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Geology of the Moreton-in-Marsh district. Sheet description of the British Geological Survey 1:50 000 Series Sheet 217 (England and Wales)

Bibliographical reference: Barron, A J M, Sumbler, M G, and Morigi, A N. 2002. Geology of the Moreton-in-Marsh district. Sheet description of the British Geological Survey, Sheet 217 (England and Wales).

Authors: A J M Barron, M G Sumbler and A N Morigi.

Contributors:

• Basement structure and geophysics: T C Pharaoh
• Engineering Geology: D C Entwisle A Forster
• Hydrogeology: M A Lewis Keyworth, Nottingham

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: GD 272191/2002. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

Nottingham: British Geological Survey 2002. © NERC 2002. All rights reserved. ISBN 085272 444 6

Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover)The face of Cleeve Cloud [SO 984 256], overlooking Cheltenham, exposes the Birdlip Limestone Formation of the Inferior Oolite Group. It was formerly a building stone quarry, and the slopes below are covered in degraded limestone spoil heaps. The escarpment here is crowned by an Iron-Age hill fort. (Aerofilms 599571).

(Back cover)

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Acknowledgements

This sheet description was compiled and largely written by A J M Barron, with assistance from M G Sumbler (who is mainly responsible for Chapters 4, 6 and 7 and Information Sources) and A N Morigi (Chapter 2). In Chapter 2, the account of engineering geology is by D C Entwisle and A Forster, and that of Hydrogeology by M A Lewis. T C Pharaoh contributed to the basement geology and structure in Chapters 3 and 8. All parts of the sheet description borrow heavily from the numerous Technical Reports listed in Information Sources (p.34).

Notes

Throughout the sheet description, the word ‘district’ refers to the area covered by the geological 1:50 000 Series Sheet 217 Moreton-in-Marsh. National Grid references are given in square brackets. Grid references of boreholes cited in the text are given in Information Sources. Enquiries concerning the availability of geological data for the district should be addressed to the Manager, National Geosciences Records Centre, British Geological Survey, Kingsley Dunham Centre, Keyworth, Notts. NG12 5GG.

Geology of the Moreton-in-Marsh district—summary

The Moreton-in-Marsh district, covered by geological 1:50 000 Series Sheet 217, includes the northernmost part of the Cotswolds, a designated Area of Outstanding Beauty. This range of hills comprises a rolling limestone plateau dissected by meandering valleys, and bordered by prominent escarpments on the west, overlooking the vales of Gloucester and Evesham, and on the east facing across the Vale of Moreton, where the town of Moreton-in-Marsh lies. There are several other attractive towns and villages, popular with tourists, and part of the large town of Cheltenham lies in the south-west.

The Cotswolds and this district derive much of their distinctive character from the use of the local limestone in the buildings, and in the dry-stone walls that border most fields and lanes in the upland areas. Thus the influence of the underlying geology is very apparent to visitors and this, coupled with the abundance of fossils and availability of rock exposures, drew attention from geological scholars from the very early days of the science. However, a full, systematic and detailed survey of the district was only completed in 1998 and this description summarises the results of this, along with information from older, published sources, and BGS archives. A section on the applied aspects of the geology is included, giving information of importance to those concerned with planning and commercial activities, such as civil engineering and mineral extraction.

The entire district is underlain at depth by deformed Precambrian volcanic rocks. These are overlain by a Lower Palaeozoic marine sedimentary sequence with intercalated basalt lavas. In the south, Devonian continental sedimentary rocks overlie these, overlain unconformably by Upper Coal Measures, present at 200–1700 m depth in the east of the district. These and the underlying strata are affected by folding and thrust faulting. The entire Palaeozoic succession is blanketed by a near-horizontal sequence of Permo-Triassic continental strata, mainly sandstone, mudstone and conglomerate, which thicken dramatically westwards.

The oldest rocks at outcrop are predominantly mudstones of the Lower Jurassic Lias Group, which floor the vales and hill slopes in the east and west. These are overlain by the Middle Jurassic Inferior Oolite Group, dominated by ooidal limestone, which forms the plateau of the north Cotswolds running through the centre of the district, and also caps the outlying hills. Mudstone and limestone of the Great Oolite Group succeed the Inferior Oolite in the centre and south of the upland area. The Jurassic sequence is generally flat-lying or gently southeasterly dipping, but locally may dip more steeply, especially where faulted.

There are no rocks of Upper Jurassic, Cretaceous or Tertiary age in the district, and the oldest Quaternary deposits are remnants of river terrace deposits. These are present in the Vale of Moreton in the east, and are followed by an Anglian glacial and glaciofluvial sequence. Younger deposits of head, river terrace gravels and alluvium lie on the slopes and lower ground, and extensive landslips mask the slopes of the escarpments.

(Table 1) Geological succession of the Moreton-in-Marsh district.

Chapter 1 Introduction

This Sheet Description describes the geological map, the 1:50 000 Series Sheet 217 Moreton-in-Marsh. The district lies largely within Gloucestershire. It includes the northernmost part of the Cotswold Hills, a broad dissected plateau extending south some 80 km to Bath and fringed on the west by a prominent escarpment (‘Cotswold Edge’), which overlooks the vales of Evesham and Gloucester. Several large outlying hills stand in the vales. In the east, a lesser scarp faces east over the vales of Moreton and Bourton.

The town of Moreton-in-Marsh lies in the Vale of Moreton in the east, and other picturesque Cotswold towns include Stow-on-the-Wold, Bourton-on-the-Water, Chipping Campden, Broadway and Winchcombe. Part of Cheltenham, known for its Georgian architecture and spa waters, lies in the south-west.

Throughout the district, buildings of all ages and purposes are constructed of local Cotswold limestone. This provides stone both for walls and roofs, and weathers to warm yellow-brown colours. The Cotswold countryside is also renowned for the dry-stone walls that border many of the fields and lanes.

The vales and plateau are predominantly arable farmland, with some areas of forestry. The steep slopes below the escarpment are generally grassland and woodlands. There are a number of limestone quarries producing building stone, roofing ‘slates’ and aggregate, and a clay pit producing bricks.

Few major roads cross the district and these are mainly restricted to the vales: the A429 following the Roman Foss Way runs north–south through Moreton-in-Marsh and is crossed by the A44 at Stow-on-the-Wold. Several ancient trackways traverse the high ground. Several railway tracks crossed the district; all but one are now disused. In the south, cuttings on a disused railway provide many excellent geological exposures.

The main escarpment, the vales of Evesham and Gloucester and the northern part of the Vale of Moreton are in the catchment of the River Severn. The remainder of the district is within the Thames catchment, and the watercourses draining the central plateau area, the southern part of the Vale of Moreton and the Vale of Bourton all run south into the Thames. These are all more or less permanent watercourses, apart from some of the upper reaches, which are intermittent or seasonal (See Chapter 2).

Geological history

Volcanic and volcaniclastic rocks inferred at depth are thought to be part of a Late Precambrian volcanic arc complex. They were deformed during the Cadomian Orogeny to form the floor of the Midland Platform, a stable crustal area throughout the early Palaeozoic. During Cambrian to earliest Ordovician times, this was buried beneath up to 1100 m of marine sediments.

By the early Ordovician, the Midland Platform (including the Moreton-in-Marsh district) was probably low-lying land, and generally remained emergent until, in the early Silurian, it was covered by a shallow sea. Volcanicity to the south generated basalt lava flows which, intercalated with sediments, formed a thick sequence that may have covered the entire region. This was followed by a considerable accumulation of marine strata through the Silurian. However, a marine regression brought continental conditions to this region, with occasional marine incursions. Fluvial Old Red Sandstone facies (red-brown mudstone and sandstone) spread across England and Wales in early Devonian times. The Acadian Orogeny interrupted deposition, such that the red-brown sandstones and conglomerates of the Upper Old Red Sandstone (mid to late Devonian) rest unconformably on the Lower Devonian sequence.

A major marine transgression in early Carboniferous times established a tropical carbonate shelf across southern Britain, on which a thick limestone sequence was deposited. However, the Variscan Orogeny caused uplift such that deposition of Namurian strata in southern England was restricted to the south-west, and the Lower Carboniferous strata were completely removed from the Moreton-in-Marsh district. By latest Carboniferous times, the district lay in an area of broad floodplains on which mud, silt and sand were accumulating, together with Carbonaceous deposits that later formed thin coals (Upper Coal Measures).

The final phase of the Variscan Orogeny caused folding and thrusting in the Carboniferous strata in the east of the district. In the Permian, the entire region was a desert in which strata at the surface suffered intense weathering and erosion. In the west of this district, Cambro-Ordovician strata were possibly exposed, with the Carboniferous succession being preserved only within the Oxfordshire Coalfield Syncline in the east. Hence, the earliest Permo-Triassic strata were deposited on a marked angular unconformity.

Permian to early Triassic crustal extension created the north–south-trending, fault-bounded Worcester Basin, in which a thick sequence of continental sediments accumulated; the axis of this basin lies in the west of the district (Figure 1). This succession thins markedly eastwards onto the London Platform. The earliest sediments were breccias and aeolian dune sands deposited in arid conditions. A substantial northward-flowing braided river in the Triassic deposited sand and pebbles of the Sherwood Sandstone Group. As the Worcester Basin filled, red-brown windblown silt and mud (Mercia Mudstone Group) were deposited in rivers and lakes on a broad arid plain across southern England. A transgression established shallow shelf seas across the region towards the end of the Triassic, represented by the grey, fossiliferous mudstones and limestones of the Penarth Group.

Continued sea level rise from latest Triassic times caused the shallow sea to extend farther, ultimately establishing marine deposition across much of England and Wales. The remaining land areas were low-lying, generating little coarse sediment, and so predominantly fine grained sediments (Lias Group) were deposited throughout the Early Jurassic. Eastward thinning of the Lias succession is a result of progressive overlap, and possibly differential subsidence across the platform margin, Vale of Moreton Axis: (Figure 1) such that the sequence in the east of the district is condensed relative to that in the west. At times, regression or uplift caused terrigenous silt and fine sand (Dyrham and Bridport Sand formations) to be added to the mud being deposited, or shallowing led to the formation of shoals, in which iron-rich ooids, sand, silt and shell debris were deposited (Marlstone Rock Formation). Open marine conditions were abruptly terminated by uplift or regression at the end of the Early Jurassic, which caused erosion of the highest part of the Lias Group.

By the earliest Mid Jurassic, a shallow shelf sea was established across the Cotswold region, and persisted through most of the Aalenian, Bajocian and Bathonian. It was bordered by landmasses to the north-west and southeast on which Lower Jurassic strata were probably exposed. A warm climate and a low input of terrigenous sediment were conducive to widespread carbonate formation and proliferation of marine life.

The basal unit of the Inferior Oolite was deposited upon the exposed Lias Group strata. Following this, a high energy, open water environment prevailed during much of the period of deposition of the Birdlip Limestone Formation, and ooid and bioclastic limestones were laid down on a steadily subsiding shelf. The greatest thickness of sediment accumulated in the Cheltenham area, over the centre of the buried Worcester Basin (Figure 1). Periodically, conditions permitted a diverse bottom dwelling fauna, and at times lagoonal or intertidal conditions were established, resulting in deposition of marl, mud or sand. Early lithification at some horizons resulted in hardgrounds, which may indicate temporary emergence or submarine cementation. Periodically, further sea level fall led to the erosion of the upper part of the Birdlip Limestone sequence. Following one of these episodes, a sea level rise in the early Bajocian led to the widespread deposition of the Aston Limestone Formation on a shallow shelf with moderate energy conditions, a diverse benthic fauna and periodic terrigenous sediment input or very high energy ooid-peloid shoal conditions.

Differential subsidence, and a slight fall in sea level with consequent erosion in the late Bajocian, resulted in the uppermost, Salperton Limestone Formation of the Inferior Oolite Group overstepping strata ranging from the Aston Limestone to the Whitby Mudstone. A decline in the influence of the Vale of Moreton Axis (Figure 4) and (Figure 5) from hereon led to the preservation of the Salperton Limestone and subsequent strata much further eastwards across the margin of the London Platform.

In early Bathonian times, the Chipping Norton Limestone Formation was deposited in very shallow water in the east of the district while in deeper water to the west, mud of the Fuller’s Earth Formation accumulated, eventually extending across the entire district. Sandy beds at the top reflect the proximity of the emergent London Platform. As sediment built up, marine carbonate deposition was re-established in the mid-Bathonian. Ooids and shell detritus were deposited in thick, cross-laminated beds (Taynton Limestone Formation) and later in thin, ripple-laminated beds, followed by very low-energy calcareous mud and micritic limestones (Hampen Formation). In an increasingly protected lagoonal environment, low-energy micritic limestones of the White Limestone Formation were deposited in the Moreton-in-Marsh district, and from time to time, sea-level fluctuations led to the formation of hardgrounds in very shallow water. Subsequently, a sea level fall resulted in submarine erosion of the upper part of the White Limestone, and the deposition of terrigenous silty clay with lignite fragments and minor limestone beds of the Forest Marble Formation.

During the next major marine transgression, shallow water limestones were deposited, and, as the water deepened, fine-grained sediments, commencing with the Kellaways and Oxford Clay formations, were deposited over an increasing area of the London Platform, continuing through the Late Jurassic. The series of tectonic events through the Mesozoic related to the opening of the North Atlantic, and grouped as the Cimmerian ‘orogeny’ culminated in regional uplift in latest Jurassic times, such that much of Britain formed an extensive landmass through the early Cretaceous. This tectonic activity may have produced most of the west-north-west-trending surface faulting seen in the district. The transgression that followed led to the deposition of a thick sequence of strata overlying this late Cimmerian unconformity, probably including the Chalk. These, and the underlying Upper Jurassic strata, in total perhaps several hundred metres thick, were subsequently completely removed from the Moreton-in-Marsh district by erosion resulting from uplift related to the commencement of the Alpine Orogeny in the early Palaeogene, coupled with a global fall in sea level. The region has remained emergent ever since.

The district’s drainage pattern may have been instigated by this uplift, but the early to mid-Pleistocene Thames–Evenlode system had a far larger catchment, until development of the Severn and the Cotswold escarpment truncated its upper part. Later, the advance of the Anglian ice-sheet deposited till as far south as Broadwell, and outwash material accumulated in the Evenlode valley in two phases, the first rich in Triassic clasts, the second containing chalk and flint. Head and fluvial sediments accumulated in the valleys. The gravelly ‘river terraces’ were later dissected, and lower terraces and modern alluvium were deposited. This process continues to a minor degree to the present day.

History of research

Study of the rocks of the Cotswolds dates back to the earliest days of English geology, when canal engineer William Smith deduced the constancy of the order of strata and their included fossils in the neighbourhood of Bath. In 1799, he compiled a geological map of the district and a ‘Table of the Order of Strata and their embedded Organic remains....’ which extended from the Coal Measures up to the Chalk. This was published in 1813 and in the following years was revised and refined, the Jurassic portion notably by Buckland (1818).

Among the earliest geological studies which included the Moreton-in-Marsh district is Murchison’s ‘Outline of the geology of the neighbourhood of Cheltenham’ (1834, second edition 1845). The variety, fossil content and accessibility of the rocks of the Cotswolds attracted many scholars, including Brodie (1850) and Wright (1856; 1860; 1868). In addition, an important related event was the founding of the Cotteswold Naturalists’ Field Club in 1846.

The Geological Survey published a map of the north Cotswolds at a scale of 1:63 360 [Old Series] Sheet 44, in 1856, which showed the solid strata and alluvium. The part covering this district was surveyed by E Hull and H H Howell. Hull also compiled the accompanying memoir (1857). The district continued to attract the attention of many geologists, including Lucy (1872, 1890), and with the opening of railways, access was improved and new exposures became available. Some of these were described by S S Buckman (1887), and figured by Woodward (1894). Buckman went on to elucidate the stratigraphy of the Inferior Oolite of the Cotswolds, deducing a number of important stratigraphical breaks (1895, 1897, 1901).

Richardson (1904) published his ‘Handbook to the geology of Cheltenham and neighbourhood’. Over the years, he described many exposures in the district (e.g. Richardson, 1912, 1913) and wrote the Geological Survey memoirs for Sheet 217 Moreton-in-Marsh and Sheet 235 Cirencester (Richardson, 1929; 1933); the former is the forerunner of the present account. The geology shown is taken from the map of 1856, with the addition of drift and the Chipping Norton Limestone by H G Dines in 1927. Arkell (1933) compiled an overview of the Jurassic of Great Britain, providing a comprehensive framework for all future work.

The Quaternary deposits of the Vale of Moreton have received much attention (Sumbler, 2001), notably by Tomlinson (1929), and Bishop (1958), with further work by Briggs (e.g. 1973, 1975).

Research interest in the Jurassic of the north Cotswolds was revived in the 1970s partly through publication of field guides (Ager et al., 1973; McKerrow and Kennedy, 1973). Parsons (1976, 1980) has refined the ammonite zonation of the Inferior Oolite Group, and Mudge (1978) studied the ‘Lower Inferior Oolite’.

The cored Batsford (or Lower Lemington) Borehole drilled for coal exploration in 1901–4, proved Palaeozoic basement (Williams and Whittaker, 1974). Knowledge of the deep geology was increased by a series of boreholes drilled by the Gas Council in the 1960s between Stow-onthe-Wold and Chipping Norton, as part of a scheme to store natural gas in the Triassic sandstones (Horton et al., 1987). Further information on the concealed strata was provided by the Guiting Power and Ash Farm boreholes, drilled for hydrocarbon exploration by Bearcat Exploration (UK) Ltd. in 1979 and 1981 respectively.

The detailed survey of the Moreton-in-Marsh district began in 1966, with mapping at 1:10 560 scale of the Bredon Hill area (Whittaker, 1972), accompanied by the drilling of two stratigraphical boreholes there. The area south from Bredon Hill to Cheltenham was mapped in 1980–82 at 1:10 000 scale as part of the survey of the adjoining Tewkesbury district. The completion of the remainder of the Moreton-in-Marsh district between 1994 and 1998 followed a full survey of the Cirencester district to the south between 1984 and 1994. Together, these projects led to the revision of the nomenclature of the Inferior Oolite Group (Barron et al., 1997). In 1997, the Gloucestershire RIGS (Regionally Important Geological and Geomorphological Sites) Group undertook conservation work in several quarries on Cleeve Hill, including the famous Rolling Bank Quarry (Angseesing et al., 2002).

Geological maps at 1:10 000 scale are available for the entire district, and corresponding descriptive reports are available for most of the maps. These items are listed in Information sources together with other pertinent BGS maps, reports and publications, and an indication of other data held by BGS.

Chapter 2 Applied Geology

The key geological factors pertinent to planning and development in the Moreton-in-Marsh district are:

• engineering geology: geotechnical properties, foundation conditions and slope stability
• hydrogeology: water resource management and conservation
• mineral and energy resources
• soils and agriculture

Engineering geology

Geotechnical properties of formations

The bedrock (solid) geological formations that crop out in the district are, in engineering terms, weak to very strong limestones, and over-consolidated, fissured, very stiff clays, weak mudstones, weak siltstones, compact silt, weak to strong sandstones and dense sands (Table 2). The natural superficial deposits comprise combinations of normally consolidated and over-consolidated clay, silt, sand and gravel. The varied nature of the geological materials and the effects of periglacial processes influence ground conditions in ways pertinent to land use and construction.

Pleistocene periglacial activity has caused the bedrock to be disrupted to a depth of several metres. Limestones are generally broken-up in the near-surface zone, and bedding and joint planes are dilated. Mudstones are similarly disrupted, with an attendant increase in moisture content, and reduction in strength to firm or occasionally soft clay. In these circumstances, strength measurements should be obtained by in-situ methods (Higginbottom and Fookes, 1971). In addition, the possible presence within formations/deposits of thin beds of material of contrasting properties highlight the importance of determining the precise nature of the materials present at a site by an appropriate site investigation.

Foundation conditions

In general, the limestones of the district offer good foundation conditions but with some important exceptions. Limestone may contain cavities formed by water charged with carbon dioxide. This process is active, but would have been more vigorous under periglacial conditions. Such voids might take the form of widened joints, perhaps tens of millimetres across. More significant voids may be found in the form of linear, tensional features called gulls which occur in cambered strata (see Chapter 8).

