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Geology of the Saffron Walden district — brief explanation of the geological map Sheet 205 Saffron Walden

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

Bibliographic reference: Moorlock, B S P, Boreham, S, Woods, M A, and Sumbler, M G. 2003. Geology of the Saffron Walden district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 205 Saffron Walden (England and Wales).

Keyworth, Nottingham: British Geological Survey

© NERC 2003 All rights reserved. Printed in the UK for the British Geological Survey by B&B Press Ltd Rotherham

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 NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) Front cover: Walden Castle [TL 541 386] built in the 12th century. The ruins are part of the original basement constructed from locally derived flints. The castle had been reduced to a ruin by the mid 18th century and turned into a 'quarry'; much of the walls had been carried off for road making and other purposes (Photographer T P D S Cullen; MN39872).

(Rear cover)

Notes

The word district refers to the area of geological Sheet 205 Saffron Walden.

National grid references are given in the form [1234 1234] or [123 123]. Unless otherwise stated all such references fall within grid square TL.

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

Acknowledgements

We would like to acknowledge the cooperation of Cambridge University and The Open University for allowing one of the authors (SB) to include unpublished research data.

Geophysical information was supplied by C P Royles. Series editor is 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 Saffron Walden district (summary from rear cover)

(Rear cover)

Modern development requires accurate geological information in order, for example, to identify resources and ensure that foundations are adequate. Modern agricultural practices also require a knowledge of the underlying geology. This Sheet Explanation and the newly published geological map that it describes provide valuable information on a range of earth science issues. This explanation is written for those who may have limited experience in the use of geological maps, and for the professional user, who may wish to be directed, via a list of references, to further geological information about the district.

The historic market town of Saffron Walden is famous for its many fine timber-framed buildings, built when sheep farming brought much local wealth. The town takes its name from the saffron crocus that used to be grown nearby during the 16th century. The district is predominantly agricultural, but it extends northwards to include the southern outskirts of Cambridge, and the town of Haverhill lies in the east. Chalk underlies most of this district and forms a gentle rolling topography; it is the main aquifer in the region. Harder beds within the Chalk are more resistant to erosion and form mappable, steepsided, scarp features. Much of the higher ground is capped by till deposited during the Anglian glaciation just over 400 000 years ago. Excavation of river terrace gravels for aggregate within the main valleys has provided important evidence of more recent interglacial episodes.

Chapter 1 Introduction

This Sheet Explanation describes the geological map Sheet 205 Saffron Walden, which includes the outskirts of southern Cambridge, Haverhill and the northern part of Saffron Walden. The map has been revised from earlier editions (p.21), and the stratigraphy of the Chalk has been revised to bring it in line with modern nomenclature. Resistivity logs are available for many of the boreholes within the district and these have greatly aided correlation of the buried Chalk formations.

The district lies on the northern margin of the London Platform, a massif of Palaeozoic rocks, which was deformed during the Variscan orogeny, and, during the Mesozoic, formed a stable structural high surrounded by areas of gradual subsidence. Mesozoic strata lapped progressively onto this platform (see cross-section on map), but parts of its interior probably remained as dry land (the so-called London Landmass) until the Cretaceous.

The regional aeromagnetic anomaly map (Figure 1) shows a major magnetic high that extends southwards from the south-western part of the district. This high, which broadly corresponds to a gravity low (Figure 2), is interpreted as a possible Devonian basin. The magnetic high near the central northern edge of the map most probably reflects the presence of Ordovician dioritic intrusions. The small magnetic high in the northern part of the Saffron Walden district may be related to these intrusions.

The north-west-trending gravity high in the north-east of the regional Bouguer gravity anomaly map (Figure 2) is most likely related to the underlying Caledonian basement fabric. An offset of this structure, which possibly reflects basement faulting, impinges on the north-east corner of the district.

Chapter 2 Geological description

Silurian to Carboniferous

In the Biggleswade district to the west, up to 350 m of undifferentiated Silurian rocks are inferred from geophysical evidence to be present at depth; these are overlain by up to 600 m of Lower and Upper Old Red Sandstone. There is no evidence that the Silurian rocks extend beneath the Saffron Walden district, but the Old Red Sandstone must be present although its thickness is not known. The Devonian rocks are overlain in the northern part of the district by Carboniferous Limestone, which has been proved in several boreholes.

Jurassic

Early Mesozoic rocks, of Triassic age, are overlapped by Lower Jurassic strata well to the north-west of the district. The Cambridge Borehole [TL 4316 5949] a few kilometres to the north of the district, proved some 12.6 m of Lower Jurassic strata (see (Figure 3) for the lithostratigraphy of the Jurassic) resting directly on Palaeozoic 'basement' (Worssam and Taylor, 1969). The beds are calcareous mudstones and argillaceous limestones of Pliensbachian age, belonging to the Charmouth Mudstone Formation of the Lias Group. The feather-edge of the Charmouth Mudstone probably extends into the north-western extremity of the district (see (Figure 4)), but farther southeast the formation is absent as a result of erosion which preceded deposition of the Great Oolite Group. During this period of erosion, a land surface was cut across a range of pre-existing strata (see Donovan et al., 1979).

The Middle Jurassic Great Oolite Group is 13.2 m thick in the Cambridge Borehole; it is thicker in boreholes to the west, and is likely to be present in the north-west of the district. It is evidently overstepped by Cretaceous sediments to the south (Figure 4), as demonstrated in the Little Chishill Borehole [TL 4528 3637] which proved Gault resting directly on Palaeozoic 'basement' (Lake and Wilson, 1990).

The basal part of the Great Oolite Group comprises the Rutland Formation, a largely non-marine succession that was deposited in land-marginal areas of the London Platform. The basal unit, known as the Stamford Member, comprises fluviatile and lacustrine sand and silt that in some accounts has been mistakenly assigned to the 'Lower Estuarine Series' (now Grantham Formation) of the Inferior Oolite Group. The latter is now believed to be overstepped by the Rutland Formation near Peterborough. Succeeding parts of the Great Oolite Group within the district probably include representatives of the Blisworth Limestone and Cornbrash formations.

