Content and licensingview original scan buy a printed copy

Geology of the Biggleswade district — brief explanation of the geological map sheet 204 Biggleswade

B S P Moorlock, M G Sumbler, M A Woods, and S Boreham

Bibliographic reference: Moorlock, B S P, Sumbler, M G, Woods, M A, And Boreham, S. 2003. Geology of the Biggleswade district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 204 Biggleswade (England and Wales).

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

Keyworth, Nottingham: British Geological Survey © NERC 2004 All rights reserved. Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham

(Front cover) St Andrews Parish Church, Biggleswade [TL 188 446]. The church is built largely from locally derived Cretaceous rocks. (Photographer T P D S Cullen; MN39873).

(Rear cover)

Notes

The word 'district' refers to the area of Sheet 204 Biggleswade. National grid references are given in the form [TL 1234 1234] or [TL 123 123]. Unless otherwise stated all such references fall within grid square TL.

Numbers at the end of photograph descriptions refer to the official collection of the British Geological Survey.

Acknowledgements

We thank landowners, companies, local authorities and others who have provided data and information. We would like to thank Cambridge University and The Open University for allowing one of the authors (SB) to include unpublished research data pertaining to the interglacial deposits within the district.

Photographs used in (Plate 1), (Plate 2), (Plate 3) were taken by R T Mogdridge and (Plate 5) by G K Lott; (Figure 2) was drawn by R J Demaine; editing was by A A Jackson.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence number GD272191/2003.

Geology of the Biggleswade district (summary from the rear cover)

(Rear cover)

(Geological succession)

This Sheet Explanation provides a summary of the geology of the district covered by geological 1:50 000 Series Sheet 204 Biggleswade. It is written for the guidance of those who use the geological map, and may wish to be directed to further geological information about the area.

The district takes its name from the small historic market town of Biggleswade in Bedfordshire situated on the right bank of the River Ivel, which flows northwards through the western part of the district. The area is renowned for its market gardening, but this has declined steadily in recent years.

Tracts of lowland underlain by mudstone of the Oxford Clay and Gault formations in the western half of the district are separated by attractive, partially wooded, higher land, such as Sandy Heath, which is underlain by the sandy, more erosionresistant deposits of the Woburn Sands Formation. In the south-east the land rises from the Gault onto the Chalk, which gives rise to a characteristic rolling landscape. The Cambridge Greensand at the base of the Chalk was worked extensively during the 19th and early 20th centuries, as a source of phospate for fertilizer, but little evidence of these workings now remain. Harder bands within the Chalk, such as the Totternhoe Stone, Melbourn Rock and Chalk Rock, form topographical features that can be traced across the countryside. At Barrington, in the east of the district, a traditional local building stone, known as Cambridgeshire Clunch, continues to be dug from the lower part of the Chalk succession. In this account and on the map, a new stratigraphical nomenclature has been used for the Chalk, and elsewhere this classification has proved to be very helpful to users.

A veneer of chalky till (boulder clay) covers much of the lower land in the west; this was deposited by ice that covered the area during the Anglian Glaciation just over 400 thousand years ago. More recently, spreads of river terrace sands and gravels were deposited along the valley of the River Ivel and its tributaries. Excavation of these terrace deposits for aggregate has revealed the presence of a complex history of interglacial events from hippopotamus, elephant, and brown bear roamed the local countryside.

Chapter 1 Introduction

This Sheet Explanation describes the geological 1:50 000 Series Sheet 204 Biggleswade that lies on the Mesozoic strata to the southwest of Cambridge. In the western part of the district is the northerly flowing River Great Ouse and its tributary the River Ivel, and the east is drained by the easterly flowing River Cam (or Rhee) with Bourn Brook farther to the north. A low discontinuous ridge of Woburn Sands Formation (Lower Greensand), locally wooded, extends from Shefford to Gamlingay and divides the Jurassic mudstone to the northwest from the Cretaceous mudstone to the south-east. Near Royston, the chalkland rises to over 130 m above sea level. The region is predominatly agricultural, with Biggleswade the focus of an important market gardening area.

The district lies on the northern margin of a concealed massif of Palaeozoic rocks known as the London Platform, which was deformed during the Variscan orogeny. During the Mesozoic, the platform remained as a high surrounded by areas in which subsidence and thus sediment accumulation was greater. Mesozoic strata gradually onlapped onto the platform, but parts of the interior probably remained as dry land (the London Landmass) throughout the Jurassic.

Chapter 2 Geological description

Concealed formations

Ordovician to Carboniferous

Ordovician rocks of Tremadoc age have been proved in the base of the Wyboston Borehole [TL 1759 5723], drilled in the north-west of the district. Up to 350 m of Silurian rocks are inferred from geophysical evidence to be present at depth in the eastern part of the district. These are overlain by up to 600 m of Lower and Upper Old Red Sandstone. Up to 22 m of Carboniferous Limestone directly overlie the Upper Old Red Sandstone in the north-east of the district.

Jurassic

The Jurassic strata are divided into three groups, and further subdivision into several formations and members is shown in (Figure 1)

Lias Group

The Wyboston Borehole [TL 1759 5773], proved that Lower Jurassic strata of the Lias Group rest directly on Old Red Sandstone 'basement' at a depth of 110.6 m (Edmonds and Dinham, 1965). In the north-east of the district, the Lias directly overlies Carboniferous Limestone. Triassic rocks are absent, but it is known that they are overlapped some distance to the north-west of the district. In Ashwell No.1 Borehole [TL 2860 3900] near the southern margin of the district, the Lias is absent, having been overlapped to the north-west. The same relationship is probably present at Barrington [TL 39 50] in the east of the district.

The Lias succession in the Wyboston Borehole as described by Edmonds and Dinham (1965) is classified into 'Lower Lias' and 'Upper Lias'. In the light of the additional information given by geophysical logs of the (uncored) Gas Council GH4 and GH5 boreholes [TL 1890 5922]; [TL 2090 6389] and the cored Great Paxton Borehole [TL 2088 6389] which lie a few kilometres to the north-east (Sheet 187 Huntingdon), the succession at Wyboston can be reinterpreted in terms of the standard lithostratigraphical formations defined by Cox et al. (1999), as follows:

Charmouth Mudstone Formation

The Charmouth Mudstone Formation (about 46 m thick) comprises grey mudstone with sporadic thin beds of shelly limestone. The upper part, in particular, is somewhat silty. The basal 1.1 m comprises limestone with pebbles of ironstone, quartz and chert ('lydite'), and a fauna dominated by bivalves and belemnites. Unfortunately, no ammonites were recovered from these basal beds, but they probably belong to the Jamesoni Zone as in the Great Paxton Borehole (see Donovan et al., 1979). Above, the greater part of the succession belongs to the Ibex Zone, which is well established by its ammonite fauna.

Dyrham Formation

The Dyrham Formation is made up of grey, very silty, and finely micaceous mudstone with sideritic nodules. The depth given for the base of the formation is somewhat arbitrary as the lithological distinction with the underlying Charmouth Mudstone is subtle, but is more easily recognised by lower gamma-ray values on the geophysical logs (of uncored boreholes). At Wyboston it is taken at the base of a thin shelly mudstone bed that lies at the base of the most silty part of the Lias succession, giving a thickness of 10.7 m for the formation. This bed is also present in the Great Paxton Borehole where the formation is 10.1 m thick. At Wyboston, ammonites prove the presence of the Margaritatus Zone down to 60 m depth; lower beds may belong to the Davoei Zone and may even extend down into the Ibex Zone as seems to be shown by the ammonite record of the Great Paxton Borehole. There is no indication of the presence of the Marlstone Rock Formation above the Dyrham Formation in this district or immediately adjoining areas.

Whitby Mudstone Formation

The Whitby Mudstone Formation (11.4 m thick) is made up largely of grey mudstone. The basal 2 to 3 m comprise interbedded mudstone and limestone, which contains scattered ferruginous ooids. These beds rest erosively on the underlying Dyrham Formation, and are highly fossiliferous, containing abundant ammonites (hence

their name the 'Cephalopod Limestone'), which demonstrate that they belong to the Falciferum Zone. The overlying mudstone extends into the succeeding Bifrons Zone. In both the Wyboston and Great Paxton boreholes, the uppermost mudstone beds have a listric structure, and contain rootlets and the mineral sphaerosiderite that is characteristic of fossil soils. These features imply terrestrial weathering before deposition of the succeeding Great Oolite Group.

