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Geology of the Newbury district and part of the Abingdon district. Sheet description of the British Geological Survey, Sheet 267 and part of Sheet 253 (England and Wales).

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, I P Wilkinson

Bibliographical reference: 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. 2010. Geology of Newbury district and part of the Abingdon district. Sheet description of the British Geological Survey, Sheet 267 and part of Sheet 253 (England and Wales).

Geology of the Newbury district and part of the Abingdon district. Sheet description for the British Geological Survey 1:50 000 Series Sheet 267 and part of Sheet 253 (England and Wales)

Authors: 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, I P Wilkinson

Keyworth, Nottingham: British Geological Survey 2010. © NERC 2010. All rights reserved. ISBN 978 0 85272 659 4

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: 1000017897/2010. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

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Keywords: Berkshire; Newbury; Berkshire Downs; River Kennet; River Lambourn; geology; bedrock geology; superficial deposits; Cretaceous; Palaeogene; Quaternary; Chalk Group; Lambeth Group; Thames Group; Bracklesham Group.

(Front cover) Typical Chalk downland scenery near Lambourn, with two large sarsen stones (weathered relicts of silicified Palaeogene sandstone) in the foreground. View from Weathercock Hill, 209 m OD [SU 2930 8223] looking north-east, with Hillbarn Clump [SU 3253 8596] (beside the Ridgeway long distance path at 226 m OD) 5 km distant on the horizon. (P535218) (C F Adkin).

(Back cover)

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Acknowledgements

Revision of the geological map of the Newbury district was carried out under the BGS Regional Mapping Programme. Financial support for geological revision of the parts of the Abingdon district described here was provided by the NERC LOCAR Project.

Palaeontological support for this work was provided by M A Woods (macrofossils) and I P Wilkinson (microfossils). Their individual reports are mentioned at relevant points in the text.

Some of the descriptions of the Chalk in areas SU58NW and SU58SW are taken from observations by B C Coppack and A Horton, compiled into unpublished notes by A Horton in about 1975. 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. A Forster reviewed the rest of the section on ground conditions and geological hazards. Computer modelling was carried out by K R Royse and B Napier under the BGS DGSM (Digital Geological Spatial Model) Project. Cartography was by Sheila Myers at BGS Keyworth. Design was by A J Hill and diagrams were prepared by P Lappage and the authors. Series editors were S G Molyneux and J E Thomas.

The authors would like to thank landowners and estate managers throughout the area for their cooperation in allowing the geologists who carried out the field survey access to private land. In particular, Mr J Gold of The Hendred Estate, East Hendred, kindly provided the opportunity to create temporary excavations at the boundary between the Chalk and Upper Greensand.

Geology of the Newbury district and part of the Abingdon district—summary

This report describes the geology of the Newbury district and the southern part of the Abingdon district, including most of the Berkshire Downs and adjacent portions of the valleys of the River Kennet and its tributaries. Together with a printed geological map of the Newbury district at 1:50 000 scale, it was produced as part of the BGS Regional Mapping Programme. A geological map at 1:50 000 scale covering the catchments of the River Pang and the River Lambourn, together with adjacent portions of the Chalk escarpment to the north, is also available from BGS as a print-on-demand product. This was produced as part of the infrastructural information provided for the LOCAR (Lowland Catchment Research) Project, under the NERC Thematic Programme.

The introduction describes the topographical and geological setting of the area. Subsequent chapters describe the bedrock (or ‘solid’) geology, the superficial (or ‘drift’) geology and the geological structure. They are followed by a section on Applied Geology. The final section, Information Sources, lists all the BGS publications relevant to the district and gives information on how to gain access to BGS collections and databases, including borehole records and geophysical, geochemical and geotechnical data.

Most of the area is underlain by the Upper Cretaceous Chalk Group, which forms the downlands (Cover picture). The Lower Cretaceous Upper Greensand and Gault, and a small area of the Jurassic Kimmeridge Clay, occur in the north. The Upper Greensand Formation also 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. At depth, the area is partly underlain by a thin discontinuous Lower Greensand sequence, attenuated Jurassic and Triassic successions, Westphalian-aged deposits of the Berkshire Coal Basin, and thin Tournaisian/Visean and thick Devonian strata resting on Lower Palaeozoic basement.

Overlying the Chalk, the southern part of the district includes the western end of the Palaeogene (Tertiary) outcrops of the synclinal London Basin. The Palaeogene succession extends from the Lambeth Group (mostly made up of clays and sands of the Reading Formation, with a thin basal Upnor Formation) up into the London Clay Formation (Thames Group), which in this district includes relatively thick beds of sand and some pebble gravels within the usual clays.

The bedrock formations are commonly obscured by superficial deposits of Quaternary age. These include river terrace deposits at nine identified levels (including the ‘plateau gravels’ of previous terminology); alluvial deposits, peat and tufa forming the modern river flood plains; clay-with-flints and sand forming a residual cover over the higher parts of the downs; and head (solifluction deposits) on slopes and valley floors in most parts of the district. Areas of worked ground, made ground, infilled ground and landscaped ground have also been delineated.

Landslides occur locally on the Palaeogene formations around Newbury. Other potential geological hazards in the area include cambering and valley bulge, ground heave, subsidence, flooding and radon emission. The possibility of risk from these factors should be taken into consideration in any construction or land development project.

The most important natural mineral resources in the area are water and aggregates (sand and gravel). In the past, clay, chalk, flint and peat were exploited on a small scale.

(Table 1) Summary of the geological succession at outcrop in the district.

Figures and plates appear at the end of the report.

Chapter 1 Introduction

Rationale and geographical setting

This report describes the geology of the Newbury district (the area corresponding to 1:50 000 scale Geological Sheet 267) and adjacent parts of the Abingdon district (Sheet 253) (Figure 1). The complementary Geological Sheet 267 (Newbury) was revised as part of the BGS Regional Mapping Programme. The geology of the part of Geological Sheet 253 (Abingdon) described here is shown on an unpublished map at 1:50 000 scale of the topographical catchments of the River Pang and the River Lambourn, together with the immediately adjoining surrounding area (Figure 1). This was produced with a complementary report (Aldiss et al., 2002) as part of the infrastructural information provided for the LOCAR (Lowland Catchment Research) Project, under the NERC Thematic Programme. At the time of writing there are no specific plans to publish a revision of Sheet 253.

The area described mostly lies within West Berkshire district, with a northern strip in Oxfordshire, a western part in Wiltshire and a small south-eastern part in Hampshire. Apart from the towns of Newbury, Thatcham and Hungerford, the district is rural, with numerous villages of all sizes. It comprises the greater part of the chalklands of the Berkshire Downs, together with parts of the Marlborough Downs (in the west), relatively low-lying hills forming part of the River Kennet valley, and a small section of the escarpment at the northern edge of the Hampshire Downs, including their highest point, Walbury Hill, at 297 m OD. The district is crossed, north to south, by part of the A34(T) road between Oxford and Newbury, and east to west by a section of the M4 motorway, between Reading and Swindon, the railway line between London and Bristol, and by the Kennet and Avon Canal.

The Berkshire Downs comprise an escarpment (or more exactly, a series of escarpments) facing north over the Vale of the White Horse, and forming the southern fringe of the upper Thames catchment. The crest of the escarpment is marked by the Ridgeway track, a long distance footpath (Figure 1). The Berkshire Downs are bounded in the east by the Goring Gap, where the River Thames passes through a breach in the escarpment. A subsidiary breach, between Chilton and Compton, carries one of the upper tributary valleys of the River Pang.

The dip slope of the downs, descending more gradually southwards to the Kennet Valley, is dissected by the River Pang, the River Lambourn and their tributaries. The River Pang, in the eastern part of the area, flows first south, then east, then north, to join the River Thames at Pangbourne, in the Reading district. The remainder of the district drains towards the River Kennet, which crosses the southern part of the Newbury district. The River Lambourn, in the west, flows south-east to join the River Kennet at Newbury. The River Enborne, in the south-east, flows east to join the Kennet about five kilometres east of the Newbury district. The Kennet joins the Thames at Reading.

In the south of the area, the chalk downland is surmounted by another, heavily dissected escarpment formed by south-dipping Palaeogene deposits in the western end of the synclinal London Basin, the axis of which approximately coincides with the line of the River Kennet. The north-dipping Palaeogene deposits on the southern limb of this syncline form a corresponding escarpment in the south-west of the district, facing south onto a series of periclinal folds lying mostly outside the district. Only one of these structures, forming the Vale of Ham and the escarpment at Inkpen Hill, lies mainly within the Newbury district. The entire Palaeogene outcrop is heavily dissected, and widely obscured by Quaternary river terrace deposits.

History of survey

The original geological survey of the whole district was by W T Aveline, B Bauerman, H W Bristow, E Hull, W Whitaker and R Trench, and was published in Old Series 1:63 360 scale Sheets 12, 13, 14, and 34 (1857–1860).

Memoirs were published describing Sheet 12 (Bristow and Whitaker, 1862), Sheet 13 (Hull and Whitaker, 1861) and Sheet 34 (Ramsay and Aveline, 1858). Portions referring to the Chalk and Palaeogene deposits were absorbed into the regional memoir by Whitaker (1872). (Note that although this work is entitled ‘Part I’, the planned sequel, intended to describe the Quaternary deposits, was never published).

Geological Sheet 267 (New Series) was surveyed at 1:10 560 scale by F J Bennett between 1886 and 1893, and published at 1:63 360 scale in 1898, being reprinted without revision in 1947 and 1971. Together with the corresponding Memoir (White, 1907), it was named after Hungerford, the town closest to the centre of the district. The district encompassed by Sheet 267 is now named after Newbury. This is the seat of the West Berkshire District Council, and greatly exceeds Hungerford in size of population and in economic and administrative significance. The new geological map of the Newbury district is based on a complete revision at 1:10 000 scale, including desk study, interpretation of 1:10 000 scale aerial photographs, and detailed geological field survey between 2000 and 2003.

The Cretaceous rocks within the Abingdon district were surveyed at 1:10 560 scale prior to 1900 by A J Jukes-Browne and F J Bennett. The whole district was resurveyed at the same scale in 1967–1969. The geological map of New Series Sheet 253 was published at 1:63 360 scale in 1971, although without a corresponding Memoir or other form of sheet description. The parts of this maps described in this report were revised at 1:10 000 scale by a combination of desk study, interpretation of aerial photographs and rapid field survey in 2000. Details of the component 1:10 000 geological maps were captured digitally and compiled for presentation at 1:50 000 scale.

The newly surveyed geological boundaries were used together with data from the interpretation of borehole records to construct a three-dimensional digital model of the Chalk of the Berkshire Downs.

Much of the district falls within the area described by Arkell (1947). The geology of the district is also discussed in a regional context by Sumbler (1996).

Geophysical logging of a number of water boreholes in the Chalk of the River Lambourn catchment were carried out by Tate et al. (1971), in support of hydrogeological investigations.

Conventions

The area covered by this report is referred to as ‘the district’: it comprises all the area covered by Geological Sheet 267 (Newbury) and by the southern part of Geological Sheet 253 (Abingdon). In some older BGS publications, the Newbury district was referred to as the Hungerford district.

In the following descriptions, estimates of the stratigraphical thickness of a particular part of the geological sequence should be taken to refer to its occurrence within the Newbury and Abingdon districts, unless stated otherwise.

National Grid References quoted in this report are given in the form [SU 2928 4352]. All lie within 100 km Grid Square SU, unless otherwise stated. Symbols in brackets, for example (SCk), refer to the symbols used on the geological map. The number given with the plate captions is the registration number in the British Geological Survey photograph collection.

This report includes interpretations of data available at the time of writing. Additional information is available in BGS files. Neither the report nor its complementary 1:10 000 scale geological maps should be taken as a substitute for detailed site investigations.

Users should note that the stratigraphical nomenclature used in this report is liable to revision.

Chapter 2 Outline of the geology

The geological succession found in this district is shown in (Table 1) and (Table 2).

The district extends across the southern edge of the Midlands Microcraton (Pharaoh et al., 1987); this was an area of relative stability throughout early Palaeozoic marine deposition and the Caledonian Orogeny that followed. During the late Palaeozoic the microcraton became part of the Wales-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.

The district is partly underlain by a section of the Carboniferous Berkshire depositional basin (Allsop et al., 1982; Foster et al., 1989; Mathers and Smith, 2000). It lies just north of the Wessex Basin, a post-Variscan depositional basin that extends across central southern England and adjacent offshore areas. The district occurs at the western end of the London Basin, which formed during Cenozoic folding and structural inversion of the Wessex Basin (Chadwick, 1986, 1993; Hawkes et al., 1998; Penn et al., 1987; Underhill and Stoneley, 1998; Whittaker, 1985).

No strata older than Carboniferous have been proven within the district, but Cambrian, Early Ordovician and Silurian rocks are thought to underlie the Mesozoic succession in the west. Based on regional considerations, especially evidence from deep boreholes in the surrounding areas, in the eastern part of the district the Lower Palaeozoic strata are thought to be overlain by thick Devonian deposits, probably nonmarine sandstones for the most part, although marine transgressions could have occurred during the Late Devonian and early Carboniferous. Tournaisian or Visean limestones, or both, were probably deposited, followed by early Variscan uplift and nondeposition during the Namurian. Deltaic and coastal marine conditions, with Coal Measures deposition, were established during Westphalian times. Uplift in the south during later Westphalian times, a precursor to the late Carboniferous to early Permian Variscan Orogeny, provided a sediment source for the thick fluvial sandstones of the Warwickshire Group that occur in part of the district, but both the foldbelt and the foreland eventually turned into an area of elevated land, leading to erosion, as reflected in the pre-Permian subcrop pattern and nondeposition during the Permian.

During the Triassic, the Newbury and Abingdon district was gradually submerged through subsidence associated with the nearby Worcester Basin to the west. Crustal extension in Triassic and in Early Jurassic times reactivated the north-vergent Variscan thrusts, with the development of east-west trending synsedimentary faults in the Wessex Basin, which extends southwards from the southern edge of the district (Chadwick, 1986, 1993; Penn et al., 1987; Whittaker, 1985). A cessation of rift subsidence in Early Cretaceous times, associated with 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 deposition of the Lower Greensand was partly controlled by north-west-south-east- trending faults. Subsequent thermal subsidence was more general, allowing the Gault and Upper Greensand to overlap the Lower Greensand onto the London Platform, part of the Variscan foreland left largely untouched by early Mesozoic basin development.

The Upper Cretaceous Chalk Group was likewise 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 Chalk succession. Uplift in the Late Cretaceous, probably associated with crustal extension in the north Atlantic, 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, as seen in this district.

Marine deposition of the Upnor Formation in the early Palaeogene was soon followed by sea-level fall, and the fluvial, deltaic and estuarine conditions during which the Reading Formation was laid down. Borehole evidence shows that thick basal sands in the Reading Formation were probably deposited in channels, perhaps marking pre-existing valleys in the underlying Chalk. The London Clay marks the return of marine conditions. It was deposited during a series of cycles of rising and falling sea level, giving rise to sequences of pebble gravel lags resting on transgressive erosion surfaces, followed by clays, with sands increasing during the regressive phase.

Continuing crustal movement in the Cenozoic led to reactivation of Variscan thrusts and reversal of the fault systems in the overlying 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.

The river terrace deposits were laid down predominantly during periods of cold climate in the Quaternary. The youngest terrace deposit is overlain by postglacial alluvium, peat and tufa. In the north and west of the district, large areas of downland carry a thin cover of ‘clay-with-flints’ and sands which, with the characteristic ‘sarsen’ stones, represent the residue of a weathered and eroded former cover of Palaeogene deposits. A discontinuous cover of head (solifluction deposits and colluvium derived locally from other bedrock or superficial deposits) is present in the district.

The geological map also shows areas of made ground (chiefly road and rail embankments), worked ground (including road and rail cuttings and quarries for chalk, gravel or other minerals), infilled ground (typically, backfilled mineral workings) and landscaped ground (where areas of worked, made or infilled ground are present but cannot be distinguished with confidence).

Also identified are springs and dissolution hollows (dolines), including swallow holes on minor streams and closed depressions with no afferent drainage channels.

North of the River Kennet, the structure is dominated by a gentle regional dip of up to about 2° to the south-south-east or south. Passing southwards across the synclinal axis within the London Basin, the regional dip reverses. It remains gentle except in a zone in the south-west of the area, where it increases to between about 15° and 30° on the northern flanks of asymmetrical periclinal folds marking the northern edge of the Wessex Basin. The axis of one of these approximately east-west-trending periclines passes through the Vale of Ham, the gently dipping southern limb forming the escarpment at Inkpen Hill, on the southern margin of the area.

Faulting is present, but minor. Drainage channels tend to follow one of several linear trends, thought to reflect the distribution of jointing in the Chalk.

Landslides occur locally on the Palaeogene formations around Newbury. Other potential geological hazards in the area include cambering and valley bulge, ground heave, subsidence, flooding and radon emission. The most important natural mineral resources in the area are water and aggregates (sand and gravel). In the past, clay, chalk, flint and peat were exploited on a small scale.

Chapter 3 Pre-Mesozoic

The geological succession present at outcrop in the district is shown in (Table 1). Much of the evidence concerning strata concealed beneath the Upper Greensand has been drawn from deep boreholes sited in and around the district (Figure 2a), (Table 2), together with interpretation of seismic reflection surveys and regional maps of gravity and magnetic anomalies.

Precambrian

Broad magnetic (Figure 3b) and gravity anomalies (Figure 3a) within the Newbury district and adjacent areas to the north 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, Precambrian rocks are buried by north-vergent Variscan thrusting.

Lower Palaeozoic

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 2a).

No Lower Palaeozoic strata have 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 deposited in a shelf sea. Thick strata of Tremadoc (Early Ordovician) age are found to the north, south and west of the Newbury district (for example in the Worcester Basin, Smith and Rushton, 1993) and therefore are likely to be present at depth. This succession probably directly underlies Triassic strata in the west (Figure 2a) and (Figure 2b). Middle to Upper Ordovician formations are rare on the Midlands Microcraton (Smith, 1987) and are probably absent from the district.

By analogy with the Bristol Coalfield, a full sequence of Silurian strata, including uppermost Silurian (Pridoli), is probably present beneath the Berkshire Coal Basin, the succession being most complete in the north-eastern half of the district (Smith, 1987, fig. 2). To the north, in Oxfordshire, only Llandovery rocks are present, overlying volcanic rocks in the Bicester Borehole [SP 5878 2081] (Cornwell et al., 1994), but this is probably in an attenuated marginal position. Any Silurian succession present would probably include volcanic rocks that are broadly related to formations exposed in the Mendip Hills (some 60 km to the west) and found in the Bicester Borehole. These volcanic units have geochemical characteristics transitional between within-plate and arc volcanic rocks (Pharaoh et al., 1991). They are thought to be part of the cause of the short-wavelength magnetic anomaly in the area between Newbury and Reading (Figure 3b), 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.

Upper Palaeozoic

Devonian

During the late Palaeozoic, the district lay at the southern margin of the Midlands Microcraton, which (except to the south) was flanked by early Palaeozoic basins. Uplift and folding during the Caledonian Orogeny transformed these basins into encircling mountains (Pharaoh et al., 1987). Devonian strata beneath the Oxfordshire and Berkshire coal basins (Figure 2a) attain a thickness in excess of 2000 m. They have been reached by several boreholes to the north of the district, but only completely penetrated near the northern margins of the basins.

Much of the Lower Devonian was deposited under fluvial, coastal or deltaic conditions. Lower Devonian strata are present in the Faringdon Borehole, drilled on an anticline between the Oxfordshire and Berkshire coal basins. These comprise 302 m of predominantly red sandstone, shale and siltstone, probably no older than Emsian (Foster et al., 1989; Mortimer, 1967). These and other intervals of Palaeozoic strata found within the district are summarised in (Table 2).

About 28 m of Devonian sandstones with some siltstones, mudstones and conglomerates were proven in the Maddle Farm Borehole [SU 305 823], where they unconformably underlie Upper Coal Measures. They comprise dominantly greenish grey, fine to coarse-grained sandstones, forming fining-upwards sequences, up to 7 m thick, with erosive bases. Conglomerates with small quartz pebbles or mud flakes occur locally, particularly near the base of the fining-upwards units. Red mudstones, with interbedded, thin, fine-grained sandstones, sometimes occur in the upper part of the fining-upwards sequence. Small lenses of carbonate occur in the mudstones. Sedimentary structures consistent with fluvial deposition were noted (Foster et al., 1989).

In the Aston Tirrold Borehole [SU 5579 8722], four distinct lithofacies occur in beds that underlie the Carboniferous Limestone either unconformably or disconformably. Mudstones, generally green in colour but locally red, have rare interlaminations of siltstone and traces of bioturbation. Cross-bedded sandstones are generally green or grey, medium to coarse-grained, commonly with poorly sorted foresets, forming cosets with erosive bases, up to 0.5 m thick. Sharply based sandstones are similarly mainly green or greenish grey, and occur in beds up to 1.5 m in thickness. Immediately above a sharply eroded base, the sandstone is poorly sorted, with scattered mudflakes and sandstone intraclasts. The rest of the sandstone tends to be fine grained, well sorted, and laminated or cross-laminated, with traces of bioturbation, particularly towards the top of the beds. Laminated sandstones are green to greenish grey, fine to very fine grained, generally well sorted, with parallel lamination or rare ripple-lamination. They are interlaminated with green mudstones 1 to 5 mm thick, some of which display apparent desiccation structures (mud curls). Bioturbation is common. A marginal marine origin is suggested (Foster et al., 1989).

Caledonian tectonism probably led to uplift of the district in the later part of the Early Devonian, so that an unconformity separates the Lower Devonian from the Upper Devonian succession.

Frasnian and Famennian strata include limestones in the Steeple Aston Borehole (Poole, 1977), whereas the sequence in the Apley Barn Borehole consists mostly of sandstones (Poole, 1969). These strata were deposited during the Frasnian marine transgression, which reached the southern Midlands from the Rheic Ocean (Butler, 1981).

Carboniferous

Thin Carboniferous Limestone strata have been proven by two local deep boreholes. Uplift at some time between the Visean and early Westphalian is suggested by the absence of strata of this age range. There is a resulting unconformity between Tournaisian/Visean and Westphalian strata, common also to the Kent Coalfield (Smith, 1993) and the margins of the Carboniferous Pennine Basin.

The thin Coal Measures sequence with lavas suggests that subsidence was limited and that the Berkshire Coal Basin did not subside significantly until Westphalian D times. This, in turn, suggests that thrusting within the Variscides did not re-occur until this time.

Tournaisian/Visean

During the early Carboniferous, a marine transgression once again introduced a shallow shelf sea into the region. In the Aston Tirrold Borehole, 15 m of Tournaisian strata were found. They comprise grey, argillaceous, bioclastic, thinly bedded limestones, interbedded with grey silty mudstones, and have been assigned to the Avon Group (Waters et al., 2007). By contrast, about 10 m of bioclastic limestones, interbedded with thin calcareous mudstones, proved in the bottom of the Foudry Bridge Borehole [SU 7063 6602], about 23 km to the east of Newbury, are of Visean (Holkerian) age (Foster et al., 1989). These have been assigned to the Pembroke Limestone Group (Waters et al., 2007).

The ages of the fauna within the limestones suggest that a thicker sequence was formerly present but eroded during Namurian times. The northern limit of Tournaisian/Visean strata south of St George’s Land (a contemporary landmass and area of nondeposition between the Pennine basins and basins in south Wales and southern England, which was part of the Wales-Brabant Massif) is near Northampton (British Geological Survey, 1985).

However, it is not known with certainty whether Tournaisian strata, or Visean, or both, are present within the Newbury district.

Westphalian

Contours on the base of the Westphalian strata (Foster et al., 1989, fig. 3) indicate that the Berkshire Coal Basin is an asymmetric syncline, with a steeper south-west limb. This interpretation is supported by the presence of steeply dipping strata in the Welford Park Station Borehole. The asymmetry is caused by the Variscan Front structure to the south-west (British Geological Survey, 1985). The base of the Westphalian strata is unconformable on older rocks. These strata accumulated in a south-westerly-deepening basin, probably flexed down by northward loading along Variscan Front thrusts, although 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 edges of the Wales-Brabant Massif, from South Wales to Kent.

Strata of Westphalian age can be subdivided into a relatively thin Coal Measures succession that is overlain by red-bed deposits with some coals comprising the Warwickshire Group (Foster et al., 1989; Powell et al., 1998). They are probably everywhere separated by an unconformity.

The Variscan Orogeny uplifted the Variscides to the south, providing a source for the thick fluvial sandstones of the Warwickshire Group. Some of the clasts indicate reworking of earlier Westphalian strata, possibly from near the original southern margin of the basin.

Coal Measures

Coal Measures strata, assigned to the South Wales Coal Measures Group (Waters et al., 2007), are present in the Foudry Bridge Borehole in the Reading district, unconformably overlying Visean limestones. In a short penetrated section of 146 m, including sills, one marine band was discovered, but could not be precisely dated. A similar sequence at nearby Strat B1 was dated as Westphalian B. The thin sequences suggest uplift prior to early Westphalian times (Smith, 1993). The Coal Measures probably thicken modestly south-westwards.

Lavas penetrated in the Aston Tirrold [SU 558 872] and Burnt Hill [SU 522 738] boreholes comprise basalts with calcite-filled vesicles and dolerites with some traces of flow structure and sharply defined bases. At Aston Tirrold they are interbedded with Lower Coal Measures. At Burnt Hill, an overlying Warwickshire Group conglomerate suggests an unconformity above the igneous rocks.

Warwickshire Group

No Etruria Marl Formation is present in this district. Most of the strata are probably equivalent to the Halesowen Formation, recognised on geophysical logs in the Coventry district (Bridge et al., 1998) and correlated by way of the Withycombe Farm Borehole. In the Aston Tirrold and Steeple Aston boreholes, these strata are intruded by a thick sill near the base, and in the Burnt Hill Borehole they rest on lavas that probably formed during early Westphalian volcanism (Foster et al., 1989). The Warwickshire Group strata are predominantly thick fluvial sandstones with sparse coals.

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

Further to the north, the youngest parts of the Warwickshire Group are of Permian age, but it is likely that no Permian strata occur in the subsurface of this district.

Variscan unconformity

The main unconformity within the district is the Variscan unconformity, resulting from folding and uplift associated with the Variscan Orogeny. Uplift of the Variscides and the foreland area in the Variscan Orogeny resulted in the district becoming elevated land.

After a long period of erosion, during which Westphalian and in some areas older strata were removed, as reflected in the pre-Permian subcrop pattern (Figure 2a) and (Figure 2b), the district was gradually submerged during Triassic to Early Cretaceous times by subsidence associated with the adjacent Wessex and Worcester basins. The depth to the Variscan unconformity (= base of Mesozoic) increases south-westwards (Figure 2a).

Chapter 4 Mesozoic

Mesozoic basin development

Crustal extension in Permian times formed the Worcester Basin along north-south-trending faults in the west. As the Tethys Ocean was opening in Jurassic times, the main synsedimentary growth faulting changed direction to east-west, following reactivation of the east-west striking thrusts within the belt of Variscan deformation, and forming the Wessex Basin adjacent to the south of the district (Figure 2b).

During the Triassic and Jurassic, the Newbury and Abingdon district was gradually submerged through subsidence associated with the nearby Worcester Basin to the west, and with the Wessex Basin to the south. 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 2a).

The Late Cimmerian unconformity (at the base of the Lower Greensand), which affected all Mesozoic basins, began in Early Cretaceous times and was caused by a reduction or cessation of rift subsidence combined with regional uplift. The more general subsidence of the later, thermal relaxation phase of basin formation caused the London Platform to subside preferentially, so that the Gault Formation thickens and overlaps the Lower Greensand to the east of the district.

The Chalk was deposited both in the basins and over the London Platform, but the succession in central southern England is incomplete (Curry, 1965). This is due to erosion at the basal Palaeogene unconformity, demonstrated within the district by variation in the age of the youngest preserved Chalk. It was caused probably by uplift in the north-west in Late Cretaceous times, associated with opening of the North Atlantic Ocean.

Triassic

The oldest Triassic strata found to occur at depth in the Newbury district are sandstones with siltstones and thin mudstones of the Sherwood Sandstone Group. Together with the overlying Mercia Mudstone Group (chiefly mudstones and siltstones) and Penarth Group (mudstones and limestones), this unit increases in thickness westwards, towards the Worcester Basin. The Triassic succession is probably absent from the eastern edge of the district, but reaches more than 450 m in the west (Whittaker, 1985).

Jurassic

Significant thicknesses of the lower part of the Lias Group are present at depth in the district, but the subsequent succession of Jurassic strata is mostly rather attenuated, as summarised in (Table 2).

Lias Group

The Lias Group onlaps eastwards from the Worcester Basin onto the London Platform, being itself overlapped by the Great Oolite Group north-east of Reading (Donovan et al., 1979) following removal of the middle and upper parts of the Lias Group (Sumbler, 1996). The thickness ranges from zero up to about 100 m. It is difficult to assign these strata to a particular formation. The Lower Lias in Strat B1 (11 m thick) comprises ferruginous, ooidal and micritic limestones interbedded with siltstones and mudstones, and lies unconformably on the Coal Measures.

Inferior Oolite and Great Oolite groups

The Inferior Oolite and Great Oolite groups comprise a complex succession of laterally variable limestones with some mudstones. A marine transgression began during deposition of the Cornbrash, the youngest formation of the Great Oolite. This introduced deeper water conditions into the district, associated with mud-dominated deposition of the following three formations.

Kellaways Formation

The Kellaways Formation can be divided into a lower clay member and an upper sandy member. The formation has a distinctive geophysical character that can be recognised in all the boreholes shown in (Table 2).

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 7.76 m of strata assigned to the Kellaways Formation to a depth of 335.4 m. The Kellaways Sand, comprising silty and muddy fine-grained sandstone, is 4.56 m thick, and the Kellaways Clay, consisting of variably sandy mudstones and a basal thin muddy limestone, is 3.2 m thick (Gallois and Worssam, 1983).

Oxford Clay and West Walton formations

Most local borehole records do not differentiate the Oxford Clay from the overlying West Walton Formation, which is probably also present throughout the district.

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 38.75 m of strata assigned to the West Walton Formation to a depth of 264.9 m. This interval is composed of fossiliferous siltstone or silty mudstone (Gallois and Worssam, 1983). It is underlain by mudstones and rare thin limestones of the Oxford Clay Formation. Strata can be assigned to the upper (5.98 m), middle (about 32 m) and lower (about 25 m) divisions of that formation.

Corallian Group

In western parts of the district the West Walton Formation may pass laterally into the lowest 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.

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 33.57 m of strata assigned to the Corallian Group to a depth of 226.15 m (Gallois and Worssam, 1983). As with other Corallian sequences in Oxfordshire, these strata can be broadly divided into an upper unit of coral-bearing and ooidal limestones, and a lower unit of sands and calcareous sandstone (Sumbler, 1996). At Harwell, the upper unit is 9.92 m thick, comprising limestones with some mudstones. The topmost 1.8 m is in ‘coral rag’ facies, interpreted as a reef apron deposit. This passes down into predominantly ooidal limestones that contain appreciable amounts of sand and shell debris and rare thin mudstone beds. The lower unit of fine-grained calcareous sandstone is 23.65 m thick (Gallois and Worssam, 1983).

Ampthill Clay and Kimmeridge Clay formations

The Kimmeridge Clay Formation, of Late Jurassic Kimmeridgian age, is seen at outcrop beneath the Vale of the White Horse in the Abingdon district. It comprises rhythmic sequences of pale calcareous mudstones, dark, shelly, carbonaceous mudstones, and silty mudstones, commonly with beds of cementstone (impure limestone) nodules. It includes some sandy and silty units in its upper parts. At Swindon, just to the west of the area, it is about 100 m thick, decreasing towards the north-east.

Local borehole records mostly do not differentiate the Kimmeridge Clay from the underlying Ampthill Clay Formation, which is probably present throughout the district. The Ampthill Clay includes some silty or ferruginous beds where it is exposed in adjacent districts to the north-west.

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 26.0 m of strata assigned to the Kimmeridge Clay Formation, to a depth of 186.95 m (Gallois and Worssam, 1983). These strata comprise soft mudstones, calcareous mudstones and bituminous mudstones. Some of the mudstones are silty, and some contain phosphatic pebbles. The upper 5.69 m were assigned to the Upper Kimmeridge Clay and the remaining 20.33 m to the Lower Kimmeridge Clay. These units are separated by a major erosion surface, and each also contains minor erosion surfaces.

The underlying interval of 5.63 m of mudstones, some silty, was assigned to the Ampthill Clay Formation, based on its lithological and faunal characters (Gallois and Worssam, 1983). It comprises three intervals separated by erosion surfaces, which can be correlated with a sequence in Fenland. At Harwell, the Ampthill Clay rests on an oyster-encrusted hardground of Corallian limestone. The length of time represented by this unconformity could not be determined.

Portland Group

During the mid-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.

This group is proved only in Strat B1, where 24 m of buff, sandy, micritic limestones overlie glauconitic sandstones resting on the Kimmeridge Clay, and in Kingsclere 1 [SU 4984 5820].

