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Geology of the Devizes district — a brief explanation of the geological map, sheet explanation 1:50 000 Sheet 282 Devizes

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

Bibliographic reference: Booth, K A, Hopson, P M, Farrant, A R, Newell, A J, Marks, R J, Bateson, L B, Woods, M A, Wilkinson, I P, and Evans, D J. 2010. Geology of the Devizes district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 282 Devizes (England and Wales).

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, email 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.

(Front cover) Caen Hill locks on the Kennet and Avon canal, Wiltshire. View looking east up the Upper Greensand escarpment from the lower basin. Photographer: P J Witney (P698535).

(Rear cover)

(Geological succession) Geological succession in the Devizes district.

Notes

National grid references quoted in this report are given in the form [ST 1234 5678]; all lie within the grid zones SU or 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heavily, and quoted extensively from BGS technical reports for the Bourne and Nine Mile River catchment areas and on an unpublished Chalk criteria document by D T Aldiss, C R Bristow, P M Hopson, M D A Samuel, C J Wood and M A Woods.

The landowners, farm managers and land agents of the district area are thanked for their cooperation in providing access to private land for the purposes of this survey. The Environment Agency (Southwest Region) is acknowledged for its contribution to part of the survey within the Bourne River Catchment. Particular thanks must go to Chris Waldren and the staff at the Defence Estates Agency, Westdown Camp and Lt Col. Nigel de Foubert and the staff at Range Control for their cooperation and tolerance in allowing access to Salisbury Plain Training Area.

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

An explanation of Sheet 282 (England and Wales) 1:50 000 Series map summary from rear cover)

(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 Devizes district extends over part of north-east Wiltshire and covers much of Salisbury Plain in the south and most of the westward-opening Vale of Pewsey in the north. The Vale of Pewsey is drained by minor tributaries of the River Avon (Hampshire). From Upavon, where these minor streams converge, the river cuts an often gorge-like southward path through the Salisbury Plain towards Bulford and beyond, into the Salisbury district. This steep-sided valley divides the Salisbury Plain Training Area into eastern and central/western portions. In the east, the Nine Mile River and River Bourne and to the south the River Till and Chitterne Brook dissect the Plain.

The main conurbation in the area is the town of Devizes, which lies to the north-west of the district. Away from Devizes, the district is largely rural. Salisbury Plain is founded on the Chalk Group. The Vale of Pewsey is underlain by the Upper Greensand with a number of notable outliers of the Grey Chalk Subgroup. In the north-west, beneath a much landslide-affected scarp made up of the Upper Greensand and Gault formations, outcrops of the Lower Greensand Group, Portland Group, Kimmeridge Clay Formation and Corallian Group form the minor interfluves and intervening valleys. In the extreme north-west beyond a major fault, the Oxford Clay Formation is present.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the Devizes district (Sheet 282), which extends over approximately 600 km2 of north-east Wiltshire (Figure 1). It covers much of Salisbury Plain in the south and most of the westward-opening Vale of Pewsey in the north.

The Vale of Pewsey is drained by minor tributaries of the River Avon in Hampshire, which cuts southward from Upavon across Salisbury Plain towards Bulford and into the Salisbury district. This steep-sided valley divides the Salisbury Plain into eastern and central/western portions. In the east, the Nine Mile River and River Bourne dissect the Plain. The River Bourne borders the eastern extremity of the Salisbury Plain Training Area (SPTA) controlled by the Ministry of Defence (MoD). In the central and western parts of the Plain, the area is drained by the southward flowing River Till and Chitterne Brook, both tributaries of the River Wylye that flows eastwards across the adjacent Salisbury district to the south.

Geological setting

Salisbury Plain is founded on the Chalk Group. Much of the area is underlain by strata within the White Chalk Subgroup, with the older Grey Chalk Subgroup cropping-out along the northward-facing scarp overlooking the Vale of Pewsey. In the eastern part of the Plain, the younger units of the White Chalk Subgroup form a much-broken secondary escarpment from Sidbury Hill southwards to Bulford and beyond. The Vale of Pewsey is underlain by the Upper Greensand with a number of notable outliers of the Grey Chalk Subgroup. In the north-west, overlooked by a landslide-affected scarp made up of the Upper Greensand and Gault formations, outcrops of the Lower Greensand Group, Portland Group, Kimmeridge Clay Formation and Corallian Group form the minor interfluves and intervening valleys. In the extreme north-west, beyond a major fault, the Oxford Clay Formation is present.

Structurally, the Devizes district lies along the north-western margin of the Channel–Wessex Basin (Figure 2). This basin, comprising a system of post-Variscan extensional sedimentary basins and highs that covered much of southern England, influenced the deposition of strata from Permo-Triassic times through to the Palaeogene. Cycles of terrigenous and marine deposition reflect the cyclic expansion and contraction of the basin in response to both the opening of the Atlantic and English Channel, and uplift associated with the Alpine Orogeny. The underlying basement consists of Palaeozoic strata that were deformed during the Variscan Orogeny — a period of mountain building that culminated at the end of the Carboniferous. Variscan deformation was followed by a long period of erosion, and a major unconformity marks the base of the Permo-Triassic succession.

The Devizes district lies between two folded and faulted structural elements. The southern part straddles the northern margins of the Pewsey sub-basin (Basin herein), and lies above the Variscan Front thrusts that controlled the development of the basin and the Pewsey Fault. The northern part is crossed by the Pewsey Anticline, an asymmetric east–west structure running through the Vale of Pewsey formed during Alpine compression (Chadwick, 1986). To the south of Salisbury, the similarly asymmetric Wardour anticline, which manifests itself partly as the complex Mere Fault (see Chadwick and Evans, 2005), runs east–west through the Vale of Wardour, passing to the east into the offset Dean Hill Anticline.

The main structure in the district is the zone of arcuate east–west trending faults forming the Pewsey Fault Zone which, as seismic reflection data illustrate, represents a complex series of en echelon normal faults (Chadwick, 1986, 1993). These structures form the boundary between the London Platform (to the north) and the Pewsey Basin.

Across the north of the district, the Pewsey Anticline is one of a number of small en echelon northerly verging anticlines affecting Jurassic and Lower Cretaceous strata.

Chapter 2 Geological description

Concealed strata

Carboniferous to Triassic

There are five deep hydrocarbons boreholes, within or adjacent to the Devizes area, which provide evidence of the strata encountered at depth. (Figure 3) shows the location of the boreholes and (Figure 4) details the stratal thicknesses within each. Seismic sections show that Ordovician to Carboniferous strata form the basement beneath the southern part of the Pewsey Basin.

The oldest basement rocks encountered at depth in boreholes and interpreted from seismic sections in the district are Ordovician (Tremadocian) through to Carboniferous (Mississippian) in age, and form an arcuate pattern centred around the Shrewton Borehole as illustrated in Smith (1985). This borehole proved a thick succession of Tremadoc-aged micaceous siltstones and interbedded mudstones, but the oldest rocks proved in the other deep boreholes are Mississippian (Carboniferous) limestones. These limestones are generally described as white, light grey, pink, red and brown limestone and mudstone, cemented to strongly cemented, slickensided and stylolitic. A more comprehensive description in Farley South (on the Winchester Sheet 299) indicates a succession of interbedded limestone and chert with numerous dolomitic limestone and dolomite units with rare beds of calcareous siltstone. These platform carbonates formed in a shelf sea on the southern margin of the emergent London–Brabant Massif.

