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Geology of the Alresford district — a brief explanation of the geological map sheet 300 Alresford

A R Farrant

Bibliographic reference: Farrant, A R. 2002. Geology of the Alresford district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 300 Alresford (England and Wales).

Keyworth, Nottingham: British Geological Survey © NERC 2003 All rights reserved Printed in the UK for the British Geological Survey by B&B Press Ltd. Rotherham

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) Cover photograph Sarsen stones, probably excavated from gravel workings in the valley head deposits, left beside the A272 near Bramdean [SU 629 272]. The top boulder is approximately a metre in diameter. Photographer A R Farrant. (GS 958).

(Rear cover)

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

Notes

The word 'district' refers to the area of Sheet 300 Alresford. National grid references are given in square brackets; unless otherwise stated all lie within the 100 km square SU. 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 (SU63NW/20). Symbols in brackets for example (LeCk) refer to symbols used on the 1:50 000 map. Numbers at the end of photograph descriptions refer to the official collection of the British Geological Survey.

Acknowledgements

This Sheet Explanation was compiled and written by A R Farrant from data held in the open file Technical Reports for this district. P M Hopson reviewed early drafts of this document. S Holloway and D Evans are thanked for their comments on the structure, concealed geology and hydrocarbon sections, and M A Woods for his review of the biostratigraphy. The manuscript was edited by A A Jackson and R D Lake. Landowners, tenants and quarry companies are thanked for permitting access to their lands.

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty's Stationery Office.

© Crown copyright reserved Ordnance Survey licence no. GD272191/2002.

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

(Rear cover)

Known more for its literary connections with Jane Austen and the gardens of the naturalist Gilbert White at Selborne, the Alresford district is typical gentile English countryside, and is fundamentally a product of the underlying geology. Commencing in the east, a journey westwards begins on the low lying sandy heaths and heavy clay pastureland around Bordon and Woolmer Forest, developed from the Lower Cretaceous sands and clays. Farther south-east around Petersfield, the characteristic ridge and vale country is founded on alternating sands and clays of the Lower Cretaceous Hythe and Sandgate Formations.

Rising steeply above the lowlands is the indented and landslipped Upper Greensand scarp, behind which the land slopes gently down to small villages such as Selborne and East Worldham before rising steeply again up the Chalk escarpment which forms perhaps the most striking feature. This scarp, running north–south across the sheet effectively divides the region into two. Above the scarp, the hills around Medstead and Four Marks, capped by, clay-with-flint drop eastwards down the long gentle dip slopes of the Chalk to the headwaters of the Itchen, around New Alresford. The majority of the East Hampshire Downs with its dry valleys and gently rolling hills is underlain by the Chalk.

The landscape seen today is the result of a very long geological history which stretches back to the Early Jurassic and beyond. The rocks at surface and those beneath the district give valuable information for the understanding of such major earth history events as the opening of the Atlantic and the Channel Basin, the drowning of most of Europe during the Cretaceous Period, the Alpine earth movements and the wide climatic variations in our most recent past.

These events have also created the conditions for the development of oil and gas and their entrapment in the rocks at depth, a feature which manifests itself in the 'nodding donkeys' pumping oil to the surface at places such as Humbly Grove, just to the north of the district.

Chapter 1 Introduction

This Sheet Explanation describes the geology of the western margin of the Weald together with the Chalk downland and primary scarp of north-east Hampshire between Winchester and Alton (Figure 1), (Figure 2).

Structurally, the district falls within the Weald Basin (Figure 3), an eastward extension of the larger Wessex Basin (sensu Underhill and Stoneley, 1998). It comprises a system of post-Variscan extensional sedimentary basins and 'highs' that covered much of southern England (and extended offshore) south of the London Platform and Mendip Hills during Permian to Mesozoic times. At greater depths lie Palaeozoic strata which were strongly deformed during the Variscan orogeny, a period of tectonic upheaval and mountain building that culminated at the end of the Carboniferous. The rocks of the 'Variscan Basement' are weakly metamorphosed sandstones and limestones of Devonian and Carboniferous age. Several major southward dipping thrust zones and north-west-orientated wrench faults have been tentatively identified in the basement, principally from seismic reflection data. These are thought to have originated during the Variscan orogeny. This deformation was followed by a long period of erosion and a major unconformity marks the base of the Permo–Triassic sequence.

In Permian times, subsidence associated with periods of tectonic extension began to affect southern England initiating the development of a number of smaller basins within the Wessex Basin, each bounded by large faults. Sedimentation in the expanding Wessex Basin began to the west of this district. Deposition gradually spread eastwards so that the earliest Mesozoic rocks present, at depth, within the district are a thin sequence of red beds thought to be of Triassic age. Crustal extension was accommodated by reactivation of existing faults in the Variscan basement which show evidence of syndepositional downthrow to the south during Permian and Mesozoic times. The largest of these faults divide the region into a series of structural provinces (Chadwick, 1986) such as the Weald and Channel Basins, separated by the Hampshire–Dieppe High (also known as the Cranborne–Fordingbridge High).

This district lies at the western edge of the Weald Basin which is bounded by the London Platform to the north and the Hampshire–Dieppe High to the south. The Hogs Back–Kingsclere structure marks the boundary between the Weald Basin and the London Platform in the Basingstoke area. The northern margin of the Hampshire–Dieppe High lies in the Portsmouth area and is marked by the Portsdown–Middleton Faults which underlie the northern flank of the Portsdown Anticline (Hopson, 2000).

Syndepositional movement on the major faults resulted in thick Jurassic sequences being laid down on the downthrow (hanging wall) sides; the beds commonly thin against the tilted fault blocks. Major periods of active extensional faulting occurred during the Jurassic and during deposition of the 'Wealden Group' of the Lower Cretaceous. During periods of tectonic quiescence, the rates of sedimentation increased evenly towards the centre of the Weald Basin.

The sea began to flood the Wessex Basin in Rhaetian (late Triassic) times, depositing the Penarth Group. The area of deposition increased gradually throughout the Jurassic, although minor periods of erosion occurred, mainly at the basin margins. By Upper Oxfordian to Kimmeridgian times, the London Platform was probably entirely submerged. Towards the end of Kimmeridgian times, the London Platform began to re-emerge, probably as a result of global sea-level fall and a reduction in the rate of tectonic subsidence. This resulted in erosion on the margins of the Wessex Basin and the beginning of the development of the Late Cimmerian unconformity. This marine regression continued into Cretaceous times, while the environment of deposition changed from offshore marine (Kimmeridge Clay Formation) to shallow marine (Portland Group), to brackish water and evaporites (Purbeck Group), to fluviatile ('Wealden Group'). The final period of extensional fault movement, marked by normal faulting, resulted in the accumulation of thick sequences of 'Wealden Group' sediments in the main fault-bounded troughs in the Weald Basin, whereas the intervening exposed highs suffered severe erosion.

A period of regional subsidence followed, and combined with eustatic sea-level rise, led to a renewed marine transgression of the Wessex Basin. The ensuing deposition of the Lower Greensand, Gault, Upper Greensand, and eventually the Chalk covered all the surrounding high areas including the London Platform.

