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Geology of the Newbury district — brief explanation of the geological map Sheet 267 Newbury

D T Aldiss, A J Newell, N J P Smith and M A Woods

Bibliographic reference: Aldiss, D T, Newell, A J, Smith, N J P, and Woods, M A. 2006. Geology of the Newbury district — a brief explanation of the geological map.Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 267 Newbury (England and Wales).

© NERC 2006 All rights reserved

Keyworth, Nottingham: British Geological Survey, 2006.

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

(Front cover) Typical Chalk downland scenery near Lambourn, with sarsen stones (weathered relicts of silicified Palaeogene sandstone) in the foreground. View from Weathercock Hill [SU 2930 8223] looking north-east (Photographer C F Adkin; P535218).

(Rear cover)

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

Notes

The area covered by geological Sheet 267 Newbury is referred to as 'the district'; in some older Geological Survey publications it was known as the Hungerford district. National Grid references are given in square brackets [3954 7264]. Unless otherwise indicated, all should be preceded by the letters SU (or the digits 4 and 1 [43954 17264]) to give a unique grid reference. The number given with the plate captions is the registration number in the British Geological Survey photograph collection. Boreholes are identified by their BGS Registration Number, given in the form SU57SW/9, where the prefix indicates the 1:10 000 scale National Grid sheet.

Acknowledgements

This Sheet Explanation is based on the account given in the corresponding Sheet Description by D T Aldiss, A J Newell, R J Marks, P M Hopson, A R Farrant, K R Royse, J A Aspden, D J Evans, N J P Smith, M A Woods and I P Wilkinson. R A Ellison and S J Mathers provided information on Palaeogene sections on the line of the Newbury bypass. J D Appleton provided text for the section on radon emissions. M Lewis improved the section on Water Resources. A Forster reviewed the rest of the section on ground conditions and geological hazards. Computer modelling of the Chalk was carried out by K R Royse and B Napier under the BGS DGSM (Digital Geological Spatial Model) Project. Cartography is by R J Demaine and P Lappage; series editor is A A Jackson.

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

© Crown copyright. All rights reserved. Licence Number: 100017897/2006.

Geology of the Newbury district (Summary from rear cover)

An explanation of sheet 267 1:50 000 Series Map (England and Wales)

Continuing development requires accurate geological information, both to identify mineral resources and to ensure appropriate design of new construction. This Sheet Explanation and the newly revised geological map that accompanies it together summarise information on many aspects of the geology of the Newbury district. The Sheet Explanation is written as an introduction both for those who may have limited geological experience, and for the professional who may wish to be directed to further geological information.

The market town of Newbury lies on the River Kennet, at the southern foot of the Berkshire Downs. The other main streams in the area are the rivers Lambourn, Enborne and Pang. Apart from Thatcham, adjacent to the east, and the small town of Hungerford, to the west, the district is rural with many villages, but it is crossed north to south by the A34(T) main road, and east to west by the M4 motorway, the railway line between London and Bristol, and by the Kennet and Avon canal.

The oldest rocks found at the surface are sandstones of the Upper Greensand of Lower Cretaceous age, forming the floor of the Vale of Ham. The rolling downlands found in the greater part of the district are underlain by the Upper Cretaceous Chalk Group, the major aquifer of the region. Relatively soft Palaeogene clays, sands and pebble gravels belonging to the Lambeth Group and the London Clay occur in the south-east, and are marked by a change to a more wooded landscape.

Periods of cold climate during the Quaternary are represented by a series of gravelly river terrace deposits. Their remnants are found at nine levels, marking the progressive down-cutting by the river systems over the past 2 million years or so. The Silchester Gravel, the most extensive of the 'high-level' gravels, caps the flat-topped ridge just to the south of Newbury, including the acid heathlands of Greenham Common. Other superficial deposits obscure large areas of the bedrock with a variably stony clayey or sandy cover.

Potentially hazardous ground conditions can arise in parts of the district from landslip, ground heave, subsidence or flooding. There are cavities in the Chalk from natural dissolution and past shallow mining.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the Newbury district, the area covered by 1:50 000 geological Sheet 267 (Newbury). Further details are given in the corresponding Sheet Description (Aldiss et al., in press) and in other literature mentioned in the text.

The Newbury district lies mostly within West Berkshire District, with a western quarter in Wiltshire and small parts in Hampshire (in the south-east), Swindon Borough (in the north-west) and Oxfordshire. The largest towns are Newbury, Thatcham and Hungerford. Newbury lies about 85 km west of central London on the River Kennet, in the western part of the Thames Basin. The district is crossed, north to south, by the A34(T) main road, and, east to west, by the M4 motorway, the main railway line between London and Bristol, and by the Kennet and Avon canal.

The surface geology of the district is dominated by the Chalk Group, which forms the downlands (Front cover). The Upper Greensand Formation appears from beneath the Chalk to form inliers in the south-west of the district, in the core of anticlinal folds aligned with the Vale of Pewsey. The southern part includes the western end of the Palaeogene outcrops of the London Basin.

The bedrock formations are commonly obscured by superficial ('drift') deposits of Quaternary age. These include river terrace deposits, alluvial deposits of the modern river flood plains, residual deposits (clay-with-flints and sand) that cover the higher parts of the Downs, and solifluction deposits (head) on slopes and valley floors in most parts of the district.

The district lay at the southern edge of the Midlands Microcraton (Pharaoh et al., 1987); this was an area of relative stability throughout early Palaeozoic basin development and the Caledonian Orogeny that followed. During the late Palaeozoic, the microcraton became part of the London–Brabant Massif, a foreland area to the north of the belt of Variscan folding and thrusting that underlies southern England. The northern limit of Variscan deformation, the Variscan Front, lies across the middle of the district (Figure 1ab).

Variscan thrusts were reactivated during a period of crustal extension in early Jurassic times that resulted in the development of the Wessex Basin, which lies to the south of the district. The London Platform (Anglo-Brabant Landmass) that lay to the east remained above sea level possibly until mid to late Jurassic times. Subsidence ceased in early Cretaceous times, and a general uplift in north-west Europe, caused by crustal extension in the north Atlantic, is marked by the widespread Late Cimmerian unconformity (at the base of the Lower Greensand Group). The Chalk was deposited across both the London Platform and the Wessex Basin, although the continuing influence of both regional and local structures can be seen in the lateral variations in the Upper Cretaceous succession. Uplift in the late Cretaceous led to differential erosion of the Chalk prior to the early Palaeogene marine transgression, so that the age of the youngest Chalk preserved beneath the sub-Palaeogene unconformity varies significantly across the district.

Continuing crustal movement in the Cainozoic led to further reactivation of the Variscan thrusts, and reversal of the fault systems seen in the Mesozoic strata. This created the broad syncline of the London Basin, and an en échelon series of strongly asymmetric monoclines and periclines on the northern margin of the Wessex Basin, the most northerly of which forms the Vale of Ham, in the south-west of the district.

Chapter 2 Geological description

The geological succession present at outcrop in the district is shown on the inside of the front cover. Much of the evidence concerning strata concealed beneath the Upper Greensand has been drawn from deep boreholes sited in and around the district (Figure 1ab), together with interpretation of seismic reflection surveys and regional maps of gravity and magnetic anomalies.

Concealed strata

Broad magnetic and gravity anomalies within the Newbury district and adjacent areas to the north (Figure 2) probably reflect the presence at relatively shallow depth of Precambrian rocks of the Midlands Microcraton (Busby and Smith, 2001). These are thought to include intermediate volcanic rocks, as found in the Withycombe Farm Borehole [SP 4319 4017] near Banbury, although none have been proved within the district itself. In the south, they are buried by north-vergent Variscan thrusting.

Palaeozoic strata occur some 400 m below Ordnance Datum in the north-east of the Newbury district, increasing in depth generally south-westwards to more than 2400 m (Figure 1ab); (Figure 3). However, Lower Palaeozoic strata have not been proved by boreholes within the district. Evidence from elsewhere in southern England indicates that the entire area is underlain by a relatively thin Cambrian and locally thick lower Ordovician succession. This succession probably directly underlies Triassic strata in the west (Figure 1ab). Mid to late Ordovician formations are probably absent but all four stages of the Silurian are likely to be present; the succession is most complete in the north-eastern half of the district (Smith, 1987).

The Silurian succession probably includes volcanic rocks, broadly related to formations exposed in the Mendip Hills (some 60 km to the west) and found in the Bicester Borehole [SP 5878 2081]. These are thought to be part of the cause of the short-wavelength magnetic anomaly in the area between Newbury and Reading (Figure 2b), although this anomaly is probably of composite origin, and is also partly attributed to Upper Carboniferous volcanic rocks and minor intrusions, and to Precambrian volcanic rocks.

The Devonian succession reaches more than 2000 m in thickness beneath the Oxfordshire and Berkshire coal basins, part of which underlies the north-east of the district (Figure 1ab); (Figure 3). About 28 m of Devonian sandstones with some siltstones, mudstones and conglomerates were proved in the Maddle Farm Borehole [SU 305 823], 1.2 km north of the district. An unconformity separates the Lower and Upper Devonian successions. Although much of the Early Devonian was deposited under fluvial, coastal or deltaic conditions, there were some more extensive marine incursions during the Late Devonian.

During the early Carboniferous, a marine transgression once again introduced a shallow shelf sea to the region. In the Aston Tirrold Borehole, about 6 km north-east of the district, 15 m of Dinantian (Tournaisian) limestones and mudstones were found. By contrast, 10 m of somewhat younger limestones of Visean (Holkerian) age were proved in the bottom of the Foudry Bridge Borehole [SU 7063 6602], about 23 km to the east of Newbury. The age span of these limestones suggests that a much thicker Lower Carboniferous succession was once present but was largely removed, probably following early Variscan uplift and erosion during Namurian times, as found in the Kent coalfield (Smith, 1993).