The Permian strata encountered within the Shrewton and Yarnbury boreholes comprise interbedded, red, micaceous mudstone and siltstone with rare thin sandstone interbeds. Above this is a widely developed Triassic succession, proved in all five deep boreholes, that shows regional southward thinning towards the Mere Fault.

Jurassic

The greater part of the marine-dominated Jurassic succession is represented at depth below the district. The relatively uniform, cyclical sequences of mudstone, sandstone and limestone provide sedimentological evidence, regionally, for an eastward shift of the area of maximum subsidence in the Wessex Basin related to the activity of faults bounding the Hampshire–Dieppe High. The topographical lows of the Weald and Channel basins thus became depocentres at this time with, in general, a regional pattern of northward thickening into the Weald Basin.

The oldest strata at outcrop in the district are of Late Jurassic age, and comprise the Oxford Clay Formation, the Corallian Group, the Kimmeridge Clay Formation and the Portland Group. These rocks occur in small areas in the north-west of the Devizes district.

Oxford Clay Formation (OxC)

The Oxford Clay succession consists of a brown, slightly calcareous, carbonaceous, siltstone passing upwards into a grey, carbonaceous mudstone. The succession is variably pyriteous with thin limestones in the higher part.

Corallian Group (Cr)

The Corallian Group represents an episode of relatively shallow marine mixed carbonate and siliciclastic sedimentation between two long periods of deeper-water argillaceous shelf sedimentation represented by the underlying Oxford Clay and overlying Kimmeridge Clay formations. The Corallian Group in this region is composed of muddy sandstone and yellow, brown and grey, very fine to medium-grained quartzose sand, with thin sandy siltstone beds in parts.

Kimmeridge Clay Formation (KC)

The Kimmeridge Clay Formation forms low-lying ground in the far north-west of the district in the small valleys south of Potterne and west of Worton and Poulshot. It consists of stiff, waxy, dark grey mudstones, with small pale grey calcareous concretions (known as 'race' or calcretes), calcareous pale grey mudstones, oil shale and silty dark grey mudstone, all are sporadically fossiliferous.

Portland Group (PL)

The Portland Group crops out in the north-west of the district between Potterne and Great Cheverell. In this district it is estimated to be up to 35 metres thick. The group consists of the Wardour Formation conformably overlain by the Portland Stone Formation.

Wardour Formation (War)

The Wardour Formation crops out over the north-western part of the district, and forms a continuous outcrop from near Coulston through to Potterne Wick and west to Worton. Worton is built on a long ridge of the Wardour Formation. The basal few metres of the Wardour Formation also crop out from beneath the Lower Greensand along the escarpment west of Poulton.

Much of the formation consists of pale brown, buff, very well-sorted, fine to medium-grained 'sugary' quartz sand, with small amounts of glauconite, giving it a speckled appearance. The base is transitional into the top of the underlying Kimmeridge Clay Formation.

Portland Stone Formation (PoSt)

The upward change from the Wardour Formation to the Portland Stone Formation is marked by the appearance of large doggers of fossiliferous, very hard, compact white to yellowish, fine- to medium-grained siliceous sandstone. The deep borehole at Urchfont (Figure 4) records 29.5 m of loose, fine- to medium-grained, mature quartz sand, occasionally glaucontic and fossiliferous, belonging to the Wardour Formation, overlain by 17 m of buff to white hard microcrystalline limestone with sparry calcite cement and fossil fragments (e.g. bivalves, ammonites and echinoids), assigned to the Portland Stone Formation.

Cretaceous

Lower Cretaceous strata in southern Britain show considerable vertical and lateral thickness and facies variation. The succession is generally most fully developed and thickest towards the basin centres (Whittaker et al., 1985). Elsewhere, as in the Devizes district, the succession is attenuated by nondeposition and erosion at various levels beneath the sub-Albian (sub-Gault) unconformity. The Cretaceous period began with brackish marine deposition, represented by the Purbeck Group at subcrop in the district.

Subsequently, and despite renewed subsidence at this time, clastic deposition in the Weald Basin was maintained in nonmarine facies by abundant sediment supply from the rising London–Brabant Massif to the north, Armorica to the south, and other land masses to the west and south-west. Thin representatives of this Wealden Group are interpreted as being present at depth in the east of the district on seismic profiles.

Following deposition of the Wealden Group, rising sea levels flooded the entire Wessex Basin. The boundary between the nonmarine Wealden Group and the overlying marine Lower Greensand Group is marked by the Late Cimmerian Unconformity. This represents a gap in the sequence that is greatest at the margins of the basin, as in the Devizes district, where much of the earliest Cretaceous is missing. This gap diminishes towards the centre of the basin and the unconformity is identified only by a number of hiatuses or breaks in deposition. Tidally-influenced shallow marine and shoreline sandstones and mudstones form the Lower Greensand succession.

Deepening of the basin continued into Albian times when the Gault and Upper Greensand formations, a succession of deeper marine mudstones and shallower water glauconitic sandstones, were deposited.

Purbeck Group (PB)

The Purbeck Group, comprising of limestones and mudstones, does not crop out in the district. However, evidence from the Yarnbury Borehole and from interpreted seismic reflection data, suggests that about 35 m of Purbeck Group occur beneath the Lower Greensand unconformity.

Wealden Group (W)

The Wealden Group, where fully developed, includes (in ascending stratigraphical order) the Ashdown, Wadhurst Clay, Tunbridge Wells Sand and Weald Clay formations. However, formation thinning, erosion and onlap at the margins of the basin prevent subdivision of the group in the Devizes district. Deposition was largely in a freshwater environment, in a large shallow lake or lagoon that occupied much of the present area of Wiltshire/Hampshire and the Weald.

Although, the Wealden Group does not crop out in the district, seismic interpretation indicates its presence at depth in the east. Its thickness and lithological makeup is unknown but it is presumed to be fluviatile sands and clays by analogy with adjacent areas.

Lower Greensand Group (LGS)

The Lower Greensand of the Vale of Pewsey typically consists of 5 to 10 m of glauconitic very fine- to medium-grained sand, with rare masses of cherty sandstone or chert and small polished pebbles, together with indurated sandy ironstones. The group includes the Seend Ironstone Formation and gives rise to a dark orange brown sandy soil with many indurated ironstone fragments. The maximum thickness of the group is about 12 m.

The Lower Greensand Group rests unconformably, by overstep, on the Jurassic and the oldest Cretaceous strata in the district and is itself overstepped by the Gault Formation, producing a discontinuous outcrop. The Lower Greensand occurs as outliers in the north-west of the district, the largest in the Poulshot area, where it forms a well-developed dip slope, approximating to the erosional plane of the Albian unconformity. Four smaller outliers occur south of Potterne. The largest is around Freith Farm [ST 994 563], a smaller one caps the ridge just south of Potterne Wick [ST 996 575], while a third and fourth, not recorded on the old Devizes sheet, occur on the hill just east of Greenlands Farm at [ST 985 554]. A further area occurs at the extreme north-west of the district, near Sells Green [ST 950 612].

Gault Formation (G)

The Gault Formation consists mainly of soft mudstone, light grey to dark grey in colour, slightly calcareous with disseminated glauconite and mica grains. It is shelly in part and is pyriteous throughout, with some bright sand-sized pyrite crystals when unweathered and pyrite nodules with a radial crystal structure. Phosphatic nodules in layers are a feature of the lower part and frequently mark the base. The Gault Formation crops out in the north-western part of the district near Devizes, in the valley north of Urchfont running westwards towards Potterne Wick and north towards Rowde. The top of the formation is frequently obscured by slumped and landslide deposits below the Upper Greensand scarp, particularly around the town of Devizes. Away from the outcrop towards the east the Gault Formation is interpreted from seismic data to underlie the remainder of the district, and is proven in the five deep boreholes mentioned above (Figure 4). Its thickness is between 33 and 58 m.