A global sea-level fall at the end of the Cretaceous resulted in erosion of parts of the higher Chalk units and the development of a pre-Cainozoic unconformity. Later, deposition in Eocene to Oligocene times was followed by the onset of the compressive tectonic regime during mid-Tertiary 'Alpine' earth movements. These movements effectively reversed the sense of movement on the major bounding faults of the Wessex and Channel basins causing inversion of the basins and highs. Uplift is estimated at about 1500 m (Simpson et al., 1989) for both the Weald and Channel depocentres. Subsequently, erosion has unroofed these inverted basins giving rise to the present-day landscape.

Cross-sections showing the main structures are presented on sheet 300 (Alresford). The major extensional faults which control the asymmetric en échelon fold structures such as the Winchester–Meon Pericline can be seen beneath Warnford (Section 2, Sheet 300) and are well picked out by the structure contours on the base of the Lower Greensand.

Chapter 2 Geological description

The stratigraphy of the rocks buried beneath the district is known from boreholes sunk primarily for the hydrocarbon industry. Those at Humbly Grove (SU64SE/19), (SU64SE/20), (SU64SE/21) [SU 6962 4487] just north of the district, Old Alresford (SU63NW/20) [SU 6244 3707], Lomer (SU52SE/18) [SU 5959 2356], Bordon (SU73NE/48) [SU 7883 3642] and East Worldham (SU73NW/28) [SU 7406 3756] form the basis of this account.

At depth, a thin Permo–Triassic sequence of limestone, siltstone, sandstone and breccia overlies beds of siltstone and orthoquartzite tentatively assigned to the Devonian–Carboniferous 'basement'. The structural contours and subcrops of the sub-Permian surface for the southern half of the United Kingdom (Smith, 1985) show a broad band of Devonian rocks flanked to the north by Carboniferous strata stretching from the Mendips south-eastwards into the eastern part of the English Channel and the Weald. A similar map (Sellwood and Scott, 1986) of the sub-Mesozoic floor beneath southern England incorporates the thin and patchy Triassic cover with the older 'basement'.

Devonian to Triassic

The Devonian and Carboniferous (D–C) rocks (Figure 4) proved beneath the district are predominantly cleaved, reddish brown, Devonian siltstones, mudstones and sandstones, and hard recrystallised Carboniferous dolomites and limestones. In the Lomer borehole, near the southern margin of the district, this pre-Variscan basement consists of beds of siltstone and orthoquartzite tentatively assigned to the Devonian. Boreholes to the south (Hopson, 2000) provide further information about these sedimentary rocks. These Devonian continental rocks were deposited on the Brabant Massif to the north of the Cornwall Basin (Zeigler, 1982), a part of the Variscan fore-deep basin. The Humbly Grove boreholes, on the northern margin of the district, proved hard recrystallised dolomitic Carboniferous Limestone, whereas to the east the East Worldham borehole penetrated siltstone, thought to be of Devonian age.

A thin Permo–Triassic sequence of limestone, siltstone, sandstone and breccia overlies the pre-Variscan basement in this district. Permo–Triassic red beds and marine Rhaetian (Penarth Group) sedimentary rocks have been identified in some boreholes locally. The Lomer borehole proved 35.4 m of limestone, dolomitic limestone and breccia. These lithologies may be the equivalent to the thicker sequence seen farther south-west in the Portland–Wight Basin (Hamblin et al., 1992), or, more likely, they represent a basal conglomerate of the Mercia Mudstone Group, analogous to the Dolomitic Conglomerate of the Mendip Hills. There is some evidence to suggest an angular discordance between these beds and the overlying strata.

Jurassic

The whole of the Jurassic System is represented in rocks at depth below the district (Figure 5). They are mainly marine in origin and were deposited within the subsiding Wessex Basin. They rest conformably on the Penarth Group, and reflect predominantly shallow marine deposition. The relatively uniform, cyclical sequences of the Jurassic provide evidence for the eastward shift of the area of maximum subsidence in the Wessex Basin as the faults bounding the Hampshire–Dieppe High became active. The Weald and Channel basin depocentres developed at this time. In general these beds thicken into the Weald Basin.

Cretaceous

Cretaceous rocks outcrop across the majority of the district. The Lower Cretaceous strata (Figure 6) in southern Britain comprise an important sequence that shows considerable vertical and lateral variation in both thickness and facies: the sequence is generally fullest and thickest towards the basin centres (Whittaker et al., 1985). In the Alresford district, the Lower Cretaceous strata are poorly understood with little published regarding their distribution and development. This is particularly so of the lowermost 'Wealden Beds', generally lying concealed at depth beneath the Lower Greensand (LGS), Gault, Upper Greensand (UGS) and Chalk of the District. However, a series of deep hydrocarbon exploration wells drilled across the Alresford and surrounding district provide valuable information on the subsurface nature and distribution of the concealed 'Wealden Group' and other Lower Cretaceous rocks (Figure 7), (Figure 8). Geophysical logs over the Lower Cretaceous interval were run in a number of these boreholes and the various stratigraphical intervals reveal characteristic log signatures that can be widely correlated between boreholes. The Cretaceous period opened with a short-lived marine transgression which produced the characteristic Cinder Bed and associated deposits of the Durlston Formation (Purbeck Group). Despite renewed subsidence at this time, clastic deposition in the Weald Basin was maintained in nonmarine facies by the abundant sediment supply derived from the rising London–Brabant Ridge to the north, Armorica to the south, and other landmasses to the west and south-west. These early Lower Cretaceous sedimentary rocks are informally called the 'Wealden Group' here and include the Hastings Group and Weald Clay Formation (WC). The latter is the lowest exposed subdivision in this district and it crops out in the south-east, near Petersfield.

The concealed Lower Cretaceous strata including the whole of the Weald Clay are summarised on (Figure 6).

Wealden 'Group'

Rocks belonging to the Hastings Group were deposited in predominantly freshwater conditions, in a large shallow lake or lagoon that occupied much of the present area of Hampshire and the Weald. Some indications of periodic erosion and shallow-water brackish conditions occur, suggesting minor flood events from the 'East Anglian Sea' to the east (Allen, 1975). Alluvial and lagoonal mud plains were periodically covered by braided rivers carrying coarser material. Some of the major siltstone–sandstone bodies are thought to have formed by lateral accretion from migrating channels, but the thickest sand units are attributed to accretion of sediment transported into the basin as a result of erosion on a rejuvenated block-faulted source area. The Weald Clay Formation is less varied than the Hastings Group, but two major sequences occur; the lower part is distinguished by thin limestones that contain small forms of Paludina and the upper part contains large forms of the same gastropod. The mudstones are pale to dark grey and yellow-brown in colour, locally variegated with greyish green and brick-red. They contain interbedded sandstones and sparse limestones (some dolomitic) particularly near the base. A thin sideritic mudstone has been worked for iron in the past, marked by a series of bell-pits. The Weald Clay has a small crop at the western end of the Vale of Fernhurst near Petersfield, where only the upper 25 m crop out.