Contours on the base of Westphalian strata indicate that the Oxfordshire–Berkshire Coal Basin is an asymmetric syncline (Figure 1ab), an interpretation supported by the presence of steeply dipping strata in the Welford Park Station Borehole [SU 407 736]. This asymmetry is attributed to thrusting at the Variscan Front. It is possible that this 'basin' is an eroded remnant of Coal Measures strata that were deposited in deltaic and coastal environments all the way along the southern and western edge of the London–Brabant massif, from Kent to south Wales. The strata of Westphalian age can be subdivided into relatively thin Coal Measures and an unconformably overlying succession of red beds and some coal-bearing strata, correlated with the Warwickshire Group (Foster et al., 1989; Powell et al., 1998). These include some clastic material derived from older Westphalian strata, presumably reworked from the southern margin of the basin as Variscan deformation progressed. The shallow-source component of the Reading–Newbury magnetic anomaly (Figure 2b) is probably due to basaltic lavas and microgabbroic minor intrusions of Westphalian age, given the range of magnetic susceptibility found in these rocks, and their widespread distribution in the region. Basalts with calcite-filled vesicles were found in the Aston Tirrold [SU 558 872] and Burnt Hill [SU 522 738] boreholes, with microgabbro sills. At Aston Tirrold these are interbedded with the Lower Coal Measures. At Burnt Hill, an overlying Warwickshire Group conglomerate suggests an unconformity above the igneous rocks.

Just to the north of the district, near the axis of the Berkshire Coal Basin, the Harwell 3 Borehole proved younger coal-bearing strata, probably the Witney Coal Group (Foster et al., 1989), which lacks thick sandstones.

It is likely that no Permian deposits occur in the district.

During the Triassic and Jurassic, the Newbury district was gradually submerged through subsidence associated with the east–west Wessex Basin, to the south (Figure 1ab), and the north–south Worcester Basin, at closest some 20 km to the west. The Mesozoic strata in the district comprise an attenuated succession deposited near the south-west margin of the London Platform, with the depth to the Variscan unconformity increasing south-westwards (Figure 1ab).

The oldest Triassic strata found at depth in the Newbury district are sandstones with siltstones and thin mudstones of the Sherwood Sandstone Group. It passes upwards into the Mercia Mudstone Group (chiefly mudstone and siltstone) and Penarth Group (mudstone and limestone). The Triassic succession increases in thickness towards the Worcester Basin and is more than 450 m thick in the west of the district; it is probably absent from the eastern edge of the district, (Whittaker, 1985).

Significant thicknesses of the Jurassic Lias Group are present at depth in the district, but the subsequent succession of Jurassic strata is mostly rather attenuated, as summarised in (Figure 3). The Lias Group onlaps eastwards from the Worcester Basin onto the London Platform. It was overlapped unconformably by the Inferior Oolite and Great Oolite Group east of the district (Donovan et al., 1979) after removal of the higher strata (Sumbler, 1996). The Inferior Oolite and Great Oolite groups comprise a complex succession of laterally variable limestones with some mudstones. A marine transgression began during the deposition of the Cornbrash, the youngest formation of the Great Oolite. This introduced deeper water conditions to the district associated with mud-dominated deposition, including the Kellaways Formation (with an upper sandy member), the Oxford Clay Formation, and the West Walton Formation. In western parts of the district, the West Walton Formation may pass laterally into the oldest unit of the Corallian Group. The rest of the Corallian probably overlies the West Walton Formation. It represents a short-lived change to shallower seas with higher energy conditions, and comprises limestones and sandstones with some mudstones, in a laterally variable succession. Mudstone deposition then resumed and is represented by the Ampthill Clay Formation, which includes some silty or ferruginous beds where it is exposed in adjacent districts to the north-west, and the Kimmeridge Clay Formation, which includes some sandy and silty units in its upper parts.

During the Late Jurassic, the sea receded from the London Platform and the upper beds of the Kimmeridge Clay were removed by erosion. The succeeding Portland Group, where present, thus rests on an unconformity. It is probably confined to the south-west and south-east parts of the district (Figure 4), comprising a thin succession of sandstones, perhaps with limestones.

The Purbeck Limestone Group is present in the Kingsclere Borehole [SU 4984 5820], 3.5 km to the south of the area and within the Weald Basin (Figure 3). In common with the Portland Group, this unit is likely to thin rapidly onto the London Platform and not to extend far north of the Mesozoic fault zones near the Variscan Front.

The concealed Cretaceous strata is thought to include a relatively thin 'Wealden Group' in the extreme south of the district: a fuller succession with Weald Clay at the top occurs within the Weald Basin just to the south, as proved in the Kingsclere Borehole [SU 4984 5820] (Figure 3); (Figure 4).

The abrupt decrease in depth to the Late Cimmerian unconformity, at the base of the Lower Greensand Group, just south of the district (Figure 4) shows the extent of inversion along faults at the northern margin of the Weald Basin. A marine transgression in mid-Aptian times led to the deposition of some 80 m of Lower Greensand Group strata around Faringdon, between about 10 and 15 km north of the district. The succession occurs in a partly fault-bounded trough trending approximately north-west and comprises sandstone, conglomerate and mudstone with some chert and two beds of fuller's earth that are exploited at Baulking [SU 323 908]. These Upper Aptian rocks rest on Jurassic strata of the Corallian Group and Kimmeridge Clay Formation (Poole et al., 1971; Ruffell, 1998). Immediately to the east, the Lower Greensand is overlapped by the Gault, but it reappears in boreholes at Harwell and Aston Tirrold (Figure 1); (Figure 3). It is likely to continue south-east at depth beneath the Newbury district in one or more fault-bounded troughs: thin units of Lower Greensand occur at Welford Park Station and Kingsclere. It is absent from the Burnt Hill Borehole, in the western part of the Reading district, but present in the Foudry Bridge and Strat B1 boreholes to the south (Figure 1ab). The Lower Greensand is overlain unconformably by the Gault.

At outcrop to the north of the district, the Gault Formation comprises grey mudstone, commonly with small dark brown phosphatic nodules. The lower part is siltier than the underlying Kimmeridge Clay, and locally sandy. Similar rock types have been found in boreholes in and around the district. It is typically about 70 m thick, increasing to more than 80 m in the south (Figure 3). The Gault passes with rapid gradation upwards into the Upper Greensand.

Cretaceous

The Upper Greensand Formation is the oldest unit to crop out in the district, occurring in the Vale of Ham and the north-eastern extremity of the Vale of Pewsey, in the south-west. There, it comprises calcareous, glauconitic, fine-grained sand and sandstone, with some lenses and layers of siliceous sandstone and chert.

At outcrop to the north of the district, the Upper Greensand comprises a variable lower unit (some 10 to 30 m thick) of sparsely glauconitic siltstone and fine-grained sandstone (in part composed principally of siliceous sponge spicules), and a more uniform upper unit (3 to 10 m thick) of dark green glauconite sand and sandstone, of youngest Albian age. There is a marked eastwards increase in thickness at Wantage [SU 39 87] presumably reflecting a measure of syndepositional tectonic control. Borehole records suggest typical thicknesses of between 20 and 25 m in the north and centre of the Newbury district, with a marked increase in the south, at the northern edge of the Weald Basin (Figure 3). The Upper Cretaceous Chalk Group underlies most of the area, extending beneath the Palaeogene outcrop in the south and south-east. The limits of the Chalk subcrop beneath the Quaternary deposits around Newbury are known only approximately, from borehole records. A typical total thickness to the top of the Seaford Chalk is estimated to be about 200 m, with as much as 42 m of the overlying Newhaven Chalk preserved locally. This is comparable with thicknesses found in boreholes in the Reading district (Mathers and Smith, 2000). Cored boreholes at North Farm [SU 3321 7971] and Winterbourne [SU 4542 7161] together provide a reasonably typical record of the local Chalk succession, except for the youngest strata, although some units, particularly the Seaford Chalk, appear thicker than average for the district (see inside front cover; (Figure 5)).

The greatest stratigraphical variations in thickness of the Chalk appear within the New Pit Chalk and the Seaford Chalk; variation of the other units is less marked. Pre-Palaeogene erosion has led to a wide variation in the preserved thickness of the Newhaven Chalk.

Traditionally, the Chalk was divided into three units: the Lower Chalk, the Middle Chalk and the Upper Chalk. The named beds forming the respective bases of these units, the Glauconitic Marl, the Melbourn Rock and the Chalk Rock, are widely recognised and are present in the Newbury district. Recent work, however, has shown that a more detailed lithostratigraphical subdivision of the Chalk Group is possible, with the boundary between an older Grey Chalk Subgroup and a younger White Chalk Subgroup being placed at the base of the Plenus Marls, slightly below the base of the traditional Middle Chalk (Figure 6). Each component formation can be recognised in the field, and in geophysical borehole records (Figure 7).

As a whole, the Chalk contains a diverse marine invertebrate fossil fauna, with the abundance and dominant types varying considerably between formations and individual beds. In different parts of the succession, ammonites, brachiopods, bivalves, belemnites, echinoids and crinoids are used for biostratigraphical zonation. A microfossil zonation, based on the distribution of foraminifera, is also used (Figure 6).

The basal member of the West Melbury Marly Chalk Formation, the Glauconitic Marl, rests non-sequentially on the Upper Greensand, and is likewise seen at the surface only in the south-west of the district. It comprises pale brownish grey clay-rich chalk (marl) with conspicuous sand-sized glauconite grains. It is typically between 0.3 and 2 m in thickness. Local records of greater thicknesses might reflect an incorrect identification of the glauconite-rich top bed of the Upper Greensand. The rest of the formation comprises numerous rhythmic alternations, of soft clay-rich chalk passing up into hard grey limestones. Some of the limestones are sparsely glauconitic and some conspicuously fossiliferous. Ammonites and sponges are especially common.

