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Geology of the country around Chelmsford. Memoir for 1:50,000 geological sheet 241 (England and Wales)

By C.R. Bristow

Bibliographical reference: Bristow, C.R. 1985. Geology of the country around Chelmsford. Memoir British Geological Survey, Sheet 241 (England and Wales)

England and Wales

• Author
• C.R. Bristow
• Contributors
• Stratigraphy R.D. Lake
• Palaeontology C.J. Wood, P.J. Bigg, A.W. Medd, M.J. Hughes and D.M. Gregory
• Geophysics R.B. Evans
• Petrology R.K. Harrison
• Economic Geology A.R. Lawrence, M.R. Clarke and M. Simmons

British Geological Survey, Natural Environment Research Council

London Her Majesty's Stationery Office 1985. © Crown Copyright 1985. Printed in the UK for HMSO Dd 699050 C.20 2/85. ISBN 0 11 884335 4. First published 1985.

• Author
• C.R. Bristow, BSc, PHD British Geological Survey, St Just, 30 Pennsylvania Road, Exeter EX4 6BX
• Contributors
• R.D. Lake, MA, R.B. Evans, MSc, A.R. Lawrence, MSc, M.R. Clarke, BSc, M.J. Hughes, MSc, M. Simmons, BSc, A.W. Medd, BSc, PhD, P.J. Biggs, MSc and D.M. Gregory, BSc British Geological Survey, Keyworth, Nottingham NG12 5GG
• C.J. Wood, BSc and R.K. Harrison, MSc British Geological Survey, London

Other publications of the Survey dealing with the geology of this and adjoining districts

Books

• British Regional Geology: London and the Thames Valley (3rd edition)
• Record of Wells in the Area of Epping (240),  Chelmsford (241) and  Brightlingsea (242) one-inch geological sheets.

Maps

• 1:625 000
• Geological (South) Quaternary (South) Aeromagnetic (South)
• 1:50 000
• Sheet 223 (Braintree) Solid and Drift edition 1982
• Sheet 240 (Epping) Solid and Drift edition 1981
• Sheet 241 (Chelmsford) Solid and Drift edition 1975
• Sheet 257 (Romford) Drift edition 1976
• Sheet 258/9 (Southend and Foulness) Solid and Drift edition 1976
• 1:25 000
• Sheet TL 81 (Witham) Solid and Drift edition 1972

Preface

This memoir describes the geology of the district covered by the 1:50 000 (Chelmsford) New Series Sheet (241) of the Geological Map of England and Wales. The district was first geologically surveyed on the one-inch scale by F.J. Bennett, W.H. Dalton, W. Boyd Dawkins, and W.H. Penning and the results published in 1868 (Old Series sheets 1 NE Solid and 2 Solid), 1883 (Sheet 48 SW Solid and Drift) and 1881 (Sheet 47 Solid). A revised drift edition, of Sheet 1NE including work of H.B. Woodward, was published in 1871, and of Sheet 2 in 1883, followed by Sheet 47 in 1884.

The first of the descriptive survey memoirs relating to this area was W. Whitaker's Geology of the London Basin published in 1872, but this dealt with the solid formations only. The Sheet Memoir for Sheet 47 describing both the solid and drift geology was published in 1878 (Whitaker, Penning, Dalton and Bennett) and that of Sheet 48 SW by Dalton in 1880. Sheets 1 and 2 (together with Sheet 7) were included in Whitaker's The Geology of London and of parts of the Thames Valley published in two volumes in 1889; Vol.1 covered the descriptive geology and Vol.2 descriptions of well sections and borings.

Several of the Geological Survey 'Water Supply' memoirs have dealt in part with the district, namely Vol.2 of Whitaker's above mentioned London memoir, and The Water Supply of Essex from Underground Sources by Whitaker and Thresh (1916). Wartime Pamphlet No 20, Part IX, compiled by Dr A. Woodland, catalogued the wells of the Chelmsford Sheet and was published in 1943. A revised well catalogue covering sheets 240 (Epping), 241 (Chelmsford) and 242 (Brightlingsea), prepared by Mrs M.C. Davies and Mr L. Standon-Batt was published in 1965.

Several publications by the Industrial Mineral Assessment Unit are directly relevant to the Chelmsford area. These include those of the following 100 km grid squares; TL 70 (Clarke, 1975), 71 (Eaton, 1973), 80 (Ambrose, 1973a), 81 (Haggard, 1972), 90 and 91 (Ambrose, 1973b) and parts of TL 80, 90, TM 00, TQ 89, 99 and TR 09 (Simmons, 1978).

The primary six-inch geological survey of the Chelmsford area was made by Dr C.R. Bristow during 1966–70 and Mr R.D. Lake during 1969–70, under the direction of Mr S.C.A. Holmes as District Geologist. A small part of the Chelmsford area had been surveyed on the six-inch scale by Mr F.H. Edmunds in 1923 as an overlap from the Romford (257) Sheet; this ground was resurveyed in the recent investigation by Dr Bristow. The 1:50 000 geological map of the area was published in 1975. Additionally the area included within the grid square TL 81, which falls partly on the Braintree (223) Sheet to the north, was published at the 1:25 000 scale in 1972.

As well as accounts by the field officers the memoir contains contributions on the macropalaeontology of the Chalk, London Clay and Claygate Beds by Mr C.J. Wood, on the foraminifera and bryozoa of the Chalk of the Witham and Kelvedon boreholes by Dr P.J. Bigg and Dr A.W. Medd respectively, and on the micropalaeontology of the Tertiary formations and Recent deposits by Mr M.J. Hughes and Miss

D.M. Gregory. Mr R.K. Harrison has supplied petrological data for part of the Stock Borehole, Mr A. R. Lawrence contributed the account on the water supply of the district and Mr M.R. Clarke and Miss M.B. Simmons provided information on sand and gravel resources. The geophysical section was written by Mr R. B. Evans.

Dr K.M. Clayton and Mr J.S. Turner generously donated to the Institute their manuscript six-inch geological maps of part of the Chelmsford sheet.

We are grateful for the assistance of Mr C. King for the identification of some of the Tertiary macrofauna of the Stock Borehole; to Mr D.J. Ward for the identification of the chondrychthian teeth, and to the late Mr F. Stinton for his work on the otoliths from this same borehole. Mr G. Ward has kindly facilitated our examination of Tertiary and Pleistocene material in the Passmore Edwards Museum and helped with bibliographical references. The memoir has been compiled by Dr Bristow.

Thanks are given to Ove Arup & Partners for permission to examine and report on material from their boreholes in the central Chelmsford area, and to George Wimpey and Co Ltd for borehole material from the Maldon area.

The surveyors are grateful for the ready co-operation which they received from the landowners and tenants of the district in facilitating access to their properties.

G.M. Brown,  Director, British Geological Survey Keyworth, Nottingham NG12 5GG 23 February 1984

List of six-inch maps

The following is a list of six-inch geological maps included wholly or in part, in the area of 1:50 000 Chelmsford (241) Geological Sheet, with the names of the surveyors and the date of the survey for each six-inch map; the surveyors were C.R. Bristow and R.D. Lake.

Manuscript copies of these maps have been deposited for public reference in the library of the British Geological Survey in Keyworth. They contain more detail than appears on the 1:50 000 map.

Geology of the country around Chelmsford

To the ornithologist on the Essex marshes or to the boating fraternity on the River Blackwater it is difficult to imagine times when the Chelmsford area was covered by a warm muddy sea, when hippopotami browsed contentedly on the lush vegetation of the river banks, when rhinoceros and mammoth roamed the cold arid steppes of the hinterland, and when the Blackwater, swollen with meltwater from the ice-caps perched on the Danbury–Tiptree ridge, had huge icebergs floating in its estuary. All these events took place during various stages in the geological evolution of the present district.

During the Pleistocene glaciation, Chelmsford lay at the margin of the maximum ice extent. The legacy of this ice are the widespread belts of commercially important sand and gravel deposits transported and winnowed by the fast flowing, heavily laden, meltwater streams, and the extensive areas of heavy chalky clay dumped in situ by the stagnant wasting ice.

In this account the geological history of the district is chronicled, the lithologies of the many stratal units described, their fossils recorded and the commercially important minerals documented. For the farmer, site engineer, hydrologist, commercial sand and gravel operator, ecologist, student, amateur geologist and many others, this memoir forms an important reference.

Geological sequence

The formations represented on the Chelmsford (241) Geological Sheet and section, some proved only in boreholes, are listed below:

The more important boreholes and wells are listed in Appendix 1. Logs of the boreholes are presented in the form of microfiche so as to keep the price of this memoir to a minimum; references in the text to the microfiche records are in the form M10.

Chapter 1 Introduction

General background

The district described in this Memoir lies within the county of Essex. The deposits preserved are listed on p. ix; their broad distribution is shown in (Figure 10). The area is underlain by Tertiary strata which occupy the central part of the London Basin, the axis of which crosses the adjacent Southend and Foulness 258 and 259 sheets in an approximately east-west direction. Obscuring much of the solid geology, especially in the north-west quadrant of the map, is an extensive spread of glacial and post-glacial deposits.

Although the Lower London Tertiaries lie directly beneath drift in the Blackwater valley, the oldest formation exposed is the London Clay, which occupies much of the outcrop and the sub-drift outcrop of the solid formations. Within the London Clay sequence the oldest beds crop out in the low-lying ground along the Chelmer and Blackwater valleys and in the coastal areas around Tollesbury and Salcott. The younger Claygate Beds and Bagshot Beds crop out principally over the higher ground in the south-west and southern part of the district, towards the main synclinal axis.

The glacial strata consist of two main units: chalky Boulder Clay, which is present over the plateau in the northwest of the district, and Glacial Sand and Gravel, which underlies much of the Boulder Clay of the plateau and crops out on the valley sides. Sand and gravel also crop out extensively along the Danbury-Tiptree Ridge. Terrace gravels lie along the Blackwater and Chelmer valleys and a broad alluvial tract occupies the major river valleys, opening out into the coastal flats, where it has been differentiated as Estuarine Alluvium. In addition heterogeneous deposits of Head irregularly mantle many of the slopes, and are extensive at a number of localities around Chelmsford.

The dominant physical feature of the area is the southwest to north-east trending Danbury-Tiptree ridge. Danbury Church which forms a prominent landmark, stands on its highest point, about 108 m OD. Immediately to the north of the ridge the River Blackwater flows south-westwards over a deep drift-filled valley, and the River Chelmer flows east-north-eastwards. Both rivers appear to be fault-guided in part, and turn to flow respectively south-eastwards and eastwards through the gap between the Danbury and Tiptree sections of the ridge. They unite a short distance above Maldon before entering the 2- to 3-km wide Blackwater Estuary. On the north side of the Chelmer and Blackwater valleys, the chalky Boulder Clay plateau rises gently northwestwards to over 70 m OD. The southern border of the district is a low ridge, capped principally by Claygate Beds, which falls eastwards from 92 m OD to 30 m OD and forms the watershed between the Blackwater and the Crouch.

The various solid and drift formations give rise to two broad soil types. Those developed on the London Clay and chalky Boulder Clay are heavy, poorly draining soils, extensively cultivated for cereals and sugar beet. The lighter and free-draining soils formed on the Glacial Sand and Gravel, Terrace Gravels and, to a lesser extent, on the Bagshot Beds, give rise to poor agricultural land which until recently has supported heaths and open woodland. Modern farming methods have produced an increasing pressure to convert these areas of lighter soils to arable farmland. This process has been accompanied by the removal of many of the hedgerows and smaller woods, and this in places has given rise to monotonous featureless terrain.

The broad Blackwater Estuary, with its numerous creeks, marshes, abandoned oyster pits and attendant wildfowl, adds an attractive wildness to the area. The port and former railhead of Maldon is situated at the head of the Estuary. In earlier days the many magnificent sailing barges, which brought such diverse goods as timber, coal and beer, were a spectacular sight. A few of the sailing barges remain, but the railways have gone. Thus, apart from a little sand and gravel which is still transported by water, the main river traffic consists of the private flotilla of dinghies, launches and holiday sailing boats.

The largest town in this area is Chelmsford, which is the county and cathedral town of Essex. Formerly an important market town it is now known for its numerous light industries. Witham, the second largest town, once had a chalybeate spa. With its housing and trading estate it now serves as an overspill town for London.

The dominant activity of the Chelmsford area is, however, still farming, and many of the other industries support it. Thus the iron factories at Maldon provide agricultural implements; fertilizers and feedstuffs are provided by agricultural sundriesmen in Chelmsford, and Tiptree has a world-wide reputation for jam. On a smaller scale, the craft of making hand-made cricket bats from local willow is practised at Danbury.

Geological history

The Palaeozoic rocks of the London Platform lie at shallow depth beneath the Chelmsford district, but data are insufficient to determine their precise attitude or to define the sub-crop of the various formations at depth. To the south, Carboniferous and Devonian (Emsian) rocks have been proved beneath the Cretaceous cover, whereas Lower Palaeozoic rocks (Llandovery or Wenlock) underlie the younger strata to the north, a distribution that may be due to folding and faulting at the end of the Palaeozoic era.

No Triassic rocks have been encountered at depth in this district, and it is probable that an uplifted London Platform was undergoing subaerial denudation at that time. The sea bordered the platform throughout most of the Jurassic, and at times may have covered it (Owen, 1971, p. 201): strata of this age have been proved in several boreholes on the margins of the Platform. During early Cretaceous times the higher ground of the London Platform was uplifted but a gradual marine transgression then progressively spread across the Platform. Consequently the Lower Greensand has a wider underground distribution than the Jurassic. The Platform was finally submerged by the Upper Gault transgression. The Gault is the oldest Mesozoic deposit thought to underlie the Chelmsford district, and marine conditions persisted throughout the rest of the Cretaceous during the deposition of the Chalk. At the close of the Mesozoic era, renewed uplift was concentrated in the area of the London Platform, and erosion of the Chalk took place. A marine planation removed more than 150 m of Chalk from the central part of the London Basin including the area of the Chelmsford Sheet (Curry, 1965, pp. 155, 163).

In the early part of the Tertiary, marine or estuarine conditions were re-established. The Thanet Beds are thought to have been laid down in a cool sea with a maximum depth of 50 m (Curry, 1965, p. 155). The Woolwich and Reading Beds represent deposition in a shallower sea that persisted in the eastern part of the London Basin, for example over the Southend and Foulness (258/9) district. Towards the basin margins this shallow sea bordered brackish mud flats. Shallow-water marine conditions returned when the Oldhaven Beds were laid down, but the sea deepened again, and most of the London Clay was deposited in water about 180 m deep. A further shallowing of the sea during the deposition of the Claygate Beds was associated with an influx of sand. These beds were probably laid down in water varying from 10 to 30 m. The succeeding Bagshot Beds are thought to have been deposited in even shallower water; Lingula-bearing beds in both the Claygate and basal Bagshot beds may be shallow sub-tidal deposits formed at a depth of less than 10 m.

On regional evidence it is assumed that folding and faulting took place during the Miocene and was responsible for the broad ripples and for the small-scale faulting that affect the district. These structures are superimposed on the broad syncline of the London Basin.

No Tertiary strata younger than these earth movements are preserved within the district. Quaternary deposition is dominated by glacial and periglacial activity. The main glacial episode was the advance of the Anglian ice sheet, which gave rise to the Chelmsford Gravels as outwash at its margin, and then overrode its own outwash. The ice stopped along the Danbury-Tiptree ridge. After deglaciation the Springfield Till remained. Outwash fluvioglacial gravels from the melting ice gave rise to a number of gravel sheets (5th, 4th and 3rd Terraces) in the Blackwater Valley. The continued amelioration of climate is demonstrated by interglacial deposits of Hoxnian age. A cold period (Wolstonian) is generally thought to intervene between the Hoxnian and Ipswichian, but within the district no deposits can definitely be assigned to this interval. Widespread solifluction deposits which mantle the interglacial deposits indicate a post-Hoxnian return to cold conditions, but there is insufficient evidence to establish whether more than one episode of periglacial activity occurred.

At the present day slow mass movement of material by land-slipping, downwash, and creep is still taking place at several localities. In the estuaries erosion and deposition are closely linked to continuing minor fluctuations in sea level caused by subsidence of the southern North Sea Basin, by isostatic readjustment after the last glaciation, and by eustatic variation in sea level. CRB

Chapter 2 Concealed strata

Palaeozoic

No deep borings within the district penetrate to the Palaeozoic basement. However, a number of boreholes in surrounding districts have proved Palaeozoic rocks at relatively shallow depth ((Figure 2) and (Table 1)). To the north-east three borings at Harwich (the first to prove Palaeozoic rocks in Essex), Weeley, and Stutton entered respectively: 'hard dark bluish grey slaty rock', 'grey sandy shale', and 'broken and jointy shaly sandstone' at the depths shown in (Figure 2). When the holes were drilled the horizon of these strata was not known with certainty though they were thought to be of Lower Palaeozoic age. It is now known that the Weeley hole terminated in beds of Llandovery age, and that at Stutton the basement rocks were either of Upper Llandovery or Wenlock age (Lister, 1971, p.93). Some 16 km to the south of the present area, boreholes at Fobbing and Canvey Island proved ?Carboniferous and Old Red Sandstone, respectively, at depths of −323.7 m and −398.5 m OD.

From this evidence, and from the structure of the later rocks, it seems likely that the upper surface of the London Platform basement under the Chelmsford district falls from just over −300 m OD in the north-west to below −400 m in the south-east. It is believed to be cut across Silurian or Old Red Sandstone strata.

Mesozoic

Jurassic

It appears unlikely that any Jurassic rocks are now preserved beneath the district, although it is possible that fault-bounded grabens with some Jurassic infilling, similar to the one proved at Cliffe (Owen, 1971; Shephard-Thorn in Dines and others, 1971, pp. 151–155), may exist.

Cretaceous

Lower Greensand

The feather edge of the Lower Greensand lies outside the confines of the district, but, as with the Jurassic rocks, some may be preserved in grabens.

Gault and Upper Greensand

The Gault is the earliest Mesozoic deposit which can confidently be predicted to underlie the district, though its presence has not been proved. It is likely to consist of grey silty or sandy marl, and regional considerations suggest that only the Upper Gault is present (Lake and others, in prep.).

At Harwich, Stutton and Weeley the combined Gault and Upper Greensand thicknesses are 18.6, 15.2 and 23.2 m respectively. The Gault is somewhat thicker at Canvey; the recorded thickness of 38.3 m includes 4.3 m of Upper Greensand. To the east, south and west the Gault thickens markedly to a maximum of 73.1 m at Crossness. The Upper Greensand was encountered in the Fobbing, Crossness, Beckton, Turnford, Ware and Loughton boreholes, but not at Cliffe.

Chalk

Chalk has been proved in many of the boreholes within the Chelmsford area, although none has penetrated the full succession. Beyond the district the total thickness of the Chalk of London, north Kent and south-western Essex varies from 189.9 m at Streatham to 206.4 m at Canvey (Table 1). At Wickham Bishops a well [TL 8427 1247] was still in Chalk at a depth of 359.7 m, having penetrated 214.3 m of the formation. However, this figure does not necessarily imply a northward thickening, because faults and steep dips are recorded in the overlying Tertiary beds (Whitaker and Thresh, 1916, pp. 311–312) and may continue in the Chalk. Nevertheless, in north-eastern Essex as a whole there is a significant thickening of the Chalk sequence, which varies from 249.9 m at Weeley to 271.3 m at Harwich, a northeasterly thickening that continues into East Anglia.

These thickness variations are thought to be partly depositional and partly due to pre-Tertiary erosion. Curry (1965, p. 163) has compiled a map showing the Chalk zones that lie directly beneath the Palaeogene in south-eastern England, and, although later information necessitates a slight revision of some of his zonal boundaries, the general picture is one of a broad denuded NW-SE-trending arch, the axis of which passes north-east of London.

At Canvey Island the Upper Chalk thickness of 84.7 m is reasonably well substantiated (Smart and others, 1964), the thicknesses of the Middle and Lower Chalk being 68.3 m and 53.3 m respectively. At Weeley the Upper Chalk appears to have thickened to 128.2 m (Whitaker and Thresh, 1916, p. 343), but the Middle and Lower Chalk (69.5 m and 52.1 m) approach the thickness proved at Canvey Island. The comparable figures from Stutton (Whitaker, 1906, p. 140), namely 167.9 m, 51.2 m and 46.9 m, also show a thicker Upper Chalk, but the lower divisions have thinned. The increase in thickness of the Upper Chalk northeastwards indicates preservation of the higher Chalk zones, possibly accompanied by a sedimentary thickening, in this direction.

The Chalk does not come to crop within the district, but detailed information has been provided by two IGS boreholes at Witham [TL 8244 1534] and Kelvedon [TL 8602 1797] where 3.61 m and 2.90 m of Chalk respectively were cored beneath Thanet Beds. In the preliminary published report (Bristow, 1971, p.20) these core lengths were both provisionally assigned to the Uintacrinus socialis Zone of the Upper Chalk (Santonian) on the basis of the identification of brachials of the index crinoid. Preparation of these critical brachials has now revealed that none of them can be attributed to Uintacrinus, and a zonal assignment on these grounds cannot, therefore, be upheld. Detailed macropalaeontological studies supported by an examination of the microfauna clearly point to an horizon in the upper part of the underlying Micraster coranguinum Zone for the Chalk of the Witham Borehole; the horizon of the Chalk in the Kelvedon Borehole is less easy to determine although micropalaeontological evidence indicates that, though it is essentially comparable with that at Witham, it may be slightly younger and lie near the boundary between the M. coranguinum Zone and the overlying Uintacrinus socialis Zone.

Details

In the Witham Borehole relatively firm white massive chalk was entered at 87.33 m beneath the Bullhead Bed of the Thanet Beds and was cored to 90.96 m; flint was present at 87.53 m and 90.50 m. The following account treats the contained fauna (DK 9315–9423) as a single assemblage. Identified species are as follows: Neoflabellina cf. praerugosa, Nodosaria sp. (i.e. giant foraminifera visible to the naked eye), Onychocella lamarcki, Glomerula gordialis^‡1 , Kingena lima, Orbirhynchia pisiformis, microbrachiopod (undescribed cancellothyrid), Atreta boehmi (adnate on Conulus), 'Chlamys (Aequipecten)' campaniensis^‡2 , Gryphaeostrea canaliculata, Inoceramus shell chips, Limea granulata, Pseudoperna sp. ( = Ostrea boucheroni sensu Woods (pars) non Coquand, 1869: Woods, 1913, pl. 60, figs. 8, 9) Pseudoptera coerulescens, aptychus indet., possibly scaphitid, Bourgueticrinus fischeri (calyx), B. granulosus (columnals), Glenotremites paradoxus (second (axillary) primibrachial and distal brachials)^‡3 ; Isocrinus? minutus (internodal), marginals of Chomataster cf. humilis and Crateraster quinqueloba, 'Cidaris' hirudo, 'C. ' perornata, Conulus sp. (incomplete specimens including one test fragment with radioles preserved), Globator? globulus, Phymosoma?, Stereocidaris sceptrifera, small fish bones and scales.

This is apparently a remarkably rich fauna from such a short length of core, but the diversity may reflect intensive collecting and the 'airbrasive' preparation technique employed rather than an unusually fossiliferous horizon. The occurrence of several specimens of Conulus in association with a large coarsely ornamented radiole of Stereocidaris sceptrifera is diagnostic of the upper part of the M. coranguinum Zone, i.e. the Santonian, for the boundary between the Coniacian and Santonian stages falls within the M. coranguinum Zone (see discussion in Rawson and others, 1978). In the key Thanet coast section Conulus occurs throughout the Santonian part of the Zone, but specimens from the lowermost horizons within the Santonian are predominantly smaller and less acutely conical than the extreme forms (C. albogalerus (Leske)) of the Conulus-rich belt with Cretirhynchia plicatilis (J. Sowerby) which marks the uppermost part of the Zone^‡4 . The Conulus from the Witham Borehole are too incomplete to determine whether they come from the equivalent of the ''plicatilis Belt' or from a lower horizon in the Santonian, although the basal position and the relatively small size of the periproct are strongly suggestive of the former. A comparatively high position within the Santonian part of the M. coranguinum Zone is also indicated by the presence of Kingena lima, and of Bourgueticrinus granulosus, which in the Thanet succession (A.S. Gale, personal communication) enters some few metres above the horizon of Whitaker's Three-Inch (Flint) Band and is more characteristic of the upper part of the zone than of the overlying U. socialis Zone. Further diagnostic evidence is provided by the presence of Pseudoperna ('Ostrea boucheroni') which is represented by the broad irregular forms characterising the M. coranguinum Zone rather than by the elongate subcylindrical forms (e.g. Woods, 1913, pl. 60, figs. 1–4) of the overlying crinoid zones and the lower part of the succeeding Offaster pilula Zone. The regular occurrence of brachials of the comatulid Glenotremites paradoxus throughout the core is also of considerable interest since this species appears to occur in belts within the M. coranguinum Zone, notably in the 'plicatilis Belt' of Thanet. Also of interest is the occurrence of the giant Coniacian – Lower Santonian benthonic foraminifer Neoflabellina cf. praerugosa. The remaining macrofauna does not provide any more detailed stratigraphical information.

In the Kelvedon Borehole relatively soft earthy white chalk was cored between 79.40 and 82.30 m, and flint was noted between 82.04 and 82.27 m. The Thanet Beds rested directly on the Chalk without a basal Bullhead Bed. The top of the Chalk is approximately planar, with irregular depressions of low amplitude.

Small branching burrows (1 mm diameter) of Chondrites type filled with glauconitic Thanet Beds sediment are present in the top 1 cm. A washed sample from a depth of about 80 m proved to be very rich in foraminifera, ostracoda and echinoid micro-radioles.

As at Witham the macrofauna has been treated as a single assemblage, the following species having been identified: Nodosaria sp., Placopsilina sp. (encrusting Bourgueticrinus columnal), Porosphaera globularis, fragmentary external mould of ventriculitid, Entalophora echinata, Nodelea durobrivensis, Vincularia cf. weybournensis, 'V. 'aff. supercilium, Gryphaeostrea canaliculata, Inoceramus chips, Pseudoperna sp., Bourgueticrinus sp.,(indet. columnals), crinoid distal secundibrachials (none definitely Uintacrinus, probably all Bourgueticrinus), Isocrinus? minutus, Crateraster quinqueloba (marginal), Ophiura serrata (lateral arm plate) Stereocidaris sceptrifera, indeterminate fish bones.

The very limited fauna is suggestive of the Uintacrinus socialis Zone, as is the relatively soft and earthy character of the Chalk. However, none of the distinctive brachial and proximal secundibrachial plates of the zonal crinoid has been observed, and the distal secundibrachials present, although superficially similar to Uintacrinus ossicles of comparable size prove, on closer examination, to be closer to Bourgueticrinus and have been tentatively attributed to that genus. The absence of Conulus contrasts with its relative frequency in Witham. Some confirmation of the tentative zonal assignment is provided by the fact that the Chalk of the U. socialis Zone exposed in the quarries at Sudbury, some 24 km to the north, is also very poorly fossiliferous (Jukes-Browne, 1904) and Uintacrinus plates are only sporadic. The micropalaeontological evidence, however, is equivocal and a very high horizon within the M. coranguinum Zone is equally possible.

Microsamples were taken from both boreholes as follows: Witham 87.35 to 87.45 m (SAG 2057); Kelvedon 79.40 to 82.30 (SAG 2058). Both samples contained numerically rich, diverse, and remarkably well-preserved foraminiferal assemblages (for faunal lists and discussion see Open File Reports PDL 76/76, 77/34 by Dr P. Bigg). Assignment of both samples to an essentially similar horizon within the Santonian is provided by the specific composition of the stratigraphically significant Stensioeina assemblage, namely S. granulata and S. exsculpta gracilis Brotzen sensu 'Leitfossilien' non Tramper, with rare S. exsculpta. In the Witham Borehole the ratio of S. granulata to S. exsculpta gracilis was approximately 1:1, whereas in Kelvedon, where the Stensioeina assemblage constituted only a relatively small proportion of the total assemblage, the ratio approximated to 2:1.

An independent micropalaeontological analysis carried out by H.W. Bailey formerly of Plymouth Polytechnic has confirmed that both sets of samples come from an essentially comparable horizon, and has placed this horizon within the Santonian with a considerable degree of accuracy, using the Dover-Thanet coast successions as a standard. The occurrence of Cibicides ex. gr. beaumontianus – ribbingi together with Eponides cf. concinnus and Reussella szajnochae praecursor indicates an horizon above Whitaker's Three-inch Flint-Band, while the relative frequency of Globotruncana bulloides Vogler ( = G. marginata (Reuss)), is diagnostic of the 'p/icatilis Belt' (i.e. above Barrois' Sponge Bed) or of even higher horizons including part at least of the overlying restricted U. socialis Zone. Bailey also records Globotruncana concavata from Kelvedon, this species being an important world-wide marker microfossil for the Lower Santonian.

Combining the micropalaeontological and macropalaeontological evidence, it is reasonable to assign the Witham samples to an horizon within the equivalent of the 'plicatilis Belt' of Thanet, which would agree with the relative frequency of Conulus and Glenotremites. The Kelvedon samples probably should be placed in the uppermost part of the same belt, i.e. at the top of the M. coranguinum Zone, or even in the thin U. socialis Zone. This new evidence supports the postulated sub-Palaeogene position of the M. coranguinum-U. socialis zonal boundary (Peake in Curry, 1965, fig. 2) in an area where hitherto there was no interpretable subcrop information. CJW

Chapter 3 Tertiary (Palaeocene and Eocene)

Nomenclature

The generally accepted subdivisions of the Tertiary strata throughout the London Basin are those made by Prestwich (1847; 1850; 1852). Although the names of Prestwich's units have been modified or changed by later authors (see (Figure 3)) they still remain easily recognisable, broad, mappable divisions throughout most of the London Basin. In recent years a plethora of local names has been introduced for poorly defined unmappable units of dubious stratigraphical value. Cooper (1976a) has attempted to formalise the nomenclature and to give group, formational or member status to the various units. He designated the Palaeocene Lower London Tertiaries as a group, composed of the Thanet, Woolwich and Reading, and Oldhaven formations^‡5 . However, the division of these formations into extremely local members is of uncertain value outside their immediate type area. In this account the Oldhaven Beds are considered not as part of the Palaeocene Lower London Tertiaries (Group), but as the lowest member of the Eocene London Clay (Formation) (Figure 3). The London Clay is succeeded by the Claygate Beds and Bagshot Beds (formations). The strata above the Oldhaven Beds, which collectively are equivalent to the London Clay Formation of Cooper (1976a), are divided by him into six, generally undefined, members. Of these members the lowest five constitute what is commonly known as the London Clay, and this name is retained in this memoir as the uppermost member of the London Clay Formation (Figure 3). Unless otherwise stated 'London Clay' in this memoir refers to the dominantly clay beds between the Oldhaven Beds below and the Claygate Beds above. Cooper's uppermost, Claygate, Member comprises Claygate Beds and Bagshot Beds as mapped by the Institute of Geological Sciences on the 1:50 000 Epping (240), Chelmsford (241) and Southend (258) sheets.

Lower London Tertiaries (Group)

The beds lying between the Chalk and the London Clay occur at depth throughout the district, and have been proved in many boreholes. They were termed the 'Lower London Tertiaries' by Prestwich (1852, p. 236); this name formerly embraced the Thanet, the Woolwich and Reading, and the Oldhaven Beds, but in this account the Oldhaven Beds are regarded as the basal bed of the overlying London Clay.

Thanet Beds (Formation)

The Thanet Beds of Whitaker (1866, p. 405), replaced the earlier term 1 hanet Sands first given by Prestwich (1852, p. 235). They were given formational status by Cooper (1976a, p. 23). Within the Chelmsford district it has not been possible to subdivide the 10 to 20 m of greenish grey fine-grained sands of the Thanet Beds, apart from the recognition of the Bullhead Bed at its base.

Woolwich and Reading Beds (Formation)

The term 'Woolwich & Reading Series' was first used by Prestwich (1854a, p. 77). Whitaker (1861, p.22) modified the name to Woolwich and Reading Beds, and Cooper (1976a, Stratigraphical Table, Part D) considered the beds to comprise a formation. The Woolwich and Reading Beds within the district can be subdivided into a lower unit consisting predominantly of green glauconitic, marine or estuarine, sands and clays (Woolwich facies), and an upper unit of mottled, fluviatile, generally unfossiliferous clays (Reading facies). There are few detailed lithological and faunal descriptions of the strata, and it is not always possible to make useful subdivisions from the borehole logs alone. The green clays and sands probably correlate in the main with the Woolwich Bottom Bed of the London area. Locally they appear to be overlain by marine shelly clays and sands which may represent the Woolwich Shell Bed.

Upper London Tertiaries (Group)

By reference to Prestwich's (1852, p. 236; 1854b, p. 437) original description of the Lower London Tertiaries and the London Clay it is apparent that the London Clay, together with what we now recognise as Claygate Beds, would have constituted the Upper London Tertiaries, although he never used this term. Cooper (1976a) extended the concept of the Upper London Tertiary Group to include the Bagshot Sands of Essex. In the present memoir this Group is considered to comprise the London Clay (Formation) (with a basal Oldhaven Beds Member), Claygate Beds (Formation) and Bagshot Beds (Formation).

London Clay (Formation)

Oldhaven Beds (Member)

The Oldhaven Beds, which in the Chelmsford area consist of shelly, greenish grey, fine- to medium-grained, locally pebbly, sands, were originally termed the Oldhaven Sands (Whitaker, 1866, p. 413); the type locality being Oldhaven Gap in north Kent. As defined by Whitaker they were virtually synonymous with the 'Basement bed of the London Clay' of Prestwich (1850), although the Oldhaven Beds also included certain strata which were grouped by Prestwich with his Woolwich and Reading Series.

London Clay (Member)

Although broad lithological divisions can be made in the main mass of the London Clay in boreholes, they are not readily applicable to field mapping, and so the formation is not divided into the named members (Highgate, Sheppey, etc.) which have been recognised in boreholes elsewhere (Cooper, 1976a, table c). In the IGS Stock Borehole [TL 7054 0045] the London Clay (Formation) was subdivided into six lithological (A–F) units and fourteen (1–14) biofacies (Figure 5), of which the lowest, arenaceous, unit (A) is regarded as the Oldhaven Beds. The Hadleigh Borehole in southern Essex duplicated the Stock Borehole and it was possible to subdivide the London Clay into the same lithological units and biofacies that were recognised at Stock. Wrigley (1924; 1940), made five broad faunal divisions of the London Clay in the London area based on a limited number of exposures. King (1970) also proposed a five-fold division of the London Clay based on five biofacies recognisable over a wide area. His units were originally named A, B, C1, C2 and C3 in ascending sequence and were later, in a modified scheme in Cooper (1976a), renamed A–E (see also Stinton, 1975, p. 6). With the exception of King's Unit A ( = Oldhaven Beds + Unit B of this account, (Figure 5)), which correlated approximately with Wrigley's Division 1, there was no exact correspondence between King's units and Wrigley's divisions.

Claygate Beds (Formation)

The term Claygate Beds was introduced by Dewey (in Leach, 1912, p. 116) for 15 m of passage beds lying between the London Clay and the Bagshot Beds around Claygate, Surrey. In Essex, Claygate Beds were distinguished and mapped for the first time in 1922–23 during the survey of the Romford (257) Sheet.

As mapped on the 1:50 000 Epping (240), Chelmsford (241) and Southend (258) sheets, the Claygate Beds comprise the strata between the top of the London Clay and the base of the Bagshot Beds (Figure 11). The base is taken at the first thick ( > 1 m) fine-grained sand in an alternating, dominantly sand/clay sequence. The top is taken at the base of the thick fine-grained sands of the Bagshot Beds. Cooper (1976a, table c) regarded his Claygate Member as the uppermost member of the London Clay Formation, but his Claygate Member included the Bagshot Beds, which in this memoir are regarded as a distinct formation. The Claygate Beds fall in the upper part of Wrigley's (1924; 1940) Division 5 and King's (1970) Division C3 ( = E). They fall within the lower part of the Wetzeliella coleothrypta dinocyst zone and also in the ostracod zone 6c of the early Eocene Ypresian Stage (Bristow and others, 1980).

Bagshot Beds (Formation)

The fine-grained buff sands of the Bagshot Beds were first recognised in Essex by Prestwich (1847, p. 380) who referred to them as the Bagshot Sands. Wood (1864) proposed the name Bagshot Pebble Bed for the overlying pebbly deposits, a formation which was not recognised during the original geological survey of Essex, but which was incorporated in later editions of the Old Series maps of Essex (see Whitaker, 1889, p. 270). The Pebble Bed and the underlying sands were generally regarded as constituting the Bagshot Beds. Monckton and Herries (1880, p. 23) were not satisfied that most of the pebble-bed outcrops were in situ Eocene deposits, and regarded many of them as residual debris of several different formations, a view shared by Wooldridge (1924, p. 362). During the field survey of the Chelmsford (241) and adjacent Southend (258) districts it was confirmed that deposits formerly mapped as the Bagshot Pebble Bed included both in situ and transported Eocene material. Because of the doubt over the classification of the Bagshot Pebble Bed in areas not then recently surveyed, it was thought best at that time to regard the pebble bed of the Chelmsford district as a unit distinct from the Bagshot Beds. Later mapping in the Epping (240) district has demonstrated that the 'Bagshot Pebble Bed' for most of that area is not an in situ Eocene deposit. It is now thought that in situ deposits of the Bagshot Pebble Bed in Essex are confined to the area around Stock in the south-west corner of the Chelmsford district and in the corner of the districts covered by the immediately adjacent sheets. In this account the Bagshot Pebble Bed is regarded as the uppermost member of the Bagshot Beds (Formation) (Figure 12). Below the Bag-shot Pebble Bed the underlying strata consist of the lowermost Bagshot Sands (Member) as previously recognised, and silts and clays, commonly referred to as 'Brickearth' (Whitaker, 1872, pp. 322, 327), but here formally named as the Stock Clay (Member) (Figure 12). The Stock Clay, which is not portrayed on the published maps, together with the Bagshot Sands, comprise the Bagshot Beds of the 1:50 000 Chelmsford (241) Sheet. Thus the term Bagshot Beds of this account differs from that of the map by including the Bagshot Pebble Bed as its uppermost member.

Age

The definition of the boundary between the Palaeocene and the Eocene is currently under debate. It is generally agreed that a date of 52 to 53 Ma approximates to this boundary (Odin and others, 1978; Fitch and others, 1978). However, there is disagreement between the above authors as to where in the English stratigraphical column such a date falls. Odin and others (1978) thought that it fell below the Oldhaven Beds, whilst Fitch and others (1978) placed it at the top of the Oldhaven Beds. Following Odin and others (1978) the top of the Palaeocene coincides, more or less, with the top of the Lower London Tertiaries as defined in this memoir.

General stratigraphy

Lower London Tertiaries (Group)

Nowhere within the Chelmsford district do the Lower London Tertiaries crop out, although a sub-drift occurrence is known along the Blackwater Valley. However, a number of boreholes penetrate the Lower London Tertiaries and, whilst the logs of some are inadequate, it is generally possible to subdivide the sequence into its component formations.

In general the gradual northward thinning of the Lower London Tertiaries across the Southend (258) district, from a maximum of 53 m^‡6  to just over 34 m along the northern edge of the district (Lake and others, in prep.), continues across the Chelmsford district to its northern boundary where the thickness is about 30 m (see (Figure 6)).

Thanet Beds (Formation)

The formation does not crop out within the district, but has a sub-drift crop along the Blackwater Valley between Witham and Kelvedon. The only modern fully cored boreholes are the IGS Witham [TL 8244 1534] and Kelvedon [TL 8602 1797] boreholes where 10.24 m and 14.91 m respectively of Thanet Beds were present between the overlying drift and underlying Chalk.

The stratigraphy of some of the existing well logs cannot now be determined in detail, but the presence of green glauconitic sand at the base of the Woolwich and Reading Beds was, in many cases, sufficiently distinctive to be noted by drillers; in such cases the thickness of the formation is known with some accuracy. The Thanet Beds appear to show a northerly thinning across the district (Figure 7) from more than 20 m in the south-west to about 10 m just beyond its northern margin. A few apparently anomalous thicknesses are probably the result of part of the Thanet Beds being wrongly classified with the Woolwich and Reading Beds or vice versa, as an unusually thick occurrence of one of these formations is generally accompanied by a thinning of the other.

The Bullhead Bed has been recorded in only some of the boreholes within the district. It is generally described as composed of green, or green and black, flints, and varies from 0.15 to 0.6 m in thickness. In the Witham Borehole one black flint pebble, 75 mm in diameter, was noted at the junction with the Chalk and another 1.4 m above. The basal flint pebble has been taken to represent the Bullhead Bed. Flint pebbles were not found at the base in the Kelvedon Borehole, but were, however, noted some 50 mm above. In this borehole the top of the Chalk was bored by organisms to a depth of 12 mm, and infilled with dark green sand identical to that of the overlying beds. In both the Witham and Kelvedon boreholes the lithology of the Thanet Beds was similar, in that the lower beds consisted of a greenish black fine-grained sand. This passed upwards into a grey or greenish brown fine-grained sand or silty sand, which was bioturbated in places.

In drillers' logs the lithologies are variously described as clay, silty clay, sandy clay, silt, clayey sand and sand; the colours would also appear to be quite variable and are said to be yellow, grey, brown, blue, black, green, dark and mottled. It is thought, however, that whereas these lithologies and colours may be present in minor proportions, the predominant lithology is that noted in the Kelvedon and Witham boreholes.

No fauna has been noted from the Thanet Beds within the area, but the presence of glauconite indicates a marine origin.

Woolwich and Reading Beds (Formation)

The Woolwich and Reading Beds do not crop out within the district. They presumably have a sub-drift crop along the Blackwater Valley between Witham and Kelvedon, but except for 0.3 m of 'dead green sand', here regarded as basal Woolwich facies, which has been recorded beneath glacial drift and resting on the Thanet Beds in a well [TL 8647 1915] at Kelvedon, no borehole has gone directly from drift into the formation.

In the Southend and Foulness (258 and 259) district to the south no beds of Reading facies have been recognised with certainty. The only strata that may possibly belong to this facies are 2.7 m of white sandy clay which were recorded immediately beneath the Oldhaven Beds at Billericay [TQ 6744 9510], and 4.6 m of 'hard dark shale', 'lignite' and 'hard chocolate clay' occurring just below the Oldhaven Beds at Burnham [TQ 9472 9707]. Farther north, however, both facies are recognisable.

The Woolwich Shell Bed has been noted in wells at Ramsden Bellhouse [TQ 7118 9648], Galleywood [TL 7037 0392] and Goldhanger [TL 8898 0815].

The boundary between the Woolwich facies and the Thanet Beds cannot always be recognised with precision from existing well logs, nor can the lower boundary of the Reading facies. The junction of the Reading facies with the overlying Oldhaven Beds, is, however, commonly well defined.

Beds of Woolwich facies are here regarded as those generally described in boreholes as green, locally shelly, clays and sands. The Reading facies comprises red, brown and mottled, unfossiliferous 'hard' clays and sand. Pebbly horizons have been noted in both facies.

The thickness of the Woolwich and Reading Beds appears to be fairly constant at between 12 and 17 m in the northeastern, south-western, and south-eastern parts of the district. In the north-west and centre consistent thicknesses of less than 10 m have been recorded (Figure 8.)

Because of the uncertainty of the well classifications the relative thicknesses of the two facies cannot always be ascertained with certainty. In general the Reading facies is thicker than the Woolwich facies in the north-east of the district.

No fossils have been identified from the Woolwich and Reading Beds within the district.

Upper London Tertiaries (Group)

London Clay (Formation)

Oldhaven Beds (Member)

On general consideration it is thought that the Oldhaven Beds are present beneath the whole district, except where they have been removed beneath drift along the Blackwater Valley between Witham and Kelvedon. The Oldhaven Beds have not been recorded in all boreholes, but this is regarded as possibly a result of non-recognition rather than non-deposition. Moreover, some of the variation in thickness recorded in the drillers' logs may result from misidentification.

Only the upper part of the Oldhaven Beds has been proved in cored boreholes in recent years in this district. The

Stock Borehole proved 7.62 + m of fine-grained, greenish grey, silty, locally Shelly and pebbly, sands, sandstones and siltstones beneath the London Clay. From the existing well logs it is thought that these beds rest on a lower unit of Oldhaven Beds consisting of non-glauconitic grey, only locally fossiliferous or pebbly, water-laden 'running' sands.

The recorded thicknesses of the Oldhaven Beds range between 1.2 and 15.4 m with the thickest development in the vicinity of Chelmsford (Figure 9).

The fauna from the Oldhaven beds consists of fully marine foraminifera, molluscs, sharks' teeth and diatoms (p. 21).

London Clay (Member)

The London Clay everywhere underlies the district, with the exception of a small tract along the Blackwater Valley between Witham and Kelvedon where it has been cut out below drift. It crops out over about 40 per cent of the area, particularly in its eastern half. In the north-west of the district it is covered by extensive spreads of the Chelmsford Gravels and the Springfield Till, and in the south-west it is generally overlain by the Claygate Beds and by irregular spreads of drift.

The thickness of the London Clay can be determined only where it is capped by the Claygate Beds, though in a few further instances, it has been possible to calculate the likely thickness because the base of the Claygate Beds is known in adjacent outcrops. Except in the Stock and Hadleigh boreholes the thicknesses quoted are imprecise because of the difficulty in defining the upper and lower limits in older logs. With these reservations in mind a surprisingly consistent picture emerges for the thickness of the London Clay along the southern margin of the district. There appears to be little variation from the 126.30 m and 127.93 m recorded at Stock and Hadleigh respectively. Such variations as do occur, from 125 to 137 m (Lake and others, in prep.) may result from the difficulty in defining the top and bottom of the London Clay in old logs and in extrapolating from the base of the Claygate Beds, itself locally imprecise. The Rectory Well [TL 7698 0132] in East Hanningfield started in 'light brown imperfect marl', 9.1 m thick, which is referable to the Claygate Beds. The underlying London Clay was 128.02 m thick. In the Danbury Hill area, where the Claygate Beds have not been recognised, there is a very thick, but incomplete, sequence of London Clay. The Nurseries Well [TL 7879 0571] proved 121.92 + m of the Formation beneath 18.29 m of Glacial Sand and Gravel, whilst farther north a well [TL 7946 0762] at Hillcrest penetrated 128.93 + m of London Clay beneath 5.18m of Glacial Sand and Gravel. The Station Well [TL 8497 0048] at Cold Norton proved 118.57 m of London Clay before entering the Lower London Tertiaries. An estimated 20 m of the uppermost London Clay are thought to be absent at this point.

Lithological and palaeontological subdivisions

The London Clay in the Stock Borehole has been subdivided into six broad lithological units; several of these, however, have gradational boundaries. In the lower part of the succession there is a broad correlation between the Lithological units A to C, and the Palaeontological Units 1 to 9, but the respective units in the higher part are not directly correlatable (see (Figure 5)).

The lithological units in the Stock Borehole are as follows:

The approximate distribution of the London Clay lithological units and Claygate Beds, both at the surface and beneath the drift deposits, is shown in (Figure 10). This has been constructed using field mapping evidence, palaeontological evidence from shallow boreholes, a contour map of the base of the London Clay (Figure 13), and by extrapolation from borehole evidence, assuming a thickness of the London Clay, based on the Stock and Hadleigh boreholes, of 126 m.

Examination of the London Clay microfaunas has been based on two sampling methods. Those examined by M.J. Hughes consisted of 4 to 5 cm of core taken at approximately 1 m intervals and washed through 75 micron mesh size sieves. C. King, who was primarily concerned with recovering mullusca, prepared a 80 cm length of core taken continuously, and washed through 250 micron mesh size sieves. King divided the beds beneath the Bagshot Beds of the Stock Borehole into 20 units, based on lithology and the distribution of a number of key fossils, including foraminifera, ostracods, molluscs, crinoids, diatoms and brachiopods. King's units, of which numbers 2 to 14 correspond to the London Clay (Member), have been used in this account and additional data on the foraminiferal distributions have been incorporated. The micropalaeontological determinations are based on the work of M.J. Hughes and the reports of C. King. C. J. Wood is responsible for the molluscan determinations in Unit 14. The otoliths have been identified by Mr F. Stinton, whilst the chondrychthian teeth have been determined by Mr D. Ward.

Claygate Beds (Formation)

The term Claygate Beds was introduced by Dewey (in Leach, 1912, p. 116) as a replacement term for the field name 'Passage Beds' which had previously been used by Geological Survey officers working in Surrey. In a subsequent paper (Dewey, 1912, p. 238) gave a brief outline of the history of research prior to the introduction of his terminology. In Essex, the Claygate Beds were distinguished and mapped for the first time by Dines and Edmunds in 1922–23 during the survey of the Romford (257) Sheet. There are, however, distinct differences between the Claygate Beds of Essex and those of the type locality. In the western part of the Romford district the beds are more loamy and the laminations not so well defined as at Claygate. Locally, for example at Shenfield (see Dines and Edmunds, 1925, pl. ii), the lithology is very similar to that at Claygate, but at nearby Brentwood the laminations of the Claygate Beds are also ill-defined and loamy. Wooldridge (1924, p. 368, fig. 29) similarly recognised two lithologies in the 'Passage Beds' of Essex; a Claygate type, very similar to that of the type-area, in the Vange, Hadleigh and Rayleigh areas, and a Brentwood type of structureless loam that occurs in all the localities falling within the Chelmsford district. He believed that this distinction was due to their positions in relation to the presumed coastline of the Eocene sea. It is known that the 'Claygate type' corresponds mainly to the upper division, (Units 19 to 20 ) of the Claygate Beds as now recognised (Bristow and others, 1980).

Within the Chelmsford district the Claygate Beds are not easily separable in the field from the London Clay, but their upper junction with the Bagshot Beds is sharp.

In the Stock Borehole (see (Figure 11)) the Claygate Beds consisted predominantly of silty, and in part sandy, clays with silt and fine-grained sand partings devoid of obvious laminations. The Claygate Beds can be divided into three upward fining units, each commencing with a fine-grained sand. Locally it has been possible to trace one or more of these sands for short distances. In the borehole, the base of the lowest bed of sand lay at a depth of 46.90 m and has been taken as the base of the Claygate Beds, giving these a total thickness of 19.42 m.

In the field it is difficult to place a precise lower boundary to the Claygate Beds; in places there is a weak spring-line at the base, and on steeper south- and west- facing scarp-like slopes capped by the Claygate Beds the base is commonly marked by a weak feature; tracing the feather-edge on dip-slopes is difficult and the basal boundary cannot be located with precision. This difficulty is aggravated by the similarity of the weathered Claygate Beds, which vary from yellow silty clays to brown silty clays, and ochreous mottled pale grey sandy and silty clays, to solifluxion deposits which were derived partly from this member. In addition field relations are complicated by landslips which are a common feature of the Claygate Beds outcrop, particularly along their base. Accordingly accurate calculations of the thickness of the Claygate Beds are impossible without adequate borehole control. In boreholes the thickness appears to be fairly constant, varying from 17 to 24 m (19.42 m at Stock; 24.69 m at Billericay; 20.12 m at Brentwood; more than 22.45 m at Hockley Heights; 23.30 m at Hockley Gattens; 17.37 m at Sand Pit Hill, Hadleigh and 17.12 m at Westley Heights).

Consequently it is suspected that several published thicknesses are incorrect, possibly because one of the clayey horizons within the Claygate Beds has been misidentified as part of the London Clay. Examples of such anomalous thicknesses are the 9.7 m recorded in the Brentwood railway cutting (Dines and Edmunds, 1925, p. 13); the 4.6 to 4.9 m or 10 m at Brentwood Brickworks (Berdinner, 1925, p. 174; Middlemiss, 1956, p. 317), and the 1.5 m in the railway cutting (?north) east of Ingatestone (Dines and Edmunds, 1925, p. 16). Prestwich (1890, p. 152), Dines and Edmunds (1925, p. 16) and Berdinner (1925, p. 176) all record areas where there appears to be an erosive break at the base of the Claygate Beds, but this may be no more than local channelling.

Eastwards of Stock there is an extensive outcrop of Claygate Beds around Hanningfield, Purleigh and Woodham Ferrers. Except at the latter locality, the overlying Bagshot Beds are absent and so the total thickness of the formation in these areas is unknown; at Woodham Ferrers it appears to be about 15 m.

A further outcrop of Claygate Beds, hitherto unsuspected, is present around Althorne in the south-east corner of the district and extends southwards into the adjacent Southend (258) district. The absence of the overlying Bagshot Beds precludes an estimate of thickness, but a well [TQ 8922 9877] at Tyle Hall, Althorne, just beyond the Chelmsford district is recorded as having passed through 10.06 m of 'brickearth' above 132.8 m of blue clay of the London Clay (Whitaker and Thresh, 1916, p. 206). Since the well commenced in strata now mapped as Claygate Beds the 'brickearth' is regarded as part of the Claygate Beds. Thicknesses in excess of 10 m are estimated to be present in the Althorne-Mayland area, but in this south-eastern area there is great difficulty in separating the Claygate Beds from the London Clay. The Claygate Beds probably die out as a separate mappable unit, towards the central, and presumably deeper, part of the basin (see Wooldridge, 1924). The base of the Claygate Beds has been taken at the feature break on the south-facing scarp overlooking the River Crouch, but inland there is little or no topographical feature and it is mapped from the yellowish brown silty clay subsoil. The base of the Claygate Beds accordingly has been shown as conjectural on the six-inch and 1:50 000 maps.

Most of the landslips recognised in the Chelmsford district are associated with the Claygate Beds outcrop (pp. 85–86). They appear usually to be related to either the spring line at the base of the Bagshot Beds, or to a weaker one at the base of the Claygate Beds. Some of the slips recorded by Hutchinson (1965, pp. 6–8) in the Langdon Hills area were related by him to the Claygate Beds outcrop; many others noted by him in the Chelmsford district are now known to fall within the Claygate Beds outcrop. In recent years the advent of large scale construction, particularly road schemes, has given rise to a greater awareness of the unstable nature of the Claygate Beds. Cuttings within them along the Brentwood By-pass encountered many constructional problems.

Depositional environment, fauna and biostrati graphical position

The fauna of the Claygate Beds is indicative of a fully marine environment. The molluscs include common browsing, suspension-feeding and carnivorous forms, and the high proportion of the first two groups suggests an environment in the photic zone, although no forms typical of very shallow water are present. The large number of pteropods, and the occurrence of fish debris (Figure 11) suggest slow or interrupted deposition leading to the concentration of fossils. The rapid and irregular changes in grain size probably reflect deposition under the influence of fluctuating tidal currents at a depth of not more than 30 m, although the basal sand of the upper division which commonly contains Lingula may be a shallow subtidal deposit formed at a depth of less than 10 m.

The distribution of the fauna from the Stock Borehole and its relationship to the lithology is demonstrated in (Figure 11). The figure is largely self-explanatory, but the following notes by Mr C. King comment on some aspects of the faunal distribution.