The correlation of the Lower Cretaceous 'Wealden Group' presented here are based upon the published geophysical log interpretations of similar sequences encountered in the Collendean Farm Borehole in the Weald Basin (Whittaker et al., 1985). North–south and east–west correlation diagrams of the Lower Cretaceous sequences (Figure 7), (Figure 8), illustrate that considerable lateral variations exist within the individual units. This is seen not only in the thinning of units to the north and west, but also in the gamma-ray and sonic log responses, which reflect lithological variations. However, despite the thinning of the 'Wealden Group' towards the basin margins, there appears to have been no northerly or westerly overstepping of the earlier 'Wealden Group' sequences by the succeeding sequences. This is illustrated by the fact the main sands and clays, including the basal Fairlight Clay unit and the (thin) Lower Tunbridge Wells Sand, can be identified in all boreholes (Figure 7), (Figure 8). Indeed, the Fairlight Clay is thought to be present as far as the Farley South and Odiham boreholes, beyond which the entire 'Wealden Group' and Lower Greensand may be truncated beneath the Gault and Upper Greensand.

The lateral change in borehole log character is most clear in the Weald Clay. In thicker more basin-ward locations the gamma-ray response of the Weald Clay is high, the log signature being of a finely and highly serrated character. This changes to the north and west, becoming more deeply serrated ('ratty'), with increasingly thick units of lower gamma-ray value giving a blocky character. There are also indications of higher gamma-ray units decreasing gradually upwards in a cyclic nature. These responses are indicative of a passage to siltier and ultimately sandier facies. This probably reflects an increasingly closer proximity of basin margins to the north and west, beyond Odiham and Stockbridge as the basin filled. It also makes the distinction between the Upper Tunbridge Wells sands and the sandier Weald Clay increasingly difficult towards the basin margins.

Rising sea level in Aptian times flooded the Wessex Basin and eventually led to the re-establishment of a marine connection with the North Sea Basin around the western end of the London–Brabant Ridge. The boundary between the lower, nonmarine sequence, the 'Wealden Group' and the upper marine sequence, the Lower Greensand Group, is marked by the Late Cimmerian Unconformity. This represents a gap in the sequence that is greatest at the margins of the Weald Basin, where much of the Lower Cretaceous is missing, and reduces progressively towards the centre of the basin. In the central Weald, the unconformity is represented by a number of closely spaced minor erosion surfaces, close to the boundary between the 'Wealden Group' and the overlying Lower Greensand Group (Chadwick, 1986; Ruffell, 1992).

Lower Greensand Group (LGS)

Tidally influenced shallow-marine and shoreline sands and clays form the Lower Greensand. Thicker sequences were deposited in the Weald Basin which subsided faster than the London–Brabant Ridge. The Lower Greensand thins towards the basin margins in a similar way to the underlying 'Wealden Group'. Thus, Strat B1 borehole (Figure 7) proves Gault and Upper Greensand lying directly upon Kimmeridge Clay, and it is clear that both the 'Wealden Group' and Lower Greensand are lost northwards, between the Odiham and Strat B1 boreholes. In a westerly direction the Lower Greensand thins considerably, so that if it is present in Farley South, it is very much thinner. The failure of the 'Wealden Group' towards the basin margin may occur through onlap. However, the marked thinning of the Weald Clay noted between the Bordon, Hesters Copse and Odiham boreholes (Figure 7), and the Borden, East Worldham, Old Alresford and Stockbridge boreholes (Figure 8) suggests that the younger part of the Weald Clay was removed prior to the deposition of the Lower Greensand (Figure 7). It is likely that thinning of the Lower Greensand is due to progressive northerly and westerly onlap rather than to erosion prior to the deposition of Lower Cretaceous strata.

Basin subsidence continued into Albian times when the Gault was deposited. This is a sequence of deeper water marine clays, and by late Albian times the Gault had overstepped the London–Brabant Ridge. The Upper Greensand is, in part, the lateral equivalent of the Gault, and was deposited in a shallow-water near-shore environment. It dominates the succession, progressively replacing the Gault towards the western part of the Wessex Basin.

Within the Weald Basin the Lower Greensand Group is divided into four formations which crop out in the south-east of the district.

Atherfield Clay Formation (AC)

This consists of stiff dark grey or brown, fossiliferous, variably glauconitic silty, locally sandy, clay of early Aptian age. The formation is about 20 m thick around Petersfield, reflecting a general thickening into the Weald.

Hythe Formation (H)

This comprises medium-grained, glauconitic sandstones with some thin siliceously cemented units. Chert beds are common. The beds span the Lower Aptian–Upper Aptian boundary and range from the deshayesi Zone up to at least the bowerbanki Zone. The thickness of the Hythe Formation varies from 10 to about 92 m. The beds thicken markedly eastwards into the Weald.

Sandgate Formation (SaB)

Around Petersfield, this formation is divided into four main subdivisions (Bristow, 1991). The lowest unit is the Rogate Member, a sequence of clayey pebbly sands interdigitated with calcareous sandstones of the Bargate Member. Locally the succeeding Pulborough Sandrock Member is separated by the Lower Marehill Clay Member (LMhC) into Lower and Upper beds (UPSk, LPSk). The Upper Marehill Clay (UMhC) caps this sequence. The two members cannot be traced farther north than Longmoor Camp, where the Sandgate Formation is mapped as undivided.

The lithologically variable Rogate Member (RoB) is characteristically a pebbly glauconite- and limonite-rich clayey coarse-grained sand. A dominantly clayey unit is thick enough to map in the middle of the unit. The member is up to 47 m thick around Greatham and probably lies within the subarcticum Subzone.

The Bargate Member (Bt) crops out in the north-east of the district and is best developed in the Guildford district (Sheet 285). It consists of friable calcareous glauconitic sandstones and cemented flagstones with a maximum thickness of 35 m.

The Pulborough Sandrock (PSk) is a grey sandstone that is friable, well sorted, uniformly fine grained, glauconitic and shows sparsely developed cross-beding; it weathers to yellowish brown. Locally, the beds are richly fossiliferous: the fauna includes Parahoplites cunningtoni, the subzonal index fossil at the top of the P. nutfieldensis Zone. In the south-east, the member divides into an upper and lower 'sandrock', separated by the Lower Marehill Clay. Around Ryefield [SU 7761 2230] the upper and lower sandstones are 12 and 20 m thick respectively.

The distinctive Marehill Clay Member (MhC) consists of dark grey to purplish grey, silty clay that is locally glauconitic. It is sparsely fossiliferous, containing only undiagnostic foraminifera. In the south-east of the district, the member is split into two parts by the Upper Pulborough Sandrock, which dies out westwards near Petersfield. The upper and lower clays are around 5 and 8 m thick, respectively, but these thicken north-eastwards.

Folkestone Formation (F)

This formation consists of sand and sandstones that are fine to coarse grained and cross-bedded. The upper part of the succession contains common white, grey or lilac clay partings. The maximum thickness is 85 m, thinning to 10 m in the south of the district. Much of the succession is exposed in the West Heath Sand Pit [SU 785 228], where it consists of 25 m of cross-bedded, yellow to yellow-brown, medium- to coarse-grained sand showing an overall dip of about 4° to the south. These sands characteristically occur in large-scale cross-beds, in units up to 3 m thick, but the upper part of the succession consists of 6 m of tabular, friable sandstones, each about 1 m thick and separated by thin (10 mm) grey clays.

At the top of the Folkestone Beds, a thin brightly coloured, sandy ironstone, the 'Iron Grit' (up to 0.1 m thick), is well developed in the Chichester district to the south-east (Aldiss, in press). The most westerly exposure in the Weald was noted near Petersfield around[SU 725 236] (White, 1910).

