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Geology of the Bedford district — brief explanation of the geological map Sheet 203 Bedford

A J M Barron, M G Sumbler, A N Morigi, H J Reeves, A J Benham, D C Entwisle, and I N Gale

Bibliographic reference: Barron, A J M, Sumbler, M G, Morigi, A N, Reeves, H J, Benham, A J, Entwisle, D C, and Gale, I N. 2010. Geology of the Bedford district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey.1:50 000 Sheet 203 Bedford (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2010.

Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham.

© NERC 2010 All rights reserved. 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.

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(Front cover) Cover Photograph Bromham Bridge on the River Great Ouse, built from local Blisworth Limestone. Photographer: A J M Barron (P700277).

(Rear cover)

(Geological succession) Summary of the geological succession in the Bedford district

Notes

The area covered by geological Sheet 203 Bedford is referred to as 'the district'. Ordnance Survey National Grid references are given in square brackets with the figures preceded by the letters SP or TL (or the digits 4 or 5 and 2: [508620 243330] in eastings and northings on maps) to indicate the 100 km grid square. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map. The serial number given with the plate captions is the registration number in the National Archive of Geological Photographs, held at the BGS. Boreholes are identified by the BGS Registration Number in the form (SP95SW/9), where the prefix indicates the 1:10 000 scale National Grid sheet.

Acknowledgements

This Sheet Explanation was written by A J M Barron, M G Sumbler, A N Morigi, H J Reeves, A J Benham, D C Entwisle and I N Gale. N J P Smith contributed to the account of the concealed geology and the structure. The plate of Oxford Clay fossils was compiled by B M Cox. The typescript was edited by M A Woods and J E Thomas, the figures were drafted by the authors and fair-drawn by P Lappage and S Ward and page-setting was by A Minks and A Hill. The authors are grateful to Hansons Brick Products for access to Stewartby brickworks, to Waste Recycling Group, Bedfordshire County Council and Bedford Council for provision of data and to M J Oates for information and discussions. The co-operation of landowners in the district is also acknowledged.

The grid, where used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright. All rights reserved. British Geological Survey 100017897/2010.

Geology of the Bedford district (summary from rear cover)

An explanation of sheet 203 (England and Wales) 1:50 000 series map

(Rear cover)

This Sheet Explanation provides a summary of the geology of the district covered by geological 1:50 000 Series Sheet 203 Bedford.

The Bedford district lies in the south-east of the English Midlands, about 45 miles north of London, and topographically is a low, undulating plateau capped by Pleistocene glacial deposits. This plateau is dissected by the valleys of the River Great Ouse and its tributaries, floored by younger fluvial deposits. The underlying sedimentary bedrock deposits, mostly of Jurassic age, crop out on the slopes of the valleys. From its confluence with the River Ouzel in the south-west, the Great Ouse valley meanders to the northern edge of the district, then south-east to Bedford. Here its valley broadens and it is joined by several right-bank tributaries, which are misfit streams draining the broad Marston Vale.

The much-expanded ancient county town of Bedford lies in the east of the district. A large part of the new city of Milton Keynes, which was established in the late 1960s, lies in the south-west. The city incorporated the existing towns of Bletchley, Wolverton and Stony Stratford and several villages including Milton Keynes. To its north-east lies the town of Newport Pagnell. The remaining area is largely agricultural land dominated by arable farming and there are many pleasant villages especially along the Great Ouse valley.

In the south-east, the low-lying Marston Vale hosts the large brickworks at Stewartby, which utilises the Oxford Clay as its raw material. This industry has contracted somewhat in the last few decades but its chimneys and huge disused pits still form major features of the Vale. To the east of Bedford there are a number of large sand and gravel pits working the river terrace deposits. The south-east side of the Marston Vale is formed by a prominent escarpment — the 'Greensand Ridge'.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology and applied geology of the district covered by geological 1:50 000 Series Sheet 203 Bedford, published as a Bedrock and Superficial Deposits edition in 2010. The district lies in the south-east of the English Midlands, about 45 miles north of London and includes parts of Bedfordshire, Northamptonshire and Buckinghamshire.

Geological history

During the Early Palaeozoic and Early Devonian, shallow marine and then continental rocks were deposited across much of southern Britain. At this time, the Bedford district lay in the foreland region of the Midland Microcraton. These rocks were deformed, uplifted and eroded during the final, Acadian phase of the Caledonian orogeny. Subsequently, subaerial conditions were established, but in Late Devonian times, a transgression from the south deposited marine sediments across at least part of the district (Figure 1).

Throughout the Carboniferous, the district formed part of the generally emergent Wales–Brabant High, and much of the Devonian succession may have been eroded. However, sea level rose at times during the early Carboniferous, and marine carbonate was deposited across the district. Extensive uplift and erosion from late Carboniferous through to Late Triassic times produced a widespread stratigraphical break known as the Variscan Unconformity.

In the Mid to Late Triassic, aeolian and alluvial deposits (Mercia Mudstone Group) accumulated around the northern and western margins of the London Platform (part of a structural high known as the Anglo–Brabant Massif, which stretched from eastern England into Belgium), and these probably extended into the western part of the district. A transgression in the Late Triassic deposited shallow marine sediments of the Penarth Group from the west, overlapping the Mercia Mudstone. However, a subsequent regression to the north-west resulted in the erosion of much of the Triassic succession, and the district may have remained emergent into the Early Jurassic.

Shallow marine conditions were reestablished in the Early Jurassic when the district formed part of the East Midlands Shelf, which extended north from the London Platform towards the Humber. The subsequent history of Jurassic and Cretaceous sedimentation in the Bedford district was profoundly influenced by the interplay of sea-level change, uplift and subsidence on the London Platform. Conditions in the district varied from open marine through shallow, protected lagoons, to paralic and alluvial environments. These led to deposition of sediments ranging from mud, silt and sand to bioclastic and peloidal carbonates, many highly fossiliferous and some iron-rich (Lias, Great Oolite, Ancholme and Lower Greensand groups). At times, subaerial erosion removed parts of the succession; there are no deposits of Aalenian to early Bajocian age, and the Late Cimmerian Unconformity between the Late Jurassic (latest Oxfordian) and Early Cretaceous (Late Aptian), represents a time gap of 40 million years (Figure 2). A protracted period of erosion was initiated by regional uplift of the British Isles in the Late Cretaceous and early Palaeogene, and later Neogene uplift and gentle deformation related to the distant effects of the Alpine Orogeny. These events removed bedrock strata down to the Early Cretaceous in the Bedford district.

Early Pleistocene climate cooling culminated in the onset of glacial conditions across Britain, resulting in the emplacement of till and deposition of glacial outwash. After the retreat of the ice-sheet, alternating episodes of fluvial downcutting and aggradation resulted in the formation of river terrace deposits. Late Pleistocene periglacial processes affected some earlier Quaternary sediments resulting in a cover of head deposits. Following postglacial sea-level rise during the Holocene, the fine-grained alluvium of the modern river floodplains was laid down in channels in the youngest river terrace deposits.

History of research

In the past, away from the extensive sections in the brick pits, the bedrock formations and superficial deposits were rarely exposed, and consequently little has been published on the geology of the Bedford district. A C G Cameron undertook the primary detailed geological survey at 1:10 560 scale at the end of the 19th century. Subsequently, H B Woodward and W J Arkell gave accounts of some railway cuttings and quarries.

For the past seventy years, fine exposures of the Oxford Clay Formation have been available in the large brick pits of the London Brick Company (now Hanson Brick Products) in Marston Vale, although the sections are subject to considerable change in extent and accessibility. Generally it is the lower part (Peterborough and Stewartby members) that is visible, but in places the upper part (Weymouth Member) is seen. Detailed descriptions have been published, notably in the 1960s by J H Callomon.

Before the new city of Milton Keynes was built in the late 1960s, the BGS (then IGS) geologically surveyed and reported on the development site. This task included processing data for thousands of boreholes. In 1986, the BGS were commissioned by UK NIREX Ltd to report on the geology of the Elstow Storage Depot. There too, many boreholes were sunk and logged, and the surrounding area was geologically mapped in detail. In 1991, Marston Vale was again the subject of a commissioned geological study by the BGS involving cored boreholes, surveying and modelling. The full resurvey of the district at 1:10 000 scale recommenced in 1999 and was completed in 2005.

Chapter 2 Geological description

Very little is known about the lithologies and thicknesses of the pre-Jurassic rocks within the district and inferences have to be drawn from the wider region. There are no boreholes deeper than 300 m within the Bedford district, and only one, Elstow A (TL04SW/317) [TL 04638 44289]), reaches probable Palaeozoic rocks (Figure 1). One seismic reflection profile was acquired by Shell within the south-east part of the Bedford district, but is not well constrained by boreholes.

The geological succession of the district can be subdivided into three major tectonostratigraphical units. The oldest comprises Precambrian and Early Palaeozoic strata, capped by the late Caledonian (Acadian) Unconformity. Above this are Devonian and Carboniferous strata, which are separated by the Variscan Unconformity from the youngest, post-Carboniferous interval.

Precambrian to Lower Palaeozoic

The Precambrian succession probably comprises deformed volcanic rocks like those seen in the Withycombe Farm Borehole ((SP44SW/9) [SP4319 4017]) in Oxfordshire. In the Bedford district, these are unconformably overlain by Lower Palaeozoic strata that on seismic evidence, include intervals belonging to the Cambrian, earliest Ordovician (Tremadoc) and younger Ordovician. South-eastward-dipping Silurian strata may occur in the south-east of the district (Figure 1). Tremadoc rocks, comprising deformed dark grey and green sandstone and siltstone, were proved in a borehole at Wyboston ((TL15NE/2) [TL 1759 5723]), and by analogy with districts to the north (Herbert et al., 2005) other parts of the Cambrian and Ordovician succession may be lithologically similar.

Upper Palaeozoic

Above the Acadian Unconformity (Figure 1), structural data suggest that more than 400 m of Upper Devonian strata may be present in the district. About 110 m are present in the Wyboston Borehole, comprising a basal conglomerate overlain by red and green mudstones, thin sandstones and minor limestones. Elstow Borehole A proves 86 m of interbedded red sandstone, siltstone and mudstone, inferred on regional evidence to belong to the Upper Devonian, but a Carboniferous or Triassic age is also possible. Lower Carboniferous dolostone and mudstone were proved in the Wellingborough district (Barron et al., 2006), and probably extend into the present district (Figure 1), perhaps attaining 50 m in thickness.

Triassic

Above the Variscan Unconformity Triassic strata are probably restricted to the west of the district, dipping and pinching out to the south-east. They may represent the Mercia Mudstone and Penarth groups, and comprise mudstone, siltstone, limestone, sandstone and conglomerate perhaps attaining 50 m in total thickness.

