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Geology of the Salisbury district — a brief explanation of the geological map Sheet 298 Salisbury

P M Hopson, A R Farrant, A J Newell, R J Marks, K A Booth, L B Bateson, M A Woods, I P Wilkinson, J Brayson and D J Evans

Bibliographic reference: Hopson, P M, Farrant, A R, Newell, A J, Marks, R J, Booth, K A, Bateson, L B, Woods, M A, Wilkinson, I P , Brayson, J, and Evans, D J. 2007. Geology of the Salisbury district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 298 Salisbury (England and Wales).

© NERC 2007 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 permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

Keyworth, Nottingham: British Geological Survey

(Front cover) Stonehenge 2004, a view looking north-east from [SU 122 421]. (Photograph C F Adkin; P535208).

Rear cover

(Geological succession) Geological succession of the Salisbury district.

Notes

National Grid References quoted in this report are given in the form [SU 1234 5678]; all lie within Grid Zone SU and ST unless otherwise stated. Boreholes mentioned in the text are identified by a BGS Borehole Registration Number in the form (SU13SE/23).

Acknowledgements

This document has drawn heavily on, the technical reports for the Bourne and Nine Mile River catchment areas and on an unpublished Chalk criteria document by D T Aldiss, C R Bristow, P M Hopson, M D A Samuel, C J Wood and M R Woods. Professor R N Mortimore is thanked for his unstinting help.

The new 1:10 000 scale survey commenced in 1999 as part of a contract partly sponsored by the Environment Agency, and was completed by BGS between 2001 and 2003. Cartography in this report is by L Noakes: pagesetting by C L Chetwyn. Series editor is A A Jackson.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2007.

Geology of the Salisbury district (summary from rear cover)

An explanation of sheet 298 (England and Wales) 1:50 000 series map

(Rear cover)

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

The historic city of Salisbury lies to the southeast of the district at the confluence of the River Nadder and River Avon valleys. Close by are the satellite conurbations of Wilton, famous for its carpet making industry, and Harnham. To the north in the Avon valley is Amesbury and to the far north and east the military areas of Salisbury Plain Training Area and the Porton Down Experimental Station. Away from Salisbury the district is largely rural. The iconic ancient monument of Stonehenge is to be found in the north adjacent to the A303, one of the major routes to the south-west of England.

The district is divided by the incised valleys of the rivers Ebble, Nadder, Wylye, Chitterne Brook, Till and Bourne all confluent with the Hampshire Avon. These valleys cut through the broad chalk plains and downlands, and in the Vale of Wardour to the south-west cut through to the top of the Jurassic. It is in this vale that much of the original building stone for Salisbury Cathedral and its most recent refurbishment were won from the Portland Group. The northern margin of the Vale of Wardour marks the location of the Mere Fault one of the principal reversed structural elements of the Wessex Basin that covers most of southern England. To the east, and north of the Mere Fault trend, softer sediments of the Reading, London Clay and Wittering formations are preserved in the Alderbury–Mottisfont Syncline.

Chapter 1 Introduction

The Salisbury district includes part of east Wiltshire and north-west Hampshire (Figure 1), covering the western margin of the Chalk outcrop from the margins of the Salisbury Plain Training Area in the north to the ridge south of the River Ebble. Palaeogene deposits are preserved in the Alderbury–Mottisfont Syncline and outcrops of Jurassic and Lower Cretaceous strata are exposed in the valley of the River Nadder.

The Chalk forms an extensive gentle south-facing dip slope that is interrupted by several small anticlines and synclines, and dissected by numerous dry valleys. A much degraded and dissected secondary Chalk escarpment is represented by the range of hills that run from Porton Down passing through Salisbury and forming the ridges south of the River Nadder and Ebble.

Geological setting

The Salisbury district lies within the Wessex Basin (Figure 2), a major structure in the south of England that influenced the deposition of strata from Permo-Triassic times through to the Palaeogene. Deposition within this basin reflects extension and compression in response to major earth movements associated with the opening of the Atlantic and English Channel, and later compression during the Alpine orogeny. The district forms the southern part of the area between two principal complex folded and faulted structural features of that basin.

To the south of Salisbury, the asymmetric Wardour anticline and associated Mere Fault runs east–west through the Vale of Wardour, passing to the east into the offset Dean Hill Anticline. To the north of the district, a similar asymmetric east–west structure, the Pewsey anticline, runs through the Vale of Pewsey (Chadwick, 1986). Lowangle thrusts of Variscan age are the main structures of the basin, but normal fault movement occurred during later periods of extension. These faults have been reactivated a number of times throughout the Jurassic and Cretaceous with the final 'Alpine' re-activation being in a reverse sense.

The sea began to flood the Wessex Basin in Rhaetian (Late Triassic) times and the Penarth Group was deposited. The area of marine deposition increased gradually throughout the Jurassic, with the London Platform remaining as land until late Oxfordian to Kimmeridgian times, when it was probably entirely submerged. Towards the end of Kimmeridgian times, the London Platform began to re-emerge, due to global sea-level fall and to 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, the environment of deposition changing from offshore marine (Kimmeridge Clay Formation) through shallow marine (Portland Group) to brackish water and eventually evaporite precipitation (Purbeck Group). Finally fluviatile precipitation conditions were established with the deposition of the 'Wealden Group'. The final period of extensional fault movement, marked by normal faulting, resulted in the accumulation of a thick succession of 'Wealden Group' sediments in the main fault-bounded troughs to the east of the Wessex Basin, while erosion occurred to the west, in this district, and on intervening exposed highs.

A period of regional subsidence followed, associated with sporadic movement on pre-existing faults. This, combined with eustatic rise in sea level, led to a renewed marine transgression of the Wessex Basin with deposition of the Lower Greensand Group, Gault Formation and Upper Greensand Formation, and eventually the Chalk Group covered all of the surrounding highs, including the London Platform. A global fall in sea level 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 compression during the mid Neogene 'Alpine' earth movements. Movement on the major bounding faults of the Wessex and Channel basins was effectively reversed causing inversion of the basins and highs. Uplift is estimated at about 1500 m (Simpson et al., 1989) for both the former Weald and Channel depocentres. Subsequent erosion during the Neogene and Quaternary has unroofed these inverted basins. Erosion, first in subtropical and later in periglacial conditions, combined with a considerable variation in sea level has given rise to the present-day landscape (Plate 1).

Chapter 2 Geological description

Concealed strata

Four deep boreholes within or close to the district penetrated below the base of the exposed strata and provide evidence of the concealed geology; a number of other boreholes have proved part of the Chalk succession and the strata that lie beneath it (Figure 3); (Figure 4).

Ordovician, Silurian and Devonian strata are suspected to occur at depth beneath parts of the district, but they have not been proved by boreholes in or near this district. Tremadoc and older Cambrian strata are proved in the core of an eroded large-scale anticline centred beneath the Shrewton and Yarnbury area, and later Ordovician to Carboniferous strata are preserved in an arcuate pattern around this core. This broad structure is terminated to the south by the Mere Fault complex, beyond which only Upper Devonian strata are known at depth from boreholes. There are numerous unconformities within this Palaeozoic succession; the most significant are post-Cambrian, post-Devonian and postCarboniferous in age. The depth to the top of the Penarth Group as determined from seismic data is shown in (Figure 5).

Jurassic

The greater part of the Jurassic succession is encountered at depth beneath the Salisbury district, the youngest units are exposed in the Vale of Wardour and are described more fully.

In general the successions described in the logs of the deep boreholes are not sufficiently detailed to allow direct comparison with strata exposed to the west and south-west; the reader is recommended to consult the memoirs for Wincanton (Bristow et al. 1999) and Shaftsbury (Bristow et al., 1995) for detailed lithostratigraphical descriptions. However, the formations can be identified within the successions of the four deep boreholes (Figure 3) but the identification of individual members is more difficult due to the presence of a number of minor unconformities and to the gaps that correspond to uncored sections. The principal characteristics of the Jurassic succession are given in (Figure 6). The depth to the top of the Inferior Oolite, which is the base of the principal hydrocarbon prospect, is given in (Figure 7).

The Kimmeridge clay Formation (KC) is the oldest unit that outcrops within the district and is described more fully. It forms low lying ground in the west within the valleys of the River Nadder and River Sem. Head and fluvial deposits obscure much of the outcrop so that there is virtually no exposure. The Tisbury Borehole (ST92NW/2) [ST 9359 2907] (Bristow et al., 1999) drilled in the bottom of the Tucking Mill Quarry west of Tisbury (just to the west of the district) proved an incomplete succession of 233.55 m within the upper part of the Kimmeridge Clay Formation. The lower part of the Kimmeridge Clay Formation is seen at Westbury quarry to the north-west in the Frome district (Sheet 281) (Birkelund et al., 1983). Correlation of the Tisbury Borehole and the Westbury quarry based on the identification of the CrussolicerasBand (Limestone E6 at Westbury) gives a formation thickness of 304 m. In the adjacent Wincanton and Shaftsbury districts, up to 280 m of Kimmeridge Clay occurs at outcrop. In the four deep boreholes, the thickness of the formation varies between 184 and 273.7 m suggesting a reduction in thickness towards the south-east.

The Kimmeridge Clay Formation comprises dark grey to black mudstone and oilshale with silty and sandy mudstone. The grey to black mudstone is calcareous, kerogen-rich and bituminous with variable small-scale rhythmical bedding. The rhythms are identified by the presence of thin siltstone and cementstone beds.

Portland Group (PL)

The Portland Group crops out in the west of the district, principally around Tisbury [ST 952 296], where the River Nadder cuts down through the Wardour Anticline and also within the tributary valley [ST 973 313], the so-called 'Chilmark Ravine'. In this district it is estimated to be up to 50 m thick at outcrop and is 32 to 50 m thick in the four deep wells.

Traditionally the group is divided into a lower arenaceous unit, the informal Lower Portland Beds, and the Upper Portland Beds that are principally limestones (Woodward, 1895). This is compared with more recent schemes in (Figure 8). A general section for the Portland strata in the Vale of Wardour is given in Reid (1903).

These beds have been extensively worked in the area and provided much of the stone used in the building of Salisbury Cathedral (indicating the antiquity of some of the workings) and other buildings both grand and humble. The antiquity and change in ownership of most of the workings, the majority long since abandoned and overgrown, has resulted in a confusion of locality names in the literature. The reports by Bristow and Lott (1994, 1995) included a valuable clarification of these locality names.

