowing stream, whilst contemporaneous deposits farther south, around Danesbrook Farm [SP 5935 1060], were deposited by the Moorbridge Brook.

River Thames

The River Thames flows across the south-westernmost corner of the district. A very small area of Second (Summertown–Radley) Terrace deposits forms a bench [SP 538 002] 4.5 m above the floodplain. To the west, First (Floodplain or Northmoor) Terrace deposits crop out on both sides of the floodplain alluvium, and as an 'island' within it. The deposit on the east bank rises to 1.3 m above the alluvium and comprises up to 1 m of sandy loam resting on gravel.

Alluvium

Alluvium occurs in all but the smallest rivers; in the headwaters of streams it may pass into Head. It is generally less than 2 m thick, but in the Thame and Thames valleys it may exceed 4 m. The alluvium in these major valleys infills a broad channel cut in the First Terrace deposits, and in places extends below their base into bedrock.

Typically, the alluvium consists of dark grey, slightly silty clay with scattered pebbles; in places, it contains seams of silt or silty sand and, less commonly, peat. Locally, a thin basal lag gravel may be present, typically with a grey silty or sandy matrix.

The district contains extensive sheets of alluvium which are disproportionate to the size of its streams, most notably in the north-west, where the alluvium of the River Ray commonly exceeds 1 km in width and at Ot Moor [SP 57 14] is 4 km wide. There, the underlying Oxford Clay was probably critical to the development of the alluvial tract; it weathers readily and during past cold periods, was particularly susceptible to landslipping and gelifluction. These processes resulted in seasonal mass movement, the material so transported being subsequently carried away by streams. Downcutting and lateral erosion eventually exposed the resistant Cornbrash limestone in the Oddington Gap. This impeded drainage, leading to the deposition of extensive tracts of alluvium on Ot Moor and in tributary valleys (Ambrose and Horton, 1991). Other extensive alluvial tracts are associated with the Oxford Clay, West Walton Formation, Ampthill Clay and Gault outcrops.

A borehole sited on the Thame floodplain at the Wheatley Gauge Station [SP 6118 0506] proved 1.05 m of soft grey clay with red mottles and scattered shell fragments, on 0.4 m of peaty clayey sand, above 3.4 m of First Terrace gravel. Farther north-west, boreholes in the centre of the Thame floodplain proved up to 4 m of brownish grey silty clay with shell fragments [SP 6204 0547]. Sand partings occur in the lower part of the alluvium in some boreholes [SP 6227 0541].

A series of boreholes associated with the Thame Bypass proved a variable thickness of alluvium. Borehole 31 [SP 6978 0638] proved 4 m of alluvial clay resting on Jurassic strata, but some 40 m away 2.2 m of alluvial sediments rest on 0.2 m of First Terrace gravel.

The alluvium of the left-bank tributaries, which drain the Chilterns escarpment, consists of pale grey calcareous silt and silty clay, commonly with scattered chalk and flint pebbles. Seams of detrital tufa may be present, particularly where springs are the source of the streams. Downstream, on the Gault outcrop, the alluvium is more generally clayey, reflecting the source rock. Basal lag gravels here are dominated by chalk and flint pebbles.

The extensive alluvial deposits of the River Ray and its tributaries consist of ochreous, and greyish brown mottled, silty clay with sand lenses and scattered pebbles. Peat or humic horizons may occur, particularly in the lower part. The deposit generally becomes more sandy with depth. Locally, it develops a thin basal lag gravel composed of locally derived shells and shell fragments, particularly Gryphaea and belemnites, pebbles of race, and rare flint and limestone fragments. Generally, the thickness is less than 2 m, although up to 3.2 m was exposed in a trial pit [SP 5795 1616] on a now abandoned course of the River Ray. The alluvium of Ot Moor and the adjacent River Ray valleys increases in height towards the edges of its outcrop; thus, between Arncott and Piddington, the margin of the alluvium rises to about 4 m above the main level of the floodplain. This reflects the extent and depth of past flooding.

Head

Deposits of head are present throughout the district. Most occur at the foot of slopes but some infill the valley bottoms of headwater streams. Head generally originated by solifluction under periglacial conditions but, downstream, can pass into fluvial sediment. Some head deposits originate by downwash and soil creep of finer-grained material. The most extensive developments are associated with the Chalk escarpment.

Several stages of head deposition can be recognised. In the south-east, the deposits have been divided into Older Head and Younger Head; the separation is based on geomorphological criteria. This classification is simplistic, since it is probable that head deposits formed during each cold phase of the Quaternary, and that preexisting deposits were remobilised as new deposits accumulated elsewhere.

Younger Head deposits grade to the alluvium and may in part be of Flandrian age, although most of them are related to the later part of the Devensian cold period. Older Head was deposited during earlier parts of the Devensian Stage, or during preceding cold stages.

Older Head

Older Head deposits are principally developed on the Gault outcrop in the south-east of the district, where they cap low interfluve ridges between the tributary streams of the River Thame. The surface levels of the deposits decrease gently to the north-west, from over 145 m above OD near Chinnor [SP 760 007] to less than 80 m above OD, for example at Kingsey [SP 743 067], with typical gradients of less than 1°. It is likely that the outcrops are the dissected remnants of an extensive apron which blanketed the face of the Chilterns escarpment, and locally extended as far as the River Thame to the north-west. The level of these north-westerly remnants suggests correlation with the Fourth Terrace of the Thame, and a possible Anglian age.

The Older Head generally consists of poorly sorted clayey sand and clay. Abundant flints occur at the surface, particularly in the outcrops to the north-east of Tetsworth. In unweathered sections, chalk-rich gravel is present at depth. In the more south-westerly outcrops, the older head comprises reddish brown, clayey sand and sandy clay with small flint pebbles. The change in lithology may, in part, reflect distance from the Chalk escarpment and increased derivation from the Upper Greensand, but may also indicate increased decalcification, and a greater antiquity for these deposits.

Some of the deposits depicted as Younger Head (for example those capping knolls around the Muswell and Brill hills) may properly belong to this group.

Younger Head

Younger Head deposits are widespread. They postdate the development of the main elements of the present topography and consequently occur on the lower slopes of valleys, marginal to the alluvium or in valley floors. Head deposits are of local origin and their lithology closely reflects that of the source material. Thus deposits below the outcrop of the Portland Formation contain a high proportion of sand and limestone fragments. Where the Portland Formation is overlain by the Whitchurch Sand Formation, ironstone fragments from the latter form a significant component of the head. On clay formations, the head commonly resembles in-situ weathered solid material, but in many cases can be distinguished by the presence of derived rock debris or pebbles, particularly at the base. Generally the deposits are less than 2 m in thickness.

Younger Head deposits on the Chalk outcrop consist of poorly sorted, chalk-flint gravel with seams of chalk silt. Such sediments are designated 'Coombe Deposits' elsewhere in the Chilterns. They are typically restricted to dry valleys, many of which have narrow, steep-sided cross-profiles, low longitudinal gradients, and flat bottoms. These characters suggest that the material has been partly reworked by fluvial processes, when the watertable was higher than at present, possibly during periods of permafrost (Williams, 1980). The valley running northwards from The Warren [SP 776 014] is one such example. It ends abruptly in a steep bluff or coombe with slopes of up to 30°, probably the result of headward erosion of a spring-fed stream.

A large embayment in the scarp at Hampton Wainhill [SP 772 011] is subcircular in outline, over 300 m across and about 75 m deep, with a cliff-like backface. Bull (1940) suggested that comparable features in the South Downs were essentially true glacial conies, formed by nivation. A valley extending north-eastwards from this feature is floored by chalk gravel Head, which grades downstream into lithologically similar river terrace deposits at Pitch Green [SP 778 032].

Peat

Small outcrops of peat occur in association with springs or streams. Plant growth and preservation of the remains occurs only where drainage is impeded and high watertables develop. Such conditions develop most commonly on the sides or floors of small streams or valleys on clay substrates. Generally, the deposits are less than 1 m thick, and may be interbedded with calcareous tufa or alluvium. Along some of the more major streams and rivers, peat occurs as thin seams within the alluvium, and represents marshy environments, possibly developed in backwater slacks, such as oxbow lakes. Pure peat, composed almost entirely of vegetable matter, is uncommon; most deposits contain a significant proportion of mineral matter and some pass into humic sands. Only the largest deposits are depicted on the 1:50 000 map.

Peat occurs in the small valleys on either side of the Churchill Hospital [SP 544 059], New Headington, Oxford. These deposits are associated with springs at the base of the Beckley Sand Member. The deposits are generally less than 1 m thick, but are up to 2 m in places. The peat is interbedded with peaty loam and minor seams of shelly calcareous tufa, and generally overlies sand or sandy gravel.

The three areas of peat at Headington Hill [SP 5312 0686] and Headington [SP 5385 0783]; [SP 5410 0800] blanket the slope below springs issuing at the base of the Beckley Sand. Similarly, two small deposits of peat [SP 5462 0930]; [SP 5460 0970] west of Wick Copse are developed as bogs on slopes beneath the basal Beckley Sand seepage line. Peat in the valley floor to the north-east may be partly the result of impeded drainage. East of the Bayswater Bridge patches of peat are associated with poor drainage on the Temple Cowley Member [SP 5619 0815] and seepage from sands in the Kimmeridge Clay [SP 5694 0837]; [SP 5710 0843]. There are four humic deposits along the floor of the Haseley Brook. The small area of peaty sand south-east of Peggs Farm [SP 6547 0058] is associated with an overflow seepage of water from the Portland Formation as it passes beneath the Gault.

An extensive area of peat occurs at Spartum Fen [SP 654 017] (Site of Special Scientific Interest), near Latchford, which overlies a 'window' in the Gault, exposing the underlying Portland Formation. Peat development was favoured by copious supplies of spring water rising from the latter.

Peat [SP 795 042] south-east of Longwick occurs where a bog has formed due to strong groundwater seepage from the base of the Upper Greensand. The poor drainage of the underlying Gault has been accentuated by the construction of the railway embankment which, but for a small culvert, effectively dams the valley.

Calcareous Tufa

Tufa is deposited from groundwater saturated with calcium carbonate as a result of percolation through calcareous rocks. Where the water issues at springs, the carbonate is precipitated, commonly on plant stems or other objects. The deposit may form an apron over which the spring water continues to flow, or it may be redistributed by the stream to form bedded sheets. The thickness of the deposits is generally less than 2 m. Although the deposition of calcareous tufa continues today, the main period of formation was probably in early Flandrian times, when the climate was warmer and more humid than at present.

An apron of calcareous tufa is associated with a series of springs [SP 5765 0960]; [SP 5772 0961]; [SP 5776 0950] issuing from the base of the Arngrove Spiculite Member in a deep-sided valley at Stanton St John. The calcium carbonate is probably derived from the Beckley Sand and Wheatley Limestone. The tufa drapes the slopes and floor of the valley, and extends downstream as a sheet grading ultimately into alluvium. It consists of pale brownish grey to white, calcareous silt. Gastropod shells and debris abound in places.

Spreads of calcareous tufa occur north-east of Elsfield. One is associated with a spring [SP 5436 1103] issuing at the base of the Beckley Sand. The other deposit [SP 551 108], in Fox Covert, lies on the West Walton Formation and Oxford Clay and is deposited from waters issuing from the Beckley Sand, which outcrops on the down-throw side of the Stow Lodge Fault.

Extensive deposits of calcareous tufa have been deposited by groundwaters issuing at the foot of the Chilterns escarpment. A deposit [SP 745 003] south-west of Chinnor infills a depression into which several streams flow; the streams are fed by springs rising from beds in the Chalk Marl. The deposits, up to 1 m thick, are creamy white or slightly ochreous calcareous silts, with many gastropod shells and shell fragments.

Sloping 'aprons' of calcareous tufa occur below springs at Wainhill [SP 769 017] and the Lyde, Bledlow [SP 778 024]. Here the calcareous sediment contains rootlets and abundant diminutive gastropod shells and locally detrital clay, silt and fine sand. The deposit downstream at Henton [SP 765 030] is largely detrital, having been transported by the Cuttle Brook from Wainhill and from a more local spring [SP 767 023] issuing from the base of the Upper Greensand. A section near the sewage works [SP 7600 0337] showed 0.5 m of pale grey, calcareous, clayey tufa overlying 1.5 m of white and yellowish mottled tufa, resting on chalk-flint gravel. Downstream, the tufa passes beneath the alluvium.

Chapter 11 Structure and geophysics

The district is situated to the north-west of the early Cainozoic London Basin. The Mesozoic rocks rest unconformably on 'basement' rocks ranging in age from Early Palaeozoic (Tremadoc) to Carboniferous (Westphalian), which lie at depths of between 50 and 140 m below OD (Figure 3).

Basement structure: Lower Palaeozoic

An area of relatively undeformed Lower Palaeozoic rocks, the Midlands Microcraton, underlies the district (Tucker and Pharaoh, 1991). The microcraton is bounded to the north-west and north-east by intensely folded, basinal Lower Palaeozoic rocks comprising the Caledonides, and to the south by the Variscan Fold Belt (Figure 26). The central core of the Midlands Microcraton, the Birmingham–Oxford Block, is characterised by high-amplitude, positive aeromagnetic anomalies indicating shallow magnetic sources, less than 2 km below ground level; it is interpreted as a north-north-west-trending mass of uplifted magnetic basement. Evidence suggests that the Midlands Microcraton had a major influence on sedimentation during the Palaeozoic, and probably also during Mesozoic times.

Geophysical lineaments based principally on the interpretation of Bouguer gravity and aeromagnetic anomalies (Figure 27) and (Figure 28), probably define major structural or stratigraphical boundaries in the concealed Palaeozoic 'basement' rocks. Other geophysical data taken into account include those from seismic reflection, resistivity sounding and detailed ground magnetic surveys (Allsop, 1988; Cornwell and Allsop, 1979). A 3-D Euler deconvolution study of aeromagnetic data (Reid et al., 1990) has supplemented the conventional interpretation of this information.

Several basement lineament trends have been recognised (Figure 27) and (Figure 28). The northern part of the district appears to be dominated by east-north-east-trending structures, notably the Charlton Axis. The southern part has fewer recognisable lineaments, but north-south and east-west trends can be recognised.

East-north-east-trending lineaments

The Charlton Axis is characterised by high-amplitude, positive aeromagnetic anomalies which are interpreted as magnetic rocks at depths of less than 1 km below ground level (Figure 28). These anomalies disappear immediately west of the district, where they seem to terminate against a north–south lineament (A–A′).

To the east-north-east, the aeromagnetic anomalies associated with the Charlton Axis are interrupted and apparently displaced southwards by a north–south-trending lineament (B–B′) (Figure 27) and (Figure 28). East of this displacement, the amplitude and frequency of the aeromagnetic anomalies are lower. The Euler solutions (Reid et al, 1990) suggest that the magnetic sources associated with the axis are deeper here, and may also result from an extensive igneous mass.

The Charlton Axis has influenced the structure, both of the uppermost part of the Palaeozoic 'basement', and the overlying Mesozoic strata. Its near-surface expression, the Charlton Anticline, corresponds with a high in the Palaeozoic 'basement' surface (Figure 3). Some details of the nature of the axis have been provided by ground magnetic surveys (Cornwell and Allsop, 1979). These indicate that it coincides with two elongated sheet-like magnetic bodies, probably representing inclined igneous intrusions or lava flows, such as those, of possible Early Silurian age, proved in the Bicester Borehole (Chapter Two), The more south-easterly of the subparallel bodies dips to the south-east and probably extends up to the basement surface, where it directly underlies the core of the Charlton Anticline. The second body converges to within 1.5 km of this and appears to increase in depth towards the south-west.

From the crest of the Charlton Anticline, the surface of the Palaeozoic 'basement' dips gently to the south-east (Figure 3), and more steeply to the north-west. Interpreted seismic reflection data suggest that deep-seated faults within the basement define both the south-east and north-west boundaries of the Charlton Axis, the north-western fault being the more clearly defined and steeper dipping.

Bouguer gravity and aeromagnetic anomaly data (Figure 27) and (Figure 28) indicate a change in the character of the basement rocks across the Charlton Axis. Negative aeromagnetic anomalies flanking the north-western boundary fault indicate a deepening of the magnetic basement. However, the Bouguer anomaly data suggest that there is no corresponding increase in the thickness of the overlying low-density sediments. Thus the change in geophysical characteristics is likely to indicate the presence of nonmagnetic rocks with similar densities to those within the core of the structure. To the south-east of the axis, similar, but much weaker aeromagnetic anomalies are present. The aeromagnetic anomaly pattern is partially obscured by the regional effect of strong, positive anomalies farther south. Within the district, the Charlton Axis is not clearly defined on the Bouguer anomaly map, but a gravity gradient, extending southwards from the structure, suggests a considerable increase in the thickness of low-density basement rocks. This may indicate a thickening of the Devonian sedimentary rocks, above a deepening magnetic basement, which may also change in character southwards.

North-to-south-trending lineaments

The two major north–south-trending lineaments, possibly faults (A–A′ and B–B′), which are coincident with the termination of the Charlton Axis to the west and displace it in the east, are expressed as gradients on the Bouguer and aeromagnetic anomaly maps (Figure 28) and (Figure 29). The western lineament A–A′ continues beyond the margins of (Figure 28) as an aeromagnetic feature, but its gravity expression is discontinuous. It is associated with a deepening of the magnetic basement to the west, and as such defines the western boundary of the uplifted Birmingham–Oxford Block (Figure 26).

East-to-west-trending lineaments

Evidence for the east–west-trending lineament D–D′ is especially clear from gradients on the aeromagnetic anomaly map (Figure 28); it probably represents magnetic variations within the upper part of the Palaeozoic 'basement'. It is related to the boundary of a series of strong, positive aeromagnetic anomalies, which occur to the south of the district; the character of these anomalies suggests several large, moderately magnetic, intrusive bodies at depths in excess of 1.5 km. The associated gravity gradients suggest that these postulated intrusions may have a lower density than the surrounding basement.

North-west-trending lineaments

The lineament C–C′ in the north-eastern part of the district is defined by strong gradients on both Bouguer anomaly and aeromagnetic anomaly maps (Figure 27) and (Figure 28). North-west-trending lineaments are a characteristic of the Palaeozoic rocks of the Eastern Caledonides, and of the eastern part of the Midlands Microcraton, including the Leighton Buzzard district (Shephard-Thorn et al., 1994).