Gault Formation (G)

The Gault consists mainly of pale to dark grey fissured soft silty clay with scattered phosphatic nodules up to 15 mm across. Its maximum thickness is 115 m at the southern edge of the district, thinning to about 100 m in the Petersfield area, around 95 m in the BGS Selborne boreholes (Hopson et al., 2001; (Figure 9)) (Boreholes (SU73SW/22), [SU 7320 3494], (SU73SE/39) [SU 7540 3435], and (SU73SE/40) [SU 7583 3400]) and about 45 to 60 m thick in the north-east of the district. The weathered profile of many natural exposures shows a gradation up into very soft, pale yellow-brown, plastic clay beneath the active soil layer.

Upper Greensand Formation (uGS)

This formation (Figure 9) consists of bedded, pale yellow-brown, pale grey and greenish grey, bioturbated siltstone and silty very fine-grained sandstone with variable amounts of mica and glauconite. A characteristically wispy-bedding structure is due to small lenses of clay and sand. In this district, there occur apparently lenticular masses of uniform siltstone that are very hard, grey to bluish grey, calcareous with a porcellanous texture and weather to a buff or white colour. These siltstones have been worked for building stone and are colloquially known as the 'bluestone' or 'malmstone'. The latter term has also been used for the local facies of the Upper Greensand as a whole. The Upper Greensand forms a distinct scarp that is commonly landslipped along its crop from Petersfield to Binstead (Plate 1). It is between 35 and 40 m thick to the south of the district but thickens markedly northward to an estimated 50 to 60 m. In the Selborne Borehole No 1. (SU73SW22 [SU 7320 3494]) the Upper Greensand has a complete thickness of 45.8 m.

Chalk Group

Upper Cretaceous chalk forms the extensive scarp that extends north–south across the district, linking the prominent scarps of the North and South Downs. It also underlies the downland to the west, and is up to about 440 m thick. The stratigraphical nomenclature for the Upper Cretaceous used in this district is based on that of Mortimore (1986a) and of Bristow et al. (1995, 1997) and was subject to a Geological Society Stratigraphical Commission review in 1999. The Chalk Group is now divided formally into White Chalk and Grey Chalk subgroups and ten formations that form the basis of the lithological mapping (Bristow et al., 1997, Rawson, 2000.) (Figure 10).

In Cenomanian times, emergent land was present in south-west England, Wales, Scotland and Northern Ireland, and in Brittany. Southern Britain lay at a latitude approximately 10° farther south than at present. Chalky sediments accumulated on the outer shelf of an epicontinental subtropical sea of normal salinity with little terrigenous input.

Grey Chalk Subgroup

Glauconitic Marl Member (GM)

This is the basal part of the West Melbury Marly Chalk Formation. It comprises up to 2 m of partly indurated fine- to medium-grained, calcareous sand with black phosphatic nodules, 2 to 20 mm in diameter. It is bright olive-green, highly glauconitic and bioturbated. This friable rock weathers to a loose, dark green clayey sand.

West Melbury Marly Chalk Formation (WMCk)

This consists predominantly of cyclic sequences of soft, pale to medium grey, marly chalks and thin, grey to brown limestones. The base of this succession is marked by a grey marl with a variable glauconite content, which rests on the eroded surface of the Glauconitic Marl and Upper Greensand. The thickness estimated from the outcrop, ranges from approximately 15 to 35 m. The basal marl with conspicuous glauconite is only a metre or so in thickness but sparse glauconite ranges up to 2 to 3 m into the overlying strata. A hard spongiferous limestone, or closely spaced series of such limestones occurs 1 to 2 m above the basal bed and these may also contain sporadic glauconite grains.

A characteristic limestone marks the middle of the West Melbury succession; it is pale greyish brown, rough textured, thin (10 to 30 cm) and is packed with the ammonite Schloenbachia. Woods (1994) tentatively equated this bed with the 'M3 limestone' at Folkestone (Gale, 1989). Above and below this limestone, a number of thin, grey poorly fossiliferous limestones occur. In general, those below the distinctive limestone contain sponges. These limestones vary in hardness. Some appear locally as 'cemented lenses' and all are laterally impersistent, so that individual beds cannot be identified reliably by 'counting up' (or down) the sequence.

The Tenuis Limestone at the top of the sequence is similar in appearance to the 'M3 limestone'. It is a pale greyish brown, rough textured calcarenitic limestone with Schloenbachia and is distinguished by the presence of Inoceramus tenuis and by its uneven hackly fracture (particularly after frost action). In places, the top of the West Melbury Marly Chalk is absent due to pre-Cast Bed erosion.

Zig Zag Chalk Formation (ZCk)

This formation is composed typically of medium hard, greyish white, blocky chalk. The lower part is more marly and contains some thin limestones. In this district the Zig Zag Chalk is estimated to be between 30 and 75 m thick. The base of this formation is taken at the Cast Bed (Bristow et al., 1997), a very fossiliferous silty chalk that occurs immediately above the Tenuis Limestone in fully developed sequences. Some 3 to 4 m higher, a pale grey, hard, splintery limestone with conspicuous ammonites (Sciponoceras) is the only other marker identified during this resurvey. Higher in the succession the formation becomes less marly and is pale cream or white in colour. This colour change is thought to occur at the level of 'Jukes-Browne Bed 7' (a calcarenite bed with phosphatic nodules). The top of the formation is taken at the base of the Plenus Marls.

White Chalk Subgroup

Holywell Nodular Chalk Formation (HCk)

The formation is composed of medium hard to very hard, nodular chalks, with flaser marls throughout. It is commonly shelly and has a gritty texture. The Plenus Marls Member at its base is a series of closely spaced, brightly coloured marl beds; these are rarely exposed but can be identified in field brash at a number of localities. The Melbourn Rock, just above the Plenus Marls, is a very hard nodular chalk but generally lacks significant shell debris. Including the Melbourn Rock (which is about 5 m thick) this formation is between 15 and 30 m thick, based on field estimates.

New Pit Chalk Formation (NPCk)

This comprises medium-hard, massive-bedded, pure white chalk with regularly spaced pairs or groups of marls, each up to 15 cm thick. It is sparsely fossiliferous, and brachiopods are dominant. In this district, flints are confined to the upper half of the succession although elsewhere they are known to occur sparsely even down to within a few metres of the base of the formation. The formation is between 25 and 45 m thick in this district.

Lewes Nodular Chalk Formation (LeCk)

Interbedded, hard to very hard, nodular chalks, with soft to medium-hard chalks and marls comprise this formation. The base coincides with the incoming of hard nodular chalks just above the top of the New Pit Chalk. The first persistent seams of flint appear near the base. The flints are typically black or bluish black with a thick white cortex. The formation is generally between 50 and 55 m thick over much of its crop, but it may be as little as 40 m over the Warnford Dome and up to 70 m thick in the north-west of the district. The Lewes Chalk is divided into two units by the Lewes Marl and the adjacent Lewes Flints, comprising a ramifying system of black cylindrical burrow-form flints. The lower unit consists of medium- to high-density chalk and conspicuously iron-stained, hard, nodular chalks. The upper unit is mainly low- to medium-density chalks with evenly spaced thin nodular beds.

