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Geology of the Leighton Buzzard–Ampthill district 1:10 000 sheets SP82NE, SE; SP92NW, NE, SW; SP93NW, NE, SW, SE; TL02NW; TL03NW, NE, SW, SE; TL13NW. Part of 1:50 000 sheets 203 (Bedford), 204 (Biggleswade), 220 (Leighton Buzzard) and 221 (Hitchin) British Geological Survey Technical Report WA/88/1

R J Wyatt, B S P Moorlock, R D Lake and E R Shephard-Thorn. Contributor: B M Cox

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British Geological Survey Technical Report WA/88/1 Onshore Geology Series

Bibliographic reference: Wyatt, RJ, Moorlock, B SP, Lake, R D, and Shepherd-Thorn, E R. 1988. Contributor B M Cox. Geology of the Leighton Buzzard–Ampthill district. British Geological Survey Technical Report WA/88/1.

© Crown copyright 1988. Keyworth, Nottingham British Geological Survey 1988

(Front cover) Cover photograph: Reach Lane Quarry, Heath and Reach [SP 933 284]. Quarry in Woburn Sands: 15 m of white 'Silver Sands', processed for industrial use, overlies 25 m of 'Brown Sands', sold for construction purposes.

(Rear cover) Rear photograph: Terraced slope of made ground, representing the former overburden of Boulder Clay and Gault. (Acknowledgement to Buckland Sand and Silica Co. Ltd [ARC]).

Subject index: Stratigraphic succession of the Leighton Buzzard area

Preface

This report describes the results of a two-year project to investigate the geology of the Leighton Buzzard–Ampthill district, in Bedfordshire, jointly funded by the British Geological Survey and the Department of the Environment. It integrates the contents of already published Open-file reports covering Phase I (1986) and Phase II (1987) of the project.

Geological surveys of the following 1:10 000 sheets, covering the districts around Leighton Buzzard, Woburn Sands, Hockliffe, Toddington, Ampthill and Shefford have been completed: SP82 NE, SE; SP92 NW, NE, SW; SP93 NW, NE, SW, SE; TL02 NW; TL03 NW, NE, SW, SE; TL13 NW. The area reported on falls mainly within 1:50 000 Geological Sheet 220 (Leighton Buzzard) but includes small parts of Sheets 203 (Bedford), 204 (Biggleswade) and 221 (Hitchin). Dr E.R. Shephard-Thorn has acted as project manager, but much of the responsibility for compiling this report has fallen upon Dr B.S.P. Moorlock and Mr R.J. Wyatt.

The area was first surveyed on the -1-inch to 1-mile scale by T. Adams, H. Bauerman, F.J. Bennett, J.R Dakyns, A.H. Green, T.McK. Hughes, E. Hull, A.J. Jukes-Browne, R.H. Tiddeman, W. Topley and W. Whitaker, and included within Old Series Geological Sheets 46 NW, Solid (published 1864); 46 NE, Drift (published 1884); 46 SW, Solid (published 1865); and 46 SE, Drift (published 1893).

Much of the area was resurveyed on the 6-inch to 1-mile scale by A.C.G. Cameron in the 1880s and 1890s, the northern part being included in the hand-coloured 1-inch to 1-mile New Series Geological Sheet 203 (Bedford). The area included within New Series Sheet 204 (Biggleswade) was resurveyed on the 6-inch scale by R.L. Sherlock in 1930–31. A 2 km overlap from the Milton Keynes Special Sheet onto Sheets SP93 NW and SW was revised at the 6-inch scale by E.R. Shephard-Thorn, R.G. Thurrell and B.J. Williams in 1967–68.

The 1:10 000 revision survey was carried out in 1985–87 by R.D. Lake (Sheets SP82 SE, SP92 NE, western half of TL02 NW), B.S.P. Moorlock (Sheets SP93 NW, NE, SW; eastern half of TL02 NW; TL03 NW, NE, eastern half of SW; TL13 NW), E.R. Shephard-Thorn (Sheets SP82 NE, 92 NW) and R.J. Wyatt (Sheets SP92 SW, SP93 SE, western half of TL03 SW).

B.M. Cox has contributed Appendix 1 giving details of the Upper Jurassic clays of the district.

Uncoloured dyeline copies of the 1:10 000 geological sheets may be obtained through the Map Sales Department, British Geological Survey, Keyworth.

F G Larminie, OBE, Director, British Geological Survey, Keyworth Nottingham NG12 5GG 1988

Executive summary

This report describes the results of a two-year project to investigate the geology of the Leighton Buzzard–Ampthill district, in Bedfordshire, jointly funded by the British Geological Survey and the Department of the Environment.

Prior to the commencement of the project, no adequate 1:10 000 or 1:50 000 scale geological maps of the Leighton Buzzard–Ampthill district were available. The district has important sand resources, currently being extensively quarried around Leighton Buzzard; and commercial fuller's earth, known from only a few localities in Britain, occurs at Woburn Sands and Clophill. Brick clay was formerly dug in the northern part of the district, but the industry is now centred on Stewart by a few kilometres farther north. Much of the district, especially the Woburn Sands escarpment in the northern part, is of great natural beauty, and thus potential conflict could arise between the amenity value of the countryside and the need for further mineral extraction. The area is largely rural, with arable and mixed farming on mainly good quality soils, which may need to be protected from loss by surface developments. The objectives of this project, therefore, have been to provide up-t6-date detailed geological maps and interpretations of them, as a basis for safe, cost-effective planning for development; for the outlining and safeguarding of mineral resources; and for land-use planning in terms of conservation requirements.

A data base was established using existing BGS holdings (mainly borehole records) supplemented by information, much of it confidential, from other sources. The data have been collated and evaluated, and are housed in the BGS National Geosciences Data Centre.

A total of fifteen 1:10 000 sheets have been surveyed, and geologists' Standard' versions of them are included in the output. This report should be studied in conjunction with these maps.

Lithological descriptions are given of solid rock formations and superficial drift deposits. Significant variations of lithology and thickness are noted, and problems of correlation and identification reviewed. Special emphasis has been placed on the Woburn Sands formation (Lower Cretaceous) which is a major source of sand and fuller's earth.

The geological structure of the district is simple. The Solid formations have a gentle regional dip of up to two degrees towards the south-south-east. Locally the dip direction is modified by gentle folding. A major unconformity beneath the Woburn Sands cuts out many of the Upper Jurassic strata over much of the district.

A short programme of trial boreholes has provided additional data on fuller's earth and sand resources, and abridged logs are included in this account; full details are presented in a separate report (Moorlock and Wyatt, 1986). It is hoped that a proposed cored borehole, to be drilled by BGS early in 1988, will provide additional stratigraphical control in the district.

Mineral resources

Sands

Near Leighton Buzzard, and locally elsewhere, sands are being extracted from the Woburn Sands formation for a variety of constructional and industrial uses. Several lithological units have been identified in the pits at Leighton Buzzard, but they are not capable of being mapped at the surface. There is no evidence to suggest that the high quality 'Silver Sands' extracted just north of the town are present in any other localities. Traced eastwards, the Woburn Sands are locally iron-cemented into moderately hard sandstones, which make them both difficult to extract and unsuitable for some purposes.

The thickness of the formation is very variable, ranging from nil to the west of Leighton Buzzard up to a maximum of over 120 m near Woburn Sands.

The potential resources of sand, the constraints upon its possible exploitation, and the relationship between sand categories and end uses are discussed in this report.

Gravels

Gravels occur within both the River Terrace Deposits and the Glacial Sand and Gravel. They consist mainly of sub-angular flints, and pebbles and cobbles of quartzite, together with less abundant pebbles of sandstone, igneous rocks and local 'carstone' (ferruginous grit). Chalk, generally absent from the River Terrace Deposits, is a common constituent of the Glacial Sand and Gravel, rendering the latter of low economic potential. In both deposits the gravels are associated with fine to coarse-grained quartz sands, and grade on average as sandy gravels or gravelly sands.

Deposits of Glacial Sand and Gravel may be several metres thick, but are generally less than 2 m. They may occur below, within, or above Boulder Clay. Their outcrops are commonly restricted to small patches a few hundreds of metres across, but more extensive spreads of thin gravel are present locally.

The River Terrace gravels occupy gently sloping benches mainly confined to the valleys of the rivers Flit and Ouzel. They locally occur below alluvial deposits in the buried channels of these rivers. The gravels are nowhere known to exceed 3 m in thickness, and in many places have been reduced by erosion to less than 1 m thick.

Fuller's earth

There are several seams of fuller's earth in the lower part of the Woburn Sands formation between Woburn and Woburn Sands. The principal one, 2.5–3.7 m thick, is worked in pits at North Wavendon Heath. A further seam, 1.25 m thick and about 17 m below the worked bed, has recently been proved beneath the floor of one of the pits in BGS Woburn Sands B Borehole (Moorlock and Wyatt, 1986). Both seams are cut out a little to the south-west against a rising surface of Oxford Clay or Corallian beds, upon which the Woburn Sands rest.

A seam of fuller's earth, 1–2 m thick, occurs about 17 m above the base of the Woburn Sands formation at Clophill. It has been sporadically worked from below the water table in several small open pits. A recent trial extraction programme was undertaken by Laporte Industries Ltd with a view to larger scale working in the near future.

A geophysical survey, using an EM31 conductivity meter in the area south of Millbrook, failed to locate any significant seams of clay (potential fuller's earth) in the Woburn Sands. The deposit at Clophill therefore appears to be separate from the seams of fuller's earth proved farther west near Woburn Sands.

The lenticular form and impersistence of most seams makes correlation between different localities difficult. 'Fingerprinting' of individual seams by identification of their residual feldspars could, however, facilitate correlation.

Brick clay

There are no active brick pits in the area. The brick-making industry is now centred on Stewartby, a few kilometres to the north, where bricks are made by the 'Fletton' process which takes advantage of the bituminous nature of the Lower Oxford Clay to produce partially self-firing bricks at low cost.

There are substantial reserves of suitable brick clay within the district, between Lidlington and Salford, but proposed extraction of the deposit in the foreseeable future is confined to the Brogborough-Lidlington district and, because production is now centred at Stewartby, it is unlikely that reserves elsewhere will be considered for exploitation for some considerable time.

Lime and cement

The marly nature of the Lower Chalk makes it particularly suitable for cement-making. The purer Middle and Upper Chalk can also be used but require the addition of clay or silt to the limestone, thus increasing manufacturing costs.

In the past, chalk has been dug and burnt as a source of lime, but the current practice is simply to dry and crush the chalk before spreading on the land.

Ground stability

Several of the steeper slopes on the clay outcrops show evidence of landslip. Such areas are outlined on the geological maps where identified. The slips appear to be mainly of the shallow translational planar type rather than the rotational variety. Movement has normally occurred above a basal shear plane.

Head deposits, comprising mixtures of clay, silt, sand or gravel, accumulated by downslope movement of weathered material under cold climatic conditions. Much of the movement may have taken place on basal or internal shear planes.

Any proposal for development on landslipped ground should recognize the latent instability of the slope. Mass movement can be reactivated by removal of material from the slip toe, by loading the slip mass, or by interfering with the natural groundwater regime. Similar difficulties could be experienced on Head deposits. Where development is deemed necessary, it should be preceded by a rigorous site investigation to define the extent, geotechnical properties and thickness of the slip mass or the mantle of Head deposits.

Made ground (fill) may be split into two categories: the first, where waste materials have been used to backfill worked out sand, clay and fuller's earth pits, and the second where materials have been dumped on an original ground surface.

Areas of Made Ground have been shown on the maps where they have been identified, with the exception of some road and railway embankments which are self-evident. Made Ground may be potentially unstable because of problems of low bearing strength and differential compaction which, in the latter, can be especially acute where a structure spans the junction between the fill and the adjacent undisturbed bedrock.

The Boulder Clay is an overconsolidated deposit and is likely, therefore, to have a low compressibility, making it a generally suitable foundation material for large structures. However, problems could be encountered from water flowing through more gravelly horizons or through deposits of Glacial Sand and Gravel within the Boulder Clay.

The valley floors of the rivers Flit and Ouzel contain alluvial silts and clays and, in the former, extensive areas of peat. These are soft, unconsolidated deposits and require detailed ground investigation prior to any development on them.

Groundwater

The groundwater resources of the district have not been actively investigated as part of the project. The hydrogeological map of the area, between Cambridge and Maidenhead, published by BGS (1984), includes the project area.

Two aquifers are of major importance, the Woburn Sands (Monkhouse, 1974) and the Chalk. Minor aquifers are present within the drift deposits, mainly the Glacial Sand and Gravel, but these are not generally used for human consumption. Extensive areas of Boulder Clay cover the Woburn Sands and thus reduce its recharge potential by infiltration of rain-water falling on the surface.

There is little risk of reduced groundwater quality from the dominantly inert wastes of landfill sites; however, two sites representing a significant risk have been identified. The application of nitrate fertilisers in a largely arable area presents a long term threat to groundwater quality in the Woburn Sands aquifer.

Introduction

Programme of research

The area described in this report is outlined in (Figure 1). The two-year programme of work was undertaken to provide up-to-date geological maps and a revised stratigraphical framework for the Leighton Buzzard–Ampthill district, as a basis for safe and cost-effective planning for development; for assessing and safeguarding mineral and other exploitable resources; and for land-use planning in terms of conservation requirements.

The work has included the establishment of a database using existing BGS holdings supplemented by information, much of it confidential, from other sources. A total of fifteen 1:10 000 geological sheets have been re-surveyed and geologists' Standard' fair copies drawn up; this report should be studied in conjunction with these maps (available as dye-line prints from BGS Map Sales, Keyworth).

An outline assessment has been made of the potential mineral resources and the constraints upon their possible exploitation; three trial boreholes were drilled to provide additional data on the fuller's earth and sand resources of the district. Areas of doubtful stability, where rigorous site investigation should be undertaken, have been determined; these include landslipped ground, made ground and outcrops of Peat and Head. A brief review of the hydrogeology is presented, with some indication of the vulnerability of aquifers to pollution.

This report gives the results of these investigations, together with descriptions of the Solid formations and Drift deposits, and of the geological structure of the district.

Geographical and geological context

The simplified topography of the area is shown in (Figure 2); the geological sequence is tabulated in Chapter 3 and the outline Solid geology is shown in (Figure 3).

The northern half of the survey area is dominated by the escarpment of the Woburn Sands formation, the prominent scarp face of which rises above the low-lying Oxford Clay and Corallian plain to the north. The latter is drained by a tributary of the River Ouzel, on the lower valley slopes of which river terrace gravels are preserved. There are also tracts of Boulder Clay and patches of Glacial Sand and Gravel.

The dissected dip-slope of the Woburn Sands escarpment is characterised by light, well-drained soils except where tracts of Boulder Clay on the higher ground give rise to heavy land. Between Woburn and Woburn Sands, deposits of fuller's earth within the sand formation are exploited in opencast workings at North Wavendon Heath [SP 934 342]. The earth has also been dug intermittently at Clophill, east of Ampthill, and has recently been the subject of a trial extraction programme by Laport Industries with a view to larger scale working in the near future.

In the north-east of the district the River Flit flows eastwards in a broad valley containing deposits of Head, River Terrace gravels, Alluvium and Peat. South of the river the Woburn Sands dip beneath the Gault clay, which produces a gentler topography with heavy soils, characterised by low hills capped with Boulder Clay. Further south the ground rises slightly onto the lighter soils of the Upper Greensand silts before the Chalk outcrop commences with an imposing scarp face. The high ground around Toddington and Hockliffe is mainly Boulder Clay plateau with valleys out through into the underlying Gault clay.

The broad open valley of the River Ouzel traverses the south-western part of the survey area. To the north-west of Leighton Buzzard, where it cuts through the Woburn Sands escarpment, the valley is constricted and characterised by river meanders, in the cores of which slip-off slopes veneered by River Terrace gravels face degraded river cliffs on the opposite bank. There are larger tracts of River Terrace gravels in and around Leighton Buzzard, which are separated from the river Alluvium by a barely perceptible feature.

A dissected plateau west of the town is covered by Boulder Clay and subordinate Glacial Sand and Gravel, which overlie Upper Jurassic clay formations and locally, north-west of Wing, a largely concealed outlier of Portland and Purbeck beds. South of Leighton Buzzard there is an extensive tract of low-lying Gault clay country with heavy clay soils, diversified by a few low, drift-covered hills.

Most of the district is rural with generally good quality soils, and is largely devoted to arable and dairy farming. The hungry soils of the Woburn Sands, however, support much woodland which contributes to the beauty of the Woburn Sands escarpment, parts of which are designated as of great landscape value. Leighton Buzzard is the focus of a major sand extraction industry. Large working pits in the Woburn Sands, commonly with an overburden of Gault clay and/or Boulder Clay, are a feature of the landscape both to the north and south of the town. The only major industrial development within the district is located here.

Geological sequence

The Solid formations and Superficial (Drift) deposits shown on the 1:10 000 geological maps are listed below, with an indication of their principal lithologies.

Superficial (Drift) Deposits

The generalised distribution of Superficial Deposits is shown in (Figure 8), (Figure 9), (Figure 10), (Figure 11) and the outcrops of Solid Formations in (Figure 3).

Outline of the geological history

The oldest known rocks within the district belong to the Upper Jurassic Oxford Clay formation. The clays of this formation, and of the Corallianand Kimmeridge Clay, were deposited in open sea environments, little disturbed by waves or currents. The bituminous shales of the Lower Oxford Clay and the oil shales of the Kimmeridge Clay formed in oxygen-deficient bottom waters, whilst shell beds of the Lower Oxford Clay and basal Corallian reflect episodes of greater current activity and oxygenation in somewhat shallower seas.

This major period of clay sedimentation was terminated by slight uplift accompanied by shallowing of the seas, restriction of the depositional basin and a period of erosion. The sandy clay at the base of the Portland Beds, containing phosphatic nodules, -probably accumulated slowly during an episode of minimal sediment input. The remainder of the Portland Beds, comprising limestones, sands and marls, were deposited in shallow marine waters. The basal limestones of the Purbeck Beds are believed to be of shallow, brackish water origin, whilst the fauna of the overlying Whitchurch Sands suggests an increasingly marine influence.

Following deposition of the Purbeck Beds, regional uplift and emergence took place, which resulted in the district becoming, for some time, part of a land mass which was probably drained by rivers contributing sediment southwards to the Wealden Basin of South-east England. Sedimentation was resumed in Lower Greensand times when a major marine transgression over the London Platform occurred. The Woburn Sands accumulated in shallow tidal environments characterised in part by large sand waves. These sands were deposited unconformably upon older beds, such that they rest on various Upper Jurassic clay formations, ranging from Oxford Clay to Kimmeridge Clay. Sporadic phosphatic nodule beds indicate episodes of minimal deposition, whilst seams of fuller's earth in the lower part of the formation represent ash falls from contemporary volcanoes.

A break in sedimentation preceded deposition, in shallow waters, of the overlying sandy, pebbly, basal 'Junction Beds' of the Gault, in which several phosphatic nodule beds represent pauses in deposition. The greater part of the formation was deposited in the deeper, quieter waters of an open sea. The Gault clays overstepped the Woburn Sands in the western part of the district, cutting out the latter and coming to rest on the Jurassic clays.

The calcareous clayey silts and siltstones of the Upper Greensand reflect the input of somewhat coarser-grained sediment, after which there was a further pause in sedimentation accompanied by considerable erosion in the eastern part of the district, where the Chalk succession commences with the Cambridge Greensand. A phosphatic nodule layer at the base of this thin bed contains fossils reworked from the underlying Gault.

The overlying Chalk represents the accumulation of vast quantities of algal skeletal plates (coccoliths) and microscopic shell detritus in an open sea with generally quiet waters. During deposition of the Lower Chalk a significant proportion of clay was introduced from neighbouring land areas. The purer Middle Chalk, however, is almost entirely made up of calcareous skeletal detritus. Any younger Tertiary strata deposited in the district were subsequently eroded away.

The superficial deposits of the district were formed during both glacial and post-glacial times. In the older (Anglian) glacial period an ice-sheet, probably moving from the north or north-east, eroded material from the landscape it traversed and later deposited it as an unsorted, heterogeneous mixture of chalky, stony, sandy clays blanketing the bedrocks of the area and filling valleys and hollows. Associated sands and gravels were deposited by meltwaters at the margin of or beneath the ice-sheet.

A warm interglacial period (Ipswichian) followed the Anglian glaciation but left no positive evidence in the superficial deposits of the study area.

In the later stage of the glacial period (Devensian) progressive downcutting of the river valleys was marked by pauses during which the sands and gravels of the river terraces were laid down. At intervals, unsorted spreads of Head and landslipped clays accumulated by doWnslope movement of material under periglacial conditions.

In post-glacial and recent times, alluvial sediments and peat were deposited in the river floodplains.

Solid formations

Oxford Clay

The Oxford Clay crops out in the northern part of the district and north-west of Leighton Buzzard, giving rise to heavy clay soils. Little local detail is known because no substantial exposures were seen during the field survey. Information from brick pits, boreholes and temporary sections in adjacent areas (Callomon 1968 and MS; Horton and others, 1974) shows that the three major lithological divisions of the Oxford Clay known elsewhere can be recognised (Figure 4): the Lower Oxford Clay, about 25 m thick, comprising bituminous shales and mudstones with pyritic shell beds; the Middle Oxford Clay, 20–25 m thick, consisting largely of slightly calcareous mudstones; and the Upper Oxford Clay, of similar thickness, characterised by slightly calcareous mudstones with thin calcareous siltstone bands.