Basement structure: Upper Palaeozoic

Silurian and Devonian strata probably underlie the greater part of the district (Figure 3), resting unconformably upon older rocks. They are succeeded unconformably by Upper Carboniferous (Westphalian) rocks in the west (Chapter Two). During the Variscan orogeny, these beds were downfolded into the deep north-northwest-trending structure which includes the Oxfordshire Coalfield and the Berkshire Coalfield synclines. A long period of erosion and uplift preceded the deposition of the Mesozoic strata.

Structures in the Mesozoic rocks

Throughout the district, the average dip of the Jurassic strata is 0.5 to 0.7° towards the south-south-east or southeast (Figure 29). Locally, the dip increases to 2°, or exceptionally 4°, as a result of local flexuring. At surface, faults show two dominant trends; north-east, parallel to the Charlton Anticline, and north-west, parallel to the Wheatley Fault Zone. Subsidiary fault trends are approximately east–west and north–south.

Charlton Anticline

The north-east-trending Charlton Anticline (the Islip Anticline of some earlier works) crosses the north-western part of the district through Charlton-on-Otmoor [SP 562 158]. It can be recognised by the outcrop of harder beds of the Great Oolite Group, which form the small hills rising above the outcrop of the Kellaways and Oxford Clay formations (Figure 1) and (Figure 29). The anticline is the surface expression of the deep-seated Charlton Axis. To the north-east, it extends to Marsh Gibbon [SP 646 230], and possibly as far as Bletchley, but south-westwards it cannot be traced beyond the River Cherwell, immediately to the west of the district. The north-west limb is slightly steeper than the south-east, but dips are generally less than 3°. Over much of its length, the anticline coincides with a horst defined by subparallel normal faults, indicated as the Merton and Street Hill faults on (Figure 29), although their continuity is unproven because of the widespread covering of river deposits and the absence of reliable markers in the Lower Oxford Clay. The anticline is cut by many sinistral wrench, and normal faults, with small displacements, which trend mostly perpendicular to the anticlinal axis. These divide the main structure into a series of small, periclinal folds. The complementary Wendlebury Syncline occurs to the north-west of the Charlton Anticline.

Wheatley Fault Zone

The Wheatley Fault Zone, a north-west-trending complex of faults and folds, can be traced for a distance of over 15 km, from near Great Haseley [SP 653 025] to near Woodeaton [SP 539 124], where it converges with the Charlton Anticline, producing the Noke Pericline (Figure 29).

The main faults within the Wheatley Fault Zone appear to be normal, and, over part of its length, define a synclinally folded graben. The bounding faults of the graben are complex, with closely spaced subsidiary faults. The large-scale and complex history of movement of the Wheatley Fault Zone suggests that it is probably related to a major feature of the Palaeozoic 'basement', although no corresponding lineament has been detected from geophysical data (Figure 27) and (Figure 28). It may be associated with the eastern margin of Carboniferous strata beneath the Mesozoic (Figure 3).

Marked thickness and facies changes within the Corallian Formation, most notably the north-eastward termination of the Wheatley Limestone Member, and overall passage of the formation into clay facies, coincide approximately with the location of the Wheatley Fault Zone (see Chapter Five). This led Arkell (1947a) to suggest that differential movements along the fault zone were important in controlling local Corallian sedimentation. Attenuated successions in the Ampthill Clay and Upper Kimmeridge Clay, and the development of coarse sands in the latter to the south-west of the Wheatley Fault Zone, suggest continued movements along the structure during late Jurassic times (Chapter Six), although there is no evidence of analogous Kimmeridgian faulting in the Thame–Brill area as envisaged by Ruffell and Wignall (1990); see Cox et al. (1990).

The differences in the stratal successions preserved on either side of the Wheatley Fault Zone, give evidence of several phases of early Cretaceous movement and inversion (Chapter Eight). Between Shotover Hill and Wheatley, the Whitchurch Sand rests on the Portland and locally on the Purbeck formations. However, across the Wheatley Fault at Forest Hill [SP 585 075], Whitchurch Sand rests directly on Kimmeridge Clay. As at Thame (see below), the absence of Portland and Purbeck at Forest Hill is best explained by movement on the Wheatley Fault, and erosion of the Portland and Purbeck Formations from the north-east, upthrown block, before the deposition of the Whitchurch Sand.

Along the Great Milton Fault (Figure 29), Whitchurch Sand, with underlying Purbeck and Portland formations, is present only to the south-west; to the north-east, Lower Greensand rests directly on Upper Kimmeridge Clay. This suggests downthrow to the south-west after the deposition of the Whitchurch Sand. Subsequent erosion removed an estimated 15 to 20 m of strata (uppermost Kimmeridge Clay, Portland, Purbeck and Whitchurch Sand formations) from the north-east (upthrown) side of the fault, prior to deposition of the Lower Greensand. In contrast, after the deposition of the Lower Greensand and before the deposition of the Gault, movement on the Great Milton Fault appears to have been reversed, with downthrow to the north-east; the Lower Greensand is thicker (up to 16 m) to the north-east of the fault than the south-west, where it is overstepped by Gault. Post-Gault movement of the Great Milton, Haseley and Latchford Copse faults is evident from the displacements of the base of the Gault Formation. These movements were probably mostly of Cainozoic date (Arkell, 1947a). However, facies and thickness changes in the uppermost Gault and Upper Greensand of the Tetsworth area may indicate Late Albian movement on an inferred extension of the Great Milton Fault, and Arkell (1942, 1947a) attributed two local earthquakes recorded in the seventeenth century to continued activity along the Wheatley Fault Zone.

Noke Pericline

Between Islip and Woodeaton, the Charlton Anticline and Wheatley Fault Zone converge to form a pericline (Figure 29), in the core of which the oldest strata to crop out in the district (White Limestone Formation) are seen. The dome is asymmetrical, with dips of 7 to 10° on the western and southern flanks, and gentler 3 to 4° dips on the northern and eastern limbs. Arkell (1944b) and Wyatt and Ambrose (1988) describe the structure in detail.

Other tectonic structures

Other faults occur throughout the district, generally with displacements of less than 5 m, although a few have throws of up to 20 m. The principal faults have a north-west to south-east orientation, subparallel to the Wheatley Fault Zone (Figure 29) and the C–C′ basement lineament (Figure 27) and (Figure 28), for example those at Coney Hill, Waddesdon, Stone, Upton, Dinton and Shabbington. A subsidiary group trends north-east to south-west, like the Charlton Anticline, and includes the Chearsley Church, Beachendon, Pollicott and Depot faults (Figure 29). A few faults show an east–west orientation, for example the Murcott and parts of the Studley and Thame fault; these parallel the D–D′ basement lineament.

Several of the north-west to south-east and east to west faults show evidence of intra-Cretaceous movements, similar to those associated with the Wheatley Fault Zone. Between the Upton and Dinton faults in the eastern part of the district (Figure 29), Gault rests variously on Lower Greensand or Portland Formation, but on either side, it rests on the Purbeck Formation. These relations are best explained by the development of a horst between the faults after deposition of the Purbeck Formation, and its subsequent erosion from the horst before deposition of the Lower Greensand (Sumbler, 1990a). Relations at the north-western end of the Dinton Fault suggest that the horst may have developed prior to deposition of the Whitchurch Sand Formation. Post-Albian inversion is indicated by the graben affecting the base of the Gault at the south-eastern end.

Within the complex of intersecting faults south of Thame [SP 71 05], different formations (Kimmeridge Clay, Portland, Whitchurch Sand or Lower Greensand) are present beneath Gault in different fault-bounded blocks. Briefly, the east-west Thame and Park Meadow faults, and the north-west-trending Moreton Fault show evidence of initial downthrow to the north after deposition of the Portland Formation but before that of the Gault. Records from a brickpit, adjoining the south side of the Park Meadow Fault (Chapter Eight), further suggest that these initial movements took place prior to deposition of the Whitchurch Sand Formation. The distribution of Lower Greensand, apparently present only between the Thame Fault and the northern branch of the Park Meadow Fault, implies an inversion, probably of early Albian age, prior to deposition of the Gault. Additionally, all the above faults show evidence of inversion after the deposition of the Gault. The north-east to southwest-trending Meadow Brook Fault appears to be entirely a post-Gault structure.

Several faults, including the Oakley and North Weston faults, have an approximately north–south orientation (cf. lineaments A–A′ and B–B′ of (Figure 27) and (Figure 28)), and several displace the Chalk near Chinnor. Multiple small-scale faults with this trend, probably accentuated by superficial effects, can be observed in Chinnor Quarry (Sumbler and Woods, 1992).

Folds are generally broad and of low amplitude, and are difficult to detect, particularly in thick clay formations. Both Davies (1907a,b) and Barrow (1908) described folds exposed during the excavation of the Great Western Railway cutting north of Dorton, with dips of 2 to 3°. These folds probably trend north-east to southwest (Cox and Sumbler, 1989), as does the shallow Thame Valley Syncline extending from the Coney Hill Fault towards Chearsley. Complementary anticlines occur to the north-west at Upper Winchendon–Chearsley and to the south-east in the Stone–Dinton area. It is probable that similar minor structures occur throughout the district.

Superficial structures

Superficial (nondiastrophic) structures are related to the present topography. They probably formed during the cold stages of the Pleistocene, when permafrost was criticial to their origin.

Cambering (Plate 21) typically occurs where resistant, permeable formations cap slopes of clay. Deformation and outward movement of the clay due to the load of the superincumbent strata causes collapse of the latter, which then extend downslope as a 'camber' of disjointed blocks separated by faults with small displacements ('dip and-fault'; Horswill and Horton, 1976). Locally, cambering is associated with the Corallian Formation near Beckley [SP 556 110] and Pans Hill [SP 616 141], and more especially with the Portland, Purbeck and Whitchurch Sand formations, for example on Shotover, Brill and Muswell hills.

In some cases, the faults between the cambered blocks open to form 'gulls' which may be infilled with overlying sediments. Probable gulls affecting the. Lower Greensand and Gault, were exposed during construction of the M40 motorway near Great Milton [SP 631 045] (Chapter Eight).

Typically, cambering is associated with 'valley bulging', which is the upward movement of clay strata in valley floors as the result of overburden pressure. This phenomenon may explain the presence, at some localities, of older beds than expected from stratigraphical considerations. Examples include the presence of the Oakley Member within the Ampthill Clay outcrop in the floor of Danes Brook [SP 6339 1394] and low Kimmeridge Clay strata within the outcrop of younger parts of the formation in Eythrope Park [SP 76 14].

Chapter 12 Economic geology

In the past, various minerals were exploited to satisfy local needs. However, as the demands on quality became more exacting, most of the local industries became uneconomic. The only resource currently being worked on a large scale is chalk, for the manufacture of cement.

Building stone

There are many stone houses in the villages in the northwest of the district. Most of the material is probably imported Wheatley Limestone (possibly from Beckley), or Great Oolite limestones from areas to the west. The local limestones of the Great Oolite Group are unlikely to have provided high-quality stone, although the Forest Marble may have yielded material for lesser buildings and walls.

By far the most important source of building stone was the Wheatley Limestone Member. Arkell (1947b) quotes Oxford college records which indicate its use as early as the thirteenth century. At first, the quarries at Wheatley supplied building stone to many Oxford colleges and to more distant localities such as Windsor Castle. However, after the end of the fourteenth century, new quarries at Headington became more important (Plate 9), and continued to be used into the early nineteenth century, by which time the poor weathering characteristics of the stone had been recognised. The earliest buildings were commonly constructed of 'random rubble' (undressed stone) or only partially dressed rock, often using Coral Rag lithologies. Later, sawn blocks of freestone from the detrital facies of the Wheatley Limestone was used for finishing exposed surfaces or for entire walls.

The oldest quarries at Wheatley were probably situated at Charlgrove [SP 5902 0659], where numerous excavations can still be recognised. A partially restored quarry to the south-west [SP 5930 0626], may date from a later period. The two large quarries in Wheatley village [SP 5943 0613]; [SP 5950 0592] may still have been active at the end of the last century (Pocock, 1908); the more easterly Gaol House Quarry is now incorporated into the Recreation Ground. Site investigation work suggested the presence of a third quarry [SP 5970 0585] close to Wheatley Church. The large Lye Hill Quarry, north-west of the village [SP 592 068], was probably the last to work in the Wheatley area, and still exposes extensive sections in the Wheatley Limestone.

Headington, now part of the Oxford suburbs, is built on a complex of ancient quarries, as indicated by the irregular topography and street names, such as Quarry High Street, Quarry Road and Pitts Road. Most of the quarries worked the detrital limestones of the Wheatley Limestone. The most extensive excavation [SP 555 067], is now a scrub thicket with numerous steep-sided hills of quarry waste. Here, the stone was worked from beneath a thick cover of Ampthill Clay and Kimmeridge Clay; elsewhere [SP 565 063], this was used for brick and tile manufacture. Of the quarries to the west of the village, the Magdalen or Workhouse Quarry [SP 551 023] was probably the last to be worked (Arkell, 1947b). There were numerous quarries at Cowley, to the south of Headington (Figure 14).The strata worked included a significant proportion of coralline limestones.

The Portland Formation was worked for building stone throughout its outcrop, and has been used for the construction of many buildings in the local villages. Much of the stone used is a sandy shell-debris limestone. Although less durable, the micritic limestones of the uppermost part of the sequence in the central and north-eastern parts of the district, together with those from the succeeding Purbeck Formation, have also been used locally for building.

There were many quarries between Little Milton, Great Milton and Great Haseley. The largest were situated at the Recreation Ground, Great Milton [SP 6321 0295], north of Rectory Road, Great Haseley [SP 6415 0195], and on both sides of the road to Milton Common [SP 6445 0250]. Former quarries also existed near Garsington [SP 5808 0206]; [SP 5780 0258]; [SP 580 024]; [SP 5800 0354]; [SP 5875 0357], and Cuddesdon, [SP 593 046]; [SP 5974 0412]; [SP 604 835]. In several places around Cuddesdon, large slabs of shelly, shell-detrital, sandy limestone, up to 1.2 m long and up to 0.15 m thick, have been used as stile stones or parish boundary markers.

The northern slopes of Brill Hill have been dug for limestone and left unrestored on Brill Common [SP 651 141] (see cover photograph). Quarrying of the Portland Stone at Long Crendon is mentioned in records from the 14th century, and a number of the stone houses in the village date from the early 16th century.

There are many pits on the Portland Formation outcrop west of Aylesbury; the best stone was obtained from the lower part of the Creamy Limestone. Most of the pits were defunct by the beginning of the 20th century; the largest were Dinton Stonepits [SP 759 114] and the Bugle Pit [SP 794 121]. Examples of the local stone can be seen in almost any of the older buildings. In many cases, giant Portlandian ammonites were used as decoration; examples can be seen in the walls around Hartwell Park [SP 795 121] (Plate 18) and in villages such as Long Crendon.

Road metal and lime

Many limestone quarries were dug to provide aggregate, and to manufacture lime for agriculture and use in mortar. Waste and poor quality material from building stone quarries was also utilised for these purposes.

The Cornbrash was intensively worked for road metal because of its rubbly nature, and the outcrop in the north-western part of the district is pockmarked with small pits. The Arngrove Spiculite was also used as a road ballast, and has been exploited in several shallow quarries between Studley and Pans Hill [SP 602 128]; [SP 608 135]; [SP 615 188]. A number of small quarries on the Chalk outcrop were presumably used as a source of lime.

The White Limestone is still worked spasmodically for aggregate at Woodeaton Quarry [SP 123 533] (Plate 2), and both White Limestone and Forest Marble were dug for fill material during construction of the M40 motorway (completed 1990) at Merton [SP 572 170] (Plate 3).

Brick clay

In past centuries, bricks were made from all of the clay formations which crop out in the district. Pits commonly supplied local needs and, in some cases, may have been opened to provide bricks for a single house or estate. However, these small local pits gradually closed in the first part of this century as brick manufacture became a larger-scale, mechanised industry.

The Lower Oxford Clay was extensively dug at Calvert [SP 690 240], 5 km to the north of the district, until 1991, but most of the brick clay pits within the district exploited the Upper Oxford Clay. Four such pits (now restored) lie within the area of urban Oxford, at Cowley Marsh [SP 545 049], Cowley Road [SP 5370 0514], St Clements [SP 5300 0584] and New Marston (Jack Straw's Pit) [SP 5303 0745]. Others were Horton-cum-Studley [SP 613 120] and Kingswood [SP 683 177]; [SP 694 184]; [SP 693 194]. A brick pit [SP 7320 1923] north-west of Quainton Station was dug through the West Walton Formation into the Upper Oxford Clay. The basal Upper Oxford Clay and uppermost Middle Oxford Clay were worked at Woodham [SP 709 185] until the late 1960s (Plate 6), and a scheme for brick manufacture at an adjacent site was proposed in the 1980s.

The Ampthill Clay and overlying Kimmeridge Clay were worked in a pit [SP 555 063] at Risinghurst, and at Rid's Hill, Brill [SP 664 152]; the latter pit closed in 1911. The uppermost part of the Kimmeridge Clay Formation was worked extensively, as the beds at this level contain silt or sand which helped to prevent shrinkage, and it could be dug in conjunction with sand and limestone from the overlying Portland Formation. Large pits were located at Kiln Lane [SP 560 067]; [SP 563 067] on the northwestern extremity of Shotover Hill (Plate 12). Both of these pits, and those to the south [SP 5664 0644], [SP 5661 0637], extended up into the Pectinatus Sand, which was used to modify the clay raw material. The Littleworth Clay Pit [SP 589 056] used the same source rocks, but old pits [SP 592 055] to the east, were entirely within the clay outcrop. Woodward (1895, p. 168) recorded a brickyard, probably in Kimmeridge Clay, between Waterstock and Great Milton possibly [SP 6360 0480] or [SP 6348 0466]. The Kimmeridge Clay was also dug at Brill [SP 650 144]; [SP 6501 1422]; here Pringle (1926) noted that the higher beds furnished a 'mild earth' (presumably silty or sandy) well adapted for brickmaking, while the clay below was stiffer and more suitable for making tiles and drain pipes. The Kimmeridge Clay was dug in several small pits around Long Crendon, the largest [SP 6975 0820] situated southeast of the village. Many small diggings mark the Kimmeridge Clay outcrop around Stone, Dinton and Cuddington. Hartwell Silt, at the top of the Kimmeridge Clay Formation, was formerly dug at the extensive Locke's Brickyard, Hartwell [SP 8035 1240] and at Dinton [SP 773 111] ; this unit was also worked at Aylesbury [SP 8220 1420], immediately east of the district.