Seaford Chalk Formation (SCk)

This is composed primarily of soft white chalk with seams of large nodular and semitabular flint. Near the base, thin harder nodular chalk beds also occur associated with seams of carious flints, giving this formation a similar appearance to the upper part of the Lewes Chalk. Hence the boundary is not clear-cut in mapping terms. Higher in the sequence, the flints are black and bluish black, mottled grey, with a thin white cortex and they commonly contain shell fragments. Typically, brash from the lower part of the Seaford Chalk contains an abundance of fragments of the bivalves Volviceramus and Platyceramus; brash from the upper part contains Cladoceramus and Platyceramus (Mortimore, 1986a). In the absence of these bivalves, the flaggy bedded nature and pure whiteness of the soft chalk serve to distinguish it from the Lewes Chalk below. In the extreme west of the district, a thin bed of extremely hard, porcellanous chalk, the Stockbridge Rock Member, occurs a few metres below the top of the formation. This hardground becomes much more extensive to the west in the Winchester district (Sheet 299). In the Alresford district the Seaford Chalk is 45 to 80 m thick.

Newhaven Chalk Formation (NCk)

Soft to medium-hard, smooth white chalk with numerous marl seams and flint bands make up this formation. Typically, the marls vary between 20 and 70 mm thick. They are much attenuated or absent locally, over positive synsedimentary features, where differentiation of the Seaford and Newhaven formations is difficult. Channels with hardgrounds and phosphatic chalks have been recorded elsewhere within the formation (Hopson, 1994; Mortimore, 1986b), but no localities were identified during this survey. In this district the Newhaven Chalk is estimated to be 40 to 70 m thick.

The brash is composed of smooth, angular, flaggy fragments of white chalk, very similar in appearance to that of the Seaford Chalk. The appearance of abundant Zoophycos (a spiral trace fossil) near the base of the formation serves as a useful marker for mapping the lower boundary. Individual thecal plates of the zonal index, Marsupites testudinarius, occur in numerous small pits and trackside exposures, but otherwise macrofossils are rare.

Tarrant Chalk Member (TCk) This is the lower part of the Culver Chalk Formation (CCk) and is composed of soft white chalk without significant marl seams, but with some very strongly developed nodular and semi-tabular flints. The maximum estimated thickness of the Tarrant Chalk is 35 m locally and only occurs in the extreme south-west of the district.

Palaeogene

Much of the once extensive cover of Palaeogene material has been removed by erosion. The Palaeocene strata are preserved only as small outliers in the north-west of the district around East Stratton. This thin sequence (less than 5 m thick) consists of clay, silt and sand. In latest Thanetian times, deposition was in a swampy, warm lowland traversed by braided rivers that deposited mainly sand.

Lambeth Group

Reading Formation (Rea)

This forms part of the Lambeth Group, consists of mottled bright red and grey clays and silty clays. The basal 15 cm of glauconitic sand with abundant small brown, black and glauconite-stained rolled flint pebbles is analogous to the Upnor Formation (or 'Bottom Bed') of the London Basin which rests unconformably on the eroded surface of the Chalk. In the area around East Stratton, similar pebbles are abundant within the clay-with-flints deposits, and indicate a local derivation.

Quaternary

About 60 Ma is estimated to have elapsed between the deposition of the youngest preserved Palaeogene and the oldest Quaternary deposits in the region. During this time younger Palaeogene and Neogene strata were deposited across the whole of southern Britain, and subsequently removed following uplift along the Wealden axis (as part of the general inversion of the Wessex Basin). During the Quaternary, a further break in deposition occurred after the accumulation of the clay-with-flints and before the deposition of the younger Pleistocene drifts.

During the Pleistocene, sea level rose and fell depending on the quantity of water that was locked up in ice caps. At times of glacial maxima, a periglacial environment was established in this district. There was enhanced erosion both by solifluction and by rivers flowing to much lower base levels. Evidence for at least three such glacial maxima can be seen in southern England; the most severe was of Anglian age. (Figure 11) summarises the principal Quaternary deposits and events of Hampshire and West Sussex.

The following descriptions of the deposits are grouped on the basis of their origin. Mass movement deposits are described first, followed by fluviatile and aeolian deposits. Their order does not imply relative age.

Clay-with-flints

This is composed typically of orange-brown or reddish brown clays and sandy clays containing abundant flint nodules and pebbles. At the base of the deposit, the matrix is stiff, waxy and fissured (slickensided), and dark brown in colour. Relatively fresh nodular flints are stained black and/or dark green by manganese compounds and/or glauconite. The deposit gives rise to a stiff, red-brown, silty clay soil strewn with flints. This is primarily a remanié deposit resulting from the modification of the original Palaeogene cover and dissolution of the underlying chalk. The thickness of the clay-with-flints is estimated at about 5 to 6 m, but may be over 10 m where dissolution of chalk is most pronounced.

The margin of the clay-with-flints is sharply defined on the scarp edge but the boundary may be diffuse on the chalk dip slope. This down-slope feather edge is locally obscured by a lateral passage into a late-stage solifluction deposit or head gravel. These deposits have a more sandy matrix and a surface brash composed principally of gravel-sized broken angular flints.

Older Head 1

This consists of soliflucted slope deposits ranging from flinty gravels to reddish brown, sandy clays containing abundant flint nodules and pebbles which are generally much more shattered than those in the clay-with-flints. They represent an earlier phase of solifluction prior to the formation of the main head deposits along the valley bottoms. Several large sheets of older head occur in this district, many are no more than a few metres thick. The deposits are most widespread on north- and east-facing slopes and commonly grade laterally into areas with only a thin flinty veneer.

Older Head 2

These deposits are comparable to Older Head 1, but generally contain fewer flints and they occur on the Lower Cretaceous strata in the south-east of the district.

Head

In general, head comprises yellow-brown, silty, sandy clay with variable proportions of coarser granular material, but all deposits have an earthy texture. Clast composition varies depending on source materials; those deposits derived mainly from the chalk were formerly mapped as 'dry valley deposits' or 'coombe deposits'. These heterogeneus deposits accumulated by solifluction, hillwash and hillcreep and are generally only a few metres thick.

River Terrace Deposits

Four sets of river terrace deposits are present in the district, associated with the major river systems; the Goldalming Wey, the Alton Wey, the Itchen and the Rother. In the north and west (Alton Wey and Itchen river systems) the gravel component is predominantly flint with subordinate quartz and rare 'exotic' clasts. In the south-east and east, associated with the rivers Rother and the Godalming Wey, the deposits are more sandy and many are graded as pebbly sands, reflecting the lithology of the source area. In places clayey and sandy silt and silty clay mask the underlying aggregate, perhaps indicating preservation of overbank or aeolian deposits at the top of each fluvial cycle. Flint again predominates, together with cherts, polished fine-grained quartz and larger fragments of pebbly 'carstone' (sandstones with a ferruginous cement) derived from the Folkestone Beds. In general the terraces are up to 5 m thick.

There is little direct evidence of the age of the terraces, but most aggradations are probably periglacial in origin. Terrace deposits above the Second Terrace show cryoturbation structures indicating that they have suffered at least one periglacial event, and thus suggest they are all pre-Devensian.