Some 54.6 m of Middle–Upper Jurassic Ancholme Group are present in the Cambridge Borehole, but this thins to the west of the district. The group is therefore presumed to be overstepped by the Cretaceous some distance to the north-west of the limit of the Great Oolite Group (Figure 4). Probably only the Kellaways Formation, comprising a few metres of fine-grained sandstone, siltstone and mudstone, and the Oxford Clay Formation (mudstone) are represented within the district.

Cretaceous

Woburn Sands Formation

The Woburn Sands Formation (formerly Lower Greensand) has been proved in many wells, dug through the Gault, in the north-west of the district, although few have penetrated the entire thickness; it does not come to crop. At Sawston, two boreholes that commenced in the Chalk, proved, respectively, 16.2 and 19.8 m of Woburn Sands, before terminating in clayey beds attributed, almost certainly in error, to the Kimmeridge Clay. From boreholes just within the Cambridge and Biggleswade districts it is estimated that the Woburn Sands of the Saffron Walden district are approximately 16 m thick. Information on the lithology of the Woburn Sands within the district is lacking.

Gault Formation

In and around the villages of Haslingfield, Barton and Grantchester in the north-west of the district, up to about 10 m of Gault Formation crop out beneath the Cambridge Greensand. Many years ago, when the Cambridge Greensand was being exploited as a source of phosphate for fertilizer, there were numerous exposures in the upper part of the Gault, but nowadays exposures are rare and of a temporary nature.

The Gault of Cambridgeshire is subject to local variations in thickness. This may be attributed to erosion prior to deposition of the Cambridge Greensand, but also may be due in part to variations in rates of sedimentation. In the Saffron Walden district the Gault (Figure 5) generally comprises between about 40 and 50 m (maximum recorded thickness of 58.5 m) of blue-grey and pale grey, stiff clay and marl (calcareous mudstone), containing many phosphatic and pyritic nodules, which may occur dispersed or in layers. Thin, discontinuous bands of hard argillaceous, shelly limestone occur sporadically. The higher beds of the Gault are noticeably micaceous, and more calcareous than the lower parts, which become arenaceous downward and include, as a basement bed, 0.6 to 3 m of loamy sand that is bluish green, glauconitic and phosphate-bearing.

The Gault is prone to shrink-swell processes which may result in 'ground heave'. The breakdown of pyrite to selenite (calcium sulphate) in the weathered zone of the Gault is associated with the development of aggressive acid sulphate groundwater conditions.

Chalk Group

The classification of the Chalk Group (Figure 6) 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 1:50 000 Series map, this account retains the Plenus Marls in the top of the Zig Zag Chalk Formation.

Grey Chalk Subgroup

West Melbury Marly Chalk Formation

The West Melbury Marly Chalk comprises interbedded limestone and marl, and marks the base of the Grey Chalk Subgroup. At its base is the Cambridge Greensand Member, a richly fossiliferous phosphatic and glauconitic sandy horizon that is characteristic of the base of the Chalk Group in the Saffron Walden district and in the Hitchin (Sheet 221) district, but dying 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 strata (S. dispar Zone) 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. The Cambridge Greensand was tentatively identified in core from the Duxford Borehole [TL 469 457] between depths of 89.92 and 89.97 m, overlain by about 30 m of West Melbury Marly Chalk. Comparison of the faunas from this borehole with the resistivity log suggests that a high resistivity peak at about 67 m depth may correlate with the Dixoni Limestone of the Leighton Buzzard district (Shephard-Thorn et al., 1994). Lower in the Duxford succession, a less pronounced resistivity high at about 84 m may equate with the Doolittle Limestone of Shephard-Thorn et al. (1994).

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. The formation comprises homogenous, pale grey and cream-coloured chalk, with a predominance of curvilinear jointing (Osborne White, 1932).

The Totternhoe Stone is a shell-rich calc-arenitic horizon, 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, markedly reducing its preserved thickness in places. In the Leighton Buzzard district, strong local erosion at the base of the Totternhoe Stone is used to infer the presence of channels (Shephard-Thorn et al., 1994, fig. 20). The base of the Totternhoe Stone is typically a phosphatic lag in the Saffron Walden district (Osborne White, 1932), and some of its fauna may be derived from the West Melbury Chalk. Directly overlying the Totternhoe Stone is a horizon characterised by an acme of the rhynchonellid brachiopod Orbirhynchia mantelliana, probably correlating with the highest of three acmes of this brachiopod recognised at Folkestone, and therefore with the middle part of the T. costatus Subzone.

In basin margin areas such as Saffron Walden, the sub-Totternhoe Stone erosion surface is interpreted as a sequence boundary, and the overlying Totternhoe Stone as a transgressive deposit (Robaszynski et al., 1998).

The Duxford Borehole [TL 469 457] proved about 32 m of Zig Zag Chalk between the top of the Plenus Marls at about 28 m depth and the base of the Totternhoe Stone at about 59.7 m depth. Faunal evidence suggests that the Totternhoe Stone is at least 0.9 m thick (approximately 58.9 to 59.7 m depth) in the borehole, corresponding with a peak in the borehole resistivity log. This resistivity peak can be correlated in several other boreholes within the district. The Plenus Marls correspond to a resistivity low in borehole logs. Within this low, a small peak is attributed to Jefferies' Bed 3, which is indurated and more massive compared with the adjacent soft marls (Jefferies, 1963).

White Chalk Subgroup

Holywell Nodular Chalk Formation

Hard, nodular, shell-rich chalk lithologies are typical of the Holywell Nodular Chalk. The base of the formation is marked by the conspicuously indurated and nodular Melbourn Rock Member, about 2 to 3 m thick, and represented by the lowest of several pronounced spikes in borehole resistivity logs. The stratotype area of the Melbourn Rock lies a few kilometres north-east of Royston, in the adjacent Biggleswade district (Sheet 204). Above the Melbourn Rock, the chalk is less massive, nodular, and contains common Inoceramus labiatus [=Mytiloides mytiloides and M. labiatus] (Osborne White, 1932), matching the typical lithology of the Holywell Nodular Chalk in southern England (Bristow et al., 1997). Osborne White's (1932) reference to

flints in the upper labiatus Zone may refer to the upper part of the Holywell Nodular Chalk. In southern England, this part of the sequence is usually flint-free, but flints occur in the top of the Holywell Nodular Chalk in the Hitchin district (Hopson et al., 1996). From faunal evidence in the Duxford Borehole, the thickness of the Holywell Nodular Chalk is about 17 m.