Great Oolite Group

Some previous accounts of the geology of this and adjoining districts (for example, Edmonds and Dinham, 1965; Horton, 1989; Shephard-Thorn et al., 1994; Hopson et al., 1996) have suggested the presence of 'Lower Estuarine Series' or Grantham Formation (Inferior Oolite Group) above the Lias. However, given the strongly erosive base of the Rutland Formation, this interpretation is considered highly unlikely. More probably the Grantham Formation is overlapped well to the northwest of the Biggleswade district, probably just south-east of where the Lincolnshire Limestone Formation is overlapped in the Peterborough district (Horton, 1989, fig. 7). Thus, the Inferior Oolite Group is absent in this district. The Great Paxton Borehole proved the Kellaways Formation, resting on 17.75 m of Great Oolite that comprises:

In comparison, in the Wyboston Borehole, the Great Oolite is 17.15 m thick. The succession is as follows:

Thus the sequences are approximately the same thickness; the absence of the Blisworth Clay Formation is probably due to overlap by the Cornbrash.

Based on geophysical evidence and core from the Ashwell Borehole, the Woburn Sands Formation rests directly on the Great Oolite Group, 18.4 m thick, at 134.6 m depth. There is no Ancholme Group here (as is incorrectly shown on the cross-section on Sheet 204) and the log of this borehole as given by Hopson et al. (1996, p.142) also requires amendment; there is no Blisworth Clay or Grantham Formation at this site. The succession is as follows:

Higher gamma-ray values on geophysical downhole logs from a borehole near Barrington indicate thin Ancholme Group beneath Woburn Sands Formation. The underlying Great Oolite Group rests directly on Palaeozoic basement, as at Ashwell.

Rutland Formation

The Rutland Formation is largely a nonmarine succession that was deposited in land-marginal areas of the London Platform following a substantial period of erosion of the land surface that exposed strata ranging from the Inferior Oolite Group, in the northwest, to Palaeozoic, in the south-east. The basal unit, the Stamford Member is of fluviatile and lacustrine origin, and comprises very pale grey siltstone and sandstone with a few mudstone horizons. The rocks contain abundant lignitic material and rootlet traces. These beds are of rather variable thickness because they infilled a somewhat irregular surface eroded into the underlying strata. The member is particularly well developed in the Ashwell Borehole where it is 4.9 m thick; at Wyboston it is 2.0 m thick.

The succeeding part of the Rutland Formation is dominated by paralic mudstones. In the Ashwell Borehole, mudstone with rootlets and lignite in the lower part is succeeded non-sequentially at a depth of 142.1 m by mudstone with a marine bivalve fauna. This mudstone contains an increasing abundance of rootlets in the upper part, and is capped by thin sandstone. Thus two shallowing-upward sedimentary rhythms are present. These rhythms represent delta progradation and marsh development; in the type area (Ketton Quarry, Rutland) of the formation, up to seven such rhythms can be recognised. In the Wyboston Borehole, four rhythms appear to be present, marked by rootlet beds overlain by marine mudstones. In Great Paxton Borehole, the Rutland Formation is thinner, and there is evidence only of the lowest rhythm, of which the Stamford Member forms a part. The younger beds are presumably absent due to overlap by the marine Blisworth Limestone Formation.

Resting sharply and non-sequentially on the Rutland Formation, the Blisworth Limestone Formation is made up of finegrained, poorly sorted, bedded limestones, with many thin mudstones or marl (calcareous mudstone) interbeds. The whole succession is very fossiliferous, with a fauna dominated by marine bivalves, notably forms such as Placunopsis, Modiolus, and oysters such as Praeexogyra.

Cornbrash Formation

The Cornbrash Formation is a hard, grey, shell fragmental limestone containing irregular patches (burrow-fills) of fine-grained micritic limestone. It also contains minor mudstone or marl seams, which are commonly very fossiliferous, particularly with the bivalve Meleagrinella. The brachiopod fauna from the Wyboston Borehole, with Cererithyris and Obovothyris confirm that Lower Cornbrash (Bathonian) is present. Edmonds and Dinham (1965) reported the presence of Upper Cornbrash (Callovian), but this has not been confirmed by evidence from the borehole. The supposed Cornbrash in the Ashwell Borehole is unlike that at Great Paxton or Wyboston in that it is much less indurated and resembles the Blisworth Limestone in its lithological characters, although its fauna supports the Cornbrash attribution. Its base is provisionally drawn at an obvious non-sequence (at 136.4 m depth), but the presence of Obovothyris and Meleagrinella in the underlying limestone at 136.7 m is also suggestive (though not diagnostic) of the Cornbrash.

Ancholme Group

The Ancholme Group comprises the Middle to Upper Jurassic succession that overlies the Cornbrash and is mudstone dominated.

The Kellaways Formation at the base was proved in both the Wyboston and Great Paxton boreholes, where it is 5 m thick. The lower part comprises dark grey mudstone, rather shelly at the base, known as the Kellaways Clay Member. The overlying strata, constituting the Kellaways Sand Member, comprise greenish grey siltstone or very fine-grained sandstone with an abundant fauna of bivalves, belemnites and ammonites such as Sigaloceras.

Exposed formations

Jurassic

Beds of the Oxford Clay, West Walton and Ampthill Clay formations are represented at surface or beneath Quaternary deposits within the district. These units all belong to the Ancholme Group. The upper part of the group, i.e. the upper part of the Ampthill Clay, and the succeeding Kimmeridge Clay Formation, which occur elsewhere in eastern England, are not represented within the district due to submarine erosion associated with the deposition of the Cretaceous Woburn Sands Formation (Lower Greensand). This formation cuts out the whole of the group at depth in the southern part of the district (Figure 2).

Oxford Clay Formation

The Oxford Clay is the oldest formation represented at surface within the district. The beds underlie the vale of the Great Ouse in the north-west, an area of low relief characterised by greyish brown clay soils where the outcrop is not covered by younger Quaternary deposits. In the subsoil, ochreous mottles and crystals of selenite (gypsum) result from the decomposition of pyrite in the presence of calcium carbonate.

Exposure is very poor, but the Wyboston Borehole penetrated the lower part of the formation, and the Gamlingay Borehole proved the upper part of the formation. However, the succession is well known from adjoining areas and the following account is necessarily based largely on information from these areas.

The total thickness of the Oxford Clay, where complete (for example north-east of Sandy) is estimated to be about 60 m. However, due to early Cretaceous erosion, it is substantially less than this in many parts of the subcrop beneath the Woburn Sands Formation. For example, to the south-west of Sandy, the base of the Woburn Sands cuts down into the upper part of the formation and at depth to the south and south-east it cuts out the Oxford Clay and indeed the Kellaways Formation so that in the Ashwell Borehole, and in the Henlow Borehole [TL 1616 3506] (see Hopson et al., 1996), it rests directly on the Great Oolite Group.

The Oxford Clay Formation has traditionally been divided into three parts, the Lower, Middle and Upper Oxford Clay, each of which has somewhat different lithological characteristics. These three units are now formalised as members of the formation, being named respectively the Peterborough Member, Stewartby Member and Weymouth Member (Cox et al., 1992). Ammonites form the basis of the standard biostratigraphical zonation of the succession. The Peterborough Member falls entirely within the Callovian Stage, but spans the upper part of the Calloviense Zone, the Jason and Coronatum zones, and the lower part of the Athleta Zone. The Stewartby Member is Upper Callovian, comprising the upper Athleta and Lamberti zones. The Weymouth Member is Lower Oxfordian, encompassing the Mariae and, in most areas, the lower part of the Cordatum Zone, although there is some evidence that the latter may be missing, at least locally, within the Biggleswade district.