Alluvium

The alluvium consists of soft, organic, mottled silty and sandy clay which generally overlies a basal lag gravel. Thin stringers of gravel may occur within the sequence, indicating channel migration or periodic increases in the flow regime of the river over time. In general the deposit is thin, usually between 1 and 3 m in the upper reaches of rivers, but at major confluences and in the lower reaches of the rivers up to 8 m have been proved. A common characteristic of streams flowing over chalk bedrock is the presence of calcareous tufa associated with peat accumulations at springs. No occurrences of calcareous tufa were sufficiently large to be mapped in the district, but thin concretionary carbonate deposits coating stream bed gravels were noted in places in the Meon and Itchen river valleys.

Peat

Small accumulations of peat and peaty material are associated with alluvium and the river terrace deposits but are generally too limited in extent to map. Several outcrops of dark, silty peat in excess of 1.5 m thick were noted east of Petersfield in the Rother valley [SU 772 226] in the extreme south-east of the district.

Blown Sand

East of West Heath Common there is a tract of hummocky ground [SU 794 227] composed of coarse-grained sand, irregularly mantling the Folkestone Beds, Marehill Clay and Pulborough Sandrock. Here it is estimated to be only a few metres thick.

Landslips

These are an ubiquitous feature of the Upper Greensand/Gault contact along much of the crop. They result from a combination of spring-head erosion and the physical properties of pore pressures and high moisture content. The resultant landforms are quite striking; generally they are composites of successional rotational slips and slab slides, with fault-like backscarps up to 30 m high, ponds trapped by slip slices, and hummocky ground commonly with a prominent toe separating the slips from the undisturbed Gault surface. The age of the slips is uncertain; probably they were initiated under periglacial conditions, but the landforms are still remarkably fresh suggesting recent movement. Elsewhere in the district landslips are uncommon.

Made ground

Extensive areas of made ground are associated with major routeways such as the A31, A3 and M3, related to the building of the recent major road network. The nature of the fill is not known in detail.

Worked ground

Only major areas of worked ground, generally associated with mineral extraction, are shown on the published maps. Several areas of infilled ground are shown; most are backfilled old railway cuttings or mineral workings. In most cases the nature of the fill is unknown. Smaller areas of worked, infilled and made ground are omitted from the 1:50 000 maps, but are shown on the larger scale geological maps held in BGS archives.

Chapter 3 Applied geology

Hydrogeology

The Chalk is the major aquifer in the district and has the largest storage capacity and catchment area. Water is also obtained from the Upper Greensand, which is in hydraulic continuity with the Chalk, and from the Lower Greensand. The latter is a separate aquifer beneath the aquiclude of the Gault. Water is also obtained in small quantities from the drift, but supplies are variable in both quantity and quality.

Yields from the Chalk Group of the Alresford district (Flett and Hearsum, 1976) vary between the formations with values of the order of 67 l/s in the White Chalk subgroup. The highest yields, up to 150 l/s, are obtained from large-diameter shafts, boreholes and headings in the upper part of the Chalk sequence. Wells in the lower, more marly Chalk generally provide lower yields. The Upper and Lower Greensand typically show maximum yields of 76 l/s. Some of the water issuing from springs at the Gault–Upper Greensand contact in the Alton Wey valley is probably derived in part from the Chalk which is locally in hydraulic continuity with the Upper Greensand. With the exception of the Itchen, the Alton Wey and Meon rivers and the lower reaches of minor tributary streams in the district, valleys are normally devoid of surface water over the Chalk.

The chalk is microporous with low intrinsic permeability and derives much of its yield from interconnected solutionally enlarged fissures. The Upper and Lower Greensand sands are porous, essentially nonfissured aquifers, although the Upper Greensand is loosely indurated with some fissuring. Wells in the Chalk are generally unlined; those in the sands require screening.

Perennial springs occur near the base of the Chalk, commonly at the top of the West Melbury Marly Chalk. Typically, they produce small yields (around 2 to 4 l/s, but yields of up to 151 l/s have been recorded). The aquifer also contributes to the baseflow of the rivers draining across the Chalk outcrop.

The hydrology of the Candover area in the north-west was the subject of a major investigation by Southern Water in the late 1970s as part of a project to use pumped boreholes to augment the flow of the River Itchen. The Candover Augmentation Scheme (Southern Water Authority, 1979) involved the construction of six boreholes in the Preston Candover area; three of these were used for production and linked via a pipeline to the outflow at Swarraton. The mean flow at the outlet was 148 l/s. The detailed hydrological observations and data are recorded in a report by the Southern Water Authority (1979).

Bulk minerals

Sand and Gravel Sand and gravel has been won from the Folkestone Beds, river terrace deposits and from the gravelly head in the major valleys. Fine- to medium-grained sands are extracted from the Folkestone Beds at West Heath [SU 785 228] in the south of the district and for building sand around Kingsley. Principally flint gravel has been extracted from the gravelly head deposits in the west of the district around Brown Candover [SU 586 399].

Clay

In the past, the Gault, Weald Clay, Reading Formations and the 'brickearth' were used in the manufacture of bricks and tiles. There is one working pit at Selborne where the Gault is worked to produce bricks and specialised brick mouldings. Many other small disused brick pits are known along the Gault clay outcrop. Brick clays have been worked from the Reading Formation around East Stratton and from many sites scattered across the clay-with-flints outcrop.

Chalk

There are many pits in the district attesting to the great historical use of the material for the liming of fields. Many of these are scattered around the clay-with-flint outcrop and may represent former dolines (or swallow holes). Most are abandoned and degraded but agricultural lime is still produced from Dudman's Quarry [SU 658 305] near Ropley Dean and from several small pits near Preston Candover.

Building stone

Building stone is not produced commercially in this district, but in the past locally derived materials have been used in construction. Only the Upper Greensand 'malmstone' or 'bluestone' was extensively worked. The best quality building stone comes from indurated calcareous siltstones ('bluestone') found as discrete beds within the malmstone. Excellent examples of houses built of this stone can be seen in all of the villages along the foot of the escarpment. Limited use has been made of the harder beds within the Chalk sequence (Melbourn Rock, nodular beds within the Lewes Chalk). Flint nodules were extensively used for building, both as dressed squared-flint and single-faced trimmed nodules, particularly in churches and larger houses. Flint, as a 'waste' product of chalk extraction and from 'field picking', has also been used to maintain farm tracks. Some of the Lower Cretaceous sandstones such as the 'Bargate Stone' and the harder cherty sandstones of the Hythe Formation (Plate 2) have been used around Liss.

Iron ore

Iron ore has been worked in the past from a thin sideritic mudstone in the Weald Clay near Petersfield. The workings are mainly shallow bell pits.

Hydrocarbons

The district was first explored in the 1930s and, more successfully, in the 1980s with the discovery of the Humbly Grove Oil Field just to the north of the district. Other oil fields have been found in the Weald basin at Horndean, Singleton, Storrington and Stockbridge. The reservoir rocks are in the Jurassic Great Oolite Formation.

The process of hydrocarbon formation, migration and entrapment is controlled by east–west, pre-Albian extensional faults. The Humbly Grove oilfield is developed on a clearly defined tilted horst block bounded by two such (now reversed) extensional faults. In the basins to the south of the major faults, and particularly in the centre of the Weald Basin, the Lias source rocks, and possibly the Kimmeridge Clay were buried sufficiently deep to generate hydrocarbons. The hydrocarbons migrated south from the centre of the Weald Basin into the Great Oolite rocks of the palaeohighs, where antithetic faults provide traps. Migration probably began in Early Cretaceous times and may have continued until uplift in mid-Tertiary times (Penn, et al., 1987, Hawkes et al., 1998). Although Cainozoic compression caused inversion of the Weald Basin, it did not destroy all the traps. Many anticlines in both the Weald and Wessex Basins, for example the Portsdown and Littlehampton anticlines, do not contain oil and gas, suggesting that primary oil migration ceased before they were formed.