Jurassic

Jurassic strata comprise mudstones, limestones and thin sandstones in the lower part and predominantly mudstones in the upper part. They represent a range of depositional environments: shallow marine in the Early Jurassic; shallow marine, lagoonal, estuarine and freshwater in the Mid Jurassic and deeper marine in the Late Jurassic. Deposition of the succession was interrupted by erosion of much of the Whitby Mudstone, and Jurassic strata post-dating the Ampthill Clay were removed by a major erosion event associated with the formation of the Late Cimmerian Unconformity.

Lias Group

The Lower Jurassic Lias Group (Li) is present at depth throughout the district (Figure 2), resting unconformably on Triassic beds in the west and Palaeozoic strata in the east. The group is about 175 m thick in the north-west but thins to 25 m towards the Islip Ridge (Figure 1) in the south-east and may overlap the ridge culmination. The lowest unit is the Charmouth Mudstone Formation, represented by grey mudstone, fossiliferous in parts and with minor limestone beds and pebbly sandstone at the base. Its thickness ranges from an estimated 120 m in the north-west to about 25 m in the south-east.

Beds of very silty grey mudstone with siltstone nodules proved in boreholes beneath the Marlstone Rock near Newport Pagnell probably represent the Dyrham Formation (formerly Middle Lias), which may attain 15 m in the district. South of Bedford similar strata in boreholes at this level are generally less silty and less distinguishable from the underlying Charmouth Mudstone.

The oldest strata at outcrop in the district belong to the Lower Jurassic Marlstone Rock Formation (MRB) seen in the floor of a gravel pit at Great Linford [SP 849 430]. Boreholes in the area show it to be between 1 and 2.5 m thick and to consist of dark greenish grey ferruginous limestone that is shell-rich and bioturbated, with a sharp basal contact marked by a pebble bed. It resembles the formation seen at outcrop to the west in Oxfordshire and in boreholes south of Bedford; in the latter area it includes limestone and intraclasts but is less than 0.5 m thick, and it may be thinner or absent further to the east and south.

The Whitby Mudstone Formation (WhM; formerly Upper Lias) overlies the Marlstone Rock throughout the district. It comprises a thin basal sequence of fossiliferous calcareous mudstones and limestones, overlain by sparsely fossiliferous grey mudstone, weathering to grey-brown clay, described in detail by Horton et al. (1974). Its thickness ranges from perhaps 40 m in the north-west to as little as 1 m proved in boreholes in the south, and it may be absent in the extreme south-east. This and local rapid thickness changes probably result mainly from erosion, including channelling, prior to the deposition of the Middle Jurassic strata.

Inferior Oolite Group

To the north and west of the Bedford district, in early Mid Jurassic (Aalenian) times, the ooidal ironstone and ferruginous limestone and sandstone of the Northampton Sand Formation were laid down under shallow marine conditions (Barron et al., 2006). The Bedford district was outside this depositional area, and instead experienced subaerial erosion of the upper beds of the Whitby Mudstone, with the development of seatearths, rootlet horizons and secondary sphaerosiderite. However, at times deposition may have extended further south to result in a thin representative of the Northampton Sand being present locally at depth.

Great Oolite Group

Throughout the Bedford district the Whitby Mudstone is non-sequentially overlain by the Great Oolite Group (GtO) — a succession of predominantly marine sedimentary rocks, divided in ascending order into the Rutland, Blisworth Limestone, Blisworth Clay, Forest Marble and Cornbrash formations (Figure 2). The thickness of the Great Oolite Group ranges between 17 and over 30 m (Figure 3), increasing westwards. In places around Bedford town it has not been possible to subdivide the Great Oolite Group, partly due to masking by superficial deposits, but also as a result of lithological and thickness changes of the component formations (see below).

Rutland Formation

The Rutland Formation (Rld; formerly 'Upper Estuarine Series'), of latest Bajocian to Bathonian age, comprises a succession of mudstone, limestone, siltstone and sandstone arranged in up to seven sedimentary rhythms. However, significant lateral variations in sedimentary environments and contemporary erosion mean that there is considerable variability in the development of individual units. The formation's total thickness lies generally between 4 and 16 m but it may exceptionally attain 22 m. The shelly macrofauna is dominated by bivalves, abundant in some beds, including low salinity-tolerant forms, but the more marine beds also contain brachiopods.

The lowest rhythm is dominated by sandstone, and sandy and non-sandy siltstone and mudstone, and is generally widely developed at the base of the formation. Ascribed to the late Aalenian 'Lower Estuarine Series' (now Grantham Formation) by Horton et al. (1974) in the Milton Keynes area, these beds are now assigned to the Stamford Member (St). The member persists at depth across the district, and is generally around 3 to 7 m thick. Locally its thickness may attain 12 m and elsewhere it is less than one metre or absent; the variation is due in part to deposition on an uneven surface of the Whitby Mudstone Formation. The member shows rapid vertical and lateral changes in lithology and thickness (Figure 3) and is characteristically pale grey, dark grey and purplish when fresh, or brownish when weathered. Root traces, lignitic debris, and secondary ferruginous nodules are also typical as is a virtual absence of any other fossil material. Together these indicate a largely nonmarine origin, probably in a fluvial or lacustrine environment.

Above the Stamford Member, the Rutland Formation is well bedded with shelly faunas at many levels and strata weather to pale to mid grey-brown. It ranges in thickness from 1 to 13, but is typically 6 to 10 m thick. The calcareous nature of many of the beds indicates a much stronger marine influence. Unlike the Stamford Member, the higher rhythms display a distinct shallowing-upward pattern from marine or brackish shelly mudstone and/or sandstone, through delta-top barren mudstone to greenish rootletted noncalcareous mudstone, representing saltmarsh deposits.

In the Milton Keynes area, the succession above the Stamford Member contains a widespread bioclastic limestone-dominated unit (formerly the 'Upper Estuarine Limestone'). This limestone, now termed the Wellingborough Limestone Member (We), formed the middle of a three-fold subdivision of the higher part of the Rutland Formation recognised by Horton et al. (1974, p.14). The Wellingborough Limestone includes subordinate calcareous mudstone and sandstone, and is typically 3 to 5.5 m thick. To the south-west (Buckingham district; Sumbler, 2002) the member becomes ooidal and lacks mudstone beds and is named the Taynton Limestone, typically developed in the Cotswolds. In the Stewartby–Bedford area, boreholes show the presence of substantial thicknesses of shelly, bioclastic and sandy limestone in the higher part of the Rutland Formation. These appear at several levels but are laterally impersistent, and none can be confidently attributed to the Wellingborough Limestone. Nonetheless, their presence suggests prolonged or repeated marine incursions into this area.

Blisworth Limestone Formation

In contrast with the mud-rich, largely nonmarine Rutland Formation the conformably overlying Blisworth Limestone Formation (BwL, formerly Great Oolite Limestone, see Woodward, 1894, pp.382–396), is a marine limestone-dominated facies that weathers to white blocky and platy brash in soil. The formation is equivalent to much of the White Limestone Formation of Oxfordshire, from which it is distinguished by its higher silicate clay and sand content, suggesting a greater terrigenous influence. The transition between the formations is regarded as occurring in the Milton Keynes area (Figure 3).

The Blisworth Limestone is 6 to 9 m thick in the south-west and 11 to perhaps 14 m thick in the north and east (Figure 3). It was formerly worked for building stone in quarries across the district (Front cover), but is now rarely exposed. When exposed, it can be divided into two members. The lower part of the succession, known as the Roade Member (Cox and Sumbler, in press) is exposed in a small pit on the edge of Olney [SP 8791 5104] where pale yellow bioclastic peloidal limestone is visible, interbedded with brown very shelly mudstone packed with small oysters and rhynchonellid brachiopods, totalling about 2 m. These are the '(Kallirhynchia) Sharpi Beds' of Torrens (1980), named after the commonly found eponymous brachiopod. The member is widely developed to the west around its type section, Roade Cutting (Sheet 202; Cox and Sumbler, 2002) and is recognised further to the south-east near Bedford. Above, the beds are dominated by pale grey to yellow, peloidal, ooidal and bioclastic packstone- and grainstone-textured limestone in thin, uneven beds with some interbedded lime-mudstone. These closely resemble the Ardley Member of the White Limestone of Oxfordshire, to which they are attributed, and are well exposed at an active quarry at Weston Underwood (Plate 1) [SP 862 514]. Here, small-scale cross-bedding is seen in some beds (as it is at Roade Cutting) and Woodward (1894, pp.393–396) also noted cross-bedded lithologies, which he likened to the Forest Marble, within the 'Great Oolite Limestone' in pits as far east as Bedford (e.g. [TL 037 490]) (but see below). The formation displays traces of bioturbation throughout, and its abundant macrofauna includes many bivalve varieties (see Horton et al., 1974; Barron et al., 2006, pl. 3) , brachiopods, gastropods, corals, and, in the Ardley Member and its lateral correlatives beyond the district, rare ammonites, indicative of a late Bathonian age.

Blisworth Clay Formation

Through most of the district, the Blisworth Limestone is overlain by a conspicuous mudstone unit named the Blisworth Clay Formation (BwC, formerly Great Oolite Clay, see Woodward, 1894, pp.384–385), typically 3 to 6 m in thickness. Brightly coloured greenish grey, purplish grey, red and yellow mottled, smooth textured, sheared mudstone is typical. The bulk of the formation is noncalcareous, devoid of marine fossils, and contains disseminated cuboidal pyrite and black carbonaceous plant material. It is interpreted as having been deposited in a saltmarsh-type environment undergoing periodic emergence and plant colonisation leading to the repeated development of soil layers. There is usually a sharp contact with the underlying Blisworth Limestone, but calcareous mudstone and fine-grained limestone beds with marine fossils are locally present at the base beneath the western part of the Marston Vale, indicating that the transition up from marine to paralic conditions was less rapid.

Forest Marble Formation

The Forest Marble Formation is a shelly marine limestone facies that regionally passes eastwards into the nonmarine mudstones of the upper part of the Blisworth Clay. The complex pattern of lateral facies change is demonstrated by newly confirmed occurrences of Forest Marble (included within the undivided Great Oolite Group). These occur from the eastern Marston Vale, through western Bedford, as far as Stevington. Across this area a sequence of limestones with minor calcareous mudstones, 0.8 to more than 7 m thick, occurs above either Blisworth Limestone or Blisworth Clay (e.g. Field Farm Borehole (TL04SW/552)), and below Cornbrash. These occurrences support Arkell's (1933, p.305) suggestion of Forest Marble at a quarry near Newton Blossomville (not identified in the current survey), and some of Woodward's (1894, pp.393–396) earlier reports of Forest Marble facies in Blisworth Limestone. Although apparently spatially isolated, the Forest Marble of the Bedford district shows many similarities with the facies of the type area. The grey-brown weathering limestones are bioclastic packstone and grainstone with subordinate peloids and ooids, and pervasive bioturbation that includes many burrows with brown ferruginous infills that are clearly distinct from those of adjacent formations.