These are very significant for land use and construction on upper valley slopes and plateaux. Gulls may be open or more or less filled by material from above with a bearing capacity and compressibility very different from that of the surrounding limestone. In some instances open gulls may be bridged, and possibly concealed, by naturally cemented limestone rubble, although in this district it is thought that most gulls show a hollow at the surface (Plate 1). Foundations which cross a gull, in whole or in part, may encounter problems due to lack of support and differential settlement and compaction. If they cannot be relocated to better ground, it may be possible to design foundations capable of bridging areas of weak ground or voids (Hawkins and Privett, 1981). The presence of gulls can be investigated by digging intersection trenches at right angles to the inferred gull direction.

In addition, the possibility of man-made voids should be considered. Backfilled quarries may no longer be apparent at the surface. In addition, the Cleeve Cloud Member (Birdlip Limestone Formation) has been worked by means of adits and galleries at Whittington [SP 007 215], Syreford [SP 033 205] Bourton-on-the-Hill [SP 170 326] and Westington [SP 139 366] and it is possible that this and other formations have been mined elsewhere.

The mudstones in the district may offer reasonable foundation conditions if suitable designs are adopted. They are generally of high plasticity but may range from intermediate to very high plasticity. In the weathered zone, strength is likely to be lower and plasticity higher. High plasticity clays are particularly prone to shrink-swell problems (Building Research Establishment, 1980a, b, 1985, 1993). Low (remoulded) strength values may be encountered in landslipped mudstones/clays (see Chapter 8). The disposal of surface water away from areas of actual or potential landslipping is particularly important. Valley bulged mudstone formations (see Chapter 8) are also likely to have suffered a loss of strength.

Geotechnical data for similar geological formations in other districts (Forster et al., 1995) indicate that conditions for sulphate attack on concrete foundations excavated within limestone units below the water table are likely to fall into Class 1 as defined in Building Research Establishment, Digest 363 (1991). Similarly, most mudstone units in the area are likely to fall into Class 1 or 2, except for the Charmouth Mudstone Formation, which is likely to be Class 3. However, unless there is a high level of dissolved sulphate in groundwater that is flowing and in contact with the concrete structure, there is unlikely to be a significant problem of sulphate attack. Geotechnical data for alluvium in other areas (Forster et al., 1995) indicate that conditions for sulphate attack on concrete foundations below the water table are likely to fall into Class 1, as defined in Building Research Establishment, Digest 363 (1991).

The sandstone units of the district should offer reasonable foundation conditions where they are sufficiently dense and are protected from erosion. Shallow foundations in sandstone may be affected by frost heave.

Of the natural superficial deposits, the most significant in the context of construction are alluvium, till and glaciolacustrine deposits. Alluvium is generally a soft to firm, compressible material, and does not usually offer good foundation conditions. Where peaty or organic layers or lenses are present, it may suffer from differential consolidation. Alluvium may also have a desiccated crust with a higher strength than the underlying material. Tills are heterogeneous deposits that range from over-consolidated, stiff to very stiff plastic clays to very dense sands and gravels. The glaciolacustrine deposits (Moreton Member) include laminated silt and clay, which are very soft near to the surface and become firm or compact with increasing depth. Like the alluvium, they do not present good foundation conditions. The variability in thickness and composition of all these deposits necessitates investigation and determination of geotechnical properties for individual sites.

Excavations

Excavation of most of the limestones of the district should be possible using mechanical excavators or rippers. However, more powerful machinery may be necessary to deal with more massive blocks. In general, the strength of most of the limestones is expected to fall in the moderately strong range, with an unconfined compressive strength of 12.5 to 50 MPa, but significantly stronger units may be present locally (Stone Industries, 1972). The stability of cut faces in the limestone units of the area may be expected to be good except where they have been affected by faulting, cambering or landslipping, or have steep dips. Excavations in the mudstone and sandstone units should be possible using mechanical excavators, except where limestone beds are present in the former.

Support in excavations in disturbed mudstone, sandstone and natural superficial deposits may be required. Groundwater inflow may need to be controlled to avoid running sand conditions in superficial deposits or softening and heave in mudstones. If a desiccated crust is present, passage of wheeled vehicles on site may be impaired if it is removed or disrupted.

Use as engineering fill

The limestones of the area should be well suited for use as engineering fill if crushed and graded; the mudstones should be suitable if the moisture content of the more plastic clays is controlled within suitable limits. Gravels may also be suitable if the particle size grading is appropriate, but alluvium, head and landslip deposits are less appropriate due to their higher moisture contents and the likely presence of organic material.

Slope stability

Landslips result when the strength of the material of the slope is too low to resist the downward shear stress due to the force of gravity. The slope changes shape until it reaches an equilibrium angle at which the material strength is in balance with the stress. Landslips may be triggered by an increase in the shear stress or by a reduction of the shear strength of the slope-forming material. The former situation is encountered where the angle of slope is increased by undercutting or by loading the top of the slope, and the latter by changes in structure and composition due to weathering or an increase in pore water pressure.

All geological formations may exhibit some form of slope instability under some circumstances, but within the Moreton-in-Marsh district, the Dyrham, Whitby Mudstone and Fuller’s Earth formations are most commonly affected (see Chapter 8). These formations include highly plastic clays, and underlie strata that act as aquifers. The clay at the interface reduces in strength as pore water pressures increase, ultimately failing and leading to relatively deep-seated rotational or translational slides. Surface water increases the moisture content in the near-surface zone, including landslipped material, leading eventually to the development of mudflows. The progression from the terraced slopes of multiple rotational failure, downhill through the hummocky ground of mud and debris-slides, to spreading, lobate mudflows, is commonly seen on slopes in the district.

Although the most intensive period of landslipping probably occurred during periglacial phases of the Pleistocene, some of the landslips within the district (Chapter 8) may still be intermittently active, as demonstrated by fresh cracks in the ground, tilted trees and buildings or roads showing structural damage or even partial burial. In Cleeve Hill village, there are recent instances of building collapse and road damage requiring frequent repairs.

Where inactive, the surface expression of landslips may be obscured within very few years (Hutchinson, 1967). However, the material may remain weak and contain shear surfaces that may be reactivated if the pore water pressure rises or the slope is undercut. Slips are prone to reactivation by Man’s activities, for example, during the recent construction of the Broadway by-pass, excavation within the landslip deposits caused the collapse of the old road [SP 112 370]. Before construction work on slopes in susceptible clay formations is started, investigations should be made for evidence of past movement, such as relict shear surfaces.

Artificial ground

Artificial ground (Worked Ground and Infilled Ground, Made Ground and Disturbed Ground) occurs in many places in the Moreton-in-Marsh district, not all of which are shown on Sheet 217. Made Ground comprises fill on the existing ground surface, and is mainly engineered, such as road and rail embankments. Worked Ground and Infilled Ground are excavations that may be partially or wholly backfilled, including road and railway cuttings. However, the majority are quarries or gravel pits. The most extensive backfilled quarries are Slade Quarry [SP 071 215], which has been entirely filled with refuse, and the limestone quarry at Saintbury Hill [SP 126 385] now partly backfilled with construction waste. The main areas of Disturbed Ground are associated with shallow tilestone workings in the Eyford Member near Naunton, and the Chipping Norton Limestone near Snowshill.

Hydrogeology and water resources

In the Moreton-in-Marsh district, the aquifer formed by the Inferior Oolite is the principal source of groundwater. Although generally at outcrop, and thus unconfined over much of the district, it is locally confined by younger beds. The mean annual rainfall in the district ranges from 650 mm on the low ground to more than 850 mm on the high ground. Recharge over much of the high escarpment is in excess of 450 mm/a. The Midlands and Thames regions of the Environment Agency administer the water resources of the district.

Groundwater abstraction and protection

The limestones of the Inferior Oolite and Great Oolite groups constitute major aquifers as defined by the Environment Agency (National Rivers Authority, 1995a, b, c; Environment Agency, 1996; Allen et al., 1997), and there are several minor aquifers (Jones et al., 2000).

Licensed groundwater abstractions within the district total 7 661 000 m³/a, the great majority (96%) being taken from the Inferior Oolite aquifer. Most is used for public supply (by Thames Water Utilities and Severn-Trent Water plc), the remainder serving agricultural, industrial and private needs.

The Sherwood Sandstone Group, present throughout the area beneath about 500 m of low permeability strata, has not been utilised as an aquifer, and any water is likely to be brackish. The outcropping Charmouth Mudstone and Whitby Mudstone formations are generally poor aquifers due to their low permeabilities. However, the Cheltenham Spa saline mineral waters, which show large variations in chemistry, derive from over 50 wells and springs in the Charmouth Mudstone (Richardson, 1930, pp.207–216), although the true chalybeate (iron-rich) springs of Cheltenham originate from superficial deposits (see below). A number of springs and seepages occur at the base of the more permeable Dyrham Formation, and the Marlstone Rock Formation is a local source of rather ironrich water (Jones et al., 2000).

The ‘Inferior Oolite Aquifer’ comprises the predominantly limestone strata of the Inferior Oolite Group, together with the underlying Bridport Sand, where present, and the overlying Chipping Norton Limestone (Great Oolite Group) which are all in hydraulic continuity. It thins from 110 m in the west to less than 15 m in the east. It is highly permeable; transmissivities are highest in the west, but very variable (Allen et al., 1997). The aquifer’s extensive outcrop offers a large area for recharge. The Great Oolite limestones are also in hydraulic continuity where the largely impermeable Fuller’s Earth is absent. Along the scarp slope, particularly in the west, the Inferior Oolite is drained by springs issuing from landslips. Elsewhere, the regional groundwater flow in the aquifers is down-dip (south-east).

There are major pumping stations at springs issuing from the base at Pinnock (Broadwater) [SP 0765 2722], [SP 0805 2705], Syreford [SP 0269 2042], Upper Swell [SP 1752 2705], Seven Springs (near Naunton) [SP 1387 2238] and Lower Swell [SP 1730 2558] and Dovedale [SP 1601 3419] and [SP 1620 3362]. The permanent head of the Dikler at Waterhead Barn [SP 166 279] and the source of the Windrush at Taddington [SP 092 315] rise at this level. In addition, smaller springs discharge at the base of the Aston Limestone Formation where it overlies the mudstones of the Harford Member.

The ‘Great Oolite Aquifer’ comprises up to 25 m of limestone formations above the Fuller’s Earth mudstones. They are brittle and fractured, and contain fissured horizons that concentrate groundwater movement. However, yields from wells are generally low due to the thin saturated thickness of the aquifer and it becomes a more important aquifer further south (Sumbler et al., 2000). Springs arise at the base of the aquifer, associated with low permeability layers within it, or rarely, at faults.

Sands and gravels within the Wolston Formation yield water to springs, wells and boreholes in the Evenlode valley. The Cheltenham Sand and Gravel yields iron-rich water in the Cheltenham to Gotherington area, although some sources are now polluted by surface contamination. Minor amounts of water were formerly obtained from river terrace and head gravel deposits in the vales.

Many springs along the escarpments issue from within landslips, but the water has actually percolated down from the interface of permeable limestone or sand on impermeable mudstone through displaced limestone strata or fissures and shears. The largest such spring is at Stanway [SP 0748 3237] which has yielded 19.4 litres/sec in a dry summer.

Groundwater in the district, although naturally of high quality, is highly vulnerable to contamination from both diffuse and point source pollutants. The soils on the aquifer outcrops have little ability to attenuate pollutants, and liquid discharges may move rapidly through them into underlying groundwater. Successful aquifer remediation is difficult, prolonged and expensive, and therefore prevention is important.

Groundwater chemistry

The major ions in water from these unconfined aquifers are calcium and bicarbonate, with tufa deposits at springs. Iron and fluoride concentrations are low but water from some springs and wells exceeds the European Community maximum admissible concentration for nitrate of 11.3 mg/l NO₃–N, due to inputs of agricultural fertiliser and contamination from septic tank discharges.

Seasonal or intermittent flow of streams and water resources conservation

In the limestone aquifers, changes in hydrostatic head may be transmitted rapidly over long distances. This may affect borehole water levels and spring flows several kilometres distant. These substantial and rapid changes can result in rivers having very high peak flows in winter that fall rapidly in summer, as groundwater levels decline. In dry years, this can cause intermittent flows in the rivers and streams traversing the plateau where they cross the aquifer outcrops, for instance in the Dikler north-east of Condicote and the Windrush around Naunton. Additionally some abstractions may be unable to operate throughout the year.

Mineral and energy resources

Up-to-date information on quarry operators is available from BGS in the form of the Directory of Mines and Quarries (latest edition 1998) or from the BRITPITS database.

Building stone

The ready availability of a wide variety of local limestones for building purposes has given the towns and villages of the district their typical Cotswold character. Pale grey to yellow-brown limestones from the Great Oolite and Inferior Oolite have been quarried in the district since Roman times. From the medieval churches and manors to modern housing projects, the limestones have been in continuous demand for building purposes (Arkell, 1947a; Clifton-Taylor, 1972).

The dry-stone field walls that are so characteristic of the Cotswolds were generally constructed from the most convenient stone that came to hand and almost every field had its quarry. However, for building construction, the masons preferred certain limestone beds. The Birdlip Limestone Formation was quarried for building stone at many localities, most notably Jackdaw Quarry, Syreford, Temple Guiting, Bourton-on-the-Hill, Saintbury Hill and Westington Hill and in galleries at Whittington, Syreford and Westington Hill. It is still quarried on a large scale at ARC Guiting and Cotswold Hill quarries at Ford, and at Stanley and Broadway quarries near Chipping Campden. The characteristic honey-coloured oolite is much in evidence in many fine buildings in the classic Cotswold towns of Broadway, Chipping Campden, Winchcombe and Bourton-on-the-Water, and in Cheltenham. Exceptional examples in the district of buildings constructed from this stone include Stanway House [SP 061 323], Sudeley Castle [SP 031 276] and Batsford Park [SP 185 336], as well as many parish churches, of which the finest is arguably St James’s, Chipping Campden.

The limestone beds of the Great Oolite Group have been worked in the past, as is evident from the abandoned quarries in the district. The Taynton Limestone is a fine building stone that is extensively quarried in the Cirencester district to the south, but in this district it is exploited only at Brockhill [SP 133 238] and Soundborough (currently dormant) [SP 053 215]. The former Slade Quarry [SP 070 215] is now a landfill site. Building stone has not been quarried from the White Limestone Formation within this district.

Roofing tilestone

The almost ubiquitous use of natural stone ‘slates’ or tilestones for roofing contributes as much to the character of local buildings as does the stone of their walls. The fissile, sandy limestones of the Eyford Member were the premier source of ‘Cotswold slate’, and were extensively quarried north and east of Naunton [SP 13 25] and [SP 13 23] and north of Whittington [SP 01 22]. Production involved exposing the quarried blocks of stone (‘pendle’) to frosts over the winter period. By the spring, the stone split readily along the bedding planes into thin sheets suitable for dressing. The Chipping Norton Limestone is also fissile and widespread shallow workings in this formation between Snowshill [SP 09 33] and Hinchwick [SP 14 30] attest to former quarrying for tilestones. Currently, tilestones are quarried on only a small scale from the Eyford Member at Brockhill Quarry [SP 133 238].

Crushed limestone aggregate

Most of the limestones have the potential for use as crushed aggregate in construction, and nearly all have been used at some time for this purpose, albeit in a piecemeal fashion. Currently, crushed aggregate is produced at ARC Guiting and Cotswold Hill quarries at Ford (Birdlip Limestone Formation), and Huntsman’s Quarry (Eyford Member) at Eyford Hill.

Sand and Gravel

The sand and gravel resources of the district lie within Quaternary deposits, namely the Cheltenham Sand and Gravel, head gravel, alluvium, river terrace deposits, and glaciofluvial deposits of the Wolston Formation. For details of composition and thickness see Chapter 7.

Extensive spreads of Cheltenham Sand and Gravel occur around and to the north of Cheltenham. Formerly used as a source of sand, virtually the entire outcrop is now sterilised by development. Head gravel deposits occur widely between Bredon Hill, Broadway and Winchcombe and have been worked near Beckford [SO 955 355]; [SO 966 358], together with underlying river terrace deposits. Similar limestone gravel has also been dug near Broadway [SP 087 379]. These deposits may constitute a potential sand and gravel resource but are likely to vary in composition and especially thickness, and may include clayey lenses.

River terrace deposits along the north side of Carrant Brook, in the north-west, have been extensively exploited and are now largely worked out or sterilised by development. The deposits to the south consist only of sandy clay. The Upper Thames Valley Formation (Sherborne and Rissington members, and suballuvial gravels) has been worked widely around Bourton-on-the-Water. All workings ceased in the 1970s and are now flooded or restored. Most of the remaining deposits are sterilised but unworked sand and gravel remains in open country close to the Dikler.

The glaciofluvial Paxford Gravel of the Wolston Formation occurs both at outcrop and beneath overburden in the Vale of Moreton, and is underlain by the Stretton Sand at Stretton and Ditchford hills. Potential resources remain at Ditchford and Todenham. The Wolford Heath Member around Moreton-in-Marsh was once worked on a small scale, but the deposits, though very extensive, are too thin and clayey to have much resource potential. South of the town, this deposit and the underlying Moreton Member were assessed by a commercial company but permission for extraction was refused. It seems unlikely that any bodies of sand and gravel in the Moreton Member will have significant economic value because of their high clay and silt content.

Clay

The Charmouth Mudstone Formation was formerly worked for brickclay at Battledown [SO 962 218] and Harp Hill [SO 967 224] in Cheltenham, and at Aston Magna [SP 199 354], where the Dyrham Formation was also utilised. The only current works is Northcot Bricks Ltd at Blockley [SP 180 370]. There is no evidence that the Fuller’s Earth Formation has ever been worked in the district, either for brick making or to provide fuller’s earth.

Coal

The eastern part of the district includes the westernmost part of the Oxfordshire Coalfield (Dunham and Poole, 1974). The Batsford Borehole was drilled for coal, proving 154 m of mudstones, sandstones and conglomerates with traces of coal (Strahan, 1913). The Upton Borehole, in the Cirencester district (Sumbler et al., 2000), proved 15 or 16 coal seams in the ‘Upper Coal Measures’ (Worssam, 1963), with a maximum thickness of 0.61 m, but none was of workable quality. Given the recent contraction of the coal industry, there is little likelihood of these being worked in the foreseeable future.

Hydrocarbons

Commercial exploration surveys, including seismic profiling and the drilling of the Guiting Power and Ash Farm boreholes, were carried out during the 1970s and 1980s in this and adjoining districts. Despite minor gas shows at some levels, the lack of suitable source rocks for much of the district indicates low prospectivity.

Soils and agriculture

The close relationship between geology, topography, soils and agriculture is particularly marked in the Moreton-in-Marsh district (Soil Survey of England and Wales, 1983, 1984); (Table 3). Rendzinas, which have developed on the limestones of the Great Oolite and Inferior Oolite groups, cover much of the Cotswold dip slope and locally, Aberford association soils occur in narrow spreads. Arable and mixed farming predominate here and ploughing brings limestone debris to the surface.

The predominant lowland soils are calcareous pelosols in the Vale of Evesham, and pelo-stagnogleys in the east. Although these soils are difficult to work being very poorly drained and subject to waterlogging, cereal cropping is common. The soils on the landslipped slopes are mainly stagnogleyic argillic brown earths and stagnogleys. These are also poorly drained and seasonally waterlogged, and are generally given over to dairying, sheep grazing and woodland.

Brown calcareous earths have formed on spreads of Cheltenham Sand and Gravel and Head Gravel, which are generally well drained with limestone gravel at shallow depths. Farming is mixed or arable (cereal) with some horticulture locally. The complex Quaternary deposits in the Vale of Moreton have given rise to a patchwork of both poorly and well drained soils.

Chapter 3 Basement geology: concealed Palaeozoic and Triassic

Information on the deep geology of the Moreton-in-Marsh district comes from two principal sources: deep boreholes, within the district and just outside, and seismic reflection data.

Precambrian and Lower Palaeozoic basement

On seismic reflection data, two highly reflective sequences are recognised in the pre-Devonian basement. The lower is interpreted as the Precambrian basement, as proved by the Kempsey No. 1 and Withycombe Farm boreholes, to the north-west and east of the district, respectively. The lower sequence comprises heterogeneous lithic sandstones and tuffs, the latter having a probable Charnian affinity (Barclay et al., 1997). Basal Cambrian quartzites are present, but the majority of the 1000 m-thick Cambrian–Tremadoc sequence comprises shales and siltstones, as proved by the Sarsden No. 2 Borehole to the east of the district.

