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Geology of the Chichester and Bognor district — brief explanation of the geological map Sheet 317/332 Chichester and Bognor

D T Aldiss abridged from the Sheet Description by A A Jackson

Bibliographic reference: Aldiss, D T. 2002. Geology of the Chichester and Bognor district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 317/332 Chichester and Bognor (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2002.

© NERC 2003 All rights reserved

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

(Front cover). Part of an archaeological excavation at Amey's Eartham Pit, Boxgrove Common [SU 92 08] in 1998. The upper part of the section has yielded a varied vertebrate fauna including the hominid 'Boxgrove Man' and Palaeolithic artefacts. The sediments are mainly raised beach deposits of possible Cromerian age, with head gravel of Devensian age at the very top of this section. Scale: the section is about 4.8 m high (A15568) (Photographer P M Hopson).

(Rear cover)

(Geological succession) Succession of the Chichester and Bognor district.

Notes

The area covered by the geological Sheet 317/332 Chichester and Bognor is referred to as 'the district'. National grid references are given in the form [SU 890 125]. Symbols in brackets, for example (NCk) 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.

Acknowledgements

This Sheet Explanation is based entirely on the account given in the Sheet Description by D T Aldiss, and abridged by A A Jackson. Contributions to the Sheet Description have been made by the following: B C Chacksfield, J D Cornwell, S Holloway contributed to the section on Pre-Cretaceous rocks and hydrocarbons; C R Bristow, P M Hopson, A A Morter, M A Woods on the Lower Cretaceous and Upper Cretaceous; C D R Evans on offshore geology; P M Harris on Mineral Resources, and P R N Hobbs on Applied Geology. C J Wood, M A Woods and I P Wilkinson assisted with palaeontology during geological mapping in the South Downs.

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

© Crown copyright reserved Ordnance. Survey licence number GD272191/2001.

Geology of the Chichester and Bognor district (summary from the rear cover)

(Rear cover)

The Chichester and Bognor district lies on the south-western edge of the Weald, stretching from the South Downs to the coast at Selsey Bill. Geologically, it lies within the Wessex basin, on the southern limb of the Wealden Anticlinorium. The oldest formations known to be present at depth are of Devonian and Carboniferous age, but the main period of basin development and sedimentary deposition occurred during the Mesozoic, and a thick sequence of Jurassic strata underlies the Cretaceous rocks that outcrop at the surface. Sedimentary deposition was eventually terminated in mid-Cainozoic times, when extensive earth movements resulted in the formation of a series of large en échelon folds, each marking major faults at depth.

This brief account describes the geology of the district. The oldest exposed strata are part of the Lower Cretaceous Wealden 'Group'. Passing south, these are successively overlain by the Lower Greensand Group, Gault and Upper Greensand formations and the Upper Cretaceous Chalk Group. Cainozoic (Palaeogene) strata underlie the broad coastal plain, and include the Reading Formation, London Clay Formation and the Bracklesham Group.

During the Quaternary, the district lay just beyond the limits of the great ice sheets. Sediments laid down during Pleistocene and post-Pleistocene times include aeolian deposits, residual deposits, mass-movement deposits, river terrace, alluvial fan, alluvium, peat, estuarine and marine deposits. On the coastal plain, ancient marine deposits occur at five levels, and up to about 43 m above the present sea level. The oldest of the raised beach deposits possibly date from the Cromerian (524 to 478 ka), and contain a varied vertebrate fauna including the hominid 'Boxgrove Man'. The preservation of these and several other interglacial deposits on the West Sussex coastal plain is of international significance.

The hydrogeology of the district is summarised. The Chalk is the major aquifer in the region, but significant water supplies are also obtained from other geological horizons. Mineral resources include sand and gravel, brick clay, building stone, chalk and hydrocarbons. Ironstone was worked in the past, and fuller's earth is recorded, but has not been worked.

Potential geohazards within the district include subsidence due to dissolution of the Chalk, cambering, landslip, swelling clay, flooding, buried clifflines, and sunken lanes. The possibility of risk from these factors should be taken into consideration in any construction or land development project.

Chapter 1 Introduction

This Sheet Explanation describes the part of West Sussex that is included in the geological map, 1:50 000 Series Sheet 317/332 Chichester and Bognor. It is based on the fuller account given in the Sheet Description (Aldiss, 2002) which also includes a full bibliography.

The Upper Cretaceous Chalk Group underlies most of the district (Figure 1), and gives rise to the characteristic long dip slopes and escarpments of the South Downs. To the north of the main Chalk escarpment, Lower Cretaceous strata underlie a series of clay vales and intervening low-lying sandstone escarpments on the southern limb of the Wealden anticlinorium. The coastal plain and the natural embayments of Chichester and Pagham Harbour are founded largely on Palaeogene strata that lies in the eastern part of the Hampshire Basin. Concealed beneath the surface strata is a thick sequence of Jurassic and early Cretaceous rocks deposited in the Weald Basin. The bedrock is largely covered by a variety of Quaternary deposits, including extensive brickearth, gravelly peri-glacial deposits, alluvium and the remnants of a complex series of river terraces. Raised marine deposits occur at several levels up to about 43 m above OD; these are associated with an important archaeological site at Boxgrove, which has yielded Palaeolithic artefacts and the earliest British hominid.

This area was a focus of early geological research; Gilbert White (1789) recognised the principal formations from the Lower Greensand upwards. The district was systematically surveyed at the scale of one inch to one mile by H W Bristow and F Drew. The resulting map (Sheet 9) was published in 1864, and a memoir The Geology of the Weald covering a part of this and adjacent sheets was published later. Large-scale surveys followed. The current map is based on a resurvey of the district at 1:10 000 scale carried out between 1979 and 1982 (partly with the support of the Department of the Environment), and 1992 to 1994. An account of the Chalk as an aquifer is given in Jones and Robins (1999). The search for oil and gas in the Wessex Basin, including this district, provided much data and a better understanding of the concealed geology of the region (Chadwick, 1993 and references therein).

Geological history

The Chichester–Bognor district lies within the Permian to Mesozoic Wessex Basin (Whittaker, 1985), on the southern margin of the Wealden Anticlinorium. It is underlain by Palaeozoic strata, which were strongly deformed during the Variscan orogeny, a period of mountain building that culminated at the end of the Carboniferous. A relatively thin sequence of red beds of Permian or Triassic age rests with marked unconformity on the Variscan basement. Crustal extension shaped the basins and highs that controlled subsequent deposition during Jurassic and Cretaceous times. The district straddles the northern margin of the Hampshire–Dieppe High and part of the Weald Basin. The boundary between these two structural domains lies along the Portsdown–Middleton faults (Figure 2), which underlie the Portsdown and Littlehampton anticlines. The London Platform (Anglo-Brabant Landmass) lay to the north and east and formed an important high that influenced sedimentation patterns throughout the Mesozoic. Thick sediments accumulated in the Weald Basin during periods of active subsidence, and thinner sequences were deposited on the intervening highs that were exposed and eroded at various times. The Lower Greensand, Gault and the Upper Greensand onlap the London Platform; the deposition of chalk sediment covered the whole of it. For much of the Paleocene, the region formed part of an area with no net deposition, but eventually a marine transgression spread from the north and at the start of the Eocene the district lay within a broad embayment that included the London, Hampshire, Belgium and Paris basins. During the Pleistocene, global sea level rose and fell according to the prevailing world climate. During glaciations, when the ice sheets were most extensive, the sea receded from the English Channel. A periglacial environment was established in southern England, in which reduced vegetation cover, intense frost activity (including the formation of permafrost) and a considerably lower drainage base level led to extensive erosion and to mass-movement of superficial deposits. Three or four major periods of cold climate affected southern England during the Quaternary, the most severe during the Anglian glaciation. During the intervening interglacial stages, when the climate was, in general, warmer than at present, rising sea levels flooded the coastal plains and the lower courses of the river systems. Beach and nearshore sediments were deposited along the margin of the English Channel.

Chapter 2 Geological description

Devonian to Lower Cretaceous rocks are buried beneath younger strata of the Chichester–Bognor district. They are known from seismic surveys and boreholes drilled for the hydrocarbon industry; a fuller description is given in Aldiss, 2002.

Sandstones tentatively assigned to the Old Red Sandstone facies of the Devonian were proved beneath the Mercia Mudstone Group in Baxter's Copse Borehole, and a Westphalian flora was identified in a succession of siltstone, sandstone and mudstone below Triassic strata in the Middleton Borehole (Figure 2). Triassic red beds assigned to the Mercia Mudstone Group have been identified in a number of boreholes and have a maximum proved thickness of 106 m in Storrington Borehole. These are overlain by fissile dark grey mudstone and pale limestone of the Penarth Group, which marks a widespread marine transgression in Rhaetian times. The whole of the Jurassic System is represented at depth below the district. Boreholes show that the sequence is 800 to 900 m thick, and that it rests conformably on the Penarth Group, but offshore it overlaps onto Devonian strata.