New Pit Chalk Formation

Above the hard, nodular lithologies that correspond with the Holywell Nodular Chalk (Bristow et al., 1997), Osborne White (1932) recorded soft, blocky chalk, with marl seams and some flints that increase in abundance upwards; the typical lithology of the New Pit Chalk Formation. The combined Holywell and New Pit Chalk formations broadly equate with the 'Middle

Chalk' of the previous survey, for which a thickness of about 62 m was estimated (Osborne White, 1932). However, borehole resistivity logs suggest that at least 80 m of combined Holywell and New Pit Chalk are present. The New Pit Chalk is recorded in 14 borehole resistivity logs in the Saffron Walden district; two of them penetrate the entire formation, and indicate a thickness of about 70 m. To the south, in the London area, and adjacent Beaconsfield district (Sheet 255), borehole resistivity log interpretations indicate that the New Pit Chalk is typically 40 to 50 m thick (Woods, 1995, 1998, 1999).

Using borehole resistivity logs as a guide, most of the increase in thickness of the New Pit Chalk in the Saffron Walden district can be attributed to the apparently higher stratigraphical base of the Lewes Nodular Chalk Formation. This can be quantified with reference to key marker horizons in the higher part of the New Pit Chalk, which have been traced through borehole resistivity logs in the district. One of these is the Mount Ephraim Marl, identified by correlation of the resistivity signature for the Great Bradley Borehole [TL 6720 5427] with that published for a borehole at North Pickenham [TF 854 706], near Swaffham, Norfolk. The Mount Ephraim Marl equates with Southerham Marl 1, near the base of the Lewes Nodular Chalk in southern England (Mortimore, 1986; Mortimore and Wood, 1986), but there it is apparently 17 m below the inferred top of the New Pit Chalk.

Lewes Nodular Chalk and Seaford Chalk formations, undivided

Hard, nodular and flinty chalk with marl seams and hardgrounds is typical of the Lewes Nodular Chalk. However, in the Saffron Walden district, there appears to be a gradational lithological change to softer and smoother textured chalk of the overlying Seaford Chalk Formation; therefore the formations are shown as 'undivided'. Nevertheless, borehole resistivity logs can locally be used to infer a thickness for the Lewes Nodular Chalk, which is at least 30 to 40 m.

In the Chilterns, to the south-west, the feature-forming beds that mark the base of the Lewes Nodular Chalk condense the upper T. lata Zone and lower S. plana Zone into an indurated horizon, traditionally marking the base of the 'Upper Chalk', and named the 'Chalk Rock'. However, in the Saffron Walden district, the 'Chalk Rock' has changed from a discrete horizon or thin group of horizons into a nodular chalk 4 to 5 m thick (Osborne White, 1932); this is shown as 'Horizon of Chalk Rock' on the earlier edition of the geological map of the district. The base of this is stratigraphically younger than the Chalk Rock of the Chilterns. It includes the correlative of the Hitchwood Hardground of Bromley and Gale (1982), but stratigraphically lower hardgrounds recognised in the 'Chalk Rock' of the Chilterns are absent. Thus strata which elsewhere belong to the lower Lewes Nodular Chalk, in the Saffron Walden district appear developed in New Pit Chalk lithofacies.

Osborne White (1932) noted that a distinctive mollusc fauna, named the Reussianum Fauna and used to trace the base of the 'Upper Chalk' in former surveys, occurs just above the base of the 'Upper Chalk'; and Bromley and Gale (1982) showed that this fauna occurs typically in a pebble bed above the Hitchwood Hardground. Locally, in the Saffron Walden district, Osborne White (1932) reported 2 to 3 m of soft chalk at the base of the S. plana Zone in a pit at Underwood Hall [TL 6090 5545], 'containing a development of tabular flint unequalled in any similar thickness of beds to be seen in the district'. Above this flinty horizon he recorded a rapid transition into nodular chalk. The thick tabular flints may equate with part of the 'Brandon Flint Series', which are a feature-forming unit below the 'Chalk Rock' in the adjacent Bury St Edmunds district (Bristow, 1990).

A stratigraphically higher bed of strongly indurated chalk, about 0.6 m thick, with glauconitised pebbles, occurs at the base of the M. cortestudinarium Zone (Osborne White, 1932) in the equivalent of the middle to upper Lewes Nodular Chalk of southern England. The latter is the 'Top Rock' of traditional usage, and probably represents the coalescence of two or more hardgrounds in the upper Lewes Nodular Chalk of southern England (Mortimore, 1983). Locally, in East Anglia, the greater part of the M. cortestudinarium Zone (Wood in Murray, 1986), and thus much of the upper Lewes Nodular Chalk is condensed to form the 'Top Rock', but Osborne White's (1932) record of about 18 m for the thickness of the cortestudinarium Zone suggests that this is not true in the Saffron Walden district. Rather soft flinty chalk seems to characterise the cortestudinarium Zone above the 'Top Rock', which is more typical of the Seaford Chalk.

Osborne White (1932) recorded that the youngest chalk of the district belonged to the M. coranguinum Zone, and that this chalk was soft, pure white, with large flint bands and scattered marls. From this description the Seaford Chalk can be inferred. Osborne White (1932) recorded Inoceramus involutus [Volviceramus involutus], which characterises the lower part of the Seaford Chalk where thin marls are typical (=Belle Tout Beds of Mortimore (1986)). The inferred thickness of the M. coranguinum Zone, which is coextensive with the Seaford Chalk in southern England, is probably less than about 45 m.