Peterborough Member

The Peterborough Member is quarried for brickmaking at Stewartby near Bedford to the west of the district, and near Peterborough. At Stewartby, the member is about 21 to 23 m thick (Callomon, 1968) but it is only 16 to 18 m thick at Peterborough (Horton, 1989). Given the intermediate location of the Biggleswade district, a thickness of around 20 m seems probable. Thus all the Oxford Clay penetrated in the Wyboston Borehole (13.6 m) must belong to the Peterborough Member. From structure contours, it can be deduced that beds belonging to the upper part of the member underlie the valley of the Great Ouse and most or all of the drift-covered ground to the northwest (Figure 2).

The Peterborough Member is made up of brownish grey, fissile, kerogen-rich mudstone ('bituminous shales') with subordinate beds of pale to medium grey, blocky mudstone. It is the organic-rich beds that render it valuable as a brick-clay resource; they do not occur to any significant degree in the higher parts of the formation. Reflecting this lithology, in those areas that are not covered by Quaternary deposits, soils are typically of a darker brown colour, and of a more crumbly texture that are typical of younger parts of the formation.

The basal beds of the member (not represented at outcrop within the district) are commonly silty, with Gryphaea-rich shell beds. Argillaceous limestone ('cementstone') nodules occur typically in beds, but are also scattered throughout the member.

The nodules, flattened spheroids which may be up to 1 m or more in diameter, consist of hard, grey, smooth-textured limestones, and are commonly septarian (fractured internally, with the fissures infilled with crystalline calcite or other minerals such as baryte). Certain nodule beds are widespread markers, most notably the Acutistriatum Band, occurring about twothirds of the way up the succession, at the base of the Athleta Zone.

While the basal boundary of the Peterborough Member (and of the Oxford Clay Formation) is a sharp lithological change from sand or sandstone of the underlying Kellaways Formation to mudstone, the top boundary is somewhat arbitrary, as the characteristic bituminous mudstones occur interbedded with pale grey, calcareous mudstones typical of the succeeding Stewartby Member over a vertical distance of some 3 m.

Stewartby Member

The Stewartby Member is also deduced to be about 20 m thick in the Biggleswade district. It consists predominantly of pale to medium grey mudstone that is smooth to slightly silty, calcareous, poorly fossiliferous and blocky. It weathers to produce a dull, mid-greyish brown soil, much heavier and stickier than that derived from the Peterborough Member. Elsewhere in the country, it has been found that the Peterborough Member tends to form ground of low relief while the Stewartby Member is characterised by somewhat steeper slopes, and this characteristic appears to be maintained in this district with the member forming the gentle south-east slopes of the Ouse valley. In the uppermost part of the member, Gryphaea-rich beds are known to occur; these may form topographical features, for example ridge-like hills such as Mox Hill [TL 127 468] and that near Tempsford [TL 166 523] that are capped by Quaternary deposits.

The uppermost few metres of the member were penetrated by the Gamlingay Borehole [TL 2320 5204], between 39.45 m and terminal depth at 44.02 m depth. These beds comprise greenish grey bioturbated mudstone (with Chondrites) and a fauna mainly of bivalves and ammonites. This bed probably represents the Lamberti Limestone, the top of which defines the top of the member, and the Callovian–Oxfordian boundary (base of the Upper Jurassic).

Weymouth Member

The Weymouth Member is about 20 m thick in the Biggleswade district; the Gamlingay Borehole proved 20.32 m (19.13 to 39.45 m depth). It consists mainly of pale grey, mostly smooth-textured mudstone, similar to that of the Stewartby Member, but with some beds of darker, more organic-rich mudstone in the upper part, in the transition to the succeeding West Walton Formation. The Weymouth Member forms ground of low relief at the foot of the Woburn Sands Formation scarp that extends north-east from Sandy, and also underlies the area to the south-west of Sandy that is covered largely by Quaternary deposits, including that around Old Warden, which was erroneously depicted as Ampthill Clay on the previous editions of Sheet 204 (1949, 1976).

The upper part of the member was formerly exposed beneath the West Walton Formation at Sandy Brick and Tile Works [TL 179 497] and in the railway cutting to the west [TL 177 496] (Edmonds and Dinham, 1965). Probably about 10 m of strata were intermittently exposed, comprising grey mudstone with 'race', selenite and ferruginous nodules and two cementstone bands about 6 and 8.5 m below the top. On the basis of the recorded ammonite fauna, the highest beds are assigned to the Mariae Zone, the apparent absence of the Cordatum Zone, perhaps suggesting localised downcutting at the base of the West Walton Formation. The uppermost beds of the Weymouth Member may also have been exposed in excavations in a brickpit at Gamlingay [TL 230 513] and penetrated by boreholes proving the Woburn Sands near Potton and Shefford.

West Walton Formation and Ampthill Clay Formation

These two formations are shown undifferentiated on the map as this part of the district has not been surveyed in sufficient detail to separate them. The two formations encompass the two units mapped as 'Clays and Limestone' including the 'Elsworth Rock' and the 'Ampthill Clay' of previous editions of Sheet 204, although much of the outcrop previously shown as Ampthill Clay probably belongs to the West Walton Formation, or in the Old Warden area, Oxford Clay (see above). The approximate distribution of these formations, as deduced from regional structure, is indicated in (Figure 2).

The West Walton Formation comprises a succession of calcareous mudstone, with cementstone or siltstone bands and nodules, which occurs between the Oxford Clay and Ampthill Clay throughout eastern England. Prior to recognition of the distinctiveness of this unit (Gallois and Cox, 1977; Cox and Gallois, 1979) the beds were generally included within the Oxford Clay or Ampthill Clay, although within the Biggleswade district, a more calcareous unit that occurs near the base was separated as the Elsworth Rock (see Edmonds and Dinham, 1965); this unit is now regarded as a member of the West Walton Formation. In the Fenland type area (West Walton Borehole (TF41SE/6)), the West Walton Formation encompasses the upper Cordatum and lower Tenuiserratum zones, but it seems possible that the Cordatum Zone is absent, at least locally in the Biggleswade district (see below).

Based on a reassessment (by B M Cox) of the record and specimens from the Gamlingay Borehole, which was drilled and logged in 1970, prior to the recognition of the formation (see Horton, 1971; Medd, 1979), the West Walton Formation is about 10 m thick in the Biggleswade district. In the borehole, the base of the formation is drawn at the base of a dark grey mudstone (at 19.13 m depth) which rests with marked interburrowing on pale grey mudstone of the Weymouth Member. Between 14.10 and 17.70 m depth, the strata consist of alternating pale calcareous mudstones and silty argillaceous limestones. This unit constitutes the Elsworth Rock Member, named from a village [TL 32 63] about 12 km north-east of Gamlingay (on Sheet 187). It contains a fairly abundant fauna of bivalves such as pectinids and Pinna, with 'oysters' such as Lopha, Nanogyra, and large Gryphaea dilatata, as well as serpulids and belemnites. This robust fauna is locally very common amongst material dug from ditches on the outcrop, particularly between Tetworth [TL 218 533] and Low Farm, Waresley [TL 233 545], as recorded by Edmonds and Dinham (1965). Above the Elsworth Rock Member in the borehole, the upper part of the formation comprises medium and dark grey mudstone, with one bed of argillaceous limestone. The top of the formation is drawn (provisionally) at about 9.6 m depth.

West Walton Formation

The West Walton Formation was also penetrated in the brickpits at Gamlingay as recorded by Roberts (1892), Woodward (1895) and Fearnsides (1904). The details are summarised by Edmonds and Dinham (1965). At Belle Vue Pit [TL 230 513] about 2 m of Elsworth Rock Member were recorded beneath 6.4 m of mudstones (Ampthill Clay and upper part of West Walton Formation); at the top of the Elsworth Rock is a serpulid-rich limestone that has been called the Gamlingay Rock.