Wardour Formation

The Wardour Formation (War) is up to 19 m thick at outcrop. Its base is taken below a bed of fine-grained, fining-upward, glauconitic sandstone. Springs frequently mark this base and this boundary can be traced with ease across unexposed ground. The bulk of the formation comprises beds of bioturbated siltstone and friable sparsely shelly, glauconitic sandstone of variable thickness. Dinoflagellates indicate that the basal 5 m of the Wardour Formation in the Tisbury Borehole fall within the youngest zones of the Kimmeridgian Stage. The succeeding beds span the Albani Zone and the base of the Glaucolithus Zone.

Portland Stone Formation

The Portland Stone Formation of this account is divided into the Tisbury, Wockley and Chilmark members. Up to 31 m of this formation is estimated from the outcrop but it is not fully developed everywhere. The Chicksgrove and Tisbury members, which Wimbledon (1976) defined in exposures, are not divisible at outcrop and they are here included within the Tisbury Member. The 'Ragstone' introduced by Woodward (1895) and an unnamed 'sand' proved during mapping to the west of this district are also included in the Tisbury Member.

Tisbury Member

The Tisbury Member (Tiy) is up to 19 m thick at outcrop in the district. It is limited to the flanks of the River Nadder from Tisbury eastward towards Upper Chicksgrove [ST 960 300] and also occurs as an inlier within the Chilmark Ravine. The member has been extensively quarried in both areas. The base of the member is taken at the abrupt change from fine-grained clayey sand of the Wardour Formation to calcareous siltstone and sandstone, sandy limestone and micritic limestone characteristic of the lower part of the Tisbury Member. Lyddite pebbles have been noted in places at this junction. These are small, well-rounded pebbles of black and grey chert and quartz.

Where exposed, the member can be divided into three units, each of variable thickness. The lower unit comprises 2 to 4 m of well bedded, pinkish grey, bioturbated, shelly micrite. The middle unit comprises porous fine-grained glauconitic sandstone with silty and sandy, bioclastic and peloidal limestone with glauconite grains. There is a gradual decrease in siliceous material and an increase in bioclastic sand up-sequence such that the higher beds become glauconitic, bioclastic sparites. At Chicksgrove Quarry [ST 9620 2960] these medial beds are about 12 m thick and throughout the outcrop constitute the principal source of building stone in the district: they were termed the Tisbury Freestone by Blake (1880). The upper unit of the member ('Ragstone') comprises a basal hard micrite with large bivalves, up to 0.85 m thick, overlain by up to 2.5 m of micritic peloidal limestone with common bivalves and ammonites. This 'Ragstone' unit is commonly quarried but is considered too shelly to be a valuable freestone.

Wockley Member

The Wockley Member (Wok) comprises 4 to 8 m of chalky micritic limestone with commonlargebivalves. Laterally, themember varies markedly, and is known from field brash to include hard, porcellanous micritic limestones with many gastropod moulds. This unit is thickest in the Chilmark Quarries where its highest part includes significant chert beds. Where the Chilmark Member is absent from the succession the junction with overlying Purbeck Group is clear-cut.

Chilmark Member

The Chilmark Member (Chk) is exposed principally in the Chilmark Ravine but recent mapping shows that it can be followed into the Tisbury area, and on the south flank of the Nadder valley it dies out east of Wallmead Farm [ST 9447 2844]. The member comprises about 4 m of cross-bedded ooid sand with some thin shell beds that include common bivalves, notably Astarte sp., together with the gastropods Ampulspira ceres and the 'Portlandian Screw' Aptyxiella portlandica. Two chert beds, up to 0.25 m thick, occur in the upper part of the member. The junction of this member with the overlying Purbeck Group is not clear-cut where the ooid sands become interbedded with stromatolitic 'tufas'.

Cretaceous

Purbeck Group (PB)

In the Salisbury district, the Purbeck Group is divided into the Lulworth Formation and overlying Durlston Formation. The Cretaceous–Jurassic boundary is now considered to be low in the Lulworth Formation although its exact location is not identified in strata exposed in the Salisbury district. The succession was not further divided during this survey as there is insufficient exposure to allow the definition of members equivalent to those seen in the type area of the Isle of Purbeck. At outcrop the group is estimated to be up to 28 m thick, but 37 m were proved in the Yarnbury Borehole.

Traditionally the Purbeck Beds have been divided into three (see (Figure 9)), and many localities have been described in the Vale of Wardour (Woodward, 1895; Reid, 1903). Manuscript field maps of Bristow include the outcrop of the Cinder Bed (within the Middle Purbeck Beds), a widespread marker in southern England, and it is this bed and its continuation into the surrounding area mapped during 2003 that marks the boundary between the Lulworth and Durlston formations of modern usage.

In general the Lulworth Formation (Lulw), up to 15 m thick at outcrop, comprises finely interbedded limestones with some shales. Within the lower part the limestones are fissile, tuffaceous and include nodules and courses of chert. These pass up into a higher part of interbedded more massive limestones with mudstones and ooidal limestones. The Durlston Formation (Durl) commences with the 'Cinder Bed', a calcareous mudstone to argillaceous limestone packed with shell debris, overlain by a succession of sandy limestone and calcareous mudstone with layers of 'beef'. A thickness of up to 13 m is estimated for this formation at outcrop.

Wealden Group (W)

Beds attributed to the Wealden Group have an arcuate outcrop on the northern and southern flanks of the Nadder valley. The outcrop is limited to the west at Teffont Lodge [ST 9985 3130] and Panter's Bridge [ST 9880 2958] by overstep of the Lower Greensand Group, and to the east the Lower Greensand Group overlies the Wealden Group as the Vale of Wardour anticline plunges beneath the valley floor. Seismic data indicate that in the subcrop to the east the Wealden Group is also restricted by erosion that occurred prior to the deposition of the Lower Greensand (Chadwick and Kirby, 1982). For much of this broad valley floor the beds are hidden beneath terrace and alluvial deposits. Estimates from outcrop suggest that 10 m, at most, of Wealden Group is present in the district.

No evidence to indicate the age of the group has been found within the district, but in Farley South Borehole a thin succession (17.68 m) questionably identified as Lower Greensand/Wealden is given a pre-Albian to post-Lower Valanginian age.

The Wealden Group comprises stiff grey clay and silty calcareous clay that weather to a yellow colour at surface. There are no extant exposures. Reid (1903) reported an exposure at Panter's Bridge [ST 9880 2958] where a deep cutting showed 3.05 m of blue-black sand with blue clay and sand showing in the road above. This succession is close to the upper boundary of the Wealden Group as mapped in 2003. The Wealden was formerly exposed in a railway cutting at Dinton, and in a well 300 m north-east of the station (Reid, 1903). JukesBrowne and Andrews (1891) identified a succession that included stiff black clay at its base. They went on to describe a 'brook section at Teffont' in which they identified a similar unit of black clay with black glauconitic sand, overlying mottled clays with yellow loamy sand below.

Lower Greensand Group (LGS)

The Lower Greensand of the Vale of Wardour consists usually of 4.5 to 6.1 m of glauconitic very fineto medium-grained sand with rare masses of cherty sandstone or chert and small polished pebbles. At depth towards the north and east of the district up to 20 m has been proved in deep wells. In the Netherhampton Borehole, for example, it is described as sandstone, very fineto medium-grained, moderately to well-sorted, angular to subangular, light grey to green and with calcite, pyrite and lignitic fragments.

The age of the group in this area is open to debate but it is considered to be equivalent to the Folkestone Formation of the Weald, and by implication is therefore of latest Aptian to earliest Albian in age. Jukes-Browne and Hill (1900) suggest that the sands at Dinton are of mammillatum Zone age.

The outcrop around the Vale of Wardour is narrow on both the northern and southern flanks of the River Nadder. Stream sinks and suffusion dolines often mark the sharp erosional contact with the underlying Purbeck strata. There are few exposures, but augering proves coarse-grained, greyish blue, glauconitic sand, which weathers to a greenish yellow colour. Surface brash commonly contains many small lydite pebbles. Locally, the Lower Greensand is cemented forming harder sandstone.

Gault Formation (G)

The Gault Formation outcrops in the Nadder valley between Fonthill and Baverstock on the northern flank, and between Hurdcott House [SU 0443 3117] and Anstey on the southern flank. The outcrop is masked by alluvium and terrace deposits as the Wardour Anticline plunges eastward. At outcrop a thickness of between 30.5 and 33.00 m was estimated by Reid (1903). At depth in the deep hydrocarbon wells the thickness varies between 33.8 and 58 m. The northern outcrop is narrow west of Teffont Evias as the steeper northerly dips on the northern flank of the Wardour Anticline are approached. On the southern flank the dips are low towards the south and the outcrop is broad.

The Gault Formation comprises soft mudstone, light grey to dark grey, slightly calcareous with disseminated glauconite and mica grains. It is pyritic throughout with some bright sand-sized pyrite crystals where unweathered, and pyrite nodules with a radial crystal structure. It is shelly in part. Phosphatic nodules in layers are a feature of the basal part and commonly mark the base. The so-called 'Basement Bed', which consists of ferruginous fineto mediumgrained sand and fine pebbles interbedded with argillaceous sand, is placed within the Gault Formation by earlier authors, although it may well be considered as part of the Lower Greensand locally.

Upper Greensand Formation (UGS)

The Upper Greensand Formation forms a significant scarp around the core of the Wardour Anticline in the Nadder valley. To the north, the formation forms a narrow outcrop with a southward-facing 25 to 35 m-high scarp. Here the dip is steep up to the faulted scarp (Mere Fault) founded in Chalk. In general these steeper northwarddipping slopes are founded on the highest Boyne Hollow Chert Member of the formation with the steep face of the scarp formed of the Shaftesbury Sandstone Member and the lower shallower slopes founded on the Cann Sand Member. To the south a similar scarp becomes higher towards the southwest, and is broken by tributary streams of the River Nadder. Southward away from this scarp a broad shallow slope founded on the Boyne Hollow Chert Member reflects the change in regional dip, which is here a few degrees towards the south.

The Upper Greensand is seen again in the extreme south-west of the district in the core of an anticline associated with the Ferne Park Fault. In the north-west it is also known from boreholes in the River Wylye and Chitterne Brook valleys.

At depth, it is thought that the Upper Greensand underlies the whole district beneath the Chalk. It is estimated to be 75 m thick in the Vale of Wardour reducing westward to about 65 m in the Wincanton district. Towards the east and north-west, the formation is known from boreholes to be 35 to 45 m in thickness. Towards the north in the Shrewton and Yarnbury boreholes 68 and 74 m were proved, respectively.

