Content and licensingview original scan buy a printed copy

Geology of the Beaconsfield district — a brief explanation of the geological map sheet 255 Beaconsfield

A N Morigi, M A Woods, H J Reeves, N J P Smith and R J Marks

Bibliographic reference: Morigi, A N, Woods, M A, Reeves, H J, Smith, N J P, and Marks, R J. 2005. Geology of the Beaconsfield district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 255 Beaconsfield (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2005. © NERC 2005. 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 NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty's Stationery Office. © Ordnance Survey licence number 100017897/2005.

(Front cover) Old brickworks at Poyle Farm, Burnham [SU 925 841] (Photograph: Roy Mogdridge; (GS1240))

(Rear cover)

(Geological succession) Geological succession of the Beaconsfield district

Notes

The word 'district' refers to the area of Sheet 225 Beaconsfield. National Grid references are given in square brackets; unless otherwise specified they lie within 100 km square SU; specific locations and boreholes are accompanied by their national grid reference at their first mention within the text. 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 SU34SE, followed by its registration number in the BGS National Geological Records Centre. Selected water wells from the BGS National Well Record Archive are prefixed by the code of the 100 km2 area upon which the site falls, for example SU24 followed by its registration number. Lithostratigraphical symbols shown in brackets in the text, for example (LeCk), are those shown on the published map. Numbers preceded by the letters GS refer to the BGS photograph collection.

Acknowledgements

The authors' thanks are due to the many landowners, local authorities, utility and site investigation companies for access to land and provision of geological information. The manuscript was edited by A A Jackson; figures were prepared in the Cartographic Section (Keyworth) by R J Demaine, P Lappage and G Tuggey. Photography by R T Mogdridge.

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

(Rear cover)

This explanation of the geology of the Beaconsfield district recounts its geological history and describes the characteristics of the rocks from early Palaeozoic times to the very recent past. The account begins with a description of the basement rocks, which, following uplift, formed a narrow east–west-trending landmass known as the London Platform. The platform remained an important influence on sedimentation in the region from the late Palaeozoic into early Cretaceous times, with the deposition of carbonate and clay beginning to encroach upon it in the later part of the Jurassic.

At the beginning of the Cretaceous, the district was inundated by the sea and the London Platform was buried first by the deposition of sand and clay and later a thick sequence of chalk. The youngest formations of the chalk are the oldest rocks that outcrop in the district: they give rise to the characteristic landscape and scenery of the Chiltern Hills. Following a period of uplift, sedimentation began anew in the Cainozoic (Tertiary) when sediments of the Lambeth and Thames groups were deposited.

By the beginning of the Quaternary, 'Britain' had moved into northern latitudes, coinciding with a prolonged cooling of the Earth's climate. This was to have a profound effect on the geology and scenery of the district. During the Middle Pleistocene, with the district once more land, cycles of 'cold' and 'warm' climatic periods superimposed upon the overall cooling trend gave rise to flights of river terrace deposits of an ancestral river Thames. At this time, the early or proto-Thames flowed across the district from the west, through the Watford area, and then north of London through the Vale of St Albans. The advance of an ice sheet as far south as Watford about 430 000 years ago blocked this course, diverting the river into its present course.

A brief account of the applied geology of district includes a description of the mineral resources, engineering geology and geohazards as well as the hydrogeogology of the important Chalk aquifer.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the 1:50 000 Series Sheet 255 Beaconsfield, and also indicates sources of additional geological information. The district lies immediately to the west of London, mainly in Buckinghamshire but includes part of Hertfordshire, the London Borough of Hillingdon, and the unitary authorities of Slough and Windsor and Maidenhead. The area is a prosperous and busy commuter belt in which the boundary between suburbs and countryside is blurred. Nevertheless, many places still retain the distinctive rural character of the Chilterns and beech woodland flourishes on many hilltops. Slough, much despised for its industrial ugliness by the poet John Betjeman ('Come, friendly bombs, and fall on Slough …'), is probably the main commercial centre, although both Watford and High Wycombe possess some small-scale industry.

The Chiltern Hills occupy the north-west of the district (Plate 1). The landscape is one of rounded hills, typical of the chalk downland, and many of the hills are capped by Quaternary deposits of clay-with-flints. The regional dip is to the south-east, and ground level falls southwards across the district, via a flight of broad flat river terrace deposits of the ancient River Thames, to the present floodplain of the river. Much of the south and east of the district is underlain by the Lambeth Group and London Clay Formation although, for the most part, these are concealed by the river terrace deposits and outcrop mainly on the relatively high ground to the east of the River Colne.

The oldest rocks proved by a borehole within the district are of late Silurian age and comprise mudstone and limestone deposited in a shallow sea. Earth movements during the Caledonian orogeny resulted in the uplift of the Wales-Brabant Massif to the north, and erosion provided sediment that was deposited on a low-lying plain which covered the district. Thus, several deep boreholes within and around the district have encountered Devonian strata consisting of mudstone and sandstone mainly of fluvial origin. In early Carboniferous times the region was covered once more by shallow sea. Intense earth movements, associated with the Variscan orogeny that culminated in late Carboniferous times, resulted in uplift of the region. The Permian and early Triassic was a period of erosion, so that Carboniferous, Permian and early Jurassic strata are absent in the district. By the end of the Palaeozoic Era the main structural elements that subsequently controlled deposition in the region were in place, and the Beaconsfield district lay on the London Platform, the southern flank of the Anglo-Brabant Massif.

As part of the London Platform, the district remained above sea level for some 200 million years, into the early Mesozoic. However, Triassic and early Jurassic sediments were deposited in adjacent basins, which, owing to changes in sea level due to regional subsidence, gradually began to encroach upon the platform. Mudstones and limestones of the Great Oolite Group have been proved in a borehole at Little Missenden where they rest on Silurian rocks, indicating that deposition did not resume in the district until mid Jurassic times. During the remainder of the Jurassic Period, deepening seas accommodated the mud-dominated sequence that makes up the Ancholme Group.

At the end of this period, Late Cimmerian earth movements, related to the opening of the northern Atlantic, raised the London Platform above sea level exposing the Jurassic and Palaeozoic strata to erosion. However, during the early Cretaceous, the sea flooded the region so that by Aptian times the Lower Greensand had been deposited over much of the district, and by Albian times mud of the Gault Formation was deposited across the whole of the London Platform. By the Late Cretaceous, sea level had risen so that the entire region, and indeed much of Europe, had been inundated. With sources of terriginous sediments far removed beyond the region, calcareous pelagic deposits of the Chalk Group dominated sedimentation.

Latest Cretaceous and early Palaeogene uplift caused widespread erosion of the Chalk. A marine incursion followed, leading to the deposition of the shallow marine and coastal deposits of the Lambeth Group and, as sea level continued to rise, the mudstone of the London Clay Formation. Tectonic movements associated with the Alpine orogeny, which began in the late Palaoegene and reached a peak in the Neogene (Miocene), resulted in uplift and once more the area was raised above sea level. The London Basin syncline was formed during this period thus providing the structural framework for the district.

During the Quaternary, the global climate cooled and oscillated between cold and warm periods. These variations are represented in the deposits of the Beaconsfield district where the climate alternated between warm temperate and glacial. An ancestral River Thames traversed the area, passing to the north of its present course, through the Vale of St Albans and across southern East Anglia to the sea. The former course is marked by a wide belt of sand and gravel deposits disposed in a complex series of river terraces. Following downcutting by the fast-flowing river at the maximum of a cold climatic stage, the waning flow of the river in the latter part of the stage spread its sediment load of sand and gravel across a wide braidplain. During the succeeding temperate stage flow rates were low, the river sinuous and sediments muddy and much less extensive. Substantially increased flow early in the next cold stage brought further deposition of sand and gravel before renewed downcutting at the stage maximum. Contemporaneous continuous gentle uplift of the region caused the river to cut down to a new lower level at the end of each depositional cycle. Many alternating cold-warm episodes resulted in many repetitions of this cycle thus forming a staircase of terraces down the valley sides above the present day floodplain, each underlain by sand and gravel deposits (Bridgland, 1994). Extensive remnants of these deposits are present throughout the district. Some 430 000 years ago, a vast ice sheet advanced south into the Vale of St Albans, ultimately causing the diversion of the Thames onto its present course. Alternating climatic phases continued through the Quaternary with further deposition of river terrace deposits in the south and east of the district. Finally, about 10 000 years ago, the climate assumed its most recent temperate phase, and the River Thames and its tributaries adopted their present slow meandering patterns. The silty clay deposits of the floodplain alluvium accumulated at this time.

Chapter 2 Geological description

Only Upper Cretaceous and younger strata outcrop in the Beaconsfield district; older strata are known only from deep boreholes. Some of these are located at Little Missenden, Slough and Langley Marshes where they reach Palaeozoic rocks (Figure 1); two other significant boreholes, Southall and Bushey, lie just to the east of the district.

Concealed geology

Palaeozoic

Information on concealed rocks is available from records of boreholes that were sited near the periphery of the district.

Upper Silurian

Upper Silurian (Přídolí) strata were proved in the Little Missenden borehole (Figure 1). The rocks consist of grey micaceous, fine-grained sandstone and sandy mudstone, with thin beds of fossiliferous limestone (Straw and Woodward, 1933).

Early Silurian volcanic rocks have been mapped widely in the subsurface of the south Midlands (Smith, 1987). They have been drilled only at Bicester [SP 5878 2081] to the north-west. A combination of seismic reflection data and aeromagnetic data (Figure 7b) suggests that Anomaly A represents the zone along which the volcanic rocks are less deeply buried. They are absent to the north-east of the district (possibly due to erosion), but concealed beneath thick Devonian and later strata to the south-west.

Lower Devonian

Lower Devonian strata have not been proved in boreholes in the district, but are likely to occur locally below the base ?Middle to Upper Devonian (Acadian) unconformity, and continue southward beneath these strata (Figure 2).