Quaternary

Lowestoft Formation

The glaciogenic deposits within the district can be attributed to the British Eastern Ice Sheet that covered much of Eastern England during the Anglian Stage, just over 400 000 years ago. Within the Lowestoft Formation, the main lithologies mapped are till, silt and sand and gravel. The till covers about one-third of the district, and is best developed in the south-east, where it attains its maximum thickness of about 55 m. It is a dark blue-grey, stiff, generally unstratified, silty clay enclosing pebbles of chalk, flint and less commonly other rocks that include well-rounded liver-coloured quartzite pebbles from the Trias, and igneous and metamorphic rocks from farther afield. Near the surface, the till is usually weathered to shades of brown, and may show grey mottling where decalcification has not been complete.

The base of the till is characterised in some exposures by a bed, less than 1 m thick, of irregularly laminated silt and sand, with scattered stones and gravel seams.

Beds directly below the till commonly show signs of disturbance; gravels are contorted, and where Chalk is present it may be brecciated with signs of thrust faulting or crumpling displayed by the layers of flint. Osborne White (1932) records that the greatest disturbance appears to occur near the top of the Chalk escarpment, in the south-west of the district. In places below the till, the top few centimetres of the Chalk has been converted into a hard, greyish buff, homogenous or brecciated limestone by deposition of calcium carbonate, partly in crystalline form.

The main mass of Lowestoft till declines south-eastwards, following the dip of the underlying solid formations. However, in places the till drops down into valleys, which demonstrates that at least some of these valleys were in existence at that time. Deposits depicted as 'Plateau Gravel' on the 1952 edition of the geological map have been reclassified as sand and gravel within the Lowestoft Formation. These gravels rest on the till around the village of Ashdon [TL 58 42]. Osborne White (1932) concluded that the gravels had been washed out from the till. In other exposures, the gravels were seen to be coarse-grained, ferruginous and generally very weathered, but locally contained evidence of bedding in the form of chalky seams.

A trench cut for archaeological investigations at Clarke's Hill [TL 4712 5349], Great Shelford, at 45.3 m OD, exposed 1.4 m of orange-brown, matrix-supported, medium to fine grade gravel with indistinct bedding, overlying a unit of chaotically bedded clast-supported cobbles (not depicted on the 1:50 000 map). A bulk sample contained many clasts between 50 and 280 mm in length, with the majority in the 64 to 128 mm size class. The largest boulder was of fossiliferous Jurassic limestone and weighed 12.5 kg. The large clasts were derived from a variety of sources that included Carboniferous and Jurassic limestone, granite, schist, 'Millstone Grit' and Whin Sill dolerite. The coarse grade of the cobbles and the large exotic component of the clasts suggests a high-energy subglacial or proglacial depositional environment. The overlying gravel contained only a few (1.9 per cent) large clasts within the 16 to 32 mm size range and most were locally derived lithologies. The large exotic component of the cobble unit is interpreted as glaciogenic in origin, while the large chalk component of the overlying gravel strongly suggests that this was derived from a relatively small stream draining a catchment dominated by chalk bedrock.

Small areas, previously depicted as 'brickearth' have been reclassified as 'silt' within the Lowestoft Formation. The silt probably accumulated in ponds and small lakes associated with the Lowestoft ice sheet.

Buried channels

In the Cam valley, proof of deep channelling has been obtained at intervals from the head of the river near Quendon (Great Dunmow Sheet 222) down to Whittlesford [TL 465 485] in this district. Boreholes suggest that the base of the channel rises northwards. The fill of the channel is composed predominantly of comminuted chalk in a silt matrix. Locally, till, and sand and gravel are present. Where the River Cam enters the district at Audley End, the valley of the Cam narrows. If the buried channel extends through this area it must be very narrow and steep sided. Towards Littlebury, the valley widens; the buried channel appears to pass through the village some 200 m west of the present Cam. Whitaker (1890) records a borehole that reached 66.4 m depth without reaching Chalk. The log of the borehole, (reproduced in Osborne White, 1932) describes the deposits as brown, blue and slate-coloured loam, sand and clay and small amounts of gravel.

Farther north at Great Chesterford a borehole to the north of the church proved 47.5 m of drift before reaching Chalk (Whitaker, 1916; Osborne White, 1932). The sequence recorded contains a higher proportion of sand and gravel than in the borehole at Littlebury.

To the north at Whittlesford, a well at the railway station proved 19 m of drift before reaching Chalk. The drift comprised blue-grey clay, with intercalations of gravel, and a bed, 4 m thick, of 'light blue clay and chalk', probably till, near the top. A borehole at 'The Cabin', Whittlesford was initially reported to prove 138.7 m of drift, but this was later reinterpreted by F H Edmunds (in Dewey, 1931, pp.37–38), as being entirely within bedrock apart from the uppermost 4.6 m. There is no evidence of any buried channel to the north of Whittlesford.

River terrace deposits

The River Cam flows northwards through the western part of the district. Along its course are widespread deposits of sand and gravel that can be assigned to several periods of terrace aggradation.

Three terraces have been distinguished within the district, and depicted as First, Second and Third on the map. In places it has not been possible to separate the First and Second Terraces and in such areas these are shown as combined. Locally it has not been possible to place terrace deposits within the numbered sequence; in these instances the terrace deposits are depicted as 'undifferentiated'.

In the north of the district, the Third Terrace barely reaches 20 m above OD. The Second Terrace is at about 13 to 14 m OD, and the First/Second Terrace is at 9 to 10 m above OD, some 2 m above the local Alluvium. Osborne White (1932) provides an account of the palaeontology of the deposits.

The most extensive outcrops of Third Terrace deposits are developed around Little and Great Chesterford, between Little Abington and Babraham, between Stapleford and Trumpington, and at Grantchester.

Gravel of the Third Terrace, comprising 'fine to medium, sandy and chalky gravel, generally evenly bedded' was formerly exposed in the railway cutting, and in pits 3 to 4 m deep on both sides of the railway, south of Trumpington (Osborne White, 1932). Kennard and Woodward (1922) recorded an extensive list of molluscs from these workings.

Interglacial deposits

Recent research by S Boreham and others at Cambridge University has resulted in a much better understanding of the depositional environments and ages of the interglacial deposits in the district. Some of this work is outlined below.