A comparison of the traditional lithostratigraphical scheme for the Upper Greensand in Wiltshire (Jukes-Browne and Hill, 1900) and that adopted for Wincanton district (Bristow et al., 1995) and for this district is given in (Figure 10).

The Melbury Sandstone Member is now considered to be equivalent to the Glauconitic Marl Member and therefore forms the basal member of the West Melbury Marly Chalk Formation of the Chalk Group.

Cann Sand Member

The Cann Sand Member (CanS) is dominantly glauconitic, poorly sorted, very sandy silt to very fine-grained micaceous sand and for the most part unconsolidated. It equates to the 'malmstone' of Jukes-Browne and Hill (1900) within which they identified some consolidated beds of sandstone. The type area of the member is around the village of Cann [ST 872 213] in the Shaftesbury district to the south-west. Bristow et al. (1999) suggest that the fauna of the Cann Sand Member is in the varicosum Subzone. Springs and seepages commonly mark the base of the member where it overlies the Gault.

Shaftesbury Sandstone Member

The Shaftesbury Sandstone Member (Shy) consists of alternating beds of coarse siltstone and fine-grained sandstone and unconsolidated sands. All are poorly to moderately sorted, weakly calcite-cemented and contain appreciable amounts of glauconite. The top of the succession is marked by the 'Ragstone' that comprises between one and two metres of hard grey to green, mediumto coarse-grained calcareous, glauconitic sandstone packed with shells. Most notable of these is Pycnodonte (Phygraea?) vesiculosum. The member falls within the top of the varicosum Subzone, with the Ragstone probably being in the auritus Subzone.

The Boyne Hollow Chert Member

The Boyne Hollow Chert Member (BHC) consists of green, highly glauconitic, quartz sand and sandstone with cherty and siliceous concretions and in places significant chert beds (Plate 2). A basal bed, recognised in the Wincanton district to the east, is about 1 m thick and includes phosphatic nodules and fossil debris. In the Salisbury district the chert beds are well developed in the lower part of the member and are conspicuous in field brash around Ridge Hill [ST 9530 3205] and north of Underhill Copse [ST 966 320], and are frequently seen elsewhere. The beds include a fauna suggesting a dispar Zone age for the member.

Chalk Group

Rocks of Late Cretaceous age, predominantly in Chalk facies, underlie most of the Salisbury district. The standard nomenclature for the Upper Cretaceous (Mortimore, 1983, 1986; Bristow et al., 1995, 1997; Rawson et al., 2001) used in this district, together with the traditional scheme, is shown in (Figure 11).

Thickness of the Chalk was estimated at about 330 m by Reid (1903), and a little over 400 m was proved in the Farley South Borehole just to the east of the district. The presence of the Portsdown Chalk Formation in the south-east of the Salisbury district suggests that the thickness encountered in the Farley South Borehole also pertains in this area, and that Reid's estimates of zonal thicknesses are too low throughout.

Grey chalk Subgroup (GCk)

This is essentially equivalent to the Lower Chalk Formation of Bristow et al. (1997), but the youngest unit, the Plenus Marls Member is now included with the overlying Holywell Nodular Chalk Formation. The Grey Chalk is divided into two formations, the West Melbury Marly Chalk and the Zig Zag Chalk.

West Melbury Marly Chalk Formation (WMCk)

In general the formation forms the shallow sloping ramp at the base of the Chalk scarp between two strongly developed negative breaks of slope. The upper negative break of slope marking the top of the formation is evident in places but may be obscured by superficial deposits. North of the River Nadder the outcrop is narrow and steeply dipping, and the formation may be absent where the Mere Fault cuts through the chalk succession.

The West Melbury Chalk consists predominantly of rhythmically bedded, pale to medium grey marly chalk with thin grey to brown limestones. The glauconite sandstone and glauconite-rich argillaceous chalk in the basal few metres of the succession in this district is the Melbury Sandstone Member as described in the Shaftsbury district (Bristow et al., 1995). It is of early Cenomanian age and directly equivalent to the Glauconitic Marl Member farther east in Hampshire and Sussex. Glauconite grains are common in the lower 3 to 5 m of the chalk. The top of the formation is taken as the top of the Tenuis Limestone where that bed is present, but normally at the base of the 'Cast Bed', a distinctive pale brown silty chalk containing abundant small brachiopods. The limestones in the lower part of the succession commonly contain sponge fragments and may contain glauconite grains. A limestone rich in Schloenbachia occurs in the middle of the formation and is thought to be equivalent to the M3 limestone identified at Folkestone. The upper limestones of the West Melbury Chalk are generally poorly fossiliferous and sponge free.

Zig Zag Chalk Formation (ZCk)

The Zig Zag Chalk is typically composed of medium-hard, pale grey, blocky chalk with some thin limestones near the base, and is 46 to 65 m thick. The lower part of the formation has a significantly higher marl content and contains some thin limestones. Some distance above the base of the formation, hard, pale grey splintery limestones with conspicuous Sciponoceras may occur. The base of the Zig Zag Chalk Formation is placed below the cast Bed, a pale brown silty chalk that commonly occurs at a strong negative slope break at the base of the Chalk escarpment. This abrupt change in slope appears to correspond with the first appearance of thick beds of firm to hard blocky chalk above the gently sloping ground underlain by the West Melbury Marly Chalk. The top of the formation is placed below the Plenus Marls Member of the succeeding formation and this is frequently identified as a weak negative slope break beneath the strong positive feature developed on the Melbourn Rock

Member. The upper part of the Zig Zag Chalk tends to be paler grey to white, firm, marly chalk with common Inoceramus atlanticus, I. pictus and the echinoid Holaster subglobosus. Elsewhere in southern England, a calcarenite with phosphatic nodules and referred to informally as the Jukes-Browne Bed 7 is present at about the level of the change upward to paler grey and white chalk. This bed has not been positively identified in the succession at outcrop in this district but it is likely to be present. No flints are recorded in this area.

White chalk Subgroup (WCk)

The White Chalk Subgroup is essentially the combined Upper Chalk and Middle Chalk formations of Bristow et al. (1997). The base of the White Chalk is taken at the base of the Holywell Nodular Chalk Formation. In general the subgroup is characterised by white chalk with numerous flint seams, and with nodular chalk in the lower part. The White Chalk is divided into seven formations, all of which occur in this district. Thickness is estimated at up to 320 m at outcrop in this district.

Holywell Nodular Chalk Formation (HCk)

The Holywell Chalk comprises generally hard, nodular chalk with flaser marl seams throughout. Three units can be identified. In ascending stratigraphical order, these are the Plenus Marls Member, the Melbourn Rock Member and an un-named succession of hard nodular and grainy chalk with abundant shell debris (most notably species is Mytiloides). The formation is between 15 and 25 m thick.

The Plenus Marls Member is rarely well exposed but is present along the whole outcrop of the White Chalk. This member consists of an alternating succession of blocky white chalk and medium grey silty marl beds, mostly between 1 and 20 cm thick, though the highest, Jefferies' Bed 8 (Jefferies, 1963), may be up to 50 cm thick. The Plenus Marls, co-extensive with the greater part of the Metoicoceras geslinianum Zone, contains common Inoceramus pictus, as well as the eponymous belemnite Praeactinocamax plenus (formerly Actinocamax plenus).

Overlying the Plenus Marls Member is the Melbourn Rock Member, a very hard, grainy nodular chalk generally lacking in shell detritus. The top of the Melbourn Rock is recognised by the appearance of abundant bivalve shell debris. The member is up to 3 m thick; it usually forms a strong positive feature and characteristic brash can be traced widely. The overlying shelldetrital and grainy chalks form a narrow outcrop in the face of the primary scarp. In places, mainly on the less steep slopes, they form a positive feature. The top of the Holywell Nodular Chalk Formation is characterised by the transition to smoother, softer New Pit Chalk, but in practice is taken at the highest recognisable shelldetrital chalk.

New Pit Chalk Formation (NPCk)

The New Pit Chalk, between 25 and 35 m thick, consists of smooth, firm, white chalk, massively bedded, with marl seams. The base of the New Pit Chalk is marked by the disappearance of inoceramid-rich nodular chalk; the top is marked by the appearance of nodularity and flints that generally occur between Glynde Marl 1 and the Southerham Marl in the standard Sussex succession.

Flints are rare in the New Pit Chalk. Where present they are small and occur in the uppermost beds. The fauna is much sparser than in the Holywell Chalk and consists mostly of brachiopods (both terebratulids and rhynchonellids) rather than abundant inoceramid bivalves. Thin shelled Mytiloides hercynicus/subhercynicus are present but tend to be flattened and preserved as chalky moulds.

Lewes Nodular Chalk Formation (LeCk)

The Lewes Chalk comprises interbedded hard to very hard nodular chalks and hardgrounds with soft to medium-hard grainy chalks and marls (Plate 3). The nodular chalks are typically lumpy and iron-stained, this iron-staining usually marking sponges. Brash is rough and flaggy and tends to be dirty. The first regular seams of flint appear near the base, and sheet flints are common. The flints are typically black or bluish black with a thick white cortex. The formation is between 40 and 45 m thick in this district. It includes the 'Chalk Rock' of traditional usage (Bromley and Gale, 1982) at its base.

In exposed sections the Lewes Nodular Chalk can be divided informally into two units. The lower is mainly mediumto high-density and conspicuously ironstained hard nodular chalks. The upper unit is mainly lowto medium-density chalks with regular thin nodular beds. The Lewes Marl and an extensive system of black cylindrical burrow-form flints called the Lewes Flints marks the boundary. The upper Lewes Nodular Chalk is further distinguished by the occurrence of the bivalve Cremnoceramus (Mortimore, 1986). There are several levels of sheet flint within an interval of 4 or 5 m in the lower part of the upper Lewes Chalk.

In this district the lower 5 to 10 m of the succession is more condensed than in Sussex, and the chalk Rock Member becomes well developed particularly in the north-west. The member comprises a number of heavily mineralised (glauconite and phosphate) nodular hardgrounds (Plate 4) above omission surfaces. In this district, the base of the Lewes Nodular Chalk is placed at the lowest nodular chalk recognisable in field brash, and this is only a short distance below the heavily mineralised nodular chalks of the Chalk Rock Member. The top of the formation is placed at the highest recognisable bed of nodular chalk below the smooth white chalks with Platyceramus. A belt of large nodular and carious (with a thin rind or skin and may be cavernous) black and grey flints mark the highest part of the succession and spans the boundary between the Lewes Nodular Chalk and Seaford Chalk formations.