Concealed, Lower Cretaceous strata of this district include the Purbeck Group to high in the Weald Clay Formation (Figure 3). The top part of the Weald Formation and the remainder of the Lower Cretaceous succession crop out in the north of the district. The Wealden 'Group' generally thickens northwards, towards the centre of the Weald Basin; this informal term includes the 'Hastings Beds' and overlying Weald Clay Formation. It is more than 500 m thick in the north-east of the district, but less than 50 m in the south-west, and seismic evidence suggests that it is absent at depth in the south. Deposition was predominantly in freshwater conditions, in a large shallow lake or lagoon that occupied much of present-day Hampshire. The 'Hastings Beds' comprises interbedded sandstone, siltstone and mudstone, and represents three major coarsening-upwards cycles.

Lower Cretaceous

Weald Clay Formation (WC)

This formation crops out in the north-east of the district, where the upper 130 to 160 m of the formation is present; total thickness at outcrop is just over 400 m. It consists mainly of noncalcareous, carbonaceous clay and silty clay with subordinate thin sandstone, clay ironstone and rare limestone. The sandstones may be traced over several kilometres; they are generally silty, very fine grained, commonly micaceous, quartzose and soft.

The limestone beds are of the 'Paludina' type, and are composed of closely packed shells of the gastropod Viviparus fluviorum, set in a matrix of broken shells and other fossil debris. The mapped beds of limestone of this type may consist of up to four layers, each up to 0.3 m thick, separated by calcareous mudstone or clay. Ironstone occurs 30 to 50 m below the base of the Hythe Formation, and can be traced by the presence of bell-pits (p.24).

The fauna of the Weald Clay includes ostracods, the fresh-water gastropod Viviparus, and the marine to brackish bivalve Filosina gregaria ('Cyrena'). Reptilian bones, fish scales and teeth and fragmentary plant remains also occur.

Lower Greensand Group (LGS)

This group consists of tidally influenced, shallow marine and shoreline sands and clays, and is divided into four formations (Figure 3). The boundary with the underlying Wealden 'Group' is marked by the Late Cimmerian unconformity, the effects of which are greater at the margin of the Wessex Basin.

The Atherfield Clay Formation (AC) crops out in the north-east of the district, and consists dominantly of stiff, shelly, clay or clayey silt of early Aptian age (Figure 4). In places it is sandy and generally contains variable proportions of glauconite.

The Hythe Formation (H) consists mainly of glauconitic, fine- to coarse-grained sand and sandstone, with some interbedded stronger sandstones and doggers that have either a calcareous or siliceous cement, the latter forming chert. Some sandstone is cross-bedded and some beds coarsen upwards. Beds of clay or sandy clay, and thin seams of fuller's earth

The Sandgate Formation is divided into members (Figure 3), but it is not clear how far these can be traced southwards at depth, where overlap on to the Portsdown Structure (the remnant of the Hampshire– Dieppe High) is likely to have occurred.

The Easebourne Member (Eb) has not been recognised to the east of Egdean [SU 99 19], where it is assumed to pass laterally into, or wedge out beneath, the Fittleworth Member. It reaches a maximum thickness of about 40 m. The member consists of glauconitic sandstone, which is fine to medium grained with alternations of uncemented and calcareously cemented beds; in places the cemented beds take the form of nodules or 'doggers'. It is likely that there is an erosional gap between the Hythe and Sandgate formations, a mid-Aptian unconformity, because there is no evidence for the martinioides Zone. A discontinuous bed of smectite-rich mudstone, up to 4.5 m thick, is present locally near the top of the Easebourne Member near Tillington.

The Fittleworth Member (FiB) is rarely exposed, but augering generally reveals weathered yellowish and orange-brown, sandy clay that gives rise to a sticky clay soil. Where weathering is less deep, bright green, glauconitic, sandy clay and clayey sand are commonly found. The base can be recognised by the appearance of conspicuous clay-rich sand and clay above the sand of the Easebourne Member or of the Hythe Formation. Generally, the top is marked by the appearance of well-sorted, fine-grained sand of the Pulborough Sandrock, but from Storrington eastward, it is marked by the appearance of silty clay of the Marehill Clay Member.

South-east of Selham [SU 86 22] to [SU 94 20], over a distance of about 8 km, the Fittleworth Member is divided by the Selham Ironshot Sands Member. Westwards from Stedham [SU 86 22], the combined Fittleworth Member and Selham Ironshot Sands Member are replaced by the Rogate Member.

The Selham Ironshot Sands Member (SIS) consists of poorly sorted ferruginous sand, mainly medium to coarse grained, and including abundant coarse-grained to granule-sized polished grains of limonite. The sand is massive and cross-bedded. It is generally poorly cemented, but with ferruginous 'iron-pan' (carstone) concretions in places. It appears to take the form of a large-scale lens or channel, up to about 25 m thick. Eastwards it passes laterally into the Fittleworth Member and westwards into the Rogate Member, although the exact relationship with these units remains unclear.

The Rogate Member (RoB) is characteristically coarse-grained to pebbly sand or calcareous-cemented sandstone, and may include pebbles of highly polished limonite, glauconite and clay; colour varies from yellowish brown to greyish orange. The member is about 8 to 15 m thick in the north-west of the district near Iping Common around [SU 845 226]. No diagnostic fauna has been found in the Rogate Member but its relationship with the overlying Pulborough Sandrock and with the Easebourne Member suggests that it probably falls within the subarcticum Subzone (Figure 4).

Throughout its outcrop, the Pulborough Sandrock Member (PSk) consists of homogeneous, friable, well-sorted, locally cross-bedded, fine-grained sand, generally grey in colour where unweathered. Grey clay seams up to 1 cm thick occur in places. The upper sands are mostly only weakly cemented, but the topmost 0.1 to 0.3 m of sandrock is usually iron-cemented, and sufficiently resistant to give rise to a distinct low scarp.

The scarp feature formed by the Pulborough Sandrock dies out eastwards at Cootham [TQ 0760 1462] and farther east the member can no longer be recognised as a distinct unit.

The collective fauna from the Pulborough Sandrock contains some very characteristic fossils that bear witness to an apparent surge of warm water during the late Aptian when exotic life forms were introduced into this area.

The Marehill Clay Member (MhC) at the top of the Sandgate Formation consists of dark grey, blocky weathering, silty clay that is locally sandy or glauconitic. Dark grey and lilac clayey silt or waxy clay also occurs in places.

In the extreme north-west of the district, and westwards to the east side of Petersfield, two units of the Marehill Clay can be mapped, separated by an upper 'leaf' of the Pulborough Sandrock. The Elsted Borehole [SU 8422 2093] proved 6 m of olive-grey sandy clay of the upper 'leaf' overlying 8 m of olive-green, clayey sand of the Upper Pulborough Sandrock, resting on at least 8 m of olive-grey sandy clay (the lower 'leaf' of the Marehill Clay). Eastwards, a lens of brown and green sand with some silty clay layers occurs within the Marehill Clay; 14.5 m was proved in the Heathend Borehole [SU 9644 1876].

The formation maintains a thickness of between 8 and 12 m over much of the outcrop, but shows a marked variation particularly in the west, near Iping Common.

Folkestone Formation (F)

The Folkestone Formation consists of yellow, fawn and orange sand and sandstone, which are clean, well sorted, moderate to well rounded, cross-bedded, medium and coarse grained; the sandstone is weakly cemented and friable. The coarser grained sands are slightly pebbly in places and contain small, well rounded quartz pebbles up to about 5 mm in diameter. Sporadic thin seams (up to 12 mm thick) of white, grey or lilac pipeclay are present; they are largely confined to the bottomset beds, but may also form drapes on the foresets. Veins of secondary ironstone (carstone) commonly form a rhomboidal trellis pattern in quarry faces. The boundary with the underlying Marehill Clay is sharp, possibly with some erosion. The cross-bedded units are up to 5 m thick, foreset beds dip at 25º to 35º and indicate palaeocurrent directions dominantly from the north-west.

A persistent bed, the 'Iron Grit', occurs at the top of the formation, from Midhurst eastwards. It is 3 to 18 cm thick, and consists of very hard, limonitic, pebbly, coarse-grained sandstone or arenaceous ironstone. This is a deep purplish colour where fresh, weathering to blood-red, mottled with black. Included pebbles are well rounded and up to 5 mm in diameter, and consist dominantly of quartz. The Iron Grit was deposited under lagoonal conditions on the northern flanks of newly emergent land, known as the Portsdown Swell.