A calcareous fauna is preserved in all but the top 3.5 m of the Claygate Beds which are decalcified. However, within the decalcified strata well preserved and identifiable pyrite moulds of molluscs are still present. The molluscan faunas are dominated by Adeorbi s, Turboella, Ringicula, Nuculana and Corbula globosa. Although the commoner species range throughout the formation, each unit shows a different faunal association. For example the unidentified Kelliellid is abundant in Unit 20, Nuculana prisca in the upper part of Unit 18 and in Unit 20, and Venericardia trinobantium in the lower part of Unit 17.The three species of pteropods are scarce from Unit 15 to Unit 17, but become very abundant in Unit 18, decreasing in abundance in Units 19 and 20. Camptoceratops prisca, which is present as isolated specimens throughout, is only common in Unit 16. Lingula is sparsely represented in most samples from Units 16 to 20, except in Unit 19 where it is abundant. The foraminiferal fauna recovered on the 60-mesh sieve consists almost exclusively of Cibicides and Lenticulina. The absence of foraminifera above Unit 19, and of ostracods above the middle of Unit 18 is to be noted.

Most of the species present in the Claygate Beds are long-ranging, and are found in both Divisions 4 and 5 of Wrigley (1924). The bivalve Venericardia trinobantium Wrigley, however, is apparently restricted to the lower and middle divisons of the Claygate Beds as here defined (Units 15–17) and is not known to occur outside the area adjacent to the Thames Estuary. The pteropod Camptoceratops prisca (Godwin-Austen) is particularly characteristic of the Claygate Beds (occurring in profusion at some horizons), but is not confined to this formation, having been recorded recently (Kirby, 1974) from an horizon in the upper part of the London Clay, west of Burnham-on-Crouch and estimated here to be 26 m below the top (Unit 13). The small bivalve provisionally assigned to the Kelliellidae is also very common in the Claygate Beds, particularly in the higher part, but its total range is not known. Essentially, therefore, the Claygate Beds fauna could be described as a Division 5 (i.e. Highgate) fauna with the addition of V. trinobantium and abundant pteropods, notably C. prisca. Full details of the faunas and their distribution are given in (Figure 11).

The Claygate Beds fall within the lower part of the Wetzeliella coleothrypta dinocyst zone, the base of this zone having been identified in the London Clay of the Sand Pit Hill Borehole, Hadleigh, some 40 m below the base of the Claygate Beds at 65 to 70 m drilled depth (R. Harland: BGS MS information). The highest part of the London Clay of the Isle of Sheppey, Kent, essentially corresponding to the Claygate Beds as understood here, has also been placed in the coleothrypta Zone (see Costa and Downie, 1976). The occurrence of the ostracod Echinocythereis reticulatissima Eager in the Claygate Beds shows that this formation can be assigned to ostracod zone 6c, of which this species is the zonal index (cf. Keen, 1978, fig. 2). As with the coleothrypta dinocyst zone, however, the base of the reticulatissima ostracod zone occurs within the restricted London Clay in the Stock Borehole at least 30 m below the base of the Claygate Beds (C. King, personal communication).

Curry and others (1978) assign the highest part of the London Clay of the area adjacent to the Thames Estuary to the international standard planktonic foraminifer zone P7 and to the nannoplankton zone NP12, but the direct evidence for this is somewhat inconclusive, and the assignment appears to have been made by extrapolation from areas outside Britain. The Claygate Beds fall within the early Eocene Ypresian Stage as originally defined, although the exact equivalence between the Claygate Beds and overlying Bagshot Beds in eastern England and the Ypres Clay and Mons-en-Pevele Sands in Belgium remains unclear at present. What is of particular interest is the fact that the total dinocyst assemblage of the topmost London Clay of Sheppey appears to suggest correlation within the lower part of the Wittering Formation of the Bracklesham Group in the Hampshire Basin (see discussion in Curry and others, 1978); thus both the Claygate Beds of Essex and the overlying Bagshot Beds are probably lateral equivalents of part or all of the Bracklesham Group.

Bagshot Beds (Formation)

Outliers of the Bagshot Beds occur in the south-western corner of the district. A large outlier around Forest Lodge, which falls on this and the three adjacent sheets, has an almost complete Bagshot Beds succession and it was here that the Stock Borehole was sited. There is a further outlier at Galleywood and another at Woodham Ferrers, which is partially hidden under Boulder Clay, and has hitherto been unrecognised.

In the Stock Borehole [TL 7054 0045] (Figure 12) the following succession was proved:

This tripartite division had earlier been noted at Stock and some of the larger nearby outliers such as Brentwood and Fryerning (Whitaker, 1889, pp. 274, 276, 278).

The 13.18 m of fine-grained sands were sharply defined from the underlying clays of the Claygate Beds and had a junction which could be followed fairly easily in the field. Earlier, Wood (1868, footnote p. 464) had adopted a similar base for his Bagshot Beds, although at Brentwood he appears to have included locally strata which are now considered to be Claygate Beds within the Bagshot Sands (see Whitaker, 1889, p. 274). In the field the base is commonly marked by a spring-line; in particular copious springs issue from the southern boundary of the Bagshot Sands at Stock in the adjacent Southend (258) district. The base of the Bagshot Beds is regular over much of the ground mapped. The local abrupt falls in its base, such as that near Ingatestone on the Epping (240) Sheet, may be attributed to cambering, though it is possible that local channels occur at the base of the Bagshot Beds, which may not everywhere rest on the same horizon of the Claygate Beds.

The Stock Clay has a much more restricted outcrop, which may be partially due to original deposition, partially to post-Tertiary erosion and partially to local channelling. Silts and silty clays are the dominant lithology, although a 3-m sand bed was present within the member at the Stock Borehole. In the Farrow's Farm Borehole [TQ 7020 9988] a 1.50-m bed of sand is also present at the top of the member. Around Stock the Stock Clay is present only locally beneath the Pebble Bed. This is thought to be probably attributable to its having been deposited in a channel (see Whitaker, 1889, p. 278) than to its erosion before the deposition of the Bagshot Pebble Bed. Post-depositional movement of the Pebble Bed has probably further restricted the outcrop of the Stock Clay. Wood (1864) noted at Warley that the Pebble Bed rested 'somewhat unconformably upon the Bagshot Sand'. A similar irregular contact between the Pebble Bed and Bagshot Sands has been noted at Billericay (French, 1968. p. 165).

The origin of the Bagshot Pebble Bed of the Chelmsford district is uncertain. Its characteristic features, first recognised by Wood (1864), are the well rounded bluish black flint pebbles, and the virtual absence of quartz pebbles (see French, 1968, p. 165).

At Langtons (Whitaker, 1889, p.273, fig. 46) the beds are in situ; here the pebbly beds are interbedded with thin seams of silty clay and fine-grained sand. This section is interesting in that 0.9 m of disturbed flint gravel (Warley Gravel) lies at the top, although the constituent pebbles differ from those of the Bagshot Pebble Bed in the presence of quartz and unworn flints. Elsewhere within the Chelmsford (241) district, some deposits which are obviously remnants of later gravel and/or Boulder Clay, but incorporate much Pebble Bed material and have subsequently been disturbed by solifluxion, have been mapped as Head (e.g. around Norsey Wood and Lodge Farm [TQ 691 956], just beyond the southern boundary of the district). Other deposits appear to be soliflucted Bagshot Pebble Bed (see also Wooldridge, 1924, p. 362). French (1968) suggested that the channels in which these pebble beds occur were initiated by densely loaded currents flowing into a regressional Pleistocene sea, and that their profiles became modified during glaciation.

The maximum recorded thickness for Bagshot Beds is 27.48 m in the Stock Borehole; incomplete thicknesses of 15.30 m were proved in the Farrow's Farm Borehole; 12.19 m at Brentwood; 11.27 m at Billericay (Buchan and others, 1940, p. 21); 12.19 m + at Rayleigh (Wooldridge, 1924, p. 362); and 10.5 m at Sandpit Hill, Hadleigh (Lake and others, in prep.).

Wooldridge (1924, p. 362, fig. 29) recognised two facies of Bagshot Beds; in the south, where all the localities within the present district lie, the beds comprise fine-grained structureless sands, loams being absent or subordinate, and the overlying pebble bed is usually absent. Wooldridge's northern facies, which lies roughly to the north of the Al2, is described by him as being predominantly loamy and culminating in a pebble bed. The change in facies was thought to be related to the 'Thames Valley–Mid Essex faults' for which he postulated movement throughout Eocene times (Wooldridge, 1923). However, no clear-cut division of the Bagshot Beds has been recognised in the outliers included in the present survey.

Reasons have been advanced (Wooldridge, 1924, p. 364) for the marine origin of the Essex Bagshot Beds, with a contemporary western shore line approximating to the present Lea Valley. This is supported by the only fauna recorded from the Bagshot Beds within the Chelmsford district which consists of numerous examples of Lingula sp.,indicative of marine shallow water deposits, found in the basal beds at the Stock Borehole. In contrast the Bagshot Beds of Surrey and

Berkshire were regarded by Wooldridge (1924, pp. 364–366, 380, fig. 32) as fluviatile. There is a marked difference in the grain-size between the two areas, those in Essex being considerably finer-grained (0.1 to 0.25 mm) than those in Surrey and Berkshire (0.25 to 1.00 mm) (Wooldridge, 1924, pp. 373–374, fig. 31). CRB

Details of stratigraphy

Woolwich and Reading Beds

In many of the boreholes it is possible to recognise only one or the other of the two facies. Where only one facies is mentioned in the account below it is because the other cannot be satisfactorily distinguished from the adjacent strata.

There appear to be no beds of Reading facies in the west at Margaretting just beyond the western margin of the district (Davies and Standon-Batt, 1965, p. 23). Similarly at Galleywood Waterworks [TL 7037 0392] the Reading facies appears to be absent; the Woolwich Shell Bed is well represented by 11.9 m of shelly clays and sands. Some 2.3 km ENE, 2 m of 'mottled clay' recorded in a well [TL 7321 0462] at Great Baddow presumably represent the feather-edge of the Reading facies. This facies is also represented by 3.66 m of 'red clay' at the Hall Street Waterworks [TL 7095 0635], Chelmsford; by 2.4 m of 'red and blue sand' at the former Wells & Perry Brewery, Chelmsford [TL 7067 0702]; by 2.1 m of 'mottled clay' at the Flour Mills, Chelmsford [TL 7068 0722], and by 0.9 m of 'red mottled clay' at the Broomfield Waterworks [TL 7096 0944], Broomfield. At this latter locality it overlay 6.71 m of green sandy silts and clays and grey clay with pebbles. At Writtle [TL 6687 0615], just beyond the district, the Reading Beds are represented by 3.6 m of 'mottled' sandy clay (Woodland, 1945).

Farther east, the most southerly well [TL 8690 0282] that yields good evidence of the Reading facies within the district is at Mundon, where 3.35 m of 'dark clay' and 2.13 m of red mottled sandy clay rest on 7.32 m of clayey green sand of Woolwich facies. Some 3 km ENE, a well at Brickhouse Farm, Mundon [TL 8983 0337], proved 9.75 m of 'coloured sands', overlying 4.88 m of grey sand with a green seam at the base.

In the Maldon area, 12 m of 'coloured sandy clay' immediately beneath the Oldhaven Beds at Heybridge Hall [TL 8596 0763] presumably includes some part of the Woolwich facies, as a well at Saltcote Mill, Heybridge [TL 8744 0781], some 2.5 km to the east, with a better log, proved only 3.96 m of 'red and green mottled clay' beneath the Oldhaven Beds and overlying 'green mottled clay' of Woolwich facies.

An even better log of a well [TL 8898 0815] at Gardener's Farm, Goldhanger, 1.6 km E of the latter well, proved 7.31 m of brown red and yellow clays (Reading facies), overlying 2.14 m of green silty clay with small stones which in turn overlay 4.87 m of green silty clay with shells (Woolwich Shell Bed).

In Maldon itself the Reading facies appears to be very thin. At the Wantz Road Waterworks [TL 8527 0662] it consists of 1.53 m of mottled red clay, overlying 4.26 m of 'bright green sand' of the Woolwich facies. At the Spital Road Waterworks [TL 8450 0666], the green sand of the Woolwich facies is 4.88 m thick, but the overlying Reading Beds cannot be separated readily from the Oldhaven Beds. A well [TL 8483 0712] at the former Maldon Brewery passed through only 0.61 m of 'mottled clay' and into 1.22 m of 'green loamy sand' before terminating. Wells at Hill Farm, Tolleshunt Major around [TL 898 111] proved 'mottled clay', with a thickness given variously as 6.1 and 9.75 m in two adjacent wells, overlying some 6.1 m 'green sandy clay' and 'green clay and sand'.

In the Tollesbury area a well [TL 9569 1074] proved 10.67 m of dappled (1.22 m), yellow (1.83 m) and mottled (7.62 m) clay beneath the Oldhaven Beds, and these overlay 3.04 m of water-bearing green sand of Woolwich facies.

In the vicinity of Hatfield Peverel three wells are consistent in showing a thinning of the Reading facies, although the total thickness of the Woolwich and Reading Beds varies from well to well; it is probable that in the Crab's Hill Farm well [TL 7892 1118] the thickness assigned to the Thanet Beds is excessive, and that of the Woolwich facies correspondingly reduced. The total thickness of the Lower London Tertiaries in all three wells is 25.6 to 30.0 m. In the Crab's Hill Farm well 0.61 m of 'clay' and an underlying 2.44 m of 'coloured sand' are assigned to the Reading facies. In the nearby well at Hatfield Place [TL 7850 1144] the Reading facies consists of 2.75 m of 'mottled clay', overlying 10.05 m of green sand of Woolwich facies. In the third well at Brakeys, Crix Lane [TL 7743 1025], some 1.5 km to the south-west of the previous two, 0.91 m of 'hard sandy clay' and an underlying 6.1 m recorded by the driller as 'Woolwich and Reading Beds' overlying 2.14 m of green sand and clay, are now regarded as together constituting the Reading facies, but the total thickness of 7.01 m may be excessive.

Near Wickham Bishops a well at Heathgate Farm [TL 8427 1247] appears to have passed through a reversed fault which duplicates part of the Lower London Tertiaries sequence. The Reading facies falls within the affected sequence, and is recorded as 10.51 m thick above, and 7.01 m thick below, the fault.

At Rivenhall End some 9.76 m of 'sandy' and 'mottled clay' and 'Woolwich and Reading Beds' which were encountered in a well [TL 8297 1754] are probably referable to the Reading facies; they rest on 4.27 m of 'Thanet Sand' and 'hard green sand' of Woolwich facies. The true Thanet Beds, 17.82 m thick, were proved below.

Just outside the district at Silver End [TL 806 194] 'mottled clay' (Reading facies) 10.97 m thick was recorded, overlying 0.91 m of 'green sand' here regarded as Woolwich facies.

A well [TL 9261 1774] at Layer Marney Hall, close to the margin of the district, passed through 9.6 m of strata referable to the Reading facies, resting on 4.88 m of green sands of the Woolwich facies. Just beyond the north-eastern corner of the district 9.75 m of mottled clay, overlying 3.66 m of grey brown and green sand of the Woolwich facies, was proved in a well at Peldon Lodge [TL 9885 1756] CRB

London Clay (Formation)

Oldhaven Beds (Member)

The only recent cored borehole in the Chelmsford district to prove Oldhaven Beds was the Stock Borehole. Here 7.62 + m of fine-grained, brownish grey, locally glauconitic and shelly, sands were proved at the base of the borehole. A calcareous siltstone at 177.72 to 177.92 m was initially considered as a possible correlative of the tuffaceous Harwich Cementstone. Mr R.K. Harrison who examined a thin section (E49749)^‡7  from this bed reports that resistate grains are patchily concentrated in a micritic calcite matrix, in a complex manner showing much disturbance and no relation to potential bedding. These grains average 0.05 mm, are moderately well sorted, show generally high orders of angularity and low sphericity, with marginal etching and replacement by the calcite cement. The principal components are clear, igneous quartz (commonly forming sharp splinters), K-feldspar (minor Na-feldspar) and conspicuous cherty silica. Minor components include yellow-brown isotropic particles, muscovite, glauconite, microframboidal pyrite, carbonaceous specks, and microfossils. Rock particles are represented by cherty silica, and while most of these are clear and colourless, others are turbid, grey-brown. Their origin is uncertain because, while they resemble the general run of cherts devitrified and igneous rocks may take on the same aspect, especially in small derived grains. Comparison with the Harwich tuff band (e.g. E49689) (Elliott, 1971) shows no similarities, for the tuff is composed of unmistakable devitrified shards, heavily chloritised microlitic lava, pumice and unaltered sodic plagioclase crystals. Thus while there is a possibility of a very minor contribution of devitrified acid igneous particles, and the highly splintery angular quartz may suggest an igneous origin, the evidence of volcanogenesis is inconclusive.

A restricted microfauna of ostracods and foraminifera characterise the Oldhaven Beds in the Stock Borehole. The most commonly recorded foraminifera are Glandulina sp. 1, G. sp. 2, Globulina inaequalis, Miliolinella subrotunda, Protelphidium sp. 3 Murray & Wright, and the ostracod Clithrocytheridea sp. 1,Cytheretta cf. scrobiculoplicata and Cytheridea unispinae. The assemblage is most common in the lower part of the unit and almost disappears towards the top.

The following macrofauna, which came principally from two horizons (a cementstone nodule at 177.72 m, and a shell bed at 180.05 to 180.16 m), has been identified in the Stock Borehole (the numbers in parentheses refer to the depths in the borehole): Dittosaria wetherelli (177.72 m), Tuccinum' concinnum (177.72 m), Cyrtodaria sp. (177.72 m, 178.82 m and 180.06 to 180.16 m), naticid indet. (178.09 m), pyramellid indet. (177.72 m), Turbonilla? (177.72 m), Arctica cf. planata (177.72 m), Ashtarotha sp. nov. ex gr. donacina, 180.12 to 180.16 m (very common)), A. sp. (juv.) (178.32, 180.75 to 180.82 m), Corbula sp. (179.65 m), Leionucula cf. proava (180.06 to 180.16 m), Mytilus sp. nov (180.06 to 180.16 m), Nemocardium sp. (180.75 to 180.82 m), Nuculana sp. (175.67 m, 177.72 m) and Thyasira? (177.72 m). Large pyritised diatoms (Coscinodiscus spp.) are common. Teeth of the shark Synodontaspis teretidens have also been found in the interval 180.06 to 180.16 m. Elements of this fauna (A. sp. nov.ex gr. donacina, Cyrtodaria sp., Nemocardium sp.) together with Ashtarotha elevata have been found in similar strata in the BGS Stanford-le-Hope Borehole on the adjacent Southend and Foulness (258/9) district (Lake and others, in prep.).

A number of boreholes across the centre of the Chelmsford district show thick sand between the top of the Reading Beds and the main clay of the London Clay. In some of these boreholes it is possible to divide the sands into an upper unit, as recognised at Stock, and a lower unit, consisting predominantly of grey sands, which have not been penetrated in recent cored borings in this district.

In the Chelmsford area a borehole [TL 7095 0635] at the Hall Street Waterworks proved 12.72 m of sand above the Reading Beds, of which the upper 8.75 m of dark sand and clay and shells may correspond to the unit at Stock. In an older well on this same site the Oldhaven Beds were recorded as only 6.7 m thick. Farther south at Great Baddow in a borehole at the waterworks [TL 7202 0560] the total sand thickness consisted of 15.40 m, predominantly of dark grey dead sand, locally pebbly and shelly. Close by in a well [TL 7321 0462] at the former Baddow Brewery, the uppermost 0.45 m of clayey sand overlying 1.07 m of pebbles of the uppermost Oldhaven Beds were originally classified as the Basement Bed of the London Clay (Whitaker and Thresh, 1916, p. 166); the underlying part of the Oldhaven Beds is 5.03 m thick.

Boreholes to the west of Maldon entered sands beneath the London Clay. Specimens from one of these [TL 8437 0677] were examined by Mr Hughes who reports that a sample at a depth of 76.20 m from the surface in a stiff dark grey sandy silt yielded Protelphidium sp. sp. 1 Murray & Wright and Globulina sp.which correlates with the Oldhaven Beds in the Stock Borehole. The silts and shelly sands which were examined between 77.11 and 81.68 m were barren of microfauna.

A disused well [TL 8483 0712] at the old Maldon Brewery proved a lower unit 3.35 m thick of loamy sands, beneath an upper unit, 4.27 m thick and probably equivalent to the Oldhaven Beds at Stock, of grey shelly sands which had a pebble bed at the base. To the south of Maldon a well [TL 8690 0282] passed through 0.15 m of sandstone and 2.74 m of 'brown clay and shell', which together are tentatively regarded as the upper Oldhaven Beds, above 2.95 m of sand which were shelly in the upper part. The 0.15 m 'sandstone' may equate with the 0.20 m calcareous siltstone at the Stock Borehole. CRB

In the Hatfield Peverel vicinity two wells [TL 7850 1144]; [TL 7891 1118] give similar figures of 5.18 m and 5.48 m respectively for an upper unit of sand, and shelly sandy clay, above 4.27 m and 3.05 m respectively of lower sand.In a third well [TL 7743 1025] to the southwest of Hatfield Peverel the upper part of the Oldhaven Beds consisted of 2.29 m of 'green sand' and 'green rock' above 3.96 m of grey sand, locally with pebbles.

At the Station Road Waterworks, Tollesbury [TL 9569 1074] it is possible that the 8.0 m of sandy clay and the overlying 0.23 m of 'sandstone' at the base of the London Clay is the upper part of the Oldhaven Beds. They overlie 3.96 m of green sand of undoubted Oldhaven Beds.

Towards the margin of the district the Oldhaven Beds in a borehole [TL 9261 1774] at Layer Marney Hall can be divided into an upper 4.11 m of dead sand, above 3.35 m of 'live sand' and pebbles.

London Clay (Member)

Lithological Unit B

(153.11 to 173.20 m in the Stock Borehole) contains sands, up to 1.50 m in thickness at a number of horizons. The base of the Unit is taken at the base of the first thick clay bed, 1.54 m thick, above the base of the borehole. The top is taken at the top of a sand, 0.46 m thick, above which follows a thick (42 m) homogeneous unit of clays and silty clays. Unit B does not crop out within the district, but has a sub-drift outcrop along the Blackwater Valley, and possibly a restricted outcrop in the Chelmer Valley (Figure 10).

Unit B may be subdivided into the following palaeontological units:

Unit 2

(162.33 to 173.20 m in the Stock Borehole) In this unit pyritised diatoms (Coscinodiscus spp., including both concave and convex forms) are abundant, together with agglutinated foraminifera at some levels. Very rare small calcareous foraminifera have also been recorded.

Unit 3

(156.06 to 162.33 m in the Stock Borehole) This dominantly clay unit is recognised by the presence of a calcareous foraminiferal assemblage which contains a proportion of individuals of planktonic acarininids and subordinate globigerinids. The benthonic assemblage is dominated by the Cibicididae. Other species recorded most frequently include Anomalina grosserugosa, Pullenia quinqueloba, Pulsiphonina prima and the agglutinated species Spiroplectammina carinata s.l.

Unit 4

(153.11 to 156.06 m in the Stock Borehole) A rich fauna of agglutinated foraminifera; Haplophragmoides sp., Reophax sp.and Ammodiscus siliceus characterises this unit. Very rare calcareous foraminifera were noted. The lithology is of alternating sandy clays and clayey sands.

In a trial borehole near Maldon [TL 8437 0677], hereafter referred to as the Maldon Borehole, the interval 57.91 to 74.67 m yielded only diatoms and rare agglutinated foraminifera which can be correlated broadly with units 4 to 2 in the Stock Borehole.

Lithological Unit C

(111.68 to 153.11 m in the Stock Borehole) is an homogeneous unit of silty clays. The base is defined by the top of the sand at the top of unit B; the top is less well defined, but is taken in the borehole at the top of a 1.12 m clay unit before the incoming of predominantly clayey silts. This Unit crops out extensively over the lower ground in the Chelmsford area (Figure 10).

Five palaeontological subdivisions have been recognised within Lithological Unit C:

Unit 5

(148.84 to 153.11 m in the Stock Borehole) This Unit cannot be distinguished easily from Unit 4 on the basis of the microfaunal assemblages. King reports that, at other localities such as Aveley, diatoms are present in both units but more frequent in Unit 5. Such a differentiation is not apparent in the residues available from Stock. It would appear that in the Stock succession, calcareous foraminifera occur rather more frequently at the base of Unit 5 than in Unit 4. Species noted include Anomalinoi des acuta, A. ypresiensis, Cibicides cassivellauni buximargo, Epistominella cf. vitrea, Marginulina enbornensis. The calcareous assemblage again becomes sparse towards the top of Unit 5 where the agglutinated foraminiferal assemblage closely resembles that of Unit 4.

In the Maldon Borehole no microfauna was recovered between 45.72 and 57.18 m except at 50.29 m where a number of specimens of the agglutinated foraminifera Ammodiscus siliceus were obtained. A similar restricted microfauna has been recorded at 149 to 150 m (base of Unit 5) and at 154 m (top of Unit 4) in Stock.

Unit 6

(145.53 to 148.84 m in the Stock Borehole) is an homogeneous unit of silty clay whose base is recognised by the appearance of a rich and diverse calcareous microfaunal assemblage which continues, with some variation to the top of Unit 9. Planktonic foraminifera reappear in this unit. Cibicididae dominate the benthonic assemblage, the most frequently occurring species being Cibicides proprius. Other benthonic species present in some numbers, in addition to those already recorded in units 3 and 5, include Gavelinella cf. danica and Turrilina brevispira. Quinqueloculina sp.also occur rarely in this unit. Species occurring frequently in the coarser residues include Karreriella danica, Marginulina enbornensis and Clavulina anglica. Ostracods are rather rare. Ossicles of the crinoid Isselicrinus subbasaltiformis which typify Division 2 of Wrigley (1924) occur towards the top of this unit. In the Maldon Borehole the lowest level at which calcareous species occur is 44.19 m and the boundary between units 6 and 5 is placed tentatively between 44.19 and 45.72 m.

Unit 7

(141.23 to 145.53 m in the Stock Borehole) This unit of sandy and silty clays is indicated by a reduction in the proportion of calcareous foraminifera and the reappearance in some numbers of the agglutinated species present in units 4 and 5, together with pyritised diatoms. The proportion of planktonic specimens in the total assemblage shows a distinct reduction throughout Unit 7. It was not possible to differentiate between units 7 and 6 at Maldon.

Unit 8

(125.05 to 141.23 m in the Stock Borehole) This unit, which consists of homogeneous silty and sandy clays with phosphatic nodules, is the oldest unit of the London Clay recognised at the surface within the Chelmsford district. It is equivalent to the lower part of Wrigley's Division 3 (Figure 5).

The reappearance of the rich and diverse calcareous microfauna together with I. subbasaltiformis indicate the base of this unit. In the lower part the planktonic assemblage, in the main, consists of Acarinina aquiensis and A. esnaensis and is gradually superseded to top by Globigerina galavisi. The benthonic calcareous species are essentially the same as those listed for Unit 6, but variations occur in the dominance of the various species throughout the unit. King has recognised the following ostracod species in this unit; Bairdia sp., Brachycythere triangulare, Cytherella londinensis, Pterygocythereis fimbriata laminosa, Trachyleberidea aranea, T. prestwichiana and Trachyleberis spp.; in addition he identified the gastropod Turboella sp., the bivalve Thyasira goodhalli and the pteropod Spiratella mercinensis and, in the lower part of the unit, the brachiopod Terebratulina wardenensis.

In the Maldon Borehole the interval between 24.84 and 39.60 m is correlated with Unit 8, although I. subbasaltiformis, which is indicative of the base of the unit at Stock, has not been recovered. The presence of the basal beds of Unit 8 can be recognised at several localities on either side of the Blackwater Estuary from the work of George and Vincent (1976).

On Northey Island [TL 907 435] I. subbasaltiformis, Hoploparia gammaroides, Linuparus scyllariformis, Portunites incerta, Teredina sp.and fish teeth have been found (George and Vincent, 1976, p. 27; G.R. Ward, 1977, personal communication).

Some 2 km to the south-east [TL 895 050] the London Clay on the foreshore yielded I. subbasaltiformis, H. gammaroides and L. scyllariformis. Additionally a specimen of the nautilus Cimomia imperialis was found nearby (George and Vincent, 1976, p. 27). A similar fauna of H. gammaroides, L. scyllariformis, Xanthilites bowerbanki and I. subbasaltiformis is recorded from the foreshore at Mundon Stone [TL 907 047], 1.5 km to the east-south-east of the above occurrence, by George and Vincent (1976, p. 27). In addition to the above species the crustaceans Portunites incerta and Dromilites lamarckii, and teeth of the sharks Otodus obliquus, Striatolamia macrota, Palaeohypotodus rutoti, Synodontaspis hopei„ 'Lamna' sp.and Trichiurides sp.were found on the east side of Lawling Creek [TL 907 035] (George and Vincent, 1976, p. 27).

H. gammaroides has also been recorded by the above authors at Steeple [TL 917 044]. The presence of the base of Unit 8 at Ordnance Datum in this vicinity appears to indicate a thickening of the underlying units 2 to 7. In the Stock Borehole these units have a combined thickness of about 32 m; at the Maldon Borehole the corresponding figure is 35.6 m. A well [TL 8983 0337] at Brickhouse Farm, which commenced at 5.2 m OD, proved 47.55 m of the London Clay above the Oldhaven Beds. By comparison with Maldon the base of Unit 8 would be expected to occur at about −7 m OD at Brickhouse Farm. A slight northward rise in base level towards the site where the fauna was found, can be expected across an anticlinal high which has been recognised in the vicinity of Osea Island (Figure 13).

On Osea Island the presence of the base of Unit 8 is indicated by I. subbasaltiformis, H. gammaroides, L. scyllariformis, Teredina sp.,gastropods, scaphopods, and fish teeth (George and Vincent, 1976, p. 27; G.R. Ward, 1977, personal communication), which were found on the foreshore [TL 913 058] 500 m W of the pier. H. gammaroides was also found at West Point [TL 903 065] by Mr G.R. Ward. The thickening of units 2 to 7 relative to Stock and Maldon, which has been suggested to occur in this general area, is confirmed by a well [TL 9147 0602] that commenced at about 5 m OD and proved 45.11 m of London Clay beneath 3.05 m of drift. The base of Unit 2 lies at about −42.5 m OD, and by comparison with Maldon, the base of Unit 8 would be expected to be about −2 m OD.

At Stansgate Abbey [TL 930 057] on the opposite side of the river were found: L. scyllariformis, H. gammaroides, I. subbasaltiformis and a nautiloid 'probably Cimomia imperialis' (George and Vincent, 1976, p. 28).

The final site in the Blackwater Estuary which yielded crustacean remains to the above authors was at The Stone [TL 953 061], on the north side of Ramsey Island, where H. gammaroides and L. scyllariformis were encountered.

The junction of units 9 and 8 was identified in three boreholes around [TL 708 068] in the Chelmsford Central Redevelopment area. Mr M. J. Hughes reports that the boundary lay at depths of between 15.39 to 16.45 m, 12.34 to 15.24 m and 13.56 to 16.15 m in the three boreholes. Correlation between the boreholes shows that the junction lies at about 11 m OD.

Three boreholes [TL 7839 0931]; [TL 7647 0790]; [TL 7908 0903] in the vicinity of Little Baddow proved the presence of Unit 8 immediately beneath Glacial Sand and Gravel. The unit was identified by the frequency of planktonic species together with Anomalina grosserugosa and the absence of Osangularia plummerae.

Unit 9

(111.68 to 125.05 m in the Stock Borehole). This unit of silty clays and clayey silts, which forms the base of King's Division C, correlates with the upper part of Wrigley's Division 3. The base of this unit is recognised on two criteria. King uses the appearance of Osangularia sp.to indicate the base of Unit 9; detailed work on the finer residues of these London Clay samples has shown that O. plummerae occurs briefly and in low numbers at a discrete level some way below the top of Unit 8 and again at the top of Unit 8 as recognised by King. A sudden increase in the percentage of O. plummerae to around 10 per cent occurs at the base of Unit 9. This pattern in the appearance of O. plummerae in Unit 8 and in Unit 9 has also been recognised in. the Maldon Borehole. At the base of Unit 9 at Stock, corresponding to the level at which O. plummerae becomes significant, the proportion of planktonic specimens, in the main G. galavisi, exceeds half the total foraminiferal population. This pattern is again present in the Maldon Borehole. The remarks concerning the benthonic calcareous microfauna in Unit 8 also apply to Unit 9. The top of Unit 9 is indicated by the presence of a stratigraphically restricted occurrence of Stilostomella palaeocenica and S. spinulosa which has also been noted in the Mahlon Borehole. King recognised the top of Unit 9 in the coarser residues by the presence of Gaudryina hiltermanni.

At Maldon the interval between 10.66 and 22.86 m is assigned to Unit 9. The presence of Unit 9 beneath the drift at the Chelmsford Central Redevelopment site has been noted above under Unit 8.

Unit 9 has also been recognised by the abundance of planktonic species together with O. plummerae under the Glacial Sand and Gravel in four boreholes [TL 7500 0961]; [TL 7355 0918]; [TL 7600 0960]; [TL 7434 0994] in the vicinity of Boreham. The brachiopod Terebratulina wardenensis was common between about 114 and 122 m at the Stock Borehole.

Lithological Unit D (92.85 to 111.68 m in Stock Borehole)

This is an homogeneous unit of silty clays and clayey silts and corresponds to the upper part of Division C of King. It crops out over the slopes below the Claygate Beds in the southern part of the district, on the higher ground of the Danbury–Tiptree Ridge, and beneath the drift in the north-west of the district. The base of the overlying Lithological Unit E is defined at the base of a 4.85-m sequence of glauconitic beds. Glauconite was also evident in the washing residues at the base of Palaeontological Unit 11. These two glauconitic beds are taken by King to mark the upper and lower limits respectively of his Unit 11. This Lithological Unit includes Palaeontological Unit 10 and most of Unit 11.

Unit 10

(100.91 to 111.68 m in Stock Borehole) There is a marked reduction in the proportion of planktonic specimens in the specimens from this borehole, but this group continues to be present throughout the unit. O. plummerae continues into Unit 10 but disappears immediately above the base, as recognised by King. The reduction in the proportion of the planktonic specimens occurs earlier than the disappearance of O. plummerae. The benthonic microfauna shows a gradual reduction in diversity from bottom to top of the unit. A tooth of Squalus minor was found between 105.66 and 106.43 m.

Unit 10 is probably represented in the decalcified strata between 2.4 and 9.75 m in the Maldon Borehole, as this is at the same height above the base of Unit 1 as Unit 10 in the Stock Borehole.

The specimens of Nautilus centralis, Xanthopsis leachi and Otodus obliquus exhibited by Mr Fitch at meetings of the Essex Field Club (1887; 1888) from the railway cutting at Maldon [TL 8420 0635] probably came from low down in Unit 10.

Unit 11

(88.15 to 100.91 m in the Stock Borehole) Abundant glauconite occurs at the base of this unit and no microfauna was recovered from these samples. Above this a fairly rich calcareous microfauna, representing a restricted number of species, was recovered from the finer residues. The assemblage is dominated by the Cibicididae and no planktonic specimens are reported. King reports only Ammodiscus sp.and Glomospira sp.from the coarser fraction in this interval. The top of the unit is marked by a further glauconitic level barren of any microfauna which has been taken as the base of lithological Unit E.

Challis (1887, p. 16) noted that the London Clay in the brickpit [TL 658 054] at Widford abounded in selenite. He also recorded the presence of sharks' teeth, fish vertebrae, crustacea, a large Nautilus, two species of Fusus, and stems and plants bored by Teredo. The pit probably lies within lithological Unit D.

Lithological Unit E (68.41 to 92.85 m in the Stock Borehole)

This extends upwards from the base of a 4.85-m thick glauconitic sequence at 92.85 m. The higher part of the unit is characterised by silty clays and fine-grained sandy silts. This lithological unit includes the top part of Palaeontological Unit 11, Unit 12 and the lower half of Unit 13.

Unit 12

(76.37 to 88.15 m in the Stock Borehole) This dominantly silt unit contains a calcareous microfaunal assemblage, rather more diverse than Unit 11, and similar in many respects to that of units 8 to 10 but without the planktonic element. King reports that the ostracod Echinocythereis reticulatissima, which appears for the first time in this unit, is characteristic of units 12 to 14. The brachiopod T. wardenensis is characteristic of, though not restricted to, this level.

Samples taken at depths of 10.3 to 10.5 m and 14.9 to 15.1 m in a borehole [TL 9253 0031] at Bovills Uplands on the southern border of the district yielded: Cibicides ex gr. ungerianus, C. westi, Lenticulina sp., Marginulina enbornensis, Ditrupa sp.,and T. wardenensis. The fauna is assigned to Unit 12, although on stratigraphical evidence a position high in Unit 13, or even within Unit 14 would be more probable. King's Unit D commences with Palaeontological Unit 12.

The lower part of Palaeontological Unit 13 falls within Lithological Unit E. It is without the sands which characterise the upper part. It was not possible to separate Unit 13 from 14 on the basis of the microfauna (see below). King notes the appearances of Spiratella tutelina at about 73 m, which persists throughout the rest of the succession. The teeth of Squalus minor and Isurolamna affinis have been recovered at depths of 72.56 to 73.33 m and 70.38 to 71.08 m respectively.

Lithological Unit F (46.90 to 68.41 m in the Stock Borehole)

Sand is common in this Unit in the borehole. It generally occurs in thin beds up to 5 mm thick, but one bed 2.03 m thick was noted between 56.29 and 58.32 m. The base of the unit is taken at the base of a 4.44 m thick silt containing large (0.5 mm + ) foraminifera. The units include the upper part of Palaeontological Unit 13 and Unit 14.

Units 13 and 14

(46.90 to 76.37 m the Stock Borehole) King divides these units on the basis of lithology; at present they are indistinguishable on the basis of the calcareous microfauna present throughout both units. The composition of the assemblage recovered from the finer residues, varies from one similar to that of Unit 12 to one very restricted in numbers of individuals and species, apparently in response to sediment type. King reports that the ostracod fauna consists almost exclusively of Cytheridea primitia and E. reticulatissima. The foraminiferal assemblages of units 13 and 14 also occur sporadically, according to sediment type, in the beds regarded by King as Claygate Beds.

Lithologically the upper part of Unit 13, which falls in Lithological Unit F, has many sandy beds and horizons and is readily separable from the lower part within Lithological Unit E. The presence of sand in Unit 13 also serves to distinguish this Unit from the silts and silty clays of Unit 14.

King records the following fauna from the upper part of Unit 13; Adeorbis lucidus, Spiratella tutelina, Corbula globosa and Striarca wrigleyi. Otoliths are common at several horizons and one, at a depth of 55.77 m is the holotype of Stolephorus productus Stinton (1977). The occurrence of teeth of Triakis wardi at depths of 53.95 to 54.61 and 59.92 to 60.62 m is of interest, as the only other known specimens, including the holotype, came from the river cliff [TQ 921 968] at Burnham-on-Crouch, on the adjacent Southend and Foulness (258 and 259) district (Capetta, 1976). This locality is regarded by the author (in Lake and others, in prep.) as also falling within Unit 13. Specimens of Ophioglypha wetherelli and the pteropod Valvatina sp.in cementstones from Cock Clarks, near Purleigh (Davis and Elliott, 1951, pp. 336–337) can be referred to a position high in Unit 13.

Unit 14 extends from 46.90 to 53.03 m at Stock where it comprises silts, clayey silts and silty clays. The molluscan fauna from this Unit includes the following: Adeorbis lucidus, Chevallieria?, Euspira glaucinoides, Daphnobela sp. nov., Eotibia sp. nov., Fusiturris aff. wetherelli, Scaphandridae gen?. nov., Seila mundula, Turboella cf. misera, Spiratella mercinensis and S. tutelina together with Abra splendens, Calpitaria sulcataria, Paranomia? scabrosa and otoliths of Gnathophis circularis and Pagurus bognoriensis.

A borehole [TQ 9181 9891] on the extreme southern border of the district at Althorne yielded the following collective fauna on samples taken at depths of 9.0 to 9.1 m, 11.7 to 11.9 m, 14.9 to 15.0 m and 17.45 to 17.6 m; Marginulina enbornensis, Nodosaria latejugata, Adeorbis cf. lucidus, Athleta nodosus, Bonellitia sp., Bullinella consors, Eotibia lucida, Ptychatractus interruptus, Ringicula turgida, Stellaxis pulcher, Spiratella tutelina, Paranomia? scabrosa, Striarca wrigleyi, Cytheridea primitia, Echinocythereis reticulatissima, Loxoconcha sp., Paijenborchella sp.,and Trachyleberis sp.The fauna is placed within Unit 14. The 8.6 m of strata in which it occurs are thicker than the corresponding unit at Stock (6.13 m). MJH, CRB

Claygate Beds (Formation)

The base of the Claygate Beds is generally marked by a spring-line in the valley north of West Hanningfield and south-west of Galleywood. Elsewhere in the Great Baddow-West Hanningfield area it is obscured by solifluxion deposits and by Boulder Clay.

In the Galleywood area, the upper part of the Claygate Beds tends to be silty and a weak spring-line lies below it, as for example south-west of Wood Farm [TL 6985 0200]. West of Gay Bowers Farm a spring [TL 7133 0171] emerges at the base of a sandy bed, some 6.0 to 7.5 m from the top of the formation, which may equate with the basal sand of the Upper Division in the Stock Borehole (Figure 11). Excavation for a gas pipeline near here proved up to 0.91 m of rusty brown fine-grained sand overlying up to 1.07 m of brownish grey silty clay. Locally the sand contained clay laminae. At a slightly higher horizon a band of sand yields weak springs [TL 7400 0016] near Linkhouse Farm. RDL

The Claygate Beds to the west of West Hanningfield weather to a yellow or orange-brown clayey silt at the surface. Locally springs mark the base; there is one [TQ 7235 9979] north-east of Slough House Farm and another [TQ 7383 9969] south-south-west of Linkhouse Farm where there was formerly a spring-fed well, associated with 1.5 m of fine-grained yellowish brown sand.

A well [TL 7698 0132] at the Rectory, East Hanningfield, penetrated 9.14 m of 'fine light-brown imperfect marl', above the London Clay. There are a few metres of Head at this point, but the rest of the 'imperfect marl' is now assigned to the Claygate Beds. A good feature marks the western boundary of the Claygate Beds, but is only locally accompanied by spring-lines or seepages. One such spring [TL 7570 0141] west of Claydons Farm, is associated with a small landslip. Other spring-lines have been noted to the east of Butt's Green [TL 7640 0274] and [TL 7695 0304] and at several points around a small outlier [TL 781 034] to the north-west of Bicnacre.

The Claygate Beds weather to a yellowish brown silty clay over much of this outcrop. Fine-grained yellow silty sand has been thrown out of a badger set [TL 7571 0113] from an horizon 7.5 to 9.0 m above the base of the formation, i.e. within the Middle Division.

Near Woodham Ferrers fine-grained, brown sand up to 1.5 m in thickness was found at several points [TL 7986 0071]; [TL 7989 0062]; [TL 8045 0057]; [TL 8014 0054]; [TL 8044 0057] in a ditch to the east of Wantz Farm. It is not known whether these occurrences are all at the same horizon, but they occur in the Middle Division of the formation.

There are numerous landslips associated with the Claygate Beds — particularly with the basal beds — to the west, south and east of Edwin's Hall [TQ 811 994] (see p.85).

Small selenite crystals have occasionally been encountered when augering e.g. at [TQ 8147 9990] and [TL 8161 0020]. Larger crystals, 100 mm or greater across, were found in the excavation for the water tower [TL 8364 0058] 300 m SE of Great Whitmans Farm. Small slips [TL 8370 0033] and [TL 839 011] in this vicinity and also on the outlier to the west of Purleigh [TL 8320 0222], and the small outlier at Purleigh [TL 838 023] are associated with the base of the formation. Springs were noted at the base some 500 m E of Great Whitmans around [TL 8397 0081]. Some 0.6m of fine sand was angered on the very small outlier [TL 8430 0005] 300 m SW of Beacon Hill.

An IGS borehole [TL 8353 0085] at Great Whitmans Farm, Cold Norton, proved 11.5 + m of Claygate Beds. The lower 4 m of the borehole were fossiliferous and the following species have been identified: Lingula sp.and Spiratella cf. tutelina between 8.60 m and 9.30 m; and Cibicides sp., Lenticulina sp., Praeglobobulimina sp., Ringicula sp., Camptoceratops prisca, Cytheridea sp. nov.,Loxoconcha sp.and Paijenborchella sp.between 9.30 and 9.70 m. The fauna is referable to Unit 16 on the abundance of C. prisca.

The western boundary of the Claygate Beds of the Mayland–Althorne outlier is fairly well defined by a feature, but the eastern boundary is defined neither by feature nor lithology. The surface indications of this formation are of silty brown, occasionally yellowish, clay. A well [TQ 8923 9878] at Tyle Hall proved 10.06 m of Claygate Beds. CRB

Bagshot Beds (Formation)

Bagshot Sands (Member)

The Bagshot Beds attain their fullest development within the district in the Stock outlier. The succession proved in the Stock Borehole (see (Figure 12)) confirm in general terms that recorded by Woodward (in Whitaker, 1889, p. 278).

Springs issue from the base of the Bagshot Sands at numerous localities and at one time were the only source of water supply for the village (Woodward in Whitaker, 1889, p. 278). The most notable within the district are the lines of springs and seepages in Scrub Wood [TQ 6905 9950], which feed a small tributary of the River Wid. Extensive downwashing by the issuing water completely obscures the underlying Claygate Beds on the eastern side of the valley. A small landslip [TQ 6905 9970] involving the uppermost Claygate Beds is associated with this spring-line to the north of Scrub Wood.

Clay and silt are not important within the Bagshot Sands, although they have been noted in the Stock Borehole and proved locally in section and by augering. C R B

Excavations for a gas pipeline trench [TL 7000 0060] to [TL 7031 0096] north-west of Forest Lodge showed up to 1.8 m of fine-grained brown sand with scattered clay laminae; the surface of the sand was locally pocketed with soliflucted material.

Disused pits near Temple Farm show similar sections in clayey sands. One such section [TL 7093 0100] reads:

At another [TL 7103 0095] — the following section was noted:

The above section showed evidence of cryoturbation. Elsewhere in the pit local iron-cementation of the sands was noted.

At Temple Farm a trial borehole, the exact site of which is unknown, showed in descending sequence: loamy sand, 2.29 m; dark brown clay, 1.07 m, resting on loamy sand 3.51 m; total depth 6.87m. RDL

A temporary section [TQ 6968 9914] 1450 m SW of Foxborough Farm, showed 0.91 m of mottled orange and grey silty clay overlying 1.83 + m of fine-grained buff sand.

The Farrow's Farm Borehole [TQ 7020 9988], sited 700 m SSW of the Stock Borehole, proved 6.0 + m of sands beneath a thick clay which is thought to be the Stock Clay. The overall stratigraphical sequence is similar to that recorded in the Stock Borehole (Figure 12), and to that in a nearby well [TQ 6917 9909]. From a surface level of 95.7 m OD this latter proved:

Bagshot Sands are mapped over the southern part of Galleywood Common. Here they appear to be more loamy and have been mistaken in the past for Glacial Drift, London Clay and Claygate Beds (see Whitaker, 1889. p. 278). These loamy sands (Brickearth of Woodward) have been worked for brick clays in the past. Mr. J. S. Turner (MS, BGS) noted 3.05 m of yellow clayey sand and clay in an old brickyard [TL 704 024], and auger evidence in this vicinity suggests that the beds are predominantly of clayey sand grade. RDL

A hitherto unknown outlier of Bagshot Sands, partially. hidden under Boulder Clay, has been mapped around Woodham Ferrers [TQ 798 996]. The base is locally well marked by a spring-line around [TQ 801 999]. There are no exposures, but fine-grained sand can be augered in the fields [TQ 7995 9965] on the opposite side of the valley. Prior to the removal of the hedges on the eastern side of the valley a badger set was established [TQ 8032 9984] in the easily worked Bagshot Sands, close to a depression thought to mark the site of an old pit opened in the sand. CRB

Stock Clay (Member)

It has not been possible at outcrop to separate the Stock Clay from the underlying sands within the district. The 10.08 m of clays and silt noted in the Stock Borehole probably represent the maximum development of this facies on the Stock outlier. In the Farrow's Farm Borehole a 2.8-m silt and clay unit may be the basal bed of the Stock Clay, but there is considerably more sand in the upper part of the sequence than in the Stock Borehole. To the south, in the adjacent Shoeburyness district, around the disused brick pits mentioned below, the 'Brickearth' has a wider outcrop, and a tentative line has been put around it on the six-inch map. The base of the 'Brickearth' is here low down in the Bagshot sequence, and probably lies within 3.0 m of the base of the formation. Whitaker (1889, p. 278) noted 'Laminated brickearth, resting obliquely on the (sand) bed below, but does not give a thickness for the strata.

A flooded pit [TL 7103 0104] at Temple Farm revealed 1.22 m of 'Brickearth' above the water line.

The brick pits [TQ 7045 9845] just beyond the southern border of the Sheet exposed a section, recorded by Mr H.G. Dines in 1923, of 1.52 to 1.83 m of laminated sand and clay. The lowest clay seam was about 100 mm thick and was of a lilac colour; the clays above were of grey and blue colour and were likened to the 'Claygate' facies. Several small faults, one known to have a displacement of 1.83 m, have been recorded in this pit.

Bagshot Pebble Bed (Member)

In the Stock Borehole it is probable that the uppermost 1.60 m, together with the underlying 1.50 m (not recovered), were not undisturbed Bagshot Pebble Bed. The various exposures of pebble beds within this member also demonstrate that some of the upper beds include gravel disturbed by cryoturbation and solifluxion. However, the practical difficulties of separating disturbed Bagshot Pebble Bed from in situ deposits has necessitated their being mapped together on this outlier. Elsewhere, for example at Norsey Wood [TQ 690 957] in the Southend district, where there is evidence of movement involving the Pebble Bed, such occurrences are regarded as Head.

The 1.12 m of coarse sharp flint grit with occasional rounded flints at the base of the pebble bed sequence in the Stock Borehole is thought to have arisen by frost shattering of the included pebbles, although drilling may account for some fragmentation.

Some 3.66 m of 'pebbly clay and gritty ballast' have also been recorded in an unlocated borehole at Temple Farm.

The Pebble Bed has been worked in the past at several localities, but no sections now remain. Wood (1868, p. 464) recorded pebbles in the form of bands embedded in laminated clay by the roadside east of the Ship Inn, probably from a now-flooded pit [TL 7065 0055].

In 1923 H.G. Dines saw two sections on the east side of the Stock Road. The first pit [TL 7031 0002] exposed 2.44 m of gravel in a coarse sandy matrix showing cross-bedding and cryoturbation. The latter phenomenon was indicated by pebbles which were preferentially vertically orientated, and by pockets full of vertically aligned smaller pebbles (Plate 2.). The second pit [TQ 7029 9980] exposed 1.52 m of gravel, and again some of the pebbles were vertical. Dines regarded the gravel in both of these pits as either 'Disturbed Bagshot Beds or Warley Gravel'.

An 'Old Gravel Pit' [TQ 6970 9965] shown on the six-inch maps is about 3 m deep, but no sections are now visible. Mr F.H. Edmunds noted in 1923 that the pebbles, which were mostly flint but included some Bunter pebbles and vein quartz, were vertically arranged. Another gravel pit [TQ 6984 9985] 250 m NE appears to have been worked to a depth of 1.8 m. CRI1

The relationship of the Pebble Bed to the underlying sand is not clear in the Galleywood outlier. Bristow (in Whitaker, 1889, p. 279) stated that 'the gravel is somewhat ferruginous and interstratified with sand', which tends to imply an in situ deposit, but Woodward (in Whitaker, 1889, p. 279) referred to pebbles with their long axes vertical. On the north-eastern side of this outlier the pebble bed appears to rest directly on the Claygate Beds, indicating at least local solifluxion, but there is little field evidence for the true relationship of the Bagshot Pebble Bed to the underlying members of the Bagshot Beds over the rest of this area.

The Pebble Bed has been worked for gravel on the north part of the Common around [TL 703 029]. RDL

Chapter 4 Structure

There is insufficient data to draw an accurate structure contour map of the top of the Chalk. There are more details pertaining to the base of the London Clay and, although the base of this formation is rarely well defined in drillers' logs, a sufficiently reliable overall picture, that incorporates information extrapolated from the base of the Claygate and Bagshot beds, can be produced to give a broad picture of the structure of this formation (Figure 13).

The overall structure is one of gentle flexuring. In general the base of the Tertiary rocks is deepest in the south-east of the district on the northern limb of a gentle syncline. Along the axis of this syncline, which is approximately coincident with the River Crouch, the base descends to below −140 m OD.

North-west of this fold, and south of the Galleyend Fault (see below), there is an area where the base lies at about 80 m OD, being somewhat lower in its eastern part (about −83 m OD) than in the west (about −70 to −79 m OD). To the east of this area and south of the Galleyend Fault a minor ridge can be detected in the Osea Island vicinity with the base of the London Clay rising along it to above −40 m OD.

Three inferred faults have been drawn on (Figure 13). The most northerly fault, here named the Witham Fault, may have determined the course of the Blackwater Valley between Kelvedon and Witham, and possibly part of the course of the River Chelmer (see (Figure 20)). The evidence for it comes from four boreholes in Witham. The top of the Chalk on the northern, upthrow, side in two of the boreholes [TL 8212 1529] and [TL 8229 1527] is at −30.5 and −26.2 m OD respectively. On the south side of the fault the top of the Chalk is recorded at −64.3 m OD in the Witham Borehole [TL 8244 1534] and at −66.14 m OD in a well at Highfields Road [TL 8146 1466]. The presence of a second fault believed to trend south-westwards, is inferred largely from the sequence recorded in the Heathgate Farm Well [TL 8427 1247], Wickham Bishops (Whitaker and Thresh, 1916, pp. 311–312). It is here named the Wickham Bishops Fault, and is believed to be a reverse fault, for in the above well the basal beds of the London Clay and the uppermost beds of the Woolwich and Reading Beds appear to be repeated by a fault recorded at a depth of 100.4 m (−29.1 m OD). The displacement of the beds is 26.8 m. A well at Messrs Thorne and Livermore's Brewery, Tiptree Heath [TL 8913 1654] was thought to have proved an abnormally thick sequence of the Lower London Tertiaries (39.95 m) (Whitaker and Thresh, 1916, p. 281).These authors suggested that the strata were affected by the Wickham Bishop 'disturbance', but a reinterpretation of the log does not indicate unusually thick Lower London Tertiaries or Oldhaven Beds.

