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Geology of the Reading district — brief explanation of the geological map sheet 268 Reading

S J Mathers and N J P Smith

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

Keyworth, Nottingham: British Geological Survey, 2000.

© NERC copyright 2000

(Front cover) Front cover The Thames valley looking south-west from Sonning. The photograph shows extensive flooded gravel workings and the town of Reading in the distance. (Aerofilms Ac 616823.)

(Rear cover)

(Geological succession) Geological succession in the Reading district.

Notes

The word 'district' refers to the area of Sheet 268 Reading. National Grid references are given in square brackets; unless otherwise specified they lie within the 100 km square SU; specific locations and boreholes are accompanied by their National Grid reference at their first mention within the text. Borehole records referred to in the text are prefixed by the code of the National Grid 25 km2 area upon which the site falls, for example SU 76 NW, followed by its registration number in the BGS National Geological Records Centre. Lithostratigraphical symbols shown in brackets in the text, for example (LMB) are those shown on the published

Acknowledgements

This Sheet Explanation was compiled by S J Mathers. N J P Smith contributed to the description of the concealed strata. The manuscript was edited by R D Lake, with specialist input from M G Culshaw and D J Allen. The authors' thanks are due to the many landowners, local authorities, utility and site investigation companies for access to land and the provision of geological information. Particular acknowledgements are due to the staff of the Babtie Group responsible for planning and environmental matters for the former Berkshire County Council area, C Fisher and J Sedman of ARC Southern and Professor P Worsley, R Goldring and colleagues at Reading University for their advice. C King of Greenwich University examined samples of the Mortimer Pit Borehole and BGS research boreholes in the western London Basin and provided a useful stratigraphical framework for the region.

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

Crown copyright reserved Ordnance Survey licence number GD 272191/2000.

Geology of the Reading district (summary from rear cover)

Continuing urban development requires accurate geological information in order, for example, to identify resources and ensure that foundations are adequate. 'this Sheet Explanation and the newly surveyed geological map that it describes provide valuable information on a wide range of earth science issues. 'this explanation is written for those who may have limited experience in the tise of geological maps and for the professional user, who may wish to be directed to further geological information about the district.

The market town of Reading lies centrally within the district which contains the confluences of the rivers Thames, Pang, Kennet, Loddon and Blackwater. Consequently, it has been the focus of transport routes leading eastwards to London. Wokingham lies to the south-east; away from these towns the district is predominantly rural.

The Chalk Group of Cretaceous age is the oldest rock at the surface in the district. It forms downlands with deeply incised valleys, in the north and north-west. Relatively softer Palaeogene strata underlie the central part of the district and include the Lambeth Group and London Clay Formation. These are predominantly clay and give rise to low-lying, fertile farmland. In the south-west and south-east, extensive heathlands are developed on the interfluves where sandier strata of the Bracklesham Group and sands and gravels of Quaternary age are widespread.

The drainage pattern of the district is complex, and has evolved considerably during the last few million years. During the late Pliocene and Quaternary, relative uplift of the district caused marked downcutting and incision of the drainage, resulting in a complex suite of river terraces. Today, the Kennet and Loddon–Blackwater rivers appear to be relatively small streams but their older deposits indicate that much larger rivers once occupied their valleys. The Kennet valley was diverted to its present course during the last period of cold climate.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 268 Reading. It is written for the professional user, and those who may have limited experience in the use of geological maps and may wish to be directed to further geological information about the district.

The district lies to the west of London, mainly within the West Berkshire, Reading and Wokingham unitary authorities. The principal urban area is the Reading conurbation, which has seen rapid expansion over the last 30 years, together with the town of Wokingham [SU 809 688] straddling the eastern border of the district. The rest of the district is predominantly rural.

In the north and north-west, chalk downlands rise to heights of about 150 m above OD and here the valleys are deeply incised. Many of the hilltops are covered by a thin veneer of Quaternary deposits. The downlands fall south-eastwards to a lower lying, gently undulating tract in the central parts of the district that is underlain by the relatively softer Palaeogene strata. Here the predominantly clayey strata of the Lambeth Group and London Clay underlie fertile land that is used mainly for arable farming. In the south-west and south-east, extensive heathlands are developed on the interfluvial areas where sandier strata of the Bracklesham Group and Quaternary sands and gravels are widespread.

The oldest rocks proved in the region are steeply dipping Ordovician and Silurian sedimentary rocks (probably within the Variscan Fold Belt) as found in deep boreholes surrounding the district. These were uplifted in mid-Devonian times when the district was situated on the Avalonian foreland of the Caledonian fold belts and deformed during the Variscan orogeny. Initial erosion of these rocks produced terrestrial red beds in the late Devonian but this uplifted area was to persist in the form of the London Platform and influence sedimentation until the Late Cretaceous.

Throughout Palaeozoic time, the area south of the district was occupied by the Rheic Ocean. A transgression from this ocean inundated the district in Late Devonian (Frasnian) times and then led to the deposition of a thinly preserved sequence of Carboniferous Limestone. Later renewed uplift and erosion created a further break in deposition which lasted until the Westphalian when Coal Measures sediments were laid down in fluvial and lacustrine environments. These are preserved in a syncline which extends south-eastwards into this district. In late Westphalian times, basaltic lavas were extruded and dolerite sills emplaced within the Coal Measure sediments. The volcanism is related to the onset of the Variscan orogeny which produced the uplifted Variscides mountain belt immediately to the south of the district, and provided a source of sediment for the youngest Westphalian fluvial sandstones in the foreland basins. Continued uplift of the Variscides together with the foreland area subjected the district once more to prolonged erosion. No Permian or Triassic strata are found in the district.

The district was gradually submerged once again in the early Jurassic while the Tethys Ocean was opening to the south. Lower Lias strata were laid down by the initial marine transgression. An attenuated succession of Jurassic strata in this district indicates deposition in relatively shallow water on the still elevated London Platform.

The Early Cretaceous is marked by the prominent Late Cimmerian unconformity caused by a cessation of local rift subsidence. Regional post-rift subsidence then occurred causing the London Platform to be inundated and the Gault Formation was deposited. By the Late Cretaceous much of Britain lay beneath a widespread relatively deep sea with little elastic input in which chalk was deposited. The thickness of the Chalk in this district is half that of many parts of England, because it was deposited as a condensed sequence in relatively shallow water on the Berkshire–Chiltern Shelf of the London Platform.

Late Cretaceous uplift and tilting, probably associated with the opening of the North Atlantic, produced significant erosion of the Chalk prior to the deposition of the Palaeogene Lambeth Group. The base of this group reflects a marked marine transgression which deposited the Upnor Formation. Later, a regression resulted in the deposition of the Reading Formation in a low-energy terrestrial and coastal environment. Renewed transgression is indicated by the marine Harwich Formation. This, in turn, was succeeded by the fluctuating cyclic sedimentation of the London Clay that reflects successive trangressive–regressive events in a marine environment.

The overlying Bracklesham Group indicates an overall shallowing of the sea. Strata of Oligocene to mid-Pliocene age are absent in the district. During the Miocene, the area was uplifted by the Alpine orogeny and the London Basin syncline was formed.

During the late Pliocene and Quaternary, southern Britain has been affected by easterly downwarping. This district has been uplifted relative to the subsiding southern North Sea Basin. This uplift has produced marked downcutting and incision of the drainage in south-east England, resulting in a complex suite of river terraces.

The global climate cooled during the late Quaternary when sea level fluctuated markedly in alternating periods of glaciation and climatic amelioration. During several of these periods, ice sheets developed across Britain but did not extend as far south as the Reading district where harsh periglacial conditions prevailed. At the end of the Anglian glaciation, the regional drainage assumed its present pattern with the Thames, Kennet and Loddon–Blackwater drainage systems becoming distinct elements. The Kennet and Loddon–Blackwater rivers today appear to be relatively small streams but their older deposits indicate that much larger rivers once occupied their valleys.