Middle Devonian

Middle Devonian rocks are rare north of the Variscan Front in England. Bushey Borehole [TQ 120 958] penetrated about 24 m of varicoloured mudstone, red sandstone and fossiliferous limestone just above terminal depth. A variety of fossils (crinoids, brachiopods, gastropods, fish fragments and Tentaculites) and spores (Mortimer and Chaloner, 1972) were dated as Givetian. This suggests that Bushey and the south-central part of the district forms part of the east-west-trending London Basin, containing thick Upper and Middle Devonian strata.

Upper Devonian

Upper Devonian Boreholes at Slough, Langley Marsh and Southall penetrated probable late Devonian strata. The Southall Borehole yielded fossil fish that were dated by Woodward (1913). These strata comprise red mudstone, containing fish and mottled claystone and sandstone with scattered beds of micaceous, cross-bedded coarse-grained sandstone.

Mesozoic

The sub-Mesozoic unconformity drops from about 225 m below OD in the north at Little Missenden to about 350 m below OD in the south (Figure 2). The overlying rocks proved in deep boreholes are Jurassic and Lower Cretaceous strata that overlap progressively onto the London Platform (Figure 3).

Jurassic

A sequence of mudstone and limestone some 51 m thick, was recorded in Little Missenden Borehole resting on rocks of Silurian age. The strata are assigned to the Great Oolite Group, but the dominance of mudstone indicates a change in facies from the marine limestones prevalent in the Cotswolds to the west to brackish mudstone north of the London Platform. A few metres of sandy limestone of possible Mid Jurassic age were also encountered in a borehole at Slough, suggesting that the Great Oolite is probably present at depth in most of the western part of the district.

At Little Missenden, 64 m of dark grey mudstone were recorded (but not cored) overlying the Great Oolite Group. These are assigned to the Ancholme Group, which is also present in boreholes at Maidenhead and Marlow. Rock-chip samples yielded Gryphaea, belemnites, pyritised nuculacean bivalves and opelliid ammonites; the fauna and its preservation both suggest that the Stewartby Member of the Oxford Clay Formation is present. Given the thickness of the mudstone, it is likely that the whole of the Oxford Clay Formation as well as the underlying Kellways Formation is present. The upper part of the Ancholme Group is absent in the Little Missenden Borehole, and is probably not present in the district because of Cretaceous overstep.

Lower Cretaceous

The Lower Greensand Group (LGS) oversteps Jurassic strata onto the London Platform. It consists of sand with subordinate sandstone and mudstone of shallow marine origin. Borehole records show that the group ranges from 18 to 54 m in thickness, and is absent from the eastern part of the district. The thick mudstone sequence (60 to 70 m thick) that comprises the succeeding Gault Formation (G) was deposited in deeper water; it is the oldest Mesozoic formation that is present throughout the district. The formation can be divided into Lower and Upper Gault; the Lower Gault is probably absent from the extreme east of the district since Bushey Borehole (just east of the district) proved Upper Gault resting directly on Devonian strata. The Upper Gault passes gradually westwards into the contiguous Upper Greensand Formation, which consists mainly of sand that is glauconitic in part, with subordinate mudstones. A biogenic sandstone known as 'malmstone' and consisting largely of siliceous sponge spicules and sand grains occurs near the top of the formation.

Upper Cretaceous

The Chalk Group is the oldest exposed unit, cropping out extensively in the north-western part of the district, and also in the south-west, west of Maidenhead. The maximum thickness of the Chalk at outcrop is about 155 m; approximately 186 m were proved in a borehole at Slough [SU 9484 8218].

Chalk is typically a very fine-grained, relatively soft, white limestone. Flints are conspicuous in the higher part of the Chalk, and give an indication of the natural bedding. Clay-rich horizons, traditionally known as marls (calcareous mudstone), occur locally and are generally a few centimetres thick in the higher part of the Chalk Group; the marl forms regular thick bands (up to 0.6 m thick) in the (unexposed) lower part of the succession. Hard, nodular beds occur in parts of the succession, and there are discrete horizons of very hard chalk which are iron-stained, glauconitised and phosphatised; they are interpreted as hardgrounds formed by enhanced sea-floor cementation associated with periods of nondeposition (Hancock, 1989).

Traditionally the Chalk has been subdivided into three: Lower Chalk, Middle Chalk and Upper Chalk. A newly agreed classification recognises two subgroups (Grey Chalk Subgroup and White Chalk Subgroup), and up to nine formations have been mapped (Rawson et al., 2001). The subdivisions recognised in the Beaconsfield district are summarised in (Figure 4) and described below.

The oldest part of the Chalk Group, belonging to the Grey Chalk Subgroup (GyCk) and basal White Chalk Subgroup, is not exposed in the district, but occurs in boreholes at Slough [SU 9470 8217], Wycombe Marsh [SU 8876 9189] and Marlow Waterworks [SU 8424 8679]. In the Slough and Marlow Waterworks boreholes, the subgroups are about 70 m thick, and the resistivity logs of both boreholes show a clear subdivision into a higher resistivity upper part, and a lower resistivity lower part (Figure 4). This may correspond with the two-fold subdivision of the Lower Chalk recognised by Bristow et al. (1997) in the Wessex Basin, where the West Melbury Marly Chalk is overlain by the Zig Zag Chalk. Typically, the Grey Chalk Subgroup is clay-rich and flint-free, with alternating bands of limestone and marl in the lower part, and massive bedded, grey chalk in the higher part. The Plenus Marls comprise a clay-rich horizon that traditionally marks the top of the Lower Chalk, but is now placed at the base of the White Chalk Subgroup. A widespread erosion surface below the Plenus Marls is the critical marker for defining the top of the Grey Chalk Subgroup. On borehole resistivity logs across the area (including Hurley [SU 8312 8395], Hughenden [SU 8642 9646] and Railway Place [SU 8707 9288], in addition to those mentioned above) the Plenus Marls are clearly recognised by a sharply defined resistivity 'low' (Figure 4).

The basal part of the White Chalk Subgroup belongs to the Holywell Nodular Chalk Formation (HCk), which is widespread, occurring in all of the boreholes mentioned above; the formation is about 15 m thick. The brachiopod Orbirhynchia cuvieri, recorded near Bradenham [SU 97 82] (Sherlock and Noble, 1922) suggests that the Holywell Nodular Chalk outcrops in that area, and this is supported by this survey. The Holywell Nodular Chalk is typically hard, nodular chalk, lacking flint and with a profusion of shell fragments of the bivalve Mytiloides spp. in the upper part. A strongly indurated basal unit, the Melbourn Rock, is conspicuous on borehole resistivity logs (Figure 4).

The New Pit Chalk Formation (NPCk) crops out widely, occurring in the upper parts of the Wye, Misbourne and Chess valleys, and in the head of valleys near Marlow. It is proved in many boreholes across the district; those at Slough and Taplow Court [SU 9043 8204] suggest a thickness of about 45 m. Typically, it comprises massively bedded, non-nodular chalk, with fairly regularly developed marl seams and sporadic flints in the upper part. A 7 m section of New Pit Chalk was recorded by Sherlock and Noble (1922) in the road cutting below Plomers Hill [SU 8485 9446] to [SU 8472 9433] where it contains few flints and fossils; the topmost 6 m of the formation were exposed on the east side of the road north of Latimer [TQ 005 993], beneath a brecciated capping of Chalk Rock (see below).

Overlying the New Pit Chalk is the Lewes Nodular Chalk Formation (LeCk). This contains a group of intensely indurated horizons, collectively named the Chalk Rock (Plate 2), at the base. It crops out on the upper flanks of the Wye, Misbourne and Chess valleys, and in the valley sides north and west of Marlow. In boreholes at Gerrards Cross [TQ 0130 8830] and Hillingdon [TQ 0765 8100] the Lewes Nodular Chalk is about 30 m thick, and about 35 m thick at Slough and Taplow Court (Mortimore and Pomerol, 1987, fig.6). The Lewes Nodular Chalk Formation is typically a hard, nodular chalk, with conspicuous, regularly developed flints, thin marls and hardgrounds.

The Chalk Rock is a group of hardgrounds that represent a very condensed part of the Chalk Group, and is the traditional marker for the base of the Upper Chalk. Detailed examination of the hardgrounds in the Chalk Rock has shown that different groups occur at different stratigraphical levels in different geographical areas, and that the Chalk Rock itself is diachronous (Bromley and Gale, 1982; Gale, 1996). One of the locally developed hardgrounds, the Blounts Farm Hardground, is characterised by large inoceramid bivalve shell fragments; the stratotype is at Blounts Farm [SU 8399 8738]. Here, Bromley and Gale (1982) recorded a temporary exposure, with the highest of the Chalk Rock hardgrounds (Hitchwood Hardground) near the top of a 3.5 m-thick succession. A marl seen near the base of the Blounts Farm exposure is named the Fognam Marl, and believed to equate with Southerham Marl 1 of southern England (Bromley and Gale, 1982; Gale, 1996). Other localities where hardgrounds have been recorded include:

Old quarry at Marlow Waterworks[SU 8425 8680]: Chatwin and Withers (1909) recorded 8 m of flinty, nodular S. plana Zone chalk above the Chalk Rock, overlain by a further 7 m of firm chalk with common nodular and tabular flints, belonging to the M. cortestudinarium Zone.

Old pit at Fern [SU 883 884]: Chalk Rock occurs beneath river terrace deposits, confirming previous reports of its suggested occurrence thereabouts (Treacher, 1916; Sherlock and Noble, 1922).

Latimer [TQ 005 993]: At Latimer there is weakening of some hardgrounds and the development of the Latimer Marl (Bromley and Gale, 1982) marks the north-eastward extension of the Chalk Rock into Hertfordshire.