The Folkestone Formation ranges in thickness from 44 m near Hardham [TQ 04 17] to 67 m in Tripphill Farm Borehole [TQ 0071 1760]; an exceptional and incomplete thickness of 71.32 m was proved in a borehole east of Hardham waterworks [TQ 0495 1755].

The Folkestone Formation spans the Upper Aptian–Lower Albian Substage boundary; the top is truncated by a widespread break in sedimentation in tardefurcata Zone.

Gault Formation (G)

The Gault was deposited in deeper marine conditions. It consists of grey and bluish grey silty mudstone, with sporadic bands of scattered phosphatic nodules up to 15 mm across. Where the Iron Grit is present at the top of the Folkestone Formation, the Gault is commonly a bright red colour, and the basal metre may be coarsely sandy (Plate 2). Elsewhere, the base may be marked by glauconitic sandy clay. The best section in the Gault was at the Marley Tile Company's pit [TQ 0953 1389] at Storrington, where basal fossiliferous mudstone yielded ammonites of the Middle Albian spathi Subzone (Figure 4). A total thickness of 91.6 m was proved in a borehole at Amberley [TQ 0282 1323], and is taken to be representative of the district.

Upper Greensand Formation (UGS)

The Upper Greensand is, in part, the lateral equivalent of the Gault and was deposited in a shallow marine, near-shore environment. The greater part of the formation consists of siltstone that is fawn, grey and greenish grey, calcareous, bedded, bioturbated and variably argillaceous. The siltstone shows a characteristic 'streaky', wispy bedded structure produced by lenticles of clay. Cross-lamination occurs locally and the siltstone is slightly glauconitic in places; it weathers to pale buff and very pale grey colours. In addition, there are scattered lenticular beds of very hard, grey or bluish grey, calcareous, uniform siltstones with a subporcellanous texture, colloquially known as 'malmstone'. The base of the formation is transistional with the Gault. Around Cocking [SU 87 17], the top appears to have been removed by erosion. The Upper Greensand is some 36 to 40 m thick in the west, thinning to about 25 m east of Cocking.

Upper Cretaceous

The Chalk Group is about 400 m thick and forms the South Downs which crosses the centre of the district. The lithostratigraphical scheme used on the geological map (Sheet 317/332) is based on the work of Mortimore (1986) and Bristow et al. (1997), although recently a new nomenclature has been introduced Rawson et al. (2001) (Figure 5).

Most of the group consists of soft, white to off-white, very fine-grained and very pure, microporous limestone (chalk) with subordinate hardgrounds and beds of calcareous mudstone (marl), calcarenite and flint. The lowest part of the succession comprises alternations of calcareous mudstone and marly limestone.

Flint occurs in nodular and tabular form in seams that parallel the bedding, and also as thin sheets along cross-cutting joints and fissures. Local porosity and permeability variations, particularly in response to burrowing (now commonly seen as the trace fossils Thalassinoides and Zoophycos) produced the characteristic burrow-fill 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 long distances.

Lower Chalk (LCk)

The Lower Chalk is about 55 to 75 m thick, and comprises three members, namely the Glauconitic Marl, the West Melbury Marly Chalk and the Zig Zag Chalk.

The Glauconitic Marl has been mapped from the River Arun westwards to near Heyshott [SU 901 175], but is absent from the succession in much of the west of the district. It is present to the east of the Arun, but was not mapped as a separate unit. It has a disconformable contact with the underlying Upper Greensand, and the discontinuous outcrop suggests that a second period of erosion occurred before the deposition of the West Melbury Marly Chalk. The member comprises between 1 and 3 m of partly indurated, calcareous, clayey, fineto medium-grained sandstone and siltstone with black phosphatic nodules and coarse granules. It is bright olive-green, highly glauconitic and bioturbated. In some exposures it is a friable rock, but generally it occurs as a dark green, loose, clayey sand. The West Melbury Marly Chalk consists of soft, pale to medium grey, marly chalk with thin grey to brown limestones in cyclical sequence. The member is exposed principally along sunken tracks and in small degraded pits; limestone debris may be seen as field brash. The base of the succession is marked by a grey calcareous mudstone (1 m thick) with variable proportions of glauconite, which rests on an eroded surface of the Glauconitic Marl or Upper Greensand. Sparse glauconite occurs in the overlying 2 to 3 m of strata. At the top of the member, the Tenuis Limestone is also a pale greyish brown limestone with the ammonite Schloenbachia, but is distinguished by the presence of the bivalve Inoceramus tenuis and an uneven hackly fracture in the limestone. The Zig Zag Chalk Member (ZCk) consists typically of medium-hard greyish blocky chalk. The lower part consists of a rhythmic alternation of calcareous mudstone and marly limestone: it equates with the upper part of the Chalk Marl of the traditional scheme (Figure 5). The base of the member is taken at the Cast Bed (Bristow et al., 1997), a brown, very fossiliferous, silty chalk that is noted for the preponderance of moulds of aragonitic mollusc shells, and marks the appearance of a distinctive assemblage of small brachiopods. The sequence passes up into more massively bedded, smooth, firm chalk with marl partings, traditionally known as the 'Grey Chalk', which constitutes the bulk of the Zig Zag Chalk. The top of the Zig Zag Chalk is here taken at the top of the Plenus Marls, estimated to be 3 to 4 m thick. An old chalk quarry south of Amberley village [TQ 0308 1233] provides the largest and most continuous exposure of the Zig Zag Chalk in the district, where up to about 40 m is exposed.

Middle Chalk (MCk)

Traditionally, the Middle Chalk includes the beds from the base of the Melbourn Rock to the base of the Chalk Rock (Figure 5). The Middle Chalk of this district is about 60 m thick and is divided into two members, the Holywell Nodular Chalk Member overlain by the New Pit Chalk.

The Holywell Nodular Chalk Member (HCk) is a medium-hard to very hard nodular chalk with interbedded flaser marls. It is generally shelly with a gritty texture, which helps to distinguish it from the chalk above and below. At the base is the Melbourn Rock, which is 3 to 5 m thick; it is a very hard, creamy white, massive to nodular chalk with marl partings, and generally lacks significant macrofossil debris. Common remains of the inoceramid bivalve Mytiloides occur locally in rock-building proportions. The sequence also contains thin interbedded plexus marls, only apparent in exposed sections. The upper limit of this member is mapped at the top of the highest occurrence of shell-detrital nodular chalk.

The Holywell Chalk is exposed in a working quarry at Cocking [SU 882 168] where about 11 m of the Holywell Chalk is overlain by almost the entire thickness of the New Pit Chalk. Exposures at the Amberley Chalk Pits Museum (by Amberley station) [TQ 0268 1182] (Mortimore, 1986, fig. 3) and in a nearby pit [TQ 0285 1185] show the full thickness, 22 m, of the Holywell Chalk.

The New Pit Chalk (NPCk) comprises pure white, medium-hard, massive-bedded chalk with regularly spaced pairs or groups of marls, each up to 15 cm thick. It is sparsely fossiliferous. Flints are confined to the upper half of the member.

The whole of the New Pit Chalk is exposed in the large chalk pits at Amberley [TQ 027 116] (Mortimore, 1986, fig. 3), and much of it is exposed in a working quarry at Cocking [SU 882 168].

Upper Chalk (UCk)

The base of the Upper Chalk is taken at the base of the Lewes Nodular Chalk, which coincides with the appearance of hard nodular chalks above the New Pit Chalk. This is somewhat lower than the base as defined in the traditional scheme (at the base of the Chalk Rock or, where this is absent, at the first appearance of abundant flint). In general, the Upper Chalk is characterised by white chalk with numerous flint seams. Here it is divided into six members. The formation is approximately 275 m thick.

The Lewes Nodular Chalk Member (LeCk) forms the top of the primary scarp of the South Downs and in places extends some way down the regional dip slope. It consists of interbedded hard to very hard nodular chalk, with soft to medium-hard chalk and marls. The first persistent seams of flint appear near the base and are conspicuous components of the member. The flints are typically black or bluish black with a thick white cortex.

The lower part of the Lewes Chalk is well exposed in Duncton Quarry [SU 951 156] and also occurs at the top of the Amberley Chalk Pits Museum quarry [TQ 027 118] (Mortimore, 1986, fig. 3).

The Seaford Chalk Member (SCk) is composed of soft white chalk with seams of large nodular and semitabular flint. Near the base, thin harder nodular chalk seams also occur, associated with seams of carious flints giving this member a similar appearance to the upper part of the Lewes Nodular Chalk. Higher in the sequence, the flints are black and bluish black, mottled grey, with a thin white cortex, and they commonly contain shell fragments. Typical fossils of the lower part of this member are the bivalves Volviceramus and Platyceramus; and the upper part contains Cladoceramus and Platyceramus (Mortimore, 1986). It is poorly exposed, but is seen in coastal exposures near Rustington [TQ 04 01].