A section, 3 m high, in gravel and sand of the Third Terrace was exposed in mining at Grantchester, and originally described by McKenny Hughes (1888). It comprised more than 2 m of 'chalky gravel and marl with pans of silty peat and bands full of land and freshwater shells', overlain by soil and decalcified gravel. The deposit was particularly rich in shells. The mammalian assemblage, which includes Equus ferus and Mammuthus primigenius, suggests that this deposit was laid down during a temperate part of Marine Isotope Stage (MIS) 7. This interpretation is supported by amino acid racemisation of Valvata shells which suggest a late 'Wolstonian' (MIS 6/7) age. However, a few elements of the 'hippopotamus fauna' characteristic of the Ipswichian Stage are also present. It is possible that bones from several horizons have been collected and grouped together, thus confusing the situation somewhat.

In 1998, excavations through a previously backfilled gravel pit at The Welding Institute's Science Park at Abington Hall [TL 5242 4909] revealed grey organic silt beneath gravels assigned to the undifferentiated First/Second Terrace. An auger-hole through these deposits proved 3.5 m of silty clay resting on gravel. The silty clay could be traced for at least 100 m to the east. Subsequent trenches dug at the eastern end of the pit proved 4 m of silty clay overlying gravel. To the south, excavations for a sports pitch exposed further sections, and as building works progressed additional sections became available. The floral, molluscan, and coleopteran assemblages from the silty clay reveal an upwards change from cool to warm conditions. Optically stimulated luminescence (OSL) dating of these deposits gave an age of 137.74 ± 41.65 ka suggesting a correlation with the early part of the Ipswichian (Marine Isotope Stage 5e) interglacial. Amino acid racemisation analyses for Valvata shells gave results consistent with this age.

Analysis of the silty clay revealed 70 to 90 per cent calcium carbonate, together with 5 to 10 per cent organic material. Such large amounts of detrital carbonate are not surprising given the chalk-dominated catchment upstream. The amount of silicate residue (10–25%) and magnetic susceptibility {0.5 to 0.3 SI units x10⁻⁸ (m³ kg⁻¹)} decreased towards the top of the unit. This suggests a reduction in the supply of clastic material, which may reflect stabilisation of local soils. The silty clay probably represents the silting up of a river channel during a phase of climatic warming. The basal part is consistent with deposition in an environment where a slowly flowing river was fringed by reed-beds, wet woodland and open meadow, and with a prevailing cool temperate climate. The top of the sequence is indicative of slow-moving fluvial conditions with emergent vegetation, in the form of nearby dry grassland and deciduous woodland in fully temperate conditions.

The base of the sequence contains a pollen assemblage dominated by up to 34 per cent Poaceae (grass), various herbs (about 35 per cent), and subordinate amounts of Betula (birch) (about 15 per cent) and Pinus (pine) (about 6 per cent). Above this is a peak in Betula pollen (about 27 per cent), an increasing amount of Pinus (about 37 per cent) and the first appearance of mixed oak woodland (Quercus (oak), Ulmus (elm), Corylus (hazel). Towards the top of the sequence, samples show declining abundance of Betula, with high proportions of Pinus (about 63 per cent). At the top of the sequence, mixed oak woodland components become more established, with Quercus (about 7 per cent) and Ulmus (about 3 per cent). This sequence is interpreted as recording a vegetational succession from an open grassland habitat with birch and pine scrub to closed canopy woodland, within a fluvial environment. Such a change in the vegetation seems consistent with climatic warming at the beginning of an interglacial.

Three bulk samples of sediment were analysed for molluscs by R C Preece of Cambridge University. One sample contained a limited, predominantly aquatic fauna including Bithynia sp. and Valvata cristata, both indicative of fluvial sedimentation in warm conditions. Another sample contained no warm elements, and was dominated by Pisidium spp. and Valvata piscinalis indicating fluvial sedimentation. The few terrestrial molluscs present in this sample are suggestive of grassland environments. The third bulk sample was also dominated by aquatic taxa, and included Planorbis obtusale and Anisus leucostoma, which are often associated with stagnant floodplain pools. Thermophilous taxa were also absent from this sample.

Coleopteran analyses of several bulk samples were undertaken by A Dixon. One sample yielded only a sparse beetle fauna indicating a temperate climate warmer than today. The presence of the dung beetles Valgus hemipterus and Onthophagus sp. is taken to indicate temperatures perhaps 3 to 4ºC higher than the present, and notably V. hemipterus is known from early Ipswichian (MIS 5e) deposits in Britain (Coope, 2000). Still or slow-flowing aquatic environments are indicated by the aquatic weevil Bagous, and the reed beetle Donacia semicuprea is found on reed sweet-grass (Glyceria maxima). There are no beetles of flowing water habitats in this sample. Dry, open grassland is suggested by the ground beetle Calathus sp., and mature deciduous woodland is suggested by V. hemipterus, the larvae of which develop in deciduous trees. The decaying dung of large mammals is suggested by the dung beetle Onthophagus sp. and mould beetle Corticarina sp. Other bulk samples have yielded a beetle fauna broadly comparable to that from southern Sweden (about 61ºN) today. However, the analogue is not an exact one, since the assemblage contains eastern European and Asian elements such as the dung beetle Aegialia kamschatica, and the rove beetle Tachinus caelatus, indicative of a cool temperate, continental and somewhat arid climate with summer temperatures slightly lower than the present day (about 15ºC), but with very cold winters. In addition, there are montane elements in the fauna, also suggestive of thermal extremes, such as the click beetle Zorochrus flavipes. The overall assemblage shows a mixture of beetles whose ranges of distribution do not overlap today. These deposits represent the very early part of an interglacial, most probably the Ipswichian (MIS 5e). The aquatic environment indicated by the insect fauna is one of a still or slowly flowing water body, with the aquatic weevil Bagous, the whirligig beetle Gyrinus opacus, the reed beetle Donacia semicuprea and a number of caddisflies (Trichoptera). This may indicate a low-energy fluvial environment, although A Dixon interprets this assemblage as representing lacustrine conditions. There is some evidence for relatively shallow water with aquatic vegetation and reed-beds, fringed by wet woodland containing willow, and expanses of open meadow. There are also some indicators of dry chalk grassland, for example the ground beetle Bembidion quadrimaculatum.