The upper highly reflective sequence is interpreted as early Silurian volcanic rocks, which are up to 700 m thick in the west. These are proved by the Netherton No. 1 and Bicester No. 1 boreholes, respectively, to the north-west and east of the district (Pharaoh et al., 1991), and inferred to lie unconformably on the Cambrian–Tremadoc strata. Seismic evidence suggests that an overlying sequence of Silurian shales with thin sandstones and limestones, penetrated by the Batsford Borehole, is up to 1000 m thick in the central part of the district, but has been largely removed by subsequent erosion in the east.

Devonian

Devonian strata are restricted to a subcrop in the southcentral part of the district. The Guiting Power No. 1 Borehole proved 172 m of fluviatile cross-bedded red sandstones and siltstones (Figure 2), for which an early Devonian age has been inferred. Ash Farm No. 1 Borehole proved at least 122 m of Devonian strata (unbottomed). The vertically dipping sandstones (Old Red Sandstone facies) at the bottom are similar to Guiting Power, and an early Devonian age and total thickness of perhaps 500 m are inferred. However, seismic data shows that the upper part of the pre-Carboniferous sequence hereabouts (up to 130 m thick) is gently dipping and more likely comprises Upper Devonian strata (Sheet 217, Palaeozoic sketch map).

Carboniferous

Late Carboniferous strata are inferred to overlie the Upper Devonian strata in the south-east of the Moreton-in-Marsh district, overstepping northwards onto Lower Devonian, then Silurian strata (Sheet 217, Palaeozoic sketch map and cross-sections 1 and 2). The Ash Farm Borehole, near the western margin of the Oxfordshire Coalfield Syncline (Sumbler et al., 2000), proved 859.6 m of Bolsovian to Westphalian D age strata overlying Old Red Sandstone. Seismic evidence suggests that over 1000 m of these strata may be present in the extreme south-east, in the axial area of the syncline, consistent with the thickness in the Apley Barn Borehole, 18 km to the south-east, which lies in a similar structural position.

It is probable that the succession at Ash Farm is comparable with that at Apley Barn, where Poole (1969) divided the Carboniferous into five formations (originally defined as ‘groups’). However, it is possible that the sequence at Ash Farm is complicated by faulting and thrusting (see Peace and Besly, 1997). The following classification should be regarded as provisional.

The Ash Farm Borehole (Figure 2) proved 171 m of the Arenaceous Coal Formation, which is dominated by sandstone with subordinate mudstone, seatearth and coal. This is overlain by 61 m of the Witney Coal Formation, comprising normal grey ‘coal measures’, made up of rhythmically bedded mudstone, siltstone and sandstone, with seatearth and some substantial coal. The 101 m of the Crawley Formation comprises brecciated and reddened mudstone, alternating with espley-type breccia and grey seatearth, sandstone and mudstone. These are overlain by 84 m of the Burford Coal Formation, similar to the Witney Coal Formation. About 443 m of the Windrush Formation ‘red measures’ - reddened and variegated mudstone, siltstone and sandstone, with minor pedogenic limestones - were also proved, comparable with the Keele Beds of the Midlands. It also includes thin units of grey measures with coal seams. Up to 600 m of these strata may be present in the extreme south-east part of the district, giving a total for the Upper Coal Measures there of perhaps 1020 m. Carboniferous strata that probably belongs to the Windrush Formation were penetrated at the base of the Stow-on-the-Wold No. 1 and No. 2 boreholes proving some 18 m and 50 m of strata, respectively.

The sequence from the Witney Coal Formation to the Windrush Formation probably equates with the Halesowen Formation of the West Midlands (Peace and Besly, 1997), now included in the Warwickshire Group (Powell et al., 2000).

Variscan structural inversion

The late Carboniferous strata are involved in a complex eastward-verging fold-thrust belt (Oxfordshire Thrust Belt of Peace and Besly, 1997) marking the eastern limit of Variscan inversion along the Worcester Anticline (Chadwick and Smith, 1988). In latest Carboniferous time, uplift led to erosion of progressively older strata westwards (Sheet 217, Palaeozoic sketch map). The overthrust belt comprises at least three linked thrusts with associated ramp anticlines. Displacement on each thrust varies considerably along strike but Peace and Besly (1997) estimate overall east–west shortening of 1600 m. Seismic evidence suggests that the thrusts flatten out at depth within the Cambrian–Tremadoc shales, before steepening again beneath the Worcester Basin.

Permian and Mesozoic structural setting

Development of the Worcester Basin

The Worcester Basin is a major north–south-trending graben produced by east–west extension in Permian and Mesozoic time. Its axis runs approximately through Bredon Hill and Cheltenham in the west of the Moreton-in-Marsh district (Figure 1) and it is here that the thickest sequence (up to 3000 m) of strata of this age are preserved. The bounding faults on the eastern margin (Moreton and Ilmington faults; Sheet 217) developed as ‘shortcut structures’ in the hanging wall of the overthrust belt (Chadwick and Smith, 1988; Corfield et al., 1996).

Permo–Triassic

The Guiting Power No. 1 Borehole (Figure 2) proved a total of 1000 m of Permian–Triassic coarse clastic strata between 1013 and 2013 m depth. A similar thickness is probably present in the footwall of the Inkberrow Fault (Sheet 217, cross-section 1), and perhaps 1500 m in its hangingwall. The sequence thins dramatically eastwards across the margin of the graben, to as little as 20 m.

Permian

The Haffield Breccia Formation and equivalents are believed to be the earliest strata to be deposited during the extensional phase of development of the Worcester Basin (Peace and Besly, 1997). Possibly 150 m are present to the west of the Ilmington Fault, unconformably overlying the Westphalian strata. The breccia is overlapped westwards by the Bridgnorth Sandstone Formation, which is of inferred aeolian origin and may reach 450 m in thickness in the west. However, in Guiting Power Borehole, it comprises just 155 m of reddish brown argillaceous fine to medium-grained sandstone, with minor amounts of siltstone and mudstone. It is overlain in the borehole by 31 m of mudstone and siltstone, which is also tentatively assigned a Permian age, but is unknown elsewhere in the region. These strata are overlapped eastwards by the Sherwood Sandstone Group, and are thought to be absent east of the Ilmington and Moreton faults.

Triassic

Sherwood Sandstone Group

About 814 m of strata attributed to the Sherwood Sandstone Group were penetrated in the Guiting Power Borehole (Figure 2). These are subdivided into three formations recognised in the Midlands: in ascending order the Kidderminster, Wildmoor Sandstone and Bromsgrove Sandstone. Farther east, only the last is present, as shown by the Batsford, Ash Farm, and Stow-on-the-Wold No. 1 and No. 2 boreholes.

The Kidderminster Formation (formerly Bunter Pebble Beds) at Guiting Power comprises 61 m of reddish brown conglomerate, sand, sandstone and mudstone, with minor amounts of siltstone and dolomite. The conglomerates comprise angular fragments and subrounded coarse to very coarse pebbles of quartz, quartzite and chert in a sandstone matrix. The sandstone ranges from very fine to very coarse grained. Although overlapped in the east, the formation may be as much as 125 m thick in the west, and reaches over 300 m in the Worcester district to the northwest (Barclay et al., 1997).

The Wildmoor Sandstone Formation (formerly ‘Upper Mottled Sandstone’), comprising reddish brown fine-grained mainly fluvial sandstones, is 258 m thick at Guiting Power (Figure 2). In the west, the formation may attain 350 m (Sheet 217, cross-section 1), and it thickens northwards to over 600 m in the Worcester district.

The highest unit of the Sherwood Sandstone is the Bromsgrove Sandstone Formation, which is probably present throughout the Moreton-in-Marsh district; it ranges from 20 to 650 m in thickness. In Guiting Power Borehole the formation is 496 m thick (Figure 2) and consists of reddish brown, fine to coarse-grained calcareous sandstone with siltstone and mudstone beds together with some very coarse-grained and conglomeratic beds. A similar sequence is seen in the centre of the Worcester Basin (Barclay et al., 1997), but the conglomerate beds are absent suggesting a more distal site of deposition by a northward-flowing predominantly braided river.

Further subdivision of the formation into the members defined in the Midlands, (Old et al., 1991) and tentatively recognised in the Stowell Park Borehole to the south (Sumbler et al., 2000), is possible by interpretation of the geophysical logs. At Guiting Power (Figure 2), the lowest, Burcot Member is inferred to comprise 230 m of high energy fluvial coarse-grained sandstones and conglomerates with minor beds of mudstone. A lower energy meandering river deposited the overlying Finstall Member, forming a 235 m-thick sequence of sandstone that is crossbedded, calcareous, micaceous, argillaceous and poorly sorted with interbedded thick silty mudstone and conglomerate beds. This passes up into a sandstone-dominated sequence 30 m thick, a coarser-grained equivalent of the Sugarbrook Member of the type area (Old et al., 1991). Similar divisions have been tentatively made in the Ash Farm and Stow-on-the-Wold No. 1 and No. 2 boreholes, showing a dramatic eastward thinning of all three members.

Mercia Mudstone Group

Reddish brown mudstone, silty mudstone and siltstone dominate the Mercia Mudstone Group. Anhydrite is present throughout most of the succession, accompanied by other evaporite minerals. The group is 446 m thick in the Guiting Power Borehole (Figure 2) although it may exceed 600 m on the downthrow side of the Inkberrow Fault (Sheet 217, cross-section 1) which is closer to the axis of the Worcester Basin. It thins eastwards across the margin of the basin, reducing to 139 m at Ash Farm, and about 110 m in the extreme south-east. The four component formations recognisable at outcrop have been tentatively identified from the logs of boreholes through the district. Thus the thinning is largely due to the sequence being condensed, although there is some evidence of overstep in the upper part.

The Eldersfield Mudstone Formation is estimated to be 313 m thick at Guiting Power; 290 m were proved at Stowell Park to the south (Sumbler et al., 2000) and 349 m at Eldersfield to the north-west which lies closer to the centre of the Worcester Basin (Barclay et al., 1997). It comprises reddish brown, calcareous, locally sandy mudstone, grading to siltstone or sandstone and includes traces of evaporite minerals. Like the Twyning Mudstone higher in the sequence, it is interpreted as having been deposited in an arid or semi-arid environment, with saline lakes and aeolian influence. In the south-east, the formation thins to about 55 m.

The Arden Sandstone Formation is inferred to lie between 690 and 700 m depth (10 m thick) in the Guiting Power Borehole (Figure 2). However, the reddish brown coarse-grained sandstone material recovered at this level does not resemble the fine-grained greenish grey Arden Sandstone in the cored Stowell Park Borehole (Sumbler et al., 2000). Similar thicknesses are inferred for the formation in the Stow-on-the-Wold No. 1 and No. 2 boreholes.

The Twyning Mudstone Formation at Guiting Power comprises reddish brown and grey mudstone that is calcareous and sandy in parts. It is about 122 m thick, identical to that proved in the Stowell Park Borehole (Sumbler et al., 2000) but thinner than the 169 m proved in the Twyning Borehole that lies closer to the centre of the Worcester Basin (Barclay et al., 1997). Eastwards, the formation thins to 84 m and 69 m in Stow-on-the-Wold No. 1 and No. 2, respectively. It is possible that the uppermost beds may have been overstepped by the Penarth Group, or more probably the Blue Anchor Formation, although it is possible that the succession is condensed in the east.

Pale greenish grey mudstones of the Blue Anchor Formation occur at the top of the Mercia Mudstone Group throughout the region and are typically 5 m thick in this district; Stowell Park Borehole proved 9 m and Twyning Borehole proved 10 m. The formation represents a transition between the continental deposits of the Mercia Mudstone and the marine deposits of the succeeding Penarth Group.

Penarth Group

The Penarth Group comprises the Westbury Formation and the succeeding Lilstock Formation. The latter is divided into lower (Cotham) and upper (Langport) members (Warrington et al., 1980). The group ranges from 13 to 15 m thick across the district, and the formation seems little affected by the Vale of Moreton Axis (Figure 1); (Sheet 217, cross-section 1), although farther east it pinches out towards an inferred shoreline on the London Platform (Horton et al., 1987).

The lowest part of the Penarth Group, the Westbury Formation, comprises dark grey to black, fissile marine shaly mudstones, rich in bivalves. It is 1.5 m thick in the Batsford Borehole and 3.7 m thick at Guiting Power. The Cotham Member of the Lilstock Formation, about 11 m thick at Batsford and 7 m at Guiting Power, comprises greenish grey, calcareous, argillaceous mudstone with subordinate limestone. The Langport Member (formerly ‘White Lias’) comprises pale grey, hard, micritic limestone ranging from less than 2 to over 4 m thick within the Moreton-in-Marsh district. Above the Langport Member in the Worcester Basin, the basal beds of the Lias Group are of Triassic age (Chapter 4).

Chapter 4 Lower Jurassic: Lias Group

The oldest strata that crop out within the district belong to the Lias Group, which underlies all of the low ground, as well as the slopes of the Cotswold escarpment. The Lias has traditionally been subdivided into Lower, Middle and Upper Lias, but is now described in terms of lithostratigraphical formations defined by Cox et al. (1999). In this district, these are, in ascending order, the Blue Lias, Charmouth Mudstone, Dyrham, Marlstone Rock, Whitby Mudstone and Bridport Sand formations (Table 4).

Most of the detailed information about the group comes from boreholes, including the cored Twyning (Barclay et al., 1997), Stowell Park, Upton (Sumbler et al., 2000) and Bredon Hill No. 1 (Lalu Barn) (Whittaker, 1972) boreholes. Supplemented by geophysical data from other boreholes, these demonstrate a dramatic south-eastward thinning of the succession, from over 500 m in the west, in the deepest part of the Worcester Basin, to less than 200 m in the south-east on the London Platform. This thinning results from loss by overlap of the basal beds, and the remainder of the succession is condensed (Sumbler et al., 2000).

Blue Lias Formation

The Blue Lias is dominated by grey, more or less calcareous, marine mudstones, but is characterized by the presence of thin beds of grey argillaceous limestone (‘cementstone’), demonstrating repetitive cycles of sedimentation. The facies is diachronous between the Worcester Basin and London Platform (Sumbler et al., 2000) such that the formation is probably substantially younger in the east than in the west (Table 4). It reaches the surface only in the neighbourhood of Sedgeberrow in the north-west of the district.

Boreholes show that the formation thins from about 75 m in thickness in the north-west, to about 10 m in the south-east. It is 64 m thick in the Guiting Power Borehole (Figure 2), comprising 25 m of mudstone beds of the Saltford Shale Member, overlain by the mudstone-limestone Rugby Limestone Member. In the westernmost part of the district, the basal Wilmcote Limestone Member may also be developed.

Charmouth Mudstone Formation

The Charmouth Mudstone Formation, which corresponds approximately to the ‘Lower Lias clay’, makes up the bulk of the Lias in terms of both its thickness and area of outcrop, as it floors the vales in the west and east of the district. It is probably about 290 m or more in thickness in the west, with some 268 m proved in the Guiting Power borehole (Figure 2). By contrast, it is only about 130 m thick in the south-east. Thus the sequence is condensed, and recognition of marker beds shows the thinning is greatest in the lower part of the formation. It ranges from the Bucklandi Zone to the Davoei Zone, although there may be an erosional nonsequence, seen across the region, between the Oxynotum and Raricostatum zones. (Table 4); (Sumbler et al. 2000).

The formation is dominated by grey mudstone, and produces a brownish grey clay soil. The uppermost beds (Ibex Zone upward) are somewhat siltier, and produce a brownish grey silty clay soil. Nodules and thin beds of argillaceous limestone are developed at some levels and some of these have been mapped at the surface locally in the east, where they form features. These are associated with a sporadic brash of grey, shell fragmental limestone yielding Gryphaea and other bivalves, and argillaceous limestone nodules.

These mapped units are believed to include the 70, 85 and 100 geophysical marker ‘members’ (Table 4) that were first described by Horton and Poole (1977) in the district to the east. It is probable that similar limestones occur at other horizons too. Fossils from the Vale of Gloucester show that zones ranging from Turneri upwards (Table 4) crop out in the extreme west of the district (M J Simms, written communications, 1998–99) and it is evident (Worssam et al., 1989, table 3, fig. 7) that some otherwise undetected (possibly superficial) structure may be present within the Charmouth Mudstone strata.

Formerly, the Charmouth Mudstone was exposed in a number of pits in Cheltenham, where it was worked for brick-making. These included the Battledown Brickworks [SO 962 218] and the Harp Hill Brickworks [SO 967 224]. Currently, the only working brick-pit in the district is at Blockley Brickworks [SP 181 370] where about 18 m of strata are exposed (Callomon, 1968; Sumbler and Barron, 1996b). The ammonite fauna indicates that the strata here belong to the Ibex Zone (Phelps, 1985).

Dyrham Formation

The Dyrham Formation approximates to the ‘Middle Lias silts and clays’ of many previous accounts or the ‘Sandy Beds’ of Richardson (1929). Typically it crops out on the lower slopes of the escarpments that are capped by the Inferior Oolite Group. It thins significantly across the district, from about 61 m in the west, to about 15 m in the east. This may be partly due to an erosive non-sequence at the top (see below).

The Dyrham Formation comprises grey mudstone, with interbeds of highly micaceous, weakly cemented siltstone or very fine-grained sandstone. At outcrop these break down to a pale bluish grey, commonly finely micaceous silt or silty clay that weathers to a pale brown colour in the soil. Small chips of limonite are locally common. The base may be marked by a weak seepage line, possibly corresponding with the so-called Capricornus Sandstone in the middle part of the Davoei Zone (see Sumbler et al., 2000).

Brown micaceous sandstone occurs at other levels too. It is widely seen close to the top of the formation and these occurrences may represent a single bed, the ‘Subnodosus Sandstone’ (Simms, 1990; Sumbler et al., 2000). On Dumbleton Hill [SP 01 35], the ‘Subnodosus Sandstone’ forms a 1.5 m-thick bed of fine-grained sandstone within the Dyrham Formation, about 3 m below the Marlstone Rock. However in the district to the south, there is evidence of erosion beneath the Marlstone Rock Formation to deeper levels (Sumbler et al., 2000).

The Dyrham Formation is generally very poorly exposed, but over 42 m were penetrated by the Bredon Hill (Lalu Barn) Borehole (Whittaker, 1972). It was formerly exposed at Aston Magna Brickworks [SP 199 354] (Richardson, 1911, 1929; Callomon, 1968; Ager et al., 1973). Currently 1.5 m of sandstone is exposed; it is fine-grained micaceous, bioturbated and calcareous, and contains lenses with ammonites, bivalves, gastropods, belemnites, echinoids, serpulids, brachiopods and fish debris.

Marlstone Rock Formation

In general, the Marlstone Rock Formation caps flat-topped spurs or forms a ledge along the Cotswolds escarpment and around the outlying hills. The Marlstone Rock is a brown to grey variably ferruginous sandstone containing a proportion of limonitic ooids, which probably represent altered berthierine. Generally, at outcrop, it produces a brash of rust-brown ferruginous, commonly sandy ironstone. Sporadic shells, mostly brachiopods occur, particularly Tetrarhynchia and Lobothyris, and the rock is locally rich in crinoid or serpulid debris. Ammonites are remarkably abundant in the Marlstone Rock on the western shoulder of Alderton Hill [at SP 004 347], where numerous moulds and some solid fragments of Pleuroceras solare occur in the field brash. Howarth (1958) also noted large numbers of P. solare at this locality as well as rare examples of P. hawskerense, indicative of the Spinatum Zone, Hawskerense Subzone.

The Marlstone Rock is about 6 m thick in the Bredon Hill (Lalu Barn) Borehole (Whittaker, 1972), but 2 to 3 m is more typical in the district. The formation is not well exposed; Simms (1990) documents a number of localities in the west and also demonstrates that though largely of Spinatum Zone age, in places the formation extends down into the Margaritatus Zone (Table 4), and that it includes at least three non-sequences and evidence of deposition in very shallow water.

In many places, the outcrop of the formation is obscured by landslipping or cambering, but locally it may be absent, for example, on the western side of the Vale of Bourton and also in places in the district to the south (Sumbler et al., 2000).

Whitby Mudstone Formation

The Whitby Mudstone Formation (Upper Lias of previous accounts) crops out on the steep upper slopes of the Cotswold scarp that are capped by the Inferior Oolite Group. Major landslips affect large areas of outcrop and in other places it is obscured by wash and debris from the overlying Inferior Oolite. The formation comprises dark grey micaceous mudstone with fossils preserved in pinkish aragonite. At outcrop these strata appear as bluish grey to fawn-weathering clays, and grey mudstone may be found in places. Argillaceous limestone nodules and small chips of limonite occur locally, the latter resulting from the weathering and disintegration of sideritic nodules.