Seaford Chalk Formation (SCk)

The Seaford Chalk is between 60 and 70 m thick, and outcrops over a wide area of the district. It underlies much of the Chalk dip slope and broad interfluves between the primary escarpment and the weak negative break of slope below the secondary Chalk escarpment. Topographically, the Seaford Chalk forms the characteristic smooth convex slopes of the major ridges, and the rounded quite steep valley sides.

The Seaford Chalk is composed primarily of soft smooth blocky white chalk (Plate 5) with abundant seams of large nodular and semi-tabular flint, and thin harder nodular chalk near the base. The flints in the lower part are usually highly carious whereas higher in the succession they are black and bluish black mottled grey with a thin white cortex. These flints commonly enclose shell fragments. Some of the large flint bands form almost continuous seams and in places create local topographical features. Thin planar sheet flints are common in parts of the succession.

The brash of the lowest part of the Seaford Chalk is similar to the upper part of the Lewes Chalk but contains an abundance of fragments of the bivalves Volviceramus and Platyceramus, whilst the upper part contains Cladoceramus and Platyceramus. In the absence of these bivalves the flaggy nature and pure whiteness of the soft chalk serves to distinguish it from the Lewes Chalk below. About 15 to 20 m from the base of the Seaford Chalk a very large semi-tabular flint (up to 30 cm thick) with characteristic brown staining is frequently found as field brash or more commonly as 'field-picked' cairns on the margins of ploughed fields. This is equated with the Seven Sister's Flint and can be distinguished from other large flints by its usual content of Platyceramus and Volviceramus bivalves.

Another characteristic semi-tabular flint occurs near the top of the Seaford Chalk in this district about 11 m below the base of the Newhaven Chalk. This flint is generally about 10 cm thick, of uniform appearance and tends to fracture vertically; it is tentatively correlated with Whittaker's Three Inch Band of the North Downs and the equivalent Rough Brow Flint of the Sussex coast (Mortimore, 1986). However, no bio-stratigraphical information has so far been found to support this correlation. Above this flint band is a thin (1–2m) horizon of intensely hard porcellanous indurated chalk (Stockbridge Rock Member), about 5 m below the Newhaven Chalk: it is shown on the 1:10 000 maps as a limestone bed. The Stockbridge Rock contains abundant sponge spicules, most commonly as moulds, together with some complete sponges. This is readily identifiable in the brash and forms a useful marker horizon. It occurs at about the level of Barrois' Sponge Bed and the Clandon Hardground of the North Downs (Robinson, 1986) and may equate with the Whitway Rock of the Newbury area (Sumbler, 1996). The Stockbridge Rock Member occurs widely between Salisbury and Winchester, but appears to be quite sporadic in the west of this district. Field evidence from the Winchester area (Farrant, 2000) suggests that there may be several thin hard horizons between 5 m and 10 m below the base of the Newhaven Chalk, each separated by white chalk.

Newhaven Chalk Formation (NCk)

Lithologically similar to the Seaford Chalk, the Newhaven Chalk is composed of soft to medium-hard, blocky smooth white chalk with numerous marl seams and flint bands, and is 55 to 70 m thick (Plate 6). Typically, the marls vary between 20 and 70 mm thick, but in this district they are generally much thinner and may be little more than a few millimetres dying out over synsedimentary positive features. The flints are generally much smaller and less continuous than those in the underlying Seaford Chalk. Tabular and sheet flints are not so well developed, but finger, horn and Zoophycos flint forms are more common. Hardgrounds and phosphatic chalks occur in a synsedimentary erosive channel noted in the boreholes drilled for the A303 dual carriageway southeast of Stonehenge.

The Newhaven Chalk outcrops extensively over the eastern part of the district occupying much of the sloping ground on and immediately below the face of the secondary Chalk escarpment. The lowest 10 m of the Newhaven Chalk generally forms the spurs extending out from the scarp foot. The base of the formation is commonly marked by an extremely faint negative break of slope.

In this district the top of the formation is placed above the Pepperbox Marls as seen in the Pepperbox Quarry (Mortimore et al., 2001) thus extending the range of the formation a little farther up-succession than described in Mortimore (1986).

Culver Chalk Formation (CCk)

The Culver Chalk is between 35 and 45 m thick in this district, and generally forms the face and crest of the secondary chalk escarpment. The Culver Chalk is composed of soft white chalk without significant marl seams, but with some very strongly developed nodular and semi-tabular flints. In this district the top of the Newhaven Chalk (Mortimore et al., 2001) is taken at the top of the Pepperbox Marls, some 4 m higher than the standard Sussex succession. This level is seen in the sections at West Harnham, East Grimstead and at Pepperbox Quarry itself. In parts of Dorset and Sussex, the Culver Chalk Formation can be divided into a lower Tarrant chalk and an upper Spetisbury chalk but current practice treats these as members of the Culver Chalk (Rawson et al., 2001).

The majority of the Culver Chalk outcrop occupies the south-east of the district but is also seen as small outliers. In the field, the base of the Culver Chalk is taken just below a strong persistent positive topographical feature coinciding with the appearance of abundant large flint nodules. Where the secondary Chalk escarpment is not so well developed, for example near Clarendon Palace [SU 182 302], the base of the Culver Chalk occurs at a prominent negative feature break at the base of a small scarp, which in effect is the upper half of the secondary Chalk escarpment.

Chalk brash from the Culver Chalk tends to be blockier than that derived from the Newhaven Chalk, but chalks of these units cannot be reliably distinguished on lithological grounds alone, even in small exposures. Some parts of the Culver Chalk (within the Applinocrinus cretaceus Subzone) are characterised by abundant bioclastic debris, especially bryozoan debris, but this abundance was not positively identified during the survey.

Micropalaeontological determinations show that the Culver Chalk has been significantly attenuated. The lower Tarrant Member is probably reduced to less than 8 m thick in places and is possibly absent in the extreme south; the overlying Spetisbury Member may well be thicker where this attenuation occurs. The members have not been mapped separately as this would require close micropalaeontological sampling over wide areas as exposure is limited and the two members do not show distinct geomorphological features in this area. Such an intensive study is beyond the remit of this survey and the Culver Chalk is in consequence shown undivided.

Portsdown Chalk (PCK)

The Portsdown Chalk outcrops in the extreme south of the district, just north of the Palaeogene outcrop in the Dean Syncline. It consists of white flinty chalk with common marl seams; the base is taken at the Portsdown Marl. Where mapped the Portsdown Chalk is less than 10 m thick having been removed by erosion prior to the deposition of the Palaeogene. No clear sections were seen and the only firm evidence for Portsdown Chalk comes from micropalaeontological evidence from a pit at Brick Kiln Copse [SU 1822 2860] that suggests the higher part of the quadrata Zone.

Palaeogene

The principal outcrop of Palaeogene strata is to be found within the Alderbury–Mottisfont Syncline in the south-east of the district. In the extreme south-east an outcrop of Reading Formation strata represents the north-western extremity of the Palaeogene in the Hampshire Basin. In this district, the Palaeogene strata were traditionally described as the Reading Beds, London Clay and Bagshot Beds. These equate more or less with the modern lithostratigraphy as shown below. In this district the succession ranges between 55 and 85 m in thickness, but this must be considered approximate as it is difficult to estimate thickness of strata within the Alderbury–Mottisfont Syncline.

Much of the succession was encountered in the Alderbury Borehole (SU12NE/7) [SU 1965 2672] where 72.85 m of strata were proved. This borehole must lie close to the axial plane of the Alderbury–Mottisfont Syncline. Whilst there is a relatively well described log of the borehole, the exact horizons dividing the three units encountered are not certain and hence thickness interpretation of these units is open to some debate. A reinterpretation suggests that the thickness of the units should be: 7.01 m for the Wittering Formation, 50.60 m for the London Clay Formation and 13.72 m for the Reading Formation. This would accord more accurately with estimates derived from outcrop, and also with thickness estimates given by Reid (1903).

Reading Formation (RB)

The principal occurrence of the Reading Formation is in a broadly oval outcrop around the Alderbury–Mottisfont Syncline, where it is the northerly extension of the main Hampshire Basin outcrop, and as small isolated outliers capping hilltops elsewhere, for example Cockey Down. The formation is estimated to be between 15 and 20 m thick within its principal outcrop. The basement bed of the Reading Formation comprises a greyish green clayey sand with abundant subangular to rounded, corroded and pitted glauconitestained flints, and brown sandy clay with well rounded flints and pockets of orange sand. Above this basal bed, the Reading Formation is lithologically highly variable and comprises mottled yellowish or lilac-brown silts and clays with some red ferruginous sands that are fine to coarse grained, crossbedded, with clay intraclasts and small well rounded patinated flint pebbles. Concentrations of large oysters are present in the basal part of the formation. Towards the southeast, the formation comprises principally fineto medium-grained sand with only minor amounts of clay; it represent a major fluvial channel within the floodplain over-bank deposits of mottled clay that otherwise make up the unit.

Small outliers of lilac-reddish brown silty pebbly clay were noted at the top of Cockey Down, Laverstock [SU 169 315]. The areas of Reading 'Beds' [SU 203 342] south of Battery Hill and on Clay Pit Hill [ST 993 424] near Codford shown on the previous edition of this map were not substantiated during this survey.

London Clay Formation (LC)

The London Clay forms the principal outcrop within the core of the Alderbury-Mottisfont Syncline. It comprises grey or brown (olive green where unweathered) silty clay that is commonly micaceous, and usually becomes more sandy and pebbly towards the base. It occupies the gentle dip slope behind the minor escarpment of the Reading Beds. The formation is approximately 35 to 50 m thick in the district. The outcrop is often under pasture. The surface drainage commonly disappears underground at the margins of the syncline, where streams flow on to the chalk or sand-rich Reading Formation.

The formation was described from the railway cutting at Clarendon Hill [SU 1827 2844] to [SU 1881 2796] (Prestwich, 1850); this was repeated in the memoir (Reid, 1903) and with further details from Elliott (1945). Later use of this cutting for part of the Alderbury Bypass (A36), constructed between 1976 and 1978, permitted King (1981) to provide a summary lithological section.

Wittering Formation (Wtt)

Above the London Clay Formation is a succession of thinly bedded sand, sandy ironstone and clay, which is attributed here to the Wittering Formation at the base of the Bracklesham Group. Reid (1903) describes them 'as false-bedded ferruginous sand, with lenticular masses of pipe-clay and with thin beds of ironstone'. There were no significant exposures noted during this survey and the outcrop in the core of the Alderbury–Mottisfont Syncline has been delimited by the occurrence of yellow and red-brown fineto medium-grained sand proved in auger holes and by the very sandy soils. In the area of Alderbury Common, this sandy soil also contains appreciable amounts of well-rounded flint pebbles. The full thickness is estimated at 5 to 15 m, based on the outcrop.