Cold phases since the Anglian an reflected by periglacial braided river deposits. Following the ultimate Devensian glaciation when ice sheets reached the English Midlands, the climate has gradually warmed and the rivers have adopted meandering style with lower gradients an. finer sediment load. The study of the alluvial deposits along the main river course makes it possible to document this amelio ration of climate and trace man's influence on the landscape of southern Britain.

Chapter 2 Geological description

The geological succession present at outcrop in the district is shown on the inside of the front cover. Much of the evidence of the concealed strata beneath the Chalk Group is drawn from deep boreholes sited in and around the district (Figure 1).

Palaeozoic

No pre-Devonian strata have been reached by drilling within the Reading district. However deep boreholes in adjacent areas and regional geophysical evidence enable some general conclusions to be reached about which strata are likely to be encountered. A magnetic anomaly that underlies the Reading—Newbury area is probably of composite origin and relates to volcanic rocks; these were found in the Withycombe Farm Borehole [SP 4319 4017] near Banbury where they are of Precambrian age, and in the Bicester Borehole [SP 5878 2081] (Cornwell et al., 1994) where they are of Silurian age. The shallow-source component of the anomaly (Ellis and Kearey, 1984) is probably due to Westphalian intrusions (Figure 2) in view of their widespread occurrence and magnetic susceptibilities.

Beneath the Berkshire Coal Basin (Figure 1) which extends into the west of the district, it is probable that a full sequence of Silurian strata, including late Silurian (Přídolí) is present (Smith, 1987). Devonian strata reach a thickness in excess of 2000 m beneath the Oxfordshire and Berkshire Coal basins (Figure 1) and are also proved in the Sonning Eye Borehole [SU 7420 7580]. Figure 2 summarises the Palaeozoic provings in this district. North of the district, 11 m of Tournaisian limestone were proved in the Aston Tirrold Borehole [SP 5581 8720]. In contrast, limestones of Viséan (Holkerian) age were proved in Foudry Bridge Borehole [SU 7063 6602]. The age span of these limestones suggests that a former, much thicker cover of Carbonifer ous Limestone has been largely removed, probably during Namurian times. The strata of Westphalian age have been subdivided into Coal Measures and an unconformably overlying sequence of both red beds and grey measures with some coals correlated with the Warwickshire Group of Powell et al. (1998).

Mesozoic

Jurassic

The earlier Mesozoic strata comprise an attenuated succession deposited near the southwest margin of the London Platform. The Jurassic strata, which reflect the effects of both onlap and the Late Cimmerian unconformity, are summarised in (Figure 3). The sub-Mesozoic unconformity deepens towards the south and west from 350 to 800 m below OD (Smith, 1985b). The supercrop map (Figure 4) shows the progressive overlap of the London Platform, which subsided during a period of thermal relaxation.

Lower Cretaceous

The Wealden Group (W) is thought to be present in the south-west (Figure 5) and is recorded in the Kingsclere [SU 4984 5820] and Hook Lane [SU 5754 5387] boreholes, adjacent to the district.

The Lower Greensand (LGS) in the Strat B1 and Foudry Bridge boreholes is a glauconitic, fine-grained, quartzose sandstone overlying coarser-grained sandstone at the base of a 12 m-thick sequence. In the Sonning Eye Borehole it comprises 6 m of greenish grey, glauconitic, calcareous, muddy sandstones and silts. The unit is absent from the Burnt Hill Borehole. Regionally the Lower Greensand dies out eastwards and is overlapped by the Gault.

Boreholes show that there are complex thickness variations in the Gault Formation (G) across the district. In the Strat B1 Borehole, the Gault comprises 53 m of dark olive-grey, silty, micaceous marlstone. The nearby Foudry-Bridge Borehole encountered about 65 m but, in the Sonning Eye Borehole to the north, the Gault is 102 m thick consisting of blue-grey to pale grey, slightly calcareous shale, with lenses of glauconitic siltstone overlying dark grey to black, silty shale and glauconitic siltstone at the base. The Gault in the Burnt Hill Borehole comprises 77 m of blue-grey mudstone.

The Upper Greensand (UGS) in the Strat B1 Borehole is a glauconitic, calcareous sandstone interbedded with limestone and underlying siltstone with a total thickness of 43 m. A similar, but slightly thinner sequence, is recorded from the Foudry Bridge Borehole. In the Sonning Eye Borehole, the Upper Greensand consists of 48 m of calcareous siltstones, silty clays and glauconitic, silty sandstones. In the Burnt Hill Borehole, some 22 m of glauconitic siltstone are present.

Upper Cretaceous

The Chalk Group (Ck) is the oldest unit exposed within the district. It crops out extensively in the north principally along the flanks of the Thames valley and also along the Kennet valley downstream of Theale. In the west, it extends as far south as the Pang valley while in the south the Chalk is overlain by Palaeogene deposits. The limits of the Chalk subcrop beneath Quaternary deposits can be accurately defined throughout much of the district by means of abundant borehole information.

Four deep boreholes, all located in the south-east of the district, prove complete sequences of the Chalk Group as follows: Strat B1: 246.3 m, Fair Cross [SU 6974 6326]: 248.0 m, Foudry Bridge: 234.0 m, Wokingham Waterworks [SU 8086 6790]: 211.9 m.

The Sonning Eye Borehole commenced in Chalk and penetrated some 165.6 m. This thickness, together with the evidence from the local outcrop, indicates a total thickness of about 190 m for this area. The variations in thickness across the district are consistent with the observed regional pattern which shows that the thickest preserved sequences lie close to the axis of the London Basin syncline (Wood, 1996).

Traditionally the Chalk Group is divided into Lower, Middle and Upper Chalk with the hard marker layers of the Melbourn Rock and Chalk Rock at the base of the Middle and Upper Chalk respectively (Figure 6). However, during this resurvey it was possible to map the Chalk Rock and the overlying nodular chalkstones as a discrete unit (Lewes Nodular Chalk) in the north-west of the district.

The Lower Chalk (LCk) crops out around Streatley [SU 592 807] on the western flank of the Thames valley and it extends beneath the adjacent alluvial deposits of the Thames. Borehole evidence and mapping in adjacent districts suggest that it is 50 to 80 m thick, comprising grey flintless chalk which is marly in its lower parts (Chalk Marl). In the Fair Cross Borehole, 55 m of Lower Chalk are present (Figure 6).

The Middle Chalk (MCk) crops out in the Thames valley between Goring [SU 600 807] and Pangbourne and in other incised valleys in the north-west of the district. It comprises white, soft and nodular chalks with subordinate marls. Rare flint seams are present in the top part. In the Fair Cross Borehole, the Middle Chalk is 61 m thick.

The lowest member of the Upper Chalk (UCk) is the Lewes Nodular Chalk (LeCk) and in the Fair Cross Borehole, this is about 39 m thick. However, chalkstone beds are present only in the lowest part of this sequence (see also Wood, 1996). At crop in the Goring–Pangbourne area, the Middle Chalk is overlain by a 10 to 15 m-thick unit of hard nodular chalkstones that can be assigned to the Lewes Chalk as defined by Bristow et al. (1997). It is probable that the mapped lithostratigraphical unit corresponds only to the lower part of the strata assigned to the Lewes Chalk in the Fair Cross Borehole (Figure 6).

The overlying Chalk has been mapped as undifferentiated Upper Chalk, comprising white, soft and nodular chalks with abundant flint seams. Faunal evidence from the outcrop in the northern parts of the district (Bromley and Gale, 1982; Hopson 1994; Woods, 1997) indicates that it belongs entirely to the Micraster coranguinum Zone, and this is equivalent to the Seaford Chalk Member of Mortimore and Pomerol (1987) and Bristow et al. (1997). Some of the faunal assemblages were collected from the Chalk close to the contact with the overlying Palaeogene Lambeth Group. Farther south however, in the Fair Cross Borehole, the uppermost 36 m of chalk belongs to the stratigraphically higher Newhaven Chalk Member characterised by Uintacrinus socialis and Marsupites testudinarius. These fossils were also reported from the uppermost Chalk beneath Palaeogene strata in the south-west of the district (Hawkins, l.953). It thus seems probable that the Lambeth Group oversteps the Newhaven Chalk northwards to rest on the Seaford Chalk (Figure 6). The latter member comprises the topmost unit of preserved Chalk at outcrop.