The clays in the Whitchurch Sand Formation were worked from a large pit [SP 753 102] at Haddenham Low, and also near Great Milton (Pringle, 1926, p. 178).

There are scattered small excavations in the Gault, which were probably worked for the making of bricks, tiles and pots. The workings are restricted to the lower part of the formation; probably the upper part is too calcareous. A pit [SP 689 053] south-west of Thame, opened during the latter part of the 19th century, was worked intermittently until about 1943. Near Thame Park [SP 709 048], basal Gault was worked in conjunction with underlying Whitchurch Sand, and other brickyards occurred at Towersey [SP 728 055] and Marsh [SP 815 086]. Gault may also have been dug for bricks in the Long Crendon area, and at Henton [SP 763 027] and Holly Green [SP 774 034].

Cement

The clay to lime ratio of the Lower Chalk makes it an ideal raw material for cement manufacture. It is exploited for this purpose at Chinnor Quarry [SP 756 000] (Rugby Portland Cement Co.). Since production began in 1908, 70 hectares of ground have been excavated. Currently beds from about 11 m below the Totternhoe Stone up to about 8 m above the Melbourn Rock are worked. Lower beds, close to the works, were formerly quarried.

The clay-rich Chalk Marl is worked separately from the more calcareous Grey Chalk and Middle Chalk. Slurries from these two sources are mixed to the correct composition, stored in tanks and then transferred as required to large rotary kilns fired by pulverised coal. The slurry is dried and then calcined at up to 1500°C. The cold clinker is mixed with a small amount of gypsum, which controls the setting time, and is then crushed to a fine powder to produce cement.

Marl

Marl (calcareous clay) has been used to make walls for houses, farm buildings or boundary walls from early times. The material is known locally as 'witchete, term probably derived from 'white earth' (Plot, 1676). The main source was the Portland and Purbeck formations, but chalk-rich head and calcareous tufa have also been used at some localities. The raw material was puddled with water and sometimes, chopped straw or horse dung was added to give strength. The mixture was placed into position, compacted and allowed to dry. The walls were then protected from the weather by being coated with an impermeable substance such as tar, and capped by tiles or stones. Examples of this construction technique can be seen in the central part of Haddenham. At Chearsley and Long Crendon, 'witchete was used as a weak mortar to bind stone.

South of Tetsworth, a belt of small pits probably exploited a particularly calcareous band in the Gault. It was probably used to improve nearby fields for agricultural purposes.

Sand and gravel

The most extensive workings exploited the deposits of the Seventh Terrace of the River Thame at Milton Pools [SP 655 031] and elsewhere on the plateau at Milton Common. Old pits occur within the Sixth Terrace at Chiselhampton Hill [SP 591 004], the Fifth Terrace at Home Farm, Rycote [SP 662 048], the Fourth Terrace at North Weston [SP 680 059], the Third Terrace at Waterstock [SP 640 059] and Little Milton [SP 612 009], and the Second Terrace near Draycot [SP 656 0621]. However, the deposits commonly contain a low proportion of gravel of small size and a significant amount of clay, and are of little economic significance. The First Terrace of the Thame does not appear to have been worked; the near-surface deposits are dominated by loams.

In the River Ray Basin, First Terrace deposits have been worked in three pits south of Wendlebury [SP 56 18].

Building sand has been obtained from the Beckley Sand in several quarries at Beckley, and from the upper part of the Kimmeridge Clay Formation at the base of Shotover Hill [SP 558 064]; [SP 562 066]; [SP 564 066], near Long Crendon [SP 688 092] and at Thame [SP 6961 0587]. Soft, fine-grained sands from Shotover were used as moulding sand (Pocock, 1908).

There were formerly numerous pits in the Whitchurch Sand, most of which were used for building sand. Fine-grained, white, silica sand from Shotover Hill has been used in the manufacture of scouring soap (Pocock, 1908), and at Stone, the Eythorpe Road Pit [SP 779 129] was worked until the 1960s for glass sand (up to 99.8 per cent silica; Boswell, 1918). The coarse, pebbly sands of the Lower Greensand have been dug in several places [SP 657 044]; [SP 6604 0402] south-west of Rycote and at Peverel Court [SP 796 118] and Curzeley Hill [SP 800 111].

Phosphate

Phosphatic nodules and clasts occur throughout the Gault, but are concentrated at some levels, notably close to the base of the Upper Gault (the orbignyi-varicosum pebble bed). These reworked nodules, traditionally called coprolites, were worked at Moreton, Marsh and Towersey between 1875–85. The main excavations were west of Moreton Farm [SP 7902 0941], where the stratum was dug from shallow workings over a wide area. Mounds of washings produced during the cleaning of the nodules are present [SP 789 084]; [SP 8080 0966]. The nodules were converted to superphosphate fertiliser by roasting, followed by treatment with sulphuric acid.

Ochre

Ochre, consisting of hydrated oxides of iron ('limonite'), was formerly used as a pigment for paints. Thin seams within the Whitchurch Sand Formation were dug from beneath a considerable thickness of overburden. The earliest records were by Plot (1676) who described seams of 'stone ochre' up to 0.18 m thick at Shotover Hill. This material consisted of relatively pure iron oxides. The less valuable 'clay ochre' probably consisted of limonitic clay. The mineral was worked in the latter part of the nineteenth century (Phillips, 1871), but working had ceased by the beginning of this century (Pocock, 1908).

Hydrogeology

The district lies entirely within the Thames catchment. The River Thame drains the central, southern and eastern sections of the district, and the River Ray, a tributary of the Cherwell, drains much of the northern and northwestern areas (Figure 2).

The average annual rainfall is about 650 mm for most of the district, rising to 800 mm over high ground in the extreme south-east. Average annual potential evaporation is about 480 mm.

The aquifers present are the Great Oolite Group, Corallian Formation, Portland Formation/Lower Greensand (including the Purbeck and Whitchurch Sand formations) and Upper Greensand/Chalk. These units are separated by relatively impermeable clay strata (Kellaways, Oxford Clay and West Walton formations; Ampthill Clay and Kimmeridge Clay; Gault), which act as aquicludes. The Hydrogeological Map of the region (Institute of Geological Sciences, 1978) covers part of the district and shows how the aquifers, particularly the Corallian and Upper Greensand/Chalk, relate to regional groundwater flow systems.

The relative importance of the various aquifers is illustrated by the abstraction licence data (Table 1). The Upper Greensand/Chalk aquifer is of greatest importance, supplying the largest total licensed abstraction mainly for industrial use. Licensed abstraction from the Portland Formation/Lower Greensand is also important, supplying agricultural requirements over a large part of the district. There are no public supply groundwater sources within the district, although water was formerly obtained from boreholes in the Portland Formation near Thame for supply to the town.

Great Oolite Group

The Great Oolite Group crops out in a belt of inliers along the Charlton Anticline and in the extreme northwest corner, where it is part of the main outcrop lying outside the district. The limited data available show that water levels are highest in the extreme north-west, and decline towards the south-east. This indicates that the principal recharge area for the aquifer is the main outcrop to the north-west. Recharge may also occur along the line of inliers. However, downward migration of percolating water is likely to be limited by mudstones in the Forest Marble.

At outcrop, water levels are rarely more than 5 m below ground level. However, where the aquifer is confined, overflowing conditions may occur and water levels may stand as much as 5 m above the ground surface. Boreholes constructed in the Great Oolite Group are commonly screened to prevent closure by squeezing marls or clays, or bridging caused by broken rock. Plain casing is required to support the overlying Kellaways and Oxford Clay formations.

Because the aquifer is predominantly composed of limestones, groundwater storage and movement is almost entirely limited to fissures. Borehole yields, which vary according to the number, width and degree of interconnection of fissures penetrated, generally range from about 0.5 to 1.0 l/s. The maximum recorded in the district was for a 150 mm diameter borehole at Weston-onthe-Green [SP 5310 1845] (in a confined section of the aquifer), which yielded 2.2 l/s for a drawdown of 16.8 m. Conversely, no water was forthcoming from the Great Oolite Group in a borehole at Upper Arncott [SP 6137 1723], where fissuring may be absent (Tiddeman, 1910).

Groundwater is generally of good quality at outcrop and in the confined aquifer north-west of the inliers, but quality declines rapidly downdip to the south-east and is unlikely to be potable more than 1 km from outcrop. A water analysis for a borehole near Bletchingdon [SP 5194 1587] (just to the west of the district), illustrates the water chemistry in the confined aquifer about 1 km north-west of the inliers (Table 2). An analysis from Westcott [SP 7100 1648] illustrates the water chemistry some 10 km down-dip of the outcrop.

Kellaways, Oxford Clay and West Walton formations

The Kellaways Sand constitutes a minor aquifer in the major aquiclude, which separates the Great Oolite Group and the Corallian Formation. Occasionally, yields up to 0.8 l/s can be obtained from the Kellaways Sand but the groundwater quality is generally poor. Saline water was recorded in a borehole [SP 5448 1674], near Oddington. Wells and boreholes in the Oxford Clay are generally dry or very low yielding.

Corallian Formation

The lithologically variable Corallian Formation is thickest in the south-western part of the district; to the northeast, it thins due to passage into West Walton Formation. Groundwater elevations are highest where the Corallian is at outcrop and aquifer recharge occurs, and lowest to the south-east, in conformity with the regional groundwater flow system (Institute of Geological Sciences, 1978). The better-yielding boreholes occur in the southwest, the highest recorded yield being 16.5 l/s for a drawdown of only 2.2 m obtained from a 250 mm diameter borehole near Cowley [SP 5513 0311]. Yields of 0.3 to 1.0 l/s are more typical, less in outcrop areas where the saturated aquifer thickness is small. Springs occur at the contact between the Corallian and underlying West Walton Formation.

Typical groundwater quality at outcrop is illustrated by an example in (Table 2). Water quality declines where the aquifer is confined by the overlying clays.

Ampthill Clay and Kimmeridge Clay formations

These beds form an aquiclude between the Corallian and Portland Formation aquifers. Small amounts of water have been obtained from shallow wells in the Ampthill Clay and small springs arise from cementstone units near the boundary between the Ampthill Clay and Kimmeridge Clay. Some of these yield iron-rich ground-waters: for example, the 'Chalybeate Spa' well at Dorton [SP 6694 1358] had a sulphate ion concentration of over 2600 mg/l and about 85 mg/l of iron (Whitaker, 1921). Some groundwater also occurs in the sands near the top of the Kimmeridge Clay Formation.

Portland, Purbeck, Whitchurch Sand and Lower Greensand formations

The Portland/Purbeck and Lower Greensand aquifers are of particular importance; although individual abstraction licenses are generally small, the annual abstraction licensed for these strata represents almost 70 per cent of all licensed groundwater abstraction for agricultural use in the district (Table 1).

In the Portland, Purbeck and Whitchurch Sand formations, groundwater storage and movement is predominantly intergranular, although fissure flow is likely in the limestone horizons. The Lower Greensand is variable in thickness and has a patchy distribution, being absent over much of the district. Groundwater storage and movement in it is dominantly intergranular.

At a few localities, the Portland Formation and Lower Greensand are in contact, and form a single aquifer, as, for example, at Spartum Fen [SP 654 016]. This fen is sustained by groundwater rising through a small 'window' in the confining Gault. In view of the calcium bicarbonate groundwater type, it is probable that most of the water originates from the Portland Formation. The hydraulic conductivity of the aquifer (calculated from rising head tests), is about 0.3 m/d for the uppermost horizon immediately below the Gault but of the order of 10 m/d for the bulk of the aquifer. The maximum summer groundwater input to the fen is of the order of 100 m³/d.

Portland/Purbeck/Whitchurch Sand outliers on high ground drain rapidly and possess a thin saturated thickness. Many springs are located at the boundary of the Portland Formation and underlying Kimmeridge Clay, for example around Brill [SP 655 140] and Long Crendon [SP 695 085].

Yields obtained from both the Portland/Purbeck and Lower Greensand are very variable and generally quite small. The Portland/Purbeck generally yields from about 0.1 to 2.0 l/s, in many cases for an appreciable amount of drawdown. Yields from the Lower Greensand range from about 0.3 to 1.0 l/s depending on saturated aquifer thickness. Specific capacities are commonly about 0.3 m²/d. Where both the Portland/Purbeck and Lower Greensand aquifers are present, yields are generally in excess of 1.0 l/s; for example 1.6 l/s from a 114 mm diameter borehole at Emmington [SP 7403 0246] causing a drawdown of 6.1 m. The highly variable nature of the Portland/Purbeck aquifer is demonstrated by wells drilled for public water supply near Thame [SP 7281 0605]. Four production boreholes of 250 mm or 300 mm diameter, penetrated the base of the Portland and entered the underlying Kimmeridge Clay. The yields ranged from 1.1 l/s to 9.3 l/s, the latter for a draw-down of only 0.9 m. Subsequently, a fifth borehole also of 300 mm diameter, was drilled about 1 km away [SP 7372 0652]; it produced a lower yield of 5.0 l/s for a draw-down of 7.3 m.

Boreholes constructed in the Lower Greensand require a well screen and gravel pack to prevent silting up of boreholes. Boreholes penetrating the Portland are often left open, apart from a short length of plain casing at surface, but some slotted or perforated screen has also been used.

No detailed water chemistry data are available for the Lower Greensand, but an example from the Portland is provided in (Table 2).

Gault Formation

The Gault forms an aquiclude between the Upper Greensand/Chalk and Lower Greensand/Portland Formation aquifers.

Upper Greensand Formation and Chalk Group

Although occupying a limited area in the extreme southeast of the district, the Chalk and Upper Greensand represent an important source of groundwater, as illustrated by the total annual licensed abstraction (Table 1).

A groundwater 'ridge' is present beneath the Chalk escarpment, indicating that aquifer recharge occurs in this zone. Groundwater levels decline both to the south-east and north-west of the ridge in conformity with the regional groundwater flow system (Institute of Geological Sciences, 1978). Groundwater levels vary on a seasonal basis, a total range in excess of 8 m having been recorded for a borehole near Chinnor [SP 7559 0017].

In the Chalk, groundwater storage occurs in both the intergranular matrix and to a lesser extent, in the fissure system, but groundwater movement is almost entirely dependent on fissure flow. Highest yields are generally obtained from the Upper and Middle Chalk rather than the more marly and less fissured Lower Chalk. In the Upper Greensand, groundwater storage and movement occur predominantly in the intergranular matrix, but fissure flow is also likely to occur in the better cemented sandstones.

To the south and east of the district, the Chalk and Upper Greensand are commonly regarded as separate aquifers separated by the low permeability Chalk Marl at the base of the Lower Chalk. Evidence from borehole logs, and the presence of springs fed from harder bands within the lower part of the Chalk Marl (for example at Chinnor [SP 747 003] and Saunderton [SP 793 015]) suggest that this is also the case within this district. However, since most boreholes and wells penetrate both the Lower Chalk and Upper Greensand, it is only possible to view the two as a single aquifer locally. A few low-yielding, shallow wells penetrate the Lower Chalk only, but all of the high-yielding boreholes and wells penetrate both the Lower Chalk and the full thickness of the Upper Greensand. Borehole completion is typically a short length of plain casing at surface with open hole below. Since no silting problems have been reported, the Upper Greensand is presumably sufficiently well cemented not to require sand screens.

Yields are variable, generally ranging up to 12.0 l/s, this for a drawdown of 6.4 m in a borehole of 240 mm diameter near Chinnor [SP 7559 0017], with the largest recorded yield in the district of 17.0 l/s from a 3.6 m diameter shaft at Princes Risborough [SP 7993 0260].

Specific capacity values range from about 1.0 to 3.5 m²/day. It would, however, appear that the contribution of Lower Chalk groundwater to the overall yield of many wells and boreholes is variable but small, most of the groundwater being derived from the Upper Greensand.

Groundwater quality is generally good, with total hardness in the range of 200 to 400 mg/l (as CaCO₃) and chloride ion concentrations less than 30 mg/l. A chemical analysis of water from a borehole at Monks Risborough [SP 8140 0464] is given in (Table 2).

Landfill and waste disposal

Most of the numerous quarries and clay pits which once existed in the district have been partly or wholly back-filled with a variety of materials. Larger-scale tipping has taken place at a number of sites, including Woodham Brickpit [SP 709 184] and Eythrope Road Sandpit, Stone [SP 779 126] which accepted industrial wastes. Only two landfill sites in the district were operational at the time of survey. That at Littleworth Mobile Homes [SP 589 057] accepts domestic waste and that at Rymans Farm [SP 657 124] is used for agricultural waste. Both are sited on the Kimmeridge Clay and consequently present little direct risk to groundwaters.

Boreholes penetrating the confined saline section of the Great Oolite Group aquifer at Westcott [SP 7100 1648] were used for the disposal of liquid waste for a short time in the late 1950s and early 1960s.

Chapter 13 Engineering geology

Most of the geological formations in the district have an unconfined compressive strength of less than 1 MPa and can be classified as soils in engineering terminology (Hencher, 1993). These are subdivided into cohesive (clay-and silt-rich) or noncohesive (sand-rich) soils (Table 3). Materials stronger than this generally are termed rocks.

The cohesive soils are mainly overconsolidated clays, i.e. clays which have been subjected to a greater overburden stress than that currently acting on them, as a result of burial by younger deposits. Alluvium is the only normally consolidated cohesive deposit in the district, never having suffered burial. Normally consolidated deposits are of very low (clays) to low (sands) strength and have a high moisture content. Overconsolidated deposits have greater strength and lower moisture contents at depth, but on exposure they have a greater ability to absorb water and swell than normally consolidated material. Close to the surface, the effects of stress relief and weathering become apparent and are shown by higher moisture content, lower density and lower strength.