The Newhaven Chalk Member (NCk) is composed of soft to medium-hard, smooth, white chalk with numerous marl seams and flint bands. Abundant Zoophycos flints occur near the base, and it yields a distinctive crinoid fauna.

The largest exposure of the Newhaven Chalk is in the Black Rabbit Quarry, Offham [TQ 024 085] (Mortimore, 1986), and it occurs in other disused quarries on the southern flanks of Lavant Down [SU 8675 0960] and Highdown Hill [TQ 092 042]. It is also exposed on the coast near Middleton-on-Sea [SU 99 00].

The Tarrant Chalk Member (TCk) is composed of soft white chalk without significant marl seams, but with some very strongly developed nodular and semitabular flints. Small exposures of the Tarrant Chalk occur in disused chalk pits, for example on the south side of Stoke Clump [SU 835 091], at the southern end of Halnaker Hill [SU 921 090] and on Highdown Hill [TQ 092 043].

The Spetisbury Chalk Member (SpCk) appears to be overstepped towards the south-east, as there is no evidence for its presence in the Highdown Hill inlier [TQ 092 043]. Coastal exposures near Middleton-on-Sea [SZ 97 99] have been assigned to the undivided Tarrant and Spetisbury Chalk and to the overlying Portsdown Chalk, so that the Spetisbury Chalk forms a subcrop around part of the Littlehampton Anticline, close to, or at, the base of the Palaeogene sequence. The member consists of firm, white chalk with large flints, including tabular, paramoudra and potstone forms. Fossils include the belemnite Gonioteuthis and distinctive forms of the echinoid Echinocorys.

The Spetisbury Chalk is exposed in the upper part of a disused quarry east of Warningcamp [TQ 0455 0760], and on the foreshore near Middleton-on-Sea [SZ 97 99] where it has been assigned to the undivided 'Tarrant and Spetisbury Chalk.'

The Portsdown Chalk Member (PCk) consists of relatively soft white chalk with common thin marl seams and some flints; in the lower part there are several seams rich in inoceramid shell debris. The base is taken at the Portsdown Marl at Farlington Redoubt [TQ 687 065] on Ports Down (Mortimore, 1986). It is exposed on the coast near Felpham, west of Middleton-on-Sea [SZ 95 99].

Archaeological excavations at Boxgrove [SU 92 08], exposing the older buried cliffline of the West Sussex coastal plain (p.15), revealed chalk debris in landslip deposits at the base of the cliff. The debris suggests that at times during the Pleistocene, the cliffs at this site exposed portions of the Chalk sequence from the Newhaven Chalk up to the Portsdown Chalk, and so were at least 60 m high (Roberts and Parfitt, 1999).

Palaeogene

Palaeogene strata are preserved in the south of the district and are largely concealed beneath younger Quaternary deposits. There are few natural exposures in these generally unconsolidated deposits. The best are found along the coast in the intertidal zone, although in general these can be seen only during the lowest tides. Quarries and pits worked in the past are now degraded and overgrown.

Lambeth Group

In latest Paleocene (late Thanetian) times, deposition was in a swampy, warm lowland, traversed by braided rivers. This gave rise to the characteristically brightly coloured sands and clays of the Reading Formation (Rea) that rests unconformably on the eroded, karstic surface of the Chalk Group. It consists mainly of red, brown, orange and grey mottled clay and silty clay. Localised lenses of sand or sandy clay, up to 8 m in thickness, occur particularly at the top and base; the sand is grey, well sorted and fine to medium grained. Lignite is common in some beds and amber has been recorded, some beds show bioturbation and evidence of root traces and palaeosoil development. In many places there is a basal bed (between 0.1 and 4.0 m thick) of green, glauconitic, fine-grained sand or interlaminated sand and clay, enclosing variable proportions of partly rounded flint gravel and chalk clasts with traces of fossil wood. It is exposed on the foreshore at Felpham where it rests on the Chalk [SZ 952 993]. Fossil fauna and flora suggest an estuarine influence.

Thames Group

The London Clay (LC) is largely undivided in this district, although two named members are present. This formation consists of blue-grey, pyritic, bioturbated, silty and fine-grained sandy clay with interbedded seams or nodules of calcareous cementstone, and beds of rounded flint pebbles. A bed of glauconitic sandy silt occurs at the base (Basement Bed), and a disconformity between this and the underlying Reading Formation is inferred. The sequence contains sheet-like and lenticular bodies of fine-grained sand or sandstone that generally mark the top of coarsening-upward sedimentary cycles. Each cycle, where complete, has a basal pebble bed or glauconitic horizon that passes up into silty clay that becomes progressively more silty and sandy upwards. The cycle is completed by cross-bedded sands. These are commonly glauconitic and some are very shelly. The sands tend to vary greatly in thickness: each is up to about 7 m thick, but they die out laterally. Although the fossil fauna is dominated by marine gastropods and bivalves, crustacean and vertebrate remains have also been found. The lithological changes in each cycle can be attributed to an initial marine transgression, followed by low-energy marine sedimentation and a final progradation of coarse sediment from the margins of the depositional basin.

On the geological map, Sheet 317/332, a pebble bed is shown at the base of the London Clay just south of Clapham [TQ 09 05]. The distribution shown on the map is based on a misinterpretation of pebbly clay-rich superficial deposits in that area.

Foreshore exposures between Bognor Regis and Pagham Harbour [SZ 942 990] to [SZ 895 970] provide a section about 90 m thick through the London Clay. The Bognor Sand Member (BoS) marks the top of one of the depositional cycles. It comprises partly cemented, bioturbated, glauconitic, cross-stratified, fine- to medium-grained sand, and is about 7 m in thickness. It is notably fossiliferous. The Bognor Sand forms the Bognor Rocks, a conspicuous subtidal ledge, cropping out on the shore near Marine Drive [SZ 9225 9856].

The Barn Rock Member marks the top of a higher depositional cycle. This sandstone is glauconitic and very fine grained. It crops out on the shore [SZ 9040 9760] and is about 2.4 m thick.

Bracklesham Group (BrB)

The Bracklesham Group occurs only in the most southerly onshore area of this district north of Selsey Bill, mainly subcropping beneath Quaternary deposits; its offshore extent is poorly known. It is a varied succession of interbedded clay, silt and sand. Shell-rich, lignitic and pebbly beds reflect deposition in transgressive or regressive sedimentary cycles. These beds were probably deposited in an offshore marine environment, but the local presence of brackish water molluscs and abundant lignite suggests that a more restricted coastal marsh environment was established at certain times. The coastal vegetation during the

Eocene thermal maximum included mangrove, dominated by the swamp-palm Nipa. The group is divided into four formations.

The Wittering Formation (Wtt) includes greyish brown laminated clay, some with wavy to lenticular sand interbeds, and sparsely glauconitic, fine- to medium-grained sand with clay flasers, laminae and interbeds. There are some intercalations of marine glauconitic sandy silt and bioturbation is common. The uppermost 13.5 m of the formation comprises a sequence of complex channel-fills of lignitic silts and fine-grained sands with flint pebbles, rich in plant debris. It outcrops on the Bracklesham Bay foreshore [SZ 765 984] to [SZ 808 961].

The Earnley Sand Formation (Ea) comprises greenish grey, glauconitic, bioturbated, silty sand and sand, generally with an abundant marine fauna, including clumps of oysters and shallow-burrowing molluscs. A basal pebble bed rests on the Wittering Formation, at a non-sequence. Pebbles and clay pellets are common near the top of the division, where brackish water molluscs also occur.

The base of the Marsh Farm Formation (MrF) is marked by an upward change from the glauconitic sand of the Earnley Formation to a sequence of thin-bedded or laminated clay and silt with sand laminae, and silty sand with clay laminae. The bulk of the Marsh Farm Formation is thus lithologically similar to the Wittering Formation. The Selsey Sand Formation (Slsy) occurs beneath Quaternary deposits in the extreme south at Selsey [SZ 85 93]. It comprises about 25 m of interbedded shelly, clayey, fine-grained silty sand, silty clay and clayey silt; it is commonly bioturbated, glauconitic and locally calcareous. The contact with the underlying formation is sharp. A shelly marine fauna is found in places, and foraminifera are locally abundant.

Quaternary

At least 40 Ma elapsed between the deposition of the youngest preserved Palaeogene and the oldest Quaternary sediments in this district. During this time younger Palaeogene strata were deposited across much of southern Britain, and subsequently removed following uplift along the Wealden axis during the Neogene.