Purbeck Group

No Purbeck Group strata have been proven in the district, but probably occur in the south-east. Some 61 m were penetrated beneath the Late Cimmerian unconformity in the Kingsclere 1 Borehole, 3.5 km to the south of the area and within the Wessex Basin (Figure 2a) & (Figure 4). 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.

Lower Cretaceous

Wealden Group

No Wealden Group has been proven to occur in the district, but a relatively thin succession is thought to be present, at least in part, in the extreme south. A more complete succession, with the Weald Clay Formation at the top, occurs within the Wessex Basin just to the south, as found in the Kingsclere 1 Borehole (Figure 2a) & (Figure 4); (Table 2).

Lower Greensand Group

The base of the Lower Greensand Group rests on the Late Cimmerian unconformity. The abrupt decrease in depth to this unconformity, just south of the district (Figure 2a) and (Figure 2b), shows the extent of the inversion along faults at the northern margin of the Wessex Basin.

A marine transgression in Mid Aptian times led to the deposition of some 80 m of Lower Greensand Group strata around Faringdon, in the north-west of the Abingdon district. The succession occurs in a partly fault-bounded trough trending approximately north-west to south-east and comprises sands, gravels and clay, with some chert and two beds of fuller’s earth, the latter being exploited at Baulking [SU 323 908].

These Upper Aptian sediments 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 2a); (Table 2). It is likely to continue to the south-east at depth beneath the Newbury district in one or more fault-bounded troughs; thin intervals occur at Welford Park Station and Kingsclere.

The Lower Greensand 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 2a). In Strat B1, the Lower Greensand is represented by a quartzitic glauconitic sandstone, coarsening at the base.

As Arkell (1947) observed, a series of north-westerly trending faulted synclines are present below the Gault outcrop at the northern edge of the Marlborough and Berkshire downs and the southern Chilterns. The Faringdon Syncline is one of these structures. It is essentially an assymetric syncline with a steeper, partly faulted north-east limb. A south-easterly continuation of this structure would pass immediately to the west of Wantage.

Another north-westerly trending syncline, with the same sense of assymetry, can be seen in the outcrop patterns of the Jurassic strata just west of the district at Bourton [SU 232 868]. A smaller syncline with the same orientation occurs some 2 km to the south-west, passing through South Marston [SU 194 880] (Arkell, 1947, fig. 23). To the south-east, within the Newbury district, the Bourton Syncline is aligned with the Lambourn Valley at Upper Lambourn, suggesting that this linear valley might be fault-controlled.

Also as noted by Arkell (1947), a Portland Group outlier at Swindon marks yet another north-westerly trending syncline. To the north-east and south-west of this structure, the Lower Greensand is cut out at the base of the Gault. The syncline also marks a slight change in the regional strike of the Cretaceous strata. To the south-east, within the Newbury district, the north-westerly trending syncline is aligned with Aldbourne. Although Arkell (1947) emphasised that these structures all predate the Lower Greensand, it seems clear that in some cases their influence persisted, exerting some control on deposition of the latter, and at Wantage also on deposition of the Upper Greensand. It can therefore be expected that evidence of tectonic control on Early Cretaceous sedimentation will be found at subcrop elsewhere in the district.

Details

SU48NE

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 5.3 m of strata assigned to the Lower Greensand (Gallois and Worssam, 1983). This interval comprised weakly cemented, mottled grey and greyish green ferruginous pebbly sandstone. Gallois and Worssam (1983) noted a resemblance to the Albian-aged Carstone of northern East Anglia. The sandstone rests on the Kimmeridge Clay at a major erosion surface, which appears as a sharp irregular contact with much burrowing.

Gault Formation

The Lower Greensand is overlain unconformably by the Gault Formation, of Mid to Late Albian age. To the east of the district, the thicker Upper Gault overlaps the Lower Gault, illustrating the composite nature of the unconformity as it progresses onto the London Platform.

At outcrop in the Abingdon district, just below the escarpment formed by the Upper Greensand, the Gault Formation comprises grey mudstones and silty mudstones, commonly with small dark brown phosphatic nodules. The lower part is siltier than the underlying Kimmeridge Clay, and locally sandy. The sequence is reported to be very sparsely fossiliferous (Arkell, 1947). Similar rock types have been found in boreholes in and around the district.

The Gault is typically about 70 m thick in this area, increasing to more than 80 m in the south (Table 2). It passes with rapid gradation upwards into the Upper Greensand.

Details

SU38NE

In 1939, a disused brickpit [SU 3577 8897], some 1.1 km north-north-west of Childrey Church, exposed about 4.6 m of grey, silty, laminated clay with some fine sand and few grey, gritty phosphatic nodules. Some of the nodules were up to 8 cm long and septarian. One yielded a specimen of the ammonite Dimorphoplites spp., assigned to the lautus Zone (Arkell, 1947).

SU48NE

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 65.6 m of strata assigned to the Gault (Gallois and Worssam, 1983). It is described as soft mudstone, some slightly silty, pale grey to olive grey in colour, commonly micaceous and calcareous, bioturbated throughout and with a sparse fossil fauna. At some levels, the mudstones are sparsely glauconitic or they contain pale brown phosphatic nodules. A 0.35 m bed of calcareous, micaceous siltstone occurs at 129.65 m. The 41 m interval to 131.05 m was assigned to the Upper Gault, and the remaining 24.58 m to the Lower Gault.

The basal 0.48 m of mudstone is sandy and pebbly, with glauconitic sand, becoming more so with depth and passing down with bioturbation into the underlying Lower Greensand (Carstone) (Gallois and Worssam, 1983).

Upper Greensand Formation

The Upper Greensand Formation, of Late Albian age, forms a minor escarpment in the north of the district, marking the southern edge of the Vale of the White Horse. It also crops out in the core of the pericline forming the Vale of Ham, and in the extreme south-west, which is at the eastern end of the Vale of Pewsey.

In much of the northern outcrop, the Upper Greensand can be divided into a thicker lower unit, and a thinner upper unit.

The lower unit is generally some 10 to 30 m thick, composed of evenly bedded, whitish, micaceous, sparsely glauconitic, calcareous siltstone and fine sandstone, with some chert and malmstone (typically sandstone composed of siliceous sponge spicules in siliceous cement). Jukes-Browne and Hill (1900) described this unit as ‘malmstone’ and sandstone, together about 11 m thick below White Horse Hill, passing east into ‘calcareous malmstone’ up to 30 m thick between Didcot and the River Thames. Recent mapping has shown a marked change in thickness from about 9 m just west of Wantage to about 40 m just to the east of that town, presumably controlled by structure. The base of the Upper Greensand is likely to be gradational with the Gault. It can be placed at the change from silty clay below to clayey silts and sands above.

This lower part of the Upper Greensand forms a subsidiary escarpment at the base of the main Chalk escarpment. The scarp slope shows a strong positive break of slope within a convex crest at the top, passing south into a planar dip slope. This dip slope is more than one kilometre long near Wantage and to the east, but decreases in length markedly to the west of the town. In the field, the base of the Upper Greensand was taken at a fairly strong, persistent negative break of slope near the base of the Upper Greensand escarpment. This is commonly a spring line. It lies as much as 10 m vertically below the level of the boundary shown on published maps of the area, such as Sheet 253 (Abingdon).

Where present, the upper unit of the Upper Greensand comprises 3 m to 10 m of dark green glauconite sand and sandstone, containing numerous small grains of glauconite and quartz sand in a clayey matrix. The base of the Chalk is also highly glauconitic and is sufficiently cemented to be impenetrable to a hand auger, suggesting that a minor nonsequence might be present.

Jukes-Browne and Hill (1900) recorded 6 m of ‘soft green sand’ at Uffington, and 4.5 m to 5 m at Wantage and Didcot. However, to the west of Wantage much of this part of the Upper Greensand outcrop is obscured by head deposits; the upper unit may be absent locally, possibly as a consequence of Late Albian or earliest Cenomanian erosion, as found for example in the Fareham district of Hampshire (Hopson, 1999). As discussed under ‘Details’ below, 4.7 m of non-fossiliferous, highly glauconitic, clayey, calcareous sand found in borehole core from the site of the Rutherford Appleton Laboratory can be assigned to the upper unit of the Upper Greensand.

Part or all of this upper unit locally forms a strong positive feature at the foot of the Chalk escarpment. Its base is approximately marked by a negative feature above the dip slope formed on the lower unit of the Upper Greensand.

Micropalaeontological examination of samples of the upper unit collected from shallow, purpose-dug pits near East Hendred found faunal assemblages indicating foraminiferal zones 6 (upper) and 6a of Hart et al. (1990), in the very youngest Albian. If a non-sequence has occurred between the Upper Greensand and the overlying Chalk Group at this locality, then it is within the limits of resolution of micropalaeontological biostratigraphy (Wilkinson, 2002a).

Relatively little is known about the Upper Greensand in its southern outcrop. It comprises calcareous, glauconitic, fine-grained sand and sandstone, with some lenses and layers of siliceous sandstone or chert.

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 Wessex Basin (Table 2); (Figure 2a). Locally the formation may be up to 50 m thick.

The Upper Greensand is overlain non-sequentially by the Chalk Group, possibly with minor unconformity in places.

Details

SU26SW

The old railway cuttings between Burbage and East Grafton are now infilled, covering the exposures recorded by Jukes-Browne and Hill (1900). The contact between the Upper Greensand and the Chalk is obscured by the canal and the railway.

SU26SE

The Upper Greensand occurs south of the Wilton-Crofton area. No exposures were noted during the recent survey. The contact between the Upper Greensand and the Chalk is obscured by the Kennet-Avon canal, the railway and Wilton Water.

SU36SW

Up to 48.8 m of Upper Greensand are seen in the Vale of Ham (White, 1907 pp. 7–8). All can be referred to Jukes-Browne’s upper ‘green sands’ division of the Vale of Pewsey, which is some 18 to 21 m thick. This consists of well-bedded, pale to rather dark greyish green to yellowish brown, speckled, calcareous, glauconitic sand and sandstone, which weather to a greyish brown light soil. Calcareous and cherty concretions occur in places.

Green sands with numerous bands of speckled calcareous sandstone are seen in the sunken lanes through Ham; in the Spray Road about half a mile [800 m] north-east of Ham church, and in the main street near the schools in Shalbourne. At the southern end of Shalbourne, ‘about a quarter of a mile [400 m] west of the smithy’, somewhat higher beds composed of grey sand passing down into sandstone with Pecten orbicularis are exposed. (The location of this particular smithy is now unclear). A similar section occurs in the sunken lane leading down to the stream on the north-west of the village [SU 3136 6322] (White, 1907, p. 8). At the time of writing, only sparse sandstone blocks are seen at any of these localities, and most are completely covered by soil and vegetation.

During fieldwork for the map revision, the Upper Greensand was seen only as rather sparse, smooth but irregular lumps, 10 to 30 cm in length, of very hard, pale grey, fine or fine to medium-grained glauconite sandstone. The soil is noticeably sandy, with sandy subsoil seen in ploughed-up material in places. The bedrock is probably rather patchily cemented. A few disused pits occur in the Upper Greensand, but none showed any exposure.

SU48NW

According to Jukes-Browne and Hill (1900), near Wantage there must be a thickness of 18 to 21 m between the lowest sandstone beds and the soft green sand at the top of the formation, but this is not wholly occupied by sandstone; the higher parts consist of micaceous marl and grey marly clay with grains of glauconite. These beds are seen only in ditches.

Eastwards, the sandstone beds increase rapidly in thickness and form a broad plateau from Ardington eastwards to Hagbourne.

Green-coloured sand (at the top of the Upper Greensand) can be seen north of Lockinge [SU 4242 8833], and in the lane 180 m east of West Hendred Church [SU 449 883].

SU48NE

In borehole core from the site of the Rutherford Appleton Laboratory, Harwell [SU 468 864], Gallois and Worssam (1983) described about 5 m of non-fossiliferous, highly glauconitic, clayey, calcareous sand, which they ascribed entirely to the Glauconitic Marl. This unit, highly glauconitic in its lowest part, rests on typical Upper Greensand siltstones at a depth of 66.2 m with an intensely bioturbated junction, and has a 0.1 m thick hardground near its top (Gallois and Worssam, 1983). As argued in the subsequent description of the West Melbury Marly Chalk, the base of the Chalk Group probably occurs at about 61.5 m (this is the deepest level at which Schloenbachia and Inoceramus occur). The 4.7 m of strata below this depth (i.e. most of the strata assigned to the Glauconitic Marl by Gallois and Worssam, 1983) can therefore be assigned to the upper unit of the Upper Greensand, which was not recognised as such in their descriptions.

The interval down to about 88 m depth was assigned to the Upper Greensand by Gallois and Worssam (1983), who described it as highly calcareous, argillaceous, quartzose siltstone, with sparse, fine-grained glauconite. Fine-grained mica occurs throughout. Burrows extend from the overlying glauconite-rich bed down to 66.9 m, and the unit is bioturbated throughout. Fossils are sparse, but demonstrate that the unit falls within the Upper Albian dispar Zone. Below 87 m, the siltstone becomes progressively more muddy with depth. It presumably passes gradually into the Gault, although the boundary interval at Harwell was not cored.

SU58NW

According to Jukes-Browne and Hill (1900), good sections of the Upper Greensand could be seen in the cuttings of the Didcot and Newbury railway line, now long disused.

Jukes-Browne (1889) stated that the ‘Glauconitic Sand’ is between 3.5 and 4.3 m thick between Upton and Harwell. It is a soft green sand, consisting of glauconite grains and fine quartz sand, with a further admixture of calcareous matter in the upper part. This is said to pass up into glauconitic marl, containing phosphatised fossils, with no erosional surface between them.

The lowest beds of the Upper Greensand were exposed near the Police Station, Didcot [SU 5277 8989], where greenish grey, possibly glauconitic micaceous bioturbated silt is interbedded with thin, cemented, fine silty sandstone and silty clay bands.

Higher beds were exposed in the railway cutting west of Hagbourne field [SU 5276 8910] where 1.2 m of soft silty sands separated two beds of massive sandstone, 0.3 and 0.9 m thick respectively.

A temporary exposure in York Road, West Hagbourne [SU 5144 8761], consisted of medium to pale grey glauconitic calcareous sandstone with mottling due to secondary recrystallisation.

Upper Cretaceous

Introduction to the Chalk Group

The Upper Cretaceous Chalk Group underlies most of the area, forming a prominent escarpment in the north and another in the extreme south, and 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.

Estimates of the thickness of the Chalk formations can be made from outcrop patterns (where the amount of dip is known), borehole records and, in this area, a computer model. The thickness of the Chalk is least well known in the steeply dipping outcrops in the south-west, partly because the amount of dip cannot be determined with sufficient accuracy. Indeed, it is possible that strike-parallel reverse faulting has reduced the apparent thickness of the succession in the steep limbs of the periclinal folds, but there is no evidence that this has occurred.

A typical total thickness from the base of the group 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], Winterbourne [SU 4542 7161] and Banterwick Barn No. 2 [SU 5134 7750] 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 in the Winterbourne borehole (Table 1) and (Figure 5).

Pearce et al. (2003) presented detailed palynological and stable isotope information for part of the White Chalk Subgroup, as sampled by core from the Banterwick Barn 2 borehole [SU 5134 7750]. They discuss their data within a regional stratigraphic framework.

The greatest stratigraphical variations in thickness occur within the New Pit Chalk and the Seaford Chalk; variation of the other units is less marked. In Dorset, some formation thickness variation is associated with the occurrence of channel-like features within the Chalk (Evans and Hopson, 2000; Evans et al., 2003) and it is possible that similar features occur within the present district. Pre-Palaeogene erosion has led to a wide variation in the preserved thickness of the Newhaven Chalk.

Formerly, the Chalk was divided into three units, effectively of formation status: the Lower Chalk, the Middle Chalk and the Upper Chalk. Named members or beds within these units, such as the Glauconitic Marl, the Melbourn Rock and the Chalk Rock (which occur at the respective bases of the three units) are widely recognised (Table 1) and are present in this district. However, following work by Mortimore (1986) and Bristow et al. (1995), a more detailed lithostratigraphical subdivision of the Chalk became possible (Bristow et al., 1997). After further discussion, it was proposed that the Chalk Group be divided into an older Grey Chalk Subgroup and a younger White Chalk Subgroup, the boundary between them being placed at the base of the Plenus Marls, slightly below the base of the Middle Chalk of previous usage (Rawson et al., 2001). The stratigraphy of the Chalk Group is described in more detail by Mortimore et al. (2001).

Each subgroup is divided into formations (Table 1) and (Table 3). The highest two do not extend as far north as Berkshire, but the remainder are present in the Berkshire Downs and adjacent parts of the Hampshire Downs. These formations are described separately, in order of decreasing age. Each can be recognised in the field and, as shown in (Figure 5) and (Figure 6), in borehole records.

Several of the formations include named members or beds. Two named members are shown on the face of the map, where their outcrop has been mapped reliably. These are the Chalk Rock, in the lower part of the Lewes Chalk, and the Stockbridge Rock, near the top of the Seaford Chalk.

As a whole, the Chalk contains a diverse marine invertebrate fossil fauna, with 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 (Table 3).

The correspondence of biostratigraphical zones with the lithostratigraphical scheme used here is shown in (Table 3), and described by Mortimore et al. (2001). The biostratigraphical significance of fossil material in the BGS collections and found during recent surveys is discussed by Woods (2000a, b, c) and Wilkinson (2001a, b, c, d, 2002a, b, c, 2003a, b, 2004). The stratigraphy of two cored boreholes in the area (Winterbourne and North Farm) is described by Woods (2001a) and Wilkinson (2001e, f). Geophysical logging of these two boreholes is discussed by Tate et al. (1971). The correlation of geophysical logs from other boreholes in the Chalk of Berkshire is discussed by Evans (1998), Woods (2001b, 2003a) and by Woods and Aldiss (2004), and the topic is explored more generally by Woods (2000a). Jukes-Browne and Hill (1903, 1904), White (1907) and Arkell (1947) described the Chalk of the area in general and at specific localities. Some fossil names and biozonal designations used in the older literature are no longer current. In the following descriptions, such non-current names are given in quotation marks.

In this district, the age of the Chalk Group ranges from the Cenomanian to the Campanian (Table 1) and (Table 3). In Late Cretaceous times, emergent landmasses were present in south-west England, Wales, Scotland and Northern Ireland, and farther afield in Brittany, the Vosges, the Ardennes and the Baltic Shield. Southern Britain lay approximately 10 degrees of latitude farther south than at present. Chalk sediment accumulated on the outer shelf of an epicontinental subtropical sea of normal salinity with little terrigenous input. Most of the group thus consists of predominantly soft, white to off-white, very fine-grained and very pure, microporous limestone (‘chalk’) with subordinate hardgrounds and beds of clay-rich chalk (marl), calcarenite and flint. The lowest part of the succession, of Cenomanian age, comprises alternations of clay-rich chalk and clayey limestone.

Chalk is composed largely of the microscopic calcareous skeletal remains of planktonic algae (coccoliths). Other coarser carbonate material is present, some in rock-building proportions; this includes foraminifera, ostracods and calcispheres, together with entire and finely comminuted echinoderm, bryozoan, coral and bivalve remains, and disaggregated prisms of inoceramids. Other, generally minor constituents of a depositional and early diagenetic origin are present (Hancock, 1975).

Mud-grade material, consisting chiefly of the clay minerals illite, smectite and kaolinite in varying relative amounts, forms an appreciable proportion (30 to 40 per cent: Destombes and Shephard-Thorn, 1971) of the marly parts of the Grey Chalk. Throughout the main body of the Chalk, above the West Melbury Marly Chalk, the proportion of argillaceous material falls progressively to less than 5 per cent in the White Chalk Subgroup and is largely concentrated in thin beds (‘marl seams’). The geochemical signature of individual marl seams in the White Chalk is proving to be sufficiently characteristic to correlate the basinal sequences of Sussex and Kent (Wray and Gale, 1993) and much farther afield (Wray, 1999).

A coarser silt and fine- to medium-grade sand fraction, predominantly of detrital quartz, forms less than 1 per cent of the chalk material. Certain beds, most notably the Glauconitic Marl in this district, contain other stable mineral species such as mica, zircon, rutile and tourmaline (Jukes-Browne and Hill, 1903). Another minor siliceous element is derived from skeletal material such as sponge spicules and radiolaria. These are commonly replaced by secondary calcite or pyrite.

The authigenic minerals glauconite and calcium phosphate occur sparsely the Chalk succession, but are most conspicuous in winnowed and condensed horizons such as the Glauconitic Marl and Jukes-Browne Bed 7 (a possible non-sequence within the Zig Zag Chalk). The phosphate occurs most commonly as reworked nodules. Hardgrounds within the Chalk commonly have concentrations of glauconite and phosphate as impregnations and coatings. Much of this material is the product of early diagenesis. Finely disseminated pyrite is a common authigenic mineral in the more argillaceous parts of the succession and pyrite nodules and burrowfills are a conspicuous feature at many levels in the Chalk. In much of the White Chalk Subgroup, the most important noncarbonate constituent is flint. As described below, this occurs in nodular and tabular form in seams which lie parallel to the bedding, and also as thin sheets along cross-cutting joints and fissures.

The diagenesis of the chalk occurred in two distinct phases. An early phase, associated with interruptions in sedimentation, affected unconsolidated sediment at or just below the sea floor. A late phase was associated with deeper burial, compaction, silicification and carbonate dissolution. Early diagenesis of the chalk in response to changing water depth, depositional rates and erosion gave rise to a variety of bedding surfaces and associated lithologies. These range from simple omission surfaces, demonstrating nondeposition, to complicated scoured, burrowed and mineralised surfaces (hardgrounds) overlying lithified chalk (chalkstone). They provide a framework of lithological markers within the Chalk Group. Many of these surfaces and lithologies are the result of basin-wide changes in depositional conditions, enabling detailed regional correlations (Bromley and Gale, 1982; Mortimore, 1986, 1987; Robinson, 1986). The development of hardgrounds was discussed by Kennedy and Garrison (1975).

Late-stage diagenesis was marked by carbonate dissolution as the result of deep burial and compaction. Microstylolites are common in hard chalks, whereas stylolites are absent in the softer chalks and anatomising residual clay seams are widespread. Where dissolution was extensive, the softer chalk takes on a ‘flaser’ appearance with ‘augen’ of white chalk enveloped by greyish clay-rich chalk. The ‘flaser’-like limestones discussed by Kennedy and Garrison (1975) have also been described as ‘griotte’ chalks (see for example Mortimore, 1979, 1986).

Analysis of superficial mineralisation found on the surface of solution cavities in the Seaford Chalk, at the base of overlying clay-with-flints, found that it is composed of gibbsite with halloysite and possibly allophane. This mineralisation is thought to have formed during the interaction of acidic, sulphate-bearing groundwater with the chalk (Kemp, 2003).

The most conspicuous diagenetic products in the chalk are flints, the result of silicification. Flint is a form of chert with a particularly well-developed conchoidal fracture. It is composed of an aggregate of ultramicroscopic quartz crystals only a few microns across. It is generally present in pure white chalks that contain an insignificant clay content, and in most areas is thus confined to the White Chalk. Flint is considered to have resulted from the precipitation of silica from pore water contained within unconsolidated chalk sediment. The silica was presumably derived by dissolution from original biogenic material such as sponges, radiolarians and diatoms. Clayton (1986) suggested that this precipitation was a multistage process and that it occurred 5 to 10 m below the sediment surface. It was promoted by an excess of dissolved sulphide (initiating acid dissolution) in the pore waters at the oxic/anoxic boundary. Local porosity and permeability variations, particularly in response to burrowing (now commonly seen as the trace fossils Thalassinoides and Zoophycos), produced the characteristic burrowfill form of many flint nodules. Silicification in beds with a more uniform permeability resulted in the formation of tabular flint bands. The most strongly developed flint bands can be traced over large distances. Apparently similar but generally thinner and less persistent sheet flints formed along near-horizontal fractures in the chalk, after lithification. Even later silicification and remobilisation of silica formed discordant sheet-like bodies along oblique joints and faults.

Eustatic changes in sea level, subsidence and variable sedimentation rates influenced the deposition of the chalk in southern England, but these factors did not have a uniform regional effect. Thick successions accumulated in basinal areas and incomplete or condensed successions on adjacent highs. The sea floor, by analogy to modern equivalents, is considered to have been generally soft, as shown by adaptations of the fauna, particularly amongst bivalves. At winnowed horizons, where the hard substrate became exposed, encrusting organisms such as oysters are common.

Evidence of small-scale cyclical deposition is seen in the West Melbury Marly Chalk Formation and the lower part of the Zig Zag Formation of the Grey Chalk Subgroup. Elsewhere in the succession, a lack of suitable markers or colour variations precludes the identification of these cycles. This cyclicity is thought to be linked with the Earth’s orbital cycles (Milankovitch Cycles) (Gale, 1995). Marl seams in some parts of the Chalk Group, particularly in the New Pit Chalk Formation, appear unrelated to sedimentary rhythms and are regarded as episodic. Their origin is not clearly understood, but Wray and Gale (1993) thought they represented an increased supply of detrital material at times of falling sea level. However, some at least represent volcanic ash accumulations, analogous to the tuffs identified in equivalent parts of the Chalk of northern Germany (Wray, 1999).

There has been much argument over the depth of the ‘Chalk Sea’, with early estimates ranging from 250 m (Cayeux, 1897) to 1280 m (Jukes-Browne and Hill, 1904). Recent estimates based on hexactinellid sponge assemblages (Reid, 1973) suggest a range of 200 to 600 m: modern sponges are most abundant and faunally diverse between these depths. Shallow-water horizons such as the limestones in the cyclically bedded West Melbury Marly Chalk and the hardgrounds of the Lewes Nodular Chalk indicate that their deposition took place in the photic mobile zone, perhaps in water depths of as little as 50 m. Kennedy and Garrison (1975) suggested a depth range of between 50 and 300 m. It can be concluded that typical white chalk is neither a deep oceanic ooze nor a deposit of shallow-water origin.

In general, relative sea level rose throughout the Late Cretaceous, until a marked fall occurred in the Maastrichtian (Hancock and Kaufman, 1979). Short-term, possibly isochronous reversals to this progression produced regionally identifiable changes in deposition. In southern Britain, the maximum transgression in Campanian times probably left only the highest parts of the Welsh Massif above sea level.

West Melbury Marly Chalk

The West Melbury Marly Chalk Formation, comprising the lower part of the Grey Chalk Subgroup, crops out in the north of the area, at the base of the main escarpment. Except in the western third of the area, this northern outcrop forms a subsidiary escarpment up to 50 m high. The dip slope is more than 1.5 km long in places. The West Melbury Chalk also crops out in the south-west of the area, where it forms an essentially similar subsidiary escarpment, although on a much smaller scale.

The West Melbury Chalk includes a basal member, the Glauconitic Marl. Certain beds, particularly the main limestone beds, have been given informal names (Sumbler, 1996; Mortimore et al., 2001) or are designated by alphanumeric codes (Gale, 1989). The Chilton Stone, a limestone bed near the top of the formation, was named after a locality in the north-east of the area, but this name has only local significance.

The Glauconitic Marl Member is composed of locally fossiliferous, pale brownish grey, clay-rich chalk (marl), with conspicuous sand-grade glauconite grains. Quartz sand, if present, is sparse. The member is between about 0.3 m and 2 m thick. It is very likely that some interpretations of borehole records have grouped the glauconite sand at the top of the Upper Greensand with the Glauconitic Marl. For example, descriptions of cored boreholes from the site of the Rutherford Appleton Laboratory, Harwell [SU 468 864], by Gallois and Worssam (1983), ascribed some 5.2 m of sequence to the Glauconitic Marl. As this is so much thicker than the interval of Glauconitic Marl found elsewhere in the district, and as the glauconite-rich upper part of the Upper Greensand was not recognised as such in the log for this borehole, it seems very likely that most of that interval would now be placed in the Upper Greensand (see also notes for the West Melbury Chalk under SU48NE, below). The Glauconitic Marl has not been identified in the southern outcrop but is probably present nonetheless.

The Glauconitic Marl can be distinguished from the upper part of the Upper Greensand of the northern outcrop by differences in body colour, in the proportion of quartz sand and in the amount of glauconite. A burrowed erosion surface would be expected to occur between the two units, and there are some indications in the older accounts that it is present but not recognised as such. Augering near the base of the Chalk in the northern outcrop encounters an impenetrable bed at or close to the base of the Chalk.

Micropalaeontological examination of samples of the Glauconitic Marl collected from shallow, purpose-dug pits near East Hendred found faunal assemblages indicating foraminiferal zone BGS1 (Table 3) in the Lower Cenomanian (Wilkinson, 2002a).

The rest of the formation comprises numerous rhythmic alternations, each consisting of soft off-white to grey clay-rich chalks (marls) passing up into grey clayey chalks and hard grey or brownish grey limestones. Glauconite grains occur in the lowest few metres of clayey chalk. Some of the limestones are nodular, some are fossiliferous, and some are sparsely glauconitic. The West Melbury Chalk includes a locally abundant, moderately diverse marine invertebrate fossil fauna dominated by ammonites, bivalves, brachiopods, and (in some limestone beds) sponges. The foraminifera low in the West Melbury Marly Chalk are typically dominated by agglutinated benthonic taxa that extend up from the Albian, but include species such as Plectina mariae and Hagenowina anglica that evolved at the beginning of the Cenomanian.

The equivalent sequences at Chinnor in Oxfordshire were described by Sumbler and Woods (1992) and at Folkestone and elsewhere by Gale (1989) and Mortimore et al. (2001). The West Melbury Marly Chalk has generally clayey soils, commonly with dense spreads of limestone debris in places. Springs occur at some of the limestones.

Good exposures of about six limestone-marl rhythms are exposed in a disused railway cutting east of Chilton [SU 5007 8628]. One of these beds was named the ‘Chilton Stone’ by BGS geologists who surveyed the area of the Abingdon Sheet (Gallois and Worssam, 1983, p. 8). This appears to form a prominent positive feature (commonly associated with numerous fragments of hard limestone in the soil) where it occurs in unexposed ground to the west. These occurrences seem to have been mistakenly interpreted as the outcrop of the Totternhoe Stone on the published 1:50 000 Sheet 253 (Abingdon). As noted in the next section, the horizon of the Totternhoe Stone is inferred to occur about 2 m higher in the stratigraphical sequence.

The northern outcrop of the West Melbury Chalk can underlie gently sloping ground at the foot of the escarpment but it mostly forms a subsidiary escarpment, in which the limestones tend to form minor positive features. Note that such positive features might be formed by different limestone beds in different sectors of the escarpment. Some of these limestones occur as outliers, creating a belt of low hills extending north-east along strike from Chain Hill (Wantage) [SU 40 87], through Roundabout Hill (Ardington) [SU 43 87] and Park Hill (East Hendred) [SU 45 88] to the Sinodun Hills [SU 56 92] and Brightwell Barrow [SU 57 91], some 4 km north-east of the area.

The base of the Chalk is typically marked by a weak negative topographical feature. This lies a short distance above the positive feature formed by the glauconite sand at the top of the Upper Greensand, or, where that sand is absent, at the lower end of the Upper Greensand dip slope.

Outcrop patterns and borehole evidence suggest that the West Melbury Chalk varies between 20 m and 50 m in thickness. It is overlain conformably or disconformably by the Zig Zag Chalk.

Details

SU36SW

The Lower Chalk (equivalent to the West Melbury Chalk together with the Zig Zag Chalk, but without the Plenus Marls) was estimated to be at most 46 m thick near Ham (White, 1907).

At the foot of the escarpment, on the west bank of the stream west of Shalbourne, at the back of the farm buildings attached to Shalbourne Mill [SU 3159 6367], F J Bennett found some very hard, blue-hearted siliceous chalk at the bottom of a bank. ‘A little to the south-west of this there is a section some 2.4 m in height in ‘marl rock’ (part of the West Melbury Chalk), with a fauna typical of the ‘varians Zone’. ‘A pit in Chalk Marl, one furlong [200 m] south-west of the church, yielded a fauna that includes S. varians’ (White, 1907, p. 12). All these sections appear to have been on the west bank of the stream passing through the Shalbourne watercress beds. No exposures were seen at the time of the survey.

A little more than one mile [1.6–1.7 km] south-west of Shalbourne church, a small pit showed grey, speckled sandy chalk ‘like Totternhoe Stone’ (White, 1907, p. 12). The only pit found that corresponds to this location appears to have been in the Upper Greensand, although possibly extending into the lowest part of the West Melbury Chalk.

Remains of S. varians were found in a small exposure of grey chalk, half a mile [800 m] south-south-west of Ham Spray House (White, 1907, p. 12). No exposure is now seen at this location, which is part of an arable field [SU 3390 6235]. It is likely that the description refers to either of two small disused pits some 700 m to 800 m south-south-east of Ham Spray House. A small specimen of S. varians was found in field brash between these two quarries [SU 3465 6242].

The Glauconitic Marl was reported by White (1907) to be ‘very attenuated’ in the Vale of Ham. This was not investigated by augering and no evidence for its presence was seen in ploughed soil.

SU38NW

According to Arkell (1947), the ‘varians Zone’ was once well exposed in a pit on the edge of Uffington Wood [SU 3064 8708], some 730 m north-east of the Uffington White Horse. Here S. varians was abundant, with Turrilites spp. and Inoceramus spp.