The noncohesive materials are mainly sands. They are dense at depth but become loose in the disturbed zone near to the surface, although some may be weakly cemented locally. Coarse noncohesive gravels are represented by the river terrace deposits. Noncohesive deposits often carry water which may affect engineering activities and structures.

The rocks, which are mainly limestones with a few sandstones, crop out only over relatively small areas. They are generally weak to moderately strong (compressive strengths of 1 to 50 MPa) and most are susceptible to weathering.

A mixed category includes formations which comprise interbedded material of contrasting properties, such as sandstone, sand, clay and silt. Also included is fill, which may contain unstructured natural and artificial components with a wide range of properties. Such deposits must be examined on a site-specific basis.

Geotechnical properties of formations

The following account is based on commercial site-investigation reports from a number of sources. Sampling and testing procedure may not be uniform and variations in the degree of weathering and mechanical disturbance of the material tested may affect the consistency of the results considered. For these reasons, only generalised descriptions of the engineering properties can be given here. Nevertheless, the data presented should assist in planning detailed site investigations.

Great Oolite Group

The Great Oolite Group is composed principally of limestone, but thin and, in some cases, discontinuous units of clay and calcareous clay are present at many levels. These may give rise to seepage in excavations and cut slopes, and settlement may be uneven.

The White Limestone Formation comprises micritic limestones interbedded with clay beds. The limestones are generally weak to moderately strong. At depth, the bed thickness is generally less than 1 m and the rock is well jointed. The weaker limestones may be frost susceptible. The Forest Marble Formation comprises oolitic, shell-detrital and sandy limestones interbedded with mudstones. Although many of the limestones are strong and thickly bedded at depth, the finer-grained limestones are often frost-susceptible. The Cornbrash limestone is thickly bedded when fresh but weathers to hard rubbly limestone at outcrop.

Kellaways Formation

The Kellaways Clay Member is an overconsolidated, fissured, silty clay. Moisture content ranges from 19 to 25 per cent; bulk density ranges from 2.00 to 2.24 Mg/m³, and plasticity is high. Undrained cohesion values range generally from 95 to 250 kPa and, exceptionally, to 340 kPa. The strength generally increases with depth. The overlying Kellaways Sand Member is a silty, clayey, fine-grained sand or, less commonly, a sandy, silty clay or clayey, sandy silt. Standard Penetration Test values indicate that it is very dense and suggest some degree of cementation. Data for the more clayey samples indicate low plasticity. Moisture content ranges from 13 to 27 per cent, with a median value of 18 per cent.

Oxford Clay Formation

The three members of the Oxford Clay are broadly similar in overall geotechnical properties but show significant differences in detail. Moisture content in the near-surface ranges from 11 to 54 per cent, but decreases to 12 to 27 per cent at depth. A comparable trend is followed by bulk density; for example, in the Middle Oxford Clay the values near the surface are 1.75 to 2.1 Mg/m³, but increase to 21.5 Mg/m³ at 20 m depth. The scatter shown by the plotted values is attributed to variation in cementation, composition and sample disturbance. The general trend is typical of the effects of weathering and stress relief on overconsolidated clay.

The plasticity charts ((Figure 30) A, B, C) show spreads from low to very high plasticity. In the Lower Oxford Clay, most values lie close above the A line, which shows a gradation from high to intermediate plasticity clay. The results for the Middle Oxford Clay indicate two distinct clay types, one of intermediate to high plasticity just above the A line, and the second of high to extremely high plasticity subparallel to the A line but further from it. The Upper Oxford Clay shows a similar wide range of values but without the distinct grouping.

Values for the undrained cohesion show a wide range from 20 kPa at the surface (Lower Oxford Clay) to at least 500 kPa at depths of more than about 20 m (Middle Oxford Clay). For each of the Oxford Clay members, there is a diffuse relationship between undrained cohesion and depth ((Figure 30)D). This suggests some differential induration and cementation in the overconsolidated clay, an interpretation supported by the relatively shallow depth (about 7 m) to which stress relief has occurred, as indicated by the moisture and density data. A steady increase in strength with depth is also shown by the Standard Penetration Test values, which in the Lower Oxford Clay increase from about N = 5 at 2 m to N = 50 at 16 m. Data for the Lower Oxford Clay indicates a slow to medium rate of consolidation and slow rates for the younger members. The Lower Oxford Clay has a low to medium compressibility whilst the other divisions have medium compressibility.

West Walton Formation

The West Walton Formation is an overconsolidated, fissured, silty clay; rarely, sand grade shell debris may be present. The moisture content and bulk density are inversely related and show the effects of weathering and stress relief on an over-consolidated clay. At the surface, the values of moisture content and density are 10 to 60 per cent and 1.6 to 2.05 Mg/m³ respectively. Below 20 m they are about 20 per cent and 2.20 Mg/m³ ((Figure 31)A). Undrained cohesion increases with depth from about 25 kPa near the surface to more than 600 kPa below 25 m ((Figure 31)B). Strength shows a change from soft at the surface to hard at 20 m depth. The material is mainly of high plasticity but with a significant spread into the intermediate and very high plasticity fields ((Figure 31)C). Up to 1 per cent of the samples exhibit extremely high plasticity. Consolidation test results indicate low to medium compressibility with consolidation taking place at a slow to medium rate.

Corallian Formation

The Corallian Formation is subdivided into several members of disparate lithology and geotechnical properties.

The Temple Cowley Member comprises laminated silts with thin beds of fine sand, sandstone and silty mudstone. No test results are available.

The Arngrove Spiculite Member comprises siliceous spiculite and spiculitic sandstone, sand, silt and sandy clay. In part, it is sufficiently cemented to be classed as a weak rock, although the only geotechnical information available for the member is for silty clay and clayey, silty sand. Standard Penetration Test values indicate a loose to very dense material (median value is medium dense). Undrained cohesion values indicate consistencies ranging from firm to hard (median value is stiff). Undrained cohesion and penetration test values show an increase in value with increasing depth. At the surface, moisture content ranges from 10 to 55 per cent, but the range decreases with depth to 19 to 34 per cent at 4 m. Similarly, bulk density shows a range near the surface of 1.65 to 2.25 Mg/ m³ which decreases below 4 m to a range of 1.85 to 2.1 Mg/m^3. Plasticity data reflect the wide range in composition of the Arngrove Spiculite, from sandy clays of low plasticity to clays of very high or extremely high plasticity. A significant number of values plot below the A line, classifying part of the material as a silt of high plasticity. Consolidation rates range from fast to slow with low to high compressibility.

The Beckley Sand Member comprises sands which may be silty or clayey, with sporadic calcareous sandstone beds and doggers. Bulk density ranges from 1.91 to 2.26 Mg/ m³ and moisture content from 16 to 30 per cent. Standard Penetration Test values for noncohesive deposits indicate a medium to very dense material. Undrained cohesion values from triaxial compression tests on the more clayey material range from 11 to 190 kPa. Consolidation is fast with low compressibility. The more clayey lithologies are of low to intermediate plasticity.

No geotechnical data are available for the Littlemore Member, but it is dominated by clays similar in composition to those of the Oakley Member.

The Oakley Member comprises fine-grained limestone, and silty clay or clay, locally sandy, gravelly (rock fragments) or calcareous. Moisture content (clays) ranges from 13 to 36 per cent. Standard Penetration Tests indicate a dense to very dense material for the non-cohesive soils of the member and undrained cohesion values indicate a stiff to very stiff consistency for the cohesive soils. The median value of bulk density is 2.06 Mg/m^3. Plasticity data show a wide scatter of values ranging from low to very high, dependent on composition. Particle size analyses show up to 50 per cent sand and 20 per cent gravel-size particles (probably shells or nodules) in a few samples.

The Wheatley Limestone Member is dominated by medium- to coarse-grained, shell-debris limestones with shelly, clayey, coral-rich limestone and silty, sandy, calcareous clay. The strongest limestone may be moderately strong and beds are generally less than 0.6 m thick. Standard Penetration Test values indicate a relatively weak material. Moisture content has a range of 7 to 28 per cent (median 15 per cent). Data from the more clayey samples indicate intermediate plasticity.

Ampthill Clay Formation

The Ampthill Clay is an overconsolidated, fissured, silty clay with thin, impersistent beds and nodules of limestone. In places, sand-grade shell debris or selenite may be present. Moisture content varies from about 11 to 43 per cent down to a depth of about 10 m, with little correlation with depth; below 10 m, values vary from about 24 to 28 per cent down to 25 m. Values of bulk density show a similar wide scatter of 1.83 to 2.15 Mg/ m³ down to a depth of about 7 m; below, to 25 m depth, values are still scattered but generally higher, ranging from 1.94 to 2.14 Mg/m^3. Undrained cohesion values ranging from 40 kPa near the surface to 255 kPa at 25 m depth indicate a firm to very stiff consistency.

The plasticity chart ((Figure 31)D) shows two distinct groups of data, the larger one of intermediate to high plasticity and the smaller of very high to extremely high plasticity. The latter samples (from near Wheatley) may relate to thin beds in the formation. If this is the case, it could be an important consideration in slope stability assessment. Consolidation data indicate a slow consolidation with low to medium compressibility.

Kimmeridge Clay Formation

The Kimmeridge Clay is an overconsolidated, fissured, locally bituminous or calcareous, silty clay. In the upper part, sand and silt are present, as are limestone nodules at some levels. Bulk density ranges from about 1.85 to 2.25 Mg/ m³ and increases with depth from the surface to about 25 m depth. Moisture content decreases with depth from a range of 18 to 38 per cent near to the surface to 14 to 25 per cent at about 20 m depth. As in the other argillaceous formations, the range of values is much greater in the near-surface zone. Values for undrained cohesion range from about 50 kPa near the surface to more than 300 kPa at depths of 15 to 25 m and show a clear relationship of increasing strength with depth ((Figure 32)C). The Kimmeridge Clay is of intermediate to high plasticity. Consolidation data indicate slow consolidation with low to medium compressibility.

Portland Formation

The Portland Formation comprises limestones, sandy limestones and sands. The available geotechnical data relate to a firm to stiff or very stiff, sandy, silty clay or sandy, clayey silt of low to intermediate plasticity. However, the formation includes sands and limestones. The limestone varies from weak to very strong, and although formerly worked for building stone, much of the material consists of fine-grained, partially cemented limestones which break down rapidly on weathering.

Purbeck Formation

The Purbeck Formation comprises fine-grained limestone with interbeds of calcareous clay. Some of the limestones are recrystallised and of high strength, but the less well-cemented limestones are unlikely to resist weathering. Minor beds of clay and marl may impede drainage and give rise to seepage on natural slopes and excavations.

Whitchurch Sand Formation

The Whitchurch Sand is a fine-grained to medium-grained, loose to dense, silty sand with thin beds of secondary ironstone and of clay, silty clay and silt. Where the sand overlies clay, it may give rise to seepage in cuttings and natural slopes. The clay beds probably have properties similar to the other clay formations of the district, but may include clays of very high plasticity.

Lower Greensand Formation

The Lower Greensand is a medium- to coarse-grained, clayey, silty and locally gravelly sand; rarely, it is cemented. Standard penetration test data indicate a loose to medium dense, in part dense material. Moisture content ranges from 8 to 25 per cent. Data for the more clayey samples indicate low to intermediate plasticity.

Gault Formation

The Gault is an overconsolidated, fissured, silty, variably calcareous clay with beds of phosphatic nodules and rare wisps of silt and silty sand. In places, sandy beds occur at its base. Moisture content may be up to 45 per cent near the surface but decreases to a range of 20 to 30 per cent at a depth of 20 m ((Figure 32)A). Dry and bulk density both increase to about 20 m depth. The Gault clay ranges from high to extremely high plasticity ((Figure 32)B) with median values of liquid limit and plasticity index of 77 and 49 per cent respectively (Forster, 1991, table 15). Undrained cohesion values show a wide range, from about 17 to 212 kPa and show a general, though scattered, increase with depth. Consolidation data indicate the Gault to be of very low to medium compressibility and to consolidate at a variable rate, depending on lithology.

Upper Greensand Formation

The Upper Greensand is a weakly cemented, clayey, calcareous, fine-grained sandstone or siltstone. Standard Penetration Test values indicate it to be medium dense to dense. Median values of dry and bulk density are 1.40 and 1.53 Mg/ m³ respectively. It contains thin beds of clayey silt of high plasticity.

Chalk Group

Geotechnical data are available only for the Lower Chalk of the district. This is a weak limestone or calcareous siltstone which weathers to a calcareous clay or silty clay. Except for a few high values in the range 25 to 30 per cent near to the surface, natural moisture content is low, between 1 and 6 per cent, and does not decrease with depth. Median values for dry density and bulk density are 1.73 and 1.81 Mg/ m³ respectively. The silts and clays are of low to high plasticity. Strength data for the weathered material indicate firm to stiff consistency. California bearing ratio values of 12 per cent and an average maximum dry density of 1.79 Mg/ m³ at an average optimum moisture content of 14.5 per cent, were reported for (presumably) relatively unweathered material.

Head

Head is a superficial material resulting from the weathering and downslope movement of solid and drift deposits. Its composition and geotechnical properties vary according to the nature of the parent material. Head is commonly thin (less than 1 m), but greater thicknesses may accumulate at the foot of slopes or in hollows. As it has been subjected to solifluction, it may contain relict shear surfaces capable of reactivation. Shears may also occur beneath granular Head where it overlies a clayey substrate. Low strength may be expected.

Alluvium

Alluvium consists predominantly of silty clay, calcareous in places, with lesser amounts of sand, and organic material. Moisture content is fairly high, with a median value of 30 per cent but ranging as high as 118 per cent in the more organic/peaty material. No correlation between depth and moisture content was found and high moisture contents (100 per cent) may be present at any level within the alluvium. Bulk density follows a similar pattern, with low-density material occurring in the lower zones of the alluvium.

Plasticity values ((Figure 32)D) show a very scattered spread from low to extremely high, with some values falling below the A line. This reflects the variable amounts of sand and organic material present in the silty clays. Standard Penetration Test data show the sands to be loose to medium dense, and the silty clays to be of very soft to firm consistency. Silty clay alluvium is generally of medium compressibility. The sandier parts are of low compressibility, and the more organic or clayey lithologies of high compressibility. Consolidation takes place at a high (organic/peaty) to medium (silty clay/sandy clay) rate.

River Terrace Deposits

The River Terrace Deposits range from gravelly and sandy clays to sand and gravel with some silt and clay. The fine materials, clay, silt and sand, have moisture contents of about 20 per cent. The more clayey samples are of low to intermediate plasticity. Standard Penetration Test data indicate the gravels to be medium dense to dense. Particle size analyses show that the gravels are rather 'dirty', with 0 to 20 per cent of clay, 0 to 40 per cent silt and up to 80 per cent sand.

Fill

The geotechnical properties of fill show a wide range of values controlled by its composition, mode of placement and compaction. Moisture contents range from at least 9 to 38 per cent, and Standard Penetration Tests indicate that they may be very loose to loose. These values may not be representative, particularly of the material infilling the large number of small quarries which occur throughout the district.

Potential site-engineering problems

Slope instability

Extensive areas of landslip have been mapped in the district. Typically, movement is shallow and translational, with rotational elements found at the backscarp and flows at the toe region. Shallow, multiple retrogressive rotational failures occur on some slopes (Plate 19). Landslips are difficult to classify in detail during mapping since the characteristic topographical features which enable the type of movement to be identified are often masked by weathering, erosion and agricultural activity. Ploughing can remove all surface traces in a few seasons but, below the depth of ploughing, shear surfaces capable of reactivation will remain.

Geological mapping has identified landslipped ground on the outcrop of the overconsolidated Gault, Kimmeridge Clay and West Walton formations and the Upper Oxford Clay. Major areas of landslipping occur at Beckley [SP 563 115], Muswell Hill [SP 640 157] (Plate 19), Pans Hill [SP 612 140] (Plate 20), Ashendon Hill [SP 702 140], Lodge Hill [SP 730 165] and Waddesdon Hill [SP 722 158]. Additional minor areas of instability have been mapped, and other unrecognised landslips may occur.

In the district, the slips are often formed as a result of the coincidence of water-bearing strata above a clay slope too steep for the strength of the saturated clay to sustain.

Slopes of West Walton Formation may be capped by the Arngrove Spiculite (for example at Pans Hill, (Plate 20)) or the Beckley Sand members, either of which may carry water. Field observations show that the average slope angle is about 7.5° on both stable and landslipped slopes, with a maximum of 9.5°.

The landslipped slopes of Kimmeridge Clay are capped by permeable sands in the upper part of the formation, or by the Portland and Purbeck formations. Natural stable slopes are at an average angle of 10° (maximum 14°), while landslipped slopes are at an average of 9.5° (maximum 12°).

Unstable Gault slopes in this district are commonly capped by gravel and sandy head, which may be water-bearing. Stable Gault slopes observed in the field seldom exceed 7°, with a maximum of 9°. The slope angles observed for landslipped Gault slopes vary from 7° to 14° with an average of about 10°.

Natural slope instability has not been identified as a problem with other clay units in the area (for example, the Lower and Middle Oxford Clay, Ampthill Clay and Kellaways Clay) because, in this district, they form low-angle slopes and are not overlain by more permeable beds. However, they are potentially unstable in artificial cut slopes.

Head deposits tend to have strengths close to residual values for the parent material and may be marginally stable even at shallow slope angles.

It is possible to estimate the angle of ultimate stability using the 'Infinite Slope' method (Skempton and De Lory, 1957). This is the inclination at which a natural slope becomes stable against any form of landslipping. By using representative geotechnical test data and assumptions about groundwater conditions, approximate values can be derived which can be used for preliminary design purposes. The method is applicable to shallow, largely planar, mass movements where groundwater moves approximately parallel to the ground surface.

Groundwater levels are partially related to rainfall and vary with both the annual cycle of weather and climatic change over a longer timescale. Therefore, different maximum stable slope angles will apply at different times. However, it may be assumed that the least stable groundwater condition, that of groundwater at the surface, will occur ultimately. The maximum stable slope angle for this condition will indicate a safe maximum for long term stability. For this groundwater condition, the Skempton and De Lory equation is:

where

β is the slope angle

γ is the bulk density of the material

γ_w is the density of the groundwater

Φr′ is the residual angle of internal friction of the slope-forming material.