The existence of an easterly trending fault farther south, here named the Galleyend Fault, is more conjectural. In the west of the district the base of the Claygate Beds falls northwards from about 53 m OD in the East Hanningfield-Cold Norton area to about 38 m in the Woodham Lodge [TL 828 043] — Overshot Bridge [TL 781 035] — Levett's Farm [TL 766 038] area. Some 1.3 km to the north, Danbury Hill rises to over 107 m OD but does not reach the base of the Claygate Beds. In such gently folded country a fault seems the most reasonable explanation of the differences in level. Near Maldon two boreholes suggest the eastward continuation of the fault. The logs of these two wells should, however, be treated with caution as they are known only in very general terms (Whitaker and Thresh, 1916, p. 219). Nevertheless, it would appear that the base of the London Clay at South House about [TL 864 055] and Northey Island about [TL 877 061] lies at −73.5 m and −71.6 m OD respectively, whereas immediately to the north of the inferred fault the base lies between −35 m and −40 m OD.

Evidence of continuing movement along some of these faults is shown by the Colchester earthquake of 1884, the isoseismal lines of which were south-westerly aligned, parallel to the Wickham Bishops Fault. This was the most destructive earthquake ever recorded in Britain. A great amount of local detail about it was assembled by Meldola and White (1885). Much of this information was incorporated by Davison (1924) in a book on the British earthquakes, and by Haining (1976) in a more popular treatment of the subject.

The Witham Fault may be replaced south-westwards by an anticline. On the north side of the Witham Fault there appears to be a major syncline, trending along an east-northeast axis which passes through Terling. Northwards from this, the base of the London Clay gradually rises to about Ordnance Datum at the northern margin of the district. CRB

Chapter 5 Pleistocene: glacial and periglacial deposits

Introduction

Glacial and periglacial deposits are widely distributed, especially in the north-west of the district. The Pleistocene deposits consist principally of a lower unit of sand and gravel, overlain by chalky Boulder Clay that is now assigned to the Anglian glaciation. In places the sand and gravel is underlain by a lower chalky Boulder Clay lithologically identical to the upper. Locally Glacial Lake Deposits are associated with deeply excavated channels: they formed near the ice margin either during, or towards the end of, the glaciation which produced the upper till. This entire suite is pre-Hoxnian in age.

Capping certain areas of the plateau are spreads of Brickearth of loessic type which are thought to have formed under extreme periglacial conditions towards the close of the Anglian glaciation. Elsewhere spreads of Brickearth which mantle the valley sides, many examples being preferentially developed on the west sides of the valleys, also probably formed under loessic conditions. However, other deposits of brickearth in similar topographical positions appear to have been derived by solifluxion from the Brickearth of the plateau. Because of the difficulty in separating the latter two categories all the occurrences in the valleys have been designated Head Brickearth. Less homogeneous deposits derived from a variety of source rocks also mantle the valley sides and have been mapped as Head. Locally the Head and Head Brickearth are distinguishable only with difficulty and they are obviously genetically related.

Because there is an apparent gradation from Brickearth to Head it is convenient to group these deposits together; it is also convenient to describe them with the glacial deposits in this chapter. It should be noted, however, that some deposits of Head overlie both Hoxnian and Ipswichian interglacial deposits and are, therefore, much younger than the glacial deposits. It has not been established whether there has been more than one phase of solifluxion within the district.

History of research and nomenclature

Wood (1867a) divided the glacial drift of East Anglia into three formations: the Upper Drift, Middle Drift and Lower Drift, a sequence typified by the cliff section at Corton, where sands lie between two boulder clays. Of these divisions only the upper two are directly relevant to the Chelmsford district (Figure 14). The name of the upper unit was subsequently modified to Upper Glacial Clay (Wood, 1867b), Chalky Clay (Wood, 1880) and Chalky Boulder Clay (Harmer, 1902), and that of the middle unit changed to Middle Glacial Beds (Wood, 1867b) and Middle Glacial sand and gravel (Wood and Harmer, 1868, p. 456).

Prestwich (1890, pp. 132–135) included most of the pre- Boulder Clay gravels within his Westleton Beds, but was careful to distinguish a sand and gravel, which overlay the Westleton Beds and underlay the Boulder Clay, as 'glacial'. Prestwich regarded gravels at Danbury as quite distinct possibly a 'Bagshot outlier invaded by Glacial gravel', whilst Gregory (1915; 1922) argued that these Danbury Gravels were of Miocene age. Gregory (1922) divided his 'Mid-Essex Gravels', which included Boulder Clay and both pre- and post-glacial strata, into seven units. He thought that his Danbury and Brain Valley Gravels were most nearly related to the Essex Westleton Beds of Prestwich, but disliked the correlation with the marine Westleton Beds of Suffolk. His Fluvio-glacial gravels are by implication (Gregory, 1922, p. 31) the equivalent of the glacial sand and gravel of Prestwich, plus possibly some of the Westleton Beds. Solomon (1935) introduced the poorly defined term Westleton Series. The name embraced a heterogeneous series of deposits; in the Chelmsford district all the pre-chalky Boulder Clay gravels were assigned to the middle of his Westleton Series.

The chalky Boulder Clay was subdivided by Baden-Powell (1948) on the basis of the nature of the erratics and colour, into the Lowestoft Till (Lower Chalky Boulder Clay) and the Gipping Till (Upper Chalky Boulder Clay), believed to be the product of two distinct glaciations that were separated by the Hoxnian Interglacial. This subdivision was supported by West and Donner (1956), who claimed that the orientation of the long axes of erratic pebbles was different within the two tills.

Clayton (1957a, b) proposed a five-fold division for the drift deposits of the Chelmsford district and introduced the following names for the ascending sequence which he recognised: Hanningfield Till, Danbury Gravels, [Hoxnian Interglacial – no deposits identified], Maldon Till, Chelmsford Gravels and Springfield Till. The three tills he later (1960) correlated respectively with the Cromer Till and Norwich Brickearth, the Lowestoft Till, and the Gipping Till.

Turner (1968; 1970) has demonstrated that at Rivenhall End the Springfield Till is overlain by Hoxnian Interglacial deposits, and, therefore, is the equivalent of the Lowestoft Till of Baden-Powell (1948).

Bristow and Cox (1973) demonstrated that there is only one widespread chalky Boulder Clay in East Anglia, and that this is pre-Hoxnian in age (i.e. equivalent to the Lowestoft Till). Nowhere in East Anglia south of Norwich could a post-Hoxnian till be recognised and they concluded that the Gipping Till of Baden-Powell is only a lithological variant of the Lowestoft Till and that no significant chronology could be based on it. The Hanningfield Till was regarded by them as scattered outliers of the Springfield Till, and the Maldon Till as a deposit of an earlier advance of the Anglian ice sheet.

Hey (1967) restricted the use of the term Westleton Beds to deposits in north-east Suffolk and south-east Norfolk. No name was given at this time to replace Prestwich's term 'Westleton Beds' in Essex.

It has been suggested recently that the Chelmsford Gravels are hybrid and composed of a thick lower unit of fluviatile sands and gravels – the Kesgrave Sands and Gravels ( = Westleton Beds of Prestwich; Essex White Ballast) separated by a reddened clay-enriched layer and associated deposits from a thin upper unit of outwash sand and gravels – the Barham Sands and Gravels ( = Glacial deposits of Prestwich) (Rose and others, 1976; Rose and Allen, 1977). Hey (1980) has since divided the Kesgrave Sands and Gravels into a high-level (Westland Green Member) and a low-level (unnamed member) quartz-rich gravel, with a tentative boundary between the two passing just north of the Chelmsford district. The Kesgrave Sands and Gravels of Chelmsford thus fall within his lower gravels, with the possible exception of outliers of the Westland Green Gravels on Danbury and Tiptree hills. The Westland Green Gravels are regarded as of pre-Pastonian a age; the lower unit of Beestonian age; the rubified sol lessive is assigned to the Cromerian Interglacial, and the Barham Sands and Gravels to the Anglian Glaciation. This chronological interpretation has been questioned by Lake, Ellison and Moorlock (1977), Lake (1977), Bristow (1977) and Read (1977).

However, without good palaeontological control, radiometric age determinations or clear stratigraphical sections the proposed chronologies and the exact stratigraphic sequence must, as yet, remain conjectural.

Within the present district the deposits portrayed as Glacial Sand and Gravel include the undivided Chelmsford Gravels, the Danbury Gravels, and gravels of uncertain origin in the Little Totham, Tolleshunt and Tollesbury area. The Boulder Clay of the present map includes both the Maldon and Springfield Till; the latter including all the deposits previously regarded as Hanningfield Till (Clayton 1957a; 1960).

Stratigraphy and lithology

Maldon Till

The existence of a boulder clay below the Chelmsford Gravels has long been in question. Clayton (1957a), after surveying a large area of Essex around Chelmsford and Harlow New Town, was satisfied that such a till existed and proposed the name Maldon Till' for it. He gave a summary of the evidence for its presence in the Chelmsford area, cited the Maldon railway cutting [TL 842 065], described by Whitaker (1889, pp. 316–317), as the type locality, and quoted some supporting evidence (Turner, 1937, p. 160; MS 6-inch maps, BGS). The deposits in the Maldon cutting were described by Whitaker as: …8 ft of brown sandy clay, with flint pebbles and flints, the bottom foot or more looking like London Clay'. This is overlain by 'stony loam (which is perhaps only a weathered Boulder Clay)… which in turn is overlain by sand and gravel. Whitaker clearly had doubts as to the true origin of this deposit. It differs from the Maldon Till mapped elsewhere within the district (see below), and it may represent a solifluction lobe, though it has been shown as Boulder Clay on the 1:50 000 Sheet. Turner (1937, p. 160) refers to a lower Boulder Clay at Broomfield Lodge [TL 7070 0876], at Widford, at Sandon, and along the valley sides of the Sandon Brook, but recent mapping has shown that all of these occurrences relate either to the London Clay, Glacial Lake Deposits, or Head.

Nevertheless the present survey has confirmed that a lower till is present though its outcrops are restricted and are generally associated with the present-day valley system. Where seen in section, or in borehole and auger holes, it is a stiff grey chalky and flinty clay, lithologically indistinguishable from the Springfield Till. This lower till is well developed in the Ter Valley near Witham, and one borehole [TL 8456 1759] on the west side of the Blackwater Valley to the north-east of Witham proved 4.6 m of it beneath 6.4 m of Springfield Till and 10.7 m of Chelmsford Gravel. Its chronology is uncertain, but the present author believes it represents a local minor episode before the advent of the ice that deposited the Springfield Till.

In addition to the Maldon section, chalky Boulder Clay has been mapped at the surface to the west of this locality [TL 832 059], and proved in one borehole [TL 8437 0677] to the east (see pp. 40, M10).

In the Danbury area, small spreads of chalky Boulder Clay occur beneath the Glacial Sand and Gravel on the south side of Danbury Hill [TL 768 043] and [TL 7810 0365], and have been recorded by Turner (1934, MS maps BGS) high up on the hill.

In this memoir the term Maldon Till is applied to all the occurrences of lower till which have been noted above.

Immediately outside the district a lower chalky Boulder Clay has been noted in boreholes to the east of Coggeshall (MS record BGS), and to the west of Braintree (Clarke and Ambrose, 1975).

Three borehole records of grey, and bluish brown silt beneath Glacial Sand and Gravel in the Great Totham area (Haggard, 1972; see also p. 40) are anomalous in that similar deposits do not appear at outcrop. Their origin is unknown, but they may be related to the lower Boulder Clay. CRB

Sand and Gravel (Chelmsford Gravels)

Sand and gravel is present at outcrop, or beneath the Springfield Till over much of the north-western part of the district and is coextensive with similar deposits which are widely distributed in East Anglia. The deposits have been termed the Chelmsford Gravels by Clayton (1957a). South of the River Chelmer extensive sand and gravel deposits form a ridge extending from Danbury to Tiptree. Their origin is uncertain, but they appear, at least in part, to predate the Springfield Till. More problematical are the spreads of sand and gravel north of the Blackwater around Little Totham, Tolleshunt and Tollesbury; these are probably related to the Danbury–Tiptree deposits, and may in part be of fluviatile origin.

The base of the Chelmsford Gravels of the plateau area falls gently south-eastwards from 60 m OD in the north-west corner of the district to about 20 m OD in the Chelmsford-Witham-Kelvedon area (Figure 15). In the Blackwater Valley between Witham and Kelvedon, and along the River Chelmer between Chelmsford and Little Baddow, Glacial Sand and Gravel is absent from the deep drift-filled channels. Minor modification to this regular south-eastwards fall can be seen in the major river valleys, i.e. the north-south trending sections of the Chelmer, Ter and Brain where the base of the sand and gravel dips down into the valleys. This probably indicates that an original valley system was filled in by the Chelmsford Gravels and has been subsequently exhumed. Cambering may have accentuated an original valleyward slope of the gravels.

The form of the base of the gravels on the Danbury-Tiptree ridge is in marked contrast to that of the plateau. Whereas post-depositional periglacial structures appear to have accentuated local channels filled with sand and gravel (p. 43), the steep regular fall from the high point of Danbury Hill, and from the crest of the Tiptree ridge (Figure 15) is remarkable. It is not known to what extent this is an original depositional feature, or to what extent it is the result of ice tectonics or Pleistocene earth movements. If it is an original feature, then the present Chelmer-Blackwater gap would appear to antedate the deposition of the Chelmsford Gravels.

The thickness of the Chelmsford Gravels beneath Springfield Till in the plateau area is quite variable (Figure 16), with marked fluctuations in thickness. Haggard (1972) noted that within the area of Sheet TL 81, the thickness varied from 0 to 17 m, with an average of 5.4 m. In the area to the west, Eaton (1973) recorded a variation of 0 to 19 m in the thickness of the sand and gravel beneath the Springfield Till of the plateau area, with an average of 6.1 m.

In the Danbury area the thickness of the sand and gravel is even more variable. This is partly due to the fact that the stratigraphical top of the deposits is not preserved, as there is no Springfield Till overburden, and partly due to frostheaving which has accentuated channel-like depressions filled with sand and gravel in which thicknesses of 18.3 and 21.3 m (Clarke, 1975) have been proved.

The physical composition of the sand and gravel is described in various IGS Mineral Assessment Unit reports (Haggard, 1972; Ambrose, 1973b; Eaton, 1973; Clarke, 1975). For example, in the area of 1:25 000 Sheet TL71 (Eaton, 1973) the gravel fraction ( + 4 mm) varies from 32 to 57 per cent, but averages 48 per cent of the total deposit, and consists predominantly of flint (over 80 per cent) with subordinate quartz and quartzite; other rock types are rare.

The majority of the flints are subangular to rounded, brown or black, more rarely white-coated; however, irregular cobbles and large pebbles are not uncommon. Vein quartz is found as well-rounded pebbles and as cobbles, and is particularly significant in the fine gravel (4 to 16 mm). Quartzite is conspicuous as subrounded pebbles and cobbles, commonly broadly tabular in shape, grey or orange-brown in colour. The sand ranges from coarse-grained to fine-grained with the medium fraction (-} to 1 mm) almost always dominant. It consists mainly of subangular quartz with subordinate well-rounded grains. Angular flint is relatively common in the coarse-grained fraction (1 to 4 mm) and commonly imparts a sharp texture to the sand. There is usually less than 10 per cent of fines (material finer than 1/16 mm), and on average the fines form about 6 per cent of the deposit; the sand and gravel fractions are in roughly equal proportions.

Eaton (1973, p. 7) found that within the area studied the proportion of gravels in the plateau area increased southwards, that is from higher to lower levels.

Iron staining is general but is usually of minor significance. Locally, however, it may cement the sand and gravel into coherent masses. There is a tendency for the upper parts of the sand and gravel deposit to be more stained than the lower. Where the latter is particularly iron-free it is referred to as 'Essex White Ballast'. The rubification of the palaeosol horizon is noted above.

In the Tiptree ridge area the deposits are predominantly sandy and are commonly divided by seams of clay (Haggard, 1972, fig. 2, p. 11).

In the Tolleshunt-Tollesbury area Ambrose (1973b) found that the proportion of gravel averages as much as 45 per cent, whereas in the Chelmsford Gravels to the north of the same assessment area (immediately beyond the district boundary) and in the Tiptree ridge (Haggard, 1972) the average is only 25 per cent, although the gravel content of the Tolleshunt-Tollesbury deposits is similar to that of the Chelmsford Gravels of the Witham area. In their high gravel content the Tolleshunt-Tollesbury gravels resemble the terrace deposits assessed farther west (Ambrose, 1973a), where average gravel proportions as high as 60 per cent have been recorded. It is possible that some of the mapped 'Glacial Sand and Gravel' in the Tolleshunt-Tollesbury vicinity includes some high-level terrace deposits. Woodward (in Whitaker, 1889, p. 320) was also uncertain of the origin of the gravel in this vicinity. It is also possible that some of the small outcrops of 'London Clay' represent reworked London Clay within a terrace sequence (see also p. 46) and may be equivalent to the interglacial deposits associated with terrace deposits recognised by Lake, Ellison, Hollyer and Simmons (1977) in the Southend-Southminster area.

There is still doubt as to the correct stratigraphical position of the Danbury Gravels. It was thought (Gregory, 1915; Clayton, 1957a) that the Danbury Gravels could be distinguished petrographically from the other glacial sand and gravel deposits, though Gregory's argument is partly invalid as he used the terrace sand and gravels (his East Essex Gravels) in the south-east of the district for his Comparison. To Gregory the high proportion of quartzites foreign to the district was of significance; Clayton (1957a, p. 7) has shown that they constitute from 9 to 15 per cent of the Danbury Gravels. Lower Greensand chert, which was said to be common in Gregory's East Essex Gravels, is rare in the Danbury Gravels. Felsites or rhyolites are absent, although Gregory did note some in gravels at Gay Bowers about [TL 795 043] in a deposit which he regarded as distinct from the Danbury Gravels. It was the absence of Jurassic sandstones, of large unrolled flints, and of basalt that led Gregory to think that the Danbury Gravels were pre-glacial, and to argue that they must have been deposited before these erratics were brought into Essex by the ice sheets. The absence of garnet (Gregory, 1922, p. 28) was also thought to be significant for it has been recorded in Pliocene deposits by Boswell (1915, p. 250), and thus a Miocene age seemed the youngest possible.

Clayton (1957a, p. 7) attempted to separate the Danbury Gravels from the Chelmsford Gravels on the basis of the relative percentages of angular or subangular flints compared to rounded ones. He included within the Danbury Gravels those deposits that had a higher proportion of angular or subangular flints (57 to 65 per cent), compared to rounded flints (22.5 to 30.5 per cent); he classed as Chelmsford Gravel those with a lower proportion of angular flints to rounded flints (48 to 53 per cent and 28 to 40 per cent, respectively). However this distinction between the Danbury and Chelmsford Gravels is not always clear. Some deposits which appear to form an integral part of the Danbury Gravels, although they lie at unusually low levels, he regarded as Chelmsford Gravel (Clayton, 1957a, table 1). Indeed only one of his localities, the Broomfield Pit [TL 720 109] can be assigned unequivocably to the Chelmsford Gravels, although the lower bed in the Baddow Hall Pit [TL 736 051] may probably also be referred to these deposits.

Clarke (1975, pp. 6–7, fig. 5) could find no major difference in the grading analyses of the Chelmsford and Danbury gravels, although the latter tend to be finer. Also his pebble counts showed that the two deposits (and also the river gravels) contain similar proportions of flint (77 per cent) and quartz/quartzite (20 per cent).

The sand and gravel of Danbury Hill is nowhere overlain by till. Locally on the south side of the hill, and in the east near Maldon, there are small patches of Boulder Clay, the Maldon Till, beneath the sand and gravel. The gravels of the Tiptree ridge pass north-eastwards beneath the Springfield Till around [TL 900 181], whilst to the south of Wickham Bishops around [TL 840 110] there is an outlier of chalky Boulder Clay resting on the sand and gravel. Rose and others (1976, fig. 1) regard the deposits at Inworth Grange, Tiptree [TL 884 158] as part of the Kesgrave Sands and Gravels, with a reddened, clay-rich layer at the top.

Rose and Allen (1977, table 2) found that the Kesgrave Sands and Gravels, and the Barham Sands and Gravels of the plateau area north of the River Chelmer were similar in their bulk lithological composition, but stressed that the two deposits have differing depositional features and palaeocurrent directions, while the upper unit has a higher percentage of non-durable rocks. These features are thought by Rose and Allen to signify marked differences in time, depositional environments, and source areas for the gravels above and below the palaeosol. The overall similarity of the two deposits is explained by the upper deriving much of its material from the lower, but additionally incorporating the non-durable material transported to the region by a 'mechanism other than water'. Whilst it is possible to recognise the two types of sand and gravel in several of the pits within the district, this is not always so. Away from the sections it is impossible to tell to which unit, if either, the sand and gravel should be assigned. Hey (1980) suggested that some of the gravels on Danbury and Tiptree hills belonged to the high-level Kesgrave Sands and Gravels [Westland Green Member], but that the bulk of the sand and gravel in these areas belonged to the low-level Kesgrave Sands and Gravels. Both types of deposits he regards as being laid down by the Proto-Thames. On the 1:50 000 map the Kesgrave Sands and Gravels, the Barham Sands and Gravels, (which collectively are the equivalent of the Chelmsford Gravels of Clayton, 1957a), together with the Danbury Gravels are portrayed as one unit – Glacial Sand and Gravel.

Chalky Boulder Clay (Springfield Till)

The Springfield Till, which crops out extensively in the north-west of the district where it overlies the Chelmsford Gravels, is part of a widespread expanse of till that covers much of East Anglia and the south-east Midlands. In the south-western part of the district scattered remnants of a lithologically similar till have been named the Hanningfield Till (Clayton, 1957a).

The Springfield Till of the plateau area of the district has, in general, a planar base which descends gently south-southeastwards from 55 m to 30 m OD. In all the major river valleys the base of the Boulder Clay is lower than on the adjacent plateau, but only by some 7 to 8 m (Figure 17). However, at two principal localities the Boulder Clay fills very deep channels. Along the Blackwater Valley between Witham and Kelvedon the base of the drift is some 90 m below the base of the drift on the plateau ((Figure 18) and p. 47), and in the Chelmsford area chalky Boulder Clay has been recorded at a level at least 30 m below its level on the nearby plateau. The deposits which fill channels are discussed in more detail below.

On the south side of the River Chelmer the gentle southerly fall in the level of the base of the chalky Boulder Clay is reversed and the scattered outliers of Boulder Clay in this vicinity (the Hanningfield Till) show evidence of a steady rise to over 76 m OD in a general southward direction on the valley sides of the River Wid and the Sandon Brook.

On the interfluves of the plateau the thickness of the Boulder Clay averages about 7 m, although locally thicknesses in excess of 18 m have been proved (Haggard, 1972; Eaton, 1973).

The Springfield Till is a grey clay with small pellets of chalk. These are generally less than 2 cm in diameter and are scattered evenly throughout the till, forming 56 to 84 per cent of the erratics. Angular fragments of flint are common, whereas rounded Eocene flint pebbles, Bunter quartzite, Jurassic, Cretaceous, Eocene and Pleistocene rocks and fossils form a small percentage of the total number of erratics. Igneous and metamorphic rocks have also been recorded from the Boulder Clay, but form less than 0.1 per cent of the erratics (see p. 47; Baden-Powell, 1948, p. 289; Clayton, 1957a, p. 3; Boswell, 1929, p. 37; Perrin and others, 1973, p. 102). Marine Pleistocene foraminifera, in association with abundant derived chalk foraminifera, were recorded by Gregory (1922, p. 13) from the Boulder Clay of the Sudbury area and were thought by him to indicate a marine sedimentary origin for the Boulder Clay of that area. However, there is no reason why these Pleistocene foraminifera could not have been derived from the Crag, or from the sea floor.

Stratified chalky Boulder Clay has been noted in the Chignall St James pit [TL 661 098] to the west of the Chelmsford district.

The age of the Springfield Till has been established at Rivenhall End as pre-Hoxnian (Turner, 1970).

Drift-filled channels

Deep drift channels infilled dominantly with chalky Boulder Clay or laminated silts with variable amounts of corn-minuted chalk are known principally beneath the River Blackwater between Kelvedon and Witham, and beneath the River Chelmer from Chelmsford and 8 km eastwards. Similar channels, designated tunnel valleys, have been described by Woodland (1970) from other areas of East Anglia.

Glacial Lake Deposits and sub-Boulder Clay laminated silts Within the Chelmsford district there are two situations in which deposits of soft, grey laminated silty clays occur.

The first is where these deposits occur either at outcrop, such as in the Sandon and Boreham areas and just beyond the western limit of the district, or beneath terrace deposits and Head along the valley of the River Chelmer between Chelmsford and some 6 km eastwards. These deposits are regarded as Glacial Lake Deposits laid down towards the end of the Anglian stage.

The second situation is where deposits of a similar lithology occupy part of the Blackwater drift-filled channel. As these laminated chalky silts are overlain by chalky Boulder Clay they must have formed early in the Anglian and are probably of sub-glacial origin. Nowhere within the Blackwater Valley do they crop out.

Because of their lithological similarity both deposits are grouped together in this chapter. Both types are principally soft to firm grey silty clays, silts or fine-grained sands, locally laminated; they have a superficial resemblance to the London Clay, and earlier authors have classified them as such in some boreholes. Small chalk pellets may be present in the laminated strata; where non-laminated strata in the buried deposits include larger chalk pellets, it is thought that they are Boulder Clay. In the Witham Borehole [TL 8244 1534] grey chalky Boulder Clay, 57.45 m thick, with little lithological variation, occupied the lower part of the channel; in the Kelvedon Borehole there was no sharp junction between the overlying chalky Boulder Clay and the underlying laminated grey silty clays (see M15). Similarly in boreholes in the Chelmsford Central Redevelopment area there was no clear-cut division between the chalky Boulder Clay and the laminated silty clays. Additionally it was extremely difficult to place the boundary between these latter clays and the London Clay. Gravel, 1.75 m thick, was recorded from the base of the deposits in the Chelmsford area, and locally it constitutes the whole of the channel fill, e.g. in the former Gray and Sons Brewery well [TL 7102 0669] where it is 19.51 m thick. Peat and unidentified shell fragments have been noted in the eastern part of the Chelmer channel (Clarke, 1975, p. 90).

Woodland (1970, pp. 559–560, fig. 3) regarded the deposition of laminated silts, silty clays and clay in the tunnel valley system of East Anglia, as resulting from the thinning of the overlying ice cap and consequent reduction in hydrostatic head for the englacial waters. At this stage the channels became filled with stagnant, or only gently flowing water. The tunnel valley system of Woodland has a markedly different geographical relationship to those within the present district being radial to the ice-margin rather than parallel to it. Nevertheless, the channels in which the buried deposits occur are thought to have been cut by water under hydrostatic pressure beneath ice banked up against the Danbury-Tiptree ridge. In view of the relationship between the laminated silts and the chalky Boulder Clay in the Blackwater Valley, the former are probably only slightly older than this till. Along the Chelmer the situation is somewhat different and the deposits are thought to have formed towards the close of the Anglian in meltwater lakes that formed behind the Danbury ridge before the River Chelmer re-established itself and drained the lake (see p. 62). The clays at outcrop in the Sandon and Boreham areas are interpreted as cold water lacustrine sediments deposited within this lake. They represent deposition in a low-energy environment, possibly during a period of very slow melting of the ice, or the winnowed transported 'fines' of an ablating ice sheet.

The relationship of the Glacial Lake Deposits at outcrop to the older drift deposits is not known with certainty. The lake deposits lie in valleys cut into the Springfield Till and Chelmsford Gravels. In the Sandon area they predate the 3rd Terrace.

Samples from the Sandon and Boreham areas which were examined by Dr. C. Turner yielded only a few poor and unidentifiable microspores. CRB

Brickearth, Head Brickearth and River Brickearth (Terrace Loam).

The term 'brickearth' has been applied in the past to materials suited for brickmaking, i.e. to sandy or silty weathered clays generally of Quaternary age but also including Tertiary strata, particularly the Claygate Beds and the higher siltier part of the London Clay. Modern usage in Quaternary deposits restricts the term to generally structureless brown clayey silts which have a subordinate sand and gravel content and which typically show columnar jointing. Confusion has arisen regarding the equivalence of the terms brickearth, loess and periglacial deposits (Head), which are riot necessarily synonymous.

Within the district brickearths occur in three principal situations: firstly overlying the chalky Boulder Clay on the plateau; secondly mantling slopes and valley floors; and thirdly overlying river terrace gravels. In the first instance it is thought that the Brickearth is a wind transported loess; it has been mapped as Brickearth. The second situation which in the plateau area occurs preferentially on east-facing slopes, may also in part be due to wind-transport, although it is believed that periglacial action is also partly responsible; these occurrences have been designated Head Brickearth on the published geological maps. In the third situation the sediments probably represent overbank deposits on the former flood plains of rivers; they have been depicted as 1st Terrace (Loam) on the map (see p. 80). RDL, CRB

Head

Heterogeneous deposits which do not fall into a specific genetic division have been grouped under the term Head. They represent accumulations due to the downhill movement by solifluxion of weathered surface material from a variety of lithological units. Soil creep and minor mass movements probably still continue the processes which were initiated under freeze-thaw periglacial conditions. The strata are broken up by ice action during the winter period and, when thawed during the Spring and Summer, they become water-saturated and easily mobilised and move downslope either fairly quickly as a discrete unit or slowly over a wider area. Probably both periglacial freeze-thaw action and mass movement have acted in the formation of the wide variety of Head deposits seen within the district.

The lithologies within the Head reflect the source material upslope. For example, Head deposits derived from the Bagshot Beds in the south-west are silty sands; Head deposits which have originated from the London Clay consist of silty clays; soliflucted chalky Boulder Clay is usually a chalk-free orange-brown stony clay. In many cases it is impossible to distinguish Head satisfactorily from its parent body. This is particularly so in the case of Head derived from the London Clay and Claygate Beds. Similarly, difficulty is experienced in separating Head from weathered in situ deposits; this problem is experienced particularly in the areas of Glacial Sand and Gravel and chalky Boulder Clay where some of the 'Head' and some of the clayey sand and gravel may represent flow tills.

Head deposits are generally confined to valleys, but may spread out to form extensive sheets. A preferential development has been noted on south- and east-facing slopes. Thicknesses vary between 0 and 7.4 m. Only deposits thicker than 1.0 to 1.5 m are shown on the geological map.

Within the district no soliflucted material has been recognised beneath either the chalky Boulder Clay or the Chelmsford Gravels. In the Blackwater Valley, Head overlies Hoxnian interglacial deposits. In the Chelmsford area it can be seen to post-date, at least locally, Ipswichian deposits and the 1st Terrace of the Chelmer. There is insufficient evidence to establish the number of post-Hoxnian solifluction events, although at least two periods have been recognised in the Widford-Chelmsford area (p.56). CRB, RDL

Chronology and correlation

Apart from Gregory (1915; 1922), who regarded the Danbury Gravels as of Miocene age, there is unanimity amongst East Anglian geologists that all the deposits discussed in the preceeding section of this chapter are of Pleistocene age. There is, however, considerable divergence of opinion in the chronological interpretation of the ages of the various deposits.

Baden-Powell (1948) was one of the earliest workers to relate the Boulder Clay of East Anglia to an interglacial period. He regarded the Hoxnian Interglacial, as it was later named, as separating an upper Gipping Till from a lower Lowestoft Till. Clayton (1957a) initially regarded his Maldon Till, Chelmsford Gravels and Springfield Till as being post-Hoxnian in age, but later (1960) he modified this view, claiming that only the Springfield Till, which he specifically correlated with the Gipping Till, was post-Hoxnian; the Chelmsford Gravels were thought to have formed during the Hoxnian, and the Maldon Till was equated with the Lowestoft Till. In both interpretations the Hanningfield Till was regarded as the oldest deposit of the sequence.

It has been demonstrated by Bristow and Cox (1973) that there is no till in southern East Anglia which overlies Hoxnian deposits, and that the 'Gipping Till' is a lithological variant of the Lowestoft Till. The latter is widely distributed in East Anglia and has been demonstrated to be overlain by Hoxnian deposits at a number of localities in this region (West, 1956; Sparks and others, 1969; Stevens, 1960). Within the present district Turner (1970) proved that the Springfield Till at Rivenhall End is pre-Hoxnian. He regarded the upper three units of Clayton (1957a) as Lowestoftian ( = Anglian age). The criteria which Clayton thought indicated that the Hanningfield Till was considerably older than the Springfield Till were shown to be invalid by Bristow and Cox (1973) who regarded the Hanningfield Till as dissected remnants of the Springfield Till.

In the field these two tills appear lithologically identical. In the Chelmsford area the Springfield Till of the plateau appears to descend into the valley of the Chelmer/Can and ascend the slope on the south. However the extensive spread of Head in this vicinity, the urban nature of the ground and the lack of borehole control prevent an accurate assessment of the relationships of the Boulder Clay at outcrop on either side of the valley. At Widford [TL 693 054] and [TL 6970 0513](see p. 49) and Galleywood [TL 7050 0385] (see p. 42) the Boulder Clay overlies small spreads of Glacial Sand and Gravel. At Great Baddow deposits of the Chelmsford Gravels occur topographically lower and in close proximity to chalky Boulder Clay, but the boundary between the two is obscured under Head and the superposition of the Boulder Clay, although inferred, is not proven. To the south and east of Galleywood the scattered outliers of Hanningfield Till' show a gradual rise on either side of the Sandon Brook to just over 70 m OD in the west and to just over 60 m in the east: the Springfield Till occurs at similar heights on the plateau north of the Chelmer (Figure 17). Whilst there is no unequivocable evidence to correlate the Hanningfield Till with either the Maldon or Springfield tills, the correlation with the latter has been adopted in this account. Nevertheless the possibility remains that the ice sheet which deposited the Maldon Till extended southwards as far as the Hanningfield area. If this were so the later ice that deposited the Springfield Till may have removed or incorporated most of the Maldon Till of the plateau area during its southwards advance; if it did not extend beyond Danbury Hill to the Hanningfields any Maldon Till to the south would not have been so severely eroded, and might now be represented by the scattered remnants of chalky Boulder Clay to which the name Hanningfield Till has been applied.

The postulated age of the Chelmsford Gravels has varied over the years. Prestwich's (1890) inclusion of the 'Essex White Ballast' within the Westleton Beds was shown to be untenable by Hey (1967). Quartzose gravels at Holton in Suffolk, which overlie the true Westleton Beds, are a correlative of similar deposits, including the Essex White Ballast, which can be found across Essex and Suffolk (Hey and Brenchley, 1977). The Kesgrave Sands and Gravels are thus younger than the Westleton Beds. However the Westleton Beds in the type area have not been reliably dated; Prestwich (1890) regarded them as a correlative of the Upper Freshwater Bed of north Norfolk, now the Cromerian stratotype. Hey (1967, p. 436) concluded that they fall somewhere within the long interval of time represented by the upper Pastonian and the whole of the Beestonian and Cromerian Stages. The Westleton Beds are currently regarded as part of the Norwich Crag (Funnell and West, 1977, p. 249).

Initially the Kesgrave Sands and Gravels were regarded as Beestonian in age, separated from the Anglian Barham Sands and Gravels by the Cromerian rubified sol lessive (Rose and others, 1976; Rose and Allen, 1977). Hey (1980) has now sub-divided the Kesgrave Sands and Gravels into a high-level unit, the Westland Green Gravels, and a low-level unit of pre-Pastonian a and Beestonian ages respectively. The interpretation of Rose and others (1976) implies that the widespread reddened horizon at the top of Kesgrave Sands and Gravels formed in a warm temperate climatic regime (a full interglacial) followed by periglacial conditions in which an Arctic structure soil developed and ice-wedge casts and involutions formed. Although the palaeosol is an important horizon, other authors (Lake, Ellison and Moorlock, 1977; Lake, 1977; Bristow, 1977; Read, 1977) have suggested that it need not necessarily indicate such a substantial time break. Indeed if the Maldon Till is of Anglian age as has been suggested by some authors (Turner in Mitchell and others, 1973; Bristow and Cox, 1973) then the suggestion by Bristow (1977) that it is overlaid by the Kesgrave Sands and Gravels would invalidate this chronology. However, this stratigraphic relationship has not been proved, and there is increasing evidence (Hey, 1980) that the Kesgrave Sands and Gravels are deposits laid down by a proto-Thames and that they are considerably older than the Anglian.

Glacial and periglacial structures

Corder (1887, p. 92) noted … a curious contortion, a quantity of London Clay being brought up at a sharp angle, and thinned out for some yards between the gravel and the sand' in Christmas's Pit [TL 776 051] to the north of Danbury Common. This appears to be one of the earliest references within the district to structures that are now regarded as of glacial or periglacial origin. This structure was one of several occurrences in the district in which a complex or unexpected contact between the London Clay and sand and gravel has been noted. Commonly this takes the form of a steep-sided, locally vertical, contact between the two lithologies. On Danbury Hill many of the elongate patches of sand and gravel occupy steep-sided 'channels', up to at least 20 m deep (Plate 4). The bedding of the basal beds of the sand and gravel show steep dips and the beds may even be overturned, but there is no evidence of slumping in these basal beds. Clearly they are not original features.

The disturbance noted by Corder (1887, p. 92) is that of a diapiric-like mass of London Clay which has been squeezed upwards through, and then along, the bedding planes of the sand and gravel. Similar structures have been noted in the Chelmsford Gravels of the plateau. Also in this area there are large (at least 120 x 250 m) 'bosses' of London Clay which have passed through the Chelmsford Gravels and the Springfield Till to the surface where they are now seen as inliers of London Clay surrounded by the Springfield Till e.g. around [TL 730 095]. Other, smaller ridges of London Clay, some 2 to 3 m high, have been noted in gravel pits e.g. Boreham pit, [TL 752 113], and in some of these cases the sand and gravel thins, or is absent, over the ridge, though the overlying Springfield Till appears to be unaffected.

In the Chelmer Valley small ridges, generally only 1 to 2 m high, occur beneath the 2nd Terrace. The terrace deposits, like the Chelmsford Gravels of the plateau, are thin or absent over these ridges.

The disturbances affecting the sand and gravel on Danbury Hill are the largest and best known within the district (Salter, 1906; Solomon, 1935; Turner, 1937; Clayton, 1957b). The detailed origin of these structures was for a long time a matter of conjecture. Salter (1906) put forward four possible hypotheses to account for them: a. Solution collapse; b. Large-scale earth movements; c. Landslipping; d. Impact of an ice sheet. Solomon (1935, p. 226) attributed them to faulting, and related the hypothetical fault to that proved in the Wickham Bishop's Borehole (see M16), and to the 1884 Colchester earthquake. In contrast Turner (1937, p. 160) was of the opinion that the bedding plane contortions were caused by landslipping, a view shared by Clayton (1957b, p. 25).

The first three of Salter's (1906) hypotheses are no longer regarded as valid, and it is now generally assumed that the structures have originated by glacial or periglacial activity. The processes within the area are not fully understood, and it is probable that the observed effects are the results of two or more differing processes.

Without exception the London Clay has been involved in the movements and it is thought that it became locally mobile under periglacial freeze-thaw conditions. On Danbury Hill the elongate channel-like structures may have been in part controlled by a waning ice margin. Probably glacial outwash accumulated in channels developed at the margin of the ice sheet which was banked up against the Danbury-Tiptree ridge. Minor local readvance of the ice sheet may have accentuated these channels by ice-shoving. On retreat of the ice, diapiric-like movements of the London Clay, aided by differential loading of the gravels within the channels, would further accentuate the structures.

Away from the Danbury area, however, the structures do not always appear to be linear and it is probable that the local mobilisation of the London Clay was controlled by minor differences in the load of the overburden. Several types of disturbances are indicated:

1. The initial development of diapirs of London Clay with a boss-shaped cross-section and a generally elongate linear profile.
2. 'Lit-par-lit' injection of the clay into the gravels – fed from the main diapirs and possibly syncontemporaneous with deposition.
3. Continued movement and dislocation of injected seams by ice movement or later cryoturbation.

It is not possible to date accurately the disturbances, which may in some cases be of two or a combination of two, differing ages. It is suggested here that the movements of bed-rock in the plateau area may have been initiated as diapirs in the Anglian and continued to be active for some time after the ice retreated. Subsequent periods of periglaciation may have reactivated the structures, or initiated others such as those affecting the 2nd Terrace in the Chelmer Valley. CRB, RDL

Details

Maldon Till

Ter Valley

The drift in a borehole [TL 7210 1628] south-east of Little Leighs falls into five units, of which the lowest, middle and uppermost have been classified as Boulder Clay (Eaton, 1973). However, it is now suspected that the uppermost 'boulder clay' and the immediately underlying sand and gravel are Head deposits, and that 1.5 m of brown chalky Boulder Clay in the middle of the sequence is the Springfield Till. The lowest Boulder Clay, 1.5 m thick, presumably corresponds to the Maldon Till, although this has not been detected at outcrop in this vicinity.

Farther downstream between Fuller Street and Terling, chalky Boulder Clay lies beneath Glacial Sand and Gravel at a number of localities. The presence of the most westerly occurrence [TL 7495 1555], too small to show on the 1:50 000 Sheet, is indicated by the presence of Old Man's Beard (Clematis vitalba) in the hedgerow; chalky Boulder Clay was augered in a field close by, while a small pond [TL 7497 1554] in Sandy Wood may have been dug as an old marl pit. Chalky Boulder Clay has also been augered a little to the east [TL 7536 1530], and it is probable that the patches are continuous, but locally obscured by Head.

On the north side of the valley a thin bed of Maldon Till can be followed for 4 km eastwards from Ridley Hall [TL 7560 1535]. Trenches showed that up to 1.5 m of sand and gravel overlies chalky Boulder Clay, and that the latter deposit rests on the London Clay. The full thickness of the till was not exposed in any one section, but it was estimated to reach a maximum of 3 m.

Chalky Boulder Clay was augered beneath Glacial Sand and Gravel on the side of the small tributary to the River Ter at Fairstead [TL 7666 1657].

Some 1 km WNW of Terling a fairly large spread of Maldon Till some 125 m wide can be found on the west bank [TL 7635 1535 7671 1508]. Farther downstream two additional outcrops [TL 7685 1500] and [TL 768 147] may be part of the same spread partially obscured by Head.

The occurrence of grey silty clay in an area of predominantly sandy loam and clay (Head) has been proved by augering at five points [TL 7747 1354], [TL 7752 1355], [TL 7776 1352], [TL 7776 1345] and [TL 7774 1337]. Chalk fragments noted at the last point, and the presence of Clematis vitalba at the first, suggest that the deposit may not be Head, as mapped, but a variant of the Maldon Till.

Two small outcrops [TL 772 103] and [TL 7830 1035] of chalky Boulder Clay to the south-west of Hatfield Peverel occur at the junction of the Glacial Sand and Gravel and London Clay, and are here referred to the Maldon Till. At the latter locality the Maldon Till was seen in a temporary section to be 1.2 m thick.

An occurrence of the Maldon Till has been noted in a borehole [TL 7999 1089] at Hatfield Peverel. Here 2.1 m of clay, brown and stone-free in the upper metre and very chalky in the bottom part, were recorded below the Glacial Sand and Gravel and above the London Clay (Eaton, 1973, p. 120).

Brain Valley

An almost continuous strip of Maldon Till can be followed north-eastwards from Chippinghill, Witham, for 2 km along the west bank of the River Brain. The junction of the Glacial Sand and Gravel and the Maldon Till is marked by a spring-line. The Maldon Till is thought to have a maximum thickness of 5 m in this vicinity.

Blackwater Valley

Along the northern margin of the Blackwater Valley chalky Boulder Clay has been proved locally beneath the Chelmsford Gravels. Because the Maldon Till is nearly always related to an existing valley system it may be that the lower part of the infilling of the deep Blackwater depression corresponds to this Till.

A borehole [TL 8368 1670] at Rivenhall End encountered 4.8 m of brownish grey clay, containing abundant cobbles and flecks of chalk or gypsum beneath the Chelmsford Gravels, and resting on the London Clay at a level of −0.6 m OD.

Some 1.3 km NE a borehole [TL 8456 1760] at Sniveller's Lane proved 4.6 m of grey chalky Boulder Clay beneath the Chelmsford Gravels. The base of the Maldon Till lay at 10.1 m OD. The local nature of the Maldon Till was demonstrated at this site as it was encountered in only the westernmost of three holes drilled within 8 m of each other.

Danbury area

Both Turner (1934, MS maps, BGS) and Clayton (1957a) mapped a large outcrop of Maldon Till along the eastern side of the Sandon Brook Valley. However, because of the lithology of grey clay and silty clay with only minor amounts of small chalk pebbles present, these occurrences have been considered as Glacial Lake Deposits (see p. 52) during the present six-inch survey.

Turner was similarly mistaken by the Glacial Lake Deposits in the Sandon area. Other spreads mapped by him on the south side of Danbury Hill have not been substantiated during the present survey although very small isolated outcrops [TL 7680 0432], [TL 7689 0409], [TL 7718 0392] and [TL 7810 0368], which coincide in part with those mapped by Turner, have been noted in this area.

Turner also mapped various deposits of the lower Boulder Clay on Danbury Hill, of which one approximately [TL 7850 0605] is recorded in the literature (Turner, 1937, p. 161). Here he described 2.4 m of greyish brown Boulder Clay resting on the London Clay and passing under the gravels.

Maldon area

Some 1.1 km SW of the type locality a narrow strip [TL 8320 0592] of chalky Boulder Clay was proved by augering beneath sand and gravel.

The type section approximately [TL 8418 0651] in the disused railway cutting at Maldon West Railway Station has been described above (p. 32). The deposit, which is referred to as a Boulder Clay at this locality, differs from that noted in a similar stratigraphical position elsewhere in the district, by the absence of chalk and by its bedded nature. It may be a flow till. However, chalk was noted in a borehole [TL 8437 0677] 300 m NE of the type locality, where 1.2 m of stiff brown bluish grey silty plastic clay with chalk fragments was proved beneath 1.2 m of soil and fill, resting on London Clay.

Great Totham area

Three boreholes in the Great Totham area encountered anomalous deposits beneath the sand and gravel: the first [TL 8514 1163] proved at least 13.1 m of brown silt, becoming bluish brown at depth, under 11.3 m of soil, Head, and sand and gravel; the second [TL 8693 1323] proved 3.6 m of grey silt below 5.5 m of soil and sand and gravel, and above the London Clay; whilst the third [TL 8741 1339] at Shrub Hill Farm, encountered 1.2 m of blue to grey silt between gravel and the top of the London Clay. CRB

Sand and Gravel (Chelmsford Gravels)

Between rivers Chelmer and Ter

In the area around Littley Green [TL 698 172], the gravels appear to be thin or absent at outcrop. In such a position the junction of the chalky Boulder Clay and the London Clay is marked by a line of weak springs and a gravel 'streak' in the soils.

Beneath the Boulder Clay of the plateau area the gravels have a variable distribution. In the north-western part of the district they appear to be absent (Figure 16). South-east of a line drawn from Great Waltham to Great Leighs a sheet-like spread is present, but its thickness is variable. In places the Chelmsford Gravels are absent, and yet close by considerable thicknesses have been proved.

The Broomfield Pit [TL 720 110] is sited close to an area where Glacial Sand and Gravel is absent. As the pit has extended eastwards the gravels have been found to thicken. A section [TL 7230 1128] measured by Mr R.A. Ellison in March 1977 showed 8.0 + m of Glacial Sand and Gravel beneath 8 to 10 m of Springfield Till (Plate 1). The London Clay was not seen at this point, but contorted ridges of London Clay have been noted previously in the base of the pit. The gravels could be divided into an upper unit 0.4 m thick, equivalent to the Barham Sands and Gravels of Rose, and others (1976), consisting of bedded chalky sand and gravel, which rested on mottled grey and orange poorly bedded clayey sand and gravel with ice-wedge casts infilled with medium to coarse-grained sand, which in turn overlay clay-free sand and gravel of the Kesgrave Sands and Gravels. The clay-rich layer presumably corresponds to the rubified sol lessive of Rose and others (1976, fig. 1) but no reddening was noted. Rose and others (1976, fig. 1) also record the superposition of an Arctic structure soil on the sol lessive in this pit. The Kesgrave Sands and Gravels contain a high proportion of rounded flint, quartz and quartzite pebbles. Secondary calcareous concretions which envelop flints are developed in the upper part of the gravels.

South of Great Waltham and around Chelmsford, Head deposits cover much of the lower valley slopes. Glacial gravels appear to be mostly thin or absent. Thick deposits, however, have been worked in the past near the railway at Springfield Green. The former pits are up to 6 m deep and apparently both the Head and the glacial deposits were worked. RDL, CRB

At Chelmsford there is an outcrop of Glacial Sand and Gravel which lies partially under the built-up area of the town around [TL 720 067]. Within the town temporary sections and borehole evidence show that this deposit is locally overlain by the Springfield Till and/or Head. Boreholes alongside the Chelmsford By-pass show that the Chelmsford Gravels are up to 9.45 m thick and rest on the London Clay. In the region of Barnes Cottages around [TL 724 069] the base of the sand and gravel varies between 21.6 and 23.8 m OD. Some 300 m NW of Brookend the Chelmsford Gravels can be proved to rest on the London Clay at a similar elevation at outcrop [TL 7340 0725].

In the Springfield area there are two old gravel pits [TL 7208 0730] and [TL 7220 0725] where up to 6 m of sand and gravel were seen by Mr J.S. Turner in 1934 (MS Map, BGS). The pits are separated by a ridge of chalky Boulder Clay [TL 7217 0730], which was proved by a borehole to be over 6 m thick.

Along the valley of the River Ter there are extensive spreads of Glacial Sand and Gravel at outcrop. They rest on the London Clay, except locally in the Terling and possibly the Great Waltham area where the Maldon Till is present. The London Clay crops out over much of the bottom of the valley and this junction is usually marked by a spring-line. The Glacial Sand and Gravel is overlain on the plateau areas by the Springfield Till. Upstream from Terling the thickness of the sand and gravel varies from 0 to 11.3 m. The deposits along the Ter Valley were found to contain 57 per cent sand. This is a higher proportion than the average (46 per cent) for the areas to the south-west and north-east of the Ter; the gravel content, 38 per cent, is correspondingly lower (Eaton, 1973).

Excavation for the Springfield-Boreham By-Pass [TL 7790 1137] and for the North Sea Gas pipeline [TL 7855 1046], [TL 7811 1026] and [TL 7788 1019] demonstrated the irregular nature of the Glacial Sand and Gravel contact with the London Clay. Diapiric structures have resulted in the London Clay rising at least 2.5 m from beneath the sand and gravel to the surface. Some of the other small mapped inliers of London Clay in the Boreham vicinity [TL 762 096] and [TL 760 094], near the railway line [TL 7335 0890], [TL 7315 0865] and [TL 7306 0853], and to the north of the line [TL 731 092], [TL 730 097] and [TL 7285 0995] near Springfield are presumed to have a similar diapiric origin.

Sand and gravel is present on the sides of the unnamed stream to the north of Boreham, but is obscured by clayey sand and gravel classed as Head, which is derived in part from this deposit and in part from the Springfield Till. The junction between the Head and the 'Essex White Ballast' was visible in the Boreham Pit [TL 7525 1155] at the time of the field survey (see (Plate 3)). During the excavation of the pit an east-west trending ridge of London Clay, presumably a heave structure, was encountered in the middle of the pit at a depth of 3.5 m, whereas adjacent boreholes showed London Clay at depth varying from 8 to 10.7 m. The absence of sand and gravel beneath the Springfield Till in nearby boreholes [TL 7487 1363], [TL 7485 1312] and [TL 7540 1091] can probably be similarly explained.

In the area surrounding the Boreham pit it was found that the base of the Chelmsford Gravels was planar and in general lay between 30 to 40 m OD (Figure 15). The sand and gravel varies in thickness from 0 to 13.1 m with an average of 4.8 m. It was found to be relatively gravel-rich on the whole, with a correspondingly low sand content. Typically the samples had an average of 6 per cent 'fines' (material finer than 1/16 mm), 38 per cent sand and 56 per cent gravel.

Between rivers Ter and Brain

The spreads of sand and gravel which crop out along the valley of the River Brain are not as extensive as those of the Ter Valley, but the available borehole evidence suggests that the average thickness is no less, and may even be greater. They rest either on the Maldon Till, which has a somewhat more extensive outcrop than in the Ter Valley, or on London Clay which crops out all the way down the bottom of the west side of the valley as far as Witham, and over much of the length of the east side of the valley. The lower boundary is commonly marked by springs; locally the formation of Calcareous Tufa at this level obscures the contact. The base of the sand and gravel appears to descend from a general level of 43 m OD on the plateau to about 30 m OD in the valley. Along the Brain Valley it was found that the sand and gravel varies in thickness from 0 to 9.5 m with an average of 6.0 m (Eaton, 1973).

In the interfluve area between the Ter and the Brain, Eaton (1973) found that the base level of the sand and gravel falls south-eastwards from 52 m OD to about 27 m OD and is possibly split into two flat areas lying at 43 to 37 m OD and 34 to 27 m OD respectively with an intervening step in the vicinity of Ardley Wood [TL 785 153]. The evidence, however, is not conclusive (Figure 15). The thickness of the sand and gravel deposit varies from 0 to 12.8 m with an average of 6 m. In two boreholes [TL 7756 1699] and [TL 7787 1600], which lie less than 1 km apart, sand and gravel was found to be absent beneath the Springfield Till, although it has been proved in surrounding boreholes and at outcrop in the shallow valley in the Fairstead vicinity to the west (see (Figure 16)). The average proportion of fines, sand and gravel were respectively 5, 47 and 48 per cent.

In the Hatfield Peverel-Witham area Springfield Till overlies the Chelmsford Gravels, but to the south Boulder Clay is absent, and irregular spreads of Head and Brickearth are present at the surface. Within this area the thickness varies from 0 to 7.0 m, with an average of 3.7 m. Fines generally formed only 1 per cent of the deposit, with sand forming 42 per cent and gravel 57 per cent.

At the time of the field survey (1967) the pit at Sandford's Farm [TL 807 120] showed up to 3 m of Springfield Till overlying Chelmsford Gravels. In the north-east corner of the pit a heaved ridge of London Clay trending south-eastwards effectively cut out the sand and gravel. A multi-coloured mass 18 m x 18 m x 2 m of cemented gravel was also visible. Rose, and others (1976, fig. 1) subdivide the Chelmsford Gravels of this pit into the lower Kesgrave Sands and Gravels and the upper Barham Sands and Gravels, separated by an Arctic-structure soil. No rubified sol lessive was developed at the top of the Kesgrave Sands and Gravels.

River Brain to northern edge of district

As in the previously described areas, sands and gravel is present over most of the area beneath the Springfield Till. The Chelmsford Gravels where present at outcrop rest everywhere on London Clay. The junction is in places obscured by calcareous tufa which forms at the springs issuing from this deposit.

In this area the thickness of the Chelmsford Gravels was found to vary from 0 to 11 m with an average of 3.7 m. The percentage of fines, sand and gravel were 5, 45 and 50, respectively (Haggard, 1972).