Palaeogene

The Lambeth Group (LMB) corresponds tc the strata formerly described as Woolwich and Reading Beds, and is shown as a single undifferentiated unit on the published map. Many excellent sections were described in brickpits around Reading and Tilehurst [SU 674 728] (Jones and King, 1875; Blake and Monckton, 1896; Hawkins, 1946). Other sections were detailed by Prestwich (1854, p.88–89), Blake (1903) and Hawkins (1954).

Boreholes have shown that the group is 19 to 28 m thick but more typical values are 21 to 25 m with no discernible systematic variation. Two formations are recognised, namely the Cpnor and Reading formations (Ellison et al., 1994). It has proved impractical to delineate these separately because the former is very thin.

The Upnor Formation comprises highly glauconitic, green, blue and grey sands and clays. Large, irrregular-shaped, glauconite-coated flint nodules and rounded flint pebbles at the base rest on a locally irregular and extensively bored chalk surface. This marine sequence is condensed and commonly contains fossiliferous material including fish, abundant oysters and other bivalves, echinoids, gastropods and scaphopods. Also recorded are rare exotic pebbles such as schist (Jones and King, 1875) that are thought to have been introduced by floating vegetation. The Upnor Formation can be recognised in most described sections. It is usually less than 1 m thickness, and exceeds 2 m only in the extreme south-west of the district, where up to 6 m of glauconitic strata have been reported (Hawkins, 1954).

Sections showing the Upnor Formation resting on Chalk were recorded during this resurvey at an old chalk pit at Rushall Farm [SU 4588 1726] and in a temporary excavation in Reading [SU 4748 1743]. The basal contact with the Chalk is commonly smooth but locally it is more irregular, and may show evidence of postdepositional solution collapse (Plate 1) and piping (see also Hawkins 1934, p.420–421). A further section at the former Pincent's Kiln [SU 6510 7200] shows up to 3 m of glauconitic sands, clays and pebble beds, with a planar basal contact (Crane and Goldring, 1991).

The overlying Reading Formation is commonly about 20 m thick and predominantly comprises colour-mottled clays, characteristically red and grey, but also in shades of purple, brown and orange. This complex mottling has been ascribed to pedogenic processes with multiple overprinting of palaeosols (Buurman, 1980). Beds of fine- to medium-grained sands, commonly up to 2 m thick but locally reaching 7 m, occur at all levels within the formation. They are most common at or near the base, but their spatial distribution is very variable and unpredictable. Parts of these sand bodies are cemented by silica to form concretions called sarsens. Blocks of silicified tree trunks have also been found, including magnolia-like species together with a piece of Sequoia from near Aldworth [SU 555 795] (BGS Collection). Beds rich in leaf remains and lignitic horizons occur within the clays, commonly towards the base of the sequence (Crane and Goldring, 1991; Newton in Blake, 1903).

The Knowl Hill pit [SU 8170 7970], on the very margin of the district, has revealed a complete sequence from the Chalk to the London Clay (Ellison and Williamson, 1999). The Chalk surface is extensively burrowed by Glyphichnus and overlain by about 2 m of the Upnor Formation, which is succeeded, by over 15 m of colour-mottled clays of the Reading Formation. These clays are, in turn, overlain by up to 8 m of planar and cross-stratified, fine- to medium-grained sands showing soft sediment deformation structures and, in the uppermost beds, burrows of Ophiomorpha (Goldring and Alghamdi, 1999). Lenticular mottled clays are preserved in depositional 'lows' in the top surface of this unit. The stratigraphical position and depositional environment of this unit are uncertain. Initially it was regarded as fluvial by Kennedy and Sellwood, (1970), but Sellwood (1974) reinterpreted this as the basal transgressive beach deposit of the London Clay. Later it was termed the Twyford Member by King (1981) and correlated with the Oldhaven Formation. Ellison and Williamson (1999) suggested that the sands belong to the Harwich Formation. Finally, in a study of the stratigraphical relationships, sedimentology and trace fossil assemblages, Goldring and Alghamdi (1999) inferred deposition in an estuarine/delta distributary environment and that the beds can best be regarded as the top part of the Reading Formation.

Overlying the Lambeth Group is the Thames Group, which includes the Harwich and London Clay formations (Ellison et al., 1994). It has not proved practical to map these separarely, so both are included with the London Clay (LC). The main outcrop extends northwards as far as Wokingham in the east and Buckleberry Common in the west. Farther north beyond the main outcrop several isolated outliers of the London Clay have been mapped east of Frilsham [SU 538 732], at Burnt Hill [SU 569 743], Tilehurst and northeast of Wargrave. One further outlier is preserved north of the Thames at Emmer Green [SU 723 769] due to downfaulting of the strata.

The thickest recorded sequence of the Thames Group is 114.3 m in the Wellington Brewery Borehole [SU 8108 6844] at Wokingham, close to the eastern margin of the district. Elsewhere in the south-east of the district, the overall thickness of the strata is 80 to 85 m; traced westwards this declines to 60 to 76 m in the Mortimer–Burghfield–Padworth area and thence to about 50 to 55 m in the Croolcham–Thatcham–Woolhampton area.

The Eocene Harwich Formation comprises a basal flint pebble bed overlain by highly glauconitic shelly sands and clayey silts. The deposits are intensely burrowed and locally the shelly beds containing abundant Glycimeris have been cemented to form calcareous sandstones. Originally described as the 'Basement Beds' of the London Clay the Harwich Formation was recorded in numerous old brickpits in and around Reading and Tilehurst (see Blake (1903, p42–52) for details of these localities and their faunas). The section at Spey Road, Tilehurst [SU 683 734] was adopted by King (1981, p.62–63) as the stratotype for his (now superseded) Tilehurst Member. At lilehurst the deposit is 3.4 m thick but, farther west in the Mortimer Pit Borehole [SU 6282 6449], 6 m of strata are assigned to the Harwich Formation suggesting a westward thickening of the unit. At Knowl Hill pit, the Harwich Formation is probably represented by a glauconitic pebbly and shelly silty clay which grades up into clay ((Plate 2); Goldring and Alghamdi, 1999); this rests on the predominantly sandy unit that these authors assign to the upper Reading Formation.

The overlying London Clay Formation comprises up to 100 m of blue-grey, silty clays and clayey silts with subordinate thin glauconitic sands and pebble beds. Five major cycles of sedimentation (A–E) have been recognised by King (1981) comprising, at their fullest development, a basal pebble bed and/or glauconitic horizon overlain by a coarsening-upward sequence of clays, silts and sands with septarian concretions. Each cycle is interpreted as a transgressive–regressive sequence. Cycles commence with a transgressive erosion surface and pebble lag, followed during a high sea-level stand by fully marine glauconitic deposits, accumulated in a deep quiet offshore environment. The succeeding progressively coarsening-upward sediments reflect gradual coastal progradation during marine regression.

The identification of these five cycles in boreholes and sections is based on faunal criteria. The boundary of cycles B and C has been interpreted (King, 1981) in sequences exposed in the M4 cuttings near Shinfield [SU 732 687] from the data of James et al. (1974). At Tilehurst the basal beds belong to unit A2, a component of the lowest (A) cycle, but farther west in the Enborne valley area the basal unit is identified as A3, the uppermost part of the cycle (King, 1981). The Mortimer Pit Borehole proved at least 73 m of London Clay including a fairly complete sequence of cycles A–C. The uppermost Pala eogene beds in this well are sands that are mapped widely in the south-west of the district. It is uncertain whether these sands are part of the London Clay Formation or a basal part of the Bagshot Formation. On the published map they have been retained as Bagshot Formation following earlier accounts (Hawkins, 1953).