For natural slopes, it may be assumed that those steeper than the maximum stable angle will have slipped until a stable slope is formed (that is, at, or below, the maximum stable angle). Thus, examination of slope angles in the field in both slipped and currently stable areas can establish the maximum slope angle for long term stability. This can be compared with the value calculated using geotechnical test data and the Skempton and De Lory equation. Slopes that, for any reason, exceed this maximum angle may still be at risk from landslipping if groundwater conditions alter, or geotechnical properties change due to weathering. Also, local variations in pore pressures and material properties will affect stability and hence slope angles will show small variations from place to place, even within the same geological formation.

The Skempton-De Lory equation requires values for density and residual strength of the slope-forming material. The latter can be measured in a triaxial compression cell, in a direct shear box and in a ring shear apparatus. Stable slope angles based on triaxial test results may be an over-estimate, because the test may neither provide sufficient displacement to reach residual values, nor take into account the effect of fissuring in the soil mass which would weaken the bulk material. Conversely, use of results from the ring shear test, which uses completely remoulded material, may under-estimate the stable slope angle because, in real slopes, strength is not necessarily reduced to the residual value on all parts of the slip plane; the slope angle based on ring shear test data is commonly 1 to 2° below the value observed in the field.

From available laboratory test data, maximum stable slope angles have been calculated for some of the landslip susceptible formations in the district. For the West Walton Formation, calculated values range from 9.5° (ring shear tests) to 14.5° (triaxial tests). For Kimmeridge Clay, the range is 12° to 14.5° and for Gault 6° to 12.5°. These values can be compared with the observed slope angles quoted above. For the West Walton Formation and the Kimmeridge Clay, observed slope angles are below the lower calculated value, suggesting that the slopes may have attained their ultimate angle of stability against shallow landslipping. However, slopes on Gault are, in places, steeper than the calculated maximum stable slope angle and may be potentially unstable where slopes exceed angles of about 7°.

Human activity is a common cause of slope instability, either by triggering first-time failures or by reactivating existing ones. Failure can be induced by undercutting the foot of a slope, by loading the top of a slope, or by altering pore pressure distributions within the slope, for example if natural drainage is impeded. It is important that the stability of clay slopes is assessed for any engineering works which affect them.

Cambering

Cambering (Chapter 11) has displaced the Portland Formation and associated beds where they cap hill slopes, leaving tension cracks (gulls) on or behind the cambered slope (Plate 21). These may be open or loosely filled with superficial material. This creates a hazard to foundations by offering poor and uneven bearing capacity. Also, the passage of water through gulls may create unstable conditions in excavations and cuttings.

Shrinking and swelling

When clays absorb or release water, their volume increases or decreases respectively. The magnitude of the change is controlled primarily by the type of clay minerals present and is indirectly measured by the plasticity of the clay. The relative magnitude of the shrinking and swelling may be determined by reference to the plasticity charts, thus Gault clay (high to extremely high plasticity) will be more prone to shrinkage and swelling problems than Kellaways Clay (high plasticity).

Problems due to shrinkage are likely to occur where foundations are placed too close to the surface, in the zone affected by annual changes in moisture content, including the effect of prolonged drought. Damage due to shrinkage may also be caused by the desiccation of ground by trees growing close to foundations, particularly at times of prolonged dry weather. Problems may be encountered where trees or hedges were removed prior to construction, because of swelling of the ground as it rehydrates. The effect of drying of subfoundation clay must be considered in the design of industrial premises where hot processes are operating.

Chemical attack on buried concrete

Chemical tests indicate soil conditions in the area to be generally near to neutral or of slightly alkaline pH. Thus acid attack on concrete or metal services below the water table is unlikely. However, peaty deposits or leachate from landfill may be acidic and be potentially corrosive to services and concrete.

Soil conditions which would require sulphate-resistant concrete mixes or other preventive measures to be used for concrete below the water table are present mainly in the cohesive soils of the area. Sulphate content ranges from class 1 to class 5 (Anon, 1981) but class 2 and 3 predominate in the Gault, Kimmeridge Clay, Ampthill Clay, West Walton Formation and Oxford Clay. Soil sulphate contents corresponding to mainly class 1 with a few of class 2 are present in the Wheatley Limestone, Arngrove Spiculite and Alluvium.

The sulphate in the clays is a product of weathering, and is present principally as selenite (hydrated calcium sulphate). This is slightly soluble in water and is dissolved near-surface and redeposited in the underlying zone of water table fluctuation. The amount of selenite decreases below this level as less weathered material is encountered. However, no clear pattern of variation in sulphate content with depth was shown by the data collected for the district, and tests must be done on a site-specific basis to establish the potential for sulphate attack. Sulphate attack should also be considered in connection with the use of lime stabilisation of clay for road sub-base. In the alkaline conditions caused by the addition of quicklime, clay minerals will release alumina, which will combine with sulphates in the presence of water to produce complex calcium aluminium-sulphate hydrates with a significant increase in volume.

Excavatibility

None of the geological formations in the area are expected to give major problems with regard to excavation; only the thicker limestones and sandstones will require ripping or pneumatic tools. Blasting is used to quarry some of the stronger, more massive limestones. Cementstone nodules up to 0.4 m thick in the clay formations (notably the Lower Oxford Clay and Kimmeridge Clay) and hard sandstone doggers in the Beckley Sand, and parts of the Kimmeridge Clay may cause problems during digging. Excavations below the water table in sandy material may experience running sand conditions and appropriate measures must be taken to control water inflow. Excavations in landslipped material may suffer rapid collapse and will need more support than would be necessary for unslipped material. Care must be taken when excavating or building embankments on or below thick head, clay slopes and especially in landslipped material, to ensure that landslipping is not reactivated or initiated.

Trafficability

The movement of site traffic on temporary roads may be difficult in wet weather on the clay formations, especially the more plastic clays such as the Gault and West Walton formations. Alluvium may be superficially desiccated, giving reasonable trafficability for light vehicles until the crust is disrupted when trafficability deteriorates rapidly in the softer, wetter material below.

Foundation conditions

The geological formations present in the district can offer reasonable foundation conditions for light structures, except for Peat, Alluvium and Calcareous Tufa which have a low bearing capacity and may contain highly compressible organic material.

The more plastic clays are subject to seasonal shrinking and swelling in response to the changes in groundwater content. Therefore, foundations must be designed with this in mind, and care should be taken over the removal or planting of large trees or bushes close to building foundations.

Unless head is removed before foundations are placed, its low strength and high compressibility may lead to excessive settlement. Periglacial processes have resulted in a very irregular base to many head deposits; characteristically, pockets of gravelly material penetrate into underlying clay formations. If unrecognised and not removed, this head could lead to excessive differential settlement of structures founded on such features, accentuating the effects of shrinking and swelling of the clays with moisture content change. The presence of shear surfaces formed during solifluction may give a potential for slope instability on low-angle head slopes.

The bearing capacities quoted for domestic landfill sites in the past may not be applicable to current or future sites because the nature of domestic refuse has altered. For example, over the last fifty years the amount of ash has decreased while the amount of paper and plastic has increased. Construction on fill should only be carried out after a detailed site investigation has determined the geotechnical properties of the site, particularly those affecting bearing capacity and settlement. Past industrial activity has sometimes left contaminated ground; thus, tests for chemical and biological contamination should be included in the site investigation of these areas to assess potential hazards to people and construction. Hazards include gases such as methane, inorganic chemicals, heavy metals and organic chemicals such as gas works waste (Eglington, 1979). Where appropriate, measures such as sealing or removal of contaminated land should be incorporated in development planning. Underground space close to methane sources may require sealing or ventilating to avoid the build up of gas. Methane in an explosive concentration (5 to 15 per cent in air) may be ignited by a spark or a shock wave, either of which may be generated during pile driving or dynamic compaction operations. A summary of the geotechnical behaviour of domestic waste has been made by Harris (1979).

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

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Appendix 1 List of constituent 1:10 000 scale geological sheets

The Thame district was surveyed between 1986 and 1990. The component.1:10 000 scale maps are listed below together with the initials of the surveyors (K Ambrose, A J M Barron, A Horton, R D Lake, M G Sumbler, M D A Samuel, R J Wyatt), and dates of survey. For those marked *, only that part which lies within the Thame district was surveyed at the date shown.

Copies of the maps are deposited for public reference in the library of the British Geological Survey, Keyworth and uncoloured dyeline copies are available for purchase.

Appendix 2 List of BGS reports and other data

The following reports are relevant to the district, and are available for public reference and purchase.

Land Survey Technical Reports

Most of the 1:10 000 maps listed in Appendix 1 have a corresponding 'open file' report which provides geological notes and local details specific to that Sheet. In the following list, the report number (in the BGS Onshore Geology Series), author's initials and date of publication are shown.

Mineral Assessment Reports etc.

HARRIES, W J R. 1977. The sand and gravel resources of the country around Eynsham, Oxfordshire: Description of 1:25 000 resource sheet SP 40 and part of SP 41. Minerals Assessment Report of the Institute of Geological Sciences, No. 28.

CORSER, C E. 1978. The sand and gravel resources of the country around Abingdon, Oxfordshire: Description of parts of 1:25 000 Resource Sheets SU 49, 59 and SP 40, 50. Mineral Assessment Report of the Institute of Geological Sciences, No. 38.

CORSER, C E. 1981. The sand and gravel resources of the country around Dorchester and Watlington, Oxfordshire: Description of 1:25 000 Resource Sheet SU 69 and part of SU 59. Mineral Assessment Report of the Institute of Geological Sciences, No. 81.

SUMBLER, M G, and SAMUEL, M D A. 1990. A preliminary study of potential resources of sand and gravel in Buckinghamshire north of the Chilterns. British Geological Survey Technical Report WA/90/50.

Borehole records

At the time of going to press, 1802 borehole records of the district are held in BGS archives, 114 of these being more than 30 m deep. Additional information is added as it becomes available.

An index to these records is available on workstation computers at each of the BGS offices and provides rapid access to the location of individual boreholes or to a set of boreholes for any defined area. A list of boreholes can be retrieved either by a place name, National Grid reference or map sheet number. The index contains information on the location and depth of the boreholes and the method of drilling. Plans to add to this database are at an advanced stage.

The full records are held in the National Geological Records Centre, BGS, Keyworth. Most are available in full for inspection by prior appointment, and copies of records may be made; a charge is made for these services, which reflects the cost of storage and maintenance of the archive.

The original records vary greatly in the quality and detail given, ranging from abstracts with no supporting information to detailed geologist's logs of core. Core specimens and geophysical logs are held for some of the boreholes.

Appendix 3 List of BGS photographs

Copies of these photographs are deposited for public reference in the Library of the British Geological Survey, Keyworth, Nottingham. Prints or slides are available on application.

Appendix 4 Petrography of the Whitchurch Sand Formations

by P Allen and A Parker

1 Sands

Samples of the Whitchurch Sand Formation from the following localities within (or just outside) the district were analysed mineralogically and petrographically:

1. Risinghurst (Monks Farm Pit) [SP 5654 0642]. Bed 7.
2. Brill (Muswell Hill) [SP 6406 1530]. Outcrop.
3. BGS Brill No. 1 Borehole [SP 6570 1412]. 14.6 m depth.
4. Brill (Excavation) [SP 6561 1365]. 1 m depth.
5. BGS Brill No. 2 Borehole [SP 6606 1403]. 1.54 m depth.
6. Ashendon (Auger hole) [SP 7094 1455]. 1.2 m depth.
7. Chearsley (Auger hole) [SP 7097 1125]. 1.7 m depth.
8. Stone Pit (Excavation) [SP 7800 1262].
9. Quainton (Conduit Hill) [SP 7500 2170]. Outcrop.

For comparative purposes samples were also analysed from:

10. Town Gardens Quarry, Swindon, Wiltshire [SU 1505 8356]. Whitchurch Sand.
11. West Dereham, Norfolk (excavation) [TL 6425 9945]. Sandringham Sand.
12. Donington Borehole, Lincolnshire [TF 2428 8182]. Upper Spilsby Sandstone.
13. Donington railway cutting, Lincolnshire [TF 2340 8220]. Mid-Spilsby Nodule Bed.
14. BGS Brill No. 1 Borehole [SP 6570 1412]. 20.2 m depth; Portland Sand.
15. Littleworth Brickpit [SP 5895 0550]. Portland Sand, Upper Lydite Bed.

From each sample, three pieces of soft sandstone were chosen to yield subsamples of 500–1000 (a) pebbles and granules (over 4 mm diameter); (b) grains of all densities (18 μm to 750 μm); (c) heavy grains (18 μm to 750 μm). Each piece was prepared first by crumbling on a 0.75 mm-mesh sieve, and the size fraction needed was then cleared of clay and limonite by boiling in sodium hexametaphosphate solution. Pebbles were identified by surface examination (after polishing where necessary) and from thin section. Grains were mounted on gridded slides; the heavy clasts were separated by settling in bromoform (specific gravity 2.9). Proportions of types of clast in the three preparations were determined by counting the whole of each sub-sample. No significant correlation between grain type and particle size was detected.

Results

The clasts from Samples 1 to 9 are overwhelmingly of stable rocks and minerals. They comprise quartz accompanied by minor amounts of chert, quartzite and acid feldspar, and traces of heavy minerals (specific gravity greater than 2.9). The assemblages are very mature and the relative frequencies of their components in the 'sand' fraction (18 to 750 μm) vary little. Amorphous 'limonite' (including goethite) varies widely in quantity, from ironstone proportions (e.g. Sample 2) to practically none (e.g. Sample 8). Traces of glauconite are present in all samples.

The pebble-granule suites (750 μm to 19 mm) are dominated by chert, but contain proportionately more quartz in the finer grades. Some of the chert (up to 19 mm diameter) from Sample 8 contains Carboniferous Endothyra and Euomphalus (Wells et al., 1947). Minor quantities of Upper Jurassic chert (with Pachustrella types) and phosphate pebbles have been recognised by Dr I R Garden (Table 4).

The 'sand' assemblages (18 to 750 p.m) are very uniform. Their siliceous components comprise 90 to 97 per cent quartz plus quartzite, 0.0 to 0.5 per cent quartz-schist and 0.0 to 3 per cent chert. Total feldspar varies from less than 0.1 per cent in Samples 2 and 4, to 5.7 per cent in Sample 9. Mostly orthoclase in the optical sense, it includes some microcline and sodic plagioclase. Traces of glauconite reach about 2 per cent in Sample 6. Heavy clasts total 0.1 to 2.3 per cent, omitting the micas.

The heavy detrital fraction is predominantly (75 to 80 per cent) zircon, black 'iron ores' (ilmenite > magnetite) and altered grains of iron ore, the last (leucoxene, anatase, brookite), with smaller amounts of rutile (6 to 9 per cent) and tourmaline (6 to 14 per cent). Biotite and muscovite, commonly altered, could not be handled quantitatively owing to technical difficulties of separation. Remaining mineral grains are mostly metamorphic: garnet (0 to 1.4 per cent, chiefly almandite), kyanite and staurolite (the total of these three ranging from 1.4 per cent in Sample 7 to 8.3 per cent in Sample 5) plus traces of sillimanite, monazite, titanite and epidote. In 6 of the 9 samples, kyanite (0.9 to 5.6 per cent) is more abundant than staurolite (0.7 to 4 per cent), but their frequencies never differ greatly. The mean percentages are shown in (Table 4). The highest proportions of less stable minerals (feldspars, glauconite, garnet, staurolite, kyanite) are found in Sample 9.

Varietal frequencies within the heavy species vary only in minor degrees. Thus the 0 to 7 per cent and 4 to 14 per cent ranges of purple and zoned zircon respectively are small in view of the low total counts. Within tourmaline, the traces of possible Cornubian types with blue/lilac/purple pleochroism (0 to 2 per cent) and those comprising fine aggregates (0 to 9 per cent), show little variation. Quartz shapes are comparatively equant in Samples 2, 4, and 8, and the proportions likely to have been highly rounded before diagenesis reach 3 per cent and 5 per cent in Samples 3, 5 and 8. Highly rounded zircons reach 7 per cent in Sample 6, and tourmalines 38 per cent in Sample 8. The degree of size sorting is high in some laminae. These features suggest extensive recycling or some aeolian inheritance, but it is difficult to make allowances for changes of shape due to diagenetic dissolution and overgrowth (see also Taylor, 1959).

Discussion

In terms of clastic composition and petrography, the sands from each of the localities sampled are essentially similar, and no statistical basis was found to distinguish between them.

The grain assemblages (Table 4) show many of the features of the marine Kimmeridgian and Portlandian sands of the immediate neighbourhood, and farther south-west as far as Dorset (Neaverson, 1925a; Latter, 1926), as shown by Taylor (1959). This suggests either recycling from them or derivation directly from the same source. Recycling is favoured because of the advanced degree of abrasion, the rarer occurrence of less stable pebbles and grains, and the absence of carbonate clasts in the Whitchurch Sand. The local Portland Formation could, in principle, have supplied all the clasts found in the Whitchurch Sand (see for example Wells et al., 1947), and there may have been large spreads of pebbly 'Lydite facies' on the London Platform shoals (Allen, 1967). The clasts in the Whitchurch Sand also show strong similarities to those in the early Cretaceous marine Lower Spilsby Sandstone and Sandringham Sands of the East Midlands, 180–190 km to the north-east. (Table 4) illustrates these similarities by means of the average frequencies of the main constituents and varieties in the 'sand' grade.

Two significant features distinguish the pre-Aptian northern sands from nearly all the nonmarine sands of the Purbeck–Wealden to the south and south-west of the London Platform. These are the presence in the northern sands of traces of titanite, pistacite-zoisite and amphibole, and the presence of regional metamorphic minerals of the garnet-kyanite-staurolite-sillimanite group, in which the proportion of kyanite characteristically exceeds or approximates to that of staurolite. South of the London Platform the relative proportion of kyanite is low. This detrital association is widespread in the North Sea (Allen, 1967), and is probably Caledonian in origin. The northern province also has a greater abundance of Carboniferous chert pebbles and grains of feldspar, with microcline and acid plagioclase more abundant than 'orthoclase'. The only known exceptions to this simple picture of distinct provinces are some 'northern' assemblages in brackish-water sediments filling small gutters in freshwater Weald Clay (Hauterivian–Barremian) of the Weald sub-basin ((Table 4), column 7). This suggests transient hydrological connections between north and south.