In brash near Mount Wood [TQ 0280 9880]: A few metres above the Chalk Rock, another bed of hard chalk, recorded by Sherlock and Noble (1922), may represent a second widespread condensed horizon in the Lewes Nodular Chalk, traditionally named the Top Rock. However, the Top Rock was not evident in the succession reported at the Marlow Waterworks quarry (Chatwin and Withers, 1909) and has not been mapped in this survey.

The youngest chalk of the district is represented by the Seaford Chalk and Newhaven Chalk (SNCk) formations, which have not been mapped separately in this district. The Seaford Chalk Formation has the greatest outcrop area of any of the formations of the Chalk Group in the district. It forms much of the downland between the valleys of the Thames, Wye, Misbourne and Chess, and extends south-eastwards towards the margin of the Palaeogene outcrop. At least 37 m of Seaford Chalk are inferred to occur between Marlow and Maidenhead (Sherlock and Noble, 1922), and about 32 m are inferred to be present in boreholes at Iver [TQ 0341 8463] and Uxbridge [TQ 0503 8380]. Newhaven Chalk Formation is much more limited, cropping out only at Taplow Court [SU 9055 8192]; it possibly occurs at the top of the boreholes at Uxbridge and Iver (see above). At Taplow Court, about 13 m of Newhaven Chalk Formation occur beneath the contact with Palaeogene deposits.

The Seaford Chalk is typically a soft, flinty chalk with local shell-rich horizons containing the bivalve Platyceramus and, in the lower part, also thick-shelled Volviceramus involutus. The topmost part of the Lewes Nodular Chalk and about 20 m of the overlying Seaford Chalk have been recorded in an old pit at Summerhouse Lane, Harefield [TQ 0435 9295], although the lower part of the succession has since been covered by infill. Here, the base of the Seaford Chalk is marked by the upper East Cliff Marl, the correlative of Shoreham Marl 2 in southern England (C J Wood, unpublished manuscript notes. Preliminary observations on the Summerhouse Lane Chalk Pit near Harefield.). Sparse fossil records from an old pit at Pinkneys Green [SU 868 829] include the echinoid Conulus albogalerus, which suggests the presence of the higher part of the Seaford Chalk. A bed of hard, nodular crystalline chalk, representing the Clandon Hardground (Robinson, 1986), marks the top of the Seaford Chalk at Taplow Court [SU 9055 8192] (Wood, 1996). The Newhaven Chalk is usually soft, white, moderately flinty chalk, but at Taplow Court it comprises two beds of brown, phosphatic chalk, deposited in small, structurally controlled erosional troughs (Wood, 1996). Each of the phosphatic chalk beds overlies a phosphatised hardground surmounted by a lag of phosphatised chalk pebbles (Wood, 1996). Fossils are common, including belemnites and the crinoids Uintacrinus socialis and Marsupites testudinarius, indicating that the Taplow Court succession is a condensed equivalent of the basal Newhaven Chalk elsewhere in southern England.

Palaeogene

Falling sea level at the beginning of the Palaeogene Period resulted in the emergence and removal by erosion of the youngest parts of the Chalk Group. Subsequently, considerable thicknesses of shallow marine, coastal and fluvial deposits were laid down in the London Basin. The Beaconsfield district lies within the central-western part of this basin.

The Lambeth Group (LMB) (formerly known as the Woolwich and Reading Beds) of the district comprises the Upnor and Reading formations. The main outcrop is confined to the centre and south-east of the district but there are about twenty outliers scattered throughout the area. The two formations are easily distinguished in borehole logs but, as in adjacent districts (Ellison and Williamson, 1999; Mathers and Smith, 2000), it has proved impractical to differentiate them at outcrop. They have a combined thickness that ranges from less than 7 m in some outliers to 28 m in the main body of the group.

The Upnor Formation is approximately co-extensive with the succeeding Reading Formation, although absent locally as, for example, recorded in the logs of boreholes at East Burnham Park. These shallow marine deposits are complex and typically comprise laminated clay, cross-bedded sand and basal conglomerate. At Harefield [TQ 048 909], an angular flint conglomerate (0.3 m thick) with a glauconitic sandy clay matrix rests on a bored surface at the top of the Chalk. This passes up into conglomerate about (1 m) thick consisting rounded, black-patinated flint pebbles in a brown matrix, which is overlain by 1.2 m of sand and clay (Bateman, 1988). This sequence is probably representative of the Upnor Formation throughout the district. In Hertfordshire, a similar conglomerate occurs as silica-cemented masses, and is known as Hertfordshire Puddingstone. Although no exposures of Puddingstone were recorded in the district, rare blocks were seen in field brash and large blocks can be seen in the walls of churches (Plate 3) and around Bradenham village green [SU 827 970], suggesting that it may occur locally. Generally, the formation is less than 2 m thick, but it may reach 6 m in thickness in some places. However, because of dissolution features in the top of the underlying Chalk, which are commonly infilled with Lambeth Group deposits, such estimates are inevitably inaccurate.

There is evidence for clay enrichment of the basal conglomerate in outliers of the Upnor Formation and a paucity of weatherable minerals in the overlying deposits may indicate post-depositional pedochemical weathering and clay translocation. Thus, these beds may have a developmental affinity with the Clay-with-flints and plateaux Head Gravel (both widespread in the district), which raises the question as to how much post-depositional modification is required before a 'Tertiary' deposit becomes a Superficial deposit (Bateman, 1988; and see Quaternary).

After the deposition of the Upnor Formation, sea level fell and alluvial mudflats developed across the district. These sediments make up the Reading Formation, which is up to 22 m thick and dominated by clay, with some silt and fine sand. The formation comprises a sequence of fining upward cycles; each begins with sand, passes up into clay and is capped by colour-mottled clay with rootlet traces. The mottling is reddish brown, pale grey or purple and is the result of repeated pedogenesis in a warm climate during periods of emergence. Large, lenticular bodies of cross-bedded sand that occur within the clay are probably the channel fill deposits of the rivers and their distributaries. An extensive sand deposit, 5 to 8 m thick and having the characteristics of a channel fill with sharp base and steep sides occurs just to the south of the Beaconsfield district stretching between Slough and Heathrow Airport (Ellison and Williamson, 1999).

In early Eocene times, a rise in sea level brought a return to marine conditions, initially shallow and confined to the eastern part of the basin, but the sea later deepened to a possible maximum depth of 200 m and extending westwards into the Hampshire Basin (Sumbler, 1996). During this period and in this environment the Thames Group was deposited. It includes the Harwich and London Clay formations. The former proved too thin to differentiate at outcrop and is not consistently recognisable in borehole records. The main outcrop of the Thames Group is confined to the south and east of the district with a few isolated small outliers in the extreme west and south-west.

The Thames Group has yielded a rich fossil biota that includes marine invertebrates, fish, birds, land mammals and the seeds, fruit and fragments of numerous species of land plants.

The Harwich Formation was laid down during the initial, shallow, phase of the transgression and comprises typically a thin bed of glauconitic, pebbly, fine-grained sand and silt that overlies the Lambeth Group in the London Basin. In this district, it includes deposits formerly assigned to the 'London Clay Basement Bed' of the London Clay, and later to the Tilehurst and Swanscombe members of the Oldhaven and London Clay formations respectively (King, 1981; Ellison and et al., 1994). The extent of the formation is not known but at Harefield [TQ 048 911], in the east of the district, Cooper (1976) recorded some 3 m of clay, silt and sand with pebble 'stringers'. The lower part of the sequence contains three layers of calcareous concretions (hardgrounds), the top-most of which had been bored by the bivalve Martesia. Bivalves were present at several horizons but were notably abundant in the upper part. Deposits at Knowl Hill Hill Pit [SU 8170 7970], just beyond the south-western limit of the district have also been assigned by various authors (although without agreement on which part of the sequence). This suggests that the Harwich Formation is represented throughout much of the district.

The London Clay Formation (LC) reaches a maximum thickness of about 150 m in the eastern part of the London Basin (Sumbler, 1996) but only 48 m have been recorded in the Beaconsfield district, at Northwood, near its eastern margin. The sequence is notably consistent, comprising dark grey stiff, fissured clay (often described as 'blue') that weathers to chocolate brown, with thin beds of glauconitic sand and pebble layers. Illite and smectite are the dominant clay minerals in the formation, and mica and glauconite are also common. Pyrite occurs as aggregates and nodules or replacing fossils but, where the deposits have been weathered, selenite has formed by reaction between the pyrite and calcium carbonate. This has important consequences for the engineering characteristics of the formation (see Applied geology).

Five sedimentary cycles (Units A to E) have been identified in the London Basin by King (1981). Each cycle begins with a thin bed containing scattered glauconite grains and perhaps sporadic well-rounded flint pebbles, which is overlain by a coarsening up sequence that begins with clay and passes up through silt and ends with fine sand. Thus each cycle records an initial transgression followed by deposition of clay in the deep water (high sea-level stand), followed in turn by gradual shallowing reflected in the gradual coarsening of the sediment to sand. Probably only the early cycles are present in this district, later cycles having been removed by erosion.

Quaternary

The district is notable for its widespread, varied and complex Quaternary (Drift) deposits (Figure 5). The most extensive of these are sand and gravel deposits disposed in a 'staircase' of terraces, or more precisely terrace remnants, rising northwards from the present day floodplain of the River Thames. They are also the most interesting of the Quaternary deposits because they record the development of the River Thames over, at least, the past 500 000 years and possibly longer. There are ten such river terrace deposits and they are most conveniently described as pre-diversionary or post-diversionary deposits. However, all of the deposits have certain aspects in common. In general they all comprise relatively thin deposits of stratified sand and gravel, commonly cross-bedded, in which the upper part shows evidence of cryoturbation, thus implying episodes of periglacial conditions. The gravels all contain, in varying amounts, far-travelled pebbles with a provenance far beyond the district, notably quartz and quartzite derived from the Kidderminster Formation (formerly Bunter Pebble Beds) of the Midlands (Figure 6). By chance, with the exception of the oldest and youngest, all of the terrace deposits have their type sites located within the district, although most of the deposits themselves extend well beyond the district. Much of the detailed description that follows is derived from the work of Gibbard (1985) and Bridgland (1994).