The severe cambering of the Inferior Oolite makes thickness estimates from outcrop highly unreliable, but boreholes confirm a dramatic thinning across the area. Some 110 m of Whitby Mudstone Formation (including ‘Cotteswold Sands’; see below) were proved in the Bredon Hill No. 1 (Lalu Barn) Borehole in the north-west (Whittaker, 1972, fig. 29; Barclay et al., 1997), and 95 m in the Guiting Power Borehole in the south. Whittaker (1972) estimates that it may be over 120 m thick at Bredon Hill, but through the central part of the district 80 to 95 m is typical. In the Stow No. 2 Borehole in the south-east it is only 43 m thick and it may be thinner locally (25 m or less). Most of the thinning appears to take place along the Vale of Moreton Axis, as in the district to the south. The eastward attenuation is due to the loss of the upper beds by overstep and probably of the basal beds by overlap (Table 4); (Sumbler et al., 2000) in addition to some thinning within the sequence.

At several localities in the east, pale commonly conglomeratic limestones and sideritic ironstones occur at the base of the formation. These beds generally contain abundant ammonites, particularly small phosphatised dactylioceratids and larger hildoceratids, indicating the Lower Toarcian Tenuicostatum or Falciferum zones. These beds correspond with the so-called Transition Bed of Horton et al. (1987) in the adjoining Chipping Norton district and rest erosively on the underlying Marlstone Rock Formation. In the west, dactylioceratid ammonites were found on the slopes of Cleeve Hill [SO 982 269] and Woolstone Hill [SO 974 308].

Bridport Sand Formation

The Bridport Sand Formation corresponds to the Cotteswold Sand of previous accounts of the Cotswold region. It is best developed in the western and central parts of the district. It is absent in the east where the formation passes into mudstone of the Whitby Mudstone Formation, partly by lateral passage and partly by interdigitation from the base upwards (Table 4); (Sumbler et al., 2000). The ‘Cotteswold Sand’ shown as widespread on earlier editions of Sheet 217 (Ammonite Sands of Hull, 1857), appears to be a generalized representation of the sandy lower beds of the Inferior Oolite Group (see also Richardson, 1929, p.28). The very widespread cambering and landslipping (Chapter 8) renders detection and mapping of any Bridport Sand Formation outcrop extremely difficult. As a result, although it is thought to be widely present in the west, it is not extensively depicted on Sheet 217, and is included here in the Whitby Mudstone outcrop. It is shown locally in the north around Chipping Campden and Blockley, and near

Snowshill and Kineton.

Bridport Sand Formation is proved in several boreholes. It is 4.0 m thick at Cotswold Hill Quarry, and 7.4 m in the Bredon Hill (Lalu Barn) Borehole where siltstone, sandstone and limestone strata were ascribed by Barclay et al. (1997) to the Cotteswold Sands. The thickest Bridport Sand appears to be in the central northern part of the district, east of Broadway, where 16.8 m of fine to medium-grained sandstone was proved at Broadway Quarry [SP 1169 3656], and 11.6 m of sand and silt at Springhill Reservoir [SP 1362 3557]. Its age range here is unknown, and the greater thickness may be a result of local preservation of younger strata beneath the unconformably overlying Inferior Oolite, rather than greater original thickness.

There are very few exposures of the formation in the district. At Sandywell Park [SP 013 204], Woodward (1893) recorded ‘micaceous blue and brown sandy loam’ overlying ‘blue micaceous clay with cement nodules, septaria and pyrites’, which yielded Bifrons Zone ammonites. In Eyford Park [SP 1449 2437], 3 m of fine to medium-grained micaceous sandstone are exposed, underlying a thin lenticular bed of ferruginous mudstone containing dark granules. The latter is similar to the Cephalopod Bed, the youngest unit of the Lias Group in the Cotswolds. Elsewhere in the district, traces of yellow sand were commonly observed at the base of the Birdlip Limestone, which may represent the Bridport Sand Formation.

Chapter 5 Middle Jurassic: Inferior Oolite Group

The Inferior Oolite Group of the Moreton-in-Marsh district is made up predominantly of shallow marine, ooidal and shell detrital limestones, with thin mudstone and sandstone beds. Its outcrop forms a broad dissected plateau, bounded by escarpments, through the centre of the district, tapering northwards from 18 km to less than 1 km, and overlain in parts by the Great Oolite Group (Figure 1). The Inferior Oolite also forms outliers; the largest are on Bredon Hill and Icomb Hill. The Inferior Oolite of the Cotswolds has been studied from the earliest days of the science of geology, resulting in a plethora of stratal names. The lithostratigraphical terminology has been revised and rationalised by Barron et al. (1997), and the group is divided, in ascending order, into the Birdlip Limestone, Aston Limestone and Salperton Limestone formations (Table 5). These lithostratigraphical units correspond with the Lower, Middle and Upper Inferior Oolite of Buckman (1905), which are essentially chronostratigraphical terms, pertaining respectively to the Aalenian Stage, and the Lower and Upper Bajocian substages. Each of the three formations is divided into a number of members, which for the most part correspond to one or more of the units traditionally recognised, by Buckman (1887, 1895, 1897, 1901), Richardson (1933) and Arkell (1933).

As with the rest of the Jurassic, the standard zonation of the Inferior Oolite is based on ammonites, although these are rare. The zonation shown in (Table 5) is based essentially on the work of Parsons (1980).

Within the district, the Inferior Oolite Group is up to about 110 m thick, the thickest at outcrop in Great Britain, attained at Cleeve Hill, in the west. Traced eastwards, the group thins dramatically, to 10 m or less in the Stow-onthe-Wold area (Figure 3) and (Figure 4). This thinning indicates continued differential subsidence between the Worcester Basin and the London Platform. Sedimentation appears largely to have kept pace with subsidence; there is no indication of deeper water facies within the Worcester Basin.

There were, however, interruptions to sedimentation, some of which are marked by bored and oyster-encrusted hardground surfaces. The most important of these nonsequences occur at the two levels which separate the three formations of the Inferior Oolite Group. Buckman (1887, 1901) demonstrated that the strata above these nonsequences variously overlap or overstep the strata beneath; the Aston Limestone Formation, for example, is overstepped by the Salperton Limestone Formation towards the Vale of Moreton Axis (Figure 4) and (Figure 7). From the distribution of the units below these non-sequences, Buckman built up a model in which episodes of ‘earth movement’ created gentle folds (‘Painswick Syncline’, ‘Cleeve Hill Syncline’ and intervening ‘Birdlip Anticline’, (Figure 3); Sumbler et al., 2000, fig 11) which were then subjected to differential erosion (Aalenian and Bajocian denudations), followed by marine transgression (Bajocian and Vesulian transgressions of Arkell, 1933). This model is broadly consistent with the evidence, but considering the modest scale of the structures and details of the thickness variation of individual units, these relationships are more compatible with a model of gentle, more or less continuous, but slightly nonuniform subsidence, producing sedimentary troughs separated by highs with thinner sequences that were eroded during sea-level lowstands (Barron et al., 1997).

Birdlip Limestone Formation

The Birdlip Limestone Formation, corresponding to the Lower Inferior Oolite [Formation] of previous accounts (e.g. Arkell, 1933; Mudge, 1978), comprises mainly ooid-limestone, shell-fragmental limestone and sandy limestone, and lesser amounts of sandstone, mudstone and subordinate calcareous mudstone. It is the thickest formation of the Inferior Oolite Group, reaching a maximum of 74 m in the west of the district, within the ‘trough’ of the ‘Cleeve Hill Syncline’ (Figure 5). It thins dramatically towards the Vale of Moreton Axis, and is absent in the east of the district but reappears farther east. Where complete, the formation is divisible into five members (Table 5) and where possible, their outcrops are indicated on the map.

Leckhampton Member

The Leckhampton Member (Barron et al., 1997) corresponds to the Scissum Beds of Richardson (1929) and others. It is the basal unit of the Inferior Oolite throughout the Cotswolds from Horton, Avon, to near Chipping Norton, Oxfordshire, resting non-sequentially on the Bridport Sand Formation in the west of the district, and overstepping onto the Whitby Mudstone Formation in the east.

The member comprises grey limestone with thin beds of calcareous siltstone. The limestone is rubbly, ferruginous, sandy, argillaceous, shelly, peloidal and ooidal. Much of the non-carbonate material is probably derived by reworking from the underlying strata. It becomes sandier eastwards, perhaps due to incorporation of material from the Bridport Sand (see Mudge, 1978), which is consequently no longer preserved here, and it passes east into the Northampton Sand Formation near Chipping Norton by an increase in the abundance of sand and ferruginous ooids (Figure 5); (Horton et al., 1987; Barron et al., 1997). The member is typically 1.5 to 6 m thick, but in parts of the district, east of Stow-onthe-Wold and at Icomb Hill, is it cut out entirely beneath the Salperton Limestone (Figure 4).

The Leckhampton Member is generally poorly exposed in the district, although its full thickness (1.5 m) is visible in Eyford Park [SP 1449 2437]. It is poorly exposed beneath the Crickley Member at Cleeve Cloud [SO 9841 2549] (Plate 2). At outcrop, the member weathers to a yellow or orange-brown colour, and commonly decalcifies to loose sand. This resembles the Bridport Sand, and indeed the greater part of the ‘Cotteswold Sands’ indicated on previous editions of Sheet 217 is now interpreted as the basal part of the Birdlip Limestone Formation, including the Leckhampton Member (see Richardson, 1929).

The Leckhampton Member is one of the more fossiliferous parts of the Birdlip Limestone, commonly containing brachiopods and large myacean bivalves. Belemnites and ammonites such as Leioceras are not uncommon; the latter indicates the lower Aalenian Opalinum and/or Scissum zones.

Crickley Member

The Crickley Member (Barron et al., 1997) corresponds to the Lower Limestone plus the overlying Pea Grit of previous accounts (e.g. Richardson, 1929), the ‘Pea Grit Series’ of some authors (but see discussion of Cleeve Cloud Member below), or the combined Crickley Limestone and overlying Crickley Oncolite members of Mudge (1978).

The Crickley Member is present only in the western half of the district (Figure 4) and (Figure 5), where it is up to about 10 m thick. The lower part is dominated by pale grey to yellow brown, peloidal, ooidal and shell-fragmental grainstone, comprising the Lower Limestone of previous accounts. This grades upwards into rubbly, poorly sorted, shelly, shell-detrital, pisoidal grainstone and packstone with thin beds of calcareous mudstone, constituting the distinctive Pea Grit. The pisoids that characterise the latter are up to 10 mm across and are subspherical or disc-shaped grains, in which a shell-fragment nucleus is surrounded by layers of micrite. Thin-section examination reveals microbial tube structures; thus the pisoids appear to be true oncoids.

The member is fully exposed at Cleeve Cloud [SO 9841 2549] where it is 6.9 m thick (Plate 2). It was proved to be at least 6.2 m thick in the Cleeve Common Borehole (Figure 6). At outcrop, it weathers to yellowish brown, very coarse-grained limestone rubble, and the pisoids may be found loose in the soil. The member contains brachiopods, bivalves, gastropods, echinoids and corals. Ammonites indicating the Murchisonae Zone are known.

Cleeve Cloud Member

The Cleeve Cloud Member is essentially equivalent to the Lower Freestone of previous accounts (but see below), and to the combined Cleeve Hill Oolite and Devils Chimney Oolite members of Mudge (1978). It is the thickest unit of the Birdlip Limestone Formation, reaching 51 m at Cleeve Hill (Figure 6), and is typically 35 to 45 m thick within the western part of the district. From there, it thins eastwards due to progressive overstep by the Scottsquar Member and the Salperton Limestone Formation, which ultimately cuts it out, so that it is absent to the east of Stow-on-the-Wold and Bourton-on-the-Water (Figure 4) and (Figure 5). The Cleeve Cloud Member is probably the highest solid stratum present on Bredon Hill.

The Cleeve Cloud Member is dominated by well bedded, well sorted medium to coarse-grained ooid-grainstone (‘Oolite’). It is well exposed in the district, illustrating its former importance as a building stone (see Chapter 2), for example, in the face of Cleeve Cloud (Cover photograph (Front cover); Cox and Sumbler, 2002), and at many disused quarries throughout the district, and in active quarries near Ford, Broadway and Blockley. In the early 19th century the member was also the source of stone for a fraudulent enterprise making water pipes.

The lower 12 to 14 m of the member in the western part of the district includes yellowish sandy ooid-grainstone with burrows (‘Sandy Beds’). Farther east, this facies forms an increasing proportion of the Cleeve Cloud Member (Figure 4), and becomes more ferruginous and markedly orange-yellow. It was termed the ‘Yellow Stone’, ‘Yellow Guiting Stone’ or ‘Pea Grit Equivalent’ by Richardson (1929, 1933).

At Cleeve Cloud, the upper part of the succession comprises about 16 to 20 m of off-white to pale grey, markedly cross-bedded ooid-grainstone. Elsewhere, these strata (‘White Guiting Stone’ of Richardson, 1929) are well exposed in Guiting Quarry [SP 080 305] and the nearby Cotswold Hill Quarry [SP 081 292], where 22 m of pale grey to cream ooid-grainstone beds can be seen. A borehole in the floor of Broadway Quarry [SP 118 367] proves the total thickness here to be about 38 m (Barron, 2000). At Oathill Quarry [SP 103 289], a reef-knoll of coralliferous carbonate mudstone occurs at the top of the ‘Yellow Stone’, and exhibits signs of subaerial emergence (karstification).

The oyster-encrusted hardground surmounting the member at Cleeve Hill is seen elsewhere in the district (e.g. Broadway and Oathill quarries and Aston Farm railway cutting [SP 144 213]) but the evidence of palaeokarst development observed at this level in the Cirencester district (Sumbler et al., 2000) were not recorded here.

Apart from varying amounts of shell debris, the member is largely unfossiliferous. Locally, in the sandy lower part, the ‘Yellow Stone’ facies, bivalves are abundant, notably the small pectinacean Propeamussium pumilum; echinoids and brachiopods such as Plectoidothyris plicata also occur. Ammonites are rare; a specimen found in the Cirencester district (Sumbler et al., 2000) may indicate the mid Aalenian Murchisonae Zone.

At outcrop, the ‘Yellow Stone’ facies weathers to various brownish yellow hues and locally decalcifies to loose sand. The overlying oolites of the White Guiting Stone produce an angular white to pale brown brash.

Scottsquar Member

The Scottsquar Member (Barron et al., 1997) is equivalent to the combined Oolite Marl (Buckman, 1842) and Upper Freestone (Hull, 1857). It has long been recognized that the distinction between these two units is essentially arbitrary (Woodward, 1894; Buckman, 1895; Richardson, 1904; Baker, 1981), and for this reason, Mudge (1978) introduced the term Scottsquar Hill Limestone Member, from which the present name is adapted.

Across the southern margin of the Moreton-in-Marsh district, the Scottsquar Member is the topmost unit of the Birdlip Formation. It is typically 4 to 8 m thick, but locally thickens to 10 m in the middle of the district and thins rapidly to zero in the east. Its extent is slightly less than the Cleeve Cloud Member partly due to overstep by the Salperton Limestone Formation (Figure 4) and (Figure 5). It is also locally absent beneath the Harford Member (see below).

The member is dominated by pale grey and brown, medium to coarse-grained peloid and ooid-packstone and grainstone (‘Upper Freestone’), in which beds of lower energy, white to mid-grey calcilutite and shell-detrital, peloidal, silty calcareous mudstone (‘Oolite Marl’) occur. The ‘Oolite Marl’ facies tends to predominate in the lower part, and ‘Upper Freestone’ facies in the upper, but the succession is extremely variable from place to place, and the two facies commonly interdigitate in a complex manner, which includes local erosion (see below). The ‘Upper Freestone’ facies may generally be distinguished from the Cleeve Cloud Member by its less well-sorted character, and by the colour contrast with the ‘Yellow Stone’ facies.

The full thickness of the Scottsquar Member is seen in several major quarries in the district. In Cotswold Hill Quarry, where the member is 7.2 m thick, peloidal and ooidal limestones dominate the sequence, and subordinate calcareous mudstone beds lie in the middle part. Local penecontemporaneous erosion is evident from hardground formation here and at the sections at the nearby Jackdaw [SP 077 310] and Guiting [SP 080 305] quarries, where Parsons (1976) reported channelling of the upper beds into the lower part of the member. At Broadway Quarry (Barron, 2000), the top of the uppermost bed is sharp. However, there is no hardground, but clear signs of a relatively continuous upward lithological passage between the Scottsquar and Harford members, which is also seen at Westington Hill Quarry some 2 km to the east and in the Whitehall Farm Borehole [SP 0090 2340]. The member is also well exposed at Notgrove Station railway cutting [SP 0939 2129], where it includes several beds of grey calcareous mudstone.

The mudstone beds produce slight hollows in the hillslopes, and the pale and highly fossiliferous material may be conspicuous in the soil. This facies has long been known for the richness and excellent preservation of its fauna (Hull, 1857; Buckman, 1895; Mudge, 1978; Baker, 1981), which is dominated by brachiopods, including the distinctive terebratulid Plectothyris fimbria. Corals, serpulids and bryozoa also occur.

Harford Member

The Harford Member (Barron et al., 1997) is the uppermost member of the Birdlip Limestone Formation. The recent survey has proved that it is much better developed and more widespread in the Moreton-in-Marsh district (Figure 5) than in the district to the south (Sumbler et al., 2000). Based on exposures, it was formerly subdivided, in ascending order, into the Naunton Clay, Harford Sands, Snowshill Clay and Tilestone (Buckman, 1897, 1901; Richardson, 1929). However, recent work shows that the succession is laterally highly variable, and that these units are of little stratigraphical value (Parsons, 1976; Mudge, 1978), although a gross ascending sequence of sandstone–mudstone–limestone is commonly applicable. The member contains a sparse fauna of bivalves and gastropods, and its lithologies exhibit characteristics that suggest deposition in a shallow-water, land marginal environment. Although no ammonites are known from the member (see Parsons, 1976), the stratigraphical position suggests that it probably belongs to the upper Aalenian Concavum Zone.

Within the district, the Harford Member is typically 4 to 8 m thick, and locally up to 14 m in the Snowshill Hill area [SP 12 32] although it thins to zero in the south and east (Figure 5). Grey to orange-brown, fine to medium-grained silicate-sand and sandstone up to 6 m thick predominates at the base, overlain by up to 8 m of grey to brown, silty, variably sandy or shell-detrital mudstone and locally up to 2.5 m of pale grey and brown sandy, shell-detrital, peloidal and ooidal limestone. These divisions are differentiated where possible on the map, mainly in the north between Taddington [SP 087 311] and Blockley [SP 165 350].

Generally, in the district, the Harford Member appears to pass up from, or rest conformably on the Scottsquar Member (see above; Parsons, 1976). However, a local nonsequence was observed between them near Lower Swell [SP 163 251] and the latter is apparently absent beneath the former south-west of Blockley, around [SP 16 34] (Figure 5), due to channelling or local lateral passage between the two.

The member is no longer well exposed at its type section, Harford railway cutting [SP 1363 2184], which is near its southern limit. However, in the Aston Farm railway cutting to the east [SP 145 213] it is seen to thin eastwards from 3.7 to 1.7 m over a distance of 120 m, as a result of overstep by the Aston Limestone. Sand and sandstone (partly as concretions) dominate over limestone and mudstone on Cleeve Hill, including in the Cleeve Common Borehole (Figure 6) and the member comprises interlaminated mudstone and sandstone in the nearby Whitehall Farm Borehole, emphasising the lateral variability.

Farther north, several exposures in large quarries demonstrate the substantial expansion of the member. At Cotswold Hill Quarry [SP 081 295], it is probably about 3.9 m thick, and Parsons (1976) reports that it has increased to 5 m at Jackdaw Quarry 1.5 km to the north [SP 077 310]. Passing north-east, the member thickens to possibly 14 m, and is 7.7 m thick at Broadway Quarry [SP 117 366] (Barron, 2000). Here, the mudstone unit is over 4 m thick (Plate 3), and its presence is also conspicuous and significant with regard to local water supply (see Chapter 2) in the surrounding area, as noted (as ‘Snowshill Clay’) by Buckman (1901) and Richardson (1929). The sandstone beds, weathering to clean sand, also formerly had a local economic importance, as indicated by the many minor sand pits along the outcrop.