Within the Whitchurch Sand of the district, there are no signs of substantial sand contributions direct from the Cornubian massif, comparable with those in the Wealden of Wessex (Allen, 1975; 1981; 1989; Garden, 1991); nor is there evidence (pace Neaverson, 1925a; Latter, 1926) of any from the Armorican massif, like those in the Wealden of the Weald. At Swindon (45 km south-west of the district), however, there is Cornubian debris in the basal 0.65 m of the Whitchurch Sand (Casey and Bristow, 1964), for example tourmaline-quartz rock pebbles, and up to 28 per cent of tourmaline in the heavy assemblages of sandy laminae (Table 4). The 'southern' detritus in the district, which includes some Carboniferous chert and subequal proportions of zircon, rutile and tourmaline grains, could have come off the north-west flank of the shared London Platform (Garden, 1991), transported from leached soils by rivers (Allen, 1989).

The clasts of the Whitchurch Sand in the district are therefore substantially northern in origin. Either they have been recycled into nonmarine, fluvial or coastal environments from older marine strata (for example the local Kimmeridgian to Portlandian beds or the Lower Spilsby Sandstone/Sandringham Sands), or they have been derived from a similar source. The sedimentological evidence is at present too meagre to distinguish between these hypotheses.

The virtual absence of Armorican and Cornubian detritus in the local Whitchurch Sand may bear on the age limits of the formation. Assuming that the northern and southern basins were connected at times (Casey and Bristow, 1964; Allen, 1989), Cornubian sand might eventually have entered the district direct from the south-west. Cornubian tourmaline-rich pebbles first appear in western Dorset, in the lower Upper Purbeck, Unio Beds (Late Berriasian) at Friar Waddon (Garden, 1988), though curiously no Cornubian sand-grade clasts have been recognised. Cornubian clasts appear in mid-Dorset (Upwey eastwards) at the base of the Wessex Formation (earliest Valanginian). By mid-Valanginian times, Cornubian-derived sediments had spread to eastern Hampshire, for the clasts occur near the middle of the Wadhurst Clay (Winchester boring, 447.4 m depth). By about mid-Hauterivian times, they had reached east Sussex and west Kent, where they are found in the mid-Lower Weald Clay; the distance from source is at least 250 km (Allen, 1975). Assuming that the ingress from Cornubia proceeded at the same rate north-eastwards, its presence in the lower Middle Purbeck at Swindon and in the mid-Wealden at Kingsclere (Taylor, 1959), less than 60 km from Thame, suggests that it could have reached the district by about mid-Valanginian times. Assuming also that there was little erosion before the deposition of the Lower Greensand, the virtual absence of Cornubian clasts in the Whitchurch Sand Formation of the district hints that its top may correlate approximately with an horizon near the top of the Wadhurst Clay (mid-Valanginian; Allen and Wimbledon, 1991).

2 Clays

Twenty-four samples from the Whitchurch Sand and Purbeck Formations of the district, and from the Whitchurch Sand of Swindon, were selected for clay mineral analysis.

• 1–10 BGS Brill No. 1 Borehole [SP 6570 1412]. 10.27 to 16.40 m depth.
• 11–14 BGS Brill No. 2 Borehole [SP 6606 1403]. 1.54 to 2.65 m depth.
• 15 BGS Brill No. 3 Borehole [SP 6629 1402]. 0.62 m depth.
• 16–20 Risinghurst (Monks Farm Pit) [SP 5654 0642].
• 21 Lower Winchendon (Auger hole) [SP 7197 1230]. 1.3 m depth.
• 22 Ashendon (Auger hole) [SP 7094 1455]. 1.1 m depth.
• 23 Chearsley (Auger hole) [SP 7097 1125]. 1.4 m depth.
• 24 Swindon (Town Gardens Quarry) [SU 1505 8356].

After ultrasonic disaggregation, the <2 μm fractions of samples were separated by sedimentation, and pipetted under vacuum onto ceramic tiles. Diffractograms were obtained after (a) air-drying, (b) glycolation and (c) heating to 375°C, using a Philips PW1380 goniometer with monochromator, and a CuKa X-ray source at 35 kv, 55 mA. Identification followed Brindley and Brown (1980), and semiquantification was by the methods of Weir et al. (1975).

Results

Analytical results are given in (Table 5). The mixed-layer mineral is invariably illite-smectite, but its precise composition (i.e. proportion of smectite) varies.

Discussion

The clays in the Purbeck Formation in the Brill Borehole are dominated by illite and illite-smectite; their ultimate source appears to have been Upper Jurassic marine sediments of either southern or eastern England, which they closely resemble. The clays are largely detrital, and lack signs of diagenetic alteration even in possible rootlet beds and seatearths (Table 5). This relates to their limited depth of burial, which probably did not exceed 400 m. The degree of mineralogical change during weathering, soil formation and erosion is uncertain, but the virtual absence of kaolinite and vermiculite suggests that weathering was minimal, and may have occurred in an arid climate.

A major change in clay mineralogy, marked by the appearance of appreciable kaolinite, occurs between 15.35 m and about 14.69 m depth (Table 5), close to the junction between the Purbeck and the Whitchurch Sand formations. The clays of the Whitchurch Sand suggest more advanced weathering, their high kaolinite contents resulting from acid leaching. This probably took place mostly in soils on the better-drained, higher ground under a warm moist climate. Evidence for some advanced pedogenesis within the basin was given by Taylor (1959), but the absence of vermiculite is unexplained. Hydrothermal and sorting processes do not appear to have operated.

A similar change in clay mineralogy is known elsewhere in southern England. Assuming that the change occurs at about the same horizon throughout this area (Sladen, 1983, 1987; Allen and Wimbledon, 1991; Hallam et al., 1991), the Whitchurch Sand Formation must be younger than the Cinder Bed of Dorset (upper Middle Purbeck), the horizon of its base lying within or above the Upper Building Stones (Allen and Wimbledon, 1991). The high kaolinite content probably resulted from a combination of climatic and tectonic changes: increased rainfall and improved drainage respectively. Uplift of the source area could have triggered both, whether or not associated with eustatic factors. If, however, the climatic change were diachronous (Knox, 1991) or local, it would restrict the use of kaolinite content for stratigraphical correlation (see Neale and Knox, 1991). Climatically influenced differences in facies between adjacent basins are possible (Allen and Wimbledon, 1991, p. 519). Nevertheless, the contemporary climate in the Wessex and Weald basins appears to have been closely similar (Sladen, 1983; 1987).

Summary

The base of the Whitchurch Sand Formation is younger than the Middle Purbeck (early Cretaceous), and its top is probably coeval with the latest Wadhurst Clay of the Weald (mid-Valanginian). The sediments were derived, ultimately, from Lower Palaeozoic rocks to the north, but may have been recycled locally from late Jurassic marine sediments. Weathering of the source material and penecontemporaneous weathering of the Whitchurch Sand took place under acid-leaching conditions in a warm, moist climate. Postdepositional diagenesis has been minimal.

Appendix 5 Geophysical logs

Geophysical downhole logs from boreholes drilled through most of the Middle and Upper Jurassic formations of the district are available. The majority are gamma-ray logs from the Oxford Clay and West Walton formations. A regional picture of the geophysical log signatures may be found in Whittaker et al. (1985).

Gamma-ray logs of many borehole sections were obtained by BGS during the field survey using a portable Mount Sopris logging instrument. Most of the boreholes logged were drilled for the M40 Waterstock to Wendlebury site investigation. Detailed lithological logs of the borehole cores were also recorded, enabling correlation between lithology and gamma-ray response to be achieved.

The following descriptions are based principally on the logs available for the district. Because of the large amount of data from the Oxford Clay, this formation is dealt with in detail below. The other formations are summarised.

Great Oolite Group

This group comprises alternating limestone-dominated and clay-dominated units, the latter typically producing high gamma-ray responses (Figure 33). Within the White Limestone Formation, gamma-ray peaks mark thin muddy partings and marl beds, and may prove to be a useful basis for regional correlation. The top of the group (i.e. the top of the Cornbrash Formation) is well defined by the rapid increase in radioactivity from the overlying Kellaways Clay Member.

Kellaways Formation

The basal Kellaways Clay Member has a very distinct high gamma-ray signature, in contrast with low values for the succeeding Kellaways Sand Member (Figure 34). The signatures are enhanced by the lithological contrast with adjacent strata.

Oxford Clay Formation

Many geophysical logs are available for the Oxford Clay. Within the district, the Westcott No. 2 Borehole [SP 7086 1648] penetrated the entire sequence but only the lower part was cored. Partial cores are available from other boreholes. Most boreholes have gamma-ray logs; the exception is Otmoor C [SP 5708 1331] which has spontaneous potential (SP) and resistivity logs only. A number of shallow (10–15 m) flight-auger boreholes were drilled during the survey, targeting key stratigraphical markers such as the base of the West Walton Formation, the Pans Hill Siltstone or the Lamberti Limestone. Gamma-ray logs were run in each of these to assist in correlation. In general, a gamma-ray log through 10 m of strata was sufficient to correlate with known sequences.

The Lower Oxford Clay gamma-ray log shows variable readings (Figure 34) but with generally high values of around 200 counts per second (CPS). The signature is usually moderately spiky with lows indicating shell-rich layers or limestone nodule beds, and silty to sandy beds in the lowermost few metres. Within a nodule bed (Chapter 4), the gamma-ray response depends on the proximity of a nodule to the borehole; it is sometimes very pronounced as illustrated by the Arncott and Blackthorn nodule beds in M40 Borehole 201 [SP 6011 1500], but more commonly is poorly defined, for example in M40 Borehole 291 [SP 5900 1603]. A moderately well-defined gamma-ray low near the top of the Jason Subzone can be traced across the area. Nodules have not been found at this level, but in M40 Borehole 291 it corresponds with a shell bed. The Wendlebury Nodule Bed has a poorly defined gamma-ray signature, but nodules were not penetrated in any of the boreholes logged. The Acutistriatum Band–Comptoni Bed produces a distinct gamma-ray low which is recognisable in all boreholes. The exact relationship of the component beds to the gamma-ray signature is not known as none of the boreholes was cored in this interval.

The SP and resistivity logs of Otmoor C also show a good match of lithology with shifts in values and the main nodule bed horizons can be identified. The resistivity log, in particular, shows the relationship of sedimentary features very well, most notably piped junctions and non-sequences which correspond to 'highs', 'lows' or points of inflection on the log.

The junction between the Lower and Middle Oxford Clay is not well defined either in borehole cores or on the gamma-ray logs (Figure 35). However, the Middle Oxford Clay shows a clear two-fold subdivision in the gamma-ray logs of Westcott No. 2 and M40 Borehole 150 [SP 6108 1398]. The lower 10 m, which consists mainly of slightly silty mudstones with some layers rich in shell debris, has relatively high gamma-ray values, and a moderately spiky signature. The upper part shows lower values, and a more pronounced spiky signature reflecting alternations of mudstone and calcareous siltstone. The Lamberti Limestone, which defines the top of the member, occurs within these mudstone–siltstone alternations, and, as all of the calcareous siltstones have low gamma-ray values, it is not possible to define the boundary on geophysical evidence alone.

The spiky zone of low gamma-ray values continues up into the lowermost 5 m of the Upper Oxford Clay to just above the Pans Hill Siltstone. The low radioactivity of the latter produces a good marker horizon. The remainder of the Upper Oxford Clay has higher gamma-ray values and a moderately spiky signature. A very prominent marker with high radioactivity occurs 5.2 to 5.8 m below the top of the member and just above the base of the Bukowskii Subzone (Figure 10) and (Figure 35). It has not been equated with a distinctive lithology; borehole cores at this level comprise pale grey, silty mudstones with thin, darker grey, silty and carbonaceous, inter-burrowed horizons.

West Walton Formation

The gamma-ray signatures of the lowermost 2 m of the West Walton Formation are indistinguishable from those of the underlying Upper Oxford Clay. Higher in the formation, the radioactivity becomes more variable producing a more serrated signature. There is a prominent gamma-ray high 2 to 3 m above the base (Figure 35), recognisable in all of the logs, and a second shift to higher values 4 to 5 m above the base. The succeeding trend is one of gradual decrease in the radioactivity to the top of the formation, with peaks and troughs reflecting alternations of paler, less fossiliferous mudstones (peaks) and darker, more silty, fossiliferous beds. The most prominent trough, about 8 m above the base (Figure 35), corresponding with the 'Black Bed', can be traced throughout the district. In the east of the district, beds typical of the West Walton Formation overlie the Corallian Formation in the Brill No. 1 Borehole [SP 6570 1412] (Figure 36).

Corallian Formation

This formation is dominated by limestones and sandstones and consequently the radioactivity levels are generally low (Figure 36). Because of the rapid lateral facies changes, there is no typical sequence. The formation dies out eastwards and is absent in the Folly Farm Borehole [SP 7958 1916].

Ampthill Clay Formation

The base of this formation is marked by a rapid increase in gamma-ray values where it overlies the Corallian Formation, and a less-marked increase where it rests on the West Walton Formation (Figure 36). In Brill No. 1 and M40 Borehole 15, the lower part shows a step-like increase in values with a prominent peak about 10 m above the base. In the Brill No. 1 Borehole, there is a double trough 12 to 14 m above the base; the upper one corresponds with the Brill Serpulite Bed (Chapter 6). In the M40 Borehole 15, there is a marked decrease in gamma-ray values at the same level but with no prominent trough. The Garsington No. 1 Borehole [SP 5726 0289] has a subdued signature with uniform radioactivity levels, apart from cementstones at around 4 m above the base, which give a very prominent trough. This trough is not so well developed in the Brill No. 1 Borehole or M40 Borehole 15 boreholes. The top of the formation is marked consistently by a second pair of troughs representing two cementstone beds.

Kimmeridge Clay Formation

Gamma-ray logs of the Kimmeridge Clay in the Brill No. 1 and Hartwell [SP 7926 1223] boreholes (Figure 36) show a similar, gradual, upward decrease in gamma-ray values, which indicate two upward-coarsening cycles. These are best developed in the Brill Borehole. The lower one shows a more spiky signature than the upper, with troughs produced by siltstones, fine-grained sandstones and cementstones. It culminates in a prominent trough marked by sandstones and sandy limestones ('Pectinatus Sand'). The Lower Lydite Bed which, in the Hartwell Borehole produces a very distinct peak, marks the base of the succeeding cycle. This comprises mudstones (Swindon Clay) passing up into siltstones and fine-grained sands (Hartwell Silt).

Portland, Purbeck and Whitchurch Sand Formations

The Portland and Purbeck formations have been proved in the Brill No. 1 and Hartwell boreholes (Figure 36). The thicker sequence in Hartwell shows a small decrease in radioactivity at the base of the Portland. There is a continued gradual overall decrease in gamma-ray values upward through the Portland and into the overlying Purbeck, with a few small peaks representing muddier beds. In the Brill No. 1 Borehole, the decrease in radioactivity continues into the Whitchurch Sand Formation (Cretaceous) but with more pronounced peaks marking mudstone interbeds.

Author citations for fossil species

To satisfy the rules and recommendations of the international codes of botanical and zoological nomenclature, authors of cited species are listed below.

Chapter 2 (Concealed rocks)

• Androgynoceras hybridiforme Spath, 1938
• Balanocrinus subteroides (Quenstedt, 1858)
• Beaniceras dundlyi Donovan, 1978
• Beaniceras crassum S S Buckman, 1919 var. costatum S S Buckman, 1919
• Cleviceras elegans (J Sowerby, 1815)
• Lingulella lepis Salter, 1866
• Tragophylloceras undulatum (Wm Smith, 1817)
• Tropidoceras acteon (d'Orbigny, 1844)
• Tropidoceras ellipticum Sowerby, 1815)

Chapter 3 (Great Oolite Group)

• Anisocardia islipensis (Lycett, 1863)
• Antiquicyprina loweana (Morris & Lycett, 1854)
• Bakevellia waltoni (Lycett, 1863)
• Camptonectes laminates Sowerby, 1818)
• Ceratomya concentrica (J de C Sowerby, 1825)
• Cererithyris intermedia (J Sowerby, 1812)
• Cererithyris magnifica S S Buckman, 1927
• Chlamys (Radulopecten) vagans de C Sowerby, 1826)
• Clydoniceras (Clydoniceras) discus Sowerby, 1813)
• Coelastarte compressiuscula (Morris & Lycett, 1855)
• Costigervillia crassicosta (Morris & Lycett, 1853)
• Cuspidaria ibbetsoni (Morris, 1853)
• Digonella digonoides S S Buckman, 1913
• Eomiodon fimbriatus (Lycett, 1863)
• Epithyris oxonica Arkell, 1931
• Epithyris bathonica S S Buckman, 1906
• Falcimytilus sublaevis (J de C Sowerby, 1823)
• Fibula phasanioides (Morris & Lycett, 1850)
• Globularia morrisi Cox & Arkell, 1950
• Isognomon isognomonoides (Stahl, 1824)
• Kallirhynchia vagans (S S Buckman, 1917)
• Kallirhynchia deliciosa S S Buckman, 1918
• Limatula cerealis Douglas & Arkell, 1932
• Liostrea undosa (Phillips, 1829)
• Meleagrinella echinata (Wm Smith, 1817)
• Modiolus imbricates (J Sowerby, 1818)
• Obovothyris obovata (J Sowerby, 1812)
• Pholadomya ovalis (J Sowerby, 1819)
• Pholodomya lirata (J Sowerby, 1818)
• Placunopsis socialis Morris & Lycett, 1853
• Plagiostoma cardiiformis (J Sowerby, 1815)
• Plagiostoma subcardiiformis (Greppin, 1867)
• Pleuromya alduini (Brongniart, 1821)
• Pleuromya uniformis (J Sowerby, 1813)
• Praeexogyra hebridica (Forbes, 1851)
• Protocardia buckmani (Morris & Lycett, 1853)
• Sphaeriola oolithica (Rollier, 1913)
• Vaugonia angulata (J de C Sowerby, 1826)

Chapter 4 (Ancholme Group Part 1)