Two important lithological markers are present within the formation: the Acutistriatum Band–Comptoni Bed within the upper part of the Lower Oxford Clay, and the Lamberti Limestone at the top of the Middle Oxford Clay (Figure 4). Both consist of hard cementstone concretions which, at outcrop, commonly give rise to a mappable topographical feature and/or a 'brash' of rock fragments in the soil. Unless these markers can be traced it is difficult to divide the Oxford Clay on surface evidence because the entire sequence weathers to similar soft greenish brown clays to depths of several metres.

The bituminous nature of the Lower Oxford Clay shales makes them particularly suitable for brick-making (Callomon, 1968) and has given rise to the important brick industry now located at Stewartby, to the north of the present district (see p 16).

The Oxford Clay is richly fossiliferous, the Lower Oxford Clay particularly so. Ammonites are common and allow a detailed zonation of the sequence to be made (Callomon, 1968); (Table 15). The microfauna has also been used to divide the Lower and Middle Oxford Clay sequence (Coleman, 1974). The detailed zonation provides a means by which the brick clay sequence of the Lower Oxford Clay can be correlated across country.

Corallian (undivided)

The Corallian has mostly a narrow outcrop at the base of the Woburn Sands scarp face. It consists mainly of clays and mudstones with thin cementstones, which can be divided in borehole cores and exposed sections into two formations, the Ampthill Clay above and the West Walton Beds below (Figure 4). At outcrop in the present survey area, however, the two cannot be differentiated and thus the Corallian is mapped undivided.

The West Walton Beds/Ampthill Clay succession was proved in the Ampthill Borehole [TL 0244 3804], just east of the survey area (for detailed log see pp 24–26). The West Walton Beds, about 12 m in thickness, consist of grey, fossiliferous, silty mudstones and calcareous mudstones, with some siltstone and cementstone bands. The basal 3 m comprise interbedded shelly mudstones and limestones with bands of oyster shells.

The overlying Ampthill Clay is made up of dark bluish grey, fossiliferous mudstones and calcareous mudstones with some phosphatized fossils and patches, totalling 15–20 m of beds. Ammonites are common in these and the underlying West Walton Beds and, together with other fossils, are characteristically bored and encrusted with worm tubes; they are rarely pyritised. These features distinguish them from the normally uncrushed, pyritised fauna of the Upper Oxford Clay below.

The Corallian is overlain unconformably by the Woburn Sands. Locally it is up to about 25m thick but, because of pre-Cretaceous erosion and folding, the thickness of preserved Corallian is variable; in places it may be absent.

On weathering the Corallian beds are indistinguishable in the field from the Oxford Clay; they cannot be separated unless fragments of the thin limestones and cementstones at the base can be found in the soil, or a topographical feature is present. North-west of Leighton Buzzard a persistent band of septarian nodules near the base has been traced for some distance and forms a useful marker.

Kimmeridge Clay

The outcrop of the Kimmeridge Clay is confined to the south-western extremity of the present area, around Wing. Elsewhere, the Kimmeridge Clay is absent because of the overstep of the Woburn Sands. Little is known of the succession, for there is minimal exposure and much of the outcrop is masked by drift deposits. However, the Hartwell Borehole [SP 7926 1223], about 10 km south-west of Leighton Buzzard, gives some guide to the regional succession.

Of the total formational thickness of 47m in the borehole, the lowest 13 m constitutes the Lower Kimmeridge Clay, comprising grey mudstones with thin dark oil shale and calcareous siltstone ('cementstone') bands. The Upper Kimmeridge Clay comprises 16 m of grey mudstone overlain by 18 m of silts and silty or sandy mudstones (the Hartwell Clay). The presence in the district of the uppermost silty and sandy beds has not been confirmed; it is probable that they were eroded before deposition of the overlying strata.

The Hartwell succession is comparable to that of the BGS Swindon Borehole [SU 1413 8349] where oil shales are largely restricted to the Lower Kimmeridge Clay and are not of sufficiently good quality to be regarded as a potential source of hydrocarbons (Gallois, 1978). It may well be that oil shale bands within the Kimmeridge Clay of the present district are similarly of doubtful commercial value, but only a programmme of trial boreholes will establish their quality.

The mudstones- of the Kimmeridge Clay weather into pale and mid grey clays which, in places, contain abundant nodular calcareous concretions ('race'), particularly near the margins of Boulder Clay spreads. These appear to have been dug for marl in the past. Locally, in the Wing area, laminated marly beds have been observed.

Portland Beds

The Portland Beds are largely hidden beneath a Boulder Clay covered plateau to the west of Wing, but small outcrops are present in the adjacent valley slopes.

The general succession was described by Bristow (1968) and comprises c.12 m of strata at the base of which is the 'Lydite Bed'. This comprises phosphatic nodules and worn fossils derived from the underlying Kimmeridge Clay, and rounded black chert pebbles ('lydites'), embedded in glauconitic silt and sandy clay. The bed locally passes laterally into limestone with 'lydites'. It reaches up to 0.6m in thickness, but is probably only 0.4 m thick in the present district. The 'Lydite Bed' rests unconformably on various zones of the underlying Kimmeridge Clay (Ballance, 1963).

Overlying the 'Lydite Bed' is the Lower Limestone, comprising a variable group of shelly limestones, sands, clays and marls, which are in part glauconitic. Next in sequence are yellow and buff silts and fine to medium-grained sands, known as the Crendon Sand, which form a persistent lithology easily traced with an auger. The succession is terminated by the Upper Limestone, a purer limestone than the Lower, and non-glauconitic.

Because of the disposition of the various drift deposits in the present district it has not been possible to establish a full succession at any one locality, and thus to confirm the sequence described by Bristow.

Purbeck Beds

The Purbeck Beds are poorly exposed in the district and have been observed in situ at only one locality (see pp 33–34). The lower part of the formation is represented by the Purbeck Limestone which consists of bedded limestones with clay and marl interbeds. There is a fissile ostracod-rich limestone marker bed at the base known as the 'Pendle'.

The Whitchurch Sands form the upper part of the Purbeck Beds in this district; they comprise fine-grained sands with a few dark clay bands. They rest unconformably on the Purbeck Limestone (Bristow, 1968). The Whitchurch Sands were formerly believed to be of Lower Cretaceous age (Casey and Bristow, 1964), but are now included within the Jurassic (Morter, 1984).

Woburn Sands

The Woburn Sands (Lower Greensand) form a prominent escarpment extending from Shefford to Leighton Buzzard and rising above the Oxford Clay and Corallian clay plain to the north. Its outcrop terminates about 2 km west of Leighton Buzzard where the sands wedge out beneath the Gault.

The sands have for a long time been exploited for building and industrial purposes, particularly around Leighton Buzzard where there are a number of large working sand pits (see pp 14–16) and (Figure 5). The sections in these pits provide the principal source of stratigraphical data for the Woburn Sands formation which is otherwise very poorly exposed. Several lithological units have been recognized, though they are not capable of being mapped at the surface (Figure 6).

North of Leighton Buzzard the lowest 45 m of the formation constitute the 'Brown Sands'. These consist of glauconitic, fine to medium-grained sands with much iron staining, abundant ferruginous nodules and sheets of 'iron pan' associated with clay partings. In pits where the 'Brown Sands' are very fine-grained and silty, they are colloquially known as 'Compo' (Figure 6) and are of only marginal economic value.

The 'Silver Sands' (Plate 2), from 6 m to 15 m thick, overlie these beds and are separated from them by a conspicuous bed of sandy pebbly ferruginous clay which forms a good marker in the sand pits. The 'Silver Sands' consist of white, pebbly, well-sorted and well-rounded, medium to coarse-grained, almost pure quartz sands with virtually no finer grained material but containing abundant fossil wood fragments.

Above the 'Silver Sands' there are up to 4.5 m of the 'Silty Beds', comprising mottled silts, silty sands and subordinate clays, all characterized by carbonaceous and ferruginous streaks and bands, and sporadic coarse pebbly lenses.

Completing the succession there are up to 5 m of 'Red Sands' which locally occupy channels cutting through the 'Silty Beds' into the 'Silver Sands' (Figure 6). Elsewhere they overlie the 'Silver Sands' and progressively replace them southwards towards Leighton Buzzard, south of which they are up to 11 m thick and directly overlie fine-grained laminated sands containing thin interbeds of black silty clay (corresponding to the 'Brown Sands Compo', described above).

The 'Red Sands' (Plate 1) are mainly medium and coarse-grained, usually slightly silty and commonly pebbly. Concentrations of goethite grains diversify the sands with dark streaks and bands. Impersistent bands of ferruginous nodules and 'boxstone' concretions are common. Rarely, thin seams of pipeclay or partings with clay flakes are present. Although typically reddish brown north of Leighton Buzzard, the 'Red Sands' are predominantly yellow, buff and orange in pits south of the town.

The Woburn Sands over the remainder of the outcrop, between Brickhill and Shefford, are probably broadly equivalent to the 'Brown Sands' of Leighton Buzzard, except that overall grain size is finer. Limonite-cemented concretions and 'boxstones' are not uncommon, and beds of hard, gritty ironstone ('carstone') occur locally and have been used in the past as a building stone.

In the lower part of the formation, between Woburn and Woburn Sands, there are several impersistent seams of fuller's earth clay, only one of which, the Main Seam, is currently worked (see pp 13–14). There is also a seam about 17 m above the base of the formation at Clophill which has been sporadically worked.

The thickness of the Woburn Sands is very variable (Figure 7). The thickest proved sequence of 85 m was recorded in a well near Woburn [SP 9411 3069]; however, drilling commenced below the top of the formation and the total thickness hereabouts is estimated to be at least 120 m. To the east of this the evidence is meagre, but the formation thins to between 20 and 30 m to the south of Ampthill before thickening again to about 60 m at Shefford. On the east side of Leighton Buzzard there are about 60 m of sands, but the thickness decreases rapidly westwards until, about 2 km west of the town, the sands abruptly wedge out and are absent at and beyond Wing.

Gault

The Gault outcrop extends from Leighton Buzzard in the west, through Toddington, to south of Shefford. South of Leighton Buzzard it gives rise to mostly low-lying featureless clay land, much of which is drift-free. Further to the east the outcrop is characterised by a more varied topography including steep-sided hills which are locally landslipped and capped with either Boulder Clay or Glacial Sand and Gravel.

The formation comprises a fairly uniform sequence, up to about 70 m thick, of grey fossiliferous clays and silty clays, with olive and olive-grey clays at some levels. Sporadic courses of phosphatic nodules occur throughout the sequence. The clays weather pale brown in the top 1 to 2 m and locally become whitish grey where exposed at the surface. The weathered clays are commonly characterised by manly streaks and scattered nodules of calcium carbonate ('race'). 'Race' seems to be most abundant near patches of Boulder Clay and can misleadingly give the Gault the appearance of chalky till.

There is little or no exposure of the Gault except for the lowest 10 metres or so, which are well displayed in the sand pits around Leighton Buzzard. In this area a group of passage beds, 1 to 2 m in thickness, which lie between the Woburn Sands and the bulk of the typical Gault clays, has generally been referred to as the 'Junction Beds'. For the purpose of the present survey they are regarded as the basal beds of the Gault formation. They are known to be variable but, in general, there are two distinctive sequences in the vicinity of Leighton Buzzard.

The first is that of a restricted area north-north-east of the town, around Shenley Hill, where a bed of hard gritty ironstone ('carstone') containing blocks and lenses of shelly limestone (Shenley Limestone) overlies up to 1.2 m of red clay (the 'Cirripede Bed') which locally passes laterally into olive and grey pebbly clay.

The other sequence, up to 1.4 m thick and of wider distribution, is found in and to the south of Leighton Buzzard, and consists of a basal conglomerate overlain by brown, sandy, gritty, fossiliferous clays and clayey sands containing bands of phosphatic nodules.

From 3 to 11 m of grey Lower Gault clays rest on the 'Junction Beds' and are, in turn, overlain by up to 5 m of paler grey Upper Gault clays which contain two conspicuous bands of fossiliferous phosphatic nodules, one at the base and the other between 1 and 2.5 m above the base.

West of Leighton Buzzard the Gault overlaps the Woburn Sands and rests on Upper Jurassic clays and, locally, Portland Beds.

Upper Greensand

This formation consists of buff to pale grey, micaceous and somewhat calcareous silts and clayey silts, which are generally deeply weathered and give rise to heavy soils. Indurated lithologies are only rarely encountered in near-surface rocks although, to the north of Houghton Regis, splintery silty limestone ('malmstone') is sufficiently well developed at the top of the formation to produce moderate topographical bench features.

The incoming of the Upper Greensand is generally marked at outcrop by a small but distinct topographical rise from the clays of the Gault. The silts extend to the base of the Chalk escarpment which in places is up to 1.5 km distant. Several outliers, concealed by drift deposits, are present to the north of the main outcrop.

It is estimated that a maximum thickness of 25 m of Upper Greensand are present in the west of the district, decreasing to between 10 and 15 m near Harlington. This variation largely reflects the differing amounts of Upper Greensand removed by erosion before deposition of the Chalk.

Lower Chalk

Unlike the Middle Chalk, which is a pure, micritic, white limestone composed of the skeletal plates of planktonic algae (coccoliths) and much other microscopic shell detritus, the Lower Chalk may contain up to about 30 per cent of detrital clay (Jeans, 1968; Perrin, 1971) which gives the rock a grey to pale buff colouration. The proportion of clay decreases upwards through the formation. Flints are absent.

In the east of the district the Lower Chalk sequence commences with the Cambridge Greensand, a bed up to 0.3 m thick, composed of glauconitic marly clay with phosphatic nodules at the base. During the latter part of the nineteenth century the phosphatic nodules were worked extensively, eastwards from Harlington [TL 030 300], as a source of fertiliser, being crushed and then treated with acid to produce 'super-phosphate'. The 'cut and fill' method of extraction left no visible evidence of the former workings, and the recent survey failed to locate the bed.

It is possible, therefore, that some of the silts mapped as Upper Greensand belong to the Lower Chalk. In the absence of positive field evidence the base of the Lower Chalk has tentatively been taken at a fairly well developed break of slope at the foot of the Chalk scarp face. Near Houghton Regis the boundary has been taken at the top of the uppermost 'malmstone' bed of the Upper Greensand. Any refinement of the position of the base of the Lower Chalk must await more detailed local stratigraphical information. It is hoped that a proposed BGS borehole to be drilled early in 1988 at Upper Sundon, near Luton, will elucidate the sequence and enable the base of the Chalk to be defined.

Within the Lower Chalk, the Totternhoe Stone, a marker bed of harder, more massive limestone with small brown phosphatic pebbles, locally up to 1 m thick, commonly forms a mappable feature. It is estimated to lie about 15 m above the base of the Lower Chalk near Chalton, increasing to about 45 m south-east of Barton-le-Clay.

Middle Chalk

Only the lowest 10 m or so of the Middle Chalk crop out in the area surveyed. To the south-east of Barton-le-Clay there are fragments of hard white chalk in the soil associated with a well-defined topographical feature which marks the position of the Melbourn Rock at the base of tle formation. Above the Melbourn Rock, which may be up to several metres thick, small pieces of softer white chalk, many containing small fragments of inoceramid shells, are common in the soil. The Middle Chalk also crops out on the higher ground around Upper Sundon, but here it is largely concealed by drift deposits.

Superficial deposits

Boulder Clay

The Boulder Clay is a heterogeneous deposit consisting mainly of firm to stiff, stony, variably silty and sandy clays which are usually bluish grey when fresh but olive-brown or khaki when weathered. It locally contains lenticular or sheet-like bodies of gravel and gravelly sand. Its thickness is variable and is probably less than 10 m in most places; locally, thicknesses in excess of 20 m occur.

Chalk in greater or lesser quantities is a common constituent of the Boulder Clay, often as fine detritus but also as pebbles up to about 0.10 m in diameter, and as sporadic larger blocks which may display glacial striae. Locally, near-surface weathering has completely decalcified the deposit.

Angular and sub-angular flints and abraded flint nodules are also prolific, as are quartzite, sandstone and grit pebbles and boulders. Carboniferous limestone pebbles are far less numerous, and coal fragments rare. Shelly limestones, cementstones, septarian nodules, 'carstone', 'boxstone' and fossil shells (notably Gryphaea) of local provenance are also common. Pebbles of igneous and metamorphic rocks have also been collected, some of which are probably of Scandinavian origin.

The Boulder Clay generally caps extensive tracts of higher land forming featureless plateaux, but it also occurs on some lower valley slopes and in valley floors, where it infills pre-glacial valleys and hollows, demonstrating that the pre-glacial topography was comparable to that of the present day (Figure 12), (Figure 13), (Figure 14).

Glacial Sand and Gravel

Outcrops of this deposit are far less extensive than those of the Boulder Clay and are commonly restricted to small isolated patches. The sediment occurs both as small lenticular masses at the base of the Boulder Clay, probably filling scour-hollows or small pre-glacial tributary valleys, and as larger irregular lenticular and sheet-like bodies below, within and above the till, or in isolation.

The deposit comprises poorly-sorted, fine to coarse-grained, locally rather clayey sands containing a variable gravel content, dominated by sub-angular flints and quartzite pebbles. Sandstone and 'carstone' fragments are also common. In places there is a proportion of chalk detritus. Minor constituents are similar to those found in the Boulder Clay (see above).

The deposit has been dug on only a small scale for domestic purposes. Its variable composition and quality probably renders it of low economic potential, the occurrence of chalk detritus locally being a particular hindrance. Most of the former pits are now obscured or ploughed over and thus exposure in the Glacial Sand and Gravel is very limited.

Head

The composition of this heterogeneous group of superficial deposits, which accumulated by a process of downslope movement of weathered surface debris, reflects that of the available source material upslope. It includes sand derived from the Woburn Sands formation, clayey gravelly sands originating from River Terrace deposits and Glacial Sand and Gravel, stony clays reworked from Boulder Clay, and silty or sandy clays derived from various clay bedrocks. The deposits are generally poorly sorted, and enclosed rock fragments are angular, except for pebbles derived from glacial drifts. Crude bedding may, however, occur locally.

Head occurs either as blanketing slope deposits or as linear deposits infilling hollows and valley floors. In the upland area around Wing and in the Ouzel valley below Leighton Buzzard, for example, it is mainly of the latter type; in the Great Brickhill and Woburn Sands area both types are recognised. Sheet-like spreads are generally no more than 2 m thick, but deposits filling the floors of major valleys may locally be up to 5 m thick.

River terrace deposits

In the eastern part of the district these deposits are restricted to the First Terrace of the River Flit, mainly between Flitton and Shefford. The terrace has a surface elevation ranging up to about 5 m above the present floodplain. Gravels which can in places be attributed to the First Terrace also occur locally beneath Alluvium.

River Terrace deposits in the valleys of the River Ouzel and its tributaries, in the western part of the district, occur at two levels, represented by the First and Second Terraces. In the Ouzel valley these have surface elevations of 0.5 to 5 m and 5 to 10 m above the present floodplain Alluvium respectively. The First Terrace gravels are invariably contiguous with the Alluvium but, as in the River Flit valley, contemporaneous gravels have also been proved below the latter.

The terrace deposits consist mainly of patinated sub-angular flints and rounded quartzite pebbles within a matrix of fine to coarse-grained sand. Fragments of 'carstone' and 'boxstone' are also common. Around Leighton Buzzard the First Terrace gravels are overlain by a veneer of slightly gravelly loam.

In general, the River Terrace deposits do not exceed 2 m in thickness, but there are thicker gravels between Clophill and Shefford.

Alluvium

Most of the Alluvium is confined to the flood plains of the Rivers Ouzel, Flit and Ivel, and their tributaries. It is largely composed of soft brown or mottled grey-brown, silty or sandy clays and subordinate silts, locally with gravelly stringers and peat layers, and commonly with a basal gravel. In the valley of the River Flit, east of Flitton, dark peaty silts predominate. Chalky detritus is typical of the Whistle Brook, a tributary of the River Ouzel, which drains from the Chalk scarp at Ivinghoe, south of the district.

The alluvial deposits rarely exceed 2 m in thickness, but in the lower part of the Ouzel valley, near Leighton Buzzard, up to 5.9 m have been recorded. However, some of the basal gravels may represent sub-alluvial First Terrace deposits.

Peat

Extensive areas of Peat have been mapped along the valley of the River Flit between Flitton and Shefford. The deposit comprises a dark brown to almost black silt, very rich in organic matter. It passes laterally into Alluvium, and may also be interbedded vertically with alluvial brown silt or clay.

At Flitwick Moor [TL 048 352], the Peat is largely undrained and supports a swamp vegetation, with abundant birch trees and many sedges and rushes. It is seen to be underlain by gravel at depths ranging from about 0.5 m up to several metres.

Landslip

Most landslips within the survey area affect slopes in the clay formations below outcrops of Glacial Sand and Gravel or Woburn Sands. They are particularly extensive along the scarp face of the Woburn Sands between Lidlington and Great Brickhill. Certain of these landslips have back scars located at the margin of the sand/gravel outcrops; others are wholly confined to the clay outcrop a little downslope. They appear to be mainly of the shallow translational planar type rather than rotational slips.

The landslips are usually characterised by uneven, hummocky surfaces, except where significantly degraded, and by poor drainage. There may be random springs within the slipped mass, as well as more organised spring lines along the slip toes and back scars.