From Richardson’s descriptions (1929) and more recent observations, his ‘Tilestone’ limestone beds are highly variable in lithology, ranging from ooid-limestone through sandy peloidal limestone to calcareous sandstone. Pebbly layers and lignite as fragments and fine disseminations are also widely noted. This may have resulted in confusion with the Scottsquar Member and the Lower Trigonia Grit Member (see below). During the current survey, ooid-limestone beds were distinguished within the Harford Member on Sheet 217 only around the Taddington area [SP 08 32] and Northwick Park [SP 15 36], although they were seen elsewhere and are exposed at Jackdaw and Cotswold Hill quarries. Additionally, a thin, lenticular blue-grey-hearted, oyster-rich limestone bed was seen in Cotswold Hill and Jackdaw quarries (Parsons, 1976, p.56, bed 11a) and near Ford Hill Farm [SP 110 303].

The sandstone and sandy limestone beds weather to a light, stoneless loamy soil, locally supporting gorse and heather (e.g. on Cleeve Common). The mudstone beds give rise to a heavy clay soil that is commonly waterlogged.

Aston Limestone Formation

The Aston Limestone Formation (Barron et al., 1997) comprises the strata of the Middle Inferior Oolite of Buckman (1905), also termed the ‘Ragstones’ (Arkell, 1933) which are up to 22 m thick. The limestone is grey and brown in colour, rubbly, variably shelly, ooidal, sandy and shell-detrital, with sandy and marly beds in parts. It is present in the western and central parts of the district (Figure 7). It is thickest in the Cleeve Hill to Snowshill area (the thickest known in the Cotswolds), thinning east and south-east partly because of attenuation of the individual members but, more importantly, because of erosion of the topmost beds beneath the Salperton Limestone Formation. The latter ultimately cuts out the Aston Limestone Formation entirely (Figure 4); (Richardson, 1929, p.80).

Where complete, the formation is divisible, in ascending order, into the Lower Trigonia Grit, Gryphite Grit, Notgrove and Rolling Bank members (Table 5), but it has not been possible to distinguish them throughout the map.

Lower Trigonia Grit Member

The Lower Trigonia Grit Member is the lowermost unit of the Aston Limestone Formation. In the Moreton-in-Marsh district, it comprises up to 2 m of grey, very shelly and shell-detrital, moderately sandy, peloidal, ooidal wackestone, packstone and grainstone in the south and west. It thins and becomes more marly and conglomeratic north-eastwards, but it is probably entirely absent beneath the Gryphite Grit Member only in the area around and to the east of Broadway Quarry (Figure 7), although elsewhere in the north it may be represented only by a thin, marly, pebbly layer. It rests sharply and non-sequentially on the Birdlip Limestone Formation, and commonly contains pebbles of derived material from that formation at the base. Many of the abundant peloids are ferruginous, producing a distinctive orange-brown speckled ‘ironshot’ appearance. The presence of a non-sequence within the member in places, as well as its northwards facies change and attenuation, indicate that it is becoming increasingly condensed, probably as a result of shoaling.

The Lower Trigonia Grit Member is fully exposed in the Harford and Aston Farm railway cuttings, in which it thins east from 0.7 to 0.3 m thick, and in Jackdaw Quarry (Parsons, 1976), where it comprises 0.8 to 1.0 m of shelly, ironshot, pebbly limestone. The member is 1.10 m thick in the Whitehall Farm Borehole (Figure 6). It weathers to brown, rubbly, shelly limestone debris in a moderately sandy soil.

The fauna includes bivalves, brachiopods and the distinctive colonial serpulid Sarcinella. Corals are common at the base on Cleeve Hill, representing the ‘Second Coral Bed’ of the Cheltenham area (Richardson, 1904). Ammonites collected from the member at Jackdaw Quarry indicate the Discites Zone (Parsons, 1976, 1980).

Gryphite Grit Member

The Gryphite Grit Member (Barron et al., 1997) includes both the Gryphite Grit and Buckmani Grit of Buckman (1895). In the Moreton-in-Marsh district, the member comprises 3 to 7 m of grainstone, packstone and wackestone that are grey and brown in colour, hard, rubbly, shelly, coarsely shell-detrital, sandy and peloidal with thin mudstone, calcareous mudstone and sandstone beds. The member generally rests non-sequentially on top of the Lower Trigonia Grit, where present, which it resembles, although it is generally more sandy and less ferruginous. At outcrop, both weather to grey-brown, shelly rubble in a sandy clay soil, and so are not shown separately on the map. Over 4 m of the Gryphite Grit is exposed in the newly conserved Pot Quarry (Angseesing et al., 2002) on Cleeve Hill [SO 9868 2667], where it is notably rich in the oyster Gryphaea. The Whitehall Farm Borehole, 4 km to the south-east, proved 4.33 m of strata, predominantly limestone (Figure 6) with thin shell-detrital mudstone beds. The member is also well exposed in the Harford and Aston Farm railway cuttings, where it includes thin calcareous mudstone beds, and as a result of erosion prior to overstep by the Salperton Limestone Formation, it ranges from 3.5 to 1.0 m in thickness. The member is fully exposed at Broadway Quarry [SP 117 366] (Plate 3); (Barron, 2000), where it comprises nearly 7 m of bedded, bioturbated, peloidal, shelly and shell-detrital limestone.

The fauna of the Gryphite Grit is distinctive, with abundant Gryphaea bilobata and pachyteuthid belemnites in the upper part, and Sarcinella and Lobothyris buckmani in the lower part. Rare ammonites collected from Harford railway cutting and elsewhere indicate that the member belongs to the Discites and Ovalis Zones (Parsons, 1976, 1980).

Notgrove Member

The Notgrove Member (Barron et al., 1997), corresponding to the Notgrove Freestone of Buckman (1887), comprises pale brownish grey, well-bedded and commonly cross-bedded, medium to coarse-grained, moderately peloidal and ooidal grainstone. It commonly appears bimodal, with large, white superficial ooids in a finer grained matrix. Shells and shell debris are rare, except in the lowest beds, which pass up from the underlying Gryphite Grit Member. The member has a less extensive distribution than the Gryphite Grit because of overstep by the Salperton Limestone Formation (Figure 4) and (Figure 7). Where it immediately underlies the latter, the top of the member is typically a well-developed, planed and oyster-encrusted hardground with abundant borings, the most conspicuous being narrow, near-vertical annelid borings which may penetrate as much as 0.3 m below the hardground surface. Where the Notgrove Member is seen beneath the Rolling Bank Member (Pot Quarry and Whitehall Farm Borehole; Angseesing et al., 2002); (Figure 6) burrows are observed in the upper part of the former, suggesting that erosion, possibly of soft-sediment, may have occurred (see below).

At outcrop, the Notgrove Member produces a distinctive blocky brash of pale grey oolite, in which pieces of the heavily bored and oyster-encrusted hardground from its top may be conspicuous. Consequently, it has proved possible to map the member separately throughout most of its outcrop. It attains its maximum thickness of perhaps 13 m in the area north of Guiting Power resulting in the increased thickness of the Aston Limestone Formation here (Figure 7), despite the absence of the Rolling Bank Member. Where the Rolling Bank Member succeeds the Notgrove Member, the latter is only 4 to 8 m thick, probably because of erosion at the non-sequence at its top.

The member is well exposed at its type section, Notgrove Railway Cutting, where the uppermost 1.8 m are visible (Plate 4). In Harford Railway Cutting it is 2.2 m thick and in Broadway Quarry 3.8 m (Barron, 2000). In Pot Quarry [SO 9868 2667] it comprises 4.4 m of peloid and ooid grainstone with shell debris, passing with increasing shell debris, ferruginous peloids and clay matrix into the overlying Rolling Bank Member.

The fauna of the Notgrove Member is generally very sparse, although the small bivalve Propeamussium may occur. Ammonites are particularly rare, but the member is inferred to belong to the Laeviuscula Zone (Parsons, 1980).

Rolling Bank Member

The Rolling Bank Member (Barron et al., 1997), the youngest unit of the Aston Limestone Formation, comprises the combined Witchellia Grit, Bourguetia Beds and Phillipsiana Beds of Buckman (1895, 1897), which he distinguished on faunal and lithological grounds. It is named after the quarry on Cleeve Hill [SO 9871 2668] where it was once fully exposed (Buckman, 1897; Cox and Sumbler, 2002). The member is present only in the western part of the Moreton-in-Marsh district (Figure 7). To the east it has been eroded by the ‘Bajocian Denudation’ of Buckman (1901) (Figure 4). Its current outcrops are restricted to the Cleeve Hill plateau where up to 8.5 m of beds are present, and a small area (previously unknown) near Taddington [SP 081 324], where the thickness is about 1 m.

The member comprises a sequence of medium-bedded packstone and wackestone limestone, variously rubbly, shelly, shell-detrital, sandy, ironshot, peloidal and ooidal. Following the RIGS conservation work (Angseesing et al., 2002) almost 5 m of the upper beds of the member are currently visible in the Rolling Bank Quarry (Plate 5). In addition, the lowest beds are exposed in the nearby Pot Quarry. Unfortunately, at present no connection can be made between these and no more accurate estimate of the full thickness can be made.

The beds visible in Pot Quarry comprise up to about 1.6 m of grey-brown, rubbly, sandy, shell-detrital peloidal and ooidal ironshot packstone, with scattered large shell fragments, ascribed here to the ‘Witchellia Grit’ of Buckman (1897). They contain a fauna of brachiopods and ammonites, including Witchellia, indicating the Laeviuscula Zone (Sumbler et al., 2000). The Witchellia Grit was also proved at the top of the Whitehall Farm Borehole, 0 to 2.57 m depth; (Figure 6). However, as Buckman (1897) estimated this unit to be about 1.2 m thick, the beds in both localities may include part of the overlying Bourguetia Beds.

Based on Buckman’s (1897) criteria, the Phillipsiana Beds comprise about 3.43 m of well-bedded, sandy, shell-detrital, shelly limestone in the Rolling Bank Quarry (Angseesing et al., 2002), according moderately well with his quoted thickness of 3.07 m. Their fauna includes bivalves, terebratulid brachiopods and ammonites, suggesting the Sauzei Zone (Parsons, 1980). The uppermost beds were also proved in the Wontley Farm Borehole [SP 0054 2469], 6.16 to 7.22 m depth; (Figure 6). Here, the upper surface of the highest, very hard and splintery bed is covered with encrusting oysters, and borings extend down about 0.20 m, indicating hardground formation beneath the Salperton Limestone Formation. Only 1.53 m of the underlying beds, ascribed here to the Bourguetia Beds, are currently exposed in the Rolling Bank Quarry (Buckman, 1897, estimated a total of 4.1 m). They comprise roughly bedded, brownish grey, shell detrital peloid and ooid wackestone, with bivalves and a gastropod, possibly Bourguetia striata.

The limestones of the Rolling Bank Member weather to a hard, yellowish grey shelly rubble with common pectiniid bivalves, and, near the top of the member, pale grey, blocky, fine-grained sandy limestone, commonly with borings, is abundant in the soil on the outcrop around West Down [SP 00 24].

Salperton Limestone Formation

The Salperton Limestone Formation (Barron et al., 1997) corresponds to the Upper Inferior Oolite of the Cotswolds. It is between 5 and 20 m thick, but is more typically 10 to 15 m, and consists of ooidal, peloidal, shelly and shell-fragmental limestone. In the west of the district, the formation rests unconformably on the Rolling Bank Member, but progressively it oversteps lower parts of the Inferior Oolite towards the east to rest on the uppermost beds of the Lias Group (Figure 4). The upper surface of the underlying limestone strata is generally a hardground, with borings and encrusting epifauna.

Except for the Leckhampton Member of the Birdlip Limestone Formation, the Salperton Limestone is the only part of the Inferior Oolite Group to extend eastwards across the Vale of Moreton Axis (Figure 4), albeit reduced in thickness. The Salperton Limestone Formation is well exposed at the type section, Notgrove Railway Cutting [SP 0845 2090] to [SP 0862 2098], near the village of Salperton. The Salperton Limestone Formation is divisible into two distinctive members, the Upper Trigonia Grit Member, and the succeeding Clypeus Grit Member. In general, they are both indicated on the map (see below).

Upper Trigonia Grit Member

In the Moreton-in-Marsh district, the Upper Trigonia Grit Member consists of very hard grey and brown, very shelly, shell-detrital, moderately peloidal and ooidal grainstone and packstone, in rubbly, medium to thick beds. At outcrop, it weathers to orange-brown, and commonly forms large, uneven slabs. Due to its hardness, it may give rise to a prominent topographical feature. The fauna is dominated by bivalves, including large myaceans and trigoniids commonly preserved as empty moulds; these are conspicuous and distinctive on weathered slabs and other fossils include brachiopods. An extensive fauna of ammonites is known from the member, and indicates the Upper Bajocian Garantiana Zone (Parsons, 1980).

The member is typically 1 to 3 m thick. It is generally absent east of a line from near Snowshill to Aston Farm (Figure 7), probably because of overstep by the Clypeus Grit Member, and may be absent locally to the west of this limit. It is fully exposed at its type section, Notgrove Railway Cutting (Plate 4), where it is 1.0 to 1.2 m thick. In Harford Railway Cutting, it thins east from 1.0 to 0.1 m and is absent at Aston Farm. A thicker sequence is visible at Rolling Bank Quarry (Plate 5); (Angseesing et al., 2002), where 2.58 m of beds are present. Over 3 m of shelly, shell-detrital peloid packstone were proved in the Wontley Farm Borehole, 3.15 to 6.16 m depth; (Figure 6). The top of the Upper Trigonia Grit is generally sharp, and is commonly a hardground.

Clypeus Grit Member

The Clypeus Grit Member is the youngest and most laterally extensive unit of the Inferior Oolite Group in the Cotswolds. It forms broad plateau outcrops throughout the district. It is generally about 9 to 13 m thick, but is rather variable with up to 18 m of beds east of Naunton [SP 12 23] and between Snowshill and Hinchwick [SP13SW area], and as little as 5 m in the east. It comprises pale grey to yellowish or pinkish brown, fine to coarse grained, shell-detrital, ooidal and peloidal packstone and grainstone with subordinate wackestone. Characteristically, it contains sporadic, orange-skinned pisoids and aggregate grains, the latter suggesting reworking of partially cemented sediment.

At outcrop, the Clypeus Grit weathers to rubble, producing a stony soil that commonly contains loose fossils. The abundant fauna includes large myacean bivalves, terebratulid brachiopods such as Stiphrothyris and, the eponymous large echinoid Clypeus ploti. The latter is particularly common in the uppermost part of the formation, and is probably largely responsible for the pervasive burrowing and consequent rubbly character of these beds. Ammonites are also relatively common and show that the member belongs largely to the Upper Bajocian Parkinsoni Zone but extends into the Lower Bathonian Zigzag Zone (Table 5).

Where overlain by the Chipping Norton Limestone (Chapter 6) in the east of the district, the top of the Clypeus Grit Member is sharp and locally marked by a planed and oyster-encrusted surface. However, no such hardground has been found where it is overlain by the Fuller’s Earth Formation, although the junction is seldom well exposed because of landslipped and downwashed material from the latter.

The full thickness of the Clypeus Grit is visible at its type section, Notgrove Railway Cutting, where it is between 10.5 and 12.0 m thick (Cox and Sumbler, 2002). The basal part of the member is exposed at Rolling Bank Quarry and Broadway Quarry, and was proved in the Wontley Farm Borehole, 0 to 3.15 m depth; (Figure 6).

Chapter 6 Middle Jurassic: Great Oolite Group and Ancholme Group

The Great Oolite Group occupies substantial areas on the plateau of the north Cotswold Hills and much of the outcrop is in the lower part of the group, notably the Chipping Norton Limestone Formation. Younger strata are preserved in fault-bound troughs, including the Kinetonhill graben (Figure 1) and there the whole of the Great Oolite succession is present (Table 6). The Great Oolite is estimated to be some 45 to 55 m in thickness (Figure 8), considerably less than the 60 to 90 m recorded in the Cirencester district to the south (Sumbler et al., 2000). Thus, it seems likely that this district was an area of reduced subsidence during Bathonian times, and the facies suggests that deposition of the Great Oolite Group occurred in shallow water with episodes of emergence. The succession is largely complete, and there is little evidence of eastward overstep. It would seem that the Vale of Moreton Axis had stabilised by the Bathonian times but was nonetheless marked by a zone of reduced deposition at the margin of the London Platform.

The Ancholme Group is represented by the Kellaways Formation — present as a small faulted outlier near Condicote.

Great Oolite Group

Chipping Norton Limestone Formation

The Chipping Norton Limestone is the most widespread unit of the Great Oolite Group in the Moreton-in-Marsh district. The formation is very variable in thickness. In the eastern and central part of the district, it is commonly 10 m and may attain 16 m in the Snowshill–Condicote area [SP 097 337] to [SP 153 283]. From there, it thins towards the south-west, by progressive lateral passage into the succeeding Fuller’s Earth Formation (Figure 8), which thickens concomitantly. It dies out along a north-west to south-east line passing close to the village of Hawling [SP 065 230]. This south-east trend continues in the Cirencester district to the south (Sumbler et al., 2000, fig. 17), and is markedly transverse to the Vale of Moreton Axis (Figure 1). This illustrates the much-reduced significance of this structure during the deposition of the Great Oolite Group.

The Chipping Norton Limestone is dominated by ooidal limestone but includes other limestone types. Most of the formation is composed of pale brown to buff or grey, variably shell-fragmental ooidal grainstone. It is generally medium grained and slightly sandy, but beds of coarser limestone occur in places. Typically it is cross-bedded and platy weathering. Fine-grained thinly bedded limestones occur and have been worked for tilestones near Upper Swell. Beds of mudstone and calcareous mudstone also occur and rarer lithologies include coral-bearing white calcilutite.

The basal beds of the formation tend to be particularly sandy and argillaceous. Commonly, the rock contains many burrows that weather out as small voids, and small mud flakes and fragments of black lignite are also a characteristic. These beds tend to decalcify and give rise to a loamy soil with little or no brash.

Oyster-rich mudstone, and/or rubbly argillaceous limestone occur at the base of the formation in places (see also Sumbler, 2000, p.55). These include the Roundhill Clay of Richardson (1929), separated on the map in a few places [SP 065 246]; [SO 082 239]. Generally the contact with the underlying Salperton Limestone is sharp and in places is marked by an eroded surface or bored and oyster-encrusted hardground.

Other than oysters (Praeexogyra) recognisable macrofossils are not common. The few ammonites known from New Park Quarry [SP 175 282] (Arkell and Donovan, 1952; Torrens, 1969) are consistent with the Zigzag Zone (Torrens, 1980). Reptile remains have been recorded from New Park Quarry (Reynolds, 1939; Benton and Spencer, 1995) and Hornsleasow Quarry [SP 131 323] (Metcalf et al., 1992). Both sites have yielded land-based forms, including crocodiles, dinosaurs, mammal-like reptiles and ‘eupanothere’ mammals.

Fuller’s Earth Formation

The Fuller’s Earth Formation is up to about 19 m thick in the south-west of the district, but thins in an eastward direction, to about 10 m in the neighbourhood of Naunton, and is further reduced to the east. This results mainly from eastward facies passage into the Chipping Norton Limestone (Figure 8) but may also be caused by erosional downcutting of the Taynton Limestone (Wyatt, 1996; Sumbler et al., 2000). Locally, the formation is absent and Taynton Limestone rests directly on Chipping Norton Limestone. The formation takes its name from the occurrence of commercial fuller’s earth (clay rich in smectite) in the upper part (Upper Fuller’s Earth) near Bath. However, in this district, the formation corresponds with the Lower Fuller’s Earth, and does not contain smectite in significant quantities.

Within the district, the Fuller’s Earth Formation is divisible into two parts, a lower, un-named part dominated by grey mudstone, and an upper part made up of sandy limestone and sandstone, known as the Eyford Member.

The mudstone of the Fuller’s Earth Formation weathers to a very heavy grey-brown clay soil, locally with thin plates of planar-laminated sandstone and siltstone. The beds are up to perhaps 15 m in thickness in the west, but thin eastwards and apparently die out near Stow-on-the-Wold. They are very prone to landslipping and are generally very poorly exposed, but the full thickness (about 10 m) was formerly exposed in the eastern part of Notgrove Station Railway Cutting [SP 096 214]. The best exposures occur at Huntsman’s Quarry [SP 125 255], where the material exposed comprises bluish to purplish grey fissile mudstone with thin slabs of planar-laminated siltstone and fine sandstone, and slabs of limestone containing abundant small oysters and other bivalves.

