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Geology of the Windsor and Bracknell district — a brief explanation of the geological map Sheet 269 Windsor

R A Ellison, and I T Williamson

Bibliographic reference: Ellison, R A, , and Williamson, I T. 1999. Geology of the Windsor and Brachnell district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey.1:50 000 Sheet 269 Windsor (England and Wales).

Keyworth, Nottingham: British Geological Survey. 1999.

ISBN 978 085272782 9 (epub) ISBN 978 085272336 4 (print)

© NERC 1999 All rights reserved

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

Your use of any information provided by the British Geological Survey (BGS) is at your own risk. Neither BGS nor the Natural Environment Research Council gives any warranty, condition or representation as to the quality, accuracy or completeness of the information or its suitability for any use or purpose. All implied conditions relating to the quality or suitability of the information, and all liabilities arising from the supply of the information (including any liability arising in negligence) are excluded to the fullest extent permitted by law.

(Front cover) Cover photograph: Oblique aerial view of Windsor Castle from the east. Windsor Castle is built upon an inlier of the Upper Chalk (Seaford Chalk Member) that owes its existence to a combination of folding and faulting at depth. Much of the surrounding urban area overlies either London Clay or much younger Quaternary Thames river terrace deposits.

(Rear cover)

(Figure 1) Summary of the geological succession in the Windsor district.

Acknowledgements

The authors thank Bracknell Forest, Runnymede and Windsor and Maidenhead district councils, the London Borough of Hounslow, and Surrey County Council and their officers for their ready co-operation in providing borehole and other information from their records. We acknowledge also the assistance of officers of the Crown Estate for organising access. We are particularly grateful to Dr C King who gave us the benefit of his wide knowledge of former exposures in the area and helped with the interpretation of stratigraphical sequences of Palaeogene (Tertiary) age.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping.

© Crown copyright reserved Ordnance Survey licence no. GD272191/1999.

Notes

The word 'district' is used here to refer to the area represented by the geological map 1:50 000 Series Sheet 269 Windsor.

National Grid references are given in the form [1234 1234] or [123 123] prefixed by either SU or TQ denoting the 100 km square within which they fall.

Lithostratigraphical symbols shown in brackets in the text, for example (CaS) are those shown on Sheet 269 Windsor.

Borehole records referred to in the text are prefixed by the code of the National Grid 25 km2 area upon which the site falls, for example SU 16 NW, followed by its registration number in the BGS National Geosciences Records Centre.

Geology of the Windsor and Bracknell district (summary from the (Rear cover))

Windsor is famous for its royal residence at Windsor Castle; other historical and traditional British institutions in the area include Eton public school, Ascot Racecourse and Sandhurst Military College; and Runneymede where King John signed the Magna Carta in 1215 AD is also here.

The natural landscape around Windsor and in this part of the Thames Valley has been subjected to considerable urban and industrial development over the centuries. Contemporary developments rely on accurate geological information in order, for example, to identify resources and ensure that foundations are adequate.

The oldest rocks seen at the surface are the Chalk, but younger rocks of Palaeogene and Quaternary age dominate the geology of the area. The re-survey of the Windsor and Bracknell district has significantly improved our understanding of the lithostratigraphy of the Palaeogene strata. It introduces a revised classification for formations above the London Clay, and for the youngest formations it establishes a secure correlation with similar clay, silt and sand sequences of the Hampshire and the North Sea basins.

The Quaternary deposits were formed during and after the time when Britain was affected by periods of glaciation. During the Pleistocene, Windsor and Bracknell lay south of the southernmost limits of the great ice sheets and experienced mainly tundra or permafrost conditions. Meltwaters issuing from these ice sheets deposited vast quantities of sand and gravel. Subsequently, geological processes have produced the terraces that now flank the Thames Valley, and have acted as a valuable source of aggregate. The river terraces have now been reclassified in accordance with a regional unified scheme.

The new geological map and this Sheet Explanation provide valuable information on a wide range of earth science issues. These include both traditional geological topics and applied aspects relevant to the suitability, siting and nature of land-use planning and future urban and industrial development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the ground covered by the 1:50 000 Series Sheet 269 Windsor. It is written for the professional user, and for those who may have limited experience in the use of geological maps and wish to be directed to further geological information about the area.

Descriptions of many important sections formerly exposed in the area can be found in the memoir for the district (Dewey and Bromehead, 1915). New information from these sections, together with newer sections, and accounts of cored boreholes recently drilled through the Palaeogene strata, are included in this account. Other detailed information is available in BGS files and Technical Reports (p.25).

Physical setting

The district lies west of London, principally in the counties of Berkshire and Surrey, but includes also parts of the London Boroughs of Hounslow and Hillingdon. The highest population density is in the Thames valley, between Windsor and Weybridge, within which Heathrow airport is situated. The main centre of population is Bracknell which has expanded rapidly since the 1960s.

In the north-east of the district, flat and comparatively low-lying ground is covered by the river deposits of the Thames and its tributaries, the Colne and Wey. Much of the remaining northern part, west of Old Windsor [SU 985 745] and including Windsor Great Park is gently undulating country underlain by the London Clay and Reading formations. The rising ground, north-west of the London–Reading railway, around Knowl Hill [SU 825 796] and Wooley Green [SU 854 801], forms the fringe of the chalk outcrops of the Chiltern Hills. Chalk also crops out on the prominent hill [SU 970 770] upon which Windsor Castle is built. South of Bracknell [SU 87 69] and Sunninghill [SU 937 680], rolling, hilly country with open heaths and pine woods marks the outcrop of the Bagshot Formation and younger Palaeogene strata. Ancient river terraces cover the extensively wooded, relatively high ground near Lightwater and Crowthorne.

Previous research

The earliest survey of the district covered by Sheet 269 Windsor was made by T R Polwhele and W Whitaker of the Geological Survey. This work concentrated on the 'solid' geology, and the geological sheets 7 and 8 were published in 1861 and 1862 (Old Series One-inch). The superficial deposits of the district were subsequently investigated by a team that included W Whitaker, F J Bennett, J H Blake and C E Hawkins; Drift editions of the two sheets were subsequently published in 1871 and 1878 respectively. The first surveys in the area at the six-inch scale were carried out by J A Howe and T I Pocock and formed part of the special one-inch map of London published in 1903. Further mapping of the entire district at the six-inch scale was carried out by H Dewey and C E N Bromehead in 1910 and 1911. A descriptive memoir was published (Dewey and Bromehead, 1915) drawing upon observations made during the mapping and on the wealth of earlier work. Publication of the accompanying 'New Series' geological map (Sheet 269) at a scale of one inch to one mile was delayed until 1920 by World War 1. It has been reprinted with minor amendments several times, and was rescaled to 1:50 000 for the edition compiled by B Young — published in 1979.

This account follows revision mapping at the 1:10 000 scale carried out in 1996 by I T Williamson, A Smith, R A Ellison, R T Mogdridge, P J Strange, A J M Barron and A J Humpage. It involved field-checking and amending the existing large-scale maps and comprehensive archive searches. As a result of this revision the principal advances in understanding the geology of the district concern the Palaeogene strata. Their succession is better known from the examination of fully cored boreholes at Bracknell and Staines, information about which is given in this account and incorporated in the new map. Additional contributions were made by T C Pharoah (interpretation of concealed geology based upon the published data of Oswald, 1993 and regional seismic data) and M A Woods (Chalk stratigraphy).

A concise account of the regional geological setting of the Windsor district is given in London and the Thames Valley (Sumbler, 1996). Information about the development of regional structures (Bevan and Hancock, 1986) is supplemented by an interpretation of the structure at Windsor based on a small amount of seismic reflection data (Oswald, 1993).

The nomenclature of the chalk in south-east England has been revised recently (Bristow et al., 1997) although this work was published after the field work in the Windsor district was complete.