Landslipping probably occurred mainly during late glacial times, when repeated freeze/thaw action weathered and disrupted the bedrock, and facilitated downslope mass movement. However, the freshness of some slip scars and toe features suggests that more recent movement has occurred. Indeed, some slips are still active.

Made ground

Made Ground (Fill) may be split into two categories: the first where waste materials have been used to backfill worked-out quarries, and the second where they have been dumped on an original ground surface. Areas of Made Ground have been shown on the maps where they have been identified, with the exception of some road and railway embankments which are self-evident.

There are various types of artificial fill materials in the study area. The vast bulk comprises rejected overburden which has been used to backfill the large clay, fuller's earth and sand pits. Locally, domestic refuse and varieties of industrial waste have also been dumped in the worked-out pits.

Details of the principal areas of Made Ground are given in pp 56–57. These should be regarded as a general description only and any surface development in these areas should be accompanied by a site investigation to give more precise information.

Structure

The district is characterised by a gentle, fairly regular regional dip to the south-east of 1° to 2°, with evidence of only minimal flexuring. The proximity of the outlier of Upper Greensand at Lower Gravenhurst to the Woburn Sands outcrop indicates a slight increase in dip to about 4°, or local thinning of the Gault or, perhaps, an undetected fault.

Careful mapping of the base of the Corallian (undivided) in the country north-west of Leighton Buzzard has allowed tentative structural contours to be drawn on this horizon (Figure 16). These clearly pick out a synclinal flexure aligned NW SE, with its axis running along the Ouzel valley, east of Stoke Hammond. The gradient on the south western limb of this fold is approximately 1 in 80, less than 1°. This is sufficient, however, to demonstrate that the Upper Jurassic formations were folded and eroded in the period represented by the Jurassic-Cretaceous unconformity.

The Cretaceous Woburn Sands rest on the Kimmeridge Clay in the south-western part of the district, but overstep onto the Corallian and Oxford Clay to the north of Leighton Buzzard. It is itself overlapped by the Gault which in turn comes to rest on the Portland Beds and Kimmeridge Clay west and south-west of Leighton Buzzard (Figure 15). The Jurassic formations have a regional dip of about 1° hereabouts, whilst the Cretaceous beds, which overstep them, have a greater dip of about 2° indicating, like the base of Corallian structural contours, that Jurassic strata were tilted before deposition of the Cretaceous rocks.

Whereas contours on the top of the Woburn Sands (Shephard-Thorn and others, 1986, Map 26) indicate a regular dip to the south-east, those on the base (Shephard-Thorn and others, 1986, Map 27) show a substantial deflection from the regional trend in the vicinity of Woburn Sands. This is confirmed by the gentle south-westerly dip of the main fuller's earth seam as observed in the two working pits. The variation from the regional dip is associated with a particularly thick succession, exceeding 120 m, which may well coincide with the most rapidly subsiding part of the depositional area in Woburn Sands times.

From a maximum thickness of over 120 m in the area around Woburn Sands the Woburn Sands formation is estimated to thin to between 20 and 30 m to the south of Ampthill, before thickening again slightly farther south. The thinning of the Woburn Sands partly results from a rise in the surface of the underlying Corallian sediments. These factors, combined with the present level of erosion, give rise to the 'inlier' of Corallian beds at Ampthill, and account for the narrowness of the Woburn Sands outcrop to the south.

The several outliers of Gault near Chicksands show evidence of having been deposited on an irregular surface of Woburn Sands or, alternatively, they may result from slight local flexuring.

Just south of Woburn Sands village several minor N-S faults, mostly with easterly downthrows, bring Oxford Clay against Woburn Sands. Similar faults may be present elsewhere within the extensive outcrops of the clay and sand formations but are unlikely to be detected by routine mapping.

Mineral resources

The principal mineral resources of the Leighton Buzzard–Ampthill district are sand and fuller's earth. The former, obtained from the Woburn Sands formation, is extensively worked in pits around Leighton Buzzard (Figure 5) and is used for a variety of constructional and industrial purposes. Fuller's earth, a raw material of restricted occurrence in Britain, is locally present as impersistent, lenticular seams within the Woburn Sands. It is a valuable resource with a wide range of applications.

The Chalk has, in the past, provided material for agricultural use and cement-making, and aggregate has been obtained on a small scale from Glacial Sand and Gravel and River Terrace deposits. Brick clay, derived from the Oxford Clay formation, was formerly exploited in the district for brick-making, but there are no workings at present. Nevertheless, considerable reserves remain.

Fuller's Earth

The name fuller's earth derives from the ability of the material to adsorb oil, grease and colouring matter and to be used, therefore, as a medium for cleansing or 'fulling' woollen cloth. This was its principal use until the 19th century when a greater range of applications was developed.

In the United Kingdom the term 'fuller's earth' is restricted to clays consisting almost entirely of clay minerals of the smectite group, chiefly calcium montmorillonite. Deposits within the survey area fall within this definition and normally contain at least 85% of calcium montmorillonite.

The occurrence of fuller's earth in the Woburn area has been known for over 300 years (Cameron, 1892, 1893), but the deposits were worked on only a small scale in the early years. In 1891 the Fuller's Earth Mining Company was set up, shafts were sunk at Aspley Heath and mining from underground galleries commenced in a systematic manner. Because of reduced demand mining subsequently ceased, but since the 1950s the earth has again been exploited, this time by opencast quarrying. Planning permission was granted in 1961 for a pit at Aspley Wood [SP 935 346] and an estimated 1 million tonnes of earth was produced before closure in 1981. At this time the Old Wavendon Heath deposits [SP 932 345], with an estimated reserve of 450 000 tonnes of fuller's earth, came into production.

Seams of fuller's earth crop out in four localities. The largest outcrop extends from Aspley Heath [SP 927 252], across the northern end of Old Wavendon Heath, through Aspley Wood, to north of Birchmoor Green [SP 943 340] (Figure 17). There are smaller outcrops on the western edge of Woburn Sands village [SP 924 354], near Aspley Guise [SP 939 363] and at Clophill [TL 096 383].

The Main Seam of the Aspley Heath-Birchmoor Green outcrop has been extensively quarried and most of the old workings have been backfilled with sand, landscaped and replanted with conifers (these areas are depicted as made ground on the map). The seam, which varies in thickness from 2.5 to 3.75 m and has a south-westerly dip, is currently being worked by Steetley Minerals Ltd west of the A5130 road [SP 932 345], under an overburden of between 25 m and 42 m of sand, and in much shallower workings east of the road.

A section through the Aspley Heath area (Cowperthwaite and others, 1972), (Figure 6), based largely on confidential borehole data, shows that the south-westerly dipping Main Seam is truncated and cut out to the west by the east-north-easterly dipping upper surface of the underlying Oxford Clay. Because of this, the seams of fuller's earth below the worked Main Seam, proved in BGS boreholes (Figure 18); (Moorlock and Wyatt, 1986), cannot extend as far south-westwards as the Main Seam. Indeed, the lowest seams proved in the Birchmoor Pumping Station Wells 113 and 117 (Figure 18), just above the base of the Oxford Clay, can extend only a very short distance south-west of Birchmoor Farm [SP 9450 3440].

Within the present workings an upper seam of, fuller's earth, 24 m above the Main Seam, is less than 1 m thick; it thins eastwards and dies out before reaching the eastern side of the quarry. It is less pure than the Main Seam and contains an appreciable quantity of sand grains.

The BGS Woburn Sands Borehole A [SP 9308 3438], sited in the bottom of this quarry (see pp 58–59), proved a third thin seam of fuller's earth about 17 m below the base of the worked Main Seam. This lower seam was also encountered in the BGS Woburn Sands Borehole B [SP 9323 3526] east of the A5130 road, together with another about 11 m lower down (Figure 18); (p 59).

At Aspley Guise there is a bed of fuller's earth about 10 m above the base of the Woburn Sands. It has been exposed in a pit primarily dug for sand [SP 9406 3632]. The bed appears to be of very variable thickness, having been described as lenticular and between 4 ft (1.22 m) and 6 ft (1.83 m) thick by Cameron (1892), and 6 inches (0.15 m) thick by Rastall (1919). Because of the presumed westerly overlap of the Woburn Sands onto the Oxford Clay, the relationship between this bed and the lower seams of fuller's earth elsewhere is uncertain.

The deposit at Clophill lies about 17m above the base of the Woburn Sands, and was discovered from the log of a borehole [TL 096 383] sunk in 1904. The fuller's earth has been dug sporadically from several shallow pits under a thin overburden of Woburn Sands. It lies below the water table and pumping is required prior to extraction. The disused pits are now filled with water. Laporte Industries have recently completed a trial extraction programme with a view to commencing large scale working in the near future.

Fuller's earth seams have been proved in a number of wells in the Birchmoor Farm-Aspley Guise area (Figure 17). Mapping suggests that those encountered in wells at Birchmoor Pumping Station [SP 944 348] are stratigraphically below the worked Main Seam which crops out just to the west at Aspley Wood. Drillers' logs of the 11 wells at the station show a great variation in the number of fuller's earth seams proved, with one log (SP39SW/113) recording 5 seams and the log of an adjacent well (SP93SW/112) failing to record any. This suggests that the variation between logs may be due to poor sample recovery or the failure of the drillers to recognise fuller's earth, rather than to abnormally rapid variations in thickness of the seams. The most informative of these logs are shown in (Figure 18). It should be noted that the driller's log of well 113 includes sand within the fuller's earth, thus over-estimating the true thickness of the latter. (Figure 18) shows the suggested correlation of seams between the working pit at Old Wavendon Heath, the Woburn Sands Boreholes A and B and the wells at Birchmoor Pumping Station.

No other occurrences of fuller's earth are known in the district. Most of the proved deposits, between Woburn Sands and Woburn, are associated with the thickest succession of the Woburn Sands formation (Figure 7). This is probably not fortuitous and suggests that, elsewhere, deposits of fuller's earth are either absent or of restricted occurrence. Thus, any as yet undiscovered seams may well be thin, particularly as they are more likely to represent deposits of direct volcanic ash falls rather than those reworked into areas of more rapid sedimentation.

The thickness of commercial fuller's earth beds, as proved in trial boreholes, may not be representative of the deposit as a whole because it usually occurs in impersistent lenticular bodies. Indeed, the occurrence of a very thin bed may well be a pointer to the possible presence of a thicker deposit nearby.

Correlation of the fuller's earth is made difficult both by their lenticular nature and by the overlapping, unconformable relationship of the base of the Woburn Sands to the underlying Oxford Clay. Thus a seam of fuller's earth near the base of the Woburn Sands at one locality may occur much higher in the overall sequence than a seam at another locality situated further from the margin of the depositional basin.

Stripping ratios in fuller's earth pits are variable. At present about 40 m of sand overburden is removed to extract up to 3 m of fuller's earth at Old Wavendon Heath. The maximum economic ratio will at any one time be dictated by current demand and market prices. The available borehole data indicate that there is a maximum overburden on the worked Main Seam of about 55 m at Old Wavendon Heath, located 0.5 km SSE of Aspley Heath [SP 927 345] (Figure 17).

Sand

Sands suitable for a range of constructional and industrial uses have been exploited in the Leighton Buzzard–Ampthill district for over a century. At first they were dug for local use only and there are numerous small sand pits throughout the outcrop which are now mostly overgrown, ploughed over or backfilled. Exploitation on a larger scale commenced in the latter half of the 1800's when sand pits became localised in the vicinity of Leighton Buzzard. There are at present 15 working sand pits hereabouts, the locations of which are shown in (Figure 5).

The mineral products derived from the Woburn Sands are constructional sands and silica (industrial) sands. The former are obtained from the 'Red' and 'Brown Sands' (see p 8) which are characterised by sand deposits with considerable variation of grain size and degree of rounding, and which contain much ferric oxide in the form of random iron staining, iron-cemented concretions and dispersed limonitic grains. A broad grain-size distribution usually includes a proportion of silt grade sediment. Although such sands constitute the bulk of the Woburn Sands succession in most localities, exploitation of them has commonly been from the upper part, beneath the Gault clays or beneath the 'Silver Sands.'

Most of the constructional sands are used for building purposes. The screening out of the finer grades produces raw material suitable for the making of asphalt, concrete and mortar. Minor quantities are used for concrete roofing tiles and other applications, such as horticulture. The current British Standards for various types of constructional sands are reviewed by Shephard-Thorn and others (1986). Sands for use in mortar are covered by BS 1200:1976, as amended in 1984 (AMD 4510), which specifies two general purpose mortar sands. However, it emphasizes that insufficient information exists to provide generalised guidance on the use of the specific sand grades. The most recent B.S. relating to sand used for asphalt (BS 594: Pt.1:1985) gives a series of grading requirements for mixtures of coarse and fine materials. Sand for concrete making is specified by BS 882:1983 in terms of three size ranges, but with some flexibility to allow use of fine sand not complying with the specification.

A simple threefold classification of sand resources in terms of end use was adopted by Shephard-Thorn and others (1986), based on the proportion of material retained on a 150pm, sieve. Sand with 75% coarser than 150pm was regarded as premium material likely to be used for concrete, mortar or asphalt. Sand with less than 50% coarser than 150pm was likely to be of little value. Sand with 50–75% coarser than 150pm was of intermediate value, and might well have some application in asphalt and, less likely, in concrete or mortar.

The silica (industrial) sands are derived from the 'Silver Sands' (see p 8) which lie near the top of the Woburn Sands succession and reach up to 15 m in thickness. These sands are restricted to a relatively small area adjacent to the village of Heath and Reach, north-east of Leighton Buzzard.

They have a silica content which exceeds 98% and a ferric oxide content which is less than 1%, except in and near sporadic carstone 'reefs'. Narrow ranges of particle size distribution are typical. Some of the silica sands are marketed as foundry sands which need to be closely grain-size graded and consistent in quality. Bands and lenses of coarse-grained, particularly well-rounded and well-sorted quartz sands are washed and screened to produce material which is ideal for filtration purposes in water and sewage treatment. The high silica content and very low ferric oxide percentage (up to 0.06%; Cameron, 1897) make the sands marginally suitable for glass-making under present economic restraints. They were formerly used for this purpose.

The specification for colourless glass sands given in BS 2975:1958 has been superseded by the Glass Manufacturers Federation which allows greater flexibility of accepted standards, although consistency in chemical composition is of overriding importance.

The sands are worked either 'dry' or 'wet' in opencast pits. The dry method involves the use of mechanical face shovels to dig the raw material from quarry faces where the water table lies below the pit floor. The wet method extracts the sand by suction dredger from below water level in flooded pits; a sand slurry is then pumped through a delivery pipe supported on pontoons to the processing plant.

The regional water table level will determine whether a pit is worked wet or dry, and whether pumping is necessary. If pumping proves to be uneconomic or aquifer protection measures apply, the water table places an effective lower limit on extraction. At present only one pit in the survey area, the Grovebury Pit [SP 923 230], uses the wet method of working.

The Woburn Sands formation has an extensive outcrop and the potential resources of sand in the survey area are theoretically large. The bulk of the succession, where known, consists of sands which are suitable for one or another of several traditional uses. Shephard-Thorn and others (1986) point out, however, that the flexibility of the British Standards for construction sand specifications, together with the range of specialised sands which are possible as a result of mixing, selective working and washing, makes it difficult to equate resources in the ground with precise end uses. Sands suitable for industrial purposes should, on the other hand, be more easily identified because of their more precisely specified characteristics. Locally there are sands, such as the 'Compo' of the Munday's Hill area (Figure 6) which, because of their fine grain size and high silt content, have limited potential uses.

Although several distinctive members within the Woburn Sands have been recognised in the pits around Leighton Buzzard, it has not been possible to specify their surface distribution or extent because their lithologies are not readily differentiated by the shallow (up to 1.3 m) hand auger method' used during the survey. Nevertheless, it is probable that the silica sands ('Silver Sands') are restricted to the small area at Heath and Reach where they are currently worked, because no unequivocal deposits of similar material of any extent have been proved elsewhere. The characteristics of potentially economic sand deposits at particular locations can only be proved by a programme of shallow diggings and/or boreholes.

Although potential resources of sand may be large, there are a number of constraints upon possible exploitation. Much of the outcrop is covered by a thick overburden of Gault clay or glacial drift deposits and, where the prospective stripping ratio exceeds the currently acceptable economic value, the sands no longer constitute a workable resource. However, changing market forces can at any time make such deposits exploitable.

The variable thickness of the Woburn Sands formation (Figure 7) also controls the available resources and location of sand deposits. A western limit, albeit approximate, is defined by the wedging out of the Woburn Sands beneath the overlying Gault to the west of Leighton Buzzard (Figure 3). The thickness to the east of this line ranges up to a maximum in excess of 120 m in the Woburn district. Lateral changes in lithology and quality may well be correlated with variations of thickness.

Urban areas such as Leighton Buzzard and Woburn Sands effectively sterilise considerable tracts of Woburn Sands outcrop, whilst areas designated as of great landscape value (e.g. the Rushmere Park–Kings Wood district near Heath and Reach), although formerly worked for sand, may now be regarded as environmentally sensitive.

Brick clay

The working of brick clays from the Oxford Clay formation has been confined to the area between Brogborough and Lidlington. At present there are no working pits in this area and the old brickworks at Ridgmont has been demolished. Clay extraction and brickmaking are now concentrated at Stewartby, 3 km to the north-east of Lidlington and just outside the survey area. However, there is a long term prospect of further extraction near Lidlington.

Brickmaking in Bedfordshire has been an important industry since the Fletton process was developed in the 1880's. The raw material is obtained largely from the Lower Oxford Clay which consists of bituminous shales and mudstones, the lowest 20 m or so being particularly favoured; the basal beds of the Middle Oxford. Clay have also been used. At one time the weathered surface clay or 'callow', up to 3 m thick, was included, but is now regarded as unsuitable for the modern Fletton process mainly because of its high moisture content.

The economics of mass-producing Fletton bricks are dependent on five main properties of the clay: the water content; the free lime content; the nature of the constituent clay minerals; the organic carbon content; and the absence of undesirable impurities (Callomon, 1968). The 'green' unweathered clay has a low lime content and mainly illite-type clay minerals; thus the bricks produced have a low porosity and relatively high strength. The highly carbonaceous nature of the clays makes them partly self-firing and consequently less fuel and a lower firing temperature are required than for many other brick clays.

Since the Lower Oxford Clay succession appears to be laterally persistent, fairly homogeneous and of reasonably uniform thickness throughout Bedfordshire, it is inferred that substantial reserves of suitable brickclay are present in the northern margin of the survey area. Much of the Oxford Clay outcrop is drift-free but, locally, there are tracts of superficial deposits which constitute an unwelcome overburden. Proposed extraction of brick clay in the foreseeable future is confined to the Brogborough-Lidlington district and, because production is now centred around Stewartby, it is unlikely that reserves elsewhere will be considered for exploitation for some considerable time.

Lime and cement

The relatively high clay content and the absence of flints in the Lower Chalk make it ideally suitable for the manufacture of cement. It was also once used for lime-burning, but the current practice is to dig and crush the chalk before spreading it on the land without it being burnt. By this means the lime content is more slowly released into the soil.

The Lower Chalk has been dug extensively for cement-making near Sundon, but the workings are now abandoned.

Aggregate

The deposits of Glacial Sand and Gravel and some of the river terraces provide potential sources of aggregate. These deposits were formerly worked on a very small scale, usually by individual farmers for their own use. More recently gravels-have been extracted to the west of Harlington and used for drainage purposes along the route of the MI Motorway, but the high chalk content of the glacial gravels severely limits their potential use in concrete manufacture.

Ground stability

A consideration of the geotechnical properties of bedrocks and superficial deposits does not form part of the present study. Nevertheless, certain superficial deposits, both natural and man-made, have been recognised as providing unsatisfactory foundation conditions for large structures. Thus it is recommended that where there is any proposed development on such materials, a thorough site investigation should be undertaken.

Landslip

Landslips, characterised by uneven surface morphology, poorly drained soil, random springs, and structurally disturbed bedrock strata, are generally only marginally stable and are a hazard to surface development. They provide foundation materials of low load-bearing capacity which usually overlie a basal shear plane along which renewed movement can readily be initiated.

Any proposal for development on landslipped ground should recognize the latent instability of the slope, on which mass downslope movement can be reactivated by removal of material from the slip toe, by loading the slip mass, or by interfering with the natural groundwater regime. Development should be preceded by a rigorous site investigation, to define the extent, geotechnical properties and thickness of the slip mass; the position of slip planes beneath and within it; and the piezometric surface or surfaces within the affected area. Structures may then be designed to avoid failure.

Head

Head deposits, comprising unsorted mixtures of clay, silt, sand or gravel derived from a range of local source rocks, originated by slow downslope mass movement of superficial materials. Much of the movement may have taken place on basal or internal shear planes. As with landslips, renewed movement along such shear planes, particularly by loading, can weaken foundations and cause structural damage to buildings and dams, for example. Again, careful site investigation is recommended.

Glacial sand and gravel

Boulder clay is an overconsolidated deposit and is likely, therefore, to have a low compressibility, making it a generally suitable foundation material for large structures. However, problems could be encountered from water flowing through gravelly horizons or bodies of Glacial Sand and Gravel within the Boulder Clay. Such deposits need to be identified, accurately located and accommodated in foundation design prior to building construction.

Peat and alluvial deposits

Soft, unconsolidated, easily compressible peat deposits have a very low load-bearing capacity and are unsuitable as a foundation material. Where present as a substantial surface spread they should either be avoided or, if appropriate, stripped off to reveal more suitable ground. Peat may also occur as thin impersistent layers or lenses within alluvial deposits, the soft clays of which commonly also provide inadequate foundation conditions.