SU38NE

Pits beside the road leading south from Childrey [SU 3610 8716] once exposed sandy glauconitic marl with nodules and S. varians, passing up gradually into marly chalk. Exposures of the ‘varians Zone’ continued southwards to the cross-roads with the Port Way (B4507) (Arkell, 1947).

SU48NE

The disused railway cutting east of Chilton [SU 4974 8585] once exposed the Totternhoe Stone, with about 4.6 m of underlying strata (Arkell, 1947).

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 66.4 m of strata assigned to the Lower Chalk. In the next section, it is argued that a bed of hard, brownish grey, silty chalk found at a depth of about 10 m (Gallois and Worssam, 1983) is the Totternhoe Stone. The underlying interval, from about 10.3 to 61.05 m depth, comprised off-white chalks, some hard, many silty or argillaceous, and commonly with S. varians amongst other fossils.

The interval between about 58.7 and 61.05 m is described as highly calcareous siltstone, chalky in part and muddy in part, with much fossil debris and bioturbation. The interval becomes increasing glauconitic with depth, and contains common sand-grade glauconite below 60.2 m. The base is marked by an irregular bioturbated surface with the underlying unit, interpreted as an erosion surface.

The underlying interval of 5.15 m, to a depth of 66.2 m, was interpreted by Gallois and Worssam (1983) as the Glauconitic Marl. It is described as an intensely bioturbated mixture of cream-coloured chalk and mud and up to 45 per cent dark green sand-grade glauconite. However, it is very probable that the base of the Glauconitic Marl (and so of the Chalk) occurs no deeper than 61.5 m, the deepest level at which Schloenbachia and Inoceramus occur (see also the general description of the West Melbury Marly Chalk). A calcitised and phosphatised hardground, with phosphatic pebbles, between 61.05 and 61.15 m (Gallois and Worssam, 1983), seems to correspond to the hard, feature-forming bed found to mark the base of the Chalk in the field and to be impenetrable to the auger.

SU48SW

According to Jukes-Browne and Hill (1903), a good exposure of the ‘Chloritic marl’ [i.e. the Glauconitic Marl] was found in the lane leading northward from Lockinge to the main road from Wantage, which here passes over a small outlier of the Chalk Marl [SU 424 884] (part of the West Melbury Marly Chalk). This lane traverses the following beds (note that fossil names are as quoted in the original work, and that these may now be obsolete):

SU58NW

Jukes-Browne (1889) stated that, between Upton and Harwell, the ‘Glauconitic Sand’ at the top of the Upper Greensand passes up into glauconitic marl, containing phosphatised fossils, with no erosional surface between them. He also stated that the ‘first cutting’ on the railway line [now disused] south-west of Upton [SU 5089 8669] is in firm marly chalk with S. varians, amongst other fossils.

The lowest beds of the Chalk, consisting of fawn-coloured glauconitic marl and rubbly marly limestone, were exposed in a roadside ditch [SU 5082 8740] and at the springs [SU 5083 8727] to the south-west of West Hagbourne.

Exposures of as many as six limestone-marl couplets in the upper part of the West Melbury Chalk can be seen on the north-west side of a disused railway cutting up to 9 m high, close to a footbridge, between Chilton and Upton [SU 5006 8627]. Jukes-Browne (1889) observed that the lower 6 m of the section in this cutting consists of blocky and marly chalk, with many fossils. The topmost limestone bed at this locality, about 0.5 m thick, is the Chilton Stone. It is pale fawn in colour, granular and massive and variably hard, with Chondrites trace fossils in places. Rare fossil shells, Inoceramus, smooth pectenoid bivalves and cirripede plates were found in the lower half of the unit.

A section about 550 m to the south-west in the same railway cutting (now buried by landfill) [SU 4974 8585] exposed a sequence up to 10 m above the Chilton Stone, including the Totternhoe Stone, here about 1.9 m above the Chilton Stone, with an erosional base.

Some of the limestone beds seen in the railway cutting, including the Chilton Stone, can be traced to the west in field debris. Geological survey records indicate that the Chilton Stone was found in the banks of the Hollow Way [SU 5123 8644], immediately west of Upton Lodge. Soft marly chalk with Schloenbachia was exposed at the cottage [SU 5123 8652] about 75 m to the north.

Zig Zag Chalk

The Zig Zag Chalk Formation, comprising the upper part of the Grey Chalk Subgroup, crops out in the north of the area, and also in the south-west. It occurs low in the main Chalk escarpment, above the dip slope formed by the West Melbury Chalk where that landform is present.

The Zig Zag Chalk is typically composed of soft to medium hard, greyish blocky chalk with some resistant limestone beds. The marl-limestone rhythms of the West Melbury Chalk continue into the lower part of the Zig Zag Chalk, but give way upwards to more massive, uniform greyish chalks. In places near Wantage, a resistant bed of grey chalk high in the Zig Zag Chalk tends to form a distinct positive feature a few metres below the similar feature formed by the Melbourn Rock. With some exceptions, the Zig Zag Chalk tends to be more sparsely fossiliferous than the West Melbury Chalk, the most usual forms being ammonites, brachiopods, bivalves and thin-tested echinoids. In the Zig Zag Chalk, the percentage of agglutinating foraminifera species suddenly drops and planktonic foraminifera become particularly common, in some samples forming over half of the foraminifera present. This change in fauna may have been caused by an increase in water depth, an event that can be recognised throughout Britain, France and Germany. Rotalipora cushmani first appears in foraminiferal zone BGS 4 (in the acutus macrofaunal zone). Of the benthonic taxa, species of Gavelinella and Plectina are present, P. cenomana first appearing at the base of zone BGS4 (Table 3).

The character of the basal bed of the Zig Zag Chalk changes regionally. In the basinal sequences of the South Downs and North Downs, it is the Cast Bed. In the Chilterns (on the London Platform), the basal bed is the Totternhoe Stone, which rests on a surface eroded into the West Melbury Marly Chalk. It seems likely that this erosion surface passes gradually basinwards into a correlative conformity, with the Totternhoe Stone concomitantly passing into the Cast Bed, but the precise nature of the relationship between the two is not known, nor where the change from one to the other occurs (Bristow et al., 1997; Mortimore et al., 2001; Sumbler, 1996). The Totternhoe Stone is known to be present in the north-east of the present area, but has not yet been observed elsewhere. The Cast Bed is probably present in part of the area.

The Cast Bed is composed of a distinctive brown silty chalk with abundant composite moulds of gastropods and other molluscs, and small brachiopods. In the North Downs, it is calcarenitic, but seems not to include phosphatic nodules. A bed of silty chalk some 0.4 m thick, at about 64 m depth in the North Farm cored borehole [SU 3321 7971], has been tentatively identified as the Cast Bed (Woods, 2001a).

The Totternhoe Stone is composed of rather friable, brownish glauconitic calcarenite, commonly with small phosphatic nodules, some of which are visible without a hand lens. It can include derived fossils, as well as fauna characteristic of the Cast Bed, but the age of the youngest included fossils varies across the outcrop, presumably reflecting diachroneity of the unit and varying condensation of this part of the succession. The Totternhoe Stone is fairly uniform and not particularly well cemented, although it hardens on exposure to air. It has been described from sections previously exposed in the disused railway cutting east of Chilton [SU 4976 8592], where it is about 0.6 m thick (Jukes-Browne and Hill, 1903). Loose fragments have been found close to Upton during recent surveys. An interval of 0.9 m of brownish grey, bioturbated, silty chalk with small dark brown phosphate nodules and phosphatic grains was found in a cored borehole at the Rutherford Appleton Laboratory Site, Harwell [SU 468 864] (Gallois and Worssam, 1983). This was ascribed to the ‘Chilton Stone’, but the presence of phosphatic material suggests it is more likely to be the Totternhoe Stone. No records of the Totternhoe Stone are known from other parts of the area, and it is possible that this unit dies out rapidly to the west and south of Chilton and Harwell.

During previous surveys, the Totternhoe Stone was mapped as far west as Wantage, as shown on the published 1:50 000 Sheet 253 (Abingdon). However, in the unexposed ground near Harwell, it appears that the ‘Chilton Stone’, a limestone forming a positive break of slope, was mapped as the Totternhoe Stone. To the west of the Hendreds, the Totternhoe Stone was placed along a spring line, whereas in the type area in the Chilterns, the Totternhoe Stone does not give rise to springs (Aldiss, 1990). Moreover, in Sussex and Hampshire, the base of the Zig Zag Chalk normally coincides with a negative break of slope, although this can be very weak.

During recent surveys of the northern outcrop, the base of the Zig Zag Chalk (and so the inferred horizon of the Totternhoe Stone or Cast Bed) was placed at a persistent, albeit locally very faint negative break of slope, some 10 m higher in the sequence than the position of the Totternhoe Stone shown on the published map. This break of slope separates fairly steep ground underlain by the Zig Zag Chalk and more gently sloping ground which merges gradually with the West Melbury Chalk dip slope. Micropalaeontological analysis of limestone debris in soil either side of this break of slope near Wantage [SU 405 861] and [SU 410 856] is consistent with this interpretation (Wilkinson, 2002b).

Mapping the base of the Zig Zag Chalk in the southern outcrop is more uncertain, through lack of evidence. The boundary was placed at a very weak negative break of slope just above a persistent positive feature taken to correspond to a limestone bed. A mappable negative break of slope occurs higher up slope, but its relative position suggests that this marks a higher level in the sequence, perhaps at or near the top of the sequence of marl-limestone couplets.

Outcrop patterns and borehole evidence suggest that the Zig Zag Chalk could vary between 25 m and 40 m in thickness. It is overlain conformably by the Holywell Nodular Chalk.

Details

SU36SW

In most parts of the area, brash is much sparser on the outcrop of the Zig Zag Chalk than on the West Melbury Chalk. Fragments of hard grey limestone or of whitish to grey-brown chalk are seen locally. Dense spreads of smooth grey-brown chalk were found at the top (uphill end) of ploughed fields west-south-west of Rivar Farm [SU 3105 6145] to [SU 3162 6156]. No evidence was seen here that the plough blades had cut into the Holywell Chalk.

Small exposures of grey-brown, rather grainy chalk were seen in erosion scars in steep grass-covered slopes south-south-east of Ham Spray House [SU 3473 6225], a short distance below an exposure of the Holywell Chalk.

Chalk assigned to the ‘Holaster subglobosus Zone’ was once exposed in the banks of the main road up the escarpment to the north of Shalbourne (White, 1907). No exposure is now seen in this part of the lane [SU 3160 6373].

SU48NW

See details under Holywell Chalk.

SU48NE

The disused railway cutting east of Chilton [SU 4974 8585] once exposed the Totternhoe Stone. This is the most south-westerly known exposure of the Totternhoe Stone, which here consists of about 0.5 m of brownish-grey stone, full of comminuted Inoceramus debris, fossils and phosphatic nodules (see further details under SU58NW below) (Arkell, 1947; Jukes-Browne, 1889).

The highest part of the Zig Zag Chalk was once exposed in chalk pits on Hagbourne Hill [SU 496 866] (Jukes-Browne, 1889).

Stratigraphical boreholes drilled at the site of the Rutherford Laboratory, Harwell [SU 468 864], passed through a total of 66.4 m of strata assigned to the Lower Chalk. A bed, about 0.9 m thick, of hard, brownish grey, silty chalk with much bioturbation, silt-grade quartz and, in parts, some small dark brown phosphatic pebbles and phosphatic grains, was found at a depth of about 10 m. The base of this bed was obscured by core loss (Gallois and Worssam, 1983). This bed was named the ‘Chilton Stone’, but from its description and its position immediately above the highest occurrence of S. varians, it resembles the Totternhoe Stone. Gallois and Worssam (1983) noted that the ‘Chilton Stone’ had been mapped in the vicinity of the Harwell site. However, the published map shows only one marker bed within the Lower Chalk, which is identified on that map as the Totternhoe Stone. Unfortunately, although it is probable that the ‘Chilton Stone’ of the Harwell boreholes can be identified with the Totternhoe Stone, and thus as the base of the Zig Zag Chalk, the bed mapped as the Totternhoe Stone has apparently been wrongly identified, and is in fact one of the limestones within the West Melbury Chalk, very possibly the Chilton Stone as exposed near Chilton.

The chalk overlying the Totternhoe Stone in the Harwell boreholes is described as soft, creamy white, slightly argillaceous with thin harder interbeds of whiter, less argillaceous chalk, becoming silty below 9.0 m and passing down into the Totternhoe Stone.

SU58NW

A section about 700 m to the east of Chilton, in the railway cutting (now buried by landfill) [SU 4974 8585], exposed the sequence up to 10 m above the Chilton Stone, including the Totternhoe Stone, here about 1.9 m above the Chilton Stone, with an erosional base. The Totternhoe Stone at this locality is 0.48 m thick (although Jukes-Browne and Hill (1903) estimated it at 0.6 m), comprising dull brown, fine-grained granular blocky chalk containing phosphatic nodules and marcasite nodules. It is rubbly and shelly, becoming coarser and more rubbly downwards, with burrows throughout. Fossils including cirripede debris, oysters, gastropods, terebratulid brachiopods, fish teeth and spines were found. The beds above the Totternhoe Stone included four limestone beds. One of these beds, about 1.4 m above the Totternhoe Stone, contained ammonite debris.

Some of the limestone beds seen in the railway cutting, including the ammonite-bearing bed and the Chilton Stone, can be traced to the west in field debris.

Holywell Nodular Chalk

The Holywell Nodular Chalk Formation, comprising the lowest part of the White Chalk Subgroup, crops out extensively in the northern half of the area, and also forms narrow outcrops in the south-west. It typically occurs in the middle part of the Chalk escarpment. Being relatively resistant, it tends to form its own subsidiary escarpment in its northern outcrop, with a few outliers. In the west of the area, several headwater tributaries of the River Lambourn lie in broad valleys floored by the Holywell Chalk, cut into the face of the escarpment. The equivalent landforms associated with the southern outcrops are very subdued by comparison.

The Holywell Chalk is lithologically varied, comprising medium hard to very hard, nodular, white to creamy white chalk with beds and laminae of clay-rich chalk (marl), including flaser-laminated marls.

The Plenus Marls, previously considered to be the topmost beds of the Lower Chalk (Table 3), are now treated as a basal member of the Holywell Chalk. They are succeeded by the Melbourn Rock, also of member status.

The Plenus Marls Member consists of alternating beds of slightly greenish grey, clay-rich chalks and clayey limestones, resting with marked colour contrast on the eroded and burrowed surface of the Zig Zag Chalk. Jefferies (1963) demonstrated that the Plenus Marls typically comprises eight beds of distinct character, which he was able to correlate over long distances, now commonly known as ‘Jefferies’ Bed 1’ and so forth, with Bed 1 being the oldest. The Plenus Marls Member appears to be between 0.5 and 1 m thick in the Berkshire Downs. A section measured by Jefferies in a chalk pit near Lockinge [SU 4246 8549], now obscured by talus and vegetation, showed the Plenus Marls as 0.82 m thick. The Plenus Marls are only about 0.3 m thick in the southern part of the Newbury district (White, 1907). No exposures were found there during the present survey.

The resistant, poorly fossiliferous, creamy white Melbourn Rock (3 to 4 m) occurs above the Plenus Marls. The upper two-thirds of the Holywell Chalk is mostly conspicuously fossiliferous; most beds contain gritty shell debris, some have mytiloid inoceramid bivalves preserved in three dimensions. Readily recognisable fragments of these rock types can be found in tilled soil on the Holywell Chalk outcrop. The straight-shelled ammonite Sciponoceras is locally abundant in the lower part, and the rhynchonellid brachiopod Orbirhynchia is locally common throughout. In the absence of shell debris, the rather grainy texture of typical Holywell Chalk distinguishes it from the smooth chalks of the succeeding New Pit Chalk.

Foraminifera are generally rare in this formation, although species of Gavelinella and Lingulogavelinella, suggesting foraminiferal zone BGS9, have been found. The planktonic species Whiteinella aprica and Dicarinella imbricata also appear in the zone.

In the north of the area, the Holywell Chalk commonly caps spurs and outliers in front of the escarpment, forming hard rubbly brash (rock fragments in the soil). Rubbly fragments of hard nodular chalk with abundant debris of fossil bivalves, and some complete, three-dimensionally preserved specimens of the bivalve Mytiloides, are widespread on the outcrop of the Holywell Chalk, in this area being found more frequently than brash of the Melbourn Rock. The Melbourn Rock tends to form a distinct positive break of slope, which in many places is associated with brash of very hard, off-white, creamy-textured, rather nodular chalk typical of the unit. The base of the Holywell Chalk is placed a short distance (less than 5 m) below the feature formed by the Melbourn Rock, a position which commonly coincides with a faint negative break of slope marking the Plenus Marls, a relative paucity of brash, and locally increased clay content in the soil. In places near Wantage, a resistant bed of grey chalk high in the Zig Zag Chalk tends to form an additional distinct positive feature a few metres below the similar feature formed by the Melbourn Rock. This can be confused with the Melbourn Rock feature.

Outcrop patterns and borehole evidence suggest that the Holywell Chalk could vary between 10 m and 40 m in thickness. It is overlain conformably by the New Pit Chalk.

Details

SU26SW

A temporary pit in a field between two old railway lines, 0.5 km east of the A346 at [SU 2256 6390] (in the Marlborough district), exposed a 7 m section of chalk at the contact between the Holywell Nodular Chalk and the Zig-Zag Chalk. The observed section exposed, from the top downwards, 2.10 m of Holywell Chalk, 9 cm of marl, 7 cm of creamy chalk, 5 cm of plastic greenish brown marl, 9 cm of harder creamy chalk, 10 cm of thick grey marl (containing belemnites), and over 4 m of massive grey chalk. Jefferies’ (1963) beds 1 to 3 are inferred to form part of the last interval, but could not be distinguished in this rather weathered, fractured section. The Chalk dips 20° due north at this site.

SU28NE and SU38SE

A quarry on Woolstone Hill [SU 2932 8656], now a car park, exposes resistant, poorly fossiliferous, creamy white chalk of the Melbourn Rock. The base of the Melbourn Rock was not observed. In the past, this quarry also exposed fossiliferous strata of the ‘labiatus Zone’ (Arkell, 1947).

Other small but disused pits were recorded just south of Letcombe Bassett [SU 3738 8470], beside the Ridgeway west-south-west of Uffington Castle [SU 2901 8576] and south of Kingston Lisle [SU 3225 8647]. These were mostly overgrown, with small exposures of shelly to very shelly nodular chalk with a gritty texture.

SU36SW

The Middle Chalk (approximately equivalent to the Holywell Chalk together with the New Pit Chalk) was estimated to be about 36.6 m thick at Ham Hill (White 1907).

Dense spreads of nodular shelly chalk were found at the top (uphill end) of ploughed fields east of Rivar Farm [SU 3192 6168] to [SU 3210 6176]; [SU 3273 6178] to [SU 3330 6179]. A small exposure of this rubbly chalk was seen in an erosion scar in steep grass-covered slopes south-south-east of Ham Spray House [SU 3473 6222], providing good local control for the base of the Holywell Chalk against nearby exposures of the Zig Zag Chalk.

Rather lumpy chalk with marl seams and a sparse fauna, referable to the middle part of the ‘cuvieri zone’, was found to a depth of 12 m in a pit at the foot of Ham Hill (White, 1907). The large pit that now exists by the road below Ham Hill [SU 3335 6175] is entirely in typical New Pit Chalk, although it is possible that Holywell Chalk was once exposed at the lowest levels. According to White (1907), Melbourn Rock was seen in the road banks ‘a little to the north’, but no exposures are seen at present.

SU38NW

The quarry on Dragon Hill [SU 3007 8687] described by Jukes-Browne and Hill (1903) has now been completely in-filled.

A quarry near the top of Blowingstone Hill [SU 322 865] once exposed about 3 m of the Holywell Chalk, being worked for the Melbourn Rock at the base of the section. The Plenus Marls have been seen at the entrance of the pit (Arkell, 1947; Jukes-Browne and Hill, 1903).

Arkell (1947) recorded a pit at about 170 m OD, south-west of Lodge Farm, Childrey, that once exposed about 4.6 m of chalk packed with ‘I. labiatus’.

SU48NW

According to Jukes-Browne and Hill (1903), the following section was exposed in 1887 in the Chalkhill Barn quarry, 1.95 km south of the church at East Lockinge [SU 4246 8548]:

They stated that there was no clear line between the basal blocky chalk and the marly layers at the base of the Plenus Marls, which seemed to occur in the upper few centimetres of that chalk. The upper marl band contained Ostrea vesicularis and a broken Actinocamax.

This section was also described by Jefferies (1963, p. 7). As illustrated in that paper, the section is estimated to be 0.82 m thick. He noted at Chalkhill Barn that the erosion surface that generally occurs at the base of his Bed 4 was very clear, although Bed 4 itself was absent. Beds 5 and 7 were not much less clay-rich than Beds 6 and 8. Contrary to Jukes-Browne and Hill’s (1903, p. 171) claim that the sub-plenus erosion surface was not present at this locality, Jefferies stated that it was, albeit obscured by bioturbation and by the relatively low clay content of Bed 1. In this section, many of the chalk intraclasts occurring at this erosion surface are phosphatised. Jefferies (1963) also provided a short list of fossils. Although the chalk pit still existed in 2003, the bedrock exposure was entirely obscured by talus and vegetation.

SU48NE

The base of the Melbourn Rock, and some underlying strata, were once exposed in chalk pits on Hagbourne Hill [SU 496 866]. The Plenus Marls were reported to be unusually thin at this locality (no more than 0.45 m), with the ‘lower marl band hardly separable from the underlying chalk’ (Jukes-Browne, 1889).

SU48SE

A disused railway cutting south-east of Chilton [SU 4982 8449] once exposed the basal part of the ‘labiatus Zone’, with the Melbourn Rock passing up into hard lumpy chalk, which is full of fragments of ‘Inoceramus mytiloides’ (Arkell, 1947; Jukes-Browne, 1889). Within this shelly chalk, a band of hard, yellowish chalk, in and below which ‘Echinoconus subrotundus’ was found (Jukes-Browne, 1889), crops out just north of a level crossing [SU 4991 8429].

SU58NW

A good section in the basal part of the Holywell Chalk was once exposed in a pit south of Upton [SU 5115 8598] (Jukes-Browne, 1889), although little chalk can now be seen there. A fault with 9 m downthrow was once exposed in this quarry.

Geological survey field records show that the basal beds of the Holywell Chalk, including the Plenus Marls, were once exposed in a small track-side pit just north of Alden Farm [SU 5049 8511] and in the vicinity of Hogtrough Bottom [SU 5490 8424]. The section near Hogtrough Bottom also exposed the Melbourn Rock.

New Pit Chalk

The New Pit Chalk Formation, in the lower part of the White Chalk Subgroup, crops out in the north and west of the area, typically forming steep ground near the top of the Chalk escarpment. It also occurs in narrow outcrops in the south-west of the area, likewise forming steep ground.

The New Pit Chalk typically comprises rather massive, blocky, soft to medium hard, smooth white chalk with regular thin beds of clay-rich chalk (‘marl seams’) and sparse small to medium-sized flints. Hard, rather gritty calcarenitic chalk occurs locally at the top of the formation, for example near Warren Down [SU 3597 7981], perhaps reflecting local condensation of the sequence or intraformational reworking. The New Pit Chalk is much less fossiliferous than the Holywell Chalk, and brachiopods tend to be more conspicuous than inoceramids in the lower and middle parts of the formation. In the lower part, the inoceramid fauna occurs mainly as flattened moulds, lacking preserved shell. Low diversity foraminiferal faunas are also present, dominated by planktonic species. Low diversity benthonic assemblages continue up into the New Pit Chalk although Globorotalites michelinianus appears for the first time in foraminiferal zone BGS10. Planktonic species are more diverse and include, for example, Marginotruncana marginata, M. pseudolinneana and Dicarinella imbricata in BGS11.

The New Pit Chalk underlies steep slopes in the upper part of the Chalk escarpment. The base is marked by a distinct negative break of slope above the Holywell Chalk outcrop. Fragments of rock (brash) from the New Pit Chalk found in the soil tend to be very uniform, smooth, brittle white chalk of medium hardness, with little fossil debris, and so are fairly easy to distinguish from the rougher, more grainy and rubbly brash characteristic of the Holywell Chalk. The New Pit Chalk is commonly quarried on a small scale.

The New Pit Chalk shows considerably more local variation in thickness than the units above and below. Outcrop patterns and borehole evidence suggest that the New Pit Chalk could vary between 20 m and 50 m in thickness. It is relatively thin (less than 30 m) in the west of the area, and relatively thick (more than 40 m) in the south, on the northern edge of the Wessex Basin, and locally in the north-east. Interpretation of borehole records suggests that this thickness variation is in part a consequence of intraformational erosion (Woods, 2001b; Woods and Aldiss, 2004). The New Pit Chalk is overlain conformably by the Lewes Chalk.

Details

SU26SE

Scrappy exposures of the New Pit Chalk occur in a pit near Wilton Hill [SU 2703 6172]. These are described by White (1907), as are the former sections along the old tramway from the clay pits on Dodsdown to Wilton.

SU36SW

A chalk pit by the road on Ham Hill [SU 3335 6176] exposes some 10–20 m of strata. Much of the face is inaccessible but the entire sequence appears to be in uniform, smooth, white chalk with thin beds of grey clay-rich chalk (marl) at intervals. These indicate a dip of about 8º towards N250º, although this appears not to be a true reflection of the local dip, possibly because of the proximity of this pit to an inferred fault.

A chalk pit by the road on Ashley Down [SU 3170 6150] exposes about 10–15 m of strata. Much of the face is inaccessible but the entire sequence appears to be in uniform, smooth, white chalk with thin beds of grey clay-rich chalk (marl) at intervals. These beds are extensively fractured and displaced by small faults, so that no dip could be safely inferred.

A small excavation just inside a field beside the main A338 road west of Shalbourne [SU 3088 6317] exposed about 30 cm of smooth, blocky, white chalk, by inference very close to the top of the New Pit Chalk.

SU38NW and SU38SE

Three pits seen within the New Pit Chalk, just south of Letcombe Bassett [SU 3738 8456], near Hillbarn Clump [SU 3256 8610] and beside Hackpen Hill [SU 3465 8527], are mostly overgrown, but do show some exposures of medium hard to soft, white blocky chalk.

The Uffington White Horse [SU 3012 8660], thought to be the oldest hill figure in Britain, is cut within the outcrop of the New Pit Chalk and basal Lewes Nodular Chalk. The image is a stylised representation of a horse, some 115 m long, and is thought to date back to 1000 BC in the late Bronze Age.

SU58SW

Beds of the New Pit Chalk crop out on Churn Hill [SU 523 845], where two flint beds can be traced locally.

Lewes Nodular Chalk and the Chalk Rock

The Lewes Nodular Chalk Formation, in the middle part of the White Chalk Subgroup, crops out in the north, west and south-west of the area, typically forming steep ground at the top of the Chalk escarpment. It is relatively resistant to erosion, forming the floor of the main valleys in the upper part of the Pang catchment, and of parts of some tributaries of the Lambourn.

The base of the Lewes Chalk is defined by the appearance of hard, nodular, gritty chalk above the smooth white chalks of the New Pit Chalk (Bristow et al., 1997). This lithological change is regionally diachronous relative to both the biostratigraphy and regionally developed marker beds such as the main marl seams (Mortimore et al., 2001). In this area it occurs between 2 m and 10 m below the Chalk Rock, a member previously taken as the basal unit of the Upper Chalk (Bromley and Gale, 1982; Jukes-Browne and Hill, 1904).

The Lewes Chalk is typically composed of hard to very hard, white to creamy or yellowish white nodular chalks and chalkstones, with interbedded soft to hard gritty white chalks and common marl seams. Regular bands of nodular flint occur more commonly than in the underlying beds. An abundant and diverse molluscan fossil fauna can be found in some beds, including ammonites, bivalves and gastropods. Echinoids and brachiopods occur throughout and are also biostratigraphically important. Benthonic foraminiferal assemblages of low diversity are present in the Lewes Chalk, although the diversity increases up the succession and Verneuilinoides muensteri and Gavelinella pertusa appear in the highest part (in BGS13). Conversely, the long-ranging Gavelinella tourainensis, originating in the Cenomanian, is not recorded above this unit. Planktonic species present include M. coronata, first seen at the base of foraminiferal zone BGS12.

The Chalk Rock is a complex sequence of hardgrounds mineralised by glauconite or calcium phosphate, each overlying a bed of chalkstone (highly indurated chalk) passing down into nodular chalk. It represents a considerable condensation, compared with basinal successions in Sussex, for example. Fragments of mineralised hardground can commonly be found in the soil on the lower part of the Lewes Chalk outcrop. However, the thickness of the unit varies significantly across the area, as does the number of hardgrounds within it (Bromley and Gale, 1982). Where this was not recognised during previous surveys, it is likely that the top and the base of the Chalk Rock have not been mapped consistently. Where the existing mapping of the Chalk Rock could be corroborated, or seemed reasonably accurate, it has been included in the revised geological maps of the area, in addition to the newly mapped boundary at the base of the Lewes Chalk. In the west and south of the area, the Chalk Rock outcrop has been omitted.

The Chalk Rock has been described in detail at two localities within the Newbury and Abingdon districts (Fognam Farm Quarry [SU 2978 7999] and Hackpen Hill, Sparsholt [SU 349 845]) and at four other localities within the vicinity (Ogbourne St George [SU 209 739], Ogbourne Maizey [SU 180 716], Burghclere [SU 4761 5905] and Harts Lock Wood [SU 621 788]) (Bromley and Gale, 1982). There is also a less detailed and less reliable description of a section near Shalbourne [SU 3157 6389] (White, 1907). At the type section at Ogbourne Maizey, and at Fognam Farm, the Chalk Rock is some 4.5 m thick, with five named hardgrounds and several subsidiary ones, grouped in three suites (Bromley and Gale, 1982; Mortimore et al., 2001). The lower hardgrounds fade out to the east. The Chalk Rock is 3.5 m in thickness at Banterwick Barn [SU 5134 7750], in the east of the area, where three of the named hardgrounds are present (see details below under SU57NW for the Lewes Chalk) (Woods and Aldiss, 2004). At Harts Lock Wood, just east of the River Thames, it is only 1.3 m thick and only two of the named hardgrounds are seen (Bromley and Gale, 1982). 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 Lewes Chalk generally underlies a broad convex slope forming the top of the main Chalk escarpment. The base of the Lewes Chalk is placed at the upwards change from smooth, white blocky chalk of the New Pit Chalk, to harder, gritty, nodular, off-white, more rubbly chalk, associated with nodular flints, which forms a more voluminous brash in the soil. Fragments of chalkstone with glauconitic or phosphatic mineralisation, or glauconite grains, are found locally, marking the position of the Chalk Rock, which is close to the base of the Lewes Chalk. This lithological change typically occurs at a positive break of slope, or locally just below it, marking the boundary between the steep, uniform slope of the New Pit Chalk outcrop to the smoothly convex slope of the Lewes Chalk outcrop.

Outcrop patterns and borehole evidence suggest that the Lewes Chalk could vary between 10 m and 30 m in thickness. It is overlain conformably by the Seaford Chalk.

Details

SU27NE

A section through the Chalk Rock in Fognam Quarry [SU 297 799], including the type example of the Fognam Marl, was described by Bromley and Gale (1982) and by Mortimore et al. (2001). It is illustrated in (Plate 1).

SU36SW

A pit at the crossroads north of Shalbourne [SU 3157 6389] once exposed low plana Zone and high lata Zone chalk (White, 1907, pp. 20–21). This pit is still open but no chalk is now exposed, the walls being obscured by soil, vegetation and piled road stone. Hard nodular chalk does appear nearby in small exposures on the opposite side of the minor road to Shalbourne. This pit section was not described by Bromley and Gale (1982). It represents the lowest part of the Lewes Chalk. White (1907) described the section as follows. Fossil names are as given in the original: note that their taxonomic classification is likely to have changed, and that modern determinations of the same specimens would very probably differ. For example, occurrences of ‘Inoceramus cf. mytiloides’ in Unit 2 are likely to refer to I. cuvieri, whereas those in Unit 6 are likely to be of other, late Turonian species. Occurrences of ‘Micraster praecursor’ might refer to M. normanniae instead. The examples of ‘Terebratula’ probably include some of Gibbithyris.

Units 1 to 3 were assigned to the lata Zone, and the remainder to the planus [plana] Zone.

By comparison with the section in the Chalk Rock exposed at Fognam Farm, Unit 5 encompasses the Top Suite and Middle Suite of Chalk Rock hardgrounds of Bromley and Gale (1982). The ‘several layers of green coated nodules’ noted by White (1907) presumably include the Fognam Farm Hardground, the Leigh Hill Hardground and (at the top) the Hitch Wood Hardground. At 3.7 m, this interval would be significantly thicker than the same beds at Fognam Farm (about 2.5 m), but comparable with the thicker sequences found at Ogbourne St George, Andover and Burghclere, amongst the next closest localities to Shalbourne where the Chalk Rock has been described in detail (Bromley and Gale, 1982).