Pre-diversionary River Terrace Deposits

The highest, and therefore oldest, river terrace deposit in the district is the Westland Green Gravel. It occurs at only two localities, Ashley Hill [SU 824 812] in the south west and Hodgemoor Wood [SU 967 939], north of the centre of the district, at elevations of about 140 m. Up to 2 m of poorly stratified gravels were seen in Chalfont Brick Pit [SU 977 942]. In addition to flint, the gravel contains a high proportion of exotic lithologies including quartz, quartzite, sandstone and Carboniferous chert. Hey (1965) suggested the presence of the chert was evidence for a 'Thames' origin for the gravels as this lithology also occurred in deposits in the Upper Thames valley.

Below this level the Chorleywood Gravel, Beaconsfield Gravel, Gerrards Cross Gravel and Winter Hill Gravel occur at successively lower levels between 115 m OD and 65 m OD. Bridgland (1994) only tentatively assigned spreads of gravel at Chorleywood [TQ 03 96] and around Micklefield Green [TQ 05 98] to his newly erected Chorleywood Gravel but this classification has been adopted for this account. Taken together, these deposits cover a wide belt across the district from south-west to north-east, well to the north of the River Thames. Nevertheless, all the evidence clearly indicates that they are the deposits of an ancestral Thames that drained to the north of London and across East Anglia, before the Anglian Glaciation some 430 000 years ago. Typically, they are fine- to medium-grained, horizontally stratified, matrix-supported gravels with thin beds of cross-bedded sand. Gibbard (1985), amongst others, recorded bodies of massive pebbly clay within some of the terrace deposits, most notably in the Gerrards Cross Gravel. He attributed them to mass flow of material derived from adjacent slopes. Palaeocurrent directions generally indicate flow towards the east-north-east. The deposits range in thickness between 3.5 and 10 m, but average about 6 m. However, solution of the underlying chalk (Plate 4) has produced irregular bases to the terrace deposits so these figures should be treated with caution. Exotic clasts occur in greater abundance in the Gerrards Cross Gravel than any other Thames terrace deposit. In particular, it contains volcanic pebbles believed to originate from north Wales. Some workers (e.g. Green and McGregor, 1978) have suggested that this indicates glaciation at this time in north Wales, as a mechanism for introducing such lithologies into the headwaters of the river. The composition of the Winter Hill Gravel also differs significantly from those of the older terraces. It reflects a marked drop (a termination according to Gibbard, 1985) in quartz and quartzite input into the bedload of the river suggesting the truncation of its headwaters, possibly because of capture by the Proto-River Soar of the Midlands.

The Winter Hill Gravel also differs from earlier terraces in that it can be divided into two subunits for part of its extent. A lower subunit exhibiting all the characteristics of the other terrace deposits, namely the sedimentary structures associated with braided river deposits and a gentle eastward slope, is present throughout but, east of Burnham Beeches, an upper subunit can be separated on the basis of its gradient. In fact, the surface of this upper unit is remarkably flat for several kilometres downstream at a more or less constant elevation of 78 to 80 m. Gibbard (1985) has interpreted this as evidence for ponding of the river downstream of Burnham Beeches when the upper part of the Winter Hill Gravel was deposited. Thus normal fluvial aggradation ceased and a lacustrine delta built out into the lake. In support of this he cites rare exposures of the upper subunit that showed large-scale cross-stratification, the identical composition of the gravels in the upper and lower units and their dissimilarity in this respect with all other terrace deposits of the district. If this interpretation is correct, the 'upper' Winter Hill Gravel can be assigned to the Anglian Stage. During this stage, ice advanced into the Vale of St Albans depositing a chalk-rich till. Beyond the north-east extremity of the district, the downstream equivalent of the lower subunit of the Winter Hill Gravel, the Westmill Gravel, passes beneath this till. Proglacial lakes formed, temporarily damming the course of the river, and resulted in deposition of the deltaic facies of the Winter Hill Gravel. Ultimately, the damming of the ancestral River Thames caused the diversion of the river southwards via a spillway near Uxbridge into its modern course.

Post-diversionary Terrace Deposits

The first post-diversionary deposit of the Thames was the Black Park Gravel. Again, for convenience of description this can be grouped with successively younger, and topographically lower, river terrace deposits that are aligned with the present-day course of the River Thames. These are, in descending order, the Boyn Hill Gravel, Lynch Hill Gravel, Taplow Gravel and Shepperton Gravel, occurring with the Black Park Gravel as a flight of terraces between 62 and about 20 m OD. In adjacent districts, the Kempton Park Gravel lies at an altitude that is intermediate between the Taplow and Shepperton gravels, but it has not been preserved within this district. These deposits were laid down between the Anglian and Late Devensian stages. The Shepperton Gravel is largely concealed beneath alluvium and is contiguous with gravels in the tributary rivers, the Wye and Colne. The terrace gravels show most of the sedimentary features of the prediversionary terrace deposits, consisting of fine- to medium-grained horizontally stratified gravels with narrow trough and tabular-bedded sand layers. But the composition of the gravels is in marked contrast to the earlier deposits, being rich in angular flint and by comparison depleted in far-travelled quartz and quartzite clasts. A small proportion of chalk is also present in places. Typically, these terrace deposits range between 4 and 6 m in thickness, but locally up to 8 m has been recorded in the Taplow Gravel.

Flint implements occur in the terrace deposits. They are common in the Boyn Hill Gravel and Lynch Hill Gravel, much less so in the others. Cannoncourt Farm Pit, located on the Lynch Hill Gravel near Maidenhead [SU 878 831] is one of the most productive localities for Palaeolithic artefacts, particularly hand axes, in Britain (Wymer, 1968, Bridgland, 1994).

Gibbard (1985) identified correlatives of the post-diversionary terrace gravels in the Colne valley, although he gave them local names. In this survey, his correlation has been applied although, in keeping with practice on adjacent maps, the nomenclature of terraces in the Thames valley has been used. However, one surveyor retains reservations about the correlation of the first terrace above the Colne floodplain with the Taplow Gravel, preferring it to be correlated with the Lynch Hill Gravel.

The Langley Silt blankets much of the Taplow Gravel and Lynch Hill Gravel, varying in thickness from a few centimetres to a maximum of 4 m, but has only been mapped in areas where it is over, approximately, 1 m thick. It consists of decalcified clayey silt and silt and commonly exhibits a columnar jointing pattern in its upper part (Gibbard, 1985). The origin of the deposit has been the subject of debate but it is now accepted that it is of both loessic and fluvial origin. Whether this involved reworking of the windblown component by water or simultaneous deposition by both agents remains uncertain; some of what has been mapped as Langley Silt may include soliflucted material. The Langley Silt was deposited during a cold climatic phase and, although it rests upon more than one terrace, Gibbard (1985) considered its main mass could be attributed to a single phase of deposition, citing evidence for a Late Devensian (17 000 BP) age.

The Alluvium of the Thames and its tributaries comprises grey to dark grey, commonly organic, silt with sporadic lenses of shelly sand and gravel that accumulated on the floodplains in postglacial times. Borehole records show that the Alluvium everywhere overlies the Shepperton Gravel and is irregularly channelled into that deposit. Its thickness therefore varies, but is commonly about 2 m and seldom exceeds 3.5 m.

Wide spreads of Clay-with-flints cap the Chalk in the north-west of the district. Typically it consists of tenacious reddish brown clay with angular nodules of flint and a proportion of rounded flint pebbles. Locally it also appears to include lenses of sand, usually at or near the base. The Clay-with-flints is a residual deposit comprising material from the Lambeth Group (clay and pebbles) and the insoluble residue of the Chalk (mainly flint nodules). The deposit is generally in the order of a few metres thick, but varies considerably since it rests upon the irregular surface of the Chalk. Development of the Clay-with-flints probably began in Neogene times and continued through the Quaternary.

During the Quaternary the district was repeatedly subjected to cold, periglacial episodes. In such conditions slope processes were extremely active, giving rise to a widespread but generally thin cover of head deposits, diamictons that reflect the composition of upslope materials from which they were derived. Only the thickest accumulations of Head are shown on the map, usually in the valley bottoms. Some of the deposits consist of stony pebbly clay, while others are gravelly and occupy the bottom of dry valleys in the Chalk: these are differentiated on the map. A separate category of Head Gravel deposits occurs as sheets of pebbly flint and quartz gravel, clay and sand on the dip slope of the Chalk. The origin of these deposits and their relationship to the adjacent Clay-with-flints and higher River Terrace Deposits is unclear but may be polygenetic, including a residuum of the Lambeth Group, Head and degraded high-level deposits of the early River Thames and its tributaries.

Isolated spreads of gravel of variable composition that occur at relatively high but differing elevations have been assigned to Sand and Gravel of unknown age and origin. They may be related to the River Terrace Deposits in some way, perhaps as fragments of the oldest terrace deposits or, as is possible in the case of those gravels grouped around Batchworth Heath, remnants of a tributary of the early Thames. Such an origin is also considered likely for the Stanmore Gravel, which comprises gravel that is sandy and clayey in part.

Deposits encountered in a solution hollow during site investigations for the M25 motorway at Slade Oak Lane [TQ 018 189] included organic silty clays, the Slade Oak Lane deposits, containing fossil pollen and plant remains that Gibbard (1985) considered indicative of the Late and Post-temperate substages of the Hoxnian ('interglacial') Stage. They are the only such deposits encountered within the district.

The only recorded landslip (p.27) in the district is at Coleshill [SU 95 95]. Here clay of the Lambeth Group has slumped along the northern slope of the hill to form a hummocky apron resting on the Chalk below.