The occurrence of Forest Marble facies in the eastern Bedford district, together with the predominance of limestone in the Rutland Formation, and thickening of the Blisworth Limestone, indicates that conditions favourable to marine carbonate deposition persisted in this area throughout the Bathonian, implying that there may have been an embayment in the coastline of the emergent part of the London Platform hereabouts.

Cornbrash Formation

The Cornbrash Formation (Cb) is a thin unit of limestone at the top of the Great Oolite Group comprising up to perhaps 3 m of pervasively bioturbated pale to mid grey bioclastic packstone, with a small proportion of peloids and subordinate beds of grey bioclastic mudstone, deposited in a shallow marine environment. Bored surfaces and small pebbles and intraclasts with dark pyritic rinds indicate repeated breaks in sedimentation with erosion at non-sequences. The slight iron content of the formation results in the rock weathering to greyish brown in a distinctive reddish brown stony (brashy) soil. There is also a profuse fauna, notably of bivalves, and of brachiopods, from which a detailed zonation has been derived. On the basis of this, some subtle lithological and weathering characteristics, and a widespread non-sequence straddling the Bathonian–Callovian boundary (Figure 2), the Cornbrash can be divided into a lower and upper unit, although these are only really distinguishable in exposures. Both units are thought to be present in the district (Arkell, 1933, p.334; Horton et al., 1974, p.22). Reduced or nondeposition and intraformational erosion associated with nonsequences has resulted in variation in thickness to the extent that the Cornbrash is thought to be very thin or even absent locally west and north-west of Bedford, a phenomenon noted by Woodward (1894, p.451) and Douglas and Arkell (1932, p.128).

Ancholme Group

Kellaways Formation

The Kellaways Formation (Kys) overlies the Cornbrash throughout the district and is typically 5 to 7 m thick, although reducing locally in the west to as little as 3.5 m. The formation is moderately fossiliferous (Plate 2), and ammonite evidence from the wider region indicates that it is entirely early Callovian in age (Figure 2).

Over most of its outcrop, and in almost all borehole logs, the Kellaways Formation can be divided into a lower mudstone division — the Kellaways Clay Member and an upper sandy division — the Kellaways Sand Member.

The Kellaways Clay Member (KlC) is a thin (0.7 to about 2 m), slightly fissile, mid to dark grey smooth mudstone, weathering to smooth pale grey selenitic clay. The highest limestone bed of the underlying Cornbrash becomes increasingly argillaceous upwards and passes over a thickness of around 0.3 of shell debris-rich mudstone into the smooth mudstone of the Kellaways Clay. Above the shell-rich basal interval, the formation as a whole has a rather sparse fauna of bivalves, gastropods and ammonites, the latter generally preserved in pyrite. Pyrite may also occur as disseminated small grains, and the member is moderately bituminous. Fine sand and silt content increases upwards, evident in burrow fills and this may suggest breaks in sedimentation, or it may be distributed in more or less sandy and silty beds which may be lenticular. These sand-rich layers of the Kellaways Clay may be confused with the overlying Kellaways Sand, and this may explain greater variation of member thicknesses given in some previous accounts (e.g. Horton et al., 1974).

The Kellaways Sand Member (KlS) is typically 2.5 to 5.5 m in thickness and comprises mid greenish grey muddy fine-grained sandstone and sandy siltstone with interbeds of fine sandy mudstone. It is sparsely fossiliferous except for local beds with belemnites, bivalves, gastropods and plant material. The sandstone beds are generally very weakly cemented, grading to uncemented sand, and weather to form pale loamy soils; in places they may be cemented by calcite into hard tabular beds or layers of nodules ('doggers'). The latter tend to occur towards the top of the member and may form weak positive features, which along with the stoneless sandy soil, facilitate the mapping of the unit. The uppermost beds of the Kellaways Sand are sometimes visible in drainage ditches and sumps in the bottom of the brick pits in the Marston Vale (Plate 3).

Oxford Clay Formation

Above the Kellaways Formation, the remainder of the Jurassic succession is almost entirely composed of mudstone. Of this, by far the greater part comprises the Oxford Clay Formation (OxC), which, from boreholes, is known to be about 65 to 70 m thick. Data from adjoining areas suggests that within this range, there is a slight thinning along the outcrop, from south-west to north-east. Given the location of the district, on the northern margins of the London Platform (see Geological history), it is likely that there is also a component of thinning south-eastwards, towards this structural high, though there is insufficient data available within the Bedford district itself to confirm this.

The Oxford Clay forms the bedrock over much of the south-eastern part of the Bedford district (Figure 4). However, a major part of the outcrop is buried beneath Quaternary deposits, and exposure is generally very poor.

Despite this, different mudstone types can be distinguished in boreholes and sections, and with some support from a study of the contained fossils, notably ammonites which underpin the standard biostratigraphical zonation of the succession, this enables subdivision of the formation into three parts, each of which has somewhat different lithological and faunal characteristics. These three units, formerly known as the Lower, Middle and Upper Oxford Clay, are named respectively the Peterborough Member, Stewartby Member and Weymouth Member (Cox et al., 1992). The distribution of the three members within the Bedford district is shown in (Figure 4); this is partly based on surveyed boundaries, and partly from the inferred geological structure and so must be regarded as approximate, particularly where the boundaries lie beneath later deposits.

Peterborough Member

The Peterborough Member (Pet) is the best-known part of the Oxford Clay Formation, as it was formerly quarried on a large scale for brick manufacture at several sites in the Midlands, providing superb exposures which have been intensively studied and collected by generations of geologists and palaeontologists. It takes its name from the brick pits near Peterborough. It has also been quarried in extensive pits at Stewartby and Kempston to the south-west of Bedford, and good exposures remain in those pits where the strata were still worked until recently (Plate 3). Here, and in nearby boreholes, the member is about 23 m thick (Callomon, 1968), and a similar thickness is known from the former Newton Longville pit in the Leighton Buzzard district (Sheet 220) to the south. It is slightly thicker, at 26 m, at Calvert to the south-west (Sheet 219), and at Peterborough is slightly thinner at 17 m (Shephard-Thorn et al., 1994; Sumbler, 2002; Horton, 1989). Thus, as with the formation as a whole, it seems probable that there is a slight thinning north-eastwards across the Bedford district.

The Peterborough Member is characterised by brownish-grey, finely laminated, fissile mudstones which are rich in organic matter (kerogen) and often have a noticeably 'bituminous' smell when fresh. These features result from anoxic conditions in the sea-floor sediment during deposition of the member, with a consequent lack of burrowing organisms. Mudstones of this type dominate the lower part of the member and are the chief reason why it has been so favoured for brickmaking (see Applied Geology). They produce soils which are of a darker brown and more crumbly texture than those developed on other parts of the Oxford Clay. Interbedded with these 'bituminous shales' are subordinate beds of paler grey, blocky (i.e. not shaly) mudstone much like those found higher in the formation; the lack of the primary bedding structure is due to pervasive burrowing, indicating more 'normal' sea-floor conditions.

The fossil fauna of the Peterborough Member is made up largely of ammonites (chiefly kosmoceratids) and bivalves (such as nuculaceans and Meleagrinella: (Plate 2)). These are typically found crushed on bedding planes in the shales, preserved in white or iridescent aragonite. Another distinctive fossil found commonly in all but the lowest beds of the Peterborough Member, is the serpulid worm tube Genicularia vertebralis (J de C Sowerby) (Plate 2).

In addition, there are a number of fossil shell beds, a few centimetres thick, in which the fauna, mainly bivalves, is preserved as uncrushed pyrite casts. Several of these beds, which weather to a rusty brown colour in the pit faces (Plate 3), are laterally persistent and form useful markers. One, about 0.5 m above the base of the member (Bed 5 of (Figure 5)), generally forms the floor of the brick pits. Beneath this, the strata comprise silty mudstone containing Gryphaea dilobotes Duff and belemnites (Plate 2); these form a transition to the underlying Kellaways Formation, and are quite unsuitable for brickmaking. Another pyritic shell bed, about 5.5 m above the base (Bed 7), is overlain by a bed of septarian cementstone nodules which can be recognised at Newton Longville and Calvert: this is probably the Wendlebury Nodule Bed of the Thame district (Horton et al., 1995). As with other marker beds, its wide geographical distribution is testimony to the stable conditions that prevailed during deposition of the Peterborough Member.

About 14 m above the base, the Comptoni Bed and the Acutistriatum Band together form a distinctive pale grey marker bed in the faces of the brick pits, often marked by a line of seepage. The Comptoni Bed (Bed 15) comprises about 0.3 m of pale shaly mudstone or weak limestone which is packed with nuculaceans and ammonites including the eponymous Binatisphinctes comptoni (Pratt) (Plate 2). It is capped by another thin, pyritic shell bed, and then the Acutistriatum Band (Bed 17), a 0.3 m bed of similar pale grey shaly mudstone locally cemented into large doggers of cementstone up to 1 m or more in diameter. These large 'cartwheel' concretions, a metre or more in diameter, are unwanted in the brickmaking process, and are often discarded on the floor of the pits; they often contain many well preserved ammonites such as Kosmoceras acutistriatum (S S Buckman) (Plate 2), from which the bed is named.

The succession above, not generally well exposed, comprises about 9 m of interbedded more or less 'bituminous' shale and paler blocky mudstone, forming a transition to the succeeding Stewartby Member. Whilst Callomon (1968) drew the top of the 'Lower Oxford Clay' at a convenient pyritic shell bed (Bed 19 of (Figure 5)), it is now defined at the top of highest dark, organic-rich mudstone within the thinly interbedded lithological transition which is developed in the top part of the Peterborough Member (Cox et al., 1992).

Stewartby Member

The Stewartby Member (Sby) is named after the Stewartby brick pits in which it tends to form the overburden above the worked brick-clay beds of the Peterborough Member. In fact only the lower beds of the member are generally exposed but, at the Rookery Pit [TL 015 410], a face also showed the top part of the member in a down-faulted block (recorded by Callomon, MS 1978) and the section there forms the type section (Figure 5). These upper beds were also exposed in excavations at Millbrook [TL 005 390] (Callomon, MS 1968; see Cox, 1988). Combining these data the Stewartby Member is 22 m thick. A somewhat greater thickness of about 27 m recorded in a nearby borehole is probably largely explained by the somewhat arbitrary base of the member (see above). It seems probable that there is a slight north-eastward thinning across the district.

The Stewartby Member typically forms slightly steeper ground, with more relief than that developed on the Peterborough Member, and locally there may be a mappable break of slope at the base, as for example to the east of Stewartby. In addition, harder beds in the upper part of the member locally form fairly prominent hills, such as those between Wilstead [TL 06 43] and Houghton Conquest [TL 04 41].