Great Baddow area

Beneath the built-up area of Great Baddow there is an outcrop of sand and gravel some 3 km long and 0.8 km wide. It had been worked in a number of pits in the past of which the Beehive Pit [TL 718 054] and the Baddow Hall Pit [TL 736 052] are the better known. Descriptions of these have been published (Corder, 1887, p. 92; Whitaker, 1889, p. 318; Salter, 1906, p. 455; Turner, 1937, p. 160; Clayton, 1957a, pp. 6–7). At the time of the field survey (1970) fresh sections were no longer visible and the Baddow Hall Pit was in the process of being filled. Other large pits [TL 706 052], [TL 715 055] on the south side of the Chelmsford By-pass have now been backfilled, but manuscript notes on them by J. S. Turner exist within the BGS. Extensive spreads of Head are present in this area and many of the old pits were dug through this deposit into the underlying Glacial Sand and Gravel.

The Chelmsford Waterworks Borehole [TL 7202 0559] at Great Baddow proved 10.8 m of sand and gravel resting on London Clay, beneath 1.0m of Head.

An old pit [TL 706 052] 800 m SW of Moulsham Lodge was worked to a depth of 6.1 m when seen by J. S. Turner in 1934. No mention of the overlying clayey gravel was made, but this was the only lithology noted during recent augering in the school grounds to the east. A trial borehole [TL 7054 0538] for a subway under the Princes Road 200 m NW of the old pit, proved the following section:

The 1.37 m of strata beneath the soil are here interpreted as Head.

In the Beehive Pit [TL 718 054], Corder (1887, p. 92) distinguished the Head (unstratified, very coarse gravel mixed with red clay) and possibly Brickearth (his loamy sand), from the glacial gravel beds. The Head and Glacial Sand and Gravel were separated by a white and purple mottled clay. The Chelmsford Gravels consisted of stratified sand and gravel which were commonly white or pale buff in colour. Masses of London Clay were noted mixed with the lower gravel. Salter (1906) noted that the gravel was worked to a depth of 13.7 m with finely stratified gravel in the lower part, and gravel 'somewhat tumultuously arranged' [ = Head] in the upper part. He recorded the presence of sarsens in this pit, one of which stood outside the Beehive Public House, together with quartz blocks, pebbles from the Bunter, small boulders of rhyolite, flints up to 0.3 m across, and blocks of conglomerate resembling Hertfordshire puddingstone. In 1970 most of the pit was overgrown, but at one point [TL 7172 0536] a section of 1.2 m of yellow brown silt with a 15 cm band of gravel at its base overlying 1 m of orange sand was still visible.

The Baddow Hall Pit was described by Turner (1937, p. 160) as being 12.2 m deep: It is known that the London Clay has been proved in the bottom of the pit. He observed red gravel and sand [? Head] resting on white cross-bedded sandy gravels [? Kesgrave Sands and Gravels]. Clayton (1957a, pp. 6–7) distinguished the upper clayey gravel [Head] from the lower Chelmsford Gravels. The Head contained 6.5 per cent more angular and subangular flints and 3.2 per cent more quartzite than the Chelmsford Gravels. At the time of the field survey (1970) the pit was largely backfilled, but irregular spreads of clayey gravel and/or brickearth up to 1.5 m thick could be seen overlying bedded sand and gravel, of which 3 m were exposed.

To the east of the pit temporary sections [TL 7400 0515] showed the gravels to overlie London Clay, but much of the Glacial Sand and Gravel/London Clay junction is obscured by Glacial Lake Deposits, or Terrace sand and gravel. CRB

J.S. Turner (1934, MS BGS) noted the following section at the Galleywood gravel pit [TL 7055 0385]: Boulder Clay, 1.5 to 3.0 m; Sand and Gravel 4.6 to 7.6 m; London Clay, 1.2 m. An old pit [TL 7235 0455] near the Galleywood Road, Great Baddow was described by Turner (1934, MS BGS) as showing: 0.9 to 1.2 m of thin red boulder clay [Head]. overlying 5.5 to 6.1 m of bedded sand and gravel.

Excavations for the Baddow By-pass exposed up to 4.6 m of sandy gravels [TL 7324 0517].

Glacial Sand and Gravel (in situ) is apparently absent or thin south of a line between Goat Hall and Pontlands.

Valley of the Sandon Brook

The gravels in the Sandon area have a complex development and morphology. The variable development is in part related to the presence of the Sandon Buried Channel. Throughout much of the area gravelly Head overlies Glacial Sand and Gravel.

Up to 10 m of cross-bedded sand and gravel was formerly exposed in the Sandon Pit [TL 7467 0435]. Field mapping and borehole data indicate that this gravel body has a sharp western extremity due to channelling, possibly further accentuated by ice action. Up to 3 m of Head overlie the gravels with a sharp, channelled contact. In the vicinity of the Sandon Buried Channel the upper part of the glacial sequence consists of sands with subordinate gravel partings and lenses of grey clay. These sands are generally well bedded and show evidence of faulting and folding by frost-heave action. They overlie glacial lake clays which' in turn overlie a lower sand and gravel unit (M. R. Clarke, personal communication 1975). RDL

Danbury area

For convenience of description the Danbury area is defined roughly as the tract of land bounded by the River Chelmer to the north, the Sandon Brook to the west, and the unnamed tributary to the Sandon Brook to the south of Danbury. The eastern boundary is arbitrary, but is approximately a north-south line to the west of Woodham Walier and Woodham Mortimer. This eastern boundary is somewhat artificial as the gravel is at least in part continuous across it; nevertheless it divides the complex frost-heaved high-level outcrops of the Danbury Hill area from the flatter, low-lying, more continuous deposits of the Woodham Walter-Woodham Mortimer area. Previous authors (Gregory, 1915; Clayton, 1957a; etc.) have confined the outcrop of the Danbury Gravels to the higher part of Danbury Hill, above 45 m OD. Hey (1980) suggested that part of the gravels on Danbury Hill represents an outlier of the high-level, Westland Green Gravel.

The sand and gravel depicted on Old Series Sheet 1 NE occurs in a nearly continuous outcrop in the Danbury area, although limited inliers of London Clay are also shown. Turner's mapping (1934, MS BGS) revealed the complex nature of the outcrop, and the outcrop pattern has been still further modified during the recent six-inch-to-the-mile survey.

The gravels rest mostly on the London Clay, although very locally e.g. [TL 7680 0433], [TL 7718 0392], and [TL 7810 0370] chalky Boulder Clay has been mapped between them and the London Clay. Turner (1934, MS BGS; 1937, p. 161) recorded 2.4m of greyish brown Boulder Clay which passed beneath the sand and gravel in the area between Firtree Lane and Wayside [TL 7845 0610].

In the western part of the area, Glacial Lake Deposits and terrace deposits mask the sand and gravel/London Clay contact; to the south and east extensive spreads of Head, Head Brickearth and Brickearth obscure this junction.

One interesting feature of the area is the presence of deep drift-filled channels whose origin has given rise to much discussion in the past (Solomon, 1935, p. 226; Turner, 1937, p. 160; Clayton, 1957b, p. 25). The presence of bedding parallel to the locally vertical channel margins demonstrates that the channels cannot be original features although they probably include depositional channels which were later modified by ice action. Their origin is discussed more fully on p. 39. In addition to the three well known structures (Wedlock Green [TL 785 075], Christmas's Pit [TL 776 051], and St Clere's Hall, [TL 767 058]) it is probable that some of the other finger-like outcrops on Danbury Hill have a channel-like form. Two such areas are near Hollybred Farm and Little Baddow Road areas where boreholes [TL 7719 0780] and [TL 7839 0592] proved 22.3 m and 17.2 m respectively of sand and gravel (Clarke, 1975). The percentage of fines, sand and gravel on Danbury Hill were 9, 49 and 42 respectively (Clarke, 1975). Figures for the thickness of the deposits in this area are so variable as to make an average thickness figure of little use. The maximum recorded thickness is more than 27.4 m in the St Clere's Hall Pit.

The St Clere's Hall Pit is divided into two sections by a ridge of London Clay which trends north-north-eastwards. The western part [TL 7645 0575] was 7.0 m deep in 1966 when the following descending sequence [TL 7646 0568] was measured within a horizontal distance of 20 m from the London Clay outcrop: cemented gravel, 1.22 m; medium- to coarse-grained orange and yellow sand, 3.05 m; gravel, 1.52 m; medium- to coarse-grained orange and yellow sand 1.22 + m. At the northern end of the eastern part of the pit [TL 767 058] the sand and the gravel/London Clay contact was formerly well displayed (Plate 4). The junction at the surface was almost vertical, although it flattened out rapidly towards the centre of the channel. At the southern end of the pit [TL 7665 0560] the contact dipped at 45° towards the centre. Small-scale faulting was noticed in the gravel in this vicinity. It may be that the less steep dips are near-depositional features and that the high dips have resulted from frost-heaving.

To the south of the pit a borehole [TL 7618 0511] in Danbury Park proved 6.1 m of Glacial Sand and Gravel beneath 2.42 m of Head. Another borehole [TL 7574 0458] encountered 0.61 m of clayey silt beneath 1.83 m of clayey sand. The lower stratum resembles the Glacial Lake Deposits which crop out to the west.

The sand and gravel in the vicinity of Mayes Farm [TL 753 043] has largely been quarried away and backfilled. A small section [TL 7507 0415] which was visible in 1970 showed an irregular spread of Head 0.8 to 2 m thick, overlying 1.5 m of fine mottled orange and grey clay with small scattered flints (Glacial Lake Deposits) which in turn overlay the sand and gravel exposed in the bottom of the pit. The gravel in this vicinity appears to have occupied a steep-sided channel, the southern boundary of which coincides with a field boundary. The gravel is reputed to have been worked to a depth of 12 m. The channel structure is presumably a continuation of that noted above to the north of Sandon Hall on the west side of the small unnamed river which separates the two localities.

A patch of sand and gravel to the south and south-east of Sandon Hall [TL 7530 0357] has been worked in the past to a depth of at least 2.4 m.

Farther east a spread of clayey sand and gravel Head appears to overlie sand and gravel in the region of Levett's Farm. In the extreme east of this outcrop [TL 771 034] sand and gravel is present at the surface; three other small outliers of gravel have been mapped in this vicinity [TL 7735 0340], [TL 7760 0340] and [TL 7720 0365]. A spring line marks the base of the deposit of the second of these outliers; at the latter locality Turner (1934, MS BGS) noted 1.8 m of sand and gravel in shallow pits. Sand and gravel may be present at the surface on the west side of the road where it has been worked in small diggings [TL 7690 0378] for domestic use in the past.

A borehole in this area [TL 7664 0377] proved 2.7 m of clayey pebbly sand beneath 2.5 m of Head, and resting on the London Clay.

Continuing eastwards, sand and gravel has been mapped as inliers beneath Head in the area between Peartree Farm [TL 786 032] and Slough House [TL 806 032]. Springs [TL 7848 0360] to [TL 7878 0370] mark the base of the deposit 400 m N of Peartree Farm. A borehole [TL 7874 0341], 250 m NE of the farm, proved only 0.8 m of sand and gravel beneath 2.3 m of pebbly silty clay (Head); elsewhere the presence of small pits [TL 7829 0331], [TL 7845 0304], [TL 7848 0335] and [TL 7878 0367] testifies to its former extraction, presumably for domestic purposes.

Gravel has also been dug at two points [TL 7953 0325] and [TL 7970 0331] in the White Elm area, but two boreholes [TL 8060 0298] and [TL 8047 0379] failed to prove significant amounts of sand and gravel (Ambrose, 1973a). Turner (1934, MS BGS) recorded 'coarse gravel' in an old pit [TL 8100 0365] 500 m WSW of the latter borehole. Westwards from this pit some 2.1 m of clayey gravel were encountered in a borehole [TL 7971 0365].

Sand and gravel become more evident westwards where the outcrops are largely free from Head. An old gravel pit [TL 7895 0430] immediately west of Gay Bowers is about 5 m deep. Only the uppermost 2 m were visible in 1970 and consisted of well rounded flints set in an orange-brown unbedded sandy matrix. This is thought to be Head. J. S. Turner in 1934 (MS BGS) noted irregularly bedded contorted sand and gravel in this pit.

The lobe of sand and gravel on which Gay Bowers Farm is situated [TL 7863 0445] on the opposite side of the valley to the above pit, may be a channel, as a borehole [TL 7864 0456] proved 9.1 m of sand and gravel beneath 2.7 m of soil and Head. The deposits have been worked in the past in the northern part of the lobe, and on the adjacent outcrop, both areas of which fall within Danbury Common.

In the area between Danbury Common [TL 783 043] and Woodhill [TL 768 044] are a number of scattered outcrops of sand and gravel which in general form small low rises on the London Clay surface. Despite the small size of the outcrops, thicknesses of up to 6.4 m of sand and gravel have been proved in boreholes [TL 7666 0442] and [TL 7788 0394] close to the edge of the deposits, which is indicative of their channel-like form.

North of the Common is the famous Christmas's Pit [TL 776 051], now partially filled and built over. It was presumably this pit in which Corder (1887, p. 92) described ' … a curious contortion, a quantity of London Clay being brought up at a sharp angle, and thinned out for some yards between the gravel and the sand'. It is assumed that Salter (1906) was also referring to this pit when he described the sand and gravel as 'much contorted'. He put forward four hypotheses (solution collapse, large-scale earth movements, landslipping, and the impact of an ice-sheet) to account for the disturbance, but did not state which he preferred (see p. 39). A maximum of 5.5 m of sand and gravel in contact with the London Clay was visible in 1966 on the eastern side of the pit [TL 7766 0500].

A borehole at the Bell Inn, Danbury [TL 7731 0533], 300 m NW of the pit, proved 7.3 m of sand and gravel beneath 2.4 m of topsoil and Head.

Approximately 1.5 km E of Christmas's Pit can be found the former Hillen's Pit [TL 791 051]. Solomon (1935, p. 226) figured a sketch section in which about 1.8 m of clayey gravel overlay 3 m of clean sand and gravel. Turner (1934, MS BGS) described the deposits as cross-bedded sand with gravel, and much contorted. Sections which were still visible in 1966 in the eastern part of the pit [TL 7925 0510] revealed up to 1.8 m of made ground and from 1.2 to 2.7 m of sand and gravel over London Clay. A nearby borehole [TL 7908 0515] encountered sand and gravel to a depth of 3.3 m. London Clay was proved at the surface in a field 100 m to the north-east.

Two pits [TL 7930 0525] and [TL 7937 0538], immediately east of this latter occurrence on the north side of the main road, appear to be separated by a ridge of London Clay, as this clay was augered in a ditch at one point [TL 7939 0534].

Some 700 m NW is the Nurseries Well [TL 7879 0571] where 18.3 m of sand and gravel were proved. A borehole [TL 7839 0592] 450 m WNW, encountered 2.5 m of Head over 14.7 m of sand and gravel, which in turn rested on London Clay. Only 100 m ENE Turner (1934, MS BGS) noted London Clay at the surface [TL 7850 0595]. To the north of this latter occurrence Turner noted Boulder Clay (? Maldon Till) passing beneath sand and gravel [TL 7844 0609].

At the eastern end of this giavel lobe, which extends as far as Woodham Walter Common, a borehole [TL 7940 0673] proved gravel to a depth of 7.2 m within 80 m of the mapped contact between sand and gravel and London Clay. Springs mark the base of the sand and gravel to the east [TL 7970 0673] and south [TL 7947 0654] of this borehole. Another borehole [TL 7813 0675] near Long Wood similarly sited close to the mapped base of the deposit encountered 5.2 m of sand and gravel beneath 0.6 m of Head.

An ice-heave structure at Wedlock Green [TL 787 077], on an outlier to the north, has been documented and figured by Clayton (1957b, pp. 24–25, fig. 1). He noted that the pit was 18 m deep and had been worked to a depth of 30 m. From his fig. 1 it is evident that the London Clay, which he thought was not in situ, but glacially redeposited, is vertical or overturned. There are two pits in this vicinity, separated by a ridge of London Clay. The westernmost one, described by Clayton, has been backfilled with London Clay, but up to 5 m of sand and gravel were still visible on the northern face in 1966. The eastern pit has similarly been filled, but again up to 5 m of sand and gravel could be seen in the old faces.

A borehole [TL 7869 0735] sited within a few metres of the mapped contact between sand and gravel and London Clay, demonstrated the steepness of the sides of another channel which lies immediately west of the one described above: it proved that sand and gravel extended down to a depth of 11.4 m beneath 1.6 m of Head. The channel is presumably filled by a finger-like lobe of sand and gravel which extends north-north-westwards for 1.5 km past Tofts. It is probable that other extended outcrops in this vicinity are also channel fills. A borehole at Hollybred [TL 7719 0780] proved one of the thickest deposits of sand and gravel (22.3 m) in the Danbury area.

Some 700 m to the west a borehole [TL 7647 0790] at Little Baddow Hall Farm encountered 2.3 m of plastic light grey silty clay resting on 6.8 m of sand and gravel, before entering the London Clay: there was 1.8 m of Head at the top of the borehole. The light grey silty clay is interpreted as Glacial Lake Deposits, although none was recognised at the surface in the vicinity.

Woodham Walter–Woodham Mortimer–Maldon area

This area differs from the preceding one in that the base of the sand and gravel is more regular. Although minor ice-heave structures presumably exist, the marked irregularities which are characteristic of Danbury Hill have not been recognised. Borehole evidence suggests, however, that such structures exist to the north of Woodham Mortimer (Ambrose, 1973a, fig. 2). There are extensive spreads of Head masking the sand and gravel deposits in the southern and eastern parts of the area, and it has not everywhere been possible to distinguish them satisfactorily. The thickest deposits of sand and gravel recorded are 18.3 m and 13.7 m in boreholes at Oak Farm [TL 8101 0599] and Lodge Farm [TL 8200 0600]; clay bands up to 1.8 m thick were noted within the sand and gravel. These clay beds are in general confined to the more southerly outcrops and may be a feature of deposition close to the ice margin. The average grading figures for the sand and gravel of this area are: fines 15 per cent; sand 47 per cent; gravel 38 per cent. These figures show that the Glacial Sand and Gravel as a whole is finer than the terrace sand and gravels within Sheet TL 80 (Ambrose, 1973a). Locally the Glacial Sand and Gravel rests on Maldon Till.

A borehole [TL 8133 0435] at Nursery Farm encountered 4.3 m of clayey gravel [Head] overlying 5.5 m of sandy gravel which in turn rested on the London Clay. Similar thicknesses were recorded in a borehole [TL 8072 0469] 700m NW.

Some 500 m farther north-west are the several Royal Oak pits. J. S. Turner (1934, MS BGS) noted 3 m of fine and coarse gravel with some sand in the old pit [TL 8060 0495] to the east of the Royal Oak. Since that time large pits have been opened to the north and northeast of the public house [TL 803 051] and [TL 806 051]. The western pit has now been restored. Generally the sections and boreholes in this vicinity are in clayey sand and gravel which can be interpreted as Head or as ice-marginal deposits. One section which was measured in the north-eastern pit [TL 8067 0520] in 1966 showed 2 m of gravel over at least a metre of brickearth. Clayton (1957b, p. 23) stated that the brickearth has been proved in boreholes to be up to 2 m thick and to overlie more than 12 m of sand and gravel. Some 700m NW of the pit a small inlier of London Clay [TL 8012 0557] has been brought to the surface possibly by ice heaving; alternatively, it may be a lens of redeposited London Clay such as was noted in the Warren Farm Pit (see below). Clayton noted the presence of a red conglomerate, a brecciated black slate and a block of puddingstone at this locality. Puddingstone was also found by the author in a small pit [TL 8117 0710] 400 m NE of Woodham Walter.

A borehole [TL 8099 0501] at Little Smith 400 m SW, proved a total of 11.9 m of silt, sand and gravel with an average of 18 per cent fines. A similar sequence with 11 per cent fines was proved in another borehole [TL 8016 0506], to a depth of 13.1 m, below which was the London Clay. A third borehole [TL 8072 0469], approximately midway between the other two, had only 2.8 m of sand and gravel of workable quality, although it proved a total of 11.0 m of sand and gravel (and Head) above the London Clay. The 8.2 m of sand and gravel encountered in a borehole [TL 8200 0523] at Woodham Mortimer likewise contained 15 per cent of fines (Ambrose, 1973a).

The Warren Farm Gravel Pit [TL 804 065] was described briefly by Clayton (1957b, p. 24). At the time of the field survey (1966) it was possible to see some 5 m of sand and gravel, but it is probable that the deposit is considerably thicker. A borehole [TL 8004 0605] on the Golf Course 400 m W of the pit proved 8.2 m of sand and gravel overlying London Clay; whilst a borehole [TL 8101 0599] 600 m ESE was still in clayey sand and gravel at a depth of 18.3 m, though it is not certain exactly how much of this should be classified as Glacial Sand and Gravel. A section [TL 8041 0620] measured in 1966 in the Glacial Sand and Gravel of the pit showed:

The lower clay was thought initially to be London Clay, but later (1967) workings showed that it was a lens with a maximum thickness of 3 m, but averaging 2 m, which was underlain by sand and gravel.

Similar lenses with volumes of as much as 20 000 m³ have been encountered in the past in this pit. Clayton (1957b, p. 24) recorded an erratic of silicified oolite and a fragment of Lower Greensand chert in the white sand and gravel of the lower part of the pit.

Locally the sand and gravel is cemented into a conglomerate. The sand fraction appears to be dominant in the deposits in the pit, but no precise figures are available. In the Golf Course Borehole (see above) it constituted 36 per cent of the deposit. To the east material of doubtful origin encountered in a borehole near Oak Farm [TL 8101 0599] contained 53 per cent sand. In the vicinity of Curling Tye Green, boreholes [TL 820 060], [TL 8168 0700], [TL 8267 0713] and [TL 8282 0753] showed a variable layer of Head, up to 6.4 m thick, which was not separable easily from the underlying beds, overlying irregular spreads of clayey gravel up to 12.8m thick.

Farther east, on the outlier around Great Beeleigh Farm, a borehole [TL 8347 0715] encountered 3.7 m of Head over 7.9 m of sand and gravel. The sand here formed 67 per cent of the deposit.

The old railway cutting at Maldon [TL 8415 0650] is the type locality of the Maldon Till, where Whitaker (1889, pp. 316–317) saw sand and gravel overlying Boulder Clay. The Hospital Well [TL 8457 0687] did not, however, encounter any Boulder Clay beneath the 8.23 m of sand and gravel.

Gravels at a lower level can be found to the south of Maldon Wick. A number of boreholes in this vicinity around [TL 843 057] show the deposit to be up to 2.4m thick; it has been variously described as resting on chalky Boulder Clay or London Clay. No Boulder Clay was found at outcrop, but the London Clay contained much race and it is thought that this has been misinterpreted as chalky Boulder Clay.

Tiptree ridge

In the Wickham Bishops–Great Totham–Great Braxted area the fairly continuous sand and gravel in the southwest gives way to thinner and more scattered outcrops in the northeast. Much of the surface is obscured by Head and, locally, chalky Boulder Clay. The conclusion that the sand and gravel in this vicinity should be classed as Chelmsford Gravels is supported by its grading characteristics (Haggard, 1972, fig. 3). The percentage of fines, sand and gravel are 5, 47 and 48 respectively. The average thickness for the sand and gravel in this block is 3.5 m.

Some 1.5 m of brown silty clay was encountered within an 11-m thick sand and gravel sequence in a borehole [TL 8274 1135] 400 m SE of Wickham Bishops Station.

An old pit [TL 8345 1137] in Wickham Hall Lane is shown as a marl pit on the Old Series Geological Sheet, but according to the local inhabitants sand was also extracted from it before it ceased to be worked in about 1925; as chalky Boulder Clay can be augered round the top of the pit it is probable that the Chelmsford Gravels were also extracted. The pit now appears to have been partially back-filled with London Clay. North of this locality the Chelmsford Gravels become fragmented into a number of small outcrops.

A borehole [TL 8606 1122] sited alongside one of the old pits at Great Totham Hall passed through 9.1 m of sand and gravel into the London Clay. A section [TL 8598 1168] visible in these pits in 1966 showed up to 5 m of sand and gravel. London Clay was augered at the surface within 60 m of the exposure.

The sand and gravel deposits in the main part of the Tiptree ridge from Great Totham to beyond Tiptree are nearly continuous. They occupy a shallow depression in which the water table tends to be high. Sand constitutes 72 per cent of the total sand and gravel deposit, with fines forming 8 per cent and gravel 20 per cent. The deposit has a maximum thickness of 10.7 m, but averages 4.8 m, and is commonly divided by seams of clay. Northwards the sand and gravel passes beneath the Springfield Till.

Pits [TL 886 159] opened in these deposits to the west of Tiptree exposed up to 3.6 m of poorly stratified clayey sand and gravel in 1967. Rose and others (1976, fig. 1) recognise the presence of a rubified sol lessive at the top of the gravels in this vicinity. Hey (1980) suggested that some of the gravels in this area might represent outliers of the high-level Westland Green Gravels.

Little Braxted–Messing

This area forms a transition zone between the plateau north-west of the River Blackwater, and the Tiptree ridge.

Sand and gravel emerges to the south-east of the Springfield Till, which occupies the drift-filled Blackwater depression. The sand and gravel can be seen to be irregular at outcrop, and this has also been proved to be true at depth. The base of the sand and gravel drops towards the Blackwater Valley, and it is on this southern flank that some of the thickest deposits are found. However the sand and gravel does not extend under the valley of the Blackwater. The small scattered outliers between the chalky Boulder Clay and the Tiptree ridge form low rises.

The thickness of the sand and gravel varies from 0 to 16.8 m with an average of 6.4 m. The grading figures for the sand and gravel showed a ratio of about 2:1 of sand to gravel (67 to 31 per cent, respectively). This contrasts with\the Chelmsford Gravels of the plateau area, where proportions of sand to gravel of 53:43 and 45:50 are recorded, but approximates to the proportions found at Tiptree (Haggard, 1972). This south-eastward fining of the deposits is presumably related to their marginal position in relation to the ice front.

Little Totham–Tolleshunt Major–Tolleshunt D'Arcy–Tollesbury

Irregular spreads of sand and gravel which are partially covered by Head rest on London Clay between Little Totham and Tollesbury. As noted above (p. 34) these deposits have a higher proportion of gravel than either the Chelmsford Gravels to the north-west, or those along the Tiptree ridge, and may include terrace sand and gravel: Woodward (in Whitaker, 1889, p. 320) was also uncertain as to their origin.

Shallow pits [TL 876 110] to the east of Jeperack's Farm exposed a variable thickness of sand and gravel. Ice-heave ridges of London Clay e.g. [TL 8741 1088] effectively cut out the sand and gravel deposits in places. A maximum thickness of 2.4 m of sand and gravel was noted in 1966, whilst 4.9 m of clayey gravel beneath 2.1 m of soil and ?Head were proved in a borehole [TL 8775 1094] near Godfrey's Farm to the east.

Farther east ditch sections and augering have revealed a number of London Clay inliers [TL 8968 1229], [TL 9018 1196], [TL 9028 1188], [TL 9117 1221], [TL 9140 1025], [TL 9236 1195] and [TL 9410 1110]. A borehole [TL 9021 1127], sited 120 m W of a 'London Clay' inlier in which a (?brick) pit had been opened [TL 9033 1129], proved 2.4 m of soil and brown clay over 5.2 m of clayey sand and gravel before entering the London Clay. It is possible that some of the other small occurrences are of reworked London Clay, and may be equivalent to the interglacial deposits recognised by Lake, Ellison, Hollyer and Simmons (1977) in the Southend-Southminster area.

Old gravel pits [TL 9197 1055] and [TL 927 113] exist alongside the Maldon-Tolleshunt D'Arcy road, but only shallow exposures remain. A borehole [TL 9264 1141] just north of the latter pit proved, in descending sequence 4 m of soil and brown clay; 0.6 m of very clayey gravel; and 3.6 m of sandy gravel on London Clay.

A coarse-grained white sand was augered at one point [TL 922 111] on the west side of the road. The surface indications on this side of the road point to very gravelly deposits which appear to have been worked in a number of shallow pits [TL 9242 1130], [TL 9228 1174] and [TL 9205 1173]. Similar very flinty soil debris can be found in the vicinity of Garland's Farm [TL 9478 1040] where old pits occur. At Garland's Farm a borehole [TL 9484 1047], 100 m NE of the farm, proved sand and gravel to a depth of 7.6 m. The average gravel content of the sand and gravel in this borehole was 52 per cent. CRB

Chalky Boulder Clay (Springfield Till)

West Side of River Chelmer

An extensive spread of chalky Boulder Clay is present on the west of the River Chelmer along the western extremity of the district. Just west of the boundary in the Great Waltham area, pits [TL 685 118] establish that the Boulder Clay exceeds 10 m in thickness. The contact of the Springfield Till with the underlying Glacial Sand and Gravel or London Clay is for the most part obscured by an extensive spread of Head which mantles both sides of the valley. Over most of this area the till is chalky at the surface, but south of Stacey's Farm [TL 697 103] surface weathering has commonly changed it to an orange-brown sandy and stony clay. To the south of Patching Hall Lane the Boulder Clay was worked for brickmaking [TL 7012 0852]. RDL

In the Chelmsford area the drift-filled channel (see p. 49), is partially filled with chalky Boulder Clay. In the northern part of Chelmsford the channel appears to lie to the west of the present River Chelmer. Boreholes [TL 7082 0739] and [TL 7061 6736] proved 10.69 and 2.4 m of 'clay and chalk' beneath 13.72 and 15.55 m of sand and gravel respectively. The levels of the base of the Boulder Clay are 11.15 and 11.58 m OD respectively. Boulder Clay is absent in a well [TL 7095 0740] 180 m to the east where 4.26 m of 'drift' and 'sand and gravel' rest directly on the London Clay at 26 m OD.

The best evidence for the presence of chalky Boulder Clay filling the channel is provided by boreholes sunk in the Chelmsford Redevelopment area around [TL 708 068], where the drift extends down to below −2.44 m OD. The deposits filling the channel lie beneath a cover of up to 6 m of made ground and sand and gravel. They consist of typical chalky and flinty clay interstratified with grey calcareous silts and small chalk pellets and have been interpreted as Glacial Lake Deposits. CRB

Between the Chelmer and Ter rivers

The base of the Springfield Till is at its highest level in the north-west of this area, where it is at about 60 m OD. Over much of the plateau the base is more or less planar at about 40 m OD, but it falls to the west into the Chelmer Valley, and to a lesser degree to the east into the streams which flow on either side of Boreham Airfield (Figure 17). In the Springfield area the configuration of the base is complicated by glaci-tectonics (see p. 58), where diapiric structures of London Clay push through the Glacial Sand and Gravel and chalky Boulder Clay to the surface.

In the area north of Little Waltham the thickness of the Springfield Till generally exceeds 10 m. Boreholes [TL 7078 1506] and [TL 7257 1454] near Chatham Green were still in Boulder Clay at a depth of 18.3 m; on the south-eastern portion of the plateau the thickness is generally less than 10 m and diminishes towards the valleys. The maximum thickness proved in this area was in a borehole [TL 7599 1267] near Porridgepot Hall where it was 17.4 m. Here the thickening of the Boulder Clay was at the expense of the underlying Glacial Sand and Gravel which is absent at this point. A similar relationship has been observed in some, but not all, of the adjacent boreholes, e.g. in boreholes [TL 7485 1312] and [TL 7487 1363] near Bird's Farm. It is especially marked in an area of thick chalky Boulder Clay to the north of Little Waltham.

North of Littley Green, the chalky Boulder Clay appears to be extensively decalcified by weathering to a depth in excess of 1.4 m. In the valley of the River Chelmer, however, where the weathered profile is reduced, the Boulder Clay was chalky at its base; this is probably due to the reduced thickness of Glacial Sand and Gravel in this area. RDL

In the Broomfield Pit a section [TL 7230 1128] at the eastern side of the workings showed 8 to 10 m of buff-brown chalky Boulder Clay overburden. The contact with the underlying Glacial Sand and Gravel was remarkably planar (Plate 1).

North of an approximate line between the Broomfield Pit, Whitehouse Farm [TL 738 145], Scarlett's Farm [TL 740 144] and Leyland's Farm [TL 750 149], chalky till can generally be found close to the surface. To the south of this line the Boulder Clay is for the most part non-chalky at the surface, probably due to decalcification. This decalcification is probably associated with the presence of thin patches of overlying permeable solifluxion gravels, which were not mapped separately but are generally present in this area. Where chalk is consistently absent from clayey surface deposits, as in the eastern suburbs of Chelmsford, these have been mapped as Head. Rose and Allen (personal communication, 1976) infer from the reddening commonly associated with this non-chalky material, especially that in the Springfield area, that it represents a post-Anglian reddened palaeosol.

The depth of decalcification is locally considerable. A borehole [TL 7599 1267], + km W of Porridgepot Hall, passed through 9.1 m of topsoil and brown Boulder Clay before entering chalky Boulder Clay. Some 1.5 km NW the whole of the 8.8 m of Boulder Clay is non-chalky [TL 7518 1386]. Another borehole [TL 7536 1412] 300m NE of the above, proved 2.4 m of 'brown Boulder Clay with sand lenses' overlying chalky Boulder Clay. At the Wallace's Farm Pit [TL 751 113] up to 1.2 m of brown sand clay was observed overlying grey chalky Boulder Clay.

From temporary exposures around [TL 720 096] to the south-west of Nabbott's Farm the following derived fauna and rock specimens were obtained Gryphaea dilatata (Oxford Clay), G. arcuata (Lower Lias), burrow of Teredina personata (Eocene–Oligocene), Neohibolites minimus (Gault), hoplitid ammonite (Gault), Holaster sp. (Chalk), Plinthosella? (Chalk), Ventriculites sp. (Chalk), cf. Ooliticia (?Cretaceous), Cardioceras spp. (upper Oxford Clay), Amoeboceras sp. (Ampthill Clay), Rasenia (lower Kimmeridge Clay), nerineid gastropod (?Lincolnshire Limestone), Pleuromya? (Lower Lias), Modiolus sp. (Lower Lias), Montlivaltia?, Ditrupa (Tetraditrupa) sp. (? Jurassic), fragment of ?Coralline Crag, piece of ?Forest Marble, and Red Chalk. Collectively they indicate derivation from the north.

On the eastern side of the Chelmsford By-pass, some 600 m ESE of the Prison, recent boreholes have shown that the Boulder Clay extends about 100 m farther east at [TL 7253 0705] than portrayed on the present printing of the 1:50 000 geological sheet.

Between the Ter and Brain Rivers

The base of the Springfield Till falls gently south-eastwards from about 65 m OD on the crest of the interfluve in the Rank's Green area to about 30 m OD on the margin of the Blackwater buried valley to the south-west of Witham. Within the buried channel boreholes have proved drift extending down to −57 m OD. In addition to the regional fall, the base of the Boulder Clay also dips down into the valleys of the major tributaries of the rivers Ter and Brain.

The thickest chalky Boulder Clay of the plateau area is 12.8 m near Troys Farm [TL 7773 1734]. An even thicker occurrence (17.1 m) is noted on the north side of Hazelton Wood just beyond the boundary of the district.

Thicker drift deposits are known to fill the south-western portion of the deep Blackwater channel. It is evident that not all the deposits underlying the terrace gravels can be referred to till as laminated chalky silts locally occupy the channel (M13). CRB

To the north and east of Great Leighs the Boulder Clay is extensively decalcified to depths which are commonly in excess of 1.4 m. This may be due in part to the presence of gravelly lenses within the till, the water from which would have assisted leaching. A temporary exposure [TL 7406 1807] at a surface level of 73 m near North Whitehouse was probably in one such lens, as it showed 1 m of sand and gravel beneath 0 to 1.2 m of orange and grey mottled sandy clay, whereas a nearby borehole [TL 7432 1841] proved the base of the chalky Boulder Clay to lie at 67 m OD. A borehole [TL 7344 1859] near Moulsham Hall just north of the district proved:

Scattered shallow pits in the Gubbion's Green area possibly formerly worked thin gravels within the chalky Boulder Clay.

It is only on the east side of the valley at Fuller Street [TL 750 160] that chalky Boulder Clay can consistently be found close to the surface. This belt of fresh chalky Boulder Clay can be followed eastwards as far as the River Brain, and south-eastwards to Hatfield Peverel.

A spread of orange and reddish brown sandy and stony clay in the vicinity of Butlers [TL 805 102], which has been mapped as Head, may represent a decalcified till. A borehole [TL 8127 1043] to the east-north-east of Butlers proved 5.5 m of 'brown Boulder Clay with much chalk in upper part, becoming less chalky with depth'. This stratum was overlain by 0.9 m of clayey loam, and underlain by the London Clay.

Between the Brain and Blackwater rivers

Within this plateau area the maximum recorded thicknesses of Boulder Clay are 10.4 m [TL 8101 1818] and 10.1 m [TL 8222 1734] on the crests of the interfluves, but the average thickness is about 6.6 m (Haggard, 1972). The base of the Boulder Clay on the plateau falls gently south-eastwards from 36 to 24 m OD on the edge of the Blackwater Valley. In the valley the Boulder Clay base drops down to below −44.5 m OD in the bottom of the tunnel valley (Figure 17) and (Figure18).

Over most of this area chalky Boulder Clay is found close to the surface. Locally the surface deposits have weathered to a brown sandy and stony clay; small patches of the weathered till have been noted [TL 812 163] to the west of Elm Hall Farm, in the area + km NE [TL 829 174] and 700m E [TL 831 172] of Stovern's Hall, and on the western margin of this area [TL 797 181], + km W of Hungry Hall.

Several small pits, presumably dug for marl, have been opened in the past in the Boulder Clay. A concentration of six such pits can be found 700 m N and NE of Hoo Hall [TL 836 180] and [TL 839 178].

Blackwater Valley

In the Blackwater Valley several boreholes have proved that chalky Boulder Clay extends below −57 m OD (p. 50, Figure 18). On the south-eastern margin of the valley the base of the Boulder Clay rises rapidly to 40 m OD and just beyond the margin of the district a more gradual rise can be traced to a high point of 60 m OD.

The deeper part of the drift-filled channel lies on the western side of the present-day Blackwater Valley, and it is here that the thickest river gravels and interglacial deposits occur. It is not always possible to decide from existing borehole logs whether the glacial infill consists wholly of Boulder Clay, or of laminated glacial silts, or both. However the 57.4 m of chalky Boulder Clay in the Witham Borehole [TL 8244 1534] is the thickest record of this lithology known in the Blackwater Valley. The Boulder Clay showed little lithological variation, except that in the lowest 6.42 m large chalk pebbles were less common, and small flints were more common, than in the upper part. In general the chalk pebbles rarely exceeded 10 cm, and were mostly less than 2.5 cm. In the Kelvedon Borehole [TL 8602 1797] the channel fill differed in that the lower part of the fill consisted of grey laminated silts and silty clays with only a minor proportion of fine chalk pellets. This unit, some 21.49m thick, passed upwards with no sharp lithological break into typical chalky Boulder Clay 34.47 m thick. These laminated silts are regarded as having formed sub-glacially (see p. 36).

Chalky Boulder Clay has been augured at the surface [TL 8263 1428] and [TL 8280 1442] to the north of Witham sewerage works. Small pits nearby were presumably dug for marl, but elsewhere more recent deposits obscure much of the outcrop of the till in this vicinity. Boreholes along the line of the Witham By-pass showed a variable thickness of 1 to 3 m of sand and gravel resting directly on the London Clay, Boulder Clay being absent.

Farther north a borehole [TL 8450 1620] near Appleford Farm proved 16.1 m of chalky Boulder Clay beneath 5.2m of river deposits, and resting on the London Clay at about −1.5 m OD. Another borehole [TL 8363 1559], near Coleman's Farm, 1 km SW, proved a similar stratigraphic sequence of 7.0 m of river deposits, over 'grey clay with chalk inclusions', 15.9 m thick, resting on London Clay at -7.3 m OD. The 'grey clay with chalk inclusions' may be the laminated glacial beds encountered in the lower part of the Kelvedon Borehole.

Boulder Clay is generally found close to the surface near Little Braxted, although locally it has lost its characteristic chalkiness and has weathered to a brown sandy and stony clay.

A borehole [TL 8375 1450] at Little Braxted proved brown chalky Boulder Clay to a depth of 4.6 m passing downwards into 6.1 m of grey chalky Boulder Clay: the base of the till lies at 20.4 OD. Another borehole [TL 8395 1399] 560 m SSE, sited close to the feather edge of the Springfield Till, was in brown chalky Boulder Clay to a depth of 3 m, and grey chalky till for an additional 1.6 m, the base of the Boulder Clay here being at 30.1 m OD. Numerous pits, all now abandoned, overgrown or flooded, have been dug on this outcrop. A well [TL 8447 1447], 100 m to the east of one such pit, proved 9.1 m of Boulder Clay over sand and gravel; the junction lay at 30.5 m OD. Some 400 m NE a borehole [TL 8473 1478] proved the base of the Boulder Clay somewhat lower than anticipated at + 23.3 m OD.

A small outlier [TL 8545 1515], partially obscured by terrace gravels, was proved by augering 300 m SW of Braxted Park House. Similar small spreads covered by irregular spreads of Head and 4th Terrace gravels occur 700 m NW and NNE of Braxted Park House. A borehole at Hill Broad Farm [TL 8472 1564] proved chalky Boulder Clay beneath terrace gravels, to have a thickness in excess of 7.6 m. Some 4 m of brown chalky Boulder Clay was encountered in a borehole [TL 8554 1608] in Great Braxted Park beneath 2.7 m of Terrace deposits; the base lay at 30.2 m OD.

A borehole [TL 8607 1708] to the north-west of Kelvedon Hall Farm proved 12.6 m of chalky Boulder Clay to a depth of 21.3 m OD above Glacial Sand and Gravel. Some 500 m to the SE the base of the Boulder Clay rises to 45 m OD. An even steeper rise in the base occurs near Highfields. A borehole here [TL 8675 1747] passed through 2 m of soil and 4th Terrace deposits, and 13.2 m of chalky Boulder Clay, before entering Glacial Sand and Gravel at a level of 22.9 m OD. London Clay can be augered at the surface at a height of about 40 m OD 150 m to the east. In a north-westerly direction the base of the Boulder Clay falls to below −44.5 m OD (Figure 17) and (Figure18).

Wickham Bishops area

A small area of Springfield Till is present in the Wickham Bishops area, but is largely obscured by Head.

Chalky Boulder Clay can be augered around the top of an old marl pit [TL 8345 1137] 200 m SW of Whitehouse Farm (see p. 45), and on the northern edge of a field [TL 8350 1116] bordering Likely Wood.

The larger spread of Boulder Clay [TL 836 106] shown km E of Wickham Hall is only doubtfully identified as such. In augering a stiff brown clay, locally with race and very similar to the London Clay, was consistently encountered. However at several points it was possible to auger through this clay into an underlying sand, and at one point [TL 8361 1059] chalky Boulder Clay was proved. The clay appears to overlie Glacial Sand and Gravel, and is itself overlain by Head. The presence, at least locally, of chalky Boulder Clay, its topographic position, and the apparent regularly stratified succession have led to this deposit being mapped as a till. Some 500 m NNE a borehole [TL 8396 1095] proved 3.4 m of 'brown Boulder Clay with a little chalk' overlying 6.4 m of gravel. Head is mapped at outcrop in this vicinity. Some 1.2 km to the east another borehole [TL 8514 1095] drilled in the north-west corner of Captain's Wood, passed through 9.1 m of brown clay with traces of gravel, here interpreted as Head, before entering 1.6 m of grey chalky Boulder Clay which in turn rested on a thin remnant, 0.9 m thick, of clayey gravel (Haggard, 1972).

South of the River Chelmer

South of the River Chelmer irregular spreads of chalky Boulder Clay have been mapped in the Widford, Galleyend, Hanningfield and Woodham Ferrers area. For the most part they were included by Clayton (1957a, fig. 1) within his Hanningfield Till, the principal exception being the till in the Little Mascalls area [TL 735 025] which he mapped as Springfield Till. Much of what Clayton regarded as deeply weathered till is now mapped as Head.

The base of the till has a general southerly rise to a maximum height of about 76 m OD, but the overall picture is masked by the fall of the base into the valleys of the Sandon Brook and Diver Wid which drain northwards across the area (see (Figure 17)). The maximum thickness proved was 14.9 m [TL 7123 0415] on the largest outcrop near Galleyend. On the other smaller outcrops, it seems unlikely that the Boulder Clay is more than 6 m thick. Mr J.S. Turner (1934, MS BGS) recorded 1.5 m to 3 m of Boulder Clay in a former gravel pit [TL 7055 0385] at Galleywood.

In the Widford area chalky Boulder Clay has been noted in a borehole [TL 6970 0513] alongside the railway line, where it was 9.1 m thick beneath Head and ? terrace gravel, and rested on the London Clay at 37.72 m OD. Farther east another borehole [TL 7002 0527] proved 1 m of soil and Head, overlying 7.75 m of chalky Boulder Clay which rested on Glacial Sand and Gravel at 35.75 m OD. Chalky Boulder Clay was seen by Mr J.S. Turner (MS BGS) in 1934 in a temporary section [TL 7009 0515] near the hospital at Widford, and by the author in 1970 in another temporary section [TL 7030 0503] some 240 m SE.

In the Galleyend area the till generally remains chalky to the surface. The area of clayey sand and gravel to the north, which is mapped as Head, may represent soliflucted decalcified till. Locally, as for instance at Reader's Corner, [TL 7202 0434], chalky Boulder Clay has been mapped as a small inlier beneath mottled orange and grey sandy and stony clay. The base of the Boulder Clay falls valleywards north, west and east from a maximum height of 76 m OD to a minimum of about 43 m OD.

The outcrop to the north-east of Little Mascalls is, for the most part, free from surface decalcification. Trenching [TL 7306 0270] to [TL 7319 0278]; [TL 7334 0290] to [TL 7352 0303] in this vicinity revealed up to about 1.3 m of chalky till, with London Clay cropping out at the surface at either end and in the intervening part of the trench. Several marl pits formerly existed on this outcrop. The base of the chalky Boulder Clay falls east-north-eastward towards the Sandon Brook from a height of about 53 m OD to about 40 m OD.

Old marl pits, which can be found on most of the eight scattered small outliers to the west and south of Little Mascalls, facilitate the identification of these small patches of chalky Boulder Clay in the field. The field relationships suggest that these occurrences are channelled into bed rock. Many of these outcrops have been completely worked out for marling purposes. There is little surface decalcification within them.

The somewhat larger spreads to the west of West Hanningfield have similarly had a number of marl pits opened in them in the past. One such pit [TQ 7033 9925] was worked to a depth of at least 5 m. For the most part the till is chalky close to the surface.

In the East Hanningfield area five pitted spreads of chalky Boulder Clay have been mapped at the surface. It is probable that the three most easterly ones are continuous beneath an irregular spread of Head which descends the valley to Bicknacre. The base of the till falls southwards from about 61 m to about 53 m OD.

To the north, some 250 m N of Great Claydons, a deposit of orange and brown sandy and stony clay has been mapped as Head. However it is probable that chalky Boulder Clay is present locally beneath the Head; indeed at one point [TL 762 019] Clayton (1957a, p. 8) proved chalky Boulder Clay at a depth of 2 m beneath orange-brown gritty pebbly clay. An old pit [TL 7643 0200], 250 m NE of Great Claydons, may have been an old marl pit.

Farther east discontinuous spreads, which are partially obscured by Head, have been mapped in the Woodham Ferrers area. As in the Hanningfield area there is a southerly fall in the level of the base of the till from about 53 m OD to about 38 m OD. The surface of the Boulder Clay is locally decalcified. Where deposits of brown and orange sandy and stony clay exceed thicknesses of 1.2 m they have been mapped separately as Head; this is most evident in the solifluxion trail which extends from the chalky Boulder Clay of the plateau area down the shallow valley towards Cock Clarks. Old marl pits can be recognised on all the spreads of chalky Boulder Clay. In places e.g. [TL 8147 0105], [TL 8047 0142] they have been worked beneath a cover of Head. The presence of Clematis vitalba in areas of extensive Head cover [TL 8150 0094] is indicative of chalky Boulder Clay at depth. CRB

Drift-filled channels

Chelmsford area

The evidence for a deep drift-filled channel in the Chelmsford area has been known since at least 1853. The Hall Street Waterworks Well [TL 7095 0635] sunk in that year penetrated 20.27 m of clay, gravel and 'quicksand' which rested on London Clay at a depth of 5.7 m OD. A later well at the same site proved 18.9 m of 'superficial deposits'. The first well at the now demolished Wells and Perry Brewery c. [TL 7067 0702] proved 20.73 m of 'Alluvium' resting on the London Clay at a depth of 5.18 m OD.

A borehole [TL 7075 0640] on the south-eastern side of the London Road, 210 m NW of the Hall Street Waterworks, passed through 7.62 m of sand and gravel and terminated in 6.71 m of 'clayey and sandy silt, and fine grey very silty sand with chalk pebbles in a light grey clay at 14.02 m' at a depth of 13.11 m OD. A well [TL 7082 0739] at the Marconi Works proved 10.67 m of 'clay and chalk' beneath 13.72 m of sand and gravel resting on the London Clay at a depth of 9.15 m OD. Another [TL 7095 0740], some 180 m to the east. appears to be outside the channel as only 4.26 m of sand and gravel were proved overlying the London Clay. A well at the former Gray and Sons Brewery [TL 7102 0669] proved a total of 19.51 m of superficial deposits, of which the lower 10.36 m was sand and gravel, resting on the London Clay at a depth of about 4.88 m.

A number of boreholes in the centre of Chelmsford around [TL 708 068] show that the base of the chalky Boulder Clay and Glacial Lake Deposits steadily falls from 8.8 m OD (10.36 m thickness of drift) in the north-western part of the site to below −2.44 m (more than 27.4 m thickness of drift) in the south-eastern corner (Figure 19).

A trial borehole [TL 7135 0621] between the River Can and the A130 was still in sand and gravel at a depth of 12.19 m (10.67 m OD).

Some 400 m downstream boreholes, e.g. [TL 7199 0623] for the proposed Chelmsford By-pass alongside the existing A12 crossing of the Chelmer floodplain, proved the channel to a depth of −5.23 m. The channel was demonstrated to be steep-sided and relatively narrow.

Sandon area

In the region of Sandford Lock [TL 740 060], boreholes e.g. [TL 7435 0658] have shown that the channel extends to below −31.95 m OD. Beyond the southern margin of the floodplain another borehole [TL 7445 0599], proved drift, resting on London Clay, to a depth of −21.62 m OD. A borehole [TL 7666 0533], 1.4 km SE, proved in descending order: 1.22 m of stony clay, 2.44m of clayey sand and gravel, and 14.63 + m of laminated grey silty clay. The base of the channel lies below 4.57 m OD. A borehole [TL 7463 0485], 250m E of the crossroads at Sandon, proved 33.30 m of laminated silts and fine-grained sand to a depth of −8.47 m OD where they rested on London Clay.

Near Sandon House School a borehole [TL 7519 0600] proved the following succession beneath 1.5 m of soil and terrace deposits: brown clay, 3 m; chalky concretions, 0.6 m; grey clay, 1.2 + m. It is not certain whether the chalky concretions are 'race' nodules within the London Clay, or whether they are genuine fragments of Chalk.

In the north-south stretch of the Chelmer downstream from Sandford Lock the drift deposits are known to occupy a channel, but they were not penetrated in the boreholes drilled in this vicinity. They are known to extend to below −3.63 m OD at one point [TL 7414 0764].

Boreham–Cardfield Farm area

Farther downstream boreholes [TL 7625 0856] and [TL 7764 0893] in the bed of the River Chelmer have proved, beneath 1.83 to 3.35 m of Alluvium and 0.91 to 1.22 m of sand and gravel, 13.72 m of soft grey silty clay, poorly laminated with traces of flint and quartz gravel. The base of the channel lies below −5.18 m OD.

On the north side of the river more than 15.9 m of plastic grey silty clay beneath 2nd Terrace gravels were proved in a borehole [TL 7668 0910]. The base of the channel-fill deposits lies below OD. A borehole [TL 7909 0944] on the north side of the floodplain 1.5 km downstream, proved 1.22 m of soft grey clay resting on London Clay at a depth of 6.40 m OD. Some 420 m to the south sub-alluvial gravel rested directly on the London Clay at 8.54m OD. As with the upstream holes it was overlain by 4.88 m of sand and gravel and Alluvium.

No evidence could be found downstream in the Maldon area for a drift-filled channel.

Witham–Kelvedon area

The presence of a drift-filled channel at Witham has been known since 1865 when a well [TL 8212 1529] was sunk at the Railway Station. Fisher (1868, p. 98) gave the first published account as follows:

The base of the drift lay 27.43 m below OD. Fisher was doubtful whether this was the 'true Boulder Drift'. A somewhat different account of the same well was published by Whitaker and Thresh (1916, p. 314), who were reluctant to accept the absence of both the London Clay and the Woolwich and Reading Beds, and thought that they must be represented in the 'glacial clay'.

In 1869 a well [TL 8197 1415] was sunk on the south side of the high road, 1.12 km SSW of the above borehole; it appeared to show drift to a depth of 74.2m ( −57.40m OD). Other classifications (e.g. Whitaker and Thresh, 1916, p. 315) showed the drift to extend to −4.17 m OD, and assigned 47.09 and 6.10 m of the underlying strata to the London Clay and Reading Beds respectively. A reexamination of the log suggests that these 47.09 m of strata are drift deposits similar to those encountered in the deeper part of the channel in the Kelvedon Borehole: the absence of the Woolwich and Reading Beds is critical to this reinterpretation.

Another well [TL 8146 1466] was sunk in 1933 just north of the railway line in Highfields Road. From the log (M17) it can be seen that drift extends certainly down to −6.11 m OD. Beneath this level 47.24 m of clay was assigned originally to the London Clay, but is almost certainly also drift; this interpretation explains the absence of the Woolwich and Reading Beds and the greatly diminished thickness (12.8 m) for the Lower London Tertiaries when considered as a whole for, in general, this unit has a fairly constant thickness of about 26 m (see (Figure 6)).

The Flour Mills Well, Witham [TL 8229 1527] proved Boulder Clay to a depth of −6.1 m, whilst the nearby BGS Witham Borehole [TL 8244 1534] penetrated the chalky Boulder Clay at −54.1 m OD.

Three boreholes to the south and east of Witham which failed to penetrate the drift were those at Pondhallow Farm [TL 8186 1355] which terminated at 9.2 m OD, at Little Braxted [TL 8332 1484] which terminated at −4.3 m OD, and at the northern end of the Witham By-pass [TL 8303 1552] where a borehole was still in 'blue clay with chalk' at 0.3 m OD.

Some 300m NE of Coleman's Farm, a borehole [TL 8363 1559] proved 'grey clay with chalk inclusions' to −7.4 m OD resting on London Clay. At Hill Broad Farm another borehole [TL 8472 1564], on the valley side, was still in Boulder Clay at 6.4 m OD. One other [TL 8450 1620], at Appleford Farm 600 m NNW of the latter, passed out of chalky Boulder Clay into London Clay at −1.5 m OD.