The Quaternary deposits in this district are described according to their mode of origin, using the classification shown on the geological map Sheet 317/332. Further details of their composition are given in BGS reports (Technical Report WA/VG/02; Mineral Assessment Report, No 138, p.29). A new system of nomenclature that divides the sequence into formations and members was introduced following detailed work carried out recently (Bates et al., 1997). A summary of the Quaternary lithostratigraphical units and chronology is shown in (Figure 6), and a brief summary of the principal events is shown in (Figure 7). The Quaternary geology of the region is discussed in Hamblin et al. (1992).

Marine sands and gravels, up to about 6 m in thickness, on the upper coastal plain have been classified as Raised Beach Deposits (Older) (Slindon Formation of Bates et al., 1997). They are mostly buried by the head gravel, and natural exposures are confined to the sides of a few river channels. Most information about the deposits has come from aggregate workings in the overlying gravels, most notably Amey's Eartham Pit on Boxgrove Common [SU 92 08]. The older raised beach deposits rest on the Chalk on a marine erosion surface that slopes from 43 m above OD to about 25 m above OD over a distance of about 2 km. Although the old cliffline that forms the northern margin of this raised beach is buried by up to 15 m of marine deposits and periglacial drifts it is nevertheless fairly well marked at the surface by a break of slope. The older raised beach deposits extend southwards to the cliffline bounding with the Aldingbourne Raised Beach (see below).

Amey's Eartham Pit (Front cover) is an important archaeological site containing the relics of 'Boxgrove Man' (Roberts and Parfitt, 1999). The sequence is described in detail elsewhere.

The Slindon Formation is divided into three members (Figure 8). The Slindon Silt and the upper levels of the Slindon Sand contain a varied vertebrate fauna including the hominid 'Boxgrove Man' and in situ Palaeolithic artefacts. The association indicates temperate, interglacial conditions but the age of the deposit remains equivocal. Mammal faunas suggest a pre-Anglian and probably late Cromerian age corresponding to oxygen isotope stage (OIS) 13, making 'Boxgrove Man' the most ancient hominid fragment found in the British Isles. However, amino-acid analysis and the composition of calcareous nannoplankton suggest instead a correlation with OIS 11 (Bates et al., 1997).

Raised Storm Beach Deposits (Older) (Aldingbourne Formation) form a low ridge at the southern margin of the older raised beach platform between Chichester and Arundel, and also occur as small outlying patches. They have been modified by cryoturbation, partly reworked into and buried by deposits of head gravel, and dissected by river channels.

Near Boxgrove [SU 909 070] a section shows:

The abundance of clasts other than flint in these gravels compared with the Slindon Gravel suggests a significant change in regional sediment transport patterns. Reworked and in situ flint tools also occur in the gravel. The Raised Beach Deposits (Younger) occur widely on the lower coastal plain, but are generally concealed by aeolian deposits and head gravel, and dissected by river channels. The thickness of the deposits is variable. They rest on the Brighton–Norton Raised Beach, a wave-cut platform up to about 15 km wide that slopes gently southwards from a buried cliffline at about 15 m above OD to close to present sea level near Selsey Bill (Figure 9). The position of the cliffline is fairly well marked at the surface by a break of slope. The younger raised beach deposits comprise a complex sequence of sand, silt, and pebbly sand, with some sandy gravel and clean flint shingle. The greater part of the deposit is of thinly bedded, calcareous, silty, fine- to medium-grained sand containing a few flint or chalk pebbles.

Raised Storm Beach Deposits (Younger) generally up to about 4 m thick, occur extensively between 2 m below OD and 4 m above OD on Selsey Bill, around Pagham Harbour, and near Bognor Regis. They are distinguished from the other raised beach deposits by their predominantly stony composition, and from storm beach deposits by their relatively elevated position. At Selsey [SZ 85 93], they form a low ridge some 3.5 km long, rising through the enclosing brickearth to a height of some 8 or 9 m above OD. Locally, they contain extensive marine faunas and reworked flint artefacts. Tidal River Deposits (formerly Marine and estuarine alluvium) includes the deposits adjacent to tidal rivers that were previously liable to tidal inundation but which now lie at up to 3 m above present high water mark, having been artificially reclaimed for agriculture. The deposits consist of soft, brown and grey mottled, laminated silty clay, silt and fine sand with a sparse shelly fauna. Boreholes indicate that up to at least 36 m of alluvial sediment fill a buried valley beneath the floodplain near the mouth of the River Arun and up to 31 m of alluvium is present near Arundel. At Fishbourne, deposits younger than about 70 AD overlie Roman remains.

Storm Beach Deposits related to present sea level have been mapped at the mouth of Pagham Harbour [SZ 88 91]; [SZ 87 89], and at intervals along the coast to Selsey Bill [SZ 85 92]. They comprise generally narrow ridges of coarse flint gravel and cobbles. The double spit at the mouth of Pagham Harbour has apparently formed by landward accretion and by longshore drift from the south-west, being subsequently breached in the middle. Nearshore and intertidal deposits grouped together as Marine Deposits, undifferentiated include tidal flat and beach deposits below present high water level. They comprise organic mud, silt, channel sand, gravel and sand shoals, together with shell banks within the inlets of the coastal plain. Areas of salt-marsh with predominantly fine-grained deposits occur fringing the tidal inlets of Chichester and Pagham harbours. Elsewhere, the beaches are chiefly sandy, with well-sorted shelly sand, locally with patches of flint shingle. Bedrock crops out intermittently in the intertidal zone.

River Terrace Deposits are widespread at various heights above the present floodplains in the valleys of the River Rother and the River Arun above Amberley [TQ 02 13]. At least seven terrace aggradations have been recognised in the district. The deposits consist of sand, sandy gravel and gravel; some are clayey. The gravel component is predominantly subangular flint with subordinate debris of chert, polished quartz grains and larger fragments of pebbly ferruginous sandstone derived from the Lower Cretaceous sequences to the north. Subrounded flint nodules are also present in places. The River Lavant terrace includes chalk and flint gravel, although this has been decalcified at the surface. Most of the terrace deposits are less than 4 m thick, but some may reach 10 m. Terrace formation occurred over a considerable time span during the Pleistocene (Figure 7). Palaeolithic flint implements are common in the fourth and lower terraces.

On the north side of Chichester, the alluvium of the River Lavant passes downstream into Alluvial Fan Deposits (formerly 'Fan Gravel'; Chichester Formation of Bates et al., 1997), a broad, low-lying sheet of coarse clayey gravels. The gravel is typically composed of subrounded pebbles of chalk and flint, but the deposit has undergone varying degrees of decalcification and little chalk survives near the surface. It contains reworked flint artefacts. The gravel represents one or more cold-climate piedmont alluvial fans, deposited immediately south of the lower buried cliffline (Figure 9). Total thicknesses of gravel may be up to 10 m.

Alluvium occurs in the floor of the valleys with flowing streams, including parts of the Lavant valley which experience only ephemeral flow. Mostly it consists of soft, grey and brown mottled, silty and sandy clay, silt and sand, locally with organic material and peat beds, which generally overlie a basal gravel that may be up to 5 m thick. Calcareous tufa is common in streams flowing over chalk bedrock, and is associated with peat at springs. The alluvium is generally less than 3 m thick, but in the larger flood plains of the Rother and Arun valleys it is typically between 8 and 12 m in thickness.

Small accumulations of peat and peaty material are commonly associated with alluvium, river terrace deposits and the tidal river deposits; most are too small to show on the 1:10 000 Series maps. Larger deposits occur on the margins of the River Arun floodplain and along the Rother. Much of the floodplain of the River Chilt near West Chiltington [TQ 076 170] is underlain by peat, and there is an extensive outcrop of peat at Amberley Wild Brooks [TQ 030 153]. The peat is 1 to 2 m thick, but up to 5 m have been recorded in boreholes in the Rother valley. Peat overlying alluvial clay at about 1.5 m above OD in Amberley Wild Brooks [TQ 0309 1413] was formed in the later part of the Flandrian (radiocarbon date 2620 ± 10 years BP, Shephard-Thorn, 1975).

Aeolian Deposits (loess) mantle large areas of the coastal plain, and are traditionally known as brickearth. They consist of silt or clayey silt, which is fairly homogeneous, yellow-brown or orange-brown, structureless and mainly noncalcareous. The deposits are commonly stoneless, but locally contain a few flint fragments, particularly near the base. In general, the brickearth is less than 2 m thick, but is up to 3.5 m in places. The presence of flint and other pebbles throughout suggests that there has been redistribution of these essentially wind-blown deposits and mixing by solifluction and cryoturbation, possibly with some fluvial input. Thermoluminescence dating of brickearth at Ferring [TQ 095 027] found an age of about 11 ka, and at Selsey [SZ 846 925] an age of about 14 ka (Parks and Rendell, 1992a, b).