Eyford Member

The upper part of the Fuller’s Earth is known as the Eyford Member, a term introduced for the ‘sands and siltstones with ooliths around Eyford, west of Stow-on-the-Wold’ (Sellwood and McKerrow, 1974), p.192). Huntsman’s Quarry, near Eyford Hill [SP 138 254], is taken as the type section.

The Eyford Member comprises grey sandy limestone or calcareous sandstone in beds typically 0.3 m thick, interbedded with sand and sporadic, very thin mudstone seams; the sand is soft, brown, fissile, poorly cemented and bituminous. The limestone is massive and bioturbated or well laminated and fissile without burrows; the latter type yields the tilestones. Rarer lithologies include fine-grained packstone or wackestone with abundant Praeexogyra acuminata. Limestone brash and olive-grey clay are common in the rather sandy soil associated with this member. It is well exposed at Huntsman’s Quarry, Brockhill Quarry [SP 135 238] and Hampen Cutting (Cox and Sumbler, 2002). The Eyford Member is up to about 9 m in thickness, with the thickest succession occurring at Sevenhampton Common [SP 015 227]. It is up to about 7 m thick at Huntsman’s Quarry, but 4 to 5 m is a more typical thickness in the central part of the district. As mapped, the member thins and dies out east of Eyford, by passage into Chipping Norton Limestone facies (Figure 8).

The strata were formerly worked extensively as a source of tilestones for roofing particularly in the Eyford and Sevenhampton Common areas. These tilestones, known as Cotswold Slates, are very similar to the Stonesfield Slates of Oxfordshire, and consequently the two units have been incorrectly equated (e.g. Hull, 1857; Woodward, 1894). However, the Stonesfield Slate, though of approximately the same age, is now known to occur within the Taynton Limestone (Boneham and Wyatt, 1993).

The Eyford Member of the district has yielded an extensive fauna that has tended to be combined with material from the Stonesfield Slate. Bivalves dominate the fauna, but the member has yielded ammonites indicating the Middle Bathonian progracilis Zone (Torrens, 1969). Of greatest interest is the vertebrate fauna, of crocodiles, dinosaurs and pterosaurs (Benton and Spencer, 1995). Fish, plants and insects have also been recorded (Savage, 1961). From the sedimentary and biogenic structures (Sarjeant, 1975), the Eyford Member evidently accumulated in very shallow water.

Taynton Limestone Formation

The Taynton Limestone is confined to the central part of the district, where it generally overlies the Eyford Member. It is dominated by white to pale buff, shell fragmental grainstone that is medium to coarse grained, generally well sorted, cross-bedded and ooidal. This generally forms a brash of rather small, crumbly fragments in the fields, but large slabs of limestone occur in places. It includes thin calcareous mudstone seams and shelly beds in places.

The formation is generally some 3 to 5 m in thickness, but is very variable ranging from 1.5 to 7 m. It is absent in the south-west (Figure 8) around Sevenhampton Common [SP 01 22], and may also be absent locally in the central part of the district [e.g. SP 131 203]. These absences are largely due to a facies passage between the Taynton Limestone and the succeeding Hampen Formation (see Sumbler et al., 2000), as to some extent are the local thickness variations of the Taynton Limestone.

The Taynton Limestone of the Moreton-in-Marsh district is an important local constructional stone, used in many walls and vernacular buildings. Most outcrops are pockmarked with disused pits and it is still worked at Brockhill and Grange Hill quarries [SP 135 238]; [SP 113 244]. It is also exposed at Huntsman’s Quarry, and in Hampen Cutting where it is about 4 m thick (Cox and Sumbler, 2002). In all exposures, the limestone is cross-bedded with foresets indicating currents from the north-east, as in the type area (Sumbler and Williamson, 1999; Sumbler et al., 2000).

Fossils from the Taynton Limestone include bivalves and brachiopods, notably Kallirhynchia. At Slade Quarry [SP 070 215], the formation yielded Procerites mirabilis, indicating the Progracilis Zone (Table 6); (Torrens, 1969; 1980).

Hampen Formation

The Hampen Formation is widespread in the central part of the district, and also occurs in the south-west where it directly overlies the Eyford Member of the Fuller’s Earth Formation (Figure 8) on Sevenhampton Common. It ranges from about 2 to 9 m in thickness, attaining its maximum thickness in the neighbourhood of the type section, the Hampen Cutting (Sumbler and Barron, 1996a; Cox and Sumbler, 2002), and generally thinning to the east or north-east. The Hampen Formation was originally named the ‘Marly Beds’ by Woodward (1894), and later ‘Hampen Marly Beds’ (Arkell, 1933) based on this cutting, but it is actually dominated by limestone, which makes up 55 per cent of the thickness of the formation there. At the surface, the formation weathers to brown sandy silty clay soil, with variable proportions of limestone brash.

The predominant lithology is grey to brown, fine to medium-grained, ooidal grainstone, variably sandy and shell detrital, with mud flakes, whole shells, and scattered coarse grained white ooids. It has an oily or bituminous smell when freshly broken. The limestone is typically flaggy due to pervasive small-scale cross-bedding, and beds of fissile ripplebedded tilestone are common, with burrows and trails on bedding surfaces. Lenses of coarse grained limestone of Taynton Limestone facies occur locally. The thicker limestone beds of the formation have been widely used for walling stone and the fissile beds may also have been employed for tiling.

The calcareous mudstone (‘marl’) beds are pale grey, shelly or finely shell-detrital, ooidal, sandy, bituminous and shaly, and grade into mudstone and argillaceous limestone. Calcareous mudstone beds tend to predominate near the top and become increasingly dominant eastwards.

The fauna of the Hampen Formation is dominated by bivalves, including the oyster Praeexogyra hebridica which may be cemented into lumachelles. Brachiopods (notably Kallirhynchia), a variety of gastropods, and rarer echinoids and corals also occur.

White Limestone Formation

The White Limestone Formation occurs as a number of faulted outliers between Hampen and Condicote. It is estimated to be between 10 and 15 m thick which is substantially less than in the Cirencester district to the south (Sumbler et al., 2000) where it is commonly over 25 m. However, in this district only the lower part of the formation, namely the Shipton and Ardley members (Sumbler, 1984) are present; the topmost Signet Member, which occurs farther south, is absent, probably because of downcutting by the Forest Marble Formation (Figure 8).

The White Limestone is generally poorly exposed in the Moreton-in-Marsh district, although the lowest part is still visible in Hampen Cutting and about 11 m were recorded there previously (Woodward, 1894; Richardson, 1929). The formation is made up largely of poorly sorted, peloidal fine to medium-grained wackestones, off-white to pale grey or pinkish brown in colour, with subordinate peloidal packstones and rarer ooidal grainstones. These limestones range from platy to massive and, at some levels, are hard and very well cemented. Thin beds of calcareous mudstone and clay also occur.

The Shipton and Ardley members have not been separated on the map, but the Excavata Bed, at the top of the Shipton Member, is believed to have been located near Swell Wold Farm [SP 142 270]. North of the farm [SP 139 276], the top of the formation, beneath the Forest Marble, is an eroded limestone surface containing gastropods; this probably represents the Bladonensis Bed at the top of the Ardley Member.

The White Limestone is generally rather sparsely fossiliferous, although nerineid gastropods are extremely abundant at certain discrete levels (see Sumbler, 1984; Barker, 1994; Cox and Sumbler, 2002). Brachiopods, corals, bivalves and echinoids occur. Ammonites are very rare, but a specimen of Tulites mustela from near Salperton [SP 0703 2026] (Richardson, 1929; Arkell and Donovan, 1952) indicates the Subcontractus Zone (Table 6); (Torrens, 1980).

Forest Marble Formation

The Forest Marble Formation crops out in the Kinetonhill graben (Figure 1) south-west of Condicote, and also as small down-faulted outliers at Flagstone Farm [SP 161 275] and between Chalk Hill and Notgrove [SP 116 252]; [SP 123 249]; [SP 125 222]; [SP 110 211]. Where complete, the formation is estimated to be about 6 m thick, substantially less than in the Cirencester district to the south, where it is typically some 20 m in thickness. The formation consists largely of mudstone, which forms a brown clay or loamy clay soil. Brash of fissile sandy limestone and flaggy, oyster-rich, shell-fragmental limestone occurs patchily, probably representing lenticular beds (Figure 8).

Cornbrash Formation

Two small, hitherto unrecorded outcrops of the Cornbrash Formation, the uppermost unit of the Great Oolite Group, are preserved adjacent to the faults on either side of the Kinetonhill graben [SP 137 281]; [SP 138 263]. The total thickness of the formation in the northernmost of the two outcrops is about 3 m. The limestone is typically very compact due to recrystallisation, and generally lacks bedding structures. It produces a brash of orange-brown, rubbly to platy, finely shell-fragmental, slightly sandy packstone.

Ancholme Group

Kellaways Formation

Loamy clay which overlies the northernmost outcrop of Cornbrash [SP 137 282] represents the Kellaways Clay Member of the Kellaways Formation, the basal unit of the Ancholme Group. Augering proved 2 m of blue and purplish grey clay with pyritic patches. It is estimated that about 3 m of Kellaways Clay is present, but it is possible that the Kellaways Sand or even the succeeding Oxford Clay Formation may be present very locally.

Chapter 7 Quaternary

Pre-glacial deposits

Northern Drift Formation

Scattered pebbles and cobbles of quartz, quartzite and other exotic lithologies (including small quantities of flint) have been recorded on the highest ground of the Cotswolds at various localities in the district. It seems possible that these are remnants of the older parts of the Northern Drift Formation, which is better preserved bordering the Evenlode valley to the south-east of the district (Sheet 236). Formerly thought to be a glacial deposit, the Northern Drift is now generally believed to represent a suite of ancient river terrace sediments (Hey, 1986), laid down during early and mid-Pleistocene times, by an ancestral Thames–Evenlode river with headwaters in the English Midlands and Welsh Borders (Whiteman and Rose, 1992). Most of the pebbles can be matched with material in the conglomerates of the Triassic Kidderminster Formation (‘Bunter Pebble Beds’) and Devonian Old Red Sandstone, which crop out in these areas.

Within the Moreton-in-Marsh district, the only deposits that can be assigned to the Northern Drift with certainty are those classified as Combe Member, which caps hilltops and spurs on the margins of the Evenlode valley south-east of Moretonin-Marsh (at up to about 150 m OD) and at Pebbly Hill near Bledington (about 137 m OD). The outcrops are characterized by a loamy soil with abundant quartz and quartzite pebbles, mostly 10 to 50 mm in diameter, with larger cobbles and boulders up to 200 mm being fairly common.

Baginton Formation

In the north-eastern corner of the district, a deposit of pink to orange cross-bedded sand with seams of small quartz and quartzite (including ‘Bunter’) pebbles has been recorded beneath the Paxford Gravel. This is the Stretton Sand Member, which is up to 9.1 m in thickness, and is inferred to be confined to a shallow channel extending from Ditchford Hill to Stretton Hill [SP 219 382] to [SP 219 372]. It has not been mapped because it was not possible to separate it from the succeeding Paxford Gravel. From its elevation at between about 110 and 120 m OD, the Stretton Sand Member is evidently substantially younger than the Combe Member, and is inferred to have been reworked from this source. It has yielded a temperate mammal fauna which suggests correlation with the Baginton Sand and Gravel (Baginton Formation) of the Coventry area, and was laid down in a southern tributary of the so-called Proto-Soar or Bytham River, which presumably truncated the ‘Northern Drift’ river (see Sumbler, 2001).

Glacial deposits

Wolston Formation

The assemblage of glacial and associated deposits that occur in the Vale of Moreton in the north-eastern part of the district were first described by Dines (1928) and Tomlinson (1929) and became known as the ‘Moreton Drift’. Bishop (1958) showed that the deposits are an outlier of a succession that is better developed in the Warwick–Coventry area, which is now known as the Wolston Formation (Sumbler, 1983), of probable Anglian age (Table 7); (Bowen, 1999).

The Moreton Drift lies at the head of a northward-trending pre-glacial valley, presumably that of the Proto-Soar/Bytham River (see above). The internal stratigraphy of the deposits suggests that ice advanced from the north or north-east down the Bytham valley, and that ice-ponded waters escaped into the Evenlode via the ‘Adlestrop Col’ (Sumbler, 2001).

The Moreton Drift is naturally divisible into several units, which are members of the Wolston Formation, in ascending order, Paxford Gravel Member, Moreton Member, Oadby Member, Wolford Heath Member. Sumbler (2001) describes the Moreton Drift succession and its significance in greater detail.

Paxford Gravel Member

The Paxford Gravel Member encompasses all the sands and gravels (other than the Stretton Sand) that occur at the base of the drift succession; it is succeeded by the Moreton Member or Oadby Till. It occurs as major outcrops on the hills in the Paxford, Stretton and Todenham areas, and underlies the Moreton Drift to the north of Moreton-in-Marsh town. Boreholes proved 8 m of Paxford Gravel beneath younger deposits at the Fire Service College, around [SP 225 327]. More generally the deposits are 3 to 5 m in thickness, but more than 10 m are inferred in places.

The Paxford Gravel is typically a poorly sorted limestone rich gravel, with subangular to subrounded pebbles composed mainly of local Inferior Oolite lithologies. Sporadic Lias fossils occur, as well as rare quartz, quartzite and gritstone pebbles, Dyrham Formation sandstone cobbles up to 0.3 m across and a negligible content of flint and chalk. Around Todenham, the deposit contains a higher proportion of quartz and quartzite pebbles. A woolly mammoth tooth was recovered from the Paxford Gravel at Ditchford, but no other indigenous fauna has been recorded. It also includes lenses of clay and silt, suggesting that it interfingers with the overlying Moreton Member. In places, glaciofluvial outwash gravel (Oadby Member) with flints and chalk probably interdigitates with limestone-rich gravel. Generally, this has not been separated and so the Paxford Gravel as mapped includes materials from substantially different phases of the glaciation, including partly reworked scree and head gravel, and outwash gravel derived from Oadby Member ice.

Moreton Member

The Moreton Member makes up the greater part of the Moreton Drift in terms of both thickness and extent. It floors the Vale of Moreton southwards to near Lower Oddington, around [SP 225 267]; this is believed to be the southern limit of glaciation in the region. Boreholes have proved the member is up to 21 m in thickness and it may slightly exceed this figure in places.

The member is composed mainly of a heterogeneous assemblage of soft silty clay, silt and silty sand, generally reddish or ochreous brown and grey in colour, and commonly with a poorly laminated structure. The material is generally free of pebbles, but a scattered few occur in places. In general, flints and chalk are rare. Locally, the deposits include brown plastic, laminated clays, typifying the ‘lake clay’ of Bishop (1958) or ‘Purple Clay’ of Briggs (1973). In many places, stonier, reddish brown diamicton (till) occurs and has been separated on the map. Typically these tills dominate the lower part in the south, and interdigitate with other Moreton Member lithologies. A few lensoid bodies of gravel occur, mainly in association with the tills e.g. [SP 190 314].

The association of laminated clay and silt with Triasrich till suggests accumulation in bodies of water ponded up in front of northern-derived glacier ice. The maximum height of the water-laid deposits (137 to 140 m OD) may relate to a col, which probably lay between Lower Oddington and Adlestrop. The ponded water drained south over the ‘Adlestrop Col’ down the Evenlode and into the Thames (Sumbler, 2001). The deposits are probably correlative with the Wolston Clay and Thrussington Till of the Wolston Formation of Warwickshire (Sumbler, 1983).

Oadby Member

The Oadby Member occurs only in the northern part of the Moreton Drift outcrop, as outliers between Paxford and Stretton, and between Aston Magna and Wolford Wood. It generally overlies the Moreton Member, but cuts down onto Paxford Gravel or bedrock in places. It is generally a few metres in thickness with a probable maximum of 15 m at Wolford Wood [SP 236 332].

Tills (Oadby Till) make up the bulk of the deposit, comprising grey to brownish diamictons with flint and chalk, which are more scarce in older parts of the Moreton Drift. They give rise to a brown flinty clay soil with chalky patches locally. Limestone, quartz, quartzite and sandstone erratics also occur, together with reworked Charmouth Mudstone fossils. The deposit is lithologically similar to the Oadby Till of the Leicester–Coventry area (Sumbler, 1983). Glaciofluvial gravel bodies within the Oadby Member have been distinguished locally on the Sheet 217 e.g. [SP 234 340].

Wolford Heath Member

The Wolford Heath Member is the youngest unit in the Moreton Drift. It consists of poorly sorted gravels, which are distinguished by common flints, with quartz/quartzite and other pebbles, usually in a rather ochreous, clayey sand matrix. The proportion of the various pebble lithologies is highly variable, and cobbles up to 20 cm diameter are quite common. In general, the gravels are highly ferruginous, and locally ironpan is developed.

The Wolford Heath Member forms a dissected more-or-less planar, sheet-like body, best preserved around Moreton-in-Marsh, where it is up to about 4 m thick, and thinning out against Oadby Till in the Wolford Wood area. It slopes gradually southwards as far as Broadwell. The deposit is interpreted as a valley sandur of outwash gravels, derived from the Oadby Member ice, and represented farther downstream by the Daylesford Member.

Northern outliers of the Wolford Heath Member occur on Paxford Hill (at 156 m OD) and near Ditchford Frary [SP 227 373]; [SP 231 375]. The elevation of the latter (96 m OD), indicates deposition from drainage to the Avon River system via the Knee Brook and River Stour. They may equate with the sandur associated with the Oadby Till in the Rugby–Coventry area (Sumbler, 1983; Old et al., 1987).

River terrace deposits

Upper Thames Valley Formation

This term is used to encompass all of the post-Northern Drift fluvial deposits of the upper River Thames and its tributaries such as the Evenlode and Windrush.

River Evenlode

Freeland Member

The Freeland Member, corresponding to the Freeland Terrace deposits of Arkell (1947b), was formerly regarded as the youngest unit of the Northern Drift Formation (Hey, 1986; Bowen, 1999). However, the recent survey of the Moreton-in-Marsh district has shown that it is unrelated to the ancestral Thames–Evenlode ‘Northern Drift River’, and is regarded as the oldest terrace unit of the modern Evenlode, and belonging to the Upper Thames Valley Formation (Sumbler, 2001). Within the Moreton-in-Marsh district, the Freeland Member occurs as a number of small outcrops in the Evenlode valley downstream from Evenlode village [SP 227 294]. Their form suggests that they represent parts of a dissected valley-fill. Overall, they fall in elevation to the south.

The deposit comprises pinkish brown sand with abundant quartz and quartzite pebbles, developing pebbly reddish brown sandy loam soils. Generally flints are rare.

The Freeland Member probably represents outwash from the Moreton Member, deposited when ponded waters drained southwards through the Adlestrop Col (Sumbler, 2001). Downstream, the Freeland Member is believed to equate with the Black Park Member of the Middle Thames, which is thought to include outwash from the Oxygen Isotope Stage 12 Anglian glaciation (Bridgland, 1994).

Daylesford Member

The Daylesford Member encompasses the terraced gravel deposits along the upper Evenlode which have previously been included variously with the Moreton Drift or Bledington Terrace (sensu Tomlinson, 1929). It is well developed downstream from the southern limit of the Moreton Drift. The deposits are rather dissected and slope gently towards the modern floodplain, although some form benches about 8 m above the floodplain. The soil on the terrace is a reddish brown loam with abundant quartz and quartzite pebbles and up to 30 per cent flint. Local limestone pebbles and ironstone are also generally present. The gravels of the Daylesford Member were formerly worked at several sites and up to 4.6 m of deposit were exposed. The profile of the deposit traced upstream, and the similarity of lithology, suggests that the Daylesford Member is the downstream equivalent of the Wolford Heath Member. The Daylesford Member is equated with the Wolvercote Terrace downstream, which is probably no older than Oxygen Isotope Stage 10 (Bridgland, 1994) (Sumbler, 1995). Since the Freeland Member is assigned to Stage 12, it follows that the Moreton Drift spans stages 12 to 10 (Table 7).

Other terrace deposits

Near Sydenham Farm [SP 221 271], three poorly defined gravel terraces are evidently associated with the Caudwell Brook. They comprise limestone-dominated gravel with quartz and quartzite pebbles.

Rivers Windrush, Dikler and Eye

Two gravel terraces have been recognised along the River Windrush and its tributaries in the neighbourhood of Bourton-in-the-Water. Correlation with the Thames succession downstream is uncertain. They are now believed to represent two separate aggradations but both were included in the ‘Bourton Terrace’ of Briggs (1973, and in Roe, 1976).