The Elsworth Rock Member was formerly exposed in the pit of the Sandy Brick and Tile Works [TL 179 497], as described by Edmonds and Dinham (1965). On the basis of the description given, it is not possible to determine the position of the base of the West Walton Formation with certainty, but it may be that the Elsworth Rock rests directly on the Weymouth Member at this locality. About 3 m of Elsworth Rock were recorded, overlain directly by Woburn Sands Formation. The beds are described as dark clays interbedded with uneven beds of argillaceous limestone, with a fauna of Nanogyra nana, Gryphaea, serpulids, echinoid spines and cardioceratid ammonites suggestive of the Tenuiserratum Zone. Debris of mudstone with large Gryphaea dilatata, N. nana and serpulids is still evident on the site of the pit, now restored. These beds are cut out entirely a short distance to the west of the pit where the erosive base of the Woburn Sands channels down into the Weymouth Member.

Ampthill Clay Formation

The distribution of the Ampthill Clay Formation within the district is very limited. In the previous account of the district, Edmonds and Dinham (1965) assigned all the Jurassic mudstones above the Elsworth Rock to the Ampthill Clay, and consequently the outcrop is greatly exaggerated. Using the current stratigraphical definitions (Gallois and Cox, 1977; Cox and Gallois, 1979), most of these mudstones should be assigned to the West Walton Formation, with only the uppermost part actually belonging to the Ampthill Clay (see above). Thus, in the Gamlingay Borehole, about 5 m of Ampthill Clay are preserved between the base of the Woburn Sands Formation at 4.17 m depth, and a burrowed surface at about 9.60 m depth. The Ampthill Clay in the borehole comprises medium grey mudstone, darker, smoother (i.e. less silty) and less calcareous than the West Walton Formation below.

Up to about 10 m of Ampthill Clay may be preserved locally, with the maximum thickness of beds cropping out on the upper part of the slope of the Woburn Sands scarp, to the north-west of Gamlingay. These strata probably represent only the lower part of the formation as developed in Fenland (Gallois, 1979). Ampthill Clay also crops out in the Jurassic 'windows' in the Woburn Sands outcrop near Gamlingay, but probably only 1 to 2 m of Ampthill Clay is preserved there. The formation is probably cut out entirely by the Woburn Sands Formation just to the south-east (Figure 2).

Cretaceous

Woburn Sands Formation

Named after the Woburn area in the adjacent Leighton Buzzard district to the west, the Woburn Sands are of late Aptian and early Albian age and equivalent of the Sandgate Beds and Folkestone Beds of the Weald. In this present district they rest with marked unconformity on the Oxford Clay and other formations. The Woburn Sands outcrop extends from Chicksands Priory, north-eastwards to Great Gransden, to the north of which it it is covered by Lowestoft Formation till. The base of the Woburn

Sands is irregular and thus the thickness varies abruptly; 76 m were proved in Crossroads Borehole [TL 2318 4904] at Potton. Where the sands are thick and free of overlying till, such as at Rowney, Warden and Sandy Warrens, rapid percolation of rain water down to a low water table allows development of ravines and steep-sided coombes, now somewhat hidden by coniferous and mixed plantations.

Sand constitutes most of the formation (Plate 1); this may be loose or cemented into sandstone by a ferruginous, or less commonly a calcareous cement. The sands are poorly graded; lenticular bodies of coarse sandrock with large 'millet-seed' grains alternate with fine-grained sands. The larger grains are highly polished, and show a rounding approaching that of desert blown sands. Most of the sandy beds are rust-coloured, orange, buff or yellow, more rarely reddish or nearly white. A pale greenish tinge, perhaps due to some admixture of fuller's earth is seen in some silty or clayey laminae. Green hues are typically seen only in fresh surface exposures, but may occur at shallow depth where glauconite has not been oxidised to limonite.

Sandy Heath and Gamlingay Heath are underlain by the upper, ferruginous sandstone division of the formation. This yields more available nutrients than the finer, looser, sands below, which crop out mainly on the escarpment at Sandy and in the coombes of Sandy Warren. The land on the upper sands has long been occupied by market gardens. The hardest beds of ferruginous sandrock make a fairly soft and easily dressed building stone, suitable for walling.

Beds of 'pale silty clay' have been recorded in boreholes within the formation. It was thought that some of these might be seams of fuller's earth, but despite further drilling during a recent appraisal of fuller's earth resources (Moorlock and Highley, 1991), no occurrences of fuller's earth have been confirmed.

Within the Woburn Sands are several pebble or nodule beds that consist of pebbles of harder rocks derived from older formations, mixed with nodules in which fossils have been preserved in calcium phosphate. It is believed that the finer sediment has been removed thus concentrating the nodules and pebbles into distinct beds. Historically, these beds have been exploited as a source of fertiliser.

The Potton Nodule Bed lies in the upper part of the formation; another seam is at least 15 m lower, but still about 30 m above the base of the Woburn Sands at Cox Hill, Sandy. The Potton Nodule Bed is lenticular and up to about 2 m thick. Pebbles and nodules occur in about equal proportions. The nodules and pebbles have a ferruginous cement, which in the lower part is sufficiently hard to retain impressions of shells. A further pebble bed is present locally at the base of the formation.

The Woburn Sands Formation was laid down in a shallow shelf sea, subjected to considerable tidal influences. This sea covered much of southern Britain in later Early Cretaceous times when a southern sea in the Wealden area flooded the south Midlands to join with the Boreal Ocean (Casey, 1963). The 'millet seed' grains noted above have presumably been carried into the sea from nearby land.

Gault Formation

The outcrop of the Gault extends from near Shefford to about 3 km south-west of Great Gransden [TL 27 56], but the lower part of the formation is extensively covered by Quaternary deposits. The Gault consists essentially of blue or grey mudstone or calcareous mudstone ('marl'). Nodules of phosphatised, commonly fossiliferous 'marl' occur sporadically, or are concentrated in thin seams that have locally been exploited for fertiliser. The thickness of the Gault varies between 30 and 70 m of which the Upper Gault greatly exceeds the Lower Gault.

Chalk Group

The classification of the Chalk Group (Figure 3) used herein largely follows Bristow et al. (1997). However, Rawson et al. (2001) proposed some modifications to this, one of the most significant of which is the reclassification of the Plenus Marls as a member at the base of the Holywell Nodular Chalk Formation, rather than forming the topmost unit of the Zig Zag Chalk Formation. For consistency with the map, this account retains the Plenus Marls in the top of the Zig Zag Chalk Formation. Rawson et al. (2001) also divided the Chalk Group into the Grey Chalk Subgroup and the White Chalk Subgroup. The former is broadly equivalent to the Lower Chalk and the latter to the combined Middle Chalk and Upper Chalk of the old classification. Bristow et al. (1997) ranked the main units within the Chalk as members, and this was used on the published map. The members were later upgraded to formation status by Rawson et al. (2001).

Grey Chalk SubGroup

West Melbury Marly Chalk Formation

The West Melbury Marly Chalk comprises interbedded limestone and marl, about 15 to 50 m thick (Edmonds and Dinham, 1965; (Figure 3)). At its base is the Cambridge Greensand Member (up to 0.9 m thick), a richly fossiliferous phosphatic and glauconitic sandy bed that is characteristic of the base of the Chalk Group in this district and the adjacent districts of Saffron Walden (Sheet 205) and Hitchin (Sheet 221) but dies out rather abruptly south-westwards near Barton-le-Clay [TL 08 32] in the Leighton Buzzard district (Sheet 220). The Cambridge Greensand has a rich phosphatised macrofauna derived from Late Albian (S. dispar Zone) strata, and an indigenous basal Cenomanian fauna. However, microfossil evidence suggests that its formation could have been (at least in part) latest Albian (Morter and Wood, 1983), rather than basal Cenomanian as conventionally assumed.

Zig Zag Chalk Formation

The base of the Zig Zag Chalk Formation is marked by the base of the Totternhoe Stone Member, and the upper contact is the top of the Plenus Marls (but see above). The formation comprises homogenous, pale grey and cream-coloured chalk, with a predominance of curvilinear jointing (Osborne White, 1932); it is about 30 m thick in this district (Edmonds and Dinham, 1965).

The Totternhoe Stone is a shell-rich calcarenitic bed (up to 6 m thick), broadly equating with the Cast Bed at the base of the Zig Zag Chalk of Bristow et al. (1997). The basal boundary is a conspicuous erosion surface that cuts down into the underlying West Melbury Marly Chalk, which markedly reduces its preserved thickness in places. The base of the Totternhoe Stone is typically a phosphatic lag, and some of its fauna might be derived from the West Melbury Marly Chalk. (Plate 2) illustrates an example of an ammonite from the Zig Zag Chalk Formation at Barrington [TL 391 510].