Above the London Clay lie the early to middle Eocene sediments of the predominantly sandy Bracklesham Group (Curry et al., 1977; Edwards and Freshney, 1987) including the Bagshot, Windlesham and Camberley Sand formations. Together they comprise about 70 m of strata. The Bagshot Formation (BgB) corresponds to the Lower Bagshot Sands of Prestwich (1847), Lower Bagshot Beds of Blake (1903) and the Bagshot Beds of Dewey and Bromehead (1915). It is widespread in the southern half of the district, comprising about 20 to 25 m of predominantly yellow-brown and grey, planar and cross-stratified, quartzose and locally glauconitic, fine-grained sands with subordinate thin clay lenses. At the base there is a discontinuous pebble bed of rounded black flint above a sharp contact.

In the south-west of the district, up to 15 m of interbedded sands, silts and clays are here assigned to the Bagshot Formation following the earlier mapping and Hawkins (1953). These beds probably cut down into the London Clay, thus explaining the attenuation of the latter in this area. An alternative explanation is that there is a lateral facies change within the London Clay at this level, from clay in the east to sand in the west. Some of the strata, regarded here on lithostratigraphical criteria as Bagshot Formation, were included on biostratigraphical evidence as the topmost part of the London Clay sequence (sensu King, 1981, p.67–68).

At Sand Martins Golf Course, in a former sand pit [SU 4797 1662] close to the base of the formation, up to 2.7 m of fine- to medium-grained sands with subordinate clay laminae are exposed. The strata display herringbone cross-bedding, flaser bedding and Ophiornorpha burrows indicating deposition in a marginal sub-intertidal sandflat environment.

The Windlesham Formation (Wi) (Ellison and Williamson, 1999) corresponds to the Middle Bagshot Sands of Prestwich (1847) and the Bracklesham Beds of Blake (1903) and Dewey and Bromehead (1915). It crops out on the high ground between Wokingham and Finchampstead [SU 795 632] in the extreme south-east of the district. There are no exposures but boreholes and evidence from the adjacent Windsor district suggest that up to 20 m of dark green to brown, highly glauconitic, bioturbated sand and clay are present. The base of this marine formation is marked by a discontinuous pebble bed of rounded flints.

The Camberley Sand Formation (CaS) (Ellison and Williamson, 1999) equates with the Upper Bagshot Sands of Prestwich (1847), the Upper Bagshot Beds of Blake (1903), and the Barton Beds of Dewey and Bromehead (1915). It crops out along the top of Finchampstead Ridges [SU 810 634] in the south-east of the district where there are up to 20 m of yellow-brown, weakly glauconitic, bioturbated, fine-grained sands with subordinate grey clay lenses and sporadic rounded flint pebbles in the basal beds. The formation is not exposed in the district.

Quaternary

Quaternary deposits of diverse origin are widespread within the district. They include extensive river terrace and periglacial mass movement deposits, together with fine-grained floodplain deposits including the Flandrian alluvium of the existing drainage system. The classification of these deposits is considerably more complex than that proposed by earlier surveyors.

Alluvial deposits

The river terrace and floodplain deposits in the Reading district are particularly complex because they contain elements belonging to the ancestral Thames, Kennet and LoddonBlackwater drainage systems. The post-Anglian sediments have distinct identities since different catchment areas have produced deposits of diverse but distinct provenance (Thomas, 1961; Walder, 1967; Hopson, 1982; see also diagram on the map). Up to ten levels of river terrace deposits can be recognised. Their correlation between drainage basins is shown by use of a common numbering system; names are also applied to the terrace deposits of the Thames and Kennet basins, largely following those in the literature. The deposits of the Loddon–Blackwater system have not been formally named, pending the complete resurvey of this catchment.

The terrace deposit correlation (Figure 7) has been established using:

The Quaternary alluvial deposits of each drainage basin are discussed separately below.

Thames valley

Four distinct high-level terrace deposits (Tenth to Seventh) are recognised within the Thames catchment in the north of the district. These are generally 3 to 8 m thick and comprise gravels and pebbly sands which are commonly clayey. These terrace deposits are closely related to spreads of head gravel generated by downslope mass-movement which produced a infix of material derived from both the terrace deposits and the adjacent bedrock. The highest and oldest deposit is the Westland Green Gravel (Tenth) which occurs as small remnant patches at elevations higher than 150 m above OD. This gravel is mainly composed of rounded flint pebbles thought to be derived locally from outcrops of the Reading Formation. This deposit corresponds to the Pebble Gravel of the early literature.

The next three levels of terrace deposits occur at elevations from 130 down to 85 m OD (Figure 7) and these comprise the Beaconsfield Gravel (Ninth), Gerrards Cross Gravel (Eighth) and Winter Hill Gravel (Seventh). These crop out north of the Thames, as dissected remnants of northeast-aligned terrace spreads, which with decreasing elevation, show evidence of river migration south-eastwards down the Chalk dip slope. Other patches are preserved around Tilehurst and Upper Basildon. These deposits are rich in quartz and quartzite pebbles, which commonly constitute 30 to 50 per cent of the clasts, together with angular and rounded flint and rare exotic clasts such as volcanic pebbles. This composition suggests that their source included the pebble beds within the Sherwood Sandstone Group (formerly Bunter Sandstone) of the English Midlands.

These deposits are all pre-early Anglian in age and, at that time, the ancestral Thames is thought to have entered the London Basin close to the present Goring Gap and then flowed north-eastwards towards East Anglia (Wooldridge, 1960; Hey, 1965; Gibbard, 1985; Bridgland, 1994).

The suite of middle level terrace deposits comprises the Black Park Gravel (Sixth), Boyn Hill Gravel (Fifth) and Lynch Hill Gravel (Fourth). These range in altitude from 80 in down to 45 m OD (Figure 7). These deposits are generally 3 to 5 m thick and comprise gravels and pebbly sands which contain noticeably fewer quartzose pebbles; in the Black Park Gravel these average less than 10 per cent (Walder, 1967). North of the Thames the Black Park Gravel (Sixth) cuts through segments of the older Winter Hill Gravel (Seventh) between Caversham [SU 715 747] and Henley. This palaeovalley has been referred to as the 'Caversham Trench' or 'ancient channel' in the literature (for example Wymer, 1961). Just beyond the northern margin of the district, significant Palaeolithic implements have been recovered from the Black Park Gravel in a pit [SU 744 813] at Highlands Farm (Bridgland, 1994). The Boyn Hill Gravel (Fifth) and Lynch Hill Gravel (Fourth) are preserved as terrace spreads between Caversham and Shiplake north of the Thames and between Coley Hill [SU 705 727] and Sonning [SU 755 755] to the south.

The Black Park Gravel (Sixth) is thought to be Anglian in age whereas the Boyn Hill Gravel (Fifth) and Lynch Hill gravel (Fourth) are thought to postdate the Anglian but predate the (Devensian) ultimate glaciation. During the Anglian glaciation, the northeasterly flow of the ancestral Thames (mentioned above) was diverted southwards to occupy its present route through the London area (Wooldridge, 1938; Gibbard, 1985). This change is reflected in the fact that the post-Anglian Boyn Hill and Lynch Hill gravel spreads in the district can be closely related to the present drainage pattern in the Thames valley whereas the distribution of the Black Park Gravel is markedly different.

The reduced quartzose component of the gravels of the middle-level Thames terrace deposits may result from the part of the upper catchment in the Midlands being lost to the newly established Severn drainage pattern, as a result of the Anglian deglaciation. Another factor may be increased dilution of the Thames gravels by tributaries such as the proto-Kennet, laden with locally derived sediment from solely within the London Basin.