Sherborne Member

The deposits of the Sherborne Member are widespread around Bourton-on-the-Water. They form a degraded terrace generally about 5 to 6 m above the adjoining present-day floodplain. The deposits are over 6 m thick, and locally extend below the level of the modern floodplains. The surface soil may be stoneless or contain limestone gravel. The deposits are mainly composed of poorly sorted, subangular, locally derived limestone gravel, with pebbles up to 8 cm across. The member probably represents head originating from the scarp to the west, which has been flushed out of the valleys by streams and has undergone further fluvial reworking.

The gravels were formerly worked for aggregate from a number of pits in which woolly mammoth (Mammuthus primigenius) and woolly rhinoceros (Coelodonta antiquitatis) remains have been found (O’Neil and Shotton, 1974). The Sherborne Member is tentatively equated with the Summertown–Radley Member of the Thames, which is a composite deposit ranging from ‘Wolstonian’ to earliest Devensian age (Sumbler, 1995).

Rissington Member

The Rissington Member borders the Windrush at Bourton-on-the-Water, and the Eye and the Dikler at Lower Slaughter. There are also two small deposits in the Windrush valley near Naunton. At Bourton, it forms a well-developed terrace up to 2 m above the floodplain, but elsewhere it slopes gently down to the floodplain. The soil on the terrace is a clayey loam, but gravel is present at shallow depth. Generally, the gravel is finer-grained than that of the Sherborne Member, with pebbles up to 4 cm across, although coarser gravels occur at depth. It is also generally better sorted and the clasts are better rounded.

The deposits of the Rissington Member were formerly worked for aggregate in the southern part of Bourton-onthe-Water village. The distribution of the now flooded pits shows that the deposits extend beneath the alluvium and records indicate that they may be up to 5 m in thickness. Palaeontological and other data show that the member includes correlatives of the Northmoor Member of the River Thames (Sumbler, 1995; Bowen, 1999).

Avon Valley Formation

Knee Brook

In the north-east of the district, fairly extensive but poorly developed terraces are developed about 1.5 m above the floodplain along the course of the Knee Brook and its southern tributary the Blockley Brook, which ultimately drain to the Severn, via the Stour and Avon. Soils are loamy clay with small flint and quartz/quartzite pebbles and limestone. Limestone-dominated gravel ploughed up suggests that this forms the bulk of the material, which comprises reworked, locally derived head gravels up to about 2 m thick. The deposits are classified as First Terrace, but this designation is purely local and no long-distance correlation is implied, although it seems probable that they represent the Bretford and/or Wasperton members of the Avon Valley Formation.

Carrant Brook

Around Beckford, in the north-west corner of the district, gravel deposits border the Carrant Brook, a tributary of the River Avon. These comprise the Beckford Terrace of Briggs, Coope and Gilbertson (1975) which they regarded as a composite deposit closely analogous to the Cheltenham Sand and Gravel (see below); that is, they are locally derived solifluction gravels and aeolian sands, all more or less fluvially reworked. This is essentially correct although their distribution and pervasive fluvial reworking requires their classification as River Terrace Deposits.

The deposits form a more or less continuous slope on the north side of the Carrant valley extending over a vertical interval of 15 to 20 m. However, on the map they have been subdivided and correlated with the Avon terraces on the basis of the stepped profile at the base of the deposits. The highest, oldest deposits are provisionally assigned to the Fourth Terrace (Cropthorne Member of the Avon). They comprise up to 4.5 m of subangular sandy limestone-dominated gravels, with medium to coarse-grained sand beds.

The lower and greater part of the terrace deposit outcrop is assigned to the Second Terrace (Wasperton Member of the Avon). On the north side of the Carrant Brook the deposits are gravels that have been extensively worked for aggregate around Beckford. Exposures show sandy well-rounded limestone gravel, somewhat finer than the Fourth Terrace deposits, with some sandy interbeds. Palaeontological and radiocarbon data (Tomlinson, 1940) (Briggs et al., 1975) are broadly consistent with the Second Terrace attribution. South of Carrant Brook, the deposits are assigned to both Second and First Terrace (Bretford Member of the Avon) and are sandy clays with scarce gravelly material.

Alluvium

Alluvial floodplains are present along the courses of all the principal rivers and streams of the district. Generally, in the vales, the floodplains are fairly broad (100–600 m wide) where the rivers cross the outcrops of less resistant mudstone rocks. In the limestone plateau area, the floodplains are very narrow because the streams are constrained within steep-sided valleys.

In the vales, the alluvium is generally brown silty clay that looks much like weathered Lias, from which it is probably in large part derived, with the exception of the Evenlode alluvium (see below). It is overlain by a dark brown, humic, loamy clay soil. This loam and clay is probably about 1 m in thickness, and excavations may encounter limestone gravel or sand below. This may be more-or-less contemporaneous with the overlying clay, which is inferred to be wholly of Holocene age. Alternatively, it may be an extension of adjacent deposits beneath the alluvium (e.g. around Bourton-on-the-Water or Cheltenham), and therefore older. The alluvium of the upper reaches of the Evenlode overlies a variety of glacial and glaciofluvial deposits, which is reflected in its composition. The alluvial deposits are still building up to a minor extent, as thin layers of mud are left on the fields after floods.

Along the higher reaches of the Coln, Windrush, Eye and Dikler and their tributaries, the floodplains are generally less than 100 m wide and the alluvium infills a shallow channel, probably no more than 3 m deep at maximum, and generally incised into bedrock. Typically, the alluvium is a brown peaty loam or brownish grey clay, commonly containing a proportion of tufa (see below) as disseminated silt or pellets. Generally no more than 1.5 m thick, it is commonly underlain by gravel of locally derived limestone, rarely more than about 1 m thick. The alluvial deposits commonly pass upstream almost imperceptibly into head (see below).

Slope deposits

Head

Head, comprising accumulations of solifluction debris of periglacial origin, together with more recent hillwash (colluvium), occurs in most of the valleys and on some slopes in the Moreton-in-Marsh district. Only the thicker, laterally more extensive deposits are indicated on the map. Their lithology varies according to the materials from which they were derived.

Extensive deposits occur on the slopes of the eastern vales at Longborough, Batsford and Bourton-on-the-Water. They comprise brown silt underlain by gravel, derived from the formations upslope; thickness rarely exceeds 3 m. Broad spreads of stony, silty, clayey head occur near Little Beckford and north-west of Dumbleton. Head blankets some of the slopes in the north-east of the district, and a spread of gravelly head covers the Birdlip Limestone outcrop south of Guiting Power.

Dry valleys

Characteristic topographical features of the plateau area of the district, and indeed of the Cotswolds in general, are the dry valleys. These are valleys which, because of the permeability of the bedrock, do not normally contain streams except when the water table is particularly high (see Chapter 2). Examples include the upper reaches of the Dikler, above Hinchwick [SP 146 301], the upper reaches of the Eye above Swell Wold Farm [SP 140 270] and numerous tributary valleys of the rivers Windrush and Coln. A flat, floodplain-like tract of brown loamy clay overlying limestone gravel, up to 2 m thick and evidently an ancient alluvium, typically floors such dry valleys. This alluvial fill is partially or wholly covered by gravelly solifluction material, and thus all of the deposits of the dry valleys are indicated as head on Sheet 217.

These valleys were probably initiated during cold periods of the Quaternary, when permafrost rendered the ground impermeable. Once established, downcutting would proceed whenever the groundwater levels were high enough to produce surface streams. The current dry state of the valleys may relate both to lower precipitation rates, and to a fall in the groundwater levels due to downcutting by rivers. In some cases, the disappearance of streams may be recent and related to current groundwater pumping practice. In other cases, the valleys may not have contained running water since Pleistocene times.

Head Gravel

Head Gravel deposits are most extensive on the low-lying ground in the north-west of the district. The Head Gravels comprise limestone gravel with sporadic bodies of sand and clay, and are smooth-topped, gently sloping and fan-like. They extend for several kilometres from the foot of the escarpment, commonly in front of embayments in it.

Tomlinson (1940) described the deposits as coarse, unstratified angular or subangular limestone gravels of local origin, overlain by a brown loam ascribed to decalcification. She noted that these unstratified gravels passed downhill into finer stratified gravels and sands that spread out into ‘fan-deltas’ and in some localities, developed into river terrace deposits. Moreover, she recognised that their bases were uneven and appeared to fill basins, a conclusion supported by geophysical investigations between Broadway and Winchcombe which indicated channels several metres deep in the underlying mudstones, radiating from the base of the escarpment. Their polyphase formation by solifluction during repeated cold climate episodes has led to a sequence of distinct levels and interdigitation with adjacent river terrace deposits, which together with their similar composition creates difficulties drawing precise boundaries between.

In general, Head Gravel deposits are not widespread in the eastern part of the district, probably because the majority of such materials have been incorporated into fluvial deposits. A number of outcrops occur in the neighbourhood of Lower Oddington. These are solifluxion debris derived mainly from the Clypeus Grit and Chipping Norton Limestone.

Cheltenham Sand and Gravel

Substantial deposits of Head Gravel between Gotherington and Cheltenham are separated as Cheltenham Sand and Gravel. This forms a gently sloping dissected apron at the foot of the Cotswold scarp. It is generally only a few metres in thickness, but up to 15 m is known and it varies widely over short distances. The form of the base indicates that it was partly laid down in deep channels running from the foot of the scarp, probably scoured by meltwater. The deposit is made up of yellowish brown, medium-grained quartz sand with beds of poorly sorted predominantly limestone gravel, the latter dominating the lower part of the deposit and also those parts close to the scarp. Decalcification has removed many of the limestone clasts from the top metre or so of the more gravelly deposits, and the soils throughout are generally composed only of sandy clay.

The deposits are evidently composite; the lower gravelly part (the Cheltenham Gravel, corresponding essentially to the Fan Gravel of Worssam et al., 1989) probably represents a partly reworked solifluction apron (Tomlinson, 1940; Briggs, 1975; Worssam et al., 1989). The overlying Cheltenham Sand is dominantly an aeolian deposit (Tomlinson, 1940), probably derived from the terraces of the River Severn a few kilometres to the west (Briggs, 1975) and partially reworked by fluvial processes.

The age of the Cheltenham Sand and Gravel is uncertain, but as with the other head gravels, it is likely to have accumulated over a considerable period of time. From the inferred depositional environment, the deposits accumulated principally during cold stages of the Quaternary, and it is probable that the Cheltenham Sand and Gravel is very largely of Devensian age (Table 7). From topographical relationships, Worssam et al. (1989) equated parts, at least, of the Cheltenham Gravel with the late Devensian Second Terrace of the Avon and Severn.

Tufa

Tufa is a calcareous deposit, precipitated from spring water with calcium carbonate that is saturated as a result of percolation through limestone. Tufa deposits are present around a number of the springs, commonly forming a coating on rocks and plants, or forming an ‘apron’. All but one of the deposits are too small to be shown on the 1:50 000 Series Sheet 217. A spread about 150 m across and perhaps 1 to 2 m thick lies at the base of the Dyrham Formation outcrop about 1.5 km south of Winchcombe [SP 023 271]. Smaller deposits were observed nearby, and all are thought to have been precipitated from water from the Marlstone Rock. Elsewhere in the district, the alluvium of the river floodplains may contain minor lenses of tufa (see Alluvium).

Peat

Small peat deposits occur in association with springs, or on poorly drained ground such as that commonly associated with landslip. However, none was sufficiently extensive to depict on the 1:50 000 scale map. Additionally, minor lenses of peat may occur within the alluvium of the floodplains in some places.

Chapter 8 Structure

Faults and folds

On a broad scale, the structure of the Jurassic rocks seen at outcrop within the district is simple, with strata dipping in a predominantly south to south-easterly direction, at an angle of about 0.5°. Locally, anomalous dip directions and values as high as 50° are encountered; these are generally caused by the superficial effects of cambering or valley bulging (see below), although dips of up to 8° were observed associated with minor tectonic folds or faults.

North-south structures

Although not the most numerous at surface, the most significant faults in terms of the deep structure are those making up the predominantly north-north-east to southsouth-west-trending belt extending from near Chipping Campden in the north to near Bourton-on-the-Water in the south (Figure 1) and on across the Cirencester district (Sumbler et al., 2000, fig. 22). Geophysical surveys suggest that these are the surface expression of the eastern margin of the Worcester Basin, a major graben structure of Permian and early Mesozoic age (Chapter 3). The fault belt thus marks the Vale of Moreton Axis, the name given to the zone of thinning in the Jurassic cover rocks which is the manifestation of the basin margin at depth. The surface fault belt is very modest structure, but gives a strong clue about fault distribution and geometry at depth. For convenience, some of the deep faults are named after their presumed surface counterparts in the following account. However, it is possible that some of the surface faults relate to differential subsidence of the thicker sediment pile in the Worcester Basin, and may not necessarily be physically connected with the faults at depth.

Much of the knowledge of the deep structure (Chapter 3) has been gained from commercial seismic data acquired for hydrocarbon exploration (Chapter 2). This has enabled the faults which bound the Worcester Basin at depth to be positioned with some accuracy (Sheet 217, Palaeozoic sketch map). However, the en echelon distribution shown is influenced by the surface fault pattern (Figure 1), from which it is offset some 0.5 to 1.5 km to the west.

The Slaughter Fault runs along the western margin of the Vale of Bourton and up the Dikler valley. Its presence is inferred from structural and stratigraphical observations, but relations could equally well be explained by a monocline. This is effectively the continuation of the Clapton Fault (Sumbler et al., 2000) and may be a manifestation of the Moreton Fault inferred at depth (Smith, 1985; Sheet 217, Palaeozoic sketch map) and accommodate some thickening of the Lias (growth faulting). It throws down to the west, with a maximum displacement of over 80 m. Passing north, it appears to die out south of Longborough, although it may be concealed by cambered strata between here and Bourton-on-the-Hill. A similarly aligned fault was traced through Blockley. This has a displacement of about 30 m on the Marlstone Rock, but less than half this at the base of the Inferior Oolite, consistent with growth faulting, and therefore also possibly related to the Moreton Fault, which is inferred to die out hereabouts. To the north, the Moreton Fault is replaced en echelon at depth by the Ilmington Fault. Unlike in the Cirencester district (Sumbler et al., 2000), no eastward-dipping antithetic faults have been inferred along the Vale of Moreton Axis.

In the north-east of the district, a substantial fault named the Ilmington Fault was traced at the surface (Figure 1). This may have a westward throw exceeding 100 m in places and is inferred to be a manifestation of a major westward-dipping basement structure of the same name, which is the southern extension of the Clopton fault system (Chadwick and Smith, 1988; Chadwick and Evans, 1995), the eastern boundary of the Worcester Basin. It shows about 700 m of downthrow at the pre-Permian basement surface, and the Permian strata terminate against it from the west (Sheet 217, cross-section 1). Additionally, the Sherwood Sandstone and Mercia Mudstone groups thin east across it, implying intra-Triassic, syndepositional movement.

Three north-south-trending surface faults were mapped near Broad Campden [SP 16 37], and Condicote [SP 15 28], [SP 15 26] (Figure 1). None has a throw exceeding 10 m and all may be of a superficial nature. However, they do lie along a projection to the surface of the Blockley Fault, postulated at depth lying parallel and some 2.5 km west of the Moreton Fault, and merging with it in the south. It has a downthrow to the west of less than 300 m at the Palaeozoic surface and is associated with an antithetic, eastward dipping, hangingwall fault within the Trias to Lias sequence (Sheet 217, Palaeozoic sketch map and crosssection 1). Although some overstep relationships in the Inferior Oolite Group take place in the vicinity of some of these faults, and are also related to the margin of the Worcester Basin, there is no definite evidence of movement on faults of this trend during deposition of the Inferior Oolite either here or elsewhere in the district.

The intra-basinal Inkberrow Fault (Sheet 217, crosssection 1) downthrows 1000 m to the west increasing northwards to 1100 m in the Stratford-upon-Avon district. This displacement is much greater than faults of the basin margin. However, the Weethley Fault of the Stratford district (Chadwick and Smith, 1988; Chadwick and Evans, 1995) also downthrows to the west and extends south into the Moreton-in-Marsh district (un-named on crosssection 1), lying 6 km east of the Inkberrow Fault; its throw decreases southwards to zero. Both are growth faults, with the Permo-Triassic sequence thickening across them from east to west (Chapter 3; Sheet 217, cross-section 1). In this district, the Weethley Fault has an antithetic, hanging-wall fault with a smaller throw.

Chadwick and Evans (1995) have presented subsidence curves for the Worcester Basin which indicate two phases of rapid subsidence, one in late Permian and the other in early Triassic times, each followed by a period of slower subsidence. The latter evidently terminated with the deposition of the Penarth Group. Renewed subsidence of the basin in the Early Jurassic is indicated by the progressive eastward overlap of successive units in the lower part of the Lias Group (Chapter 4; Sumbler et al., 2000, fig. 10). This phase of basin infill terminated with deposition of the shallow-water Marlstone Rock Formation.

Minor subsidence continued in the area of the Worcester Basin, accounting for a thicker and more complete Whitby Mudstone Formation and Inferior Oolite succession there than to the east. The presence of many breaks in the Inferior Oolite succession (marked by hardground beds) accompanied by erosion of the pre-existing strata towards the Vale of Moreton Axis, has been taken to indicate that this subsidence was episodic, but these phenomena may alternatively relate to periods of lowered sea level, while slow subsidence continued. At the end of Inferior Oolite times, the Upper Bajocian Salperton Limestone Formation was deposited across the region, without any major influence from the Vale of Moreton Axis. In the Great Oolite too, the effects appear to have been minimal, although there is some indication of local increased downcutting at the base of the Taynton Limestone in the neighbourhood of the axis.

West-north-west to east-south-east structures

This is the predominant fault trend in the district (Figure 1). These faults generally have throws of a few metres, but some showing displacements of over 20 m and locally up to 40 m may be traced for several kilometres. Downthrows to both the north and south are seen, and in some cases form simple graben structures, as in the Hornsleasow graben, Sevenhampton graben, and those near Condicote and north of Naunton.

Elsewhere, many of the structures are more complex. West of Lower Swell is a belt of alternating horsts and graben separated by west-north-west-trending faults [SP 15 28] to [SP 16 25] which may have influenced sedimentation of the Harford and Scottsquar members of the Birdlip Limestone Formation. South-west of Condicote, the Kinetonhill graben, enclosing the youngest solid strata in the district (see Chapter 6), is complicated by additional faults, one of which is highly arcuate, resulting in local dips of 8º.

The effects at depth of the west-north-west-trending faults are unclear. They may be associated with the latest Jurassic to early Cretaceous phase of regional uplift preceding the development of the Late Cimmerian unconformity (Chapter 1).

Superficial (non-diastrophic) structures

The strata at outcrop in the Moreton-in-Marsh district are extensively affected by a variety of superficial structures, including valley bulging, cambering and gulls, which are deformation phenomena that are related essentially to the topography of the land surface, and landslipping (landsliding). In many cases they may be on a larger scale than diastrophic structures and are largely a result of periglacial (permafrost) conditions, in force in this area during the cold ‘glacial’ phases of the Pleistocene (Quaternary Period). They are generally not currently active processes except along escarpments. Many of their effects must be considered potential engineering hazards (see Chapter 2).

Landslips are shown on the 1:50 000 Series Sheet 217. An inset diagram at 1:250 000 scale indicates some of the locations of cambering and surface gulls, although the areas affected by cambering are likely to be more extensive. Valley bulging is not depicted as the extent within the vales is unknown.

Valley bulging

Within the Moreton-in-Marsh district, both valley bulging and cambering are caused fundamentally by the deformation and consequent movement of the clays of the Charmouth Mudstone and Whitby Mudstone formations near outcrop, where these are subject to the load of overlying strata. In the Vale of Gloucester, no specific instances of valley bulging were observed, but the inferred eastward dip of the Charmouth Mudstone strata in the Cheltenham–Oxenton area may be in part a result of valley bulging (see Chapter 4). Valley bulging has probably taken place in the Vale of Moreton, and may account for the relatively narrow outcrop of the Dyrham and Whitby Mudstone formations in places, and the anomalous dips on the mapped outcrops of the 85 and 100 markers east of Icomb.

Valley bulging is inferred to have affected the Whitby Mudstone Formation along the incised valleys of the Dikler, Windrush and Eye. In places the outcrops occur at elevations up to 25 m higher than might be expected, and the adjoining strata have been disrupted such that locally they dip steeply.