Geotechnical considerations

There are four principal hazards inherent in the strata of this district. The following statements should be taken only as a guide to likely or possible problems and should not replace site-specific studies.

The relatively loose sand of the Folkestone Beds provide unreliable foundations on steep slopes. Freshly ploughed fields or exposed ground can become gullied during heavy rainfall. The Gault contains highly shrinkable clays with a high smectite content. Consequently, they may move and crack during extreme drought conditions. Suitable precautions should be taken during construction. Peat, and other alluvial deposits which contain thin beds of peat, may be liable to compression and differential compaction when the ground is subject to loading.

Landslip and foundering of strata along the springline between the Upper Greensand and the Gault is a known hazard along stretches of the crop. Most other natural slopes are thought to be stable in the district but this can be strongly influenced by human activity, particularly where over-steepened slopes are created during construction work (Plate 3). The Chalk may be affected by solution phenomena which result in small surface depressions (dolines) that range in size up to 50 m across and up to 6 m deep. As a consequence of this solution, fractures naturally occurring in the chalk are enlarged. The resultant pipes that may be filled with clay-with-flints, continue to provide sumps for excess surface water, and may be liable to further subsidence and differential settlement. Solution features are likely to be common on the outcrop of the higher Chalk formations, particularly where a thin clay-with-flints or Palaeogene cover occurs nearby. Chalk has a high natural water content and may lead to slurrying if over compacted. The stability of excavations in the chalk is largely controlled by the frequency and direction of natural cavities and joints.

In addition to the naturally occurring hazards, man has had considerable influence on the landscape. Many of the abandoned sand and gravel, chalk and clay pits in the area have been used as landfill sites, particularly adjacent to the urban areas. Records are held by the local authorities, but old areas of fill may be poorly documented. Cuttings and embankments for major road and rail links are commonplace

Information sources

in the district. Further geological information held by the British Geological Survey relevant to the Alresford district is listed below. 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 Programme Manager, Integrated Geoscience Surveys (South) BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, photographs, geophysical, geochemical and hydrogeological data. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries, and BGS is providing increased online access to its national data sets. Web address: http://www.bgs.ac.uk

Books

• British Regional Geology
• The Hampshire Basin and adjoining areas, Fourth edition, 1982
• The Wealden district, Fourth edition, (1965) Fourth impression 1992
• Memoirs and Sheet Explanations
• Geology of the country around Andover, (Sheet 283), 1908*
• Geology of the country around Basingstoke, (Sheet 284), 1909*
• Geology of the country around Aldershot, (Sheet 285), 1929*
• Geology of the country around Winchester and Stockbridge (Sheet 299), 1912* Geology of the country around Alresford, (Sheet 300), 1910*
• Geology of the country around Haslemere, (Sheet 301), 1968
• Geology of the country around Southampton, (Sheet 315), 1987
• Geology of the country around Fareham, (Sheet 316), 1913*
• Geology of the country around Chichester, (Sheet 317), 1903*
• Geology of the country around Bognor, (Sheet 332), 1897*
• Geology of the Fareham and Portsmouth district - a brief explanation (Sheet 316 and part of 331), 2000
• Geology of the Chichester and Bognor District, a description (Sheet 317 and part of 332), in press.
• * out of print; photocopies may be purchased from BGS Library, Keyworth at a price that is set to cover copying cost.
• Technical Reports
• Technical Reports relevant to the district are described below. Most are not widely available but may be purchased from BGS or consulted at BGS and other libraries
• Geology
• Reference numbers for the technical reports covering the geology of individual or combined 1:10 000 scale geological sheets are given with the list of 1:10 000 Series maps.
• Biostratigraphy
• There are 17 biostratigraphical reports covering the Alresford district. These are held as internal open file reports by the Biostratigraphy Group of BGS under the prefix WH. Readers are recommended to visit the BGS web site at http://www.bgs.ac.uk for details of how to access to these reports and to the Palaeontological collections.

Maps

• Geology
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom 1985
• 1:625 000
• Solid geology of the United Kingdom (south sheet) 1979
• Quaternary map of the United Kingdom (south sheet) 1977
• 1:250 000
• Sheet 51N 02W Chilterns, Solid geology 1991
• 1:50 000 (Solid and Drift)
• Sheet 283 Andover 1975
• Sheet 284 Basingstoke 1974
• Sheet 285 Aldershot 1976
• Sheet 299 Winchester 1976*
• Sheet 300 Alresford 1999
• Sheet 301 Haslemere 1981
• Sheet 315 Southampton 1987
• Sheet 316 Fareham 1997
• Sheet 317/332 Chichester and Bognor 1996
• * remapping recently completed or in progress.

1:10 000/1:10 560

Details of the original geological surveys are listed on editions of the 1:50 000 geological sheets. Copies of the fair-drawn maps of the earlier surveys may be consulted at the BGS Library, Keyworth.

The maps covering the 1:50 000 Series Sheet 300 Alresford are listed below together with the surveryors' initials and the date of survey. The surveyors were P M Hopson, A R Farrant, C R Bristow, R K Westhead, A Pedley and D T Aldiss.

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 at the BGS Sales Desk.

• Geochemistry maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain (south sheet) 1995. Radon potential based on solid geology, Great Britain (south sheet) 1995.
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (south sheet) 1995.
• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997. Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998.
• 1:625 000
• United kingdom aeromagnetic map (south sheet) 1965
• 1:250 000
• Sheet 51N 02W Chilterns, 1980, aeromagnetic anomaly
• Sheet 51N 02W Chilterns, 1983, Bouguer gravity anomaly
• 1:50 000
• A geophysical information map (GIM) at a scale of 1:50 000 is available for this district. This shows information held in
• BGS digital databases, including Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.
• Hydrogeological maps
• 1:625 000
• Sheet 1 England and Wales, 1997
• 1:100 000
• South Downs and part of the Weald, 1978 Hampshire and Isle of Wight, 1979 Groundwater vulnerability of north-west Hampshire, Sheet 44, 1995
• Groundwater vulnerability of south Hampshire and Isle of Wight, Sheet 52, 1996
• Minerals Maps
• 1:1 000 000
• Industrial minerals resources map of Britain, 1996

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 sheets. For further information contact: The Manager, National Geological Records Centre, BGS Keyworth.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheet 300 Alresford 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.

Material collections

Palaeontological collections

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

Petrological collections

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

Borehole core collection

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

BGS photographs

BGS holds a large collection of photographs including those shown in this book (Plate 4). The photographs date back to the early part of the 20th century and may be viewed in BGS libraries in Keyworth and Murchison House, Edinburgh. Colour or black and white prints and transparencies can be supplied at a fixed tariff.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites Information on licensed abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and the extent of Washlands and licensed landfill sites are held by the Environment Agency Southern Region, Guildbourne House, Chatsworth Road, Worthing, West Sussex, BN11 1LD.