By contrast with the Peterborough Member, the Stewartby Member is composed very largely of variably pale to medium grey, smooth or slightly silty, slightly calcareous, blocky mudstone. At outcrop, these mudstones produce a mid greyish brown soil, much heavier and stickier than that on the Peterborough Member. The member is generally sparsely fossiliferous, although sporadic ammonites occur, generally preserved in pyrite (not aragonite); amongst the bivalve fauna, Bositra buchii (Roemer) is abundant at some levels (Plate 2).

The uppermost approximately 10 m of the member contain a number of thin (a few centimetres) beds of nodular cementstone or weak, calcareous siltstone (Figure 5) which seem to be responsible for the minor hills mentioned above. The strata at this level often contain Gryphaea lituola Lamarck (Plate 2), which may be found in the soil on the outcrop. Another important faunal marker within this succession is the button coral Trochocyathus(Plate 2), which occurs about 7.5 m below the top of the member at Millbrook, and has been recorded in boreholes elsewhere. The topmost bed of the Stewartby Member is the Lamberti Limestone, which is stratigraphically important in that it also marks the Callovian–Oxfordian stage boundary, and the top of the Middle Jurassic Series. It is a pale grey to cream, weakly cemented 'marly' limestone 0.2 to 0.3 m thick. It is generally packed with solid internal moulds of bivalves, gastropods and ammonites often with brownish pyritous coatings. The fossils include the zonal ammonite Quenstedtoceras lamberti (J Sowerby) (Plate 2), after which it is named. Within the Bedford district, the outcrop of the overlying Weymouth Member (Wey) is almost entirely restricted to a narrow strip on the higher and steeper slopes of the Marston Vale (Figure 4). Additionally, there is a very small downfaulted outlier at Rookery Pit [TL 020 413]. Where not obscured by solifluxion material from upslope, the Weymouth Member forms a grey and fawn clay soil.

The whole of the member was exposed during construction of the Millbrook test track [TL 005 390] (Callomon, MS 1968; see Cox, 1988), in which it proved to be 21.3 m thick (Figure 5), much the same as in adjoining districts, in which it generally seems to be about 20 m. Intriguingly, however, the BGS Ampthill Borehole [TL 0244 3804] only 2 km to the south-east, proved 27.4 m of Weymouth Member without bottoming the unit ((Figure 5); Cox, 1988; Shephard-Thorn et al., 1994). In the absence of any evidence of repetition of the succession in the borehole by faulting, this anomalous thickness is hard to explain. It may be a localised phenomenon, perhaps related to syndepositional growth-faulting, but another possibility is that part of the succession, probably in the lower part of the member, is unrepresented in the Millbrook section as a result of landsliding, which is common on the steep slopes of the Marston Vale.

Weymouth Member

The Weymouth Member is largely composed of pale grey, mostly smooth-textured, rather calcareous blocky mudstones, in which thin, nodular weakly cemented calcareous siltstones or cementstones are developed. As such it is much like the Stewartby Member, but, by contrast, it also contains fairly common, thin units of darker grey, more organic-rich mudstone. These form the basal unit of sedimentary rhythms (cf. the succeeding West Walton Formation), and typically rest sharply on pale mudstones, the contact often exhibiting striking colour-mottling due to interburrowing. They pass upwards gradually into the more usual pale grey mudstone. Fossils are not generally abundant: ammonites and most bivalves are preserved in pyrite and soon weather away in exposures. However, belemnites, notably small Hibolithes (Plate 2), and Gryphaea, may be quite common, and being composed of calcite they are much more resistant to weathering and may sometimes be found in the soil. Gryphaea fossils are particularly common in the upper part of the member, they belong to the species G. dilatata J Sowerby (Plate 2), which is substantially larger and less incurved than the species found lower in the succession.

West Walton Formation

The West Walton Formation occurs above the Oxford Clay throughout eastern England. With the lower part of the succeeding Ampthill Clay, it is the approximate equivalent of the limestone and sandstone succession of the Corallian Group developed to the south-west between Oxford and Swindon. The West Walton and Ampthill Clay formations (WWAC) cannot be differentiated on the map.

The lower part (6.4 m) of the formation was exposed beneath Early Cretaceous Woburn Sands Formation in the Millbrook test track section [TL 005 390], and the whole formation, 11.92 m thick, was cored in the BGS Ampthill Borehole (Callomon, MS 1968; Cox, 1988; Shephard-Thorn et al., 1994; (Figure 5)). This thickness is intermediate between the approximately 15 m typical in the Thame and Buckingham districts (Horton et al. 1995; Sumbler 2002) and the 10 m in the adjoining Biggleswade district (Moorlock et al., 2003), again suggesting a gradual north-eastward thinning of the succession across the Bedford district.

The West Walton Formation comprises a succession of calcareous, typically rather silty mudstones. Pale grey cementstone or calcareous siltstone bands and nodules are particularly well developed in the lower 5 or 6 m. Mudstones at this level are often packed with Lopha, Nanogyra nana (J Sowerby) and serpulids , which often form cemented aggregates, together with large Gryphaea dilatata (Plate 2). This abundant calcitic fauna may be found in the rather 'marly' soil of the outcrop. These basal beds (beds WWF 1–12 of Cox and Gallois, 1979) correspond with the Elsworth Rock Member of the Biggleswade and Huntingdon districts (Moorlock et al., 2003). Some beds of dark carbonaceous mudstone also occur, notably at the base of the formation. The higher parts (WWF 13–16), as recorded in the BGS Ampthill borehole (Figure 5), are mainly alternating pale and medium grey mudstones, with a few bivalves and ammonites, but generally lacking the distinctive fauna of the lower beds. The strata weather to pale grey brown silty clay.

Ampthill Clay Formation

The youngest Jurassic strata present in the Bedford district belong to the Ampthill Clay Formation. This unit is very poorly exposed; the distribution shown in (Figure 4) is at best an approximation. However the formation is known to occur in the Ampthill railway cutting and tunnel [TL 020 384], the cutting being the type locality, where the formation's presence was first indicated by collections of fossils made by Seeley (1869) during its construction (see Woodward, 1895).

The type section was never described in detail and is now obscured, and little was known about it until the cored BGS Ampthill Borehole [TL 0244 3804] was drilled in 1970 (Cox, 1988; Shephard-Thorn et al., 1994). This proved 12.62 m of Ampthill Clay beneath Woburn Sands Formation representing the lower part (AmC 1–15) of the full succession (AmC 1–42; Cox and Gallois, 1979).

The Ampthill Clay, as proved in the borehole, comprises interbedded dark and medium grey mudstone and silty mudstone, often showing interburrowing much as in the West Walton Formation. In the upper few metres, pale grey calcareous, generally smooth mudstone dominates. The fauna is mainly of bivalves and ammonites, but belemnites and oysters, notably large Gryphaea dilatata, also occur. The strata weather to pale grey-brown clay.

Cretaceous

The succession in adjacent regions (e.g. Horton et al., 1995) suggests that marine sedimentation continued in the Bedford district throughout the Late Jurassic with deposition of a substantial thickness of Kimmeridge Clay and possibly of Portland and Purbeck strata. However, uplift in latest Jurassic and earliest Cretaceous times led to regression of the sea, and subaerial erosion of the Jurassic of the interior and margins of the London Platform. With rising sea levels in late Early Cretaceous times, marine sedimentation became re-established around the London Platform, whilst the central part, now stripped down to the Palaeozoic basement, provided a source of sandy sediment which was transported by rivers into the surrounding shallow seas to form the Lower Greensand Group.

Woburn Sands Formation

The local representative of the Lower Greensand Group is the Woburn Sands Formation (WbS), named from the town of Woburn Sands [SP 92 35] in the adjoining Leighton Buzzard district, where the formation is up to 120 m thick (Shephard-Thorn et al., 1994). Up to 35 m of Woburn Sands occur in the Bedford district although the presence of inliers of Jurassic strata south of Haynes, near the eastern margin, suggest that the total thickness beneath the Gault there may be as little as 15 m. The formation rests on the Ampthill Clay or West Walton Formation, but apparently cuts down to the Weymouth Member of the Oxford Clay, both west of Millbrook and probably also in the south-east corner of the district (Figure 2) and (Figure 4).

The lower part of the succession ('Lower Woburn Sands', Shephard-Thorn et al.,1994) is made up of fine-grained generally uncemented quartz sand which at depth (as in the BGS Ampthill Borehole), retains a greenish colour due to glauconite which coats the sand grains. At outcrop, the sand invariably weathers to produce a yellow or ochreous quartz sand, often patchily cemented by dark brown limonite into a more or less hard sandstone. The limonite (derived from oxidation of glauconite) tends to occur in thin layers and lenticles, and fragments of this material occur commonly in the soil of the outcrop. The basal bed of the formation is much coarser, and often contains quartzose and phosphatic pebbles, which again are common in the soil. The phosphatic material includes internal casts of Jurassic fossils reworked from the subjacent strata. In the Millbrook section, this pebbly bed was 0.3 m thick with sand-infilled fissures extending up to 0.9 m in the subjacent Ampthill Clay. Fossils included bivalves and ammonites such as Pavlovia (Callomon, MS), indicating derivation from the Kimmeridge Clay, which is otherwise unrepresented in the district.

Beds which form the ground south of Haynes, perhaps rather higher in the succession, are very different in character. They are generally very coarse, gritty sands which are locally cemented by limonite into a hard 'carstone', much like the 'Upper Woburn Sands' of the Leighton Buzzard district (Shephard-Thorn et al., 1994).

Gault Formation

Grey mudstones of the Gault Formation (G), up to 10 m thick, rest sharply and somewhat discordantly (Shephard-Thorn et al., 1994) upon the Woburn Sands. The basal beds occur in the south-east corner of the Bedford district [TL 110 396] and where seen by augering, comprise pale to dark grey, slightly silty clay with reddish brown ferruginous streaks or laminae (oxidised glauconitic silt), and chips of dark brown phosphatic material. Such silty mudstone beds are typical of the base of the Lower Gault (mid Albian). They pass up into dark, fissile, less silty mudstone. The strata weather to grey to brown clay.

Quaternary

Pre-Anglian fluvial deposits in the adjacent districts of Northampton, Wellingborough and Towcester are known as the Milton Formation (Belshaw et al., 2005; Barron et al., 2006). They comprise locally sourced gravel mixed with sand derived from the Triassic sandstones of the West Midlands. The deposits have the characteristics of braided stream sediments and infill channels cut into the bedrock. The trend of some of these channels is such that they may be expected to enter the Bedford district from the west or north-west near Yardley Hastings (Barron et al. 2006, fig. 4) but they have not been recorded. Instead, thick glacial deposits infill a buried valley in that region, suggesting that Milton Formation sediments may have been removed by glacial erosion.