Blown sand occurs at the mouth of the River Arun [TQ 02 01], where it is associated with sandy and gravelly deposits along the present-day coastline. At the coast there is a narrow strip of high unstable sand dunes, behind which is a broader area of lower, stabilised dunes. The deposits typically comprise well-sorted medium-grained sands with a proportion of fine-grained shell debris, blown from the intertidal sand flats.

Clay-with-flints occurs as a discontinuous cap to the youngest parts of the Upper Chalk that forms much of the high ground of the South Downs. It is a remanié deposit derived mainly by the modification of the original Palaeogene cover and partly from dissolution of the underlying chalk.

The basal surface of the deposit approximates to the sub-Palaeogene unconformity, but it may occur some distance below that level in solution pipes within the Chalk. Small areas of clay-with-flints are known to exist in addition to those shown on Sheet 317/332. These are of limited extent and for the most part represent the eroded remnants of solution pipe-fills, as seen in the sides of some chalk pits. The thickness of the clay-with-flints is estimated at about 5 to 6 m as a general maximum, but where dissolution of the underlying chalk is most pronounced, it may exceed 10 m.

Clay-with-flints is composed typically of orange-brown or reddish brown clay and sandy clay containing abundant flint nodules and pebbles. At the base of the clay-with-flints, the matrix is typically dark brown, stiff and waxy; it includes relatively fresh nodular flints stained black and/or dark green by manganese compounds and glauconite. It gives rise to a stiff, silty clay soil strewn with flints. The margin of the clay-with-flints is sharply defined on the scarp edge but tends to be diffuse on the chalk dip slope, as, in many places, the deposit passes laterally down-slope into head.

A broad sheet of head gravel covers the northern part of the coastal plain to the west of the River Arun, below a section of the South Downs that has little cover of clay-with-flints. The deposit overlies both the older and the younger raised beach deposits in places but locally these have been incorporated within the head gravel. The more distal portions of the head gravel are overlain by reworked aeolian deposits. Channels floored by head (formerly Dry Valley Deposits) cross the head gravel outcrop. The deposit is composed mainly of coarse angular flint gravel set in a stiff matrix of sandy, silty or chalky clay, although in the upper few metres of the deposit any component of chalk has generally been lost by decalcification. The head gravel is thickest and coarsest along the older buried cliffline (Figure 9), where it is generally between 5 and 7 m thick, and locally up to 12 m. It becomes thinner and more fine grained southwards. The top metre or so has typically been affected by Devensian cryoturbation. Silts from the head gravel at Boxgrove have yielded minimum thermoluminescence ages of between 150 and 225 ka (Parks and Rendell, 1992a), suggesting deposition during the Wolstonian glacial, that is during Oxygen Isotope Stage (OIS) 6–8, but it is likely that head gravel formation continued in Devensian times (Figure 7).

Head occurs widely in valley floors, on valleysides and on interfluves. It is probably much more widespread than shown on the map. In general, clay-with-flints passes downslope into head, locally without a separating break of slope. Head accumulated largely by solifluction and hillwash, mainly under periglacial conditions during the Quaternary glaciations. Its composition varies according to the local sources of material and mode of landscape evolution. It is typically composed of reddish brown to yellow-brown, gravelly silty, sandy clay or diamicton, commonly with variable proportions of coarser material (chiefly broken flint nodules). Borehole records suggest that the head is generally less than 3 m thick, but may be over 5 m locally.

Landslip

Landslips occur along the scarps in the north of the district where the Upper Greensand overlies the Gault, and where the Hythe Formation overlies the Atherfield Clay and Weald Clay. They also occur locally within other formations notably where steep slopes have been eroded at spring-heads or by rivers. Disturbance or loading of the ground in or near landslips is likely to adversely affect their stability.

Worked and Made Ground

Relatively extensive areas of made ground shown around Chichester and between Boxgrove and Eartham are mostly in exhausted gravel workings. There are small areas of made ground associated with coastal land reclamation, for example in Pagham Harbour [SZ 85 96] and in the Chichester Yacht Basin [SU 83 01]. Part of the Chichester Canal near Runcton [SU 88 02] has been backfilled. There are some embankments associated with major road and rail routes, but they are mostly small and not all are shown on Sheet 317/332. In each case, the nature of the fill is not known.

There are significant areas of worked ground associated with mineral extraction around Chichester (p.22). Many areas of worked ground are not delineated on the 1:50 000 Series Sheet 317/332, but are shown on the larger scale geological maps held in BGS archives.

Chapter 3 Applied geology

Hydrogeology

The Hythe Formation has numerous productive wells and there is a spring line at its basal contact with the Atherfield Clay. The Folkestone Formation is also a productive aquifer, particularly in the Wiggonholt Syncline. The Sandgate Formation is regarded as an aquitard. In the north, water is obtained from the Upper Greensand, which is in hydraulic continuity with the Chalk. The Greensands are essentially porous nonfissured aquifers, although parts of the Upper Greensand are weakly indurated with some fissuring. Wells in the uncemented sands require screening. The Upper and Lower Greensands typically show maximum yields of 0.4 litre/second (l/s).

The Chalk is the major aquifer in the district in terms of catchment area, storage capacity and yield (Jones and Robins, 1999). Wells in the Chalk are generally unlined in firm bedrock. Maximum yields from boreholes vary up to 100 l/s, being greatest in the Upper Chalk and least in the Lower Chalk. Chalk groundwater is rather hard, though otherwise of generally good quality. Perennial springs occur near the base of the Chalk Group and commonly near the top of the West Melbury Marly Chalk, but also higher in the sequence. They tend to occur at limestone beds, marl seams or flint beds. Typically along this part of the South Downs, springs at the base of the Chalk yield up to 4 l/s, but yields of up to 150 l/s have been recorded from springs in dry valleys in the Chalk. The aquifer also supplies the baseflow to the rivers draining southwards across the district.

The Palaeogene sands are essentially porous nonfissured aquifers, but the supply is variable in both quality and quantity. Supplies from the sand bodies within the Reading and London Clay formations are small and there may be problems associated with high concentrations of sulphate and iron. Springs may also occur at the base of the Reading Formation, at sand beds in the Reading Formation, or at the base of the head gravel or brickearth where it overlies the Reading Formation.

Limited water supplies have been obtained from the alluvium and river terrace deposits but they are vulnerable to pollution from surface sources. The main hydrogeological significance of permeable superficial deposits is where they overlie impermeable bedrock formations and so support perched aquifers. These may give rise to small springs and water seepages, and create obstacles to aggregate extraction or hazards to construction.

Surface drainage characteristics vary considerably across the district. Perennial streams and rivers occur on the Gault and older Cretaceous strata in the north. Of these, only the River Arun crosses the Chalk outcrop, through a gap in the escarpment. Surface water abstraction for public supply takes place at the Hardham pumping station [TQ 0328 1775], just above the limit of potential saline contamination by tidal waters.

With the exception of the River Arun and the lower reaches of the River Lavant, valleys on the main Chalk crop lie above the seasonal water table and are normally devoid of surface water.

Mineral resources

Information on the mineral resources of this district and adjacent areas is given in Hopson et al. (1998). Minerals have been worked in the past from a large number of small pits and quarries in the district (Figure 10), but as economic interest generally requires large-scale production many of these operations have been abandoned. A large quarry at Bognor Common, near Petworth [TQ 008 213] provides material for construction fill. Chalk is quarried at two sites: Duncton Quarry [SU 951 157] and Cocking [SU 882 168]. The latter calcined chalk to produce quicklime (CaO) for a variety of industrial purposes, but now, as at Duncton, the chalk is mainly crushed for use in agriculture. Where it is sufficiently hard it may also be used for construction fill. The Cocking Pit works the upper part of the Holywell Chalk and much of the New Pit Chalk, while at Duncton the topmost part of the New Pit Chalk and the Lewes Nodular Chalk are taken.

Extensive working of head gravel has taken place at Slindon Bottom [SU 950 082], Valdoe Wood [SU 877 081], Lavant [SU 85 07] and Eartham [SU 92 08] either for washed, graded materials or for untreated 'hoggin'.

Fuller's earth is a clay composed essentially of smectite, specifically in its most commonly occurring form calcium montmorillonite. BGS investigated the occurrence in this area (BGS Technical Report, WA/91/75; IGS Open File Report, 1979/1). A thickness of over 4 m was proved at two localities west of Tillington [SU 9280 2225]; [SU 9419 2178] within the Easebourne Member of the Sandgate Formation, but evidence from boreholes suggests that the bed is lenticular and of limited extent.