Made Ground

Made Ground (Fill), whether of spoil backfill in clay, fuller's earth and sand pits, construction waste or domestic refuse, may be potentially unstable and of low bearing strength because they generally have heterogeneous geotechnical properties. In particular, ground composed predominantly of domestic refuse offers an unsatisfactory foundation of low strength for large surface structures. Buildings at such sites require special foundations, such as piles driven through the fill to an underlying firm base.

Elsewhere, the mixed character and properties of the fill make differential compaction under loading possible. This may be especially acute where a structure spans the junction between fill and adjacent undisturbed bedrock.

Hydrogeology

The groundwater resources of the area have not been actively investigated as part of the present study. The following remarks are therefore based on currently available data and information acquired during field surveys.

The district is included within the 1:100 000 hydrogeological map of the area between Cambridge and Maidenhead published by the British Geological Survey in 1984. The groundwater resources of the Lower Greensand (Woburn Sands) between Leighton Buzzard and Ely have been assessed by Monkhouse (1974).

Two aquifers are of importance in the district, the Woburn Sands and the Chalk, which are physically separated by an aquiclude, the Gault clay. Minor quantities of groundwater are obtained from the Drift deposits, mainly from the Glacial Sand and Gravel, and the River Terrace Deposits, but they are not generally used for human consumption.

The Woburn Sands is overstepped by the Gault clay from the south-east, and becomes a confined aquifer down dip of its exposed outcrop. Extensive deposits of Boulder Clay occur on the Woburn Sands outcrop, giving rise to local confined conditions and reducing the potential recharge of the aquifer by infiltration of rain water falling on the surface. Other factors which adversely affect the recharge are low rainfall, high evapo-transpiration, and the relatively narrow outcrop of the aquifer.

The total thickness of the Woburn Sands ranges from below 30 m west of Leighton Buzzard, where it dies out abruptly, to over 100 m in the Woburn area. The sands vary considerably in lithology, particularly in grain size and in the proportion of iron minerals and clays, and this affects both their permeability and the chemistry of the groundwater. Thin interbedded clays and seams of fuller's earth locally give rise to perched water tables in the formation; these can be of significance in quarrying operations, where drainage problems may result.

There is a general paucity of detailed information on the aquifer properties of the Woburn Sands. In the Leighton Buzzard area transmissivities of over 1000 m2/d have been recorded. Average yields range from 0.5 to 3.0 l/s from boreholes up to 150 mm in diameter, to 20 l/s for diameters over 380mm. The eight pumping stations for public supply in the area are: Birchmoor [SP 944 348], Sandhouse [SP 937 300], Stanbridge Road [SP 935 247], Milebush [SP 907 255], Battlesden [SP 960 281], Clophill [TL 096 383], Meppershall [TL 150 370] and Barton-le-Clay [TL 080 336]. In addition several sand quarries are licensed to abstract water for washing purposes, but this is normally returned to the aquifer.

Water quality is generally good in the area. Carbonate hardness values are low in the Leighton Buzzard area; total ionic concentrations of 150 mg/l and 285 mg/l have been recorded from Birchmoor and Sandhouse pumping stations respectively. Higher values are usually noted under Boulder Clay cover, and a reduction down dip beneath Gault cover. At outcrop nitrates are typically less than 2 mg/l, but may exceed 25 mg/l; they are usually absent at depth.

Because of the friable uncemented lithology of the Woburn Sands care has to be taken in well construction to avoid silting up and collapse. The best results are obtained by the use of perforated casing and gravel packs in the zone of extraction in a borehole.

Groundwater contours in the Woburn Sands reflect surface topography and geological structure. Seasonal fluctuations in water level are typically small (less than 1 m) even in the outcrop areas. Overflowing artesian conditions were formerly encountered in the confined aquifer, but have ceased after recent large scale abstraction. Springs commonly emerge at the contact of the base of the Woburn Sands with underlying Jurassic clays and are sometimes exploited for farm use. Springs also occur where Glacial Sand and Gravel deposits overlie impermeable Jurassic clays or Boulder Clay, and can have considerable capacity as, for example, in Liscombe Park [SP 891 261].

The Chalk has a restricted outcrop in the study area but is the most important aquifer in this part of the South Midlands. Its groundwater flow is mainly through fissures; thus well yields are dependent on intersecting them. Yields can be increased by 'acidising' the wells with hydrochloric acid to open out fissures. The highest yields are obtained from the Upper Chalk, which is less marly than the Lower and Middle Chalk.

Chalk water is hard and usually of excellent quality. Nitrate concentrations at outcrop are generally in the range 30–40 mg/l; levels in the confined aquifer are minimal.

No public supply wells extract water from the Chalk within the survey area.

Groundwater in an aquifer is generally less vulnerable to pollution than surface sources because of the filtering and attenuating effects of the unsaturated zone above the aquifer. Consequently it commonly receives little treatment before being pumped into supply. It is important, therefore, that aquifers are protected from potential pollutants.

There are two main sources of pollution, namely point and diffuse sources. Point sources include landfill and other waste disposal sites such as sewage treatment works and storage tanks for silage, fuels, industrial solvents and other chemicals. In the present district most of the areas of made ground comprise old quarries backfilled with inert wastes which represent little risk to groundwater quality. However, there are two landfill sites currently accepting household and putrescible wastes at [TL 038 280] on the Lower Chalk and at [SP 912 236] on the Woburn Sands; the latter only in baled form. These, plus several small tips that have taken domestic refuse in the past, represent a significant risk to groundwater quality. Storage tanks, particularly where poorly constructed or in bad repair, also represent a potential risk.

The study area is primarily arable and the diffuse sources of pollution are nitrates, applied to the ground in the form of fertiliser, and biocides (particularly herbicides); both are widely used in agriculture. The use of these products has increased significantly since the war. The potential for nitrate applied on the land surface to reach the aquifer depends on the crop, timing of the application, agricultural practice, climate, soil type and presence of any overlying impermeable deposits.

Leaching losses of 30–50% of the original applications are common, with a consequent increase in the nitrate concentration of both surface and groundwaters. The significant quantities of nitrate, present in the pore-waters of the unsaturated zone of most aquifers, indicate that its concentration in groundwaters will continue to rise for a period of time even if the use of fertilisers is discontinued. Within the area under consideration the Woburn Sands are most vulnerable at outcrop or where overlain by permeable sands and gravels. At Birchmoor Pumping Station [SP 944 348] nitrate concentrations in water from one borehole, no longer used for supply, already exceed the maximum admissible concentration of 50 mg/l as NO₃ (11.3 mg/l as N) specified in the European Communities Drinking Water Directive (EC 1980); the concentrations at other sites are increasing with time. Where the aquifer is overlain by more than a metre or so of Boulder Clay or Gault, nitrates are generally very low (typically less than 1 mg/l as N) and are likely to remain so.

The British Geological Survey and the Soil Survey and Land Research Centre are currently compiling a map indicating the vulnerability of aquifers to pollution for the part of the district administered by Anglian Water Authority. The risk ranges from extreme, where well-drained sandy or coarse loamy soils directly overlie the Woburn Sands, to low where clayey, low permeability soils occur on the Chalky Boulder Clay or the Gault and Upper Jurassic clays.

The transport from the soil to the water table of organic biocides (that have been sprayed on the land to control weeds and pests) is likely to be significantly different from that of nitrates. Little is known about their behaviour within or below the soil zone but they are known to be adsorbed onto clay minerals, to be biodegradable and to be less readily leached than nitrates. Therefore, aquifers at little risk from nitrate pollution are likely to be protected also from organic biocides, but the converse is not necessarily true.

Conclusions

1. The study area, for which only inadequate 19th century 6 inch to 1 mile geological sheets were available, has been re-surveyed on the 1:10 000 scale to produce more accurate and informative maps for land-use planning. In particular, the distribution of superficial deposits and landslips has been considerably clarified. This report is perhaps the first integrated account of the geology of the district, for which neither a 1:50 000 geological sheet nor a descriptive memoir has yet been published.

2. An updated map showing variations of thickness of the Woburn Sands formation, an important aquifer and sand resource, has been compiled for the Woburn-Leighton Buzzard area; and there is now a better understanding of the relationships between various members of the formation exposed in pits around the latter town. It has not been possible, however, to map these members at the surface because of drift cover and hillwash; thus the several sand units have not been localised at outcrop.

Some uncertainty remains about the position of the base of the Chalk, the definition of the Upper Greensand and the occurrence of the Cambridge Greensand. It is hoped that a proposed borehole near Luton, to be drilled by BGS early in 1988, will help resolve these problems.

Little is known of the Upper Jurassic succession, other than the Oxford Clay-Corallian sequence at the northern margin of the district. It is recommended that, when appropriate, a cored borehole be drilled towards the western edge of the study area to investigate the succession in detail and, in particular, to establish whether oil shales occur in the Kimmeridge Clay.

3. Head deposits have been recognised and mapped for the first time in this district, and extensive tracts of peat outlined in the valley floor of the River Flit. Landslips have been identified and shown to be particularly common along the scarp face of the Woburn Sands outcrop. Made Ground proves to be dominantly backfill in worked-out clay, fuller's earth and sand pits.

4. The geological structure is confirmed as being essentially simple, with a very gentle south-easterly regional dip subject to only minor flexuring.

5. It is concluded that economically workable resources of sand suitable for constructional purposes are large, despite the overburden of Gault clay or glacial drift over much of the Woburn Sands outcrop. No evidence has been found to indicate the occurrence elsewhere of the industrial 'Silver Sands' extracted near Leighton Buzzard.

6. Boreholes drilled during the study have confirmed the occurrence of at least two, seams of fuller's earth below the worked Main Seam at Old Wavendon Heath, near Woburn Sands. A tentative correlation of the proved seams between this locality and Birchmoor Green is given. Work by the BGS Stratigraphy and Sedimentology Research Unit on 'fingerprinting' fuller's earth seams to facilitate correlation is still under way; preliminary results are encouraging.

7. The extent of gravel deposits have been defined; those of glacial origin are generally of low economic potential because of their chalk content.

8. Substantial resources of brick clay are considered to be present in the northern part of the district, although exploitation of them is unlikely in the foreseeable future.

9. Certain Superficial deposits provide unsatisfactory foundation conditions for large structures, and rigorous site investigation is recommended before there is any development. Soft, unconsolidated peat has a low load-bearing capacity; made ground commonly has heterogeneous geotechnical properties, allowing differential compaction under load; landslip and Head deposits are likely to fail by downslope mass movement on shear planes.

10. The Woburn Sands and Chalk are the principal aquifers. The former provides good quality water with low carbonate hardness values. Potential recharge is reduced by an extensive cover of impermeable Boulder Clay and by a relatively narrow outcrop. Springs commonly emerge at the base where it overlies Jurassic clays. Groundwater from the Chalk is plentiful, hard and of good quality, and is obtained from water-bearing fissures.

Most landfill sites accommodate inert waste and present little risk to groundwater quality; however, two sites are identified as representing a significant risk. The application of nitrates as fertiliser to soils may constitute a long term threat to groundwater on the Woburn Sands where they are at outcrop or overlain by permeable sands and gravels. The risk of pollution where low permeability soils rest on clay formations is small.

11. There is one known SSSI within the district at Rushmere Park/King's Wood. From the geological standpoint the sections in the 'Junction Beds' (including the Shenley Limestone and Cirripede Bed) at the base of the Gault are of particular interest for the stratigraphical significance of their faunas. It is recognised that conservation of such sections in an area of working pits would probably present problems. However, when worked out, and before backfilling, consideration might well be given to the conservation of selected sections displaying these strata. It should be understood that preservation of such soft, friable beds will demand regular maintenance.

Acknowledgements

We acknowledge the kind assistance of the local landowners, quarry operators and councils in permitting access to their lands and undertakings. The quarry owners and councils have given additional help by way of borehole and other technical data, some of which is confidential and thus not directly quoted in this report. These operators have also provided information on the landfill materials used in restoring worked-out quarries. We are particularly indebted to Joseph Arnold and Sons Ltd, P. Bennie Ltd, Buckland Sand and Silica Co. Ltd (A.R.C. Group), George Garside (Sand) Ltd (E.C.C. Group), Hall Aggregates (Eastern Counties) Ltd. (R.M.C. Group), Steetley Minerals Ltd, Laporte Industries Ltd, Blue Circle Industries, Bedfordshire County Council and Mr Richard Carter of Silsoe College.

Our colleague Dr C.R. Bristow (BGS Exeter) has generously given free access to the information in his unpublished Ph.D. thesis (1963), which includes much useful data on former sections around Leighton Buzzard.

Stratigraphical palaeontology (chiefly of the Upper Jurassic formations) for this report has been carried out by Dr B.M. Cox. Native sulphur and certain iron minerals occurring at Mundays Hill Quarry have been identified by Mr B.R. Young using X-ray methods.

We acknowledge the contributions of Dr R.W.O. Knox who is currently working on the residual feldspars of fuller's earth seams as an aid to correlation; Miss C.R. Hallsworth who has provided grain size profiles and clay mineral determinations for the Woburn Sands of the BGS boreholes; and Mrs M.A. Lewis for observations on the vulnerability of aquifers to pollution.

Appendix 1 Upper Jurassic (Callovian-Oxfordian) Clays of the Leighton Buzzard–Ampthill district (by B M Cox)

In common with other areas of central and eastern England, the Upper Jurassic clays of the Leighton Buzzard–Ampthill district are poorly exposed, largely covered by superficial deposits and give rise to relatively featureless terrain. Nevertheless, the Oxford Clay, West Walton Beds and Ampthill Clay are all recognised in the study area where they have been proved in excavations and a borehole. At surface outcrop, however, it has not been possible to separate the West Walton Beds and Ampthill Clay; they have been mapped as Corallian (undivided). The Kimmeridge Clay is also present but its outcrop is restricted to a small area near Wing [SP 88 22]; there is no exposure and details of the sequence remain largely unknown. The lithological and macrofaunal characteristics of these formations are summarised in (Table 1).

There are three important localities in or adjacent to the study area which provide stratigraphical reference sections. The Lower-Middle Oxford Clay sequence is exposed in the London Brick Company pits near Stewartby [TL 01 42] and [SP 96 39]; there is a published description for most of this section (Callomon1968, p 281–2). The Middle-Upper Oxford Clay with the overlying West Walton Beds were exposed in a major temporary excavation at Millbrook [TL 005 390] in 1968. The section, on the north side of Heydon Hill, was measured and collected by Dr J H Callomon but a full description has never been published (Rood, Hay and Barnard, 1971, p.247; Callomon in Wright 1980, 70–1). The Ampthill Borehole [TL 0244 3804], about 2km south-east of Millbrook, was drilled by the BGS in 1970 in order to prove the stratigraphy and thickness of the Ampthill Clay of the type locality but, like the Millbrook section, there is as yet no proper published account (Horton, 1971). Descriptive summaries of the Millbrook section and Ampthill Borehole are therefore given below. The Ampthill Clay succession can be related to the standard bed-numbered sequence of Fenland, descriptions of which are given in Gallois and Cox (1977) and Cox and Gallois (1979).

Temporary section (1968) at Millbrook [TL 005 390]

Excavations for Vauxhall (now General Motors) proving track, south slope, on the north side of Heydon Hill.

The following simplified descriptive summary is based on the detailed lithological and faunal manuscript log prepared by J.H.Callomon (Figure 4). The lithostratigraphy and zonal stratigraphy of the upper part of the section have been revised by B.M.Cox.

Ampthill Borehole [TL 0244 3804]

6.50 seen 6.10 The following summary log is based on the detailed original prepared by A. Horton. The stratigraphical classification has been modified by B.M.Cox. AC and WWB numbers refer to the standard Fenland sequence.

Ampthill railway cutting [TL 020393] to [TL 022373]

The section in the Ampthill railway cutting is the type locality for the Ampthill Clay. Woodward (1895, p.136) recorded a section through about 21 m of Jurassic clays exposed in the cuttings north of Ampthill Station; however, the section was never measured adequately, nor the fauna precisely located. It apparently showed a sequence (dipping south) from the Upper Oxford Clay (Lower Oxfordian, Mariae Zone) through the West Walton Beds to the Ampthill Clay (Upper Oxfordian, Glosense Zone). There is no Kimmeridge Clay.

Specimens alleged to have come from 'impure limestone bands in clay' at the northern end of the tunnel include serpulids, rhynchonellids, Pleurotomaria, bivalves (arcids, Gryphaea, Lopha, Modiolus, Oxytoma, Nanogyra nana, nuculoids, pectinids, Pholadomya, Pinna, Plicatula), cardioceratid ammonites including Cardioceras (S.) praecordatum Douville (cited Arkell 1947, p.356), Hibolites, a crinoid and cidarids. A trigoniid and the ammonites Decipia decipiens (J. Sowerby) and Perisphinctes (Ampthillia) ampthillensis Arkell were collected from a 'shaly clay' above the tunnel (cited Arkell 1947, pp.356, 361, 373), but these may be from the Drift (Wright 1980, p.70).

Dicroloma, arcids, serpulid-encrusted oysters, Thracia, cardioceratid ammonites, Decipia decipiens (cited Arkell 1947, 356, 373) and belemnites including Pachyteuthis, are recorded from the southern end of the tunnel and unlocalised material includes serpulids, gastropods, an arcid, Gryphaea dilatata encrusted with Plicatula, Thracia and Cardioceras (Cawtoniceras) spp. Nearly all the specimens from the cutting were collected in the 1880's and 1890's.

A more recent trial excavation in the cutting reported by Wright (1980, p.70) showed "Of the Ampthill Clay proper at most 10 m preserved below the Lower Greensand". According to Wright, the ammonites found suggested the Glosense Zone, Ilovaiskii Subzone. This is compatible with the Ampthill Borehole (see above), where the youngest Ampthill Clay belongs to Bed 15 of the standard sequence.

Ampthill sewage works [TL 0390 3640]

Specimens collected from spoil heaps during excavations in 1970 for a new sewage works at Ampthill indicate the Upper Oxford Clay and West Walton Beds. Many of the ammonites (identified by Prof. J.H.Callomon) are heavily bored, worn and encrusted. They include Euaspidoceras akantheen Buckman, E. cf. catena (J. de C. Sowerby), E. cf. crebricostis Arkell, E. paucituberculatum Arkell, Goliathiceras cf. titan Arkell and G. (Pachycardioceras) sp., Perisphinctes (Arisphinctes) cf. cotovui Simionescu, P. (A.) helenae de Riaz, P. (A.) ingens (Young and Bird), P. (A.) cf. kingstonensis Arkell.

Also present is a rich fauna of Cardioceras spp. including C. cf. costicardia Buckman, C. falcatum Arkell, C. cf. harmonicum Maire, C. cf. reesidei Maire and C. sequanicum Maire, together with Peltoceras sp. Belemnites (Hibolites hastatus (Blainville) and Pachyteuthis sp.), Dicroloma, serpulids, Rhynchonella, and bored/encrusted Gryphaea dilatata are also present.

The Euaspidoceras, Goliathiceras and Perisphinctes clearly come from a bed equivalent to Callomon's Bed 32 at Millbrook (see below). The Cardioceras fauna comes from the underlying clays, but no lower than Callomon's Bed 26 at Millbrook.

(Table 1). The lithological and macrofaunal characteristics, and thicknesses, of the Upper Jurassic formations.

Appendix 2: Supplementary general remarks; solid formations

Woburn sands

The Woburn Sands commonly contain small, well-rounded pebbles of quartz, quartzite and chert, and also concentrations of polished, dark brown goethite ooids at the base of foreset laminae in cross-bedded strata (Plate 1). A local abundance of black shiny pebbles has been noted, however, in Woburn Park [SP 9630 3505]. A thin section of one such pebble showed it to be a quartz-schorlite hornfels similar to that found within the aureoles of granite intrusions in the south-west of England (Catt and others, 1974).

Many beds are strongly bioturbated, imparting a characteristic mottled appearance. Additionally, discrete burrow traces of various sizes (up to 5 cm in diameter) stand out boldly on weathered sand faces. Much larger, vertically disposed columns with U-shaped distortions of the bedding are locally numerous; they may represent crustacean burrows (Diplocraterion type).

Although strong cross-bedding with depositional dips up to 25° is characteristic of much of the Woburn Sands, planar bedding and gently cross-bedded tabular sets, from 0.5 m to 3.0m in thickness, are not uncommon. Planar cross-bedded sands with depositional dips up to 25° are very well represented, but trough cross-bedded sediments are less common. Locally, cross-bedded strata are disposed in wedge-bedded units. Foreset beds often comprise tabular and tongue-shaped sand flow cross-strata (Buck, 1985).

The moderately good sorting and rounding of the sands, the presence of well-rounded pebbles, low percentage of fines and the common occurrence of cross-bedding and wedge-bedding are generally regarded as being indicative of deposition in shallow waters subject to vigorous tidal currents. It is envisaged by Allen (1982) and Bridges (1982) that the bulk of the sediment was transported and deposited as large sand waves migrating across the area in a tidal gulf. Johnson and Levell (1980) regard the alternation of cross-bedded channel-fill sands and thinner bedded bioturbated sands as being typical of estuary-mouth deposits. They suggest that the tabular cross-bedded sets which overlie concave-upward erosion surfaces indicate inner estuarine channel/shoal deposits.

The goethite ooids, referred to above, are considered to have formed in an adjacent shallow water, nearshore environment, after which they were swept away by tidal currents into deeper water and hydraulically segregated into the lower parts of foreset laminae (Schiavon, 1988).