Glacial deposits

Two glacial tills have been mapped in this district: a distinctive lower till (the 'Lower Boulder Clay' of Hollingworth and Taylor, 1946) that has been named the Bozeat Till (Barron et al., 2006) and an upper till that is correlated with the Oadby Member, (formerly known as the Oadby Till) of the Wolston Formation. In Rutland, the Bozeat Till shows a transport direction from the north, and is separated from the Oadby Member transported from the east by a palaeosol containing a cool temperate flora (Prof. J Rose, Royal Holloway College, oral communication, June 2008). The two tills clearly represent discrete glacial events although their identities remain uncertain (see below).

Bozeat Till

The Bozeat Till is a dark bluish grey diamicton consisting of silty clay with clasts mainly of Jurassic limestone and ironstone, some quartz and quartzite, derived Jurassic fossils, rare flint and, very rarely, chalk. It occurs extensively in the adjacent Wellingborough district (Barron et al, 2006), and also in the Buckingham and Towcester districts where a thickness of over 24 m, including sizable rafts of Jurassic strata, was proved in a borehole (Horton, 1970; Horton et al., 1974; Sumbler, 2002). It has been mapped only in the north-western part of the Bedford district around Yardley Hastings [SP 86 57] and in the south-west near Haversham [SP 82 43] to [SP 82 46]. Horton (1970) recorded similar till up to 6 m thick below 'Chalky Boulder Clay' in boreholes north-west of Newport Pagnell.

Oadby Member

The Oadby Member is typical of what used to be known as the 'Chalky Boulder Clay' of central and eastern England. Traditionally it has been referred to the Anglian Stage, Marine Isotope Stage (MIS) 12 (Bowen, 1999), but other workers have suggested a younger MIS 10 age (e.g. Sumbler, 1995). The till is an olive-grey to dark grey diamicton that weathers to yellowish brown. It comprises silty, variably sandy clay with abundant clasts of chalk, flint, Jurassic limestone, sandstone and ironstone, quartz, quartzite, and Carboniferous limestone and sandstone, the latter often as large rounded clasts. More exotic clasts have also been recorded including dolerite, tuff, schist, gneiss and granulites (Sabine, 1949). The silt and clay content of the till is derived almost wholly from Jurassic mudstone formations. The till is present throughout the district with its base generally lying between 80 and 85 m OD. In the central and north-eastern parts of the district the till is about 20 to 25 m thick whilst in the south-eastern sector it is a little thinner at about 10 to 15 m. However, in the north-west at Yardley Chase a borehole near Horton ((SP85NW/41) [SP 8294 5540]) proved about 60 m of till (and associated glaciofluvial deposits) with an unusually low base at about 51 m OD. A nearby borehole (SP85SW 6 [SP 8442 5470]) encountered the base of the till at 71 m OD resting on 14 m of glaciofluvial deposits. Taken together these boreholes indicate the presence of a buried valley (see below).

Glaciolacustrine deposits

Glaciolacustrine deposits have been mapped in the south-west in the Ouzel valley, and have been proved in boreholes at Caldecote [SP 878 423] where they infill a depression (Horton et al., 1974). They also occur some 25 metres higher on the slopes near Great Linford [SP 852 417]. Glacial lakes beneath the fluvial sediments of the Great Ouse valley, just to the west of the district at Stony Stratford, and in the Ouzel valley south of Newport Pagnell, have been interpreted as infilling a buried channel network (Horton, 1970; Horton et al., 1974). The succession is up to 12 m thick and includes thick units of grey and brown laminated (varved) clay and silt or fine sand, interbedded with diamicton. Similar deposits further south are more than 40 m thick (Shephard-Thorn et al., 1994).

Glaciofluvial deposits

Glaciofluvial deposits comprise brownish yellow sand and gravel with clasts of Jurassic limestone, sandstone and ironstone, flint, quartz and quartzite and locally chalk. They occur as sparsely distributed discontinuous bodies resting on, within and beneath the glacial tills, and represent outwash deposits of the ice sheet that deposited the tills.

Buried channels

Horton (1970) considered that the varied sequences of glaciolacustrine deposits, till and glaciofluvial deposits described above (Glaciolacustrine deposits) filling buried channels, represented a complex sequence of ice marginal events in which proglacial lakes formed between repeated minor advances of the ice sheet. However, the fact that they are discontinuous suggests that they are infilled tunnel valleys. At Yardley Chase, several boreholes encountered exceptional thicknesses of glacigenic deposits filling a depression beneath the plateau, including over 60 m of deposits at Tarts Barn (SP85NW 41 [SP 8294 5540]) reaching to a level some 30 to 40 m below what might be expected from outcrop evidence.

River terrace deposits

Sand and gravel deposits occurring along the valleys of the River Great Ouse, its major tributary the River Ouzel and their minor tributaries represent river terrace deposits. They make up the Ouse Valley Formation, and have been mapped on the principle that they underlie flattish 'benches' or terraces on the valley sides that can be grouped into one of three levels on the basis of their elevation above the present-day floodplain. Traditionally it has been assumed that the highest of these terraces is the oldest and the lowest the youngest, and although it remains generally true, this simplicity belies the complexities revealed by detailed studies of the deposits of the Great Ouse. These have shown that deposition was almost certainly multiphase and probably multistage through a range of climatic conditions. Sand and gravel units were deposited by braided rivers under periglacial conditions while silt and clay typically fill channels cut by meandering rivers during temperate climate episodes. Put simply, the result is a sand and gravel 'sandwich' with silt and clay 'filling' (for a full account of river terrace formation see Bridgland, 2000). The surface of the lowest terrace, the Felmersham Member ( Fe ), is between 0.6 m and 2 m above the floodplain. It comprises planar-bedded brownish yellow sand and gravel, about 3 m thick, in which the gravel fraction is composed mainly of flint and limestone with quartzite, sandstone, chalk, ironstone and chert. Bones of horse, deer, ox, mammoth and rhinoceros were found in the terrace deposits exposed in excavations related to the railway near Radwell [TL 005 575] (Wyatt, 1861) and a similar assemblage plus Hippopotamus amphibus Linné has been recorded from the Felmersham Member some 3 km east of Bedford [TL 084 497] (Stuart, 1982). In a former pit near Radwell, a fossiliferous sandy silt infilling a channel cut into the Felmersham Member, named the Radwell Member, contains plant, mollusc, ostracod and beetle remains indicative of cool temperate conditions (Rogerson et al., 1992). The Radwell Member is overlain by a further metre of sand and gravel. Rogerson et al. (1992) interpreted this sequence as accumulating under periglacial conditions punctuated by a warmer interstadial phase during which the channel-fill was deposited. The age of the Radwell Member is uncertain because of conflicting evidence, but it is thought to date from late in MIS 5 (Bowen, 1999). It seems probable that both the Felmersham and Radwell members were deposited during the Early Devensian (Marine Isotope Substage (MIS 5d–a). The surface of the terrace underlain by the Stoke Goldington Member lies between 5 and 7 m above the floodplain and is up to 8 m thick (Bowen, 1999). The composition of the gravel is similar to that of the Felmersham Member, but with a significantly higher proportion of chalk in the upper part of the terrace. At the type locality, the basal unit of the deposit comprises planar-bedded sand and gravel. This was presumably deposited under periglacial conditions prior to MIS 7, since the Hartigan's Pit Member, comprising fossiliferous clay and sandy clay infilling a channel cut into it, has been referred to the warm MIS 7. This is overlain by a further sand and gravel unit assumed to have accumulated during the cold MIS 6, as the clay and sandy clay fill of a channel, the Ravenstone Member, that has cut into it, has been assigned to the warm temperate substage MIS 5e (Green et al., 1996). A chalk-rich sand and gravel unit that was probably deposited in an Early Devensian interstadial caps the sequence. Pollen, plant macrofossils, molluscs, insects, ostracods and rare vertebrate remains have been found in the terrace deposits (Green et al., 1996). The highest terrace, the Biddenham Member ( d ), some 11 to 13 m above the floodplain, is underlain by sand and gravel with a similar composition to the lower terraces and is up to 7 m thick. At the type site, the Spinney Pit [TL 023 503], fossiliferous clay layers within the sand and gravel deposits contained a temperate molluscan fauna including Belgrandia marginata (Michaud) (Bowen, 1999). Abundant Palaeolithic artefacts in mixed assemblages have been found towards the base of the terrace deposits (Prestwich, 1861; Harding et al., 1991). As with the younger river terrace deposits the Biddenham Member is the result of multistage deposition that probably occurred from late in MIS 10 to MIS 8.

Alluvium

The present-day floodplains of the rivers are underlain by alluvium comprising brown clay and silt up to 4 m thick, with peat locally. In the main river valleys, the alluvium occupies channels cut into the lowest river terrace deposit. Bowen (1999) has classified the alluvium as a member (Ouse Member) of the Ouse Valley Formation but on the map and in this account the term alluvium has been retained.

Head

Head represents superficial deposits formed by solifluction processes: mainly downslope mass movement of unconsolidated materials in past periglacial environments, but including rain wash and soil creep. It rarely exceeds 3 m in thickness and at least two generations of head are present in the Marston Vale. Isolated spreads of head that cap knolls and ridges are gravel-dominated and include some chalk, reflecting their partial derivation from till of the Oadby Member. These represent older head deposits that postdate the glacial deposits but predate the development of the present day drainage. They are probably dissected remnants of formerly more extensive, sheets of head. Wider spreads of younger head lie at a lower level than adjacent tracts of older head where they fill hollows and drape slopes. Younger head is generally finer-grained than the older head, consisting largely of brown slightly gravelly sandy clay, and may be crudely stratified. Where the deposits overlie soft-weathering clay-rich formations, such as the Oxford Clay, freeze-thaw processes during Quaternary periglacial episodes have commonly caused pods of the clay to migrate upwards into the head and pods of the coarser material to be displaced downwards (cryoturbation), resulting in a very uneven contact. The older deposits are likely to be more pervasively affected by this process. During the survey it was not always possible to separate the different generations of head: in such instances the deposits were mapped as head, undivided .

Elsewhere, head comprising mainly gravelly sandy clay, commonly floors the upper reaches of tributary valleys, and probably forms a veneer of deposits (too thin to be mapped) on lower valley slopes. Extensive spreads of head mantle the slopes of the Ouzel valley where they merge almost imperceptibly with the youngest river terrace deposit, the Felmersham Member (Horton et al., 1974).

Calcareous tufa

An apron-like spread of Calcareous tufa consisting of pale calcareous silt up to 2 m thick, was mapped to the south-west of Haversham at [SP 823 422], where springs issue from the base of the Blisworth Limestone or Wellingborough Limestone. The tufa was deposited from water saturated with calcium bicarbonate that percolated through the limestone formations and precipitated calcium carbonate upon emerging at these springs. Calcareous tufa is typically very soft and unstable if loaded.