Unit 3 is taken to be the Fognam Marl (Bromley and Gale, 1982). However, at Fognam Farm the interval between the top of the Hitch Wood Hardground and the Fognam Marl is only some 3.1 m (Mortimore et al., 2001) or less (Bromley and Gale, 1982). The corresponding interval represented at Shalbourne by Units 4 and 5 is apparently 5.8 m. Assuming that the measurements recorded by White (1907), who noted that the beds dip at 22º northwards and that their thickness ‘cannot be exactly measured, as the face of the working is irregular and much obscured by talus’, are not seriously inaccurate, the interval between the Fognam Farm Hardground and the Fognam Marl is considerably expanded compared with the sections found at other localities.

Unit 2 can be interpreted as the interval including the Pewsey Hardground, which at Fognam Farm is overlain by chalk containing common brown phosphatised intraclasts. Shalbourne is to the east of the known extent of the Pewsey Hardground itself (Bromley and Gale, 1982) and White’s description seems consistent with this, as the thickness of this unit is somewhat greater (2.1 m) than the equivalent elsewhere, for example at Fognam Farm where it is 1.5 m, but consistent with the expansion inferred for Unit 4.

The mineralised bed at the top of Unit 1 can be correlated with the Ogbourne Hardground, which is characteristically strongly glauconitised with the underlying chalkstone being tinted a rusty orange (Bromley and Gale, 1982).

Thus the total thickness of the Chalk Rock from the Hitch Wood Hardground to the Ogbourne Hardground is about 8 m, perhaps as much as 8.3 m if a possible interval of hard cemented chalk immediately beneath the Ogbourne Hardground is included.

According to White (1907), the Chalk Rock was also seen at intervals along the ridge north of Ham, and in the sides of sunken roads and cart tracks on the higher part of the main escarpment between Ham Hill and Walbury Camp. During the recent survey, small rubbly exposures of nodular chalk and chalkstone, including glauconitised intraclasts, representing the Chalk Rock, were seen on the west side of the road, some 1.4 km south of the church at Ham [SU 3317 6154]. It was previously recorded at an equivalent position south of Shalbourne [SU 3148 6133].

A small exposure of hard, nodular chalk at the base of the roadside bank at the entrance to Newton Dairy [SU 3101 6340] is representative of the Lewes Chalk.

Numerous fossils were found loose in association with fragments of hard nodular chalk in soil just inside the field beside the A338 west of Shalbourne [SU 3118 6358]. These include the ammonite Subprionocyclus, characteristic of the lower part of the Lewes Nodular Chalk, and a fragment of a gastropod (also common in the Lewes Chalk, and in particular the Chalk Rock). Inoceramus cuvieri is also present, plus a strangely geniculate inoceramid, apparently an undescribed species that occurs in the plana Zone (M A Woods, written communication, 2003). This fauna is consistent with the mapped position near the base of the Lewes Chalk.

SU37NW

The Chalk Rock forms a bench-like feature just above the valley floor, south-west of Lambourn [SU 317 780].

SU37NE

A small cutting about 200 m south of Grange Farm [SU 3594 7981] exposed gritty, hard, blocky chalk. Micropalaeontological analysis indicates that this falls within the lata Zone, but the lithology indicates a development of Lewes Chalk facies some metres below the mapped outcrop of the Chalk Rock.

A small chalk pit on the eastern side of a dry valley at Cockcrow Bottom, 575 m north-east of Warren Farm, [SU 3619 8223], exposed similar gritty chalk, some metres below the Chalk Rock.

Micropalaeontological analysis of a chalk sample taken from field brash 400 m west of Eastbury Grange [SU 3545 7990] indicates that the fauna is no younger than the cortestudinarium Zone and no older than the upper plana Zone (Wilkinson, 2001a).

SU38NW, SU38SW and SU38SE

Numerous small chalk pits expose the Lewes Nodular Chalk, commonly at or about the level of the Chalk Rock, for example at [SU 3551 8452], [SU 3738 8440], [SU 3419 8555], [SU 3463 8425], [SU 3610 8115], [SU 3620 8225] and [SU 3015 8595]. The Chalk Rock tends to form flat-topped hills in this area, for example at Woolstone Down [SU 304 845], Sparsholt Down [SU 332 844] and elsewhere ([SU 310 834], [SU 312 827], [SU 321 824], [SU 302 814]).

SU38SE

A small chalk pit on the east side of a dry valley at Nutwood Down [SU 3620 8225] exposes up to about 2 m of rather slabby, gritty chalk a few metres below the Chalk Rock. Fragments of mineralised hardground occur in head at the top of the face. Bivalves (I. cuvieri) are abundant in some beds.

SU48NW

The Lewes Chalk (above the Chalk Rock) can be seen in rather scrappy exposures up to 2 m high and 10 m wide in a disused (and partially backfilled) quarry, about 3 km south-east of Lockinge [SU 444 852].

SU48NE

Quarries on Knob Down [SU 452 855], north-west of Cuckhamsley Hill, exposed the Chalk Rock, including a richly fossiliferous bed about 6–10 cm thick. Material from this pit was used in an earlier study of the Chalk Rock fauna (Davey and Hudleston, 1877; Woods, 1896, 1897).

SU57NW

There are rather scrappy, discontinuous exposures of about 2 m of hard nodular gritty chalks exposed in an old railway cutting just north of what was Compton Station. The site is now occupied by a small business park. As mapped, this section occurs some 1 to 3 m above the Chalk Rock and 15 to 17 m below the Seaford Chalk. An excavation at the base of the cutting [SU 5238 7992] yielded a specimen of M. cortestudinarium.

Several mineralised hardgrounds and a thick, mid grey marl seam are clearly seen on Makivision optical videologs of the Banterwick Barn boreholes No. 2 (BB2) [SU 5134 7750] and No. 3 (BB3) [SU 51185 77775], about 250 m apart (Plate 1). These can be correlated with regionally developed beds in the Chalk Rock by reference to information in Bromley and Gale (1982), as discussed below, and Woods and Aldiss (2004).

Banterwick Barn is near Compton in the eastern part of the Berkshire Downs. The closest sections described by Bromley and Gale (1982) are those at Sparsholt (17.9 km to the west-north-west), Fognam Farm (21.9 km to the west), Harts Lock Wood (10.9 km to the east), Ewelme (18.4 km north-east) and Henley (25.3 km east-north-east).

Correlation between beds seen in the Banterwick optical logs and in the sections described by Bromley and Gale (1982) can be made on the basis of relative position and physical appearance. It is convenient to describe the beds in sequence from the top.

The first (topmost) hardground seen at Banterwick (40.3 m depth in BB2) is highly convolute, with glauconitic and phosphatic mineralisation extending through an interval of about 94 mm. Pebbles of glauconitised chalk rest on the hardground. There is an irregular karstic void between about 20 and 100 mm in height immediately above the mineralised hardground surface. The chalk between this surface and all the hardgrounds below contains a network of broad (20–40 mm) thallassinoid burrows. Although there is no evidence of the distinctive nodular flint bed that typically overlies this surface, this hardground is correlated by position and appearance with the Hitch Wood Hardground, the topmost hardground recorded at Sparsholt, Fognam Farm, Harts Lock Wood, Ewelme and Henley.

The second hardground at Banterwick (40.8 m depth in BB2) is even more deeply convolute, with phosphatic mineralisation extending through about 120 mm depth. Glauconitic staining is less than in the topmost hardground, here being confined to the tops of the mineralised bosses.

Bromley and Gale (1982) noted that the second hardground from the top of the Chalk Rock is everywhere strongly developed in the sections that they described. However, they were uncertain about the correlation of the hardgrounds at this stratigraphic level either side of the 34 km gap between Fognam Farm and Harts Lock Wood, and so treated them as different beds. To the south-west of the gap, they named this second hardground the Leigh Hill Hardground, while to the north-east they named it the Blounts Farm Hardground (Bromley and Gale, 1982, fig. 8). Banterwick falls within this gap between Bromley and Gale’s sections, being rather closer to Harts Lock Wood than to Fognam Farm. The second hardground at Banterwick is thus a candidate for correlation with either the Leigh Hill or the Blounts Farm hardgrounds, or possibly with both.

The morphology and mineralisation style of the Leigh Hill Hardground is rather variable (Bromley and Gale, 1982, p. 228), but the style of the second Banterwick hardground seems similar to that hardground as depicted at Sparsholt (Bromley and Gale, 1982, fig. 12). Also, the position of the second Banterwick hardground relative to the hardgrounds above and below is similar to that at Sparsholt and Fognam Farm, although the intervals are smaller at Banterwick.

At Henley, the Blounts Farm Hardground has a seemingly distinctive double-mineralised surface (Bromley and Gale, 1982, p. 288 and fig. 14), whereas nearby at Ewelme it has a single surface. At Harts Lock Wood, however, it has faded to ‘an inconspicuous nodular chalk’ and Bromley and Gale (1982) considered that ‘it is therefore doubtful if it may be correlated further west with the Leigh Hill Hardground’. Nevertheless, the relative position of the Blounts Farm Hardground at Ewelme is similar to that of the second Banterwick Hardground, suggesting that they can be correlated. This in turn suggests a correlation between the Blounts Farm Hardground and the Leigh Hill Hardground. However, there remain grounds for caution, as noted below.

A pair of moderately phosphatised convolute hardgrounds occurs a short distance below the Leigh Hill Hardground (at depths of 41.1 and 41.2 m in BB2). In BB3 these are underlain by a faint but distinct convolute hardground, the fifth from the top. This is barely visible in the videolog for BB2, but probably occurs at 41.5 m depth. Bromley and Gale (1982) noted the presence of two or three hardgrounds below the Leigh Hill Hardground (between that hardground and the Fognam Farm Hardground), which are similar to each other but generally weak, and which show much local variation. They are most fully developed at Ewelme and Henley. Bromley and Gale (1982) did not name these hardgrounds. For the purposes of this description they are herein referred to as Fognam Farm 2, 3 and 4, counting upwards. That is, the fifth hardground from the top of the Chalk Rock at Banterwick Barn is here referred to Fognam Farm 2.

Below Fognam Farm 2 is a strongly mineralised hardground (the sixth from the top) at 41.65 m depth in BB2. This is relatively thin (up to about 40 mm in mineralised thickness) but is overlain by a distinct bed of strongly glauconitised and phosphatised pebbles. This pebble bed is characteristic of the Fognam Farm Hardground, which occurs in an analogous position in the Fognam Farm and Sparsholt sections (Bromley and Gale, 1982, fig. 12).

Below the Fognam Farm Hardground at Banterwick, there is a discontinuous bed of mid grey marl, up to about 64 mm in maximum thickness, within nodular chalk (at a depth of about 42.05 m in BB2). The position of this marl relative to the Fognam Farm Hardground at Banterwick, compared with the section at Fognam Farm, suggests correlation with the Fognam Marl. However, none of the distinctive lithological characteristics described by Bromley and Gale (1982, p. 285–286) can be confirmed in the videolog images. The Fognam Marl of the Berkshire Downs can be correlated with the Southerham Marl 2 of Mortimore (1986, see also below).

No other hardgrounds are seen below the Fognam Marl at Banterwick, although nodular chalk characteristic of the Lewes Chalk continues down to a marl plexus, the base of which is at about 43.8 m depth. This plexus is assumed to correlate with the two Glynde Marls of Mortimore (1986, see also below).

Comparison of the intervals between the beds of the Chalk Rock in the five sections described by Bromley and Gale (1982) and in the Banterwick Borehole sections shows some consistent variations from west to east, but also suggests that there are some localised, marked changes between Banterwick and Harts Lock Wood, close to the Goring Gap.

The interval between the Hitch Wood and Leigh Hill/Blounts Farm hardgrounds decreases fairly consistently from 100 cm at Fognam Farm to 15 cm at Henley.

The interval between the Leigh Hill/Blounts Farm and the Fognam Farm hardgrounds also decreases from west to east, but with a marked step from 91.2 to 85.1 cm, west of the Goring Gap, to 49.3 to 42.8 cm, east of the Goring Gap.

The interval between the Fognam Farm Hardground and the marl below it decreases from Fognam Farm to Banterwick, but then apparently increases markedly across the Goring Gap.

This last point suggests that the Fognam Marl (as identified by Bromley and Gale, 1982) east of the Goring Gap correlates not with the Fognam Marl of the type section (at Fognam Farm) and as identified here in the Banterwick boreholes, but instead with the marl plexus seen at 43.8 m in BB2, at the base of the Lewes Chalk (i.e. the probable correlative of the Glynde Marl complex). This would imply that the type Fognam Marl is cut out of the sequence near the Goring Gap. This alternative is supported by the correlation of the ‘Fognam marl’ at Ewelme with the Glynde Marl (Gale, 1996, p. 190, fig. 10), while the ‘type Fognam marl’ is correlated with the Southerham Marl (Gale, 1996, p. 189, C J Wood pers. comm. to M A Woods, 2000). As implied by Gale (1996, p. 190), erosion beneath the Fognam Farm Hardground has cut the Southerham Marl/type Fognam Marl out of the sequence within the Goring Gap.

Pearce et al. (2003, fig. 3) correlated the thick grey marl seam immediately below the hardgrounds of the Chalk Rock with the Glynde Marls. However, a downhole videolog of the Banterwick Barn borehole (Woods and Aldiss, 2004, fig. 6) permits the identification of three of the regionally developed Chalk Rock hardgrounds named by Bromley and Gale (1982), and the accurate measurement of the interval between each of them and the underlying marl. This shows that the marl correlates with Southerham Marl 2 of Sussex, rather than with the Glynde Marl(s) (Woods and Aldiss, 2004, fig. 5). This correlation implies that the δ¹³C excursion named the ‘Pewsey Event’ should have been preserved at Banterwick; although possibly represented only by the sample taken at 43.02 m (Pearce et al., 2003, Table 1, fig. 3).

These localised changes found in the Chalk Rock near the Goring Gap suggest that the correlation between the Leigh Hill and Blounts Farm hardgrounds inferred here should still be treated with caution.

SU58SW

Notes on the field slip for 1:10 560 scale sheet Berkshire 21SW(E) (held in BGS archives), annotated by A J Jukes-Browne in 1887, record that a pit west of Compton [SU 505 809] exposed 8 feet [c. 2.4 m] of rubbly chalk, hard and nodular in places with green-coated nodules, overlying 2 feet [c. 0.6 m] of hard, massive, cream-coloured chalk. This is part of the Chalk Rock.

The Chalk Rock was once exposed in an industrial site at Compton [SU 5203 8028], but only about 0.3 m of the Hitch Wood Hardground was seen there.

Seaford Chalk

The Seaford Chalk Formation, in the upper part of the White Chalk Subgroup, underlies most of the centre and south of the area, typically extending from the crest of the Chalk escarpments down to where it is succeeded by the Newhaven Chalk, or where the chalk dip slope is covered by Palaeogene and Quaternary deposits.

The base of the Seaford Chalk is taken at the upward limit of nodularity and grittiness of the Lewes Chalk. This is gradational and can be difficult to locate in the field and in boreholes. The Stockbridge Rock Member, which occurs near the top of the Seaford Chalk, can be mapped in some places in the south of the area.

The Seaford Chalk is composed mainly of soft to medium hard white chalk, with common seams of small to very large flint nodules, and sporadic beds of semitabular flint. The flints commonly contain shell fragments, and in some cases echinoids. Carious (spongy-textured) flints are common near the base, but can also occur in the upper part of the Lewes Chalk. Otherwise, the flints are typically black to bluish-black, and mottled grey with a thin white cortex. Thin beds of clay-rich chalk (‘marl seams’) occur in the lowest Seaford Chalk, but in general are rare. Phosphatic chalks and hardgrounds occur in the upper part of the Seaford Chalk near Boxford [SU 4308 7195] (Jarvis and Woodroof, 1981), and might be more widespread.

Many beds within the Seaford Chalk contain macrofossils, of which inoceramid bivalves and echinoids are most significant biostratigraphically. For example, the lower part of the Seaford Chalk contains abundant fragments of the thick-shelled bivalves Volviceramus and Platyceramus, whilst the middle part contains thin-shelled, pinkish Cladoceramus; the echinoid Conulus is locally common at the top of the formation (Mortimore, 1986; Bristow et al., 1997).

A more rapid upwards increase in foraminiferal abundance and diversity is found in the Seaford Chalk, compared with underlying formations, enabling subdivision of the formation into a number of zones and subzones. Biostratigraphically important first appearances include those of Stensioeina granulata granulata (in BGS14), S. exsculpta exsculpta (BGS15) and Loxostomum eleyi (BGS 16). Planktonic forms are less common than in the Turonian (the Holywell, New Pit and lower part of the Lewes Nodular chalk formations), but long ranging species of Marginotruncana, Dicarinella, Whiteinella and Heterohelix are found, together with the first species of planispiral Globigerinelloides. The concurrent range of Stensioeina granulata polonica and Lingulogavelinella arnagerensis (foraminiferal zone BGS17) and the appearance of Gavelinella stelligera and Gavelinella cristata (foraminiferal zone BGS18) are characteristic of the upper part of the formation.

In the type area of Sussex and in other parts of southern England, the base of the Seaford Chalk is defined by Shoreham Marl 2, and coincides with the base of the M. coranguinum Zone (Mortimore, 1986; Bristow et al., 1997; Mortimore et al., 2001). Lithological, geophysical and biostratigraphical evidence from the Newbury district demonstrates significant divergence from this standard succession. Core samples and logs from the Banterwick Barn Borehole No. 2 [SU 5134 7750], in the east of the area, show that the base of the Seaford Chalk there occurs some 10 m below the apparent correlative of Shoreham Marl 2, within the higher part of the underlying M. cortestudinarium Zone (Woods and Aldiss, 2004) (see also notes for Seaford Chalk under SU57NW, below). By contrast, at Stitchcombe Farm [SU 2270 6925], just to the west of the district, an exposure in a disused chalk pit exposes hard nodular chalks typical of the Lewes Chalk within the low coranguinum Zone (Woods, 2003b). This possibly indicates a greater duration of Lewes Chalk deposition, although it might just represent an isolated bed within the Seaford Chalk. Such 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, locally porcellanous, creamy-white chalk (the ‘Stockbridge Rock’) occurs in the topmost Seaford Chalk, about 5 m below the base of the Newhaven Chalk. It contains abundant sponge spicules, most commonly as moulds, together with some complete sponges. Its thickness is difficult to estimate (no exposures have been seen in the present area) but is probably about 1–3 m. The Stockbridge Rock is widely present in Hampshire and Wiltshire, but appears sporadically (Farrant, 1999, 2000). It is at about the level of Barrois’ Sponge Bed and the Clandon Hardground of the North Downs (Robinson, 1986), and may equate with the Whitway Rock of the Burghclere area, south of Newbury (Sumbler, 1996).

In the old Hungerford Memoir, White (1907, p. 22) recorded a bed of ‘very hard yellow chalk, from 3 inches to 18 inches thick [7.6 cm to 45 cm], and possessing a sharply defined upper limit’, which he thought occurred some 60 or 70 feet [18 to 21 m] below the top of the coranguinum Zone [that is, the top of the Seaford Chalk]. He considered that the various occurrences of this type of chalk probably all occurred at the same horizon, although pointing out that they might not, which (if White’s estimate of the level of this bed is correct) now seems to be the case and that some occurrences refer to a different bed of hard chalk, lower in the succession than the Stockbridge Member.

The Seaford Chalk typically forms the long ‘dip slopes’ behind the crest of the main chalk escarpment. The base of the Seaford Chalk is marked by the upwards change from rubbly, hard nodular chalks of the Lewes Chalk to smooth, white soft chalks. Individual fragments of typical Seaford Chalk are smaller and more equant; flints are generally larger and more abundant. Flint nodules, some very large, are common and cultivated fields on the Seaford Chalk typically have piles of such flints at their margins. Some include fragments of thick-shelled inoceramids. This change in lithology typically occurs at a faint (locally exceedingly faint) negative break of slope where the steeply convex slope marking the Lewes Chalk gives way to a much flatter convex slope, rising to a ridge crest, associated with the Seaford Chalk outcrop. The Stockbridge Rock is associated with a slight positive topographical feature in places.

Chalk subdivisions in the Faircross Borehole [SU 6974 6326] can be correlated with the Burnt Hill and Foudry Bridge boreholes (Figure 2a). The Seaford Chalk is thickest in the Faircross Borehole, with the unconformable Reading Beds probably resting on a slightly older Chalk horizon in the Burnt Hill Borehole (Mathers and Smith, 2000).

Outcrop patterns and borehole evidence suggest that the Seaford Chalk typically varies between 50 m and 75 m in thickness. 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 to west-south-west zone up to 7 km wide between Great Bedwyn [SU 27 64] and Chieveley [SU 47 73], it appears to be thicker than usual, generally exceeding 80 m. This zone is just to the north of the fault zone bounding the Wessex Basin, and the anomalous chalk succession exposed at Boxford is on its northern edge, suggesting a measure of tectonic control.

The Seaford Chalk is locally overlain conformably by the Newhaven Chalk, but more generally it is unconformably overlain by the Lambeth Group.

Details

SU26NW

Two good exposures of lower Seaford Chalk occur on the escarpment overlooking the Kennet valley near Axford (both in the Marlborough district). One at Brick Hill [SU 2360 6930] exposes the Belle Tout Beds with abundant Platyceramus and Volviceramus. Many good examples of large tabular flints, including one which may be the Seven Sisters Flint, sheet flints and thin marl seams occur. A superb dissolution pipe, 6–8 m deep, 4 m wide and infilled with clay-and-flint, is exposed in the south-east corner of this pit.

A second pit occurs adjacent to the road at Stitchcombe Farm [SU 2270 6925] also exposes the lower Seaford Chalk (Belle Tout Beds) with abundant fragments of Platyceramus and Volviceramus. A hard nodular chalk exposed at the base of pit, which contains Platyceramus debris, (i.e. typical of the Seaford Chalk), possibly represents the topmost part of the Lewes Chalk. Alternatively, the nodular horizon may be a sponge bed within Seaford Chalk facies (Woods, 2003b).

SU26NE

Up to 45–50 m of Seaford Chalk crops out over much of the northern half of this area, but there are very few good exposures. Fossils characteristic of the M. coranguinum Zone were noted from two sites, a small pit with large flints just south-west of Upper Horsehall Hill Farm [SU 2588 6670], and a pit near Almshouse Copse [SU 2845 6729].

The Stockbridge Rock Member has been recorded only from the valley north-west of Bedwyn Common at about 150 m OD [SU 2505 6600]. Here, brash consisting of very hard blocky yellowish chalk was noted. It is not laterally persistent and can be traced for only about 0.5 km.

SU27SE

An exposure of typical Seaford Chalk occurs at Southward Lane Farm [SU 2655 7477]. Chalk is exposed for some 50 m along a quarry face, cross-cut by several, widely spaced, steep normal faults, each showing variable (up to 5 cm) vertical displacement of a tabular flint horizon.

SU36NW

Chalk with a large number of thin-shelled globular flints in the sparsely fossiliferous part of the coranguinum Zone was once exposed in the side of the Station yard at Hungerford [SU 340 685] (White, 1907, p. 27). White (1907) also reported that chalk with many carious flints occurred in the railway cutting near the Barracks, one mile west of Hungerford. He provided a faunal list from these localities.

According to White (1907), chalk with many layers of nodular flint was worked in a large quarry at Furze Hill, Chilton Foliat (on the south side of the River Kennet) [SU 321 698]. A very irregular junction of the higher (although not the highest) beds of the coranguinum Zone with the Reading Formation was visible near the kiln (of the old Hop Grass Brick Works) in Brick Kiln Copse [SU 3205 6931], a little further to the south. No fossils of interest were found in the pinnacles of dirty chalk that rise into the Eocene sands and clays.

On the west side of the cart track leading from that kiln south to the Bath Road near Hop Grass Farm, probably 20 or 30 feet [6 to 9 m] lower in the coranguinum Zone, the chalk was worked in a vertical shaft with lateral galleries or headings [SU 3217 6914]. Fossils found include Echinocorys scutatus, ‘varying much in shape, but all referable to the ovate variety’, Spondylus spinosus, spines of Cidaris, and fairly common asteroid ossicles. A 6 inch [15 cm] band of hard yellow chalk was seen in the lower part of the shaft, about 15 feet [4.6 m] from the top (White, 1907, p. 27).

Higher beds than those seen at Brick Kiln Copse are exposed in a field pit on the eastern side of Littlecote Park Farm [SU 302 693]. The chalk is rather soft and blocky and contains small scattered flint nodules with pale violet bands. Fossils including Conulus albogalerus and Lima hoperi are fairly common, the forms noted being almost the same as those seen in the higher beds near Wickham. One side of the pit intersects a very large pipe of Eocene clay and pebbles (White, 1907).

Chalk with large, elongate flint nodules and many spines of ‘Cidaris clavigera’ is well shown at Standen Manor [SU 323 663] (White, 1907, p. 28).

SU36SW

According to White (1907, p. 27), a small quarry at the crossroads on the ridge south-west of Shalbourne [SU 3050 6271] exposed chalk with numerous flint bands, doubtfully near the base of the coranguinum Zone. The flint bands indicate a dip of 20º north-westward. Field survey suggests that this pit could equally lie in the upper part of the Lewes Chalk, but no exposures were found to corroborate this.

White (1907, p. 28) stated that some of the highest beds of the coranguinum Zone chalk, with a few common fossils, occurred on the northern slope of the same ridge, 3 furlongs [600 m] north of Shalbourne Church [SU 3153 6405]. This is taken to refer to exposures in a quarry, now disused and backfilled, close to Royal Oak Cottages.

A group of pits on and near the Ridgeway south-east of Sadler’s Farm, Inkpen [SU 353 641], yielded a small but characteristic fauna indicating the coranguinum Zone (White, 1907, p. 28).

A cutting on the south and east sides of Newton Dairy [SU 3082 6328], up to 2 m high, exposes uniform, smooth, white, soft chalk with some large flints. Fragments of thick-shelled inoceramid occur in places.

Soft white chalk collected from beside Sadler’s Road, Inkpen [SU 3489 6421], yielded a microfauna indicating the coranguinum Zone, from which the Seaford Chalk can be inferred.

SU37SW

A chalk sample from near Chilton Foliat was analysed micropalaeontologically. The contained fauna was undiagnostic but suggested a position in the middle part of the Seaford Chalk (Woods, 2003b).

SU37NE

Field brash typical of the Seaford Chalk, consisting of firm to soft fragments of white tabular chalk with many large nodular flints, occurs in many places, for example near Whatcombe Farm [SU 3973 7874]. Seaford Chalk, containing many fragments of the thick-shelled bivalves Volviceramus and Platyceramus, was seen north of the pig farm on East Garston Down [SU 3600 7980].

Several chalk pits are located in the Seaford Chalk, for example at [SU 3550 7507], [SU 3532 7840] and [SU 3928 7872].

SU37SE

A pit just north of Weston, close to Elton Farm [SU 3991 7424], exposes 1–2 m of very flinty, soft white Chalk, with Micraster coranguinum, Platyceramus and Volviceramus, indicating lower Seaford Chalk.

A pit north of Coldridge Copse [SU 3712 7430] exposes a short section in the middle to upper Seaford Chalk.

A pit just west of Stibbs Wood [SU 3671 7046], 1.5 km south-east of Hungerford Newtown, exposes 4–6 m of upper Seaford Chalk with large tabular flints and Conulus albogalerus. The pit is overlain by about 1 m of clay-with-flints with good examples of involutions and small dissolution pipes.

The Stockbridge Rock Member forms a small feature and occurs as field brash along a track just east of Orpenham Farm [SU 3920 7056] at 123 m OD. The brash consists of very hard, smooth, blocky chalk. It also crops out near Clapton [SU 3856 7017]. In this area, it is estimated to be about 2–3 m thick, but is laterally impersistent.

White (1907, p. 25) noted Seaford Chalk with a thin marl band at the top exposed in a small roadside pit [SU 3891 7451], a little north-west of ‘East Shefford Church’ (presumably Holy Innocents Church, which once stood about 100 m east of Shefford House, then the Rectory), [SU 3908 7440]. He noted the same marl band in the middle of a quarry [SU 3838 7495] in barren chalk with regular courses of tabular and nodular flint, a little west of the smithy [SU 3843 7506] of West Shefford (now Great Shefford).

White (1907, p. 26) reported that, by the side of the Roman Road, just north of the line of the M4 (400 m north-west of the site of Nicnocks) [SU 385 722], a ‘little harsh chalk with violet banded flints is seen’. This was assigned to the coranguinum Zone. He also reported that ‘hard yellowish chalk with hollow casts of sponge spicules, and Parasmilia centralis’ occurred in a hedge bank 800 m south-west of Wickham Church [SU 392 709]. This description corresponds with the Stockbridge Rock.

In the bottom of the dry valley east of Clapton and 600 m west of Elcot Lower Farm, a small pit in flinty beds yielded M. coranguinum, Conulus albogalerus and other fossils (White, 1907).

A pit in a narrow wood, about 1.5 km south of Great Shefford [SU 3848 7362], exposes up to 4 m of the upper Seaford Chalk, with bands of large flints. This appears to be a pre-existing pit that had been enlarged shortly before the survey. Another pit in the upper Seaford Chalk occurs 1.5 km south-west of Great Shefford [SU 3713 7428], exposing some 4 to 5 m of chalk with large flints. The upper Seaford Chalk is also exposed in a pit about 1.5 km south-east of Hungerford Newtown [SU 3671 7045]. This pit shows up to 8 m of chalk, with a major flint band and other large flints.

A similar pit in the lower Seaford Chalk (Belle Tout Beds), exposing white chalk with large tabular flints, occurs 250 m north-east of Elton Farm [SU 3992 7422].

SU46NE

White (1907, p. 23) noted that half a mile [800 m] east of Shaw Church, a pit just east of the (now-disused) Didcot to Newbury railway [near 4829 6829] exposed 15 feet [4.6 m] of soft white chalk with scattered grey flints. A varied fauna was obtained, including a band of Conulus albogalerus about 6 feet [1.8 m] from the bottom of the pit. The highest beds here are within a few feet (about 1 m or less) of the upper limit of the coranguinum Zone. Previously, as much as 20 feet [6.1 m] of chalk with approximately horizontal beds of nodular flint had been visible. The upper surface of the chalk was described as ‘perfectly level’, but perforated by innumerable burrows to a depth of ‘a foot [0.3 m] and more’ ‘leaving vertical and oblique pipes, about half-an-inch [0.013 m] in diameter, now filled with the dark sandy clay that overlies the Chalk; the perforations being very close at the top, and becoming wider and wider apart to the depth of 16 inches [0.4 m]’ (Jones and Whitaker, 1878).

At Donnington, an old pit on the west side of the Oxford Road (now the B4494), (presumed to be at [SU 4670 6888]), and some excavations for water and drain pipes in that road and in Shaw Lane (now Love Lane) nearby, exposed a more flinty chalk, at a slightly lower level in the coranguinum Zone, containing many of the same fossils ‘with a few unimportant additions’ (White, 1907, p. 23).

A sample of chalk from a pit by the kiln at Shaw Brickyard (a little to the south-west of and at a lower level than that of the old pits showing the junction of ‘Uintacrinus chalk’ [Newhaven Chalk] and the Reading beds [Reading Formation] yielded Scalpellum maximum? (J. de C. Sow) and Eschara acis d’Orbigny (White, 1907, p. 23).

There are two field pits, in beds with regular flint bands, north of Bagnor [SU 4544 6950] and [SU 4554 6955]. They have yielded Micraster coranguinum, Cidaris clavigera, Terebratula semiglobosa and a few other common fossils (White, 1907, p. 23).

SU47NE

A disused chalk pit just north of Stanmore [SU 4788 7900] exposes white chalk with flints, typical of the Seaford Chalk, in a face about 2.5 m high and 10 m long.

SU47SW

Treacher and White (1906, p. 391) stated that the Boxford Chalk Pit [SU 4307 7195] exposed chalk with a strong dip to the south-east, all in the upper part of the coranguinum Zone, although the socialis Zone must come in not far above the pit. There are two rock bands, the upper band containing many small oysters. The chalk is distinctly phosphatic in places.

White (1907, p. 24), described the section in the Boxford Pit and its fauna in some detail (following the description of locality ‘u’ of White and Treacher, 1906), noting that the beds dip at 23º to 25º to the south-south-east. He stated that hard chalks, like those seen in the pit, could be traced for 275 m to 365 m southward and north-eastward from the pit along the eastern side of the lane beside the pit. He stated that other phosphatic beds, lower in the coranguinum Zone, are known to exist beneath the cultivated ground on the western side of the road, about 540 m north-north-east of Boxford Church, and presumably within about 200 m north of Boxford Pit. White (1906, p. 353) and Hawkins (1924) also described Boxford Pit.

The sections exposed in the Boxford Pit were also described in detail by Jarvis and Woodroof (1981) and Mortimore et al. (2001).

White (1907, p. 24) recorded a small pit 200 m west of Wyfield Manor Farm [SU 4384 7283], about 7.6 m below the base of the Palaeogene. This showed 2.4 m of rather soft white chalk with one seam of tabular flints dipping 5º north-eastward. Fossils are very scarce, but White deduced that the beds lie close to the top of the coranguinum Zone, for Uintacrinus occurs in the chalk just beneath the base of the Lambeth Group a little to the south. Small cylindrical pipes of green sand with pieces of tabular flint traverse the section from top to bottom. Fossils listed include inoceramid debris.