Several regional studies of the Palaeogene successions in the London Basin have drawn upon localities within the Windsor district. For example, the lithofacies distribution and lithostratigraphical classification of the early Palaeogene strata formed part of wider ranging studies by Ellison (1983) and Ellison et al. (1994). King (1981) carried out a detailed study of the London Clay of the London and Hampshire basins, including localities in the Windsor district. There has been no recent research on the Bracklesham Group of the district, but descriptions of the geology can be found in Dewey and Bromehead, 1915. The borehole drilled at Bracknell (see p.10 and (Figure 2)) is the first fully cored hole to prove the Bracklesham Group sequence and has shown that it may be correlated with the Hampshire Basin succession described in Edwards and Freshney (1987) and Plint (1988). In the past 20 years considerable research has been undertaken to determine the lithostratigraphy of the Quaternary deposits. The deposits of the modern-day Thames valley have been studied principally by Gibbard (1985), whose correlation of the river terrace deposits, alluvium and 'brickearths' has been the subject of recent discussion (Bridgland, 1994). A small area of the Thames valley gravels in the north of the district was included in a borehole programme as part of a mineral assessment study (Dunkley, 1979). A larger assessment, of the older river terrace deposits in the south of the district and in the Lodden and Blackwater valleys (Clarke, Dixon and Kubala, 1979), led to a re-evaluation of the deposits formerly known as 'Plateau Gravel' (Clarke and Dixon, 1981). Their lithostratigraphical scheme was amended by Gibbard (1982).

Geological history

The oldest rocks of the district, proved in boreholes, are desert, lacustrine and fluvial sedimentary rocks of Devonian age. They are similar to the Old Red Sandstone strata seen at outcrop in the Welsh Borderland. Seismic reflection data and borehole data from south of the district suggest that the Old Red Sandstone is underlain by Silurian rocks. These rocks were subjected to episodes of folding, faulting and uplift, eventually forming the London Platform in Upper Palaeozoic time. It became part of a stable landmass that extended to Belgium, and was an area of erosion rather than deposition. Consequently, a period of over 200 million years is not represented in the rock record of the district.

It was not until relatively late in the Mesozoic, about 200 million years ago, that the sea surrounding the London Platform once again gradually transgressed across the Windsor district. Even then, the area lay only on the fringe of the deepening Wessex Basin to the south. As a result, only a thin sequence of shallow estuarine deposits of Jurassic age was deposited. These were easily eroded away from much of the platform during late Jurassic tectonic uplift, once again exposing Palaeozoic strata. Subsidence of the Wessex Basin continued, and by early Aptian time, about 140 million years ago, a deepening sea flooded the whole of southern England, including the London Platform. The Lower Greensand, Gault and Upper Greensand were laid down in this sea.

In late Cretaceous time, sea level rose and the whole of Europe was progressively inundated. Decreasing amounts of terrigenous sediment were supplied from the shrinking landmass and increasingly pure calcareous pelagic sediments, that now form the Chalk, were deposited. The beginning of the Palaeogene Era saw a period of uplift in which considerable thickness of the Upper Chalk was eroded away. The succeeding shallow marine and coastal Palaeogene sediments were laid down at the margins of a new basin centred on the North Sea. The youngest preserved sediments that were laid down in this basin are part of the Bracklesham Group. In the 45 to 50 million years that elapsed between its deposition and the Quaternary, minor flexuring and considerable erosion of the Palaeogene strata took place. During this time, the London Basin syncline was formed, and a small fault-bounded uplift structure brought Chalk close to the surface at Windsor.

During the Quaternary, Windsor lay to the south of the great ice sheets that advanced to cover much of Britain. The area was drained by a precursor of the River Thames, which at that time flowed well to the north of Windsor and across East Anglia into the North Sea. Fragments of the terrace deposits laid down in a south bank tributary of this early Thames river are preserved on the higher ground in the south-west of the district.

In the period between the Anglian glaciation and the end of the Devensian glaciation, about 10 000 years ago, the Thames, Wey and Colne rivers became established in their modern-day valleys. The terraces laid down during this time are generally well preserved. The main periods of gravel deposition and intervening phases of downcutting took place during cold episodes when rivers were swollen with glacial meltwaters and erosion was more intense. The river alluvium, the most recent of the river deposits, has been deposited in the last 8000 years or so during an interval of relatively low river discharges and periodic flooding.

Chapter 2 Geological description

Summary information from the deeper boreholes in the district (Figure 2) are given in (Figure 3). Location information for boreholes other than these, and referred to in the text, is given in the section on Information Sources at the end of this sheet explanation. Additional information is available in BGS files.

Palaeozoic rocks

The oldest rocks proved in the district are red sandstones and mudstones at depths of 343.3 and 367.29 m respectively in boreholes at Langley Marsh and Harmondsworth These have been referred to the Devonian (LDv and UDv) Old Red Sandstone on the basis of lithology, although few details survive of the rocks obtained. Probably similar grey and purple sandstones and shales of proven Middle Devonian age have been recorded from a borehole at Slough Trading Estate, a short distance north of the district. There is no information as to the nature of the suspected Silurian (Sil) rocks beneath the Devonian.

Mesozoic rocks

On the basis of regional seismic evidence and deep boreholes in adjacent areas, a thin sequence of Jurassic strata is inferred at depth in the south of the district.

The oldest proven Mesozoic rocks are the Lower Greensand (LGS) which has been penetrated by several deep boreholes (Figure 3) that proved it consists of sands and sandy clays. The upper beds closely resemble the sands of the Folkestone Formation in the Weald.

The Lower Greensand is conformably overlain by the Gault Formation (G). Typically, this consists of grey mudstone with scattered phosphate nodule beds. The range of thickness proved is 37 to 102 m (Figure 3). Such a great variation is almost certainly due to inclusion of strata above and below the Gault, due to the difficulty of identifying the boundaries in borehole cuttings or from incomplete borehole cores. For this reason, thicknesses of all Mesozoic formations quoted in individual borehole logs must be regarded with caution.

The Upper Greensand (UGS) log of the Winkfield Borehole is probably the most reliable in the district and records 9.45 m of fine-grained, calcareous, glauconitic, silty malmstone (an impure calcareous rock) and sandstone resembling that seen at outcrop near Wallingford about 35 km north-west. The Upper Greensand, seen some 27 km distance to the south around Farnham and Guildford, comprises calcareous sandy siltstone and silty limestones.

The topmost marly beds of the Lower Chalk (LCk), the Plenus Marls, are overlain by hard nodular chalk of the Melbourn Rock at the base of the Middle Chalk (MCk). Boreholes that have penetrated the Middle Chalk show it to consist mainly of white chalk with some flints in the upper beds. The basal bed of the Upper Chalk (UCk), the Chalk Rock, was identified in the Winkfield Borehole, but in other holes in the district its position is uncertain. The Upper Chalk consists mainly of soft white chalk with flint nodules generally lying in distinct beds. Thin beds of tabular flint also occur. The fauna of the U. socialis and M. testudinarius Zones identified in boreholes (Curry, 1965), and gamma-ray log signatures of chalk above the Plenus Marls marker both indicate that the Newhaven Chalk Member (Bristow et al., 1997) is present, and likely to occur widely, directly below the Palaeogene strata. In the order of 25 to 30 m of Upper Chalk crop out. It has been identified as part of the M. coranguinum Zone (Dewey and Bromehead, 1915), and thus within the Seaford Chalk Member (Bristow et al., 1997).