Analyses of cross-bedding by various authors gives conflicting conclusions about current directions and provenance of sediment. According to Cowperthwaite and others (1972) cross-bedding in sands immediately above the main worked fuller's earth seam at Aspley Heath indicates that they were derived from the south. They also described continuous and bifurcating sand waves below the fuller's earth which indicate the same current direction. These findings contrast with those of Narayan (1963) and Bridges (1982) who demonstrated that the overall direction of sediment transport during deposition of the Woburn Sands was from the north and north-west. Johnson and Levell (1980) found evidence for bi-directional palaeocurrents indicating a dominant north-easterly flood direction.

The sedimentological characteristics of the 'Silver Sands' suggest that they were subjected to an extended period of movement and winnowing on the sea floor, in shallow waters within the zone of wave activity, possibly as a belt of continuous shifting shoals.

The fuller's earth seams in the lower part of the Woburn Sands consist of bluish or greenish, yellowish weathering clay or mudstone having a smooth, soapy texture. When dry it becomes hard and brittle, with a sub-conchoidal fracture. Distinctively, it disintegrates rapidly into a clay slurry when immersed in water.

The presence of unworn crystals of zircon, sphene and biotite in the fuller's earth clays leaves little doubt that they derive from a volcanic source (Cowperthwaite and others, 1972; Jeans and others, 1977). Whilst thinner (less than 1 m thick), more laterally extensive seams may result from primary falls of volcanic ash, the thicker lenticular and localised seams of commercial earth, which commonly show planar bedding and gentle cross-bedding, were almost certainly produced by the reworking and concentration of volcanic ash into shallow lagoons', perhaps as a result of its rapid erosion from an adjoining landmass. The volcanic vents from which the ash was derived have not been positively identified. Cowperthwaite and others (1972) suggested the Wolf Rock in the Western Approaches as a possible source, whilst recent work on North Sea sediments (Dixon and others, 1981) favours an easterly derivation. As a result of their lenticular form fuller's earth deposits are difficult to correlate and thus any means of 'fingerprinting' individual seams would be of considerable value in improving correlation. Because of alteration and varying degrees of detrital contamination, chemical analysis is unlikely to detect subtle variations in original ash composition. However, studies on early Tertiary bentonites from N.W. Europe have shown that igneous feldspars survive the process of montmorillonitization, and that they may show sufficiently distinctive compositions to allow correlation between individual ash layers (Knox, 1984).

A study has accordingly been started by B.G.S. Stratigraphy and Sedimentology Research Unit on the residual feldspars of Lower Cretaceous fuller's earth seams from Surrey and Bedfordshire. Preliminary results show that the feldspars are anorthoclases, indicating an original trachytic composition. Variation in composition between the three Surrey (Redhill) fuller's earths was small, but the two from the Woburn Sands showed distinctly different compositions, indicating a potential for regional correlation.

Bands of phosphatic nodules commonly occur in the basal few metres of the Woburn Sands. Locally, as at Ridgmont and Leighton Buzzard, they are concentrated in a single bed at the base of the formation (Casey, 1961; Middlemiss, 1962). They were formerly worked under the name 'coprolites' and used as a phosphatic fertiliser. A rich suite of derived Upper Jurassic and indigenous fossils has been collected from these nodules. The nodule bed is nowhere exposed at present in the north of the district, although the nodules have been found at a number of localities in the soil. The bed has been recorded from several boreholes near Birchmoor Farm [SP 944 348].

The Woburn Sands rest unconformably on various Upper Jurassic formations; much of the Lower Cretaceous is therefore absent. The known indigenous fauna is restricted to the basal beds (Teall, 1875; Keeping, 1883; Casey, 1961) and indicates that they belong to the upper part of the Upper Aptian Stage (Parahoplites nutfieldiensis Zone). There is no confirmed dating of the upper part of the formation which is directly overlain by the 'Junction Beds' (see p 9) of Lower Albian Leymeriella tardefurcata Zone (regularis subzone) age. Thus the unfossiliferous upper sands span the interval occupied by the Hypacanthoplites jacobi Zone and the lower part of the L. tardefurcata Zone.

Gault

Although the 'Cirripede Bed', which overlies the Shenley Limestone north-north-east of Leighton Buzzard, was formerly included in the 'Junction Beds' at the base of the Gault, it is now regarded by Owen (1972) as being the bottom bed of his Lower Gault, overlying the 'Junction Beds'.

The basal stratum of the 'Junction Beds' south of Leighton Buzzard contains scattered tabular 'carstone' or 'boxstone' fragments, a few fragments of Shenley Limestone, and sporadic far-travelled pebbles of quartz and quartzite in a gritty matrix. Locally the pebbles are sufficiently numerous to form a basal conglomerate. Brown, sandy, gritty fossiliferous clays and clayey sands containing bands of phosphatic nodules overlie the basal bed. Lenticles of Shenley Limestone have been recorded at the base of similar clayey sands in a former sand pit at Southcott Mill [SP 9045 2453] (Lamplugh, 1922; Owen, 1971) and in the Gault basement bed at Littleworth [SP 881 233], in Wing (Owen, 1971).

During the accumulation of the 'Junction Beds', a restricted supply of terrigenous material and slow deposition resulted in a thin condensed sequence in which phosphatic nodule beds mark pauses in sedimentation. There was still an input of sandy sediment and pebbles at this time and locally, on slight elevations of the sea floor, beds of iron pan or of shelly limestone were formed, the latter where colonies of epifaunal organisms were able to colonise the sea bed. Subsequently, clays and silty clays dominated sedimentation in the deeper, quieter waters of an open sea environment in which there were only sporadic pauses in deposition, as indicated by phosphatic nodule bands. The most significant of these breaks is at the base of the Upper Gault where sediments of the late Euhoplites loricatus and E. lautus zones are lacking. The Gault probably accumulated in water between 125 and 200 m deep (Hancock, 1975) although a depth of up to 335 m has been suggested by Khan (1950) based on foraminiferal evidence.

The Gault yields an abundance of ammonites which allow a detailed zonation to be made (Wright and Wright, 1947; Casey, 1961; Owen, 1972). The formation is assigned to the Albian Stage of the Lower Cretaceous. The thin condensed 'Junction Beds' represent the upper part of the Leymeriella tardefurcata Zone and the lower part of the Douvilleiceras mammillatum Zone; the top of the latter is missing. The overlying Lower Gault is assigned to the Hoplites dentatus and E. lautus zones, and the Upper Gault to the Mortoniceras in flatum and Stoliczkaia dispar zones. The absence of several subzones throughout the sequence reflects the occurrence of depositional pauses and erosional episodes.

The 'Junction Beds' correspond in age to the upper part of the Folkestone Beds of the Weald and are thus regarded as part of the Lower Greensand by some authors (e.g. Wright and Wright, 1947; Casey, 1961).

Lower Chalk

The phosphatic nodules in the Cambridge Greensand, at the base of the Lower Chalk, are reported (Reed, 1897) to range in size from sand grade up to several centimetres across, and are composed mainly of the casts and moulds of fossils, in particular sponges derived from the Upper Gault and Upper Greensand. Other nodules may represent phosphatized mud. Pebbles of far-travelled rocks such as gneiss, granite, basalt and limestone have also been recorded from the deposit (Jukes-Browne, 1900; Hawkes, 1943), having perhaps been transported in floating tree roots. Reed (1897) suggested, however, that many of these rocks have been derived from the glacial drift, or may have been brought into the area as ballast or for paving purposes.

The age of the Cambridge Greensand has been the subject of some debate. Penning and Jukes-Browne (1881) divided the phosphatized fauna into derived Upper Albian types and indigenous Lower Cenomanian forms, but their findings were disputed by Spath (1942–3) who concluded that all the forms could be assigned to the Upper Albian. More recently, a study of the foraminifera within the Cambridge Greensand (Hart, 1973) indicates a Lower Cenomanian age for the deposit, although all the datable macro-fauna is of Upper Albian age and derived. Hart also demonstrated a major non-sequence at the base of the Cambridge Greensand, with the lower part of the Cenomanian and the upper part of the Upper Albian missing as compared with the succession in east Kent.

Appendix 3 Local details of solid formations

Oxford Clay

In the General Motors Vehicle Proving Ground at Millbrook [TL 005 390], Callomon (MS, 1970) recorded a section (now overgrown and landslipped) through the Middle and Upper Oxford Clay, the Corallian and the basal beds of the Woburn Sands. The uppermost 13m of the Middle Oxford Clay were exposed, with the 0.23 m-thick Lamberti Limestone at the top. The Upper Oxford Clay was 21.7 m thick and overlain by the basal bed of the Corallian, the 'Black Bed' (Bed 31) of Callomon.

In the nearby Ampthill Borehole [TL 0244 3802] the Upper Oxford Clay has thickened to more than 27.43 m (base not reached). The mainly pale grey mudstones are much burrowed and contain abundant pyritic trails and pyritized ammonites. Full details of this borehole and the section at Millbrook are given in Appendix 1 (pp 22–27).

Within the outcrop of the Oxford Clay near Millbrook there are three low rounded hills [TL 013 399], [TL 017 395] and [TL 022 395]. It was thought these might owe their existence to a thin capping of harder basal Corallian beds, but no evidence was found to substantiate this. One of the hills is, however, capped by a veneer of gravel, and it is therefore probable that the others may also have originally been surmounted by a similar deposit.

Corallian (undivided)

The type section for the Ampthill Clay, the Ampthill railway cutting [TL 021 378], is now completely overgrown. It was figured and briefly described by Woodward (1895); (Figure 63). The BGS Ampthill Borehole [TL 0244 3804], sunk in 1970, proved the complete Corallian sequence. A summary log of the borehole and details of the Ampthill cutting are given in Appendix 1 (p 26); also details of the section recorded by Callomon (MS, 1970) at Millbrook [TL 005 390] (pp 22–24).

About 0.5 km to the south-east of the Proving Ground a topographical feature in a field [TL 015 387] at the Equitation Centre may mark the base of the Corallian, although no limestone or cementstone fragments have been found in the soil. About 1 km farther east a strong feature [TL 021 390] is associated with ploughed-up blocks of cementstone and large oysters, and again probably marks the approximate position of the base of the Corallian. East of this the boundary becomes less clear, but may be equated with a weak feature passing through Park Farm [TL 023 390].

A fauna indicative of the Corallian (see p 26) has been collected in spoil heaps from deep excavations dug during the construction of the sewage works [TL 039 364] east of Flitwick, where the Corallian is overlain by Head deposits.

North of Leighton Buzzard the Ampthill Clay has been proved at only one locality, at the Woburn Experimental Farm to the west of Woburn village [SP 966 358]. There, samples from a shallow bore have yielded (Whatley, 1974) foraminifera, ostracoda, and holothurian spicules characteristic of the Perisphinctes cautisnigrae and upper P. plicatilis zones of the Middle and Upper Oxfordian Stages (equivalent broadly to the new zones of C. tenuiserratum and A. glosense).

Catt and others (1974) compared the clay mineralogy of the Ampthill Clay from the above borehole with samples from the Oxford Clay from elsewhere. They found that in the Ampthill Clay smectite is dominant over illite in the ratio 76:24, whilst in the samples of Oxford Clay illite is the dominant component. Also, the Ampthill Clay contains appreciable quantities of kaolinite, whereas there is little or none in the Oxford Clay.

From the mineralogy of the samples, Catt and others (1974) estimated the thickness of Ampthill Clay in the area around Woburn Experimental Farm to be about 4 m only. They made no reference to the underlying West Walton Beds, implying that the Ampthill Clay rests directly on Oxford Clay.

Although the Corallian has been proved to underlie the Experimental Farm, its outcrop away from the farm cannot be traced by conventional field methods; nor can it be constructed theoretically because the geological structure of the Oxford Clay and Corallian beneath the major unconformity at the base of the Woburn Sands is unknown. Thus the two formations are not differentiated between Lidlington and Great Brickhill.

Kimmeridge Clay

The Kimmeridge Clay is recorded as underlying 'Gault' in the former brick pit at Littleworth, Wing [SP 881 233] (Woodward, 1895; Davies, 1901, 1915). A fauna collected in 1862 from the 'south end of Stewkley' [SP 854 253], now in the BGS archive, indicates a Lower Kimmeridge Clay horizon. An old well at Milebush Farm [SP 9037 2586], west of Linslade, apparently entered the formation at depth, but the record is too brief for detailed interpretation. A recently drilled BGS borehole at Ascott Farm Sand Pit [SP 9078 2408] (see p 58) proved the basal 1.80 m of the Kimmeridge Clay beneath the Woburn Sands and overlying the Ampthill Clay. It consists of pale and mid grey, shelly mudstone with phosphatic nodules at the base. The ammonite fauna comprises fragmentary raseniid ammonites, probably indicative of the Cymadoce Zone.

Portland Beds

The following section is an abridged version of that recorded by Davies (1915) at the Warren Farm quarry, Stewkley [SP 851 242], then better exposed:

Fitton (1836) recorded 0.6 m of Lower Limestone at this quarry. The section exposed at the present day is:

An auger hole [SP 8515 2241] sited close to the base of the Crendon Sand near Lockharts proved the sequence:

The limestone encountered in this auger hole was 0.23 formerly dug nearby [SP 8538 2218], and solifluction debris from this bed was noted in a new ditch cut to the east [SP 8546 2211]:

An auger hole [SP 8676 2258] to the west of Oldpark Farm proved the following sequence:

A somewhat fuller sequence was recorded by Bristow (1963), as follows:

Limestone was reputedly dug from two small pits nearby [SP 8695 2273] and [SP8698 2267] from the basal beds of the succession. Abundant limestone debris from the same horizon is present in the soil [SP 8765 2145] to the north of Lower Wingbury Farm, close to the Gault outcrop. It is not clear if this locality marks the site of a former pit which has been bulldozed over. The brash extends over the outcrop of the Kimmeridge Clay.

Purbeck Beds

Beds of Purbeck age have been recognised in Warren Farm quarry, Stewkley [SP 851 242], where about 2 m are poorly exposed. An abridged version of the section by Bristow (1968, p.310) from beneath Drift deposits in this quarry is as follows:

The lower part of the sequence is at present somewhat disturbed and covered with talus in part; the section is:

Fragments of oolitic limestone and ferruginous sandstone noted in field brash by Bristow (1963) at Oldpark Farm [SP 869 227] were considered by him possibly to indicate an outcrop of Purbeck Beds. It is unlikely that Purbeck Beds are preserved elsewhere except beneath Boulder Clay in the area immediately south-east of Warren Farm quarry.

Woburn Sands

Sand was formerly dug by ARC from a large pit at Clophill [TL 0800 3800]. After a period of inactivity it has recently been reopened by P Bennie Ltd. The sand is dark orange-brown, cross-bedded, stained by limonite, and locally cemented into sandstone. Orange-brown, medium to coarse-grained, cross-bedded sandstone is exposed for 3 m behind a public house [TL 0838 3811] in the village. In a pit to the south of Clophill [TL 082 372] sand is dug by London Brick (Hanson Trust) and stock-piled for use in the manufacture of sand-faced bricks.

North of Clophill, very fine-grained sand and silt was seen in a deep drainage ditch [TL 0835 3939] to [TL 0857 3912] and augered nearby. It is covered by up to 1 m of pebbly clayey sand wash.

Cross-bedded, yellow to orange-brown, fine to coarse-grained sand and sandstone have been dug from two small pits [TL 0891 3853] and [TL 0895 3837] east of Great Lane, Clophill.

North of Green End, medium-grained sands are much cemented with limonite and, because of their relative hardness, they form topographic plateaux. Sandstone, commonly cross-bedded, is exposed along several sunken lanes and tracks [TL 0684 3811], [TL 0667 3773] and [TL 0725 3849]. Variably cemented sand and sandstone are exposed in places in the deep cutting of the A6 to the north of Ampthill, and steeply dipping, planar cross-bedded, orange-brown sandstone is exposed alongside a track [TL 0756 3813] leading from the A6.

Just west of Snowhill, up to 5 m of orange, medium –grained, cross -bedded sands with 'carstone' horizons and silty stringers up to 2 cm thick are exposed in a small pit [TL 0435 3795].

South of the River Flit, at Flitton, 6 m of orange brown, medium-grained, moderately-cemented sand form a face behind a house [TL 0628 3592]. Similar sand is exposed in a disused quarry about 50 m to the east on the opposite side of the road.

Near the village, of Millbrook there are several disused, mostly overgrown sandpits; 1.2 m of medium-grained orange sand can be seen in one of them. [TL 0154 3861].

Buff phosphatic nodules and pebbly, limonite-cemented sandstone fragments from the basal beds of the Woburn Sands are locally abundant in the soil to the south of Millbrook, both east [TL 0175 3743] to [TL 0175 3682] and west [TL 0168 3652] of the Fordfield Road. The nodules, up to about 5 cm in diameter, are commonly composed of the broken whorls of ammonites.

Farther south, near Steppingley, small polished, rounded, black pebbles, also from the basal part of the Woburn Sands, are common in the soil [TL 0128 3602] and [TL 0170 3547]. Similar pebbles, derived from a higher stratigraphical level, occur west [TL 0232 3509] of Flitwick.

South and east of Steppingley, strong topographic features are produced by hard layers of 'carstone', blocks of which are commonly ploughed up in the soil.

In a mostly overgrown and degraded sand 'carstone' pit [SP 9965 3404] 850 m north-east of Water End Farm, Eversholt, only the uppermost 2 to 3 m of beds are now visible. The exposure shows yellow, buff, orange and rusty brown, strongly planar cross-bedded, medium to coarse-grained sands and sandstones (dips up to 25°). The latter range from weakly iron-cemented and friable to well-cemented and hard. They include bands of hard, gritty, pebbly 'carstone'. The uppermost 2 m at the north-west corner of the pit are orange and buff sands with irregular patchy iron-cementation and contain vertical/highly inclined burrow traces about 0.01 m in diameter.

A composite section in the fuller's earth pit at Old Wavendon Heath [SP 932 345] (Plates 3 and 4) was measured over several faces in September 1985; as the pit is active and constantly changing, the section is intended to identify only the broad units within the sequence.

Two boreholes were drilled in the Old Wavendon Heath-Aspley Wood area during the spring of 1986 to further investigate the stratigraphy of the lower part of the Woburn Sands (Moorlock and Wyatt, 1986). The boreholes were of 35 m depth and commenced just above the main worked fuller's earth seam. One [SP 9308 3438] was sited in the bottom of the deep workings west of the A5130 road, the other [SP 9323 3526] east of the road. Both penetrated a sequence of mainly medium to fine-grained sands and proved a thin seam of fuller's earth about 17 m below the base of the Main Seam. Limitations of the drilling equipment led to both boreholes being terminated whilst still in the Woburn Sands.

Between the A5 road and the town of Leighton Buzzard, sand has been and is still extensively quarried for a variety of constructional and industrial purposes. The working quarries have been examined and recorded in some detail during the present study. General sections for each quarry are illustrated in (Figure 6) with suggested correlations between them. In the following text the composite general sections, as recorded in 1985 for these quarries, are presented in simplified form for the sake of brevity.

The most northerly quarry examined is the A5 Pit of P Bennie Ltd [SP 932 304], which is worked for building sand only. The following section in the Woburn Sands was recorded, beneath a Boulder Clay overburden up to 3.5 m thick:

It is estimated that 5 to 6 m of the 'Silver Sands' latterly the pit has been deepened and extended to are absent due to erosion. The clay band separating the 'Silver' and 'Brown Sands' gives rise to a perched water table in the former.

There are two pits at Fox Corner. The larger [SP 926 291], operated by Hall Aggregates (Eastern Counties) Ltd., has been worked for building sand, but is now nearly exhausted and is in the process of restoration by the tipping of approved waste. At the northern end of the workings, Boulder Clay overburden up to 6 m thick is present and infills channels cut into the sands. The following section in the Woburn Sands was recorded, probably starting a few metres below the top of the formation:

Immediately adjacent is a smaller pit [SP 928 292] operated by Buckland Sand and Silica Co Ltd on an intermittent basis, to supply material to their main quarry at Reach Lane. Boulder Clay infills a small channel up to 2 m deep cut into the 'Silver Sands'. The latter are here exposed to a thickness of up to 6 metres and comprise medium-grained, white, cross-bedded sands which are locally iron-cemented to 'carstone' near the top. The lower 2 to 3 m of the sands are vividly coloured in shades of brick red and ochreous yellow, which may be related to the presence of the glacial channel mentioned above. Traces of lignitic material occur in the sands.

Nearby, to the south, is the Stone Lane Quarry [SP 929 289] of Joseph Arnold and Sons which has been worked for many years. Initially, the 'Silver Sands' were hand dug for industrial use, but latterly the pit has been deepened and extended to exploit the 'Brown Sands' for building purposes.

The overburden here comprises Boulder Clay and Glacial Sand and Gravel totalling 16 metres in thickness. An infilled glacial channel is currently exposed in the eastern face. The following section has been recorded:

The marker bed between the 'Silver' and 'Brown Sands' is virtually planar as seen in the quarry and gives an indication of the true dip locally, which is 3° to the south-east. It marks a major change in sedimentary style between the two sand groups.