The low-level terrace deposits of the Thames system comprise the Taplow Gravel (Third), the Kempton Park Gravel (Second) and the Shepperton Gravel (First); the last occurs beneath the modern floodplain alluvium and does not crop out in this district. The terrace surface of the Kempton Park Gravel lies only 1 to 2 m above the modern floodplain (Figure 7) whereas that of the Taplow Gravel rises to about 10 m above that datum. These gravel spreads are all closely confined to the present-day valley. Boreholes indicate that the deposits commonly exceed 5 m in thickness. Locally, close to the confluences of the major rivers, they are over 10 m thick. This suggests that they are infilled hollows that were scoured into the Chalk bedrock by complex flow patterns near the points of mixing.

The gravel clasts are predominantly angular and rounded flint, together with limestone derived from the Jurassic strata in the present Upper Thames catchment. This limestone is prone to both mechanical and chemical erosion so that its content, which is as high as 50 per cent in the Goring Gap, falls across the district to about 20 per cent around Wargrave (Hopson, 1982). Dilution by sediment from the Kennet and Loddon–Blackwater systems may also have caused this decrease. Correlation with the well-documented Kennet valley sequence suggests that the Taplow Gravel is a cold-phase deposit between the Anglian and Devensian stages, whereas the lower two levels are Devensian in age.

Sandy and clayey silt deposits, commonly 1 to 2 m thick, overlie the Kempton Park Gravel downstream from Pangbourne. These sediments have been named the Langley Silt (Gibbard, 1985) and represent an older Devensian floodplain alluvium with a possible loessic component.

The postglacial Thames alluvium comprises grey silty clays with minor gravel lenses, up to 3 m thick, and overlies the Shepperton Gravel (Gibbard, 1985) as a continuous sheet along the entire valley floor.

Kennet valley

The Kennet drainage basin contains three high terrace deposits: the Bucklebury Common Gravel (Eighth), the Beenham Stocks Gravel (Seventh) and the Silchester Gravel (Sixth). These occur at elevations of 140 m down to 85 m OD (Figure 7) forming the extensive interfluvial heathlands north and south of the present valley, principally on Bucklebury Common [SU 545 686], between Beenham [SU 590 687] and Englefield [SU 625 719], and in the Crookham–Silchester–Burghfield area. These deposits are predominantly flint gravels, generally less than 5 m thick, and variably clayey. The terrace surface of the Silchester Gravel has a gradient of about 1.5 m /km falling north-eastwards (Collins, 1994). These deposits represent extensive former braidplains, probably developed on a surface of considerably less relief than at present. They are thought to be pre-Anglian to Anglian in age (Bridgland, 1994).

Within the Kennet valley, the remnants of three low-level terrace deposits are preserved — the Thatcham Gravel (Third), the Beenham Grange Gravel (Second) and the Heales Lock Gravel (First). The last occurs beneath the modern floodplain deposits and is not exposed. The Beenham Grange Gravel has a terrace surface 1 to 3 m above the flood-plain along the whole valley. The Thatcham Gravel is preserved as isolated patches rising to about 10 m above the floodplain.

These three terrace deposits are variably sandy and silty flint-gravels, generally up to 5 m thick, containing rare silt, clay and peat lenses. The sequence has been the subject of detailed study mainly in pit sections (Cheetham, 1975; Holyoak, 1980; Bryant, 1982; Collins 1994). Unusually thick sequences of the Heales Lock Gravel, locally over 10 m thick, are preserved in depressions. The gravels show evidence of syndepositional collapse into the depressions that are probably of periglacial origin. One major depression occurs near Woolhampton [SU 573 668] and was investigated in detail by Collins (1994); another overlies a brecciated chalk diapir at Ashford Hill [SU 557 623] (Hill, 1985) which is also probably of periglacial origin.

The Beenham Grange Gravel (Second) has been shown to be mainly Late Devensian in age (after 25 000 years BP) while the Heales Lock Gravel (First) represents the Dimlington Chronozone to Loch Lomond Stadial at approximately 16 000 to 10 000 years BP. These conclusions are based on radiocarbon dating and pollen analysis (Collins, 1994; Worsley and Collins, 1995; see also (Figure 8)).

The Langley Silt occurs between Theale and Pangbourne, where it is up to 1.5 m thick and rests on the Beenham Grange Gravel.

The alluvium of the River Kennet comprises complexly interbedded silty clay, shell marl, peat, thin gravel seams and reworked tufa. A detailed informal stratigraphy for this postglacial sequence was proposed by Collins (1994). The detailed relationships of the lower Kennet terraces and alluvium are shown in (Figure 8) and based largely on Worsley and Collins (1995).

The gravel component in the terraces of the Kennet system is composed predominantly of clasts of angular and rounded flint with rare pebbles of sarsen and quartz. This reflects their derivation from the Chalk and Palaeogene strata in the upper reaches of the catchment that comprises the axial drainage element of the western end of the London Basin. The fine sand and silt fraction of many of the surficial terrace deposits appears to be partly derived from beyond the catchment. This prompted Chartres (1981) to suggest the introduction of a Late Devensian loessic component. For much of its history the Kennet appears to have joined the Thames in the Tilehurst–Pangbourne area. However, it was later diverted to follow its present course eastwards from Theale towards Reading (Hawkins, 1926) where it flows through a narrow gap northwards into the Thames. This gap probably represents the former lower reaches of the Foudry Brook which were exploited by the much larger Kennet. A possible periglacial cause of diversion was provided by Worsley (1986 p. 272). He suggested that icings formed at the surface by the freezing of groundwater issuing from springs developed between Theale and Pangbourne. These ice masses may have blocked the Kennet during the deposition of the Beenham Grange Gravel which is found along both drainage routes and has been shown to be mainly of Late Devensian age (Worsley and Collins, 1995). A detailed terrace profile of the former Kennet route to Pangbourne suggests correlation of the Beenham Grange Gravel with the Kempton Park Gravel of the Thames system in contrast to the correlation with the Shepperton Gravel proposed by Bridgland (1994). This latter unit probably equates with the Heales Lock Gravel of the Kennet valley.

Loddon–Blackwater valley

This area is extensively dissected, and only isolated small patches of the older Eighth and Sixth Terrace Deposits remain. These are generally less than 4 m thick and comprise variably clayey flint-rich gravels. These older terrace remnants appear to bear little relation to the present drainage pattern and occur at elevations above 75 m OD (Figure 7).

The Fifth, Fourth and Third Terrace Deposits occur as patches throughout the basin and indicate that the present-day drainage pattern was broadly established at this stage. These various deposits comprise flint-rich gravels up to 5 m thick. Their distribution between Sonning [SU 756 756] and Shinfield shows that the ancestral Loddon migrated progressively south-eastwards in this tract. The Fifth Terrace Deposits have a confluence with the Boyn Hill Gravel of the Thames system at Earley [SU 746 722]. In general, the Fifth Terrace Deposits lies 25 to 30 m above the modern floodplain, the Fourth Terrace 10 to 20 m, and the Third Terrace rises up to 10 m above that datum (Figure 7).

The widespread, lowest exposed Second Terrace Deposits lie at 1 to 3 m above the floodplain, whereas the First Terrace Deposits are buried beneath its alluvium. Both of these deposits are predominantly flint-rich gravels up to 5 m in thickness. Downstream of Swallowfield [SU 727 650], up to 2 m of brown silty clay rests on parts of the Second Terrace Deposits, especially at the back of this terrace where the slopes beyond are formed of London Clay. Whereas some of these deposits may be alluvial in origin, and so probably equivalent to the Langley Silt of the Thames system, it is probable that a significant proportion of this sediment was derived by downslope mass-movement of London Clay. This material is depicted as Brickearth, retaining the original terminology of the earlier survey. The alluvium of the Loddon–Blackwater flood-plain is generally silty clay of restricted extent and thickness (less than 2 m thick), particularly in the Loddon valley.

The terrace deposits of the Loddon–Blackwater catchment are compositionally distinct from others in the district. The gravels are dominated by angular and rounded flint but, in contrast to the Kennet terrace deposits, contain a proportion of chert derived from the Lower Greensand in the upper parts of the catchment (Clarke and Dixon, 1981; Gibbard, 1982).