Alluvial fan deposits

These are the 'Taele gravels' of Osborne White (1932). They form undulating ridges and small plateaux on the lower parts of the Chalk escarpment, and range in altitude from about 60 m down to 25 m above O D. Former small pits in the alluvial fans have revealed up to about 3 m of deposits that range in composition from chalky gravel with seams of sand to chalky sand with scattered flints. Boulders of quartzite, basalt and other rocks are also reported to be present.

Shell-bearing clays (small intercalated patches of loam) were first described by Penning and Jukes-Browne (1881) from within gravels at West Heath Farm, near Thriplow. Some seventy years later, new workings in the gravels at this locality allowed Sparks (1957) to re-examine the 'Taele Gravels' in greater detail. He described less than a metre of 'rusty, non-calcareous weathered gravel' of irregular thickness resting on impersistent beds of mixed sand, silt and clay, reaching almost a metre in thickness and much disturbed by frost action. This was seen to overlie a 'tumultuously-bedded gravel' with flints and erratics. Sparks recorded a range of aquatic and terrestrial molluscs from the silty clay. He concluded that the gravel was derived from the Lowestoft Till under tundra conditions, and that the silty clay represented accumulation in pools within a dry grassland environment.

The essentially water-laid character of these deposits and their connection with the drainage of the Chalk escarpment were earlier 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 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. Recent research by one of the present authors favours the earlier conclusion of Penning and Jukes-Browne, that the deposits are related to 'ancient river systems'.

Alluvium

Many of the streams are bordered and underlain by tracts of alluvium that consist in the main, of material reworked from 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 (see below). True peat with logs of wood has been found in some of the deeper excavations. Thicknesses of alluvium are generally unproved, but typically are less than about 3 m, although Osborne White (1932) records up to 12.2 m in the Cam valley to the south-west of Cambridge.

Peat

Peat has been shown in that part of the district that has been geologically surveyed since 1970. Thus it has been mapped in the central northern part of the district just east of Fulbourn, where it is associated with deposits of the First and Second River Terraces. It is also present within the alluvium in places. Peat is highly compressive and its presence, therefore, needs to be determined prior to any development taking place above. Acid sulphate groundwater conditions may also prevail.

Head

In the present district, Head has been mapped only in areas that have been surveyed since 1970. In other areas, Head may be presumed to be present as a veneer on most lower valley sides, and on the floor 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. 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.

Although not depicted on the map, most of the valleys within the Chalk outcrop contain elements of Head that were formerly referred to as 'dry valley deposits'. These have been described in detail from Eastwood Pit, at Barrington by Sparks (1952) and Norris (1962). Hoare and Connell (1981) examined new exposures and reviewed the previous evidence. The latter concluded that Anglian chalky till is overlain by soliflucted 'putty' chalk of probable late Devensian age. An overlying clay and gravel unit appears to represent soil and colluvial slopewash of Flandrian age. Molluscs within the 'putty' chalk (Sparks, 1952) indicate climatic conditions similar to those in northern Scandinavia or perhaps central Europe today. In contrast, molluscs within the overlying clay and gravel are indicative of both open ground and woodland and a climate at least as warm as today.

Head deposits commonly contain internal shear planes, or a basal shear plane, which may fail when loaded. Thus its presence or absence should be determined prior to any development taking place.

Gravelly Head

This comprises sandy gravel and gravelly sand, the gravel component being dominated by angular, shattered flint. It is believed to have formed during periglacial periods, by sheet-flood and stream-flood processes, rather than by solifluction. There is little information on the thickness of Gravelly Head deposits, but they are likely to be more than 3 m thick.

Artificially modified ground

Categories under this heading include land modified by human agency.

Worked ground

This category depicts areas where minerals have been dug, and where subsequent back-fill is minimal or absent. In this district 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. There is now little evidence to indicate the extent of these former workings and they are not depicted on the map.

Made ground

This category depicts areas where material has been deposited by human agency on an original land surface. Road and rail embankments have generally been omitted from the 1:50 000 map, except where they are very extensive, although they are depicted on the component 1:10 000 scale maps.

Infilled ground

This category is used to depict areas where minerals have been extracted and the area partially or wholly back-filled. Areas depicted as such include former gravel, sand and chalk pits.

Chapter 3 Applied geology

Chalk

In the past Chalk has been dug for agricultural lime from numerous small pits.

Building stone

On, and adjacent to, the Chalk outcrop, local flint has been used extensively as a building stone. The flints have been used whole or split (knapped) to produce fresh black surfaces as seen in the railway station building at Great Shelford [TL 464 522] (Plate 1).

Elsewhere, away from the Chalk outcrop, rounded cobbles and boulders derived from the local Woburn Sands Formation and river terrace deposits, and from far travelled glacial deposits have been picked from the fields for use as building stone, for example in Haslingfield Church [TL 403 521] (Plate 2); (Plate 3).

Sand and gravel aggregate

The River Terrace Deposits form the main resource of sand and gravel aggregate within the district, and are concentrated within the north-western part of the district. There are extensive workings for sand and gravel at Barrington [TL 3960 5040] which extend into the adjacent Biggleswade district. Sand and gravel has also been dug [TL 4870 4650] near Hinxton.

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 has been worked from 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 underlying Gault were also dug on a smaller scale.

Water supply

The Chalk is the main aquifer in the district; the Woburn Sands is a minor aquifer in the extreme north-west of the district where it is confined by the overlying Gault Formation.

Groundwater within the Chalk may be stored both in the intergranular matrix and also within microfissures and macrofissures, although groundwater flow relies on flow within fissures. The density of the fissures is greatest in valleys and other low-lying areas, where stress release from the removal of overburden and the enlargement of fissures by solution offer the best hydraulic conditions for high-yielding boreholes. The West Melbury Marly Chalk Formation is composed predominantly of marls and the overlying Zig Zag Chalk Formation is also relatively marly. The succeeding Holywell Nodular Chalk Formation and the New Pit Chalk Formation also contain a number of thin, but persistent marl seams. Thus, in general, these formations may offer slightly inferior hydraulic properties to those in the Lewes Nodular Chalk Formation and the Seaford Chalk Formation (formerly Upper Chalk). The yield-drawdown characteristics of boreholes in the Chalk are very variable, and depend on the diameter of the well and the intersection of fissures. Groundwater within the Chalk is of the calcium bicarbonate type. The aquifer is vulnerable to diffuse pollution, particularly where there is no cover of till.