The Bryants Lane Quarry [SP 929 286] of L B Silica Sand Ltd is immediately adjacent to the Stone Lane Quarry on the south, the two being separated only by a narrow lane. This quarry is chiefly concerned with the production of sand for building purposes. The overburden comprises up to 10 m of Boulder Clay with interbedded gravel and up to 4 m of Gault. The section is as follows:

To the west of the main road through Heath and Reach, L B Silica Sand Cooperate a pit in Bakers Wood [SP 923 285] where up to 27 m of the 'Brown Sands', forming the lower part of the Woburn Sands locally, have been exploited for building purposes. In the floor of the pit a basal nodule bed, up to 0.15 m thick and comprising 'coprolites' and phosphatic casts of perisphinctid ammonites and bivalves (derived from Upper Jurassic rocks) in a sandy ferruginous matrix, rests on the Ampthill Clay. The approximate level of the base of the formation here is 93.5 m OD. Worked-out pits to the north of the present operation have been restored after use as land-fill sites.

The Reach Lane Quarry of the Buckland Sand and Silica Co [SP 933 284] (see front cover) is immediately south of and partly contiguous with the Bryants Lane Quarry described above. It produces foundry sands from the 'Silver Sands' and building sand from the underlying 'Brown Sands'. Some 20 m of overburden are currently being removed, comprising up to 12 m of Boulder Clay with pockets of gravel and about 8 m of Gault. The section is as follows:

On the eastern side of the Boulder Clay capped ridge, into which the preceding quarries are dug, there is an extensively worked area of about 1 km2, operated in part by Joseph Arnold and Sons Ltd as their Double Arches Quarry (now exhausted) and also by George Garside (Sand) Ltd as the Churchways Quarry, from which small amounts of sand are still extracted. Few large sections are now visible; those seen in the early 1960s were described by Bristow (1963).

Garsides also operate a quarry at Mundays Hill [SP 937 282] where the coarse-grained development of the 'Silver Sands' is a valuable source of industrial sand. Up to 16 m of overburden, comprising 6 m of Boulder Clay and up to 10 m of Gault, is removed to expose the 'Silver Sands'. The sequence is variable in detail and the units described below are not necessarily present everywhere in the quarry.

The occurrence of 'calstone reefs' at the top of the 'Silver Sands' at Mundays Hill (Plate 2), and as formerly seen at Double Arches, have been discussed by Bristow (1963). The reefs are linear features formed of iron-cemented 'Silver Sands'; in cross section they may be 10 to 25 m in width and 2 to 3 m in height. They are apparently laterally continuous over distances of a kilometre or more and seem to be related to the margins of remnant banks of the 'Silty Beds' (Figure 16); (p.280 in Bristow, 1963). The 'Silver Sands' beneath the remnants of 'Silty Beds' are white and relatively free of iron whereas, where the 'Silty Beds' have been removed and 'Red Sands' directly overlie the 'Silver Sands', the latter have ochreous staining to a greater or less degree. The change from clean to iron-stained 'Silver Sands' is quite abrupt and coincides with the position of the 'carstone reefs'. This relationship is important in quarrying as the iron-stained 'Silver Sands' are of much less value for industrial use.

Contiguous with Mundays Hill Quarry on the south is the Nine Acres Pit [SP 940 277] of Joseph Arnold and Sons Ltd., where an overburden of up to 3 m of badly slipped Gault obscures its junction with the Woburn Sands. Traces of Shenley Limestone have been noted in an abandoned part of this pit adjacent to Mundays Hill Quarry. The highest division of the Woburn Sands currently exposed, the 'Silver Sands', is here seen chiefly in ferruginous preservation as a 'carstone' reef, 1.5 m high and 25 m in width. The 'reef' comprises red-brown, medium to coarse-grained, pebbly sandstone, with clay galls and fossil wood included. Some small patches of clean sand remain near the base, which is marked by a conglomerate of 'boxstone' nodules resting on an erosion surface. Beneath this, very fine-grained micaceous silty sands, equivalent to the 'Compo' at the base of the Mundays Hill section, are exposed for up to 10 m. They are planar cross-bedded with frequent silty clay partings and much bioturbation, and contain abundant fossil wood and sporadic 'boxstone' nodules. Sand from this pit is taken to Arnold's plant at Double Arches for processing with other sands for a variety of uses.

Shenley Hill, capped by Boulder Clay, lies south-west of Nine Acres and has some worked-out and restored ground on its eastern flank. On the west of the hill lies Arnold's New Trees Quarry [SP 931 275], where Woburn Sands are exploited beneath about 4 m of Gault overburden. The following sequence has been recorded:

To the south-west of Shenley Hill lies Arnold's Chamberlains Barn Quarry [SP 931 268], which has been worked for many years and has been extensively backfilled with Gault overburden material. The Gault overburden in the current workings on the east is up to 11 m thick. The section in the working part of the quarry in 1985 was as follows:

This section clearly demonstrated the overlapping relationship of the 'Red Sands' on the 'Silver Sands'. The output of this quarry goes almost entirely to the adjacent concrete tile works.

Away from the quarries exposures in the Woburn Sands are rare and the outcrop area is generally mantled with a sandy wash, which locally becomes thick enough to map as Head. A few metres of ochreous cross-bedded sandstones are exposed near the north [SP 911 262] and south [SP 911 259] portals of the Linslade Railway Tunnel.

The Woburn Sands die out abruptly to the west of Great Brickhill (approximately at easting 895) and between Leighton Buzzard and Wing. In the latter area the last evidence of their presence is apparently given by a well log situated about 1.5 km north-east of Wing [SP 8935 2420] which revealed the following sequence:

The sands could be glacial deposits, but are more likely to represent the feather edge of the Woburn Sands. The sands have been worked in a number of pits to the south of Leighton Buzzard. The working face of Ascott Farm Pit of Hall Aggregates Ltd [SP 9090 2405], just south of Linslade, affords a good exposure of the 'Red Sands'. Up to 11 m of beds are present, comprising mainly yellow and fawn, medium and coarse-grained, commonly pebbly sands, which occur as planar beds or gently cross-bedded tabular sets from 1.5–2.5 m thick. Dark streaks and layers contain tiny, polished goethite grains. Bioturbation is common and, at certain levels, there are sub-vertical burrow traces up to 1 cm in diameter. Larger Diplocraterion type burrow traces are also common. Sporadic iron cementation produces irregular toxstones' and doggers. The top surface of the sands beneath the Gault is sharp and planar. A few large blocks of white, very hard, calcite-cemented, gritty, pebbly sandstone lying loose in the pit are presumably doggers excavated from the sands. This lithology is similar to that of the gritty nodules in the overlying 'Junction Beds' of the Gault which have, however, been phosphatized.

In the floor of this pit, at [SP 9094 2406], a trial excavation proved 1 m of very dark grey, thinly-bedded clay with pale silt partings and pyritic nodules. This clay is comparable to the one exposed in Tiddenfoot New Pit (see below) on the other side of the railway line. BGS Ascott Farm Sandpit Borehole, 150 m to the west [SP 9078 2408] (see p 58), proved 1.05 m of the same clay below 7.75 m of 'Red Sands'. Underlying the clay there are 2.95 m of pale to mid grey, slightly glauconitic, well-sorted sands with sporadic thin bands of black laminated silty clay. These sands are thought to correspond to the 'Brown Sands Compo' of the sand pits north of Leighton Buzzard; they rest on Kimmeridge Clay.

At the north-east corner of Tiddenfoot New Pit [SP 9120 2396], also worked by Hall Aggregates Ltd., the following section was recorded beneath 1.2 m of river terrace deposits:

At the base of the pit face, c100 m to the west-south-west, there is a lenticular body of very dark grey clay which wedges out at the parting between the two major sand units described above. The section in the clay is:

The clay appears to be barren of macrofossils. It is said to have extended over the greater part of the worked out area of the sand pit, with a maximum proved thickness of about 2.5 m

Trial boreholes about 150 m E.N.E. of Grove Hospital [SP 9139 2338] proved up to 14.3 m of yellow-brown, medium to coarse-grained sands Cited Sands') beneath the Ouzel valley alluvium, overlying from 3.6 to 4.3 m of interbedded pale grey, fine-grained, laminated sand and very dark grey silty clay ('Brown Sands').

The section in the small working Brickyard Pit (George Garside Ltd), adjacent to Grovebury Farm [SP 9245 2373], excludes the topmost c2 m of the Woburn Sands which have been stripped off:

The only readily accessible face in Arnold's Pratts Pit (Billington Road) at the time of survey was at the north-eastern end [SP 9326 2408]:

The other working faces showed up to 8 m of strongly cross-bedded striped sands with numerous thin, dark layers of goethite grains (Plate 1).

The lengthy working face of Garside's flooded Grovebury Pit [SP 925 229] is inaccessible because the 'Red Sands' are dredged from below water level. The exposed face, 10–12 m high, consists dominantly of cross-bedded sands which appear to be similar to those recorded in nearby pits. Up to 5 m of sands are worked from below the water line. Beyond the working face the overlying Gault and drift deposits have been stripped off to reveal a bench cut in orange, buff, yellow and rusty brown, medium and coarse-grained, slightly pebbly sands.

Gault

Jukes-Browne (1900) records that a seam of phosphatic nodules was worked in the Lower Gault at Campton near Shefford. A well at the works was stated to be 28 ft (8.5 m) deep, through clay into sand, indicating that the nodule bed must lie about 7 m above the base of the Gault. The nodules occurred in a bed of light grey clay, and were scattered through a thickness varying between 9 in (0.23 m) and 2 ft (0.6 m). Calcareous shields of Belemnites minimus were so abundant that they had to be picked out by handfrom the washed nodules.

Jukes-Browne (1900) also mentions that a small pit (presumably [TL 0913 3880]) near the old Church at Clophill revealed in 1884, 'several feet of grey clay with phosphatic nodules passing down through sandy clay into clayey sand, the last enclosing large arenaceo-phosphatic nodules like those which occur in the zone of Ammonites mammillaris at Folkestone and elsewhere. This bed rests on yellow and brown sand.' The clay and sand with nodules almost certainly correspond to the 'Junction Beds' at Leighton Buzzard.

A nodule bed overlain by light grey marly clay was recorded (Jukes-Browne, 1900) in a shallow excavation by the roadside NW of Grange Mill near Sharpenhoe. The same bed was seen at the surface 'north-west of Great Faldo Farm, and has been found by trial borings about 18 ft (5.5 m) below the surface in the area near Brookend, and not far from the outcrop of the nodule bed at the base of the Chalk Marl, which was formerly dug for 'coprolites' at this place.'

Around Heath and Reach, just north of Leighton Buzzard, Gault is exposed as overburden in the following quarries, which exploit the Woburn Sands: Bryants Lane [SP 929 286], Reach Lane [SP 933 284], Mundays Hill [SP 937 282]. Chamberlains Barn [SP 931 268] and New Trees [SP 931 275]. Brief accounts of the Gault sections have been included with descriptions of the Woburn Sands above. Their stratigraphy has been recorded in some detail by Owen (1972) (except New Trees) and is not repeated here.

To the south of Linslade the following section was recorded in the north face of Ascott Farm Pit [SP 9090 2409]:

Gault is exposed in the south-west corner [SP 9110 2340] of the worked-out Tiddenfoot New Pit but is inaccessible because of flooding. An estimated 6 m of Gault, including the 'Junction Beds' can be seen to overlie about 8 m of cross-bedded Woburn Sands above the water-line. Owen (1972) recorded a section in this pit at [SP 9114 2355] of which the following is an abridged version:

The basal beds of the Gault are poorly exposed in a small working sand pit adjacent to Grovebury Farm [SP 9245 2368]. About: 6 m of grey clays of the Lower Gault containing sporadic phosphatic nodules can be seen in a degraded slope above the working sand face. The 'Junction Beds' are currently obscured by talus.

There is a rather better exposure of the same beds in Pratt's Pit (also known as Billington Cross Pit) [SP 9322 2400]. Numbers of nodule beds are those of Wright and Wright (1947).

Sections of the Gault and lists of its fauna in this pit have been published at various dates (e.g. Lamplugh, 1915; Wright and Wright, 1947; Hancock, 1958; Casey, 1961; Middlemiss, 1962; Owen, 1972). Wright and Wright distinguished four ill-defined bands of phosphatic nodules in the 'Junction Beds', the bottom three corresponding with those at Chamberlains Barn Pit [SP 931 268]. The present exposure, in which only two discrete nodule beds can be recognised, suggests that the nodule horizons are not everywhere easily distinguished.

The only clear section of Gault currently visible in the large Grovebury Sand Pit is at the south-east corner [SP 9275 2305]:

Owen (1972) recorded a much fuller section in this pit, abridged as follows:

Sections in a number of other abandoned, backfilled or overgrown sand pits immediately south of Leighton Buzzard are recorded in the literature (see especially Lamplugh, 1922; Bristow, 1963). These showed much the same sequences as those of present day exposures. Of particular interest, however, is a section recorded by Lamplugh (1922, p.38) of a small sand pit just east of Southcott mill pond [SP 9045 2435]. Within gritty clayey ferruginous loam containing phosphatic nodules (i.e. 'Junction Beds') there were lenticular bodies of fossiliferous limestone resembling the Shenley Limestone of the type locality. In this pit there was thus an association of the two major facies characteristic of the 'Junction Beds'.

Upper Greensand

No details

Lower Chalk

The Lower Chalk is well exposed in the former Blue Circle Cement workings at Sundon. The best exposures [TL 041 267] are to be found in the most recently abandoned part of a quarry, just north of the Chalton-Sundon road. Buff-grey (pale buff when dry), silty, marly, micaceous chalk is exposed for several metres at the base of the quarry face; in the quarry floor it is covered by a veneer of soft, weathered muddy chalk. The Totternhoe Stone, a pale buff, hard, massive limestone up to 1 m thick, overlies this marly chalk and forms a pronounced feature in the quarry face. It contains small pale brown phosphatic nodules and shell debris. The base of the Totternhoe Stone is relatively well-defined, but the top is indistinct, passing up into softer pale grey to pale buff soft marly limestones, which continue for several metres to the top of the workings. The Totternhoe Stone can be seen in several other exposures [TL 0366 2746], [TL 0383 2737], [TL 0407 2724] and [TL 0385 2764] farther north in the older workings. It can be traced south-westwards from the quarry along a poorly defined topographical feature to a road cutting with small exposures [TL 0380 2647] of harder limestone.

Between Chalton and Houghton Regis the position of the Totternhoe Stone as mapped is conjectural and may need to be revised after the area to the south has been surveyed.

To the north of Upper Sundon, blocks of soft fossiliferous buff-coloured marly limestone are ploughed up on some of the steeper slopes [TL 0435 2883], [TL 0445 2902], [TL 0476 2896] and [TL 0465 2912]. A small disused quarry [TL 050 290], worked in strata above the Totternhoe Stone, revealed about 1 m of soft buff manly limestone overlain by up to 2 m of harder more massively-bedded limestone believed to be the Totternhoe Stone.

Middle Chalk

No details

Appendix 4 Local details of superficial deposits

Boulder clay

Boulder Clay covers the plateau which extends from north-east of Ampthill eastwards to near Chicksands. Much of it lies above 100 m OD but, locally, for example around Beadlow [TL 105 385], its base descends about 50 m into the valley of the River Flit. It falls a similar amount into the valley south-east of Upper Gravenhurst.

At Kiln Farm [TL 0845 3860], to the north of Clophill, the basal beds of the Boulder Clay appear to have been dug in the past as a brick clay.

To the north and north-east of Silsoe [TL 080 345] the Boulder Clay is thin, with only the basal few metres being preserved. It consists of rather gravelly clay, which locally grades into small patches mapped as Glacial Sand and Gravel.

South-east of Pulloxhill the Boulder Clay occurs only as isolated small patches, a few hundreds of metres across, capping gentle ridges and hills on the Gault outcrop.

To the north of Steppingley an E-W aligned narrow strip of Boulder Clay occupies a channel cut into the underlying Woburn Sands.

Boulder Clay overlies the Gault on the high ground to the north and north-east of Hockliffe. Small isolated outcrops also occur at lower heights of about 100 m OD in the valley of the Clipstone Brook to the west of Hockliffe, demonstrating either that the valley was eroded by the advancing ice sheet or that it is of pre-glacial origin.

At Upper Sundon [TL 045 284] the Boulder Clay to the north of Home Farm is draped over the margin of the Glacial Sand and Gravel.

An extensive spread [TL 040 260] of Boulder Clay has been mapped to the south-east of Chalton. The deposit comprises an orange to red-brown pebbly clay, apparently chalk-free. The presence of derived Jurassic fossils and exotic pebbles demonstrates that it is not weathered bedrock. Locally the deposit becomes quite gravelly. Evidence from the adjacent railway cutting suggests that the clay is less than 2 m thick.

Most of the Ml motorway cuttings through the Boulder Clay are now completely grown over. Trial borehole logs for the road show that the thickness of the deposit on the plateaux is less than 10 m.

Exposures in the banks of the ponds in Woburn Park show heterogeneous bluish grey and brown weathered, variably sandy, stony clays with gravelly sand intercalations. Near the Safari Park [SP 9739 3385] the basal 1 m or so consists of similar material overlying crudely interbedded, slightly chalky clay and orange-brown slightly gravelly sand, possibly a flow-till incorporating local Woburn Sands bedrock.

South of Woburn Abbey there is an extensive tract of Boulder Clay carrying a typical suite of erratic and local rocks (see p 10). However, about 600 m WNW of Brook End, Eversholt [SP 978 328], there is a preponderance of Jurassic cementstones, septarian nodules and shelly limestones. A water well in Woburn Park [SP 9657 3133] encountered 32.3 m of Boulder Clay containing beds of sand.

The higher ground around Heath and Reach, and north to the A5, is capped by Boulder Clay, which is exposed as overburden in some of the working sand quarries. Thin remnants of Boulder Clay also occur on Shenley Hill [SP 933 269]. The till is typically a stiff blue-grey clay with clasts of chalk, flint, 'Bunter' pebbles, Jurassic rocks and occasional far-travelled erratics. Locally, layers or lenticular pockets of brown sandy chalky gravel occur within it.

In the A5 pit of P. Bennie Ltd [SP 932 304] Boulder Clay is up to 3.5 m thick, increasing from north to south. The thickest sequence is as follows:

On the north-west face, beneath the beds described above, a small channel about 3 m wide by 1 m is cut into the Woburn Sands; it is filled with sandy stony clay. The base of the Boulder Clay undulates slightly in this pit.

At the northern limit of the workings in Hall's Fox Corner pit [SP 9277 2936] typical blue-grey, chalky Boulder Clay infills a steep-sided channel, 25 m wide by 6 m deep, cut into the Woburn Sands. On the eastern face there is another channel (or a continuation of the same) [SP 9279 2929] of similar dimensions which can be traced into the adjacent Buckland's Fox Corner Pit [SP 9278 2924] where it is 2 m deep and filled with grey brown sandy clay containing chalk and flint clasts.

In Arnold's Stone Lane Quarry [SP 929 289], the overburden comprises up to 10 m of Boulder Clay and up to 6 m of Glacial Sand and Gravel. The latter fills a steep-sided channel about 50 m wide, cut into the Woburn Sands and trending approximately WNW–ESE. It is well seen in the eastern part of the workings. The Boulder Clay comprises typical blue-grey chalky till with clasts of flint, 'Bunter' pebbles and Jurassic limestones. The Glacial Sand and Gravel infilling the channel comprises medium brown sandy clayey gravel interbedded with layers of till. In some parts of the pit its top is caicreted to a depth of 0.5 m, producing massive concrete-like slabs with clasts of chalk, flint and Jurassic rocks in a hard sandy matrix. Beneath this channel the 'Brown Sands' of the Woburn Sands are stained crimson. This may result from the oxidation of iron minerals in the sands by waters percolating from the gravels. Similar staining occurs at Buckland's Fox. Corner Pit, where it affects the 'Silver Sands.'

In the L B Silica Sand Co's Bryants Lane Pit [SP 929 286] up to 10 m of typical Boulder Clay is removed as overburden. It includes an interbedded layer of sandy gravel 1 to 2 m thick, 1 m above the base. Further south in Buckland's Reach Lane Quarry [SP 933 284] recent earth-moving has exposed up to 12 m of Boulder Clay at the eastern extreme of the workings. The Boulder Clay includes irregular pockets and layers of brown sandy gravel, which are sometimes water-bearing.

Boulder Clay is also exposed at Garside's Mundays Hill Quarry [SP 937 282] where it comprises up to 6 m of blue-grey and grey-brown chalky/flinty till, resting with a sharp planar junction on the Gault.

West of the Ouzel valley at Leighton Buzzard, Boulder Clay, associated in a complex fashion with Glacial Sand and Gravel, is present on much of the higher ground; there are, however, few exposures of any size. In some cases near Stoke Hammond and Soulbury, the Boulder Clay has steep contacts with the underlying solid rocks, suggesting the presence of buried channels related to that of the Ouzel valley in the Milton Keynes area (Horton and others, 1974). There is, however, no borehole information whatever to provide supporting evidence for such channels.

South and east of Leighton Buzzard there is an extensive area of almost drift-free Gault clay outcrop with only a few small scattered patches of Boulder Clay. A few of these cap ridges of Gault clay, on which the drift base is between 100 m and 125 m OD. The patch at Billington contains small bodies of gravelly sand (e.g. SP 9385 2230) which have not been separately mapped.

In the Ouzel valley there are lower-lying patches of chalky till at an elevation of 85 m to 90 m OD. The most northerly of these [SP 921 241] adjoins the flooded sand pit close to Grovebury Road. This is- almost certainly the 'Rockley Hill' pit referred to by Lamplugh (1915) in which he recorded stiff bluish boulder clay filling a steep-sided 'ravine' in the Woburn Sands. The thickness of Boulder Clay exceeded 6 m and it contained much chalk, flint and 'Oolitic' (i.e. Jurassic) detritus, including Gryphaea shells; ice-scratched boulders of Carboniferous Limestone and basalt were also noted.