White (1907, p. 25) recorded a small pit east of Westbrook Farm [SU 4272 7242], in very flinty poorly fossiliferous chalk, lower in the coranguinum Zone than near Boxford Pit. The dip is 5º or 6º to the south-south-east, probably, he considered, due to the same disturbance as at Boxford Pit. He inferred that flexure predated the Lambeth Group.

White (1907, p. 25) recorded a field pit 700 m north-west of Sole Farm, near Wickham kiln [SU 4057 7161], which showed 4.6 m of rather soft, white, closely jointed chalk, with a few layers of nodular flints, assigned to the coranguinum Zone, and capped by the green, sandy ‘bottom bed’ (Upnor Formation) of the Lambeth Group.

White (1907, p. 25) recorded a roadside pit about 400 m west-north-west of Sole Farm [SU 4068 7123] that exposed 5.5 m of similar chalk (of the highest part of the coranguinum Zone). He listed fossils, with Conulus being common in both this and the last pit, and ‘many hundreds of small crinoid and asteroid fragments were obtained’, including Bourgueticrinus.

Treacher and White (1906, p. 391) and White (1907) stated that all sections [in the Chalk] seen on the north-east border of the elongate Palaeogene outlier between Speen [SU 460 678] and Wickham [SU 717 395] are in the coranguinum Zone.

Beds lower in the coranguinum Zone were reported by White (1907) to be exposed in a copse about 1 km north-east of Wickham Church. This is probably just south of the M4 at the south end of Buck’s Copse [SU 4015 7222], but could be slightly further north in Mantclose Copse. The beds include large carious flints and many remains of Cidaris sceptrifera, C. perornata and Inoceramus.

SU47SE

A disused chalk quarry south-west of Winterbourne Holt [SU 4572 7138] exposes Seaford Chalk in a face about 2 m high and 10 m long.

SU57NW

Notes on the field slip for 1:10 560 scale sheet Berkshire 27NE(E) (held in BGS archives), annotated by F J Bennett in 1887, record that the base of the pit about 1 km north-east of Hampstead Norreys [SU 5396 7728] exposed 20 feet [c. 6 m] of chalk, with six rows of large flints. Several headings (adits) were driven in the chalk for lime. The abundance of flints is consistent with a level in the Seaford Chalk.

Pearce et al. (2003) identified a marl seam about 17 m above the Top Rock at Banterwick (that is, at about 22 m depth) as the East Cliff Marl, undoubtedly referring to the upper East Cliff Marl (or East Cliff Marl 2) of Robinson (1986), which is equivalent to Shoreham Marl 2 of Mortimore (1986). They make this correlation by comparison with the section at Dover, where the upper East Cliff Marl lies some 19 m above the Navigation Hardground, the local equivalent of the Top Rock. In southern England, the upper East Cliff Marl/Shoreham Marl 2 marks the base of the Seaford Chalk and of the coranguinum Zone. The bed at Banterwick identified by Pearce et al., (2003) as the East Cliff Marl was thus assumed by them to mark the base of the coranguinum Zone.

However, as noted by Woods and Aldiss (2004), evidence from microfossils (Wilkinson, 2000a), macrofossils and trace fossils indicates that the base of coranguinum Zone lies above about 18 m depth in the Banterwick borehole. A marl seam at 15.8 m (figs. 2 and 3 of Pearce et al. 2003) is the most likely correlative of Shoreham Marl 2/East Cliff Marl 2, and therefore the most likely position of the base of the coranguinum Zone.

Like Pearce et al. (2003) (who use the stratigraphical terminology developed for the Chalk of Kent by Robinson, 1986), Woods and Aldiss (2004) found that the appearance at Banterwick of soft, fine, white chalks (Broadstairs Member/Seaford Chalk Formation) above the hard, gritty, nodular chalks of the St Margarets Member/Lewes Chalk Formation occurs within the cortestudinarium Zone, rather than at the base of the coranguinum Zone as found in Sussex and Kent (Mortimore et al., 2001). This lithological change is typically difficult to locate with precision, as it is gradual and is generally expressed by an interval of alternating soft and hard chalks. Pearce et al. (2003) placed this lithostratigraphical boundary at a marl seam at about 33 m depth, following Murphy et al. (1997). By using both lithological and geophysical evidence, however, Woods and Aldiss (2004) placed it at a marl seam at 25.8 m depth. This level coincides with a marked inflection in downhole resistivity logs (Murphy et al., 1997, fig. 5; Woods and Aldiss, 2004, fig. 3) and is within the upper part of the interval of lithological change found in the core and seen in a downhole videolog (Woods and Aldiss, 2004). This position is also consistent with the findings of large-scale geological surveys in the area and with a marked upwards increase in strontium content (Pearce et al., 2003, fig. 2). This implies that the topmost 10 m of the cortestudinarium Zone chalk is in Seaford Chalk facies at Banterwick Barn.

SU57SW

The Hungerford memoir (White, 1907, p. 23) referred to a pit [SU 5143 7482] adjacent to the disused railway line 1 km north of Hermitage, and now partially backfilled. This site is just below the junction with the Lambeth Group and the text refers to a face White describes thus: ‘about 8 feet [2.4 m] of chalk with pipes of sand and clay. Near the middle of the section there is a yellow rock band, 6 inches [15 cm] thick, bearing a few impressions of a sponge (Coscinopora?) and containing concretions of decomposed pyrite. The surface of the rock is uneven and above it there are small nodules of a similar hard chalk. Bourgueticrinus, Cidaris sceptrifera Forbes, Echinocorys, and other common forms occur in a fragmentary condition.’ This hard chalk may represent the Whitway Rock described by Jukes-Browne (1908, p. 30) in the Whitway pit [SU 4575 5914], 10 km south of Newbury. Pipes in the surface of the chalk at this locality are discussed below under the Lambeth Group.

A small pit in the area of [SU 462 726] once exposed a hard band of yellow chalk, 5 to 7 cm thick, with concretions of iron pyrite (White, 1907). It is probably the same bed as the hard band found north of Hermitage.

White (1907) referred to a pit [SU 4584 7217] east of Winterbourne (now partially backfilled), which exposed blocky chalk with cavernous flints of large sizes and containing a variety of fossils, including ‘Inoceramus cuvieri, Cidaris sceptrifera, Echinocorys scutatus and Micraster coranguinum’. He also referred to a pit [SU 4532 7225] west of Winterbourne with softer chalk, bands of smaller nodular flints and a similar range of fossils.

Micropalaeontological analysis (Wilkinson, 2001b) was made of three chalk samples collected in the area, described in this and the following two paragraphs. A sample from the top of a disused pit [SU 5298 7155] south-west of Marlston Farm, near Hermitage indicates Seaford Chalk immediately above Whitaker’s 3 Inch Flint (or lateral equivalent) and below the Chartham Flint of Robinson (1986) (or lateral equivalent) (Robinson, 1986). This bed is located about 6 m below the contact with the Lambeth Group.

A disused pit on Newhouse Farm [SU 5089 7498], north of Oare, has a 3 m face of soft white chalk with subhorizontal bedding planes about 20 cm apart. It contains nodular flint bands and the microfauna was interpreted as early Santonian in age, in the upper part of the coranguinum Zone. From this, a position in the middle to upper Seaford Chalk can be inferred. This site is located about 5 m below the contact with the Lambeth Group in this area. ‘Pipes’ filled with orange brown sandy clay are present in the face.

A chalk sample collected from a disused pit [SU 4571 7139] south-west of Winterbourne Holt contained a microfauna of early Santonian age, from the upper part of the coranguinum Zone, which equates to the middle to upper Seaford Chalk. The current chalk face is about 4 m high and 30 m long, and is very weathered, showing subhorizontal bedding with nodular flint bands, jointing and minor faulting. The contact with the Lambeth Group is about 5 m above the sampled horizon.

Frilsham Chalk Pit [SU 5396 7292], in the Reading district just west of Frilsham and about 1.3 km east of the Newbury district, is now disused and partially backfilled. It has a number of chalk faces (each up to 2 m high and 10 m long) on its east and south sides, and the Lambeth Group is present above in the east. The chalk is a soft white limestone with bands of nodular flint. A palaeontological investigation of chalk samples suggested a position in the middle Seaford Chalk. A prominent flint band with associated Cladoceramus undulatoplicatus perhaps represents Bedwell’s Columnar Flint of the Kent coast (Robinson, 1986; Woods, 2000c). ‘Pipes’ of material derived from the Lambeth Group extend down into the Chalk. The material generally consists of orange brown fine- to medium-grained sand.

Newhaven Chalk

The Newhaven Chalk Formation, the youngest part of the White Chalk Subgroup present in Berkshire, crops out in the south of the area, within the core of the London Basin syncline, where it is largely covered by Palaeogene deposits. Four small outliers near the northern margin of the main outcrop occupy ground standing slightly above the main chalk dip slope. None occurs within the Abingdon district.

The Newhaven Chalk is typically composed of soft to medium hard, smooth white chalk with numerous thin beds of clay-rich chalk (‘marl seams’) and widely spaced, rather sparse flint bands. Phosphatic chalk and hardgrounds occur in the Newhaven Chalk near Winterbourne [SU 4477 7223] (Jarvis and Woodroof, 1981), and might be more widespread.

Many beds within the Newhaven Chalk contain macrofossils, of which crinoids and echinoids are most significant biostratigraphically. These fossils can be found in rock fragments in the soil (brash), as well as in exposed bedrock. Biostratigraphically, the Newhaven Chalk is co-extensive with the ‘crinoid zones’ (the zones of Uintacrinus socialis, Marsupites testudinarius and Uintacrinus anglicus), together with most if not all of the Offaster pilula Zone (Table 3). Note that in older accounts, the three crinoid zones are treated together as a single ‘Marsupites Zone’.

Gavelinella cristata rapidly increases in numbers to become the most dominant element of the benthonic foraminiferal community in the Newhaven Chalk. Stensioeina granulata perfecta and Bolivinoides strigillatus appear a little above the base of the formation, respectively in the ‘middle’ and upper U. socialis macrofaunal Zone. Planktonic foraminifera are of little biostratigraphical significance in this formation. However, just below the base of the Campanian, in the middle part of the Newhaven Chalk, several species occur for the first time including Rugoglobigerina pilula and Heterohelix striata. The inception of Bolivinoides culverensis and the extinction of Stensioeina exsculpta exsculpta characterise the basal part of foraminiferal Zone BGS19 (basal O. pilula macrofossil Zone).

A clay pit at Shaw [SU 4837 6805], just north of Newbury, once exposed about 2.5 m of chalk of the Uintacrinus Zone (indicative of the Newhaven Chalk Formation) beneath the basal Palaeogene (Treacher and White, 1906; White, 1907, p. 29). The extent of the crinoid zones beneath Palaeogene cover east of Newbury is not known.

The base of the Newhaven Chalk is defined by the incoming of common marl seams above the flinty, marl-free Seaford Chalk (Bristow et al., 1997). In practice this is taken at the incoming of a particular assemblage of bioclastic debris (mainly comprising indeterminate crinoid brachials and thin-shelled oysters, but including calyx plates of U. socialis). These changes in the lithological assemblage commonly coincide with a negative break of slope (although this is exceedingly slight in some areas), assumed to mark a persistent marl seam. This break of slope is not always present, especially if the boundary occurs close to the base of the Palaeogene.

A fairly conservative approach has been taken to mapping the Newhaven Chalk in places where a thin capping (up to 5 m) might be present, especially where this would be partly obscured by clay-with-flints. It is thus possible that the Newhaven Chalk is significantly more extensive than shown on the map, particularly on the northern margins of the outliers immediately to the east and west of Winterbourne.

Outcrop patterns suggest that up to about 14 m of the Newhaven Chalk are present in the outliers around Boxford and Winterbourne. More than 40 m is present in boreholes in the south of the area.

The Newhaven Chalk is unconformably overlain by the Lambeth Group. The stratigraphical level of the youngest Chalk preserved at this unconformity varies considerably across the area. The youngest chalk seen in the district belongs to the pilula Zone (Table 3), once exposed at Kintbury [SU 3874 6658], near the axis of the London Basin syncline (White, 1907, although assigned by him to the ‘A. quadratus’ Zone), 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, around Thatcham for example. The oldest Chalk of known age found at the basal Palaeogene unconformity in the area belongs to the middle of the coranguinum Zone. This occurs at Frilsham Quarry [SU 5400 7294], about 1.3 km east of the district.

In places, the Chalk just below the Palaeogene unconformity has become cemented. This ‘Bedwyn Stone’ can be seen capping the Chalk spurs between Chisbury and Great Bedwyn. White (1907, p. 37) described this an ‘an exceedingly tough, compact, white or yellowish limestone, with a conchoidal fracture. The rock contains a few small greyish flint-nodules, occasional seams of clear calcite, and some brown, crystalline-granular, branching bodies - possibly infilled borings’.

Details

SU26NW

The Newhaven Chalk probably occurs over much of the southern part of the area, but there is very little exposure or field brash due to extensive superficial cover and forestry. Up to 15 m of Newhaven Chalk are thought to be present beneath the Palaeogene, including all of the socialis Zone and part of the testudinarius Zone. No good sections occur in this area and no calyx plates of U. socialis or M. testudinarius were found during the recent survey.

SU26NE

The Newhaven Chalk occurs over much of the southern part of the area. Up to 25 m of Newhaven Chalk is present beneath the Palaeogene, including all of the socialis Zone and much of the testudinarius Zone, but it thins to less than 10 m north of Chisbury. In the north of the area the Palaeogene unconformity is on the upper Seaford Chalk (see sheet SU37SW). No good exposures were seen in this area.

Calyx plates of U. socialis and M. testudinarius have also been found at several localities in the Little Bedwyn-Chisbury area. Ossicles of U. socialis were found on the hilltop just east of Little Bedwyn [SU 2964 6608], in a pit near the Old Vicarage, Little Bedwyn [SU 2864 6636], and at the edge of Chisbury Wood [SU 2771 6551]. M. testudinarius fragments were found north-east of Chisbury Manor Farm [SU 2815 6551], on the ridge near Parlow Bottom [SU 2925 6526] and near Chisbury pumping station [SU 2849 6638].

Other sites are mentioned by Treacher and White (1906) and White (1907).

SU26SE

The Newhaven Chalk occurs over much of the northern part of the area. Up to 25 m are present beneath the Palaeogene, including all of the socialis Zone and much of the testudinarius Zone. The outcrop of Newhaven Chalk extends down to the valley floor between Crofton and Great Bedwyn, in the core of the Bedwyn Syncline. No good sections occur in this area, although several metres of very weathered Newhaven Chalk with M. testudinarius can be seen in a pit near Great Bedwyn [SU 2880 6427].

Calyx plates of U. socialis have been found at a pit by Church Lock, Great Bedwyn [SU 2790 6411], along with possible fragments of M. testudinarius. Fragments of the last-named crinoid were also found near Crofton Farm [SU 2581 6260], [SU 2550 6288], near Mill Bridge [SU 2699 6332], [SU 2725 6372], and along the margins of Wilton Brail and Bedwyn Brail, e.g. [SU 2751 6310], and in Tottenham Park [SU 2605 6470] and [SU 2537 6420].

According to White (1907, p. 36), two shallow workings on the northern edge of Burridge Heath [SU 297 652] near Little Bedwyn are close to the base of the Lambeth Group and both are in the U. socialis Zone. Other sites are mentioned by Treacher and White (1906) and White (1907).

The ‘Bedwyn Stone’ (described above) caps the Chalk spurs between Crofton and Great Bedwyn. Large quantities of this stone can also be seen in field brash along the western side of Wilton Brail [SU 2709 6272].

SU26SW

The Newhaven Chalk occurs in the northern part of the area. Up to 25 m of Newhaven Chalk is present beneath the Palaeogene, including all of the socialis Zone and much of the testudinarius Zone. No good sections occur in this area, and over much of the outcrop, the Newhaven Chalk is covered by clay-with-flints. Calyx plates of U. socialis have been recorded from several sites in Tottenham Park. M. testudinarius fragments were found near Durley [SU 2340 6393]. No exposures of the ‘Bedwyn Stone’ were observed and it may not exist in this area, although the Chalk immediately below the unconformity was not seen.

SU36NW

Shallow workings at the north-western end of Long Walk, in Stype Copse [SU 306 667] and on the western edge of that copse [SU 306 664], are close to the base of the Lambeth Group and are in the U. socialis Zone, according to White (1907, p. 36).

Chalk samples from the area north of Bagshot [SU 3144 6674] and at Anvilles [SU 3424 6561] were analysed micropalaeontologically. Both yielded faunas diagnostic of the lower part of the Newhaven Chalk, above the level of Peake’s Sponge Bed (Wilkinson, 2003c).

SU36NE

Treacher and White (1906, p. 390) reported that the ‘Uintacrinus band’ is exposed in a small pit, 12 feet [3.7 m] deep, close to the Palaeogene boundary, near the hamlet of Elcot [SU 393 697]. The chalk is soft and white, and flints are small and scarce. Both Uintacrinus and Bourgueticrinus are common fossils here. White (1907, p. 30) likewise stated that a small pit on the west side of the hamlet of Elcot, close to the base of the Lambeth Group, showed 3 to 4 m of soft beds with a few small thin rinded flints. He listed several fossils including Uintacrinus. About 3.5 m of soft white blocky chalk with scattered small flints was visible at the time of survey in 2002.

SU36SW

The presence of the Newhaven Chalk in the north of the area can be inferred from several records demonstrating the crinoid zones. No evidence for the presence of the Newhaven Chalk was found in the south of the area.

The base of the Newhaven Chalk is difficult to place with much confidence in this area. In general it has been taken at the upwards change from soft white chalk with many large flints to similar chalk with few flints. Those flints that do occur commonly include large cylindrical forms, typically about 10 cm in diameter, and irregular nodules preserving the trace fossil Zoophycos. The Zoophycos flints most typically occur in the soil close to the inferred base of the Newhaven Chalk. This change in lithology is, in some places, associated with a weak negative break of slope. In much of the area, between about Newton Dairy [SU 398 634] and the area north of Ham [SU 326 644], the base of the Newhaven Chalk was mapped entirely by reference to the mapped position of the base of the Seaford Chalk, and to historical records of the crinoid zone fauna. If more time had been available for fieldwork in this area, the position of this boundary could probably have been mapped more accurately by searching the locally abundant brash for fossil crinoid debris.

White (1907, p. 36) noted that Marsupites was found in almost flintless chalk at the base of the Lambeth Group by Slope End, on the eastern side of the road between Hungerford and Shalbourne, inferred to be near [SU 3275 6495]. He also noted a small excavation in iron-stained chalk, yielding plates of Marsupites, by the mill north-west of Slope End [SU36NW]. This is also close to the base of the Lambeth Group.

By contrast, shallow workings at the north-western end of Long Walk, in Stype Copse [SU 306 667], another on the western edge of that copse [SU 306 664], and two more on the northern edge of Burridge Heath [SU 297 652] near Little Bedwyn, are all close to the base of the Lambeth Group and are all in the U. socialis Zone, according to White (1907, p. 36). Indeed, outcrop patterns recently mapped in the Shalbourne valley near Eastcourt Farm [SU 317 644] imply that the base of the Lambeth Group locally oversteps the Newhaven Chalk entirely.

According to White (1907, p. 37), Marsupites occurs in the rubble marking the site of an old pit 3 furlongs [600 m] north-west of Shalbourne Church. This is taken to refer to a pit some 500 m north-east of Newton Dairy [SU 3110 6382]. Although now mostly infilled, about 2 m of uniform, flintless white chalk were exposed in the north-east corner at the time of the survey, but inaccessible to close inspection.

White (1907, p. 37) noted that in the yard at Prosperous Farm, now known as Mount Prosperous [SU 3440 6443], an old working showed about 6 m of soft white chalk with ‘fine greyish venules’ [perhaps marl seams] and a few irregularly shaped flints, dipping at 27º to the north. He found that fossils were scarce, but that plates of Uintacrinus occur in a seam about 15 cm thick in the middle of the sequence. No exposure was seen in this pit during the present survey.

An old chalk pit, extended about a year before the present survey to provide hardcore for estate track maintenance, occurs just south of Sadlers Copse [SU 3440 6448]. Up to 4 m of strata are thus newly exposed over an area some 10 to 15 m across, all in soft, smooth, blocky, white chalk. No marl seams were seen. There are a few sparsely defined beds of flint nodules; the nodules range from small to very large (the latter appearing elongate in the plane of bedding), spaced approximately 50 cm apart along the bed, but sufficient to indicate bedding dip of N033/14ºNE. Flints are sparser than would be expected for Seaford Chalk.

Two examples of Echinocorys were collected from the same horizon, several metres apart, and about 1 m from the top of the exposed section. These appear to be forms typical of the testudinarius Zone (M A Woods, written communication, 2003). A single unornamented Marsupites plate was found in chalk taken from this pit. By inference, the quarry is in Newhaven Chalk.

A similar chalk pit some 350 m due west [SU 3404 6448] exposed very similar brittle white chalk with few flints.

SU37SE

The Stockbridge Rock Member (Seaford Chalk) forms a small feature along a track just east of Orpenham Farm [SU 3920 7056] at 123 m OD. Ossicles of Uintacrinus socialis were found in the brash 250 m further east along the track.

The Newhaven Chalk is not seen except as field brash. A sample collected at [SU 3841 7042] was shown by micropalaeontological analysis to belong to the socialis Zone. The maximum thickness of Newhaven Chalk in this area is probably less than 10 m. No Marsupites testudinarius plates were found and the Reading Formation probably rests directly on socialis Zone chalk south of Wickham. North of Wickham, the Palaeogene unconformity probably rests directly on Seaford Chalk.

SU47SW

Treacher and White (1906, p. 391) and White (1907) reported a small outlier of the ‘zones of Marsupites and Actinocamax quadratus’ (in modern terms, from the socialis Zone to the pilula Zone) beneath the Palaeogene outlier between the Lambourn and Winterbourne valleys. The chalk is in varying degrees phosphatic and includes several rock bands.

Treacher and White (1906, p. 391) also reported that a small roadside pit, 200 m east of Boxford Rectory, exposed 2.4 m of soft crushed chalk of the ‘Uintacrinus band’, with few flints and very few fossils. At that time, Boxford Rectory was the house now shown on large scale maps as ‘Boxford House’. This places the roadside pit a little east of the south end of School Lane [SU 4319 7151]. White (1907, p. 31) also mentioned this locality, stating that the soft white chalk included a very small proportion of phosphatic matter (coprolites, fish bones, etc.) and the remains of Uintacrinus, ‘Kingena lima’ and bryozoa. This is locality ‘s’ of White and Treacher (1906). White (1907, p. 31) recorded chalk with the same fossils 540 m farther east, at Iremongers Cottages [SU 4371 7156] (at the base of the Lambeth Group) and at Wyfield Farm [SU 4440 7284].

White (1907, p. 31) also reported that a group of soft to very hard phosphatic chalks containing Marsupites, Offaster pilula and A. quadratus cropped out in a somewhat irregular manner between Iremongers Cottages and Wyfield Farm, on both sides of the Borough Hill Ridge, both south-east and south-west of the hill fort [SU 434 725]. These beds seem to have a maximum thickness of 10 to 12 m at outcrop. They include flints containing fossils, and at least one band rich in A. quadratus. Their harder beds have numerous rusty sponges.

Treacher and White (1906, p. 391) noted that the Winterbourne pit [SU 4477 7223] showed about 3.7 m of chalk in the ‘Marsupites’ [crinoid] and ‘quadratus’ [pilula] zones. Both are more or less phosphatic and very fossiliferous. No flints were seen in place. There are two rock bands, the higher of which marks the junction between the two zones. In the higher band and for a few centimetres above it, A. quadratus is ‘extraordinarily abundant’, and is associated with many specimens of E. scutatus. White (1907, p. 32) gave a detailed description of this pit, and White and Treacher (1906, p. 501) also described it (their locality ‘a’).

Treacher and White (1906, p. 391) stated that many other patches of rocky chalk had been detected on both sides of the hill between Boxford and Winterbourne.

In addition to the records of White (1907) and White and Treacher (1906), calyx plates of the crinoid Uintacrinus socialis were found during the recent field survey in field brash about 900 m north of Winterbourne Church [SU 4502 7282], and in a shallow excavation about 900 m north-west of Winterbourne Church [SU 4452 7267].

The section exposed in the Winterbourne Pit was described in detail by Jarvis and Woodroof (1981) and by Mortimore et al. (2001).

SU47SE and SU57SW

Bearing in mind the general thickness of the Seaford Chalk, the Newhaven Chalk might occur much more extensively than shown on the geological map, albeit in thicknesses of less than 10 m. A relatively conservative approach was taken to mapping the extent of this ‘feather edge’.

Chapter 5 Palaeogene

Introduction to the 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.

Lambeth Group

The Lambeth Group, of Paleocene age and the oldest Palaeogene unit present in the Newbury district, crops out in the south of the area. It does not occur within the Abingdon district. There are several outliers of the Lambeth Group, but the largest outcrop continues through the axis of the London Basin. It corresponds to the ‘Woolwich and Reading Beds’ of earlier work.

Regionally, the Lambeth Group includes a thin basal unit, the Upnor Formation (previously known as the Bottom Bed), but in the western part of the London Basin it mostly comprises the Reading Formation (Ellison et al., 1994). It is not practical to differentiate the Upnor Formation from the Reading Formation except in exposed sections, and so on the map they are shown together.

The base of the Lambeth Group, and so of the Upnor Formation, is marked by a distinct, persistent negative break of slope and a change to poorly drained, clay-rich soil (locally with lime-hating plants such as Rhododendron), compared with that of the adjacent chalk outcrop.

The Upnor Formation and parts of the overlying Reading Formation tend to be impervious, so that solution hollows occur at the margin of the Palaeogene outcrop, where its surface drainage first meets the Chalk.

Outcrop patterns and borehole evidence suggest that the Lambeth Group varies between about 20 m and 35 m in thickness. It is conformably overlain by the London Clay Formation.

Upnor Formation

The Upnor Formation was deposited during an early Palaeogene marine transgression. It consists of highly glauconitic, green, blue and grey sands and clays. It contains large, irregular, glauconite-coated flint nodules and flint pebbles at the base, resting on a locally irregular, bored Chalk surface (Plate 2) and (Plate 3). It also contains a varied marine fauna. In West Berkshire, it is generally less than 1 m in thickness (Mathers and Smith, 2000), but some 2.2 m were observed in sections on the line of the A34(T) Newbury bypass (BGS unpublished records), 2.4 m at Shaw (Jones and Whitaker, 1878) and up to 4 m in boreholes at Curridge and Cold Ash. Elsewhere in the area, the presence of the Upnor Formation can be demonstrated locally by the occurrence of glauconitic sandy clays ploughed up from the subsoil.

Details

SU36NW

According to White (1907), a very irregular junction of the Chalk with the Reading Formation was visible at a pit near the kiln (of the previous Hop Grass Brick Works) in Brick Kiln copse [SU 3205 6931], a little to the south of Chilton Foliat. Pinnacles of dirty chalk rise into the Eocene sands and clays.

SU36NE

A temporary section seen in 2002 in Horn Copse [SU 3949 6608], about 1 km south-east of Kintbury, exposed clays and sand of the Reading Formation overlying about 0.5 m of green glauconitic shelly sands of the Upnor Formation (Plate 3).

SU36SW

Green-coated flint nodules were found in soil at a few localities close to the base of the Lambeth Group in the Shalbourne area. No other evidence for the presence of the Upnor Formation was noted during the recent survey, although no augering was carried out here.

SU37SE

White (1907, p. 56) stated that the hill on which Wickham Church sits [SU 3947 7152] seems to consist almost entirely of sand, with a few flint pebbles in parts, and with olive green sand of the ‘bottom bed’. He reported that the boundary of the Wickham Palaeogene outlier was locally marked by swallow holes, and that the bottom bed occurs west of Wickham, where it is seen in the fields.

SU46NE

Sections in the large road cutting on the Newbury Bypass on the north side of the Lambourn valley, about 1 km north-west of Donnington [SU 458 696] to [SU 459 699] were examined by R A Ellison and S J Mathers during road construction in October 1997. A ditch section beside the same road near Speen, about 2 km to the south-south-west [SU 450 678], was also examined.

The basal 2.2 m of these sections comprise highly glauconitic sediments of the Upnor Formation (Ellison et al., 1994). The basal bed contains pebbles and cobbles of well-rounded and nodular flint and phosphate resting on a smooth but burrowed Chalk surface. Above it are interbedded sands and clays that are extensively burrowed and bioturbated. Oyster shells occur at several horizons.

Hawkins (1924) briefly described a large gravel pit immediately north of Newbury Station [SU 4719 6678]. He reported that glauconitic clayey sands were seen in several places below river terrace gravels. These occurrences of the Upnor Formation suggest that the base of the Lambeth Group lies further north than shown on the geological map, although possibly within outliers.

A large clay pit at Shaw, north of Newbury and just east of the now-disused Didcot to Newbury railway line [near 4829 6829], previously exposed a section from the Seaford Chalk to the basal London Clay. Here the Upnor Formation was described as comprising a basal bed of six inches [0.15 m] of dark green sandy clay with green-coated flints in it, one foot [0.3 m] of similar greenish sandy clay (loam) in thin beds and full of large and small fossil oyster shells. Some green-stained pebbles occur in this bed and also some shark’s teeth. ‘A similar laminated greenish loam, but without oyster shells, succeeds, with an irregular thickness of a foot and a little more; then a few inches of oyster-bed as before, with irregular surface, covered with similar green sandy shale, about 18 inches thick [0.45 m], pebbly, and passing upwards into ash-coloured and blue shale, with sandy loam, about five feet [1.5 m], and darker below than above’ (Jones and Whitaker, 1878). Unless the grey and blue shale is part of the Reading Formation, the Upnor Formation is about 2.4 m thick at Shaw.

SU46NW

White (1907, p. 56) reported that the boundary of the Wickham Palaeogene outlier was locally marked by swallow holes. The bottom bed was seen in a road cutting at the southern end of the spur south of Hoe Benham [SU 4084 6903] and in various other places nearby.

SU47SW

White (1907, p. 25) recorded a field pit 700 m north-west of Sole Farm, near Wickham kiln [SU 4057 7161], in which flinty chalk was capped by the green sand ‘bottom bed’ of the Lambeth Group.

White (1907, p. 24) recorded a little pit 200 m west of Wyfield Manor Farm [SU 438 728], about 7.6 m below the base of the Palaeogene, as showing 2.4 m of rather soft white chalk. Small cylindrical pipes of green sand with pieces of tabular flint traverse the section from top to bottom.

White (1907, p. 55) saw the Upnor Formation 800 m west-north-west of North Heath [SU 449 748] stating that ‘there is a road section showing the green clayey sand of the bottom bed above the chalk; and just to the west of the common is another small outlier of sand [on the edge of 47SW and 47SE]. Both are much covered with drift’.

White (1907, p. 56) records that in 1858, the pits at the Wickham brickyard [SU 404 717] exposed sections with about 1 m of olive green sand (the ‘bottom bed’) resting on chalk at the base.

The basal olive green sand is also to be seen in the fields north-east of the brickyard.

SU47SE

The old Hungerford memoir (White, 1907, p. 51) states that the ‘Bottom Bed’ is exposed in the valley either side of the road between Angel Hill [SU 4920 7035] and Longlane [SU 499 717]. In this area, the base of the valley is up to one kilometre wide and the contact between the Seaford Chalk and the Upnor Formation occurs at the edge. This is a complex contact with low hills of Lambeth Group isolated in the base of the valley. The memoir details a section in a chalk pit (no longer visible) [SU 491 709], as ‘over 1.5 m thick and consisting of slightly laminated grey clay and clayey sand, with green grains scattered more or less throughout; at the base there are green-coated flints and oyster shells, much broken up. The junction with the chalk is even and shows a slight dip in a south-easterly direction.’

At a now disused pit at Longlane [SU 4984 7166], White (1907) described the Upnor Formation as 2.1 m of ‘bluish-grey sand, loamy at the top, and containing pebbles of grey clay, soft concretions of ironstone, and lenticles of carbonaceous matter near the bottom.’ During the current survey, the Upnor Formation was identified at numerous points in this area by an abundance of well-rounded flint pebbles in soil brash. Notable locations were Longlane [SU 4984 7178], Cold Ash Farm [SU 4983 7109], Hill Top [SU 4936 7021] and numerous locations in the area of Grange Farm [SU 4865 7076].

The Old Kiln Sand Pit, north-west of Curridge [SU 487 725], abstracted sand from the Reading Formation. Spoil from pits in the base of the quarry show the presence of chalk at depth, overlain by well-rounded pebbles of ironstone and flint pebbles in a matrix of orange brown (weathered) sand and clay. Records for 26 investigation boreholes show the Upnor Formation as a clay and silt band, described as light grey, mauve and dark brown with thin beds of sand (orange and orange-brown) and pebbles, beneath the main sand (orange to buff) sequence of the Reading Formation. Here, the Upnor Formation has an average thickness of 2.9 m, and ranges from 1 to 2 m thick in the north to 3 to 4 m in the south. These records illustrate the relatively rapid lateral variation in the lithology and variation in thickness of the formation. The base shows a considerable range from 92 m to 114 m OD, but in most cases it is between 104 m OD in the south-west to 108 m OD in the north-east.