Upper Greensand Formation (UGS)

The Upper Greensand Formation forms a significant scarp above the lower ground occupied by the Gault. In the Devizes district, the outcrop runs from the Coulstons eastward by Great and Little Cheverell to Market Lavington and Urchfont. To the north, a continuous outcrop through Potterne and Devizes forms much of the landslide-affected scarp and dip slope eastwards towards Stert. The higher part of the formation has an extensive outcrop across the Vale of Pewsey, eastward to Burbage and beyond, outside the district to the east.

This formation consists of pale, yellow-brown, grey and greenish grey, bioturbated siltstone and silty, very fine-grained sandstone, with variable amounts of mica and glauconite. The thickness is relatively uniform, up to 56 m over much of the outcrop within the district, but it thins rapidly south-eastwards.

During the recent survey of the Devizes district the component members of the formation became increasingly difficult to differentiate towards the north-east. As a result, the divisions have not been shown in this survey east of West Lavington. There is, in particular, a notable absence of chert-bearing Upper Greensand within the upper part of the formation.

The Potterne Rock, in the higher part of the Upper Greensand, forms a widely recognisable horizon at outcrop in the Devizes district (Plate 1). This horizon is typically a hard, nodular, well-burrowed unit, usually with phosphatic clasts, and always underlain by a few metres of grey-green glauconitic sand containing scattered phosphatic clasts. It can clearly be seen in a number of cuttings around Potterne itself, for example, in Coxhill Lane [SU 004 584]. A series of sections were logged and recorded and their correlation is shown in (Figure 5).

Chalk Group

Approximately 300 m of Upper Cretaceous chalk underlies the Devizes district, forming extensive dip slopes, undulating ground and the dissected primary and secondary escarpments that cross the area.

Most of the lithostratigraphical units typically exhibit surface topographical features, these include the primary and secondary escarpments, and subsidiary escarpments developed on dip slopes. Generally, the West Melbury to Seaford Chalk formations forms the primary escarpment, whilst the Newhaven to Portsdown Chalk formations form secondary excarpments.

The nomenclature for the Upper Cretaceous used in this district is shown in (Figure 6), together with its relationship to the traditional scheme. The current nomenclature is a development of the schemes devised by Mortimore (1983, 1986), Bristow et al. (1995, 1997) and adopted by the Geological Society Stratigraphy Committee in 1999 (Rawson et al., 2001; Hopson, 2005).

Chalk accumulated on the outer shelf of an epicontinental subtropical sea of normal salinity and, generally, with little terrigenous input. In Cenomanian times, landmasses were present in south-west England, Wales, Scotland and Northern Ireland, and farther afield in Brittany. However, much of this land was submerged by later sea-level rise, especially from the earlier Turonian onwards.

Grey Chalk Subgroup (GCk)

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

West Melbury Marly Chalk Formation (WMCk)

In the Devizes district the West Melbury Marly Chalk is 15–30 m thick, and crops out in the valley along the southern edge of the Vale of Pewsey. In general, the formation forms the shallow sloping ramp at the base of the Chalk scarp, between two strongly developed negative breaks of slope. This geomorphological featuring is clearly seen on the southern flank of the Vale of Pewsey, between West Lavington [SU 0066 5299] and Upavon [SU 1350 5494].

The formation is a repetitive sequence of hard limestone and softer marl (calcareous mudstone) couplets, each pair being between 0.5 and 2 m thick at most. The base of the formation is marked by the Glauconitic Marl Member; an arenaceous, glauconitic, marly sandstone which provides a distinctive positive gamma-ray peak in borehole geophysical logs across southern England.

The West Melbury Marly Chalk generally forms an aquitard between the Upper Greensand and the Zig Zag Chalk due to its high clay content.

Zig Zag Chalk Formation (ZCk)

The Zig Zag Chalk Formation typically comprises 25 to 55 m of medium–hard, pale grey, blocky chalk, with some thin limestones near the base. The lower part of the formation has a greater marl content and some distance above the base, hard, pale grey splintery limestones with conspicuous Sciponoceras may occur. The upper part of the Zig Zag Chalk tends to be pale grey to white, firm, marly chalk with common Inoceramus atlanticus (Heinz), I. pictus J de C Sowerby and the echinoid Holaster subglobosus (Leste). In the Devizes district the formation is exposed as a narrow outcrop along the Chalk escarpment, forming the southern flank of the Vale of Pewsey, and forms outliers around Etchilhampton, Alton Priors, and just to the south of Burbage.

The base of the formation is commonly at a strong negative slope break near the base of the Chalk escarpment. This abrupt change in slope appears to correspond with the incoming of thick beds of firm to hard, blocky chalk, above the gently sloping ground underlain by the West Melbury Marly Chalk. The upper limit of the Zig Zag Chalk is taken at the base of the Plenus Marls Member.

White Chalk Subgroup (WCk)

The White Chalk Subgroup is essentially the combined Middle and Upper Chalk formations of Bristow et al. (1997). It is divided into seven formations (Figure 6), of which, the lower six occur in the Devizes district. The base of the White Chalk Subgroup is taken at the base of the Holywell Nodular Chalk Formation, which in present practice includes the Plenus Marls Member (Plate 2). In general, the subgroup is characterised by pure white chalk, hard and nodular at some levels, with common flint horizons in the middle and higher parts. A maximum of 260 m of White Chalk is estimated to crop out in the district. The Netheravon Borehole (SU14NE/1) [SU 169 483] proved an estimated 138 m of White Chalk Subgroup, with the youngest chalk belonging to the highest Seaford or lowest Newhaven Chalk.

Holywell Nodular Chalk Formation (HCk)

The Holywell Nodular Chalk generally comprises hard, nodular chalks 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 unnamed succession of hard, nodular and grainy chalks with abundant shell debris, most notably species of Mytiloides. In the Devizes district it crops out along the face of the primary Chalk escarpment (Plate 3), in re-entrant valleys and as an inlier in Water Dean Bottom. The formation is also present around Knook in the south-west of the district, and more extensively outside the district towards Warminster.

The Plenus Marls Member is rarely well exposed, but is present along the whole outcrop of the Holywell Nodular Chalk Formation. It consists of an alternating succession of blocky white chalk and medium grey silty marl beds, mostly between 1 cm and 20 cm thick.

Overlying the Plenus Marls 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 incoming of abundant bivalve shell debris. It is up to 3 m thick and often forms a strong positive feature and characteristic brash that can be traced around the outcrop. The overlying unnamed unit of shell detrital and grainy chalks forms 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 shell detrital chalk during surveying. The Holywell Nodular Chalk Formation is between 15 and 25 m thick in the Devizes district.

New Pit Chalk Formation (NPCk)

The New Pit Chalk Formation, between 10 and 35 m thick, consists of smooth, firm, massively bedded, white chalks with marl seams. In general the New Pit Chalk forms moderately to steeply sloping ground, often with a characteristic buttressed form, above the first positive feature indicating the Holywell Chalk (Plate 3), and below another strong positive feature indicating the base of the overlying Lewes Nodular Chalk. It is usually softer than both the underlying Holywell Nodular Chalk and the overlying Lewes Nodular Chalk, and often forms a slight negative feature in the scarp.