The marl–limestone rhythms continue into the lower part of the Zig Zag Chalk Formation, but pass upwards into more massive, uniform greyish chalk. On the whole, the unit is more sparsely fossiliferous than the West Melbury Chalk. The character of the basal bed of the Zig Zag Chalk changes regionally. Over the greater part of the Newbury district, this is probably the Cast Bed, which also occurs in the basinal successions of the North Downs and South Downs. The Cast Bed is composed of brown silty chalk with abundant moulds of gastropods and other molluscs, and small brachiopods. In the Chilterns, and perhaps in north-eastern parts of the Newbury district, the basal bed is the Totternhoe Stone. This is composed of glauconitic calcarenitic chalk, commonly with small phosphatic nodules. Its closest known occurrence is at Chilton [SU 4976 8592], about 5 km north of the district. Unlike the Cast Bed, it rests on an erosion surface and can include derived fossils. The base of the Zig Zag Chalk is nowhere exposed in the district, and has proved difficult to map consistently.

In contrast, the base of the Holywell Nodular Chalk Formation is clearly marked by the relatively soft Plenus Marls. Typically, the Plenus Marls comprise eight beds, alternately of clay-rich chalk and clayey limestone, altogether between about 0.5 and 1 m in thickness in this area. The overlying Melbourn Rock is some 3 or 4 m thick, and consists of very hard, poorly fossiliferous creamy white chalk. The upper two thirds of the Holywell Chalk is composed mostly of hard nodular chalk with conspicuous remains of mytiloid bivalves. Readily recognisable fragments of these rock-types can be found in tilled soil on the Holywell Chalk outcrop.

The rather massive, blocky, white chalk of the New Pit Chalk Formation tends to form the steepest parts of the Chalk escarpments, although where it crops out in the valleys in the north-west of the district this characteristic is somewhat subdued. This unit includes numerous thin beds of clay-rich chalk ('marl seams'). It is much less fossiliferous than the Holywell Chalk. The New Pit Chalk also shows considerably more local variation in thickness. It is relatively thin (less than 30 m) in the west of the area, because of intraformational erosion or nondeposition (Figure 7). It is relatively thick (more than 40 m) in the south, on the northern edge of the Weald Basin, and locally in the north-east.

The base of the Lewes Nodular Chalk Formation is defined by the appearance of hard nodular, gritty chalk above the smooth chalk of the New Pit Chalk. This change is regionally diachronous. In this district it occurs between about 2 and 10 m below the base of the Chalk Rock, a member taken as the base of the Upper Chalk (Figure 6). An abundant and diverse invertebrate fossil fauna occurs in some beds of the Lewes Chalk. The Chalk Rock is a complex and variable succession of hardgrounds mineralised by glauconite or phosphate, each overlying a bed of chalkstone (highly indurated chalk) passing down into nodular chalk. It represents a sequence that is considerably condensed in comparison with basinal successions in Sussex, for example. At Fognam Farm Quarry [SU 2978 7999], the Chalk Rock includes five named hardgrounds and several subsidiary ones and is about 4.5 m thick (Plate 1). The lower hardgrounds fade out to the east (Bromley and Gale, 1982). In the Banterwick Barn Borehole [SU 5134 7750], only three of the named hardgrounds are present and the Chalk Rock is only 3.5 m thick. Conversely, to the south at Shalbourne [SU 3157 6389], four of the named hardgrounds are present and the unit could be as much as 8.5 m thick.

The gradational upward limit of nodularity and grittiness of the Lewes Chalk, marking the base of the Seaford Chalk Formation, is also regionally diachronous. In the Banterwick Barn Borehole, in the east, it is seen in the higher part of the cortestudinarium Zone, rather earlier than in Sussex, for example. Conversely, at Stichcombe Farm [SU 2270 6925] just to the west of the district, an occurrence of hard nodular chalk within the low coranguinum Zone possibly indicates a greater duration of Lewes Chalk deposition. This variation could reflect both tectonic and sedimentological control on local chalk deposition. Although the general aspect of the Seaford Chalk is of typically uniform 'white chalk with flints' (Plate 2), as seen in its broad outcrop underlying most of the Newbury district, the presence of phosphatic chalks and hardgrounds in the Seaford Chalk near Boxford [SU 4308 7195] (Jarvis and Woodroof, 1981) also demonstrates probable local tectonic control of chalk facies. A bed of very hard creamy-white chalk containing abundant sponge spicules, about 1 m in thickness, the Stockbridge Rock, occurs about 5 m from the top of the formation in places. Beds containing abundant coarse bivalve shell debris occur at some levels.

In general, the thickness of the Seaford Chalk in the Newbury district is between 50 and 75 m. Similar thicknesses have been estimated for chalk of the coranguinum Zone in the adjacent districts of Reading, Basingstoke, Andover, and Devizes. However, in an east-north-east-trending zone up to 7 km wide between Great Bedwyn [SU 27 64] and Chieveley [SU 47 73], it appears to be thicker, generally exceeding 80 m. This zone is just to the north of the fault zone bounding the Weald Basin, and the anomalous chalk succession exposed at Boxford is on its northern edge, suggesting a measure of tectonic control.

The Newhaven Chalk Formation, the youngest part of the Chalk present in Berkshire, crops out in the core of the London Basin syncline, and in four small outliers north of the River Kennet. Where it can be observed, the Newhaven Chalk appears rather similar to the Seaford Chalk but contains crinoid remains, less flint and more seams of clay-rich chalk. Phosphatic chalks and hardgrounds occur near Winterbourne [SU 4476 7223] implying further persistence of local tectonic control (Jarvis and Woodroof, 1981). Up to 14 m of strata are present in the outliers around Boxford, and more than 40 m are present beneath the Palaeogene to the south-west of Newbury.

The unconformity at the base of the Palaeogene cuts across the Chalk stratigraphy. The youngest chalk seen in the district is of pilula Zone age (Figure 6), once exposed at Kintbury [SU 3874 6658], near the axis of the London Basin syncline (White, 1907), and in the anomalous sequence near Winterbourne (Jarvis and Woodroof, 1981). In most of the area south of the Kennet, the Palaeogene lies on testudinarius Zone chalk, cutting down to the socialis Zone between Shalbourne and Little Bedwyn, and locally into the coranguinum Zone. The Newhaven Chalk has been completely removed by early Palaeogene erosion in much of the outcrop north of the Kennet, as near Chieveley [SU 479 731] (Plate 2), although its subcrop extent is poorly known in the east, for example around Thatcham. The oldest Chalk of known age found locally at the basal Palaeogene unconformity is from the middle of the coranguinum Zone, at Frilsham Quarry [SU 5400 7294], about 1.3 km east of the district.

Palaeogene

Palaeogene deposits of the Lambeth Group and the Thames Group are present in the south of the area, forming the western tip of the London Basin. Together they form an escarpment standing conspicuously above the main chalk dip slope. This escarpment tends to be wooded, in marked contrast to the open fields of typical chalk downland.

The Lambeth Group, of Paleocene age, consists mainly of the Reading Formation, with a thin basal unit, the Upnor Formation. It corresponds to the 'Woolwich and Reading Beds' of earlier work. In the western part of the London Basin, it is not practical to separate the two except in exposed sections, and so the map shows the combined unit. The Lambeth Group generally varies between about 20 and 25 m in thickness, but thicknesses of up to 35 m occur locally due to the appearance of thick basal sands.

The Upnor Formation was deposited during an early Palaeogene marine transgression. It consists of highly glauconitic, green, blue and grey sands and clays. At the base it contains large, green- or black-coated, irregularly shaped flint nodules and well-rounded flint pebbles, resting unconformably on a locally irregular Chalk surface, bored by marine organisms and further modified by karstic dissolution (Plate 2). The formation also contains a varied marine fauna. It is generally less than 1 m in thickness, but as much as 2.2 m was found in sections on the line of the A34(T) Newbury bypass [SU 4495 6778].

The Reading Formation represents deposition in a predominantly nonmarine, deltaic and fluvial environment, probably with some minor marine incursions. It is made up of colour-mottled clays (mainly red and grey, but also green, purple, brown or orange), some silty or sandy, with common beds of sand that is grey, brown, yellow or orange in colour and fine to coarse grained. A bed of black organic clay up to about 30 cm in thickness commonly occurs within the lower half of the formation, and is marked by a peak on many natural gamma-ray borehole logs (Figure 8). The sand is locally and patchily cemented by silica, thus forming a very hard sandstone. Relict masses and fragments of such sandstone are highly resistant to weathering and erosion, and commonly occur in the soil and in some superficial deposits of the area: these relicts are known as sarsens (Front cover). Fossil leaves, seeds, lignite and silicified wood occur widely, and are locally abundantly in the Reading Formation, as found in a section at Cold Ash, for example (Crane and Goldring, 1991). Some of the sands, particularly near the base, include the trace fossil Ophiomorpha, flaser bedding and clay drapes suggesting some tidally influenced deposition (Plate 2).

The sand beds occur at all levels but especially near the base. They are mostly up to 2 m in thickness, but locally reach as much as 10 m. The sand is commonly cross-bedded or cross-laminated, some with clay flasers. Some sand bodies are very steep-sided and upwards-fining, representing river channel deposits within an otherwise clay-dominated floodplain. Borehole correlation shows that some thick sand bodies near the base of the sequence occupy depressions, presumably channels, in the top of the Chalk. Borehole logs also suggest the presence of upwards-coarsening sequences as much as 5 m in thickness, perhaps representing channel-mouth bars or crevasse-splay deposits (Figure 8).

Clay-with-flints (see below) is regarded as a residual deposit derived, in the main, through prolonged weathering, in situ disturbance and partial erosion of Palaeogene deposits that once extended across the district. The spatial relationship between typical clay-with-flints, in which few if any relicts of the Palaeogene bedding can be discerned, and undisturbed outcrops of the Lambeth Group is largely unknown, but it is likely to be complex. The transition possibly occurs within a zone some hundreds of metres in width. The mapped boundary between these two deposits should be regarded as highly approximate.