Palaeogene rocks

Thanet Sand Formation (TS)

This formation occurs at depth in theeast but is overstepped in a westerly direction by the Upnor Formation and does not come to crop. Proved equivocally only in the Staines (No. 5) Borehole (Figure 5), (Figure 6) and (Figure 7), it consists of 3.63 m of brownish or greenish grey, bioturbated, fine- to medium-grained sand. At the base is the Bullhead Bed: a thin, glauconitic, sandy clay with glauconite-coated, mainly unworn, flint pebbles and cobbles. Thanet Sand is probably present in other boreholes in the east of the district: for example 3.35 m of sand overlying the chalk in the Chertsey (Guildford Street) Borehole, but this has not been divided from the Reading Formation. In gamma-ray logs it has a distinctive coarsening-upwards signature.

Lambeth Group (LMB)

The group may be divided into the Upnor and Reading formations (Ellison et al., 1994). In the Windsor district these are not generally separated in borehole logs, the strata being classified as 'Reading Beds'. Their combined thickness is typically 22 to 28 m, but is probably less in the west where 18 m are proved in a borehole at Shurlock Row and an estimated 18 m also crop out at Knowl Hill. Selected borehole logs illustrating lithological and thickness variations within the group are shown in (Figure 4) and (Figure 5).

Upnor Formation (UPR)

This formation was formerly known as the 'Reading Formation Bottom Bed'. It is probably present throughout the district, and though readily identified in borehole core it has not proved possible to map it at outcrop.

It consists of medium-grained, variably pebbly, glauconitic sand from which oysters and fish teeth have been recorded. Locally there may be units of thinly interbedded and burrowed sand and grey clay in which leaf impressions have been recorded. Where it rests on the Chalk, a bed of glauconitic, sandy clay with glauconite-coated, unworn flints is present. This is in general indistinguishable from the Bullhead Bed at the base of the Thanet Sand and is probably reworked from it. The formation is generally up to 6 m thick. The only boreholes in which it is certain that the Upnor Formation occurs are the Shurlock Row Borehole (2.13 m), the M4 motorway site investigation boreholes [SU 817 723] to [SU 885 785] between White Waltham and Bray where 3 m are proved, and the Staines No. 5 Borehole (Figure 6) which proved 0.37 m. There are no exposures though it was previously seen near Knowl Hill (Geological Survey photographs A869–70). Core from 129.74 to 130.11 m depth in the Staines No. 5 Borehole, held in the BGS archive, is a reference section.

Reading Formation (RB)

Present throughout the district, the most extensive outcrops are in the north-west around Knowl Hill [SU 825 795] and Waltham St Lawrence [SU 830 765]. They give rise to gently undulating topography with dark brown clay and loam soils.

Colour-mottled clays are the dominant lithology, with subordinate silt and fine- to medium-grained sand. The sequence is arranged in a series of fining-upwards cycles; ideally, a complete cycle starts with a cross-bedded sand, grades upwards into sporadically burrowed, thinly laminated silt and sand, and culminates in mottled clays with rootlet traces and white carbonate nodules of pedogenic origin. A fluctuating water table during and immediately after deposition of the clays led to emergent conditions and precipitation of these nodules, which range up to the order of 30mm in diameter. It also resulted in the development of iron minerals in various oxidation states giving rise to the colour mottling characteristic of the formation. Colours vary from pale dull brown with pale grey-blue reduction patches to bright yellow-brown with crimson and purple patches. In addition, beds of dark purple to black clay occur locally; such beds were a prominent feature in the temporary sections seen during this survey at Knowl Hill.

Beds of well-sorted sand appear to be thickest and most prominent in the middle part of the formation. They have sharp bases, and are laterally impersistent; they probably represent channel fills and are likely to have steep sides, but there is little detail of them in this district. One such sand, 5 to 8 m thick, is particularly widespread between Slough and Heathrow Airport (Figure 4). Other lenticular sands are present at similar stratigraphical levels in the M4 investigation boreholes between White Waltham and Bray.

Apart from Knowl Hill there are no exposures of the Reading Formation, but the core from Staines No. 5 Borehole provides a reference section (Figure 6).

Thames Group (TGp)

This comprises two formations, the Harwich Formation and the London Clay Formation.

Harwich Formation (Har)

Throughout the London Basin the Harwich Formation comprises the dominantly sandy beds between the Lambeth Group and the London Clay. In the Windsor district, these were formerly known as the 'London Clay Basement Bed' (Dewey and Bromehead, 1915; Ellison et al., 1994). The formation is, in general, too thin to be mapped but former exposures prove up to 3 m of glauconitic and pebbly fine-grained sand and silt. A bed or beds of cemented calcareous sandstone, up to 0.3 m thick, have been described as forming ledges in The Cut at Paley Street [SU 868 759] (Dewey and Bromehead, 1915; Bromehead, 1918).

Above the mottled clays of the Reading Formation in the pit at Knowl Hill [SU 817 796] are about 6 m of sand-dominated strata (Figure 4); (Plate 1). These beds have been variously ascribed to the 'Twyford member' (Oldhaven Formation) (King, 1981, p.18), the 'Reading Beds' (Kennedy and Sellwood, 1970; S Mathers and R Goldring, personal communication) and the basal part of the London Clay (Sellwood, 1974). Here we suggest that they represent the Harwich Formation. The lowest beds are medium-grained, cross-stratified sands with clay intraclasts and scattered deeply weathered flint pebbles. Similar sands with soft-sediment deformation structures occur in the middle part of the sequence, and the highest 1.8m of the section consists of thinly interbedded fine sand and clay with Ophiomorpha burrows. There is a lateral facies change to interbedded clays and sands with Beaconites burrows in these uppermost beds.

There are no other exposures of the Harwich Formation. It may have been encountered in boreholes, especially in the west, for example at Shurlock Row, where 2 m are noted.

London Clay Formation (LC)

The thickest London Clay is 116 m, proved in a well at Chobham Place (SU96SE/4) [SU 9646 6377] in the east of the district. Although the logs of many old wells are not easily interpretable, it seems that the London Clay is thinner in the west where 105 m are proved, for example in the Wellington College Borehole [SU 831 634].

The London Clay consists of dark grey clay, weathering brown, with subordinate silt and fine-grained sand. Sand is particularly abundant at the base and top of the formation. Beds with calcareous cementstone concretions up to 0.4 m in diameter occur sporadically; some of these contain septarian calcite veins. Phosphate nodules occur rarely. Glauconite, in the form of small pellets and microcrystalline grains, is quite common in some of the more sandy beds and at other horizons. Pyrite is disseminated throughout the rock, occurring as a replacement of fossil shell debris, and as nodules up to 30 mm in diameter. In the weathered zone, it reacts with acidic groundwater and carbonates to produce crystals of selenite (calcium sulphate).

Five sedimentary cycles, denoted units A to E in an ascending sequence, have been recognised in the London Clay throughout the London Basin by King (1981). An idealised cycle commences with a thin bed of clay containing coarse sand grains, scattered, well-rounded, black flint pebbles and glauconite grains. This is overlain by clay in which the proportion of silt and fine sand gradually increases upwards, culminating in a fine sand. In regional terms, the sand content of each cycle increases towards the west, although there is insufficient evidence in the Windsor district alone to illustrate this.

Numerous seams of calcareous nodules, up to 100 mm in diameter, are present in the lowest 20 m of the London Clay, and are probably potentially important for regional correlation. Thin beds containing rounded black flint pebbles occur at the base of units B, D and E. The one at the base of unit D seems to be particularly widespread in the Windsor district, having been recorded in boreholes at Ottershaw Park and Chertsey, and at Bracknell (King, 1981, fig. 17). The base of unit B is similarly well defined in boreholes at Wellington College, Holloway Sanatorium and Brookwood (King, 1981).

Two boreholes (Figure 6), Staines No. 5 and Bracknell, provide virtually a complete succession through the London Clay Formation. The cores are retained in the BGS collections.

The only significant exposures noted during this survey were those in the uppermost sections of the workings at Knowl Hill.