In this area, the records of site investigation boreholes on the line of the M4 motorway describe the Upnor Formation briefly. It appears not to be present at all localities. Where present, the records show varying thicknesses of sandy clay with silt and beds of sand, and the presence of gravel. The colour ranges from brown to grey.

Few of the Newbury Bypass investigation boreholes penetrated the full thickness of the Upnor Formation. Typically they record shells and flint gravel at the base of the sequence and reference is made to glauconite sand in the overlying, dark coloured, sandy clay deposit.

About 3 m of the basal part of the Lambeth Group was exposed during construction of a new road at the A34/M4 junction in 2004 [SU 47940 73294]. This included a well-defined flint pebble and glauconitic sand at the base. Above a sharp contact, weakly bioturbated grey clays with thin fine-grained sand lenses and beds (Plate 2) are present.

To the west of North Heath [SU 4533 7457], the base of the Upnor Formation was identified by the presence of abundant, well-rounded flint pebbles in a sandy soil. Spoil from a borrow pit just north of Winterbourne Holt [SU 4592 7184] showed evidence of the basal pebble bed set in an orange-brown (weathered) silty clay matrix.

SU57SW

Cold Ash Farm Sand Pit (Hoveringham Gravels) [SU 501 713] is now backfilled, but boreholes drilled to investigate the hydrogeology proved the presence of the Upnor Formation, ranging from 1.2 to 3.2 m thick. The borehole records described it as a sandy clay with beds of clay and sand, and it contains well-rounded flint pebbles in most boreholes, particularly at the base of the sequence. The colours are described as grey with greenish, brownish and bluish tints. There is considerable variation between the boreholes showing the lateral variation in the lithology.

Just north of the M4 motorway at Oare, spoil from a new farm track cutting [SU 5100 7435] illustrates the section through the Upnor Formation and into the surface of the chalk. The base of the Upnor Formation is characterised by abundant well-rounded flint pebbles in an orange brown (weathered) clayey sand matrix. Nodular flints occur at the basal contact. The pebbles become less numerous upwards.

Reading Formation

The Reading Formation is made up predominantly of colour-mottled clays, some silty or sandy, mainly red and grey, but also green, purple, brown and orange (Plate 3) and (Plate 4). A bed of black organic clay, up to about 30 cm thick, commonly occurs within the lower half of the formation, visible as a peak on many natural gamma borehole logs (Figure 7).

Beds of grey, buff or orange-coloured, generally fine- to medium-grained but locally coarse- or very coarse-grained sands are also present. Some contain sparse very well-rounded flint pebbles in stringers or thin beds. These are commonly up to 2 m thick, but locally attain as much as 10 m. These sand beds occur at all levels, but especially near the base of the formation. The sequence is laterally variable. The sand is locally cemented by silica, forming an intensely hard sandstone. Fragments of such sandstone commonly occur in soils and in some superficial deposits on the Chalk of the area; these fragments are known as sarsens. Similarly cemented pebbly sandstone or gravel is known as ‘puddingstone’. 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.

Fossil leaves, seeds, lignite and silicified wood occur widely and locally abundantly in the Reading Formation, as found for example in a section at Cold Ash (Crane and Goldring, 1991).

Springs can occur at the sand beds in the Reading Formation.

The Reading Formation represents deposition in a predominantly nonmarine, deltaic and fluvial environment, probably with some minor marine incursions. 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).

As noted below, the clay-with-flints 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 expected to be complex. It possibly occurs within a zone some hundreds of metres in width. The mapped boundary between these two deposits should be regarded as highly approximate. It is possible that relicts of the Lambeth Group occur within the clay-with-flints, but these cannot be mapped.

Within this district, the Reading Formation is between about 20 and 30 m in thickness.

Details

SU26NE

The Reading Formation here consists mainly of mottled red-orange clay. It is estimated to be about 10–15 m thick. However a patch of very sandy clay-with-flints, possibly an infilled pipe, was noted near Little Bedwyn [SU 2961 6605]. The Reading Formation has been used for brickmaking and old clay pits occur near Lower Farm, Chisbury [SU 2788 6682].

SU26SW

The Reading Formation consists mainly of mottled red-orange clay in this district. It is estimated to be about 10–15 m thick. The Reading Formation was not observed in any sections in this area and is covered by forestry, but sections are recorded by White (1907, p. 64) from old clay pits [SU 2270 6474] near Leigh Hill (Marlborough district). These are now very overgrown.

SU26SE

The Reading Formation here consists mainly of mottled red-orange clay. It is estimated to be about 10–15 m thick. However, sandier facies occur in places and there is a major sand unit in the Bedwyn Brail-Harding Farm area, bounded by grid references [SU 287 630], [SU 295 632] and [SU 293 637]. Several sand pits, some now infilled, occur around Folly Farm. In particular, a large sand pit occurs near Harding Farm, adjacent to the road 2.2 km south-east of Great Bedwyn, where approximately 8 m of fine- to coarse-grained pebbly sand is exposed [SU 2955 6320].

SU36NW

White (1907, p. 61) reported a composite section through the Palaeogene of Bagshot Hill [SU 317 653]. The portion of this section in the Lambeth Group is as follows:

SU36NE

A temporary section seen in 2002 in Horn Copse [SU 3949 6608], about 1 km south-east of Kintbury, exposed about 2 m of multicoloured, waxy clays and yellow sand of the Reading Formation, overlying about 0.5 m of green glauconitic shelly sands of the Upnor Formation (Plate 3) and (Plate 4).

SU36SW

No good exposures of the Lambeth Group were seen in the Shalbourne area during the recent survey. Historical records indicate that the sequence includes both clays and sands, and this is reflected in local soil types. Red-brown and grey mottled clays are seen locally in ploughed fields or in ditches. Blocks of ferruginous medium-grained sandstone up to about 80 cm in diameter occur locally on the Lambeth Group outcrop.

Springs occur locally at the base of the sequence, and also within it, presumably marking the local position of sand beds resting on clays.

A disused pit in the Lambeth Group some 900 m north of Shalbourne church [SU 314 644] marks the site of a brick and tile works.

An outlier of the Lambeth Group, up to about 200 m in diameter, possibly occurs in the vicinity of the junction between the A338 main road and the side road to Ham [SU 324 646]. If present, this outlier is unlikely to be more than a few metres in thickness.

SU37SE

In the Wickham area, the Reading Formation consists mainly of fine- to medium-grained, reddish-orange sand, with a few flint pebbles in parts. Much of the area is covered in a thick deposit of clay-with-flints. It is possible that small outliers of Reading Formation may occur within the clay-with-flints, but cannot be mapped.

SU46NE

Sections in the large road cutting on the Newbury bypass, on the north side of the Lambourn valley, about 1 km north-west of Donnington [SU 458 696] to [SU 459 699] were examined by R A Ellison and S J Mathers during road construction in October 1997.

A virtually complete section through the Lambeth Group was seen, with a total thickness of 24.8 m. This accords well with borehole evidence in the Newbury area, and also in the Reading district to the east, where 19–27 m are preserved (Mathers and Smith, 2000). It indicates that there is no regional westward thinning of the Lambeth Group strata along the axis of the London Basin at least as far as Newbury.

Up to 2.2 m at the base of the section were assigned to the Upnor Formation. Overlying the Upnor Formation are 22.6 m of strata assigned here to the Reading Formation (Ellison et al., 1994). The basal part (2.2–8.2 m above the base of the section) is dominated by fine- to medium-grained sands with subordinate clay layers. Flaser bedding occurs in the more clayey strata whereas herringbone cross-stratification and Ophiomorpha burrows occur in the thicker sand units. Close to the base of the formation are beds rich in leaf fragments and lignitic debris. Similar strata have been described at several locations in the western London Basin, at Reading (Newton in Blake, 1903, p. 40–41), Cold Ash north-east of Newbury, and Pincent’s Kiln near Tilehurst (Crane and Goldring, 1991). The bedding styles in the ‘basal sands’ and the presence of Ophiomorpha burrows indicate that this sequence was deposited in a sand-dominated, intertidal-subtidal setting.

The overlying 6.6 m of strata (8.2–14.8 m) are dominated by clays. They include strongly colour-mottled clays, including the medium grey and red mottling characteristic of the Reading Formation. A prominent, thin, but laterally extensive bed of black, illite-rich clay occurs at 12.9–13.0 m. The top of this clay-dominated sequence is extensively burrowed. The highly mottled nature of these clays is a characteristic of the Reading Formation and has been interpreted as the multiple overprinting of palaeosols, implying periodic emergence of a low-energy, mud-dominated environment.

The uppermost beds of the Reading Formation (14.8–24.8 m) exhibit a complex lateral interdigitation of fine- to medium-grained sands with brown clayey silts and mottled clays. A body of sand up to 40 m across and 3–4 m thick passes laterally into finer grained lithologies. The edges of this sand body are generally sharp and steeply inclined or vertical. At several levels, the sand broadens into horizontal thin beds, forming offshoots that extend laterally for up to 10–15 m into clay-dominated lithologies. The sands appear to be planar bedded and there is no evidence of large scale cross-stratification and disturbance, loading or faulting of the strata. The deposit appears to have retained its original depositional geometry. This sand body is interpreted as the deposit of an anatomising channel. These are characteristically relatively stable channels embedded within a muddy floodplain.

The Lambeth Group is overlain by thin Quaternary flint-rich gravels in the road cutting, but Cenozoic strata overlying the Lambeth Group were proved in a nearby site investigation borehole [SU 4589 6982]. This revealed clay with pebbles directly beneath the Quaternary gravel, interpreted as the basal bed of the London Clay.

Prestwich (1854, p. 86–87) describes a complete section through the Lambeth Group at Clay Hill, Shaw [SU 485 681], north of Newbury and just east of the now-disused Didcot to Newbury railway line. The section was also described by Jones and Whitaker (1878), who saw a basal layer of red, grey, green and brownish laminated clayey sands and sandy clays about 4 feet [1.2 m] thick, with fossil plants in the sandy clay beds. Ochreous laminated sands and clay bands followed next for about 12 feet [3.7 m], forming the slope between the lower, chalk pit and the upper, clay pit. In the upper pit, Jones and Whitaker (1878) saw 25 feet [7.6 m] of the mottled clays of the Reading Formation, including a thin bed of white sand about 6 to 8 feet [1.8 to 2.4 m] above the base of the interval.

SU46SW

Borehole records from this area suggest that the Lambeth Group probably varies between 29 and 32 m thick.

SU47SW

White (1906, p. 351) recorded a pit on the north side of the road near the eastern edge of Winterbourne Wood [SU 4472 7171] that exposed red-brown, laminated, ferruginous sand and sandstone of the Lambeth Group, overlain by a thick gravelly wash. A lens of very coarse quartz grit occurred in the sands. He reported that the Palaeogene in this outlier is thickly covered by flint gravel, but was seen in sand pits at Borough Hill [SU 439 725] and just west of Winterbourne Manor [SU 449 719], and on top of the hill about halfway between them. The sand is brown, buff, yellow and white. Some is very coarse.

White (1907, p. 56) records that in 1858, the pits at the Wickham brickyard [SU 404 717] exposed the following section:

Whitaker (1872), p. 180, also gave details for Wickham Pit. He (p. 181) noted observations near Wormstal and around Winterbourne (p. 192).

SU47SE

The Oare Sand Pit [SU 499 740] has a 5 m face, mostly in fine- to medium-grained sand, coloured pale grey, yellow brown, orange brown and rusty brown with cross-bedding. The sand contains stringers of black and brown, subangular pebbles of flint and quartz. In the lower part, there were also well-rounded intraclasts of light grey clay. At the base, there are beds of stiff light grey clay interdigitated with beds of sand and occasional beds of ironstone.

The Old Kiln Sand Pit, west of Curridge, [SU 487 725], shows a sequence of cross-bedded, fine- to medium-grained sand, mostly light grey with some orange brown, with seams of silt and clay. There are beds of light grey clay ‘gravel’ (comprising mudstone intraclasts) in a matrix of sand. This dominantly sand sequence passes laterally into clays and silts on the eastern edge of the pit. Investigation borehole records show that the sequence has a maximum thickness of 14.3 m, and the thickest sand bed is 9.2 m. In places, a silt and clay bed between 1 m and 2 m thick is developed within the sand sequence, and a similar bed can occur above the sand.

Some of the site investigation boreholes for the M4 motorway and the Newbury bypass record the lowest part of the Reading Formation. Where it is present, the records briefly describe the rocks as medium- and fine-grained sand or stiff clays, with interbedded sequences and beds of clayey sand and sandy clay, with colour ranges from brown to grey.

SU57NW

Whitaker (1872, p. 194) noted that, in a small wood some 400 m north of Pibworth Farm [SU 5482 7975], there was a sandpit on the slope of the hill, in a hollow in the Chalk (this is within an area mapped as clay-with-flints). In a saw pit at the crossroads near the farm [SU 5484 7911] there was brown and light-coloured sand, with pale blue and lilac clay, apparently drifted from the Reading Formation.

Geological survey records note the discovery in 1960 of a piece of fossilised Sequoia, about 0.4 m across, found in Reading Formation clay between Pibworth Farm and Dumworth Farm [SU 549 793] at a depth of 0.5 to 1 m.

SU57NE

Notes on the back of a field slip for 1:10 560 sheet Berkshire 28 SW/E, written by F J Bennett in 1887, state that a chalk well at Basildon Kiln (i.e. a shaft for the extraction of chalk for lime-burning) presumed [SU 5861 7630] was 60 feet [18.3 m] deep, being about 20 feet [6 m] to the top of the Chalk. The ground (nearby) rises 10 feet [3 m] more, therefore the local outcrop of the Reading Formation is about 30 feet [9 m] thick. The section through the Reading Formation shows 1.5 m to 3 m of brown clay overlying about the same thickness of fine white sand. Red mottled clay occurs just at the top, irregularly capped by pebbly gravel.

SU57SW

A sand pit at Cold Ash Farm [SU 501 713], now backfilled, proved at least 20 m of sands (Crane and Goldring, 1991), with beds of clay-pebbles in a sand matrix and silty clay lenses. Mottled red, cream and grey clays, typical of the upper part of the formation were proved in borings above the quarry. Loose, fine- to coarse-grained sand, sometimes weakly silica-cemented, make up the bulk of section. The sand consists predominantly of angular to subangular quartz with occasional flint fragments. Large scale planar cross-bedding is the most common sedimentary structure with sets up to 1 m thick. Dip directions for the co-sets indicate a general trend from westward flow at the base to south-easterly flow higher in the sequence. Beds with clay pebbles occur at many levels and in intervals up to 1 m thick. Lenses of silt and clay from a few centimetres to over 10 m long occur throughout the sands. Plant debris associated with the silt and clay lenses have been identified. The fossil flora consists of well-preserved angiosperm leaves, and there is evidence for the activity of contemporary leaf-miner insects. As a result of the importance of the site, it has been given SSSI status, and the section has been covered for protection.

The record of a borehole [SU 505 711] at Down House, just to the north-east of the Cold Ash pit, proved a sequence of 25.3 m in the Reading Formation, largely of sand with some clay and silt in the top 4.4 m. The record of a public supply borehole [SU 5024 7040], 0.5 km south-south-east of the pit, shows a similar thickness, but the lithology has changed, with almost 10 m of clay in the middle of the sequence, with sand below and above.

SU57SE

Notes on the back of a field slip for 1:10 560 sheet Berkshire 28 SW/E, written by F J Bennett in about 1887, state that a sand pit [in the Lambeth Group] south of Mapleton’s Farm [SU 5822 7424] exposed a thin lensoid upper layer of brown loam, with the greater underlying part of the section in coarse, ferruginous yellow sand with cross-bedding.

Thames Group

The Thames Group comprises the London Clay Formation underlain by the Harwich Formation, previously recognised as the ‘London Clay Basement Bed’ (Ellison et al., 1994). In the Reading district, the Harwich Formation is less than 10 m thick. It has not been mapped separately from the London Clay (Mathers and Smith, 2000). The same practice is followed in this report and on the accompanying map.

The Thames Group marks the return of marine deposition to the region, although around Newbury it is in a relatively proximal sandy facies.

Harwich Formation

The Harwich Formation is up to 6 m thick in the east of the present area. It comprises a basal flint pebble bed overlain by highly glauconitic shelly sands and clayey silts. These are intensely burrowed and locally cemented. The formation can appear intergradational with the Reading Formation where the latter is sandy. Springs occur at the Harwich Formation outcrop.

Well-developed sandy pebble gravel at the base of the London Clay in the south-west, where it forms several outliers, is presumably part of the Harwich Formation. See also the following details for the London Clay.

Details

SU47SW

White (1907, p. 66) states that the ‘bottom bed’ of the London Clay could be observed north-east of Harrod Farm (presumably Harrod’s Barn [SU 4072 7015]) and at Sole Common (north side of Sole Plantation, [SU 4095 7072]).

London Clay Formation

The London Clay Formation, of Eocene age, crops out in the south of the area. The largest outcrop marks the axis of the London Basin, but there are several small outliers to the north and west.

In this district, the London Clay comprises blue-grey silty clays and clayey silts, together with extensive beds of glauconitic sand that form 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. 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 clay weathers to a rusty brown colour. Calcareous concretions and pyrite nodules occur. The London Clay contains a shelly marine fauna.

The sands in 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 of each sand unit is commonly marked by a sharply defined, possibly erosive contact, in places with a discontinuous bed of black flint pebbles. The sand units tend to coarsen upwards, with a gradual diminution in the proportion of clay. Although this change is gradational, the base of the sandy top is commonly marked by springs or by a negative break of slope.

Beds of sand up to 20 m thick occur in the London Clay in this district, the largest of which have been shown separately on the geological map. The interval of the London Clay below the lowest mappable sand body gradually decreases 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 largely comprise sandy clayey pebble gravels and gravelly sandy clays, locally overlain by sand. The gravel typically consists of very well-rounded flint pebbles and rare cobbles, commonly black coated, and bearing crescentic ‘chatter marks’ characteristic of flint beach pebbles (Plate 5). To the east, around Inkpen, a thin basal pebble bed is overlain in turn by 5 to 10 m of grey silty micaceous clay, then some 8 to 10 m of silty fine sands and sandy silts, followed by a bed of very sandy silty clay packed with flint pebbles. This upper pebble bed has a sharply defined, possibly erosional base. It locally attains 5 m in thickness, but rapidly thins and disappears 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 these 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).

In its type area in the Windsor district, the base of the Bagshot Formation is abrupt and probably erosional: there, the lower parts of the sand infill channels within the London Clay (Ellison and Williamson, 1999). The Bagshot Formation is typically composed of fine- to medium-grained, pale yellow-brown to pale grey sand with planar and cross-bedding. An impersistent flint pebble bed occurs at the base. Interbeds and lenticular bodies of pale grey clay occur, up to 0.3 m thick. In geophysical borehole logs, the gamma signature is characteristically blocky, with a sharply defined base.

By contrast, the lower contact of the sand bodies forming the local top of the Palaeogene sequence in the Newbury district is generally indistinct or gradational, although sometimes marked by a pebble bed or spring line (Blake, 1903; Hawkins, 1953). They are typically composed of yellow and reddish silty or clayey sands with thin stringers of pipe clay. The sands are commonly current bedded and micaceous, and contain sparse thin beds of flint pebbles. The scarcity of sand pits in the outcrop (Blake, 1903) suggests that clean sands are unusual. This inference is supported by the appearance of gamma borehole logs, which develop ‘funnel-shaped’ profiles, indicating coarsening-upwards sequences.

Outcrop patterns and borehole evidence suggest that up to about 70 m of the London Clay is present in the east of the district, with the preserved thickness diminishing northwards and westwards.

The biostratigraphical significance of fossil material held in the BGS collections and that found during recent surveys is discussed by Wilkinson (2003d, e).

The London Clay Formation is the youngest bedrock formation seen in the area. It is conformably overlain by the Bracklesham Group to the east of the district.

Details

SU26NE

The base of the London Clay is marked by a prominent pebble bed, which forms a marked feature and is well displayed at Burridge Heath. The London Clay is at least 10 m thick, but no good exposures were seen.

Fine-grained, red-brown and yellow micaceous sand about 10 m above the base of the London Clay can be seen in a road bank near Stoke Manor [SU 266 651] and in the road cutting south of Chisbury [SU 2780 6623].

SU26SE

The base of the London Clay is marked by a prominent pebble bed, which forms a marked feature, well-displayed at Burridge Heath.

The London Clay is at least 10 m thick, but no good exposures were seen. About 10 m above the base of the London Clay, fine-grained, red-brown and yellow micaceous sand occurs. This is thought to be a sand unit within the London Clay.

SU36SW

The base of the London Clay is placed at the base of a conspicuous pebble bed. This forms fairly extensive plateaux, abruptly bounded by a positive break of slope, in the north-west of the area. Arable fields on these plateau areas have very gravelly sandy soils, with very numerous, well-rounded flint pebbles (‘Tertiary pebbles’), typical of the Palaeogene pebble beds. A strong spring line marks the base of this pebble bed.

The basal London Clay pebble bed (strictly, the Harwich Formation) is exposed in a small pit about 30 cm deep, about 550 m north of Newton Farm [SU 3015 6390]. This showed very abundant, well-rounded pebbles, clast-supported, in a sandy clay matrix. The thickness of this basal bed is difficult to assess, due in part to the spread of pebbles downslope from the outcrop, but is likely to be about 1 to 2 m.

Strata overlying the basal pebble bed form two low hillocks, each bounded by a negative break of slope at the base, at the western edge of the area, north of Newtown Farm. These hillocks are covered by very light sandy soils, with some fine sand in the subsoil. Some of the intervening ground has sandy clay soils with few pebbles, suggesting that a short interval of clays or sandy clays occurs between the sand and the pebble beds.

White (1907, p. 61) described a composite section through the Palaeogene of Bagshot Hill [SU 317 653].

SU46NE

A large clay pit at Shaw, north of Newbury and just east of the now-disused Didcot to Newbury railway line [near 4829 6829], previously exposed a section from the Seaford Chalk to the basal London Clay. Here the London Clay was described as comprising a basal thin and irregular band of flint pebbles, and ten feet [3.0 m] of reddish sandy clay with fossil shells, and including a bed of concretions, forming the cap of the hill (Jones and Whitaker, 1878).

SU46SW

Sand bodies within the London Clay in this area were previously mapped as part of the Bagshot Beds. Borehole records suggest that the London Clay is up to 66 m thick here.

Samples of London Clay taken from a temporary excavation near Wash Water [SU 45060 63525] proved to date from the Early Eocene and possibly from the basal Division E of King (1981) (Wilkinson, 2003d).

SU47SW

White (1907, p. 56) records that in 1858, the pits at the Wickham brickyard [SU 404 717] exposed the following section:

Whitaker (1872), p. 180, also gave details for Wickham Pit. He reported (p. 299) that there is a patch of London Clay above the brickyard at Wickham, consisting of a few feet of the basement bed ‘in a very confused state’. He added: ‘There may be other patches, but hidden by the gravel, which thickly covers this outlier.’

Chapter 6 Quaternary

Overview of Quaternary deposits

The superficial (or ‘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 occur 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. Silts of aeolian origin occur locally on the younger river terraces.

The alluvial deposits of the Newbury district were mostly laid down 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.

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 sands. A discontinuous cover of mass-movement deposits is present on slopes and in valley floors throughout the district. These comprise mainly head (solifluction deposits and colluvium), but landslide deposits occur locally on the Palaeogene formations around Newbury.

Residual deposits

Clay-with-flints

Clay-with-flints occurs on interfluves within the area of the Seaford Chalk and Newhaven Chalk outcrops. It is typically composed of orange-brown or reddish brown clays and sandy clays containing abundant, matrix-supported flint nodules and pebbles. These clays are thought to be predominantly cryoturbated remanié deposits derived mainly from Palaeogene sequences and by dissolution of the Chalk. It is recognised that these deposits have been modified by periglacial processes but they are thought to have undergone little lateral movement compared with head. Thus the base of the clay-with-flints can be expected to correspond to the original basal Palaeogene transgressive surface. In some parts of the area, for example north of Oakhouse Farm [SU 510 770] near Hampstead Norreys, the surface forms a recognisable landscape facet, but lacks a consistently mappable deposit. However, the contact with the underlying Chalk is typically of extremely irregular shape as a consequence of dissolution of the Chalk by groundwater, with concomitant collapse of the overlying deposits.

Three main types of flint are present in typical clay-with-flints, in variable proportions. These are: nodules recognisable as the same types found in the Chalk, although more or less broken; ‘Tertiary pebbles’, that is very well rounded, subspherical flint pebbles derived from Palaeogene deposits, typically with crescentic percussion fractures, ranging up to cobble size; and angular shards of frost-shattered flint. The clay-with-flints often also includes fragments of hard sandstone (sarsen) and ironstone derived from Palaeogene deposits.

Clay-with-flints can be expected to pass laterally into undisturbed Palaeogene deposits with a complex intergradation, perhaps over a broad area. By way of simplifying this relationship, in places the clay-with-flints has been shown with a narrow overlap on the Reading Formation.

Details

SU26NW

A pit at Brick Hill [SU 2360 6930] near Axford (in the Marlborough district) exposes a superb dissolution pipe in the south-east corner, 6–8 m deep and 4 m wide in Seaford Chalk and infilled with clay-with-flints.

SU26NE

Much of the Chalk is covered by clay-with-flints. Towards the Froxfield valley, the clay-with-flints on the hill spurs becomes increasingly flinty north of a line approximating to Northing 6700. The flints become more abundant, smaller and rounded, and the deposit tends to resemble river terrace deposits. However, there is no clear boundary or feature demarking the two types of deposit in this area.

A patch of very sandy clay-with-flints, possibly an infilled pipe, was noted near Little Bedwyn [SU 2961 6605].

SU27SW

Older surveys indicate an outcrop of clay-with-flints on a hilltop west-north-west of House Farm. Observations of chalk brash [SU 2397 7086] to [SU 2411 7086] during the recent survey indicate that any such deposit is confined to ground above 165 m OD, and to the west of the Newbury district.

SU37NE

In this area, the clay-with-flints caps the high ground underlain by the Seaford Chalk, forming extensive sheet-like bodies. Typical showings of the clay-with-flints, consisting of brown to reddish brown, stiff, slightly sandy clay, containing unworn flints and a small proportion of unbroken flint nodules were seen, for example, in a field 400 m south of Hasham Copse [SU 3815 7804]. A borehole at Goodings Farm [SU 3525 7520] indicates around 2 m of brown clay with flints overlying chalk.

SU37SW

The base of the clay-with-flints, lying on Seaford Chalk, is well seen from brash and features on the valley side south of Membury [SU 302 737]. The lower part of the clay-with-flints forms a positive break of slope on the top of valley side while the base of the deposit occurs at a negative break of slope about 20 m further down the valley side.

SU37SE

A pit just west of Stibbs Wood [SU 3671 7046], 1.5 km south-east of Hungerford Newtown, exposes 4–6 m of upper Seaford Chalk overlain by about 1 m of clay-with-flints with good examples of involutions and small dissolution pipes.

White (1907) noted that sandy sarsens and pebbly sarsens were rather abundant at Wickham.

SU38SW

An isolated patch of red-brown subsoil [SU 340 848] indicates the local presence of clay-with-flints, perhaps in a karstic solution feature.

A pasture field at Old Warren, which is a SSSI [SU 300 816], contains an unusual density of sarsen stones, inferred to lie in their natural position. (The SSSI was designated to afford protection to the lichens growing on the sarsens).

See also area details under Head.

SU57NW

Notes made on the field slip for 1:10 560 scale sheet Berkshire 27NE(E) (held in BGS archives) by F J Bennett in 1887 record that at the ‘kiln near Buttonshaw Farm [SU 5438 7774] clay is dug in irregular holes in chalk’. This pit was about 1 km north-east of Hampstead Norreys [SU 5396 7728]. Bennett saw an upper layer of clay with large unworn flints and sarsens overlying rusty brown clay, with black-stained joints, on a very irregular surface. The lower clay possibly represents little-disturbed Lambeth Group, although no evidence is given for the presence of the Upnor Formation at its base. He continues: ‘A hole near the kiln shows a jumble of plastic clay and coarse red sand with clay-with-flints at the top. The clay seems in the process of conversion into the rusty clay of the section’ [i.e. the lower layer]. The base of the pit exposed 20 feet [c. 6 m] of chalk with large flints.

Whitaker (1872, p. 194) noted that there was a sandpit on the slope of the hill, in a hollow in the Chalk, in a small wood some 400 m north of Pibworth Farm [SU 5482 7975]; this is within an area mapped as clay-with-flints.

Sand in clay-with-flints

In the area between Hermitage, Chieveley, Beedon and Hampstead Norreys, in the east of the Newbury district, the clay-with-flints appears to pass laterally into clean, medium- to fine-grained sands, here classified as ‘sand in clay-with-flints’.

The nearby outcrops of the Lambeth Group include relatively thick beds of sand. As mapped, these coincide with relatively upstanding ground, whereas the ‘sand in clay-with-flints’ forms a low-lying blanket over the Chalk. Otherwise, in the absence of exposures, this category of superficial deposit cannot be distinguished from the in situ Palaeogene sequence; indeed, part or all of what has been shown as ‘sand in clay-with-flints’ may comprise essentially undisturbed Lambeth Group sands.

Mass-movement deposits

Landslide

Several previously unrecorded landslides have been observed 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 all associated with the emergence of water from minor perched aquifers within the Palaeogene sands. These landslides are generally of translational type. They typically underlie areas up to 150 m wide and 500 m long, marked by irregular, ill-drained hummocky ground. Some are partly covered by dense woodland, where their extent cannot be mapped as accurately as in open ground. Other examples might be present, either on steep wooded slopes or where the original landform was subdued or has become degraded.

Head

‘Head’ refers to superficial deposits formed by solifluction processes, mainly downslope 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.

Head occurs on some slopes and in valley floors throughout the area. It tends to be more widespread on slopes facing east or north. It 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 topographical situation.

Head is very variable in composition, depending on local sources of material and details of landscape evolution. It is typically composed of very stony, sandy and silty clays, or clayey gravels. It can be clast-supported or matrix-supported. It tends to include a large proportion of angular, frost-shattered flint gravel (Plate 6).

The head deposits comprise diamicton, more or less stony, but predominantly matrix-supported. In comparison with the clay-with-flints, head has undergone lateral movement, and tends to be more sandy, with a greater proportion of shattered flints. It can be derived from (and overlie) clay-with-flints, but it commonly includes admixed material from other deposits as well. In much of the area, head has been derived from the clay-with-flints and the Chalk, in many places together with sand and water-worn flints from river terrace deposits. Head is locally separated from the clay-with-flints by a break of slope, but elsewhere a clear boundary cannot be recognised.

Head with a large proportion of material derived from river terrace deposits gives rise to conspicuously gravelly soils, but such head deposits lack the flat-topped landforms characteristic of river terraces.

Head composed of clayey sand, with little admixed gravel, occurs on parts of the Palaeogene outcrop. Such deposits can be difficult to distinguish with certainty from landslide deposits. Head on the scarp face tends to have a large proportion of chalk fragments, together with broken flint and perhaps some clay.

Head that occurs in the steeper, upstream parts of dry valleys on the Chalk outcrop is assumed to be composed of a similar diamicton to that found on adjacent slopes. The downstream sections are expected to be underlain by more flinty deposits, perhaps including 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, where the influence of fluvial processes predominates over that of solifluction.

Details

SU27SE

A temporary section in Ramsbury village [SU 2770 7190] exposed clast-supported gravelly head, comprising large carious flints (typical of the Seaford Chalk) set in a clay matrix. Towards the base of the section the flints tend to be unbroken, and irregular balls and pinnacles of very soft plastic chalk are also present.

SU27SW

A 3 m section through gravelly head was seen in a small quarry east of Axford [SU 2449 7034]. The deposit is a clast-supported gravel of broken flints with a clay matrix and irregular clasts of very soft white plastic chalk towards the base.

SU36NW

Gravelly head capping a hill top south of Froxfield [SU 300 670] appears to represent the residuum from a Palaeogene pebble bed.

SU36SW

In this area, some head was mapped in valley floors, but it also occurs on slopes and interfluves. For example, thick flinty wash on a low ridge west of Ham must predate the formation of the valley to the west.

The head in this area is mostly flinty, even on the Upper Greensand outcrop. The coarser components comprise predominantly angular flint fragments, but also include Palaeogene sandstone (sarsen) and Tertiary pebbles.

SU38SW

Some of the higher ground carries a thin capping of a flinty superficial deposit [SU 342 838], [SU 316 824], [SU 341 818]. No exposures were found and the deposit was recognised only by a very dense concentration of coarse, angular, white-coloured flint gravel and pebbles. This deposit occupies a similar position to the clay-with-flints, except that it is significantly lower than the projected level of the basal Palaeogene unconformity. There is no evidence for a significant clay or sand component in the deposit.