The largest outcrop of Boulder Clay in this area is just north of Slapton [SP 933 212] where a thin spread of very chalky clay, probably no more than 2 m thick at most, fools the Ouzel valley.

In the vicinity of Wing the Boulder Clay contains extensive beds of Glacial Sand and Gravel. The basal part contains much clay of local derivation and is thus difficult to differentiate from the underlying bedrock.

The following sequence was recorded by Davies (1915, p.92) at the former brickworks in Wing [SP 881 232]:

It was noted that to the north the 'lowest beds rose up and the lower Boulder Clay disappeared altogether, while on the other side of the brook they apparently dipped down again.' The valleyside succession [SP 880 229] nearest to this site shows a simpler sequence of gravels (about 8 m) overlying Boulder Clay (about 2.5 m) which in turn rests on bedrock. Thus both of the boulder clays may be lenticular in form.

An unsited well (SP82SE/9) in Wing proved 3.7 m of gravel overlying about 4 m of Boulder Clay, again resting on bedrock. Another well (SP82SE/10), to the north-east of the village [SP 8935 2420], penetrated 9.1 m of Boulder Clay overlying at least 10.4m of Glacial Sand and Gravel.

In the vicinity of Ascott Home Farm [SP 990 231] a fourfold succession of drift is present i.e. Boulder Clay, Glacial Sand and Gravel, Boulder Clay, Glacial Sand and Gravel in descending order. The bedrock surface is lower in this area and there may be a genetic link between the presence of such multi-layer sequences and of broad scale channelling of the bedrock surface.

Buff stoneless silts occur locally, particularly in the area [SP 878 219] to the south of Wing where they apparently show interdigitating relationships. Although there are no exposures it is thought that these sediments may be bedded and perhaps represent phases of subaqueous deposition.

On the higher ground in this general area the soils developed on the Boulder Clay locally contain abundant pebbly material, suggesting the former presence of bodies of Glacial Sand and Gravel. In such tracts, the Boulder Clay has suffered decalcification.

It has been shown (Horton, 1970) that some of the major river valleys of the south-east Midlands have buried channels filled with glacial deposits. It is inferred that these depressions form parts of a relict pre-glacial drainage system which is being progressively exhumed. One of the buried channels has been traced up the Ouzel valley to Stoke Hammond (Horton and others, 1974) at the northern margin of the present survey area. It is filled with Boulder Clay associated with considerable thicknesses of glacial lake sediments.

Upstream from Leighton Buzzard the only evidence of a possible buried channel is obtained from boreholes at the back of the First Terrace, to the south of Leighton Buzzard railway station. A well at the former pumping station [SP 9097 2445] proved 8.23 m of drift, mainly sand and clay with some gravel at the base, of which no more than the top 1.5 m is likely to be First Terrace material. Boreholes over a distance of 400 m to the south of the well record sequences in grey, brown and yellow clays and sandy clays up to a maximum of 18 m. Further to the south, however, there is no evidence of a continuation of a buried channel.

Glacial sand and gravel

Chalky gravel with angular and cobble-size rounded nodular flints have been dug to a depth of about 8 m from a now overgrown pit [TL 1163 3622] about 400 m north-east of Upper Gravenhurst Church. About 700 m to the east, a spread of Glacial Sand and Gravel has been mapped down the valley side from the edge of the Boulder Clay. Augering of the deposit indicates a composition ranging from a hard chalky silt to a sandy gravel. Its thickness is unknown.

Gravels composed predominantly of poorly-sorted angular and nodular flints, and chalk pebbles, occur in the valley west of Standalone Farm [TL 131 373].

Patches of sand and gravel on the higher ground at Meppershall appear to underlie the Boulder Clay.

To the north-east of Flitwick [TL 035 350] an extensive area of Glacial Sand and Gravel caps much of the higher ground, masking the junction between the underlying Woburn Sands and the Corallian clays. A partially degraded section [TL 0353 3521], with over 2 m of poorly-sorted sand and gravel composed mainly of angular and sub-angular flints, was noted off Brookes Road, Flitwick. The sand and gravel hereabouts probably nowhere exceeds a few metres in thickness.

An E-W train of gravel is present along the south side of the stream to the north of Steppingley [TL 012 354]. The gravels, although following the course of the present day stream, occur at variable heights above the floodplain and are thus believed not to be River Terrace Deposits, but more likely to represent the eroded basal infill of a pre-glacial valley along the line of the present valley.

At Millbrook Golf Club, Glacial Sand and Gravel crops out from below the margins of a small patch of Boulder Clay [TL 002 383). The gravel consists mainly of sub-angular flints with subordinate rounded pebbles of 'Bunter' quartzite. Similar patches of sand and gravel are present around the margin of the more extensive area of Boulder Clay to the north-east of Ampthill. About 800 m ESE of Houghton Park Farm a section in a small disused pit [TL 0498 3922] revealed up to 3 m of poorly-sorted gravel and sand, cemented by silica to form a hard conglomeratic rock.

In the Hockliffe area the Glacial Sand and Gravel consists mainly of poorly-sorted, fine to medium-grained, silty sands with lenses of chalk and flint gravel, and of buff silt. The deposit occurs in lenses at the base of the Boulder Clay and above it as isolated patches on the higher ground. One occurrence [SP 977 278] near Watergate Farm apparently occupies a channel within the basal part of the Boulder Clay. The maximum thickness of the Glacial Sand and Gravel in this area is about 4 m; the high level examples are very flinty at the surface, probably degraded and of variable thickness due to the effects of cryoturbation.

In the area between Tingrith and Toddington an impersistent bed of sand and gravel lies about 5 m above the base of the Boulder Clay. In places it has an extensive outcrop, but this may not reflect its true thickness; it is possible that the sand and gravel and the overlying drift may be 'draped' over an inclined bedrock surface. To the north-east of Toddington, and locally elsewhere, this sand and gravel rests directly on the Gault. Field relationships in the vicinity of old workings [TL 019 281] near White Hart Farm indicate that the base of the dominantly sandy lithology here may be markedly channelled.

Capping the higher ground at Toddington, and about 15 m higher in the succession, there is an extensive spread of sandy gravels which have been dug sporadically in the past. It is possible that, towards the margins of this spread, 'draping' may also have occurred, and thus the outcrop pattern may suggest an exaggerated thickness. It is estimated that there may be up to 10 m present.

On the evidence of a gravelly soil in allotments [TL 036 347] and private gardens, Glacial Sand and Gravel has been mapped underlying much of the southern part of Flitwick.

Around Westoning an extensive spread of sand and gravel occupies the low ground adjacent to the River Flit. Although some of these deposits occupy positions normally associated with river terraces, they do not display a typical terrace morphology and locally are overlain by boulder clay. In this area gravels and gravelly sands predominate, the gravel fraction consisting mainly of rounded 'Bunter' pebbles and sub-angular flints up to about 5 cm in diameter.

An extensive tract of sand and gravel caps much of the higher ground around Upper Sundon. A pit section [TL 0442 2747] about 10 m high revealed a cross-bedded sequence of gravels composed predominantly of rounded 'Bunter' pebbles, sub-angular flints and chalk pebbles, with diameters up to about 5 cm, although larger pebbles up to about 10 cm were also noted. The gravels have a variably sandy matrix. Several beds up to about 10 cm thick of slightly pebbly grey-brown clay occur within the sequence. Discontinuities within the deposit, emphasized by the cross-bedding, cannot be satisfactorily explained by changes in sediment transport direction or solution of the underlying chalk, and may result from glacio- tectonic deformation caused by an over-riding ice-sheet.

The surface expression of the sands and gravel around Upper Sundon might suggest that they have the form of a sheet-like body, but an examination of the east face [TL 0430 2710] of the Sundon Cement Works reveals a series of gravel-filled channels and pipes cut into the chalk. One channel [TL 0425 2683] contains a gravel with boulders of flint, sandstone, limestone and igneous rocks, interbedded with sands and thin seams of grey-brown clay.

Between Lidlington and Salford, Glacial Sand and Gravel is largely restricted to a number of small outcrops which lie about 1 km north of the M1 motorway. Most of the sand and gravel at outcrop is probably less than 2 m thick. However, a deposit north-west of Hulcote Farm [SP 950 387] was formerly dug to depths of 3 to 4 metres and is unusual in containing a high proportion of 'carstone' fragments derived from the Woburn Sands.

South of Eversholt several small lenticular bodies of slightly gravelly sand occur within the Boulder Clay, in the vicinity of Oakhill Spinney [SP 994 311]. A larger body of similar material is present within Woburn Park at and near Speedwell Belt [SP 960 317]; small exposures are visible around Upper Hopgarden Pond [SP 964 319].

Small spreads of gravelly sand, locally rather clayey, occur at and near Milton Bryan [SP 963 303], [SP 970 311] and [SP 972 303]. Similar, but better sorted material caps the ridge on which Berry End Farm stands [SP 983 343]. Small bodies of sand and gravel at the base of the Boulder Clay occur at Hills End [SP 977 332] and north-east of Eversholt [SP 993 338].

Remnants of Glacial Sand and Grayel cap some of the higher ridges in the Woburn Sands outcrop, south of Great Brickhill, where it may be up to 10 m in thickness. They appear to be at levels comparable to local deposits of Boulder Clay, and are genetically related to them. There are no exposures of any size in these deposits at present; the few former pits are overgrown and degraded. The general composition of these deposits is brown sandy gravel, with fine to coarse-grained clasts of flint, chalk and 'Bunter' pebbles.

East of Leighton Buzzard small isolated remnants occur on low hills in the valley of the Clipstone Brook and its triburaries. In the area west and north-west of Leighton Buzzard, Glacial Sand and Gravel occurs extensively in association with Boulder Clay. It may occur at the base, within or on top of the till. It generally gives rise to fairly light, sandy gravelly soils, in contrast to the heavier clay soils characteristic of the Boulder Clay. Springs often issue where these deposits rest on impermeable Boulder Clay or bedrock.

Between Linslade and Stewkley North End the deposits occur in an irregular swathe up to 2 km in width with an approximate WNW–ESE trend. Several old pits north-east of Stewkley mark the former local use of the gravels and sands [SP 854 271], [SP 857 269], [SP 859 269] and [SP 862 268]. The results of a commercial investigation for gravel near Vicarage Farm, Stewkley [SP 860 268], showed between 10 and 20 m of assorted sands, chalky gravel, sandy gravel and clays occupying a channel cut through the Boulder Clay to bedrock. The channel varies in width between 60 and 150 m and has a WNW–ESE trend.

The deposits have also been worked around Hill Farm, Hollingdon [SP 873 272], where an old pit up to 5.5 m deep is still evident though largely overgrown. In Soulbury, a large erratic block of Carboniferous Limestone (presumably from Derbyshire), 1.1 by 0.7 m in dimensions, is preserved [SP 8829 2709] in a minor lane off High Road. It may have come from a former pit north-west of the church [SP 882 271], but has reportedly been in its present position at least since Cromwellian times.

There are sporadic small patches of gravel and gravelly sand east and south of Leighton Buzzard, capping low hills of varying elevation. The gravels at Redborough Farm are rather clayey. Small lenses of slightly gravelly sand within the Boulder Clay, which caps the ridge at Billington [SP 9385 2230], were noted but not separately mapped.

Large outcrops of gravelly sands occur around Wing [SP 882 239] and [SP 882 228], forming sheet-like bodies within the Boulder Clay. Locally, in this area, small pockets of sand were detected within the till which could not be delineated with confidence.

South of Leighton Linslade the Glacial Sand and Gravel lies below the Boulder Clay and forms outcrops at the margin of the higher plateau area. It varies from reddish and orange-brown medium-grained sands with little gravel to very gravelly sands. Just north-east of Ascott Farm [SP 903 234] the deposit contains much chalk. Boreholes hereabouts proved up to 6.8 m of gravel. In one [SP 9050 2365], 4.5 m of gravel were found to overlie 3.7 m of Boulder Clay, in turn resting on Gault. The Boulder Clay here is probably a local lens at the base of the glacial gravels.

Glacial Sand and Gravel deposits at the extreme south-west corner of the survey area [SP 851 203], south of Vicarage Farm, consist mainly of orange-brown, medium-grained sand with only a small gravel content. The thickness is estimated to be 8 m.

Head

Head locally drapes the escarpment of the Woburn Sands in the area to the north of Ampthill. It consists of brown clayey sands, which become increasingly clayey downslope with the incorporation of material derived from the Corallian and Oxford Clay. There are few sections in this area, but about 1 m of clayey silty sand containing scattered small pebbles, mainly of angular and sub-angular flints, is exposed behind the tuition ring at the Equitation Centre [TL 0114 3875].

Extensive deposits of Head cover the lower ground between Ampthill and Flitwick. Here they are variable in composition, having been derived from several different sources, and include sandy clay, pebbly clayey sand and clayey gravel.

In the Hockliffe area the Head deposits are dominantly soft to firm, brown, flinty, sandy clays and sandy loarns; peaty silts are present below spring lines. Clayey flint gravels are common near the base of the Head, which is typically 2 to 3 m thick. Lenses of redistributed Gault clay occur in places.

The gravelly, sandy nature of the Head deposits along the lower slopes of the valley of the River Flit between Flitwick and its source near Chalton reflects their derivation mainly from the adjacent outcrops of Glacial Sand and Gravel.

South of Toddington M1 Services the Head deposits are derived mainly from the Gault and Upper Greensand and therefore slightly pebbly clays and silts predominate.

Between Great Brickhill and Lidlington, Head composed of sand with variable admixtures of clay and containing 'carstone' fragments commonly extends downslope from the escarpment of the Woburn Sands, obscuring the junction between the latter and the Oxford Clay. The resulting tract of Head is generally between 100 m and 150 m wide, but is locally more extensive as, for example, south-east of Husborne Crawley [SP 962 358] and west of Woburn Sands [SP 905 353].

The Head is probably thickest adjacent to the Woburn Sands outcrop where it may be several metres thick but, more distant from the outcrop, thicknesses of less than 2 m are usual.

Head consisting of wash derived from the Second Terrace occurs north of Wavendon [SP 901 392] and just south-west of Ridgmont Brickworks [SP 960 374]. It is characterised by pebbly sandy clays and clayey sands, usually between 1 m and 2 m thick.

Deposits of brown sandy loam with 'carstone' debris and pebbles derived from the glacial deposits occur in the floors of valleys falling away from the Woburn Sands outcrop between Great Brickhill and Leighton Buzzard.

In the Ouzel valley, the Head is derived more from glacial deposits and has a larger proportion of flints and 'Bunter' pebbles, set in a brown sandy clay matrix. It merges downslope with River Terrace deposits where these are present. West of the R. Ouzel, similar Head deposits occur in the valleys of the several north-easterly draining tributaries.

In the Wing area the Head was largely derived from the Boulder Clay, Gault and Kimmeridge Clay, and thus consists dominantly of firm brown and grey flinty clays. Locally there are soft stoneless grey clays derived solely from bedrock lithologies. Where Portland Beds crop out on valley slopes the Head includes a proportion of angular limestone debris.

These deposits fill valley bottoms and grade down to the river alluvium at Ledburn. Up to 5 m may be present in the major valleys and they may be composite, comprising multiple solifluction lobes of variable derivation. A basal gravelly bed is commonly present.

The outcrop east of Ascott Farm [SP 906 232] consists of orange-brown clayey sand derived from Boulder Clay and Glacial Sand and Gravel. Variable clayey, sandy and silty loams occur west of Southcourt Stud Farm, Leighton Linslade [SP 901 244].

River Terrace Deposits

Second Terrace Deposits

Second Terrace deposits near Salford are separated from those of the First Terrace by a valley slope of Oxford Clay. They attain a height of 2 to 4 m above the present floodplain. They have been dug for gravel, but there are no current exposures.

In the Ouzel valley there are small patches of Second Terrace deposits near Nares Glady Farm [SP 258 280] and at Oak Farm, Stoke Hammond [SP 887 293]. The largest deposit, in Leighton Buzzard between Page's Park [SP 929 243] and the industrial estate [SP 938 248], is built over except at Page's Park. A small exposure in the face of an old sand pit [SP 9273 2426] shows about 1 m of fine to medium sandy gravel with bands of bedded fine-grained sand containing gravelly strings.

First Terrace Deposits

Small relict patches of a formerly more extensive First Terrace occur along the north side of the River Flit between Maulden and Clophill. In many places the surrounding Head is probably underlain by Terrace Deposits at shallow depth.

The Terrace Deposits have been cut into by the Maulden to Clophill Bypass where a section [TL 0510 3611] reveals up to 1.5 m of poorly-sorted sand and gravel resting on Woburn Sands. To the north of the cutting the terrace deposits merge upslope with lithologically similar Glacial Sand and Gravel.

Larger areas of Terrace Deposits between Clophill and Shefford are poorly exposed but again give rise to broad flats with gravelly soils. They were formerly worked to a depth of several metres in a now overgrown pit [TL 1130 3838] to the north of the Ampthill Road.

Two small benches underlain by sand to the north-east of Tingrith have been assigned to the First Terrace.

The First Terrace has an extensive outcrop just west of Salford [SP 922 390] where it reaches a maximum height of about 2 m above the level of the floodplain. Ditch and stream sections indicate that it is generally less than 2 m thick. The deposit consists mainly of patinated sub-angular flints and rounded 'Bunter' quartzite pebbles with a matrix of medium to coarse-grained sand. Just south of Husborne Crawley the terrace appears, from surface indications, to be composed mainly of pebbly sand, the sand fraction presumably being derived from the nearby outcrop of the Woburn Sands.

There are extensive outcrops of the First Terrace in and around Leighton Buzzard, in the valleys of the R. Ouzel and its tributary the Clipstone Brook. Downstream from Leighton Buzzard a number of separate outcrops occupy the slip-off slopes of meander cores in the Ouzel- valley, facing degraded river cliffs on the opposite side of the floodplain.

In the river bank just north of Ledburn [SP 9052 2252] 1.2 m of medium gravel overlies Gault clay. It consists mainly of sub-angular flints and quartzite pebbles, with subordinate quartz, 'carstone' and jasper pebbles. The matrix is a poorly sorted, clayey, medium to coarse-grained sand.

South-east of Grove Lock [SP 9201 2269] a temporary excavation revealed the following:

The terrace deposit is exposed in the northern face of Tiddenfoot New Pit [SP 9120 2396] where the section is:

A trench section at Leighton Buzzard [SP 2427] showed:

Alluvium

A tributary of the River Flit flows along the E-W valley separating Ampthill and Flitwick, joining the Flit to the west of Flitton. The present day stream is small and obviously a misfit. Patches of Glacial Sand and Gravel along this valley to the north of Steppingley demonstrate its antiquity and suggest that in pre-glacial times this might have been the main river valley. Alluvial silts and clays have been deposited along the valley bottom. The floodplain is usually less than 200 m wide, but locally, for example to the south-west and south-east of Ampthill, it widens considerably. This may have resulted from ponding-up of the valley by Glacial Sand and Gravel to the north of Ruxox Farm [TL 0475 3595]. The alluvial deposits, mainly silts and clays, are generally less than 2 m thick and locally become sandy or gravelly [TL 0425 3665], or have a gravelly base [TL 0430 3646].

The floodplain of the River Flit is typically several hundreds of metres across. Between Flitton and Shefford the floodplain deposits are composed largely of dark brown to almost black silts, very rich in organic matter. These have been differentiated from the general Alluvium and are shown as Peat on the map. The junction between the Alluvium and the Peat is gradational and arbitrary. In places, gravel underlies the alluvial silts and clays; this may represent a gravelly base to the alluvium or, alternatively, be part of an older terrace deposit underlying the Alluvium.

An un-named stream, with an alluvial tract some 100 to 150 m wide, flows north-east through Lower Gravenhurst to Shefford. Its alluvial deposits consist of dark brown silts, rich in organic matter. In many places the silts are seen to overlie gravel at depths of between 1 and 1.5 m. It is thought that the gravels form an integral part of the alluvial sequence as there are no terrace gravels adjacent to the margins of the floodplain.

Narrow strips of alluvial silts and clays, up to about 100 m in width, -occupy the bottoms of several broad NE-SW trending valley to the north-east of Barton-le-Clay. The edge of the alluvial deposits against the surrounding weathered Gault clay is in places difficult to determine as it is not always associated with a distinct break of slope.

Alluvial deposits consisting mainly of brown silty clays, commonly with pebbles of flint and quartzite in 'stringers' near their base, floor the valley of the un-named tributary of the River Ouzel downstream from Husborne Crawley. In the upper reaches of this valley, just north-west of Woburn, clayey sand and sandy clay lithologies reflect the composition of the Woburn Sands over which the stream flows. Peat has been recorded within the deposits east of Birchmoor Farm [SP 9495 3448].

Near Old Linslade Manor the floodplain is notably boggy and three trial boreholes [SP 913 272] proved up to 5.5 m of Alluvium. An area north of the Manor has been reclaimed by tipping of waste.

West of Nares Glady Farm several river bank exposures provide useful sections in the Alluvium. One [SP 9070 2761] shows 1.6 m of grey-brown sandy clay resting on 0.4 m of peat, with 0.1 m of gravel at the base overlying Oxford Clay. A little to the north [SP 9071 2770], another section shows 0.9m of grey-brown silty clay on up to 1.0 m of fine to medium gravel, with clasts of flint, 'Bunter' pebbles and 'carstone' from the Woburn Sands, resting on Oxford Clay. The gravel has cobbles up to 0.2 m in diameter and some ferruginous staining and cementation. It probably represents a First Terrace remnant for the most part.