Artificial deposits and worked ground

The Beaconsfield district is a populous area on the outskirts of one of the major cities of the world, so it is unsurprising that change in the landscape and ground wrought by human activity is profound. By their very nature, the extent and composition of artificial deposits and worked ground may change substantially over short periods of time. The distribution shown on the map is essentially that known at the date of the survey, compiled from observation, aerial photographs, historical maps and records, and information provided by local authorities and the Environment Agency. Unrecorded sites without surface expression, particularly small sites, may remain undetected.

Made Ground is shown in areas where material has been deposited by man upon the natural ground surface. It falls into three broad categories, although these are not distinguished on the map: constructional fill used for road, rail, canal and similar embankments; spoil from mineral workings or other excavations; waste in raised landfill sites.

Worked Ground is shown where natural materials have been removed as, for example, from quarries and pits, and road, rail and canal cuttings. There are extensive active and disused pits in the district, mainly sand and gravel workings in fluvial deposits. Many old pits in the river valleys, especially along the floodplain of the Colne, have flooded and are now used for recreation.

Infilled Ground is where natural materials have been removed and the void partly or wholly back-filled with waste the composition of which, at any particular site, is usually not known. The categories of fill are generally the same as those for Made Ground, but with the exception of constructional fill. Many former quarries and pits of the district have been used for the disposal of domestic and industrial waste. Perhaps the most active and extensive of these is located on the north side of the M40 to the west of Gerrards Cross (see Gas emissions p.27).

Structure

The Palaeozoic rocks that underlie the district include Silurian sedimentary rocks proved in Little Missenden Borehole. In the west, they overlie early Silurian volcanic rocks that form a component of the magnetic anomalies recorded in the district, together with deeper magnetic Precambrian rocks (Figure 7b). Devonian rocks were proved in a number of boreholes (p.4), and the Lower Devonian strata are separated from Middle and Upper Devonian by the Acadian unconformity (Figure 1). These Palaeozoic rocks lie north of the Variscan fold belt within the Midlands Microcraton, and were not greatly affected either by early Devonian (Acadian) or Variscan deformation. The limit of deformation of the Caledonian chain, bordering the Midlands Microcraton lies farther north-east (approximately on the line of the Thringstone Fault; Smith 1985). The gravity and aeromagnetic anomaly maps (Figure 7a), (Figure 7b) show the north-west-trending tectonic strike of the Midlands Microcraton. The Acadian unconformity lies about 1000 m below OD in the south of the district (Figure 1). It represents uplift of the area, and a period of erosion before subsidence and further deposition, probably within a small foreland basin that extended from the Thames estuary to Berkshire, north of the 'Variscan front'.

Extensional faulting and subsidence in late Palaeozoic and early Mesozoic times produced deep basins in the surrounding area including the Weald and the southern North Sea. However, this district lay on the southern flank of the Anglo-Brabant Massif, a long-standing positive feature formed by the amalgamation of the Midlands Microcraton and the East Anglian-Brabant Caledonian chain (to the north-east). This landmass was not fully submerged until later in the Cretaceous with the deposition of Gault Formation, the Upper Greensand and Chalk groups. These units successively onlap on to the massif to rest unconformably on Palaeozoic rocks (Figure 3), and represent the feather-edge of a thick sequence of sediments that were deposited to the south. In the south-west, some minor north-west-trending faults are probably associated with the Windsor Anticline. The anticline was formed during the Alpine orogeny, but the original faulting may be related to Mesozoic basin formation.

Chapter 3 Applied geology

Water resources and hydrology

The main aquifer is the White Chalk Subgroup, which underlies the entire district; the underlying Grey Chalk Subgroup is of less importance as a result of its higher clay content. The sand and clay facies of the Lambeth Group Beds in the south and east form a banded aquifer and aquiclude respectively. There are also potential aquifers in the Upper and Lower Greensand with the Gault forming an aquiclude between them. The Quaternary deposits range from low to high permeability, but the low lying river terrace deposits generally have a high permeability and can form a minor aquifer.

The district falls within the Hydrogeological Map of the area between Cambridge and Maidenhead (British Geological Survey, 1984) and lies within the area administered by the Thames Region of the Environment Agency.

The rainfall generally decreases to the south-east with a high on the Chiltern Hills of over 800 mm/a, but most of the district falls in the range 650 to 750 mm/a (Meteorological Office, 1977). The effective rainfall, which is the rainfall less evapotranspiration is about 200 to 300 mm/a (British Geological Survey, 1984). This water is available for runoff or recharge to aquifers. Where the aquifer is exposed or present beneath permeable Quaternary deposits a high proportion could be recharge. Recharge in the unconfined Chalk aquifer is believed to be of the order of 150 to 180 mm/a (Water Resources Board, 1972) and occurs during the winter months once there is no soil moisture deficit.

The River Thames drains the south-west and southern part of the area with a tributary, the River Wye, draining the north-west. The River Colne and its tributaries the Alder Bourne and rivers Misbourne, Chess and Gade flow across the east of the area joining the River Thames to the south of the area. This drainage system is formed of effluent rivers on the chalk and is based largely on groundwater flow mainly through seepages but also through springs that form the baseflow for the groundwater regime. The River Wye rises in West Wycombe in the summer but moves upstream about 1.5 km in the winter, coincident with higher flows occurring downstream. The Alder Bourne rises in the area of Fulmer and the Misbourne rises above Little Missenden; the Chess and Gade rise north of the district. The Chalk has a moderate to high permeability, which allows relatively rapid infiltration of overland flow and this gives rise to numerous dry valleys in the upper regions of the Chiltern Hills.

Hydrogeology of the Lower Cretaceous Strata

The Lower Greensand Formation is an important confined aquifer, which has been developed in the area of Slough where it is more than 250 m below OD. Borehole yields can exceed 40 l/s and the water quality is good with a total hardness of less than 200 milligrams per litre (mg/l) and a chloride ion concentration of less than 25 mg/l (British Geological Survey, 1984). Radiometric dating (14C) indicates that the majority of this water comes from storage, with a small amount of replenishment feasible from the outcrop to the north of the district (Mather et al, 1978).

The Gault Formation is an aquiclude that overlies the Lower Greensand and is hydrogeologically important because it forms a lower limit to the major Chalk aquifer, and it isolates the underlying Lower Greensand aquifer.

The Upper Greensand Formation is present beneath the south-east of the district and although a minor confined aquifer it is of little importance as a groundwater resource due to its limited thickness and relatively low permeability. Where present it appears to be isolated from the main White Chalk aquifer by the basal clay-rich Grey Chalk (Jones, 2000); water is of a similar quality to that from the Chalk, although tends to be less hard.

Hydrogeology of the Chalk

The Grey Chalk Subgroup is not exposed in the area and has a higher clay content than the overlying White Chalk. As a result it is generally not developed, although it does have some aquifer potential.

The White Chalk Subgroup is unconfined in the north and west, although extensively covered by Quaternary deposits and is confined beneath the Lambeth Group and London Clay Formation in the south-east. The water table in the unconfined Chalk mostly follows a subdued version of the surface topography and has a low season range from about 130 m above OD beneath the Chiltern Hills in the north-west to about 20 m above OD at the River Colne (British Geological Survey, 1984) in the south-east. The hydraulic gradient in the unconfined Chalk averages about 1:150 to the south-south-east. The seasonal variation of the water table in response to recharge is most marked beneath the interfluves on the Chiltern Hills were it can be over 20 m, but it is usually restricted to a few metres in the valleys and the confined aquifer.

The aquifer properties of the White Chalk Subgroup are based largely on fracture flow with limited intergranular movement. It is a microporous aquifer with low intrinsic permeability; although the porosity is mostly in the range 25 to 45 per cent with an average of 30 per cent (Bloomfield et al., 1995). It derives much of its groundwater storage and flow from solution-enlarged interconnected fissures that are developed by water movement. These karst features form conduits that can develop along flint beds, hard bands, marl horizons, bedding and fault planes or joints. They are generally most developed in the valley areas and in the zone of the water table fluctuation. The transmissivity of the White Chalk in the valleys is usually between 500 and 2000 m2/d (information from Thames Region, Environment Agency, 2002) but can exceed 25 000 m2/d. Beneath interfluves it is usually between 15 to 100 m2/d and a similar decrease in transmissivity occurs in the confined aquifer where values are about 200 m2/d (Allen, 1997). The storage coefficient in the valleys is usually in excess of 0.02, in the interfluves it is about 0.01 and in the confined environment it is less than 10−4 (Allen, 1997).

Where the surface drainage crosses the junction from low permeability Lambeth Group onto the underlying White Chalk swallow holes can develop where surface drainage flows underground at times of recharge. Examples of these karst features are seen at Lane End [SU 82 92] and Cookham Dean [SU 86 84]. Solution collapse hollows (dolines) are also present where a recharge point has a closed base that allows seepage to the underlying aquifer. These karst features are commonly associated with the edge of the low permeability Clay-with-flints where it overlies Chalk and are seen for example in the area between Bradenham [SU 83 97] and Hughenden [SU 86 95]. All these features can give rise to high velocity groundwater flow, with the potential for rapid contaminant transport. Periodically collapse can occur where such solution features have caused the ground to become unstable.

The White Chalk groundwater is very hard and generally of excellent quality, with a total dissolved solids content generally between 250 and 400 mg/l (Water Resources Board, 1972). It is a calcium-bicarbonate type water in the unconfined and edge of the confined aquifers, moving to soft, sodium rich water in the confined aquifer in the south-east of the area. This change is a result of cation exchange and is coincident with an increase in total dissolved solids, but not to impotable limits. The carbonate hardness is between 170 and 250 mg/l (as CaCO3) in the unconfined Chalk in the west, but rises to over 250 mg/l in the east and in the confined aquifer. Non-carbonate hardness is mostly between 15 and 50 mg/l. The chloride ion concentration is usually less than 30 mg/l at outcrop and reaches over 50 mg/l where confined and the sulphate (as SO4) is usually less than 50 mg/l at outcrop and reaches 100 mg/l where confined. Nitrate values at outcrop are generally in the range 30 to 40 mg/l (NO3), but they are minimal in the confined aquifer, and iron is less than 0.2 mg/l in both environments.