It is likely that this deposit represents a residuum derived by dissolution of the Chalk over a very long period. It is thus equivalent to clay-with-flints, but without a residual component derived from the Palaeogene. Flint nodules released from chalk bedrock by dissolution have been shattered by prolonged exposure to frost during the Quaternary and bleached by other weathering processes.

To the west [SU 305 804] and north-west [SU 303 828] of Upper Lambourn, head deposits occur on broad valley-side benches or terraces, extending at least 1 km along the valley respectively on the north side and the south-west side. These presumably represent solifluction deposits formed in the upper reaches of the catchment during a time of higher river base-level, and can perhaps be correlated with the Beenham Grange Gravel.

SU47SW

The field slips of F J Bennett (recording field observations of c. 1887) note an old gravel pit intersected by a cutting on the Lambourn Valley Railway, to the north of the road skirting Welford Park (north and west of Welford) [SU 406 736]. This pit showed about 12 feet [3.7 m] of gravel with sarsens at the eastern end. The section included fine chalky sand or loam (on an irregular gently dipping contact) with seams and pockets of coarse gravel or a chalk and flint gravel. The chalk gravel seems to be composed of Chalk Rock. This was originally interpreted as a terrace gravel but is here regarded as gravelly head derived from a degraded terrace.

Bennett also noted coarse sand and many sarsens in a shallow cutting just north of Easton Farm [SU 4185 7247], similarly interpreted as head derived from degraded river terrace deposits.

He recorded that the chalk was almost bare at the north end [SU 423 720] of the railway cutting west of Westbrook, with many large flints resting on an irregular surface of chalk. In the southern part [SU 424 719], the drift becomes thicker in deep pipes of clay-with-flints and very coarse gravel with many large sarsens. South of the path (about midway) ‘the drift gets more loamy gravel, in places very chalky and almost made of it.’

SU48NE

Jukes-Browne (1889) observed that the ‘gravel and loam’ seen overlying the Chalk in the Chilton railway cuttings sets in about 140 m north of the bridge 800 m east-south-east of Chilton Church. It is here a yellowish gravel, he stated, consisting of angular flints and chalk rubble in irregularly stratified beds, with occasional layers of brown loam. This continues south for about 450 m with a thickness of 3.6 m to 4.9 m, and then thickens to more than 9 m, that being the depth of the railway cutting. He found that the surface of this gravelly deposit sloped gently from north-west through Chilton towards the south-east, and considered that the presence of broken, unworn flints was incompatible with prolonged transport by water. The predominance of chalk and flint debris, with a minor component of sarsen, demonstrates local derivation, with a catchment entirely within the Chalk outcrop.

Numerous mammalian remains were found near the base of the deposit, near its northern end. A deposit of white and buff marly loam with numerous molluscan shells (mainly terrestrial but including some aquatic species) occurs near the more southerly bridge [SU 4958 8517] (Jukes-Browne, 1889).

Organic and fluvial deposits

Peat

At Newbury and for some distance upstream, the flood-plain deposits of the River Lambourn and the River Kennet are dominated by peat. White (1907) noted that this comprised a black or dark-brown deposit consisting of decomposed moss, sedge and bracken, with the leaves and branches of alder and other trees. It ranges in thickness to more than 4.5 m. During the 18th and 19th centuries, the peat near Newbury was extensively dug and burnt; the ash being used as fertiliser. In addition, much of the floodplain has been modified by drainage works, sluices and the like.

In addition, unmapped deposits of peat are known to be present within the alluvium. For example, site investigation boreholes on the line of the M4 demonstrated the presence of several metres of peat within the River Lambourn floodplain.

Alluvium

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

Holyoak (1980) divided the floodplain deposits of the Kennet valley into 2 broad groups: the floodplain gravels (gravels underlying the modern floodplain, and now treated as the Heales Lock Gravel), and other floodplain deposits (mainly fen peat and tufa, with minor organic mud, clay, silt, sand). He noted that some of the ‘Valley Gravels’ of the old Hungerford sheet could be assigned to the ‘floodplain gravels’, whereas some belong to low terraces. He also noted that the Hungerford sheet shows the peats as ‘alluvium’, and that tufas, which occur in most parts of the Kennet valley between Hungerford and Reading, ‘are very inadequately mapped by the Geological Survey’. The recent survey recognised a much greater extent of peat deposits within the River Kennet flood plain. It was otherwise not practical to subdivide the alluvial deposits.

Alluvium can be expected to pass upstream into river terrace deposits (as in the Pang valley) or head.

SU46SW

In general, the alluvial sequence of the River Enborne is particularly gravelly. Borehole (SU46SW/1) [SU 4365 6336] proved a 1.2 m thickness of ‘clayey’ material with 62 per cent gravel consisting entirely of nodular flint.

Tufa

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.

River terrace deposits

River terrace deposits occur widely in the south of the area and have long been noted as a significant feature of the landscape around Newbury. Until recently, a single outcrop of the Silchester Gravel, sloping eastwards at about 1 in 700, supported the longest (c. 3.6 km) airfield runway in Europe, at Greenham Common. The terraces 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 (Table 4). 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, and 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 floors 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, although some boreholes prove as much as 7.5 m. Typical proved thicknesses are in the range 3 to 4 m. Their composition is dominated by water-worn flint, with minor amounts of sandstone, vein quartz and ironstone, in a matrix of fine- to coarse-grained sand, which can be silty and clayey in places. The river terrace deposits are characterised by a high proportion of weathered, broken, nodular flints, up to about 50 mm across, which are mostly white or grey and subrounded to rounded. Well-rounded, black flint pebbles derived from the Palaeogene are also common, typically forming between 5 and 20 per cent of the deposit (White, 1907), but locally up to 50 per cent. With the exception of a wind-blown fine-grained fraction, it is likely that material was derived entirely from formations presently found within the Kennet catchment, at least in the lowest six terraces (Chartres, 1981). The structure of these gravels is variable and they are often unstratified, although the thicker deposits can be stratified (White, 1907). Cross-bedding can be seen locally.

River terrace deposits normally underlie what appears to be uniformly level ground (which is, in reality, very gently sloping). The limit of a particular terrace deposit against the valley-side (the ‘outer’ edge) will generally be mapped at a negative break of slope (i.e. an upwards increase in slope gradient) marking the edge of the corresponding area of level ground. In some places this break of slope is very clear and the boundary to the river terrace deposit can be mapped accurately and consistently. In other places, it is more subdued, and perhaps masked by head, so the accuracy of mapping the boundary diminishes accordingly. Locally, as west [SU 3983 7316] and south [SU 4075 7178] of Welford, the topography at a terrace margin has been reversed by preferential erosion of what was the valley side, leaving the gravelly deposit standing on a slight eminence.

The ‘inner’ edge of the terrace deposit, if any, where it has been eroded following a fall in river base level, is commonly marked by a distinct positive break of slope (i.e. an upwards decrease in slope gradient), this being generally less well defined where older terrace landforms have become degraded by soil creep, solifluction and erosion. The base of the terrace deposit is some distance below this break of slope. It is rarely exposed and commonly masked by gravelly head formed by downslope mass movement of the terrace deposit. It is marked locally by a spring line or a break of slope, but otherwise can be mapped accurately only if the thickness of the undisturbed terrace deposit is known from boreholes or exposures. In practice, the outcrop of a terrace deposit is often drawn to show it abutting a younger one (or alluvium), even where the two might be separated by a narrow outcrop of bedrock. The assumption that two adjacent terrace deposits are contiguous usually makes little difference to the mapped area of the older deposit, but it might cause its base to be shown at a lower altitude than it is in reality.

During the early surveys, river terrace deposits were classified according to the height of their surface above sea level. There has been a gradual shift to modern practice, which recognises not only that river terrace deposits slope parallel to the axis of the river that deposited them, but also that the most important level for correlation is the base of the terrace deposit, relative to the surface of the corresponding modern flood plain. Although this realisation has been accompanied by an increase in borehole and other records proving the base of river terrace deposits, coverage of such data remains patchy. Moreover, as explained, the accuracy with which the base of a terrace deposit can be mapped is limited.

The older terrace deposits probably represent extensive former alluvial plains of subdued relief, over which flowed braided rivers. They have been extensively dissected by subsequent erosion of the river valleys to progressively lower levels. Those remnants that have escaped erosion can be expected to 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 is 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. In general, if a gravelly deposit retains a recognisable ‘terrace’ landform, with a flattish top and a base at a reasonably consistent level, it has been mapped as a ‘terrace’. Otherwise similar gravelly deposits with no associated terrace landform have been mapped as head.

Recent resurvey of the Newbury and Reading districts, on 1:10 000 scale Landplan base maps with a 5 m contour interval, has enabled a systematic reinterpretation of the Kennet terrace system. The river terrace deposit correlation (Figure 8) 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. Thomas (1961) produced a map that resembles the present one more closely than that of any subsequent work. Differences in interpretation are mostly minor, as noted in the following text. 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 ‘Fourth to Seventh, undivided’, can be indicated.

Where a range of numbers is given, e.g. T5T7, this implies that elements of the numbered terraces, possibly with any or all of the intervening terraces are present, but that they cannot be subdivided. For example, west of Boxford, gentle planar slopes are present between 140 m and 123 m OD, across the span predicted for the Fifth, Sixth and Seventh terraces. These deposits are likely to include some gravelly head also, for example where the lower margins of the older terrace elements have been degraded, so that they merge into the younger terrace elements.

Areas designated as undifferentiated terrace deposits cannot be assigned to a particular terrace or range of terraces, typically because they have been extensively degraded and remobilised by solifluction. They are therefore very likely to include a significant component of head gravel.

In general, the terraces developed on the Palaeogene outcrop indicate the previous existence of broad valleys, which tend to migrate southwards. However, once the valley incision reached the Chalk, it became narrower, perhaps entrenched, and tended not to migrate. This is taken to reflect the propensity for surface water to drain into the Chalk, but not through the Palaeogene succession.

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 drainages. However, elements of the Bucklebury Common 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 the 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 Lambourn Valley seems then to have migrated slightly northwards, contrary to the regional dip. This argues strongly for structural control of the valley.

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, whose 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.

Occurrences of Thomas’s (1961) Upper Winter Hill Terrace at Beenham and to the east can be correlated with Terrace 7 (Beenham Stocks Gravel). However, his occurrences of the same terrace from just north of Woolhampton and to the west, correlate with Terrace 6 (Silchester). This difference seems to have arisen through a miscorrelation between Beenham and Woolhampton by Thomas, who was using an aneroid barometer to determine altitude. Similarly, two western occurrences of the Silchester terrace shown by Thomas (1961) south of the Kennet are now found to be parts of Terrace 5.

Five low-level terrace deposits are preserved in the valleys of the Kennet, the Lambourn and the Pang.

The base of the Hamstead Marshall Gravel (Fifth terrace), which is up to 3 m thick, rests on bedrock some 35 to 40 m above the modern flood plain. It was correlated by Bridgland (1994, p. 145) with the Silchester Terrace, but long-valley profiles show clearly that it is younger. It is preserved in a series of outliers on a section of the watershed between the Kennet and the Enborne, like the Greenham Common spread.

Small areas representing an unnamed Fourth Terrace Deposit are recognised tentatively in only a few places, all in the Lambourn catchment. The deposit is therefore not named. It is not equivalent to the Boxford Member of Collins (1999), which is said to be ‘confined to the Lambourn valley’. The height range (123–127 m OD) of the deposit at the given locality (a previous shallow quarry west of Boxford, [SU 415 717]) is consistent with that of Terrace 5, rather than Terrace 4. Moreover, the same deposit continues westwards upslope to a well-marked erosional back-feature at 140 m OD [SU 407 718], consistent with the lower levels of Terrace 7.

The deposits of the Second (Beenham Grange Gravel) and Third terraces (Thatcham Gravel) can locally be divided into upper and lower levels, separated at the surface by breaks of slope. Of the post-Anglian terraces, only the Second and (in the Reading district) Third have been specifically identified in the Enborne valley.

Terrace 2 (Beenham Grange) has been divided at Newbury and locally upstream into a lower 2a and an upper 2b, separated by a weak topographical feature. A similar feature occurs locally downstream of Newbury, in the vicinity of Thatcham, for example, but could not be mapped consistently. Moreover, borehole records provide no evidence for any consistent difference in height between the base of the deposits underlying Terraces 2a and 2b. The two facets are treated as a single terrace (2) in many places, and the difference in height is tentatively attributed to erosional processes rather than aggradation.

However, Terrace 2a seems to correspond to the Beenham Grange Terrace of Cheetham (1980), and Terrace 2b to his Thatcham Terrace I. Cheetham’s (1980) Thatcham Terrace II correlates with Thatcham Gravel (Terrace 3) of this work.

Occurrences of Thomas’s (1961) Lower Taplow Terrace west of Newbury are here identified as Terrace 2b, and his Upper Taplow Terrace as Terrace 3. Occurrences of the Lower Taplow Terrace noted by Thomas (1961) east of Newbury, however, correspond to Terrace 3.

In common with the other terrace deposits, the Beenham Grange Gravel is mainly composed 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).

The Heales Lock Gravel (First Terrace) 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, sand and gravel extend as much as 11 m below the flood plain surface in places, more than twice the usual depth.

The Beenham Grange Gravel has been shown by radiocarbon analysis and molluscan faunas 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). Holyoak (1983) described a large mud clast, inferred to be of Ipswichian interglacial age, excavated from low in the Heales Lock Gravel at Thatcham [SU503 666], suggesting that in situ deposits of this age might occur nearby.

Much information on the river terrace systems around Reading is given by Bridgland (1994), Gibbard, (1982, 1985, 1999) and Mathers and Smith (2000).

Details

SU36NW

Undifferentiated river terrace deposits occur in isolated patches on both flanks of the Kennet valley [SU 327 694], [SU 344 697], [SU 345 680]. These deposits are characterised by a brash of nodular flint. The gradient on the terrace towards the valley is relatively shallow in comparison to that seen on similar deposits nearby to the north (SU37SW). The deposits have clearly been degraded by solifluction and weathering, and although planar terrace features are present in places, the surface is commonly gently rounded. The base of the deposit is between 113 and 140 m OD where it overlies Seaford Chalk. In the north, the terrace deposit is cut into segments by minor tributary valleys.

SU37SW

Undifferentiated river terrace deposits on the northern flank of the Kennet valley, north of Chilton Foliat [SU 305 715], are characterised by a heavy brash of very coarse, angular, nodular flint. They abut the clay-with-flints above an elevation of 151 to 156 m OD. The terrace feature has a steep surface gradient of about 1:15 towards the valley, and has clearly been degraded by weathering and solifluction. Although typical planar terrace features can be discerned, the ground surface is commonly gently rounded. A weak negative break of slope is present in some places a little over half way down the terrace at 140 to 145 m OD. The base is between 118 and 126 m OD where the undifferentiated river terrace deposits overlie Seaford Chalk. The River Kennet is somewhat lower, at 100 to 105 m OD, and the First Terrace reaches only 110 m OD. This outcrop of undifferentiated river terrace deposits is cut into segments by minor tributary valleys.

Similar deposits to the west have been classified as head.

SU46NE

Hawkins (1924) briefly described a large gravel pit immediately north of Newbury Station [SU 4719 6678], noting that the river terrace deposit (here the Beenham Grange Gravel) consists almost exclusively of flints and sarsens embedded in seams of greenish clayey sand. The pebbles are very coarse, with some sarsen fragments measuring more than 1 foot [0.3 m] across. Most of the flint pebbles are very well-rounded, with a green coating, characteristic of the Upnor Formation. The base of the gravel was found to be notably uneven, attributed to channelling of the underlying Upnor Formation. A well-marked channel, infilled with brown sand, was seen in the gravel itself. Hawkins (1924) also noted that remains of mammoth and reindeer have been found in neighbouring exposures of the same terrace, although no definite flint implements had been found.

SU46NW

Collins (1999, p. 51) described a locality at Furze Hill (near Stockcross) [SU 426 687] as the stratotype of the ‘Bucklebury Member’ (= Bucklebury Common Gravel, Terrace 8). However, it lies too low for this to be sustained, and must correlate instead with Terrace 6.

SU46SW

Isolated outcrops of undifferentiated river terrace deposits are present on the tops of many of the hills south of the Enborne valley. They form flat-topped hills with a north-easterly slope of about 1 in 60. Many of the outcrops have a long sinuous form up to 3 km long and ranging in elevation from 110 to 140 m OD. These deposits are characterised by a brash of weathered white and brown nodular flint. Fractured edges on the flint have been worn to form subrounded pebbles. The lithology of these deposits thus differs markedly from the river terrace deposits on the northern flank of the Kennet valley (SU37SW and SU36NW), which are formed of coarser nodular flint that shows little evidence of weathering.

SU47SW

White (1906, p. 351) recorded 2.4 to 3 m of well-stratified and cross-bedded gravel, consisting of pebbly and subangular flints, worn pieces of sarsen, small quartz pebbles and bits of ironstone, in the roadside pits on the summit of Basford Hill [SU 439 716]. White had collected a worked flint from this pit.

White (1906, p. 351) recorded that pits some 2 m deep near the cottages by the road in Winterbourne Wood [SU 445 718] exposed well-stratified and pebbly gravel forming a shelf on the southern flank of the ridge. This also yielded a worked flint.

Notes from F J Bennett’s field slips (dated about 1887) record a gravel pit about 100 m north-west (not north-east, as was recorded) of Tulloch Farm [SU 4078 7230] that exposed about 2 m of coarse to fine, subangular gravel with some pebbles and sarsens, and a pipe of fine buff sand (other notes for this locality are illegible).

The high level remnant north of Hoe Benham could have been assigned to Terrace 8 (Bucklebury Common) rather than Terrace 9 (Cold Ash). The preferred interpretation requires a slightly more gentle gradient for Terrace 9, rather than a slightly steeper gradient that would be required if it were Terrace 8. This interpretation is also more consistent with the height difference of the terrace remnant of at least 11 m above the nearest occurrence of Terrace 7. If this high-level remnant were part of Terrace 8, the height difference of only 2 to 4 m found elsewhere in the district between Terraces 7 and 8 would be expected. Conversely, in the east of the district, Terrace 9 is between about 14 and 20 m above Terrace 7.

SU56NW

Holyoak (1983) noted the occurrence of fossiliferous silts of Late Devensian age within the Heales Lock Gravel at Thatcham [SU503 666].

SU57SW

One of the main terrace deposits occurs on the prominent flat-topped ridge between Cold Ash Common and Grimsbury Castle. It consists of an unstratified flint gravel. It has a shallow surface slope from 158 m OD at Grimsbury Castle to 153 m OD on the southern margin of the area, which equates to a gradient of about 1:420. Two mineral assessment boreholes (SU57SW/29) and (SU57SW/30) prove it to contain about 65 per cent gravel, 20 per cent sand and 15 per cent silt and clay, and to have a thickness of about 3 m.

A similar deposit at Snelsmore Common slopes from 148 m OD at Bussocks Camp to 135 m OD in the south. Mineral assessment borehole (SU47SW/31) shows it to be 2.4 m thick and to consist of about 60 per cent gravel, 20 per cent sand and 20 per cent silt and clay. There is a general lack of stratification (White, 1907), except that attributable to cryoturbation and mass movement.

The two deposits adjacent to the Winterbourne Stream at Red Barn Cottage [SU 4633 7355] have a similar lithology, being dominated by a coarse nodular flint gravel, but have a steeper slope towards the stream of 1:23. The lower part is only a few metres above the stream.

The previous map of the district shows outcrops of ‘Plateau Gravel’ on the east side of the valley just south of Hermitage [SU 505 726] and on the Yattendon road just north of the M4 motorway [SU 524 742]. Following resurvey, these areas are now included in the Lambeth Group. Superficial gravel, which may have been observed during the original survey, is here thought to represent thin head deposits.

SU57SE

Notes on the back of a field slip for 1:10 560 sheet Berkshire 28 SW/W, written by F J Bennett in about 1887, state that the Burnt Hill Gravel Pit [SU 5678 7409] shows 7 feet [2.1 m] of pebbles and fine ?subangular gravel. The notes provide a sketch and some rather unclear details.

SU58SW

The surface of gravelly superficial deposits in the floor of the Compton Gap slopes southwards more gently than the thalweg south of Compton. It grades instead towards occurrences of Terrace 3 in the Pang Valley.

Dissolution hollows, sinks and springs

Closed topographical depressions are common in parts of the south and east of the area, particularly close to 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 the overlying deposits.

These depressions are typically in the range 25 m to 50 m in diameter, and 1 m to 10 m deep, being generally either circular or oval in plan and saucer- or cone-shaped in vertical section. The amalgamation of contiguous hollows can lead to the formation of larger structures of more complex shape.

The largest and deepest of these structures occur at the base of the Lambeth Group, penetrating the top of the underlying Chalk. These act as sinks for minor streams draining the Palaeogene outcrop. However, most such depressions, including the smaller, shallower ones, have no afferent drainage channels, and many occur in otherwise fairly level ground on interfluves. In general, these are interpreted as dolines, or dissolution hollows, marking the site of gradual dissolution or collapse of the underlying chalk. Excavations that intersect such structures show them to be typically infilled by clay-with-flints, perhaps with recognisable remnants of Palaeogene deposits.

Most of the sinks and dolines occur in the lower part of the Lambeth Group, although some are in the Chalk. The lower part of the Lambeth Group consists largely of sands with a moderate to high permeability. Groundwater moving down through these sands may concentrate at low points on the Chalk surface and where there are fractures or joints, the latter being developed by karst activity. Lithological variations within the Lambeth Group, giving rise to different permeability characteristics, may also influence the development of karst features. Collapse of the Chalk due to the development of karst features will be transmitted through overlying sands, giving rise to collapse features at the surface.

Locally, as in the Pang valley, successive stream sinks occur in a chain of dolines. The initial stream flow is normally lost in the upper sink, but as the flow increases this becomes flooded. Excess water then flows on down to the next sink where it goes underground. This situation will repeat itself into lower sinks and ultimately flow would continue overland to the Pang. In periods of very high flow, sinks can convert to discharge points.

Most springs develop in the London Clay and the Lambeth Group, where permeable horizons underlain by clay bring groundwater to the surface.

Some dolines may have been enlarged by small-scale working for chalk, clay or sand. Indeed, some closed depressions shown as dolines on the accompanying map may instead mark the sites of ‘chalk-mining’ (described below under ‘Worked ground’).

Details

SU26NE

Numerous small stream sinks and dolines occur around the margins of the Palaeogene outcrop. One of the best examples of karstic features in the area is a complex of stream sinks and dolines in a copse, east-south-east of Bewley Farm, Great Bedwyn [SU 269 653]. Here, two large stream sinks occur. One at [SU 2690 6530] is a very large closed depression up to 10 m deep and 100 m across. At least two small streams sink into the Chalk at this point. Another large stream sink and blind valley, 4 to 6 m deep and 80 m across, occurs 150 m to the south. Other examples occur around Bewley Farm [2645 6528, 2665 6563], in Little Bonning’s Copse south-east of Little Bedwyn [SU 2975 6542], and north of Chisbury.

The Froxfield stream rises in the old gravel pits west of Harrow Farm [SU 267 680].

SU26SE

Numerous small stream sinks and dolines occur around the margins of the Palaeogene outcrop, especially near Wilton Brail and Bedwyn Brail. A complex of stream sinks and dolines occurs in a copse east-south-east of Stokke Manor [SU 267 648]. Other examples occur around Folly Farm at [SU 2938 6378], [SU 296 635] and [SU 296 630], Foxbury Wood [SU 294 646] where three small streams sink, and at several sites around the margins of the spur near Bloxham Lodge [SU 263 637].

SU36NW

A large spring (estimated at about 300 l/s) emerges from the Seaford Chalk adjacent to the flood plain, about 2 m above river level, on the south side of the River Dun, west of Hungerford [SU 3187 6797]. This presumably represents the emergence of conduit flow within the chalk.

SU37NW

A major spring at Lambourn [SU 328 793] is presumably the perennial head of the River Lambourn.

SU37NE

Solution features are common on the outcrop of the Seaford Chalk, particularly where a thin clay-with-flints cover is present or is nearby. Examples were seen at [SU 3696 7521], [SU 3802 7788] and [SU 3835 7971].

SU37SE

Several stream sinks were noted in New Copse, south of Wickham. The largest of these occurs at [SU 3905 7145] where a small stream sinks in a blind valley 3 m deep. White (1907) observed several strong springs in New Copse, from one of which pipes were laid to supply a nearby farm.

A series of small discrete springs occurs in the Lambourn valley at Weston, emerging from gravel in the left (northern) bank between Elton Farm and Weston Mill. These springs combine to create a sizeable stream.

SU38SW

At high flow levels, the River Lambourn can exhibit flashy flow from within the village of Upper Lambourn [SU 314 805] after heavy rainfall.

SU57SW

There are numerous dolines in the area between Hermitage and Curridge, perhaps a consequence of the absence of any significant clay bed above the Chalk. However, two lakes in the south-west of Hermitage [SU 5017 7254], [SU 5049 7290] appear to represent perched water tables approximately 20 m above the regional water table. A long-term local resident confirmed that these hollows did not represent old workings and reported that he had watched the more north-easterly one grow from nothing to being 1 m deep and 30 m in diameter over the past 40 years. He also reported difficulties with running sand during construction at a local housing development [SU 5053 7289]. This suggests that a perched water table is present in the local Lambeth Group sands. South of Curridge, the valley becomes wider, with a flat floor, and with numerous sinks and dolines on both the Seaford Chalk and the Lambeth Group. A small flow across fields at the southern margin of the area was observed following wet weather in February 2001.

Chieveley Service Station [SU 480 726] lies in a dry valley with numerous karst features on the Seaford Chalk, the Lambeth Group and the London Clay. A number of springs and sinks to the south of the M4 motorway became active during the winter of 2000/1. These mostly occur within the Lambeth Group, with some of the springs in the lower part of the London Clay and some of the sinks in the top of the Seaford Chalk. Flowing springs issued from the London Clay in the Gully [SU 479 703] in February 2001, with flow continuing across the Lambeth Group to the south of the area. In the tributary valley on the southern margin of the area, to the south of Snelsmore Common, springs were rising from high in the Lambeth Group in February 2001, with sinks in the Chalk on the edge of the area.

There is a series of springs and sinks in a pasture field with much undulating ground just below Frilsham village [SU 5440 7303] (visited on 23rd August 2000). There is a small upper spring with little flow in the upper part of the Reading Formation. Just below, a second spring joins the flow giving a total of about 1 l/s, but this dissipates after about 30 m with dry ground below. Below is a steep gradient with a dry hollow at the base, possibly indicating the site of a further spring and sink. Below this there is a spring, which flows into a narrow channel and discharges down an open sink adjacent to a hedge, which is on the high rim of the sink. Drainage pipes bring flow from the surrounding area, possibly from other springs, and discharge directly into the sink. The catchment of these springs must be a fairly limited area, underlain by low permeability London Clay.

A string of open sinks occurs over a distance of about 700 m in the dry sand and pebble bed of the stream adjacent to Holly Lane [SU 531 707], about 2 km east of Cold Ash, and in tributary valleys. The sinks are all in the Lambeth Group, from close to the Chalk contact up to about 5 m below the London Clay contact and with an elevation range of about 10 m. The occupier of Coles Farm [SU 5363 7115] confirmed that the system flowed each winter, and some years the overland flow reached the River Pang.

A number of spring and sinks sites have been identified in the Wellhouse area [SU 521 725]. Some form ponds, and there are also numerous dolines. A similar cluster of features was also found south and south-west of Boars Hole Farm [SU 522 719]. Both clusters occur in the lower part of the Lambeth Group. An active spring and sink system was surveyed in Briff Lane [SU 547 705] in August 2000, with spring flow of <1 l/s from the Lambeth Group and the sink occurring in the top of the Chalk outcrop. According to the occupier, the pond at Hawksridge Farm [SU 5476 7239] (in the Lambeth Group) is spring fed and higher springs are active during the winter. The disused pit in Seaford Chalk to the south-east would appear also to be a sink in this system and there is a further significant subsidence feature in the field to the south.

Artificial deposits and worked ground

Worked ground

Two broad categories of worked ground are represented in the area: engineered road or railway cuttings and surface mineral workings.

Engineered cuttings are found along most of the major roads and the railway lines. Disused railway lines lie between Didcot and Newbury (via Chilton, Hampstead Norreys and Shaw) and in the Lambourn Valley. The latter once had a branch line leading to RAF Welford.

Pits and quarries, mostly small and mostly disused, were opened for a variety of mineral commodities: chalk, flint, building stone, sand, gravel, clay and peat. Relatively extensive and deep gravel pits occur east of Newbury.

In addition to open workings, some chalk was extracted by underground working. This mode of working typically involved the sinking of a shaft (or ‘chalk well’) about 1 m to 1.5 m in diameter and 5 m to 20 m deep, and the excavation of inclined ‘headings’ or adits from the base of the shaft. This enabled fresh, unweathered chalk to be won for lime burning or for spreading on agricultural land to ameliorate clay-rich or acidic soils. Such shafts might be sunk into the Chalk through a thin cover of superficial deposits, such as clay-with-flints, or the Lambeth Group (White, 1907, p. 116). It is supposed that gradual subsidence at the site of such excavations, even after backfilling, would lead to the formation of a closed depression of similar appearance to a natural doline (see preceding section).

Observations on the construction of ‘chalk wells’ for the excavation of agricultural lime were noted by F J Bennett on the back of 1:10 560 field slip Berkshire 35NW(W), dated April 1885. He stated that the deepest wells were 40 feet [12.2 m], and that the depth was according to the extent of land to be dressed. A well 20 feet deep [6 m] would dress 8 acres. The wells were enlarged at the bottom and three or four headings were driven. In one observed by Bennett, each heading was started just wide enough, 3 foot [0.9 m] across, for a man to enter (Figure 9). The heading was then worked towards the right and left (tangentially to the well shaft) and ascending up steeply, possibly at as much as 50 degrees. Each tangential heading was intended to meet its neighbour at what was known as an ‘angle’. This type of ‘chalk well’ was hence known as a ‘chalk angle’. The supports between the shaft and the headings are pillars, or ‘quoins’, about 7 feet [2.1 m] wide. When all the chalk wanted had been dug out the ‘quoins’ were knocked away as much as possible. The well then fell in, possibly after some time, leaving a ‘dell’ in the field.

Chalk wells intended to supply kilns were constructed more carefully as they were meant to have more long term use. They were often lined at the top.

Details

SU26NW

Valley head deposits have been worked for gravel at several points, principally in the Froxfield valley around Puthall Farm Gate, where old workings can be seen extending from [SU 2320 6785] to [SU 22247 6778]. These were worked for gravel and are typically up to 3 m deep. Gravel was also worked in Savernake Forest at various points.

SU26NE

Valley head deposits have been worked for gravel at several points, principally in the Froxfield valley around Golden Arrow bungalow where old workings can be seen extending from Harrow Farm [SU 2715 6787] to Knowle Farm [SU 2582 6786]. These were worked from gravel and are typically up to 3 m deep. Gravel was also worked near Upper Horsehall Hill Fm at [SU 2580 6669]. Some of these old workings have now been infilled.

SU26SE

The Reading Formation has been used for brickmaking, and old clay pits occur near Jockey Green [SU 290 642] where remains of an old brick kiln can be seen, and at Dodsdown [SU 273 622] where many old pits are now flooded.

SU27SE

An exposure of typical Seaford Chalk occurs at Southward Lane Farm [SU 2655 7477]. Chalk is exposed for some 50 m along a quarry face, cross-cut by several, widely spaced, steep normal faults, each showing variable (up to 5 cm) vertical displacement of a tabular flint horizon.

SU36NW

According to White (1907, p. 27), Seaford Chalk has been worked in a large quarry at Furze Hill, Chilton Foliat (on the south side of the River Kennet) [SU 321 698]. A very irregular junction of the Seaford Chalk with the Reading Formation was visible in a pit near the kiln (of the old Hop Grass Brick Works) in Brick Kiln Copse [SU 3205 6931], a little further to the south.

On the west side of the cart track leading from that kiln south to the Bath Road near Hop Grass Farm, the chalk was worked in a vertical shaft with lateral galleries or headings [SU 3217 6914].

SU36SW

The Kennet and Avon Canal was ingeniously constructed to follow the side of the Dun valley, close to the edge of the alluvial plain. The construction was presumably achieved by digging the canal into the valley side and using the spoil to build the lower side; thereby having to move the material only across the width of the canal. If this is the case in general, then the canal itself is largely worked ground and its downslope side is made ground. However, variations on this will occur locally and it may be that in places the canal is excavated wholly in the valley side, with the excess material used for locks, etc. Where the canal crosses the flood plain, both sides are largely made ground and the canal is mostly worked ground, but is made ground in the proximity of the aqueduct west of Hungerford.

Such variations are found in many other parts of the canal within the Newbury district. In many places they are beyond the limits of resolution at 1:10 000 scale, or cannot be identified with confidence.

SU57NE

Notes on the back of a field slip for 1:10 560 sheet Berkshire 28SW/E, written by F J Bennett in 1887, state that a chalk well at Basildon Kiln (i.e. a shaft for the extraction of chalk for lime-burning) [presumed 5861 7630] was 60 feet [18.3 m] deep, being about 20 feet [6 m] to the top of the Chalk. Bennett noted two horizontal headings in the well, respectively 8 feet [2.4 m] and 15 feet [4.6 m] long.