Quaternary

The Quaternary deposits include all the superficial deposits in the district, principally the clay-with-flints, various fluviatile sediments and an assortment of periglacial head deposits. There is a considerable time gap (about 50 Ma) between the deposition of the Palaeogene strata and the oldest Quaternary deposits in this district. The gap represents the time during which the Palaeogene strata were deposited across the whole of southern Britain and subsequently removed, following uplift along the Wealden axis. Within the Quaternary there is a shorter but no less significant time gap between the formation of the clay-with-flints and of the younger drift deposits. A general relationship between the Superficial Deposits is given as a marginal sketch section on the geological sheet.

Clay-with-flints

The clay-with-flints is primarily a remanié deposit created by modification of the original Palaeogene cover and by solution of the underlying Chalk. It is typically composed of orange-brown or reddish brown clay and sandy clay containing abundant flint nodules and pebbles. At the base of the deposit the matrix becomes stiff, waxy and fissured and is dark brown in colour; the nodular flints are relatively fresh and stained black or dark green by manganese compounds and glauconite. In places, notably on the high ground around Salisbury, the deposit includes very gritty hard coarse-grained sandstone pebbles and small rounded sarsens (pebbles of very hard fine-grained sandstone). There were few good exposures of this deposit within the district, and it has been mapped on the basis of its characteristic reddish brown sticky clayey soil with nodular, often stained (orange), flints. Its distribution reflects the remnants of the pre-Palaeogene erosion surface. Deposits are estimated to be between 2 and 8 m thick, but will be much thicker over the solution pipes that may extend 10 m or more into the underlying Chalk. Within these pipes it is not uncommon to find disturbed fineto medium-grained multicoloured sand and stone-free clay, commonly thought to be Palaeogene in origin. The deposit is closely associated with older head which is a solifluction deposit derived directly from the clay-with-flints. Evidence from outside this district suggests that the clay-with-flints was developed in a number of phases and closely associated with periglacial climates.

Head

Older head consists of a series of deposits ranging from flinty gravel to orange-brown or reddish brown clay and sandy clay containing abundant flint nodules and pebbles. It is derived from the clay-with-flints deposits and the underlying bedrock by solifluction and solution. It is found on upper valley slopes below the plateau. The flints in older head are generally much more shattered than those in the clay-with-flints due to solifluction processes and frost shattering. The downhill boundary is taken where the deposit thins and Chalk becomes apparent in the soil, or at the negative break of slope bounding the relatively flat-lying ground underlain by finer grained valley head deposits. Older head commonly grades imperceptibly into head gravel, and then there is no marked break of slope. The uphill boundary is taken at a positive break of slope at the edge of the clay-withflints or the plateau from which the clay-withflints has been removed, and again is often a transitional boundary. This deposit represents the earliest periglacial material in the area and was probably much more extensive in the immediately post-Devensian period. It is thought to be the source from which most of the younger Quaternary deposits are derived and its development and subsequent removal probably covered a considerable time span.

Head Gravel

This deposit is very similar lithologically to gravelly head but occurs on lower valley sides. The deposit is a coarseor very coarsegrained, poor to moderately sorted flint gravel, with an admixture of fluvial rounded to subangular rolled worn flints and rare angular large, commonly broken nodular and coarse gravel-sized flints, set in a greyish brown to orange-brown clayey, silty, fineto coarsegrained sand matrix. Its gravelly nature serves to distinguish it from head and its occurrence on significant slopes distinguishes it from terrace deposits. The deposit is a mix of periglacial solifluction deposits derived from the Chalk, Palaeogene, clay-with-flints and older head deposits, intermixed with fluvial gravels derived from older degraded river terrace or head deposits.

It is most frequently identified in valleys above the perennial stream-head and may be regarded as valley infill that has not been consistently reworked by fluvial processes to create well defined terrace aggradations (it is effectively immature terrace).

Gravelly Head

The gravelly head is essentially alluvial and head materials in valley bottoms from which the fine-grained silt and clay material has been flushed by periodic water flow, either during the depositional process or later by ephemeral stream flow. The resulting deposit is a flint gravel that is coarse or very coarse grained, poor to moderately sorted, clast-supported, subangular to subrounded and with generally little or no fine-grained material. In this district, this deposit occurs in the floor of the River Till near Winterbourne Stoke and in the Bourne River upstream of the perennial spring. In both cases at a position in the valley where ephemeral winter runoff flushes finer material out. The valley floor where this deposit occurs usually contains a well defined, often dry, stream channel. Downstream of the perennial springs, the gravel is usually overlain by over-bank alluvial deposits of silt, sand and peat. The deposit is interpreted as the product of both solifluction and fluvial transport in a periglacial environment.

Head

Head is a heterogeneous group of superficial deposits that have accumulated by solifluction, hillwash and hillcreep. Composition ranges from very gravelly silty, sandy clay to clayey sandy gravel, with variable proportions of coarser granular material, and an earthy texture. The clasts are primarily of large nodular and coarse gravel-sized flint. It is regarded as a periglacial deposit resulting from the solifluction of Chalk, Palaeogene and clay-with-flints material. The term includes the chalky, flinty materials that were formerly mapped as 'dry valley deposits'. Most of the dry valleys on the Chalk have a head deposit covering the valley floor (Plate 1). This is usually thickest and most prevalent in the lower reaches of the dry valley network, where the gradient is markedly reduced, but it may be absent where the valley is narrow or steep. In many cases, the lower limit of the head deposits occur at the highest springhead, where it becomes reworked and merges into the alluvial deposits and the coarser less clayey gravelly head. Where seen in section, head commonly shows cryoturbation structures in the uppermost few metres (Plate 7). The thickness of most head deposits in the area is not known. Borehole records show that it is generally less than 2 m thick, but may locally attain 5 m or more.

The map user should be aware that large parts of the area shown as bedrock with no overlying superficial deposit do actually carry a thin (generally less than 1.0 m), extensive, but discontinuous blanket of head of varying age.

River Terrace Deposits

This term is reserved for the deposits formerly called Brickearth or in some papers the Fisherton Deposits or Fisherton Brickearth. These were exposed in a number of brickpits between Quidhampton [SU 113 310] and the city centre Old Manor Hospital site [SU 136 304], north of Salisbury Railway station. The outcrop shown on the map has been identified on the basis of its generally flat surface or low southerly slope, and associated silty clay soil that contains some pebbles; it lies between the gravelrich soils of the flat fourth terrace to the south and the clayey gravels on the steeper slopes to the north. The deposits rest on the fourth terrace (see below). The unit thins up slope to the north where progressively more soliflucted material interdigitates with the deposit. The brick-making industry in this area ceased around 1900 and descriptions by Reid (1903) essentially reiterate earlier works. Since that time all of the sites have degraded and the majority were built over as Salisbury expanded north-westwards into the Bemerton district. It is not clear whether all of the occurrences mentioned in the early literature are related.

Delair and Shackley (1979) gave a valuable account of the Fisherton Brickpits including their known stratigraphy and faunal lists, and published the first locality map placing the former named sites in the context of the present road network. Green et al. (1983) gave an account of sediments and gave the name Fisherton Terrace to the underlying coarse gravel materials; Delair and Shackley (1979) referred to the same gravel as the Bemerton Terrace.

The 'brickearths' yielded a rich fauna of mammals and mollusca while they were being worked in the early to late 19th century, and the beds achieved some notoriety because of the distinct 'Arctic' character of the mammals identified. They are considered to be a late interglacial (Ipswichian) or early Devensian in age.

River Terrace Deposits

River terrace deposits are associated with all of the major rivers in the district. With the exception of the younger terraces (first and second) associated with the River Dun in the west, all the rivers are confluent with the Hampshire Avon and with which they must share the history of terrace development. Outside the district to the south, a full suite of terraces has been designated including the highest that are topographically associated with the development of the proto-Solent and form gravelly spreads on the interfluves. Younger terraces found within the present valley topography are related to base levels of the River Avon system. Higher terraces exist but are not geomorphologically very distinct (with the exception of those of the lower Avon and River Dun) and have been labelled as undifferentiated. Many of these higher terraces have undergone weathering and degradation by solifluction and grade both upslope and down slope into spreads of gravelly head deposits of various types.

The topographically highest terraces in this district are designated as the eight and ninth and are between 30 and 45 m, respectively, above the floodplain on the left or eastern valley side of the lower Avon. Their designation follows that developed for the lower Avon outside the district to the south around Ringwood and Bournemouth. In this district the very limited outcrops are characterised by clayey sandy angular flint gravel soils. There are no exposures or boreholes that indicate the thickness of deposits of the two terraces, but they can be no more than 5 m at most.

River terrace deposits undifferentiated is used within this district to identify gravel spreads on the lower valley slopes that show some crude or degraded terrace surface. It is this generally flat surface that differentiates this deposit from other gravelly slope head deposits, but the surface brash may well have the same appearance.

Only the fourth terrace has been identified extensively in the district, and occurs at around 1.5 to 5 m above river level and separated from the alluvial flat by a small bluff. No sections have been seen in this terrace, but field brash consists of abundant well rounded to subangular and broken flint gravel, with clasts of varying sizes. Some are stained and rubified, and are probably derived from the clay-withflints. They are best developed within the Wylye–Nadder–Avon confluence area around Wilton and Salisbury.

The lowest terrace of the River Avon in this district is the third, mapped around Charlton-All-Saints [SU 177 238]. South of the district in the Ringwood and Bournemouth districts, this terrace becomes more extensive and two further and younger terraces are also designated.

Within the River Dun catchment (a tributary of the River Test to the east) two levels of terrace accumulation have been identified on the basis of two positive breaks of slope adjacent to the floodplain and some 5 m above. These terraces may well be a single accumulation with erosional features relating to a down-cutting event. The soil comprises very flinty sandy silty clay.

Alluvium

The alluvium in the district comprises a complex interdigitation of three distinct lithologies: sandy gravel (in places chalky), fine-grained sandy mud and peat. In places a fourth unit of chalky, gravelly, sandy, silty clay is regarded as solifluction material derived from the steeper valley sides; it interdigitates with the other floodplain deposits.