The White Chalk is a major aquifer that is developed extensively for water supplies. The highest yields are obtained in the chalk valleys, where most abstractions take place, and the majority of groundwater is obtained within 75 m of the surface of the chalk. Typical borehole yields are between 2.5 and 20 l/s (British Geological Survey, 1984), but some boreholes yield more than 80 l/s. Boreholes in the confined chalk have lower yields which average about 5 l/s. There is considerable variation in potential yield between boreholes with successful boreholes in this karst aquifer normally penetrating a few significant inflow horizons. Where there is thin drift cover or where the aquifer is in hydraulic continuity with the river and the water table is close to the ground surface, the aquifer is vulnerable to pollution from induced river flow and surface sources, but in the confined environment the aquifer is protected from pollution by the low permeability of the confining rocks. Within the district there are a total of 25 licensed abstractions for over 2 Mm3/a, 27 licensed abstractions for between 0.5 and 2 Mm3/a, which includes a total of 34 public supply abstraction sites (British Geological Survey, 1984). The Environment Agency licenses abstractions and discharges and has a total of 11 observation boreholes for monitoring the quantity and quality of the resource.

Hydrogeology of the Lambeth Group

The sandy facies of the Lambeth Group forms a potential aquifer while the interlayered clayey facies form an aquiclude. This can give rise to a multi-layered aquifer and perched water tables. Apart from the south-east corner of the district where the Lambeth Group is confined by the London Clay, most of the sandy facies is unsaturated and only thin perched water tables and seepages at the interface with the underlying clay beds are present. This environment can give rise to a series of springs and sinks as the groundwater is bought to the surface by impermeable clay beds and sinks into the top of lower sand sequences. Most of these features are ephemeral, but some are perennial and an example of this is seen at Burnham Beeches [SU 95 85]. Where a significant thickness of the sandy facies is saturated as is the case in the confined area in the south-east, small water supplies can be obtained, but the groundwater frequently contains high concentrations of sulphate, calcium and magnesium (British Geological Survey, 1984).

Hydrogeology of the London Clay Formation

The London Clay Formation is an aquiclude; although very small yields of poor quality water can sometimes be obtained from shallow excavations in the weathered zone near to the ground surface and from the more arenaceous basement bed. The water tends to be extremely hard with sulphate ion concentrations that can exceed 2000 mg/l and high concentrations of iron. The main hydrogeological significance of this formation is in confining the White Chalk aquifer.

Hydrogeology of the Quaternary Deposits

The river terrace deposit and alluvial aquifers are directly underlain by the chalk aquifer over much of the area and they can be in hydraulic continuity. However, only the alluvium and lower terraces are likely to be saturated. These sand and gravel bodies have a high permeability, which, coupled with relatively shallow water tables, makes them vulnerable to surface sources of pollution. Where saturated they can be developed for water supplies, but pumping this resource is likely to give rise to induced flow both from the Chalk and the river, with consequent risk of pollution. Where these deposits overlie the London Clay, as is the case in the lower part of the Colne valley, or clay sequences in the Lambeth Group they can form an isolated minor aquifer. This is seen at Uxbridge where two adjacent boreholes (TQ08/101A) and (TQ08/101B) [TQ 0485 8272] with a saturated gravel thickness of 3.6 m have a licensed abstraction of 25 l/s.

The Clay-with-flints aquitard locally reduces infiltration to the chalk, but causes increased surface run-off, which recharges the aquifer around the edge of the deposit.

Mineral resources

The potential bulk mineral resources of the district are considerable and comprise mainly sand and gravel, and chalk. The main historical and current mineral resources and their uses are summarised in (Figure 8).

Surface mineral workings

Throughout the district, the most actively exploited mineral resource is sand and gravel. There are many active and disused pits and quarries, the majority of them for working sand and gravel from the River Terrace Deposits. There has also been some extraction of sands and gravel from the Lambeth Group, usually worked in conjunction with overlying River Terrace Deposits as, for example, near Harefield [TQ 06 88]. Many of the former sand and gravel pits in the district have been reused for landfill or are used for recreation and conservation (see Artificial Deposits and Worked Ground). There has been considerable urban development on the River Terrace Deposits which has 'sterilised' some deposits. However, extensive resources remain and have been quantified and described in Mineral Assessment Reports of the British Geological Survey (Squirrel, 1974; Dunkley, 1979 and Harries et al., 1982).

The Chalk of the district has also been worked to extract chalk for marling, for agricultural lime, as a source of flints for building and as aggregate and fill. Large disused quarries can be found in the valley sides near Harefield and Taplow. Wartime investigations showed that the Newhaven Chalk at Taplow contains between 7 and 15 per cent of phosphate, but this has not been commercially exploited. Chalk is currently intermittently extracted at Pinkneys Green [SU 868 829].

The Langley Silt (formerly known as 'brickearth') was once extensively used for brick making but is now largely worked out or built over. The Lambeth Group has also been worked on a small scale for brick making, most notably at Harefield. Both the Lambeth Group and the London Clay constitute a theoretical resource for this purpose but the fact that they are not widely used suggests that the resource is not economic.

Underground mining

The only known occurrence of mining in the district is at West Wycombe [SU 8275 9483] where chalk was extracted in the late 18th century from an underground cavern. Sir Francis Dashwood used the chalk to construct a new road at West Wycombe, a two-mile stretch of the London to Oxford road connecting High Wycombe and West Wycombe, between 1748 to 1752. Dashwood also used these tunnels for meetings of the infamous Hell Fire Club (Plate 5).

Engineering ground conditions

Three of the most important ground conditions relating to construction and development are the suitability of the ground for supporting structural foundations, its ease of excavation and its use in engineering works. These issues are summarised for the main engineering geological units in the district in (Figure 9). Some important geohazards within the district include ground heave and subsidence, slope stability and mass-movement, chalk dissolution and gas emissions related to both the underlying geology and landfills.

Ground heave and subsidence

Clays, some of which have a relatively high smectite content, dominate the London Clay Formation and Lambeth Group. Such clays undergo significant volume changes in response to variations in moisture content. During the winter months, the clay absorbs a large quantity of water, which is lost during dry periods, leading to extensive shrinkage and cracking. The alternating processes of expansion and contraction may cause structural damage to buildings and roads.

Unweathered London Clay and Lambeth Group clay also contain pyrite (iron sulphide), which on weathering is oxidised to yield sulphate ions in solution. In clay formations like the London Clay, any calcium carbonate present may react with the sulphate to precipitate selenite crystals. This involves an eight-fold increase in volume compared to the original pyrite and can cause disruption and weakening of the strata. The weathering of selenite produces sulphuric acid, which causes high concentrations of sulphate in the ground or groundwater and can weaken concrete foundations that are not designed to resist this chemical attack.

Slope stability and landslip

During the geological survey of this district only one landslip was identified, at Coleshill [SU 95 95] in the Lambeth Group. Slopes over 7°, which are cut into the London Clay and Lambeth Group, are prone to failure, and slopes exceeding 3° should also be considered as potentially unstable as a result of periglacial processes (Culshaw and Crummy, 1991). Thinly interbedded sequences of sand and clay are also prone to landslip due to the presence of springs and high confined pore pressures that may lead to loss of strength. Such sequences are found in the London Clay Formation and Lambeth Group.

Chalk dissolution

Dissolution features (solution hollows, pipes or dolines) are commonly encountered within the Chalk, and this district is no exception. Such features can occur anywhere in the Chalk but tend to be most abundant at the base of the overlying Lambeth Group, near the margins of Clay-with-flints and, particularly, beneath Quaternary sand and gravel deposits (Plate4). The scale of dissolution features is generally of the order of a few metres, but a solution pipe 37.5 m deep has been recorded at Slade Oak Lane [TQ 018 897] (Gibbard, 1985). Chalk is prone to dissolution because of the reaction between acidic rainwater or groundwater and the calcium carbonate content of the chalk. This process leads to the dissolution and enlargement of naturally occurring discontinuities, causing the formation of cavities and pipes. Dissolution was enhanced during the prolonged periods of periglacial conditions of the Pleistocene. Where these features collapse, they may be infilled with overlying deposits such as Clay-with flints, Head Gravel, River Terrace Deposits and Lambeth Group. They could, potentially, cause ground instability and differential settlement.

Gas emissions

Some of the landfills within the district produce methane, a potentially explosive gas. At certain localities, the methane is flared-off as, for example, beside the M40 motorway near Hedgerley [SU 974 886].

Natural radon emissions

Radon is a naturally occurring radioactive gas produced by the radioactive decay of radium, which in turn, is derived from the radioactive decay of uranium. Uranium is found in small quantities in all soils and rocks, although the amount varies from place to place. Radon is released from rocks and soils, and is quickly diluted in the atmosphere. Concentrations in the open air are normally very low and do not present a hazard. Radon that enters poorly ventilated enclosed spaces such as basements, buildings, caves, mines and tunnels may reach high concentrations in some circumstances. The construction method and the degree of ventilation can influence radon levels in individual buildings. 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 the radon levels between different parts of the country is mainly controlled by the underlying geology. Within the Beaconsfield district radon potential is generally low, with higher radon potential associated with the White Chalk Subgroup. Information on the radon potential of specific sites may be obtained from BGS, Keyworth (addresses on the back cover) or email enquiries@bgs.ac.uk

Flooding

Most of the areas vulnerable to flooding are on the floodplains of the rivers, which are defined on the geological map by the distribution of Alluvium.

The River Thames has flooded many times, particularly at Maidenhead. The last significant flood was in 1990 (a 1 in 5 to 7 year event), but catastrophic floods occurred in 1947 (a 1 in 56 year event) when the Thames expanded to a mile wide, more-or-less corresponding to the width of the floodplain. The Maidenhead, Windsor and Eton Flood Alleviation Scheme has recently been put in place by the Environment Agency in order to cope with flood events with a return of 1 in 65 years. It comprises a large flood relief channel — known as the Jubilee River (Plate 6) — that has been excavated from upstream of Maidenhead to downstream of Windsor (outside the district), and associated earthworks. A by-product of the construction was the extraction from the channel of very large quantities of sand and gravel.