White Chalk SubGroup

Holywell Nodular Chalk Formation

In the Biggleswade district the base of the Holywell Nodular Chalk is marked by the conspicuously indurated and nodular Melbourn Rock Member (Figure 3), about 2 to 3 m thick. The stratotype locality of this horizon lies within the district, north-east of Royston, and the contact with the underlying Plenus Marls can be seen at Ashwell [TL 2687 3945] (Hopson et al., 1996). Above the Melbourn Rock, the chalk is less massively nodular, and contains common Inoceramus labiatus [ =Mytiloides mytiloides and M. labiatus], matching the typical lithology of the Holywell Nodular Chalk in southern England (Bristow et al., 1997). This interval is well exposed in the quarries at Steeple Morden [TL 298 402], [TL 296 401], in the south of the district (Hopson et al., 1996). Here, the Holywell Nodular Chalk contains a bed of very hard chalk above the Melbourn Rock, named the Morden Rock by Hopson et al. (1996), and the higher part of the formation is conspicuously flinty, and therefore unlike its typical flint-free development across much of southern England.

New Pit Chalk Formation

Above the hard, nodular, shell-rich lithologies that correspond with the Holywell Nodular Chalk (Bristow et al., 1997), the chalk is soft and blocky, with marl seams and flints, the typical lithology of the New Pit Chalk Formation. In the quarries at Steeple Morden, the lower part of the formation contains common sponge beds, and at least 36 m of succession occur in the Royston Bypass cutting [TL 372 410] (Hopson et al., 1996). The combined Holywell and New Pit Chalk formations broadly equate with the 'Middle Chalk' of the previous survey.

Lewes Nodular Chalk Formation

The hard, nodular and flinty chalk lithology that typifies this formation has a limited outcrop to the south of Royston. The base of the Lewes Nodular Chalk contains an indurated horizon, traditionally named the 'Chalk Rock', wich was used in previous surveys to map the base of the 'Upper Chalk'. This marker-bed, seen at Reed [TL 3595 3704], just to the south of the district, occurs in the lower S. plana Zone and equates with the Hitchwood Hardground of Bromley and Gale (1982) (Hopson et al., 1996). Stratigraphically lower hardgrounds that form part of the basal Lewes Nodular Chalk in the Chilterns are not developed in the Biggleswade district. A pebble bed associated with the Hitchwood Hardground has a distinctive mollusc fauna (Reussianum Fauna) that was often used in former surveys to identify the base of the Upper Chalk in poorly featured ground. The exposure at Reed shows that the base of the Lewes Nodular Chalk occurs about 3 m below the Hitchwood Hardground, rather than being coincident with it. Another bed of indurated chalk, traditionally named the 'Top Rock', occurs about 4 m above the Hitchwood Hardground at Reed (Hopson et al., 1996), and probably represents the coalescence of two or more hardgrounds that occur in the middle and upper Lewes Nodular Chalk in southern England (Mortimore, 1983).

Quaternary

Lowestoft Formation

Glaciogenic deposits of the Lowestoft Formation, deposited by the Anglian ice sheet some 400 000 years ago, cover much of the western and northern part of the district, and occur sporadically elsewhere. The formation comprises till and associated outwash sands and gravels, depicted separately on the map.

The till, previously known as the Chalky Boulder Clay, is typically a bluish grey, overconsolidated, variably sandy, silty, clay containing clasts of chalk, flint, quartz and quartzite derived from the Triassic Sherwood Sandstone Group, and blocks of Jurassic limestone, and fossils, (notably Gryphaea shells and belemnite guards). Igneous erratics of northerly provenance occur less commonly. On weathering, the till becomes decalcified and ochreous in colour. It gives rise to heavy stiff clay soils, which were once left to pasture, but now are under intense arable culture. The thickness of the till is extremely variable, in part because of pre-Anglian relief and also due to post-Anglian erosion. One of the thickest recorded sequences of till is in a well [TL 316 546] just south of Longstowe, where 50.0 m were proved resting directly on Woburn Sands. Here the till occupies a northeast–south-west-aligned buried channel, the Hatley Channel of Edmonds and Dinham (1965). Good exposures of the till are rare, although in the past it has been worked in numerous small pits, some for brick-making and others as a source of marl for spreading on the adjacent acid soils.

Several small patches of glaciofluvial sand and gravel have been mapped within the district. The larger spreads depicted on the map near Biggleswade may possibly be river terrace deposits. The main spread of gravel extends from Stanford to Caldecote. The gravel falls gently eastwards towards the River Ivel. In the north-east the gravel merges imperceptibly into that of the first and second river terraces. The gravel contains about 60 per cent chalk clasts, 30 per cent flint with the remainder quartz and quartzite derived from the Triassic Sherwood Sandstone Group.

In a pit at Roxton [TL 158 538] Gao (1997) described glaciofluvial sands and gravels containing numerous chalk pebbles and cobbles overlain by chalky till and terrace gravels, filling a channel-form below the level of the present valley of the Great Ouse. A pit [TL 171 391] about 450 m east-southeast of Clifton church, showed (in 1946) up to 2 m of well-bedded chalky gravel overlain by brown decalcified flint gravel which extended downwards into the chalk gravel as long irregular pockets. This type of irregular junction between decalcified and chalky gravel is typical of the area.

River terrace deposits

Three distinct river terraces have been recognised, although over much of the district the first and second terrace deposits have been combined as a single unit. The highest terrace deposits of the River Great Ouse generally lie at a height of 14.6 to 17.6 m above the alluvium, although Edmonds and Dinham (1965) included gravels at a height of 12.2 m above alluvium within this terrace.

Deposits of the Third Terrace occur at Blunham [TL 145 510] and Wyboston [TL 155 565] in the west of the district where they overlie till of the Lowestoft Formation. As mentioned above, it is possible that the large spread of gravel to the west of the River Ivel near Biggleswade, should be assigned to this terrace rather than the Lowestoft Formation. Small spreads of gravel also attributed to the third terrace occur in the east of the district near Toft, Comberton and Barrington, associated with the Bourn Brook and the River Rhee (Cam), both tributaries of the River Great Ouse.

Extensive spreads of gravel of the First and Second Terrace extend down the western side of the district along the valleys of the Great Ouse and its tributary the River Ivel. In New Town church pits [TL 182 393], Edmonds and Dinham (1965) recorded 1.21 m of coarse gravel, overlain by 1.36 m of grey gravel, capped by 1.21 m of sandy ochreous gravel of varied size. In the 0.5 inch (12.7 mm) size fraction, chalk made up 50 per cent, flint 45 per cent and quartzite 5 per cent. A well [TL 197 455] to the north of Biggleswade proved 10.05 m of sand and gravel resting on Woburn Sands. Investigations by Gao et al. (1998) in a small quarry [TL 1817 4750] to the south of Sandy excavated into deposits of the First Terrace have revealed that deposition occurred as bars and sheets in a braided river regime. Fossiliferous silt lenses within the gravel indicate that their deposition occurred under slow moving or still water in abandoned meander channels. The palaeontological data from plant macrofossils, Mollusca and Coleoptera reveal a harsh climate similar to that of the tundra of present day arctic Russia. Radiocarbon ages (Gao et al., 1998) of 34 055 + 330-310 (Q-2936) and 29 250 + 460-420 (Q-2935) yrs BP suggest dates for the lower part of the gravelly sequence, in the Middle Devensian, at the end of the Upton Warren Interstadial Complex.