The sandy gravel is generally found at depth below fine-grained over-bank deposits, and is of variable thickness. It is composed of fineand coarse-grained clasts of subangular to rounded flints with subordinate amounts of chalk, quartz, quartzite and sandstone, and rare exotic rock types. The fineto medium-grained sand matrix has a variable silt and clay content. Within the headwaters of the River Nadder the gravel fraction contains appreciable quantities of local Jurassic limestone. This sandy gravel unit represents the bed-load of the stream probably laid down in cold or cooler phases of climate, and generally therefore likely to have been deposited within a braided stream environment with numerous channels separated by low-lying gravel bars.

The principal unit of the alluvial tract is the fine-grained sandy mud, which represents the mature over-bank flood deposits that have gradually built up the floodplain above the level of the gravelly braided stream deposits. The deposit usually comprises pale grey or silvery grey, wet, sticky mud with varying proportions of very fineand fine-grained sand and may contain shell material either as individuals or in thin shell beds. It contains peat and disseminated organic material and commonly includes very thin fine-grained gravel beds of flint and chalk.

The peat is intimately associated with the finegrained over-bank deposits laid down within a mature stream environment. It occurs in beds, lenses and as disseminated fragments a darkbrown or black, fibrous, spongy organic material with varying admixtures of silt and clay. Its presence indicates areas of slow-flowing waters and significant plant growth. The intimate relationship between the over-bank deposits and the organic beds is complex and they have not been delimited separately. Thin peat may be enclosed within the other deposits of the alluvium as mapped in this district.

The alluvium in the district is generally around 4 to 5 m thick, of which the top 2 to 3 m is of the over-bank deposit type. In places, particularly where rivers merge, a thicker gravel unit at the base increases the overall thickness of the deposit. The cross-section (Figure 12) gives the general relationship seen in the larger streams across the district.

Peat

Peat is the term used for deposits of an organic nature that are generally fibrous and contain discernable organic material, but the term can also cover richly organic finegrained sediments that demonstrate a fibrous nature. In general the peat deposits found within chalk streams in southern England are of the alder-carr or reed-bed type. They represent accumulations of fibrous organic material within floodplain marginal woodland or reed/sedge beds in slow flowing backwater situations. They often contain appreciable amounts of trapped fine-grained organic mud and notable shell-detrital beds. In special circumstances where carbonaterich groundwater infiltrates the peat units, deposits of calcareous tufa or nodules of this carbonate precipitate occur. The presence of this precipitate is controlled by relative concentrations of carbonate and the chemistry of the water within the sediment.

Within the district there is only one area of peat delimited by mapping (see below) but beds of fibrous, black, peat and peaty silty clay are known to be present within the mature river valleys closely associated with the alluvium (see above).

Artificial Ground

The major occurrences of made, worked, infilled and landscaped ground are noted on the 1:10 000 scale maps within the district. Not all are transferred onto the published 1:50 000 scale Sheet 298 Salisbury. Within urban areas the amount of artificial ground is often difficult to determine and its limits often masked by the built environment. The artificial ground shown on the maps within those areas is probably an underestimate.

Worked ground delimits areas where natural resources have been extracted. Made ground is a term used to denote areas where additional material foreign to the site has been deposited above the natural ground surface. For the most part occurrences are related to road and rail embankments and archaeological sites (commonly identified by a symbol on the base maps). Infilled ground is used to delimit areas where former sites of extraction have been utilised for landfill. The type of fill is often difficult to determine but sites are known to have been used for both household waste and inert fill.

Structure

The major structural elements within the district are shown in a marginal diagram on the geological sheet. Regionally the district falls within the southern part of the Pewsey Basin and to the south forms the northern margin of the Cranborne–Fordingbridge High (both elements of the larger Wessex Basin). The boundary between these two structures is placed at the Mere Fault. South of the Vale of Pewsey, the chalk dips gently southwards at generally less than 2°, at least as far south as Netheravon and Tidworth in the east and Chitterne in the west where the dip is less.

In the extreme north and east of the district, a broad low-amplitude east–west-oriented syncline extends across the district, through Cholderton and plunging east towards Thruxton. To the west, this broad syncline splits into two, one branch extending north of this district; the other just north of Boscombe Down Airfield is the Amesbury Syncline. A small east–west anticline (unnamed) separates the two, running parallel with the A303, but dies out eastwards near Beacon Hill just north of the district. These structures are asymmetric with the anticline having a much steeper northern limb (dip up to 5º) and amplitude of about 10 to 15 m.

Farther to the south is a northwardfacing pericline or dome structure centred on High Post [SU 151 365]. This broad low amplitude (about 15 to 20 m) structure is best seen around Middle Woodford in the Avon valley where the Lewes Chalk is brought up in the core of the anticline (Woodford Anticline). To the east, and slightly offset to the north, a similar anticline (Boscombe Down anticline) trends through High Post, Boscombe Down and extends on to the northern part of Porton Down, just south of Grateley. To the west, the Woodford Anticline swings round to a north-westerly orientation and merges with the Wylye Anticline, again with a slight offset, but this time towards the south. It appears that these structures have an en échelon relationship, which may be determined by faulting at depth. These structures are probably the continuation of the Stockbridge anticline to the east.

South of the Wylye–Woodford structure to the east of the River Avon, the dip increases up to 3º to the south; farther east the dip is much gentler at between 0.5º and 2º to the south or south-east. Westward a shallow syncline (unnamed) separates the Wylye Anticline from the Great Ridge Anticline.

South of Great Ridge, the dip again becomes southerly into the very tight Barford St Martin Syncline created as a 'drag-structure' immediately to the north of the Mere Fault. This syncline broadens and fades eastward into the Wylye valley but matches with the much broader structure of the Mottisfont–Alderbury Syncline east of the River Avon. This may well reflect a deep-seated north-westerly orientated structure in the basement as the reversed Mere Fault Structure also fades eastward into the periclinal Dean Hill Anticline.

In the west of the district the Wardour Monocline, with its steep northerly dipping limb, affects the Jurassic and Lower Cretaceous strata exposed in the River Nadder valley. This structure fades towards the east where it also broadens into a symmetrical anticline.

The Coombe Bissett Syncline appears to be similar in its formation to the Barford St Martin syncline as it has the Coombe Bissett Fault on its southern flank.

Faulting

There are a number of recognisable, mappable faults at surface in the area, and most of these occur on the primary Chalk scarp and associated with the Mere Fault complex within the River Nadder valley. The Mere Fault is the principal fault of the district. it runs from the western margin along the Nadder valley north of the Upper Greensand ridge, is concealed beneath the floodplain of the river near Barford St Martin, and continues south-east to Odstock where it dies beneath the Avon valley. At surface the Mere Fault is generally a single, southerly dipping, steeply inclined or vertical reverse fault, downthrowing to the north. The maximum throw is estimated to be around 100 m, but the throw is variable along its length. The western portion of the Mere Fault is complex and associated with a strongly developed set of normal faults. At Baverstock, a small north–south-oriented fault appears to cut the Mere Fault, and corresponds to a marked increase in the dip on either side of the fault. To the east of Baverstock, the fault is less well marked and much of the displacement is taken up by folding associated with the tight Barford St Martin Syncline. Farther east, south of Wilton, the Mere Fault becomes less distinct and in part difficult to trace across the similar lithologies of the White Chalk Subgroup. Here, in addition to field observation of gross formational lithologies, it has been detected with the aid of macroand microfossils that clearly show a variable maximum throw on the fault in the range 50 to 70 m. Farther east through Odstock, a throw of as little as 20 m is indicated and the fault dies beneath the Quaternary deposits of the River Avon and cannot be detected east of the valley. Here, however, the asymmetric Dean Hill Anticline is thought to demonstrate the line of this structure at depth. The reverse movement is associated with postCretaceous compression. At depth the nullpoint (at which the sense of movement on the fault cannot be detected) is within the lowermost Jurassic and Permo-Triassic.

Medium-scale faulting is noted in the headwater regions of the Wylye, Nadder and Ebble rivers, where the Cretaceous strata are thin due to erosion. These faults probably reflect early Cretaceous re-activation of deep-seated normal faulting and perhaps attest to the continued expansion of the Wessex Basin at this time.

Many smaller faults with throws of less than 1 m were seen in several of the disused pits, but could not be traced beyond. This minor faulting may well be a ubiquitous feature of the Chalk. Instead of discrete faults, the strata may be displaced several metres by numerous minor shear zones over a distance of several tens of metres, thus smearing the 'fault' zone over quite a wide area with little or no surface expression.

Chapter 3 Applied geology

Hydrogeology

The principal aquifer within the district is within the Chalk Group. Public water supplies are also won from the Upper Greensand Formation although this resource is more important to the west. Local supplies are extracted from the Portland Group (and possibly the Purbeck Group) and to a lesser extent the Palaeogene and Quaternary strata. This latter source probably taps the underlying bedrock aquifers with which the deposits are in hydraulic continuity. Brief notes on the hydrogeology are given below but readers are recommended to the reports on minor aquifers (Jones et al., 2000) and major aquifers (Allen et al., 1997) that give valuable overviews of the water resources.

The Chalk is a microporous limestone and water flow is generally along fissures and joints that can become enlarged due to solution. As a consequence of its nature the hydraulic properties of the Chalk are complex. There is a high storage potential in saturated chalk but its microporous character with pore throat sizes measured in microns, means that unfractured chalk has a high porosity but low transmisivity rate, and is therefore slow to release its resources. Its value as a water resource comes from its ability to release and transport water along bedding planes, joints and through macro and micro fractures, which give the rock mass a high permeability (and provide much of its usable storage). The Chalk is often regarded therefore as a dual porosity aquifer. Permeability is generally developed only towards the top of the Chalk, through the unsaturated and into the top of the saturated chalk where fracturing is prevalent and groundwater circulates above the local base-level. With depth, fracturing declines due to increased overburden, change in lithology and a general reduction in circulating groundwater. The lithological properties of each of the formations in the Chalk and the geological structure will have an important influence on how the streams behave and the aquifer functions.

The Portland Group limestones tend to be cemented and intergranular permeability is low. Water movement is through fractures that have been enlarged by solution. High yields can be obtained where these openings are closely interconnected. In this district their limited surface outcrop and highly fissured and fractured nature, which compartmentalises the aquifer, make the aquifer vulnerable to fluctuations in the water table and to contamination. In general, because of the deeply incised valleys wells are prone to seasonal drying unless they penetrate the whole thickness of the limestones down to the Kimmeridge Clay Formation.

In this district, the Purbeck Group is a poor aquifer because of its generally more argillaceous nature with interbedded shales, limestones, sandstones and evaporitic beds. The water resources are limited, not least because of their small outcrop area. To the east, at the closure of the Vale of Wardour, the Purbeck Group (and underlying Portland Group) is probably in hydraulic continuity with the thin Wealden and Lower Greensand strata (both of which are poor aquifers providing poor quality water) and can be regarded here as a single low-yield concealed aquifer.