The youngest part of the London Clay, corresponding to the top part of unit E, is known as the Claygate Member (ClgB). It forms a transition between clay-dominated London Clay and the succeeding sand of the Bagshot Formation. It gives rise to loamy soils and in places forms a small positive feature. The thickness is up to about 10 m at outcrop but there is little information about its extent or thickness at depth. The Claygate Member comprises a variable sequence of finely laminated fine-grained sand and clayey silt, with thinly interbedded clayey fine-grained sand. The base is commonly taken at the bottom of a sand, generally about 0.3 to 0.5 m thick. Currently there are no exposures. At Bracknell town centre, King (1981, p.62) recorded a 1.5 m-thick bed of laminated silty fine sand at the top of the member.

Bracklesham Group (BrB)

The Bracklesham Group is a lithostratigraphical unit of late Early and Middle Eocene age defined in the Hampshire Basin (Curry et al., 1977; Edwards and Freshney, 1987). Curry (1992) uses the term Bagshot Formation to describe its London Basin equivalents, comprising the Bagshot Beds, Bracklesham Beds and Barton Beds as used by Dewey and Bromehead (1915).

In this account, it is considered to be represented by the Bagshot, Windlesham and Camberley Sand Formations. These terms replace (with some adjustment of boundaries) the Bagshot, Bracklesham and Barton Beds of Dewey and Bromehead.

Bagshot Formation (BgB)

These strata were formerly known as Lower Bagshot Sands (Prestwich, 1847) and Bagshot Beds (Dewey and Bromehead, 1915). The term Virginia Water Formation was introduced by King (1981) but is not used here. It may correlate in part with units of the Wittering Formation in the Hampshire Basin (Curry et al., 1977; Edwards and Freshney, 1987; Plint, 1988), but as yet there is no conclusive proof of this.

The main outcrop of Bagshot Formation forms an escarpment, with small outliers (for example at Popeswood [SU 840 690], Cranbourne Chase [SU 935 743] and Virginia Water [SU 960 695]). The thickness of the formation is variable, ranging from about 40 m in the type area between Bagshot and Chertsey, to 20 m in the south-west. The base is sharp and probably erosional; the lower beds of the thicker sequences probably infill channels in the London Clay.

At the base is an impersistent bed containing rounded black flint pebbles, proved in the Bracknell area and around Virginia Water (King, 1981, pp.40 and 60; Dewey and Bromehead, 1915, p.33). The majority of the formation consists of fine-grained, pale yellow brown to pale grey quartzose sand with planar and cross-bedding. Subsidiary interbeds and lenticular-shaped bodies of pale grey clay, up to 0.3 m thick, occur sporadically. Good examples of these were exposed in the Kitsmead Lane Landfill site [SU 994 662] (Plate 2). These merge into thicker clay-dominated units, up to 9 m thick, in the top part of the Bagshot Formation, and were formerly assigned to the lower part of the Bracklesham Beds. They occur principally at Row Town [TQ 037 637], on the west side of St George's Hill [TQ 0805 6290] and at Hatch Farm [TQ 052 655]. A similar clay, the Swinley Clay Member (SwC), is mapped south and west of Ascot and proved in the Bracknell Borehole (Figure 6). It consists of 3.4 m of brown organic laminated clay with flasers and laminae of fine-grained sand and silt, and roots in the top part originating at a disconformity; above the disconformity is about 1 m of brecciated grey clay with burrows and clay intraclasts. The disconformity, marked by evidence of emergence, has regional significance and can probably be traced into the Wittering Formation in the Hampshire Basin (Curry et al., 1977; Edwards and Freshney, 1987; Plint, 1988).

There are several scattered minor exposures of the Bagshot Formation; the main ones are a 4 to 5 m section at Trumps Mill [TQ 0069 6757], and in railway cuttings north of Virginia Water Station [TQ 0010 6796] and east of Weybridge Junction [TQ 071 633].

Windlesham Formation (Wi)

As defined here, the Windlesham Formation comprises all but the lowermost parts of the former Bracklesham Beds of Dewey and Bromehead (1915) or the Middle Bagshot Sands (Prestwich, 1847).

The Bracknell Borehole proved 11.54 m of Windlesham Formation. Other boreholes typically prove 20 m, although many of the records for these are unreliable and interpretations made from them equivocal. The dominant lithology is dark green and brown, bioturbated sand and clay with coarse sand grade glauconite pellets that may constitute up to 70 per cent of the sediment. It is overlain by organic dark grey clay with lenticles of fine sand. At the base in the east, is the St Anne's Hill Pebble Bed (Plate 3). This is an impersistent bed (up to 3.5 m thick) of cross-bedded and imbricate, rounded, black flint pebbles and cobbles generally 2 to 10 cm in diameter, set in a matrix of medium-grained sand. The best exposure is in a disused quarry at The Dingle [TQ 0270 6743]. A similar bed of pebbles and glauconitic sand and silt (up to about 4 m thick), the Stanners Hill Pebble Bed, occurs at the top of the formation, also in the east of the district. It is not exposed, but was formerly dug at Stanners Hill [SU 997 630] and was proved in boreholes at the junction of the M3 and A322 at Bagshot [SU 919 627].

The only good exposures of Windlesham Formation, other than the pebble beds, are in a pit in the top part of the formation at Fox Hills [TQ 0138 6491].

Camberley Sand Formation (CaS)

This is equivalent to the former terms Barton Beds (Dewey and Bromehead, 1915) or Upper Bagshot Sands (Prestwich, 1847). It is thought to be equivalent to the lower parts of the Selsey Formation of the Hampshire Basin.

The formation crops out on the highest ground south of Bracknell to Sandhurst and Bagshot, where it forms prominent steep-sided features capped by river terrace deposits. The base is generally drawn at a marked break of slope caused by a spring line. Up to 70 m are present. It consists of a remarkably uniform sequence of yellow- brown, sparsely to moderately glauconitic, bioturbated, fine-grained sand. Sporadic pebbles are recorded in the basal metre or so. It is not clear whether this is a separate pebble bed or part of the Stanners Hill Pebble Bed. Locally, there may also be thin pale grey 'pipe' clays.

Core from the Bracknell Borehole that proved the lowest 16 m of Camberley Sand is the best reference material available from the Windsor districtA borehole drilled by BGS at Mytchett, near Farnborough in 1997 provides an additional reference section of some 35m. This provides direct evidence for correlation with the Selsey Formation of the Hampshire Basin.. A section of about 6 m was seen at Tower Hill [SU 9052 6664] and there are numerous small exposures between Caesar's Camp [SU 864 657], Surrey Hill [SU 890 640], Broadmore [SU 855 635] and Bagshot Heath [SU 910 615].

Quaternary deposits (Drift)

Drift (superficial) deposits that comprise postglacial and periglacial deposits cover about 30 per cent of the district. The outcrops of those deposits were, in general, determined during previous surveys but they have been altered locally on the evidence of new field observations such as open sections, topographical features, auger holes and borehole data.

The deposits of a precursor of the Thames river system that formed the main route of drainage from the London Basin into the North Sea from early Pleistocene time are River Terrace Deposits (Ninth Terrace to Sixth Terrace). They form a series of highly degraded river terrace gravels, formerly classified as 'Plateau Gravel', in the south of the district (Figure 8). The ancestral River Thames at the time of their formation was flowing well to the north of the district and the river terrace deposits in this district were laid down in precursors of the present Kennet and Blackwater systems (Gibbard, 1982) whose confluence probably lay to the west, between Reading and Wokingham. Erosional benches beneath the gravel deposits represent successive levels of rivers that progressively cut down through the bedrock during the slow uplift and tilting of southern Britain. The wide outcrop of the oldest of these deposits, the Surrey Hill Gravel (Ninth Terrace), was deposited in a particularly broad valley by a braided river carrying a great volume of sediment, mainly flints eroded from the Hampshire chalk downs. Such volumes can only have been made available as a result of intense periglacial activity causing erosion in the headwater valleys. In common with the other terrace deposits, the clasts also include a proportion of rounded flint (derived from the Palaeogene strata) and chert (from the Lower Greensand) brought in by smaller tributaries draining from the south (Figure 7). One of them probably deposited the Greensand chert-rich gravel, mapped as Sand and Gravel of Uncertain Origin capping St Georges's Hill.