Information sources

Further geological information held by the British Geological Survey relevant to the Beaconsfield district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the District Geologist, South Midlands, BGS, Keyworth. BGS hydrogeology enquiry service for wells, springs and water borehole records can be contacted at: BGS Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB Telephone 01491 838800 Fax 01491 692345.

Other information sources include borehole records, fossils, rock samples, thin sections and photographs. Searches of indexes to some of the collections can be made on the Geoscience Data Index system in BGS libraries data and on the BGS web site (addresses on back cover).

Maps

Books

British Regional Geology

London and the Thames Valley, fourth edition, 1996

Memoirs

Sheet 269 Windsor and Chertsey, 1915†

Sheet 254 Henley-on-Thames and Wallingford, 1908†

Sheet 255 Beaconsfield, 1922†

Sheet 238 Aylesbury and Hemel Hemstead, 1922†

† out of print; facsimile copies are available at a tariff that is set to cover the cost of copying

Sheet Explanations

Sheet 269 Geology of the Windsor and Bracknell district, 1998

Sheet 268 Geology of the Reading district, 2000

Mineral Assessment Reports

Dunkley, P N. 1979. The sand and gravel resources of the country around Maidenhead and Marlow: Resource Sheet SU 88 and parts 87, 97, 98. Mineral Assessment Report of the Institute of Geological Sciences, No. 42.

Harries, W J R, Hollyer, S E, and Hopson, P M. 1982. The sand and gravel resources of the country around Hemel Hempstead, St Albans and Watford, Hertfordshire: Resource Sheet TL 00,TL 10 and parts of TQ 09 and TQ 19. Mineral Assessment Report of the Institute of Geological Sciences, No. 71.

Squirrell, H C. 1974. The sand and gravel resources of the country around Gerrards Cross, Buckinghamshire: Resource sheet SU 99, TQ 08 and TQ 09. Report of the Institute of Geological Sciences. No. 74/14

BGS Technical Reports

Allen, D J, Brewerton, L J, Coleby, L M, Gibb, S B R, Lewis, M A, MacDonald, A M, Wadstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication. No. 8.

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

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, A M, Coleby, L M, Talbot, J C, McKenzie A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication. No. 68.

Mathers, S J, and Smith, N J P. 2000. Geology of the Reading district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 268 Reading (England and Wales). (Keyworth, Nottingham: British Geological Survey.)

Water Supply Papers

Water supply of Berkshire, 1902

Water supply of Buckinghamshire and Hertfordshire from underground sources, 1921

Documentary collections

Boreholes

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

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series 255 Beaconsfield Sheet are held in the Lexicon database. This is available on web site http://www.bgs.ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

Copies of the photographs used in this report are deposited for reference in the BGS Library, Keyworth.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

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

Earth science conservation sites

Information on the Sites of Special Scientific Interest and other conservation sites present within the Beaconsfield district is held by English Nature, Headquarters and Eastern Region, Northminster House, Peterborough.

References

Most of the references listed below are held in the libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright legislation.

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wadstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication 8.

Bateman, R M. 1988. Relationship of the Woolwich and Reading Formation (Late Palaeocene) to the Upper Chalk (Late Cretaceous) and Clay-with-flints sensu lato (Quaternary) in the Chiltern Hills,southern England. Tertiary Research, Vol. 10, 53–63.

Bridgland, D R. 1994. Quaternary of the Thames. Geological Conservation Review Series, No. 7.

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

British Geological Survey. 1984. Hydrogeological map of the area between Cambridge and Maidenhead. Scale 1:100 000.

Bloomfield, J P, Brewerton, L J, and Allen, D J. 1995. Regional trends in matrix porosity and bulk density of the Chalk of England. Quarterly Journal of Engineering Geology, Vol. 28, 131–142.

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

Chatwin, C P, and Withers, T H. 1909. On the Chalk section at the Waterworks Quarry, Marlow. Geological Magazine, Vol. 6, 123–125.

Cocks, L R M, Holland, C H, and Rickards, R B. 1992. A revised correlation of Silurian rocks in the British Isles. Geological Society of London Special Report, No. 21.

Cooper, J. 1976. Report of the field meeting to Harefield, Middlesex, 14th March 1976. Tertiary Research, Vol. 1, 31–36.

Culshaw, M G, and Crummy, J A. 1991. SW Essex — M25 Corridor: engineering geology. British Geological Survey Technical Report, WN/90/2.

Dunkley, P N. 1979. The sand and gravel resources of the country around Maidenhead and Marlow: resource Sheet SU 88 and parts 87, 97, 98. Mineral Assessment Report of the Institute of Geological Sciences, No. 42.

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

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

Gale, A S. 1996. Turonian correlation and sequence stratigraphy of the Chalk in southern England. 177–195 in Sequence stratigraphy in British geology. Hesselbo, S P, and Parkinson, D N (editors). Geological Society Special Publication, No. 103.

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

Goldring, R, and Alghamdi, J A. 1999. The stratigraphy and sedimentology of the Reading Formation (Palaeocene to Eocene) at Knowl Hill, near Reading (southern England). Tertiary Research, Vol. 19, 111–120.

Green, C P, and McGregor, D F M. 1978. Pleistocene gravel trains of the River Thames. Proceedings of the Geolologists' Association, Vol. 89, 143–156.

Hancock, J M. 1989. Sea-level changes in the British region during the Late Cretaceous. Proceedings of the Geologists' Association, Vol. 100, 565–594.

Harries, W J R, Hollyer, S E, and Hopson, P M. 1982. The sand and gravel resources of the country around Hemel Hempstead, St Albans and Watford, Hertfordshire: Resource Sheet TL 00, TL 10 and parts of TQ 09 and TQ 19. Mineral Assessment Report of the Institute of Geological Sciences, No. 71.

Hey, R W. 1965. Highly quartzose gravels in the London Basin. Proceedings of the Geologists' Association, Vol. 76, 403–420

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication No. 68.

King, C. 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper, No. 6.

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.

Mather, J D, Gray, D A, Allen, R A, and Smith, D B. 1973. Groundwater recharge in the Lower Greensand of the London Basin — results of tritium and carbon 14 determinations. Quarterly Journal of Engineering Geology, Vol. 6, 141–152.

Mathers, S J, and Smith, N J P. 2000. Geology of the Reading district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 268 Reading (England and Wales).

Meteorological Office. 1977. Average annual rainfall, southern Britain. 1916 to 1950. Meteorological Office. 886 (SB). Scale 1:625 000.

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.

Mortimer, M G, and Chaloner, W G. 1972. The palynology of the concealed Devonian rocks of southern England. Bulletin of the Geological Survey of Great Britain, No. 39, 1–56.

Mortimore, R N, and Pomerol, B. 1987. Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association, Vol. 98, 97–143.

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

Robinson, N D. 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proceedings of the Geologists' Association, Vol. 97, 141–170.

Ruffell, A H, and Wignall, P B. 1990. Depositional trends in the Upper Jurassic–Lower Cretaceous of the northern margin of the Wessex Basin. Proceedings of the Geologists' Association, Vol. 101, 279–288.

Sherlock, R L, and Noble, A H. 1922. The geology of the country around Beaconsfield. Memoir of the Geological Survey (England and Wales, Sheet 255).

Squirrel, H C. 1974. The sand and gravel resources of the country around Gerrards Cross, Buckinghamshire: Resource Sheet SU 99, TQ 08 and TQ 09. Report of the Institute of Geological Science, No. 74/14

Smith, N J P (compiler). 1985. Pre-Permian geology of the United Kingdom (south). 1:1 000 000 scale. (Keyworth: British Geological Survey.)

Smith, N J P. 1987. Deep geology of central England: prospectivity of the Palaeozoic rocks. 217–224 in Petroleum geology of north-west Europe. Brooks, J, and Glennie, K W (editors). (London: Graham & Trotman.)

Straw, S H, and Woodward, A S. 1933. The fauna of the Palaeozoic rocks of the Little Missenden boring. Summary of Progress of the Geological Survey, for 1932, 112–141.

Sumbler, M G. 1996. British regional geology: London and the Thames valley. Fourth edition. (London: HMSO for British Geological Survey.)

Treacher, L. 1916. Excursion to Bourne End. Proceedings of the Geologists' Association, No. 27, 107–109.

Water Resources Board. 1972. Thehydrogeology of the London basin.

Whitaker, W. 1921. Water supply of Buckingham- shire and Hertfordshire fromunderground sources. Memoir of the Geological Survey.

Whittaker, A (editor). 1985. Atlas of onshoresedimentary basins in England and Wales. (London: Blackie & Son.)

Wood, C J. 1996. Upper Cretaceous: the Chalk Group. 76–91 in British regional geology: London and the Thames valley. Fourth edition. Sumbler, M G (editor). (London: HMSO for British Geological Survey.)

Woodward, A S. 1913. Note on the fish remains from the Upper Devonian (at Southall). Quarterly Journal of the Geological Society, Vol. 69, 81–84.

Wymer, J J. 1968. Lower Palaeolithicarchaeology in Britain as represented by the Thames valley. (London: John Baker).

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.

(Index map)

Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Depth to the early Devonian (Acadian) unconformity.

(Figure 2) Subcrop map and depths to the sub-Mesozoic (Variscan ) unconformity.

(Figure 3) Supercrop map showing the distribution of strata resting on the sub-Mesozoic unconformity.

(Figure 4) Lithostratigraphical correlation of selected borehole resistivity logs in the Chalk Group (approximate depth in metres shown against each geophysical log).

(Figure 5) Chronology of the Quaternary deposits of the district.