Peat

Peat comprising dark brown to almost black organic silt occurs along the floodplain of the River Flit in the extreme southeastern corner of the district. It is part of an area of peat that extends along the valley between Flitton and Chicksands, in the adjacent Leighton Buzzard and Biggleswade districts (Shephard-Thorn et al., 1994). Borehole records indicate that 3 to 4 m of peat may overlie about 5 m of gravel.

Artificial ground

Mapped deposits of artificial ground are principally the legacy of industrial-scale brick manufacturing that worked the Oxford Clay Formation between Brogborough [SP 96 39] and Elstow [TL 04 46], and sand and gravel extraction from the Ouse Valley Formation, although there are a few old and generally infilled limestone quarries and smaller brick pits. The former brick pits are mostly disused or infilled with waste while some of the former sand and gravel workings now form lakes that are used for recreation and nature conservation.

Made ground is material that has been deposited by man on the pre-existing ground surface. It includes engineered fill such as brickyards, spoil heaps and landfill. Worked ground is indicated on the map where natural material has been removed by man and includes mineral workings (quarries and pits) and significant excavations for construction. Some areas of worked ground may have been partially back-filled. Road and rail embankments and cuttings are not shown. Infilled ground is worked ground that has been largely or completely backfilled with spoil or landfill to, or above, the level of the natural land surface, and may be restored. Some ill-defined areas of sand and gravel workings in the Great Ouse valley, which may be restored, landscaped or flooded, are shown as disturbed ground.

Structure

The two major stratigraphical breaks seen at depth in the district, the Acadian and Variscan unconformities, occur widely across the UK, and are the results of major episodes of orogenesis (mountain building). Thus strata above and below these surfaces are generally not only considerably different in age, but also in structure. Beneath the Acadian Unconformity, Precambrian and Lower Palaeozoic rocks are generally reported in the wider region as cleaved and sheared, with steep dips. Contours on the unconformities reveal the Islip Ridge (Charlton Axis), which is prominent as far north-east as Bletchley, but is concealed by the Upper Devonian basin extending west-north-west to east-south-east across the district (Figure 1). The ridge is also apparent on the gravity data (Figure 6) identified as GA25 by Busby et al. (2006), and may incorporate magnetic rocks ((Figure 7), part of MA24) close to the surface towards Milton Keynes. These are either intrusions in Tremadoc strata, or are Precambrian volcanic rocks. The Islip Ridge may extend as far as the line of the seismic reflection profile, based on a predicted level for the Variscan Unconformity in a borehole at Deadman's Cross ((TL14SW/6) [TL 112 419]). The gravity high which occurs at the south-west margin of the district (Figure 6), extends north-west to Nuneaton, and is caused by the Tremadoc and older rocks which lie closer to the surface compared to those areas (Northampton–Bedford) where Upper Palaeozoic strata are present (Figure 1). The relative gravity low that extends across the centre of the district reflects the Upper Devonian and Lower Carboniferous basin.

The structure of the rocks at outcrop is generally fairly simple; the Jurassic sequence is flat-lying or displays a gentle regional dip to the south-east of about 0.2 degrees, modified locally by faulting or flexuring. Faults shown on the 1:50 000 map have been located by mapping and/or proved by boreholes and geophysical investigations. A predominant north-north-west trend is evident in the west, including a prominent block-faulted structure through Milton Keynes [SP 87 38 to 85 43], showing fault throws of up to 13 m (Horton et al., 1974, pp.5–6, fig. 12). Further east, a well developed graben affects the Oxford Clay at Stewartby [TL 01 41], with fault displacements of up to 25 m. The positions of these two structures are locally well constrained by numerous borehole logs, and in the case of the Stewartby graben where excavation of the brick pits terminated at the fault planes. Elsewhere, trends are more variable, and a major east–west graben structure downthrows Kellaways and Oxford Clay formations north-west of Olney [SP 87 54 area]. Near Warrington a fault has been mapped at surface [SP 895 539], but further west, where the presence of the structure beneath thick glacial deposits is inferred from borehole data, the fault alignment is poorly constrained.

The base of the Woburn Sands Formation is part of the regional Late Cimmerian Unconformity (Whittaker, 1985). The unconformity deepens south-eastwards from about 80 m OD at crop to about 25 m OD below the Gault outcrop.

Strata at outcrop may be affected locally by superficial structures, including cambering and valley bulging (p.28).

Chapter 3 Applied geology

Hydrogeology

In the Bedford district, usable quantities of groundwater are found in the Jurassic sandstones and limestones (both at outcrop and where confined by the Oxford Clay Formation) and in the overlying Woburn Sands, but mainly in the more widely distributed superficial deposits. The majority of groundwater is taken from fluvial sands and gravels, the only major abstraction from the bedrock being for process water in the brewing industry. Abstraction from gravel pits for washing and dust suppression forms the biggest use of groundwater in the district (Figure 8). Public supply, by far the biggest water user, is from surface water sources.

The area is drained by the River Great Ouse and its tributaries. The river flows on superficial deposits of river terraces and alluvium. Where the river is in hydraulic connectivity with aquifer units then borehole yields are improved through induced recharge. This applies to both superficial deposits and the thin limestone and sandstone aquifers in the Middle and Lower Jurassic.

The Lower Jurassic Whitby Mudstone Formation sometimes provides small sources of water, generally at outcrop. The underlying Marlstone Rock Formation is an aquifer of local significance but groundwater yields and quality vary considerably, even over short distances.

The Jurassic limestones and sandstones form a complex series of aquifers (Jones et al., 2000). The Kellaways Formation sandstone and the limestones of the Cornbrash and Blisworth Limestone formations are aquifers at or close to their outcrops, but as they are thin and become more deeply buried below the confining Oxford Clay Formation, yields and quality of groundwater deteriorate. Where small supplies of water are found the quality is generally very hard, and not always suitable for potable use without treatment.

Around Bedford, the Kellaways Sand contains limited quantities of groundwater due to partial saturation, but is likely to be in hydraulic continuity with the river via the river terrace deposits. The underlying Cornbrash and the Blisworth Limestone are water-bearing strata, the latter being the most productive, particularly where in hydraulic continuity with the Great Ouse. Yields from the Blisworth Limestone ranging from 0.8 to 12.4 l/s have been obtained, the latter during a pumping test at Clapham Pumping Station [TL 0382 5151].

Since 1868, the town of Bedford was supplied from a large diameter shallow well in the Blisworth Limestone at Clapham alongside the Great Ouse, upstream of the town. The yield was increased by borings to about 30 m and headings, up to 250 m long, into the limestone (Woodward et al., 1909). The limestone is in hydraulic continuity with the overlying river gravels which contribute to the yield. This source continued to supply the town until about 2000 when highway construction necessitated closure. Water is now obtained from the Great Ouse and Grafham Water.

Water quality in the Jurassic limestone aquifers varies greatly, but these can provide good quality, hard water. However, total hardness values in excess of 1500 mg/l, sulphate ion concentrations of greater than 500 mg/l and chloride ion concentrations of around 90 mg/l have been recorded. Iron concentrations may also be elevated and require treatment such as aeration. The mudstones of the Ampthill Clay, West Walton and Oxford Clay formations are commonly regarded as non-aquifers, although they can sometimes provide small quantities of groundwater. Groundwater in the Oxford Clay is often of poor quality, with high total dissolved solids (salinity) and high dissolved iron concentrations.

Transmissivity values from the Woburn Sands Formation range from 23 to 6100 m²/d with a geometric mean of 260 m²/d (Allen et al., 1997) although no values were measured in the district. Highest values are measured in the confined section of the aquifer within two kilometres of the outcrop as they have greater saturated thickness than sites on the outcrop. The sands become increasingly well cemented towards the top and where the formation is thin (Monkhouse, 1974), reducing groundwater flow. The specific capacity (yield/drawdown) of the Woburn Sands aquifer varies from below 1 l/s/m to greater than 5 l/s/m, the saturated thickness being important in determining the yield that can be obtained. The capacity of the aquifer in the district is limited by its saturated thickness.

Groundwater quality in the Woburn Sands is likely to be hard but good. Total dissolved solids of up to 500 mg/l are expected, with total hardness in the region of 200–300 mg/l and chloride ion concentration of less than 50 mg/l. Groundwater quality is usually satisfactory for domestic supply but iron concentrations can exceed the EC guideline concentration of 0.1 mg/l, and are commonly over 2.0 mg/l.

The glacial till is generally considered to be an aquiclude and to restrict recharge to the underlying formations. It may be possible to obtain small quantities of water from any sand and gravel present within the till, but the water quality from these deposits is often poor.

Sand and gravel aquifers are found along river courses which are used for small domestic and agricultural sources of groundwater but also provide storage and hydraulic connectivity between the river and the underlying aquifers.

Mineral resources

Brick clay

The major source of brick clay in the Bedford area is the Peterborough Member of the Oxford Clay Formation known locally as 'knotts', comprising some 23 m of bituminous shales and calcareous mudstones. It contains about 5 per cent of finely divided carbonaceous matter, which burns off during firing, and so significantly reduces the costs of brick manufacture. Other advantages include the thickness and uniform composition of the deposit, and a comparatively high plasticity that allows the clay to be pressed into moulds by a semi-dry procedure known as the Fletton process. The weathered mantle of the Peterborough Member, known as 'callow', and the more calcareous Stewartby and Weymouth members are unsuited to Fletton brick production and are removed as overburden, some of which may be used for sealing landfill sites.

Brick clay resources in the Bedford district are very large. However, the outcrop of the Peterborough Member is covered by significant thicknesses of superficial deposits, mainly till. Consequently, because of this overburden, and the reduced thickness of the Peterborough Member over much of its outcrop, the most promising area for future development remains the Marston Vale, and it is within this area that resources should be protected against sterilisation by other developments. Current permitted reserves here are large and sufficient for some 70 years. The workings are already extensive and the resulting voids are of regional importance for waste disposal. However, there has been a significant reduction in brickmaking capacity in the Vale during the last two decades, and brick clay production in Bedfordshire declined from 2.6 million tonnes in 1980 to 0.42 million tonnes in 1999. Up until recently, only the large Stewartby works [TL 01 42] continued to produce Fletton bricks (Plate 4). Together with those near Peterborough (Cambridgeshire) it supplied about 30 per cent of Britain's brick output in 1991. Products included stock bricks and a wide range of facing bricks. However, following recent legislation on emissions, Stewartby closed in February 2008.

Sand and gravel

The most significant resources of sand and gravel in the Bedford area are associated with the River Great Ouse, although extensive deposits have been sterilised by urban development, notably around Bedford. In many other places the sand and gravel may be covered by a thin overburden of alluvium. The River Ouse gravels are generally dominated by pebbles of Jurassic limestone and flint derived from glacial deposits.