In the Loddon–Blackwater system seven levels of terrace deposits are recognised (Eighth, and Sixth to First), compared to eight proposed in this part of the catchment by Clarke and Dixon (1981). Gibbard (1982) included some of the oldest deposits in a regional synthesis and reconstructed their drainage patterns.

Other drainage systems

There are only a few isolated remnants of several terrace deposits preserved in the straight and deeply incised valley of the River Pang. The Foudry Brook catchment includes isolated remnants of middle-level terrace deposits and a widespread development of Second Terrace Deposits. In both the Pang and Foudry Brook valleys, the alluvium flooring the valley is discontinuous. Where it is absent, gravel deposits of the First Terrace, which elsewhere underlie the alluvium, have been mapped.

Mass-movement deposits

Throughout much of the Quaternary, this district was subjected to prolonged periglacial conditions. In such environments, vegetation cover is generally scant or absent, and this, together with annual freeze-thaw of the surficial layer and increased surface runoff, lead to considerable downslope mass-movement of weathered material. Some such deposits of disturbed gravel commonly mixed with clayey and sandy material have been classified as Head Gravel. These generally comprise up to 5 m of poorly sorted, matrix-supported, very stony, sandy and silty clays. They are best developed along the edges of the high terrace deposits which, because of their antiquity, were subjected to recurrent periglaciation.

The other Head deposits of the district have a variable composition reflecting the parent materials upslope. Those deposits present in the floors of dry valleys on the Chalk comprise a mixture of chalk and flint rubble in a silty clay matrix (formerly named Coombe Deposits). Head has also accumulated on some lower valley slopes cut into Palaeogene strata. These deposits, predominantly sandy and pebbly clays, are com monly poorly preserved in the larger valleys, where surface drainage has reworked the slope deposits into alluvial sediments. A large patch of head lies at the foot of a long slope in London Clay at Whiteknights Park [SU 735 717] comprising remobilised London Clay and Quaternary deposits. This was identified from numerous shallow boreholes and other similar deposits may occur elsewhere but, in the absence of data, have not been delineated.

Clay-with-flints comprises reddish orange, sandy clay in which abundant, nodular and rounded flint pebbles are embedded. These deposits on the Chalk in the north-west of the district are believed to result from both the reworking of small outliers of the Lambeth Group (Bateman, 1988) and from dissolution of the Chalk. The Clay-with-flints appears to grade into, and locally interdigitate with, Head Gravel. The two deposits may have a common origin but the latter derives more from the higher river terrace deposits.

Artificial deposits and worked ground

The more extensive areas of artificial deposits and worked ground are shown on the map but many minor occurrences have been omitted for clarity The 1: 10 000 scale maps of the district, listed in the Information sources show a more detailed distribution.

Made Ground

Made Ground is shown in areas where material was deposited by man upon the natural ground surface. There are two main categories:

Worked Ground

Worked Ground is shown where natural materials are known to have been removed, for example in quarries and pits, road and rail cuttings and general landscaping. There are extensive tracts in this district, particularly associated with the extraction of sand and gravel, chalk and brickclay. Along the river valleys, many abandoned sand and gravel workings are now flooded due to the high water table and some of these have been adapted for recreational uses.

Infilled Ground

Infilled Ground comprises areas where the natural ground has been removed and the void, so created, has been wholly or partially backfilled with man-made deposits which may be either natural or waste materials, or a combination of both. Many former sand and gravel pits within the district, particularly in and around Reading, have been used for the dumping of domestic and industrial inert waste materials.

Landscaped Ground

Landscaped Ground consists of areas which have been extensively remodelled or landscaped, with complex patterns of cut and fill, too small to be identified separately. Such areas commonly include parkland, golf courses and major construction sites.

Structure

Caledonian

The north-north-west-trending aeromagnetic anomalies, attributable at least in part to Precambrian and early Palaeozoic igneous rocks, reflect the probable structural strike.

Variscan

The westward continuation of the Variscan Front (Smith, 1985a), just to the south of this district is indicated by the southward termination of the Berkshire Coal Basin (Figure 1), and the unusually steep dips in the Warwickshire Group in the Welford Park Station Borehole [SU 4065 736] west of the district. The thrusts at the Variscan Front, which emplaced early Carboniferous and older Palaeozoic rocks from a southerly direction (Smith, 1985a) are known to lie immediately to the south of the district.

The Berkshire Coal Basin, (Figure 1) lies en échelon to, and south-east of, the larger Oxfordshire Coal Basin (Peace and Besly, 1997) and is aligned north-west–south-east. The Berkshire Coal Basin was truncated to the south by later uplift along the Variscan Front, forming a syncline with a steeper south-western limb.

Alpine

The east–west synsedimentary growth faulting, which bounded the Wessex–Weald basin, reactivated Variscan thrusts and continued spasmodically from Early Jurassic to Early Cretaceous times (Chadwick, 1993). At the base of the Lower Greensand lies the Late Cimmerian unconformity. The shallower depth of this unconformity, south of the district (Figure 5), shows the extent of the inversion along the Wessex–Weald Basin margin. The map of the subcrops (Figure 5) beneath the unconformity shows the influence of the Wessex–Weald Basin, represented by a more complete Jurassic sequence in the south-east (Strat B1 Borehole) than elsewhere.

During the Alpine orogeny, which culminated in the Miocene inversion of the Wessex–Weald Basin, the Variscan thrusts were reactivated again as reverse faults in the Mesozoic cover (Whittaker, 1985). This inversion and subsequent erosion restricted the distribution of Palaeogene strata to the London and Hampshire basins.

In the north of the district, the surface strata dip gently south-eastwards at 1 to 2°. Towards the southern margin, the beds are almost horizontal along the axis of the London Basin syncline. The Palaeogene strata are only gently folded and cut by near-vertical faults aligned northwards with throws up to 20 m.

Post-Alpine

The regional distribution of the shallow marine (late Pliocene–Pleistocene) Red and Norwich Crag formations in southern England provides evidence for the easterly downtilting of the London Basin during the Quaternary, probably in response to subsidence in the southern North Sea Basin (Mathers and Zalasiewicz, 1988; Moffat and Catt, 1986). The tilt was estimated at about 1 m per km for these sediments. This feature is also reflected by the incised easterly flowing drainage of the region; many phases of its evolution are preserved within the district as river terrace deposits.

Chapter 3 Applied geology

Geological factors have a bearing on the siting and nature of future urban and industrial developments. By giving consideration to geological conditions at an early stage in the planning process, it may be possible to mitigate some of the problems commonly encountered during construction work. The diverse local geology gives rise to variable ground conditions and some significant aspects of this are discussed below. Other important factors are water resources, mineral workings, gas emissions and the risk of flooding.

Water resources

The Chalk is the most important aquifer in the region and provides a significant source of potable water. The Chalk groundwater source at Gatehampton, near Goring, is one of the largest in England and Wales. The aquifer is confined by Palaeogene clays except where it is exposed. In the unconfined aquifer, the water table broadly follows the surface topography in subdued form, with subsurface flows away from recharge areas on high ground. Natural annual fluctuations in the unconfined aquifer exceed ten metres under the higher parts of the downs. In the confined aquifer, natural fluctuations of the potentiometric surface decrease away from outcrop.

The hydraulic properties of the Chalk aquifer are complex and result from a combination of matrix and fracture properties. The intergranular porosity of the Chalk is high, often around 35 per cent for Upper Chalk, falling to around 25 per cent for the Lower Chalk in the Thames region (Bloomfield et al., 1995). However the pore sizes are so small that the permeability of the rock is minimal. The high transmissivity of the aquifer is provided by fractures, which are commonly enlarged by solution.