(Figure 6) Proportions of component lithologies within the gravel fraction of the River Terrace Deposits (other minor constituents may also be present). Percentages are for the entire Middle Thames basin. Source: Gibbard, 1985.

(Figure 7a) Colour shaded relief gravity map of the district shows the north-west trend of the Palaeozoic and Precambrian basement; Devonian and Mesozoic strata increase in thickness towards the south-west. A variable reduction density has been used. The shaded topographic effect has been created using an imaginary light source, located to the north. Contour interval 1 mGal (milligal).

(Figure 7b) Colour shaded relief magnetic map of the district. This shows that within the area of the gravity low (Figure 7a) a magnetic anomaly trends north-north-west. This is attributed to Silurian volcanic rocks, proved in the Bicester borehole and mapped on seismic reflection profiles to the west (Smith, 1987). The shaded topographic effect has been created using an imaginary light source, located to the north. Anomalies in nanotesla to a local variant of IGRF90. Contour interval 25 nT.

(Figure 8) Summary of the mineral resources of the district.

(Figure 9) Engineering characteristics of the bedrock formations and superficial deposits of the district.

Plates

(Plate 1) Chiltern Hills downland, West Wycombe: Clay-with-flints caps Seaford Chalk and Newhaven Chalk formations, overlying Lewes Nodular Chalk Formation [SU 832 949] (GS1241).

(Plate 2) Chalk Rock Member, Lewes Nodular Chalk Formation at Fern Pit [SU 884 884] (GS1242).

(Plate 3) Hertfordshire Puddingstone in the wall of Sarratt Church [TQ 039 984] (GS1244).

(Plate 4) Beaconsfield Gravel and Lambeth Group rest on the irregular surface of the Chalk, the result of dissolution, at Springfield Pit, near Beaconsfield [SU 930 894]. The pit is being prepared for landfill — note the liner on the graded slope in the background (GS1245).

(Plate 5) Entrance to West Wycombe cave, constructed mainly from flint. The cave was once the meeting place of the infamous 18th century Hell Fire Club [SU 8275 9483] (GS1246).

(Plate 6) Jubilee River at Taplow, part of the Maidenhead, Windsor and Eton floodalleviation scheme [SU 904 818] (GS1247).

(Front cover)Old brickworks at Poyle Farm, Burnham [SU 925 841] (Photograph: Roy Mogdridge; (GS1240))

(Rear cover)

(Geological succession) Geological succession of the Beaconsfield district

Figures

(Figure 6) Proportions of component lithologies within the gravel fraction of the River Terrace Deposits

(other minor constituents may also be present). Percentages are for the entire Middle Thames basin. Source: Gibbard, 1985.

River terrace deposit Compositional range of main components (%)
Angular flint Rounded flint Quartz Quartzite Greensand chert Other
Post-diversionary Shepperton Gravel 76.5–92 3.5–9 1.5–5.5 0.7–3 0.5–5.5 chalk: 0–2
Taplow Gravel 85–91 2.5–7.5 1.5–6.5 1–3.5 1–5
Lynch Hill Gravel 80–87 4.5–9 2.5–7 1–4 0.7–7.5
Boyn Hill Gravel 77–81 5–10 4–7 1.5–5 2.5–4
Black Park Gravel 75–89 3–9 4–10 1–6 present
Pre-diversionary Winter Hill Gravel 59–80 5–10 7–21 3–13 present
Gerrards Cross Gravel 38–61 3–19 18–30 7–15 present Carboniferous chert: present
Beaconsfield Gravel 34-51 8–12 23–43 8–17 0–2
Chorleywood Gravel Not available
Westland Green Gravel 75–84.5 5–13 7–8 2.5–5 0 Carboniferous chert: present

(Figure 8) Summary of the mineral resources of the district

Mineral resource Source Activity Principal use
sand and gravel River terrace deposits numerous working quarries and pits; formerly worked extensively. concrete aggregate; building and asphalt sand
clay and sand Lambeth Group no activity; former sites at Harefield brickmaking; reservoir and landfill lining
clay London Clay Formation no activity; may have been worked locally in the past brickmaking
clayey silt and silt Langley Silt no activity; widespread historical extraction in the Thames valley brickmaking
chalk Newhaven Chalk Formation

Seaford Chalk Formation

Lewes Nodular Chalk Formation

 only active quarry at Pinkneys Green; former sites at Springwell Farm and Weybeards Farm, near Harefield; and near Taplow agricultural lime, locally used as aggregate
flint Chalk

River terrace deposits

 no activity; former sites at Springwell Farm and Weybeards Farm, near Harefield; and near Taplow building and decorative stone and gun flints

(Figure 9) Engineering characteristics of the bedrock formations and superficial deposits of the district

Engineering geological units Geological units Description/ Characteristics Engineering considerations
Foundations Excavation Engineering fill Site investigation
Highly variable artificial deposits (includes landfill and non-engineering) Worked Ground

Made Ground

Infilled Ground

 Highly variable composition, depth and geotechnical properties Highly variable. May be highly compressible. Hazardous waste may be present causing leachate and methane production Usually diggable Highly variable. Some material may be suitable Essential to determine depth, extent condition and type of fill and chemistry of groundwater
Non-cohesive River Terrace Deposits

Sand Gravel of unknown age

 Medium dense to dense SAND AND GRAVEL. May contain buried channels and lenses of clay, silt, and peat. Variable thickness Generally good. Thick deposits in buried channels may be significant in foundation design Diggable. Trench support may be required. May be water bearing Suitable as granular fill Important to identify the presence and dimensions of buried channels and characteristic of infilling deposits. Geophysical methods may be advisable
Mixed (Cohesive/ non-cohesive) Alluvium Soft grey to brown CLAY occasional sand and peat. High moisture content and variable composition Soft highly compressible zones may be present; risk of differential settlement Diggable. Moderate stability, decreasing with increasing moisture content. Running sand conditions possible below the water table and in pockets at perched water tables. Risk of flooding. Generally unsuitable Determine the presence, depth and extent of soft compressible zones and depth to sound strata
Langley Silt Soft to firm yellow brown sandy CLAY occasional gravel Generally stable, but when saturated has the potential to collapse Diggable. Trench support required when strata saturated Generally unsuitable Important that good quality sampling undertaken so testing for collapse can be undertaken
Clay-with-flints Variable composition. Ranging from a stiff inorganic CLAY or intermediate plasticity to a medium dense clayey GRAVEL with cobbles Generally stable. Dissolution features close to contact with chalk Diggable. Trench support may be required in gravel. May be water bearing. Generally suitable Essential to determine depth, extend and type of strata
London Clay Formation London Clay Formation Stiff bluish grey CLAY with occasional thin sily and fine gravel beds Generally good provided suitable design is adopted. Liable to ground heave and subsidence due to high proportion of smectite in clays. High concentration of sulphate in groundwater could weaken concrete foundations. Presence of perched water table and springs in sand layers Easily diggable. Trench support may be required depending on the proportion of sand and clay. Difficulty dewatering. Suitable for general fill (mixing or screening required) Essential to determine depth and extent of strata, as well as the chemistry of groundwater
Reading Formation Reading Formation Stiff mottled grey, red, purple, and reddish-brown CLAY with lenticular beds of fine to medium sand and gravel. Weathers to yellow-brown colour. Variable due to the presence of sand bodies that may vary considerably in both lateral and vertical extent and subject to the effect of ground heave and subsidence due to high proportion of smectite in clays. High concentrations of sulphate in groundwater could weaken concrete foundations. Presence of perched water table and springs in sand layers. Dissolution features close to contact with chalk Easily diggable. Trench support may be required depending on the proportion of sand and clay. Difficulty dewatering. Suitable for general fill (mixing or screening required) Essential to determine depth and extent of strata, as well as the chemistry of groundwater
Chalk Newhaven Chalk

Seaford Chalk Formation

Lewis Nodular Chalk Formation

 Moderately weak to (rarely) moderately strong LIMESTONE. Properties dependant on weathering grade. Usually food dependant on weathering grade and fracture spacing. Dissolution features cause an irregular rock head and could cause problems with foundations if not detected and designed for Hard digging to easy ripping depending on weathered state Suitable for general fill when dry Important to assess weathering grade of rock and the presence and extent of groundwater protection

(Geological succession) Geological succession of the Beaconsfield district

Thickness (m)
Quaternary Holocene
Alluvium silt, shelly sand and gravel 2 to 3
Pleistocene
Head stony pebbly clay; gravel variable
Head gravel pebbly flint and quartz gravel variable
Clay-with-flints clay, nodular and rounded flints up to 10
Langley Silt silt up to 4
Interglacial deposits organic clay
River terrace deposits sand and gravel up to 10 ?
Stanmore Gravel sand and gravel ?
Sand and gravel of unknown age and origin sand and gravel ?
Palaeogene THAMES GROUP
London Clay Formation mudstone 48
Harwich Formation glauconitic pebbly sand; clay 3
LAMBETH GROUP
Reading Formation clay and sand 22
Upnor Formation clay; sand; gravel 2 to 6
Upper Cretaceous CHALK GROUP
White Chalk Subgroup
Newhaven Chalk Formation chalk with layers of flints; locally phosphatic 13
Seaford Chalk Formation chalk with layers of flints 37
Lewes Nodular Chalk Formation nodular chalk with layers of flint, thin marls and hardgrounds 30–35
New Pit Chalk Formation chalk with sporadic flints; thin marls and nodular chalk 45
Holywell Nodular Chalk Formation nodular chalk 15
Grey Chalk Subgroup limestone and marl interbedded 70
Lower Cretaceous Upper Greensand Formation sand, partly glauconitic, biogenic sandstone 60 to 70
Gault Formation mudstone
LOWER GREENSAND GROUP sand with sandstone and mudstone 18 to 54
Upper Jurassic ANCHOLME GROUP
Oxford Clay Formation mudstone 64
Kellaways Formation mudstone, sand and sandstone
Middle Jurassic GREAT OOLITE GROUP mudstone and limestone 51