Sand

The Woburn Sands Formation (Lower Greensand), which crops out in the south-east, is an important source of construction sand, and locally, where of higher purity, silica (industrial) sand. The formation shows marked variations in quality and particle-size from place to place, but the distribution of these different qualities of sand is generally poorly known. Extraction is currently centred on the Leighton Buzzard and Potton areas, the latter being an important source of building and asphalting sand. There are no currently active workings in the Bedford district.

Limestone

Limestones of the Great Oolite Group crop out widely in the north-west of the district and were formerly extracted in many places for local building stone (Front cover). Currently the Blisworth Limestone is worked on a small scale at Weston Underwood for building purposes (Plate 1).

Engineering ground conditions and hazards

Three of the most important ground conditions relevant to construction and development are the suitability of the ground to support structural foundations, the ease of excavation and the use in engineering works. These issues are summarised in (Figure 10) for the main engineering geological units found in the district. Short descriptions of important geohazards that may occur within the district are given below.

Ground subsidence and heave (shrink-swell)

The Whitby Mudstone, Blisworth Clay, Oxford Clay, West Walton, Ampthill Clay and Gault formations are prone to volume change close to the surface in response to changes in moisture content. During wet weather they absorb water and swell, but during dry weather they lose water, shrink and may crack. These alternating processes may cause structural damage to buildings and roads. Subsidence is increased, and to greater depths, by trees growing nearby. Swelling may occur for a number of years after trees and hedgerows are removed. Foundation design must take volume change, in particular differential volume change, into consideration (BRE, 1993; NHBC, 2000). Glacial till and glaciolacustrine deposits may also shrink and swell but generally to a lesser extent.

Chemical attack on buried concrete

The soils in this area are generally neutral or slightly alkaline, so acid attack on concrete or metal services below the water table is unlikely. However, peaty deposits or leachate from landfill may be acidic and potentially corrosive.

Some ground conditions require sulphate-resistant concrete or other preventative measures. The weathering of the Whitby Mudstone, Oxford Clay, West Walton, Ampthill Clay and Gault formations and the Kellaways Clay Member leads to the oxidation of pyrite, which produces sulphuric acid. The subsequent reaction with calcium carbonate, often present as fossils or as disseminated particles, forms gypsum, which is commonly seen in the weathered zone.

Cambering and valley bulging

Cambering, the extension and lowering of near-surface strata induced by gravity, occurs on valley sides where strong beds (such as limestone) overlie weaker clay or mudstone. This is attributed to processes including erosion-related stress relief, frost-induced heave and creep processes during periglacial conditions in the Pleistocene (Parks, 1991). As a result, the thickness of the clay or mudstone beds is reduced, and the overlying strong, cap rock is lowered as a 'camber'. The lowered strata separate along vertical joints, on which minor vertical displacements may take place. These joints may open forming wide fissures ('gulls'), which may be empty, but are more likely to be filled with loose rock and soil. However, gulls may also be dissolution cavities in limestone. It is unlikely that cambering is active in the Bedford district.

Valley bulging is another consequence of the stress relief in narrow valleys with clay strata. In this process, weaker material below the valley floor is forced upwards, becoming folded and possibly faulted, above its normal stratigraphical position.

Slope stability and mass movement

Slopes over 10° in the Whitby Mudstone, Blisworth Clay, Kellaways, Oxford Clay, West Walton, Ampthill Clay or Gault formations are prone to failure, particularly where adjoining permeable layers give rise to springs, and high confined pore pressures or softening leads to loss of strength. Failure may also occur along pre-existing shears formed during periglacial conditions. Slopes may become unstable if the upper part of the slope is loaded, water is introduced into the slope (such as from drains, soakaways or water pipes) or the lower part of the landslide (the toe) is cut away. Man-made cuts into the clays and mudstone containing shear planes may also be unstable and require some form of engineering to stabilise. Till, glaciolacustrine deposits and head may contain thinly interbedded sequences of sands and clays, which may also be prone to landsliding in cut slopes such as along rivers.

Natural radon emissions

Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium. Radon is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Miles and Appleton, 2005).

Less than five per cent of the Bedford district is classified as a radon affected area (Miles et al., 2007). The government recommends that houses in radon affected areas should be tested for radon. Radon protective measures will be required in new buildings, extensions, conversions and refurbishments in small parts of the district (Scivyer, 2007).

A study of geological radon potential in this area indicates that the radon affected areas include parts of those areas underlain by the following combinations of bedrock and superficial geological units: Great Oolite Group limestones; clays of the Blisworth Clay Formation and Ancholme Group; Woburn Sands Formation; and sand and gravel deposits overlying Great Oolite Group limestones.

Flooding

Low-lying alluvial ground adjacent to active stream and river courses may be prone to flooding during periods of exceptional rainfall. Flood protection measures may be required in these areas but consideration must be given to the overall hydrological situation. It is important that all drainage courses are regularly maintained.

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Bedford district and adjacent areas are listed here.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

• Geological maps
• 1:250 000
• 52N 02W East Midlands (Solid, 1983)
• 1:50 000
• Sheet 185, Northampton (Solid and Drift, 1980)
• Sheet 186, Wellingborough (Bedrock and Superficial Deposits, 2007)
• Sheet 187, Huntingdon (Solid and Drift, 1975)
• Sheet 202, Towcester (Solid and Drift, 1:63 360-scale, 1969)
• Sheet 203, Bedford (Bedrock and Superficial Deposits, 2010)
• Sheet 204, Biggleswade (Solid and Drift, 2001)
• Sheet 219, Buckingham (Solid and Drift, 2002)
• Sheet 220, Leighton Buzzard (Solid and Drift, 1992)
• Sheet 221, Hitchin (Solid and Drift, 1995)
• 1:25 000
• Classical Area Map 10, Milton Keynes (Solid and Drift, 1971)
• 1:10 000

The maps listed below were geologically surveyed at the 1:10 560 or 1:10 000 scale by G W Green (1947); K Taylor (1961); A Horton and E R Shephard-Thorn (1967–68); A J M Barron, B S P Moorlock, J Pattison (1985–91); A J M Barron, A D Gibson, R J O Hamblin, C Herbert, A N Morigi, H J Reeves and M G Sumbler (1999–2004). For details of dates and surveyors contact the BGS. Copies of maps from these and earlier large-scale surveys are available for reference in the BGS Libraries at Keyworth and Edinburgh, and at the BGS London Office in the Natural History Museum Earth Galleries. Copies for purchase are produced on a print-on-demand basis and are available from the BGS Sales Desk.

Digital geological map data

• In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geo- logical map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.
• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas (1997) Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas (1998)
• 1:625 000
• Gravity anomaly map of the UK: South sheet (2007)
• Magnetic anomaly map of the UK: South sheet (2007)
• 1:250 000
• 52N 02W East Midlands, Bouguer gravity anomaly (1982)
• 52N 02W East Midlands, aeromagnetic anomaly (1980)
• Geochemical maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain — south sheet (1995) Radon potential based on solid geology, Great Britain — south sheet (1995) Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain — south sheet (1995)
• Hydrogeological maps
• 1:625 000
• England and Wales (1977)
• 1:100 000
• Hydrogeology between Cambridge & Maidenhead (Sheet 14, 1984) Groundwater Vulnerability Map, Bedfordshire (Sheet 31) (1990); compiled for and published by the Environment Agency.
• Minerals and resources maps
• 1:1 000 000
• Industrial minerals resources map of Britain (1996)
• Building Stone resources of the UK (2001)

Books

• British regional geology guides
• Central England (Third edition, 1969) East Anglia and adjoining areas (Fourth edition, 1968)
• London and Thames Valley (Fourth Edition, 1996)
• Memoirs and sheet explanations
• Geology of the Wellingborough district — a brief explanation of the geological map (Sheet 186), (Sheet Explanation, 2006)
• Geology of the country around Huntingdon and Biggleswade (Sheets 187 and 204), (Memoir, 1965)
• Geology of the Biggleswade district—a brief explanation of the geological map (Sheet 204), (Sheet Explanation, 2003)
• Geology of the Buckingham district—a brief explanation of the geological map (Sheet 219), (Sheet Explanation, 2003)
• Geology of the country around Leighton Buzzard (Sheet 220), (Memoir, 1994)
• Geology of the country around Hitchin (Sheet 221), (Memoir, 1996)
• Hydrogeology reports
• Allen, D J and seven others. 1997. Jones, H K and twelve others. 2000.
• Mineral resources reports
• Bedfordshire: resources and constraints WF/95/02
• Northamptonshire: resources and constraints WF/00/04 Buckinghamshire and Milton Keynes: resources and constraints CR/03/077N samuel, m d a. 1982. A preliminary assessment of the sand and gravel deposits of part of the Ouse Valley in Bedfordshire, Buckinghamshire and Northamptonshire (1:25 000 sheets SP 84, 85, 95 and TL 05):
• British Geological Survey Technical Report,WF/MN/82/7.
• Minerals website www.mineralsUK.com

Documentary records collections

Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.

Borehole and trial pit records

Borehole records for the district are catalogued in the NGDC at the BGS Keyworth. Index information, which includes site references, names and depths

for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at the BGS, Keyworth.

Hydrogeological data

Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.

Geophysical data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Indexes can be consulted on the BGS website.

BGS Lexicon of named rock units Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 203 (Bedford), are held in the BGS Stratigraphical Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at the BGS in Keyworth and Edinburgh. Part of the collection can be viewed on the BGS website. Copies of the photographs can be purchased from BGS.

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and extent of Washlands and licensed landfill sites are held by the Environment Agency.

Earth science conservation sites Information on the Sites of Special Scientific Interest and other conservation sites in the Bedford district is held by Natural England, Headquarters and Eastern Region, Northminster House, Peterborough, PE1 1UA. Tel: 01733 455000.

References

Most of the references listed here can be consulted at the BGS Library, Keyworth. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wagstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication, 8.

Arkell, W J. 1933. The Jurassic System in Great Britain. (Oxford: Clarendon Press.)

Barron, A J M, Morigi, A N, and Reeves, H J. 2006. Geology of the Wellingborough district. Sheet Explanation of the British Geological Survey. Sheet 186 (England and Wales).

Belshaw, R K, Hackney, G D, and Smith, K A. 2005. The evolution of the drainage pattern of the English Midlands from the late Tertiary to the early Pleistocene: the significance of the Milton Formation. Quaternary Newsletter, Vol. 105, 16–31.

Bowen, D Q. 1999. A revised correlation of Quaternary deposits in the British Isles. Special Report of the Geological Society of London, No. 23.

BRE. 1993. Low-rise buildings on shrinkable clay soils. Building Research Establishment, B R E Digest 240: Part 1.

Bridgland, D R. 2000. River terrace systems in north-west Europe: an archive of environmental change, uplift and early human occupation. Quaternary Science Reviews, Vol. 19, 1293–1303.