The Upper Greensand Formation is an important minor aquifer in southern England. In this district, the formation comprises three members all sandy in nature. The formation is highly permeable and intergranular flow predominates. It is often found in hydraulic continuity with the overlying Chalk aquifer and when this occurs they are usually considered together as a single aquifer unit. However, where it is at outcrop, such as in the Vale of Wardour, and just outside the district in the upper Wylye valley, the formation is an aquifer in its own right. The Gault Clay Formation below acts as an aquiclude.

The Wittering Formation and the Reading Formation both provide poor low-yield aquifers but the water has generally a high iron content.

Shallow wells intercept the water table along most of the river valleys. They take water from the sub-alluvial gravels that for the greater part are in hydraulic continuity with the underlying bedrock (principally Chalk in this district). Yields normally reflect those of the underlying bedrock but the shallow wells are prone to surface contamination and the majority are no longer used for supply.

Karstic solution features

Solution features are widespread within the Chalk of the Hampshire Basin and typically densities of between 10 and 50 per km2 occur across Hampshire and Wiltshire. Their location is unpredictable, but by assessing the geology and geomorphical setting, it is possible to highlight areas with greater potential for solution features.

A wide variety of solution features occur but only two, 'buried' and 'subsidence' sinkholes (dolines) are common on the Chalk. Buried sinkholes are typified by 'pipe' or cone-like cavities within the chalk (Plate 8); (Plate 9). Subsidence sinkholes are closed surface depressions, usually either bowl, pipe or cone-like in shape. They can occur as isolated examples or as groups, and may coalesce into large composite dolines. They can form rapidly as a dropout failure. Most occur in covers of unconsolidated sediment between 1 and 10 m thick, such as the claywith-flints and older head.

The presence of these solution features is dependent on several variables including rock lithology, fracture style, geomorphic setting, geological structure and even anthropogenic factors. The wide variety in chalk lithology, fracture style, geological structure, flint content, porosity and fissure permeability significantly affects the style and degree of karst weathering, both at surface and underground.

Karstic features are also known in the Purbeck strata in the Vale of Wardour, both in the Salisbury district and to the west in the Wincanton district. Many stream sinks have been noted around Tisbury and Sutton Mandeville as well as a few small phreatic caves and resurgences. None of these have been traced to any specific resurgence.

The distribution of observed solution features are held in a BGS database. Many sinkholes have been ploughed in or landscaped so the distribution of solution features marked on the updated geological maps is certain to be an underestimate of the true density. Others have been worked as chalk pits and some 'dolines' may simply be small, degraded marl pits. Furthermore, many solution features such as the infilled 'pipes' often have no surface expression and cannot be identified by surface mapping.

The highest density of solution features is close to the Palaeogene outcrop and around the extensive clay-with-flints cover of the Great Ridge, on the interfluve between the Lower Avon and the River Wylye and on the interfluves to the north and south of the River Ebble. Here the present land surface is close to the sub-Palaeogene peneplain and both recent active and relict solution hollows derived from a former Palaeogene cover occur. Other outcrops of clay-withflints are associated with dolines but elsewhere, the land surface has undergone greater dissection and these relict solution hollows have been eroded.

Solution features ('bourne holes') can be expected to occur along the middle and upper reaches of the Bourne, Till and Chitterne Brook where significant recharge into the aquifer occurs. These may act as either sinks or springs depending on relative groundwater levels.

Several stream sinks are developed at the contact between the Lower Greensand and the Durlston Formation.

Soil type

The soils of the district are delimited and described on the 1:250 000 scale Soils of England and Wales Sheet 5 (South West England) published by the Soil Survey of England and Wales and the compendium volume, Bulletin No. 14 (Findley et al., 1984). The soils of Sheet SU03 (Wilton) are dealt with in more detail in the explanatory booklet, Soil Survey Record No. 32, Soils in Wiltshire 1 Sheet SU03 (Wilton), (Cope, 1976). Within the district, there are 22 Soil Associations and each is closely associated with a number of ancillary subgroups and soil series. Full descriptions and representative soil profiles are found in numerous publications of the Soil Survey (see Findley et al., 1984).

Building stone

Many buildings use stone imported from the Upper Jurassic succession in the Vale of Wardour, west of Salisbury. Here the Wardour or Tisbury Stone and the Chilmark Stone have been quarried widely around Tisbury, Wockley, Chicksgrove, Chilmark and Teffont and mined in extensive galleries, chiefly from quarries in the Chilmark Ravine. Traditionally the lower part of the Portland Group, the Wardour Formation, herein, is the major source of freestone. The principal beds quarried have the local workmen's names of the Trough Bed, the Green Bed, the Pinney Bed and the Fretting Bed. The freestone has been used for the construction of many buildings including the cathedrals at Salisbury, Rochester and Chichester and in other notable buildings such as Wardour Castle, Longford Castle, Fonthill Abbey, Wilton Abbey, Romsey Abbey, Westminster Abbey (the Chapter House), Christchurch Priory, Balliol College Oxford. The upper part of the group, the Chilmark Formation provides oolitic freestone (the Chilmark Stone) that has been used in the construction of the west front of Salisbury Cathedral.

The Upper Greensand has been quarried in many places for building material and was formerly much sought after. Evidence of degraded workings are frequently found throughout the outcrop of the Shaftesbury Sandstone and Boyne Hollow Chert members of the formation but little now remains of this former industry other than the many local houses built from the stone. The weathered blocks were much prized, as it was not susceptible to frost once 'hardened' and withstood immersion making it a valuable stone for the building of foundations and copings. The pit at Upper Hurdcott Farm [SU 0503 2997] is still active producing a small amount of soft sandstone for the restoration of local buildings.

Extensive use is made of the flints from the Chalk for building, particularly in churches and the larger houses and farms. The flint is used both as knapped squared blocks and as single-faced trimmed nodules. Flint shards derived from the knapping of dressed flint are often seen pressed into the wet mortar for decoration, a process known a 'galletting'. Flint, as a 'waste' product of chalk extraction and from 'field picking', has also been used to maintain farm tracks.

The harder chalks from the Melbourn Rock Member and the Lewes Nodular Chalk Formation are incorporated into buildings to a small extent in this area. Their source is unknown but both dressed blocks and 'field picked' clasts are seen in older buildings.

A loosely bound mixture of chalk rubble, mud and straw has been used traditionally in the construction of the older cottages and garden walls in villages in the district. Such walls (cob walls) are very susceptible to damage by the weather and so have to be sheltered by thatch or tile roofs and a surface rendering.

Bulk minerals

Chalk and limestone

Apart from the uses of the freestone derived from the Portland Group the limestones and calcareous shales of the Jurassic strata in the Vale of Wardour (predominantly the Purbeck Group) were used locally for the production of lime mortar and cements. There are numerous sites where kilns are noted on both historical and current Ordnance Survey maps but the industry has long been abandoned and the sites are degraded and in many cases the original kilns removed.

The cement industry was based around the three major pits at East and West Harnham and Quidhampton. Only the latter is still worked for high purity calcium carbonate from the Seaford Chalk Formation. Flints are a by-product of the extraction and are used locally to restore buildings or to add architectural features to new buildings. Many disused chalk pits are found throughout the district, but are particularly common in the east where the chalk was used to 'marl' the heavier clay soils. Elsewhere in the district, the chalk is only won on a local and 'at need' basis as hardcore.

Flint mines are known in an area of Porton Down that perhaps attest to the winning of flints by our ancient cousins, but there is also evidence that these same areas were a centre for the production of gunflints.

Sand and gravel

There is no large-scale extraction of aggregate from any of the deposits within the district. Sand resources exist within the Upper Greensand, Reading Formation, Wittering Formation, but either they are not exploited or only on a local scale by individual farmers. Their grade and potential as a source of aggregate has not been tested. Several small gravel pits occur in the Avon valley to the west of Amesbury, but none are active. There are former workings for sand and gravel within the Wylye valley near Langford [SU 040 370] (now a nature reserve) and at Croucheston [SU 066 256] in the River Ebble. There are numerous small sites elsewhere within the floodplains of the principal river valleys.

Brick clay

The London Clay and Reading formations in the south-east of the district and the Gault Clay in the Vale of Wardour were used for brick making. Old clay pits within the London Clay occur near Brick Kiln Copse [SU 186 288] on the Clarendon Estate. Reid (1903) describes a brickyard in the Gault Formation at Ridge [ST 9526 3173] and the formation was formerly used for the production of tiles and bricks at Dinton [SU 018 318].

Within the Quaternary the clay-with-flints, and alluvial clays have been worked on a small scale but the largest industry locally was based on the 'brickearth' around Fisherton. This industry effectively finished in 1900 but was declining before that as reserves became exhausted and the expansion of Salisbury encroached over the remaining deposits.

Hydrocarbons

The district was first explored for hydrocarbons in the 1930s and again in the 1980s. None of the deep wells produced viable resources but the legacy of seismic profiles and the detail from the wells provide much of the detail discussed herein.

Geological hazards

The following statements should be taken only as a guide to likely or possible problems and should not replace site-specific studies. The Chalk is locally affected by solution phenomena and a very irregular rockhead is created. Solution can result in the formation of small surface depressions (dolines) that range in size up to some 50 m across, and up to 6 m deep. Such depressions may be liable to further subsidence. Differential compaction under load can occur across such structures. Dissolution phenomena are also present in the Purbeck strata in the Vale of Wardour.

Map users should be aware that thin and extensive, but discontinuous deposits of head are much more widespread than indicated by the geological map.

Planning for future construction should allow for the possible existence of small areas of Made Ground, Infilled Ground or Landscaped Ground. Such areas may be liable to differential settlement.

Peat is a compressible material and will compact when loaded or give rise to differential settlement when partially built over. Care should be taken to identify peat units within the major floodplains where they have not been delimited by surface mapping.

Excavations within units comprising sand are liable to failure if unsupported, particularly where groundwater is present.

Areas of landfill or older areas of made ground may be subject to differential compaction. Frequently the nature of the fill is unknown. In the case of landfill sites, the presence of gas derived from the breakdown of the buried waste may constitute a problem.

Areas of landslipped ground are not common in the Salisbury district. Minor areas (not shown on the published map, but identified on the larger scale survey maps) exist on the Palaeogene deposits around the Alderbury–Mottisfont syncline and are associated with the steep scarp of the Upper Greensand Formation. In most cases the area of slip is obvious from the disruption of the surface sediments.