• Binatisphinctes comptoni (Pratt, 1841)
• Bositra buchii (Roemer, 1836)
• Cardioceras (Cardioceras) costicardia S S Buckman, 1926
• Cardioceras maltonense (Young & Bird, 1822)
• Cardioceras (Scarburgiceras) scarburgense (Young & Bird, 1822)
• Cardioceras (Vertebriceras) vertebrale (J Sowerby, 1817)
• Cardioceras tenuiserratum (Oppel, 1863)
• Cardioceras (Plasmatoceras) tenuistriatum Borissjak, 1908
• Creniceras rengerri (Oppel, 1863)
• Cylindroteuthis puzosiana (d'Orbigny, 1860)
• Genicularia vertebralis (J. de C Sowerby, 1829)
• Gryphaea dilatata J. Sowerby, 1816
• Gryphaea dilobotes Duff, 1978
• Gryphaea lituola Lamarck, 1819
• Hibolithes hastatus Montfort, 1808
• Kosmoceras (Spinikosmoceras) acutistriatum (S S Buckman, 1924)
• Kosmoceras obductum (S S Buckman, 1925)
• Kosmoceras phaeinum (S S Buckman, 1924)
• Kosmoceras proniae Teisseyre, 1884
• Kosmoceras spinosum (J de C Sowerby, 1826)
• Meleagrinella braamburiensis (Phillips, 1829)
• Oxytoma inequivalve (J Sowerby, 1819)
• Procerithium damonis (Lycett, 1860)
• Quenstedtoceras henrici (Douville, 1912)
• Quenstedtoceras woodhamense Arkell, 1939
• Taramelliceras richei (de Loriol, 1898)
• Trochocyathus magnevillianus Michelin, 1840

Chapter 5 (Corallian Formation)

• Bathrotomaria reticulata (J Sowerby, 1821)
• Bourguetia saemanni (Oppel, 1856)
• Camptonectes giganteus Arkell, 1926
• Cardioceras (Cawtoniceras) cawtonense (Blake & Hudleston, 1877)
• Cardioceras (Subvertebriceras) densiplicatum Boden, 1911
• Cardioceras (Scoticardioceras) excavatum (J Sowerby, 1815)
• Cardioceras (Maltoniceras) highworthense Arkell, 1941
• Cardioceras (Vertebriceras) rachis (S S Buckman, 1920)
• Cardioceras (Subvertebriceras) sowerbyi Arkell, 1936
• Cardioceras tenuiserratum (Oppel, 1863)
• Cardioceras (Vertebriceras) vertebrale (J Sowerby, 1817)
• Cardioceras (Subvertebriceras) zenaidae Ilovaisky, 1904
• Chlamys (Radulopecten) fibrosa (J Sowerby, 1816)
• Chlamys nattheimensis (de Loriol, 1894)
• Chlamys splendens (Dollfuss, 1863)
• Cucullaea contracta Phillips, 1829
• Cydoserpula intestinalis (Phillips, 1829)
• Deltoideum delta (Wm Smith, 1817)
• Diplopodia versipora (Phillips MS; Wright, 1857)
• Discomiltha lirata (Phillips, 1829)
• Euaspidoceras crebricostis (Arkell, 1927)
• Euaspidoceras perarmatum (J Sowerby, 1822)
• Gervillella aviculoides (J Sowerby, 1814)
• Goliathiceras chamoussetiforme Arkell, 1943
• Goliathiceras gorgon Arkell, 1943
• Goliathiceras microtrypa S S Buckman, 1923
• Gryphaea dilatata J Sowerby, 1816
• Hyboclypeus wrighti Etallon, 1860
• Isastraea explanata (Goldfuss, 1829)
• Isocyprina cyreniformis (Buvignier, 1852)
• Isognomon promytiloides Arkell, 1931
• Isognomon subplana (Etallon, 1862)
• Liostrea quadrangularis (Arkell, 1927)
• Lopha gregarea (J Sowerby, 1816)
• Mactromya aceste (d'Orbigny, 1850)
• Meleagrinella ovalis (Phillips, 1829)
• Miticardioceras mite S S Buckman, 1923
• Modiolus bipartitus J Sowerby, 1818
• Myophorella hudlestoni (Lycett, 1877)
• Nanogyra nana (J Sowerby, 1822)
• Natica arguta Phillips, 1829
• Neocrassina ovata (Wm Smith, 1816)
• Nucleolites scutatus (Lamarck, 1816)
• Ochetoceras (Campylites) henrici (d'Orbigny, 1850)
• Oxytoma expansa (Phillips, 1829)
• Pachyteuthis abbreviata (Miller, 1823)
• Paracenoceras hexagonus (J de C Sowerby, 1826)
• Perisphinctes (Dichotomosphinctes) antecedens Salfeld, 1914
• Perisphinctes (Liosphinctes) apolipon (S S Buckman, 1925)
• Perisphinctes (Arisphinctes) ariprepes (S S Buckman, 1924)
• Perisphinctes (Dichotomosphinctes) auriculatus Arkell, 1935
• Perisphinctes (Dichotomosphinctes) buckmani Arkell, 1936
• Perisphinctes (Kranaosphinctes) bullingdonensis Arkell, 1939
• Perisphinctes (Perisphinctes) chloroolithicus (Gimbel, 1865)
• Perisphinctes (Arisphinctes) cotovui Simionescu, 1907
• Perisphinctes (Arisphinctes) cowleyensis S S Buckman, 1926
• Perisphinctes (Kranaosphinctes) cymatophorus (S S Buckman, 1923)
• Perisphinctes (Kranaosphinctes) decurrens (S S Buckman, 1923)
• Perisphinctes (Arisphinctes) headingtonensis Arkell, 1936
• Perisphinctes (Arisphinctes) helenae de Riaz, 1898
• Perisphinctes (Arisphinctes) ingens (Young & Bird, 1822)
• Perisphinctes (Arisphinctes) kingstonensis Arkell, 1939
• Perisphinctes (Liosphinctes) laevipickeringius Arkell, 1939
• Perisphinctes (Liosphinctes) linki Choffat, 1893
• Perisphinctes (Dichotomosphinctes) magnouatius Arkell, 1938
• Perisphinctes (Dichotomosphinctes) maltonensis Arkell, 1938
• Perisphinctes (Arisphinctes) maximus (Young & Bird, 1828)
• Perisphinctes (Dichotomosphinctes) ouatius (S S Buckman, 1926)
• Perisphinctes (Perisphinctes) parandieri de Loriol, 1903
• Perisphinctes (Arisphinctes) parandiformis Arkell, 1940
• Perisphinctes (Arisphinctes) pickeringius (Young & Bird, 1822)
• Perisphinctes (Arisphinctes) plicatilis (J Sowerby, 1817)
• Perisphinctes (Kranaosphinctes) promiscuus Bukowski, 1887
• Perisphinctes (Dichotomosphinctes) rotoides Ronchadze, 1917
• Perisphinctes (Kranaosphinctes) trifidus Sowerby, 1821)
• Perisphinctes (Perisphinctes) tumulosus S S Buckman, 1927
• Perisphinctes (Arisphinctes) vorda Arkell, 1939
• Plagiostoma mutabilis (Arkell, 1926)
• Plagiostoma rigidum J Sowerby, 1816
• Plegiocidaris florigemma (Phillips, 1829)
• Pleuromya alduini (Brongniart, 1821)
• Pleuromya uniformis Sowerby, 1813)
• Plicatula weymouthiana Damon, 1860
• Procerithium muricatum (J. de C Sowerby, 1825)
• Pseudomelania heddingtonensis (J Sowerby, 1813)
• Pygaster semisulcata (Phillips, 1829)
• Rhaxella perforata Hinde, 1890
• Thecosmilia annularis (Fleming, 1828)
• Trigonia reticulata Agassiz, 1840

Chapter 6 (Ancholme Group Part 2)

• Amoeboceras serratum (J Sowerby, 1813) A
• Ammonites biplex J Sowerby, 1821
• Aulacostephanus autissiodorensis (Cotteau, 1853)
• Aulacostephanus eudoxus (d'Orbigny, 1847)
• Aulacostephanus eulepidus (Schneid, 1939)
• Aulacostephanus linealis (Quenstedt, 1888)
• Aulacostephanus mutabilis J de C Sowerby, 1823)
• Aulacostephanus volgensis (Vischniakoff, 1875)
• Cardioceras kokeni Boden, 1911
• Cardioceras tenuiserratum (Oppel, 1863)
• Deltoideum delta (Wm Smith, 1817)
• Gryphaea dilatata J Sowerby, 1816
• Isocyprina minuscula (Blake, 1875)
• Isocyprina (Venericyprina) pellucida Casey, 1952
• Nanogyra nana Sowerby, 1822)
• Nanogyra virgula (Defrance, 1821)
• Pachyteuthis abbreviate (Miller, 1823)
• Pavlovia pallasioides (Neaverson, 1924)
• Pectinatites (Pectinatites) naso (S S Buckman, 1926)
• Pectinatites (Pectinatites) pectinatus (Phillips, 1871)
• Pectinatites (Virgatosphinctoides) pseudoscruposus (Spath, 1936)
• Pectinatites (Virgatosphinctites) wheatleyensis Neaverson, 1925
• Pectinatites (Virgatosphinctoides) woodwardi (Neaverson, 1925)
• Pholadomya acuticosta J de C Sowerby, 1827
• Rasenia evoluta Spath, 1935
• Rasenia moeschi (Oppel, 1863)
• Serkula tetragona J de C Sowerby, 1829
• Torquirhynchia inconstans J. Sowerby, 1821)

Chapter 7 (Portland Formation and Purbeck Formation)

• Ampullospira ceres (de Loriol, 1867)
• Aptyxiella portlandica J de C Sowerby, 1836)
• Behemoth megasthenes S S Buckman, 1922
• Briarites polymeles S S Buckman, 1921
• Camptonectes lamellosus (J Sowerby, 1819)
• Galbanites galbanus S S Buckman, 1922
• Holcostephanus pallasianus (d'Orbigny, 1845)
• Isognomon bouchardi (Oppel, 1858)
• Isocyprina pringlei Cox, 1925
• Laevitrigonia gibbosa Sowerby, 1819)
• Lima rustica J. Sowerby, 1822)
• Lucina portlandica J de C Sowerby, 1836
• Ostrea expansa J Sowerby, 1819
• Modiolus hudlestoni Cox, 1925
• Mytilus suprajurensis Cox, 1925
• Nanogyra nana (J Sowerby, 1822)
• Nanogyra virgula (Defrance, 1821)
• Pleuromya telling Agassiz, 1845
• Pleuromya uniformis (J Sowerby, 1813)
• Protocardia dissimilis J de C Sowerby, 1827)
• Titanites glottodes S S Buckman, 1923
• Titanites polymeles S S Buckman, 1921
• Titanites pseudogigas (Blake, 1880)
• Titanites titan S S Buckman, 1921
• Titanites trophon S S Buckman 1922
• Trigonia damoniana de Loriol, 1866

Chapter 8 (Lower Cretaceous)

• Aetostreon latissimum (Lamarck, 1819)
• Ammonites auritus J Sowerby, 1816 var. catillus J de C Sowerby, 1827
• Ammonites cristatus J de C Sowerby, 1823
• Ammonites interruptus Bruguiere, 1868
• Ammonites planulatus J de C Sowerby, 1827
• Anahoplites (Leptohoplites) falcoides Spath, 1925
• Anisoceras saussureanum (Pictet, 1847)
• Arrhaphoceras precoupei Spath, 1928 Aucellina gryphaeoides (J de C Sowerby, 1836)
• Aucellina uerpmanni Polutoff, 1933
• Birostrina concentrica (Parkinson, 1819)
• Birostrina sulcata (Parkinson, 1819)
• Birostrina sulcata (Parkinson, 1819)
• Birostrina subsulcata Wiltshire, 1869
• Burrirhynchia shenleyensis (Lamplugh & Walker, 1903)
• Callihoplites auritus Sowerby, 1816)
• Callihoplites vraconensis (Pictet & Campiche, 1860)
• Capillithyris diversa (Cox & Middlemiss, 1978)
• Corbula inflexa Roemer, 1836
• Cyclas medius auctt.
• Cypridea aculeata Jones, 1885
• Cypridea bispinosa Jones, 1878
• Cypridea valdensis (J de C Sowerby, 1836)
• Cypridea verrucosa Jones, 1878
• Cyrena media auctt.
• Dimorphoplites niobe Spath, 1925
• Dipoloceras cristatum (J de C Sowerby, 1823)
• Endogenites erosa Mantell, 1833
• Euhoplites inornatus Spath, 1930
• Euhoplites meandrinus Spath, 1930
• Euhoplites sublautus Spath, 1928
• Euhoplites monacantha Spath, 1930
• Exogyra sinuata (J Sowerby, 1822)
• Gemmarcula menardi Lamarck, 1819 var. pterygotos (Lamplugh & Walker,1903)
• Hoplites (Hoplites) dentatus (J Sowerby, 1812)
• Hoplites (Hoplites) spathi (Breistroffer, 1940)
• Hysteroceras orbignyi (Spath, 1922)
• Hysteroceras varicosum (J de C Sowerby, 1824)
• Idiohamites elegantulus elegantulus Spath, 1939
• Impardecispora apiverrucata (Couper 1958)
• Venkatachala, Kar & Raza, 1965
• Laevitrigonia gibbosa (J Sowerby, 1819)
• Lepidotes mantelli Agassiz, 1833
• Mantelliana phillipsiana (Jones, 1888)
• Mortoniceras rostratum (J Sowerby, 1817)
• Neohibolites minimus (Miller, 1826)
• Neomiodon medius (J de C Sowerby, 1826)
• Neomiodon sublaevis (Roemer, 1836)
• Pleurohoplites subvarians Spath, 1928
• Prohysteroceras (Goodhallites) goodhalli (J Sowerby, 1820)
• Protocardia purbeckensis (de Loriol, 1865)
• Pseudonodosaria vulgata (Bornemann, 1854)
• Rectithyris shenleyensis (Lamplugh & Walker, 1903)
• Septifer lineatus (J de C Sowerby, 1836)
• Serpula coacervata Blumenbach, 1803
• Terebrirostra arduennensis (d'Orbigny, 1847)
• Toucasia lonsdalei (J de C Sowerby, 1836)
• Unio stricklandii Phillips, 1858
• Viviparus antiquus Huckriede, 1967
• Viviparus cariniferus (J de C Sowerby, 1826)
• Viviparus ornata (Phillips, 1871)
• Viviparus subangulata (Phillips, 1871 non Roemer, 1839)

Chapter 9 (Upper Cretaceous: Chalk Group)

• Acanthoceras rhotomagense (Brongniart, 1822)
• Acanthoceras sussexiense (Mantell, 1822)
• Actinocamax plenus (Blainville, 1825)
• Concinnithyris subundata (J Sowerby, 1813)
• Exanthensis labrosus (T Smith, 1848)
• Gryphaeostrea canaliculata (J Sowerby, 1816)
• Hypoturrilites gravesianus (d'Orbigny, 1842)
• Hypoturrilites tuberculatus (Bosc, 1801)
• Inoceramus atlanticus (Heinz, 1936)
• Inoceramus crippsi Mantell, 1822
• Inoceramus labiatus (Schlotheim, 1813)
• Inoceramus pictus J de C Sowerby, 1829
• Inoceramus virgatus Schluter, 1877
• Lima subovalis J de C Sowerby, 1836
• Mantelliceras saxbii (Sharpe, 1857)
• Mariella lewesiensis (Spath, 1926)
• Monticlarella rectifrons (Pictet, 1871)
• Mytiloides labiatus (Schlotheim, 1813)
• Orbirhynchia cuvieri (d'Orbigny, 1847)
• Orbirhynchia mantelliana (J de C Sowerby, 1826)
• Orbirhynchia multicostata (Pettitt, 1954)
• Ornatothyris sulczfera (Morris, 1847)
• Ostrea vesicularis Lamarck, 1806
• Plagiostoma globosum J de C Sowerby, 1836
• Plicatula inflata J de C Sowerby, 1823
• Scaphites obliquus J Sowerby, 1813
• Schloenbachia varians (J Sowerby, 1817)
• Terebratula semiglobosa J Sowerby, 1813
• Turrilites scheuchzerianus Bosc, 1925

Chapter 10 (Quaternary)

• Ancylus fluviatilis (Muller, 1774)
• Unio littoralis Lamarck, 1801

Figures, plates and tables

(Figure 1) Simplified solid geology of the district.

(Figure 2) Physiography of the district.

(Figure 3) Geology and structure of Palaeozoic 'basement' surface, showing location of boreholes mentioned in the text. Contour values are in metres;

(Figure 4) Distribution of Triassic strata and location of principal boreholes. Based in part on Horton et al. (1987, figs. 7, 8, 11, 12).

(Figure 5) Thickness of the Lias Group and extent of component formations beneath the Middle Jurassic unconformity.

(Figure 6) Lias stratigraphy in the district. Only the zones indicated with an asterisk (*) have been proved locally. Hettangian and earlier Sinemurian, and later Toarcian strata are absent.

(Figure 7) Inferior and Great Oolite groups: lithostratigraphical correlation of borehole sections and thickness variation in the district and adjoining areas.

(Figure 8) Lithological variation within the Rutland Formation and the equivalent Hampen Manly Formation. Inset map indicates arbitrary limits of these formations, which are dominated by nonmarine and marine facies respectively. 'W' indicates upper limit of Wellingborough Rhythm.

(Figure 9) Borehole and quarry sections through the Great Oolite Group in the north-western part of the district.

(Figure 10) Generalised vertical section through the Oxford Clay Formation of the district. Inset shows the distribution of the formation. The gamma-ray log is based on M40 boreholes 150 and 201.

(Figure 11) Sections showing the relationship between the West Walton and Corallian formations within the district.

(Figure 12) Stratigraphical relationships within the Corallian Formation of the district.

(Figure 13) Borehole sections through the Corallian Formation of the district and adjoining areas.