The London Clay Formation, of Eocene age, crops out in the axis of the London Basin, in the south-east, with several outliers to the north and west. Regionally, the London Clay is underlain by the Harwich Formation, previously recognised as its 'Basement Bed': together they comprise the Thames Group (Ellison et al., 1994). The Thames Group marks a return to marine deposition in the region, although around Newbury it is in a relatively proximal sandy facies.

In the Newbury district, the Harwich Formation is represented by a basal pebble bed of variable thickness, up to 3 m, typically overlain by highly glauconitic shelly sands and clayey silts of marine origin. The total thickness apparently does not exceed 6 m, seen in the east of the district. As in the Reading district (Mathers and Smith, 2000), this basal unit has been mapped as part of the London Clay.

The London Clay comprises blue-grey silty clay and clayey silt, together with extensive beds of sand forming upwards-coarsening cycles, each with a basal pebble gravel lag. Beds of sandy gravel occur in the west. The gravels overlie a transgressive marine erosion surface, and the clays are taken to mark the ensuing period of high sea level, with the succeeding sandy deposits reflecting gradual marine regression and coastal progradation. The clays contain a shelly marine fauna, and calcareous concretions and pyrite nodules also occur. They weather to a rusty brown colour.

The sands within the London Clay are generally fine grained, plane bedded or cross-stratified, slightly micaceous and locally glauconitic, with subordinate thin lenses or beds of silt or clay. They are typically yellow-brown and grey in colour, varying to white or brownish red. The base is commonly marked by a sharply defined, possibly erosive contact, in places with a discontinuous bed of black flint pebbles. Beds of sand up to 20 m in thickness occur in the London Clay in this district, the largest of which have been shown separately on the geological map. The lower part of the London Clay, below the lowest mappable sand body, gradually decreases in thickness north and west from about 40 m just north-east of Newbury to some 25 m near Hermitage, about 10 m near Hamstead Marshall and about 4 m west of Little Bedwyn. This reflects the increasing dominance of the progradational, proximal sand facies.

The westernmost outliers of the London Clay comprise sandy clayey, pebble gravel and gravelly sandy clay, locally overlain by sand. Typically, the gravel consists of very well-rounded flint pebbles and rare cobbles, commonly black coated, and bearing crescentic 'chatter marks' characteristic of flint beach pebbles (Plate 3). To the east, around Inkpen, a thin basal pebble bed is overlain, in turn, by 5 to 10 m of grey silty micaceous clay, some 8 to 10 m of silty fine sand and sandy silt, and a bed of very sandy silty clay packed with flint pebbles. This upper pebble bed has a sharply defined, possibly erosional base. Locally, it attains 5 m in thickness, but rapidly thins and dies out laterally.

The sand beds forming the local top of the Palaeogene sequence were previously correlated with the Bagshot Formation. Evidence from the stratigraphy and distribution of these deposits shows that they are instead part of the London Clay Formation. It is likely that the Bagshot Formation does not extend west of Reading, a possibility recognised by Mathers and Smith (2000, p.13).

Up to about 50 m of the London Clay is present in the east of the district, with the preserved thickness diminishing northwards and westwards.

Quaternary

The superficial (drift) deposits of Quaternary age are classified by mode of origin. In the south-east of the district, they are dominated by a complex sequence of river terrace deposits, which range in a disjointed 'staircase' from some ridge-tops down to beneath the floor of the main valleys, where they are overlain by alluvium and peat. Silt of aeolian origin occurs locally on the younger river terraces. In the north and west of the district, large areas of downland carry a cover of residual deposits conventionally known as 'clay-with-flints', although in this district this cover includes some sand. A discontinuous cover of mass-movement deposits is present on slopes and in valley floors throughout the district. These consist mainly of head (solifluction deposits and colluvium), but landslips occur locally on the Palaeogene formations around Newbury.

River terrace and floodplain deposits

The alluvial deposits of the Newbury district were laid down mostly in the River Kennet drainage system (including the River Enborne and the River Lambourn), although some elements belonging to the River Pang are also present in the north-east. Both rivers flow to the Thames.

River terrace deposits occur widely in the south-east of the area. They were laid down predominantly during periods of cold climate in the Quaternary. Nine principal levels of river terrace deposit can be recognised, with two of them being locally subdivided. Local names are applied to these deposits in the Kennet catchment but their correlation within the rest of the Thames basin is indicated by a common number. The older terraces (Ninth to Sixth), dating from the Anglian and earlier Quaternary times, were laid down by rivers following somewhat different courses to the modern ones. These 'high-level' deposits occur at various altitudes on valley sides and interfluves. These correspond to the aptly named 'plateau gravel' of the older literature. The post-Anglian, 'low-level', terraces (Fifth to First) follow the present-day river valleys. The youngest, the First Terrace, was deposited in the floor of the modern valleys at a time of lower sea level than at present. It is generally overlain by postglacial alluvium and associated deposits including peat and tufa.

The river terrace deposits typically comprise sandy gravels and gravelly sands, commonly clayey or silty. They are generally less than 5 m thick. Their composition is dominated by water-worn flint, with minor amounts of sandstone, vein quartz and ironstone. With the exception of a wind-blown fine-grained fraction, the material is likely to have been derived entirely from formations presently found within the Kennet catchment, at least in the lowest six terraces (Chartres, 1981).

The older terrace deposits probably represent extensive former alluvial plains of subdued relief, over which flowed braided rivers. They have been dissected by subsequent erosion to progressively lower levels, and in post-Anglian times within entrenched river valleys. Those terrace remnants that have escaped erosion have undergone cryoturbation and solifluction during the periods of cold climate following their deposition. These processes led to a gradual degradation of the original landform as their component material was reworked into head, or into younger river terraces. There is a general tendency for the terrace landforms to lose definition in an upstream direction, with the river terrace deposits passing progressively into blankets of gravelly head.

Correlation of the river terrace deposits (Figure 9) was established by resurvey at 1:10 000 scale, followed by construction of along-valley profiles. Correlation of individual remnants is based partly on relative height above the modern flood plains, following Bridgland (2003). The names largely follow Collins (1994, 1999) and Mathers and Smith (2000), with some modification. Where the individual terrace deposits have begun to merge into head, it is not possible to subdivide them with confidence and only the probable range of terrace levels, such as the Fourth to Seventh, can be indicated.

The highest and therefore oldest remnants belong to the Cold Ash Gravel (Ninth terrace), occurring between about 140 and 165 m OD, and typically comprising 3 m of sandy gravel. They are confined to ridges on the watersheds between the Kennet and, respectively, the Pang and the Lambourn, and so may have predated the establishment of these rivers as separate drainage basins. However, elements of the Bucklebury Gravel (Eighth terrace) and, in the Reading district, younger terraces occur both to the north and the south of the Cold Ash ridge, implying separation of the Kennet from the Pang–Thames streams by this time at latest. There is similar evidence for the separate identity of the Kennet and Lambourn by the time the Beenham Stocks Gravel (Seventh terrace) was deposited. The youngest high-level terrace deposit, the Silchester Gravel (Sixth terrace) occurs between about 100 and 130 m OD. A remarkably extensive outcrop forms the flat-topped ridge south of Newbury, including the heathlands of Greenham Common, where the surface slopes gently eastwards at a gradient of about 1.4 m/km. Sandy gravels, some 5 m in thickness at Furze Hill quarry [SU 426 687], are here assigned to the Silchester Gravel.

Five low-level terrace deposits are preserved in the valleys of the Kennet, the Lambourn and the Pang: the Hamstead Marshall Gravel (Fifth terrace), small areas representing an unnamed Fourth Terrace Deposit, the Thatcham Gravel (Third terrace), the Beenham Grange Gravel (Second terrace) and the Heales Lock Gravel (First terrace). The last is largely covered by alluvium and peat and is not exposed, except, probably, in the upper reaches of the River Pang. Unusually thick sequences of the Heales Lock Gravel occur locally, probably infilling closed depressions in the bedrock of periglacial or karstic origin. For example, where the A34(T) main road crosses the River Kennet west of Newbury, in places sand and gravel extend as much as 11 m below the flood plain surface, more than twice the usual depth. The deposits of the second and third terraces can locally be divided into upper and lower levels, separated at the surface by breaks of slope. The base of the Hamstead Marshall Gravel, which is up to 3 m thick, rests on bedrock some 35 to 40 m above the modern flood plain.

Of the post-Anglian terraces, only the Second and (in the Reading district) Third have been specifically identified in the Enborne valley.

The Beenham Grange Gravel has been shown by radiocarbon analysis to be mainly of Late Devensian age (younger than 25 000 years BP), while the Heales Lock Gravel was deposited at times of low sea level from about 16 000 to 10 000 years BP (Worsley and Collins, 1995).

In common with the other terrace deposits, the Beenham Grange Gravel is composed mainly of sandy, silty and clayey flint gravels, but at Halfway [SU 40 68] the gravels are overlain by silty clays and silts of aeolian origin. Similar but thinner aeolian deposits, or silts admixed with gravel by cryoturbation, occur at several other levels and were presumably originally very widespread (Chartres, 1981).

Alluvium comprises the predominantly fine-grained valley-floor deposits (including silt, clay, shell marl, peat and tufa) closely associated with the rivers Kennet, Lambourn, and Enborne, and underlies their flood plains. It can include gravel lenses and commonly has a basal gravel lag, or overlies gravelly river terrace deposits.

At Newbury and for some distance upstream, the flood-plain deposits of the River Lambourn and the River Kennet are dominated by peat, mainly sedge peat but with some coarser, more woody varieties. It ranges in thickness to more than 4.5 m. During the 18th and 19th centuries, the peat near Newbury was worked extensively, and used either as fuel or burnt for ash to be used as fertiliser. In addition, unmapped deposits of peat are known to be present within the alluvium.