The Woburn Sands aquifer between Leighton Buzzard and Ely has been described by Binnie and Partners (1982). Groundwater flow in the aquifer is intergranular, and there is virtually no fissure flow. Groundwater in the Woburn Sands is of the calcium bicarbonate type. The water is less mineralised than water in the Chalk, although much of the water is rich in iron and sulphate due to the dissolution of pyrite.

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 km² of the Ordnance Survey National Grid.

The variation in radon levels between different parts of the country is controlled mainly by the underlying geology. Within the Saffron Walden 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 (bedrock) formations and superficial deposits are outlined in (Figure 8) and (Figure 9).

The Gault Formation is dominated by clay. On account of the high smectite content the 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 clays absorb high quantities of water during wet periods and lose it again during droughts. Drying clays 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 sulphate) 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.

Information sources

Further geological information held by the British Geological Survey relevant to the Saffron Walden 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, Keyworth. Geological advice for the area should be sought initially from the BGS Enquiries Service, Keyworth.

• The Geological Data Index (GDI) is now available on the internet at: http://www.bgs.ac.uk. This includes the following themes:
• borehole records
• water wells
• site investigation reports
• drillcore
• samples
• geophysical logs
• well water levels
• aquifer properties
• geochemistry
• topography
• outline of BGS maps at 1:50 000 and 1:10 000 scale and 1:10 560 scale County Series
• aeromagnetic and gravity data recording stations

Maps

• 1:250 000
• 52N 00 East Anglia, Solid, 1986; Quaternary, 1991; Sea bed sediments, 1988
• 1:50 000
• The map is based largely on the earlier edition at 1:63 360 scale (one inch to one mile), which, in turn, was based on earlier editions at 1:63 360 and 1:10 560 scales (six inches to one mile). The distribution of the glaciogenic deposits is based mainly on late 19th century mapping (at 1:63 360 scale), and 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 resurveyed at 1:10 000 scale during the 1970s.
• Maps of adjacent districts include sheets:
• 187 Huntingdon, 1975
• 189 Cambridge, 1981
• 189 Bury St Edmunds, 1982
• 204 Biggleswade, 2001
• 206 Sudbury, 1991
• 221Hitchin, 1995
• 222 Great Dunmow, 1990
• 223 Braintree, 1982
• 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
• Geophysics
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996 Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:250 000
• 52N 00 East Anglia, Aeromagnetic anomaly, 1982
• 52N 00 East Anglia, Bouguer gravity anomaly, 1981
• 1:50 000
• A geophysical information map (GIM) at a scale of 1:50 000 is available for this district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.
• Geochemical maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain (South Sheet) 1995 Radon potential based on solid geology, Great Britain (South Sheet) 1995 Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet) 1995
• Minerals Maps
• 1:1 000 000
• Industrial minerals resources map of Britain, 1996
• Hydrogeology maps
• 1:125 000
• Northern East Anglia† Southern East Anglia, 1981

Books

• British regional geology: East Anglia and adjoining areas
• Memoirs and Sheet Explanations* of this and adjacent districts:
• 187/204 Huntingdon and Biggleswade, 1965
• 188 Cambridge, 1969
• 189 Bury St Edmunds, 1990
• 204 Biggleswade, 2003*
• 205 Saffron Walden, 1932†
• 221 Hitchin, 1996
• 222 Great Dunmow, 1990
• 223 Braintree, 1986
• † out of print; available as a facsimile copy at a tariff that covers the cost of copying

Documentary collections

Boreholes

Borehole data for the district is catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For further information contact: Chief Curator, National Geological Records Centre, BGS, Keyworth.

Geophysics

Gravity and aeromagnetic data are held digitally in the National Gravity Databank and the National Gravity Aeromagnetic Databank at BGS Keyworth.

Minerals

MINGOL is a GIS based minerals information system, from which hard copy and digital products can be obtained, tailored to the requirements of individual clients.

Hydrogeology

BGS hydrogeology enquiry service; wells and springs and water borehole records are held at the British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB. Telephone 01491 838800. Fax 01491 692345.

BGS Lexicon of named rock unit definitions Definitions of the named rock units shown on the 1:50 000 Series Sheet 205 Saffron Walden are held in the Lexicon database. This is available on the BGS web site. Further information on the database can be obtained from the Lexicon Manager, BGS, Keyworth.

Material collections

Palaeontological collection

Macrofossils and micropalaeontological samples collected from the district are held at BGS Keyworth. Enquiries concerning all the macrofossil material should be directed to the Chief Curator, Biostratigraphy Collections, BGS, Keyworth.

BGS Photographs

Copies of photographs used in this report are deposited for reference in the BGS Library, Keyworth. BGS maintains a large collection of photographs that can be purchased. Some of the collection can be viewed on our web site.

Other relevant collections

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

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references can be purchased subject to the current copyright legislation.

BINNIE and PARTNERS. 1982. Report on the hydrogeology of the Lower Greensand aquifer (Leighton Buzzard - Ely). (London: Binnie and Partners.)

BRISTOW, C R. 1990. Geology of the country around Bury St Edmunds. Memoir of the British Geological Survey, Sheet 189 (England and Wales).

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. 108, 293–315.

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

COOPE, G R. 2000. The climatic significance of coleopteran assemblages from the Eemian deposits in southern England. Geologie en Mijnbouw, Vol. 79, No. 2/3, 257–268.

DEWEY, H. 1931. Summary of Progress of the Geological Survey of Great Britain and the Museum of Practical Geology for the year 1930. (London: HMSO.)

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.

HOARE, P G, and CONNELL, E R. 1981. The chalky till at Barrington, near Cambridge, and its connection with other Quaternary deposits in southern Cambridgeshire and adjoining areas. Geological Magazine, Vol. 118, 463–476.

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).

JEFFERIES, R P S. 1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association, Vol. 74, 1–33.

KENNARD, A S, and WOODWARD. 1922. The post-Pliocene non-marine mollusca of the east of England. Proceedings of the Geologists' Association, Vol. 33 104–142.