A trial borehole at Rivenhall End [TL 8400 1655] failed to penetrate 'stiff bluish clay and chalk' at −2.44m OD. Some 370m NW of the latter, a borehole [TL 8368 1670] provided evidence of the possible occurrence of the Maldon Till to −0.6 m OD, at which height it rested on London Clay (Haggard, 1972).

Further evidence of the existence of the channel to the north-east can be found in several boreholes in the vicinity of Kelvedon. One at the western end of the Kelvedon By-pass [TL 8531 1761] proved 14.02 m of 'grey/blue clay with small pieces of chalk' beneath 7.32 m of sand and gravel of the 3rd Terrace. The base of the Boulder Clay was not proved, but lies below OD. A bridge foundation borehole [TL 8559 1769] 130m farther east, penetrated 28.65 m of 'blue clay, chalk and stones' beneath 3.05 m of 3rd Terrace deposits. Again the base was not proved, but lay below −13.72 m OD.

The best documented evidence of the deep channel is from the IGS Kelvedon Borehole [TL 8602 1797], where it extends down to −44.5 m OD. Some 210 m to the south-south-east a borehole [TL 8613 1777] was still in 'blue clay with traces of chalk and other stones' at about 3.0m OD.

Just north of the boundary of the district there are a number of borehole records that have been included in this account in order to delimit the channel pattern along the margin of the district. A well [TL 8758 1902] sited on the Alluvium of the Domsey Brook passed into the London Clay beneath 1.8 m of 'drift' and sand and gravel, according to the published log (Sayer and Harvey, 1965). However, reference to the original log shows that 49.07 m of strata assigned to the London Clay by Sayer and Harvey were described as 'clay and chalk stones'. Additional notes relating to this well also record clay and chalk down to 62.18 m. Therefore, the drift deposits, which rest on Woolwich and Reading Beds or Thanet Beds, extend down to at least −39.61 m. A borehole [TL 8764 1894] 100 m to the ESE passed through 10.67 m of chalky Boulder Clay into more than 13.72 m of sand and gravel and stopped at 0.3 m OD without penetrating the latter deposit.

A well sunk in 1887 at the now demolished Fuller's Brewery [TL 8647 1915] clearly proved Boulder Clay to a level of −34.22 m OD. However, the anomalously thin Lower London Tertiaries (16.46 m) in this borehole (Figure 6) suggest that 5.79 m of strata below the Boulder Clay, which were formerly included within the

London Clay, are glacial deposits, and probably equate with the commonly chalk-free laminated silts proved in the lower part of the Kelvedon Borehole. If this is so then the base of the channel extends to −39.01 m OD.

To the east of the brewery a borehole [87W 1926] proved 10.36 m of 3rd Terrace deposits overlying more than 14.02m of chalky Boulder Clay. The base of the borehole lay at 5.79 m OD. Some 500m S of the brewery, another borehole [TL 8639 1867] similarly failed to penetrate the Boulder Clay at a depth of 7.92 m OD. A third hole [TL 8556 1829] was still in terrace sand and gravel at 1.22m OD.

The extent of the channel to the east is not known with certainty. A borehole for the Kelvedon By-pass [TL 8773 1956] proved Boulder Clay to a depth of 18.90 m (14.63 m OD), and two other boreholes [TL 8831 1989] and [TL 8956 1940] stopped without penetrating the chalky Boulder Clay at 18.29 m (9.1 and 16.4 m OD respectively). CRB

Glacial Lake Deposits

Chelmsford area

Outside the limits of the Chelmsford district, but relevant in terms of glacial palaeogeography, Glacial Lake Deposits have been mapped on the west slope of the valley, north of Warren Farm [TL 680 080]. Cream clay with 'race' was also observed south of the River Can [TL 6875 0688]. Sections in the Chignall's gravel pit [TL 6611 0982] formerly showed up to 7 m of chalky Boulder Clay with a colour-banded appearance suggesting a water-lain origin. The deposits at Warren Farm apparently pass laterally westward into chalky Boulder Clay, suggesting a limit for the lake deposits in this area. RDL

The Hall Street Well [TL 7095 0635] which proved 20.27 m of drift to a depth of 5.70 m OD makes no reference to lake clay deposits, only to sand and gravel. Some 200 m WNW a borehole [TL 7075 0640] proved 6.71 m of clayey and sandy silt, with a seam of light grey clay and chalk, beneath 7.62 m of sand and gravel.

Boreholes at the Chelmsford Central Development site around [TL 708 068], proved stiff grey, locally laminated, clayey silt with some fine- to coarse-grained sand, fine- to coarse-grained gravel, chalk fragments and scattered cobbles down to at least −2.44 m OD in the south-east corner of the site. North-westwards across the site the base of this drift rises until it is cut out beneath the overlying 1st Terrace deposits of the River Chelmer.

The best evidence for Glacial Lake Deposits in the buried channel downstream from Chelmsford is provided by the trial boreholes which were drilled across the floodplain of the River Chelmer alongside the Chelmer road on the south-east side of the town. The deepest proved part of the channel [TL 7199 0623] lay at a depth of −5.23 m OD; the edited log of this borehole [TL 7199 0623] is given below:

Adjacent boreholes proved similar thick drift sequences. Some 300m SW a borehole [TL 7174 0605]. was still in dense grey silty fine sand and gravel at 3.85 m below OD, and the gravel at the base of the channel-fill in this vicinity appears not to have been reached.

Less than 200 m SW, just beyond the floodplain of the Chelmer, no channel was proved. Here terrace deposits, or Glacial Sand and Gravel overlain by Head, rest directly on the London Clay at about 19 m OD. The northern margin of the channel is equally steep-sided: two boreholes [TL 7213 0635] and [TL 7207 0631] proved that the surface of the London Clay rises more than 16.0 m from below −0.34m OD in a horizontal distance of 45.0m.

Upstream from the bridge crossing more than 3.96m of light grey silty sand were proved at the bottom of a borehole [TL 7135 0621] beneath 8.23 m of Alluvium and sub-alluvial gravel.

Sandon area

The deposits in the Sandon area are divided into two by the Alluvium of the Sandon Brook and a tract of Head which occurs principally on the west side of the Brook. There are now no permanent exposures although in the past several pits were dug in the deposit, possibly to extract marl. A buff or creamy silt is generally proved by augering. A trench [TL 7400 0522] 600 m NW of Sandon, showed up to 5 m of yellow silt and laminated grey white and brown clay with 'race'. At one point in the trench these deposits rested on 75 mm of chalky Boulder Clay which in turn rested on the London Clay. A pylon hole [TL 7419 0522] 190m E of the above section showed 3.0 m of yellow silty clay beneath 0.6 m of soil.

Some 250 m E of the crossroads in Sandon a trial borehole [TL 7463 0485] penetrated the following sequence:

The drift thus extends down to 8.47 m below OD. However, another borehole [TL 7465 0448] 300 m to the south, sited on the Alluvium of the Sandon Brook, proved 3.50m of Alluvium resting directly on the London Clay. A third hole [TL 7430 0460] was sited close to the edge of the channel in which the Lake Deposits occur. Here 3.5 m of stiff grey clay, stony at the base, rested on the London Clay at 26.5 m OD (Clarke, 1975).

On the east side of the Sandon Brook chalky Boulder Clay in the lower part of the Glacial Lake Deposits has been proved by augering at three points [TL 7536 0446], [TL 7553 0446] and [TL 7543 0457]. Gravel which was encountered 300 m ESE of Bridge Farm [TL 7540 0475] has been interpreted as a small outlier of the 3rd Terrace of the River Chelmer overlying the lake deposits, but may be an outlier of the fine stratified sands which overlie the Glacial Lake Deposits in the Sandon pit.

A borehole [TL 7571 0457] sited 600 m ESE of Bridge Farm, penetrated in descending sequence: made ground, 0.61 m; silty clay, 0.30 m; clayey sand, 2.74 m; clayey silt, 0.61 m; London Clay 0.91 + m. The 0.61 − m of clayey silt at the bottom of the drift is here regarded as the feather-edge of the Glacial Lake Deposits: the overlying deposits may represent a remnant of a terrace, or Head, or the upper sands noted in the Sandon pit. Another borehole [TL 7618 0511] some 700m NE of the above, appeared to enter London Clay at a depth of 27.3 m OD (Clarke, 1975). However an examination of this clay by M. J. Hughes suggests that it is a Quaternary deposit derived from the London Clay. To the northeast of this locality there are three quite large abandoned pits in the Glacial Lake Deposits [TL 7575 0500]: [TL 7600 0545]; [TL 7605 0569].

Some 170 m ESE of Sandon Bridge, a borehole [TL 7566 0533] proved 14.63 + m of laminated grey silty clay without penetrating it. The clay was overlain by 3.66 m of 2nd Terrace deposits. The downstream continuation of this channel and its associated deposits has not been proved. However, overlying the gravel deposit and underlying ?Head in a borehole to the north [TL 7647 0790], a very light grey silty clay 2.3 m thick with its base at 23.1 m OD was encountered. London Clay appears to crop out 100 m to the west of this borehole.

Downstream of Sandford Mill Bridge boreholes have proved the deepest part of the channel so far known (to below −31.95 m OD). The log of the trial borehole [TL 7435 0658] which reached this level is included below in an abridged form:

One of the interesting features of this borehole is the 37.9 m of grey silty clay, the written description and the lithology of which at first sight are suggestive of London Clay. The presence of chalk fragments and gravel in the lower 6.45 m of the section, however, confirm that it is drift.

Another borehole [TL 7443 0636] 500 m SSE had a thinner clay bed, 13.20 m thick, below the alluvial gravel, with a more variable lower sequence more than 10.30m thick, which consisted of layers of fine-grained sand and silty clays with chalk. The 3 -m of 'stiff grey silty clay with partings of fine sand' at the base of a nearby borehole [TL 7445 0599] is interpreted as London Clay below a sequence, 43.5 m thick, of alternating fine-grained sands and silty clays classified as Glacial Lake Deposits. The base of the drift lies at 21.62 m below OD. A borehole close by [TL 7443 0690] was drilled to 0.9 m below OD, at which depth it was still in soft light grey silty clay. The 8.70 -m of 'firm grey silty clay with some partings and pockets of fine sand' penetrated to 5.59 OD in an adjacent borehole [TL 7427 0705], are interpreted as Glacial Lake Deposits in a report accompanying the logs. Deposits in the lower portions of trial boreholes to the north [TL 7412 0723], [TL 7414 0764] and [TL 7405 0799] are also so classified: the Glacial Lake Deposits, which were not penetrated, extend down to lower than 3.63 m below OD.

A borehole [TL 7472 0810] 400 m WSW of Stoneham's Lock, proved 4.7 m of soft light grey laminated silty clay beneath alluvial deposits, resting on 'London Clay' at + 4.8 m OD (Clarke, 1975). Work by M.J. Hughes suggests that in the absence of any Tertiary microfauna, fine sand, pyrite, glauconite or mica that the 'London Clay' is a Quaternary clay. Some 500 m NE of Stoneham's Lock another borehole [TL 7540 0858] encountered 7.6 m of Glacial Lake Deposits beneath Alluvium and gravel. The underlying 1.0 m of clay was initially regarded as London Clay, but Mr M.J. Hughes has again suggested that it is a Quaternary clay.

Boreham House area

At Boreham Hall a borehole [TL 7541 0890] proved 1.22 m of 2nd Terrace Gravels overlying 4.27 m of grey silty clay which in turn rested on the London Clay; the base of Glacial Lake Deposits was at 16.7 m OD. Temporary sections in this vicinity showed up to 1.52 m of terrace gravel overlying creamy and grey silts which locally contained chalk pebbles. These deposits do not extend north of the unnamed stream that lies some 250 m N of Boreham Hall. To the west and north-west an extensive spread of Glacial Lake Deposits has been mapped at the surface. In the vicinity of the now demolished Lonebarn Farm [TL 7370 0818] a low feature marks the contact between the Glacial Lake Deposits and the Chelmsford Gravels.

Augering and temporary sections show that the lithology is predominantly laminated grey or white clay or silty clay. At one point [TL 7386 0892] 1.5 m of grey clay were seen to overlie chalky Boulder Clay. A borehole [TL 7475 0931], sited 200 m NE of Boreham House, proved 5.79 m of these deposits beneath 0.61 m of soil, and overlying the London Clay.

Some 900 m ESE of Boreham Hall a borehole [TL 7626 0857] recorded Alluvium of the River Chelmer, 3.35 m thick, overlying 0.91 m of gravel, which in turn overlay 14.02 m of poorly laminated grey silty clay with traces of fine flint and quartz gravel within which the borehole stopped; these last deposits extended down to more than 3.67 m below OD. To the north of the river another borehole [TL 7668 0911], proved in descending sequence: topsoil, 0.3 m; clayey gravel, 2.13 m; brown sandy clay, 0.30 m; grey silty clay, more than 14.94m thick. The base of the channel in this vicinity must lie below OD.

At Husketts Mills a borehole [TL 7664 0893] sited on the Alluvium, proved 1.83 m of sandy clay, on 2.74m of sand and gravel, on 13.72 + m of soft grey silty clay. The base of the channel lay deeper than 5.18m below OD. The 1.67m of clay with gravel, peat and shell debris, which was encountered between 14.7 m of alluvium and gravel and the top of the London Clay at 6.40 m OD in another borehole [TL 7839 0931] may also be referable to the Glacial Lake Deposits.

The lowest point downstream at which Glacial Lake Deposits have been noted is 720 m farther east in a borehole [TL 7909 0945] sited 300 m SW of Cardfield Farm. Here 1.22 m of soft grey clay, which rested on London Clay at 6.40 m OD, was proved beneath Alluvium and sub-alluvial gravel. No Glacial Lake Deposits were recorded in another borehole [TL 7908 0903] sited 400 m to the south. Trial boreholes downstream in the Maldon area have similarly failed to provide evidence of a buried channel and associated lake clays.

Osea Island

A little known occurrence of clay and silts, overlain by deposits of the 2nd Terrace, has been shown as 2nd Terrace Loam at the western end of Osea Island [TL 908 064] on the published Geological Sheet. A borehole [TL 9077 0640] sited close to the west of a low rise at a level of 6.9 m OD proved 4.0 m of silts and clays, resting on 1 m of gravel which in turn rested on the London Clay (Simmons, 1978). Their relationship, if any, to the deposits described above is unknown: it is possible that they may be interglacial clays.

Kelvedon area

In the Kelvedon Borehole the lower part of the drift sequence consisted predominantly of laminated grey silts and silty clays with only minor amounts of small chalk pellets. There was no sharp distinction between these laminated beds, which were approximately 21.49m thick, and the overlying 34.47m of chalky Boulder Clay. The base of the channel at 44.5 m below OD was cut in Thanet Beds (M15).

Brickearth

West of the Ter Valley

The Brickearth of the plateau is typically developed on the higher parts. In augering a light sandy loam was consistently found, which was in marked contrast to the heavy clay soils developed over the chalky Boulder Clay. In general only thicknesses in excess of 1 m have been mapped; the maximum proved thickness was 4.4 m in a borehole [TL 7604 1289] to the west of Porridgepot Hall (Eaton, 1973). Exposures are rare and there appears to have been little large-scale exploitation for brickmaking. Some of the small pits within the outcrop may have been dug for this purpose and/or for the extraction of 'marl' from the underlying chalky Boulder Clay.

A section [TL 7470 1432] to [TL 7496 1425] to the south-east of Scarlett's Farm showed more than 1.2 m of structureless brown silty loam overlying chalky till.

An old brickpit [TL 7680 1095] appears to have utilised the brickearth, but no sections were visible in 1966; the pit was subsequently filled in during the construction of the Springfield-Boreham By-pass. Boreholes sunk in connection with the above by-pass proved thicknesses of 1 to 3.2 m of Brickearth.

The 2.4 m of 'brown clay with a few pebbles' in the upper part of a borehole [TL 7752 1056] to the south of Crix Farm (Eaton, 1973) is probably Brickearth. Trenches [TL 7706 0991] to [TL 7738 1003], some 600 m S of the borehole, revealed up to 1.8 m of structureless brown sandy loam. At one point [TL 7724 0996] 1.5 m of sandy loam overlay 0.3 m a creamy and yellowish brown silt. Several flooded old pits in Brakey Wood around [TL 772 097] may have been old brickpits.

Between the rivers Ter and Brain

No exposures were noted in most of the spreads of Brickearth between the Rivers Ter and Brain. Thicknesses in excess of 1.2 m were consistently augered.

Boreholes around [TL 8100 1284] sunk in connection with the Witham By-pass proved 1.5 m of 'dry brown silty clay' over chalky Boulder Clay.

To the south the small outcrop in the Sandford's Farm area [TL 810 120] has been largely removed by quarrying operations. Sections visible in 1967 showed up to 2.1 m of structureless brown sandy loam overlying either Glacial Sand and Gravel or chalky Boulder Clay.

To the east of Butlers another small spread of brown sandy loam, greater than 1.2 m in thickness, has been mapped. An old flooded pit [TL 8100 0987] may have been an old brickpit.

South of the River Chelmer

To the west of Oak Farm occurs an outlier of Brickearth [TL 810 060]. It was exposed in the faces of the Warren Farm sand and gravel pit where it was seen to consist of up to 3.6 m of unbedded mottled orange and grey silty and sandy clay.

Head Brickearth

Chelmer Valley (west of the River Ter)

A small tract of Head Brickearth [TL 780 094] has been mapped on the north bank of the River Chelmer and on the west bank of a small tributary to the west of Botter's Farm, upstream of which it merges with Brickearth. It was proved by augering to consist of brown silty and sandy loam, and to locally exceed 1.5 m in thickness.

Ter Valley

Three small tracts [TL 7815 1350], [TL 784 130] and [TL 788 128] of Head Brickearth have been mapped on the east side of the Ter Valley below Terling. Thicknesses in excess of 1.5 m have been proved by augering. The Devil's Pit [TL 785 130] may have utilised the brickearth for brickmaking: alternatively the underlying chalky Boulder Clay was possibly extracted for marl.

A much more extensive spread, some 4 km long, has been mapped on the opposite side of the valley as far as the confluence of the Ter and Chelmer. A borehole [TL 7781 1298] at the northern end of the outcrop proved 2 m of brown clay with scattered pebbles (Eaton, 1973).

The Brick and Tile Works [TL 7770 1235] at Hatfield Wick was disused long prior to 1965. Dalton (in Whitaker and others, 1878, p. 65) noted that in 1872 the pit was 3 m deep in 'brown brickearth over finely laminated grey and buff loam'. A borehole [TL 7784 1212] 200 m S of the brickpit, proved 1.5 m of brickearth overlying Glacial Sand and Gravel (Eaton, 1973). Dalton (in Whitaker and others, 1878, pp. 64–65) noted 4.2 to 4.6 m of fine brown loam, the lower part stony in a brickyard about [TL 7800 1145] 'east of the 34th milepost'. The spread of Head Brickearth extends southwards from the brickyard to the River Chelmer. A trench [TL 7860 1050] to [TL 7885 1065] to the north of Fairfields exposed up to 2.7 m of structureless sandy loam overlying sand and gravel. The brickearth has been dug from an old pit [TL 795 100] 300 m N of Cardfield's Farm. Several shallow pits to the north and south of the old workings were presumably also dug to work the brickearth.

Chelmer Valley (east of River Ter)

Immediately east of the Ter/Chelmer confluence a spread of Head Brickearth is more clayey than those described above, and may include a component derived from the London Clay, on which it rests. The thickness exceeds 1.2 m. The eastwards continuation of this tract extends past Ulting Hall and into the shallow valley on the east side of the Hall. There are no exposures in the brown sandy loam, ‘,.hich was augered consistently and was found to exceed 1.2 m in thickness.

On the east side of the River Blackwater an extensive spread of Head Brickearth mantles the London Clay slopes and extends on to the 2nd Terrace of the River Chelmer. Upslope the brickearth covers the floors and sides of three shallow valleys in the Langford Park area around [TL 845 105]. No exposures were seen in this tract; the brickearth exceeds 1.2 m in thickness.

Blackwater and Brain valleys

Three valleys on the west side of the River Brain have a fill of Head Brickearth. In part this may have originated by solifluxion from the spreads of Brickearth which are present close by on the plateau. No sections were seen in any of the valleys. In augering it was noticed that the brown sandy loam tended to become clayey downwards; this is thought to indicate proximity to the underlying chalky Boulder Clay. Thicknesses in excess of 1.2 m were augered over most of the outcrops.

Upstream from the Brain/Blackwater confluence a small spread of Head Brickearth [TL 834 162] which partially obscures Hoxnian interglacial deposits, has been mapped to the south-west of Rivenhall End. On the opposite side of the valley a more extensive tract [TL 844 155] has been worked for brickmaking. Wood and Harmer (1877, p. 111) noted an upward passage from chalky Boulder Clay into sandy bedded brickearth, into fine-bedded yellow sand (more loamy downwards ), and finally into red sandy gravel (?Head). Wood's figure and notes are suggestive of a water-lain deposit possibly related to the Terrace Loam described below, but because of its general attitude mantling the south-east bank of the river it has been mapped as Head Brickearth on the published Geological Maps.

Danbury–Cock Clarks area

Head Brickearth to the south-east of Danbury is separated only with difficulty from Head. The mapping criteria adopted for the Head Brickearth have been the less clayey nature of the deposit and the presence of fewer flints on the surface. The Head Brickearth has been worked in two large pits around [TL 806 045] to the east of Tyndales.

The 'brickearth' recorded in the Royal Oak pits [TL 8070 0515] within the Glacial Sand and Gravel (Clayton, 1957b, p. 23) has clearly a different origin to that of the surrounding deposits. It may represent reworked London Clay, such as that noted in the Warren Farm Pit [TL 8040 0621] (see p. 45).

Head Brickearth has been worked in several pits around Cock Clarks. The Slough Brick Works [TL 807 027] had been disused for several years prior to 1966 when the area was geologically surveyed. Mr J.S. Turner (1934, MS map BGS) noted a few sections 1 m deep in 'brown brickearth'. In the pit the brickearth seems to have been worked down to the underlying Head. A borehole [TL 8135 0255] to the east of the pit proved 5.2 m of soil and brown sandy clay.

Large shallow pits around [TL 8185 0300] on the north-east side of Cock Clarks were only about 1 m deep. Thicknesses of brown, yellow and grey silty loam, in excess of 1.5 m, were augered.

Chelmsford area

A number of pits in the Chelmsford area were formerly opened to work 'brickearth'. At least one of these occurrences is known to overlie an Ipswichian Interglacial deposit (see p.68). The other deposits in this vicinity have been mapped as Head on the 1:50 000 Sheet, although Mr J.S. Turner (1934, MS map BGS) depicted these same deposits as Brickearth. The lack of exposures and borehole records in this built-over area, and the ubiquitous cover of Head has not allowed the exact stratigraphical or genetic relationship of the deposits in this vicinity to be determined. CRB

Head

Chelmer Valley

In the Chelmer Valley north of Great Waltham, the Head deposits comprise brown silts and clayey silts with a variable content of angular flints. Much of this material was generated at the spring-line present at the London Clay/drift boundary.

In the Great Waltham area an extensive development of silty loam with gravel intercalations is present. In places these deposits are related to the spring-line at the base of the drift; elsewhere they blanket the chalky Boulder Clay on the lower slopes of the valley. Locally the gravelly deposits have been dug in the past from shallow pits.

In the Chelmer Valley north of Chelmsford, the Head deposits consist of clayey gravels and flinty loams. The gravels are ill-sorted and variably angular to rounded in nature. Up to 2.4 m of ochreous and pale grey mottled flinty clay was noted in the Pump Lane area [TL 724 084]. Similar stony clays up to 3 m thick were observed near Duke's Farm [TL 727 079]. At the former locality [TL 7233 0850] a sharp angular contact was recorded between the Head and the chalky Boulder Clay.

In the Springfield area up to 3 m of mottled flinty clays are again present. They mask the boundary between the glacial lacustrine deposits and other glacial material. Both the Head and the glacial deposits have been affected by ice-heaving in this area; in places vertical contacts were noted between the various lithologies. RDL

Unnamed river valley to the north of Boreham

The presence of Head, developed preferentially on the west side of the valley, is best demonstrated in the Wallace's Farm pit [TL 752 114]. On the western side of the pit the Springfield Till is decalcified to a depth of 1.2 m. Towards the valley side on the east, the Boulder Clay is absent but the Glacial Sand and Gravel is overlain by up to 4 m of mottled orange and grey sandy and clayey gravel (Plate 3). Southwards from the pit it was not always possible to distinguish the Head from the Glacial Sand and Gravel by augering. Borehole and well evidence [TL 7572 1110] and [TL 7575 1059] locally show that the gravelly surface deposits overlie chalky Boulder Clay. On the opposite side of the valley the Boulder Clay of the plateau is extensively decalcified, and there is again difficulty in distinguishing this decalcified till from Head which is largely derived from it. The available evidence from the disused pits on the east side of the valley, and from the very gravelly soil around [TL 760 110] suggests that Head in this area is probably less than 1 m in thickness.

Head is also present in the shallow tributary valley on the southwest side of the disused Boreham airfield. There are no exposures in the deposit, but augering revealed a silty and sandy loam greater than 1 m thick which, at least locally, directly overlies the London Clay [TL 735 113].

Southwards beyond the confluence of these two valleys, where the respective Head deposits merge, the outcrop broadens to form a sheet-like spread to the west of Boreham. This spread is coextensive with that along the Chelmer Valley, which has been described above. Boreholes for the Springfield-Boreham By-pass showed that the Head in this vicinity is up to 3.0 m thick and indiscriminately overlies Glacial Sand and Gravel, Glacial Lake Deposits and London Clay.

Around Boreham House [TL 745 090] the Head consists predominantly of a sandy loam. A borehole [TL 7502 0889] proved 2.1 m of 'brown clayey silt' beneath 0.6 m of topsoil, and overlying 2nd Terrace deposits. Trenching nearby showed that thicknesses of 0.6 to 2.0 m of sandy loam, with a gravelly base up to 150 mm thick, overlie Glacial Lake Deposits.

Ter Valley

The sides of the Ter Valley for the most part appear to be free of Head. Among the minor occurrences it has been suggested above (p. 39) that the upper 0.9 m of 'brown boulder clay' and 2.8 m of 'clayey sand and gravel' in the borehole [TL 7210 1628] at Little Leighs (Eaton, 1973) is Head. Other small patches of Head have been recognised in this vicinity e.g. [TL 7185 1620], but it has not been possible consistently to differentiate them from the Glacial Sand and Gravel.

A small tract of stony loam has been mapped on the north bank of the valley [TL 732 158] near Lyons Hall. Farther east similar small spreads of sandy loam have been recognised on both sides of the valley in the Terling area, and up the unnamed tributary to Fairstead. In this latter valley the Head is confined to the western side.

Below Terling the Head is also preferentially developed on the west side of the valley. Up to 1 m of sandy loam or sandy clay was commonly augered. A different lithology was found to the east of Terling Hall. At five points [TL 7747 1354]; [TL 7752 1355]; [TL 7776 1352]; [TL 7776 1345] and [TL 7774 1337] grey silty clay was augered. Chalk was noted at the last locality and the presence of Clematis vitalba at the first suggests that the lithology may in fact be a variant of the Maldon Till.

Farther downstream the deposits which mantle the valley sides are of brickearth lithology and have been mapped as Head Brickearth.

Ulting area

A spread of Head has been mapped to the north of Ulting, capped by an outlier of Brickearth around [TL 810 102]. In augering it was found to consist of mottled red, orange and grey sandy and gravelly clay. This lithology can be seen in the pits [TL 803 100] to the south-west of Butlers. Sections visible in 1967 showed up to 1.5 m of Head overlying Glacial Sand and Gravel. It is known from trial boreholes to the north that the Head is up to 2.4 m thick.

To the south-east of Ulting a borehole [TL 8084 0965] proved 3.4 m of 'soil and brown clay with sandy lenses' above Glacial Sand and Gravel.

It is not certain, however, whether all the deposits mapped as Head should be so classified, for some may represent decalcified, but non-mobilised, chalky Boulder Clay. For example a borehole [TL 8127 1043] alongside the Ulting road proved 0.9 m of clayey loam over 5.5 m of brown Boulder Clay with much chalk.

Blackwater Valley and tributaries

In the Brain Valley a small patch [TL 805 164] of sandy loam up to 1 m thick, overlying gravel, has been mapped to the east of Faulkbourne Hall. On the opposite side of the valley a small tributary of the River Brain has its sides mantled with up to 1.2 m of brown sandy loam.

In the vicinity of Witham an extensive tract of brown clayey gravel has been mapped. The trail of Head appears to originate in the small tributary valley to the south of Stovern's Hall, where it consists of sandy loam and clay and has probably been derived principally from the London Clay. Southwards down the valley, and also spreading up to the plateau sides, the Head becomes more gravelly and incorporates decalcified Boulder Clay, Glacial Sand and Gravel, and ? Terrace sand and gravel. In this wider outcrop an area of very flinty gravel, which can be proved to overlie the Springfield Till [TL 823 158], may represent a reworked Terrace deposit. The Witham Borehole [TL 8244 1534] some 500 m south-south-east, passed through 3.56 m of yellow, streaked with grey, sandy and stony clay, into Hoxnian lake deposits. A similar sequence of Head, 3.4 m thick, overlay Hoxnian deposits in a borehole [TL 8250 1544] 100 m NE.

In the vicinity of Rivenhall End [TL 840 165] the Head similarly partially masks Hoxnian deposits. Here the Head was probably derived from the 3rd Terrace gravels and appears to be up to 3 m thick. It consists of gravel in a sandy matrix. At the bottom a chocolate brown clay up to 0.6 m thick was noted in a temporary section [TL 8441 1693] cryoturbated into the underlying Hoxnian clays.

South of Witham an extensive tract of Head has been mapped on the west side of the Blackwater Valley. Over much of its outcrop the parent material appears to be either London Clay or Terrace (mainly 3rd) deposits. A borehole [TL 8186 1355] near Pondhallow Farm proved 6.0 m of clay, clay with fine gravel, and sandy clay beneath 0.5 m of topsoil, and above Terrace deposits. The surface strata appear to be sufficiently clayey to have been utilised for brickmaking. A number of old pits [TL 8195 1321]; [TL 8198 1332]; [TL 8207 1327]; [TL 8210 1340]; [TL 8228 1342] and [TL 8225 1330] in the vicinity of Home Farm appear to have been former brickpits.

Trenching [TL 8141 1278] to [TL 8180 1295] on the south side of the Witham By-pass revealed up to 1.5 m of brown sandy clay which either overlies the London Clay directly; or is separated from it by up to 0.9 m of sand and gravel.

A borehole [TL 8188 1234], 650 m S of the By-pass, penetrated 2.1 m of soil and brown clay with 'chalk' (?race) fragments above Terrace deposits. When traced southwards the Head is found to consist of sandy clay or loam with only minor amounts of gravel, and is principally derived from the London Clay around [TL 820 1151. It is probable that a thin remnant of Head extends on to the Terraces mapped to the north and south of this last locality. The contact was taken at a low feature break.

On the east side of the Blackwater Valley, in the Wickham Bishops area [TL 836 113] and extending eastwards through Little Totham [TL 890 120] to Tolleshunt D'Arcy [TL 930 120] and to near Tollesbury [TL 945 105], a spread of higher level Head has been recognised. That around Wickham Bishops was at least partly derived from local outcrops of chalky Boulder Clay. As in the Ulting area mentioned above some of the deposits mapped as Head may be decalcified, but non-mobilised, chalky till, as suggested by a borehole [TL 8514 1095] in Captain's Wood, where 9.1 m of soil, brown clay, and brown clay with traces of gravel overlay grey chalky Boulder Clay, and by a borehole [TL 8396 1095] to the south-east of Likely Wood, where 4.6 m of soil and 'brown Boulder Clay with a little chalk' overlay Glacial Sand and Gravel. The surface indications of the deposit are sandy loam, or mottled orange and grey sandy clay, with a minor proportion of gravel.

The Head occupying the valley to the east of Great Totham was for the most part derived from the London Clay and consists of silty and sandy loam. This becomes more gravelly where the trail of Head crosses an outcrop of Glacial Sand and Gravel. A borehole [TL 8704 1111] at Jeperack's Farm penetrated 4.6 m of topsoil, and brown clay with chalk ( = ?race) inclusions, above the London Clay. At Voucher's Farm [TL 8816 1140] the Head was 0.9 m thick and overlay sand and gravel.

An isolated pocket of Head, 3 to 4 m thick, within an outcrop of Glacial Sand and Gravel was encountered in boreholes [TL 8514 1163] and [TL 8421 1123] 700 m E and 620 m NW of Crabb's Farm respectively.

In Strowling Wood to the north of Great Totham, an isolated outcrop of Head, appears to be quite thick in one borehole record [TL 8582 1334]. Some 2.4 m of brown sandy clay overlay 2.2 m of very clayey gravel; the latter being classified as Glacial Sand and Gravel. Similarly at Porter's Farm, Great Braxted, a borehole [TL 8638 1409] proved a thick isolated occurrence of Head. Here 6.4 m of topsoil and brown sandy clay rested directly on the London Clay.

Eastwards of Great Totham a number of shallow valleys have their sides and bottoms mantled with Head derived from London Clay. The Head consists principally of a sandy clay but some gravel has been incorporated.

Around Tolleshunt Major the Head occupies two principal positions. The higher position comprises scattered outliers, possibly the dissected remnants of a more widespread sheet, which commonly rest on Glacial Sand and Gravel; the lower position comprises the sides and bottoms of the shallow valley system. The higher deposits consists principally of sandy and clayey gravel, although it is probable that most of the incorporated clayey material was derived directly from the London Clay. It is suggested above (p. 34) that some of the apparent 'London Clay inliers' represent London Clay reworked and redeposited during an interglacial stage. Subsequently this reworked London Clay has been remobilised. At Tolleshunt D'Arcy a borehole [TL 9242 1230] proved 1.8 m of topsoil and brown silty clay above the Glacial Sand and Gravel. In general the Head in the valley system contains little gravel, especially in those valleys which drain only London Clay. In places the two types of Head merge, such as on the west side of Tolleshunt Major and to the west of Tollesbury. In the vicinity of the former, a borehole [TL 8932 1209] showed the Head, 4.9 m thick, to consist of brown clay with a little sand and gravel.

Much of the Head is of Brickearth lithology, and it is probable that Brickearth has become incorporated in the Head. Outcrops predominantly of this type can be found to the west of Highams Farm [TL 910 093] and around White House Farm [TL 937 105]. At the southern end of the valley to the east of White House Farm, the Head in a borehole [TL 9359 0949] consisted of 2.7 m of silty clay overlying 4.9 m of 'brown sandy clay' (?London Clay).

In the Oxley Green vicinity [TL 911 144] a spread of Head 3 km in length occurs on the side of the hill. The lithology comprises principally sandy clay, but coarse-grained clayey sand is common, together with minor amounts of gravel. The exact genesis of this deposit is unknown but it may represent either a reworked high-level terrace, or a low-level spread of Glacial Sand and Gravel. CRB

Valley of the River Wid

To the east of the River Wid, between Crondon Hall and Lodge Farm and north of Goat Hall, the Head deposits comprise clayey silts and clays with pockets of gravelly material. For the most part, these sediments are derived from the Tertiary clays although isolated pockets of glacial material, now masked by Head, may have contributed to the localised, more gravelly, patches. Extensive solifluxion has taken place from the outcrop of the Bagshot gravels northwards and downslope westwards into the Wid Valley. The northern boundary of the Bagshot Beds is marked by clayey gravels, and these have been worked in small pits north-east of Lodge Farm [TL 6958 0295] and [TL 6977 0312] on the Claygate Beds crop.

On Chelmsford Golf Course the ochreous pale grey mottled silts and clays are apparently almost stone-free and were utilised for brick-making. Locally the clays are brown in colour and superficially resemble the London Clay, from which they were derived.

River Wid–Sandon Brook

An extensive spread of soliflucted material, which on the higher ground overlies the Springfield Till, and on the lower the Chelmsford Gravels and river-terrace gravels, occurs in this locality. It is coextensive with the sheet of Head which extends along the River Wid. At least two phases of solifluxion are indicated by the field evidence. A trench section [TL 6963 0626] near Waterhouse Lane showed up to 2 m of clayey gravels overlying 0.6 m of white sand with clayey gravel lenses. A nearby excavation [TL 6990 0623], however, proved 1.5 m of silty loam on 1.2 m of clayey gravel. Loams in the Beeches Road area [TL 6925 0695] and in the Beach's Drive area [TL 690 074] have been worked in the past to a depth of 3 m for brickmaking. RDL

The deposits between Widford [TL 700 055] and Sandon [TL 743 047] are now largely built over and the exposures are poor. However, trial boreholes close to the existing Chelsmford By-pass give a fairly detailed picture of the thickness and lithological variation of the deposits. At the western end of the By-pass in Widford the Head, consisting predominantly of stiff sandy clay with pockets of sand and fine gravel, was 2.05 m thick [TL 6970 0513]. Farther eastwards the Head comprised some 7.40 m of brown sandy clay, and overlay Glacial Sand and Gravel [TL 7003 0531]. At the intersection of Longstamps Avenue and the By-pass [TL 7041 0538] the soliflucted material consisted of 2.45 m of stiff mottled grey and brown sandy clay with some gravel. Some 500 m eastwards, Head of similar lithology had increased in thickness to 7.40 m [TL 7089 0544], although in one intervening borehole [TL 7053 0542] Glacial Sand and Gravel was present at the surface. In the vicinity of the Baddow Road/By-pass junction the Head varies from 1 to 2.2 m in thickness [TL 7155 0590].

In the built up area of Moulsham [TL 7080 0615] trial boreholes showed that the Head varied in thickness from 1.8 m to 3.0 m, and overlay sand and gravel.

The old Moulsham Brick pit [TL 703 061] is now known as Haig Gardens. Sections by Woodward (in Whitaker, 1889, p. 452) clearly show an upper irregular unit of Head overlying brickearth (see p. 80).

From the published descriptions of some of the former gravel pits in the Great Baddow area it is possible to distinguish Head from the underlying Glacial Sand and Gravel. In the Beehive Pit [TL 718 054] Corder (1887 p. 92) noted that the upper deposits (Head) consisted of unstratified, very coarse gravel mixed with red clay. Salter (1906, p. 455) noted that the gravel (Head) in the upper part was 'somewhat tumultuously arranged.' A poor section seen in 1970 in this pit, which is now largely overgrown, showed 1.2 m of yellowish brown silt with a 15 -cm band of gravel at the base, overlying 1 m of orange sand.

The Baddow Hall Pit [TL 736 052] was described by Turner (1937, p. 160) as 12.2 m deep: he observed red gravel and sand (Head) resting on white cross-bedded sandy gravels. Clayton (1957a, pp. 6–7) distinguished an upper clayey gravel (Head), regarded by him as outwash from the Springfield Till, from the lower Chelmsford Gravels. In 1970 all that was visible in the pit, which was partly backfilled, was a number of irregular spreads of clayey gravel and/or brickearth up to 1.5 m thick overlying bedded sand and gravel.

To the north of the pit a spread of sandy loam extends from the Glacial Sand and Gravel down the valley side and on to the 1st Terrace of the River Chelmer around [TL 734 057]. About 1 km NE of the above occurrence another spread of brown sandy loam was mapped. A borehole [TL 7445 0599] proved 4.0 m of sandy clay and clayey silt with traces of sand and gravel overlying gravel of the 1st Terrace of the River Chelmer.

Sandon Brook Valley

Head is developed principally on the west side of the Sandon Brook. In the upper reaches the parent material is principally London Clay and Claygate Beds, although the Bagshot Beds, small outliers of Glacial Sand and Gravel, and chalky Boulder Clay have contributed minor amounts of detritus. Downstream from Sandon, Terrace deposits have been an important contributory source. In certain places, springs at the London Clay/Claygate Beds boundary have clearly been responsible for the formation of the Head. Beyond the district margin a trench section, exposed during the construction of the Hanningfield Reservoir, was recorded by Clayton (1957a, pp. 8–9, fig. 3) at Frennells [TQ 722 971]. Here 3 to 3.6 m of orange sandy clay, with pebbles of flint and quartzite, were noted overlying the chalky Boulder Clay. It was the presence of the thick Head cover in this area, regarded by Clayton as weathered till, which led him to suggest that the Boulder Clay in the Hanningfield area had undergone a long period of weathering and was, therefore, considerably older than the Springfield Till recognised farther north where the thick weathered mantle is absent.

Downstream from the reservoir the Head consists of up to 1.5 m of mottled orange and grey silty and sandy clay. In the vicinity of The Grove, trench sections [TL 7385 0367] to [TL 7393 0414] showed the Head varied in thickness from 0 to 3 m and rested on London Clay with a very irregular cryoturbated base.

In the vicinity of Pontlands [TL 733 041] the Head at surface consists of ochreous clayey sand and gravel. In the Sandon gravel pit [TL 7451 0429] reddish brown clayey gravels are up to 3 m thick and have an erosive base on the Glacial Sand and Gravel; it is thought that this is due to channelling. Clayey gravel containing angular flints is apparently subordinate in this region to the pebbly type of Head.

From Sandon to the confluence of the Sandon Brook with the River Chelmer, an extensive tract of Head has been mapped on the west side of the valley. The lithology is principally brown sandy loam with only a minor amount of gravel. The contact with the underlying Terrace deposits is not well defined. A borehole [TL 7534 0553] to the east of Potash Farm (now Sandon School) proved 2.4 m of topsoil and brown clay above Terrace gravels. Another borehole [TL 7546 0662] passed through 3.0 m of soil and brown silty clay directly into the London Clay, and yet another [TL 7518 0670] encountered 0.6 m of soil, 1.2 m of brown silty clay, and 5.2 m of soft yellow and grey sand resting directly on the London Clay. A borehole alongside Grace's Walk [TL 7499 0671] proved 1.8 m of topsoil and brown silty clay overlying 2nd Terrace deposits. Some 500 m farther south the Head was 3 4 m thick [TL 7499 0616].

East Hanningfield–Bicnacre–Danbury–Woodham Mortimer area

An extensive tract of Head mantles the floor and sides of a small valley from East Hanningfield to Bicnacre, beyond which it merges with other sheets from the south and north. The parent material is mainly London Clay and Claygate Beds, although chalky Boulder Clay has contributed some debris in the East Hanningfield area. The dominant lithology is a mottled orange and grey silty and sandy clay. Within this tract the Head appears everywhere to exceed 1.2 m in thickness. A borehole [TL 7772 0220] to the south-east of South Gibcracks penetrated 1.2 m of topsoil and 4.2 m of brown clay, silty in parts, before entering the London Clay.

Pebbles, which are common in the soil at several localities [TL 7775 0145], [TL 777 025], [TL 7790 0165] and [TL 780 030], may indicate the presence of Glacial Sand and Gravel partially obscured by the Head and probably continuous with the spread at outcrop in the Peartree Farm area [TL 785 033]. To the north of this farm a borehole [TL 7846 0348] proved 2.7 m of very clayey gravel. Even where the gravel outcrop appears to be comparatively free from Head, thicknesses of up to 1 m of mottled orange and grey sandy and stony clay were proved by augering to overlie the gravel; in a borehole [TL 7874 0341] near Peartree Farm, the Head, a hard fawn silty clay and a mottled orange grey silty clay with some flints and quartzite pebbles, was found to be 2.3 m thick. It is probable that all the mapped deposits of Glacial Sand and Gravel in this vicinity have a thin, but variable cover of soliflucted material.

Two tongues of Head that originate just north of Woodham Ferrers unite in the area [TL 807 017] to the south of Wickham's Farm, and then merge farther south with the spread described above. The parent material is principally the London Clay and Claygate Beds, with minor amounts of chalky Boulder Clay. The lithology accordingly is a mottled orange and grey silty and sandy clay with minor amounts of gravel. Nodules of race were noted at several localities. Where it was possible to auger through the Head, as to the north of Flambird's Farm [TL 820 017], a thin gravel lay at the base of the soliflucted material.

A borehole [TL 7994 0296] to the west of Jacklett's Farm encountered 4.6 m of brown clay with silty patches, not readily differentiated from the underlying London Clay. To the east of the farm another borehole [TL 8060 0298] proved a similar clay sequence (3.7 m) above the London Clay, but here the presence of flint pebbles in the upper unit allowed it to be recognised as Head. Around Cock Clarks [TL 813 027] the complete absence of flint from the Head made it suitable for brickmaking in the past. Where surface evidence and augering indicate an absence of flint, the Head has been separated as Head Brickearth.

On the north side of the tributary valley which runs through Cock Clarks an extensive tract of mottled red, orange and grey clay with only a minor proportion of flint, extends down from the Glacial Sand and Gravel outcrop to stream level. In augering a thickness consistently greater than 1 m was found.

Brown clay with some gravel, 4.6 m thick, was proved to overlie the London Clay in a borehole [TL 8152 0337] to the north of Cock Clarks. Southwards from the borehole the soliflucted deposits appear to merge with the Head Brickearth. To the north-west, a borehole [TL 8047 0379] sited close to the mapped limit of the Head, nevertheless penetrated 3.7 m of brown clay with scattered flints, before entering very clayey red brown gravel. Another borehole [TL 7971 0365] in a similar stratigraphical position, passed through 2.1 m of brown flinty clay before entering very clayey gravel. Northwards from the boreholes the surface indications are that the Head becomes slightly more gravelly. However, a borehole [TL 7952 0461] to the north of Gay Bowers showed 4.6 m of brown and red sandy clay above the Glacial Sand and Gravel. To the east of it the Head passes into Head Brickearth.

Farther east lies the Royal Oak Pit [TL 803 052]. The stratigraphy of the deposits here is not clear. There were no deep sections visible in 1966, indeed part of the pit had been backfilled. In general up to 3 m of mottled orange and grey clayey gravel were noted in the upper part of the pit, with an observed maximum of 5 m. One section [TL 8067 0520] measured in 1966, showed 2 m of clayey gravel over more than 1 m of brickearth. Clayton (1957b, p. 23) recorded that the 5 m or so of gravels which were worked in the pit overlay 2 m of brickearth which in turn rested on more than 12 m of gravel. This suggests that the brickearth and overlying gravel should be referred to as Head. A borehole [TL 8016 0506] just south of the pit penetrated a total of 13.1 m of very clayey gravel, in which the fines formed 11 per cent of the total deposit. A 1.8 -m bed of brown sandy clay at a depth of 6.7 m may correspond to the brickearth noted in the pit; lower down a 1 -m brown silty clay was encountered at a depth of 9.4 m. Other boreholes [TL 8099 0501], [TL 8200 0523] and [TL 8072 0469] all proved very clayey gravel sequences. The high clay content is thought to be a result of the deposition of the Glacial Sand and Gravel close to the ice margin.

A gravel pit [TL 799 053] has been dug through the Head to the north-west of Whitehouse Farm. In the shallow valley to the north the Head was proved to be 6.7 m thick in a borehole [TL 7965 0590]. A narrow area of soliflucted brown sandy loam has been mapped on either side of the valley for about 2 km downstream.

Difficulty was found in distinguishing Head from Glacial Sand and Gravel to the east of Woodham Walter. In general, where Head has been mapped, the surface indication is of a mottled orange and grey sandy clay, comparatively free from flints. However, one or two areas where the soil is very flinty have been proved by boreholes to be underlain by Head. Some 6.4 m of brown silty clay becoming increasingly sandy with depth was encountered in a borehole [TL 8168 0700] to the east of Woodham Walter. About 1 km to the east another borehole [TL 8267 0713] proved 3.7 m of greyish brown sandy and silty clay overlying clayey gravel. A similar thickness of Head was found in a third borehole [TL 8347 0715] 800 m farther east.

Purleigh–Latchingdon–Mayland area

Head mantles the valley sides of the Mundon Wash and its tributaries. In the upper reaches the parent material is the basal Claygate Beds and the uppermost London Clay. Locally [TL 8320 0222], [TL 8300 0240], [TL 8395 0115] the Head is associated with small landslips which have contributed to its formation. A similar situation occurs in the valleys draining southwards from Woodham Ferrers around [TQ 790 995]. The spreads to the north and south of Purleigh occupy flattish ground and pass eastwards into Alluvium. The dominant lithology is a brown silty clay, which exceeds 1.2 m in thickness. At one point [TL 8355 0009] to the west of Cold Norton the ground level on the northern upslope side of a hedge was 0.6 to 1.2 m higher than on the other side of the hedge. This represents recent hillwash and was not sufficiently extensive to be mapped.

In the vicinity of Mundon the Head occupies a flat area and appears to be thinner than farther inland, as it was possible at a number of points to auger through the deposit into the underlying London Clay. Commonly a thin gravel seam up to 0.30 m thick was encountered at the junction. The parent material is the London Clay. A section [TL 8634 0242] on the west side of Mundon Hill revealed 0.9 to 1.2 m of orange and brown silty clay with a 0 to 15 cm thick gravel bed at the base resting on London Clay. An isolated patch of Head [TL 858 020] to the south-west of this site was stony at the surface.

Ditch sections around [TL 8830 0235] to the south-east of Mundon Hill exposed 0.9 to 1.5 m of yellowish brown silty clay with a gravelly base up to 0.3 m thick, overlying the London Clay with cementstones.

In the Mayland area the contributory material to the Head is principally the London Clay, and the soliflucted deposits are accordingly composed of yellowish brown silty clay. A thin bed of gravel is usually present at the base. The Head is confined mostly to the valley sides and bottoms.

Southminster–Asheldham area

Head, derived principally from the London Clay, but incorporating terrace sand and gravel, occupies the floors and lower valley sides of the streams draining eastwards and southward. The lithology is a brown or yellowish brown silty clay, commonly with a gravelly base.

To the south of West Hyde a tongue of Head is continuous with a much more extensive spread which extends along the Tillingham Brook in the Brightlingsea (242) district, where it has been proved (Lake, Ellison, Hollyer and Simmons, 1977) to overlie deposits in the Burnham Buried Channel. Most of the lobe within the present district, which consists of a yellowish brown silty and sandy clay up to 1.5 thick and with a thin gravelly base, is thought not to overlie the buried channel deposits. However, the presence of grey clay at two points [TL 9743 0331] and [TL 9762 0321] may indicate a local occurrence of the channel clays.

A trial borehole about [TQ 969 993], 500 mm ESE of Southminster Hall, proved about 6 m of firm sandy clay, with sand lenses in the lower part, overlying about 1.8 m of sand and gravel which in turn overlay the London Clay. CRB

Glacial and periglacial structures

Details of all the occurrences listed below are distributed throughout the text dealing with the Glacial Sand and Gravel and the Terrace Gravels. For convenience they are here assembled together, but to minimise duplication only brief descriptions appear in this section. Fuller descriptions are available by cross references.

Boulder Clay plateau area

In the Broomfield Pit [TL 720 110] ridges of London Clay, which cause a thinning of the overlying Chelmsford Gravels, have been noted at differing stages of the pit's development.

To the south-east of the pit, several structures bringing London Clay to the surface were noted during trenching of the North Sea gas pipe line [7.28 099], [TL 730 097], by augering [TL 731 092], and in excavation for the Springfield-Boreham By-pass [TL 7307 0853], [TL 7315 0866], and [TL 7335 0890]: most of these London Clay inliers are large enough to portray on the 1:50 000 map. In those disturbances to the north of the railway line the London Clay is either wholly, or partially, in contact with the Springfield Till, and the Chelmsford Gravels are either absent or considerably reduced in thickness.

An east-west trending ridge was noted in the base of the Boreham Pit [TL 752 112]. The thickness of the overlying 'Essex White Ballast' is reduced to 3.5 m over the ridge, compared to the 8 to 10 m of gravels encountered in nearby boreholes. The absence of sand and gravel in several boreholes [TL 7487 1363], [TL 7485 1312] and [TL 7540 1091] is probably also due to glaci-tectonic action.

In the Sandford Pit [TL 807 120] a south-eastward trending ridge of London Clay effectively cuts out the Chelmsford Gravels in the north-eastern part of the pit. Along this ridge the Springfield Till rested directly on the London Clay.

Danbury–Sandon area

To the south of the village of Sandon sand and gravel is worked in a number of pits (see p. 43). These gravels, which have an abrupt westerly and southerly termination, overlie the Sandon buried channel. The steep dips, folds and faults noted in these deposits (see Clarke, 1975, pl. 2) may in part be explained by collapse or compaction of the sediments within the lower part of the channel. The structures, which affect Glacial Lake Deposits as well as the Glacial Sand and Gravel, were probably accentuated by subsequent periglacial activity.

On the main part of Danbury Hill, in addition to the three well-documented channel-like structures (Wedlock Green, [TL 787 077];

Christmas's Pit [TL 775 051], and St Clere's Hall, (Plate 4) [TL 767 058], it is thought that many, if not all, of the finger-like outcrops have a channel form. The structure in the St Clere's Hall Pit has been best exposed in recent years (see (Plate 4)). Details are given on p. 43.

On the north side of Danbury Common is the former Christmas's Pit [TL 776 051], now partially filled in and built over. It was here that Corder (1887, p. 92) noted his 'curious contortions' and where Salter (1906) recorded that the sand and gravel was 'much contorted'. The pit is also mentioned by Turner (1937, p. 160), but nothing is now visible of the disturbances.

Turner (1934, MS BGS) also described the deposits in one of the former Hillen's Pits [TL 791 051] as cross-bedded sand and gravel much contorted. The sand and gravel in this pit appears to occupy a fairly steep-sided channel, as London Clay was mapped at the surface 100 m away from a borehole proving 3.3 m of sand and gravel (see p. 44).

A ridge [TL 7939 0534] of London Clay appears to separate two small gravel pits lying to the east of Hillen's Pit (see p. 44).

The lobe of sand and gravel on which Gay Bowers Farm [TL 7863 0445] is situated, appears to lie in a channel (p. 44). Many of the smaller outliers of Glacial Sand and Gravel to the west (p. 44) are probably also steep sided.

The Nurseries Well [TL 7878 0571] proved 18.3 m of sand and gravel at a point close to the outcrop of the London Clay (p.44).

Some 0.5 km to the north is a SW-NE-trending spread of gravel which again appears to be confined in a channel (p. 44). A similar trending channel is also recognised a further 0.5 km to the north (p. 44). Northwards from this latter occurrence the lobes of sand and gravel have a general north-north-westerly trend down the north slope of Danbury Hill towards the Chelmer Valley.

One of the better documented disturbances is recorded by Clayton (1957b, pp. 24–25, fig. 1) at Wedlock Green [TL 787 077]. Additional details can be found on p. 44.

Immediately to the west, the next outlier of gravel appears to be confined within channels (p. 44), as does the outlier on which Hollybred Farm is situated (p. 44).

Tiptree Ridge

In the shallow pits [TL 876 110] to the east of Jeperack's Farm, a SW-NE-trending ridge of London Clay was noted at one point [TL 874 109]. The London Clay, which appears to have risen almost to the ground surface, effectively cuts out some 2 to 3 m of sand and gravel.