The base of the New Pit Chalk is marked by the disappearance of inoceramid-rich nodular chalk. The upper limit is marked by the incoming of common flints and hard nodular chalks, the latter typically between Glynde Marl 1 and Southerham Marl 2 in the standard Sussex succession. Flints are rare in the New Pit Chalk Formation. Where present they are small and occur in the uppermost beds. The macrofossil fauna is much sparser than in the Holywell Nodular Chalk, and comprises brachiopods (both terebratulids and rhynchonellids) and local horizons of the bivalves Mytiloides (at the base) and Inoceramus. Thin-shelled Mytiloides hercynicus/subhercynicus in the lowest beds tend to be flattened and preserved as chalky moulds.

Lewes Nodular Chalk Formation (LeCk)

The Lewes Nodular Chalk comprises interbedded, hard to very hard, nodular chalks, with soft to medium-hard chalks and marls. The first persistent seams of flint occur near the base. The flints are typically black or bluish black with a thick white cortex. The formation is generally between 15 and 40 m thick. The Lewes Nodular Chalk is divided into two informal units by the Lewes Marl and associated underlying Lewes Flints, the latter comprising a ramifying system of black cylindrical burrow-form flints. The lower unit consists of medium to high-density chalk and conspicuous, iron-stained, hard, nodular chalks. The upper unit is mainly of low to medium-density chalks with evenly spaced, thin, hard, nodular beds.

The Lewes Nodular Chalk Formation forms the highest steep slopes at the top of the primary Chalk scarp, and the dip slopes beyond onto the major interfluves. In the Devizes district the basal part of the succession is more condensed, compared to expanded sequences seen in Sussex, and the Chalk Rock Member becomes well developed particularly in the Vale of Pewsey.

The Lewes Nodular Chalk Formation outcrops over much of the northern downs area. The hardgrounds at the base of the Lewes Nodular Chalk (Chalk Rock) can be identified in the valleys either side of the primary escarpment (Plate 4). These hardgrounds, and the hard nodular beds form good positive breaks of slope. The upper part of the Lewes Nodular Chalk comprises soft to crisp, flaky, white chalk with horizons of the bivalve Cremnoceramus sp. The contact between this and the base of the overlying Seaford Chalk is gradational, and can be difficult to identify. Within the Salisbury Plain Training Area many of the exposures are limited to the impact craters, which reveal the underlying chalk. The rocks found in such craters are intensely shattered.

Seaford Chalk Formation (SCk)

The Seaford Chalk Formation is between 55 and 65 m thick, and crops out over a wide area of the Devizes Sheet. It underlies much of the Chalk dip slope and the broad interfluves between the primary escarpment and the negative break of slope below the secondary Chalk escarpment. Topographically, the Seaford Chalk Formation forms the characteristic smooth convex slopes of the major ridges between the dry valleys across much of the Salisbury Plain Training Area.

The Seaford Chalk Formation is composed primarily of soft, smooth, blocky, white chalk with abundant seams of large nodular and semitabular flint, and thin harder nodular chalk near the base. The flints in the lower part of the unit are often highly carious, whereas higher in the succession the flints are black and bluish black, mottled grey, with a thin white cortex. These flints commonly enclose shell fragments. Some of the large flint bands, notably the Seven Sisters Flint (Mortimore, 1986) (15–20 m above the base of the formation), form almost continuous seams and in places create local topographical features. Examples can be seen in the valleys around Shrewton. The Seven Sisters Flint can often be distinguished from other large flints by its usual inoceramid bivalve content of Platyceramus and Volviceramus which are typical of the lower part of the Seaford Chalk. Platyceramus also occurs in the upper part of the Seaford Chalk, associated with Cladoceramus (Mortimore, 1986). Thin, planar, sheet flints, are also common in parts of the succession.

Another particularly characteristic semitabular flint occurs near the top of the Seaford Chalk in the Devizes district, about 11 m below the base of the Newhaven Chalk Formation. This flint is generally about 10 cm in thickness, of uniform appearance, and tends to fracture vertically. The blocks thus formed are up to 50 cm across and are fairly conspicuous in ploughed fields. This flint bed is tentatively correlated with Whittaker's Three Inch Band of the North Downs (described by Robinson, 1986), which is probably equivalent to the Rough Brow Flint of the Sussex coast (Mortimore, 1986). However, no biostratigraphical information has so far been found to support this correlation.

Above the inferred Whittaker's Three Inch Flint, and to about 5 m below the Newhaven Chalk, is a thin (1–2 m) horizon of intensely hard porcellanous indurated chalk (the 'Winchester Hardground' of Farrant, 1999). This interval is now formally called the Stockbridge Rock Member, and is shown on the 1:10 000 maps in the south-east of the district as a limestone unit. It 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 in the Bourne River valley to the east of the district. 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). In Kent, Rowe's Echinoid Band, a bed 0.3 m thick containing an acme occurrence of Conulus sp. with other echinoids, occurs just above Barrois' Sponge Bed (Robinson, 1986) and mapping demonstrates that this occurs just above the Stockbridge Rock Member. The Stockbridge Rock Member occurs widely between Salisbury and Winchester, but appears to be quite sporadic and intermittent in the west and north of that district, and has not been recorded north of Tidworth nor within the outcrops to the west in the Devizes district. Its patchy distribution may be partly explained by the state of plough, but is more likely to be due to variations in the degree of cementation. As yet it has not been seen in section, so its true origin is open to debate. Field evidence from the Winchester area (Farrant, 2000), suggests that there might be several thin hard bands between 5 m and 10 m below the base of the Newhaven Chalk in those areas, each separated by thin intervals of softer chalk.

Newhaven Chalk Formation (NCk)

The Newhaven Chalk Formation is composed of soft to medium–hard, blocky, smooth, white chalks with regular marl seams and flint bands, and is between 55 and 70 m thick in the Devizes district. Typically, the marls vary between 20 and 70 mm thick but, in this area, they are generally little more than a few millimetres. This thinning of marl seams probably reflects the influence of positive synsedimentary structural features within the basin. (Mortimore, 1986; Mortimore and Pomerol, 1987; Mortimore and Pomerol, 1991). 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. Channels with hardgrounds and phosphatic chalks occur locally in the succession elsewhere, and were noted in the boreholes drilled for the A303 dual carriageway south-east of Stonehenge in the adjacent Salisbury sheet area to the south.

The Newhaven Chalk crops out extensively over the far eastern part of the district, occupying much of the sloping ground on and immediately below the face of the secondary Chalk escarpment. In the classic South Downs succession of Sussex, the base of the Newhaven Chalk typically forms a prominent double negative feature break at the base of this scarp. Although the secondary escarpment is well developed in the Devizes district, only rarely does the base of the Newhaven Chalk correspond with the most prominent negative break of slope. Instead this break usually occurs within the M. testudinarius Zone, about 10 m above the base of the formation. The lowest ten metres of the Newhaven Chalk generally caps the spurs extending out from the scarp foot. The base of the formation is commonly marked by an extremely faint negative break of slope a short way above a rounded positive break of slope, which in some parts of the district seems to be caused by the indurated horizon at the top of the Seaford Chalk.

There are three isolated outliers of Newhaven Chalk in the district. The first, occurs on the crest of the ridge between Larkhill Barracks and Larkhill Racecourse. Although there are no exposures, fragments of soft, smooth, low-density white chalk with small spiky 'finger' and Zoophycos flints were seen in the many rabbit scrapes. Ossicles of Uintacrinus socialis, the zonal fossil for the lowest zone in the formation, were found at [SU 1319 4535]. The lower boundary was confirmed at a positive feature by micropalaeontology.