Tufa and calcareous 'shell-marl' are commonly found interbedded with the alluvium. Deposits of mappable extent occur in association with the peat deposits between Kintbury and Marsh Benham.

Residual deposits

Clay-with-flints occur on the interfluves within the Seaford Chalk and Newhaven Chalk outcrops, approximately marking the extent of the sub-Palaeogene erosion surface. Typically, it is composed of orange—brown or reddish brown clay and sandy clay containing abundant matrix-supported flint nodules and pebbles. These clays are thought to be predominantly remanie deposits derived mainly from Palaeogene formations and by dissolution of the Chalk. Three main types of flint are present, in varying proportions:

The clay-with-flints may also include fragments of very hard silicified sandstone (sarsen) and ironstone derived from Palaeogene deposits.

The clay-with-flints can be expected to pass laterally into undisturbed Palaeogene deposits with a complex intergradation, perhaps over a broad area. In the same way that the Lambeth Group north of Newbury includes some relatively thick beds of sand, the clay-with-flints locally appears to pass laterally into clean, medium- to fine-grained sand, here classified as 'sand in clay-with-flints'.

Mass-movement deposits

'Head' refers to superficial deposits formed by solifluction processes: mainly down-slope mass movement of unconsolidated materials under the prolonged influence of freezing and thawing in a periglacial environment, but including rain wash and soil creep in more temperate climatic conditions. It occurs on some slopes and in valley floors throughout the area, tending to be more widespread on slopes facing east or north. It is derived from local bedrock and superficial deposits, and is assumed to have formed gradually during the successive Quaternary glacial periods, presumably most recently during the Devensian, and to be of fairly uniform age irrespective of-topographic situation.

Head is very variable in composition depending on local sources of material and details of landscape evolution. Typically, it is composed of very stony, sandy and silty clay, or clayey gravel. It may be clast supported or matrix supported, and tends to include a large proportion of angular, frost-shattered flint gravel (Plate 4).

Gravelly head includes a large proportion' of coarse material derived from river terrace deposits. In the lower reaches of the main river valleys, it tends to occur downslope of preserved terrace remnants. Upstream these become progressively more degraded, and their flat-topped landforms pass into an amorphous blanket of head.

Head that occurs in the floors of dry valleys on the Chalk outcrop is expected to include some coarse, clast-supported gravels containing fragments of both flint and chalk. The near-surface layers of such a deposit can be expected to have been decalcified by the passage of water, leaving a flint gravel. 'Valley-bottom head' of this type is thought to pass imperceptibly downstream into river terrace deposits, as the influence of fluvial processes predominates over that of solifluction.

Several previously unrecorded landslips have been mapped during the recent geological surveys in the area. These all occur on relatively steep slopes underlain by the sands and clays of the Reading Formation or the London Clay. They are presumably associated with the emergence of water from minor perched aquifers within the sand beds. These landslips are generally of translational type and typically underlie areas up to 150 m wide and 500 m long. Other examples might be present, particularly on wooded slopes.

Worked ground and Artificial deposits

Two broad categories of worked ground occur in the Newbury district: engineered road or railway cuttings, and surface mineral workings. Made ground consists mainly of engineered road, railway or canal embankments. Considerable use of locally excavated material was made in the construction of screening embankments along the A34(T) Newbury bypass (Perry et al., 2000). Made ground also occurs within some archaeological sites. Areas delineated as infilled ground are mostly either disused mineral workings or disused railway cuttings in which waste material has been deposited. Landscaped ground includes some worked ground or made ground, or both, in some cases together with infilled ground, but in situations where these categories cannot be reliably recognised or delineated.

Structure

No structures of Caledonian or greater age are known in the district. However, north-westerly trending magnetic anomalies seen to the north-east of the district (Figure 2b) may be attributed in part to Silurian and Precambrian igneous rocks, reflecting the probable structural grain in the local basement. The presence of the Variscan Front across the centre of the district is suggested by the southward termination of the concealed Oxfordshire–Berkshire Coal Basin, the presence of steeply dipping strata in the Welford Park Station Borehole, and the limit of east–west folding and faulting at the northern edge of the Weald Basin (Figure 1) (Busby and Smith, 2001). From mid-Carboniferous to early Permian times, Variscan folding and thrusting emplaced Lower Carboniferous and older strata towards the Variscan Front from the south, although no individual Variscan structures have been identified locally. North of the fold belt, Palaeozoic strata were only gently folded.

The small Berkshire Coal Basin is aligned north-west–south-east, en échelon to and south of the Oxfordshire Coal Basin (Figure 1ab). It was probably initiated in compensation to Variscan uplift in the Worcester Basin area in the west and to thrust loading in the south. The basin was then truncated from the south as Variscan deformation progressed, forming a syncline with a steeper south-western limb.

Synsedimentary east–west-trending growth faults bounding the Weald Basin developed through reactivation of Variscan thrusts during the early Jurassic, and were spasmodically active until early Cretaceous times (Chadwick, 1993). The most northerly of these faults, within the Pewsey–London Platform fault system, occur at depth in the south-west of the district (Figure 1); (Figure 4). North-west-trending faults that occur both on the London Platform (Arkell, 1947; Mathers and Smith, 2000) and in the Wessex Basin (Karner et al., 1987) also date from this period. Faults of this trend are likely to have exerted local control on the deposition of the Lower Greensand, and possibly on the south-western extent of the Berkshire Coal Basin.

Opening of the North Atlantic Ocean during the Late Cretaceous and Cainozoic caused compressive stresses oblique to the Mesozoic basins, leading to reactivation of the Variscan thrusts and reversal of the fault systems in the overlying Mesozoic successions (Penn et al., 1987; Underhill and Stoneley, 1998). This created the broad syncline of the London Basin, the western end of which lies in the south of the district, in an analogous manner to the formation of the Berkshire Coal Basin. The presence of a zone of anomalously thick Seaford Chalk, and the local development of hardgrounds and phosphatic chalks in an immediately adjacent part of the Seaford and Newhaven chalks suggests that some movement in the Pewsey–London Platform fault zone occurred during Chalk deposition.

In most of the district, the regional dip varies between about 0.5° and 1° (locally increasing to 2°). In the north the dip is generally towards the south-south-east, but in the south it is to the south, towards the axis of the London Basin syncline. South of the London Basin synclinal axis, similarly gentle northerly dips abruptly steepen to as much as 30° on the northern side of the Vale of Ham, in the south-west of the district, bringing the local bedrock formations to the surface in rapid succession. This zone of steeply dipping strata is the northern limb of the most northerly of an en échelon series of strongly asymmetric, north-facing periclines formed above reverse faults on the north-east margin of the Pewsey Basin, in a similar manner to the Hogs Back structure of Surrey (Ellison et al., 2002). South of an anticlinal axis passing close to Ham, the gentle regional southerly dip resumes. Part of a similar fold structure forming the Vale of Pewsey is seen in the extreme south-west of the district. The northern limbs of these folds are not continuously curved, but instead comprise a series of planar segments, the strike changing between one and the next within only a few hundred metres, at most. These inflections may be positioned at the intersection of northerly trending faults.

The few faults that can be demonstrated by geological mapping can be traced for only a limited distance. Most are orientated approximately north-west to south-east, subparallel to a trend followed by some Jurassic faulting, with downthrows either to the north-east or the south-west. A smaller number of faults trend between north-east and east-north-east, and so probably represent a conjugate set, with one north–south-trending fault. Faults of both principal orientations displace strata up to and including the Lambeth Group.

Computer modelling of the structure of the Chalk suggests that faulting on north-westerly and northerly trends is more extensive than revealed by surface mapping (see structural map in the margin of geological Sheet 267). Faults offsetting the axis of the London Basin syncline probably reach the surface, at least for part of their length, but they do so in places where field evidence for their existence is obscured by a lack of contrast in the displaced strata, or by superficial deposits.

The regional distribution of late Pliocene to Pleistocene formations provides evidence for the easterly down-tilting of the London Basin during the Quaternary, probably following subsidence in the southern North Sea Basin. The tilt has been estimated at about 1 m per km (Mathers and Zalasiewicz, 1988). It is reflected by the incised easterly flowing drainage of the region; many phases of its evolution are preserved within the district as river terrace deposits.

Chapter 3 Applied geology

The most important geological material resources being exploited in the Newbury district at present are water and aggregates (sand and gravel). In the past, chalk, flint and brick clay were worked widely on a small scale; peat was once dug from the River Kennet flood plain. Coal exists at depth but has not been exploited.

The composition and structure of the ground influence the stability of what is constructed upon it, and so have an important bearing on the siting, type and design of developments of many kinds. If appropriate consideration is given to local geological conditions, it may be possible to mitigate some problems that might otherwise be encountered during construction, or afterwards. The diverse geology of the Newbury district gives rise to a range of ground conditions, and to a variety of potential geological hazards.

Water resources

The Chalk is the most important aquifer in the region in terms of catchment area, storage capacity and yield, with most water being taken from the White Chalk. The hydraulic properties of the Chalk are complex. The primary, intergranular porosity of typical chalk in this region is high, averaging between about 25 and 40 per cent, depending on stratigraphical level (Bloomfield et al., 1995). However, the typical pore throat diameter is so small that the permeability of unfractured chalk is minimal, allowing an exceedingly slow passage of water. Conversely, the Chalk is fractured, both parallel to and across bedding surfaces. Fractures which occur in the Chalk are commonly enlarged by solution of the fracture wall in groundwater ((Plate 1), inset), creating zones of locally very high secondary porosity and transmissivity. For example, flow velocities as high as 6 km per day have been demonstrated by tracer studies near Stanford Dingley, beside the River Pang in the Reading district (Banks et al., 1995). As a consequence, the level of the water table can change rapidly in response to rainfall, and the spring heads of chalk streams correspondingly move up and down their valley. For example, following heavy rain during the winter of 2000–2001, historically high groundwater levels led to stream flow in the River Pang from East Ilsley [SU 4924 8144] and groundwater flooding at the watershed in the Compton Gap [SU 518 826].