Busby, J P, Walker, A S D, and Rollin, K E. 2006. Regional Geophysics of South-east England. Version 1 on C D-R O M. Keyworth, British Geological Survey.

Callomon, J H. 1968. The Kellaways Beds and the Oxford Clay. 264–290 in The geology of the East Midlands. Sylvester-Bradley, P C, and Ford, T D (editors). (Leicester: Leicester University Press.)

Cox, B M. 1988. Upper Jurassic (Callovian–Oxfordian) clays of the Leighton Buzzard–Ampthill district. Appendix 1 in Wyatt, R J, Moorlock, B S P, Lake, R D, and Shephard-Thorn, E R. Geology of the Leighton Buzzard–Ampthill district. British Geological Survey Technical Report, WA/88/1.

Cox, B M, and Gallois, R W. 1979. Description of the standard stratigraphical sequences of the Upper Kimmeridge Clay, Ampthill Clay and West Walton Beds. 6–21 in Geological investigations for the Wash Water Storage Scheme. Gallois, R W (editor). Report of the Institute of Geological Sciences, 78/19.

Cox, B M, and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series. No. 26. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.)

Cox, B M, and Sumbler, M G. In press. Bathonian–Callovian correlation chart. A correlation of Jurassic rocks in the British Isles. Cope, J C W (editor). Special Report of the Geological Society of London.

Cox, B M, Hudson, J D, and Martill, D M. 1992. Lithostratigraphic nomenclature of the Oxford Clay (Jurassic). Proceedings of the Geologists' Association, Vol. 103, 343–345.

Douglas, J A, and Arkell, W J. 1932. The stratigraphical distribution of the Cornbrash: II. The north-eastern area. Quarterly Journal of the Geological Society of London, Vol. 88, 112–170.

Green, C P, Coope, G R, Currant, A P, Holyoak,D T, Ivanovitch, M, Robinson, J E, Rogerson, R J,and Young, R C. 1996. Pleistocene deposits at Stoke Goldington, in the valley of the Great Ouse, U K. Journal of Quaternary Science, Vol. 11, 59–87.

Harding, P, Keen, D H, Bridgland, D R, and Rogerson, R J. 1991. A Palaeolithic site rediscovered at Biddenham, Bedfordshire. Bedfordshire Archaeology, Vol. 19, 87–90.

Herbert, C, Barron, A J M, Reeves, H J, and Morigi, A N. 2005. Geology of the Kettering district. Sheet Explanation of the British Geological Survey. Sheet 171 (England and Wales).

Hollingworth, S E, and Taylor, J H. 1946. An Outline of the geology of the Kettering District. Proceedings of the Geologists' Association, Vol. 57, 204–233.

Horton, A. 1970. The drift sequence and subglacial topography in parts of the Ouse and Nene basin. Report of the Institute of Geological Sciences, No. 70/9.

Horton, A. 1989. Geology of the Peterborough district. Memoir of the British Geological Survey, Sheet 158 (England and Wales).

Horton, A, Shephard-Thorn, E R, and Thurrell, R G. 1974. The geology of the new town of Milton Keynes. Report of the Institute of Geological Sciences, No. 74/16.

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

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication, 68.

Miles, J C H, and Appleton, J D. 2005. Mapping variation in radon potential both between and within geological units. Journal of Radiological Protection, Vol. 25, 257–276.

Miles, J C H, Appleton, J D, Rees, D M, Green, B M R, Adlam, K A M, and Myers, A H. 2007. Indicative Atlas of Radon in England and Wales. Health Protection Agency and British Geological Survey, H P A-R P D-033.

Monkhouse, R A. 1974. An assessment of the groundwater resources of the Lower Greensand in the Cambridge–Bedford region. (Reading: Water Resources Board.)

Moorlock, B S P, Sumbler, M G, Woods, M A,and Boreham, S. 2003. Geology of the Biggleswade district. Sheet Explanation of the British Geological Survey. Sheet 204 (England and Wales).

N H B C. 2000. Building near trees. Standards Chapter 4.2. (Amersham, Buckinghamshire: National House Builders Council.)

Parks, C D. 1991. A review of the mechanisms of cambering and valley bulging. 373–380 in Quaternary Engineering Geology. Forster, A, Culshaw, M G, Cripps, J C, Little, J A, and Moon, C F (editors). Engineering Geology Special Publication of the Geological Society of London,No, 7.

Prestwich, J. 1861. Notes on some further discoveries of flint implements in beds of post-Pliocene gravel and clay, with a few suggestions to search elsewhere. Quarterly Journal of the Geological Society of London, Vol. 17, 362–368.

Rogerson, R J, Keen, D H, Coope, G R, Robinson, E,Dickson, J, and Dickson, C A. 1992. The fauna, flora and palaeoenvironmental significance of deposits beneath the low terrace of the River Great Ouse at Radwell, Bedfordshire, England. Proceedings of the Geologists' Association, Vol. 103, 1–13.

Sabine, P A. 1949. The source of some erratics from north-eastern Northamptonshire and adjacent parts of Huntingdonshire. Geological Magazine, Vol. 86, 255–260.

Scivyer, C R. 2007. Radon: Guidance on protective measures for new buildings (including supplementary advice for extensions, conversions and refurbishments). Building Research Establishment Report, No. B R211. (Watford: I HS BR E Press.)

Seeley, H G. 1869. Index to the fossil remains of Aves, Ornithosauria and Reptilea from the secondary System of Strata, arranged in the Woodwardian Museum of the University of Cambridge. (Cambridge: Deighton, Bell and Co.)

Shephard-Thorn, E R, Moorlock, B S P, Cox, B M,Allsop, J M, and Wood, C J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England and Wales).

Stuart, A J. 1982. Pleistocene vertebrates in the British Isles. (London: Longman.)

Sumbler, M G. 1995. The terraces of the rivers Thame and Thames and their bearing on the chronology of glaciation in central and eastern England. Proceedings of the Geologists' Association, Vol. 106, 93–106.

Sumbler, M G. 2002. Geology of the Buckingham district. Sheet Explanation of the British Geological Survey. Sheet 219 (England and Wales).

Torrens, H S. 1980. Bathonian Correlation chart. 21–45 in A correlation of Jurassic rocks in the British Isles. Part 2: Middle and Upper Jurassic. Cope, J C W (editor). Special Report of the Geological Society of London, No. 15.

Whittaker, A (editor). 1985. Atlas of onshore sedimentary basins in England and Wales: post-Carboniferous tectonics and stratigraphy. (Glasgow: Blackie.)

Woodward, H B. 1894. The Jurassic rocks of Britain. Vol. 4. The Lower Oolitic rocks of England (Yorkshire excepted). Memoir of the Geological Survey of the United Kingdom.

Woodward, H B. 1895. The Jurassic rocks of Britain. Vol. 5. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Memoir of the Geological Survey of the United Kingdom.

Woodward, H B, Thompson, B, and Mill, H R. 1909. The water supply of Bedfordshire and Northamptonshire, from underground sources: with records of sinkings and borings. Memoir (Water Supply) of the Geological Survey of Great Britain.

Wyatt, J. 1861. Flint implements in the Drift. Bedfordshire Architectural and Archaeological Society Transactions, 1–17.

Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.

(Index map)

The area described in this sheet explanation is indicated by a solid block.

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Office at the Natural History Museum, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Acadian and Variscan subcrop map, showing contours on the unconformities and the locations of deep boreholes and the seismic reflection line.

(Figure 2) Main subdivisions of the Mesozoic succession of the district and adjacent area. Vertical ruling indicates non-sequences. LGS–Lower Greensand, Sel–Selborne Group. Not to scale.

(Figure 3) Generalised south-west to north-east cross-section through the Great Oolite Group illustrating lithostratigraphical relationships and thickness variation.

(Figure 4) Approximate distribution of the Oxford Clay, West Walton and Ampthill Clay formations, and of the component members of the Oxford Clay within the Bedford district. Based partly on mapping, but largely inferred from structural and topographical data (see also 1: 50 000 Sheet 203).

(Figure 5) The geological succession proved at Stewartby brick pits, Millbrook and in the BGS Ampthill Borehole. Based on Callomon 1968 MS (summarised by Cox, 1988), Cox, 1988 and Shephard-Thorn et al., 1994.

(Figure 6) Bouguer gravity anomaly map shown as a colour shaded relief illuminated from the north.

(Figure 7) Total field aeromagnetic anomaly map shown as a colour shaded relief illuminated from the north.

(Figure 8) Licensed groundwater abstraction (m³/year) in the district (data supplied by Anglian Region, Environment Agency. March 2007). Numbers in brackets refer to the number of licences.

(Figure 9) Mineral resources of the district

(Figure 10) Engineering considerations

Plates

(Plate 1) The Blisworth Limestone Formation exposed at Weston Underwood Quarry [SP 862 514] (P5933140).

(Plate 2) Characteristic fossils from the Oxford Clay Formation (P708315). 1. Cylindroteuthis puzosiana (d'Orbigny); mainly Peterborough Member. 2. Gryphaea dilatata J Sowerby; Weymouth Member (and West Walton and lower Ampthill Clay formations). 3. Quenstedtoceras lamberti (J Sowerby); Stewartby Member.4. Hibolithes hastatus Montfort; Stewartby and Weymouth members. 5 a, 5b Gryphaea lituola Lamarck; Stewartby Member. 6. Kosmoceras acutistriatum (S S Buckman); Peterborough Member. 7. Gryphaea dilobotes Duff; Kellaways Formation and basal Peterborough Member. 8. Trochocyathus magnevillianus Michelin; Stewartby Member. 9. Meleagrinella braamburiensis (Phillips); Kellaways Formation and (mainly) Peterborough Member. 10. Genicularia vertebralis (J de C Sowerby); Peterborough Member and basal Stewartby Member. 11. Binatisphinctes comptoni (Pratt); Peterborough Member. 12. Bositra buchii (Roemer); all members of the Oxford Clay Formation. All fossils illustrated natural size, except 2, 3, 5a, 5b, 7 and 11 which are x ½, 6 which is x 2/3 and 12 which is x 2 ½.

(Plate 3) The Peterborough Member exposed at Quest Pit, Stewartby [TL 035 430]. Beds 4 to 10 of (Figure 5) are visible (P671628).

(Plate 4) Unfired bricks ready to load into the Hoffman kiln at Stewartby brickworks [TL 018 428] (P671548).

(Front cover) Cover Photograph Bromham Bridge on the River Great Ouse, built from local Blisworth Limestone. Photographer: A J M Barron (P700277).

(Rear cover)

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey

Figures

Summary of the geological succession in the Bedford district

(Geological succession)

Summary of the geological succession in the Bedford district

(Geological succession)

Figure 9 Mineral resources of the district

Figure 10 Engineering considerations