The second outlier occurs at Netheravon airfield [SU 1656 4908], to the east of the Avon Valley, and a third, larger outcrop, caps Silk Hill [SU 1850 4687] and the adjoining ridge to the south-west.

The Newhaven Chalk Formation also forms the expanse of higher ground in the far south-east of the district, from Weather Hill [SU 2047 5171] at its most northerly limit, southwards to Sidbury Hill [SU 21600 50600], Tidworth, and following the ridge crest towards Bulford Camp. Exposures are generally poor, being within military grassland, and restricted to a few deeply cut tank tracks. Abundant Zoophycos flints were found in one such track around the base of Sidbury Hill [SU 22200 51030], with pipe flints a few metres below, indicating a very low level near to the base the Newhaven Chalk.

Culver Chalk Formation (CCk)

This formation, up to 25 m thick in the Dervizes district, forms small isolated outliers on the crest of the secondary Chalk escarpment. The Culver Chalk is composed of soft, white chalks without significant marl seams, but with some very strongly developed nodular and semitabular flints. A particular concentration of large flints, the Castle Hill Flints, occurs near the base of the unit as defined (at the Castle Hill Marls) by Mortimore (1986).

The Culver Chalk Formation has very limited development in the Devizes district. Three small outliers occur on the hilltops at Sidbury Hill [SU 21600 50600], Beacon Hill [SU 2093 4434] and south-east of Bulford [SU 1949 4275] on the southern margin of the district.

Palaeogene

The Palaeogene succession usually consists of a number of sedimentary cycles each commencing with a marine transgression commonly marked by a thin bed of flint pebbles. These pebbles are overlain by sediments which were deposited as the coastline advanced (Edwards and Freshney, 1987). Each cycle probably lasted between 1 and 2 million years. The Reading Formation, of the Lambeth Group, is the only Palaeogene deposit present in the district.

Lambeth Group

The Lambeth Group corresponds to the strata formerly described as the Woolwich and Reading Beds. In general, the Lambeth Group consists of two units, the Upnor and Reading formations (Ellison et al., 1994). The Upnor Formation is very thin and is mapped and described as part of the Reading Formation.

Reading Formation (RB)

The Reading Formation rests unconformably on the eroded surface of the Chalk, and is probably less than 5 m thick in the Devizes district. The basement bed of the Reading Formation (equivalent to the Upnor Formation) comprises reddish brown sand or interbedded sand and clay with abundant rounded to well-rounded, stained flint pebbles with locally glauconitic sandy clays, analogous to the 'Bottom Bed' of the London Basin. This basal bed is usually less than 1 m thick, at maximum up to 2 m in places.

The remainder of the formation consists of mottled, bright red and grey clay and silty clay, but also in shades of purple, brown and orange. The complex mottling has been ascribed to pedogenic processes with multiple overprinting of palaeosols (Buurman, 1980). Lenticular bodies of well-sorted, fine to medium-grained sand occur locally at various levels, particularly at the top and base.

In the Devizes district, Palaeogene strata occur as a single isolated outlier capping Sidbury Hill [SU 2160 5060], north of Tidworth. Here the basement bed of the Reading Formation unconformably overlies the Culver Chalk, and comprises a greyish green, clayey sand with abundant subangular to rounded, corroded and pitted glauconite-stained flints and brown sandy clay with well-rounded flints with pockets of orange sand.

Quaternary

In the Devizes district, about 50 million years is estimated to have elapsed between the deposition of the youngest preserved Palaeogene and the oldest Quaternary deposits. During this time younger Palaeogene and Neogene strata were deposited across much of southern Britain, and subsequently were removed following uplift along the Wealden axis (as part of the general inversion of the Wessex Basin). During the Quaternary, a further significant break in deposition occurred after the initial accumulation of the clay-with-flints and before the deposition of the younger Pleistocene deposits.

During the Pleistocene, sea levels rose and fell according to the quantity of water locked up in ice caps. At times of glacial maxima, a periglacial environment was established in this district. There was enhanced erosion both by solifluction and by an extensive river system flowing to much lower base levels (up to 100 m below present sea level in the most extreme glacial episodes).

The following descriptions of the deposits are grouped on the basis of their origin. Mass movement deposits are described first, followed by fluviatile deposits. Their order does not imply relative age. (Figure 7) schematically indicates the relationships of these deposits in the Devizes district.

Clay-with-flints

The clay-with-flints is primarily a remanié deposit created by the twin agencies of the weathering and erosion of the original Palaeogene cover and dissolution of the underlying Chalk. It is typically composed of orange-brown or reddish brown clays and sandy clays containing abundant flint nodules and pebbles. At the base of the deposit the matrix becomes stiff, waxy and fissured, and of a dark brown colour with relatively fresh nodular flints stained black and/or dark green by manganese compounds and glauconite.

The clay-with-flints is most widespread on the high ground, underlain by the Seaford Chalk Formation, between Chitterne and Shrewton. It also covers the higher ground east of Upavon Airfield, forms small deposits near Everleigh Ashes [SU 195 563] and caps Sidbury Hill [SU 216 506]. There are no significant exposures of this deposit in the district and it was mapped on the basis of its characteristic reddish brown, sticky, clayey soil with nodular, often stained (orange) flints. In general, it forms the flat tops of hills and long dip-slope spurs.

Head

Head generally comprises yellow-brown, silty, sandy clay with variable proportions of coarser granular material, but all deposits have an earthy texture. Clast composition varies depending on source materials; those deposits derived mainly from the chalk were formerly mapped as 'dry valley deposits' or 'coombe deposits'. These heterogeneous deposits accumulated in valley bottoms by solifluction, hillwash and hillcreep and are generally only a few metres thick. Head deposits occur as soliflucted slope deposits and gravelly valley bottom deposits. The slope deposits are thought to represent an earlier phase of deposition before the valley bottom head.

Slope head

Slope head ('older head') deposits range from flinty gravels to reddish brown, sandy clays containing abundant flint nodules and pebbles that are generally much more shattered than those in the clay-with-flints. Several large sheets occur in this district, generally no more than a few metres thick. The deposits are most widespread on north and east-facing slopes and commonly grade laterally into areas with only a thin flinty veneer.

Gravelly head

Gravelly head is essentially alluvial, and comprises 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 coarse or very coarse, poor to moderately sorted, clast-supported, subangular to subrounded, flint gravel, with generally little or no fine-grained material. In the Devizes 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 run-off 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 overbank alluvial deposits of silt, sand and peat.

In the south-west of the district, gravelly head is mapped in the middle course of the Berril Valley–Chitterne Brook (Plate 5) above the perennial spring and below the most significant area where the stream rises during excessive rainfall periods. The valley bottom tends to be very gravelly and hummocky (perhaps reflecting areas where the groundwater breaks the surface).

Head gravel

This deposit is very similar lithologically to gravelly head, but occurs on lower valley sides. The deposit is a coarse or very coarse, poor to moderately sorted flint gravel, with an admixture of fluvial rounded to subangular rolled worn flints and rare angular-large, often broken nodular and coarse gravel-sized flint set in a greyish brown to orange-brown clayey, silty, fine- to coarse-grained sand matrix. Its gravelly nature serves to distinguish it from head and its occurrence on significant slopes distinguishes it from terrace deposits.