LAKE, R D, and WILSON, D. 1990. Geology of the country around Great Dunmow. Memoir of the British Geological Survey, Sheet 222 (England and Wales).

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, (London: HMSO.)

MCKENNY HUGHES, T. 1888. On the mollusca of the Pleistocene gravels in the neighbourhood of Cambridge. Geological Magazine, Vol. 13, 193–207.

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.

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.

MORTIMORE, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97, 97–139.

MORTIMORE, R N, and WOOD, C J. 1986. The distribution of flint in the English Chalk, with particular reference to the 'Brandon flint series' and the high Turonian flint maximum in The scientific study of flint and chert; Proceedings of the Fourth International Flint Symposium. SIEVEKING, G D G, and HART, M B (editors). Vol. 1. (Cambridge: Cambridge University Press.)

MURRAY, K H. 1986. Correlation of electrical resistivity marker bands in the Cenomanian and Turonian Chalk from the London Basin to east Yorkshire. British Geological Survey Report, Vol. 17, No. 8.

NORRIS, G. 1962. Some glacial deposits and their relation to the Hippopotamus-bearing beds at Barrington, Cambridgeshire. Geological Magazine, Vol. 99, No. 2, 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.

RAWSON, P F, ALLEN, P, and GALE, A. 2001. The Chalk Group — a revised stratigraphy. Geoscientist, Vol. 11, 21.

ROBASZYNSKI, F, GALE, A, JUIGNET, P, AMEDRO, F, and HARDENBOL, J. 1998. Sequence stratigraphy in the Upper Cretaceous series of the Anglo-Paris Basin: exemplified by the Cenomanian Stage. Society of Economic Palaeontologists and Mineralogists, Special Publication, No. 60, 362–386.

SHEPHARD-THORN, E R, MOORLOCK, B S P, Cox, B M, ALLSOP, J M, and WOOD, C J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England and Wales).

SPARKS, B W. 1957. The evolution of the relief of the Cam valley. Geographical Journal, Vol. 123, 189–207.

WHITAKER, W. 1890. A deep channel in the valley of the Cam, Essex. Quarterly Journal of the Geological Society of London, Vol. 46, 336–338.

WHITAKER, W. 1916. Water Supply of Essex. Memoir of the Geological Survey of Great Britain.

WOODS, M A. 1995. Interpretation of geophysical well logs from the Chalk of the London Basin. British Geological Survey Technical Report, WH/95/184C.

WOODS, M A. 1998. A review of the stratigraphy of the Chalk Group (Upper Cretaceous) of the Beaconsfield Sheet (255). British Geological Survey Technical Report, WH/98/2R.

WOODS, M A. 1999. Lithostratigraphical correlation of borehole resistivity logs from the Chalk of the Beaconsfield district and adjoining areas. British Geological Survey Technical Report, WH/99/66R.

WORSSAM, B C, and TAYLOR, J H. 1969. Geology of the country around Cambridge. Memoir of the Geological Survey of Great Britain, Sheet 188 (England and Wales).

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 BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

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

Figures and plates

Figures

(Figure 1) Colour shaded relief aeromagnetic anomaly map. Total field magnetic anomalies in nanotesla (nT) relative to a local variant of IGRF90. The anomalies are shown as a colour shaded relief presentation using the BGS COLMAP Package. The shaded topographic effect has been created using an imaginary light source, located to the north. Contour interval 10nT.

(Figure 2) Colour shaded relief Bouguer gravity anomaly map. Bouguer gravity in milligals (mGal) calculated against the Geodetic Reference System 1967, referred to the National Gravity Reference Net 1973. A variable reduction density has been used. The anomalies are shown as a colour shaded relief presentation using the BGS COLMAP Package. The shaded topographic effect has been created using an imaginery light source, located to the north. Contour interval 1 mGal (1 mGal = 1 3 10⁻⁵ m/s²).

(Figure 3) Lithostratigraphy of the Jurassic strata within the districtGault Formation

(Figure 4) Limits of Jurassic strata in the district.

(Figure 5) Thickness variations in the Gault of the Saffron Walden district recorded by Osborne White (1932).

(Figure 6) Lithostratigraphy of the Chalk Group in the district.

(Figure 7) Satellite image of the Saffron Walden district. The darker hues in the north-west coincide with the Zig Zag Chalk and West Melbury Marly Chalk members of the Grey Chalk Subgroup. The lighter colours which extend north-eastwards from the south-west of the district reflect the outcrop of the overlying New Pit Chalk and Holywell Nodular Chalk members of the White Chalk Subgroup. The darker hues over much of the eastern part of the district coincide with the outcrop of the clay tills of the Lowestoft Formation. Winter Landsat TM satellite image displayed in bands 4, 5, 7 red, green, blue.

(Figure 8) Engineering characteristics of the bedrock formations.

(Figure 9) Engineering characteristics of the superficial deposits

Plates

(Plate 1) Knapped flints, revealing dark inner layers and thin white cortex, used as a building stone in Great Shelford Railway Station [TL 464 522] (GS1228).

(Plate 2) Haslingfield Church built with stone picked from the local fields [TL 403 521] (GS1229).

(Plate 3) Close up view of the cobbles and boulders used in Haslingfield Church. Many are derived from the Lower Greensand Woburn Sands Formation, but some have probably been derived from nearby glacial and river terrace deposits [TL 403 521] (GS1230).

(Front cover) Front cover: Walden Castle [TL 541 386] built in the 12th century. The ruins are part of the original basement constructed from locally derived flints. The castle had been reduced to a ruin by the mid 18th century and turned into a 'quarry'; much of the walls had been carried off for road making and other purposes (Photographer T P D S Cullen; MN39872).

(Rear cover)

(Geological succession) Geological succession of the Saffron Walden district

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

Figures

(Geological succession) of the Saffron Walden district

(Figure 5) Thickness variations in the Gault of the Saffron Walden district recorded by Osborne White (1932)

(Figure 6) Lithostratigraphy of the Chalk Group in the district

(Figure 8) Engineering characteristics of the bedrock formations

(Figure 9) Engineering characteristics of the superficial deposits