Earth Science conservation sites

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

References

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

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

Allen, J R L, and Narayan, J. 1964. Cross-stratified units, some with silt bands, in the Folkestone Beds (Lower Greensand) of southeast England. Geologie en Mijnbouw, Vol. 43, 451–461.

Allen, P. 1975. Wealden of the Weald: a new model. Proceedings of the Geologists' Association, Vol. 86, 389–437.

Bristow, C R. 1991. Geology of the Petersfield district, Hampshire. British Geological Survey Technical Report, WA/91/24

Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivemey-Cook, H I, and Taylor, R T. 1994. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313, (England and Wales).

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, 293–316.

Brydone, R M. 1912. The Stratigraphy of the Chalk of Hants. (London: Dulau.)

Butler, M, and Pullen, C P. 1990. Tertiary structures and hydrocarbon entrapment in the Weald Basin of Southern England. 371–391 in Tectonic events responsible for Britain's oil and gas reserves. Hardman, R F P, and Brooks, J (editors). Geological Society Special Publication, No. 55.

Casey, R. 1961. The stratigraphical palaeontology of the Lower Greensand. Palaeontology, Vol. 3, 487–621.

Casey, R. 1963. The dawn of the Cretaceous period in Britain. Bulletin of the South-Eastern Union of Scientific Societies, Vol. 17, 1–15.

Chadwick, R A. 1985. End Jurassic–early Cretaceous sedimentation and subsidence (late Portlandian to Barremian), and the late-Cimmerian unconformity. 52–56 in Atlas of onshore sedimentary basins in England and Wales. Post Carboniferous tectonics and stratigraphy.

Whittaker, A (Editor). (Glasgow: Blackie.)

Chadwick, R A. 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London, Vol. 143, 465–488.

Clayton, C J. 1986. The chemical environment of flint formation in the Upper Cretaceous. 43–54 in The scientific study of flint and chert. Proceedings of the Fourth International Flint Symposium held at Brighton Polytechnic 10–15 April 1983. Sieveking, G de G, and Hart, M B (editors). (Cambridge: Cambridge University Press.)

Clements, R G. 1993. Type-section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of the Dorset Natural History and Archaeological Society, Vol. 88, 243–254.

Edwards, R A, and Freshney, E C. 1987. Geology of the country around Southampton. Memoir of the British Geological Survey, Sheet 315 (England and Wales).

Flett, A G, and others. 1976. Records of wells in the area around Alresford. Metric well inventory of the Institute of Geological Sciences, Sheet 300.

Gale, A S. 1989. Field meeting at Folkstone Warren, 29th November 1987. Proceedings of the Geologists' Association, Vol. 100, 73–82.

Gaster, C T A. 1944. The stratigraphy of the Chalk of Sussex, Part III. Western area — Arun gap to the Hampshire border, with zonal map. Proceedings of the Geologists' Association, Vol. 55, 173–188.

Gradstein, F M, and Ogg, J. 1996. A Phanerzoic time scale. Episodes, Vol. 19, 3–4.

Grant, S F, Coe, A L, and Armstrong, A. 1999. Sequence stratigraphy of the Coniacian succession of the Anglo–Paris Basin. Geological Magazine, Vol. 136, 17–38.

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: HMSO for the British Geological Survey.)

Hancock, J M. 1975. The petrology of the Chalk. Proceedings of the Geologists' Association, Vol. 86, 499–535.

Hawkes, P W, Fraser, A J, and Einchcomb, C C G. 1998. The tectono-stratigraphic development and exploration history of the Weald and Wessex Basins, Southern England. 39–66 in Development, evolution and petroleum geology of the Wessex Basin. Underhill, J R (editor). Geological Society Special Publication, No. 133.

Hester, S W. 1965. Stratigraphy and palaeogeography of the Woolwich and Reading Beds. Bulletin of the Geological Survey of Great Britain, Vol. 23, 117–137.

Hodgson, J M, Catt, J A, and Weir, A H. 1967. The origin and development of clay-with-flints and associated soil horizons on the South Downs. Journal of Soil Science, Vol. 18, 85–102.

Hopson, P M. 1994. Geology of the Treyford, Cocking and Chilgrove district, West Sussex. British Geological Survey Technical Report, WA/94/48.

Hopson, P M. 2000. Geology of the Fareham and Portsmouth district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey 1:50 000, Sheet 316 Fareham and part of Sheet 331 Portsmouth (England and Wales).

Hopson, P M, Farrant, A R, and Booth, K A. 2001. Lithostratigraphy and regional correlation of the basal chalk, Upper Greensand, Gault and uppermost Folkestone formations (Mid-Cretaceous) from cored boreholes near Selborne, Hampshire. Proceedings of the Geologists' Association, Vol. 112.

Humphries, D W. 1964. The stratigraphy of the Lower Greensand of the south-west Weald. Proceedings of the Geologists' Association, Vol. 75, 39–59.

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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 BGS-approved stockists and agents.

Figures, plates

Figures

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

(Figure 2) The main geomorphical features of the district.

(Figure 3) a Structure of the Wessex Basin 3 b Cross-section 3c Depth to the Variscan basement.

(Figure 4) The major subdivisons of the concealed pre-Jurassic strata of the district.

(Figure 5) The major subdivisions of the concealed Jurassic strata of the district.

(Figure 6) The principal elements of the concealed Lower Cretaceous strata of the district.

(Figure 7) Correlation of boreholes along a north-south transect.

(Figure 8) Correlation of boreholes along an east-west transect.

(Figure 9) Lithological logs of the Gault Formation and Upper Greensand ('Selbornian') from the Selborne boreholes.

(Figure 10) Correlation of zonal schemes for the Chalk of southern England.

(Figure 11) Principal deposits and events of the Quaternary of the Hampshire Basin. Arrow indicates the time span for the development of periglacial deposits.

Plates

(Plate 1) Many of the minor roads that cross the escarpment are incised into the soft sandstones of the Upper Greensand: escarpment near Selborne [SU 752 359] (GS 961).

(Plate 2) Section through part of the Hythe Formation on the Rogate–Rake road [SU 8012 2582] showing orange, cross-bedded, medium-grained sandstones with evidence of slumping and dewatering structures in the lower part of the section, which is approximately 1.5 to 2 m high.

(Plate 3) View along the primary Chalk escarpment near Steep [SU 734 263]. The crest of the escarpment on the left is capped by Lewes Chalk. The fields on low ground in the centre lies on the West Melbury Marly Chalk, to the right, in the trees, is the Upper Greensand escarpment (GS 962).

(Plate 4) Water spout at Selborne [SU 330 744] is dedicated to Gilbert White, 1894, a famous naturalist who lived in the village. The water supply comes from a small stream rising at the foot of the chalk scarp (GS 965).

(Front cover) Cover photograph Sarsen stones, probably excavated from gravel workings in the valley head deposits, left beside the A272 near Bramdean [SU 629 272]. The top boulder is approximately a metre in diameter. Photographer A R Farrant. (GS 958).

(Rear cover)

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

Figures

(Figure 4) The major subdivisons of the concealed pre-Jurassic strata of the district

(Figure 5) The major subdivisions of the concealed Jurassic strata of the district

(Figure 6) The principal elements of the concealed Lower Cretaceous strata of the district