During the 1970s, the Chalk aquifer of the Berkshire Downs (and the Marlborough Downs, to the west) was the subject of the Thames Groundwater Scheme, a large investigation into the possibility of augmenting the flow of the River Thames and its tributaries with groundwater (Thames Water Authority, 1978). This study showed that although some preferred flow horizons (for example at the Chalk Rock, the Melbourn Rock and other hard or impermeable beds) are stratigraphically controlled, the vertical variation in aquifer properties in the Chalk is more closely related to the topography than to the stratigraphy. The fracture systems providing the productive permeability of the unconfined aquifer tend to occur within about 60 m of the surface, typically within the zone of water table fluctuation. Beneath valleys the fractures are better developed and extend over a greater depth interval, so that transmissivity is typically up to 40 times greater there than beneath the interfluves. Storage coefficients are also larger beneath the valleys (Allen et al., 1997).

Water from the chalk is normally of very good quality. Near the surface, it is hard to very hard, with carbonate hardness predominating (British Geological Survey, 1978). In the south-east of the district where the Chalk is confined by younger deposits, the composition may start to change due to cation exchange to a softer, sodium bicarbonate water composition.

Local water supplies have also been obtained from the Upper Greensand in the south-west, from sand beds in the Lambeth Group and London Clay, and from river terrace deposits. Of these, only the Upper Greensand and river terrace deposits are capable of producing significant supplies. The Upper Greensand and Lambeth Group sands may be in partial hydraulic continuity with the Chalk. However, south and east of Newbury, the Chalk is confined by overlying beds and overflowing artesian conditions occur. Where river terrace deposits overlie clays, they can form local perched aquifers (in which water levels are unrelated to those in the Chalk) but where they overlie permeable formations, they are generally in hydraulic continuity with the aquifer below.

Mineral workings

Sand and gravel have been extracted from the river terrace deposits in many places. The largest workings are those in the youngest terrace deposits close to the River Kennet, just to the east of Newbury. Smaller pits occur in the older terraces, both on the valley sides and on the interfluves, notably at Welford (now restored to agriculture) and Hamstead Marshall. Gravelly head deposits in the valley floor have also been worked locally. The gravel composition is dominated by flint.

Building sand has been dug on a commercial scale from the Reading Formation around Hermitage.

Chalk and flint were once extracted from numerous small open pits found in many parts of the outcrop. Harder chalks, particularly the Chalk Rock, were quarried as hard core. The softer varieties of chalk were used in brick manufacture, to make quicklime, or as agricultural lime. Some chalk used for these purposes was mined on a small scale from short shafts and adits, particularly north and north-east of Newbury. Pure chalk from the Newhaven Chalk was once quarried at Kintbury for 'whiting'.

Flint is widely used as a building material, with knapped flint giving a characteristic appearance to many of the local churches and older buildings. Blocks of chalk from the New Pit Chalk have been used very locally as building stone. Sarsen (silicified Palaeogene sandstone) has also been used locally for building, either dressed or untrimmed, but this would have been collected opportunistically.

Clay and sand were extracted for brick and tile making from numerous small pits in the Reading Formation, and to a much lesser extent from the London Clay and the clay-with-flints.

Peat was once dug over considerable areas of the River Kennet floodplain; it was used as fuel or burnt to produce ash for use as fertiliser.

Deep boreholes have proved the existence of thin coal seams beneath the Newbury district but no attempt has been made to exploit them.

Many of the disused quarries opened for these commodities have now been used for landfill. A few have been restored to agriculture and some of those on the flood plain have been left as bodies of open water.

Foundation conditions and geological hazards

Ground stability can be adversely influenced by a variety of geological circumstances present in the Newbury district. A very general guide to potential problems associated with each of the geological units is shown in (Figure 10). Requests for geological reports on specific sites can be addressed to the BGS Enquiry Service (see Information Sources).

Slope stability and mass-movement

Several previously unrecognised landslips were identified during the recent resurvey of the district. These all occur where steep slopes cut into the Lambeth Group or London Clay outcrop, mostly on the sides of minor tributary valleys around Newbury. Springs associated with interbedded sand bodies appear to have caused high pore water pressures, promoting failure by rotation and translation.

Other landslips could be present, particularly where slopes on the London Clay or Lambeth Group exceed 7°. Slopes exceeding 3° on these formations, and in clay-rich head, should also be considered as potentially unstable due to periglacial weathering processes. Low-angle slip planes may be present in head, significantly reducing its bearing strength.

Cambering and valley bulge

Evidence for cambering of the sands in the London Clay and valley bulge in the underlying clays has been found in the Enborne valley in the south-west of the Reading district (Hawkins, 1954). Disturbance of the strata occurred to a depth of more than 30 m in places, with local dips up to 40°. Similar effects may be found within the Newbury district higher in the Enborne valley, and in other places where valleys are cut into Palaeogene strata.

Ground heave and subsidence

Parts of the Lambeth Group and, particularly, the London Clay are dominated by clays. Some of these have a high smectite content and so tend to change volume significantly with changes in moisture content. The clays generally absorb water during the winter, and then lose it during dry summer periods. The consequent cycles of swelling and shrinking can lead to structural damage, particularly where drought accentuates normal seasonal movements, or the uptake of water by trees and other vegetation creates conditions for uneven movement (Driscoll, 1983). The clay-with-flints is prone to shrink and swell behaviour, depending on its local clay content and thickness.

In their unweathered state, many of the Palaeogene strata contain pyrite. On weathering, this is oxidised, releasing sulphate into solution. In the clay-rich formations, especially the London Clay, this sulphate can react with any calcium carbonate present to form selenite crystals. This involves an eight-fold increase in volume compared with the unweathered state and can cause disruption of the strata. Moreover, high concentrations of sulphate in groundwater can weaken concrete foundations if these have not been designed to resist this type of chemical attack.

Chalk dissolution and mining

Closed topographical depressions between 25 and 50 m in diameter and from 1 to 10 m deep are common where the Chalk is overlain by the Lambeth Group, or by clay-with-flints. The depressions occur on the outcrops of both the Chalk and of the overlying deposits. They are interpreted as dolines, marking the site of gradual karstic dissolution of the underlying chalk by groundwater. The largest and deepest act as sinks for minor streams draining the Palaeogene outcrop but many such depressions occur in otherwise level ground on interfluves. Excavations that intersect such structures show they are typically steep-sided 'pipes' within an uneven karstic Chalk rockhead. They are infilled by clay-with-flints, locally with recognisable remnants of Palaeogene deposits.

Some dolines may have been enlarged by small-scale working for chalk, clay or sand.

Indeed, some closed depressions may mark the site of 'chalk-mining', which apparently was widespread in arable land where clay soils overlie the Chalk at no great depth. Underground chalk workings are also known to occur in the vicinity of former brick kilns, for example at Hermitage.

In some instances, collapse into natural karstic features or old underground workings can occur spontaneously, typically following heavy rain. It might also be induced by loading during construction, leading to differential subsidence, or changes in the local flow of groundwater, for example from soakaways or leaking pipes. Enlarged fissures might also occur in the Chalk beneath alluvium and so there is a small possibility that subsidence could occur in flood plain areas.

Flooding

Low-lying ground adjacent to rivers, typically underlain by alluvium or peat, is liable to flood following heavy rain. The larger dry valleys within the Chalk outcrop may also flood, depending on the prevailing groundwater levels. Localised flooding can also occur at springs from sand or pebble beds in the Palaeogene succession. During periods of very high groundwater level, stream sinks in the Chalk can become discharge points.

Gas emission

Decomposition of organic refuse in landfill can lead to the generation of methane. Modern landfills are designed accordingly, with impermeable linings and the facility to collect or burn the methane. Lateral underground seepage might occur from older landfills in contact with permeable strata, and depending on local ground conditions could give rise to hazards at some distance.

Natural radon emission

Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium. It is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995). Relatively high levels of radon emissions are associated with particular types of bedrock and unconsolidated deposits. The highest radon potential in the Newbury district is associated with interbedded sandstones and siltstones of the Upper Greensand Formation, and it is found in some areas where river terrace sand and gravel overlie the Seaford Chalk. Slightly elevated radon potential is associated with some ground underlain by the Lewes Nodular Chalk, New Pit Chalk and Seaford Chalk and where the Seaford Chalk is overlain by clay-with-flints.

Normally, radon released from the soil is quickly dispersed in the atmosphere. However, radon that enters poorly ventilated enclosed spaces such as some basements, buildings, caves, mines, and tunnels may reach high concentrations in some circumstances. Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer.

Radon Affected Areas have been declared by the National Radiological Protection Board (NRPB) where it is estimated that the radon concentration exceeds the Action Level (200 Bq m−3) in one per cent or more of homes (Green et al., 2002). Approximately 30 per cent of the Newbury district has been identified by the NRPB as Radon Affected.

Radon protective measures may need to be installed in new dwellings (and extensions to existing ones) in areas where it is estimated that the radon concentration exceeds the Action Level in three per cent or more of homes (BRE, 1999).

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Newbury district and adjacent areas are listed below. Information on BGS publications is given in the current BGS catalogue of geological maps and books, available on request, and at the BGS web site (see below). Enquiries concerning unpublished geological data should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological enquiries and requests for geological reports on specific sites should be addressed to the BGS Enquiry Service at Keyworth. Addresses are given on the back cover.

Searches of the indexes to some of the material and records collections can be made on the Geoscience Data Index (GDI) in BGS libraries and online at http://www.bgs.ac.uk. This web site also provides access to the BGS Enquiry Service.