Following the main Anglian glaciation, about 0.5 million years ago, the River Thames was diverted into its present valley and the modern river system was initiated. Aggradation of a newer series of river terrace deposits, Post-diversionary River Terrace Deposits, was initiated in the area. These deposits (Fifth Terrace to First Terrace) are generally well preserved in a 'staircase' caused by the progressive downcutting and southwards migration of the river channel, thought to be a reflection of continuing gentle uplift. The steeper gradients of the tributary Colne and Wey rivers gives rise to gently sloping fan-like spreads of terrace deposits at their confluence with the Thames. The terrace deposits in the present Thames valley are named (Figure 7) in accordance with the regional work of Gibbard (1985) and Bridgland (1994). Those in other valleys are numbered to indicate their relative ages, the highest number being allocated to the oldest deposits.

There are also a few terrace deposits for which there is insufficent data to make correlations with those of the major rivers. These are denoted River Terrace Deposits, undifferentiated on the map. The main outcrops are those associated with the River Bourne near Chobham [SU 975 620] and scattered examples in the Virginia Water [SU 980 680] area.

Correlation of the river terrace deposits is based on reconstruction of the former river profiles by plotting the levels of the base of the deposits. The history of their deposition is related to alternating warm and cold periods throughout the Quaternary. Deposition is thought to have taken place mainly in cold periods when river bedload increased dramatically due to intense periglacial weathering and high seasonal runoff. A braided river system became established in the valley and aggradation of sand and gravel took place, waning at the onset of the next warm period (Bridgland, 1994). In warm periods, a meandering river with a relatively low bed load lowered the river base level. Meander channels cut into bedrock and abandoned by the river may contain interglacial organic silt and clay deposits.

More detailed evidence of the development of the Kempton Park Gravel just to the east of the district (Gibbard et al., 1987) show that an organic silt within the gravels is about 35 000 years old. The silt was laid down at a time of climatic deterioration from a warm (probably the Upton Warren interstadial) to a continental cold climate.

Similar plant-bearing organic beds, indicative of cold and damp or swampy conditions, were described by Gibbard and Hall (1982) from within gravel deposits below the alluvium in the Colne Valley at Colnbrook [TQ 026 776] and West Drayton [TQ 053 792]. The gravels are continuous with the Shepperton Gravel of the Thames valley although they differ slightly in clast composition. The organic deposits have been dated as 11 000 to 13 500 BP, and are interpreted as being interstadial deposits of the last cold period, the Devensian.

Following the Devensian, the major rivers became established in their present valleys. The Alluvium represents the alluvial deposits of the current floodplain. Older Alluvium is a similar deposit found in abandoned floodplains, slightly elevated above present flooding levels. It has been mapped between Bray and Dorney [SU 930 790] and also in the valley of The Cut west of Jealott's Hill [SU 8685 7340].

In abandoned channels, thin deposits of Peat, usually less than 1 m thick, accumulated within the alluvium, for example at Bray [SU 9067 7890] and Windsor Marina [SU 9300 7728]. In the Windle Brook valley [SU 920 635] to [SU 925 630] peat is currently accumulating in marshy ground fed by springs in the Windlesham and Bagshot formations.

The Langley Silt (Gibbard, 1985; Gibbard et al., 1987) blankets much of the post-diversionary River Terrace Deposits; it consists largely of brown silt or 'brickearth'. Thicknesses are variable, ranging from a few centimetres to over 2 m. They are generally thought to be wind-blown deposits, some of which have been reworked during contemporary river flooding. Harding et al. (1991) have shown that the Langley Silt overlying the Lynch Hill Gravel at Maidenhead around [SU 876 832] is not a primary Pleistocene loess. Though there may be a wind-blown component to the deposit, it is described as colluvium, formed mainly through 'slope processes', and derived from deposits of Palaeogene age. A fossil soil within this Langley Silt was probably formed during an interglacial (temperate) period.

Periglacial activity and downwash since the Devensian was responsible for the formation of Head deposits. They are ubiquitous in the district but have not been mapped extensively. Thicknesses vary from a veneer of only a few centimetres to over 2 m. On lower, concave valley slopes head generally consists of sand and clay with variable amounts of gravel derived largely from sediments of Palaeogene age. On higher ground in the south of the district, head containing a high proportion of gravel occurs in solifluction lobes formed during periglacial erosion of the older, high-level river terrace deposits, for example at Caesar's Camp [SU 860 660] and Bagshot [SU 903 630].

Landslips are likely to develop on clay slopes of 7° or greater in the London Clay and Windlesham formations. They may occur also on slopes of lower angle, where ground that is saturated by springs has high pore pressures and consequent low strength.

Landslips on slopes oversteepened by river undercutting occur at Cooper's Hill [SU 995 720] and St Ann's Hill [TQ 0265 6775]. A locality at Ashley Hill [SU 824 811] was described as a landslip of London Clay by Dewey and Bromehead (1915, p.81) but no unequivocable evidence was seen during this survey (p.19).

Artificial Deposits and Worked Ground

Made Ground is shown in areas where material is known to have been deposited by man upon the natural ground surface. The main categories are spoil from mineral extraction, building and demolition rubble, and waste in raised landfill sites. The composition of the deposit is not known at most localities. Excepting the widespread use of made ground in the landscaping of urban and industrial areas, the most extensive areas of made ground are road and railway embankments and reservoir retaining banks; for clarity many of these have been omitted from the 1:50 000 Series map.

Worked Ground is where natural materials are known to have been removed, for example in quarries and pits, excavations for roads and railways and in general landscaping. In the district, some of the large former gravel pits of the Thames valley are not backfilled, and are shown as Worked Ground. Due to the water table being close to the ground surface these pits are now lakes used for leisure purposes.

Infilled Ground is shown where the natural ground has been removed and the void partly of wholly backfilled with man-made deposits, the composition of which is generally not known. The types of fill are the same as those making Made Ground. Many former gravel pits in the Thames valley have been used for the disposal of waste materials. The largest of these are at Holyport [SU 906 777] and Stanwell Moor [TQ 035 745]. Sites operating at the time of the resurvey are at Trumps Farm [SU 995 664], Downfield Lane [SU 8375 7625] and Longhill [SU 8925 6940].

Where quarries and pits have been filled, the ground restored and landscaped, built on or returned to agricultural use, there may be no surface indication of the extent of the backfilled area. In such cases, the boundaries of these sites is taken from archival sources such as local authority records and old topographical and geological maps.

Disturbed Ground is associated with areas thought to be former excavations and tips that are now ill-defined, for example on the outcrop of Lambeth Group at Knowl Hill [SU 8235 7945] and [SU 825 793].

Structure

Structures in the concealed strata are poorly known. The district lies to the north of the main Variscan Front that is characterised by east–west-trending faults, although it is possible that such faults may occur in the south of the district.

The exposed strata lie on the northern limb of the London Basin, a broad synclinorium with a roughly east–west axis. The regional dip of the Palaeogene strata is in the order of 0.6° towards the south-east. A regional set of north-west-trending normal faults, several of which are mapped in the Windsor district, is known in the London Basin (Bevan and Hancock, 1986; Oswald, 1993). They are inferred to have been initiated by movements along faults within the London Platform 'basement' and to have propagated upwards through the Mesozoic strata. Movement on them was contemporary with development of the periclines with dips up to 6° in the Chalk and Palaeogene strata at Windsor Castle [SU 97 77] and Staines [TQ 045 720]. The London Clay involved in the latter structure and nearby zones of relatively steep dip exhibits bedding-parallel shear zones (Chandler et al., 1998).