River terrace deposits

The River Avon has a history of terrace development in response to base level changes of the river system. Terrace deposits associated with this river in the Devizes district correspond to the 'fourth' level in the scheme of terrace numbering for this catchment. Higher terraces exist outside the district but are not generally geomorphologically very distinct. These are principally associated with the development of the proto-Solent, and from gravelly spreads on interfluves. Terraces in the Avon headwaters have undergone weathering and degradation by solifluction and grade both upslope and down slope into spreads of gravelly head deposits of various types. These terraces have been labelled as undifferentiated.

The term river terrace deposits (undifferentiated) is used in the district to identify gravel spreads on the lower valley slopes that show some crude or degraded terrace surface. It is the generally flat surface formed by this deposit that differentiates this deposit from other gravelly slope head deposits but the surface brash may well have the same appearance.

Alluvium

The alluvium in the district comprises a complex interdigitation of three distinct lithologies; sandy gravel (in places chalky), peat and fine-grained sandy muds (and muddy sands). In places a fourth unit of chalky, gravelly, sandy, silty clay is regarded as solifluction material derived from the steeper valley sides. This unit, mapped as part of the alluvium, is generally buried by fine-grained overbank deposits along the margins of the alluvial tract in the broader streams. The soils associated with the alluvium are generally pale greyish brown silty clay, and can be very flinty in places. These overbank deposits can be seen to rest on a flint and chalk gravel base, for example, in the deep channel particularly south of the village of Chitterne.

The principal outcrop of the alluvium is associated with the River Avon and its headwaters in the Vale of Pewsey. Narrower outcrops are associated with minor valleys in the north-west of the district.

A small area of alluvium is mapped in the south-west of the district in the Chitterne Brook valley beneath the perennial spring. There are no exposures other than the shallow bank of the canalised brook where overbank silty clay with sporadic pebbles can be seen resting on a gravel stream bed. The Chitterne Brook flows over a narrow floodplain between 100 and 200 m wide.

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 fine-grained sediments that exhibit 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 in floodplain marginal woodland or reed/sedge beds in slow flowing backwater situations. They often contain appreciable amounts of trapped fine-grained organic muds and notable shell detrital beds. In special circumstances where carbonate-rich 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.

In the north-west of the district, the upper reaches of the River Avon system east of Coate are founded on the Upper Greensand at the level of the groundwater interface and consequently sandy peat areas are found in these marshy areas. East and south-east of Coate, the proportion of clay and silt increases such that the deposit becomes a humic alluvium at surface. Similar peat deposits also occur in the valley south towards Patney and Marden and as isolated deposits around Woodborough and north of Pewsey to the east.

Artificial ground

The major occurrences of made, worked, infilled and landscaped ground are noted on the 1:10 000 scale maps within the district. Only the larger areas are transferred onto the published 1:50 000 scale Devizes Sheet 282. In urban areas the amount of artificial ground is often difficult to determine, and its limits often masked by the built environment. Whilst most of the villages are on natural ground, the larger urban areas (particularly within industrial estates, development parks and post-war housing estates) have suffered a large amount of landscaping and the degree of 'cut and fill' is often impossible to determine. The artificial ground shown on the maps in those areas is probably an underestimate

Made ground

Made ground is a term used to denote areas where additional material, foreign to the site, has been deposited above the natural ground surface. Occurrences are mainly related to road and rail embankments and archaeological sites (commonly identified by a symbol on the base maps). Modern road and rail developments are generally made up of 'engineered fill' designed to carry the loads expected, and therefore considered to be more stable than that created by the excavations for archaeological earthworks and other features.

The two main categories of made ground are waste in landfill sites and natural materials produced either as spoil from mineral extraction, or dug for the construction of various embankments and raised areas, including bunds for flood defence. Recycling of waste construction materials is leading to their increased usage in urban and industrial landscaping

Worked ground

Worked ground is shown where natural materials are known to have been removed, for example in quarries and pits, road and rail cuttings and general landscaping. In the Devizes district, chalk is the most commonly extracted material, principally for use as an agricultural lime but also for the manufacture of cement, as a filler and whitening agent.

Outside the district to the south and west, the Portland and Purbeck groups are a source for architectural stone, with extraction since Roman times. However, the Portland outcrops on the Devizes Sheet have not been similarly exploited.

There are few sand and gravel workings, perhaps reflecting the generally poorer quality of the local aggregate. In the past most villages had a small brick and tile quarry to supply local needs. In the Devizes district, the Gault Formation and a number of the Quaternary deposits have been used in the past for such material.

Infilled ground

Infilled ground comprises areas where the natural ground has been removed and the void, wholly or partly backfilled with man-made deposits, which may be either natural or waste material, or a combination of both. Where quarries and pits have been filled, the ground restored and landscaped, built on or returned to agricultural use, there may be no surface indication of the extent of the backfilled area. In such cases, the boundaries of these sites is taken from archival sources such as earlier aerial photographs, local authority records and old topographical and geological maps.

Landscaped ground

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

Chapter 3 Applied geology

Hydrogeology

The principal aquifer within the district is within the Chalk Group. Public water supplies are also derived from the Upper Greensand Formation, although this resource is more important to the south-west in the Wincanton and Shaftesbury areas. Local supplies come from the Portland Group (and possibly the Purbeck Group) and to a lesser extent the Palaeogene and Quaternary strata. The 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 for detailed information readers are advised to consult the reports on minor aquifers (Jones et al., 2000) and major aquifers (Allen et al., 1997) that give valuable overviews of the water resources. Stream sinks are known at the margin of the Palaeogene strata and at the boundary of the Gault Formation.

The hydraulic properties of the Chalk aquifer are complex and result from a combination of matrix and fracture properties. The Chalk is microporous with low intrinsic permeability. The intergranular porosity of the Chalk is high, usually around 35 per cent for the White Chalk Subgroup, falling to around 25 per cent for the Grey Chalk Subgroup (Bloomfield et al., 1995). However, the pore sizes are so small that the permeability of the rock is minimal. The high transmissivity of the aquifer is provided by fractures, which are commonly enlarged by dissolution.

The hydrogeology of numerous streams within the district (e.g. the Bourne River, Nine Mile River, River Till, Chitterne Brook) are dominated by groundwater flow from the Chalk. Hence the lithological properties of the Chalk and the geological structure will have an important influence on how the streams behave and the aquifer functions.

Each Chalk formation has differing aquifer properties resulting from the lithological control on fracture style and spacing, the presence or absence of marl seams, and the frequency and style of flint bands. Marl seams, bedding planes, sheet flints and tabular flints are all horizons where downward percolation of water may be impeded. Dissolution occurs and conduits often form where flow is concentrated along these horizons. The strength of the chalk is also important. Fractures in very soft chalk are often sealed by remoulded chalk putty, and thus form aquitards or even aquicludes. Joints in harder, nodular chalks often remain open and thus solution cavities can develop more readily.

Numerous springs occur over the Devizes district. Some springs are small trickles, but some issue a significant amount of water. In the northern part of the district, around Devizes and in the Vale of Pewsey, there are supplies of water obtainable from both the Upper Greensand and the Chalk. To the west of Devizes, near Whistley Farm, Jukes-Browne (1905) recorded ferruginous water issuing from the Lower Greensand near its junction with the Gault. There are a series of springs that issue from the base of the Upper Greensand at or near the Gault contact across the district. These springs occur across the outcrop from Coulston to Urchfont and to the north, around Stert and Potterne. A strong spring was recorded at this contact in Peppercombe Lane, [SU 039 573] Urchfont. Numerous springs were also recorded along the steep-sided valley south of Devizes [SU 007 596], again associated with the Upper Greensand–Gault boundary.