During the 1970s the Chalk aquifer in Berkshire and Marlborough Downs was subjected to a large investigation as part of the Thames groundwater scheme (Thames Water Authority, 1978). The purpose of this scheme was to augment the flow of the River Thames in times of drought by pumping groundwater from the aquifer and transferring it to the perennial sources of the Chalk streams. As a result of these studies a good understanding of the aquifer properties of the Chalk in this area was obtained. It was found that in the Upper and Middle Chalk the vertical distribution of aquifer properties is largely independent of the stratigraphy. The hydraulically productive fractures providing the permeability of the aquifer tend to be concentrated within the top 60 m of the exposed Chalk, or below the layer where the Chalk is confined. Laterally, transmissivity varies significantly in the unconfined Chalk in the Berkshire Downs: values range from commonly less than 50 m2/d under interfluves to around 2000 m2/d under valleys (Allen et al., 1997). Storage coefficients are also larger beneath the valleys.

Rapid groundwater flows are sometimes found in the unconfined Chalk aquifer where karstic-type development has taken place; this is commonly associated with the proximity of thin cover, such as Palaeogene deposits, or Clay-with-flints. For example tracer studies near Stanford Dingley on the floodplain of the River Pang have revealed rapid flows (velocities of over 6 km/d) between swallow holes and a spring known as the Blue Pool (Banks et al., 1995).

Chalk water is normally of very good chemical quality. At outcrop it is hard to very hard, with carbonate hardness predominating. However, at depth in the confined aquifer, the water changes to a soft sodium bicarbonate type as a result of ion exchange. At the same time chloride ion and total dissolved solids increase (though not to impotable limits), fluoride increases and anaerobic conditions set in, where nitrate is replaced by ammonia (Institute of Geological Sciences, 1978).

Surface mineral workings

Throughout the district sand and gravel is worked from the river terrace deposits that occupy the interfluves, valley sides and floors. Sand has also been dug sporadically from the Bagshot Formation. There has been widespread extraction of valley floor peat in the Kennet valley around Woolhampton. This was used for fuel and burnt to produce ash for use as a fertiliser.

The digging of brick clay was once commonplace in the district, most notably from large pits in the mottled clays of the Reading Formation, situated mainly near Reading and Tilehurst. Some of these pits were worked for at least 200 years. Other local sources of brick clay have included the London Clay, 'plastic clays' within the Bagshot Formation, and the Langley Silt (formerly mapped as brickearth). The London Clay is presently dug at Knowl Hill for use as a lining for landfill sites.

The widespread digging of Chalk for marling adjacent loamy land, for burning to produce agricultural lime and as a source of flints for building goes back at least to Roman times. Some of the abandoned pits produced by these various activities have been subsequently utilised for the disposal of inert domestic and industrial waste.

Foundation conditions

There is a range of potential problems relevant to ground stability in the district. (Figure 9) tabulates potential ground constraints and the deposits with which they are commonly associated. In particular, care should be taken when excavating deposits which are of heterogeneous lithological character, such as parts of the Palaeogene sequence where clays are variably interbedded with sands. These conditions can lead to instability of sidewalls and groundwater ingress requiring pumping.

Ground heave and subsidence

The London Clay and Reading formations are dominated by clays, some of which have a relatively high smectite content. These undergo significant volume changes in response to variations in moisture content. Seasonal effects, in which vegetation, especially trees, play a dominant role, were identified by Driscoll (1983) as the most important influences on the amount of shrinking and swelling. During winter months, the clays absorb large quantities of water which is lost during dry periods, leading to extensive cracking. The alternating processes of expansion and contraction may cause structural damage to buildings and roads.

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

Cambering and valley bulges

Cambering of the near-surface strata is a gravitational effect produced by a lack of lateral confinement, commonly on valley shoulders, and occurs where competent beds such as limestones and sandstones overlie clays or mudrocks. The deposits affected move downslope either detaching by brittle fracture or deforming plastically, depending on their character. The formation of valley bulges involves the arching and disruption of strata in valley floors due to lateral pressures consequent on camber processes and unloading that can result when rapid valley incision occurs.

In a detailed study for a proposed reservoir, 79 closely spaced boreholes were sunk in the Enborne catchment. One line of boreholes traversed the Enborne valley between Brimpton [SU 558 647] and Brimpton Common, while a second line crossed a tributary valley at Ashford Hill (Hawkins, 1954). Detailed correlation of the Palaeogene strata encountered in these boreholes showed deformation of these strata caused by both cambering and valley bulging. Disturbance of the strata was noted to a depth of about 30 m; dips of up to 40° were recorded. Comparable effects can be expected near the floors of other valleys cut into the Palaeogene deposits.

Slope stability and mass-movement

No landslips were identified during the geological surveys of this district, which is perhaps surprising given the steep nature of some of the incised valley flanks. Where such slopes are cut into the clays of the Palaeogene strata, those over 7° are prone to failure, and slopes exceeding 3° should also be considered as potentially unstable due to weathering by periglacial processes (Culshaw and Crummy, 1991). Thinly interbedded sequences of sands and clays are also prone to landslip due to the presence of springs and high confined pore pressures leading to a loss of strength. Such sequences are most commonly found near the top of the London Clay sequence and near the base of the overlying Bagshot Formation. Slope stability may also be affected by the presence of weakened strata such as the low-angle, bedding-parallel shear zones that were observed to cause slope failure in borrow pits in the London Clay in the adjacent Windsor district (Chandler et al., 1998; Northmore et al., 1999).

Sandy beds such as those of the Bagshot Formation are prone to gully erosion, especially below springs near the base of the formation.

Chalk dissolution

Chalk is prone to dissolution because of the action of acidic rainwater and groundwater, especially in the near-surface vadose zone. This process was more enhanced during prolonged periods of periglacial conditions in the Pleistocene. The resultant swallow or sink holes are common features of Chalk outcrop in southern Britain, but distinguishing them from small, old chalk pits can be difficult.

In recent years, several dissolutional collapse hollows have been noted in the district, for example at Palmer Park in Reading [SU 738 732] in the mid 1990s (P Worsley, personal communication). Many of these were quickly filled leaving little trace of their existence. Collapse structures have been observed at the contact between the Lambeth Group and the Chalk in several pit sections. Hawkins (1934) recorded a highly irregular contact between the two units at the former Englefield Chalk Pit [SU 625 732]. Here, extensive solution piping and numerous cavities are developed within the Chalk, some of which are partially filled by sediment washed in from the overlying strata. These cavities are interpreted as former underground watercourses. Other collapse structures were observed during this resurvey in the otherwise planar top of the Chalk surface, for example at Rushall Farm Pit [SU 4588 1726] where a depression 2 m deep and up to 3.5 m wide is infilled with collapsed sediments of the Lambeth Group. The Chalk adjacent to the feature is rubbly in texture indicating preferential dissolution along fractures and joints. Many borehole logs record a comparable upper rubbly top to the Chalk, particularly beneath the lower river terrace deposits where the Chalk is commonly in contact with flowing groundwater.

Further evidence of the (unusual) geotechnical properties of Chalk under periglacial conditions is afforded by a diapiric mass of brecciated Chalk that pierces the overlying

Palaeogene strata at Ashford Hill in the south-west of the district. The diapir, detected as the result of closely spaced drilling (Hawkins, 1953), was subsequently investigated by Hill (1985) who considered it was initiated during permafrost degradation. It is likely that similar structures exist elsewhere in the valleys of the district.

Gas emissions

The backfilling of former pits and quarries with domestic and industrial refuse produces methane, a potentially explosive gas. Modern landfills are lined with barriers that prevent the lateral seepage of methane and normal practice is to flare-off accumulations of the gas. Older landfills tend to be prone to seepage: these should be monitored with extra care especially where they overlie permeable layers that could store accumulations of the gas.

Natural radon emissions

Radon is a naturally occurring radioactive gas that is produced by the progressive decay of uranium, which is found in small but variable quantities in all soils and rocks. Radon released from rocks and soils is normally quickly diluted in the atmosphere and does not present a hazard. However, radon that enters poorly ventilated spaces such as basements can be potentially hazardous. and can lead to an increased risk of serious illness including lung cancer.