Chelmer Valley

During the construction of the reservoir [TL 733 066] 700 m SW of Brookend, the thickness of the terrace sand and gravel was shown to vary markedly. At one point [TL 7332 0659] the London Clay was within 0.9 m of the surface, whereas at another point [TL 7335 0657] 60 m SE the sand and gravel was 4.0 m thick. The irregularities are attributed to periglacial effects.

Similar variations in the thickness of the sand and gravel were noted in a trench about 1 km to the north-east (see p. 72).

Farther downstream, in the vicinity of Saltcote Hall, an irregular base to the 2nd Terrace was noted in a number of pits [TL 870 085]. The sand and gravel thickness varied from 0.6 to 5 m. The intervening ridges of London Clay are attributed to glaci-tectonic activity. CRB

Chapter 6 Pleistocene and Recent: interglacial and postglacial

Evolution of the river system

It is generally postulated that a proto-Thames flowed northeastwards across central Essex in what is loosely defined as the mid-Essex Depression. This depression is bounded to the south, at least in part, by the high ground of the Billericay-Danbury-Tiptree ridge; the northern boundary is not so well defined but lies to the south of a line drawn from Dunmow to Braintree.

A line of drainage to the north-east along the mid-Essex Depression was first suggested by Salter (1905, pp. 28–33), although he stated that it had no connection with the present Thames. Gregory (1922, p. 25, fig. 3) adopted Salter's north-easterly flowing river and regarded it as a precursor of the Thames, but suggested that the early Thames flowed to the south of Danbury Hill. Rose and Allen (1977) believe that the Kesgrave Sands and Gravels were deposited by a major north-eastwards flowing river. Hey (1980) regarded this river as the proto-Thames and assigned the high-level Westland Green Member of the Kesgrave Sands and Gravels, thought by him to be the oldest surviving deposits of the Thames, to a pre-Pastonian a age, and the low-level Kesgrave Sands and Gravels to a Beestonian age. After the Cromerian Interglacial the river system is thought to have changed, and the Barham Sands and Gravels were deposited by east-south-eastward flowing streams draining from the encroaching Anglian ice-sheet. This chronological interpretation is not accepted by all authors (see p. 31), some regarding the Chelmsford Gravels ( = Barham Sands and Gravels + Kesgrave Sands and Gravels) as representing a single episode within the Anglian Glaciation.

Wooldridge and Linton (1955, pp. 128–133) in a much wider evaluation of the evolution of the Thames thought that the Lower Gravel Train, which they regarded as a product of the First Chiltern Glaciation [ = early Anglian (Mitchell and others, 1973, p. 46)], was deposited by the proto-Thames flowing along the mid-Essex Depression. They inferred (Wooldridge and Linton, 1955, p. 132) that the east-flowing section of the present River Chelmer within the Chelmsford district approximates to part of the position of the former Thames, and that the south-flowing section of this same river, together with the Brain and Blackwater, were north bank tributaries to it: the River Wid and Sandon Brook were regarded as the principal south bank tributaries within the Chelmsford area. They were uncertain whether this proto-Thames entered the sea via the present Stour or via the Blackwater Estuary, but thought the latter more probable.

The course of the Thames during the later Winter Hill Terrace Stage, which is equivalent to the first interglacial of Wooldridge and Linton (1955) and the middle Anglian of Mitchell and others (1973, p. 46), was not located by these authors, but they suggested that it was graded to a sea level about 60 m higher than the present day: the outlet to the sea was not stated. The Thames was diverted from its mid-Essex course to its present line by the advance of ice into valleys in the Romford and Hornchurch area.

Clayton (1957a, p. 18) correlated the till at Romford and Hornchurch with the Maldon Till, which the present author believes to be the product of a local minor glacial episode before the advent of the more widespread ice that deposited the Springfield Till. Both tills are currently assigned to the Anglian Glaciation.

Although the early chronology of an inferred proto-Thames within this area still remains speculative, it is possible to be more precise about the later evolution of the Blackwater, Chelmer and Thames-Medway rivers. Their proposed chronologies are outlined below.

River Blackwater

The evolution of the River Blackwater and associated tributaries is indicated diagrammatically in (Figure 20). It is suggested that the following sequence of events took place.

1. Pre-Anglian The River Brain was the principal south-eastwards draining river and had already cut a gap through the Danbury–Tiptree ridge. The River Chelmer ( = ? former proto-Thames) and Domsey Brook were fault-guided tributaries to the River Brain.
2. Anglian Glaciation The Anglian ice-sheet, which was probably close to its maximum extension, failed to surmount the Danbury-Tiptree ridge, but plugged the gap cut by the River Brain. Glacial meltwater continued to flow along the course of the River Brain to the south of the plug. Water under hydrostatic pressure beneath the ice banked against the north side of the Tiptree ridge carved an enclosed 'tunnel valley' down to at least −54 m OD.
3. Late-Anglian The ice began to wane and pulled back from the Tiptree ridge between Great Braxted and Messing with the formation of a lake between the ice and the Tiptree ridge. The 5th and 4th terraces were formed during successive stages during the lake's short existence before the ice had melted sufficiently to allow the Domsey Brook to resume its old course and flow through the Brain gap in the Danbury-Tiptree ridge. At the same time the Chelmer became re-established in its old course on the north side of the Danbury ridge.
4. Late Anglian/early Hoxnian When the ice finally vacated that part of what is now the Blackwater Valley between Witham and Kelvedon, an irregular hollow was left which rapidly became filled with the coarse fluvio-glacial detritus which was to form the 3rd Terrace.
5. Hoxnian Ponding occurred in irregular hollows on the 3rd Terrace surface, the hollows possibly having formed over melting stagnant ice at depth. The River Brain had reoccupied its old valley and the Domsey Brook was its principal tributary. The Domsey Brook had its own north bank tributary, namely a stream draining the area from Coggeshall Hamlet to Kelvedon.
6. Post-Hoxnian time The basic river pattern was the same as that during the Hoxnian, with the principal exception that the north bank Domsey tributary was actively undergoing headwater erosion and succeeded in capturing the River Blackwater in the vicinity of Coggeshall. The lower part of the former River Blackwater continues today as the misfit Roman River, and its former course between Coggeshall and the Roman River is marked by a prominent depression which passes east-northeastwards from Coggeshall past Houchin's Farm [TL 871 236] and East Gores [TL 880 240] to Teybrook Farm [TL 892 248]. There appears to be no evidence to support the suggestion by Gregory (1922, fig. 2) that the River Blackwater continued its southeastwards course below Kelvedon to enter the sea at Salcott [TL 950 137].

River Chelmer

It has been suggested above that in pre-Anglian times the River Chelmer below Chelmsford was a fault-guided, east-north-eastwards flowing tributary to the River Brain which reached the sea at Maldon via the gap between the Danbury and Tiptree hills. The River Wid and Sandon Brook were the two principal tributaries on the south, the River Ter the main one on the north, and in the vicinity of Chelmsford the north-south section of the present Chelmer combined with the east-south-easterly flowing River Can before joining the ? proto Thames.

During the Anglian Glaciation the ice was banked against the north side of the Danbury Hills. Farther west the main mass of ice was stopped by the high ground in the Great Baddow and Danbury areas, but it is thought that local tongues invaded the valleys of the Wid and Sandon Brook and surmounted the watershed of the Blackwater and Crouch rivers (see (Figure 17) and 1:50 000 Southend and Foulness (258 and 259) Geological Sheet). Hydrostatic pressure of the water under this ice was responsible for the erosion of the deep channels, or, tunnel valleys, at the ice margin in the Chelmsford-Sandon-Boreham area.

Clayton (1957a, p. 11) suggested that the Glacial Sand and Gravel in the area to the south of Ramsden Heath around [TQ 711 951], [TQ 708 944] and [TQ 705 940] within the Southend (258) district, represents the outwash gravel of an overflow channel from the ice  margin. The absence of thick or widespread Glacial Sand and Gravel here or elsewhere along the ridge (small spreads can be found to the east of West Hanningfield [TQ 746 999] and to the south-west of Woodham Ferrers [TQ 787 992] and [TQ 793 995]), or of an extensive terrace along the River Crouch, indicated either that there was very little outwash gravel, possibly because the ice had become stagnant and separated from the main Anglian sheet soon after its emplacement, or that the outwash had an alternative outlet to the sea via the Blackwater Estuary. The topography from Woodham Ferrers via Cold Norton and Purleigh to Woodham Mortimer, where the ground level does not drop below about 50 m OD, and the complete absence of sand and gravel deposits over both this high ground and the lower ground to the east, suggests that there was no significant escape of meltwater to the east. It is possible that the present westward-draining unnamed brook on the south side of Danbury Hill, and the eastward-draining Mundon Wash, which are separated by a low col at about 50 m OD in the vicinity of Scott's Farm [TL 827 027], may have formed a temporary eastward-draining spillway.

The virtual absence of outwash material suggests that the ice in this area was thin and short-lived and melted in situ leaving only scattered remnants of chalky Boulder Clay. The main drainage appears to have been northwards along the River Wid and Sandon Brook. The extensive sand and gravel deposits which lie principally on the east side of the Brook, and which flank the west side of Danbury Hill, may relate to this outwash phase. More probably the Glacial Lake Deposits recognised to the west of Chelmsford in the Epping (240) district, and the more extensive deposits in the Sandon-Boreham area, represent deposits in former ice-dammed lakes which formed between the waning ice sheet and the higher ground to the south, and occupied the sites of the deeply cut 'tunnel valleys'. The outlet for the lake in late Anglian times was eastwards from Boreham along the former course of the Chelmer. Continued melting of the ice allowed the rivers to re-occupy their former position and, with the increase in meltwater, much coarse detritus was brought into the area. This detritus was probably reworked in post-Hoxnian times to form the 1st and 2nd terraces, which are coeval with those of the Blackwater.

Thames-Medway river across Dengie Peninsula

The arguments for a proto-Thames which flowed across central Essex in pre-Anglian times have been outlined above. Various authors (Whitaker, 1889, p. 476; Gruhn and others, 1974, p. 70; Lake, Ellison, Hollyer and Simmons, 1977) have postulated that the proto-Thames-Medway system had a northerly or north-easterly course across eastern Essex in late and post-Anglian times. It has been proposed by Lake, Ellison, Hollyer and Simmons (1977) that the former alignment of the Thames-Medway marked the maximum westward extension of an Anglian ice-lobe which lay in what is now the southern North Sea. These authors suggested that the excavation, and subsequent filling in, of buried channels along this line took place at the end of the Anglian Glaciation. Continued deposition in the channels occurred during the Hoxnian Interglacial. The Hoxnian deposits are thought by them to be overlain by the 3rd Terrace deposits of the Roach, although the Hoxnian deposits in the present district appear to occupy a channel cut in the 3rd Terrace of the Blackwater. In neither area is the evidence wholly unequivocal, and the correlation between the terrace suites in the two rivers remains debatable. The lower two terraces of the Dengie Peninsula, which may correlate with the 1st and 2nd of the Blackwater and Chelmer, were laid down before the Thames adopted its present course. It is difficult, however, to envisage the mechanism by which the two lower terraces could have been laid down after the disappearance of the ice which formed the eastern boundary to the Thames-Medway river during Anglian times. CRB

River Terraces (Gravels)

River Terrace Gravels are found principally along the River Blackwater, the River Chelmer and its tributary the Sandon Brook, and in the south-east corner of the district where they appear to be related to a former course of the Thames-Medway river system.

The terraces of the Chelmer and Blackwater are best developed near the present confluences with their major tributaries. This suggests that there was a local ponding where any two rivers met with a consequent reduction in the loads which could be carried by the rivers, and the formation of gravel sheets which were later dissected to form the present-day terraces. Generally a marked thickening of the 1st Terrace takes place where the major rivers (Can, Ter and Blackwater) join the River Chelmer (see (Figure 21)).

In the Blackwater Valley five terraces have been recognised. Along the Chelmer three terraces have been mapped but the status of the highest, 3rd Terrace, is not certain. Four terraces occur across the Dengie Peninsula, although only the highest three are present within the district. The chronology of these different terrace deposits is still uncertain and the numbering of the terraces in the different valleys adopted on the 1:50 000 Geological sheet is not meant to infer direct correlation between the deposits. Hoxnian deposits partly rest in a channel cut into the 3rd Terrace of the River Blackwater, although Turner (in Rose and Turner, 1973) regarded the underlying sands and gravels as of subglacial origin and considered that the 3rd Terrace was younger than the Hoxnian deposits. On Osea Island the 2nd Terrace rests on deposits of possible Hoxnian age (see p. 74). It is also evident that the 1st and 2nd terraces of the Blackwater and the Chelmer are post-Hoxnian. It is not possible to be more precise in dating these lower terraces, as their relationship to the only known Ipswichian deposits within the district at Moulsham has not been established, though there is a suggestion (p. 80) that the 1st Terrace overlies the Ipswichian deposits.

The general distribution of the terraces is shown diagramatically in (Figure 21), (Figure 22) and (Figure 23): it should be noted in (Figure 21) and (Figure 22) that the representation of the upper surface of the terraces relates to the elevation of the back feature of that terrace, In compiling these two figures the profile data from the individual terraces have been superimposed. These figures are not meant to imply that the whole sequence of five terraces and sub-alluvial gravel would be encountered at every point, represented by a line within the plane of the diagram, although all or most, of these deposits are locally present at different elevations and in differing areas across the valley.

Chelmer Valley and Sandon Brook

Extensive spreads of terrace gravel are present along the River Chelmer from close to its confluence with the River Can in the west, to the vicinity of Ramsey Island [TL 950 060] in the east (Figure 21). The terrace gravels are low lying and have been subdivided into a 1st and 2nd Terrace. Locally, as to the north and east of Sandon [TL 746 057] and [TL 7540 0475] and to the west of Chappel Farm [TL 880 090], a 3rd Terrace has been mapped, but there is some doubt about its origin.

The terrace gravels are similar in composition to the Glacial Sand and Gravel deposits, being composed of angular to subrounded flints and quartzite pebbles and angular to subrounded quartz sands. The terrace deposits have been assessed for sand and gravel (Clarke, 1975; Ambrose, 1973a). In general they contain more fines than the Glacial Sand and Gravel. Paradoxically the grading percentages in all grades show an increase in grain size down stream. Resource Block C of Clarke (1975) and blocks B and C of Ambrose (1973a) correspond to the terraces of the upper, middle and lower reaches respectively of the River Chelmer. The percentages by weight of sand and gravel from these blocks which pass the various sieve sizes are tabulated below:

No basic difference was noted in the grading characteristics of 1st, 2nd and 3rd terraces, although Ambrose (1973a, pp. 8, 11) noted that the gravel of the 1st and 3rd terraces was somewhat coarser and less sandy than that of the 2nd Terrace.

Blackwater Valley

Within the Blackwater Valley it has been generally possible to subdivide the terrace sequence into five distinct groups, based on their heights above OD. Locally, however, it has neither been possible satisfactorily to separate the 5th and 4th terraces (around [TL 890 190] on the Braintree 1:50 000 sheet) nor to recognise the individual components within an undifferentiated spread of terrace sand and gravel around [TL 850 148].

It has been suggested by Turner (in Rose and Turner, 1973) that the two highest terraces may not be river terraces, but kame terraces. As can be seen in (Figure 22) the bases of these two terraces are approximately horizontal at 38 and 30 m OD respectively, and do not appear to be graded to the present River Blackwater. They are thought to have been deposited in a body of water ponded between the ice front and the Tiptree ridge.

The 3rd Terrace is the most widespread of the Blackwater terraces and is developed to the north of the Tiptree ridge principally on the northern side of the river. The towns of Witham and Kelvedon are situated on it. There is no consistent fall in level of the back feature of the terrace, although there is an overall fall from about 30 m OD in the vicinity of Kelvedon to about 15 m OD to the south of Witham (Figure 22). At the latter locality the surface of the 3rd Terrace descends to Alluvium level, and the surface profile of the terrace merges with that of the Alluvium. The bulk of the sand and gravel comprising the 3rd Terrace is thought to represent deposition in a very irregular basin vacated by the melting ice (see p. 60). Hoxnian Interglacial deposits have been found at a number of localities apparently overlying the 3rd Terrace.

The grading figures for the sand and gravel in three boreholes [TL 8556 1829], [TL 8250 1544] and [TL 8450 1620] which penetrated the 3rd Terrace in the area north of Witham are all very similar; they vary from 16 to 18 per cent gravel, 76 to 79 per cent sand, and 3 to 7 per cent fines (Haggard, 1972).

The 2nd Terrace can be found over much of the length of the River Blackwater within the district. Downstream the 2nd Terrace merges with the 2nd Terrace of the River Chelmer. The back feature of the terrace falls from about 21 m OD in the vicinity of Ashman's Farm [TL 856 173] to about 12 m OD at the Blackwater-Chelmer confluence.

The 1st Terrace is developed sporadically along the course of the River Blackwater. In the higher reaches, just to the south of Kelvedon, the back feature of the terrace is at about 20 m OD, and this falls to about 8 m OD near the confluence of the rivers Blackwater and Chelmer.

Between the Crouch and Blackwater estuaries

A flight of terraces aligned SW-NE has been recognised from the vicinity of Southend, in the Southend (258) district to the south, through Burnham-on-Crouch and Southminster as far as Bradwell-on-Sea on the Blackwater Estuary (see (Figure 23)). These terraces have not been recognised beyond Bradwell, although it has been suggested above (p. 34) that some of the sand and gravel in the Tollesbury area may be fluvioglacial spreads rather than of glacial origin, as indicated on the 1:50 000 sheet.

A well defined sequence of four terraces and associated loams has been recognised, of which the upper three occur within the present district. The distribution and height range of the terraces are shown in (Figure 23) and discussed in more detail below.

It has been suggested (Lake, Ellison, Hollyer and Simmons, 1977) that these terraces relate to the former drainage of the Thames-Medway river, and that the course of the river was determined by the maximum westward extension of an Anglian ice-lobe. Tentative correlations were suggested between the 'Upper Floodplain Terrace' of the River Thames and the 1st Terrace of this area, between the 'Taplow' or 'Ilford' Terrace and the 2nd Terrace, and between the 'Boyn Hill Terrace and the 3rd Terrace. The 4th Terrace has no direct correlative with the Thames terraces. These correlations disagree with that of Gruhn, Bryan and Moss (1974, p. 70) who regarded their Rochford Terrace ( = 2nd Terrace), with a surface level of 12 m OD, as equivalent to the (Hoxnian) 'Boyn Hill Terrace'.

The following sequence of events is proposed by Lake, Ellison, Hollyer and Simmons (1977) for the evolution of the terraces and associated deposits in the Dengie Peninsula:

1. Westward extension of an Anglian ice-lobe and the diversion of the Thames-Medway river across this area and the deposition of the 4th Terrace.
2. Excavation and partial filling during Anglian times of channels which lie beyond the district.
3. Polycyclic lacustrine deposition in the upper part of the channels in early Hoxnian times.
4. Initiation of 3rd Terrace deposition along the channel alignment during a cold phase within the Hoxnian.
5. Continued deposition of terrace deposits.

Within the district there is a gradual fall in the base-levels of the terraces in a north-easterly direction. A somewhat steeper fall of base level towards the former valley can also be recognised (see (Figure 23)).

Stone counts from the 3rd Terrace by Gruhn and others, (1974, pp. 60 and 61) in the pits at Asheldham (their Asheldham Terrace), and at Southminster and Tillingham (their Southminster Terrace), revealed respectively 13, 10 and 17 per cent of Lower Greensand pebbles. They note that in general quartz pebbles form less than one per cent of the gravel constituents of the Dengie terraces.

Terrace Loam

Locally deposits of 'brickearth' have been mapped overlying the various terrace gravels. Lithologically they are similar to the glacial and Head brickearths, but they are thought to represent overbank deposits on the former floodplains of river. CRB

Lake Deposits (Hoxnian)

The deposits formed during the Hoxnian Interglacial arc shown as Lake Deposits on the 1:50 000 sheet. Interglacial deposits have been known at IIoxne, Suffolk for many years. Baden-Powell (1948, p. 280) thought that they occurred stratigraphically between his Lowestoft and Gipping tills, as did West and Donner (1956, p. 86) when they introduced the term Hoxnian for the interglacial period which they correlated with the Holstein (Elster / Saale) Interglacial of the Continent. This interglacial is generally considered to be distinct from the later Ipswichian interglacial. This stratigraphy and chronology has been adopted by most subsequent workers, although lately it has been questioned by Bristow and Cox (1973) who have suggested that the Hoxnian and Ipswichian interglacials may be closer in time than thought hitherto. The discrepancy between the 'short' sequence of post-Hoxnian deposits proposed by Bristow and Cox and the 'long' sequence of Evans (1970; 1971) is discussed by Sutcliffe (1975). Evans (1971, p. 280) postulated five climatic cycles in post-Hoxnian time, each beginning with a cold phase. The half-cycles, which he designated by either 'w' or 'c' to indicate warm or cold, had each an approximate duration of 20 000 years. The Hoxnian (and Holstein) Interglacial he placed in his half-cycle 6 w, which Shackleton and Turner (1967, p. 1081) have demonstrated at Marks Tey had a duration of some 30 000 to 50 000 years, and the Ipswichian in half-cycle 3 w. Sutcliffe, on the mammalian evidence, favoured the longer sequence, and this is supported by the radiometric dates of Szabo and Collins (1975). Taking the radiometric dates at face value shows that the 'Hoxnian' deposits at Clacton (245 000 years BP) fall within half-cycle 7 w; the 'Hoxnian' Swanscombe deposits ( > 272 000 years BP) are older than half-cycle 8 w; the 'Ipswichian' deposits at Sutton (125 000 years BP) can be assigned to half-cycle 4 w; and the deposits at Brundon (174000 years BP), which are only tentatively correlated with the Ipswichian, relate to half-cycle 5 w. Evans (in discussion of Bristow and Cox, 1973, p. 36) suggested that some deposits attributed to the Hoxnian could belong to half-cycle 5 w. It is unfortunate that the critical Hoxnian deposits at Hoxne and Marks Tey have not been satisfactorily dated. Published radiocarbon dates from Hoxne (26 930 years BP) and Stanway Bypass (32 500 and 28 170 years BP) (Bristow and Cox, 1973, p. 17) are thought by these authors to be inaccurate because of contamination. Similarly dates of 24 500 and 24 100 years BP at Hoxne, and other 'young' Hoxnian dates obtained by Page (1972) are regarded by Shotton (1973) as unreliable for the same reason. Shotton (1973, p. 393) recorded dates of more than 37 000, more than 47 600, more than 48 500 and more than 43 000. years from samples at Hoxne.

Hoxnian lacustrine deposits within the Chelmsford district are known from the Blackwater Valley (Turner, 1968, p. 333; 1970, pp. 377, 433). Deposits of suspected Hoxnian age, shown as Older Estuarine Alluvium on the 1:50 000 geological maps, are present in the south-east of the district, and in the adjacent Southend and Foulness (258/9) district. The distribution of this latter occurrence is demonstrated and discussed by Lake, Ellison, Hollyer and Simmons (1977).

Dalton (in Whitaker and others, 1878, p. 67) was the first to record the presence of the deposits now known to be of Hoxnian age in the Rivenhall End area. Boreholes sunk in the site investigation for the widening of the Al2 at Rivenhall End in 1962 confirmed their presence and extended their distribution. During the six-inch survey it was possible to map separately part of the outcrop to the south of Rivenhall End and also in the area to the east of Witham. The drilling of the Witham Borehole, and the boreholes sunk as part of the IMAU sand and gravel resource survey (Haggard, 1972) in the Kelvedon area, extended the known occurrence of these deposits and provided the first reliable evidence of the altitude and form of the base of the deposits, together with their thickness.

The dominant lithology is white, creamy or buff silts, silty clays and clays, locally laminated. Wood fragments and shells have been noted in some borehole samples. The maximum recorded thicknesses are 13.41, 8.84 and 6.02 m in boreholes [TL 8556 1829, 8250 1544] and [TL 8400 1655] at Kelvedon, Witham and Rivenhall End respectively. At Witham the surface level of the top of the deposits varies from 19.44 to 20.77 m OD and the base from 11.29 to 19.21 m OD. From the limited evidence available the base of the deposits appears to rise towards the east. Farther northeastwards, in the vicinity of Rivenhall End, the respective levels are 21.96 and 12.81 m OD, and in the Kelvedon area the figures are 25.62 and 10.67 m OD. At each locality investigated in the Witham-Kelvedon area the interglacial deposits are separated from the underlying Springfield Till by a unit of sand and gravel, which varies in thickness from 2.7 to 11.48 m, and are overlain by Head. The underlying sand and gravel is interpreted as 3rd Terrace on the published Geological sheet. Turner (in Rose and Turner, 1973), however, disagrees and regards the sand and gravel as of subglacial origin considering that the 3rd Terrace gravels abut against, and are therefore younger, than the interglacial deposits.

The Hoxnian age of the Rivenhall End deposits is based on provisional work by Turner (1968, p. 333; 1970, pp. 377, 433), who has made a detailed study of the more widely known and better exposed deposits at Marks Tey, some 6 km NE of Kelvedon. A detailed study of the Rivenhall End deposits is necessary in order to establish if the same full climatic sequence is represented here as at Marks Tey. On stratigraphical grounds it would appear that deposition of the chalky Boulder Clay ceased and was followed by a slight amelioration of climate which resulted in the deposition of a widespread deposit of sand and gravel by high-energy meltwater streams in the basin vacated by the ice-sheet, prior to the establishment of the quiet lacustrine conditions in which the interglacial clays and silts were deposited.

The post-Hoxnian chronology is uncertain within the district as only one deposit referable to the Ipswichian Interglacial is known. As with the many other known interglacial sites in East Anglia deposits of the two interglacials do not occur in close proximity and there is no direct stratigraphical evidence for the relative positions. Both sets of deposits are associated with the present-day valleys systems in areas where deep drift-filled channels exist, and both are overlain by soliflucted material. It is, however, not known whether the Head covering both deposits is a product of the same periglacial episodes. CRB Boreholes drilled in south-east Essex have proved the existence of drift-filled depressions from Clacton to Bradwell and thence, by way of Burnham, Rochford and Shoeburyness, to Hoo on the north side of the Medway estuary. Similar deposits noted on Osea Island are probably related to the above. The sedimentary fill of these channels consists of a lower gravelly sand unit and an upper argillaceous unit. The deposits which fill these channels are probably all of Anglian-Hoxnian age, but have been considerably affected by subsequent episodes of erosion and aggradation during the development of the river terrace sequence and during the Flandrian transgression.

The lower gravelly sand unit (3rd Terrace) consists of cross-bedded sands with subordinate silts and gravels and is similar in appearance to the glacial gravels present to the north. The upper argillaceous unit consists of greenish grey to brownish grey, soft to firm, silty clays with intercalations of silty sand. Locally carbonaceous inclusions and shell detritus are present. Calcareous 'race' concretions are common. At the surface these clays weather to a brown silty consistency which renders them locally indistinguishable from weathered London Clay or Head derived from the London Clay.

Where these argillaceous sediments are overlain by terrace deposits, evidence of reworking and cryoturbation is common. The upper clay unit appears to have been particularly susceptible to cryoturbation and locally contains gravel incorporated from the overlying drift. RDL

It is possible that some of the small inliers of 'London Clay' in the Tolleshunt D'Arcy-Tollesbury area (see p. 34) may represent reworked London Clay, and be equivalent to the above mentioned interglacial deposits in the Southminster area. CRB

Interglacial deposits (Ipswichian)

At only one locality within the district have interglacial deposits of this age been recognised. This is at Moulsham within the built-up area of Chelmsford (see p.80). Dawkins (1869, p. 198) mentioned Hippopotamus major, Bos primigenius and possibly Bison from Moulsham near Chelmsford; Hippopotamus bones from Moulsham, which are preserved in the British Museum (Natural History), appear to be those he listed. They are currently regarded as H. amphibius; the metatarsal of Bos is identified as B. cf. primigenius. The specimens, which were presented by Mr Roper in 1864, came from below Brickearth at a depth of about 6 m. According to Stuart (1976, pp. 240–241) the presence of H. amphibius is indicative of zones IIb and early IIIa of the Ipswichian Interglacial, and is commonly associated with B. primigenius. Woodward in Whitaker (1889, p. 452) gave more details of the strata in which the fauna occurred. He described a section in a pit [TL 7030 0605] on the east side of the railway line from where wood, shells and Hippopotamus have been found in a sandy clay with chalk fragments at the bottom of the pit. Later sections in Mr Brown's brickpit [TL 7005 0630] on the west side of the railway line (see p. 80) yielded the lower jaw of Mammuthus primigenius in a ferruginous gravel (1st Terrace) overlying a sandy clay with chalk fragments (Holmes, 1896; Newton, 1896) and freshwater shells (Webb, 1896) from the overlying brickearth.

The apparent faunal succession of Hippopotamus followed by Mammuthus; is in accord with Stuart (1976) who shows that M. primigenius is characteristic of the later part of the Ipswichian (zones IIIb to IV). Further support is provided by the record of an elephant tusk and the rhinoceras Coelodonta antiquitatis (see details below, p. 80) apparently from the same pit as and from strata overlying, the Hippopotamus bed. C. antiquitatis is only known from zone IV of the Ipswichian. A molluscan fauna listed by Webb (1896) appears to have come from the brickearth which overlies the gravel containing Mprimigenius, at a depth of about 4 m from the original ground surface. The fauna included Limnaea palustris, L. pereger, L. truncatula, Planorbis marginatus, P. spirorbis, Pupa muscorum, Succinea elegans and S. oblonga.

As mentioned below (p. 80) there is the possibility that the gravel with Mammuthus and Coelodonta is the 1st Terrace of the River Chelmer. CRB

Alluvium and Sub-Alluvial Gravel

Alluvium is present in all the major, and many of the minor river valleys. Boreholes along the courses of the Chelmer and Blackwater rivers have demonstrated that it is possible to subdivide it into an upper clayey unit and a lower gravelly one. This lower unit is referred to as 'Sub-alluvium gravels' by Ambrose (1973a) and Clarke (1975).

A generalised profile of the Alluvium, including the sub-alluvial gravel, is portrayed in (Figure 21) and (Figure 22). From (Figure 21) it can be seen that the sub-alluvial gravel forms a unit which is topographically distinct from the 1st Terrace with which they have been tentatively correlated (Clayton, 1957a, p.12; Ambrose, 1973a, p. 13), although locally there is a slight overlap of their respective height ranges. A similar distinction has been noted in the Blackwater Valley, but for clarity the base of the 1st Terrace has been omitted from (Figure 22). For this reason, combined with the facts that the gravels are confined to the floodplain and are present everywhere under the alluvial clays even where the 1st Terrace is absent and the rivers are flanked with London Clay, they are regarded as an integral part of the present-day flood plain deposits.

The thickness of the Alluvium of the River Blackwater varies from 2.4 to 8.54 m and averages 4.43 m. The variation in thickness of the clay and gravel units are from 1.0 to 3.05 m with an average of 1.79 m and 1.52 to 5.49 m, with an average of 2.77 m, respectively.

The general distribution of the alluvial clay and sub-alluvial gravel of the River Chelmer can be seen in (Figure 21). Although locally there is marked variation in thickness from borehole to borehole, it would appear that the clays show an overall decrease in thickness downstream, and that the sub-alluvial gravel shows an overall increase in this same direction. Average figures for these two units for three stretches of the River Chelmer, namely the upper reaches above Chelmsford, Chelmsford to Ulting, and Ulting to Maldon, are 2.09, 1.84 and 1.56 m, and 1.85, 2.42 and 3.01 m, respectively.

Grading figures which are available for the sub-alluvial gravel in the stretch of river between Sandford and Ulting show 48 per cent of gravel, 40 per cent of sand and 12 per cent of fines (Clarke, 1975). These grading figures are similar to those of the several terraces in this vicinity, and also to the overall grading figures for the Glacial Sand and Gravel. The figures contrast with those from the sub-alluvial gravel downstream from Ulting which show 75 per cent gravel, 20 per cent sand and 5 per cent fines (Ambrose, 1973a). However, if the individual borehole records for the Sandford to Ulting stretch are consulted there appears to be a general downstream increase of the gravel fraction, and a corresponding decrease of the fines. The lowermost four boreholes within Resource Sheet TL 70 give average proportions of gravel, sand and fines as 62.5, 29 and 7.5 per cent respectively (Clarke, 1975).

The grading figures for the sub-alluvial gravel of the River Chelmer given in Ambrose (1973a) are very similar to the ones obtained from the sub-alluvial gravel in the lower reaches of the Blackwater [TL 8282 1051] (see p. 82) (Haggard, 1972). As the boreholes from which the Chelmer data were obtained lie close to the confluence of the Chelmer and Blackwater rivers, it may be that the Blackwater was the dominant influence in contributing material to the sub-alluvial gravel in this sector. The similarity of the grading figures of the sub-alluvial gravel to those of the 1st Terrace led Ambrose (1973a, p. 13) to suggest that the two deposits might be continuous.

There is no independent evidence within the district to date the Alluvium, but on regional evidence it is considered that deposition commenced in the late Devensian with the sub-alluvial gravel and continued during Flandrian times to the present day with the alluvial clays and silts. CRB

Older Estuarine Alluvium

In the extreme south-east. of the district, and extending into the adjacent Brightlingsea (242) district, there is a spread of grey, brown and fawn silty clay which forms a low rise about 1 to 2 m above the Estuarine Alluvium. It has been termed Older Estuarine Alluvium. There is no well marked topographic feature between the two deposits, but the Older Estuarine Alluvium is sufficiently elevated for trees and hedges to grow and to develop roots that are above the influence of the saline water normally present in the Estuarine Alluvium. More than 7.9 m of the clay have been proved, and the deposit extends down to below −3.55 m OD. Its molluscan fauna has not yet been studied.

An ostracod fauna has been recovered from samples in one borehole [TQ 9773 9986]. A mixture of fresh-water and oligohaline species was present. On general consideration it may possibly correlate with the 1st Terrace (Gravel) of the Dengie Peninsula, or it may be intermediate in age between this gravel terrace and the Estuarine Alluvium. CRB

Estuarine Alluvium

Estuarine Alluvium is present only sporadically along the Blackwater Estuary. A more extensive tract lies to the east of Tollesbury along the reclaimed Tollesbury Wick and Old Hall marshes.

In the adjacent Southend and Foulness (258/9) district to the south, the composition, lateral variation and age of many of the components which comprise the Marine and Estuarine Alluvium are known in some detail from borehole evidence (Lake and others, in prep.). In general a complex of fluvial gravels (Basal Sand and Gravel) overlain by clays and sand of marine or estuarine origin can be recognised. Locally the marine and estuarine deposits may overlie or interdigitate with fresh-water deposits from which they are not readily separable. Above the Basal Sand and Gravel five lithostratigraphical units were recognised by the above authors, of which only the upper three were present at outcrop. On Foulness Island, and on the eastern part of the Dengie Peninsula, the oldest unit that crops out, the Upper Tidal Flat deposits, passes seawards beneath the Tidal Flat Sands. The uppermost unit of shell and gravel beach ridge deposits overlies both the Upper Tidal Flat deposits and the Tidal Flat Sands. In terms of the classification there adopted, the Estuarian Alluvium of the Chelmsford district comprises the Upper Tidal Flat deposits which consist in the main of silty clays. It is known from limited borehole evidence that the Basal Sand and Gravel is present at depth. It would appear that these gravels are continuous with the fluviatile sub-alluvial gravel which is invariably present beneath the major rivers draining into the Blackwater Estuary. The Basal Sand and Gravel in the adjacent Southend district is thought to be late Pleistocene by Lake and others (in prep.).

The Flandrian sea level rise that inundated the area and gave rise to the tidal flat succession, is generally regarded as commencing at about 10 000 BP. It is suggested by D'Olier (1972) that the sea level stood at around −45 m OD at that time. Flandrian sedimentation seems to have commenced around 9000 to 8000 BP (Greensmith and Tucker, 1973, p. 193; D'Olier, 1972, pp. 127–128, figs. 6–9) with the inundation of various parts of the area. A time-depth curve of mean sea level is given by Lake and others (in prep.). It shows that initially, sea level rose sharply in the Boreal and Atlantic zones; during the late Atlantic this rate reduced and there were periods of slightly lowered sea level or still-stands during the Sub-Boreal. An overall Sub-Atlantic transgression is recorded by Greensmith and Tucker (1969, p. 424). Two principal transgressions are thought to have occurred between 1550 and 1200 BP and between 1000 and 700 BP (Greensmith and Tucker, 1975, p. 261), at which time sea levels were higher than at the present day. The inland shell banks are suggested by the above authors to have formed during these periods.

The history of the reclamation of the marshes, and the continual struggle to protect the reclaimed land from the sea, is outlined by Grieve (1959). Recent evidence of phases of salt marsh erosion and accretion are mentioned by Greensmith and Tucker (1969, p. 417; 1975, p. 262).

Although essentially similar to the Estuarine Alluvium, the present-day Tidal Flat Deposits are those deposits that are still subject to twice-daily inundations by the sea. The sea walls, therefore, form arbitrary geological boundaries which, with fluctuating sea levels, may lead to reclassification of these deposits. There is not the detailed information available, as on the Dengie Peninsula and Foulness Island, to chronicle the successive stages of marsh reclamation within the Chelmsford district. However, it is worth quoting

Northey Island [TL 880 060] as one example: the marsh of Northey Island was reclaimed at some unknown date; on November 11th 1897 the sea wall was breached; the wall has not been repaired and only its crest remains visible at high tide. The inundated area is, therefore, mapped at the present day as Tidal Flat Deposits. CRB

Storm Gravel Beach Deposits

Ridges of sand and gravel can be found at the retreating marsh edges at several localities within the district. Similar ridges composed of shells have been mapped in the area to the east. Such deposits are referred to as cheniers by Greensmith and Tucker (1969; 1975).

Most of the gravel ridges are situated close to outcrops of terrace sand and gravel from which their components are presumably derived. Greensmith and Tucker (1975, p. 235) suggest that the gravel 'terrace', equivalent to the Basal Sand and Gravel of the Estuarine Alluvium, exposed in the intertidal zone in the lower reaches of the Blackwater Estuary, provides important source material for the ridges in this vicinity. They point out that the present condition along the Essex coast is one of marine transgression. In addition earlier transgressions, such as those at about 1434 ± 110 BP to 1265 ± 100 BP and 1000 to 700 BP, were responsible for the inland cheniers of the Dengie Peninsula (Greensmith and Tucker, 1975, pp. 254, 261, 267).

The main effect of the reworking of the gravel deposits is the formation of asymmetric gravel banks on the lower and middle tidal flats. The width of the banks varies up to 75 m; they are up to 1.5 m higher than the surrounding material and reach lengths of up to 500 m. The larger bodies are thought to have formed by coalescence of several smaller ones. The movement of these bodies across the flats is very slow, and marked movement occurs only during prolonged storm conditions. The end result is the formation of the cheniers at the upper extremities of tidal action on the upper and supratidal flats. The longshore drift of the constituents is variable in direction and depends largely on the fetch. In the mature chenier stage, progressive growth, extension and landward movement of the cheniers, with an increase of complexity of structure occurs. Locally creeks become blocked, or beheaded, as the ridges move into, or across them (Greensmith and Tucker, 1975). The old age chenier stage is not visible within the district, but is well illustrated in the Dengie Peninsula. Here the cheniers have become isolated by salt marsh extension and today form 'fossil' shell banks within the Estuarine Alluvium.

Peat

Small spreads of peat have been mapped locally. The peat occurs where springs issue from the base of the Chelmsford Gravels, and is intimately associated with the Calcareous Tufa which forms in the same stratigraphical position.

Only three spreads of peat have been mapped, in the Chelmer and Ter valleys and at Rivenhall End, although peat in differing stratigraphical positions has been noted in boreholes outside these areas (e.g. Clarke, 1975, p. 90). In augering all that is generally found is a soft black waterlogged humic mass, which has been proved locally to exceed 1.4 m in thickness.

As with the Calcareous Tufa it is probable that these deposits are still forming at the present day and do not have any great antiquity. They are probably entirely Holocene in age, but may extend back into the Pleistocene. CRB

Calcareous tufa

Deposits of soft unconsolidated calcareous tufa occur at several localities along the valleys of the rivers Ter and Brain in the northern part of the district. They are nearly all related to calcium-laden springs which issue from the base of the Chelmsford Gravels. The deposits consist of soft porous calcareous spongy-textured material. The texture is presumably related to the former positions of plant stems and roots, as the deposit can be seen to encrust the present day flora. No contained fauna was noted in the field samples. For most of the spreads their full thickness is not known, but some have been proved by augering to exceed 1.4 m.

Many of the deposits are still forming at the present day and they are not thought to have any great antiquity. They are probably entirely Holocene in age, but may extend back into the Pleistocene. CRB

Landslips

Landslips have been mapped on several slopes facing north, south and west. Most occur close to the boundary between the London Clay and Claygate Beds, or locally to that between the Claygate Beds and the Bagshot Beds. The former junction is commonly marked by springs which emanate from the sandy more permeable layers within the Claygate Beds. Most of the landslipped slopes are probably the result of over-steepening caused by fluviatile action undercutting the foot of the slope, combined with saturation of the clay by spring water. Hutchinson (1967, p. 117), however, did not think that fluviatile erosion was important but believed that the slides were probably initiated by a combination of progressive reduction in shear strength through weathering, and an increase in pore-water pressure arising from deforestation or climatic deterioration. Landslips at Maldon, where the slopes represent the degraded cliff line of the River Blackwater, are entirely within the London Clay. To the south, in the region of Bushy Hill [TQ 812 990] near Woodham Ferrers, a landslip which straddles the district boundary lies entirely within the Claygate Beds. Here sand seams cause springs to rise upslope from the boundary between the London Clay and Claygate Beds.

All the landslips recognised within the district lie close to the approximate limit of the Anglian glaciation. This is particularly evident in the case of those slips in the vicinity of Woodham Ferrers where chalky Boulder Clay is present. The landslips are thought to have originated in succeeding periods of periglacial activity, during which times extensive sheets of Head also formed.

Movement of many of the slips continues at the present day. No slips have been noted which affect the Springfield Till on the sides of the river valleys which cross the plateau area in the north-west of the district. Older Pleistocene landslips may, however, exist in such areas, and may now be concealed by drift. Alternatively, in the south, older landslips may not now have any topographic expression and Hutchinson (1967, p. 117) has suggested that some ancient landslips may be concealed under Head.

Hutchinson (1965; 1967) has made an extensive study of the landslipped areas of Essex and south Suffolk. He noted that shallow landslips predominate under present climatic conditions, and that the inclination of slopes may be broadly correlated with the type of slip involved, as follows: shallow rotational (R); successive or stepped rotational (S); undulating (U); marked non-circular shallow rotational (N); or predominantly translational (L). Of these five types of slips all but the rotational type were recognised by Hutchinson within the district. He believes rotational slips were initiated only under conditions of fairly strong marine erosion in areas where the initial slope was in excess of 13°. The present author thinks that slips at Maldon may have initiated as rotational slips caused by undercutting of the Blackwater river cliffs.

On a slope of less than about 13°, successive rotational slips are developed, e.g. at Maldon west, Bushy Hill west and Martin's Farm west [TQ 822 993], 1 km W of Stow Maries. Successive rotational slipping proceeds until the slope angle reaches 8°. Hutchinson found that most of the inland slips are of N or L type. These slides occur typically in the steepest part of an otherwise smooth and stable slope.

The N-type slides such as Mill Hill south [TQ 798 987], Loddart's Hill north-east [TL 826 042], and Woodham Ferrers north-west are associated with slopes ranging between about 12 and 9°. The shallow L-type slab slides, such as Martin's Farm south-south-east, Bushy Hill north-east, Steeple [TL 940 025], Purleigh north-west [TL 832 024], Kits Hill west [TQ 856 991], Great Canney [TL 837 003], Bushy Hill north-west, Purleigh west, Martin's Farm south-east and Loddart's Hill north are confined to flatter slopes of between 10 and 8° inclination. The U-type slides, namely Maldon east and Bushy Hill south-west, like the S slides, have an inland slope angle of between 12 and 8+°. This suggests that an angle of 8° is required for ultimate stability on London Clay and Claygate Beds. Hutchinson also found that the steepest stable slope are up to 10°, and that there was therefore a 2° overlap between the flattest unstable slope and the steepest stable slope, a difference he attributed to varying lithological and ground-water conditions. CRB

Details

River Terrace deposits of the River Chelmer

3rd Terrace

In the area [TL 747 057] to the north of Grace's Cross there is a widespread flat developed over sand and gravel which varies in height from about 30 m OD in the south to 26.5 m OD in the north. A poor break-in-slope at about 23 m OD on the eastern side of the Chelmer Valley is taken as the base of the terrace deposits, but the contact between the terrace and the underlying deposits is obscured by Head. Away from the river the available borehole evidence indicates that the base of the sand and gravel is irregular. A borehole [TL 7454 0543] drilled just south of the main Chelmsford-Maldon road close to the feather edge of the terrace deposits, proved the following: soil, and brown silty clay (Head), 1.5 m; clayey gravel of the 3rd Terrace, 1.5 m; London Clay. The base of the terrace deposit was at 27.5 m OD. Some 500 m E another borehole [TL 7500 0560] near Potash Farm penetrated 1.2 m of soil and Head, which overlay 6.4 m of very clayey sand (3rd Terrace), which in turn overlay the London Clay at 17.4 m OD.

Some-350 m NE of Mayes Farm to the east of the Sandon Brook, a small spread of sand and gravel at about 30 m OD [TL 7540 0475] overlies Glacial Lake Deposits. It is some 4.5 m higher than the 2nd Terrace 200 m to the west.

Farther downstream a deposit of sand and gravel to the west of Chappel Farm [TL 880 090] has a surface at 8 to 9 m OD and lies some 1 to 2 m higher than the back feature of the 2nd Terrace: a low feature separates the 3rd and 2nd terraces. A borehole [TL 8762 0906] proved 2.8 m of gravel beneath 1.2 m of soil and brown sandy clay (Head); the junction of the sand and gravel lay at 5.1 m OD (Ambrose, 1973a). Another borehole [TL 8821 0911], some 250 m to the east, penetrated 3.7 m of soil and brown silty clay (Head) overlying 2.1 m of clayey gravel, the base of which lay at 2.5 m OD.

2nd Terrace

In the Chelmsford area the interval between the surfaces of the 2nd and 1st terraces is only about 1 m, although a distinct low feature break separates the two deposits. Downstream in the Heybridge area [TL 860 080] the surface of the 2nd Terrace is about 2 to 3 m higher than that of the 1st Terrace, and a distinct feature again marks the boundary between them.

The highest point upstream at which the 2nd Terrace has been recognised is in the vicinity of Brookend [TL 737 070]. The back of the terrace rests on the London Clay at about 23 m OD some 250 m W of Brookend. When traced south-westwards the terrace sand and gravel abuts against Glacial Sand and Gravel and the two deposits are indistinguishable lithologically, though the terrace flat is separated from the Glacial Sand and Gravel by a low feature. The surface of the 2nd Terrace falls across the terrace to about 20 m towards Sandford Lock. Downstream the surface level falls to about 18 m to the north of Brookend.

The thickness of the sand and gravel appears to vary quite markedly, although this may partly be caused by ice-heave structures within the London Clay which bring the London Clay close to the surface. An example of this variability was seen during the construction of the reservoir [TL 733 066] 700 m SW of Brookend. At one point [TL 7332 0659] the London Clay was within 0.9 m of the surface; at another point [TL 7335 0657] 60 m SE the thickness of the overlying sand and gravel was 4.0 m, whereas sections on the west side of the reservoir revealed 2.4 to 3 m of terrace deposits overlying the London Clay. A borehole [TL 7343 0650] 100 m SE of the reservoir was still in sand and gravel at a depth of 4.88 m. Another borehole [TL 7371 0662] at Pease Hall failed to penetrate sand and gravel at a depth of 3.35 m, and a third borehole [TL 7387 0673] proved more than 4.27 m of sand and gravel (Clarke, 1975).

In a trench which passed to the west of Brookend a further example of the cryoturbated contact between the terrace deposits and the London Clay was observed. Over most of the length of the trench, which was about 1.8 m deep, the base of the sand and gravel was not seen. However, at two points [TL 736 071] and [TL 7364 0729] the London Clay was within 0.9 and 1.2 m respectively of the surface. Two boreholes [TL 7372 0727] and [TL 7369 0717] on the east side of the trench each proved 1.5 m of sand and gravel, beneath 0.6 and 5.5 m respectively of made ground, soil and Boulder Clay (Clarke, 1975). Farther north-east a trial borehole [TL 7412 0723] penetrated 1.65 m of Head above 1.60 m of terrace gravel which in turn overlay Glacial Lake Deposits.

On the opposite side of the Chelmer Valley, sand and gravel of the 2nd Terrace is present on the east side of the Sandon Brook, and on either side of the unnamed tributary which joins it at Sandon. The surface level of the deposit falls from about 30 m OD at the highest point at which terraces have been mapped to about 22 or 23 m OD in the vicinity of Great Graces [TL 758 066]. In the south, the 2nd Terrace rests on London Clay. Downstream, the more widespread occurrence overlies Glacial Lake Deposits.

A borehole [TL 7567 0534] at Sandon Bridge proved in descending sequence: brown clay with stones [Head], 1.2 m; clayey gravel of the 2nd Terrace, 2.4 m; Glacial Lake Deposits (Clarke, 1975).

A spread of 2nd Terrace has been mapped between Hammond's Farm [TL 750 070] and just north of Phillow's Farm [TL 755 080]. Within this tract the surface level falls gently northwards from about 21 m OD to about 17 m OD. A widespread, but generally thin, irregular cover of brickearth, varying in thickness from 0.6 to 2.1 m, overlies the sand and gravel. The maximum proved thickness of the sand and gravel in this vicinity was 3.0 m in the borehole [TL 7528 0744] at Hurrells Lane.

Discontinuous spreads of 2nd Terrace are present downstream from the Sandon Brook. The most extensive extends from the Sandon Brook to the Little Baddow-Hatfield road [TL 777 086]. The back edge of the terrace, which everywhere abuts against the London Clay, appears to be approximately horizontal over this length, at about 25 m OD, some 10 to 11 m above the present flood plain of the River Chelmer. At the eastern end of this outcrop the base of the 2nd Terrace deposits overlies the London Clay at about 16 m OD, which is some 2 m above the level of the flood plain. A borehole [TL 7703 0843] drilled within this deposit showed that the sand and gravel was only 0.9 m thick, and that it was overlain by 2.4 m of soil and brickearth. The base of the terrace lay at 15.8 m OD.

In the vicinity of Boreham Hall [TL 753 090] on the north side of the River Chelmer a small spread of 2nd Terrace overlies Glacial Lake Deposits. The maximum elevation of the terrace is about 22 m OD. Trench sections [TL 7519 0878]–[TL 7553 0882]–[TL 7569 0908] showed that the sand and gravel varies in thickness from 1 m to more than 2.1 m, and that its base is correspondingly irregular.

A more extensive terrace can be found on the east side of the unnamed stream which flows to the south of Boreham. The back feature of this terrace, which abuts either the London Clay or Glacial Sand and Gravel, falls gently eastwards from about 23 m OD to about 21 m OD. The base was not seen at outcrop, but in one borehole [TL 7668 0910] at Culvert's Farm it was at 15.9 m OD, and overlay Glacial Lake Deposits.

Downstream from the Little Baddow to Hatfield road on the south side of the River Chelmer the back of the 2nd Terrace falls only gently to about 23 m OD and its base falls from about 16 m in the west to about 14 m in the east over a horizontal distance of 2 km. The borehole [TL 7885 0878] at Bassett's Farm proved only 0.9 m of sand and gravel which was overlain by 0.6 m of brown sandy clay. The base of the deposit was at 15.0 m OD.

In the vicinity of Hoemill Barns [TL 810 080] there is a prominent flat of sand and gravel. It would appear that the sand and gravel is composite, its upper part being a terrace deposit and its lower part Glacial Sand and Gravel: the two deposits are not lithologically separable in some boreholes including those at Hoemill Barns [TL 8100 0809] and Manor Farm [TL 8194 0802].

Farther east, in the vicinity of Beeleigh Abbey a trial borehole [TL 8400 0766] showed that the base of the 2nd Terrace gravels lay at 6.5 m OD; the upper surface of the 2nd Terrace is at approximately 11.0 m OD and is approximately 1.5 m higher than that of the 1st Terrace. Springs [TL 8351 0810] and [TL 8361 0805] issue from the lower part of a low feature that marks the base of the 2nd Terrace, and may originate from the sand and gravel/London Clay contact which is obscured by a thin layer of Head.

On the north side of the River Chelmer there is a very extensive outcrop of 2nd Terrace between Ulting in the west and Goldhanger in the east. Beyond Goldhanger the outcrop becomes fragmented and only small scattered outliers remain.

In the vicinity of the confluence of the Chelmer and Blackwater rivers the respective terraces of these rivers merge and it is probable that the 1st and 2nd terraces of the Blackwater correlate directly with the 1st and 2nd terraces of the Chelmer.

The surface of the 2nd Terrace on the north side of the Chelmer west of the Blackwater is at about 10 or 12 m OD in the lower reaches of the River Blackwater at Stock Hall. The borehole [TL 8287 0980] at Stock Hall Farm penetrated 4.3 m of soil and sand and gravel. The junction of the terrace deposits and the London Clay lay at 6.4 m OD. On the east side of the River Blackwater the back end of the 2nd Terrace falls gently eastward from about 12 m OD at Langford to about 3 m OD to the east of Goldhanger. Farther east the terrace level appears to descend to Alluvium level around [TL 930 090] and [TL 945 085]; downstream it may pass below the Estuarine Alluvium as the Basal Sand and Gravel (but see p. 82). There appears to be little regional fall in its base, which varies from 0.3 to 4 m OD with an average of about 2 m OD. In none of the boreholes have the deposits of the 2nd Terrace been proved to extend below sea level as have those of the 1st Terrace, although towards the east of the deposit, they presumably do so.

Within Resource Block C of Ambrose (1973a), which covers most of the terraces east of the River Blackwater, no notable variation in composition between the 1st and 2nd terrace deposits could be observed; the average thickness of 2nd Terrace deposits in this area was 2.8 m. Irregular patches of brickearth up to 2.1 m in thickness have been noted in some of the boreholes and exposures within this area.

The sand and gravel of the 2nd Terrace are exploited commercially in workings [TL 870 085] to the north of Salcote Hall. Sections visible during 1966–67 (Plate 5), in pits which have now been backfilled, showed that the junction between the Terrace Deposits and the London Clay was irregular. The thickness of the sand and gravel varied from 0.6 m [TL 8667 0835] to more than 3.0 m [TL 8703 0854]. Boreholes in the excavated area showed a maximum original thickness Of up to 5.2 m of sand and gravel.

Beyond the Saltcote Hall workings there are no exposures in the 2nd Terrace Deposits. Boreholes in the vicinity of Cobb's Farm [TL 893 084] showed a consistent sequence of 0.9 to 1.8 m of brickearth overlying 1.5 to 1.8 m of terrace sand and gravel, which in turn rested on the London Clay. The surface of the terrace shows a fall in level from about 7.6 m OD at the back, to about 3.7 m OD close to the Alluvium. The base of the deposit shows a corresponding fall from 4.2 to 1.0 m OD.