(Figure 14) Sketch map showing the outcrops of the calcareous members of the Corallian Formation in the vicinity of Oxford. (1. Littlemore railway cutting; 2. Sandford underpass (Henley Road); 3. Sandford Sewage Works; 4. Garsington Road substation; 5. BGS Garsington No. 1 Borehole; 6. M40 Borehole 010; 7. M40 Borehole 008A; 8. M40 Borehole 012; 9. M40 Borehole 15; 10. M40 sections near Waterperry overbridge; 11. Pylon excavations west of Worminghall; 12. Worminghall sewage works; 13. Field Farm; 14. A40 road cutting near Wheatley-Holton underpass; 15. Gaolhouse Quarry; 16. Lye Hill Quarry; 17. Cowley Industrial School South Quarry; 18. Cowley Industrial School North Quarry; 19. Brittleton Barn Quarry; 20. Horspath Road Quarry; 21. Horspath Road-Eastern By-pass junction; 22. Cowley Barracks Quarry; 23. Wingfield Hospital South Quarry; 24. Wingfield Hospital North Quarry; 25. Windmill Quarry; 26. Cross Roads Quarry; 27. Vicarage Quarry; 28. Magdalen Quarry; 29. Stanton St John Schoolhouse Quarry; 30. Benfield & Loxley's Quarry; 31. Horton Road Quarry; 32. Woodperry Road Quarry).

(Figure 15) Ampthill Clay (AmC) sections in the district. Bed numbers relate to the standard Fenland succession of Cox and Gallois (1979).

(Figure 16) Sections through the Ampthill Clay–Kimmeridge Clay boundary. Bed numbers relate to the standard Fenland succession of Cox and Gallois (1979).

(Figure 17) Kimmeridge Clay (KC) sections in the district. Bed numbers relate to the standard succession of Gallois and Cox (1976), and Cox and Gallois (1979; 1981).

(Figure 18) Lithostratigraphical nomenclature, selected sections and locality map for the Portland Formation of the district.

(Figure 19) Sketch map showing the distribution of Whitchurch Sand Formation, Lower Greensand and Gault. Broken line indicates approximate inferred overstep of the Whitchurch Sand by the Lower Greensand and Gault; to the south and east of the line, the Whitchurch Sand was almost entirely removed by erosion prior to deposition of the latter formations, although minor remnants may remain (e.g. at Thame).

(Figure 20) Upper Aptian and Albian (Lower Cretaceous) stratigraphy in the district. Zonation after Casey (1961) and Owen (1971; 1975; 1988a; 1988b; 1992; 1994). Positions of major non-sequences are indicated by vertical ruling.

(Figure 21) Schematic section illustrating thickness and facies variation in the Gault and Upper Greensand of the district and surrounding area, showing major erosive non-sequences.

(Figure 22) Upper Cretaceous stratigraphy in the district, showing the approximate relationship of the local lithostratigraphical sequence to the traditional British biozonation, the north-west European standard ammonite zonation and chronostratigraphical stages and substages. Ammonite zonation based largely on Hancock (1991). Notes 1 obsolete zones; the ammonite standard is now used for classification of the British Cenomanian. 2 formerly the zonal index was Calycoceras naviculare. 3 formerly the zonal index was Sciponoceras gracile. 4 = [Inoceramus labiatus]. Mytiloides is absent from the basal part of this zone. 5 = [Holaster planus]. 6 = [Reesideoceras petrocoriensis].7 subdivided into Neostlingoceras carcitanense and Mantelliceras saxbii subzones. 8 subdivided into Turrilites costatus and Turrilites acutus subzones.

(Figure 23) Composite section of Chinnor Quarry 3 [SU 758 998] after Sumbler and Woods (1992, fig. 2).

(Figure 24) Older fluvial deposits of the region and the limit of Anglian glaciation.

(Figure 25) Longitudinal profile of the River Thame showing the classification of terrace surfaces by height, and their relationship to the local glacial deposits.

(Figure 26) Sketch map showing major structural elements in the Palaeozoic basement of central England and the location of the district in the Midlands Microcraton (stippled). In part based on Reid et al., 1990); Tucker and Pharaoh (1991).

(Figure 27) Bouguer Gravity anomaly map of the district and surrounding areas, showing main structural lineaments of the Palaeozoic 'basement'. Anomalies calculated against the International Gravity Formula, 1967, and referred to the National Gravity Reference Net, 1973, using a surface density of 2.7 Mgm⁻³. Contour interval 1 mGal; thicker lines at 5 mGal intervals (1 mGal = 10 μms^-2). Taken from the BGS 1:250 000 UK and Continental Shelf Series Bouguer gravity anomaly map, Chilterns, Sheet 51°N-02°W.

(Figure 28) Aeromagnetic anomaly map of the district and surrounding areas, showing main structural lineaments of the Palaeozoic basement. Total force magnetic anomalies in nanotesla (nT) above a computed regional field for the British Isles. Contour interval 10 nT; thicker lines at 50 nT intervals (1 nT = 1 gamma). Flown at 550 m (1800 ft) mean terrain clearance with N–S flight lines at 2 km spacing. Taken from the BGS 1:250 000 UK and Continental Shelf Series aeromagnetic anomaly map, Chilterns, Sheet 51°N–02°W.

(Figure 29) Distribution of surface faults and other main structures in the district, and generalised contours on top of the Great Oolite Group (Jurassic), illustrating south-easterly dip. Contour values in metres relative to Ordnance Datum (OD).

(Figure 30) Geotechnical data for the Oxford Clay Formation.

(Figure 31) Geotechnical data for the West Walton and Ampthill Clay formations.

(Figure 32) Geotechnical data for the Kimmeridge Clay Formation, Gault Formation and Alluvium.

(Figure 33) Gamma-ray and resistivity logs of the Great Oolite Group in M40 Borehole 231 [SP 5715 1718].

(Figure 34) Correlation of geophysical and lithological marker beds in the Kellaways Formation and Lower Oxford Clay Member. Geophysical correlations are based on gamma-ray logs except for Otmoor C Borehole (electric logs).

(Figure 35) Gamma-ray log correlation of the upper part of the Oxford Clay and West Walton formations.

(Figure 36) Gamma-ray log signatures of Upper Jurassic and lowest Cretaceous formations.

Plates

(DeLory) De Lory equation

Frontispiece Ammonites from the Portland Formation. This plate originally appeared as Plate VITA in the Geological Survey memoir for Weymouth, Swanage, Corfe and Lulworth published in 1947. The original caption implied that the figured specimens came from Dorset; in fact, they are all from the Thame district. All the species are characteristic of the Kerberus Zone. The scale bar is marked in inches (25.4 mm). Top row, left to right: Titanites pseudogigas (Blake), Barrel Hill, Long Crendon (BGS GSM47178); Titanites trophon S S Buckman (holotype), Barrel Hill, Long Crendon (BGS GSM47139); Titanites glottodes S S Buckman (holotype), Barrel Hill, Long Crendon (BGS GSM47192); Galbanites galbanus S S Buckman (holotype), Haddenham (BGS GSM47155) Bottom row, left to right: Titanites polymeles S S Buckman (holotype), Scotsgrove, Haddenham (BGS GSM32058); Titanites titan S S Buckman (holotype), near Baggle Hill, Haddenham (BGS GSM32044)

(Front cover) Cover photograph Brill Windmill [SP 6519 1415], a famous local landmark, is a reminder of days gone by. The hummocks and hollows in the foreground are all that remain of another vanished industry; they result from centuries of quarrying of Portland and Purbeck strata for building stone, lime and sand (A15366).

Rear cover

Geological succession Geological succession in the Thame district

(Plate 1) Rutland Formation, Woodeaton Quarry [SP 5328 1235] (1991). Detail of rootlet bed (Bed 8; see text) at the top of the Rutland Formation (A15362).

(Plate 2) Woodeaton Quarry [SP 5313 1228] (1991). Flaggy limestones of the Forest Marble Formation, above the topmost grey marly band, overlie an almost complete sequence of the White Limestone (beds 3 to 18; see text) (A15357).

(Plate 3) Merton Quarry [SP 572 170] (1991). Brown-weathered Cornbrash caps the distant face. The Forest Marble Formation and underlying Bladon Member of the White Limestone Formation, form the main face in the middle distance. The closest face shows almost the entire 6 m thickness of the Ardley Member of the White Limestone (A15348).

(Plate 4) Fossils from the Oxford Clay (all X 1) 1 Trochocyathus magnevillianus Michelin; Middle Oxford Clay; Athleta Zone; Chalgrove Borehole, 183.05 m (BGS BDY4905). 2a–b Gryphaea lituola Lamarck; Middle Oxford Clay; Ludgershall (BGS Zn2540). 3 Genicularia vertebralis de C Sowerby); Lower Oxford Clay; M40 motorway Borehole 150 (1986), 52.86 m (BGS BDZ8144). 4 Kosmoceras acutistriatum (S S Buckman) with Procerithium damonis (Lycett) and nuculoid bivalve fragments; Lower Oxford Clay; Athleta Zone, Phaeinum Subzone; Otmoor Borehole B, 10.71 m (BGS BDG2544). 5 Binatisphinctes comptoni (Pratt); Lower Oxford Clay, Comptoni Bed; Calvert brickpit (BGS FR1020). 6 Cylindroteuthis puzosiana (d'Orbigny); Lower Oxford Clay; Calvert brickpit (BGS MGS2167). 7 Meleagrinella braamburiensis (Phillips); Lower Oxford Clay; Otmoor Borehole C, 20.10 m (BGS BDG2978). 8 Hibolithes hastatus Montfort; Middle Oxford Clay; Otmoor Borehole A, 4.0 m (BGS BDG2144). 9 Bositra buchii (Roemer); Lower Oxford Clay; Otmoor Borehole C, 22.28 m (BGS BDG2995). 10 Lamberti Limestone with ammonites, including Quenstedtoceras and Kosmoceras, and bivalves; Middle Oxford Clay; Woodham Brickpit (BGS FRC1). m

(Plate 5) Fossils from the Oxford Clay, West Walton and Corallian formations (all x 1). 1 Lopha gregarea (J Sowerby); Beckley Sand Member; Littlemore (BGS AH5137). 2–3 Isastraea explanata (Goldfuss); Wheatley Limestone Member (Coral Rag facies); Barton (AHH281 (Fig.2) and AHH282 (Fig.3)). 4 Cemented aggregate of Nanogyra nana (J Sowerby); Oakley Member; Boarstall (BGS AH1572). 5 Gryphaea dilatata J Sowerby; West Walton Formation; Quainton (BGS MGS409). 6 Bored relic of Gryphaea valve; West Walton Formation; Oakley (BGS AHH283). 7 Cardioceras (Subvertebriceras) sowerbyi Arkell; Arngrove Spiculite Member; Arngrove (BGS AH1507). 8 Taramelliceras richei (de Loriol); Upper Oxford Clay; Ludgershall (BGS GSM27772). 9 Creniceras rengerri (Oppel); Upper Oxford Clay; Ludgershall (BGS GSM27791). 10 Quenstedtoceras woodhamense Arkell; Upper Oxford Clay; Ludgershall (BGS GSM27677). 11 a–b Cardioceras (Scarburgiceras) scarburgense (Young Bird); Upper Oxford Clay; Ludgershall (BGS GSM27751). 12a–b Cardioceras (Cardioceras) costicardia S S Buckman (holotype); Upper Oxford Clay; Horton-cum-Studley (BGS GSM47833). 13 Cardioceras (Plasmatoceras) tenuistriatum Borissjak; West Walton Formation; M40 motorway Borehole 012 (1978), 13.55 m (BGS BDU5127)

(Plate 6) Woodham Brickpit [SP 709 185] (1972); Middle and Upper Oxford Clay The calcareous mudstone ribs in the lower part of the face are rich in Gryphaea lituola. The Lamberti Limestone (topmost Middle Oxford Clay), is the main rib in the middle of the face, about 2.75 m above the base of the section (B M Cox). GS1

(Plate 7) Littlemore Railway Cutting [SP 5328 0276] (1925). Planar bedded Beckley Sand with doggers of hard calcareous sandstone. The paler beds at the top of the distant face belong to the Littlemore Member (A3204).

(Plate 8) Littlemore Railway Cutting [SP 5314 0277] (1925). The Littlemore Member, comprising interbedded mudstones, marls and limestones, with a prominent bed of sandy limestone at the base, overlies sands of the Beckley Sand Member (A 3205A).

(Plate 9) Cross Roads Quarry [SP 5505 0650]; Wheatley Limestone Member. Rubbly coralliferous shell-detrital limestone interbedded with poorly cemented shell sand (A15369).

(Plate 10) Fossils from the Ampthill Clay and Kimmeridge Clay (all X 1) 1 Nanogyra virgula (Defrance); Lower Kimmeridge Clay (KC30); M40 motorway cutting, Waterstock (BGS FR2012) 2a-c Torquirhynchia inconstans (J Sowerby); Ampthill Clay (Inconstans Bed); Wormstone, near Waddesdon (BGS MGS649) 3–4 Ammonite aptychal plates (Laevaptychus); Lower Kimmeridge Clay; Garsington (Oxford University Museum J21072a (Fig. 3) and J21072b (Fig. 4)) 5 Pachyteuthis abbreviata (Miller); Ampthill Clay; Westcott (BGS MGS621) 6 Deltoideum delta (Wm Smith); Ampthill Clay; Shotover (BGS GSa3240) 7 Brill Serpulite Bed with abundant Serpula tetragona J de C Sowerby, other shells include Isocyprina (Venericyprina) pellucida Casey; Ampthill Clay; M40 motorway Trial Pit 23, Waterstock (BGS AH2659) 8 Cemented lumachelle of Deltoideum delta (Wm Smith); Ampthill Clay; Waddesdon (BGS MGS1141)

(Plate 11) Fossils from the Kimmeridge Clay (all X 1) 1 Pectinatites (Pectinatites) naso (S S Buckman); Upper Kimmeridge Clay (Pectinatus Sand); Shotover (BGS GSM49357) 2 Pectinatites (Virgatosphinctoides) woodwardi? (Neaverson); Upper Kimmeridge Clay (Wheatley Nodule Bed); M40 motorway cutting, Waterstock (BGS FR2039) 3 Pavlovia pallasioides (Neaverson) (holotype); Upper Kimmeridge Clay (Hartwell Silt); Hartwell (BGS GSM30721) 4 Aulacostephanus volgensis (Vischniakoff); Lower Kimmeridge Clay; Autissiodorensis Zone; BGS Chalgrove Borehole, 101.62 m (BGS BDY3627) 5 Pliosaur vertebra; ?Upper Kimmeridge Clay; Hardwick, near Aylesbury (BGS GSM85796)

(Plate 12) Shotover Brickyard [SP 561 066] (1925); Kimmeridge Clay Formation. Pectinatus Sand (Shotover Grit Sand) containing large calcareous sandstone doggers ('Giant's Marbles'), overlain by Swindon Clay. The uneven junction between the two is probably a result of landslipping (see text) (A3195).

(Plate 13) Fossils from the Portland, Purbeck and Whitchurch Sand formations (all X 1). 1 'Unio'sp.; Whitchurch Sand; Combe Wood, Wheatley (BGS GSM111048). 2 Pleurotomaria sp.; Portland Formation; Garsington (BGS Geol. Soc. Coll. 2907). 3 Aptyxiella portlandica de C Sowerby) (the 'Portland Screw'); Portland Stone Member; Waddesdon (BGS MGS2168). 4 Neomiodon sublaevis (Roemer); Whitchurch Sand; Wheatley (BGS GSM52371). 5 'Ostrea' expansa J Sowerby; Portland Formation; Hartwell (BGS GSM43557). 6 Algal limestone (stromatolite); Purbeck Formation; Stone (BGS MGS2169). 7 Laevitrigonia gibbosa Sowerby); Portland Stone Member; Brill (BGS GSM27815)

(Plate 14) Bugle Pit SSSI, Hartwell [SP 7932 1205]; Purbeck Formation. Almost the whole of the Purbeck Formation is exposed at this locality. The prominent overhanging limestone near the bottom of the section is Bed 3 (see text). Graduated rule is 1 m long (A15373).

(Plate 15) Windmill Pit, Wheatley [SP 589 053] (1925); Whitchurch Sand Formation. Thinly bedded sand with layers of dark ironstone and clay lenses. The folding and the numerous microfaults are a result of cambering (A3201).

(Plate 16) Windmill Quarry, Long Crendon [SP 693 093] (1899). The photograph shows the section recorded by Davies (1899a, p. 22). The pale beds at the bottom of the face are limestones of the Purbeck Formation. They are overlain by clays with ironstone lenses, assigned to the Whitchurch Sand Formation. Overlying beds are Gault; a thin unit of pebbly sand at the base (on which the hammer rests) is probably 'Junction Beds' of early Albian age (Brittsh Association 2416).

(Plate 17) Chinnor Quarry [SU 7607 9994]; Lower and Middle Chalk. The Plenus Marls, about 1 m thick form the lowest part of the face; the distinctive marl band corresponds with beds 6 to 8 of Jefferies (1963). It is overlain by the hard, nodular chalk of the Melbourn Rock, the basal unit of the Middle Chalk. Graduated rule is 1 m long (A14989).

(Plate 18) Boundary wall of Hartwell Park [SP 7950 1208]. The wall is built principally of Portland Stone (Creamy Limestone), probably obtained from the nearby Bugle Pit. The decorative panels feature giant ammonites from the Portland Stone and siliceous concretions ('bowel-stones') from the local Whitchurch Sand, as well as flints from the Upper Chalk (A15374).

(Plate 19) Landslipped Kimmeridge Clay, Muswell Hill [SP 640 157]. Shallow, multiple, retrogressive landslips degrading to lobate mudflows at the toe. The topographical expression of these landslips is commonly suppressed by agriculture (A 15347).

(Plate 20) Landslipped West Walton Formation; north-western slope of Pans Hill [SP 612 140]; aerial view. The lobate form of mudflows is clearly displayed in the lower (northern) half of the photograph. The hill is capped by Arngrove Spiculite. (Sir William Halcrow & Partners; reproduced by permission of the Director, Department of Transport, South East Construction Programme Division).

(Plate 21) Windmill Pit, Wheatley [SP 5890 0522] (1925); cambering with dip and fault structures in the Whitchurc Sand Formation (A3203).

Tables

(Table 1) Groundwater abstraction licence data for the district.

(Table 2) Typical chemical analyses of groundwater in the district.

(Table 3) Geotechnical classification of geological formations in the district.

(Table 4) Lithological composition of the Whitchurch Sand Formation compared with other late Jurassic and early Cretaceous sands.

(Table 5) Clay mineralogy of the Whitchurch Sand and Purbeck formations.

Tables

(Table 1) Groundwater abstraction licence data for the district