Interglacial deposits

Hughes (1916) recorded a section from Cardo's Pit [TL 3826 4914], Barrington where a bed about 1 m thick and exceptionally rich in mammalian remains was overlain by nearly 5 m of silty clay. Although there has been debate with regard to the stratigraphical position of these 'Barrington Beds' (Sparks, 1952; Norris, 1962), most authors believe that they represent an interglacial, probably the Ipswichian (Marine Isotope Stage 5e), followed by an episode of cooler climatic conditions. The basal bone bed appears to have been laid down under low-energy fluvial conditions, and has probably been disturbed by large animals, thus incorporating gravel, shells and bones into the deposit. The overlying clay unit is interpreted as an overbank deposit. Gibbard and Stuart (1975) analysed pollen scraped from two hippopotamus teeth and a rhinoceros jaw from the Sedgwick Museum collection. Despite poor preservation, sufficient pollen was counted to enable a palaeoenvironmental reconstruction. The assemblage indicated a treeless floodplain, with mixed oak woodland at some distance on the higher ground. Gibbard and Stuart (1975) assign the flora to the early part of the Ipswichian interglacial (Substage IP II). These authors also re-examined the large collection of Barrington vertebrate material in the Sedwick Museum, Cambridge, and smaller collections in the British Museum and in the then Geological Museum, London. The faunal assemblage of Hippopotamus amphibius (hippopotamus), Palaeoloxodon antiquus (straight tusked elephant) Dama dama (fallow deer), Cervus elephas (red deer), Canis lupus (wolf), and Ursus arctos (brown bear) characterises the 'hippopotamus fauna' associated with the Ipswichian Stage (MIS 5e). Records of Mammuthus [Elephas] Primignius and Equus ferus [caballus] from the 'Barrington Beds' are probably in error and may have been collected from nearby sites of differing age. Amino acid recemisation of Trichia spp. shells associated with Hippopotamus produced mixed dates that are not incompatible with an Ipswichian age, but suggest that the deposits contain significant reworked older material. A collection of molluscs made by C E Gray from the very basal part of the Barrington deposits yielded several pristine specimens of Cobicula together with other aquatic taxa including Bithynia tentaculata. The presence of Corbicula has been taken to indicate an MIS 7 age at other sites (Keen, 1990; Preece 1995), and amino acid recemisation of Bithynia shells from this collection produced comparable age estimates. Therefore, it appears that two interglacial deposits of different ages, Ipswichian (MIS 5e) and MIS 7, may occur at Barrington.

Alluvial fan deposits

The alluvial fan deposits to the east of Royston (fomerly depicted as Taele Gravel) are more extensively developed in the adjacent Saffron Walden district. The essential water-laid character and connection with the drainage of the Chalk escarpment were recognised by Penning and Jukes-Browne (1881), who included them under the heading of 'Gravels of the ancient river system'. Osborne White (1932) considered the gravels not to be terraces left by winding rivers, but rather as remnants of widespread fans of waste brought down and distributed by torrents from the higher ground of Chalk and till to the south-east. Former small pits have revealed up to about 3 m of alluvial fan deposits which range from chalky gravels with seams of sand to chalky sands with scattered flints. Boulders of quartzite, basalt and other rocks are also reported to be present (White, 1932).

Alluvium

Many of the streams are bordered and underlain by tracts of alluvium, comprised in the main from reworking of earlier drift deposits and the Chalk bedrock. The alluvium also contains an appreciable quantity of organic material, including peat that has not been mapped separately. Peat is shown on the map in those areas that have been geologically surveyed since 1970. True peat with logs of wood has been found in some of the deeper excavations. Thickness of alluvium is usually unproved, but is generally likely to be less than about 3 m. In many places, thin gravel underlies the alluvial silts and clays.

Peat

Peat has been mapped in the south-west of the district near Chicksands [TL 12 39] associated with alluvial deposits of the River Flit. It may also be present within alluvium at other localities (see above), but has not been distinguished separately.

Head

Head has been mapped only in the small areas that have been surveyed since 1970. Elsewhere, it may be presumed that head is present as a veneer on most lower valley sides, and on floors of smaller valleys where alluvial deposits are absent.

Head is typically a heterogeneous deposit derived from the mass downslope movement of material under periglacial conditions. In practice no distinction has been made between deposits formed under periglacial conditions and those formed by hill wash and soil creep, processes still operating at present. Head deposits formed in this way were described from Eastwood's Pit, Barrington Works [TL 394 511] by Sparks (1952). Because of its local derivation, the composition of Head is closely related to that of the upslope deposits from which it has formed; thus head derived from till within the area will be composed of silt and clay whereas that derived from the underlying Chalk will be calcareous. Head deposits commonly contain shear planes that may fail when loaded, thus its presence or absence needs to be determined prior to any development taking place.

Landslip

Areas of landslip are present along the scarp of the Woburn Sands Formation between Sandy and Gamlingay, at its junction with mudstone of the West Walton and Ampthill Clay formations.

Buried Channel

A north–south aligned buried channel is present in the south of the district between Clifton [TL 17 39] and Broom [TL 17 42] in the south-west of the district. The channel is infilled with Anglian glaciogenic deposits.

Artificially modified ground

Worked ground is shown where minerals have been dug, and where subsequent back-fill is minimal or absent. Worked minerals included chalk, and sand and gravel aggregate.

During the latter part of the 19th and early part of the 20th centuries, phosphatic nodules were dug from the Cambridge Greensand at the base of the Chalk Group and from within the Gault Formation. There is now little evidence to indicate the extent of these former workings, and they are not shown on the map.

Made ground is material that has been deposited by human agency on an original land surface; such areas include road and rail embankments. These have generally been omitted from the 1:50 000 map although they are depicted on the component 1:10 000 scale maps.

Infilled ground

This indicates where clay, chalk and sand and gravel have been extracted and the workings partially or wholly back-filled.

Chapter 3 Applied geology

Chalk

Generally, the small pits excavated in chalk have been dug for spreading directly as chalk on the more acid soils developed on the nearby land that is underlain by sand and gravel. Some chalk has been burned for lime. Chalk from the lower part of the group, around Shepreth and Barrington, has also been used in the manufacture of cement. Powdered whiting has been produced from chalk quarried at Melbourn.

There is one active quarry [TL 305 390] at Steeple Morden working the Holywell Nodular Chalk Formation.

Cement

Chalk from the Grey Chalk Subgroup and Gault are quarried at Barrington [TL 390 510] for use in the manufacture of cement.

Sand and gravel aggregate

Sand from the Woburn Sands Formation in the Potton area and sand and gravel from the extensive River Terrace Deposits in the western part of the district, particularly around Sandy, form the main resource. Three active quarries are currently (2002) working sand from the Woburn Sands Formation, two [TL 212 488]; [TL 225 503] at Potton and one [TL 203 492] at Sandy Heath. A further quarry [TL 180 480] at Sandy formerly worked sand and gravel from river terrace deposits. A quarry [TL 173 440] at Broom worked sand and gravel from glaciofluvial deposits within the Lowestoft Formation.

Building stone

Stone suitable for wall building can be obtained from the more cemented horizons within the Woburn Sands Formation. 'Cambridgeshire Clunch' derived from the Grey Chalk Subgroup is dug at Barrington [TL 340 510] (Plate 3) for building stone (Plate 4).

Phosphate

The Cambridge Greensand at the base of the Chalk was extensively worked for phosphate nodules from the mid 19th century up to the early years of the 20th century. The nodules provided a source of calcium phosphate for the manufacture of superphosphate fertiliser. Although there are few visible signs of the former workings, much of the outcrop was worked in pits up to several metres deep; the spoil was replaced in the worked areas, and the land restored to agricultural use immediately after the working. Phosphate-rich horizons within the Gault were also dug on a smaller scale.

Water supply

The district lies within the boundaries of the River Great Ouse catchment, which is one of the driest in the UK. The mean rainfall lies between 500 and 640 mm, and the average annual potential evaporation is believed to be about 430 mm. The amount of precipitation available for the maintenance of river flow and underground storage amounts to between 70 and 200 mm per year.

Limited supplies of groundwater have been obtained locally at Blunham [TL 150 523] and Ickwell Bury [TL 146 460] from the Great Oolite Group which underlies the Oxford Clay Formation. Because of the confining nature of the overlying clay, artesian condition may occur. The probability of encountering saline water increases with depth and distance from the outcrop.