To the south of Goldhanger up to 1.5 m of sand and gravel can be seen in the ditches behind the sea walls e.g. [TL 908 084]. The London Clay was not encountered by augering in any of these ditches. As noted above the scattered spreads of 2nd Terrace farther east appear to descend to, and may pass below, the Alluvium.

Small scattered relics of the 2nd Terrace can be found south of the Blackwater Estuary to the east of Northey House [TL 860 060] and on Ramsey Island [TL 945 056]. In the middle of the Estuary, deposits have been assigned to this terrrace on Osea Island [TL 920 063]. The Northey House occurrence has a base at some 4 to 5 m OD. In augering up to 1.2 m of sandy and flinty gravel were proved overlying the London Clay. To the west and south-west the terrace is obscured by up to 0.2 m of yellowish brown silty clay. At Ramsey Island the 2nd Terrace Deposits also rest on the London Clay at about 4 to 5 m OD. The higher ground is capped by an irregular spread of up to 1.3 m of yellowish grey sandy clay (Head). Storm Gravel Beach Deposits are banked up against the terrace at the river. In a borehole [TL 9424 0582] at Haynes Green some 1.15 m of topsoil and brickearth, overlay 2.35 m of sand and gravel, which in turn overlay the London Clay. On Osea Island the junction of the 2nd Terrace and the London Clay can be found at outcrop only on the northern part and on the foreshore on the southern parts of the Island. On the western part of the Island the 2nd Terrace rests on grey silty clays of uncertain origin, which have been shown as 2nd Terrace (Loam) on the 1:50 000 sheet; they may, however, be Hoxnian Interglacial, or Glacial Lake Deposits. The surface level of the terrace varies from 4 to 7 m OD. Its base on the foreshore, is only about 2 to 3 m above OD; it lay at 0.7 m OD in a borehole [TL 9203 0629] towards the eastern end of the island where the sand and gravel was 2.2 m thick (Simmons, 1978). Up to 1 m of brown sandy loam is present over the surface of the deposit.

1st Terrace

Chelmer Valley and Sandon Brook

Only one terrace has been recognised in the Chelmer Valley above Chelmsford. It forms a low rise about 1 to 1.5 m above the present flood plain. At the back end a low feature marks the contact with the London Clay, although this is largely obscured under Head. A trench [TL 7117 1096] to [TL 7131 1087] which crossed this area exposed the junction of the back of the terrace with the London Clay in the west, and also the contact with the London Clay close to the Alluvium in the east. Over the main section of the trench the terrace deposits were more than 1.2 m of clayey sand and gravel.

Deposits of the 1st Terrace have been mapped in Chelmsford near the confluence of the Chelmer and Can rivers. Site investigation boreholes around [TL 708 067] for the Chelmsford Central Redevelopment proved up to 4.7 m of sand and gravel overlying London Clay, chalky Boulder Clay or Glacial Lake Deposits. The surface of the deposit varied from 22 to 25 m OD, and its base from 20 to 21 m OD. Up to 2 m of made ground was present over the site. Farther to the west around [? 7055 0680] Bristow (in Whitaker, 1889, p. 453) noted some 1.5 m of brickearth overlying 3.0 m of gravel which in turn overlay the London Clay.

On the northern side of the railway line 1.5 m of 'drift', overlying 2.7 m of gravel, which in turn rested on London Clay, was proved in the Maltings Well [TL 7095 0740]. A temporary exposure seen in 1970 immediately to the west of the well showed a similar, but thinner, drift sequence as follows: made ground, 0.3 m; brown silt (brickearth), 0.75 m; sand and gravel, 3.0 m; London Clay. The base of the terrace deposit lay at about 23 to 24 m OD.

To the south of the River Chelmer in the Moulsham area, sand and gravel of the 1st Terrace has been proved in a number of boreholes. One of these boreholes [TL 7075 0640], which commenced at about 25.5 m OD, proved in descending sequence: topsoil, 0.61 m; brown sandy clay (brickearth), 0.91 m; clayey sand and a little gravel, 0.76 m; gravel, 5.33 m; silts (Glacial Lake Deposits), more than 6.70 m. The base of the terrace deposits lay at about 18 m OD.

A somewhat thinner terrace sequence was encountered in the Hall Street Waterworks Well [TL 7095 0635] to the east, where 1.67 m of soil and yellow clay (Head and/or brickearth) overlay 3.81 m of gravel (1st Terrace), which in turn rested at about 21 m OD on 13.57 m of 'quicksand' (Glacial Lake Deposits). Shallow boreholes between this well and a point some 300 m to the southeast proved a consistent picture of brickearth up to 2.4 m thick, overlying sand and gravel, the base of which was not penetrated.

A thin remnant of 1st Terrace lies on the north side of the River Chelmer near the Chelmsford By-pass [TL 721 065]. A low feature at 22 m OD marks the contact between the terrace deposit and Glacial Sand and Gravel. A similar low feature separates the 1st and 2nd terraces near Barnes Farm [TL 726 067] 500 m E.

On the opposite side of the valley a low-lying gravel flat [TL 730 060], only about 1 to 1.5 m above the floodplain, has a covering of up to at least 1 m of yellowish brown sandy loam. The back margin of the terrace is obscured by Head. A little farther downstream a 1.5 m step separates a small spread of 1st Terrace sand and gravel [TL 742 060] beneath 1 m or more of brickearth, from the Alluvium. The back margin of the terrace is covered by Head.

A more widespread occurrence of the 1st Terrace extends for about 3 km downstream from the Lock at Sandford [TL 740 064]. The surface of the terrace falls downstream from about 19 to 17 m OD. A borehole [TL 7399 0651] proved 0.6 m of made ground and topsoil above 3.5 m of terrace gravel, which in turn overlay more than 4.9 m of Glacial Lake Deposits (Clarke, 1975). The base of the terrace deposits lay at 15.1 m OD, and the percentages of gravel, sand and fines were 56, 30 and 14 respectively. Farther downstream a borehole [TL 7412 0723] which commenced at 18.35 m OD penetrated the base of the terrace sand and gravel at 15.65 m OD. There is an ubiquitous cover of brickearth up to at least 1.2 m thick over the terrace.

Spreads of 1st Terrace occur to the west of Phillow's Farm, and to the north-east of it, on either side of the Sandon Brook [TL 750 080], [TL 758 082] and [TL 763 083]. The back margin of the terrace falls from about 17 m OD in the westernmost of these three deposits to about 16 m in the easternmost. A borehole [TL 7583 0820] near the Lock at Little Baddow, which commenced at 14.9 m OD, proved 0.9 m of brown clay (brickearth), over 2.7 m of gravel, which in turn overlay the London Clay at a height of 11.3 m OD. The percentages of gravel, sand and fines in the terrace deposits were 71, 23 and 6, respectively.

On the opposite side of the valley, to the south of Culvert's Farm, a further occurrence of the 1st Terrace has been recognised [TL 770 090]. The back of the terrace, which is partially obscured under Head, lies at about 15 to 14 m OD. Farther downstream a small area of 1st Terrace is present to the south of Botter's Farm [TL 784 095] and has its back margin at about 14 m OD. A spread of sufficient extent and thickness to be exploited commercially lies at the confluence of the Ter and Chelmer rivers. The back of the terrace is covered by Head Brickearth, but a low feature at 12 m OD is regarded as its limit. The thickest sand and gravel was 6.7 m at a point [TL 7961 0907] close to the river where its base lay at an estimated 5.5 m OD. At the eastern margin of the pit [TL 7975 0904] it was only 4.6 m thick, and boreholes have shown that it thins rapidly beyond this point.

In the vicinity of Langford [TL 835 090] the 1st Terrace is well developed at the confluence of the Chelmer and Blackwater rivers. A low feature separates the 1st Terrace from the 2nd Terrace to the north-west, and from the Alluvium to the south. The back margin of the terrace is at about 6 to 7 m OD. Boreholes [TL 8297 0894] and around [TL 832 090] have proved that the sand and gravel varies from 4.5 to 8.5 m in thickness, and that the base of the 1st Terrace varies from −1.5 m to −2.9 m OD with an average for the five boreholes of −2.1 m OD. The percentages of gravel, sand and fines in the westernmost borehole [TL 8297 0894] was 71, 27 and 2 respectively.

On the south side of the River Chelmer there is a spread of 1st Terrace north and west of Beeleigh Grange around [TL 830 083]. The surface of this terrace varies from 4 to 5 m OD and is approximately 1.2 to 1.5 m higher than the present Alluvium. The base of the terrace lies between 2.6 and 3 m OD. A distinct low feature to the south denotes the contact with the 2nd Terrace. The thickness of the sand and gravel in this vicinity varies from 1 to 1.8 m.

To the west of Heybridge the 1st Terrace has been mapped at three discrete localities [TL 845 085], [TL 848 080] and [TL 854 077]. The surface of the middle one of these spreads is about 3.6 to 4.0 m OD. The former Maldon Railway Station was sited on the third spread which, prior to development, must have formed a low gravelly 'island' above the Alluvium of the estuary.

The most extensive spread of 1st Terrace within the Chelmer Valley extends from Heybridge eastwards as far as Heybridge Basin [TL 873 073]. The surface of the terrace falls downstream from about 4.0 to 2.4 m OD; the base is more variable but nevertheless falls eastwards from about 2.5 to 1.6 m OD. The thickness of the sand and gravel within this tract varies from 1.3 to 5.5 m. The grading figures for the three boreholes drilled in this terrace show the following percentages of gravel, sand and fines 58 to 62, 25 to 35 and 7 to 15, respectively (Ambrose, 1973a).

Beyond the Heybridge Basin only scattered remnants of the 1st Terrace are preserved. A low spread on Northey Island [TL 877 063] has a maximum thickness of sand and gravel of at least 1.2 m; similar thicknesses were proved by augering in the ditches dug into the terrace deposits to the south and south-west of Joyce's Farm [TL 916 087] and [TL 903 078]. CRB

River Blackwater Terrace Gravels

5th Terrace

Deposits of the 5th Terrace have been recognised only along the south side of the River Blackwater and its tributary, the Domsey Brook, in the adjacent Braintree (223) district.

The base of the terrace appears to be fairly constant at about 38.0 m OD over the whole of its length. The height of the back margin of the terrace is similarly uniform at about 40 m OD, although in the vicinity of Messing just beyond the district boundary, it rises to about 50 m OD.

Within the district the deposits of the 5th Terrace are represented by 1 m or more of sandy clay and gravel 600 m NE of Highfields [TL 872 179]. A thin spread of clayey sand and gravel has also been mapped to the west of Kelvedon Hall Farm, where the outcrop is divided by a shallow valley. Locally the surface indications are of a very gravelly substratum around [TL 8625 1685], and it is possible that the small pit [TL 8632 1695] was an old gravel pit.

No sections were visible in the spread of 5th Terrace around All Saints Church, Great Braxted [TL 852 154], but it appears to be less clayey than the other occurrences noted above. A small pit [TL 8518 1527] was presumably opened to extract sand and gravel for domestic use.

4th Terrace

The deposits of the 4th Terrace have a similar distribution to those of the 5th and are confined to the southern side of the Blackwater Valley. The base of the terrace is roughly horizontal at about 30 m OD.

No sections are now visible in the spread of 4th Terrace around [TL 870 182] 800 m N of Highfields. In augering, up to 1 m of ferruginous sand has been proved. During the construction of the Kelvedon By-pass, sections in this spread were visible just beyond the district margin. Up to 0.6 m of sand and gravel overlay 0.6 m of sand which in turn overlay the chalky Boulder Clay [TL 8709 1841].

At Highfields [TL 867 175] the 4th Terrace was proved in a borehole [TL 8675 1747] to consist of 0.91 m of very clayey gravel beneath 1.06 m of soil and overburden. Augering in this outlier indicated a clayey sand to be the dominant lithology.

Some 700 m W of Highfields coarse-grained ferruginous sand was proved. At one point [TL 8611 1752] the surface indications are of a very gravelly substratum, and the small pit immediately to the north may have been opened to exploit this deposit.

Clayey sand was noted in the vicinity of Brickhouse Farm [TL 854 166] and on the spread [TL 856 168] 300 m NE.

In the area to the south of the shallow valley which runs through Fabian's Plantation clayey sand and gravel also typified the 4th Terrace. A borehole [TL 8554 1608] proved 1.5 m of soil and gravelly clay overburden, above 1.2 m of pebbly sand which in turn rested on the Springfield Till.

3rd Terrace

Kelvedon–Rivenhall End

Near Kelvedon, just beyond the district boundary the surface of the 3rd Terrace forms a wide tract of flat-lying ground. This tract narrows southwards where it passes into the Chelmsford district and, in the Rivenhall End area on the north side of the River Blackwater, the terrace is absent or obscured under Head or Hoxnian Interglacial deposits. A similar southwards tapering of the outcrop can be followed on the south side of the valley. Within this area the back of the terrace appears to fall gradually from 30 to 23 m OD.

Several boreholes have been sunk in the deposit in this northern area and demonstrate the irregular base to the sand and gravel. Immediately north of the district the borehole [TL 8556 1823] at Church Hall Farm, which commenced at 25.6 m OD, proved in descending sequence: topsoil and Head, 1.5 m; lacustrine clays and silts, 13.4 m; more than 9.5 m of pebbly sand. The base of the sand and gravel here lies below 1.2 m OD. Sand and gravel deposits, more than 18.6 m thick, beneath 3.35 m of superficial deposit, have also been encountered beneath the bed of the river [TL 8564 1774] 550 m to the south. Here the base lay below about −1 m OD. Another borehole [TL 8533 1764], which was sited some 300 m WSW of the above, close to the edge of the terrace, proved more than 19.8 m of sand and gravel. The variable thickness of the sand and gravel was illustrated by a fourth borehole [TL 8531 1761], only 80 m SW of the above borehole, which proved 7.31 m of sand and gravel with a base at about 16 m OD, above chalky Boulder Clay.

On the opposite side of the valley the 3rd Terrace sand and gravel towards the back margin of the terrace varied from 2.4 to 4 m in thickness, above chalky Boulder Clay in the road cutting for the Kelvedon By-pass [TL 8691 1794]. In the bottom of the shallow valley to the south-west the chalky Boulder Clay is within 1.37 m of the surface beneath Alluvium and 3rd Terrace deposits [TL 8631 1784]. Some 100 m to the west and a few metres higher, the sand and gravel is 3.35 m thick and has its base at about 21 m OD [TL 8620 1783]. The thickness increases to 7.92 m a further 100 m to the SW [TL 8613 1777] where the terrace deposits have their base at 18 m OD and rest on chalky Boulder Clay, as at the previous site.

The 3rd Terrace around Appleford Farm [TL 846 161] has a surface level which varies from about 21 to 22 m OD. A borehole at Appleford Farm [TL 8450 1620], which commenced at 21.3 m OD, proved 5.2 m of topsoil and sand and gravel overlying chalky Boulder Clay at 16.1 m OD. In the vicinity of Rivenhall End to the north-west, Head and Hoxnian lacustrine deposits appear to cover the 3rd Terrace. A borehole [TL 8400 1655] at Rivenhall End, with a surface level about 22 m OD, proved the following descending sequence: soil and Head, 3.12 m; Hoxnian lacustrine deposits, 6.02; brown sand and grit (3rd Terrace), 3.96 m; chalky Boulder Clay (at 8.9 m OD).

Rivenhall End–River Brain

At the northern end of the Witham By-pass a similar sequence of Head, interglacial deposits, and sand and gravel has been proved in boreholes, where it overlay 'blue silty clay', here interpreted as Glacial Lake Deposits such as those encountered in the lower part of the Kelvedon Borehole. The base of the sand and gravel varies from 9.45 to 16.45 m OD, but if the extreme figures are excluded the base falls within the range of 13 to 16 m OD.

The Witham Borehole [TL 8244 1534] was drilled some 500 m W of the By-pass boreholes and a similar stratigraphical sequence was proved. The sand and gravel beneath the Hoxnian lacustrine clays was 11.48 m thick and rested on chalky Boulder Clay at a height of 3.34 m OD. Another borehole [TL 8250 1544], sited 100 m NE of the Witham Borehole, proved the same sequence but with differing thicknesses: the sand and gravel was only 2.7 m thick and rested on chalky Boulder Clay at 8.6 m OD.

Up to 3 m of sand and gravel could be seen in 1967 in the railway cutting [TL 8210 1526] at Witham. Fisher (1868) saw a fresher section and examined samples from a nearby well [TL 8212 1529] which showed that the 'coarse gravel' was 6.1 m thick and overlay 'glacial clay'. The contact of the two deposits lay at about 18.3 m OD.

A well [TL 8175 1543] at the Woolpack Inn, 400 m NW of the station, proved about 4.7 m of gravel, with a base at about 24 m OD, above Boulder Clay.

A gravel pit [TL 818 147] on the east side of Brain Valley is now disused, but 2.1 m of clean sand and gravel were noted in 1967. Farther south along the River Brain the Old Waterworks well [TL 8197 1415] encountered 3.5 m of coarse gravel beneath 0.9 m of 'mould' and made ground, and which rested on 'silt' at a height of 12.4 m OD.

On the eastern side of the disused Maldon railway line sections were observed during extensions to the sewage farm around [TL 8280 1395]. The sand and gravel varied in thickness from 1.8 m to more than 3.0 m and overlay the London Clay. Trial boreholes for the Witham By-pass on the north side of the River Brain proved a similar sequence with the sand and gravel varying in thickness from 1.2 m to more than 3.7 m; the level of the base of the sand and gravel varied correspondingly from below 11.3 to 14 m OD. London Clay was proved beneath the terrace deposits as far north as a point [TL 8300 1423] 400 m N of the River Brain. Beyond this point to the north the terrace deposits maintained a thickness of about 2.4 m to more than 5.5 m, with a base on chalky Boulder Clay, which varies in level from 11.6 to 13.4 m OD.

On the opposite side of the valley, near Little Braxted around [TL 835 147] the thickness of the sand and gravel is variable. At one point [TL 8335 1440] chalky Boulder Clay was augered beneath the sand and gravel, whereas at two other points [TL 8347 1440] and [TL 8352 1466] London Clay appeared to underlie the terrace. The back margin of the terrace is at about 27 m OD in this vicinity.

West and south of the River Brain

On the west side of the River Brain the 3rd Terrace has been built over in recent years. An old pit [TL 8165 1497] indicates the former exploitation of this deposit.

The Co-operative Wholesale Society Ltd Well [TL 8146 1466] proved 17.99 m of sand and gravel resting on chalky Boulder Clay at a height of about 1.8 m OD. On the south side of Hatfield Road [81.70 1385] the soil is very flinty. Farther south a borehole [TL 8186 1355] at Pondhallow Farm encountered 2.2 m of sandy gravel beneath 6.0 m of Head. The sand and gravel rested on chalky Boulder Clay at 14.7 m OD. Head obscures much of the 3rd Terrace in this vicinity.

Boreholes for the southern part of the Witham By-pass in the Howbridge Hall Road vicinity around [TL 820 130] proved up to 4.9 m or more of sand and gravel beneath an irregular spread of Head. Where the By-pass crosses the Maldon road [TL 8235 1325] the 3rd Terrace deposits are 4.27 m thick and rest on the London Clay at 14.6 m OD. At the railway line crossing, two boreholes in close proximity encountered 3.2 and 5.33 m of sand and gravel, which rested on the London Clay at 13 and 11.1 m OD. Only 60 m NE of these two boreholes the thickness of the sand and gravel had increased to 6.55 m and its base is correspondingly lower at 9.3 m

OD. The variable thickness of this deposit was again illustrated by two boreholes [TL 8281 1368] and [TL 8286 1378] close to the River Brain where the thickness of the sand and gravel above the London Clay was 6.55 and 1.67 m respectively, with its base at 8.68 and 12.64 m OD.

South of the By-pass there are no exposures. The back of the terrace, which descends to Alluvium level at about 15 m OD near Wickham Place Farm [TL 8210 1185], is covered by Head. Beyond Wickham Place Farm the 3rd Terrace has not been recognised. A borehole [TL 8188 1234], sited on the Head close to the back end of the terrace, proved 1.8 m of Head above 3.5 m of gravel which in turn rested on the London Clay at 10.2 m OD. Some 400 m SE of the borehole the surface indications are of a very gravelly substratum, and this is in accord with the grading figures for the above borehole which were 73, 27 and 0 per cent for the gravel, sand and fines, respectively.

2nd Terrace

In the northern part of the district a 2-km strip of 2nd Terrace can be traced southwards from just north of Ashman's Farm [TL 856 173] to Park Cottages [TL 849 159]. The fall in the surface level of the terrace within this tract is from about 21 m OD to about 18 m OD. A pit [TL 8540 1695] was opened before the war to supply material for the Feering dual-carriageway; it ceased operation in about 1947 and was backfilled in 1967. The pit was said to have been 6 m deep, but all that was visible in 1967 was 0.6 m of topsoil over 2.4 m of sand and gravel. An irregular spread of 0.61 to 1.2 m of brickearth covers much of the surface of the terrace,.

On the opposite side of the valley a wide tract of flat ground with a surface level of some 15 to 16 m OD lies north of Coleman's Farm around [TL 840 160]. A borehole [TL 8363 1559] 300 m NE of Coleman's Farm, which commenced at 15.8 m OD, proved 7.0 m of soil and 2nd Terrace deposits above chalky Boulder Clay. The sand and gravel in this borehole comprised 35 per cent gravel, 58 per cent sand and 7 per cent fines. Trenches some 300 m to the south-west of the farm, revealed up to 2.4 m of sand and gravel which locally [TL 8326 1517] rested on a grey clayey silt of similar lithology to the Glacial Lake Deposits recognised elsewhere in the Blackwater Valley.

A borehole [TL 8299 1515] some 250 m W of the trenches proved 3.0 m of sand and gravel above 'clay and chalk', the base of the terrace deposit being at 13.5 m OD.

The next occurrence of the 2nd Terrace is to the south of Benton Hall [TL 827 131], where the surface level of the terrace is at about 13 m OD. A borehole [TL 8275 1310] alongside the road leading to Wickham Bishops proved 2.4 m of topsoil and terrace sand and gravel above the London Clay. Thicknesses of 0.6 m to more than 1.2 m of brickearth are present over much of the terrace.

To the south, and on the opposite side of the valley, the back margin of the terrace is at about 13 m OD. A trench [TL 8371 1265] to [TL 8272 1234] revealed the irregular thickness of the terrace deposit. At one point [TL 8371 1260] the deposit consisted of 1.5 m of rounded and subrounded gravel and sand overlying London Clay, whereas 70 m to the south the sand and gravel was more than 4.6 m thick. The southern part of this outcrop is obscured by more than 1.8 m of Head deposits which consist of banded brown and grey clayey sand.

A further strip of 2nd Terrace has been mapped on the east side of the Blackwater to the south of Wickham Place. The back margin of the terrace is about 11 to 12 m OD. A borehole [TL 8226 1151] sited close to the front edge of the terrace, proved topsoil to a depth of 0.6 m and clayey gravel to 1.8 m, at which depth it rested on London Clay at an elevation of 7.46 m OD.

The next occurrence downstream of the 2nd Terrace lies close to the confluence with the River Chelmer and has already been described (p. 73).

1st Terrace

In the area [TL 848 169] to the east of Durwards Hall the surface of the 1st Terrace is 1.5 m higher than the Alluvium. From borehole evidence to the south of Durwards Hall it is known that some 1.2 to 2.4 m of Head, consisting of brown clay and stones, overlies 1.2 to 3.35 m of sand and gravel.

On the east side of the river below Appleford Bridge the surface indications are of a very gravelly deposit around [TL 843 156].

In the vicinity of Coleman's Farm [TL 835 153] the back margin of the terrace is at about 15 m OD. A borehole [TL 8332 1484], sited close to the front edge of the terrace at an elevation of about 14.0 m OD, encountered clay and silt to a depth of 2.0 m which rested on 3.5 m of clayey gravels, which in turn rested on either Glacial Lake Deposits or chalky Boulder Clay at 8.5 m OD. Glacial Lake Deposits were locally noted beneath the 1st Terrace deposits in trenches on the west side of the terrace. In one section [TL 8323 1507] to [TL 8321 1500] the sand and gravel varied from 1.8 m to 2.4 m in thickness and overlay a grey silt. The southward continuation of this trench to [TL 8315 1482] was not so deep, and only gravel was seen to a depth of 1.5 m to 1.8 m.

The small area of terrace to the south of Benton Hall around [TL 829 131] has its back at about 12 m OD. The surface indications are of a very flinty deposit. Some 0.9 m of sand and gravel were noted above London Clay in the small area of terrace around [TL 8215 1100] to the north-east of Small-lands Farm. The surface of the terrace in this vicinity is at about 9.0 m OD. Farther downstream the 1st Terrace of the River Blackwater merges with the 1st Terrace of the River Chelmer.

River Terrace deposits between the Crouch and Blackwater estuaries

4th Terrace

The 4th Terrace has its maximum elevation at Althorne Barn [TQ 905 990] on the margin of the district, where its base lies at about 45 m OD. North-eastwards a gradual fall in its base to about 18 m OD can be traced to Drinkwaters [TL 983 058] just beyond the district boundary (see (Figure 23)). There is also a fall in the base of about 10 to 15 m towards the former valley. At Althorne Barn the 4th Terrace deposits consist of more than 1.2 m of silty and clayey sands and clayey gravel. Another spread of this terrace can be found 1 km to the east at Mansion House [TQ 917 989]. A borehole [TQ 9181 9891] to the east of the house proved only 0.3 m of gravelly and sandy clay. Elsewhere in this vicinity up to 1 m of clayey sand was augered.

A more extensive spread of 4th Terrace deposits occurs a further 1 km to the east. It is of sufficient thickness to have been exploited for domestic use for example around [TQ 9400 9925]. At High House [TQ 9320 9894] some 3 to 4 m of gravel have been proved in an old pit. A borehole [TQ 9323 9901] on the north side of the road by the house penetrated 1.7 m of sand and gravel which rested on the London Clay at 36.3 m OD. At the eastern end of this outcrop the terrace deposits, consisting of an upper unit of gravelly silty clay 0.25 m thick resting on a middle unit of fine- to medium- and coarse-grained sand 1.55 m thick, overlay 0.25 m of very sandy silty clay. The base of the terrace lay at 27 m OD. Coarse-grained sand, presumably of the middle unit, could be augered consistently in the fields for some 0.5 km to the south-west of Caidge.

There were no exposures of the 4th Terrace deposits around Bovills Upland [TL 925 000], but coarse-grained sand was augered to a depth of 1.2 m at a number of localities.

No exposure could be found in the outcrop which extends from just east of Mayland Court [TL 937 002] to Theadhams Farm [TL 949 003]. Coarse-grained sand and gravel were proved by augering to a depth of at least 1.5 m. Similarly, there were no sections in the spreads of 4th Terrace at Lunendales [TL 948 010] or to the north-east and south-west of Foxhall Farm [TL 940 014] and [TL 935 009]. At the last locality an old pit [TL 936 009] may have been worked for sand and gravel.

The base of the 4th Terrace around Batt's Farm is marked by a line of springs [TL 9430 0246]. A small pond [TL 9440 0236] to the north of the farm was presumably an old sand-pit. Elsewhere on this spread coarse-grained sand with minor amounts of gravel up to 1.2 m have been proved by augering.

The spread of 4th Terrace which extends westwards from Badnocks [TL 956 017] appears to have an irregular base as the London Clay was proved beneath 1.2 m of sand and gravel at a number of points, but elsewhere the terrace deposits, consisting mostly of coarse-grained sands, were not penetrated at this depth.

A small pit [TL 9563 0284] on the 4th Terrace to the south-east of West Newlands was probably opened for the extraction of sand which appears to be the dominant constituent of this outlier. Sand was also augered on the East Newlands outcrop [TL 963 030] and on the outcrops around St Lawrence Hall [TL 966 044] and [TL 968 040]. Small pits on this latter spread were probably worked for domestic sand and gravel. The 4th Terrace deposits were 1.6 m thick beneath 0.9 m of topsoil and superficial deposits in the borehole at St Lawrence Hall [TL 9665 0412]: their base lay at 34.1 m OD. These occurrences of the 4th Terrace are the most northerly of those which occur in the Chelmsford district, but small outliers at Bradwell Wick [TL 978 053] and just north of Drinkwaters [TL 983 058] at about 23 m and 18 m OD respectively, lie in the adjacent Brightlingsea (242) district.

3rd Terrace

The 3rd Terrace is the most widespread of the terraces in this region and has been exploited commercially for sand and gravel. It has a maximum elevation of about 25 m OD in the Southend (258) district; it falls gradually north-eastwards from about 20 to 15 m OD in the Chelmsford (241) district to about 10 m OD in the Brightlingsea (242) district. Across the terrace the fall is more marked, and in the Southend (258) district it is from about 25 m to about 10 m OD (see (Figure 23)).

The Southminster gravel pits [TQ 960 990] and [TQ 962 990] have mostly been worked out. The easternmost pit is known to have worked gravel to a depth of 6 m. The pit has now been partially backfilled, and partly filled up with slurry. Consequently only poor sections were visible at the time of the field survey in 1968. At that time it was noted that up to 1.2 m of brickearth overlay more than 1.8 m of crudely bedded sand and gravel.

On the western side of the railway line a section [TQ 9598 9930] was observed by Miss M. Simmons in 1973 in 0.9 m of brickearth overlying more than 5.8 m of sand and gravel. Gravel had been dug out from the foundations of some buildings [TQ 9596 9947] some 170 m N of this section.

A borehole [TQ 9583 9952] to the west of these sections proved 3.8 m of buried channel deposits covered by 1.2 m of Head, and resting on 2.4 m of sand and gravel. Because of the height of its base (14.5 m OD) the sand and gravel is here regarded as an erosional remnant of the 3rd Terrace, but it is considered by Lake, Ellison, Hollyer and Simmons (1977) to represent an early stage of filling of the buried channel. Farther south, a borehole [TQ 9563 9921] failed to prove any terrace sand and gravel beneath buried channel deposits.

There is an old sand-pit [TQ 961 998] on the north side of Station Road at Southminster. All that was visible in 1968 was 0.9 m of brickearth overlying more than 0.15 m of sand and gravel. At Northend some 3 to 4 m of sand and gravel were encountered in a temporary excavation [TL 9604 0004]. Farther east and north-east old gravel pits [TL 9662 0007], [TL 966 004] and [TL 9687 0050] occur at a number of localities in the vicinity of Newmoor, but no sections are visible. A borehole [TL 9664 0035], 200 m NNE of Newmoor, which commenced at 21.3 m OD, proved 10 m of sand and gravel above London Clay. A line of springs [TL 9650 0062]–[TL 9656 0070] mark the base of the terrace to the west of Oldmoor.

Small outliers of what appear to be predominantly sandy 3rd Terrace deposits are present around Sheepcote [TL 959 008] and 800 m E of Badnocks [TL 964 018] at about 15 and 18 m OD respectively.

Around Asheldham the sand and gravel of the widespread 3rd Terrace has been exploited in a number of pits, mostly now abandoned, worked out or restored. A borehole [TL 971 013] on the southern part of this terrace spread proved 7.92 m of sand and gravel above the London Clay. The old gravel pits to the east are either flooded [TL 9730 0115] or partially backfilled [TL 975 012]. The former is 2.4 m deep to the water level, which presumably reflects the proximity of the top of the London Clay. Springs issue from the base of the sand and gravel immediately to the south [TL 9725 0100], to the north-east [TL 9703 0111] and [TL 9684 0136] and to the south-east [TL 9647 0073] and [TL 9763 0080] of the pits. An exposure [TL 9750 0098] seen by Miss M. Simmons in 1973 revealed 0.9 m of brickearth overlying more than 4 m of sand and gravel.

The large abandoned pit [TL 973 018] to the north of Asheldham is about 5.5 m deep. Poor exposures of coarse-grained ferruginous sand and gravel are visible in the degraded banks. A more recent excavation [TL 9725 0192] to the west revealed 4.5 to 5.5 m of sand and gravel. Gruhn, Bryan and Moss (1974, p. 60) noted that the upper 3 m of gravel consisted of a reddish brown sandy loam with evidence of frost action in the form of festooning, frost cracks and a fossil ice-wedge cast, overlying about 3 m of cross-bedded sandy gravel. Some 2.4 m of sand and gravel were noted in the small flooded pit [TL 9713 0158] to the south-west of the large one. Springs were noted at two points [TL 9702 0169] and [TL 9705 0183] at the base of the sand and gravel on the west side of the outlier.

Northwards from the pit the surface indications are of a very gravelly substratum around [TL 973 023]. The borehole [TL 9730 0269] to the east of High House proved 3.5 m of sand and gravel, lying beneath 0.7 m of soil and overburden, and resting on the London Clay at a height of 18.5 m OD.

Small outliers of the 3rd Terrace can be found to the west [TL 9650 0255] and [TL 970 028] and north [TL 973 033] of High House. The last is the most extensive, and thicknesses of sand and gravel exceeding 1.5 m have been proved by augering. On this outlier the sand and gravel has been exploited in the past in two small pits [TL 9733 0330] and [TL 974 033].

2nd Terrace

Within the Chelmsford district the 2nd Terrace has been recognised at only one locality, which lies some 700 m E of Newmoor. The base of the terrace is at about 10 m OD. There are now no exposures within this outcrop but large [TL 9715 0017] and small [TL 9722 0060] pits testify to its former exploitation for sand and gravel. Augering indicates that coarse-grained sand, exceeding 1.2 m thickness, is the dominant lithology. A spring [TL 9731 0058] marks the junction of the terrace and the London Clay on the east side of the spread. CRB

Lake deposits (Hoxnian)

In the Witham Borehole [TL 8244 1534] 4.62 m of silts and clay, were proved beneath 3.56 m of Head deposits, and resting on sand and gravel of the 3rd Terrace. The base of the silts and clays lies at 14.82m OD. It is at 11.28m in a nearby borehole [TL 8250 1544] which encountered 8.83 m of clay resting on gravel of the 3rd Terrace, and covered by Head. Together with other similar occurrences these silts and clays have been mapped as Lake deposits and are considered to be of Hoxnian age.

Some 150m E [TL 8260 1534] of the Witham Borehole temporary sections in the Witham Trading Estate showed 1 to 2.5 m of clayey sand and gravel (Head) overlying 2.0 + m of white clay.

East of the main road, sections in the grounds of Marconi Microelectrics [TL 8275 1524] to [TL 8282 1520] revealed a maximum thickness of 0.6 m of clayey sand and gravel (Head) resting on 1.2 m of white creamy clay which in turn rested on a coarse-grained brown sand (0.2 + m). To the south of the above localities much of the ground is built over but creamy silt was augered at one point [TL 8280 1466].

Auger samples and temporary sections in an area of disturbed ground 400 m farther south showed buff creamy clay and chalky white clay at least 1.8 + m thick beneath 1 m of Brickearth. Chalky Boulder Clay was augered at a number of localities around old pits [TL 8280 1442] and [TL 8270 1426], which may have been old marl pits; a nearby cutting in the Witham By-pass [TL 8294 1442] exposed 3.35 m of chalky Boulder Clay. The relationship of the Lake Clays to the Boulder Clay could not be established in this vicinity.

Dalton (in Whitaker and others, 1878, p. 68) recorded brick-earth (no thickness given), gravel 5 m, shell marl 5 m, and running sand, at the bottom of a well at a brickyard south of Witham Station. He also noted a similar sequence in wells 180 m E of the church at Chipping Hill where the marl and the overlying gravel were each about 6 m thick. Marl was also encountered in Witham High Street (Whitaker and others, 1878), and from it the following fossils have been recorded: 'Carychium minimum, Clausilia rugosa, Limnaea truncatula, Pupa marginata, P. palustris, Vertigo pusilla and V. pygmaea'.

At the northern end of the Witham By-pass the Lake Clays were proved in a number of trial boreholes, and also were encountered by augering in the ditch on the west side of the old road [TL 8289 1576], [TL 8301 1568]. Some 180 m N of this latter locality augering in the ditch [TL 8299 1586] proved 1.2 m of creamy silt in very close proximity to chalky Boulder Clay.

Many boreholes were sunk as part of the site investigation for the Witham By-pass. Two holes [TL 8307 1559] and [TL 8307 1569] proved successions of Head, on Lake Deposits, on Terrace Gravels, on chalky Boulder Clay, similar to that recorded in the Witham Borehole, with the base of the lake clays at + 14.9 and + 17.2 m respectively. The interglacial Lake Deposits are described in the first borehole above as 'soft, off-white clay, mainly redeposited chalk with some stones' in the upper part, and as 'firm very crumbly greenish grey clayey silt or silty clay' in the lower 0.76 m. Deposits of similar lithology, but underlying sand and gravels of the 3rd Terrace and extending down to at least −9.7 m OD, have also been proved in several of the adjacent site investigation boreholes. However, these particular silts and clays are considered to be Glacial Lake Deposits and chalky Boulder Clay infilling the Blackwater Valley deep channel, similar to the deposits recorded in the Kelvedon Borehole.

Towards Rivenhall End creamy silt was augered in ditches [TL 8343 1623], [TL 8367 1636] on the north side of the main road. The outcrop in this vicinity is locally obscured by Head. South of the main road the deposits have been worked in small pits [TL 8385 1631 and [TL 8402 1634] to the south and east of Matchyn's Farm. A temporary section [TL 8403 1637] just north of the latter pit revealed in descending sequence: clayey sand and gravel (Head), 0.6 to 0.9 m; white marl with shells and small flint pebbles, 1.2 m; greyish brown marl, 0.3 m; sand and gravel, 0.6 m.

A borehole [TL 8400 1655] at the underpass at Rivenhall End proved the following sequence:

The base of the interglacial deposits lies at c. + 12.86 m OD.

In another borehole 50 m NNE [TL 8395 1665] 4 m + of grey, brown and bluish silty clay was proved beneath 2.13 m of Head.

It was in this vicinity that Dalton (in Whitaker and others, 1878, p. 67) recorded a section in shell marl beneath 1.5 m of peat, and overlain by irregular spreads of sand and gravel. A reservoir had been sunk 6 m into the marl (between 6 and 12 m from the surface). The beds at the bottom of the section are described as pale and laminated. From the debris the following fossils were collected: 'Bithynia tentaculata, ?Helix, ?Limnaea, Succinea, Valvata piscinalis, Pisidium and vertebrae of fish'. The peat noted by Dalton may be continuous with that mapped at the surface immediately north of the railway on the east side of the valley, where it is related to springs issuing from the base of the Glacial Sand and Gravel.

The next exposures to the north-east, in the cuttings for the new road, are separated from those of Rivenhall End by a spread of Head and Alluvium. An extended auger hole [TL 8426 1686] just north of the road proved:

In the road cuttings it was seen that the Head, up to 2 m thick, was irregularly channelled into the clays and marls.

Farther north-east a pond [TL 8484 1724] is shown as a 'Marl Pit' on the Old Series geological Sheet. This is presumably the locality referred to by Dalton (in Whitaker and others, 1878, p. 67). Dalton also noted that the marl had been proved in a well at Durwards Hall, 350 m to the south-west, under 1.5 m of sand. White clay was noted in a ditch [TL 8503 1783]-[TL 8506 1779] just west of Crabb's Farm. A borehole [TL 8519 1784] to the south-east of the farm proved, beneath 2.9 of Head, 4.1 m of clayey, shelly silt resting on sands of the 3rd Terrace Deposits. CRB

In 1975, the Southminster gravel pits [TQ 955 982], just beyond the district boundary, were extended. They exposed the following section:

The clay sequence is generally affected by cryoturbation and rootlet activity so that original sedimentary structures are only rarely observable. In places the topmost clay unit, where the brown coloration is dominant, closely resembled weathered London Clay. Elsewhere this unit displays the characteristic columnar jointing of a brickearth.

Argillaceous deposits similar to those of the upper channel fill have also been recorded at surface outcrop adjacent to the 3rd Terrace gravels near Tillingham [TL 982 040]. RDL

It is suggested above (see p. 34) that some of the apparent inliers of 'London Clay' in the Tolleshunt D'Arcy–Tollesbury area may be reworked London Clay and equivalent to the Southminster deposits. The outcrop of 'London Clay' east of Southminster Hall, when augered, was seen to consist of brown silty clay, and it was not certain at the time of mapping whether this was in situ or reworked London Clay. A borehole about [TQ 969 993] sited close to the mapped 'London Clay' contact proved 6.4 m of sandy clay with sand lenses, on 1.83 m of sand and gravel, overlying more than 3.66m of London Clay. It would appear now, with the new evidence from the Southminster gravel pits, that it is possible that this 'London Clay' outcrop is in fact the topmost clay unit recognised in the pits.

At two points [TL 9745 0312] and [TL 9762 0322] some 400m NNE of Hill House, grey clay was augered at a depth of 1 m beneath Head, and may represent Lacustrine Deposits.

An anomalous deposit on Osea Island, which has been shown as 2nd Terrace (Loam) on the 1:50 000 Chelmsford (241) Sheet, is here included with these interglacial deposits, but it is possible that it represents a downstream continuation of the Glacial Lake Deposits described on p. 53: It crops out in the western part of the Island where it is overlain by terrace deposits, Head and Brickearth. Beach sections [TL 9034 0645], [TL 9038 0636] and a borehole [TL 9077 0640] demonstrate that the clays and silts rest on sand and gravel. A grey silt or silty clay was found by augering. In the above borehole Simmons (1978) recorded 1.75m of stiff silty clay, which lay beneath 1.45 of silt and sand, and rested on 0.8 m of silt; below this silt occurred 1.0 m of sand and gravel resting on London Clay.

The base of the clay lay at −1.2 m OD. CRB

Interglacial deposits (Ipswichian)

A brickpit [TL 7005 0630] on the western side of the railway line near Moulsham is described by Holmes (1896) as follows:

Bed b is possibly the feather edge of the 1st Terrace of the Chelmer which has been mapped in this vicinity. Some 2 to 2.4 m of brickearth had previously been stripped off. A specimen of M. primi genius came from Bed b, and molluscs listed by Webb (1896) appear to have come from Bed c. The probable Ipswichian age of this deposit is discussed on pp. 68–69.

A pit on the eastern side of the railway line extended from just south of Bradford Street [TL 7020 0620] to opposite the gates of the cemetery [TL 7030 0605]. The pit is now largely restored and built over; all that was visible in 1970 was 1.5 m of gravelly clay and sand (Head), overlying 0.9 m of banded fine-grained yellow sand and buff creamy silty clay which in turn overlay 1 m of brown silty clay. It is indicated as (Glacial) Lake Deposits on the 1:50 000 sheet. Woodward (in Whitaker, 1889, p. 452, figs. 94 and 95) recorded the following section in this pit:

• c. Brick-earth
• b. Gravel of pebbles, sometimes in a matrix of loose sand, sometimes in pale clay.
• a. Pale bluish, sandy clay with small pieces of chalk, and iron pyrites with wood, shells and mammalian bones, amongst others Hippopotamus. White bricks are made from this clay.

It is evident that beds a, b, and c in each of the two pits are equivalent. Irregular deposits of gravelly Head are figured by Woodward (in Whitaker, 1889, figs. 94 and 95) overlying the brickearth.

It would appear that Mr Brown's pit 'near Lower Anchor Street', in which the tusk of an elephant, and a lower jaw borie of Coelodonta antiquitatis were found (Cole, 1894, pp. 155, 219; Newton, 1896), is the same as that described by Woodward. The bones are said to occur on top of the 'White clay', which presumably corresponds to Woodward's bed a.

Some 1.5 m of 'loam' and 'brickearth' was noted by Mr J.S. Turner in 1934 (MS BGS) at several points [TL 7044 0615], [TL 7055 0616], [TL 7070 0607] and [TL 7073 0601] to the east of the brickpit. This 'loam' and 'brickearth' may be the same as the 'soft to firm brown silty clay' up to 2.4 m thick, resting on, sand and gravel, which was noted in trial boreholes around [TL 7080 0615] between Hamlet and Grove roads. The deposits in this area have been mapped as Head (see p. 57).

It is not known if the brickearth of any of the following localities correlates with that of the Moulsham brickpits for the lack of exposures and borehole records in this built-over area and the ubiquitous cover of Head have not allowed the stratigraphical relationship of the deposits at Moulsham to be determined. For convenience the brickearth is described in this section.

Bristow (in Whitaker, 1889, p. 453) recorded brickearth in the brickpits on the north side of the River Can south of the railway line. Here the brickearth was 1.5 m thick and overlay 3.0 m of (1st Terrace) gravel which in turn overlay the London Clay. It was suggested that it was a fairly small deposit which had largely been worked out. A somewhat similar sequence was noted in 1970 in temporary exposures [TL 706 068] near Market Road. The brickearth had a maximum thickness of 0.9 m.

To the west of the railway line, Mr J.S. Turner noted large excavations [TL 6925 0700] where brickearth was worked to a depth of 1.2 m. In the vicinity of the present Ash Tree Crescent [TL 6965 0673] he saw brickearth to a depth of 1.8 m in trial pits. At the old brick-pits [TL 690 072] on the northside of the River Can, brickearth was dug down to 3.0 m.

1st Terrace deposits (Loam)

A small spread of sandy and silty loam up to 1.2 m thick has been mapped as 1st Terrace (Loam) overlying 3rd Terrace deposits and chalky Boulder Clay [TL 827 143] in the Witham area. As noted on p. 54 it may be related to part of the brickearth upstream at Appleford Bridge and has been referred to as 'river brickearth'.

Irregular spreads of brickearth are present over much of the terraces of the River Chelmer, but it has only been practicable to distinguish one such spread [TL 758 080] over the 1st Terrace to the north-east of Phillow's Farm.

A further tract of 1st Terrace (Loam) has been mapped on Osea Island [TL 910 062], where up to 1.8 m of grey, brown and yellow silts and silty clays overlie terrace sand and gravels, and deposits of uncertain origin that have been mapped as 2nd Terrace loams, but are possibly of Hoxnian Interglacial age (see p. 68). CRB

Alluvium and sub-alluvial gravel

River Chelmer

Within the district the Alluvium and sub-alluvial gravel are present; except locally, over the whole course of the River Chelmer. The outcrop width varies from about 100 m in the upper reaches to about 200 m in the vicinity of the confluence with the River Can. Downstream the floodplain widens from about 500 m immediately below Chelmsford to about 700 m where the rivers Blackwater and Chelmer combine at Langford. A well defined tract of Alluvium continues as far as Maldon, below which the Alluvium is mapped as Estuarine Alluvium. It is probable that the Alluvium between Maldon and Langford also includes some Estuarine Alluvium as this is within the present day tidal range, but it has not proved practicable to differentiate it. Prior to the installation of locks at Beeleigh [TL 839 084] it is probable that the tidal influence extended farther upstream.

The highest point upstream within the district for which there are geological observations of the Alluvium is to the north of Great Waltham where boreholes around [TL 6970 1555] in the bed of the river proved 1.3 to 2.7 m of topsoil and soft brown or grey silty clay with peat, overlying 1.7 to 2.4 m of sub-alluvial gravel composed of coarse gravel with some sand.

In the vicinity of Great Waltham the Chelmer is joined by the Waltham Brook and another unnamed stream, with the resultant formation of a somewhat wider floodplain.

Some 7 km SSE a number of boreholes have been drilled in the Alluvium to the north-east of Chelmsford. The most northerly borehole [TL 7155 0914], sited very close to the river, proved 2.60 m of soft brown and grey silty clay with peat above 0.30 m of sub-alluvial gravel. In a borehole some 80 m WSW [TL 7149 0912] the sub-alluvial gravel was absent, although the lower part of the overlying 2.60 m of brown and grey mottled clay contained some gravel. However the sub-alluvial gravel, some 1.85 m thick, was present in an adjacent borehole [TL 7146 0902] 60 m farther south-west, beneath 0.95 m of topsoil and brown silty and sandy clay. A fourth borehole [TL 7126 0895] drilled immediately north of the river encountered 3.0 m of topsoil and brown silty clay with peat, above 2 m of soft grey silty clay with sand and gravel, which in turn rested on the London Clay.

Continuing southwards the respective thicknesses of the upper clay unit and lower gravelly unit were 1.3 and 2.5 m in a borehole [TL 7102 0855]. Boreholes around [TL 710 085], for a proposed bridge crossing the Chelmer south of Broomfield, demonstrated that the Alluvium varied from 2.85 to 4.50 m in thickness (average thickness 4 m), and that the upper unit, predominantly of silts and clays, and the lower sub-alluvial gravels, ranged from 1.25 to 4 m (average 2.75 m) and from 0.75 to 3.85 m (average 1.54 m) respectively. The most southerly of the boreholes [TL 7095 0828] drilled in the Alluvium of this area was sited some 200 m SSW of the river crossing. Here 1.5 m of brown clayey silt overlay more than 1.5 m of gravel.

At one point [TL 6931 0384] in the River Wid, a south bank tributary of the Can, the Alluvium included an upper unit of clay, 1.2 m thick, overlying sub-alluvial gravel.

A number of boreholes have been drilled into the Alluvium of the urban area of Chelmsford. One such borehole [TL 7089 0659], sited immediately south of the River Can at the London Road crossing, proved in descending sequence: fill, 1.5 m; soft greyish black organic slightly sandy clayey silt with shell fragments, 2.44m; medium gravel, 1.0 m; Glacial Lake Deposits.

Farther downstream two boreholes [TL 7111 0642] and [TL 7112 0647] drilled on either side of the River Can, but within the area of the combined Can-Chelmer floodplain, differed in detail. The southern borehole proved 1.67 m of made ground above 3.80 m of peaty soil and silty clay, and 1.83 m of sub-alluvial gravel, which in turn rested on Glacial Lake Deposits. In the northern borehole, the made ground (1 m thick) and alluvial clay extended down only to a depth of 3.50 m and the sub-alluvial gravel was 1.52 m thick.

Some 200m ENE a borehole [TL 7130 0652] drilled on the north side of the Chelmer penetrated 0.9 m of made ground, on 1.37 m of blue and brown clay, above 1.06 m of 'ballast'. A temporary section [TL 7128 0625] 270 m S of the above borehole, revealed 0.9 to 1.2 m of made ground, on 0.9 m of bluish black clay, overlying more than 1.2 m of grey sand and gravel. Another trial borehole [TL 7135 0620] 90 m SE showed a similar sequence, but with differing thicknesses: 2.13 m of brown sandy clay with gravel, above 1.83 m of sandy gravel. The absence of gravel in the underlying 1.2 m of medium-grained sand and 3.96 m of light grey silty sand suggests that they are Glacial Lake Deposits. A third borehole in this vicinity [TL 7143 0624] proved 2.44 m of soft silty clay overlying more than 1.52 m of sand and gravel.

A number of boreholes around [TL 719 062] have been drilled across the floodplain of the River Chelmer alongside the Chelmsford By-Pass. The Alluvium in this vicinity was found to vary from 4.25 to 6.30 m (average of six boreholes, 5.24 m), with the alluvial clays and silts varying from 1.00 to 2.80 m thick (average 2.1 m), and the sub-alluvial gravel from 1.90 to 5.30 m thick (average 3.18 m). The log of one of these boreholes is reproduced on microfiche.

Downstream, trench sections [TL 7241 0624] to [TL 7283 0636] showed that the alluvial clays and silts varied from 0.9 to 2.1 m in thickness and everywhere overlay gravel. Additional trenches [TL 7300 0621 7344 0604] farther east on the south side of the flood plain revealed a somewhat thinner upper unit varying from 0.3 to 1.2 m in thickness, above gravel.

Downstream from these trench sections as far as Ulting, boreholes have been drilled in the Alluvium at fairly regular spacings. A tabular summary of the thickness of the two principal corn ponents is presented below.

Ditch sections [TL 7468 0828] revealed a somewhat thinner clay unit, 0.3 to 0.9 m thick, above the sub-alluvial gravel.

In the vicinity of Ulting the sub-alluvial gravel has been exploited commercially. Boreholes have been drilled on either side of the Ulting to Woodham Walter road around [TL 806 085]. The average thickness of the Alluvium in this vicinity was 3.8 m, and varied from 2.58 to 4.57 m, whereas the alluvial clays and silts, and sub-alluvial gravel varied from 0.3 to 2.13 m (average 0.97 m) and 1 to 3.96 m (average 2.84 m) respectively.

A borehole [TL 8262 0862] alongside the first lock upstream from Beeleigh Abbey, proved 2.44m of firm and grey silty clay overlying 3.7 m of sub-alluvial gravel. Immediately downstream from the lock, gravel has been dredged from the river bed. Gravel was also dredged from a 170-m long stretch of the river above [TL 8362 0841] above Beeleigh Abbey. Boreholes around [TL 838 088] were drilled on either side of the Langford cut. The Alluvium varied in thickness from 2.59 to 6.10 m (average 4.27 m) in thickness with the upper clay unit ranging from 0.6 to 2.28 m (average 1.23 m) and the sub-alluvial gravel 1.97 to 4.57m (average 3.05 m).

Boreholes around [TL 846 077] where the Alluvium is crossed by the former railway line showed a varied alluvial sequence. On the north side of the river 1.22 m of 'Alluvium' overlay 2.13 m of gravel, whereas on the south side no alluvium was recorded, and the gravel was 3.05 m thick.

Wells in the Heybridge area [TL 8587 0770] and [TL 8537 0817] show thicker than usual sub-alluvial gravel. The upstream borehole passed through 3.35 m of soil and brown and black clay above 5.56 m of ballast. The downstream borehole penetrated 0.15 m of 'soil' and 6.17m of 'ballast'.

Downstream from Maldon the Alluvium of the River Chelmer merges with the Estuarine Alluvium. The limited borehole evidence (see p. 82) suggests that a bipartite division into an upper clay unit and a lower gravel unit can still be made.

River Ter

The Alluvium of the River Ter within the district has an outcrop width which varies from about 40 to 250 m, but averages about 80 m. The only details available are from a borehole [TL 7835 1150] at Hatfield Bridge, near Hatfield Peverel, where 1.68 m of made ground overlay 0.61 m of brown silty soil, which in turn overlay 1.06 m of stiff brown silty and sandy gravelly clay. This last unit is probably equivalent to the sub-alluvial gravel.

Downstream a trench [TL 7885 1064] to [TL 7893 1067] across the Alluvium of the Ter revealed more than 1.5 m of grey and brown clay.

River Brain

Alluvium is present along the whole course of the River Brain within the district; it varies from 40 to 200 m in width and averages about 80 m. Lithological details are available only for the lower reaches of the river. Here boreholes around [TL 8283 1375] proved 1.37 m of hard brown silty clay overlying 2.14 to 5.34 m of sandy gravel.

Sandon Brook

Alluvium, some 100m wide, is present along the whole course of the Sandon Brook and is divisible into an upper clayey unit and a lower gravelly unit. A trench [TL 7397 0429] across the Brook to the south of Sandon revealed 0.6 to 1.2 m of mottled grey and brown clay overlying gravel. Some 700 m downstream a borehole [TL 7471 0455] proved 1.2 m of mottled brown and grey silty clay overlying 6.30 m of sand and gravel.