Of historic interest is the ironstone that was formerly worked from beds within the Weald Clay, now marked by lines of bellpits (p.25). Wealden iron working is known from pre-Roman times and reached a peak of activity in the 16th century. It died out about the end of the 18th century. These ironstones have no economic significance today. Neolithic flint mines are known to occur locally (Edmonds et al., 1987a), for example on Blackpatch Hill [TQ 0900 0900]. These workings comprise shafts of a few metres depth, with horizontal galleries, some of which interconnect.

Extensive use has been made of the local building stone that gives a particular character to the buildings in the district. Beds of 'Paludina' limestone, known as Petworth Marble, Sussex Marble or 'winklestone', were once worked from a small pit north of Broadford Bridge [TQ 0965 2222]. Indurated calcareous siltstone, known as 'bluestone' occurs as discrete beds within the Upper Greensand. It weathers to a pale greenish white, but retains its blue-grey core. Excellent examples of houses built of this stone can be seen in all of the villages along the base of the chalk escarpment; the most notable are Sutton and West Burton where a majority of the older buildings and many boundary walls are made of trimmed rectangular blocks. Stone from the Upper Greensand was also used in the construction of Amberley Castle (Plate 3) and Parham House [TQ 0601 1420]. Phosphatic chalk from within the Newhaven Chalk near Stoke Clump [SU 835 098] was quarried as building stone, now known as 'Lavant Stone' (Plate 4). In the 12th and 14th centuries it was used in the construction of Chichester Cathedral [SU 859 048], Boxgrove Priory [SU 909 075] and at least 64 other buildings within a 15 km radius of Chichester.

Flint nodules occurring within the Chalk have been used both as dressed squared-flint and single-faced trimmed nodules and give a singular appearance to churches, vicarages and the larger estate houses and farms. Goodwood House provides a particularly fine example of this use of flint.

Hydrocarbons

The district was first explored in the 1930s and, more successfully, in the 1980s with the discovery of the Singleton and Storrington oilfields. The reservoir rocks are in the Middle Jurassic Great Oolite Formation. Significant, but undeveloped, discoveries have also been made at Lidsey and Baxter's Copse. Production at Singleton [SU 885 155] started in 1991, with recoverable reserves then estimated at 0.36 Mt. Production at Storrington [TQ 069 148] started in 1998, with recoverable reserves estimated at 0.154 Mt.

In the Weald Basin, organic-rich shales forming petroleum source rocks occur in the Lower Lias, the Oxford Clay and the Kimmeridge Clay. These beds were buried sufficiently deeply in the axial part of the basin to generate hydrocarbons. Migration probably began in Early Cretaceous times and may have continued until uplift in mid-Cainozoic times, when regional compression caused inversion of the Weald Basin. The hydrocarbons migrated from the centre of the Weald Basin towards its margins. On the southern margin of the basin they accumulated mainly in the Great Oolite carbonate reservoir rocks, in traps formed by a combination of dip and faulting.

Geotechnical considerations and geohazards

Potential restraints to development inherent in the geology of this district include subsidence, cambering, landslip, swelling clays, flooding and buried clifflines. While not a natural hazard, sunken lanes also present some geotechnical problems. In addition, some areas of artificial ground (made ground) may create obstacles to construction works. All forms of land-use planning should take account of these factors, and it is important to carry out proper site-specific studies for any construction. A very general guide to likely or possible problems is shown in (Figure 11).

Subsidence

Lines of bell-pits occur on outcrops of ironstone within the Weald Clay. A typical bellpit was up to about 10 m deep and about 2 m in diameter at the top, widening downwards. Most were backfilled with clay, and settlement then led to the formation of a characteristic crater-like depression. Some of these bell-pits may still be liable to settlement. Although much of the area dug for ironstone is now woodland, some bellpits may have been disguised by agricultural activity or landscaping.

The Chalk is commonly affected by dissolution, which enlarges naturally occurring fissures and fractures. Close to the surface, such fissures tend to develop into steep-sided 'pipes' below an uneven karstic rockhead. This process tends to be more pronounced on the older erosion surfaces, most notably at the base of the clay-with-flints and the Palaeogene, partly as a consequence of acidic groundwaters which issue from the latter (Edmonds et al., 1987a). The solution pipes tend to be steep-sided and filled with clay-with-flints, head, head gravel or recognisable relicts of Palaeogene strata, sometimes with relict pinnacles of Chalk within them (Edmonds et al., 1987b). Some filled solution pipes are marked by small surface depressions (dolines). These typically range in size up to 50 m across and up to 6 m deep, but they can be deeper and much wider, especially where several have joined together. They continue to provide sumps which drain surface water, and they may be liable to further subsidence. There are records of solution hollows newly appearing at the surface, typically after periods of heavy rainfall. Building structures which cross solution pipes may suffer from differential subsidence. Some of the solution hollows near the margin of the Reading Formation act as springs at times of high groundwater levels.

Shallow underground working of chalk or flint was once common in parts of Kent, Essex and Surrey. Most of these shallow mines ('dene holes' or 'bell-pits') are in areas where the Chalk is covered by Palaeogene or superficial deposits, above the watertable. It is believed that dene holes do not occur in the South Downs, although Neolithic flint mines are known (p.24).

Cambering

Cambering, and the associated dilation of near-vertical fissures to form 'gulls', tends to occur alongside valleys or escarpments where competent strata such as the Lower Cretaceous sandstones overlie plastic clay formations. In this district, this may be expected in parts of the Hythe Formation, the Folkestone Formation and the Upper Greensand, and possibly in some parts of the Pulborough Sandrock. Local steepening of dip close to the eroded edges of sandstone outcrops may reflect the presence of cambering, especially where the local dip direction is towards the edge of the outcrop. (Where anomalously steepened strata dip away from an escarpment, rotational landslipping may be suspected).

Landslip

Landslips are particularly prevalent in the Gault, but also occur in the Weald Clay, Atherfield Clay, Reading Formation, London Clay, and in head (especially that derived from clay-rich formations). Where the landform has not been degraded by subsequent erosion, the upper limit of the landslips is generally marked by a steep slip-scar and the lower limit by a 'toe' which separates the slipped material from the smooth adjacent terrain underlain by undisturbed ground. Landslips may be naturally occurring or be initiated, or re-activated, by excavation work. It may take decades to establish equilibrium of pore-water pressure within clay slopes. Landslips may thus occur in cuttings long after their construction.

Swelling clays

The Gault has a significant smectite content, and so is susceptible to swelling and shrinkage with increasing or decreasing moisture content. This can result in the disruption of roads and of buildings with light, shallow foundations. The Atherfield Clay and some parts of the Palaeogene (mainly the London Clay Formation) are also liable to shrink-swell behaviour.

Flooding

The alluvial plains of the River Arun and its major tributaries are liable to flood after winter rains. The larger valleys in the Chalk outcrop may also flood, sometimes after very local rainfall, depending on the prevailing groundwater levels. Some coastal areas, such as around Selsey Bill, are vulnerable to inundation by the sea, especially when high tides coincide with storms.

Buried clifflines

In parts of the West Sussex coastal plain, raised beach deposits at up to five levels are concealed beneath a cover of head gravel and brickearth. Typically, the northern edge of each raised beach level is marked by the degraded remnants of a cliffline, now obscured by superficial deposits. The clifflines consist of a step up to 10 m high cut into the bedrock, typically with raised beach gravels banked against it (p.15). The thickness and composition of the superficial deposits tend to change markedly across these structures. Civil engineering excavation at these clifflines would encounter significant variations in substrate conditions, possibly creating problems of excavatability or trench stability. Water seepage is likely from the base of superficial deposits where these overlie Palaeogene clays.

Unless marked by a break of slope at the surface, the position of these buried clifflines is not known exactly, although resistivity surveys may help to locate them. Borehole investigation or trenching may be needed to provide additional information.

Sunken lanes

Many minor roads which cross the outcrops of the Cretaceous sandstones, and some of those across the Chalk, are confined to narrow, steep-sided cuttings, several metres deep, commonly with rock outcrops in their sides. Some of these 'sunken lanes' are too narrow for the easy passage of two-way motor traffic. They were formed prior to modern road construction by the progressive break-up of the soil and subsoil by passing traffic and by frost, and removal of debris by surface drainage.

The sides of sunken lanes are prone to continuing erosion, rockfalls or landslip, creating potential obstructions or hazards to road-users. They also present significant obstacles to further infrastructure development, either along or across the line of the road, compounded by the difficulty of carrying out thorough site investigations.

Information sources

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Information on BGS products is listed in the current Catalogue of geological maps and books (available on request) and the Geological Data Index may be interrogated at the BGS web site (address on back cover).

Maps

Geological maps of the district

1:50 000

317/332 Chichester and Bognor, 1996

Large-scale geological maps

The 1:10 000 and 1:10 560 scale revision survey of the district was carried out in 1979 to 1982 and in 1992 to 1994. These maps and the accompanying reports give a detailed description of the area and form the basis of this account and of the more detailed account given in the Sheet Description (Aldiss, 2002).