Chapter 3 Applied geology

Geological factors may be important in influencing the suitability, siting and nature of future urban and industrial developments. By giving consideration to geological matters at an early stage in the planning process it may be possible to mitigate some of the problems commonly encountered. The key earth-science related factors relevant to the Windsor district are discussed briefly below.

Geology has played a significant role in the industrial heritage of the district. In particular the area has been extensively worked for sand and gravel; many of the resultant voids are filled with water and used for recreation and sites of wildlife conservation, others for landfill. The rather variable consolidated clays, silts and sands that dominate the outcrops in the district give rise to variable foundation conditions. Other important factors influenced by the geology are the water resources, the risk of flooding along the major rivers, ground subsidence, slope stability and pollution potential.

Water resources

The Chalk is the most important aquifer and locally may be in hydrogeological continuity with the Thanet Sand and Upnor formations. Water supplies are typically hard. The fine-grained nature of the Chalk gives it a high porosity but a low, intrinsic permeability. A secondary porosity related to interconnected fracture systems within the upper part of the aquifer gives a high permeability and transmissivity resulting in correspondingly low hydraulic gradients and a generally flat water table. Over most of the district it is confined by the London Clay Formation and the Lambeth Group which are effective aquicludes.

Water is obtained in small quantities from the sands of the Palaeogene, most notably the Bagshot Formation, and locally also from perched water tables within the Quaternary deposits. Supplies from these minor aquifers are generally not particularly reliable, being for the most part intermittent and of variable quality and quantity. Some of the Lower Cretaceous strata, especially the Lower Greensand, may yield significant supplies of soft groundwater.

Water supplies for the district, and much of London, are also abstracted from the River Thames and stored in the large reservoirs around Staines. The Thames is also a major source of recharge for the Chalk aquifer. The aquifer is recharged directly from the Thames through the river gravels at Bray [SU 895 803].

Information about the wells and abstraction boreholes in the district are given in Murray et al. (1966), and general aspects of the hydrogeology of the London Basin, including the Windsor district, are dealt with in Downing et al. (1972). The general characteristics of the chalk aquifer are described by Allen et al. (1997).

Rising groundwater

Groundwater levels fluctuate with time and the degree of extraction. Rising levels are not identified as a significant problem in the district at the present time.

Groundwater pollution

Leakage and spillage of contaminants may cause pollution of surface water courses and groundwater, and lead to costly remedial programmes. An example of contamination by jet fuel (kerosene) at Heathrow Airport [TQ 075 755] is reported by Clark and Sims (1998). Sites of former industries, such as gas works, tanners and sewage works, are potential sources of chemical pollution. The overuse of artificial fertilisers is a major source of nitrate pollution.

Surface mineral workings

These include former or active quarries or pits which have not been backfilled. They are an important resource that may be suitable for waste disposal, for re-opening as a source of minerals, or may be developed as sites of educational, recreational or wildlife conservation value. Quarries and pits are distributed throughout the district; the great majority of them are excavations for sand and gravel. Most options for landfill sites have already been used and there is increased pressure on current and future workings, most of which are close to urban development.

The main resources of current and historical importance are described in (Figure 8).

Foundation conditions

There are relatively few potential problems connected with ground stability in the district. (Figure 9) lists some of the potential ground constraints commonly associated with each deposit.

In valleys, alluvium, head and peat can pose problems because of variability and low bearing capacity. The most significant areas of peat are in the Windle valley [SU 917 633] to [SU 942 621] and the valley between Ascot and Sunninghill [SU 927 681] to [SU 941 691].

The locally unpredictable nature of the Palaeogene strata may present problems during construction, for example small bodies of clay within units dominated by sands and vice versa. Loose sands, prone to erosion and gullying, are a feature of the Bagshot and Camberley Sand formations. The development and reactivation of springs at the base of these formations, especially after periods of heavy rain, may cause erosion and burst-out of the sands.

Ground Heave

The London Clay and Reading formations are dominated by clays. These, on account of their high smectite content, may undergo significant volume change with variation in moisture content. Seasonal effects, in which vegetation, especially trees, plays a dominant role, were identified by Driscoll (1983) as the most important controls on shrinking and swelling. The clays absorb high quantities of water during wet periods and lose it again during droughts. Drying clays may crack and swell causing structural damage to roads and buildings.

High concentrations of sulphate in the ground or groundwater can weaken concrete foundations that are not designed to resist this chemical attack. Unweathered Palaeogene formations contain pyrite (iron sulphide). In the weathered zone near the surface the pyrite is oxidised to give sulphate ions in solution. In clay-dominated formations, particularly the London Clay, calcium carbonate present may react with the sulphate to precipitate selenite crystals. This involves an eight-fold increase in volume over the original pyrite and can cause disruption and weakening of the strata. When the selenite itself is subjected to weathering, sulphuric acid is produced which reacts with cement not designed to be sulphate resistant, causing it to break down.

Mass-movement and slope stability

Few landslips are mapped in the district. Slopes of 7° or greater on clay are prone to ground failure. These are likely to have been oversteepened by recent or ancient river undercutting. Landslips on slopes oversteepened by river undercutting are mapped on the northern slopes of St. Ann's Hill [TQ 0265 6775], Lynwood Farm [TQ 0190 6585], near Cranbourne Chase [SU 9264 7449] and at Cooper's Hill [SU 995 720]. A locality at Ashley Hill [SU 824 811] was described as a landslip of London Clay by Dewey and Bromehead (1915, p.81) but no unequivocable evidence was seen during this survey. Between Cruchfield Manor House [SU 878 741] and New Lodge [TQ 912 750] the London Clay slopes are oversteepened but no landslips are recorded. Instability may also be present on lower-angle slopes where ground is saturated by springs giving rise to high pore pressures and loss of strength. Such situations may occur on the outcrops of the Claygate Member and at the base of the Bagshot and Camberley Sand formations.

Many slopes on the London Clay Formation are covered with a veneer of Head deposits that may not be shown on the geological maps. Culshaw and Crummy (1991) suggested that those slopes greater than 3° should also be considered as potentially unstable. The Head, composed of redeposited London Clay, including the Claygate Member, and an element of sandy and gravelly wash from overlying formations, accumulated by downslope solifluction and soil creep. It may contain relict shear surfaces and the deposits are likely to have shear strengths at, or close to, their residual value. Reactivation of the shear surfaces may occur if the slopes are undercut, loaded or saturated, or if the water table rises. Dewey and Bromehead (1915, pp.81–82) describe solifluction and cryoturbation-induced structures affecting the London Clay and Bagshot Formations. River Terrace Gravels may also be affected.

Slope stability may also be affected by the presence of beds of relatively weak strata. For example, low-angle, bedding-parallel shear zones within the London Clay have caused slope failure in borrow pits at Prospect Park [TQ 057 782] (Chandler et al., 1998).

Chalk dissolution

Swallow or sink holes are a feature of Chalk valleys in the Chiltern Hills and North Downs, but none have been identified on the Chalk outcrop of the Windsor district. However, irregularities on the Chalk surface, especially along the boundary between the Chalk and the Lambeth Group, may in part be due to these. The reaction between acid surface run-off and the Chalk (Water Resources Board, 1972) causes dissolution which enlarges naturally occurring fractures within the Chalk leading to the formation of cavities or pipes, generally less than 2 m deep and infilled with Chalk breccia or Quaternary gravel and silt. These are sources of potential ground instability.