Maps

Books

Documentary collections and indexes

Borehole records

Borehole records for the district are catalogued in the BGS archive at Keyworth. For information contact: The Manager, National Geological Records Centre, BGS Keyworth. An index to the borehole records can be assessed at the BGS web site.

For information on water wells, springs, aquifer properties and water borehole records, contact the BGS Hydrogeology Enquiry Service, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB. Telephone 01491 838800; fax 01491 692345.

BGS Lexicon of named rock units Definitions of the named rock units shown on the Sheet 267 Newbury are held in the BGS Stratigraphic Lexicon, which can be consulted on the BGS web site. Further information on this database can be obtained from the Lexicon Manager, BGS Keyworth.

Material collections

Borehole samples

Samples, including core, from a small number of boreholes in the Newbury district are held by the National Geosciences Records Centre, BGS, Keyworth.

BGS Photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs held at BGS, Keyworth and Edinburgh. Part of this image-database can be viewed on line. Copies of the photographs may be purchased from BGS.

Palaeontological collections

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

References

British Geological Survey holds most of the references listed below and copies may be obtained via the library service subject to copyright legislation (contact libuser@bgs.ac.uk for details). The library catalogue is available at: http://geolib.bgs.ac.uk

Aldiss, D T, Newell, A J, Marks, R J, Hopson, P M, Farrant, A R, Royse, K R, Aspden, J A, Evans, D J, Smith, N J P, Woods, M A, and Wilkinson, I P. in press. Geology of the Newbury district and part of the Abingdon district. British Geological Survey Research Report.

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wagstaff, 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.

Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and the relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Arkell, W J. 1947. The geology of Oxford. (Oxford: Clarendon Press.)

Banks, D, Davies, C, and Davies, W. 1995. The Chalk as a karstic aquifer: evidence from a tracer test as Stanford Dingley, Berkshire, U K. Quarterly Journal of Engineering Geology, Vol. 28, S31–S38.

Bloomfield, J P, Brewerton, L J, and Allen, D J. 1995. Regional trends in matrix porosity and dry density of the chalk of England. Quarterly Journal of Engineering Geology, Vol. 28, S131–S142.

B R E. 1999. Radon: guidance on protective measures for new dwellings. Building Research Establishment Report, B R 211 (1999).

Bridgland, D R. 2003. The evolution of the River Medway, S E England, in the context of Quaternary palaeoclimate and the Palaeolithic occupation of N W Europe. Proceedings of the Geologists' Association, Vol. 114, 23–48.

British Geological Survey. 1978. Hydrogeological map of the south-west Chilterns and the Berkshire and Marlborough Downs, Sheet 7. (London: Institute of Geological Sciences.)

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

Busby, J P, and Smith, N J P. 2001. The nature of the Variscan basement in southeast England: evidence from integrated potential field modelling. Geological Magazine, Vol. 138, 669–685.

Carter, D J, and Hart, M B. 1977. Aspects of mid-Cretaceous stratigraphical micropalaeontology. Bulletin of the British Museum (Natural History)(Geology), Vol. 29, 1–135.

Chadwick, R A. 1993. Aspects of basin inversion in southern Britain. Journal of the Geological Society of London, Vol. 150, 311–322.

Chartres, C J. 1981. The mineralogy of Quaternary deposits in the Kennet Valley, Berkshire. Proceedings of the Geologists' Association, Vol. 92, 93–103.

Collins, P E F. 1994. Floodplain environmental change since the last glacial maximum in the lower Kennet valley, south-central England. PhD thesis, University of Reading.

Collins, P E F. 1999. Kennet and Pang Valleys. 51-53 in A revised correlation of Quaternary deposits in the British Isles. Bowen, D Q (editor). Geological Society of London Special Report, No. 23.

Crane, P R, and Goldring, R. 1991. The Reading Formation (late Palaeocene to early Eocene) at Cold Ash and Pincent's Kiln (Berks) in the western London Basin. Tertiary Research, Vol. 12, 147–158.

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

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

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

Ellison, R A, Williamson, I T, and Humpage, A. 2002. Geology of the Guildford district —a brief explanation of the geological map. Sheet Explanation of the British Geological Survey, 1:50 000 Sheet 285 Guildford (England and Wales).

Foster, D, Holliday, D W, Jones, C M, Owens, B, and Welsh, A. 1989. The concealed Upper Palaeozoic rocks of Berkshire and south Oxfordshire. Proceedings of the Geologists' Association, Vol. 100, 395–407.

Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon atlas of England and Wales. National Radiological Protection Board Report, NRPB–W26.

Hart, M B, Bailey, H W, Crittenden, S,Fletcher, B N, Price, R J, and Swiecicki, A. 1989. Cretaceous. 273–371 in Stratigraphical atlas of fossil Foraminifera (Second edition). Jenkins, D G, and Murray, J W (editors). (Chichester: Ellis Horwood.)

Hawkins, H L. 1954. The Eocene succession in the eastern part of the Enborne valley, on the borders of Berkshire and Hampshire. Quarterly Journal of the Geological Society of London, Vol. 110, 409–430.

Jarvis, I, and Woodroof, P. 1981. The phosphatic chalks and hardgrounds of Boxford and Winterbourne, Berkshire — two tectonically controlled facies in the late Coniacian to early Campanian (Cretaceous) of southern England. Geological Magazine, Vol. 118, 175–187.

Jukes-Browne, A J, and Hill, W. 1903. The Cretaceous rocks of Britain. Vol. II. The Lower and Middle Chalk of England. Memoir of the Geological Survey of the United Kingdom.

Jukes-Browne, A J, and Hill, W. 1904. The Cretaceous rocks of Britain. Vol. III. The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom.

Karner, G D, Lake, S D, and Dewey, J F. 1987. The thermal and mechanical development of the Wessex Basin, southern England. 517–536 in Continental extensional tectonics. Coward, M P, Dewey, J F, and Hancock, P L (editors). Geological Society of London Special Publication, No. 28.

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).

Mathers, S J, and Zalasiewicz, J A. 1988. The Red Crag and the Norwich Crag formations of southern East Anglia. Proceedings of the Geologists' Association, Vol. 99, 261–278.

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

Mortimore, R N, Wood, C J, and Gallois, R W. 2001. British Upper Cretaceous stratigraphy. Geological Conservation Review Series. No. 23. (Peterborough: Joint Nature Conservation Committee.)

Penn, I E, Chadwick, R A, Holloway, S, Roberts, G, Pharaoh, T C, Allsop, J M, Hulbert, A G, and Burns, I M. 1987. Principal features of the hydrocarbon prospectivity of the Wessex-Channel Basin, U K. 109–118 in Petroleum geology of north west Europe. Brooks, J, and Glennie, K (editors). (London: Graham & Trotman.)

Perry, J, Field, M, Davidson, W, and Thompson, D. 2000. The benefits from geotechnics in construction of the A34 Newbury Bypass. Proceedings of the Institution of Civil Engineers Geotechnical Engineering, Vol. 143, 83–92.

Pharaoh, T C, Merriman, R J, Webb, P C, and Beckinsale, R D. 1987. The concealed Caledonides of eastern England: preliminary results of a multidisciplinary study. Proceedings of the Yorkshire Geological Society, Vol. 46, 355–369.

Poole, E G, Kelk, B, Bain, J A, and Morgan, D J. 1971. Calcium montmorillonite (fuller's earth) in the Lower Greensand of the Fernham area, Berkshire. British Geological Survey Report,No. 71/12.

Powell, J H, Chisholm, J I, Bridge, D M, Rees, J G, Glover, B W, and Besly, B M. 1998. Stratigraphical framework for Westphalian to early Permian red-bed successions of the Pennine Basin. British Geological Survey Research Report, RR/00/01.

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

Ruffell, A H. 1998. Tectonic accentuation of sequence boundaries: evidence from the Lower Cretaceous of southern England. 331–348 in Development, evolution and petroleum geology of the Wessex Basin. Underhill, J R (editor). Geological Society of London Special Publication, No. 133.

Simpson, I R, Gravestock, M, Ham, D, Leach, H, and Thompson, S D. 1989. Notes and cross-sections illustrating inversion tectonics in the Wessex Basin. 123–129 in Inversion Tectonics. Cooper, M A, and Williams, G D (editors). Geological Society of London Special Publication, No. 44.

Smith, N J P. 1985. Structure contours and subcrops of the pre-Permian surface of the United Kingdom (South) (British Geological Survey 150th Anniversary Publication edition). (Keyworth, Nottingham: British Geological Survey.)

Smith, N J P. 1987. The 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.)

Smith, N J P. 1993. The case for exploration of deep plays in the Variscan fold belt and its foreland. 667–675 in Petroleum geology of north west Europe: Proceedings of the 4th Conference. Parker, J R (editor). (Geological Society, London.)

Sumbler, M G. 1996. British regional geology: London and the Thames Valley (Fourth edition). (London: H MS O for the British Geological Survey.)

Swiecicki, A. 1980. A foraminiferal biostratigraphy of the Campanian and Maastrichtian chalks of the United Kingdom. PhD thesis, Plymouth Polytechnic.

Thames Water Authority. 1978. The Thames groundwater scheme. (London: Institution of Civil Engineers.)

Underhill, J R, and Stoneley, R. 1998. Introduction to the development, evolution and petroleum geology of the Wessex Basin. 1–18 in Development, evolution and petroleum geology of the Wessex Basin. Underhill, J R (editor). Geological Society of London Special Publication, No. 133.

White, H J O. 1907. The geology of the country around Hungerford and Newbury. Memoir of the Geological Survey of England and Wales, Sheet 267 (England and Wales).

Whittaker, A (editor). 1985. Atlas of onshore sedimentary basins in England and Wales. (Glasgow: Blackie.)