The precise location of this chalk well is unknown. Unless it was backfilled with carefully compacted material, it will probably remain a subsidence hazard.

SU57SE

Notes on the back of a field slip for 1:10 560 sheet Berkshire 28SW/W, written by F J Bennett in about 1887 state that ‘Chalkangles Copse’ [SU 5632 7396] illustrates the two methods of chalk extraction, one by the up-sloping ‘levels’ or ‘angles’ for land chalk, the other by the horizontal heading for kiln chalk [for quicklime]. This note implies that the copse is the site of a chalk mine.

A chalk mine in a depression in Larch Copse, Yattendon [SU 561 743] is described in the Records of the Chelsea Speleological Society, Volume 22, as having more than 100 m of passageways at a depth of less than about 10 m.

Made ground

Made ground refers chiefly to engineered embankments on the main roads, on railway lines, and along the Kennet and Avon Canal. 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.

Infilled ground

Areas delineated as infilled ground are mostly either disused mineral workings or disused railway cuttings in which waste material has been deposited. The nature of the fill and the degree to which it has been compacted are generally unknown.

Details

Waste disposal has been recorded at the following sites. This list is not necessarily complete.

SU26NE

Knowle Tip, Froxfield [SU 260 680].

SU27SW

Stock Close Tip, Stock Close, Aldbourne [SU 242 732].

SU36NE

Dark Lane, Denford, Hungerford [SU 354 689]. A disused chalk quarry.

SU37NW

Hill Drop Farm, Lambourn Woodlands, Hungerford [SU 323 753].

SU48SW

Tinken Corner, Farnborough, Wantage [SU 426 810].

SU56NW

Lower Way Lane Tip, Newbury [SU 504 670].

SU56SW

Limbercast Farm, Thatcham, Newbury [SU 534 648].

SU57NW

Haw Farm, Hampstead Norreys [SU 540 773].

Landscaped ground

Areas that are known to include worked ground or made ground or both, in some cases together with infilled ground, but where these categories cannot be reliably recognised or delineated, have been categorised as landscaped ground.

Details

SU46SE and SU56SW

The area previously occupied by the Greenham Common airbase has been extensively modified during original construction, demolition and removal of infrastructure, remediation of contaminated ground, and restoration of stony acid heathland.

Chapter 7 Structure

Structural setting

The area is underlain at depth by Precambrian and Palaeozoic formations, including part of the coal-bearing Berkshire Carboniferous depositional basin (Allsop et al., 1982; Foster et al., 1989; Mathers and Smith, 2000; Sumbler and Woods, 1992). The area lies at the southern edge of the London Platform, part of the Variscan foreland, with the northern limit of late Carboniferous Variscan deformation in the south. Within the Variscan foldbelt, deformation caused extensive north-vergent folding and thrusting of the Palaeozoic sequences, including the formation of east–west striking, high-angle reverse faults or thrusts at depth in the vicinity of Newbury. North of the Variscan front, there was broad, large-scale folding, but the region was otherwise relatively little affected (Peace and Besly, 1997; Sumbler and Woods, 1992).

The area lies at the northern margin of the Wessex Basin, a post-Variscan depositional basin that extends across central southern England and adjacent offshore areas. Faults on the northern margin of the basin are marked by periclinal folds in the Chalk to the south of Newbury that formed during Cenozoic structural inversion of the Wessex Basin (Chadwick, 1986, 1993; Hawkes et al., 1998; Underhill and Stoneley, 1998; Whittaker, 1985). The east–west-trending axis of the synclinal London Basin, which also formed at this time, passes just south of Newbury. The district thus lies at the western end of the London Basin.

Caledonian

No structures of Caledonian or greater age are known in the district, although north-westerly trending magnetic anomalies seen to the north-east of the district (Figure 3b), attributable in part to Silurian and Precambrian igneous rocks, reflect the probable structural grain in the local basement.

Variscan

The presence of the Variscan Front across the centre of the district is suggested by the southward termination of the 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 Wessex Basin (Figure 2a) and (Figure 2b) (Busby and Smith, 2001). From mid Carboniferous to early Permian times, Variscan folding and thrusting emplaced early Carboniferous and older strata towards the front from the south, although no individual Variscan structures have been identified locally. They are presumed to lie south of the late Westphalian strata of the Berkshire Coal Basin.

The small Berkshire Coal Basin, en echelon to the south-east of the Oxfordshire Coal Basin and aligned north-west to south-east was probably initiated by early Variscan uplift of the Worcester basement (Smith, 1993). The uplift of the Worcester basement flexed down the margin of the Midlands Microcraton providing accommodation space for the basin sediments. The basin was truncated to the south-east by later Variscan uplift along the Variscan Front, forming a syncline with a steeper southern limb.

The configuration of the Devonian and early Carboniferous basin prior to this deformation is unclear. Early uplift within the Variscides to the south may have caused the Frasnian (Late Devonian) marine transgression to encroach onto the subsiding foreland.

Alpine

Formation of the north-south trending Worcester Basin to the west of the district and the east–west Wessex Basin, to the south, began a new cycle of crustal development (Whittaker, 1985). The synsedimentary growth faulting, which bounds these basins, reactivated Variscan thrusts and continued spasmodically from Permian to 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 2b) & (Figure 4). A footwall shortcut developed by some of these faults means they reach the surface to the south of the district (e.g. the Vale of Pewsey Fault).

Faulting also took place on north-west to south-east axes during this period, both on the London Platform (Arkell, 1947; Mathers and Smith, 2000) and in the Wessex Basin (Karner et al., 1987). 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.

By Early Cretaceous times the basins had filled with syn-rift sediments, and a phase of thermal relaxation subsidence caused more widespread subsidence of the London Platform as well as the basins. An unconformity developed at the base of the Lower Greensand in the district reflecting this change in basin subsidence. Further east the Lower Greensand is overlapped by an easterly thickening Gault Clay (Table 2).

Opening of the North Atlantic Ocean, oblique to the early Mesozoic basin trends, caused compressive stresses within the European Plate, which led to inversion of the main structures. During the Alpine Orogeny, inversion of the Wessex Basin was achieved by again reactivating the Variscan thrusts, as reverse faults in the Mesozoic cover (Whittaker, 1985).

The inversion of the Wessex Basin to form the Wealden Anticline 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. The marginal faults to the Wessex Basin mostly lie to the south of the district.

Bedding orientation

There are very few bedrock exposures anywhere in the area. Even where a reliable estimate of dip can be made at such an exposure, it is only of local significance. Therefore the following observations of bedding orientation in the Chalk are based on a three-dimensional computer model (Figure 10), (Figure 11), (Figure 12) and (Figure 13). This model was created from lithostratigraphical interpretations of geophysical logs from approximately 60 boreholes. The derivation of lithostratigraphical interpretations of the Chalk from borehole logs of resistivity and natural gamma is described by Woods (Woods, 2001b). These data were used to model surfaces representing the base of each of the Chalk formations, together with the base of the Palaeogene. Outcrop patterns from the previously published maps were used to control the intersection of these modelled geological surfaces with the digital terrain model representing the ground surface.

Structure contours for the surfaces representing the respective basal surfaces of the Lewes Nodular Chalk, the Holywell Nodular Chalk and the West Melbury Marly Chalk are shown in (Figure 11), (Figure 12) and (Figure 13). The surface model for the base of the Lewes Chalk (Figure 11) is considered to give the best overall impression of the local structure as it is based on a larger number of data points than the others. Moreover, identification of the Chalk Rock in borehole logs (either by the original drillers or in subsequent interpretations) is probably more reliable than for any other horizon in the Chalk.

In the north of the area, the regional dip direction is generally towards the south-south-east (N155º). Towards the south, this changes to a more southerly direction (N180º), normal to the axis of the London Basin syncline. The amount of regional dip generally varies between about 0.5º and 1º, locally increasing to as much as 2º.

South of the London Basin synclinal axis, similarly gentle northerly dips abruptly steepen to as much as 30º 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 notably 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 several kilometres long, the strike changing between one and the next within only a few hundred metres, at most. These inflections might occur at the intersection of northerly trending faults.

The estimated dip in the steep northern limb of the Vale of Ham fold is based on direct observation in disused chalk pits. The amount of dip cannot be determined at all accurately from the outcrop patterns, partly because the thickness of the Chalk formations is not well known in this area. Indeed, it is possible that strike-parallel reverse faulting has reduced the apparent thickness of the succession in the steep limbs of the folds. There is, however, no evidence that this has occurred, either in fracture patterns seen in exposures or in discontinuities in the topographical features marking the formation boundaries.

The outliers of Newhaven Chalk between Boxford and Winterbourne apparently mark a synclinal axis. However, there is no apparent expression of this structure at the base of the Seaford Chalk or of the Lewes Chalk. The outcrop pattern could instead reflect faulting, perhaps a reactivation of faults that controlled deposition of the anomalous chalk facies in this area.

Details

SU26SW

Chalk of the Holywell Chalk and the Zig Zag Chalk dipping 20º due north was observed in a temporary pit in the field between two old railway lines, 0.5 km east of the A346 at [SU 2256 6390] (Marlborough district).

SU36SW

In the area west of the easting of Ham Spray House [SU 345 641] and east of a point north-west of Shalbourne [SU 313 637], the Chalk dips consistently a little west of north (N350º) at about 20º. To the east of this section, it dips equally consistently to the north-east (N035º) and to the west, it dips to the north-west (N310º). In each case, the change in dip direction occurs within a strike length of only a few hundred metres.

Faulting and jointing

Few faults have been noted during geological surveying of the area, and those can be traced for only a limited distance. Those that are shown on the map mostly trend 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. The larger ones downthrow to the south-west, by between 5 m and 20 m. 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 as young as the Lambeth Group.

However, detection of small- to medium-scale faulting in unexposed ground is not always possible, especially where (as in this area) the bedrock units are relatively thick and superficial cover is extensive. Although small faults would be apparent in exposed sections, they could remain undetected or equivocal during even a detailed field survey of unexposed ground. Therefore, faulting could be more common than is apparent from the map.

Various medium-scale and small-scale perturbations are superimposed on the overall pattern of dip direction found within the 3D geological model. Medium-scale variations are expressed by local changes in strike direction, reflecting offsets between adjoining sections of the modelled formation surfaces. Within the resolution of the model, these offsets apparently persist through successive layers, indicating a series of north-west to south-east trending zones of displacement that offset the axis of the London Basin syncline; Lines A to D on (Figure 11), (Figure 12) and (Figure 13).

The density of data available is insufficient to show conclusively whether these offsets reflect fault zones or gentle monoclinal folds. However, coincident faulting can be demonstrated at outcrop in some places. Elsewhere, if these inferred faults reach the surface, 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.

However, even if the zones of displacement do mark a series of very gentle north-west to south-east trending monoclinal folds in the Chalk of this area, folds of this kind are likely to form above faults in the underlying formations, and to be marked by zones of enhanced fracturing within the Chalk. Such zones could include faults with displacements of up to, say, 5 m, which would be undetectable by field survey.

The significance of the small-scale perturbations in the modelled surface is difficult to assess; they appear to reflect either local structures or changes in formation thickness that cannot be delineated from the available data.

Tectonically controlled thickness variation within the Chalk is known from other areas of southern England (Evans and Hopson, 2000; Evans et al., 2003; Mortimore and Pomerol, 1991), and might be found in the Berkshire Downs. Faulting in the pre-Cretaceous formations could have been active during Cretaceous sedimentation, so inducing local thickness variations. For example, recent mapping has shown a marked change in thickness in the Upper Greensand from about 9 m just west of Wantage to about 40 m just to the east of that town. This coincides approximately with a marked reduction or local disappearance of the glauconite sand at the top of the Upper Greensand. To the west of this line, the outcrop patterns in the Chalk change, reflecting marked changes in the topography; in general, the Chalk escarpment becomes steeper. These changes might be controlled by faulting in a zone aligned with Line B on (Figure 11), with downthrow to the north-east. The occurrence of phosphatic chalks and hardgrounds in the Chalk near Boxford and Winterbourne (see sections on Seaford and Newhaven Chalk formations) might also reflect proximity to this structure.

To the north of the area, Line B is aligned with faults near Faringdon that appear to have exerted local control on the distribution of Lower Greensand strata (British Geological Survey, 1971; Ruffell, 1998), although these strata are preserved to the south-west of the fault zone, implying contemporary downthrow in that direction. The Berkshire Syncline, which preserves Carboniferous strata at depth beneath the Berkshire Downs, is elongated in a north-west to south-east direction (Allsop et al., 1982; Foster et al., 1989), possibly reflecting structural control by deep-seated fault zones.

It is assumed that linear valleys within the Chalk outcrop reflect zones of relatively dense jointing, although not necessarily accompanied by significant faulting. Identification of linear trends on digital terrain models of the area suggests there is a dominant fracture set oriented north-west to south-east. Fracture sets of this orientation appear to control the linear Lambourn valley, and linear valleys extending south-east from Chilton and Aldbourne, for example. Fracture systems of this orientation are widely developed in the Chalk of southern England (Bevan and Hancock, 1986). Subsidiary fracture sets in the Pang-Lambourn area are indicated by linear landscape elements oriented north-east to south-west and north to south.

Details

SU27SE

An exposure of typical Seaford Chalk occurs at Southward Lane Farm [SU 2655 7477]. Chalk is exposed for some 50 m along a quarry face, cross-cut by several, widely spaced, steep normal faults, each showing variable (up to 5 cm) vertical displacement of a tabular flint horizon. The orientation of the faults is difficult to determine but is probably east-south-east–west-north-west to south-east–north-west.

SU36SW

Three north-west-trending faults are inferred to occur in the south-east quadrant of the area, based on consistent displacements of the succession of geological boundaries. All downthrow to the west, one by about 20 m and the others by about 10 m and 5 m respectively. Although they were mapped mostly on the basis of breaks of slope, these were surveyed using hand-held GPS. They also coincide with very distinct steps in the otherwise smoothly profiled ridge.

A north-easterly fault of unknown throw between Buttermere and Inkpen Hill is inferred entirely from a similar step in the ridge, just west of Long Covert. This step is unlikely to be the consequence of the same north-westerly fault seen in the scarp face, assuming that both faults are approximately planar.

These faults are aligned with north-west-oriented linear valleys, near Buttermere in the south-east corner of the area, and passing through the village of Ham. The most westerly is also aligned with the valley through which the Shalbourne stream passes across the Chalk outcrop, but there is no evidence that the chalk has been displaced by faulting in this valley.

An old chalk pit just south of Sadler’s Copse [SU 3440 6448] exposes about 4 m of Newhaven Chalk. There are several strong, persistent joints subparallel to bedding, but none show evidence of slip. A fracture zone with slip planes dipping N057/60ºW occurs in the west end of the exposure.

SU38NW

A small fault mapped on Hackpen Hill [SU 3462 8523] downthrows 20 m to the west.

Chapter 8 Applied Geology

The most important geological 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 taken widely on a small scale, and peat was once dug from the River Kennet flood plain. Coal is present at depth, but has not been exploited (Table 5).

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 subsequently. 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 Subgroup. 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 in the Chalk are commonly enlarged by solution of the fracture wall in groundwater (Plate 1), creating zones of locally very high secondary porosity and transmissivity. For example, flow velocities as high as 6 kilometres 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 1960s and 1970s, the Chalk aquifer of the Berkshire Downs (and the Marlborough Downs, to the west) was the subject of a large investigation into the possibilities of augmenting the surface flow in the River Thames and its tributaries by pumping groundwater into natural streams (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 topography than to stratigraphy. The fracture systems providing the productive permeability of the unconfined aquifer tend to occur within about 60 m of the surface, extending to the greatest depths below valleys. Indeed, the main concentrations of fractures tend to coincide with valleys, 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).

Chalk water is normally of very good quality, in terms of chemical composition. Where the Chalk crops out, the groundwater is hard to very hard, with carbonate hardness predominating. However, at depth in the confined aquifer, the composition changes to a soft, sodium bicarbonate type as a consequence of ion exchange. The content of chloride ion and of total dissolved solids increases concomitantly (although not above the levels appropriate for drinking water). Fluoride also increases and anaerobic conditions set in, with nitrate being replaced by ammonia (British Geological Survey, 1978).

In the past, local water supplies were also obtained from the Upper Greensand, in the south-west, from sand beds in the Lambeth Group and London Clay, and from river terrace deposits. These minor aquifers tend to have small yields.

The Lambeth Group and London Clay comprise an interbedded set of high and low permeability layers, associated with the sand and clay beds respectively. The sandy facies are largely unsaturated, as they generally occur on the interfluves between the valleys, but perched water tables are likely to be present. Surface drainage on the clay sequences can disappear underground in a sink or swallow hole where it crosses onto sand beds. Down gradient, this water can be brought back to the surface as a spring where an underlying clay bed crops out. Where the dip of the beds is contrary to that of the topography, perched water tables with associated springs can occur in sand beds supported by underlying clay beds. It is possible to have a succession of these in some areas, and where the topographical gradient is high, the ground can be unstable and result in landsliding.

Where clay beds are thin, they can be breached by joints and fractures, opening a route for the groundwater to move down into the underlying sand bed. The Upnor Formation is known to have a clayey composition, in contrast to the overlying sandy facies of the Reading Formation, but this potential aquitard would appear to be breached in numerous places. The Lambeth Group sands may thus have partial hydraulic continuity with the Chalk.

Where river terrace deposits overlie clays, they can form discrete local aquifers (which could be perched a considerable distance above the regional water table), 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 taken 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’. Flintless, smooth-textured chalks, sawn into blocks about 30 cm across or more, have been used locally as building stone, as in Woolstone and some nearby villages (Arkell, 1947).

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 was taken 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, then burnt to produce ash for use a fertiliser or used as fuel.

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.

Details

SU26NW

In places, the clay-with-flints has been worked for brick making, e.g. at Brick Hill [SU 2375 6920].

Valley-floor head deposits have been worked for gravel in the Froxfield valley around Puthall Farm Gate. Here, old workings, typically as much as 3 m deep, can be seen extending from [SU 2320 6785] to [SU 22247 6778]. Gravel was also worked in Savernake Forest at various points.

SU26NE

The Reading Formation has been used for brickmaking, and old clay pits occur near Lower Farm, Chisbury [SU 2788 6682].

Valley-floor head deposits have been worked for gravel at several points, principally in the Froxfield valley around Golden Arrow bungalow, where old workings can be seen extending from Harrow Farm [SU 2715 6787] to Knowle Farm [SU 2582 6786]. These are typically up to 3 m deep. Gravel was also worked near Upper Horsehall Hill Farm at [SU 2580 6669]. Some of these old workings have now been infilled.

SU26SE

The Reading Formation has been used for brickmaking, and old clay pits occur near Jockey Green [SU 290 642], where remains of an old brick kiln can be seen, and at Dodsdown [SU 273 622], where many old pits are now flooded.

Several sand pits, some now infilled, occur in the Reading Formation around Folly Farm. In particular, a large sand pit occurs near Harding Farm, adjacent to the road, 2.2 km south-east of Great Bedwyn, where approximately 8 m of fine- to coarse-grained pebbly sand is exposed [SU 2955 6320].

SU47SE

The Old Kiln Pit, west of Curridge [SU 488 725], is an active sandpit abstracting from the Reading Formation. At the time of survey (2000), it was approximately half way through a planned seven-year life. Spoil from the current abstractions is used to backfill sections already worked.

There are numerous small, old, disused pits and quarries in the Reading Formation, used for brick and tile making, in the Seaford Chalk Formation, used for the extraction of chalk and flint, and in the river terrace deposits, used for gravel extraction. Most of these are completely degraded and some have been partly backfilled as waste tips.

SU57NW

Victorian large-scale topographical maps of the area show a brick and tile works about 1 km north-east of Hampstead Norreys [SU 5396 7728]. By 1910, however, the site was marked only as a ‘chalk pit’. Notes on the field slip for 1:10 560 scale sheet Berkshire 27NE(E) (held in BGS archives), annotated by F J Bennett in 1887, record that the base of the pit exposed 20 feet [c. 6 m] of chalk (Section 4.5.7). Several headings (adits) were driven in the chalk for lime. Clay was taken from the overlying clay-with-flints. Lime and bricks were burnt in the same kiln.

The fine- and medium-grained sands of the Reading Formation were being extracted at the Old Kiln Pit north-west of Curridge [SU 483 722] during 2000. In the past, they have been extracted at the Oare pit [SU 499 742], closed a short time prior to 2000, at the Cold Ash Farm Pit [SU 501 713], the Hermitage Pit [SU 493 732], the pit beneath the Curridge Industrial Estate [SU 502 719] and numerous old small-scale workings. All the disused major workings have been backfilled with the exception of the Oare pit, which is currently being used as a waste disposal site. The material is utilised as a soft building sand, but there can be difficulties working these sands because of the presence of clay and silt lenses of various scales within the deposit.

In the past, there have been several small pits working the sandy clays of the Reading Formation and the London Clay for brick and tile making using local kilns.

There are numerous, small, disused quarries in the chalk, mostly in the Pang valley. Some of these are now backfilled or partially backfilled. The largest is the Frilsham Chalk Pit [SU 539 729].

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 (Table 6). 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 landslides 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 landslides could be present, particularly where slopes on the London Clay or Lambeth Group exceed 7º. Slopes exceeding 3º 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 of as much as 40º. Similar effects might 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 also likely to be 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 m and 50 m in diameter and from 1 m to 10 m deep are common where the Chalk is overlain by the Lambeth Group or by clay-with-flints. The depressions occur both on the outcrops of 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.

In periods of very high groundwater level, sinks can become discharge points. For example, a sink used to dispose of water from part of the Newbury Bypass [SU 4703 7075] became a spring during a period of high groundwater levels in the winter of 2000/2001. Consequently the road is subject to flooding.

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 previous 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. Many springs show a seasonal variation in flow and some can dry up in the summer months. Conversely, spring flow can commence suddenly or increase sharply following a significant rainfall event.

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

The radioactive gas radon is a natural product of the radioactive decay of radium and uranium, which occurs widely in rocks and soils at very low concentrations, 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. Interbedded sandstones and siltstones of the Upper Greensand Formation and some areas where river terrace sand and gravel overlie the Seaford Chalk have the highest radon potential in the Newbury district. 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 NRPB where it is estimated that the radon concentration exceeds the Action Level (200 Bq m^-3) in 1 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 3 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. The full address is 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 www.bgs.ac.uk. This website also provides access to the BGS Enquiry Service.

Published information sources

Books

• British Regional Geology
• London and the Thames Valley, 4th edition, 1996 (Sumbler, 1996).

Memoirs and sheet descriptions

• The geology of the country around Henley-on-Thames and Wallingford (Sheet 254)
• The geology of the country around Marlborough (Sheet 266)
• The geology of the country around Hungerford and Newbury (Sheet 267)
• The geology of the country around Reading (Sheet 268)
• The geology of the country south and east of Devizes (Sheet 252)
• The geology of the country around Andover (Sheet 253)
• The geology of the country around Basingstoke (Sheet 254)
• Geology of the Devizes district (Sheet 282) - sheet description
• Note that all these memoirs are out of print but photocopies may be purchased from the BGS Library, Keyworth.

Sheet explanations

• Geology of the Reading district - a brief explanation of the geological map (Sheet 268)
• Geology of the Devizes district - a brief explanation of the geological map (Sheet 282)

Mineral assessment reports

• Reports of the Institute of Geological Sciences (now BGS) describing the sand and gravel resources of this district are available for the following 1:25 000 scale sheets:
• SU46 and 57, and parts of 36, 37 and 47 - Newbury. MAR 59 (1981).
• SU56 and 66 - Aldermaston. MAR 24 (1976).

Water supply papers

• The water supply of Berkshire from underground sources (1902).
• The water supply of Hampshire including the Isle of Wight from underground sources (1910).
• The water supply of Wiltshire from underground sources (1925).
• Water supply from underground sources of Reading-Southampton district (quarter-inch geological sheets 19 and 23, eastern halves): part 1 Well catalogue for new series one-inch sheets 253 eastern half (Abingdon), 254 (Henley-on-Thames) and 267 eastern half (Newbury) (1942).

Maps

• Geology
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• United Kingdom (South Sheet) Solid geology, 2007; Quaternary geology, 1977.
• 1:250 000
• Chilterns 51N 02W, Solid geology, 1991
• 1:50 000 and 1:63 360
• Sheet 252 Swindon (Solid & Drift), 1974
• Sheet 253 Abingdon (Solid & Drift), 1971
• Sheet 254 Henley-on-Thames (Solid & Drift), 1980
• Sheet 266 Marlborough (Solid & Drift), 1974
• Sheet 267 Hungerford (Solid & Drift), 1898
• Sheet 268 Reading (Solid & Drift), 2000
• Sheet 282 Devizes (Solid & Drift), 2008
• Sheet 283 Andover (Solid & Drift), 1975
• Sheet 284 Basingstoke (Solid & Drift), 1981
• 1:10 000
• Details of the previous surveys are given on the 1:50 000 or 1:63 360 scale geological sheets. Copies of the fair-drawn maps arising from these earlier surveys may be consulted at the BGS Library, Keyworth.
• During the latest resurvey, the relevant parts of the component 1:10 000 scale National Grid sheets were geologically surveyed by:

These maps have not been published but are available for public reference in the Libraries of the BGS at Keyworth and Edinburgh, and at the BGS London Information Office in the Natural History Museum, South Kensington, London. Copies for purchase are produced according to demand and are available from the BGS Sales Desk.

Geophysics

• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996.
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996.
• 1:250 000
• 51N 02W Chilterns, Bouguer gravity anomaly, 1991.
• 51N 02W Chilterns, Aeromagnetic anomaly, 1991.
• 1:50 000
• A geophysical information map (GIM) at the scale of 1:50 000 is available for the district. This shows information held in BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.

Geochemistry

• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain (South Sheet) 1995.
• Radon potential based on solid geology, Great Britain (South Sheet) 1995.
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet) 1995.

Hydrogeology

• 1:625 000
• Hydrogeological map of England and Wales, 1977.
• 1:100 000
• Hydrogeological map of the south-west Chilterns and the Berkshire and Marlborough Downs, Sheet 7, 1978.
• Groundwater vulnerability map, Upper Thames and Berkshire Downs, Sheet 38, 1995.

Mineral resources

• 1:1 000 000
• Industrial minerals resources map of Britain, 1996.

Unpublished information sources

Documentary collections and indexes

Borehole records

Borehole records for the district are catalogued in the BGS archives at Keyworth, according to 1:10 000 scale sheet areas. For further information contact: The Manager, National Geological Records Centre, BGS Keyworth. An Index to these borehole records can be consulted through the Geoscience Data Index at www.bgs.ac.uk/geoindex/home.html.

For information on water wells, springs, aquifer properties and water borehole records, please 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 BGS maps, including those shown on the 1:50 000 geological sheet 267 (Newbury) are held in the BGS Stratigraphical Lexicon. This can be consulted on the BGS web site: http://www.bgs.ac.uk. Further information on this database can be obtained from the Lexicon Manager, BGS Keyworth.

BGS Photographs

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

A11067 1967

A11070 1967

A11071 1967

A11936 1970

A11937 1970

P535213–280 2003

Material collections

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.

Petrological collections

Hand specimens and thin sections of rocks from the district, if any, are held in the England and Wales Sliced Rocks collection at BGS Keyworth. A collection database is maintained by the Mineralogy and Petrology Group at BGS Keyworth. Enquiries concerning petrological material should be directed to the Manager, Petrological Collections, BGS Keyworth.

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.

Some sources of relevant information held outside BGS

Environment Agency

The Environment Agency holds information on licensed water abstraction sites, groundwater resources, springs, reservoirs, Catchment Management Plans with surface water quality maps, aquifer protection policy and licensed landfill sites. The Newbury district lies within the Thames Region of the Environment Agency, Kings Meadow House, Kings Meadow Road, Reading, Berkshire, RG1 8DQ; telephone: 0118 9535000.

Natural England

Natural England (previously known as English Nature) holds information on Sites of Special Scientific Interest (SSSI), Regionally Important Geological Sites (RIGS) and Geological Conservation Review (GCR) sites within the Newbury district.

References

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

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Figures, plates and tables

Figures

(Figure 1) Location of Newbury and Abingdon districts.

(Figure 2a) Regional tectonic setting and Palaeozoic subcrop map. Palaeozoic formations beneath the sub-Mesozoic unconformity, and location of 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; SBI StratB1; WPS Welford Park Station

(Figure 2b) Regional tectonic setting and Palaeozoic subcrop map. Part of southern England, with the sub-basins and main fault systems of the Wessex Basin separated from the Midlands Microcraton by the Pewsey-London Platform Fault zone (PLPF), just south of the Variscan Front.

(Figure 3a) Regional potential fields. Bouguer gravity anomalies shown as a colour shaded relief illuminated from the north. Contour interval 1mGal (1mGal=1x10−5m/s2).

(Figure 3b) Regional potential fields. Total field magnetic anomalies shown as a colour relief illuminated from the north. Contour interval 10nT. See Busby and Smith (2001) for a broader context for this data.

(Figure 4) Subcrop map at the mid Cretaceous ‘Late Cimmerian’ unconformity.

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

(Figure 6) Correlation of Chalk Group through geophysical borehole logs.

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

(Figure 8) River terrace correlation and terminology.

(Figure 9) Plan of chalk well. Field sketch by F J Bennett, 1885. (County Series 1:10 560 map sheet 35NW(W)), A = ‘angles’, q = ‘quoins’.

(Figure 10) 3D perspective view of the Pang-Lambourn area.

(Figure 11) Structure contours on the base of the Lewes Chalk Formation.

(Figure 12) Structure contours on the base of the Holywell Chalk Formation.

(Figure 13) Structure contours on the base of the West Melbury Marly Chalk Formation.

Plates

(Front cover) Typical Chalk downland scenery near Lambourn, with two large sarsen stones (weathered relicts of silicified Palaeogene sandstone) in the foreground. View from Weathercock Hill, 209 m OD [SU 2930 8223] looking north-east, with Hillbarn Clump [SU 3253 8596] (beside the Ridgeway long distance path at 226 m OD) 5 km distant on the horizon. (P535218) (C F Adkin).

(Plate 1) The Chalk Rock. Fognam Farm Quarry [SU 297 799] is a Site of Special Scientific Interest (SSSI) and a Geological Conservation Review (GCR) site. It exposes the very top of the New Pit Chalk and the lower part of the Lewes Nodular Chalk, including the Chalk Rock (Mortimore et al., 2001). 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. The inset shows the Chalk Rock as seen in a videoscan log in the Banterwick Barn Borehole [SU 5134 7750], 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. (A11936).

(Plate 2) Seaford Chalk and Lambeth Group. (A) The topmost 3 m of the Seaford Chalk exposed in a newly excavated cutting for realignment of the A34(T) near Chieveley [SU 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 of the Upnor Formation, with a basal clayey, sandy pebble bed, as seen in (B), resting on the tabular flint. Divisions on ranging pole are each 50 cm (P535262); (P535264) (C F Adkin).

(Plate 3) Lambeth Group (Reading Formation and Upnor Formation). Temporary section within Horn Copse, 1 km south-east of Kintbury [SU 3949 6608]. Basal multicoloured clays and sand of the Reading Formation overlying the green shelly sands of the Upnor Formation (below mid-section join in auger handle). Auger is 1.3 m long. (P710848) (P M Hopson).

(Plate 4) Lambeth Group (Reading Formation). Temporary section within Horn Copse, 1 km south-east of Kintbury [SU 3949 6608]. The colour-mottled stiff and waxy clays are of the upper part of the Reading Formation beneath the strong dark grey/black humic and pebbly subsoil. Hammer is 28 cm long. (P710852) (P M Hopson).

(Plate 5) Stony soil on ‘pebble bed’ in the London Clay. Arable field south of Kintbury [SU 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 mostly 4 to 64 mm diameter, rarely up to 128 mm (maximum in view is about 50 mm); mostly composed of flint, with about 15 per cent quartz or quartzite. Hammer is 28 cm long. (P710847) (P M Hopson).

(Plate 6) 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 (Plate 3) and a few relatively complete flint nodules. (P608887) (A J Newell).

(Back cover)

Tables

(Table 1) Summary of the geological succession at outcrop in the district.

(Table 2) Strata found in deep boreholes in and near the Newbury district.

(Table 3) Chalk stratigraphy. # Traditional Chalk subdivisions after Jukes-Browne and Hill (1903, 1904, for example). U: Upper; M: Middle; L: Lower; s.l. = sensu lato. *Foraminiferal zones after Carter and Hart (1977), Swiecicki (1980), Hart et al. (1989) (UKB zones) and Wilkinson (2000) (BGS zones). Not to scale.

(Table 4) Classification of river terrace deposits in the Pang-Lambourn catchment. Terraces not named are not present or have not yet been differentiated. Correlations after Mathers and Smith (2000).

(Table 5) Mineral resources by geological formation.

(Table 6) Potential ground constraints.

Tables

(Table 4) Classification of river terrace deposits in the Pang-Lambourn catchment

Terraces not named are not present or have not yet been differentiated. Correlations after Mathers and Smith (2000).

(Table 5) Mineral resources by geological formation

(Table 6) Potential ground constraints