An irregular chalk surface is likely beneath River Terrace Deposits in the north-east of the district, probably due to dissolution of chalk after deposition of the terrace gravels (Harding et al., 1991).

Gas emissions

The backfilling of former pits and quarries with domestic and industrial refuse produces methane, a potentially explosive gas. Large quantities of methane are flared-off at the Kitsmead Lane [SU 9940 6620] landfill site beside the M3 motorway near Virginia Water and to a lesser extent at London Road [SU 895 691], Bracknell. Methane being generated at older landfill sites is closely monitored, for example at Longhill [SU 8925 6940], Callow Hill [SU 995 691] and near Amen Corner [SU 850 692]. Migration of methane from the sites is prevented by barriers. However, the areas around sites located on permeable formations such as the Bagshot and Camberley Sand formations require particularly close monitoring.

Natural radon emissions

Radon is a naturally occurring radioactive gas that is produced by the radioactive decay of radium which, in turn, is derived from the radioactive decay of uranium. Uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. Radon is released from rocks and soils, and is quickly diluted in the atmosphere. Concentrations in the open air are normally very low and do not present a hazard. Radon that enters poorly ventilated enclosed spaces such as some basements, buildings, caves, mines and tunnels may reach high concentrations in some circumstances. Radon levels in individual buildings can be influenced by the construction method and the degree of ventilation. Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer. If individuals are exposed to high concentrations for significant periods of time, there may be cause for concern. In order to limit the risk to individuals, the Government has adopted an 'Action Level' for radon in dwellings of 200 becquerels per cubic metre (Bq m⁻³). The National Radiological Protection Board (NRPB) (Lomas et al., 1996; Miles et al., 1996) has drawn up maps of radon-affected areas which are those areas of the UK with a probability of 1 per cent or more of homes being above the Action Level. The NRPB maps show the estimated proportion of homes exceeding the Action Level for each 5 km square of the Ordnance Survey National Grid.

The variation in radon levels between different parts of the country is mainly controlled by the underlying geology. Within the Windsor district radon potential is generally low, with the highest radon concentration values identified on areas of the Chalk outcrop. In these areas, between 1 to 3 per cent of properties are likely to exceed the UK 'Action Level'.

Flooding

Low-lying ground adjacent to active stream courses may be prone to flooding during periods of exceptional rainfall. A major flood alleviation scheme is currently in progress to prevent flooding of the Thames between Maidenhead and Eton. It involves the excavation of a new, straightened river course and construction of new flood embankments.

Information sources

BGS publications relevant to the Windsor district and adjoining areas are listed below.

Books

• British Regional Geology
• London and the Thames Valley, 4th edition, 1996
• Memoirs
• Geology of the country around Windsor and Chertsey (Sheet 269), 1915
• Geology of the country around Aldershot and Guildford (Sheet 285), 1929
• Geology of South London (Sheet 270),1921
• Geology of the country around Beaconsfield (Sheet 255), 1922
• Water supply papers of the Geological Survey of Great Britain, Well Catalogue Series
• Records of wells in the area of New Series One-Inch (Geological) Windsor (269) Sheet. K H Murray et al., 1966 (NERC)

Maps

• Geological maps
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• United Kingdom (South Sheet) Solid Geology, 1979; Quaternary Geology, 1977
• 1:250 000
• Chilterns 51N 02W, Solid Geology, 1991
• 1:50 000 and 1:63 360
• Sheet 269 Windsor (Solid & Drift) 1999
• Sheet 255 Beaconsfield (Solid & Drift) 1974
• Sheet 270 South London (Solid & Drift) 1998
• Sheet 285 Aldershot (Solid & Drift) 1928–1976
• Sheet 268 Reading (Solid & Drift) 1971
• 1:10 000
• For the most recent revision of the 1:50 000 Series Sheet 269 Windsor, the component 1:10 000 National Grid maps are listed below, together with the initials of the surveyors and dates of survey.

• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas (1996)
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas (1996)
• 1:250 000
• Magnetic anomaly map Chilterns 51N 02W (1991)
• Bouguer anomaly map Chilterns 51N 02W (1991)
• 1:50 000
• A geophysical information map (GIM) at the scale of 1:50 000 is available for this district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.
• Geochemistry maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain — south (1995)
• Radon potential based on solid geology, Great Britain — south (1995)
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain — south (1995)
• Hydrogeological maps
• 1:100 000
• Hydrogeological map of the south-west Chilterns and the Berkshire and Marlborough Downs (Sheet 7) (1978)
• Hydrogeological map of the area between Cambridge and Maidenhead (Sheet 14) (1984)
• Groundwater vulnerability of west London (1994)
• Minerals maps
• 1:1 000 000
• Industrial minerals resources map of Britain (1996)

BGS reports

Ellison, R A. 1997. Geological information on the Chertsey and Weybridge area. British Geological Survey Technical Report, WA/97/37.

Clarke, M R, Dixon, A J, and Kubala, M. 1979. The sand and gravel resources of the Blackwater Valley (Aldershot) area: Description of 1:25 000 sheets SU85, 86 and parts of SU84, 94, 95 and 96. Mineral Assessment Report Institute of Geological Sciences, No. 39.

Dunkley, P N. 1979. The sand and gravel resources of the country around Maidenhead and Marlow: Resource Sheet SU88 and parts 87, 97, 98. Mineral Assessment Report Institute of Geological Sciences, No. 42.

BGS photographs

The BGS Photographic Collection houses a number of photographs from the Windsor district. The relevant numbers are listed below along with the dates they were taken. Copies of these may be viewed at BGS libraries in Keyworth and Edinburgh.

Document collections and databases

The collections of the National Geological Records Centre at BGS Keyworth hold large numbers of borehole and other records relevant to the district. These may be consulted by contacting The Manager, NGRC, at BGS Keyworth.

Data on water boreholes, wells and springs and aquifer properties are held in the BGS (Hydrogeology Group) database at Wallingford.

Location details of additional boreholes referred to in the text

Name Number Grid Reference

References

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

Allen, D J, Bloomfield, J P, and Robinson, V K (editors). 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34.

Bevan, T G, and Hancock, P L. 1986. A late Cenozoic regional mesofracture system in southern England and northern France. Journal of the Geological Society of London, Vol. 143, 355–362.

Blake, J H. 1902. The water supply of Berkshire from underground sources. Memoir of the Geological Survey of Great Britain.

Bridgland, D R. 1994. The Quaternary of the Thames. Geological Conservation Review Series: Joint Nature Conservation Committee. 441pp. (London: Chapman and Hall.)

Bristow, C R, Mortimore, R N, and Woods, C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 108, 293–315.

Bromehead, C E N. 1918. Excursion to Maidenhead and Bray Cut. Proceedings of the Geologists' Association, Vol. 29, 137–139.

Chandler, R J, Willis, M R, Hammilton, P S, and Andreou, I. 1998. Tectonic shear zones in the London Clay Formation. Geotechnique, Vol. 48, No. 2, 257–270.

Clark, L, and Sims, P A. 1998. Investigation and clean-up of jet-fuel contaminated groundwater at Heathrow International Airport, UK. 147–157 in Groundwater contaminants and their migration. Mather, J, Dumpleton, S, and Fermor, M (editors). Geological Society of London Special Publication, No. 128.

Clarke M R, Dixon, A J, and Kubula, M. 1979. The sand and gravel resources of the Blackwater Valley (Aldershot) area. Description of 1:25 000 sheets SU85, 86 and parts of SU84, 94, 95 and 96. Mineral Assessment Report of the Institute of Geological Sciences, No. 39.

Clarke, M R, and Dixon, A J. 1981. The Pleistocene braided river deposits in the Blackwater Valley area of Berkshire and Hampshire, England. Proceedings of the Geologists' Association, Vol. 92, 139–157.