Cambering

Cambering affects the resistant limestone or sandstone strata (‘cap-rock’) overlying the mudstone formations in several situations:

• associated with valley bulging
• near escarpment edges
• on plateaux
• thin limestone beds within mudstone sequences

Where mudstone beds have bulged, the overlying cap-rock strata may be disrupted, laterally extended and lowered as a ‘camber’, comprising blocks separated by ‘dip and fault structures’ as described by Horswill and Horton (1976, p.434, fig. 6) in cap-rock up to 17 m thick. The faults show small displacements (1 to 3 m), and are closely spaced (up to 15 m).

Creep and plastic flow under periglacial conditions also affects mudstone on slopes that have not undergone classic valley bulging. As a result, the vertical thickness reduces (possibly to almost zero in places), and any cap-rock strata may be cambered. These slopes are generally almost planar. Cambering of thick (20 to perhaps 90 m) cap-rock sequences on escarpments may result in outward movement causing parallel vertical linear voids (gulls) to open. These may contain blocks of strata, or sustain cavities or even caves (see below, Self, 1995). Surface hollows (also known as gulls) may develop over these structures (Plate 1) and are common in the north Cotswolds.

A study of sections and a geophysical survey (Raines et al., 1999) near Bourton-on-the-Water investigated gull structures. These showed that they may be graben with steep bounding faults extending down to the underlying mudstone, or shallower complex half-graben. The infills include step-faulted or largely intact blocks of strata, overlain by rubble and deep soil.

Bredon Hill, the flanks of Cleeve Hill and Stanley Hill are extensively cambered throughout. As a result, they are capped by broad smooth slopes of Birdlip Limestone strata reaching downhill across the Whitby Mudstone Formation. Surface gulls are found on many of the slopes, forming linear hollows up to 8 m deep, 60 m wide and 500 m long, including a gull known locally as Happy Valley [SO 991 239]. Whittaker (1972) estimates that up to 53 m of strata may be missing beneath the tips of the cambers on Bredon Hill. In addition, he cites downslope extension of the cambers of over 200 m laterally for the concealment of a diastrophic fault on the south side of the hill.

Along the main escarpment, where a much greater thickness of Inferior Oolite Group strata is present, intense gull formation has taken place. Near Farmcote Wood Farm, at Shenberrow Hill, and around Broadway Tower (Plate 1) up to five surface gulls may lie side-by-side, parallel to the escarpment edge. These are up to 1.2 km long, 30 m across and 5 m deep with subsurface structures up to 80 m deep. At the latter, it was reported that a line of hollows 200 m long had opened in a wood [SP 115 359] and an underlying gull cave had been explored for some distance.

Overlooking the Vale of Moreton, cambering and intense gull formation was observed in the Inferior Oolite from Chipping Campden to Longborough. Southwards from Bourton-on-the-Hill, the Cleeve Cloud Member forms an almost planar slope, dipping eastward at about 5º. Exposures hereabouts show that the beds actually comprise ‘dip-and-fault’ structures with easterly dips of 30º to 50º. In addition, it is possible that the cambered strata here may conceal a north–south diastrophic fault (see p.30), similar to that on Bredon Hill (see above). West of Bourton-onthe-Water, multiple blocks of Cleeve Cloud Member dipping at 40º to 55º are visible in a railway cutting, and linear depressions on Icomb Hill may be gulls.

Many of the slopes of Inferior Oolite and Great Oolite strata adjoining the valley bulges along the Windrush, Eye and Dikler (see above) are cambered. The large gulls generally occur on the upper slopes; the lower slopes are smooth. Near Aston Farm, gulls form a grid-like pattern on both sides of the Windrush overlying structures 20 to 60 m deep. Gulls up to 1000 m in length have formed on the higher slopes of the Windrush valley at Guiting Power and Kineton and the Eye valley at Rockcliffe.

Sets of subparallel surface gulls occur on plateaux at Pinnock Wood Farm, Temple Guiting, Ford and above Postlip. These are up to 800 m long, 70 m wide and 10 m deep and are underlain by up to 70 m of Inferior Oolite. Cambering generally lacking surface gulls also affects the Great Oolite limestone units overlying the Fuller’s Earth Formation at Naunton and Hawling, and some outcrops of the Marlstone Rock Formation along the main escarpment and on Woolstone Hill, Bredon Hill and Dumbleton Hill, where a gull is present. South of Icomb, dip-and-fault structure was observed in cambered Marlstone Rock.

Cambering on a smaller scale may also affect the limestone beds within the Charmouth Mudstone Formation in the east. Faulting in the drift deposits at Stretton-on-Fosse may be associated with faulting in the underlying Charmouth Mudstone bedrock, further emphasising that cambering and related effects took place under periglacial regimes.

Landslips

Landslips have occurred widely throughout the district wherever mudstone or, to a lesser extent, sandstone formations crop out on a slope. These were recognised and mapped during the survey in part using aerial photographs. The mechanisms are outlined in Chapter 2.

The largest landslips have been recorded along the main Cotswold escarpment between Saintbury Hill in the north, and Dowdeswell Wood in the south-west, and around Bredon Hill in the north-west, where they principally affect the Whitby Mudstone Formation. Less extensive slipping was observed along the slopes of the vales of Moreton and Bourton between Chipping Campden and Bourton-on-the-Hill, around Stow-on-the-Wold and Icomb Hill. The upper limits are marked by large, subtly arcuate scars, generally in the basal beds of the Inferior Oolite. Below these, rotated masses of limestone-capped mudstone form multiple terrace-like features with their tops sloping slightly towards the hillside. Farther downslope, the rough and broken ground reflects the complex rotational and translational processes within the landslips. Springs are common hereabouts. Below, lobate mudflows may override the Marlstone Rock and descend across the slopes of the Dyrham and Charmouth Mudstone formations. In addition, less extensive landslips affect the outcrops of the Dyrham Formation around Winchcombe and on the outlying hills. The most intensive period of landslipping probably occurred during periglacial phases of the Pleistocene, but symptoms of recent activity (see Chapter 2) were observed in many places.

Landslips also affect the Fuller’s Earth outcrops around Hawling and Brockhampton commonly overriding the Salperton Limestone outcrops. The general morphology is very similar to those that have affected the Lias Group.

Information sources

BGS publishes an extensive range of geoscience maps, memoirs, regional geology guides, offshore regional reports, technical reports and other publications for the United Kingdom and adjacent continental shelf. Those that relate to the Moreton-in-Marsh district or immediately adjoining areas are listed below. Also listed are unpublished reports, an index of selected boreholes, and other relevant information. BGS maintains a catalogue comprising over two hundred datasets, including some digital databases, which give index-level access to its large, and diverse, collections of material. Further information can be obtained from the Manager, British Geological Survey, Keyworth, Nottingham NG12 5GG. A publicly accessible data catalogue is available at the BGS World Wide Web site (http://www. bgs.ac.uk).

Maps

(*An asterisk denotes that the maps are also available as digital datasets under licence)

Geology

• 1:2 500 000
• British Islands and adjacent continental shelf Sub-Pleistocene geology, 1979
• 1:1 584 000
• British Islands Solid geology, 1969
• 1:1 500 000
• *Britain, Ireland and adjacent areas Tectonic, 1996
• 1:1 000 000
• United Kingdom, Ireland and adjacent continental shelf (South Sheet)
• Solid geology, 1991 United Kingdom (South) Pre-Permian geology, 1985
• 1:625 000
• *Solid geology map UK South Sheet, 2001 Quaternary geology, 1977
• 1:250 000
• *Chilterns (Sheet 51N 02W), 1991 Solid geology, 1991
• *Bristol Channel (Sheet 51N 04W)
• Solid geology, 1988; Quaternary geology and sea-bed sediments, 1986
• 1:50 000 or 1:63 360
• *Moreton-in-Marsh (Sheet 217) Solid and Drift, 2000
• 1:10 000 and 1:10 560
• The primary survey of the district was carried out at the scale of one inch to one mile (1:63 360) by E Hull and H H Howell in the 1850s, and published as part of [Old Series] geological Sheet 44, in 1857 and 1879. New Series one-inch geological Sheet 217. Moreton-in-Marsh was first published in 1929; it was essentially a re-issue of the original survey with the addition of Drift deposits and the Chipping Norton Limestone Formation, which had been added by H G Dines in the 1927 survey at the scale of six inches to one mile (1:10 560). A small amount of survey work in connection with adjoining districts took place between 1959 and 1966, and again in 1980–82 and the early 1990s. The greater part of the district was resurveyed at 1:10 000 scale between 1994 and 1998 (by AJMB, ANM and MGS), and the earlier maps were also updated. These new maps, covering the whole of the district, were used to compile the new (2000) edition of 1:50 000 Series Sheet 217 Moreton-in-Marsh.
• The 1:10 000 scale maps which, either wholly or in part, fall within 1:50 000 Series Sheet 217 are listed below, together with the surveyor’s initials and the dates of the survey. The surveyors were: A J M Barron, A N Morigi, M G Sumbler, A Whittaker, I T Williamson, B C Worssam and R J Wyatt.
• The 1:10 000 scale maps are available for consultation in the Library, British Geological Survey, Kingsley Dunham Centre, Nicker Hill, Keyworth, Nottingham NG12 5GG, at BGS Edinburgh and the BGS London Information Office (Natural History Museum, South Kensington, London). Dye-line copies may be purchased from Customer Services. Corresponding technical reports are available for most of these maps (see Books and reports).

Geophysical maps

• ·         1:1 584 000
• ·         Great Britain and Northern Ireland Smooth aeromagnetic, 1970
• ·         1:1 500 000
• ·         Earthquakes, 1980–1994. 1995
• ·         1:1 000 000
• ·         Southern Britain, Ireland and adjacent areas; Bouguer gravity anomaly, 1992
• ·         1:625 000
• ·         Great Britain and Northern Ireland (South Sheet); Aeromagnetic anomaly, 1965
• ·         British Isles (South Sheet); Bouguer gravity anomaly, 1986
• ·         1:250 000
• ·         Chilterns Sheet 51N 02W; Aeromagnetic anomaly, 1980 Chilterns Sheet 51N 02W; Bouguer gravity anomaly, 1983 Bristol Channel Sheet 51N 04W; Aeromagnetic anomaly, 1988 Bristol Channel Sheet 51N 04W; Bouguer gravity anomaly, 1986
• ·         1:50 000
• ·         Sheet 217 Moreton-in-Marsh. Geophysical information map (GIM); plot-on-demand map summarising graphically the publicly available geophysical information held for the sheet in the BGS digital databases. Features include Bouguer gravity and aeromagnetic anomalies and location of data points, gravity and magnetic fields plotted to show correlation between anomalies, location of geophysical surveys, location of public domain seismic reflection and refraction surveys and location of deep boreholes and those with geophysical logs.

Hydrogeology maps

• ·         1:625 000
• ·         Sheet 1 (England and Wales) 1977
• ·         1:100 000
• ·         Groundwater vulnerability maps (Prepared for the Environment Agency by BGS and the Soil Survey and Land Research Centre) Sheet 29 (Worcestershire), 1995
• ·         Sheet 30 (Northern Cotswolds), 1996
• ·         Sheet 37 (Southern Cotswolds), 1995
• ·         Sheet 38 (Upper Thames and Berkshire Downs), 1995

Economic geology

• ·         1:1 000 000
• ·         Britain; Industrial Mineral Resources, 1996
• ·         1:1 500 000
• ·         Britain and Ireland; Metallogenic Map

Books and reports

Onshore Geology Series

Reports in this series are specifically related to the component 1:10 000 geological maps included within the Moreton-in-Marsh district. They give details of the geology of the area and records of sections and important cored boreholes. Though not widely available, they can be consulted at the BGS Library, or may be purchased through BGS.

• The Cheltenham (East) area (Sheet SO 92 SE), WA/99/04. The Bishop’s Cleeve area (Sheet SO 92 NE), WA/99/01. The Teddington area (Sheet SO 93 SE), WA/DM/82/20. The Beckford area (Sheet SO 93 NE), WA/DM/82/18.
• Sheet SO 93 (Explanation of 1:10 000 sheets SO 93 NW, NE, SW and SE with special emphasis on potential resources of sand and gravel, WA/VG/82/10.
• The Andoversford area (Sheet SP 01 NW). WA/95/107. The Compton Abdale area (Sheet SP 01 NE), WA/95/4. The Hawling area (Sheet SP 02 SE), WA/98/26.
• The Temple Guiting area (Sheet SP 02 NE), WA/00/38. The Farmington area (Sheet SP 11 NW), WA/95/3.
• The Naunton area (Sheet SP 12 SW), WA/98/27. The Chalk Hill area (Sheet SP 12 NW), WA/99/41.
• The Snowshill Hill area (Sheet SP 13 SW), IR/01/080. The Rissingtons area (Sheet SP 11 NE), WA/95/17.
• The Bourton-on-the-Water area (Sheet SP 12 SE), WA/99/07. The Stow-on-the-Wold area (Sheet SP 12 NE), WA/00/12.
• The Bourton-on-the-Hill area (Sheet SP 13 SE), WA/99/27. The Paxford area (Sheet SP 13 NE), IR/01/098.
• The Fifield area (Sheet SP 21 NW), WA/95/79. The Icomb area (Sheet SP 22 SW), WA/98/15.
• The Oddington area (Sheet SP 22 NW), WA/99/15.
• The Moreton-in-Marsh area (Sheet SP 23 SW), WA/98/53. The Stretton-on-Fosse area (Sheet SP 23 NW), WA/99/19.

Books

• British Regional Geology: Bristol and Gloucester region, 3rd edition, 1992.
• British Regional Geology: London and the Thames valley, 4th edition, 1996.

Memoirs

• Listed by sheet number
• 199 Worcester, 1997.
• 200 Stratford-upon-Avon and Evesham, 1974
• 201Banbury and Edge Hill, 1965.
• 216 Tewkesbury, 1989
• 218 Chipping Norton, 1987.
• 235Cirencester, 2000.
• 236 Witney, 1946.
• Wells and springs of Warwickshire. 1928.
• Wells and springs of Gloucestershire. 1930.

Other publications

The geology of the country between Sherborne, Gloucestershire and Burford, Oxfordshire. Bulletin of the Geological Survey of Great Britain (for 1961), No. 17, 75–115.

Biostratigraphy and petrography

There are 20 reports that contain information specifically pertinent to the Moreton-in-Marsh district. They are not generally available to the public, but in some cases they may be examined on application to BGS.

Documentary collections

Borehole records

BGS holds the records of approximately 315 wells and boreholes for the Moreton-in-Marsh district (March 2001). Most of these are descriptive, written logs but a proportion are downhole geophysical logs. The records are mainly stored on paper but can be accessed via a digital database containing index information. For further information contact the Records Officer, BGS Keyworth.

Hydrogeology

Data on water boreholes, wells and springs are held in the BGS (Hydrogeology Group) databases at BGS Wallingford.

Materials collections

Geological Survey photographs

Copies of the 79 photographs held for the Moreton-in-Marsh district are deposited for reference in the Library, BGS, Keyworth, Nottingham. Prints and transparencies can be supplied at a fixed tariff, but note that the earlier prints are available only in black and white.

The British Association for the Advancement of Science collection of geological photographs is held at BGS Keyworth. The following pertain to sites within Sheet 217 Moreton-in-Marsh.

Petrological collection

The registered petrological collection for the Moreton-in-Marsh district comprises 132 hand specimens and thin sections of rocks from quarries. Access to the collection is facilitated through a digital database, the BGS Petmin Database, which holds index data for England, Wales and Scotland. Further information can be obtained from the Curator, Petrography and Mineralogy Collections, BGS Keyworth.

Biostratigraphy collections

Macrofossils from some 188 localities are held in the BGS Biostratigraphy Collections. As well as material collected by Geological Survey staff during the various phases of work in the Moreton-in-Marsh district, it includes specimens donated by non-Survey personnel. Enquiries should be directed to the Curator, Biostratigraphy Collections, BGS Keyworth.

Mineral industry operators (2001)

Twenty-six active, closed or dormant quarries in the Moreton-in-Marsh district are listed in the BGS Directory of Mines and Quarries (1998 edition).

Earth science conservation

Eleven earth science Sites of Special Scientific Interest (SSSI) lie wholly or partly within the Moreton-in-Marsh district. For further information contact English Nature.

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Figures, plates and tables

Figures

(Figure 1) Simplified geological map showing the distribution of principal faults and generalised dip directions at surface in the Moreton-in-Marsh district.

(Figure 2) Geophysical logs of the Guiting Power and Ash Farm boreholes.

(Figure 3) Thickness of the Inferior Oolite Group in the district and adjoining areas. Outcrop and subcrop shown coloured. Contours in metres. Heavy line indicates possible syndepositional fault, where thickness changes across the fault. The ‘Cleeve Hill Syncline’ (Buckman, 1901) is better regarded as a sedimentary trough than a fold structure (see text).

(Figure 4) Schematic cross-section of the Inferior Oolite Group of the district and the area to the east. Vertical exaggeration x200.

(Figure 5) Thickness of Birdlip Limestone Formation and extent of component members in the district and adjoining areas. Outcrop and subcrop is coloured. Contours in metres. Heavy dashed line indicates possible syndepositional fault, where thickness changes across the fault. * indicates location cited in the text.

(Figure 6) Correlation of cored boreholes on Cleeve Hill. See (Figure 3) for location.

(Figure 7) Thickness of the Aston Limestone Formation and extent of the component members in the district and adjoining areas. Outcrop and subcrop shown coloured. Contours in metres. Extent of Upper Trigonia Grit Member of Salperton Limestone Formation also shown. * indicates location cited in the text.

(Figure 8) Generalised cross-section from west to east through the Great Oolite Group, illustrating lithostratigraphical and facies relationships and thickness variations.

Plates

(Front cover) The face of Cleeve Cloud [SO 984 256], overlooking Cheltenham, exposes the Birdlip Limestone Formation of the Inferior Oolite Group. It was formerly a building stone quarry, and the slopes below are covered in degraded limestone spoil heaps. The escarpment here is crowned by an Iron-Age hill fort. (Aerofilms 599571).

(Plate 1) Surface gull seen from the top of Broadway Tower. (GS 1047).

(Plate 2) The basal Birdlip Limestone Formation strata exposed at Cleeve Cloud (1981). The Leckhampton Member (0.6 m seen) forms the notch at the base, overlain by the lower part of the Crickley Member (total 6.9 m). (GS 1049).

(Plate 3) Broadway Quarry [SP 117 366]. Eastern face, showing the Gryphite Grit Member (about 6 m) on the Harford Member (7.7 m), on the Scottsquar Member (about 2 m seen). The conspicious dark beds are mudstone at the top of the Harford Member. (GS 1045).

(Plate 4) Notgrove Railway Cutting [SP 0845 2090]. Upper Trigonia Grit (about 1 m) resting on Notgrove Member (1.8 m). Staff in 1 m long. (GS 1046).

(Plate 5) Rolling Bank Quarry. Section exposed following 1998 restoration and installation of information board; rubbly Clypeus Grit (about 3.9 m) resting on Upper Trigonia Grit (2.58 m), resting on Rolling Bank Member (4.96 m). (GS 1048).

(Back cover)

Tables

(Table 1) Geological succession of the Moreton-in-Marsh district.

(Table 2) General classification of bedrock formations in terms of engineering behaviour.

(Table 3) Typical soil associations of the district.

(Table 4) Lithostratigraphical classification of the Lias Group of the Moreton-in-Marsh district and relationship to the standard chronostratigraphical framework. Not to scale. Vertical ruling indicates non-sequence.

(Table 5) Lithostratigraphical classification of the Inferior Oolite Group of the Moreton-in-Marsh district, and the relationship to the standard chronostratigraphical framework, based on Barron et al. (1997, fig. 2). Not to scale. Vertical ruling indicates non-sequence. Ammonite symbol indicates zonally indicative ammonite fauna (see text).

(Table 6) Lithostratigraphical classification of the Great Oolite Group of the Moreton-in-Marsh district, and the relationship to the standard chronostratigraphical framework. Not to scale. Vertical ruling indicates non-sequence. Ammonite symbol indicates zonally indicative ammonite fauna (see text).

(Table 7) Chronostratigraphy of the Quaternary deposits of the Moreton-in-Marsh district.

Tables

(Table 1) Geological succession of the Moreton-in-Marsh district

(Table 2) General classification of bedrock formations in terms of engineering behaviour