South of West Lavington, at the head of the valley known as 'The Warren', a strong spring issues from part of the Zig Zag Chalk Formation owing to the presence of marly beds below. All the springs that issue from the White Chalk, such as at Chitterne and at Imber, are intermittent springs or bournes, which only flow in the winter months or after periods of prolonged rainfall. The water is probably associated with the top of the Lewes Nodular Chalk Formation that occurs at or just below rockhead in the valley floor. Here water can be seen issuing from very coarse poorly sorted flint gravel at many places. This is a classic location for karstic development. In winter, these springs may form the perennial head of the stream, but in very dry periods this stretch of the river is dry and water does not emerge until further down the valley.

Bulk minerals

Bulk mineral extraction is confined to two deposits in this district: sand and gravel, and brick clay.

Sand and gravel

There is no large-scale extraction of aggregate from any of the deposits in the district. Resources exist within the Upper Greensand (sand) and within the various Quaternary deposits (sand and gravel), but they are either not exploited or only worked on a local scale to support farms. Their grade and potential as a source of aggregate has not been tested.

Brick clays

The Gault in around Devizes was used for brickmaking, and further afield outside the district, the Kimmeridge Clay Formation is still used as a resource. There are no pits in evidence today.

Building stone

Chalk

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 as '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 (suggesting some form of quarrying) and 'field-picked' clasts are seen in older buildings.

Upper Greensand

A bed of compact calcareous sandstone occurs in the Upper Greensand near Potterne (Jukes-Browne, 1905). This stone was used in the construction of Blount's Court and other buildings in Potterne. This stone was occasionally dug for walling stone but quantity was limited to a 0.5 m thick bed, commonly seen in the track cuttings east of Potterne. This same rock bed appears near Market Lavington, but it is not known whether any quarrying has occurred in this area.

Geotechnical considerations and hazards

The following statements should be taken only as a guide to likely or possible problems and should not replace site-specific studies. (Figure 8) tabulates potential ground constraints and the deposits with which they are commonly associated.

Chalk dissolution

The Chalk is locally affected by solution phenomena and as a consequence, fractures naturally occurring in the Chalk are enlarged 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 4 m to 6 m deep. These generally overlie pipes filled with Palaeogene materials, clay-with-flints, or in some places, head. Such depressions continue to act as sumps for surface drainage, and may be liable to further subsidence. Differential compaction under load can occur across such structures. Stream sinks may be locally present.

Head deposits

Map users should be aware that thin deposits of head are much more widespread than indicated by the geological map. In particular, large parts of the White Chalk outcrop, which are shown with no overlying superficial deposit, do actually carry a thin and extensive, but discontinuous, blanket of head. Head, especially where clay-rich, can contain gently dipping shear planes that can fail when loaded.

Artificial ground

Planning for future construction should allow for the possible existence of small areas of made, infilled or landscaped ground. Such areas might be liable to differential settlement.

Peat

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.

Landslide deposits

Areas of landslide-affected ground are relatively common in the north-west of the district. These areas are mainly associated with the steep scarp slopes adjacent to the major valleys, for example, along the face of the Upper Greensand Formation scarp around Devizes. In most cases the area of slip is obvious from the disruption of the surface sediments.

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

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Devizes district and adjacent areas are listed here.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

Geological maps
1:1 500 000
Tectonic map of Britain, Ireland and adjacent areas (1996)
1:1 000 000
Industrial mineral resources map of Britain (1996)
Pre-Permian geology of the United Kingdom (South) (1985)
1:625 000
Bedrock geology of the United Kingdom: South Map (2007)
Quaternary geology: South sheet (1977)
1:250 000
Chilterns. Sheet 51N-02W. Solid geology (1991).
Bristol Channel. Sheet 51N-04W. Solid geology (1988).
1:10 000
For the most recent revision of the 1:50 000 Series Sheet 282 Devizes, the component 1:10 000 series geological maps are available for purchase from the BGS Sales Desk.
1:10 560 field slips: The original 1:10 560 field slips from the 1885–1898 primary survey are archived in the BGS collections.
Details of the original geological surveys are listed on editions of the 1:50 000 or 1:63 360 geological sheets. Relevant parts of the component 1:10 000 scale maps for the Devizes district were surveyed between 1996 and 2005.

1:10K tile	Name	Geologist	Date
ST96SE*	Devizes West	PMH	Spring 05
ST95NE	Potterne	ARF	Spring 05
ST95SE	Little Cheverell	PMH, LB, RJM	Autumn 05
ST94NE	Imber	RJM, AJN, PMH	August 04
ST94SE	Chitterne	PMH	2003/04
SU06SW*	Devizes East	PMH	Spring 05
SU05NW	Urchfont	KAB	Spring 05
SU05SW	The Lavingtons	KAB, PMH	August 04
SU04NW	Tilshead	ARF, PMH, AJN	August 04
SU04SW	Yarnbury Castle	ARF	Spring 04
SU06SE*	All Cannings	RJM	Spring 05
SU05NE	Chirton	RJM	Spring 05
SU05SE	Charlton Down	RJM, PMH, ARF, LB	August 04
SU04NE	Orcheston St Mary	LB, AJN	August 05
SU04SE	Shrewton	AJN, ARF	Spring 03
SU16SW*	Wilcot	RJM, AJN	June 05
SU15NW	Upavon	LB	Spring 05
SU15SW	Enford	KAB, ARF	Autumn 99/Spring 05
SU14NW	Netheravon	KAB, AJN, ARF, PMH	Autumn 99/August 04
SU14SW	Durrington Down	ARF	Spring 03
SU16SE	Milton Lilbourne	AJN	Spring 05
SU15NE	Pewsey	KAB	Autumn 99
SU15SE	Everleigh	KAB	Autumn 99
SU14NE	Figheldean	KAB, PMH, ARF	Autumn 99
SU14SE	Bulford	ARF	Autumn 99/Spring 03
SU26SW	Burbage	AJN	Spring 05
SU25NW	Easton	KAB	Autumn 99
SU25SW	Sidbury	KAB	Autumn 99
SU24NW	Tidworth	PMH	Autumn 99
SU24SW	Cholderton	ARF, LB	Autumn 99
ST96SW*	Seend	PMH	Spring 05
ST95NW*	Steeple Ashton	ARF	Spring 05
ST95SW*	Bratton	LB, ARF, RJM	August 04
ST94NW*	Battlesbury	RJM, AJN	August 04
ST94SW*	Heytesbury	PMH, CRB	1994/Spring 2004
* indicates part mapped sheet. Geologists: AJN – A J Newell, ARF – A R Farrant, KAB – K A Booth, PMH – P M Hopson, RJM – R J Marks, LB – L Bateson, CRB – C R Bristow.

Copies of maps from these and earlier large-scale surveys are available for reference in the BGS Libraries at Keyworth and Edinburgh, and at the BGS London Information Office in the Natural History Museum, South Kensington. Copies for purchase are produced on a print-on-demand basis and are available from the BGS Sales Desk.

Digital geological map data
In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.
Geophysical maps
Geophysical colour shaded relief maps of gravity and magnetic anomalies for Britain, Ireland and adjacent areas are available at a scale of 1:1 500 000.
1:625 000
Gravity anomaly map of the UK: South sheet (2007)
Magnetic anomaly map of the UK: South sheet (2007)
Hydrogeological maps
1:625 000
England and Wales (1977)
1:100 000
North-west Hampshire (1990)
East Somerset and south-west Wiltshire *

Books

British Regional Geology Guides

South-west England (Fourth Edition) (1975)

Memoirs and Sheet Explanations

282 South and East of Devizes (1905)*

*these items are out of print but facsimile copies may be purchased from the BGS Sales Desk.