Culshaw, M G, and Crummy, J A. 1991. S W Essex–M25 Corridor: Engineering geology. British Geological Survey Technical Report, WN 90/2.

Curry, D. 1965. The Palaeogene beds of south-east England. Proceedings of the Geologists' Association, Vol. 76, 151–173.

Curry, D. 1992. Tertiary. 389–411 in Geology of England and Wales. Duff, P McL D, and Smith, A J (editors). (London: Geological Society of London.)

Curry, D, King, A D, King, C, and Stinton, F C. 1977. The Bracklesham Beds (Eocene) of Bracklesham Bay and Selsey, Sussex. Proceedings of the Geologists' Association, Vol. 88, 243–254.

Dewey, H, and Bromehead, C E N. 1915. The geology of the country around Windsor and Chertsey. Memoir of the Geological Survey of Great Britain. Sheet 269 (England and Wales).

Dewey, H, and Bromehead, C E N. 1921. The geology of South London. Memoir of the Geological Survey of Great Britain. Sheet 270 (England and Wales).

Dewey, H, Pringle, J, and Chatwin, C P. 1925. Some recent deep borings in the London Basin. Summary of Progress of the Geological Survey of Great Britain: for 1924, 127–137.

Dines, H G, and Edmunds, F H. 1929. The geology of the country around Aldershot and Guildford — Explanation of Sheet 285 (England and Wales). Memoir of the Geological Survey of Great Britain.

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

Dunkley, P N. 1979. The sand and gravel resources of the country around Maidenhead and Marlow: Resource Sheet SU88 and parts 87, 97, 98. Mineral Assessment Report of the Institute of Geological Sciences, No. 42.

Edwards, R A, and Freshney, E C. 1987. Lithostratigraphic classification of the Hampshire Basin Palaeogene Deposits (Reading Formation to Headon Formation). Tertiary Research, Vol. 8, 43–73.

Ellison, R A. 1983. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proceedings of the Geologists' Association, Vol. 94, No. 4, 311–319.

Ellison, R A, Knox, R W O'B, Jolley, D W, and King, C. 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists' Association, Vol. 105, 187–197.

Gardner, J S, Keeping, H, and Monkton, H W. 1888. The Upper Eocene, comprising the Barton and Upper Bagshot Formations. Quarterly Journal of the Geological Society of London, Vol. 44, 578–635.

Gibbard, P L. 1982. Terrace stratigraphy and drainage history of the Plateau Gravels of north Surrey, south Berkshire and north Hampshire, England. Proceedings of the Geologists' Association, Vol. 93, 369–384.

Gibbard, P L. 1985. The Pleistocene history of the Middle Thames Valley. (Cambridge: Cambridge University Press.)

Gibbard, P L, Coope, G R, Hall, A R, Preece, R C, and Robinson, J E. 1981. Middle Devensian deposits beneath the 'Upper Floodplain' terrace of the River Thames at Kempton Park, Sunbury, England. Proceedings of the Geologists' Association, Vol. 93, 275–289.

Gibbard, P L, and Hall, A R. 1982. Late Devensian river deposits in the Lower Colne valley, west London, England. Proceedings of the Geologists' Association, Vol. 93, 291–299.

Gibbard, P L, Wintle, A G, and Catt, J A. 1987. Age and origin of clayey silt 'brickearth' in west London, England. Journal of Quaternary Science, Vol. 2, 3–9.

Harding, P, Bridgland, D R, Madgett, P A, and Rose, J. 1991. Recent investigations of Pleistocene sediments near Maidenhead, Berkshire, and their archaeological content. Proceedings of the Geologists' Association, Vol. 102, No. 1, 25–63.

Kennedy, W J, and Sellwood, B W. 1970. Ophiomorpha nodosa Lundren, a marine indicator from the Sparnacian of South-East England. Proceedings of the Geologists' Association, Vol. 81, 99–110.

King, C. 1981. The stratigraphy of the London Clay and associated deposits. (Rotterdam: W Backhuys.)

Lomas, P R, Green, B M R, Miles, J C H, and Kendall, G M. 1996. Radon atlas of England. Report of the National Radiological Protection Board, No. R290.

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

Murray, K H, and others. 1966. Records of wells in the area of New Series One-Inch (Geological) Windsor (269) Sheet. Water Supply Papers of the Geological Survey of Great Britain, Well Catalogue Series. (London: Natural Environment Research Council.)

Oswald, D H. 1993. Here are geology, potential of England's Windsor anticline. Oil and Gas Journal, Vol. 91, No. 47, 85–87.

Plint, A G. 1988. Global eustacy and the Eocene sequence in the Hampshire Basin. Basin Research, Vol. 1, 1–11.

Potter, J F. 1977. Eocene, lower Bracklesham Beds iron workings in Surrey. Proceedings of the Geologists' Association, Vol. 88, 229–241.

Prestwich, J. 1847. On the main points of structure and the probable age of the Bagshot Sands. Quarterly Journal of the Geological Society of London, Vol. 3, 378–409.

Sellwood, B W. 1974. Tertiary beach deposits east of Reading associated with the London Clay transgression. Geological Magazine, Vol. 111, 80–83.

Sumbler, M G (compiler). 1996. British regional geology: London and the Thames Valley (4th edition). (London: HMSO for the British Geological Survey.)

Water Resources Board. 1972. The hydrogeology of the London Basin — with special reference to artificial recharge. (Reading: Water Resources Board.)

Whitaker, W. 1872. The geology of the London Basin. Memoir of the Geological Survey of Great Britain, Part 1.

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Summary of the geological succession in the Windsor district.

(Figure 2) Location of boreholes proving strata below the Chalk.

(Figure 3) Summary information of boreholes proving strata below the Chalk

(Figure 4) Selected boreholes and section (Knowl Hill) proving the Lambeth Group.

(Figure 5) Simplified geological map showing the location of selected boreholes proving the Lambeth Group (see Figures 4 and 6).

(Figure 6) Graphic sections of Staines No. 5 and Bracknell boreholes. Key to (Figure 6)

(Figure 7) Classification and clast composition of River Terrace Deposits.

(Figure 8) Mineral resources of the district.

(Figure 9) Potential ground constraints

Plates

(Plate 1) Knowl Hill quarry. The Harwich Formation comprises up to 6 m of cross-bedded and laminated sands with soft sediment deformation structures. The highest 1.8 m of the section consists of thinly interbedded fine sand and clay with Ophiomorpha burrows. There is a lateral facies change to interbedded clays and sands with Beaconites in these uppermost beds. A thin, lenticular clay is seen below (GS369).

(Plate 2) Bagshot Formation at Kitsmead Lane landfill site [TQ 9945 6615]. Here a bed of clay occurs within the otherwise sand-dominated sequence (GS553).

(Plate 3) St Anne's Hill Pebble Bed exposed at the base of the Windlesham Formation [TQ 0165 6770] (GS554).

(Plate 4) Sarsen blocks have been used in this bridge, and for facing the adjacent dam at Virginia Water [SU 9785 6855] (GS555).

(Plate 5) Photograph from the archives showing the construction of Queen Mary Reservoir [TQ 07 70] in 1921. Bedded gravel (river terrace deposits) on the right of the picture and London Clay in the foreground was removed in benches (A1898).

(Front cover) Cover photograph: Oblique aerial view of Windsor Castle from the east. Windsor Castle is built upon an inlier of the Upper Chalk (Seaford Chalk Member) that owes its existence to a combination of folding and faulting at depth. Much of the surrounding urban area overlies either London Clay or much younger Quaternary Thames river terrace deposits.

(Rear cover)

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

Figures

(Figure 3) Summary information of boreholes proving strata below the Chalk

(Figure 7) Classification and clast composition of River Terrace Deposits

(Figure 8) Mineral resources of the district

(Figure 9) Potential ground constraints