Within the Reading district, radon potential is generally low. The highest radon concentration values identified occur on the Chalk crop, where between 1 to 3 per cent of properties are likely to exceed the UK 'Action Level' of 200 becquerels per cubic metre.

Flooding

Low-lying alluvial ground in valleys adjacent to active river courses is liable to periodic flooding during periods of heavy rainfall. Flood protection measures include the construction of bunds, the regular maintenance of drainage courses and the embankment of land to be used for the construction of buildings and transport infrastructure.

Information sources

Further geological information held by the British Geological Survey relevant to the Reading district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Regional Geologist, Southern and Eastern England, BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries. This features a topographical backdrop based on 1:250 000 scale maps. The indexes are:

Books

Maps

During this resurvey the relevant parts of the component 1:10 000 National Grid maps were surveyed by:

S J Mathers SU 56, 57, 58, 66, 67, 68, 76, 77, 78 1996–1997,

A J Humpage SU 86 NW 1997,

P J Strange SU 86 SW 1996,

I T Williamson SU 87 NW, SU88SW 1996,

A J M Barron SU 87 SW 1996.

The maps are not published but are available for public reference in the libraries of the British Geological Survey at Keyworth and Edinburgh and the BGS London Information Office in the Natural History Museum, South Kensington, London. Uncoloured dyeline sheets or photographic copies are available for purchase from the BGS Sales Desk.

Documentary collections

Boreholes

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

BGS hydrogeology enquiry service; wells, springs and water borehole records.

British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series 268 Reading Sheet are held in the Lexicon database. This is available on Web Site http://www.bgs.ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

Copies of these photographs are deposited for reference in the BGS Library, Keyworth. Those prefixed with BAAS derive from the British Association collection lodged with BGS.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

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

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.

ALLEN, D J, BLOOMFIELD, J P, and ROBINSON, V K. 1997. The physical properties of the major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34.

BANKS, D, DAVIES, C, and DAVIES, W. 1995. The Chalk as a karstic aquifer: evidence from a tracer test at Stanford Dingley, Berkshire, UK. Quarterly Journal of Engineering Geology, Vol. 28, S31–38

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

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

(Index map)

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

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

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

Figures and plates

Figures

(Figure 1) Deep borehole locations in the region and contoured depth to the top of the Lower Palaeozoic strata. Deep boreholes AB–Apley Barn; AT–AstonTirrold; BH–Burnt Hill; FB–Foudry Bridge; HL–Hook Lane; KC–Kingsclere SA1–Strat A1;B1–Strat B1; SE–Sonning Eye; SH–Shalford; WPS–Welford Park Station WW–Wokingham Waterworks O–Other deep boreholes

(Figure 2) Palaeozoic strata proved in the district.

(Figure 3) Major subdivisions of the concealed Jurassic strata.

(Figure 4) Supercrop map and depths to the sub-Mesozoic unconformity.

(Figure 5) Contoured depth to the Late Cimmerian unconformity and subcrop map. W–Wealden Group; Pu–Purbeck Group; Pl–Portland Group; KC–Kimmeridge Clay Formation; Cr–Corallian Group; OxC–Oxford Clay Formation; GtO–Great Oolite Group; InO–Inferior Oolite Group

(Figure 6) Chalk Group strata at crop and in the Fair Cross Borehole.

(Figure 7) River terrace correlation, chronology and terminology within the district.

(Figure 8) Late Quaternary stratigraphy in the Woolhampton area of the Kennet valley, based on Worsley and Collins (1995).

(Figure 9) Potential ground constraints.

Plates

(Plate 1) Rushall Farm Pit, dissolution collapse into the Chalk of the highly glauconitic Upnor Formation with a basal layer of flint nodules; this small hollow is 30 cm across.

(Plate 2) The basal glauconitic shelly clays of the Harwich Formation resting on mottled clays at the top of the Reading Formation at Knowl Hill pit. Graduations on the spade handle are at 10 cm intervals.

(Front cover) The Thames valley looking south-west from Sonning. The photograph shows extensive flooded gravel workings and the town of Reading in the distance. (Aerofilms Ac 616823.)

(Rear cover)

(Geological succession) Geological succession in the Reading district.

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

Figures

(Figure 2) Palaeozoic strata proved in the district

Lithostratigraphical division Maximum thickness in metres Map Code Borehole proving Principal lithologies Subsidiary components Notes
Igneous intrusion D Strat B1 altered olivine dolerite probably late Westphalian
Igneous intrusions D Foudry Bridge dolerite as above
Igneous intrusion D Sonning Eye olivine basalt as above
W ESTPHALIAN
Warwickshire Group About 300 WG Burnt Hill grey sandstones, siltstones, mudstones thin coals
Lavas 123 B Burnt Hill basalts, dolerites
Coal Measures 146 CM Foudry Bridge (also Strat B1) grey mudstones, siltstones, sandstones thin coals
DINANTIAN
Carboniferous Limestone 100 CL Foudry Bridge bioclastic limestones calcareous mudstones Holkerian age
DEVONIAN 454+ Dv Smiling Eye red, grey and purple silty mudstones

(Figure 3) Major subdivisions of the concealed Jurassic strata

Lithostratigraphical division Maximum thickness in metres Map Code Borehole proving Divisions Principal lithologies Subsidiary components Notes
Purbeck Limestone Group PB unrecorded but possibly present south of Strat B1 Duriston and Lulworth formations basal evaporites pass up into mudstone and shelly limestone Durlston Formation of Cretaceous age
Portland Group 24 Pl Strat B1 Portland Stone and Portland Sand formations glauconitic sandstone passes up into micritic limestone
Kimmeridge Clay

Formation

 About 80 KC Foudry Bridge Strat B1 grey

mudstones

 micritic limestones
Corallian Group About 30 Cr Burnt Hill Foudry Bridge Strat B1 ?Sonning Eye argillaceous limestones mudstone interbeds
Oxford Clay Formation About 85 OxC Burnt Hill Foudry Bridge Sonning Eye Strat B1 grey sandy and silty mudstones, locally calcareous skeletal limestone. Cannel coals
Kellaways Formation 17 Kys as above Kellaways Sand and Clay members silty mudstone passes up into fine-grained sandstone single coarsening-up sequence
Great Oolite Group 54 GtO as above ooidal, detrital, micritic and skeletal limestones mudstones, cannel coals; siltstones in lower part Sonning Eye sequence with cannel was assigned by some to the Coal Measures
Inferior Oolite Group 14 InO Burnt Hill Strat B1; probably absent in others limestone mudstone
Lias Group 11 LLi Strat B1 ?Burnt Hill Lower Lias only interbedded limestones, siltstones and mudstones nonsequence at top

(Figure 9) Potential ground constraints

Geological unit Potential ground constraints
Worked Ground variable foundation conditions; unstable sides of old workings
Made Ground variable foundation conditions; leachate and methane production from waste
Infilled Ground as for Made Ground
Head variable foundation conditions
Alluvium compressible strata; risk of flooding; variable foundation conditions
Langley Silt and Brickearth metastable when saturated
River Terrace Deposits high water table in valleys; perched water table on interfluves; possible undocumented former workings
Windlesham Formation loose sand prone to gully erosion
Bagshot Formation local perched water tables and springs; ground heave and subsidence in clays; loose sand prone to gully erosion
London Clay Formation ground heave, landslip and subsidence in clays; high sulphate content groundwater; perched water table and springs in sand layers
Reading Formation variable foundation conditions; ground heave and subsidence in clays; perched water table and springs in sand layers; dissolution collapse close to Chalk contact
Chalk Group slightly elevated natural radon emissions; groundwater protection requirement; undocumented and infilled former workings; dissolution cavities and sink holes
Cambering and valley bulging is a potential ground constraint in Palaeogene strata within narrowly incised valleys