Copies of the fairdrawn maps are deposited for public reference in the library of the British Geological Survey at Keyworth. Copies of the completely surveyed sheets may be purchased from BGS either as uncoloured dyeline sheets, photographic copies, or as published sheets, subject to availability.

Books and reports

Books

British Regional Geology

The Hampshire Basin and adjoining areas, fourth edition, 1982

The Wealden district, fourth edition, 1965, fourth impression, 1992*

Memoirs

317 Chichester, 1903, out of print*

332 Bognor, 1897, out of print*

The geology of the Weald, 1875, out of print*

Facsimiles may be obtained from BGS library at a tariff that is set to cover the cost of copying

BGS reports

Geology

Copies of the Technical Reports and Open–file Reports for the constituent large-scale maps of the 1:50 000 Series Sheet 317/332 Chichester and Bognor may be ordered from the British Geological Survey, Keyworth, Nottingham.

Mineral assessment

Mineral resource information for development plans: Phase One West Sussex: resources and constraints. BGS Technical Report, WF/98/5.

The sand and gravel resources of the country around Chichester and north of Bognor Regis, Sussex. Description of 1:25 000 resource sheet SU 80 and 90. Mineral Assessment Report, No. 138, 1983.

Preliminary examination of possible fuller's earth clays from Tillington, Sussex. Mineralogy Unit Report, No. 162.

Calcium montmorillonite (fuller's earth) deposits in the Lower Greensand of the Tillington area, West Sussex. IGS Open-file Report, No. 1979/1.

An appraisal of fuller's earth resources in England and Wales. BGS Technical Report, WA/91/75.

Mineralogy of clays from the Petworth– Pulborough area, Sussex. BGS Technical Report, GD 24.44/2.

Petrological report on glauconite-bearing sediments from the Lower Greensand and base of the Chalk in Hampshire and Sussex. Un-numbered report of the Petrology Unit.

Biostratigraphical reports and palaeontological collections

There are 41 unpublished biostratigraphical reports relevant to the district. Enquiries concerning access to the reports and to the palaeontological collections should be addressed to the Chief Curator, BGS, Keyworth.

Hydrogeology

Chalk aquifer of the South Downs. British Geological Survey Hydrogeological Report Series. 1999

BGS Collections

Data collections include over 2230 borehole records from the district, extensive geophysical, geochemical, minerals and lexicon data. Enquiries concerning thin sections and hand specimens of rocks from the district should be directed to the Manager, Petrological Collections, BGS Keyworth. BGS maintains a large photographic collection. Photographs can be purchased; some of the collection can be viewed on our web site http://www.bgs.ac.uk

References

This account draws on BGS memoirs and reports from this and adjacent areas listed on p.29. A full bibliography for this district is given in Aldiss, 2002. Most of the references listed here are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies can be purchased from the Library, subject to the current copyright legislation.

Aldiss, D T. 2002. Geology of the Chichester and Bognor district. Sheet Description of the British Geological Survey, 1:50 000 Series Sheet 317 and Sheet 332 (England and Wales).

Bates, M R, Parfitt, S A, and Roberts, M B. 1997. The chronology, palaeogeography and archaeological significance of the marine Quaternary record of the West Sussex coastal plain, southern England, U K. Quaternary Science Reviews, Vol. 16, 1227–1252.

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

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

Edmonds, C N, Green, C P, and Higginbottom, I E. 1987a. Subsidence hazard prediction forlimestone terrains, as applied to the English Cretaceous Chalk. 283–293 in Planning andengineering geology. Culshaw, M G, Bell, F G, Cripps, J C, and O'Hara, M (editors). Engineering Geology Special Publication, No. 4.

Edmonds, C N, Kennie, T J M, and Rosenbaum, M S. 1987b. The application of airborne remote sensing to the detection of solution features in limestone. 125–131 in Planning andengineering geology. Culshaw, M G, Bell, F G, Cripps, J C, and O'Hara, M (editors). Engineering Geology Special Publication, No. 4.

Hamblin, R J O, Crosby, A, Balson, P S, Chadwick, R A, Penn, I E, and Arthur, M J. 1992. United Kingdom offshore regional report: the geology of the English Channel. (London: H MS O for the British Geological Survey.) I SB N 0 11 884490 3

Hopson, P M, Bloodworth, A J, Harrison, D J, Highley, D E, and Holloway, S. 1998. Mineral resource information for development plans: Phase One West Sussex Resources and constraints. British Geological Survey Technical Report, WF/98/5.

Jones, H K, and Robins, N S. 1999. The Chalk aquifer of the South Downs. Hydrogeological Report Series of the British Geological Survey.

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

Parks, D A, and Rendell, H M. 1992a. T L geochronology of brickearths from south-east England. Quaternary Science Reviews, Vol. 11, 7–12.

Parks, D A, and Rendell, H M. 1992b. Thermoluminescence dating and geochemistry of loessic deposits in southeast England. Journal of Quaternary Science, 99–107.

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

Roberts, M B, and Parfitt, S A. 1999. A Middle Pleistocene hominid site at Eartham Quarry, Boxgrove, West Sussex. English Heritage Archaeological Report, 17.

Ruffell, A H, and Owen, H G. 1995. The Sandgate Formation of the M20 Motorway near Ashford, Kent and its correlation. Proceedings of the Geologists' Association, Vol. 106, 1–9.

Shephard-Thorn, E R. 1975. The Quaternary of the Weald — a review. Proceedings of the Geologists' Association, Vol. 86, 537–547.

White, G. 1789. The natural history and antiquities of Selborne in the county of Southampton. (London.)

Whittaker, A, (editor) 1985. Atlas of onshore sedimentary basins in England and Wales. (Glasgow: Blackie.) I SB N 02169 17883

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

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

(Index map)

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

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

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

Figures and plates

Figures

(Figure 1) Geology of the district.

(Figure 2) Basement structure and location of boreholes. Contours show the sub-Permian–Mesozoic surface.

(Figure 3) Lower Cretaceous strata.

(Figure 4) Aptian–Albian lithostratigraphy and biostratigraphical zonation.* see Aldiss, 2002 for data sources

(Figure 5) Upper Cretaceous succession and comparison with other schemes of nomenclature. The nomenclature used here and on Sheet 317/332 follows Bristow et al. (1997).

(Figure 6) Pliocene and Quaternary deposits in the district.

(Figure 7) Pliocene and Quaternary events.

(Figure 8) Slindon Formation at Amey's Eartham pit [SU 92 08]. The sequence is illustrated on the front cover.

(Figure 9) Deposits of the coastal plain.

(Figure 10) Mineral resources worked in the district.

(Figure 11) Potential engineering ground constraints.

Plates

(Plate 1) Hythe Formation in Bognor Common Quarry [TQ 008 213]. Fuller's earth occurs as a lens 1.8 m thick by 45 m long with the sandstones of the Hythe Formation; it can be seen in the centre of this photograph just below the tipped debris. Several near-vertical open joints are visible in the sandstone. These are thought to indicate cambering of the surrounding ground; some are filled from above by fuller's earth. Main face is approximately 3 m high. (A13582).

(Plate 2) Gault overlying Iron Grit and Folkestone Formation, Heyshott Green [SU 8980 1865]. Scale: section is about 80 cm high (A15571).

(Plate 3) Upper Greensand Stonework at Amberley Castle [TQ 027 132]. The walls are constructed mainly of the local 'malmstone', a hard calcareous siltstone (A14086).

(Plate 4) Lavant Stone used at Boxgrove Priory. Blocks of this distinctive pale brown, bioclastic, phosphatic chalk have been found in at least 65 medieval buildings in the Chichester area, including Chichester Cathedral. Hammer handle is about 40 cm long. (A15567).

(Front cover) Front cover. Part of an archaeological excavation at Amey's Eartham Pit, Boxgrove Common [SU 92 08] in 1998. The upper part of the section has yielded a varied vertebrate fauna including the hominid 'Boxgrove Man' and Palaeolithic artefacts. The sediments are mainly raised beach deposits of possible Cromerian age, with head gravel of Devensian age at the very top of this section. Scale the section is about 4.8 m high (A15568) (Photographer P M Hopson).

(Rear cover)

(Geological succession) Succession of the Chichester and Bognor district.

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

Figures

(Figure 6) Pliocene and Quaternary deposits in the district

(Figure 7) Pliocene and Quaternary events

(Figure 8) Slindon Formation at Amey's Eartham pit [SU 92 08]. The sequence is illustrated on the front cover

(Figure 10) Mineral resources worked in the district

Figure 11 Potential engineering ground constraints