Reports

Technical reports relevant to the district, including biostratigraphical reports, may be consulted at the BGS library or purchased from the BGS Sales Desk.

Geology
WA/00/11	Aldiss	2000
WA/00/18	Booth	2000
WA/00/23	Hopson	2000
WA/00/24	Farrant	2000
IR/01/157	Farrant, et al.	2001
Biostratigraphy
WH/99/138R	Woods	1999a
WH/99/142R	Woods	1999b
IR/04/052R	Woods	2004a
IR/04/146	Wilkinson	2004a
IR/04/148	Wilkinson	2004b
IR/04/149	Woods	2004b
IR/04/156	Wilkinson	2004c
IR/06/003	Wilkinson	2006
IR/05/107	Woods	2005

Documentary records collections

Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.

Borehole and trial pit records

Borehole records for the district are catalogued in the NGDC at BGS Keyworth. Index information, which includes site references, names and depths for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at BGS Keyworth.

Hydrogeological data

Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.

Geophysical data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data from coal and hydrocarbon exploration programmes is available for the north of the district. Indexes can be consulted on the BGS website.

BGS Lexicon of named rock units

Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 282 (Devizes), are held in the BGS Stratigraphical Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS Photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collection can be viewed at BGS libraries at Keyworth and Edinburgh, and on the BGS website. Copies of the photographs can be purchased from BGS.

Aerial photographs

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

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.

References

Most of the references listed here can be consulted at the BGS Library, Keyworth. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.

Aldiss, D T. 2000. Geology of the Cholderton and Grateley area, Hampshire and Wiltshire, part of 1:50 000 geological sheets 282 (Devizes), 283 (andover), 298 (Salisbury) and 299 (Winchester). British Geological Survey Technical Report, WA/00/11.

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, 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.

Bloomfield, J P, Brewerton, L J, and Allen, D J. 1995. Regional trends in matrix porosity and dry density of the Chalk of England. Quarterly Journal of Engineering Geology, Vol. 28, S131–142.

Booth, K A. 2000. Geology of the Bourne River Catchment, Netheravon to Pewsey, Hampshire. British Geological Survey Technical Report, WH/00/18.

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.

Buurman, P. 1980. Palaeosols in the Reading Beds (Palaeocene) of Alum Bay, Isle of Wight, U K. Sedimentology, Vol. 27, 593–606.

Carter, D J, and Hart, M B. 1977. Aspects of mid Cretaceous stratigraphical micro-palaeontology. Bulletin of the British Museum (Natural History) (Geology), Vol. 29, 1–135.

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

Chadwick, R A. 1993. Aspects of basin inversion in Southern Britain. Journal of the Geological Society of London, Vol. 150, 893–911.

Chadwick, R A, and Evans, D J. 2005. A seismic atlas of southern Britain – images of subsurface structure. Keyworth, Nottingham. British Geological Survey Occasional Publication, No. 7.

Edwards, R A, and Freshney, E C. 1987. Lithostratigraphical classification of the Hampshire Basin Palaeogene Deposits (Reading Formation to Headon Formation). Tertiary Research, Vol. 8, 43–73.

Ellison, R A, Knox, R WO’B, Jolley, D W, and King, C. 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists’ Association, Vol. 105, 187–197.

Farrant, A R. 1999. Geology of the Kings Somborne–Winchester district, Hampshire. British Geological Survey Technical Report, WH/99/06.

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

Farrant, A R, Hopson P M, Booth K A, and Aldiss D T. 2001. Geology of the Bourne River Catchment, Wiltshire. British Geological Survey Internal Report, I R/01/157.

Gradstein, F M, Ogg J G, and Smith A G. 2004. A Geologic Time Scale. (Cambridge: Cambridge University Press.)

Hart, M B, Bailey H W, Crittenden, S, Fletcher, B N, Price, R J, and Swiecicki, A. 1989. Cretaceous. 273–371 in Stratigraphical atlas of fossil foraminifera (second edition). Jenkins, D G, and Murray, J W (editors). (Chicester: Ellis Harwood.)

Hopson, P M. 2000. The geology of the area around North Tidworth, Ludgershall, Netheravon, Tidcombe and Porton Down, Wiltshire and west Hampshire. British Geological Survey Technical Report, WA/00/23.

Hopson, P M. 2005. A Stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the U K Offshore Sector. British Geological Survey Research Report, RR/05/01.

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, 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. 1905. The Geology of the country south and east of Devizes. Memoir of the Geological Survey of the United Kingdom, Sheet 282 (England and Wales).

Jukes-Browne, A J. 1908. The geology of the country around andover. Memoir of the Geological Survey of Great Britain, Sheet 283 (England and Wales).

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.

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.

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. 1987. Upper Cretaceous Chalk in the North and South Downs, England: a correlation. Proceedings of the Geologists’ Association, Vol.98, 77–86.

Mortimore, R N. 1993. Chalk water and engineering geology. 65–92 in The hydrogeology of the Chalk of north-west Europe. Downing, R A, Price, M, and Jones, G P (editors). (Oxford: Clarendon Press.)

Mortimore, R N, and Pomerol, B. 1987. Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo- Paris Basin. Proceedings of the Geologists’ Association, Vol. 98, 97–143.

Mortimore, R N, and Pomerol, B. 1991. Upper Cretaceous tectonic disruptions in a placid chalk sequence in the Anglo-Paris Basin. Journal of the Geological Society of London, Vol. 148, 391–404.

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

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

Smith, N J P. 1985. Pre-Permian Geology of the United Kingdom (South Sheet) (Keyworth: British Geological Survey.)

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

Swiecicki, A. 1980. A foraminiferal biostratigraphy of the Campanian and Maastrichtian Chalks of the United Kingdom. Unpublished PhD thesis, Plymouth Polytechnic.

Whittaker, A, Holliday, D W H, and Penn, I EP. 1985. Geophysical logs in British Stratigraphy. Special Report of the Geological Society of London, No. 18.

Wilkinson, I P. 2000a. Late Cretaceous Foraminiferal biostratigraphy of the Hampshire Basin (50K sheets 282, 283, 298 and 299). British Geological Survey Technical Report, WH/00/43R.

Wilkinson, I P. 2000b. Biostratigraphical analysis of the calcareous microfaunas from the Netheravon Borehole and related sequences. British Geological Survey Technical Report, WH/00/43R.

Wilkinson, I P. 2004a. Biostratigraphical analysis of a suite of Chalk samples from the Devizes district. British Geological Survey Internal Report, I R/04/146.

Wilkinson, I P. 2004b. Foraminiferal biostratigraphy of the Seaford Chalk in the Devizes district. British Geological Survey Internal Report, I R/04/148.

Wilkinson, I P. 2004c. Chalk biostratigraphy around Tilshead and Larkhill, Salisbury Plain M OD training area, Wiltshire. British Geological Survey Internal Report, I R/04/156.

Wilkinson, I P. 2006. Biostratigraphy of a suite of samples from the Devizes area. British Geological Survey Internal Report, I R/06/003.

Woods, M A. 1999a. Preliminary report on chalk macrofossils from the Salisbury, Devizes, andover and Winchester districts. British Geological Survey Technical Report, WH/99/138R.

Woods, M A. 1999b. Chalk macrofossils from the Devizes (282), andover (283), Salisbury (298) and Winchester (299) districts. British Geological Survey Technical Report, WH/99/142R.

Northern Ireland and the U K Offshore Sector. British Geological Survey Research Report, RR/05/01.