The sandstone of the Kellaways Formation may yield small supplies, but large amounts of water are unlikely within the district. The 'Elsworth Rock' within the West Walton Formation was formerly of local importance as a source of domestic and farm supplies. A number of small springs issue along its outcrop.

The outcrop north of Biggleswade forms the greater part of the Woburn Sands Formation catchment and is free of Quaternary deposits, but substantial replenishment must infiltrate the overlying till in the area to the north-east of Gamlingay. The Woburn Sands Formation has been exploited for ground water supplies over wide areas below Gault and Chalk cover. Artesian supplies have been obtained in the low-lying Gault Formation and lowermost Chalk areas where the confining action of the overlying Gault holds the Woburn Sands water under pressure. In recent years heavy abstraction has meant that many formerly artesian wells no longer overflow.

Wells in the Woburn Sands need to be specially constructed to prevent the ingress of fine-grained sand during pumping. There appears to be a general improvement in the quality of Woburn Sands water in a south-westerly direction along the outcrop. There is a considerable variation in the ratio of non-carbonate to carbonate hardness, and the iron content may be appreciable. Water from the Dunton Pumping Station has a total dissolved solid content of 275 ppm, chlorides 14 ppm and total hardness 180 ppm (non-carbonate 20 ppm, carbonate 160 ppm).

The West Melbury Marly Chalk, Zig Zag Chalk, Holywell Nodular Chalk and the New Pit Chalk members (since the map was published these members have all been raised to formation status p.12), which form almost the entire outcrop of the Chalk in the district, do not yield large supplies. The yield from Chalk wells depends upon the degree of fissuring, and the latter depends largely upon the lithology of the Chalk (including its flint content) and the presence of structural features. In the present district the Chalk dips gently and uniformly to the south-east or south-south-east and is devoid of any major structures. The harder Melbourn Rock at the base of the Holywell Nodular Chalk probably offers good prospects, but is present only south of a line between Melbourn [TL 38 45] and Ashwell [TL 26 39]. The highest yields within the district are from a 90 m borehole of 380 mm diameter at Royse Grove Pumping Station [TL 358 400], Royston. After acid treatment, this well tested at 67 330 litres for a drawdown of 24 m. A number of springs issue from the Totternhoe Stone at the base of the Zig Zag Chalk. At Ashwell one of these springs used to supply a brewery with 15 500 litres per day. Water from the Chalk is usually very hard, most of the hardness being due to carbonate ions. Few analyses are available within the district, but the records from Ashwell Pumping Station indicate 330 ppm total dissolved solids, 15 ppm chlorides (as chlorine) and 275 ppm total hardness (non-carbonate 60, carbonate 215). At Royse Grove, the total hardness is 271 ppm (non-carbonate 71, carbonate 200). River terrace deposits are well developed in the western part of the district and form the only sources of water where the River Great Ouse crosses the Jurassic mudstones in the north-west of the district. Storage in the gravels is limited, and in prolonged dry weather, yields tend to diminish, and pumping may affect the flow of the river, although most of the groundwater abstracted is returned as effluent.

The Lowestoft till is likely to yield only very small quantities of water.

Natural radon emissions

Radon is a naturally occurring radioactive gas that is produced by the radioactive decay of radium which, in turn, is derived from the radioactive decay of uranium. Uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. Radon is released from rocks and soils, and is quickly diluted in the atmosphere. Concentrations in the open air are normally very low and do not present a hazard. Radon that enters poorly ventilated enclosed spaces such as basements, buildings, caves, mines and tunnels may reach high concentrations in some circumstances. Radon levels in individual buildings can be influenced by the construction method and the degree of ventilation. Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer. If individuals are exposed to high concentrations for significant periods of time, then there may be cause for concern. In order to limit the risk to individuals, the Government has adopted an 'Action Level' for radon in dwellings of 200 becquerels per cubic metre (Bq m⁻³).

The National Radiological Protection Board (NRPB) (Lomas et al., 1996; Miles et al., 1996) has drawn up maps of radon affected areas which are those areas of the UK with a probability of 1 per cent or more homes being above the Action Level. The NRPB maps show the estimated proportion of homes exceeding the Action Level for each 5 km square of the Ordnance Survey National Grid. The variation in radon levels between different parts of the country is mainly controlled by the underlying geology. Within the Biggleswade district radon potential is generally low, with the highest radon concentration values identified on areas of Chalk outcrop. In these areas, between 1 to 3 per cent of properties are likely to exceed the UK 'Action Level'.

Engineering geology

The basic engineering geology characteristics of the solid and Quaternary formations are outlined in (Figure 4) and (Figure 5).

The Gault, Ampthill, West Walton and Oxford Clay formations are dominated by mudstone, weathering to clay. On account of the high smectite content, clay may undergo significant volume change with variation in water content. Seasonal effects, in which vegetation, especially trees, plays a dominant role, were identified by Driscoll (1983) as the most important controls on shrinking and swelling. The clay absorbs high quantities of water during wet periods and loses it again during droughts. Drying clay may crack and swell, causing structural damage to roads and buildings.

High concentrations of sulphate in the ground or groundwater can weaken concrete foundations that are not designed to resist this chemical attack. Pyrite (iron sulphide) in the clays becomes oxidised in the weathered zone to form sulphate ions in solution. Calcium carbonate present in the deposit may react with the sulphate ions to precipitate selenite (calcium carbonate) crystals. This involves an eight-fold increase in volume over the original pyrite and may cause disruption or weakening of strata. When the selenite itself is weathered, sulphuric acid is produced which reacts with cement not designed to be sulphate resistant, causing it to break down. The surface of the Chalk is prone to solution which may take the form of approximately cylindrical pipes or more irregular areas. The voids may remain open or become filled with overlying deposits. If built upon, differential subsidence may result.

Areas of landslip are particularly common along the outcrop of the Ampthill Clay and West Walton formations at their junction with the overlying Woburn Sands Formation.

Information sources

Information on BGS products is listed in the current BGS Catalogue of geological maps and books and much additional information is held by the BGS; to help access this data, a comprehensive Geological Data Index (GDI) is available on the BGS web site (addresses on back cover). The index includes information on geology, hydrogeology, geochemistry and geophysics.

Some examples of data available are given below:

• index of boreholes and water wells
• water levels in boreholes
• outline of BGS maps at 1:50 000 and 1:10 000 scale, 1:10 560 scale and County Series.
• geochemical sample locations on land
• aeromagnetic and gravity data recording stations
• land survey records and site investigation reports
• drillcore
• geophysical logs
• magnetic anomalies (grey and colour shaded)
• gravity readings
• gravity anomalies (grey and colour shaded)
• map and book products
• mineral information

Further geological information held by the British Geological Survey relevant to the district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Geological Enquiry Service, BGS, Keyworth.

Maps

• Geological maps
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• Solid geology map UK South Sheet, 2001; Quaternary geology 1977)
• 1:250 000
• 52N00 East Anglia, Solid Geology, 1991
• 1:50 000
• The 1:50 000 Series Sheet 204 Biggleswade is based largely on the former 1:63 360 scale map (one inch to one mile), which in turn was based on earlier maps at 1:10 560 (six inches to one mile) and 1:63 360 scales. The distribution of the glaciogenic deposits is based mainly on late 19th century 1:63 360 mapping, whereas the river terrace deposits were largely resurveyed at 1:10 560 scale during the early 1930s. The area to the south of Northing 40 was surveyed at the 1:10 000 scale between 1986 and 1992. The area to the north was partly revised in 1998–1999. The stratigraphy of the Chalk has also been revised to bring it in line with modern nomenclature. Maps of adjacent areas are listed below:
• Sheet 186 Wellingborough 1974
• Sheet 187 Huntingdon 1975
• Sheet 188 Cambridge 1981
• Sheet 205 Saffren Walden 2002
• Sheet 220 Leighton Buzzard 1992
• Sheet 221 Hitchin 1995
• Sheet 222 Great Dunmow 1990

Digital geological map data

In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets are:

• DiGMapGB-625 (1:625 000 scale)
• DiGMapGB-250 (1:250 000 scale)
• DiGMapGB-50 (1:50 000 scale)
• DiGMapGB-10 (1:10 000 scale)

The current availability of these can be checked on the BGS web site at: http://www.bgs.ac.uk/products/digitalmaps/digmapgb.html

Books and reports

• British regional geology
• East Anglia and surrounding areas, 1961, reprint 1982
• Memoirs
• Sheet 188 Cambridge, 1960
• Sheet 205 Saffron Walden, 1932†
• Sheet 203/204 Huntingdon and Biggleswade, 1965†
• Sheet 205 Saffron Walden, 1932†
• Sheet 220 Leighton Buzzard, 1994
• Sheet 221 Hitchin, 1996
• Sheet 222 Great Dunmow, 1990
• † out of print, but facsimile copies are available at a tariff that is set to cover copying costs

Sheet Explanation

Sheet 205 Saffron Walden, 2003

References

Most of the references listed below are held in the libraries of the British Geological Survey at Murchison House, Edinburgh and at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyrightlegislation.