Wilkinson, I P. 2000. A preliminary foraminiferal biozonation of the Chalk Group. British Geological Survey Internal Report, I R/00/13.

Worsley, P, and Collins, P E F. 1995. The geomorphological context of the Brimpton late Pleistocene succession (south central England). Proceedings of the Geologists' Association, Vol. 106, 39–45.

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

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

(Index map)

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

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

Figures and plates

Figures

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

(Figure 1ab) Palaeozoic formations beneath the sub-Mesozoic unconformity, and location of deep boreholes. 1b Part of the Wessex Basin (with sub-basins) separated from the Midlands Microcraton by the Pewsey–London Platform Fault zone (PLPF). VF Variscan Front. Deep boreholes: AT Aston Tirrold; BH Burnt Hill; F Faringdon; FB Foudry Bridge; HM Ham 1; HW Harwell 3; KC Kingsclere 1; MF Maddle Farm; SB1 Strat B1; WPS Welford Park Station

(Figure 2a) Bouguer gravity anomalies shown as colour shaded relief illuminated from the north. Contour interval (1mGal=1 3 10-5 m/s2).

(Figure 2b) Total field magnetic anomalies shown as a colour relief illuminated from the north. Contour interval 10nT.

(Figure 4) Subcrop map at the mid-Cretaceous 'Late Cimmerian' unconformity (adapted from Whittaker, 1985).

(Figure 5) Lithostratigraphy and correlation of key cored boreholes in the Chalk.

(Figure 6) Chalk stratigraphy.

(Figure 7) Correlation of the Chalk Group through geophysical borehole logs.

(Figure 8) Correlation of the Lambeth Group through geophysical borehole logs. The Newbury Bypass section is from unpublished notes by R A Ellison and S J Mathers.

(Figure 9) River terrace correlation and terminology.

Plates

(Plate 1) The Chalk Rock: Fognam Farm Quarry [297 799] is a Siteof Special Scientific Interest (SSSI) and a Geological ConservationReview site. The very top of the New Pit Chalk and the lower part of the of the Lewes Chalk including the Chalk Rock (Mortimore et al., 2001) is exposed. As seen in this plate, taken in 1970, four of the hardgrounds named by Bromley and Gale (1982) are present, together with the Fognam Marl at its type locality(A11936).The inset shows the Chalk Rock as seen in a videoscan log in the Banterwick Farm borehole (SU57NW37), some 20 km due east, where the Ogbourne Hardground is absent and the interval from the Hitch Wood Hardground to the Fognam Marl is reduced from about 3 m to 2.75 m. Note the enlarged karstic fissure at the top of the relatively impermeable Chalk Rock.

(Plate 2) Seaford Chalk and Lambeth Group: the topmost 3 m of the Seaford Chalk exposed in a newly excavated cutting for re-alignment of the A34(T) near Chieveley [478 733]. Several beds of flint nodules are present in well-jointed white chalk, with a well-developed tabular flint lying at a slight angle to the basal unconformity of the overlying Lambeth Group. This is composed mainly of thinly bedded clayey fine-grained sands, with a basal clayey, sandy pebble bed, as seen in the inset, resting on the tabular flint. Divisions on ranging pole are each 50 cm (P535262; P535264).

(Plate 3) Stony soil on 'pebble bed' in the London Clay: arable field south of Kintbury [3875 6580] with a typically stony soil developed on the clayey, sandy gravel beds occurring in the London Clay in the south-west of the district. Very well-rounded pebbles are generally 4 to 64 mm in diameter but rarely up to 128 mm, composed mainly of flint with about 15 per cent quartz or quartzite (GS1299).

(Plate 4) Head deposits: Sandy stony clay with flint fragments in weak alignment, typical of head deposits derived from the clay-with-flints. Note the preponderance of angular, frost-shattered flints, with some well-rounded flint pebbles (compare with Plate 3) and a few relatively complete flint nodules (P608887).

(Front cover) Typical Chalk downland scenery near Lambourn, with sarsen stones (weathered relicts of silicified Palaeogene sandstone) in the foreground. View from Weathercock Hill [SU 2930 8223] looking north-east (Photographer C F Adkin; P535218).

(Rear cover)

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

Figures

Figure 1 Summary of the geological succession in the district

Age Deposits Thickness (metres)
QUATERNARY HOLOCENE Artificial (man made) deposits: Made ground, infilled ground Landscaped ground
Alluvium Peat

Tufa

 Mass-movement deposits: Head; Gravelly head

Residual deposits:

Clay-with-flints, Sand in clay-with-flints
Post Anglian (including Devensian) River terrace deposits Fifth to First
Anglian and pre-Anglian River terrace deposits Ninth to Sixth
PALAEOGENE EOCENE Thames Group London Clay Formation clay, some silty, and sand Thin pebble beds up to 50
Harwich Formation sand, silt, clay, glauconitic and shelly, with a pebble bed 0.3–6
PALAEOCENE Lambeth Group Reading Formation clay and sand 15–35
Upnor Formation sand and clay, very glauconitic; green-coated flint nodules and flint gravel at base 0–22
CRETACEOUS UPPER CRETACEOUS SANTONIAN TO CAMPANIAN Chalk Group WHITE CHALK SUBGROUP Newhaven Chalk Formation chalk, white, soft to medium-hard, uniform; some flint nodules; seams of clay-rich chalk. Phosphatic chalk and hardgrounds occur locally 0–40
CONIACIAN TO SANTONIAN Seaford Chalk Formation chalk, white, soft to medium-hard, uniform; many flint nodules and some tabular flints, a few seams of clay-rich chalk 55–90
TURONIAN TO CONIACIAN Lewes Nodular Chalk Formation chalk, white to yellowish, hard to very hard, nodular, gritty, flinty, some seams of clay-rich chalk. Includes Chalk Rock near base: mineralised hardgrounds and very hard chalks 10–30
TURONIAN New Pit Chalk Formation chalk, white, soft to medium-hard, uniform; seams of clay-rich chalk; sparse small flint nodules 20–50
CENOMANIAN AND TURONIAN Holywell Nodular Chalk Formation chalk, off-white, some hard to very hard, commonly nodular, seams of clay-rich chalk 10–40
CENOMANIAN GREY SUBGROUP Zig Zag Chalk Formation chalk, grey, soft to medium-hard, some clay-rich,some hard limestones 25–40
West Melbury Marly Chalk Formation chalk, grey, soft, clay-rich and hard grey limestones 20–50
LOWER ALBIAN Upper Greensand Formation glauconitic sand and sandstone, some siliceous, and chert 20–50

(Figure 3) Strata found in deep boreholes in and near the Newbury district

Thickness of each stratigraphical interval is given in metres

Borehole name Faringdon Harwell 3 Aston Tirrold Maddle Farm Welford Park Station Kingsclere 1 Ham 1
1:50 000 sheet 253 254 254 253 267 283 267
Grid Reference [SU 3224 9398] [SU 4680 8644] [SU 5579 8722] [SU 3050 8230] [SU 4066 7364] [SU 4984 5820] [SU 3310 6247]
BGS registration number (SU39SW/1) (SU48NE/92) (SU58NE/42) (SU38SW/13) (SU47SW/59) (SU45NE/1) (SU36SW/1)
CRETACEOUS Chalk Group Spudded in Chalk >66 Spudded in Chalk >5 Spudded in Chalk >102.2 Spudded in Chalk >193.6 Spudded in Chalk >18.3
Upper Greensand Formation 25 20.2 21.8 20.6 Spudded in UGS >33.8 42.4
Gault Formation 68 70.3 66.2 74.8 86.6 84.5
Lower Greensand Group 6 45 6.2 9.4 13.7 4.5
Wealden Group absent absent absent absent 282.3 >11.2
JURASSIC Purbeck Group 61.2
Portland Group 48.2
Kimmeridge Clay and Ampthill Clay Formations 26 44.5 10.3 58 280
Corallian Group Spudded in Corallian

>33.2

 77 29.3 22.3 24.6 34.5
West Walton, Oxford Clay and Kellaways Formations 136.5 70 101.5 145.8 121.1 124
Great Oolite Group 55 49 50.6 75 62.3 73.8
Inferior Oolite Group 14 15 10.4

(faulted?)

 10.9 12.6 120.1
Lias Group 199.6 46.8 15.5 213.3 135.6 360.4
TRIASSIC Penarth Group 18.3 absent absent 22.8 12.9 22.3
Mercia Mudstone Group 190.5 291.8 19.7 >22
Sherwood Sandstone Group absent 3.67 139.9 21.2
CARBONIFEROUS Westphalian >101.33 362.4 128.12 >533.99 (in steeply dipping beds
Namurian absent absent
Dinantian 14.9
Upper Devonian 154 >11.77 >28
Lower Devonian >149

(Figure 10) Potential ground constraints

These types of constraint on construction are not necessarily present or significant at individual sites but are possibilities that should be considered, if relevant

Geological unit Potential ground constraint
Worked ground unstable sides of old workings; local contamination
Made ground and infilled variable foundation conditions; fill might be compressible; leachate
ground and methane generation from waste
Head variable foundation conditions; possible low-angle shear planes
Clay-with-flints subsidence associated with dissolution of underlying chalk; ground heave
Alluvium and peat compressible strata (especially where peat is present); risk of flooding; variable foundation conditions, including running sand below water table
River terrace deposits local perched water tables, running conditions below water table; possible unrecorded workings or infill
London Clay ground heave and landslip in clays; high sulphate content in groundwater; perched water table with associated springs and running conditions in sands
Lambeth Group variable foundation conditions; ground heave and landslip in clays; perched water table with associated springs and running conditions in sands; subsidence associated with dissolution of underlying chalk
Chalk Group dissolution cavities and sink holes, especially in areas close to the Lambeth Group or clay-with-flints; undocumented and infilled workings at surface and underground; groundwater protection requirement
Upper Greensand possible unrecorded workings or infill