River Blackwater

Alluvium and sub-alluvial gravel are present along the whole of the course of the River Blackwater within the district. The width of the outcrop varies from 70 to 400 m.

The Kelvedon Borehole [TL 8602 1797] proved 3.05 m of mottled greyish brown clay, locally with silty patches, above 5.48 m of gravel, sand and 'grit', which rested on chalky Boulder Clay at 11.47 m OD. The sub-alluvial gravel could be subdivided into an upper gravel, 1.52 m thick, a middle sand 1.83 m thick, and a lower quartz and flint 'grit', 2.13 m thick. The 'grit' components varied in size from 2 mm to 10 mm; the smaller grains were subangular to rounded, whereas the grains greater than 2 mm mostly consisted of angular flints, though some rounded quartzites were noted. At the base of the 'grit' a thin pebble bed with pebbles up to 50 mm in diameter was present.

Some 450m downstream a borehole [TL 8559 1769] proved, in descending sequence: soil, 0.3 m; brown clay, 0.6 m; blue clay, 2.1 m; sand and gravel, 1.8m; chalky Boulder Clay.

To the east of Durwards Hall [TL 8485 1684] 1.2m of dark brown clay was proved by augering to overlie gravel.

Trench sections [TL 8315 1482] to [TL 8300 1428] across the Alluvium to the west of Little Braxted revealed 0.6 to 1.5 m of grey and brown, locally organic clay, over more than 1.2 m of gravel.

A trial borehole [TL 8301 1382] at the Witham Sewage works proved 0.6 m of topsoil above 1.8 m of silty and sandy clay with a little gravel, above the London Clay. There seemed to be no well defined occurrence of sub-alluvial gravel.

A borehole [TL 8282 1051] sited on the Alluvium to the west of Wickham Hall encountered the following descending sequence: soil, 0.3 m; brown silty clay, 0.9 m; grey silt and peat, 0.9 m; gravel 3.4 m; black silt and peat 1.2 m; London Clay. The percentages of gravel, sand and fines of the sub-alluvial gravel were 73, 26 and 1 respectively.

Boreholes around [TL 8367 0910] close to the confluence of the rivers Chelmer and Blackwater, showed that the topsoil and clayey alluvium varied in thickness from 1.2 to 1.67 m and overlay 1.52 to 2.43 m of sub-alluvial gravel.

Farther downstream the Alluvium and sub-alluvial gravels of the Blackwater merge with those of the River Chelmer and are discussed above. CRB

Older Estuarine Alluvium

The most complete sequence in this deposit is known from a borehole [TQ 9773 9986] 2 km E of Southminster church which proved the following succession:

A sample taken at a depth of 7.45 to 7.9 m yielded the ostracods: Candona candida? [rare], Cyprideis torosa [abundant], Cytheromorpha fuscata [rare], Darwinula stevensoni [very rare] and Limnocythere inopinata [very rare]. Miss D. Gregory who identified the fauna remarks that C. torosa is the most abundantly occurring species in this association. Both males and females occur and many juveniles are found. Commonly the individuals are noded, a feature which seems to be linked with very low salinity (Kilenyi, 1972). C. candida?, D. stevensoni and L. inopinata inhabit fresh or oligohaline water. It is the presence of C. fuscata that makes this fauna unusual. C. fuscata typically inhabits the brackish waters of estuaries and shallow marine areas at temperate latitudes. It is known to live at salinities as low as one or two parts per thousand but only rarely has it been recorded in association with fresh-water ostracods (Delorme and Neale, personal communication 1976). Normally C. fuscata is found associated with other euryhaline species of ostracods. Its association with C. candida?, C. torosa and D. stevensoni is, therefore, remarkable.

In the field, silty, sandy and gravelly deposits are commonly encountered close to the surface, but may only represent a thin veneer of Head. Augering in the bottom of ditches for example [TQ 9678 9997] to [TQ 9757 9960] at depths generally between one to two metres from the surface, usually proved grey silty clay with thin-shelled mollusca and pellets of race. CRB

Estuarine Alluvium

Within the district there is only one borehole [TL 8898 0748] in the Estuarine Alluvium. This proved 3.7 m of soil and clay, above 4.8 m of clayey gravel, which in turn overlay the London Clay. The base of the Basal Sand and Gravel lay at −5.5 m OD. This level is considerably lower than the base of either the 1st or 2nd Terrace (see (Figure 21)), but it is apparently continuous with that of the Sub-alluvial gravel which has been recognised beneath the Chelmer and Blackwater Alluvium. Sand was proved by augering beneath grey clay at a depth of 0.9 m at a point [TL 8877 0741] only 200 m WSW of the above borehole. An additional auger hole [TL 8867 0724] 400 m SW of the borehole, encountered gravel beneath 0.9 m of brown clay. A 'red hill' [TL 8870 0745] (see p. 93) in this vicinity dated at 239 ± 63BC by Switsur (1974), indicates that Tidal Flat Deposits were in existence in this vicinity over 2200 years ago.

Upstream near Heybridge 1.5 m of dark grey mud were seen to overlie gravel in a drainage ditch [TL 8561 0785]. Farther downstream the present day Tidal Flat Deposits are about 1.5 m higher than the reclaimed Estuarine Alluvium [TL 8680 0565 and around 8680 0495].

The Estuarine Alluvium bordering Limbourne Creek does not appear to be very thick and is without the basal gravel unit. London Clay was augered in the bottom of a ditch [TL 8704 0490] at a depth of 1.2 m. An exposure [TL 8604 0493] 1 km upstream from the ditch section, revealed 0.9 to 1.2 m of greyish brown clay with flints overlying the London Clay. The absence of the Basal Sand and Gravel can be explained by a recent overlap of the Tidal Flat Deposits caused by either transgression or subsidence. This extension inland of the tidal influence of the sea is very evident along the River Crouch; the river is now tidal to Wickford, over 1 km farther upstream than indicated on the Ordnance Survey topographic map. The natural tidal limits along the River Blackwater are indeterminable because of the presence of locks and sluices at the mouths of the rivers and tributaries.

The Estuarine Alluvium does not seem to exceed 2 m in thickness in the old creek to the west of Brick House Farm. London Clay was proved at a depth of 1.2 to 1.5 m at several points. A borehole in this creek [TL 8921 0354] proved 1.2 m of soft, slightly silty clay with plant fragments, above 0.6 m of clayey silt which in turn overlay the London Clay. A sample from the interval 1.5 to 1.7 m yielded the following foraminifera: Ammonia batavus, Elphidium williamsoni, Jadammina macrescens, Protelphidium anglicum, and Trochammina inflata, together with the ostracod Loxoconcha sp.indet. and diatoms of the type Coscinodiscaceae. Mr Hughes reports that the fauna is typical of a Quaternary hyposaline marsh. However, the lack of plant debris and the clean appearance of the residue suggests a sand bar, possibly deposited in brackish or fresh water. The numbers are too low to regard this as in situ assemblage and the fauna is assumed to have drifted into the area together with reworked London Clay species. Current action was moderate and the unimodal size of the Quaternary foraminifera supports the hypothesis of drifting. The species are typically found living in contemporary British marshes and there are no cold forms. Their age is, therefore, regarded as Flandrian.

The Estuarine Alluvium of the stream which drains into Mundon Creek is similarly thin and directly overlies the London Clay. A ditch section [TL 8916 0260] to [TL 8923 0246] revealed 0.6 to 1.5 m of dark grey mud, locally with a thin gravelly base, above the London Clay. Another ditch section [TL 8895 0216] to [TL 8903 0199] exposed only 1.8 m of dark brown and orange mud. A third ditch [TL 8863 0198] to [TL 8865 0190] showed 0.6 to 1.2 m of dark brown and grey clay resting on the London Clay. A gravelly seam, 0.3 m thick, underlay 1.2 m of mottled yellow and grey clay at a point [TL 8850 0169] 170 m S of the last ditch.

The creek to the west of Ramsey Island [TL 940 055] was formerly known as the 'Wade'. It was 'finned' by building a sea wall across its mouth about 1800 and has virtually silted up. It is now known as Ramsey Marsh. Auger holes proved at least 1.8 m of soft grey and brown mud to the south of the marsh around [TL 8395 0535].

The Estuarine Alluvium on the north side of Goldhanger Creek is composed in part of silt, underlying 0.5 to 1.2 m of grey clay. Silt was augered at three points [TL 9212 0835], [TL 9208 0855] and [TL 9266 0894]. Gravel has been dredged out of an old pond [TL 926 083]. A 'red hill' [TL 926 087], shown on the six-inch-to-the-mile map, indicates extensive salt working before the marshes were enclosed.

On the marsh [TL 946 083] to the south of Rolls Farm generally only grey mud to a depth of 1.2 m has been proved by augering. Some 800 m E of the farm a ditch section [TL 9516 0847] revealed 0.6 to 0.9 m of grey clay, overlying 0 to 0.9 m of gravel, which in turn overlay the London Clay.

On the southern part of Tollesbury Wick Marshes, 1.5 m of grey mud with Ostrea was noted [TL 9724 0907]. Dark grey clay was dredged out of a ditch [TL 9617 1141] to [TL 9668 1136] in the marshes to the north of Tollesbury.

The Old Hall Marshes [TL 970 130] still retain stretches of open water (Joyce's Head Fleet [TL 947 124], and Pennyhole Fleet [TL 986 121]), but many others appear to have silted up in recent years. All that was found by augering was dark greyish brown, or bluish grey mud, locally up to at least 1.8 m thick. On the north side of the marshes a ditch section [TL 9642 1295] to [TL 9664 1313] proved London Clay at a depth of 1.2 to 1.5 m over the whole section, beneath brown silty clay. Similarly London Clay was noted in a ditch at two points [TL 9576 1319] and [TL 9583 1316] 700 m to the west-north-west, beneath the Alluvium.

To the east of Marsh Farm, soft brown or black mud was augered in the ditch sides around [TL 959 135]. From the ditch [TL 9540 1367] immediately north of Marsh Farm gravel and dark bluish grey clay had been dug.

Red clay, indicative of salt pan working prior to the enclosure of the marshes, can be found at a number of points in this vicinity [TL 9569 1294], [TL 9573 1314], [TL 9630 1345], [TL 9637 1351], [TL 9657 1367], [TL 9703 1377], [TL 9707 1380], and [TL 9775 1408].

A small tract of the extensive Southminster Marshes occurs in the south-east corner of the district. No details are available for the Alluvium, but a well [TQ 9912 9830] about 1.4 km SE of the district, proved 2.42 m of clay, above 2.90 m of gravel (Basal Sand and Gravel), which in turn overlay the London Clay.

A ditch section [TQ 9757 9848] about 400 m S of the margin of the district revealed 1.8 m of brown and grey silt with Ostrea, Cardium and thin shelled bivalves. Cardium was found at several other localities in the Alluvium hereabouts. A nearby section [TQ 9753 9857] proved 0.9 m of grey silt overlying more than 1.2 m of grey clay. CRB

Storm Gravel Beach Deposits

South side of the Blackwater Estuary

Small patches of sand and gravel overlie the Tidal Flat Deposits on the north side of Lawling Creek. The more westerly occurrence [TL 9097 0461] consisted in 1969 of a spit of sand and gravel, some 20 x 70 m, with a surface elevation above the Tidal Flat Deposits of 0.3 to 0.6 m. The easterly occurrence [TL 9142 0468] is known as Mundon Stone Point. It is a north-south trending spit, slightly convex to the east, measuring about 20 x 70 m, and it has been mapped from the 1963 aerial photographs.

A hammer-headed shingle spit was mapped on the south side of Steeple Creek [TL 9257 0467] in 1969. The maximum dimensions were 80 x 80 m. A small area of saltings [TL 9265 0482] shown on the six-inch-to-the mile map, some 200 m NE of the spit, may also have a cap of gravel. On the north side of the creek, a ridge of shingle 110 x 20 m, is banked against the sea wall [TL 930 050].

One of the most extensive occurrences within the district lies just north of the above deposit and extends along the coast for about 3.5 km. This spit appears to predate the construction of the sea-wall and has a maximum inland extension of 200 m. In the south the London Clay can be found at depths of 1.2 to 1.3 m below sand and gravel [TL 9300 0533] and [TL 9300 0546]. At Stansgate Abbey Farm behind the sea-wall the sand and gravel exceeds 4.5 m in thickness [TL 9307 0574]. London Clay was encountered in ditches at two points [TL 9318 0565] and [TL 9320 0584] 130 m SE and 150 NE, respectively, of the above occurrence. Continuing eastwards for 250 m along the ditch to the north-east of the Abbey Farm, London Clay was encountered mostly at depths between 1.2 and 1.6 m, although locally [TL 9338 0587] sand and gravel extended down to at least 1.8 m. Sand and gravel can be found on the seaward side of the sea-wall in the vicinity of this ditch. Farther eastwards only sand and gravel was found in the ditch behind the sea-wall, where depths of at least 2 m were recorded, and for at least 150 m inland. The Wade, which was reclaimed about 1800, now has shingle spit across the sea-wall and thus Ramsey Island is joined naturally to the mainland.

An extensive gravel ridge borders Ramsey Island. This has derived most of its material from the 2nd Terrace of the Chelmer which caps the higher part of the Island. Gravel was seen to be up to at least 2.1 m in thickness in the ditch behind the sea-wall. A gravel bank [TL 9430 0612] about 190 x 20 m was present in 1969 between the low and high-water levels offshore from Ramsay Island. In the eastern part of the Island this chenier only extends about halfway across the former creek between the island and the mainland. At its eastern extremity it bifurcates, one branch curving northwards for about 80 m, and the main extension curving east-south-eastwards over about 130 m.

Approximately midway between the Stone and the end of the spit, the London Clay crops out in the foreshore [TL 954 060], and only a thin skin of gravel is banked against the sea-wall. For about 100 m SE of this locality the gravel is banked against a low cliff of London Clay.

North side of the Blackwater Estuary

A thin strip of gravel extends along 800 m of the sea-wall [TL 8764 0768–8832 0743] in the vicinity of Mill Beach Hotel.

To the south of Goldhanger, gravel is shown on the foreshore on the six-inch OS map in an area bordered by terrace deposits, but it was not considered to be sufficiently extensive to depict on the geological map. On the east side of the creek which extends to Goldhanger, a ridge, 900 m long, has been mapped [TL 9085 0832–9165 0813]. For the most part the shingle is banked against the sea-wall, but at the eastern end it extended in 1966 for 300 m eastwards across the Tidal Flat Deposits.

A more extensive chenier borders Gore Marsh [TL 918 083] to [TL 9283 0812]. At its western end the chenier abuts the sea-wall for 400 m, but the remaining 900 m are deposited over the saltings. A narrow strip of salting is preserved between the chenier and the seawall in this latter tract. Greensmith and Tucker (1975, fig 10.4) found evidence for two periods of growth; during the later period the chenier transected the old filled-in channel which originally drained the area to its rear (north).

Shingle banks were visible near the low water marks on the 1955 aerial photographs on the north [TL 9587 0775] to [TL 9647 0790] and south side [TL 9540 0713] to [TL 9580 0733] of Thistly Creek.

Osea Island

Cheniers are present almost continuously around the island. Only a small tract in the centre of the northern and southern part of the island appears to be free of cheniers.

The western occurrence is for the most part thin and overlies either London Clay or, additionally in the south, the loams of the 2nd Terrace. Close to West Point [TL 9034 0641] balls of gravel up to 0.9 m in diameter could be seen enclosed in mottled brownish grey silty clay up to 1.5 m thick, which overlay sand and gravel, (possibly old chenier, or terrace gravel). On the north side of West Point a short spur 110 m long extends east-north-eastwards from the main chenier. In the first 200 to 300 m of the ditch behind the sea-wall the gravel was proved to be at least 1.3 m thick. In the eastern section, London Clay was augered at a depth of 1.2 m.

The easternmost chenier is more extensive and extends inland for at least 100 m. Where the chenier is banked against the London Clay the contact can be fixed fairly precisely in the field. Elsewhere the contact between the chenier and the 2nd Terrace is somewhat arbitrary.

On the south side of the island the chenier is banked against a low cliff of London Clay [TL 9160 0585]. To the east of this point the chenier locally [TL 9190 0596] encloses an old creek. Continuing eastwards, 1.2 m of sand and gravel rests on a green and grey silty clay at one point [TL 9227 0605] behind the sea-wall.

East Point [TL 9265 0627] represents a 300-m east-north-eastwards seaward extension of the chenier from the sea wall. About halfway along its length there is a 170 m curving spur to the north. Between this spur and the sea-wall is a sheltered area of saltings some 150 m across where the Tidal Flat Deposits appear to rest on sand and gravel. It is not known whether this latter represents a degraded chenier, the seaward extension of the 2nd Terrace, or an outcrop of the Basal Sand and Gravel. Some 700 m NE of East Point is Goldhanger Spit, where a 150-m strip of shingle is visible at low tide on the 1963 aerial photographs.

Peat

A 300 -m strip of peat at Fuller Street is mentioned below together with the Calcareous Tufa with which it is associated. Peat is also associated with Calcareous Tufa on the north side of the River Ter lower downstream [TL 7516 1573] to [TL 7558 1540] near Ridley Hall. At Rivenhall End there is a strip of peat some 450 m long on the east side of the valley where it was at least 1.4 m thick. The peat recorded by Dalton in this vicinity, associated with shell marl, may be related to the above occurrence (see p. 79). Higher up this same valley peat was noted intermixed with Calcareous Tufa on the west side of the valley at Rivenhall [TL 8300 1787].

A patch of peat on the alluvial plain [TL 737 058] of the River Chelmer at Sandford Lock has its back margin against the feature which limits the Alluvium. Its origin is unknown, but is thought to be related to springs issuing from the base of terrace and/or Glacial Sand and Gravel, which have been noted some 200 m to the east [TL 7394 0582] CRB

Calcareous Tufa

River Ter

A strip of calcareous tufa associated with much peat is present along the east side of the valley [TL 7460 1642] to [TL 7468 1610] at Fuller Street. A spring-line is present at the contact of the deposit with the Chelmsford Gravels. At one point [TL 7464 1622] 1.4 m of tufa was proved by augering to overlie gravel. RDL

Lower down the valley a more extensive mass of tufa, locally associated with peat, occurs on the north side of the River Ter [TL 7516 1573] to [TL 7558 1540] near Ridley Hall. Springs are present along the upper margin of the deposit, and it is in this vicinity that peat can be found. The calcicole Old Man's Beard (Clematis vitalba) grows at numerous points on the outcrop.

River Brain

Blocks of calcareous tufa up to 0.2 m in size occur over a small area on the surface of the London Clay in a field [TL 7926 1822] 750 m E of White Notley. They are presumably related to springs at the base of the Chelmsford Gravels, although no springs were observed in the field.

On the opposite side of the river, 600 m to the south, there is a similarly situated thin spread up to 0.6 m thick around [TL 7932 1765]. Some 350 m ESE a slightly larger occurrence has been noted [TL 796 175].

Lower downstream in the vicinity of Chippinghill is one of the most extensive deposits within the district. It has an outcrop length of 900 m [TL 8100 1624] to [TL 8156 1557] with a maximum width of 60 m. Springs are present everywhere along the back margin, and more locally at the base. The maximum thickness augered was 1 m, but it probably exceeds this figure in certain areas.

Opposite the above occurrence is a shallow valley floored for 200 m by tufa [TL 8106 1575] to [TL 8125 1583]. The tufa is here related to a spring issuing from the contact of the Chelmsford Gravels with the Maldon Till.

Another extensive spread occurs in the Caperner's Green vicinity where it completely floors the valley and obscures the underlying geology [TL 8079 1454] to [TL 8155 1472]. The deposit exceeds 1.2 m in thickness.

Rivenhall

Calcareous tufa, associated with peat occurs on the west side of the unnamed valley at Rivenhall [TL 8300 1787]. It is possible that some of the peat and shell marl recorded by Dalton (in Whitaker and others, 1878, p. 67) at Rivenhall End, which are described as part of the Hoxnian interglacial deposit in this memoir, are of Recent origin. CRB

Landslips

In the extreme south-west corner of the district a small slip measuring 250 by 60 m [TQ 690 998] is associated at its back margin with springs issuing from the base of the Bagshot Beds. It possibly continues southwards into Scrub Wood, but the characteristic hummocky surface visible in the fields was not readily discernible in the wood.

Two small slips [TL 7565 0150] and [TL 7570 0143], measuring 50 by 50 m and 100 by 40 m respectively, are associated with springs issuing from the base of the Claygate Beds 500 m NNW of Great Claydons Farm.

Several small slips have been mapped in the vicinity of Woodham Ferrers. Most of them have been documented by Hutchinson. One small slip [TQ 7895 9980] of the N-type (Hutchinson, 1967, p. 116) measures 50 by 60 m and lies 800 m W of the village. Its upper limit is the London Clay/Claygate Beds junction. Hutchinson (1965, p. 25, fig. 11) suggested that the slope [TQ 792 995] 400 m SE of the above occurrence appears now to be virtually stable at an inclination of about 8°. He classifies it as an N-type slip. A spring issuing from a thin cap of Glacial Sand and Gravel marks the upper limit of the steep slope.

Immediately on the west side of the main road in the village [TQ 7975 9960] hummocky ground 140 by 60 m characterises a small slip. An additional slip not noted by Hutchinson lies 1 km SE of the village, where a crescent-shaped tract of hummocky ground 370 m by 50 m [TQ 803 992] is associated with the London Clay/Claygate Beds junction. Another small area of instability, 110 m by 30 m [TQ 809 993] and some 600 m E of the above, lies entirely within the Claygate Beds.

To the south and south-south-east is the better known slip of Bushy Hill [TQ 8107 9915] to [TQ 8110 9854] which extends into the Southend (258) district. The area of disturbance is confined almost entirely to the Claygate Beds, except at the northern end where the London Clay is also affected. Hutchinson (1967, p. 116) classifies this as the successive or stepped rotational type, passing lower down into the translational type. He recognised (1965, pp. 26–27) a series of slab slides with toes between 0.3 and 0.9 m high and rear scarps 0.6 to 0.9 m high. Both the scarps and toes are grassed over and appear to be relatively old.

Another fairly extensive landslip has been mapped on the hill to the east and south-east [TQ 8214 9943] to [TQ 8249 9930] of Martin's Farm. The slip on the west side Hutchinson (1967, p. 116) classified as S-type with some features of the N-and L-types. He noted (1965, pp. 28–29) mature trees 0.6 m diameter growing in the face of the rear scarp of the southern L-type slip. The grassy toe is about 0.5 m high.

Some 1.75 km NE, to the south of Great Canney Farm, an unstable area [TL 8370 0034], 100 by 40 m in extent, has been mapped. It is recorded by Hutchinson (1965, p. 29) that this slide has been in existence since at least 1925 and appears to have slowly doubled in size between that date and 1965. The rear scarp and toe are grassy and about 0.5 and 0.3 m high respectively. Only the basal beds of the Claygate Beds are involved. A nearby slide [TL 835 002] was not recognised during mapping as its diagnostic features had been obscured by ploughing (Hutchinson, 1965, p. 29). Similarly on the south-facing slope of Kit's Hill [TQ 8555 9910] to [TL 8610 8890] the levelling of the ground in 1968 was necessitated by the irregular profile caused by slipping. It was classified by Hutchinson (1967, p. 116) as a translational slab slide.

Some 300 m WNW of Howegreen Farm a small landslip, 100 by 50 m, has been mapped [TL 8393 0114] on a north-facing slope. Its back scarp coincides with the base of the Claygate Beds. Hutchinson (1965., p. 32, fig. 14) suggests that it may be more extensive than it now appears, but that its features have been subdued by weathering and ploughing.

At Purleigh prominent slips have affected the uppermost London Clay and the basal Claygate Beds over a 320 m length of outcrop [TL 8387 0243] to [TL 8392 0212]. The toe of the slip, some 0.3 to 0.6 m high, extends down almost to Alluvium level. The more southerly part of this disturbance has, according to local knowledge, been in existence since at least 1920, and there is evidence of continual slow movement of the toe up to at least 1965 (Hutchinson, 1965, p. 33). Another slide, 100 by 20 m, [TL 832 022] associated with the basal Claygate Beds and uppermost London Clay can be found 900 m WNW of Purleigh.

Loddart's Hill, 4 km SW of Maldon, comprises a small outlier of Claygate Beds overlying London Clay. Two small slips [TL 8233 0413] and [TL 8255 0430] have been mapped on the west and north-facing slopes of the hill, at the top of the London Clay. The grassy toes, about 0.6 m high at each site, are well marked. At the latter site Hutchinson (1965, p. 34) noted evidence of at least two periods of movement.

A small area 90 m x 90 m of hummocky ground [TL 8045 0805] has been noted on the south bank of the River Chelmer near Raven's Farm. A spring [TL 8043 0807] which issues from the base of the slip is presumably related to the junction of the Glacial Sand and Gravel with the London Clay, although this junction was not observed in the field and is probably hidden by a thin layer of Head.

Three slips have been mapped on the north-north-east facing river cliff of the Blackwater at Maldon. The most westerly and smallest [TL 842 076] measures 170 by 20 m and occurs where a small outcrop of London Clay beneath 2nd Terrace deposits previously formed a low river cliff. The slip extends down to the present level of the saltings and may pass under them. Springs were noted at the back of the slip and the ground is generally boggy. The middle and most obvious slip [TL 8442 0742] to [TL 8471 0737] is referred to as the West Maldon slip by Hutchinson (1965, pp. 35–36; 1967). Hutchinson, however, thought that this landslip is more widespread than the present survey suggests and that it extends south-southeastwards as far as the main road, although he notes that the eastern end is more stable. He regarded it as having been formed by successive rotational slips. The component slips, which are grassed over, extend down to the level of the saltings and appear to be quite old, although small fresh scars indicate current instability in some places. Presumably the most active period of slipping took place when the River Blackwater was directly eroding the toes of the landslips. The presence of the saltings indicates that erosion has not taken place for a number of years. The surface of the slip area is characteristically hummocky and covered with brambles (see (Plate 7)), with pools occupying some of the hollows. A spur of London Clay between the above slip and the East Maldon slip appears to be stable and this may have determined the course of the main road which descends the London Clay slope to the river crossing. As with the western slip it is regarded by Hutchinson (1965, p. 34, fig. 14) as more extensive than has been portrayed on the geological maps. The eastern end, if it really is a landslip, is built over and appears relatively stable. The surface of the western portion undulates markedly and has small ponds on its surface. The toe is now partially protected by a river wall, but presumably river action prior to its construction was responsible for periodic movements until quite recently.

A small area [TL 9390 0240] to [TL 9416 0256] of instability 900 m SE of Steeple appears to involve only the uppermost beds of the London Clay. Somewhat higher up the slope is an outlier of 4th Terrace river gravels. It is of the translational slab slide type (Hutchinson, 1967, p. 116). In 1969 there was a well defined, but degraded grassy toe.

The Steeple slip is on a well defined north-facing feature which extends north-eastward from the Althorne-Mayland area, where the Claygate Beds crop out, to the south of Steeple and beyond to the vicinity of Beacon Hill [TL 963 051], where a small slip measuring 140 by 50 m has been mapped, and beyond to the margin of the district where the feature descends to river level. Although no other areas of instability were detected along this feature they can nevertheless be expected. It is probable that ploughing has obliterated their more obvious topographical features. CRB

Chapter 7 Economic geology

Sand and gravel

The following account outlines the results of drilling and sampling surveys undertaken by the then Mineral Assessment Unit of the Institute in the Chelmsford district between 1966 arid 1974, and forming the basis of six reports (Ambrose, 1973a, b; Clarke, 1975; Eaton, 1973; Haggard, 1972, and Simmons, 1978).

For the purposes of these surveys, sand and gravel ('mineral') is regarded as potentially workable (that is as 'resources') if the following four arbitrary physical criteria are met:

1. The deposit should average at least 1 m in thickness
2. The ratio of overburden to sand and gravel should be no more than 3:1
3. The proportion of fines (particles passing the 0.063 mm sieve) should not exceed 40 per cent
4. The deposit must lie within 25 m of the surface

The major resources of aggregate are the extensive spreads of Glacial Sand and Gravel (the Chelmsford Gravels) and the 'River Terrace Deposits of the Blackwater and Chelmer valleys (see (Table 3)).

Glacial Sand and Gravel (Chelmsford Gravels)

There is great variation in the sampled thickness of mineral. On Danbury Hill [TL 780 050] the sand and gravel is preserved in steep-sided trough-like bodies (p.44; (Plate 4)) where the maximum recorded thickness is 21.3 m in a borehole near

Little Baddow [TL 771 078]. Elsewhere, however, the Glacial Sand and Gravel has a fairly uniform thickness (see (Table 3)), ranging commonly from 5.1 to 7.0m, though a greater thickness was recorded to the south-east of Danbury Hill (for example 13.7 m was proved in a borehole [TL 820 060] near Woodham Walter) and also to the north-east of Tiptree where a borehole [TL 903 186] proved 14.0 m. The recorded thickness variations of the mineral show no apparent regional trends.

The Glacial Sand and Gravel is composed essentially of subangular to angular flint gravel ( + 4 mm size) with occasional subrounded and rounded pebbles and subangular to subrounded fine- (0.063 to 0.25 mm) and medium- (0.25 to 1 mm) grained quartz sand, with some angular and subangular flint in the coarse sand fraction (1 to 4 mm). Subrounded and rounded pebbles of vein quartz and quartzite are a characteristic component of these deposits and comprise up to 20 per cent by weight of the gravel fraction.

Although the mean gradings of the mineral proved in each borehole show no systematic variation over the district as a whole (Figure 24), the deposit appears to be sandier locally to the north-west of Terling where interbedded sand occurs within the Glacial Sand and Gravel, to the north-east of Witham and on the south-eastern side of the Danbury-Tiptree ridge [TL 874 139]; [TL 795 046]. The particle size distribution of the Glacial Sand and Gravel in the district approximates to 50 per cent gravel, 40 per cent sand and 10 per cent fines. Medium sand is predominant in the sand grades and accounts for between 20 and 30 per cent of the deposit.

River Terrace Deposits and sub-alluvial gravel

The main spreads of mineral within the River Terrace Deposits occur in the valleys of the rivers Chelmer and Blackwater and on the Dengie peninsula.

In the Chelmer and Blackwater valleys the sand and gravel occurs in three terraces and as sub-alluvial gravel. The deposits in the Chelmer Valley have a mean thickness of 2.3 m and appear to thicken downstream where the mean thicknesses calculated for each resource block range from 3.0 to 4.6 m near Maldon.

The composition of the River Terrace Deposits in the Chelmer-Blackwater valley is similar to that of the surrounding Glacial Sand and Gravel from which they are almost certainly derived: the gravel fraction makes up approximately 50 per cent of the deposit, with 40 per cent sand and 10 per cent fines. To the south of the Blackwater Estuary, the River

Terraces range in height from 7 to 45 m above OD and have been proved to have a sand and gravel mean thickness of 2.7 m. No regional trend in thickness variation is discernible, but the greater recorded thicknesses of mineral appear to be within channels cut into the bedrock. In common with the mineral found elsewhere in the Chelmsford district, the sands and gravels in the Dengie peninsula comprise fine to coarse subangular to subrounded flint and subrounded and rounded vein-quartz-and quartzite-gravel, and quartz-rich sand. However, they also contain subangular and subrounded Greensand pebbles, which distinguish them from the other deposits.

The 2nd and 3rd terrace deposits on the Dengie peninsula have identical mean particle size distributions of gravel 32 per cent, sand 57 per cent, and fines 11 per cent, and are noticeably different in grading from the 4th Terrace (gravel 21, sand 60 and fines 19 per cent), and from the terrace deposits in the vicinity of the Blackwater Valley which have a mean grading of gravel 62, sand 35 and fines 3 per cent. Thin soil and Alluvium ranging in thickness from 1.2 to 1.6 m, comprise the main overburden for all the River Terrace Deposits in the district, but locally greater thicknesses of Head and channel-fill deposits may occur.

Overburden

Chalky Boulder Clay, found mainly to the north of the Chelmer-Blackwater valley, forms the major overburden. It is thickest on the interfluves in the northern part of the district where it has an average thickness of 3 to 4 m. In some places, for example near Little Waltham [TL 710 145], the sand and gravel is too deeply buried beneath the Boulder Clay to be considered potentially workable. Locally, thin seams of Boulder Clay occur within and beneath the sand and gravel deposits. Head, which comprises pebbly silty clays and silts and occurs throughout the district, commonly attains a thickness of several metres; for example a borehole [TL 816 070] proved 6.4 m of soil and Head.

Production and resources

Although working pits (see (Table 4)) in Glacial Sand and Gravel are found at scattered localities throughout the district, workings in the River Terrace Deposits are current, ly concentrated in the Blackwater Valley near Maldon.

In 1975 the Chelmsford Service Area produced 2 569 000 tonnes of sand and gravel (Department of the Environment, 1976). Of this total, the production of gravel and hoggin, concreting sand, and building sand was in the ratio of 6:3:1 respectively.

It is estimated that total resources of the Chelmsford district amount to approximately 1200 million cubic metres (1800 million tonnes). Of this total, five-sixths, 1000 m3 (1500 million tonnes) are found in the Glacial Sand and Gravel and the remainder in the River Terrace Deposits. These estimates exclude deposits lying within the urban areas of Chelmsford, Maldon and Witham, but no allowance has been made for any other factors which might inhibit workings. MRC, MBS

Geophysical investigations

Regional gravity and aeromagnetic surveys have been made over the entire area of the Chelmsford (241) Sheet, and detailed seismic, electrical and airborne electromagnetic surveys have been undertaken over selected areas to evaluate sand and gravel resources.

Gravity Surveys

Falcon and Tarrant (1951) presented the results of a gravity survey of Southern England, made by the Anglo-Iranian Oil Company (AIOC) now incorporated in the British Petroleum Company. This AIOC survey, with about 2 gravity stations per square mile (1 station per 1.3 km2), covered all except the north-east corner of the district. This remaining part was later covered at a similar gravity station density by the Geological Survey of Great Britain (GSGB). The results of the AIOC and GSGB surveys in south-east England have been collected, compiled and published by the GSGB as part of the Quarter-inch to One Mile Gravity Survey Overlay Map Series and the Chelmsford district is covered partly by Sheet 16 (Pugh, 1959) and partly by Sheets 20 and 24 (Stubblefield, 1964a). To relate the gravity values to the new National Gravity Reference Net 1973 (see Masson Smith and others, 1974) and refer the anomalies to the 1967 International Gravity Formula as recommended by Morelli (1976), approximately 2.26 mGal should be subtracted from the Bouguer anomalies on the overlay maps. For the Bouguer correction a uniform rock density of 2.0 g cm -3 was used for the AIOC survey, whereas a density of 1.9 g cm -3 was used in the GSGB survey. As the elevation on the Chelmsford district nowhere exceeds about 110 m this change in density will cause a maximum discrepancy between the two surveys of about 0.5 mGal only in the Bouguer anomaly. A density of between 1.9 and 2.0 g cm -3 is a reasonable mean density for the total column of Quaternary and Tertiary rocks above sea level in the Chelmsford district.

The main feature of the gravity survey overlays in the district is a NW-SE alignment of the Bouguer anomaly contours with values increasing from about −9 mGal in the south-west to slightly over + 1 mGal in the Tiptree area of the north-east, with maximum gradients of over + 1 mGal per km in a NNE direction occurring in the Purleigh area. This regional positive gradient towards the north-east is considered to be largely caused by a north-eastward thinning in this area of the Tertiary and underlying Mesozoic sediments of the London Basin as the denser basement of the London Platform rises to the north. A small part of this gradient may be due to isostatic thinning of the crust in a north-easterly direction in response to the considerable thickness of low density sediments beneath the North Sea. Minor deviations from this regional gradient, particularly in the north-east of the district might be caused by variations in thicknesses of light Estuarine Alluvium and, in places, river Alluvium. It might be possible to trace old river courses by the gravity method provided a sufficient density contrast exists between the Alluvium and the rock in which it is emplaced.

Aeromagnetic surveys

The GSGB aeromagnetic survey has been published as contoured magnetic anomaly maps and the Chelmsford district is covered partly by Sheet 3 and partly by Sheet 6 of the 1:250 000 Series (Stubblefield, 1964b and c). There are very weak magnetic anomaly gradients of under 3 nanotesla (nT), or gamma, per km and within the district the maximum anomaly fluctuation is less than 40 nT. This indicates that there are no shallow magnetic rocks in the district, and that the magnetic contour pattern is caused either by sources deep within the basement beneath Chelmsford or by a coalescing of magnetic anomalies from distant sources well outside the area.

Seismic and electrical surveys

In 1967 staff of the Applied Geophysics Unit of the Institute undertook seismic and electrical surveys to assist with the evaluation of sand and gravel resources over an area of about 8 km² north of the River Blackwater downstream from Maldon, and two small areas totalling about 3 km² near Terling. Resistivity depth probes revealed that the London Clay has a very uniform resistivity of about 10 ohm metres, but the overlying sand and gravel resistivities were very variable in the range 10 to 1000 ohm metres, without any marked peaks in histograms of resistivity values. Seismic refraction profiles enabled accurate depths to be determined to the London Clay which had a very uniform velocity of 1.53 km s⁻¹. Sand and gravel velocities were much more variable in the range 0.6 to 1.5 km s ⁻¹ and there was a poor velocity contrast with the overlying Boulder Clay. From comparison of the geophysical results with borehole information it was concluded that, in the limited area investigated, particle size distribution of the sand and gravel was not related to its seismic velocity or resistivity and that current geophysical methods were of little value as a means of assessing sand and gravel reserves.

Airborne electromagnetic surveys

In 1971 and 1973 an airborne electromagnetic survey was made by Barringer Research Ltd, under contract to the Natural Environment Research Council, of an area of about 160 km² around Witham and Tiptree in order to evaluate the application of this new survey method to prospecting for sand and gravel. The 1971 helicopter-borne Radiophase Survey used transmissions on 19.6 KHz from a 'Very Low Frequency' radio station at Oswestry. In the hope of measuring resistivity variations of the Quaternary deposits in greater detail part of the area was reflown in 1973 with a multifrequency Barringer E-PHASE system. This system employs 'Low Frequency' (LF) and 'Medium Frequency' (MF) waves and for this survey signals were received from Droitwich, UK on 200 KHz, Burg Magdeburg, Eastern Germany on 263 KHz, and Daventry, UK on 647 KHz. The direction to these transmitting stations from Chelmsford required approximately north-south flight lines for maximum electromagnetic response to ground anomalies in the mobile airborne receiver. Flight lines were spaced at about 0.1 mile (0.16 km) and the normal flying height was between 60 and 80 m.

The airborne resistivity results at LF and MF were presented to the Institute as contoured 1:25 000 maps. No systematic correlation has been proved so far between E-PHASE resistivity highs on these maps and proven sand and gravel reserves. This airborne resistivity method, which has been applied successfully in prospecting for sand and gravel elsewhere in the world, may be unsuitable in the Chelmsford area surveyed because of variable sand and gravel resistivities and substantial thicknesses of conductive clay and overburden. RBE

Brickmaking

In an area largely devoid of natural building stones reliance has had to be placed on the manufacture of local bricks from a variety of deposits. Brickmaking was carried out in the Roman period and from the late Middle Ages until the middle of the 20th century. No brickmaking is currently carried out in this area, the last pits having closed down in the period 1950–60.

A survey was carried out by Tyler (Tyler and Denney, 1893) of the brickfields of Mr Wm. Clover at Boreham [TL 7680 1095], Widford c. [TL 698 054] and Hatfield Peverel [TL 7800 1145]: Tyler noted that at Boreham, the upper red brickearth, 3 m thick, was utilised for making 'Boreham Reds'. Below lay a brickearth from which white bricks similar to 'Suffolk Whites', were made. In addition to bricks, the following wares were produced: extra hard bricks for paving, ornamental and ordinary chimney pots, vases, ridge tiles, pan tiles, ornamental garden tiles and land drains from 50 to 230 mm diameter. The annual production was 600 000 bricks in two kilns. At Widford the brickearth was estimated to be 3.6 m thick and suitable for making pipes and tiles. The one kiln could produce 800 000 bricks yearly. The Hatfield Peverel pit could then produce about 800 000 red bricks per annum.

The annual production from all three pits in 1893 was 1 300 000 bricks. The best bricks, the Boreham Reds, were sent to London by rail.

The following deposits, listed in stratigraphical order have been worked in the Chelmsford district (grid references are given only where the pits have been accurately located):

• London Clay Broomfield [TL 701 085]; Barrack Lane, Chelmsford (Wood Street Pottery) [TL 700 048], and Widford [?698 054] (Challis, 1885, p. 68; 1887, p. 16; Whitaker, 1889, p. 259).
• Claygate Beds Galleywood ( = possibly the Bagshot Beds) (Whitaker, 1889, p. 259).
• Bagshot Beds Galleywood [TL 7035 0245]; Stock [TQ 704 984]; this latter pit which utilised the Stock Clay, lies just beyond the Sheet boundary (Whitaker, 1889, p. 259).
• Chalky Boulder Clay near Brownings, Chelmsford [TL 701 085]. This pit, which is marked as a pottery on the old six-inch maps of the area, appears to have worked the chalky Boulder Clay, at least in the upper part. It is possible that this is the Broomfield Pit of Whitaker (1889), in which case the London Clay would have been worked at the bottom of the pit.
• Head Brickearth Hatfield Wick [TL 7770 1235]; Boreham [TL 7680 1095] (Tyler and Denney, 1893); Hatfield Peverel [TL 7800 1145]; Appleford Bridge about [TL 844 156] (Wood and Harmer, 1877, p. 111; Whitaker and others, 1878, p. 67); Woodham Mortimer [TL 806 045]; Slough Brickworks [TL 807 027], and east of Cock Clarks [TL 818 030]; south of Witham Station (Whitaker and others, 1878, p. 68); Widford [?698 054] (Challis, 1885, p. 68; Tyler and Denney, 1893).
• Head (probably including some Head Brickearth) near Pondhallow Farm, Witham [TL 820 134]; Moulsham area of Chelmsford [TL 6925 0700], [TL 690 072], [TL 7005 0630], [TL 7020 0610], c. ? [TL 704 068].
• Ipswichian lacustrine deposits (Head and Head Brickearth also worked in the same pits) Moulsham [TL 7020 0610], [TL 7005 0630] (Whitaker, 1889, p.452; Holmes, 1896; Cole, 1894, pp. 155, 219; Newton, 1896).
• Hoxnian lacustrine deposits (possibly also worked for marl) ? Hole Farm, near Rivenhall End [TL 8484 1724], [TL 8485 1731], [TL 8402 1734]. Estuarine Alluvium Whitaker (1889, p. 501) refers to brickworks 'in the parish of Latchingdon' which utilise marsh clay. This probably refers to the Estuarine Alluvium of the River Crouch, in the adjacent Southend and Foulness (258/9) district, where there were known to be brickworks on Bridgemarsh Island.

Marl

Numerous small pits, now overgrown or flooded, occur on the outcrop of the chalky Boulder Clay. Each farm appears to have had at least one such pit, and it is probable that the chalky clay was dug for local use in marling the soil. Locally, as at Hole Farm [TL 8481 1724] near Rivenhall End, the Hoxnian lacustrine clays have been dug for marl. The practice probably died out with the establishment of good road and rail communications when better quality lime could be brought in from the chalk pits further afield.

Building stone

The Chelmsford district is devoid of good quality building stone. Reliance in the past has been placed on timber and bricks for building construction. Flints from the glacial or terrace deposits, and occasionally cementstones from the London Clay, have been used (Plate 8). A few of the more important buildings are built of imported Kentish Ragstone; Latchingdon Church [TL 886 005] is one such example.

Salt

Salt produced by the evaporation of sea water has a recorded history in Essex since at least the time of Edward the Confessor (Christy, 1906, p. 193). Indeed a radiometric date of 2130 + 40 BP from the 'red hill', Osea Road [TL 8870 0745], Maldon, indicates extensive salt production prior to the Roman occupation (Switsur, 1974).

In the early days it is thought that the sea water was let into shallow ponds where it slowly evaporated. The resultant salt was known as 'bay salt'. Later, artificial heat was applied to the pans to speed the evaporation and purer 'white salt' resulted. A by-product of this process was the production of red earth by the fires. This characteristic red soil can be found at a number of localities (see p. 83 of the details of the Estuarine Alluvium), and indicates the sites of former salt pans. They are particularly common in the vicinity of Salcott which probably derives its name from the industry.

A description of the manufacture of the salt of the Maldon Crystal Salt Company' is given by Christy (1906, p. 202; see also Reader, 1908; 1910). The amount produced at that time was about 80 tons annually. The industry still continues at Maldon, and some 200 to 250 kg a day of high quality table salt is produced by evaporation (Notholt and Highley, 1973, p. 20).

Water supply

The area of the Chelmsford (241) sheet, which is administered by the Essex Rivers Division of the Anglian Water Authority and constitutes part of Hydrometric Area 37, is one of gentle relief, the higher ground (maximum elevation c. 107 m) lying to the west around Danbury and falling gradually eastwards to sea level on the flat plains and marshes along the coast. The River Chelmer flows across the district from the west to the Blackwater Estuary in the east and is joined by a number of tributaries from both the north and south. These rivers drain predominantly impermeable catchments of Boulder Clay or London Clay.

This area has one of the driest climates in England. The average annual rainfall varies from 585 mm on the higher ground around Danbury to a little over 535 mm along the coast with winter and summer precipitation being generally equal. The average annual potential evaporation amounts to some 530 mm with little variation throughout the area (Penman, 1948). Evaporation is greatest during the summer months and, as a consequence, effective precipitation (rainfall minus actual evaporation) is much lower during this period. As this is a feature which is accentuated in Essex because of its lower rainfall and higher evaporation rates, soil moisture deficits built up during the summer months are often not eliminated until mid-winter or later. The effective precipitation can change rapidly from year to year with extremes of 350 mm and 0 mm being recorded during the period 1931 to 1968, although the annual mean is somewhat less than 150 mm.

Nevertheless, surface water is the chief source of supply with the Abberton Reservoir [TL 970 180] providing storage for the Chelmsford area. Water is also pumped from near the confluence of the rivers Chelmer and Blackwater at Langford for storing in the reservoir at Hanningfield [TQ 730 990).

Boulder Clay

This is generally impermeable although it may locally yield small quantities of sulphate-rich water. At Elbows Farm, Good Easter [TL 639 138], about 3 km to the west of the district, a yield of 3 m3/d was obtained from a shallow well.

Glacial Sand and Gravel

These underlie the Boulder Clay and are quite widespread in the district. Around Danbury the Glacial Sand and Gravel is not capped by Boulder Clay. At the junction between these deposits and the underlying London Clay a strong spring-line is often present. Because of the wide variation in lithology and extent of these deposits aquifer characteristics are variable but yields up to 5000 m3/d can be obtained.

Terrace sands and gravels

The gravels are an important aquifer especially for local agricultural purposes and, around Maldon, provide water for public supply. The waters are of the calcium bicarbonate type with high values of sulphate and low chloride content. A representative analysis from a gravel spring in Admirals Park, Chelmsford, is given below:

Bagshot Beds

The Bagshot Beds in their lower part consist of fine-grained sands and locally form a good aquifer. In the Stock area, just beyond the south-western corner of the district, copious springs issue from the base of this formation.

Claygate Beds

These fine-grained sands and clays overlying the London Clay do not constitute an important source of supply.

London Clay

The London Clay, excluding the Oldhaven Beds, which covers the greater part of the district, is present in such thickness that it can be regarded as impermeable, preventing infiltration to the Chalk. Although silty and sandy horizons have yielded small quantities of water, this deposit is unimportant as an aquifer. The Oldhaven Beds consists of sand and coarse gravel about 4 m thick which can yield up to 436 m3/d.

Lower London Tertiaries

The Lower London Tertiaries are divided into the Woolwich and Reading Beds and the Thanet Beds. Of this group the Thanet Beds is the more important, although it is difficult to assess yields from these beds alone for the Lower London Tertiaries are probably in hydraulic continuity with the Chalk and yields are comparable to those obtained from the Chalk.

Chalk

Because the Chalk and Lower London Tertiaries are overlain by a considerable thickness of impermeable Tertiary and Recent deposits and are therefore not subject to secondary weathering processes, the Chalk is not such an important aquifer in this part of Essex as it is farther north towards its outcrop around north west Essex.

The primary permeability is low (3 x 10⁻⁵–1 x 10⁻³ m/d); secondary permeability due to fissuring governs the yield of Chalk boreholes. Little is known of the nature and distribution of fissuring with depth in the district, although from first principles well developed fissuring is not to be expected. Transmissivity values are less than 100 m2/d in this part of Essex.

As the Chalk groundwater moves downdip in the direction of the hydraulic gradient, which is to the south-east, a number of chemical changes occur, principally cationic exchange and sulphate reduction (Ineson and Downing, 1963); the initially calcium bicarbonate type water at outcrop passing to a sodium bicarbonate type beneath clay cover. A representative analysis (Guiver, 1972) of the 'Main Inland Group' is given below. Furthermore along the coast around Maldon, waters from Chalk boreholes have higher sodium chloride and fluoride concentrations; this may be a result of mixing with residual connate water: a representative analysis from the Maldon area is also given.

Water levels have remained fairly constant over the last few years with the declining abstraction from chalk wells.

A yield of 209 m³/d for a drawdown of 12.5 m was obtained from a 169-m borehole of 0.324 m diameter at Galleywood Road (241/14) [TL 703 039].

The amount of water abstracted within the area for industry and agriculture from each of the three principal sources — surface, Chalk and gravel is given below. (Values obtained from Anglian Water Authority licences.)

Values in million litres per year

Most of the public water supply is obtained from surface water sources, although there is a minor contribution from gravel boreholes. ARL

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Appendix 1 List of boreholes and wells

The more important boreholes and wells are listed below. Logs for these are given on the microfile that is enclosed in the pocket at the back of this volume. Most of these logs have been summarised, but those for the BGS Witham, Kelvedon and Stock boreholes are given in full. Details of boreholes drilled by the Industrial Minerals Assessment Unit can be found in the following BGS Reports or Mineral Assessment Reports: Ambrose (1973 a,b), Clarke (1975), Eaton (1973), Haggard (1972) and Simmons (1978).

The logs are arranged in order of 1:10 000 or 1:10 560-scale National Grid quarter sheets and the BGS Registration Number of each borehole or well record is given.

Appendix 2 List of Geological Survey photographs

Copies of these photographs are deposited for reference in the British Geological Survey library at Exhibition Road, South Kensington, London SW7 2DE, and at the Keyworth Office, Keyworth, Nottingham NG12 5GG. Black and white prints and slides can be supplied at a fixed tariff, and in addition colour prints and transparencies are available for all the photographs with the exception of the first two. The photographs were taken by Messrs J. Rhodes, C. Friend and S. Curtis.

Abstracts of some of the more important borehole and well logs

The logs are arranged by 6-inch sheet order: the full BGS reference number is obtainable by appending the number given to each borehole to the 6.inch sheet number, e.g. Sandford Mill Water. works  (TL70NW/2).

Logs of the BGS Witham, Kelvedon and Stock boreholes are given in full. Details of boreholes drilled by the Industrial Minerals Assessment Unit can be found in the Institute reports of Ambrose (1973 a,b). Clarke (1975), Eaton (1973). Haggard (1972) and Simmons (1978)

Sheet TL70NW

Sandiford Mill Waterworks [TL 7285 0600]. Surface level + 20.42 OD; Date 1955* level * 20.0 et OD; Date 1955

(TL70NW/119) Hammond Road [TL 7445 0599]. Surface level +21.88m OD; Date 1972.

(TL70NW/127) River Chelmer [TL 7435 0658] Surface level + 18.05 m OD; Date 1972

River Chelmer/A12 crossing [TL 7199 0623]. Surface level +21.97 m; Date 1972.

(TL70NW/263/12) Chelmsford Centre Redevelopment c. [TL 7085 0668] Surface level +24.93 m OD; Date 1968

TL70NW265 Broomfield Waterworks [TL 7096 0945] Surface level + 33.53m OD; Date 1911

(TL70NW/269) T.D. Ridley & Sons, Flour Mills, Clelmsford  [TL 7068 0722] Surface level  +27.43 m OD; Date 1900

(TL70NW/270) Marconi's. Telegraph Works, Chelmsford [TL 7082 0739]. Surface level +33.53 m OD; Date 1912

(TL70NW/271) Marconi's Telegraph Works, Chelmsford [TL 7061 0736] Surface level + 29.56 m OD; Date 1912

(TL70NW/272) Wells & Perry’s Brewery, Chelmsford [TL 7067 0702] Surface level, 25.90m OD; Date 1898

(TL70NW/273a) Hall Street Waterworks, Chelmsford [TL 7095 0635] Surface level + 25.98 m OD; Date 1853

(TL70NW/273b) Hall Street Waterworks, Chelmsford [TL 7095 0635] Surface level +25.98 m OD; Date 1901

(TL70NW/274) Waterworks, Great Baddow [TL 7202 0560] Surface level + 29.56 m OD; Date 1901

(TL70NW/276) The Maltings, Chelmsford [TL 7095 0740] Surface level + 30.48 in OD; Date 1948

Sheet TL70NE

(TL70NE/59) Hillcrest Danbury [TL 7946 0762] Surface level about + 71.6 m OD; Date 1958

*The London Clay is abnormally thick and probably includes some Lower London Tertiaries which are much thinner than anticipated.

(TL70NE/60) The Nurseries, Danbury [TL 7879 0571] Surface level about + 94.5 m OD; Date 1934

Sheet TL70SW

(TL70SW/1) BGS Stock Borehole [TL 7054 0045] Surface level + 93 m OD; Date 1970

(TL70SW/142) Woodhill Road Sandon [TL 7463 0485] Surface level + 24.83 m OD; Date 1972

(TL70SW/176a) Galleywood Waterworks A [TL 7037 0392] Surface level + 58.98 m OD; Date 1912

(TL70SW/176b) Galleywood Waterworks.[TL 7037 0391]. Surface level+.58.98 m OD; Date 1913

(TL70SW/177) Baddow Brewery, Great Baddow [TL 7321 0462] Surface level +40.54 m OD; Date c.1806

Sheet TL70SE

(TL70SE/16) The Rectory, East Hasulinglield [TL 7698 0132] Surface level + 57.91 mOD; Date 1791

Sheet TL71SE

(TL71SE/33) Hatfield Place, Hatfield Peverel [TL 7850 1144] Surface level + 30.48 in OD; Date 1900

(TL71SE/36) Crab's Hill Farm, Hatfield Peverel [TL 7891 1118] Surface level +33.83 m OD; Date 1916

(TL71SE/38) Brakeys, Crix Lane, Hatfield Peverel [TL 7743 1025] Surface level +36.56 m(?) OD; Date 1928

Sheet TL80NW

(TL80NW/65) Trial borehole near Maldon [TL 8437 0677] Surface level + 39.93 m OD; Date 1965

(TL80NW/137) Angula Engineering Co. Ltd., Maldon [TL 8483 0712] Surface level + 28.95 m OD; Date 1891