North of Paper Mill Farm a river bank section [SP 8924 2951] shows up to 2 m of grey-brown sandy silty clay; the base is not seen. Approximately 0.5km east of Stoke Hammond lock another river bank exposure [SP 8913 2990] shows 1.3 m of grey-brown silty clay on up to 1.3m of brown, medium to coarse sandy gravel.

The headwaters of the River Ouzel, east of Slaptonbury Mill [SP 9368 2139], drain the Gault clay outcrop. Its Alluvium consists of up to 1.5 m of brown silty and sandy clays containing freshwater gastropod shells, scattered small flints and a few small chalk pebbles. At the base there is a thin flinty gravel. A similar sequence occurs in the tributary valley passing through Slapton Lock [SP 9285 2015].

The tributary known as Whistle Brook drains from the chalk scarp at Ivinghoe and is confluent with the River Ouzel near Slaptonbury Mill. Its floodplain and that of the River Ouzel below the confluence are underlain by Alluvium containing abundant chalk debris. This Alluvium typically consists of a thin surface spread of brown silty clay, overlying white clayey chalk detritus containing scattered small angular flints, and quartzite and chalk pebbles. At the base there is commonly a thin gravel bed containing flint, quartzite and chalk clasts.

A total thickness of 2.5 m was proved in a borehole just north-west of Slaptonbury Mill c [SP 9343 2147]. Further downstream c [SP 9145 2335], below Grove Lock, boreholes have proved up to 5.9 m of Alluvium consisting mainly of grey and brown, silty and sandy clays with scattered chalk, flint and quartzite pebbles and some sand and gravel layers. The deposit becomes more gravelly towards the base. One of these boreholes [SP 9151 2341] encountered 3.5 m of Alluvium resting on 1.2 m of gravelly sand with thin clayey layers, overlying Woburn Sands. The gravelly sand probably represents a sub-alluvial First Terrace deposit.

In the tributary valley through Ledburn to Grove Lock up to 1 m of brown sandy clay with scattered small flints overlies up to 0.5 m of poorly sorted medium to coarse-grained gravel, containing mainly sub-angular flints and flint pebbles, and quartzite and quartzitic sandstone pebbles up to 0.10 m in diameter. Quartz and 'carstone' pebbles were also recorded.

The south-east face of Grovebury Sand Pit [SP 9151 2341] intersects a tract of Alluvium in which dark brown loamy soil (0.5 m) overlies brown, slightly sandy clay with sporadic gravelly strings (up to 1.0 m), resting on a basal fine to medium-grained flint and quartzite gravel (up to 0.9 m).

Peat

No details

Landslip

About 1 km west of Upper Gravenhurst small translational slips occur in the Gault clay along a steep-sided ENE-WSW trending ridge [TL 101 356] capped with Boulder Clay.

At Pulloxhill, slipped, hummocky and uneven scrubland is present on a steep slope [TL 064 337] in Gault clay to the south-east of the village church.

Within the General Motors Proving Ground at Millbrook construction of the deep cutting [TL 004 390] housing the circular test track has resulted in oversteepening of the slope composed of Woburn Sands, Corallian and Oxford Clay, causing slipping. The area of slipped ground consists of a mixture of sand and clay.

At the nearby Equitation Centre landslipping has occurred on a natural slope [TL 011 386] at about the same stratigraphical level as the slips in the Proving Ground. Slipping may also have occurred in the past in an adjacent field [TL 0125 3870].

An area of slip [SP 959 286] near Battlesden Park, which affects mainly Gault and Boulder Clay, may have been induced by the impounding of the adjacent lake to the north.

A shallow translational slip of the Upper Greensand and Gault on the steep slope about 100 m south-east of Samshill Farm, Westoning, has given rise to an area of extremely hummocky ground.

Large areas of landslipped Oxford Clay and Ampthill Clay are present to the south and south-west of Lidlington, and to the south of Bow Brickhill, both on slopes of about 8°. There are also two small landslips in these clays to the north of Little Brickhill [SP 906 329] and [SP 912 327]. The slips appear to be mainly of the translational type with the development of many small back scars, some more than 1 m high. Much of the slipping is still active, although some areas now appear to be stable. At Lidlington some parts of the slip have recently been bulldozed to produce a more uniform slope, but these areas are already showing considerable signs of renewed movement.

Other potentially unstable areas are present around the margins of the disused brick clay pits at Lidlington and Brogborough where steep quarry faces have been left.

There is a small landslip in Boulder Clay on the valley slope immediately north-west of Grange Farm, Milton Bryan [SP 970 315]. It is characterised by uneven hummocky ground, with a degraded back scar.

A small slip in Oxford Clay and Ampthill Clay, below the outcrop of the Woburn Sands, occurs on a 6° slope, 350 m north-east of Stapleford Farm [SP 897 286].

About 1 km south of Linslade [SP 906 237] there is a degraded landslip in Gault clay on a slope of about 6°. A prominent scar at the back of the slip is aligned with the margin of the Glacial Sand and Gravel outcrop, from whence numerous springs emerge, giving rise to saturated ground. The freshness of the back scar suggests that slipping has occurred in relatively recent times.

An area of the valleyside [SP 8775 2380] at Burcott shows evidence of landslipping in Boulder Clay. The angle of slope here is only 7°, but the instability of the ground is probably exacerbated by springs which emanate from the Glacial Sand and Gravel upslope.

Slopes of up to 10° are present on the valleyside [SP 877 221] to the south-west of Wing, but these show no evidence of instability. Probably the presence of silts in the Boulder Clay here provides better drainage and hence stable conditions.

In an intervening tract [SP 877 225], where Boulder Clay overlies Kimmeridge Clay, a scar-like feature may have resulted from landslipping or from disturbance of the ground by man. The average angle of slope is here less than 6°.

Made ground

The land enclosed within the General Motors Vehicle Proving Ground [TL 005 395] at Millbrook has been extensively landscaped and now includes many areas of Made Ground, generally composed of clay or sand moved from elsewhere on the site; in places concrete, brick and other rubble have been dumped. Within the site there are almost certainly other small areas of made ground that have not been delimited during the survey.

At the now demolished brickworks at Ridgmont [SP 968 380] much brick debris and concrete rubble remains; large quantities of reject brick have been dumped to the north of the old works and, in the extreme north-east of the site, up to 20 m of brick, ash and concrete are present.

There are a number of scattered small tracts of made ground in which the fill material has not been identified. Embankments along railways and the M1 Motorway are assumed to be largely composed of material dug from adjacent cuttings.

Appendix 5 Principal areas of made ground

Appendix 6 Abridged logs of BGS boreholes

Ascott Farm Sandpit Borehole [SP 9078 2408]

BGS registration number (SP92SW/142) Surface level c+102m above O.D. Drilled by Anglia Drilling Ltd in 1986. Samples examined by B.M. Cox and R.J. Wyatt

Woburn Sands A Borehole [SP 9308 3438]

BGS registration number (SP93SW/174). Surface level (base of working pit) c+96m above O.D. Drilled by Anglia Drilling Ltd in 1986. Samples examined by B.S.P. Moorlock

Woburn Sands B Borehole [SP 9323 3526]

BGS registration number (SP93NW/62) Surface level +c123m above O.D. Drilled by Anglia Drilling Ltd in 1986 Samples examined by B.S.P. Moorlock

Appendix 7: Glossary

ANAEROBIC Lacking oxygen

AQUICLUDE A body of relatively impermeable strata that does not readily transmit ground water

AQUIFER A bed or group of strata which yields water, because it is either porous or pervious

BIOGENIC Material produced by the action of living organisms

BIOTURBATION The disturbance of a sediment by burrowing organisms

BITUMINOUS Containing bitumen, a substance composed of a mixture of hydrocarbons

BIVALVE A mollusc which has a shell consisting of two valves which open and shut by movement along a hinge line

'BOXSTONE' A hollow concretion composed of iron compounds which commonly has a banded structure

BUNTER PEBBLES Pebbles derived from Lower Triassic sediments which were formerly known as the Bunter Pebble Beds.

CALCAREOUS Having a significant percentage of calcium carbonate

CALCITE A crystalline variety of calcium carbonate (CaCO3) common in sedimentary rocks and in certain fossil shells

'CARSTONE' A limonite cemented, hard, gritty sandstone

CEMENTSTONE A clayey limestone that is suitable for cement making

CHERT A rock composed mainly of a very finely crystalline form of silica

CHONDRITIC MOTTLING Colour mottling caused by the contrast between the host sediment and that filling the branching burrows of the trace fossil Chrondrites

CLAST Rock fragment or pebble

COCCOLITH A microscopic calcite plate, many of which together form the skeleton of marine algae

CONCHOIDAL Describes a form of curved, concentrically ribbed fracture of certain rocks

CONCRETION A hard, rounded or irregular mass formed by the aggregation of mineral matter, commonly around a nucleus, by precipitation from an aqueous solution

CONDENSED SEQUENCE A term used for a series of beds which show a much thinner development than equivalent beds elsewhere

CONFINED AQUIFER An aquifer bounded above and below by impermeable strata

CONGLOMERATE A coarse-grained rock consisting of rounded rock fragments, generally in a matrix of finer sediment

COPROLITES Phosphatic nodules

CROSS-BEDDING The internal layering of a bed of sediment, characterised by inclined minor beds disposed at various angles to the principal bedding planes

DECALCIFIED Said of soils or rocks from which calcium carbonate has been leached out

DIORITE A coarse-grained igneous rock

DIP The maximum angle of inclination of a bed measured relative to the horizontal

DIP SLOPE See ESCARPMENT

EPIFAUNA Fauna living on rather than below the surface of the sea floor; also organisms living attached to other organisms

ERRATIC A rock which has been transported some distance from its source, commonly by moving ice sheets

ESCARPMENT An asymmetrical ridge with one long gentle slope (dip slope) conforming to the dip of the strata, and an opposite steep slope (scarp face) cutting across the bedding

EUSTATIC Pertaining to world-wide changes in sea level

EXOTIC See ERRATIC

FACIES The sum of all primary lithological and palaeontological characteristics displayed by a sedimentary rock from which its origin may be inferred

FAULT A fracture in rock along which relative movement has taken place

FAULT-PLANE Plane of movement of a fault

FEATURE Topographical ridge or hollow formed by surface weathering of relatively hard or soft rocks respectively

FELDSPAR A common rock forming aluminium silicate mineral

FERRUGINOUS Stained or cemented with iron oxide

FORAMINIFERA Very small to microscopic, mainly marine organisms having a calcite shell

FORESET BED One of the inclined, internal layers of sediment forming a cross-bedded unit

FULLER'S EARTH A clay rich in the clay mineral montmorillonite and capable of adsorbing oil and grease

GEOTECHNICAL PROPERTIES The mechanical properties of rocks which can be applied to civil engineering projects

GLAUCONITE A dull green iron silicate mineral found in marine sediments

HOLOTHURIA Marine organisms related to sea urchins which have a skeleton composed of calcite spicules

HORNFELS A granular metamorphic rock IGNEOUS Derived from molten rock (magma)

ILLITE A clay mineral which is essentially a potassium aluminium silicate

INLIER A limited outcrop of rocks completely surrounded by younger rocks

KAOLINITE A clay mineral, essentially an aluminium silicate

LIMONITE A hydrated iron oxide mineral, of which there are several varieties

LITHOLOGY The characteristics of a rock such as colour, grain size, mineralogy, structure, etc

LITHOSTRATIGRAPHICAL Based upon variations in composition and texture of a rock

LUMACHELLE A rock composed almost entirely of fossil shells or shell debris

LYDITE A dense black variety of chert

MARKER BED An easily recognised bed, distinctive enough to serve as a reference datum over long distances

MARL A calcareous mudstone

METAMORPHIC Describes rocks which have been altered mineralogically and structurally by excessive pressure and temperature resulting from deep burial

MICACEOUS Containing flakes of the mineral mica

MICRITE Micro-crystalline calcite (CaCO₃)

MONTMORILLONITE A clay mineral belonging to the smectite group which has the property of adsorbing water and oil

OIL SHALE A dark grey or black shale containing kerogen, an organic substance which yields liquid hydrocarbons on distillation

OSTRACODA A group of small water fleas with an external skeleton consisting of two valves

OUTCROP The area over which a particular rock formation or lesser unit occurs at the surface, whether visibly exposed or not

OUTLIER An outcrop of strata surrounded by outcrops of older rocks

OVERCONSOLIDATED Describes sediments that have been compressed more than the existing overburden would suggest, normally because of removal of former overburden by erosion

OVERLAP A relationship in which each successive unit in a stratigraphical sequence extends geographically further, such as to come to rest on an older group of beds

PIEZOMETRIC SURFACE An imaginary surface above or within the ground at which the water level would settle in a tube whose lower end passes below the water table

PERIGLACIAL Said of an environment beyond the periphery of ice sheets in which frost action is an important factor

PLANKTONIC Forms which float passively in surface waters

POORLY-SORTED Consisting of particles of various sizes

PORPHYRY An igneous rock containing well-formed crystals

PYRITIC Incorporating the iron sulphide mineral pyrite

QUARTZITE Rock composed predominantly of cemented grains of the mineral quartz (SiO₂)

'RACE' Small calcium carbonate concretions

ROTATIONAL SLIP A landslip in which there is downslope movement of material on a curved shear surface that is concave upwards

SCARP FACE See ESCARPMENT

SCHIST A metamorphic rock characterized by a parallel arrangement of the constituent minerals

SCHORL Black tourmaline, a complex silicate mineral

SELENITE The crystalline form of gypsum (CaSO₄.2H₂O)

SEPTARIAN NODULE A concretion with radial and concentric cracks commonly lined with crystals

SERPULID A worm that secretes an external tube in which it resides

SLIP-OFF SLOPE The long gentle slope on the inside of a river meander

SLIP TOE The lower margin of a landslip, commonly marked by a feature

SMECTITE A group of clay minerals which have the property of adsorbing water and oil

SYNCLINE A concave upward fold in the core of which are the stratigraphically younger rocks

SOLIFLUCTION The viscous downslope flow of waterlogged surface material especially over frozen ground

STRATIGRAPHY The form, geographical distribution, chronological sequence, classification, correlation and mutual relationships of rock strata

TILL Unsorted material deposited from an ice sheet (boulder clay)

TOPOGRAPHY The relief of the landscape

TRANSGRESSION The invasion of a large area of land by an advancing sea

TRANSLATIONAL SLIP A landslip in which there is downslope movement of material on a shear surface that parallels the ground surface

TRANSMISSIVITY The rate at which ground-water is transmitted through a unit width under a unit hydraulic gradient

UNCONFORMITY A break in the history of sedimentation represented by missing strata and often by a structural contrast between superimposed sets of strata

WATER TABLE The upper surface of a body of unconfined groundwater i.e. top of the saturated zone

WELL-SORTED Consisting of particles of more or less one size

ZONATION The arrangement of strata in groups of beds each of which is characterized by a diagnostic fauna

Appendix 8: Previous research

Callomon (1968) was the first to publish work on the stratigraphy of the Oxford Clay, as interpreted from sections in the brickpits at Bletchley and Stewartby. He also compiled a detailed record (MS, 1970) of the Corallian (West Walton Beds and Ampthill Clay), as revealed in temporary sections at Milibrook [TL 005 390] (see p 22–24). A section through the now overgrown Ampthill railway cutting (the type section for the Ampthill Clay) was figured and briefly described by Woodward (1895). Knowledge of the Corallian sequence was advanced by the drilling of the BGS Ampthill Borehole in 1970, logged by A. Horton (see p 22–24).

The Kimmeridge Clay is very poorly exposed within the survey area and little has been published on it. The regional succession of Portland Beds and Purbeck Beds for the South Midlands was described by Bristow (1963, 1968). The only satisfactory exposure of these beds in the Leighton Buzzard district is at Warren Farm, Stewkley [SP 851 242], first recorded by Davies (1901, 1915).

The Woburn Sands (Lower Greensand) have attracted much attention because of their good exposure in sand and fuller's earth pits. Stopes (1915) compiled a catalogue of fossil plants, and brachiopod faunas near the base of the succession received early attention from Keeping (1883) and later interpretation by Middlemiss (1962). The indigenous fauna of the basal nodule bed was described by Middlemiss (1961) and the stratigraphical palaeontology of the Lower Greensand as a whole was the subject of a major review by Casey (1961). The stratigraphy of the Woburn Sands has been considered by Kirkaldy (1939) and Bristow (1963), whilst their sedimentology has been the subject of studies by Allen (1982), Bridges (1982), Buck (1985) and Schiavon (1988). Descriptions of the fuller's earth deposits near Woburn Sands were given by Cameron (1892) and Cowperthwaite and others (1972); the latter, together with Jeans and others (1977), confirmed the volcanic origin of the montmorillonitic clays.

Jukes-Browne (1900) gave a brief account of the Gault and Upper Greensand of Bedfordshire, and also described the method of working the phosphatic nodules which occur in the Gault and the overlying Cambridge Greensand in the east of the district. The lowest few metres of the Gault, so well exposed in the sand pits around Leighton Buzzard, have attracted a disproportionate interest because of an abundant fauna and because the basal 1–2m represent a notably condensed sequence (see, for example, Cameron, 1897; Kitchin and Pringle, 1920, 1921 and 1922; Lamplugh, 1922; Toombs, 1935; Owen, 1962; Bristow, 1963; Hancock, 1967). Wright and Wright (1947) and Casey (1961) emphasised the faunal aspects, whilst Owen (1972) gave a detailed account of the stratigraphy and correlation of sections in the basal Gault.

The stratigraphy of the Lower and Middle Chalk was described by Jukes-Browne (1903). The Cambridge Greensand, a 0.3m-thick glauconitic sandy marl with phosphatic nodules ('coprolites') occurring in the east of the district and continuing into Cambridgeshire, has been the subject of considerable attention (Jukes-Browne, 1875, 1903; Sollas, 1873; and Spath, 1942–43) largely on account of its phosphatized fauna. Its foraminifera have been studied by Hart (1973). Pebbles of far-travelled exotic rocks within the bed, such as basalts and granites, were described by Hawkes (1943).

Little has been published that bears directly on the drift deposits of the Leighton Buzzard–Ampthill district. Horton (1970) discussed the glacial and post-glacial history of the Ouse basin, of which the Ouzel valley forms a part; and Horton and others (1974) described glacial sediments and more recent superficial deposits from the adjacent Milton Keynes district, which are contemporaneous with those of the survey area.

A review of the groundwater resources of the Lower Greensand in Bedfordshire and Cambridgeshire was given by Monkhouse (1974). The area is also included in a hydrogeological map of the area between Cambridge and Maidenhead, published in 1984 by BGS. Although less important than the Chalk of the Chilterns, the Woburn Sands of the present district make a major contribution to the local water supply.

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Figures, plates and tables

Figures

(Figure 1) Leighton Buzzard Project Area

(Figure 2) Simplified topography of the area

(Figure 3) Solid geology of the area (for cross section A—B see (Figure 8))

(Figure 4) Correlation between the sections at Ampthill, Millbrook and Stewartby

(Figure 5) Location of principal sand pits in the Leighton Buzzard area

(Figure 6) Lithostratigraphic correlation of pits in the Woburn Sands of the Leighton Buzzard area

(Figure 7) Isopachytes of the Woburn Sands in the Leighton Buzzard—Woburn Sands area

(Figure 8) Cross section showing the structure south of Leighton Buzzard (for line of section see Figure 3; key as in Figure 9)

(Figure 9) Sketch structural contours on the base of the Corallian to illustrate the effects of pre-Cretaceous folding and the magnitude of the Jurassic-Cretaceous unconformity

(Figure 10) Generalised distribution of Drift deposits: sheets TL 03 NW and NE, TL 13 NW

(Figure 11) Generalised distribution of the Drift deposits: sheets SP 92 NE, TL 02 NW, TL 03 SW and SE

(Figure 12) Generalised distribution of Drift deposits: sheets SP 93 NW, NE, SW and SE

(Figure 13) Generalised distribution of Drift deposits: sheets SP 82 NE and SE, SP 92 NW and SW

(Figure 14) Contours on base of Glacial Drift: sheets TL 03 NW and NE, TL 13 NW

(Figure 15) Contours on base of the Glacial Drift: sheets SP 92 NE, TL 02 NW, TL 03 SW and SE

(Figure 16) Contours on base of the Glacial Drift: sheets SP 93 NW, NE, SW and SE

(Figure 17) Occurrence of fuller's earth in the Woburn Sands area

(Figure 18) Tentative correlation of fuller's earth seams within the Woburn Sands

Plates

(Plate 1) Pratts Quarry, Leighton Buzzard. Cross-bedded 'Red Sands' (Woburn Sands)

(Plate 2) Mundays Hill Quarry, Heath and Reach. 'Carstone reef' and 'Silver Sands' (Woburn Sands).

(Plate 3) Old Wavendon Heath Quarry, near Woburn Sands. Woburn Sands overlying fuller's earth (Main Seam)

(Plate 4) Old Wavendon Heath Quarry, near Woburn Sands. Fuller's earth bed (Main Seam)

Tables

(Table 1) Lithological and microfaunal characteristics, and thicknesses, of the Upper Jurassic formations

Tables

(Table 1) Lithological and microfaunal characteristics, and thicknesses, of the Upper Jurassic formations