Bristow, C R, Mortimore, R N, and Wood, C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 109, 293–315.

Bromley, R G, and Gale, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273–306.

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

Casey, R. 1963. The dawn of the Cretaceous period in Britain. Bulletin of the South Eastern Union of Scientific Societies, No. 117.

Cox, B M, and Gallois, R W. 1979. Description of the standard stratigraphical sequences of the Upper Kimmeridge Clay, Ampthill and West Walton Beds. 68–72 in Geological investigations for the Wash Water Storage Scheme. Gallois, R W. Report of the Institute of Geological Sciences, 78/19.

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

Cox, B M, Sumbler, M G, and Ivimey-Cook, H C. 1999. A formational framework for the Lower Jurassic of England and Wales (Onshore Area). British Geological Survey Research Report, RR/99/01.

Donovan, D T, Horton, A, and Ivimey-Cook, H C. 1979. The transgression of the Lower Lias over the northern flank of the London Platform. Journal of the Geological Society of London, Vol. 136, 165–173.

Driscoll, R. 1983. The influence of vegetation on swelling and shrinking of clay soils in Britain. Geotechnique, Vol. 33, 93–105.

Edmonds, E A, and Dinham, C H. 1965. Geology of the country around Huntingdon and Biggleswade. Memoir of the Geological Survey of Great Britain, Sheets 187 and 204 (England and Wales).

Fearnsides, W G. 1904. The geology of Cambridgeshire. 9–50 in Handbook to the natural history of Cambridgeshire. Marr, J E, and Shipley, A E (editors). (Cambridge: Cambridge University Press.)

Gallois, R W. 1979. The Kimmeridge Clay in Norfolk. Bulletin of the Geological Society of Norfolk, Vol. 31, 45–68.

Gallois, R W, and Cox, B M. 1977. The stratigraphy of the Middle and Upper Oxfordian sediments of Fenland. Proceedings of the Geologists' Association, Vol. 88, 207–228.

Gao, C, Coope, G R, Keen, D H, and Pettit, M E. 1998. Middle Devensian deposits of the Ivel valley at Sandy, Bedfordshire, England. Proceedings of the Geologists' Association, Vol. 109, 127–137.

Gibbard, P L, and Stuart, A J. 1975. Flora and vertebrate fauna of the Barrington Beds. Geological Magazine, Vol. 112, No. 5, 493–501.

Hopson, P M, Aldiss, D T, and Smith, A. 1996. Geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England and Wales).

Horton, A. 1971. Gamlingay Borehole. Annual Report of the Institute of Geological Sciences for 1970. Vol. 88, 207–228.

Horton, A. 1989. The geology of the country around Peterborough. Memoir of the British Geological Survey, Sheet 158 (England and Wales).

Hughes, T McK. 1916. The gravels of East Anglia. (Cambridge: Cambridge University Press.)

Keen, D H. 1990. Significance of the record provided by Pleistocene fluvial deposits and their included molluscan faunas for palaeo-environmental reconstruction and stratigraphy: case studies from the English Midlands. 25–34 in Methods for the study of stratigraphical records. Rousseau Denis, D (editor), Vol. 80; 1. (Amsterdam, Netherlands: Elsevier.)

Lomas, P R, Green, B M R, Miles, J C H, and Kendall, G M. 1996. Radon atlas of England. National Radiological Protection Board Report, R290.

Medd, A W. 1979. The Upper Jurassic coccoliths from the Haddenhgam and Gamlingay boreholes. Eclogae Geologicae Helvetiae, Vol. 12, 19–109.

Miles, J C H, Green, B M R, and Lomas, P R. 1996. Radon affected areas: England, Wales. Documents of the National Radiological Protection Board, Vol. 7, No. 2.

Moorlock, B S P, and Highley, D E. 1991. An appraisal of fuller's earth resources in England and Wales. British Geological Survey Technical Report, WA/91/75.

Morter, A A, and Wood, C J. 1983. The biostratigraphy of Upper Albian–Lower Cenomanian Aucellina in Europe. Zitteliana, Vol. 10, 515–529.

Mortimore, R N. 1983. The stratigraphy and sedimentation of the Turonian–Campanian in the southern province of England. Zitteliana, Vol. 10, 27–41.

Norris, G. 1962. Some glacial deposits and their relation to the Hippopotamus-bearing beds at Barrington, Cambridgeshire. Geological Magazine, Vol. 99, 97–118.

Osborne White, H J. 1932. The geology of the country near Saffron Walden. Memoir of the Geological Survey, Sheet 205 (England and Wales).

Penning, W H, and Jukes-Browne, A J. 1881. The geology of the neighbourhood of Cambridge. Memoir of the Geological Survey of Great Britain.

Preece, R C. 1995. Mollusca from interglacial sediments at three critical sites in the lower Thames. 53–60 in The Quaternary of the lower reaches of the Thames: field guide. Bridgland, D R, Allen, P, and Haggart, B A (editors). (Durham, United Kingdom: Quaternary Research Association.)

Rawson, P F, Allen, P, and Gale, A. 2001. The Chalk Group — a revised stratigraphy. Geoscientist, Vol. 11, 21.

Roberts, T. 1892. The Jurassic rocks of the neighbourhood of Cambridge. (Sedgewick Prize Essay for 1886, Cambridge.)

Sparks, B W. 1952. Notes on some Pleistocene sections at Barrington, Cambridgeshire [England]. Geological Magazine, Vol. 89, 163–174.

Shephard-Thorn, E R, Moorlock, B S P, Cox, B M, Allsop, J M, and Wood, C J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 222 (England and Wales).

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

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Classification of the Jurassic strata in the district.

(Figure 2) Extent of Jurassic formations within the district.

(Figure 3) Lithostratigraphy of the Chalk Group.

(Figure 4) Engineering characteristics of the bedrock formations.

(Figure 5) Engineering characteristics of the Quaternary deposits.

Plates

(Plate 1) General aspect of the Woburn Sands Formation, Potton [TL 2135 4875]; the structures within the sands are highlighted by variations in the amount of ferruginous cement present (GS1231).

(Plate 2) Acanthoceras, an ammonite with numerous large tubercles and strong ribs that is characteristic of the lower and middle parts of the Zig Zag Chalk, Barrington Pit [TL 390 510] (GS1232).

(Plate 3) General view of Barrington Pit [TL 390 510], which produces 'Cambridgeshire Clunch' from the Grey Chalk Subgroup. The quarried rock is used locally as a building stone. (GS1233).

(Plate 4) Barrington Church [TL 397 499] is built using 'Cambridgeshire Clunch', a local building stone quarried from the Grey Chalk Subgroup (GS1234).

(Front cover) St Andrews Parish Church, Biggleswade [TL 188 446]. The church is built largely from locally derived Cretaceous rocks. (Photographer T P D S Cullen; MN39873).

(Rear cover)

(Geological succession) Geological succession of the Biggleswade district.

(Index map) Index to the1:50 000 Series maps of the British Geological Survey.

Figures

(Geological succession) Geological succession of the Biggleswade district

(Figure 1) Classification of the Jurassic strata in the district

(Figure 3) Lithostratigraphy of the Chalk Group

(Figure 4) Engineering characteristics of the bedrock formations

(Figure 5) Engineering characteristics of the Quaternary deposits