Information sources

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, British Geological Survey, Keyworth. For information on wells, springs and water borehole records contact: BGS Hydrogeological Enquiries, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB; Telephone 01491 838800; Fax 01491 692345

Other geological information held by BGS include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries and on the website; BGS catalogue of maps and books is available on request (see back cover for addresses).

1:10 000 field slips

The field slips for this survey are archived in the BGS collections. They were completed between 1989 and 2004 by D T Aldiss, C M Barton, K A Booth, C R Bristow, A R Farrant, P M Hopson, R J Marks, A J Newell and K R Royse.

Digital coloured computer printed copies of the 1:10 000 maps can be purchased from BGS, Keyworth, where records of the boreholes may also be consulted. This report includes interpretations of data available at the time of writing. Additional information is available in BGS files. Neither the report nor its complementary 1:10 000 scale geological maps should be taken as a substitute for detailed site investigations. Users should note that the stratigraphical nomenclature used in this report is likely to be revised.

1:10 560 field slips

The original 1:10 560 field slips for the 1903 primary survey are archived in the BGS collections.

BGS Technical reports

• WH/99/57R Woods, 1999b
• WH/99/138R Woods, 1999a
• WH/00/42C Woods, 2000
• WH/00/36R Wilkinson, 2000a
• WH/00/43R Wilkinson, 2000b
• WA/00/11 Aldiss, 2000
• WA/00/18 Booth, 2000
• WA/00/23 Hopson, 2000
• WA/00/24 Farrant, 2000

Borehole records

The registered borehole records for the individual sheets may be consulted through the National Geosciences Information Service, BGS Keyworth and are filed according to 1:10 000 quarter sheet.

Aerial photographs

NRSC (National Remote Sensing Centre) nominal scales 1:25 000 (1993) and 1:10 000 (1991).

Other reports and publications are listed under References.

These can be consulted at the BGS Library, Keyworth.

References

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

Allen, D J, Brewerton, L J, Coleby, L M, Gibb s, B R, Lewis, M A, MacDonald, A M, Wagstaff , S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication 8.

Andrews, W R, and Jukes-Browne, A J. 1885. The Purbeck Beds of the Vale of Wardour. Quarterly Journal of the Geological Society of London, Vol. 50, 44–71. Birkeland, T, Callomon, J H, Clausen, C K, Nohr Hansen, H, and Salinas, I. 1983. The Lower Kimmeridge Clay at Westbury, Wiltshire, England. Proceedings of the Geologists' Association, Vol. 94, 289–309.

Blackmore, H P. 1864. Remains of birds eggs found at Fisherton, near Salisbury. Edinburgh New Philosophical Journal, Vol. 19, 74–75 Blake, J F. 1880. On the Portland rocks of England. Quarterly Journal of the Geological Society of London, Vol. 36, 199–233.

Bristow, C R. 1995. Geology of the Tisbury district (Wiltshire). British Geological Survey Technical Report, WA/95/82. Bristow, C R, and Lott, G K. 1994. The stratigraphy and building stone potential of the Portland Beds in the western part of the Vale of Wardour. Report of the British Geological Survey for the Dean and Chapter of Salisbury Cathedral. Unpublished

Bristow, C R, and Lott, G K. 1995. The stratigraphy and building stone potential of the Portland Beds between Tisbury and Chilmark in the Vale of Wardour. British Geological Survey Technical Report, WA/95/15C.

Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H I, and Taylor, R T. 1995. 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, 293–315. Bristow, CR, Barton, C M, Westhead, R K, Freshney, E C, Cox, B M, and Woods, M A. 1999. The Wincanton district — a concise account of the geology. Memoir of the British Geological Survey, Sheet 297 (England and Wales).

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

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

Chadwick, R A, and Kirby, G A. 1982. The geology beneath the Lower Greensand/Gault surface in the Vale of Wardour area. Report of the Institute of Geological Sciences, No. 82/1, 15–18.

Cope, D W. 1976. Soils in Wiltshire I: Sheet SU03 (Wilton). Soil Survey Record, No. 32.

Delair, J B, and Shackley, M L. 1979. The Fisherton Brickpits; their stratigraphy and fossil contents. Wiltshire Archeaological and Natural History Magazine, Vol. 72, 3–16.

Elliott, G F. 1945. Faunal horizons in the London Clay of Clarendon, Wilts. Proceedings of the Geologists' Association, Vol. 56, 151–152.

Farrant, A R. 2000. Geology of the Bourne River, Salisbury to Bulford Camp, Wiltshire. Technical Report of the British Geological Survey, WA/00/24.

Findley, D C, Colborne, G J N, Cope, D W, Harrod, T R, Hogan D V, and Staines, S J. 1984. Soils and their use in south west England. Bulletin of the Soil Survey of Great Britain, No. 14.

Gradstein, F M, Ogg, J G, and Smith, A G (editors). A geological time scale 2004. (Cambridge: Cambridge University Press.)

Green, C P, Keen, D H, McGregor, D F M, Robinson, J E, and Williams, R B G. 1983. Stratigraphy and environmental significance of Pleistocene deposits at Fisherton, near Salisbury, Wiltshire. Proceedings of the Geologists' Association, Vol. 94, 17–22.

Jefferies, R P S. 1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association, Vol. 74, 1–34.

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, PD, Lewis, M A, Mac Donald, A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication 68.

Jukes-Browne, A J, and Andrews, W R. 1891. The Lower Cretaceous Series of the Vale of Wardour. Geological Magazine, Vol. 38, 292–294. Jukes-Browne, A J, and Hill, W. 1900. The Cretaceous rocks of Britain. Vol. 1. The Gault and Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. (London: HMSO.)

Jukes-Browne, A J, and Hill, W. 1903. The Cretaceous rocks of Britain. Vol. 2. The Lower and Middle Chalk of England. Memoir of the Geological Survey of the United Kingdom. (London: HMSO.)

Jukes-Browne, A J, and Hill, W. 1904. The Cretaceous rocks of Britain. Vol. 3. The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom. (London: HMSO.)

King, C. 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper, No. 6.

Mortimore, R N. 1983. The stratigraphy and sedimentation of the Turonian–Campanian in the southern province of England. Zitteliana, Vol. 10, 27–41.

Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97(2), 97–139.

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

Prestwich, J. 1850. On the structure of the strata between the London Clay and the Chalk in the London and Hampshire Tertiary systems. Proceedings of the Geological Society, Vol. 6 (1), 252–281.

Rawson, P F, Allen, P W, and Gale, A S. 2001. A revised lithostratigraphy for the Chalk Group. Geoscientist, Vol. 11(1), 21.

Reid, C. 1903. The geology of the country around Salisbury. Memoir of the Geological Survey of Great Britain, Sheet 298 (England and Wales).

Robinson, N D. 1986. Lithostratigraphy of the Chalk Group of the North Downs, southeast England. Proceedings of the Geologists' Association, Vol. 97(2), 141–170.

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

Smith, N J P. 1985. Pre-Permian geology of the United Kingdom (South Sheet). (Keyworth, Nottingham: British Geological Survey.) Soil Survey of England, and Wales. 1983. Soils of England and Wales. Sheet 5 South West England.

Sumbler, M G. 1996. British Regional Geology: London and the Thames Valley. (London: HMSO for the British Geological Survey.)

Wimbledon, W A. 1976. The Portland Beds (Upper Jurassic) of Wiltshire. The Wiltshire Archaeological and Natural History Magazine, Vol. 71, 3–11.

Woodward, H B. 1895. The Jurassic rocks of Britain. The Middle and Upper oolitic rocks of England (Yorkshire excepted). Memoir of the Geological Survey. Vol. 5. (London: HMSO.)

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Map of the Salisbury district.

(Figure 2) The Wessex Basin showing major faults and other named structures.

(Figure 3) Summary of the four principal boreholes within and adjacent to the district. Thickness in metres.

(Figure 4) Concealed Cambrian to Triassic strata proved in boreholes.

(Figure 5) Depth to the top of the Penarth Group relative to OD.

(Figure 6) Summary of the concealed Jurassic strata.

(Figure 7) Depth to the top of the Inferior Oolite Group relative to OD.

(Figure 8) Portland Stone Formation, comparison of nomenclature used by authors.

(Figure 9) Comparison of nomenclature used for the Purbeck Group.

(Figure 10) Comparison of nomenclature used in the Upper Greensand Formation.

(Figure 11) Chalk Group correlation chart for the Southern Chalk Province.

(Figure 12) Valley fill of the River Avon.

Plates

(Plate 1) Dry valley head fan, Foxhole Bottom, Codford St Mary [ST 9804 3951] (P598775).

(Plate 2) Hurdcott Farm, view of contact of Shaftesbury Sandstone and Boyne Hollow Chert members (Upper Greensand Formation) [SU 0504 2990] (P584757).

(Plate 3) Small pit in Lewes Nodular Chalk Formation, Wylye valley near Stapleford. Close-up of a thick marl seam (Fognam Marl) [SU 05517 37260] (P584673).

(Plate 4) Section HP64. Close-up of nodular glauconitised chalk hardgrounds within the Chalk Rock Member [ST 98088 36894] (P598790).

(Plate 5) Seaford Chalk, view looking towards the north-west of Quidhampton Quarry [SU 1141 3157] (P598771).

(Plate 6) Newhaven Chalk Formation: West Harnham Chalk Pit, looking north-east over Salisbury Cathedral. (Photograph: R N Mortimore, 2005) [SU 128 287]. Telscombe Marls and abundant Offaster piliula planata beds. Meeching Marls and Echinocorys cincta beds in lower exposures

(Plate 7) Solifluction features in flinty head, Harnham [SU 12433 28917] (P584737).

(Plate 8) Typical solution feature shown in cross-section within the Quidhampton Quarry [SU 1140 3151] (P598768).

(Plate 9) Sediment-filled solution feature, Britford Quarry [SU 15113 28054] (P584721).

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

(Front cover) Stonehenge 2004, a view looking north-east from [SU 122 421]. (Photograph C F Adkin; P535208).

Rear cover

(Geological succession) Geological succession of the Salisbury district.

Figures

(Figure 3) Summary of the four principal boreholes within and adjacent to the district. Thickness in metres

(Figure 4) Concealed Cambrian to Triassic strata proved in boreholes

(Figure 6) Summary of the concealed Jurassic strata

(Figure 8) Portland Stone Formation, comparison of nomenclature used by authors

(Figure 10) Comparison of nomenclature used in the Upper Greensand Formation